Autophagic degradation of active caspase-8: a crosstalk mechanism between autophagy and apoptosis

New frontiers in promoting tumour cell death: targeting apoptosis, necroptosis and autophagy

Cancer is a multifaceted disease comprising a combination of genetic, metabolic and signalling aberrations, which severely disrupt the normal homeostasis of cell growth and death. Many oncogenic events while promoting tumour development also increase the sensitivity of cells to cell death stimuli including chemotherapeutic drugs. As a result, tumour cells often acquire the ability to evade death by inactivating cell death pathways that normally function to eliminate damaged and harmful cells. The impairment of cell death function is also often the reason for the development of chemotherapeutic resistance encountered during treatment. It is therefore necessary to achieve a comprehensive understanding of existing cell death pathways and the relevant regulatory components involved, with the intention of identifying new strategies to kill cancer cells. This review provides an insightful overview of the common forms of cell death signalling pathways, the interactions between these pathways and the ways in which these pathways are deregulated in cancer. We also discuss the emerging therapies targeted at activating or restoring cell death pathways to induce tumour cell death, which are currently being tested in clinical trials.

Attenuation of TNFSF10/TRAIL-induced apoptosis by an autophagic survival pathway involving TRAF2- and RIPK1/RIP1-mediated MAPK8/JNK activation

Although it is known that tumor necrosis factor-related apoptosis-inducing ligand (TNFSF10/TRAIL) induces autophagy, the mechanism by which autophagy is activated by TNFSF10 is still elusive. In this report, we show evidence that TRAF2- and RIPK1-mediated MAPK8/JNK activation is required for TNFSF10-induced cytoprotective autophagy. TNFSF10 activated autophagy rapidly in cancer cell lines derived from lung, bladder and prostate tumors. Blocking autophagy with either pharmacological inhibitors or siRNAs targeting the key autophagy factors BECN1/Beclin 1 or ATG7 effectively increased TNFSF10-induced apoptotic cytotoxicity, substantiating a cytoprotective role for TNFSF10-induced autophagy. Blocking MAPK8 but not NFκB effectively blocked autophagy, suggesting that MAPK8 is the main pathway for TNFSF10-induced autophagy. In addition, blocking MAPK8 effectively inhibited degradation of BCL2L1/Bcl-xL and reduction of the autophagy-suppressing BCL2L1-BECN1complex. Knockdown of TRAF2 or RIPK1 effectively suppressed TNFSF10-induced MAPK8 activation and autophagy. Furthermore, suppressing autophagy inhibited expression of antiapoptosis factors BIRC2/cIAP1, BIRC3/cIAP2, XIAP and CFLAR/c-FLIP and increased the formation of TNFSF10-induced death-inducing signaling complex (DISC). These results reveal a critical role for the MAPK8 activation pathway through TRAF2 and RIPK1 for TNFSF10-induced autophagy that blunts apoptosis in cancer cells. Thus, suppression of MAPK8-mediated autophagy could be utilized for sensitizing cancer cells to therapy with TNFSF10.

A role for c-FLIP(L) in the regulation of apoptosis, autophagy, and necroptosis in T lymphocytes

Caspase 8 plays a dual role in the survival of T lymphocytes. Although active caspase 8 mediates apoptosis upon death receptor signaling, the loss of caspase 8 activity leads to receptor-interacting protein (RIP)-1/RIP-3-dependent necrotic cell death (necroptosis) upon TCR activation. The anti-apoptotic protein c-FLIP (cellular caspase 8 (FLICE)-like inhibitory protein) suppresses death receptor-induced caspase 8 activation. Moreover, recent findings suggest that c-FLIP is also involved in inhibiting necroptosis and autophagy. It remains unclear whether c-FLIP protects primary T lymphocytes from necroptosis or regulates the threshold at which autophagy occurs. Here, we used a c-FLIP isoform-specific conditional deletion model to show that c-FLIP(L)-deficient T cells underwent RIP-1-dependent necroptosis upon TCR stimulation. Interestingly, although previous studies have only described necroptosis in the absence of caspase 8 activity, we found that pro-apoptotic caspase 8 activity and apoptosis were also enhanced in c-FLIP(L)-deficient T lymphocytes. Furthermore, c-FLIP(L)-deficient T cells exhibited enhanced autophagy, which served a cytoprotective function. Together, these findings indicate that c-FLIP(L) plays an important antinecroptotic role and is a key regulator of apoptosis, autophagy, and necroptosis in T lymphocytes.

