Autophagy,an accomplice or antagonist of drug resistance in HCC?

Hepatocellular carcinoma (HCC) is a highly lethal malignancy characterized by poor prognosis and a low 5-year survival rate. Drug treatment is proving to be effective in anti-HCC. However, only a small number of HCC patients exhibit sensitive responses, and drug resistance occurs frequently in advanced patients. Autophagy, an evolutionary process responsible for the degradation of cellular substances, is closely associated with the acquisition and maintenance of drug resistance for HCC. This review focuses on autophagic proteins and explores the intricate relationship between autophagy and cancer stem cells, tumor-derived exosomes, and noncoding RNA. Clinical trials involved in autophagy inhibition combined with anticancer drugs are also concerned.Subject terms: Cancer metabolism, Cancer therapeutic resistance     

Intersection of autophagy regulation and circadian rhythms in the heart

Autophagy is a vital cellular mechanism that controls the removal of damaged or dysfunctional cellular components. Autophagy allows the degradation and recycling of damaged proteins and organelles into their basic constituents of amino acids and fatty acids for cellular energy production. Under basal conditions, autophagy is essential for the maintenance of cell homeostasis and function. However, during cell stress, excessive activation of autophagy can be destructive and lead to cell death. Autophagy plays a crucial role in the cardiovascular system and helps to maintain normal cardiac function. During ischemia- reperfusion, autophagy can be adaptive or maladaptive depending on the timing and extent of activation. In this review, we highlight the molecular mechanisms and signaling pathways that underlie autophagy in response to cardiac stress and therapeutic approaches to modulate autophagy by pharmacological interventions. Finally, we also discuss the intersection between autophagy and circadian regulation in the heart. Understanding the mechanisms that underlie autophagy following cardiac injury can be translated to clinical cardiology use toward improved patient treatment and outcomes.     

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.     

Survival or death: disequilibrating the oncogenic and tumor suppressive autophagy in cancer

Autophagy (macroautophagy) is an evolutionarily conserved lysosomal degradation process, in which a cell degrades long-lived proteins and damaged organelles. Recently, accumulating evidence has revealed the core molecular machinery of autophagy in carcinogenesis; however, the intricate relationship between autophagy and cancer continue to remain an enigma. Why does autophagy have either pro-survival (oncogenic) or pro-death (tumor suppressive) role at different cancer stages, including cancer stem cell, initiation and progression, invasion and metastasis, as well as dormancy? How does autophagy modulate a series of oncogenic and/or tumor suppressive pathways, implicated in microRNA (miRNA) involvement? Whether would targeting the oncogenic and tumor suppressive autophagic network be a novel strategy for drug discovery? To address these problems, we focus on summarizing the dynamic oncogenic and tumor suppressive roles of autophagy and their relevant small-molecule drugs, which would provide a new clue to elucidate the oncosuppressive (survival or death) autophagic network as a potential therapeutic target.     

Freeze-fracture replica immunolabelling reveals human WIPI-1 and WIPI-2 as membrane proteins of autophagosomes

Autophagy defines the lifespan of eukaryotic organisms by ensuring cellular survival through regulated bulk clearance of proteins, organelles and membranes. Pathophysiological consequences of improper autophagy give rise to a variety of age-related human diseases such as cancer and neurodegeneration. Rational therapeutic implementation of autophagy modulation remains problematic, as fundamental molecular details such as the generation of autophagosomes, unique double-membrane vesicles formed to permit the process of autophagy, are insufficiently understood. Here, freeze-fracture replica immunolabelling reveals WD-repeat protein interacting with phosphoinositides 1 and 2 (WIPI-1 and WIPI-2) as membrane components of autophagosomes and the plasma membrane (PM). In addition, WIPI-1 is also present in membranes of the endoplasmic reticulum (ER) and WIPI-2 was further detected in membranes close to the Golgi cisternae. Our results identify WIPI-1 and WIPI-2 as novel protein components of autophagosomes, and of membrane sites from which autophagosomes might originate (ER, PM, Golgi area). Hence therapeutic modulation of autophagy could involve approaches that functionally target human WIPI proteins.     

