Background: Previous work has suggested that an increase in expression of cyclooxygenase-2, concomitant formation of E-type prostanoids, and in turn intracellular cAMP conveys macrophage resistance against apoptosis.
Materials and methods: We analyzed the effects of lipophilic cAMP analogs on nitric oxide (NO)-induced apoptosis in RAW 264.7 macrophages and human primary monocyte-derived macrophages. Parameters comprised DNA fragmentation (diphenylamine assay), annexin V staining of phosphatidylserine, caspase activity (quantitated by the cleavage of a fluorogenic caspase-3-like substrate Ac-DEVD-AMC), and mitochondrial membrane depolarization (DeltaPsi), analyzed using DiOC(6)(3). Western blots detected accumulation of the tumor suppressor protein p53, relocation of cytochrome c, and expression of the antiapoptotic protein Bcl-X(L). A cAMP response-element decoy approach confirmed cAMP-dependent gene induction.
Results: We verified resistance of murine and human macrophages against NO donors such as S-nitrosoglutathione or spermine-NO by pre-exposing cells to lipophilic cAMP analogs or by pretreatment with lipopolysaccaride, interferon-gamma, and N(G)-nitroarginine-methylester for 15 hr. Cellular prestimulation decreased NO-evoked apoptosis, as apoptotic parameters were basically absent. Macrophage protection was not achieved during a short period of preexposure, i.e., 1 hr. To verify gene induction as the underlying protective principle, we treated RAW cells with oligonucleotides containing a cAMP-responsive element in order to scavenge cAMP response element-binding protein prior to its promoter-activating ability. Decoy oligonucleotides, but not an unrelated control oligonucleotide, weakened cAMP-evoked protection and re-established a p53 response following NO addition.
Conclusion: Gene induction by cAMP protects macrophages against apoptosis that occurs as a result of excessive NO formation. Decreasing programmed cell death of macrophages may perpetuate inflammatory conditions in humans when macrophages become activated in close association with innate immune responses.