Apical, but not basolateral, endotoxin preincubation protects alveolar epithelial cells against hydrogen peroxide-induced loss of barrier function: the role of nitric oxide synthesis

Relaxing effect of a new ruthenium complex nitric oxide donor on airway smooth muscle of an experimental model of asthma in rats

NO is a potent bronchodilator and NO-donor compounds have demonstrated clinical significance for obstructive airway diseases. This study evaluated the relaxation mechanisms of two NO donors, a ruthenium compound (TERPY), and sodium nitroprusside (SNP), in rat tracheas with ovalbumin-induced asthma (OVA group) and in another control group. The effect of TERPY and SNP was evaluated in tracheal rings in an isolated organ chamber. The contribution of K(+) channels, sGC/cGMP pathway, phosphodiesterases, and extra and intracellular Ca(2+) sources were analyzed. The TERPY and SNP-induced tracheal smooth muscle relaxation in both groups. However, the maximum effect induced by TERPY was higher than that of SNP in both control (110.2 ± 3.2% vs 68.3 ± 3.1%, P < 0.001) and OVA groups (106.1 ± 1.5% vs 49.9 ± 2.7%, P < 0.001). In the control group, TERPY relaxation was induced by the activation of K(+) channels and reduction of the calcium influx, while in the OVA group, these same effects were also brought about by TERPY, but with participation of the sGC/cGMP pathway. In both groups, SNP-induced relaxation occurred through the activation of K(+) channels, sGC/cGMP pathway and reduction of calcium influx. However, the activation of sGC pathway and reticular Ca(2+) -ATPase seemed to be reduced in the OVA group. Furthermore, TERPY is capable of reversing the contraction of carbachol in asthmatic bronchioles. Finally, TERPY and SNP relaxation mechanisms were modified by asthma. SNP presented less relaxation than TERPY, which induced full relaxation with greater participation of K(+) and Ca(2+) fluxes through the membrane, thereby making TERPY a promising drug for reversing the narrowing of airways.

Induction of nitric oxide synthase expression by lipopolysaccharide is mediated by calcium-dependent PKCα-β1 in alveolar epithelial cells

Nitric oxide (NO) plays an important role in innate host defense and inflammation. In response to infection, NO is generated by inducible nitric oxide synthase (iNOS), a gene product whose expression is highly modulated by different stimuli, including lipopolysaccharide (LPS) from gram-negative bacteria. We reported recently that LPS from Pseudomonas aeruginosa altered Na⁺ transport in alveolar epithelial cells via a suramin-dependent process, indicating that LPS activated a purinergic response in these cells. To further study this question, in the present work, we tested whether iNOS mRNA and protein expression were modulated in response to LPS in alveolar epithelial cells. We found that LPS induced a 12-fold increase in iNOS mRNA expression via a transcription-dependent process in these cells. iNOS protein, NO, and nitrotyrosine were also significantly elevated in LPS-treated cells. Ca²⁺ chelation and protein kinase C (PKCα-β1) inhibition suppressed iNOS mRNA induction by LPS, implicating Ca²⁺-dependent PKC signaling in this process. LPS evoked a significant increase of extracellular ATP. Because PKC activation is one of the signaling pathways known to mediate purinergic signaling, we evaluated the hypothesis that iNOS induction was ATP dependent. Although high suramin concentration inhibited iNOS mRNA induction, the process was not ATP dependent, since specific purinergic receptor antagonists could not inhibit the process. Altogether, these findings demonstrate that iNOS expression is highly modulated in alveolar epithelial cells by LPS via a Ca²⁺/PKCα-β1 pathway independent of ATP signaling.

