Bcl-2 expression restores the leukemogenic potential of a BCR/ABL mutant defective in transformation

Growth factor-dependent hematopoietic cell lines expressing the BCR/ABL onc aprotein of the Ph chromosome show growth factor-independent proliferation and resistance to apoptosis. Apoptosis resistance of BCR/ABL-expressing cel ls may depend on enhanced expression of anti-apoptotic proteins as well as reduced expression and/or inactivation of pro-apoptotic proteins. Compared to myeloid precursor 32DcI3 cells expressing wild type BCR/ABL, cells expre ssing a BCR/ABL mutant lacking amino acids 176-426 in the BCR domain (p185 Delta BCR) are susceptible to apoptosis induced by interleukin-8 (IL-3) dep rivation. These cells exhibited the hypophosphorylated apoptotic BAD and ma rkedly reduced levels of Bcl-2, Upon ectopic expression of Bcl-2, these cel ls showed no changes in BAD phosphorylation, but they became apoptosis-resi stant and proliferated in the absence of IL-3, albeit more slowly than cell s expressing wild type BCR/ABL. Moreover, the p185 Delta BCR/Bcl-2 double t ransfectants were leukemogenic when injected into immunodeficient mice, but Bcl-2 expression did not restore the leukemia-inducing effects of p185 Del ta BCR to the levels of wild type BCR/ABL. Leukemic cells recovered from th e spleen of mice injected with p185 Delta BCR/Bcl-2 cells did not show rear rangements in the Bcl-2 genomic locus, but they exhibited enhanced prolifer ation in culture and induced a rapidly fatal disease process when inoculate d in secondary recipient mice. Together, these data support the importance of anti-apoptotic pathways for BCR/ABL-dependent leukemogenesis and suggest that Eel-2 expression promotes secondary changes leading to a more aggress ive tumor phenotype. (C) 2000 by The American Society of Hematology.