Leukotriene B4 omega-oxidation by human polymorphonuclear leukocytes is inhibited by pyocyanin, a phenazine derivative produced by Pseudomonas aeruginosa

Phenazines as potential biomarkers of Pseudomonas aeruginosa infections: synthesis regulation, pathogenesis and analytical methods for their detection

Infectious diseases are still a worldwide important problem. This fact has led to the characterization of new biomarkers that would allow an early, fast and reliable diagnostic and targeted therapy. In this context, Pseudomonas aeruginosa can be considered one of the most threatening pathogens since it causes a wide range of infections, mainly in patients that suffer other diseases. Antibiotic treatment is not trivial given the incidence of resistance processes and the fewer new antibiotics that are placed on the market. With this scenario, relevant quorum sensing (QS) molecules that regulate the secretion of virulence factors and biofilm formation can play an important role in diagnostic and therapeutic issues. In this review, we have focused our attention on phenazines, as possible new biomarkers. They are pigmented metabolites that are produced by diverse bacteria, characterized for presenting unique redox properties. Phenazines are involved in virulence, competitive fitness and are an essential component of the bacterial QS system. Here we describe their role in bacterial pathogenesis and we revise phenazine production regulation systems. We also discuss phenazine levels previously reported in bacterial isolates and in clinical samples to evaluate them as putative good candidates to be used as P. aeruginosa infection biomarkers. Moreover we deeply go through all analytical techniques that have been used for their detection and also new approaches are discussed from a critical point. Graphical abstract.

Pyocyanin-induced mucin production is associated with redox modification of FOXA2

Background

The redox-active pyocyanin (PCN) is a toxic, secondary metabolite secreted by the respiratory pathogen Pseudomonas aeruginosa (PA). Previously, we have shown that mouse lungs chronically exposed to PCN develop goblet cell hyperplasia and metaplasia (GCHM) and mucus hypersecretion, fibrosis and emphysema. These pathological features are commonly found in the airways of several chronic lung diseases, including cystic fibrosis (CF), as well as in mouse airways deficient in the forkhead box A2 (FOXA2), a transcriptional repressor of goblet GCHM and mucus biosynthesis. Furthermore, PCN inhibits FOXA2 by activating the pro-GCHM signaling pathways Stat6 and EGFR. However, it is not known whether PCN-generated reactive oxygen (ROS) and nitrogen (RNS) species posttranslationally modify and inactivate FOXA2.

Methods

We examined the posttranslational modifications of FOXA2 by PCN using specific antibodies against oxidation, nitrosylation, acetylation and ubiquitination. Electrophoretic mobility shift assay (EMSA) was used to examine the ability of modified FOXA2 to bind the promoter of MUC5B mucin gene. In addition, we used quantitative real time PCR, ELISA, immunofluorescence and mouse lung infection to assess whether the loss of FOXA2 function caused GCHM and mucin overexpression. Finally, we examined the restoration of FOXA2 function by the antioxidant glutathione (GSH).

Results

We found that PCN-generated ROS/RNS caused nitrosylation, acetylation, ubiquitination and degradation of FOXA2. Modified FOXA2 had reduced ability to bind the promoter of the MUC5B gene. The antioxidant GSH alleviated the modification of FOXA2 by PCN, and inhibited the overexpression of MUC5AC and MUC5B mucins.

Conclusion

These results suggest that PCN-mediated posttranslational modifications of FOXA2 are positively correlated with GCHM and overexpression of airway mucins. Furthermore, antioxidant treatment restores the function of FOXA2 to attenuate GCHM and mucus hypersecretion.

Pyocyanin-induced toxicity in A549 respiratory cells is causally linked to oxidative stress

Pyocyanin, a virulence factor produced by Pseudomonas aeruginosa, has many damaging effects on mammalian cells. Several lines of evidence suggest that this damage is primarily mediated by its ability to generate ROS and deplete host antioxidant defence mechanisms. However, a causal role for oxidative stress has not yet been demonstrated conclusively. Parallel measures of ROS production, antioxidant levels and cytotoxicity provide convincing evidence that pyocyanin-induced cytotoxicity in A549 respiratory cells is mediated by acute ROS production and subsequent oxidative stress. Pyocyanin increased ROS levels in A549 cells as measured by the fluorescent H(2)O(2) probes Amplex Red and DCFH-DA. These effects were attenuated by the antioxidant N-acetylcysteine. Furthermore, pyocyanin-induced depletion of intracellular GSH levels 24h after exposure was also prevented by pre-treatment of cells with NAC. Under these conditions, NAC protected cells against pyocyanin-induced cytotoxicity as measured by resazurin reduction to resorufin and viable cell counts, strongly supporting a causal role for oxidative stress. Finally, we also show that pyocyanin-induced activation of the immune and inflammatory transcription factor NF-κB in A549 cells is likely mediated by increased ROS. This increased understanding of mechanisms underlying pyocyanin-induced cytotoxicity may ultimately lead to better strategies for reducing the virulence associated with chronic P. aeruginosa infection.

