autophagy is generally considered to be a cell survival mechanism and may also contribute to cell death , depending on its various biological contexts.sphingolipids were first discovered in brain extracts in 1876 , and within a century of intensive research , the chemical structures of thousands of individual sphingolipids had been elucidated.sphingolipid-associated autophagy contributes to a range of diseases , including neurodegeneration , tumors , metabolic disorders , and heart diseases .
autophagy is generally considered to be a cell survival mechanism and may also contribute to cell death , depending on its various biological contexts .
sphingolipids were first discovered in brain extracts in 1876 , and within a century of intensive research , the chemical structures of thousands of individual sphingolipids had been elucidated .
sphingolipid - associated autophagy contributes to a range of diseases , including neurodegeneration , tumors , metabolic disorders , and heart diseases .
sphingolipids are abundant lipid components of eukaryotic plasma membranes that function in a wide range of biological processes .
are sphingolipids involved in the composition of autophagic membranes?why do perturbations of sphingolipid metabolism by chemotherapy and nutrition starvation trigger lethal autophagy and protective autophagy , respectively?sphingolipid - mediated signals that have long been known to induce apoptosis are now also recognized to cause autophagy .
are sphingolipid - associated apoptosis and autophagy interlinked?how does sphingolipid - associated autophagy occur and to what extent do alterations in autophagy contribute to disease phenotype?what drugs or compounds that target sphingolipid metabolism are attractive candidates for therapeutic development ?
sphingolipids are abundant lipid components of eukaryotic plasma membranes that function in a wide range of biological processes . are sphingolipids involved in the composition of autophagic membranes
? why do perturbations of sphingolipid metabolism by chemotherapy and nutrition starvation trigger lethal autophagy and protective autophagy , respectively ?
sphingolipid - mediated signals that have long been known to induce apoptosis are now also recognized to cause autophagy .
how does sphingolipid - associated autophagy occur and to what extent do alterations in autophagy contribute to disease phenotype ?
autophagy is generally considered to be a cell survival mechanism , and this process also contributes to cell death in several situations .
sphingolipids ( sls ) are ubiquitous components of membrane structures , and renewed interest in sls has focused on sl - induced intracellular and extracellular signaling .
moreover , studies have uncovered that a dynamic balance among sl metabolites is significant in cell fate determination .
here we review the key aspects of bioactive sls that have emerged as important effectors in regulating the autophagic pathway , mediating the cross talk between apoptosis and autophagy , and determining the associated cell fate .
a deeper understanding of the relationship between sl metabolism and autophagy explains how intact or impaired sl - related autophagic pathways are involved in the malfunctions associated with neurodegeneration , cancer , and other diseases .
we also discuss how autophagy - regulating drugs work via cells ' sl metabolism and the methods that could be used to monitor sl - related autophagy .
present and future investigations regarding sl - related autophagy will help to develop novel treatment strategies to control autophagy - related diseases .
sls represent a significant class of lipids that contain a backbone of sphingoid bases and are ubiquitous constituents of membranes in eukaryotes .
sls were first discovered in brain extracts in 1876 , and within a century of intensive research , the chemical structures of thousands of individual sls had been elucidated .
the sl metabolic pathway displays an intricate network of reactions that result in the formation of multiple sls , including ceramide ( cer ) , dihydroceramide ( dhcer ) , and sphingosine-1-phosphate ( s1p ) .
first , a substantial portion of sls are derived from de novo biosynthesis in most organisms , with the condensation of serine and palmitoyl - coa catalyzed by serine palmitoyl transferase to generate dehydrosphinganine .
dehydrosphinganine is subsequently reduced to form dihydrosphingosine ( sphinganine ) , which is then n - acylated by dhcer synthase to produce dhcer or cer .
second , through hydrolysis via a complex lipid - turnover pathway , several specific hydrolases are involved in the turnover process , including sphingomyelinases ( smases ) and glucocerebrosidase ( gcase ) .
in addition to the above two sources , cer is also recycled . in this pathway
, sphingosine is recycled into cer via cer synthases ( cers ) ; therefore , this pathway is also called the salvage pathway . the cellular metabolic homeostasis of the sphingolipidome is often achieved through the coordination of the biosynthesis and removal of sl species , which requires a delicate balance between de novo biosynthesis , turnover , and recycling .
