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(Circulation Research. 2004;95:957.)
© 2004 American Heart Association, Inc.

Review

The Mitochondrial Death Pathway and Cardiac Myocyte Apoptosis

Michael T. Crow, Kartik Mani, Young-Jae Nam, Richard N. Kitsis

From the Department of Medicine (M.T.C.), Johns Hopkins University School of Medicine, Baltimore Md; and the Departments of Medicine and Cell Biology, Cardiovascular Research Center (K.M., Y.-J.N., R.N.K.), and Cancer Center (R.N.K.), Albert Einstein College of Medicine, Bronx, NY.

Correspondence to Michael T. Crow, Department of Medicine, Johns Hopkins University School of Medicine, 5501 Hopkins Bayview Circle, Rm 5A.58, Baltimore, MD 21224; E-mail mcrow1{at}jhmi.edu; and to Richard N. Kitsis, Cardiovascular Research Center, Albert Einstein College of Medicine, 1300 Morris Park Ave, Bronx, NY 10461. E-mail kitsis{at}aecom.yu.edu

This Review is part of a thematic series on Mitochondrial Dysfunction in Ischemia, which includes the following articles:

Role of the Mitochondrial Permeability Transition in Myocardial Disease

Primary and Secondary Signaling Pathways in Early Preconditioning That Converge on the Mitochondria to Produce Cardioprotection

Evidence for Mitochondrial K⁺ Channels and their Role in Cardioprotection

The Mitochondrial Death Pathway and Cardiac Myocyte Apoptosis
Elizabeth Murphy, Guest Editor; Roberto Bolli, Editor

	Abstract

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
Apoptosis has been causally linked to the pathogenesis of myocardial infarction and heart failure in rodent models. This death process is mediated by two central pathways, an extrinsic pathway involving cell surface receptors and an intrinsic pathway using mitochondria and the endoplasmic reticulum. Each of these pathways has been implicated in myocardial pathology. In this review, we summarize recent advances in the understanding of the intrinsic pathway and how it relates to cardiac myocyte death and heart disease.

Key Words: apoptosis • necrosis • cell death • mitochondria • Bcl-2 • caspase • death-inducing signaling complex • apoptosome • ischemia • heart failure

	Introduction

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
Over the past decade, interest in cell death has intensified among scientists in multiple areas of biology and medicine. This fascination has been driven by the discovery that apoptosis is mediated by an ancient program that is hard-wired into all metazoan cells. Renewed attention in the cardiovascular field has been fueled by the notion that cell death is often an active process that, in principle, can be inhibited in various disease states.

	Types of Cell Death

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
Three modes of death are currently defined by morphological criteria: apoptosis, autophagy, and necrosis. Work over the past 20 years has provided a molecular framework for apoptosis,¹ the subject of this review. Autophagy is an important cellular process in which proteins and organelles are degraded in the lysosomal pathway so that their constituents can be used as energy substrates by the cell.² There exists considerable debate, however, as to whether autophagy functions as a death process.³ The mechanisms that mediate necrosis in mammals are poorly understood. Recent studies in Caenorhabditis elegans, however, demonstrate that necrosis is carried out by specific cellular pathways,^4–6 suggesting that this form of cell death may be more "programmed" than initially thought. With further understanding of the mechanisms that mediate various types of cell death, it is likely that future classification systems will be based on molecular, rather than morphological, criteria.

	Apoptosis

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
Apoptosis is an evolutionarily conserved suicide process that plays critical roles in embryonic development and in the homeostasis, remodeling, surveillance, and host defenses of postnatal tissues.⁷ The pathways that mediate apoptosis are more than one billion years old and are central to such fundamental biological processes as growth, proliferation, differentiation, death, inflammation, and immunity. Hence, apoptosis is essential for life itself. Conversely, dysregulation of apoptosis, resulting in either too little or too much cell death, has been implicated in human disease.⁸ For example, insufficient apoptosis may contribute to carcinogenesis, whereas excessive apoptosis may be a component in the pathogenesis of stroke, myocardial infarction, and heart failure. It is interesting to speculate why a pathological role for apoptosis has persisted in the face of evolutionary change. One possibility is that the importance of apoptosis in basic biological functions outweighs its potentially detrimental effects. Another likely explanation is that most diseases involving excessive apoptosis occur in postreproductive life.

Cardiac myocytes undergo apoptosis in response to a myriad of stimuli including hypoxia,⁹ especially followed by reoxygenation,¹⁰ acidosis,¹¹ oxidative stress,¹² serum deprivation,^13,14 glucose deprivation and metabolic inhibition,^15,16 ß₁-adrenergic agonists,^17–19 stretch,²⁰ angiotensin II,²¹ tumor necrosis factor-,²² Fas ligand,^23,24 and anthracyclines.²⁵ In intact animals, cardiac myocyte apoptosis occurs during myocardial infarction,^26–29 especially followed by reperfusion,^30,31 heart failure^32–38 and various cardiomyopathic states,³⁹ myocarditis,⁴⁰ and transplant rejection.⁴¹ The strength of the data differ for these syndromes but are most compelling for ischemia-reperfusion injury and heart failure.

During ischemia-reperfusion in rodents and humans, 5% to 30% of cardiac myocytes in the area at risk undergo apoptosis within 16 hours.^26,29,31,42 Although controversy exists as to whether this cell death occurs within the central infarct or border zones, most data suggest that both areas are involved. In contrast to this large burst of death during ischemia-reperfusion that takes place over a short period of time, heart failure involves low, but abnormal, rates of cardiac myocyte apoptosis that persist for months (0.023% in rodents and 0.08% to 0.25% in humans compared with 0.001% to 0.002% in normal rodent and human controls).^32,35–37

To elucidate whether cardiac myocyte apoptosis plays a causal role in the pathogenesis of any cardiac syndrome, it is necessary to determine whether inhibiting this death process ameliorates the expected pathological changes in cardiac structure and function. Genetic approaches (discussed below) have established that inhibition of cardiac myocyte apoptosis reduces infarct size 50% to 70% and decreases cardiac dysfunction after ischemia-reperfusion.^23,43–46 Although pharmacological studies have been less consistent, caspase inhibitors have reduced infarct size by 21% to 52%.^47–50 These data suggest that cardiac myocyte apoptosis plays a critical role in the pathogenesis of ischemia-reperfusion injury.

