An understudied variable that could potentially alter insulin signaling during metabolic disease is the binding of proinsulin to IR-A and its possible downstream effect. In fact, it is still not clear if this binding results in a signal transduction, or if the IR acts as a "sponge" for proinsulin resulting in a decrease in insulin binding. This aspect could play an important role during T2DM development, where the proinsulin/insulin ratio is increased in plasma [37].

In summary, much work needs to be done to understand basic questions regarding the IR isoform expression and signaling in specific cell types and during disease. However, essential tools to discriminate between the two IR isoforms are lacking and critical to address these important issues and provide breakthroughs in the field.

#### **6.3 The need for new tools**

Up until now, measuring IR isoform mRNA has been useful to investigate the isoforms at tissue level, and more recently at cellular resolution *in situ*. However, IR mRNA expression might not be linked to actual protein levels in cells and tissue [70]. One intriguing future possibility to identify the existence of IR isoforms in single cells would be to utilize single-cell RNA sequencing (RNA-Seq). However, given the low levels of IR mRNA expression and that only a few base-pairs distinguish the two isoforms, this method is still technically challenging but probably possible in the near future [109].

Pivotal to understanding the dynamics and the regulation of the equilibrium of the IR isoforms (and therefore signaling) in health and disease is to develop tools with the possibility to visualize the amount and localization of the mature isoform receptors. Up to now, attempts to develop an isoform-specific antibody for Western blotting or immunostaining have failed, probably due to the small difference between the two denatured proteins and to the fact that the 12 differential amino acids reside in a poorly accessible area of the receptor. Means to visualize the IR isoform in live cells would allow studying the binding of ligands to the IR isoforms at the cell membrane. Additionally, other small molecules such as aptamers, nucleotides oligomers that could differentially bind to the IR isoforms could conjugate with fluorescent dyes and allow the visualization of endogenous IR receptor monomers or dimers in living cells. Such visualization would allow to track the receptors *in situ* and understand more about their intracellular dynamics during health and disease. That aptamers can block IR-mediated signaling in an isoform-specific manner *in vitro* has been demonstrated [106] and shows the potential use of these molecules for IR isoform detection.

Alternatively, the results of overexpression studies can be useful in understanding the possible segregation of the IRs due to different lipid membrane requirements, and possibly signaling [110]. However, a spill over of signaling and changes in IR distribution could take place due to overexpression and the results would need to be confirmed based on the endogenous receptor.

#### **6.4 Therapeutic potential**

Let us imagine having the available methods to visualize and measure the IR at the protein level as well as the cellular distribution of IR-A/IR-B monomers, dimers and hybrids receptors. With these tools it would be possible to study which signals are transduced by the IR isoforms in a determined cell type and in response to insulin, proinsulin, IGF1 and IGF2. In this scenario, new discoveries could be used to develop targeted treatments for metabolic diseases. For example, we could screen for selective activators and inhibitors of the different IR isoforms by measuring downstream activation of selective pathways in specific healthy or diseased cells.

We have discussed the necessity of being able to understand the specific signaling cascades initiated by the different receptors upon ligand binding. The two IR isoforms could be considered as targets to selectively increase or decrease signaling pathways driven by one of the two receptors in specific cell types. Focusing on diabetes, and provided that signaling pathways downstream of IR-B are the ones modulating glucose metabolism, the generation of selective IR-B activators could be beneficial in comparison to the current insulin sensitizers or insulin analogues, which do not target either isoform specifically. On the contrary, selectively targeting IR-A, using specific antibodies or other therapeutic molecules, could be directed to treat tumor proliferation by blunting its mitogenic signaling pathways and thus hamper cell growth and survival. Newly developed insulin analogues that preferentially bind one or the other IR isoform [111] could be pivotal for improving insulin therapy, but to define this feature we would need to have tools to measure selective downstream signaling in specific cells and tissues to understand the potency and effect of such molecules. Not only peptides could be used to modulate the IR isoform activity in a selective manner, but other small molecules such as aptamers could also prove useful. One promising publication showed that these molecules can be IR isoform-specific and can facilitate or block the signaling selectively, working as allosteric regulators [112].

Another intriguing therapeutic possibility to modulate IR splice isoforms would be to use a selective splice switcher, which has recently been tested (reviewed in [113]). This would also be a better strategy to study the IR isoforms *in vitro*, since the overexpression of specific isoforms might produce a spill over of signaling cascades, as explained above.

#### **7. Conclusions**

In this chapter, we aimed to summarize the state of the art research involving the IR isoforms, especially in the area of metabolic disease. We also wanted to draw attention to how important it is to understand the full implications of having two IR isoforms. The majority of the research focused on insulin signaling refers to the IR as one receptor, without considering that there are two structurally and functionally distinct isoforms in play. In our view, future research in this field would benefit from a focus on cell type-specific IR isoform signal transduction pathways, what differences there may be between cells of the same type but different localization in a tissue, and finally, what changes occur during disease. The development of the tools needed to address these questions would pave the way for important breakthroughs in comprehending the ubiquitous, but diverse, IR signaling. Finally, these tools will be essential for the development and testing of new therapeutic strategies to counteract diseases affected by IR signaling dysfunction.

#### **Acknowledgements**

We would like to thank Ingo and Barbara Leibiger for the critical reading of this chapter and their contribution as part of the Per-Olof Berggren lab in the field of IR isoforms in pancreatic islets.

#### **Conflicts of interest**

The authors declare no conflict of interest.

*Insulin Receptor Isoforms in Physiology and Metabolic Disease DOI: http://dx.doi.org/10.5772/intechopen.103036*

### **Author details**

Noah Moruzzi\* and Francesca Lazzeri-Barcelo The Rolf Luft Research Center for Diabetes and Endocrinology, Department of Molecular Medicine and Surgery, Karolinska Institute, Stockholm, Sweden

\*Address all correspondence to<EMAIL_ADDRESS>
© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **References**

[1] Kimura KD, Tissenbaum HA, Liu Y, et al. daf-2, an insulin receptor-like gene that regulates longevity and diapause in *Caenorhabditis elegans*. Science. 1997;**277**:942-946

[2] Abbott AM, Bueno R, Pedrini MT, et al. Insulin-like growth factor I receptor gene structure. The Journal of Biological Chemistry. 1992;**267**:10759-10763

[3] Renteria ME, Gandhi NS, Vinuesa P, et al. A comparative structural bioinformatics analysis of the insulin receptor family ectodomain based on phylogenetic information. PLoS One. 2008;**3**:e3667

[4] Hernandez-Sanchez C, Mansilla A, de Pablo F, et al. Evolution of the insulin receptor family and receptor isoform expression in vertebrates. Molecular Biology and Evolution. 2008;**25**: 1043-1053

[5] Seino S, Seino M, Nishi S, et al. Structure of the human insulin receptor gene and characterization of its promoter. Proceedings of the National Academy of Sciences of the United States of America. 1989;**86**:114-118

[6] Ebina Y, Ellis L, Jarnagin K, et al. The human insulin receptor cDNA: The structural basis for hormone-activated transmembrane signalling. Cell. 1985;**40**:747-758

[7] Ullrich A, Bell JR, Chen EY, et al. Human insulin receptor and its relationship to the tyrosine kinase family of oncogenes. Nature. 1985;**313**:756-761

[8] De Meyts P, Whittaker J. Structural biology of insulin and IGF1 receptors: Implications for drug design. Nature Reviews. Drug Discovery. 2002;**1**:769-783 [9] Alarcon C, Cheatham B, Lincoln B, et al. A Kex2-related endopeptidase activity present in rat liver specifically processes the insulin proreceptor. The Biochemical Journal. 1994;**301**(Pt 1):257-265

[10] Gorden P, Arakaki R, Collier E, et al. Biosynthesis and regulation of the insulin receptor. The Yale Journal of Biology and Medicine. 1989;**62**:521-531

[11] Bravo DA, Gleason JB, Sanchez RI, et al. Accurate and efficient cleavage of the human insulin proreceptor by the human proprotein-processing protease furin. Characterization and kinetic parameters using the purified, secreted soluble protease expressed by a recombinant baculovirus. The Journal of Biological Chemistry. 1994;**269**:25830-25837

[12] Bailyes EM, Nave BT, Soos MA, et al. Insulin receptor/IGF-I receptor hybrids are widely distributed in mammalian tissues: Quantification of individual receptor species by selective immunoprecipitation and immunoblotting. The Biochemical Journal. 1997;**327**(Pt 1):209-215

[13] Yamaguchi Y, Flier JS, Benecke H, et al. Ligand-binding properties of the two isoforms of the human insulin receptor. Endocrinology. 1993;**132**:1132-1138

[14] Treadway JL, Morrison BD, Goldfine ID, et al. Assembly of insulin/insulin-like growth factor-1 hybrid receptors in vitro. The Journal of Biological Chemistry. 1989;**264**:21450-21453

[15] Frattali AL, Pessin JE. Relationship between alpha subunit ligand occupancy and beta subunit autophosphorylation in insulin/insulin-like growth

*Insulin Receptor Isoforms in Physiology and Metabolic Disease DOI: http://dx.doi.org/10.5772/intechopen.103036*

factor-1 hybrid receptors. The Journal of Biological Chemistry. 1993;**268**:7393-7400

[16] Hubbard SR, Wei L, Ellis L, et al. Crystal structure of the tyrosine kinase domain of the human insulin receptor. Nature. 1994;**372**:746-754

[17] Lou M, Garrett TP, McKern NM, et al. The first three domains of the insulin receptor differ structurally from the insulin-like growth factor 1 receptor in the regions governing ligand specificity. Proceedings of the National Academy of Sciences of the United States of America. 2006;**103**:12429-12434

[18] Croll TI, Smith BJ, Margetts MB, et al. Higher-resolution structure of the human insulin receptor ectodomain: Multi-modal inclusion of the insert domain. Structure. 2016;**24**:469-476

[19] Scapin G, Dandey VP, Zhang Z, et al. Structure of the insulin receptor-insulin complex by single-particle cryo-EM analysis. Nature. 2018;**556**:122-125

[20] Uchikawa E, Choi E, Shang G, et al. Activation mechanism of the insulin receptor revealed by cryo-EM structure of the fully liganded receptorligand complex. eLife. 2019;**8**:e48630

[21] Gutmann T, Schafer IB, Poojari C, et al. Cryo-EM structure of the complete and ligand-saturated insulin receptor ectodomain. The Journal of Cell Biology. 2020;**219**(1):e201907210

[22] Mynarcik DC, Yu GQ, Whittaker J. Alanine-scanning mutagenesis of a C-terminal ligand binding domain of the insulin receptor alpha subunit. The Journal of Biological Chemistry. 1996;**271**:2439-2442

[23] Whittaker J, Whittaker L. Characterization of the functional insulin binding epitopes of the full-length insulin receptor. The Journal of Biological Chemistry. 2005;**280**:20932-20936

[24] Kristensen C, Wiberg FC, Andersen AS. Specificity of insulin and insulin-like growth factor I receptors investigated using chimeric minireceptors. Role of C-terminal of receptor alpha subunit. The Journal of Biological Chemistry. 1999;**274**:37351-37356

[25] Kristensen C, Andersen AS, Ostergaard S, et al. Functional reconstitution of insulin receptor binding site from non-binding receptor fragments. The Journal of Biological Chemistry. 2002;**277**:18340-18345

[26] Menting JG, Ward CW, Margetts MB, et al. A thermodynamic study of ligand binding to the first three domains of the human insulin receptor: Relationship between the receptor alpha-chain C-terminal peptide and the site 1 insulin mimetic peptides. Biochemistry. 2009;**48**:5492-5500

[27] Menting JG, Whittaker J, Margetts MB, et al. How insulin engages its primary binding site on the insulin receptor. Nature. 2013;**493**:241-245

[28] Hao C, Whittaker L, Whittaker J. Characterization of a second ligand binding site of the insulin receptor. Biochemical and Biophysical Research Communications. 2006;**347**:334-339

[29] Menting JG, Lawrence CF, Kong GK, et al. Structural congruency of ligand binding to the insulin and insulin/type 1 insulin-like growth factor hybrid receptors. Structure. 2015;**23**:1271-1282

[30] Whittaker J, Sorensen H, Gadsboll VL, et al. Comparison of the functional insulin binding epitopes of the A and B isoforms of the insulin receptor.

The Journal of Biological Chemistry. 2002;**277**:47380-47384

[31] Mosthaf L, Grako K, Dull TJ, et al. Functionally distinct insulin receptors generated by tissue-specific alternative splicing. The EMBO Journal. 1990; **9**:2409-2413

[32] Kellerer M, Lammers R, Ermel B, et al. Distinct alpha-subunit structures of human insulin receptor A and B variants determine differences in tyrosine kinase activities. Biochemistry. 1992;**31**:4588-4596

[33] Yamaguchi Y, Flier JS, Yokota A, et al. Functional properties of two naturally occurring isoforms of the human insulin receptor in Chinese hamster ovary cells. Endocrinology. 1991;**129**:2058-2066

[34] McClain DA. Different ligand affinities of the two human insulin receptor splice variants are reflected in parallel changes in sensitivity for insulin action. Molecular Endocrinology. 1991;**5**:734-739

[35] Frasca F, Pandini G, Scalia P, et al. Insulin receptor isoform A, a newly recognized, high-affinity insulin-like growth factor II receptor in fetal and cancer cells. Molecular and Cellular Biology. 1999;**19**:3278-3288

[36] Sacco A, Morcavallo A, Pandini G, et al. Differential signaling activation by insulin and insulin-like growth factors I and II upon binding to insulin receptor isoform A. Endocrinology. 2009;**150**:3594-3602

[37] Malaguarnera R, Sacco A, Voci C, et al. Proinsulin binds with high affinity the insulin receptor isoform A and predominantly activates the mitogenic pathway. Endocrinology. 2012;**153**:2152-2163

[38] Slaaby R, Schaffer L, Lautrup-Larsen I, et al. Hybrid receptors formed by insulin receptor (IR) and insulin-like growth factor I receptor (IGF-IR) have low insulin and high IGF-1 affinity irrespective of the IR splice variant. The Journal of Biological Chemistry. 2006;**281**:25869-25874

[39] Slaaby R. Specific insulin/IGF1 hybrid receptor activation assay reveals IGF1 as a more potent ligand than insulin. Scientific Reports. 2015;**5**:7911

[40] Benyoucef S, Surinya KH, Hadaschik D, et al. Characterization of insulin/IGF hybrid receptors: Contributions of the insulin receptor L2 and Fn1 domains and the alternatively spliced exon 11 sequence to ligand binding and receptor activation. The Biochemical Journal. 2007;**403**:603-613

