Cleavage of Atg3 protein by caspase-8 regulates autophagy during receptor-activated cell death

Autophagy Function and Regulation in Kidney Disease

Autophagy is a dynamic process by which intracellular damaged macromolecules and organelles are degraded and recycled for the synthesis of new cellular components. Basal autophagy in the kidney acts as a quality control system and is vital for cellular metabolic and organelle homeostasis. Under pathological conditions, autophagy facilitates cellular adaptation; however, activation of autophagy in response to renal injury may be insufficient to provide protection, especially under dysregulated conditions. Kidney-specific deletion of Atg genes in mice has consistently demonstrated worsened acute kidney injury (AKI) outcomes supporting the notion of a pro-survival role of autophagy. Recent studies have also begun to unfold the role of autophagy in progressive renal disease and subsequent fibrosis. Autophagy also influences tubular cell death in renal injury. In this review, we reported the current understanding of autophagy regulation and its role in the pathogenesis of renal injury. In particular, the classic mammalian target of rapamycin (mTOR)-dependent signaling pathway and other mTOR-independent alternative signaling pathways of autophagy regulation were described. Finally, we summarized the impact of autophagy activation on different forms of cell death, including apoptosis and regulated necrosis, associated with the pathophysiology of renal injury. Understanding the regulatory mechanisms of autophagy would identify important targets for therapeutic approaches.

Prognostic implications of autophagy-associated gene signatures in non-small cell lung cancer

Autophagy, a highly conserved cellular proteolysis process, has been involved in non-small cell lung cancer (NSCLC). We tried to develop a prognostic prediction model for NSCLC patients based on the expression profiles of autophagy-associated genes. Univariate Cox regression analysis was used to determine autophagy-associated genes significantly correlated with overall survival (OS) of the TCGA lung cancer cohort. LASSO regression was performed to build multiple-gene prognostic signatures. We found that the 22-gene and 11-gene signatures could dichotomize patients with significantly different OS and independently predict the OS in TCGA lung adenocarcinoma (HR=2.801, 95% CI=2.252-3.486, P<0.001) and squamous cell carcinoma (HR=1.105, 95% CI=1.067-1.145, P<0.001), respectively. The prognostic performance of the 22-gene signature was validated in four GEO lung cancer cohorts. Moreover, GO, KEGG, and GSEA analyses unveiled several fundamental signaling pathways and cellular processes associated with the 22-gene signature in lung adenocarcinoma. We also constructed a clinical nomogram with a concordance index of 0.71 to predict the survival possibility of NSCLC patients by integrating clinical characteristics and the autophagy gene signature. The calibration curves substantiated fine concordance between nomogram prediction and actual observation. Overall, we constructed and verified a novel autophagy-associated gene signature that could improve the individualized outcome prediction in NSCLC.

Mechanisms and Pathophysiological Roles of the ATG8 Conjugation Machinery

Since their initial discovery around two decades ago, the yeast autophagy-related (Atg)8 protein and its mammalian homologues of the light chain 3 (LC3) and γ-aminobutyric acid receptor associated proteins (GABARAP) families have been key for the tremendous expansion of our knowledge about autophagy, a process in which cytoplasmic material become targeted for lysosomal degradation. These proteins are ubiquitin-like proteins that become directly conjugated to a lipid in the autophagy membrane upon induction of autophagy, thus providing a marker of the pathway, allowing studies of autophagosome biogenesis and maturation. Moreover, the ATG8 proteins function to recruit components of the core autophagy machinery as well as cargo for selective degradation. Importantly, comprehensive structural and biochemical in vitro studies of the machinery required for ATG8 protein lipidation, as well as their genetic manipulation in various model organisms, have provided novel insight into the molecular mechanisms and pathophysiological roles of the mATG8 proteins. Recently, it has become evident that the ATG8 proteins and their conjugation machinery are also involved in intracellular pathways and processes not related to autophagy. This review focuses on the molecular functions of ATG8 proteins and their conjugation machinery in autophagy and other pathways, as well as their links to disease.

Autophagy, apoptosis, and mitochondria: molecular integration and physiological relevance in skeletal muscle

Apoptosis and autophagy are processes resulting from the integration of cellular stress and death signals. Their individual importance is highlighted by the lethality of various mouse models missing apoptosis or autophagy-related genes. In addition to their independent roles, significant overlap exists with respect to the signals that stimulate these processes as well as their effector consequences. While these cellular systems exemplify the programming redundancies that underlie many fundamental biological mechanisms, their intertwined relationship means that dysfunction can promote pathology. Although both autophagic and apoptotic signaling are active in skeletal muscle during various diseases and atrophy, their specific roles here are somewhat unique. Given our growing understanding of how specific changes at the cellular level impact whole-organism physiology, there is an equally growing interest in pharmacological manipulation of apoptosis and/or autophagy for altering human physiology and health.

Insulin-like growth factor 1 exhibits the pro-autophagic and anti-apoptotic activity on T cells of oral lichen planus

BACKGROUND

Oral lichen planus (OLP) is an autoimmune mucocutaneous disease characterized by T cell infiltrating in microenvironment. T cell-mediated immune dysfunctions are of importance in the pathogenesis of OLP. Insulin-like growth factor 1 (IGF1) has profound effects on maintenance of immune functions; however, its specific mechanism in OLP remains unknown. This study aims to explore how IGF1 regulates T-cell immune functions in OLP.

METHODS

IGF1 in OLP lesions was stained by immunohistochemistry and immunofluorescence. Additionally, proliferation, apoptosis and autophagy of T cells were examined after stimulation with IGF1 for 24 h, respectively. Z-VAD-FMK, a pan-caspase inhibitor, was used to explore IGF1-mediated crosstalk between apoptosis and autophagy. The modulation of IGF1 on ERK and PI3K/mTOR pathway was also analyzed.

RESULTS

IGF1 was increased in OLP lesions and was remarkably co-located with T cells. IGF1 significantly enhanced T-cell proliferation, suppressed apoptosis and induced autophagic flux. Moreover, autophagy was induced by apoptosis inhibitor, Z-VAD-FMK, thereby reducing death of T cells. IGF1 could facilitate Z-VAD-FMK-induced autophagy and then decrease proportion of apoptotic T cells. IGF1-treated T cells also showed elevated phosphorylation of ERK, PI3K and mTOR.

CONCLUSIONS

IGF1 inhibited apoptosis and promoted autophagy in T cells, potentially contributing to the pathogenesis of OLP.

