Metformin is widely used in the treatment of type-2 diabetes. The pleotropic effects of metformin on glucose and lipid metabolism have been proposed to be mediated by the activation of AMP-activated protein kinase (AMPK) and the subsequent up-regulation of small heterodimer partner (SHP). SHP suppresses the functions of several nuclear receptors involved in the regulation of hepatic metabolism, including pregnane X receptor (PXR), which is referred to as a "master regulator" of drug/xenobiotic metabolism. In this study, we hypothesize that metformin suppresses the expression of CYP3A4, a main detoxification enzyme and a target gene of PXR, due to SHP up-regulation. We employed various gene reporter assays in cell lines and qRT-PCR in human hepatocytes and in Pxr(-/-) mice. We show that metformin dramatically suppresses PXR-mediated expression of CYP3A4 in hepatocytes. Consistently, metformin significantly suppressed the up-regulation of Cyp3a11 mRNA in the liver and intestine of wild-type mice, but not in Pxr(-/-) mice. A mechanistic investigation of the phenomenon showed that metformin does not significantly up-regulate SHP in human hepatocytes. We further demonstrate that AMPK activation is not involved in this process. We show that metformin disrupts PXR's interaction with steroid receptor coactivator-1 (SRC1) in a two-hybrid assay independently of the PXR ligand binding pocket. Metformin also inhibited vitamin D receptor-, glucocorticoid receptor- and constitutive androstane receptor (CAR)-mediated induction of CYP3A4 mRNA in human hepatocytes. We show, therefore, a suppressive effect of metformin on PXR and other ligand-activated nuclear receptors in transactivation of the main detoxification enzyme CYP3A4 in human hepatocytes.

Department of Pharmacology and Toxicology Faculty of Pharmacy in Hradec Kralove Charles University Prague Heyrovskeho 1203 Hradec Kralove CZ 500 05 Czech Republic

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