The role of intestinal Cytochrome P450 in vitamin D metabolism

Background and objectives: Physiological vitamin D status is important for bone health. Vitamin D hydroxylation in the liver/kidney results in conversion to its physiologically active form of "1,25-dihydroxyvitamin D" (1,25D). 1,25D controls gene expression through the nuclear vitamin D receptor (VDR) mainly expressed in intestinal epithelial. Activation of VDR by 1,25D binding results in heterodimerization with the retinoid X receptor (RXR) and association with vitamin D responsive elements (VDREs) in the promoter region of vitamin D target genes. Cytochrome P450 (CYP) 24A1 is a catabolic enzyme expressed in kidney that is important for 1,25D inactivation and maintaining 1,25D homeostasis. Interestingly, a recently identified mutation in CYP3A4 (gain of function) caused vitamin D-dependent rickets, type III. Unlike CYP24A1, CYP3A4 is mainly expressed in liver and is a known xenobiotic enzyme. CYP3A4 has been reported to convert 25-hydroxyvitamin D to 4beta,25-dihydroxyvitamin D, in vitro. The CYP3A family in humans (3A4, 3A5, and 3A7) possesses high amino acid homology. These CYP3As and CYP24A1 are also expressed in intestine, but their hydroxylation activities towards vitamin D substrates are unknown. Methods: We constructed human CYP3A4, 3A5, 3A7 and 24A1 expression plasmids, and evaluated their CYP activities on vitamin D action using VDRE-transcriptional assays, mammalian two-hybrid assay (VDR-RXR), or real-time PCR in human colon cultured cells. Moreover, we examined the expression level of CYPs in the intestine, kidney or liver from mouse by real-time PCR or western blotting. Results: Expression of CYP3A4, 3A5, 3A7, and 24A1 all significantly reduced 1,25D-VDRE activity in HCT116. Moreover, CYP3A4, 3A5, and 3A7 significantly reduced VDR-RXR interaction/heterodimerization. In C2BBe1 cells, CYP3A4 also inhibited 1,25D-mediated induction of Transient Receptor Potential Vanilloid (TRPV6), a membrane calcium channel which is involved in Ca2+ absorption in the intestine. Finally, in mice, Cyp3a mRNA or protein was more expressed than Cyp24a1 in the intestine. Conclusions: Therefore, CYP3As attenuate 1,25D action in intestine, and CYP3A inducers like phenytoin or phenobarbital, carbamazepine (anti-epileptic medicines), pioglitazone (hypoglycemic medicine), etc. could compete with vitamin D treatment in osteoporosis patients. Further studies are needed to clarify the physiological role of intestinal CYP3A and/or CYP24A1 in metabolism of 1,25D, in vivo.