Endothelial LAT1 (SLC7A5) Mediates S-Nitrosothiol Import and Modulates Respiratory Sequelae of Red Blood Cell Transfusion In Vivo.

BACKGROUND: Increased adhesivity of red blood cells (RBCs) to endothelial cells (ECs) may contribute to organ dysfunction in malaria, sickle cell disease, and diabetes. RBCs normally export nitric oxide (NO)-derived vascular signals, facilitating blood flow. S-nitrosothiols (SNOs) are thiol adducts formed in RBCs from precursor NO upon the oxygenation-linked allosteric transition in hemoglobin. RBCs export these vasoregulatory SNOs on demand, thereby regulating regional blood flow and preventing RBC-EC adhesion, and the large (system L) neutral amino acid transporter 1 (LAT1; SLC7A5) appears to mediate SNO export by RBCs. METHODS: To determine the role of LAT1-mediated SNO import by ECs generally and of LAT1-mediated SNO import by ECs in RBC SNO-dependent modulation of RBC sequestration and blood oxygenation in vivo, we engineered LAT1fl/fl; Cdh5-Cre+ mice, in which the putative SNO transporter LAT1 can be inducibly depleted (knocked down, KD) specifically in ECs ("LAT1ECKD"). RESULTS: We show that LAT1 in mouse lung ECs mediates cellular SNO uptake. ECs from LAT1ECKD mice (tamoxifen-induced LAT1fl/fl; Cdh5-Cre+) import SNOs poorly ex vivo compared with ECs from wild-type (tamoxifen-treated LAT1fl/fl; Cdh5-Cre-) mice. In vivo, endothelial depletion of LAT1 increased RBC sequestration in the lung and decreased blood oxygenation after RBC transfusion. CONCLUSION: This is the first study showing a role for SNO transport by LAT1 in ECs in a genetic mouse model. We provide the first direct evidence for the coordination of RBC SNO export with EC SNO import via LAT1. SNO flux via LAT1 modulates RBC-EC sequestration in lungs after transfusion, and its disruption impairs blood oxygenation by the lung.

ERK-dependent proteasome degradation of Txnip regulates thioredoxin oxidoreductase activity.

Dynamic control of thioredoxin (Trx) oxidoreductase activity is essential for balancing the need of cells to rapidly respond to oxidative/nitrosative stress and to temporally regulate thiol-based redox signaling. We have previously shown that cytokine stimulation of the respiratory epithelium induces a precipitous decline in cell S-nitrosothiol, which depends upon enhanced Trx activity and proteasome-mediated degradation of Txnip (thioredoxin-interacting protein). We now show that tumor necrosis factor-α-induced Txnip degradation in A549 respiratory epithelial cells is regulated by the extracellular signal-regulated protein kinase (ERK) mitogen-activated protein kinase pathway and that ERK inhibition augments both intracellular reactive oxygen species and S-nitrosothiol. ERK-dependent Txnip ubiquitination and proteasome degradation depended upon phosphorylation of a PXTP motif threonine (Thr349) located within the C-terminal α-arrestin domain and proximal to a previously characterized E3 ubiquitin ligase-binding site. Collectively, these findings demonstrate the ERK mitogen-activated protein kinase pathway to be integrally involved in regulating Trx oxidoreductase activity and that the regulation of Txnip lifetime via ERK-dependent phosphorylation is an important mediator of this effect.

Thioredoxin-mediated denitrosylation regulates cytokine-induced nuclear factor κB (NF-κB) activation.

S-nitrosylation of nuclear factor κB (NF-κB) on the p65 subunit of the p50/p65 heterodimer inhibits NF-κB DNA binding activity. We have recently shown that p65 is constitutively S-nitrosylated in the lung and that LPS-induced injury elicits a decrease in SNO-p65 levels concomitant with NF-κB activation in the respiratory epithelium and initiation of the inflammatory response. Here, we demonstrate that TNFα-mediated activation of NF-κB in the respiratory epithelium similarly induces p65 denitrosylation. This process is mediated by the denitrosylase thioredoxin (Trx), which becomes activated upon cytokine-induced degradation of thioredoxin-interacting protein (Txnip). Similarly, inhibition of Trx activity in the lung attenuates LPS-induced SNO-p65 denitrosylation, NF-κB activation, and airway inflammation, supporting a pathophysiological role for this mechanism in lung injury. These data thus link stimulus-coupled activation of NF-κB to a specific, protein-targeted denitrosylation mechanism and further highlight the importance of S-nitrosylation in the regulation of the immune response.

