Autophagy in airway diseases: a new frontier in human asthma?

The study of autophagy (‘self‐eating’), a fundamental cell fate pathway involved in physiological and pathological subcellular processes, opens a new frontier in the continuous search for novel therapies for human asthma. Asthma is a complex syndrome with different disease phenotypes. Autophagy plays a central role in cell physiology, energy and metabolism, and cell survival. Autophagy's hallmark is the formation of double‐membrane autophagic autophagosomes, and this process is operational in airway epithelial and mesenchymal cells in asthma. Genetic associations between autophagy genes and asthma have been observed including single nucleotide polymorphisms in Atg5 which correlate with reduced lung function. Immune mechanisms important in asthma such as Th2 cells and eosinophils also manifest autophagy. Lastly, we address the role of autophagy in extracellular matrix deposition and fibrosis in asthmatic airways remodeling, a pathologic process still without effective therapy, and discuss potential pharmacologic inhibitors. We end by offering two opposing but plausible hypotheses as to how autophagy may be directly involved in airway fibrosis.     

Basal autophagy decreased during the differentiation of human adult mesenchymal stem cells

Autophagy plays an important role in homeostasis, development, and disease, functioning both as a survival and cell death pathway. However, despite its importance in cell physiology, there is little information about the role of autophagy in stem cells and, in particular, on its implication in their survival and/or cell death. We describe here that in vitro, human mesenchymal stem cells (hMSCs) exhibited a high level of constitutive autophagy. Inhibitors of autophagy such as Bafilomycin A1 (Baf-A1) inhibited the proteolytic degradation associated with autophagy in these cells. In addition, we show that a knockdown in the expression of Bcl-xL is accompanied by a loss of autophagic proteolytic ability. Indeed, Bcl-xL seems to exert a tight control on autophagy regulation, since its reintroduction by a protein construct PTD-Bcl-xL resulted in the reacquisition of autophagy. We show that the suppression of autophagy through the knockdown of Bcl-xL influenced hMSC survival and differentiation. This study expands our knowledge on the control exerted by Bcl-xL on autophagy and illustrates the important role of autophagy in the maintenance and differentiation of adult hMSCs.     

Pyroptosis: An inflammatory cell death implicates in atherosclerosis

Cell death and inflammation are the fundamental biological processes in both normal physiology and pathology. Apoptosis is the most well-studied process of cell death, but there are also many other forms of cell death such as necrosis, autophagy and pyroptosis. Cell death could be observed throughout atherosclerosis and plays an important role in determining the fate of atherosclerotic lesion. Inflammation, the primary response of innate immunity, is considered essential in initiating and driving atherosclerosis. Apoptosis and autophagy had been reported in atherosclerosis, however, the mechanism of cell death involved in atherosclerosis still remain largely unknown. Cell death and inflammation are inextricably linked with their effectors modulating the process of atherosclerosis. Therefore, we proposed hypothesis that pyroptosis, an inflammatory form of cell death, may be implicated in atherosclerosis and play an important role in lesion instability.     

Autophagy in the immune system

Autophagy is an intracellular homeostatic mechanism important for the degradation of waste components from the cytoplasm in acidic lysosomal compartments. Originally, surplus parts of the cytoplasm that acted as targets for autophagy were thought to comprise cellular organelles and proteins, but this has now extended to include a range of pathogens with particular emphasis on intracellular bacteria. The finding that autophagy can sequester intracellular bacteria and mediate their destruction has opened the door to a wider role for autophagy as an effector arm of the immune system. In innate immunity, autophagy works downstream of pattern recognition receptors where it facilitates a number of effector responses, including cytokine production and phagocytosis. Autophagy is also able to intersect pathways of innate and adaptive immunity through its potential to deliver antigens for antigen presentation. Autophagy provides a substantial source of antigens for loading onto MHC class II molecules and it may be important in dendritic cells for cross‐priming to CD8⁺ T cells. In lymphocytes, autophagy is essential for cell survival and homeostasis, particularly in T cells. In the thymus, autophagy can modulate the selection of certain CD4⁺ T‐cell clones while in the bone marrow autophagy is needed for B‐cell development at specific stages. However, large holes exist in our knowledge as to how autophagy regulates, and is regulated by, the immune system and it is important to now apply what we have gleaned from in vitro studies to how autophagy operates in vivo in the setting of natural infection.     

