Evidence for the presence of a functional pregnane X receptor response element in the CYP3A7 promoter gene.

Pregnane X Receptor (PXR) has been recently shown to regulate the inducible expression of CYP3A genes in response to xenobiotics and steroids. PXR forms a heterodimer with the retinoic acid receptor (RXR) and this complex binds to and transactivates an 18bp region containing two everted repeats TGA(A/C)CT separated by 6 nucleotides (ER6) and located at approximately -150 in the CYP3A4 promoter. In this work we have isolated and sequenced the proximal 5'-flanking region of CYP3A7 from two different human genomic libraries. In contrast to a previously reported sequence (Itoh et al., 1992), we did not observe any mutation in the 3'-half of the CYP3A7 ER6 element. Using electrophoretic mobility shift assays and cotransfection experiments we show that this element is able to bind the PXR:RXR complex and transactivates the expression of a down stream promoter in response to rifampicin, clotrimazole, and RU-486, three compounds known to specifically activate the human PXR. This is consistent with the fact that CYP3A7 mRNA is inducible in several primary cultures of human hepatocytes from different patients, as well as in two hepatocarcinoma cell lines HuH7 and HepG2, in response to these compounds. In contrast to a previous report (Blumberg et al., 1998), based on the sequence published by Itoh et al., we conclude that CYP3A7, like CYP3A4, is inducible in response to xenobiotics and presumably in a large proportion of the population.