Mitochondrial Proteins Bnip3 and Bnip3L Are Involved in Anthrax Lethal Toxin-induced Macrophage Cell Death

MicroRNA-210 Enhances Cell Survival and Paracrine Potential for Cardiac Cell Therapy While Targeting Mitophagy

The therapeutic potential of presumed cardiac progenitor cells (CPCs) in heart regeneration has garnered significant interest, yet clinical trials have revealed limited efficacy due to challenges in cell survival, retention, and expansion. Priming CPCs to survive the hostile hypoxic environment may be key to enhancing their regenerative capacity. We demonstrate that microRNA-210 (miR-210), known for its role in hypoxic adaptation, significantly improves CPC survival by inhibiting apoptosis through the downregulation of Casp8ap2, a ~40% reduction in caspase activity, and a ~90% decrease in DNA fragmentation. Contrary to the expected induction of Bnip3-dependent mitophagy by hypoxia, miR-210 did not upregulate Bnip3, indicating a distinct anti-apoptotic mechanism. Instead, miR-210 reduced markers of mitophagy and increased mitochondrial biogenesis and oxidative metabolism, suggesting a role in metabolic reprogramming. Furthermore, miR-210 enhanced the secretion of paracrine growth factors from CPCs, with a ~1.6-fold increase in the release of stem cell factor and of insulin growth factor 1, which promoted in vitro endothelial cell proliferation and cardiomyocyte survival. These findings elucidate the multifaceted role of miR-210 in CPC biology and its potential to enhance cell-based therapies for myocardial repair by promoting cell survival, metabolic adaptation, and paracrine signalling.

Urolithin A attenuates auditory cell senescence by activating mitophagy

Aging of sensory organs is associated with a decline in mitochondrial function and the accumulation of dysfunctional mitochondria. Impaired mitophagy blocks the turnover of dysfunctional mitochondria and leads to their accumulation. Urolithin A (UA) induces mitophagy in various mammalian cells. This study was aimed at investigating the effect of the mitophagy activator, UA, on premature senescent auditory cells. The levels of cellular senescence-associated p53 and p21 significantly increased in H₂O₂-induced senescent House Ear Institute-Organ of Corti 1 (HEI-OC1) cells and cochlear explants. However, the levels of mitophagy-related molecules significantly decreased. UA significantly decreased the expression of senescence-associated p53 and p21, and increased the expression of mitophagy-related proteins, in H₂O₂-induced senescent cells and cochlear explants. The percentage of β-galactosidase-stained senescent cells also reduced in H₂O₂-treated cells and cochlear explants upon UA pre-treatment. The formation of mitophagosomes and mitophagolysosomes was restored upon UA pre-treatment of H₂O₂-induced senescent cells. The knockdown of mitophagy-related genes (Parkin and Bnip3) resulted in annulment of UA-induced anti-senescent activity. UA significantly increased the ATP content, mitochondrial DNA (mtDNA) integrity, and mitochondrial membrane potential in senescent HEI-OC1 cells. These findings indicate that UA counteracted mitophagy decline and prevented premature senescence in auditory cells. Hence, UA administration might be a promising strategy for preventing mitochondrial dysfunction in patients with age-related hearing loss.

Uncoupling Protein 2 Increases Blood Pressure in DJ -1 Knockout Mice

Background The redox-sensitive chaperone DJ -1 and uncoupling protein 2 are protective against mitochondrial oxidative stress. We previously reported that renal-selective depletion and germline deletion of DJ -1 increases blood pressure in mice. This study aimed to determine the mechanisms involved in the oxidative stress-mediated hypertension in DJ -1 ^-/- mice. Methods and Results There were no differences in sodium excretion, renal renin expression, renal NADPH oxidase activity, and serum creatinine levels between DJ -1 ^-/- and wild-type mice. Renal expression of nitro-tyrosine, malondialdehyde, and urinary kidney injury marker-1 were increased in DJ -1 ^-/- mice relative to wild-type littermates. mRNA expression of mitochondrial heat shock protein 60 was also elevated in kidneys from DJ -1 ^-/- mice, indicating the presence of oxidative stress. Tempol-treated DJ -1 ^-/- mice presented higher serum nitrite/nitrate levels than vehicle-treated DJ -1 ^-/- mice, suggesting a role of the NO system in the high blood pressure of this model. Tempol treatment normalized renal kidney injury marker-1 and malondialdehyde expression as well as blood pressure in DJ -1 ^-/- mice, but had no effect in wild-type mice. The renal Ucp2 mRNA expression was increased in DJ -1 ^-/- mice versus wild-type and was also normalized by tempol. The renal-selective silencing of Ucp2 led to normalization of blood pressure and serum nitrite/nitrate ratio in DJ -1 ^-/- mice. Conclusions The deletion of DJ -1 leads to oxidative stress-induced hypertension associated with downregulation of NO function, and overexpression of Ucp2 in the kidney increases blood pressure in DJ -1 ^-/- mice. To our knowledge, this is the first report providing evidence of the role of uncoupling protein 2 in blood pressure regulation.