Pseudomonas aeruginosa exotoxin pyocyanin causes cystic fibrosis airway pathogenesis

The cystic fibrosis (CF) airway bacterial pathogen Pseudomonas aeruginosa secretes multiple virulence factors. Among these, the redox active exotoxin pyocyanin (PCN) is produced in concentrations up to 100 mumol/L during infection of CF and other bronchiectatic airways. However, the contributions of PCN during infection of bronchiectatic airways are not appreciated. In this study, we demonstrate that PCN is critical for chronic infection in mouse airways and orchestrates adaptive immune responses that mediate lung damage. Wild-type FVBN mice chronically exposed to PCN developed goblet cell hyperplasia and metaplasia, airway fibrosis, and alveolar airspace destruction. Furthermore, after 12 weeks of exposure to PCN, mouse lungs down-regulated the expression of T helper (Th) type 1 cytokines and polarized toward a Th2 response. Cellular analyses indicated that chronic exposure to PCN profoundly increased the lung population of recruited macrophages, CD4(+) T cells, and neutrophils responsible for the secretion of these cytokines. PCN-mediated goblet cell hyperplasia and metaplasia required Th2 cytokine signaling through the Stat6 pathway. In summary, this study establishes that PCN is an important P. aeruginosa virulence factor capable of directly inducing pulmonary pathophysiology in mice, consistent with changes observed in CF and other bronchiectasis lungs.

Polyphenol cytotoxicity induced by the bacterial toxin pyocyanin: role of NQO1

Pyocyanin is an important bacterial redox-active toxin produced by the opportunistic human pathogen Pseudomonas aeruginosa. The bacterium is a cause of serious infections of the respiratory tract, particularly for those with cystic fibrosis and for those with burn injuries. Pyocyanin induces oxidative stress and causes cells to become prematurely senescent, which compromises tissue remodeling and wound repair. A diverse range of antioxidants have been found useful in preventing oxidant-induced cellular senescence, including quercetin, a common dietary polyphenol. This study evaluated the effectiveness of three common polyphenols (quercetin, (+)-catechin, and (-)-epicatechin) as potential inhibitors of pyocyanin-induced senescence. Whereas at the lowest concentration the polyphenols maintained cellular replicative capacity, in the presence of pyocyanin they unexpectedly displayed concentration-dependent cytotoxicity with a rank order of quercetin>epicatechin>>catechin. On oxidation, polyphenols with B-ring catechol functionality form toxic alkylating quinones that are normally inactivated by cellular antioxidant defense and redox maintenance systems, including reduction by ascorbate and NAD(P)H:quinone oxidoreductase 1 (NQO1). Pyocyanin inhibited cellular NQO1 activity at low micromolar concentrations, but the presence of exogenous ascorbate eliminated pyocyanin-induced polyphenol cytotoxicity. These data indicate that pyocyanin compromises cellular redox maintenance systems, leaving cells susceptible to the adverse effects of otherwise nontoxic redox-active compounds.

Redox warfare between airway epithelial cells and Pseudomonas: dual oxidase versus pyocyanin

The importance of reactive oxygen species-dependent microbial killing by the phagocytic cell NADPH oxidase has been appreciated for some time, although only recently has an appreciation developed for the partnership of lactoperoxidase with related dual oxidases (Duox) within secretions of the airway surface layer. This system produces mild oxidants designed for extracellular killing that are effective against several airway pathogens, including Staphylococcus aureus, Burkholderia cepacia, and Pseudomonas aeruginosa. Establishment of chronic pseudomonas infections involves adaptations to resist oxidant-dependent killing by expression of a redox-active virulence factor, pyocyanin, that competitively inhibits epithelial Duox activity by consuming intracellular NADPH and producing superoxide, thereby inflicting oxidative stress on the host.

