9 Adiponectin and the Cardiovascular System

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From: Contemporary Cardiology: Diabetes and Cardiovascular Disease, Second Edition Edited by: M. T. Johnstone and A. Veves © Humana Press Inc., Totowa, NJ

9 Adiponectin and the Cardiovascular System

Suketu Shah, ^MD , Alina Gavrila, ^MD , and Christos S. Mantzoros, ^MD

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INTRODUCTION

Adiponectin, a recently discovered protein produced exclusively by adipocytes, is thought to be a possible mediator between obesity, insulin resistance, and cardiovascular disease (CVD). Although its function is not entirely known, body fat distribution, insulin, sex hormones, tumor necrosis factor (TNF)-F, and peroxisome proliferator-activated receptor (PPAR)-F may contribute to its regulation. Along with being associated with cardiovascular risk factors such as diabetes and dyslipidemia, deficiency in adiponectin may also directly compromise endothelial action and promote atherosclerosis.

Our understanding of the function of fat cells has changed dramatically with the

realization of the endocrine function of adipose tissue. Initially thought to serve only as

a repository for energy via storage of triglycerides, adipocytes are now known to secrete

a variety of proteins with diverse metabolic functions. These proteins include leptin,

TNF-F, plasminogen activator inhibitor-1, acylation-stimulating protein, resistin, and

adiponectin (1,2). Adiponectin has received much attention for its putative role in diabetes

and CVD. Besides being associated with the development of diabetes, it may also have

a direct role in modulating inflammation and atherosclerosis and thereby be one of the

factors that links obesity to CVD.

STRUCTURE OF ADIPONECTIN

In the mid-1990s, four different research groups, using either human or mouse-derived samples, concurrently discovered adiponectin, alternatively termed Acrp30, apM1 pro- tein, adipoQ, and GBP28 (3–6). Produced exclusively by differentiated adipocytes, adiponectin is a 30 kDa protein composed of 244 amino acids (3,4). One of the most abundantly produced fat hormones, it comprises approx 0.01% of the plasma proteins in humans, with plasma levels ranging from 2 to 20 μg/mL (7). Slightly increasing with age, adiponectin levels have a diurnal variation with nadir at night and peak in the morning (8,9).

The adiponectin molecule has four distinct parts, with the amino terminal having two short regions consisting of a secretory signal sequence and a domain unlike any other known protein (3–6). The next two regions, a collagen-like fibrous structure followed by a globular domain at the carboxy terminal, share homology with complement factor C1q and collagen VIII and X (10). Despite different amino acid sequences, the overall tertiary structure of the globular domain has similarity to TNF- F, another protein secreted by adipocytes but having opposing actions (11,12).

REGULATION OF ADIPONECTIN

Although its structure and source are known, the regulation of adiponectin remains to be determined. The various factors thought to be involved in controlling adiponectin production and secretion include obesity, nutritional status, hormones such as insulin, leptin, glucocorticoids, sex hormones, and catecholamines, TNF-F, and PPAR-F.

Obesity and Nutritional Intake

Obesity, in general, is associated with decreased adiponectin expression in adipose tissue and plasma levels (7,13). In both men and women, overall obesity, assessed by parameters such as body mass index (BMI) and fat mass, is negatively correlated to adiponectin, although prolonged weight reduction leads to increased adiponectin levels (7,14–17). Nutritional intake does not seem to explain this relationship. Although fasting decreases adiponectin messenger ribonucleic acid (mRNA) levels in mice, serum levels remain unchanged (18). In humans, short-term fasting also does not change plasma levels of adiponectin, although prolonged caloric restriction does result in weight loss and increased adiponectin levels (14,19). Additionally, daily caloric intake, macronutrient intake, or a high-fat meal is not related to any immediate change in circulating adiponectin levels in humans except possibly in obese individuals (20–22).

Instead of food intake, the distribution of adipose tissue may be more closely associated with adiponectin. There is a strong inverse correlation between adiponectin levels and visceral or central fat, compared to subcutaneous fat (9,19). In contrast to subcutaneous adipocytes, human omental adipose tissue had a significant negative correlation with BMI, and only it responded to insulin and PPAR- F agonist administration with increased adiponectin production (23). These findings suggest that adipose tissue, particularly in the visceral distribution, may have an inhibitory mechanism for its own production of adiponectin, perhaps mediated by other factors produced by fat cells such as TNF-F (13).

Hormone Regulation

Hormones have also been suggested to regulate adiponectin. Insulin likely has a role

in regulating adiponectin, but its exact role remains controversial. In vitro studies have

shown conflicting results on whether insulin has an inhibitory or stimulatory effect on

adiponectin production and secretion (23–25), whereas in an in vivo study involving

humans, hyperinsulinemic euglycemic dosing for at least 2 hours led to a decrease in adiponectin levels (26). Therefore, further studies would be helpful to resolve the rela- tionship.

