CYCLIC-AMP ANALOG AS A TRIGGERING SIGNAL FOR THE INDUCTION OF NITRIC-OXIDE SYNTHESIS IN MURINE PERITONEAL-MACROPHAGES

To understand the role of cAMP during macrophage activation, we invest igated the effects of various cAMP analogues in the induction of nitri c oxide (NO) synthesis in murine peritoneal macrophages. Intracellular cAMP modulators such as N-6,O-2'-dibutyryl cyclic AMP (DB-cAMP), 8-br omo-cAMP, or 8-chloro-cAMP had no effect on NO synthesis by themselves , whereas cAMP analogues in combination with interferon-gamma (IFN-gam ma had a marked cooperative induction of NO synthesis in a dose-depend ent manner. This increase in NO synthesis was reflected as an increase d amount of inducible NO synthase mRNA, as determined by Northern blot ting. To find the point in the signaling pathways of macrophage activa tion at which cAMP is involved, we carried out several of the followin g experiments. Although DB-cAMP showed synergistic action with rIFN-ga mma to induce NO synthesis when the cells were treated with DB-cAMP af ter or with simultaneous treatment with rIFN-gamma, there is no synerg istic induction of NO synthesis when the cells were treated with DB-cA MP 6 hr before treatment with rIFN-gamma, In addition, when phorbol la -myristate 13-acetate (PMA), which is known to provide a triggering si gnal in the induction of NO synthesis in murine macrophages, was added to the cells 6 hr after the treatment with DB cAMP, PMA showed no syn ergistic cooperation with DB-cAMP. On the other hand, DB-cAMP alone in duced the release of NO to the incubation medium from bacillus Calmett e-Guerin-infected peritoneal macrophages just as lipopolysaccharide (L PS) did. However, DB-cAMP, unlike LPS, decreased the secretion of tumo r necrosis factor-alpha from IFN-gamma-treated macrophages. Based on t he results obtained in this study, we suggest that cAMP analogue could give a ''triggering'' signal which might be different from one given by LPS in the production of NO by primed macrophages. (C) 1997 Academi c Press.