Placental vitamin D metabolism and its associations with circulating vitamin D metabolites in pregnant women

Abstract

Background: Little is known about placental vitamin D metabolism and its impact on maternal circulating vitamin D concentrations in humans.Objective: This study sought to advance the current understanding of placental vitamin D metabolism and its role in modulating maternal circulating vitamin D metabolites during pregnancy.Design: Nested within a feeding study, 24 healthy pregnant women (26-29 wk of gestation) consumed a single amount of vitamin D (511 IU/d from diet and a cholecalciferol supplement) for 10 wk. Concentrations of placental and blood vitamin D metabolites and placental messenger RNA (mRNA) abundance of vitamin D metabolic pathway components were quantified. In addition, cultured human trophoblasts were incubated with 13C-cholecalciferol to examine the intracellular generation and secretion of vitamin D metabolites along with the regulation of target genes.Results: In placental tissue, 25-hydroxyvitamin D3 [25(OH)D3] was strongly correlated (r = 0.83, P < 0.001) with 24,25-dihydroxyvitamin D3 Moreover, these placental metabolites were strongly correlated (r ≤ 0.85, P ≤ 0.04) with their respective metabolites in maternal circulation. Positive associations (P ≤ 0.045) were also observed between placental mRNA abundance of vitamin D metabolic components and circulating vitamin D metabolites [i.e., LDL-related protein 2 (LRP2, also known as megalin) with 25(OH)D3 and the C3 epimer of 25(OH)D3 [3-epi-25(OH)D3]; cubilin (CUBN) with 25(OH)D3; 25-hydroxylase (CYP2R1) with 3-epi-25(OH)D3; 24-hydroxylase (CYP24A1) with 25(OH)D3, 3-epi-25(OH)D3, and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]; and 1α-hydroxylase [(CYP27B1) with 3-epi-25(OH)D3 and 1,25(OH)2D3]. Notably, in vitro experiments with trophoblasts showed increased production and secretion of 25(OH)D3 and higher CYP24A1 gene transcript abundance in response to cholecalciferol treatment.Conclusions: The numerous associations of many of the placental biomarkers of vitamin D metabolism with circulating vitamin D metabolites among pregnant women [including a CYP27B1-associated increase in 1,25(OH)2D3] and the evidence of trophoblast production and secretion of vitamin D metabolites, especially 25(OH)D3, suggest that the placenta may play an active role in modulating the vitamin D metabolite profile in maternal circulation in human pregnancy. This trial was registered at clinicaltrials.gov as NCT03051867.

Keywords: 25-hydroxyvitamin D; CYP24A1; CYP27B1; CYP2R1; megalin/cubilin; placenta; pregnancy; trophoblasts; vitamin D; vitamin D metabolism.

© 2017 American Society for Nutrition.

Figures

FIGURE 1

The relations of placental and circulating vitamin D metabolites in pregnant women (n = 24) by study time point: (A) placental 25(OH)D3 and serum 25(OH)D3 concentrations; (B) placental 25(OH)D3 and plasma 24,25(OH)2D3 concentrations; (C) placental 25(OH)D3 and circulating 24,25(OH)2D3:25(OH)D3 ratio; (D) placental 25(OH)D3 and circulating free 25(OH)D3 concentrations; (E) placental 25(OH)D3 and serum 3-epi-25(OH)D3 concentrations; (F) placental 24,25(OH)2D3 and serum 25(OH)D3 concentrations; (G) placental 24,25(OH)2D3 and plasma 24,25(OH)2D3 concentrations; and (H) placental 24,25(OH)2D3 and serum 3-epi-25(OH)D3 concentrations. Data were derived from Pearson and Spearman correlation tests. Study baseline values are presented in open squares with dashed line slopes, and study-end values are presented in closed squares with solid line slopes. 3-epi-25(OH)D3, C3 epimer of 25-hydroxyvitamin D3; 24,25(OH)2D3, 24,25-dihydroxyvitamin D3; 25(OH)D3, 25-hydroxyvitamin D3.

FIGURE 2

The responses of CYP24A1 mRNA abundance in HTR-8/SVneo cells in response to vitamin D (unlabeled) treatment at 72 h. Values are means ± SEs. Data were obtained from 3 separate experiments, with each experiment containing 3 replicates/vitamin D treatment, and were analyzed after log10 transformation with the use of ANOVA. Means without a common lowercase letter differ, P < 0.05. CYP24A1, 24-hydroxylase gene; HTR-8/SVneo, immortalized human placental extravillous trophoblast cell line; 1,25(OH)2D3, 1,25-dihydroxyvitamin D3; 25(OH)D3, 25-hydroxyvitamin D3.

FIGURE 3

Intracellular and extracellular concentrations of 13C-labeled vitamin D metabolites from HTR-8/SVneo cells incubated with 13C5-25(OH)D3 (A–C) or 13C2-vitamin D3 (cholecalciferol) (D–G) for 24-, 72-, and 96-h: (A) 13C5-25(OH)D3; (B) 13C5-1,25(OH)2D3; (C) 13C5-24,25(OH)2D3; (D) 13C2-cholecalciferol; (E) 13C2-25(OH)D3; (F) 13C2-1,25(OH)2D3; (G) 13C2-24,25(OH)2D3. All concentrations are predicted geometric means (95% CIs) derived from the covariate-adjusted linear mixed models. Values were obtained from 3 separate experiments with each experiment containing 3 replicates/vitamin D treatment at a time point. Different letters denote changes in concentrations through time within cells or media, P < 0.05. Asterisks denote differences in concentrations between cells and media at a culture time point, P < 0.05. None of the 13C-labeled vitamin D metabolites were detected in the ethanol control treatment. HTR-8/SVneo, immortalized human placental extravillous trophoblast cell line; 1,25(OH)2D3, 1,25-dihydroxyvitamin D3; 24,25(OH)2D3, 24,25-dihydroxyvitamin D3; 25(OH)D3, 25-hydroxyvitamin D3.

FIGURE 4

The product to precursor ratios of 13C-1,25(OH)2D3 and 13C-24,25(OH)2D3 in HTR-8/SVneo cells incubated with 13C5-25(OH)D3 (A) or 13C2-vitamin D3 (cholecalciferol) (B) for 24, 72, and 96 h. All ratios are predicted geometric means (95% CIs) derived from the covariate-adjusted linear mixed models. Values were obtained from 3 separate experiments with each experiment containing 3 replicates/vitamin D treatment at a time point. Different letters denote changes within 1,25(OH)2D3:25(OH)D3 ratio or 24,25(OH)2D3:25(OH)D3 ratio through time, P < 0.05. Asterisks denote the differences between the 1,25(OH)2D3:25(OH)D3 ratio and the 24,25(OH)2D3:25(OH)D3 ratio at a culture time point, P < 0.05. None of the 13C-labeled vitamin D metabolites were detected in the ethanol control treatment. HTR-8/SVneo, immortalized human placental extravillous trophoblast cell line; 1,25(OH)2D3, 1,25-dihydroxyvitamin D3; 24,25(OH)2D3, 24,25-dihydroxyvitamin D3; 25(OH)D3, 25-hydroxyvitamin D3.