Autophagy is required for stem cell mobilization by G-CSF

Autophagy prevents graft failure during murine graft-versus-host disease

ABSTRACT

Autophagy is an intracellular survival process that has established roles in the long-term survival and function of hematopoietic stem cells (HSC). We investigated the contribution of autophagy to HSC fitness during allogeneic transplantation and graft-versus-host disease (GVHD). We demonstrate in vitro that both tumor necrosis factor and IL-1β, major components of GVHD cytokine storm, synergistically promote autophagy in both HSC and their more mature hematopoietic progenitor cells (HPC). In vivo we demonstrate that autophagy is increased in donor HSC and HPC during GVHD. Competitive transplant experiments demonstrated that autophagy-deficient cells display reduced capacity to reconstitute the hematopoietic system compared to wild-type counterparts. In a major histocompatibility complex-mismatched model of GVHD and associated cytokine dysregulation, we demonstrate that autophagy-deficient HSC and progenitors fail to establish durable hematopoiesis, leading to primary graft failure and universal transplant related mortality. Using several different models, we confirm that autophagy activity is increased in early progenitor and HSC populations in the presence of T-cell-derived inflammatory cytokines and that these HSC populations require autophagy to survive. Thus, autophagy serves as a key survival mechanism in HSC and progenitor populations after allogeneic stem cell transplant and may represent a therapeutic target to prevent graft failure during GVHD.

CD200Rhigh neutrophils with dysfunctional autophagy establish systemic immunosuppression by increasing regulatory T cells

Distinct neutrophil populations arise during certain pathological conditions. The generation of dysfunctional neutrophils during sepsis and their contribution to septicemia-related systemic immune suppression remain unclear. In this study, using an experimental sepsis model that features immunosuppression, we identified a novel population of pathogenic CD200Rhigh neutrophils that are generated during the initial stages of sepsis and contribute to systemic immune suppression by enhancing regulatory T (Treg) cells. Compared to their CD200Rlow counterparts, sepsis-generated CD200Rhigh neutrophils exhibit impaired autophagy and dysfunction, with reduced chemotactic migration, superoxide anion production, and TNF-α production. Increased soluble CD200 blocks autophagy and neutrophil maturation in the bone marrow during experimental sepsis, and recombinant CD200 treatment in vitro can induce neutrophil dysfunction similar to that observed in CD200Rhigh neutrophils. The administration of an α-CD200R antibody effectively reversed neutrophil dysfunction by enhancing autophagy and protecting against a secondary infection challenge, leading to increased survival. Transcriptome analysis revealed that CD200Rhigh neutrophils expressed high levels of Igf1, which elicits the generation of Treg cells, while the administration of an α-CD200R antibody inhibited Treg cell generation in a secondary infection model. Taken together, our findings revealed a novel CD200Rhigh neutrophil population that mediates the pathogenesis of sepsis-induced systemic immunosuppression by generating Treg cells.

Efficiently normalizing leukopoiesis by gadofullerene nanoparticles to ameliorate radiation-triggered myelosuppression

Myelosuppression is a predominant side-effect of radiotherapy, which manifests as the lower activity of blood cell precursors in bone marrow. Though progress in anti-myelosuppression has been made by the application of growth factors e.g., the granulocyte colony-stimulating factor (G-CSF), the side-effects (e.g., bone-pain, liver injury, and lung toxicity) limit their applications in clinic. Herein, we developed a strategy of efficiently normalizing leukopoiesis using gadofullerene nanoparticles (GFNPs) against myelosuppression triggered by radiation. Specifically, GFNPs with high radical-scavenging abilities elevated the generation of leukocytes and alleviated the bone marrow's pathological state under myelosuppression. Notably, GFNPs potentiated the differentiation, development, and maturation of leukocytes (neutrophils, lymphocytes) in radiation bearing mice even better than what G-CSF did. In addition, GFNPs had little toxicity towards the main organs including the heart, liver, spleen, lung, and kidney. This work provides an in-depth understanding of how advanced nanomaterials mitigate myelosuppression by regulating leukopoiesis.

