Response to myocardial ischemia/reperfusion injury involves Bnip3 and autophagy

When apoptosis meets autophagy: deciding cell fate after trauma and sepsis

Apoptotic cell death is considered to be an underlying mechanism in immunosuppression and multiple organ dysfunction after trauma-hemorrhage and sepsis. Although studied intensively over the last decade, the role of other cell death mechanisms under similar pathophysiological conditions has remained elusive. Recently, autophagy has emerged as an important mediator of programmed cell death pathways. Here, we review recent advances in our understanding of apoptosis and autophagy and the crosstalk between these processes. We explore the coexistence of these two processes and the effects of autophagy on apoptosis after trauma-hemorrhage and sepsis. The inter-relationship between autophagy and apoptosis might unveil novel therapeutic approaches for the detection and treatment of trauma-hemorrhage and sepsis.

Role of BNIP3 and NIX in cell death, autophagy, and mitophagy

BNIP3 and NIX are proteins related to the BH3-only family, which induce both cell death and autophagy. Consistent with their ability to induce cell death, BNIP3 and NIX are implicated in the pathogenesis of cancer and heart disease. In tumor cells, BNIP3 and NIX are regulated by hypoxia, and the deregulation of BNIP3 or NIX expression is associated with tumor growth. In heart muscle, BNIP3 and NIX are regulated by hypoxia and Galphaq-dependent signaling, respectively, and their expression is associated with decreased myocardial function. Apart from their role in cell death, BNIP3 and NIX are also implicated in the induction of autophagy. In erythroid cells, NIX is required for a specialized type of autophagy that targets mitochondria for elimination (mitophagy). Similarly, BNIP3 regulates mitophagy in response to hypoxia. In this review, we will discuss possible mechanisms by which BNIP3 and NIX induce cell death and mitophagy. We will also consider the potential relationship between cell death pathways and autophagy in development and homeostasis.

Apoptotic and non-apoptotic programmed cardiomyocyte death in ventricular remodelling

A defining cellular event in the transition from compensated hypertrophy to dilated cardiomyopathy is cardiomyocyte drop-out due to apoptosis, programmed necrosis, and autophagy. The importance of apoptosis in heart failure has been recognized for over a decade, while other forms of programmed cell death have more recently been appreciated, and their pathophysiological roles continue to be defined in experimental and clinical heart failure. The major focus of this review is on apoptosis in heart failure, with a discussion of molecular cross-talk between apoptosis, autophagy, and programmed necrosis.

Autophagy in ischemic heart disease

Autophagy is a major catabolic pathway by which mammalian cells degrade and recycle macromolecules and organelles. It plays a critical role in removing protein aggregates, as well as damaged or excess organelles, to maintain intracellular homeostasis and to keep the cell healthy. In the heart, autophagy occurs at low levels under normal conditions, and defects in this process cause cardiac dysfunction and heart failure. However, this pathway is rapidly upregulated under environmental stress conditions, including ATP depletion, reactive oxygen species, and mitochondrial permeability transition pore opening. Although autophagy is enhanced in various pathophysiological conditions, such as during ischemia and reperfusion, the functional role of increased autophagy is not clear and is currently under intense investigation. In this review, we discuss the evidence for autophagy in the heart in response to ischemia and reperfusion, identify factors that regulate autophagy, and analyze the potential roles autophagy might play in cardiac cells.

Autophagy: a target for therapeutic interventions in myocardial pathophysiology

BACKGROUND

Autophagy is a major degradative and highly conserved process in eukaryotic cells that is activated by stress signals. This self-cannibalisation is activated as a response to changing environmental conditions, cellular remodelling during development and differentiation, and maintenance of homeostasis.

OBJECTIVE

To review autophagy regarding its process, molecular mechanisms and regulation in mammalian cells, and its role in myocardial pathophysiology.

RESULTS/CONCLUSION

Autophagy is a multistep process regulated by diverse, intracellular and/or extracellular signalling complexes and pathways. In the heart, normally, autophagy occurs at low basal levels, where it represents a homeostatic mechanism for the maintenance of cardiac function and morphology. However, in the diseased heart the functional role of the enhanced autophagy is unclear and studies have yielded conflicting results. Recently, it was shown that during myocardial ischemia autophagy promotes survival by maintaining energy homeostasis. Also, rapamycin was demonstrated to prevent cardiac hypertrophy. In heart failure, upregulation of autophagy acts as an adaptive response that protects cells from hemodynamic stress. In addition, sirolimus-eluting stents have been shown to lower re-stenosis rates in patients with coronary artery disease after angioplasty. Thus, this mechanism can become a major target for therapeutic intervention in heart pathophysiology.

Anti- and pro-tumor functions of autophagy

Autophagy constitutes one of the major responses to stress in eukaryotic cells, and is regulated by a complex network of signaling cascades. Not surprisingly, autophagy is implicated in multiple pathological processes, including infection by pathogens, inflammatory bowel disease, neurodegeneration and cancer. Both oncogenesis and tumor survival are influenced by perturbations of the molecular machinery that controls autophagy. Numerous oncoproteins, including phosphatidylinositol 3-kinase, Akt1 and anti-apoptotic members of the Bcl-2 family suppress autophagy. Conversely, several tumor suppressor proteins (e.g., Atg4c; beclin 1; Bif-1; BH3-only proteins; death-associated protein kinase 1; LKB1/STK11; PTEN; UVRAG) promote the autophagic pathway. This does not entirely apply to p53, one of the most important tumor suppressor proteins, which regulates autophagy in an ambiguous fashion, depending on its subcellular localization. Irrespective of the controversial role of p53, basal levels of autophagy appear to inhibit tumor development. On the contrary, chemotherapy- and metabolic stress-induced activation of the autophagic pathway reportedly contribute to the survival of formed tumors, thereby favoring resistance. In this context, autophagy inhibition would represent a major therapeutic target for chemosensitization. Here, we will review the current knowledge on the dual role of autophagy as an anti- and pro-tumor mechanism.

