Mechanisms of TNF-alpha stimulation of amiloride-sensitive sodium transport across alveolar epithelium

Acute lung injury edema fluid decreases net fluid transport across human alveolar epithelial type II cells

Most patients with acute lung injury (ALI) have reduced alveolar fluid clearance that has been associated with higher mortality. Several mechanisms may contribute to the decrease in alveolar fluid clearance. In this study, we tested the hypothesis that pulmonary edema fluid from patients with ALI might reduce the expression of ion transport genes responsible for vectorial fluid transport in primary cultures of human alveolar epithelial type II cells. Following exposure to ALI pulmonary edema fluid, the gene copy number for the major sodium and chloride transport genes decreased. By Western blot analyses, protein levels of alphaENaC, alpha1Na,K-ATPase, and cystic fibrosis transmembrane conductance regulator decreased as well. In contrast, the gene copy number for several inflammatory cytokines increased markedly. Functional studies demonstrated that net vectorial fluid transport was reduced for human alveolar type II cells exposed to ALI pulmonary edema fluid compared with plasma (0.02 +/- 0.05 versus 1.31 +/- 0.56 microl/cm2/h, p < 0.02). An inhibitor of p38 MAPK phosphorylation (SB202190) partially reversed the effects of the edema fluid on net fluid transport as well as gene and protein expression of the main ion transporters. In summary, alveolar edema fluid from patients with ALI induced a significant reduction in sodium and chloride transport genes and proteins in human alveolar epithelial type II cells, effects that were associated with a decrease in net vectorial fluid transport across human alveolar type II cell monolayers.

Alveolar fluid clearance in acute lung injury: what have we learned from animal models and clinical studies?

Background

Acute lung injury and the acute respiratory distress syndrome continue to be significant causes of morbidity and mortality in the intensive care setting. The failure of patients to resolve the alveolar edema associated with these conditions is a major contributing factor to mortality; hence there is continued interest to understand the mechanisms of alveolar edema fluid clearance.

Discussion

The accompanying review by Vadász et al. details our current understanding of the signaling mechanisms and cellular processes that facilitate clearance of edema fluid from the alveolar compartment, and how these signaling processes may be exploited in the development of novel therapeutic strategies. To complement that report this review focuses on how intact organ and animal models and clinical studies have facilitated our understanding of alveolar edema fluid clearance in acute lung injury and acute respiratory distress syndrome. Furthermore, it considers how what we have learned from these animal and organ models and clinical studies has suggested novel therapeutic avenues to pursue.

Differential roles of p55 and p75 tumor necrosis factor receptors on stretch-induced pulmonary edema in mice

Ventilator-induced lung injury plays a crucial role in the outcome of patients with acute lung injury. Previous studies have shown a role for the cytokine tumor necrosis factor-alpha (TNF) in stretch-induced alveolar neutrophil recruitment, but the involvement of TNF in stretch-induced pulmonary edema is unclear. We investigated the effects of TNF through its individual p55 and p75 receptors on early pulmonary edema formation during high stretch ventilation, before neutrophil infiltration. Anesthetized wild-type or TNF receptor single/double knockout mice were ventilated with high tidal volume ( approximately 38 ml/kg) for 2 h or until they developed arterial hypotension. Pulmonary edema was assessed by physiological parameters including respiratory mechanics and blood gases, and by lavage fluid protein, lung wet:dry weight ratio, and lung permeability measurements using fluorescence-labeled albumin. High stretch ventilation in wild-type and TNF receptor double knockout animals induced similar pulmonary edema, and only 25-30% of mice completed the protocol. In contrast, the p55 receptor knockout mice were strongly protected from edema formation, with all animals completing the protocol. Myeloperoxidase assay indicated that this protective effect was not associated with decreased pulmonary neutrophil sequestration. The p75 receptor knockout mice, however, displayed increased susceptibility to edema formation, and no animals survived the full 2 h. These results demonstrate a novel role for TNF signaling (independent from its effects on neutrophil recruitment) specifically through the p55 receptor, in promoting high stretch-induced pulmonary edema, whereas p75 signaling may play an opposing role.