Rodent studies have also implicated low rates of cardiac myocyte apoptosis in the pathogenesis of heart failure. An apoptotic rate as low as 0.023% is sufficient to cause a lethal, dilated cardiomyopathy within 8 to 24 weeks in transgenic mice with cardiac-restricted expression of an inducible caspase-8 allele.³² Of note, this apoptotic rate is 5- to 10-fold lower than that measured in cardiac tissue from patients with end-stage heart failure, suggesting that apoptosis may also be a causal mechanism of human heart failure.^35–37 In addition, the structural and functional abnormalities in this model can largely be rescued by caspase inhibition. Caspase inhibition has also been demonstrated to decrease apoptosis, improve cardiac function, and, most strikingly, ablate mortality in the peripartum cardiomyopathy resulting from myocardial Gq overexpression,⁵¹ a model of hypertrophic signaling.⁵² These data suggest that cardiac myocyte apoptosis plays a causal role in the pathogenesis of heart failure. A caveat pertaining to both the ischemia-reperfusion and heart failure studies is that they are limited to rodent models. Whether the data "translate" to humans remains to be shown. If so, however, cardiac myocyte apoptosis may constitute a novel target for therapies directed against myocardial infarction and heart failure.

	Central Apoptotic Pathways

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
Apoptosis is mediated by two central pathways (Figure 1): the extrinsic (or death receptor) pathway and the intrinsic (or mitochondrial) pathway.¹ The immediate objectives of apoptotic signaling are the activation of procaspases and the disabling of mitochondrial function. Caspases, a subclass of cysteine proteases that cleave substrates after aspartic acid residues, are central to the execution of apoptosis.⁵³ These proteases are synthesized as largely inactive zymogens (procaspases) containing an N-terminal prodomain and a C-terminal catalytic domain consisting of 20 kDa (p20) and 10 kDa (p10) subdomains. Human caspases are subdivided into upstream (apical, signaling) caspases (caspases 2, 8, 9, 10, and 12) and downstream (effector, executioner) caspases (caspases 3, 6, and 7). Upstream procaspases are activated by dimerization.^54–56 In contrast, downstream procaspases, which exist as preformed inactive dimers, are activated by proteolytic cleavage usually performed by already activated upstream caspases. The cleavage events take place after aspartic acid residues located in small linker regions between the prodomain, p20, and p10 subunits.⁵⁷ This allows two p20 and two p10 subunits to reassemble noncovalently into the active downstream caspase. After activation, downstream caspases perform the proteolytic destruction of the cell.⁵⁸

View larger version (26K): [in this window] [in a new window]   Figure 1. Central apoptosis pathways. In the extrinsic pathway, death receptor activation stimulates DISC assembly (see text) leading to procaspase-8 activation. Activated caspase-8 then cleaves downstream procaspase-3, which proteolyses cellular substrates killing the cell. In the intrinsic pathway, intracellular and extracellular death signals are transmitted to the mitochondria through BH3-only proteins (eg, Bid) and Bax, which translocates to the outer mitochondrial membrane. Bax and Bak (not shown) stimulate the release of cytochrome c (green), Smac/DIABLO (blue), and other apoptogens (not shown). This process is opposed by Bcl-2 and Bcl-xL (not shown). Cytochrome c, dATP, Apaf-1, and procaspase-9 assemble into the apoptosome (see text) leading to procaspase-9 activation, which subsequently activates procaspase-3. XIAP inhibits activation of procaspase-9 as well as active caspases-9, 3, and 7 (not shown). Smac/DIABLO and Omi/HtrA2 (not shown) bind XIAP, relieving this inhibition. Bid, a direct substrate of caspase-8, connects the extrinsic and intrinsic pathways. After cleavage, its C-terminus translocates to and inserts into the outer mitochondrial membrane triggering activation of Bax and Bak and cytochrome c release.

	Extrinsic Pathway

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
Although the focus of this review is the intrinsic pathway, the extrinsic pathway will be considered briefly because the two pathways are intimately entwined. Extrinsic signaling is initiated by the binding of a death ligand trimer to its cognate cell surface receptor,⁵⁹ which also exists as a preformed trimer.⁶⁰ The ligand may be an integral membrane protein on the surface of a second cell (eg, Fas [CD95/Apo-1] ligand) or a soluble extracellular protein (eg, tumor necrosis factor-). Ligand binding initiates the formation of a multiprotein complex termed the death-inducing signaling complex (DISC).^61–63 Using the Fas death receptor as an example, the binding of Fas ligand is presumed to induce a conformational change in Fas. This results in the recruitment of the adaptor protein FADD (Fas-associated via death domain) to Fas through interactions involving death domains in each of the proteins.^64,65 FADD, in turn, recruits procaspase-8 through death effector domains in each protein.^62,63 Thus, the net result of death receptor activation is the recruitment of procaspase-8 into the DISC. This, in turn, results in procaspase-8 dimerization and activation.⁵⁶ Once activated, caspase-8 cleaves and activates downstream procaspase-3 and Bid (BH3 [B cell leukemia/lymphoma-2 {Bcl-2} homology domain 3] interacting domain death agonist),^66–68 a proapoptotic Bcl-2 protein, which links the extrinsic and intrinsic pathways.

	Intrinsic Pathway: Premitochondrial Events

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
In contrast to the extrinsic pathway that transduces a specialized set of death stimuli, the intrinsic pathway integrates a broad spectrum of extracellular and intracellular stresses. Extracellular stimuli include deficiencies in survival/trophic factors/nutrients, radiation, and other chemical (eg, drugs) and physical stresses, whereas intracellular stimuli include oxidative stress, DNA damage, and protein misfolding. Intrinsic death signaling is mediated by a complex interplay between stimulus, upstream pathways, and sometimes the endoplasmic reticulum. This myriad of signals converges on the mitochondria leading to dysfunction of this organelle, the release of apoptogenic proteins, and the activation of caspases.

