Simultaneous induction of apoptotic and survival signaling pathways in macrophage-like THP-1 cells by Shiga toxin 1

Tissue-resident macrophages mediate neutrophil recruitment and kidney injury in shiga toxin-induced hemolytic uremic syndrome

Enterohaemorrhagic E. coli cause major epidemics worldwide with significant organ damage and very high percentages of death. Due to the ability of enterohaemorrhagic E. coli to produce shiga toxin these bacteria damage the kidney leading to the hemolytic uremic syndrome. A therapy against this serious kidney disease has not been developed yet and the impact and mechanism of leukocyte activation and recruitment are unclear. Tissue-resident macrophages represent the main leukocyte population in the healthy kidney, but the role of this important cell population in shiga toxin-producing E. coli-hemolytic uremic syndrome is incompletely understood. Using state of the art microscopy and mass spectrometry imaging, our preclinical study demonstrated a phenotypic and functional switch of tissue-resident macrophages after disease induction in mice. Kidney macrophages produced the inflammatory molecule TNFα and depletion of tissue-resident macrophages via the CSF1 receptor abolished TNFα levels in the kidney and significantly diminished disease severity. Furthermore, macrophage depletion did not only attenuate endothelial damage and thrombocytopenia, but also activation of thrombocytes and neutrophils. Moreover, we observed that neutrophils infiltrated the kidney cortex and depletion of macrophages significantly reduced the recruitment of neutrophils and expression of the neutrophil-attracting chemokines CXCL1 and CXCL2. Intravital microscopy revealed that inhibition of CXCR2, the receptor for CXCL1 and CXCL2, significantly reduced the infiltration of neutrophils and reduced kidney injury. Thus, our study shows activation of tissue-resident macrophages during shiga toxin-producing E. coli-hemolytic uremic syndrome leading to the production of disease-promoting TNFα and CXCR2-dependent recruitment of neutrophils.

Pathogenic functions and diagnostic utility of cytokines/chemokines in EHEC-HUS

Hemolytic - uremic syndrome (HUS) is a severe complication of infection by Shiga toxin (STx)-producing enterohemorrhagic Escherichia coli. Hemolytic - uremic syndrome is defined clinically as a triad of non-immune microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injuries. Neurologic complications such as acute encephalopathy are also observed. In humans, endothelial cells, proximal tubular epithelial cells, mesangial cells, podocytes, intestinal epithelial cells, and monocytes / macrophages are susceptible to STx-mediated injury. Shiga toxin induces the secretion of inflammatory cytokines and chemokines from susceptible cells, including tumor necrosis factor-α interleukin (IL)-1, IL-6, and IL-8. These cytokines and chemokines contribute to the pathogenesis of HUS and encephalopathy by enhancing STx-induced cytotoxicity and inducing inflammatory cell infiltration. Serum cytokine/chemokine levels are therefore useful as indicators of disease activity and predictors of progression from acute kidney injury to chronic kidney disease. Anti-inflammation therapy combined with apheresis to remove excessive cytokines / chemokines and methylprednisolone pulse therapy to suppress cytokine/chemokine production may be an effective treatment regimen for severe E. coli-associated HUS. However, this regimen requires careful monitoring of potential side effects, such as infections, thrombus formation, and hypertension.

Pathogenic role of inflammatory response during Shiga toxin-associated hemolytic uremic syndrome (HUS)

Hemolytic uremic syndrome (HUS) is defined as a triad of noninmune microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. The most frequent presentation is secondary to Shiga toxin (Stx)-producing Escherichia coli (STEC) infections, which is termed postdiarrheal, epidemiologic or Stx-HUS, considering that Stx is the necessary etiological factor. After ingestion, STEC colonize the intestine and produce Stx, which translocates across the intestinal epithelium. Once Stx enters the bloodstream, it interacts with renal endothelial and epithelial cells, and leukocytes. This review summarizes the current evidence about the involvement of inflammatory components as central pathogenic factors that could determine outcome of STEC infections. Intestinal inflammation may favor epithelial leakage and subsequent passage of Stx to the systemic circulation. Vascular damage triggered by Stx promotes not only release of thrombin and increased fibrin concentration but also production of cytokines and chemokines by endothelial cells. Recent evidence from animal models and patients strongly indicate that several immune cells types may participate in HUS physiopathology: neutrophils, through release of proteases and reactive oxygen species (ROS); monocytes/macrophages through secretion of cytokines and chemokines. In addition, high levels of Bb factor and soluble C5b-9 (sC5b-9) in plasma as well as complement factors adhered to platelet-leukocyte complexes, microparticles and microvesicles, suggest activation of the alternative pathway of complement. Thus, acute immune response secondary to STEC infection, the Stx stimulatory effect on different immune cells, and inflammatory stimulus secondary to endothelial damage all together converge to define a strong inflammatory status that worsens Stx toxicity and disease.

