The autophagy protein Atg7 is essential for hematopoietic stem cell maintenance

Dynamic regulation of macroautophagy by distinctive ubiquitin-like proteins

Autophagy complements the ubiquitin-proteasome system in mediating protein turnover. Whereas the proteasome degrades individual proteins modified with ubiquitin chains, autophagy degrades many proteins and organelles en masse. Macromolecules destined for autophagic degradation are 'selected' through sequestration within a specialized double-membrane compartment termed the phagophore, the precursor to an autophagosome, and then are hydrolyzed in a lysosome- or vacuole-dependent manner. Notably, a pair of distinctive ubiquitin-like proteins (UBLs), Atg8 and Atg12, regulate degradation by autophagy in unique ways by controlling autophagosome biogenesis and recruitment of specific cargos during selective autophagy. Here we review structural mechanisms underlying the functions and conjugation of these UBLs that are specialized to provide interaction platforms linked to phagophore membranes.

Ulk1-mediated Atg5-independent macroautophagy mediates elimination of mitochondria from embryonic reticulocytes

Macroautophagy is a highly conserved intracellular process responsible for the degradation of subcellular constituents. Macroautophagy was recently suggested to be involved in the removal of mitochondria from reticulocytes during the final stage of erythrocyte differentiation. Although Atg5 and Atg7 are indispensable for macroautophagy, their role in mitochondrial clearance remains controversial. We recently discovered that mammalian cells use conventional Atg5/Atg7-dependent macroautophagy as well as an alternative Unc-51-like kinase 1 (Ulk1)-dependent Atg5/Atg7-independent macroautophagy process. We hypothesized that the latter may be involved in mitochondrial clearance from reticulocytes during erythrocyte differentiation. Here we report that fetal definitive reticulocytes from Ulk1-deficient and Ulk1/Atg5 double-deficient mice retain their mitochondria, whereas the mitochondria are engulfed and digested within autophagic structures in wild-type and Atg5-deficient mice. Mitochondrial retention by Ulk1-deficient reticulocytes is far less marked in primitive and adult definitive reticulocytes. These data indicate that Ulk1-dependent Atg5-independent macroautophagy is the dominant process of mitochondrial clearance from fetal definitive reticulocytes.

Ribosomal protein mutations induce autophagy through S6 kinase inhibition of the insulin pathway

Mutations affecting the ribosome lead to several diseases known as ribosomopathies, with phenotypes that include growth defects, cytopenia, and bone marrow failure. Diamond-Blackfan anemia (DBA), for example, is a pure red cell aplasia linked to the mutation of ribosomal protein (RP) genes. Here we show the knock-down of the DBA-linked RPS19 gene induces the cellular self-digestion process of autophagy, a pathway critical for proper hematopoiesis. We also observe an increase of autophagy in cells derived from DBA patients, in CD34⁺ erythrocyte progenitor cells with RPS19 knock down, in the red blood cells of zebrafish embryos with RP-deficiency, and in cells from patients with Shwachman-Diamond syndrome (SDS). The loss of RPs in all these models results in a marked increase in S6 kinase phosphorylation that we find is triggered by an increase in reactive oxygen species (ROS). We show that this increase in S6 kinase phosphorylation inhibits the insulin pathway and AKT phosphorylation activity through a mechanism reminiscent of insulin resistance. While stimulating RP-deficient cells with insulin reduces autophagy, antioxidant treatment reduces S6 kinase phosphorylation, autophagy, and stabilization of the p53 tumor suppressor. Our data suggest that RP loss promotes the aberrant activation of both S6 kinase and p53 by increasing intracellular ROS levels. The deregulation of these signaling pathways is likely playing a major role in the pathophysiology of ribosomopathies.

Diseases linked to mutations affecting the ribosome, ribosomopathies, have an exceptionally wide range of phenotypes. However, many ribosomopathies have some features in common including cytopenia and growth defects. Our study aims to clarify the mechanisms behind these common phenotypes. We find that mutations in ribosomal protein genes result in a series of aberrant signaling events that cause cells to start recycling and consuming their own intracellular contents. This basic mechanism of catabolism is activated when cells are starving for nutrients, and also during the tightly regulated process of blood cell maturation. The deregulation of this mechanism provides an explanation as to why blood cells are so acutely affected by mutations in genes that impair the ribosome. Moreover, we find that the signals activating this catabolism are coupled to impairment of the highly conserved insulin-signaling pathway that is essential for growth. Taken together, our in-depth description of the pathways involved as the result of mutations affecting the ribosome increases our understanding about the etiology of these diseases and opens up previously unknown avenues of potential treatment.

Metabolic requirements for the maintenance of self-renewing stem cells

A distinctive feature of stem cells is their capacity to self-renew to maintain pluripotency. Studies of genetically-engineered mouse models and recent advances in metabolomic analysis, particularly in haematopoietic stem cells, have deepened our understanding of the contribution made by metabolic cues to the regulation of stem cell self-renewal. Many types of stem cells heavily rely on anaerobic glycolysis, and stem cell function is also regulated by bioenergetic signalling, the AKT-mTOR pathway, Gln metabolism and fatty acid metabolism. As maintenance of a stem cell pool requires a finely-tuned balance between self-renewal and differentiation, investigations into the molecular mechanisms and metabolic pathways underlying these decisions hold great therapeutic promise.

Autophagy in chronic myeloid leukaemia: stem cell survival and implication in therapy

The insensitivity of Chronic Myeloid Leukaemia (CML) stem cells to Tyrosine Kinase Inhibitor (TKI) treatment is now believed to be the main reason for disease persistence experienced in patients. It has been shown that autophagy, an evolutionarily conserved catabolic process that involves degradation of unnecessary or harmful cellular components via lysosomes, is induced following TKI treatment in CML cells. Of clinical importance, autophagy inhibition, using the anti-malarial drug hydroxychloroquine (HCQ), sensitised CML cells, including primitive CML stem cells, to TKI treatment. In this review we discuss the role of autophagy in the maintenance and survival of stem cells in more detail, with a focus on its role in survival of CML stem cells and the possibility to inhibit this pathway as a way to eliminate persistent CML stem cells in vitro and in patients.

