How do BCL-2 proteins induce mitochondrial outer membrane permeabilization?

Abstract

The mitochondrial pathway of apoptosis proceeds when molecules sequestered between the outer and inner mitochondrial membranes are released to the cytosol by mitochondrial outer membrane permeabilization (MOMP). This process is controlled by the BCL-2 family, which is composed of both pro- and anti-apoptotic proteins. Although there is no disagreement that BCL-2 proteins regulate apoptosis, the mechanism leading to MOMP remains controversial. Current debate focuses on what interactions within the family are crucial to initiate MOMP. Specifically, do the BH3-only proteins directly engage BAX and/or BAK activation or do these proteins solely promote apoptosis by neutralization of anti-apoptotic BCL-2 proteins? We describe these models and contend that BH3-only proteins must perform both functions to efficiently engage MOMP and apoptosis.

Figure 1

The BCL-2 family of proteins is divided into three functional groups based on their composition of BCL-2 homology domains. The anti-apoptotic members include BCL-2, BCL-xL, BCL-w, A1 and MCL-1 and contain four BCL-2 homology domains (designated BH1–4). The pro-apoptotic multi-domains (BAX, BAK and BOK*) contain BH1–3 domains (*there is little evidence that BOK is a functional effector molecule). The BH3-only proteins are structurally diverse and contain only one conserved region, the BH3 (e.g. leucine-x-x-x-x-aspartic acid, where x is any amino acid). Often, the BH3-only proteins are subdivided into direct activators (e.g. BID an d BIM) and de-repressors/sensitizers (e.g. BAD, BIK, BMF, bNIP3, HRK, Noxa and PUMA). The α helices of each protein are designated and the regions contained within each BH domain are illustrated by bold lines under each protein. The hydrophobic carboxyl terminal transmembrane domain (TM) of each protein is based on in silico predictions and/or structural data and is not necessarily present in each member. Also, a typical BH3 domain might not be absolutely required for every BH3-only protein to induce cell death; for example, deletion of the BH3 domain in bNIP3 does not alter its anti-apoptotic binding or pro-apoptotic activity [52].

Figure 2

The balance of anti-apoptotic and pro-apoptotic BCL-2 proteins dictates cellular fate. (a) The rheostat model. In a hypothetical basal state, the number of anti-apoptotic and pro-apoptotic molecules is equal; tipping this balance dictates cellular fate. If a stress (e.g. DNA damage) is applied, the induction of pro-apoptotic molecules provides the signal to engage MOMP. On the contrary, growth factor addition would promote cellular survival by increasing the amount of anti-apoptotic proteins. (b) The anti-apoptotic protein neutralization model. The BH3-only proteins BID, BIM and PUMA engage MOMP because they bind and neutralize all anti-apoptotic BCL-2 members. A combination of other BH3-only proteins is required to promote apoptosis because each neutralizes only a subset of anti-apoptotic BCL-2 proteins (e.g. BAD and Noxa neutralize BCL-2/BCL-xL/BCL-w and MCL-1/A1, respectively). This model contends that the BCL-2 effector molecules are sufficiently active to oligomerize and promote MOMP once anti-apoptotic proteins are neutralized.

Figure 3

The direct activation model: mechanisms of pro-apoptotic effector activation by BH3-only proteins. (a) Central to MOMP is the activation and oligomerization of BAX or BAK. These proteins, once activated by a BH3-only protein, create oligomeric proteolipid pores in the outer mitochondrial membrane that permit the release of intermembrane space proteins to the cytosol. (b) Direct activator BH3-only proteins (e.g. BIM and BID) induce the oligomerization and activation of BAX or BAK in the absence of other proteins. Through a transient interaction with BAX or BAK, the direct activator BH3-only proteins (BID is shown in this example), or peptides derived from the BH3 region, induce MOMP and cytochrome c release. This is often referred to as the ‘hit and run’ mechanism for effector activation. (c) A subset of BH3-only proteins, the de-repressors/sensitizers, cannot induce the activation of BAX or BAK alone. In this scenario, a direct activator BH3-only protein is sequestered by an anti-apoptotic BCL-2 protein. Following stress, a de-repressor/sensitizer BH3-only protein is induced, either by transcriptional up-regulation or by post-translat ional modification, and this protein then binds to an anti-apoptotic BCL-2 protein, promoting the release of a sequestered, direct activator BH3-only protein. In this example, B IM istonically sequestered by MCL-1, and the induction of Noxa enables the release of BIM to engage MOMP. If cells constitutively harbor a sequestered direct activator protein, they are referred to as being ‘primed for death’ or ‘BCL-2 addicted’. Not shown in this figure is the potential influence of Noxa-induced MCL-1 degradation after binding, which might have important implications in maintaining anti-apoptotic levels to preserve outer mitochondrial membrane integrity [53]. (d) Cells are sensitized to undergo MOMP when de-repressor/sensitizer BH3-only proteins are constitutively inhibiting anti-apoptotic BCL-2 proteins, and any future induction of BID or BIM cannot be tolerated. This scenario is referred to as ‘sensitized for death’. In this example, BCL-xL is inhibited by BAD, and the induction of BIM engages MOMP; in the absence of B AD expression, the MOMP signal would have been inhibited by BCL-xL.