Glucocorticoid-Induced Insulin Resistance in Vitro: Evidence for Both Receptor and Postreceptor Defects*

Abstract

Glucocorticoid excess in vivo has been shown to cause decreases in insulin sensitivity and insulin receptor binding in target tissues. It has not been previously possible to produce all of these changes in vitro because of limitations of the isolated rat adipocyte as an experimental model. We have studied the effects of glucocorticoid hormones on insulin binding and insulin action in differentiated mouse 3T3-L1 fatty fibroblasts, in an attempt to develop a complete in vitro model of the insulin resistance associated with glucocorticoid excess. Exposure of 3T3-L1 cells to dexamethasone, a synthetic glucocorticoid, resulted in decreased binding of insulin to its receptor and decreased insulin stimulation of 2-deoxyglucose uptake. Although some effect of dexamethasone on these parameters was observed after 24 h, more than 3 days were required for maximal inhibition of both binding and biological response. The magnitude of these changes was concentration dependent over a range of 0.1 –100 nM dexamethasone. Exposure to hydrocortisone produced similar changes, although higher concentrations were required. Cortexolone and deoxycorticosterone, compounds which weakly interact with the glucocorticoid receptor, caused partial inhibition of insulin action. Estradiol, testosterone, and methyl testosterone were inactive. When analyzed by Scatchard plots, the defect in insulin binding was identical to that seen when rodents were treated in vivo with glucocorticoids, namely a decrease in the affinity of the receptor for insulin, with little change in receptor concentration. A marked decrease in the sensitivity of dexamethasonetreated cells to insulin was seen, as well as a reduction in the ability of insulin to maximally stimulate 2-deoxyglucose uptake. When the binding and biological activity data were compared, the decreased affinity of the insulin receptor appeared to account for much, but not all, of the decrease in insulin sensitivity. Some postreceptor changes must also occur to a count for the decrease in both maximal responsiveness and insulin sensitivity. In summary, we have been able to demonstrate in cultured cells each of the components of glucocorticoid-induced insulin resistance seen in vivo. Since the effects of glucocorticoid hormones in these cells are first seen within physiological concentrations, these changes may help explain the derangements in glucose metabolism seen in both hypo- and hyperadrenocortical states.