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In vivo cells receive simultaneous signals from multiple extracellular ligands and must integrate and interpret them to respond appropriately. Here we investigate the interplay between pathways downstream of two transforming growth factor ß (TGF-ß) superfamily members, bone morphogenetic protein (BMP) and TGF-ß. We show that in multiple cell lines, TGF-ß potently inhibits BMP-induced transcription at the level of both BMP-responsive reporter genes and endogenous BMP target genes. This inhibitory effect requires the TGF-ß type I receptor ALK5 and is independent of new protein synthesis. Strikingly, we show that Smad3 is required for TGF-ß's inhibitory effects, whereas Smad2 is not. We go on to demonstrate that TGF-ß induces the formation of complexes comprising phosphorylated Smad1/5 and Smad3, which bind to BMP-responsive elements in vitro and in vivo and mediate TGF-ß-induced transcriptional repression. Furthermore, loss of Smad3 confers on TGF-ß the ability to induce transcription via BMP-responsive elements. Our results therefore suggest that not only is Smad3 important for mediating TGF-ß's inhibitory effects on BMP signaling but it also plays a critical role in restricting the transcriptional output in response to TGF-ß. [ABSTRACT FROM AUTHOR]

Transforming growth factor β (TGF-β) signals predominantly through a receptor complex comprising ALK5 and TβRII to activate receptor-regulated Smads (R-Smads) Smad2 and Smad3. In endothelial cells, however, TGF-β can additionally activate Smad1 and Smad5. Here, we report that TGF-β also strongly induces phosphorylation of Smad1/5 in many different normal epithelial cells, epithelium-derived tumor cells, and fibroblasts. We demonstrate that TβRII and ALK5, as well as ALK2 and/or ALK3, are required for TGF-β-induced Smad1/5 phosphorylation. We show that the simultaneous activation of the R-Smads Smad2/3 and Smad1/5 by TGF-β results in the formation of mixed R-Smad complexes, containing, for example, phosphorylated Smad1 and Smad2. The prevalence of these mixed R-Smad complexes explains why TGF-β-induced Smad1/5 phosphorylation does not result in transcriptional activation via bone morphogenetic protein (BMP)-responsive elements, which bind activated Smad1/5-Smad4 complexes that are induced by BMP stimulation. Thus, TGF-β induces two parallel pathways: one signaling via Smad2-Smad4 or Smad3-Smad4 complexes and the other signaling via mixed R-Smad complexes. Finally, we assess the function of the novel arm of TGF-β signaling and show that TGF-β-induced Smad1/5 activation is not required for the growth-inhibitory effects of TGF-β but is specifically required for TGF-β-induced anchorage-independent growth. [ABSTRACT FROM AUTHOR]

Transforming growth factor β (TGF-β) superfamily members signal via complexes of activated Smads, comprising phosphorylated receptor-regulated Smads, such as Smad2 and Smad3, and Smad4. These complexes are recruited to DNA by specific transcription factors. The forkhead/winged-helix transcription factors, XFast-1/XFoxH1a and XFast-3/XFoxH1b, bind an activated Smad heterotrimer comprising two Smad2s and one Smad4. Here we identify a novel Smad2 interaction motif, the Fast/FoxH1 motif (FM), present in all known Fast/FoxH1 family members, N-terminal to the common Smad interaction motif (SIM). The FM is necessary and sufficient to bind active Smad2/Smad4 complexes. The FM differs from the SIM since it discriminates between Smad2 and Smad3, and moreover only binds phosphorylated Smad2 in the context of activated Smad complexes. It is the first Smad interaction motif with this property. Site-directed mutagenesis indicates that the binding site for the FM on a Smad2/Smad4 heterotrimer is a hydrophobic pocket that incorporates the Smad/Smad interface. We demonstrate that the presence of an FM and SIM in the Fast/FoxH1 proteins allows them to compete efficiently for activated Smad2/Smad4 complexes with transcription factors such as Mixer that only contain a SIM. This establishes a hierarchy of Smad-interacting transcription factors, determined by their affinity for active Smad complexes. [ABSTRACT FROM AUTHOR]