Modulation by dexamethasone of the pyruvate dehydrogenase-complex activity in 3T3-L1 adipocytes

Induction of dihydrolipoamide dehydrogenase in 3T3-L1 cells during differentiation

The activity and turnover of dihydrolipoamide dehydrogenase (E3), the common component of the three 2-oxoacid dehydrogenase complexes, were measured during the differentiation of 3T3-L1 preadipocytes into 3T3-L1 adipocytes. The specific activity of E3 increased approx. 3-4-fold in 3T3-L1 adipocytes differentiated under a regimen of insulin, dexamethasone and 3-isobutyl-1-methylxanthine for 48 h, followed by insulin alone thereafter. A rabbit antibody to pig heart E3 quantitatively precipitated the enzyme from 3T3-L1 adipocytes. By using immunoprecipitation and gel electrophoresis, a 3.3-fold increase was observed in E3 protein in 3T3-L1 adipocytes as compared with 3T3-L1 preadipocytes, on a DNA basis. Pulse-labelling experiments with L-[35S]methionine revealed a 3.5-fold increase in the rate of synthesis of E3 in 3T3-L1 adipocytes compared with that observed in 3T3-L1 preadipocytes. In contrast, the apparent half-lives of the E3 in 3T3-L1 preadipocytes (43 h) and 3T3-L1 adipocytes (33 h) were not significantly different. Therefore, the 3-4-fold increase in the specific activity of E3 in 3T3-L1 adipocytes resulted from an increased rate of synthesis of the enzyme.

Glucocorticoid binding during the differentiation of 3T3-F442A fibroblasts into adipocytes. A possible regulatory effect of insulin

Until recently, few studies had been carried out on receptors for glucocorticoids in adipocytes, although the role of these steroids is considerable. In the present studies, we chose the pre-adipocyte line 3T3-F442A, which constitutes an excellent model for investigating the differentiation and function of adipocytes. Using a whole cell assay system, we showed the existence of a homogenous class of sites with the characteristics of glucocorticoid receptors, that is, high-affinity binding which is reversible, specific and saturable. Whatever the state of cellular differentiation, the affinity of the receptor for dexamethasone did not vary, although we observed an increase in the number of sites during differentiation. When cells were differentiated in the presence of insulin, there was a further increase in the binding capacity; moreover, insulin deprivation of such adipocytes caused a decrease in the number of sites. Our results therefore suggest that factors other than the glucocorticoids themselves influence dexamethasone binding. It is suggested that insulin plays a role in the regulation of the number of glucocorticoid receptors.