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952 FIBROBLAST GROWTH FACTOR 23 ACCELERATES PHOSPHATE-INDUCED CALCIFICATION OF VASCULAR SMOOTH MUSCLE CELLS

Background: Fibroblast growth factor 23 (FGF23) is a phosphate (P) -regulating hormone. Clinical studies suggest that increased level of FGF23 is associated with vascular calcification among chronic kidney disease patients but its direct effect on vasculature is unknown. Methods: Vascular smooth muscle cells (VSMCs) were extracted from Sprague-Dawley rats. The protein expressions of Klotho, co-receptor essential for FGF23 signalling, in VSMCs and the aorta were analyzed by western blot and immunohistochemistry. ERK 1/2 phosphorylation in normal VSMCs or Klotho-overexpressed VSMCs (Klotho-VSMCs) by recombinant hFGF23 was analyzed by western blot. Calcification of Klotho-VSMCs was induced by high-P medium in the presence or absence of FGF23. The calcium depositions were visualized by calcification staining and quantified by methylxylenol blue method. The mRNA expressions of osteoblastic transcription factors and sodium dependent P transporters were evaluated by real-time RT-PCR. Results: Klotho protein was detected in the medial layer of the aorta but not in the VSMCs. ERK 1/2 phosphorylation by FGF23 was increased in Klotho-VSMCs in a dose dependent manner and inhibited by FGFR1 inhibitor or MEK inhibitor U0126. FGF23 accelerated P-induced calcification of Klotho-VSMCs in a dose dependent manner. P induced Msx2 and osterix expressions were enhanced by FGF23 and U0126 inhibited the additive effect of FGF23 on the expression of these osteoblastic markers. The expressions of Pit1 and Pit 2 were not affected by either increased P level or FGF23. Conclusions: FGF23 enhances P-induced calcification in Klotho-VSMCs by promoting osteoblastic differentiation and its effect might be mediated by the ERK 1/2 pathway.