Autophagy and signaling: their role in cell survival and cell death

Macroautophagy is a vacuolar, self-digesting mechanism responsible for the removal of long-lived proteins and damaged organelles by the lysosome. The discovery of the ATG genes has provided key information about the formation of the autophagosome, and about the role of macroautophagy in allowing cells to survive during nutrient depletion and/or in the absence of growth factors. Two connected signaling pathways encompassing class-I phosphatidylinositol 3-kinase and (mammalian) target of rapamycin play a central role in controlling macroautophagy in response to starvation. However, a considerable body of literature reports that macroautophagy is also a cell death mechanism that can occur either in the absence of detectable signs of apoptosis (via autophagic cell death) or concomitantly with apoptosis. Macroautophagy is activated by signaling pathways that also control apoptosis. The aim of this review is to discuss the signaling pathways that control macroautophagy during cell survival and cell death.     

Autophagy and cell death

Autophagy exerts dual functions in cancer, acting as both a tumor suppressor, for example, by preventing the accumulation of damaged proteins and organelles, and as a tumor promoter that supports tumor growth. Many anticancer therapies engage autophagy as part of a cellular response. However, the question of whether or not autophagic activity in cells undergoing cell death is the cause of death or whether it is actually an attempt to support survival in response to cellular stress conditions has been discussed with great controversy.     

Autophagy is a survival force via suppression of necrotic cell death

Macroautophagy or autophagy is a self-digesting mechanism that the cellular contents are engulfed by autophagosomes and delivered to lysosomes for degradation. Although it has been well established that autophagy is an important protective mechanism for cells under stress such as starvation via provision of nutrients and removal of protein aggregates and damaged mitochondria, there is a very complex relation between autophagy and cell death. At present, the molecular cross-talk between autophagy and apoptosis has been well discussed, while the relationship between autophagy and programmed necrotic cell death is less understood. In this review we focus on the role of autophagy in necrotic cell death by detailed discussion on two important forms of necrotic cell death: (i) necroptosis and (ii) poly-(ADP-ribose) polymerase (PARP)-mediated cell death. It is believed that one important aspect of the pro-survival function of autophagy is achieved via its ability to block various forms of necrotic cell death.     

Autophagy functions in programmed cell death

Autophagic cell death is a prominent morphological form of cell death that occurs in diverse animals. Autophagosomes are abundant during autophagic cell death, yet the functional role of autophagy in cell death has been enigmatic. We find that autophagy and the Atg genes are required for autophagic cell death of Drosophila salivary glands. Although caspases are present in dying salivary glands, autophagy is required for complete cell degradation. Further, induction of high levels of autophagy results in caspase-independent autophagic cell death. Our results provide the first in vivo evidence that autophagy and the Atg genes are required for autophagic cell death and confirm that autophagic cell death is a physiological death program that occurs during development.     

Autophagy functions in programmed cell death

Autophagic cell death is a prominent morphological form of cell death that occurs in diverse animals. Autophagosomes are abundant during autophagic cell death, yet the functional role of autophagy in cell death has been enigmatic. We find that autophagy and the Atg genes are required for autophagic cell death of Drosophila salivary glands. Although caspases are present in dying salivary glands, autophagy is required for complete cell degradation. Further, induction of high levels of autophagy results in caspase-independent autophagic cell death. Our results provide the first in vivo evidence that autophagy and the Atg genes are required for autophagic cell death and confirm that autophagic cell death is a physiological death program that occurs during development.

Addendum to: Berry DL, Baehrecke EH. Growth arrest and autophagy are required for programmed salivary gland cell degradation in Drosophila. Cell 2007; 131:1137-48.     

Autophagy promotes T-cell survival through degradation of proteins of the cell death machinery

Autophagy is implicated in regulating cell death in activated T cells, but the underlying mechanism is unclear. Here, we show that inhibition of autophagy via Beclin 1 gene deletion in T cells leads to rampant apoptosis in these cells upon TCR stimulation. Beclin 1-deficient mice fail to mount autoreactive T-cell responses and are resistant to experimental autoimmune encephalomyelitis. Compared with Th17 cells, Th1 cells are much more susceptible to cell death upon Beclin 1 deletion. Cell death proteins are highly increased in Beclin 1-deficient T cells and inhibition of caspases and genetic deletion of Bim reverse apoptosis. In addition, p62/sequestosome 1 binds to caspase-8 but does not control levels of procaspase-8 or other cell death-related proteins. These results establish a direct role of autophagy in inhibiting the programmed cell death through degradation of apoptosis proteins in activated T cells.     

