Discussion

More fundamentally than regulating specific functions or cascades, PTM affords proteins the mechanical complexity necessary to make human cells function. We believe that specifically modified forms of proteins are what conduct the cells business, and that by monitoring these '?proteoforms' we can gain unique insight into cellular physiology. To test this, we treated HepG2 hepatocellular carcinoma cells with the multi-kinase inhibitor Sorafenib, and observed pharmacodynamic changes to PTM across the cellular proteome. Our novel approach, referred to as Snapshot Proteomics, revealed changes in phosphorylation to known targets of this drug. Intriquingly, we also observed changes to proteins whose roles make them strong candidates for mediating the clinical side effects of Sorafenib treatment. We also observed that the changes mediating the therapeutic response were far from the most dramatic ones observed, suggesting other, unexpected proteins as better putative pharmacodynamic biomarkers. Finally, we found that manipulating cellular phosphorylation pharmacologically had dramatic and interesting effects on other PTM pathways as well, ubiquitin in particular. This confirmed the already established crosstalk between these pathways, and supports our contention that no PTM occurs in a vacuum.