Iron deficiency, elevated erythropoietin, fibroblast growth factor 23, and mortality in the general population of the Netherlands: A cohort study

Background
Emerging data in chronic kidney disease (CKD) patients suggest that iron deficiency and higher circulating levels of erythropoietin (EPO) stimulate the expression and concomitant cleavage of the osteocyte-derived, phosphate-regulating hormone fibroblast growth factor 23 (FGF23), a risk factor for premature mortality. To date, clinical implications of iron deficiency and high EPO levels in the general population, and the potential downstream role of FGF23, are unclear. Therefore, we aimed to determine the associations between iron deficiency and higher EPO levels with mortality, and the potential mediating role of FGF23, in a cohort of community-dwelling subjects.
Methods and findings
We analyzed 6,544 community-dwelling subjects (age 53 ± 12 years; 50% males) who participated in the Prevention of Renal and Vascular End-Stage Disease (PREVEND) study—a prospective population-based cohort study, of which we used the second survey (2001–2003)—and follow-up was performed for a median of 8 years. We measured circulating parameters of iron status, EPO levels, and plasma total FGF23 levels. Our primary outcome was all-cause mortality. In multivariable linear regression analyses, ferritin (ß = –0.43), transferrin saturation (TSAT) (ß = −0.17), hepcidin (ß = −0.36), soluble transferrin receptor (sTfR; ß = 0.33), and EPO (ß = 0.28) were associated with FGF23 level, independent of potential confounders. During median (interquartile range [IQR]) follow-up of 8.2 (7.7–8.8) years, 379 (6%) subjects died. In multivariable Cox regression analyses, lower levels of TSAT (hazard ratio [HR] per 1 standard deviation [SD], 0.84; 95% confidence interval [CI], 0.75–0.95; P = 0.004) and higher levels of sTfR (HR, 1.15; 95% CI 1.03–1.28; P = 0.01), EPO (HR, 1.17; 95% CI 1.05–1.29; P = 0.004), and FGF23 (HR, 1.20; 95% CI 1.10–1.32; P < 0.001) were each significantly associated with an increased risk of death, independent of potential confounders. Adjustment for FGF23 levels markedly attenuated the associations of TSAT (HR, 0.89; 95% CI 0.78–1.01; P = 0.06), sTfR (HR, 1.08; 95% CI 0.96–1.20; P = 0.19), and EPO (HR, 1.10; 95% CI 0.99–1.22; P = 0.08) with mortality. FGF23 remained associated with mortality (HR, 1.15; 95% CI 1.04–1.27; P = 0.008) after adjustment for TSAT, sTfR, and EPO levels. Mediation analysis indicated that FGF23 explained 31% of the association between TSAT and mortality; similarly, FGF23 explained 32% of the association between sTfR and mortality and 48% of the association between EPO and mortality (indirect effect P < 0.05 for all analyses). The main limitations of this study were the observational study design and the absence of data on intact FGF23 (iFGF23), precluding us from discerning whether the current results are attributable to an increase in iFGF23 or in C-terminal FGF23 fragments.
Conclusions and relevance
In this study, we found that functional iron deficiency and higher EPO levels were each associated with an increased risk of death in the general population. Our findings suggest that FGF23 could be involved in the association between functional iron deficiency and increased EPO levels and death. Investigation of strategies aimed at correcting iron deficiency and reducing FGF23 levels is warranted.

### 研究背景
现有针对慢性肾脏病（chronic kidney disease, CKD）患者的新兴研究数据显示，铁缺乏与循环中较高的促红细胞生成素（erythropoietin, EPO）水平可刺激骨细胞源性、磷酸盐调节激素成纤维细胞生长因子23（fibroblast growth factor 23, FGF23）的表达及其伴随的裂解过程，而该激素是过早死亡的危险因素。迄今为止，普通人群中铁缺乏与高促红细胞生成素水平的临床意义，以及成纤维细胞生长因子23的潜在下游作用仍不明确。因此，本研究旨在针对社区居住人群队列，明确铁缺乏与高促红细胞生成素水平和死亡率之间的关联，以及成纤维细胞生长因子23的潜在中介作用。

### 研究方法与结果
本研究分析了参与「肾脏与终末期血管疾病预防（Prevention of Renal and Vascular End-Stage Disease, PREVEND）」研究的6544名社区居住受试者（年龄53±12岁，男性占50%）。该研究为基于人群的前瞻性队列研究，我们采用其第二次随访调查（2001–2003年）的数据，中位随访时间为8年。本研究检测了受试者的循环铁状态参数、促红细胞生成素水平以及血浆总成纤维细胞生长因子23水平，主要结局为全因死亡率。
在多变量线性回归分析中，铁蛋白（β=-0.43）、转铁蛋白饱和度（transferrin saturation, TSAT，β=-0.17）、铁调素（β=-0.36）、可溶性转铁蛋白受体（soluble transferrin receptor, sTfR，β=0.33）以及促红细胞生成素（β=0.28）均与成纤维细胞生长因子23水平显著相关，且不受潜在混杂因素影响。
中位随访时间为8.2（四分位距7.7~8.8）年，期间共有379名（6%）受试者死亡。在多变量Cox回归分析中，校正潜在混杂因素后，较低的转铁蛋白饱和度[每1个标准差的风险比（hazard ratio, HR）=0.84；95%置信区间（confidence interval, CI）0.75~0.95；P=0.004]、较高的可溶性转铁蛋白受体（HR=1.15；95%CI 1.03~1.28；P=0.01）、促红细胞生成素（HR=1.17；95%CI 1.05~1.29；P=0.004）以及成纤维细胞生长因子23（HR=1.20；95%CI 1.10~1.32；P<0.001）均与死亡风险升高显著相关。
校正成纤维细胞生长因子23水平后，转铁蛋白饱和度（HR=0.89；95%CI 0.78~1.01；P=0.06）、可溶性转铁蛋白受体（HR=1.08；95%CI 0.96~1.20；P=0.19）以及促红细胞生成素（HR=1.10；95%CI 0.99~1.22；P=0.08）与死亡率之间的关联均显著减弱。而校正转铁蛋白饱和度、可溶性转铁蛋白受体以及促红细胞生成素水平后，成纤维细胞生长因子23仍与死亡率显著相关（HR=1.15；95%CI 1.04~1.27；P=0.008）。
中介分析显示，成纤维细胞生长因子23可解释转铁蛋白饱和度与死亡率之间31%的关联；类似地，成纤维细胞生长因子23可解释可溶性转铁蛋白受体与死亡率之间32%的关联，以及促红细胞生成素与死亡率之间48%的关联（所有分析的间接效应P均<0.05）。
本研究的主要局限性为观察性研究设计，且缺乏完整成纤维细胞生长因子23（intact FGF23, iFGF23）的数据，因此无法明确当前研究结果是源于完整成纤维细胞生长因子23的升高还是C端成纤维细胞生长因子23片段的增加。

### 结论与意义
本研究发现，普通人群中的功能性铁缺乏与高促红细胞生成素水平均与死亡风险升高相关。研究结果提示，成纤维细胞生长因子23可能参与了功能性铁缺乏、促红细胞生成素水平升高与死亡之间的关联通路。未来有必要探索旨在纠正铁缺乏并降低成纤维细胞生长因子23水平的干预策略。