Shiga Toxins Induce Apoptosis and ER Stress in Human Retinal Pigment Epithelial Cells

Shiga toxins (Stxs) produced by Shiga toxin-producing bacteria Shigella dysenteriae serotype 1 and select serotypes of Escherichia coli are the most potent known virulence factors in the pathogenesis of hemorrhagic colitis progressing to potentially fatal systemic complications such as acute renal failure, blindness and neurological abnormalities. Although numerous studies have defined apoptotic responses to Shiga toxin type 1 (Stx1) or Shiga toxin type 2 (Stx2) in a variety of cell types, the potential significance of Stx-induced apoptosis of photoreceptor and pigmented cells of the eye following intoxication is unknown. We explored the use of immortalized human retinal pigment epithelial (RPE) cells as an in vitro model of Stx-induced retinal damage. To the best of our knowledge, this study is the first report that intoxication of RPE cells with Stxs activates both apoptotic cell death signaling and the endoplasmic reticulum (ER) stress response. Using live-cell imaging analysis, fluorescently labeled Stx1 or Stx2 were internalized and routed to the RPE cell endoplasmic reticulum. RPE cells were significantly sensitive to wild type Stxs by 72 h, while the cells survived challenge with enzymatically deficient mutant toxins (Stx1A⁻ or Stx2A⁻). Upon exposure to purified Stxs, RPE cells showed activation of a caspase-dependent apoptotic program involving a reduction of mitochondrial transmembrane potential (Δψ_m), increased activation of ER stress sensors IRE1, PERK and ATF6, and overexpression CHOP and DR5. Finally, we demonstrated that treatment of RPE cells with Stxs resulted in the activation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38MAPK), suggesting that the ribotoxic stress response may be triggered. Collectively, these data support the involvement of Stx-induced apoptosis in ocular complications of intoxication. The evaluation of apoptotic responses to Stxs by cells isolated from multiple organs may reveal unique functional patterns of the cytotoxic actions of these toxins in the systemic complications that follow ingestion of toxin-producing bacteria.

Shiga Toxins Activate the NLRP3 Inflammasome Pathway To Promote Both Production of the Proinflammatory Cytokine Interleukin-1β and Apoptotic Cell Death

Shiga toxin (Stx)-mediated immune responses, including the production of the proinflammatory cytokines tumor necrosis-α (TNF-α) and interleukin-1β (IL-1β), may exacerbate vascular damage and accelerate lethality. However, the immune signaling pathway activated in response to Stx is not well understood. Here, we demonstrate that enzymatically active Stx, which leads to ribotoxic stress, triggers NLRP3 inflammasome-dependent caspase-1 activation and IL-1β secretion in differentiated macrophage-like THP-1 (D-THP-1) cells. The treatment of cells with a chemical inhibitor of glycosphingolipid biosynthesis, which suppresses the expression of the Stx receptor globotriaosylceramide and subsequent endocytosis of the toxin, substantially blocked activation of the NLRP3 inflammasome and processing of caspase-1 and IL-1β. Processing and release of both caspase-1 and IL-1β were significantly reduced or abolished in Stx-intoxicated D-THP-1 cells in which the expression of NLRP3 or ASC was stably knocked down. Furthermore, Stx mediated the activation of caspases involved in apoptosis in an NLRP3- or ASC-dependent manner. In Stx-intoxicated cells, the NLRP3 inflammasome triggered the activation of caspase-8/3, leading to the initiation of apoptosis, in addition to caspase-1-dependent pyroptotic cell death. Taken together, these results suggest that Stxs trigger the NLRP3 inflammasome pathway to release proinflammatory IL-1β as well as to promote apoptotic cell death.

Shiga toxin type 2dact displays increased binding to globotriaosylceramide in vitro and increased lethality in mice after activation by elastase

Shiga toxin type 2dact (Stx2dact), an Stx2 variant originally identified from Escherichia coli O91:H21 strain B2F1, displays increased cytotoxicity after activation by elastase present in intestinal mucus. Activation is a result of cleavage of two amino acids from the C-terminal tail of the A2 subunit. In this study, we hypothesized that activation leads to increased binding of toxin to its receptor on host cells both in vitro and in vivo. To test this theory, Stx2dact was treated with elastase or buffer alone and then each toxin was assessed for binding to purified globotriaosylceramide (Gb3) in an enzyme-linked immunosorbent assay, or cells in culture by immunofluorescence, or flow cytometry. Elastase- and buffer-treated Stx2dact were also evaluated for binding to mouse kidney tissue and for relative lethality in mice. We found that activated Stx2dact had a greater capacity to bind purified Gb3, cells in culture, and mouse kidney tissue and was more toxic for mice than was non-activated Stx2dact. Thus, one possible mechanism for the augmented cytotoxicity of Stx2dact after activation is its increased capacity to bind target cells, which, in turn, may cause greater lethality of elastase-treated toxin for mice and enhanced virulence for humans of E. coli strains that express Stx2dact.

