Biphasic modulation of α-ENaC expression by lipopolysaccharide in vitro and in vivo

Repressing cytokine storm-like response in macrophages by targeting the eIF2α-integrated stress response pathway

Cytokine storm is a life-threatening systemic hyper-inflammatory state caused by different etiologies, in which the bulk production of pro-inflammatory cytokines from activated macrophages has a central role. Integrated stress response (ISR) comprises several protective signaling pathways, leading to phosphorylation of eukaryotic initiation factor 2α (eIF2α) and repression of protein translation. Emerging evidence suggests that ISR induction may elicit anti-inflammatory effects. Currently, however, it is unclear whether targeting eIF2α phosphorylation is sufficient to inhibit the cytokine storm-like response in macrophages. Here we carried out a proof-of-concept study, employing two approaches: (1) ectopic expression of the eIF2α-S51D mutant (mimicking the phosphorylated eIF2α); (2) treatment with salubrinal, a small molecule inhibitor of eIF2α dephosphorylation. Experiments were performed in lipopolysaccharides (LPS)-stimulated macrophages and in murine models with LPS-induced acute endotoxemia. We demonstrated that in macrophages, ectopic expression of eIF2α-S51D, treatment with salubrinal, and gene silencing of PP1/GADD34 (the phosphatase holoenzyme mediating eIF2α dephosphorylation) significantly inhibited LPS-induced cytokine productions without changing their mRNA levels. Polysome PCR and puromycin incorporation assays confirmed that salubrinal suppressed de novo protein translation of the cytokines. In vivo, salubrinal pre-treatment mitigated LPS-induced acute lung injury and significantly reduced the concentration of circulating TNF-α. Salubrinal did not exhibit any effects on the Toll-like receptor 4-mediated signaling or the activation of mammalian target of rapamycin (mTOR). Our data suggest that direct manipulation of eIF2α phosphorylation, thereby bypassing all associated upstream signaling events, may suppress the cytokine storm-like response in activated macrophages, likely by decoupling the gene transcription and protein translation. Inhibiting eIF2α dephosphorylation with small molecule inhibitors may be a viable therapeutic strategy to treat disorders involving cytokine storm-like responses.

Systemic Effects of mitoTEMPO upon Lipopolysaccharide Challenge Are Due to Its Antioxidant Part, While Local Effects in the Lung Are Due to Triphenylphosphonium

Mitochondria-targeted antioxidants (mtAOX) are a promising treatment strategy against reactive oxygen species-induced damage. Reports about harmful effects of mtAOX lead to the question of whether these could be caused by the carrier molecule triphenylphosphonium (TPP). The aim of this study was to investigate the biological effects of the mtAOX mitoTEMPO, and TPP in a rat model of systemic inflammatory response. The inflammatory response was induced by lipopolysaccharide (LPS) injection. We show that mitoTEMPO reduced expression of inducible nitric oxide synthase in the liver, lowered blood levels of tissue damage markers such as liver damage markers (aspartate aminotransferase and alanine aminotransferase), kidney damage markers (urea and creatinine), and the general organ damage marker, lactate dehydrogenase. In contrast, TPP slightly, but not significantly, increased the LPS-induced effects. Surprisingly, both mitoTEMPO and TPP reduced the wet/dry ratio in the lung after 24 h. In the isolated lung, both substances enhanced the increase in pulmonary arterial pressure induced by LPS observed within 3 h after LPS treatments but did not affect edema formation at this time. Our data suggest that beneficial effects of mitoTEMPO in organs are due to its antioxidant moiety (TEMPO), except for the lung where its effects are mediated by TPP.

Mesenchymal Stem Cell-Conditioned Medium Rescues LPS-Impaired ENaC Activity in Mouse Trachea via WNK4 Pathway

BACKGROUND

Airway epithelium plays an essential role in maintaining the homeostasis and function of respiratory system as the first line of host defense. Of note, epithelial sodium channel (ENaC) is one of the victims of LPS-induced airway injury. Regarding the great promise held by mesenchymal stem cells (MSCs) for regenerative medicine in the field of airway injury and the limitations of cell-based MSCs therapy, we focused on the therapeutic effect of MSCs conditioned medium (MSCs-CM) on the ENaC activity in mouse tracheal epithelial cells.

METHODS

Ussing chamber apparatus was applied to record the short-circuit currents in primary cultured mouse tracheal epithelial cells, which reflects the ENaC activity. Expressions of α and γ ENaC were measured at the protein and mRNA levels by western blot and real-time PCR, respectively. The expression of with-no-lysinekinase- 4 (WNK4) and ERK1/2 were measured at protein levels, and the relationship between WNK4 and ERK1/2 was determined by WNK4 knockdown.

