Oncogenic Ras-induced expression of Noxa and Beclin-1 promotes autophagic cell death and limits clonogenic survival

Autophagy and the invisible line between life and death

For a considerable time cell death has been considered to represent mutually exclusive states with cell death modalities that are governed by their inherent and unique mode of action involving specific molecular entities and have therefore been studied primarily in isolation. It is now, however, becoming increasingly clear that these modalities are regulated by similar pathways and share a number of initiator and effector molecules that control both cell death as well as cell survival mechanisms, demanding a newly aligned and integrative approach of cell death assessment. Frequently cell death is triggered through a dual action that incorporates signaling events associated with more than one death modality. Apoptosis and necrosis regularly co-operate in a tightly balanced interplay that involves autophagy to serve context dependently either as a pro-survival or a pro-death mechanism. In this review we will assess current cell death modalities and their molecular overlap with the goal of clarifying the controversial role of autophagy in the cell death response. By dissecting the key molecular pathways and their positioning within a network of regulatory signalling hubs and checkpoints we discuss a distinct approach that integrates autophagy with a resultant cell death manifestation. In doing so, former classifications of cell death modalities fade and reveal the intricate molecular proportions and complexities of the cell death response that may contribute towards an enhanced means of cell death control.

Inhibition of autophagosome-lysosome fusion by ginsenoside Ro via the ESR2-NCF1-ROS pathway sensitizes esophageal cancer cells to 5-fluorouracil-induced cell death via the CHEK1-mediated DNA damage checkpoint

Modulation of autophagy has been increasingly regarded as a promising cancer therapeutic approach. In this study, we screened several ginsenosides extracted from Panax ginseng and identified ginsenoside Ro (Ro) as a novel autophagy inhibitor. Ro blocked the autophagosome-lysosome fusion process by raising lysosomal pH and attenuating lysosomal cathepsin activity, resulting in the accumulation of the autophagosome marker MAP1LC3B/LC3B and SQSTM1/p62 (sequestosome 1) in various esophageal cancer cell lines. More detailed studies demonstrated that Ro activated ESR2 (estrogen receptor 2), which led to the activation of NCF1/p47(PHOX) (neutrophil cytosolic factor 1), a subunit of NADPH oxidase, and subsequent reactive oxygen species (ROS) production. Treatment with siRNAs or inhibitors of the ESR2-NCF1-ROS axis, such as N-acetyl-L-cysteine (NAC), diphenyleneiodonium chloride (DPI), apocynin (ACN), Tiron, and Fulvestrant apparently decreased Ro-induced LC3B-II, GFP-LC3B puncta, and SQSTM1, indicating that ROS instigates autophagic flux inhibition triggered by Ro. More importantly, suppression of autophagy by Ro sensitized 5-fluorouracil (5-Fu)-induced cell death in chemoresistant esophageal cancer cells. 5-Fu induced prosurvival autophagy, and by inhibiting such autophagy, siRNAs against BECN1/beclin 1, ATG5, ATG7, and LC3B enhanced 5-Fu-induced autophagy-associated and apoptosis-independent cell death. We observed that Ro potentiates 5-Fu cytotoxicity via delaying CHEK1 (checkpoint kinase 1) degradation and downregulating DNA replication process, resulting in the delayed DNA repair and the accumulation of DNA damage. In summary, these data suggest that Ro is a novel autophagy inhibitor and could function as a potent anticancer agent in combination therapy to overcome chemoresistance.

Cell death in the pathogenesis of systemic lupus erythematosus and lupus nephritis

Nephritis is one of the most severe complications of systemic lupus erythematosus (SLE). One key characteristic of lupus nephritis (LN) is the deposition of immune complexes containing nucleic acids and/or proteins binding to nucleic acids and autoantibodies recognizing these molecules. A variety of cell death processes are implicated in the generation and externalization of modified nuclear autoantigens and in the development of LN. Among these processes, apoptosis, primary and secondary necrosis, NETosis, necroptosis, pyroptosis, and autophagy have been proposed to play roles in tissue damage and immune dysregulation. Cell death occurs in healthy individuals during conditions of homeostasis yet autoimmunity does not develop, at least in part, because of rapid clearance of dying cells. In SLE, accelerated cell death combined with a clearance deficiency may lead to the accumulation and externalization of nuclear autoantigens and to autoantibody production. In addition, specific types of cell death may modify autoantigens and alter their immunogenicity. These modified molecules may then become novel targets of the immune system and promote autoimmune responses in predisposed hosts. In this review, we examine various cell death pathways and discuss how enhanced cell death, impaired clearance, and post-translational modifications of proteins could contribute to the development of lupus nephritis.

Colorectal cancer-related mutant KRAS alleles function as positive regulators of autophagy

The recent interest to modulate autophagy in cancer therapy has been hampered by the dual roles of this conserved catabolic process in cancer, highlighting the need for tailored approaches. Since RAS isoforms have been implicated in autophagy regulation and mutation of the KRAS oncogene is highly frequent in colorectal cancer (CRC), we questioned whether/how mutant KRAS alleles regulate autophagy in CRC and its implications. We established two original models, KRAS-humanized yeast and KRAS-non-cancer colon cells and showed that expression of mutated KRAS up-regulates starvation-induced autophagy in both. Accordingly, KRAS down-regulation inhibited autophagy in CRC-derived cells harboring KRAS mutations. We further show that KRAS-induced autophagy proceeds via up-regulation of the MEK/ERK pathway in both colon models and that KRAS and autophagy contribute to CRC cell survival during starvation. Since KRAS inhibitors have proven difficult to develop, our results suggest using autophagy inhibitors as a combined/alternative therapeutic approach in CRCs with mutant KRAS.

Inhibition of colorectal cancer stem cell survival and invasive potential by hsa-miR-140-5p mediated suppression of Smad2 and autophagy

Colorectal cancer (CRC) is the third highest mortality cancer in the United States and frequently metastasizes to liver and lung. Smad2 is a key element downstream of the TGF-β signaling pathway to regulate cancer metastasis by promoting epithelial to mesenchymal transition and maintaining the cancer stem cell (CSC) phenotype. In this study, we show that hsa-miR-140-5p directly targets Smad2 and overexpression of hsa-miR-140-5p in CRC cell lines decreases Smad2 expression levels, leading decreased cell invasion and proliferation, and increasing cell cycle arrest. Ectopic expression of hsa-miR-140-5p in colorectal CSCs inhibited CSC growth and sphere formation in vitro by disrupting autophagy. We have systematically identified targets of hsa-miR-140-5p involved in autophagy. Furthermore, overexpression of hsa-miR-140-5p in CSCs abolished tumor formation and metastasis in vivo. In addition, there is a progressive loss of hsa-miR-140-5p expression from normal colorectal mucosa to primary tumor tissues, with further reduction in liver metastatic tissues. Higher hsa-miR-140 expression is significantly correlated with better survival in stage III and IV colorectal cancer patients. The functional and clinical significance of hsa-miR-140-5p suggests that it is a key regulator in CRC progression and metastasis, and may have potential as a novel therapeutic molecule to treat CRC.

