Lung Edema Clearance: Relevance to Patients with Lung Injury

Elevated arginine vasopressin levels surrogate acute lung injury in acute decompensated heart failure

Activated arginine vasopressin (AVP) pathway worsens congestion in heart failure (HF), but its potential to relieve pulmonary congestion is also reported. The pathophysiological role and prognostic utility of AVP elevation in acute decompensated HF (ADHF) are poorly understood. We prospectively enrolled 52 hospitalized patients for ADHF to investigate the association between acute lung injury (ALI) in ADHF and AVP levels on admission. ALI was defined as respiratory failure leading to death, or requiring a respirator or a more than 12-h non-invasive intermittent positive pressure ventilation (NIPPV) support. In addition, we investigated the prognostic value of AVP levels on admission for cardiovascular death or recurrence of ADHF after discharge. ALI was documented in 7 patients (13.5%) during a median hospital stay of 14 days. And the patients with ALI demonstrated significantly higher AVP levels than those without (32.5 ± 21.6 vs. 6.4 ± 8.7 pg/ml, p = 0.018). Besides, the patients with ALI demonstrated significantly higher heart rates (HR) and lower E/e' on admission (HR: 127 ± 24 vs. 97 ± 28 bpm; E/e': 10.6 ± 3.7 vs. 17.4 ± 6.2, all p < 0.05, respectively). Of note, significant hemodilution assessed by hemoglobin and hematocrit values were observed in the patients with ALI 48 h after admission. A receiver operating characteristic curve analysis showed that higher than 7.2 pg/ml surrogate ALI in ADHF (AUC: 0.897, p = 0.001, Sensitivity: 85.7%, and Specificity: 77.8%). In contrast, increased AVP levels on admission could not predict cardiovascular events after discharge. Elevated AVP levels on admission are associated with ALI in ADHF but not cardiovascular events after discharge.

Inhaled β2 Adrenergic Agonists and Other cAMP-Elevating Agents: Therapeutics for Alveolar Injury and Acute Respiratory Disease Syndrome?

Inhaled long-acting β-adrenergic agonists (LABAs) and short-acting β-adrenergic agonists are approved for the treatment of obstructive lung disease via actions mediated by β2 adrenergic receptors (β2-ARs) that increase cellular cAMP synthesis. This review discusses the potential of β2-AR agonists, in particular LABAs, for the treatment of acute respiratory distress syndrome (ARDS). We emphasize ARDS induced by pneumonia and focus on the pathobiology of ARDS and actions of LABAs and cAMP on pulmonary and immune cell types. β2-AR agonists/cAMP have beneficial actions that include protection of epithelial and endothelial cells from injury, restoration of alveolar fluid clearance, and reduction of fibrotic remodeling. β2-AR agonists/cAMP also exert anti-inflammatory effects on the immune system by actions on several types of immune cells. Early administration is likely critical for optimizing efficacy of LABAs or other cAMP-elevating agents, such as agonists of other Gs-coupled G protein-coupled receptors or cyclic nucleotide phosphodiesterase inhibitors. Clinical studies that target lung injury early, prior to development of ARDS, are thus needed to further assess the use of inhaled LABAs, perhaps combined with inhaled corticosteroids and/or long-acting muscarinic cholinergic antagonists. Such agents may provide a multipronged, repurposing, and efficacious therapeutic approach while minimizing systemic toxicity. SIGNIFICANCE STATEMENT: Acute respiratory distress syndrome (ARDS) after pulmonary alveolar injury (e.g., certain viral infections) is associated with ∼40% mortality and in need of new therapeutic approaches. This review summarizes the pathobiology of ARDS, focusing on contributions of pulmonary and immune cell types and potentially beneficial actions of β2 adrenergic receptors and cAMP. Early administration of inhaled β2 adrenergic agonists and perhaps other cAMP-elevating agents after alveolar injury may be a prophylactic approach to prevent development of ARDS.

Alcohol inhibits alveolar fluid clearance through the epithelial sodium channel via the A2 adenosine receptor in acute lung injury

