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 phosphatid ylinositol-3 kinase PI-3k) --> Akt serine/threonine kinase pathway and of t he 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 Delta SH2 mutant (SH2 domain deleted) and of STAT5 by BCR /ABL Delta SH3 + Delta SH2 mutant (SH3 and SH2 domains deleted) is not perm anent and both Akt and STAT5 could be 'reactivated' by in vitro culture. Th is phenomenon was responsible for increased resistance to apoptosis, growth factor-independent proliferation and leukemogenesis in SCID mice. Incubati on 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 ac tivity. The immediate upstream activators of Akt and STAT5 such as PI-3k an d Jak-2 were also activated. In addition, the common beta subunit of IL-3/I L-5/GM-CSF receptor was tyrosine phosphorylated in the clones in which 'rea ctivation' was dependent on the BCR/ABL kinase activity, These results sugg ested that 're-activation' of Akt and STAT5, in the absence of functional B CR/ABL SH3+SH2 domains, may be achieved by two different mechanisms: (i) BC R/ABL kinase-dependent activation of alternative pathway(s) and (ii) additi onal genetic changes stimulating Akt and STAT5 independently of BCR/ABL.