The unfolded protein response protects human tumor cells during hypoxia through regulation of the autophagy genes MAP1LC3B and ATG5

Autophagy is essential to support skeletal muscle plasticity in response to endurance exercise

Physical exercise is a stress that can substantially modulate cellular signaling mechanisms to promote morphological and metabolic adaptations. Skeletal muscle protein and organelle turnover is dependent on two major cellular pathways: Forkhead box class O proteins (FOXO) transcription factors that regulate two main proteolytic systems, the ubiquitin-proteasome, and the autophagy-lysosome systems, including mitochondrial autophagy, and the MTORC1 signaling associated with protein translation and autophagy inhibition. In recent years, it has been well documented that both acute and chronic endurance exercise can affect the autophagy pathway. Importantly, substantial efforts have been made to better understand discrepancies in the literature on its modulation during exercise. A single bout of endurance exercise increases autophagic flux when the duration is long enough, and this response is dependent on nutritional status, since autophagic flux markers and mRNA coding for actors involved in mitophagy are more abundant in the fasted state. In contrast, strength and resistance exercises preferentially raise ubiquitin-proteasome system activity and involve several protein synthesis factors, such as the recently characterized DAGK for mechanistic target of rapamycin activation. In this review, we discuss recent progress on the impact of acute and chronic exercise on cell component turnover systems, with particular focus on autophagy, which until now has been relatively overlooked in skeletal muscle. We especially highlight the most recent studies on the factors that can impact its modulation, including the mode of exercise and the nutritional status, and also discuss the current limitations in the literature to encourage further works on this topic.

The integrated stress response in lung disease

Lungs are repeatedly exposed to inhaled toxic insults, such as smoke, diesel exhaust, and microbes, which elicit cellular stress responses. The phosphorylation of eukaryotic translation initiation factor 2α by one of four stress-sensing kinases triggers a pathway called the integrated stress response that helps protect cellular reserves of nutrients and prevents the accumulation of toxic proteins. In this review, we discuss how activation of the integrated stress response has been shown to play an important role in pulmonary pathology, and how its study may help in the development of novel therapies for diverse conditions, from hypoxia to cancer.

Tumor vessel normalization by chloroquine independent of autophagy

Chloroquine (CQ) has been evaluated as an autophagy blocker for cancer treatment, but it is unknown if it acts solely by inhibiting cancer cell autophagy. We report that CQ reduced tumor growth but improved the tumor milieu. By normalizing tumor vessel structure and function and increasing perfusion, CQ reduced hypoxia, cancer cell invasion, and metastasis, while improving chemotherapy delivery and response. Inhibiting autophagy in cancer cells or endothelial cells (ECs) failed to induce such effects. CQ's vessel normalization activity relied mainly on alterations of endosomal Notch1 trafficking and signaling in ECs and was abrogated by Notch1 deletion in ECs in vivo. Thus, autophagy-independent vessel normalization by CQ restrains tumor invasion and metastasis while improving chemotherapy, supporting the use of CQ for anticancer treatment.

ER stress, autophagy, and RNA viruses

Endoplasmic reticulum (ER) stress is a general term for representing the pathway by which various stimuli affect ER functions. ER stress induces the evolutionarily conserved signaling pathways, called the unfolded protein response (UPR), which compromises the stimulus and then determines whether the cell survives or dies. In recent years, ongoing research has suggested that these pathways may be linked to the autophagic response, which plays a key role in the cell's response to various stressors. Autophagy performs a self-digestion function, and its activation protects cells against certain pathogens. However, the link between the UPR and autophagy may be more complicated. These two systems may act dependently, or the induction of one system may interfere with the other. Experimental studies have found that different viruses modulate these mechanisms to allow them to escape the host immune response or, worse, to exploit the host's defense to their advantage; thus, this topic is a critical area in antiviral research. In this review, we summarize the current knowledge about how RNA viruses, including influenza virus, poliovirus, coxsackievirus, enterovirus 71, Japanese encephalitis virus, hepatitis C virus, and dengue virus, regulate these processes. We also discuss recent discoveries and how these will produce novel strategies for antiviral treatment.

Ursolic acid induces autophagy in U87MG cells via ROS-dependent endoplasmic reticulum stress

Malignant gliomas are the most common primary brain tumors, and novel ways of treating gliomas are urgently needed. Ursolic acid (UA), a pentacyclic triterpenoid, has been reported to exhibit promising antitumor activity. Here, we evaluated the effects of UA on U87MG cells and explored the underlying molecular mechanisms. The results demonstrated that both G1-phase arrest and autophagy were induced by UA in U87MG cells. Evidence of UA-induced autophagy included the formation of acidic vesicular organelles, increase of autophagolysosomes and LC3-II accumulation. UA was also found to induce ER stress and an increase in intracellular calcium accompanied by ROS production. The increase in free cytosolic calcium induced by UA activated the CaMKK-AMPK-mTOR kinase signaling cascade, which ultimately triggered autophagy. Western blot analysis showed that UA promoted the phosphorylation of PERK and eIF2α; this was followed by the upregulation of the downstream protein CHOP, implying the involvement of the ER stress-mediated PERK/eIF2α/CHOP pathway in glioma cells. Meanwhile, UA activated IRE1α and subsequently increased the levels of phosphorylated JNK and Bcl-2, resulting in the dissociation of Beclin1 from Bcl-2. Furthermore, TUDCA and the silencing of either PERK or IRE1α partially blocked the UA-induced accumulation of LC3-II, suggesting that ER stress precedes the process of autophagy. Additionally, NAC attenuated the UA-induced elevation in cytosolic calcium, ER stress markers and autophagy-related proteins, indicating that UA triggered ER stress and autophagy via a ROS-dependent pathway. Collectively, our findings revealed a novel cellular mechanism triggered by UA and provide a molecular basis for developing UA into a drug candidate.

The unfolded protein response as a target for cancer therapy

Various physiological and pathological conditions generate an accumulation of misfolded proteins in the endoplasmic reticulum (ER). This results in ER stress followed by a cellular response to cope with this stress and restore homeostasis: the unfolded protein response (UPR). Overall, the UPR leads to general translational arrest and the induction of specific factors to ensure cell survival or to mediate cell death if the stress is too severe. In multiple cancers, components of the UPR are overexpressed, indicating increased dependence on the UPR. In addition, the UPR can confer resistance to anti-cancer treatment. Therefore, modification of the UPR should be explored for its anti-cancer properties. This review discusses factors associated with the UPR that represent potential therapeutic targets.

Getting ready for building: signaling and autophagosome biogenesis

Autophagy is the main cellular catabolic process responsible for degrading organelles and large protein aggregates. It is initiated by the formation of a unique membrane structure, the phagophore, which engulfs part of the cytoplasm and forms a double-membrane vesicle termed the autophagosome. Fusion of the outer autophagosomal membrane with the lysosome and degradation of the inner membrane contents complete the process. The extent of autophagy must be tightly regulated to avoid destruction of proteins and organelles essential for cell survival. Autophagic activity is thus regulated by external and internal cues, which initiate the formation of well-defined autophagy-related protein complexes that mediate autophagosome formation and selective cargo recruitment into these organelles. Autophagosome formation and the signaling pathways that regulate it have recently attracted substantial attention. In this review, we analyze the different signaling pathways that regulate autophagy and discuss recent progress in our understanding of autophagosome biogenesis.