PKCδ and Tissue Transglutaminase are Novel Inhibitors of Autophagy in Pancreatic Cancer Cells

Apoptosis (type I) and autophagy (type II) are both highly regulated forms of programmed cell death and play crucial roles in physiological processes such as the development, homeostasis and selective, moderate to massive elimination of cells, if needed. Accumulating evidence suggests that cancer cells, including pancreatic cancer cells, in general tend to have reduced autophagy relative to their normal counterparts and premalignant lesions, supporting the contention that defective autophagy provides resistance to metabolic stress such as hypoxia, acidity and chemotherapeutics, promotes tumor cell survival and plays a role in the process of tumorigenesis. However, the mechanisms underlying the reduced capability of undergoing autophagy in pancreatic cancer remain elusive. In a recent study, we demonstrated a novel mechanism for regulation of autophagy in pancreatic ductal carcinoma cells. We found that protein kinase C-delta (PKCδ) constitutively suppresses autophagy through induction of tissue transglutaminase (TG2). Inhibition of PKCδ/TG2 signaling resulted in significant autophagic cell death that was mediated by Beclin 1. Elevated expression of TG2 in pancreatic cancer cells has been implicated in the development of drug resistance, metastatic phenotype and poor patient prognosis. In conclusion, our data suggest a novel role of PKCδ/TG2 in regulation of autophagy, and that TG2 may serve as an excellent therapeutic target in pancreatic cancer cells.

Addendum to:

Tissue Transglutaminase Inhibits Autophagy in Pancreatic Cancer Cells

U. Akar, B. Ozpolat, K. Mehta, J. Fok, Y. Kondo and G. Lopez-Berestein

Mol Cancer Res 2007; 5:241-9     

Autophagy: a barrier or an adaptive response to cancer

Macroautophagy or autophagy is a degradative pathway terminating in the lysosomal compartment after the formation of a cytoplasmic vacuole that engulfs macromolecules and organelles. The recent discovery of the molecular controls of autophagy that are common to eukaryotic cells from yeast to human suggests that the role of autophagy in cell functioning is far beyond its nonselective degradative capacity. The involvement of proteins with properties of tumor suppressor and oncogenic properties at different steps of the pathway implies that autophagy must be considered in tumor progression. Autophagy as a stress response mechanism protects cancer cells from low nutrient supply or therapeutic insults. Autophagy is also involved in the elimination of cancer cells by triggering a non-apoptotic cell death program, suggesting a negative role in tumor development. These two aspects of autophagy will be discussed in this review.     

Autophagy in Neurodegenerative Diseases: From Mechanism to Therapeutic Approach

Autophagy is a lysosome-dependent intracellular degradation process that allows recycling of cytoplasmic constituents into bioenergetic and biosynthetic materials for maintenance of homeostasis. Since the function of autophagy is particularly important in various stress conditions, perturbation of autophagy can lead to cellular dysfunction and diseases. Accumulation of abnormal protein aggregates, a common cause of neurodegenerative diseases, can be reduced through autophagic degradation. Recent studies have revealed defects in autophagy in most cases of neurodegenerative disorders. Moreover, deregulated excessive autophagy can also cause neurodegeneration. Thus, healthy activation of autophagy is essential for therapeutic approaches in neurodegenerative diseases and many autophagy-regulating compounds are under development for therapeutic purposes. This review describes the overall role of autophagy in neurodegeneration, focusing on various therapeutic strategies for modulating specific stages of autophagy and on the current status of drug development.     

Autophagy,a guardian against neurodegeneration

Autophagy is an intracellular degradation process responsible for the clearance of most long-lived proteins and organelles. Cytoplasmic components are enclosed by double-membrane autophagosomes, which subsequently fuse with lysosomes for degradation. Autophagy dysfunction may contribute to the pathology of various neurodegenerative disorders, which manifest abnormal protein accumulation. As autophagy induction enhances the clearance of aggregate-prone intracytoplasmic proteins that cause neurodegeneration (like mutant huntingtin, tau and ataxin 3) and confers cytoprotective roles in cell and animal models, upregulating autophagy may be a tractable therapeutic strategy for diseases caused by such proteins. Here, we will review the molecular machinery of autophagy and its role in neurodegenerative diseases. Drugs and associated signalling pathways that may be targeted for pharmacological induction of autophagy will also be discussed.     

