Canonical Wnt signaling skews TGF-β signaling in chondrocytes towards signaling via ALK1 and Smad 1/5/8

Background: Both Wnt signaling and TGF-beta 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-beta signaling via activin-like kinase (ALK)5, resulting in Smad 2/3 phosphotylation, inhibits chondrocyte hypertrophy. In contrast TGF-beta 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-beta signaling in chondrocytes from the protective Smad 2/3 towards the Smad 1/5/8 pathway. Results: Stimulation with Wnt3a, either alone or in combination with its downstream protein WISPI, decreased TGF-beta-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-beta signaling via Smad 2/3, measured by the functional CAGA(12)-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-beta signaling towards dominant signaling via the ALKI/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. (C) 2014 Elsevier Inc. All rights reserved.