Down‐regulation of mammalian 3‐hydroxy‐3‐methylglutaryl coenzyme A reductase activity with highly purified liposomal cholesterol

Chinese hamster ovary‐215 cells (CHO‐215) cannot synthesize C 27 and C 28 sterols because of a defect in the reaction that decarboxylates 4‐carboxysterols [Plemenitas, A., Havel, C.M. & Watson, J.A. (1990) J. Biol. Chem. 265 , 17012–17017]. Thus, CHO‐215 cell growth is dependent on an exogenous metabolically functional source of cholesterol. We used CHO‐215 cells to (a) determine whether highly purified (> 99.5%) cholesterol, in egg lecithin liposomes, could down‐regulate derepressed 3‐hydroxy‐3‐methylglutaryl coenzyme A (HMG‐CoA) reductase activity and if so (b) determine whether the loss in reductase catalytic activity correlated kinetically with the synthesis and accumulation of detectable oxycholesterol derivatives. Liposomal cholesterol (26–39 µ m ) supported maximum CHO‐215 growth and initiated suppression of HMG‐CoA reductase activity at concentrations greater than 50 µ m . Maximum suppression (50–60%) of reductase activity was achieved with 181.3 µ m liposomal cholesterol in 6 h. Also, regulatory concentrations of highly purified liposomal [ 3 H]cholesterol were not converted (biologically or chemically) to detectable levels of oxy[ 3 H]cholesterol derivatives during 3–6 h incubations. Lastly, a broad‐spectrum cytochrome P450 inhibitor (miconazole) had no effect on liposomal cholesterol‐mediated suppression of HMG‐CoA reductase activity. These observations established that (a) highly purified cholesterol, incorporated into egg lecithin liposomes, can signal the down‐regulation of derepressed mammalian cell HMG‐CoA reductase activity and (b) if oxycholesterol synthesis was required for liposomal cholesterol‐mediated down‐regulation, the products had to be more potent than 24‐, 25‐, or 26‐/27‐hydroxycholesterol.