RETRACTED ARTICLE: The α7 Nicotinic Acetylcholine Receptor Agonist GTS-21 Improves Bacterial Clearance in Mice by Restoring Hyperoxia-Compromised Macrophage Function

Abstract

Mechanical ventilation with supraphysiological concentrations of oxygen (hyperoxia) is routinely used to treat patients with respiratory distress. However, prolonged exposure to hyperoxia compromises the ability of the macrophage to phagocytose and clear bacteria. Previously, we showed that the exposure of mice to hyperoxia elicits the release of the nuclear protein high mobility group box-1 (HMGB1) into the airways. Extracellular HMGB1 impairs macrophage phagocytosis and increases the mortality of mice infected with Pseudomonas aeruginosa (PA). The aim of this study was to determine whether GTS-21 [3-(2,4 dimethoxy-benzylidene)-anabaseine dihydrochloride], an α7 nicotinic acetylcholine receptor (α7nAChR) agonist, could inhibit hyperoxia-in-duced HMGB1 release into the airways, enhance macrophage function and improve bacterial clearance from the lungs in a mouse model of ventilator-associated pneumonia. GTS-21 (0.04, 0.4 and 4 mg/kg) or saline was systemically administered via intraperitoneal injection to mice that were exposed to hyperoxia (≥99% O₂) and subsequently challenged with PA. We found that systemic administration of 4 mg/kg GTS-21 significantly increased bacterial clearance, decreased acute lung injury and decreased accumulation of airway HMGB1. To investigate the cellular mechanism of these observations, RAW 264.7 cells, a macrophagelike cell line, were incubated with different concentrations of GTS-21 in the presence of 95% O₂. The phagocytic activity of macrophages was significantly increased by GTS-21 in a dose-dependent manner. In addition, hyperoxia-induced hyperacetylation of HMGB1 was significantly reduced in macrophages incubated with GTS-21. Furthermore, GTS-21 significantly inhibited the cytoplasmic translocation and release of HMGB1 from these macrophages. Our results indicate that GTS-21 is effective in improving bacterial clearance and reducing acute lung injury by enhancing macrophage function via inhibiting the release of nuclear HMGB1. Therefore, the α7nAChR represents a possible pharmacological target to improve the clinical outcome of patients on ventilators by augmenting host defense against bacterial infections.

Introduction

Oxygen therapy, by using mechanical ventilation with supraphysiological concentrations of oxygen (hyperoxia), is a lifesaving intervention for patients with respiratory distress. However, patients on ventilators become highly susceptible to lung infections and have a greater likelihood of developing ventilator-associated pneumonia (VAP). Pseudomonas aeruginosa (PA), a gramnegative aerobic bacterium, has been reported to be associated with 21% of all VAP cases (1). VAP accounts for up to 60% of all deaths from hospital-acquired infections in the United States and continues to be a major cause of high morbidity and mortality for patients on ventilators (2–4).

Alveolar macrophages are the first line of defense against invading pathogens and are the earliest effectors of the phagocytic response against microbial infections in the distal airways (5). Alveolar macrophage isolated from animals and cultured macrophages exposed to hyperoxia exhibit impaired phagocytosis of pathogens such as PA and Klebsiella pneumoniae, as well as paraffin oil droplets (6–12). Impaired macrophage functions have been associated with increased susceptibility and severity of bacterial infections in animals exposed to hyperoxia (7,9).

Prolonged exposure to hyperoxia also induces the accumulation of high mobility group box-1 protein (HMGB1) in the airways of mice and in the media of cultured macrophages (7). High levels of airway HMGB1 were reported in patients with cystic fibrosis (CF) and patients on ventilators (13,14). Extracellular HMGB1 in the airways is sufficient to impair the phagocytic function of alveolar macrophages (13). Furthermore, HMGB1-compromised macrophage functions can result in decreased host defenses against bacterial infection in animal models of CF and VAP (7,13). Thus, reducing the accumulation of HMGB1 in the airways of patients with CF and VAP may provide an important therapeutic strategy for these patients.

Numerous studies have been directed toward elucidating the mechanisms underlying the release of nuclear HMGB1 into the extracellular milieu to develop treatments or interventions that attenuate the adverse effects of extracellular HMGB1 (15,16). The activation of α7 nicotinic acetylcholine receptors (α7nAChRs) plays a critical role in the release of nuclear HMGB1 into the extracellular milieu (17,18). Macrophages express high levels of α7nAChR, which may be a target to reduce the accumulation of extracellular HMGB1 (19,20). GTS-21 [3-(2,4-dimethoxybenzylidene)-anabaseine dihydrochloride], an agonist of α7nAChR (21,22), was reported to inhibit endotoxin-induced HMGB1 release from RAW 264.7 cells (21). The aim of this study was to determine the effects of GTS-21 on (a) the accumulation of extracellular HMGB1 in the airways of animals subjected to prolonged exposure to hyperoxia, (b) attenuating hyperoxia-induced suppression of macrophage phagocytosis and (c) improving hyperoxia-reduced host defense to clear PA infection in a mouse model of VAP. In this article, we show that GTS-21 can significantly improve bacterial clearance in these mice and enhance macrophage function by specifically reducing the hyperacetylation and translocation of HMGB1 and its subsequent extracellular release.

