A large number of studies have demonstrated that the enhanced secretion of proinflammatory cytokines is an important factor in the initiation and perpetuation of inflammation in different tissues.1–4 These cytokines recruit other immune cells, including neutrophils, thereby increasing leukocyte trafficking and lung injury.5,6 Neutrophils can release superoxide anions and proteolytic enzymes, which diffuse across the endothelium and injure parenchymal cells or, alternatively, neutrophils can leave the microcirculation, migrate to and adhere to matrix proteins or other cells.7,8
Hemorrhagic shock results in excessive production of proinflammatory mediators, which play a significant role in the development of multiple organ dysfunction.9 Studies have shown that neutrophils are activated after hemorrhagic shock5 and the subsequent accumulation of neutrophils in the lung is associated with lung injury.9 Interleukin (IL)-6 seems to be an essential component of the inflammatory cascade that is associated with lung injury in hemorrhagic shock.10 IL-6-deficient mice showed less neutrophil infiltration and organ damage when compared with wild-type mice under those conditions.11
There is also a growing body of evidence indicating that heme degradation products (i.e., carbon monoxide, biliverdin, bilirubin) may counteract the detrimental consequences of decreased organ perfusion and ischemia-reperfusion-related injury after low-flow conditions.12 Carbon monoxide acts as a vasodilator agent via the activation of guanylate cyclase, whereas biliverdin and bilirubin have immunomodulatory and antioxidant capacities. Heme is predominantly catabolized by a class of enzymes known as hemeoxygenases (HOs). The HO-1 is also known as heat shock protein-32, and seems to act as a protective agent in many organs, protecting against insults such as ischemia and oxidative stress.13 HO-1 is induced after various pathophysiological conditions, such as ischemia, oxidative stress and endotoxemia, in which redox stress is induced,14 and its induction seems to play a critical role in protection against the deleterious pathological effects of low-flow states, as its induction results in protection but its inhibition leads to exacerbation of the injury.6,15 Previous studies have also demonstrated that upregulation of HO-1 causes a reduction of neutrophil accumulation after trauma-hemorrhage.6,15
Sirtinol, an inhibitor of the sirtuin family of nicotinamide adenine dinucleotide -dependent deacetylases in Saccharomyces cerevisiae, has been shown to be protective after shock-like states in male rats.16–18 Our recent studies have shown that sirtinol can reduce neutrophil and cytokine production after trauma-hemorrhage.18 Nonetheless, it remains unknown whether administration of sirtinol in male rats after trauma-hemorrhage will modulate HO-1 expression and attenuate lung injury. We therefore hypothesized that sirtinol upregulates HO and that upregulation of HO is responsible for attenuating lung injury after trauma-hemorrhage.
The current study was approved by the Institutional Animal Care and Use Committee of Chang Gung Memorial Hospital. All animal experiments were performed according to the guidelines of the Animal Welfare Act and the Guide for Care and Use of Laboratory Animals from the National Institutes of Health.