BAX-BAK1-independent LC3B lipidation by BH3 mimetics is unrelated to BH3 mimetic activity and has only minimal effects on autophagic flux

Inhibition of prosurvival BCL2 family members can induce autophagy, but the mechanism is controversial. We have provided genetic evidence that BCL2 family members block autophagy by inhibiting BAX and BAK1, but others have proposed they instead inhibit BECN1. Here we confirm that small molecule BH3 mimetics can induce BAX- and BAK1-independent MAP1LC3B/LC3B lipidation, but this only occurred at concentrations far greater than required to induce apoptosis and dissociate canonical BH3 domain-containing proteins that bind more tightly than BECN1. Because high concentrations of a less-active enantiomer of ABT-263 also induced BAX- and BAK1-independent LC3B lipidation, induction of this marker of autophagy appears to be an off-target effect. Indeed, robust autophagic flux was not induced by BH3 mimetic compounds in the absence of BAX and BAK1. Therefore at concentrations that are on target and achievable in vivo, BH3 mimetics only induce autophagy in a BAX- and BAK1-dependent manner.

The renewed battle against RAS-mutant cancers

The RAS genes encode for members of a large superfamily of guanosine-5'-triphosphate (GTP)-binding proteins that control diverse intracellular signaling pathways to promote cell proliferation. Somatic mutations in the RAS oncogenes are the most common activating lesions found in human cancers. These mutations invariably result in the gain-of-function of RAS by impairing GTP hydrolysis and are frequently associated with poor responses to standard cancer therapies. In this review, we summarize key findings of past and present landmark studies that have deepened our understanding of the RAS biology in the context of oncogenesis. We also discuss how emerging areas of research could further bolster a renewed global effort to target the largely undruggable oncogenic RAS and/or its activated downstream effector signaling cascades to achieve better treatment outcomes for RAS-mutant cancer patients.

KRAS Mutant Pancreatic Cancer: No Lone Path to an Effective Treatment

Pancreatic ductal adenocarcinoma (PDAC) is among the deadliest cancers with a dismal 7% 5-year survival rate and is projected to become the second leading cause of cancer-related deaths by 2020. KRAS is mutated in 95% of PDACs and is a well-validated driver of PDAC growth and maintenance. However, despite comprehensive efforts, an effective anti-RAS drug has yet to reach the clinic. Different paths to inhibiting RAS signaling are currently under investigation in the hope of finding a successful treatment. Recently, direct RAS binding molecules have been discovered, challenging the perception that RAS is an “undruggable” protein. Other strategies currently being pursued take an indirect approach, targeting proteins that facilitate RAS membrane association or downstream effector signaling. Unbiased genetic screens have identified synthetic lethal interactors of mutant RAS. Most recently, metabolic targets in pathways related to glycolytic signaling, glutamine utilization, autophagy, and macropinocytosis are also being explored. Harnessing the patient’s immune system to fight their cancer is an additional exciting route that is being considered. The “best” path to inhibiting KRAS has yet to be determined, with each having promise as well as potential pitfalls. We will summarize the state-of-the-art for each direction, focusing on efforts directed toward the development of therapeutics for pancreatic cancer patients with mutated KRAS.

Plant Homeo Domain Finger Protein 8 Regulates Mesodermal and Cardiac Differentiation of Embryonic Stem Cells Through Mediating the Histone Demethylation of pmaip1

Histone demethylases have emerged as key regulators of biological processes. The H3K9me2 demethylase plant homeo domain finger protein 8(PHF8), for example, is involved in neuronal differentiation, but its potential function in the differentiation of embryonic stem cells (ESCs) to cardiomyocytes is poorly understood. Here, we explored the role of PHF8 during mesodermal and cardiac lineage commitment of mouse ESCs (mESCs). Using a phf8 knockout (ph8(-/Y) ) model, we found that deletion of phf8 in ESCs did not affect self-renewal, proliferation or early ectodermal/endodermal differentiation, but it did promote the mesodermal lineage commitment with the enhanced cardiomyocyte differentiation. The effects were accompanied by a reduction in apoptosis through a caspase 3-independent pathway during early ESC differentiation, without significant differences between differentiating wide-type (ph8(+/Y) ) and ph8(-/Y) ESCs in cell cycle progression or proliferation. Functionally, PHF8 promoted the loss of a repressive mark H3K9me2 from the transcription start site of a proapoptotic gene pmaip1 and activated its transcription. Furthermore, knockdown of pmaip1 mimicked the phenotype of ph8(-/Y) by showing the decreased apoptosis during early differentiation of ESCs and promoted mesodermal and cardiac commitment, while overexpression of pmaip1 or phf8 rescued the phenotype of ph8(-/Y) ESCs by increasing the apoptosis and weakening the mesodermal and cardiac differentiation. These results reveal that the histone demethylase PHF8 regulates mesodermal lineage and cell fate decisions in differentiating mESCs through epigenetic control of the gene critical to programmed cell death pathways. Stem Cells 2016;34:1527-1540.

Rapamycin Inhibits Cardiac Hypertrophy by Promoting Autophagy via the MEK/ERK/Beclin-1 Pathway

Rapamycin, also known as sirolimus, is an antifungal agent and immunosuppressant drug used to prevent organ rejection in transplantation. However, little is known about the role of rapamycin in cardiac hypertrophy and the signaling pathways involved. Here, the effect of rapamycin was examined using phenylephrine (PE) induced cardiomyocyte hypertrophy in vitro and in a rat model of aortic banding (AB) - induced hypertrophy in vivo. Inhibition of MEK/ERK signaling reversed the effect of rapamycin on the up-regulation of LC3-II, Beclin-1 and Noxa, and the down-regulation of Mcl-1 and p62. Silencing of Noxa or Beclin-1 suppressed rapamycin-induced autophagy, and co-immunoprecipitation experiments showed that Noxa abolishes the inhibitory effect of Mcl-1 on Beclin-1, promoting autophagy. In vivo experiments showed that rapamycin decreased AB-induced cardiac hypertrophy in a MEK/ERK dependent manner. Taken together, our results indicate that rapamycin attenuates cardiac hypertrophy by promoting autophagy through a mechanism involving the modulation of Noxa and Beclin-1 expression by the MEK/ERK signaling pathway.

The presence of LC3B puncta and HMGB1 expression in malignant cells correlate with the immune infiltrate in breast cancer

Several cell-intrinsic alterations have poor prognostic features in human breast cancer, as exemplified by the absence of MAP1LC3B/LC3B (microtubule-associated protein 1 light chain 3 β)-positive puncta in the cytoplasm (which indicates reduced autophagic flux) or the loss of nuclear HMGB1 expression by malignant cells. It is well established that breast cancer is under strong immunosurveillance, as reflected by the fact that scarce infiltration of the malignant lesion by CD8(+) cytotoxic T lymphocytes or comparatively dense infiltration by immunosuppressive cell types (such as FOXP3(+) regulatory T cells or CD68(+) tumor-associated macrophages), resulting in low CD8(+):FOXP3(+) or CD8(+):CD68(+) ratios, has a negative prognostic impact. Here, we reveal the surprising finding that cell-intrinsic features may influence the composition of the immune infiltrate in human breast cancer. Thus, the absence of LC3B puncta is correlated with intratumoral (but not peritumoral) infiltration by fewer CD8(+) cells and more FOXP3(+) or CD68(+) cells, resulting in a major drop in the CD8(+):FOXP3(+) or CD8(+):CD68(+) ratios. Moreover, absence of HMGB1 expression in nuclei correlated with a general drop in all immune effectors, in particular FOXP3(+) and CD68(+) cells, both within the tumor and close to it. Combined analysis of LC3B puncta and HMGB1 expression allowed for improved stratification of patients with respect to the characteristics of their immune infiltrate as well as overall and metastasis-free survival. It can be speculated that blocked autophagy in, or HMGB1 loss from, cancer cells may favor tumor progression due to their negative impact on anticancer immunosurveillance.