[41] Pandini G, Frasca F, Mineo R, et al. Insulin/insulin-like growth factor I hybrid receptors have different biological characteristics depending on the insulin receptor isoform involved. The Journal of Biological Chemistry. 2002;**277**:39684-39695

[42] Vogt B, Carrascosa JM, Ermel B, et al. The two isotypes of the human insulin receptor (HIR-A and HIR-B) follow different internalization kinetics. Biochemical and Biophysical Research Communications. 1991;**177**:1013-1018

[43] Leibiger B, Leibiger IB, Moede T, et al. Selective insulin signaling through A and B insulin receptors regulates transcription of insulin and glucokinase genes in pancreatic beta cells. Molecular Cell. 2001;**7**:559-570

[44] Uhles S, Moede T, Leibiger B, et al. Isoform-specific insulin receptor signaling involves different plasma membrane domains. The Journal of Cell Biology. 2003;**163**:1327-1337

*Insulin Receptor Isoforms in Physiology and Metabolic Disease DOI: http://dx.doi.org/10.5772/intechopen.103036*

[45] Uhles S, Moede T, Leibiger B, et al. Selective gene activation by spatial segregation of insulin receptor B signaling. The FASEB Journal. 2007; **21**:1609-1621

[46] Leibiger B, Moede T, Uhles S, et al. Insulin-feedback via PI3K-C2alpha activated PKBalpha/Akt1 is required for glucose-stimulated insulin secretion. The FASEB Journal. 2010;**24**:1824-1837

[47] Mitrofanova A, Mallela SK, Ducasa GM, et al. SMPDL3b modulates insulin receptor signaling in diabetic kidney disease. Nature Communications. 2019;**10**:2692

[48] Mosthaf L, Vogt B, Haring HU, et al. Altered expression of insulin receptor types A and B in the skeletal muscle of non-insulin-dependent diabetes mellitus patients. Proceedings of the National Academy of Sciences of the United States of America. 1991;**88**:4728-4730

[49] Moller DE, Yokota A, Caro JF, et al. Tissue-specific expression of two alternatively spliced insulin receptor mRNAs in man. Molecular Endocrinology. 1989;**3**:1263-1269

[50] Seino S, Bell GI. Alternative splicing of human insulin receptor messenger RNA. Biochemical and Biophysical Research Communications. 1989;**159**:312-316

[51] Sesti G, Tullio AN, D'Alfonso R, et al. Tissue-specific expression of two alternatively spliced isoforms of the human insulin receptor protein. Acta Diabetologica. 1994;**31**:59-65

[52] Louvi A, Accili D, Efstratiadis A. Growth-promoting interaction of IGF-II with the insulin receptor during mouse embryonic development. Developmental Biology. 1997;**189**:33-48

[53] Pomytkin I, Costa-Nunes JP, Kasatkin V, et al. Insulin receptor in the brain: Mechanisms of activation and the role in the CNS pathology and treatment. CNS Neuroscience & Therapeutics. 2018;**24**:763-774

[54] Phy JL, Conover CA, Abbott DH, et al. Insulin and messenger ribonucleic acid expression of insulin receptor isoforms in ovarian follicles from nonhirsute ovulatory women and polycystic ovary syndrome patients. The Journal of Clinical Endocrinology and Metabolism. 2004;**89**:3561-3566

[55] Neuvians TP, Gashaw I, Hasenfus A, et al. Differential expression of IGF components and insulin receptor isoforms in human seminoma versus normal testicular tissue. Neoplasia. 2005;**7**:446-456

[56] Vella V, Milluzzo A, Scalisi NM, et al. Insulin receptor isoforms in cancer. International Journal of Molecular Sciences. 2018;**19**:3615

[57] Muller D, Huang GC, Amiel S, et al. Identification of insulin signaling elements in human beta-cells: Autocrine regulation of insulin gene expression. Diabetes. 2006;**55**:2835-2842

[58] Vienberg SG, Bouman SD, Sorensen H, et al. Receptor-isoformselective insulin analogues give tissuepreferential effects. The Biochemical Journal. 2011;**440**:301-308

[59] Norgren S, Arner P, Luthman H. Insulin receptor ribonucleic acid levels and alternative splicing in human liver, muscle, and adipose tissue: Tissue specificity and relation to insulin action. The Journal of Clinical Endocrinology and Metabolism. 1994;**78**:757-762

[60] Escribano O, Guillen C, Nevado C, et al. Beta-cell hyperplasia induced by

hepatic insulin resistance: Role of a liver-pancreas endocrine axis through insulin receptor A isoform. Diabetes. 2009;**58**:820-828

[61] Amessou M, Tahiri K, Chauvet G, et al. Age-related changes in insulin receptor mRNA and protein expression in genetically obese Zucker rats. Diabetes & Metabolism. 2010;**36**:120-128

[62] McGrattan PD, Wylie AR, Bjourson AJ. A partial cDNA sequence of the ovine insulin receptor gene: Evidence for alternative splicing of an exon 11 region and for tissue-specific regulation of receptor isoform expression in sheep muscle, adipose tissue and liver. The Journal of Endocrinology. 1998;**159**:381-387

[63] Besic V, Shi H, Stubbs RS, et al. Aberrant liver insulin receptor isoform a expression normalises with remission of type 2 diabetes after gastric bypass surgery. PLoS One. 2015;**10**:e0119270

[64] Hribal ML, Perego L, Lovari S, et al. Chronic hyperglycemia impairs insulin secretion by affecting insulin receptor expression, splicing, and signaling in RIN beta cell line and human islets of Langerhans. The FASEB Journal. 2003;**17**:1340-1342

[65] Kaminska D, Hamalainen M, Cederberg H, et al. Adipose tissue INSR splicing in humans associates with fasting insulin level and is regulated by weight loss. Diabetologia. 2014;**57**:347-351

[66] Malakar P, Chartarifsky L, Hija A, et al. Insulin receptor alternative splicing is regulated by insulin signaling and modulates beta cell survival. Scientific Reports. 2016;**6**:31222

[67] Norgren S, Zierath J, Galuska D, et al. Differences in the ratio of RNA encoding

two isoforms of the insulin receptor between control and NIDDM patients. The RNA variant without exon 11 predominates in both groups. Diabetes. 1993;**42**:675-681

[68] Sell SM, Reese D, Ossowski VM. Insulin-inducible changes in insulin receptor mRNA splice variants. The Journal of Biological Chemistry. 1994;**269**:30769-30772

[69] Muller D, Huang GC, Amiel S, et al. Gene expression heterogeneity in human islet endocrine cells in vitro: The insulin signalling cascade. Diabetologia. 2007;**50**:1239-1242

[70] Moruzzi N, Lazzeri-Barcelo F, Valladolid-Acebes I, et al. Tissue-specific expression of insulin receptor isoforms in obesity/type 2 diabetes mouse models. Journal of Cellular and Molecular Medicine. 2021;**25**:4800-4813

[71] Huang Z, Bodkin NL, Ortmeyer HK, et al. Hyperinsulinemia is associated with altered insulin receptor mRNA splicing in muscle of the spontaneously obese diabetic rhesus monkey. The Journal of Clinical Investigation. 1994;**94**:1289-1296

[72] Serrano R, Villar M, Martinez C, et al. Differential gene expression of insulin receptor isoforms A and B and insulin receptor substrates 1, 2 and 3 in rat tissues: Modulation by aging and differentiation in rat adipose tissue. Journal of Molecular Endocrinology. 2005;**34**:153-161

[73] Vidal H, Auboeuf D, Beylot M, et al. Regulation of insulin receptor mRNA splicing in rat tissues. Effect of fasting, aging, and diabetes. Diabetes. 1995;**44**:1196-1201

[74] Westermeier F, Salomon C, Farias M, et al. Insulin requires normal expression and signaling of insulin receptor A to

reverse gestational diabetes-reduced adenosine transport in human umbilical vein endothelium. The FASEB Journal. 2015;**29**:37-49

[75] Garwood CJ, Ratcliffe LE, Morgan SV, et al. Insulin and IGF1 signalling pathways in human astrocytes in vitro and in vivo; characterisation, subcellular localisation and modulation of the receptors. Molecular Brain. 2015;**8**:51

[76] Entingh AJ, Taniguchi CM, Kahn CR. Bi-directional regulation of brown fat adipogenesis by the insulin receptor. The Journal of Biological Chemistry. 2003;**278**:33377-33383

[77] Avnet S, Perut F, Salerno M, et al. Insulin receptor isoforms are differently expressed during human osteoblastogenesis. Differentiation. 2012;**83**:242-248

[78] Ziegler AN, Schneider JS, Qin M, et al. IGF-II promotes stemness of neural restricted precursors. Stem Cells. 2012;**30**:1265-1276

[79] Andres SF, Simmons JG, Mah AT, et al. Insulin receptor isoform switching in intestinal stem cells, progenitors, differentiated lineages and tumors: Evidence that IR-B limits proliferation. Journal of Cell Science. 2013;**126**:5645-5656

[80] Sciacca L, Prisco M, Wu A, et al. Signaling differences from the A and B isoforms of the insulin receptor (IR) in 32D cells in the presence or absence of IR substrate-1. Endocrinology. 2003;**144**:2650-2658

[81] Kosaki A, Webster NJ. Effect of dexamethasone on the alternative splicing of the insulin receptor mRNA and insulin action in HepG2 hepatoma cells. The Journal of Biological Chemistry. 1993;**268**:21990-21996

[82] Accili D, Drago J, Lee EJ, et al. Early neonatal death in mice homozygous for a null allele of the insulin receptor gene. Nature Genetics. 1996;**12**:106-109

[83] Okamoto H, Nakae J, Kitamura T, et al. Transgenic rescue of insulin receptor-deficient mice. The Journal of Clinical Investigation. 2004;**114**:214-223

[84] Lin HV, Accili D. Reconstitution of insulin action in muscle, white adipose tissue, and brain of insulin receptor knock-out mice fails to rescue diabetes. The Journal of Biological Chemistry. 2011;**286**:9797-9804

[85] Diaz-Castroverde S, Gomez-Hernandez A, Fernandez S, et al. Insulin receptor isoform A ameliorates longterm glucose intolerance in diabetic mice. Disease Models & Mechanisms. 2016;**9**:1271-1281

[86] Lopez-Pastor AR, Gomez-Hernandez A, Diaz-Castroverde S, et al. Liver-specific insulin receptor isoform A expression enhances hepatic glucose uptake and ameliorates liver steatosis in a mouse model of diet-induced obesity. Disease Models & Mechanisms. 2019;**12**((2):dmm036186

[87] Nevado C, Valverde AM, Benito M. Role of insulin receptor in the regulation of glucose uptake in neonatal hepatocytes. Endocrinology. 2006;**147**:3709-3718

[88] Diaz-Castroverde S, Baos S, Luque M, et al. Prevalent role of the insulin receptor isoform A in the regulation of hepatic glycogen metabolism in hepatocytes and in mice. Diabetologia. 2016;**59**:2702-2710

[89] Mosthaf L, Eriksson J, Haring HU, et al. Insulin receptor isotype expression correlates with risk of non-insulindependent diabetes. Proceedings of the National Academy of Sciences of the United States of America. 1993;**90**:2633-2635

[90] Kellerer M, Sesti G, Seffer E, et al. Altered pattern of insulin receptor isotypes in skeletal muscle membranes of type 2 (non-insulin-dependent) diabetic subjects. Diabetologia. 1993;**36**:628-632

[91] Benecke H, Flier JS, Moller DE. Alternatively spliced variants of the insulin receptor protein. Expression in normal and diabetic human tissues. The Journal of Clinical Investigation. 1992;**89**:2066-2070

[92] Sesti G, Marini MA, Tullio AN, et al. Altered expression of the two naturally occurring human insulin receptor variants in isolated adipocytes of non-insulin-dependent diabetes mellitus patients. Biochemical and Biophysical Research Communications. 1991;**181**:1419-1424

[93] Savkur RS, Philips AV, Cooper TA. Aberrant regulation of insulin receptor alternative splicing is associated with insulin resistance in myotonic dystrophy. Nature Genetics. 2001;**29**:40-47

[94] Savkur RS, Philips AV, Cooper TA, et al. Insulin receptor splicing alteration in myotonic dystrophy type 2. American Journal of Human Genetics. 2004;**74**:1309-1313

[95] Sen S, Talukdar I, Liu Y, et al. Muscleblind-like 1 (Mbnl1) promotes insulin receptor exon 11 inclusion via binding to a downstream evolutionarily conserved intronic enhancer. The Journal of Biological Chemistry. 2010;**285**:25426-25437

[96] Laustriat D, Gide J, Barrault L, et al. In vitro and in vivo modulation of alternative splicing by the Biguanide Metformin. Molecular Therapy--Nucleic Acids. 2015;**4**:e262

[97] Backdahl J, Franzen L, Massier L, et al. Spatial mapping reveals human adipocyte subpopulations with distinct sensitivities to insulin. Cell Metabolism. 2021;**33**:2301

[98] Halpern KB, Shenhav R, Matcovitch-Natan O, et al. Single-cell spatial reconstruction reveals global division of labour in the mammalian liver. Nature. 2017;**542**:352-356

[99] Haring HU, Kellerer M, Mosthaf L. Modulation of insulin receptor signalling: Significance of altered receptor isoform patterns and mechanism of hyperglycaemia-induced receptor modulation. Diabetologia. 1994;**37** (Suppl. 2):S149-S154

[100] Duarte N, Coelho IC, Patarrao RS, et al. How inflammation impinges on NAFLD: A role for Kupffer cells. BioMed Research International. 2015;**2015**:984578

[101] Heni M, Hennige AM, Peter A, et al. Insulin promotes glycogen storage and cell proliferation in primary human astrocytes. PLoS One. 2011;**6**:e21594

[102] Spencer B, Rank L, Metcalf J, et al. Identification of insulin receptor splice variant B in neurons by in situ detection in human brain samples. Scientific Reports. 2018;**8**:4070

[103] Taniguchi CM, Emanuelli B, Kahn CR. Critical nodes in signalling pathways: Insights into insulin action. Nature Reviews. Molecular Cell Biology. 2006;**7**:85-96

[104] Pollak M. The insulin and insulinlike growth factor receptor family in neoplasia: An update. Nature Reviews. Cancer. 2012;**12**:159-169

*Insulin Receptor Isoforms in Physiology and Metabolic Disease DOI: http://dx.doi.org/10.5772/intechopen.103036*

[105] Leibiger IB, Leibiger B, Berggren PO. Insulin feedback action on pancreatic beta-cell function. FEBS Letters. 2002;**532**:1-6