Pseudomonas pyocyanin inhibits wound repair by inducing premature cellular senescence: role for p38 mitogen-activated protein kinase

Pseudomonas aeruginosa is an important nosocomial pathogen of burn wounds. Pyocyanin, a virulence factor produced by the bacterium, induces persistent intracellular oxidative stress and premature senescence in mammalian cells. Our aims were to evaluate pyocyanin levels in infected wound dressings and the potential of the toxin to influence wound repair. Surgical dressings from infected burn patients were examined for pyocyanin and normal primary human diploid fibroblasts (HDFs) were treated with comparable concentrations and their replicative ability examined. Pyocyanin was detected in the exudate of infected wound dressings in amounts up to 5.3 microg/g (mean: 2.0+/-2.3 microg/g). HDFs exposed to pyocyanin (1-50 microM; 0.2-10.5 microg/ml) underwent growth arrest at all concentrations and developed morphological characteristics associated with cellular senescence, including expression of senescence-associated beta-galactosidase. Using an in vitro wound repair model, a single exposure to pyocyanin inhibited wound repair in a concentration-dependent manner. Prior treatment with a specific p38(MAPK) inhibitor allowed cells to maintain their replicative ability and pre-senescent morphology indicating pyocyanin operates through the Erk/p38(MAPK) senescence pathway. These data support the hypothesis that bacterial virulence factors capable of inducing persistent low-level oxidative stress play a pivotal role in modulating the tissue repair response to infection by inducing premature cellular senescence.

Liver sinusoidal endothelial cells and acute non-oxidative hepatic injury induced by Pseudomonas aeruginosa pyocyanin

The liver sinusoidal endothelial cell (LSEC) is damaged by many toxins, including oxidants and bacterial toxins. Any effect on LSECs of the Pseudomonas aeruginosa virulence factor, pyocyanin, may be relevant for systemic pseudomonal infections and liver transplantation. In this study, the effects of pyocyanin on in vivo rat livers and isolated LSECs were assessed using electron microscopy, immunohistochemistry and biochemistry. In particular, the effect on fenestrations, a crucial morphological aspect of LSECs was assessed. Pyocyanin treatment induced a dose-dependent reduction in fenestrations in isolated LSECs. In the intact liver, intraportal injection of pyocyanin (11.9 microM in blood) was associated with a reduction in endothelial porosity from 3.4 +/- 0.2% (n = 5) to 1.3 +/- 0.1% (n = 7) within 30 min. There were decreases in both diameter and frequency of fenestrations in the intact endothelium. There was also a decrease in endothelial thickness from 175.8 +/- 5.8 to 156.5 +/- 4.0 nm, an endothelial pathology finding previously unreported. Hepatocyte ultrastructure, liver function tests and immunohistochemical markers of oxidative stress (3-nitrotyrosine and malondialdehyde) were not affected. Pyocyanin induces significant ultrastructural changes in the LSEC in the absence of immunohistochemical evidence of oxidative stress or hepatocyte injury pointing to a novel mechanism for pyocyanin pathogenesis.

Premature cellular senescence induced by pyocyanin, a redox-active Pseudomonas aeruginosa toxin

Pseudomonas aeruginosa is an important nosocomial pathogen that can cause acute and chronic infection, particularly of the respiratory system. Pyocyanin is a major P. aeruginosa virulence factor that displays redox activity and induces oxidative stress in cellular systems. The effect of pyocyanin on replicating human pulmonary epithelial (A549) cells was investigated. Cells were exposed to pyocyanin for 24 h and their subsequent growth and development were followed for 7 days. Pyocyanin (5-10 microM) arrested cell growth and resulted in the development of a morphological phenotype consistent with cellular senescence, that is, an enlarged and flattened appearance. The senescent nature of these cells was supported by positive staining for increased lysosomal content and senescence-associated beta-galactosidase activity. All cells treated with pyocyanin (10 microM) converted to the senescent phenotype, which remained stable for up to 7 days. Exposure to pyocyanin at 25 microM or greater resulted in cell death due to apoptosis. A549 cells exposed to pyocyanin generated hydrogen peroxide in a dose-dependent manner and the senescence-inducing effect of pyocyanin was inhibited by the antioxidant, glutathione, suggesting the involvement of reactive oxygen species. The induction of premature cellular senescence by redox-active bacterial toxins may be a hitherto unrecognized aspect of infection pathology and a limiting factor in the tissue repair response to infection.