Nix/BNIP3L-dependent mitophagy accounts for airway epithelial cell injury induced by cigarette smoke

Cigarette smoke-induced airway epithelial cell mitophagy is an important mechanism in the pathogenesis of chronic obstructive pulmonary disease (COPD). Mitochondrial protein Nix (also known as BNIP3L) is a selective autophagy receptor and participates in several human diseases. However, little is known about the role of Nix in airway epithelial cell injury during the development of COPD. The aim of the present study is to investigate the effects of Nix on mitophagy and mitochondrial function in airway epithelial cells exposed to cigarette smoke extract (CSE). Our present study has found that CSE could increase Nix protein expression and induce mitophagy in airway epithelial cells. And Nix siRNA significantly inhibited mitophagy and attenuated mitochondrial dysfunction and cell injury when airway epithelial cells were stimulated with 7.5% CSE. In contrast, Nix overexpression enhanced mitophagy and aggravated mitochondrial dysfunction and cell injury when airway epithelial cells were incubated with 7.5% CSE. These data suggest that Nix-dependent mitophagy promotes airway epithelial cell and mitochondria injury induced by cigarette smoke, and may be involved in the pathogenesis of COPD and other cigarette smoke-associated diseases.

Hypoxia Induces Apoptosis through HIF-1α Signaling Pathway in Human Uterosacral Ligaments of Pelvic Organ Prolapse

The purpose of this study is to evaluate the expression of hypoxia-inducible factor-1α (HIF-1α) in women uterosacral ligament tissues with pelvic organ prolapse and women with normal uterine support structures and illuminate its relationship with apoptosis. Samples were collected from 38 women with pelvic organ prolapse and 31 age matched controls. The expression levels of HIF-1α and BNIP3 in the uterosacral ligaments were measured using immunohistochemistry, qRT-PCR, and Western blot. To assess apoptosis we performed TUNEL assay and Western blot analyses. Lastly, the short form of the Pelvic Floor Impact Questionnaire-7 (PFIQ-7) was used to evaluate prognosis of surgical patients and twenty patients finished the follow-up. The expressions of HIF-1α and BNIP3 in the uterosacral ligaments were significantly higher in patients with pelvic organ prolapse than in control group. Pearson's correlation test revealed significant positive correlations between HIF-1α and apoptosis index. Similarly, Western blot analysis showed the expression of proapoptosis proteins (Bax and Bad), Cytochrome-c, cleaved caspase-3, and caspase-9 in patients with pelvic organ prolapse was upregulated. The PFIQ-7 scores were higher in HIF-1α positive group than in the negative group. Hypoxia may contribute to the pathological process of pelvic organ prolapse by increasing apoptosis via activating HIF-1α signaling pathway.

HDAC8 Prevents Anthrax Lethal Toxin-induced Cell Cycle Arrest through Silencing PTEN in Human Monocytic THP-1 Cells

Anthrax lethal toxin (LeTx) is a cytotoxic virulence factor that causes cell cycle arrest and cell death in various cell types. However, susceptibility to the cytotoxic effects varies depending on cell types. In proliferating monocytes, LeTx has only transient cytotoxic effects due to activation of the phosphoinositide 3-kinase (PI3K)-AKT-mediated adaptive responses. To date, the mechanism of LeTx in activating PI3K-AKT signaling axis is unknown. This study shows that the histone deacetylase 8 (HDAC8) is involved in activating PI3K-AKT signaling axis through down-regulating the phosphatase and tensin homolog 1 (PTEN) in human monocytic THP-1 cells. The HDAC8-specific activator TM-2-51 and inhibitor PCI-34051 enhanced and prevented, respectively, AKT activation and cell cycle progression in LeTx-treated cells. Furthermore, HDAC8 induced tri-methylation of histone H3 lysine 27 (H3K27me3), which is known to suppress PTEN expression, through at least in part down-regulating the H3K27me3 eraser Jumonji Domain Containing (JMJD) 3. Importantly, the JMJD3-specific inhibitor GSK-J4 induced AKT activation and protected cell cycle arrest in LeTx-treated cells, regardless the presence of HDAC8 activity. Collectively, this study for the first time demonstrated that HDAC8 activity determines susceptibility to cell cycle arrest induced by LeTx, through regulating the PI3K-PTEN-AKT signaling axis.

