BACKGROUND: Although alveolar macrophages are considered to be the primary cellular mediators of host defence in the lung, there is increasing evidence that type II cells may also play an active role in host defence. A study was undertaken to investigate whether type II cells generate O2-. and H2O2 via an NADPH oxidase-like system and whether exposure of the type II cells to soluble or particulate stimuli known to activate NADPH oxidase in macrophages also leads to increased production of H2O2. METHODS: Rat type II cells and alveolar macrophages were exposed to 10, 100, or 1000 nM phorbol-12-myristate-13-acetate (PMA) and the production of O2-. and H2O2 was determined by chemiluminescence. Thirty minutes before stimulation with 1 microM PMA type II cells were also exposed to the same concentrations of a protein kinase C (PKC) antagonist GF109203x, the non-selective protein kinase inhibitor staurosporine (1, 10, or 100 nM), or the NADPH oxidase inhibitor diphenyliodonium chloride (DPI) (1, 10, 100, or 1000 microM). The effects of arachidonic acid, zymosan and Staphylococcus aureus on H2O2 production were determined. Cell membrane fractions from type II cells and macrophages were assayed for NADPH oxidase activity. RESULTS: After exposure to 1 microM PMA, O2-. and H2O2 generation increased 6.3-fold and 9.0-fold, respectively, in type II cells and 2.4-fold and 5.2-fold, respectively, in macrophages. In contrast to the macrophages, the increase in O2-. and H2O2 generation by type II cells was completely prevented by 1 mM KCN. Preexposure to GF109203x, staurosporine, or DPI completely prevented the rise in O2-. and H2O2 generation. Mean (SD) NADPH oxidase activity of 138 (38) nmol O2-./min/mg protein was found in membrane fraction I of the type II cells, and 102 (31) nmol O2-./min/mg protein in fraction II. Macrophages showed higher NADPH oxidase activity in membrane fraction II. In type II cells exposure to arachidonic acid led to a significant 5.3-fold increase in H2O2 generation, exposure to zymosan increased H2O2 generation 46-fold, and exposure to S aureus 25-fold with a maximum 30-50 minutes after addition of the bacteria. CONCLUSIONS: Type II cells generate O2-. and H2O2 via a PKC-mediated activation of an NAD(P)H oxidase-like membrane bound enzyme. Arachidonic acid, zymosan, and bacteria also give rise to increased H2O2 production. Type II cells might thus play an active role in host defence.
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