PDIA6 modulates apoptosis and autophagy of non-small cell lung cancer cells via the MAP4K1/JNK signaling pathway

BACKGROUND

Non-small cell lung cancer (NSCLC) is the most common type of lung cancer with a poor prognosis. We previously found that protein disulfide isomerase family 6 (PDIA6) is upregulated in lung squamous cell carcinoma (LSCC). This study aimed to elucidate the clinical relevance, biological functions, and molecular mechanisms of PDIA6 in NSCLC.

METHODS

The expression of PDIA6 in NSCLC was assessed using the TCGA database, western blotting, and immunohistochemistry. Correlations of PDIA6 expression with clinicopathological and survival features were evaluated. The functions of PDIA6 in regulating NSCLC cell growth, apoptosis, and autophagy were investigated using gain-and loss-of-function strategies in vitro or in vivo. The underlying molecular mechanisms of PDIA6 function were examined by human phospho-kinase array and co-immunoprecipitation.

FINDINGS

PDIA6 expression was upregulated in NSCLC compared with adjacent normal tissues, and the higher PDIA6 expression was correlated with poor prognosis. PDIA6 knockdown decreased NSCLC cell proliferation and increased cisplatin-induced intrinsic apoptosis, while PDIA6 overexpression had the opposite effects. In addition, PDIA6 regulated cisplatin-induced autophagy, and this contributed to PDIA6-mediated apoptosis in NSCLC cells. Mechanistically, PDIA6 reduced the phosphorylation levels of JNK and c-Jun. Moreover, PDIA6 interacted with MAP4K1 and inhibited its phosphorylation, ultimately inhibiting the JNK/c-Jun signaling pathway.

INTERPRETATION

PDIA6 is overexpressed in NSCLC and inhibits cisplatin-induced NSCLC cell apoptosis and autophagy via the MAP4K1/JNK/c-Jun signaling pathway, suggesting that PDIA6 may serve as a biomarker and therapeutic target for NSCLC patients. FUND: National Natural Science Foundation of China and Institutions of higher learning of innovation team from Liaoning province.

Autophagy: An Essential Degradation Program for Cellular Homeostasis and Life

Autophagy is a lysosome-dependent cellular degradation program that responds to a variety of environmental and cellular stresses. It is an evolutionarily well-conserved and essential pathway to maintain cellular homeostasis, therefore, dysfunction of autophagy is closely associated with a wide spectrum of human pathophysiological conditions including cancers and neurodegenerative diseases. The discovery and characterization of the kingdom of autophagy proteins have uncovered the molecular basis of the autophagy process. In addition, recent advances on the various post-translational modifications of autophagy proteins have shed light on the multiple layers of autophagy regulatory mechanisms, and provide novel therapeutic targets for the treatment of the diseases.

The effect of curcumin on the differentiation, apoptosis and cell cycle of neural stem cells is mediated through inhibiting autophagy by the modulation of Atg7 and p62

Curcumin is an orange-yellow colored, lipophilic polyphenol substance derived from the rhizome of Curcuma longa that is widely used in many countries. Curcumin has many reported functions, including antioxidant and anti‑inflammatory effects. Autophagy removes damaged organelles and protein aggregates in the cell. However, whether curcumin mediates its effects on neural stem cell (NSC) differentiation, cell cycle and apoptosis through autophagy is unknown. In the present study, the effects of curcumin and 3‑methyladenine (3MA; an autophagy inhibitor, as a positive control) on the autophagy, differentiation, cell cycle progression and apoptosis of NSCs in different culture states were examined. In order to confirm the role of autophagy in these processes of NSC behavioral change, the protein expression level changes of markers of autophagy, such as autophagy‑related protein 7 (Atg7), light chain (LC)3 and p62, were assessed. When NSCs were in an adherent state, 10 µM curcumin inhibited their differentiation into GFAP+ astrocytes or DCX+ immature neurons, while Atg7 and p62 protein expression were also reduced compared with the untreated control group. When NSCs were in a suspended state, 10 µM curcumin inhibited the cell cycle progression and apoptosis of NSCs as determined by western blotting, which was associated with a decreased autophagic flux and Atg7 expression. In addition, the curcumin‑treated group trended in a similar direction to the 3MA‑treated group. Thus, the data suggest that curcumin can inhibit differentiation, promote cell survival and inhibit cell cycle progression from G1 to S in NSCs, and that these effects are mediated through the regulation of Atg7 and p62.

BmCalpains are involved in autophagy and apoptosis during metamorphosis and after starvation in Bombyx mori

Apoptosis and autophagy play crucial roles during Bombyx mori metamorphosis and in response to various adverse conditions, including starvation. Recently, calpain, one of the major intracellular proteases, has been reported to be involved in apoptosis and autophagy in mammals. BmATG5 and BmATG6 have been identified to mediate apoptosis following autophagy induced by 20-hydroxyecdysone and starvation in B. mori. However, B. mori calpains and their functions remain unclear. In this study, phylogenetic analysis of calpains from B. mori, Drosophila melanogaster and Homo sapiens were performed and the results showed distinct close relationships of BmCalpain-A/B with DmCalpain-A/B, BmCalpain-C with DmCalpain-C, and BmCalpain-7 with HsCalpain-7. Then, the expression profiles of BmCalpains were analyzed by quantitative real-time polymerase chain reaction, and results showed that expression of BmCalpain-A/B, BmCalpain-C and BmCalpain-7 was significantly increased during B. mori metamorphosis and induced in the fat body and midgut of starved larvae, which is consistent with the expression profiles of BmAtg5, BmAtg6 and BmCaspase-1. Moreover, the apoptosis-associated cleavage of BmATG6 in Bm-12 cells was significantly enhanced when BmCalpain-A/B and BmCalpain-7 were induced by starvation, and was partially inhibited by the inhibitor of either calpain or caspase, but completely inhibited when both types of inhibitors were applied together. Our results indicated that BmCalpains, including BmCalpain-A/B, -C and -7, may be involved in autophagy and apoptosis during B. mori metamorphosis and after starvation, and may also contribute to the apoptosis-associated cleavage of BmATG6.

Till Death Do Us Part: The Marriage of Autophagy and Apoptosis

Autophagy is a widely conserved catabolic process that is necessary for maintaining cellular homeostasis under normal physiological conditions and driving the cell to switch back to this status quo under times of starvation, hypoxia, and oxidative stress. The potential similarities and differences between basal autophagy and stimulus-induced autophagy are still largely unknown. Both act by clearing aberrant or unnecessary cytoplasmic material, such as misfolded proteins, supernumerary and defective organelles. The relationship between reactive oxygen species (ROS) and autophagy is complex. Cellular ROS is predominantly derived from mitochondria. Autophagy is triggered by this event, and by clearing the defective organelles effectively, it lowers cellular ROS thereby restoring cellular homeostasis. However, if cellular homeostasis cannot be reached, the cells can switch back and choose a regulated cell death response. Intriguingly, the autophagic and cell death machines both respond to the same stresses and share key regulatory proteins, suggesting that the pathways are intricately connected. Here, the intersection between autophagy and apoptosis is discussed with a particular focus on the role ROS plays.

