biochemical cascade biochemical cascade a biochemical cascade, also known as a signaling cascade or signaling pathway, is a series of chemical reactions that occur within a biological cell when initiated by a stimulus. this stimulus, known as a first messenger, acts on a receptor that is transduced to the cell interior through second messengers which amplify the signal and transfer it to effector molecules, causing the cell to respond to the initial stimulus. most biochemical cascades are series of events, in which one event triggers the next, in a linear fashion. at each step of the signaling cascade, various controlling factors are involved to regulate cellular actions, in order to respond effectively to cues about their changing internal and external environments. an example would be the coagulation cascade of secondary hemostasis which leads to fibrin formation, and thus, the initiation of blood coagulation. another example, sonic hedgehog signaling pathway, is one of the key regulators of embryonic development and is present in all bilaterians. signaling proteins give cells information to make the embryo develop properly. when the pathway malfunctions, it can result in diseases like basal cell carcinoma. recent studies point to the role of hedgehog signaling in regulating adult stem cells involved in maintenance and regeneration of adult tissues. the pathway has also been implicated in the development of some cancers. drugs that specifically target hedgehog signaling to fight diseases are being actively developed by a number of pharmaceutical companies. introduction signaling cascades cells require a full and functional cellular machinery to live. when they belong to complex multicellular organisms, they need to communicate among themselves and work for symbiosis in order to give life to the organism. these communications between cells triggers intracellular signaling cascades, termed signal transduction pathways, that regulate specific cellular functions. each signal transduction occurs with a primary extracellular messenger that binds to a transmembrane or nuclear receptor, initiating intracellular signals. the complex formed produces or releases second messengers that integrate and adapt the signal, amplifying it, by activating molecular targets, which in turn trigger effectors that will lead to the desired cellular response. transductors and effectors signal transduction is realized by activation of specific receptors and consequent production/delivery of second messengers, such as ca2+ or camp. these molecules operate as signal transducers, triggering intracellular cascades and in turn amplifying the initial signal. two main signal transduction mechanisms have been identified, via nuclear receptors, or via transmembrane receptors. in the first one, first messenger cross through the cell membrane, binding and activating intracellular receptors localized at nucleus or cytosol, which then act as transcriptional factors regulating directly gene expression. this is possible due to the lipophilic nature of those ligands, mainly hormones. in the signal transduction via transmembrane receptors, the first messenger binds to the extracellular domain of transmembrane receptor, activating it. these receptors may have intrinsic catalytic activity or may be coupled to effector enzymes, or may also be associated to ionic channels. therefore, there are four main transmembrane receptor types: g protein coupled receptors (gpcrs), tyrosine kinase receptors (rtks), serine/threonine kinase receptors (rstks), and ligand-gated ion channels (lgics). second messengers can be classified into three classes: hydrophilic/cytosolic – are soluble in water and are localized at the cytosol, including camp, cgmp, ip3, ca2+, cadpr and s1p. their main targets are protein kinases as pka and pkg, being then involved in phosphorylation mediated responses. hydrophobic/membrane-associated – are insoluble in water and membrane-associated, being localized at intermembrane spaces, where they can bind to membrane-associated effector proteins. examples: pip3, dag, phosphatidic acid, arachidonic acid and ceramide. they are involved in regulation of kinases and phosphatases, g protein associated factors and transcriptional factors. gaseous – can be widespread through cell membrane and cytosol, including nitric oxide and carbon monoxide. both of them can activate cgmp and, besides of being capable of mediating independent activities, they also can operate in a coordinated mode. cellular response the cellular response in signal transduction cascades involves alteration of the expression of effector genes or activation/inhibition of targeted proteins. regulation of protein activity mainly involves phosphorylation/dephosphorylation events, leading to its activation or inhibition. it is the case for the vast majority of responses as a consequence of the binding of the primary messengers to membrane receptors. this response is quick, as it involves regulation of molecules that are already present in the cell. on the other hand, the induction or repression of the expression of genes requires the binding of transcriptional factors to the regulatory sequences of these genes. the transcriptional factors are activated by the primary messengers, in most cases, due to their function as nuclear receptors for these messengers. the secondary messengers like dag or ca2+ could also induce or repress gene expression, via transcriptional factors. this response is slower than the first because it involves more steps, like transcription of genes and then the effect of newly formed proteins in a specific target. the target could be a protein or another gene. examples of biochemical cascades in biochemistry, several important enzymatic cascades and signal transduction cascades participate in metabolic pathways or signaling networks, in which enzymes are usually involved to catalyze the reactions. for example, the tissue factor pathway in the coagulation cascade of secondary hemostasis is the primary pathway leading to fibrin formation, and thus, the initiation of blood coagulation. the pathways are a series of reactions, in which a zymogen (inactive enzyme precursor) of a serine protease and its glycoprotein co-factors are activated to become active components that then catalyze the next reaction in the cascade, ultimately resulting in cross-linked fibrin. another example, sonic hedgehog signaling pathway, is one of the key regulators of embryonic development and is present in all bilaterians. different parts of the embryo have different concentrations of hedgehog signaling proteins, which give cells information to make the embryo develop properly and correctly into a head or a tail. when the pathway malfunctions, it can result in diseases like basal cell carcinoma. recent studies point to the role of hedgehog signaling in regulating adult stem cells involved in maintenance and regeneration of adult tissues. the pathway has also been implicated in the development of some cancers. drugs that specifically target hedgehog signaling to fight diseases are being actively developed by