Role of FGF-23 in Regulation of Phosphate Homeostasis

描述(由申请人提供)： 肿瘤性骨软化(OOM)、X连锁低磷血症(XLH)和常染色体显性遗传性低磷血症(ADHR)是以低磷血症、肾脏磷酸盐重吸收降低、骨化三醇合成缺陷和骨骼矿化缺陷为特征的表型相似的疾病。OOM是由间充质肿瘤引起的，这种肿瘤产生了一种磷酸因子。XLH是由PHEX基因突变引起的，该基因编码一种内肽酶。ADHR与编码成纤维细胞生长因子-23的基因突变有关。在我的K08奖开始的工作中，我比较了来自OOM患者的间叶性肿瘤的基因表达谱(SAGE)，发现成纤维细胞生长因子-23高表达。此外，我还证明了成纤维细胞生长因子-23抑制了近端肾小管上皮细胞的磷酸盐转运。我进一步证明了成纤维细胞生长因子-23是PHEX底物，而且成纤维细胞生长因子-23 R179Q(ADHR突变)不被PHEX切割。综上所述，这些发现支持了这样的假设，即在正常的生理状态下，成纤维细胞生长因子-23是通过依赖于苯丙氨酸的蛋白分解来调节的。然而，当PHEX处于非活性状态时，FGF-23抵抗切割或异位产生，FGF-23积聚，从而导致磷酸盐尿。本研究的目的是了解成纤维细胞生长因子-23在正常磷酸盐稳态中的作用，并阐明成纤维细胞生长因子-23与磷酸盐重吸收减少之间的分子联系。首先，我建议定义人体内成纤维细胞生长因子-23的正常日变化，并检测磷酸盐稳态紊乱的受试者的血清成纤维细胞生长因子-23。其次，我将确定暴露于成纤维细胞生长因子-23是否改变了近端肾小管上皮细胞系2a型钠-磷酸转运体(NPT-2)的转录、蛋白表达或内吞。在小鼠中，我将探讨注射成纤维细胞生长因子-23对NPT-2mRNA和蛋白表达及肾刷状缘膜磷酸盐转运的影响。阐明成纤维细胞生长因子-23导致肾脏磷酸盐消耗的分子机制将有助于我们理解成纤维细胞生长因子-23在磷酸盐稳态中的作用。

DESCRIPTION (provided by applicant): Oncogenic osteomalacia (OOM), X linked hypophosphatemic rickets (XLH) and autosomal dominant hypophosphatemic rickets (ADHR) are phenotypically similar disorders characterized by hypophosphatemia, decreased renal phosphate reabsorption, defective calcitriol synthesis, and defective skeletal mineralization. OOM is caused by mesenchymal tumors that elaborate a phosphaturic factor. XLH results from mutations in the PHEX gene, that encodes an endopeptidase. ADHR is associated with mutations of the gene encoding FGF-23. In work initiated in my K08 award, I compared gene expression profiles (SAGE) of mesenchymal tumors derived from patients with OOM and found that FGF-23 was highly-expressed. Moreover, I demonstrated that FGF-23 inhibited phosphate transport in a model of proximal renal tubular epithelia. I further demonstrated that FGF-23 is a PHEX substrate and that FGF-23 R179Q (ADHR mutation), was not cleaved by PHEX. Taken together, these findings support the hypothesis that in normal physiologic states, FGF-23 is regulated via PHEX-dependent proteolysis. However, when PHEX is inactive, FGF-23 is resistant to cleavage or FGF-23 is ectopically produced, FGF-23 accumulates and thus leads to phosphaturia. The objective of the proposed studies is to understand the role of FGF-23 in normal phosphate homeostasis and to elucidate the molecular link between FGF-23 and reduced phosphate reabsorption. First, I propose to define the normal diurnal variation of FGF-23 in humans and to measure serum FGF-23 in subjects with disorders of phosphate homeostasis. Second, I will determine if exposure to FGF-23 alters the type 2a sodium-phosphate transporter (Npt-2) transcription, protein expression or endocytic retreival from the membrane in proximal renal tubular cell lines. In mice, I will explore the effect of FGF-23 infusion on Npt-2 mRNA and protein expression and renal brush border membrane phosphate transport. Elucidating the molecular mechanism by which FGF-23 causes renal phosphate wasting will contribute substantially to our understanding of the role of FGF-23 in phosphate homeostasis.