BRAF associated autophagy exploitation: BRAF and autophagy inhibitors synergise to efficiently overcome resistance of BRAF mutant colorectal cancer cells

Autophagy is the basic catabolic mechanism that involves cell degradation of unnecessary or dysfunctional cellular components. Autophagy has a controversial role in cancer--both in protecting against tumor progression by isolation of damaged organelles, or by potentially contributing to cancer growth. The impact of autophagy in RAS induced transformation still remains to be further analyzed based on the differential effect of RAS isoforms and tumor cell context. In the present study, the effect of KRAS/BRAF/PIK3CA oncogenic pathways on the autophagic cell properties and on main components of the autophagic machinery like p62 (SQSTM1), Beclin-1 (BECN1) and MAP1LC3 (LC3) in colon cancer cells was investigated. This study provides evidence that BRAF oncogene induces the expression of key autophagic markers, like LC3 and BECN1 in colorectal tumor cells. Herein, PI3K/AKT/MTOR inhibitors induce autophagic tumor properties, whereas RAF/MEK/ERK signalling inhibitors reduce expression of autophagic markers. Based on the ineffectiveness of BRAFV600E inhibitors in BRAFV600E bearing colorectal tumors, the BRAF related autophagic properties in colorectal cancer cells are further exploited, by novel combinatorial anti-cancer protocols. Strong evidence is provided here that pre-treatment of autophagy inhibitor 3-MA followed by its combination with BRAFV600E targeting drug PLX4720 can synergistically sensitize resistant colorectal tumors. Notably, colorectal cancer cells are very sensitive to mono-treatments of another autophagy inhibitor, Bafilomycin A1. The findings of this study are expected to provide novel efficient protocols for treatment of otherwise resistant colorectal tumors bearing BRAFV600E, by exploiting the autophagic properties induced by BRAF oncogene.

Use of Anti-Noxa Antibody for Differential Diagnosis between Epithelioid Mesothelioma and Reactive Mesothelial Hyperplasia

OBJECTIVES

The histological differential diagnosis between epithelioid mesothelioma (EM) and reactive mesothelial hyperplasia (RMH) is not always straightforward. The aim of the present study was to search for new immunohistochemical markers to distinguish EM from RMH.

METHODS

We evaluated and compared the expression of apoptosis-related genes in EM and RMH by real-time RT-PCR array analysis followed by clustering of significant gene expression. Immunohistochemical staining and statistical analysis of Noxa expression in 81 cases of EM and 55 cases of RMH were performed and compared with the utility of other previously reported antibodies such as Desmin, EMA, GLUT-1, IMP-3 and CD146.

RESULTS

Noxa mRNA expression levels were found to be increased in EM when compared to RMH by RT-PCR array analysis. In the immunohistochemical analysis, Noxa showed sensitivity of 69.0%, specificity of 93.6% and positive predictive value of 93.0% as a positive marker of EM in distinguishing it from RMH, and these values were almost similar to IMP-3.

CONCLUSION

Noxa is a marker with relatively high specificity, and can be used to distinguish EM from RMH. It would be a valuable addition to the current antibody panel used for the differential diagnosis of EM and RMH.

Macroautophagy is dispensable for growth of KRAS mutant tumors and chloroquine efficacy

Macroautophagy is a key stress-response pathway that can suppress or promote tumorigenesis depending on the cellular context. Notably, Kirsten rat sarcoma (KRAS)-driven tumors have been reported to rely on macroautophagy for growth and survival, suggesting a potential therapeutic approach of using autophagy inhibitors based on genetic stratification. In this study, we evaluated whether KRAS mutation status can predict the efficacy to macroautophagy inhibition. By profiling 47 cell lines with pharmacological and genetic loss-of-function tools, we were unable to confirm that KRAS-driven tumor lines require macroautophagy for growth. Deletion of autophagy-related 7 (ATG7) by genome editing completely blocked macroautophagy in several tumor lines with oncogenic mutations in KRAS but did not inhibit cell proliferation in vitro or tumorigenesis in vivo. Furthermore, ATG7 knockout did not sensitize cells to irradiation or to several anticancer agents tested. Interestingly, ATG7-deficient and -proficient cells were equally sensitive to the antiproliferative effect of chloroquine, a lysosomotropic agent often used as a pharmacological tool to evaluate the response to macroautophagy inhibition. Moreover, both cell types manifested synergistic growth inhibition when treated with chloroquine plus the tyrosine kinase inhibitors erlotinib or sunitinib, suggesting that the antiproliferative effects of chloroquine are independent of its suppressive actions on autophagy.

Squaramides with cytotoxic activity against human gastric carcinoma cells HGC-27: synthesis and mechanism of action

A series of squaramates and squaramides have been synthesized and their cytotoxic activity has been investigated in different cancer cell lines.

Cell Death Signaling

In multicellular organisms, cell death is a critical and active process that maintains tissue homeostasis and eliminates potentially harmful cells. There are three major types of morphologically distinct cell death: apoptosis (type I cell death), autophagic cell death (type II), and necrosis (type III). All three can be executed through distinct, and sometimes overlapping, signaling pathways that are engaged in response to specific stimuli. Apoptosis is triggered when cell-surface death receptors such as Fas are bound by their ligands (the extrinsic pathway) or when Bcl2-family proapoptotic proteins cause the permeabilization of the mitochondrial outer membrane (the intrinsic pathway). Both pathways converge on the activation of the caspase protease family, which is ultimately responsible for the dismantling of the cell. Autophagy defines a catabolic process in which parts of the cytosol and specific organelles are engulfed by a double-membrane structure, known as the autophagosome, and eventually degraded. Autophagy is mostly a survival mechanism; nevertheless, there are a few examples of autophagic cell death in which components of the autophagic signaling pathway actively promote cell death. Necrotic cell death is characterized by the rapid loss of plasma membrane integrity. This form of cell death can result from active signaling pathways, the best characterized of which is dependent on the activity of the protein kinase RIP3.

How does metabolism affect cell death in cancer?