Protective effect of Dl-3n-butylphthalide on learning and memory impairment induced by chronic intermittent hypoxia-hypercapnia exposure

Cognitive impairment is a common finding in patients with chronic obstructive pulmonary disease (COPD), but little attention has been focused on therapeutic intervention for this complication. Chronic intermittent hypoxia hypercapnia (CIHH) exposure is considered to be responsible for the pathogenesis of COPD. Dl-3n-Butylphthalide (NBP), extracted from Apium graveolens Linn, has displayed a broad spectrum of neuroprotective properties. Our study aimed to investigate the potential of NBP on CIHH-induced cognitive deficits. The cognitive function of rats after CIHH exposure was evaluated by the Morris water maze, which showed that the NBP treated group performed better in the navigation test. NBP activated BDNF and phosphorylated CREB, the both are responsible for neuroprotection. Additionally, NBP decreased CIHH induced apoptosis. Moreover, NBP further induced the expression of HIF-1α, accompanied by the up-regulation of the autophagy proteins Bnip3, Beclin-1 and LC3-II. Finally, NBP also reversed the decreased expression of SIRT1 and PGC-1α, but the expression of Tfam, Cox II and mtDNA remained unchanged. These results suggested that the neuroprotective effects of NBP under CIHH condition possibly occurred through the inhibition of apoptosis, promotion of hypoxia-induced autophagy, and activation of the SIRT1/PGC-1α signalling pathway, while stimulation of mitochondrial biogenesis may not be a characteristic response.

Free radicals in cross talk between autophagy and apoptosis

SIGNIFICANCE

Oxidative (reactive oxygen species [ROS]) and nitrosative (reactive nitrogen species [RNS]) stress affects many physiological processes, including survival and death. Although high levels of ROS/RNS mainly causes cell death, low levels of free radicals directly modulate the activities of transcriptional factors, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), p53, and nuclear factor (erythroid-derived) 2-like (Nrf2), and regulate numerous protein kinase cascades that participate in the regulation of the cross talk between autophagy and apoptosis.

RECENT ADVANCES

Low levels of ROS modify Atg4 and high mobility group box 1 (HMGB1) proteins, activate AMP-activated protein kinase (AMPK) and apoptosis signal-regulating kinase/c-Jun N-terminal kinase (JNK) pathways, or transactivate various proteins that could upregulate autophagy, leading to reductions in apoptosis. Transactivation of antioxidant genes blocks apoptosis and serves as a feedback loop to reduce autophagy. Free radicals could also activate protein kinase B (PKB, or Akt), preventing both autophagy and apoptosis. Stimulation of nitric oxide formation causes S-nitrosylation of several kinases, including JNK1 and IκB kinase β, which blocks autophagy and could promote apoptosis. However, S-nitrosylation of some proapoptotic proteins could block apoptosis.

CRITICAL ISSUES

Endoplasmic reticulum and mitochondria are the main sources of free radicals, which play an essential role in the regulation of apoptosis and autophagy. Oxidation of cardiolipin promotes cytochrome c release and apoptosis that potentially could be inhibited by autophagic clearance of damaged mitochondria. Elimination of damaged mitochondria reduces ROS accumulation, creating a feedback loop that causes inhibition of autophagy. Low levels of RNS could inhibit fission of mitochondria, which would block their degradation by autophagy and spare cells from apoptosis.

FUTURE DIRECTIONS

Understanding of mechanisms that regulate the cross talk between cell fates is essential for discovery of therapeutic tools in the strenuous fight against various disorders, including neurodegeneration and cancer.

Dual role for CHOP in the crosstalk between autophagy and apoptosis to determine cell fate in response to amino acid deprivation

CHOP encodes a ubiquitous transcription factor that is one of the most important components in the network of stress-inducible transcription. In particular, this factor is known to mediate cell death in response to stress. The focus of this work is to study its pivotal role in the control of cell viability according to the duration of a stress like amino acid starvation. We show that during the first 6h of starvation, CHOP upregulates a number of autophagy genes but is not involved in the first steps of the autophagic process. By contrast, when the amino acid starvation is prolonged (16-48h), we demonstrated that CHOP has a dual role in both inducing apoptosis and limiting autophagy through the transcriptional control of specific target genes. Overall, this study reveals a novel regulatory role for CHOP in the crosstalk between autophagy and apoptosis in response to stress.

Autophagy: detection, regulation and its role in cancer and therapy response

PURPOSE

Macroautophagy is a catabolic pathway that degrades cellular components through the lysosomal machinery. Cytoplasmic components are sequestered in double-membrane autophagosomes. They fuse with lysosomes where their cargo is delivered for degradation and recycling. Autophagy acts as a survival mechanism under stress by producing energy and as an intracellular quality management system by clearing damaged organelles like mitochondria and proteins. In this review, the regulation and the role of autophagy in cancer and therapy response are discussed. Furthermore, we will summarize methods for detecting autophagy in vitro and in vivo.

CONCLUSION

During the early and late stages of cancer development, the role of autophagy differs. In the very early stages of carcinogenesis, autophagy has an important function by reducing cancer initiating genetic instability and aberrant protein aggregates as well as promoting anti-cancer immune response. In established malignant tumors autophagy confers resistance against metabolic stress caused by nutrient deprivation and the rapid proliferation of carcinoma cells. This function of autophagy is also important for radiation and chemotherapy resistance in cancer. Our laboratory has found that Neuropilin-2-induced autophagy is a potent mediator of therapy resistance in different cancer types. Autophagy not only promotes the survival of tumor cells, but also leads to autophagic cell death. During dysfunctional apoptosis this form of cell death mainly sensitizes cancer cells for therapy such as ionizing radiation. Therefore, the functions of autophagy during cancer progression and therapy are two-sided and further research is needed to understand these in more detail.

Therapeutic targeting of autophagy in cancer. Part II: pharmacological modulation of treatment-induced autophagy

Autophagy, the catabolic pathway in which cells recycle organelles and other parts of their own cytoplasm, is increasingly recognised as an important cytoprotective mechanism in cancer cells. Several cancer treatments stimulate the autophagic process and when autophagy is inhibited, cancer cells show an enhanced response to multiple treatments. These findings have nourished the theory that autophagy provides cancer cells with a survival advantage during stressful conditions, including exposure to therapeutics. Therefore, interference with the autophagic response can potentially enhance the efficacy of cancer therapy. In this review we examine two approaches to modulate autophagy as complementary cancer treatment: inhibition and induction. Inhibition of autophagy during cancer treatment eliminates its cytoprotective effects. Conversely, induction of autophagy combined with conventional cancer therapy exerts severe cytoplasmic degradation that can ultimately lead to cell death. We will discuss how autophagy can be therapeutically manipulated in cancer cells and how interactions between the conventional cancer therapies and autophagy modulation influence treatment outcome.

Assessment of ER Stress and autophagy induced by ionizing radiation in both radiotherapy patients and ex vivo irradiated samples

Acute radiation leads to several toxic clinical states and triggers some molecular pathways. To shed light on molecular mechanisms triggered by ionizing radiation (IR), we examined the expression profiles of endoplasmic reticulum (ER) stress and autophagy-related genes in individuals who were exposed to IR. Blood samples were collected from 50 cancer patients before radiotherapy and on the 5th, 15th, and 25th days of the treatment. Peripheral blood samples from 10 healthy volunteers were also obtained for ex vivo irradiation, divided into five and irradiated at a rate of 373 kGy/h to 0, 0.1, 0.5, 1, and 3Gy γ-rays using a constant gamma source. GRP78, ATG5, LC3, ATF4, XBP1, and GADD153 genes were analyzed by quantitative real-time polymerase chain reaction (QRT-PCR) using beta 2 microglobulin (B2M) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as references. In both groups, expressions of the selected genes have increased. It can be concluded that IR induces ER stress and related authophagy pathway in the peripheral lymphocyte cells proportionally by dose.

