Comitogenicity of eicosanoids and the peroxisome proliferator ciprofibrate in cultured rat hepatocytes

Several hypolipidemic drugs and environmental contaminants induce hepatic peroxisome proliferation and hepatic tumors when administered to rodents. These chemicals increase the expression of the peroxisomal β-oxidation pathway and the cytochrome P-450 4A family, which metabolize lipids, including eicosanoids and their precursor fatty acids. We previously found that the peroxisome proliferator ciprofibrate decreases the level of eicosanoids in the liver and in cultured hepatocytes. In this study, we examined the effect of prostaglandins E₂ and F(2α) (PGE₂ and PGF(2α)), leukotriene C₄ (LTC₄) and the peroxisome proliferator ciprofibrate on DNA synthesis in cultured hepatocytes. Primary rat hepatocytes were cultured on collagen gels in serum-free L-15 medium with varying concentrations of eicosanoids and ciprofibrate, and the absence or presence of growth factors. Ciprofibrate lowered hepatocyte eicosanoid concentrations; the addition of eicosanoids restored their levels. After a 48-h exposure with [³H]-thymidine, DNA synthesis was determined by measuring [³H]-thymidine incorporation into DNA. The addition of PGE₂, PGF(2α), and LTC₄ to cultures along with ciprofibrate increased DNA synthesis, whereas treatment with ciprofibrate or eicosanoids alone resulted in a much smaller increase. The addition of epidermal growth factor (EGF) to the eicosanoid-ciprofibrate combination increased DNA synthesis more than EGF or the eicosanoid-ciprofibrate combination alone. The PGF(2α)-ciprofibrate combination also was comitogenic with transforming growth factor-α and hepatocyte growth factor. The addition of both ciprofibrate and prostaglandins also blocked the growth inhibitory effect of transforming growth factor-β on DNA synthesis induced by EGF. These results show that the eicosanoids PGE₂, PGF(2α), and LTC₄ are comitogenic with the peroxisome proliferator ciprofibrate in cultured rat hepatocytes.