Transcriptional up-regulation of ULK1 by ATF4 contributes to cancer cell survival

Overexpression of ULK1 Represents a Potential Diagnostic Marker for Clear Cell Renal Carcinoma and the Antitumor Effects of SBI-0206965

Background

Uncoordinated 51-like kinase 1 (ULK1) plays a vital role in autophagy. ULK1 dysregulation has recently been found in several human cancers.

Methods

mRNA expression levels of ULK1 and clinical information were analysed from The Cancer Genome Atlas data. ULK1 expression levels were verified in 36 paired fresh ccRCC tissue specimens by western blot analysis. Expression of ULK1 was knockdown by shRNA lentivirus. ULK1 activity was inhibited by SBI-0206965. The effect of inhibition of ULK1 was measured by detecting the apoptotic rate, autophagy, and the ratio of ROS and NADPH. The efficacy of SBI-0206965 in vivo was assessed by the murine xenograft model.

Findings

ULK1 mRNA expression was significantly upregulated in clear cell renal cell carcinoma (ccRCC) and overexpression of ULK1 correlated with poor outcomes. We found that ULK1 was highly expressed in 66.7% of ccRCC tumours (p < 0·05). Knockdown of ULK1 and selective inhibition of ULK1 by SBI-0206965 induced cell apoptosis in ccRCC cells. We demonstrated that SBI-0206965 triggered apoptosis by preventing autophagy and pentose phosphate pathway (PPP) flux. Furthermore, blocking the kinase activity of ULK1 with SBI-0206965 resulted in a level of anticancer effect in vivo.

Interpretation

Taken together, our results suggested that ULK1 was upregulated in ccRCC tumours and may be a potential therapeutic target. Therefore, SBI-0206965 should be further considered as an anti-ccRCC agent.

Fund

This work was supported in part by The National Natural Science Foundation of China (No. 81570748) and Natural Science Foundation of Fujian Province (No. 2018J01345, 2017XQ1194).

The endoplasmic reticulum co-chaperone ERdj3/DNAJB11 promotes hepatocellular carcinoma progression through suppressing AATZ degradation

AIM

The co-chaperone ERdj3/DNAJB11 is involved in the endoplasmic reticulum stress response observed in cancer cells. We hypothesized that ERdj3 functions as a hepatocellular carcinoma (HCC) oncogene by inhibiting AATZ degradation.

MATERIALS & METHODS

ERdj3 and AATZ expressions were analyzed in 84 HCC patients. Cell proliferation, epithelial-mesenchymal transition marker expression, migration and invasiveness were assessed in HepG2 and Huh-7 cells. A murine xenograft tumor model was constructed.

RESULTS

ERdj3 is upregulated in HCC tumors and cell lines. Tumor ERdj3 levels are positively associated with cirrhosis, enhanced HCC status, inferior survival outcomes and AATZ levels. ERdj3 suppresses AATZ degradation. ERdj3 overexpression enhances proliferation, epithelial-mesenchymal transition marker expression, migration, invasiveness and xenograft tumor growth in an AATZ-dependent manner.

CONCLUSION

ERdj3 enhances HCC progression through suppressing AATZ degradation.

An Overview of Unfolded Protein Response Signaling and Its Role in Cancer

Secretory and transmembrane proteins undergo post-translational modifications and folding in the subcellular organelle, that is, endoplasmic reticulum (ER) to become functionally active. Various factors such as high oxidative stress, low glucose, calcium imbalance, and viral infections interfere with the ER protein folding functions, leading to accumulation of unfolded and misfolded proteins that activate downstream signal transduction pathways, termed as unfolded protein response (UPR). This UPR signaling is adaptive and restored the normal function of cells by decreasing protein synthesis, increasing the folding capacity of ER and degradation of misfolded proteins. If the stress condition is overwhelmed, then UPR signaling shifts to apoptotic pathways. However, cancer cells utilized these UPR signaling for their survival and progression as an adaptive mechanism. In this review, the authors discuss about the overview of ER stress and subsequent UPR signaling and various aspects of cancer as survival, proliferation, and angiogenesis in relation to UPR. Understanding the UPR signaling in relation to cancer will be further helpful in designing therapeutics against cancer.

Hypoxia-induced autophagy promotes gemcitabine resistance in human bladder cancer cells through hypoxia-inducible factor 1α activation

Overcoming the chemoresistance of bladder cancer is a pivotal obstacle in clinical treatments. Hypoxia widely exists in solid tumors and has been demonstrated to contribute to chemoresistance through hypoxia-inducible factor 1α (HIF‑1α)-mediated autophagy in several types of cancer. However, it is unclear whether HIF‑1α-mediated autophagy and chemoresistance occur in bladder cancer. The present study demonstrated that HIF‑1α was overexpressed in 20 bladder cancer tissues compared with matched paracarcinoma tissues. Gemcitabine-induced apoptosis during hypoxia was significantly reduced compared with that observed under normoxic conditions. In addition, hypoxia activated autophagy and enhanced gemcitabine-induced autophagy. Combined treatment using gemcitabine and an autophagy inhibitor (3-methyladenine) under hypoxia significantly increased gemcitabine cytotoxicity. Furthermore, it was demonstrated that hypoxia-activated autophagy depended on the HIF‑1α/BCL2/adenovirus E1B 19 kDa protein-interacting protein 3 (BNIP3)/Beclin1 signaling pathway. Suppressing HIF‑1α inhibited autophagy, BNIP3 and Beclin1, as well as enhanced gemcitabine-induced apoptosis in bladder cancer cells under hypoxic conditions. Consequently, the results of the present study demonstrated that hypoxia-induced cytoprotective autophagy counteracted gemcitabine-induced apoptosis through increasing HIF‑1α expression. Therefore, targeting HIF‑1α-associated pathways or autophagy in bladder cancer may be a successful strategy to enhance the sensitivity of bladder cancer chemotherapy.

mTOR independent alteration in ULK1 Ser758 phosphorylation following chronic LRRK2 kinase inhibition

Unc-51 Like Kinase 1 (ULK1) is a critical regulator of the biogenesis of autophagosomes, the central component of the catabolic macroautophagy pathway. Regulation of ULK1 activity is dependent upon several phosphorylation events acting to repress or activate the enzymatic function of this protein. Phosphorylation of Ser758 ULK1 has been linked to repression of autophagosome biogenesis and was thought to be exclusively dependent upon mTOR complex 1 kinase activity. In the present study, a novel regulation of Ser758 ULK1 phosphorylation is reported following prolonged inhibition of the Parkinson’s disease linked protein leucine rich repeat kinase 2 (LRRK2). Here, modulation of Ser758 ULK1 phosphorylation following LRRK2 inhibition is decoupled from the repression of autophagosome biogenesis and independent of mTOR complex 1 activity.

Ulk1 over-expression in human gastric cancer is correlated with patients' T classification and cancer relapse

Ulk1 is a key autophagy protein. Here, we tested expression and potential function of Ulk1 in human gastric cancer. Ulk1 mRNA and protein were significantly elevated in multiple fresh human gastric cancer tissues. Its level was relatively low in surrounding normal epithelial tissues. Ulk1 over-expression was also observed in several gastric cancer cell lines (AGS, HGC-27, and SNU601). Remarkably, Ulk1 knockdown by targeted-shRNA inhibited AGS gastric cancer cell survival and proliferation. On the other hand, exogenous Ulk1 over-expression could further promote AGS cell survival and proliferation. Immunohistochemistry (IHC) staining assay of 145 paraffin-embedded gastric cancer tissues showed that Ulk1 was over-expressed in majority (114 out of 145) of gastric cancer tissues. Importantly, high Ulk1 expression in gastric cancer was correlated with patients' T classification and cancer relapse. Together, we demonstrate that Ulk1 over-expression in human gastric cancer is pro-survival. Its over-expression is associated with patients' T classification and cancer relapse.

