Abstract
Cartilage calcification induces the synthesis of degrading enzymes, such as matrix metalloproteinases (MMPs) and prostaglandin E2 leading to tissue degeneration. The aim of the study was to investigate the effect of vitamin D on the calcification process in osteoarthritic cartilage.
We evaluated the effect of vitamin D on klotho (KL), Fibroblast Growth Factor 23 (FGF23) and Fibroblast Growth Factor Receptor 1c (FGFR1c) mRNA and protein expression levels by real-time PCR and western blot analysis, respectively. Possible interactions between klotho and FGF23 on the receptor FGFR1c in normal chondrocytes were investigated using immunoprecipitation assay. The direct effect of 1,25 dihydroxyvitamin D3 (1,25D) on KL, FGF23 and FGFR1c promoter was also evaluated.
We found that FGF23 and FGFR1c mRNA expression levels were significantly increased in osteoarthritic chondrocytes compared to normal, while KL mRNA levels were decreased (p=0.001 for all genes). We showed that klotho-FGF23-FGFR1c form complexes in normal chondrocytes and confirmed the participation of klotho in the initiation of FGF23-FGFR1c signalling. Treatment of normal chondrocytes with 1,25D resulted in a significant dose and time dependent increase of FGF23 and FGFR1c mRNA levels and in an increase of KL mRNA levels in osteoarthritic chondrocytes compared to untreated (p=0.001). We revealed, for the fist time, the presence of conserved, canonical VDREs in the proximal promoters of KL, FGF23 and FGFR1c.
We propose a common regulatory scheme of mineral homeostasis and aging in osteoarthritic chondrocytes evidenced by the positive/negative feedback actions by KL, FGF23, FGFR1c and 1,25D, through binding of vitamin D receptor (VDR) on the promoters of the above mentioned genes.
Correspondence should be addressed to Anastasia C. Tilentzoglou MD, General Secretary of the Board of Directors of HAOST, 20 A. Fleming Str. (N.Filothei), Gr. 15123 Maroussi, Athens Greece. E-mail: info@eexot.gr
