Prostaglandin E2 Inhibits Alveolar Macrophage Phagocytosis through an E-Prostanoid 2 Receptor-Mediated Increase in Intracellular Cyclic AMP

Abstract

Prostaglandin E2 is a potent lipid mediator of inflammation that effects changes in cell functions through ligation of four distinct G protein-coupled receptors (E-prostanoid (EP)1, EP2, EP3, and EP4). During pneumonia, PGE2 production is enhanced. In the present study, we sought to assess the effect of endogenously produced and exogenously added PGE2 on FcRγ-mediated phagocytosis of bacterial pathogens by alveolar macrophages (AMs), which are critical participants in lung innate immunity. We also sought to characterize the EP receptor signaling pathways responsible for these effects. PGE2 (1–1000 nM) dose-dependently suppressed the phagocytosis by rat AMs of IgG-opsonized erythrocytes, immune serum-opsonized Klebsiella pneumoniae, and IgG-opsonized Escherichia coli. Conversely, phagocytosis was stimulated by pretreatment with the cyclooxygenase inhibitor indomethacin. PGE2 suppression of phagocytosis was associated with enhanced intracellular cAMP production. Experiments using both forskolin (adenylate cyclase activator) and rolipram (phosphodiesterase IV inhibitor) confirmed the inhibitory effect of cAMP stimulation. Immunoblot analysis of rat AMs identified expression of only EP2 and EP3 receptors. The selective EP2 agonist butaprost, but neither the EP1/EP3 agonist sulprostone nor the EP4-selective agonist ONO-AE1-329, mimicked the effects of PGE2 on phagocytosis and cAMP stimulation. Additionally, the EP2 antagonist AH-6809 abrogated the inhibitory effects of both PGE2 and butaprost. We confirmed the specificity of our results by showing that AMs from EP2-deficient mice were resistant to the inhibitory effects of PGE2. Our data support a negative regulatory role for PGE2 on the antimicrobial activity of AMs, which has important implications for future efforts to prevent and treat bacterial pneumonia.

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

“…6). Consistent with our previously published results [2], pretreatment of AMs with 1 μM PGE 2 for 5 min produced a significant decrease in the number of captured microbeads/phagosomes (data not shown). However, when equal numbers of untreated and PGE 2 -treated microbead/phagosomes were analyzed, flotillin increased similarly in both conditions, suggesting that PGE 2 treatment did not alter the rate of phagosome maturation.…”

“…5A, B). Peripheral phagosomes that formed on PGE 2 -pretreated AMs were clearly delimited by Epac-1-containing membrane, as was seen in AMs not receiving PGE 2 (Figs. 1, 2).…”

“…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].…”

“…EP1 receptor activation provokes G q -coupled increases in intracellular Ca 2+ , EP2 and EP4 receptors signal predominantly through G s , increasing cAMP, and the EP3 receptor most commonly reduces cAMP via G i coupling after PGE 2 ligation [7]. It is through its ability to stimulate the production of intracellular cAMP (via EP2 and/or EP4) that PGE 2 impairs FcγR-mediated phagocytosis [2]. And though cAMP is a well known counterregulator of FcγR phagocytosis [2,[8][9][10], the precise mechanisms downstream of cAMP remain to be fully defined.…”

“…It is through its ability to stimulate the production of intracellular cAMP (via EP2 and/or EP4) that PGE 2 impairs FcγR-mediated phagocytosis [2]. And though cAMP is a well known counterregulator of FcγR phagocytosis [2,[8][9][10], the precise mechanisms downstream of cAMP remain to be fully defined.…”