The tubulin inhibitor MG-2477 induces autophagy-regulated cell death, ROS accumulation and activation of FOXO3 in neuroblastoma

Neuroblastoma is the most frequent extra-cranial solid tumor in children with still high mortality in stage M. Here we studied the tubulin-inhibitor MG-2477 as a possible therapeutic agent for neuroblastoma therapy and uncovered that MG-2477 induces death in neuroblastoma cells independent of PKB-activation status and stage. MG-2477 triggers within 30 minutes extensive autophagosome-formation that finally leads to cell death associated with mitotic catastrophe. Autophagy is critical for MG-2477-induced death and is regulated by the BH3-only protein PMAIP1/NOXA which sequesters the anti-apoptotic BCL2-protein BCLXL and thereby displaces and activates the autophagy-regulator BECN1/beclin1. Knockdown of NOXA or overexpression of its pro-survival binding partners MCL1 and BCLXL counteracts MG-2477-induced cell death. MG-2477 also rapidly induces the repression of the anti-apoptotic protein Survivin, which promotes autophagy and cell death. We further observed the accumulation of reactive oxygen species (ROS) that triggers autophagy induction suggesting a change of the PI3 kinase-III/BECN1 complex and activates the transcription factor FOXO3, which contributes to final cell death induction. The combined data suggest that MG-2477 induces a sequential process of ROS-accumulation, autophagy and FOXO3-activation that leads to cell death in neuroblastoma cells.

Late-stage inhibition of autophagy enhances calreticulin surface exposure

Calreticulin (CRT) exposure on the cell surface is essential for inducing immunogenic cell death by chemotherapy. Recent studies have shown conflicting effects of chemotherapy-induced autophagy on CRT exposure in cancer cells. Our data revealed that surface-exposed CRT (Ecto-CRT) emission was attenuated by inhibition of autophagy at early stages; however, inhibition of autophagy at late stages resulted in increased Ecto-CRT. Furthermore, neither autophagy activation nor endoplasmic reticulum (ER) stress induction alone was sufficient for CRT surface exposure. Moreover, chemotherapeutic agents that only activated autophagy without inducing ER stress could not increase Ecto-CRT; therefore, combined use of an autophagy activator and ER stress inducer could effectively promote CRT translocation to the plasma membrane. Together, our results highlight the potential of the combined use of ER stress inducers and autophagy late-stage inhibitors to reestablish and strengthen both the CRT exposure and immunogenicity of chemotherapeutic agents induced death cells.

BAY 11-7085 induces glucocorticoid receptor activation and autophagy that collaborate with apoptosis to induce human synovial fibroblast cell death

Inhibition of proapoptotic pathways in synovial fibroblasts is one of the major causes of synovial proliferation and hyperplasia in rheumatic diseases. We have shown previously that NF-κB inhibitor BAY 11-7085, through inactivation of PPAR-γ, induces apoptosis in human synovial fibroblasts. In this work we showed that BAY 11-7085 induced autophagy that preceded BAY 11-7085-induced apoptosis. Of interest, BAY 11-7085 induced Serine 211 phosphorylation and degradation of glucocorticoid receptor (GR). Glucocorticoid prednisolone induced both activation and degradation of GR, as well as autophagy in synovial fibroblasts. BAY 11-7085-induced cell death was significantly decreased with glucocorticoid inhibitor mifepristone and with inhibitors of autophagy. Both BAY 11-7085-induced autophagy and GR activation were down regulated with PPAR-γ agonist, 15d-PGJ2 and MEK/ERK inhibitor UO126. Inhibition of autophagy markedly decreased endogenous and BAY 11-7085-induced ERK phosphorylation, suggesting a positive feed back loop between ERK activation and autophagy in synovial fibroblasts. Co-transfection of MEK1 with PPAR-γ1 in HEK293 cells caused known inhibitory phosphorylation of PPAR-γ1 (Serine 112) and enhanced GR degradation, in the absence or presence of prednisolone. Furthermore, GR was both phosphorylated on Serine 211 and down regulated in synovial fibroblasts during serum starvation induced autophagy. These results showed that GR activation and PPAR-γ inactivation mediated BAY 11-7085-induced autophagy.

Thyroid hormone inhibits lung fibrosis in mice by improving epithelial mitochondrial function

Thyroid hormone (TH) is critical for the maintenance of cellular homeostasis during stress responses, but its role in lung fibrosis is unknown. Here we found that the activity and expression of iodothyronine deiodinase 2 (DIO2), an enzyme that activates TH, were higher in lungs from patients with idiopathic pulmonary fibrosis than in control individuals and were correlated with disease severity. We also found that Dio2-knockout mice exhibited enhanced bleomycin-induced lung fibrosis. Aerosolized TH delivery increased survival and resolved fibrosis in two models of pulmonary fibrosis in mice (intratracheal bleomycin and inducible TGF-β1). Sobetirome, a TH mimetic, also blunted bleomycin-induced lung fibrosis. After bleomycin-induced injury, TH promoted mitochondrial biogenesis, improved mitochondrial bioenergetics and attenuated mitochondria-regulated apoptosis in alveolar epithelial cells both in vivo and in vitro. TH did not blunt fibrosis in Ppargc1a- or Pink1-knockout mice, suggesting dependence on these pathways. We conclude that the antifibrotic properties of TH are associated with protection of alveolar epithelial cells and restoration of mitochondrial function and that TH may thus represent a potential therapy for pulmonary fibrosis.

Modulating autophagy in cancer therapy: Advancements and challenges for cancer cell death sensitization

Autophagy is a major protein degradation pathway capable of upholding cellular metabolism under nutrient limiting conditions, making it a valuable resource to highly proliferating tumour cells. Although the regulatory machinery of the autophagic pathway has been well characterized, accurate modulation of this pathway remains complex in the context of clinical translatability for improved cancer therapies. In particular, the dynamic relationship between the rate of protein degradation through autophagy, i.e. autophagic flux, and the susceptibility of tumours to undergo apoptosis remains largely unclear. Adding to inefficient clinical translation is the lack of measurement techniques that accurately depict autophagic flux. Paradoxically, both increased autophagic flux as well as autophagy inhibition have been shown to sensitize cancer cells to undergo cell death, indicating the highly context dependent nature of this pathway. In this article, we aim to disentangle the role of autophagy modulation in tumour suppression by assessing existing literature in the context of autophagic flux and cellular metabolism at the interface of mitochondrial function. We highlight the urgency to not only assess autophagic flux more accurately, but also to center autophagy manipulation within the unique and inherent metabolic properties of cancer cells. Lastly, we discuss the challenges faced when targeting autophagy in the clinical setting. In doing so, it is hoped that a better understanding of autophagy in cancer therapy is revealed in order to overcome tumour chemoresistance through more controlled autophagy modulation in the future.

