Identification of a third pathway for arachidonic acid mobilization and prostaglandin production in activated P388D macrophage-like cells

Abstract

Previous studies have demonstrated that P388D1 macrophages are able to mobilize arachidonic acid (AA) and synthesize prostaglandins in two temporally distinct phases. The first phase is triggered by platelet-activating factor within minutes, but needs the cells to be previously exposed to bacterial lipopolysaccharide (LPS) for periods up to 1 h. It is thus a primed immediate phase. The second, delayed phase occurs in response to LPS alone over long incubation periods spanning several hours. Strikingly, the effector enzymes involved in both of these phases are the same, namely the cytosolic group IV phospholipase A2 (cPLA2), the secretory group V phospholipase A2, and cyclooxygenase-2, although the regulatory mechanisms differ. Here we report that P388D1 macrophages mobilize AA and produce prostaglandins in response to zymosan particles in a manner that is clearly different from the two described above. Zymosan triggers an immediate AA mobilization response from the macrophages that neither involves the group v phospholipase A2 nor requires the cells to be primed by LPS. The group VI Ca2+-independent phospholipase A2 is also not involved. Zymosan appears to signal exclusively through activation of the cPLA2, which is coupled to the cyclooxygenase-2. These results define a secretory PLA2-independent pathway for AA mobilization in the P388D1 macrophages, and demonstrate that, under certain experimental settings, stimulation of the cPLA2 is sufficient to generate a prostaglandin biosynthetic response in the P388D1 macrophages.