Rapamycin and mTOR-independent autophagy inducers ameliorate toxicity of polyglutamine-expanded huntingtin and related proteinopathies

The formation of intra-neuronal mutant protein aggregates is a characteristic of several human neurodegenerative disorders, like Alzheimer's disease, Parkinson's disease (PD) and polyglutamine disorders, including Huntington's disease (HD). Autophagy is a major clearance pathway for the removal of mutant huntingtin associated with HD, and many other disease-causing, cytoplasmic, aggregate-prone proteins. Autophagy is negatively regulated by the mammalian target of rapamycin (mTOR) and can be induced in all mammalian cell types by the mTOR inhibitor rapamycin. It can also be induced by a recently described cyclical mTOR-independent pathway, which has multiple drug targets, involving links between Ca(2+)-calpain-G(salpha) and cAMP-Epac-PLC-epsilon-IP(3) signalling. Both pathways enhance the clearance of mutant huntingtin fragments and attenuate polyglutamine toxicity in cell and animal models. The protective effects of rapamycin in vivo are autophagy-dependent. In Drosophila models of various diseases, the benefits of rapamycin are lost when the expression of different autophagy genes is reduced, implicating that its effects are not mediated by autophagy-independent processes (like mild translation suppression). Also, the mTOR-independent autophagy enhancers have no effects on mutant protein clearance in autophagy-deficient cells. In this review, we describe various drugs and pathways inducing autophagy, which may be potential therapeutic approaches for HD and related conditions.     

Unfolding the role of autophagy in the cancer metabolism

Autophagy is considered an indispensable process that scavenges toxins, recycles complex macromolecules, and sustains the essential cellular functions. In addition to its housekeeping role, autophagy plays a substantial role in many pathophysiological processes such as cancer. Certainly, it adapts cancer cells to thrive in the stress conditions such as hypoxia and starvation. Cancer cells indeed have also evolved by exploiting the autophagy process to fulfill energy requirements through the production of metabolic fuel sources and fundamentally altered metabolic pathways. Occasionally autophagy as a foe impedes tumorigenesis and promotes cell death. The complex role of autophagy in cancer makes it a potent therapeutic target and has been actively tested in clinical trials. Moreover, the versatility of autophagy has opened new avenues of effective combinatorial therapeutic strategies. Thereby, it is imperative to comprehend the specificity of autophagy in cancer-metabolism. This review summarizes the recent research and conceptual framework on the regulation of autophagy by various metabolic pathways, enzymes, and their cross-talk in the cancer milieu, including the implementation of altered metabolism and autophagy in clinically approved and experimental therapeutics.     

自噬与肿瘤关系的研究与进展

自噬是一个高度发达而且十分保守的生物学分解代谢过程。自噬与肿瘤的关系十分密切,在肿瘤发生发展的过程中,自噬活性的改变却是一把双刃剑。自噬,它既能够使肿瘤细胞耐受不同的应激条件而使其获得更好的生存,也可以通过各种信号途径减轻许多不良应激条件下的细胞损伤,如慢性炎症、慢性细胞死亡及基因组损伤等,从而而减少肿瘤的发生。再者,一方面,某些肿瘤的发生和发展过程中也同样依赖于自噬,并且肿瘤细胞可以利用自噬来对抗抗癌药物的一定的细胞毒性。而另一方面,有些癌症却需要利用自噬的作用来杀死肿瘤细胞。虽然自噬与肿瘤的关系是十分复杂的,也存在不少的分歧,但总的来说自噬在癌症中的作用是至关重要的。结合近年来国内外研究的发展,我们这篇综述重点讨论的是自噬在癌症中的作用,并且探讨其潜在的作用机制,以及目前自噬在癌症治疗中的应用。     