The link between apoptotic stimuli and the central apoptotic machinery is provided by a multitude of peripheral signaling pathways, which are not the subjects of this review. Moreover, in many instances, the precise connections linking these pathways to the central death machinery remain sketchy. It is clear, however, that most, if not all, apoptotic stimuli eventually funnel into the proapoptotic Bcl-2 proteins.¹ The Bcl-2 family can be divided into the following: (1) antiapoptotic members (eg, Bcl-2 and Bcl-x_L [Bcl-x protein long isoform]) and (2) proapoptotic members, which are further subdivided into (a) multidomain proapoptotics (eg, Bax [Bcl-2–associated X protein] and Bak [Bcl-2–antagonist/killer]) and (b) BH3-only proapoptotics (eg, Bid, Bad [Bcl-2-antagonist of cell death], Bim [Bcl-2 interacting mediator of cell death], Bmf [Bcl-2 modifying factor], Noxa [for damage], Puma [p53 upregulated modulator of apoptosis], BNip3 [Bcl-2/adenovirus E1B nineteen kDa-interacting protein 3], and Nix [Nip3-like protein X]/BNip3L [BNip3-like protein]). The antiapoptotics and multidomain proapoptotics contain BH1–3 domains. In addition, some, but not all, of the antiapoptotics contain a BH4 domain. The BH3-only proteins are homologous to these two groups only in the 10 to 16 residue BH3 domain. Surprisingly, the three-dimensional structures of all 3 groups of proteins are similar, with each containing 8 to 9 -helices.⁶⁹

Multidomain Proapoptotic Bcl-2 Proteins
Bax⁷⁰ and Bak^71–73 are multidomain proapoptotic Bcl-2 proteins. Either Bax or Bak is required for all instances of apoptosis mediated via the intrinsic pathway.⁷⁴ Cells lacking both proteins are resistant to activators of this pathway. Thus, these proteins control access of upstream apoptotic signals to the mitochondria. Conformational changes regulate the activation of both Bax and Bak. Bax is held in an inactive state in the cytosol by the insertion of its most C-terminal -helix (9) into a hydrophobic cleft created by its BH1–3 domains.⁷⁵ In response to apoptotic stimuli, this helix is presumed to move out of the cleft causing a conformational change that also affects the N-terminus. These events trigger Bax translocation to mitochondria, oligomerization, and insertion into the outer mitochondrial membrane via its C-terminal tail, which includes 9. The precise sequence of these events is unclear. Bax then stimulates the release of cytochrome c and other apoptogenic mitochondrial proteins into the cytosol. Although the precise molecular events that mediate Bax conformational activation are not understood, caspase-2⁷⁶ and several Bax-binding proteins are involved. The latter include proteins that inhibit Bax activation (Ku70,⁷⁷ Humanin,⁷⁸ and ARC [apoptosis repressor with a CARD {caspase recruitment domain}^79,80), as well as promote Bax activation (ASC [apoptosis-associated speck-like protein containing a CARD]⁸¹). In addition, p53, whose abundance increases in response to genotoxic insults, hypoxia, and other cellular stresses, conformationally activates Bax through mechanisms that are not understood because the two proteins do not appear to interact.⁸² In addition to these examples of nonproteolytic conformational activation, Bax activity may also be augmented by removal of its inhibitory N-terminus by calpain.^83,84 As calpain has been implicated in necrosis,³ this event may provide a connection between necrotic and apoptotic pathways. Bax may also be transcriptionally upregulated during apoptosis by p53.⁸⁵ Bax plays a critical role in ischemia-reperfusion injury as illustrated by the 50% reductions in infarct size exhibited by isolated hearts from Bax knockout mice.⁸⁶

Bak also undergoes conformational activation, but the mechanism differs from that of Bax. In the case of Bak, the C-terminal tail does not mediate intramolecular inhibition because it is already involved in anchoring Bak constitutively to the outer mitochondrial membrane. Instead, Bak inhibition is provided by VDAC 2 (voltage-dependent anion channel 2), an outer mitochondrial membrane protein that may be a component of the mitochondrial permeability transition pore (MPTP),⁸⁷ and Mcl-1 (myeloid cell leukemia sequence 1 isoform 2), an antiapoptotic Bcl-2 protein.⁸⁸ On initiation of apoptosis, VDAC2 is displaced from Bak by activated (truncated) Bid, Bim, and Bad, whereas Mcl-1 binding to Bak is disrupted by p53. Bak conformational activation stimulates its intramembranous homoligomerization and triggers the release of apoptogenic mitochondrial proteins into the cytoplasm.

BH3-Only Bcl-2 Proteins
BH3-only proteins function as sentinels that survey upstream apoptotic signals and transmit them directly or indirectly to Bax and Bak. In contrast to Bax and Bak, however, the BH3-only proteins are often specialized to transduce only a subset of specific stimuli. For example, Bim senses cytokine deprivation in lymphocytes,⁸⁹ whereas Puma responds to p53.^90,91 In general, the ability of BH3-only proteins to kill is dependent on the BH3 domain. The activation of the BH3-only proteins involves diverse mechanisms. Bid⁹² is cleaved by caspase-8^66–68 in the death receptor pathway, calpain,⁹³ and granzyme B^94,95 to expose its BH3 domain. Caspase-3 can also cleave and activate Bid after the onset of apoptosis as part of a positive feedback loop.⁹⁶ After cleavage, the truncated C-terminal portion of Bid (tBid) translocates to the mitochondria and inserts into the outer membrane via its tail.^68,97 This leaves its BH3 domain facing the cytosol, where it can bind to Bak,⁹⁷ resulting in the displacement of VDAC2 from Bak⁸⁷ and Bak activation.⁹⁷ The BH3 domain of Bid also interacts with Bax,⁹² facilitating its insertion into the outer mitochondrial membrane.⁹⁸ Bid thus functions as a direct conduit between the extrinsic and intrinsic pathways. This link is important in the pathogenesis of myocardial infarction as evidenced by 53% reductions in infarct size and improved cardiac function in Bid^–/– mice subjected to ischemia-reperfusion (P. Lee, S.J. Korsmeyer, and R.N. Kitsis, unpublished data).

