Adipokines, Insulin Resistance and Coronary Artery Calcification

doi:10.1016/j.jacc.2008.04.016

J Am Coll Cardiol. Author manuscript; available in PMC 2010 April 12.

Published in final edited form as:

J Am Coll Cardiol. 2008 July 15; 52(3): 231–236.

doi: 10.1016/j.jacc.2008.04.016

PMCID: PMC2853595

NIHMSID: NIHMS66001

Adipokines, Insulin Resistance and Coronary Artery Calcification

Atif Qasim, MD, Nehal N. Mehta, MD, Mahlet G. Tadesse, ScD, Megan L. Wolfe, BS, Thomas Rhodes, MSPH, Cynthia Girman, DrPH, and Muredach P Reilly, MB

Atif Qasim, Cardiovascular Division and Center for Experimental Therapeutics, Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA;

Contributor Information.

Address for Correspondence: Muredach P. Reilly, Cardiovascular Division, University of Pennsylvania Medical Center, 909 BRB 2/3, 421 Curie Blvd., Philadelphia, PA 19104-6160. Tel: 215-573-1214, Fax: 215-573-9004. E-mail: muredach/at/spirit.gcrc.upenn.edu

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Abstract

Objectives

We evaluated the hypothesis that plasma levels of adiponectin and leptin are independently but oppositely associated with coronary calcification (CAC), a measure of subclinical atherosclerosis. In addition, we assessed which biomarkers of adiposity and insulin resistance are the strongest predictors of CAC beyond traditional risk factors, the metabolic syndrome and plasma C-reactive protein (CRP).

Background

Adipokines are fat-secreted biomolecules with pleiotropic actions that converge in diabetes and cardiovascular disease.

Methods

We examined the association of plasma adipocytokines with CAC in 860 asymptomatic, non-diabetic participants in the Study of Inherited Risk of Coronary Atherosclerosis (SIRCA).

Results

Plasma adiponectin and leptin levels had opposite and distinct associations with adiposity, insulin resistance and inflammation. Plasma leptin was positively (top vs. bottom quartile) associated with higher CAC after adjusting for age, gender, traditional risk factors and Framingham Risk Scores (FRS) [tobit regression ratio 2.42 (95% CI 1.48–3.95, p=0.002)] and further adjusting for metabolic syndrome and CRP [ratio 2.31 (95% CI 1.36–3.94, p=0.002)]. In contrast, adiponectin levels were not associated with CAC. Comparative analyses suggested that levels of leptin, IL-6 and sol-TNFR2 as well as HOMA-IR predicted CAC scores but only leptin and HOMA-IR provided value beyond risk factors, the metabolic syndrome and CRP.

Conclusion

In SIRCA, while both leptin and adiponectin levels were associated with metabolic and inflammatory markers, only leptin was a significant independent predictor of CAC. Of several metabolic markers, leptin and the HOMA-IR index had the most robust, independent associations with CAC.

Condensed Abstract

Adipokines are fat-secreted biomolecules with pleiotropic actions and represent novel markers for cardiovascular risk. We examined the association of plasma adipocytokines with CAC in 860 asymptomatic, non-diabetic Caucasians. Leptin was positively (top vs. bottom quartile) associated with higher CAC even after adjustment for age, gender, traditional risk factors, Framingham Risk Score, metabolic syndrome, and CRP [ratio 2.31 (95% CI 1.36–3.94, p=0.002)]. Adiponectin levels were not associated with CAC. Comparative analyses suggested that levels of leptin, IL-6 and sol-TNFR2 as well as HOMA-IR predicted CAC scores, but only leptin and HOMA-IR provided value beyond risk factors, the metabolic syndrome and CRP.

Keywords: Adiponectin, Leptin, Coronary Artery Calcification, Atherosclerosis, Inflammation

Introduction

Adipokines are fat-secreted biomolecules with diverse signaling effects that modulate insulin resistance, hepatic lipoprotein production and vascular inflammation (1). Two in particular, adiponectin and leptin, are almost exclusively fat-derived and have antithetical actions in insulin resistance and in vascular signaling (2). Because of these properties, adiponectin and leptin have been proposed as biomarkers of adipose function that may add value in predicting cardiovascular disease (CVD) risk and provide targets for therapeutic interventions.

Levels of adiponectin, an insulin sensitizing hormone with anti inflammatory properties (3), are reduced in obesity, type 2 diabetes and coronary artery disease (CAD) compared to controls (4–6). Indeed, several (7,8) but not all (9,10), epidemiological studies suggest that reduced plasma adiponectin levels are independent predictors of CVD. Leptin, on the other hand, is a pleiotropic adipokine that modulates innate immune functions and vascular signaling in addition to its central role in regulation of appetite and energy expenditure (11). In contrast to adiponectin, leptin levels directly correlate with insulin resistance, obesity (12,13) and several CVD risk factors (14). Leptin levels have been associated with CVD beyond BMI, in some (15–17), but not all studies (18).