Microparticles and autophagy: a new frontier in the understanding of atherosclerosis in rheumatoid arthritis

Microparticles (MPs) are small membrane vesicles released by many cell types under physiological and pathological conditions. In the last years, these particles were considered as inert cell debris, but recently many studies have demonstrated they could have a role in intercellular communication. Increased levels of MPs have been reported in various pathological conditions including infections, malignancies, and autoimmune diseases, such as rheumatoid arthritis (RA). RA is an autoimmune systemic inflammatory disease characterized by chronic synovial inflammation, resulting in cartilage and bone damage with accelerated atherosclerosis increasing mortality. According to the literature data, also MPs could have a role in endothelial dysfunction, contributing to atherosclerosis in RA patients. Moreover many researchers have shown that a dysregulated autophagy seems to be involved in endothelial dysfunction. Autophagy is a reparative process by which cytoplasmic components are sequestered in double-membrane vesicles and degraded on fusion with lysosomal compartments. It has been shown in many works that basal autophagy is essential to proper vascular function. Taking into account these considerations, we hypothesized that in RA patients MPs could contribute to atherosclerosis process by dysregulation of endothelial autophagy process.     

Autophagy and lipids: tightening the knot

The degradation of intracellular components in lysosomes, also known as autophagy, participates in a broad range of cellular functions from cellular quality control to cellular remodeling or as mechanism of defense against cellular aggressors. In this review, we focus on the role of autophagy as an alternative source of cellular energy, particularly important when nutrients are scarce. Almost since the discovery of autophagy, it has been known that amino acids obtained through the breakdown of proteins in lysosomes are essential to maintaining the cellular energetic balance during starvation. However, it is only recently that the ability of autophagy to mobilize intracellular lipid stores as an additional source of energy has been described. Autophagy contributes thus to modulating the amount of cellular lipids and allows cells to adapt to lipogenic stimuli. Interestingly, this interplay between autophagy and lipid metabolism is bidirectional, as changes in the intracellular lipid content also contribute to modulating autophagic activity. In this review, we describe the recent findings on the contribution of autophagy to lipid metabolism in different tissues and the consequences that impairments in autophagy have on cellular physiology. In addition, we comment on the regulatory role that lipid molecules and their modifying enzymes play on different steps of the autophagic process.     

Targeting autophagy in osteoporosis: From pathophysiology to potential therapy

Osteoporosis is a highly prevalent disorder characterized by the loss of bone mass and microarchitecture deterioration of bone tissue, attributed to various factors, including menopause (primary), aging (primary) and adverse effects of relevant medications (secondary). In recent decades, knowledge regarding the etiological mechanisms underpinning osteoporosis emphasizes that bone cellular homeostasis, including the maintenance of cell functions, differentiation, and the response to stress, is tightly regulated by autophagy, which is a cell survival mechanism for eliminating and recycling damaged proteins and organelles. With the important roles in the maintenance of cellular homeostasis and organ function, autophagy has emerged as a potential target for the prevention and treatment of osteoporosis. In this review, we update and discuss the pathophysiology of autophagy in normal bone cell life cycle and metabolism. Then, the alternations of autophagy in primary and secondary osteoporosis, and the accompanied pathological process are discussed. Finally, we discuss current strategies, limitations, and challenges involved in targeting relevant pathways and propose strategies by which such hurdles may be circumvented in the future for their translation into clinical validations and applications for the prevention and treatment of osteoporosis.     

