Osteocyte produced fibroblast growth factor 23 (FGF23) is the key regulator of serum phosphate (Pi) homeostasis. The interplay between parathyroid hormone (PTH), FGF23 and other proteins that regulate FGF23 production and serum Pi levels is complex and incompletely characterised. Evidence suggests that the protein product of the SOST gene, sclerostin (SCL), also a PTH target and also produced by osteocytes, plays a role in FGF23 expression, however the mechanism for this effect is unclear. Part of the problem of understanding the interplay of these mediators is the complex multi-organ system that achieves Pi homeostasis in vivo. In the current study, we sought to address this using a cell line model of the osteocyte, IDG-SW3, known to express FGF23 at both the mRNA and protein levels. In cultures of differentiated IDG-SW3 cells, both PTH_1-34 and recombinant human (rh) SCL remarkably induced Fgf23 mRNA expression dose-dependently within 3 h. Both rhPTH_1-34 and rhSCL also strongly induced C-terminal FGF23 protein secretion. Secreted intact FGF23 levels remained unchanged, consistent with constitutive post-translational cleavage of FGF23 in this cell model. Both rhPTH_1-34 and rhSCL treatments significantly suppressed mRNA levels of Phex, Dmp1 and Enpp1 mRNA, encoding putative negative regulators of FGF23 levels, and induced Galnt3 mRNA expression, encoding N-acetylgalactosaminyl-transferase 3 (GalNAc-T3), which protects FGF23 from furin-like proprotein convertase-mediated cleavage. The effect of both rhPTH_1-34 and rhSCL was antagonised by pre-treatment with the NF-κβ signalling inhibitors, BAY11 and TPCK. RhSCL also stimulated FGF23 mRNA expression in ex vivo cultures of human bone. These findings provide evidence for the direct regulation of FGF23 expression by sclerostin. Locally expressed sclerostin via the induction of FGF23 in osteocytes thus has the potential to contribute to the regulation of Pi homeostasis.

骨细胞产生的成纤维细胞生长因子 23 (FGF23) 是血清磷酸盐 (Pi) 稳态的关键调节因子。甲状旁腺激素 (PTH)、FGF23 和其他调节 FGF23 产生和血清 Pi 水平的蛋白质之间的相互作用是复杂且不完全表征的。有证据表明，SOST的蛋白质产物基因 sclerostin (SCL) 也是 PTH 靶点，也由骨细胞产生，在 FGF23 表达中起作用，但这种作用的机制尚不清楚。理解这些介质相互作用的部分问题是在体内实现 Pi 稳态的复杂多器官系统。在目前的研究中，我们试图使用骨细胞 IDG-SW3 的细胞系模型来解决这个问题，该模型已知在 mRNA 和蛋白质水平上都表达 FGF23。在分化的 IDG-SW3 细胞培养物中，PTH _1-34和重组人 (rh) SCL 在 3 小时内显着诱导Fgf23 mRNA 表达，且呈剂量依赖性。rhPTH _1-34和 rhSCL 也强烈诱导 C 末端 FGF23 蛋白分泌。分泌的完整 FGF23 水平保持不变，与该细胞模型中 FGF23 的组成型翻译后切割一致。rhPTH _1-34和 rhSCL 处理均显着抑制Phex、Dmp1和Enpp1 mRNA 的 mRNA 水平，编码 FGF23 水平的推定负调节因子，并诱导Galnt3 mRNA 表达，编码 N-乙酰半乳糖胺基转移酶 3 (GalNAc-T3)，保护 FGF23来自弗林蛋白酶样前蛋白转化酶介导的切割。用 NF-κβ 信号通路抑制剂 BAY11 和 TPCK 预处理可拮抗 rhPTH _1-34和 rhSCL 的作用。RhSCL 也刺激人骨离体培养物中的FGF23 mRNA 表达。这些发现为硬化蛋白直接调节 FGF23 表达提供了证据。因此，通过在骨细胞中诱导 FGF23 局部表达的硬化素有可能有助于调节 Pi 稳态。