Extracellular sodium regulates fibroblast growth factor 23 (FGF23) formation

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone that has recently received much attention due to its association with the progression of chronic kidney disease, cardiovascular disease, and associated mortality. Extracellular sodium ion concentration ([Na+]) plays a significant role in bone metabolism. Both hyponatremia (low serum [Na+]) and hypernatremia (high serum [Na+]) have been shown to affect bone remodeling. However, nothing is known about the impact of [Na+] on FGF23 production. Here, we show that elevated [Na+] (by +20 mM) suppressed FGF23 formation, whereas low [Na+] (by -20 mM) led to an increase in FGF23 synthesis in the osteoblast-like cell line UMR-106. Similar bidirectional changes in FGF23 were observed when osmolality was altered by mannitol but not by urea, suggesting a role of tonicity in FGF23 formation. Moreover, these changes in FGF23 were inversely proportional to the expression of NFAT5 (nuclear factor of activated T cells-5), a transcription factor responsible for tonicity-mediated cellular adaptations. On the other hand, arginine vasopressin (AVP), which is often responsible for hyponatremia, did not affect FGF23 production. Next, comprehensive and unbiased RNA-seq analysis of UMR-106 cells exposed to low vs. high [Na+] revealed several novel genes involved in cellular adaptation to altered tonicity. Additional analysis of cells with Crisp-Cas9 mediated NFAT5 deletion indicated that NFAT5 controls numerous genes associated with FGF23 synthesis, thereby confirming its role in [Na+]-mediated FGF23 regulation. In line with these findings, in a pilot study, we found that human hyponatremic patients have higher FGF23 levels. Our results suggest that [Na+] is a critical regulator of FGF23 synthesis.成纤维细胞生长因子23（Fibroblast growth factor-23, FGF23）是一种骨源性激素，近年来因其与慢性肾脏病、心血管疾病进展及相关死亡的关联而受到广泛关注。细胞外钠离子浓度（[Na+]）在骨代谢中发挥重要调控作用。低钠血症（血清[Na+]水平降低）与高钠血症（血清[Na+]水平升高）均被证实可影响骨重塑过程。然而，目前尚不清楚[Na+]对FGF23生成的具体影响。本研究发现，升高[Na+]（增幅20 mM）会抑制成骨细胞样细胞系UMR-106中的FGF23形成，而降低[Na+]（降幅20 mM）则可促进该细胞系中FGF23的合成。当以甘露醇而非尿素改变细胞渗透压时，同样观察到FGF23的此类双向变化，提示细胞张力在FGF23形成过程中发挥调控作用。此外，FGF23的上述变化与活化T细胞核因子5（nuclear factor of activated T cells-5, NFAT5）的表达呈负相关，NFAT5是介导张力适应性细胞反应的关键转录因子。另一方面，常与低钠血症相关的精氨酸血管加压素（arginine vasopressin, AVP）并不会对FGF23的生成产生影响。随后，本研究对暴露于高、低[Na+]环境的UMR-106细胞开展了全面且无偏倚的RNA测序（RNA-seq）分析，鉴定出若干参与细胞应对张力改变的新型功能基因。通过CRISPR-Cas9介导的NFAT5敲除细胞进行的额外分析表明，NFAT5可调控诸多与FGF23合成相关的基因，从而证实了其在[Na+]介导的FGF23调控通路中的核心作用。与上述实验发现一致的是，在一项预试验中，我们观察到人类低钠血症患者的血清FGF23水平更高。本研究结果表明，细胞外钠离子浓度[Na+]是FGF23合成的关键调控因子。