Overexpression of bovine growth hormone in transgenic mice is associated with changes in hepatic insulin receptors and in their kinase activity.

Abstract

Transgenic mice expressing a hybrid gene produced by linking the promoter regulatory region of phosphoenolpyruvate carboxykinase (PEPCK) gene to the bovine growth hormone (bGH) gene, were used to investigate the effects of GH on insulin binding and insulin dependent tyrosine kinase activity of hepatic insulin receptors. Transgenic mice had normal levels of blood glucose, despite hyperinsulinemia, indicating that these animals were insulin resistant. The number of insulin receptors in the liver of transgenic mice was significantly decreased in both the particulate fraction (25%) and the solubilized membranes (40%) indicating that expressed (functional) and non-expressed (cryptic) receptors were affected. Scatchard analysis of competitive binding curves for insulin indicated that the affinity of the receptor did not differ between transgenic and normal mice. Insulin dependent tyrosine kinase activity in insulin receptors partially purified by wheat germ agglutin (WGA) agarose chromatography from solubilized liver membranes, was measured. The stimulatory action of insulin on phosphorylation of the synthetic substrate (a copolymer Glu-Tyr, 4:1) was increased 100% in transgenic, as compared to normal mice, using the same binding activity. Since transgenic mice are hyperinsulinemic, it is likely that the decreased insulin binding in this group reflects down regulation of the expressed and non-expressed insulin receptors, and the increased kinase activity represents a compensatory mechanism. We conclude that alterations in the insulin receptor number and in the tyrosine kinase activity develop in response to changes in insulin levels. Thus, insulin resistance detected in the liver of transgenic mice overexpressing GH may be due to post receptor defects.