Materials and Methods

Cell Culture and Reagents

Murine macrophagelike RAW 264.7 cells (ATCC TIB-71, American Type Culture Collection, Manassas, VA, USA) were cultured in RPMI 1640 medium (Gibco/BRL, Life Technologies, Carlsbad, CA, USA) supplemented with 10% fetal bovine serum (Atlanta Biologicals, Lawrenceville, GA, USA), 1% penicillin and 1% streptomycin (Life Technologies). The cells were maintained at 37°C under normoxic conditions for 24 h (5% CO₂/ 21% O₂) and allowed to grow to 70–80% confluency and subcultured every 3 d. GTS-21 was obtained from Abcam (Cambridge, MA, USA).

Bronchoalveolar Lavage

Murine bronchoalveolar lavage (BAL) fluid was obtained as described previously (7). Briefly, mice were anesthetized by an intraperitoneal injection of sodium pentobarbital (120 mg/kg). After a 1- to 2-cm incision was made on the neck, the trachea was dissected, and a 20-gauge × 1.25-inch intravenous catheter was inserted caudally into the lumen of the exposed trachea. The lungs were gently lavaged twice with 1 mL sterile, nonpyrogenic phosphate-buffered saline (PBS) solution (Mediatech, Herndon, VA, USA). The BAL samples were centrifuged, and the resultant supernatants were stored in a freezer at −80°C for analyzing concentrations of extracellular HMGB1 and the total protein content by using Western blot analysis and a bicinchoninic acid assay.

Animal Studies

C57BL/6 mice (male, 8 to 12 wks old; The Jackson Laboratory, Bar Harbor, ME, USA) were used in this study in accordance with the Institutional Animal Care and Use Committees of St. John’s University. The mice were housed in a specific pathogen-free environment maintained at 22°C in ≈50% relative humidity and with a 12-h light/dark cycle. All mice had ad libitum access to standard rodent food and water. Mice were randomized to receive either GTS-21 (0.04, 0.4 and 4 mg/kg) or saline, administered by intraperitoneal injection every 8 h, starting 32 h after the onset of hyperoxic exposure. After 48 h of exposure, the mice were inoculated with 0.1 × 10⁸ colonyforming units (CFUs) of PA by making a 1- to 2-cm incision on the neck to expose the trachea after anesthetization with sodium pentobarbital (60 mg/kg). Eighteen hours after bacterial inoculation, mice were euthanized with intraperitoneal sodium pentobarbital (120 mg/kg) to obtain BAL and lung tissues as described previously (7). After lavaging with PBS, the lungs were excised and immediately placed into 1 mL cold PBS and homogenized.

Exposure to Hyperoxia

Male C57BL/6 mice and cultured macrophages were exposed to hyperoxia as previously described (7). Briefly, animals were placed in microisolator cages (Allentown Caging Equipment, Allentown, NJ, USA), which were kept in a Plexiglas chamber (BioSpherix, Lacona, NY, USA) and exposed to ≥99% O₂ for up to 48 h. The exposure of murine macrophagelike RAW 264.7 cells was achieved in sealed, humidified Plexiglas chambers (Billups-Rothenberg, Del Mar, CA, USA) flushed with 95% O₂/5% CO₂ at 37°C. An oxygen analyzer (MSA; Ohio Medical Corporation, Gurnee, IL, USA) was used to monitor the O₂ concentration in the chamber.