A non-heparinized rat model of trauma-hemorrhage was used in this study.19 Forty-eight male Sprague-Dawley rats (275–325 g) obtained from the National Science Council were divided into 6 groups of 8 animals each, according to a table of random numbers. They were housed in an air-conditioned room under a reversed light-dark cycle and allowed at least 1 wk to adapt to the environment. Before initiation of the experiment, they were fasted overnight but were allowed water ad libitum. The rats were anesthetized by isoflurane (Attane, Minrad, Bethlehem, PA) inhalation before the performance of a 5-cm midline laparotomy in the abdomen. The abdomen was closed in layers, and catheters were placed in both femoral arteries and the right femoral vein (polyethylene-50 tubing; Becton Dickinson & CO, Sparks, MD). The wounds were bathed with 1% lidocaine (Elkins-Sinn, Cherry Hill, NJ) throughout the surgical procedure to reduce postoperative pain. Rats were then allowed to awaken, and bled to, and maintained at, a mean arterial blood pressure of 40 mm Hg. This level of hypotension persisted until the animals could not maintain mean arterial blood pressure of 40 mm Hg unless additional fluid in the form of Ringer’s lactate solution was administered. This time was defined as maximum bleed-out, and the amount of withdrawn blood was noted. After this, the rats were maintained at a mean arterial blood pressure of 40 mm Hg until 40% of the maximum bleed-out volume was returned in the form of Ringer’s lactate solution. The animals were then resuscitated with four times the volume of the shed blood over 60 min with Ringer’s lactate solution. The time required for maximum bleed-out was approximately 45 min, the volume of maximum bleed-out was approximately 60% of the calculated circulating blood volume,20 and the total hemorrhage time was approximately 90 (84–95) min. Thirty minutes before the end of the resuscitation period, the rats received sirtinol (1 mg/kg, IV),18 the combination of sirtinol and a specific HO inhibitor chromium-mesoporphyrin (CrMP) (2.5 mg/kg intraperitoneally; Frontier Scientific, Logan, UT), or an equal volume of the vehicle (approximately 0.2 mL, 10% DMSO, Sigma). The catheters were then removed, the vessels ligated and the skin incisions closed with sutures. Sham-operated animals underwent the surgical procedure, which included a laparotomy in addition to the ligation of the femoral artery and vein, but neither hemorrhage nor resuscitation was performed. Vehicle or sirtinol was also administered in sham-operated rats after catheters were placed. The animals were then returned to their cages and were allowed food and water ad libitum. The animals were killed 24 h after the end of resuscitation.
Preparation of Lung Tissue and Collection of Bronchoalveolar Lavage Fluid
Twenty-four hours after the completion of fluid resuscitation or sham operation, the animals were anesthetized with isoflurane and then killed. The chest was opened and the left side of the lung was obtained after clamping the hilum. Excess blood was blotted and the left upper lobe of the lung was stored at −80°C until analysis. The trachea was then cannulated and bronchoalveolar lavage fluid was obtained by washing the airways 4 times with 5 mL of phosphate-buffered saline (PBS). The bronchoalveolar lavage fluid was centrifuged at 1200g at 4°C for 10 min. The supernatant was collected and stored at −80°C until analyzed.9
Protein Assay in Lung Lavage
Cell-free bronchoalveolar lavage fluid was evaluated for total protein content (Bio-Rad DC Protein Assay, Bio-Rad, Hercules, CA).9
Measurement of Myeloperoxidase (MPO) Activity
Myeloperoxidase (MPO) activity in homogenates of whole lung was determined as described previously.5,9 All reagents were purchased from Sigma. Briefly, equal weights (100 mg wet weight) of lung from various groups were suspended in 1 mL buffer (0.5% hexadecyltrimethylammonium bromide in a 50 mM phosphate buffer, pH 6.0) and sonicated at 30 cycles, twice, for 30 s on ice. Homogenates were cleared by centrifuging at 2000g at 4°C, and the supernatants were stored at −80°C. Protein content in the samples was determined using the Bio-Rad (Hercules, CA) assay kit. The samples were incubated with a substrate o-dianisidine hydrochloride. This reaction was performed in a 96-well plate by adding 290 μL 50 mM phosphate buffer, 3 μL substrate solution (containing 20 g/L o-dianisidine hydrochloride), and 3 μL H2O2 (20 mM). Sample (10 μL) was added to each well to start the reaction. Standard MPO (Sigma, St. Louis, MO) was used in parallel to determine MPO activity in the sample. The reaction was stopped by adding 3 μL sodium azide (30%). Light absorbance at 460 nm was read. MPO activity was determined using the curve obtained from the standard MPO.