In cancer research, identifying a specificity of tumor cells compared with 'normal' proliferating cells for targeted therapy is often considered the Holy Grail for researchers and clinicians. Although diverse in origin, most cancer cells share characteristics including the ability to escape cell death mechanisms and the utilization of different methods of energy production. In the current paradigm, aerobic glycolysis is considered the central metabolic characteristic of cancer cells (Warburg effect). However, recent data indicate that cancer cells also show significant changes in other metabolic pathways. Indeed, it was recently suggested that Kreb's cycle, pentose phosphate pathway intermediates, and essential and nonessential amino acids have key roles. Renewed interest in the fact that cancer cells have to reprogram their metabolism in order to proliferate or resist treatment must take into consideration the ability of tumor cells to adapt their metabolism to the local microenvironment (low oxygen, low nutrients). This variety of metabolic sources might be either a strength, resulting in infinite possibilities for adaptation and increased ability to resist chemotherapy-induced death, or a weakness that could be targeted to kill cancer cells. Here, we discuss recent insights showing how energetic metabolism may regulate cell death and how this might be relevant for cancer treatment.

Discontinuing MEK inhibitors in tumor cells with an acquired resistance increases migration and invasion

BACKGROUND

Development of small molecular inhibitors against BRAF and MEK has been a breakthrough in the treatment of malignant melanoma. However, the long-term effect is foiled in virtually all patients by the emergence of resistant tumor cell populations. Therefore, mechanisms resulting in the acquired resistance against BRAF and MEK inhibitors have gained much attention and several strategies have been proposed to overcome tumor resistance, including interval treatment or withdrawal of these compounds after disease progression.

METHODS

Using a panel of cell lines with an acquired resistance against MEK inhibitors, we have evaluated the sensitivity of these cells against compounds targeting AKT/mTOR signaling, as well as novel ERK1/2 inhibitors. Furthermore, the effects of withdrawal of MEK inhibitor on migration in resistant cell lines were analyzed.

RESULTS

We demonstrate that withdrawal of BRAF or MEK inhibitors in tumor cells with an acquired resistance results in reactivation of ERK1/2 signaling and upregulation of EMT-inducing transcription factors, leading to a highly migratory and invasive phenotype of cancer cells. Furthermore, we show that migration in these cells is independent from AKT/mTOR signaling. However, combined targeting of AKT/mTOR using MK-2206 and AZD8055 efficiently inhibits proliferation in all resistant tumor cell lines analyzed.

CONCLUSIONS

We propose that combined targeting of MEK/AKT/mTOR or treatment with a novel ERK1/2 inhibitor downstream of BRAF/MEK suppresses proliferation as well as migration and invasion in resistant tumor cells. We provide a rationale against the discontinuation of BRAF or MEK inhibitors in patients with an acquired resistance, and provide a rationale for combined targeting of AKT/mTOR and MEK/ERK1/2, or direct targeting of ERK1/2 as an effective treatment strategy.

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.

The potential of Beclin 1 as a therapeutic target for the treatment of breast cancer

INTRODUCTION

Beclin 1 plays a crucial role in autophagy via the Beclin 1 interactome, and is involved in various biological processes such as protein sorting, chemokinesis, and cell death. Via these biologic functions, Beclin 1 contributes to both tumor suppression and tumor progression.

AREAS COVERED

Beclin 1 plays a key biologic function on cell homeostasis and affects tumorigenesis. In this review, detailing up-to-date knowledge on the tumorigenic role of Beclin 1, its implication in breast cancer, and its utility as a breast cancer-specific drug target is discussed.

EXPERT OPINION

Because Beclin 1 is expressed in breast cancer cells, Beclin 1 could be a unique, effective drug target for the prevention and treatment of breast cancer. However, the expression of Beclin 1 varies according to cancer molecular subtypes, and Beclin 1 is involved in both breast cancer suppression and tumor progression; therefore, the decision of using a Beclin 1 inducer or inhibitor should be made based on breast cancer stage and subtype.

Autophagy in Cell Life and Cell Death

Macroautophagy (hereafter referred to as autophagy) is a process used by the cell to deliver cytoplasmic components to the lysosome for degradation. Autophagy is most often associated with cell survival, as it provides cells with molecular building blocks during periods of nutrient deprivation and also aids in the elimination of damaged organelles and protein aggregates. However, autophagy has also been implicated in cell death. Here, we review what is known about autophagy, its regulation, its role both in cell life and cell death, and what is known about autophagic cell death in vivo.

Noxa upregulation by oncogenic activation of MEK/ERK through CREB promotes autophagy in human melanoma cells

Reduction in the expression of the anti-survival BH3-only proteins PUMA and Bim is associated with the pathogenesis of melanoma. However, we have found that the expression of the other BH3-only protein Noxa is commonly upregulated in melanoma cells, and that this is driven by oncogenic activation of MEK/ERK. Immunohistochemistry studies showed that Noxa was expressed at higher levels in melanomas than nevi. Moreover, the expression of Noxa was increased in metastatic compared to primary melanomas, and in thick primaries compared to thin primaries. Inhibition of oncogenic BRAF^V600E or MEK downregulated Noxa, whereas activation of MEK/ERK caused its upregulation. In addition, introduction of BRAF^V600E increased Noxa expression in melanocytes. Upregulation of Noxa was due to a transcriptional increase mediated by cAMP responsive element binding protein, activation of which was also increased by MEK/ERK signaling in melanoma cells. Significantly, Noxa appeared necessary for constitutive activation of autophagy, albeit at low levels, by MEK/ERK in melanoma cells. Furthermore, it was required for autophagy activation that delayed apoptosis in melanoma cells undergoing nutrient deprivation. These results reveal that oncogenic activation of MEK/ERK drives Noxa expression to promote autophagy, and suggest that Noxa has an indirect anti-apoptosis role in melanoma cells under nutrient starvation conditions.

RIP3 Regulates Autophagy and Promotes Coxsackievirus B3 Infection of Intestinal Epithelial Cells

Receptor interacting protein kinase-3 (RIP3) is an essential kinase for necroptotic cell death signaling and has been implicated in antiviral cell death signaling upon DNA virus infection. Here, we performed high-throughput RNAi screening and identified RIP3 as a positive regulator of coxsackievirus B3 (CVB) replication in intestinal epithelial cells (IECs). RIP3 regulates autophagy, a process utilized by CVB for viral replication factory assembly, and depletion of RIP3 inhibits autophagic flux and leads to the accumulation of autophagosomes and amphisomes. Additionally, later in infection, RIP3 is cleaved by the CVB-encoded cysteine protease 3C(pro), which serves to abrogate RIP3-mediated necrotic signaling and induce a nonnecrotic form of cell death. Taken together, our results show that temporal targeting of RIP3 allows CVB to benefit from its roles in regulating autophagy while inhibiting the induction of necroptotic cell death.