Transfection of macrophages by collagen hollow spheres loaded with polyplexes: a step towards modulating inflammation

Macrophages are key orchestrators of inflammation as they secrete proteases and inflammatory cytokines. To date, therapies aimed at modulating macrophage phenotype have failed due to the short half-life of biomolecules in the body. Therefore, inhibition of inflammation by gene therapy constitutes a new hope. In the present study, we have assessed collagen hollow spheres as a reservoir system for polyplexes in order to transfect human macrophages while preserving cell viability. Polyplexes were formed by complexing G-Luc plasmid with a poly(2-dimethylaminoethyl methacrylate) poly(ethylene glycol) based hyperbranched polymer. Several ratios of polymer/pDNA (5:1, 8:1, 10:1w/w) complexes in two different sphere sizes (1.24 and 4.5μm) were tested. Collagen hollow spheres were loaded with polyplexes up to 80μg of pDNA per mg of microspheres. The release of polyplexes from the spheres was delayed and prolonged i.e. 20% of the initial amount released in 5days. Following incubation with polyplex-loaded microspheres, macrophages were transfected (polyplex pDNA:polymer ratio 1:10w/w). In addition, collagen hollow spheres maintained cell viability as more than 80% of cells were viable after 4days in culture. In contrast, when used alone, polyplexes were seen to be toxic, while there was no transfection detected. Taken together, these results show that collagen hollow spheres may be used as a reservoir for controlled gene delivery to macrophages. Unlike existing gene delivery systems, this system allows for macrophage transfection with minimal toxicity. Hence, this system has a potential for the delivery of a therapeutic gene in order to modulate inflammation.

Shiga toxin 1 induces on lipopolysaccharide-treated astrocytes the release of tumor necrosis factor-alpha that alter brain-like endothelium integrity

The hemolytic uremic syndrome (HUS) is characterized by hemolytic anemia, thrombocytopenia and renal dysfunction. The typical form of HUS is generally associated with infections by Gram-negative Shiga toxin (Stx)-producing Escherichia coli (STEC). Endothelial dysfunction induced by Stx is central, but bacterial lipopolysaccharide (LPS) and neutrophils (PMN) contribute to the pathophysiology. Although renal failure is characteristic of this syndrome, neurological complications occur in severe cases and is usually associated with death. Impaired blood-brain barrier (BBB) is associated with damage to cerebral endothelial cells (ECs) that comprise the BBB. Astrocytes (ASTs) are inflammatory cells in the brain and determine the BBB function. ASTs are in close proximity to ECs, hence the study of the effects of Stx1 and LPS on ASTs, and the influence of their response on ECs is essential. We have previously demonstrated that Stx1 and LPS induced activation of rat ASTs and the release of inflammatory factors such as TNF-α, nitric oxide and chemokines. Here, we demonstrate that rat ASTs-derived factors alter permeability of ECs with brain properties (HUVECd); suggesting that functional properties of BBB could also be affected. Additionally, these factors activate HUVECd and render them into a proagregant state promoting PMN and platelets adhesion. Moreover, these effects were dependent on ASTs secreted-TNF-α. Stx1 and LPS-induced ASTs response could influence brain ECs integrity and BBB function once Stx and factors associated to the STEC infection reach the brain parenchyma and therefore contribute to the development of the neuropathology observed in HUS.

Hemolytic-uremic syndrome (HUS) is generally caused by Shiga toxin (Stx)-producing Escherichia coli but bacterial lipopolysaccharide (LPS) and neutrophils (PMN) contribute to the pathophysiology. Acute renal failure is the main feature of HUS, but in severe cases, patients develop neurological complications, which are usually associated with death. Although the mechanisms of neurological damage remain uncertain, alterations/injury of brain endothelial cells (ECs) which constitute the blood-brain barrier (BBB) is clear. Astrocytes (ASTs) are inflammatory cells enclosing ECs and are responsible of the normal function of the barrier. We have recently demonstrated that Stx1, one of the most common types of Stx, induce an inflammatory response in LPS-treated ASTs. We then study the effects of factors released by ASTs in response to LPS and/or Stx1 on brain-like ECs. We demonstrate that Stx1 induces in LPS-treated ASTs the release of factors that alter brain properties in ECs, including the permeability; turning them more susceptible to Stx1 toxic effects. Furthermore, they activate ECs, neutrophils (PMN) and platelets and render ECs into a proagregant state promoting PMN and platelet adhesion. Our results suggest that ASTs could influence brain ECs integrity and BBB function once Stx in combination with bacterial factors reach the brain parenchyma.

