Transforming growth factor beta signaling through Smad1 in human breast cancer cells.

Previous results have suggested that Smad1 transduces signals in response to bone morphogenetic proteins (BMPs), but not in response to transforming growth factor beta (TGF-beta). Here we investigated the ability of TGF-beta to regulate Smad1 phosphorylation, hetero-oligomerization with Smad4, translocation to the nucleus, and transcriptional activation of 3TP-luciferase reporter activity in TGF-beta- and BMP-responsive Hs578T human breast cancer cells. We demonstrate that Smad1 was rapidly phosphorylated in vivo in response to both TGF-beta3 and BMP2 as determined using an antibody against the epitope-tagged Smad1 being expressed. In addition, both TGF-beta3 and BMP2 increased Smad1-Smad4 hetero-oligomerization in Hs578T cells. Visualization of Smad1 nuclear translocation with the aid of green fluorescent protein (GFP) in live cells demonstrated nuclear accumulation of GFP-Smad1 fluorescence in response to either TGF-beta or BMP2 stimulation. After ligand stimulation, approximately 60-70% of transfected cells displayed prominent nuclear fluorescence. Expression of Smad1 in Hs578T cells increased the activity of the TGF-beta-responsive reporter 3TP-Lux. Moreover, TGF-beta treatment further potentiated the effect of Smad1 on 3TP-luciferase activity. Collectively, our results demonstrate that TGF-beta as well as BMP can signal through Smad1.