Insulin Signaling Pathway

Insulin is a peptide hormone produced by beta cells of the pancreatic islets. It is the major hormone controlling critical energy functions such as glucose and lipid metabolism. Insulin signaling plays an important role in the physiological action of insulin. Insulin binds to its receptor, inducing intracellular signal transduction through a series of intracellular signaling molecules, activating signaling pathways, reaching the effector, and finally producing various physiological effects.

Insulin action is mediated through the insulin receptor, which is composed of two extracellular α subunits and two transmembrane β subunits linked together by disulphide bonds. Insulin activates the insulin receptor tyrosine kinase (IR) that phosphorylates and absorbs different substrate adaptors. Among them, PI3-kinase (Phosphoinositide 3-kinase) leads to crucial metabolic functions such as synthesis of lipids, proteins and glycogen mainly via the activation of the PKB(AKT) and the PKCζ cascades. AKT, a serine kinase, which can deactivate glycogen synthesis kinase 3 (GSK3), leads to the activation of glycogen synthase (GYS) and thus glycogen synthesis. it can influence the synthesis of protein via mTOR. Insulin activates several other protein kinases, such as p70 ribosomal S6 kinase (S6K), belonging to the same protein kinase subfamily as PKB, and also participates in the synthesis, growth and proliferation of protein. In addition, PKB can inhibit multiple pro-apoptotic factors (such as Bad, GSK-3, Caspase 9 and mTOR) to ensure cell survival. Insulin signaling also has growth and mitogenic effects, which are mostly mediated by activation of the Ras/MAPK pathway. This pathway involves the tyrosine phosphorylation of IRS proteins and/or Shc. With the adapter protein Grb2, recruiting the Son-of-sevenless (SOS) exchange protein to the plasma membrane for activation of Ras. Once activated, Ras operates as a molecular switch, stimulating a serine kinase cascade through the stepwise activation of Raf, MEK and ERK. Activated ERK can translocate into the nucleus, where it catalyses the phosphorylation of transcription factors that leads to cellular proliferation or differentiation. Insulin signal transduction promotes glucose uptake by activating intracellular signal transduction pathways that promote transports of GLUT4 glucose to the plasma membrane. In addition, binding of insulin to its receptor also causes it to phos-phorylate the protein Cbl in a complex with the adaptor protein CAP. At this location Cbl interacts with the adaptor protein Crk which is constitutively associated with the Rho-family guanine nucleotide exchange factor. C3G then activates members of the GTP-binding protein family, TC10, which activates unknown effector molecules to promote GLUT4 translocation. Insulin stimulates glucose uptake in muscle and adipocytes via translocation of GLUT4 vesicles to the plasma membrane.

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