Involvement of p38 in signal switching from autophagy to apoptosis via the PERK/eIF2α/ATF4 axis in selenite-treated NB4 cells

Selenite has emerged as an optional chemotherapeutic agent for hematological malignancies. Autophagy and apoptosis are both engaged in selenite-induced cell death. In a previous report, we have identified heat shock protein 90 (Hsp90) as a critical modulator of the balance between autophagy and apoptosis in selenite-treated leukemia cells. However, the mechanisms by which selenite mediates the crosstalk between autophagy and apoptosis remain largely unknown. Herein, we demonstrate that the endoplasmic reticulum (ER) stress-related PERK/eIF2α/ATF4 pathway and p38 are core modules for the selenite-induced switch to apoptosis from autophagy. We found that selenite activated PERK and eIF2α/ATF4 downstream to promote apoptosis. During this progression, p38 was dissociated from PERK-inhibiting Hsp90 and became autophosphorylated. Then, activated p38 further enhanced the docking of activating transcription factor 4 (ATF4) onto the CHOP (CCAAT/enhancer-binding protein homologous protein) promoter via eIF2α to enhance apoptosis. We also found that activated p38 suppressed the phosphorylation of eIF4E that directed ATF4 to bind to the MAP1LC3B (microtubule-associated protein 1 light chain 3B) promoter. Because of the deactivation of eIF4E, the association of ATF4 with the MAP1LC3B promoter was inhibited, and autophagy was compromised. Intriguingly, p53 played important roles in mediating the p38-mediated regulation of eIF2α and eIF4E. When activated by p38, p53 induced the phosphorylation of eIF2α and the dephosphorylation of eIF4E, particularly in the nucleus where the ATF4 transcription factor was modulated, ultimately resulting in differential expression of CHOP and LC3. Moreover, selenite exhibited potent antitumor effects in vivo. In an NB4 cell xenograft model, selenite induced apoptosis and hampered autophagy. In addition, related signaling proteins demonstrated similar changes to those observed in vitro. These data suggest that selenite may be a candidate drug for leukemia therapy.

Therapeutic targeting of autophagy in cancer. Part I: molecular pathways controlling autophagy

Autophagy is a process in which cells can generate energy and building materials, by degradation of redundant and/or damaged organelles and proteins. Especially during conditions of stress, autophagy helps to maintain homeostasis. In addition, autophagy has been shown to influence malignant transformation and cancer progression. The precise molecular events in autophagy are complex and the core autophagic machinery described to date consists of nearly thirty proteins. Apart from these factors that execute the process of autophagy, several signalling pathways are involved in converting internal and external stimuli into an autophagic response. In this review we provide an overview of the signalling pathways that influence autophagy, particularly in cancer cells. We will illustrate that interference with multiple of these signalling pathways can have significant effects on cancer cell survival.

Endoplasmic reticulum stress and protein quality control in diabetic cardiomyopathy

Endoplasmic reticulum (ER) stress, together with the unfolded protein response (UPR), is initially considered an adaptive response aiming at maintenance of ER homeostasis. Nonetheless, ER stress, when in excess, can eventually trigger cell apoptosis and loss of function. UPR is mediated by three major transmembrane proteins, including inositol-requiring enzyme 1 (IRE1), protein kinase RNA-like ER kinase (PERK), and activating transcription factor (ATF) 6. A unique role has been speculated for ER stress in the pathogenesis of diabetes mellitus (DM) and its complications. Recent studies have shown that ER stress is an early event associated with diabetic cardiomyopathy, and may be triggered by hyperglycemia, free fatty acids (FFAs) and inflammation. In this mini-review, we attempted to discuss the activation machinery for ER stress in response to these triggers en route to disrupted ER function and cellular autophagy or apoptosis, ultimately insulin resistance and development of diabetic cardiomyopathy. This article is part of a Special Issue entitled: Autophagy and protein quality control in cardiometabolic diseases.

Autophagy inhibitor 3-methyladenine potentiates apoptosis induced by dietary tocotrienols in breast cancer cells

INTRODUCTION

Tocomin® represents commercially available mixture of naturally occurring tocotrienols (T3s) and tocopherols extracted from palm oil/palm fruits that possess powerful antioxidant, anticancer, neuro/cardioprotective and cholesterol-lowering properties. Cellular autophagy represents a defense mechanism against oxidative stress and several anticancer compounds. Recently, we reported that T3s induce apoptosis and endoplasmic reticulum stress in breast cancer cells.

METHODOLOGY

We studied the effects of Tocomin® on MCF-7 and MDA-MB 231 breast cancer cells and non-tumor MCF-10A cells.

RESULTS

Tocomin® inhibited cell proliferation and induced apoptosis in both MCF-7 and MDA-MB 231 breast cancer cell lines without affecting the viability of MCF-10A cells. We also showed that Tocomin® negatively modulates phosphoinositide 3-kinase and mTOR pathways and induces cytoprotective autophagic response in triple negative MDA-MB 231 cells. Lastly, we demonstrate that autophagy inhibitor 3-methyladenine (3-MA) potentiated the apoptosis induced by Tocomin® in MDA-MB 231 cells.

CONCLUSION

Together, our data indicate anticancer effects of Tocomin® in breast cancer cells, which is potentiated by the autophagy inhibitor 3-MA.

Activation of the MAPK11/12/13/14 (p38 MAPK) pathway regulates the transcription of autophagy genes in response to oxidative stress induced by a novel copper complex in HeLa cells

Transition metal copper (Cu) can exist in oxidized or reduced states in cells, leading to cytotoxicity in cancer cells through oxidative stress. Recently, copper complexes are emerging as a new class of anticancer compounds. Here, we report that a novel anticancer copper complex (HYF127c/Cu) induces oxidative stress-dependent cell death in cancer cells. Further, transcriptional analysis revealed that oxidative stress elicits broad transcriptional changes of genes, in which autophagy-related genes are significantly changed in HYF127c/Cu-treated cells. Consistently, autophagy was induced in HYF127c/Cu-treated cells and inhibitors of autophagy promoted cell death induced by HYF127c/Cu. Further analysis identified that the MAPK11/12/13/14 (formerly known as p38 MAPK) pathway was also activated in HYF127c/Cu-treated cells. Meanwhile, the MAPK11/12/13/14 inhibitor SB203580 downregulated autophagy by inhibiting the transcription of the autophagy genes MAP1LC3B, BAG3, and HSPA1A, and promoted HYF127c/Cu-induced cell death. These data suggest that copper-induced oxidative stress will induce protective autophagy through transcriptional regulation of autophagy genes by activation of the MAPK11/12/13/14 pathway in HeLa cells.

A curated census of autophagy-modulating proteins and small molecules: candidate targets for cancer therapy

Autophagy, a programmed process in which cell contents are delivered to lysosomes for degradation, appears to have both tumor-suppressive and tumor-promoting functions; both stimulation and inhibition of autophagy have been reported to induce cancer cell death, and particular genes and proteins have been associated both positively and negatively with autophagy. To provide a basis for incisive analysis of those complexities and ambiguities and to guide development of new autophagy-targeted treatments for cancer, we have compiled a comprehensive, curated inventory of autophagy modulators by integrating information from published siRNA screens, multiple pathway analysis algorithms, and extensive, manually curated text-mining of the literature. The resulting inventory includes 739 proteins and 385 chemicals (including drugs, small molecules, and metabolites). Because autophagy is still at an early stage of investigation, we provide extensive analysis of our sources of information and their complex relationships with each other. We conclude with a discussion of novel strategies that could potentially be used to target autophagy for cancer therapy.