Determination of Lung Tumor Necrosis Factor α (TNF-α), IL-6, and IL-10 Levels
Lung TNF-α, IL-6, and IL-10 levels were determined using ELISA kits (R&D, Minneapolis, MN) according to the manufacturer’s instructions and as described previously.9 Briefly, the samples were homogenized in PBS (1:10 weight: volume) (pH 7.4) containing protease inhibitors (Complete Protease Inhibitor Cocktail, Boehringer Mannheim, Germany). The homogenates were centrifuged at 2000g for 20 min at 4°C and the supernatants were assayed for TNF-α, IL-6, and IL-10 levels. An aliquot of the supernatant was used to determined protein concentration.
Western Blot Assay
Rat lung tissues were homogenized in a buffer containing 10 mM Tris (pH 7.5), 150 mM NaCl, 1 mM ethylene-diamine-tetra-acetic acid, 1 mM EGTA, 50 mM NaF, 0.5 mM phenylmethylsulfonyl fluoride, 1 mM sodium vanadate, 1% Triton X-100, 0.5% Nonidet P-40, and 1 mg/L of aprotinin. The homogenates were centrifuged at 12,000g for 15 min at 4°C. An aliquot of the supernatant was used to determined protein concentration. Protein aliquots were mixed with 4× sample buffer and were electrophoresed on 4%–12% sodium dodecyl sulfate-polyacrylamide gels (Invitrogen, Carlsbad, CA) and transferred electrophoretically onto nitrocellulose transfer membranes (Invitrogen, Carlsbad, CA). The membranes were then incubated with anti-HO-1 (1: 2000) (Chemicon International, Temecula, CA) in 5% nonfat dry milk overnight at 4°C and then washed with TBST. The membranes were later incubated with horseradish peroxidase-conjugated goat anti-rabbit antibody (1:5000 dilution in 5% nonfat dry milk for HO-1) for 1.5 h at room temperature and washed with TBST. The blots were immersed for 5 min in Super Signal West Pico detection reagent and then exposed to film. Signals were quantified using ChemiImager 5500 imaging software (Alpha Innotech Corp., San Leandro, CA).
Histological Analysis of Lung
As described previously,21 three pieces of left lung were fixed in 10% formalin in PBS for 24 h and were sent to the histology laboratory at the Chang Gung University for further processing. Briefly, the sections were embedded in paraffin. These were then cut (4–5 μM) and mounted on glass slides. Lung sections were stained with hematoxylin-eosin, observed under the microscope (Nikon Eclipse TS100) at a magnification ×100 for changes in lung morphology, and photographed using a camera (SPOT, RTcolor, Diagnostic Instrument, Iowa City, IA) attached to the microscope.
For statistical analysis we used the InStat 3.0 biostatistics program (Graph Pad Software, San Diego, CA). Results are presented as mean ± sd (n = 8 rats/group). The data were analyzed using one-way analysis of variance and Tukey’s test, and differences were considered significant at P ≤ 0.05.
HO-1 Protein Expression in Lung
Trauma-hemorrhage induced a significant increase in lung HO-1 protein expression as compared with shams (Fig. 1). Administration of sirtinol after trauma-hemorrhage induced a further significant increase in lung HO-1 protein expression as compared with trauma-hemorrhage vehicle-treated rats. The results indicated that trauma-hemorrhage-induced increase in HO-1 protein expression was increased further by sirtinol administration.
HO-1 and Total Protein Content in Bronchoalveolar Lavage Fluid
In sham-operated animals, no significant differences in bronchoalveolar lavage fluid total protein content were found between vehicle- and sirtinol-treated groups (Fig. 2). Trauma-hemorrhage significantly increased total protein content in bronchoalveolar lavage fluid. Sirtinol treatment attenuated the trauma-hemorrhage-induced increase in bronchoalveolar lavage fluid total protein content; however, the level remained higher than in shams. To determine the role of HO-1 in sirtinol-induced decreases in total protein content in bronchoalveolar lavage fluid after trauma-hemorrhage, rats were treated with the HO inhibitor CrMP along with sirtinol. The results indicate that administration of CrMP with sirtinol prevented the sirtinol-induced decrease in total protein content in bronchoalveolar lavage fluid (Fig. 2).
