Involvement of Adenosine A2A Receptors in Engulfment-Dependent Apoptotic Cell Suppression of Inflammation

Abstract

Efficient execution of apoptotic cell death followed by efficient clearance mediated by professional macrophages is a key mechanism in maintaining tissue homeostasis. Removal of apoptotic cells usually involves three central elements: (1) attraction of phagocytes via soluble `find me' signals, (2) recognition and phagocytosis via cell surface presenting `eat me' signals, and (3) suppression or initiation of inflammatory responses depending on additional innate immune stimuli. Suppression of inflammation involves both direct inhibition of pro-inflammatory cytokine production and release of anti-inflammatory factors, which all contribute to the resolution of inflammation. In the present study, using wild type and adenosine A2A receptor (A2AR) null mice, we investigated whether A2ARs, known to mediate anti-inflammatory signals in macrophages, participate in the apoptotic cell-mediated immunosuppression. We found that macrophages engulfing apoptotic cells release adenosine in sufficient amount to trigger A2ARs, and simultaneously increase the expression of A2ARs, possibly via activation of activation of liver X receptor and peroxisome proliferators activated receptor δ. In macrophages engulfing apoptotic cells, stimulation of A2ARs suppresses the NO-dependent formation of neutrophil migration factors, such as macrophage inflammatory protein-2, using the adenylate cyclase / protein kinase A pathway. As a result, loss of A2ARs results in elevated chemoattractant secretion. This was evident as pronounced neutrophil migration upon exposure of macrophages to apoptotic cells in an in vivo peritonitis model. Altogether our data indicate that adenosine is one of the soluble mediators released by macrophages that mediate engulfment-dependent apoptotic cell suppression of inflammation.

Efferocytosis and Lung Disease

“…18 , 19 Both apoptotic and necrotic human neutrophils (but not other cell types) inhibit Mø infl ammatory response through release of a -defensins; the absence of this effect in mice, in which neutrophils lack a -defensins, is an important species difference that should be considered in extrapolating murine models. 18 The dominant Mø response to efferocytosis is antiinfl ammatory due to release of transforming growth factor-b and PGE 2 , 3 , 20 activation of Mø adenosine receptors, 21 and production by both apoptotic neutrophils and ingesting Mø of specialized proresolving lipid mediators, including resolvins and lipoxins. 22 Additionally, the inhibitory intracellular proteins SOCS1, SOCS3, and Twist, upregulated by signaling through TAM receptors, dampen Toll-like receptor signaling and reduce proinfl ammatory cytokine release.…”

Transcriptional control of transglutaminase 2 expression in mouse apoptotic thymocytes

“…Adenosine formation was detected only when these cells were incubated together (Koroskenyi et al, 2011). Thus adenosine formation could be the result of either intracellular degradation of apoptotic cell-derived nucleotides by engulfing macrophages, or that of extracellular breakdown of adenine nucleotides released by either apoptotic cells or engulfing macrophages.…”

“…It has been shown by our lab and others that various molecules, mediators are secreted by macrophages in the thymic environment during the engulfment, including transforming growth factor beta (TGF-β) (Fadok et al, 1998), adenosine (Koroskenyi et al, 2011) and retinoids . These molecules can act on macrophages in an autocrine manner, but importantly, double positive thymocytes express receptors for all the above mentioned molecules.…”

“…Since our previous studies indicated that macrophage-derived TGF-β and retinoids are involved in the regulation of TGM2 expression in dying thymocytes, we wanted to test whether adenosine, which is also produced during engulfment of apoptotic cells (Koroskenyi et al, 2011), could also affect TGM2 expression in thymocytes. Adenosine can be efficiently metabolized by adenosine deaminase of thymocytes, we applied NECA, a non-metabolisable analogue of adenosine known to act on all subtypes of adenosine receptors.…”

“…Since in our previous studies indicated that 9-cis retinoic acid (9cRA) is the most effective retinoid in inducing TGM2 at the mRNA level , 9cRA was selected to be used in this study. 9cRA and TGF-β were applied in a concentration that was shown to be effective to affect apoptosis of thymocytes (Szondy et al, 1997b;Szondy et al, 2003), while CGS21680 was administered at 1 µM concentration, because we have shown previously by using A2AR null cells that CGS21680 administered at this concentration selectively activates A2ARs (Koroskenyi et al, 2011). Exposure to TGF-β 9cRA or CGS21680 alone all increased the mRNA expression of TGM2 in cultured thymocytes, while no increase was found in the endogenous mRNA expression of TGM2 during the 6h culture period (Fig.…”