Decreased autophosphorylation of EGF receptor in insulin-deficient diabetic rats

Abstract

We have previously reported that despite an increase in receptor concentration, there is a decrease in autophosphorylation and tyrosine kinase activity of the insulin receptor in insulin-deficient diabetic rats. To determine if other tyrosine kinases might be altered, we have studied the epidermal growth factor (EGF) receptor kinase in wheat germ agglutinin-purified, Triton X-100-solubilized liver membranes from streptozotocin (STZ)-induced diabetic rats and the insulin-deficient BB rat. We find that autophosphorylation of EGF receptor is decreased in proportion to the severity of the diabetic state in STZ rats with a maximal decrease of 67% (P < 0.01). A similar decrease in autophosphorylation was observed in diabetic BB rats that was partially normalized by insulin treatment. Separation of tryptic phosphopeptides by reverse-phase high-performance liquid chromatography revealed a decrease in labelling at all sites of autophosphorylation. A parallel decrease in EGF receptor phosphorylation was also found by immunoblotting with an antiphosphotyrosine antibody. EGF receptor concentration, determined by Scatchard analysis of 125I-labeled EGF binding, was decreased by 39% in the STZ rat (P < 0.05) and 27% in the diabetic BB rat (not significant). Thus autophosphorylation of EGF receptor, like that of the insulin receptor, is decreased in insulin-deficient rat liver. In the case of EGF receptor, this is due in part to a decrease in receptor number and in part to a decrease in the specific activity of the kinase. Because tyrosine kinases are involved in the regulation of cellular growth and metabolism, alterations in the EGF and insulin receptor kinases may reflect a generalized change in this important class of proteins in diabetic animals.