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-hydro xy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and activation o f cholesterol synthesis by gemfibrozil shown in whole body (Hashimoto F,, Ishikavva T, Hamada S. and Hayashi H., Biochemical. Pharm., 49, 12 13-1221 (1995)) is also detected at a culture cell level, and we made a comparative analysis of the effects of clofibric acid. Gemfibrozil a t 0.25 mM increased the activity of some peroxisomal enzymes (catalase and the cyanide-insensitive fatty acyl-CoA oxidizing system) after in cubation for 72 h. However, contrary to whole body experiments, gemfib rozil decreased the activity of HMG-CoA reductase and cholesterol synt hesis from [C-14]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, indicati ng 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 res pect 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 in hibited 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 an d over. That is, gemfibrozil directly inhibited the activity more stro ngly than clofibric acid. The direct inhibition of the enzyme itself r equired higher concentrations of both agents than did inhibition at th e culture cell level. These results suggest that the cytotoxicity of g emfibrozil is greater than that of clofibric acid, and that gemfibrozi l, as well as clofibric acid, can induce peroxisomal enzymes in the cu lture cell level. In contrast to whole body results, gemfibrozil may s uppress cholesterol synthesis from [C-14]acetate through the inhibitio n of HMG-CoA reductase at the culture cell level. The decreases in the reductase activity caused by gemfibrozil and clofibric acid at the cu lture cell level may not be caused by the direct inhibition of the enz yme.