Adipose tissue in the pathophysiology of cardiovascular disease: Who is guilty?

Abstract

Epidemiological evidence has shown how abdominal obesity is closely associated with the development of cardiovascular disease. It has been demonstrated that patients with extensive adipose tissue usually have other concomitant cardiovascular risk factors, such insulin resistance, hypertension and dyslipidemia. Moreover, obese patients have a significantly higher risk of developing thrombophilic events compared with the non-obese. Thus, obesity is actually considered an independent cardiovascular risk factor. The pathophysiological mechanisms responsible for the association between obesity and cardiovascular disease remain largely unknown. However, it has been postulated that obese patients have an “inflammatory milieu” responsible for their metabolic disorders and vascular disease. In this context, adipocyte-derived molecules with inflammatory activity might play a pivotal role in the development of these mechanisms. In the present report, we provide an updated overview on the molecules produced by adipose tissue that are potentially involved in cardiovascular pathophysiology.

Key Words: Adipokines, Atherosclerosis, Cardiovascular disease, Inflammation, Obesity, Thrombosis

INTRODUCTION

Nowadays, obesity is considered an emerging and rapidly expanding disease, mainly in industrialized countries. In fact, it can be considered a typical “disease of the affluent”[1]. The World Health Organization estimates that more than 1 billion people are overweight and 300 million are obese in the world[2]. For many years, epidemiological evidence has shown that abdominal obesity is closely associated with the development of cardiovascular disease[3,4]. Specifically, patients with extensive adipose tissue have a higher incidence of other cardiovascular risk factors, such as insulin resistance, hypertension and dyslipidemia[5]. Interestingly, obese patients have a thrombophilic risk 1.5 to 2.5 times higher than the non-obese. Taken together, these observations have suggested that obesity might be considered as an independent cardiovascular risk factor[6]. Despite these clinical observations, the pathophysiological mechanisms responsible for the association between obesity and cardiovascular disease remain largely unknown. However, it has been recently postulated that obese patients have an “inflammatory milieu” responsible for their metabolic disorders and vascular disease[7]. In this context, adipocyte-derived molecules with inflammatory activity might play a pivotal role in the development of these inflammatory mechanisms. In the present report, we provide an updated overview of the molecules produced by adipose tissue potentially involved in cardiovascular pathophysiology.

ADIPO(CYTO)KINES

The cells of adipose tissue, adipocytes, are no longer considered only as fat storage cells. They are considered as cells able to produce and secrete several substances with biological activity, known as “adipokines”[8,9]. The biological functions of adipokines are still partially unknown; however, they seem involved in the regulation of many physiological processes, such as appetite regulation and energy balance, lipid metabolism, blood pressure, insulin sensitivity, inflammation, haemostasis and angiogenesis[10]. It is known that, in the plasma of patients with obesity or metabolic syndrome, increased levels of some adipokines can be measured, suggesting that these adipocyte-derived substances might be considered as novel biomarkers and regulators of the metabolic syndrome[11].

It has been observed that adipocytes belonging to the adipose tissue of obese patients can synthesize and secrete several adipokines. Moreover, this tissue appears to be infiltrated by inflammatory cells[7]. Interestingly, adipokines secreted by visceral fat have a more remarkable biological activity than adipokines released by subcutaneous fat[7]. It was also shown that weight loss and exercise could ameliorate the inflammatory milieu of patients with metabolic syndrome by modulating their adipokine profile[11,12]. To date, the adipokines actually discovered can be divided into four groups: (1) adipokines with metabolic functions; (2) adipokines with pro-inflammatory functions; (3) adipokines components of the extracellular matrix; and (4) adipokines with pro- angiogenic and pro-mitogenic action[11]. However, some of them fall out of this schematization and can be placed transversely across multiple categories. In addition, only some of these molecules appear to play an active role in cardiovascular pathophysiology.