Inhibition of Interleukin 1β (IL-1β) Expression by Anthrax Lethal Toxin (LeTx) Is Reversed by Histone Deacetylase 8 (HDAC8) Inhibition in Murine Macrophages

Many pathogenic microbes often release toxins that subvert the host's immune responses to render the environment suitable for their survival and proliferation. LeTx is one of the toxins causing immune paralysis by cleaving and inactivating the mitogen-activated protein kinase (MAPK) kinases (MEKs). Here, we show that inhibition of the histone deacetylase 8 (HDAC8) by either the HDAC8-specific inhibitor PCI-34051 or small interference (si)RNAs rendered LeTx-exposed murine macrophages responsive to LPS in pro-IL-1β production. HDAC8 selectively targeted acetylated histone H3 lysine 27 (H3K27Ac), which is known to associate with active enhancers. LeTx induced HDAC8 expression, in part through inhibiting p38 MAPK, which resulted in a decrease of H3K27Ac levels. Inhibition of HDAC8 increased H3K27Ac levels and enhanced NF-κB-mediated pro-IL-1β enhancer and messenger RNA production in LeTx-exposed macrophages. Collectively, this study demonstrates a novel role of HDAC8 in LeTx immunotoxicity and regulation of pro-IL-1β production likely through eRNAs. Targeting HDAC8 could be a strategy for enhancing immune responses in macrophages exposed to LeTx or other toxins that inhibit MAPKs.

Structure, function, and epigenetic regulation of BNIP3: a pathophysiological relevance

BCL-2 [B-cell leukemia/lymphoma 2]/adenovirus E1B 19KD interacting protein 3 (BNIP3) is an atypical BH3 domain only containing member of Bcl2 family of proteins. BNIP3 is known to be involved in various cellular processes depending on the cell type and conditions and also shown to play a role in various disease conditions including myocardial ischemia, autophagy and apoptosis. Though its role in autophagy and its pro-death activity have been reported in various studies, recent findings have shown its contradictory role in the regulation of these cellular processes. The various studies have shown its epigenetic regulation in disease development and progression and also found to be cytoprotective. In this review, we have focused on the structural and functional aspects of BNIP3 in relation to recent advances of its role in autophagy and apoptosis. Also its role of epigenetic regulation of several genes involved in various diseases was also discussed.

HDAC8-mediated epigenetic reprogramming plays a key role in resistance to anthrax lethal toxin-induced pyroptosis in macrophages

Macrophages pre-exposed to a sublethal dose of anthrax lethal toxin (LeTx) are refractory to subsequent high cytolytic doses of LeTx, termed toxin-induced resistance (TIR). A small population of TIR cells (2-4%) retains TIR characteristics for up to 5-6 wk. Through studying these long-term TIR cells, we found that a high level of histone deacetylase (HDAC)8 expression was crucial for TIR. Knocking down or inhibition of HDAC8 by small interfering RNAs or the HDAC8-specific inhibitor PCI-34051, respectively, induced expression of the mitochondrial death genes Bcl2 adenovirus E1B 19 kDa-interacting protein 3 (BNIP3), BNIP3-like and metastatic lymph node 64, and resensitized TIR cells to LeTx. Among multiple histone acetylations, histone H3 lysine 27 (H3K27) acetylation was most significantly decreased in TIR cells in an HDAC8-dependent manner, and the association of H3K27 acetylation with the genomic regions of BNIP3 and metastatic lymph node 64, where HDAC8 was recruited to, was diminished in TIR cells. Furthermore, overexpression of HDAC8 or knocking down the histone acetyltransferase CREB-binding protein/p300, known to target H3K27, rendered wild-type cells resistant to LeTx. As in RAW264.7 cells, primary bone marrow-derived macrophages exposed to a sublethal dose of LeTx were resistant to LeTx in an HDAC8-dependent manner. Collectively, this study demonstrates that epigenetic reprogramming mediated by HDAC8 plays a key role in determining the susceptibility of LeTx-induced pyroptosis in macrophages.

Hijacking multivesicular bodies enables long-term and exosome-mediated long-distance action of anthrax toxin

Anthrax lethal toxin is a classical AB toxin comprised of two components: protective antigen (PA) and lethal factor (LF). Here, we show that following assembly and endocytosis, PA forms a channel that translocates LF, not only into the cytosol, but also into the lumen of endosomal intraluminal vesicles (ILVs). These ILVs can fuse and release LF into the cytosol, where LF can proteolyze and disable host targets. We find that LF can persist in ILVs for days, fully sheltered from proteolytic degradation, both in vitro and in vivo. During this time, ILV-localized LF can be transmitted to daughter cells upon cell division. In addition, LF-containing ILVs can be delivered to the extracellular medium as exosomes. These can deliver LF to the cytosol of naive cells in a manner that is independent of the typical anthrax toxin receptor-mediated trafficking pathway, while being sheltered from neutralizing extracellular factors of the immune system.