Western Blot Analysis

To determine the levels of extracellular HMGB1, RAW 264.7 cells were cultured in serum-free Opti-MEM I medium (Gibco/BRL, Life Technologies) in 12-well plates and were exposed to either 95% O₂ alone or 95% O₂ in the presence of GTS-21 for 24 h. After hyperoxic exposure, the levels of HMGB1 in the culture media of treated cells and BAL samples obtained from mice were measured by Western blot analysis. C57BL/6 mice were exposed to ≥99% O₂ for 48 h and then inoculated with PA (0.1 × 10⁸ CFUs/mouse) and returned to 21% O₂ after inoculation. Mice were randomized to receive either GTS-21 or saline, administrated by intraperitoneal injection every 8 h starting at 32 h during hyperoxia. For determining the levels of nuclear factor (NF)-κB and IκB in the nucleus and cytoplasm of lung cells in these mice, nuclear and cytoplasmic extract was prepared by using the NE-PER Nuclear and Cytoplasmic Extraction Reagents kit (Thermo Fisher Scientific, Waltham, MA, USA), according to the manufacturer’s protocol. A total of 15 µg nuclear extract, 30 µg cytoplasmic extract and an equal volume of BAL samples and culture media were loaded on to sodium dodecyl sulfate-polyacrylamide gels (10 and 13%) and then transferred to Immobilon-P membranes (Millipore, Billerica, MA, USA). Nonspecific binding sites on the membrane were blocked by using 5% nonfat dry milk (Bio-Rad, Hercules, CA, USA) in Tris-buffered saline containing 1% Tween 20 (TBST) for 1 h at room temperature. The membranes were rinsed three times with TBST and incubated overnight at 4°C with rabbit antiHMGB1 polyclonal primary antibody (1:500; Sigma-Aldrich, St. Louis, MO, USA), anti-NF-κB p65 rabbit polyclonal antibody (1:500; Santa Cruz Biotechnology, Dallas, TX, USA) and anti-IκB antibody (1:1,000; Sigma-Aldrich) diluted in 5% nonfat dry milk. After three washes in TBST, the membranes were incubated with anti-rabbit horseradish peroxide-coupled secondary antibody (1:5,000; GE Healthcare, Piscataway, NJ, USA) for 1 h at room temperature. After washing the membranes thrice in TBST, the immunoreactive proteins were visualized by using the enhanced chemiluminescence reagent kit (GE Healthcare Bio-Sciences, Pittsburgh, PA, USA) per the manufacturer’s instructions. The image was developed by using a BioSpectrum 600 Imaging system (UVP, Upland, CA, USA).

Immunofluorescence Analysis

RAW 264.7 cells were seeded in 12-well plates and allowed to adhere overnight at 37°C. RAW 264.7 cells were exposed to either 95% O₂ alone or 95% O₂ in the presence of GTS-21 for 24 h. After incubation for 24 h, cells were fixed with 2% phosphate-buffered formaldehyde (pH 7.4) for 15 min and washed three times with PBS. Cells were then permeabilized with 0.2% Triton X-100 (Sigma-Aldrich), and nonspecific binding sites were blocked with 10% normal goat serum (Chemicon, Temecula, CA, USA) for 20 min. Next, cells were washed with 1% bovine serum albumin in PBS and incubated with anti-HMGB1 (1:200; Sigma-Aldrich) or NF-κB p65 (1:200; Santa Cruz Biotechnology) primary antibodies overnight at 4°C. The incubation with the secondary antibody, a goat anti-rabbit immunoglobulin G (IgG) conjugated with Alex Fluor 594 (1:200; Molecular Probes/Life Technologies), was performed for 1 h. Normal blocking serum without primary antibody was used as a negative control. To visualize the nuclei, cells were counterstained with DAPI (4′,6-diamidino-2-phenylindole). HMGB1 and NF-κB translocation was observed under an immunofluorescence microscope (Nikon, Melville, NY, USA).

Analysis of HMGB1 by Liquid Chromatography Tandem Mass Spectrometry

All chemicals and solvents were of the highest available grade (Sigma-Aldrich). Samples were precleared with 50 µL protein G-Sepharose beads for 1 h at 4°C. Supernatant HMGB1 was immunopre-cipitated with 5 µg rabbit anti-HMGB1 (Abcam, Cambridge, UK) for 16 h at 4°C as previously described (23). Free thiol groups within HMGB1 were alkylated for 90 min with 10 mmol/L iodoacetamide at 4°C. Cysteine residues in disulfide bonds were then reduced with 30 mmol/L dithiothreitol at 4°C for 1 h, followed by alkylation of newly exposed thiol groups with 90 mmol/L N-ethylmaleimide (NEM) at 4°C for 10 min. Samples were subjected to GluC (New England Biolabs, Hitchin, Herts, UK) digestion according to the manufacturer’s instructions and desalted by using Zip-Tip C18 pipette tips (Millipore, Consett, UK). The characterization of acetylated lysine residues within HMGB1 was determined as described previously (24,25) by using an AB Sciex TripleTOF 5600 (AB Sciex UK Limited, Warrington, Cheshire, UK).