Autophagy and Its Lineage-Specific Roles in the Hematopoietic System

Autophagy is a dynamic process that regulates the selective and nonselective degradation of cytoplasmic components, such as damaged organelles and protein aggregates inside lysosomes to maintain tissue homeostasis. Different types of autophagy including macroautophagy, microautophagy, and chaperon-mediated autophagy (CMA) have been implicated in a variety of pathological conditions, such as cancer, aging, neurodegeneration, and developmental disorders. Furthermore, the molecular mechanism and biological functions of autophagy have been extensively studied in vertebrate hematopoiesis and human blood malignancies. In recent years, the hematopoietic lineage-specific roles of different autophagy-related (ATG) genes have gained more attention. The evolution of gene-editing technology and the easy access nature of hematopoietic stem cells (HSCs), hematopoietic progenitors, and precursor cells have facilitated the autophagy research to better understand how ATG genes function in the hematopoietic system. Taking advantage of the gene-editing platform, this review has summarized the roles of different ATGs at the hematopoietic cell level, their dysregulation, and pathological consequences throughout hematopoiesis.

The role of autophagy in acute myeloid leukemia development

INTRODUCTION

Autophagy is a highly conservative self-degradative process. It aims at elimination-impaired proteins and cellular organelles. Previous research confirmed the autophagy role in cancer pathogenesis.

AREAS COVERED

This article discusses the role of autophagy in the development of AML. Autophagy seems to be a 'double-sword' mechanism, hence, either its suppression or induction could promote neoplasm growth. This mechanism could also be the aim of the 'molecular targeted therapy.' Chemo- and radiotherapy induce cellular stress in neoplasm cells with subsequent autophagy suppression. Simultaneously, it is claimed that the autophagy suppression increases chemosensitivity 'in neoplastic cells. Some agents, like bortezomib, in turn could promote autophagy process, e.g. in AML (acute myeloid leukemia). However, currently there are not many studies focusing on the role of autophagy in patients suffering for AML. In this review, we summarize the research done so far on the role of autophagy in the development of AML.

EXPERT OPINION

The analysis of autophagy genes expression profiling in AML could be a relevant factor in the diagnostic process and treatment 'individualization.' Autophagy modulation seems to be a relevant target in the oncological therapy - it could limit disease progression and increase the effectiveness of treatment.

Unraveling the mystery: How bad is BAG3 in hematological malignancies?

BAG3, also known as BIS and CAIR-1, interacts with Hsp70 via its BAG domain and with other molecules through its WW domain, PXXP repeats and IPV motifs. BAG3 can participate in major cellular pathways including apoptosis, autophagy, cytoskeleton structure, and motility by regulating the expression, location, and activity of its chaperone proteins. As a multifunctional protein, BAG3 is highly expressed in skeletal muscle, cardiomyocytes and multiple tumors, and its intracellular expression can be stimulated by stress. The functions and mechanisms of BAG3 in hematological malignancies have recently been a topic of interest. BAG3 has been confirmed to be involved in the development and chemoresistance of hematological malignancies and to act as a prognostic indicator. Modulation of BAG3 and its corresponding proteins has thus emerged as a promising therapeutic and experimental target. In this review, we consider the characteristics of BAG3 in hematological malignancies as a reference for further clinical and fundamental investigations.

Autophagy-driven neutrophil extracellular traps: The dawn of sepsis

Sepsis is a systemic inflammatory syndrome caused by infection disorders. The core mechanism of sepsis is immune dysfunction. Neutrophils are the most abundant circulating white blood cells, which play a crucial role in mediating the innate immune response. Previous studies have shown that an effective way to treat sepsis is through the regulation of neutrophil functions. Autophagy, a highly conserved degradation process, is responsible for removing denatured proteins or damaged organelles within cells and protecting cells from external stimuli. It is a key homeostasis process that promotes neutrophil function and differentiation. Autophagy has been shown to be closely associated with inflammation and immunity. Neutrophils, the first line of innate immunity, migrate to inflammatory sites upon their activation. Neutrophil-mediated autophagy may participate in the clinical course of sepsis. In this review, we summarized and analyzed the latest research findings on the changes in neutrophil external traps during sepsis, the regulatory role of autophagy in neutrophil, and the potential application of autophagy-driven NETs in sepsis, so as to guide clinical treatment of sepsis.

Metabolic regulation of somatic stem cells in vivo

Metabolism has been studied mainly in cultured cells or at the level of whole tissues or whole organisms in vivo. Consequently, our understanding of metabolic heterogeneity among cells within tissues is limited, particularly when it comes to rare cells with biologically distinct properties, such as stem cells. Stem cell function, tissue regeneration and cancer suppression are all metabolically regulated, although it is not yet clear whether there are metabolic mechanisms unique to stem cells that regulate their activity and function. Recent work has, however, provided evidence that stem cells do have a metabolic signature that is distinct from that of restricted progenitors and that metabolic changes influence tissue homeostasis and regeneration. Stem cell maintenance throughout life in many tissues depends upon minimizing anabolic pathway activation and cell division. Consequently, stem cell activation by tissue injury is associated with changes in mitochondrial function, lysosome activity and lipid metabolism, potentially at the cost of eroding self-renewal potential. Stem cell metabolism is also regulated by the environment: stem cells metabolically interact with other cells in their niches and are able to sense and adapt to dietary changes. The accelerating understanding of stem cell metabolism is revealing new aspects of tissue homeostasis with the potential to promote tissue regeneration and cancer suppression.

