Vitamin D3 (VD3) is a multifunctional nutrient which can be either synthesized or absorbed from the diet.It plays a pivotal role in systemic calcium and phosphate homeostasis,as well as in various physiological and pathological processes.VD3 is converted to the active form,1α,25-dihydroxyvitamin D3 (1,25-D3),by cytochrome P450 2R1 (CYP2R1)/CYP27A1 and CYP27B1 sequentially,and deactivated by multiple enzymes including CYP3A4.On the other hand,1,25-D3 is capable of activating the transcription of CYP3A genes in humans,mice and rats.The vitamin D receptor (VDR)-mediated transactivation of human CYP3A4 and CYP3A5 resembles that known for pregnane X receptor (PXR).Activated VDR forms a heterodimer with retinoid X receptor α (RXRα),recruits co-activators,translocates to the cell nucleus,binds to the specific vitamin D responsive elements (VDRE),and activates the gene transcription.In mice,intestinal Cyp3a11 mRNA levels,but not those of hepatic CYP3As,were induced by in vivo administration of VDR and PXR agonists.In rats,intestinal Cyp3a1 and Cyp3a2 mRNAs were induced by 1,25-D3 or lithocholic acid (LCA),whereas hepatic Cyp3a2,but not Cyp3a1 and Cyp3a9,was modulated to 1,25-D3 treatment.In general,the VDR-mediated regulation of CYP3A presents species and organ specificity.