The induction of autophagy by mechanical stress

The ability to respond and adapt to changes in the physical environment is a universal and essential cellular property. Here we demonstrated that cells respond to mechanical compressive stress by rapidly inducing autophagosome formation. We measured this response in both Dictyostelium and mammalian cells, indicating that this is an evolutionarily conserved, general response to mechanical stress. In Dictyostelium, the number of autophagosomes increased 20-fold within 10 min of 1 kPa pressure being applied and a similar response was seen in mammalian cells after 30 min. We showed in both cell types that autophagy is highly sensitive to changes in mechanical pressure and the response is graduated, with half-maximal responses at ~0.2 kPa, similar to other mechano-sensitive responses. We further showed that the mechanical induction of autophagy is TOR-independent and transient, lasting until the cells adapt to their new environment and recover their shape. The autophagic response is therefore part of an integrated response to mechanical challenge, allowing cells to cope with a continuously changing physical environment.     

The induction of autophagy by mechanical stress

The ability to respond and adapt to changes in the physical environment is a universal and essential cellular property. Here we demonstrated that cells respond to mechanical compressive stress by rapidly inducing autophagosome formation. We measured this response in both Dictyostelium and mammalian cells, indicating that this is an evolutionarily conserved, general response to mechanical stress. In Dictyostelium, the number of autophagosomes increased 20-fold within 10 min of 1 kPa pressure being applied and a similar response was seen in mammalian cells after 30 min. We showed in both cell types that autophagy is highly sensitive to changes in mechanical pressure and the response is graduated, with half-maximal responses at ~0.2 kPa, similar to other mechano-sensitive responses. We further showed that the mechanical induction of autophagy is TOR-independent and transient, lasting until the cells adapt to their new environment and recover their shape. The autophagic response is therefore part of an integrated response to mechanical challenge, allowing cells to cope with a continuously changing physical environment.     

力学刺激诱导细胞自噬的研究进展

自噬是细胞重要的自我保护机制,多种伤害性刺激激活的自噬具有维持细胞稳态和正常功能的作用.此外,自噬还参与调控恶性肿瘤、动脉粥样硬化等多种疾病的发生发展过程.体内细胞处于复杂的力学微环境中,力学刺激参与调控细胞自噬,如压力可诱导心肌细胞的自噬、牵张力调控运动系统多种细胞的自噬、流体剪切力可激活血管内皮细胞和肿瘤细胞的自噬.力学刺激诱导的细胞自噬依赖众多信号通路.细胞骨架作为重要的调节因子,不仅参与细胞力学信号转导,同时可参与调控细胞自噬.因此,细胞骨架与力学刺激诱导的细胞自噬密切相关.本文结合最新的研究成果,综述力学刺激对细胞自噬的影响及其分子机制,以期为研究力学刺激对细胞生物学行为的影响提供新的视角,进而为相关疾病的治疗提供新思路和分子靶点.     

Liver autophagy: physiology and pathology

Autophagy has long been thought of as a bulk degradation system in which cytoplasmic components are sequestered by double-membrane structures called autophagosomes, and the contents are then degraded after autophagosomes fuse with lysosomes. Genetic experiments in yeast identified a set of Autophagy-related (ATG) genes that are essential for autophagy. We have since elucidated many of the molecular underpinnings of autophagy and the physiologic roles of these processes in various systems. This review summarizes the physiologic roles of autophagy with a particular focus on liver autophagy based on analyses of knockout mice lacking Atg genes.     

Cell death pathology: cross-talk with autophagy and its clinical implications

Autophagy is a self-digesting mechanism that cells adopt to respond to stressful stimuli. Morphologically, cells dying by autophagy show multiple cytoplasmic double-membraned vacuoles, and, if prolonged, autophagy can lead to cell death, “autophagic cell death”. Thus, autophagy can act both as a temporary protective mechanism during a brief stressful episode and be a mode of cell death in its own right. In this mini-review we focus on recent knowledge concerning the connection between autophagy and programmed cell death, evaluating their possible implications for therapy in pathologies like cancer and neurodegeneration.     

Role of plant autophagy in stress response

Autophagy is a conserved pathway for the bulk degradation of cytoplasmic components in all eukaryotes. This process plays a critical role in the adaptation of plants to drastic changing environmental stresses such as starvation, oxidative stress, drought, salt, and pathogen invasion. This paper summarizes the current knowledge about the mechanism and roles of plant autophagy in various plant stress responses.     