Other post-translational mechanisms that activate BH3-only proteins include release from the microtubule and actin cytoskeletons in the case of Bim⁹⁹ and Bmf,¹⁰⁰ respectively. Bad¹⁰¹ is regulated by phosphorylation of specific serine residues by a variety of survival kinases, leading to its sequestration by the 14 to 3-3 protein.¹⁰² Loss of survival factors, such as insulin-like growth factor-1, lead to Bad dephosphorylation, resulting in its translocation to the mitochondria, where Bad activates Bak by displacing VDAC2.⁸⁷ Interestingly, a dominant-negative mutant of 14 to 3-3 predisposes cardiac myocytes to pressure overload-induced apoptosis in vivo,¹⁰³ although this effect cannot be attributed specifically to Bad release, because 14 to 3-3 also binds other proapoptotic molecules.

Transcriptional induction of gene expression is central to the regulation of other BH3-only proteins including Noxa, Puma, BNip3, and Nix/BNip3L. The transcription factor p53 activates the expression of Noxa¹⁰⁴ and Puma,^90,91 whereas the transcription of BNip3^105,106 and Nix/BNip3L¹⁰⁵ is upregulated by hypoxia-inducible factor-1. Levels of BNip3 increase in response to hypoxia and acidosis in cardiac myocytes and in failing hearts.^107,108 Moreover, BNip3 is required for hypoxia–acidosis-induced cardiac myocyte death.¹⁰⁷ The abundance of Nix/BNip3L is increased by hemodynamic overload and the transgenic overexpression of Gq,¹⁰⁹ a signaling molecule important in the hypertrophic program.⁵² Furthermore, Nix/BNip3L is necessary for the peripartum cardiomyopathy of Gq transgenic mice.¹⁰⁹ Both BNip3 and Nix/BNip3L are situated at the mitochondria during apoptosis, and the transmembrane domain is required for this localization and the subsequent killing.¹¹⁰ In contrast to other BH3-only proteins, however, the "BH3-like" domain of BNip3 is not needed for killing,¹¹⁰ raising the possibility that another effector motif or distinct mechanism mediates its cytotoxicity.

Antiapoptotic Bcl-2 Proteins
Multidomain and BH3-only proapoptotic Bcl-2 proteins are antagonized by antiapoptotic family members, including Bcl-2^111–113 and Bcl-x_L.¹¹⁴ Although the latter were among the first apoptotic regulators to be discovered, their precise mechanisms of action, including how they inhibit mitochondrial cytochrome c release,^115,116 remain unclear. Bcl-2 is constitutively localized at the outer mitochondrial membrane, endoplasmic reticulum (ER), and nuclear envelope,¹¹⁷ and is thought to face the cytoplasm. Although Bcl-x_L is also found at these membranes, a significant pool is not membrane-bound.¹¹⁸ The rheostat hypothesis holds that the ratio of Bcl-2 or Bcl-x_L to Bax or Bak determines whether the cell lives or dies.⁷⁰ The physical basis for this hypothesis was thought to be the direct interaction of Bcl-2 or Bcl-x_L with Bax or Bak. Some data, however, raise the possibility that previously demonstrated interactions between these proteins may be an artifact of the nonionic detergents in the buffers used.¹¹⁸ Moreover, genetic data show that Bcl-2 and Bax are able to exert their effects independently of one another.¹¹⁹ These observations suggest that Bcl-2 and Bcl-x_L may exert their prolife effects through mechanisms other than direct interactions with Bax and Bak. One postulated mechanism is that Bcl-2 and Bcl-x_L function as "sinks" that sequester BH3-only proteins, such as tBid, preventing these BH3-only proteins from activating Bax and Bak.¹²⁰ Consistent with this model, a Bcl-x_L mutant, which is defective for Bax and Bak binding but competent to bind tBid, is still cytoprotective. In contrast, a mutant defective for binding tBid is unable to protect.¹²⁰ Other BH3-only proteins, such as Bad, may facilitate apoptosis by binding to Bcl-2 and displacing tBid.¹²¹ Antiapoptotic Bcl-2 proteins can markedly affect myocardial ischemia-reperfusion injury. For example, transgenic mice with cardiac-restricted overexpression of Bcl-2 demonstrate 48% to 64% decreases in infarct size.^43,44

	Tumor Suppressor p53

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
The transcription factor p53 mediates apoptosis in response to diverse stimuli including hypoxia, oxidative stress, and DNA damage.¹²² The abundance and activity of p53 are controlled at multiple levels by these and other stimuli. In turn, p53 induces apoptosis by transactivating the expression of multiple proapoptotic genes, including bax, noxa, puma, bid, asc, apaf-1, caspase-6, and fas.^81,122 It is also well-known that transcriptionally defective p53 mutants retain the ability to kill, potentially through their activation of Bax⁸² and Bak⁸⁸ proteins, as described.

In cardiac myocytes, hypoxia increases p53 levels, and p53 alone suffices to kill normoxic cardiac myocytes.¹²³ Despite this, p53 deficiency does affect ischemia-reperfusion–induced cardiac myocyte apoptosis.^11,29 One important caveat concerning this conclusion, however, is that changes in infarct size, the bottom-line parameter of myocardial damage, were not measured in these early studies. For this reason, a re-evaluation of the role of p53 in ischemia-reperfusion injury might be warranted. Stretch of cardiac myocytes has also been noted to increase p53 levels through a mechanism involving secreted angiotensin II and activation of the type 1 angiotensin II receptor.¹²⁴ In turn, p53 transactivates the expression of the genes encoding angiotensinogen and the type I angiotensin II receptor.

