A novel role of 1,25(OH)2 vitamin D3 in the regulation of intestinal Npt2b and renal Npt2c in hypophosphatemia
Authors: Thomas Linto , Xue Jianxiang , Rieg Jessica A. Dominguez , Rieg Timo 
Published: May 2023

1,25(OH)2 vitamin D3 (Vit-D3) is a hormone that is formed by the coordinated actions of the skin and kidneys and is involved in the regulation of Ca2+ and phosphate (Pi) homeostasis. Of note, the regulation of these minerals is complex and involves other hormones including parathyroid hormone and fibroblast growth factor 23. Of major importance is the renal Na+/phosphate co-transporter (Npt2a) for long-term Pi homeostasis and Npt2a knockout (Npt2a-/-) mice are hypophosphatemic. However, the impact of Vit-D3 on Pi and Ca2+ in the absence of Npt2a has never been assessed. We hypothesized that exogenous application of Vit-D3 might modulate impaired Pi homeostasis by regulating their Pi transporters in the absence of Npt2a. Here we compared the effects of subcutaneous administration of Vit-D3 (300,000 IU/kg for 4 days, q.d.) or vehicle (5% Ethanol,5% Cremophore EL, and 90% water) in male wild-type (WT) and Npt2a-/- mice (n=6/genotype and treatment). Blood samples were collected before (baseline) and after Vit-D3 treatment. In WT mice, Vit-D3 or vehicle treatment did not change plasma Pi levels compared to baseline. In contrast, Vit-D3 significantly increased (~1.7-fold) plasma Pi levels in Npt2a-/- mice compared to baseline (1.5±0.09 vs. 0.9±0.04 mmol/L, P<0.05). Vehicle treatment did not significantly change plasma Ca2+ levels in either genotype. In WT, Vit-D3 significantly increased plasma Ca2+ levels compared to baseline (4.8±0.1 vs 2.4±0.03 mmol/L, P<0.05). Similarly, in Npt2a-/- mice, Vit-D3 significantly increased plasma Ca2+ levels compared to baseline (5.5±0.14 vs 3.0±0.06 mmol/L, P<0.05). Ca2+ and Pi contents in the bone were unchanged in WT or Npt2a-/- mice after Vit-D3 treatment compared to vehicle. Npt2b protein expression levels in the proximal small intestine were not different in WT and Npt2a-/- mice upon Vit-D3 treatment. Similarly, Vit-D3 did not change the expression of Npt2b in the distal small intestine of WT mice. In contrast to WT mice, a significantly lower (~67%, P<0.05) Npt2b protein expression was observed in the distal small intestine of Npt2a-/- mice in response to Vit-D3 treatment. Renal Npt2a expression in WT mice was significantly lower (~65%, P<0.05) in response to Vit-D3 treatment compared to vehicle. Consistent with compensatory upregulation, renal Npt2c expression was significantly greater (~240%, P<0.05) in Npt2a-/- compared to WT mice. Upon Vit-D3 treatment, lower (~40%, P<0.05) Npt2c expression was observed in WT mice compared to vehicle, whereas expression of Npt2c was almost completely abolished (~98%, P<0.05) in Npt2a-/- mice. In summary, our study demonstrates that Vit-D3 has a distinct role on Npt2a, b and c regulation; some of them depend on the presence of functional Npt2a. This might indicate a so far unknown regulator affecting the hormonal feedback loops. Funding to Dr. Rieg was provided by the NIDDK grant 1R01DK110621, VA Merit Review Award IBX004968A, AHA Transformational Research Award 19TPA34850116, and NIDDK Diabetic Complications Consortium grants DK076169 and DK115255. Dr. Thomas was supported by an AHA postdoctoral grant (828731). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.