The role of autophagy in liver cancer: Molecular mechanisms and potential therapeutic targets

Autophagy is an evolutionarily conserved pathway for degradation of cytoplasmic proteins and organelles via lysosome. Proteins coded by the autophagy-related genes (Atgs) are the core molecular machinery in control of autophagy. Among the various biological functions of autophagy identified so far, the link between autophagy and cancer is probably among the most extensively studied and is often viewed as controversial. Autophagy might exert a dual role in cancer development: autophagy can serve as an anti-tumor mechanism, as defective autophagy (e.g., heterozygous knockdown Beclin 1 and Atg7 in mice) promotes the malignant transformation and spontaneous tumors. On the other hand, autophagy functions as a protective or survival mechanism in cancer cells against cellular stress (e.g., nutrient deprivation, hypoxia and DNA damage) and hence promotes tumorigenesis and causes resistance to therapeutic agents. Liver cancer is one of the common cancers with well-established etiological factors including hepatitis virus infection and environmental carcinogens such as aflatoxin and alcohol exposure. In recent years, the involvement of autophagy in liver cancer has been increasingly studied. Here, we aim to provide a systematic review on the close cross-talks between autophagy and liver cancer, and summarize the current status in development of novel liver cancer therapeutic approaches by targeting autophagy. It is believed that understanding the molecular mechanisms underlying the autophagy modulation and liver cancer development may provoke the translational studies that ultimately lead to new therapeutic strategies for liver cancer.     

The role of cancer metabolism in defining the success of oncolytic viro-immunotherapy

Oncolytic viruses infect, replicate in, and kill cancer cells selectively without harming normal cells. The rapidly expanding clinical development of oncolytic virotherapy is an exciting interdisciplinary field that provides insights into virology, oncology, and immunotherapy. Recent years have seen greater focus on rational design of cancer-selective viruses together with strategies to exploit their immunostimulatory capabilities, ultimately to develop powerful oncolytic cancer vaccines. However, despite great interest in the field, many important experiments are still conducted under optimum conditions in vitro, with many nutrients present in excess and with cellular stress kept to a minimum. Whilst this provides a convenient platform for cell culture, it bears little relation to the typical conditions found within a tumour in vivo, where cells are often subject to a range of metabolic and environmental stresses. Viral infection and cancer will both lead to production of metabolites that are also not present in media in vitro. Understanding how oncolytic viruses interact with cells exposed to more representative metabolic conditions in vitro represents an under-explored area of study that could provide valuable insight into the intelligent design of superior oncolytic viruses and help bridge the gap between bench and bedside. This review summarises the major metabolic pathways altered in cancer cells, during viral infection and highlights possible targets for future studies.     

Gadd34 induces autophagy through the suppression of the mTOR pathway during starvation

Several types of cellular stress induce expression of growth arrest and DNA damage protein 34 (Gadd34). Autophagy occurs under both basal conditions and conditions of stress, such as starvation. Gadd34 and autophagy are both induced under starvation conditions. In this study we found that starvation induced the expression of Gadd34, reduced mTOR activity, and induced autophagy in wild type mice, but not Gadd34 KO mice. Gadd34 bound to and dephosphorylated pTSC2 at Thr1462. Dephosphorylation of TSC2 during the starvation time period leads to the suppression of mTOR, which is a potent inhibitor of autophagy. We concluded that starvation-induced Gadd34 suppresses mTOR and, thereby, induces autophagy.     

缺氧环境下自噬对肠黏膜屏障的影响

肠黏膜屏障是机体屏障系统的重要组成部分,可有效阻止肠道寄生菌及其毒素向肠腔外组织移位,防止机体受内源性微生物及其毒素的侵害.自噬在各种生命活动中发挥着重要作用.在缺血缺氧等应激状态下,自噬对细胞存活、清除细胞内衰老细胞器等起重要作用.缺氧可诱导自噬.多数情况下自噬被认为是细胞的一种保护作用,然而在某些条件下细胞过度自噬也能导致细胞凋亡.肠黏膜屏障损伤的研究是目前医学研究领域的一个重要课题,本文就自噬在缺氧环境下对肠黏膜屏障的影响做一综述.     