ADIPONECTIN

Adiponectin is a 247 amino acid protein with a globular carboxyl-terminal domain and an amino-terminal collagen-like domain[13]. This adipocytokine and complement factor 1q has a similar structure[14]. In humans, the adiponectin gene is located on chromosome 3q27[15]. This adipokine seems to exert protective effects on the cardiovascular system[16]. In fact, patients with a high atherosclerotic burden have low plasma levels of adiponectin[17]. Moreover, it has been demonstrated that low plasma levels of this adipokine are closely related to the progression of coronary atherosclerosis in patients with angina pectoris[18]. Finally, it has been observed that women with low plasma levels of adiponectin have impairment of the coronary flow reserve[19]. Although adipocytes are the main source of adiponectin, patients in which adipose tissue are largely represented, such as the obese and those affected by metabolic syndrome or diabetes mellitus, have low measurable plasma levels of this adipocytokine[20]. In the plasma, three different oligomers of adiponectin have been isolated, each one with a specific biological function[21]. We can identify: (1) low molecular weight (LMW) oligomers, constructed by three molecules of adiponectin; (2) middle molecular weight (MMW) oligomers, formed by six adiponectin fractions; and (3) high molecular weight (HMW) oligomers constituted by 12 -18 molecules of adiponectin[22]. Then, another oligomer has been recently isolated in which three molecules of adiponectin are bound to albumin (Alb-LMW)[23]. In humans, MMW and LMW adiponectin represent 25% while HMW adiponectin represents 50% of whole circulating adiponectin[23]. Since plasma levels of HMW appear to be closely related to insulin sensitivity, it has been suggested that HMW is biologically active[24,25]. Two specific adiponectin receptors have been identified: AdipoR1 and AdipoR2. Binding of adiponectin to the AdipoR2 receptor increases energy consumption and improves fatty acid oxidation. Moreover, when adiponectin binds the AdipoR2 receptor, pro-atherosclerotic processes such as oxidative stress and inflammation are significantly inhibited[26]. In particular, in the atherosclerotic plaques, adiponectin should modulate the inflammatory response by down-regulating the expression of pro-inflammatory mediators, such as tumor necrosis factor (TNF)-α, interleukin (IL)-6 and interferon (IFN)-c, and by up-regulating anti-inflammatory molecules, such as the antagonist receptor for IL-1[27]. As reported above, in vivo, adiponectin increases energy consumption and oxidation of fatty acids in the liver and muscles. These phenomena contribute to reducing triglycerides levels in these tissues and improving insulin sensitivity[28]. In endothelial cells, adiponectin induces the activation of endothelial nitric oxide synthase (eNOS) and stimulates nitric oxide production[29]. Moreover, adiponectin inhibits the oxygen free radical production and ameliorates the endothelial function in mice genetically modified to develop hyperlipidemia and atherosclerosis[30].

Several experimental studies have clearly demonstrated that adiponectin plays a protective role for the cardiovascular system since it is able to interfere with the early steps of atherosclerotic disease. In particular, it has been shown that adiponectin deficiency increases leukocyte-endothelium interactions via up-regulation of endothelial cell adhesion molecules (CAMs) in vivo[31]. Conversely, the expression of adhesion molecules is reduced when increased levels of adiponectin are measurable[32]. Moreover, adiponectin suppresses smooth muscle cells proliferation[33] and inhibits lipopolysaccharide-induced adventitial fibroblast migration and transition to myofibroblasts via the AdipoR1-AMPK-iNOS pathway[34]. Again, adiponectin suppresses lipid accumulation and class A scavenger receptor expression in human monocyte-derived macrophages[35]. Finally, this adipocytokine reduces lipid accumulation in macrophage foam cells[36] and is able to reduce atherosclerosis in apolipoprotein E-deficient mice[37]. Of note, adiponectin seems to play an important role in modulating the inflammatory network involved in the pathophysiology of cardiovascular disease because it regulates the expression of several important chemokines, such as IP-10, Mig and I-TAC, which bind to the chemokine receptor CXCR3, an important regulator of chemotaxis of lymphocytes within the atherosclerotic plaque[38]. The protective role played by adiponectin has also been confirmed by recent studies showing that the adiponectin receptor is detectable on platelets, thus suggesting that this adipocytokine acts as an endogenous antithrombotic factor[39].