Surviving change: the metabolic journey of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are a rare population of somatic stem cells that maintain blood production and are uniquely wired to adapt to diverse cellular fates during the lifetime of an organism. Recent studies have highlighted a central role for metabolic plasticity in facilitating cell fate transitions and in preserving HSC functionality and survival. This review summarizes our current understanding of the metabolic programs associated with HSC quiescence, self-renewal, and lineage commitment, and highlights the mechanistic underpinnings of these changing bioenergetics programs. It also discusses the therapeutic potential of targeting metabolic drivers in the context of blood malignancies.

Hypoxia and metabolic properties of hematopoietic stem cells

SIGNIFICANCE

The effect of redox signaling on hematopoietic stem cell (HSC) function is not clearly understood.

RECENT ADVANCES

A growing body of evidence suggests that adult HSCs reside in the hypoxic bone marrow microenvironment or niche during homeostasis. It was recently shown that primitive HSCs in the bone marrow prefer to utilize anaerobic glycolysis to meet their energy demands and have lower rates of oxygen consumption and lower ATP levels. Hypoxia-inducible factor-α (Hif-1α) is a master regulator of cellular metabolism. With hundreds of downstream target genes and crosstalk with other signaling pathways, it regulates various aspects of metabolism from the oxidative stress response to glycolysis and mitochondrial respiration. Hif-1α is highly expressed in HSCs, where it regulates their function and metabolic phenotype. However, the regulation of Hif-1α in HSCs is not entirely understood. The homeobox transcription factor myeloid ecotropic viral integration site 1 (Meis1) is expressed in the most primitive HSCs populations, and it is required for primitive hematopoiesis. Recent reports suggest that Meis1 is required for normal adult HSC function by regulating the metabolism and redox state of HSCs transcriptionally through Hif-1α and Hif-2α.

CRITICAL ISSUES

Given the profound effect of redox status on HSC function, it is critical to fully characterize the intrinsic, and microenvironment-related mechanisms of metabolic and redox regulation in HSCs.

FUTURE DIRECTIONS

Future studies will be needed to elucidate the link between HSC metabolism and HSC fates, including quiescence, self-renewal, differentiation, apoptosis, and migration.

Retinoid receptor signaling and autophagy in acute promyelocytic leukemia

Retinoids are a family of signaling molecules derived from vitamin A with well established roles in cellular differentiation. Physiologically active retinoids mediate transcriptional effects on cells through interactions with retinoic acid (RARs) and retinoid-X (RXR) receptors. Chromosomal translocations involving the RARα gene, which lead to impaired retinoid signaling, are implicated in acute promyelocytic leukemia (APL). All-trans-retinoic acid (ATRA), alone and in combination with arsenic trioxide (ATO), restores differentiation in APL cells and promotes degradation of the abnormal oncogenic fusion protein through several proteolytic mechanisms. RARα fusion-protein elimination is emerging as critical to obtaining sustained remission and long-term cure in APL. Autophagy is a degradative cellular pathway involved in protein turnover. Both ATRA and ATO also induce autophagy in APL cells. Enhancing autophagy may therefore be of therapeutic benefit in resistant APL and could broaden the application of differentiation therapy to other cancers. Here we discuss retinoid signaling in hematopoiesis, leukemogenesis, and APL treatment. We highlight autophagy as a potential important regulator in anti-leukemic strategies.

Targeting of preexisting and induced breast cancer stem cells with trastuzumab and trastuzumab emtansine (T-DM1)

The antibody trastuzumab (Herceptin) has substantially improved overall survival for patients with aggressive HER2-positive breast cancer. However, about 70% of all treated patients will experience relapse or disease progression. This may be related to an insufficient targeting of the CD44^highCD24^low breast cancer stem cell subset, which is not only highly resistant to chemotherapy and radiotherapy but also a poor target for trastuzumab due to low HER2 surface expression. Hence, we explored whether the new antibody-drug conjugate T-DM1, which consists of the potent chemotherapeutic DM1 coupled to trastuzumab, could improve the targeting of these tumor-initiating or metastasis-initiating cells. To this aim, primary HER2-overexpressing tumor cells as well as HER2-positive and HER2-negative breast cancer cell lines were treated with T-DM1, and effects on survival, colony formation, gene and protein expression as well as antibody internalization were assessed. This revealed that CD44^highCD24^lowHER2^low stem cell-like breast cancer cells show high endocytic activity and are thus particularly sensitive towards the antibody-drug conjugate T-DM1. Consequently, preexisting CD44^highCD24^low cancer stem cells were depleted by concentrations of T-DM1 that did not affect the bulk of the tumor cells. Likewise, colony formation was efficiently suppressed. Moreover, when tumor cells were cocultured with natural killer cells, antibody-dependent cell-mediated cytotoxicity was enhanced, and EMT-mediated induction of stem cell-like properties was prevented in differentiated tumor cells. Thus our study reveals an unanticipated targeting of stem cell-like breast cancer cells by T-DM1 that may contribute to the clinical efficacy of this recently approved antibody-drug conjugate.

Hexokinase II inhibitor, 3-BrPA induced autophagy by stimulating ROS formation in human breast cancer cells

Hexokinase II (HKII), a key enzyme of glycolysis, is widely over-expressed in cancer cells. 3-bromopyruvate (3-BrPA), an inhibitor of HK II, has been proposed as a specific antitumor agent. Autophagy is a process that regulates the balance between protein synthesis and protein degradation. Autophagy in mammalian systems occurs under basal conditions and can be stimulated by stresses, including starvation, oxidative stress. Therefore, we hypothesized that 3-BrPA could induce autophagy. In the present study, we explored the mechanism of 3-BrPA and its combined action with chloroquine. Our results demonstrate that in MDA-MB-435 and in MDA-MB-231 cells, 3-BrPA induces autophagy, which can be inhibited by chloroquine. Furthermore, the combined treatment synergistically decreased the number of viable cells. Interestingly, the combined treatment triggered apoptosis in MDA-MB-435 cells, while it induced necroptosis in MDA-MB-231 cells. ROS mediated cell death when 3-BrPA and CQ were co-administered. Finally, CQ enhanced the anticancer efficacy of 3-BrPA in vivo. Collectively, our results show that 3-BrPA triggers autophagy, increasing breast cancer cell resistance to 3-BrPA treatment and that CQ enhanced 3-BrPA-induced cell death in breast cancer cells by stimulating ROS formation. Thus, inhibition of autophagy may be an innovative strategy for adjuvant chemotherapy of breast cancer.human skeletal muscle. Efficient Mirk depletion in SU86.86 pancreatic cancer cells by an inducible shRNA decreased expression of eight antioxidant genes. Thus both cancer cells and differentiated myotubes utilize Mirk kinase to relieve oxidative stress.

