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-dihydroxy vitamin D-3 [1,25(OH)(2)D-3/calcitriol] through the activity of 25-hydroxyvitamin D-3-1 alpha-hydroxylase (CYP27B1) and 1,25(OH)(2)D-3-24-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)(2)D-3 upon the expression of CYP27B1 and CYP24A1 were also investigated. 1,25(OH)(2)D-3 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. (c) 2006 Elsevier Ltd. All rights reserved.