Abstract
OBJECTIVE: Wnt signaling plays a pivotal role in skeletal development and in the control of cartilage and bone turnover. We have recently shown that the secreted Wnt antagonist Wnt inhibitory factor 1 (WIF-1) is mainly expressed in the upper layers of epiphyseal and articular cartilage and, to a lesser extent, in bone. Nevertheless, WIF-1(-/-) mice develop normally. In light of these findings, we undertook this study to analyze the role of WIF-1 in arthritis. METHODS: Expression analyses for WIF-1 were performed by real-time reverse transcription-polymerase chain reaction (RT-PCR). WIF-1(-/-) and tumor necrosis factor (TNF)-transgenic mice were crossbred, and the progression of arthritis in TNF-transgenic WIF-1(-/-) mice and littermate controls was evaluated. Structural joint damage was analyzed by histologic staining, histomorphometry, and micro-computed tomography. Wnt/beta-catenin signaling was investigated by real-time RT-PCR and immunofluorescence on primary chondrocytes. RESULTS: WIF-1 expression was repressed by TNFalpha in chondrocytes and osteoblasts and down-regulated in experimental arthritis and in articular cartilage from patients with rheumatoid arthritis. WIF-1 deficiency partially protected TNF-transgenic mice against bone erosion and loss of trabecular bone, probably as a result of less osteoclast activity. In contrast, arthritis-related cartilage damage was aggravated by WIF-1 deficiency, while overexpression of WIF-1 attenuated cartilage degradation in TNF-transgenic mice. In chondrocytes, TNFalpha stimulated canonical Wnt signaling, which could be blocked by WIF-1, indicating a direct effect of TNFalpha and WIF-1 on Wnt signaling in this system. CONCLUSION: These data suggest that WIF-1 may take part in the fine-tuning of cartilage and bone turnover, promoting the balance of cartilage versus bone anabolism.
