Signal transducer and activator of transcription (STAT)5 activation by BCR/ABL is dependent on intact Src homology (SH)3 and SH2 domains of BCR/ABL and is required for leukemogenesis

Signal transducer and activator of transcription (STAT)5 is constitutively activated in BCR/ABL-expressing cells, but the mechanisms and functional co nsequences of such activation are unknown. We show here that BCR/ABL induce s phosphorylation and activation of STAT5 by a mechanism that requires the BCR/ABL Src homology (SH)2 domain and the proline-rich binding site of the SH3 domain. Upon expression in 32Dcl3 growth factor-dependent myeloid precu rsor cells, STAT5 activation-deficient BCR/ABL SH3+SH2 domain mutants funct ioned as tyrosine kinase and activated Ras, but failed to protect from apop tosis induced by withdrawal of interleukin 3 and/or serum and did not induc e leukemia in severe combined immunodeficiency mice. In complementation ass ays, expression of a dominant-active STAT5B mutant (STAT5B-DAM), but not wi ld-type STAT5B (STAT5B-WT), in 32Dcl3 cells transfected with STAT5 activati on-deficient BCR/ABL SH3+SH2 mutants restored protection from apoptosis, st imulated growth factor-independent cell cycle progression, and rescued the leukemogenic potential in mice. Moreover, expression of a dominant-negative STAT5B mutant (STAT5B-DNM) in 32Dcl3 cells transfected with wild-type BCR/ ABL inhibited apoptosis resistance, growth factor-independent proliferation , and the leukemogenic potential of these cells. In retrovirally infected m ouse bone marrow cells, expression of STAT5B-DNM inhibited BCR/ABL-dependen t transformation. Moreover, STAT5B-DAM, but not STAT5B-WT, markedly enhance d the ability of STAT5 activation-defective BCR/ABL SH3+SH2 mutants to indu ce growth factor-independent colony formation of primary mouse bone marrow progenitor cells. However, STAT5B-DAM did not rescue the growth factor-inde pendent colony formation of kinase-deficient K1172R BCR/ABL or the triple m utant Y177F+R522L+ Y793F BCR/ABL, both of which also fail to activate STAT5 . Together, these data demonstrate that STAT5 activation by BCR/ABL is depe ndent on signaling from more than one domain and document the important rol e of STAT5-regulated path ways in BCR/ABL leukemogenesis.