Autophagy in parasitic protists: unique features and drug targets

Eukaryotic cells can degrade their own components, cytosolic proteins and organelles, using dedicated hydrolases contained within the acidic interior of their lysosomes. This degradative process, called autophagy, is used under starvation conditions to recycle redundant or less important macromolecules, facilitates metabolic re-modeling in response to environmental cues, and is also often important during cell differentiation. In this review, we discuss the role played by autophagy during the life cycles of the major parasitic protists. To provide context, we also provide an overview of the different forms of autophagy and the successive steps in the autophagic processes, including the proteins involved, as revealed in recent decades by studies using the model organism Saccharomyces cerevisiae, methylotrophic yeasts and mammalian cells. We describe for trypanosomatid parasites how autophagy plays a role in the differentiation from one life cycle stage to the next one and, in the case of the intracellular parasites, for virulence. For malarial parasites, although only a limited repertoire of canonical autophagy-related proteins can be detected, autophagy seems to play a role in the removal of redundant organelles important for cell invasion, when sporozoites develop into intracellular trophozoites inside the hepatocytes. The complete absence of a canonical autophagy pathway from the microaerophile Giardia lamblia is also discussed. Finally, the essential role of autophagy for differentiation and pathogenicity of some pathogenic protists suggests that the proteins involved in this process may represent new targets for drug development. Opportunities and strategies for drug design targeting autophagy proteins are discussed.     

自噬在免疫及免疫相关疾病中的研究进展

自噬是细胞内溶酶体/内体参与的,涉及细胞增殖,分化及稳态平衡调节的降解过程.遗传学研究发现其在物种进化过程中较保守,从酵母到哺乳动物细胞中均有自噬相关基因的存在,提示自噬广泛参与各类生物正常的生理过程.近年来,随着研究的不断深入,人们发现自噬在许多疾病尤其在免疫相关疾病中亦扮演着重要角色,在感染,自身免疫,肿瘤免疫中所起的作用可为今后研究提供依据.     

Autophagy in Plasma Cell Ontogeny and Malignancy

Autophagy is a highly conserved pathway that recycles cytosolic material and organelles via lysosomal degradation. Once simplistically viewed as a non-selective survival strategy in dire straits, autophagy has emerged as a tightly regulated process ensuring organelle function, proteome plasticity, cell differentiation and tissue homeostasis, with key roles in physiology and disease. Selective target recognition, mediated by specific adapter proteins, enables autophagy to orchestrate highly specialized functions in innate and adaptive immunity. Among them, the shaping of plasma cells for sustainable antibody production through a negative control on their differentiation program. Moreover, memory B cells and long-lived plasma cells require autophagy to exist. Further, the plasma cell malignancy, multiple myeloma deploys abundant autophagy, essential for homeostasis, survival and drug resistance.     

IL-37 induces autophagy in hepatocellular carcinoma cells by inhibiting the PI3K/AKT/mTOR pathway

Autophagy is an intracellular “self-eating” process that is closely related to inflammation and cellular immunity. New studies indicate that autophagy is also involved in tumor suppression. The anti-inflammatory cytokine interleukin-37 (IL-37) has been shown to have tumor-suppressive abilities in hepatocellular carcinoma (HCC). Notably, autophagy appears to play a dual role in the development of HCC and may be involved in both tumorigenesis and tumor suppression. However, the potential role of IL-37 in autophagy is currently unknown. In this study, we investigated the effect of IL-37 on autophagy in multiple HCC cell lines. In doing so, we found that IL-37 inhibits proliferation in HCC cells and also induces autophagy and apoptosis in the SMMC-7721 and Huh-7 cell lines. Further experiments revealed that IL-37 treatment reduced the levels of phosphorylated protein kinase B (p-AKT), phosphorylated mammalian target of rapamycin (p-mTOR), phosphorylated p70 ribosomal protein s6 kinase (p-p70S6K) and phosphorylated 4E-binding protein 1 (4E-BP1). Moreover, treatment with an AKT agonist, insulin-like growth factor 1 (IGF-1), reversed these IL-37-mediated effects on autophagy, and treatment with an phosphoinositide-3-kinase (PI3K)/AKT inhibitor, LY294002, mimicked the effects of IL-37. Taken together, these results indicate that IL-37 regulates autophagy in SMMC-7721 and Huh-7 cells via inhibition of the PI3K/AKT/mTOR signaling pathway.     