Acute denervation alters the epithelial response to adrenoceptor activation through an increase in alpha1-adrenoceptor expression on villus enterocytes

Loss of sympathetic input due to intestinal denervation results in hypersensitivity and increased intestinal secretion. It is unknown whether denervation-induced alterations in intestinal epithelial physiology are the result of changes in adrenoceptors on enterocytes (ENTs). The purpose of this study was to examine adrenoceptor distribution and pharmacology on small intestinal ENTs following acute intestinal denervation. Lewis rats underwent small bowel transplantation (SBT) or sham operation and proximal small intestinal segments were harvested 1, 2 and 4 weeks postoperatively. Intestinal electrolyte movement was assessed using short-circuit current (Isc) measurements of stripped epithelial sheets following stimulation with phenylephrine (PE), an alpha(1)-adrenoceptor agonist. The presence of adrenoceptor subtypes on separated villus and crypt ENTs was assessed using flow cytometry. Alpha(1)-adrenoceptors were found on approximately 27% of jejunal villus ENTs, but not crypt ENTs, following acute extrinsic denervation. ENTs from the Lewis rat have few beta-adrenoceptors. Alpha(1)-adrenoceptor stimulation of acutely denervated intestinal epithelial sheets decreased Isc by -13.45%. This effect was mediated by a reduction in chloride (Cl(-)) secretion; the absence of Cl(-) reversed the Isc to +13.79%. In conclusion, loss of sympathetic innervation to the gastrointestinal epithelium causes acute upregulation of alpha(1)-adrenoceptors on villus ENTs, leading to inhibition of Cl(-) secretion at the villus tip. The increase in adrenoceptors may reflect a compensatory mechanism to combat the increased secretory state of the bowel due to the loss of the sympathetic innervation and tonic control over intestinal secretion.

Dexamethasone inhibits the action of TNF on ENaC expression and activity

We have reported that TNF, a proinflammatory cytokine present in several lung pathologies, decreases the expression and activity of the epithelial Na(+) channel (ENaC) by approximately 70% in alveolar epithelial cells. Because dexamethasone has been shown to upregulate ENaC mRNA expression and is well known to downregulate proinflammatory genes, we tested if it could alleviate the effect of TNF on ENaC expression and activity. In cotreatment with TNF, we found that dexamethasone reversed the inhibitory effect of TNF and upregulated alpha, beta, and gammaENaC mRNA expression. When the cells were pretreated for 24 h with TNF before cotreatment, dexamethasone was still able to increase alphaENaC mRNA expression to 1.8-fold above control values. However, in these conditions, beta and gammaENaC mRNA expression was reduced to 47% and 14%, respectively. The potential role of TNF and dexamethasone on alphaENaC promoter activity was tested in A549 alveolar epithelial cells. TNF decreased luciferase (Luc) expression by approximately 25% in these cells, indicating that the strong diminution of alphaENaC mRNA must be related to posttranscriptional events. Dexamethasone raised Luc expression by fivefold in the cells and augmented promoter activity by 2.77-fold in cotreatment with TNF. In addition to its effect on alphaENaC gene expression, dexamethasone was able to maintain amiloride-sensitive current as well as the liquid clearance abilities of TNF-treated cells within the normal range. All these results suggest that dexamethasone alleviates the downregulation of ENaC expression and activity in TNF-treated alveolar epithelial cells.

Leflunomide prevents alveolar fluid clearance inhibition by respiratory syncytial virus

RATIONALE

Previously, we demonstrated that intranasal infection of BALB/c mice with respiratory syncytial virus (RSV) resulted in an early 40% reduction in alveolar fluid clearance (AFC), an effect mediated via P2Y purinergic receptors.

OBJECTIVES

To confirm that RSV-induced inhibition of AFC is mediated by uridine triphosphate (UTP), and to demonstrate that inhibition of de novo pyrimidine synthesis with leflunomide prevents increased UTP release after RSV infection, and thereby also prevents inhibition of AFC by RSV.

METHODS

BALB/c mice were infected intranasally with RSV strain A2. AFC was measured in anesthetized, ventilated mice by instillation of 5% bovine serum albumin into the dependent lung. Some mice were pretreated with leflunomide or 6-mercaptopurine.

