Loss of sensitivity to insulin at early events of the insulin signaling pathway in the liver of growth hormone-transgenic mice

Growth hormone (GH) excess is associated with secondary hyperinsulinemia, b ut the molecular mechanism and consequences of this alteration are poorly u nderstood. To address this problem we have examined the levels and phosphor ylation state of the insulin receptor (IR) and the insulin receptor substra te-1 (IRS-1), the association between IRS-1 and the p85 subunit of phosphat idylinositol 3-kinase (PI3-kinase) as well as the PI 3-kinase activity in t he 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 immuno blotting with an anti-IR beta-subunit antibody. IR and IRS-1 phosphorylatio n as determined by immunoblotting with antiphosphotyrosine antibody were in creased in basal conditions by 315% and 560% respectively. After a bolus ad ministration of insulin in vivo, IR phosphorylation increased by 40% while IRS-1 phosphorylation did not change. Insulin administration to control (no rmal) mice produced 670% and 300% increases in the IR and IRS-1 phosphoryla tion 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 administrat ion of insulin in vivo, indicating a complete insensitivity to insulin at t hese levels. In conclusion, GH excess and the resulting secondary hyperinsu linemia were associated with alterations at the early steps of insulin acti on in liver. IR concentration was reduced, while IR and IRS-1 phosphorylati on, IRS-1/PI 3-kinase association, and PI 3-kinase activity appeared to be maximally activated under basal conditions, thus making this tissue insensi tive to further stimulation by exogenous insulin in vivo.