1α,25-dihydroxyvitamin D₃ triggered vitamin D receptor and Farnesoid X Receptor-like effects in rat intestine, liver, and kidney in vivo

1α,25-Dihydroxyvitamin D3 Triggered Vitamin D Receptor and Farnesoid X Receptor-like Effects in Rat Intestine, Liver, and Kidney In Vivo E. C. Chow 1, H-J. Maeng 1, A. A. Khan 2, G. M. Groothuis 2, K. S. Pang 1 1 Department of Pharmaceutical Sciences, Leslie Dan Faculty of Pharmacy, University of Toronto, Canada, 2 Pharmacokinetics and Drug Delivery, Department of Pharmacy, University of Groningen, The Netherlands Purpose. The effects of 1α,25dihydroxyvitamin D3 (calcitriol), the natural ligand of the vitamin D receptor (VDR), on intestinal, hepatic, and renal enzymes, transporters, and nuclear receptors were investigated in the rat in vivo. Methods. Rats were treated with calcitriol (0.64, 1.28, 2.56 nmol/kg/day) intraperitoneally for 4 days. Protein and mRNA levels of nuclear receptors, transporters, and enzymes of the small intestine [enterocytes from duodenum (S1), jejunum (S2-S7), and ileum (S8)], liver, and kidney were assayed by Western blotting and qPCR, respectively. The intestinal everted sac technique was used to appraise transporter functions. Cyp7a1 functional activity in liver microsomes was estimated from cholesterol metabolite formation in vitro and assayed by HPLC. Results. In intestine, protein levels of Mrp2, Mrp4, and PepT1 in S1 and S2 and ileal Asbt (S8) were induced with calcitriol. The rate of 5-(and-6)-carboxy-2',7'’dichlorofluorescein (CDF) efflux from dosing of diacetate precursor by Mrp2 in S2 everted sac was higher in the calcitriol-treated group. Portal bile acid concentration increased by 50% with calcitriol treatment (65.1 ?14.9 vs. 41.9 ?7.8 μM, P < .05), which was commensurate with elevated expression of hepatic FXR and the short heterodimer partner SHP. In S8, mRNA levels of the fibroblast growth factor (FGF 15), a hormonal signaling molecule that decreases Cyp7a1 levels in the liver, was also induced. Concomitantly, > 50% reduction in Cyp7a1 protein and Cyp7a1 metabolic activity, likely consequences of the bile acid-FXR-SHP cascade and activation of c-Jun N-terminal kinase signaling pathway of FGF15, were observed. In the kidney, calcitriol treatment significantly induced Cyp24, the catabolic enzyme of calcitriol, and Mdr1a mRNA and P-gp protein. Conclusion. Calcitriol treatment elicited significant changes in the expression of transporters and enzymes in the rat intestine, liver, and kidney. However, in view of the low levels of VDR in liver, the down-regulation of Cyp7a1 level observed in the liver appeared to be indirect, secondary effects due to elevated portal bile acids and increased hepatic FXR and FGF15