RESULTS

MSCs-CM restored the LPS-impaired ENaC activity, as well as enhanced the mRNA and protein expressions of ENaC in primary cultured mouse tracheal epithelial cells. Meanwhile, WNK4 and ERK1/2, both negative-regulators of ENaC, were suppressed accordingly after the administration of MSCs-CM in LPS-induced airway injury. After WNK4 gene was knocked down by siRNA, the level of ERK1/2 phosphorylation decreased.

CONCLUSION

In light of the key role of ENaC in fluid reabsorption and the beneficial effects of MSCs-CM in the injury of airway epithelium, our results suggest that MSCs-CM is effective in alleviating LPS-induced ENaC dysfunction through WNK4-ERK1/2 pathway, which will provide a potent direction for the therapy of airway injury.

Impact of Bacterial Toxins in the Lungs

Bacterial toxins play a key role in the pathogenesis of lung disease. Based on their structural and functional properties, they employ various strategies to modulate lung barrier function and to impair host defense in order to promote infection. Although in general, these toxins target common cellular signaling pathways and host compartments, toxin- and cell-specific effects have also been reported. Toxins can affect resident pulmonary cells involved in alveolar fluid clearance (AFC) and barrier function through impairing vectorial Na⁺ transport and through cytoskeletal collapse, as such, destroying cell-cell adhesions. The resulting loss of alveolar-capillary barrier integrity and fluid clearance capacity will induce capillary leak and foster edema formation, which will in turn impair gas exchange and endanger the survival of the host. Toxins modulate or neutralize protective host cell mechanisms of both the innate and adaptive immunity response during chronic infection. In particular, toxins can either recruit or kill central players of the lung's innate immune responses to pathogenic attacks, i.e., alveolar macrophages (AMs) and neutrophils. Pulmonary disorders resulting from these toxin actions include, e.g., acute lung injury (ALI), the acute respiratory syndrome (ARDS), and severe pneumonia. When acute infection converts to persistence, i.e., colonization and chronic infection, lung diseases, such as bronchitis, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) can arise. The aim of this review is to discuss the impact of bacterial toxins in the lungs and the resulting outcomes for pathogenesis, their roles in promoting bacterial dissemination, and bacterial survival in disease progression.

RAGE inhibition reduces acute lung injury in mice

The receptor for advanced glycation end-products (RAGE) is involved in inflammatory response during acute respiratory distress syndrome (ARDS). Growing body of evidence support strategies of RAGE inhibition in experimental lung injury, but its modalities and effects remain underinvestigated. Anesthetised C57BL/6JRj mice were divided in four groups; three of them underwent orotracheal instillation of acid and were treated with anti-RAGE monoclonal antibody (mAb) or recombinant soluble RAGE (sRAGE), acting as a decoy receptor. The fourth group served as a control. Lung injury was assessed by the analysis of blood gases, alveolar permeability, histology, AFC, and cytokines. Lung expression and distribution epithelial channels ENaC, Na,K-ATPase, and aquaporin (AQP)−5 were assessed. Treatment with either anti-RAGE mAb or sRAGE improved lung injury, arterial oxygenation and decreased alveolar inflammation in acid-injured animals. Anti-RAGE therapies were associated with restored AFC and increased lung expression of AQP-5 in alveolar cell. Blocking RAGE had potential therapeutic effects in a translational mouse model of ARDS, possibly through a decrease in alveolar type 1 epithelial cell injury as shown by restored AFC and lung AQP-5 expression. Further mechanistic studies are warranted to describe intracellular pathways that may control such effects of RAGE on lung epithelial injury and repair.

Gene expression of the concentration-sensitive sodium channel is suppressed in lipopolysaccharide-induced acute lung injury in mice

PURPOSE

The concentration-sensitive sodium channel (Na_C) is expressed in alveolar type II epithelial cells and pulmonary microvascular endothelial cells in mouse lungs. We recently reported that Na_C contributes to amiloride-insensitive sodium transport in mouse lungs (Respiratory Physiology & Neurobiology, 2016). However, details regarding its physiological role in the lung remain unknown. To examine whether Na_C is involved in alveolar fluid clearance during an acute lung injury (ALI), we analyzed the relationship between Na_C gene expression in the lung and the development of pulmonary edema in lipopolysaccharide (LPS)-induced ALI mice.

