Cutting Edge: Macrophage Inhibition by Cyclic AMP (cAMP): Differential Roles of Protein Kinase A and Exchange Protein Directly Activated by cAMP-1

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Abstract
CAMP has largely inhibitory effects on components of macrophage activation, yet downstream mechanisms involved in these effects remain incompletely defined. Elevation of cAMP in alveolar macrophages (AMs) suppresses FcγR-mediated phagocytosis. We now report that protein kinase A (PKA) inhibitors (H-89, KT-5720, and myristoylated PKA inhibitory peptide 14–22) failed to prevent this suppression in rat AMs. We identified the expression of the alternative cAMP target, exchange protein directly activated by cAMP-1 (Epac-1), in human and rat AMs. Using cAMP analogs that are highly specific for PKA (N6-benzoyladenosine-3′,5′-cAMP) or Epac-1 (8-(4-chlorophenylthio)-2′-O-methyladenosine-3′,5′-cAMP), we found that activation of Epac-1, but not PKA, dose-dependently suppressed phagocytosis. By contrast, activation of PKA, but not Epac-1, suppressed AM production of leukotriene B4 and TNF-α, whereas stimulation of either PKA or Epac-1 inhibited AM bactericidal activity and H2O2 production. These experiments now identify Epac-1 in primary macrophages, and define differential roles of Epac-1 vs PKA in the inhibitory effects of cAMP.