Natural agents mediated autophagic signal networks in cancer

Recent studies suggested that natural compounds are important in finding targets for cancer treatments. Autophagy (“self-eating”) plays important roles in multiple diseases and acts as a tumor suppressor in cancer. Here, we examined the molecular mechanism by which natural agents regulate autophagic signals. Understanding the relationship between natural agents and cellular autophagy may provide more information for cancer diagnosis and chemoprevention.

Cytoplasmic fragment of CD147 generated by regulated intramembrane proteolysis contributes to HCC by promoting autophagy

Hepatocellular carcinoma (HCC) is one of the most lethal and prevalent cancers worldwide. CD147 (EMMPRIN or basigin) is a leading gene relating to hepatocarcinogenesis and metastasis, and is detected in transmembrane, exosome or circulating forms in HCC patients. The endosome recycling of CD147 further enhances the function of this oncoprotein from a dynamic perspective. However, previous studies about CD147 mainly focused on one separate form, and little attention has been paid to how the different forms of tumor-derived CD147 changes. Moreover, uncovering the roles of the residual C-terminal portion of CD147 after shedding is inevitable to fully understand CD147 promoting tumor progression. In this study, we discovered that under low-cholesterol condition, CD147 endocytosis is inhibited but its shedding mediated by ADAM10 is enhanced. Further procession of residual CD147 in the lysosome produces nuclear-localized CD147-ICD (intracellular domain of CD147), which contributes to autophagy through NF-κB–TRAIL–caspase8–ATG3 axis. As autophagy endows cancer cells with increased adaptability to chemotherapy, and HAb 18 (a specific antibody targeting CD147) inhibits CD147 shedding and sequential CD147-ICD enhances autophagy, we found the combination of HAb 18 and cisplatin exhibited marked antitumor efficiency.

Quercetin simultaneously induces G0 /G1 -phase arrest and caspase-mediated crosstalk between apoptosis and autophagy in human leukemia HL-60 cells

Quercetin is a plant-derived bioflavonoid with high anticancer activity in various tumors. Herein, the molecular mechanisms by which quercetin exerts its anticancer effects against HL-60 acute myeloid leukemia (AML) cells were investigated. Results showed that quercetin suppressed cell proliferation in the HL-60 cell line in vitro and in vivo. Quercetin-induced G₀ /G₁ -phase arrest occurred when expressions of cyclin-dependent kinase (CDK)2/4 were inhibited and the CDK inhibitors, p16 and p21, were induced. Moreover, quercetin treatment not only activated proapoptotic signaling like poly (ADP ribose) polymerase (PARP)-1 cleavage and caspase activation but also triggered autophagy events as shown by the increased expression of light chain 3 (LC3)-II, decreased expression of p62, and formation of acidic vesicular organelles. Interestingly, it was found that use of the autophagy inhibitor, 3-methyladenine, significantly enhanced quercetin-mediated apoptotic cell death as analyzed by MTS and DNA fragmentation assays. Moreover, pretreatment of HL-60 cells with the pan-caspase inhibitor, Z-VAD-fmk, dramatically reversed quercetin-mediated apoptotic and autophagic cell death. Although apoptosis and autophagy are two independent cell death pathways, our findings indicated that quercetin can activate caspases to trigger these two pathways, and both pathways played contrary roles in quercetin-mediated HL-60 cell death. In conclusion, besides promoting apoptosis, quercetin also induced cytoprotective autophagy in HL-60 cells, and inhibition of autophagy may be a novel strategy to enhance the anticancer activity of quercetin in AML.

Caspase involvement in autophagy

Caspases are a family of cysteine proteases widely known as the principal mediators of the apoptotic cell death response, but considerably less so as the contributors to the regulation of pathways outside cellular demise. In regards to autophagy, the modulatory roles of caspases have only recently begun to be adequately described. In contrast to apoptosis, autophagy promotes cell survival by providing energy and nutrients through the lysosomal degradation of cytoplasmic constituents. Under basal conditions autophagy and apoptosis cross-regulate each other through an elaborate network of interconnections which also includes the interplay between autophagy-related proteins (ATGs) and caspases. In this review we focus on the effects of this crosstalk at the cellular level, as we aim to concentrate the main observations from research conducted so far on the fine-tuning of autophagy by caspases. Several members of this protease-family have been found to directly interact with key ATGs involved in different tiers across the autophagic cascade. Therefore, we firstly outline the core mechanism of macroautophagy in brief. In an effort to emphasize the importance of the intricate cross-regulation of ATGs and caspases, we also present examples of autophagy's contribution to apoptotic cell death during development.

Aging-related Atg5 defect impairs neutrophil extracellular traps formation

Formation of neutrophil extracellular traps (NETs) is an important function of the innate immune system against infections. It has been proven that aging dysregulates immunity and impairs neutrophil function. However, the influence of aging on the ability to produce NETs has yet to be fully addressed. In this study, we tested the hypothesis that a lower level of autophagy in neutrophils from aged mice was responsible for the decrease in NET formation. We demonstrated that a broad range of Toll-like receptor 2 (TLR2) ligands could efficiently induce reactive oxygen species (ROS) -dependent NET release in young mice, but not in aged ones. We further explored that the difference between young and aged mice in TLR2 ligand-induced NETosis is the result of an Atg5 defect and subsequent impaired autophagy. Furthermore, we found that lower autophagy capacity led to not only reduced NET formation, but also increased apoptosis. Our results suggest an important role of Atg5 and autophagy in maintaining the function of NETs formation in response to infection and in regulating neutrophil death. Targeting autophagy-promoted NETs may present a therapeutic strategy to improve infection defence in an aged population.

Changes in cIAP2, survivin and BimEL expression characterize the switch from autophagy to apoptosis in prolonged starvation

BACKGROUND

Autophagy is a catabolic process involving the engulfment of cytoplasmic content within autophagosomes followed by their delivery to lysosomes. This process is a survival mechanism, enabling cells to cope with nutrient deprivation by degradation and recycling of macromolecules. Yet during continued stress such as prolonged starvation, a switch from autophagy to apoptosis is often detected.

OBJECTIVE

In this work, we characterized the temporal dynamics of the transition from autophagy towards apoptosis with the aim of elucidating the molecular mechanism regulating the switch from survival autophagy to apoptotic cell death.

