Regulation of calbindin-D9k expression by 1,25-dihydroxyvitamin D₃ 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)₂D₃. Both CaBP-D9k mRNA and protein were highly induced by 1,25(OH)₂D₃ 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)₂D₃, confirming the requirement of VDR for CaBP-D9k expression. Treatment of the PRTCs with 1,25(OH)₂D₃ also increased VDR protein abundance, suggesting that enhanced VDR transactivation is involved in the CaBP-D9k upregulation. Moreover, PTH had a synergistic effect on the 1,25(OH)₂D₃ induction of CaBP-D9k. These data demonstrate that CaBP-D9k is highly regulated by 1,25(OH)₂D₃ and PTH in mouse PRTCs, which provides a suitable in vitro system for further investigating the molecular mechanisms involved in CaBP-D9k gene regulation.