Dandelion root extract affects colorectal cancer proliferation and survival through the activation of multiple death signalling pathways

Dandelion extracts have been studied extensively in recent years for its anti-depressant and anti-inflammatory activity. Recent work from our lab, with in-vitro systems, shows the anti-cancer potential of an aqueous dandelion root extract (DRE) in several cancer cell models, with no toxicity to non-cancer cells. In this study, we examined the cancer cell-killing effectiveness of an aqueous DRE in colon cancer cell models. Aqueous DRE induced programmed cell death (PCD) selectively in > 95% of colon cancer cells, irrespective of their p53 status, by 48 hours of treatment. The anti-cancer efficacy of this extract was confirmed in in-vivo studies, as the oral administration of DRE retarded the growth of human colon xenograft models by more than 90%. We found the activation of multiple death pathways in cancer cells by DRE treatment, as revealed by gene expression analyses showing the expression of genes implicated in programmed cell death. Phytochemical analyses of the extract showed complex multi-component composition of the DRE, including some known bioactive phytochemicals such as α-amyrin, β-amyrin, lupeol and taraxasterol. This suggested that this natural extract could engage and effectively target multiple vulnerabilities of cancer cells. Therefore, DRE could be a non-toxic and effective anti-cancer alternative, instrumental for reducing the occurrence of cancer cells drug-resistance.

Endoplasmic reticulum chaperone glucose-regulated protein 78 in gastric cancer: An emerging biomarker

The endoplasmic reticulum (ER) is the principal organelle responsible for the synthesis, initial post-translational modification, folding, export and secretion of proteins. It is also responsible for the maintenance of cellular homeostasis. In response to cellular stress conditions including glucose deprivation, hypoxia and changes in calcium homeostasis, ER stress machinery is activated and triggers the unfolded protein response, resulting in the restoration of homeostasis or activation of cell death. Glucose-regulated protein 78 (GRP78), a molecular chaperone, may be induced by ER stress at the transcriptional and translational level. A number of studies have demonstrated that GRP78 serves an important role in tumor cell proliferation, metastasis, angiogenesis and drug-resistance. The present review systematically describes the association between GRP78 expression and gastric cancer pathogenesis, and emphasizes that GRP78 is a novel diagnostic and therapeutic biomarker of gastric cancer.

Design of Small Molecule Autophagy Modulators: A Promising Druggable Strategy

Autophagy is a lysosome-dependent mechanism of intracellular degradation for maintaining cellular homeostasis. Dysregulation of autophagy has been verified to be closely linked to a number of human diseases. Consequently, targeting autophagy has been highlighted as a novel therapeutic strategy for clinical utility. Mounting efforts have been done in recent years to elucidate the mechanisms of autophagy regulation and to identify potential modulators of autophagy. However, most of the compounds target complex and multifaceted pathway and proteins, which may limit the evaluation of therapeutic value and in depth studies as chemical tools. Therefore, the development of specific and active autophagy modulators becomes most desirable. Here, we briefly review the regulation of autophagy and then summarize the recent development of small molecules targeting the core autophagic machinery. Finally, we put forward our viewpoints on the current problems, with the aim to provide reference for future drug discovery and potential therapeutic perspectives on novel, potent, selective autophagy modulators.

The mammalian ULK1 complex and autophagy initiation

Autophagy is a vital lysosomal degradation pathway that serves as a quality control mechanism. It rids the cell of damaged, toxic or excess cellular components, which if left to persist could be detrimental to the cell. It also serves as a recycling pathway to maintain protein synthesis under starvation conditions. A key initial event in autophagy is formation of the autophagosome, a unique double-membrane organelle that engulfs the cytosolic cargo destined for degradation. This step is mediated by the serine/threonine protein kinase ULK1 (unc-51-like kinase 1), which functions in a complex with at least three protein partners: FIP200 (focal adhesion kinase family interacting protein of 200 kDa), ATG (autophagy-related protein) 13 (ATG13), and ATG101. In this artcile, we focus on the regulation of the ULK1 complex during autophagy initiation. The complex pattern of upstream pathways that converge on ULK1 suggests that this complex acts as a node, converting multiple signals into autophagosome formation. Here, we review our current understanding of this regulation and in turn discuss what happens downstream, once the ULK1 complex becomes activated.

Surviving Stress: Modulation of ATF4-Mediated Stress Responses in Normal and Malignant Cells

Activating transcription factor 4 (ATF4) is a stress-induced transcription factor that is frequently upregulated in cancer cells. ATF4 controls the expression of a wide range of adaptive genes that allow cells to endure periods of stress, such as hypoxia or amino acid limitation. However, under persistent stress conditions, ATF4 promotes the induction of apoptosis. Recent advances point to a role for post-translational modifications (PTMs) and epigenetic mechanisms in balancing these pro- and anti-survival effects of ATF4. We review here how PTMs and epigenetic modifiers associated with ATF4 may be exploited by cancer cells to cope with cellular stress conditions that are intrinsically associated with tumor growth. Identification of mechanisms that modulate ATF4-mediated transcription and its effects on cellular metabolism may uncover new targets for cancer treatment.

MicroRNA-93 Regulates Hypoxia-Induced Autophagy by Targeting ULK1

The expression of the core autophagy kinase, Unc51-like kinase 1 (ULK1), is regulated transcriptionally and translationally by starvation-induced autophagy. However, how ULK1 is regulated during hypoxia is not well understood. Previously, we showed that ULK1 expression is induced by hypoxia stress. Here, we report a new ULK1-modulating microRNA, miR-93; its transcription is negatively correlated with the translation of ULK1 under hypoxic condition. miR-93 targets ULK1 and reduces its protein levels under hypoxia condition. miR-93 also inhibits hypoxia-induced autophagy by preventing LC3-I to LC3-II transition and P62 degradation; these processes are reversed by the overexpression of an endogenous miR-93 inhibitor. Re-expression of ULK1 without miR-93 response elements restores the hypoxia-induced autophagy which is inhibited by miR-93. Finally, we detected the effects of miR-93 on cell viability and apoptosis in noncancer cell lines and cancer cells. We found that miR-93 sustains the viability of MEFs (mouse embryonic fibroblasts) and inhibits its apoptosis under hypoxia. Thus, we conclude that miR-93 is involved in hypoxia-induced autophagy by regulating ULK1. Our results provide a new angle to understand the complicated regulation of the key autophagy kinase ULK1 during different stress conditions.

Downregulation of ULK1 by microRNA-372 inhibits the survival of human pancreatic adenocarcinoma cells

Dysregulation of microRNA (miRNA) expression in various cancers and their role in cancer progression is well documented. The purpose of this study was to investigate the biological role of miR‐372 in human pancreatic adenocarcinoma (HPAC). We collected 20 pairs of HPAC tissues and adjacent non‐cancerous tissues to detect miR‐372 expression levels. We transfected BXPC‐3 and PANC‐1 cells with miR‐372 inhibitor/mimics to study their effect on cell proliferation, apoptosis, invasion, migration and autophagy. In addition, miR‐372 mimics and a tumor protein UNC51‐like kinase 1 (ULK1) siRNA were co‐transfected into BXPC‐3 and PANC‐1 cells to explore the mechanism of miR‐372 and ULK1 on HPAC tumorigenesis. We found that the expression of miR‐372 was markedly downregulated in HPAC cells compared to adjacent normal tissues. Furthermore, functional assays showed that miR‐372 inhibited cell proliferation, invasion, migration and autophagy in BXPC‐3 and PANC‐1 cells. An inverse correlation between miR‐372 expression and ULK1 expression was observed in HPAC tissues. Downregulation of ULK1 inhibited the overexpression effects of miR‐372, and upregulation of ULK1 reversed the effects of overexpressed miR‐372. Finally, we found that silencing ULK1 or inhibiting autophagy partly rescued the effects of miR‐372 knockdown in HPAC cells, which may explain the influence of miR‐372/ULK1 in HPAC development. Taken together, these results revealed a significant role of the miR‐372/ULK1 axis in suppressing HPAC cell proliferation, migration, invasion and autophagy.