sls are found in cellular membranes , lipoproteins , and other lipid - rich structures ; however , they are synthesized in the endoplasmic reticulum ( er ) and golgi apparatus .
the scheme in figure 1 depicts the subcellular localization of sl biosynthesis , turnover , and recycling .
in addition , sls may be incorporated into other intracellular compartments , such as mitochondria or autophagosomes .
the turnover and recycling of sls also occur at several intracellular locations , including the endosome , phagosome , and lysosome .
thus , sls travel between organelles , and this transport occurs via either transport vesicles or transfer proteins .
cert ( cer transfer protein ) and fapp2 ( four - phosphate adaptor protein 2 ) have proved to be two critical sl - trafficking proteins that regulate the trafficking of sls to specific compartments within cells .
lipids are increasingly implicated in the control of the membrane remodeling and vesicle transport that underlie the biogenesis of autophagosomes . however
, our knowledge of whether and how sl trafficking is associated with the autophagic process remains incomplete .
sls are considered to primarily have roles as components of membranes and other biological structures .
however , several sl metabolites , including cer , dhcer , and s1p , have drawn attention as bioactive signaling molecules that mediate cell growth , differentiation , senescence , apoptosis , and autophagy .
 autophagy , ' which is derived from greek and means  to eat oneself , ' ensures the synthesis , degradation , and recycling of cellular components in eukaryotic cells ranging from yeasts to mammals . at present , the following three modes of autophagy have been identified : macroautophagy ( which is commonly called  autophagy , ' including in this review ) , microautophagy , and chaperone - mediated autophagy . during the autophagic process , nonspecific or targeted cytoplasmic constituents
the formation of the autophagosome is a multistep process that includes the biogenesis of the isolation membrane , followed by its elongation and closure .
most autophagy - related genes ( atgs ) contribute to autophagosome formation , and many are well conserved from yeasts to mammals .
sls are abundant lipid components of eukaryotic plasma membranes that function in a wide range of biological processes , which make sls strong candidates as possible autophagosomal lipids .
several studies have implied that sls , and especially cer , are autophagosomal membrane components .
in addition , sls formed by de novo biosynthesis in the er might be a driving force for the formation of the autophagosomal vacuole , in what has been referred to as the  membrane extension ' step , which occurs after many of the associated autophagosomal proteins have been recruited . because the enzymes required for de novo cer biosynthesis reside in the er , it is possible that they might be recruited into autophagosomes and perhaps continue to produce sls there . the best - characterized pathway regulating autophagy includes a class i phosphatidyl inositol 3-kinase ( pi3k ) and mammalian target of rapamycin ( mtor ) , which act to inhibit autophagy .
although how autophagy is suppressed by these molecular signals remains to be revealed , mtor complex 1 ( mtorc1 ) is known to phosphorylate the autophagy regulatory complex , ulk1 ( containing unc-51-like kinase 1 ) , the mammalian atg13 protein , and focal adhesion kinase - interacting protein of 200  kd ( fip200 ) .
indeed , different bioactive sl species have been shown to mediate distinct autophagic pathways , described as protective autophagy and autophagy - associated cell death ( figure 2 ) , which have opposing functions in cellular life - or - death decisions .
cer , which is known to induce cell cycle arrest and has been implicated in important physiological roles in cell differentiation , senescence , migration , adhesion , and inflammatory responses , is the key intermediate in sl metabolism .
exogenous cer - related autophagy was shown to trigger autophagy - associated cell death , which occurs in several malignant cell types , including colon cancer , breast cancer , cervical cancer , nasopharyngeal cancer , and glioma cells .
in addition to exogenous cer , endogenous cer species are critical for the induction of autophagy .
tamoxifen , an estrogen receptor antagonist used to treat several types of breast cancer , has been shown to induce autophagy - associated cell death by increasing endogenous cer levels .
for example , cer has been found to stimulate autophagy by regulating classic or atypical autophagic pathways and signals . via these signals ,
class i pi3k and akt negatively regulate autophagy , but a class iii pi3k is needed for the activation of autophagy .
