ATP release and purinergic signaling in NLRP3 inflammasome activation

Abstract

The NLRP3 inflammasome is a protein complex involved in IL-1β and IL-18 processing that senses pathogen- and danger-associated molecular patterns (PAMPs and DAMPs). One step- or two step-models have been proposed to explain the tight regulation of IL-1β production during inflammation. Moreover, cellular stimulation triggers adenosine triphosphate (ATP) release and subsequent activation of purinergic receptors at the cell surface. Importantly some studies have reported roles for extracellular ATP, in NLRP3 inflammasome activation in response to PAMPs and DAMPs. In this mini review, we will discuss the link between active ATP release, purinergic signaling and NLRP3 inflammasome activation. We will focus on the role of autocrine or paracrine ATP export in particle-induced NLRP3 inflammasome activation and discuss how particle activators are competent to induce maturation and secretion of IL-1β through a process that involves, as a first event, extracellular release of endogenous ATP through hemichannel opening, and as a second event, signaling through purinergic receptors that trigger NLRP3 inflammasome activation. Finally, we will review the evidence for ATP as a key pro-inflammatory mediator released by dying cells. In particular we will discuss how cancer cells dying via autophagy trigger ATP-dependent NLRP3 inflammasome activation in the macrophages engulfing them, eliciting an immunogenic response against tumors.

Keywords: ATP; NLRP3; P2R; autophagic cell death; danger signal; inflammasome; purinergic signaling.

FIGURE 1

Schematic diagram illustrating the specific cascade and signaling pathway in LPS-primed macrophages stimulated with MSU, silica, or alum salt crystals. LPS priming induces transcription of pro-IL-1β gene and other genes in the nucleus upon activation of the transcription factor NFkB and subsequent production of pro-IL-1β protein in the cytosol (1). Particle internalization (2), fusion to lysosome (3) and further phagolysosome destabilization may lead to cathepsin leakage (4) which precedes pannexin/connexin (Panx/Conx) hemichannel and purinergic signaling-dependent intracellular ATP release (5). Extracellular ATP may act through P2X7 receptor to amplify ATP release in a P2X7 receptor-dependent way (6). ATP, UTP, or their derived degradation products such as ADP, UDP, and adenosine, generated by ecto-endonucleases, may act through an autocrine loop on other purinergic receptor P2X, P2Y, and/or P1 receptors (7) leading to NLRP3 receptor activation (8). This allows inflammasome complex formation and maturation of pro-IL-1β to IL-1β production (9) and IL-1β secretion (10).

FIGURE 2

Schematic diagram illustrating the mechanisms of NLRP3 inflammasome activation by autophagic dying cells through ATP leakage and purinergic signaling in murine macrophages or dendritic cells. Autophagic dying cells release ATP through pannexin-1 (Panx-1) hemichannel resulting in activation of the purinergic receptor P2X7R on macrophages or dendritic cells. Phagocytosis of autophagic dying cells and K⁺ efflux are required for NLRP3 inflammasome activation in macrophages or dendritic cells. Since extracellular ATP is rapidly degraded in ADP, AMP, and adenosine, ATP metabolites could also act through other purinergic receptors and in particular, ATP and ADP could signal through P2YR. Purinergic signaling and K⁺ efflux result in inflammasome assembly and caspase-1 activation leading to maturation of pro-IL-1β in IL-1β and IL-1β secretion.