We previously examined the association of plasma levels of CRP, resistin, interleukin-6 (IL-6) and soluble TNF receptor 2 (sol-TNFR2), as well as the metabolic syndrome, with coronary artery calcification (CAC) in the Study of Inherited Risk of Coronary Atherosclerosis (SIRCA) (19–22). In this report, we examined the association of adiponectin and leptin with CVD risk factors and CAC in SIRCA and then compared the relative value of all measured biomarkers of adiposity and insulin resistance in predicting CAC scores beyond traditional risk factors, metabolic syndrome and plasma CRP.

Methods

Study participants

The Study of Inherited Risk of Coronary Atherosclerosis (SIRCA) is a single center community based cross-sectional study of factors associated with Coronary Artery Calcium (CAC) (21,23). Participants were healthy adults aged 30–75 with a family history of premature CVD, but without evidence of clinical CAD (defined as myocardial infarction, coronary revascularization, angiographic evidence of CAD, or ischemia seen on a cardiac stress test), diabetes, elevated serum creatinine >3.0 mg/dl or elevated total cholesterol (>300mg/dL). This report focuses on 860 unrelated, non-diabetic SIRCA participants.

Evaluated parameters

Study subjects were evaluated in a fasting state at the GCRC at the Hospital of the University of Pennsylvania (21,23). Plasma levels of adiponectin, leptin, resistin and insulin (Linco, St Charles MO), as well as IL-6 and sol-TNFR2 (R+D Systems, Minneapolis) were measured by ELISAs. CRP levels were assayed as described (21). The intra- and inter-assay c.v.’s for pooled human plasma were 5.7% and 9.9% for adiponectin; 5.5% and 12.4% for leptin; 4.6% and 4.3% for resistin; 4.1% and 11.6% for insulin; 8.7% and 10.9% for IL-6; 5.3% and 12.1% for sol-TNFR2; and 8.0% and 8.3% for CRP respectively. Framingham Risk Scores (FRS), were calculated as described by Wilson et. al. (24). Participants were classified as having the metabolic syndrome using the National Cholesterol Education Program (NCEP) definition (25). The homeostasis model assessment (HOMA-IR index = fasting glucose (mmol/L) × fasting insulin (µU/mL)/22.5) (26) was used as a measure of insulin resistance. Global Agatston CAC scores (27), measured at electron beam tomography (Imatron, San Francisco, CA) were determined as previously described (21,23).

Statistical analysis

Data are reported as median with first and third quartiles (Q₁ = 25^th percentile, Q₃ = 75^th percentile), or mean ± SD, for continuous variables, and as proportions for categorical variables. Spearman correlations of plasma adiponectin and leptin with other continuous variables are presented. Crude associations of adipokine levels with categorical variables were examined using the Kruskal-Wallis rank test. Tobit regression, using natural log (CAC+1) as the outcome, was used for the analysis of CAC data because of its marked right skewed distribution and the presence of many zero scores (28). The tobit model is designed to assess the relationship between explanatory variables and a censored dependent variable at one end, where many observations are clustered. We chose this modeling since CAC scores are censored at zero and the use of ordinary least-squares regression on such a non normal distribution would produce biased estimates and invalid inference. Tobit modeling has otherwise similar assumptions about error distributions as the linear regression model.

Because of potential gender differences in adipose associations with CVD, models are presented for each gender separately and combined when appropriate. The association between CAC and highest vs. lowest quartile of adiponectin and leptin were assessed in incremental models including the variables age (age and age²), race, gender, family history of CAD, exercise (none versus any), medications (aspirin, statins, angiotensin converting enzyme inhibitors), Framingham Risk Score, metabolic syndrome, and CRP. Gender differences in the association of adipokines with CAC were assessed using the likelihood-ratio test (LRT). A priori, BMI data was not included in the models because adipokines may be intermediate in the causal pathway between adiposity and sub-clinical atherosclerosis.

We used the LRT in nested models to assess the incremental value of each biomarker of adiposity and insulin resistance, adiponectin, leptin, resistin, IL-6, sol-TNFR2, HOMA-IR data (all included as log transformed variables) and metabolic syndrome, in predicting CAC scores beyond established risk factors. Statistical analyses were performed using Stata 9.0 software (Stata Corp, College Station, TX. A tobit regression model was fit in Stata (http://www.ats.ucla.edu/stat/stata/dae/tobit.htm) using the tobit command with the ll(0) option to indicate left-censoring at a CAC score of zero.

