Alveolar macrophages release mediators upon stimulation of bacterial toxins, which inhibit fluid reabsorption from the alveolar space, increase barrier leakage, and cause pulmonary edema and alveolar hypoxia, which further impairs barrier function. Hypoxia can also cause a pro-inflammatory response. Here we test whether hypoxia aggravates the pro-inflammatory response in alveolar epithelial cells and whether this increased response involves HIF-1α.

Rat primary alveolar epithelial cells (AEC), in which HIF-1α was silenced, were exposed to conditioned media from normoxic, LPS-stimulated macrophages (MM-LPS) for 24h in normoxia and hypoxia (1.5% O₂). Transepithelial electrical resistance (TEER), Na-reabsorption, NO production and cytokine mRNA expression were measured. Our results indicate that hypoxia aggravated the decrease in TEER and Na-transport induced by MM-LPS. Effects on permeability and Na-transport were not affected by HIF-1α silencing. Exposure to MM-LPS increased the expression of cytokines in AEC, which was enhanced in hypoxia. Silencing of Hif-1α prevented the increased IL-1beta and IL-6 mRNA expression in hypoxic AEC. In contrast to macrophages, AII cells do not produces nitrite upon stimulation although iNOS expression was increased.

These results indicate that signals from stimulated macrophages induce a pro-inflammatory response in alveolar epithelium, which is enhanced by hypoxia in a Hif-1α dependent manner. The increased permeability and the decreased ion transport in hypoxic AEC exposed to stimulated macrophages seem not to be caused by the cytokines from alveolar epithelium, because they were not prevented by HIF-1α silencing.