The effect of vitamin D on placental development and pregnancy success

Abstract

Vitamin D₃ deficiency is prevalent around the world, with 30-60% of Australians having 25(OH)D₃ levels below 50 nmol/L. There has been a resurgence of interest in vitamin D₃ in the last decade as its deficiency has been shown to be associated with an increasing number of diseases, including pregnancy complications. The role of vitamin D₃ in pregnancy is unclear, although vitamin D₃ metabolism genes are expressed in the placenta and circulating active vitamin D₃ increases 3-fold during pregnancy. As many pregnancy complications are associated with poor placental development, we hypothesise that vitamin D₃ deficiency may impair placental development and thereby contribute to the pathogenesis of pregnancy complications. To determine the effect of dietary vitamin D₃ and calcium deficiency on the placenta we used a mouse model. Mice were fed diets deficient in vitamin D₃ and/or calcium, mated with normal males and killed at d18.5 post-coitus. Pregnant and non-pregnant mice had altered vitamin D₃ metabolism, namely alterations in biochemistry and kidney gene expression. During pregnancy serum 1,25(OH)₂D₃ levels are elevated. The level of expression of the gene for the enzyme responsible for activation of vitamin D, vitamin D₃ 25-hydroxylase (CYP27B1), was highest in kidneys of pregnant mice compared to levels in non-pregnant mice as well as to levels in the placenta of pregnant mice, indicating that increased renal 1,25(OH)₂D₃ production is a feature of pregnancy. Pregnant mice consuming diets deficient in both vitamin D₃ and calcium resulted in higher incidence of preterm birth (PTB) as defined by a delivery before d18.5 of gestation. While pregnant mice had comparable placental weights regardless of vitamin D and calcium deficient diets, the placental morphometry was altered such that there was increased capacity for feto-maternal exchange. Consistent with this, the average fetal weight was significantly greater in those dams consuming a low calcium diet regardless of the dietary vitamin D level, suggesting that placental adaptions allowed greater fetal growth. We next examined the human placental expression profile of vitamin D₃ metabolism and the interacting insulin-like growth factor (IGF) pathway genes across gestation. We found that vitamin D₃ pathway genes (VDR, CYP2R1) increase while IGF genes (IGF1R, IGF2) decrease from early to late gestation, reflecting the most important timeframes in gestation for the action of these pathways. Correlations in gene expression were found between IGF2 and VDR, as well as IGF1R and VDR, suggesting new interactions between the pathways. Immunohistochemistry revealed reducing CYP27B1, VDR and IGF2 protein across gestation, while CYP24A1 was not altered. As VDR protein is reduced, with high mRNA levels, this indicates a high turnover of VDR protein in term placentas. Human placental gene expression from pregnancies with medical complications was also compared to placentas from normal term-delivered controls. The mRNA level for VDR was reduced in placentas of pregnancies with preeclampsia and spontaneous preterm deliveries. In addition, CYP24A1 mRNA levels were reduced in placentas from pregnancy cases with gestational diabetes and fetuses that were considered small for gestational age. This suggests that placental vitamin D₃ metabolism is altered both across gestation and in pregnancy complications. Despite these changes in gene expression for vitamin D related genes, there was only one altered IGF-family gene, with IGF1 increased in placentas from pregnancies with preterm deliveries. As IGF1 interacts with the vitamin D pathway, this could be an interacting pathway in pathology of preterm birth. In conclusion, vitamin D₃ and calcium metabolism are altered by pregnancy, with deficiency of both vitamin D₃ and calcium resulting in PTB in mice. Dietary vitamin D and calcium deficiency increased fetal weight which was associated with changes in both placental morphometry and gene expression. In human placenta, the expression of vitamin D₃ metabolic pathway components is altered both across gestation and between complicated and uncomplicated pregnancies. These studies together strongly indicate a role for vitamin D₃ action in normal and complicated pregnancy.