Autophagy induced by ionizing radiation promotes cell death over survival in human colorectal cancer cells

Autophagy is commonly described as a cell survival mechanism and has been implicated in chemo- and radioresistance of cancer cells. Whether ionizing radiation induced autophagy triggers tumor cell survival or cell death still remains unclear. In this study the autophagy related proteins Beclin1 and ATG7 were tested as potential targets to sensitize colorectal carcinoma cells to ionizing radiation under normoxic, hypoxic and starvation conditions. Colony formation, apoptosis and cell cycle analysis revealed that knockdown of Beclin1 or ATG7 does not enhance radiosensitivity in HCT-116 cells. Furthermore, ATG7 knockdown led to an increased survival fraction under oxygen and glutamine starvation, indicating that ionizing radiation indeed induces autophagy which, however, leads to cell death finally. These results highlight that inhibition of autophagic pathways does not generally increase therapy success but may also lead to an unfavorable outcome especially under amino acid and oxygen restriction.     

Autophagy in the control of programmed cell death

Programmed cell death (PCD) is essential for plant development and immunity. Localized PCD is associated with the hypersensitive response (HR), which is a constituent of a successful plant innate immune response. Plants have developed mechanisms to meticulously prevent HR-PCD lesions from spreading. Our understanding of these mechanisms is still in its incipient stages. A recent study demonstrated that autophagy, a universally conserved process of macromolecule turnover, plays a pivotal role in controlling HR-PCD. The molecular identity of the mediators between the PCD and HR pathways is still obscure, but recent work has begun to shed light on the relationship between HR-PCD and autophagy and to suggest possible mechanisms for the regulation of these pathways.     

Autophagy contributes to caspase-independent macrophage cell death

Macrophage cell death plays a role in many physiological and pathophysiological conditions. Previous work has shown that macrophages can undergo caspase-independent cell death, and this process is associated with Nur77 induction, which is involved in inducing chromatin condensation and DNA fragmentation. Here we show that autophagy is a cytosolic event that controls caspase-independent macrophage cell death. Autophagy was induced in macrophages treated with lipopolysaccharides (LPSs) and the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp (Z-VAD), and the inhibition of autophagy by either chemical inhibitors or by the RNA interference knockdown of beclin (a protein required for autophagic body formation) inhibited caspase-independent macrophage cell death. We also found an increase in poly(ADP-ribose) (PAR) polymerase (PARP) activation and reactive oxygen species (ROS) production in LPS + Z-VAD-treated macrophages, and both are involved in caspase-independent macrophage cell death. We further determined that the formation of autophagic bodies in macrophages occurs downstream of PARP activation, and PARP activation occurs downstream of ROS production. Using macrophages in which receptor-interacting protein 1 (RIP1) was knocked down by small interfering RNA, and macrophages isolated from Toll/interleukin-1 receptor-domain-containing adaptor inducing IFN-beta (TRIF)-deficient mice, we found that TRIF and RIP1 function upstream of ROS production in LPS + Z-VAD-treated macrophages. We also found that Z-VAD inhibits LPS-induced RIP1 cleavage, which may contribute to ROS over-production in macrophages. This paper reveals that TRIF, RIP1, and ROS production, as well as PARP activation, are involved in inducing autophagy, which contributes to caspase-independent macrophage cell death.     

Autophagy, mitochondria and cell death in lysosomal storage diseases

Lysosomal storage diseases (LSDs) are debilitating genetic conditions that frequently manifest as neurodegenerative disorders. They severely affect eye, motor and cognitive functions and, in most cases, abbreviate the lifespan. Postmitotic cells such as neurons and mononuclear phagocytes rich in lysosomes are most often affected by the accumulation of undegraded material. Cell death is well documented in parts of the brain and in other cells of LSD patients and animal models, although little is known about mechanisms by which death pathways are activated in these diseases, and not all cells exhibiting increased storage material are affected by cell death. Lysosomes are essential for maturation and completion of autophagy-initiated protein and organelle degradation. Moreover, accumulation of effete mitochondria has been documented in postmitotic cells whose lysosomal function is suppressed or in aging cells with lipofuscin accumulation. Based upon observations in the literature and our own data showing similar mitochondrial abnormalities in several LSDs, we propose a new model of cell death in LSDs. We suggest that the lysosomal deficiencies in LSDs inhibit autophagic maturation, leading to a condition of autophagic stress. The resulting accumulation of dysfunctional mitochondria showing impaired Ca2+ buffering increases the vulnerability of the cells to pro-apoptotic signals.     