LPS-induced autophagy is mediated by oxidative signaling in cardiomyocytes and is associated with cytoprotection

Bacterial endotoxin lipopolysaccharide (LPS) is responsible for the multiorgan dysfunction that characterizes septic shock and is causal in the myocardial depression that is a common feature of endotoxemia in patients. In this setting the myocardial dysfunction appears to be due, in part, to the production of proinflammatory cytokines. A line of evidence also indicates that LPS stimulates autophagy in cardiomyocytes. However, the signal transduction pathway leading to autophagy and its role in the heart are incompletely characterized. In this work, we wished to determine the effect of LPS on autophagy and the physiological significance of the autophagic response. Autophagy was monitored morphologically and biochemically in HL-1 cardiomyocytes, neonatal rat cardiomyocytes, and transgenic mouse hearts after the administration of bacterial LPS or TNF-alpha. We observed that autophagy was increased after exposure to LPS or TNF-alpha, which is induced by LPS. The inhibition of TNF-alpha production by AG126 significantly reduced the accumulation of autophagosomes both in cell culture and in vivo. The inhibition of p38 MAPK or nitric oxide synthase by pharmacological inhibitors also reduced autophagy. Nitric oxide or H(2)O(2) induced autophagy in cardiomyocytes, whereas N-acetyl-cysteine, a potent antioxidant, suppressed autophagy. LPS resulted in increased reactive oxygen species (ROS) production and decreased total glutathione. To test the hypothesis that autophagy might serve as a damage control mechanism to limit further ROS production, we induced autophagy with rapamycin before LPS exposure. The activation of autophagy by rapamycin suppressed LPS-mediated ROS production and protected cells against LPS toxicity. These findings support the notion that autophagy is a cytoprotective response to LPS-induced cardiomyocyte injury; additional studies are needed to determine the therapeutic implications.

The autophagy effector Beclin 1: a novel BH3-only protein

BH3 domains were originally discovered in the context of apoptosis regulators and they mediate binding of proapoptotic Bcl-2 family members to antiapoptotic Bcl-2 family members. Yet, recent studies indicate that BH3 domains do not function uniquely in apoptosis regulation; they also function in the regulation of another critical pathway involved in cellular and tissue homeostasis called autophagy. Antiapoptotic Bcl-2 homologs downregulate autophagy through interactions with the essential autophagy effector and haploinsufficient tumor suppressor, Beclin 1. Beclin 1 contains a BH3 domain, similar to that of Bcl-2 proteins, which is necessary and sufficient for binding to antiapoptotic Bcl-2 homologs and required for Bcl-2-mediated inhibition of autophagy. This review will summarize the evidence that the BH3 domain of Beclin 1 serves as a key structural motif that enables Bcl-2 to function not only as an antiapoptotic protein, but also as an antiautophagy protein.

Autophagy protects against hypoxic injury in C. elegans

Macroautophagy has been implicated in a variety of pathological processes. Hypoxic/ischemic cellular injury is one such process in which autophagy has emerged as an important regulator. In general, autophagy is induced after a hypoxic/ischemic insult; however, whether the induction of autophagy promotes cell death or recovery is controversial and appears to be context dependent. We have developed C. elegans as a genetically tractable model for the study of hypoxic cell injury. Both necrosis and apoptosis are mechanisms of cell death following hypoxia in C. elegans. However, the role of autophagy in hypoxic injury in C. elegans has not been examined. Here, we found that RNAi knockdown of the C. elegans homologs of beclin 1/Atg6 (bec-1) and LC3/Atg8 (lgg-1, lgg-2), and mutation of Atg1 (unc-51) decreased animal survival after a severe hypoxic insult. Acute inhibition of autophagy by the type III phosphatidylinositol 3-kinase inhibitors, 3-methyladenine and Wortmannin, also sensitized animals to hypoxic death. Hypoxia-induced neuronal and myocyte injury as well as necrotic cellular morphology were increased by RNAi knockdown of BEC-1. Hypoxia increased the expression of a marker of autophagosomes in a bec-1-dependent manner. Finally, we found that the hypoxia hypersensitive phenotype of bec-1(RNAi) animals could be blocked by loss-of-function mutations in either the apoptosis or necrosis pathway. These results argue that inhibition of autophagy sensitizes C. elegans and its cells to hypoxic injury and that this sensitization is blocked or circumvented when either of the two major cell-death mechanisms is inhibited.

Cyclin-dependent kinase 2 signaling regulates myocardial ischemia/reperfusion injury

Ischemia/reperfusion (I/R) injury to the heart is accompanied by the upregulation and posttranslational modification of a number of proteins normally involved in regulating cell cycle progression. Two such proteins, cyclin-dependent kinase-2 (Cdk2) and its downstream target, the retinoblastoma gene product (Rb), also play a critical role in the control of apoptosis. Myocardial ischemia activates Cdk2, resulting in the phosphorylation and inactivation of Rb. Blocking Cdk2 activity reduces apoptosis in cultured cardiac myocytes. Genetic or pharmacological inhibition of Cdk2 activity in vivo during I/R injury led to a 36% reduction in infarct size (IFS), when compared to control mice, associated with a reduction in apoptotic myocytes. To confirm that Rb was the critical target in Cdk2-mediated I/R injury, we determined the consequences of I/R injury in cardiac-specific Rb-deficient mice (CRb(L/L)). IFS was increased 140% in CRb(L/L) mice compared to CRb+/+ controls. TUNEL positive nuclei and caspase-3 activity were augmented by 92% and 36%, respectively, following injury in the CRb(L/L) mice demonstrating that loss of Rb in the heart significantly exacerbates I/R injury. These data suggest that Cdk2 signaling pathways are critical regulators of cardiac I/R injury in vivo and support a cardioprotective role for Rb.

