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 C27 and C28 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 microM) supported maximum CHO-215 growth and initiated suppression of HMG-CoA reductase activity at concentrations greater than 50 microM. Maximum suppression (50-60%) of reductase activity was achieved with 181.3 microM liposomal cholesterol in 6 h. Also, regulatory concentrations of highly purified liposomal [3H]cholesterol were not converted (biologically or chemically) to detectable levels of oxy[3H]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.
Plemenitas
A
A
Institute of Biochemistry, Medical Faculty of the University of Ljubljana, Slovenia.
Watson
J A
JA
D000818:Animals; D016466:CHO Cells; D002455:Cell Division; D002784:Cholesterol / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D006224:Cricetinae; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D006888:Hydroxycholesterols / Q000378:metabolism; D006903:Hydroxymethylglutaryl CoA Reductases / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D008081:Liposomes / Q000378:metabolism*; D008825:Miconazole / Q000494:pharmacology; D011485:Protein Binding; D013997:Time Factors
D006888:Hydroxycholesterols; D008081:Liposomes; C007997:25-hydroxycholesterol; D008825:Miconazole; D003577:Cytochrome P-450 Enzyme System; D002784:Cholesterol; D006903:Hydroxymethylglutaryl CoA Reductases