GM-CSF impairs erythropoiesis by disrupting erythroblastic island formation via macrophages

Anemia is a significant complication of chronic inflammation and may be related to dysregulated activities among erythroblastic island (EBI) macrophages. GM-CSF was reported to be upregulated and attracted as a therapeutic target in many inflammatory diseases. Among EBIs, we found that the GM-CSF receptor is preferentially and highly expressed among EBI macrophages but not among erythroblasts. GM-CSF treatment significantly decreases human EBI formation in vitro by decreasing the adhesion molecule expression of CD163. RNA-sequence analysis suggests that GM-CSF treatment impairs the supporting function of human EBI macrophages during erythropoiesis. GM-CSF treatment also polarizes human EBI macrophages from M2-like type to M1-like type. In addition, GM-CSF decreases mouse bone marrow (BM) erythroblasts as well as EBI macrophages, leading to a reduction in EBI numbers. In defining the molecular mechanism at work, we found that GM-CSF treatment significantly decreases the adhesion molecule expression of CD163 and Vcam1 in vivo. Importantly, GM-CSF treatment also decreases the phagocytosis rate of EBI macrophages in mouse BM as well as decreases the expression of the engulfment-related molecules Mertk, Axl, and Timd4. In addition, GM-CSF treatment polarizes mouse BM EBI macrophages from M2-like type to M1-like type. Thus, we document that GM-CSF impairs EBI formation in mice and humans. Our findings support that targeting GM-CSF or reprogramming EBI macrophages might be a novel strategy to treat anemia resulting from inflammatory diseases.

Loss of Atg2b and Gskip impairs the maintenance of the hematopoietic stem cell pool size

A germ line copy number duplication of chromosome 14q32, which contains ATG2B and GSKIP, was identified in families with myeloproliferative neoplasm (MPN). Here, we show that mice lacking both Atg2b and Gskip, but not either alone, exhibited decreased hematopoiesis, resulting in death in utero accompanied by anemia. In marked contrast to MPN patients with duplication of ATG2B and GSKIP, the number of hematopoietic stem cells (HSCs), in particular long-term HSCs, in double-knockout fetal livers was significantly decreased due to increased cell death. Although the remaining HSCs still had the ability to differentiate into hematopoietic progenitor cells, the differentiation efficiency was quite low. Remarkably, mice with knockout of Atg2b or Gskip alone did not show any hematopoietic abnormality. Mechanistically, while loss of both genes had no effect on autophagy, it increased the expression of genes encoding enzymes involved in oxidative phosphorylation. Taken together, our results indicate that Atg2b and Gskip play a synergistic effect in maintaining the pool size of HSCs.

Prognosis and Characterization of Immune Microenvironment in Acute Myeloid Leukemia Through Identification of an Autophagy-Related Signature

Acute myeloid leukemia (AML) is one of the most common hematopoietic malignancies that has an unfavorable outcome and a high rate of relapse. Autophagy plays a vital role in the development of and therapeutic responses to leukemia. This study identifies a potential autophagy-related signature to monitor the prognoses of patients of AML. Transcriptomic profiles of AML patients (GSE37642) with the relevant clinical information were downloaded from Gene Expression Omnibus (GEO) as the training set while TCGA-AML and GSE12417 were used as validation cohorts. Univariate regression analyses and multivariate stepwise Cox regression analysis were respectively applied to identify the autophagy-related signature. The univariate Cox regression analysis identified 32 autophagy-related genes (ARGs) that were significantly associated with the overall survival (OS) of the patients, and were mainly rich in signaling pathways for autophagy, p53, AMPK, and TNF. A prognostic signature that comprised eight ARGs (BAG3, CALCOCO2, CAMKK2, CANX, DAPK1, P4HB, TSC2, and ULK1) and had good predictive capacity was established by LASSO–Cox stepwise regression analysis. High-risk patients were found to have significantly shorter OS than patients in low-risk group. The signature can be used as an independent prognostic predictor after adjusting for clinicopathological parameters, and was validated on two external AML sets. Differentially expressed genes analyzed in two groups were involved in inflammatory and immune signaling pathways. An analysis of tumor-infiltrating immune cells confirmed that high-risk patients had a strong immunosuppressive microenvironment. Potential druggable OS-related ARGs were then investigated through protein–drug interactions. This study provides a systematic analysis of ARGs and develops an OS-related prognostic predictor for AML patients. Further work is needed to verify its clinical utility and identify the underlying molecular mechanisms in AML.