To breathe or not to breathe: the haematopoietic stem/progenitor cells dilemma

Adult haematopoietic stem/progenitor cells (HSPCs) constitute the lifespan reserve for the generation of all the cellular lineages in the blood. Although massive progress in identifying the cluster of master genes controlling self-renewal and multipotency has been achieved in the past decade, some aspects of the physiology of HSPCs still need to be clarified. In particular, there is growing interest in the metabolic profile of HSPCs in view of their emerging role as determinants of cell fate. Indeed, stem cells and progenitors have distinct metabolic profiles, and the transition from stem to progenitor cell corresponds to a critical metabolic change, from glycolysis to oxidative phosphorylation. In this review, we summarize evidence, reported in the literature and provided by our group, highlighting the peculiar ability of HSPCs to adapt their mitochondrial oxidative/bioenergetic metabolism to survive in the hypoxic microenvironment of the endoblastic niche and to exploit redox signalling in controlling the balance between quiescence versus active cycling and differentiation. Especial prominence is given to the interplay between hypoxia inducible factor-1, globins and NADPH oxidases in managing the mitochondrial dioxygen-related metabolism and biogenesis in HSPCs under different ambient conditions. A mechanistic model is proposed whereby 'mitochondrial differentiation' is a prerequisite in uncommitted stem cells, paving the way for growth/differentiation factor-dependent processes. Advancing the understanding of stem cell metabolism will, hopefully, help to (i) improve efforts to maintain, expand and manipulate HSPCs ex vivo and realize their potential therapeutic benefits in regenerative medicine; (ii) reprogramme somatic cells to generate stem cells; and (iii) eliminate, selectively, malignant stem cells.

LINKED ARTICLES

This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

Lentiviral vector-mediate ATG3 overexpression inhibits growth and promotes apoptosis of human SKM-1 cells

Based on the nested case-control study cohort and gene expression profile, we have picked up a subset of six genes to distinguish the leukemia group and control group stably. ATG3 is the only down regulated gene. This research is to investigate the effect of ATG3 gene over expression by lentivirus on SKM-1 cell line and myelodysplastic syndrome to leukemic transformation. Human SKM-1 cells were transfected with ATG3-GFP recombinant lentiviral vectors and compared with cells transfected with GFP lentiviral vectors. Western blot was performed to detect the ATG3 protein. Cell proliferation was assessed by cell counting kit-8. Cell vitality was tested by Trypan Blue. Cell apoptosis was determined by Annexin V Apoptosis Detection Kit APC. Observe and compare the changes on growth curve, cell vitality and cell apoptosis. After 72 h of transfection, satisfactory transfection efficiency (> 90 %) was observed. SKM-1 cell line showed a statistically significant (P < 0.05) overexpression of ATG3, parallel to significantly (P < 0.05) inhibited cell proliferation. The cell vitality of ATG3 overexpression was significantly (P < 0.05) lower than negative control. Cell apoptosis analysis by flow cytometer demonstrated decreased proportion of early apoptosis and increased that of late apoptosis and death (P < 0.05). Over expressed ATG3 gene and protein, the SKM-1 cell line was inhibited in proliferation and cell vitality. It was promoted from early apoptosis to late apoptosis and death. The malignancy of SKM-1 cell line was decreased after transfection. ATG3 gene and its gene family may play an important role in transformation of myelodysplastic syndrome.

Cellular mechanisms of somatic stem cell aging

Tissue homeostasis and regenerative capacity rely on rare populations of somatic stem cells endowed with the potential to self-renew and differentiate. During aging, many tissues show a decline in regenerative potential coupled with a loss of stem cell function. Cells including somatic stem cells have evolved a series of checks and balances to sense and repair cellular damage to maximize tissue function. However, during aging the mechanisms that protect normal cell function begin to fail. In this review, we will discuss how common cellular mechanisms that maintain tissue fidelity and organismal lifespan impact somatic stem cell function. We will highlight context-dependent changes and commonalities that define aging, by focusing on three age-sensitive stem cell compartments: blood, neural, and muscle. Understanding the interaction between extrinsic regulators and intrinsic effectors that operate within different stem cell compartments is likely to have important implications for identifying strategies to improve health span and treat age-related degenerative diseases.

Neuroprotective effects of autophagy induced by rapamycin in rat acute spinal cord injury model

BACKGROUND/AIMS

To explore the effects of rapamycin-induced autophagy on apoptosis in a rat model of acute spinal cord injury (SCI), and to explore the effect of rapamycin on apoptosis in primary spinal cord cell culture.

METHODS

SCI was induced at T10 in female adult Sprague-Dawley rats. After injury was induced, the rats were injected with rapamycin and/or methylprednisolone and were sacrificed at various days after injury. Apoptosis and autophagy were examined with TUNEL staining and electron microscopy. Hind limb function was assessed by the Gale scale.

RESULTS

The expression of the apoptosis-related protein caspase-3 did not significantly increase until 21 days following injury, while increases in LC3II and LC3I began 10 days after injury, but then declined. TUNEL staining and electron microscopy confirmed that following injury autophagy occurred before apoptosis, but by 14 days after the injury, the level of autophagy had decreased significantly while the level of apoptosis showed a continued increase. Following treatment with rapamycin, apoptosis was significantly higher than in the vehicle control group, but significantly lower than in the sham-operated group, showing a protective effect of rapamycin. Gale scale grades in rats treated with rapamycin were significantly higher compared with the vehicle control group, suggesting a functional effect of rapamycin-induced inhibition of apoptosis.

CONCLUSIONS

The results indicate that rapamycin significantly improved the prognosis of acute SCI in rats by inhibiting cell apoptosis. Rapamycin might be useful as a therapeutic agent for acute SCI.

