25-hydroxyvitamin D3-1α-hydroxylase is expressed in human vascular smooth muscle cells and is upregulated by parathyroid hormone and estrogenic compounds

Background - 1,25(OH)(2) vitamin D-3 exerts multiple effects in human vascular smooth muscle cells (VSMCs). We therefore tested the possibility that VSMCs possess an endogenous 25-hydroxyvitamin D-3-1 alpha-hydroxylase system, the final enzyme in the biosynthetic pathway of 1,25(OH)(2)D-3. Methods and Results - We assessed the expression and activity of 25-hydroxyvitamin D-3-1 alpha-hydroxylase by real-time polymerase chain reaction and the conversion of 25( OH) D-3 into 1,25(OH)(2)D-3. First, 25-hydroxyvitamin D-3- 1 alpha-hydroxylase mRNA was identified in cultured VSMCs by real-time polymerase chain reaction. Second, in cells treated daily ( 3 days) with parathyroid hormone ( 66 nmol/L), estradiol-17 beta ( 30 nmol/L), raloxifene ( 3 mu mol/L), and the phytoestrogens genistein ( 3 mu mol/L), biochainin A ( 3 mu mol/L), and 6-carboxy biochainin A ( 30 nmol/L), 25-hydroxyvitamin D-3- 1 alpha-hydroxylase mRNA increased by 43 +/- 13%, ( P < 0.05) 7 +/- 24% ( P = NS), 176 +/- 28% ( P < 0.01), 65 +/- 11% ( P < 0.05), 152 +/- 24% ( P < 0.01), and 71 +/- 9% ( P < 0.05), respectively. Third, production of 1,25( OH)(2)D-3 from 25(OH)D-3 was seen with a Km of 25 ng/mL and increased dose dependently after treatment with parathyroid hormone, genistein, and the phytosetrogen derivative 6-carboxy biochainin A. Estradiol-17 beta and biochainin A also increased the generation of 1,25(OH)(2)D-3 by 40 +/- 23% ( P < 0.05) and 55 +/- 13% ( P < 0.05), respectively. Conclusions - We provide here the first evidence for the expression of an enzymatically active 25(OH) D-3-1 alpha-hydroxylase system in human VSMCs, which can be upregulated by parathyroid hormone and estrogenic compounds. Because exogenous vitamin D inhibits VSMC proliferation, the role of this system as an autocrine mechanism to curb changes in VSMC proliferation and phenotype is a subject for future investigation.