DEXAMETHASONE DOWN-REGULATION OF INSULIN-RECEPTOR SUBSTRATE-1 IN 3T3-L1 ADIPOCYTES

Insulin resistance resulting from prolonged exposure of intact animals or cultured cells to glucocorticoids is often attributed to postreceptor signaling defects. To better understand the specific effects of glucocorticoids on insulin signaling, we have characterized the effect of dexamethasone on the expression of an insulin signaling intermediate, the insulin receptor substrate-1 (IRS-1) in 3T3-L1 adipocytes. Addition of dexamethasone resulted in a 40-70% decline in steady-state IRS-1 protein over 24-48 h of treatment. Dexamethasone did not significantly change the degradation rate of IRS-1 protein but decreased the net rate of amino acid incorporation into IRS-1 by 87%. Between 1 and 2.5 h of treatment with dexamethasone, actinomycin D, or both drugs given simultaneously, the concentration of IRS-1 mRNA declined with a half-life of 0.7-1.0 h. However, after 4 h of dexamethasone treatment, IRS-1 mRNA concentrations stabilized at approximately 35% of the control level. The dexamethasone-induced decline in IRS-1 protein could be prevented by simultaneous administration of the glucocorticoid antagonist mifepristone, RU38486. These results suggest that in 3T3-L1 adipocytes the loss of IRS-1 protein after dexamethasone treatment can be accounted for chiefly by inhibition of the synthesis of IRS-1 mRNA.