MEASUREMENTS AND MAIN RESULTS

RSV-mediated inhibition of AFC is associated temporally with a 20-nM increase in UTP and ATP content of bronchoalveolar lavage fluid, hypoxemia, and altered nasal potential difference. RSV-mediated nucleotide release, AFC inhibition, and physiologic sequelae thereof can be prevented by pretreatment of mice with the de novo pyrimidine synthesis inhibitor leflunomide, which is not toxic to the mice, and which does not affect RSV replication in the lungs. In contrast, pretreatment of mice with 6-mercaptopurine, an inhibitor of de novo purine synthesis, has no beneficial effect on AFC or other indicators of disease progression. Finally, RSV-mediated inhibition of AFC is prevented by volume-regulated anion channel inhibitors.

CONCLUSION

Pyrimidine synthesis or release pathways may provide novel therapeutic targets to counter the pathophysiologic sequelae of impaired AFC in RSV disease.

Mechanisms of pulmonary edema clearance

The mechanisms of pulmonary edema resolution are different from those regulating edema formation. Absorption of excess alveolar fluid is an active process that involves vectorial transport of Na+out of alveolar air spaces with water following the Na+osmotic gradient. Active Na+transport across the alveolar epithelium is regulated via apical Na+and chloride channels and basolateral Na-K-ATPase in normal and injured lungs. During lung injury, mechanisms regulating alveolar fluid reabsorption are inhibited by yet unclear pathways and can be upregulated by pharmacological means. Better understanding of the mechanisms that regulate edema clearance may lead to therapeutic interventions to improve the ability of lungs to clear fluid, which is of clinical significance.

Bench-to-bedside review: the role of the alveolar epithelium in the resolution of pulmonary edema in acute lung injury

Clearance of pulmonary edema fluid is accomplished by active ion transport, predominantly by the alveolar epithelium. Various ion pumps and channels on the surface of the alveolar epithelial cell generate an osmotic gradient across the epithelium, which in turn drives the movement of water out of the airspaces. Here, the mechanisms of alveolar ion and fluid clearance are reviewed. In addition, many factors that regulate the rate of edema clearance, such as catecholamines, steroids, cytokines, and growth factors, are discussed. Finally, we address the changes to the alveolar epithelium and its transport processes during acute lung injury (ALI). Since relevant clinical outcomes correlate with rates of edema clearance in ALI, therapies based on our understanding of the mechanisms and regulation of fluid transport may be developed.

First Encounter: How Pathogens Compromise Epithelial Transport

Pathogenic organisms trigger numerous signaling pathways that ultimately lead to drastic changes in physiological functions. Apart from altering structure and function of the epithelial tight junction barrier and activating inflammatory cascades, they induce changes in fluid and electrolyte transport. Pathogens do so by activating or by inhibiting ion channels and transporters, and the result might be to their benefit or to their disadvantage.

IκB Kinase-β (IKKβ) Modulation of Epithelial Sodium Channel Activity

Using the yeast two-hybrid system, we identified a number of proteins that interacted with the carboxyl termini of murine epithelial sodium channel (ENaC) subunits. Initial screens indicated an interaction between the carboxyl terminus of beta-ENaC and IkappaB kinase-beta (IKKbeta), the kinase that phosphorylates Ikappabeta and results in nuclear targeting of NF-kappaB. A true two-hybrid reaction employing full-length IKKbeta and the carboxyl termini of all three subunits confirmed a strong interaction with beta-ENaC, a weak interaction with gamma-ENaC, and no interaction with alpha-ENaC. Co-immunoprecipitation studies for IKKbeta were performed in a murine cortical collecting duct cell line that endogenously expresses ENaC. Immunoprecipitation with beta-ENaC, but not gamma-ENaC, resulted in co-immunoprecipitation of IKKbeta. To examine the direct effects of IKKbeta on ENaC activity, co-expression studies were performed using the two-electrode voltage clamp technique in Xenopus oocytes. Oocytes were injected with cRNAs for alphabetagamma-ENaC with or without cRNA for IKKbeta. Co-injection of IKKbeta significantly increased the amiloride-sensitive current above controls. Using cell surface ENaC labeling, we determined that an increase of ENaC in the plasma membrane accounted for the increase in current. The injection of kinase-dead IKKbeta (K44A) in ENaC-expressing oocytes resulted in a significant decrease in current. Treatment of mpkCCD(c14) cells with aldosterone increased whole cell amounts of IKKbeta. Because this result suggested that aldosterone might activate NF-kappaB, mpkCCD(c14) cells were transiently transfected with a luciferase reporter gene responsive to NF-kappaB activation. Both aldosterone and tumor necrosis factor-alpha (TNFalpha) stimulation caused a similar and significant increase in luciferase activity as compared with controls. We conclude that IKKbeta interacts with ENaC by up-regulating ENaC at the plasma membrane and that the presence of IKKbeta is at very least permissive to ENaC function. These studies also suggest a previously unexpected interaction between the NF-kappaB transcription pathway and steroid regulatory pathways in epithelial cells.