Sex hormones may also affect secretion of adiponectin, because women have higher plasma levels of adiponectin than men, independent of body composition (14). Of the sex hormones, estrogen does not seem to account for the gender-related difference in adiponectin level, because premenopausal women have higher estrogen levels and lower adiponectin concentrations than postmenopausal females and estradiol levels actually have a strong negative correlation with serum adiponectin levels, females would be expected to have lower adiponectin concentrations than men (19). Testosterone may lower adiponectin levels by possibly inhibiting its secretion, however. In mice, removal of the testes led to an increase in adiponectin, although administration of testosterone reduced adiponectin levels (27). Although one study has demonstrated no association between adiponectin and free testosterone concentrations in women, this relationship remains to be explored in men (19).

Leptin and glucocorticoids have also been thought to be involved in adiponectin regulation, because leptin is also secreted by adipose tissue and both hormones affect insulin sensitivity (28,29). Although a cross-sectional study reported a strong inverse relationship between serum adiponectin and leptin levels (30), leptin given exogenously to rodents or humans had no significant effect on the plasma concentration of adiponectin (18,19). In vitro studies show that dexamethasone suppresses adiponectin gene expres- sion (24,25), but in human studies, cortisol was found to have no correlation with circulat- ing levels of adiponectin (19). Further studies are necessary to evaluate if glucocorticoids have a local effect on adiponectin production not reflected by their serum concentrations.

Catecholamines may also suppress expression of adiponectin, because G-adrenergic agonists reduced adiponectin gene expression in cultured mouse fat cells and human adipose tissue and decreased plasma levels in mice (31). Stimulation of cultured adipocytes by isoproteronolol, a G1 and G2 agonist, leads to reduced expression of adiponectin, an effect that propranolol, a nonselective G-antagonist, can inhibit (32).

Another study in animals confirmed that peripheral injection of a G3-adrenergic agonist suppressed adiponectin mRNA expression in adipose tissue (18).

TNF-F

As another factor produced by adipocytes, TNF- F may also be involved in the regu- lation of adiponectin. TNF- F and adiponectin inhibit each other’s production in adipose tissue, in addition to having opposing actions. TNF- F decreases expression and secretion of adiponectin in mouse and human adipocytes (25,33,34) and adiponectin-knockout mice have elevated serum TNF-F levels that decrease with adiponectin administration (35). Because these two molecules share, in part, similar tertiary structure, they may exert opposite actions by acting on the same cellular receptors (11,13).

PPAR-F

PPAR- F is a transcription factor that enhances insulin sensitivity in adipose and other

tissues (36). In a randomized, double blind, placebo-controlled trial, patients with type

2 diabetes given 6 months of rosiglitazone, a PPAR-F agonist, had increased levels of

adiponectin (37), with a similar change being seen even in humans without insulin resis-

tance (38). PPAR-F agonists may mediate their effect by directly promoting adiponectin

transcription or by inhibiting the actions of TNF-F (34,39).

ADIPONECTIN AND CARDIOVASCULAR DISEASE RISK FACTORS Although the role of adiponectin has not been definitively established, evidence is mounting that it is involved in insulin resistance, diabetes, inflammation, and atheroscle- rosis (40). Because, among the various adipocytokines, it decreases with increasing body fat (7), its low levels may lead to the development of pathological states associated with obesity such as insulin resistance and CVD.

Diabetes and Insulin Resistance

Adiponectin’s involvement in CVD is likely multifactorial, but one of its main roles is likely in affecting traditional risk factors associated with coronary artery disease (CAD), particularly diabetes. As one of the diabetes susceptibility genes and the adiponectin gene both localize to 3q27, mutation at this locus has been associated with both type 2 diabetes and decreased adiponectin (41).

The majority of data for animal studies thus far suggest that adiponectin acts as an insulin-sensitizing hormone. Adiponectin-knockout mice develop insulin resistance either independently of diet or only after high-fat and high-sucrose diet, and treating these mice with adiponectin ameliorates their insulin resistance (35,42). The insulin resistance in adiponectin-deficient lipoatrophic and obese mice can partially be reversed via adiponectin administration and fully restored with both leptin and adiponectin supple- mentation (29). Furthermore, in a longitudinal study analyzing the progression of type 2 diabetes in obese monkeys, decrease in adiponectin closely parallels the observed reduc- tion in insulin sensitivity, and the obese monkeys with greater plasma levels of adiponectin had less severe insulin resistance (43).