[106] Leibiger B, Moede T, Paschen M, et al. PI3K-C2alpha knockdown results in rerouting of insulin signaling and pancreatic beta cell proliferation. Cell Reports. 2015;**13**:15-22

[107] Brown MS, Goldstein JL. Selective versus total insulin resistance: A pathogenic paradox. Cell Metabolism. 2008;**7**:95-96

[108] Tan SX, Fisher-Wellman KH, Fazakerley DJ, et al. Selective insulin resistance in adipocytes. The Journal of Biological Chemistry. 2015;**290**:11337-11348

[109] Arzalluz-Luque A, Conesa A. Single-cell RNAseq for the study of isoforms-how is that possible? Genome Biology. 2018;**19**:110

[110] Gerdes JM, Christou-Savina S, Xiong Y, et al. Ciliary dysfunction impairs beta-cell insulin secretion and promotes development of type 2 diabetes in rodents. Nature Communications. 2014;**5**:5308

[111] Chrudinova M, Zakova L, Marek A, et al. A versatile insulin analog with high potency for both insulin and insulin-like growth factor 1 receptors: Structural implications for receptor binding. The Journal of Biological Chemistry. 2018;**293**:16818-16829

[112] Yunn NO, Koh A, Han S, et al. Agonistic aptamer to the insulin receptor leads to biased signaling and functional selectivity through allosteric modulation. Nucleic Acids Research. 2015;**43**:7688-7701

[113] Havens MA, Hastings ML. Spliceswitching antisense oligonucleotides as therapeutic drugs. Nucleic Acids Research. 2016;**44**:6549-6563

#### **Chapter 3**

## Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges

*Atamjit Singh, Nikhita Ghai and Preet Mohinder Singh Bedi*

#### **Abstract**

Insulin resistance (IR) is a condition in which insulin-mediated regulation of glucose metabolism in body tissues (primarily liver, adipose tissue and skeletal muscle) becomes disrupted. IR is a characteristic marker of type 2 diabetes and cardiovascular diseases. IR is generally associated with metabolic abnormalities, including hyperinsulinemia, impaired glucose homeostasis, hyperlipidemia and obesity. IR can arise from pathological, genetic and environmental factors or from a combination of these factors. Studies conducted in recent decades showcase the important role of adipose tissue in the development of IR via release of lipids and different circulating factors. These extracellular factors influence the intracellular levels of intermediates including ceramide and various lipids that influence the cell responsiveness to insulin. These intermediates are suggested to promote IR via inhibition of one or more components of insulin signaling pathway (e.g., insulin receptor, insulin receptor substrate proteins). This chapter will shed light on various molecular mechanisms and factors contributing to IR, which will help the researchers to design potential therapeutic strategies and interventions for efficiently managing IR and its related disorders.

**Keywords:** insulin, insulin receptor, insulin resistance, glucose uptake, glucose metabolism

#### **1. Introduction**

Following a meal, pancreatic β-cells produce insulin in response to increasing blood glucose and other metabolite levels for regulating systemic glucose homeostasis. Tissue insulin sensitivity, which characterizes the ability of a given concentration of insulin to correct blood glucose levels, is the driving force behind this homeostasis. Multiple processes in several organs are involved in this typically well-regulated homeostatic mechanism, including decreased glucose output from the liver (hepatic glucose output), increased glucose uptake into skeletal muscle and adipose tissue (where glucose is stored as glycogen), suppression of free fatty acid (FFA) release from adipocytes (suppression of lipolysis), and increased lipid accumulation in the liver and adipocytes. A sophisticated insulin-dependent signal transduction cascade controls these metabolic processes. Insulin resistance (IR) is defined as decreased insulin-stimulated glucose uptake into muscle and adipocytes and faulty insulin

regulation of hepatic glucose production in patients with type 2 diabetes (T2D) and in many subjects affected by other conditions characterized by insulin resistance, such as obesity and polycystic ovary syndrome. The term insulin resistance was first coined to explain the considerable variability in the insulin dose necessary to lower high glucose levels in people with T2D, and then to characterize the magnitude of change in blood glucose level when a given amount of insulin and glucose was administered. The "defined quantity of insulin" is crucial because people with insulin resistance often have hyperinsulinemia, a condition in which insulin levels in the blood are higher than normal relative to the amount of blood glucose concentration under both fasting and fed conditions; this hyperinsulinemia compensates for IR in peripheral tissues to bring blood glucose levels back to normal [1].

When pancreas fails to supply excess insulin in humans with insulin resistance, a major defect in whole-body glucose homeostasis occurs, resulting in hyperglycemia and glucose intolerance (the latter including impaired fasting glucose and impaired glucose tolerance), which are the defining features of T2D. It is worth noting that, somewhat counterintuitively, patients with T2D frequently maintain "relative hyperinsulinemia" until the condition is at an advanced stage. IR is defined by insulin's inability to induce glucose uptake into muscle and adipose cells due to a failure of the glucose transport mechanism mediated, at the molecular level, by glucose transporter type 4 (GLUT4) in those tissues. Furthermore, one of the hallmarks of IR is the inability to decrease hepatic glucose production, which is mostly due to a persistent increase in hepatic gluconeogenesis. IR has been linked to a variety of diseases. Indeed, IR represents a risk factor for various conditions, such as metabolic disorders (including T2D and obesity), heart disease, liver diseases (e.g., non-alcoholic fatty liver disease and non-alcoholic steatohepatitis), cancer, neurodegenerative diseases and frailty [2–4]. Despite the fact that IR is inextricably linked to T2D, an important factor involved in T2D pathophysiology is represented by the pancreas' incapacity to function properly to compensate for the significant rise in blood glucose levels by secreting enough insulin to meet the increasing demand and help get blood glucose levels back to normal. IR is a key risk factor for T2D, yet it is not commonly recognized or treated in people without diabetes. The main reason for this phenomenon is that many people with insulin resistance do not have abnormal blood glucose levels. Therefore, diagnosis of IR is based on measuring insulin levels, which is not commonly done in clinical practice. Furthermore, only a small fraction of subjects with IR develop T2D, which is likely due to a propensity to β-cell failure in these subjects. There are no procedures to identify this susceptible subpopulation at this time. Individuals with IR are predisposed to significant disorders linked to T2D, including retinopathy, neuropathy and kidney disease, even if they do not have T2D [5]. In this chapter, the association between the early possible causes of IR is first discussed. Obesity is common in people with IR, but it is unclear whether concomitant hyperinsulinemia contributes to obesity development or whether it is a consequence of obesity-associated IR. We then look at how different metabolic tissues, such as muscle, adipose tissue, and the liver, communicate with one another. The mechanisms of impaired insulin signaling and the role of abnormal GLUT4 trafficking in the development of IR are also discussed. Extracellular factors that may contribute to IR are postulated. This discussion is then followed by a discussion of various intracellular molecular factors that contribute to IR. These factors have been considered as involved in processes that lead to IR. There are several ways for determining insulin action. Many laboratories have lately resorted to employing surrogate markers of insulin sensitivity and IR [6].

The "traditional" definition of IR is a condition in which blood glucose levels are abnormally high and insulin concentration needed to maintain glucose homeostasis is greater than predicted [7, 8].

#### **2. Pathway to insulin resistance**

Despite years of research, there is still a lot of uncertainty about the causative and temporal link between obesity, hyperinsulinemia, and IR. The proximal and distal parts of the insulin signaling system, which governs metabolism, can be arbitrarily partitioned. The classical components—which comprise the insulin receptor, insulin receptor substrate (IRS) proteins, phosphoinositide 3-kinase (PI3K) and AKTconstitute the proximal segment of the insulin signaling system. A common trait of the proximal components is their sparseness, which means that just a little part of each element is necessary to elicit a physiological signal. This guarantees signal amplification across the network. The proximal portion is also susceptible to very complex feedforward and feedback control, and is incorporated into a broader network that is dynamically regulated by combinatorial signaling inputs. The AKT substrates that are intimately related to the many physiological activities of insulin and are typically specialized to a particular cell type are referred to as the "distal segment" of the insulin signaling pathway. The distal elements are generally phosphorylated, which is a common trait. Insulin signaling begins with the hormone binding to its surface receptor, followed by activation of the receptor tyrosine kinase and tyrosine phosphorylation. IRS proteins are phosphorylated, causing them to create a signaling complex, which contains proteins with Src homology domains such as PI3K. As a result, phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3 or PIP3] is produced. Serine/ threonine (Ser/Thr) protein kinases like PDK1 and AKT, for example, are recruited to the inner leaflet of the plasma membrane. AKT is phosphorylated by PDK1 at one of its phosphorylation sites. Partially phosphorylated AKT activates mTORC2, while phosphorylation of AKT specifically at Ser473 results in complete AKT activation. Thus, AKT is a critical node in the insulin signaling pathway. AKT performs a variety of biological roles and is involved in the majority, if not all, of physiological metabolic processes. The Rab GTPase-Activating Protein (GAP) is an AKT substrate, which activates TBC1D4 (TBC1 Domain Family Member 4), a protein that regulates GLUT4 trafficking within the plasma membrane. The activation of glucose transport by insulin is the key mechanism that is disrupted in insulin-resistant muscle and fat cells. The GLUT4 is a facilitative glucose transporter, which is found in skeletal muscle, heart, adipocytes, and insulin-responsive neurons; it regulates muscle/fat glucose transfer. Unlike other transporters (like GLUT1), GLUT4 has a set of specific trafficking cues that let it migrate from endosomes and the trans-Golgi network (TGN) to a special intracellular population of vesicles known as "GLUT4 storage vesicles" (GSVs) [9–11]. GSVs act as a distinct controlled exocytic compartment that distributes GLUT4 to the cell surface in response to insulin and serves as a storage depot assuring low rates of glucose absorption in the fasting state. Although exercise increases GLUT4 translocation in muscle cells, it does so through a different mechanism than that regulated by insulin. AKT plays a critical role in the insulin-regulated GLUT4 translocation [12, 13]. These characteristics typically coexist, and there is strong evidence that each can cause the other two branches of the triad to emerge: obesity, hyperinsulinemia, and IR are caused by overnutrition in humans and animals; in humans, IR and obesity may also be caused by continuous insulin administration or by genetic factors; in

addition, IR in humans may be caused by pharmacological interventions resulting in hyperinsulinemia [14, 15].

#### **3. The trio-axis of obesity-hyperinsulinemia-insulin resistance**

Obesity and IR are two topics that come up frequently. The long period during which obesity, IR and hyperinsulinemia develop, makes the determination of causative links between these conditions (which usually coexist in most persons with T2D at the time of diagnosis) particularly difficult. Obesity is common in people with IR, although it is unclear whether simultaneous hyperinsulinemia plays a role in obesity development or it is predominantly a result of obesity-dependent IR [16]. The study of first-degree relatives of people with T2D who only show some of these traits has shown to be one of the most effective strategies for addressing some unanswered questions in humans. As a result, a trait seen in relatives is more likely to appear early in the course of the disease. It has been found that these subjects can have considerable IR in skeletal muscle and liver (and possibly fat), along with modest hyperinsulinemia, even if they are not obese or glucose-intolerant [15, 17, 18]. Individuals who are lean and glucose-tolerant but exhibit IR have been identified in larger crosssectional studies [19]. In these instances, obesity is unlikely to be the primary cause of tissue IR. However, the term "obesity" is defined differently depending on race and genetic background, and it should therefore used with caution. Body mass index may be more important in determining the risk of IR. Body weight, in general, and visceral fat (but not subcutaneous fat), in particular, should be considered for evaluation [20].

First-degree relatives of people with T2D had greater levels of circulating FFAs and intramuscular lipids than healthy control subjects [21], suggesting that intramyocellular lipid content represents an early abnormality in the pathogenesis of insulin resistance and that it may contribute to the impaired glucose uptake in skeletal muscle of insulin-resistant subjects to a greater extent than overall adiposity. This is in line with severe IR observed in patients with lipodystrophy syndromes, which are a heterogeneous group of diseases characterized by selective absence of adipose tissue, loss of functional adipocytes, ectopic steatosis, and severe dyslipidemia and IR [22, 23]. On the other hand, individuals with moderate or severe obesity can be "metabolically healthy" [24]. While it appears acceptable, based on this research, to conclude that obesity is not a risk factor essential for the development of IR, it is vital to highlight that the majority of subjects with IR are obese. As we will discuss later in the text, the amount and location of adiposity required to create IR varies greatly between subjects.

#### **4. Insulin resistance and hyperinsulinemia**

Defining the temporal link between hyperinsulinemia and IR is difficult since, as far as we know, IR does not exist in the absence of hyperinsulinemia in humans, and vice versa. Hyperinsulinemia can produce obesity and IR in humans, as evidenced by trials in which insulin is administered to induce hyperinsulinemia in otherwise healthy individuals or as it occurs naturally in people with insulinomas [25, 26]. Transgenic expression of multiple copies of the normal insulin gene causes hyperinsulinemia in mice, resulting in IR and glucose intolerance [27]. Inhibition of insulin secretion has also been shown to improve insulin sensitivity and to decrease body

#### *Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

weight in rodents [28–30]. In mice, deletion of one copy of the insulin gene resulted in a reduction of the Western diet-induced hyperinsulinemia and in an improvement of insulin sensitivity [31]. Overall, the hypotheses that hyperinsulinemia causes IR and promotes obesity, or that IR associated with obesity causes hyperinsulinemia, both remain acceptable for the initial events involved in T2D pathophysiology. In actuality, IR and hyperinsulinemia coexist and lead to T2D in almost all cases [32]. Several data suggest a concept in which hyperinsulinemia is responsible for, or at least partly contributes to, many of the negative effects of IR; this implies that IR is a state in which many of the insulin actions are preserved, a condition known as "selective IR" [33–35]. This was first observed in the liver, where increased insulin levels are unable to decrease hepatic glucose output in people with T2D, although lipogenesis (a canonical insulin action in the liver) remains elevated [36, 37]. One explanation for this selectivity is that insulin signaling pathway in the liver splits into two arms, with IR affecting only the arm regulating hepatic gluconeogenesis but not the arm regulating lipid metabolism. Hepatic de novo lipogenesis is essentially a cell-autonomous phenomenon, whereas cell-nonautonomous suppression of hepatic glucose production by insulin depends upon the insulin-mediated decrease of adipocyte lipolysis and circulating FFAs [38]. There has also been evidence of selective IR in muscle and adipose tissue. Those insulin-regulated activities which are not affected by IR—such as lipogenesis, protein synthesis, or transcriptional control mediated by FOXO proteins—are hyperactivated in the context of hyperinsulinemia and are likely to worsen IR or its consequences [33, 34, 39, 40].