The role of the RB tumour suppressor pathway in oxidative stress responses in the haematopoietic system

Exposure to pro-oxidants and defects in the repair of oxidative base damage are associated with disease and ageing and also contribute to the development of anaemia, bone marrow failure and haematopoietic malignancies. This Review assesses emerging data indicative of a specific role for the RB tumour suppressor pathway in the response of the haematopoietic system to oxidative stress. This is mediated through signalling pathways that involve DNA damage sensors, forkhead box O (Foxo) transcription factors and p38 mitogen-activated protein kinases and has downstream consequences for cell cycle progression, antioxidant capacity, mitochondrial mass and cellular metabolism.

Control of autophagy by oncogenes and tumor suppressor genes

Multiple oncogenes (in particular phosphatidylinositol 3-kinase, PI3K; activated Akt1; antiapoptotic proteins from the Bcl-2 family) inhibit autophagy. Similarly, several tumor suppressor proteins (such as BH3-only proteins; death-associated protein kinase-1, DAPK1; the phosphatase that antagonizes PI3K, PTEN; tuberous sclerosic complex 1 and 2, TSC1 and TSC2; as well as LKB1/STK11) induce autophagy, meaning that their loss reduces autophagy. Beclin-1, which is required for autophagy induction acts as a haploinsufficient tumor suppressor protein, and other essential autophagy mediators (such as Atg4c, UVRAG and Bif-1) are bona fide oncosuppressors. One of the central tumor suppressor proteins, p53 exerts an ambiguous function in the regulation of autophagy. Within the nucleus, p53 can act as an autophagy-inducing transcription factor. Within the cytoplasm, p53 exerts a tonic autophagy-inhibitory function, and its degradation is actually required for the induction of autophagy. The role of autophagy in oncogenesis and anticancer therapy is contradictory. Chronic suppression of autophagy may stimulate oncogenesis. However, once a tumor is formed, autophagy inhibition may be a therapeutic goal for radiosensitization and chemosensitization. Altogether, the current state-of-the art suggests a complex relationship between cancer and deregulated autophagy that must be disentangled by further in-depth investigation.

Bnip3 functions as a mitochondrial sensor of oxidative stress during myocardial ischemia and reperfusion

Bcl-2/adenovirus E1B 19-kDa protein-interacting protein 3 (Bnip3) is a member of the Bcl-2 homology domain 3-only subfamily of proapoptotic Bcl-2 proteins and is associated with cell death in the myocardium. In this study, we investigated the potential mechanism(s) by which Bnip3 activity is regulated. We found that Bnip3 forms a DTT-sensitive homodimer that increased after myocardial ischemia-reperfusion (I/R). The presence of the antioxidant N-acetylcysteine reduced I/R-induced homodimerization of Bnip3. Overexpression of Bnip3 in cells revealed that most of exogenous Bnip3 exists as a DTT-sensitive homodimer that correlated with increased cell death. In contrast, endogenous Bnip3 existed mainly as a monomer under normal conditions in the heart. Screening of the Bnip3 protein sequence revealed a single conserved cysteine residue at position 64. Mutation of this cysteine to alanine (Bnip3C64A) or deletion of the NH2-terminus (amino acids 1-64) resulted in reduced cell death activity of Bnip3. Moreover, mutation of a histidine residue in the COOH-terminal transmembrane domain to alanine (Bnip3H173A) almost completely inhibited the cell death activity of Bnip3. Bnip3C64A had a reduced ability to interact with Bnip3, whereas Bnip3H173A was completely unable to interact with Bnip3, suggesting that homodimerization is important for Bnip3 function. A consequence of I/R is the production of reactive oxygen species and oxidation of proteins, which promotes the formation of disulfide bonds between proteins. Thus, these experiments suggest that Bnip3 functions as a redox sensor where increased oxidative stress induces homodimerization and activation of Bnip3 via cooperation of the NH2-terminal cysteine residue and the COOH-terminal transmembrane domain.

Protection of cardiomyocytes from ischemic/hypoxic cell death via Drbp1 and pMe2GlyDH in cardio-specific ARC transgenic mice

The ischemic death of cardiomyocytes is associated in heart disease and heart failure. However, the molecular mechanism underlying ischemic cell death is not well defined. To examine the function of apoptosis repressor with a caspase recruitment domain (ARC) in the ischemic/hypoxic damage of cardiomyocytes, we generated cardio-specific ARC transgenic mice using a mouse alpha-myosin heavy chain promoter. Compared with the control, the hearts of ARC transgenic mice showed a 3-fold overexpression of ARC. Langendoff preparation showed that the hearts isolated from ARC transgenic mice exhibited improved recovery of contractile performance during reperfusion. The cardiomyocytes cultured from neonatal ARC transgenic mice were significantly resistant to hypoxic cell death. Furthermore, the ARC C-terminal calcium-binding domain was as potent to protect cardiomyocytes from hypoxic cell death as ARC. Genome-wide RNA expression profiling uncovered a list of genes whose expression was changed (>2-fold) in ARC transgenic mice. Among them, expressional regulation of developmentally regulated RNA-binding protein 1 (Drbp1) or the dimethylglycine dehydrogenase precursor (pMe(2)GlyDH) affected hypoxic death of cardiomyocytes. These results suggest that ARC may protect cardiomyocytes from hypoxic cell death by regulating its downstream, Drbp1 and pMe(2)GlyDH, shedding new insights into the protection of heart from hypoxic damages.

Crosstalk between autophagy and apoptosis in heart disease

Autophagy is a cell survival mechanism that involves degradation and recycling of cytoplasmic components, such as long-lived proteins and organelles. In addition, autophagy mediates cell death under specific circumstances. Apoptosis, a form of programmed cell death, has been well characterized, and the molecular events involved in apoptotic death are well understood. Damaged cardiomyocytes that show characteristics of autophagy have been observed during heart failure. However, it remains unclear whether autophagy is a sign of failed cardiomyocyte repair or is a suicide pathway for the failing cardiomyocytes. Although autophagy and apoptosis are markedly different processes, several pathways regulate both autophagic and apoptotic machinery and autophagy can cooperate with apoptosis. This review summarizes the evidence for crosstalk between autophagy and apoptosis.