Regulation of longevity by FGF21: Interaction between energy metabolism and stress responses

Fibroblast growth factor 21 (FGF21) is a hormone-like member of FGF family which controls metabolic multiorgan crosstalk enhancing energy expenditure through glucose and lipid metabolism. In addition, FGF21 acts as a stress hormone induced by endoplasmic reticulum stress and dysfunctions of mitochondria and autophagy in several tissues. FGF21 also controls stress responses and metabolism by modulating the functions of somatotropic axis and hypothalamic-pituitary-adrenal (HPA) pathway. FGF21 is a potent longevity factor coordinating interactions between energy metabolism and stress responses. Recent studies have revealed that FGF21 treatment can alleviate many age-related metabolic disorders, e.g. atherosclerosis, obesity, type 2 diabetes, and some cardiovascular diseases. In addition, transgenic mice overexpressing FGF21 have an extended lifespan. However, chronic metabolic and stress-related disorders involving inflammatory responses can provoke FGF21 resistance and thus disturb healthy aging process. First, we will describe the role of FGF21 in interorgan energy metabolism and explain how its functions as a stress hormone can improve healthspan. Next, we will examine both the induction of FGF21 expression via the integrated stress response and the molecular mechanism through which FGF21 enhances healthy aging. Finally, we postulate that FGF21 resistance, similarly to insulin resistance, jeopardizes human healthspan and accelerates the aging process.

Transcriptional regulation of mammalian autophagy at a glance

Macroautophagy, hereafter referred to as autophagy, is a catabolic process that results in the lysosomal degradation of cytoplasmic contents ranging from abnormal proteins to damaged cell organelles. It is activated  under diverse conditions, including nutrient deprivation and hypoxia. During autophagy, members of the core autophagy-related (ATG) family of proteins mediate membrane rearrangements, which lead to the engulfment and degradation of cytoplasmic cargo. Recently, the nuclear regulation of autophagy, especially by transcription factors and histone modifiers, has gained increased attention. These factors are not only involved in rapid responses to autophagic stimuli, but also regulate the long-term outcome of autophagy. Now there are more than 20 transcription factors that have been shown to be linked to the autophagic process. However, their interplay and timing appear enigmatic as several have been individually shown to act as major regulators of autophagy. This Cell Science at a Glance article and the accompanying poster highlights the main cellular regulators of transcription involved in mammalian autophagy and their target genes.

Assessing Autophagic Flux by Measuring LC3, p62, and LAMP1 Co-localization Using Multispectral Imaging Flow Cytometry

Autophagy is a catabolic pathway in which normal or dysfunctional cellular components that accumulate during growth and differentiation are degraded via the lysosome and are recycled. During autophagy, cytoplasmic LC3 protein is lipidated and recruited to the autophagosomal membranes. The autophagosome then fuses with the lysosome to form the autolysosome, where the breakdown of the autophagosome vesicle and its contents occurs. The ubiquitin-associated protein p62, which binds to LC3, is also used to monitor autophagic flux. Cells undergoing autophagy should demonstrate the co-localization of p62, LC3, and lysosomal markers. Immunofluorescence microscopy has been used to visually identify LC3 puncta, p62, and/or lysosomes on a per-cell basis. However, an objective and statistically rigorous assessment can be difficult to obtain. To overcome these problems, multispectral imaging flow cytometry was used along with an analytical feature that compares the bright detail images from three autophagy markers (LC3, p62 and lysosomal LAMP1) and quantifies their co-localization, in combination with LC3 spot counting to measure autophagy in an objective, quantitative, and statistically robust manner.

Physiological/pathological ramifications of transcription factors in the unfolded protein response

Numerous environmental, physiological, and pathological insults disrupt protein-folding homeostasis in the endoplasmic reticulum (ER), referred to as ER stress. Eukaryotic cells evolved a set of intracellular signaling pathways, collectively termed the unfolded protein response (UPR), to maintain a productive ER protein-folding environment through reprogramming gene transcription and mRNA translation. The UPR is largely dependent on transcription factors (TFs) that modulate expression of genes involved in many physiological and pathological conditions, including development, metabolism, inflammation, neurodegenerative diseases, and cancer. Here we summarize the current knowledge about these mechanisms, their impact on physiological/pathological processes, and potential therapeutic applications.

Differences in the starvation-induced autophagy response in MDA-MB-231 and MCF-7 breast cancer cells

Breast cancer is a heterogeneous disease with distinct subtypes that have made targeted therapy of breast cancer challenging. Previous studies have demonstrated that an altered autophagy capacity can influence the development of breast cancer. However, the molecular differences in starvation-induced autophagic responses in MDA-MB-231 and MCF-7 cells have not been fully elucidated. In this study, we found that an increase of LC3B-II protein expression level and a decrease of the p62 protein expression level in both cells treated by Earle’s balanced salt solution. Meanwhile, we observed an increase of autophagosome using transmission electron microscopy and an enhancement in the green fluorescence intensity of LC3B protein by confocal microscopy. Furthermore, we detected the expression of 13 autophagy-related (ATG) genes and 11 autophagy signaling pathway-related genes using qPCR. Among 13 ATG genes, we found that 6 genes were up-regulated in treated MDA-MB-231 cells, while 4 genes were up-regulated and 1 gene was down-regulated in treated MCF-7 cells. In addition, among 11 autophagy signaling pathway-related genes, 7 genes were up-regulated in treated MDA-MB-231 cells, while 5 genes were up-regulated and 1 gene was down-regulated in treated MCF-7 cells. These findings suggest that the autophagic response to starvation was different in the two treated cell lines, which will contribute to further study on the molecular mechanism of starvation-induced autophagy and improve the targeted therapy of breast cancer.

SBI0206965, a novel inhibitor of Ulk1, suppresses non-small cell lung cancer cell growth by modulating both autophagy and apoptosis pathways

Lung cancer is a major public health problem worldwide. Non-small cell lung cancer (NSCLC) accounts for 85% of lung cancer cases. Autophagy has recently sparked great interest, and it is thought to participate in a variety of diseases, including lung cancer. Uncoordinated (Unc) 51-like kinase 1 (Ulk1), a serine/threonine kinase, plays a central role in the autophagy pathway. However, the role of Ulk1 in NSCLC remains unclear. We report that NSCLC cell lines exhibited high expression of Ulk1 and that Ulk1 was negatively correlated with prognosis in lung cancer patients. Knockdown of Ulk1 or the inhibition of Ulk1 by the selective inhibitor SBI0206965, inhibited cell proliferation, induced cell apoptosis and enhanced the sensitivity of cisplatin against NSCLC cells. Moreover, we demonstrated that Ulk1 exerted oncogenic activity in NSCLC by modulating both autophagy and apoptosis pathways. Inhibition of autophagy by SBI0206965 sensitized NSCLC cells to cisplatin by inhibiting cisplatin induced cell-protective autophagy to promote apoptosis. Furthermore, SBI0206965 promoted apoptosis in NSCLC cells independent of autophagy, which was partly mediated by destabilization of Bcl2/Bclxl. In summary, our results show that inhibition of Ulk1 suppresses NSCLC cell growth and sensitizes NSCLC cells to cisplatin by modulating both autophagy and apoptosis pathways, and that Ulk1 might be a promising target for NSCLC treatment.

Identification and validation of a novel autophagy gene expression signature for human bladder cancer patients

We sought to identify and validate a novel urinary autophagy transcript signature in patients with bladder cancer and evaluate its clinical utility. We performed an initial screening for seven autophagy transcript-based panel (autophagy-related protein 12 (ATG12); WD repeat domain, phosphoinositide interacting 2 (WIPI2); FYVE and coiled-coil domain-containing protein 1 (FYCO1); microtubule-associated protein light chain (MAPLC3); RB1-inducible coiled-coil 1 (RB1CC1); tachylectin-II-like beta-propeller domain 1 (TECPR1); and Unc-51-like kinase (ULK1)) that was identified based on bioinformatics analysis followed by SYBR Green-based polymerase chain reaction array validation in paired tissue and urine samples. Afterward, we evaluated the expression of differentially expressed autophagy transcripts in an independent validation set with reverse transcription quantitative real-time polymerase chain reaction in urine sediments of 140 patients with bladder cancer, 68 patients with benign urological lesions, and 74 healthy controls (age and sex matched). The expression levels of ATG12, FYCO1, TECPR1, and ULK1 in paired bladder tissue and urine samples were significantly lower in bladder cancer than in control group (p < 0.001). In the validation set, the receiver-operating characteristic curve analyses demonstrated that each urinary autophagy transcripts showed high sensitivity and specificity for distinguishing bladder cancer from non-bladder cancer patients (ATG12, 75.4% and 86.1%; FYCO1, 87% and 75.7%; ULK1, 85.5% and 75.6%; and TECPR1, 90% and 81.9%). We document and validate a novel autophagy transcript signature for human bladder cancer diagnosis: bilharzial and non-bilharzial types.