AMPK-mTOR-ULK1/2信号通路与自噬

自噬(autophagy)又称Ⅱ型细胞死亡,是细胞利用溶酶体降解自身受损的细胞器和大分子物质的过程.研究发现,自噬与多种疾病的发生发展密切相关,包括阿尔兹海默症、心肌病、肿瘤.AMPK-mTOR-ULK1/2信号通路对自噬具有重要的调控作用,对该通路的深入研究将有助于更好地理解细胞自噬过程,并为自噬相关疾病的治疗提供新的靶点.该文总结了AMPK-mTOR-ULK1/2信号通路中各分子的作用及其对自噬的调控机制及相关研究进展.     

Autophagy contributes to BMP type 2 receptor degradation and development of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is characterised by an increase in mean pulmonary arterial pressure which almost invariably leads to right heart failure and premature death. More than 70% of familial PAH and 20% of idiopathic PAH patients carry heterozygous mutations in the bone morphogenetic protein (BMP) type 2 receptor (BMPR2). However, the incomplete penetrance of BMPR2 mutations suggests that other genetic and environmental factors contribute to the disease. In the current study, we investigate the contribution of autophagy in the degradation of BMPR2 in pulmonary vascular cells. We demonstrate that endogenous BMPR2 is degraded through the lysosome in primary human pulmonary artery endothelial (PAECs) and smooth muscle cells (PASMCs): two cell types that play a key role in the pathology of the disease. By means of an elegant HaloTag system, we show that a block in lysosomal degradation leads to increased levels of BMPR2 at the plasma membrane. In addition, pharmacological or genetic manipulations of autophagy allow us to conclude that autophagy activation contributes to BMPR2 degradation. It has to be further investigated whether the role of autophagy in the degradation of BMPR2 is direct or through the modulation of the endocytic pathway. Interestingly, using an iPSC-derived endothelial cell model, our findings indicate that BMPR2 heterozygosity alone is sufficient to cause an increased autophagic flux. Besides BMPR2 heterozygosity, pro-inflammatory cytokines also contribute to an augmented autophagy in lung vascular cells. Furthermore, we demonstrate an increase in microtubule-associated protein 1 light chain 3 beta (MAP1LC3B) levels in lung sections from PAH induced in rats. Accordingly, pulmonary microvascular endothelial cells (MVECs) from end-stage idiopathic PAH patients present an elevated autophagic flux. Our findings support a model in which an increased autophagic flux in PAH patients contributes to a greater decrease in BMPR2 levels. Altogether, this study sheds light on the basic mechanisms of BMPR2 degradation and highlights a crucial role for autophagy in PAH. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.     

Dual roles of autophagy in the survival of Caenorhabditis elegans during starvation

Autophagy is a major pathway used to degrade long-lived proteins and organelles. Autophagy is thought to promote both cell and organism survival by providing fundamental building blocks to maintain energy homeostasis during starvation. Under different conditions, however, autophagy may instead act to promote cell death through an autophagic cell death pathway distinct from apoptosis. Although several recent papers suggest that autophagy plays a role in cell death, it is not known whether autophagy can cause the death of an organism. Furthermore, why autophagy acts in some instances to promote survival but in others to promote death is poorly understood. Here we show that physiological levels of autophagy act to promote survival in Caenorhabditis elegans during starvation, whereas insufficient or excessive levels of autophagy contribute to death. We found that inhibition of autophagy decreases survival of wild-type worms during starvation, and that muscarinic signaling regulates starvation-induced autophagy, at least in part, through the death-associated protein kinase signaling pathway. Furthermore, we found that in gpb-2 mutants, in which muscarinic signaling cannot be down-regulated, starvation induces excessive autophagy in pharyngeal muscles, which in turn, causes damage that may contribute to death. Taken together, our results demonstrate that autophagy can have either prosurvival or prodeath functions in an organism, depending on its level of activation.     

