The transforming growth factor-β system, a complex pattern of cross-reactive ligands and receptors

A new homodimer form of transforming growth factor-β (TGF-β), TGF-β2, has been identified in porcine blood platelets. TGF-β2 is homologous to ordinary TGF-β (TGF-β1), which is also present in platelets. TGF-β1.2, a heterodimer containing one TGF-β1 chain and one TGF-β2 chain, has also been isolated. TGF-β1 and TGF-β2 interact differently with a family of receptors in target cells. A 280 kd receptor displays high affinity for both TGF-β1 and TGF-β2. Occupancy of this receptor by TGF-β1 or TGF-β2 correlates with the ability of these TGF-βs to inhibit cell proliferation. In contrast, 65 kd and 85 kd receptors have high affinity for TGF-β1 but lower affinity for TGF-β2. The existence of distinct forms of TGF-β that interact differently with a family of TGF-β receptors could provide flexibility to the regulation of tissue growth and differentiation by the TGF-β system.