Pyocyanin induces oxidative stress in human endothelial cells and modulates the glutathione redox cycle

Pyocyanin is a redox active virulence factor produced by the human pathogen Pseudomonas aeruginosa. Treatment of endothelial cells with pyocyanin (1-50 microM) resulted in the dose-dependent formation of hydrogen peroxide that was detected in the extracellular medium. Total intracellular glutathione levels decreased in response to pyocyanin in a dose-dependent manner from a control value of 19.9 +/- 2.7 nmol/mg protein to 10.0 +/- 2.4 nmol/mg protein. Prior treatment of cells with catalase afforded complete protection against loss of glutathione. Total intracellular soluble thiols decreased from 95.0 +/- 6.2 nmol/mg protein to 78.6 +/- 2.3 nmol/mg protein at the highest test dose. Intracellular levels of NADPH increased up to 2.4-fold in response to pyocyanin exposure. It is concluded that pyocyanin exposes endothelial cells to oxidative stress by the generation of hydrogen peroxide, which subsequently depletes intracellular glutathione and increases intracellular levels of mixed disulfides.

Modulation of neutrophil superoxide response and intracellular diacylglyceride levels by the bacterial pigment pyocyanin

Low concentrations of pyocyanin are reported to enhance superoxide production by human neutrophils exposed to various stimuli, yet the mechanism remains unknown. Using lucigenin-enhanced chemiluminescence, we examined the kinetics of the neutrophil superoxide response in the presence of pyocyanin. At all concentrations (12.5 to 200 microM), pyocyanin decreased the peak superoxide response while prolonging the duration of the response. The prolonged response may be associated with an observed increase in intracellular diacylglyceride levels due to pyocyanin exposure.

Scavenging of neutrophil-derived superoxide anion by 1-hydroxyphenazine, a phenazine derivative associated with chronic Pseudomonas aeruginosa infection: relevance to cystic fibrosis

The airways of cystic fibrosis patients colonised by Pseudomonas aeruginosa contain the redox active phenazine derivative, 1-hydroxyphenazine (OHP). As the presence of reactive oxygen species is of importance to tissue damage in cystic fibrosis, OHP was investigated for its ability to reduce molecular oxygen to superoxide. In the presence of NADPH, OHP reduced cytochrome c in a dose-dependent manner. This effect was not inhibited by superoxide dismutase and demonstrates an electron transport role for OHP. The OHP/NADPH system was unable to reduce molecular oxygen to superoxide as judged by an inability to oxidase epinephrine to adrenochrome. However, using lucigenin-enhanced chemiluminescence to detect superoxide, it was found that pathophysiologically relevant concentrations of OHP (5-25 microM) effectively scavenged superoxide from a xanthine/xanthine oxidase system. Similarly, in the presence of OHP, superoxide availability from contact-activated neutrophils was substantially reduced. It is concluded that OHP is an efficient scavenger of superoxide and that electron transfer from superoxide to OHP represents a major mechanism for reduction of OHP in vivo. Reduced OHP has the potential to alter cellular function by participating in the reduction of iron-containing proteins and in this manner contribute to the pathogenesis of P. aeruginosa infection in cystic fibrosis.

Pyocyanin from Pseudomonas aeruginosa inhibits prostacyclin release from endothelial cells

Pseudomonas aeruginosa pneumonia causes a vasculitis of small pulmonary arteries. While the fully developed lesion demonstrates vessel wall necrosis, the early lesion is remarkable for preservation of viable endothelium despite vessel wall invasion by bacteria. Pyocyanin, an exoproduct of P. aeruginosa, markedly inhibited prostacyclin production by pulmonary artery endothelial cells without causing cell lysis. Pyocyanin might after vascular homeostasis in the absence of cytolysis.

Inhibition of the human platelet cyclooxygenase response by the naturally occurring phenazine derivative, 1-hydroxyphenazine

The phenazine derivative, 1-hydroxyphenazine (OHP), is produced in vivo by Pseudomonas aeruginosa, an organism that colonises the airways of patients with cystic fibrosis. While known to inhibit leukotriene production by human neutrophils, the effects of OHP on cyclooxygenase pathways have not previously been reported. We used [3H] arachidonic acid (AA) under conditions of concurrent labelling-stimulation or pre-labelling for one hour followed by stimulation to determine the effects of OHP on the production of cyclooxygenase metabolites by human platelets stimulated with the calcium ionophore, A23187. Thromboxane B2 (TxB2) and 12-hydroxyheptadecatrienoic acid (HHT) production was inhibited in a dose-dependent manner by OHP using either pre-labelled or concurrently labelled platelets. However, production of 12-hydroxyeicosatetraenoic acid (12-HETE) was not diminished. Determination of the amount of total free label (AA+non-esterified AA metabolites) after stimulation of pre-labelled platelets indicated a dose-dependent inhibition of the release of AA from phospholipid by OHP. This was reflected in a corresponding increase in phospholipid AA content. These data indicate that phenazine derivatives of bacterial origin exhibit complex interactions with pathways of arachidonic acid metabolism in host cells. These effects may prove to be of pharmacological importance.