Canonical and noncanonical functions of ULK/Atg1

Mammalian Unc-51-like kinases 1 and 2 (ULK1 and ULK2) belong to the ULK/Atg1 family of serine/threonine kinases, which are conserved from yeast to mammals. Although ULK/Atg1 is best known for regulating flux through the autophagy pathway, it has evolutionarily conserved noncanonical functions in protein trafficking that are essential for maintaining cellular homeostasis. As a direct target of energy- and nutrient-sensing kinases, ULK/Atg1 is positioned to regulate the distribution and use of cellular resources in response to metabolic cues. In this review, we provide an overview of the molecular mechanisms through which ULK/Atg1 carries out its canonical and noncanonical functions and the signaling pathways that link its function to metabolism. We also highlight potential contributions of ULK/Atg1 in human diseases, including cancer and neurodegeneration.

Phosphoglycerate Kinase 1 Phosphorylates Beclin1 to Induce Autophagy

Autophagy is crucial for maintaining cell homeostasis. However, the precise mechanism underlying autophagy initiation remains to be defined. Here, we demonstrate that glutamine deprivation and hypoxia result in inhibition of mTOR-mediated acetyl-transferase ARD1 S228 phosphorylation, leading to ARD1-dependent phosphoglycerate kinase 1 (PGK1) K388 acetylation and subsequent PGK1-mediated Beclin1 S30 phosphorylation. This phosphorylation enhances ATG14L-associated class III phosphatidylinositol 3-kinase VPS34 activity by increasing the binding of phosphatidylinositol to VPS34. ARD1-dependent PGK1 acetylation and PGK1-mediated Beclin1 S30 phosphorylation are required for glutamine deprivation- and hypoxia-induced autophagy and brain tumorigenesis. Furthermore, PGK1 K388 acetylation levels correlate with Beclin1 S30 phosphorylation levels and poor prognosis in glioblastoma patients. Our study unearths an important mechanism underlying cellular-stress-induced autophagy initiation in which the protein kinase activity of the metabolic enzyme PGK1 plays an instrumental role and reveals the significance of the mutual regulation of autophagy and cell metabolism in maintaining cell homeostasis.

Quantifying autophagy: Measuring LC3 puncta and autolysosome formation in cells using multispectral imaging flow cytometry

The use of multispectral imaging flow cytometry has been gaining popularity due to its quantitative power, high throughput capabilities, multiplexing potential and its ability to acquire images of every cell. Autophagy is a process in which dysfunctional organelles and cellular components that accumulate during growth and differentiation are degraded via the lysosome and recycled. During autophagy, cytoplasmic LC3 is processed and recruited to the autophagosomal membranes; the autophagosome then fuses with the lysosome to form the autolysosome. Therefore, cells undergoing autophagy can be identified by visualizing fluorescently labeled LC3 puncta and/or the co-localization of fluorescently labeled LC3 and lysosomal markers. Multispectral imaging flow cytometry is able to collect imagery of large numbers of cells and assess autophagy in an objective, quantitative, and statistically robust manner. This review will examine the four predominant methods that have been used to measure autophagy via multispectral imaging flow cytometry.

Tumor suppressor BTG1 promotes PRMT1-mediated ATF4 function in response to cellular stress

Cancer cells are frequently exposed to physiological stress conditions such as hypoxia and nutrient limitation. Escape from stress-induced apoptosis is one of the mechanisms used by malignant cells to survive unfavorable conditions. B-cell Translocation Gene 1 (BTG1) is a tumor suppressor that is frequently deleted in acute lymphoblastic leukemia and recurrently mutated in diffuse large B cell lymphoma. Moreover, low BTG1 expression levels have been linked to poor outcome in several solid tumors. How loss of BTG1 function contributes to tumor progression is not well understood. Here, using Btg1 knockout mice, we demonstrate that loss of Btg1 provides a survival advantage to primary mouse embryonic fibroblasts (MEFs) under stress conditions. This pro-survival effect involves regulation of Activating Transcription Factor 4 (ATF4), a key mediator of cellular stress responses. We show that BTG1 interacts with ATF4 and positively modulates its activity by recruiting the protein arginine methyl transferase PRMT1 to methylate ATF4 on arginine residue 239. We further extend these findings to B-cell progenitors, by showing that loss of Btg1 expression enhances stress adaptation of mouse bone marrow-derived B cell progenitors. In conclusion, we have identified the BTG1/PRMT1 complex as a new modifier of ATF4 mediated stress responses.

Happily (n)ever after: Aging in the context of oxidative stress, proteostasis loss and cellular senescence

Aging is a complex phenomenon and its impact is becoming more relevant due to the rising life expectancy and because aging itself is the basis for the development of age-related diseases such as cancer, neurodegenerative diseases and type 2 diabetes. Recent years of scientific research have brought up different theories that attempt to explain the aging process. So far, there is no single theory that fully explains all facets of aging. The damage accumulation theory is one of the most accepted theories due to the large body of evidence found over the years. Damage accumulation is thought to be driven, among others, by oxidative stress. This condition results in an excess attack of oxidants on biomolecules, which lead to damage accumulation over time and contribute to the functional involution of cells, tissues and organisms. If oxidative stress persists, cellular senescence is a likely outcome and an important hallmark of aging. Therefore, it becomes crucial to understand how senescent cells function and how they contribute to the aging process. This review will cover cellular senescence features related to the protein pool such as morphological and molecular hallmarks, how oxidative stress promotes protein modifications, how senescent cells cope with them by proteostasis mechanisms, including antioxidant enzymes and proteolytic systems. We will also highlight the nutritional status of senescent cells and aged organisms (including human clinical studies) by exploring trace elements and micronutrients and on their importance to develop strategies that might increase both, life and health span and postpone aging onset.

Autophagy-Dependent Secretion: Contribution to Tumor Progression

Autophagy is best known as a lysosomal degradation and recycling pathway to maintain cellular homeostasis. During autophagy, cytoplasmic content is recognized and packed in autophagic vacuoles, or autophagosomes, and targeted for degradation. However, during the last years, it has become evident that the role of autophagy is not restricted to degradation alone but also mediates unconventional forms of secretion. Furthermore, cells with defects in autophagy apparently are able to reroute their cargo, like mitochondria, to the extracellular environment; effects that contribute to an array of pathologies. In this review, we discuss the current knowledge of the physiological roles of autophagy-dependent secretion, i.e., the effect on inflammation and insulin/hormone secretion. Finally, we focus on the effects of autophagy-dependent secretion on the tumor microenvironment (TME) and tumor progression. The autophagy-mediated secreted factors may stimulate cellular proliferation via auto- and paracrine signaling. The autophagy-mediated release of immune modulating proteins changes the immunosuppresive TME and may promote an invasive phenotype. These effects may be either direct or indirect through facilitating formation of the mobilized vesicle, aid in anterograde trafficking, or alterations in homeostasis and/or autonomous cell signaling.

The Lysosome as a Regulatory Hub

The lysosome has long been viewed as the recycling center of the cell. However, recent discoveries have challenged this simple view and have established a central role of the lysosome in nutrient-dependent signal transduction. The degradative role of the lysosome and its newly discovered signaling functions are not in conflict but rather cooperate extensively to mediate fundamental cellular activities such as nutrient sensing, metabolic adaptation, and quality control of proteins and organelles. Moreover, lysosome-based signaling and degradation are subject to reciprocal regulation. Transcriptional programs of increasing complexity control the biogenesis, composition, and abundance of lysosomes and fine-tune their activity to match the evolving needs of the cell. Alterations in these essential activities are, not surprisingly, central to the pathophysiology of an ever-expanding spectrum of conditions, including storage disorders, neurodegenerative diseases, and cancer. Thus, unraveling the functions of this fascinating organelle will contribute to our understanding of the fundamental logic of metabolic organization and will point to novel therapeutic avenues in several human diseases.

