The 'hallmarks of cancer' are generally accepted as a set of genetic and epigenetic alterations that a normal cell must accrue to transform into a fully malignant cancer. It follows that therapies designed to counter these alterations might be effective as anti-cancer strategies. Over the past 30 years, research on the BCL-2-regulated apoptotic pathway has led to the development of small-molecule compounds, known as 'BH3-mimetics', that bind to pro-survival BCL-2 proteins to directly activate apoptosis of malignant cells. This Timeline article focuses on the discovery and study of BCL-2, the wider BCL-2 protein family and, specifically, its roles in cancer development and therapy.

Escape from apoptosis is a key attribute of tumour cells and facilitates chemo-resistance. The 'BCL-2-regulated' or 'intrinsic' apoptotic pathway integrates stress and survival signalling to govern whether a cancer cell will live or die. Indeed, many pro-apoptotic members of the BCL-2 family have demonstrated tumour-suppression activity in mouse models of cancer and are lost or repressed in certain human cancers. Conversely, overexpression of pro-survival BCL-2 family members promotes tumorigenesis in humans and in mouse models. Many of the drugs currently used in the clinic mediate their therapeutic effects (at least in part) through the activation of the BCL-2-regulated apoptotic pathway. However, initiators of this apoptotic pathway, such as p53, are mutated, lost or silenced in many human cancers rendering them refractory to treatment. To counter such resistance mechanisms, a novel class of therapeutics, 'BH3-mimetics', has been developed. These drugs directly activate apoptosis by binding and inhibiting select antiapoptotic BCL-2 family members and thereby bypass the requirement for upstream initiators, such as p53. In this review, we discuss the role of the BCL-2 protein family in the development and treatment of cancer, with an emphasis on mechanistic studies using well-established mouse models of cancer, before describing the development and already recognised potential of the BH3-mimetic compounds.

Intrinsic apoptosis, reliant on BAX and BAK, has been postulated to be fundamental for morphogenesis, but its precise contribution to this process has not been fully explored in mammals. Our structural analysis of BOK suggests close resemblance to BAX and BAK structures. Notably, BokBaxBak animals exhibited more severe defects and died earlier than BaxBak mice, implying that BOK has overlapping roles with BAX and BAK during developmental cell death. By analyzing BokBaxBak triple-knockout mice whose cells are incapable of undergoing intrinsic apoptosis, we identified tissues that formed well without this process. We provide evidence that necroptosis, pyroptosis, or autophagy does not substantially substitute for the loss of apoptosis. Albeit very rare, unexpected attainment of adult BokBaxBak mice suggests that morphogenesis can proceed entirely without apoptosis mediated by these proteins and possibly without cell death in general.

Although tumor development requires impaired apoptosis, we describe a novel paradigm of apoptosis-dependent tumorigenesis. Because DNA damage triggers apoptosis through p53-mediated induction of BH3-only proteins Puma and Noxa, we explored their roles in γ-radiation-induced thymic lymphomagenesis. Surprisingly, whereas Noxa loss accelerated it, Puma loss ablated tumorigenesis. Tumor suppression by Puma deficiency reflected its protection of leukocytes from γ-irradiation-induced death, because their glucocorticoid-mediated decimation in Puma-deficient mice activated cycling of stem/progenitor cells and restored thymic lymphomagenesis. Our demonstration that cycles of cell attrition and repopulation by stem/progenitor cells can drive tumorigenesis has parallels in human cancers, such as therapy-induced malignancies.

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Impaired apoptosis is a cancer hallmark, and some types of lymphomas and other cancers harbor mutations that directly affect key cell death regulators, such as Bcl-2 family members. However, because the majority of tumors seem to lack such mutations, we are examining the hypothesis that tumorigenesis can be sustained at least initially by the normal expression of specific endogenous pro-survival Bcl-2 family members. We previously demonstrated that the lymphomagenesis in ⌭-myc transgenic mice, which constitutively overexpress the c-Myc oncoprotein in B-lymphoid cells and develop pre-B and B-cell lymphomas, does not require endogenous Bcl-2. In striking contrast, we report here that loss in these mice of its close relative Bcl- IntroductionImpaired apoptosis is considered a prerequisite for the development of most, if not all, cancers, 1-3 yet the mechanisms that promote the survival of most nascent malignant cells during the process of neoplastic transformation remain unknown. Apoptosis is regulated by opposing factions of the Bcl-2 family, which include both proteins essential for cell survival and those that drive cell death. 4,5 Despite very similar biochemical functions, the pro-survival family members (Bcl-2, Bcl-x L , Bcl-w, Mcl-1, and A1) have proven to have essential functions in specific cell types. 5 The critical initiators of apoptosis are the "BH3-only" proteins (eg, Bim, Puma, Bid), so-called because they share only a single Bcl-2 homology (BH) domain with other family members. They are activated by developmental cues and diverse stress stimuli, including cytokine deprivation, DNA damage and activation of oncogenes, such as c-Myc. 6-10 A second proapoptotic Bcl-2 sub-family, principally represented by the multi-BH domain proteins Bax and Bak, mediates the pivotal downstream step of mitochondrial outer membrane permeabilization (MOMP), which evokes activation of the caspase cascade that demolishes the cell. 4,11 The BH3-only proteins may provoke activation of Bax and Bak by their direct engagement, by sequestering pro-survival relatives or both ways. 4,5,[12][13][14] Studies using transgenic mice have established that overexpression of Bcl-2 15-18 or its antiapoptotic relatives, including and 20,21 or loss of proapoptotic Bcl-2 family members, such as Bim 10 or Puma, 22-24 contribute to tumor development, particularly in conjunction with mutations that deregulate cell cycle control, such as enforced c-Myc expression. Importantly, lymphomas evoked by combined overexpression of Bcl-2 and c-Myc require sustained Bcl-2 overexpression, 25 most likely to counter the apoptosis promoted by c-Myc overexpression under stress conditions, such as limiting supply of growth factors. [26][27][28][29] However, because only a proportion of cancers contain cytogenetic alterations (chromosomal translocations or somatically acquired copy number alterations) that directly promote the overexpression of Bcl-2 or one of its homologs, 2,3,30 we surmise that most cells undergoing neoplastic transformation are sustained (at ...

Understanding the critical factors that govern recovery of the hematopoietic system from stress, such as during anticancer therapy and bone marrow transplantation, is of clinical significance. We investigated the importance of the prosurvival proteins myeloid cell leukemia-1 (MCL-1) and B-cell lymphoma–extra large (BCL-XL) in stem/progenitor cell survival and fitness during hematopoietic recovery from stress. Loss of a single Mcl-1 allele, which reduced MCL-1 protein levels, severely compromised hematopoietic recovery from myeloablative challenge and following bone marrow transplantation, whereas BCL-XL was dispensable in both contexts. We identified inhibition of proapoptotic p53 upregulated modulator of apoptosis (PUMA) as the key role of MCL-1 in both settings, with Mcl-1+/−;Puma−/− mice completely protected from the deleterious effects of loss of 1 Mcl-1 allele. These results reveal the molecular mechanisms that govern cell survival during hematopoietic recovery from stress.

Key Points MCL-1 is critical for thymic lymphoma development mediated by loss of p53. MCL-1 is essential for sustained growth of p53-deficient thymic lymphoma cells.