Phagocytosis Assay

The phagocytosis assay was performed as previously described with minor modifications (6). RAW 264.7 cells were seeded in 48-well plates and allowed to adhere overnight at 37°C. RAW 264.7 cells were exposed to either 95% O₂ alone or 95% O₂ in the presence of GTS-21 for 24 h. After incubation, RAW 264.7 cells were kept at 37°C with fluorescein isothiocyanate (FITC)-labeled latex minibeads (Polysciences, Warrington, PA, USA) at a ratio of 100:1 (beads/cell). Macrophages were then washed with ice-cold PBS to stop phagocytosis, fixed with 4% paraformaldehyde for 10 min and washed with PBS. The fluorescence of the beads that were not phagocytosed by cells was quenched by incubating cells for 10 min with 0.4% trypan blue in PBS. The cytoskeleton was visualized by staining with Texas Red X-phalloidin (Molecular Probes/Life Technologies) in 1% bovine serum albumin. Phagocytosis or the uptake of the latex beads was assessed by using an immunofluorescence microscope (Nikon) by counting 250 consecutive individual macrophages per well in duplicates from three independent experiments for each treatment group.

Statistical Analysis

The data are presented as the mean ± standard error of the mean (SEM) of at least two independent experiments. The integrated area density of immunoreactive bands was measured by using ImageJ software, and the data were analyzed by using a Student t test and Microsoft Excel software (Microsoft, Redmond, WA). A p value of <0.05 was considered statistically significant.

Results

Systemic administration of GTS-21 significantly enhances bacterial clearance and decreases acute lung injury in a mouse model of VAP. To investigate whether GTS-21 can enhance bacterial clearance under hyperoxic conditions, male C57BL/6 mice were exposed to ≥99% O₂ for 48 h as described previously (7) and given either GTS-21 (0.04, 0.4 and 4 mg/kg) or normal saline (as control) intraperitoneally every 8 h, starting at 32 h after the onset of hyperoxic exposure. The mice were then inoculated with PA as described previously (7). Bacterial load, both in the airways and lung tissue, was significantly reduced in a dose-dependent manner in GTS-21-treated animals compared with that of controls (Figure 1). GTS-21, at 4 mg/kg, significantly reduced bacterial counts in the lungs (4.85 ± 0.48 log CFUs/lung versus controls 6.39 ± 0.34 log CFUs/lung, p < 0.05; Figure 1A) and in the airways (4.66 ± 0.45 log CFUs/mL versus controls 5.94 ± 0.26 log CFUs/mL BAL, p < 0.05; Figure 1B). Mice that received 4 mg/kg GTS-21 also had significantly lower total protein content in the lung lavage samples (a marker of lung injury) compared with that of mice given normal saline (2,755.34 ± 827.78 versus 5,204.70 ± 553.03 µg/mL, p < 0.05; Figure 1C). These data suggest that GTS-21 is effective in improving bacterial clearance and decreasing acute lung injury in the mouse model of VAP.

Figure 1

Systemic administration of GTS-21 increases bacterial clearance and decreases acute lung injury in mice exposed to hyperoxia and challenged with PA. C57BL/6 mice were exposed to ≥99% O₂ for 48 h and then inoculated with PA (0.1 × 10⁸ CFUs/mouse) and returned to 21% O₂ after inoculation. Mice were randomized to receive either GTS-21 (0.04, 0.4 and 4 mg/kg) or saline, administrated by intraperitoneal injection every 8 h starting at 32 h during hyperoxia. BAL and lung tissue were harvested 18 h after inoculation. Viable bacteria in the airways and lungs were quantified by plating serial dilutions of homogenized lung (A) and BAL (B) and are expressed as log CFUs/lung and log CFUs/mL of BAL, respectively. The total protein content in the BAL (C) was measured as a marker of lung injury. Data represent the mean ± SEM from two independent experiments (n = 5 for control, n = 6 for all GTS-21-treated mice). *p < 0.05, compared with mice receiving normal saline.

GTS-21 restores hyperoxia-compromised phagocytic activity of macrophages in hyperoxia. Previous studies in our lab indicate that hyperoxic exposure can compromise macrophage phagocytic activity and decrease the clearance of invading bacteria (7,13). Next, we examined whether GTS-21-improved bacterial clearance in the mouse model of VAP results from an increase in the ability of hyperoxic macrophages to phagocytose microorganisms. As shown previously (6,7), the phagocytic ability of hyperoxia-exposed macrophages was significantly reduced compared with macrophages cultured at room air, 21% O₂ (42.8 ± 1.9 versus 100.05 ± 2.06%, p < 0.001; Figure 2). GTS-21, at 5, 25 and 50 µmol/L, significantly increased phagocytic activity of macrophages to 70.3 ± 5.8, 84.5 ± 7.3 and 80.1 ± 11.6%, respectively, from that of macrophages exposed to 95% O₂ alone (42.8 ± 1.9%, p < 0.05). These data suggest that GTS-21 can improve the phagocytic ability of hyperoxic macrophages.

