Probing the role of homomeric and heteromeric receptor interactions in TGF-β signaling using small molecule dimerizers

Background: Transforming growth factor beta (TGF-beta) arrests many cell types in the G1 phase of the cell cycle and upregulates plasminogen activator inhibitor 1 (PAI-I). The type I (TGF-beta RI) and II (TGF-beta RII) TGF-beta receptors mediate these and other effects of TGF-beta on target cells. TGF-beta initially binds to TGF-beta RII and subsequently TGF-beta RI is recruited to form a heteromeric complex. TGF-beta RI phosphorylates the downstream effecters Smad2 and Smad3, leading to their translocation into the nucleus. Here, we explored the role of receptor oligomerization in TGF-beta signaling. Results: We constructed fusion proteins containing receptor cytoplasmic tails linked to binding domains for small-molecule dimerizers. In COS-I cells, recruitment of a soluble TGF-beta RII tail to a myristoylated TGF-beta RI tail promoted Smad2 nuclear translocation. In mink lung cells, homo-oligomerization of a myristoylated TGF-beta RI tail in the presence of a myristoylated TGF-beta RII tail activated the PAI-1 promoter. Oligomerization of an acidic mutant of the TGF-beta RI tail in the absence of TGF-beta RII activated the PAI-1 promoter and inhibited the growth of mink lung cells. Conclusions: Non-toxic, small molecules designed to oligomerize cytoplasmic tails of TGF-beta receptors at the plasma membrane can activate TGF-beta signaling. Although TGF-beta normally signals through two receptors that are both necessary for signaling, in one small-molecule system, a dimerizer activates signaling through a single type of receptor that is sufficient to induce TGF-beta signaling. These methods of activating TGF-beta signaling could be extended to signaling pathways of other TGF-beta superfamily members such as activin and the bone morphogenetic proteins. (C) Current Biology Ltd.