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


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.
Experimental Rheumatology, Radboud university medical center, Geert Grooteplein 28, 6525 GA Nijmegen, The Netherlands. Electronic address: martijn.vandenbosch@radboudumc.nl.
Martijn H
MH
van den Bosch
Experimental Rheumatology, Radboud university medical center, Geert Grooteplein 28, 6525 GA Nijmegen, The Netherlands. Electronic address: arjen.blom@radboudumc.nl.
Arjen B
AB
Blom
Experimental Rheumatology, Radboud university medical center, Geert Grooteplein 28, 6525 GA Nijmegen, The Netherlands. Electronic address: peter.vanlent@radboudumc.nl.
Peter L
PL
van Lent
Experimental Rheumatology, Radboud university medical center, Geert Grooteplein 28, 6525 GA Nijmegen, The Netherlands. Electronic address: henk.vanbeuningen@radboudumc.nl.
Henk M
HM
van Beuningen
Experimental Rheumatology, Radboud university medical center, Geert Grooteplein 28, 6525 GA Nijmegen, The Netherlands. Electronic address: esmeralda.blaneydavidson@radboudumc.nl.
Esmeralda N
EN
Blaney Davidson
Experimental Rheumatology, Radboud university medical center, Geert Grooteplein 28, 6525 GA Nijmegen, The Netherlands. Electronic address: peter.vanderkraan@radboudumc.nl.
Peter M
PM
van der Kraan
Experimental Rheumatology, Radboud university medical center, Geert Grooteplein 28, 6525 GA Nijmegen, The Netherlands. Electronic address: wim.vandenberg@radboudumc.nl.
Wim B
WB
van den Berg
D030201:Activin Receptors, Type I / Q000378:metabolism*; D030301:Activin Receptors, Type II; D000818:Animals; D055511:CCN Intercellular Signaling Proteins / Q000378:metabolism; D002460:Cell Line; D019902:Chondrocytes / Q000166:cytology / Q000378:metabolism; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D000077293:Receptor, Transforming Growth Factor-beta Type I; D018125:Receptors, Transforming Growth Factor beta / Q000378:metabolism; D015398:Signal Transduction; D051898:Smad1 Protein / Q000378:metabolism*; D051902:Smad5 Protein / Q000378:metabolism*; D051904:Smad8 Protein / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000378:metabolism*; D051153:Wnt Proteins / Q000378:metabolism*; D060509:Wnt3A Protein / Q000378:metabolism; D051176:beta Catenin / Q000378:metabolism
D055511:CCN Intercellular Signaling Proteins; C492067:CCN4 protein, mouse; D036341:Intercellular Signaling Peptides and Proteins; D011518:Proto-Oncogene Proteins; D018125:Receptors, Transforming Growth Factor beta; D051898:Smad1 Protein; D051902:Smad5 Protein; D051904:Smad8 Protein; D016212:Transforming Growth Factor beta; D051153:Wnt Proteins; D060509:Wnt3A Protein; C507016:Wnt8a protein, mouse; D051176:beta Catenin; D017346:Protein Serine-Threonine Kinases; D030201:Activin Receptors, Type I; D030301:Activin Receptors, Type II; C498931:Acvrl1 protein, mouse; D000077293:Receptor, Transforming Growth Factor-beta Type I; C000625841:TGFBR1 protein, human; C000625842:Tgfbr1 protein, mouse
Chondrocyte hypertrophy; Osteoarthritis; Smad signaling; TGF-β; Wnt signaling