Prolyl hydroxylase domain enzymes and their role in cell signaling and cancer metabolism

The prolyl hydroxylase domain (PHD) enzymes regulate the stability of the hypoxia-inducible factor (HIF) in response to oxygen availability. During oxygen limitation, the inhibition of PHD permits the stabilization of HIF, allowing the cellular adaptation to hypoxia. This adaptation is especially important for solid tumors, which are often exposed to a hypoxic environment. However, and despite their original role as the oxygen sensors of the cell, PHD are currently known to display HIF-independent and hydroxylase-independent functions in the control of different cellular pathways, including mTOR pathway, NF-kB pathway, apoptosis and cellular metabolism. In this review, we summarize the recent advances in the regulation and functions of PHD in cancer signaling and cell metabolism.     

The paradox of autophagy and its implication in cancer etiology and therapy

Autophagy is a cellular self-catabolic process in which cytoplasmic constituents are sequestered in double membrane vesicles that fuse with lysosomes where they are degraded. As this catabolic activity generates energy, autophagy is often induced under nutrient limiting conditions providing a mechanism to maintain cell viability and may be exploited by cancer cells for survival under metabolic stress. However, progressive autophagy can be cytotoxic and autophagy can under certain settings substitute for apoptosis in induction of cell death. Moreover, loss of autophagy is correlated with tumorigenesis and several inducers of autophagy are tumor-suppressor genes. Thus, the relation of autophagy to cancer development is complex and depends on the genetic composition of the cell as well as on the extra-cellular stresses a cell is exposed to. In this review we describe the intricate nature of autophagy and its regulators, particularly those that have been linked to cancer. We discuss the multifaceted relation of autophagy to tumorigenesis and highlight studies supporting a role for autophagy in both tumor-suppression and tumor-progression. Finally, various autophagy-targeting therapeutic strategies for cancer treatment are presented. This review is dedicated to the memory of Dr. Avner Eisenberg 1953–2004.     

Molecular modulation of autophagy: New venture to target resistant cancer stem cells

Autophagy is a highly regulated catabolic process in which superfluous,damaged organelles and other cytoplasmic constituents are delivered to the lysosome for clearance and the generation of macromolecule substrates during basal or stressed conditions. Autophagy is a bimodal process with a context dependent role in the initiation and the development of cancers. For instance,autophagy provides an adaptive response to cancer stem cells to survive metabolic stresses, by influencing disease propagation via modulation of essential signaling pathways or by promoting resistance to chemotherapeutics. Autophagy has been implicated in a cross talk with apoptosis. Understanding the complex interactions provides an opportunity to improve cancer therapy and the clinical outcome for the cancer patients. In this review, we provide a comprehensive view on the current knowledge on autophagy and its role in cancer cells with a particular focus on cancer stem cell homeostasis.     

Prognostic significance of autophagy related genes in estrogen receptor positive tamoxifen treated breast cancer

Resistance to Tamoxifen constitutes a major therapeutic challenge in treating hormone sensitive breast cancer. The induction of autophagy has been shown to be involved as one of the mechanism responsible for Tamoxifen resistance. Autophagy related gene (ATG) members are the regulators and effectors of Macroautophagy process in the cellular systems. In this study, we evaluated the prognostic significance of ATGs in Tamoxifen treated breast cancer. The "Kaplan- Meier plotter" database was utilized to analyze the relevance and significance of ATGs mRNA expression to Relapse Free Survival in breast cancer patients. We used the data of patients who are Estrogen receptor positive and are treated with Tamoxifen. Hazard ratio and log-rank p-value were calculated using KM survival plots for various ATGs. Overexpressed ATG3, ATG 5, ATG 8B and PIK3R4 resulted in a poor prognosis. A gene signature of these ATGs predicts deteriorated RFS (p-value=8.3e-05 and HR=1.84 (1.35-2.51) and Distant Metastasis Free Survival (p value = 0.0027 and HR=2.03 (1.27-3.26). We report the distinct prognostic values of ATGs in patients of breast cancer treated with Tamoxifen. Thus, better understandings of the induction of autophagy pathway may potentially form the basis for use of autophagy inhibitors in the Tamoxifen treated breast cancer.     