Interactions between Autophagy, Proinflammatory Cytokines, and Apoptosis in Neuropathic Pain: Granulocyte Colony Stimulating Factor as a Multipotent Therapy in Rats with Chronic Constriction Injury

Our previous studies have shown that early systemic granulocyte colony-stimulating factor (G-CSF) treatment can attenuate neuropathic pain in rats with chronic constriction injury (CCI) by modulating expression of different proinflammatory cytokines, microRNAs, and proteins. Besides the modulation of inflammatory mediators' expression, previous studies have also reported that G-CSF can modulate autophagic and apoptotic activity. Furthermore, both autophagy and apoptosis play important roles in chronic pain modulation. In this study, we evaluated the temporal interactions of autophagy, and apoptosis in the dorsal root ganglion (DRG) and injured sciatic nerve after G-CSF treatment in CCI rats. We studied the behaviors of CCI rats with or without G-CSF treatment and the various levels of autophagic, proinflammatory, and apoptotic proteins in injured sciatic nerves and DRG neurons at different time points using Western blot analysis and immunohistochemical methods. The results showed that G-CSF treatment upregulated autophagic protein expression in the early phase and suppressed apoptotic protein expression in the late phase after nerve injury. Thus, medication such as G-CSF can modulate autophagy, apoptosis, and different proinflammatory proteins in the injured sciatic nerve and DRG neurons, which have the potential to treat neuropathic pain. However, autophagy-mediated regulation of neuropathic pain is a time-dependent process. An increase in autophagic activity in the early phase before proinflammatory cytokines reach the threshold level to induce neuropathic pain can effectively alleviate further neuropathic pain development.

Uncovering the multifaceted roles played by neutrophils in allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (alloHSCT) is a life-saving procedure used for the treatment of selected hematological malignancies, inborn errors of metabolism, and bone marrow failures. The role of neutrophils in alloHSCT has been traditionally evaluated only in the context of their ability to act as a first line of defense against infection. However, recent evidence has highlighted neutrophils as key effectors of innate and adaptive immune responses through a wide array of newly discovered functions. Accordingly, neutrophils are emerging as highly versatile cells that are able to acquire different, often opposite, functional capacities depending on the microenvironment and their differentiation status. Herein, we review the current knowledge on the multiple functions that neutrophils exhibit through the different stages of alloHSCT, from the hematopoietic stem cell (HSC) mobilization in the donor to the immunological reconstitution that occurs in the recipient following HSC infusion. We also discuss the influence exerted on neutrophils by the immunosuppressive drugs delivered in the course of alloHSCT as part of graft-versus-host disease (GVHD) prophylaxis. Finally, the potential involvement of neutrophils in alloHSCT-related complications, such as transplant-associated thrombotic microangiopathy (TA-TMA), acute and chronic GVHD, and cytomegalovirus (CMV) reactivation, is also discussed. Based on the data reviewed herein, the role played by neutrophils in alloHSCT is far greater than a simple antimicrobial role. However, much remains to be investigated in terms of the potential functions that neutrophils might exert during a highly complex procedure such as alloHSCT.

The Role of Metabolic Enzymes in the Regulation of Inflammation

Immune cells undergo dramatic metabolic reprogramming in response to external stimuli. These metabolic pathways, long considered as simple housekeeping functions, are increasingly understood to critically regulate the immune response, determining the activation, differentiation, and downstream effector functions of both lymphoid and myeloid cells. Within the complex metabolic networks associated with immune activation, several enzymes play key roles in regulating inflammation and represent potential therapeutic targets in human disease. In some cases, these enzymes control flux through pathways required to meet specific energetic or metabolic demands of the immune response. In other cases, key enzymes control the concentrations of immunoactive metabolites with direct roles in signaling. Finally, and perhaps most interestingly, several metabolic enzymes have evolved moonlighting functions, with roles in the immune response that are entirely independent of their conventional enzyme activities. Here, we review key metabolic enzymes that critically regulate inflammation, highlighting mechanistic insights and opportunities for clinical intervention.

