Transforming Growth Factor (cid:2) -Induced Smad1/5 Phosphorylation in Epithelial Cells Is Mediated by Novel Receptor Complexes and Is Essential for Anchorage-Independent Growth (cid:1) †

Transforming growth factor (cid:1) (TGF- (cid:1) ) signals predominantly through a receptor complex comprising ALK5 and T (cid:1) RII to activate receptor-regulated Smads (R-Smads) Smad2 and Smad3. In endothelial cells, however, TGF- (cid:1) can additionally activate Smad1 and Smad5. Here, we report that TGF- (cid:1) also strongly induces phosphorylation of Smad1/5 in many different normal epithelial cells, epithelium-derived tumor cells, and fibroblasts. We demonstrate that T (cid:1) RII and ALK5, as well as ALK2 and/or ALK3, are required for TGF- (cid:1) induced Smad1/5 phosphorylation. We show that the simultaneous activation of the R-Smads Smad2/3 and Smad1/5 by TGF- (cid:1) results in the formation of mixed R-Smad complexes, containing, for example, phosphorylated Smad1 and Smad2. The prevalence of these mixed R-Smad complexes explains why TGF- (cid:1) -induced Smad1/5 phosphorylation does not result in transcriptional activation via bone morphogenetic protein (BMP)- responsive elements, Smad1/5-Smad4 BMP Thus, TGF- (cid:1) induces two parallel pathways: one signaling via Smad2-Smad4 or Smad3-Smad4 complexes and the other signaling via mixed R-Smad complexes. Finally, we assess the function of the novel arm of TGF- (cid:1) signaling and show that TGF- (cid:1) -induced Smad1/5 activation is not required for the growth-inhibitory effects of TGF- (cid:1) but is specifically required for TGF- (cid:1) -induced anchorage-independent growth.