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Arachidonic Acid Release Is Closely Related to the Fcγ Receptor‐Mediated Superoxide Generation in Macrophages

Stimulation of macrophages with IgG2 immune complexes induced dose‐dependently the O2− generation and the release of arachidonic acid and its metabolites. This FcγR‐mediated O2− generation was inhibited by a phospholipase A2 inhibitor, 4‐p‐bromophenacyl bromide (4‐pBPB), in parallel to the dose‐dependent inhibition of arachidonic acid release. The main arachidonic acid metabolites released were shown to be prostaglandin E2 and thromboxane B2 and blocking of the production of these metabolites by indomethacin did not inhibit the O2− generation. Inhibition of the FcγR‐mediated O2− generation and the arachidonic acid release by the C‐kinase inhibitor, 1‐(5‐isoquinolinesulfonyl)‐2‐methylpiperazine (H‐7), was less intense than by 4‐pBPB. These results support the previously proposed hypothesis that arachidonic acid acts as an intracellular activator of the FcγR‐mediated O2− generation in macrophages. Although the C‐kinase activation may also contribute to the activation of the O2−‐generating system, arachidonic acid release appears to play a major role in FcγR‐mediated O2− generation. In contrast, activation of C‐kinase seems to be contributing mainly in the induction of both the arachidonic acid release and O2− generation by 12‐o‐tetradecanoylphorbol 13‐acetate (TPA). Furthermore, suboptimal concentrations of TPA and arachidonate were found to act synergistically to stimulate O2− generation and the inhibition study suggested a positive synergism between C‐kinase and arachidonic acid release to induce O2− generation. This synergistic action may have general importance in receptor‐mediated O2− generation.