Bcl-2-regulated apoptosis: mechanism and therapeutic potential

https://doi.org/10.1016/j.coi.2007.05.004Get rights and content

Apoptosis is essential for tissue homeostasis, particularly in the hematopoietic compartment, where its impairment can elicit neoplastic or autoimmune diseases. Whether stressed cells live or die is largely determined by interplay between opposing members of the Bcl-2 protein family. Bcl-2 and its closest homologs promote cell survival, but two other factions promote apoptosis. The BH3-only proteins sense and relay stress signals, but commitment to apoptosis requires Bax or Bak. The BH3-only proteins appear to activate Bax and Bak indirectly, by engaging and neutralizing their pro-survival relatives, which otherwise constrain Bax and Bak from permeabilizing mitochondria. The Bcl-2 family may also regulate autophagy and mitochondrial fission/fusion. Its pro-survival members are attractive therapeutic targets in cancer and perhaps autoimmunity and viral infections.

Introduction

Programmed cell death plays a critical role in both development and tissue homeostasis. Indeed, in the immune system, apoptosis shapes the immune repertoire and refines and terminates immune responses [1]. As impaired apoptosis can lead to either autoimmunity or malignancy, there is intense interest in uncovering its control mechanisms and exploring the therapeutic options they provide.

Apoptosis is precipitated by sequential activation of cysteine proteases of the caspase family, in two distinct but converging pathways [2, 3]. The extrinsic pathway activates Caspase-8 (and Caspase-10 in humans) when ligand-mediated trimerization of ‘death receptors’ of the tumor necrosis factor family on the plasma membrane recruits the adaptor protein FADD and the caspase into multi-protein complexes. The intrinsic pathway (also termed the ‘mitochondrial’ or ‘stress’ pathway) activates Caspase-9 on the scaffold protein Apaf-1 when cytochrome c is released from damaged mitochondria in response to diverse stresses, including cytokine deprivation and DNA damage. These initiator caspases can cleave and activate the effector caspases (Caspase-3, Caspase-6, and Caspase-7) that mediate cellular demolition by cleaving multiple critical cellular proteins.

The intrinsic pathway is controlled by the Bcl-2 protein family, which constitutes a tri-partite regulatory cassette [4]. Bcl-2 itself, the oncoprotein discovered via the chromosome translocation that hallmarks human follicular lymphoma, inhibits apoptosis, as do its close homologs Bcl-xL, Bcl-w, Mcl-1, A1, and (in humans) Bcl-B. In contrast, two other subgroups promote apoptosis: one comprises Bax and Bak (and the little-studied Bok), which also resemble Bcl-2, particularly in three ‘BH’ (Bcl-2 homology) domains, whereas the other group, which includes Bim, Bad, Bid, Bik, Bmf, Puma, Noxa, and Hrk, shares only the BH3 domain. When activated by stress signals, these ‘BH3-only’ proteins insert that domain, an amphipathic α-helix, into a hydrophobic groove on the pro-survival relatives [5]. This coupling primes the cell for apoptosis, but commitment requires activation of Bax and/or Bak [6, 7], which then form oligomers on intracellular membranes, including the mitochondrial outer membrane, and thereby perturb their integrity.

In reviewing the role of the Bcl-2 family in apoptosis, we will focus primarily on two areas receiving great attention: the vexed issue of how interactions between the warring Bcl-2 factions commit the cell to apoptosis and the exciting prospect of targeting this family as a new approach to the therapy of cancer and perhaps autoimmune and virally induced diseases. We also briefly discuss recent evidence that the family also influences autophagy and mitochondrial morphology. Further background of the Bcl-2 family and apoptosis is provided by recent reviews [1•, 4•, 8, 9•], including the accompanying ones.

