Interleukin (IL)-1 and IL-18 are structurally similar proteins that require caspase-1 processing for activation. Both proteins are released from the cytosol by unknown pathway(s). To better characterize the release pathway(s) for IL-1 and IL-18 we evaluated the role of lipopolysaccharide priming, of interleukin-1-converting enzyme (ICE) inhibition, of human purinergic receptor (P2X 7 ) function, and of signaling pathways in human monocytes induced by ATP. Monocytes rapidly processed and released both IL-1 and IL-18 after exogenous ATP. Despite its constitutive cytosolic presence, IL-18 required lipopolysaccharide priming for the ATPinduced release. Neither IL-1 nor IL-18 release was prevented by ICE inhibition, and IL-18 release was not induced by ICE activation itself. Release of both cytokines was blocked completely by a P2X7 receptor antagonist, oxidized ATP, and partially by an antibody to P2X 7 receptor. In evaluating the signaling components involved in the ATP effect, we identified that the protein-tyrosine kinase inhibitor, AG126, produced a profound inhibition of both ICE activation as well as release of IL-1/IL-18. Taken together, these results suggest that, although synthesis of IL-1 and IL-18 differ, ATP-mediated release of both cytokines requires a priming step but not proteolytically functional caspase-1. IL-11 and IL-18 are proinflammatory cytokines that require processing by the IL-1-converting enzyme, caspase-1, at specific aspartic acid residues to generate functional molecules (1-3). Although lacking in sequence homology, IL-1 and IL-18 share significant structural homology (4, 5), bind to receptors from the IL-1 family of receptors (6), exist in the cytosol without a classic signal sequence (3, 7), and are released from cells in a noncanonical fashion (8). Both molecules are regulated posttranscriptionally, and a particularly important aspect of regulation occurs at the level of release (9 -14). In this context, recent studies show that this release pathway can be rapidly induced by stimulation with exogenous ATP (15-18). In macrophages, exogenous ATP works via the recently cloned P2X 7 receptor, a member of the P2X family of nucleotide-gated channels that are activated by extracellular ATP (19). However, it has been suggested that monocytes do not express P2X 7 receptors (20, 21). P2X 7 is a 595-amino acid polypeptide with two membrane-spanning domains and intracellular N-and C-terminal domains (22,23). P2X 7 receptor is the only pore-forming P2X family member, and its activation results in the opening of a cationic channel with increased permeability to calcium and intracellular depolarization (20). The present work focuses on release mechanisms by taking advantage of the dramatic ability of ATP to telescope the processing and release down to a 15-30-min interval while enhancing the overall release signal (15).In this report we examine the mechanism of ATP-induced release of IL-18 and compare it to IL-1 in human monocytes. We show that fresh monocytes express functional P2X 7 rece...
Macrophages and their precursors, monocytes, are key cells involved in the innate immune response. Although both monocytes and macrophages produce caspase-1, the key enzyme responsible for pro-IL-1β processing; macrophages are limited in their ability to activate the enzyme and release functional IL-1β. In this context, because mutations in the pyrin gene (MEFV) cause the inflammatory disorder familial Mediterranean fever, pyrin is believed to regulate IL-1β processing. To determine whether variations in pyrin expression explain the difference between monocytes and macrophages in IL-1β processing and release, pyrin was studied in human monocytes and monocyte-derived macrophages. Although monocytes express pyrin mRNA and protein, which is readily inducible by endotoxin, monocyte-derived macrophages express significantly less pyrin mRNA and protein. Pyrin levels directly correlated with IL-1β processing in monocytes and macrophages; therefore, we asked whether pyrin might promote IL-1β processing and release. HEK293 cells were transfected with pyrin, caspase-1, apoptotic speck protein with a caspase recruitment domain, and IL-1β. Pyrin induced IL-1β processing and release in a dose-dependent manner. Conversely, pyrin small interference RNA suppressed pro-IL-1β processing in both THP-1 cells and fresh human monocytes. In summary, both pyrin expression and IL-1β processing and release are diminished upon the maturation of monocytes to macrophages. When pyrin is ectopically expressed or silenced, IL-1β processing and release parallels the level of pyrin. In conclusion, in the context of endotoxin-induced activation of mononuclear phagocytes, pyrin augments IL-1β processing and release.
These investigations are the first to demonstrate a protective effect of cyclosporin A against mitochondrial injury in a representative systemic organ during acute endotoxemia. We propose that mitochondrial injury likely related to induction of the mitochondrial permeability transition may participate in the pathogenesis of systemic organ injury and organ failures during acute sepsis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.