1α,25-Dihydroxyvitamin D₃ enhances TRPV6 transcription through p38 MAPK activation and GADD45 expression

The active form of vitamin D, 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], acts as a ligand for the vitamin D receptor (VDR), and regulates various physiological processes, including calcium and bone metabolism, cellular growth and differentiation, immunity and cardiovascular function. A number of vitamin D derivatives have been synthesized for the treatment of cancer and inflammatory disease, but the adverse effect of hypercalcemic activity due to intestinal calcium absorption has limited wide clinical application. The VDR target gene product TRPV6 is essential for intestinal calcium absorption. Our prior study has demonstrated that 1,25(OH)₂D₃ induces TRPV6 mRNA expression at lower concentrations than for induction of CYP24A1, a VDR target gene involved in vitamin D inactivation, in intestinal SW480 cells, suggesting an additional mechanism for vitamin D signaling on TRPV6 induction. By searching for a signal transduction pathway involved in 1,25(OH)₂D₃-induced expression of TRPV6, we found that a p38 mitogen-activated protein kinase (MAPK) inhibitor reduces the expression of TRPV6 but not CYP24A1 in 1,25(OH)₂D₃-treated SW480 cells. Knockdown experiments showed that p38α is involved in 1,25(OH)₂D₃-induced expression of TRPV6 but not CYP24A1. Treatment with a de novo protein synthesis inhibitor suppressed 1,25(OH)₂D₃-induced TRPV6 expression. Finally, we found that 1,25(OH)₂D₃ treatment induced expression of GADD45A, which encodes the GADD45α MAPK kinase kinase activator, earlier than TRPV6 expression and that GADD45A knockdown reduced TRPV6 induction by 1,25(OH)₂D₃. These findings indicate that p38α and GADD45α are involved in an enhanced vitamin D signaling on TRPV6 expression.