Activation of the heat shock response attenuates the interleukin 1β-mediated inhibition of the amiloride-sensitive alveolar epithelial ion transport

Ion transport mechanisms for smoke inhalation-injured airway epithelial barrier

Smoke inhalation injury is the leading cause of death in firefighters and victims. Inhaled hot air and toxic smoke are the predominant hazards to the respiratory epithelium. We aimed to analyze the effects of thermal stress and smoke aldehyde on the permeability of the airway epithelial barrier. Transepithelial resistance (R_TE) and short-circuit current (I_SC) of mouse tracheal epithelial monolayers were digitized by an Ussing chamber setup. Zonula occludens-1 tight junctions were visualized under confocal microscopy. A cell viability test and fluorescein isothiocyanate-dextran assay were performed. Thermal stress (40 °C) decreased R_TE in a two-phase manner. Meanwhile, thermal stress increased I_SC followed by its decline. Na⁺ depletion, amiloride (an inhibitor for epithelial Na⁺ channels [ENaCs]), ouabain (a blocker for Na⁺/K⁺-ATPase), and CFTRinh-172 (a blocker of cystic fibrosis transmembrane regulator [CFTR]) altered the responses of R_TE and I_SC to thermal stress. Steady-state 40 °C increased activity of ENaCs, Na⁺/K⁺-ATPase, and CFTR. Acrolein, one of the main oxidative unsaturated aldehydes in fire smoke, eliminated R_TE and I_SC. Na⁺ depletion, amiloride, ouabain, and CFTRinh-172 suppressed acrolein-sensitive I_SC, but showed activating effects on acrolein-sensitive R_TE. Thermal stress or acrolein disrupted zonula occludens-1 tight junctions, increased fluorescein isothiocyanate-dextran permeability but did not cause cell death or detachment. The synergistic effects of thermal stress and acrolein exacerbated the damage to monolayers. In conclusion, the paracellular pathway mediated by the tight junctions and the transcellular pathway mediated by active and passive ion transport pathways contribute to impairment of the airway epithelial barrier caused by thermal stress and acrolein. Graphical abstract Thermal stress and acrolein are two essential determinants for smoke inhalation injury, impairing airway epithelial barrier. Transcellular ion transport pathways via the ENaC, CFTR, and Na/K-ATPase are interrupted by both thermal stress and acrolein, one of the most potent smoke toxins. Heat and acrolein damage the integrity of the airway epithelium through suppressing and relocating the tight junctions.

Regulation of Lung Epithelial Sodium Channels by Cytokines and Chemokines

Acute lung injury leading to acute respiratory distress (ARDS) is a global health concern. ARDS patients have significant pulmonary inflammation leading to flooding of the pulmonary alveoli. This prevents normal gas exchange with consequent hypoxemia and causes mortality. A thin fluid layer in the alveoli is normal. The maintenance of this thin layer results from fluid movement out of the pulmonary capillaries into the alveolar interstitium driven by vascular hydrostatic pressure and then through alveolar tight junctions. This is then balanced by fluid reabsorption from the alveolar space mediated by transepithelial salt and water transport through alveolar cells. Reabsorption is a two-step process: first, sodium enters via sodium-permeable channels in the apical membranes of alveolar type 1 and 2 cells followed by active extrusion of sodium into the interstitium by the basolateral Na⁺, K⁺-ATPase. Anions follow the cationic charge gradient and water follows the salt-induced osmotic gradient. The proximate cause of alveolar flooding is the result of a failure to reabsorb sufficient salt and water or a failure of the tight junctions to prevent excessive movement of fluid from the interstitium to alveolar lumen. Cytokine- and chemokine-induced inflammation can have a particularly profound effect on lung sodium transport since they can alter both ion channel and barrier function. Cytokines and chemokines affect alveolar amiloride-sensitive epithelial sodium channels (ENaCs), which play a crucial role in sodium transport and fluid reabsorption in the lung. This review discusses the regulation of ENaC via local and systemic cytokines during inflammatory disease and the effect on lung fluid balance.

Conditioned media from mesenchymal stromal cells restore sodium transport and preserve epithelial permeability in an in vitro model of acute alveolar injury

Mesenchymal stromal cells (MSCs) or their media (MSC-M) were reported to reverse acute lung injury (ALI)-induced decrease of alveolar fluid clearance. To determine the mechanisms by which MSC-M exert their beneficial effects, an in vitro model of alveolar epithelial injury was created by exposing primary rat alveolar epithelial cells (AECs) to hypoxia (3% O2) plus cytomix, a combination of IL-1β, TNF-α, and IFN-γ. MSC-M were collected from human MSCs exposed for 12 h to either normoxia (MSC-M) or to hypoxia plus cytomix (HCYT-MSC-M). This latter condition was used to model the effect of alveolar inflammation and hypoxia on paracrine secretion of MSCs in the injured lung. Comparison of paracrine soluble factors in MSC media showed that the IL-1 receptor antagonist and prostaglandin E2 were markedly increased while keratinocyte growth factor (KGF) was twofold lower in HCYT-MSC-M compared with MSC-M. In AECs, hypoxia plus cytomix increased protein permeability, reduced amiloride-sensitive short-circuit current (AS-Isc), and also decreased the number of α-epithelial sodium channel (α-ENaC) subunits in the apical membrane. To test the effects of MSC media, MSC-M and HCYT-MSC-M were added for an additional 12 h to AECs exposed to hypoxia plus cytomix. MSC-M and HCYT-MSC-M completely restored epithelial permeability to normal. MSC-M, but not HCYT-MSC-M, significantly prevented the hypoxia plus cytomix-induced decrease of ENaC activity and restored apical α-ENaC channels. Interestingly, KGF-deprived MSC-M were unable to restore amiloride-sensitive sodium transport, indicating a possible role for KGF in the beneficial effect of MSC-M. These results indicate that MSC-M may be a preferable therapeutic option for ALI.