Crosstalk mechanisms between the mitogen-activated protein kinase pathways and Smad signaling downstream of TGF-β: implications for carcinogenesis

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Abstract
Transforming growth factor-β (TGF-β) superfamily members signal via membrane-bound heteromeric serine–threonine kinase receptor complexes. Upon ligand-binding, receptor activation leads to phosphorylation of cytoplasmic protein substrates of the Smad family. Following phosphorylation and oligomerization, the latter move into the nucleus to act as transcription factors to regulate target gene expression. TGF-β responses are not solely the result of the activation Smad cascade, but are highly cell-type specific and dependent upon interactions of Smad signaling with a variety of other intracellular signaling mechanisms, initiated or not by TGF-β itself, that may either potentiate, synergize, or antagonize, the rather linear TGF-β/Smad pathway. These include, (a), regulation of Smad activity by mitogen-activated protein kinases (MAPKs), (b), nuclear interaction of activated Smads with transcriptional cofactors, whether coactivators or corepressors, that may be themselves be regulated by diverse signaling mechanisms, and (c), negative feedback loops exerted by inhibitory Smads, transcriptional targets of the Smad cascade. This review focuses on how MAPKs modulate the outcome of Smad activation by TGF-β, and how cross-signaling mechanisms between the Smad and MAPK pathways may take place and affect cell fate in the context of carcinogenesis.