In humans, type 2 diabetes has been associated with decreased levels of adiponectin (14). In several studies, adiponectin has a negative correlation with fasting glucose, insulin, and insulin resistance and a positive association with insulin sensitivity, indepen- dent of BMI (9,14,44). One study demonstrated that adjusting for central obesity renders the negative correlation between adiponectin and insulin resistance no longer significant, suggesting that adiponectin may mediate the relationship between central obesity and insulin resistance (19). In studies involving Pima Indians, Japanese people, and Europe- ans, subjects with lower adiponectin were more likely to develop type 2 diabetes, inde- pendent of adiposity parameters (45–47). In contrast, type 1 diabetic patients have elevated adiponectin levels compared to nondiabetic individuals, and chronically admin- istered insulin does not have an effect on adiponectin levels (48).

Although not entirely known, the cellular and molecular mechanisms linking adiponectin to improved insulin sensitivity are also likely multifactorial. In rodents, adiponectin administration enhances insulin-stimulated glucose uptake into fat and skel- etal muscle cells (49–51). By increasing fatty acid oxidation, adiponectin can also lower circulating free fatty acids (FFAs), which may improve insulin action (51,52). Another important function of adiponectin is enhancement of insulin-induced suppression of hepatic glucose production (53,54). By generating nitric oxide (NO) formation, adiponectin may also augment vascular blood flow to promote glucose uptake (55).

Taken together, all these effects could explain why giving adiponectin to mice on a high- fat and high-sucrose diet will induce weight loss and reduction in FFA, triglycerides, and glucose levels (56).

Adiponectin may also improve insulin sensitivity by promoting activation of the

insulin-signaling system (58). The main enzyme implicated in adiponectin’s action is

adenosine monophosphate-activated protein kinase (AMPK) (49–51). A recent study demonstrates that binding of adiponectin to two distinct adiponectin receptors increases the levels of this enzyme (57). AMPK prevents activation of other enzymes involved in gluconeogenesis and may stimulate enzymes contributing to fatty acid oxidation (49,50,54,56).

Dyslipidemia

Besides diabetes and insulin resistance, adiponectin is also related to dyslipidemia, another risk factor for CVD. Adiponectin is a strong independent positive predictor of high-density lipoprotein levels and is negatively associated with serum triglycerides (14,59,60). In contrast, low-density lipoprotein and total cholesterol do not have signifi- cant independent relationships to adiponectin levels (19).

Adiponectin may lead to favorable lipid profiles by stimulating fatty acid oxidation.

The administration of adiponectin to rodents has been associated with increased fatty acid oxidation in skeletal muscle, both in vitro and in vivo, an effect probably mediated by AMPK (49,50,56). However, in one study, fatty acid oxidation in muscle cells was found to be increased in adiponectin-knockout mice (61), and in a single cross-sectional study in humans, plasma levels of adiponectin did not have any correlation with lipid oxidation, as measured by energy expenditure and respiratory quotient (62). Thus, further studies are needed to clarify adiponectin’s effects on fatty acid oxidation in humans.

Hypertension

Adiponectin has also been associated with hypertension. In adiponectin-deficient mice, a high-fat and -sucrose diet led to increased blood pressure (BP) (63). Although an initial study in humans reported that hypertensive males had increased plasma levels of adiponectin (64), subsequent studies reported that BP has a negative correlation to adiponectin (65–67). However, more recent data adjusting for insulin sensitivity did not show any significant correlation with hypertension and adiponectin, indicating that insulin resistance may mediate the potential association between adiponectin and BP (68). However, adiponectin has been associated with a vasodilatory response (63), with recent evidence suggesting that adiponectin increases NO formation through AMPK (55). Further studies are needed to elucidate more completely adiponectin’s role in regu- lating BP levels.

Cigarette Smoking

Smoking and even a history of smoking have been associated with decreased levels of circulating adiponectin. Among patients with heart disease, current and former smokers had lower adiponectin levels than nonsmokers, after adjusting for BMI and insulin resis- tance (69). Possible explanations for this decrease include smoking inducing an increase in catecholamines that suppress adiponectin or consumption of adiponectin by endothe- lium injured by cigarette toxins (69,70).

ADIPONECTIN’S DIRECT VASCULAR EFFECTS

Although development of insulin resistance and alterations in lipid profile may

account for part of adiponectin’s role in CVD, low adiponectin has also been associated

with CAD independent of these risk factors, suggesting that it may have its own direct

effect on the vascular system (14,71,72).

Adiponectin might have a protective role against atherosclerosis, because increasing adiponectin levels of mice deficient in apolipoprotein (apo)-E slows the rate of athero- sclerosis and reduces lipid accumulation in arterial plaques (73). In adiponectin-knock- out mice, injury induced to a femoral artery resulted in greater neointimal thickening compared to control, independent of degree of glucose intolerance (42), but overexpres- sion of adiponectin attenuated neointimal proliferation in these mice (74).

