Probing tyrosine kinase inhibitors specificity

Using a three-pronged MS-based proteomic approach, we assessed the mode-of-action of four clinically-relevant tyrosine kinase inhibitors in an epithelial cancer cell line. Here we defined the: (I) semi-quantitative tyrosine kinome; (II) drug interactome; and (III) drug-induced effects on the tyrosine signalome. We first examined the entire proteome and kinome profile expressed in the A431 cell line, and determined the relative abundance of all possible protein targets of the tyrosine kinase inhibitors. We therefore utilized a ‘shot-gun’ approach that involved a multidimensional separation-based proteomic strategy. To ensure maximal protein identification, we reduced sample complexity by strong cation exchange (SCX) chromatography, and analyzed all fractions by LC-MS/MS. To determine the direct targets of the four selected TKIs, we subjected the proteins extracted from the A431 cancer cell line to drug-coupled affinity matrices. By comparing the pull-down results from a lysate with a second identical lysate to which 20 uM of the inhibitor had been added to block binding to the beads, we were able to identify selective binding interactors using LC-MS/MS. This differential affinity pull-downs approach allowed us to also quantitatively evaluate the binding affinity of the tyrosine kinases to each of the drugs. To evaluate the global tyrosine phosphorylation dynamics that occurs in A431 cells upon treatment with each of the four inhibitors, phosphotyrosine peptides were enriched and identified by quantitative mass spectrometry. For this purpose, we treated the cells with each inhibitor for 2 h at appropriate concentrations adopted from reported studies. Cells were lysed, and the proteins digested with the proteases Lys-C and trypsin. To allow quantitation of the phosphorylated peptides originating from the three different peptide pools (control/imatinib/dasatinib or control/bosutinib/nilotinib), we used stable isotope dimethyl labeling. Subsequently, tyrosine-phosphorylated peptides were enriched by immunoprecipitation and analyzed by LC-MS/MS.