a number of pharmaceutical companies. most biochemical cascades are series of events, in which one event triggers the next, in a linear fashion. biochemical cascades include: the complement system the insulin signaling pathway the sonic hedgehog signaling pathway the wnt signaling pathway the jak-stat signaling pathway the adrenergic receptor pathways the acetylcholine receptor pathways the mitogen-activated protein kinase cascade conversely, negative cascades include events that are in a circular fashion, or can cause or be caused by multiple events. negative cascades include: ischemic cascade cell-specific biochemical cascades epithelial cells adhesion is an essential process to epithelial cells so that epithelium can be formed and cells can be in permanent contact with extracellular matrix and other cells. several pathways exist to accomplish this communication and adhesion with environment. but the main signalling pathways are the cadherin and integrin pathways. the cadherin pathway is present in adhesion junctions or in desmosomes and it is responsible for epithelial adhesion and communication with adjacent cells. cadherin is a transmembrane glycoprotein receptor that establishes contact with another cadherin present in the surface of a neighbour cell forming an adhesion complex. this adhesion complex is formed by β-catenin and α-catenin, and p120cas is essential for its stabilization and regulation. this complex then binds to actin, leading to polymerization. for actin polymerization through the cadherin pathway, proteins of the rho gtpases family are also involved. this complex is regulated by phosphorylation, which leads to downregulation of adhesion. several factors can induce the phosphorylation, like egf, hgf or v-src. the cadherin pathway also has an important function in survival and proliferation because it regulates the concentration of cytoplasmic β-catenin. when β-catenin is free in the cytoplasm, normally it is degraded, however if the wnt signalling is activated, β-catenin degradation is inhibited and it is translocated to the nucleus where it forms a complex with transcription factors. this leads to activation of genes responsible for cell proliferation and survival. so the cadherin-catenin complex is essential for cell fate regulation. integrins are heterodimeric glycoprotein receptors that recognize proteins present in the extracellular matrix, like fibronectin and laminin. in order to function, integrins have to form complexes with ilk and fak proteins. for adhesion to the extracellular matrix, ilk activate the rac and cdc42 proteins and leading to actin polymerization. erk also leads to actin polymerization through activation of cpla2. recruitment of fak by integrin leads to akt activation and this inhibits pro-apoptotic factors like bad and bax. when adhesion through integrins do not occur the pro-apoptotic factors are not inhibited and resulting in apoptosis. hepatocytes the hepatocyte is a complex and multifunctional differentiated cell whose cell response will be influenced by the zone in hepatic lobule, because concentrations of oxygen and toxic substances present in the hepatic sinusoids change from periportal zone to centrilobular zone10. the hepatocytes of the intermediate zone have the appropriate morphological and functional features since they have the environment with average concentrations of oxygen and other substances. this specialized cell is capable of: regulate glucose metabolism via camp/pka/torc (transducers of regulated creb)/cre, pip3 /pkb and plc /ip3 expression of enzymes for synthesis, storage and distribution of glucose synthesis of acute phase proteins via jak /stat /apre (acute phase response element) expression of c-reactive protein, globulin protease inhibitors, complement, coagulation and fibrinolytic systems and iron homeostasis regulate iron homeostasis (acute phase independent) via smads /hamp hepcidin expression regulate lipid metabolism via lxr /lxre (lxr response element) expression of apoe cetp, fas and lpl exocrine production of bile salts and other compounds via lxr /lxre expression of cyp7a1 and abc transporters degradate of toxic substances via lxr /lxre expression of abc transporters endocrine production via jak/stat /ghre (growth hormone response element) igf-1 and igfbp-3 expression via thr/thre (thyroid hormone response element) angiotensinogen expression regenerate itself by hepatocyte mitosis via stat and gab1: ras/mapk, plc/ip3 and pi3k/fak cell growth, proliferation, survival, invasion and motility the hepatocyte also regulates other functions for constitutive synthesis of proteins (albumin, alt and ast) that influences the synthesis or activation of other molecules (synthesis of urea and essential amino acids), activate vitamin d, utilization of vitamin k, transporter expression of vitamin a and conversion of thyroxine. neurons purinergic signalling has an essential role at interactions between neurons and glia cells, allowing these to detect action potentials and modulate neuronal activity, contributing for intra and extracellular homeostasis regulation. besides purinergic neurotransmitter, atp acts as a trophic factor at cellular development and growth, being involved on microglia activation and migration, and also on axonal myelination by oligodendrocytes. there are two main types of purinergic receptors, p1 binding to adenosine, and p2 binding to atp or adp, presenting different signalling cascades. the nrf2/are signalling pathway has a fundamental role at fighting against oxidative stress, to which neurons are especially vulnerable due to its high oxygen consumption and high lipid content. this neuroprotective pathway involves control of neuronal activity by perisynaptic astrocytes and neuronal glutamate release, with the establishment of tripartite synapses. the nrf2/are activation leads to a higher expression of enzymes involved in glutathione syntheses and metabolism, that have a key role in antioxidant response. the lkb1/nuak1 signalling pathway regulates terminal axon branching at cortical neurons, via local immobilized mitochondria capture. besides nuak1, lkb1 kinase acts under other effectors enzymes as sad-a/b and mark, therefore regulating neuronal polarization and axonal growth, respectively. these kinase cascades implicates also tau and others map. an extended knowledge of these and others neuronal pathways could provide new potential therapeutic targets for several neurodegenerative chronic diseases as alzheimer's, parkinson's and huntington's disease, and also amyotrophic lateral sclerosis. blood cells the blood cells (erythrocytes, leukocytes and platelets) are produced by hematopoiesis. the erythrocytes have as main function the o2 delivery to the tissues, and this transfer occurs by diffusion and is determined by the o2 tension (po2). the erythrocyte is able to feel the tissue need for o2 and cause a change in vascular caliber, through the pathway of atp release, which requires an increase in camp, and are regulated by the phosphodiesterase (pde). this pathway can be triggered via two mechanisms: physiological stimulus (like