Expression of CYP3A4, CYP2B6, and CYP2C9 is regulated by the vitamin D receptor pathway in primary human hepatocytes

The fully active dihydroxylated metabolite of vitamin D-3 induces the expression of CYP3A4 and, to a lesser extent, CYP2B6 and CYP2C9 genes in normal differentiated primary human hepatocytes. Electrophoretic mobility shift assays and cotransfection in HepG2 cells using wild-type and mutated oligonucleotides revealed that the vitamin D receptor (VDR) binds and transactivates those xenobiotic-responsive elements (ER6, DR3, and DR4) previously identified in CYP3A4, CYP2B6, and CYP2C9 promoters and shown to be targeted by the pregnane X receptor (PXR) and/or the constitutive androstane receptor (CAR). Full VDR response of various CYP3A4 heterologous/homologous promoter-reporter constructs requires both the proximal ER6 and the distal DR3 motifs, as observed previously with rifampicin-activated PXR. Cotransfection of a CYP3A4 homologous promoter-reporter construct (including distal and proximal PXR-binding motifs) and of PXR or CAR expression vectors in HepG2 cells revealed the ability of these receptors to compete with VDR for transcriptional regulation of CYP3A4. In conclusion, this work suggests that VDR, PXR, and CAR control the basal and inducible expression of several CYP genes through competitive interaction with the same battery of responsive elements.