INDUCTION OF SISTER CHROMATID EXCHANGE AND MICRONUCLEI IN PRIMARY CULTURES OF RAT AND HUMAN HEPATOCYTES BY THE PEROXISOME PROLIFERATOR, WY-14,643

The ability of peroxisome proliferators to induce hepatocellular carcinomas in rodents has been known since the mid 1970's, but the mechanism of tumor formation is still poorly understood. In this study, we have used primary cultures of both rat and human hepatocytes to address the question of whether the peroxisome proliferator, [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio] acetic acid (Wy-14,643), causes genotoxic damage in hepatocytes as measured by sister chromatid exchange (SCE), micronuclei formation, and chromosomal aberrations. We have found that in rat hepatocytes the number of SCEs per chromosome increased in a dose-dependent manner from a background level of 0.7 to a maximum of 1.1 in cells exposed for 48 h to 100 muM of Wy-14,643. In contrast, no increase in SCE frequency was observed in rat hepatocytes exposed to Wy-14,643 for 3 h. A dose-dependent increase in micronuclei formation was also seen in the 48 h but not in the 3 h cultures. The maximum frequency of micronuclei formation after a 48 h exposure occurred at 20 muM Wy-14,643 and was 2.3 times that for control cells. At this concentration of Wy-14,643, the frequency of chromosomal aberrations was increased by more than 10-fold. A 48 h exposure to Wy-14,643 also significantly increased micronuclei formation in human hepatocytes, but it was less effective than in rat hepatocytes. To investigate the potential role of peroxisome proliferation in these genotoxic responses, we measured the activities of palmitoyl-CoA beta-oxidase in hepatocytes exposed for 48 h to Wy-14,643. A dose-dependent increase in palmitoyl-CoA beta-oxidase activity was observed in rat hepatocytes, but not in human hepatocytes. The SCE frequency in rat hepatocytes correlated well with the degree of peroxisome proliferation, however, the increased formation of micronuclei in both rat and human hepatocytes occurred by a mechanism that appeared to be independent of peroxisome induction. In summary, these results demonstrate that the peroxisome proliferator, Wy-14,643, causes genotoxic damage in primary cultures of both rat and human hepatocytes.