Transforming growth factor β family ligands → type II + type I serine/threonine kinase receptors → R-SMAD (SMAD2/3) phosphorylation → R-SMAD + co-SMAD (SMAD4) complex → nuclear translocation → transcription of ECM genes (collagen, fibronectin, integrins) + EMT genes. Master driver of FIBROSIS in IPF, hepatic, renal, scleroderma, post-surgical scarring. Pharmacology: pirfenidone (multimodal antifibrotic, partly anti-TGF-β; approved for IPF + slows FVC decline); nintedanib (multi-kinase inhibitor PDGFR + FGFR + VEGFR; reduces IPF FVC decline). Direct TGF-β neutralization (fresolimumab, etc.) trialed in fibrosis + oncology with mixed results. Bone morphogenetic proteins (BMPs) are TGF-β family but signal through SMAD1/5/8.
Organ Systems
immune-hematologic
respiratory
integumentary
Pathway Steps
tgf-beta-ligand → tgfbr1-tgfbr2-activation — via ligand binds type II receptor → recruits + phosphorylates type I. TGF-β signaling begins when the ligand binds the type II receptor (TGFBR2), which recruits and transphosphorylates the type I receptor (TGFBR1/ALK5). This receptor pairing is the activation switch — and its context-dependence (the same ligand can suppress or promote tumors) is a hallmark of the pathway.
tgfbr1-tgfbr2-activation → smad2-smad3-phosphorylation — via type I receptor kinase domain phosphorylates R-SMAD. Activated TGFBR1 phosphorylates the receptor-regulated SMADs, SMAD2 and SMAD3, on their C-terminal tails. This is the canonical transduction step, tunable by inhibitory SMAD7 and by receptor turnover — providing built-in negative feedback.
smad2-smad3-phosphorylation → smad2-3-4-complex-nuclear — via R-SMAD + SMAD4 oligomerization → nuclear translocation. Phospho-SMAD2/3 partner with the common mediator SMAD4 and translocate to the nucleus as a complex. The requirement for SMAD4 (a frequently deleted tumor suppressor) explains why its loss disables the growth-suppressive arm of TGF-β signaling in cancer.
smad2-3-4-complex-nuclear → ecm-gene-transcription — via collagen + fibronectin + integrin gene induction → fibrosis. In the nucleus the SMAD complex, with cofactors, drives target genes — prominently extracellular-matrix components (collagens) and regulators of growth and EMT. The ECM-inducing output is central to tissue fibrosis, making TGF-β a key antifibrotic target.