Effect of the peroxisome proliferator ciprofibrate on hepatic cyclooxygenase and phospholipase A₂ in rats

Peroxisome proliferators, which include several hypolipidemic drugs, plasticizers and other chemicals, induce hepatic tumors in rodents. These chemicals alter the expression of enzymes involved in lipid metabolism, such as the cytochrome P450 4A family and peroxisomal β-oxidation enzymes. Previous studies have shown that the peroxisome proliferator ciprofibrate reduces eicosanoid concentrations in rat livers and primary hepatocyte cultures, yet the mechanism is still unclear. In this study we examined cyclooxygenases 1 and 2 (COX-1 and COX-2) and cytosolic phospholipase A₂ (cPLA₂) to determine whether the rate-limiting enzymes in the eicosanoid synthetic pathway are altered by ciprofibrate. Rats were fed 0.01% ciprofibrate for 3, 6, or 10 days. Western analysis revealed that COX-2 protein was induced by ciprofibrate (up to 13-fold at day 10), but that calcium-dependent (Ca-D) cPLA₂ protein was not different from controls. The enzyme activity of calcium-independent (Ca-I) cPLA₂ in ciprofibrate-treated rats was increased 2-fold, whereas Ca-D cPLA₂ and total COX activities were not affected. Using enzyme kinetics, we found that COX-1 (K(i) = 143 μM) and Ca-I cPLA₂ (K(i) = 121 μM) were competitively inhibited by ciprofibrate, but the inhibition was not physiologically significant. COX-2 and Ca-D cPLA₂ were not inhibited by ciprofibrate. These results show that ciprofibrate increases Ca-I cPLA₂ enzyme activity and COX-2 protein expression.