Identification of BCR-ABL Dependent Gene Regulation by Using a Phosphoproteomics Approach.

Introduction: The selective tyrosine kinase inhibitor imatinib (formerly STI571, Glivecâ) has been shown to block phosphorylation of tyrosine residues by occupying the ATP binding site of the Abl tyrosine kinases Bcr-Abl, c-Abl, v-Abl and Abl-related gene (ARG), as well as platelet-derived growth factor receptor (PDGF) alpha and beta and of the receptor for human stem cell factor (SCF) c-kit. We chose a large scale phospho-proteomics approach to identify novel downstream targets of imatinib which could possibly be utilized for combined treatment strategies.

Material and Methods: Phospho-proteomics was performed by comparison of large scale phosphotyrosine-immunoprecipitation of imatinib (10 mM/2 hours) versus DMSO treated K562 cells separated by one-dimensional polyacrylamide gelelectrophoresis. In addition enriched CD34+ cells (>70%) of a newly diagnosed Bcr-Abl positive CML patient were used immediately after purification and treated in the same way as described above. Resulting differentially immuno-precipitated proteins were analyzed using matrix-assisted laser desorption/ionization - time of flight mass spectrometry (MALDI-TOF) and nano electrospray ionization tandem mass spectrometry (ESI-MS/MS). Protein identification via peptide mass-fingerprinting and peptide sequencing was performed using Mascot search tool and NCBI nr database. Differential phosphorylation was confirmed by combined immunoprecipitation and western blot analysis of selected candidate proteins.

Results: Phospho-proteomics of K562 cells revealed 8 differentially phosphorylated proteins after a two hour treatment with imatinib including the recently identified c-cbl, and Bcr-Abl itself, the latter confirming autophosphorylation. Ship2 which was originally identified as beeing constitutively phosphorylated in chronic myelogenous leukemia progenitor cells showed reduced, imatinib sensitive phosphorylation. Remaining candidates could be classified as being involved in protein folding or in ATPase activities associated with a variety of functions (type II AAA). The analysis of primary CD34+ cells from a CML patient showed a predominance of the nonmuscular myosin heavy chain protein in different molecular weight forms.

Discussion: We detected significant imatinib-dependent differences in protein phosphotyrosine-immunoreactivity of the Bcr-Abl -dependent cell line K562. Previously identified down-stream targets of Bcr-Abl could be confirmed and novel candidate proteins were identified. Phosphorylation of Ship2, a previously identified down-stream target of Bcr-Abl, was found to be inhibited by imatinib treatment. Ongoing studies are aimed at the characterization of the role of the identified phospho-proteins, particularly type II AAAs for Bcr-Abl induced signal transduction as well as for the development of resistance to imatinib.