Matrine, a novel autophagy inhibitor, blocks trafficking and the proteolytic activation of lysosomal proteases

Autophagy has been referred to as a double-edged sword in tumorigenesis and tumor progression. Emerging evidence suggests that pharmacological modulation of autophagy is a promising therapeutic strategy for cancer. However, few autophagy-modulating compounds are currently approved for clinical use in humans. Matrine is a natural compound extracted from traditional Chinese medicine that is widely used for treatment of a variety of diseases without any obvious side effects. Recently, matrine has been reported to induce autophagy and autophagic cell death in cancer cells, although the underlying mechanisms have yet to be elucidated. Here, we systematically examined the autophagic events induced by matrine in SGC7901 cells. The accumulation of autophagic vacuoles in matrine-treated cells was verified by the conversion of microtubule-associated protein light chain 3 as well as confocal and transmission electron microscopy. Furthermore, we demonstrated that matrine blocked autophagic degradation by impairing the activities of lysosomal proteases. Moreover, confocal microscopy and gradient ultracentrifugation revealed that the trafficking processes and proteolytic activation of cathepsins were inhibited by matrine. Using a pH sensor probe, we found elevated pH values in endosomes/lysosomes in response to matrine treatment. Therefore, matrine seems to be a novel autophagy inhibitor that can modulate the maturation process of lysosomal proteases.

Neem oil limonoids induces p53-independent apoptosis and autophagy

Azadirachta indica, commonly known as neem, has a wide range of medicinal properties. Neem extracts and its purified products have been examined for induction of apoptosis in multiple cancer cell types; however, its underlying mechanisms remain undefined. We show that neem oil (i.e., neem), which contains majority of neem limonoids including azadirachtin, induced apoptotic and autophagic cell death. Gene silencing demonstrated that caspase cascade was initiated by the activation of caspase-9, whereas caspase-8 was also activated late during neem-induced apoptosis. Pretreatment of cancer cells with pan caspase inhibitor, z-VAD inhibited activities of both initiator caspases (e.g., caspase-8 and -9) and executioner caspase-3. Neem induced the release of cytochrome c and apoptosis-inducing factor (AIF) from mitochondria, suggesting the involvement of both caspase-dependent and AIF-mediated apoptosis. p21 deficiency caused an increase in caspase activities at lower doses of neem, whereas p53 deficiency did not modulate neem-induced caspase activation. Additionally, neem treatment resulted in the accumulation of LC3-II in cancer cells, suggesting the involvement of autophagy in neem-induced cancer cell death. Low doses of autophagy inhibitors (i.e., 3-methyladenine and LY294002) did not prevent accumulation of neem-induced LC3-II in cancer cells. Silencing of ATG5 or Beclin-1 further enhanced neem-induced cell death. Phosphoinositide 3-kinase (PI3K) or autophagy inhibitors increased neem-induced caspase-3 activation and inhibition of caspases enhanced neem-induced autophagy. Together, for the first time, we demonstrate that neem induces caspase-dependent and AIF-mediated apoptosis, and autophagy in cancer cells.

Mitochondrial dynamics and mitophagy in the 6-hydroxydopamine preclinical model of Parkinson's disease

We discuss the participation of mitochondrial dynamics and autophagy in the 6-hydroxidopamine-induced Parkinson's disease model. The regulation of dynamic mitochondrial processes such as fusion, fission, and mitophagy has been shown to be an important mechanism controlling cellular fate. An imbalance in mitochondrial dynamics may contribute to both familial and sporadic neurodegenerative diseases including Parkinson's disease. With special attention we address the role of second messengers as the role of reactive oxygen species and the mitochondria as the headquarters of cell death. The role of molecular signaling pathways, for instance, the participation of Dynamin-related protein 1(Drp1), will also be addressed. Furthermore evidence demonstrates the therapeutic potential of small-molecule inhibitors of mitochondrial division in Parkinson's disease. For instance, pharmacological inhibition of Drp1, through treatment with the mitochondrial division inhibitor-1, results in the abrogation of mitochondrial fission and in a decrease of the number of autophagic cells. Deciphering the signaling cascades that underlie mitophagy triggered by 6-OHDA, as well as the mechanisms that determine the selectivity of this response, will help to better understand this process and may have impact on human treatment strategies of Parkinson's disease.