IL-17A and TNF-α exert synergistic effects on expression of CXCL5 by alveolar type II cells in vivo and in vitro

CXCL5, a member of the CXC family of chemokines, contributes to neutrophil recruitment during lung inflammation, but its regulation is poorly understood. Because the T cell-derived cytokine IL-17A enhances host defense by triggering production of chemokines, particularly in combination with TNF-α, we hypothesized that IL-17A would enhance TNF-α-induced expression of CXCL5. Intratracheal coadministration of IL-17A and TNF-α in mice induced production of CXCL1, CXCL2, and CXCL5, which was associated with increased neutrophil influx in the lung at 8 and 24 h. The synergistic effects of TNF-α and IL17A were greatly attenuated in Cxcl5(-/-) mice at 24 h, but not 8 h, after exposure, a time when CXCL5 expression was at its peak in wild-type mice. Bone marrow chimeras produced using Cxcl5(-/-) donors and recipients demonstrated that lung-resident cells were the source of CXCL5. Using differentiated alveolar epithelial type II (ATII) cells derived from human fetal lung, we found that IL-17A enhanced TNF-α-induced CXCL5 transcription and stabilized TNF-α-induced CXCL5 transcripts. Whereas expression of CXCL5 required activation of NF-κB, IL-17A did not increase TNF-α-induced NF-κB activation. Apical costimulation of IL-17A and TNF-α provoked apical secretion of CXCL5 by human ATII cells in a transwell system, whereas basolateral costimulation led to both apical and basolateral secretion of CXCL5. The observation that human ATII cells secrete CXCL5 in a polarized fashion may represent a mechanism to recruit neutrophils in host defense in a fashion that discriminates the site of initial injury.

Nitric oxide and airway epithelial barrier function: regulation of tight junction proteins and epithelial permeability

Acute airway inflammation is associated with enhanced production of nitric oxide (NO(.)) and altered airway epithelial barrier function, suggesting a role of NO(.) or its metabolites in epithelial permeability. While high concentrations of S-nitrosothiols disrupted transepithelial resistance (TER) and increased permeability in 16HBE14o- cells, no significant barrier disruption was observed by NONOates, in spite of altered distribution and expression of some TJ proteins. Barrier disruption of mouse tracheal epithelial (MTE) cell monolayers in response to inflammatory cytokines was independent of NOS2, based on similar effects in MTE cells from NOS2-/- mice and a lack of effect of the NOS2-inhibitor 1400W. Cell pre-incubation with LPS protected MTE cells from TER loss and increased permeability by H2O2, which was independent of NOS2. However, NOS2 was found to contribute to epithelial wound repair and TER recovery after mechanical injury. Overall, our results demonstrate that epithelial NOS2 is not responsible for epithelial barrier dysfunction during inflammation, but may contribute to restoration of epithelial integrity.

The lectin-like domain of tumor necrosis factor-alpha improves alveolar fluid balance in injured isolated rabbit lungs

OBJECTIVE

Identification of mechanisms that preserve optimal alveolar fluid balance during pulmonary edema is of great clinical importance. This study was performed to determine whether the lectin-like domain of tumor necrosis factor-alpha (designated TIP) can improve fluid balance in experimental lung injury by affecting alveolocapillary permeability and/or fluid clearance.

DESIGN

Prospective, randomized laboratory investigation.

SETTING

University-affiliated laboratory.

SUBJECTS

Adult male rabbits.

INTERVENTIONS

TIP, a scrambled peptide (scrTIP), dibutyryl cyclic adenosine monophosphate (db-cAMP), or saline was applied to isolated, ventilated, and buffer-perfused rabbit lungs by ultrasonic nebulization, after which hydrostatic edema or endo/exotoxin-induced lung injury was induced and edema formation was assessed. In studies evaluating the resolution of alveolar edema, 2.5 mL of excess fluid was deposited into the alveolar space of isolated lungs by nebulization in the absence or presence of TIP, scrTIP, amiloride, or ouabain or combinations thereof.