cer was found to promote the interaction of class iii pi3k with other regulators of autophagy and to inhibit akt by activating phosphoprotein phosphatase 2a . moreover , exogenously added cer and the accumulation of endogenous cer due to treatment with certain chemotherapeutic drugs , such as tamoxifen , inhibit the mtor signaling pathway , which has a central role in inducing autophagy and increasing the expression of beclin1 , an upstream regulator of autophagy .
a short - chain cer analog has also been associated with the induction of autophagy - associated cell death by increasing the transcription of the bh3-only protein , a mitochondria - associated protein that induces nonapoptotic cell death .
a recent study showed that cer directly interacts with microtubule - associated protein light chain 3 ( lc3 ) on mitochondrial membranes to induce deadly autophagy via an increase in intracellular mitophagy .
s1p has been found to induce survival - mediated or protective autophagy under nutrient starvation , distinct from cer - associated autophagy - associated cell death .
in contrast to cer - related autophagy , s1p - mediated autophagy has not been shown to be related to the accumulation of beclin1 protein or to the suppression of class i pi3k or akt
. however , mtor was still inhibited by increased s1p levels , which suggests that s1p induces autophagy via inhibiting mtor and is independent of the class i pi3k signal .
dhcer has been used as a negative control for cer treatments to induce autophagy because dhcer has long been thought to be biologically inactive .
however , dhcer was recently identified as a novel sl - based mediator of autophagy .
endogenously added dhcer induced a transient , early increase in dhcer levels via inhibition of dhcer desaturase , such as xm462 , which successively promoted autophagy and reduced etoposide toxicity in gastric and colon cancer cells . however , anticancer agents such as fenretinide ( 4-hydroxy ( phenyl ) retinamide or 4-hpr ) and gamma tocotrienol ( a lipophilic antioxidant of vitamin e ) induced cancer cell death through the elevation of intracellular dhcer levels .
interestingly , none of the reported cer - mediated autophagic pathways is induced by both cer and dhcer .
intriguingly , the accumulation of dhcer may serve as an additional  switch ' to regulate cell fate ; however , the biological activity of dhcer remains controversial and unclear .
in addition to the major sls that induce autophagy , gangliosides and several rare sls are also involved in autophagy - related cell fate decisions .
an increase in the amount of lc3-ii and an accumulation of autophagic vacuoles were observed in cells that underwent ganglioside treatment .
cer methylaminoethyl phosphonate and sphingadienes are two rare sls that promote autophagy - associated cell death through the downregulation of pi3k / akt and the activation of beclin1 , similar to cer .
overall , although cer , dhcer , and s1p are all able to induce autophagy , the effects of s1p - mediated autophagy are markedly mild compared with the effects elicited by cer .
dhcer - related autophagic effects are moderate , and their intensity level lies between the levels caused by cer and s1p .
treatment with chemotherapeutic agents often promotes the biosynthesis of intracellular cer and leads to autophagy - associated cell death ; however , elevation in s1p levels is typically a cell response to nutrition starvation and mediates cytoprotective autophagy .
furthermore , cells maintain a dynamic equilibrium between the levels and the effects of cer , dhcer , and s1p .
the conversion of cer to s1p simultaneously accumulates the survival effects and removes the death signals .
this observation has led to the concept of a so - called  sl rheostat ' or  sl biostat , ' based on the relative amounts and reciprocal roles of these antagonistic metabolites , which are critical in guiding the destiny of cells .
apoptosis generally initiates cell death , whereas autophagy is primarily a protective process for the cell that may also contribute to cell death .
there is a sophisticated and not yet fully understood association between autophagy - related cell death and survival that depends on various biological situations . in certain cellular contexts , autophagy functions as a stress response to suppress apoptosis and promote cell survival .
however , in other cases , autophagy may serve as a mechanism for caspase - dependent or -independent cell death .
certain sls are now thought to elicit autophagy ; however , they have also been known to induce apoptosis .
cer is a well - established inducer of apoptosis via activation of the mitochondrial pathway . in a number of different cell types ,
dhcer has also been suggested to exhibit antiapoptotic effects because the lipid inhibits cer channel assembly in isolated mitochondria .