Mechanisms of beta-receptor stimulation-induced improvement of acute lung injury and pulmonary edema

Acute lung injury (ALI) and the acute respiratory distress syndrome are complex syndromes because both inflammatory and coagulation cascades cause lung injury. Transport of salt and water, repair and remodeling of the lung, apoptosis, and necrosis are additional important mechanisms of injury. Alveolar edema is cleared by active transport of salt and water from the alveoli into the lung interstitium by complex cellular mechanisms. Beta-2 agonists act on the cellular mechanisms of pulmonary edema clearance as well as other pathways relevant to repair in ALI. Numerous studies suggest that the beneficial effects of beta-2 agonists in ALI include at least enhanced fluid clearance from the alveolar space, anti-inflammatory actions, and bronchodilation. The purposes of the present review are to consider the effects of beta agonists on three mechanisms of improvement of lung injury: edema clearance, anti-inflammatory effects, and bronchodilation. This update reviews specifically the evidence on the effects of beta-2 agonists in human ALI and in models of ALI. The available evidence suggests that beta-2 agonists may be efficacious therapy in ALI. Further randomized controlled trials of beta agonists in pulmonary edema and in acute lung injury are necessary.

IL-1beta and TNFalpha regulate sodium absorption in rat distal colon

The epithelial Na+ channel (ENaC) provides the main absorptive pathway of the distal large intestine. This study aimed to characterize regulatory influences of cytokines in rat late distal colon. After 6 h incubation with either IL1beta, TNFalpha, IFNgamma, or combinations of TNFalpha and IFNgamma, ENaC was measured as electrogenic Na+ transport after 8 h induction by 3 nM aldosterone (JNa) in totally stripped specimens in the Ussing chamber. Subsequently, alpha-, beta-, and gamma-ENaC subunit mRNAs were analyzed by Northern blotting. The gamma-ENaC promoter was cloned and characterized by reporter gene assays. IL-1beta and TNFalpha, but not interferon-gamma, decreased JNa. In parallel, beta- and gamma-ENaC transcription was inhibited, whereas alpha-ENaC was unaffected. gamma-ENaC promoter activity was inhibited by IL-1beta and TNFalpha but not by IFNgamma. We conclude that the pro-inflammatory cytokines IL-1beta and TNFalpha inhibit electrogenic sodium absorption in rat distal colon by mRNA expression regulation of the beta- and gamma-ENaC subunits.

Mouse prostasin gene structure, promoter analysis, and restricted expression in lung and kidney

Human prostasin is a membrane-anchored serine peptidase hypothesized to regulate lung epithelial sodium transport. It belongs to a unique family of genes on chromosome 16p11.2/13.3. Here we describe genomic cloning, promoter analysis, and expression of prostasin's mouse ortholog. The 4.3-kb mouse prostasin gene (prss8) has a six-exon organization identical to human prostasin. Prss8 spans two signal tagged-sites localized to chromosome 7. Multiple mRNA transcripts arise from two consensus initiator elements of a TATA-less promoter and an alternatively spliced, 5' untranslated region intron. Reporter assay establishes that the initiator elements and a GC-rich domain comprise the core promoter and identifies 5' flanking regions with strong enhancer and repressor activity. The 3' untranslated region overlaps the 3' untranslated region of the Myst1 gene oriented tail-to-tail at this locus. Prss8 is highly transcribed in pancreas, kidney, submaxillary gland, lung, thyroid, prostate, and epididymis, and is developmentally regulated. Using selective riboprobes and antibodies to mouse prostasin, we localized its expression to lung airway epithelial and alveolar type II cells and kidney cortical tubule epithelium. Mouse prostasin highly resembles its human ortholog in gene organization and tissue specificity, including strong expression in pulmonary epithelium, suggesting that mice will be useful for probing prostasin's functions in vivo.