Repression of Mcl-1 and disruption of the Mcl-1/Bak interaction in myeloma cells couple ER stress to mitochondrial apoptosis

As myeloma cells actively produce and secrete immunoglobulins, they are prone to ER stress, which if unresolved leads to apoptosis. We found that myeloma cell death induced by the ER stressor Thapsigargin was highly variable, ranging from 2 to 89%. Induction of ATF4 and CHOP was observed in myeloma cells under Thapsigargin independently of cell death. The decrease in Mcl-1 was associated with protein translation inhibition and identified as a crucial factor in Thapsigargin sensitivity, since it was the only Bcl-2 family protein differentially modified between sensitive and resistant myeloma cells. Bak but not Bax was found to contribute to Thapsigargin-induced apoptosis. Appropriately, a basal Mcl-1/Bak interaction was demonstrated in Thapsigargin-sensitive cells. Of note, the only pro-apoptotic protein freed from Mcl-1 under Thapsigargin was Bak, whereas Mcl-1/Noxa or Mcl-1/Bim complexes were simultaneously increased. Thus, the disruption of the basal Mcl-1/Bak complex in Thapsigargin-sensitive cells seemed to be an essential event in cell death induction, probably favored by the induced Noxa and Bim BH3-only proteins. These findings underscore the implication of the Mcl-1/Bak axis in myeloma cell death triggered by Thapsigargin.

Elevated levels of autophagy-related marker ULK1 and mitochondrion-associated autophagy inhibitor LRPPRC are associated with biochemical progression and overall survival after androgen deprivation therapy in patients with metastatic prostate cancer

AIM

To evaluate the expression levels and prognostic significance of autophagy-related markers, UNC-51-like kinase1 (ULK1), Beclin1, microtubule-associated protein light chain 3 (LC3), autophagy-related gene 5 (ATG5) and mitochondrion-associated autophagy inhibitor, LRPPRC, in patients with metastatic prostate cancer (PCa) after androgen deprivation therapy (ADT).

METHODS

Expressions of ULK1, Beclin1, LC3, ATG5 and LRPPRC were assessed by immunohistochemical examination in 198 patients with metastatic PCa who were receiving ADT (goserelin and bicalutamide).

RESULTS

High expression levels of LRPPRC and ULK1were significantly associated with Gleason score, serum prostate-specific antigen (PSA) levels, PSA levels after ADT and number of metastatic sites. High expression of ULK1 in patients with concomitant high expression of LRPPRC was significantly associated with multiple metastases, shorter biochemical progression (BCP)-free survival and shorter overall survival (OS). ULK1 expression, LRPPRC expression, Gleason score, PSA levels after ADT and number of metastatic sites were independently associated with shorter BCP-free survival and OS on multivariate analysis. Furthermore, two-year BCP rate of patients with ≥3 risk factors was found to be significantly higher as compared with that of patients with ≤1 and 2 risk factors. Three-year OS rate in patients with ≥3 risk factors was significantly lower than that of those with ≤1 and 2 risk factors.

CONCLUSIONS

High expression of ULK1 concomitant with high expression of LRPPRC may serve as useful markers for shorter BCP-free survival and OS in patients with metastatic PCa after ADT.

Emerging tale of UPR and cancer: an essentiality for malignancy

A set of cellular response to counter any alteration in homeostasis of a cell originating at endoplasmic reticulum is collectively termed as unfolded protein response (UPR). It initially is adaptive in nature as to restore cellular normalcy failing in course often activates pro-apoptotic signaling pathway resulting in cell death. UPR has emerged as an essential adaptation mechanism that cross talk with various cellular processes for cancer pathogenesis. Interestingly, it plays diverse role in plethora of signaling pathways instrumental in transformation, cell invasion, cell migration, metastasis, neovascularization, proliferation, and maintenance of energy metabolism of cancerous cells. In cancerous cells, it is triggered by change in microenvironment of a cell usually driven by hypoxia, acidosis, and nutrient deprivation, which often leads to positive selection pressure involving the reprogramming of energy metabolism which promotes channelization of limited metabolites into the hexosamine biosynthetic pathway (HBP). Substantial evidences suggest the role of UPR in oncogene (Myc, mTOR, RAS, HER2) driven cancer transformation and progression. In this review, we have comprehensively underlined the role played by UPR in adaptation, transformation, proliferation, invasion, and metastasis of cancerous cells.

MicroRNA-106a targets autophagy and enhances sensitivity of lung cancer cells to Src inhibitors

OBJECTIVES

Src tyrosine kinase inhibitors (TKIs) significantly inhibit cell migration and invasion in lung cancer cell lines with minor cytotoxic effects. In clinical trials, however, they show modest activity in combination with chemotherapeutic agents. Possible resistance mechanisms include the induction of cytoprotective autophagy upon Src inhibition. Autophagy is a cellular recycling process that allows cell survival in response to a variety of stress stimuli including responses to various treatments.

MATERIAL AND METHODS

We screened autophagic activity in A549, H460, and H1299 NSCLC cell lines treated with two different Src-TKIs (saracatinib, dasatinib) or shRNA targeting SRC. The autophagy response was determined by LC3B-I to -II conversion, increased ULK1 epxression and increased GFP-LC3B dot formation. Autophagy was inhibited by pharmacological (bafilomycin A, chloroquine) or genetic (ULK1 shRNA) means. Expression of miR-106a and miR-20b was analyzed by qPCR, and we used different lentivral vectors for ectopic expression of either miR-106a mimetics, anti-sense miR-106a or different miR-106a-363 cluster constructs.

RESULTS

In the current study we found that Src-TKIs induce autophagy in lung adenocarcinoma cell lines and that a combination of autophagy and Src tyrosine kinase inhibition results in cell death. Moreover, Src-TKI induced autophagy depends on the induction of the key autophagy kinase ULK1. This ULK1 upregulation is caused by downregulation of the ULK1-targeting microRNA-106a. An inverse correlation of miR-106a and ULK1 expression was seen in lung adenocarcinoma. Accordingly, ectopic expression of miR-106a in combination with Src-TKI treatment resulted in significant cell death as compared to control transduced cells.

CONCLUSIONS

Autophagy protects lung adenocarcinoma cells from Src-TKIs via a newly identified miR-106a-ULK1 signaling pathway. The combined inhibition of Src and ULK1/autophagy might represent a promising treatment option for future clinical trials. Lastly, our data might challenge the term "oncogenic" miR-106a as it can promote sensitivity to Src-TKIs thereby underlining the context-dependent function of miRNAs.

Dengue-induced autophagy, virus replication and protection from cell death require ER stress (PERK) pathway activation

A virus that reproduces in a host without killing cells can easily establish a successful infection. Previously, we showed that dengue-2, a virus that threatens 40% of the world, induces autophagy, enabling dengue to reproduce in cells without triggering cell death. Autophagy further protects the virus-laden cells from further insults. In this study, we evaluate how it does so; we show that dengue upregulates host pathways that increase autophagy, namely endoplasmic reticulum (ER) stress and ataxia telangiectasia mutated (ATM) signaling followed by production of reactive oxygen species (ROS). Inhibition of ER stress or ATM signaling abrogates the dengue-conferred protection against other cell stressors. Direct inhibition of ER stress response in infected cells decreases autophagosome turnover, reduces ROS production and limits reproduction of dengue virus. Blocking ATM activation, which is an early response to infection, decreases transcription of ER stress response proteins, but ATM has limited impact on production of ROS and virus titers. Production of ROS determines only late-onset autophagy in infected cells and is not necessary for dengue-induced protection from stressors. Collectively, these results demonstrate that among the multiple autophagy-inducing pathways during infection, ER stress signaling is more important to viral replication and protection of cells than either ATM or ROS-mediated signaling. To limit virus production and survival of dengue-infected cells, one must address the earliest phase of autophagy, induced by ER stress.