Figure 2

GTS-21 restores hyperoxia-compromised phagocytic ability of macrophages. RAW 264.7 cells were either exposed to 95% O₂ alone (black bar) or 95% O₂ in the presence of different concentrations of GTS-21 (gray bars) or remained at 21% O₂ (white bar) for 24 h. Cells were then incubated with FITC-labeled latex minibeads for 1 h and stained with phalloidin to visualize the cells. (A) Immunofluorescent images of RAW 264.7 cells (red: actin cytoskeleton; green: minibeads). At least 250 cells per well from three independent experiments were counted, and the number of beads per cell was represented as percent beads phagocytosed in the bar graph (B). Each value represents mean ± SEM of three independent experiments for each group. *p < 0.05 and **p < 0.01 versus 95% O₂ alone (black bar). Bar = 10 um.

GTS-21 inhibits the accumulation of extracellular HMGB1 induced by hyperoxic exposure. The exposure of macrophages to hyperoxia induces the release of nuclear HMGB1 into the extracellular milieu, which compromises the phagocytic activity of alveolar macrophages (13,26). To determine whether the GTS-21-induced increase in host defense and macrophage function is due to decreased accumulation of extracellular HMGB1, RAW 264.7 cells were exposed to either 95% O₂ alone or 95% O₂ in the presence of GTS-21 (5, 25 and 50 µmol/L). Consistent with our previous observations (7,27), exposure of RAW 264.7 cells to 95% O₂ for 24 h induced a significant release of nuclear HMGB1 into the extracellular milieu (Figure 3A). GTS-21, at 25 and 50 umol/L, significantly inhibited hyperoxia-induced HMGB1 release (61.76 ± 6.57 and 42.22 ± 11.24, respectively, versus 97.43 ± 1.45, p < 0.01; Figure 3A).

Figure 3

GTS-21 inhibits the accumulation of extracellular HMGB1. (A) RAW 264.7 cells were either exposed to 95% O₂ alone (black bar) or 95% O₂ in the presence of a series of concentration of GTS-21 (gray bars) or remained at 21% O₂ (white bar) for 24 h. HMGB1 levels in cell culture media were analyzed by Western blot analysis. A representative image of the immunoreactive bands on Western blots is shown. Bar graph shows the integrated density value of HMGB1 bands. The data are expressed as mean ± SEM of three independent experiments. **p < 0.01 versus control 95% O₂ alone (black bar). (B) C57BL/6 mice were exposed to ≥99% O₂ for 48 h and then inoculated with PA (0.1 × 10⁸ CFUs/mouse) and returned to 21% O₂ after inoculation. Mice were randomized to receive either GTS-21 or saline, administrated by intraperitoneal injection every 8 h starting at 32 h during hyperoxia. A representative image of Western blot immunoreactive bands of HMGB1 in the BAL of these mice is shown. Bar graph shows the integrated density value of HMGB1 bands in BAL of mice that received saline-treated (black bar, n = 5) and GTS-21-treated (4 mg/kg) mice (gray bar, n = 6).

Recent studies in our lab show that hyperoxia-suppressed bacterial clearance in PA pneumonia is associated with a substantial accumulation of extracellular HMGB1 in the airways (7). To determine whether the levels of extracellular HMGB1 in the airways of hyperoxic animals are altered in GTS-21-treated mice, we determined the concentrations of airway HMGB1 in lung lavage fluids from mice exposed to hyperoxia and treated with GTS-21. Figure 3B shows that mice treated with 4 mg/kg GTS-21 had a decrease in the accumulation of extracellular HMGB1 in the airways, compared with mice treated with normal saline (34.9 ± 12.23 versus 100.0 ± 31.24, p = 0.106, n = 5 for saline and n = 6 for GTS-21-treated mice). These data suggest that GTS-21 is effective in reducing accumulation of high levels of HMGB1 in the airways of hyperoxic animals by reducing the release of nuclear HMGB1 from lung cells induced by prolonged exposure to hyperoxia.

GTS-21 inhibits hyperoxia-induced HMGB1 release via blocking HMGB1 translocation from the nucleus to the cytoplasm and attenuating its hyperacetylation. Before its release, HMGB1 translocates from the nucleus to the cytoplasm, a critical step in its extracellular secretion (28,29). To test whether GTS-21 can inhibit HMGB1 translocation in hyperoxic macrophages, HMGB1 was visualized by using immunofluorescence in RAW 264.7 cells that were exposed to either 95% O₂ alone or 95% O₂ in the presence of GTS-21. Exposure to hyperoxia induced translocation of HMGB1 from the nucleus to the cytoplasm (Figure 4A, 95% O₂). In contrast, cells incubated with GTS-21 (50 µmol/L) retained HMGB1 in the nuclei, yielding a reduced stain in the cytoplasm compared with the hyperoxia control group. These data indicate that GTS-21 is effective in inhibiting hyperoxia-induced translocation of HMGB1.