Results

Characteristics of Participants

As previously described, (19,21) the SIRCA sample is predominantly Caucasian (Table 1). Plasma levels of adiponectin and leptin were significantly higher in women than in men (p< 0.001 for both). Almost 40% had CAC scores above the 70^th percentile, consistent with accelerated atherosclerosis most likely related to recruitment strategy based on family history of CVD.

Table 1

Characteristics of the SIRCA Study Sample

Differential Association of Adiponectin and Leptin with Cardiovascular Risk Factors

Adiponectin and leptin correlated only modestly (and inversely) with each other while associations with lipid, metabolic and inflammatory variables were greater for both adipokines in women than men (Appendix Table A). Among all factors, adiponectin’s strongest (direct) correlation was with plasma levels of HDL cholesterol, whereas it was only modestly and inversely correlated with HOMA-IR, adiposity and inflammatory markers. In contrast, leptin was strongly associated with BMI, waist circumference and HOMA, moderately correlated with inflammatory markers, blood pressure and apoB lipoproteins but only weakly inversely related to HDL cholesterol.

Plasma Levels of Leptin, but not of Adiponectin, are Associated with Coronary Artery Calcification

In simple models, adiponectin levels were not significantly correlated with CAC in either men or women while leptin had strong direct associations with CAC across gender (Table 2). There were suggestive gender differences in the association of adipokines with CAC in cruder models, which were significant for adiponectin in age and race adjusted models (p<0.01). In fully adjusted models even after controlling for FRS, metabolic syndrome and plasma CRP levels, the top quartile of leptin, but not adiponectin, was significantly associated with CAC scores (Table 2).

Table 2

Association of Plasma Levels of (A) Adiponectin and (B) Leptin with Coronary Artery Calcification in Tobit Multivariable Models

Comparison between Adipocytokines, HOMA-IR, Metabolic Syndrome and CRP in Prediction of CAC Scores

In SIRCA there was no evidence for major gender differences in the association of metabolic syndrome, HOMA-IR, IL-6, sol-TNFR2, and resistin with CAC in adjusted analyses (e.g., p=0.10, 0.37, 0.68, 0.18 respectively for gender difference in age and race adjusted models). Except for adiponectin therefore, results of these analyses are presented for both genders combined. The HOMA-IR index, plasma levels of leptin, IL-6 and sol-TNFR2 as well as the NCEP defined metabolic syndrome (glucose cut-point >110 mg/dL), provided significant improvements in the association with CAC beyond traditional risk factors and the FRS (Table 3). After further adjustment for the metabolic syndrome and CRP data, only HOMA-IR (LRT χ ² 10.39, p<0.01) and plasma leptin levels (LRT χ² 6.87, p<0.01) significantly improved model prediction of CAC (Table 3).

Table 3

Incremental Value of Metabolic Syndrome, C Reactive Protein, Adipocytokines or Homeostasis Model Assessment of Insulin Resistance in Predicting Coronary Calcium Scores beyond Established Risk Factors

Discussion

Adiponectin and leptin are fat secreted hormones with opposing actions on insulin resistance and vascular inflammation. While plasma leptin and adiponectin had opposite correlations with lipid, metabolic and inflammatory risk factors, we found that only plasma leptin levels were independently associated with CAC. Further, in a comparison of several metabolic biomarkers, leptin and the HOMA-IR index of insulin resistance had the most robust associations with CAC scores beyond traditional risk factors, NCEP defined metabolic syndrome and plasma levels of CRP.

Leptin is an important negative regulator of body weight (11). Paradoxically, obesity is associated with increased plasma leptin levels, most likely due to resistance to its actions in the setting of increased production by adipose tissue (29). Leptin activates the endothelium, induces smooth muscle cell proliferation and its receptors are expressed in atherosclerotic plaques (30). Recent studies suggest an association between plasma leptin levels and atherosclerotic CVD in humans including angiographic CAD (31) and CVD events (32). In a case (n=377) control (n=783) study nested within the WOSCOPS clinical trial, plasma leptin levels predicted CVD even after adjusting for traditional risk factors, BMI and plasma CRP levels (15). However, in a nested case-control study from the Quebec Cardiovascular Study Cohort, plasma leptin levels were not related to CVD events (18).

Few data are available on the association between leptin and direct measures of atherosclerosis in humans. Van den Beld et. al. found no association between plasma leptin levels with carotid intima-media thickness (IMT) in 403 healthy elderly men (33), while Ciccone et. al. reported an association of leptin with IMT in 126 healthy Italians (34). We previously reported that leptin levels were associated with CAC in a type-2 diabetic sample even after controlling for establish risk factors including CRP and measures of sub-clinical vascular disease (16). Recently, Iribarren et. al. reported an association of plasma leptin levels with CAC in older women in the ADVANCE study, but this association was not significant after controlling for metabolic risk factors and BMI data (35). In SIRCA, we found an association of plasma leptin with CAC even after controlling for metabolic syndrome and CRP.

