Moreover, the sumoylation of SMURF2 by SUMO E3 ligase PIAS3 enhances its ability to degrade TGF- receptors and thereby suppresses TGF- effects, e.g., EMT . To get a broader overview of the currently known regulatory mechanisms of TGF- signaling, the reader is referred to the elegant review by Derynck and Budi . Finally, we discuss new approaches to target the TGF- pathway, name current clinical trials, and explain promises and drawbacks that deserve to be properly resolved. mice spontaneously developed severe liver fibrosis with huge TGF-/Smad3 and subsequent HSC activation. The animals pass away between 8 and 12 weeks of age. This phenotype could be rescued by adenoassociated computer virus (AAV) mediated expression of ECM1 or by interfering with TGF- signaling using AAV expressing soluble TRII. Moreover, carbon tetrachloride (CCl4)-induced liver damage was blunted by ECM1 overexpression . Active TGF- starts signaling by binding to the TGF- type II receptor (TRII) resulting in recruitment of the TGF- type I receptor (TRI). Next, TRII phosphorylates TRI at a Gly-SerCrich (GS) domain leading to a conformational modulation in TRI and sensitizing it to bind and phosphorylate its substrates, i.e., SMAD2 and SMAD3 proteins (also called receptor-activated SMADs or R-SMADs). After C-terminal SMAD phosphorylation, pSMAD2 and pSMAD3 form heterocomplexes with the common SMAD4, which thereafter translocates to the nucleus to bind DNA and regulate the transcription of multiple target genes, e.g., (Physique 2) [13,26]. AMG 837 Two important facts deserve to be highlighted here. First, SMAD2 does not bind to DNA, while SMAD3 possesses a poor DNA binding affinity. Therefore, SMAD2/3/4 complexes generally recruit additional transcriptional coactivators to stabilize transactivation complexes [13,27]. Second, several TGF- target genes can be activated by R-SMADs without the requirement of SMAD4 . Open in a separate window Physique 2 SMAD- and Non-SMAD-dependent TGF- signaling. Upon liver damage associated signaling, TGF- molecules are freed AMG 837 from the large latent complex (LLC) through the conversation of integrins with the latent association protein (LAP). Binding of released TGF- to TRII results in the formation of a heterotetramer with TRI, which then initiates the canonical signaling pathway KLRD1 through phosphorylation of R-SMADs, i.e., SMAD2 (S2) and SMAD3 (S3). TGF- can also activate non-canonical SMAD-independent pathways, as exemplified here by MAPK, mTOR, PI3K/AKT, and Rho/GTPase pathways. Alongside other mechanisms, SMAD7 negatively regulates TGF- signaling through competing with R-SMADs for TRI binding. TF: Transcription factors, P: phosphate group, LTBP: latent TGF- binding protein. Canonical R-SMAD-mediated TGF- signaling does not explain all observed effects of TGF-. Many studies identified other signaling pathways that could be activated by TGF-, such as mitogen-activated protein kinase (MAPK), mammalian target of rapamycin (mTOR), phosphatidylinositol-3-kinase/AKT, and Rho GTPase pathways (Physique 2). TGF- non-canonical pathways provide a broad windows for intracellular cross-talk [29,30,31] and can be classified into three major groups : (I) R-SMADs interact with other pathways instead of directly transmitting the transmission to the nucleus. Such conversation is usually illustrated by the ability of SMAD2 and SMAD3 to activate ERK and PKA [32,33]. (II) TheTR complex can activate intracellular substrates other than SMADs, such as Daxx, a proapoptotic adaptor protein, leading to JNK activation and apoptosis . (III) R-SMADs could be activated by TR-independent mechanisms. The latter mechanism is best exemplified by phosphorylation of the linker domain name of R-SMADs, e.g., by ERK, which interferes with R-SMAD nuclear translocation . Non-canonical pathways provide one explanation for the versatile effects of TGF- signaling and its dichotomal functions, as for example explained in carcinogenesis . In fibrosis, however, such events have not yet been thoroughly investigated, with exception of linker phosphorylation . It should be emphasized here that results obtained from SMAD4 AMG 837 cells or specific kinase inhibitor treatments should be cautiously attributed to non-SMAD signaling for several reasons [29,30]. Firstly, as previously mentioned, SMAD4 is not required for transcription of several specific R-SMAD dependent genes such as . Secondly, chemical inhibitors can block several kinases AMG 837 dose-dependently . Therefore, in our opinion, specific SMAD2 and SMAD3 models represent the best way to characterize non-SMAD pathways downstream to TGF- treatment . Signaling kinetics can also be utilized to shed light on SMAD and non-SMAD-dependent effects. For example, in some cells, e.g., mast cells, TGF- mediated ERK phosphorylation occurs within 10 min in comparable kinetics to EGF-induced ERK activation, which suggests a direct non-SMAD mechanism. In contrast, the same effect requires several hours in other cells, e.g., pancreatic acinar cells, highlighting.