Colchicine down-regulates cytochrome P4502B6, 2C8, 2C9, and 3A4 in human hepatocytes by affecting their glucocorticoid receptor-mediated regulation

The xenobiotic-mediated induction of three major human liver cytochrome P450 genes, CYP2B6, CYP2C9, and CYP3A4, is known to be regulated by the constitutive androstane receptor ( CAR) and the pregnane X receptor (PXR). CAR and PXR are regulated, at least in part, by the glucocorticoid receptor (GR) and the hypothesis of a signal transduction cascade GR-[CAR/PXR]P450 has been proposed. This study was aimed at testing this hypothesis in primary human hepatocytes by using the tubulin network disrupting agent colchicine. Colchicine ( COL) decreased both basal and rifampicin- and phenobarbital-inducible expression of CYP2B6, CYP2C8/9, and CYP3A4. A parallel down-regulation of mRNA expression of CAR, PXR, and tyrosine aminotransferase, a prototypic gene directly regulated by GR, was observed. COL affected neither the level of GR mRNA nor ligand binding to GR. To evaluate the effect of colchicine on GR-mediated gene transactivation, HeLa cells stably or transiently transfected with a GR-responsive element-dependent luciferase reporter gene were used. COL decreased the dexamethasone- induced luciferase expression in stably transfected cell line by 50%, whereas GR transactivation in transiently transfected cells was not affected by COL. In contrast, ligand-dependent GR translocation in the human embryonic kidney 293 cell line transiently transfected with GFP-GR was inhibited by COL. We conclude that alteration of the signal transduction mediated through the GR-[CAR/PXR]-P450 cascade by colchicine is responsible for the down-regulation of CYP2C9 and CYP3A4, implicating cytoskeleton as necessary for correct functioning of this cascade under physiological conditions.