High sodium reduced the expression of PTH1R and Klotho by inhibiting 1,25(OH)(2)D(3) synthesis in cultured proximal tubule epithelial cells

BACKGROUND: The proximal tubule is the sensing site of sodium and phosphate and the main place for the synthesis and metabolism of 1,25(OH)(2)D(3). We aimed to investigate the effects of high sodium on the synthesis and function of active vitamin D and local phosphate regulation in proximal tubular epithelial cells. METHODS: Human proximal tubule epithelial (HK-2) cells were treated with different concentrations of sodium/phosphate. The expression of 1α-OHase and 24-OHase was determined. Liquid chromatography/mass spectrometry (LC/MS) and enzyme-linked immunosorbent assay (ELISA) were used to detect the levels of 1,25(OH)(2)D(3.) RNA sequencing and bioinformatics analysis was used to probe into the possible pathways. Chromatin samples were immunoprecipitated with antibodies against parathyroid receptor 1 (PTH1R) and Klotho. RESULTS: We found that high sodium decreased the expression of 1,25(OH)(2)D(3) by reducing 1α-OHase and 24-OHase, reduced the expression of PTH1R and Klotho, and increased the intracellular calcium concentration. These effects were reversed by sodium phosphate transporter inhibitor, sodium hydrogen transporter inhibitor, and a chelator of the extracellular calcium, whereas enhanced by ouabain. Vitamin D receptor (VDR) agonists significantly increased the recruitment of VDR to the vitamin D response element (VDRE) of PTH1R and Klotho promoter, thus increasing the expression of PTH1R and Klotho. CONCLUSIONS: High sodium can decrease the synthesis of active vitamin D in the proximal tubules, affect the gene regulation of 1,25(OH)(2)D(3)/VDR, and significantly reduce the expression of PTH1R and Klotho. It revealed the influence of a high-sodium diet on mineral metabolism and the core role of vitamin D in kidney mineral metabolism.