Activation of cell stress response pathways by Shiga toxins

Shiga toxin-producing bacteria cause widespread outbreaks of bloody diarrhoea that may progress to life-threatening systemic complications. Shiga toxins (Stxs), the main virulence factors expressed by the pathogens, are ribosome-inactivating proteins which inhibit protein synthesis by removing an adenine residue from 28S rRNA. Recently, Stxs were shown to activate multiple stress-associated signalling pathways in mammalian cells. The ribotoxic stress response is activated following the depurination reaction localized to the α-sarcin/ricin loop of eukaryotic ribosomes. The unfolded protein response (UPR) may be initiated by toxin unfolding within the endoplasmic reticulum, and maintained by production of truncated, misfolded proteins following intoxication. Activation of the ribotoxic stress response leads to signalling through MAPK cascades, which appears to be critical for activation of innate immunity and regulation of apoptosis. Precise mechanisms linking ribosomal damage with MAPK activation require clarification but may involve recognition of ribosomal conformational changes and binding of protein kinases to ribosomes, which activate MAP3Ks and MAP2Ks. Stxs appear capable of activating all ER membrane localized UPR sensors. Prolonged signalling through the UPR induces apoptosis in some cell types. The characterization of stress responses activated by Stxs may identify targets for the development of interventional therapies to block cell damage and disease progression.

Interactions with M cells and macrophages as key steps in the pathogenesis of enterohemorrhagic Escherichia coli infections

Enterohemorrhagic Escherichia coli (EHEC) are food-borne pathogens that can cause serious infections ranging from diarrhea to hemorrhagic colitis (HC) and hemolytic-uremic syndrome (HUS). Translocation of Shiga-toxins (Stx) from the gut lumen to underlying tissues is a decisive step in the development of the infection, but the mechanisms involved remain unclear. Many bacterial pathogens target the follicle-associated epithelium, which overlies Peyer's patches (PPs), cross the intestinal barrier through M cells and are captured by mucosal macrophages. Here, translocation across M cells, as well as survival and proliferation of EHEC strains within THP-1 macrophages were investigated using EHEC O157:H7 reference strains, isogenic mutants, and 15 EHEC strains isolated from HC/HUS patients. We showed for the first time that E. coli O157:H7 strains are able to interact in vivo with murine PPs, to translocate ex vivo through murine ileal mucosa with PPs and across an in vitro human M cell model. EHEC strains are also able to survive and to produce Stx in macrophages, which induce cell apoptosis and Stx release. In conclusion, our results suggest that the uptake of EHEC by M cells and underlying macrophages in the PP may be a critical step in Stx translocation and release in vivo. A new model for EHEC infection in humans is proposed that could help in a fuller understanding of EHEC-associated diseases.

Nitric oxide-mediated apoptosis in rat macrophages subjected to Shiga toxin 2 from Escherichia coli

Shiga toxin-producing Escherichia coli are important food-borne pathogens. The main factor conferring virulence on this bacterium is its capacity to secrete Shiga toxins (Stxs), which have been reported to induce apoptosis in several cell types. However, the mechanisms of this apoptosis have not yet been fully elucidated. In addition, Stxs have been shown to stimulate macrophages to produce nitric oxide (NO), a well-known apoptosis inductor.The aim of this study was to investigate the participation of NO in apoptosis of rat peritoneal macrophages induced by culture supernatants or Stx2 from E. coli. Peritoneal macrophages incubated in the presence of E. coli supernatants showed an increase in the amounts of apoptosis and NO production. Furthermore, inhibition of NO synthesis induced by addition of aminoguanidine (AG) was correlated with a reduction in the percentage of apoptotic cells, indicating participation of this metabolite in the apoptotic process. Similarly, treatment of cells with Stx2 induced an increase in NO production and amount of apoptosis, these changes being reversed by addition of AG. In summary, these data show that treatment with E. coli supernatants or Stx2 induces NO-mediated apoptosis of macrophages.

Concurrent proinflammatory and apoptotic activity of a Helicobacter pylori protein (HP986) points to its role in chronic persistence

Helicobacter pylori induces cytokine mediated changes in gastroduodenal pathophysiology, wherein, the activated macrophages at the sub-mucosal space play a central role in mounting innate immune response against the antigens. The bacterium gains niche through persistent inflammation and local immune-suppression causing peptic ulcer disease or chronic gastritis; the latter being a significant risk factor for the development of gastric adenocarcinoma. What favors persistence of H. pylori in the gastric niches is not clearly understood. We report detailed characterization of a functionally unknown gene (HP986), which was detected in patient isolates associated with peptic ulcer and gastric carcinoma. Expression and purification of recombinant HP986 (rHP986) revealed a novel, ∼29 kDa protein in biologically active form which associates with significant levels of humoral immune responses in diseased individuals (p<0.001). Also, it induced significant levels of TNF-α and Interleukin-8 in cultured human macrophages concurrent to the translocation of nuclear transcription factor-κB (NF-κB). Further, the rHP986 induced apoptosis of cultured macrophages through a Fas mediated pathway. Dissection of the underlying signaling mechanism revealed that rHP986 induces both TNFR1 and Fas expression to lead to apoptosis. We further demonstrated interaction of HP986 with TNFR1 through computational and experimental approaches. Independent proinflammatory and apoptotic responses triggered by rHP986 as shown in this study point to its role, possibly as a survival strategy to gain niche through inflammation and to counter the activated macrophages to avoid clearance.

