The BH3-mimetic ABT-737 and an orally bioavailable compound of the same class, navitoclax (ABT-263), have shown promising antitumor efficacy in preclinical and early clinical studies. Although both drugs avidly bind Bcl-2, Bcl-x L , and Bcl-w in vitro, we find that Bcl-2 is the critical target in vivo, suggesting that patients with tumors overexpressing Bcl-2 will probably benefit. In human non-Hodgkin lymphomas, high expression of Bcl-2 but not Bcl-x L predicted sensitivity to ABT-263. Moreover, we show that increasing Bcl-2 sensitized normal and transformed lymphoid cells to ABT-737 by elevating proapoptotic Bim. In striking contrast, increasing Bcl-x L or Bcl-w conferred robust resistance to ABT-737, despite also increasing Bim. Cell-based protein redistribution assays unexpectedly revealed that ABT-737 disrupts Bcl-2/Bim complexes more readily than Bcl-x L /Bim or Bcl-w/Bim complexes. These results have profound implications for how BH3-mimetics induce apoptosis and how the use of these compounds can be optimized for treating lymphoid malignancies. (Blood. 2012;119(24):5807-5816) IntroductionDefects in the mitochondrial apoptotic pathway regulated by the Bcl-2 family of proteins play a major role in cancer development and in conferring chemoresistance. 1 Within the family, Bax and Bak are essential for mitochondrial membrane permeabilization and cell death. 2 Prosurvival proteins (Bcl-2, Bcl-x L , Bcl-w, Mcl-1, A1) oppose Bax and Bak and ensure mitochondrial integrity and cell survival. 1 These prosurvival proteins also interact with distant relatives that share only 1 Bcl-2 Homology region, BH3, that is critical for their proapoptotic function. The BH3-only proteins such as Bim, Bad, Puma, and Noxa act as stress sensors and relieve the inhibition of Bax and Bak by the prosurvival proteins.The clinical efficacy of most anticancer therapeutics primarily reflects their ability to induce apoptosis. Resistance to conventional anticancer therapeutics (eg, etoposide) is often because of a failure to activate BH3-only proteins, for example because of mutation of the tumor suppressor p53, which is critical for transcriptional induction of Puma and Noxa after DNA damage. 3 Overexpression of prosurvival Bcl-2 proteins, or silencing of BH3-only protein expression, are also associated with inferior therapeutic outcomes. 4,5 BH3-mimetic drugs, such as ABT-737, were developed to directly counter such apoptotic blocks. ABT-737 binds avidly to Bcl-2, Bcl-x L , and Bcl-w, but not Mcl-1 or A1. 6,7 In preclinical studies, it demonstrated single agent efficacy against tumors with low Mcl-1 or A1 levels, such as follicular lymphoma (FL), chronic lymphocytic leukemia (CLL), and small cell lung carcinoma (SCLC). ABT-737 shows limited toxicity toward normal cells, although there is transient reduction of platelets and lymphocytes. 6,8 , an orally bioavailable compound in the same class with similar target specificity, also exhibited efficacy in various cancer-derived cell lines both in vitro and in vivo, 6,9,10 and is undergoing p...

The transcriptional regulator c-MYC is abnormally overexpressed in many human cancers. Evasion from apoptosis is critical for cancer development, particularly c-MYC-driven cancers. We explored which anti-apoptotic BCL-2 family member (expressed under endogenous regulation) is essential to sustain c-MYC-driven lymphoma growth to reveal which should be targeted for cancer therapy. Remarkably, inducible Cre-mediated deletion of even a single Mcl-1 allele substantially impaired the growth of c-MYC-driven mouse lymphomas. Mutations in p53 could diminish but not obviate the dependency of c-MYC-driven mouse lymphomas on MCL-1. Importantly, targeting of MCL-1 killed c-MYC-driven human Burkitt lymphoma cells, even those bearing mutations in p53. Given that loss of one allele of Mcl-1 is well tolerated in healthy tissues, our results suggest that therapeutic targeting of MCL-1 would be an attractive therapeutic strategy for MYC-driven cancers.

Interleukin (IL)-7 receptor (R) signaling is essential for T and B lymphopoiesis by promoting proliferation, differentiation, and survival of cells. Mice lacking either IL-7 or the IL-7Rα chain have abnormally low numbers of immature as well as mature T and B lymphocytes. Transgenic expression of the apoptosis inhibitor Bcl-2 rescues T cell development and function in IL-7Rα–deficient mice, indicating that activation of a proapoptotic Bcl-2 family member causes death of immature and mature T cells. BH3-only proteins such as Bim, which are distant proapoptotic members of the Bcl-2 family, are essential initiators of programmed cell death and stress-induced apoptosis. We generated Bim/IL-7Rα double deficient mice and found that loss of Bim significantly increased thymocyte numbers, restored near normal numbers of mature T cells in the blood and spleen, and enhanced cytotoxic T cell responses to virus infection in IL-7Rα−/− mice. These results indicate that Bim cooperates with other proapoptotic proteins in the death of IL-7–deprived T cell progenitors in vivo, but is the major inducer of this pathway to apoptosis in mature T cells. This indicates that pharmacological inhibition of Bim function might be useful for boosting immune responses in immunodeficient patients.

Deregulated over-expression of MYC is implicated in the development and malignant progression of most (~70%) human tumors. MYC drives cell growth and proliferation but also, at high levels, promotes apoptosis. Here, we report that the proliferative capacity of MYC-driven normal and neoplastic B lymphoid cells depends on MNT, a MYC-related transcriptional repressor. Our genetic data establish that MNT synergises with MYC by suppressing MYC-driven apoptosis and that it does so primarily by reducing the level of pro-apoptotic BIM. In Em-Myc mice, which model the MYC/IGH chromosome translocation in Burkitt's lymphoma, homozygous Mnt deletion greatly reduced lymphoma incidence by enhancing apoptosis and markedly decreasing premalignant B lymphoid cell populations. Strikingly, by inducing Mnt deletion within transplanted fully-malignant Em-Myc lymphoma cells, we significantly extended transplant recipient survival. The dependency of lymphomas on MNT for survival suggests that drugs inhibiting MNT could significantly boost therapy of MYC-driven tumors by enhancing intrinsic MYC-driven apoptosis.

The pro-apoptotic BH3-only protein Bim has a major role in hematopoietic homeostasis, particularly in the lymphocyte compartment, where it strongly affects immune function. The three major Bim isoforms (Bim EL , Bim L and Bim S ) are generated by alternative splicing. Bim EL , the most abundant isoform, contains a unique sequence that has been reported to be the target of phosphorylation by several MAP kinases. In particular, Erk1/2 has been shown to interact with Bim EL through the DEF2 domain of Bim EL and specifically phosphorylate this isoform, thereby targeting it for ubiquitination and proteasomal degradation. To examine the physiological importance of this mechanism of regulation and of the alternative splicing of Bim, we have generated several Bim knock-in mouse strains and analyzed their hematopoietic system. Although mutation in the DEF2 domain reduces Bim EL degradation in some circumstances, this mutation did not significantly increase Bim's pro-apoptotic activity in vivo nor impact on the homeostasis of the hematopoietic system. We also show that Bim EL and Bim L are interchangeable, and that Bim S is dispensable for the function of Bim. Hence, we conclude that physiological regulation of Bim relies on mechanisms independent of its alternative splicing or the Erk-dependent phosphorylation of Bim EL .