Evidence for post-transcriptional regulation by insulin of 3-hydroxy-3-methylglutaryl coenzyme A reductase and sterol synthesis in human mononuclear leucocytes

Abstract

Incubation of freshly isolated human mononuclear leucocytes in lipid-depleted serum for 4 h resulted in a two-fold increase in 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity. Insulin, when added to the incubation medium at concentrations of 10 and 100 nmol/l at zero time, caused additional increases in the enzyme activity of 30% and 37%, respectively. The hormone action was not immediate because no effect was observed when insulin was added at 4 h and activity examined thereafter. Under these conditions sterol synthesis from 14C-acetate and tritiated water was strictly proportional to the activity of HMG-CoA reductase. Cycloheximide (20 micrograms/ml), a translational inhibitor of protein synthesis, prevented the insulin-mediated increase in the enzyme activity and the incorporation of 14C-acetate into sterols. Cordycepin (50 micrograms/ml) inhibited messenger RNA synthesis by greater than 50%, but had no inhibitory effect on the induction of HMG-CoA reductase and sterol synthesis. Low density lipoprotein (80 micrograms protein/ml) and complete serum blocked the induction of the enzyme and sterol synthesis from 14C-acetate caused by lipid-depleted serum. The insulin-effect, however, remained unchanged. The results suggest that insulin may regulate the de novo synthesis of HMG-CoA reductase and accordingly sterol synthesis at a post-transcriptional level.