Disruption of neuronal autophagy by infected microglia results in neurodegeneration

There is compelling evidence to support the idea that autophagy has a protective function in neurons and its disruption results in neurodegenerative disorders. Neuronal damage is well-documented in the brains of HIV-infected individuals, and evidence of inflammation, oxidative stress, damage to synaptic and dendritic structures, and neuronal loss are present in the brains of those with HIV-associated dementia. We investigated the role of autophagy in microglia-induced neurotoxicity in primary rodent neurons, primate and human models. We demonstrate here that products of simian immunodeficiency virus (SIV)-infected microglia inhibit neuronal autophagy, resulting in decreased neuronal survival. Quantitative analysis of autophagy vacuole numbers in rat primary neurons revealed a striking loss from the processes. Assessment of multiple biochemical markers of autophagic activity confirmed the inhibition of autophagy in neurons. Importantly, autophagy could be induced in neurons through rapamycin treatment, and such treatment conferred significant protection to neurons. Two major mediators of HIV-induced neurotoxicity, tumor necrosis factor-alpha and glutamate, had similar effects on reducing autophagy in neurons. The mRNA level of p62 was increased in the brain in SIV encephalitis and as well as in brains from individuals with HIV dementia, and abnormal neuronal p62 dot structures immunoreactivity was present and had a similar pattern with abnormal ubiquitinylated proteins. Taken together, these results identify that induction of deficits in autophagy is a significant mechanism for neurodegenerative processes that arise from glial, as opposed to neuronal, sources, and that the maintenance of autophagy may have a pivotal role in neuroprotection in the setting of HIV infection.

Pivotal role of Bcl-2 family proteins in the regulation of chondrocyte apoptosis

During endochondral ossification, chondrocytes undergo hypertrophic differentiation and die by apoptosis. The level of inorganic phosphate (P(i)) elevates at the site of cartilage mineralization, and when chondrocytes were treated with P(i), they underwent rapid apoptosis. Gene silencing of the proapoptotic Bcl-2 homology 3-only molecule bnip3 significantly suppressed P(i)-induced apoptosis. Conversely, overexpression of Bcl-xL suppressed, and its knockdown promoted, the apoptosis of chondrocytes. Bnip3 was associated with Bcl-xL in chondrocytes stimulated with P(i). Bcl-xL was expressed uniformly in the growth plate chondrocytes, whereas Bnip3 expression was exclusively localized in the hypertrophic chondrocytes. Finally, we generated chondrocyte-specific bcl-x knock-out mice using the Cre-loxP recombination system, and we provided evidence that the hypertrophic chondrocyte layer was shortened in those mice because of an increased apoptosis of prehypertrophic and hypertrophic chondrocytes, with the mice afflicted with dwarfism as a result. These results suggest the pivotal role of Bcl-2 family members in the regulation of chondrocyte apoptosis.

Adriamycin-induced autophagic cardiomyocyte death plays a pathogenic role in a rat model of heart failure

BACKGROUND

The mechanisms underlying heart failure induced by adriamycin are very complicated and still unclear. The aim of this study was to investigate whether autophagy was involved in the progression of heart failure induced by adriamycin, so that we can develop a novel treatment strategy for heart failure.

METHODS

3-methyladenine (3MA), a specific inhibitor on autophagy was used in a heart failure model of rats induced by adriamycin. Neonatal cardiomyocytes were isolated from Sprague-Dawley rat hearts and randomly divided into controls, an adriamycin-treated group, and a 3MA plus adriamycin-treated group. We then examined the morphology, expression of beclin 1 gene, mitochondrial permeability transition (MPT), and Na+-K+ ATPase activity in vivo. We also assessed cell viability, mitochondrial membrane potential changes and counted autophagic vacuoles in cultured cardiomyocytes. In addition, we analyzed the expression of autophagy associated gene, beclin 1 using RT-PCR and Western blotting in an animal model.

RESULTS

3MA significantly improved cardiac function and reduced mitochondrial injury. Furthermore, adriamycin induced the formation of autophagic vacuoles, and 3MA strongly downregulated the expression of beclin 1 in adriamycin-induced failing heart and inhibited the formation of autophagic vacuoles.

CONCLUSION

Autophagic cardiomyocyte death plays an important role in the pathogenesis of heart failure in rats induced by adriamycin. Mitochondrial injury may be involved in the progression of heart failure caused by adriamycin via the autophagy pathway.

The rationale for cardiomyocyte resuscitation in myocardial salvage

Clinical heart failure results from the cumulative loss of functioning myocardium from any cause. At the cellular level, cardiac myocytes die from three causes, individually or in combination: Necrosis occurs when external conditions are not sufficient to sustain minimal cellular functions, as with ischemia, and there is a general and unorganized breakdown of cell organelles, engendering an inflammatory response that may have harmful collateral tissue effects. Apoptosis, or cell suicide, occurs when specific external or internal conditions provoke a highly structured sequence of events to shut down cellular functions and remove the cell, with minimal consequences to surrounding tissue. Autophagy is a normal response to cell starvation that is induced under conditions of chronic metabolic or other stress. Current therapeutics, such as early myocardial revascularization after myocardial infarction, are focused exclusively upon minimizing cardiac myocyte necrosis and may even contribute to secondary apoptosis and autophagy. This review explores possible approaches to bring cardiac myocytes that are destined to die, back to life, i.e., cellular resuscitation. Two pro-apoptotic proteins in particular, Bnip3 and Nix, are transcriptionally upregulated specifically in response to myocardial ischemia and pathological hypertrophy and have been examined as therapeutic targets. In Bnip3 and Nix genetic mouse models, prevention of cardiac myocyte apoptosis in ischemic and hemodynamically overloaded hearts salvaged myocardium, minimized late ventricular remodeling, and enhanced ventricular performance. Cardiomyocyte resuscitation by preventing programmed cell death shows promise as an additive approach to minimizing necrosis for long-term prevention of heart failure.