Polyphenol-rich extract of Pimenta dioica berries (Allspice) kills breast cancer cells by autophagy and delays growth of triple negative breast cancer in athymic mice

Bioactive compounds from edible plants have limited efficacy in treating advanced cancers, but they have potential to increase the efficacy of chemotherapy drugs in a combined treatment. An aqueous extract of berries of Pimenta dioica (Allspice) shows promise as one such candidate for combination therapy or chemoprevention. An aqueous extract of Allspice (AAE) was tested against human breast cancer (BrCa) cells in vitro and in vivo. AAE reduced the viability and clonogenic growth of several types of BrCa cells (IC50 ≤ 100 μg/ml) with limited toxicity in non-tumorigenic, quiescent cells (IC50 >200 μg/ml). AAE induced cytotoxicity in BrCa was inconsistent with apoptosis, but was associated with increased levels of autophagy markers LC3B and LC3B-positive puncta. Silencing the expression of autophagy related genes (ATGs) prevented AAE-induced cell death. Further, AAE caused inhibition of Akt/mTOR signaling, and showed enhanced cytotoxicity when combined with rapamycin, a chemotherapy drug and an inhibitor of mTOR signaling. Oral administration (gavage) of AAE into athymic mice implanted with MDA-MB231 tumors inhibited tumor growth slightly but not significantly (mean decrease ~ 14%, p ≥ 0.20) if mice were gavaged post-tumor implant. Tumor growth showed a significant delay (38%) in tumor palpability and growth rate (time to reach tumor volume ≥ 1,000 mm3) when mice were pre-dosed with AAE for two weeks. Analysis of tumor tissues showed increased levels of LC3B in AAE treated tumors, indicating elevated autophagic tumor cell death in vivo in treated mice. These results demonstrate antitumor and chemo-preventive activity of AAE against BrCa and potential for adjuvant to mTOR inhibition.

Transcriptional Repressor DAXX Promotes Prostate Cancer Tumorigenicity via Suppression of Autophagy

The DAXX transcriptional repressor was originally associated with apoptotic cell death. However, recent evidence that DAXX represses several tumor suppressor genes, including the DAPK1 and DAPK3 protein kinases, and is up-regulated in many cancers argues that a pro-survival role may predominate in a cancer context. Here, we report that DAXX has potent growth-enhancing effects on primary prostatic malignancy through inhibition of autophagy. Through stable gene knockdown and mouse subcutaneous xenograft studies, we demonstrate that DAXX promotes tumorigenicity of human ALVA-31 and PC3 prostate cancer (PCa) cells in vivo. Importantly, DAXX represses expression of essential autophagy modulators DAPK3 and ULK1 in vivo, revealing autophagy suppression as a mechanism through which DAXX promotes PCa tumorigenicity. Furthermore, DAXX knockdown increases autophagic flux in cultured PCa cells. Finally, interrogation of the Oncomine(TM) database suggests that DAXX overexpression is associated with malignant transformation in several human cancers, including prostate and pancreatic cancers. Thus, DAXX may represent a new cancer biomarker for the detection of aggressive disease, whose tissue-specific down-regulation can serve as an improved therapeutic modality. Our results establish DAXX as a pro-survival protein in PCa and reveal that, in the early stages of tumorigenesis, autophagy suppresses prostate tumor formation.

Autophagic activity in neuronal cell death

As post-mitotic cells with great energy demands, neurons depend upon the homeostatic and waste-recycling functions provided by autophagy. In addition, autophagy also promotes survival during periods of harsh stress and targets aggregate-prone proteins associated with neurodegeneration for degradation. Despite this, autophagy has also been controversially described as a mechanism of programmed cell death. Instances of autophagic cell death are typically associated with elevated numbers of cytoplasmic autophagosomes, which have been assumed to lead to excessive degradation of cellular components. Due to the high activity and reliance on autophagy in neurons, these cells may be particularly susceptible to autophagic death. In this review, we summarize and assess current evidence in support of autophagic cell death in neurons, as well as how the dysregulation of autophagy commonly seen in neurodegeneration can contribute to neuron loss. From here, we discuss potential treatment strategies relevant to such cell-death pathways.

Molecular mechanisms of mitochondrial autophagy/mitophagy in the heart

Mitochondrial quality is a crucial determinant of cell viability, and mitochondrial autophagy plays a central role in this control mechanism. Based on studies in yeast, numerous investigations of this process have been conducted, and the framework of mammalian mitochondrial autophagy is progressively appearing. However, many enigmas about the molecular mechanisms involved remain unsolved. Furthermore, the pathological significance of mitochondrial autophagy in the heart remains largely unclear. In this review, we discuss the current understanding of mitochondrial autophagy in mammals with reference to that in yeast. Regarding the process in yeast, some points of uncertainty have arisen. We also summarize recent advances in the research of autophagy and mitochondrial autophagy in the heart. This article is a part of a review series on Autophagy in Health and Disease.

Autophagy-Regulated ROS from Xanthine Oxidase Acts as an Early Effector for Triggering Late Mitochondria-Dependent Apoptosis in Cathepsin S-Targeted Tumor Cells

Cathepsin S (CTSS), which is highly expressed in various malignant tumor cells, has been proposed to promote tumor progression, migration, and invasion. CTSS inhibition not only blocks tumor cell invasion and endothelial tube formation but also induces cellular cytotoxicity. In our previous studies, we have observed that CTSS inhibition induces autophagy, which is responsible for up-regulating xanthine oxidase for early ROS generation and consequent cell death. However, whether the autophagy-regulated early ROS triggers apoptosis remains unclear. We conducted a long-term follow-up study to investigate the relationship between early autophagy and late mitochondria-dependent apoptosis. We demonstrated that early ROS generation is critical for mitochondria damage and the activation of intrinsic apoptotic pathway. Attenuating the early ROS level diminished later mitochondrial damage and downstream apoptotic signaling. Collectively, mitochondria-dependent apoptosis is regulated by autophagy-regulated early ROS, which serves as an early effector that triggers mitochondrial signaling for late apoptosis. The data emphasize the essential role of autophagy-regulated early ROS in triggering late apoptotic signaling.

Autophagy and cancer metabolism

The metabolism of malignant cells is profoundly altered in order to maintain their survival and proliferation in adverse microenvironmental conditions. Autophagy is an intracellular recycling process that maintains basal levels of metabolites and biosynthetic intermediates under starvation or other forms of stress, hence serving as an important mechanism for metabolic adaptation in cancer cells. Although it is widely acknowledged that autophagy sustains metabolism in neoplastic cells under duress, many questions remain with regard to the mutual relationship between autophagy and metabolism in cancer. Importantly, autophagy has often been described as a "double-edged sword" that can either impede or promote cancer initiation and progression. Here, we overview such a dual function of autophagy in tumorigenesis and our current understanding of the coordinated regulation of autophagy and cancer cell metabolism in the control of tumor growth, progression, and resistance to therapy.

Co-activation of AMPK and mTORC1 Induces Cytotoxicity in Acute Myeloid Leukemia

AMPK is a master regulator of cellular metabolism that exerts either oncogenic or tumor suppressor activity depending on context. Here, we report that the specific AMPK agonist GSK621 selectively kills acute myeloid leukemia (AML) cells but spares normal hematopoietic progenitors. This differential sensitivity results from a unique synthetic lethal interaction involving concurrent activation of AMPK and mTORC1. Strikingly, the lethality of GSK621 in primary AML cells and AML cell lines is abrogated by chemical or genetic ablation of mTORC1 signaling. The same synthetic lethality between AMPK and mTORC1 activation is established in CD34-positive hematopoietic progenitors by constitutive activation of AKT or enhanced in AML cells by deletion of TSC2. Finally, cytotoxicity in AML cells from GSK621 involves the eIF2α/ATF4 signaling pathway that specifically results from mTORC1 activation. AMPK activation may represent a therapeutic opportunity in mTORC1-overactivated cancers.

