Phosphatidylinositol 3-Kinase p85 Subunit-Dependent Interaction with BCR/ABL: Molecular Mechanism and Biological Consequences.

BCR/ABL fusion tyrosine kinase is responsible for the initiation and maintenance of the Philadelphia chromosome (Ph¹)-positive chronic myelogenous leukemia (CML) and a cohort of acute lymphocytic leukemias (ALL). Our previous studies showed that a signaling protein phosphatidylinositol-3 kinase (PI-3k) is essential for the growth of CML cells, but not of normal hematopoietic cells, and that p85 subunit of PI-3k co-immunoprecipitates with BCR/ABL (Skorski et al., (1995) Blood 86, 726–36; Skorski et al., (1997) Embo J 16, 6151–61; Klejman et al., (2002) Oncogene 21, 5868–76). Therefore, we made an attempt to better characterize the p85 - BCR/ABL interactions. Mutagenesis approach combined with pull-down assays indicated that both N-terminal and C-terminal SH2 domains (nSH2 and cSH2, respectively) and SH3 domain of p85 play an important role in the interaction with BCR/ABL. SH2 domains exerted their function through binding to the tyrosine-phosphorylated motifs, and SH3 domain recognized proline-rich regions. Disruption of these functions by introduction of point-mutations in nSH2, cSH2 and SH3 domains abrogated their interaction with BCR/ABL. p85 mutant protein (p85-mut) bearing these mutations was not able to interact with BCR/ABL, while its binding to the p110 catalytic subunit of PI-3k was intact. In addition, binding of Shc and Gab2, but not Crk-L, to p85-mut was abrogated. When expressed in BCR/ABL-transformed hematopoietic cells p85-mut diminished activation of Akt kinase, the downstream effector of PI-3k. This effect was associated with the inhibition of BCR/ABL-dependent growth of hematopoeitic cells line and murine bone marrow cells. Interestingly, addition of IL-3 rescued BCR/ABL-transformed cells from the inhibitory effect of p85-mut. SCID mice injected with BCR/ABL-positive hematopoietic cells expressing p85-mut survived longer in comparison to the animals inoculated with BCR/ABL-transformed counterparts transfected with empty plasmid. In conclusion, we have identified the domains of p85 responsible for the interaction with BCR/ABL. Moreover, we demonstrated that expression of p85-mut, which binds to the p110 catalytic subunit of PI-3k but is not able to interact with BCR/ABL, affected the growth ability of BCR/ABL-positive leukemia cells.