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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.

The aims of the present study were (1) to determine the cytotoxicity of colchiceine (EIN) in comparison with that of colchicine (COL); (2) to evaluate the effect of EIN on cytochrome P450 (CYP) expression and activity. Primary human hepatocytes were the model of choice for cytotoxicity and CYP expression experiments. LDH leakage and albumin secretion served as cytotoxicity parameters. EIN was less toxic than COL based on both parameters within the concentration range 1-100 microM. 10 microM concentration of EIN did not induce the expression of CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2E1 and CYP3A4 isoforms, which were evaluated at the levels of mRNAs, proteins and specific activities in culture. EIN in concentrations up to 200 microM had no effect on marker activities of CYP1A2, 2C9, 2E1 and 3A4 in human liver microsomes. It was concluded that EIN in concentrations up to 10 microM is not cytotoxic in primary human hepatocytes as revealed by albumin secretion and LDH leakage. Possible drug-drug interactions of EIN due to effects on cytochromes P4501A2, 2C9, 2E1 and 3A4 isoforms are unlikely because inhibition/induction studies show any lack of such effects. As EIN was shown to have better antifibrotic properties than COL (European Journal of Clinical Investigation 1997, 2, 77), it can be used as a COL substitute with anticipated fewer side-effects.