HO-1 and Lung MPO Activity
Lung MPO activity in sham-operated or trauma-hemorrhaged animals with and without sirtinol treatment is shown in Figure 3. In sham-operated rats, sirtinol did not alter lung MPO activity. Trauma-hemorrhage resulted in a significant increase in lung MPO activity in vehicle-treated animals. Furthermore, sirtinol treatment attenuated the increase in lung MPO activity. To evaluate the role of HO-1 in sirtinol-induced decrease in lung MPO activity in trauma-hemorrhaged rats, rats were treated with the HO inhibitor CrMP along with sirtinol after trauma-hemorrhage. The results indicate that administration of CrMP with sirtinol abolished the sirtinol-induced decrease in lung MPO activity (Fig. 3).
HO-1 and Lung TNF-α, IL-6, and IL-10 Levels
As shown in Figure 4, lung TNF-α, IL-6, and IL-10 levels were not influenced by sirtinol administration in sham animals compared with shams receiving vehicle. Trauma-hemorrhage significantly increased lung TNF-α, IL-6, and IL-10 levels compared with sham animals. However, sirtinol administration after trauma-hemorrhage significantly reduced the elevated lung TNF-α and IL-6 levels (Figs. 4A and B). Moreover, co-administration of the HO inhibitor CrMP with sirtinol prevented the sirtinol-induced reduction in lung TNF-α and IL-6 levels in the trauma-hemorrhage group (Figs. 4A and B). No statistically significant difference in IL-10 levels in the lung was found in trauma-hemorrhaged rats (Fig. 4C).
Histological Analysis of Lung
The representative photomicrographs of lung are presented for sham animals treated with vehicle (Fig. 5A), sham animals treated with sirtinol (Fig. 5B), trauma-hemorrhage animals treated with vehicle (Fig. 5C), trauma-hemorrhage animals treated with sirtinol (Fig. 5D), trauma-hemorrhage animals treated with sirtinol and CrMP (Fig. 5E), and trauma-hemorrhage animals treated with CrMP (Fig. 5F). Similar results were obtained from four or more animals in each group. Together, these results, as presented in Figure 5, suggest that sirtinol ameliorated the trauma-hemorrhage-induced damage in the lung; however, the damage remained higher than in shams. Co-administration of the HO inhibitor CrMP with sirtinol prevented the sirtinol-induced reduction of lung damage in the trauma-hemorrhage group.
Our studies collectively suggest that the sirtinol-mediated lung protection after trauma-hemorrhage is in part mediated via upregulation of HO-1. To our knowledge, this is the first report demonstrating the protective effects of sirtinol on trauma-hemorrhage-induced lung injury. Induction of HO-1 plays an important role in organ protection against the deleterious pathophysiological conditions, such as trauma-hemorrhage, ischemia, oxidative stress and endotoxemia.6,14,15 Consistent with these findings, our results indicate that sirtinol treatment after trauma-hemorrhage leads to a further increase in HO-1 expression and attenuates the trauma-hemorrhage-induced increase in MPO activity, and TNF-α, and IL-6 levels in the lung, protein concentrations in bronchoalveolar lavage fluid and tissue histology. However, we did not find a statistically significant difference in IL-10 levels in the lungs of trauma-hemorrhaged rats. Furthermore, sirtinol-induced protective effects on the lung after trauma-hemorrhage were prevented by co-administration of the HO inhibitor CrMP.