Small-molecule inhibitors of lethal factor protease activity protect against anthrax infection

Bacillus anthracis, the causative agent of anthrax, manifests its pathogenesis through the action of two secreted toxins. The bipartite lethal and edema toxins, a combination of lethal factor or edema factor with the protein protective antigen, are important virulence factors for this bacterium. We previously developed small-molecule inhibitors of lethal factor proteolytic activity (LFIs) and demonstrated their in vivo efficacy in a rat lethal toxin challenge model. In this work, we show that these LFIs protect against lethality caused by anthrax infection in mice when combined with subprotective doses of either antibiotics or neutralizing monoclonal antibodies that target edema factor. Significantly, these inhibitors provided protection against lethal infection when administered as a monotherapy. As little as two doses (10 mg/kg) administered at 2 h and 8 h after spore infection was sufficient to provide a significant survival benefit in infected mice. Administration of LFIs early in the infection was found to inhibit dissemination of vegetative bacteria to the organs in the first 32 h following infection. In addition, neutralizing antibodies against edema factor also inhibited bacterial dissemination with similar efficacy. Together, our findings confirm the important roles that both anthrax toxins play in establishing anthrax infection and demonstrate the potential for small-molecule therapeutics targeting these proteins.

Cellular adaptation to anthrax lethal toxin-induced mitochondrial cholesterol enrichment, hyperpolarization, and reactive oxygen species generation through downregulating MLN64 in macrophages

Cellular adaptation to different stresses related to survival and function has been demonstrated in several cell types. Anthrax lethal toxin (LeTx) induces rapid cell death, termed "pyroptosis," by activating NLRP1b/caspase-1 in murine macrophages. We and others (S. D. Ha et al., J. Biol. Chem. 282:26275-26283, 2007; I. I. Salles et al., Proc. Natl. Acad. Sci. U. S. A. 100:12426 -12431, 2003) have shown that RAW264.7 cells preexposed to sublethal doses of LeTx become resistant to subsequent high cytolytic doses of LeTx, termed toxin-induced resistance (TIR). To date, the cellular mechanisms of pyroptosis and TIR are largely unknown. We found that LeTx caused NLRP1b/caspase-1-dependent mitochondrial dysfunction, including hyperpolarization and generation of reactive oxygen species, which was distinct from that induced by stimuli such as NLRP3-activating ATP. In TIR cells, these mitochondrial events were not detected, although caspase-1 was activated, in response to LeTx. We identified that downregulation of the late endosomal cholesterol-transferring protein MLN64 in TIR cells was involved in TIR. The downregulation of MLN64 in TIR cells was at least in part due to DNA methyltransferase 1-mediated DNA methylation. In wild-type RAW264.7 cells and primary bone marrow-derived macrophages, LeTx caused NLRP1b/caspase-1-dependent mitochondrial translocation of MLN64, resulting in cholesterol enrichment, membrane hyperpolarization, reactive oxygen species (ROS) generation, and depletion of free glutathione (GSH). This study demonstrates for the first time that MLN64 plays a key role in LeTx/caspase-1-induced mitochondrial dysfunction.

Anthrax and the inflammasome

Anthrax lethal toxin (LT), a major virulence determinant of anthrax disease, induces vascular collapse in mice and rats. LT activates the Nlrp1 inflammasome in macrophages and dendritic cells, resulting in caspase-1 activation, IL-1β and IL-18 maturation and a rapid cell death (pyroptosis). This review presents the current understanding of LT-induced activation of Nlrp1 in cells and its consequences for toxin-mediated effects in rodent toxin and spore challenge models.

Role of chondroitin sulfate C in the action of anthrax toxin

Anthrax toxin is produced by Bacillus anthracis, the causative agent of anthrax, and is responsible for the majority of disease symptoms. The toxin consists of 3 proteins, protective antigen (PA), lethal factor (LF), and edema factor (EF), which combine to form lethal and edema toxin. Glycosaminoglycans, which are present on the surface of cells, were investigated with regard to their role in toxicity resulting from anthrax toxin exposure. Lethal toxin-induced cytotoxicity of the RAW 264.7 cells was significantly inhibited by the addition of chondroitin sulfate C as determined by the MTT assay. By contrast, several other glycosaminoglycans, including heparin, heparan sulfate, and dermatan sulfate did not show significant levels of inhibition. Studies utilizing fluorescence-labeled PA demonstrated decreased PA binding to RAW 264.7 cells with the addition of chondroitin sulfate C. Formation of PA oligomers at the surface of cells after binding was also inhibited by chondroitin sulfate C. Interestingly, enzymatic degradation of endogenous chondroitin sulfate C from the cell surface with chondroitinase ABC was accompanied by increased sensitivity to the toxin. These findings were further confirmed by pretreating cells with sodium chlorate to reduce the degree of cell surface glycosaminoglycans sulfation. In addition, chondroitin sulfate C effectively inhibits edema toxin-induced cAMP accumulation in cells. Our results indicate that chondroitin sulfate C may play an important role in the toxicity of anthrax toxin.

