Intracellular dynamics of Smad‐mediated TGFβ signaling

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Abstract
The transforming growth factor‐β (TGFβ) family represents a class of signaling molecules that plays a central role in morphogenesis, growth, and cell differentiation during normal embryonic development. Members of this growth factor family are particularly vital to development of the mammalian secondary palate where they regulate palate mesenchymal cell proliferation and extracellular matrix synthesis. Such regulation is particularly critical since perturbation of either cellular process results in a cleft of the palate. While the cellular and phenotypic effects of TGFβ on embryonic craniofacial tissue have been extensively catalogued, the specific genes that function as downstream mediators of TGFβ action in the embryo during palatal ontogenesis are poorly defined. Embryonic palatal tissue in vivo and murine embryonic palate mesenchymal (MEPM) cells in vitro secrete and respond to TGFβ. In the current study, elements of the Smad component of the TGFβ intracellular signaling system were identified and characterized in cells of the embryonic palate and functional activation of the Smad pathway by TGFβ1, TGFβ2, and TGFβ3 was demonstrated. TGFβ‐initiated Smad signaling in cells of the embryonic palate was found to result in: (1) phosphorylation of Smad 2; (2) nuclear translocation of the Smads 2, 3, and 4 protein complex; (3) binding of Smads 3 and 4 to a consensus Smad binding element (SBE) oligonucleotide; (4) transactivation of transfected reporter constructs, containing TGFβ‐inducible Smad response elements; and (4) increased expression of gelatinases A and B (endogenous genes containing Smad response elements) whose expression is critical to matrix remodeling during palatal ontogenesis. Collectively, these data point to the presence of a functional Smad‐mediated TGFβ signaling system in cells of the developing murine palate. J. Cell. Physiol. 197: 261–271, 2003.