in addition , other sl metabolites , such as sphingosine and ganglioside , are involved in the modulation of apoptosis and autophagy .
sl - associated apoptosis and autophagy are interlinked . the conversion of cer to s1p switches the cell fate from apoptosis to autophagy - induced survival . several exogenous stimuli , and particularly the activation of the enzymes that interconvert cer and s1p , such as sphingosine kinases , lead to an increase in s1p levels , a corresponding reduction in cer levels , and the antagonistic effects of cer .
in addition , dhcer has been proposed to exhibit antiapoptotic effects to promote cell survival during hypoxia through the induction of autophagy , while also serving as a lipid reserve for the rapid production of cer to address cellular damage on reperfusion .
dhcer serves as a unique regulator of cell fate , controlling the  switch ' between cytoprotective autophagy and cer - mediated apoptosis in response to stress .
intriguingly , cer promotes both apoptosis and autophagy through the inhibition of class i pi3k and akt signaling or the mtor pathway .
furthermore , suppressors of apoptosis , such as the bcl-2 family , also directly bind and control beclin1 .
disruption of the beclin1bcl-2 complex has also emerged as a common mechanism in cer - associated autophagy .
in addition to altering the balance between beclin1 and bcl-2 protein levels , endogenous cer liberates beclin1 for autophagy induction through the jnk - mediated phosphorylation of bcl-2 .
notably , bcl-2 has emerged as a critical regulator of the cer - mediated cross talk between autophagy and apoptosis .
although the primary function of autophagy is to aid in adaptation to cellular stress under adverse conditions by enabling cells to degrade cytosolic proteins and organelles to generate a supply of essential nutrients , autophagy has also emerged in the pathological process of many disorders , including neurodegeneration , tumors , immunity responses , and heart diseases .
sphingolipidoses are a collection of more than 40 genetically distinct disorders caused by inherited deficiencies of lysosomal hydrolytic activities or lipid transport .
these deficiencies result in intracellular accumulations of cholesterol and lipids in the endosomal / lysosomal network , usually leading to signs of neurodegeneration .
the biochemical abnormalities in sphingolipidoses are complex and lead to the accumulation of sl metabolites by diverse pathways ( table 1 ) .
pick disease type c ( npc ) is a complex neurodegenerative sphingolipidosis characterized by the accumulation of unesterified cholesterol , sls , and complex gangliosides in late endosomes and lysosomes .
the disease is caused by mutations in either the npc1 or the npc2 gene , which disturbs not only the regular transport of endocytosed lipids but also the autophagic flux , leading to an accumulation of autophagic vacuoles in cells . 

npc1 and npc2 are genes whose protein products mediate proper intracellular lipid transport through pathways that are incompletely understood .
the brains of afflicted humans and npc - deficient animal models are marked by a loss of neurons .
histological studies and cell culture experiments have shown that defective lysosomal degradation of autophagosomes may contribute to the abnormal autophagic flux .
the deficiency present in npc promotes both the generation of autophagosomes and the impairment of autophagic flux .
an imbalance between induction and flux through the autophagic pathway contributes to cell stress and neuronal loss in npc .
gaucher disease ( gd ) , a kind of sphingolipidosis , is characterized by the accumulation of glucosylceramide or glucosylsphingosine in the lysosomes of the cells of the monocyte / macrophage system . in gd ,
the point mutations within the gba ( glucosidase , beta , acid ) gene lead to the production of acid -glucosidase with functional , kinetic , trafficking , and/or stability defects and a resultant decrease in lysosomal function and increase in the accumulation of glucosylceramide and glucosylsphingosine .
gd can be classed into three subsets , based on the age of onset and the presence of central nervous system abnormalities .
type 1 is known as the non - neuronopathic form , and types 2 and 3 are differentiated from type 1 by neurodegeneration of the central nervous system with either rapid or chronic progression .
reported that mutation of saposin ( sap ) c , and not a direct gba gene mutation , was associated with the type 3 or type 1 phenotype in cases of gd .
the decrease in / absence of sap c affected gcase intracellular localization , resulting in lysosomal lipid accumulation and enhanced autophagy .
