BCL-2 proteins are critical for cell survival and are overexpressed in many tumors. ABT-737 is a small-molecule BH3 mimetic that exhibits single-agent activity against lymphoma and small-cell lung cancer in preclinical studies. We here report that ABT-737 effectively kills acute myeloid leukemia blast, progenitor, and stem cells without affecting normal hematopoietic cells. ABT-737 induced the disruption of the BCL-2/BAX complex and BAK-dependent but BIM-independent activation of the intrinsic apoptotic pathway. In cells with phosphorylated BCL-2 or increased MCL-1, ABT-737 was inactive. Inhibition of BCL-2 phosphorylation and reduction of MCL-1 expression restored sensitivity to ABT-737. These data suggest that ABT-737 could be a highly effective antileukemia agent when the mechanisms of resistance identified here are considered.

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Certain proteins or activities are present in mitotic cells but not in interphase cells. These proteins may be synthesized or activated, or both, just prior to mitosis and are responsible for the breakdown of the nuclear envelope and the condensation of chromosomes. To learn more about the nature of these proteins, we raised monoclonal antibodies to mitotic cells. Spleen cells from mice immunized with a 0.15 M NaCl extract of synchronized mitotic HeLa cells were fused with SP2/0-Agl4 mouse myeloma cells, and hybrids were selected in medium containing hypoxanthine, methotrexate, thymidine, and glycine. Two different hybridoma clones secreting antibodies reactive with mitotic and meiotic cells from every species tested were isolated. Chromosomes as well as cytoplasm in mitotic cells reacted with the antibodies, as detected by indirect immunofluorescence. The proteins from mitotic cells were separated by electrophoresis in NaDodSO4/polyacrylamide slab gels, transferred to nitrocellulose sheets, and stained immunochemically. The two antibodies, designated MPM-1 and MPM-2, recognize a family of polypeptides with apparent molecular masses of 0.40 to >200 kilodaltons (kDa). Both antibodies reacted strongly with three polypeptide bands of 182 kDa, 118 kDa, and 70 kDa. Only mitotic cells exhibited the protein bands that were recognized by the antibodies. All these bands were found to be phosphoproteins as shown by 32P labeling and autoradiography and their removal by alkaline phosphatase treatment.The nucleus of a mammalian cell undergoes profound reorganization when the cell enters mitosis. The nuclear envelope breaks down, and the chromatin condenses into discrete chromosomes. Although the initiation of mitosis is dependent on both RNA and protein synthesis until 2 hr and 1 hr, respectively, before mitosis, the addition of inhibitors of RNA and protein synthesis to cells after these times does not affect either initiation or completion of mitosis (1, 2).The fusion of a mitotic cell with an interphase cell can induce nuclear envelope breakdown and premature chromosome condensation of the interphase nucleus (3). During this process, [3H]tryptophan-labeled proteins from mitotic cells labeled at G2 phase become associated with the prematurely condensed chromosomes, whereas the grain density on the metaphase chromosomes is not changed (4). Chromosome condensation and germinal vesicle (nucleus) breakdown can be induced in frog oocytes by the injection of extracts from mitotic but not interphase cells (5). The activity is labile to heat, sensitive to protease, insensitive to nuclease, and stabilized by phosphatase inhibitors and is found both in the cytoplasm and on the chromosomes of mitotic cells (6). These results suggest that certain nonhistone proteins, possibly phosphoproteins, of mitotic cells are not present in interphase cells. Al-Bader et al. (7) have reported the presence of such mitotic-specific proteins by using two-dimensional gel electrophoresis.The objective of this study was to raise monoclonal antib...