Effects of STZ-induced diabetes and fasting on insulin receptor mRNA expression and insulin receptor gene transcription in rat liver.

Insulinopenic states in rodents are known to cause an increase in the number of hepatic insulin receptors. To determine if this change is related to an abnormality in insulin receptor gene expression, insulin receptor binding, insulin receptor mRNA levels, and insulin receptor gene transcription rates have been measured in livers from rats rendered hypoinsulinemic by STZ administration (65 mg/kg) or fasting. In the two groups of experimental rats, insulin binding to liver plasma membranes was increased (approximately 40 and 25%, respectively) relative to control, normoinsulinemic animals. Northern blot analysis of either total or poly (A)+ RNA from livers of hypo- and normoinsulinemic rats revealed two major insulin receptor mRNA species of 9.5 and 7.5 kbs. In hypoinsulinemic rats, insulin receptor mRNA levels were increased > or = 10-fold, with similar effects on the two mRNA species. The effects of STZ administration and fasting on insulin receptor binding and insulin receptor mRNA levels were fully reversed by insulin treatment or refeeding, respectively. Injection of ACT D, an inhibitor of gene transcription, decreased insulin receptor mRNA levels by > or = 80% in control and diabetic rats and suppressed the overexpression of mRNA seen in diabetic rats. In vitro nuclear transcription assays showed that the rate of transcription of the insulin receptor gene was increased 2-fold in STZ-induced diabetic rats and fasted rats relative to control animals. Taken together, these results suggest that the upregulation of the insulin receptor induced by chronic insulinopenia results, at least in part, from an increase in insulin receptor gene transcription.