2001 Volume 16 Issue 5 Pages 485-490
Cytochrome P450s included in CYP3A subfamily are involved in metabolism of numbers of clinically relevant drugs as well as numerous endogenous compounds. CYP3A forms are induced by treatment with various drugs, but clear differences are observed on induction profiles between experimental animals and humans. Recently, a novel orphan receptor, pregnane X receptor (PXR) has been isolated and reported to make a heterodimer with retinoid X recepterα (RXRα). In our experiment, co-expression of human PXR (hPXR) with RXRα did not show the CYP3A1 gene activation by treatment with rifampicin. When hPXR was co-expressed with other orphan receptors, COUP-TFs, apolipoprotein AI regulatory protein 1 (ARP-1) or v-ErbA-related protein 3, the CYP3A1 gene expression was activated by rifampicin in HepG2 cells. On the contrary, neither co-expression of PXR with RXRα nor with COUP-TFs showed the CYP3A4 gene activation by the treatment, although the activation was observed on the single expression of hPXR in HepG2 cells. On the other hand, both CYP3A1 and CYP3A4 inductions through hPXR activation by rifampicin are basically similar at the level of their cis-elements. The transactivation of the CYP3A1 gene by the drug is mediated through B site binding with hPXR and enhanced through C site. The transactivation of the CYP3A4 gene is mainly mediated through dNR-1 locating around at its -7600 bp and enhance through ER-6. B site and dNR-1 or C site and ER-6 conserve high homologies in those nucleotide sequences. To further verify the in vivo activation of the CYP3A4 gene expression by drugs, adenovirous reporter vector, AdCYP3A4-364, was constructed with a proximal promoter region ( -362 to + 11) of the CYP3A4 gene and injected into mouse. Induction profile of the CYP3A4 gene expression agreed well with that of the mouse CYP3A activity. The mutation study of receptor binding region (ER-6) diminished the induction of the CYP3A4 gene expression, but not the mouse CYP3A activity, testosterone 6β-hydroxylase. These results strongly suggest that the in vivo study of the gene transcriptional activation is very useful to assess the induction profile of the CYP3A4 gene expression by drugs.