Epigenetic regulation of autophagy: A key modification in cancer cells and cancer stem cells

Aberrant epigenetic alterations play a decisive role in cancer initiation and propagation via the regulation of key tumor suppressor genes and oncogenes or by modulation of essential signaling pathways. Autophagy is a highly regulated mechanism required for the recycling and degradation of surplus and damaged cytoplasmic constituents in a lysosome dependent manner. In cancer, autophagy has a divergent role. For instance, autophagy elicits tumor promoting functions by facilitating metabolic adaption and plasticity in cancer stem cells (CSCs) and cancer cells. Moreover, autophagy exerts pro-survival mechanisms to these cancerous cells by influencing survival, dormancy, immunosurveillance, invasion, metastasis, and resistance to anti-cancer therapies. In addition, recent studies have demonstrated that various tumor suppressor genes and oncogenes involved in autophagy, are tightly regulated via different epigenetic modifications, such as DNA methylation, histone modifications and non-coding RNAs. The impact of epigenetic regulation of autophagy in cancer cells and CSCs is not well-understood. Therefore, uncovering the complex mechanism of epigenetic regulation of autophagy provides an opportunity to improve and discover novel cancer therapeutics. Subsequently, this would aid in improving clinical outcome for cancer patients. In this review, we provide a comprehensive overview of the existing knowledge available on epigenetic regulation of autophagy and its importance in the maintenance and homeostasis of CSCs and cancer cells.     

中华绒螯蟹亲蟹的饥饿代谢研究

1998年 10～ 12月 ,对体重为 5 4 .5 9(± 2 .37) g的亲蟹在 2 0 (± 0 .5 )℃温度条件下进行饥饿实验 ,研究了饥饿对中华绒螯蟹亲蟹代谢的影响 .结果表明 ,经过 30d的饥饿处理 ,亲蟹的耗氧率降低为摄食状态的 5 0 .0 % ;CO2 排出率减小为摄食状态的 6 3.4 % ;NH3 N排泄率减小为 5 9.1% .耗氧率和CO2 排出率、NH3 N排泄率的变化差别较大 ,耗氧率的减小可分为 4个阶段 ,而CO2 排出率和NH3 N排泄率的减小仅可分 3个阶段 .与此同时 ,亲蟹的标准代谢水平同样受到饥饿的影响 ,饥饿 30d后 ,由 4 .4 5J·g-1·h-1减小为 2 .36J·g-1·h-1;在饥饿亲蟹的代谢中 ,脂肪消耗最多 ,其次是碳水化合物 .     

The role of autophagy in doxorubicin-induced cardiotoxicity

Doxorubicin (Dox) is an effective chemotherapeutic agent, however, its use is limited by cardiotoxicity. The mechanisms causing cardiotoxicity have not been clearly elucidated, but known to involve, at least in part, oxidative stress, mitochondrial dysfunction and apoptosis. More recently, it has been suggested that dysregulation of autophagy may also play an important role in Dox-induced cardiotoxicity. Autophagy has dual functions. Under physiological conditions, autophagy is essential for optimal cellular function and survival by ridding the cell of damaged or unwanted proteins and organelles. Under pathological conditions, autophagy may be stimulated in order to protect the cell from stress stimuli or, alternatively, to contribute to cell death. Thus, appropriate regulation of autophagy can be a matter of life or death. The role of autophagy in Dox-induced cardiotoxicity has recently been explored, however, conflicting reports on the effects of Dox on autophagy and its role in cardiotoxicity exist. Most, but not all, of the studies conclude that Dox upregulates cardiac autophagy and contributes to the pathogenesis of Dox-induced toxicity. Dox may induce autophagy by suppressing the expression of GATA4 and/or S6K1, which may directly or indirectly regulate expression of essential autophagy genes such as Atg12, Atg5, Beclin1 and Bcl-2. Interestingly, the Dox-induced autophagic response may be species specific as Dox treatment has been shown to stimulate autophagy in rat models, but suppress autophagy in mouse models. Additional studies will elucidate this possibility.     

The induction of host cell autophagy triggers defense mechanisms against Trypanosoma cruzi infection in vitro