Autophagy-Sirt3 axis decelerates hematopoietic aging

Autophagy suppresses mitochondrial metabolism to preserve hematopoietic stem cells (HSCs) in mice. However, the mechanism by which autophagy regulates hematopoietic aging, in particular in humans, has largely been unexplored. Here, we demonstrate that reduction of autophagy in both hematopoietic cells and their stem cells is associated with aged hematopoiesis in human population. Mechanistically, autophagy delays hematopoietic aging by activating the downstream expression of Sirt3, a key mitochondrial protein capable of rejuvenating blood. Sirt3 is the most abundant Sirtuin family member in HSC‐enriched population, though it declines as the capacity for autophagy deteriorates with aging. Activation of autophagy upregulates Sirt3 in wild‐type mice, whereas in autophagy‐defective mice, Sirt3 expression is crippled in the entire hematopoietic hierarchy, but forced expression of Sirt3 in HSC‐enriched cells reduces oxidative stress and prevents accelerated hematopoietic aging from autophagy defect. Importantly, the upregulation of Sirt3 by manipulation of autophagy is validated in human HSC‐enriched cells. Thus, our results identify an autophagy‐Sirt3 axis in regulating hematopoietic aging and suggest a possible interventional solution to human blood rejuvenation via activation of the axis.

The NAD-Booster Nicotinamide Riboside Potently Stimulates Hematopoiesis through Increased Mitochondrial Clearance

It has been recently shown that increased oxidative phosphorylation, as reflected by increased mitochondrial activity, together with impairment of the mitochondrial stress response, can severely compromise hematopoietic stem cell (HSC) regeneration. Here we show that the NAD⁺-boosting agent nicotinamide riboside (NR) reduces mitochondrial activity within HSCs through increased mitochondrial clearance, leading to increased asymmetric HSC divisions. NR dietary supplementation results in a significantly enlarged pool of progenitors, without concurrent HSC exhaustion, improves survival by 80%, and accelerates blood recovery after murine lethal irradiation and limiting-HSC transplantation. In immune-deficient mice, NR increased the production of human leucocytes from hCD34+ progenitors. Our work demonstrates for the first time a positive effect of NAD⁺-boosting strategies on the most primitive blood stem cells, establishing a link between HSC mitochondrial stress, mitophagy, and stem-cell fate decision, and unveiling the potential of NR to improve recovery of patients suffering from hematological failure including post chemo- and radiotherapy.

Autophagy-mediated regulation of neutrophils and clinical applications

Autophagy, an adaptive catabolic process, plays a cytoprotective role in enabling cellular homeostasis in the innate and adaptive immune systems. Neutrophils, the most abundant immune cells in circulation, are professional killers that orchestrate a series of events during acute inflammation. The recent literature indicates that autophagy has important roles in regulating neutrophil functions, including differentiation, degranulation, metabolism and neutrophil extracellular trap formation, that dictate neutrophil fate. It is also becoming increasingly clear that autophagy regulation is critical for neutrophils to exert their immunological activity. However, evidence regarding the systematic communication between neutrophils and autophagy is insufficient. Here, we provide an updated overview of the function of autophagy as a regulator of neutrophils and discuss its clinical relevance to provide novel insight into potentially relevant treatment strategies.

Autophagy in the renewal, differentiation and homeostasis of immune cells

Across all branches of the immune system, the process of autophagy is fundamentally important in cellular development, function and homeostasis. Strikingly, this evolutionarily ancient pathway for intracellular recycling has been adapted to enable a high degree of functional complexity and specialization. However, although the requirement for autophagy in normal immune cell function is clear, the mechanisms involved are much less so and encompass control of metabolism, selective degradation of substrates and organelles and participation in cell survival decisions. We review here the crucial functions of autophagy in controlling the differentiation and homeostasis of multiple immune cell types and discuss the potential mechanisms involved.

Elevated VMP1 expression in acute myeloid leukemia amplifies autophagy and is protective against venetoclax-induced apoptosis

Vacuole membrane protein (VMP1) is a putative autophagy protein, which together with Beclin-1 acts as a molecular switch in activating autophagy. In the present study the role of VMP1 was analysed in CD34⁺ cells of cord blood (CB) and primary acute myeloid leukemia (AML) cells and cell lines. An increased expression of VMP1 was observed in a subset of AML patients. Functional studies in normal CB CD34⁺ cells indicated that inhibiting VMP1 expression reduced autophagic-flux, coinciding with reduced expansion of hematopoietic stem and progenitor cells (HSPC), delayed differentiation, increased apoptosis and impaired in vivo engraftment. Comparable results were observed in leukemic cell lines and primary AML CD34⁺ cells. Ultrastructural analysis indicated that leukemic cells overexpressing VMP1 displayed a reduced number of mitochondrial structures, while the number of lysosomal degradation structures was increased. The overexpression of VMP1 did not affect cell proliferation and differentiation, but increased autophagic-flux and improved mitochondrial quality, which coincided with an increased threshold for venetoclax-induced loss of mitochondrial outer membrane permeabilization (MOMP) and apoptosis. In conclusion, our data indicate that in leukemic cells high VMP1 is involved with mitochondrial quality control.