Figure 4

GTS-21 inhibits hyperacetylation of HMGB1 and cytoplasmic translocation of nuclear HMGB1 in macrophages. (A) RAW 264.7 cells were either exposed to 21% O₂ or 95% O₂ in the absence or presence of GTS-21 (50 µmol/L) for 24 h. HMGB1 localization was visualized by immunofluorescence microscopy with anti-HMGB1 antibody (red). Counterstaining with DAPI was used to visualize nuclei (blue). Bar = 10 µm. Multiple pictures were taken using an immunofluorescence microscope to visualize HMGB1. The immunofluorescence images shown are representative of three independent experiments. (B) Representative spectra of the liquid chromatography mass spectrometric (LC-MS) characterization of peptides produced from HMGB1 derived from macrophage cell lysates enzymatically cleaved with endopeptidase GluC. Macrophages were exposed to 95% O₂ in the presence or absence of GTS-21. The presence of the peptides with molecular weights of 1,750 and 1,343 Da indicates the hyper-acetylation of lysine residues within NLS1 and NLS2, respectively. The presence of the peptides with molecular weights of 1,624 and 1,133 Da indicate the hypoacetylation of lysine residues within NLS1 and NLS2, respectively. (C) Representative spectra of the liquid chromatography tandem mass spectrometric (LC-MS/MS) characterization of a peptide (amino acids 180–188) covering the lysine (K) residues within NLS2 of HMGB1 to confirm the presence or absence of acetyl modifications on specific K residues. Acetyl modifications are represented as (ac) on specific lysine residues (K181, K182, K183 and K184) when required, and b and y ions are highlighted were appropriate.

The acetylation status of key lysines on two nuclear localization signal (NLS) sites of HMGB1 plays a critical role in its translocation between the nucleus and cytoplasm (28). The two sites, NLS1 and NLS2, contain four and five lysine residues, respectively. Hyperacetylation of NLS1 and NLS2 lysines mediates the active release of HMGB1 into the extracellular milieu from stimulated macrophages (28,30). To determine the acetylation status of HMGB1, RAW 264.7 cell lysates were analyzed using liquid chromatography tandem mass spectrometric (LC-MS/MS) as previously described (31,32). Under normoxic conditions, HMGB1 is in a hypoacetylated state (28), which is associated with predominant localization of HMGB1 in the nucleus (Figure 4A). In contrast, HMGB1 under hyperoxic conditions was found to be hyperacetylated (Figure 4B). However, HMGB1 in RAW 264.7 cells incubated with 50µmol/L GTS-21 was hypoacetylated (Figure 4B). Figure 4C shows representative images of the MS/MS data for hypoacetylation (top) and hyperacetylation (bottom) of HMGB1. These data suggest that GTS-21 is effective in blocking HMGB1 translocation and release via inhibiting acetylation of HMGB1.

GTS-21 inhibits hyperoxia-induced NF-κB activation. Recent observations in our lab suggest that NF-κB activation affected acetylation status of HMGB1 (unpublished data). To determine the underlying mechanism of GTS-21-induced HMGB1 hypoacetylation, the localization of NF-κB p65 subunit, a marker for NF-κB activation status, was determined in cells incubated with GTS-21 under hyperoxic conditions, as described previously (33). Profound staining of the NF-κB p65 subunit was mainly found in the cytoplasm in RAW cells that remained in room air (Figure 5A, 21% O₂), while in RAW cells exposed to hyperoxia, the profound stain was in the nuclei (an indicator of NF-κB activation) (Figure 5A, 95% O₂). Many of the cells incubated with GTS-21 (50 µM) showed a decreased nuclear staining of NF-κB p65 subunit in comparison to macrophages exposed to hyperoxia alone. Similarly, reduced levels of nuclear NF-κB p65 subunit were observed in lung cells of mice treated with GTS-21 (4 mg/kg) (Figure 5B). In addition, elevated levels of IκB, an inhibitor of NF-κB activation, were found in lung cytoplasmic extracts of mice treated with GTS-21, compared with that of the mice that received saline (Figure 5B). These results suggest that GTS-21 is effective in blocking hyperoxia-induced NF-κB activation.

