Fibroblast growth factor 23 (FGF23) is produced and secreted by osteocytes and is essential for maintaining phosphate homeostasis. One of the main regulators of FGF23, 1,25-dihydroxyvitamin D (1,25(OH)2D3), is primarily synthesized in the kidney from 25-hydroxyvitamin D (25(OH)D) by 1α-hydroxylase (encoded by CYP27B1). Hitherto, it is unclear whether osteocytes can convert 25(OH)D and thereby allow for 1,25(OH)2D3 to induce FGF23 production and secretion locally. Here, we differentiated MC3T3-E1 cells toward osteocyte-like cells expressing and secreting FGF23. Treatment with 10-6 M 25(OH)D resulted in conversion of 25(OH)D to 150 pmol/L 1,25(OH)2D3 and increased FGF23 expression and secretion, but the converted amount of 1,25(OH)2D3 was insufficient to trigger an FGF23 response, so the effect on FGF23 was most likely directly caused by 25(OH)D. Interestingly, combining phosphate with 25(OH)D resulted in a synergistic increase in FGF23 expression and secretion, likely due to activation of additional signaling pathways by phosphate. Blockage of the vitamin D receptor (VDR) only partially abolished the effects of 25(OH)D or 25(OH)D combined with phosphate on Fgf23, while completely inhibiting the upregulation of cytochrome P450 family 24 subfamily A member 1 (Cyp24a1), encoding for 24-hydroxylase. RNA sequencing and in silico analyses showed that this could potentially be mediated by the nuclear receptors Retinoic Acid Receptor β (RARB) and Estrogen Receptor 2 (ESR2). Taken together, we demonstrate that osteocytes are able to convert 25(OH)D to 1,25(OH)2D3, but this is insufficient for FGF23 activation, implicating a direct effect of 25(OH)D in the regulation of FGF23, which occurs at least partially independent from its cognate VDR. Moreover, phosphate and 25(OH)D synergistically increase expression and secretion of FGF23, which warrants investigating consequences in patients receiving a combination of vitamin D analogues and phosphate supplements. These observations help us to further understand the complex relations between phosphate, vitamin D, and FGF23.

中文翻译：

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骨细胞样 MC3T3-E1 细胞合成的 25-羟基维生素 D 和 1,25-二羟基维生素 D 对成纤维细胞生长因子 23 的体外调节。

成纤维细胞生长因子 23 (FGF23) 由骨细胞产生和分泌，对于维持磷酸盐稳态至关重要。FGF23 的主要调节因子之一是 1,25-二羟基维生素 D (1,25(OH)2D3)，主要在肾脏中通过 1α-羟化酶（由 CYP27B1 编码）从 25-羟基维生素 D (25(OH)D) 合成）。迄今为止，尚不清楚骨细胞是否可以转化25(OH)D，从而允许1,25(OH)2D3局部诱导FGF23产生和分泌。在这里，我们将 MC3T3-E1 细胞分化为表达和分泌 FGF23 的骨细胞样细胞。用 10-6 M 25(OH)D 处理导致 25(OH)D 转化为 150 pmol/L 1,25(OH)2D3，并增加 FGF23 表达和分泌，但 1,25(OH) 的转化量2D3 不足以触发 FGF23 反应，因此对 FGF23 的影响很可能是由 25(OH)D 直接引起的。有趣的是，磷酸盐与 25(OH)D 结合导致 FGF23 表达和分泌协同增加，这可能是由于磷酸盐激活了其他信号通路。维生素D受体（VDR）的阻断仅部分消除了25（OH）D或25（OH）D与磷酸盐组合对Fgf23的影响，同时完全抑制了细胞色素P450家族24亚家族A成员1（Cyp24a1）的上调，编码24-羟化酶。RNA 测序和计算机分析表明，这可能是由核受体视黄酸受体 β (RARB) 和雌激素受体 2 (ESR2) 介导的。综上所述，我们证明骨细胞能够将 25(OH)D 转化为 1,25(OH)2D3，但这不足以激活 FGF23，这表明 25(OH)D 在 FGF23 的调节中具有直接作用，这至少部分独立于其同源 VDR 发生。此外，磷酸盐和 25(OH)D 协同增加 FGF23 的表达和分泌，这值得研究接受维生素 D 类似物和磷酸盐补充剂组合的患者的后果。这些观察结果有助于我们进一步了解磷酸盐、维生素 D 和 FGF23 之间的复杂关系。