RESULTS AND CONCLUSIONS

We defined an inverse relationship between apoptosis and autophagy spanning a period of 72 h, manifested by the sequential reduction in LC3 lipidation and the activation of caspase-3. The transition to apoptosis correlated with a selective decline in the mRNA and protein levels of two anti-apoptotic IAP family proteins, survivin and cIAP2 and a selective increase in the BH3-only protein, BimEL. This 'molecular signature' was common to several cell lines undergoing the switch from autophagy to apoptosis during prolonged starvation. Mechanistically, the increased BimEL protein levels resulted from its reduced binding to its specific E3 ligase, βTrCP, leading to protein stabilization. Consistent with this, BimEL showed decreased phosphorylation at critical sites previously reported to be essential for binding to the E3 ligase. The decrease in the anti-apoptotic IAPs and the increase in the pro-apoptotic BimEL may thus constitute a molecular switch from autophagy to apoptosis during prolonged starvation.

Crosstalk of autophagy and apoptosis: Involvement of the dual role of autophagy under ER stress

Endoplasmic reticulum (ER) stress is a common cellular stress response that is triggered by a variety of conditions that disturb cellular homeostasis, and induces cell apoptosis. Autophagy, an important and evolutionarily conserved mechanism for maintaining cellular homeostasis, is closely related to the apoptosis induced by ER stress. There are common upstream signaling pathways between autophagy and apoptosis induced by ER stress, including PERK/ATF4, IRE1α, ATF6, and Ca²⁺ . Autophagy can not only block the induction of apoptosis by inhibiting the activation of apoptosis-associated caspase which could reduce cellular injury, but also help to induce apoptosis. In addition, the activation of apoptosis-related proteins can also inhibit autophagy by degrading autophagy-related proteins, such as Beclin-1, Atg4D, Atg3, and Atg5. Although the interactions of different autophagy- and apoptosis-related proteins, and also common upstream signaling pathways have been found, the potential regulatory mechanisms have not been clearly understood. In this review, we summarize the dual role of autophagy, and the interplay and potential regulatory mechanisms between autophagy and apoptosis under ER stress condition.

Role of Autophagy and Apoptosis in Non-Small-Cell Lung Cancer

Non-small-cell lung cancer (NSCLC) constitutes 85% of all lung cancers, and is the leading cause of cancer-related death worldwide. The poor prognosis and resistance to both radiation and chemotherapy warrant further investigation into the molecular mechanisms of NSCLC and the development of new, more efficacious therapeutics. The processes of autophagy and apoptosis, which induce degradation of proteins and organelles or cell death upon cellular stress, are crucial in the pathophysiology of NSCLC. The close interplay between autophagy and apoptosis through shared signaling pathways complicates our understanding of how NSCLC pathophysiology is regulated. The apoptotic effect of autophagy is controversial as both inhibitory and stimulatory effects have been reported in NSCLC. In addition, crosstalk of proteins regulating both autophagy and apoptosis exists. Here, we review the recent advances of the relationship between autophagy and apoptosis in NSCLC, aiming to provide few insights into the discovery of novel pathogenic factors and the development of new cancer therapeutics.

IBMPFD Disease-Causing Mutant VCP/p97 Proteins Are Targets of Autophagic-Lysosomal Degradation

The ubiquitin-proteasome system (UPS) degrades soluble proteins and small aggregates, whereas macroautophagy (autophagy herein) eliminates larger protein aggregates, tangles and even whole organelles in a lysosome-dependent manner. VCP/p97 was implicated in both pathways. VCP/p97 mutations cause a rare multisystem disease called IBMPFD (Inclusion Body Myopathy with Paget's Disease and Frontotemporal Dementia). Here, we studied the role IBMPFD-related mutants of VCP/p97 in autophagy. In contrast with the wild-type VCP/p97 protein or R155C or R191Q mutants, the P137L mutant was aggregate-prone. We showed that, unlike commonly studied R155C or R191Q mutants, the P137L mutant protein stimulated both autophagosome and autolysosome formation. Moreover, P137L mutant protein itself was a substrate of autophagy. Starvation- and mTOR inhibition-induced autophagy led to the degradation of the P137L mutant protein, while preserving the wild-type and functional VCP/p97. Strikingly, similar to the P137L mutant, other IBMPFD-related VCP/p97 mutants, namely R93C and G157R mutants induced autophagosome and autolysosome formation; and G157R mutant formed aggregates that could be cleared by autophagy. Therefore, cellular phenotypes caused by P137L mutant expression were not isolated observations, and some other IBMPFD disease-related VCP/p97 mutations could lead to similar outcomes. Our results indicate that cellular mechanisms leading to IBMPFD disease may be various, and underline the importance of studying different disease-associated mutations in order to better understand human pathologies and tailor mutation-specific treatment strategies.

Mechanisms for autophagy modulation by isoprenoid biosynthetic pathway inhibitors in multiple myeloma cells

Multiple myeloma (MM) is characterized by the production of monoclonal protein (MP). We have shown previously that disruption of the isoprenoid biosynthetic pathway (IBP) causes a block in MP secretion through a disruption of Rab GTPase activity, leading to an enhanced unfolded protein response and subsequent apoptosis in MM cells. Autophagy is induced by cellular stressors including nutrient deprivation and ER stress. IBP inhibitors have been shown to have disparate effects on autophagy. Here we define the mechanisms underlying the differential effects of IBP inhibitors on autophagic flux in MM cells utilizing specific pharmacological inhibitors. We demonstrate that IBP inhibition induces a net increase in autophagy as a consequence of disruption of isoprenoid biosynthesis which is not recapitulated by direct geranylgeranyl transferase inhibition. IBP inhibitor-induced autophagy is a cellular defense mechanism as treatment with the autophagy inhibitor bafilomycin A1 enhances the cytotoxic effects of GGPP depletion, but not geranylgeranyl transferase inhibition. Immunofluorescence microscopy studies revealed that IBP inhibitors disrupt ER to Golgi trafficking of monoclonal light chain protein and that this protein is not a substrate for alternative degradative pathways such as aggresomes and autophagosomes. These studies support further development of specific GGTase II inhibitors as anti-myeloma agents.

