Negative functional interaction of retinoic acid and TGF-beta signaling mediated by TG-interacting factor during chondrogenesis.

We previously showed that atRA (all-trans Retinoic Acid, atRA) inhibites chondrogenesis by downregulation of TGF-beta/Smad signaling. However, the molecular link between RA and TGF-beta signaling is unknown. Using a mouse embryo palate mesenchyme micomass cultures (MMCs) system, we explored interactions of RA and TGF-beta signaling during chondrogenesis. We found that atRA suppressed chondrogenesis and Smad2/3 phosphorylation regardless of the presence of TGF-beta3. Functional assays indicated that TGF-beta3 treatment or co-transfection of expressing Smad2/3 vectors suppressed atRA-induced RARE-tk-Luc activity. Conversely, atRA or RAR-overexpression repressed TGF-beta3-induced transactivation of the TGF-beta-responsive reporter, p3TP-Lux. ChIP assay revealed the binding of the Smad transcriptional co-repressor TGIF (TG-interacting factor, TGIF) to RARbeta promoter in control MMCs, but this association was decreased by the addition of RA and increased by TGF-beta3, respectively. Further examinations revealed that TGIF exerted a pivotal role in regulating crosstalk of RA and TGF-beta signaling, since siRNA knockdown of TGIF partially abolished the ability of atRA to suppress TGF-beta3-induced chondrogenesis, whereas forced expression of TGIF blocked the ability of TGF-beta3 to relieve atRA-mediated the suppression of chondrogenesis. Furthermore, we demonstrated that the effects of atRA on TGF-beta-dependent gene activation and of TGF-beta on RA-dependent gene activation are mediated by TGIF with siRNA to downregulate TGIF. Collectively, these findings indicated a negative functional interplay of RA and TGF-beta signaling mediated by TGIF to modulate chondrogenesis in MMCs.