Section snippets

Flipping the life–death switch

Activated BH3-only proteins were thought to bind indiscriminately to all their pro-survival counterparts until quantitative studies revealed marked differences [10••]. Bim, Puma, and tBid (the activated, truncated form of Bid) do bind avidly to all the pro-survival proteins, but the others associate only with subsets [10••]. For example, Noxa engaged only Mcl-1 and A1, and Bad engaged only Bcl-2, Bcl-xL, and Bcl-w. More importantly, the promiscuous binders killed much more potently than the

Death by default

In the indirect activation model, apoptosis is the default pathway and the Bcl-2 pro-survival proteins function mainly by constraining activation of Bax and/or Bak (Figure 2). The diverse phenotypes observed in mice on inactivation of individual pro-survival genes (see [4] and the review by Opferman in this issue) presumably indicate that specific pro-survival proteins are the dominant guards on Bax/Bak in different cell types. For example, a Bcl-xL/Bak switch primarily controls the life span

The prospect of BH3 mimetic drugs

As most tumors have defects in the p53 pathway or overexpress a Bcl-2 homolog, interest is surging in the potential of anticancer drugs that, like the BH3 domain, bind one or more Bcl-2 homologs and trigger apoptosis [4•, 9•]. Such ‘BH3 mimetics’ should be more effective than anti-tumor agents acting far upstream. Furthermore, BH3 selectivity [5, 10••] heralds the prospect of specifically targeting the Bcl-2 homolog(s) required to maintain a particular tumor type, sparing more of the normal

Links of the Bcl-2 family with autophagy and mitochondrial morphology

Autophagy is an ancient mechanism by which starved cells produce energy and stave off death by gradually targeting their organelles and cytoplasmic elements to lysosomes for digestion. Excessive self-cannibalization may represent a second form of programmed cell death [49•, 50]. Significantly, Bcl-2 and Bcl-xL associate with the evolutionarily conserved autophagy inducer Beclin-1, a haplo-insufficient tumor suppressor [51], and inhibit autophagy [52••]. The inhibition may require Bcl-2

Conundrums and controversies

Surprisingly, most Bax molecules probably translocate from the cytosol to membranes after caspases are activated [62•, 63, 64]. Bax translocation and cytochrome c release were slower in cells lacking both Caspase-3 and Caspase-7, implicating a proteolytic amplification loop [62]. Conceivably, these activated caspases augment the initial response by cleaving Bcl-2 pro-survival proteins or by generating tBid from full-length Bid. In any case, such findings imply that commitment to apoptosis

Concluding remarks

Both biochemical and genetic evidence now strongly supports the hypothesis that the BH3-only proteins trigger apoptosis primarily by engaging their pro-survival relatives (Figure 1b), freeing Bax and Bak (Figure 2) [10••, 19••, 20••, 21]. Nevertheless, in vitro data from liposome disruption [12•, 14•] and from some mutant proteins [16] argue that certain BH3-only proteins, such as tBid, can directly activate Bax/Bak (Figure 1a). Hence, direct activation may still have a role, perhaps in

Update

How the ER stress induced by misfolded proteins or certain drugs induces apoptosis has been poorly understood. New work shows that Bim is required in diverse cells, including thymocytes [71]. ER stress seemed to activate Bim in two ways: its transcription was induced directly by the CHOP-C/EBPα transcription factors, whereas Bim ubiquination and degradation was prevented via its dephosphorylation by protein phosphatase-2A. This work re-enforces the concept that ER stresses act predominantly

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

We thank our colleagues, particularly David Huang, Andreas Strasser, Peter Colman, Mark Hinds, Philippe Bouillet, Ruth Kluck, Jamie Fletcher, Simon Willis and Mark van Delft for valuable discussions on the issues addressed here. Our research is supported by a National Health and Medical Research Council Program Grant (257502), a Specialized Center of Research Grant from the Leukemia and Lymphoma Society, and project grants from the US National Cancer Institute (CA80188 and CA43540).

References (77)

  • E. White

    Mechanisms of apoptosis regulation by viral oncogenes in infection and tumorigenesis

    Cell Death Differ

    (2006)
  • M. Kvansakul et al.

    A structural viral mimic of pro-survival Bcl-2: a pivotal role for sequestering pro-apoptotic Bax and Bak

    Mol Cell

    (2007)
  • Y.-T. Hsu et al.

    Bax in murine thymus is a soluble monomeric protein that displays differential detergent-induced conformations

    J Biol Chem

    (1998)
  • S.J. Gardai et al.

    Phosphorylation of Bax Ser184 by Akt regulates its activity and apoptosis in neutrophils

    J Biol Chem

    (2004)
  • J.D. Sadowsky et al.