ONC201 kills solid tumor cells by triggering an integrated stress response dependent on ATF4 activation by specific eIF2α kinases

ONC201 (also called TIC10) is a small molecule that inactivates the cell proliferation- and cell survival-promoting kinases Akt and ERK and induces cell death through the proapoptotic protein TRAIL. ONC201 is currently in early-phase clinical testing for various malignancies. We found through gene expression and protein analyses that ONC201 triggered an increase in TRAIL abundance and cell death through an integrated stress response (ISR) involving the transcription factor ATF4, the transactivator CHOP, and the TRAIL receptor DR5. ATF4 was not activated in ONC201-resistant cancer cells, and in ONC201-sensitive cells, knockdown of ATF4 or CHOP partially abrogated ONC201-induced cytotoxicity and diminished the ONC201-stimulated increase in DR5 abundance. The activation of ATF4 in response to ONC201 required the kinases HRI and PKR, which phosphorylate and activate the translation initiation factor eIF2α. ONC201 rapidly triggered cell cycle arrest, which was associated with decreased abundance of cyclin D1, decreased activity of the kinase complex mTORC1, and dephosphorylation of the retinoblastoma (Rb) protein. The abundance of X-linked inhibitor of apoptosis protein (XIAP) negatively correlated with the extent of apoptosis in response to ONC201. These effects of ONC201 were independent of whether cancer cells had normal or mutant p53. Thus, ONC201 induces cell death through the coordinated induction of TRAIL by an ISR pathway.

Hypoxia signaling pathways: modulators of oxygen-related organelles

Oxygen (O2) is an essential substrate in cellular metabolism, bioenergetics, and signaling and as such linked to the survival and normal function of all metazoans. Low O2 tension (hypoxia) is a fundamental feature of physiological processes as well as pathophysiological conditions such as cancer and ischemic diseases. Central to the molecular mechanisms underlying O2 homeostasis are the hypoxia-inducible factors-1 and -2 alpha (HIF-1α and EPAS1/HIF-2α) that function as master regulators of the adaptive response to hypoxia. HIF-induced genes promote characteristic tumor behaviors, including angiogenesis and metabolic reprogramming. The aim of this review is to critically explore current knowledge of how HIF-α signaling regulates the abundance and function of major O2-consuming organelles. Abundant evidence suggests key roles for HIF-1α in the regulation of mitochondrial homeostasis. An essential adaptation to sustained hypoxia is repression of mitochondrial respiration and induction of glycolysis. HIF-1α activates several genes that trigger mitophagy and represses regulators of mitochondrial biogenesis. Several lines of evidence point to a strong relationship between hypoxia, the accumulation of misfolded proteins in the endoplasmic reticulum, and activation of the unfolded protein response. Surprisingly, although peroxisomes depend highly on molecular O2 for their function, there has been no evidence linking HIF signaling to peroxisomes. We discuss our recent findings that establish HIF-2α as a negative regulator of peroxisome abundance and suggest a mechanism by which cells attune peroxisomal function with O2 availability. HIF-2α activation augments peroxisome turnover by pexophagy and thereby changes lipid composition reminiscent of peroxisomal disorders. We discuss potential mechanisms by which HIF-2α might trigger pexophagy and place special emphasis on the potential pathological implications of HIF-2α-mediated pexophagy for human health.

Nuclear ULK1 promotes cell death in response to oxidative stress through PARP1

Reactive oxygen species (ROS) may cause cellular damage and oxidative stress-induced cell death. Autophagy, an evolutionarily conserved intracellular catabolic process, is executed by autophagy (ATG) proteins, including the autophagy initiation kinase Unc-51-like kinase (ULK1)/ATG1. Although autophagy has been implicated to have both cytoprotective and cytotoxic roles in the response to ROS, the role of individual ATG proteins, including ULK1, remains poorly characterized. In this study, we demonstrate that ULK1 sensitizes cells to necrotic cell death induced by hydrogen peroxide (H2O2). Moreover, we demonstrate that ULK1 localizes to the nucleus and regulates the activity of the DNA damage repair protein poly (ADP-ribose) polymerase 1 (PARP1) in a kinase-dependent manner. By enhancing PARP1 activity, ULK1 contributes to ATP depletion and death of H2O2-treated cells. Our study provides the first evidence of an autophagy-independent prodeath role for nuclear ULK1 in response to ROS-induced damage. On the basis of our data, we propose that the subcellular distribution of ULK1 has an important role in deciding whether a cell lives or dies on exposure to adverse environmental or intracellular conditions.

ATF4-dependent induction of heme oxygenase 1 prevents anoikis and promotes metastasis

The integrated stress response (ISR) is a critical mediator of cancer cell survival, and targeting the ISR inhibits tumor progression. Here, we have shown that activating transcription factor 4 (ATF4), a master transcriptional effector of the ISR, protects transformed cells against anoikis - a specialized form of apoptosis - following matrix detachment and also contributes to tumor metastatic properties. Upon loss of attachment, ATF4 activated a coordinated program of cytoprotective autophagy and antioxidant responses, including induced expression of the major antioxidant enzyme heme oxygenase 1 (HO-1). HO-1 upregulation was the result of simultaneous activation of ATF4 and the transcription factor NRF2, which converged on the HO1 promoter. Increased levels of HO-1 ameliorated oxidative stress and cell death. ATF4-deficient human fibrosarcoma cells were unable to colonize the lungs in a murine model, and reconstitution of ATF4 or HO-1 expression in ATF4-deficient cells blocked anoikis and rescued tumor lung colonization. HO-1 expression was higher in human primary and metastatic tumors compared with noncancerous tissue. Moreover, HO-1 expression correlated with reduced overall survival of patients with lung adenocarcinoma and glioblastoma. These results establish HO-1 as a mediator of ATF4-dependent anoikis resistance and tumor metastasis and suggest ATF4 and HO-1 as potential targets for therapeutic intervention in solid tumors.

Molecular Connections between Cancer Cell Metabolism and the Tumor Microenvironment

Cancer cells preferentially utilize glycolysis, instead of oxidative phosphorylation, for metabolism even in the presence of oxygen. This phenomenon of aerobic glycolysis, referred to as the “Warburg effect”, commonly exists in a variety of tumors. Recent studies further demonstrate that both genetic factors such as oncogenes and tumor suppressors and microenvironmental factors such as spatial hypoxia and acidosis can regulate the glycolytic metabolism of cancer cells. Reciprocally, altered cancer cell metabolism can modulate the tumor microenvironment which plays important roles in cancer cell somatic evolution, metastasis, and therapeutic response. In this article, we review the progression of current understandings on the molecular interaction between cancer cell metabolism and the tumor microenvironment. In addition, we discuss the implications of these interactions in cancer therapy and chemoprevention.

Cell intrinsic and extrinsic activators of the unfolded protein response in cancer: Mechanisms and targets for therapy

A variety of cell intrinsic or extrinsic stresses evoke perturbations in the folding environment of the endoplasmic reticulum (ER), collectively known as ER stress. Adaptation to stress and re-establishment of ER homeostasis is achieved by activation of an integrated signal transduction pathway called the unfolded protein response (UPR). Both ER stress and UPR activation have been implicated in a variety of human cancers. Although at early stages or physiological conditions of ER stress, the UPR generally promotes survival, when the stress becomes more stringent or prolonged, its role can switch to a pro-cell death one. Here, we discuss historical and recent evidence supporting an involvement of the UPR in malignancy, describe the main mechanisms by which tumor cells overcome ER stress to promote their survival, tumor progression and metastasis and discuss the current state of efforts to develop therapeutic approaches of targeting the UPR.

Emerging strategies to effectively target autophagy in cancer

Autophagy serves a dichotomous role in cancer and recent advances have helped delineate the appropriate settings where inhibiting or promoting autophagy may confer therapeutic efficacy in patients. Our evolving understanding of the molecular machinery responsible for the tightly controlled regulation of this homeostatic mechanism has begun to bear fruit in the way of autophagy-oriented clinical trials and promising lead compounds to modulate autophagy for therapeutic benefit. In this manuscript we review the recent preclinical and clinical therapeutic strategies that involve autophagy modulation in cancer.

