Transforming growth factor beta (TGF-β)

Transforming growth factor beta (TGF-β) is a secreted signaling protein — a multifunctional cytokine that exists in mammals as three closely related isoforms (TGF-β1, TGF-β2, TGF-β3). It is synthesized as a latent complex that must be activated before it can bind its receptors; release of the mature ligand from this latent form is a key regulatory step, mediated in part by integrins such as αvβ6, which is induced on epithelial cells during chronic tissue injury. Active TGF-β binds type II and type I serine/threonine kinase receptors, and the activated type I receptor phosphorylates receptor-regulated Smad proteins, which complex with Smad4 and accumulate in the nucleus to control transcription. Non-canonical branches, including MAPK signaling, operate alongside this canonical Smad route.

Biologically, TGF-β governs cell proliferation, differentiation, extracellular matrix deposition, immune regulation, and wound repair, and its effects are strongly context-dependent. In fibrosis, TGF-β/Smad signaling drives activation of stellate and other mesenchymal cells, collagen production, and epithelial-mesenchymal transition — processes central to liver fibrosis in metabolic dysfunction-associated steatohepatitis, intestinal fibrosis in chronic colitis, myocardial and renal fibrosis, and airway scarring; Sirt1-linked metabolic regulation has been implicated upstream of this axis. In cancer, TGF-β acts both as a tumor suppressor in early epithelial lesions and as a driver of invasion, metastasis, and immune evasion in established tumors, where it remodels the tumor microenvironment, promotes regulatory T cell differentiation, and suppresses the cytotoxic function of natural killer cells and CD8+ T cells, contributing to resistance to checkpoint inhibitors such as anti-Programmed cell death 1 (PD-1) and anti-PD-L1 agents. This dual biology makes TGF-β both a mechanistic readout of fibrotic and immunosuppressive disease states and an active therapeutic target: strategies span small-molecule ATP-competitive inhibitors of the type I receptor kinase, ligand- and integrin-directed blockade, targeted protein degradation of pathway components, and cell-engineering approaches that render adoptively transferred immune cells insensitive to TGF-β-mediated suppression.