The Role of Interleukin-18 in the Metabolic Syndrome Marius Trøseid1*, Ingebjørg Seljeflot1,2, Harald Arnesen1,2
Total Page:16
File Type:pdf, Size:1020Kb
View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Springer - Publisher Connector Trøseid et al. Cardiovascular Diabetology 2010, 9:11 CARDIO http://www.cardiab.com/content/9/1/11 VASCULAR DIABETOLOGY REVIEW Open Access The role of interleukin-18 in the metabolic syndrome Marius Trøseid1*, Ingebjørg Seljeflot1,2, Harald Arnesen1,2 Abstract The metabolic syndrome is thought to be associated with a chronic low-grade inflammation, and a growing body of evidence suggests that interleukin-18 (IL-18) might be closely related to the metabolic syndrome and its conse- quences. Circulating levels of IL-18 have been reported to be elevated in subjects with the metabolic syndrome, to be closely associated with the components of the syndrome, to predict cardiovascular events and mortality in populations with the metabolic syndrome and to precede the development of type 2 diabetes. IL-18 is found in the unstable atherosclerotic plaque, in adipose tissue and in muscle tissue, and is subject to several regulatory steps including cleavage by caspase-1, inactivation by IL-18 binding protein and the influence of other cytokines in modulating its interaction with the IL-18 receptor. The purpose of this review is to outline the role of IL-18 in the metabolic syndrome, with particular emphasis on cardiovascular risk and the potential effect of life style interventions. Introduction developing CVD is approximately doubled in the meta- The metabolic syndrome is a cluster of risk factors that bolic syndrome [10]. In a meta-analysis including identifies a population with increased risk for developing 43 cohorts, the relative risk for cardiovascular events type 2 diabetes mellitus and cardiovascular disease and death was 1.78, with the highest risk in women [11]. (CVD). The syndrome has received increased attention There is increasing evidence that the metabolic syn- after practical definitions by the Adult Treatment Panel drome is associated with a chronic, low-grade inflamma- III and International Diabetes Federation [1,2]. The tion [2]. Several pro-inflammatory cytokines have been diagnostic criteria vary slightly, in particular concerning shown to be elevated in parallel with an increasing the meausure for central obesity, but in a recent state- number of components of the syndrome, whereas the ment it is agreed that the presence of any three of the anti-inflammatory and adipocyte-specific substance adi- following five components is diagnostic for the meta- ponectin is consistently lower [12-15]. Some investiga- bolic syndrome: Central obesity, elevated blood pressure, tors have discussed that both type 2 diabetes, metabolic raised fasting glucose, raised levels of trigycerides and syndrome and atherosclerosis are multifactorial condi- low levels of high-density lipoprotein cholesterol [3]. tions which appear to have a common inflammatory More than 25% of the US population can be classified basis [16], and it is currently discussed if a measure of as having the metabolic syndrome, and the prevalence is inflammation should be included in the definition of the increasing with age, affecting >40% of US adults above syndrome [2,17]. So far, CRP has been the most likely the age of 60 years [4-6]. candidate [17,18]. The metabolic syndrome is a strong predictor of type However, a growing body of evidence suggests that IL- 2 diabetes, with an increased incidence rate of 5 to 18 is closely associated with the metabolic syndrome and 7-fold [7,8]. Indeed, the increased cardiovascular risk its consequences. The purpose of this review is to outline might develop as a continuum in parallel with increasing the role of IL-18 in the metabolic syndrome, with parti- fasting glucose, from the normal range via impaired fast- cular emphasis on CVD risk and life style interventions. ing glucose to overt diabetes mellitus [9]. The risk of Interleukin-18: a unique proinflammatory cytokine * Correspondence: [email protected] IL-18 is a member of the IL-1 family of cytokines and 1Center for Clinical Heart Research, Department of Cardiology, Oslo University Hospital, Ullevål, Oslo, Norway was originally described as an interferon gamma (IFN-g) © 2010 Trøseid et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Trøseid et al. Cardiovascular Diabetology 2010, 9:11 Page 2 of 8 http://www.cardiab.com/content/9/1/11 inducing factor [19]. The cytokine is produced constitu- levels of IL-18, impaired insulin sensitivity and increased tively in many different cell types, including macro- risk of having the metabolic syndrome [45], suggesting phages, endothelial cells, vascular smooth muscle cells, that IL-18 might be involved in the pathogenesis of the dendritic cells and Kupffer