Figure 5

GTS-21 inhibits hyperoxia-induced NF-κB activation. (A) RAW 264.7 cells were exposed either to 95% O₂ alone or 95% O₂ in the presence of GTS-21 (50 µmol/L) or remained at 21% O₂ for 24 h. After oxygen exposure, cells were washed with PBS, fixed, permeabilized and stained to localize the NF-κB p65 subunit (red). Multiple pictures were taken using an immunofluorescence microscope to visualize the localization of the p65 subunit of NF-κB. Counterstaining with DAPI was used to visualize nuclei (blue). The immunofluorescence images shown are representative of three independent experiments. (B) C57BL/6 mice were exposed to ≥99% O₂ for 48 h and then inoculated with PA (0.1 × 10⁸ CFUs/mouse) and returned to 21% O₂ after inoculation. Mice were randomized to receive either GTS-21 (4 mg/kg) or saline, administered by intraperitoneal injection every 8 h starting at 32 h during hyperoxia. Lungs of these mice were used to prepare nuclear and cytoplasmic extracts. Representative images are shown of Western blot immunoreactive bands of NF-κB p65 in the nuclear extract and IκB in the cytoplasmic extract of lungs of these mice. Bar graph shows the integrated density value of NF-κB p65 in the nuclear extract and IκB in the cytoplasmic extract of mice that received saline (black bar) and in GTS-21-treated (4 mg/kg) mice (gray bar) (n = 5 for saline, and n = 6 for GTS-21-treated mice). β-Actin expression was measured as a loading control.

Discussion

We have previously shown that extracellular HMGB1, released from the nuclei of hyperoxia-exposed lung cells, compromises macrophage function and bacterial clearance in a mouse model of VAP (7). In this study, we demonstrate that GTS-21, an α7nAChR agonist, inhibits hyperoxiainduced accumulation of HMGB1 in the airways of mice in a model of VAP, by attenuating the release of nuclear HMGB1. The inhibition of HMGB1 release resulted from decreased translocation of HMGB1 from the nucleus to the cytoplasm. The reduced HMGB1 translocation is associated with an attenuation of hyperoxia-induced hyperacetylation of HMGB1 and activation of NF-κB. Importantly, systemic administration of GTS-21 dose-dependently increased bacterial clearance from the airways and the lungs and decreased acute lung injury in the mouse model of VAP, whereas hyperoxia-compromised macrophage function of phagocytosis was restored in cells treated with GTS-21. These results suggest that GTS-21 increases bacterial clearance by improving hyperoxia-compromised phagocytic function of macrophages via inhibiting HMGB1 release.

Extracellular HMGB1, released from either the nuclei of intact immune cells or necrotic cells, has been implicated in the pathophysiology of a variety of diseases (29,34). For example, HMGB1 has been postulated to play a role in the pathogenesis of inflammatory diseases such as sepsis and rheumatoid arthritis (16,25,35,36). In addition, extracellular HMGB1 has been shown to compromise macrophage function of phagocytosis (13,26) and host defense against bacterial infection in mouse models of VAP and CF (7,13). Therefore, inhibiting the accumulation of extracellular HMGB1 may significantly attenuate the adverse effects of extracellular HMGB1 in excessive inflammatory responses and compromised innate immunity. In this article, we show that GTS-21 significantly enhances bacterial clearance from the lungs of mice exposed to hyperoxia and challenged with PA (Figure 1). The improved clinical outcomes in GTS-21-treated animals with bacterial infection are associated with reduced accumulation of airway HMGB1 (Figure 3). Others have shown that GTS-21 can inhibit HMGB1 release from LPS-stimulated immune cells (21,37) and decrease serum HMGB1 levels in a mouse model of endotoxemia (21). Nicotine, another α7nAChR agonist, was shown to inhibit HMGB1 release from LPS-stimulated macrophages and improve the survival of animals in a CLP model of sepsis with polymicrobial infection (18). Thus, these data suggest that the activation of the α7nAChR with agonists, such as GTS-21 or nicotine, can be an effective approach to combat gram-negative bacterial infections in organisms subjected to oxidative stress (13,16,38,39).

A critical step in the release of nuclear HMGB1 is its translocation from the nucleus into the cytoplasmic endolysosomes (40,41). Under normoxic conditions, HMGB1 regularly shuttles between the nucleus and the cytoplasm, but primarily resides in the nucleus (28). This nucleocytoplasmic shuttling is mainly regulated by various posttranslational modifications such as methylation, acetylation and phosphorylation (42–45). Recent findings suggest that acetylation of lysine residues on HMGB1 regulates its active release from activated monocytes and macrophages (28,30). Here, we showed that hyperoxia induces translocation of nuclear HMGB1 into the cytoplasm, which is associated with an increase in acetylation and accumulation of extracellular HMGB1 in cultured media (Figures 3 and 4). This translocation and hyperacetylation of HMGB1 were significantly inhibited by GTS-21 (Figure 4). Our results suggest that acetylation of HMGB1 leads to its release into the extracellular milieu under hyperoxic conditions. Inhibiting hyperoxia-induced acetylation of HMGB1 may provide a critical step for preventing its cytoplasmic translocation and subsequent release (Figure 4).