	ER Pathway

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
The mitochondria are appropriately viewed as the central organelle in the intrinsic pathway. In some circumstances, however, the ER (or sarcoplasmic reticulum in muscle cells) plays an important role in the mitochondrial death pathway, as well as perhaps mediating cell death independently of mitochondria. For example, stimuli, including ceramide, arachidonic acid, and, interestingly, oxidative stress, appear to require the ER death pathway for efficient killing.¹²⁵

Although the mechanisms by which the ER brings about cell death are poorly understood, increases in intracellular Ca²⁺ appear to be central. ER Ca²⁺ stores are thought to be increased by Bax and Bak,¹²⁵ which are located at ER, as well as mitochondrial, membranes.¹²⁶ Conversely, Bcl-2, which also resides at the ER membrane,¹¹⁷ decreases steady-state ER Ca²⁺ stores.^127,128 Increased ER Ca²⁺ facilitates a more robust release of Ca²⁺ into the cytoplasm on delivery of an apoptotic stimulus.

Increased cytoplasmic Ca²⁺ may activate several apoptotic mechanisms. First, mitochondrial Ca²⁺ overload can trigger MPTP opening and cytochrome c release.¹²⁹ Interestingly, this mechanism may be amplified by a positive feedback loop in which cytochrome c binds the inositol 1,4,5-trisphosphate (IP3) receptor, one of the ER Ca²⁺ release channels, to further stimulate Ca²⁺ release.¹³⁰ Second, increased intracellular Ca²⁺ can activate calpain. As discussed previously, calpain can cleave Bid, providing another mechanism for cytochrome c release.⁹³ An additional consequence of calpain activation is cleavage of procaspase-12.¹³¹ Caspase-12, which is present in some, but not all, human populations (attributable to polymorphisms with non-sense mutations¹³²), has been shown in knockout mice to be required for apoptosis induced specifically by ER stress.¹³³ How caspase-12 signals downstream to bring about cell death is not certain, but recent data suggest that cleaved caspase-12 translocates to the cytoplasm and activates caspase-9 independently of apoptosome formation.^134,135 These events provide a mitochondria-independent mechanism for ER-mediated apoptosis. In addition to its multiple roles in apoptotic signaling, increased intracellular Ca²⁺ has also been implicated in necrotic pathways through its effects on mitochondria¹³⁶ and calpains.³ Thus, Ca²⁺ may be a point of convergence between apoptotic and necrotic pathways. The putative role of Ca²⁺ in cell death raises an important question, however. How do cardiac myocytes avoid mitochondrial dysfunction and cell death in the face of the relentless wide cyclic swings in cytoplasmic Ca²⁺ concentrations (10^–7 M in diastole to 10^–6 M in systole) with each contraction?¹³⁷

Some signals that activate the ER death pathways originate within this organelle itself, where a complex array of pathways mediate the unfolded protein and other ER stress responses.¹³⁸ In addition, given their roles in carrying upstream apoptotic stimuli to Bax and Bak at the mitochondria, BH3-only proteins would be anticipated to perform an analogous function in the ER pathway. In fact, the BH3-only proteins Bik (Bcl-2–interacting killer)¹³⁹ and Puma¹⁴⁰ have been implicated in the ER death pathway. It remains unclear, however, whether these proteins function to relay signals from the periphery to the ER and/or from the ER to mitochondria. However, upstream signals originating in the extrinsic pathway are known to be linked with the ER by Bap31 (B-cell receptor–associated protein 31), an integral ER membrane protein that is cleaved by caspase-8 resulting in ER Ca²⁺ release.¹⁴¹

Relatively few studies have explored the role of the ER pathway in cardiac myocyte apoptosis. Transgenic overexpression of heat shock protein-70 (Hsp-70) in the heart decreases infarct size after ischemia-reperfusion.¹⁴² It remains to be determined whether cardioprotection results from the role of Hsp-70s as a chaperone (reducing ER stress) and/or to its inhibition of apoptosome assembly^143,144 or apoptosis-inducing factor (AIF).¹⁴⁵ Transverse aortic constriction has been shown to induce an ER stress response in the heart that is associated with apoptosis, and inhibition of the angiotensin II type 1 receptor decreases both the ER stress response and apoptosis.¹⁴⁶ Future work will be required to precisely define the role of the ER death pathway in heart disease.

	Intrinsic Pathway: Intramitochondrial Events

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
Mitochondria play an important role in transmitting and amplifying death signals. In the intrinsic pathway, they function as the interface between upstream apoptotic pathways and the caspases and other downstream death machinery. The most critical mitochondrial events during apoptosis are the structural and functional remodeling of this organelle and subsequent release of apoptogenic proteins into the cytosol. These proteins include cytochrome c,^116,147 Smac/DIABLO (second mitochondria-derived activator of caspase/direct IAP-binding protein with low pI),^148,149 Omi/HtrA2 (high temperature requirement protein A2),^150,151 AIF,¹⁵² and EndoG (Endonuclease G).¹⁵³ The release of these proteins activates apoptotic events in the cytosol and nucleus. Although cytochrome c participates in respiration, the functions of the other apoptogens under normal conditions have not been established definitively.