MEASUREMENTS AND MAIN RESULTS

Microvascular permeability was largely increased during hydrostatic edema and endo/exotoxin-induced lung injury in saline-treated lungs, or lungs that received scrTIP, as assessed by capillary filtration coefficient (K(f,c)) and fluorescein isothiocyanate-labeled albumin flux across the alveolocapillary barrier. In contrast, TIP- or db-cAMP-treated lungs exhibited significantly lower vascular permeability upon hydrostatic challenge. Similarly, extravascular fluid accumulation, as assessed by fluid retention, wet weight to dry weight ratio, and epithelial lining fluid volume measurements, was largely inhibited by TIP or db-cAMP pretreatment. Furthermore, TIP increased sodium-potassium adenosine triphosphatase (Na,K-ATPase) activity 1.6-fold by promoting Na,K-ATPase exocytosis to the alveolar epithelial cell surface and increased amiloride-sensitive sodium uptake, resulting in a 2.2-fold increase in active Na+ transport, and hence improved clearance of excess fluid from the alveolar space.

CONCLUSIONS

Aerosolized TIP improved alveolar fluid balance by both reducing vascular permeability and enhancing the absorption of excess alveolar fluid in experimental lung injury. These data may suggest a role for TIP as a potential therapeutic agent in pulmonary edema.

Inflammatory levels of nitric oxide inhibit airway epithelial cell migration by inhibition of the kinase ERK1/2 and activation of hypoxia-inducible factor-1 alpha

Increased synthesis of NO during airway inflammation, caused by induction of nitric-oxide synthase 2 in several lung cell types, may contribute to epithelial injury and permeability. To investigate the consequence of elevated NO production on epithelial function, we exposed cultured monolayers of human bronchial epithelial cells to the NO donor diethylenetriaamine NONOate. At concentrations generating high nanomolar levels of NO, representative of inflammatory conditions, diethylenetriaamine NONOate markedly reduced wound closure in an in vitro scratch injury model, primarily by inhibiting epithelial cell migration. Analysis of signaling pathways and gene expression profiles indicated a rapid induction of the mitogen-activated protein kinase phosphatase (MPK)-1 and decrease in extracellular signal-regulated kinase (ERK)1/2 activation, as well as marked stabilization of hypoxia-inducible factor (HIF)-1alpha and activation of hypoxia-responsive genes, under these conditions. Inhibition of ERK1/2 signaling using U0126 enhanced HIF-1alpha stabilization, implicating ERK1/2 dephosphorylation as a contributing mechanism in NO-mediated HIF-1alpha activation. Activation of HIF-1alpha by the hypoxia mimic cobalt chloride, or cell transfection with a degradation-resistant HIF-1alpha mutant construct inhibited epithelial wound repair, implicating HIF-1alpha in NO-mediated inhibition of cell migration. Conversely, NO-mediated inhibition of epithelial wound closure was largely prevented after small interfering RNA suppression of HIF-1alpha. Finally, NO-mediated inhibition of cell migration was associated with HIF-1alpha-dependent induction of PAI-1 and activation of p53, both negative regulators of epithelial cell migration. Collectively, our results demonstrate that inflammatory levels of NO inhibit epithelial cell migration, because of suppression of ERK1/2 signaling, and activation of HIF-1alpha and p53, with potential consequences for epithelial repair and remodeling during airway inflammation.

Nitric oxide attenuates hydrogen peroxide-induced barrier disruption and protein tyrosine phosphorylation in monolayers of intestinal epithelial cell

The intestinal epithelium provides a barrier to the transport of harmful luminal molecules into the systemic circulation. A dysfunctional epithelial barrier is closely associated with the pathogenesis of a variety of intestinal and systemic disorders. We investigated here the effects of nitric oxide (NO) and hydrogen peroxide (H(2)O(2)) on the barrier function of a human intestinal epithelial cell line, Caco-2. When treated with H(2)O(2), Caco-2 cell monolayers grown on permeable supports exhibited several remarkable features of barrier dysfunction as follows: a decrease in transepithelial electrical resistance, an increase in paracellular permeability to dextran, and a disruption of the intercellular junctional localization of the scaffolding protein ZO-1. In addition, an induction of tyrosine phosphorylation of numerous cellular proteins including ZO-1, E-cadherin, and beta-catenin, components of tight and adherens junctions, was observed. On the other hand, combined treatment of Caco-2 monolayers with H(2)O(2) and an NO donor (NOC5 or NOC12) relieved the damage to the barrier function and suppressed the protein tyrosine phosphorylation induced by H(2)O(2) alone. These results suggest that NO protects the barrier function of intestinal epithelia from oxidative stress by modulating some intracellular signaling pathways of protein tyrosine phosphorylation in epithelial cells.