LEPTIN

Leptin is a polypeptide consisting of 167 amino acids, encoded by the “ob” gene and implicated in the regulation of body weight and energy balance[40]. Experimental and clinical evidence has shown that this peptide might be involved in the pathophysiology of metabolic syndrome[41]. In fact, elevated leptin plasma levels are usually detectable in the plasma of obese patients[42]. In this context, the relationship between obesity, elevated plasma levels of leptin and cardiovascular disease appears of particular interest. Several clinical studies have shown that patients with increased plasma concentrations of leptin are at high risk of developing myocardial infarction[43] and stroke[44]. In addition, elevated serum levels of leptin were measured in patients with myocardial infarction with ST elevation[45]. Finally, a large prospective study on leptin and cardiovascular risk, the West of Scotland Coronary Prevention Study, confirmed that leptin is an independent predictor of coronary events[46]. This adipokine has recently been identified as a good prognostic marker of future cardiovascular events in patients with angiographically proven atherosclerosis[47]. Elevated leptin baseline levels are associated with increased risk of cardiac death, new myocardial infarction, stroke and coronary revascularization, even in patients without diabetes[47]. Hyperleptinemia is closely associated with in-stent restenosis in patients undergoing coronary stenting[48]. Moreover, the ratio of leptin:adiponectin seems to be directly correlated with the magnitude of the intima-media thickness of the common carotid artery, a good index of subclinical atherosclerosis[49]. Finally, it has been suggested that leptin, by stimulating the sympathetic nervous system, might also play an important role in the pathophysiology of hypertension[50]. These clinical observations have been supported by experimental studies which have strongly reinforced the hypothesis that leptin might be involved in the pathophysiology of cardiovascular disease. Leptin seems to be able to modulate platelet aggregation[51] and arterial thrombosis[52,53]. In a recent paper, it has been demonstrated that leptin, at concentrations usually measurable in the plasma of patients with acute coronary syndrome, induces a pro-atherothrombotic phenotype in human coronary endothelial cells through the expression of tissue factor (TF) and CAMs[54]. Moreover, leptin can induce the expression of TF in human peripheral blood mononuclear cells[55].

The progression of atherosclerotic plaques might be modulated by leptin. Treatment of apo-lipoprotein E deficient mice with leptin causes faster progression of vessel atherosclerosis and increases the amount of calcium in the vessel wall[56]. In addition, it has been shown that treatment of hyper-lipidemic mice with recombinant leptin increases the atherosclerotic burden and promotes a faster thrombus formation[53]. Interestingly, leptin deficiency suppresses progression of atherosclerosis in apoE-deficient mice[57]. Recently, it has been demonstrated that leptin stimulates the production of C-reactive protein (CRP) in human coronary endothelial cells[58]. Since it has been previously evidenced that CRP induces vascular thrombosis[59], taken together, these observations suggest that leptin might promote the development of the atherosclerotic disease and that it might be involved in the pathophysiology of acute coronary syndromes.