Endoplasmic reticulum stress in malignancy

The combination of relative nutrient deprivation and dysregulation of protein synthesis make malignant cells especially prone to protein misfolding. Endoplasmic reticulum stress, which results from protein misfolding within the secretory pathway, has a profound effect on cancer cell proliferation and survival. In this review, we examine the evidence implicating endoplasmic reticulum dysfunction in the pathology of cancer and discuss how recent findings may help to identify novel therapeutic targets.

Autophagy enhances hepatocellular carcinoma progression by activation of mitochondrial β-oxidation

BACKGROUND

Several types of cancers, including hepatocellular carcinoma (HCC), show resistance to hypoxia and nutrient starvation. Autophagy is a means of providing macromolecules for energy generation under such stressed-conditions. The aim of this study was to clarify the role of autophagy in HCC development under hypoxic conditions.

METHODS

The expression of microtubule-associated protein 1 light chain 3 (LC3), which is a key gene involved in autophagosome formation, was evaluated in human HCC using immunohistochemistry and western blot. The relationship between LC3 and hypoxia-induced factor 1α (HIF1α) expression was examined using real-time PCR. In addition, human HCC cell line Huh7 was treated with pharmacological autophagy-inhibitor and inactive mutant of Atg4B (Atg4B(C74A)) under hypoxic condition to evaluate the effects of hypoxia-induced autophagy on cell survival, intracellular ATP, and mitochondrial β-oxidation.

RESULTS

LC3 was significantly highly expressed in HCC as compared with noncancerous tissues. LC3 expression, correlated with HIF1α expression, was also significantly correlated with tumor size, and only in the context of large tumors, was an independent predictor of HCC recurrence after surgery. In addition, Huh7 treated with autophagy-inhibitor under hypoxia had lower viability, with low levels of intracellular ATP due to impaired mitochondrial β-oxidation.

CONCLUSIONS

Autophagy in HCC works to promote HIF1α-mediated proliferation through the maintenance of intracellular ATP, depending on the activation of mitochondrial β-oxidation. These findings demonstrated the feasibility of anti-autophagic treatment as a potential curative therapy for HCC, and improved understanding of the factors determining adaptive metabolic responses to hypoxic conditions.

The roles of reactive oxygen species and autophagy in mediating the tolerance of tumor cells to cycling hypoxia

Tumor hypoxia (low oxygenation) causes treatment resistance and poor patient outcome. A substantial fraction of tumor cells experience cycling hypoxia, characterized by transient episodes of hypoxia and reoxygenation. These cells are under a unique burden of stress, mediated by excessive production of reactive oxygen species (ROS). Cellular components damaged by ROS can be cleared by autophagy, rendering cycling hypoxic tumor cells particularly vulnerable to inhibition of autophagy and its upstream regulatory pathways. Activation of the PERK-mediated signaling arm of the unfolded protein response during hypoxia plays a critical role in the defense against ROS, both by stimulating glutathione synthesis pathways and through promoting autophagy.

Overexpression of Atg5 in mice activates autophagy and extends lifespan

Autophagy has been implicated in the ageing process, but whether autophagy activation extends lifespan in mammals is unknown. Here we show that ubiquitous overexpression of Atg5, a protein essential for autophagosome formation, extends median lifespan of mice by 17.2%. We demonstrate that moderate overexpression of Atg5 in mice enhances autophagy, and that Atg5 transgenic mice showed anti-ageing phenotypes, including leanness, increased insulin sensitivity and improved motor function. Furthermore, mouse embryonic fibroblasts cultured from Atg5 transgenic mice are more tolerant to oxidative damage and cell death induced by oxidative stress, and this tolerance was reversible by treatment with an autophagy inhibitor. Our observations suggest that the leanness and lifespan extension in Atg5 transgenic mice may be the result of increased autophagic activity.

Changes in autophagy have been shown to modulate lifespan in lower organisms. Here, Pyo et al. show that mice globally overexpressing the autophagy protein Atg5 live longer and are leaner than normal mice, providing the first evidence that increased autophagy extends lifespan in mammals.

Overexpression of smooth muscle myosin heavy chain leads to activation of the unfolded protein response and autophagic turnover of thick filament-associated proteins in vascular smooth muscle cells

Duplications spanning nine genes at the genomic locus 16p13.1 predispose individuals to acute aortic dissections. The most likely candidate gene in this region leading to the predisposition for dissection is MYH11, which encodes smooth muscle myosin heavy chain (SM-MHC). The effects of increased expression of MYH11 on smooth muscle cell (SMC) phenotypes were explored using mouse aortic SMCs with transgenic overexpression of one isoform of SM-MHC. We found that these cells show increased expression of Myh11 and myosin filament-associated contractile genes at the message level when compared with control SMCs, but not at the protein level due to increased protein degradation. Increased expression of Myh11 resulted in endoplasmic reticulum (ER) stress in SMCs, which led to a paradoxical decrease of protein levels through increased autophagic degradation. An additional consequence of ER stress in SMCs was increased intracellular calcium ion concentration, resulting in increased contractile signaling and contraction. The increased signals for contraction further promote transcription of contractile genes, leading to a feedback loop of metabolic abnormalities in these SMCs. We suggest that overexpression of MYH11 can lead to increased ER stress and autophagy, findings that may be globally implicated in disease processes associated with genomic duplications.

Unfolded Protein Response and Cancer

Physiological stresses, such as hypoxia and oxidative stress, induce protein misfolding in the endoplasmic reticulum (ER). If proteasome degradation fails to remove the misfolded proteins, these proteins accumulate in the ER, triggering the unfolded protein response (UPR). UPR involves a series of responses, such as the suppression of global protein synthesis and the select expression of a set of proteins to reduce ER stress and restore the homeostasis of ER. In different stages of tumor development, hypoxia occurs and UPR is initiated. The roles of UPR in cancer development are complex, involving angiogenesis, cell survival and proliferation. The current knowledge of the molecular mechanisms involved in UPR, particularly its role in the development of cancer, is discussed.

Increased autophagy reduces endoplasmic reticulum stress after neonatal hypoxia-ischemia: role of protein synthesis and autophagic pathways

The endoplasmic reticulum (ER) stress can result from several pathological conditions that perturb ER homeostasis and is characterized by accumulation of unfolded proteins in the ER lumen. To cope with ER stress, cells activate the unfolded protein response (UPR), a protein quality control mechanism aimed at restoring homeostasis. The present study was undertaken to characterize the UPR after neonatal hypoxia/ischemia (HI) and its crosstalk with autophagy. After HI, there was a significant increase of GRP78 and Hsp70 expression, phosphorylation of eIF2α, Xbp-1 mRNA splicing and CHOP expression, revealing severe ER stress and UPR. Increasing autophagy with rapamycin (Rap) significantly reduced the UPR. Rap did not further increase the eIF2α phosphorylation and p70S6 kinase (p70S6K) inactivation induced by HI. After autophagy activation, however, there was a clear co-localization between monodansylcadaverine (MDC)-positive autophagosome-like structures and the ribosomal protein S6 (RPS6), indicating the presence of ribosomes in autophagosomes (ribophagy). We found that the autophagy inhibitor 3-methyladenine administered after Rap treatment completely reverted the increased phosphorylation of eIF2α and p70S6K inactivation, and blocked the formation of autophagosome-like structures restoring the UPR. These results demonstrate that the UPR is strongly activated after neonatal HI. Over-activation of autophagy significantly reduces this response, highlighting the relevance of the cross-talk between ER and the autophagy machinery in this important pathological condition. Furthermore, the presence of ribosome subunits in autophagosome-like structures suggests that increased ribosome turnover through autophagy (ribophagy) may represent an additional mechanism involved in the neuroprotective effect observed after autophagy over-activation.