It is possible that GTS-21 suppresses HMGB1 release through a mechanism that resembles vagus nerve stimulation (19). Vagus nerve stimulation releases acetylcholine, which acts on the α7nAChR to inhibit NF-κB signaling and prevent tumor necrosis factor (TNF)-α production during endotoxemia (46,47). Both nicotine (a nonselective α7nAChR agonist) and GTS-21 inhibit endotoxin-induced NF-κB activation (19,21). Studies in our lab indicate that NF-κB activation plays a critical role in hyperoxia-induced HMGB1 release (unpublished data). NF-κB can activate histone acetyltransferases, which can lead to acetylation of HMGB1 and subsequent release (48). Figure 5 indicates that hyperoxia-induced NF-κB activation in both cultured macrophages and mouse lung cells can be inhibited by GTS-21, suggesting possible involvement of NF-κB in mediating an inhibitory effect of GTS-21 on hyperoxia-induced HMGB1 release (Figure 6), although further studies are required to confirm these results.

Figure 6

Hypothesized pathway of GTS-21-inhibited hyperoxia-induced HMGB1 release (A) and improved hyperoxia-compromised macrophage function in bacterial clearance (B). Hyperoxia induces the hyperacetylation of NLS1 and NLS2 sites on HMGB1, thus causing its cytoplasmic translocation and subsequent release from cells. GTS-21 inhibits hyperoxia-induced HMGB1 cytoplasmic translocation by inhibiting the hyperacetylation of HMGB1 (A). Under room air conditions, alveolar macrophages maintain normal phagocytic activity and efficiently clear bacteria. In macrophages exposed to hyperoxia, NF-κB is translocated into the nucleus, whereas HMGB1 translocates from the nucleus into the cytoplasm and subsequently into the extracellular milieu, leading to the impairment of phagocytosis and bacterial clearance by macrophages. GTS-21 can efficiently attenuate hyperoxia-impaired bacterial clearance by suppressing hyper-acetylation of HMGB1, which leads to its translocation into the cytoplasm and subsequent accumulation of extracellular HMGB1. GTS-21 also inhibits NF-κB translocation into the nucleus, which may prevent hyperacetylation of HMGB1 and subsequent translocation and release into the extracellular milieu.

To our knowledge, this study is the first to report that GTS-21 affects macrophage phagocytic function that is essential in combating pulmonary bacterial infections. In this study, GTS-21 significantly increased the phagocytic activity of hyperoxic macrophages (Figure 2) and successfully improved bacterial clearance from hyperoxia-exposed mice with PA pneumonia (Figure 1). The restoration of macrophage phagocytosis by GTS-21 occurred at concentrations of 5–50 umol/L and is at least partly due to the inhibition of HMGB1 release. In addition, extracellular HMGB1 can impair the ability of macrophages to clear apoptotic neutrophils, which may worsen bacterial infections by exacerbating inflammatory tissue injury (7,26). Thus, results shown in this article suggest that GTS-21 restores the phagocytic function of macrophages by inhibiting hyperacetylation of HMGB1 and its subsequent extracellular accumulation via attenuating NF-κB activation (Figure 6). Furthermore, GTS-21 has been shown to significantly attenuate the levels of proinflammatory cytokines, such as TNF and improves survival of animals subjected to polymicrobial infections (21). Taken together, targeting pathways to attenuate the accumulation of extracellular HMGB1 by GTS-21 may be a novel approach to develop therapies to treat bacterial infections in patients with VAP.

Conclusion

In summary, this article shows that the α7nAChR agonist GTS-21 can significantly inhibit hyperoxia-induced HMGB1 release from hyperoxic macrophages and lung cells, most likely by inhibiting its hyperacetylation and decreasing its translocation into the cytoplasm from nuclei. Importantly, GTS-21 significantly improved bacterial clearance in a mouse model of VAP, likely via enhancing hyperoxia-suppressed phagocytic ability of macrophages. These results suggest that α7nAChR may represent a pharmacological target for the improvement of the clinical outcome in patients on ventilators by augmenting host defense against bacterial infections.

Disclosure

The authors declare that they have no competing interests as defined by Molecular Medicine, or other interests that might be perceived to influence the results and discussion reported in this paper.