Autophagy: A double-edged sword in Alzheimer’s disease

Autophagy is a major protein degradation pathway that is essential for stress-induced and constitutive protein turnover. Accumulated evidence has demonstrated that amyloid-beta (A beta) protein can be generated in autophagic vacuoles, promoting its extracellular deposition in neuritic plaques as the pathological hallmark of Alzheimer's disease (AD). The molecular machinery for A beta generation, including APP, APP-C99 and beta-/gamma-secretases, are all enriched in autophagic vacuoles. The induction of autophagy can be vividly observed in the brain at early stages of sporadic AD and in an AD transgenic mouse model. Accumulated evidence has also demonstrated a neuroprotective role of autophagy in mediating the degradation of aggregated proteins that are causative of various neurodegenerative diseases. Autophagy is thus widely regarded as an intracellular hub for the removal of the detrimental A beta peptides and Tau aggregates. Nonetheless, compelling data also reveal an unfavorable function of autophagy in facilitating the production of intracellular A beta. The two faces of autophagy on the homeostasis of A beta place it in a very unique and intriguing position in AD pathogenesis. This article briefly summarizes seminal discoveries that are shedding new light on the critical and unique roles of autophagy in AD and potential therapeutic approaches against autophagy-elicited AD.     

Autophagy: A New Mechanism of Prosurvival and Drug Resistance in Multiple Myeloma

Autophagy is an intracellular self-degradative process that balances cell energy source and regulates tissue homeostasis. In physiological condition, autophagy funnels cytoplasmic constituents to autophagolysosomes for degradation and is an alternative way for cell-death behavior. Here, we inspected autophagy as a prosurvival mechanism essential for drug resistance in multiple myeloma (MM). Accordingly, autophagy inhibitors used in association to conventional anti-MM drugs might enforce the effect against resistant MM plasma cells and render autophagy a new therapeutic target.     

Novel targets for Huntington's disease in an mTOR-independent autophagy pathway

Autophagy is a major clearance route for intracellular aggregate-prone proteins causing diseases such as Huntington's disease. Autophagy induction with the mTOR inhibitor rapamycin accelerates clearance of these toxic substrates. As rapamycin has nontrivial side effects, we screened FDA-approved drugs to identify new autophagy-inducing pathways. We found that L-type Ca2+ channel antagonists, the K+ATP channel opener minoxidil, and the G(i) signaling activator clonidine induce autophagy. These drugs revealed a cyclical mTOR-independent pathway regulating autophagy, in which cAMP regulates IP3 levels, influencing calpain activity, which completes the cycle by cleaving and activating G(s)alpha, which regulates cAMP levels. This pathway has numerous potential points where autophagy can be induced, and we provide proof of principle for therapeutic relevance in Huntington's disease using mammalian cell, fly and zebrafish models. Our data also suggest that insults that elevate intracytosolic Ca2+ (like excitotoxicity) inhibit autophagy, thus retarding clearance of aggregate-prone proteins.     

Autophagy proteins promote hepatitis C virus replication

Autophagy is a fundamental process for anti-viral defense. Not surprisingly, viruses have developed strategies to subvert or use autophagy for their own benefit. In cell culture, autophagy proteins are proviral factors that favor initiation of hepatitis C virus (HCV) infection. Autophagy proteins are required for translation of incoming viral RNA. We propose that autophagy factors might support the delivery of incoming RNA to the translation apparatus and/or the recruitment of cellular factors required to initiate HCV translation.     

Spermidine promotes nucleus pulposus autophagy as a protective mechanism against apoptosis and ameliorates disc degeneration

Spermidine has therapeutic effects in many diseases including as heart diastolic function, myopathic defects and neurodegenerative disorders via autophagy activation. Autophagy has been found to mitigate cell apoptosis in intervertebral disc degeneration (IDD). Accordingly, we theorize that spermidine may have beneficial effects on IDD via autophagy stimulation. In this study, spermidine's effect on IDD was evaluated in tert‐butyl hydroperoxide (TBHP)‐treated nucleus pulposus cells of SD rats in vitro as well as in a puncture‐induced rat IDD model. We found that autophagy was actuated by spermidine in nucleus pulposus cells. In addition, spermidine treatment weakened the apoptotic effects of TBHP in nucleus pulposus cells. Spermidine increased the expression of anabolic proteins including Collagen‐II and aggrecan and decreased the expression of catabolic proteins including MMP13 and Adamts‐5. Additionally, autophagy blockade using 3‐MA reversed the beneficial impact of spermidine against nucleus pulposus cell apoptosis. Autophagy was thus important for spermidine's therapeutic effect on IDD. Spermidine‐treated rats had an accentuated T2‐weighted signal and a diminished histological degenerative grade than vehicle‐treated rats, showing that spermidine inhibited intervertebral disc degeneration in vivo. Thus, spermidine protects nucleus pulposus cells against apoptosis through autophagy activation and improves disc, which may be beneficial for the treatment of IDD.     