Apolipoprotein L1, a novel Bcl-2 homology domain 3-only lipid-binding protein, induces autophagic cell death

The Bcl-2 family proteins are important regulators of type I programmed cell death apoptosis; however, their role in autophagic cell death (AuCD) or type II programmed cell death is still largely unknown. Here we report the cloning and characterization of a novel Bcl-2 homology domain 3 (BH3)-only protein, apolipoprotein L1 (apoL1), that, when overexpressed and accumulated intracellularly, induces AuCD in cells as characterized by the increasing formation of autophagic vacuoles and activating the translocation of LC3-II from the cytosol to the autophagic vacuoles. Wortmannin and 3-methyladenine, inhibitors of class III phosphatidylinostol 3-kinase and, subsequently, autophagy, blocked apoL1-induced AuCD. In addition, apoL1 failed to induce AuCD in autophagy-deficient ATG5(-/-) and ATG7(-/-) mouse embryonic fibroblast cells, suggesting that apoL1-induced cell death is indeed autophagy-dependent. Furthermore, a BH3 domain deletion construct of apoL1 failed to induce AuCD, demonstrating that apoL1 is a bona fide BH3-only pro-death protein. Moreover, we showed that apoL1 is inducible by p53 in p53-induced cell death and is a lipid-binding protein with high affinity for phosphatidic acid (PA) and cardiolipin (CL). Previously, it has been shown that PA directly interacted with mammalian target of rapamycin and positively regulated the ability of mammalian target of rapamycin to activate downstream effectors. In addition, CL has been shown to activate mitochondria-mediated apoptosis. Sequestering of PA and CL with apoL1 may alter the homeostasis between survival and death leading to AuCD. To our knowledge, this is the first BH3-only protein with lipid binding activity that, when overproduced intracellularly, induces AuCD.

Bcl-2 family members: dual regulators of apoptosis and autophagy

The essential autophagy protein and haplo-insufficient tumor suppressor, Beclin 1, interacts with several cofactors (Ambra1, Bif-1, UVRAG) to activate the lipid kinase Vps34, thereby inducing autophagy. In normal conditions, Beclin 1 is bound to and inhibited by Bcl-2 or the Bcl-2 homolog Bcl-X(L). This interaction involves a Bcl-2 homology 3 (BH3) domain in Beclin 1 and the BH3 binding groove of Bcl-2/Bcl-X(L). Other proteins containing BH3 domains, called BH3-only proteins, can competitively disrupt the interaction between Beclin 1 and Bcl-2/Bcl-X(L) to induce autophagy. Nutrient starvation, which is a potent physiologic inducer of autophagy, can stimulate the dissociation of Beclin 1 from its inhibitors, either by activating BH3-only proteins (such as Bad) or by posttranslational modifications of Bcl-2 (such as phosphorylation) that may reduce its affinity for Beclin 1 and BH3-only proteins. Thus, anti-apoptotic Bcl-2 family members and pro-apoptotic BH3-only proteins may participate in the inhibition and induction of autophagy, respectively. This hitherto neglected crosstalk between the core machineries regulating autophagy and apoptosis may redefine the role of Bcl-2 family proteins in oncogenesis and tumor progression.

Mitochondrial autophagy is an HIF-1-dependent adaptive metabolic response to hypoxia

Autophagy is a process by which cytoplasmic organelles can be catabolized either to remove defective structures or as a means of providing macromolecules for energy generation under conditions of nutrient starvation. In this study we demonstrate that mitochondrial autophagy is induced by hypoxia, that this process requires the hypoxia-dependent factor-1-dependent expression of BNIP3 and the constitutive expression of Beclin-1 and Atg5, and that in cells subjected to prolonged hypoxia, mitochondrial autophagy is an adaptive metabolic response which is necessary to prevent increased levels of reactive oxygen species and cell death.

Recycle or die: the role of autophagy in cardioprotection

Autophagy is a highly conserved cellular process responsible for the degradation of long-lived proteins and organelles. Autophagy occurs at low levels under normal conditions, but is upregulated in response to stress such as nutrient deprivation, hypoxia, mitochondrial dysfunction, and infection. Upregulation of autophagy may be beneficial to the cell by recycling of proteins to generate free amino acids and fatty acids needed to maintain energy production, by removing damaged organelles, and by preventing accumulation of protein aggregates. In contrast, there is evidence that enhanced autophagy can contribute to cell death, possibly through excessive self-digestion. In the heart, autophagy has an essential role for maintaining cellular homeostasis under normal conditions and increased autophagy can be seen in conditions of starvation, ischemia/reperfusion, and heart failure. However, the functional significance of autophagy in heart disease is unclear and controversial. Here, we review the literature and discuss the evidence that autophagy can have both beneficial and detrimental roles in the myocardium depending on the level of autophagy, and discuss potential mechanisms by which autophagy provides protection in cells.

Guidelines for the use and interpretation of assays for monitoring autophagy in higher eukaryotes

Research in autophagy continues to accelerate,(1) and as a result many new scientists are entering the field. Accordingly, it is important to establish a standard set of criteria for monitoring macroautophagy in different organisms. Recent reviews have described the range of assays that have been used for this purpose.(2,3) There are many useful and convenient methods that can be used to monitor macroautophagy in yeast, but relatively few in other model systems, and there is much confusion regarding acceptable methods to measure macroautophagy in higher eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers of autophagosomes versus those that measure flux through the autophagy pathway; thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from fully functional autophagy that includes delivery to, and degradation within, lysosomes (in most higher eukaryotes) or the vacuole (in plants and fungi). Here, we present a set of guidelines for the selection and interpretation of the methods that can be used by investigators who are attempting to examine macroautophagy and related processes, as well as by reviewers who need to provide realistic and reasonable critiques of papers that investigate these processes. This set of guidelines is not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to verify an autophagic response.

