Role of FGF-23 in Regulation of Phosphate Homeostasis

描述（由申请人提供）： 瘤源性骨软化症（OOM）、X连锁低磷血症性佝偻病（XLH）和常染色体显性低磷血症性佝偻病（ADHR）是表型相似的疾病，其特征为低磷血症、肾磷酸盐重吸收减少、骨化三醇合成缺陷和骨骼矿化缺陷。OOM是由产生磷酸尿因子的间叶肿瘤引起的。XLH由编码内肽酶的PHEX基因突变引起。ADHR与编码FGF-23的基因突变有关。在我的K 08奖开始的工作中，我比较了来自OOM患者的间充质肿瘤的基因表达谱（SAGE），发现FGF-23高表达。此外，我证明了FGF-23在近端肾小管上皮细胞模型中抑制磷酸盐转运。我进一步证明了FGF-23是PHEX底物，并且FGF-23 R179 Q（ADHR突变）不被PHEX切割。总之，这些发现支持了这样的假设，即在正常生理状态下，FGF-23通过PHEX依赖性蛋白水解调节。然而，当PHEX失活时，FGF-23对切割有抗性或FGF-23异位产生，FGF-23积累并因此导致磷酸尿症。这些研究的目的是了解FGF-23在正常磷酸盐稳态中的作用，并阐明FGF-23与磷酸盐重吸收减少之间的分子联系。首先，我建议定义正常昼夜变化的FGF-23在人类和测量血清FGF-23与磷酸盐稳态障碍的主题。其次，我将确定是否暴露于FGF-23改变2a型钠磷酸转运蛋白（Npt-2）的转录，蛋白质表达或内吞retreival从近端肾小管细胞系的膜。在小鼠中，我将探索FGF-23输注对Npt-2 mRNA和蛋白表达以及肾刷状缘膜磷酸盐转运的影响。阐明FGF-23导致肾磷酸盐消耗的分子机制将大大有助于我们理解FGF-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.