Molecular mechanisms of mTOR regulation by stress

Tumors are prime examples of cell growth in unfavorable environments that elicit cellular stress. The high metabolic demand and insufficient vascularization of tumors cause a deficiency of oxygen and nutrients. Oncogenic mutations map to signaling events via mammalian target of rapamycin (mTOR), metabolic pathways, and mitochondrial function. These alterations have been linked with cellular stresses, in particular endoplasmic reticulum (ER) stress, hypoxia, and oxidative stress. Yet tumors survive these challenges and acquire highly energy-demanding traits, such as overgrowth and invasiveness. In this review we focus on stresses that occur in cancer cells and discuss them in the context of mTOR signaling. Of note, many tumor traits require mTOR complex 1 (mTORC1) activity, but mTORC1 hyperactivation eventually sensitizes cells to apoptosis. Thus, mTORC1 activity needs to be balanced in cancer cells. We provide an overview of the mechanisms contributing to mTOR regulation by stress and suggest a model wherein stress granules function as guardians of mTORC1 signaling, allowing cancer cells to escape stress-induced cell death.

A novel dual kinase function of the RET proto-oncogene negatively regulates activating transcription factor 4-mediated apoptosis

The RET proto-oncogene, a tyrosine kinase receptor, is widely known for its essential role in cell survival. Germ line missense mutations, which give rise to constitutively active oncogenic RET, were found to cause multiple endocrine neoplasia type 2, a dominant inherited cancer syndrome that affects neuroendocrine organs. However, the mechanisms by which RET promotes cell survival and prevents cell death remain elusive. We demonstrate that in addition to cytoplasmic localization, RET is localized in the nucleus and functions as a tyrosine-threonine dual specificity kinase. Knockdown of RET by shRNA in medullary thyroid cancer-derived cells stimulated expression of activating transcription factor 4 (ATF4), a master transcription factor for stress-induced apoptosis, through activation of its target proapoptotic genes NOXA and PUMA. RET knockdown also increased sensitivity to cisplatin-induced apoptosis. We observed that RET physically interacted with and phosphorylated ATF4 at tyrosine and threonine residues. Indeed, RET kinase activity was required to inhibit the ATF4-dependent activation of the NOXA gene because the site-specific substitution mutations that block threonine phosphorylation increased ATF4 stability and activated its targets NOXA and PUMA. Moreover, chromatin immunoprecipitation assays revealed that ATF4 occupancy increased at the NOXA promoter in TT cells treated with tyrosine kinase inhibitors or the ATF4 inducer eeyarestatin as well as in RET-depleted TT cells. Together these findings reveal RET as a novel dual kinase with nuclear localization and provide mechanisms by which RET represses the proapoptotic genes through direct interaction with and phosphorylation-dependent inactivation of ATF4 during the pathogenesis of medullary thyroid cancer.

The fate of chemoresistance in triple negative breast cancer (TNBC)

BACKGROUND

Treatment options for women presenting with triple negative breast cancer (TNBC) are limited due to the lack of a therapeutic target and as a result, are managed with standard chemotherapy such as paclitaxel (Taxol®). Following chemotherapy, the ideal tumour response is apoptotic cell death. Post-chemotherapy, cells can maintain viability by undergoing viable cellular responses such as cellular senescence, generating secretomes which can directly enhance the malignant phenotype.

SCOPE OF REVIEW

How tumour cells retain viability in response to chemotherapeutic engagement is discussed. In addition we discuss the implications of this retained tumour cell viability in the context of the development of recurrent and metastatic TNBC disease. Current adjuvant and neo-adjuvant treatments available and the novel potential therapies that are being researched are also reviewed.

MAJOR CONCLUSIONS

Cellular senescence and cytoprotective autophagy are potential mechanisms of chemoresistance in TNBC. These two non-apoptotic outcomes in response to chemotherapy are inextricably linked and are neglected outcomes of investigation in the chemotherapeutic arena. Cellular fate assessments may therefore have the potential to predict TNBC patient outcome.

GENERAL SIGNIFICANCE

Focusing on the fact that cancer cells can bypass the desired cellular apoptotic response to chemotherapy through cellular senescence and cytoprotective autophagy will highlight the importance of targeting non-apoptotic survival pathways to enhance chemotherapeutic efficacy.

How to control self-digestion: transcriptional, post-transcriptional, and post-translational regulation of autophagy

Macroautophagy (hereafter autophagy), literally defined as a type of self-eating, is a dynamic cellular process in which cytoplasm is sequestered within a unique compartment termed the phagophore. Upon completion, the phagophore matures into a double-membrane autophagosome that fuses with the lysosome or vacuole, allowing degradation of the cargo. Nonselective autophagy is primarily a cytoprotective response to various types of stress; however, the process can also be highly selective. Autophagy is involved in various aspects of cell physiology, and its dysregulation is associated with a range of diseases. The regulation of autophagy is complex, and the process must be properly modulated to maintain cellular homeostasis. In this review, we focus on the current state of knowledge concerning transcriptional, post-transcriptional, and post-translational regulation of autophagy in yeast and mammals.

Proteotoxic stress induces phosphorylation of p62/SQSTM1 by ULK1 to regulate selective autophagic clearance of protein aggregates

Disruption of proteostasis, or protein homeostasis, is often associated with aberrant accumulation of misfolded proteins or protein aggregates. Autophagy offers protection to cells by removing toxic protein aggregates and injured organelles in response to proteotoxic stress. However, the exact mechanism whereby autophagy recognizes and degrades misfolded or aggregated proteins has yet to be elucidated. Mounting evidence demonstrates the selectivity of autophagy, which is mediated through autophagy receptor proteins (e.g. p62/SQSTM1) linking autophagy cargos and autophagosomes. Here we report that proteotoxic stress imposed by the proteasome inhibition or expression of polyglutamine expanded huntingtin (polyQ-Htt) induces p62 phosphorylation at its ubiquitin-association (UBA) domain that regulates its binding to ubiquitinated proteins. We find that autophagy-related kinase ULK1 phosphorylates p62 at a novel phosphorylation site S409 in UBA domain. Interestingly, phosphorylation of p62 by ULK1 does not occur upon nutrient starvation, in spite of its role in canonical autophagy signaling. ULK1 also phosphorylates S405, while S409 phosphorylation critically regulates S405 phosphorylation. We find that S409 phosphorylation destabilizes the UBA dimer interface, and increases binding affinity of p62 to ubiquitin. Furthermore, lack of S409 phosphorylation causes accumulation of p62, aberrant localization of autophagy proteins and inhibition of the clearance of ubiquitinated proteins or polyQ-Htt. Therefore, our data provide mechanistic insights into the regulation of selective autophagy by ULK1 and p62 upon proteotoxic stress. Our study suggests a potential novel drug target in developing autophagy-based therapeutics for the treatment of proteinopathies including Huntington’s disease.

Accumulation of misfolded proteins deposited in the form of inclusion bodies is a common pathological hallmark for many human genetic diseases, particularly for the neurodegenerative disorders. The aggregation of the disease related proteins suggests a failure of the cellular machineries that maintain the protein homeostasis or proteostasis. The cellular clearance pathways, e.g. autophagy-lysosomal pathway, may not be of high efficiency in the face of rapid formation of misfolded protein aggregates. Thus, understanding of intrinsic mechanism whereby autophagy offers protection to cells by removing toxic protein aggregates is important. Here we report that a signaling transduction event that chemically modifies autophagy receptor protein p62/SQSTM1 regulates the receptor’s binding affinity to small molecule called ubiquitin(essential for marking the protein for degradation), as well as the selective degradation of targeted proteins. Furthermore, we find that expression of Huntington’s disease (HD) associated protein aggregates (containing polyglutamine or polyQ expansion) triggers the same modification of p62, which is dependent on the length of the polyQ expansion, suggesting a protective response of the cell by activating autophagy toward degradation of toxic aggregates. The modification of p62 also occurs in HD model brains in an age-dependent manner. Our study sheds light on the regulation of selective autophagy and provides a rationale for targeting p62 modification to treat aggregate diseases including HD.

