Progressive changes in the leukemogenic signaling in BCR/ABL-transformed cells

Our previous study indicated that BCR/ABL SH2 domain and BCR/ABL SH3 domain + SH2 domain complex are required for immediate activation of the phosphatidylinositol-3 kinase PI-3k)→ Akt serine/threonine kinase pathway and of the signal transducer and activator of transcription 5 (STAT5), respectively, in hematopoietic cells. We show here that the defect in activation of PI-3k/Akt by BCR/ABL ΔSH2 mutant (SH2 domain deleted) and of STAT5 by BCR/ABL ΔSH3 + ΔSH2 mutant (SH3 and SH2 domains deleted) is not permanent and both Akt and STAT5 could be 'reactivated' by in vitro culture. This phenomenon was responsible for increased resistance to apoptosis, growth factor-independent proliferation and leukemogenesis in SCID mice. Incubation of cells with BCR/ABL tyrosine kinase inhibitor STI571 abrogated the 're-activation' of Akt or STAT5 by BCR/ABL SH3 + SH2 mutants in some clones, in the others Akt and STAT5 activation became independent on BCR/ABL kinase activity. The immediate upstream activators of Akt and STAT5 such as PI-3k and Jak-2 were also activated. In addition, the common β subunit of IL-3/IL-5/GM-CSF receptor was tyrosine phosphorylated in the clones in which 'reactivation' was dependent on the BCR/ABL kinase activity. These results suggested that 're-activation' of Akt and STAT5, in the absence of functional BCR/ABL SH3 + SH2 domains, may be achieved by two different mechanisms: (i) BCR/ABL kinase-dependent activation of alternative pathway(s) and (ii) additional genetic changes stimulating Akt and STAT5 independently of BCR/ABL.