Mechanisms Underlying Mitochondrial Apoptogen Release
The mechanisms that mediate the release of the apoptogenic mitochondrial proteins are poorly understood. Cytochrome c has been studied most intensively in this context. Although it has been known for some time that cytochrome c release is stimulated by proapoptotic Bcl-2 proteins (eg, Bax) and inhibited by antiapoptotic Bcl-2 proteins (eg, Bcl-2), the precise mechanisms by which this apoptogen gains access to the cytosol are not known. A number of models have been proposed. One of the earliest postulated that cytochrome c release is linked to the mitochondrial permeability transition (MPT) (Figure 2A), an event triggered by changes in the permeability of the inner mitochondrial membrane. In healthy cells, the inner mitochondrial membrane is relatively impermeable, as it must be to maintain the proton gradient driving respiration and the osmotic gradient created by the high concentration of metabolites in the mitochondrial matrix. Under stress conditions (eg, death signals), the MPTP opens to allow the passage of molecules <1.5 kDa, including protons and water. The proton gradient and electrical potential across the inner mitochondrial membrane collapses, leading to uncoupling of oxidative phosphorylation. The influx of water, attributable to the osmotic pressure resulting from the protein- and metabolite-dense mitochondrial matrix, leads to swelling of the matrix and expansion of the highly convoluted inner mitochondrial membrane. Although the inner mitochondrial membrane possesses sufficient surface area to accommodate this expansion without rupturing, the ability of the outer mitochondrial membrane to expand is limited. Thus, expansion of the inner mitochondrial membrane causes the outer mitochondrial membrane to rupture and release cytochrome c and other apoptogenic proteins into the cytosol.

View larger version (59K): [in this window] [in a new window]   Figure 2. Models of cytochrome c release. A, MPT model. In healthy cells (left panel), the MPTP functions as a nucleotide exchanger. In response to stress signals, ANT undergoes a conformational change that converts it from a ligand-specific translocase to a nonspecific pore that collapses the inner mitochondrial membrane (IMM) potential and allows the entry of water down its osmotic gradient (middle panel). The subsequent swelling of the IMM ruptures the outer mitochondrial membrane (OMM), releasing cytochrome c (blue) (right panel). D denotes cyclophilin D. B, Bax–VDAC model. In healthy cells (left), VDAC functions as a component of the nucleotide exchange complex and restricts the passage of cytochrome c. During apoptosis (right), Bax binds VDAC changing its properties to permit cytochrome c release. C, Bax pore models. In healthy cells (left), 15% of cytochrome c is in the mitochondrial intermembrane space and 85% in cristae that are sequestered from the intermembrane space because of narrow junctions. During apoptosis, remodeling of cristae widens the junctions, allowing the majority of cytochrome c to access the intermembrane space. Bax is postulated to form protein pores or to stimulate the formation of lipid pores in the OMM, through which cytochrome c translocates into the cytosol.

Although the precise molecular composition of the MPTP is currently undergoing investigation, its core components are thought to be the adenine nucleotide translocase (ANT) located in the inner mitochondrial membrane and cyclophilin D, a peptidyl prolyl cis-trans isomerase that interacts with ANT.¹²⁹ Other components may include VDAC and the peripheral benzodiazepine receptor, which are located in the outer mitochondrial membrane. In healthy mitochondria, the close association of VDAC and ANT create a macromolecular complex that shuttles adenine nucleotides between the ATP-producing matrix and ATP-consuming cytosol. MPTP opening can be triggered under stress conditions, however, by increases in Ca²⁺, oxidative stress, depletion of adenine nucleotides, increases in inorganic phosphate, and depolarization of the inner mitochondrial membrane, stimuli that operate during ischemia-reperfusion.¹²⁹ Although proapoptotic (Bax and Bak) and antiapoptotic (Bcl-2 and Bcl-x_L) Bcl-2 proteins have been reported to bind ANT and VDAC,^87,154–157 the functional significance of these interactions remains controversial.

Cytochrome c release caused by MPT-mediated rupture of the outer mitochondrial membrane may occur during necrosis and forms of apoptosis associated with mitochondrial calcium overload including ischemia-reperfusion.¹³⁶ It fails, however, to account for many instances of apoptosis in which cytochrome c release occurs before or without mitochondrial swelling or collapse of the inner mitochondrial membrane potential.^158,159 In addition, whereas MPT is often blocked by caspase inhibition, cytochrome c release is not.¹⁵⁸ In fact, caspase activation resulting from cytochrome c release can stimulate MPT,¹⁶⁰ which acts as an amplifying mechanism for further cytochrome c release. These data argue against a role for MPT in the initiation of cytochrome c release in many instances of apoptosis.

Given the limitations of the MPT to explain the first steps in cytochrome c release, other models have focused primarily on more selective permeabilization of the outer mitochondrial membrane. Bax and Bak play central roles in these models by inducing other proteins to form channels for cytochrome c release, forming channels themselves, or inducing lipid channel formation. Regarding the first of these models, an interaction between Bax and VDAC has been noted previously.¹⁵⁷ Moreover, addition of Bax to VDAC-containing liposomes changes the properties of VDAC from that of a voltage-dependent and anion-selective channel to a voltage-independent and nonselective pore sufficient in size to allow the release of cytochrome c (Figure 2B).¹⁵⁶ Questions about the importance of VDAC in this process have been raised, however, by yeast studies in which the absence of VDAC does not affect Bax-stimulated cytochrome c release.¹⁶¹ In addition, as discussed, VDAC2 may actually be antiapoptotic through its interaction with and inhibition of Bak.⁸⁷ Of course, the possibility remains that Bax may modulate cytochrome c release via other yet to be identified proteins, an area of active investigation.

In the second model, Bax and Bak themselves are postulated to form pores in the outer mitochondrial membrane (Figure 2C). The first hint that Bcl-2 proteins might be capable of pore formation was provided by the three dimensional structure of Bcl-x_L, which resembles diphtheria toxin and the colicins.¹⁶² In fact, Bax, Bcl-2, and Bcl-x_L are able to form ion channels in artificial membranes,^163–165 and under some conditions, Bax channels are inhibited by Bcl-2.¹⁶³ Moreover, Bax can form pores in liposomes capable of releasing cytochrome c.¹⁶⁶

Some studies, however, demonstrate that Bax-induced pores do not show discrete conductance states characteristic of protein-lined pores^167,168 and are sensitive to the lipid composition of the planar membrane in which they reside.¹⁶⁹ These properties suggest a third model in which Bax induces the formation of lipid pores (Figure 2C). In support of this model, Bax reduces the linear tension of membranes, a property that could facilitate the rearrangement of membrane phospholipids into pores or pore-like structures.¹⁶⁷ Monomeric Bax and tBID together or oligomerized Bax on its own have been shown to produce pores in outer mitochondrial membranes and liposomes that allow the release of molecules up to 2 MDa in size.¹⁶⁹ These Bax-induced pores are also sensitive to the source of phospholipids used to produce the liposomes, with lipids from the outer mitochondrial membrane markedly more effective than those from microsomal membranes. This preference may be attributable to the presence of cardiolipin. When incorporated into chemically defined liposomes, cardiolipin markedly increases Bax-induced permeabilization.¹⁶⁹ These data suggest a role for Bax-induced lipid remodeling in the permeabilization of the outer mitochondrial membrane.

