The regulation of hydroxymethylglutaryl-CoA reductase in cultured cells

Growth-stimulated synchronized cells exhibit a rapid increase in 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase, EC 1.1.1.88) activity prior to the onset of DNA synthesis. Under normal culture conditions, HMG-CoA reductase activity exhibits wide variations among experiments. To determine whether this phenomenon is dependent on cell replication, we used J774 macrophage-like cells to compare changes in reductase activity in cells synchronized by serum deprivation and then growth-stimulated by fresh media containing serum to unsynchronized cells treated with fresh media and serum. Under these conditions, no increase in [3H]thymidine incorporation into cell DNA was seen in unsynchronized cells, but a large increase was observed in synchronized cells 10-12 h after media change. Although the growth characteristics differed between the cells, reductase activity was low at the time of media change and increased 10 to 20-fold 5-10 h after media change, returning to basal levels by 24 h in both synchronized and unsynchronized cells. This pattern of reductase activity was observed in unsynchronized cells from a variety of cell lineages, although the magnitude of the changes varied. Fluctuations of [14C]acetate incorporation into cholesterol were observed in parallel to alterations in reductase activity. LDL receptor expression also paralleled the changes in reductase activity, but scavenger receptor expression was not affected. Addition of lipoproteins at the time of media change inhibited the rise in reductase activity by 80-90%. The increase in reductase activity was not due to a stimulation of cholesterol efflux into the medium, but evidence for the secretion into the media of an inhibitory factor was obtained. These results suggest that cell requirements for cholesterol are not always directly related to replication, and that standard culture conditions induce transient fluctuations in reductase activity and lipoprotein receptor expression.