Serum amyloid A protects murine macrophages from lethal toxin-mediated death

Lethal toxin, a key virulence factor produced by Bacillus anthracis, induces cell death, in part by disrupting numerous signaling pathways, in mouse macrophages. However, exposure to sublethal doses of lethal toxin allows some cells to survive. Because these pro-survival signaling events occur within a few hours after exposure to sublethal doses, we hypothesized that acute phase proteins might influence macrophage survival. Our data show that serum amyloid A (SAA) is produced in response to lethal toxin treatment. Moreover, pre-treatment of macrophages with exogenous SAA protected macrophages from lethal toxin-mediated death. Exogenous SAA activated the p38 mitogen activated protein kinase (MAP) kinase pathway, while lethal toxin mutants incapable of p38 activation were incapable of causing cell death. Chemical inhibition of the p38 activation pathway abrogated the protective effects of SAA. These data show that SAA affords protection against lethal toxin in mouse macrophages and link this response to the p38 pathway.

Constitutive ERK activity induces downregulation of tristetraprolin, a major protein controlling interleukin8/CXCL8 mRNA stability in melanoma cells

Most melanoma cells are characterized by the V600E mutation in B-Raf kinase. This mutation leads to increased expression of interleukin (CXCL8), which plays a key role in cell growth and angiogenesis. Thus CXCL8 appears to be an interesting therapeutic target. Hence, we performed vaccination of mice with GST-CXCL8, which results in a reduced incidence of syngenic B16 melanoma cell xenograft tumors. We next addressed the molecular mechanisms responsible for aberrant CXCL8 expression in melanoma. The CXCL8 mRNA contains multiples AU-rich sequences (AREs) that modulate mRNA stability through the binding of tristetraprolin (TTP). Melanoma cell lines express very low TTP levels. We therefore hypothesized that the very low endogenous levels of TTP present in different melanoma cell lines might be responsible for the relative stability of CXCL8 mRNAs. We show that TTP is actively degraded by the proteasome and that extracellular-regulated kinase inhibition results in TTP accumulation. Conditional expression of TTP in A375 melanoma cells leads to CXCL8 mRNA destabilization via its 3' untranslated regions (3'-UTR), and TTP overexpression reduces its production. In contrast, downregulation of TTP by short hairpin RNA results in upregulation of CXCL8 mRNA. Maintaining high TTP levels in melanoma cells decreases cell proliferation and autophagy and induces apoptosis. Sorafenib, a therapeutic agent targeting Raf kinases, decreases CXCL8 expression in melanoma cells through reexpression of TTP. We conclude that loss of TTP represents a key event in the establishment of melanomas through constitutive expression of CXCL8, which constitutes a potent therapeutic target.

Sublethal doses of anthrax lethal toxin on the suppression of macrophage phagocytosis

Background

Lethal toxin (LT), the major virulence factor produced by Bacillus anthracis, has been shown to suppress the immune system, which is beneficial to the establishment of B. anthracis infections. It has been suggested that the suppression of MEK/MAPK signaling pathways of leukocytes contributes to LT-mediated immunosuppressive effects. However, the involvement of MAPK independent pathways has not been clearly elucidated; nor has the crucial role played by LT in the early stages of infection. Determining whether LT exerts any pathological effects before being enriched to an MEK inhibitory level is an important next step in the furtherance of this field.

Methodology/Principal Findings

Using a cell culture model, we determined that low doses of LT inhibited phagocytosis of macrophages, without influencing MAPK pathways. Consistent low doses of LT significantly suppressed bacterial clearance and enhanced the mortality of mice with bacteremia, without suppressing the MEK1 of splenic and peripheral blood mononuclear cells.

Conclusion/Significance

These results suggest that LT suppresses the phagocytes in a dose range lower than that required to suppress MEK1 in the early stages of infection.