溶酶体途径在细胞自噬过程中的功能意义

溶酶体具有高度保守的异质性,是细胞自噬的关键细胞器。细胞质中的蛋白质和细胞器最终在溶酶体降解,故溶酶体在维持细胞结构和功能的平衡方面起着重要生理作用。通过自噬溶酶体途径,细胞可清除某些病原体并参与抗原呈递。细胞自噬与异噬经溶酶体密切联系。自噬过程中溶酶体功能障碍与某些疾病和衰老等相关。对细胞自噬的溶酶体途径及其功能意义作了概述。     

Links between ER stress and autophagy in plants

Autophagy is a major pathway for the delivery of proteins or organelles to be degraded in the vacuole and recycled. It can be induced by abiotic stresses, senescence, and pathogen infection. Recent research has shown that autophagy is activated by ER stress. Here we review the major progress that has been made in the study of autophagy and ER stress in plants, and describe the links between ER stress and autophagy to guide further study on how autophagy is regulated in response to ER stress.     

ER stress inhibits neuronal death by promoting autophagy

Endoplasmic reticulum (ER) stress has been implicated in neurodegenerative diseases but its relationship and role in disease progression remain unclear. Using genetic and pharmacological approaches, we showed that mild ER stress ("preconditioning") is neuroprotective in Drosophila and mouse models of Parkinson disease. In addition, we found that the combination of mild ER stress and apoptotic signals triggers an autophagic response both in vivo and in vitro. We showed that when autophagy is impaired, ER-mediated protection is lost. We further demonstrated that autophagy inhibits caspase activation and apoptosis. Based on our findings, we conclude that autophagy is required for the neuroprotection mediated by mild ER stress, and therefore ER preconditioning has potential therapeutic value for the treatment of neurodegenerative diseases.     

整合素在细胞响应机械应力中的作用

机械应力在细胞生长、分化和基因表达等生理学过程和某些病理学过程中起了重要的作用.细胞粘附分子——整合素是机械信号转导中重要的跨膜分子.细胞通过整合素与胞外基质蛋白、细胞骨架蛋白以及聚焦粘附激酶等的反应,将感应的力信号转化为化学信号,从而调节细胞的生理机能,其中整合素与胞外基质蛋白之间的动态和特异性反应在细胞的机械信号转导过程中起了功能性作用.     

ER stress inhibits neuronal death by promoting autophagy

Endoplasmic reticulum (ER) stress has been implicated in neurodegenerative diseases but its relationship and role in disease progression remain unclear. Using genetic and pharmacological approaches, we showed that mild ER stress (“preconditioning”) is neuroprotective in Drosophila and mouse models of Parkinson disease. In addition, we found that the combination of mild ER stress and apoptotic signals triggers an autophagic response both in vivo and in vitro. We showed that when autophagy is impaired, ER-mediated protection is lost. We further demonstrated that autophagy inhibits caspase activation and apoptosis. Based on our findings, we conclude that autophagy is required for the neuroprotection mediated by mild ER stress, and therefore ER preconditioning has potential therapeutic value for the treatment of neurodegenerative diseases.     

Hyperosmotic stress induces autophagy and apoptosis in recombinant Chinese hamster ovary cell culture

During recombinant Chinese hamster ovary (rCHO) cell culture, various events, such as feeding with concentrated nutrient solutions or the addition of base to maintain an optimal pH, increase the osmolality of the medium. To determine the effect of hyperosmotic stress on two types of programmed cell death (PCD), apoptosis and autophagy, of rCHO cells, two rCHO cell lines, producing antibody and erythropoietin, were subjected to hyperosmotic stress resulting from NaCl addition (310–610 mOsm/kg). For both rCHO cell lines, hyperosmolality up to 610 mOsm/kg increased cleaved forms of PARP, caspase‐3, caspase‐7, and fragmentation of chromosomal DNA, confirming the previous observation that apoptosis was induced by hyperosmotic stress. Concurrently, hyperosmolality increased the level of accumulation of LC3‐II, a widely used autophagic marker, which was determined by Western blot analysis and confocal microscopy. When glucose and glutamine concentrations were measured during the cultures, glucose and glutamine concentrations in the culture medium at various osmolalities (310–610 mOsm/kg) showed no significant differences. This result suggests that induction of PCD by hyperosmotic stress occurred independently of nutrient depletion. Taken together, autophagy as well as apoptosis was observed in rCHO cells subjected to hyperosmolality. Biotechnol. Bioeng. 2010;105: 1187–1192. © 2009 Wiley Periodicals, Inc.     

COST1 balances plant growth and stress tolerance via attenuation of autophagy

ABSTRACT

In plants, macroautophagy/autophagy has been reported to function in various biotic and abiotic stress-response pathways, but few direct regulators linking stress and autophagy have yet been identified. Other than the conserved nutrient sensing kinase TOR (Target of Rapamycin), negative regulators that can directly modulate plant autophagy are unknown. We recently identified a mutant, termed cost1 (Constitutively Stressed 1), which has strong drought tolerance with constitutive induction of autophagy and broad expression of normally stress-responsive genes. The COST1 protein negatively regulates autophagy by direct interaction with the key autophagy adaptor ATG8E, thus directly linking autophagy and drought tolerance. Moreover, plant growth and development in a cost1 mutant is greatly retarded, suggesting that COST1 controls the tradeoff between growth and stress tolerance.     