H-Ras regulation of TRAIL death receptor mediated apoptosis

TNF-related apoptosis-inducing ligand (TRAIL) induces apoptosis through the death receptors (DRs) 4 and/or 5 expressed on the cell surface. Multiple clinical trials are underway to evaluate the antitumor activity of recombinant human TRAIL and agonistic antibodies to DR4 or DR5. However, their therapeutic potential is limited by the high frequency of cancer resistance. Here we provide evidence demonstrating the role of H-Ras in TRAIL receptor mediated apoptosis. By analyzing the genome wide mRNA expression data of the NCI60 cancer cell lines, we found that H-Ras expression was consistently upregulated in TRAIL-resistant cell lines. By contrast, no correlation was found between TRAIL sensitivity and K-Ras expression levels or their mutational profiles. Notably, H-Ras upregulation associated with a surface deficiency of TRAIL death receptors. Selective inhibition of H-Ras activity in TRAIL-resistant cells restored the surface expression of both DR4 and DR5 without changing their total protein levels. The resulting cells became highly susceptible to both TRAIL and agonistic DR5 antibody, whereas K-Ras inhibition had little or no effect on TRAIL-induced apoptosis, indicating H-Ras plays a distinct role in the regulation of TRAIL death receptors. Further studies are warranted to determine the therapeutic potential of H-Ras-specific inhibitors in combination with TRAIL receptor agonists.

Oncogenic KRAS sensitizes premalignant, but not malignant cells, to Noxa-dependent apoptosis through the activation of the MEK/ERK pathway

KRAS is mutated in about 20-25% of all human cancers and especially in pancreatic, lung and colorectal tumors. Oncogenic KRAS stimulates several pro-survival pathways, but it also triggers the trans-activation of pro-apoptotic genes. In our work, we show that G13D mutations of KRAS activate the MAPK pathway, and ERK2, but not ERK1, up-regulates Noxa basal levels. Accordingly, premalignant epithelial cells are sensitized to various cytotoxic compounds in a Noxa-dependent manner. In contrast to these findings, colorectal cancer cell sensitivity to treatment is independent of KRAS status and Noxa levels are not up-regulated in the presence of mutated KRAS despite the fact that ERK2 still promotes Noxa expression. We therefore speculated that other survival pathways are counteracting the pro-apoptotic effect of mutated KRAS and found that the inhibition of AKT restores sensitivity to treatment, especially in presence of oncogenic KRAS. In conclusion, our work suggests that the pharmacological inhibition of the pathways triggered by mutated KRAS could also switch off its oncogene-activated pro-apoptotic stimulation. On the contrary, the combination of chemotherapy to inhibitors of specific pro-survival pathways, such as the one controlled by AKT, could enhance treatment efficacy by exploiting the pro-death stimulation derived by oncogene activation.

Self-assembled autophagy-inducing polymeric nanoparticles for breast cancer interference in-vivo

A peptide-conjugated poly(β-amino ester) that self-assembles into micelle-like nanoparticles is prepared by a convenient and modular supramolecular approach. The polymer-beclin-1 (P-Bec1) nanoparticles display enhanced cytotoxicity to breast cancer cells through induction of autophagy. This approach overcomes two major limitations of the haploinsufficient tumor suppressor Bec1 compared to small-molecule drugs: poor delivery to tumors owing to enzymatic degradation, and unstable, non-specific bio-distribution and targeting in the tumor tissues.

Die another way--non-apoptotic mechanisms of cell death

Regulated, programmed cell death is crucial for all multicellular organisms. Cell death is essential in many processes, including tissue sculpting during embryogenesis, development of the immune system and destruction of damaged cells. The best-studied form of programmed cell death is apoptosis, a process that requires activation of caspase proteases. Recently it has been appreciated that various non-apoptotic forms of cell death also exist, such as necroptosis and pyroptosis. These non-apoptotic cell death modalities can be either triggered independently of apoptosis or are engaged should apoptosis fail to execute. In this Commentary, we discuss several regulated non-apoptotic forms of cell death including necroptosis, autophagic cell death, pyroptosis and caspase-independent cell death. We outline what we know about their mechanism, potential roles in vivo and define outstanding questions. Finally, we review data arguing that the means by which a cell dies actually matters, focusing our discussion on inflammatory aspects of cell death.

Live and Let Die: Roles of Autophagy in Cadmium Nephrotoxicity

The transition metal ion cadmium (Cd²⁺) is a significant environmental contaminant. With a biological half-life of ~20 years, Cd²⁺ accumulates in the kidney cortex, where it particularly damages proximal tubule (PT) cells and can result in renal fibrosis, failure, or cancer. Because death represents a powerful means by which cells avoid malignant transformation, it is crucial to clearly identify and understand the pathways that determine cell fate in chronic Cd²⁺ nephrotoxicity. When cells are subjected to stress, they make a decision to adapt and survive, or—depending on the magnitude and duration of stress—to die by several modes of death (programmed cell death), including autophagic cell death (ACD). Autophagy is part of a larger system of intracellular protein degradation and represents the channel by which organelles and long-lived proteins are delivered to the lysosome for degradation. Basal autophagy levels in all eukaryotic cells serve as a dynamic physiological recycling system, but they can also be induced by intra- or extracellular stress and pathological processes, such as endoplasmic reticulum (ER) stress. In a context-dependent manner, autophagy can either be protective and hence contribute to survival, or promote death by non-apoptotic or apoptotic pathways. So far, the role of autophagy in Cd²⁺-induced nephrotoxicity has remained unsettled due to contradictory results. In this review, we critically survey the current literature on autophagy in Cd²⁺-induced nephrotoxicity in light of our own ongoing studies. Data obtained in kidney cells illustrate a dual and complex function of autophagy in a stimulus- and time-dependent manner that possibly reflects distinct outcomes in vitro and in vivo. A better understanding of the context-specific regulation of cell fate by autophagy may ultimately contribute to the development of preventive and novel therapeutic strategies for acute and chronic Cd²⁺ nephrotoxicity.

Chemoresistance is associated with increased cytoprotective autophagy and diminished apoptosis in bladder cancer cells treated with the BH3 mimetic (-)-Gossypol (AT-101)

Background

Acquired resistance to standard chemotherapy causes treatment failure in patients with metastatic bladder cancer. Overexpression of pro-survival Bcl-2 family proteins has been associated with a poor chemotherapeutic response, suggesting that Bcl-2-targeted therapy may be a feasible strategy in patients with these tumors. The small-molecule pan-Bcl-2 inhibitor (−)-gossypol (AT-101) is known to induce apoptotic cell death, but can also induce autophagy through release of the pro-autophagic BH3 only protein Beclin-1 from Bcl-2. The potential therapeutic effects of (−)-gossypol in chemoresistant bladder cancer and the role of autophagy in this context are hitherto unknown.

