ARDS is the most frequent cause of death in intensive care medicine, yet effective pharmacological treatments are lacking. Loss of alveolar-capillary barrier function due to increased alveolar epithelial cell (AEC) apoptosis is a hallmark of ARDS and acute lung injury (ALI). We reported earlier that the Na+-coupled neutral amino acid (AA) transporter SNAT2, which transports one neutral AA along with one Na+, promotes alveolar fluid clearance by AEC-mediated Na+ uptake. SNAT2´s contribution to AEC homeostasis in ARDS/ALI by its role as AA transporter is, however, unknown. We hypothesize that SNAT2 is lost under inflammatory conditions, enhancing AEC apoptosis and lung permeability, thus promoting ALI.
In vitro, SNAT2 was downregulated in AEC stimulated with cytomix (IFN-?, TNF-?, IL-1? and LPS, 50 ng/ml) or pneumolysin (0,2 ?g/ml) mimicking the inflammatory or infectious milieu of ARDS, respectively. Inhibition of SNATs by ?-methylaminoisobutiric acid (MeAIB, 10mM) and specific SNAT2 knock-down in AEC increased the abundance of apoptotic cells, and the expression of pro-apoptotic CHOP compared to untreated cells. Precision-cut lung slices (PCLS) from LC3-GFP mice showed an increase in LC3-GFP+ cells when stimulated with MeAIB as compared to untreated PCLS, indicating regulation of autophagy by SNAT2. In vivo, 24h after LPS-induced ALI, Slc38a2+/- (partial SNAT2 deficient) mice had increased BALF protein content and increased CHOP expression in lung tissue compared to Slc38a2+/+ WT mice.
Our findings suggest that in ALI/ARDS, inflammation and/or infection downregulate SNAT2, promoting excessive autophagy and apoptosis of AEC, thus exacerbating ALI.