Expression of PPAR(alpha) in human hepatocytes and activation by trichloroacetate and dichloroacetate.

Dichloroacetate (DCA) and trichloroacetate (TCA) are carcinogenic metabolites of trichloroethylene (TCE), a known hepatocarcinogen in B6C3F1 mice. This hepatocarcinogenesis is believed to result from peroxisome proliferation via PPAR(alpha) and/or stimulation of hepatocyte replication. In this study hPPAR(alpha) levels in six human liver tissues and in a long-term human hepatocyte cell line are compared. PPAR(alpha) levels varied significantly between individual tissues and are generally lower than PPAR(alpha) levels detected in mouse liver. Long-term cultured human hepatocytes display PPAR(alpha) levels only slightly lower than cultured mouse hepatocytes. Transfection studies examining the endogenous hPPAR(alpha) activity revealed little or no receptor activation, even following treatment with high concentrations of peroxisome proliferators. In contrast human hepatocytes transfected with mPPAR(alpha) and mRXR(alpha) display increased expression of PPAR(alpha), and increased PPRE-reporter activity when treated with WY-14,643, TCA, and DCA. This human hepatocyte transfection system is a promising tool for examinin the regulation of genes by PPAR(alpha) from different species.

The effect of the trichloroethylene metabolites trichloroacetate and dichloroacetate on peroxisome proliferation and DNA synthesis in cultured human hepatocytes.

Dichloroacetate (DCA) and trichloroacetate (TCA) are metabolites of the environmental contaminant trichloroethylene (TCE) that are thought to be responsible for its hepatocarcinogenicity in B6C3F1 mice. TCA and DCA induce peroxisomal proliferation and are mitogenic in rodent liver. The susceptibility of humans to TCA- and DCA-induced hepatocarcinogenesis is unknown. The current studies were aimed at using both primary and long-term human hepatocyte cultures to study the effects of TCA, DCA, and a potent peroxisome, proliferator, WY-14,643, on peroxisomal activity and DNA synthesis in human hepatocytes. Peroxisome proliferation, as assessed by palmitoyl-CoA oxidation activity, was below the limit of detection in all human cell lines tested. However, the human cell lines did display small but significant increases in CYP450 4A1 1 levels following treatment with WY-14,643 (0.1 mmol/L), indicating that the CYP 4A11 gene may be regulated by peroxisome proliferator-activated receptor alpha in humans. Similarly to their effect in rodent hepatocyte cultures, TCA and DCA were not complete mitogens in human hepatocyte cultures. In fact, DNA synthesis tended to be significantly decreased following treatment of the cells with WY-14,643, TCA, or DCA. In contrast to rodent hepatocyte responses, TCA and DCA did not increase palmitoyl-CoA oxidation and caused a decrease in DNA synthesis in human hepatocyte cultures, suggesting that humans may not be susceptible to TCA- and DCA-induced hepatocarcinogenesis.