RESISTIN

Resistin is a cysteine-rich protein of 12.5 kDa, consisting in humans of 108 amino acids: 17 amino acids form the N-terminal signal sequence, 37 in the variable portion and the remaining in the constant area at the C-terminal. Its gene is located on chromosome 19. Recently, a family of resistin-like molecules (RELMs) have been described. These polypeptides consist of 105-114 amino acids and are composed of three domains: a signal sequence at the N-terminal, a variable central portion and a highly conserved C-terminal. RELM α is secreted mainly by adipose tissue, while RELM-β is expressed only in the gastrointestinal tract and in neoplastic cells, suggesting a possible role in cell proliferation. RELM-γ, recently discovered, was found in hematopoietic tissue, where it is supposed to have a cytokine-like activity. In rodents, adipocytes are the main source of resistin, while in humans, macrophages have this function[60]. Initially, this adipokine was proposed as a potential link between obesity and diabetes by modulating the mechanisms responsible for insulin resistance[61]. Then, experimental evidence in vivo and in vitro has shown that resistin is able to trigger the mechanisms involved in inflammation[60]. In addition, plasma levels of resistin appear to be closely correlated with other markers of inflammation, such as TNF-α, type 2 soluble receptor for TNF-α and IL-6[62-64]. Of note, patients with acute coronary syndrome have resistin plasma levels significantly higher than patients with stable angina and healthy controls[65]. Then, it was also shown that resistin might be an independent predictor of coronary atherosclerosis in humans[62-64]. Finally, since plasma resistin is associated with myocardium injury in patients with acute coronary syndrome, it has been proposed as a marker of ischemic injury[66]. Recent experimental evidence has indicated that resistin promotes endothelial dysfunction. In fact, it induces expression of adhesion molecules and cytokine in human endothelial cells[67]. In addition, it influences the expression of PI3Kp85 and stimulates the release of plasminogen activator inhibitor 1, von Willebrand factor and endothelin[68]. Again, resistin inhibits the expression of eNOS. Finally, recent studies have shown that this molecule is able to promote proliferation and migration of smooth muscle cells through the activation of ERKs and PI3K kinase[69,70]. In a recent in vitro study, it has also been shown that resistin induces the synthesis and expression of active TF in human coronary artery cells[71].

TNF-α

TNF-α is another important chemical mediator involved in the progression of atherosclerotic disease[72]. It has pro-inflammatory activity and is produced mainly by inflammatory cells, such as monocytes and macrophages. This molecule is actively involved in the modulation of inflammatory and autoimmune diseases. Adipose tissue of animals used as experimental models of obesity and type 2 diabetes has increased expression of TNF-α[73]. Similarly, elevated levels of TNF-α have been detected in adipose tissue and plasma of obese patients. Interestingly, TNF-α plasma levels are closely related to the extent of visceral adipose tissue, while weight loss and lifestyle changes reduce these levels[74]. TNF-α has been isolated in atherosclerotic plaques and it seems to be a marker of plaque growth[75-77]. The role played by this pro-inflammatory molecule was also highlighted by elegant experiments in mice genetically modified to be deficient in expression of TNF-α: these animals showed less atherosclerosis compared to the same animals not genetically modified. In addition, these animals showed a poor inflammatory profile with low levels of IL-1β, IFN-γ, ICAM-1, VCAM-1, MCP-1, GM-CSF and nuclear factor (NF)-κB[78]. TNF-α also appears to be able to regulate the expression of lectin-like type 1 receptor for oxidized LDL, an important facilitating atherosclerosis mediator in various cell types, including endothelial cells and macrophages[79].

IL-6

IL-6 is another pro-inflammatory cytokine with plasma levels closely related to the amount of visceral fat[80]. In fact, obese patients have elevated plasma levels of IL-6 which can be reduced by weight loss[74]. It is estimated that the adipose tissue is responsible for the production of about one third of the circulating levels of IL-6[80] and it has been hypothesized that the secretion of IL-6 in obese individuals contributes to metabolic dysfunction. In fact, plasma levels of IL-6 can be considered predictive for the development of type 2 diabetes and, in turn, high levels of this cytokine are detectable in diabetics[81]. IL-6 plays an important role in the pathophysiology of atherothrombosis: it modulates the release of pro-inflammatory cytokines and of other pro-thrombotic mediators, promotes lipoprotein oxidation, activates matrix metalloproteinases and, finally, it stimulates secretion of the acute phase proteins[82]. IL-6 is secreted directly into the portal circulation; thus, it reaches the liver where it stimulates release of CRP[11], an important predictor of future cardiovascular events both in patients with documented cardiovascular damage and in apparently healthy people[83].