Phenotype based functional gene screening using retrovirus-mediated gene trapping in quasi-haploid RAW 264.7 cells

In vitro random mutagenesis, followed by phenotype screening, provides a rapid and convenient tool for identifying novel genes involved in the phenotype of interest. However, the forward mutagenic approach in mammalian somatic cells is seriously limited by the diploidic nature of the genome. To overcome this impediment, we developed a method that allows functional screening for both haploid insufficient and sufficient genes involved in the phenotype of interest, utilizing a retrovirus gene trap mutagenesis in chemical mutagen-generated quasi-haploid cells. This method was used to identify novel host genes that are required for macrophage sensitivity to anthrax lethal toxin.

BNIP3 subfamily BH3-only proteins: mitochondrial stress sensors in normal and pathological functions

The BNIP3 subfamily of BH3-only proteins consists of BNIP3 and BNIP3-like (BNIP3L) proteins. These proteins form stable homodimerization complexes that localize to the outer membrane of the mitochondria after cellular stress. This promotes either apoptotic or non-apoptotic cell death such as autophagic cell death. Although the mammalian cells contain both members of this subfamily, the genome of Caenorhabditis elegans codes for a single BNIP3 ortholog, ceBNIP3, which shares homology in the transmembrane (TM) domain and in a conserved region close to the BH3 domain of mammalian BNIP3 protein. The cell death activities of BNIP3 and BNIP3L are determined by either the BH3 domain or the C-terminal TM domain. The TM domain of BNIP3 is unique, as it is capable of autonomous stable dimerization and contributes to mitochondrial localization of BNIP3. In knockout mouse models, BNIP3L was shown to be essential for normal erythrocyte differentiation and hematopoietic homeostasis, whereas BNIP3 plays a role in cellular responses to ischemia/reperfusion injury in the heart. Both BNIP3 and BNIP3L play a role in cellular responses to stress. Under hypoxia, both BNIP3 and BNIP3L expression levels are elevated and contribute to hypoxia-induced cell death. In addition, these proteins play critical roles in disease states. In heart disease, both BNIP3 and BNIP3L play a critical role in cardiomyocyte cell death following ischemic and non-ischemic injuries. In cancer, expression of BNIP3 and BNIP3L is downregulated by promoter hypermethylation or by homozygous deletion of the gene locus in certain cancers, whereas their expression was increased in other cancers. In addition, BNIP3 expression has been correlated with poor prognosis in some cancers. The results reviewed here suggest that BNIP3 and BNIP3L may be novel therapeutic targets for intervention because of their pathological roles in regulating cell death in disease states.

ATP Depletion Via Mitochondrial F1F0 Complex by Lethal Factor is an Early Event in B. Anthracis-Induced Sudden Cell Death

Bacillus anthracis’ primary virulence factor is a tripartite anthrax toxin consisting of edema factor (EF), lethal factor (LF) and protective antigen (PA). In complex with PA, EF and LF are internalized via receptor-mediated endocytosis. EF is a calmodulin-dependent adenylate cyclase that induces tissue edema. LF is a zinc-metalloprotease that cleaves members of mitogen-activated protein kinase kinases. Lethal toxin (LT: PA plus LF)-induced death of macrophages is primarily attributed to expression of the sensitive Nalp1b allele, inflammasome formation and activation of caspase-1, but early events that initiate these processes are unknown. Here we provide evidence that an early essential event in pyroptosis of alveolar macrophages is LF-mediated depletion of cellular ATP. The underlying mechanism involves interaction of LF with F₁F₀-complex gamma and beta subunits leading to increased ATPase activity in mitochondria. In support, mitochondrial DNA-depleted MH-S cells have decreased F₁F₀ ATPase activity due to the lack of F₀6 and F₀8 polypeptides and show increased resistance to LT. We conclude that ATP depletion is an important early event in LT-induced sudden cell death and its prevention increases survival of toxin-sensitive cells.

Expression of Nlrp1b inflammasome components in human fibroblasts confers susceptibility to anthrax lethal toxin

Anthrax lethal toxin causes macrophages and dendritic cells from some mouse strains to undergo caspase-1-dependent cell death. Central to this process is the NOD-like receptor Nlrp1b (Nalp1b), which detects intoxication and then self-associates to form a complex, termed an inflammasome, that is capable of activating the procaspase-1 zymogen. The nature of the signal detected directly by Nlrp1b is not known, and the mechanisms of inflammasome assembly are poorly understood. Here, we demonstrate that transfection of human fibroblasts with plasmids encoding murine Nlrp1b and procaspase-1 was sufficient to confer susceptibility to lethal toxin-mediated death on the cells. As has been observed in murine macrophages, the enzymatic activities of lethal toxin and the proteasome were both required for activation of the Nlrp1b inflammasome and this activation led to prointerleukin-1 beta processing. Release of interleukin-1beta from cells was not dependent on cell lysis, as its secretion was not affected by an osmoprotectant that prevented the appearance of lactate dehydrogenase in the culture medium. We generated constitutively active mutants of Nlrp1b by making amino-terminal deletions to the protein and observed that the ability to activate procaspase-1 was dependent on the CARD domain, which bound procaspase-1, and a region adjacent to the CARD domain that promoted self-association. Our results demonstrate that lethal toxin can activate Nlrp1b in a nonmyeloid cell line and are consistent with work that suggests that activation induces proximity of procaspase-1.

