Abstract 13830: Soluble Klotho Suppresses Human Aortic Valve Interstitial Cell Fibrocalcification Induced by Fibroblast Growth Factor 23

Abstract

Aortic valve interstitial cells (AVIC) play a major role in the pathogenesis of calcific aortic valve disease (CAVD). Higher fibroblast growth factor 23 (FGF23) levels are associated with chronic kidney disease (CKD), an important risk factor of CAVD. The role of FGF23 in AVIC pathobiology associated with CAVD remains unclear. Klotho (KL) is an anti-aging protein present in membrane (mKL) and soluble (sKL) forms. While mKL is a coreceptor for FGF23, sKL may function as a FGF23 antagonist. We have found that KL inhibits osteogenic activity in AVIC exposed to CKD-associated high phosphate condition. We hypothesized that FGF23 mediates AVIC fibrocalcification, and sKL inhibits the effect of FGF23. Methods and Results: Protein levels of FGF23 and KL were analyzed in AVIC and aortic valves from CAVD patients. Diseased aortic valves and AVIC had higher FGF23 and lower KL levels than normal valves and AVIC, and diseased AVIC secreted greater amount of FGF23. Knockdown or neutralization of FGF23 in diseased AVIC suppressed their spontaneous fibrocalcification. Treatment of normal AVIC with recombinant FGF23 (40 ng/ml) for 72h enhanced the expression of collagens I and IV, Runx2, and alkaline phosphatase. Prolonged treatment of normal AVIC with FGF23 resulted in collagen and calcium deposition. Elevated levels of FGF receptor (FGFR) 1 and 4 were detected in normal AVIC after FGF23 stimulation. Inhibition of FGFR1 attenuated FGF23-induced fibrogenic response, while inhibition of FGFR4 abrogated FGF23-induced osteogenic response. Recombinant KL (0.5 μg/ml) reduced FGFR expression and fibrocalcification induced by FGF23. Further, KL attenuated FGF23-induced collagen and calcium deposition in normal AVIC. In vitro incubation of KL and FGF23 led to the formation of a complex. Conclusion: Diseased aortic valves and AVIC have higher levels of FGF23 and lower levels of KL. FGF23 is capable of inducing fibrocalcification in normal AVIC via FGFR1 and FGFR4. sKL suppresses FGF23 activities by inhibiting FGFR upregulation and direct interaction with FGF23. Our novel findings suggest that FGF23 may accelerate CAVD progression. sKL suppresses the pro-fibrocalcific effect of FGF23 and may offer therapeutic potential for slowing CAVD progression, especially in CKD patients.