Microvesicles released from pneumolysin-stimulated lung epithelial cells carry mitochondrial cargo and suppress neutrophil oxidative burst

Microvesicles (MVs) are cell-derived extracellular vesicles that have emerged as markers and mediators of acute lung injury (ALI). One of the most common pathogens in pneumonia-induced ALI is Streptococcus pneumoniae (Spn), but the role of MVs during Spn lung infection is largely unknown. In the first line of defense against Spn and its major virulence factor, pneumolysin (PLY), are the alveolar epithelial cells (AEC). In this study, we aim to characterize MVs shed from PLY-stimulated AEC and explore their contribution in mediating crosstalk with neutrophils. Using in vitro cell and ex vivo (human lung tissue) models, we demonstrated that Spn in a PLY-dependent manner stimulates AEC to release increased numbers of MVs. Spn infected mice also had higher levels of epithelial-derived MVs in their alveolar compartment compared to control. Furthermore, MVs released from PLY-stimulated AEC contain mitochondrial content and can be taken up by neutrophils. These MVs then suppress the ability of neutrophils to produce reactive oxygen species, a critical host-defense mechanism. Taken together, our results demonstrate that AEC in response to pneumococcal PLY release MVs that carry mitochondrial cargo and suggest that these MVs regulate innate immune responses during lung injury.

ID: 109: PARKIN MEDIATES ENDOTHELIAL PRO-INFLAMMATORY RESPONSES IN ACUTE LUNG INJURY

Introduction

Acute lung injury (ALI) and its more severe form, the Acute Respiratory Distress Syndrome (ARDS), are serious conditions resulting from direct or indirect lung injury that occur in critically ill patients and are associated with an unacceptable mortality of up to 40%. A key biological event in the pathogenesis of ALI/ARDS is the dysfunction of the lung endothelium (EC), which is triggered by a variety of inflammatory insults leading to damaged EC, vascular leak, and excessive inflammation. Recently, we demonstrated that an Abl family tyrosine kinase inhibitor, imatinib, protects against LPS-induced endothelial dysfunction by inhibiting c-Abl kinase through mechanisms that remain largely unknown. In the present study, we identified parkin, a novel c-Abl substrate, as a critical mediator of endothelial dysfunction in ALI.

Methods

In vitro Human pulmonary artery endothelial cells (EC) were transfected with siRNA for parkin and then challenged with LPS (1 µg/ml, 3 hrs). Inflammatory mediators were determined in cell lysates and supernatants by Western blotting and ELISA respectively. In vivo C57BL/6 (WT) and parkin deficient (PARK2 KO) male mice (8–12 wks, n=5–8) were subjected to LPS (intratracheally, 1 mg/kg) or PBS (controls), and allowed to recover prior to harvest 18 hrs later. Leakage of proteins into the alveolar space was assessed by measuring the protein levels in the bronchoalveolar lavage (BAL). To assess lung inflammation, neutrophil cell counts, myeloperoxidase (MPO) activity, and IL-6 levels were determined in BAL.

Results

In human lung EC, down-regulation of parkin by siRNA reduces LPS-induced VCAM-1 expression (adhesion molecule involved in neutrophil adhesion to EC) (by 35%, p<0.05), IL-8 (neutrophil chemoattractant) (by 59%, p<0.01), and IL-6 (inflammatory cytokine) release (by 79%, p<0.01). PARK2 KO mice exhibit less ALI after LPS compared to WT. In PARK2 KO, BAL protein levels were reduced by 27% (p=0.0024) compared to WT mice. LPS-induced neutrophil recruitment into the alveoli of PARK2 KO was attenuated by 47% compared to WT (p=0.0019). BAL MPO activity (marker of neutrophil activation) and BAL IL-6 levels were also significantly lower in PARK2 KO by 52% (p=0.03) and 28% (p=0.0061) respectively.

Conclusion

These results suggest that endothelial parkin mediates EC activation and neutrophil adhesion/migration after LPS, and therefore it may represent a new potential therapeutic target in ALI/ARDS.

