Elevation of HeLa cell 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by glucocorticoids: possible...

Elevation of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase, EC 1.1.1.34) activity by glucocorticoids was shown to be dependent on the concentration of hormone in the medium over a range of 5 X 10(-10) to 1 X 10(-8) M, although the presence of steroid in the assay at 10(-5) M elicited no increase in activity. There was a demonstrated time dependence for the addition of dexamethasone i.e., from zero to six hours after serum removal, addition of hormone resulted in the same peak acitvity; addition at 12 hours gave slight elevation but resulted in an extended maintenance of the peak level of activity; addition at 24 hours showed no effect. When cycloheximide was added to the above times, subsequent kinetics showed identical decay of the enzyme activities from control and treated cultures at 6 and 24 hours, but at 12 hours the activity from dexamethasone treated cells exhibited an extended lag before the onset of decay, which then proceeded at the same rate as the control. The continuous presence of the hormone was not necessary for the induction to continue and the addition of Actinomycin D to cultures incubated in the presence of hormone resulted in an immediate decay of catalytic activity without evidence of "superinduction." The addition of progesterone at the same time as dexamethasone resulted in a concentration-dependent inhibition of the augmentation, suggesting the involvement of the glucocorticoid receptor in the elevation of HMG-CoA reductase activity. Flow microfluorometric (FMF) analysis of hormone treated cells indicated a delayed entrance into the DNA synthesis (S) phase of the cell cycle. The temporal relationships between this cell cycle perturbation and HMG-CoA reductase elevation are discussed.