Multiple pathways for signal transduction in the regulation of arachidonic acid metabolism in rat peritoneal macrophages

Abstract

The roles of guanine nucleotide-binding proteins (G-proteins) and cyclic AMP (cAMP) in signal-transduction of inflammatory stimuli leading to arachidonic acid metabolism in resident rat peritoneal macrophages (RPM) were investigated. Opsonized zymosan, targeting multiple receptors and latex particles coated with IgG (latex-IgG), targeting Fc receptors, were used as models of in vivo inflammatory stimuli encountered by macrophages. A comparison of the patterns of eicosanoid products produced in response to these stimuli showed differences: opsonized zymosan stimulated production of more leukotriene B 4 (LTB 4) than prostaglandin E 2 (PGE 2), while latex-IgG stimulated production of more PGE 2 than LTB 4. Non-selective stimulation of G-proteins by GTP and non-hydrolyzable analogs of GTP also stimulated arachidonic acid metabolism; these agents were not selective for PGE 2 or LTB 4 production. Cholera toxin, however, selectively stimulated production of PGE 2 rather than LTB 4 and also increased intracellular cAMP concentrations. The increased cAMP did not appear to mediate cholera toxin stimulation since forskolin, which also increased cAMP, was inhibitory to PGE 2 production. This suggests that latex-IgG and cholera toxin may activate arachidonic acid metabolism through a G-protein other than Gs to induce PGE 2 production specifically. The effects of pertussis toxin were biphasic: a partial inhibitory effect was observed at a low concentration of pertussis toxin (1 ng/ml) on opsonized zymosan or latrix-IgG stimulated arachidonic acid metabolism, while a high concentration of pertussis toxin (100 ng/ml) augmented the stimuli. A pertussis toxin-sensitive G-protein, possibly Gi, may therefore mediate a portion of the stimulatory signals. We have concluded that multiple pathways probably exist for opsonized zymosan and latex-IgG stimulation of arachidonic acid metabolism potentially involving multiple G-proteins.