Abstract: Osteoarthritis (OA) is the most common form of chronic joint disease, characterized by cartilage degeneration that results from complex changes in the chondrocyte phenotype. The presence of phosphate-containing microcrystals in the injured cartilage areas suggests the contribution of the phosphocalcic metabolism in the hypertrophic phenotype-like appearance of some chondrocytes during the disease. As Fibroblast Growth Factor 23 (FGF23) plays a major role in regulating concentrations of inorganic phosphate (Pi), a major inducer of mineralization, FGF23 is an attractive candidate to participate in the phenotype switch of the articular chondrocyte observed in OA. To address this hypothesis, we first compared the expression of FGF23, its receptors (FGFR) and coreceptor (Klotho) in cartilage samples obtained from healthy or OA individuals, then studied the consequences of in vitro FGF23 exposure on chondrocyte phenotype biomarkers and signaling pathways. Immunohistochemistry and gene expression studies were performed on human cartilage samples. Phenotypic biomarkers were studied by quantitative RT-PCR in human OA chondrocytes stimulated with up to 100 ng/ml of FGF23. Collagenase 3 activity was measured by a fluorescent assay. MAPK signaling was investigated by phosphoprotein array, immunoblotting and the use of selective inhibitors. RNA silencing was performed to confirm the respective contribution of FGFR1 and Klotho.In the present study, we showed that FGF23 and FGFR1 expression was 2-fold and 1.4-fold higher in OA chondrocytes than in normal ones, respectively. These overexpressions were confirmed by immunohistochemical analysis of OA cartilage specimens. In addition, Klotho mRNA level was higher in OA chondrocytes. cartilage. When stimulated with inducing concentrations of FGF23, human OA chondrocytes displayed a sustained expression of FGF23 and markers of hypertrophy such as COL10A1, VEGF and MMP13. Collagenase 3 activity was also increased. We demonstrated further, that FGF23 expression was both FGFR1- and Klotho-dependent whereas MMP13 expression was mainly dependent on FGFR1 alone. Finally, we showed that FGF23-induced MMP13 was strongly regulated by the MEK/ERK cascade and to a lesser extent by the PI-3K/AKT pathway. Altogether, the results obtained in this study demonstrate that FGF23 promotes differentiation of OA chondrocytes towards a hypertrophic phenotype and may therefore be considered as an aggravating factor for OA.