Eat your heart out: Role of autophagy in myocardial ischemia/reperfusion

Autophagy is an important process in the heart which is responsible for the normal turnover of long lived proteins and organelles. Inhibition of autophagy leads to the accumulation of protein aggregates and dysfunctional organelles which can cause cell death. Autophagy occurs at low basal levels under normal conditions in the heart, but is rapidly upregulated in response to stress such as nutrient deprivation, hypoxia, and pressure overload. Autophagy is a prominent feature of myocardial ischemia and reperfusion. Although enhanced autophagy is often seen in dying cardiac myocytes, the functional significance of autophagy under these conditions is not clear. Upregulation of autophagy has been reported to protect cardiac cells against death as well as be the cause of it. Here, we review the evidence that autophagy can have both beneficial and detrimental roles in the myocardium, and discuss potential mechanisms by which autophagy provides protection in cells.

Molecular mechanisms and physiological significance of autophagy during myocardial ischemia and reperfusion

Autophagy is an intracellular bulk degradation process whereby cytoplasmic proteins and organelles are degraded and recycled through lysosomes. In the heart, autophagy plays a homeostatic role at basal levels, and the absence of autophagy causes cardiac dysfunction and the development of cardiomyopathy. Autophagy is induced during myocardial ischemia and further enhanced by reperfusion. Although induction of autophagy during the ischemic phase is protective, further enhancement of autophagy during the reperfusion phase may induce cell death and appears to be detrimental. In this review we discuss the functional significance of autophagy and the underlying signaling mechanism in the heart during ischemia/reperfusion.

Nix-mediated apoptosis links myocardial fibrosis, cardiac remodeling, and hypertrophy decompensation

BACKGROUND

Pathological cardiac hypertrophy inevitably remodels, leading to functional decompensation. Although modulation of apoptosis-regulating genes occurs in cardiac hypertrophy, a causal role for programmed cardiomyocyte death in left ventricular (LV) remodeling has not been established.

METHODS AND RESULTS

We targeted the gene for proapoptotic Nix, which is transcriptionally upregulated in pressure overload and Gq-dependent hypertrophies, in the mouse germ line or specifically in cardiomyocytes (knockout [KO]) and conditionally overexpressed it in the heart (transgenic [TG]). Conditional forced Nix expression acted synergistically with the prohypertrophic Gq transgene to increase cardiomyocyte apoptosis (0.8+/-0.1% in GqTG versus 7.8+/-0.6% in GqTG+NixTG; P<0.001), causing lethal cardiomyopathy with LV dilation and depressed systolic function (percent fractional shortening, 39+/-4 versus 23+/-4; P=0.042). In the reciprocal experiment, germ-line Nix ablation significantly reduced cardiomyocyte apoptosis (4.8+/-0.2% in GqTG+NixKO versus 8.4+/-0.5% in GqTG; P=0.001), which improved percent fractional shortening (43+/-3% versus 27+/-3%; P=0.017), attenuated LV remodeling, and largely prevented lethality in the Gq peripartum model of apoptotic cardiomyopathy. Cardiac-specific (Nkx2.5-Cre) Nix KO mice subjected to transverse aortic constriction developed significantly less LV dilation by echocardiography and magnetic resonance imaging, maintained concentric remodeling, and exhibited preserved LV ejection fraction (61+/-2% in transverse aortic constriction cardiac Nix KO versus 36+/-6% in transverse aortic constriction wild-type mice; P=0.003) at 9 weeks, with reduced cardiomyocyte apoptosis at day 4 (1.70+/-0.21% versus 2.73+/-0.35%; P=0.032).

CONCLUSIONS

Nix-induced cardiomyocyte apoptosis is a major determinant of adverse remodeling in pathological hypertrophies, a finding that suggests therapeutic value for apoptosis inhibition to prevent cardiomyopathic decompensation.

Hydrophobic statins induce autophagy in cultured human rhabdomyosarcoma cells

Statins are widely used to treat hypercholesterolemia, but they are associated with muscle-related adverse events, by as yet, inadequately resolved mechanisms. In this study, we report that statins induced autophagy in cultured human rhabdomyosarcoma A204 cells. Potency differed widely among the statins: cerivastatin induced autophagy at 0.1muM, simvastatin at 10muM but none was induced by pravastatin. Addition of mevalonate, but not cholesterol, blocked induction of autophagy by cerivastatin, suggesting that this induction is dependent on modulation of isoprenoid metabolic pathways. The statin-induced autophagy was not observed in other types of cells, such as human hepatoma HepG2 or embryonic kidney HEK293 cells. Muscle-specific abortive induction of autophagy by hydrophobic statins is a possible mechanism for statin-induced muscle-related side effects.

FoxO3 controls dangerous proteolytic liaisons

FoxO3 regulates the transcription of critical components of the ubiquitin-proteasome system in muscle wasting. Two reports (Mammucari et al., 2007; Zhao et al., 2007) now implicate FoxO3 in the transcription of autophagy-related genes and provide the first direct evidence for a coordinated role of autophagy in muscle atrophy.