Autophagy protects the rotenone-induced cell death in α-synuclein overexpressing SH-SY5Y cells

Loss of dopaminergic cells induced by α-synuclein accumulation in substantia nigra causes the development of Parkinson's disease (PD). To date, although autophagy has been implicated in the pathology of PD, the molecular mechanism is still unclear. To study the role of autophagy in PD pathogenesis, we established stable SH-SY5Y cell lines overexpressing wild-type or mutant α-synuclein proteins (A30P or A53T). Overexpression of mutant α-synuclein induced some protein aggregates and cell death in the absence of drug. LC3-II protein, a critical marker for autophagy, was produced in an autophagy-dependent manner. The rotenone-induced cell death was interrupted by autophagy stimulation. Autophagy activation also restored the mitochondrial membrane potential (MMP) impaired by rotenone in mutant α-synuclein expressing cells. Additionally, autophagy activation significantly relieved rotenone-induced ROS accumulation and HIF-1α expression in neuronal cells expressing mutant α-synuclein proteins. These findings indicate that autophagy plays an important scavenger role against harmful influence of toxic protein aggregates produced in rotenone-treated cells.     

Autophagy in stem cell maintenance and differentiation

Autophagy is a lysosome-dependent degradation pathway that allows cells to recycle damaged or superfluous cytoplasmic content, such as proteins, organelles, and lipids. As a consequence of autophagy, the cells generate metabolic precursors for macromolecular biosynthesis or ATP generation. Deficiencies in this pathway were associated to several pathological conditions, such as neurodegenerative and cardiac diseases, cancer, and aging. The aim of this review is to summarize recent discoveries showing that autophagy also plays a critical role in stem cell maintenance and in a variety of cell differentiation processes. We also discuss a possible role for autophagy during cellular reprogramming and induced pluripotent stem (iPS) cell generation by taking advantage of ATP generation for chromatin remodeling enzyme activity and mitophagy. Finally, the significance of autophagy modulation is discussed in terms of augmenting efficiency of iPS cell generation and differentiation processes.     

Autophagy Is a Renoprotective Mechanism During in Vitro Hypoxia and in Vivo Ischemia-Reperfusion Injury

Autophagy mediates bulk degradation and recycling of cytoplasmic constituents to maintain cellular homeostasis. In response to stress, autophagy is induced and may either contribute to cell death or serve as a cell survival mechanism. Very little is known about autophagy in renal pathophysiology. This study examined autophagy and its pathological role in renal cell injury using in vitro and in vivo models of ischemia−reperfusion. We found that hypoxia (1% O₂) induced autophagy in cultured renal proximal tubular cells. Blockade of autophagy by 3-methyladenine or small-interfering RNA knockdown of Beclin-1 and ATG5 (two key autophagic genes) sensitized the tubular cells to hypoxia-induced apoptosis. In an in vitro model of ischemia−reperfusion, autophagy was not induced by anoxic (0% O₂) incubation in glucose-free buffer, but was induced during subsequent recovery/reperfusion period. In this model, suppression of autophagy also enhanced apoptosis. In vivo, autophagy was induced in kidney tissues during renal ischemia−reperfusion in mice. Autophagy was not obvious during the ischemia period, but was significantly enhanced during reperfusion. Inhibition of autophagy by chloroquine and 3-methyladenine worsened renal ischemia/reperfusion injury, as indicated by renal function, histology, and tubular apoptosis. Together, the results demonstrated autophagy induction during hypoxic and ischemic renal injury. Under these pathological conditions, autophagy may provide a protective mechanism for cell survival.Autophagy is a cellular process of “self-eating” wherein various cytoplasmic constituents are broken down and recycled through the lysosomal degradation pathway.¹ This process consists of several sequential steps, including sequestration of cytoplasmic portions by isolation membrane to form autophagosome, fusion of the autophagosome with lysosome to create an autolysosome, and degradation of the engulfed material to generate monomeric units such as amino acids.² Identification of the autophagy-related genes (ATG) in yeast and their orthologs in other organisms including mammals demonstrates that autophagy is evolutionarily conserved in all eukaryotic cells. The ATG genes constitute the core molecular machinery of autophagy and function at the different levels to regulate autophagy induction, progression, and completion.¹Autophagy occurs at basal level in most cells and contributes to the turnover of long-lived proteins and organelles to maintain intracellular homeostasis. In response to cellular stress, autophagy is up-regulated and can provide an adaptive strategy for cell survival, but may also directly or indirectly lead to cell demise.^3–6 With the dual role in life and death, autophagy is involved in various physiological processes, and more importantly, linked to the pathogenesis of a wide array of diseases, such as neurodegeneration, cancer, heart disease, aging, and infections.^1,2,6,7 However, it remains largely unknown how autophagy makes the life and death decisions of a stressed cell. Moreover, the conundrum is further complicated by the cross talk and coordinated regulation between autophagy and apoptosis.^4,5,8Despite rapid progress of autophagy research in other organ systems, the role of autophagy in the pathogenesis of renal diseases was not recognized until very recently. In 2007, Chien et al⁹ suggested the first evidence of autophagy during renal ischemia−reperfusion in rats. Subsequent work by Suzuki et al¹⁰ further showed autophagy in ischemic mouse kidneys and notably, in transplanted human kidneys. In nephrotoxic models of acute kidney injury, we and others have demonstrated autophagy during cisplatin nephrotoxicity and have suggested a role for autophagy in renoprotection.^11,12 A prosurvival role of autophagy was also shown in tubular cells during cyclosporine A nephrotoxicity.¹³ In contrast, Gozuacik et al¹⁴ suggested that autophagy may serve as a second cell killing mechanism that acts in concert with apoptosis to trigger kidney damage in tunicamycin-treated mice. A cell killing role for autophagy was also suggested by Suzuki et al¹⁰ during H₂O₂-induced renal tubular cell injury. As a result, whether autophagy is a mechanism of cell death or survival in renal pathology remains unclear.In this study, we have determined the role of autophagy in renal tubular cell injury using in vitro and in vivo models of renal ischemia−reperfusion. We show that autophagy is induced in these models. Importantly, blockade of autophagy sensitizes renal cells and tissues to injury by hypoxia and ischemia−reperfusion, suggesting a prosurvival role for autophagy.     