Leukotriene D4Activates Alveolar Epithelial Na,K-ATPase and Increases Alveolar Fluid Clearance

Cysteinyl leukotrienes are increased during acute lung injury in animals and humans. In this study, we determined the effect of leukotriene D4 (LTD4) on the function of Na,K-ATPase in alveolar epithelial cells and on alveolar fluid clearance in rat lungs. LTD4 (1 x 10(-7) M) increased Na,K-ATPase activity at 1 and 5 minutes by 14% (p < 0.05) and 31% (p < 0.001), respectively, in A549 alveolar epithelial cells. This was accompanied by recruitment of Na,K-ATPase alpha1 subunits from intracellular compartment(s) to the basolateral plasma membrane. LTD4-induced alpha1 Na,K-ATPase membrane translocation was blocked by the dual cysteinyl LT1 (cysLT1)/ cysteinyl LT3 (cysLT3) receptor antagonist BAY-u9773, but not by the cysLT1 antagonist MK571, implicating the cysLT3 receptor. Expression of mRNA for cysLT2, but not cysLT1, was confirmed in A549 cells and rat alveolar type 2 cells by reverse transcriptase-polymerase chain reaction. Finally, compared with control, LTD4 (1 x 10(-11) M) increased alveolar fluid clearance by 41% (p < 0.001) in isolated, perfused rat lungs; this was also blocked by BAY-u9773 but not MK571. By activating alveolar epithelial Na,K-ATPase and increasing alveolar fluid reabsorption, cysteinyl leukotrienes may, in part, have a beneficial role in the acute respiratory distress syndrome.

Hyperosmolarity enhances the lung capillary barrier

Although capillary barrier deterioration underlies major inflammatory lung pathology, barrier-enhancing strategies are not available. To consider hyperosmolar therapy as a possible strategy, we gave 15-minute infusions of hyperosmolar sucrose in lung venular capillaries imaged in real time. Surprisingly, this treatment enhanced the capillary barrier, as indicated by quantification of the capillary hydraulic conductivity. The barrier enhancement was sufficient to block the injurious effects of thrombin, TNF-alpha, and H2O2 in single capillaries, and of intratracheal acid instillation in the whole lung. Capillary immunofluorescence indicated that the hyperosmolar infusion markedly augmented actin filament formation and E-cadherin expression at the endothelial cell periphery. The actin-depolymerizing agent latrunculin B abrogated the hyperosmolar barrier enhancement as well as the actin filament formation, suggesting a role for actin in the barrier response. Furthermore, hyperosmolar infusion blocked TNF-alpha-induced P-selectin expression in an actin-dependent manner. Our results provide the first evidence to our knowledge that in lung capillaries, hyperosmolarity remodels the endothelial barrier and the actin cytoskeleton to enhance barrier properties and block proinflammatory secretory processes. Hyperosmolar therapy may be beneficial in lung inflammatory disease.

Up-regulation of activated macrophages in response to degeneration in the taste system: Effects of dietary sodium restriction

