Effect of hypoxia on release of IL-1 and TNF by human alveolar macrophages

Our previous work demonstrated that hypoxia decreases transcription of the human prostaglandin H synthase-2 (PGHS-2) gene during exposure to lipopolysaccharide (LPS), resulting in decreased prostaglandin E2 (PGE2) synthesis (J. Biol. Chem. 269:32979-32984, 1994). Because PGE2 is reported to inhibit interleukin 1 (IL-1) and tumor necrosis factor (TNF), it is likely that hypoxia, through changes in PGE2, will alter IL-1 and TNF release from the human alveolar macrophage. In addition, like PGHS-2, the TNF and IL-1 promoters contain oxidant-sensitive elements which might be altered by hypoxia. Therefore, we hypothesized that LPS-induced release of TNF and IL-1 would be altered by hypoxia. To test this, human alveolar macrophages were cultured for 24 h with 0 to 1 microgram/ml LPS in a room-air incubator with 5% CO2 or a hypoxia incubator continuously perfused with 5% CO2/95% N2 (O2 < 0.05%). With room air, LPS increased IL-1 beta mRNA and increased IL-1 beta protein release into the culture medium in a dose-dependent manner. Hypoxia increased the LPS-stimulated release of IL-1 beta 30% above that of room-air controls. However, immunoblots showed that hypoxia caused no change in intracellular IL-1 beta compared with room-air controls. There was also no change in LPS-induced IL-1 beta message with hypoxia. The inhibitor of IL-1, IL-1RA, was apparently decreased by hypoxia, but this decrease was not statistically significant. TNF-alpha mRNA and release of protein also increased during LPS exposure in room air. Hypoxia markedly increased LPS-induced TNF-alpha message and release of TNF-alpha compared with LPS-exposed room-air controls. Consistent with our prior observations, hypoxia decreased LPS-induced PGHS-2 message and protein, and also the PGHS-2 product, PGE2. Because PGE2 is reported to inhibit the expression of IL-1 and TNF genes, we inhibited PGE2 synthesis with indomethacin during culture in room air; the result was an increase in the release of IL-1 and TNF. In additional studies, adding PGE2 inhibited TNF release from the hypoxia cells to values near those of room-air controls. In summary, hypoxia increases the release of the cytokines IL-1 beta and TNF-alpha. This increase may be due to decreased PGE2 synthesis during hypoxia. These results demonstrate that the response of the human alveolar macrophage to hypoxia is complex. Hypoxia increases the LPS-stimulated release of the inflammatory cytokines IL-1 and TNF, whereas synthesis of PGHS-2, which generates the anti-inflammatory prostaglandin PGE2 is decreased.