1α, 25-Dihydroxyvitamin D₃, and 24,25-dihydroxyvitamin D₃ in vitro synthesis by human decidua and placenta

Abstract

The biologically active metabolite of vitamin D, 1α, 25-dihydroxyvitamin D (1α, 25(OH)₂D), is synthesised by successive hydroxylation of vitamin D in the liver and kidney¹. The latter organ is the only known site of 1α-hydroxylation of 25-hydroxyvitamin D (25-OH-D)² and is also a major site of production of 24,25-dihydroxyvitamin D (24,25(OH)₂D), a metabolite involved in bone ossification₃. However, 24-hydroxylation of 25-OH-D may also occur in rat intestinal homogenates and in cartilage cells^4,5. Accumulating data show that, during pregnancy, 1α, 25(OH)₂D increases in the maternal circulation⁶ but is absent or present only in small amounts in the fetus^7,8. In comparison, 24,25(OH)₂D accumulates in fetal tissues, particularly in the skeleton⁸. Little is known, however, about the independent metabolism of vitamin D in the fetoplacental unit. It has been reported⁹ that the nephrectomised pregnant rat can synthesise 1α, 25(OH)₂D₃ and 24,25(OH)₂D₃ and that the fetoplacental unit is the most likely site of production of such metabolites. Those data suggested that there may be independent vitamin D metabolism in the fetoplacental unit. We now report in vitro synthesis of 24,25(OH)₂D₃ by human placenta and of 1α,25(OH)₂D₃ and 24,25(OH)₂D₃ by human decidua. The human decidua is a development of the endometrium that is unique to pregnancy, originating from both epithelial and stromal cells and intimately attached to fetal structures, that is, the placenta and chorion.