Dietary sodium restriction combined with unilateral chorda tympani nerve section leads to a rapid and specific decrease in neurophysiological taste responses to sodium in the contralateral, intact chorda tympani (Hill and Phillips [1994] J. Neurosci. 14:2904-2910). Previous work demonstrated that dietary sodium restriction may induce these early functional deficits by inhibiting immune activity after denervation (Phillips and Hill [1996] Am. J. Physiol. 271:R857-R862). However, little is known about the leukocyte response to denervation of taste buds in fungiform papillae. In the current study, it was hypothesized that T cells and macrophages are increased in the tongue after unilateral denervation in control-fed but not sodium-restricted animals. Adult, specified pathogen-free rats received unilateral chorda tympani nerve section or sham section followed by dietary sodium restriction or maintenance on control diet. At day 1, 2, 5, 7, or 50 postsectioning, immunostaining was used to detect the percentage of staining for activated macrophages, the number of alpha beta T cells, and the number of delta gamma epithelial T cells in the tongue. The number of lingual T cells did not significantly differ between treatment groups following denervation. However, there was a dramatic bilateral increase in ED1(+) staining for activated macrophages in control-fed rats that peaked at day 2 postsectioning. In contrast, sodium-restricted rats did not show an increase in activated macrophages above baseline at any time postsectioning. Further analysis of extralingual macrophages indicated that the deficit in immune activity in sodium-restricted rats is localized to the tongue and is not widespread. A model for immune modulation of taste receptor cell function is proposed based on these novel findings.

Functional identification of the alveolar edema reabsorption activity of murine tumor necrosis factor-alpha

Tumor necrosis factor-alpha (TNF-alpha) activates sodium channels in Type II alveolar epithelial cells, an important mechanism for the reported fluid resorption capacity of the cytokine. Both TNF-alpha receptor-dependent and -independent effects were proposed for this activity in vitro, the latter mechanism mediated by the lectin-like domain of the molecule. In this study, the relative contribution of the receptor-dependent versus receptor-independent activities was investigated in an in situ mouse lung model and an ex vivo rat lung model. Fluid resorption due to murine TNF-alpha (mTNF-alpha) was functional in mice that were genetically deficient in both types of mTNF-alpha receptor, establishing the importance of mTNF-alpha receptor-independent effects in this species. In addition, we assessed the capacity of an mTNF-alpha-derived peptide (mLtip), which activates sodium transport by a receptor-independent mechanism, to reduce lung water content in an isolated, ventilated, autologous blood-perfused rat lung model. The results show that in this model, mLtip, in contrast to mTNF-alpha, produced a progressive recovery of dynamic lung compliance and airway resistance after alveolar flooding. There was also a significant reduction in lung water. These results indicate that the receptor-independent lectin-like domain of mTNF-alpha has a potential physiological role in the resolution of alveolar edema in rats and mice.

Downregulation of ENaC activity and expression by TNF-alpha in alveolar epithelial cells

Sodium absorption by an amiloride-sensitive channel is the main driving force of lung liquid clearance at birth and lung edema clearance in adulthood. In this study, we tested whether tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine involved in several lung pathologies, could modulate sodium absorption in cultured alveolar epithelial cells. We found that TNF-alpha decreased the expression of the alpha-, beta-, and gamma-subunits of epithelial sodium channel (ENaC) mRNA to 36, 43, and 16% of the controls after 24-h treatment and reduced to 50% the amount of alpha-ENaC protein in these cells. There was no impact, however, on alpha(1) and beta(1) Na(+)-K(+)-ATPase mRNA expression. Amiloride-sensitive current and ouabain-sensitive Rb(+) uptake were reduced, respectively, to 28 and 39% of the controls. A strong correlation was found at different TNF-alpha concentrations between the decrease of amiloride-sensitive current and alpha-ENaC mRNA expression. All these data show that TNF-alpha, a proinflammatory cytokine present during lung infection, has a profound influence on the capacity of alveolar epithelial cells to transport sodium.

Measurement of nasal potential difference in mild asthmatics

STUDY OBJECTIVE

To determine whether ion transport or barrier function across the nasal epithelium are altered in asthmatics.

DESIGN

In this pilot study, the nasal potential difference (PD) was measured using the technique established by Knowles and colleagues. A flowing agar bridge made electrical contact with the surface of the nasal epithelium along the floor of the nose. This bridge was referenced to a cutaneous electrode to determine the PD across the nasal epithelium. Changes in nasal PD in response to amiloride, chloride-free medium, and chloride-free medium containing isoproterenol were measured, and responses of asthmatics and healthy control subjects were compared.

PARTICIPANTS

Measurements were made in eight adult nonasthmatic subjects and 6 adult asthmatic subjects. All asthmatics had mild intermittent asthma.