Autophagy mitigates metabolic stress and genome damage in mammary tumorigenesis

Autophagy is a catabolic process involving self-digestion of cellular organelles during starvation as a means of cell survival; however, if it proceeds to completion, autophagy can lead to cell death. Autophagy is also a haploinsufficient tumor suppressor mechanism for mammary tumorigenesis, as the essential autophagy regulator beclin1 is monoallelically deleted in breast carcinomas. However, the mechanism by which autophagy suppresses breast cancer remains elusive. Here we show that allelic loss of beclin1 and defective autophagy sensitized mammary epithelial cells to metabolic stress and accelerated lumen formation in mammary acini. Autophagy defects also activated the DNA damage response in vitro and in mammary tumors in vivo, promoted gene amplification, and synergized with defective apoptosis to promote mammary tumorigenesis. Therefore, we propose that autophagy limits metabolic stress to protect the genome, and that defective autophagy increases DNA damage and genomic instability that ultimately facilitate breast cancer progression.     

Autophagy plays a protective role as an anti‐oxidant system in human T cells and represents a novel strategy for induction of T‐cell apoptosis

Autophagy is an intracellular degradation system that plays an important role in T‐cell survival. However, the precise mechanism linking autophagy and cell death in primary human T cells is unclear because methods for monitoring autophagy in small numbers of primary human cells remain controversial. We established a novel method for assessing autophagy in activated human T cells using a retroviral GFP–LC3 expression system. We found that autophagy was induced after TCR stimulation and that autophagy‐defective naïve CD4⁺ T cells were susceptible to apoptosis through the intrinsic apoptotic pathway. Enhanced apoptosis of autophagy‐defective T cells resulted from accumulation of ROS due to impaired mitophagy. We also demonstrated that effector memory CD4⁺ T cells had lower autophagic activity than naïve CD4⁺ T cells, which contributed to their enhanced apoptosis due to increased ROS. Moreover, blocking autophagy increased intracellular mitochondrial volume and ROS levels in activated T cells. These results suggest a protective role of autophagy as an anti‐oxidant system in activated human T cells. The combination of an autophagy blocker and a mitochondrial electron transport chain inhibitor has a synergistic effect on T‐cell death, which could be a novel strategy for induction of T‐cell apoptosis.