Dexamethasone-induced changes in phosphorylation of the insulin and epidermal growth factor receptors and their substrates in intact rat hepatocytes

Dexamethasone-induced changes in insulin and epidermal growth factor (EGF) receptor number, autophosphorylation, and kinase activity were studied in intact rat hepatocytes. Hepatocytes were freshly isolated from Sprague-Dawley rats treated with dexamethasone (1 mg/kg) for 4 days and from untreated littermates. Dexamethasone had no effect on insulin receptor number, while EGF receptor binding was slightly increased (21.3% vs. 17.2% binding/10⁶ cells) after dexamethasone treatment. In hepatocytes from both control and dexamethasone- treated animals labeled with ³²P, insulin induced tyrosine phosphorylation of the β-subunit of the insulin receptor as well as of a 175K protein believed to be its endogenous substrate. The degree of phosphorylation of the insulin receptor was decreased 34% by dexamethasone treatment compared to the control value when studied in fasted animals. In contrast, phosphorylation was increased to a similar extent by dexamethasone treatment in fed animals. In addition, the β-subunit of the insulin receptor extracted from dexamethasone-treated animals migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a slightly increased mobility compared to normal (89 ± 1.2K vs. 92.5 ± 0.4K). EGF induced tyrosine phosphorylation of its own receptor and of a 120K protein in intact hepatocytes. Their degree of phosphorylation was decreased by 30% as a result of dexamethasone treatment in the fasted animal and was unchanged in the fed animals. Our data indicate that glucocorticoids modulate insulin and EGF receptor kinase activity, but the nature of their effect depends on other factors, including the dietary state of the animal. These studies also suggest that postreceptor changes account for a major component of glucocorticoid-induced insulin resistance.