1,25(OH)2D3 Induces Placental Vascular Smooth Muscle Cell Relaxation by Phosphorylation of Myosin Phosphatase Target Subunit 1Ser507: Potential Beneficial Effects of Vitamin D on Placental Vasculature in Humans1

Placental vascular dysfunction has been linked to insufficiency/deficiency of maternal vitamin D levels during pregnancy. In contrast, sufficient maternal vitamin D levels have shown beneficial effects on pregnancy outcomes. To study the role of vitamin D in pregnancy, we tested our hypothesis that vitamin D exerts beneficial effects on placental vasculature. We examined expression of CYP2R1, CYP27B1, vitamin D receptor (VDR), and CYP24A1 in placental vascular smooth muscle cells (VSMCs) in response to 1,25(OH)2D3 We found that VDR expression was inducible, CYP27B1 expression was dose-dependently down-regulated, and CYP24A1 expression was dose-dependently up-regulated in cells treated with 1,25(OH)2D3 These data suggest a feedback autoregulatory system of vitamin D existing in placental VSMCs. Using a VSMC/collagen-gel contraction assay, we evaluated the effect of 1,25(OH)2D3 on placental VSMC contractility. We found that, similar to losartan, 1,25(OH)2D3 could diminish angiotensin II-induced cell contractility. The mechanism of 1,25(OH)2D3-mediated VSMC relaxation was further explored by examination of Rho-associated protein kinase 1 (ROCK1)/phosphorylation of myosin phosphatase target subunit 1 (MYPT1) pathway molecules. Our results showed that p-MYPT1(Thr853) and p-MYPT1(Thr696) were undetectable. However, p-MYPT1(Ser507), but not p-MYPT1(Ser668), was significantly up-regulated in cells treated with losartan plus angiotensin II. Similar effects were also seen in cells treated with 1,25(OH)2D3 plus angiotensin II or 1,25(OH)2D3 plus losartan plus angiotensin II. Because MYPT1 serine phosphorylation could activate myosin light chain phosphatase (MLCP), and MLCP activation is an important regulatory machinery of smooth muscle cell relaxation, up-regulation of MYPT1(Ser507) phosphorylation could be a mechanism of vitamin D and/or losartan mediated placental VSMC relaxation.