this study suggested that sap c might have a role in intralysosomal sl transport , as indicated by the accumulation of glucosylceramide and cer in the lysosomes of the sap c - mutant cells .
in addition , deficient degradation of autophagic substrates in cells in gd can lead to an increased risk of parkinson disease .
for npc , gd , and other sphingolipidoses , the relevance of the impairment of autophagic flux to disease pathogenesis remains poorly defined .
in addition , the extent to which the accumulation of autophagic substrates contributes to neuron dysfunction needs to be determined in future studies .
dysregulated sl metabolism occurs in numerous cancers and has been shown to contribute to cancer progression and chemoresistance .
several species of sls have been shown to have aberrant expression or metabolism in cancer cells .
the tumor suppressor cer and the tumor promoter s1p are generally recognized to trigger autophagy ; however , cer and s1p have different outcomes regarding cell death and survival .
nevertheless , recent studies have suggested that de novo - generated cer - associated autophagy can be lethal as well as protective for cells .
for example , beljansky et al . determined that enhancement of cer levels through treatment with a sphk 2-selective inhibitor suppresses tumor growth by leading to autophagy - associated cell death .
conversely , park et al . suggested that cer - cd95-perk signaling promotes cell death via the cascade activation of caspase proteins ; however , gene silencing of atg5 further strengthened the cell death process , indicating that the autophagy was protective . the contradictory activities may correspond to variations among cers in their carbon chain lengths , double bond numbers , subcellular distributions , and versatile targets .
in addition , cer is metabolized into s1p by sphk 1 or sphk 2 , which are associated with protective autophagy and other biological behaviors , including survival , infiltration , angiogenesis , metastasis , and resistance to anticancer drugs in cancer cells .
intact and impaired sl - related autophagy has significant implications for many other diseases and pathological processes .
for example , cers 5-mediated autophagy has recently been implicated in lipotoxic cardiomyopathy and hypertrophy .
in addition , kdo 2-lipid a induced substantial alterations in sl metabolism and composition in raw264.7 cells , a mouse macrophage - like cell line .
these changes apparently promote the de novo sl biosynthesis that is required for autophagosome generation , which is suggested to have an essential role in the processes of the innate immune response .
cer also mediates the augmentation of interleukin-1(il-1 ) levels and the release of tumor necrosis factor ( tnf ) alpha that are induced by toll - like receptor 4 , and this process may contribute to the enhanced inflammatory response in metabolic diseases , including obesity and diabetes , which are characterized by dyslipidemia .
overall , the aberrations in the bioactive sls that mediate autophagy have been associated with diverse pathological conditions , including neurodegeneration , carcinogenesis , metabolic diseases , and inflammatory responses .
understanding the specific mechanism connecting sl - related autophagy and these diseases has substantial implications for revealing these diseases ' biochemical characteristics and for designing and developing new therapeutic strategies .
many human disease states may be caused by the aberrant regulation of sl metabolism - associated autophagy ; therefore , drugs and compounds that target sl metabolism , including chemotherapeutic drugs , sphingomimetics , and enzyme inhibitors , are attractive candidates for therapeutic development ( table 2 ) .
many chemotherapeutic drugs elicit autophagy and exert their anticancer properties by increasing the intracellular levels of sls .
a large number of malignant cells often acquire genetic deletions or mutations that render them resistant to the classic apoptotic cell death that is induced by many anticancer therapies , which ultimately results in poor prognosis . in these cases
, the use of autophagy - inducing drugs offers an alternative way to induce cell death in apoptosis - resistant tumors .
as reviewed in the present paper , one of the major results of the chemotherapy - mediated increase in the levels of intracellular sls , such as cer and dhcer , is cell death induction by autophagy in various human cancer cells .
autophagy is initiated by tamoxifen , an antagonist of the estrogen receptor , leading to an increase in endogenous cer levels that acts against breast cancer cells .
arsenic trioxide , a potent antineoplastic agent , has been used clinically for the treatment of certain types of leukemia .
arsenic trioxide was shown to induce the biosynthesis of cer in both acute promyelocytic leukemia and adult t - cell leukemia cells , and the cytotoxic effects were attributed to the induction of both apoptosis and autophagy .