ID: 124: ABL FAMILY KINASES MEDIATE LUNG VASCULAR PERMEABILITY AND INFLAMMATION IN ACUTE LUNG INJURY

Rationale

Acute respiratory distress syndrome (ARDS) is a life-threatening disease process in which overwhelming inflammation causes disruption of the pulmonary endothelial cell (EC) barrier, leading to leakage of fluid and inflammatory cells from the blood stream into the airspaces. Current research aims to identify agents that could both decrease inflammation and increase pulmonary vascular barrier integrity. Recently published work suggests that imatinib, an FDA-approved Abl family kinase inhibitor, may attenuate vascular leak and inflammation; however the mechanism underlying these effects is not completely understood. In the present study we explored the effects of LPS on the expression of the Abl family kinases, c-Abl and Arg, as well as the effects of Abl family kinases on LPS-induced vascular permeability and inflammation.

Methods

In silico analyses of the promoter regions of Abl1 (encodes c-Abl) and Abl2 (encodes Arg) were analyzed for potential responsive elements using the online programs Genomatix, TFsearch, and Jaspar. Cultured human pulmonary artery ECs were challenged with LPS (100 ng/mL, 24 hrs), harvested using RNeasy kit and reverse transcribed to cDNA. RT-PCR was performed to assess alterations in the expression of Abl1 and Abl2 after LPS challenge. In separate studies, siRNA was used to selectively silence either c-Abl or Arg and inter-endothelial gap formation was assessed by measuring FITC-dextran binding to a biotinylated avidin substrate. Complementary immunofluorescence studies were carried out to assess effects on adherens junction distributions. Western blotting was used to assess the effects of c-Abl and Arg silencing on NFkB phosphorylation.

Results

In silico analyses revealed that c-Abl contains two antioxidant responsive elements, whereas Arg contains two mechanical stress responsive elements. LPS treatment caused an increase in the mRNA expression of c-Abl (1.5 fold, p<0.05), without effecting Arg expression. Silencing c-Abl, but not Arg, attenuated LPS induced NFkB phosphorylation. However, silencing Arg, but not c-Abl attenuated inter-endothelial gap formation (41%, p<0.05) and adherens junction dissociation (figure 1).

Conclusions

The Abl family kinases c-Abl and Arg play complementary but distinct roles in mediating vascular permeability and inflammation following LPS challenge. The promoter of Abl1 (c-Abl) contains antioxidant response elements and LPS causes an increase in c-Abl expression. Additionally, LPS increases the mRNA expression of c-Abl, but not Arg. C-Abl contributes to LPS-induced NFκB signaling; whereas Arg contributes to inter-endothelial gap formation and adherens junction stability. Inhibition of both of these kinases may be of benefit in patients with ARDS.

Differential and opposing effects of imatinib on LPS- and ventilator-induced lung injury

Endothelial dysfunction underlies the pathophysiology of vascular disorders such as acute lung injury (ALI) syndromes. Recent work has identified the Abl family kinases (c-Abl and Arg) as important regulators of endothelial cell (EC) barrier function and suggests that their inhibition by currently available pharmaceutical agents such as imatinib may be EC protective. Here we describe novel and differential effects of imatinib in regulating lung pathophysiology in two clinically relevant experimental models of ALI. Imatinib attenuates endotoxin (LPS)-induced vascular leak and lung inflammation in mice but exacerbates these features in a mouse model of ventilator-induced lung injury (VILI). We next explored these discrepant observations in vitro through investigation of the roles for Abl kinases in cultured lung EC. Imatinib attenuates LPS-induced lung EC permeability, restores VE-cadherin junctions, and reduces inflammation by suppressing VCAM-1 expression and inflammatory cytokine (IL-8 and IL-6) secretion. Conversely, in EC exposed to pathological 18% cyclic stretch (CS) (in vitro model of VILI), imatinib decreases VE-cadherin expression, disrupts cell-cell junctions, and increases IL-8 levels. Downregulation of c-Abl expression with siRNA attenuates LPS-induced VCAM-1 expression, whereas specific reduction of Arg reduces VE-cadherin expression in 18% CS-challenged ECs to mimic the imatinib effects. In summary, imatinib exhibits pulmonary barrier-protective and anti-inflammatory effects in LPS-injured mice and lung EC; however, imatinib exacerbates VILI as well as dysfunction in 18% CS-EC. These findings identify the Abl family kinases as important modulators of EC function and potential therapeutic targets in lung injury syndromes.