Peroxisome Proliferator-activated Receptor Alpha (PPAR alpha) Agonists Induce Constitutive Androstane Receptor (CAR) and Cytochrome P450 2B in Rat Primary Hepatocytes

The constitutive androstane receptor (CAR; NR1I3) is a key transcriptional factor that regulates genes encoding drug-metabolizing enzymes and drug transporters. However, studies on regulation of CAR target genes via up-or down-regulation of CAR are limited. In this study, we examined the effects of PPAR alpha agonists (ciprofibrate, bezafibrate, fenofibrate and WY14643) on the expression of CAR and its target gene CYP2B1/2 in rat primary hepatocytes. Results from real-time PCR analysis showed that CAR and CYP2B1/2 mRNAs exhibit increases in response to all PPAR alpha agonists studied (5 to 10-folds of control). Pretreatment of cells with cycloheximide, an inhibitor of protein synthesis, completely suppressed increase in CYP2B1/2 mRNA in response to ciprofibrate, suggesting that protein synthesis is required in this process. In addition, the induction of CAR by ciprofibrate on the protein level was observed with nuclear extracts as well as total cell lysates. These results indicate that CYP2B1/2 mRNAs are induced by PPAR alpha agonists and that this effect is accompanied by increase in the expression of CAR gene at both mRNA and nuclear protein levels. Activated PPAR alpha may increase functional CAR protein, which can induce the expression of CAR target genes such as CYP2B.