Neuronal Tsc1/2 complex controls autophagy through AMPK-dependent regulation of ULK1

Tuberous sclerosis complex (TSC) is a disorder arising from mutation in the TSC1 or TSC2 gene, characterized by the development of hamartomas in various organs and neurological manifestations including epilepsy, intellectual disability and autism. TSC1/2 protein complex negatively regulates the mammalian target of rapamycin complex 1 (mTORC1) a master regulator of protein synthesis, cell growth and autophagy. Autophagy is a cellular quality-control process that sequesters cytosolic material in double membrane vesicles called autophagosomes and degrades it in autolysosomes. Previous studies in dividing cells have shown that mTORC1 blocks autophagy through inhibition of Unc-51-like-kinase1/2 (ULK1/2). Despite the fact that autophagy plays critical roles in neuronal homeostasis, little is known on the regulation of autophagy in neurons. Here we show that unlike in non-neuronal cells, Tsc2-deficient neurons have increased autolysosome accumulation and autophagic flux despite mTORC1-dependent inhibition of ULK1. Our data demonstrate that loss of Tsc2 results in autophagic activity via AMPK-dependent activation of ULK1. Thus, in Tsc2-knockdown neurons AMPK activation is the dominant regulator of autophagy. Notably, increased AMPK activity and autophagy activation are also found in the brains of Tsc1-conditional mouse models and in cortical tubers resected from TSC patients. Together, our findings indicate that neuronal Tsc1/2 complex activity is required for the coordinated regulation of autophagy by AMPK. By uncovering the autophagy dysfunction associated with Tsc2 loss in neurons, our work sheds light on a previously uncharacterized cellular mechanism that contributes to altered neuronal homeostasis in TSC disease.

Autophagy and radiosensitization in cancer

Autophagy is a natural self-degradative process by which cells eliminate misfolded proteins and damaged organelles. Autophagy has been shown to have multiple functions in tumor cells that may be dependent on the tumor type and the treatment conditions. Autophagy can have a cytoprotective role and be thought of as a survival mechanism or be cytotoxic in nature and mediate cell death. Radiation, one of the primary treatments for many different types of cancer, almost uniformly promotes autophagy in tumor cells. While autophagy produced in response to radiation is often considered to be cytoprotective, radiation-induced autophagy has also been shown to mediate susceptibility to radiation. This review addresses the complexity of autophagy in response to radiation treatment in three different cancer models, specifically lung cancer, breast cancer and glioblastoma. A deeper understanding of the different roles played by autophagy in response to radiation should facilitate the development of approaches for enhancing the therapeutic utility of radiation by providing strategies for combination treatment with unique radiosensitizers as well as preventing the initiation of strategies which are likely to attenuate the effectiveness of radiation therapy.

Targeting ER stress-induced autophagy overcomes BRAF inhibitor resistance in melanoma

Melanomas that result from mutations in the gene encoding BRAF often become resistant to BRAF inhibition (BRAFi), with multiple mechanisms contributing to resistance. While therapy-induced autophagy promotes resistance to a number of therapies, especially those that target PI3K/mTOR signaling, its role as an adaptive resistance mechanism to BRAFi is not well characterized. Using tumor biopsies from BRAF(V600E) melanoma patients treated either with BRAFi or with combined BRAF and MEK inhibition, we found that BRAFi-resistant tumors had increased levels of autophagy compared with baseline. Patients with higher levels of therapy-induced autophagy had drastically lower response rates to BRAFi and a shorter duration of progression-free survival. In BRAF(V600E) melanoma cell lines, BRAFi or BRAF/MEK inhibition induced cytoprotective autophagy, and autophagy inhibition enhanced BRAFi-induced cell death. Shortly after BRAF inhibitor treatment in melanoma cell lines, mutant BRAF bound the ER stress gatekeeper GRP78, which rapidly expanded the ER. Disassociation of GRP78 from the PKR-like ER-kinase (PERK) promoted a PERK-dependent ER stress response that subsequently activated cytoprotective autophagy. Combined BRAF and autophagy inhibition promoted tumor regression in BRAFi-resistant xenografts. These data identify a molecular pathway for drug resistance connecting BRAFi, the ER stress response, and autophagy and provide a rationale for combination approaches targeting this resistance pathway.

Hepatitis C virus core protein activates autophagy through EIF2AK3 and ATF6 UPR pathway-mediated MAP1LC3B and ATG12 expression

HCV infection induces autophagy, but how this occurs is unclear. Here, we report the induction of autophagy by the structural HCV core protein and subsequent endoplasmic reticular (ER) stress in Huh7 hepatoma cells. During ER stress, both the EIF2AK3 and ATF6 pathways of the unfolded protein response (UPR) were activated by HCV core protein. Then, these pathways upregulated transcription factors ATF4 and DDIT3. The ERN1-XBP1 pathway was not activated. Through ATF4 in the EIF2AK3 pathway, the autophagy gene ATG12 was upregulated. DDIT3 upregulated the transcription of autophagy gene MAP1LC3B (LC3B) by directly binding to the -253 to -99 base region of the LC3B promoter, contributing to the development of autophagy. Collectively, these data suggest not only a novel role for the HCV core protein in autophagy but also offer new insight into detailed molecular mechanisms with respect to HCV-induced autophagy, specifically how downstream UPR molecules regulate key autophagic gene expression.

LAMP3 is involved in tamoxifen resistance in breast cancer cells through the modulation of autophagy

Lysosome-associated membrane protein 3 (LAMP3) is a member of the LAMP-family of proteins, which are involved in the process of autophagy. Autophagy is induced by tamoxifen in breast cancer cells and may contribute to tamoxifen resistance. In this study, the significance of LAMP3 for tamoxifen resistance in breast cancer was examined. The methods employed included use of clonogenic assays to assess the survival of MCF7 breast cancer cells with LAMP3 knockdown after tamoxifen treatment and of quantitative real-time PCR of LAMP3 to evaluate its predictive value for first-line tamoxifen treatment in patients with advanced breast cancer. Results show that tamoxifen treatment of MCF7 cells induced LAMP3 mRNA expression. LAMP3 knockdown in these cells increased tamoxifen sensitivity. Evaluation of expression of the autophagy markers, LC3B and p62, after LAMP3 knockdown showed increased expression levels, indicating that cells with LAMP3 knockdown have a suppressed ability to complete the autophagic process. In addition, knockdown of autophagy-associated genes resulted in sensitization to tamoxifen. Next, tamoxifen-resistant MCF7 cells were cultured. These cells had a sevenfold higher LAMP3 mRNA expression, showed elevated basal autophagy levels, and could be significantly resensitized to tamoxifen by LAMP3 knockdown. In patients treated with first-line tamoxifen for advanced disease (n=304), high LAMP3 mRNA expression was associated with shorter progression-free survival (P=0.003) and shorter post-relapse overall survival (P=0.040), also in multivariate analysis. Together, these results indicate that LAMP3 contributes to tamoxifen resistance in breast cancer. Tamoxifen-resistant cells are resensitized to tamoxifen by the knockdown of LAMP3. Therefore, LAMP3 may be clinically relevant to countering tamoxifen resistance in breast cancer patients.