Chronic alcohol abuse increases the risk of mortality and poor outcomes in patients with acute respiratory distress syndrome. However, the underlying mechanisms remain to be elucidated. The present study aimed to investigate the effects of chronic alcohol consumption on lung injury and clarify the signaling pathways involved in the inhibition of alveolar fluid clearance (AFC). In order to produce rodent models with chronic alcohol consumption, wild‑type C57BL/6 mice were treated with alcohol. A2a adenosine receptor (AR) small interfering (si)RNA or A2bAR siRNA were transfected into the lung tissue of mice and primary rat alveolar type II (ATII) cells. The rate of AFC in lung tissue was measured during exposure to lipopolysaccharide (LPS). Epithelial sodium channel (ENaC) expression was determined to investigate the mechanisms underlying alcohol‑induced regulation of AFC. In the present study, exposure to alcohol reduced AFC, exacerbated pulmonary edema and worsened LPS‑induced lung injury. Alcohol caused a decrease in cyclic adenosine monophosphate (cAMP) levels and inhibited α‑ENaC, β‑ENaC and γ‑ENaC expression levels in the lung tissue of mice and ATII cells. Furthermore, alcohol decreased α‑ENaC, β‑ENaC and γ‑ENaC expression levels via the A2aAR or A2bAR‑cAMP signaling pathways in vitro. In conclusion, the results of the present study demonstrated that chronic alcohol consumption worsened lung injury by aggravating pulmonary edema and impairing AFC. An alcohol‑induced decrease of α‑ENaC, β‑ENaC and γ‑ENaC expression levels by the A2AR‑mediated cAMP pathway may be responsible for the exacerbated effects of chronic alcohol consumption in lung injury.

Sodium Toxicity in the Nutritional Epidemiology and Nutritional Immunology of COVID-19

Dietary factors in the etiology of COVID-19 are understudied. High dietary sodium intake leading to sodium toxicity is associated with comorbid conditions of COVID-19 such as hypertension, kidney disease, stroke, pneumonia, obesity, diabetes, hepatic disease, cardiac arrhythmias, thrombosis, migraine, tinnitus, Bell's palsy, multiple sclerosis, systemic sclerosis, and polycystic ovary syndrome. This article synthesizes evidence from epidemiology, pathophysiology, immunology, and virology literature linking sodium toxicological mechanisms to COVID-19 and SARS-CoV-2 infection. Sodium toxicity is a modifiable disease determinant that impairs the mucociliary clearance of virion aggregates in nasal sinuses of the mucosal immune system, which may lead to SARS-CoV-2 infection and viral sepsis. In addition, sodium toxicity causes pulmonary edema associated with severe acute respiratory syndrome, as well as inflammatory immune responses and other symptoms of COVID-19 such as fever and nasal sinus congestion. Consequently, sodium toxicity potentially mediates the association of COVID-19 pathophysiology with SARS-CoV-2 infection. Sodium dietary intake also increases in the winter, when sodium losses through sweating are reduced, correlating with influenza-like illness outbreaks. Increased SARS-CoV-2 infections in lower socioeconomic classes and among people in government institutions are linked to the consumption of foods highly processed with sodium. Interventions to reduce COVID-19 morbidity and mortality through reduced-sodium diets should be explored further.

Aldosterone alleviates lipopolysaccharide-induced acute lung injury by regulating epithelial sodium channel through PI3K/Akt/SGK1 signaling pathway

Reduced alveolar fluid clearance (AFC) is a major pathological feature of acute lung injury (ALI). Epithelial sodium channel (ENaC) plays a key role in regulating the transport of Na⁺ and clearing alveolar edema fluid effectively. ENaC has been reported to be regulated by aldosterone in the distal collecting tube of the kidney. We hypothesized whether aldosterone regulated ENaC in alveolar epithelium and correspondingly played a role in ALI. In this study we found that the expression of aldosterone synthesis encoding gene, CYP11B2, and ENaC were decreased in the lung tissue of LPS-induced ALI mice. Furthermore, aldosterone alleviated ALI by increasing the expression of ENaC-α and relieving pulmonary edema. Besides, we found that aldosterone upregulated ENaC-α through PI3K/Akt/SGK1 pathway. In conclusion, our study demonstrated that aldosterone attenuated pulmonary edema by upregulating ENaC-α through the PI3K/Akt/SGK1 pathway in LPS-induced ALI, indicating that aldosterone might be a promising adjuvant drug for ALI treatment.

Insulin ameliorates pulmonary edema through the upregulation of epithelial sodium channel via the PI3K/SGK1 pathway in mice with lipopolysaccharide‑induced lung injury

Epithelial sodium channel (ENaC) provides the driving force for the removal of edema from the alveolar spaces in acute lung injury (ALI). Our previous study reported that insulin increased the expression of α‑ENaC, possibly via the serum/glucocorticoid‑inducible kinase‑1 (SGK1) pathway in ALI; however, the upstream regulator of SGK1 activity remains unclear. In the current study, C3H/HeN mice were subjected to lipopolysaccharide (LPS)‑induced lung injury without hyperglycemia. Exogenous insulin was administered intravenously using a micro‑osmotic pump, and intratracheal delivery of SGK1 small interfering RNA (siRNA) was performed. Furthermore, alveolar epithelial type II cells transfected with phosphatidylinositol 3‑kinase (PI3K) siRNA or SGK1 siRNA were incubated with insulin. Insulin protected the pulmonary epithelial barrier, reduced the apoptosis of alveolar epithelial cells, attenuated pulmonary edema, improved alveolar fluid clearance, and increased the expression levels of α‑, β‑ and γ‑ENaC in mice. In addition, in alveolar epithelial cells, insulin increased the expression levels of α‑, β‑ and γ‑ENaC, as well as the level of phosphorylated SGK1, which were then inhibited by the selective targeting of PI3K or SGK1 by siRNA. Taken together, the results of the present study demonstrated that insulin protected the lung epithelium and attenuated pulmonary edema through the upregulation of ENaC via the PI3K/SGK1 pathway in LPS‑induced lung injury.

