Evidence indicating that inactivation of 3-hydroxy-3-methylglutaryl coenzyme A reductase by low density lipoprotein or by 25-hydroxycholesterol requires mediator protein(s) with rapid turnover rate.

The half-life (t 1/2) of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase of Chinese hamster ovary cells grown in fetal calf serum medium is approximately 2 h. When cells are switched to grow in delipidated serum medium (DeL-M) for more than 24 h, the t 1/2 of the enzyme is found to be drastically altered to approximately 13 h. Exposure of low density lipoprotein (LDL) (100 micrograms of protein/ml) or 25-hydroxycholesterol (1 microgram/ml) to cells grown in DeL-M suppresses reductase activity more rapidly than would be expected solely if reductase synthesis were suppressed, showing that inactivation of reductase activity by sterols, previously demonstrated using only analogs of cholesterol, is a normal mechanism for regulation of HMG-CoA reductase activity by the physiologically important sterol source (LDL). This inactivation effect by LDL or by 25-hydroxycholesterol is shown to be at least in part due to acceleration of reductase degradation rate. Furthermore, the inactivation effect by sterols is shown to be largely abolished if cycloheximide (250 micrograms/ml) is added simultaneously to the growth medium, indicating that continuous synthesis of a class of mediator protein(s) is necessary in mediating the effect of LDL or 25-hydroxycholesterol. Two different protein synthesis inhibitors (emetine and puromycin) were used and gave essentially identical results. Preincubation of cell culture with cycloheximide for 2 h essentially completely abolishes the effect of 25-hydroxycholesterol, indicating that the mediator protein(s) turns over rapidly, with t 1/2 less than 3 or 4 h.