The lung is considered to be a critical organ in the development of the delayed organ dysfunction in patients suffering from traumatic injuries and severe blood loss.6 Multiple organ failure or dysfunction secondary to a systemic inflammatory response is the major cause of mortality and morbidity after trauma.10 Neutrophils are the principal cells involved in host defense against acute bacterial and fungal infections,22 and thus these cells have a protective effect. However, under conditions such as those described in this study, the infiltration of these cells may cause tissue damage.5,23
There is now considerable evidence demonstrating a role for sirtinol in mediating the production of proinflammatory cytokines.18,24 Our recent studies have reported that sirtinol reduced levels of IL-6, chemokines, and adhesion molecules after trauma-hemorrhage.18 The cytokines IL-1, IL-6, and TNF-α are important early mediators in the lung, and are required for expression of adhesion molecules and chemokines.9,25 The ability of sirtinol to mediate expression of inflammatory cytokines suggests a role for sirtinol in the regulation of lung inflammation.
The present study is the first to examine the protective effects of sirtinol in the lung after trauma-hemorrhage and to indicate that sirtinol administration after trauma-hemorrhage decreases TNF-α and IL-6 levels. The dose for administration of sirtinol in the present study was obtained from our previous study.18 In addition, we also administered sirtinol at doses of 0.1, 0.3, 3, and 5 mg/kg to evaluate its salutary effects in attenuation of lung injury after trauma-hemorrhage. We found fewer beneficial effects when sirtinol was administered at a dosage of 0.1 or 0.3 mg/kg and similar results when sirtinol was administered at a dosage of 3 or 5 mg/kg (data not shown).
Our studies also suggest that the salutary effects of sirtinol are mediated via HO-1. A growing body of evidence indicates that HO-1 expression is upregulated after trauma-hemorrhage, and that the HO byproduct carbon monoxide plays a central role in the preservation of tissue microcirculation under such conditions.15,26 In addition, induction of HO-1 has been shown not only to improve local tissue circulation, but also to attenuate lung injury after trauma-hemorrhage.6 It has also been reported that HO-1 can reduce the expression of adhesion molecules and may therefore also prevent subsequent leukocyte-endothelial cell interactions.6,15
Our results showed that sirtinol administration in male rats attenuates the trauma-hemorrhage-induced increases in lung MPO activity, TNF-α, and IL-6 levels. However, co-administration of sirtinol and the HO inhibitor CrMP abolished the beneficial effects of sirtinol administration in male rats after trauma-hemorrhage. These findings suggest that the protective effects in the sirtinol-treated male rats were mediated via upregulation of HO-1 expression in the lung. Furthermore, our study showed that inhibition of HO-1 prevents flutamide-induced improvement in organ function after trauma-hemorrhage.15 These findings therefore suggest that HO-1 may play an important role in improving organ function after trauma-hemorrhage.
Administration of a single dose of sirtinol was chosen for the present study. It is unknown whether sirtinol treatment in multiple doses has further beneficial effects in trauma-hemorrhaged rats. It is also not known if the potential protective effects of sirtinol after trauma-hemorrhage are species-specific or variable.
In addition, further in vitro study of the cellular mechanisms of sirtinol on HO-1 expression would be helpful to strengthen the in vivo data. In vitro data on the cellular mechanisms of sirtinol were not collected in the current study. Thus, additional studies are needed to completely elucidate the mechanism by which sirtinol leads to attenuation of trauma-hemorrhage-induced lung injury.
In conclusion, our study indicates that sirtinol administration upregulates HO-1 expression and attenuates lung injury after trauma-hemorrhage. Blockade of HO pathways and the associated deterioration of the examined variables suggest that the reduction of neutrophil accumulation in the lung is mediated via HO. Although the precise mechanism of the salutary effects of sirtinol administration in male rats on organ functions and the contribution of HO pathways in reducing organ injuries after trauma-hemorrhage remain unclear, our study provides evidence that upregulation of HO-1 serves as a significant effecter mechanism in the reduction of lung injury after trauma-hemorrhage. Since sirtinol administration after trauma-hemorrhage decreased lung injury and upregulated HO-1 expression in male Sprague-Dawley rats, this drug might be a novel adjunct for improving the depressed lung function in humans after adverse circulatory conditions.
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