The mechanisms that mediate release of the other mitochondrial apoptogens, all of which are larger than cytochrome c, are even less well-understood. Caspase inhibition differentially affects the release of these proteins suggesting the involvement of different release mechanisms. In contrast to cytochrome c release, which is caspase-independent,¹⁵⁸ translocation of AIF and EndoG is sensitive to caspase inhibition.^170,171 Smac/DIABLO release was also originally thought to be caspase-dependent,¹⁷² but additional work suggests that its mitochondrial release as well as that of Omi/HtrA2 is caspase-independent.¹⁷¹ Clearly, further work is needed to understand the mechanisms that regulate the release of these apoptogens.

Mechanisms of Mitochondrial Remodeling During Apoptosis
Although permeabilization of the outer mitochondrial membrane could explain how cytochrome c gains access to the cytosol, it fails to account for the completeness of its release observed in many apoptotic models. This is because only 15% of cytochrome c resides in the mitochondrial intermembrane space, where it is freely available for discharge.¹⁷³ The remainder of the cytochrome c is located in the cristae. This pool of cytochrome c is prevented from accessing the mitochondrial intermembrane space under normal conditions because of the narrowness of cristae junctions. During apoptosis, mitochondria undergo a remodeling process in which these junctions widen from 19 to 57 nm (shown schematically in Figure 2C).¹⁵⁹ In addition, remodeling results in increased interconnectivity among cristae. These changes facilitate a redistribution of the majority of cytochrome c to the mitochondrial intermembrane space so that after outer membrane permeabilization, most cytochrome c can be released into the cytoplasm.¹⁷⁴

Remodeling of the cristae is mediated by a mechanism that is distinct from that responsible for permeabilization of the outer mitochondrial membrane.¹⁵⁹ Using tBid to induce apoptosis, remodeling has been shown to be inhibited by cyclosporin A but to occur independently of Bax, Bak, and the BH3 domain of tBid. Because cyclopsporin A inhibits MPTP opening, these characteristics suggest involvement of the MPTP in the mobilization of intracristal cytochrome c stores. In fact, transient opening of the MPTP occurs in this process but does not progress to mitochondrial swelling or loss of the inner mitochondrial membrane potential. In contrast to remodeling of the inner mitochondrial membrane, tBid-induced permeabilization of the outer mitochondrial membrane requires the BH3 domain of tBid, as well as Bax or Bak.

Similar remodeling changes have been reported in cardiac myocytes treated with hydrogen peroxide.¹⁷⁵ Despite this, the rapidity and completeness of cytochrome c release in cardiac myocytes may be less than that in cell lines induced with ultraviolet radiation¹⁷⁴ or in isolated mitochondria stimulated with tBid.¹⁵⁹ Further work will be required to define the dynamics of cytochrome c release in cardiac myocytes.

	Intrinsic Pathway: Postmitochondrial Events

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Apoptosome Formation
Once cytochrome c is released into the cytoplasm, it binds WD40 repeats in the C-terminus of the adaptor protein Apaf-1 (apoptotic protease activating factor-1).^176,177 In addition, dATP and/or ATP, which are already present in the cytoplasm, interact with the nucleotide binding domain in the central portion of Apaf-1. These events are thought to stimulate a conformational change in Apaf-1 that results in its homo-oligomerization (mediated through the nucleotide binding domain) and the recruitment of procaspase-9 (mediated through caspase recruitment domains [CARD] in Apaf-1 and procaspase-9).^177–187 This >1 MDa complex, termed the apoptosome, appears as a wheel with heptad symmetry (Figure 1).^181,187 The forced approximation of the upstream procaspase-9 in the apoptosome leads to its activation. As with procaspase-8 activation in the DISC, dimerization of procaspase-9 appears to be the major activating event, with the subsequent proteolytic processing playing a less important role.^54–56 Once activated, caspase-9 cleaves and activates downstream procaspase-3, which is also recruited into the apoptosome.¹⁸⁸ Caspase-3, in turn, feed backs on and further activates procaspase-9.¹⁸⁹

Cells that are genetically deficient for cytochrome c,¹⁹⁰ Apaf-1,¹⁹¹ or procaspase-9^192,193 show marked resistance to activators of the mitochondrial pathway. Transgenic mice with cardiac-restricted overexpression of either of two procaspase-9 dominant-negative mutations exhibit 53% and 68% reductions in infarct size, respectively, and improved cardiac function after ischemia-reperfusion (C.-F. Peng, G. Tremp, A. Silberstein, and R.N. Kitsis, unpublished data). These data demonstrate that postmitochondrial events in the intrinsic pathway are important in the pathogenesis of myocardial ischemia-reperfusion injury.

Inhibitors of Apoptosis
Several endogenous proteins, which contain baculovirus inhibitor of apoptosis repeats (BIR),¹⁹⁴ antagonize the postmitochondrial steps in the intrinsic pathway. One such protein, XIAP (X-linked inhibitor of apoptosis), binds to and inhibits caspases-9, 3, and 7.¹⁹⁵ These interactions involve BIR3 in the case of caspase-9, and the BIR1–2 linker in the case of caspases-3 and 7.^196–199 Inhibition of these already activated caspases is achieved by blocking substrate access to the active sites. XIAP also inhibits procaspase-9 activation in the apoptosome by preventing its dimerization.²⁰⁰ In this manner, XIAP protects against the destruction of the cell by the accidental activation of caspases. Other BIR proteins cIAP1 and 2 (cellular inhibitor of apoptosis 1 and 2) act in a similar fashion. Thus, inhibitor of apoptosis (IAP) proteins inhibit both upstream caspase-9, the major initiator caspase in the intrinsic pathway, and downstream caspases-3 and 7.