#### **5. Heterogeneity in the development of insulin resistance and progression of metabolic disease and T2D**

T2D patients are divided into different phenotypic clusters based on their symptoms and clinical features. Individuals in one of these groups share phenotypic traits. As a result, performing a comprehensive analysis of these groups will be of great importance in clinical settings. Phenotype data analysis and combination of phenotype data with genetic data are essential to gain a better understanding of the variability in the development and presentation of IR in humans [10, 11, 41].

#### **6. Tissue-specific progression to insulin resistance**

The appearance of IR occurs in various tissues in a specific order. The development of IR in several tissues—including skeletal muscle, liver, and adipose tissue—is a hallmark of fully developed T2D in humans [18, 19, 39, 40, 42]. Evidence shows a hierarchical progression of IR in skeletal muscle, liver and adipose tissue, whereby IR develops in one tissue and then spreads to other tissues via systemic circulating components. For example, IR in the liver and adipose tissue appears to occur prior to IR in muscle in C57Bl/6 mice fed a high-fat diet [43–46]. An equivalent pattern in humans is unlikely, since first-degree relatives of persons with T2D who are in the early stages of the disease already have IR in both muscle and liver (and possibly fat) [47]. Since insulin sensitivity in humans is often measured as whole-body glucose consumption (to which adipose tissue contributes only to a small extent), the temporal development of IR in adipose tissue in humans is less obvious. Interestingly, multiple investigations show that insulin modulates hepatic glucose production via

reducing adipocyte lipolysis in a non-cell-autonomous manner [45]. Given these findings, it is reasonable to believe that adipose tissue IR is a precursor to metabolic disease and T2D. However, there is a clear distinction between insulin action on the liver and insulin action on muscle: even in people with T2D, the defect in insulin sensitivity in the liver can be almost completely overcome by sufficiently high levels of insulin, whereas muscle (and fat) insulin sensitivity defects persist at higher insulin concentrations [40, 48, 49]. This indicates that the processes that cause IR in muscle and liver are distinct.

Tissue-specific insulin receptor gene knockouts in mice have provided persuasive evidence that IR in a particular tissue can at least spread to other organs. Experimenting with a specific deficiency in insulin action in muscle, fat, or liver has resulted in the spread of IR to other tissues in a number of cases [50]. However, depending on the tissue that is first targeted and/or in which a specific gene deletion occurs, the mechanism of inter-tissue communication varies. The deletion of GLUT4, which is essential for glucose uptake in adipose tissue and skeletal muscle, is one of the best examples of this inter-tissue communication. In mice, deletion of GLUT4 resulted in IR not only in the tissue from which the transporter was removed, but also in all metabolic tissues, including the liver. Surprisingly, normalization of blood glucose levels reverses IR in the liver and adipose tissue in muscle-specific Glut4 gene-knockout mice. This shows that glucotoxicity generated IR in this animal model, which is not the case in many other IR models, including the Western diet-fed C57BL/6J mice, which do not show considerable hyperglycemia [48–51]. As a result, while these animal studies have been useful in uncovering mechanisms of IR in specific tissues, their clinical applicability is less evident because complete deletion of a gene preferentially in one tissue does not occur in humans. Nonetheless, these experiments have provided persuasive evidence that metabolic or signaling changes in one tissue can have systemic effects by influencing insulin activity in other organs, a phenomenon that has been well-validated by clinical findings [50, 51].

#### **7. Impaired insulin signaling in insulin resistance**

Over the past 40 years, much research has resulted in a precise understanding of the insulin signaling system, which mediates the insulin's physiological activities. One popular theory is that IR is caused by a defect in one or more of these signaling components. Another viewpoint is that IR is only caused by a shift in metabolic flux. For example, since the 1960s fatty acids have been proven to impede cells' ability to utilize carbohydrate by allosterically modifying crucial rate-limiting steps in carbohydrate metabolic pathways. Several pieces of evidence, however, refute this claim. IR can be seen in cells or tissues long after the animal tissues have been removed, implying that changes that contribute to IR are long-lasting and cannot be explained by the acute action of a systemic factor. Fatty acids decrease the insulin-dependent translocation of GLUT4 to the plasma membrane and limit glucose uptake, there is no indication that this inhibition is caused by an allosteric change of GLUT4. Finally, IR can persist even after significant changes in dietary intake and after changes in metabolic state induced by pharmacological interventions. Thus, based on this information, it is reasonable to believe that IR is caused by an alteration in insulin signaling, although the exact location of the defect in the insulin signaling pathway remains unknown. Many essential components of the insulin signaling system have been identified. These components are divided into two parts: (i) the proximal part, which represents

#### *Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

the core canonical signaling pathway, which includes the insulin receptor, IRS, PI3K and AKT; and (ii) the distal part, which includes TBC1D4, GSK3 (glycogen synthase kinase-3) and PDE3B (phosphodiesterase 3B). IR has been linked to defects in proximal insulin signaling system, that are associated with cellular stress. Many of the intracellular stressors discussed in the next sections activate a variety of intracellular Ser/Thr kinases, including novel PKCs (protein kinase C), JNK (c-Jun amino-terminal kinase), mTOR (mammalian target of rapamycin)and S6 kinase, which phosphorylate either the insulin receptor or the insulin receptor-related protein (INSRR). This could be a negative-feedback route that inhibits insulin signaling, according to the theory. However, as it will be discussed later, mounting evidence suggests that proximal insulin signaling system is unaffected in IR, implying that IR is caused by abnormalities in distal components of the insulin signaling network [52–54].

#### **8. Insulin resistance and insulin signaling at the proximal level**

The current focus on proximal insulin signaling abnormalities as a cause of IR stems from research into rare, monogenic severe types of IR that were discovered to be caused by mutations in the insulin receptor gene or by the development of insulin receptor blocking antibodies. Because of the superficial parallels between these rare conditions and T2D, it is reasonable to conclude that both diseases are caused by abnormalities in insulin receptor function, with the degree of receptor failure varying only slightly. Despite early enthusiasm for this theory, subsequent research found that IR in most forms of T2D was caused by neither impaired insulin receptor activity nor changes in the expression or quantity of insulin receptors. Insulin-binding experiments in rat adipocytes found that only 2.4% of total insulin receptors are required for a full biological response, implying that metabolic cells like muscle, fat and liver cells have an abundance of insulin receptors; this finding became known as the "spare insulin receptor" hypothesis. Insulin-mediated glucose uptake is reduced in insulinresistant skeletal muscle cells and adipocytes. Since a slight decrease in the number of insulin receptors could only diminish insulin sensitivity and not the maximal insulin response [54–58]. While some studies contradict the "spare insulin receptor" hypothesis, recent genetic studies in mice support the idea that insulin signaling is preserved when the number of insulin receptors is reduced: mice with heterozygous loss of the insulin receptor had normal glucose and insulin tolerance and no impairment in AKT signaling in muscle or adipose tissue [59–62].

The concept of spare insulin receptors shifted focus to a "postreceptor defect", which is represented by defects in signaling downstream intermediates of the insulin receptor as the cause of IR [57, 58, 63]. Loss-of-function mutations in a number of signaling genes—including TBC1D4, AKT2, and IRS1 in humans—have been linked to severe forms of IR and T2D; moreover, cancer treatments that block PI3K or AKT have been linked to IR and T2D in humans. IR is caused in mice by targeted deletion of these genes. In addition, IR results in reduction of skeletal muscle AKT phosphorylation in response to insulin stimulation [34, 64, 65].

Given evidence of "spareness" for IRS, PI3K and AKT, the possibility that abnormalities in proximal insulin signaling might be responsible for IR has to be questioned, in the same way that the "spare receptor" theory has to be questioned. Homozygous deletion of AKT2, the most prevalent AKT gene isoform, resulted in a 90% reduction in insulin-stimulated AKT phosphorylation, but with no discernible defect in phosphorylation of the AKT substrate, or protein synthesis in response to insulin.

In this situation, there was a tiny quantity of AKT1 expression that was not influenced by the gene deletion and was enough to deliver a completely functional message as response to insulin [66, 67]. Similarly, whereas AKT2 accounts for 85% of total AKT in the liver, its ablation does not result in significant glucose intolerance because the remaining AKT1 compensates for this defect [68]. The insulin dose-response curve in adipocytes, where the curve for AKT phosphorylation is "shifted to the right" compared to that for AKT substrate phosphorylation or insulin action, indicates that partial phosphorylation of AKT is sufficient for maximal biological responses, providing additional evidence for "spareness" in proximal insulin signaling network. At "normal" insulin concentrations, phosphorylation of AKT substrates requires only 1% of the entire AKT pool to be activated [69–71]. Furthermore, AKT phosphorylation is reduced in muscle from T2D patients, while downstream substrate phosphorylation is unaffected. Importantly, studies in animals fed a Western diet have indicated that IR begins before any detectable insulin signaling defect. Only 42 days of Western diet feeding resulted in reduced insulin-stimulated AKT phosphorylation, but TBC1D4 phosphorylation remained normal. As a result, minor changes in phosphorylation of proximal insulin signaling components may result in insulin sensitivity, but they are unlikely to result in a reduction in the maximal physiologic response [53].

Thus, how can the predominance of abnormalities in proximal insulin signaling components observed in diverse IR models, such as lower AKT phosphorylation, be reconciled? It is possible that these defects are a result of defective glucose metabolism rather than the cause. This could be a direct effect secondary to compensatory hyperinsulinemia, a typical hallmark of IR (since persistent hyperinsulinemia can lead to degradation of proximal insulin signaling components); alternatively, it may be a cell-autonomous effect due to a reduction in AKT phosphorylation as a result of defective glycolysis. Many studies used insulin-stimulated AKT phosphorylation in mice (sometimes in response to a maximal, pharmacological dosage of insulin) as an indicator of insulin sensitivity [72–75].

However, under physiological settings such as the response to a meal (with minimal insulin release), AKT phosphorylation is barely detectable, due to the non-linearity between AKT phosphorylation and phosphorylation of its substrates. As a result, when evaluating the physiological importance of insulin signaling, it is critical to look at the phosphorylation of a variety of AKT substrates to determine if there is a major deficiency in "AKT activity" in vivo. These findings suggest that a minor impairment in proximal insulin signaling network is unlikely to account for the significant reduction in insulin-stimulated glucose uptake observed in patients with T2D. Furthermore, these findings underline that lower AKT2 phosphorylation should not be used as a direct marker or even as a proxy measure of IR [71].

Negative feedback loops originating from Ser/Thr kinases that phosphorylate and limit the action of IRS proteins have also been proposed as a cause of IR. This theory is refuted by a number of studies. Since Platelet-derived growth factor (PDGF) by-passes these proteins to activate glucose uptake, mice bred to overexpress PDGF receptor (PDGFR) in muscle presented an ideal model to explore whether deficiencies in insulin receptor or IRS were implicated in experimental IR. In these mice, PDGF treatment resulted in increased glucose uptake in muscle [76]. Notably, when PDGFR transgenic rats were fed a Western diet, muscle glucose uptake in response to PDGF was decreased to the same degree as insulin-mediated uptake. This refutes a role for inhibitory Ser/Thr phosphorylation of the insulin receptor or IRS as a cause of IR, indicating that the deficiency in glucose uptake or IR does not involve the insulin receptor or IRS [8, 53, 54, 77, 78].

*Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

Furthermore, in mice, targeted mutation of one of the major putative inhibitory sites in IRS1 (Ser307), deletion of potential mediators of IR, such as PKC (which is reported to phosphorylate insulin receptor), and pharmacological blockade of key negative feedback pathways, such as mTOR (which is activated by insulin signaling and inhibits signaling by phosphorylating IRS through a negative feedback mechanism) [78–81].

Finally, investigations in humans with IR or T2D revealed that insulin-stimulated muscle glucose uptake is reduced by 50–100% even at maximum insulin dosages [82–85], with no change or reduction in AKT phosphorylation [86–88]. Only a few of these studies addressed the mechanism of AKT substrate phosphorylation in depth, and those that did found no deficiency or poorly linked with IR. These findings support the theory that the proximal insulin signaling network in human tissues has enough "spareness" to overcome even a moderate deficiency in AKT phosphorylation [87–89], and that lowered AKT phosphorylation is adequate to ensure a normal signal transduction. As previously stated, faulty proximal insulin signaling is most likely a result of IR rather than a cause of IR [90].

#### **9. GLUT4 and insulin resistance**

Insulin stimulates the transfer of intracellular GLUT4 storage vesicles to the cell surface, resulting in glucose uptake in skeletal muscle cells and adipocytes (**Figure 1**) [91–94]. Insulin-dependent GLUT4 translocation has been linked to IR in both skeletal muscle and adipose tissue. This decrease in GLUT4 availability at the plasma membrane causes a reduced glucose uptake, which can lead to other IR-related consequences like reduced AKT phosphorylation, protein synthesis defects, and increased lipolysis [72, 95, 96]. GLUT4 does not show spareness, unlike proximal insulin signaling components such as IRS1 and AKT. The fact that heterozygous GLUT4 gene-knockout mice acquire metabolic disease exemplifies this concept [97].

However, while GLUT4 levels are lowered by 50% in human adipose tissue from patients with T2D, such levels remain unaltered in skeletal muscle, implying that GLUT4 levels cannot explain IR development in skeletal muscle [98].

#### **Figure 1.**

*Translocation of glucose transporter type 4 (GLUT4) from GLUT4 storage vesicles (GSVs) to the plasma membrane of normal adipocytes and skeletal muscle cells (a). This process is altered in conditions characterized by insulin resistance (b).*

Despite normal GLUT4 levels, insulin-stimulated GLUT4 translocation to the cell surface in skeletal muscle is faulty in both individuals with T2D [92] and in several rodent models of IR [99, 100]. Importantly, while exercise-modulated GLUT4 translocation to the cell surface is unaffected [101], the impairment in muscle GLUT4 trafficking in T2D is insulin signaling-specific. Insulin and exercise both cause GLUT4 translocation to the cell surface from discrete intracellular compartments [102].