VISFATIN

Visfatin, identified in 2004, derives its name from the fact that it is produced mainly in visceral fat. It is composed of 491 amino acids and has a molecular weight of 52 kDa. Visfatin gene corresponds to the gene of the Pre-B cell colony enhancing factor (PBEF). PBEF was described in 1994 as a cytokine produced by lymphocytes, involved in regulation of inflammatory mechanisms.

Interestingly, this adipokine is produced by macrophages resident in adipose tissue and not directly by adipocytes. In this regard, the levels of visfatin are believed to be the expression of macrophages infiltrating the adipose tissue, where they produce it in response to inflammatory signals[84]. Circulating levels of visfatin are closely correlated with white adipose tissue accumulation[9]. However, the relationship between circulating visfatin levels and anthropometric and metabolic parameters of obesity and type 2 diabetes is still not completely understood. The role of visfatin in cardiovascular disease is not clear. Increased expression of visfatin has been demonstrated in macrophages of human unstable carotid and coronary atherosclerotic plaques[85]. Again, endothelial dysfunction has been described in those patients with elevated plasma levels of visfatin[86]. Moreover, it has been observed that visfatin is able to increase the expression of adhesion molecules by activating the transcription factor NF-κB[87]. Taken together, these data suggest that visfatin might play a role in plaque destabilization.

“OTHER” ADIPOKINES

Several “other” adipokines with paracrine/endocrine activities are produced by adipose tissue. Some of these molecules have been partially characterized: apelin is a bioactive peptide produced by adipocytes, stromal vascular cells and cardiac myocytes. In obese patients with hyperinsulinemia, increased plasma levels of apelin are measurable[88]. In animal models of heart failure, cardiac apelin is down-regulated by angiotensin II, while its production is restored after treatment with an angiotensin receptor 1 antagonist[89]. In rats, the cardiac production of apelin is increased by hypoxia[90] and ischemia[91]. In spontaneously hypertensive rats, exercise has also been shown to stimulate the production of apelin[92]. This molecule also has a positive hemodynamic effect, acting with an inotropic mechanism in rats with heart failure, as well as in isolated cardiomyocytes[93]. In humans, low plasma apelin was observed in patients with chronic heart failure[94] and in patients with atrial fibrillation[95]. In these, cardiac resynchronization therapy is accompanied by increases in the concentrations of apelin[96]. Despite the available evidence, it is still unclear whether the form of metabolically active apelin is produced by adipose or by cardiac tissue. Thus, more studies are needed to clarify the role of this interesting peptide in the pathophysiology of cardiovascular disease and its potential therapeutic use.

Omentin is a secretory protein selectively expressed in the visceral adipose tissue, where it is synthesized by the visceral stromal vascular cells[97,98]. It has also been found in the human lung, intestine, ovaries, placenta and heart[98]. Omentin plasma levels are reduced in obese patients[99]. Conversely, elevated levels of omentin are measurable in the plasma of lean subjects with increased levels of adiponectin and high-density lipoproteins[99,100]. Omentin increases insulin-stimulated glucose uptake in both omental and subcutaneous adipocytes and promotes Akt/PKB phosphorylation[101]. Interestingly, it has been shown that low levels of omentin are measurable in patients with severe coronary atherosclerotic disease[102-104]. Finally, the association of circulating omentin level with arterial stiffness and carotid plaque in patients with diabetes has been demonstrated[105]. Recently, it has been proposed that omentin might be an early marker of metabolic dysfunction[106].

Vaspin is a novel adipokine expressed in the visceral and subcutaneous adipose tissue, involved in the development of obesity and insulin resistance[107-110]. The relationship between vaspin and cardiovascular disease is still obscure. It has been demonstrated that patients with unstable angina have reduced plasma and mRNA levels of this adipokine[111]. In addition, vaspin serum concentrations were significantly low in patients with atherosclerosis of carotid arteries[112].

Table 1: Overview of adipocitokines and of their potential cardiovascular functions

Table 1 Sources and potential role in cardiovascular disease of key adipokines.

