Calcium channel modulators of the dihydropyridine family are human pregnane X receptor activators and inducers of CYP3A, CYP2B, and CYP2C in human hepatocytes

The expression of three cytochromes P450 (CYP3A4, CYP2C9, and CYP2B6) was i nvestigated in primary human hepatocyte cultures following treatment with f our calcium channel modulators (CCM) of the dihydropyridine family, three a ntagonists (nifedipine, nicardipine, and isradipine), and one agonist (BK86 44). Induction of CYP3A4 was studied by Northern blot, Western blot, and en zymatic activity. Induction began between 1 and 10 muM CCM and was dependen t on the presence of dexamethasone (100 nM) in the medium. CYP3A4 mRNA accu mulation started only after 16 h of treatment because pregnane X receptor ( hPXR) synthesis was needed. Cotransfection experiments showed that the prox imal and the distal PXR response elements of the CYP3A4 promoter and hPXR ( HepG2 cells) or dexamethasone-induced hPXR (primary hepatocytes) were neces sary to obtain full induction. Furthermore, glutathione S-transferase pull- down assays demonstrated that the CCM tested can act as hPXR ligands. In ad dition, cotransfection experiments in CV1 cells showed that these compounds failed to reverse CAR (constitutively activated receptor) inactivation by androstenol. Finally, 10 muM CCM induced both CYP2C9 and CYP2B6, strengthen ing the evidence that hPXR is involved in the regulation of these genes. Al l together, these results widen the field of hPXR activators to a new class of ligand, namely the CCM of the dihydropyridine family.