Short Communication: Differences Between Macrophages and Dendritic Cells in the Cyclic AMP-Dependent Regulation of Lipopolysaccharide-Induced Cytokine and Chemokine Synthesis

Abstract

Cyclic adenosine monophosphate (cAMP) is an intracellular signaling molecule responsible for directing cellular responses to extracellular signals. Once believed to signal exclusively through its ability to bind protein kinase A (PKA), recent research has revealed alternative cAMP-binding targets involved in PKA-independent processes. In this study we addressed the hypothesis that the guanine nucleotide exchange protein directly activated by cAMP (Epac-1) and PKA differentially regulate inflammatory mediator production in distinct phagocytic cell types. To accomplish this, we compared the release of cAMP-regulated polypeptide inflammatory mediators in both macrophages (obtained from the lung and peritoneum) and bone marrow-derived dendritic cells (DCs) stimulated with bacterial endotoxin. Using the highly selective Epac-1 and PKA activating cAMP analogs 8-pCPT-2 -O-Me-cAMP and 6-Bnz-cAMP, respectively, we found that macrophages differ from DCs in the involvement of these distinct cAMP pathways in modulating inflammatory mediator release in response to endotoxin. Whereas the regulation of cytokine and chemokine production in macrophages by cAMP was solely dependent on PKA, we found that both Epac-1 and PKA activation could regulate mediator production in DCs. This finding may be important in the pharmacologic regulation of immune responses through manipulation of cAMP signaling cascades and contributes to our understanding of the differences between these cell types.

Synthetic Prostacyclin Analogs Differentially Regulate Macrophage Function via Distinct Analog-Receptor Binding Specificities

“…First, IL-6 is well recognized to be regulated by cAMP and, in particular, both PGE 2 and PGI 2 have been shown to modulate its synthesis during LPS stimulation (34,35). Second, in the rat model both AM and PM IL-6 synthesis are profoundly regulated by PGE 2 under conditions of LPS treatment (13). Lastly, as discussed below, IL-6 production by human AMs has been shown to be regulated by treprostinil during LPS exposure (29).…”

“…Our treatments were brief (minutes) in the studies of cAMP elevation, FcR-mediated phagocytosis, and bacterial killing, whereas PPAR␤ is a transcription factor whose effects would be expected to require a number of hours. Though it remains possible that the effect of treprostinil on cytokine synthesis involved the PPAR␤ system because those experiments were conducted in the presence of treprostinil for Ͼ16 h, we have previously shown that cAMP signaling mechanisms are sufficient to explain such results (13). The finding that treprostinil is more promiscuous in binding G protein-coupled receptors than previously thought warns us against assuming that the pharmacological effects of this drug in vivo result from IP-mediated signaling exclusively.…”

“…Acute increases in cAMP have profound anti-inflammatory effects on a number of different cell types involved in both innate and acquired immunity (10). Our laboratory has studied the effects of cAMP-elevating compounds on the innate immune system, particularly in the lung (11)(12)(13). Recent work demonstrated that PGE 2 suppressed critical antimicrobial defense functions of alveolar macrophages (AMs), including phagocytosis, bacterial killing, and inflammatory mediator production, all in a cAMP-dependent manner, mediated via the G ␣s -coupled EP2 and EP4 receptors (11,12).…”

“…Cells were treated according to a previously published protocol (13), including a 60-min preincubation (37°C with 5% CO 2 ) with compounds of interest in serum-free medium followed by incubation (37°C with 5% CO 2 ) for an additional 16 h in the presence of LPS 100 ng/ml (or vehicle) and 1% FCS. After this time, cell-free supernatants were collected and analyzed using commercially available ELISA kits for IL-6 (Assay Designs) or TNF-␣ (R&D Systems) per the manufacturer's instructions.…”

Effects of prostaglandin E2 on the subcellular localization of Epac-1 and Rap1 proteins during Fcγ-receptor-mediated phagocytosis in alveolar macrophages

“…Such metabolites, known as eicosanoids, are generated in abundance at sites of inflammation (e.g. the host-microbial interface) and influence key components of the innate immune system, namely, phagocytosis [2,3], intracellular microbial killing [4,5], and inflammatory mediator generation [6].…”