Adipokine	Primary source(s)	Potential role in CVD
Adiponectin	Adipocytes	↑Insulin sensitivity, energy consumption, fatty acid oxidation
		↓Oxidative stress
		Anti-inflammatory activity
		↓TNF-α, IL-6, interferon-c ↑IL-1R antagonist
		Modulation of chemokine expression
		Improved endothelial function
		eNOS induction ↑NO ↓ROS
		regulation of adhesion molecules
		Regulation of macrophage function
		Antiaggregants effects
		Decreased progression of atherosclerotic lesions
Leptin	Adipocytes	Appetite regulation and modulation of energy expenditure
		Independent prognostic marker of ACS
		Modulation of blood pressure
		Regulation of platelet aggregation and induction of arterial thrombosis
		TF, CRP and adhesion molecules induction in endothelial cells
		TF induction in peripheral blood mononuclear cells
		Maintenance of progression of atherosclerotic disease
Resistin/RELMs	RELM-α: Adipose tissue macrophages (human), adipocytes (mice)	Induction of insulin resistance
		Cell proliferation (RELM-β)
		Cytokine-like functionality (RELM-γ)
	RELM-β: Tumor and gastro-intestinal cells	Pro-inflammatory activity
		Independent predictive marker of atherosclerosis and severity of ischemic injury
	RELM-γ: Hematopoietic tissue	Effects on endothelial cells
		↑Adhesion molecules, cytokines, TF, plasminogen activator inhibitor, von Willebrand factor, endothelin
		↓e-NOS expression
		Smooth muscle cell proliferation and migration
TNF-α	Inflammatory cells, monocytes, macrophages, adipocytes	Reduction of insulin signaling
		Induction of insulin resistance
		Maintaining proinflammatory state and atherosclerosis
IL-6	Inflammatory cells, stromal vascular fraction cells, adipocytes, liver, muscle	Induction of insulin resistance
		Maintaining pro-inflammatory status
		Modulation of pro-inflammatory cytokines and pro-thrombotic mediators release
		Promotion of lipoproteins oxidation
		Activation of matrix metalloproteinase
		Induction of CRP production by the liver
Visfatin	Lymphocytes, macrophages, adipocytes, other cells	Monocyte chemotactic activity
		Endothelial dysfunction
		Atherosclerotic plaque destabilization
		TF induction
Other adipokines
Apelin	Adipocytes, stromal vascular cells and cardiac myocytes	Cardiomyocyte function regulation
Omentin	Stromal vascular cells of visceral adipose tissue	↑Insulin-stimulated glucose uptake in both Low levels in severe coronary atherosclerotic disease and arterial stiffness and carotid plaque in patients with diabetes
		Possible marker of metabolic dysfunction
Vaspin	Visceral and subcutaneous adipose tissue	Reduced plasma and mRNA levels in patients with unstable angina
		Low serum concentrations in patients with carotid atherosclerosis

ACS: Acute coronary syndrome; CRP: C-Reactive protein; eNOS: Endothelial nitric oxide synthase; IL: Interleukin; IL-1R: Interleukin-1 receptor; NO: Nitric oxide; RELM: Resistin like molecule; ROS: Reactive oxygen species; TF: Tissue factor; TNF: Tumor necrosis factor.

CONCLUSION

It is well established that obesity and cardiovascular events are closely related. However, although it has been clearly demonstrated that obesity is also associated with other conditions that may accelerate atherosclerosis, the molecular mechanisms underlying these phenomena are not fully defined yet. In this context, adipose tissue, abundantly represented in obese individuals, might play a role by producing substances, called adipokines, with an active role in the pathophysiology of cardiovascular disease. Although some of these molecules, such as adiponectin, leptin and resistin, now have a well defined position in the complex network between inflammation, obesity and cardiovascular diseases, many others are still poorly defined functionally. It seems clear that the adipokines panorama needs new experimental evidence and clinical trials to compose the complex puzzle that link a “modern” and wellness disease like obesity with cardiovascular disease, the leading cause of mortality in industrialized world. As in a complex crime scene, we have several potentially guilty candidates.