METHODS

LPS-induced ALI mice were prepared by the intratracheal administration of LPS. Bronchoalveolar lavage (BAL) neutrophils and lung water content (LWCs) were used as a marker of ALI and pulmonary edema, respectively. Na_C protein production in the lung was detected by immunoblotting and immunofluorescence. The gene expressions of Na_C and the epithelial sodium channel (ENaC) of LPS-induced ALI mice were examined by quantitative RT-PCR over a time course of 14 days.

RESULTS

The BAL neutrophil count increased until day 2 after LPS administration and had nearly recovered by day 6. LWCs in LPS-induced mice gradually increased until day 8 and had recovered by day 14. The expression of the Na_C protein in the lungs of LPS-induced mice dramatically decreased from day 2 to day 6, but recovered by day 8. The mRNA expression of Na_C decreased in the lung, as well as those for α-, β-, and γ-ENaC during ALI. Thus, Na_C expression is suppressed during the development stage of pulmonary edema and then recovers in the convalescent phase.

CONCLUSION

Our results suggest that suppression of the gene expression of Na_C is involved in the development of pulmonary edema in ALI.

Effects of baicalin on alveolar fluid clearance and α-ENaC expression in rats with LPS-induced acute lung injury

Baicalin has been reported to attenuate lung edema in the process of lung injury. However, the effect of baicalin on alveolar fluid clearance (AFC) and epithelial sodium channel (ENaC) expression has not been tested. Sprague-Dawley rats were anesthetized and intratracheally injected with either 1 mg/kg lipopolysaccharide (LPS) or saline vehicle. Baicalin with various concentrations (10, 50, and 100 mg/kg) was injected intraperitoneally 30 min before administration of LPS. Then lungs were isolated for measurement of AFC, cyclic adenosine monophosphate (cAMP) level, and cellular localization of α-ENaC. Moreover, mouse alveolar type II (ATII) epithelial cell line was incubated with baicalin (30 μmol/L), adenylate cyclase inhibitor SQ22536 (10 μmol/L), or cAMP-dependent protein kinase inhibitor (PKA) KT5720 (0.3 μmol/L) 15 min before LPS (1 μg/mL) incubation. Protein expression of α-ENaC was detected by Western blot. Baicalin increased cAMP concentration and AFC in a dose-dependent manner in rats with LPS-induced acute lung injury. The increase of AFC induced by baicalin was associated with an increase in the abundance of α-ENaC protein. SQ22536 and KT5720 prevented the increase of α-ENaC expression caused by baicalin in vitro. These findings suggest that baicalin prevents LPS-induced reduction of AFC by upregulating α-ENaC protein expression, which is activated by stimulating cAMP/PKA signaling pathway.

Inhalation therapy with the synthetic TIP-like peptide AP318 attenuates pulmonary inflammation in a porcine sepsis model

BACKGROUND

The lectin-like domain of TNF-α can be mimicked by synthetic TIP peptides and represents an innovative pharmacologic option to treat edematous respiratory failure. TIP inhalation was shown to reduce pulmonary edema and improve gas exchange. In addition to its edema resolution effect, TIP peptides may exert some anti-inflammatory properties. The present study therefore investigates the influence of the inhaled TIP peptide AP318 on intrapulmonary inflammatory response in a porcine model of systemic sepsis.

METHODS

In a randomized-blinded setting lung injury was induced in 18 pigs by lipopolysaccharide-infusion and a second hit with a short period of ventilator-induced lung stress, followed by a six-hour observation period. The animals received either two inhalations with the peptide (AP318, 2×1 mg kg(-1)) or vehicle. Post-mortem pulmonary expression of inflammatory and mechanotransduction markers were determined by real-time polymerase chain reaction (IL-1β, IL-6, TNF-α, COX-2, iNOS, amphiregulin, and tenascin-c). Furthermore, regional histopathological lung injury, edema formation and systemic inflammation were quantified.

RESULTS

Despite similar systemic response to lipopolysaccharide infusion in both groups, pulmonary inflammation (IL-6, TNF-α, COX-2, tenascin-c) was significantly mitigated by AP318. Furthermore, a Western blot analysis shows a significantly lower of COX-2 protein level. The present sepsis model caused minor lung edema formation and moderate gas exchange impairment. Six hours after onset pathologic scoring showed no improvement, while gas exchange parameters and pulmonary edema formation were similar in the two groups.

CONCLUSION

In summary, AP318 significantly attenuated intrapulmonary inflammatory response even without the presence or resolution of severe pulmonary edema in a porcine model of systemic sepsis-associated lung injury. These findings suggest an anti-inflammatory mechanism of the lectin-like domain beyond mere edema reabsorption in endotoxemic lung injury in vivo.