Effect of FR194738, a potent inhibitor of squalene epoxidase, on cholesterol metabolism in HepG2 cells

(E)-N-ethyl-N-(6,6-dimethyl-2-hepten-4-ynyl)-3-[2-methyl-2-(3-thienylmethoxy)propyloxy]benzylamine hydrochloride (FR194738) inhibited squalene epoxidase activity in HepG2 cell homogenates with an IC50 value of 9.8 nM. In the study using intact HepG2 cells, FR194738 inhibited Cholesterol synthesis from [14C]acetate with an IC50 value of 4.9 nM, and induced intracellular [14C]squalene accumulation. On the other hand, the 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitor simvastatin reduced both Cholesterol and squalene synthesis from [14C]acetate. Incubation with simvastatin for 18 h produced increases in HMG-CoA reductase activity in HepG2 cells, which was related to the degree of reduction in Cholesterol synthesis. The HMG-CoA reductase activity increased by 13- and 19-fold at the concentrations of simvastatin that inhibited Cholesterol synthesis by 65% and 82%, respectively. In contrast, FR194738 did not increase HMG-CoA reductase activity at the concentrations that inhibited Cholesterol synthesis by 24% and 69%, and moderate increase (4.6-fold) was observed at the concentration that inhibited Cholesterol synthesis by 90%. These results suggest that non-sterol metabolite(s) derived from mevalonate prior to the squalene epoxidation step in the Cholesterol synthetic cascade have a regulatory role in the suppression of HMG-CoA reductase activity. We speculate that FR194738 inhibits Cholesterol synthesis with a minimal change of the regulator(s) and would be highly effective in the treatment of hypercholesterolemia.