FoxO3 controls autophagy in skeletal muscle in vivo

Autophagy allows cell survival during starvation through the bulk degradation of proteins and organelles by lysosomal enzymes. However, the mechanisms responsible for the induction and regulation of the autophagy program are poorly understood. Here we show that the FoxO3 transcription factor, which plays a critical role in muscle atrophy, is necessary and sufficient for the induction of autophagy in skeletal muscle in vivo. Akt/PKB activation blocks FoxO3 activation and autophagy, and this effect is not prevented by rapamycin. FoxO3 controls the transcription of autophagy-related genes, including LC3 and Bnip3, and Bnip3 appears to mediate the effect of FoxO3 on autophagy. This effect is not prevented by proteasome inhibitors. Thus, FoxO3 controls the two major systems of protein breakdown in skeletal muscle, the ubiquitin-proteasomal and autophagic/lysosomal pathways, independently. These findings point to FoxO3 and Bnip3 as potential therapeutic targets in muscle wasting disorders and other degenerative and neoplastic diseases in which autophagy is involved.

Inhibition of ischemic cardiomyocyte apoptosis through targeted ablation of Bnip3 restrains postinfarction remodeling in mice

Following myocardial infarction, nonischemic myocyte death results in infarct expansion, myocardial loss, and ventricular dysfunction. Here, we demonstrate that a specific proapoptotic gene, Bnip3, minimizes ventricular remodeling in the mouse, despite having no effect on early or late infarct size. We evaluated the effects of ablating Bnip3 on cardiomyocyte death, infarct size, and ventricular remodeling after surgical ischemia/reperfusion (IR) injury in mice. Immediately following IR, no significant differences were observed between Bnip3(-/-) and WT mice. However, at 2 days after IR, apoptosis was diminished in Bnip3(-/-) periinfarct and remote myocardium, and at 3 weeks after IR, Bnip3(-/-) mice exhibited preserved LV systolic performance, diminished LV dilation, and decreased ventricular sphericalization. These results suggest myocardial salvage by inhibition of apoptosis. Forced cardiac expression of Bnip3 increased cardiomyocyte apoptosis in unstressed mice, causing progressive LV dilation and diminished systolic function. Conditional Bnip3 overexpression prior to coronary ligation increased apoptosis and infarct size. These studies identify postischemic apoptosis by myocardial Bnip3 as a major determinant of ventricular remodeling in the infarcted heart, suggesting that Bnip3 may be an attractive therapeutic target.

Autophagy in cardiovascular disease

Autophagy is a major cytoprotective pathway that eukaryotic cells use to degrade and recycle cytoplasmic contents. Recent evidence indicates that autophagy under baseline conditions represents an important homeostatic mechanism for the maintenance of normal cardiovascular function and morphology. By contrast, excessive induction of the autophagic process by environmental or intracellular stress has an important role in several types of cardiomyopathy by functioning as a death pathway. As a consequence, enhanced autophagy represents one of the mechanisms underlying the cardiomyocyte dropout responsible for the worsening of heart failure. Successful therapeutic approaches that regulate autophagy have been reported recently, suggesting that the autophagic machinery can be manipulated to treat heart failure or to prevent rupture of atherosclerotic plaques and sudden death.

Self-eating and self-killing: crosstalk between autophagy and apoptosis

The functional relationship between apoptosis ('self-killing') and autophagy ('self-eating') is complex in the sense that, under certain circumstances, autophagy constitutes a stress adaptation that avoids cell death (and suppresses apoptosis), whereas in other cellular settings, it constitutes an alternative cell-death pathway. Autophagy and apoptosis may be triggered by common upstream signals, and sometimes this results in combined autophagy and apoptosis; in other instances, the cell switches between the two responses in a mutually exclusive manner. On a molecular level, this means that the apoptotic and autophagic response machineries share common pathways that either link or polarize the cellular responses.

The role of the hypoxia-inducible BH3-only proteins BNIP3 and BNIP3L in cancer

In this review, we summarize current knowledge of the biological functions of the atypical BH3-only proteins BNIP3 and BNIP3L, focusing on the role of these proteins in cancer. Hypoxia increases the expression of BNIP3 through the transcription factor HIF-1, but despite a considerable number of investigations, it has proven difficult to establish a clear role for BNIP3 in the cellular hypoxic response. BNIP3 can induce a form of cell death that shows features of both necrosis and apoptosis, but unusually for a BH3-only protein, death occurs independently of the BH3 domain and is critically dependent on a C-terminal transmembrane domain, which also localizes the protein to the mitochondria. BNIP3 expression does not always result in cell death, suggesting that additional factors may suppress BNIP3 or cooperate with it to induce death. BNIP3 is highly expressed in some tumors, including those of the breast, lung and cervix. However, in colorectal and pancreatic cancers BNIP3 is frequently epigenetically silenced, possibly reflecting different functions for BNIP3 in different tissues. Recent reports have shown that BNIP3 can induce autophagy and there is some evidence to suggest this may represent an emerging role for BH3-only proteins in general. However, the mechanism through which BNIP3 induces autophagy and the cellular consequences of this are yet to be established.

Regulation of Bnip3 death pathways by calcium, phosphorylation, and hypoxia-reoxygenation

Bnip3 is a proapoptotic member of the Bcl-2 family of death-regulating proteins that promote the intrinsic pathway of programmed cell death. The Bnip3 death program requires membrane insertion through an N-terminal transmembrane domain that directs the protein to mitochondrial and endoplasmic reticular (ER) membranes. We have reported that simulated ischemia induces transcription of the Bnip3 gene, and Bnip3 protein is stabilized by acidosis. Bnip3 programmed death is atypical, with features of both apoptosis and necrosis. Here we demonstrate that hypoxia-reoxygenation and agents that activate protein kinase C, including calcium ionophore, phorbol 12-myristate 13-acetate, and okadaic acid, also induce Bnip3. The molecular size of Bnip3 predicted from the amino acid sequence is 21.5 kDa, but the protein typically migrates in SDS-PAGE as a 31-kDa monomer and 60-kDa dimer. Treatment of cell extracts containing Bnip3 with phosphatase yielded a series of rapidly migrating species, the smallest of which corresponded with the theoretic molecular size of Bnip3. Conversely, treatment of cells with okadaic acid eliminated the rapidly migrating species, suggesting that Bnip3 phosphorylation is a dynamic process. Elevated levels of the phosphoprotein correlated with initiation of Bnip3-dependent death, whereas the dephosphorylated species correlated with extreme acidosis.

