Energy sensing factors PGC-1α and SIRT1 modulate PXR expression and function.

The pregnane X receptor (PXR), a xenobiotic-sensing nuclear receptor plays a major role in regulation of drug metabolism but also modulates hepatic energy metabolism. PXR interacts with and represses several important transcription factors and coactivators regulating key enzymes in energy metabolism. Much less is known about how energy sensing cellular factors regulate PXR function. In this study we have investigated the effect of two major regulators of hepatic energy homeostasis, the transcriptional coactivator, peroxisome proliferator-activated receptor γ coactivator 1 alpha (PGC-1α) and the NAD-dependent deacetylase protein, sirtuin 1 (SIRT1) on PXR expression and function. Fasting induces PXR expression in liver. Furthermore, glucagon and PGC-1α overexpression upregulate PXR expression level in mouse primary hepatocytes suggesting that PGC-1α, in addition to coactivation of PXR, also transcriptionally regulates PXR gene. Knockdown of peroxisome proliferator-activated receptor α by siRNA attenuates PGC-1α mediated induction of PXR mRNA. PGC-1α overexpression alone has no effect on cytochrome P450 (CYP) 3A11 expression but potentiates induction by pregnenolone-16α-carbonitrile (PCN). Pyruvate, a nutrient signal activating SIRT1 abolishes synergistic induction of CYP3A11 by PCN and PGC-1α. Knockdown of SIRT1 prevented this effect of pyruvate. Downregulation of CYP7A1 by PCN was not affected by PGC-1α or pyruvate. Mammalian two hydrid assays indicate that pyruvate and SIRT1 interfere with interaction of PXR and PGC-1α. This may be mediated by well established PGC-1α deacetylation by SIRT1. However, we show by immunoprecipitation that SIRT1 also interacts with PXR. Thus we show that two fasting activated pathways PGC-1α and SIRT1 differentially modify PXR expression and function.