SIGNAL TRANSDUCTION PATHWAYS IN STABLE PHASE CHRONIC MYELOID LEUKEMIA CELLS

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The Philadelphia chromosome translocation generates a chimeric oncogene in which the BCR gene and the c-ABL genes are fused. The product of this oncogene, p210BCR/ABL, has elevated ABL tyrosine kinase activity, relocates to the cytoskeleton, and phosphorylates several cellular proteins. BCR/ABL transforms hematopoietic cells, induces factor-independence, reduction of apoptosis, and alters adhesion of CML cells. However, at a biochemical level, the mechanisms by which BCR/ABL transforms myeloid cells are poorly understood. Several substrates of the BCR/ABL kinase have been identified, including c-BCR, p120rasGAP, c-CBL, p52SHC, p93FES, p95VAV, p125FAK, p68paxillin, and p72SHPTP2. Also, BCR/ABL has been shown to bind directly to GRB2 at Y177 of BCR, and therefore potentially activating p21 ras. However, it has been difficult to determine the significance of any of these potential BCR/ABL substrates, in part due to the complexity of studying a large protein with many potential signaling motifs. One approach to simplifying BCR/ABL biology has been to examine primary human CML cells, rather than cell lines made to overexpress BCR/ABL. Interestingly, in primary leukemic cells, there are only a few proteins which are phosphorylated by BCR/ABL. This suggests that studies in primary CML cells, rather than tissue culture cell lines, may be more reliable in terms of identifying important signaling pathways. In preliminary studies we found that there is only a single tyrosine phosphoprotein complexed with BCR/ABL in CML neutrophils, recently identified as CRKL, AN sh2/sh3 "adapter" protein. CRKL binds to BCR/ABL through its SH3 domain. In additional studies, we have identified two cellular proteins which bind to the CRKL S2 domain in CML cells. The first protein is a component of focal adhesions, p68 paxillin, and the second is a 130 kDa protein termed CAS for "CRK associated substrate". In preliminary studies, we cloned the human and chicken pavilion genes, and identified sites for binding CRKL SH2 and other proteins. The central hypothesis to be tested here is that the interaction of BCR/ABL with the CRKL adapter protein is important in the pathogenesis of stable phase CML. Our preliminary data suggest that CRKL, through binding to paxillin and or CAS, may be activating a pathway which regulates integrin function, viability, or proliferation, and these hypotheses will be tested. It is anticipated that these results will improve our understanding of the pathogenesis of CML.