Apelin inhibits the proliferation and migration of rat PASMCs via the activation of PI3K/Akt/mTOR signal and the inhibition of autophagy under hypoxia

Apelin is highly expressed in the lungs, especially in the pulmonary vasculature, but the functional role of apelin under pathological conditions is still undefined. Hypoxic pulmonary hypertension is the most common cause of acute right heart failure, which may involve the remodeling of artery and regulation of autophagy. In this study, we determined whether treatment with apelin regulated the proliferation and migration of rat pulmonary arterial smooth muscle cells (SMCs) under hypoxia, and investigated the underlying mechanism and the relationship with autophagy. Our data showed that hypoxia activated autophagy significantly at 24 hrs. The addition of exogenous apelin decreased the level of autophagy and further inhibited pulmonary arterial SMC (PASMC) proliferation via activating downstream phosphatidylinositol-3-kinase (PI3K)/protein kinase B (Akt)/the mammalian target of Rapamycin (mTOR) signal pathways. The inhibition of the apelin receptor (APJ) system by siRNA abolished the inhibitory effect of apelin in PASMCs under hypoxia. This study provides the evidence that exogenous apelin treatment contributes to inhibit the proliferation and migration of PASMCs by regulating the level of autophagy.

Emerging evidence of the physiological role of hypoxia in mammary development and lactation

Hypoxia is a physiological or pathological condition of a deficiency of oxygen supply in the body as a whole or within a tissue. During hypoxia, tissues undergo a series of physiological responses to defend themselves against a low oxygen supply, including increased angiogenesis, erythropoiesis, and glucose uptake. The effects of hypoxia are mainly mediated by hypoxia-inducible factor 1 (HIF-1), which is a heterodimeric transcription factor consisting of α and β subunits. HIF-1β is constantly expressed, whereas HIF-1α is degraded under normal oxygen conditions. Hypoxia stabilizes HIF-1α and the HIF complex, and HIF then translocates into the nucleus to initiate the expression of target genes. Hypoxia has been extensively studied for its role in promoting tumor progression, and emerging evidence also indicates that hypoxia may play important roles in physiological processes, including mammary development and lactation. The mammary gland exhibits an increasing metabolic rate from pregnancy to lactation to support mammary growth, lactogenesis, and lactation. This process requires increasing amounts of oxygen consumption and results in localized chronic hypoxia as confirmed by the binding of the hypoxia marker pimonidazole HCl in mouse mammary gland. We hypothesized that this hypoxic condition promotes mammary development and lactation, a hypothesis that is supported by the following several lines of evidence: i) Mice with an HIF-1α deletion selective for the mammary gland have impaired mammary differentiation and lipid secretion, resulting in lactation failure and striking changes in milk compositions; ii) We recently observed that hypoxia significantly induces HIF-1α-dependent glucose uptake and GLUT1 expression in mammary epithelial cells, which may be responsible for the dramatic increases in glucose uptake and GLUT1 expression in the mammary gland during the transition period from late pregnancy to early lactation; and iii) Hypoxia and HIF-1α increase the phosphorylation of signal transducers and activators of transcription 5a (STAT5a) in mammary epithelial cells, whereas STAT5 phosphorylation plays important roles in the regulation of milk protein gene expression and mammary development. Based on these observations, hypoxia effects emerge as a new frontier for studying the regulation of mammary development and lactation.

Detection of WIPI1 mRNA as an indicator of autophagosome formation

Autophagy is a cellular bulk degradation system for long-lived proteins and organelles that operates during nutrient starvation and is thus a type of recycling system. In recent years, a series of mammalian orthologs of yeast autophagy-related (ATG) genes have been identified; however, the importance of the transcriptional regulation of ATG genes underlying autophagosome formation is poorly understood. In this study, we identified several ATG genes, including the genes ULK1, MAP1LC3B, GABARAPL1, ATG13, WIPI1, and WDR45/WIPI4, with elevated mRNA levels in thapsigargin-, C2-ceramide-, and rapamycin-treated as well as amino acid-depleted HeLa cells except for MAP1LC3B mRNA in rapamycin-treated HeLa cells. Rapamycin had a weaker effect on the expressions of ATG genes. The increase in WIPI1 and MAP1LC3B mRNA was induced prior to the accumulation of the autophagy marker protein MAP1LC3 in the thapsigargin- and C2-ceramide-treated A549 cells. By counting the puncta marked with MAP1LC3B in HeLa cells treated with different autophagy inducers, we revealed that the time-dependent mRNA elevation of a specific set of ATG genes was similar to that of autophagosome accumulation. The transcriptional attenuation of WIPI1 mRNA using RNA interference inhibited the puncta number in thapsigargin-treated HeLa cells. Remarkably, increases in the abundance of WIPI1 mRNA were also manifested in thapsigargin- and C2-ceramide-treated human fibroblasts (WI-38 and TIG-1), human cancer cells (U-2 OS, Saos-2, and MCF7), and rodent fibroblasts (Rat-1). Taken together, these results suggest that the detection of WIPI1 mRNA is likely to be a convenient method of monitoring autophagosome formation in a wide range of cell types.

The role of autophagy in the pathogenesis of diabetic nephropathy

Diabetic nephropathy is a leading cause of end-stage renal disease worldwide. The multipronged drug approach targeting blood pressure and serum levels of glucose, insulin, and lipids fails to fully prevent the onset and progression of diabetic nephropathy. Therefore, a new therapeutic target to combat diabetic nephropathy is required. Autophagy is a catabolic process that degrades damaged proteins and organelles in mammalian cells and plays a critical role in maintaining cellular homeostasis. The accumulation of proteins and organelles damaged by hyperglycemia and other diabetes-related metabolic changes is highly associated with the development of diabetic nephropathy. Recent studies have suggested that autophagy activity is altered in both podocytes and proximal tubular cells under diabetic conditions. Autophagy activity is regulated by both nutrient state and intracellular stresses. Under diabetic conditions, an altered nutritional state due to nutrient excess may interfere with the autophagic response stimulated by intracellular stresses, leading to exacerbation of organelle dysfunction and diabetic nephropathy. In this review, we discuss new findings showing the relationships between autophagy and diabetic nephropathy and suggest the therapeutic potential of autophagy in diabetic nephropathy.

EGFR signaling and autophagy dependence for growth, survival, and therapy resistance

The epidermal growth factor receptor (EGFR) is amplified or mutated in various human epithelial tumors. Its expression and activation leads to cell proliferation, differentiation, and survival. Consistently, EGFR amplification or expression of EGFR variant 3 (EGFRvIII) is associated with resistance to conventional cancer therapy through activation of pro-survival signaling and DNA-repair mechanisms. EGFR targeting has successfully been exploited as strategy to increase treatment efficacy. Nevertheless, these targeting strategies have only been proven effective in a limited percentage of human tumors. Recent knowledge indicates that EGFR deregulated tumors display differences in autophagy and dependence on autophagy for growth and survival and the use of autophagy to increase resistance to EGFR-targeting drugs. In this review the dependency on autophagy and its role in mediating resistance to EGFR-targeting agents will be discussed. Considering the current knowledge, autophagy inhibition could provide a novel strategy to enhance therapy efficacy in treatment of EGFR deregulated tumors.

The return of the nucleus: transcriptional and epigenetic control of autophagy

Autophagy is a conserved process by which cytoplasmic components are degraded by the lysosome. It is commonly seen as a cytoplasmic event and, until now, nuclear events were not considered of primary importance for this process. However, recent studies have unveiled a transcriptional and epigenetic network that regulates autophagy. The identification of tightly controlled transcription factors (such as TFEB and ZKSCAN3), microRNAs and histone marks (especially acetylated Lys16 of histone 4 (H4K16ac) and dimethylated H3K9 (H3K9me2)) associated with the autophagic process offers an attractive conceptual framework to understand the short-term transcriptional response and potential long-term responses to autophagy.