The ultimate defect that defines IR is the impaired GLUT4 translocation to the plasma membrane. However, it is unknown how the numerous potential intracellular IR mediators mentioned later affect GLUT4 trafficking. Three options are discussed here. First, GLUT4 translocation requires that GLUT4 is localized in the appropriate intracellular compartment, the so-called GLUT4 storage vesicles (GSVs); GLUT4 targeting to GLUT4 GSVs has been hypothesized to be altered in IR [91, 100]. However, whereas this would likely result in GLUT4 degradation, GLUT4 levels in skeletal muscle from patients with IR remain unaffected. Second, given the importance of protein phosphorylation in insulin action [101–104], it is possible that the defect is caused by a distal component of the insulin-regulated phosphorylation network such as TBC1D4, which regulates GLUT4 trafficking, although there is no convincing evidence for defective TBC1D4 phosphorylation in IR [105]. TBC1D4 is unlikely to be the only AKT target causing GLUT4 translocation, as cells lacking TBC1D4 still have some insulin-sensitive glucose transport [106]. Recent phosphoproteomics studies have revealed the existence of a wide range of insulin-responsive phosphoproteins in metabolic cells, allowing for the identification of insulin signaling targets in the distal part of the insulin signaling pathway that may be involved in the development of IR [104]. Indeed, IR is associated with massive alterations in the architecture of the entire insulin signaling pathway, according to examination of muscle cells from T2D patients [107]. Finally, a direct alteration of GLUT4 or a defect in a yet undiscovered protein that interacts with GLUT4 could cause the abnormalities in GLUT4 trafficking. This could include carbonylation and oxidation-induced inactivation of GLUT4, which have been observed in humans as a response to short-term overnutrition [108]. Protein carbonylation is linked to H2O2 production, lipid peroxidation and IR, suggesting a link between such molecular processes and the development of IR [109].

#### **10. Adipose tissue and insulin resistance**

While IR is regularly seen in lean first-degree relatives of patients with T2D, it is also found in many lean "healthy" individuals, suggesting that IR is more common than previously thought. In this regard, dietary habits, physical activity level and genetics are important factors that can significantly contribute to IR. Adipose tissue makes a significant contribution to the development of IR. Limitations in peripheral adipose tissue storage capacity and expansion in response to over nutrition (as it occurs in overweight and obesity) lead to increased circulating lipids, subsequent lipid accumulation in non-adipose tissues (ectopic lipid in liver, skeletal muscle, heart, and pancreas) and development of lipid-induced IR and metabolic derangements [110, 111]. Because of this, and since there is a clear link between IR and increased adipose tissue mass, we will discuss the role of adipose tissue mass and lipotoxicity as significant drivers of IR, as well as the emerging mechanisms by which adipocytes contribute to systemic IR.

*Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

#### **10.1 Adipose tissue dysfunction**

IR in adipocytes could be the first step in the progression of adipose tissue dysfunction, similar to IR in muscle and liver. In adipocytes from first-degree relatives of patients with T2D, there is a low expression of markers of insulin sensitivity such as GLUT4 and adiponectin (a crucial systemic insulin-sensitizing adipokine produced by adipose tissue), supporting this theory [112]. Furthermore, adipocyte hypertrophy (increase in adipocyte size) appears to precede T2D onset in Pima Indians, a group of Native Americans with a high incidence of IR and T2D [113]. Additionally, mouse models with adipose-specific IR also have IR in their muscle and liver. Notably, IR in the muscle of adipose-specific Glut4 gene-knockout mice was only present in vivo but not when muscles were isolated and assessed in vitro, implying a role for systemic factors (which did not include circulating FFAs or inflammatory cytokines) in the progression of IR from adipose tissue-specific pathology [114, 115].

Human genetic research has also suggested that adipose tissue plays a significant role in IR. Studies in identical twins or first-degree relatives of T2D patients have shown that inheritance has a substantial influence in IR and T2D [116].  While these investigations have generally discovered genes linked to beta-cell function and insulin secretion, deeper analysis of phenotypes more closely aligned with IR have begun to uncover genetic drivers of IR in other organs. Surprisingly, several of these drivers are involved in the function of adipose tissue [118]. Although subclinical lipodystrophy is a rare cause of severe IR, it has been suggested that milder forms of lipodystrophy are responsible for IR in general, supporting a model in which excessive lipid spillover into circulation is a proximal, mechanistic cause of altered insulin action. Specifically, when the individual's capacity to store lipids in adipose tissue has been exceeded, lipid spillover into circulation leads to elevated plasma FFAs and triglyceride levels, which result in increased ectopic storage of these molecules in non-adipose tissues—such as liver and skeletal muscle and subsequent metabolic derangements via lipotoxicity (lipid-induced toxicity). Surprisingly, genes in the insulin signaling system linked to IR (IRS1 and GRB14) are also linked to familial partial lipodystrophy [119].

PPARG (Peroxisome Proliferator-Activated Receptor Gamma, a master positive regulator of adipogenesis) and CCDC92, DNAH10, and L3MBTL3 (regulators of adipocyte differentiation) were among the 53 loci discovered in a study employing an integrated genomic approach to find genes related to IR. Thiazolidinediones are insulin-sensitizing peroxisome proliferator-activated receptor gamma agonists that are used in the treatment of T2D and act by promoting adipogenesis and adipose tissue growth (through cell size and cell number increase or adipocyte hypertrophy and hyperplasia) [119]. The availability of additional lipid storage induced by thiazolidinediones may therefore promote insulin sensitivity by alleviating lipotoxicity [120]. These drugs also improve insulin sensitivity in first-degree relatives of T2D patients, implying that adipose tissue hypertrophy and "unhealthy" lipid storage are critical regulators of insulin action and contributors to IR [121].

Adipose tissue's primary function is to store fat and release it into circulation when needed, and it has the unique capacity to expand in response to nutrient overload. Lipids can be released into the bloodstream when the adipocyte capacity to store lipids has been exceeded [39]. There is compelling evidence that the accumulation of excess lipids in non-adipose tissues (e.g., skeletal muscle and liver), known as lipotoxicity (a.k.a. lipid-induced toxicity), plays a role in the development of muscle

and liver IR [122]. As a result, studies aimed at understanding the cause and magnitude of increased circulating lipid levels in IR are now being pursued. Furthermore, intracellular lipid accumulation in cells and tissues—including pancreatic beta cells and liver—has been linked to the onset of cellular dysfunctions, such as secretory abnormalities and inflammation (**Figure 2**). Elevated circulating FFA levels have been linked to IR, and this has been proposed as a possible cause of lipotoxicity [123].

In humans and animals, lipid infusion causes muscle IR and enhanced hepatic gluconeogenesis, the latter attributable to changes in metabolic fluxes rather than to fat accumulation [123–125]. Furthermore, animals with increased circulating FFA levels due to increased lipolysis develop muscle and hepatic IR, whereas obese mice with reduced fat cell lipolysis are protected from glucose intolerance [126]. It is worth noting that, as discussed elsewhere [127], circulating FFA levels in patients with IR or T2D usually are not elevated. However, there are several confounders in this measurement, including the wide range of FFA levels in healthy adults and the fact that fasting FFAs are typically assessed rather than the more relevant postprandial FFAs. Nonetheless, there is strong evidence that serum FFA levels are elevated in first-degree relatives of patients with T2D [127, 128], implying that this elevation represents an early stage of the disease. It is unclear if the rise in circulating FFA levels is related to defects in insulin-mediated regulation of lipolysis, to alterations in fat

#### **Figure 2.**

*Excessive adipocyte lipid storage in response to overnutrition, resulting in adipocyte hypertrophy, inflammation and increased release of free fatty acids (FFAs) into circulation, leading to ectopic fat accumulation, lipotoxicity and development of insulin resistance in non-adipose tissues, such as liver and skeletal muscle.*

*Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

storage capacity, or to an increase in adipose tissue mass without defects in lipolysis. Lipolysis per gram of adipose tissue mass is considerably lower in obese subjects, suggesting that enlargement of adipose tissue mass is the principal driver of abnormal FFA homeostasis [129].

Adipose tissue can grow in size by either hypertrophy, which involves the enlargement of existing adipocytes, or hyperplasia, which involves the generation of new fat cells from preadipocytes via adipogenesis, resulting in an increase in the number of tiny adipocytes [130]. Subcutaneous adipose tissue is more expandable than visceral adipose tissue in humans, whereas the opposite is true in C57BL/6J male mice [131]. Female mice, interestingly, show expandability of both adipose tissue depots in response to Western diet feeding, suggesting that sex hormones and other sex-dependent elements play a role in this process [131, 132]. Pathological adipose tissue expandability under situations of overnutrition, particularly adipose tissue hypertrophy, has got a lot of attention as a likely cause of IR. Indeed, firstdegree relatives of patients with T2D have greater amounts of hypertrophic adipose tissue, implying that changes in cell size—presumably due to defective adipogenesis—represent an early event in the pathophysiology of T2D. Hypertrophic large adipocytes are linked to poor metabolic outcomes when compared to hyperplastic adipocytes [39, 133], which have been shown to confer metabolic health in obesity [134–136]. More importantly, hypertrophic adipocytes may contribute to an increase in circulating FFA levels due to their reduced FFA storage capacity. Reduced preadipocyte differentiation, diminished de novo lipogenesis or FFA uptake in hypertrophic adipocytes, and/or reduced adipose tissue expandability due to physical limits on expanding cell size may all contribute to decreased lipid storage capacity by the hypertrophic adipose tissue. Furthermore, adipogenesis abnormalities may result in decreased generation of beige adipocytes, thereby contributing to higher circulating FFA levels; indeed, beige adipocytes differentiate from a subpopulation of progenitors resident in white adipose tissue and have the ability to promote FFA oxidation through thermogenesis [137, 138].

#### **10.2 Circulatory factors released from adipocytes**

Adipose tissue secretes a number of factors (termed "adipokines") into the bloodstream that regulate energy metabolism. These factors include cytokines, hormones, extracellular matrix proteins, as well as growth and vasoactive factors. The type of adipose tissue expansion has been demonstrated to impact the secretion of certain of these factors under IR conditions. Since the discovery of leptin as the first adipokine [139], a growing list of adipose tissue-secreted factors implicated in IR has been discovered, with roles in IR that are either protective or causative [20, 140].

Leptin, for example, regulates whole-body energy metabolism by acting on feeding centers in the brain to suppress food intake and increase energy expenditure; leptin deficiency causes obesity, hyperinsulinemia, IR and impaired glucose homeostasis [141]. Adiponectin, another well-known adipokine secreted from adipocytes, has been linked to regulation of cell insulin sensitivity. In humans, circulating adiponectin levels are favorably linked with whole-body insulin sensitivity; additionally, physical training increases circulating adiponectin levels and the expression of its receptors in muscle, which may mediate the improvement of IR in response to exercise [142]. Surprisingly, small and subcutaneous adipocytes release more adiponectin than visceral or large adipocytes [143]. Anti-atherogenic, anti-inflammatory, and insulin-sensitizing effects of adiponectin have also been discovered [144].

It is worth mentioning, however, that while adiponectin's positive benefits in rats are outstanding, the role of this adipokine in humans is less obvious, and Mendelian randomization studies on adiponectin's relationship with metabolic disease in humans have generated inconsistent results [145, 146].

Adipocytes release a variety of substances, including metabolites like lipids and extracellular vesicles that contain proteins and microRNAs. Branched fatty acid esters of hydroxy fatty acids (FAHFAs) are a unique class of lipids synthesized in adipocytes that have been shown to increase insulin sensitivity and reduce inflammation; accordingly, individuals with IR have lower circulating FAHFA levels [147]. As a result, further research into this metabolite class is necessary. Adipocytes, for example, release tiny lipid-encapsulated extracellular vesicles into the bloodstream. These vesicles may alter metabolic processes in other target tissues, such as the liver, according to increasing evidence based on mouse studies. MicroRNAs represent one of the components found in extracellular vesicles that have been linked to this mechanism. While investigations on microRNAs are intriguing, many fundamental aspects about the mechanism of their controlled secretion and their tissue targeting and entry into target cells remain unknown [148, 149].

Many circulating factors are also produced by other adipose tissue-resident cells, such as immune or vascular cells, rather than by adipocytes themselves (the so-called "stromal vascular fraction" of adipose tissue). Some of these adipokines, such as tumor necrosis factor (TNF), resistin or vascular endothelial growth factor (VEGF), are important regulators of tissue homeostasis and may be secreted as a result of adipose tissue enlargement during the development of obesity [150]. Nonetheless, inflammatory cytokines have been widely suggested as possible IR-inducing adipokines, and several of these factors have significant proinflammatory activities [151–153].

#### **11. Inflammation and insulin resistance**

It is now well recognized that cells of both innate and adaptive immunity, notably macrophages, infiltrate hypertrophic adipose tissue in most obesity models, and that this is accompanied by a loss of immunosuppressive regulatory T cells in visceral fat depots [154]. When macrophages in adipose tissue are activated in response to overnutrition, they polarize towards a proinflammatory phenotype and release cytokines that may trigger IR in all metabolic tissues [155]. Diet-induced obesity in mice and humans is unmistakably linked to elevated levels of systemic inflammatory markers, including C-reactive protein (CRP) and enhanced immune cell infiltration of adipose tissue and other organs [156]. In addition, inflammatory cytokines, such as TNF, can elicit IR in metabolic tissues when infused in humans [157]. Although macrophage infiltration into hypertrophic adipose tissue is well documented, the role of inflammation in IR is convoluted and controversial; for example, inflammatory markers are not elevated in first-degree relatives of T2D patients [158]. Furthermore, in Western diet-fed mice, tissue IR occurs before the adipose tissue infiltration by a considerable number of immune cells, and genetic or pharmacological anti-inflammatory methods do not prevent the development of Western diet-induced IR [159, 160]. The administration of a neutralizing antibody against interleukin-1 (IL-1), a proinflammatory cytokine implicated in IR, to approximately 4000 patients with T2D and almost 5000 subjects with prediabetes resulted in a significant decrease in CRP levels, as well as in a modest positive effect on cardiovascular outcomes, but without reducing the frequency of new-onset T2D or increasing fasting glucose levels [161–163].

#### *Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

Overall, evidence suggests that adipose tissue infiltration by macrophages is unlikely to be the major cause of IR. Macrophage infiltration into the growing adipose tissue may affect its function in addition to systemic inflammation, but the exact impact of this infiltration is unknown [164]. Anti-inflammatory macrophages (M2), on the other hand, have been shown to promote angiogenesis and preadipocyte differentiation, which aids adipose tissue expansion [165, 166]. The diversity of cytokines, their concentrations, and the timing of their release into the tissue are likely to have a considerable impact on the final biological response, contributing to the observed inconsistent results. The ability of genetically induced adipocyte IR to elicit adipose tissue inflammation adds to the growing body of evidence that inflammation may be a consequence rather than a cause of IR. Hyperinsulinemia has been shown to induce adipose tissue inflammation, implying that the latter is a late event in the IR pathophysiology [30].

#### **12. Intracellular mediators and insulin resistance**

Many extrinsic stimuli and genetic alterations can antagonize insulin action in vitro and in vivo, and their investigation has led to the identification of a series of molecules as putative intracellular mediators of IR. In the sections that follow, we will look at the role of a few intracellular components that have got a lot of attention as drivers of IR. It is worth noting that mechanisms of action of these components are not well-established yet, and further research is needed to better understand their role in IR development.

