Figure 1710

Diagram of intracellular signaling pathways mediated by the insulin-like growth factor I (IGF-IR) receptor. IGF-IR when bound to IGF-I undergoes autophosphorylation on its tyrosine residues. This enhances its intrinsic tyrosine kinase activity and phosphorylates multiple substrates, including insulin receptor substrate 1 (IRS-1), IRS-2, and Src homology/collagen (SHC). IRS-1 upon phosphorylation associates with the p85 subunit of the PI3-kinase (PI3K) and phos-phorylates PI3-kinase. PI3K upon phospho-rylation converts phosphoinositide-3 phosphate (PI-3P) into PI-3,4-P2, which in turn activates a serine-thronine kinase Akt (protein kinase B). Activated Akt kinase phos-phorylates the proapoptotic factor Bad on a serine residue, resulting in its dissociation from B-cell lymphoma-X (Bc1-Xl) . The released Bc1-Xl is then capable of suppressing cell death pathways that involve the activity of apoptosis protease activating factor (Apaf-1), cytochrome C, and caspases. A number of growth factors, including platelet-derived growth factor (PDGF) and IGF 1 promotes cell survival. Activation of the PI3K cascade is one of the mechanisms by which growth factors mediate cell survival. Phosphorylated IRS-1 also associates with growth factor receptor bound protein 2 (Grb2), which bind son of sevenless (Sos) and activates the ras-raf-mitogen activated protein (ras/raf-MAP) kinase cascade. SHC also binds Grb2/Sos and activates the ras/raf-MAP kinase cascade. Other substrates for IGF-I are phosphotyrosine phos-phatases and SH2 domain containing tyro-sine phosphatase (Syp). Figure 17-7 has details on the other signaling pathways in this figure. MBP—myelin basic protein; nck—an adaptor protein composed of SH2 and SH3 domains; TF—transcription factor.

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