Two phases of disulfide bond formation have differing requirements for oxygen

Disulfide bonds introduced during or shortly after protein synthesis can occur without oxygen, whereas those introduced during post-translational folding or isomerization are oxygen dependent.

Most proteins destined for the extracellular space require disulfide bonds for folding and stability. Disulfide bonds are introduced co- and post-translationally in endoplasmic reticulum (ER) cargo in a redox relay that requires a terminal electron acceptor. Oxygen can serve as the electron acceptor in vitro, but its role in vivo remains unknown. Hypoxia causes ER stress, suggesting a role for oxygen in protein folding. Here we demonstrate the existence of two phases of disulfide bond formation in living mammalian cells, with differential requirements for oxygen. Disulfide bonds introduced rapidly during protein synthesis can occur without oxygen, whereas those introduced during post-translational folding or isomerization are oxygen dependent. Other protein maturation processes in the secretory pathway, including ER-localized N-linked glycosylation, glycan trimming, Golgi-localized complex glycosylation, and protein transport, occur independently of oxygen availability. These results suggest that an alternative electron acceptor is available transiently during an initial phase of disulfide bond formation and that post-translational oxygen-dependent disulfide bond formation causes hypoxia-induced ER stress.

Hypoxic activation of the PERK/eIF2α arm of the unfolded protein response promotes metastasis through induction of LAMP3

PURPOSE

Conditions of poor oxygenation (hypoxia) are present in many human tumors, including cervix cancer, and are associated with increased risk of metastasis and poor prognosis. Hypoxia is a potent activator of the PERK/eIF2α signaling pathway, a component of the unfolded protein response (UPR) and an important mediator of hypoxia tolerance and tumor growth. Here, the importance of this pathway in the metastasis of human cervix carcinoma was investigated.

EXPERIMENTAL DESIGN

Amplification and expression of LAMP3, a UPR metastasis-associated gene, was examined using FISH and immunofluorescence in a cohort of human cervix tumors from patients who had received oxygen needle electrode tumor oxygenation measurements. To evaluate the importance of this pathway in metastasis in vivo, we constructed a series of inducible cell lines to interfere with PERK signaling during hypoxia and used these in an orthotopic cervix cancer model of hypoxia-driven metastasis.

RESULTS

We show that LAMP3 expression in human cervix tumors is augmented both by gene copy number alterations and by hypoxia. Induced disruption of PERK signaling in established orthotopic xenografts resulted in complete inhibition of hypoxia-induced metastasis to the lymph nodes. This is due, in part, to a direct influence of the UPR pathway on hypoxia tolerance. However, we also find that LAMP3 is a key mediator of hypoxia-driven nodal metastasis, through its ability to promote metastatic properties including cell migration.

CONCLUSION

These data suggest that the association between hypoxia, metastasis, and poor prognosis is due, in part, to hypoxic activation of the UPR and expression of LAMP3. Clin Cancer Res; 19(22); 6126-37. ©2013 AACR.

Paneth cells as a site of origin for intestinal inflammation

The recognition of autophagy related 16-like 1 (ATG16L1) as a genetic risk factor has exposed the critical role of autophagy in Crohn's disease. Homozygosity for the highly prevalent ATG16L1 risk allele, or murine hypomorphic (HM) activity, causes Paneth cell dysfunction. As Atg16l1(HM) mice do not develop spontaneous intestinal inflammation, the mechanism(s) by which ATG16L1 contributes to disease remains obscure. Deletion of the unfolded protein response (UPR) transcription factor X-box binding protein-1 (Xbp1) in intestinal epithelial cells, the human orthologue of which harbours rare inflammatory bowel disease risk variants, results in endoplasmic reticulum (ER) stress, Paneth cell impairment and spontaneous enteritis. Unresolved ER stress is a common feature of inflammatory bowel disease epithelium, and several genetic risk factors of Crohn's disease affect Paneth cells. Here we show that impairment in either UPR (Xbp1(ΔIEC)) or autophagy function (Atg16l1(ΔIEC) or Atg7(ΔIEC)) in intestinal epithelial cells results in each other's compensatory engagement, and severe spontaneous Crohn's-disease-like transmural ileitis if both mechanisms are compromised. Xbp1(ΔIEC) mice show autophagosome formation in hypomorphic Paneth cells, which is linked to ER stress via protein kinase RNA-like endoplasmic reticulum kinase (PERK), elongation initiation factor 2α (eIF2α) and activating transcription factor 4 (ATF4). Ileitis is dependent on commensal microbiota and derives from increased intestinal epithelial cell death, inositol requiring enzyme 1α (IRE1α)-regulated NF-κB activation and tumour-necrosis factor signalling, which are synergistically increased when autophagy is deficient. ATG16L1 restrains IRE1α activity, and augmentation of autophagy in intestinal epithelial cells ameliorates ER stress-induced intestinal inflammation and eases NF-κB overactivation and intestinal epithelial cell death. ER stress, autophagy induction and spontaneous ileitis emerge from Paneth-cell-specific deletion of Xbp1. Genetically and environmentally controlled UPR function within Paneth cells may therefore set the threshold for the development of intestinal inflammation upon hypomorphic ATG16L1 function and implicate ileal Crohn's disease as a specific disorder of Paneth cells.

Pushing tumor cells towards a malignant phenotype: stimuli from the microenvironment, intercellular communications and alternative roads

The tumor microenvironment produces different types of stimuli capable of endowing tumor cells with an aggressive behavior that is characterized by increased motility, invasiveness and propensity to metastasize, gain of a tumor-initiating phenotype, and drug resistance. The following classes of stimuli have been reported to promote such a malignant phenotype: (i) solid- or fluid-induced stress; (ii) altered composition of the extracellular matrix; (iii) hypoxia and low pH; (iv) innate and adaptive immune responses; (v) antitumor drugs. The simultaneous presence of more than one of these stimuli, as likely occurs in vivo, may lead to synergistic interactions in the induction of malignant traits. In many cases, the gain of a malignant phenotype is not the result of a direct effect of the stimuli on tumor cells but, rather, a stimulus-promoted cross-talk between tumor cells and other cell types within the tumor microenvironment. This cross-talk is mainly mediated by two classes of molecules: paracrine factors and adhesion receptors. Stimuli that promote a malignant phenotype can promote additional outcomes in tumor cells, including autophagy and cell death. We summarize here the available evidence about the variables that induce tumor cells to take one or the other of these roads in response to the same stimuli. At the end of this review, we address some unanswered questions in this domain and indicate future directions of research.

MicroRNA 23b regulates autophagy associated with radioresistance of pancreatic cancer cells

BACKGROUND & AIMS

Tumor resistance to radiation is a challenge in the treatment of patients with pancreatic cancer. Improving our understanding of the mechanisms of radioresistance could lead to strategies to increase patients' response to therapy. We investigated the roles of microRNAs (miRNAs) involved in radioresistance of pancreatic cancer cells.

METHODS

We established radioresistant pancreatic cancer cell lines and used array analysis to compare levels of different miRNAs between radioresistant cell lines and the parental cell lines from which they were derived. We transfected pancreatic cancer cells with miRNA mimics or inhibitors and evaluated their effects on cell radiosensitivity using a clonogenic survival assay. The effects of miRNA on autophagy were determined by transmission electron microscopy and immunoblot analysis. We used a luciferase reporter assay to identify messenger RNA targets of specific miRNAs.