A microtubule inhibitor, ABT-751, induces autophagy and delays apoptosis in Huh-7 cells

The objective was to investigate the upstream mechanisms of apoptosis which were triggered by a novel anti-microtubule drug, ABT-751, in hepatocellular carcinoma-derived Huh-7 cells. Effects of ABT-751 were evaluated by immunocytochemistry, flow cytometric, alkaline comet, soft agar, immunoblotting, CytoID, green fluorescent protein-microtubule associated protein 1 light chain 3 beta detection, plasmid transfection, nuclear/cytosol fractionation, coimmunoprecipitation, quantitative reverse transcription-polymerase chain reaction, small-hairpin RNA interference and mitochondria/cytosol fractionation assays. Results showed that ABT-751 caused dysregulation of microtubule, collapse of mitochondrial membrane potential, generation of reactive oxygen species (ROS), DNA damage, G₂/M cell cycle arrest, inhibition of anchorage-independent cell growth and apoptosis in Huh-7 cells. ABT-751 also induced early autophagy via upregulation of nuclear TP53 and downregulation of the AKT serine/threonine kinase (AKT)/mechanistic target of rapamycin (MTOR) pathway. Through modulation of the expression levels of DNA damage checkpoint proteins and G₂/M cell cycle regulators, ABT-751 induced G₂/M cell cycle arrest. Subsequently, ABT-751 triggered apoptosis with marked downregulation of B-cell CLL/lymphoma 2, upregulation of mitochondrial BCL2 antagonist/killer 1 and BCL2 like 11 protein levels, and cleavages of caspase 8 (CASP8), CASP9, CASP3 and DNA fragmentation factor subunit alpha proteins. Suppression of ROS significantly decreased ABT-751-induced autophagic and apoptotic cells. Pharmacological inhibition of autophagy significantly increased the percentages of ABT-751-induced apoptotic cells. The autophagy induced by ABT-751 plays a protective role to postpone apoptosis by exerting adaptive responses following microtubule damage, ROS and/or impaired mitochondria.

Atg3 Overexpression Enhances Bortezomib-Induced Cell Death in SKM-1 Cell

Background

Myelodysplastic syndrome (MDS) is a group of heterogeneous hematopoietic stem cell malignancies with a high risk of transformation into acute myeloid leukemia (AML). Clonal evolutions are significantly associated with transformation to AML. According to a gene expression microarray, atg3 is downregulated in MDS patients progressing to leukemia, but less is known about the function of Atg3 in the survival and death of MSD/AML cells. Moreover, the role of autophagy as a result of bortezomib treatment is controversial. The current study was designed to investigate the function of Atg3 in SKM-1 cells and to study the effect of Atg3 on cell viability and cell death following bortezomib treatment.

Methods

Four leukemia cell lines (SKM-1, THP-1, NB4 and K562) and two healthy patients’ bone marrow cells were analyzed for Atg3 expression via qRT-PCR and Western blotting analysis. The role of Atg3 in SKM-1 cell survival and cell death was analyzed by CCK-8 assay, trypan blue exclusion assay, DAPI staining and Annexin V/PI dual staining with or without bortezomib treatment. Western blotting analysis was used to detect proteins in autophagic and caspase signaling pathways. Electron microscopy was used to observe ultrastructural changes after Atg3 overexpression.

Results

Downregulation of Atg3 expression was detected in four leukemia cell lines compared with healthy bone marrow cells. Atg3 mRNA was significantly decreased in MDS patients’ bone marrow cells. Overexpression of Atg3 in SKM-1 cells resulted in AKT-mTOR-dependent autophagy, a significant reduction in cell proliferation and increased cell death, which could be overcome by the autophagy inhibitor 3-MA. SKM-1 cells overexpressing Atg3 were hypersensitive to bortezomib treatment at different concentrations via autophagic cell death and enhanced sensitivity to apoptosis in the SKM-1 cell line. Following treatment with 3-MA, the sensitivity of Atg3-overexpressing cells to bortezomib treatment was reduced. Atg3 knockdown blocked cell growth inhibition and cell death induced by bortezomib.

Conclusion

Our preliminary study of Atg3 in the high-risk MDS cell line suggests that Atg3 might be possibly a critical regulator of autophagic cell death and a gene target for therapeutic interventions in MDS.

MicroRNA-495 regulates starvation-induced autophagy by targeting ATG3

The functions of some essential autophagy genes are regulated by microRNAs. However, an ATG3-modulating microRNA has never been reported. Here we show that the transcription of miR-495 negatively correlates with the translation of ATG3 under nutrient-deprived or rapamycin-treated conditions. miR-495 targets ATG3 and regulates its protein levels under starvation conditions. miR-495 also inhibits starvation-induced autophagy by decreasing the number of autophagosomes and by preventing LC3-I-to-LC3-II transition and P62 degradation. These processes are reversed by the overexpression of an endogenous miR-495 inhibitor. Re-expression of Atg3 without miR-495 response elements restores miR-495-inhibited autophagy. miR-495 sustains cell viability under starvation conditions but has no effect under hypoxia. Moreover, miR-495 inhibits etoposide-induced cell death. In conclusion, miR-495 is involved in starvation-induced autophagy by regulating Atg3.

Crosstalk between Autophagy and Apoptosis: Potential and Emerging Therapeutic Targets for Cardiac Diseases

Autophagy is a cell survival process which is related to breaking down and reusing cytoplasm components. Moreover, autophagy regulates cell death under certain conditions. Apoptosis has the characteristics of chromatin agglutination and the shrinking of nuclear and apoptosis body form. Even if the mechanisms of autophagy and apoptosis have differences, some proteins modulate both autophagy and apoptosis. Crosstalk between them exists. This review highlights recent advances in the interaction of autophagy and apoptosis and its importance in the development of cardiovascular diseases.

Autophagy and apoptosis in liver injury

Apoptosis is a primary characteristic in the pathogenesis of liver disease. Hepatic apoptosis is regulated by autophagic activity. However, mechanisms mediating their interaction remain to be determined. Basal level of autophagy ensures the physiological turnover of old and damaged organelles. Autophagy also is an adaptive response under stressful conditions. Autophagy can control cell fate through different cross-talk signals. A complex interplay between hepatic autophagy and apoptosis determines the degree of hepatic apoptosis and the progression of liver disease as demonstrated by pre-clinical models and clinical trials. This review summarizes recent advances on roles of autophagy that plays in pathophysiology of liver. The autophagic pathway can be a novel therapeutic target for liver disease.

Autophagy in acute kidney injury

Autophagy is a conserved multistep pathway that degrades and recycles damaged organelles and macromolecules to maintain intracellular homeostasis. The autophagy pathway is upregulated under stress conditions including cell starvation, hypoxia, nutrient and growth-factor deprivation, endoplasmic reticulum stress, and oxidant injury, most of which are involved in the pathogenesis of acute kidney injury (AKI). Recent studies demonstrate that basal autophagy in the kidney is vital for the normal homeostasis of the proximal tubules. Deletion of key autophagy proteins impaired renal function and increased p62 levels and oxidative stress. In models of AKI, autophagy deletion in proximal tubules worsened tubular injury and renal function, highlighting that autophagy is renoprotective in models of AKI. In addition to nonselective sequestration of autophagic cargo, autophagy can facilitate selective degradation of damaged organelles, particularly mitochondrial degradation through the process of mitophagy. Damaged mitochondria accumulate in autophagy-deficient kidneys of mice subjected to ischemia-reperfusion injury, but the precise mechanisms of regulation of mitophagy in AKI are not yet elucidated. Recent progress in identifying the interplay of autophagy, apoptosis, and regulated necrosis has revived interest in examining shared pathways/molecules in this crosstalk during the pathogenesis of AKI. Autophagy and its associated pathways pose potentially unique targets for therapeutic interventions in AKI.

