Smad2 functions as a co-activator of canonical Wnt/β-catenin signaling pathway independent of Smad4 through histone acetyltransferase activity of p300

Both canonical Wnt/β-catenin and TGFβ/Smad signaling pathways coordinately regulate pattern formation during embryogenesis as well as tumor progression. Evidence of cross-talk between these two pathways has been reported. Here we demonstrated that the Activin-like kinase 4 (Alk4)/Smad2 pathway facilitates the transcriptional activity of the oncogenic Wnt/β-catenin/Tcf4 pathway through a novel Smad4-independent mechanism. Upon activation, Smad2 physically interacted with Tcf4, β-catenin and the co-activator p300 to enhance transcriptional activity of β-catenin/Tcf4 through the histone acetyltransferase activity of p300. Transactivation by Smad2 was independent of a Smad-binding element (SBE) and Smad4. Indeed, the enhancement of β-catenin/Tcf4 transcriptional activity by activated Smad2 was negatively regulated by the presence of Smad4. Moreover, a tumor-derived missense mutant of Smad2, lacking the ability to bind to Smad4 was still able to enhance the Tcf4 transcriptional reporter in the presence of β-catenin and Tcf4. Our findings suggest that Smad2 may function as an activator of canonical Wnt/β-catenin/Tcf4 signaling through a SBE/Smad4-independent pathway.