Regulation of FGF23 in Chronic Kidney Disease (CKD) by iron and inflammation

 描述（由申请人提供）：成纤维细胞生长因子23（FGF 23）由骨细胞分泌，调节磷酸盐和维生素D稳态。血清FGF 23水平在慢性肾脏病（CKD）病程早期升高，高水平与CKD进展、心血管疾病和死亡的风险增加独立相关。这些发现激发了人们对设计降低FGF 23水平的疗法的兴趣，但该方法受到对早期CKD开始刺激FGF 23产生的分子机制理解不足的限制。铁缺乏是FGF 23转录的新刺激物，并且可能是CKD中FGF 23水平升高的关键因素。虽然明显的缺铁在早期CKD中并不常见，但由于慢性炎症导致的网状内皮铁螯合导致的功能性缺铁非常普遍。在本申请中提供的初步数据中，我们证明了高水平的铁螯合蛋白、中性粒细胞明胶酶相关脂质运载蛋白（NGAL）降低骨细胞中的细胞内铁浓度并刺激FGF 23产生。在进一步的初步数据中，我们报告了NGAL介导的细胞铁减少稳定缺氧诱导因子（HIF）1a，其通过结合FGF 23启动子启动FGF 23转录。由于NGAL表达增加，以响应肾损伤和炎症，血清水平长期升高，慢性肾病，我们建议，肾损伤刺激NGAL生产，诱导铁流出骨细胞，创造细胞内铁缺乏，稳定HIF 1a和促进FGF 23的生产。在这项创新的建议中，我们将研究炎症，NGAL，以及真正的和功能性的铁缺乏对健康和CKD中FGF 23产生的调节作用。在目的1中，我们将通过将野生型和NGALko小鼠暴露于三种全身性炎症模型来定义炎症和NGAL对FGF 23产生的调节作用，并且我们将表明内源性和外源性NGAL增加FGF 23产生。在目标2中，我们将研究HIF 1a作为FGF 23调节的分子介质，其是NGAL，铁，炎症和FGF 23之间的机械串扰的基础。我们将进行CHIP测定和定点诱变以测试HIF 1a是否结合FGF 23启动子，并且我们将删除骨细胞中的HIF 1a以证明HIF 1aOc-cko小鼠对NGAL和铁缺乏诱导的FGF 23增加具有抗性。在目标3中，我们将通过研究NGAL和HIF 1a缺失对进行性CKD的Col 4a 3 ko小鼠模型的影响，确定NGAL和HIF 1a通路在CKD中的临床相关性和治疗潜力。我们将研究Col 4a 3 ko/NGALko和Col 4a 3 ko/HIF 1aOc-cko复合突变体相对于具有完整NGAL和HIF 1a系统的Col 4a 3 ko小鼠是否显示出FGF 23产生的钝性增加，并且我们还将在Col 4a 3 ko小鼠中测试NGAL阻断抗体和HIF 1抑制剂对FGF 23产生的影响。该项目将为FGF 23在健康和CKD中的分子调控提供新的见解，并支持我们开发新的治疗方法以改善CKD结局的最终目标。

 DESCRIPTION (provided by applicant): Fibroblast growth factor 23 (FGF23) is secreted by osteocytes and regulates phosphate and vitamin D homeostasis. Serum FGF23 levels rise early in the course of chronic kidney disease (CKD), and high levels are independently associated with greater risks of CKD progression, cardiovascular disease and death. These findings have stimulated interest in designing therapies to lower FGF23 levels, but the approach is limited by poor understanding of the molecular mechanisms that stimulate FGF23 production beginning in early CKD. Iron deficiency is a novel stimulator of FGF23 transcription and may be a critical contributor to elevated FGF23 levels in CKD. Although overt iron deficiency is uncommon in early CKD, functional iron deficiency due to reticuloendothelial iron sequestration as a result of chronic inflammation is highly prevalent. In preliminary data presented in this application, we demonstrate that high levels of the iron sequestering protein, neutrophil gelatinase associated lipocalin (NGAL), reduce intracellular iron concentrations in osteocytes and stimulate FGF23 production. In further preliminary data, we report that NGAL-mediated reductions in cellular iron stabilize hypoxia inducible factor (HIF)1a, which initiates FGF23 transcription by binding the FGF23 promoter. Since NGAL expression is increased in response to kidney injury and inflammation, and serum levels are chronically elevated in CKD, we propose that kidney injury stimulates NGAL production, which induces iron efflux from osteocytes, creating intracellular iron deficiency that stabilizes HIF1a and promotes FGF23 production. In this innovative proposal, we will examine the regulatory effects of inflammation, NGAL, and true and functional iron deficiency on FGF23 production in health and in CKD. In Aim 1, we will define the regulatory effects of inflammation and NGAL on FGF23 production by exposing wild type and NGALko mice to three models of systemic inflammation, and we will show that endogenous and exogenous NGAL increases FGF23 production. In Aim 2, we will investigate HIF1a as a molecular mediator of FGF23 regulation that underlies the mechanistic crosstalk between NGAL, iron, inflammation and FGF23. We will perform CHIP assays and site-directed mutagenesis to test whether HIF1a binds the FGF23 promoter, and we will delete HIF1a in osteocytes to demonstrate that HIF1aOc-cko mice are resistant to NGAL and iron deficiency-induced increases in FGF23. In Aim 3, we will determine the clinical relevance and therapeutic potential of the NGAL and HIF1a pathways in CKD by studying the impact of NGAL and HIF1a deletion in the Col4a3ko mouse model of progressive CKD. We will investigate if Col4a3ko/NGALko and Col4a3ko/HIF1aOc-cko compound mutants display blunted increases in FGF23 production relative to Col4a3ko mice with intact NGAL and HIF1a systems, and we will also test the effects on FGF23 production of NGAL blocking antibodies and HIF1 inhibitors in Col4a3ko mice. The project will contribute new insights into the molecular regulation of FGF23 in health and in CKD, and support our ultimate goal of developing novel therapeutic approaches to improve outcomes in CKD.