Cellular and systemic effects of anthrax lethal toxin and edema toxin

Anthrax lethal toxin (LT) and edema toxin (ET) are the major virulence factors of anthrax and can replicate the lethality and symptoms associated with the disease. This review provides an overview of our current understanding of anthrax toxin effects in animal models and the cytotoxicity (necrosis and apoptosis) induced by LT in different cells. A brief reexamination of early historic findings on toxin in vivo effects in the context of our current knowledge is also presented.

Anthrax toxins: a weapon to systematically dismantle the host immune defenses

Successful colonization of the host by bacterial pathogens relies on their capacity to evade the complex and powerful defenses opposed by the host immune system, at least in the initial phases of infection. The two toxins of Bacillus anthracis, lethal toxin and edema toxin, appear to have been shaped by evolution to assist the microorganism in this crucial function, in addition to act as general toxins acting on almost all cell types. Edema toxin causes a consistent elevation of cAMP, an important second messenger the production of which is normally strictly controlled in mammalian cells, whereas lethal toxin cleaves most isoforms of mitogen-activated protein kinase kinases. By disrupting or subverting central modules common to all the principal signaling networks which control immune cell activation, effector function and migration, the anthrax toxins effectively and systematically dismantle both the innate and the adaptive immune defenses of the host. Here, we review the specific effects of the lethal and edema toxins of B. anthracis on the activation and function of phagocytes, dendritic cells and lymphocytes. We also discuss some open issues which should be addressed to gain a comprehensive insight into the complex relationship that B. anthracis establishes with the host.

Lung epithelial injury by B. anthracis lethal toxin is caused by MKK-dependent loss of cytoskeletal integrity

Bacillus anthracis lethal toxin (LT) is a key virulence factor of anthrax and contributes significantly to the in vivo pathology. The enzymatically active component is a Zn²⁺-dependent metalloprotease that cleaves most isoforms of mitogen-activated protein kinase kinases (MKKs). Using ex vivo differentiated human lung epithelium we report that LT destroys lung epithelial barrier function and wound healing responses by immobilizing the actin and microtubule network. Long-term exposure to the toxin generated a unique cellular phenotype characterized by increased actin filament assembly, microtubule stabilization, and changes in junction complexes and focal adhesions. LT-exposed cells displayed randomly oriented, highly dynamic protrusions, polarization defects and impaired cell migration. Reconstitution of MAPK pathways revealed that this LT-induced phenotype was primarily dependent on the coordinated loss of MKK1 and MKK2 signaling. Thus, MKKs control fundamental aspects of cytoskeletal dynamics and cell motility. Even though LT disabled repair mechanisms, agents such as keratinocyte growth factor or dexamethasone improved epithelial barrier integrity by reducing cell death. These results suggest that co-administration of anti-cytotoxic drugs may be of benefit when treating inhalational anthrax.

Shigella induces mitochondrial dysfunction and cell death in nonmyleoid cells

Shigella rapidly kills myeloid cells via a caspase-1 inflammasome-dependent cell death mechanism. However, despite a critical role for nonmyeloid cells in the physiopathology of Shigella infection, the mechanism by which Shigella kills nonmyeloid cells remains uncharacterized. Here we demonstrate that, in nonmyeloid cells, Shigella infection induces loss of mitochondrial inner membrane potential, mitochondrial damage, and necrotic cell death through a pathway dependent on Bnip3 and cyclophilin D, two molecules implicated in the host oxidative stress responses. This mitochondrial cell death mechanism was potently counterbalanced by a Nod1-dependent Rip2/IKKbeta/NF-kappaB signaling pathway activated by the pathogen in the first hours of infection. Our results suggest that in nonmyeloid cells, oxidative stress pathways and signaling triggered by an intracellular bacterial pathogen are tightly linked and demonstrate the existence of specific Shigella-induced prodeath and prosurvival pathways converging at the mitochondria to control a necrotic cell death program.

