The Wnt/b-catenin signaling pathway participates in normal adult bone and cartilage biology and seems to be involved in cartilage degeneration and subsequent OA progression. The aim of this study was to investigate the activation of Wnt/b-catenin pathway in osteoarthritis and the role of LRP5, a coreceptor of Wnt/b-catenin pathway, in human osteoarhritic chondrocytes. Human cartilage was obtained from 11 patients with primary osteoarthritis (OA) undergoing total knee and hip replacement surgery. Normal cartilage was obtained from 5 healthy individuals. b-catenin and LRP5 mRNA and protein levels were investigated using real time PCR and western blot analysis, respectively. Blocking LRP5 expression was performed using small interfering (siRNA) against LRP5 and subsequent MMP-13 mRNA and protein levels were evaluated by real time RCR and western blot analysis, respectively. We confirmed the activation of Wnt/b-catenin pathway in osteoarthritis, as we observed significant upregulation of b-catenin mRNA and protein expression in osteoarthritic chondrocytes. We also observed that LRP5 mRNA and protein expression was significantly up-regulated in osteoarthritic cartilage compared to normal. Also, blocking LRP5 expression using siRNA against LRP5 resulted in a significant decrease in MMP-13 mRNA and protein expressions. Our findings suggest that the upregulation of LRP5 mRNA and protein expression in osteoarthritic chondrocytes results in an increased activation of Wnt/b-catenin pathway in osteoarthritis. The observed reduction of MMP-13 expression after blocking LRP5 expression in osteoarthritic chondrocytes, suggests the involvement of LRP5 in the progression and pathogenesis of osteoarthritis.
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.