The dual role of autophagy under hypoxia-involvement of interaction between autophagy and apoptosis

Hypoxia is one of severe cellular stress and it is well known to be associated with a worse outcome since a lack of oxygen accelerates the induction of apoptosis. Autophagy, an important and evolutionarily conserved mechanism for maintaining cellular homeostasis, is closely related to the apoptosis caused by hypoxia. Generally autophagy blocks the induction of apoptosis and inhibits the activation of apoptosis-associated caspase which could reduce cellular injury. However, in special cases, autophagy or autophagy-relevant proteins may help to induce apoptosis, which could aggravate cell damage under hypoxia condition. In addition, the activation of apoptosis-related proteins-caspase can also degrade autophagy-related proteins, such as Atg3, Atg4, Beclin1 protein, inhibiting autophagy. Although the relationship between autophagy and apoptosis has been known for rather complex for more than a decade, the underlying regulatory mechanisms have not been clearly understood. This short review discusses and summarizes the dual role of autophagy and the interaction and molecular regulatory mechanisms between autophagy and apoptosis under hypoxia.

Redox signaling: Potential arbitrator of autophagy and apoptosis in therapeutic response

Redox signaling plays important roles in the regulation of cell death and survival in response to cancer therapy. Autophagy and apoptosis are discrete cellular processes mediated by distinct groups of regulatory and executioner molecules, and both are thought to be cellular responses to various stress conditions including oxidative stress, therefore controlling cell fate. Basic levels of reactive oxygen species (ROS) may function as signals to promote cell proliferation and survival, whereas increase of ROS can induce autophagy and apoptosis by damaging cellular components. Growing evidence in recent years argues for ROS that below detrimental levels acting as intracellular signal transducers that regulate autophagy and apoptosis. ROS-regulated autophagy and apoptosis can cross-talk with each other. However, how redox signaling determines different cell fates by regulating autophagy and apoptosis remains unclear. In this review, we will focus on understanding the delicate molecular mechanism by which autophagy and apoptosis are finely orchestrated by redox signaling and discuss how this understanding can be used to develop strategies for the treatment of cancer.

Posttranslational modification of autophagy-related proteins in macroautophagy

Macroautophagy is an intracellular catabolic process involved in the formation of multiple membrane structures ranging from phagophores to autophagosomes and autolysosomes. Dysfunction of macroautophagy is implicated in both physiological and pathological conditions. To date, 38 autophagy-related (ATG) genes have been identified as controlling these complicated membrane dynamics during macroautophagy in yeast; approximately half of these genes are clearly conserved up to human, and there are additional genes whose products function in autophagy in higher eukaryotes that are not found in yeast. The function of the ATG proteins, in particular their ability to interact with a number of macroautophagic regulators, is modulated by posttranslational modifications (PTMs) such as phosphorylation, glycosylation, ubiquitination, acetylation, lipidation, and proteolysis. In this review, we summarize our current knowledge of the role of ATG protein PTMs and their functional relevance in macroautophagy. Unraveling how these PTMs regulate ATG protein function during macroautophagy will not only reveal fundamental mechanistic insights into the regulatory process, but also provide new therapeutic targets for the treatment of autophagy-associated diseases.

Cell Death Conversion under Hypoxic Condition in Tumor Development and Therapy

Hypoxia, which is common during tumor progression, plays important roles in tumor biology. Failure in cell death in response to hypoxia contributes to progression and metastasis of tumors. On the one hand, the metabolic and oxidative stress following hypoxia could lead to cell death by triggering signal cascades, like LKB1/AMPK, PI3K/AKT/mTOR, and altering the levels of effective components, such as the Bcl-2 family, Atg and p62. On the other hand, hypoxia-induced autophagy can serve as a mechanism to turn over nutrients, so as to mitigate the adverse condition and then avoid cell death potentially. Due to the effective role of hypoxia, this review focuses on the crosstalk in cell death under hypoxia in tumor progression. Additionally, the illumination of cell death in hypoxia could shed light on the clinical applications of cell death targeted therapy.

Recent insights into cell death and autophagy

Macroautophagy (hereafter autophagy) is an evolutionarily-ancient mechanism by which cellular material is delivered to lysosomes for degradation. Autophagy and cell death are intimately linked. For example, both processes often use the same molecular machinery and recent work suggests that autophagy has great influence over a cell's decision to live or die. However, this decision-making is complicated by the fact that the role of autophagy in determining whether a cell should live or die goes both ways: autophagy inhibition can result in more or less cell death depending on the death stimulus, cell type or context. Autophagy may also differentially affect different types of cell death. In the present review, we discuss the recent literature that helps make sense of this apparently inconsistent role of autophagy in influencing a cell to live or die.

Recent advances in autophagy-based neuroprotection

Macroautophagy is a highly regulated intracellular process that, under certain circumstances, delivers cytoplasmic components to the lysosomes for degradation. It consists of several sequential steps including initiation and nucleation, double membrane formation and elongation, formation and maturation of autophagosomes and finally autophagosomes/lysosomes fusion and degradation of intra-autophagosomal contents by lysosomal enzymes. After decades of considering autophagy as a cell death pathway, it has recently been shown to have a survival function through clearing of protein aggregates and damaged cytoplasmic organelles in response to a variety of stress conditions. Most recently, there is increasing evidence from literature revealing that autophagy induction may combat neurodegeneration. In the light of this, our current review tried to address the recent advances in the role of induced autophagy in neuroprotection with a particular focus on its contribution in the most common neurodegenerative disorders like Alzheimer's disease, Parkinson's disease and Huntington's disease.

miR-Let7A Modulates Autophagy Induction in LPS-Activated Microglia

Microglia regulate the secretion of various immunomediators in central nervous system diseases. Microglial autophagy is the crucial process for cell's survival and cytokine productions. Recent studies have reported that several microRNAs are involved in the autophagy system. miR-Let7A is such a microRNA that plays a role in various inflammation responses, and is magnified as a key modulator particularly in the autophagy system. In present study, we investigated whether miR-Let7A is involved in autophagy in activating microglia. Overexpression of miR-Let7A in LPS-stimulated BV2 microglial cells promoted the induction of the autophagy related factors such as LC3II, Beclin1, and ATG3. Our results suggest a potential role of miR-Let7A in the autophagy process of microglia during CNS inflammation.