ULK1: a promising biomarker in predicting poor prognosis and therapeutic response in human nasopharygeal carcinoma

Plenty of studies have established that dysregulation of autophagy plays an essential role in cancer progression. The autophagy-related proteins have been reported to be closely associated with human cancer patients’ prognosis. We explored the expression dynamics and prognostic value of autophagy-related protein ULK1 by immunochemistry (IHC) method in two independent cohorts of nasopharygeal carcinoma (NPC) cases. The X-tile program was applied to determine the optimal cut-off value in the training cohort. This derived cutoff value was then subjected to analysis the association of ULK1 expression with patients’ clinical characteristics and survival outcome in the validation cohort and overall cases. High ULK1 expression was closely associated with aggressive clinical feature of NPC patients. Furthermore, high expression of ULK1 was observed more frequently in therapeutic resistant group than that in therapeutic effective group. Our univariate and multivariate analysis also showed that higher ULK1 expression predicted inferior disease-specific survival (DSS) (P<0.05). Consequently, a new clinicopathologic prognostic model with 3 poor prognostic factors (ie, ULK1 expression, overall clinical stage and therapeutic response) could significantly stratify risk (low, intermediate and high) for DSS in NPC patients (P<0.001). These findings provide evidence that, the examination of ULK1 expression by IHC method, could serve as an effective additional tool for predicting therapeutic response and patients’ survival outcome in NPC patients.

Transcription factors and cognate signalling cascades in the regulation of autophagy

Autophagy is a process that maintains the equilibrium between biosynthesis and the recycling of cellular constituents; it is critical for avoiding the pathophysiology that results from imbalance in cellular homeostasis. Recent reports indicate the need for the design of high-throughput screening assays to identify targets and small molecules for autophagy modulation. For such screening, however, a better understanding of the regulation of autophagy is essential. In addition to regulation by various signalling cascades, regulation of gene expression by transcription factors is also critical. This review focuses on the various transcription factors as well as the corresponding signalling molecules that act together to translate the stimuli to effector molecules that up- or downregulate autophagy. This review rationalizes the importance of these transcription factors functioning in tandem with cognate signalling molecules and their interfaces as possible therapeutic targets for more specific pharmacological interventions.

The secreted antigen, HP0175, of Helicobacter pylori links the unfolded protein response (UPR) to autophagy in gastric epithelial cells

Autophagy is an intracellular catabolic process that is required to maintain cellular homeostasis. Pathogen-elicited host cell autophagy may favour containment of infection or may help in bacterial survival. Pathogens have developed the ability to modulate host autophagy. The secreted antigen HP0175, a peptidyl prolyl cis,trans isomerase of Helicobacter pylori, has moonlighting functions with reference to host cells. Here we show that it executes autophagy in gastric epithelial cells. Autophagy is dependent on the unfolded protein response (UPR) that activates the expression of PKR-like ER kinase (PERK). This is accompanied by phosphorylation of eukaryotic initiation factor 2α (eIF-2α) and transcriptional activation of ATF4 and CHOP. Knockdown of UPR-related genes inhibits the conversion of LC3I to LC3II, a marker of autophagy. The autophagy-inducing ability of H. pylori is compromised when cells are infected with an isogenic hp0175 mutant. Autophagy precedes apoptosis. Silencing of BECLIN1 augments cleavage of caspase 3 as well as apoptosis. Increased apoptosis of gastric epithelial cells is known to be linked to H. pylori-mediated gastric inflammation and carcinogenesis. To the best of our knowledge, this study provides the first demonstration of how HP0175 endowed with moonlighting functions links UPR-dependent autophagy and apoptosis during H. pylori infection.

The unfolded protein response and cellular senescence. A review in the theme: cellular mechanisms of endoplasmic reticulum stress signaling in health and disease

The endoplasmic reticulum (ER) is a multifunctional organelle critical for the proper folding and assembly of secreted and transmembrane proteins. Perturbations of ER functions cause ER stress, which activates a coordinated system of transcriptional and translational controls called the unfolded protein response (UPR), to cope with accumulation of misfolded proteins and proteotoxicity. It results in ER homeostasis restoration or in cell death. Senescence is a complex cell phenotype induced by several stresses such as telomere attrition, DNA damage, oxidative stress, and activation of some oncogenes. It is mainly characterized by a cell enlargement, a permanent cell-cycle arrest, and the production of a secretome enriched in proinflammatory cytokines and components of the extracellular matrix. Senescent cells accumulate with age in tissues and are suspected to play a role in age-associated diseases. Since senescence is a stress response, the question arises of whether an ER stress could occur concomitantly with senescence and participate in the onset or maintenance of the senescent features. Here, we described the interconnections between the UPR signaling and the different aspects of the cellular senescence programs and discuss the implication of UPR modulations in this context.

Stress-mediated translational control in cancer cells

Tumor cells are continually subjected to diverse stress conditions of the tumor microenvironment, including hypoxia, nutrient deprivation, and oxidative or genotoxic stress. Tumor cells must evolve adaptive mechanisms to survive these conditions to ultimately drive tumor progression. Tight control of mRNA translation is critical for this response and the adaptation of tumor cells to such stress forms. This proceeds though a translational reprogramming process which restrains overall translation activity to preserve energy and nutrients, but which also stimulates the selective synthesis of major stress adaptor proteins. Here we present the different regulatory signaling pathways which coordinate mRNA translation in the response to different stress forms, including those regulating eIF2α, mTORC1 and eEF2K, and we explain how tumor cells hijack these pathways for survival under stress. Finally, mechanisms for selective mRNA translation under stress, including the utilization of upstream open reading frames (uORFs) and internal ribosome entry sites (IRESes) are discussed in the context of cell stress. This article is part of a Special Issue entitled: Translation and Cancer.

Targeting tumour hypoxia to prevent cancer metastasis. From biology, biosensing and technology to drug development: the METOXIA consortium

The hypoxic areas of solid cancers represent a negative prognostic factor irrespective of which treatment modality is chosen for the patient. Still, after almost 80 years of focus on the problems created by hypoxia in solid tumours, we still largely lack methods to deal efficiently with these treatment-resistant cells. The consequences of this lack may be serious for many patients: Not only is there a negative correlation between the hypoxic fraction in tumours and the outcome of radiotherapy as well as many types of chemotherapy, a correlation has been shown between the hypoxic fraction in tumours and cancer metastasis. Thus, on a fundamental basis the great variety of problems related to hypoxia in cancer treatment has to do with the broad range of functions oxygen (and lack of oxygen) have in cells and tissues. Therefore, activation-deactivation of oxygen-regulated cascades related to metabolism or external signalling are important areas for the identification of mechanisms as potential targets for hypoxia-specific treatment. Also the chemistry related to reactive oxygen radicals (ROS) and the biological handling of ROS are part of the problem complex. The problem is further complicated by the great variety in oxygen concentrations found in tissues. For tumour hypoxia to be used as a marker for individualisation of treatment there is a need for non-invasive methods to measure oxygen routinely in patient tumours. A large-scale collaborative EU-financed project 2009-2014 denoted METOXIA has studied all the mentioned aspects of hypoxia with the aim of selecting potential targets for new hypoxia-specific therapy and develop the first stage of tests for this therapy. A new non-invasive PET-imaging method based on the 2-nitroimidazole [(18)F]-HX4 was found to be promising in a clinical trial on NSCLC patients. New preclinical models for testing of the metastatic potential of cells were developed, both in vitro (2D as well as 3D models) and in mice (orthotopic grafting). Low density quantitative real-time polymerase chain reaction (qPCR)-based assays were developed measuring multiple hypoxia-responsive markers in parallel to identify tumour hypoxia-related patterns of gene expression. As possible targets for new therapy two main regulatory cascades were prioritised: The hypoxia-inducible-factor (HIF)-regulated cascades operating at moderate to weak hypoxia (<1% O(2)), and the unfolded protein response (UPR) activated by endoplasmatic reticulum (ER) stress and operating at more severe hypoxia (<0.2%). The prioritised targets were the HIF-regulated proteins carbonic anhydrase IX (CAIX), the lactate transporter MCT4 and the PERK/eIF2α/ATF4-arm of the UPR. The METOXIA project has developed patented compounds targeting CAIX with a preclinical documented effect. Since hypoxia-specific treatments alone are not curative they will have to be combined with traditional anti-cancer therapy to eradicate the aerobic cancer cell population as well.