    Chimeric (alpha/beta + alpha)-peptide ligands for the BH3-recognition cleft of Bcl-XL: critical role of the molecular scaffold in protein surface recognition

    J Am Chem Soc

    (2005)
  • M.F. van Delft et al.

    The BH3 mimetic ABT-737 targets selective Bcl-2 proteins and efficiently induces apoptosis via Bak/Bax if Mcl-1 is neutralized

    Cancer Cell

    (2006)
  • S.K. Tahir et al.

    Influence of Bcl-2 family members on the cellular response of small-cell lung cancer cell lines to ABT-737

    Cancer Res

    (2007)
  • S. Chen et al.

    Mcl-1 down-regulation potentiates ABT-737 lethality by cooperatively inducing Bak activation and Bax translocation

    Cancer Res

    (2007)
  • X. Lin et al.

    ‘Seed’ analysis of off-target siRNAs reveals an essential role of Mcl-1 in resistance to the small-molecule Bcl-2/Bcl-X(L) inhibitor ABT-737

    Oncogene

    (2007)
  • V. Del Gaizo Moore et al.

    Chronic lymphocytic leukemia requires BCL2 to sequester prodeath BIM, explaining sensitivity to BCL2 antagonist ABT-737

    J Clin Invest

    (2007)
  • A.R. Shoemaker et al.

    A small-molecule inhibitor of Bcl-XL potentiates the activity of cytotoxic drugs in vitro and in vivo

    Cancer Res

    (2006)
  • X. Qu et al.

    Promotion of tumorigenesis by heterozygous disruption of the beclin 1 autophagy gene

    J Clin Invest

    (2003)
  • S. Pattingre et al.

    Bcl-2 antiapoptotic proteins inhibit Beclin 1-dependent autophagy

    Cell

    (2005)
  • Y. Zhu et al.

    Bax does not have to adopt its final form to drive T cell death

    J Exp Med

    (2006)
  • O. Kepp et al.

    Bak and Bax are non-redundant during infection- and DNA damage-induced apoptosis

    EMBO J

    (2007)
  • I. Kamer et al.

    Proapoptotic BID is an ATM effector in the DNA-damage response

    Cell

    (2005)
  • T. Kaufmann et al.

    The pro-apoptotic BH3-only protein Bid is dispensable for DNA damage and replicative stress-induced apoptosis and cell cycle arrest

    Cell

    (2007)
  • Puthalakath H, O’reilly LA, Gunn P, Lee L, Kelly PN, Huntington ND, Hughes PD, Michalak EM, McKimm-Breschkin J,...
  • Kuroda J, Kimura S, Strasser A, Andreeff M, O’Reilly LA, Ashihara E, Kamitsuji Y, Yokota A, Kawata E, Takeuchi M, et...
  • R. Mathew et al.

    Autophagy suppresses tumor progression by limiting chromosomal instability

    Genes Dev

    (2007)
  • A. Strasser

    The role of BH3-only proteins in the immune system

    Nat Rev Immunol

    (2005)
  • J.M. Adams

    Ways of dying: multiple pathways to apoptosis

    Genes Dev

    (2003)
  • Y. Shi

    Mechanical aspects of apoptosome assembly

    Curr Opin Cell Biol

    (2006)
  • M.G. Hinds et al.

    Regulation of apoptosis: uncovering the binding determinants

    Curr Opin Struct Biol

    (2005)
  • E.H. Cheng et al.

    BCL-2, BCL-xL sequester BH3 domain-only molecules preventing BAX- and BAK-mediated mitochondrial apoptosis

    Mol Cell

    (2001)
  • S.W. Fesik

    Promoting apoptosis as a strategy for cancer drug discovery

    Nat Rev Cancer

    (2005)
  • A. Letai et al.

    Distinct BH3 domains either sensitize or activate mitochondrial apoptosis, serving as prototype cancer therapeutics

    Cancer Cell

    (2002)
  • T. Kuwana et al.

    BH3 domains of BH3-only proteins differentially regulate Bax-mediated mitochondrial membrane permeabilization both directly and indirectly

    Mol Cell

    (2005)
  • Cited by (555)

    View all citing articles on Scopus
    View full text