The role of autophagy in mediating cell survival and death during ischemia and reperfusion in the heart

Autophagy is a major mechanism for degrading long-lived cytosolic proteins and the only known pathway for degrading organelles. Autophagy is activated by many forms of stress, including nutrient and energy starvation, oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress, and infections. Although autophagy recycles amino acids and fatty acids to produce energy and removes damaged organelles, thereby playing an essential role in cell survival, inappropriate activation of autophagy leads to cell death. In the heart, activation of autophagy can be observed in response to nutrient starvation, ischemia/reperfusion, and heart failure. In this review, the signaling mechanism and the functional significance of autophagy during myocardial ischemia and reperfusion are discussed.

Necrostatin-1 protects against glutamate-induced glutathione depletion and caspase-independent cell death in HT-22 cells

Glutamate, a major excitatory neurotransmitter in the CNS, plays a critical role in neurological disorders such as stroke and Parkinson's disease. Recent studies have suggested that glutamate excess can result in a form of cell death called glutamate-induced oxytosis. In this study, we explore the protective effects of necrostatin-1 (Nec-1), an inhibitor of necroptosis, on glutamate-induced oxytosis. We show that Nec-1 inhibits glutamate-induced oxytosis in HT-22 cells through a mechanism that involves an increase in cellular glutathione (GSH) levels as well as a reduction in reactive oxygen species production. However, Nec-1 had no protective effect on free radical-induced cell death caused by hydrogen peroxide or menadione, which suggests that Nec-1 has no antioxidant effects. Interestingly, the protective effect of Nec-1 was still observed when cellular GSH was depleted by buthionine sulfoximine, a specific and irreversible inhibitor of glutamylcysteine synthetase. Our study further demonstrates that Nec-1 significantly blocks the nuclear translocation of apoptosis-inducing factor (a marker of caspase-independent programmed cell death) and inhibits the integration of Bcl-2/adenovirus E1B 19 kDa-interacting protein 3 (a pro-death member of the Bcl-2 family) into the mitochondrial membrane. Taken together, these results demonstrate for the first time that Nec-1 prevents glutamate-induced oxytosis in HT-22 cells through GSH related as well as apoptosis-inducing factor and Bcl-2/adenovirus E1B 19 kDa-interacting protein 3-related pathways.

Regulation of mitochondrial integrity, autophagy and cell survival by BNIP3

Understanding the role of BNIP3 in the systemic response to hypoxia has been complicated by conflicting results that indicate on the one hand that BNIP3 promotes cell death, and other data, including our own that BNIP3 is not sufficient for cell death, but rather plays a critical role in hypoxia-induced autophagy. This work suggests that rather than promoting death, BNIP3 may actually allow survival either by preventing ATP depletion or by eliminating damaged mitochondria. However, the function of BNIP3 may be subverted under unusual conditions associated with acidosis that arise following extended periods of hypoxia and anaerobic glycolysis. Despite this novel insight into BNIP3 function, much remains to be done in terms of pinning down a molecular activity for BNIP3 that explains both its role in autophagy and how this may be subverted to induce cell death. As a target of the RB tumor suppressor, our work also places BNIP3 at the center of efforts to exploit autophagy to better treat human cancers in which tumor hypoxia is implicated as a progression factor.

Bnip3 mediates mitochondrial dysfunction and cell death through Bax and Bak

Bnip3 is a pro-apoptotic member of the Bcl-2 family that is down-regulated in pancreatic cancers, which correlates with resistance to chemotherapy and a worsened prognosis. In contrast, Bnip3 is up-regulated in heart failure and contributes to loss of myocardial cells during I/R (ischaemia/reperfusion). Bnip3 exerts its action at the mitochondria, but the mechanism by which Bnip3 mediates mitochondrial dysfunction is not clear. In the present study, we have identified Bax and Bak as downstream effectors of Bnip3-mediated mitochondrial dysfunction. Bnip3 plays a role in hypoxia-mediated cell death, but MEFs (mouse embryonic fibroblasts) derived from mice deficient in Bax and Bak were completely resistant to hypoxia even with substantial up-regulation of Bnip3. These cells were also resistant to Bnip3 overexpression, but re-expression of Bax or Bak restored susceptibility to Bnip3, suggesting that Bnip3 can act via either Bax or Bak. In contrast, Bnip3 overexpression in wild-type MEFs induced mitochondrial dysfunction with loss of membrane potential and release of cytochrome c. Cell death by Bnip3 was reduced in the presence of mPTP (mitochondrial permeability transition pore) inhibitors, but did not prevent Bnip3-mediated activation of Bax or Bak. Moreover, overexpression of Bnip3DeltaTM, a dominant-negative form of Bnip3, reduced translocation of GFP (green fluorescent protein)-Bax to mitochondria during sI/R (simulated I/R) in HL-1 myocytes. Similarly, down-regulation of Bnip3 using RNA interference decreased activation of Bax in response to sI/R in HL-1 myocytes. These results suggest that Bnip3 mediates mitochondrial dysfunction through activation of Bax or Bak which is independent of mPTP opening.

The interplay between pro-death and pro-survival signaling pathways in myocardial ischemia/reperfusion injury: apoptosis meets autophagy

INTRODUCTION

Programmed cell death of cardiac myocytes occurs following a bout of ischemia/reperfusion (I/R), which results in reduced function of the heart. Numerous studies, including in vivo, have shown that cell death occurs via necrosis and apoptosis following I/R. Recently, autophagy has emerged as a powerful mediator of programmed cell death, either opposing or enhancing apoptosis, or acting as an alternative form of programmed cell death distinct from apoptosis.

AIM

Here we review the apoptotic and autophagic signaling pathways, their influences on each other, and we discuss the relevance of autophagy in the heart.