Eicosanoids, fatty acids and neutrophils: their relevance to the pathophysiology of disease

PUFA and their eicosanoid metabolites are potent biological modifiers. They have beneficial effects in a number of diseases, which may result in part from their direct actions on neutrophils as well as from their ability to modulate eicosanoid biosynthesis. A consideration of their interactions with other cell types, e.g. lymphocytes and macrophages, is beyond the scope of this review. Small alterations in structure can result in large changes in the neutrophil response. This will have important implications for the further development and use of fatty acids for therapeutic purposes.

Eicosapentaenoic acid in cystic fibrosis: evidence of a pathogenetic role for leukotriene B4

Much of the lung damage that limits the life of young adults with cystic fibrosis is due to proteases and oxygen metabolites generated by neutrophils, which are recruited into the airway by the interaction between Pseudomonas aeruginosa and pulmonary macrophages. Leukotriene B4 (LTB4) has been proposed as a local mediator of this process; its production is susceptible to specific modulation with dietary eicosapentaenoic acid (EPA). We carried out a placebo-controlled trial of EPA (2.7 g daily for 6 weeks) to assess its effects on markers of clinical state, peripheral neutrophil function, and lung inflammation in sixteen patients with cystic fibrosis colonised with P aeruginosa. EPA was well tolerated and resulted in a significant reduction in sputum volume (median change with EPA -10 mL/day, placebo 0; p = 0.015), and improvements in Schwachman score (EPA 5%, placebo 0; p = 0.034), forced expiratory volume in 1 s (EPA 0.25 L, placebo -0.1 L; p = 0.006), and vital capacity (EPA 0.6 L, placebo 0; p = 0.011). Relative chemotaxis of circulating neutrophils to LTB4 increased from a subnormal baseline of 4 (median; range 0-10) microns/30 min before treatment, to a near normal value of 11 (5-18) microns/30 min after EPA. Relative chemotaxis to LTB4 of patients taking placebo did not change: the difference in response was highly significant (p = 0.001). Specific reduction of neutrophil chemotaxis to LTB4 is a sensitive assay of chronic in-vivo exposure to LTB4. Our results suggest that LTB4 has a pathogenetic role in the lung damage of cystic fibrosis. Longer-term clinical trials of EPA are warranted in a larger number of cystic fibrosis patients.

Sputum tumour necrosis factor-alpha and leukotriene concentrations in cystic fibrosis

It is postulated that a vigorous host inflammatory response in the cystic fibrosis lung contributes to lung injury. Tumour necrosis factor-alpha (TNF-alpha) may play a part in that process and in the generation of leukotrienes. Therefore, the relationships between sputum TNF-alpha, leukotriene concentration, and lung function abnormalities in 16 children with cystic fibrosis were investigated. Each subject provided sputum samples and performed spirometry. TNF-alpha was measured by enzyme linked immunosorbent assay; individual leukotrienes were separated using high performance liquid chromatography and quantified by radioimmunoassay. The geometric mean concentration of TNF-alpha was 129.7 pg/ml and 95% confidence interval 48.2 to 348.3. Mean (SEM) leukotriene B4 (LTB4) was 97.8 (22.9) pmol/g and total cysteinyl leukotrienes were 60.9 (14.8) pmol/g. Mean (SD) forced expiratory volume in one second (FEV1) of the group was 53 (15)% of predicted and forced vital capacity (FVC) was 65 (14)% of predicted. There was a significant positive correlation between TNF-alpha and both LTB4 and the total cysteinyl leukotriene sputum content. An inverse relationship existed between TNF-alpha and FEV1 and FVC. Moreover, a negative correlation was observed between sputum LTB4 and FEV1 and FVC. These results suggest that TNF-alpha and the leukotrienes may participate in the airways inflammation and airflow obstruction observed in cystic fibrosis subjects and support the hypothesis that TNF-alpha upregulates the 5-lipoxygenase pathway in vivo.