Wnt Inhibitory Factor 1 Deficiency Uncouples Cartilage and Bone Destruction in Tumor Necrosis Factor α–Mediated Experimental Arthritis
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.
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/β-catenin signaling was investigated by real-time RT-PCR and immunofluorescence on primary chondrocytes.
WIF-1 expression was repressed by TNFα 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, TNFα stimulated canonical Wnt signaling, which could be blocked by WIF-1, indicating a direct effect of TNFα and WIF-1 on Wnt signaling in this system.
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.