#### **12.1 Accumulation of ceramides**

Ceramides have been implicated as IR mediators by a large body of research. Ceramides are essential precursors of most of the complex sphingolipids localized in lipid bilayers, including sphingosine, sphingomyelins, and glucosylceramides. Ceramides accumulate in muscle, liver and adipose tissue of subjects with IR, according to human and animal studies [167–170]. In insulin-resistant tissues, the levels of 16- or 18-carbon chain-length ceramides are raised, whereas the levels of other chain-length ceramides are not consistently changed [171, 172]. Indeed, in adipose tissue from obese subjects, the level of ceramide synthase isoform 6 (CERS6), which synthesizes C16 ceramide, is raised [171]. Surprisingly, the presence of a double bond in the ceramide backbone promotes IR, as ablation of the enzyme responsible for its formation (dihydroceramide desaturase 1) abrogates IR [173]. While it is unclear how specific extrinsic mediators of IR cause increased intracellular ceramide levels, it is possible that excess FFAs serve as a crucial substrates for ceramide biosynthesis [174–176].

Another theory connects intracellular ceramide to levels of circulating adiponectin. Ceramidase activity is found on adiponectin receptors, and lower adiponectin levels in IR may lead to decreased ceramidase activity and, consequently, to higher ceramide levels [177, 178]. AMP-activated protein kinase (AMPK), a major metabolic sensor that regulates mitochondrial biogenesis and metabolism, is activated by adiponectin, potentially regulating ceramide via increased mitochondrial lipid oxidation [179]. By using small-molecule inhibitors or genetic deletion of ceramide-producing enzymes to neutralize ceramide accumulation in metabolic organs, researchers were able to reverse or prevent IR induced by the Western diet in C57BL/6 mice with

diet-induced obesity [122]. The relationship between ceramide and decreased insulin action is not univocal, as it is for many possible intracellular mediators of IR. In fact, ceramide suppresses AKT activity, although IR is unlikely to be caused by defects in AKT, which is a proximal arm of insulin signaling (as it has previously been mentioned). Ceramide could be part of a wider, IR-related stress mechanism that leads to mitochondrial dysfunction and to the production of reactive oxygen species (ROS). Ceramide has also been connected to the release of pro-inflammatory cytokines, which have been involved in IR, as it has previously been described [180, 181].

#### **12.2 Accumulation of diacylglycerol (DAG)**

Another popular theory for the cause of IR is the accumulation of diacylglycerols (DAGs) in muscle, adipocytes and liver, as a result of elevated serum FFA levels [182, 183]. Protein kinase C (PKC) is recruited to the plasma membrane by DAGs, where it phosphorylates and inhibits insulin receptor kinase activity. While it is quite plausible that DAG levels are elevated in insulin-resistant tissues, a scenario in which DAG-dependent phosphorylation of the insulin receptor is the major cause of IR raises a number of questions. Given the "spareness" of the insulin receptor and proximal signaling intermediates, it is doubtful that IR is caused solely by abnormalities in these components, at least in muscle. In contrast to other insulin-responsive proteins, the stoichiometry of insulin receptor phosphorylation at the region implicated in DAG-mediated IR is low and not detectable by conventional phosphopeptide analysis [79, 104, 184]. PKC deletion in the liver had little effect on whole-body insulin sensitivity in mice, indicating against PKC being a key target of DAG-induced IR in that tissue [79, 104], although this has since been challenged by studies in rats showing that acute knockdown of PKC in the liver protected animals from IR. However, antisense oligonucleotides were delivered systemically, which could target PKC expression in other organs. While technical differences between these studies and others have been suggested as a reason for the discrepancies observed [183], there appears to be enough disagreement about the role of the DAG-PKC-insulin receptor pathway in IR to warrant further investigation and, in particular, validation by multiple independent laboratories [185].

#### **12.3 Mitochondrial dysfunction and reactive oxygen species (ROS)**

IR has been linked to a decrease in mitochondrial function. Mitochondrial dysfunction is a term that has been used to describe a variety of mitochondrial phenotypes, including decreased respiratory capacity and ATP production, decreased number of mitochondria, accumulated mitochondrial damage due to defects in mitophagy, and altered mitochondrial morphology caused by changes in mitochondrial fission-fusion dynamics. Many of these alterations are also linked to an increase in mitochondrial ROS generation, which has long been linked to IR [186–188].

It is not unexpected that IR is linked to higher levels of reactive oxygen species (ROS). This is due to the fact that IR is frequently accompanied by a positive energy balance, which leads to an excess of reducing equivalents (NADH and FADH2). This determines a reductive stress on the mitochondrial respiratory electron transport chain, which invariably results in the formation of free electrons and, as a result, in an increased production of various forms of ROS [189]. Furthermore, enhanced ROS production has been found in response to a variety of extracellular stressors linked to IR, including inflammation [190]. Superoxide, H2O2, reactive nitrogen andoxidized

*Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

lipids accumulate in insulin-resistant cells or tissues, and a mitochondria-targeted small molecule transiently produced mitochondrial ROS in muscle and adipocytes, causing IR. As a result, attempts to reduce ROS levels have been proven to reverse or prevent IR in mice [191–194].

Reduced levels of coenzyme Q (CoQ ) have recently been linked to IR in humans [44]. In mitochondria, CoQ is a key component of the electron transport chain, transferring electrons from complex I or II to complex III. Furthermore, unlike complex I, CoQ receives electrons directly from the electron-transferring flavoprotein, and this is unrelated to proton pumping or mitochondrial membrane potential, relying only on the availability of oxidized CoQ. Reduced CoQ accumulates, causing reductive stress in complex I, complex II and other dehydrogenases that feed electrons into the CoQ pool, resulting in increased ROS production [195]. As a result, lowering the total CoQ pool [44] will most likely lower the ROS production threshold at a given energy demand-supply ratio. It is also worth noting that FFA oxidation produces far more ROS than carbohydrate oxidation [195]. This is because the electron-transferring flavoprotein feeds a higher proportion of reducing equivalents straight into the CoQ pool during FFA oxidation. Therefore, as lipid metabolism increases, the supply of reducing equivalents outnumbers the demand, lowering the ratio of oxidized to reduced CoQ. This is likely worsened when total CoQ levels are low, as seen in IR [44], resulting in reductive stress and increased ROS production. The mechanism that regulates CoQ levels in IR is unknown. Intriguingly, statins, which are commonly used as cholesterol-lowering drugs, have been linked to IR in humans [196], with the possibility that this relationship is related to the statin-induced reductions in CoQ biosynthesis [44]. Unfortunately, given the low bioavailability of CoQ, oral supplements, which are frequently recommended as an antioxidant strategy, are unlikely to be successful in replenishing the mitochondrial CoQ pool in patients with IR or even in individuals who take statins. Other hazardous intermediates can be generated, in addition to ROS, as a result of mitochondrial respiration abnormalities. Acylcarnitine is an example of incompletely oxidized lipids produced by lipid overload. Acylcarnitine has been reported to accumulate in IR, indicating a deficiency in or an overabundance of the mitochondrial oxidative ability. In this regard, it has been postulated that lipid-induced mitochondrial stress mediates IR, although the exact mechanisms remain elusive [197].

#### **12.4 Insulin resistance associated with stress pathway**

Many of the pathways involved in IR pathophysiology, such as those involving ceramides, DAGs or ROS, are now being linked as part of what we call an "intracellular IR stress pathway", according to new evidence. Ceramide, for example, promotes mitochondrial fission and ROS production [198, 199]. In subjects with IR, the quantity of mitochondrial ceramide is higher, and enzymes involved in ceramide biosynthesis have been found in mitochondria [185, 200–203]. Ceramide is involved in apoptosis triggered by mitochondria in some cells, including insulin-producing pancreatic beta cells, but not in other metabolic tissues [204–206]. Ceramide also contributes to endoplasmic reticulum stress, which frequently co-occurs with mitochondrial stress and has been proposed as a driver of IR, where endoplasmic reticulum stress causes JNK activation, which, as previously described, affects the insulin signaling pathway via inhibitory IRS1 Ser/Thr phosphorylation [204–206]. Ceramide also induces PKC, a DAG-regulated kinase, to translocate to mitochondria, activating it and causing mitochondrial damage through an unknown mechanism [207].

Ceramides and DAGs are also biochemically connected; sphingomyelin synthase, for example, converts ceramide to DAG. Finally, in rats, reducing mitochondrial ROS levels with mitochondria-targeted catalase improved insulin sensitivity while lowering muscle DAG levels [208]. The potential connection of many of these suspected IR-causing elements into a dynamic network should help to resolve some of the current debates on this topic.

#### **12.5 Signals from the mitochondria**

Despite the interest in mitochondrial dysfunction in IR, it is unclear how intramitochondrial signals, like ceramide or ROS, may cause changes in insulin action, such as impaired GLUT4 translocation, which occurs mostly in the cytosol. The mitochondrial permeability transition pore (mPTP), a multiprotein complex located in the inner mitochondrial membrane, is a promising candidate for "inside-out" mitochondrial signaling because it opens under conditions of mitochondrial stress—most notably involving mitochondrial ROS—to allow molecules to be transported from mitochondria to the cytoplasm [209]. In L6 myotubes, inhibiting mPTP prevented ceramide- or palmitate-induced IR, and mice with defective mPTP opening were protected from diet-induced IR in skeletal muscle [210]. Although at least a part of the impact is attributable to its anti-obesogenic effect, deletion of mPTP in the liver has been shown to protect mice from liver steatosis and IR [211].

#### **13. Conclusions and perspectives**

The rising frequency of IR, as well as its crucial involvement in a variety of diseases, demands a greater understanding of the processes behind IR pathogenesis and how they interact with genetics and various surroundings, notably dietary factors. We have attempted to offer an overview of the main mechanisms hypothesized to contribute to IR in this chapter, highlighting both supportive and non-confirmatory evidence when appropriate. Many of the molecules and processes studied as causative in IR, in our opinion, function in series as a connected pathway or a loop rather than acting independently. Unfortunately, there has been a recent trend to describe IR as a dysfunction of insulin signaling, regardless of whether a simultaneous examination of insulin action on glucose metabolism has identified a defect in the latter process. We feel that this method has produced significant problems in the field, and we wish to send a message that simple, unitary errors in proximal insulin signaling are unlikely to be a major cause of IR. Rather, IR develops as a result of a variety of challenges that disrupt cellular homeostasis, resulting in cellular stress that can have a variety of deleterious consequences on insulin signal sensing and transmission.

The difficulty in translating findings from model organisms to humans, particularly in terms of differentiating IR causation from the multiplicity of effects, is a key roadblock in investigating the underpinnings of IR. By discovering causal genetic variants, human genetics holds a lot of promise for tackling this problem. However, genetics can only explain a portion of the pathophysiology of IR. Environmental variables play a crucial role in determining susceptibility to IR development and interact with genetics. Furthermore, the heterogeneity of metabolic diseases like T2D demands detailed phenotyping. Focusing on phenotypes that has better track with IR has proven difficult to achieve in the large cohorts. It is required to identify genetic polymorphisms that only explain a small proportion of disease in the human

#### *Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

population. Despite these limitations, a number of genetic loci linked to human IR have been discovered, leading to a renewed focus on adipose tissue enlargement as a critical aspect of IR. However, since IR is a systemic condition, we expect future investigations to discover variations in genes governing multiple cellular processes throughout organs as linked to IR pathophysiology.

A more systematic approach involving large-scale omics to analyze the molecular landscape rather than relying on individual components as causal would be required to gain a better understanding of IR. Moreover, while knockout mice have been critical in characterizing the biochemistry of insulin action, they have also sparked numerous debates. One reason for this is that gene deletions typically result in adaptive processes that are difficult to define and may have limited physiological value, as indicated in a recent study with muscle-specific Akt gene-knockout mice [58]. In animals with both insulin and insulin-like growth factor 1 (IGF-1) receptors removed in muscle, similar adaptation mechanisms have been reported [212].

The ultimate goal of understanding mechanisms behind IR is to develop new, effective anti-IR therapeutic strategies. One key point to consider in this endeavor is whether such therapies would be beneficial if the initial insult—nutritional overload—persists. While IR is typically considered abnormal, as it is linked to a variety of disease outcomes, it is also a prevalent component of many normal physiological states, such as starvation, pregnancy, and hibernation. IR is believed to play a protective or adaptive role in such conditions, supporting survival by saving glucose for the brain and other vital tissues and organs or for the fetus during pregnancy. It is possible that IR has a similar function in metabolic disease. Since the primary metabolic tissues are frequently exposed to potentially harmful quantities of nutrients, IR could be a protective mechanism that helps to prevent tissue nutrition overload [190]. However, this comes at a price, namely concomitant hyperinsulinemia, which is the most serious pathophysiological consequence of IR. Insulin-sensitizing drugs may thus act as a "circuit breaker", reducing hunger, inflammation and IR by suppressing hyperinsulinemia. As a result, we believe there is still a strong need to describe the molecular characteristics that drive IR in order to identify appropriate targets that can break the IR vicious cycle.

#### **Acknowledgements**

The authors are also thankful to Guru Nanak Dev University (Amritsar, Punjab, India) for providing various facilities to carry out the present work.

#### **Conflict of interest**

The authors declare no conflict of interest.

