EXPRESSION OF THE TYPE-II PHOSPHOLIPASE A(2) IN ALVEOLAR MACROPHAGES - DOWN-REGULATION BY AN INFLAMMATORY SIGNAL

We have shown previously that guinea pig alveolar macrophages (AM) synthesize a secretory phospholipase A(2) (PLA(2)) during in vitro incubation. Here, we report the molecular cloning of this enzyme and show that it has structural features closely related to all known mammalian type-II PLA(2). The mRNA and PLA(2) activity were undetectable in freshly collected AM, but their levels increased dramatically to reach maximal values after 16 h of culture. Thereafter, the PLA(2) activity remained constant with a parallel secretion in the medium, in contrast to mRNA level which returned to near basal values after 32 h. Incubation of AM for 16 h with the inflammatory secretagogue peptide f-Met-Leu-Phe (fMLP) markedly reduced the PLA(2) activity and mRNA levels. This inhibition was prevented by preexposure of AM to pertussis toxin, an inhibitor of G-protein. In contrast, when AM were first cultured for 16 h and then incubated with fMLP, no significant change was observed in their PLA(2) activity. In conditions where the type-II PLA(2) was completely abrogated by fMLP, the latter did not alter the lipopolysaccharide-induced accumulation of tumor necrosis factor ru mRNA or the release of arachidonic acid induced by the subsequent addition of the calcium ionophore A23187. These studies show that the inflammatory peptide fMLP down-regulates the expression of the type-II PLA(2) by AM through a process mediated by G-protein. A possible negative control of the type-II PLA(2) expression during AM activation is suggested.