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.