Induction of autophagy by anthrax lethal toxin

Autophagy is an evolutionary conserved intracellular process whereby cells break down long-lived proteins and organelles. Accumulating evidences suggest increasing physiological significance of autophagy in pathogenesis of infectious diseases. Anthrax lethal toxin (LT) exerts its influence on numerous cells and herein, we report a novel effect of LT-induced autophagy on mammalian cells. Several autophagy biochemical markers including LC3-II conversion, increased punctuate distribution of GFP-LC3 and development of acidic vesicular organelles (AVO) were detected in cells treated with LT. Analysis of individual LT component revealed a moderate increase in LC3-II conversion for protective antigen-treated cells, whereas the LC3-II level in lethal factor-treated cells remained unchanged. In addition, our preliminary findings suggest a protective role of autophagy in LT intoxication as autophagy inhibition resulted in accelerated cell death. This study presents a hitherto undescribed effect of LT-induced autophagy on cells and provides the groundwork for future studies on the implication of autophagy in anthrax pathogenesis.

Inhibition of anthrax lethal toxin-induced cytolysis of RAW264.7 cells by celastrol

Background

Bacillus anthracis is the bacterium responsible for causing anthrax. The ability of B. anthracis to cause disease is dependent on a secreted virulence factor, lethal toxin, that promotes survival of the bacteria in the host by impairing the immune response. A well-studied effect of lethal toxin is the killing of macrophages, although the molecular mechanisms involved have not been fully characterized.

Methodology/Principal Findings

Here, we demonstrate that celastrol, a quinone methide triterpene derived from a plant extract used in herbal medicine, inhibits lethal toxin-induced death of RAW264.7 murine macrophages. Celastrol did not prevent cleavage of mitogen activated protein kinase kinase 1, a cytosolic target of the toxin, indicating that it did not inhibit the uptake or catalytic activity of lethal toxin. Surprisingly, celastrol conferred almost complete protection when it was added up to 1.5 h after intoxication, indicating that it could rescue cells in the late stages of intoxication. Since the activity of the proteasome has been implicated in intoxication using other pharmacological agents, we tested whether celastrol blocked proteasome activity. We found that celastrol inhibited the proteasome-dependent degradation of proteins in RAW264.7 cells, but only slightly inhibited proteasome-mediated cleavage of fluorogenic substrates in vitro. Furthermore, celastrol blocked stimulation of IL-18 processing, indicating that celastrol acted upstream of inflammasome activation.

Conclusions/Significance

This work identifies celastrol as an inhibitor of lethal toxin-mediated macrophage lysis and suggests an inhibitory mechanism involving inhibition of the proteasome pathway.

Defensive strategies of Bacillus anthracis that promote a fatal disease

Bacillus anthracis is a Gram-positive bacterium that causes anthrax. Bacterial spores that enter the host germinate into metabolically active bacilli that disseminate throughout the body and replicate to high numbers. Two virulence factors are essential for this unrestrained growth. The first is a weakly immunogenic poly gamma-D-glutamic acid capsule that surrounds the bacilli and confers resistance to phagocytosis. The second virulence factor, anthrax toxin, disrupts multiple host functions to diminish the immune response.

Critical role of the phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase-3 signaling pathway in recovery from anthrax lethal toxin-induced cell cycle arrest and MEK cleavage in macrophages

Anthrax lethal toxin (LeTx) is a virulence factor causing immune suppression and toxic shock of Bacillus anthracis infected host. It inhibits cytokine production and cell proliferation/differentiation in various immune cells. This study showed that a brief exposure of LeTx caused a continual MEK1 cleavage and prevented tumor necrosis factor-alpha (TNF) production in response to lipopolysaccharide (LPS) in non-proliferating cells such as human peripheral blood mononuclear cells or mouse primary peritoneal macrophages. In human monocytic cell lines U-937 and THP-1, LeTx induced cell cycle arrest in G0-G1 phase by rapid down-regulation of cyclin D1/D2 and checkpoint kinase 1 through MEK1 inhibition. However, THP-1 cells adaptively adjusted to LeTx and overrode cell cycle arrest by activating the phosphatidylinositol 3-kinase/Akt signaling pathway. Inhibitory Ser-9 phosphorylation of glycogen synthase kinase 3beta (GSK3beta) by Akt prevented proteasome-mediated cyclin D1 degradation and induced cell cycle progress in LeTx-intoxicated THP-1 cells. Recovery from cell cycle arrest was required before recovering from on-going MEK1 cleavage and suppression of TNF production. Furthermore, pretreatment with LeTx or the GSK3-specific inhibitor SB-216763, or transfection with dominant active mutant Akt or degradation-defected mutant cyclin D1 protected cells from LeTx-induced cell cycle arrest, on-going MEK1 cleavage and suppression of TNF production. These results indicate that modulation of phosphatidylinositol 3-kinase/Akt/GSK3beta signaling cascades can be beneficial for protecting or facilitating recovery from cellular LeTx intoxication in cells that depend on basal MEK1 activity for proliferation.