Calcineurin suppresses AMPK-dependent cytoprotective autophagy in cardiomyocytes under oxidative stress

Calcineurin signalling plays a critical role in the pathogenesis of many cardiovascular diseases. Calcineurin has been proven to affect a series of signalling pathways and to exert a proapoptotic effect in cardiomyocytes. However, whether it is able to regulate autophagy remains largely unknown. Here, we report that prolonged oxidative stress-induced activation of calcineurin contributes to the attenuation of adaptive AMP-activated protein kinase (AMPK) signalling and inhibits autophagy in cardiomyocytes. Primary cardiomyocytes exhibited rapid formation of autophagosomes, microtubule-associated protein 1 light chain 3 (LC3) expression and phosphorylation of AMPK in response to hydrogen peroxide (H₂O₂) treatment. However, prolonged (12 h) H₂O₂ treatment attenuated these effects and was accompanied by a significant increase in calcineurin activity and apoptosis. Inhibition of calcineurin by FK506 restored AMPK function and LC3 expression, and decreased the extent of apoptosis caused by prolonged oxidative stress. In contrast, overexpression of the constitutively active form of calcineurin markedly attenuated the increase in LC3 induced by short-term (3 h) H₂O₂ treatment and sensitised cells to apoptosis. In addition, FK506 failed to induce autophagy and alleviate apoptosis in cardiomyocytes expressing a kinase-dead K45R AMPK mutant. Furthermore, inhibition of autophagy by 3-methylanine (3-MA) or by knockdown of the essential autophagy-related gene ATG7 abrogated the protective effect of FK506. These findings suggest a novel role of calcineurin in suppressing adaptive autophagy during oxidative stress by downregulating the AMPK signalling pathway. The results also provide insight into how altered calcineurin and autophagic signalling is integrated to control cell survival during oxidative stress and may guide strategies to prevent cardiac oxidative damage.     

Deficiency in the mitochondrial apoptotic pathway reveals the toxic potential of autophagy under ER stress conditions

Endoplasmic reticulum (ER) stress-induced cell death is normally associated with activation of the mitochondrial apoptotic pathway, which is characterized by CYCS (cytochrome c, somatic) release, apoptosome formation, and caspase activation, resulting in cell death. In this study, we demonstrate that under conditions of ER stress cells devoid of CASP9/caspase-9 or BAX and BAK1, and therefore defective in the mitochondrial apoptotic pathway, still undergo a delayed form of cell death associated with the activation of caspases, therefore revealing the existence of an alternative stress-induced caspase activation pathway. We identified CASP8/caspase-8 as the apical protease in this caspase cascade, and found that knockdown of either of the key autophagic genes, ATG5 or ATG7, impacted on CASP8 activation and cell death induction, highlighting the crucial role of autophagy in the activation of this novel ER stress-induced death pathway. In line with this, we identified a protein complex composed of ATG5, FADD, and pro-CASP8 whose assembly coincides with caspase activation and cell death induction. Together, our results reveal the toxic potential of autophagy in cells undergoing ER stress that are defective in the mitochondrial apoptotic pathway, and suggest a model in which the autophagosome functions as a platform facilitating pro-CASP8 activation. Chemoresistance, a common problem in the treatment of cancer, is frequently caused by the downregulation of key mitochondrial death effector proteins. Alternate stress-induced apoptotic pathways, such as the one described here, may become of particular relevance for tackling the problem of chemoresistance in cancer cells.     

Deficiency in the mitochondrial apoptotic pathway reveals the toxic potential of autophagy under ER stress conditions

Endoplasmic reticulum (ER) stress-induced cell death is normally associated with activation of the mitochondrial apoptotic pathway, which is characterized by CYCS (cytochrome c, somatic) release, apoptosome formation, and caspase activation, resulting in cell death. In this study, we demonstrate that under conditions of ER stress cells devoid of CASP9/caspase-9 or BAX and BAK1, and therefore defective in the mitochondrial apoptotic pathway, still undergo a delayed form of cell death associated with the activation of caspases, therefore revealing the existence of an alternative stress-induced caspase activation pathway. We identified CASP8/caspase-8 as the apical protease in this caspase cascade, and found that knockdown of either of the key autophagic genes, ATG5 or ATG7, impacted on CASP8 activation and cell death induction, highlighting the crucial role of autophagy in the activation of this novel ER stress-induced death pathway. In line with this, we identified a protein complex composed of ATG5, FADD, and pro-CASP8 whose assembly coincides with caspase activation and cell death induction. Together, our results reveal the toxic potential of autophagy in cells undergoing ER stress that are defective in the mitochondrial apoptotic pathway, and suggest a model in which the autophagosome functions as a platform facilitating pro-CASP8 activation. Chemoresistance, a common problem in the treatment of cancer, is frequently caused by the downregulation of key mitochondrial death effector proteins. Alternate stress-induced apoptotic pathways, such as the one described here, may become of particular relevance for tackling the problem of chemoresistance in cancer cells.