Background: Transforming growth factor Beta (TGF-Beta) arrests many cell types in the G1 phase of the cell and upregulates plasminogen activator inhibitor 1 (PAI-1). The type 1 (TGF-Beta RI) an 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 effectors 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-1 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 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 absence of TGF-Beta RII activated the PAI-A 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.