cells [20-22]. IL-18 is also syndrome. produced in adipocytes [23], but non-adipocyte cells Circulating levels of IL-18 have consistently been have been identified as the main source of IL-18 in adi- reported to be elevated in patients with type 2 diabetes pose tissue [24]. In contrast to most other cytokines, mellitus in cross sectional studies [40,46,47], and have but in a similar way to IL-1b, IL-18 is expressed as a also been suggested to contribute to microangiopathy precursor, pro-IL18, which is inactive until cleaved by such as nephropathy in type 2 diabetes [48]. Moreover, the enzyme caspase-1 [25]. Notably, caspase-1 itself in two prospective cohorts, elevated levels of IL-18 have exists as an inactive precursor which requires the assem- been shown to predict the development of type 2 dia- bly of multi-unit complexes, known as inflammasomes, betes (Table 1) [49,50]. However, a major limitation of to be activated [26,27]. both studies is that fasting glucose or oral glucose toler- Once secreted, IL-18 is bound and inactivated by IL- ance tests were not performed at baseline evaluation: 18 binding protein, which is enhanced as a negative Elevated fasting glucose increases the risk of diabetes, feedback mechanism in response to increased IL-18 pro- and experimental hyperglycemia might increase circulat- duction, ensuring protection from tissue damage due to ing levels of IL-18 [51]. uncontrolled proinflammatory activity [28,29]. IL-18 binds to its receptor, consisting of an a chain which is Interleukin-18 and cardiovascular disease responsible for extracellular binding of IL-18, and a b Studies regarding assocations between IL-18 and stable chain which is responsible for intracellular signal trans- atherosclerosis have yielded conflicting results. One duction [25,30,31]. Although both free and protein- study showed that elevated levels of IL-18 were asso- bound IL-18 may bind to the a chain, only the free frac- ciated with the presence of subclinical atherosclerosis tion is able to activate the b chain [28,29]. evaluated with intima media thickness of the carotid IL-18 is a potent proinflammatory cytokine which artery, also after adjustment for traditional risk factors, enhances T cell and natural killer cell maturation, as CRP and IL-6 [52]. On the other hand, in two large stu- well as the production of cytokines, chemokines and cell dies elevated levels of IL-18 were associated with carotid adhesion molecules [32,33]. Of note, in CD4+ lympho- intima media thickness in univariate analyses, but not cytes IL-18 can stimulate both type 1 helper T (Th1) after adjustment for traditional risk factors [38,53]. and Th2 responses depending on its cytokine milieu: IL- Furthermore, in a study of patients with type 2 diabetes, 18 may stimulate a Th2 response in combination with both carotid intima media thickness and brachial-ankle IL-2, and may act synergistically with IL-12 to stimulate pulse wave velocity were significantly associated with a Th1 response with production of IFN-g [25], a central serum levels of IL-18, although not in multivariate ana- feature in the atherosclerotic lesion (Figure 1). One lyses [54]. However, we recently showed that arterial mechanism underlying this synergistic effect is that IL- stiffness measured by brachial pulse wave propagation 12 may induce the a chain of the IL-18 receptor in lym- was associated with IL-18 levels and components of the phocytes [34,35], whereas in non-T cells such as macro- metabolic syndrome, even in multivariate analyses, both phages, the IL-18 receptor is constitutively expressed. cross sectionally and during three years follow up [55]. Also data regarding IL-18 as a potential predictor of Interleukin-18 in the metabolic syndrome and type 2 future cardiovascular events have so far been conflicting diabetes (Table 1). In patients with known coronary artery dis- IL-18 has in several studies been associated with obesity ease, circulating IL-18 levels as well as polymorphisms [14,36-38], insulin resistance [39-41], hypertension [42] in the IL-18 gene were associated with future cardiovas- and dyslipidemia [14,37]. Furthermore, IL-18 has been cular mortality [56,57]. In two prospective studies, ele- shown to be elevated in subjects with the metabolic syn- vated IL-18 levels were associated with future drome [43] and to increase in parallell with an increas- cardiovascular disease in previously healthy men [58] ing number of components of the syndrome [38]. In a and women [59]. However, another large population large cross sectional study, elevated IL-18 levels were based study with a follow up of 11 years showed that associated with increasing number of components also increased levels of IL-6 and CRP, but not IL-18 were after adjustment for insulin resistance, obesity, IL-6 and associated with future coronary events [60].