Growth hormone (GH) is a growth-promoting and metabolic hormone that also exhibits features common to cytokines. Metabolically, GH acutely exhibits insulin-like effects (eg., glucose uptake), but with chronic exposure promotes a state of cellular and whole-body insulin resistance (a diabetogenic effect). Like many cytokines, in certain cells GH activates transcription factors called STATs that rapidly induce particular genes. One of these STATs, activated by GH in some cells, but not others, is STAT3, an important mediator of acute inflammatory responses. At a molecular level, GH signalling is initiated by activation of the cytoplasmic kinase, JAK2, which promotes tyrosine phosphorylation of various cellular proteins including insulin receptor substrate-1 (IRS-1) -- a key component of the insulin signalling pathway -- and STATs. We propose to extend our established expertise in studies of tyrosine phosphorylation and GH receptor signalling to investigate molecular mechanisms underlying: a) GH- induced insulin resistance; and b) regulation of GH-induced STAT3 activation.
Our specific aims are: 1. Map regions of the GHR, JAK2, and IRS2, and IRS-1 (and IRS-2) necessary for IRS physical and functional interaction with the GHR- JAK2 complex. 2. Using cell lines stably expressing the GHR and JAK2 with or without IRS-2, assess the impact of the presence of IRS proteins on GH-induced phosphatidylinositol-3 kinase (PI-3 K) activation, JAK2 activation, cell proliferation, and glucose transporter translocation. 3. Develop a cell-based model of GH-induced insulin resistance in murine adipocytes and in stably transfected non-adipocyte cell lines expressing combinations of the GHR, JAK2, IRS molecules, and insulin receptor (IR) to test the effects of GH-induced tyrosine (and perhaps serine/threonine) phosphorylation events on IR signalling. 4. Assess the degree to which GH-induced STAT3 activation is attributable to autophosphorylation of particular JAK2 tyrosine residues and whether this process can be specifically influenced by the level of protein tyrosine phosphatase activation. These studies are relevant to understanding: a) the insulin resistance common to NIDDM; and b) the nature of the inflammatory state and body's response to tissue injury.
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