Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCA129

Poster Communications

Placental uptake and metabolism of vitamin D

C. Simner1, R. Lewis1, C. Cooper2, N. Harvey2, J. K. Cleal1

1. Institute of Developmental Sciences, University of Southampton, Southampton, United Kingdom. 2. MRC Lifecourse Epidemiology Unit, University of Southampton, Southampton, United Kingdom.

Low vitamin D levels are common in pregnancy and are linked to suboptimal fetal growth (Harvey et al, 2008). This is important as poor fetal growth is associated with increased risk of chronic disease in adulthood (Barker, 1998). Supplementation with inactive 25-hydroxyvitamin D (25(OH)D) is often recommended. However, it is unclear whether 25(OH)D or the active 1,25-dihydroxyvitamin D (1,25(OH)2D) are both taken up into the placenta. In addition, serum vitamin D is bound to carrier proteins, vitamin D binding protein (DBP) and albumin (Chun et al, 2014), but their role in vitamin D uptake is uncertain. We aimed to establish whether both 25(OH)D and 1,25(OH)2D are taken up into the human placenta, and whether the presence of carrier proteins impacts uptake. Term human placentas were collected within 30 min of delivery with ethical approval and informed consent. To investigate placental uptake of vitamin D, placental villous fragments were cultured for 8 h in Tyrodes buffer containing 20 µM 25(OH)D (n=6), 25(OH)D + 0.7 mM albumin (n=6), 50 nM 1,25(OH)2D (n=11), 1,25(OH)2D + 0.7 mM albumin (n=11) or 1,25(OH)2D + 5 µM DBP (n=6). Controls were incubated with vehicle and albumin. Endocytic processes for 1,25(OH)2D uptake were investigated by adding 5 mM amiloride (n=5) or 80 µM dynasore (n=5). Samples were snap frozen and RNA extracted. mRNA expression of CYP24A1, a vitamin D responsive gene, was measured by quantitative real-time PCR. To investigate carrier protein uptake into the human placenta, placental villous fragments were incubated with 150 nM FITC-albumin (n=5) with or without endocytic blockers (n=3) for ≤1 h at 4 and 37°C. Samples were fixed, stained with lectins, and viewed on the confocal microscope. Data were analysed by one- and two-way ANOVA. CYP24A1 mRNA expression increased with 25(OH)D (p<0.001) compared to controls indicating 25(OH)D uptake and conversion to 1,25(OH)2D within the placenta. The magnitude of the effect was greater with 25(OH)D and albumin (p=0.01), suggesting it facilitates vitamin D uptake. Incubation with 1,25(OH)2D increased CYP24A1 mRNA expression (p<0.001) compared to controls, which did not increase further with albumin (p=0.16) or DBP (p=0.88). Uptake of FITC-albumin increased proportionately with time (p=0.08) at 37°C but not at 4°C (p=0.004). Dynasore did not alter CYP24A1 expression compared to 1,25(OH)2D with (p=0.9) or without albumin (p=1). Amiloride significantly reduced CYP24A1 mRNA expression compared to 1,25(OH)2D with (p<0.001) and without albumin (p=0.006) and also reduced FITC-albumin uptake (p=0.03). These data suggest that 25(OH)D and 1,25(OH)2D are taken up into the placenta and can induce vitamin D dependent gene expression, implying the placenta converts 25(OH)D to 1,25(OH)2D. Furthermore uptake of 25(OH)D may be enhanced by albumin, while amiloride inhibited both albumin uptake and 1,25(OH)2D stimulated CYP24A1 expression.

Where applicable, experiments conform with Society ethical requirements