Shiga toxins induce autophagy leading to differential signalling pathways in toxin-sensitive and toxin-resistant human cells

The bacterial virulence factors Shiga toxins (Stxs) are expressed by Shigella dysenteriae serotype 1 and certain Escherichia coli strains. Stxs are protein synthesis inhibitors and induce apoptosis in many cell types. Stxs induce apoptosis via prolonged endoplasmic reticulum stress signalling to activate both extrinsic and intrinsic pathways in human myeloid cells. Studies have shown that autophagy, a lysosome-dependent catabolic process, may be associated with activation of pro-survival or death processes. It is currently unknown if autophagy contributes to apoptosis or protects cells from Stxs. To study cellular responses to Stxs, we intoxicated toxin-sensitive cells (THP-1 and HK-2 cells), and toxin-resistant cells (primary human monocyte-derived macrophages) and examined toxin intracellular trafficking and autophagosome formation. Stxs translocated to different cell compartments in toxin-resistant versus toxin-sensitive cells. Confocal microscopy revealed autophagosome formation in both toxin-resistant and toxin-sensitive cells. Proteolytic cleavage of Atg5 and Beclin-1 plays pivotal roles in switching non-cytotoxic autophagy to cell death signalling. We detected cleaved forms of Atg5 and Beclin-1 in Stx-treated toxin-sensitive cells, while cleaved caspases, calpains, Atg5 and Beclin-1 were not detected in toxin-resistant primary human monocytes and macrophages. These findings suggest that toxin sensitivity correlates with caspase and calpain activation, leading to Atg5 and Beclin-1 cleavage.

The induction of apoptosis by Shiga toxins and ricin

Shiga toxins and ricin are ribosome-inactivating proteins which share the property of inhibiting protein synthesis by catalytic inactivation of eukaryotic ribosomes. There is now abundant evidence that Shiga toxins and ricin induce apoptosis in epithelial, endothelial, lymphoid and myeloid cells in vitro, and in multiple organs in animals when administered these toxins. Many studies suggest that protein synthesis inhibition and apoptosis induction mediated by Shiga toxins and ricin may be dissociated. In some cells, non-enzymatic toxin components (Shiga toxin B-subunits, ricin B-chain) appear capable of inducing apoptosis. The toxins appear capable of activating components of both the extrinsic or death receptor-mediated and intrinsic or mitochondrial-mediated pathways of apoptosis induction. Although the toxins have been shown to be capable of activating several cell stress response pathways, the precise signaling mechanisms by which Shiga toxins and ricin induce apoptosis remain to be fully characterized. This chapter provides an overview of studies describing Shiga toxin- and ricin-induced apoptosis and reviews evidence that signaling through the ribotoxic stress response and the unfolded protein response may be involved in apoptosis induction in some cell types.

Hyperglycemia magnifies Schwann cell dysfunction and cell death triggered by PA-induced lipotoxicity

Lipid overload resulting in lipotoxicity is prominent in a number of chronic diseases and has been associated with cellular dysfunction and cell death. This study characterizes palmitic acid-induced lipotoxicity (PA-LTx) in Schwann cell cultures grown in normal and high glucose concentrations. The study shows for the first time that Schwann cell (SC) cultures exposed to elevated levels of PA exhibit a dose- and time-dependent loss in cell viability. Hoescht and Annexin V/7AAD staining confirmed cell death through apoptosis and the lipotoxic effect was more dramatic in SC cultures grown under high glucose conditions. The first indication of cellular dysfunction in treated SC cultures was a decrease in Ca(++) levels in the endoplasmic reticulum (ER, [Ca(++)](ER)) observed five minutes following the initial challenge with PA. This decrease in [Ca(++) ](ER) was followed by a significant increase in the expression of ER stress signature genes CHOP, Xbp1 and GRP78. The early ER stress response induced by PA-LTx was followed by a strong mitochondrial membrane depolarization. Flow cytometry using 2', 7'-dichlorodihydrofluorescein diacetate (H(2)DCFDA) showed an increase in oxidative stress within three to six hours after PA treatment. Treatment of cultures undergoing PA-LTx with the calcium chelator BAPTA-AM and the anti-oxidant MCI-186 significantly reversed the lipotoxic effect by decreasing the generation of ROS and significantly increasing cell viability. We conclude that lipotoxicity in Schwann cells results in cellular dysfunction and cell death that involves a robust ER stress response, mitochondrial dysfunction and an augmented state of cellular oxidative stress (ASCOS).