-tetrahydrocannabinol , the major active component of marijuana , was reported to induce the activation of autophagy - mediated apoptotic cell death by increasing de novo cer biosynthesis . moreover
, sl metabolism may be involved in the regulation of autophagy by other chemotherapeutic drugs , including , but not limited to , etoposide , doxorubicin , and daunorubicin .
resveratrol is a natural polyphenolic phytoalexin produced by grapes and other berries that exerts its cancer - preventive properties in several animal models and exhibits its potent anticancer activities in leukemia and many solid tumors .
the capacity to trigger dhcer biosynthesis contributes to resveratrol - induced autophagy - associated cell death .
puissant et al . found that resveratrol initiates autophagy in chronic myelogenous leukemia cells through the amp - activated protein kinase ( ampk)-dependent induction of autophagy and that p62 is also required for resveratrol - mediated autophagy .
whether other polyphenolic compounds , such as curcumin , which has two phenolic hydroxyl groups : genistein , which has three phenolic hydroxyl groups ; and quercetin , which has four phenolic hydroxyl groups , are implicated in sl metabolism and sl - mediated autophagy remains to be determined . in clinical trials ,
the promise of 4-hpr as a therapeutic agent is enhanced by its unique ability to activate the autophagy - associated cell death pathway when the apoptotic pathway is deregulated , which is a characteristic that is not shared by other chemotherapeutic agents , including cisplatin and etoposide .
furthermore , the accumulation of dhcer following treatment with 4-hpr was found to induce cell cycle arrest in neuroblastoma cells .
sl metabolism is an exploitable target for the discovery of novel chemotherapeutics , and this target has been validated through the development of several sphingomimetics .
safingol , a synthetic sphinganine , triggers autophagy in several tumor cell types by the suppression of class i pi3k and akt signaling .
safingol has been assessed in phase i clinical trials alone or in combination with cisplatin for adults with advanced solid tumors and for children with neuroblastoma .
safingol treatment was shown to trigger a dose - dependent decrease in s1p levels , and safingol is a promising representative drug whose primary mechanism for promoting tumor cell death is autophagy .
targeting enzymes in sl metabolism is another attractive avenue to provide therapeutic benefits in sl - related human diseases .
acid smases ( asmases ) and acid ceramidases ( acdases ) are crucial enzymes that modulate the synthesis and degradation of cer in sl metabolism .
acdase overexpression has been observed in cancer cell lines and primary tumors and contributes to resistance to chemotherapy and radiation .
the consequence of acdase overexpression is the ability to convert cer , which is often produced as a proapoptotic response to stress , to sphingosine , which can then be converted to s1p .
in addition to the ability to metabolize the cer produced in response to stress , turner et al . found that prostate cancer cell lines overexpressing acdase also have an increased lysosomal density and increased levels of autophagy .
identified an analog of cyclosporin a , psc 833 ( valspodar , novartis pharma ag , basel , switzerland ) , a second - generation , non - immunosuppressive p - glycoprotein antagonist that inhibits acdase and generates autophagy - associated cytotoxicity in pancreatic cancer cells .
b13 was found to be a potent cdase inhibitor that induced colon cancer cell death and the inhibition of colon tumor growth in a xenograft model .
furthermore , lcl385 , a b13 analog , enhanced the sensitivity of prostate cancer cells to radiation and inhibited tumor growth in a nude mouse model .
another novel b13 analog , lcl204 , has been shown to overcome the resistance of head and neck squamous carcinoma cells to fas - induced cell death in both in vitro and in vivo experiments .
most acdase inhibitors are lysosomotropic agents and cause lysosomal destabilization and a change in sl metabolism enzymes in the lysosomal compartment .