Adiponectin’s reduction of atherosclerosis may occur through its actions on inflam- matory mediators, macrophages, smooth muscle cells, and endothelium (40). With its association with inflammatory markers, lack of adiponectin may foster an inflammatory milieu related to developing atherosclerosis and diabetes (75). In patients with or without CAD, serum C-reactive protein, an inflammatory marker, was inversely related to adiponectin (75–77). Other inflammatory markers such as phospholipase A2, interleukin- 6, and soluble E (SE)-selectin, are also negatively correlated with adiponectin in one study (75).

As an anti-inflammatory agent, adiponectin may inhibit inflammatory mediators involved in atherosclerosis, particularly TNF-F. An in vivo study in mice demonstrated that administration of adiponectin decreased serum TNF-F levels (35). Although no significant correlation occurred between adiponectin and serum TNF-F receptors 1 (sTNFR1) and 2 (sTNFR2), which are markers of activation of the TNF-F system, a study in humans found lower sTNFR2 in the highest quartile of circulating adiponectin, sug- gesting a threshold effect instead of a dose-dependent relationship (78). A different study, however, found a significant negative correlation between plasma adiponectin and TNF- F mRNA expression (13). Besides inhibiting the production of TNF-F, adiponectin may impede TNF-F’s involvement in atherosclerosis by reducing TNF-F-induced expression of endothelial cell adhesion molecules such as vascular cell adhesion molecule-1, SE-selectin, and intracellular adhesion molecule-1, that otherwise recruit monocytes and macrophages involved in atherosclerosis development (79,80). Through a cyclic adenosine monophosphate (cAMP)-dependent pathway, adiponectin may prevent TNF- F from inducing stimulation of nuclear factor PB, a transcriptional factor that promotes gene expression of endothelial adhesion molecules (80).

The anti-inflammatory effects of adiponectin may also directly involve macrophages and monocytes, an integral aspect of atherosclerotic lesions. By decreasing the level of expression of class A scavenger receptors on macrophages, adiponectin suppressed macrophage-to-foam cell transformation (81). It also attenuated the phagocytic action of macrophages and inhibited expression and secretion of TNF-F from macrophages (82).

Additionally, adiponectin may decrease proliferation of precursors of monocytes and macrophages by suppressing bone marrow production of these cells (82).

In the pathogenesis of atherosclerosis, adiponectin may also affect the proliferation of smooth muscle cells in the vascular wall by inhibiting growth factors that promote hyperplasia. Adiponectin binds to and inhibits a subtype of platelet-derived growth factor produced by platelets and foam cells (83) and blocks the proliferative action of heparin- binding epidermal growth factor (HB-EGF)-like growth factor (74). It also prevents TNF-F from inducing increase in HB-EGF mRNA production (74).

Finally, adiponectin may mediate endothelial vasodilatation, because plasma

adiponectin was independently correlated with peak forearm blood flow and vasodilator

response to reactive hyperemia (36,84), and stimulate production of the vasodilatory

agent NO in vascular endothelial cells (55).

FUTURE THERAPEUTIC DIRECTIONS

Because the adverse cardiovascular events associated with obesity may be related to the relative decrease in adiponectin, supplementing this protein exogenously, increasing endogenous production, or designing agonists for its receptor need to be tested in relation to cardiovascular outcomes. PPAR-F agonists have been shown to increase adiponectin levels in lean, obese, and diabetic humans (38). Because their ability to improve insulin resistance may be mediated by adiponectin, they may also prove to have an added indication for CVD. Angiotensin-converting enzyme inhibitors and angiotensin receptor blockers also may increase adiponectin (68), which may mediate in part their beneficial effects in insulin-resistant states including CVD. G-Adrenergic antagonists may also have similar use, as they prevent catecholamine-induced suppression of adiponectin production (32). Finally, with the discovery of adiponectin receptors, ago- nists at these sites may allow for targeted augmentation of adiponectin’s effects in meta- bolically active tissues.

CONCLUSION

Obesity has long been associated with insulin resistance, hypertension, and CAD, but the mechanism has remained largely unknown. Adiponectin may be one of the factors that explains these associations. Because deficiencies in adiponectin may result in the development of these processes, increased endogenous production or exogenously administered adiponectin or its agonists may contribute to restoring insulin sensitivity and preventing atherosclerosis by increasing fatty acid oxidation and insulin-mediated glucose uptake, and decreasing the endothelial inflammatory process associated with atherosclerotic plaque development. Although animal studies have demonstrated ben- efits, clinical trials are needed to determine whether the beneficial effects of adiponectin can also be observed in humans and whether either adiponectin or adiponectin receptor agonists represent a novel treatment option for type II diabetes and CAD.

ACKNOWLEDGMENT

This chapter was supported by NIDDK grant DK 58785 and NIH grant K30 HL04095.

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