Imaging Flow Cytometry to Assess Antigen-Presenting-Cell Function

This unit describes methods for quantifying phagocytosis and imaging the immunological synapse between T cells and antigen-presenting cells (APCs), with both techniques delivering valuable information about APC function. These aspects of APC biology have traditionally been challenging to quantify, and imaging flow cytometry, which harnesses the high-throughput nature of flow cytometry combined with the capacity of microscopy to deliver spatial localization, facilitates analysis of these APC functions in a fashion that was previously not possible. Imaging flow cytometry allows large numbers of events to be captured and large amounts of fluorescence data to be quantified at the physical location of markers of interest, both on the cell surface and in intracellular compartments, combining key features of traditional flow cytometry and fluorescence microscopy. © 2019 by John Wiley & Sons, Inc.

Tumor-derived exosomes induce N2 polarization of neutrophils to promote gastric cancer cell migration

Background

Exosomes are extracellular vesicles that mediate cellular communication in health and diseases. Neutrophils could be polarized to a pro-tumor phenotype by tumor. The function of tumor-derived exosomes in neutrophil regulation remains unclear.

Methods

We investigated the effects of gastric cancer cell-derived exosomes (GC-Ex) on the pro-tumor activation of neutrophils and elucidated the underlying mechanisms.

Results

GC-Ex prolonged neutrophil survival and induced expression of inflammatory factors in neutrophils. GC-Ex-activated neutrophils, in turn, promoted gastric cancer cell migration. GC-Ex transported high mobility group box-1 (HMGB1) that activated NF-κB pathway through interaction with TLR4, resulting in an increased autophagic response in neutrophils. Blocking HMGB1/TLR4 interaction, NF-κB pathway, and autophagy reversed GC-Ex-induced neutrophil activation. Silencing HMGB1 in gastric cancer cells confirmed HMGB1 as a key factor for GC-Ex-mediated neutrophil activation. Furthermore, HMGB1 expression was upregulated in gastric cancer tissues. Increased HMGB1 expression was associated with poor prognosis in patients with gastric cancer. Finally, gastric cancer tissue-derived exosomes acted similarly as exosomes derived from gastric cancer cell lines in neutrophil activation.

Conclusion

We demonstrate that gastric cancer cell-derived exosomes induce autophagy and pro-tumor activation of neutrophils via HMGB1/TLR4/NF-κB signaling, which provides new insights into mechanisms for neutrophil regulation in cancer and sheds lights on the multifaceted role of exosomes in reshaping tumor microenvironment.

Electronic supplementary material

The online version of this article (10.1186/s12943-018-0898-6) contains supplementary material, which is available to authorized users.

Influence of diet and metabolism on hematopoietic stem cells and leukemia development following ionizing radiation exposure

PURPOSE

The review aims to discuss the prominence of dietary and metabolic regulators in maintaining hematopoietic stem cell (HSC) function, long-term self-renewal, and differentiation.

RESULTS

Most adult stem cells are preserved in a quiescent, nonmotile state in vivo which acts as a "protective state" for stem cells to reduce endogenous stress provoked by DNA replication and cellular respiration as well as exogenous environmental stress. The dynamic balance between quiescence, self-renewal and differentiation is critical for supporting a functional blood system throughout life of an organism. Stress-conditions, for example ionizing radiation exposure can trigger the blood forming HSCs to proliferate and migrate through extramedullary tissues to expand the number of HSCs and increase hematopoiesis. In addition, a wealth of investigation validated that deregulation of this balance plays a critical pathogenic role in various different hematopoietic diseases including the leukemia development.

CONCLUSION

The review summarizes the current knowledge on how alterations in dietary and metabolic factors could alter the risk of leukemia development following ionizing radiation exposure by inhibiting or even reversing the leukemic progression. Understanding the influence of diet, metabolism, and epigenetics on radiation-induced leukemogenesis may lead to the development of practical interventions to reduce the risk in exposed populations.