Reprogramming and carcinogenesis--parallels and distinctions

Rapid progress made in various areas of regenerative medicine in recent years occurred both at the cellular level, with the Nobel prize-winning discovery of reprogramming (generation of induced pluripotent stem (iPS) cells) and also at the biomaterial level. The use of four transcription factors, Oct3/4, Sox2, c-Myc, and Klf4 (called commonly "Yamanaka factors") for the conversion of differentiated cells, back to the pluripotent/embryonic stage, has opened virtually endless and ethically acceptable source of stem cells for medical use. Various types of stem cells are becoming increasingly popular as starting components for the development of replacement tissues, or artificial organs. Interestingly, many of the transcription factors, key to the maintenance of stemness phenotype in various cells, are also overexpressed in cancer (stem) cells, and some of them may find the use as prognostic factors. In this review, we describe various methods of iPS creation, followed by overview of factors known to interfere with the efficiency of reprogramming. Next, we discuss similarities between cancer stem cells and various stem cell types. Final paragraphs are dedicated to interaction of biomaterials with tissues, various adverse reactions generated as a result of such interactions, and measures available, that allow for mitigation of such negative effects.

Identification of a small molecule preventing BMSC senescence in vitro by improving intracellular homeostasis via ANXA7 and Hmbox1

ABO was discovered to be a novel anti-aging chemical in cultured BMSCs by improving intracellular homeostasis.

Autophagy in infection, inflammation and immunity

Autophagy is a fundamental eukaryotic pathway that has multiple effects on immunity. Autophagy is induced by pattern recognition receptors and, through autophagic adaptors, it provides a mechanism for the elimination of intracellular microorganisms. Autophagy controls inflammation through regulatory interactions with innate immune signalling pathways, by removing endogenous inflammasome agonists and through effects on the secretion of immune mediators. Moreover, autophagy contributes to antigen presentation and to T cell homeostasis, and it affects T cell repertoires and polarization. Thus, as we discuss in this Review, autophagy has multitiered immunological functions that influence infection, inflammation and immunity.

Autophagy-mediated regulation of macrophages and its applications for cancer

Autophagy is a highly conserved homeostatic pathway that plays an important role in tumor development and progression by acting on cancer cells in a cell-autonomous mechanism. However, the solid tumor is not an island, but rather an ensemble performance that includes nonmalignant stromal cells, such as macrophages. A growing body of evidence indicates that autophagy is a key component of the innate immune response. In this review, we discuss the role of autophagy in the control of macrophage production at different stages (including hematopoietic stem cell maintenance, monocyte/macrophage migration, and monocyte differentiation into macrophages) and polarization and discuss how modulating autophagy in tumor-associated macrophages (TAMs) may represent a promising strategy for limiting cancer growth and progression.

Suppression of autophagy by FIP200 deletion leads to osteopenia in mice through the inhibition of osteoblast terminal differentiation

Autophagy is a conserved lysosomal degradation process that has important roles in both normal human physiology and disease. However, the function of autophagy in bone homeostasis is not well understood. Here, we report that autophagy is activated during osteoblast differentiation. Ablation of focal adhesion kinase family interacting protein of 200 kD (FIP200), an essential component of mammalian autophagy, led to multiple autophagic defects in osteoblasts including aberrantly increased p62 expression, deficient LC3-II conversion, defective autophagy flux, absence of GFP-LC3 puncta in FIP200-null osteoblasts expressing transgenic GFP-LC3, and absence of autophagosome-like structures by electron microscope examination. Osteoblast-specific deletion of FIP200 led to osteopenia in mice. Histomorphometric analysis revealed that the osteopenia was the result of cell-autonomous effects of FIP200 deletion on osteoblasts. FIP200 deletion led to defective osteoblast terminal differentiation in both primary bone marrow and calvarial osteoblasts in vitro. Interestingly, both proliferation and differentiation were not adversely affected by FIP200 deletion in early cultures. However, FIP200 deletion led to defective osteoblast nodule formation after initial proliferation and differentiation. Furthermore, treatment with autophagy inhibitors recapitulated the effects of FIP200 deletion on osteoblast differentiation. Taken together, these data identify FIP200 as an important regulator of bone development and reveal a novel role of autophagy in osteoblast function through its positive role in supporting osteoblast nodule formation and differentiation.

Autophagy: cancer's friend or foe?

The functional relevance of autophagy in tumor formation and progression remains controversial. Autophagy can promote tumor suppression during cancer initiation and protect tumors during progression. Autophagy-associated cell death may act as a tumor suppressor, with several autophagy-related genes deleted in cancers. Loss of autophagy induces genomic instability and necrosis with inflammation in mouse tumor models. Conversely, autophagy enhances survival of tumor cells subjected to metabolic stress and may promote metastasis by enhancing tumor cell survival under environmental stress. Unraveling the complex molecular regulation and multiple diverse roles of autophagy is pivotal in guiding development of rational and novel cancer therapies.

The immune system GTPase GIMAP6 interacts with the Atg8 homologue GABARAPL2 and is recruited to autophagosomes

The GIMAPs (GTPases of the immunity-associated proteins) are a family of small GTPases expressed prominently in the immune systems of mammals and other vertebrates. In mammals, studies of mutant or genetically-modified rodents have indicated important roles for the GIMAP GTPases in the development and survival of lymphocytes. No clear picture has yet emerged, however, of the molecular mechanisms by which they perform their function(s). Using biotin tag-affinity purification we identified a major, and highly specific, interaction between the human cytosolic family member GIMAP6 and GABARAPL2, one of the mammalian homologues of the yeast autophagy protein Atg8. Chemical cross-linking studies performed on Jurkat T cells, which express both GIMAP6 and GABARAPL2 endogenously, indicated that the two proteins in these cells readily associate with one another in the cytosol under normal conditions. The GIMAP6-GABARAPL2 interaction was disrupted by deletion of the last 10 amino acids of GIMAP6. The N-terminal region of GIMAP6, however, which includes a putative Atg8-family interacting motif, was not required. Over-expression of GIMAP6 resulted in increased levels of endogenous GABARAPL2 in cells. After culture of cells in starvation medium, GIMAP6 was found to localise in punctate structures with both GABARAPL2 and the autophagosomal marker MAP1LC3B, indicating that GIMAP6 re-locates to autophagosomes on starvation. Consistent with this finding, we have demonstrated that starvation of Jurkat T cells results in the degradation of GIMAP6. Whilst these findings raise the possibility that the GIMAPs play roles in the regulation of autophagy, we have been unable to demonstrate an effect of GIMAP6 over-expression on autophagic flux.