Apoptosis and autophagy in breast cancer cells following exemestane treatment

Aromatase inhibitors (AIs), which block the conversion of androgens to estrogens, are used for hormone-dependent breast cancer treatment. Exemestane, a steroidal that belongs to the third-generation of AIs, is a mechanism-based inhibitor that binds covalently and irreversibly, inactivating and destabilizing aromatase. Since the biological effects of exemestane in breast cancer cells are not totally understood, its effects on cell viability, cell proliferation and mechanisms of cell death were studied in an ER-positive aromatase-overexpressing breast cancer cell line (MCF-7aro). The effects of 3-methyladenine (3-MA), an inhibitor of autophagy and of ZVAD-FMK, an apoptotic inhibitor, in exemestane treated cells were also investigated. Our results indicate that exemestane induces a strong inhibition in MCF-7aro cell proliferation in a dose- and time-dependent manner, promoting a significant cell cycle arrest in G(0)/G1 or in G(2)/M phases after 3 and 6 days of treatment, respectively. This was accompanied by a decrease in cell viability due to activation of cell death by apoptosis, via mitochondrial pathway and the occurrence of autophagy. Inhibition of autophagy by the autophagic inhibitor, 3-MA, resulted in a reduction of cell viability and activation of caspases. All together the results obtained suggest that exemestane induced mitochondrial-mediated apoptosis and autophagy, which act as a pro-survival process regulating breast cancer cell apoptosis.

Autophagy and the immune system

Stressors ranging from nutrient deprivation to immune signaling can induce the degradation of cytoplasmic material by a process known as autophagy. Increasingly, research on autophagy has begun to focus on its role in inflammation and the immune response. Autophagy acts as an immune effector that mediates pathogen clearance. The roles of autophagy bridge both the innate and adaptive immune systems and include functions in thymic selection, antigen presentation, promotion of lymphocyte homeostasis and survival, and regulation of cytokine production. In this review, we discuss the mechanisms by which autophagy is regulated, as well as the functions of autophagy and autophagy proteins in immunity and inflammation.

The protein factor-arrest 11 (Far11) is essential for the toxicity of human caspase-10 in yeast and participates in the regulation of autophagy and the DNA damage signaling

The heterologous expression of human caspase-10 in Saccharomyces cerevisiae induces a lethal phenotype, which includes some hallmarks of apoptosis and autophagy, alterations in the intra-S checkpoint, and cell death. To determine the cellular processes and pathways that are responsible of the caspase-10-induced cell death we have designed a loss-of-function screening system to identify genes that are essential for the lethal phenotype. We observed that the ER-Golgi-localized family of proteins Far, MAPK signaling, the autophagy machinery, and several kinases and phosphatases are essential for caspase-10 toxicity. We also found that the expression of caspase-10 elicits a simultaneous activation of the MAP kinases Fus3, Kss1, and Slt2. Furthermore, the protein Far11, which is a target of MAP kinases, is essential for the dephosphorylation of Atg13 and, consequently, for the induction of autophagy. In addition, Far11 participates in the regulation of the DNA damage response through the dephosphorylation of Rad53. Finally, we have also demonstrated that Far11 is able to physically interact with the phosphatases Pph21, Pph22, and Pph3. Overall, our results indicate that the expression of human caspase-10 in S. cerevisiae activates an intracellular death signal that depends on the Far protein complex and that Far11 may function as a regulator subunit of phosphatases in different processes, thus representing a mechanistic link between them.

Antitumor effects of cytoplasmic delivery of an innate adjuvant receptor ligand, poly(I:C), on human breast cancer

