Divergence of epidermal growth factor-transforming growth factorß signaling in embryonic orofacial tissue

Summary

The epidermal growth factor (EGF) and transforming growth factor β (TGFβ) families of signaling molecules play a major role in growth and development of embryos. Abrogation of either signaling pathway results in defects in embryogenesis, including cleft palate. In the developing palate, both EGF and TGFβ regulate cellular proliferation, extracellular matrix synthesis, and cellular differentiation but often in an opposing manner. Evidence from various adult cell types suggests the existence of cross talk between the EGF and TGFβ signaling pathways, although it is unclear whether such cross talk exists in murine embryonic maxillary mesenchymal cells, from which the developing palate is derived. In this study, embryonic maxillary mesenchymal cells in culture were treated with EGF and TGFβ, either singly or in combination, and the cells were subsequently examined for signaling interactions between these two pathways. Immunoblot analyses of nuclear extracts of embryonic maxillary mesenchymal cells revealed that TGFβ-induced nuclear translocation of Smad 2 and Smad 3 proteins was not affected by EGF. Conversely, immunoblot analyses of whole-cell extracts of these cells indicated that EGF-induced phosphorylation of extracellular signal-regulated kinase proteins, ERK1 and ERK2, was not affected by TGFβ. Expression of a transfected luciferase reporter gene driven by a promoter with Smad binding elements was induced by TGFβ in these cells but was not affected by EGF. Last, TGFβ was found to induce expression of the endogenous gelatinase B gene in embryonic maxillary mesenchymal cells; however, this effect was independent of any interaction of EGF. Collectively, data from this study suggest that the EGF and TGFβ signal transduction pathways do not converge in murine embryonic maxillary mesenchymal cells.