Loss of sensitivity to insulin at early events of the insulin signaling pathway in the liver of growth hormone-transgenic mice
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Growth hormone (GH) excess is associated with secondary hyperinsulinemia, but the molecular mechanism and consequences of this alteration are poorly understood. To address this problem we have examined the levels and phosphorylation state of the insulin receptor (IR) and the insulin receptor substrate-1 (IRS-1), the association between IRS-1 and the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) as well as the PI 3-kinase activity in the livers of GH-transgenic mice. As expected, IR levels were reduced in the liver of GH-transgenic mice (55% of normal values) as determined by immunoblotting with an anti-IR beta-subunit antibody. IR and IRS-1 phosphorylation as determined by immunoblotting with antiphosphotyrosine antibody were increased in basal conditions by 315% and 560% respectively. After a bolus administration of insulin in vivo, IR phosphorylation increased by 40% while IRS-1 phosphorylation did not change. Insulin administration to control (normal) mice produced 670% and 300% increases in the IR and IRS-1 phosphorylation respectively. In the GH-transgenic animals, basal association of PI 3-kinase with IRS-1 as well as PI 3-kinase activity in liver was increased by 200% and 280% respectively, and did not increase further after administration of insulin in vivo, indicating a complete insensitivity to insulin at these levels. In conclusion, GH excess and the resulting secondary hyperinsulinemia were associated with alterations at the early steps of insulin action in liver. IR concentration was reduced, while IR and IRS-1 phosphorylation, IRS-1/PI 3-kinase association, and PI 3-kinase activity appeared to be maximally activated under basal conditions, thus making this tissue insensitive to further stimulation by exogenous insulin in vivo.
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