Soluble guanylyl cyclase expression is reduced in LPS-induced lung injury

Soluble guanylyl cyclase (sGC) is a cGMP-generating enzyme implicated in the control of smooth muscle tone that also regulates platelet aggregation. Moreover, sGC activation has been shown to reduce leukocyte adherence to the endothelium. Herein, we investigated the expression of sGC in a murine model of LPS-induced lung injury and evaluated the effects of sGC inhibition in the context of acute lung injury (ALI). Lung tissue sGC alpha1 and beta1 subunit protein levels were determined by Western blot and immunohistochemistry, and steady-state mRNA levels for the beta1 subunit were assessed by real-time PCR. LPS inhalation resulted in a decrease in beta1 mRNA levels, as well as a reduction in both sGC subunit protein levels. Decreased alpha1 and beta1 expression was observed in bronchial smooth muscle and epithelial cells. TNF-alpha was required for the LPS-triggered reduction in sGC protein levels, as no change in alpha1 and beta1 levels was observed in TNF-alpha knockout mice. To determine the effects of sGC blockade in LPS-induced lung injury, mice were exposed to 1H-[1,2,4]oxodiazolo[4,3-a]quinoxalin-l-one (ODQ) prior to the LPS challenge. Such pretreatment led to a further increase in total cell number (mainly due to an increase in neutrophils) and protein concentration in the bronchoalveoalar lavage fluid; the effects of ODQ were reversed by a cell-permeable cGMP analog. We conclude that sGC expression is reduced in LPS-induced lung injury, while inhibition of the enzyme with ODQ worsens lung inflammation, suggesting that sGC exerts a protective role in ALI.

Nitric oxide promotes airway epithelial wound repair through enhanced activation of MMP-9

The airway epithelium provides a protective barrier against inhaled environmental toxins and microorganisms, and epithelial injury initiates a number of processes to restore its barrier integrity, including activation of matrix metalloproteinases such as MMP-9 (92-kD gelatinase B). Airway epithelial cells continuously produce nitric oxide (NO), which has been linked to cell migration and MMP-9 regulation in several cell types, but the importance of epithelial NO in mediating airway epithelial repair or MMP-9 activation is unknown. Using primary or immortalized human bronchial epithelial cells, we demonstrate that low concentrations of NO promote epithelial cell migration and wound repair in an in vitro wound assay, which was associated with increased localized expression and activation of MMP-9. In addition, in HBE1 cells that were stably transfected with inducible NOS (NOS2), to mimic constitutive epithelial NOS2 expression in vivo, NOS inhibition decreased epithelial wound repair and MMP-9 expression. The stimulatory effects of NO on epithelial wound repair and MMP-9 expression were dependent on cGMP-mediated pathways and were inhibited by 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of soluble guanylyl cyclase. Inhibition of cGMP-dependent protein kinase (PKG) attenuated NO-mediated epithelial wound closure, but did not affect MMP-9 expression. However, pharmacologic MMP inhibition and siRNA knockdown of MMP-9 expression demonstrated the contribution of MMP-9 to NO-mediated wound closure. Overall, our results demonstrate that NOS2-derived NO contributes to airway epithelial repair by both PKG-dependent and -independent mechanisms, and involves NO-dependent expression and activation of MMP-9.