Autophagy induction and autophagic cell death in effector T cells

The population size of the T cells is tightly regulated. The T cell number drastically increases in response to their specific antigens. Upon antigen clearance, the T cell number decreases over time. Apoptosis, also called type I programmed cell death, plays an important role in eliminating T cells. The role of autophagic cell death, also called type II programmed cell death, is unclear in T cells. Our recent work demonstrated that autophagy is induced in both Th1 and Th2 cells. Both TCR signaling and IL-2 increase autophagy in T cells, and JNK MAP kinases are required for the induction of autophagy in T cells, whereas caspases and mTOR inhibit autophagy in T cells. Autophagy is required for mediating growth factor withdrawal-dependent cell death in T cells. Here, we hypothesize that autophagic cell death plays an important role in T cell homeostasis.     

Autophagy is a cell death mechanism in Toxoplasma gondii

Nutrient sensing and the capacity to respond to starvation is tightly regulated as a means of cell survival. Among the features of the starvation response are induction of both translational repression and autophagy. Despite the fact that intracellular parasite like Toxoplasma gondii within a host cell predicted to be nutrient rich, they encode genes involved in both translational repression and autophagy. We therefore examined the consequence of starvation, a classic trigger of autophagy, on intracellular parasites. As expected, starvation results in the activation of the translational repression system as evidenced by elevation of phosphorylated TgIF2α (TgIF2α-P). Surprisingly, we also observe a rapid and selective fragmentation of the single parasite mitochondrion that leads irreversibly to parasite death. This profound effect was dependent primarily on the limitation of amino acids and involved signalling by the parasite TOR homologue. Notably, the effective blockade of mitochondrial fragmentation by the autophagy inhibitor 3-methyl adenine (3-MA) suggests an autophagic mechanism. In the absence of a documented apoptotic cascade in T. gondii, the data suggest that autophagy is the primary mechanism of programmed cell death in T. gondii and potentially other related parasites.     

Autophagy and cell death: no longer at odds

Autophagy has been associated with both cell survival and cell death, but the role of autophagy in cell death has been controversial. In this issue, Berry and Baehrecke (2007) report that autophagy is involved in physiological cell death during Drosophila development and is controlled by similar mechanisms as those that control its function in cell survival.     

Autophagy promotes caspase-dependent cell death during Drosophila development

The relationship between autophagic cell death and apoptosis is a poorly understood aspect of programmed cell death (PCD). We have examined this relationship by studying the elimination of an extra-embryonic tissue, known as the amnioserosa (AS), during Drosophila development. The AS becomes autophagic during the final stages of embryogenesis; ultimately, however, the elimination of the AS involves caspase-dependent nuclear fragmentation, tissue dissociation, and engulfment by phagocytic macrophages. Mutants that are defective in the activation or execution of caspase-dependent PCD fail to degrade and eliminate the AS but show no abatement in AS autophagy. Sustained autophagy does not, therefore, necessarily result in cell death. Surprisingly, the down-regulation of autophagy also results in a persistent AS phenotype and reduced cell death. Conversely, up-regulation of autophagy results in caspase-dependent premature AS dissociation. These observations are consistent with the interpretation that autophagy is a prerequisite for caspase-dependent cell death in the AS.     

Autophagy promotes cell survival by maintaining NAD levels

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Autophagy is required for necrotic cell death in Caenorhabditis elegans

Autophagy is the main process for bulk protein and organelle recycling in cells under extracellular or intracellular stress. Deregulation of autophagy has been associated with pathological conditions such as cancer, muscular disorders and neurodegeneration. Necrotic cell death underlies extensive neuronal loss in acute neurodegenerative episodes such as ischemic stroke. We find that excessive autophagosome formation is induced early during necrotic cell death in C. elegans. In addition, autophagy is required for necrotic cell death. Impairment of autophagy by genetic inactivation of autophagy genes or by pharmacological treatment suppresses necrosis. Autophagy synergizes with lysosomal catabolic mechanisms to facilitate cell death. Our findings demonstrate that autophagy contributes to cellular destruction during necrosis. Thus, interfering with the autophagic process may protect neurons against necrotic damage in humans.