Neutrophils activate alveolar macrophages by producing caspase-6-mediated cleavage of IL-1 receptor-associated kinase-M

Alveolar macrophages (AMs) are exposed to respirable microbial particles. Similar to phagocytes in the gastrointestinal tract, AMs can suppress inflammation after exposure to nonpathogenic organisms. IL-1R-associated kinase-M (IRAK-M) is one inhibitor of innate immunity, normally suppressing pulmonary inflammation. During pneumonia, polymorphonuclear neutrophils (PMNs) are recruited by chemotactic factors released by AMs to produce an intense inflammation. We report that intact IRAK-M is strongly expressed in resting human AMs but is cleaved in patients with pneumonia via PMN-mediated induction of caspase-6 (CASP-6) activity. PMN contact is necessary and PMN membranes are sufficient for CASP-6 induction in macrophages. PMNs fail to induce TNF-α fully in macrophages expressing CASP-6 cleavage-resistant IRAK-M. Without CASP-6 expression, PMN stimulation fails to cleave IRAK-M, degrade IκBα, or induce TNF-α. CASP-6(-/-) mice subjected to cecal ligation and puncture have impaired TNF-α production in the lung and decreased mortality. LPS did not induce or require CASP-6 activity demonstrating that TLR2/4 signaling is independent from the CASP-6 regulated pathway. These data define a central role for CASP-6 in PMN-driven macrophage activation and identify IRAK-M as an important target for CASP-6. PMNs de-repress AMs via CASP-6-mediated IRAK-M cleavage. This regulatory system will blunt lung inflammation unless PMNs infiltrate the alveolar spaces.

Verotoxin-1 treatment or manipulation of its receptor globotriaosylceramide (gb3) for reversal of multidrug resistance to cancer chemotherapy

A major problem with anti-cancer drug treatment is the development of acquired multidrug resistance (MDR) of the tumor cells. Verotoxin-1 (VT-1) exerts its cytotoxicity by targeting the globotriaosylceramide membrane receptor (Gb3), a glycolipid associated with multidrug resistance. Gb3 is overexpressed in many human tumors and tumor cell lines with inherent or acquired MDR. Gb3 is co-expressed and interplays with the membrane efflux transporter P-gp encoded by the MDR1 gene. P-gp could act as a lipid flippase and stimulate Gb3 induction when tumor cells are exposed to cancer chemotherapy. Recent work has shown that apoptosis and inherent or acquired multidrug resistance in Gb3-expressing tumors could be affected by VT-1 holotoxin, a sub-toxic concentration of the holotoxin concomitant with chemotherapy or its Gb3-binding B-subunit coupled to cytotoxic or immunomodulatory drug, as well as chemical manipulation of Gb3 expression. The interplay between Gb3 and P-gp thus gives a possible physiological approach to augment the chemotherapeutic effect in multidrug resistant tumors.

Shiga toxins: intracellular trafficking to the ER leading to activation of host cell stress responses

Despite efforts to improve hygenic conditions and regulate food and drinking water safety, the enteric pathogens, Shiga toxin-producing Escherichia coli (STEC) and Shigella dysenteriae serotype 1 remain major public health concerns due to widespread outbreaks and the severity of extra-intestinal diseases they cause, including acute renal failure and central nervous system complications. Shiga toxins are the key virulence factors expressed by these pathogens mediating extra-intestinal disease. Delivery of the toxins to the endoplasmic reticulum (ER) results in host cell protein synthesis inhibition, activation of the ribotoxic stress response, the ER stress response, and in some cases, the induction of apoptosis. Intrinsic and/or extrinsic apoptosis inducing pathways are involved in executing cell death following intoxication. In this review we provide an overview of the current understanding Shiga toxin intracellular trafficking, host cellular responses to the toxin and ER stress-induced apoptosis with an emphasis on recent findings.

Signaling through C/EBP homologous protein and death receptor 5 and calpain activation differentially regulate THP-1 cell maturation-dependent apoptosis induced by Shiga toxin type 1

Shiga toxins (Stxs) induce apoptosis via activation of the intrinsic and extrinsic pathways in many cell types. Toxin-mediated activation of the endoplasmic reticulum (ER) stress response was shown to be instrumental in initiating apoptosis in THP-1 myeloid leukemia cells. THP-1 cells responded to Shiga toxin type 1 (Stx1) in a cell maturation-dependent manner, undergoing rapid apoptosis in the undifferentiated state but reduced and delayed apoptosis in differentiated cells. The onset of apoptosis was associated with calpain activation and changes in expression of C/EBP homologous protein (CHOP), Bcl-2 family members, and death receptor 5 (DR5). Ligation of DR5 by tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) activates the extrinsic pathway of apoptosis. We show here that expression of TRAIL and DR5 is increased by Stx1 treatment. Addition of exogenous TRAIL enhances, and anti-TRAIL antibodies inhibit, Stx1-induced apoptosis of THP-1 cells. Silencing of CHOP or DR5 expression selectively prevented caspase activation, loss of mitochondrial membrane potential, and Stx1-induced apoptosis of macrophage-like THP-1 cells. In contrast, the rapid kinetics of apoptosis induction in monocytic THP-1 cells correlated with rates of calpain cleavage. The results suggest that CHOP-DR5 signaling and calpain activation differentially contribute to cell maturation-dependent Stx1-induced apoptosis. Inhibition of these signaling pathways may protect cells from Stx cytotoxicity.