Trypanosoma cruzi causes Chagas disease, a neglected illness that affects millions of people worldwide, especially in Latin America. The balance between biochemical pathways triggered by the parasite and host cells response will ultimately define the progression of a life-threatening disease, justifying the efforts to understand cellular mechanisms for infection restrain. In this interaction, parasite and host cells are affected by different physiological responses as autophagy modulation, which could be under intense cellular stress, such as nutrient deprivation, hormone depletion, or infection. Autophagy is a constitutive pathway that leads to degradation of macromolecules and cellular structures and may induce cell death. In Trypanosoma cruzi infection, the relevance of host autophagy is controversial regarding in vitro parasite intracellular life cycle. In the present study, we evaluated host cell autophagy during T. cruzi infection in phagocytic and non-professional phagocytic cells. We described that the presence of the parasite increased the number of LC3 puncta, a marker for autophagy, in cardiac cells and peritoneal macrophages in vitro. The induction of host autophagy decreased infection in macrophages in early and late time-periods. We suggest that starved phagocytic cells reduced internalization, also confirmed by inert particles and dead trypomastigotes. Whereas, in cardiac cells, starvation-induced autophagy decreased lipid droplets and infection in later time-point, by reducing parasite differentiation/proliferation. In ATG5 knockout MEF cells, we confirmed our hypothesis of autophagy machinery activation during parasite internalization, increasing infection. Our data suggest that host autophagy downregulates T. cruzi infection through impairing parasite intracellular life cycle, reducing the infection in primary culture cells.     

Brain-derived neurotrophic factor: Its role in energy balance and cancer cachexia

Brain-derived neurotrophic factor (BDNF) plays an important role in the development of the central and peripheral nervous system during embryogenesis. In the mature central nervous system, BDNF is required for the maintenance and enhancement of synaptic transmissions and the survival of neurons. Particularly, it is involved in the modulation of neurocircuits that control energy balance through food intake, energy expenditure, and locomotion. Regulation of BDNF in the central nervous system is complex and environmental factors affect its expression in murine models which may reflect to phenotype dramatically. Furthermore, BDNF and its high-affinity receptor tropomyosin receptor kinase B (TrkB), as well as pan-neurotrophin receptor (p75^NTR) is expressed in peripheral tissues in adulthood and their signaling is associated with regulation of energy balance. BDNF/TrkB signaling is exploited by cancer cells as well and BDNF expression is increased in tumors. Intriguingly, previously demonstrated roles of BDNF in regulation of food intake, adipose tissue and muscle overlap with derangements observed in cancer cachexia. However, data about the involvement of BDNF in cachectic cancer patients and murine models are scarce and inconclusive. In the future, knock-in and/or knock-out experiments with murine cancer models could be helpful to explore potential new roles for BDNF in the development of cancer cachexia.     

Oxygen metabolism and a potential role for cytochrome c oxidase in the Warburg effect

By manipulating the physical properties of oxygen, cells are able to harvest the large thermodynamic potential of oxidation to provide a substantial fraction of the energy necessary for cellular processes. The enzyme largely responsible for this oxygen manipulation is cytochrome c oxidase, which resides at the inner mitochondrial membrane. For unknown reasons, cancer cells do not maximally utilize this process, but instead rely more on an anaerobic-like metabolism demonstrating the so-called Warburg effect. As the enzyme at the crossroads of oxidative metabolism, cytochrome c oxidase might be expected to play a role in this so-called Warburg effect. Through protein assay methods and metabolic studies with radiolabeled glucose, alterations associated with cancer and cytochrome c oxidase subunit levels are explored. The implications of these findings for cancer research are discussed briefly.     

Molecular regulation of autophagy in a pro-inflammatory tumour microenvironment: New insight into the role of serum amyloid A

Chronic inflammation, systemic or local, plays a vital role in tumour progression and metastasis. Dysregulation of key physiological processes such as autophagy elicit unfavourable immune responses to induce chronic inflammation. Cytokines, growth factors and acute phase proteins present in the tumour microenvironment regulate inflammatory responses and alter crosstalk between various signalling pathways involved in the progression of cancer. Serum amyloid A (SAA) is a key acute phase protein secreted by the liver during the acute phase response (APR) following infection or injury. However, cancer and cancer-associated cells produce SAA, which when present in high levels in the tumour microenvironment contributes to cancer initiation, progression and metastasis. SAA can activate several signalling pathways such as the PI3K and MAPK pathways, which are also known modulators of the intracellular degradation process, autophagy. Autophagy can be regarded as having a double edged sword effect in cancer. Its dysregulation can induce malignant transformation through metabolic stress which manifests as oxidative stress, endoplasmic reticulum (ER) stress and DNA damage. On the other hand, autophagy can promote cancer survival during metabolic stress, hypoxia and senescence. Autophagy has been utilised to promote the efficiency of chemotherapeutic agents and can either be inhibited or induced to improve treatment outcomes. This review aims to address the known mechanisms that regulate autophagy as well as illustrating the role of SAA in modulating these pathways and its clinical implications for cancer therapy.     

