Abstract
Purpose
To identify genes showing altered expression in osteoarthritic (OA) cartilage and synovium. Dkk3, a member of the Dickoppf family of Wnt signalling inhibitors was overexpressed and this work highlights the potential function of Dkk3 in OA.
Methods
Real-time PCR was used to compare the expression of 270 cytokines, chemokines and their receptors in cartilage and synovium from OA and non-OA patients. Expression of Dkk3 was also measured in ATDC5 cells and in bovine nasal cartilage (BNC) explants treated with inflammatory cytokines. The effect of Dkk3 on hydroxyproline and GAG release was measured in BNC explant cultures.
To assess the distribution of Dkk3 in OA cartilage immunohistochemistry was carried out on anteromedial gonarthrosis specimens. The level of Dkk3 in synovial fluid tricompartmental and unicompartmental cartilage lesions was measured using ELISA.
Results
Dkk3 expression was increased 10- in OA cartilage (p=0.00011) and 3.5-fold increase in OA synovium (p=0.007) when compared to respective control tissues. Dkk3 expression was shown to decrease during chondrogenic differentiation of ATDC5 cells and to be increased by interleukin 1 and oncostatin-M in BNC explants. Dkk3 inhibited the release of hydroxyproline and proteoglycan from BNC explants co-treated with interleukin-1 and oncostatin-M.
Immunohistochemistry of anteromedial gonarthrosis specimens demonstrated increased Dkk3 in superficial zone chondrocytes in damaged compared to undamaged cartilage from within the same knee. Increased Dkk3 protein was found in the synovial fluid of individuals with tricompartmental OA (n=4) versus unicompartmental cartilage lesions (n=10) (182ng/ml v 116 ng/ml, p<0.01, a single non-OA control synovial fluid measured 43ng/ml.
Conclusions
Dkk3 is a molecule with poorly ascribed function, especially within the musculoskeletal system. In contrast to other members of the Dkk family, Dkk3 does not act consistently as a Wnt inhibitor. Literature on a number of tumour-derived cells have shown that Dkk3 can regulate Wnt, TGFβ, BMP, FGF and Activin signaling and cell proliferation and apoptosis. These cellular processes are highly relevant to OA. In this preliminary study we have shown that Dkk3 is overexpressed in OA cartilage and synovial tissues
The decreased expression of Dkk3 during chondrogenesis, and its increase in inflammatory cytokine stimulated BNC explants is suggestive of a role of Dkk3 not only in articular cartilage maintenance but also in development. The ability of Dkk3 to regulate collagen and proteoglycan breakdown (hallmarks of OA) is further evidence for a role in OA pathogenesis. Dkk3 is a compelling molecule that shows potential to further our understanding of OA and to be used as a biomarker of disease or as a target in OA therapeutics.