The release of Smac/DIABLO and Omi/HtrA2 from mitochondria to the cytosol opposes the inhibition of caspases by XIAP.^148,149,151 This relief of inhibition is mediated by binding of Smac/DIABLO and Omi/HtrA2 to XIAP, displacing the caspases. In addition to this mechanism, Omi/HtrA2 cleaves and irreversibly inactivates XIAP through its serine protease activity.²⁰¹ Thus, Smac/DIABLO and Omi/HtrA2 act as "inhibitors of the inhibitor," permitting activation of these caspases. The roles of Smac/DIABLO and Omi/HtrA2 emphasize that although release of cytochrome c is necessary, it is not sufficient in many situations to bring about cell death.²⁰² Inactivation of the "cell death brakes" by these other apoptogens is also often necessary for apoptosis to occur.

Interestingly, the IAPs also possess a RING domain, which functions as a ubiquitin E3 ligase that mediates ubiquitin-dependent protein degradation.²⁰³ Targets of this ligase include proapoptotic proteins such as caspases-3 and 7, as well as Smac.^204–207 This provides an additional mechanism for IAP-mediated cytoprotection. IAPs, however, also target themselves for degradation through self-ubiquitination,²⁰³ presumably as a mechanism to remove themselves rapidly and efficiently, so as to allow apoptosis to proceed in the face of an overwhelming cellular insult. The mechanisms that differentially direct IAP E3 ligase activity toward proapoptotic versus antiapoptotic proteins remain to be determined.

	Apoptosis-Inducing Factor

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Another mitochondrial protein that promotes cell death when released into the cytoplasm is AIF, a flavoprotein with oxidoreductase activity.¹⁵² In response to death signals, AIF translocates from the mitochondria to the nucleus in a manner dependent on PARP (poly[ADP-ribose] polymerase) activation.^152,208 Moreover, AIF is required for cell death mediated by PARP, which is activated by genotoxic and oxidative stresses.²⁰⁸ In the nucleus, AIF triggers the degradation of DNA to 50 Kb fragments,¹⁵² which subsequently undergo internucleosomal cleavage by endonucleases. AIF itself does not have endonuclease activity.¹⁵² Wah-1 (worm apoptosis-inducing factor homolog-1), the C elegans ortholog of AIF, binds to and cooperates with Cps-6 (Ced-3 [cell death abnormality gene-3] protease suppressor-6), the worm ortholog of the mammalian endonuclease EndoG, a protein that is also released from mitochondria during apoptosis.^153,209 These observations suggest that AIF may cooperate with EndoG or another endonuclease, but this mechanism remains to be demonstrated. It has been shown, however, that AIF interacts with cyclophilin A to degrade DNA, and the peptidyl prolyl cis-trans isomerase activity of cyclophilin A is not needed for this function.²¹⁰ In addition to its nuclear effects, AIF also triggers mitochondria to release cytochrome c.¹⁵² AIF^–/– embryonic stem cells exhibit defects in apoptosis, and embryoid bodies generated from these cells undergo inadequate cavitation attributable to decreased cell death.²¹¹ Although most data support a role for AIF in apoptosis, cells derived from the harlequin mouse, a naturally occurring mutant with 80% reduction in AIF levels, exhibit increased sensitivity to oxidative stress.²¹² The explanation for this seemingly paradoxical observation may relate to a dual function of AIF in the metabolism of reactive oxygen species as well as its role in apoptosis. Although AIF release from mitochondria is caspase-dependent in some models,^170,171 its actions after release occur independently of caspase activation.¹⁵² Moreover, AIF-induced killing does not require Apaf-1 or procaspase-9.²¹¹ Thus, the AIF pathway is distinct from, but intricately involved with, caspase-mediated cell death. The reduction in infarct size noted in PARP knockout mice subjected to ischemia-reperfusion suggests that AIF may play a role in this process.²¹³

The net result of apoptotic signaling is mitochondrial dysfunction and caspase activation, the consequences of which have been considered here. The activation of downstream caspases leads to the cleavage of numerous structural and regulatory cellular proteins.⁵⁸ These proteolytic events bring about the implosion of the cell in ways that are incompletely understood. This execution phase and the engulfment of apoptotic bodies by phagocytes have been the subjects of recent reviews.^58,214

	Conclusion

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:... Intrinsic Pathway:... Apoptosis-Inducing Factor Conclusion References

 
The preceding discussion highlights progress over the past decade in defining the molecular circuitry that mediates the mitochondrial apoptotic pathway, as well as its relevance to aspects of heart disease. In contrast, relatively little is known about necrosis at the molecular level. Future efforts to further delineate necrotic pathways and their points of convergence with the apoptotic program will be critical to achieve a more complete understanding of the role of the mitochondrial death pathway in disease pathogenesis.

	Acknowledgments

 
This work was funded by grants from the National Institutes of Health (R01s HL73935 to M.T.C. and HL60665 and HL61550 to R.N.K.). R.N.K. is the Dr Gerald and Myra Dorros Chair in Cardiovascular Disease of the Albert Einstein College of Medicine and the recipient of the Monique Weill-Caulier Career Scientist Award. Y.-J.N. was supported by a National Institutes of Health Training Grant in Cellular and Molecular Biology and Genetics (T32 GM07491). We acknowledge Chang-Fu Peng, Alan Thoms-Chesley, and Eric Bind for discussions and critical reading of the manuscript.

	Footnotes

 
Original received July 29, 2004; resubmission received October 12, 2004; accepted October 12, 2004.

	References

Top Abstract Introduction Types of Cell Death Apoptosis Central Apoptotic Pathways Extrinsic Pathway Intrinsic Pathway:... Tumor Suppressor p53 ER Pathway Intrinsic Pathway:...