Cross-talk between the TGFβ and Wnt signaling pathways in murine embryonic maxillary mesenchymal cells

The transforming growth factor beta (TGFβ) and Wnt signaling pathways play central roles regulating embryogenesis and maintaining adult tissue homeostasis. TGFβ mediates its cellular effects through types I and II cell surface receptors coupled to the nucleocytoplasmic Smad proteins. Wnt signals via binding to a cell surface receptor, Frizzled, which in turn activates intracellular Dishevelled, ultimately leading to stabilization and nuclear translocation of β-catenin. Previous studies have demonstrated several points of cross-talk between the TGFβ and Wnt signaling pathways. In yeast two-hybrid and GST-pull down assays, Dishevelled-1 and Smad 3 have been shown to physically interact through the C-terminal one-half of Dishevelled-1 and the MH2 domain of Smad 3. The current study demonstrates that co-treatment of murine embryonic maxillary mesenchyme (MEMM) cells with Wnt-3a and TGFβ leads to enhanced reporter activity from TOPflash, a Wnt-responsive reporter plasmid. Transcriptional cooperation between TGFβ and Wnt did not require the presence of a Smad binding element, nor did it occur when a TGFβ-responsive reporter plasmid (p3TP-lux) was transfected. Overexpression of Smad 3 further enhanced the cooperation between Wnt and TGFβ while overexpression of dominant-negative Smads 2 and 3 inhibited this effect. Co-stimulation with TGFβ led to greater nuclear translocation of β-catenin, providing explanation for the effect of TGFβ on Wnt-3a reporter activity. Wnt-3a exerted antiproliferative activity in MEMM cells, similar to that exerted by TGFβ. In addition, Wnt-3a and TGFβ in combination led to synergistic decreases in MEMM cell proliferation. These data demonstrate a functional interaction between the TGFβ and Wnt signaling pathways and suggest that Wnt activation of the canonical pathway is an important mediator of MEMM cell growth.