Targeting Inflammatory Pathways in Cardiovascular Disease: The Inflammasome, Interleukin-1, Interleukin-6 and Beyond

Abstract

Recent clinical trials have now firmly established that inflammation participates causally in human atherosclerosis. These observations point the way toward novel treatments that add to established therapies to help stem the growing global epidemic of cardiovascular disease. Fortunately, we now have a number of actionable targets whose clinical exploration will help achieve the goal of optimizing beneficial effects while avoiding undue interference with host defenses or other unwanted actions. This review aims to furnish the foundation for this quest by critical evaluation of the current state of anti-inflammatory interventions within close reach of clinical application, with a primary focus on innate immunity. In particular, this paper highlights the pathway from the inflammasome, through interleukin (IL)-1 to IL-6 supported by a promising body of pre-clinical, clinical, and human genetic data. This paper also considers the use of biomarkers to guide allocation of anti-inflammatory therapies as a step toward realizing the promise of precision medicine. The validation of decades of experimental work and association studies in humans by recent clinical investigations provides a strong impetus for further efforts to target inflammation in atherosclerosis to address the considerable risk that remains despite current therapies.

Keywords: acute coronary syndromes; cytokines; innate immunity; ischemic heart disease; myocardial infarction; thrombosis; vascular biology.

Figure 1

Vascular cells can produce IL-1. (A). This Northern blot shows induction of the messenger RNA encoding interleukin-1β by Gram-negative bacterial endotoxin. The top panel shows IL-1β transcript. The bottom panel shows β-Tubulin expression in a re-hybridization of the same blot. The left-hand panel (B) shows interleukin-1 activity as measured by thymocyte co-stimulation. The supernatants of the cultures of the cells probed for messenger RNA in the left-hand panel were analyzed for biological activity showing the induction of release of activity with incubation with endotoxin. Addition of polymyxin B, an LPS inhibitor, to the medium during exposure to LPS blunted the rise in thymocyte co-stimulatory activity. (open circles). From Am J Path. Libby et al. 1986, 124:179-185.

Figure 2

Cytokines can act through autocrine, paracrine, juxtacrine, or endocrine pathways. Cytokine signaling can operate in different ways. Autocrine signaling, shown on the upper left, can involve IL-1-induced IL-1 expression. Paracrine signaling denotes exchange of cytokines such as IL-1 between neighboring cells as exemplified here by endothelial cell and smooth muscle cell crosstalk. Juxtacrine signaling involves cell contact. Two examples pertinent to cytokine signaling in vascular pathophysiology include IL-1α expressed on the surface of smooth muscle cells that are activated or are undergoing death. IL-1α can signal to macrophages by contact. IL-1α also associates with neutrophil extracellular traps (NETs) and can activate endothelial cell pro-inflammatory functions such as adhesion molecule expression and tissue factor generation in a manner that requires contact. Finally, the circulation can carry secreted cytokines to distant organs, denoted endocrine signaling. For example, IL-6 secreted by vascular cells as well as leukocytes can circulate to encounter hepatocytes and trigger the acute phase response as shown in Figure 3.

Figure 3

Amplification loops in cytokine signaling. IL-1 induces its own gene expression, auto-induction as shown on the left-hand of this figure. IL-1 can in turn induce IL-6 from many cell types, an amplification loop. IL-6 triggers the acute phase response in hepatocytes which augments fibrinogen, the precursor of clots, and plasminogen activator inhibitor-1 (PAI-1), proteins that favor clot formation and resist fibrinolysis. The acute phase reactants C-reactive protein (CRP) and serum amyloid A (SAA) can serve as biomarkers of the inflammatory response. The inflammasome can process the inactive forms of IL-1β and IL-18 to their biologically mature forms instigating this inflammatory cytokine cascade. This series of amplification steps can occur locally in a chronic disease such as atherosclerosis, more acutely locally in many diseases including the vasculitides. The positive feedback loop can lead to cytokine storm associated with sepsis or acute viral infections such as SARS-CoV-2.

Figure 4

Autoinduction of IL-1 amplifies inflammatory signaling. IL-1 induces IL-1 gene expression. Either recombinant IL-1β (top) or recombinant IL-1α (bottom) induced the messenger RNA encoding IL-1β. Messenger RNA as shown by Northern blotting. Either isoform of IL-1 induced the biological activity as well (not shown.) The concentration of IL-1β was 100 ng/mL and IL-1α was 10 ng/mL. From J Exp Med 165:1316-1331; 1987.

Figure 5

IL-1 induces IL-6, a further amplification of cytokine signaling. This Northern blot shows the expression of the messenger RNA encoding interleukin-6 in human smooth muscle cells incubated with interleukin-1 isoforms, bacterial lipopolysaccharide (LPS), platelet-derived growth factor (PDGF) or IL-1 itself. IL-1 strongly induces IL-6 messenger RNA and in other experiments not shown induced IL-6 protein synthesis as shown by metabolic labeling and biological activity as determined by thymidine incorporation by B9 cells. From J Clin Invest 85:731-738, 1990.

Figure 6

Inflammasomes operate in human atherosclerosis. Human atherosclerotic plaques express the messenger RNA encoding caspase-1 or interleukin-1β converting enzyme (ICE). Lane 1 shows an RT-PCR reaction without template. Lanes 2–5 represent RNA from extracts of human atherosclerotic plaques. Lanes 6–9 depict analysis of extracts of a normal aorta as control. Lane 10 shows size markers. The protein product was also demonstrated in the paper cited [22]. From Geng and Libby. Am J Path 147:251-266; 1995.

Figure 7

IL-1α activates smooth muscle cells via juxtacrine signaling. Smooth muscle cell monolayers were lightly fixed with paraformaldehyde after stimulation with IL-1β or tumor necrosis factor. A responder layer of smooth muscle cells was seeded above the fixed smooth muscle cells. Thymidine incorporation into D10S cells assessed IL-1 activity. The responder cells also elaborated IL-6 in other experiments not shown. From Exp Cell Res 198:283-290;1992.

Figure 8

IL-6 signaling is complex and subject to multiple levels of regulation. As described in the text, classical IL-6 signaling involves binding of the ligand to the membrane bound IL-6 receptor (CD 126.) Hepatocytes and leukocytes express this receptor, which associates with gp130 to initiate transmembrane signaling. However, the surface IL-6 receptor can also undergo limited proteolytic cleavage by ADAM 17, releasing a soluble form of this receptor. This soluble IL-6 receptor can bind to the IL-6 and form a binary complex. This binary complex can bind to gp130, which is expressed ubiquitously, effecting trans IL 6 signaling to multiple cell types. A soluble form of gp130 can combine with the binary complex, forming a ternary complex that sequesters IL-6 bound to its receptor, providing a buffer for trans IL-6 signaling. A common variant in the IL-6 receptor can promote its shedding, depleting the hepatocyte or leukocyte surface of this receptor (CD126), limiting classical signaling, but increasing formation of the binary complex by mass action, and thus favoring trans IL-6 signaling.