RESULTS

Radioresistant pancreatic cancer cells had reduced levels of the miRNA miR-23b and increased autophagy compared with cells that were not radioresistant. Overexpression of miR-23b inhibited radiation-induced autophagy, whereas an inhibitor of miR-23b promoted autophagy in pancreatic cancer cells. Overexpression of miR-23b sensitized pancreatic cancer cells to radiation. The target of miR-23b, ATG12, was overexpressed in radioresistant cells; levels of ATG12 protein correlated with the occurrence of autophagy. Expression of miR-23b blocked radiation-induced autophagy and sensitized pancreatic cancer cells to radiation. We observed an inverse correlation between the level of miR-23b and autophagy in human pancreatic cancer tissue samples.

CONCLUSIONS

In pancreatic cancer cells, reduced levels of the miRNA miR-23b increase levels of ATG12 and autophagy to promote radioresistance. miR-23b might be used to increase the sensitivity of pancreatic cancer cells to radiation therapy.

Tumour hypoxia determines the potential of combining mTOR and autophagy inhibitors to treat mammary tumours

BACKGROUND

Hypoxia can activate autophagy, a self-digest adaptive process that maintains cell turnover. Mammalian target of rapamycin (mTOR) inhibitors are used to treat cancer but also stimulate autophagy.

METHODS

Human mammary cancer cells and derived xenografts were used to examine whether hypoxia could exacerbate autophagy-mediated resistance to the mTOR inhibitor rapamycin.

RESULTS

Rapamycin exerted potent antitumour effects in MCF-7 and MDA-MB-231 mammary tumours through a marked inhibition of angiogenesis, but the autophagy inhibitor chloroquine (CQ) failed to further sensitise tumours to mTOR inhibition. Rapamycin treatment actually led to tumour reoxygenation, thereby preventing the development of autophagy. Chloroquine alone, however, blocked the growth of MCF-7 tumours and in vitro blunted the hypoxia-induced component of autophagy in these cells. Finally, when initiating CQ treatment in large, hypoxic tumours, a robust antitumour effect could be observed, which also further increased the antiproliferative effects of rapamycin.

CONCLUSION

The mTOR inhibitor rapamycin significantly contributes to tumour growth inhibition and normalisation of the tumour vasculature through potent antiangiogenic effects. The resulting reduction in hypoxia accounts for a lack of sensitisation by the autophagy inhibitor CQ, except if the tumours are already at an advanced stage, and thus largely hypoxic at the initiation of the combination of rapamycin and CQ treatment.

Higher activation of autophagy in skeletal muscle of mice during endurance exercise in the fasted state

Activation of autophagy in skeletal muscle has been reported in response to endurance exercise and food deprivation independently. The purpose of this study was to evaluate whether autophagy was more activated when both stimuli were combined, namely when endurance exercise was performed in a fasted rather than a fed state. Mice performed a low-intensity running exercise (10 m/min for 90min) in both dietary states after which the gastrocnemius muscles were removed. LC3b-II, a marker of autophagosome presence, increased in both conditions, but the increase was higher in the fasted state. Other protein markers of autophagy, like Gabarapl1-II and Atg12 conjugated form as well as mRNA of Lc3b, Gabarapl1, and p62/Sqstm1 were increased only when exercise was performed in a fasted state. The larger activation of autophagy by exercise in a fasted state was associated with a larger decrease in plasma insulin and phosphorylation of Akt(Ser473), Akt(Thr308), FoxO3a(Thr32), and ULK1(Ser757). AMPKα(Thr172), ULK1(Ser317), and ULK1(Ser555) remained unchanged in both conditions, whereas p38(Thr180/Tyr182) increased during exercise to a similar extent in the fasted and fed conditions. The marker of mitochondrial fission DRP1(Ser616) was increased by exercise independently of the nutritional status. Changes in mitophagy markers BNIP3 and Parkin suggest that mitophagy was increased during exercise in the fasted state. In conclusion, our results highlight a major implication of the insulin-Akt-mTOR pathway and its downstream targets FoxO3a and ULK1 in the larger activation of autophagy observed when exercise is performed in a fasted state compared with a fed state.

The eIF2α kinases: their structures and functions

Cell signaling in response to an array of diverse stress stimuli converges on the phosphorylation of the α-subunit of eukaryotic initiation factor 2 (eIF2). Phosphorylation of eIF2α on serine 51 results in a severe decline in de novo protein synthesis and is an important strategy in the cell's armory against stressful insults including viral infection, the accumulation of misfolded proteins, and starvation. The phosphorylation of eIF2α is carried out by a family of four kinases, PERK (PKR-like ER kinase), PKR (protein kinase double-stranded RNA-dependent), GCN2 (general control non-derepressible-2), and HRI (heme-regulated inhibitor). Each primarily responds to a distinct type of stress or stresses. Thus, while significant sequence similarity exists between the eIF2α kinases in their kinase domains, underlying their common role in phosphorylating eIF2α, additional unique features determine the regulation of these four proteins, that is, what signals activate them. This review will describe the structure of each eIF2α kinase and discuss how this is linked to their activation and function. In parallel to the general translational attenuation elicited by eIF2α kinase activation the translation of stress-induced mRNAs, most notably activating transcription factor 4 (ATF4) is enhanced and these set in motion cascades of gene expression constituting the integrated stress response (ISR), which seek to remediate stress and restore homeostasis. Depending on the cellular context and concurrent signaling pathways active, however, translational attenuation can also facilitate apoptosis. Accordingly, the role of the kinases in determining cell fate will also be discussed.

Autophagy in tumour cell death

In every moment of a cell's existence one key question is always asked, "To be or not to be"? Cells constantly weigh up signals from their environment against their own integrity and metabolic status and decide whether to live or die. Such cell death decisions are central to the progression and treatment of cancer. The term autophagy describes three processes that deliver cytoplasmic macromolecules and organelles to lysosomes for degradation, the difference between each form being the method of delivery. The most extensively studied form is macroautophagy (hereafter referred to as autophagy) where cytosolic components are engulfed by double membraned autophagosomes. Autophagosomes fuse with lysosomes to form structures called autolysosomes, within which organelles, proteins and other macromolecules are degraded by catabolic enzymes in the acidic lysosome environment. Autophagy, which normally occurs at low levels in unstressed cells, is widely regarded as having a positive effect on cell health as potentially harmful protein aggregates and damaged organelles can be recycled. During periods of nutrient shortage autophagy is enhanced to provide, albeit temporarily, an internal energy source. Autophagy is also enhanced by other stresses encountered by tumour cells and this may protect the cell or aid its demise. In this review we examine the effect of autophagy on cell death decisions in tumour cells.

The many faces of C/EBPδ and their relevance for inflammation and cancer

The CCAAT/enhancer binding protein delta (CEBPD, C/EBPδ) is a transcription factor that modulates many biological processes including cell differentiation, motility, growth arrest, proliferation, and cell death. The diversity of C/EBPδ's functions depends in part on the cell type and cellular context and can have opposing outcomes. For example, C/EBPδ promotes inflammatory signaling, but it can also inhibit pro-inflammatory pathways, and in a mouse model of mammary tumorigenesis, C/EBPδ reduces tumor incidence but promotes tumor metastasis. This review highlights the multifaceted nature of C/EBPδ's functions, with an emphasis on pathways that are relevant for cancer and inflammation, and illustrates how C/EBPδ emerged from the shadow of its family members as a fascinating “jack of all trades.” Our current knowledge on C/EBPδ indicates that, rather than being essential for a specific cellular process, C/EBPδ helps to interpret a variety of cues in a cell-type and context-dependent manner, to adjust cellular functions to specific situations. Therefore, insights into the roles and mechanisms of C/EBPδ signaling can lead to a better understanding of how the integration of different signaling pathways dictates normal and pathological cell functions and physiology.