Mitophagy in hematopoietic stem cells: the case for exploration

Hematopoietic stem cells (HSCs) are inherently quiescent and self-renewing, yet can differentiate and commit to multiple blood cell types. Intracellular mitochondrial content is dynamic, and there is an increase in mitochondrial content during differentiation and lineage commitment in HSCs. HSCs reside in a hypoxic niche within the bone marrow and rely heavily on glycolysis, while differentiated and committed progenitors rely on oxidative phosphorylation. Increased oxidative phosphorylation during differentiation and commitment is not only due to increased mitochondrial content but also due to changes in mitochondrial cytosolic distribution and efficiency. These changes in the intracellular mitochondrial landscape contribute signals toward regulating differentiation and commitment. Thus, a functional relationship exists between the mitochondria in HSCs and the state of the HSCs (i.e., stemness vs. differentiated). This review focuses on how autophagy-mediated mitochondrial clearance (i.e., mitophagy) may affect HSC mitochondrial content, thereby influencing the fate of HSCs and maintenance of hematopoietic homeostasis.

Autophagy prevents irradiation injury and maintains stemness through decreasing ROS generation in mesenchymal stem cells

Stem cells were characterized by their stemness: self-renewal and pluripotency. Mesenchymal stem cells (MSCs) are a unique type of adult stem cells that have been proven to be involved in tissue repair, immunoloregulation and tumorigenesis. Irradiation is a well-known factor that leads to functional obstacle in stem cells. However, the mechanism of stemness maintenance in human MSCs exposed to irradiation remains unknown. We demonstrated that irradiation could induce reactive oxygen species (ROS) accumulation that resulted in DNA damage and stemness injury in MSCs. Autophagy induced by starvation or rapamycin can reduce ROS accumulation-associated DNA damage and maintain stemness in MSCs. Further, inhibition of autophagy leads to augment of ROS accumulation and DNA damage, which results in the loss of stemness in MSCs. Our results indicate that autophagy may have an important role in protecting stemness of MSCs from irradiation injury.

Resilient and resourceful: genome maintenance strategies in hematopoietic stem cells

Blood homeostasis is maintained by a rare population of quiescent hematopoietic stem cells (HSCs) that self-renew and differentiate to give rise to all lineages of mature blood cells. In contrast to most other blood cells, HSCs are preserved throughout life, and the maintenance of their genomic integrity is therefore paramount to ensure normal blood production and to prevent leukemic transformation. HSCs are also one of the few blood cells that truly age and exhibit severe functional decline in old organisms, resulting in impaired blood homeostasis and increased risk for hematologic malignancies. In this review, we present the strategies used by HSCs to cope with the many genotoxic insults that they commonly encounter. We briefly describe the DNA-damaging insults that can affect HSC function and the mechanisms that are used by HSCs to prevent, survive, and repair DNA lesions. We also discuss an apparent paradox in HSC biology, in which the genome maintenance strategies used by HSCs to protect their function in fact render them vulnerable to the acquisition of damaging genetic aberrations.

BMP4 is involved in the chemoresistance of myeloid leukemia cells through regulating autophagy-apoptosis balance

This study showed that silencing BMP4 expression significantly activated caspase-2, 3, and 9, while decreasing Matrigel colony formation in Cytarabine (Ara-C)-treated leukemia HL-60 cells. In contrast, Ara-C significantly upregulated Atg5 and Beclin-1 expression, the ratio of LC3-II/LC3-I, and CDK1 and cyclin B1 expression in leukemia cells expressing BMP4. BafA significantly sensitized the apoptotic effect of Ara-C in leukemia cells. Injection of Ara-C significantly inhibited tumor growth in mice inoculated with leukemia cells with BMP4 silenced. In conclusion, BMP4 plays a crucial role in the chemoresistance of leukemia cells through the activation of autophagy and subsequent inhibition of apoptosis.

FLVCR is necessary for erythroid maturation, may contribute to platelet maturation, but is dispensable for normal hematopoietic stem cell function

Heme is a pleiotropic molecule that is important for oxygen and oxidative metabolism, most notably as the prosthetic group of hemoglobin and cytochromes. Because excess free intracellular heme is toxic, organisms have developed mechanisms to tightly regulate its concentration. One mechanism is through active heme export by the group C feline leukemia virus receptor (FLVCR). Previously, we have shown that FLVCR is necessary for embryonic and postnatal erythropoiesis. However, FLVCR is also expressed in numerous other tissues, including hematopoietic stem cells (HSCs). To explore a possible role for FLVCR in HSC function, we performed serial, competitive repopulation transplant experiments using FLVCR-deleted and control bone marrow cells, along with wild-type competitor cells. Loss of FLVCR did not impact HSC function under steady-state or myelotoxic stress conditions (such as arsenic or radiation exposure), nor did FLVCR deletion result in alterations in the various progenitor compartments. However, even when 95% of the donor bone marrow cells lacked FLVCR, all red cells in recipient mice were wild type. This is due to the increased apoptosis of FLVCR-deleted proerythroblasts. Also, remarkably, loss of FLVCR increased megakaryocyte ploidy. Together, these findings show FLVCR is redundant in stem cells but has critical and contrasting stage-specific roles in discrete hematopoietic lineages.