The critical molecular interconnections in regulating apoptosis and autophagy

Apoptosis and autophagy are both highly regulated biological processes that have important roles in development, differentiation, homeostasis, and disease. These processes may take place independently, with autophagy being cytoprotective for preventing cells from apoptosis and apoptosis blocking autophagy. But in most circumstances, both may be induced sequentially with autophagy preceding apoptosis. The simultaneous activation of both processes has been observed not only in experimental settings but also in pathophysiological conditions. In fact, these two pathways are tightly connected with each other by substantial interplays between them, enabling the coordinated regulation of cell fates by these two pathways. They share some common upstream signaling components, and some components of one pathway may play important roles in the other, and vice versa. Such proteins represent the critical interconnections of the two pathways, which seem to determine the cell for survival or death. Here several critical molecular interconnections between apoptosis and autophagy pathways are reviewed, with their action mechanisms being highlighted.

Autophagy and apoptosis: where do they meet?

Autophagy and apoptosis are two important cellular processes with complex and intersecting protein networks; as such, they have been the subjects of intense investigation. Recent advances have elucidated the key players and their molecular circuitry. For instance, the discovery of Beclin-1's interacting partners has resulted in the identification of Bcl-2 as a central regulator of autophagy and apoptosis, which functions by interacting with both Beclin-1 and Bax/Bak respectively. When localized to the endoplasmic reticulum and mitochondria, Bcl-2 inhibits autophagy. Cellular stress causes the displacement of Bcl-2 from Beclin-1 and Bax, thereby triggering autophagy and apoptosis, respectively. The induction of autophagy or apoptosis results in disruption of complexes by BH3-only proteins and through post-translational modification. The mechanisms linking autophagy and apoptosis are not fully defined; however, recent discoveries have revealed that several apoptotic proteins (e.g., PUMA, Noxa, Nix, Bax, XIAP, and Bim) modulate autophagy. Moreover, autophagic proteins that control nucleation and elongation regulate intrinsic apoptosis through calpain- and caspase-mediated cleavage of autophagy-related proteins, which switches the cellular program from autophagy to apoptosis. Similarly, several autophagic proteins are implicated in extrinsic apoptosis. This highlights a dual cellular role for autophagy. On one hand, autophagy degrades damaged mitochondria and caspases, and on the other hand, it provides a membrane-based intracellular platform for caspase processing in the regulation of apoptosis. In this review, we highlight the crucial factors governing the crosstalk between autophagy and apoptosis and describe the mechanisms controlling cell survival and cell death.

Cancer-type-specific crosstalk between autophagy, necroptosis and apoptosis as a pharmacological target

Cell death plays an essential role in the development of organs, homeostasis, and cancer. Apoptosis and programmed necrosis are two major types of cell death, characterized by different cell morphology and pathways. Accumulating evidence shows autophagy as a new alternative target to treat tumor resistance. Besides its well-known pro-survival role, autophagy can be a physiological cell death process linking apoptosis and programmed necrosis cell death pathways, by various molecular mediators. Here, we summarize the effects of pharmacologically active compounds as modulators of different types of cancer cell death depending on the cellular context. Indeed, current findings show that both natural and synthetic compounds regulate the interplay between apoptosis, autophagy and necroptosis stimulating common molecular mediators and sharing common organelles. In response to specific stimuli, the same death signal can cause cells to switch from one cell death modality to another depending on the cellular setting. The discovery of important interconnections between the different cell death mediators and signaling pathways, regulated by pharmacologically active compounds, presents novel opportunities for the targeted treatment of cancer. The aim of this review is to highlight the potential role of these compounds for context-specific anticancer therapy.

The molecular mechanisms between autophagy and apoptosis: potential role in central nervous system disorders

Autophagy involves degradation of dysfunctional cellular components through the actions of lysosomes. Apoptosis is the process of programmed cell death involving a series of characteristic cell changes. Autophagy and apoptosis, as self-destructive processes, play an important role in the pathogenesis of neurological diseases; and a crosstalk between "self-eating" (autophagy) and "self-killing" (apoptosis) plays an important role in pathological cellular adaptation. Expert knowledge of autophagy and apoptosis has increased in recent years, particularly in regards to cellular and molecular mechanisms. The crosstalk between autophagy and apoptosis was partially uncovered and several key molecules, including Bcl-2 family members, Beclin 1, and p53 were identified. However, the precise mechanisms of such a crosstalk remain to be elucidated. This current review article aims to summarize key mediators of the autophagy-apoptosis crosstalk in pathological conditions, and to highlight recent advances in the field, as well as to discuss further investigations and therapeutic potentials of manipulating those mechanisms in central nervous system diseases.

Old, new and emerging functions of caspases

Caspases are proteases with a well-defined role in apoptosis. However, increasing evidence indicates multiple functions of caspases outside apoptosis. Caspase-1 and caspase-11 have roles in inflammation and mediating inflammatory cell death by pyroptosis. Similarly, caspase-8 has dual role in cell death, mediating both receptor-mediated apoptosis and in its absence, necroptosis. Caspase-8 also functions in maintenance and homeostasis of the adult T-cell population. Caspase-3 has important roles in tissue differentiation, regeneration and neural development in ways that are distinct and do not involve any apoptotic activity. Several other caspases have demonstrated anti-tumor roles. Notable among them are caspase-2, -8 and -14. However, increased caspase-2 and -8 expression in certain types of tumor has also been linked to promoting tumorigenesis. Increased levels of caspase-3 in tumor cells causes apoptosis and secretion of paracrine factors that promotes compensatory proliferation in surrounding normal tissues, tumor cell repopulation and presents a barrier for effective therapeutic strategies. Besides this caspase-2 has emerged as a unique caspase with potential roles in maintaining genomic stability, metabolism, autophagy and aging. The present review focuses on some of these less studied and emerging functions of mammalian caspases.

Autophagy as a pro-death pathway

The evolutionarily conserved catabolic process of autophagy involves the degradation of cytoplasmic components through lysosomal enzymes. Basal levels of autophagy maintain cellular homeostasis and under stress conditions high levels of autophagy are induced. It is often under such stress conditions that high levels of autophagy and cell death have been observed, leading to the idea that autophagy may act as an executioner of cell death. However the notion of autophagy as a cell death mechanism has been controversial and remains mechanistically undefined. There is now growing evidence that in specific contexts autophagy can indeed facilitate cell death. The pro-death role of autophagy is however complicated due to the extensive cross-talk between different signalling pathways. This review summarises the examples of where autophagy acts as a means of cell death and discusses the association of autophagy with the different cell death pathways.