Aberrant hypertrophy in Smad3-deficient chondrocytes is rescued by restoring TAK1-ATF-2 signaling: a potential clinical implication for osteoarthritis

Objective— To investigate the biological significance of Smad3 in the progression of osteoarthritis (OA), the crosstalk between Smad3 and ATF-2 in the TGF- β signaling pathway, and the effects of ATF-2 overexpression and p38 activation in chondrocyte differentiation. Methods— Joint disease in Smad3 knockout (Smad3 − / − ) mice was examined by micro-CT and histology. Numerous in vitro methods including immunostaining, real-time PCR, Western blotting, an ATF-2 DNA-binding assay and a p38 kinase activity assay were used to study the various signaling responses and protein interactions underlying the altered chondrocyte phenotype in Smad3 − / − mice. Results— Smad3 − / − mice gradually developed an end-stage OA phenotype. TGF- β -induced TAK1-ATF-2 signaling was disrupted in Smad3 − / − chondrocytes at the level of p38 MAP kinase activation resulting in reduced ATF-2 phosphorylation and transcriptional activity. Re-introduction of Smad3 into the Smad3 − / − cells restored the normal p38 response to TGF- β . Phospho-p38 formed a complex with Smad3 by binding to the Smad3 MH1-linker domains. the of to of signaling in and pathways using / − and to examine the effects of TAK1-ATF-2 signaling on chondrocyte