Paradoxical roles of autophagy in different stages of tumorigenesis: protector for normal or cancer cells

Autophagy serves as a dynamic degradation and recycling system that provides biological materials and energy in response to stress. The role of autophagy in tumor development is complex. Various studies suggest that autophagy mainly contributes to tumor suppression during the early stage of tumorigenesis and tumor promotion during the late stage of tumorigenesis. During the tumorization of normal cells, autophagy protects genomic stability by retarding stem cells-involved damage/repair cycle, and inhibits the formation of chronic inflammatory microenvironment, thus protecting normal cell homeostasis and preventing tumor generation. On the other hand, autophagy also protects tumor cells survival during malignant progression by supporting cellular metabolic demands, decreasing metabolic damage and supporting anoikis resistance and dormancy. Taken together, autophagy appears to play a role as a protector for either normal or tumor cells during the early or late stage of tumorigenesis, respectively. The process of tumorigenesis perhaps needs to undergo twice autophagy-associated screening. The normal cells that have lower autophagy capacity are prone to tumorization, and the incipient tumor cells that have higher autophagy capacity possibly are easier to survival in the hash microenvironment and accumulate more mutations to promote malignant progression.

Hif-2α is not essential for cell-autonomous hematopoietic stem cell maintenance

Local hypoxia in hematopoietic stem cell (HSC) niches is thought to regulate HSC functions. Hypoxia-inducible factor-1 (Hif-1) and Hif-2 are key mediators of cellular responses to hypoxia. Although oxygen-regulated α-subunits of Hifs, namely Hif-1α and Hif-2α, are closely related, they play overlapping and also distinct functions in nonhematopoietic tissues. Although Hif-1α-deficient HSCs lose their activity on serial transplantation, the role for Hif-2α in cell-autonomous HSC maintenance remains unknown. Here, we demonstrate that constitutive or inducible hematopoiesis-specific Hif-2α deletion does not affect HSC numbers and steady-state hematopoiesis. Furthermore, using serial transplantations and 5-fluorouracil treatment, we demonstrate that HSCs do not require Hif-2α to self-renew and recover after hematopoietic injury. Finally, we show that Hif-1α deletion has no major impact on steady-state maintenance of Hif-2α-deficient HSCs and their ability to repopulate primary recipients, indicating that Hif-1α expression does not account for normal behavior of Hif-2α-deficient HSCs.

Autophagy in blood cancers: biological role and therapeutic implications

Autophagy is a cell recycling process the molecular apparatus of which has been identified over the past decade. Autophagy allows cells to survive starvation and inhospitable conditions and plays a key role in numerous physiological functions, including hematopoiesis and immune responses. In hematologic malignancies, autophagy can either act as a chemo-resistance mechanism or have tumor suppressive functions, depending on the context. In addition, autophagy is involved in other important aspects of blood cancers as it promotes immune competence and anti-cancer immunity, and may even help enhance patient tolerance to standard treatments. Approaches exploiting autophagy, either to activate or inhibit it, could find broad application in hematologic malignancies and contribute to improved clinical outcomes. These aspects are discussed here together with a brief introduction to the molecular machinery of autophagy and to its role in blood cell physiology.

Inhibition of GATE-16 attenuates ATRA-induced neutrophil differentiation of APL cells and interferes with autophagosome formation

Autophagy is an intracellular bulk degradation process involved in cell survival upon stress induction, but also with a newly identified function in myeloid differentiation. The autophagy-related (ATG)8 protein family, including the GABARAP and LC3 subfamilies, is crucial for autophagosome biogenesis. In order to evaluate the significance of the GABARAPs in the pathogenesis of acute myeloid leukemia (AML), we compared their expression in primary AML patient samples, CD34(+) progenitor cells and in granulocytes from healthy donors. GABARAPL1 and GABARAPL2/GATE-16, but not GABARAP, were significantly downregulated in particular AML subtypes compared to normal granulocytes. Moreover, the expression of GABARAPL1 and GATE-16 was significantly induced during ATRA-induced neutrophil differentiation of acute promyelocytic leukemia cells (APL). Lastly, knocking down GABARAPL2/GATE-16 in APL cells attenuated neutrophil differentiation and decreased autophagic flux. In conclusion, low GABARAPL2/GATE-16 expression is associated with an immature myeloid leukemic phenotype and these proteins are necessary for neutrophil differentiation of APL cells.

Mechanisms that regulate stem cell aging and life span

Mammalian aging is associated with reduced tissue regeneration, increased degenerative disease, and cancer. Because stem cells regenerate many adult tissues and contribute to the development of cancer by accumulating mutations, age-related changes in stem cells likely contribute to age-related morbidity. Consistent with this, stem cell function declines with age in numerous tissues as a result of gate-keeping tumor suppressor expression, DNA damage, changes in cellular physiology, and environmental changes in tissues. It remains unknown whether declines in stem cell function during aging influence organismal longevity. However, mechanisms that influence longevity also modulate age-related morbidity, partly through effects on stem cells.

Autophagy and genomic integrity

DNA lesions, constantly produced by endogenous and exogenous sources, activate the DNA damage response (DDR), which involves detection, signaling and repair of the damage. Autophagy, a lysosome-dependent degradation pathway that is activated by stressful situations such as starvation and oxidative stress, regulates cell fate after DNA damage and also has a pivotal role in the maintenance of nuclear and mitochondrial genomic integrity. Here, we review important evidence regarding the role played by autophagy in preventing genomic instability and tumorigenesis, as well as in micronuclei degradation. Several pathways governing autophagy activation after DNA injury and the influence of autophagy upon the processing of genomic lesions are also discussed herein. In this line, the mechanisms by which several proteins participate in both DDR and autophagy, and the importance of this crosstalk in cancer and neurodegeneration will be presented in an integrated fashion. At last, we present a hypothetical model of the role played by autophagy in dictating cell fate after genotoxic stress.

Molecular regulation of stem cell quiescence

Subsets of mammalian adult stem cells reside in the quiescent state for prolonged periods of time. This state, which is reversible, has long been viewed as dormant and with minimal basal activity. Recent advances in adult stem cell isolation have provided insights into the epigenetic, transcriptional and post-transcriptional control of quiescence and suggest that quiescence is an actively maintained state in which signalling pathways are involved in maintaining a poised state that allows rapid activation. Deciphering the molecular mechanisms regulating adult stem cell quiescence will increase our understanding of tissue regeneration mechanisms and how they are dysregulated in pathological conditions and in ageing.

