Regulation of Vitamin D hydroxylases gene expression by 1,25-dihydroxyvitamin D3 and cyclic AMP in cultured human syncytiotrophoblasts

Human placenta synthesizes and metabolizes 1,25-dihydroxyvitamin D3 [1,25(OH)2D3/calcitriol] through the activity of 25-hydroxyvitamin D3-1α-hydroxylase (CYP27B1) and 1,25(OH)2D3-24-hydroxylase (CYP24A1), the two key enzymes for Vitamin D metabolism. In this study, calcitriol rapidly generated intracellular cAMP accumulation in cultured human syncytiotrophoblast cells, which in turn enhanced hCG secretion, a marker of trophoblast endocrine activity. The effects of 1,25(OH)2D3 upon the expression of CYP27B1 and CYP24A1 were also investigated. 1,25(OH)2D3 and activators of the PKA signaling system decreased the expression of CYP27B1, whereas increased CYP24A1 gene transcription. The use of a selective inhibitor of PKA (H-89) prevented the effects of calcitriol on CYP27B1 gene and hCG secretion, but not on CYP24A1 transcription. Addition of ZK 159222, a Vitamin D receptor (VDR) antagonist, blocked the calcitriol-mediated upregulation of 24-hydroxylase gene expression but did not affect calcitriol-induced downregulation of CYP27B1 gene or hCG stimulation. In addition, our study also demonstrated a role of calcitonin on Vitamin D hydroxylases gene regulation in placenta. The overall data suggest that calcitriol downregulates CYP27B1 expression via a cAMP-dependent signaling pathway, whereas upregulates 24-hydroxylase gene expression through a VDR-dependent mechanism.