Innate adjuvant receptors are expressed in immune cells and some types of cancers. If antitumor therapies targeting these receptors are established, it is likely that they will be therapeutically beneficial because antitumor effects and immune-cell activation can be induced simultaneously. In this study, we tested this possibility of using an innate adjuvant receptor ligand, polyinosinic-polycytidylic acid [poly(I:C)], to treat human breast cancer cell lines. Three breast cancer cell lines (MCF-7, MDA-MB-231, and BT-549) were used in this study. Poly(I:C) was transfected into these cancer cells to stimulate melanoma differentiation-associated gene (MDA) 5, which is a cytoplasmic adjuvant receptor. Poly(I:C) transfection significantly reduced the viability of all cell lines in a manner partially dependent on MDA5. Flow cytometeric analyses and immunoblot assays revealed that the antitumor effect depended on both caspase-dependent apoptosis and c-Myc- and cyclinD1-dependent growth arrest. Interestingly, poly(I:C) transfection was accompanied by autophagy, which is thought to protect cancer cells from apoptosis after poly(I:C) transfection. In a xenograft mouse model, local transfection of poly(I:C) significantly inhibited the growth of xenografted MDA-MB-231 cells. Our findings indicate that cytoplasmic delivery of poly(I:C) can induce apoptosis and growth arrest of human breast cancer cells, and that therapy-associated autophagy prevents apoptosis. The results of this study suggest that the innate adjuvant receptors are promising targets and that their ligands could serve as antitumor reagents, which have the potential to simultaneously induce antitumor effects and activate immune cells.

Emerging role of autophagy in kidney function, diseases and aging

Autophagy is a highly conserved process that degrades cellular long-lived proteins and organelles. Accumulating evidence indicates that autophagy plays a critical role in kidney maintenance, diseases and aging. Ischemic, toxic, immunological, and oxidative insults can cause an induction of autophagy in renal epithelial cells modifying the course of various kidney diseases. This review summarizes recent insights on the role of autophagy in kidney physiology and diseases alluding to possible novel intervention strategies for treating specific kidney disorders by modifying autophagy.

Proteolysis of Ambra1 during apoptosis has a role in the inhibition of the autophagic pro-survival response

Under stress conditions, pro-survival and pro-death processes are concomitantly activated and the final outcome depends on the complex crosstalk between these pathways. In most cases, autophagy functions as an early-induced cytoprotective response, favoring stress adaptation by removing damaged subcellular constituents. Moreover, several lines of evidence suggest that autophagy inactivation by the apoptotic machinery is a crucial event for cell death execution. Here we show that apoptotic stimuli induce a rapid decrease in the level of the autophagic factor Activating Molecule in Beclin1-Regulated Autophagy (Ambra1). Ambra1 degradation is prevented by concomitant inhibition of caspases and calpains. By both in vitro and in vivo approaches, we demonstrate that caspases are responsible for Ambra1 cleavage at the D482 site, whereas calpains are involved in complete Ambra1 degradation. Finally, we show that Ambra1 levels are critical for the rate of apoptosis induction. RNA interference-mediated Ambra1 downregulation further sensitizes cells to apoptotic stimuli, while Ambra1 overexpression and, more efficiently, a caspase non-cleavable mutant counteract cell death by prolonging autophagy induction. We conclude that Ambra1 is an important target of apoptotic proteases resulting in the dismantling of the autophagic machinery and the accomplishment of the cell death program.

Phospho-ΔNp63α-dependent regulation of autophagic signaling through transcription and micro-RNA modulation

Cisplatin was shown to induce the ataxia telangiectasia mutated (ATM)-dependent phosphorylation of tumor protein p63 isoform, (ΔNp63α), leading to a transcriptional regulation of specific genes implicated in the control of cell death of squamous cell carcinoma (SCC) cells. We previously observed that the cisplatin-induced phosphorylated (p)-ΔNp63α transcriptionally regulates the expression of specific microRNAs (miRNAs) in SCC cells. We found here that cisplatin exposure of SCC cells led to modulation of the members of the autophagic pathway, such as Atg1/Ulk1, Atg3, Atg4A, Atg5, Atg6/Becn1, Atg7, Atg9A and Atg10, by a direct p-ΔNp63α-dependent transcriptional regulation. We further found that specific miRNAs (miR-181a, miR-519a, miR-374a and miR-630), which are critical downstream targets of the p-ΔNp63α, modulated the protein levels of ATG5, ATG6/BECN1, ATG10, ATG12, ATG16L1 and UVRAG, adding another level of expression control for autophagic pathways in SCC cells upon cisplatin exposure. Our data support the notion that the cisplatin-induced p-ΔNp63α could regulate key pathways implicated in response of cancer cells to chemotherapeutics.

The crosstalk between autophagy and apoptosis: where does this lead?

