Synthesis of Bcl-2 in response to anthracycline treatment may contribute to an apoptosis-resistant phenotype in leukemic cell lines

Background: Some forms of chemoresistance in leukemia may start from failur e of tauour cells to successfully undergo apoptosis and Bcl-2 may play a ro le in this defect. Therefore, are evaluated the Bcl-2 content and synthesis in relation with the apoptotic potential in leukemic cell lines after anth racycline treatment. Methods: U937, HL60, and K562 cells and their drug resistant (DR) variants were treated with varying concentrations of Idarubicin (IDA). Apoptosis was evaluated by fluorescence microscopy after acridine orange staining. Bcl-2 and Pax content were evaluated either by flow cytometry after indirect imm unolabelling or by Western blot. Results: High Bcl-2 contents were not related to a poor ability to undergo apoptosis in U937, HL60, K562 and their DR variants. IDA induced a concentr ation-dependent: increase in Bcl-2 content in all cell lines as long as the y do not perform apoptosis. Enhanced Bcl-2 expression was inhibited by cycl oheximide, actinomycin D, or antisense oligonucleotide directed against bcl -2; mRNA. Bcl-2 expression was also increased in the resistant U937 variant after sei run deprivation or C2-ceramide treatment. The synthesis of Bcl-2 led to an increased Bcl-2/Bax ratio solely in the cells with an apoptosis- resistance phenotype. Conclusions: These data suggest chat exposure to IDA induces Bcl-2 expressi on in leukemic cell lines, and that this mechanism could contribute to apop tosis resistance and participate in the acquisition of chemoresistance. The y also confirm bra the evolution of the Bcl-2/Bax ratio reflects apoptotic ability better than eht steady state level of Bcl-2 expression. Cytometry 3 6:140-149, 1999. (C) 1999 Whiley-Liss,Inc.