*Evolving Concepts in Insulin Resistance*

### **Author details**

Atamjit Singh1 , Nikhita Ghai1 and Preet Mohinder Singh Bedi1,2\*

1 Department of Pharmaceutical Sciences, Guru Nanak Dev University, Amritsar, Punjab, India

2 Drug and Pollution Testing Lab, Guru Nanak Dev University, Amritsar, Punjab, India

\*Address all correspondence to<EMAIL_ADDRESS>
© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

#### **References**

[1] Himsworth HP. Diabetes mellitus: Its differentiation into insulin-sensitive and insulin-insensitive types. 1936. International Journal of Epidemiology. 2013;**42**:1594-1598

[2] Jee SH, Kim HJ, Lee J. Obesity, insulin resistance and cancer risk. Yonsei Medical Journal. 2005;**46**(4):449-455

[3] Suzanne M. Insulin resistance and neurodegeneration: Progress towards the development of new therapeutics for Alzheimer's disease. Drugs. 2017;**77**(1):47-65

[4] Pérez-Tasigchana RF, León-Muñoz LM, Lopez-Garcia E, Gutierrez-Fisac JL, Laclaustra M, Rodríguez-Artalejo F, et al. Metabolic syndrome and insulin resistance are associated with frailty in older adults: A prospective cohort study. Age and Ageing. 2017;**46**(5):807-812

[5] Chawla A, Chawla R, Jaggi S. Microvasular and macrovascular complications in diabetes mellitus: Distinct or continuum? Indian Journal of Endocrinology and Metabolism. 2016;**20**(4):546

[6] Singh B, Saxena A. Surrogate markers of insulin resistance: A review. World Journal of Diabetes. 2010;**1**(2):36

[7] Philipson LH. Harnessing heterogeneity in type 2 diabetes mellitus. Nature Reviews. Endocrinology. 2020;**16**:79-80

[8] Ahlqvist E et al. Novel subgroups of adult-onset diabetes and their association with outcomes: A data-driven cluster analysis of six variables. The Lancet Diabetes and Endocrinology. 2018;**6**:361-369

[9] James DE, Strube M, Mueckler M. Molecular cloning and characterization of an insulin-regulatable glucose transporter. Nature. 1989;**338**:83-87

[10] Birnbaum MJ. Identification of a novel gene encoding an insulinresponsive glucose transporter protein. Cell. 1989;**57**:305-315

[11] Bryant NJ, Govers R, James DE. Regulated transport of the glucose transporter GLUT4. Nature Reviews. Molecular Cell Biology. 2002;**3**:267-277

[12] Tunduguru R, Thurmond DC. Promoting glucose transporter-4 vesicle trafficking along cytoskeletal tracks: PAK-Ing them out. Frontiers in Endocrinology. 2017;**20**:329

[13] Yeh JI, Gulve EA, Rameh L, Birnbaum MJ. The effects of wortmannin on rat skeletal muscle. Dissociation of signaling pathways for insulin- and contraction-activated hexose transport. The Journal of Biological Chemistry. 1995;**270**:2107-2111

[14] Clemmensen C et al. Gut-brain cross-talk in metabolic control. Cell. 2017;**168**:758-774

[15] Rizza RA, Mandarino LJ, Genest J, Baker BA, Gerich JE. Production of insulin resistance by hyperinsulinaemia in man. Diabetologia. 1985;**28**:70-75

[16] Erion KA, Corkey BE. Hyperinsulinemia: A cause of obesity? Current Obesity Reports. 2017;**6**:178-186

[17] Lillioja S et al. Insulin resistance and insulin secretory dysfunction as precursors of non-insulin-dependent diabetes mellitus. Prospective studies of Pima Indians. The New England Journal of Medicine. 1993;**329**:1988-1992

[18] Vaag A, Henriksen JE, Beck-Nielsen H. Decreased insulin activation of glycogen synthase in skeletal muscles in young nonobese Caucasian first-degree relatives of patients with non-insulindependent diabetes mellitus. The Journal of Clinical Investigation. 1992;**89**:782-788

[19] Hollenbeck C, Reaven GM. Variations in insulin-stimulated glucose uptake in healthy individuals with normal glucose tolerance. The Journal of Clinical Endocrinology and Metabolism. 1987;**64**:1169-1173

[20] Ghaben AL, Scherer PE. Adipogenesis and metabolic health. Nature Reviews. Molecular Cell Biology. 2019;**20**:242-258

[21] Jacob S et al. Association of increased intramyocellular lipid content with insulin resistance in lean nondiabetic offspring of type 2 diabetic subjects. Diabetes. 1999;**48**:1113-1119

[22] Lim K, Haider A, Adams C, Sleigh A, Savage D. Lipodystrophy: A paradigm for understanding the consequences of 'overloading' adipose tissue. Physiological Reviews. 2020;**101**:907-993

[23] Akinci B, Sahinoz M, Oral E. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Lipodystrophy Syndromes: Presentation and Treatment. South Dartmouth (MA): MDText.com, Inc.; 2000

[24] Blüher M. Metabolically healthy obesity. Endocrine Reviews. 2020;**41**:405-420

[25] Thomas DD, Corkey BE, Istfan NW, Apovian CM. Hyperinsulinemia: An early indicator of metabolic dysfunction. Journal of the Endocrine Society. 2019;**3**(9):1727-1747

[26] Pontiroli AE, Alberetto M, Capra F, Pozza G. The glucose clamp technique for the study of patients with hypoglycemia: Insulin resistance as a feature of insulinoma. Journal of Endocrinological Investigation. 1990;**13**:241-245

[27] Marbán SL, Roth J. Transgenic hyperinsulinemia: A mouse model of insulin resistance and glucose intolerance without obesity. In: Shafrir E, editor. Lessons from Animal Diabetes VI: 75th Anniversary of the Insulin Discovery. Boston, MA: Birkhäuser; 1996. pp. 201-224

[28] Gray SL, Donald C, Jetha A, Covey SD, Kieffer TJ. Hyperinsulinemia precedes insulin resistance in mice lacking pancreatic beta-cell leptinsignaling. Endocrinology. 2010;**151**:4178-4186

[29] Alemzadeh R, Slonim AE, Zdanowicz MM, Maturo J. Modification of insulin resistance by diazoxide in obese Zucker rats. Endocrinology. 1993;**133**:705-712

[30] Pedersen DJ et al. A major role of insulin in promoting obesity-associated adipose tissue inflammation. Molecular Metabolism. 2015;**4**:507-518

[31] Templeman NM et al. Reduced circulating insulin enhances insulin sensitivity in old mice and extends lifespan. Cell Reports. 2017;**20**:451-463

[32] Czech MP. Insulin action and resistance in obesity and type 2 diabetes. Nature Medicine. 2017;**23**:804-814

[33] Tan S-X et al. Selective insulin resistance in adipocytes. The Journal of Biological Chemistry. 2015;**290**:11337-11348

*Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

[34] Tonks KT et al. Impaired Akt phosphorylation in insulin-resistant human muscle is accompanied by selective and heterogeneous downstream defects. Diabetologia. 2013;**56**:875-885

[35] Brown MS, Goldstein JL. Selective versus total insulin resistance: A pathogenic paradox. Cell Metabolism. 2008;**7**:95-96

[36] Hillgartner FB, Salati LM, Goodridge AG. Physiological and molecular mechanisms involved in nutritional regulation of fatty acid synthesis. Physiological Reviews. 1995;**75**:47-76

[37] Hellerstein MK, Schwarz JM, Neese RA. Regulation of hepatic de novo lipogenesis in humans. Annual Review of Nutrition. 1996;**16**:523-557

[38] Titchenell PM et al. Direct hepatocyte insulin signaling is required for lipogenesis but is dispensable for the suppression of glucose production. Cell Metabolism. 2016;**23**:1154-1166

[39] Hammarstedt A, Gogg S, Hedjazifar S, Nerstedt A, Smith U. Impaired adipogenesis and dysfunctional adipose tissue in human hypertrophic obesity. Physiological Reviews. 2018;**98**:1911-1941

[40] Kolterman OG, Insel J, Saekow M, Olefsky JM. Mechanisms of insulin resistance in human obesity: Evidence for receptor and postreceptor defects. The Journal of Clinical Investigation. 1980;**65**:1272-1284

[41] Anjana RM, Pradeepa R, Unnikrishnan R, Tiwaskar M, Aravind SR, Saboo B, et al. New and unique clusters of type 2 diabetes identified in Indians. The Journal of the Association of Physicians of India. 2021;**69**(2):58-61

[42] Jia Q, Morgan-Bathke ME, Jensen MD. Adipose tissue macrophage burden, systemic inflammation, and insulin resistance. American Journal of Physiology. Endocrinology and Metabolism. 2020;**319**:E254-E264

[43] Turner N et al. Distinct patterns of tissue-specific lipid accumulation during the induction of insulin resistance in mice by high-fat feeding. Diabetologia. 2013;**56**:1638-1648

[44] Fazakerley DJ et al. Mitochondrial CoQ deficiency is a common driver of mitochondrial oxidants and insulin resistance. eLife. 2018;**7**:e32111

[45] Perry RJ et al. Hepatic acetyl CoA links adipose tissue inflammation to hepatic insulin resistance and type 2 diabetes. Cell. 2015;**160**:745-758

[46] Van der Heijden RA, Sheedfar F, Morrison MC, Hommelberg PP, Kor D, Kloosterhuis NJ, et al. High-fat diet induced obesity primes inflammation in adipose tissue prior to liver in C57BL/6j mice. Aging (Albany NY). 2015;**7**(4):256

[47] Ali O. Genetics of type 2 diabetes. World Journal of Diabetes. 2013;**4**(4):114

[48] Minokoshi Y, Kahn CR, Kahn BB. Tissue-specific ablation of the GLUT4 glucose transporter or the insulin receptor challenges assumptions about insulin action and glucose homeostasis. The Journal of Biological Chemistry. 2003;**278**:33609-33612

[49] Kim JK et al. Glucose toxicity and the development of diabetes in mice with muscle-specific inactivation of GLUT4. The Journal of Clinical Investigation. 2001;**108**:153-160

[50] Gancheva S, Jelenik T, Álvarez-Hernández E, Roden M. Interorgan metabolic crosstalk in human insulin resistance. Physiological Reviews. 2018;**98**:1371-1415

[51] Severinsen MCK, Pedersen BK. Muscle-organ crosstalk: The emerging roles of myokines. Endocrine Reviews. 2020;**41**:594-609

[52] Burchfield JG et al. High dietary fat and sucrose results in an extensive and time-dependent deterioration in health of multiple physiological systems in mice. The Journal of Biological Chemistry. 2018;**293**:5731-5745

[53] Hoehn KL et al. IRS1-independent defects define major nodes of insulin resistance. Cell Metabolism. 2008;**7**:421-433

[54] Copps KD, White MF. Regulation of insulin sensitivity by serine/threonine phosphorylation of insulin receptor substrate proteins IRS1 and IRS2. Diabetologia. 2012;**55**:2565-2582

[55] Kahn CR et al. The syndromes of insulin resistance and acanthosisnigricans. Insulin-receptor disorders in man. The New England Journal of Medicine. 1976;**294**:739-745

[56] Kono T, Barham FW. The relationship between the insulin-binding capacity of fat cells and the cellular response to insulin. Studies with intact and trypsintreated fat cells. The Journal of Biological Chemistry. 1971;**246**:6210-6216

[57] Kahn CR. Insulin resistance, insulin insensitivity, and insulin unresponsiveness: A necessary distinction. Metabolism. 1978;**27**: 1893-1902

[58] Olefsky JM, Kolterman OG, Scarlett JA. Insulin action and resistance in obesity and noninsulindependent type II diabetes mellitus. The American Journal of Physiology. 1982;**243**:E15-E30

[59] Camps M et al. Evidence for the lack of spare high-affinity insulin receptors in skeletal muscle. The Biochemical Journal. 1992;**285**:993-999

[60] Gumà A et al. Effect of benzyl succinate on insulin receptor function and insulin action in skeletal muscle: Further evidence for a lack of spare high-affinity insulin receptors. Molecular and Cellular Endocrinology. 1993;**91**:29-33

[61] Fehlmann M, Morin O, Kitabgi P, Freychet P. Insulin and glucagon receptors of isolated rat hepatocytes: Comparison between hormone binding and amino acid transport stimulation. Endocrinology. 1981;**109**:253-261

[62] Merry TL et al. Impairment of insulin signalling in peripheral tissue fails to extend murine lifespan. Aging Cell. 2017;**16**:761-772

[63] Czech MP. Cellular basis of insulin insensitivity in large rat adipocytes. The Journal of Clinical Investigation. 1976;**57**:1523-1532

[64] Melvin A, O'Rahilly S, Savage DB. Genetic syndromes of severe insulin resistance. Current Opinion in Genetics & Development. 2018;**50**:60-67

[65] Crouthamel M-C et al. Mechanism and management of AKT inhibitorinduced hyperglycemia. Clinical Cancer Research. 2009;**15**:217-225

[66] Nandi A, Kitamura Y, Kahn CR, Accili D. Mouse models of insulin resistance. Physiological Reviews. 2004;**84**:623-647

[67] Jaiswal N et al. The role of skeletal muscle Akt in the regulation of muscle mass and glucose homeostasis. Molecular Metabolism. 2019;**28**:1-13

*Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

[68] Lu M et al. Insulin regulates liver metabolism in vivo in the absence of hepatic Akt and Foxo1. Nature Medicine. 2012;**18**:388-395

[69] Tan S-X et al. Amplification and demultiplexing in insulin-regulated Akt protein kinase pathway in adipocytes. The Journal of Biological Chemistry. 2012;**287**:6128-6138

[70] Ng Y et al. Cluster analysis of insulin action in adipocytes reveals a key role for Akt at the plasma membrane. The Journal of Biological Chemistry. 2010;**285**:2245-2257

[71] Larance M et al. Global phosphoproteomics identifies a major role for AKT and 14-3-3 in regulating EDC3. Molecular & Cellular Proteomics. 2010;**9**:682-694

[72] Trefely S et al. Kinome screen identifies PFKFB3 and glucose metabolism as important regulators of the insulin/insulin-like growth factor (IGF)-1 signaling pathway. The Journal of Biological Chemistry. 2015;**290**:25834-25846

[73] Ricort JM, Tanti JF, Van Obberghen E, Le Marchand-Brustel Y. Alterations in insulin signalling pathway induced by prolonged insulin treatment of 3T3-L1 adipocytes. Diabetologia. 1995;**38**:1148-1156

[74] Kurowski TG et al. Hyperglycemia inhibits insulin activation of Akt/protein kinase B but not phosphatidylinositol 3-kinase in rat skeletal muscle. Diabetes. 1999;**48**:658-663

[75] Oku A et al. Inhibitory effect of hyperglycemia on insulin-induced Akt/ protein kinase B activation in skeletal muscle. American Journal of Physiology. Endocrinology and Metabolism. 2001;**280**:E816-E824

[76] Yuasa T et al. Platelet-derived growth factor stimulates glucose transport in skeletal muscles of transgenic mice specifically expressing platelet-derived growth factor receptor in the muscle, but it does not affect blood glucose levels. Diabetes. 2004;**53**:2776-2786

[77] Draznin B. Molecular mechanisms of insulin resistance: Serine phosphorylation of insulin receptor substrate-1 and increased expression of p85alpha: The two sides of a coin. Diabetes. 2006;**55**:2392-2397

[78] Copps KD et al. Irs1 serine 307 promotes insulin sensitivity in mice. Cell Metabolism. 2010;**11**:84-92

[79] Brandon AE et al. Protein kinase C epsilon deletion in adipose tissue, but not in liver, improves glucose tolerance. Cell Metabolism. 2019;**29**:183-191.e7

[80] Deblon N et al. Chronic mTOR inhibition by rapamycin induces muscle insulin resistance despite weight loss in rats. British Journal of Pharmacology. 2012;**165**:2325-2340

[81] Lamming DW et al. Rapamycininduced insulin resistance is mediated by mTORC2 loss and uncoupled from longevity. Science. 2012;**335**:1638-1643

