Mathematical modeling and experimental analyses reveal that TGF-β ligand depletion has an important role in converting short-term graded signaling responses to long-term switch-like responses.

Abbreviations: AIP4 (atrophin 1-interacting protein 4); BMP (bone morphogenetic protein); CHIP (carboxyl terminus of Hsc70-interacting protein); GSK3b (glycogen synthase kinase 3-b); HECT (homologous to E6-AP carboxyl terminus); MAPK (mitogen-activated protein kinase); NEDD4-2 (neural precursor cell expressed, developmentally downregulated 4-2); PIAS (protein inhibitor of activated Stat); Smurf (Smad ubiquitylation regulatory factor); STRAP (serine-threonine kinase receptorassociated protein); SUMO (small ubiquitin-like modifier); TbRI/TbRII (TGFb receptor type I and II); TGFb (transforming growth factor b); WWP1 (WW domain-containing protein 1); Ub (ubiquitin) npg Transforming growth factor β (TGFβ) controls cellular behavior in embryonic and adult tissues. TGFβ binding to serine/threonine kinase receptors on the plasma membrane activates Smad molecules and additional signaling proteins that together regulate gene expression. In this review, mechanisms and models that aim at explaining the coordination between several components of the signaling network downstream of TGFβ are presented. We discuss how the activity and duration of TGFβ receptor/Smad signaling can be regulated by post-translational modifications that affect the stability of key proteins in the pathway. We highlight links between these mechanisms and human diseases, such as tissue fibrosis and cancer. Regulating the stability of TGFβ receptors and Smads

The versatile cytokine transforming growth factor β (TGF-β) regulates cellular growth, differentiation, and migration during embryonic development and adult tissue homeostasis. Activation of TGF-β receptors leads to phosphorylation of Smad2 and Smad3, which oligomerize with Smad4 and accumulate in the nucleus where they recognize gene regulatory regions and orchestrate transcription. Termination of Smad-activated transcription involves Smad dephosphorylation, nuclear export, or ubiquitin-mediated degradation. In an unbiased proteomic screen, we identified poly(ADP-ribose) polymerase-1 (PARP-1) as a Smad-interacting partner. PARP-1 dissociates Smad complexes from DNA by ADP-ribosylating Smad3 and Smad4, which attenuates Smad-specific gene responses and TGF-β-induced epithelial-mesenchymal transition. Thus, our results identify ADP-ribosylation of Smad proteins by PARP-1 as a key step in controlling the strength and duration of Smad-mediated transcription.