Methods

Cisplatin (5637^rCDDP¹⁰⁰⁰, RT4^rCDDP¹⁰⁰⁰) and gemcitabine (5637^rGEMCI²⁰, RT4^rGEMCI²⁰) chemoresistant sub-lines of the chemo-sensitive bladder cancer cell lines 5637 and RT4 were established for the investigation of acquired resistance mechanisms. Cell lines carrying a stable lentiviral knockdown of the core autophagy regulator ATG5 were created from chemosensitive 5637 and chemoresistant 5637^rGEMCI²⁰ and 5637^rCDDP¹⁰⁰⁰ cell lines. Cell death and autophagy were quantified by FACS analysis of propidium iodide, Annexin and Lysotracker staining, as well as LC3 translocation.

Results

Here we demonstrate that (−)-gossypol induces an apoptotic type of cell death in 5637 and RT4 cells which is partially inhibited by the pan-caspase inhibitor z-VAD. Cisplatin- and gemcitabine-resistant bladder cancer cells exhibit enhanced basal and drug-induced autophagosome formation and lysosomal activity which is accompanied by an attenuated apoptotic cell death after treatment with both (−)-gossypol and ABT-737, a Bcl-2 inhibitor which spares Mcl-1, in comparison to parental cells. Knockdown of ATG5 and inhibition of autophagy by 3-MA had no discernible effect on apoptotic cell death induced by (−)-gossypol and ABT-737 in parental 5637 cells, but evoked a significant increase in early apoptosis and overall cell death in BH3 mimetic-treated 5637^rGEMCI²⁰ and 5637^rCDDP¹⁰⁰⁰ cells.

Conclusions

Our findings show for the first time that (−)-gossypol concomitantly triggers apoptosis and a cytoprotective type of autophagy in bladder cancer and support the notion that enhanced autophagy may underlie the chemoresistant phenotype of these tumors. Simultaneous targeting of Bcl-2 proteins and the autophagy pathway may be an efficient new strategy to overcome their “autophagy addiction” and acquired resistance to current therapy.

Autophagy in malignant transformation and cancer progression

Autophagy plays a key role in the maintenance of cellular homeostasis. In healthy cells, such a homeostatic activity constitutes a robust barrier against malignant transformation. Accordingly, many oncoproteins inhibit, and several oncosuppressor proteins promote, autophagy. Moreover, autophagy is required for optimal anticancer immunosurveillance. In neoplastic cells, however, autophagic responses constitute a means to cope with intracellular and environmental stress, thus favoring tumor progression. This implies that at least in some cases, oncogenesis proceeds along with a temporary inhibition of autophagy or a gain of molecular functions that antagonize its oncosuppressive activity. Here, we discuss the differential impact of autophagy on distinct phases of tumorigenesis and the implications of this concept for the use of autophagy modulators in cancer therapy.

YAP enhances autophagic flux to promote breast cancer cell survival in response to nutrient deprivation

The Yes-associated protein (YAP), a transcriptional coactivator inactivated by the Hippo tumor suppressor pathway, functions as an oncoprotein in a variety of cancers. However, its contribution to breast cancer remains controversial. This study investigated the role of YAP in breast cancer cells under nutrient deprivation (ND). Here, we show that YAP knockdown sensitized MCF7 breast cancer cells to nutrient deprivation-induced apoptosis. Furthermore, in response to ND, YAP increased the autolysosome degradation, thereby enhancing the cellular autophagic flux in breast cancer cells. Of note, autophagy is crucial for YAP to protect MCF7 cells from apoptosis under ND conditions. In addition, the TEA domain (TEAD) family of growth-promoting transcription factors was indispensable for YAP-mediated regulation of autophagy. Collectively, our data reveal a role for YAP in promoting breast cancer cell survival upon ND stress and uncover an unappreciated function of YAP/TEAD in the regulation of autophagy.

Casein kinase 1α-dependent feedback loop controls autophagy in RAS-driven cancers

Activating mutations in the RAS oncogene are common in cancer but are difficult to therapeutically target. RAS activation promotes autophagy, a highly regulated catabolic process that metabolically buffers cells in response to diverse stresses. Here we report that casein kinase 1α (CK1α), a ubiquitously expressed serine/threonine kinase, is a key negative regulator of oncogenic RAS-induced autophagy. Depletion or pharmacologic inhibition of CK1α enhanced autophagic flux in oncogenic RAS-driven human fibroblasts and multiple cancer cell lines. FOXO3A, a master longevity mediator that transcriptionally regulates diverse autophagy genes, was a critical target of CK1α, as depletion of CK1α reduced levels of phosphorylated FOXO3A and increased expression of FOXO3A-responsive genes. Oncogenic RAS increased CK1α protein abundance via activation of the PI3K/AKT/mTOR pathway. In turn, elevated levels of CK1α increased phosphorylation of nuclear FOXO3A, thereby inhibiting transactivation of genes critical for RAS-induced autophagy. In both RAS-driven cancer cells and murine xenograft models, pharmacologic CK1α inactivation synergized with lysosomotropic agents to inhibit growth and promote tumor cell death. Together, our results identify a kinase feedback loop that influences RAS-dependent autophagy and suggest that targeting CK1α-regulated autophagy offers a potential therapeutic opportunity to treat oncogenic RAS-driven cancers.

Inhibition of Hedgehog signaling pathway impedes cancer cell proliferation by promotion of autophagy

Multiple lines of evidence implicate that aberrant activation of Hedgehog (Hh) signaling is involved in a variety of human cancers. However, the molecular mechanisms underlying how cancer cells respond to Hh inhibition remain to be elucidated. In this study, we found that blockade of Hh signaling suppresses cell proliferation in human cancer cells. Microarray analysis revealed that differentially expressed genes (DEGs) in human cancer cells are enriched in autophagy pathway in response to the inhibition of Hh signaling. Interestingly, inhibition of Hh signaling induced autophagy, whereas activation of Hh signaling by ligand treatments prevented the induction of autophagy. In addition, inhibition of autophagy by 3-methyladenine (3-MA) partially suppressed cytotoxicity induced by inhibition of Hh signaling. Finally, in autophagy deficient cells, cytotoxic effect triggered by inhibition of Hh signaling was partially reversed, indicating the modulation of autophagy by Hh signaling is autophagy-specific. These results suggest that inhibition of Hh signaling impedes cancer cell proliferation in part through induction of autophagy.

Pirarubicin induces an autophagic cytoprotective response through suppression of the mammalian target of rapamycin signaling pathway in human bladder cancer cells

Pirarubicin is widely used in intravesical chemotherapy for bladder cancer, but its efficacy is limited due to drug resistance; the mechanism has not been well studied. Emerging evidence shows that autophagy can be a novel target for cancer therapy. This study aimed to investigate the role of autophagy in pirarubicin-treated bladder cancer cells. Bladder cancer cells EJ and J82 were treated with pirarubicin, siRNA, 3-methyladenine or hydroxychloroquine. Cell proliferation and apoptosis were tested by cell survival assay and flow cytometric analysis, respectively. Autophagy was evaluated by immunoblotting before and after the treatments. The phosphorylated mammalian target of rapamycin, serine/threonine kinase p70 S6 kinase, and eukaryotic translation initiation factor 4E binding protein 1 were also investigated by immunoblotting. We found that pirarubicin could induce autophagy in bladder cancer cells. Inhibition of autophagy by 3-methyladenine, hydroxychloroquine or knockdown of autophagy related gene 3 significantly increased apoptosis in pirarubicin-treated bladder cancer cells. Pirarubicin-induced autophagy was mediated via the mTOR/p70S6K/4E-BP1 signaling pathway. In conclusion, autophagy induced by pirarubicin plays a cytoprotective role in bladder cancer cells, suggesting that inhibition of autophagy may improve efficacy over traditional pirarubicin chemotherapy in bladder cancer patients.

