Production of leukotrienes and thromboxane by resident and activated rat alveolar macrophages: A possible role of protein kinase C

Arachidonic acid metabolism in resident rat alveolar macrophages and in those activated with complete Freund's adjuvant (CFA) was studied. Adult Sprague-Dawley rats were injected with 0.05 ml CFA, and macrophages were harvested 10 days later. Macrophages were labeled overnight with carbon 14-labeled arachidonic acid, washed, and then stimulated with calcium ionophore A23187 (IoA), phorbol myristate acetate (PMA), or zymosan for 30 minutes. Prostaglandins, thromboxane, and leukotrienes were extracted from the medium and analyzed by radioimmunoassay or radio high-pressure liquid chromatography. Cell lipids were analyzed by radio thin-layer chromatography. Medium and cell β-glucuronidase activity and protein kinase C activity of the membrane fraction were also assayed. We found (1) lower leukotriene B₄ (LTB₄) production in stimulated resident macrophages when compared with resident macrophages after IoA stimulation-the suppressed LTB₄ production was reversed by PMA; (2) unchanged or higher LTB₄ production in activated macrophages when compared with resident macrophages after zymosan stimulation; (3) inhibition of zymosan-stimulated LTB₄ production by staurosporine, a protein kinase C inhibitor, in both groups; and (4) lower diacylglycerol (DAG) production in activated macrophages when compared with resident macrophages after IoA stimulation, but not after zymosan stimulation. These results suggest that the reduced response of activated macrophages to IoA is due to decreased production of an endogenous protein kinase C activator. This hypothesis was further supported by the observation that protein kinase C activation in response to IoA was lower in activated macrophages than in resident macrophages. In contrast, zymosan stimulation resulted in higher protein kinase C activation in activated macrophages when compared with resident cells. We hypothesize that protein kinase activation is necessary for leukotriene production and that the preserved ability of zymosan to activate PKC via DAG accounts for the high leukotriene production in zymosan-activated macrophages. We also found that stimulated thromboxane production was higher in activated than resident cells, regardless of the stimulus, and that thromboxane production was not affected by staurosporine. Thus alterations of eicosanoid metabolism in immunologically activated macrophages depend on the stimulus used and the type of eicosanoid examined. Furthermore, leukotriene biosynthesis in rat alveolar macrophages may be regulated by protein kinase C.