, asmase seems to reside in classic lysosomes , where it mediates the catabolism of sphingomyelin .
asmase activity is essential for lysosomal stability and the survival of cancer cells , as well as for the multidrug - resistant phenotype .
because of their ability to inhibit autophagic flux , certain asmase inhibitors , such as chloroquine , siramesine , and clomipramine , are currently being tested as anticancer agents in several clinical trials and laboratory studies .
asmase was also found to be a target for the treatment of niemann  pick disease type a ( npa ) , which is caused by loss - of - function mutations in the asmase gene and is a lysosomal storage disorder leading to neurodegeneration .
fibroblasts from npa patients and asmase - knockout mouse brains show similar autophagolysosome accumulation and impaired autophagy .
a recent study has suggested that the control of lysosome trafficking and fusion by asmase is essential to normal autophagic flux in coronary arterial smooth muscle cells and has a protective role in atherosclerosis .
other chemotherapeutic treatments , such as several histone deacetylase inhibitors , were also demonstrated to lead to leukemic cell death by enhancing cer production via the degradation of sphingomyelin .
one of these histone deacetylase inhibitors is suberoylanilide hydroxamic acid , which was demonstrated to trigger autophagy - associated cell death in apoptosis - deficient tumor cells , such as chondrosarcoma cells .
numerous well - developed and convenient experimental methods and techniques that can be used to detect autophagy in different species systems exist . to establish that an autophagic response is occurring in sl - related autophagy , multiple methods have been used , including cellular ultrastructure studies by transmission electron microscopy , an lc3-puncta formation assay combined with immunoblots for autophagic cargoes ( for example , p62 and nbr1 ) , and the detection of autophagy - related molecules .
in addition , performing a large - scale and comprehensive sphingolipidomic analysis has been challenging in the study of sl - associated autophagy .
both nets and hooks are indispensable tools to allow efficient  fishing ' for multiple intermediates and products directly from the sophisticated process of sl metabolism . because there is an obvious overlap between the subcellular localization of enzymes needed for sl biosynthesis and autophagosome formation , sls
autophagy - associated cell death and cell survival represent the so - called  yin - yang ' regulatory mechanism .
the balance has been exemplified by interconvertible sls that produce opposite autophagic effects on cells that were termed as  sl rheostat ' or  sl biostat ' .
in addition , the increase in intracellular cer levels usually results from chemotherapy and often stimulates autophagy - associated cell death , although this increase has also been found to be cytoprotective .
in contrast , starvation treatment triggers protective autophagy via increasing s1p levels . however , the underlying mechanisms by which chemotherapy and nutrient deprivation mediate different regulation are still largely unknown .
sls mediate the cross talk between apoptosis and autophagy because certain sls share several common kinase signaling pathways that regulate cell fate that affect both apoptosis and autophagy or execute a seesaw type of regulation of key molecules .
increasing new knowledge on the biochemistry and cell biology of sl - mediated autophagy is beneficial for deepening our understanding of sl - associated diseases , including neurodegeneration , cancer pathogenesis , and inflammatory responses .
an imbalance between induction of and flux through the autophagic pathway contributes to cell stress and neuronal loss in npc .
intact and impaired sl - related autophagy has significant implications for the development of malignant tumors .
in addition , the current understanding of the effects of multiple sls on the modulation of various autophagy - related cell fates has primarily arisen from exogenous sls or pharmacological perturbations of sl metabolism .
the identification of sl - mediated cytoprotective autophagy that can be shifted to cell death may provide a novel strategy for cancer therapy , including chemotherapy , sphingomimetics , sl metabolic enzyme inhibitors , and histone deacetylase inhibitors .
 the autophagic process involves encompassing damaged proteins and organelles within double- or multi - membraned structures and delivering these molecules to the lytic compartments of vacuoles . 
 sphingolipids ( sls ) , which are ubiquitous membrane lipids in eukaryotes , participate in the generation of various membrane structures , including rafts , caveolae , and cytosolic vesicles . 
 sls are a complex family of molecules that have a growing number of members , including ceramide , sphingosine-1-phosphate , and dihydroceramide , which have been associated with the essential cellular process of autophagy . 
 this review highlights recent studies focusing on the regulation and function of sl - associated autophagy and its role in cell fate , diseases , and therapeutic interventions . 
Facts
Open Questions
The SL Family
SLs Involved in the Autophagic Pathway
Possible SL Involvement in the Cross Talk Between Autophagy and Apoptosis
SL-Associated Autophagy and Diseases
Compounds and Drugs that Might Trigger Autophagy-Associated Cell Death Through Regulation of SL Metabolism
Conclusion