Recent advances in the understanding of the molecular processes contributing to autophagy have provided insight into the relationship between autophagy and apoptosis. In contrast to the concept of "autophagic cell death," accumulating evidence suggests that autophagy serves a largely cytoprotective role in physiologically relevant conditions. The cytoprotective function of autophagy is mediated in many circumstances by negative modulation of apoptosis. Apoptotic signaling, in turn, serves to inhibit autophagy. While the mechanisms mediating the complex counter-regulation of apoptosis and autophagy are not yet fully understood, important points of crosstalk include the interactions between Beclin-1 and Bcl-2/Bcl-xL and between FADD and Atg5, caspase- and calpain-mediated cleavage of autophagy-related proteins, and autophagic degradation of caspases. Continued investigation of these and other means of crosstalk between apoptosis and autophagy is necessary to elucidate the mechanisms controlling the balance between survival and death both under normal conditions and in diseases including cancer.

The adaptor protein p62/SQSTM1 in osteoclast signaling pathways

Paget's disease of bone (PDB) is a skeletal disorder characterized by focal and disorganized increases in bone turnover and overactive osteoclasts. The discovery of mutations in the SQSTM1/p62 gene in numerous patients has identified protein p62 as an important modulator of bone turnover. In both precursors and mature osteoclasts, the interaction between receptor activator of NF-κB ligand (RANKL) and its receptor RANK results in signaling cascades that ultimately activate transcription factors, particularly NF-κB and NFATc1, promoting and regulating the osteoclast differentiation, activity, and survival. As a scaffold with multiple protein-protein interaction motifs, p62 is involved in virtually all the RANKL-activated osteoclast signaling pathways, along with being implicated in numerous other cellular processes. The p62 adaptor protein is one of the functional links reported between RANKL and TRAF6-mediated NF-κB activation, and also plays a major role as a shuttling factor that targets polyubiquitinated proteins for degradation by either the autophagy or proteasome pathways. The dysregulated expression and/or activity of p62 in bone disease up-regulates osteoclast functions. This review aims to outline and summarize the role of p62 in RANKL-induced signaling pathways and in ubiquitin-mediated signaling in osteoclasts, and the impact of PDB-associated p62 mutations on these processes.

Autophagy: for better or for worse

Autophagy is a lysosomal degradation pathway that degrades damaged or superfluous cell components into basic biomolecules, which are then recycled back into the cytosol. In this respect, autophagy drives a flow of biomolecules in a continuous degradation-regeneration cycle. Autophagy is generally considered a pro-survival mechanism protecting cells under stress or poor nutrient conditions. Current research clearly shows that autophagy fulfills numerous functions in vital biological processes. It is implicated in development, differentiation, innate and adaptive immunity, ageing and cell death. In addition, accumulating evidence demonstrates interesting links between autophagy and several human diseases and tumor development. Therefore, autophagy seems to be an important player in the life and death of cells and organisms. Despite the mounting knowledge about autophagy, the mechanisms through which the autophagic machinery regulates these diverse processes are not entirely understood. In this review, we give a comprehensive overview of the autophagic signaling pathway, its role in general cellular processes and its connection to cell death. In addition, we present a brief overview of the possible contribution of defective autophagic signaling to disease.

Neuronal deletion of caspase 8 protects against brain injury in mouse models of controlled cortical impact and kainic acid-induced excitotoxicity

Background

Acute brain injury is an important health problem. Given the critical position of caspase 8 at the crossroads of cell death pathways, we generated a new viable mouse line (Ncasp8^−/−), in which the gene encoding caspase 8 was selectively deleted in neurons by cre-lox system.

Methodology/Principal Findings

Caspase 8 deletion reduced rates of neuronal cell death in primary neuronal cultures and in whole brain organotypic coronal slice cultures prepared from 4 and 8 month old mice and cultivated up to 14 days in vitro. Treatments of cultures with recombinant murine TNFα (100 ng/ml) or TRAIL (250 ng/mL) plus cyclohexamide significantly protected neurons against cell death induced by these apoptosis-inducing ligands. A protective role of caspase 8 deletion in vivo was also demonstrated using a controlled cortical impact (CCI) model of traumatic brain injury (TBI) and seizure-induced brain injury caused by kainic acid (KA). Morphometric analyses were performed using digital imaging in conjunction with image analysis algorithms. By employing virtual images of hundreds of brain sections, we were able to perform quantitative morphometry of histological and immunohistochemical staining data in an unbiased manner. In the TBI model, homozygous deletion of caspase 8 resulted in reduced lesion volumes, improved post-injury motor performance, superior learning and memory retention, decreased apoptosis, diminished proteolytic processing of caspases and caspase substrates, and less neuronal degeneration, compared to wild type, homozygous cre, and caspase 8-floxed control mice. In the KA model, Ncasp8^−/− mice demonstrated superior survival, reduced seizure severity, less apoptosis, and reduced caspase 3 processing. Uninjured aged knockout mice showed improved learning and memory, implicating a possible role for caspase 8 in cognitive decline with aging.