Epithelial Sodium Channel-α Mediates the Protective Effect of the TNF-Derived TIP Peptide in Pneumolysin-Induced Endothelial Barrier Dysfunction

BACKGROUND

Streptococcus pneumoniae is a major etiologic agent of bacterial pneumonia. Autolysis and antibiotic-mediated lysis of pneumococci induce release of the pore-forming toxin, pneumolysin (PLY), their major virulence factor, which is a prominent cause of acute lung injury. PLY inhibits alveolar liquid clearance and severely compromises alveolar-capillary barrier function, leading to permeability edema associated with pneumonia. As a consequence, alveolar flooding occurs, which can precipitate lethal hypoxemia by impairing gas exchange. The α subunit of the epithelial sodium channel (ENaC) is crucial for promoting Na⁺ reabsorption across Na⁺-transporting epithelia. However, it is not known if human lung microvascular endothelial cells (HL-MVEC) also express ENaC-α and whether this subunit is involved in the regulation of their barrier function.

METHODS

The presence of α, β, and γ subunits of ENaC and protein phosphorylation status in HL-MVEC were assessed in western blotting. The role of ENaC-α in monolayer resistance of HL-MVEC was examined by depletion of this subunit by specific siRNA and by employing the TNF-derived TIP peptide, a specific activator that directly binds to ENaC-α.

RESULTS

HL-MVEC express all three subunits of ENaC, as well as acid-sensing ion channel 1a (ASIC1a), which has the capacity to form hybrid non-selective cation channels with ENaC-α. Both TIP peptide, which specifically binds to ENaC-α, and the specific ASIC1a activator MitTx significantly strengthened barrier function in PLY-treated HL-MVEC. ENaC-α depletion significantly increased sensitivity to PLY-induced hyperpermeability and in addition, blunted the protective effect of both the TIP peptide and MitTx, indicating an important role for ENaC-α and for hybrid NSC channels in barrier function of HL-MVEC. TIP peptide blunted PLY-induced phosphorylation of both calmodulin-dependent kinase II (CaMKII) and of its substrate, the actin-binding protein filamin A (FLN-A), requiring the expression of both ENaC-α and ASIC1a. Since non-phosphorylated FLN-A promotes ENaC channel open probability and blunts stress fiber formation, modulation of this activity represents an attractive target for the protective actions of ENaC-α in both barrier function and liquid clearance.

CONCLUSION

Our results in cultured endothelial cells demonstrate a previously unrecognized role for ENaC-α in strengthening capillary barrier function that may apply to the human lung. Strategies aiming to activate endothelial NSC channels that contain ENaC-α should be further investigated as a novel approach to improve barrier function in the capillary endothelium during pneumonia.

Involvement of Cytokines in the Pathogenesis of Salt and Water Imbalance in Congestive Heart Failure

Congestive heart failure (CHF) has become a major medical problem in the western world with high morbidity and mortality rates. CHF adversely affects several systems, mainly the kidneys and the lungs. While the involvement of the renin-angiotensin-aldosterone system and the sympathetic nervous system in the progression of cardiovascular, pulmonary, and renal dysfunction in experimental and clinical CHF is well established, the importance of pro-inflammatory mediators in the pathogenesis of this clinical setting is still evolving. In this context, CHF is associated with overexpression of pro-inflammatory cytokines, such as tumor necrosis factor-α, interleukin (IL)-1, and IL-6, which are activated in response to environmental injury. This family of cytokines has been implicated in the deterioration of CHF, where it plays an important role in initiating and integrating homeostatic responses both at the myocardium and circulatory levels. We and others showed that angiotensin II decreased the ability of the lungs to clear edema and enhanced the fibrosis process via phosphorylation of the mitogen-activated protein kinases p38 and p42/44, which are generally involved in cellular responses to pro-inflammatory cytokines. Literature data also indicate the involvement of these effectors in modulating ion channel activity. It has been reported that in heart failure due to mitral stenosis; there were varying degrees of vascular and other associated parenchymal changes such as edema and fibrosis. In this review, we will discuss the effects of cytokines and other inflammatory mediators on the kidneys and the lungs in heart failure; especially their role in renal and alveolar ion channels activity and fluid balance.