Update in acute lung injury and mechanical ventilation 2011

Propofol Protects Lung Endothelial Barrier Function by Suppression of High-Mobility Group Box 1 (HMGB1) Release and Mitochondrial Oxidative Damage Catalyzed by HMGB1

Background

The processes of mechanical ventilation-induced lung injury (VILI) triggers the release of high-mobility group box 1 (HMGB1), a prominent damage-associated molecular pattern (DAMP) family member, which can cause damage to pulmonary vascular endothelial cells. We aimed to determine whether propofol protected against endothelial cell injury induced by HMGB1 in vitro and in vivo.

Material/Methods

ICR mice (male) were mechanically ventilated for 4 h after anesthetization at both low tidal volume (LVT, 6 ml/kg) and high tidal volume (HVT, 30 ml/kg). A propofol bolus (10 mg/kg) was administered to the animals prior to the onset of ventilation, followed by infusion at 5 mg/(kg·h). We obtained confluent cultures of mouse lung vascular endothelial cells (MLVECs) and then performed cyclic stretching at 20% stretch for 4 h with or without propofol.

Results

HMGB1 reduced the expression of tight junctions between endothelial cells, including VE-cadherin and ZO-1, and increased endothelial permeability, and both were blocked by propofol. We found that MLVECs exhibited mitochondrial oxidative damage by HMGB1, which was successfully suppressed through administration of MnTBAP as well as propofol. Propofol ameliorated HVT-associated lung vascular hyperpermeability and HMGB1 production in vivo. Propofol also inhibited HMBG1 release caused by cyclic stretching in MLVECs in vitro.

Conclusions

Our results prove that the cyto-protective function of propofol protects against lung ventilation-induced dysfunction of the lung endothelial barrier. This function of propofol is mediated through inhibition of HMGB1 release caused by mechanical stretching and mitochondrial oxidative damage triggered by HMGB1.

Capturing the multifactorial nature of ARDS - "Two-hit" approach to model murine acute lung injury

Severe acute respiratory distress syndrome (ARDS) presents typically with an initializing event, followed by the need for mechanical ventilation. Most animal models of ALI are limited by the fact that they focus on a singular cause of acute lung injury (ALI) and therefore fail to mimic the complex, multifactorial pathobiology of ARDS. To better capture this scenario, we provide a comprehensive characterization of models of ALI combining two injuries: intra tracheal (i.t.) instillation of LPS or hypochloric acid (HCl) followed by ventilator‐induced lung injury (VILI). We hypothesized, that mice pretreated with LPS or HCl prior to VILI and thus receiving a (“two‐hit injury”) will sustain a superadditive lung injury when compared to VILI. Mice were allocated to following treatment groups: control with i.t. NaCl, ventilation with low peak inspiratory pressure (PIP), i.t. HCl, i.t. LPS, VILI (high PIP), HCl i.t. followed by VILI and LPS i.t. followed by VILI. Severity of injury was determined by protein content and MPO activity in bronchoalveolar lavage (BAL), the expression of inflammatory cytokines and histopathology. Mice subjected to VILI after HCl or LPS instillation displayed augmented lung injury, compared to singular lung injury. However, mice that received i.t. LPS prior to VILI showed significantly increased inflammatory lung injury compared to animals that underwent i.t. HCl followed by VILI. The two‐hit lung injury models described, resulting in additive but differential acute lung injury recaptures the clinical relevant multifactorial etiology of ALI and could be a valuable tool in translational research.

Selenium supplementation of lung epithelial cells enhances nuclear factor E2-related factor 2 (Nrf2) activation following thioredoxin reductase inhibition