Brd4 regulates the expression of essential autophagy genes and Keap1 in AML cells

We have recently reported that activation of Brd4 is associated with the presence of autophagy in NPMc+ and MLL AML cells. In order to determine the mechanisms underlying this relationship, we have examined the role of Brd4 in regulating the expression of several genes that are central to the process of autophagy. We found that Brd4 binds to the promoters of ATG 3, 7 and CEBPβ, and expression of these genes is markedly reduced by inhibitors of Brd4, as well as by Brd4-shRNA and depletion of CEBPβ. Inhibitors of Brd4 also dramatically suppress the transcription of Keap1, thereby increasing the expression of anti-oxidant genes through the Nrf2 pathway and reducing the cytotoxicity induced by Brd4 inhibitors. Elimination of ATG3 or KEAP1 expression using CRISPR-cas9 mediated genomic editing markedly reduced autophagy. We conclude that Brd4 plays a significant role in autophagy activation through the direct transcriptional regulation of genes essential for it, as well as through the Keap1-Nrf2 axis in NPMc+ and MLL-fusion AML cells.

Autophagy dictates metabolism and differentiation of inflammatory immune cells

The role of macroautophagy/autophagy, a conserved lysosomal degradation pathway, during cellular differentiation has been well studied over the last decade. In particular, evidence for its role during immune cell differentiation is growing. Despite the description of a variety of dramatic immune phenotypes in tissue-specific autophagy knockout models, the underlying mechanisms are still under debate. One of the proposed mechanisms is the impact of autophagy on the altered metabolic states during immune cell differentiation. This concept is strengthened through novel molecular insights into how AMPK and MTOR signaling cascades affect both autophagy and metabolism. In this review, we discuss direct and indirect evidence linking autophagy, metabolic pathways and immune cell differentiation including T, B, and innate lymphocytes as well as in myeloid cells that are direct mediators of inflammation. Herein, we propose a model for autophagy-driven immunometabolism controlling immune cell differentiation.

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

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

Autophagy maintains the metabolism and function of young and old stem cells

With age, hematopoietic stem cells (HSCs) lose their ability to regenerate the blood system, and promote disease development. Autophagy is associated with health and longevity, and is critical for protecting HSCs from metabolic stress. Here, we show that loss of autophagy in HSCs causes accumulation of mitochondria and an activated metabolic state, which drives accelerated myeloid differentiation mainly through epigenetic deregulations, and impairs HSC self-renewal activity and regenerative potential. Strikingly, the majority of HSCs in aged mice share these altered metabolic and functional features. However, ~ 1/3 of aged HSCs exhibit high autophagy levels and maintain a low metabolic state with robust long-term regeneration potential similar to healthy young HSCs. Our results demonstrate that autophagy actively suppresses HSC metabolism by clearing active, healthy mitochondria to maintain quiescence and stemness, and becomes increasingly necessary with age to preserve the regenerative capacity of old HSCs.

Metabolic effects of fasting on human and mouse blood in vivo

Starvation is a strong physiological stimulus of macroautophagy/autophagy. In this study, we addressed the question as to whether it would be possible to measure autophagy in blood cells after nutrient deprivation. Fasting of mice for 48 h (which causes ∼20% weight loss) or starvation of human volunteers for up to 4 d (which causes <2% weight loss) provokes major changes in the plasma metabolome, yet induces only relatively minor alterations in the intracellular metabolome of circulating leukocytes. White blood cells from mice and human volunteers responded to fasting with a marked reduction in protein lysine acetylation, affecting both nuclear and cytoplasmic compartments. In circulating leukocytes from mice that underwent 48-h fasting, an increase in LC3B lipidation (as assessed by immunoblotting and immunofluorescence) only became detectable if the protease inhibitor leupeptin was injected 2 h before drawing blood. Consistently, measurement of an enhanced autophagic flux was only possible if white blood cells from starved human volunteers were cultured in the presence or absence of leupeptin. Whereas all murine leukocyte subpopulations significantly increased the number of LC3B⁺ puncta per cell in response to nutrient deprivation, only neutrophils from starved volunteers showed signs of activated autophagy (as determined by a combination of multi-color immunofluorescence, cytofluorometry and image analysis). Altogether, these results suggest that white blood cells are suitable for monitoring autophagic flux. In addition, we propose that the evaluation of protein acetylation in circulating leukocytes can be adopted as a biochemical marker of organismal energetic status.