Conclusions

Neuron-specific deletion of caspase 8 reduces brain damage and improves post-traumatic functional outcomes, suggesting an important role for this caspase in pathophysiology of acute brain trauma.

Lysosomes and lysosomal cathepsins in cell death

Lysosomes are the key degradative compartments of the cell. Lysosomal cathepsins, which are enclosed in the lysosomes, help to maintain the homeostasis of the cell's metabolism by participating in the degradation of heterophagic and autophagic material. Following the targeted lysosomal membrane's destabilization, the cathepsins can be released into the cytosol and initiate the lysosomal pathway of apoptosis through the cleavage of Bid and the degradation of the anti-apoptotic Bcl-2 homologues. Cathepsins can also amplify the apoptotic signaling, when the lysosomal membranes are destabilized at a later stage of apoptosis, initiated by other stimuli. However, the functional integrity of the lysosomal compartment during apoptosis enables efficient autophagy, which can counteract apoptosis by providing the energy source and by disposing the damaged mitochondria, which generate the ROS. Impairing autophagy by disabling the lysosome function is being investigated as an adjuvant therapeutic approach to sensitize cells to apoptosis-inducing agents. Destabilization of the lysosomal membranes by the lysosomotropic detergents seems to be a promising strategy in this context as it would not only disable autophagy, but also promote apoptosis through the initiation of the lysosomal pathway. In contrast, the impaired autophagy and lysosomal degradation linked with the increased oxidative stress underlie degenerative changes in the aging neurons. This further suggests that lysosomes and lysosomal cathepsins have a dual role in cell death. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Autophagy and apoptosis: what is the connection?

The therapeutic potential of autophagy for the treatment cancer and other diseases is beset by paradoxes stemming from the complexity of the interactions between the apoptotic and autophagic machinery. The simplest question of how autophagy acts as both a protector and executioner of cell death remains the subject of substantial controversy. Elucidating the molecular interactions between the processes will help us understand how autophagy can modulate cell death, whether autophagy is truly a cell death mechanism, and how these functions are regulated. We suggest that, despite many connections between autophagy and apoptosis, a strong causal relationship wherein one process controls the other, has not been demonstrated adequately. Knowing when and how to modulate autophagy therapeutically depends on understanding these connections.

Autophagy activation in the injured photoreceptor inhibits fas-mediated apoptosis

PURPOSE

To examine the activation of autophagy and its relationship to Fas-mediated photoreceptor apoptosis during experimental retinal detachment.

METHODS

Retina-retinal pigment epithelium (RPE) separation was created in Brown-Norway rats by subretinal injection of 1% hyaluronic acid and the intraretinal levels of the autophagy proteins LC3 and Atg5, the time course of LC3-I to LC3-II conversion, and the activation of cathepsins B and D were assayed with Western blot analysis and immunohistochemistry. We measured the ability of a Fas-activating antibody to induce LC3-I to LC3-II conversion in 661W cells, and the in vivo effect of Met12, a small molecule inhibitor of the Fas receptor, on LC3-I to LC3-II conversion and Atg5 expression. Autophagy activation was inhibited using 3-methyladenine (3-MA) or siRNA knockdown of Atg5 and the effect on apoptosis was measured using a caspase 8 activity assay, caspase 8 immunoblots, and photoreceptor TUNEL staining.

RESULTS

Retina-RPE separation resulted in a Fas-dependent activation of autophagy, with increased Atg5 levels and intraphotoreceptor conversion of LC3-I to LC3-II. Detached retinas had increased levels of autophagosome-associated lysosomal proteases, cathepsins B and D. Inhibition of autophagy by 3-MA or siAtg5 accelerated the time course of caspase 8 activation and photoreceptor TUNEL staining.