The trace element selenium (Se) contributes to redox signaling, antioxidant defense, and immune responses in critically ill neonatal and adult patients. Se is required for the synthesis and function of selenoenzymes including thioredoxin (Trx) reductase-1 (TXNRD1) and glutathione peroxidases (GPx). We have previously identified TXNRD1, primarily expressed by airway epithelia, as a promising therapeutic target to prevent lung injury, likely via nuclear factor E2-related factor 2 (Nrf2)-dependent mechanisms. The present studies utilized the TXNRD1 inhibitor auranofin (AFN) to test the hypothesis that Se positively influences Nrf2 activation and selenoenzyme responses in lung epithelial cells. Murine transformed Club cells (mtCCs) were supplemented with 0, 10, 25, or 100 nM Na2SeO3 to create a range of Se conditions and were cultured in the presence or absence of 0.5 μM AFN. TXNRD1 and GPX2 protein expression and enzymatic activity were significantly greater upon Se supplementation (p < 0.05). AFN treatment (0.5 μM AFN for 1 h) significantly inhibited TXNRD1 but not GPx activity (p < 0.001). Recovery of TXNRD1 activity following AFN treatment was significantly enhanced by Se supplementation (p < 0.041). Finally, AFN-induced Nrf2 transcriptional activation was significantly greater in mtCCs supplemented in 25 or 100 nM Na2SeO3 when compared to non-supplemented controls (p < 0.05). Our novel studies indicate that Se levels positively influence Nrf2 activation and selenoenzyme responses following TXNRD1 inhibition. These data suggest that Se status significantly influences physiologic responses to TXNRD1 inhibitors. In conclusion, correction of clinical Se deficiency, if present, will be necessary for optimal therapeutic effectiveness of TXNRD1 inhibitors in the prevention of lung disease.

Metabolomics and Its Application to Acute Lung Diseases

Metabolomics is a rapidly expanding field of systems biology that is gaining significant attention in many areas of biomedical research. Also known as metabonomics, it comprises the analysis of all small molecules or metabolites that are present within an organism or a specific compartment of the body. Metabolite detection and quantification provide a valuable addition to genomics and proteomics and give unique insights into metabolic changes that occur in tangent to alterations in gene and protein activity that are associated with disease. As a novel approach to understanding disease, metabolomics provides a "snapshot" in time of all metabolites present in a biological sample such as whole blood, plasma, serum, urine, and many other specimens that may be obtained from either patients or experimental models. In this article, we review the burgeoning field of metabolomics in its application to acute lung diseases, specifically pneumonia and acute respiratory disease syndrome (ARDS). We also discuss the potential applications of metabolomics for monitoring exposure to aerosolized environmental toxins. Recent reports have suggested that metabolomics analysis using nuclear magnetic resonance (NMR) and mass spectrometry (MS) approaches may provide clinicians with the opportunity to identify new biomarkers that may predict progression to more severe disease, such as sepsis, which kills many patients each year. In addition, metabolomics may provide more detailed phenotyping of patient heterogeneity, which is needed to achieve the goal of precision medicine. However, although several experimental and clinical metabolomics studies have been conducted assessing the application of the science to acute lung diseases, only incremental progress has been made. Specifically, little is known about the metabolic phenotypes of these illnesses. These data are needed to substantiate metabolomics biomarker credentials so that clinicians can employ them for clinical decision-making and investigators can use them to design clinical trials.

Hypoxia signaling during acute lung injury

Acute lung injury (ALI) is an inflammatory lung disease that manifests itself in patients as acute respiratory distress syndrome and thereby contributes significantly to the morbidity and mortality of patients experiencing critical illness. Even though it may seem counterintuitive, as the lungs are typically well-oxygenated organs, hypoxia signaling pathways have recently been implicated in the resolution of ALI. For example, functional studies suggest that transcriptional responses under the control of the hypoxia-inducible factor (HIF) are critical in optimizing alveolar epithelial carbohydrate metabolism, and thereby dampen lung inflammation during ALI. In the present review we discuss functional roles of oxygenation, hypoxia and HIFs during ALI, mechanisms of how HIFs are stabilized during lung inflammation, and how HIFs can mediate lung protection during ALI.

Lung endothelial barrier protection by resveratrol involves inhibition of HMGB1 release and HMGB1-induced mitochondrial oxidative damage via an Nrf2-dependent mechanism

