The signal transduction mechanisms underlying the pathophysiological activities of transforming growth factor-β (TGF-β) have been extensively studied since its discovery nearly 30 years ago. TGF-β ligands belong to a large superfamily of cytokines that bears its name (TGF-β Superfamily) and includes bone morphogenic proteins, activins, inhibin, growth/differentiation factors, Mullerian inhibiting substance, Nodal, and several other structurally-related polypeptides. Mammals express three TGF-β isoforms (i.e., TGF-β1, TGF-β2, and TGF-β3) that are encoded by distinct genes in a tissue-specific and developmentally-regulated manner. TGF-β was identified originally via its stimulation of morphological transformation and anchorage-independent growth in fibroblasts; however, this cytokine is now recognized as being a potent tumor suppressor that prevents the dysregulated growth and survival of epithelial, endothelial, and hematopoietic cells. In addition, numerous studies have clearly established TGF-β as a multifunctional cytokine that plays essential roles in regulating virtually all aspects of mammalian development and differentiation, and in maintaining mammalian tissue homeostasis. The pleiotropic nature of TGF-β is highlighted by the fact that every cell in the metazoan body can produce and respond to this cytokine. Even more remarkably, malignant cells have evolved a variety of complex mechanisms capable of circumventing the tumor suppressing activities of TGF-β, and in doing so, typically convert the functions of TGF-β to that of a tumor promoter, particularly the induction of carcinoma epithelial-mesenchymal transition, invasion, and dissemination to distant organ sites. This peculiar conversion in TGF-β function is known as the "TGF-β Paradox", which underlies the lethality of TGF-β in metastatic cancer cells. Thus, elucidating the effectors and signaling modules activated by TGF-β may offer new insights into the development of novel neoadjuvants capable of effectively targeting the TGF-β pathway to significantly improve the clinical course of patients with cancer, fibrosis, or immunologic disorders. TGF-β is secreted from cells as a latent homodimeric polypeptide that becomes tethered to the extracellular matrix by latent-TGF-β-binding proteins. Mature TGF-β isoforms are activated and liberated from extracellular matrix depots by a variety of mechanisms, including proteolysis, reactive oxygen species, changes in pH, and physical interactions with integrins, thromobspondin-1, or SPARC. Once activated, mature TGF-β initiates transmembrane signaling by binding to two distinct transmembrane Ser/Thr protein kinases, termed TGF-β type I (TβR-I) and type II (TβR-II) receptors. In some cells and tissues, TGF-β also binds to a third cell surface receptor, TGF-β type III (TβR-III), which transfers TGF-β to TβR-II and TβR-I. Full activation of these cytokine:receptor ternary complexes transpires upon TβR-II-mediated transphosphorylation and activation of TβR-I, which then phosphorylates and activates the latent transcription factors, Smad2 and Smad3. Afterward, phosphorylated Smad2/3 interact physically with Smad4, with the resulting heterotrimers translocating into the nucleus to regulate the expression of TGF-β-responsive genes. These Smad-dependent events are subject to fine-tuning and crosstalk regulation in the cytoplasm by their interaction with a variety of adapter molecules, including SARA, Hgs, PML and Dab2, and with Smad7, whose inhibitory activity is modulated by STRAP, AMSH2, and Arkadia; and in the nucleus by their interaction with a variety of transcriptional activators and repressors that occur in a gene- and cell-specific manner. In addition to activating canonical Smad2/3-dependent signaling, accumulating evidence clearly links the development of a variety of human pathologies to aberrant coupling of TGF-β to its noncanonical effector molecules. Included in this ever expanding list of noncanonical signaling molecules stimulated by TGF-β are PI3K, AKT, mTOR, integrins and focal adhesion kinase, and members of the MAP kinase (e.g., ERK1/2, JNK, and p38 MAPK small GTP-binding proteins (e.g., Ras, Rho, and Rac1). The interactions and intersections between canonical and noncanonical TGF-β signaling systems are depicted in the pathway map.
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Kandasamy, K., Mohan, S. S., Raju, R., Keerthikumar, S., Kumar, G. S. S., Venugopal, A. K., Telikicherla, D., Navarro, J. D., Mathivanan, S., Pecquet, C., Gollapudi, S. K., Tattikota, S. G., Mohan, S., Padhukasahasram, H., Subbannayya, Y., Goel, R., Jacob, H. K. C., Zhong, J., Sekhar, R., Nanjappa, V., Balakrishnan, L., Subbaiah, R., Ramachandra, Y. L., Rahiman, B. A., Prasad, T. S. K., Lin, J., Houtman, J. C. D., Desiderio, S., Renauld, J., Constantinescu, S. N., Ohara, O., Hirano, T., Kubo, M., Singh, S., Khatri, P., Draghici, S., Bader, G. D., Sander, C., Leonard, W. J. and Pandey, A. (2010). NetPath: A public resource of curated signal transduction pathways. Genome Biology. 11:R3d3aMediates epithelial-mesenchymal transitionGO:0045217Type your comment hereType your comment hereUbiquitination of SMAD2 by ITCH promotes phosphorylation of SMAD2transforming growth factor-beta superfamily mediated signaling pathwayPW:0000329Pathway Ontology20067622PubMedNetPath: a public resource of curated signal transduction pathways.Genome Biol2010Kandasamy KMohan SSRaju RKeerthikumar SKumar GSVenugopal AKTelikicherla DNavarro JDMathivanan SPecquet CGollapudi SKTattikota SGMohan SPadhukasahasram HSubbannayya YGoel RJacob HKZhong JSekhar RNanjappa VBalakrishnan LSubbaiah RRamachandra YLRahiman BAPrasad TSLin JXHoutman JCDesiderio SRenauld JCConstantinescu SNOhara OHirano TKubo MSingh SKhatri PDraghici SBader GDSander CLeonard WJPandey A