Autophagic control of cell 'stemness'

Stem cells have the ability to self-renew and differentiate into various cell types. Both cell-intrinsic and extrinsic factors may contribute to aging-related decline in stem cell function and loss of stemness. The maintenance of cellular homeostasis requires timely removal of toxic proteins and damaged organelles that accumulate with age or in pathological conditions. Autophagy is one of the main strategies to eliminate unwanted cytoplasmic materials thereby ultimately preventing cellular damage. Here, we shall discuss the accumulating evidence suggesting that autophagy plays a critical role in the homeostatic control of stem cell functions during aging, tissue regeneration, and cellular reprogramming.

Basic biology of skeletal aging: role of stress response pathways

Although a decline in bone formation and loss of bone mass are common features of human aging, the molecular mechanisms mediating these effects have remained unclear. Evidence from pharmacological and genetic studies in mice has provided support for a deleterious effect of oxidative stress in bone and has strengthened the idea that an increase in reactive oxygen species (ROS) with advancing age represents a pathophysiological mechanism underlying age-related bone loss. Mesenchymal stem cells and osteocytes are long-lived cells and, therefore, are more susceptible than other types of bone cells to the molecular changes caused by aging, including increased levels of ROS and decreased autophagy. However, short-lived cells like osteoblast progenitors and mature osteoblasts and osteoclasts are also affected by the altered aged environment characterized by lower levels of sex steroids, increased endogenous glucocorticoids, and higher oxidized lipids. This article reviews current knowledge on the effects of the aging process on bone, with particular emphasis on the role of ROS and autophagy in cells of the osteoblast lineage in mice.

Inhibition of the autophagic flux by salinomycin in breast cancer stem-like/progenitor cells interferes with their maintenance

Breast cancer tissue contains a small population of cells that have the ability to self-renew; these cells are known as cancer stem-like cells (CSCs). We have recently shown that autophagy is essential for the tumorigenicity of these CSCs. Salinomycin (Sal), a K (+) /H (+) ionophore, has recently been shown to be at least 100 times more effective than paclitaxel in reducing the proportion of breast CSCs. However, its mechanisms of action are still unclear. We show here that Sal blocked both autophagy flux and lysosomal proteolytic activity in both CSCs and non-CSCs derived from breast cancer cells. GFP-LC3 staining combined with fluorescent dextran uptake and LysoTracker-Red staining showed that autophagosome/lysosome fusion was not altered by Sal treatment. Acridine orange staining provided evidence that lysosomes display the characteristics of acidic compartments in Sal-treated cells. However, tandem mCherry-GFP-LC3 assay indicated that the degradation of mCherry-GFP-LC3 is blocked by Sal. Furthermore, the protein degradation activity of lysosomes was inhibited, as demonstrated by the rate of long-lived protein degradation, DQ-BSA assay and measurement of cathepsin activity. Our data indicated that Sal has a relatively greater suppressant effect on autophagic flux in the ALDH (+) population in HMLER cells than in the ALDH (-) population; moreover, this differential effect on autophagic flux correlated with an increase in apoptosis in the ALDH (+) population. ATG7 depletion accelerated the proapoptotic capacity of Sal in the ALDH (+) population. Our findings provide new insights into how the autophagy-lysosomal pathway contributes to the ability of Sal to target CSCs in vitro.

The response to DNA damage during differentiation: pathways and consequences

Damage to genomic DNA triggers a prompt set of signaling events known as the DNA damage response (DDR) which coordinates DNA repair, cell cycle arrest and ultimately cell death or senescence. Although activation of adequate DNA damage signaling and repair systems depends on the type of lesion and the cell-cycle phase in which it occurs, emerging evidence indicates that DNA repair and DDR function differently in different cellular contexts. Depending on the time maintenance and function of a specific cell type the risk of accumulating DNA damage may vary. For instance, damage to stem cells if not repaired can lead to mutation amplification or propagation through the processes of self-renewal and differentiation, respectively, whereas damage to post-mitotic cells can affect mostly tissue homeostasis. Stem cells are therefore expected to address DNA damage differently from their somatic counterparts. In this review the information available on the common and distinct mechanisms of control of genome integrity utilized by different cell types along the self-renewal/differentiation program will be reviewed, with special emphasis on their roles in the prevention of aging and disease.

Pharmacological agents with inherent anti-autophagic activity improve the cytotoxicity of imatinib

Resistance to tyrosine kinase inhibitors (TKIs) remains a limitation to the treatment of chronic myeloid leukaemia (CML), due in part, to the induction of autophagy. We examined whether disruption of autophagy with the pharmacological agents, brefeldin A, vincristine and chloroquine, improves the cytotoxicity of imatinib. In K562 CML cells, all drugs tested, in combination with imatinib impaired the expression or cellular distribution of LC3 and Beclin 1 (autophagy markers) and reduced the recovery of cells following drug withdrawal. The combination of imatinib and an agent that impedes autophagy demonstrates impressive potential as a more curative regime for CML.

FIP200 is required for maintenance and differentiation of postnatal neural stem cells

Despite recent studies showing depletion of hematopoietic stem cells (HSCs) pool accompanied by increased intracellular ROS upon autophagy inhibition, it remains unknown whether autophagy is essential in the maintenance of other stem cells. Moreover, it is unclear whether and how the aberrant ROS increase causes depletion of stem cells. Here, we report that ablation of FIP200, an essential gene for autophagy induction in mammalian cells, results in a progressive loss of neural stem cells (NSCs) pool and impairment in neuronal differentiation specifically in the postnatal brain, but not the embryonic brain, in mice. The defect in maintaining the postnatal NSC pool was caused by p53-dependent apoptotic responses and cell cycle arrest. However, the impaired neuronal differentiation was rescued by anti-oxidant NAC treatment, but not by p53 inactivation. These data reveal a role of FIP200-mediated autophagy in the maintenance and functions of NSCs through regulation of oxidative state.

