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

Abstract

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 PPARalpha 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 PPARalpha 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 PPARalpha agonists and that this effect is accompanied by increase in the expression of CAR gene at both mRNA and nuclear protein levels. Activated PPARalpha may increase functional CAR protein, which can induce the expression of CAR target genes such as CYP2B.

Clofibric acid induces hepatic CYP 2B1/2 via constitutive androstane receptor not via peroxisome proliferator activated receptor alpha in rat

“…The results of this study are in agreement with recent data reporting that the PPARα ligand, di-(2-ethylhexyl) phthalate are able to activate CAR. 35) Our assumption of CAR activation by CFA is in contrast to the explanation of CYP2B induction by PPARα ligands assumed by Saito et al 36) who proposed that PPARα activation was the cause of CAR protein induction however, that proposal still need stronger evidences. Moreover, in our study HepG2 cells transfected with PPARα failed to exhibit any CYP2B reporter activity before or after CFA treatment (Fig.…”

An update on the constitutive androstane receptor (CAR)

“…Different isoforms of HNF4 α appear to either activate (isoform 1) or suppress (isoform 7) the expression of CAR in a co-activatordependent manner [ 86 ]. The integration of CAR to many physiological processes controlled by other NRs gains support from the findings that CAR expression and/or CYP inducibility is increased by the glucocorticoid receptor [ 87 ] and the retinoic acid receptor [ 88 ]. CAR expression is also activated by PXR agonists (e.g., PCN, dexamethasone [ 87 , 89 ]), potentially by peroxisome proliferators (e.g., fibrates [ 88 ]) and is dependent on thyroid hormones [ 90 ].…”

Differential Regulation of Gene Expression by Cholesterol Biosynthesis Inhibitors That Reduce (Pravastatin) or Enhance (Squalestatin 1) Nonsterol Isoprenoid Levels in Primary Cultured Mouse and Rat Hepatocytes

“…A species-dependent interaction was also observed among treatments with respect to gene regulation by the nuclear receptors CAR and PPAR. These changes may reflect species-dependent regulation of CAR responses after PPAR activation as discussed elsewhere (Guo et al, 2006;Wieneke et al, 2007;Saito et al, 2010;Rondini et al, 2016). In mouse hepatocytes, PPAR activation by the prototypical agonist Cipro decreased expression of the CAR target gene Fig.…”