[82] Friedman JE, Caro JF, Pories WJ, Azevedo JL Jr, Dohm GL. Glucose metabolism in incubated human muscle: Effect of obesity and noninsulin-dependent diabetes mellitus. Metabolism. 1994;**43**:1047-1054

[83] Shulman GI et al. Quantitation of muscle glycogen synthesis in normal subjects and subjects with noninsulindependent diabetes by 13C nuclear magnetic resonance spectroscopy. The New England Journal of Medicine. 1990;**322**:223-228

[84] Baron AD, Laakso M, Brechtel G, Edelman SV. Reduced capacity and affinity of skeletal muscle for insulin-mediated glucose uptake in noninsulindependent diabetic subjects. Effects of insulin therapy. The Journal of Clinical Investigation. 1991;**87**:1186-1194

[85] Friedman JE et al. Restoration of insulin responsiveness in skeletal muscle of morbidly obese patients after weight loss. Effect on muscle glucose transport and glucose transporter GLUT4. The Journal of Clinical Investigation. 1992;**89**:701-705

[86] Meyer MM, Levin K, Grimmsmann T, BeckNielsen H, Klein HH. Insulin signalling in skeletal muscle of subjects with or without type II-diabetes and first degree relatives of patients with the disease. Diabetologia. 2002;**45**:813-822

[87] Kim YB, Nikoulina SE, Ciaraldi TP, Henry RR, Kahn BB. Normal insulindependent activation of Akt/protein kinase B, with diminished activation of phosphoinositide 3-kinase, in muscle in type 2 diabetes. The Journal of Clinical Investigation. 1999;**104**:733-741

[88] Ramos PA et al. Insulin-stimulated muscle glucose uptake and insulin signaling in lean and obese humans. The Journal of Clinical Endocrinology and Metabolism. 2020;**106**:e1631-e1646

[89] Vind BF et al. Impaired insulininduced site-specific phosphorylation of TBC1 domain family, member 4 (TBC1D4) in skeletal muscle of type 2 diabetes patients is restored by endurance exercise-training. Diabetologia. 2011;**54**:157-167

[90] Caro JF et al. Insulin receptor kinase in human skeletal muscle from obese subjects with and without noninsulin dependent diabetes. The Journal of Clinical Investigation. 1987;**79**:1330-1337

[91] Garvey WT, Maianu L, Zhu JH, Hancock JA, Golichowski AM. Multiple defects in the adipocyte glucose transport system cause cellular insulin resistance in gestational diabetes. Heterogeneity in the number and a novel abnormality in subcellular localization of GLUT4 glucose transporters. Diabetes. 1993;**42**:1773-1785

[92] Ryder JW et al. Use of a novel impermeable biotinylated photolabeling reagent to assess insulin and hypoxiastimulated cell surface GLUT4 content in skeletal muscle from type 2 diabetic patients. Diabetes. 2000;**49**:647-654

[93] Garvey WT et al. Evidence for defects in the trafficking and translocation of GLUT4 glucose transporters in skeletal muscle as a cause of human insulin resistance. The Journal of Clinical Investigation. 1998;**101**:2377-2386

[94] Gumà A, Zierath JR, Wallberg-Henriksson H, Klip A. Insulin induces translocation of GLUT-4 glucose transporters in human skeletal muscle. The American Journal of Physiology. 1995;**268**:E613-E622

[95] Dills WL Jr, McDonough GM, Kingsley PB. Glucose-stimulated protein synthesis in rat testis slices: Substrate specificity and effects of insulin and substrate analogs. Biology of Reproduction. 1981;**25**:466-474

[96] Chlouverakis C. The action of glucose on lipolysis. Metabolism. 1967;**16**:469-472

[97] Li J, Houseknecht KL, Stenbit AE, Katz EB, Charron MJ. Reduced glucose uptake precedes insulin signaling defects in adipocytes from heterozygous GLUT4

*Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

knockout mice. The FASEB Journal. 2000;**14**:1117-1125

[98] Shepherd PR, Kahn BB. Glucose transporters and insulin action— Implications for insulin resistance and diabetes mellitus. The New England Journal of Medicine. 1999;**341**:248-257

[99] Etgen GJ Jr et al. Exercise training reverses insulin resistance in muscle by enhanced recruitment of GLUT-4 to the cell surface. The American Journal of Physiology. 1997;**272**:E864-E869

[100] Klip A et al. Recruitment of GLUT-4 glucose transporters by insulin in diabetic rat skeletal muscle. Biochemical and Biophysical Research Communications. 1990;**172**:728-736

[101] Kennedy JW et al. Acute exercise induces GLUT4 translocation in skeletal muscle of normal human subjects and subjects with type 2 diabetes. Diabetes. 1999;**48**:1192-1197

[102] Ploug T, van Deurs B, Ai H, Cushman SW, Ralston E. Analysis of GLUT4 distribution in whole skeletal muscle fibers: Identification of distinct storage compartments that are recruited by insulin andmuscle contractions. The Journal of Cell Biology. 1998;**142**:1429-1446

[103] Stöckli J et al. The RabGAP TBC1D1 plays a central role in exercise-regulated glucose metabolism in skeletal muscle. Diabetes. 2015;**64**:1914-1922

[104] Humphrey SJ et al. Dynamic adipocyte phosphoproteome reveals that Akt directly regulates mTORC2. Cell Metabolism. 2013;**17**:1009-1020

[105] Kjøbsted R et al. Intact regulation of the AMPK signaling network in response to exercise and insulin in skeletal muscle of male patients with type 2 diabetes:

Illumination of AMPK activation in recovery from exercise. Diabetes. 2016;**65**:1219-1230

[106] Eguez L et al. Full intracellular retention of GLUT4 requires AS160 RabGTPase activating protein. Cell Metabolism. 2005;**2**:263-272

[107] Batista TM et al. A cell-autonomous signature of dysregulated protein phosphorylation underlies muscle insulin resistance in type 2 diabetes. Cell Metabolism. 2020;**32**:844-859.e5

[108] Boden G et al. Excessive caloric intake acutely causes oxidative stress, GLUT4 carbonylation, and insulin resistance in healthy men. Science Translational Medicine. 2015;**7**:304re7

[109] Hauck AK, Huang Y, Hertzel AV, Bernlohr DA. Adipose oxidative stress and protein carbonylation. The Journal of Biological Chemistry. 2019;**294**:1083-1088

[110] Snel M, Jonker JT, Schoones J, Lamb H, de Roos A, Pijl H, et al. Ectopic fat and insulin resistance: Pathophysiology and effect of diet and lifestyle interventions. International Journal of Endocrinology. 2012;**983814**:1-18

[111] Samuel VT, Petersen KF, Shulman GI. Lipid-induced insulin resistance: Unravelling the mechanism. The Lancet. 2010;**375**(9733):2267-2277

[112] Hammarstedt A, Graham TE, Kahn BB. Adipose tissue dysregulation and reduced insulin sensitivity in non-obese individuals with enlarged abdominal adipose cells. Diabetology and Metabolic Syndrome. 2012;**4**:42

[113] Weyer C, Foley JE, Bogardus C, Tataranni PA, Pratley RE. Enlarged subcutaneous abdominal adipocyte

size, but not obesity itself, predicts type II diabetes independent of insulin resistance. Diabetologia. 2000;**43**:1498-1506

[114] Abel ED et al. Adipose-selective targeting of the GLUT4 gene impairs insulin action in muscle and liver. Nature. 2001;**409**:729-733

[115] Vazirani RP et al. Disruption of adipose Rab10-dependent insulin signaling causes hepatic insulin resistance. Diabetes. 2016;**65**:1577-1589

[116] Poulsen P et al. Heritability of insulin secretion, peripheral and hepatic insulin action, and intracellular glucose partitioning in young and old Danish twins. Diabetes. 2005;**54**:275-283

[117] Flannick J et al. Exome sequencing of 20,791 cases of type 2 diabetes and 24,440 controls. Nature. 2019;**570**:71-76

[118] Dimas AS et al. Impact of type 2 diabetes susceptibility variants on quantitative glycemic traits reveals mechanistic heterogeneity. Diabetes. 2014;**63**:2158-2171

[119] Lotta LA et al. Integrative genomic analysis implicates limited peripheral adipose storage capacity in the pathogenesis of human insulin resistance. Nature Genetics. 2017;**49**:17-26

[120] MacKellar J, Cushman SW, Periwal V. Differential effects of thiazolidinediones on adipocyte growth and recruitment in Zucker fatty rats. PLoS One. 2009;**4**(12):e8196

[121] Levin K, Hother-Nielsen O, Henriksen JE, Beck-Nielsen H. Effects of troglitazone in young first-degree relatives of patients with type 2 diabetes. Diabetes Care. 2004;**27**:148-154

[122] Chaurasia B, Summers SA. Ceramides in metabolism: Key lipotoxic players. Annual Review of Physiology. 2021;**83**:303-330

[123] Boden G. Free fatty acids (FFA), a link between obesity and insulin resistance. Frontiers in Bioscience. 1998;**3**:d169-d175

[124] Pereira S et al. Resveratrol prevents insulin resistance caused by short-term elevation of free fatty acids in vivo. Applied Physiology, Nutrition, and Metabolism. 2015;**40**:1129-1136

[125] Felber JP, Vannotti A. Effects of fat infusion on glucose tolerance and insulin plasma levels. Medicina Experimentalis. International Journal of Experimental Medicine. 1964;**10**:153-156

[126] Wang L et al. Adipocyte Gi signaling is essential for maintaining wholebody glucose homeostasis and insulin sensitivity. Nature Communications. 2020;**11**:2995

[127] Karpe F, Dickmann JR, Frayn KN. Fatty acids, obesity, and insulin resistance: Time for a reevaluation. Diabetes. 2011; **60**:2441-2449

[128] Perseghin G, Ghosh S, Gerow K, Shulman GI. Metabolic defects in lean nondiabetic offspring of NIDDM parents: A cross-sectional study. Diabetes. 1997;**46**:1001-1009

[129] McQuaid SE et al. Downregulation of adipose tissue fatty acid trafficking in obesity: A driver for ectopic fat deposition? Diabetes. 2011;**60**:47-55

[130] Jeffery E, Church CD, Holtrup B, Colman L, Rodeheffer MS. Rapid depotspecific activation of adipocyte precursor cells at the onset of obesity. Nature Cell Biology. 2015;**17**:376-385

[131] Jeffery E et al. The adipose tissue microenvironment regulates *Molecular Mechanisms Involved in Insulin Resistance: Recent Updates and Future Challenges DOI: http://dx.doi.org/10.5772/intechopen.104806*

depot-specific adipogenesis in obesity. Cell Metabolism. 2016;**24**:142-150

[132] Arner P, Arner E, Hammarstedt A, Smith U. Genetic predisposition for type 2 diabetes, but not for overweight/ obesity, is associated with a restricted adipogenesis. PLoS One. 2011;**6**:e18284

[133] Ye J. Regulation of PPARgamma function by TNF-alpha. Biochemical and Biophysical Research Communications. 2008;**374**:405-408

[134] Shao M et al. De novo adipocyte differentiation from Pdgfrβ preadipocytes protects against pathologic visceral adipose expansion in obesity. Nature Communications. 2018;**9**:890

[135] Kim J-Y et al. Obesity-associated improvements in metabolic profile through expansion of adipose tissue. The Journal of Clinical Investigation. 2007;**117**:2621-2637

[136] Shepherd PR et al. Adipose cell hyperplasia and enhanced glucose disposal in transgenic mice overexpressing GLUT4 selectively in adipose tissue. The Journal of Biological Chemistry. 1993;**268**:22243-22246

[137] Carobbio S, Pellegrinelli V, Vidal-Puig A. Adipose tissue function and expandability as determinants of lipotoxicity and the metabolic syndrome. Advances in Experimental Medicine and Biology. 2017;**960**:161-196

[138] Czech MP. Mechanisms of insulin resistance related to white, beige, and brown adipocytes. Molecular Metabolism. 2020;**34**:27-42

[139] Zhang Y et al. Positional cloning of the mouse obese gene and its human homologue. Nature. 1994;**372**:425-432

[140] Kahn CR, Wang G, Lee KY. Altered adipose tissue and adipocyte function in

the pathogenesis of metabolic syndrome. The Journal of Clinical Investigation. 2019;**129**:3990-4000

[141] Friedman JM. Leptin and the endocrine control of energy balance. Nature Metabolism. 2019;**1**:754-764

[142] Blüher M et al. Circulating adiponectin and expression of adiponectin receptors in human skeletal muscle: Associations with metabolic parameters and insulin resistance and regulation by physical training. The Journal of Clinical Endocrinology and Metabolism. 2006;**91**:2310-2316

[143] Meyer LK, Ciaraldi TP, Henry RR, Wittgrove AC, Phillips SA. Adipose tissue depot and cell size dependency of adiponectin synthesis and secretion in human obesity. Adipocytes. 2013;**2**:217-226

[144] Yamauchi T et al. Targeted disruption of AdipoR1 and AdipoR2 causes abrogation of adiponectin binding and metabolic actions. Nature Medicine. 2007;**13**:332-339

[145] Chen Z et al. Effects of adiponectin on T2DM and glucose homeostasis: A mendelian randomization study. Diabetes, Metabolic Syndrome and Obesity. 2020;**13**:1771-1784

[146] Ortega Moreno L et al. Evidence of a causal relationship between high serum adiponectin levels and increased cardiovascular mortality rate in patients with type 2 diabetes. Cardiovascular Diabetology. 2016;**15**:17

[147] Yore MM et al. Discovery of a class of endogenous mammalian lipids with anti-diabetic and antiinflammatory effects. Cell. 2014;**159**:318-332

[148] Mori MA, Ludwig RG, Garcia-Martin R, Brandão BB, Kahn CR. Extracellular miRNAs: From biomarkers to mediators of physiology and disease. Cell Metabolism. 2019;**30**(4):656-673

[149] Agbu P, Carthew RW. MicroRNAmediated regulation of glucose and lipid metabolism. Nature Reviews. Molecular Cell Biology. 2021;**22**:425-438

[150] Crewe C, An YA, Scherer PE. The ominous triad of adipose tissue dysfunction: Inflammation, fibrosis, and impaired angiogenesis. The Journal of Clinical Investigation. 2017;**127**:74-82

[151] Hotamisligil GS. Inflammation and metabolic disorders. Nature. 2006;**444**:860-867

[152] Kammoun HL, Kraakman MJ, Febbraio MA. Adipose tissue inflammation in glucose metabolism. Reviews in Endocrine & Metabolic Disorders. 2014;**15**:31-44

[153] McNelis JC, Olefsky JM. Macrophages, immunity, and metabolic disease. Immunity. 2014;**41**:36-48