Role of peroxisome proliferator-activated receptor α in altered cell cycle regulation in mouse liver

The mechanisms underlying peroxisome proliferator-induced hepatocarcinogenesis are unclear but are mediated by the peroxisome proliferator-activated receptor α (PPARα). To determine the role of PPARα in the mechanisms of hepatocarcinogenesis, the effect of Wy-14,643 on expression patterns of acyl CoA oxidase (ACO) and proteins involved in cell proliferation in the PPARα-null mouse were evaluated. ACO, CDK-1, CDK-2, CDK-4, PCNA and c-myc proteins were significantly increased in wild-type mice fed Wy-14,643 for 5 weeks or 11 months, as compared with controls. This effect was not observed in Wy-14,643-treated PPARα-null mice. Expression patterns of cyclin B1, cyclin D, cyclin E and p53 were not different in any of the groups, mRNAs encoding CDK-1, CDK-4, cyclin D1 and c-myc were also increased in wild-type mice fed Wy-14,643 but not in PPARα-null mice. These results indicate that the increase in CDK-1, CDK-4 and c-myc may be caused by an increase in transcription that is mediated directly or indirectly by PPARα. Thus PPARα-dependent alterations in cell cycle regulatory proteins induced by peroxisome proliferators are likely to contribute to the hepatocarcinogenicity of peroxisome proliferators.