MEASUREMENTS AND RESULTS

Continuous measurements of nasal PD were obtained while the nasal surface was perfused consecutively with saline solution (NaCl-containing solution), saline solution plus 100 micro mol/L amiloride, chloride-free solution plus amiloride, and chloride-free solution with amiloride plus 10 micro mol/L isoproterenol (a beta-adrenergic agonist). No significant differences in baseline PD or change in PD in response to changes in perfusate were found between the two groups.

CONCLUSIONS

Our results suggest that ion transport and barrier function of patients with mild asthma are normal. Therefore, by contrast to cystic fibrosis, changes in salt and water transport across airway epithelium may not contribute to accumulation of mucous secretions in asthma.

Functional convergence of invertebrate and vertebrate cytokine-like molecules based on a similar lectin-like activity

It is generally accepted that the action of cytokines results from their binding to specific receptors. However, many cytokines possess lectin-like activity that may be essential for the expression of their full biological activities. This review focuses on the physiological relevance of the lectin-like activity of cytokines during the innate immune response in mammals, using TNF as an illustrative example. Moreover, we will show that TNF displays functional analogies with a defense molecule from the earthworm Eisenia foetida termed CCF. These analogies are not reflected by primary sequence homology between CCF and TNF but are particularly based on a similar lectin-like activity/domain. Hence, from a phylogenetic point of view, the lectin-like activity/domain of CCF and TNF may represent an essential recognition mechanism that has been functionally conserved during the innate immune response of invertebrates and vertebrates as a result of convergent evolution.

Lung edema clearance: 20 years of progress: invited review: alveolar edema fluid clearance in the injured lung

Resolution of pulmonary edema involved active transepithelial sodium transport. Although several of the cellular and molecular mechanisms involved are relatively well understood, it is only recently that the regulation of these mechanisms in injured lung are being evaluated. Interestingly, in mild-to-moderate lung injury, alveolar edema fluid clearance is often preserved. This preserved or enhanced alveolar fluid clearance is mediated by catecholamine-dependent or -independent mechanisms. This stimulation of alveolar liquid clearance is related to activation or increased expression of sodium transport molecules such as the epithelial sodium channel or the Na(+)-K(+)-ATPase pump and may also involve the cystic fibrosis transmembrane conductance regulator. When severe lung injury occurs, the decrease in alveolar liquid clearance may be related to changes in alveolar permeability or to changes in activity or expression of sodium or chloride transport molecules. Multiple pharmacological tools such as beta-adrenergic agonists, vasoactive drugs, or gene therapy may prove effective in stimulating the resolution of alveolar edema in the injured lung.

Sensitive periods for the effect of dietary sodium restriction on intact and denervated taste receptor cells

Unilateral chorda tympani nerve (CT) section combined with dietary sodium restriction leads to striking alterations in sodium taste function. The regenerated rat CT exhibits deficits in sodium sensitivity, and surprisingly, there are also functional alterations in the intact, contralateral nerve. The studies presented here describe the functional "sensitive periods" for these aberrations and the number of taste buds present during corresponding stages. The regenerated CT is sensitive to dietary sodium restriction during the first 2 wk after denervation, whereas the intact CT is sensitive to dietary manipulation during the first week postsection. Therefore, distinct mechanisms are responsible for the effects of sodium restriction combined with denervation, because separate sensitive periods exist for the regenerated and intact CT nerves. Identification of mature taste buds with an antibody directed at anti-keratin 19 revealed that there is a loss of ~85% of taste buds on the denervated side of the tongue under control and low-sodium diets within the first week postsection. Thus, sodium restriction does not differentially affect the loss of taste buds following denervation.

