Molecular Mechanism of Growth Hormone - Involvement of Janus Kinase 2, Insulin Receptor Substrate-1 and -2 and Phosphatidylinositol 3-Kinase In the Acute Insulin-Like Effects of Growth Hormone In Primary Rat Adipocytes

Sammanfattning: Originally studying the molecular mechanisms for the insulin-like effects of growth hormone (GH), lipogenesis and antilipolysis, in isolated rat adipocytes we found that the GH receptor (GHR) was tyrosine phosphorylated in response to GH in cells that were responsive to these effects. Then, as Janus kinase 2 (JAK2) was described as a GH-stimulated and GHR-associated tyrosine kinase, we found that JAK2 was tyrosine phosphorylated in response to GH in responsive cells. These phosphorylations were only marginally observed in response to GH in cells that were refractory to the insulin-like effects. Subsequent to the discovery of phosphatidylinsositol 3-kinase (PI3-kinase) being essential for the insulin stimulated uptake of glucose in adipocytes, PI3-kinase-inhibitor wortmannin was shown to inhibit GH-induced lipogenesis and antilipolysis. Tyrosine phosphorylated proteins binding and thereby activating PI3-kinase in response to GH were identified as insulin receptor substrate-1 and -2 (IRS-1 and IRS-2). As was the case for the tyrosine phosphorylation of GHR and JAK2, these effects dominated in responsive as compared to refractory cells. The activity of PI3-kinase was increased in IRS-1 immunoprecipitates from GH treated cells. In contrast, equally in both responsive and in refractory cells, GH stimulated the tyrosine phosphorylation of a 95 kDa protein (pp95) which we identified as Signal Transducer and Activator of Transcription 5 (STAT5), or a protein immunologically related to STAT5. In addition, while inhibiting both other GH-stimulated cellular tyrosine phosphorylations as well as the insulin-like effects, the tyrosine phosphorylation of STAT5 was not inhibited by tyrosine kinase- and JAK2-inhibitor staurosporine. Nor was the ability of GH to stimulate tyrosine phosphorylation of pp95/STAT5 desensitized by prior treatment with GH as is the case for the tyrosine phosphorylation of GHR, JAK2, IRS-1 and IRS-2 as well as the insulin-like effects. In conclusion, the data presented here suggest a role for JAK2 activation and thereby tyrosine phosphorylation of GHR, JAK2, IRS-1 and IRS-2 in the insulin-like effects of GH but also that there possibly are actions of GH mediated by kinases other than JAK2. Alternatively the kinetics of JAK2 activation are different in refractory as compared to responsive cells and a model supporting this hypothesis is outlined.