Western blotting was subsequently performed using anti-pGSK3 and anti-GSK3 antibodies, the latter to demonstrate equal loading. Primary cortical neurons were treated with insulin (50 nM) and lysates were prepared at the indicated time points. Following phosphorylation the Cbl-CAP complex translocates to lipid rafts in the plasma membrane.Ĭbl then interacts with the adaptor protein Crk, which is constitutively associated with the Rho-family guanine nucleotide exchange factor, C3G.Ĭ3G in turn activates members of the GTP-binding protein family, TC10, which promote GLUT4 translocation to the plasma membrane through the activation of as yet unknown adaptor molecules.įig 2: GSK3 phosphorylation after insulin treatment In this pathway, insulin receptor activation leads to the phosphorylation of Cbl, which is associated with the adaptor protein CAP. In addition, a PI3-kinase independent pathway provides a second cue for GLUT4 recruitment to the plasma membrane (Saltiel and Kahn 2001). PI3-kinase and AKT are known to play a role in GLUT4 translocation (Lizcano and Alessi 2002). Recent evidence suggests that forkhead transcription factors, which are excluded from the nucleus following phosphorylation by AKT, play a role in hepatic enzyme regulation by insulin (Schmoll et al., 2000 Barthel et al., 2001).Ī key action of insulin is to stimulate glucose uptake into cells by inducing translocation of the glucose transporter, GLUT4, from intracellular storage to the plasma membrane. Insulin directly controls the activities of a set of metabolic enzymes by phosphorylation and dephosphorylation events and also regulates the expression of genes encoding hepatic enzymes involved in gluconeogenesis. In addition to promoting glucose storage, insulin inhibits the production and release of glucose by the liver by blocking gluconeogenesis and glycogenolysis (Saltiel and Kahn 2001). Phosphorylation of glycogen synthase by GSK3 inhibits glycogen synthesis therefore the inactivation of GSK3 by AKT promotes glucose storage as glycogen. A major substrate of GSK3 is glycogen synthase, an enzyme that catalyzes the final step in glycogen synthesis. Once active, AKT enters the cytoplasm where it leads to the phosphorylation and inactivation of glycogen synthase kinase 3 (GSK3) (Figure 3). Activation of AKT also requires the protein kinase 3-phosphoinositide dependent protein kinase-1 (PDK1), which in combination with an as yet unidentified kinase leads to the phosphorylation of AKT (Figure 2). A key downstream effector of Ptd(3,4,5)P 3 is AKT, which is recruited to the plasma membrane. The catalytic subunit of PI3-kinase, p110, then phosphorylates phosphatidylinositol (4,5) bisphosphate leading to the formation of Ptd(3,4,5)P 3. Receptor activation leads to the phosphorylation of key tyrosine residues on IRS proteins, some of which are recognized by the Src homology 2 (SH2 ) domain of the p85 regulatory subunit of PI3-kinase (a lipid kinase ). These residues are recognized by phosphotyrosine-binding (PTB ) domains of adaptor proteins such as members of the insulin receptor substrate family (IRS) (Saltiel and Kahn 2001 Lizcano and Alessi 2002). Binding of insulin to the α subunit induces a conformational change resulting in the autophosphorylation of a number of tyrosine residues present in the β subunit (Van Obberghen et al., 2001). The insulin receptor is composed of two extracellular α subunits and two transmembrane β subunits linked together by disulphide bonds (Figure 1). View our interactive insulin signaling pathway. Type-1 diabetes is characterized by the inability to synthesize insulin, whereas in type-2 diabetes the body becomes resistant to the effects of insulin, presumably because of defects in the insulin signaling pathway. Failure to uptake and store nutrients results in diabetes. Insulin triggers the uptake of glucose, fatty acids and amino acids into liver, adipose tissue and muscle and promotes the storage of these nutrients in the form of glycogen, lipids and protein respectively. Insulin is a hormone released by pancreatic beta cells in response to elevated levels of nutrients in the blood.
0 kommentar(er)