High-mobility group box 1 (HMGB1) contributes to lung vascular hyperpermeability during ventilator-induced lung injury. We aimed to determine whether the natural antioxidant resveratrol protected against HMGB1-induced endothelial hyperpermeability both in vitro and in vivo. We found that HMGB1 decreased vascular endothelial (VE)-cadherin expression and increased endothelial permeability, leading to mitochondrial oxidative damage in primary cultured mouse lung vascular endothelial cells (MLVECs). Both the mitochondrial superoxide dismutase 2 mimetic MnTBAP and resveratrol blocked HMGB1-induced mitochondrial oxidative damage, VE-cadherin downregulation, and endothelial hyperpermeability. In in vivo studies, anesthetized male ICR mice were ventilated for 4h using low tidal volume (6 ml/kg) or high tidal volume (HVT; 30 ml/kg) ventilation. The mice were injected intraperitoneally with resveratrol immediately before the onset of ventilation. We found that resveratrol attenuated HVT-associated lung vascular hyperpermeability and HMGB1 production. HVT caused a significant increase in nuclear factor-erythroid 2-related factor 2 (Nrf2) nuclear translocation and Nrf2 target gene expression in lung tissues, which was further enhanced by resveratrol treatment. HMGB1 had no effect on Nrf2 activation, whereas resveratrol treatment activated the Nrf2 signaling pathway in HMGB1-treated MLVECs. Moreover, Nrf2 knockdown reversed the inhibitory effects of resveratrol on HMGB1-induced mitochondrial oxidative damage and endothelial hyperpermeability. The inhibitory effect of resveratrol on cyclic stretch-induced HMGB1 mRNA expression in primary cultured MLVECs was also abolished by Nrf2 knockdown. In summary, this study demonstrates that resveratrol protects against lung endothelial barrier dysfunction initiated by HVT. Lung endothelial barrier protection by resveratrol involves inhibition of mechanical stretch-induced HMGB1 release and HMGB1-induced mitochondrial oxidative damage. These protective effects of resveratrol might be mediated through an Nrf2-dependent mechanism.

Advances in ventilator-associated lung injury: prevention is the target

Mechanical ventilation (MV) is the main supportive treatment in respiratory failure due to different etiologies. However, MV might aggravate ventilator-associated lung injury (VALI). Four main mechanisms leading to VALI are: 1) increased stress and strain, induced by high tidal volume (VT); 2) increased shear stress, i.e. opening and closing, of previously atelectatic alveolar units; 3) distribution of perfusion and 4) biotrauma. In severe acute respiratory distress syndrome patients, low VT, higher levels of positive end expiratory pressure, long duration prone position and neuromuscular blockade within the first 48 hours are associated to a better outcome. VALI can also occur by using high VT in previously non injured lungs. We believe that prevention is the target to minimize injurious effects of MV. This review aims to describe pathophysiology of VALI, the possible prevention and treatment as well as monitoring MV to minimize VALI.

The thioredoxin reductase-1 inhibitor aurothioglucose attenuates lung injury and improves survival in a murine model of acute respiratory distress syndrome

AIMS

Inflammation and oxygen toxicity increase free radical production and contribute to the development of acute respiratory distress syndrome (ARDS), which is a significant cause of morbidity and mortality in intensive care patients. We have previously reported increased glutathione (GSH) levels in lung epithelial cells in vitro and attenuated adult murine hyperoxic lung injury in vivo after pharmacological thioredoxin reductase-1 (TrxR1) inhibition. Using a murine ARDS model, we tested the hypothesis that aurothioglucose (ATG) treatment increases pulmonary GSH levels, attenuates lung injury, and decreases mortality in a GSH-dependent manner.

RESULTS

Adult mice received a single intratracheal dose of 0.375 μg/g lipopolysaccharide (LPS) 12 h before a single intraperitoneal injection of 25 mg/kg ATG. Control mice received intratracheal and/or intraperitoneal saline. Mice were then exposed to room air or hyperoxia (>95% O2). Lung injury was assessed by bronchoalveolar lavage protein concentrations. Expression of glutamate-cysteine ligase modifier subunit (GCLM), GSH, cytokines, and chemokines was determined. Exposure to LPS/hyperoxia induced inflammation and lung injury. ATG treatment significantly attenuated lung injury, increased lung GCLM expression and GSH levels, and decreased mortality. GSH depletion completely prevented the protective effects of ATG in LPS/hyperoxia-exposed mice.

INNOVATION

ATG treatment significantly attenuates lung injury and enhances survival in a clinically relevant murine model of ARDS. The protective effects of ATG are GSH dependent.

CONCLUSION

Augmentation of GSH systems by TrxR1 inhibition could represent a promising therapeutic approach to attenuate oxidant-mediated lung injury and improve patient outcomes.