Activating and Inhibitory Functions of WNT/b-Catenin in the Induction of Cytochromes P450 by Nuclear Receptors in HepaRG Cells

The WNT/b-catenin signaling pathway has been identified as an important endogenous regulator of hepatic cytochromeP450 (P450) expression inmouse liver. In particular, it is involved in the regulation of P450 expression in response to exposure to xenobiotic agonists of the nuclear receptors constitutive androstane receptor (CAR), aryl hydrocarbon receptor (AhR), and Nrf2. To systematically elucidate the effect of the WNT/b-catenin pathway on the regulation and inducibility of major human P450 enzymes, HepaRG cells were treated with either the WNT/b-catenin signaling pathway agonist, WNT3a, or with small interfering RNA directed against b-catenin, alone or in combination with a panel of activating ligands for AhR [2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)], CAR [6-(4-chlorophenyl) imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl) oxime (CITCO)], pregnane X receptor (PXR) [rifampicin], and peroxisome proliferator–activated receptor (PPAR) a [4-chloro-6(2,3-xylidino)-2-pyrimidinylthioacetic acid (WY14,643)]. Assessment of P450 gene expression and enzymatic activity after downregulation or activation of the WNT/b-catenin pathway revealed a requirement of b-catenin in the AhR-, CAR-, and PXR-mediated induction of CYP1A, CYP2B6 and CYP3A4 (for CAR and PXR), and CYP2C8 (for PXR) gene expression. By contrast, activation of the WNT/b-catenin pathway prevented PPARa-mediated induction of CYP1A, CYP2C8, CYP3A4, and CYP4A11 genes, suggesting a dominant-negative role of b-catenin in PPARa-mediated regulation of these genes. Our data indicate a significant effect of theWNT/ b-catenin pathway on the regulation of P450 enzymes in human hepatocytes and reveal a novel crosstalk between b-catenin and PPARa signaling pathways in the regulation of P450 expression.