Comparison of the effects of gemfibrozil and clofibric acid on peroxisomal enzymes and cholesterol synthesis of rat hepatocytes

We studied whether the peroxisomal proliferation, induction of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and activation of cholesterol synthesis by gemfibrozil shown in whole body (Hashimoto F., Ishikawa T., Hamada S. and Hayashi H., Biochemical. Pharm., 49, 1213-1221 (1995)) is also detected at a culture cell level, and we made a comparative analysis of the effects of clofibric acid. Gemfibrozil at 0.25 mM increased the activity of some peroxisomal enzymes (catalase and the cyanide-insensitive fatty acyl-CoA oxidizing system) after incubation for 72 h. However, contrary to whole body experiments, gemfibrozil decreased the activity of HMG-CoA reductase and cholesterol synthesis from [14C]acetate. At 1 mM, gemfibrozil decreased not only the activity of HMG-CoA reductase and cholesterol synthesis, but also the protein content of the cells and peroxisomal enzyme activity, indicating nonspecific inhibition at this concentration. Clofibric acid (0.25 and 1 mM) increased the activity of peroxisomal enzymes, but decreased the activity of HMG-CoA reductase and cholesterol synthesis. With respect to the direct effect on HMG-CoA reductase in the cell homogenate, gemfibrozil at 0.25 mm did not affect the activity, but it clearly inhibited the activity at 2 mM and above. Clofibric acid at 2 mM hardly affected the activity, but it clearly decreased the activity at 5 mM and over. That is, gemfibrozil directly inhibited the activity more strongly than clofibric acid. The direct inhibition of the enzyme itself required higher concentrations of both agents than did inhibition at the culture cell level. These results suggest that the cytotoxicity of gemfibrozil is greater than that of clofibric acid, and that gemfibrozil, as well as clofibric acid, can induce peroxisomal enzymes in the culture cell level. In contrast to whole body results, gemfibrozil may suppress cholesterol synthesis from [14C]acetate through the inhibition of HMG-CoA reductase at the culture cell level. The decreases in the reductase activity caused by gemfibrozil and clofibric acid at the culture cell level may not be caused by the direct inhibition of the enzyme.