Selected contribution: long-term effects of beta(2)-adrenergic receptor stimulation on alveolar fluid clearance in mice

Stimulation of active fluid transport with beta-adrenergic receptor (betaAR) agonists can accelerate the resolution of alveolar edema. However, chronic betaAR-agonist administration may cause betaAR desensitization and downregulation that may impair physiological responsiveness to betaAR-agonist stimulation. Therefore, we measured baseline and terbutaline- (10(-3) M) stimulated alveolar fluid clearance in mice that received subcutaneously (miniosmotic pumps) either saline or albuterol (2 mg. kg(-1). day(-1)) for 1, 3, or 6 days. Continuous albuterol administration increased plasma albuterol levels (10(-5) M), an effect that was associated with 1) a significant decrease in betaAR density and 2) attenuation, but not ablation, of maximal terbutaline-induced cAMP production. Forskolin-mediated cAMP-release was unaffected. Continuous albuterol infusion stimulated alveolar fluid clearance on day 1 but did not increase alveolar fluid clearance on days 3 and 6. However, terbutaline-stimulated alveolar fluid clearance in albuterol-treated mice was not reduced compared with saline-treated mice. Despite significant reductions in betaAR density and agonist-mediated cAMP production by long-term betaAR-agonist exposure, maximal betaAR-agonist-mediated increase in alveolar fluid clearance is not diminished in mice.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Interleukin-13 induces a hypersecretory ion transport phenotype in human bronchial epithelial cells

Interleukin (IL)-13 has been associated with asthma, allergic rhinitis, and chronic sinusitis, all conditions where an imbalance in epithelial fluid secretion and absorption could impact upon the disease. We have investigated the effects of IL-13 on the ion transport characteristics of human bronchial epithelial cells cultured at an apical-air interface. Ussing chamber studies indicated that 48 h pretreatment with IL-13 or IL-4 significantly reduced the basal short-circuit current (I(sc)) and inhibited the amiloride-sensitive current by >98%. Furthermore, the I(sc) responses were increased by more than six- and twofold over control values when stimulated with UTP or forskolin, respectively, after cytokine treatment. The IL-13-enhanced response to UTP/ionomycin was sensitive to bumetanide and DIDS and was reduced in a low-chloride, bicarbonate-free solution. Membrane permeablization studies indicated that IL-13 induced the functional expression of an apical Ca(2+)-activated anion conductance and that changes in apical or basolateral K(+) conductances could not account for the increased I(sc) responses to UTP or ionomycin. The results indicate that IL-13 converts the human bronchial epithelium from an absorptive to a secretory phenotype that is the result of loss of amiloride-sensitive current and an increase in a DIDS-sensitive apical anion conductance.

Amiloride blockades lipopolysaccharide-induced proinflammatory cytokine biosynthesis in an IkappaB-alpha/NF-kappaB-dependent mechanism. Evidence for the amplification of an antiinflammatory pathway in the alveolar epithelium

It has been previously reported that amiloride suppresses inflammatory cytokine biosynthesis. However, the molecular mechanism involved has yet to be ascertained. Therefore, the immunoregulatory potential mediated by amiloride and the underlying signaling transduction pathway was investigated. Exposure of alveolar epithelial cells to amiloride or its analog, 5-(N,N-hexamethylene)-amiloride (HMA), reduced, in a dose-dependent manner, lipopolysaccharide (LPS)-induced secretion of interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha. This inhibitory effect was associated with the augmentation of a counter antiinflammatory response, mediated by IL-6 and IL-10. Analysis of the mechanism implicated revealed the involvement of an inhibitory kappaB (IkappaB-alpha)/nuclear factor kappaB (NF- kappaB)-sensitive pathway. Amiloride and HMA suppressed the phosphorylation of IkappaB-alpha mediated by LPS, thereby allowing its cytosolic accumulation. Furthermore, both inhibitors interfered with the nuclear translocation of selective NF-kappaB subunits, an effect associated with blockading the DNA-binding activity of NF-kappaB. Recombinant IL-10 blockaded LPS-induced biosynthesis of IL-1beta and TNF-alpha and reduced NF-kappaB activation. Immunoneutralization of endogenous IL-10 reversed the inhibitory effect of amiloride on proinflammatory cytokines and restored the DNA-binding activity of NF-kappaB. These results indicate that amiloride acts as a novel dual immunoregulator in the alveolar epithelium: it downregulates an inflammatory signal and at the same time upregulates an antiinflammatory response. This biphasic effect is IL-10 sensitive and is associated with the selective targeting of the IkappaB-alpha/NF-kappaB signaling transduction pathway.