CONCLUSIONS

Autophagy activation occurs in the photoreceptors after retina-RPE separation. This appears to be, at least in part, dependent on Fas receptor activation, and plays a role in regulating the level of photoreceptor apoptosis.

Apolipoprotein L6, induced in atherosclerotic lesions, promotes apoptosis and blocks Beclin 1-dependent autophagy in atherosclerotic cells

Inflammatory cytokine-regulated apoptosis and autophagy play pivotal roles in plaque rupture and thrombosis of atherosclerotic lesions. However, the molecular interplay between apoptosis and autophagy in vascular cells has not been investigated. Our prior study showed that human apolipoprotein L6 (ApoL6), a pro-apoptotic BH3-only member of the Bcl-2 family, was one of the downstream targets of interferon-γ (INFγ), which sensitizes atherosclerotic lesion-derived cells (LDCs) to Fas-induced apoptosis. To investigate whether ApoL6 plays a causal role in atherosclerotic apoptosis and autophagy, in this study, we demonstrate that IFNγ treatment itself strongly induces ApoL6, and ApoL6 is highly expressed and partially co-localized with activated caspase 3 in activated smooth muscle cells in atherosclerotic lesions. In addition, overexpression of ApoL6 promotes reactive oxygen species (ROS) generation, caspase activation, and subsequent apoptosis, which can be blocked by pan caspase inhibitor and ROS scavenger. Knockdown of ApoL6 expression by siApoL6 suppresses INFγ- and Fas-mediated apoptosis. Further, ApoL6 binds Bcl-X(L), one of the most abundant anti-death proteins in LDCs. Interestingly, forced ApoL6 expression in LDCs induces degradation of Beclin 1, accumulation of p62, and subsequent attenuation of LC3-II formation and translocation and thus autophagy, whereas siApoL6 treatment reverts the phenotype. Taken together, our results suggest that ApoL6 regulates both apoptosis and autophagy in SMCs. IFNγ-initiated, ApoL6-induced apoptosis in vascular cells may be an important factor causing plaque instability and a potential therapeutic target for treating atherosclerosis and cardiovascular disease.

Responses of two insect cell lines to starvation: autophagy prevents them from undergoing apoptosis and necrosis, respectively

The responses of insect cells to starvation and the characteristics of cell death after the depletion of nutrients remain largely unknown. In the present study, we investigated autophagy, apoptosis and necrosis in two Lepidoptera insect cell lines in response to amino acid starvation. Our data demonstrated that starvation induced a significant increase in autophagy in Spodoptera litura SL-ZSU-1 cells, and cell apoptosis followed autophagy after starvation of more than 48h. However, at an early stage of starvation, inhibition of autophagy with 3-MA rapidly triggered apoptosis of SL-ZSU-1 cells, suggesting autophagy inhibits cell apoptosis. By contrast, Bombyx mori SPC Bm36 cells died by a non-apoptotic pathway if the starvation was prolonged for more than 48 h. At the early stage of starvation, inhibition of autophagy with 3-MA did not trigger apoptosis in Bm36 cells, but resulted in necrotic-like cell death. Under starvation pressure, autophagy in SL-ZSU-1 cells was much more active than in Bm36 cells. The activity of caspase-9-like in apoptotic SL-ZSU-1 cells also was much higher than in apoptotic Bm36 cells. RT-PCR analyses revealed that transcriptional levels of saposin-like (Bm109) and Atg6 were undetectable in Bm36 cells, but expression level of saposin-like in SL-ZSU-1 was high. Expression of Atg6 in SL-ZSU-1 cells was not analyzed because its sequence was unknown. These data indicate that autophagy prevents Lepidoptera insect cells from death at an early stage of starvation, but prolonged starvation results in cell death. The pathways of cell death might be dependent on the abundance of caspase-9-like, saposin-like and Atg6.

