MJ. FGF23 directly impairs endothelium-dependent vasorelaxation by increasing superoxide levels and reducing nitric oxide bioavailability. Am J Physiol Endocrinol Metab 307: E426 -E436, 2014. First published July 15, 2014 doi:10.1152/ajpendo.00264.2014.-Fibroblast growth factor 23 (FGF23) is secreted primarily by osteocytes and regulates phosphate and vitamin D metabolism. Elevated levels of FGF23 are clinically associated with endothelial dysfunction and arterial stiffness in chronic kidney disease (CKD) patients; however, the direct effects of FGF23 on endothelial function are unknown. We hypothesized that FGF23 directly impairs endothelial vasorelaxation by hindering nitric oxide (NO) bioavailability. We detected expression of all four subtypes of FGF receptors (Fgfr1-4) in male mouse aortas. Exogenous FGF23 (90 -9,000 pg/ml) did not induce contraction of aortic rings and did not relax rings precontracted with PGF 2␣. However, preincubation with FGF23 (9,000 pg/ml) caused a ϳ36% inhibition of endothelium-dependent relaxation elicited by acetylcholine (ACh) in precontracted aortic rings, which was prevented by the FGFR antagonist PD166866 (50 nM). Furthermore, in FGF23-pretreated (9,000 pg/ml) aortic rings, we found reductions in NO levels. We also investigated an animal model of CKD (Col4a3 Ϫ/Ϫ mice) that displays highly elevated serum FGF23 levels and found they had impaired endothelium-dependent vascular relaxation and reduced nitrate production compared with age-matched wild types. To elucidate a mechanism for the FGF23-induced impairment, we measured superoxide levels in endothelial cells and aortic rings and found that they were increased following FGF23 treatment. Crucially, treatment with the superoxide scavenger tiron reduced superoxide levels and also restored aortic relaxation to ACh. Therefore, our data suggest that FGF23 increases superoxide, inhibits NO bioavailability, and causes endothelial dysfunction in mouse aorta. Together, these data provide evidence that high levels of FGF23 contribute to cardiovascular dysfunction. fibroblast growth factor 23; chronic kidney disease; nitric oxide; superoxide; and cardiovascular disease IT IS WELL KNOWN THAT PATIENTS with chronic kidney disease (CKD) have an increased risk of cardiovascular disease (CVD). Modification of the traditional risk factors for CVD (e.g., dyslipidemia, hypertension, anemia, and hyperhomocysteinemia) does not improve cardiovascular function in patients with CKD (32), suggesting that other factors may be responsible. Fibroblast growth factor 23 (FGF23) is a hormone secreted by osteocytes that serves as an important regulator of serum phosphate and vitamin D via direct actions on the kidney and parathyroid (6,8). Recently, high circulating levels of FGF23 have been clinically associated with the development of CVD (3,9,33,47,55,72) especially during CKD where serum FGF23 is substantially increased 10-to 1,000-fold (30, 37). Nevertheless, despite these clinical associations, there have been relatively few studies to determine whethe...