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

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

Autophagy-dependent regulatory T cells are critical for the control of graft-versus-host disease

Regulatory T cells (Tregs) play a crucial role in the maintenance of peripheral tolerance. Quantitative and/or qualitative defects in Tregs result in diseases such as autoimmunity, allergy, malignancy, and graft-versus-host disease (GVHD), a serious complication of allogeneic stem cell transplantation (SCT). We recently reported increased expression of autophagy-related genes (Atg) in association with enhanced survival of Tregs after SCT. Autophagy is a self-degradative process for cytosolic components that promotes cell homeostasis and survival. Here, we demonstrate that the disruption of autophagy within FoxP3⁺ Tregs (B6.Atg7^fl/fl-FoxP3cre⁺ ) resulted in a profound loss of Tregs, particularly within the bone marrow (BM). This resulted in dysregulated effector T cell activation and expansion, and the development of enterocolitis and scleroderma in aged mice. We show that the BM compartment is highly enriched in TIGIT⁺ Tregs and that this subset is differentially depleted in the absence of autophagy. Moreover, following allogeneic SCT, recipients of grafts from B6.Atg7^fl/fl-FoxP3cre⁺ donors exhibited reduced Treg reconstitution, exacerbated GVHD, and reduced survival compared with recipients of B6.WT-FoxP3cre⁺ grafts. Collectively, these data indicate that autophagy-dependent Tregs are critical for the maintenance of tolerance after SCT and that the promotion of autophagy represents an attractive immune-restorative therapeutic strategy after allogeneic SCT.

Cytoprotective autophagy maintains leukemia-initiating cells in murine myeloid leukemia

Autophagy is required for maintenance of AML-initiating cells and peripheral myeloblast survival. Loss of autophagy potentiates the therapeutic effects of AraC in vivo.

G-CSF maintains controlled neutrophil mobilization during acute inflammation by negatively regulating CXCR2 signaling

Luo et al. report that CXCR2 ligands are responsible for rapid neutrophil mobilization during early-stage acute inflammation and that G-CSF suppresses this mobilization by negatively regulating CXCR2-mediated intracellular signaling.

Cytokine-induced neutrophil mobilization from the bone marrow to circulation is a critical event in acute inflammation, but how it is accurately controlled remains poorly understood. In this study, we report that CXCR2 ligands are responsible for rapid neutrophil mobilization during early-stage acute inflammation. Nevertheless, although serum CXCR2 ligand concentrations increased during inflammation, neutrophil mobilization slowed after an initial acute fast phase, suggesting a suppression of neutrophil response to CXCR2 ligands after the acute phase. We demonstrate that granulocyte colony-stimulating factor (G-CSF), usually considered a prototypical neutrophil-mobilizing cytokine, was expressed later in the acute inflammatory response and unexpectedly impeded CXCR2-induced neutrophil mobilization by negatively regulating CXCR2-mediated intracellular signaling. Blocking G-CSF in vivo paradoxically elevated peripheral blood neutrophil counts in mice injected intraperitoneally with Escherichia coli and sequestered large numbers of neutrophils in the lungs, leading to sterile pulmonary inflammation. In a lipopolysaccharide-induced acute lung injury model, the homeostatic imbalance caused by G-CSF blockade enhanced neutrophil accumulation, edema, and inflammation in the lungs and ultimately led to significant lung damage. Thus, physiologically produced G-CSF not only acts as a neutrophil mobilizer at the relatively late stage of acute inflammation, but also prevents exaggerated neutrophil mobilization and the associated inflammation-induced tissue damage during early-phase infection and inflammation.

Autophagy Proteins ATG5 and ATG7 Are Essential for the Maintenance of Human CD34(+) Hematopoietic Stem-Progenitor Cells

Autophagy is a highly regulated catabolic process that involves sequestration and lysosomal degradation of cytosolic components such as damaged organelles and misfolded proteins. While autophagy can be considered to be a general cellular housekeeping process, it has become clear that it may also play cell type-dependent functional roles. In this study, we analyzed the functional importance of autophagy in human hematopoietic stem/progenitor cells (HSPCs), and how this is regulated during differentiation. Western blot-based analysis of LC3-II and p62 levels, as well as flow cytometry-based autophagic vesicle quantification, demonstrated that umbilical cord blood-derived CD34(+) /CD38(-) immature hematopoietic progenitors show a higher autophagic flux than CD34(+) /CD38(+) progenitors and more differentiated myeloid and erythroid cells. This high autophagic flux was critical for maintaining stem and progenitor function since knockdown of autophagy genes ATG5 or ATG7 resulted in reduced HSPC frequencies in vitro as well as in vivo. The reduction in HSPCs was not due to impaired differentiation, but at least in part due to reduced cell cycle progression and increased apoptosis. This is accompanied by increased expression of p53, proapoptotic genes BAX and PUMA, and the cell cycle inhibitor p21, as well as increased levels of cleaved caspase-3 and reactive oxygen species. Taken together, our data demonstrate that autophagy is an important regulatory mechanism for human HSCs and their progeny, reducing cellular stress and promoting survival. Stem Cells 2016;34:1651-1663.