Calcium- and G-protein-dependent activation of arachidonic acid release by concanavalin-A-stimulated mouse macrophages

Abstract

In this work, signaling mechanisms put into function by concanavalin A in macrophages and its relationship to arachidonic acid release were investigated. After a lag period of approx. 3 min, concanavalin A induced the release of arachidonic acid from macrophages in a time- and dose-dependent manner. Removal of calcium from the extracellular medium led to a strong inhibition of the response. However, down-regulation of protein kinase C by prolonged treatment of macrophages with phorbol myristate acetate did not affect concanavalin-A-induced arachidonic acid release, suggesting that protein kinase C does not mediate the concanavalin A response. The role of G proteins in mediating the concanavalin A response was also investigated. Concanavalin-A-stimulated arachidonic acid release was inhibited by treatment with pertussis toxin but was enhanced by preincubation with cholera toxin. An increase of cAMP did not appear to mediate the stimulatory effect of cholera toxin since non-hydrolyzable cAMP derivatives or agents which raise cAMP levels, such as prostaglandin E 2 and forskolin, were without effect on Con-A-stimulated arachidonate release. The direct G-protein activator fluoroaluminate was able to stimulate arachidonic acid release in a Ca 2+-dependent manner. Combined treatment with fluoroaluminate and concanavalin A resulted in a greater than additive effect on arachidonic acid release. Altogether, these results suggest that concanavalin-A-induced arachidonic acid release in macrophages is co-ordinately regulated by Ca 2+ and G proteins, but not by protein kinase C.