The regulation of autophagy - unanswered questions

Autophagy is an intracellular lysosomal (vacuolar) degradation process that is characterized by the formation of double-membrane vesicles, known as autophagosomes, which sequester cytoplasm. As autophagy is involved in cell growth, survival, development and death, the levels of autophagy must be properly regulated, as indicated by the fact that dysregulated autophagy has been linked to many human pathophysiologies, such as cancer, myopathies, neurodegeneration, heart and liver diseases, and gastrointestinal disorders. Substantial progress has recently been made in understanding the molecular mechanisms of the autophagy machinery, and in the regulation of autophagy. However, many unanswered questions remain, such as how the Atg1 complex is activated and the function of PtdIns3K is regulated, how the ubiquitin-like conjugation systems participate in autophagy and the mechanisms of phagophore expansion and autophagosome formation, how the network of TOR signaling pathways regulating autophagy are controlled, and what the underlying mechanisms are for the pro-cell survival and the pro-cell death effects of autophagy. As several recent reviews have comprehensively summarized the recent progress in the regulation of autophagy, we focus in this Commentary on the main unresolved questions in this field.

The Beclin 1 network regulates autophagy and apoptosis

Beclin 1, the mammalian orthologue of yeast Atg6, has a central role in autophagy, a process of programmed cell survival, which is increased during periods of cell stress and extinguished during the cell cycle. It interacts with several cofactors (Atg14L, UVRAG, Bif-1, Rubicon, Ambra1, HMGB1, nPIST, VMP1, SLAM, IP(3)R, PINK and survivin) to regulate the lipid kinase Vps-34 protein and promote formation of Beclin 1-Vps34-Vps15 core complexes, thereby inducing autophagy. In contrast, the BH3 domain of Beclin 1 is bound to, and inhibited by Bcl-2 or Bcl-XL. This interaction can be disrupted by phosphorylation of Bcl-2 and Beclin 1, or ubiquitination of Beclin 1. Interestingly, caspase-mediated cleavage of Beclin 1 promotes crosstalk between apoptosis and autophagy. Beclin 1 dysfunction has been implicated in many disorders, including cancer and neurodegeneration. Here, we summarize new findings regarding the organization and function of the Beclin 1 network in cellular homeostasis, focusing on the cross-regulation between apoptosis and autophagy.

Combined modality therapy with TRAIL or agonistic death receptor antibodies

Molecularly targeted therapies, such as antibodies and small molecule inhibitors have emerged as an important breakthrough in the treatment of many human cancers. One targeted therapy under development is tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) due to its ability to induce apoptosis in a variety of human cancer cell lines and xenografts, while lacking toxicity in most normal cells. TRAIL and apoptosis-inducing agonistic antibodies to the TRAIL death receptors have been the subject of many preclinical and clinical studies in the past decade. However, the sensitivity of individual cancer cell lines of a particular tumor type to these agents varies from highly sensitive to resistant. Various chemotherapy agents have been shown to enhance the apoptosis-inducing capacity of TRAIL receptor-targeted therapies and induce sensitization of TRAIL-resistant cells. This review provides an overview of the mechanisms associated with chemotherapy enhancement of TRAIL receptor-targeted therapies including modulation of the apoptotic (death receptor expression, FLIP, and Bcl-2 or inhibitors of apoptosis (IAP) families) as well as cell signaling (NFκB, Akt, p53) pathways. These mechanisms will be important in establishing effective combinations to pursue clinically and in determining relevant targets for future cancer therapies.

Autophagy and cancer therapy

Although autophagy has been shown to have a clear role as a tumor suppressor mechanism, its role in cancer treatment is still controversial. Because autophagy is a survival pathway activated during nutrient deprivation and other stresses, it is reasonable to think that autophagy can function as a tumor cell survival mechanism activated after cancer treatment. Such a mechanism could be widely important because most cancer treatments induce autophagy in tumor cells. Indeed, many papers have presented data suggesting that tumor cell autophagy induced by anti-cancer treatment inhibits tumor cell killing. However, it has also been proposed that autophagy is a cell death mechanism that could function as a backup when apoptosis is disabled. The fact that there are active clinical trials in patients both using autophagy inhibitors or inducers together with other cancer treatments underscores the importance of understanding and distinguishing between these opposing ideas. Here we discuss some of the recent work studying the role of autophagy with different cancer therapies.

Apoptosis promotes early tumorigenesis

One hallmark of cancer is intrinsic or acquired resistance to apoptosis. Surprisingly, recent studies demonstrate that CD95/Fas/Apo1 and p53 upregulated mediator of apoptosis/PUMA (potent inducers of the death receptor and the mitochondrial apoptotic pathways, respectively) promote tumorigenesis. These findings provide important insights into the multifaceted roles of apoptosis in tumorigenesis.