Evidence for the suppression of apoptosis by the peroxisome proliferator activated receptor alpha (PPAR alpha).

Abstract

Peroxisome proliferators (PPs) are a class of nongenotoxic rodent hepatocarcinogens. We have demonstrated previously that PPs suppress both spontaneous rat hepatocyte apoptosis and that induced by exogenous stimuli such as transforming growth factor-beta1 (TGFbeta1). PPs transcriptionally activate the peroxisome proliferator activated receptor-alpha (PPAR alpha), a member of the nuclear hormone receptor superfamily. Here, we investigate whether activation of PPAR alpha mediates the suppression of rat hepatocyte apoptosis induced by PPs. We isolated a naturally occurring variant form of PPAR alpha (hPPAR alpha-6/29) from human liver by PCR cloning. Electrophoretic mobility shift assays (EMSA) demonstrated that hPPAR alpha-6/29 shared the ability of mPPAR alpha to heterodimerise with the retinoid X receptor (RXR) and bind to DNA. When hPPAR alpha-6/29 was transfected into Hepa1c1c7 cells together with a reporter plasmid containing a PPAR response element (PPRE), hPPAR alpha-6/29, unlike mPPAR alpha, could not be activated by PPs. Furthermore, hPPAR alpha-6/29 could act as a dominant negative regulator of PPAR-mediated gene transcription since increasing concentrations of hPPAR alpha-6/29 abrogated the activation of co-transfected mPPAR alpha. When introduced into primary rat liver cell cultures by transient transfection, hPPAR alpha-6/29 prevented the suppression of hepatocyte apoptosis by the PP nafenopin, but not that seen in response to phenobarbitone (PB), a nongenotoxic carcinogen whose action does not involve PPAR alpha. The suppression of hepatocyte apoptosis was abrogated completely even though only 30% of hepatocytes were transfected, suggesting the involvement of a soluble factor. These data indicate that activation of rat liver PPAR alpha provides a survival signal for hepatocytes, preventing their death in response to apoptotic stimuli.