Magnesium Deficiency Enhances Hydrogen Peroxide Production and Oxidative Damage in Chick Embryo Hepatocyte In Vitro

Magnesium deficiency and oxidative stress have been identified as correlative factors in many diseases. The origin of free radicals correlated with oxidative damage resulting from Mg-deficiency is unclear at the cellular level. To investigate whether hydrogen peroxide (H2O2) is associated in the oxidative stress induced by Mg-deficiency, the effect of Mg2+ deficiency (0, 0.4, 0.7 mM) on the metabolism of H2O2 was investigated in cultured chick embryo hepatocytes. After being cultured in the media with various concentrations of Mg2+ for 1, 2, 4, 6 and 10 days, parameters of H2O2 production, catalase activity, lipid peroxidation, intracellular total Mg and cell viability were analyzed. Results demonstrated that long-term incubation of chick embryo hepatocyte in extracellular Mg2+-deprivative and Mg2+-deficient (0.4 mM) states significantly enhanced the production of H2O2 (approximately twofold, respectively) and lipid peroxidation in the cell cultures, while decreasing the cell viability. Additionally, the reversing action of Mg2+ re-added to 1.0 mM and the partial reversing action of dimethylthiourea suggested that (i) [Mg2+]e deficiency induced the increase of H2O2 production, (ii) [Mg2+]e deficiency decreased catalase activity in chick embryo hepatocyte in vitro, subsequently causing oxidative stress and cell peroxidative damage.

Role of hypoxia-inducible factor-1alpha in hypoxia-induced apoptosis of primary alveolar epithelial type II cells

Hypoxia affects alveolar homeostasis and may induce epithelial injury, which has been implicated in lung diseases such as fibrosis. The underlying cellular and molecular mechanisms are, however, largely unknown. Primary rat alveolar epithelial type II cells (ATII) exposed to graded hypoxia for 24 and 48 h caused a dose-dependent induction of cell cycle arrest and suppression of proliferation, which were comparable to the effects of angiotensin II, a potent inducer of ATII cell death. Hypoxia-induced changes in ATII homeostasis are thought to proceed primarily via activation of hypoxia inducible-factor (HIF)-1alpha, because hypoxia increased HIF-1alpha protein expression, nuclear translocation, and transactivation of its specific DNA binding domain, the hypoxia responsive element (HRE). Under hypoxic conditions, expression of the proapoptotic protein Bnip3L, which belongs to the Bcl 2 family and is known to be one of the HIF-1-dependent target genes, was upregulated. Suppression of HIF-1alpha or Bnip-3L with small interfering RNA (siRNA) fully blocked the hypoxia-induced apoptosis and Bnip3L expression. In line with these data, overexpression of HIF-1alpha by transient transfection enhanced the hypoxia-induced apoptosis. Thus, we conclude that hypoxia suppresses alveolar epithelial cell proliferation and enhances ATII apoptosis through activation of the HIF-1alpha/HRE axis and a mechanism that involves Bnip3L. Targeting HIF-1alpha may represent a new strategy that could impede the alveolar denudation that is observed in several lung diseases.

Respiratory epithelial cells regulate lung inflammation in response to inhaled endotoxin

To determine the role of respiratory epithelial cells in the inflammatory response to inhaled endotoxin, we selectively inhibited NF-kappa B activation in the respiratory epithelium using a mutant I kappa B-alpha construct that functioned as a dominant negative inhibitor of NF-kappa B translocation (dnI kappa B-alpha). We developed two lines of transgenic mice in which expression of dnI kappa B-alpha was targeted to the distal airway epithelium using the human surfactant apoprotein C promoter. Transgene expression was localized to the epithelium of the terminal bronchioles and alveoli. After inhalation of LPS, nuclear translocation of NF-kappa B was evident in bronchiolar epithelium of nontransgenic but not of transgenic mice. This defect was associated with impaired neutrophilic lung inflammation 4 h after LPS challenge and diminished levels of TNF-alpha, IL-1 beta, macrophage inflammatory protein-2, and KC in lung homogenates. Expression of TNF-alpha within bronchiolar epithelial cells and of VCAM-1 within peribronchiolar endothelial cells was reduced in transgenic animals. Thus targeted inhibition of NF-kappa B activation in distal airway epithelial cells impaired the inflammatory response to inhaled LPS. These data provide causal evidence that distal airway epithelial cells and the signals they transduce play a physiological role in lung inflammation in vivo.