Induction of apoptosis by Shiga toxins

Shiga toxins comprise a family of structurally and functionally related protein toxins expressed by Shigella dysenteriae serotype 1 and multiple serotypes of Escherichia coli. While the capacity of Shiga toxins to inhibit protein synthesis by catalytic inactivation of eukaryotic ribosomes has been well described, it is also apparent that Shiga toxins trigger apoptosis in many cell types. This review presents evidence that Shiga toxins induce apoptosis of epithelial, endothelial, leukocytic, lymphoid and neuronal cells. Apoptotic signaling pathways activated by the toxins are reviewed with an emphasis on signaling mechanisms that are shared among different cell types. Data suggesting that Shiga toxins induce apoptosis through the endoplasmic reticulum stress response and clinical evidence demonstrating apoptosis in humans infected with Shiga toxin-producing bacteria are briefly discussed. The potential for use of Shiga toxins to induce apoptosis in cancer cells is briefly reviewed.

Cisplatin-induced expression of Gb3 enables verotoxin-1 treatment of cisplatin resistance in malignant pleural mesothelioma cells

Background:

A major problem with cisplatin treatment is the development of acquired-drug resistance of the tumour cells. Verotoxin-1 (VT-1) exerts its cytotoxicity by targeting the membrane glycolipid globotriasosylceramide (Gb3), a molecule associated with drug resistance. Cisplatin- and VT-1-induced apoptosis involves mitogen-activated protein kinase (MAPK) activation, and deactivation of MAPKs is associated with cisplatin resistance. This study aimed to investigate whether a sub-toxic concentration of VT-1 could enhance cisplatin-induced apoptosis and overcome acquired-cisplatin resistance in cultured cancer cell lines.

Method:

P31 and H1299 cells with corresponding cisplatin-resistant sub-lines (P31res/H1299res) were incubated with VT-1 and/or cisplatin followed by determination of Gb3 expression, cell viability, apoptosis, and signalling pathways.

Results:

Cells from the resistant sub-lines had elevated Gb3 expression compared with the parental cell lines, and cisplatin further increased Gb3 expression, whereas VT-1 reduced the percentage of Gb3-expressing cells. Combination of cisplatin and sub-toxic concentrations of VT-1 led to a super-additive increase of cytotoxicity and TUNEL staining, especially in the cisplatin-resistant sub-lines. Blockade of Gb3 synthesis by a Gb3 synthesis inhibitor not only led to eradicated TUNEL staining of P31 cells, but also sensitised P31res cells to the induction of apoptosis by cisplatin alone. Cisplatin- and VT-1-induced apoptosis involved the MAPK pathways with increased C-Jun N-terminal kinase and MAPK kinase-3 and -6 phosphorylation.

Conclusions:

We show the presence of Gb3 in acquired-cisplatin resistance in P31res and H1299res cells. Cisplatin up-regulated Gb3 expression in all cells and thus sensitised the cells to VT-1-induced cytotoxicity. A strong super-additive effect of combined cisplatin and a sub-toxic concentration of VT-1 in cisplatin-resistant malignant pleural mesothelioma cells were observed, indicating a new potential clinical-treatment approach.

Bcl-2 regulates the onset of shiga toxin 1-induced apoptosis in THP-1 cells

Shiga toxins (Stxs), which are proteins expressed by the enteric pathogens Shigella dysenteriae serotype 1 and some serotypes of Escherichia coli, are potent protein synthesis inhibitors. Stx-producing organisms cause bloody diarrhea with the potential to progress to acute renal failure and central nervous system complications. Studies using animal models of these diseases have shown that Stxs are major virulence factors, and purified toxins have been shown to be capable of killing many types of cells in vitro. We showed that Stx type 1 (Stx1) rapidly induced apoptosis in undifferentiated, monocytic THP-1 cells through a mechanism involving the endoplasmic reticulum (ER) stress response. Rapid apoptosis correlated with increased expression of C/EBP homologous protein (CHOP), TRAIL, and DR5, while expression of the antiapoptotic factor Bcl-2 was downregulated. Stx1 treatment of differentiated, macrophage-like THP-1 cells was associated with cytokine production and delayed apoptosis. The mechanisms contributing to cell maturation-dependent differences in responses to Stx1 are unknown. We show here that in macrophage-like cells, Stx1 activated the proximal ER stress sensors RNA-dependent protein kinase-like ER kinase and inositol-requiring ER signal kinase 1alpha but did not activate activating transcription factor 6. Proapoptotic signaling pathways mediated by CHOP and by Bax and Bak were activated by Stx1. However, the toxin also activated prosurvival signaling through increased expression, mitochondrial translocation, and alternative phosphorylation of Bcl-2.

