Laboratory for Calcium and Bone metabolism and Erasmus MC Bone Centre, Department of Internal Medicine, Erasmus University Medical Center, PO Box 2040, Rotterdam 3015 CN, The Netherlands.
Eur J Endocrinol. 2023 Oct 17;189(4):448-459. doi: 10.1093/ejendo/lvad131.
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.
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 的主要调节因子之一，1,25-二羟维生素 D（1,25(OH)2D3），主要由肾脏中的 1α-羟化酶（由 CYP27B1 编码）从 25-羟维生素 D（25(OH)D）合成。迄今为止，尚不清楚骨细胞是否可以将 25(OH)D 转化为 1,25(OH)2D3，从而允许 1,25(OH)2D3 诱导 FGF23 的局部产生和分泌。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 10-6M 25(OH)D 处理可将 25(OH)D 转化为 150pmol/L 的 1,25(OH)2D3，并增加 FGF23 的表达和分泌，但转化的 1,25(OH)2D3 量不足以引发 FGF23 反应，因此对 FGF23 的影响很可能直接由 25(OH)D 引起。有趣的是，将磷酸盐与 25(OH)D 结合可协同增加 FGF23 的表达和分泌，这可能是由于磷酸盐激活了其他信号通路。阻断维生素 D 受体（VDR）仅部分消除了 25(OH)D 或 25(OH)D 与磷酸盐对 Fgf23 的作用，而完全抑制细胞色素 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，这可能是由核受体维甲酸受体 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，表明 25(OH)D 对 FGF23 的调节具有直接作用，至少部分独立于其同源 VDR。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联合治疗的患者的后果。这些观察结果帮助我们进一步了解磷酸盐、维生素 D 和 FGF23 之间的复杂关系。
