Quantitative phosphoproteomics of transforming growth factor-β signaling in colon cancer cells

The transforming growth factor-beta (TGF-beta) signaling pathway progresses through a series of protein phosphorylation regulated steps. Smad4 is a key mediator of the classical TGF-beta signaling pathway; however, reports suggest that TGF-beta can activate other cellular pathways independent of Smad4. By investigating the TGF-beta-regulated phosphoproteome, we aimed to uncover new functions controlled by TGF-beta. We applied titanium dioxide to enrich phosphopeptides from stable isotope labeling with amino acids in cell culture (SILAC)-labeled SW480 cells stably expressing Smad4 and profiled them by mass spectrometry. TGF-beta stimulation for 30 min resulted in the induction of 17 phosphopeptides and the repression of 8 from a total of 149 unique phosphopeptides. Proteins previously not known to be phosphorylated by TGF-beta including programmed cell death protein 4, nuclear ubiquitous casein and cyclin-dependent kinases substrate, hepatoma-derived growth factor and cell division kinases amongst others were induced following TGF-beta stimulation, while the phosphorylation of TRAF2 and NCK-interacting protein kinase are examples of proteins whose phosphorylation status was repressed. This phosphoproteomic screen has identified new TGF-beta-modulated phosphorylation responses in colon carcinoma cells.