Intracellular signalling by the insulin receptor

The insulin receptor transduces insulin\u27s biological signal through the tyrosine kinase present in the receptor\u27s B subunit. The activated insulin receptor kinase then phosphorylates a series of intracellular substrate including insulin receptor substrate 1 (IRS-1), which has been shown to be the pivotal substrate for insulin receptor signal transduction. The phosphorylated tyrosine residues in IRS-1 can bind and activate the downstream effectors, many of which are SH2 domain containing proteins such as phosphotidylinositol 3-kinase, growth factor binding protein 2, and SH2 phosphotyrosine phosphatase 2. Phosphorylated synthetic IRS-1 peptides with the corresponding sequences of the IRS-1 have been shown to associate and activate their respective SH2 domain containing proteins. Another important event happening during insulin binding with the insulin receptor is that the insulin receptor rapidly undergoes internalization. However, the insulin receptor signalling and the receptor endocytosis have been studied as two independent processes. The hypothesis of the present thesis is that the insulin receptor endocytosis is involved in insulin receptor signalling and signal termination. The results of the present investigation demonstrate that insulin receptors in the earliest stage of endocytosis contain significantly greater kinase activity towards IRS-1 peptides than the receptors localized at the plasma membrane, indicating that they are potentially more capable of transducing signals. On the other hand, insulin receptors in the middle and late stage of endocytosis lose their kinase activity, suggesting that insulin receptor kinase activity inactivation and signal termination might take place in the late phase of the insulin receptor internalization. In addition, this study also found that the increased insulin receptor kinase activity in the endosomes is related to the tyrosyl phosphorylation of the specific domains of the receptor\u27s $\beta$ subunit