Fatty Acid Labeling from Glutamine in Hypoxia Can Be Explained by Isotope Exchange without Net Reductive Isocitrate Dehydrogenase (IDH) Flux

Acetyl-CoA is an important anabolic precursor for lipid biosynthesis. In the conventional view of mammalian metabolism, acetyl-CoA is primarily derived by the oxidation of glucose-derived pyruvate in mitochondria. Recent studies have employed isotope tracers to show that in cancer cells grown in hypoxia or with defective mitochondria, a major fraction of acetyl-CoA is produced via another route, reductive carboxylation of glutamine-derived α-ketoglutarate (catalyzed by reverse flux through isocitrate dehydrogenase, IDH). Here, we employ a quantitative flux model to show that in hypoxia and in cells with defective mitochondria, oxidative IDH flux persists and may exceed the reductive flux. Therefore, IDH flux may not be a net contributor to acetyl-CoA production, although we cannot rule out net reductive IDH flux in some compartments. Instead of producing large amounts of net acetyl-CoA reductively, the cells adapt by reducing their demand for acetyl-CoA by importing rather than synthesizing fatty acids. Thus, fatty acid labeling from glutamine in hypoxia can be explained by spreading of label without net reductive IDH flux.     

Proteasome inhibitors in cancer therapy

The ubiquitin proteasome pathway plays a critical role in regulating many processes in the cell which are important for tumour cell growth and survival. Inhibition of proteasome function has emerged as a powerful strategy for anti-cancer therapy. Clinical validation of the proteasome as a therapeutic target was achieved with bortezomib and has prompted the development of a second generation of proteasome inhibitors with improved pharmacological properties. This review summarises the main mechanisms of action of proteasome inhibitors in cancer, the development of proteasome inhibitors as therapeutic agents and the properties and progress of next generation proteasome inhibitors in the clinic.     

Resistin: A journey from metabolism to cancer

Resistin, a small secretory molecule, has been implicated to play an important role in the development of insulin resistance under obese condition. For the past few decades, it has been linked to various cellular and metabolic functions. It has been associated with diseases like metabolic disorders, cardiovascular diseases and cancers. Numerous clinical studies have indicated an increased serum resistin level in pathological disorders which have been reported to increase mortality rate in comparison to low resistin expressing subjects. Various molecular studies suggest resistin plays a pivotal role in proliferation, metastasis, angiogenesis, inflammation as well as in regulating metabolism in cancer cells. Therefore, understanding the role of resistin and elucidating its’ associated molecular mechanism will give a better insight into the management of these disorders. In this article, we summarize the diverse roles of resistin in pathological disorders based on the available literature, clinicopathological data, and a compiled study from various databases. The article mainly provides comprehensive information of its role as a target in different treatment modalities in pre as well as post-clinical studies.     

Taxol-induced unfolded protein response activation in breast cancer cells exposed to hypoxia: ATF4 activation regulates autophagy and inhibits apoptosis

Understanding the mechanisms responsible for the resistance against chemotherapy-induced cell death is still of great interest since the number of patients with cancer increases and relapse is commonly observed. Indeed, the development of hypoxic regions as well as UPR (unfolded protein response) activation is known to promote cancer cell adaptive responses to the stressful tumor microenvironment and resistance against anticancer therapies. Therefore, the impact of UPR combined to hypoxia on autophagy and apoptosis activation during taxol exposure was investigated in MDA–MB-231 and T47D breast cancer cells. The results showed that taxol rapidly induced UPR activation and that hypoxia modulated taxol-induced UPR activation differently according to the different UPR pathways (PERK, ATF6, and IRE1α). The putative involvement of these signaling pathways in autophagy or in apoptosis regulation in response to taxol exposure was investigated. However, while no link between the activation of these three ER stress sensors and autophagy or apoptosis regulation could be evidenced, results showed that ATF4 activation, which occurs independently of UPR activation, was involved in taxol-induced autophagy completion. In addition, an ATF4-dependent mechanism leading to cancer cell adaptation and resistance against taxol-induced cell death was evidenced. Finally, our results demonstrate that expression of ATF4, in association with hypoxia-induced genes, can be used as a biomarker of a poor prognosis for human breast cancer patients supporting the conclusion that ATF4 might play an important role in adaptation and resistance of breast cancer cells to chemotherapy in hypoxic tumors.