25-Hydroxyvitamin D 3 -1α-Hydroxylase Is Expressed in Human Vascular Smooth Muscle Cells and Is Upregulated by Parathyroid Hormone and Estrogenic Compounds

Abstract

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 -1α-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 -1α-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 -1α-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-17β (30 nmol/L), raloxifene (3 μmol/L), and the phytoestrogens genistein (3 μmol/L), biochainin A (3 μmol/L), and 6-carboxy biochainin A (30 nmol/L), 25-hydroxyvitamin D 3 -1α-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-17β 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 -1α-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.