Chapter 9
Dual-Action Inhibitors of Cholesterol Biosynthesis Lanosterol Analogs That Inhibit Lanosterol 14α-Methyl Demethylase and Suppress 3-Hydroxy-3methylglutaryl—Coenzyme A Reductase Activity 1
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Downloaded by COLUMBIA UNIV on August 2, 2012 | http://pubs.acs.org Publication Date: July 2, 1992 | doi: 10.1021/bk-1992-0497.ch009
Leah L. Frye and Deborah A. Leonard 1
Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180-3590 Biology Department, West Virginia University, Morgantown, WV 26506-6057 2
Lanosterol 14α-methyl demethylase (P-450DM) is the cytochrome P450 monooxygenase which oxidatively removes the 14α-methyl group of lanosterol. This demethylation is the rate limiting step in the conversion of lanosterol to cholesterol. The intermediates in this transformation are known to bind very tightly to P-450DM and have been implicated in the regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) activity, the rate limiting enzyme in overall cholesterol biosynthesis. Lanosterol analogs 32a, 32b, and 33, which are methylated analogs of the intermediates generated during the removal of the 14α-methyl group by P-450DM, have been prepared and their biological activities assessed. All three compounds were found to inhibit P-450DM and to cause the suppression of HMGR activity. Studies with a P-450DM-deficient mutant suggest that the mechanism of suppression of HMGR by these compounds may be the competitive inhibiton of P-450DM causing the build-up of the natural intermediates generated during the removal of the 14αmethyl group which in turn suppress HMGR activity. Early epidemiological studies suggested a graded relationship between serum cholesterol levels and the risk o f coronary heart disease ( C H D ) which is a major medical problem in the U S (1). M o r e recently, the results of numerous clinical studies have indicated that the lowering serum cholesterol levels may indeed reduce the risk o f C H D and even foster the regression o f atherosclerotic lesions (2-4). Serum cholesterol levels can often be controlled by restricting their dietary intake of cholesterol; however, with a significant number of patients, this approach does not result in the reduction o f serum cholesterol concentrations to beneficial levels. In these patients, it is imperative that other methods for the control of serum cholesterol levels be available. A potential approach to the treatment of hypercholesterolemia in these patients is the development of inhibitors of cholesterol biosynthesis. Background Regulation of H M G R Activity. T h e rate-limiting enzyme o f cholesterol biosynthesis is 3-hydroxy-3-methylglutaryl coenzyme A reductase ( H M G R , E C 0097-6156/92/0497-0094S06.00/0 © 1992 American Chemical Society
In Regulation of Isopentenoid Metabolism; Nes, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
Downloaded by COLUMBIA UNIV on August 2, 2012 | http://pubs.acs.org Publication Date: July 2, 1992 | doi: 10.1021/bk-1992-0497.ch009
9.
FRYE & LEONARD
Dual-Action
Inhibitors of Cholesterol
Biosynthesis
95
1.1.1.34). H M G R catalyzes the reductive deacylation of 3-hydroxy-3-methylglutaryl coenzyme A 1 ( H M G - C o A ) to yield mevalonate 2 (Scheme 1) (5-7). It has been suggested that in cultured mammalian cells the activity of H M G R is regulated tlirough a multivalent feedback mechanism mediated by end product sterols along with an additional metabolite of mevalonate 2 (8-10). Addition of highly purified cholesterol to cultured cells does not affect the activity of H M G R or the rate o f sterol synthesis; however, numerous oxygenated sterols have been shown to be potent suppressors of H M G R activity resulting in marked reductions of cholesterol production (11-13). These observations led Kandutsch and Chen (14) to hypothesize that oxysterols, rather than cholesterol, may function as the natural regulators of H M G R activity and sterol synthesis. The structures of representative oxysterols known to suppress H M G R activity are depicted in Figure 1. The oxysterol used most frequently in the study of H M G R regulation is 25hydroxycholesterol 3 (Figure 1). This oxysterol is a potent inhibitor o f H M G R activity (IC50
