Background
Both Wnt signaling and TGF-β signaling have been implicated in the regulation of the phenotype of many cell types including chondrocytes, the only cell type present in the articular cartilage. A changed chondrocyte phenotype, resulting in chondrocyte hypertrophy, is one of the main hallmarks of osteoarthritis. TGF-β signaling via activin-like kinase (ALK)5, resulting in Smad 2/3 phosphorylation, inhibits chondrocyte hypertrophy. In contrast, TGF-β signaling via ALK1, leading to Smad 1/5/8 phosphorylation, has been shown to induce chondrocyte hypertrophy. In this study, we investigated the capability of Wnt3a and WISP1, a protein downstream in canonical Wnt signaling, to skew TGF-β signaling in chondrocytes from the protective Smad 2/3 towards the Smad 1/5/8 pathway.
Conclusions
Our data show that canonical Wnt signaling is able to skew TGF-β signaling towards dominant signaling via the ALK1/Smad 1/5/8 pathway, which reportedly leads to chondrocyte hypertrophy. In this way canonical Wnts and WISP1, which we found to be increased during experimental osteoarthritis, may contribute to osteoarthritis pathology.
Results
Stimulation with Wnt3a, either alone or in combination with its downstream protein WISP1, decreased TGF-β-induced C-terminal phosphorylation of Smad 2/3. In addition, both Wnt3a and WISP1 increased Smad 1/5/8 phosphorylation at the C-terminal domain in both murine and human chondrocytes. DKK-1, a selective inhibitor of canonical Wnt signaling, abolished these effects. TGF-β signaling via Smad 2/3, measured by the functional CAGA12-Luc reporter construct activity, was decreased by stimulation with Wnt3a in accordance with the decrease in Smad 2/3 phosphorylation found on Western blot. Furthermore, in vivo overexpression of the canonical Wnt8a decreased Smad 2/3 phosphorylation and increased Smad 1/5/8 phosphorylation. Conclusions: Our data show that canonical Wnt signaling is able to skew TGF-β signaling towards dominant signaling via the ALK1/Smad 1/5/8 pathway, which reportedly leads to chondrocyte hypertrophy. In this way canonical Wnts and WISP1, which we found to be increased during experimental osteoarthritis, may contribute to osteoarthritis pathology.