Autophagy in stem cells

Autophagy is a highly conserved cellular process by which cytoplasmic components are sequestered in autophagosomes and delivered to lysosomes for degradation. As a major intracellular degradation and recycling pathway, autophagy is crucial for maintaining cellular homeostasis as well as remodeling during normal development, and dysfunctions in autophagy have been associated with a variety of pathologies including cancer, inflammatory bowel disease and neurodegenerative disease. Stem cells are unique in their ability to self-renew and differentiate into various cells in the body, which are important in development, tissue renewal and a range of disease processes. Therefore, it is predicted that autophagy would be crucial for the quality control mechanisms and maintenance of cellular homeostasis in various stem cells given their relatively long life in the organisms. In contrast to the extensive body of knowledge available for somatic cells, the role of autophagy in the maintenance and function of stem cells is only beginning to be revealed as a result of recent studies. Here we provide a comprehensive review of the current understanding of the mechanisms and regulation of autophagy in embryonic stem cells, several tissue stem cells (particularly hematopoietic stem cells), as well as a number of cancer stem cells. We discuss how recent studies of different knockout mice models have defined the roles of various autophagy genes and related pathways in the regulation of the maintenance, expansion and differentiation of various stem cells. We also highlight the many unanswered questions that will help to drive further research at the intersection of autophagy and stem cell biology in the near future.

Chloroquine in cancer therapy: a double-edged sword of autophagy

Autophagy is a homeostatic cellular recycling system that is responsible for degrading damaged or unnecessary cellular organelles and proteins. Cancer cells are thought to use autophagy as a source of energy in the unfavorable metastatic environment, and a number of clinical trials are now revealing the promising role of chloroquine, an autophagy inhibitor, as a novel antitumor drug. On the other hand, however, the kidneys are highly vulnerable to chemotherapeutic agents. Recent studies have shown that autophagy plays a protective role against acute kidney injury, including cisplatin-induced kidney injury, and thus, we suspect that the use of chloroquine in combination with anticancer drugs may exacerbate kidney damage. Moreover, organs in which autophagy also plays a homeostatic role, such as the neurons, liver, hematopoietic stem cells, and heart, may be sensitive to the combined use of chloroquine and anticancer drugs. Here, we summarize the functions of autophagy in cancer and kidney injury, especially focusing on the use of chloroquine to treat cancer, and address the possible side effects in the combined use of chloroquine and anticancer drugs.

Mitochondrial fusion by pharmacological manipulation impedes somatic cell reprogramming to pluripotency: new insight into the role of mitophagy in cell stemness

Recent studies have suggested a pivotal role for autophagy in stem cell maintenance and differentiation. Reprogramming of somatic cells to induced pluripotent stem cells (iPSCs) has been also suggested to bio-energetically take advantage of mitochondrial autophagy (mitophagy). We have preliminary addressed how mitophagy might play a role in the regulation of induced pluripotency using mdivi-1 (for mitochondrial division inhibitor), a highly efficacious small molecule that selectively inhibits the self-assembly of DRP1, a member of the dynamin family of large GTPases that mediates mitochondrial fission. At mdivi-1 concentrations that rapidly induced the formation of mitochondrial net-like or collapsed perinuclear mitochondrial structures, we observed that the reprogramming efficiency of mouse embryonic fibroblasts transduced with the Yamanaka three-factor cocktail (OCT4, KLF4, and SOX2) is drastically reduced by more than 95%. Treatment of MEFs with mdivi-1 at the early stages of reprogramming before the appearance of iPSC colonies was sufficient to completely inhibit somatic cell reprogramming. Therefore, the observed effects on reprogramming efficiencies were due likely to the inhibition of the process of reprogramming itself and not to an impairment of iPSC colony survival or growth. Moreover, the typical morphology of established iPSC colonies with positive alkaline phosphatase staining was negatively affected by mdivi-1 exposure. In the presence of mdivi-1, the colony morphology of the iPSCs was lost, and they somewhat resembled fibroblasts. The alkaline phosphatase staining was also significantly reduced, a finding that is indicative of differentiation. Our current findings provide new insight into how mitochondrial division is integrated into the reprogramming factors-driven transcriptional network that specifies the unique pluripotency of stem cells.

Autophagy in Mycobacterium tuberculosis infection: a passepartout to flush the intruder out?

Tuberculosis is a global health calamity. The causative agent, Mycobacterium tuberculosis (M. tuberculosis), has evolved elaborate survival mechanisms in humans, allowing it to remain in a clinically latent infection state, constantly engaging the immune system, with the possibility to progress to active disease. Autophagy is a cellular process responsible for the degradation of intracellular components, including invading pathogens, playing an important role in both innate and adaptive immunity. In this review, we describe the molecular mechanisms employed by M. tuberculosis to avoid autophagic degradation and exploit this process to its own advantage. Moreover, we discuss the multiple roles played by autophagy in the immune responses to M. tuberculosis, and its unforeseen contribution to the antibacterial activity of tuberculosis-specific drugs.

FOXO3A directs a protective autophagy program in haematopoietic stem cells

Blood production is ensured by rare self-renewing hematopoietic stem cells (HSCs). How HSCs accommodate the diverse cellular stresses associated with their life-long activity remains elusive. Here, we identify autophagy as an essential mechanism protecting HSCs from metabolic stress. We show that HSCs, in contrast to their short-lived myeloid progeny, robustly induce autophagy following ex vivo cytokine withdrawal and in vivo caloric restriction. We demonstrate that FoxO3a is critical to maintain a gene expression program that poise HSCs for rapid induction of autophagy upon starvation. Notably, we find that old HSCs retain an intact FoxO3a-driven pro-autophagy gene program, and that ongoing autophagy is needed to mitigate an energy crisis and allow their survival. Our results demonstrate that autophagy is essential for the life-long maintenance of the HSC compartment and for supporting an old, failing blood system.

The contribution of autophagy to lymphocyte survival and homeostasis

Over the life span of a T lymphocyte, from thymic development to death, it is subjected to a variety of stresses and stimuli. Upon receipt of each stress or stimulus, a potentially life-changing fate decision must be made, namely, whether to commit to a form of programmed cell death or to make the necessary adaptations to effectively deal with the changing environment. In our laboratory, we have identified several stresses that a T lymphocyte will encounter during a normal life span. Our studies have focused on how T cells utilize autophagy to get a grasp on the situation, or in cases in which survival is untenable, how T cells use autophagy to hasten their demise. This review focuses on the functions of T-cell autophagy in maintaining homeostasis, eliminating excess or dangerous levels of mitochondria, trimming levels of endoplasmic reticulum, and promoting a healthy metabolic level to allow cells to perform as productive components of the immune system. In addition, the use of autophagy signaling molecules to perform autophagy-independent tasks involved in the maintenance of immune homeostasis is discussed.