NOS2 regulation of LPS-induced airway inflammation via S-nitrosylation of NF-{kappa}B p65.

Inducible nitric oxide synthase (NOS2) expression is increased in the airway epithelium in acute inflammatory disorders although the physiological impact remains unclear. We have previously shown that NOS2 inhibits NF-κB (p50-p65) activation in respiratory epithelial cells by inducing S-nitrosylation of the p65 monomer (SNO-p65). In addition, we have demonstrated that mouse lung SNO-p65 levels are acutely depleted in a lipopolysaccharide (LPS) model of lung injury and that augmenting SNO-p65 levels before LPS treatment results in decreased airway epithelial NF-κB activation, airway inflammation, and lung injury. We now show that aerosolized LPS induces NOS2 expression in the respiratory epithelium concomitant with an increase in lung SNO-p65 levels and a decrease in airway NF-κB activity. Genetic deletion of NOS2 results in an absence of SNO-p65 formation, persistent NF-κB activity in the respiratory epithelium, and prolonged airway inflammation. These results indicate that a primary function of LPS-induced NOS2 expression in the respiratory epithelium is to modulate the inflammatory response through deactivation of NF-κB via S-nitrosylation of p65, thereby counteracting the initial stimulus-coupled denitrosylation.

Protection from lipopolysaccharide-induced lung injury by augmentation of airway S-nitrosothiols.

RATIONALE: S-Nitrosothiols (SNO) inhibit immune activation of the respiratory epithelium and airway SNO levels are decreased in inflammatory lung disease. Ethyl nitrite (ENO) is a gas with chemical properties favoring SNO formation. Augmentation of airway SNO by inhaled ENO treatment may decrease lung inflammation and subsequent injury by inhibiting activation of the airway epithelium. OBJECTIVES: To determine the effect of inhaled ENO on airway SNO levels and LPS-induced lung inflammation/injury. METHODS: Mice were treated overnight with inhaled ENO (10 ppm) or air, followed immediately by exposure to aerosolized LPS or saline. Parameters of inflammation and lung injury were quantified 1 hour after completion of the aerosol exposure and correlated to lung airway and tissue SNO levels. MEASUREMENTS AND MAIN RESULTS: Aerosolized LPS induced a decrease in airway and lung tissue SNO levels including S-nitrosylated NF-kappaB. The decrease in lung SNO was associated with an increase in lung NF-kappaB activity, cytokine/chemokine expression (keratinocyte-derived chemokine, tumor necrosis factor-alpha, and IL-6), airway neutrophil influx, and worsened lung compliance. Pretreatment with inhaled ENO restored airway SNO levels and reduced LPS-mediated NF-kappaB activation thereby inhibiting the downstream inflammatory response and preserving lung compliance. CONCLUSIONS: Airway SNO serves an antiinflammatory role in the lung. Inhaled ENO can be used to augment airway SNO and protect from LPS-induced acute lung injury.

NOS2 regulation of NF-kappaB by S-nitrosylation of p65.

Signal transduction in the NF-kappaB transcription factor pathway is inhibited by inducible nitric oxide synthase (NOS2) activity, although the molecular mechanism(s) are incompletely understood. We have previously shown that nitric oxide (NO), derived from NOS2 consequent upon cytokine stimulation, attenuates NF-kappaB p50-p65 heterodimer DNA binding and have identified the p50 monomer as a locus for inhibitory S-nitrosylation. We now show that the binding partner of p50, NF-kappaB p65, is also targeted by NO following cytokine stimulation of respiratory epithelial cells and macrophages and identify a conserved cysteine within the Rel homology domain that is the site for S-nitrosylation. S-Nitrosylation of p65 inhibits NF-kappaB-dependent gene transcription, and nuclear levels of S-nitrosylated p65 correlate with decreased DNA binding of the p50-p65 heterodimer. NOS2 regulates cytokine-induced S-nitrosylation of p65, resulting in decreased NF-kappaB binding to the NOS2 promoter, thereby inhibiting further NOS2 expression. Collectively, these findings delineate a mechanism by which NOS2 modulates NF-kappaB activity and regulates gene expression in inflammation.