Role of autophagy in breast cancer

Autophagy is an evolutionarily conserved process of cytoplasm and cellular organelle degradation in lysosomes. Autophagy is a survival pathway required for cellular viability during starvation; however, if it proceeds to completion, autophagy can lead to cell death. In neurons, constitutive autophagy limits accumulation of polyubiquitinated proteins and prevents neuronal degeneration. Therefore, autophagy has emerged as a homeostatic mechanism regulating the turnover of long-lived or damaged proteins and organelles, and buffering metabolic stress under conditions of nutrient deprivation by recycling intracellular constituents. Autophagy also plays a role in tumorigenesis, as the essential autophagy regulator beclin1 is monoallelically deleted in many human ovarian, breast, and prostate cancers, and beclin1(+/-) mice are tumor-prone. We found that allelic loss of beclin1 renders immortalized mouse mammary epithelial cells susceptible to metabolic stress and accelerates lumen formation in mammary acini. Autophagy defects also activate the DNA damage response in vitro and in mammary tumors in vivo, promote gene amplification, and synergize with defective apoptosis to accelerate mammary tumorigenesis. Thus, loss of the prosurvival role of autophagy likely contributes to breast cancer progression by promoting genome damage and instability. Exploring the yet unknown relationship between defective autophagy and other breast cancer promoting functions may provide valuable insight into the pathogenesis of breast cancer and may have significant prognostic and therapeutic implications for breast cancer patients.     

Autophagy is a therapeutic target in anticancer drug resistance

Autophagy is a type of cellular catabolic degradation response to nutrient starvation or metabolic stress. The main function of autophagy is to maintain intracellular metabolic homeostasis through degradation of unfolded or aggregated proteins and organelles. Although autophagic regulation is a complicated process, solid evidence demonstrates that the PI3K-Akt-mTOR, LKB1-AMPK-mTOR and p53 are the main upstream regulators of the autophagic pathway. Currently, there is a bulk of data indicating the important function of autophagy in cancer. It is noteworthy that autophagy facilitates the cancer cells' resistance to chemotherapy and radiation treatment. The abrogation of autophagy potentiates the re-sensitization of therapeutic resistant cancer cells to the anticancer treatment via autophagy inhibitors, such as 3-MA, CQ and BA, or knockdown of the autophagy related molecules. In this review, we summarize the accumulation of evidence for autophagy's involvement in mediating resistance of cancer cells to anticancer therapy and suggest that autophagy might be a potential therapeutic target in anticancer drug resistance in the future.     

Autophagy Modulation for Alzheimer’s Disease Therapy

Autophagy is an essential and conserved lysosomal degradation pathway that controls the quality of cytoplasm by eliminating the intracellular aggregated proteins and damaged organelles. Autophagy works in mammalian target of rapamycin (mTOR)-dependent pathway or mTOR-independent pathway to keep the neuronal homeostasis. Mounting evidence has implicated the importance of defective autophagy in the pathogenesis of aging and neurodegenerative diseases, especially in Alzheimer’s disease (AD). It has also demonstrated a neuroprotective role of autophagy in mediating the degradation of amyloid beta and tau which are major factors of AD. Amounts of molecules function in either mTOR-dependent pathway or mTOR-independent pathway to induce autophagy, which maybe a potential treatment for AD. In this review, we summarize the latest studies concerning the role of autophagy in AD and explore autophagy modulation as a potential therapeutic strategy for AD. However, to date, little of the researches on autophagy have been performed to investigate the modulation in AD; more investigations need to be confirmed in the future.