Expression of a glucose transporter gene cloned from brain in cellular models of insulin resistance: dexamethasone decreases transporter mRNA in primary cultured adipocytes

Abstract

In two cellular models of insulin resistance we measured glucose transport activity, total glucose transporter number using the cytochalasin B binding assay, and expression of a transporter mRNA species specifically hybridizing with cDNA cloned from brain. In primary cultured adipocytes, chronic exposure to glucose plus insulin (24 h), but neither agent alone, markedly decreased (less than 50%) glucose transport activity; however, neither glucose nor insulin regulated the number of glucose transporters or levels of transporter mRNA whether normalized per total RNA, RNA per cell, or as a fraction of CHO-B mRNA. On the other hand, chronic treatment with 30 nM dexamethasone (24 h) decreased basal and maximal transport rates (approximately 75%), led to a 40% depletion in total cellular glucose transporters, and decreased transporter mRNA by 57-59% (t 1/2 = 10 h; ED50 = 4-5 nm). Dexamethasone's effects to decrease transport rates, transporter protein, and mRNA were inhibited by coincubation with insulin. Dexamethasone did not alter the degradation rate of transporter mRNA relative to that in control cells indicating a lack of effect on mRNA stability. Also, suppression of transporter mRNA did not appear to require ongoing protein synthesis since the effect was observed when dexamethasone was added to cycloheximide-treated cells; however, cycloheximide per se specifically increased transporter mRNA 4-fold. We conclude in adipocytes: 1) glucose and insulin (24 h) do not regulate the total number of glucose transporters or expression of mRNA encoding a transporter species cloned from brain. 2) Long-term dexamethasone treatment reduces the cellular abundance of both glucose transporters and the specific transporter mRNA; these effects may be due to inhibition of gene transcription since dexamethasone does not influence transporter mRNA stability. 3) Insulin heterologously inhibits regulation of the glucose transport system by dexamethasone. 4) Dexamethasone-mediated insulin resistance is due in part to regulation of a glucose transporter species encoded by cDNA cloned from brain. These observations may be relevant to mechanisms of insulin resistance in clinical states of hypercortisolism.