Bcl-2-regulated apoptosis: mechanism and therapeutic potential
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).
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