Phagocytosis of Staphylococcus aureus by macrophages exerts cytoprotective effects manifested by the upregulation of antiapoptotic factors

It is becoming increasingly apparent that Staphylococcus aureus are able to survive engulfment by macrophages, and that the intracellular environment of these host cells, which is essential to innate host defenses against invading microorganisms, may in fact provide a refuge for staphylococcal survival and dissemination. Based on this, we postulated that S. aureus might induce cytoprotective mechanisms by changing gene expression profiles inside macrophages similar to obligate intracellular pathogens, such as Mycobacterium tuberculosis. To validate our hypothesis we first ascertained whether S. aureus infection could affect programmed cell death in human (hMDMs) and mouse (RAW 264.7) macrophages and, specifically, protect these cells against apoptosis. Our findings indicate that S. aureus-infected macrophages are more resistant to staurosporine-induced cell death than control cells, an effect partly mediated via the inhibition of cytochrome c release from mitochondria. Furthermore, transcriptome analysis of human monocyte-derived macrophages during S. aureus infection revealed a significant increase in the expression of antiapoptotic genes. This was confirmed by quantitative RT-PCR analysis of selected genes involved in mitochondria-dependent cell death, clearly showing overexpression of BCL2 and MCL1. Cumulatively, the results of our experiments argue that S. aureus is able to induce a cytoprotective effect in macrophages derived from different mammal species, which can prevent host cell elimination, and thus allow intracellular bacterial survival. Ultimately, it is our contention that this process may contribute to the systemic dissemination of S. aureus infection.

Efficient non-viral transfection of THP-1 cells

Macrophages are an important part of the cellular immune system and play a key role during immune responses. Thus, macrophages are interesting targets in basic and clinical research. Primary monocytes or monocyte-derived macrophages do not proliferate on a suitable scale so that their use for functional studies in vitro is limited. Immortal proliferating cell lines, such as the human THP-1 monocytic leukemia cell line, are therefore often used instead of primary cells. Transfection is a useful tool to study the function of gene products, but transfection of THP-1 monocytes and pre-differentiated THP-1 macrophages with subsequent differentiation into mature THP-1 macrophages using phorbol esters is usually accompanied by a progressive loss of cell viability. In this study, we describe a simple and rapid approach for efficient transfection of THP-1 monocytes and pre-differentiated THP-1 macrophages using a modified Nucleofection-based approach. The protocol maintains cell viability and functionality, thus allowing efficient transfection of THP-1 cells combined with subsequent differentiation of transfected THP-1 cells into mature macrophages.

Survival of monocytes and macrophages and their role in health and disease

Macrophages are versatile cells involved in health and disease. These cells act as scavengers to rid the body of apoptotic and senescent cells and debris through their phagocytic function. Although this is a primary function of these cells, macrophages play vital roles in inflammation and repair of damaged tissue. Macrophages secrete a large number of cytokines, chemokines and growth factors that recruit and activate a variety of cell types to inflamed tissue compartments. These cells are also critical in cell-mediated immunity and in the resolution of inflammation. Since macrophages, and their precursors, blood monocytes, are important in regulating and resolving inflammation, prolonged cellular survival in tissue compartments could be detrimental. Thus, factors that regulate the fate of monocyte and macrophage survival are important in cellular homeostasis. In this article, we will explore stimuli and the intracellular pathways important in regulating macrophage survival and implication in human disease.

Proapoptotic activity and chemosensitizing effect of the novel Akt inhibitor perifosine in acute myelogenous leukemia cells

The serine/threonine kinase Akt, a downstream effector of phosphatidylinositol 3-kinase (PI3K), is known to play an important role in antiapoptotic signaling and has been implicated in the aggressiveness of a number of different human cancers including acute myelogenous leukemia (AML). We have investigated the therapeutic potential of the novel Akt inhibitor, perifosine, on human AML cells. Perifosine is a synthetic alkylphospholipid, a new class of antitumor agents, which target plasma membrane and inhibit signal transduction networks. Perifosine was tested on THP-1 and MV 4-11 cell lines, as well as primary leukemia cells. Perifosine treatment induced cell death by apoptosis in AML cell lines. Perifosine caused Akt and ERK 1/2 dephosphorylation as well as caspase activation. In THP-1 cells, the proapoptotic effect of perifosine was partly dependent on the Fas/FasL system and c-jun-N-kinase activation. In MV 4-11 cells, perifosine downregulated phosphorylated Akt, but not phosphorylated FLT3. Moreover, perifosine reduced the clonogenic activity of AML, but not normal, CD34(+) cells, and markedly increased blast cell sensitivity to etoposide. Our findings indicate that perifosine, either alone or in combination with existing drugs, might be a promising therapeutic agent for the treatment of those AML cases characterized by upregulation of the PI3K-Akt survival pathway.