Regulation of calbindin-D9k expression by 1,25-dihydroxyvitamin D(3) and parathyroid hormone in mouse primary renal tubular cells.

Calbindin (CaBP)-D9k is a major vitamin D target gene involved in calcium homeostasis. However, studies on the molecular mechanisms of CaBP-D9k gene regulation have been hampered by the lack of an appropriate cell culture system. In the present study, we used mouse primary renal tubular cell (PRTC) cultures to investigate the regulation of CaBP-D9k expression by 1,25(OH)(2)D(3). Both CaBP-D9k mRNA and protein were highly induced by 1,25(OH)(2)D(3) in a time- and dose-dependent manner in PRTCs, and new RNA and protein synthesis was required for the induction. Transfection of VDR(-/-) cells derived from VDR null mice with human VDR restored the induction of CaBP-D9k expression by 1,25(OH)(2)D(3), confirming the requirement of VDR for CaBP-D9k expression. Treatment of the PRTCs with 1,25(OH)(2)D(3) also increased VDR protein abundance, suggesting that enhanced VDR transactivation is involved in the CaBP-D9k up-regulation. Moreover, PTH had a synergistic effect on the 1,25(OH)(2)D(3) induction of CaBP-D9k. These data demonstrate that CaBP-D9k is highly regulated by 1,25(OH)(2)D(3) and PTH in mouse PRTCs, which provides a suitable in vitro system for further investigating the molecular mechanisms involved in CaBP-D9k gene regulation.