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.

Non-apoptotic cell death associated with perturbations of macropinocytosis

Although macropinocytosis is widely recognized as a distinct form of fluid-phase endocytosis in antigen-presenting dendritic cells, it also occurs constitutively in many other normal and transformed cell types. Recent studies have established that various genetic or pharmacological manipulations can hyperstimulate macropinocytosis or disrupt normal macropinosome trafficking pathways, leading to accumulation of greatly enlarged cytoplasmic vacuoles. In some cases, this extreme vacuolization is associated with a unique form of non-apoptotic cell death termed “methuosis,” from the Greek methuo (to drink to intoxication). It remains unclear whether cell death related to dysfunctional macropinocytosis occurs in normal physiological contexts. However, the finding that some types of cancer cells are particularly vulnerable to this unusual form of cell death has raised the possibility that small molecules capable of altering macropinosome trafficking or function might be useful as therapeutic agents against cancers that are resistant to drugs that work by inducing apoptosis. Herein we review examples of cell death associated with dysfunctional macropinocytosis and summarize what is known about the underlying mechanisms.

Autophagy, cell death, and cancer

Autophagy is an evolutionarily conserved intracellular catabolic process that is used by all cells to degrade dysfunctional or unnecessary cytoplasmic components through delivery to the lysosome. Increasing evidence reveals that autophagic dysfunction is associated with human diseases, such as cancer. Paradoxically, although autophagy is well recognized as a cell survival process that promotes tumor development, it can also participate in a caspase-independent form of programmed cell death. Induction of autophagic cell death by some anticancer agents highlights the potential of this process as a cancer treatment modality. Here, we review our current understanding of the molecular mechanism of autophagy and the potential roles of autophagy in cell death, cancer development, and cancer treatment.

BH3-only proteins: a 20-year stock-take

BH3-only proteins are the sentinels of cellular stress, and their activation commits cells to apoptosis. Since the discovery of the first BH3-only protein BAD almost 20 years ago, at least seven more BH3-only proteins have been identified in mammals. They are regulated by a variety of environmental stimuli or by developmental cues, and play a crucial role in cellular homeostasis. Some are considered to be tumor suppressors, and also play a significant role in other pathologies. Their non-apoptotic functions are controversial, but there is broad consensus emerging regarding their role in apoptosis, which may help in designing better therapeutic agents for treating a variety of human diseases.

Cell death by autophagy: emerging molecular mechanisms and implications for cancer therapy

Autophagy is a tightly-regulated catabolic process of cellular self-digestion by which cellular components are targeted to lysosomes for their degradation. Key functions of autophagy are to provide energy and metabolic precursors under conditions of starvation and to alleviate stress by removal of damaged proteins and organelles, which are deleterious for cell survival. Therefore, autophagy appears to serve as a pro-survival stress response in most settings. However, the role of autophagy in modulating cell death is highly dependent on the cellular context and its extent. There is an increasing evidence for cell death by autophagy, in particular in developmental cell death in lower organisms and in autophagic cancer cell death induced by novel cancer drugs. The death-promoting and -executing mechanisms involved in the different paradigms of autophagic cell death (ACD) are very diverse and complex, but a draft scenario of the key molecular targets involved in ACD is beginning to emerge. This review provides an up-to-date and comprehensive report on the molecular mechanisms of drug-induced autophagy-dependent cell death and highlights recent key findings in this exciting field of research.

Ras and autophagy in cancer development and therapy

Autophagy, a process of self-degradation and turnover of cellular components, plays a complex role in cancer. Evidence exists to show that autophagy may support tumor growth and cell survival, whereas it can also contribute to tumor suppression and have anti-survival characteristics in different cellular systems. Numerous studies have described the effects of various oncogenes and tumor suppressors on autophagy. The small GTPase Ras is an oncogene involved in the regulation of various cell-signaling pathways, and is mutated in 33% of human cancers. In the present review, we discuss the interplay between Ras and autophagy in relation to oncogenesis. It appears that Ras can upregulate or downregulate autophagy through several signaling pathways. In turn, autophagy can affect the tumorigenicity driven by Ras, resulting in either tumor progression or repression, depending on the cellular context. Furthermore, Ras inhibitors were shown to induce autophagy in several cancer cell lines.

Araguspongine C induces autophagic death in breast cancer cells through suppression of c-Met and HER2 receptor tyrosine kinase signaling

Receptor tyrosine kinases are key regulators of cellular growth and proliferation. Dysregulations of receptor tyrosine kinases in cancer cells may promote tumorigenesis by multiple mechanisms including enhanced cell survival and inhibition of cell death. Araguspongines represent a group of macrocyclic oxaquinolizidine alkaloids isolated from the marine sponge Xestospongia species. This study evaluated the anticancer activity of the known oxaquinolizidine alkaloids araguspongines A, C, K and L, and xestospongin B against breast cancer cells. Araguspongine C inhibited the proliferation of multiple breast cancer cell lines in vitro in a dose-dependent manner. Interestingly, araguspongine C-induced autophagic cell death in HER2-overexpressing BT-474 breast cancer cells was characterized by vacuole formation and upregulation of autophagy markers including LC3A/B, Atg3, Atg7, and Atg16L. Araguspongine C-induced autophagy was associated with suppression of c-Met and HER2 receptor tyrosine kinase activation. Further in-silico docking studies and cell-free Z-LYTE assays indicated the potential of direct interaction between araguspongine C and the receptor tyrosine kinases c-Met and HER2 at their kinase domains. Remarkably, araguspongine C treatment resulted in the suppression of PI3K/Akt/mTOR signaling cascade in breast cancer cells undergoing autophagy. Induction of autophagic death in BT-474 cells was also associated with decreased levels of inositol 1,4,5-trisphosphate receptor upon treatment with effective concentration of araguspongine C. In conclusion, results of this study are the first to reveal the potential of araguspongine C as an inhibitor to receptor tyrosine kinases resulting in the induction of autophagic cell death in breast cancer cells.

Development of autophagy inducers in clinical medicine

Defects in autophagy have been linked to a wide range of medical illnesses, including cancer as well as infectious, neurodegenerative, inflammatory, and metabolic diseases. These observations have led to the hypothesis that autophagy inducers may prevent or treat certain clinical conditions. Lifestyle and nutritional factors, such as exercise and caloric restriction, may exert their known health benefits through the autophagy pathway. Several currently available FDA-approved drugs have been shown to enhance autophagy, and this autophagy-enhancing action may be repurposed for use in novel clinical indications. The development of new drugs that are designed to be more selective inducers of autophagy function in target organs is expected to maximize clinical benefits while minimizing toxicity. This Review summarizes the rationale and current approaches for developing autophagy inducers in medicine, the factors to be considered in defining disease targets for such therapy, and the potential benefits of such treatment for human health.