Expression of cytochrome P450 3A4 (CYP3A4) is induced by 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) in Caco-2 cells. However, since a typical vitamin D responsive element has not been found in the 5'-flanking region of the CYP3A4 gene, the mechanism of 1,25 (OH) A-induced CYP3A4 mRNA expression is poorly understood. In the present study, we demonstrated that vitamin D receptor (VDR) is a critical factor for the induction using the antisense oligonucleotide technique. In addition, we found that treatment of Caco-2 cells with the protein kinase C (PKC) inhibitors, staurosporine and GF109203X, and the tyrosine kinase inhibitor, genistein, but not with the protein kinase A inhibitor, H-89, suppressed CYP3A4 mRNA induction by 1,25(OH)(2)D-3. The depletion of PKC by prolonged treatment with phorbol ester abolished the induction. On the other hand, protein kinase inhibitors used had no effects on the constitutive expression of VDR mRNA. Therefore, these observations suggest that 1,25 (OH)(2)D-3-induced CYP3A4 mRNA expression might be involved in phosphorylation events in addition to transcriptional regulation via VDR. However, 1,25(OH)(2)D-3 did not rapidly activate PKC in the Caco-2 cells used, while the treatment with staurosporine and GF109203X, but not genistein, decreased basal PKC activity by similar to30% of the controls. Taken together, these findings suggest that the change in the phosphorylation state via PKC and tyrosine kinase might, at least in part, modulate 1,25(OH)(2)D-3-induced CYP3A4 mRNA expression via VDR. (C) 2002 Elsevier Science (USA). All rights reserved.