Transforming growth factor β (TGF-β) is a potent naturally occurring inhibitor of cell growth. It exerts its action by binding to type I (TGFBR1) and type II (TGFBR2) transmembrane receptors located on the cell membrane. Intracellular signaling begins once TGF-β has bound to the TGFBR1/TGFBR2 complex. TGFBR2 activates TGFBR1, which acts as the initiator of intracellular responses. Mothers against decapentaplegic homolog 2 (SMAD2) and SMAD3 are subsequently activated by TGFBR1 and form complexes with SMAD4. Activated SMAD complexes enter the nucleus where they regulate the activity of target genes.
1 There is evidence that TGF-β related proteins activate not only SMADs but also other signaling pathways.
2