Adiponectin has emerged as a unique fat secreted hormone that regulates insulin sensitivity (36). Atheroprotective effects may be directed through inhibition of the NF-κB inflammatory pathway in vascular cells (37) and by attenuation of foam cell formation (38). Plasma levels are depressed in patients with CAD (39) and are associated with clinical CVD in diabetics (7). A nested case control study by Maahs and colleagues suggested low plasma adiponectin predicted short term CAC progression, more so in non diabetics (40). Several recent prospective studies of clinical CVD, however, have been negative. In a nested case-control study from the Strong Heart Study, there was no association with incident CAD events (10). Similarly, in the British Women's Heart and Health Cohort Study, adiponectin levels were not associated with CVD (9). More recently, Sattar et. al looked at 589 men with fatal and non fatal CAD and 1231 controls and found no difference in median adiponectin levels despite adiponectin associations with HDL and CRP. A seven-study meta-analysis by the same authors failed to demonstrate a consistent relationship of adiponectin with CAD events (41).

Despite correlations with lipids, metabolic factors and insulin resistance, we also did not find an inverse association of adiponectin levels with CAC. The reasons for conflicting study findings are uncertain but may reflect differences in study design and populations as well heterogeneous outcomes including sub-clinical atherosclerotic and different CVD outcomes. In fact, Steffes and colleagues unexpectedly found a positive association of adiponectin with CAC in a study of over 3,000 young adults aged 33 to 45 years (42). Finally, several studies suggest that the high molecular weight adiponectin complex, but not the lower molecular weight hexamer, may be the active signaling molecule (43,44). Few epidemiological studies, however, have assayed the different circulating forms of adiponectin.

We also determined which of several metabolic biomarkers predicted CAC scores beyond established clinical CVD risk factors. Leptin, HOMA-IR, and, to a lesser extent, IL-6 and sol-TNFR2, provided incremental value beyond FRS, metabolic syndrome and CRP. Our finding that HOMA-IR levels are associated with CAC beyond all other risk factors is consistent with most (22) but not all (45) studies which found hyperinsulinemia and insulin resistance indices were independently associated with atherosclerosis and CVD. The clinical application of insulin-based measures, however, is challenging given the lack of assay standardization and because of ultradian and circadian variation in circulating insulin.

This study has several limitations. The study sample is cross-sectional and is not capable of determining causal relationships. Moreover, it is a study of a population consisting primarily of Caucasians with a family history of premature CVD who are otherwise deemed to be at low risk, therefore the generalizability of our findings across other populations and ethnic groups is uncertain. In addition, CAC is not a direct measure of coronary atherosclerosis. In autopsy studies, however, CAC has been shown to be a quantitative estimate of coronary atherosclerosis (46). It has also been shown to be an independent predictor of CVD (47).

In summary, we found that plasma levels of leptin but not adiponectin were associated with CAC after controlling for traditional cardiovascular risk factors, metabolic syndrome and CRP levels. Whether leptin signaling promotes human atherosclerotic CVD directly remains to be established. Finally, leptin levels and the HOMA-IR index had stronger associations with CAC scores than other adipocytokines in this asymptomatic sample. A systematic comparison of multiple adipocytokine and insulin resistance biomarkers across diverse clinical settings is warranted in order to establish which provide utility as metabolic biomarkers of clinical CVD.

Supplementary Material

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Acknowledgments

This work was supported by a Clinical and Translational Science Award (RFA-RM-06-002) from the National Center for Research Resources (NCRR) and by a Diabetes Endocrinology Research Award (P30 DK-019525) from the NIH (Bethesda, MD) to the University of Pennsylvania, by RO1 HL-073278, RO1 DK-021224, P50 HL-083799 (SCCOR) and W.W. Smith Charitable Trust (West Conshohocken, PA; Grant #H0204) awards to MR.

Selected Abbreviations of Common Terms


GCRC	GENERAL CLINICAL RESEARCH CENTER

BMI	BODY MASS INDEX

ELISA	ENZYME LINKED IMMUNOSORBENT ASSAY

HOMA-IR	HOMEOSTASIS MODEL ASSESSMENT of INSULIN RESISTANCE

FRS	FRAMINGHAM RISK SCORE

CRP	C-REACTIVE PROTEIN

CAC	CORONARY ARTERY CALCIUM

CVD	CARDIOVASCULAR DISEASE

LRT	LIKELIHOOD RATIO TEST

IMT	INTIMA-MEDIA THICKNESS

Footnotes

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There are no conflicts of interest.

Contributor Information

Atif Qasim, Cardiovascular Division and Center for Experimental Therapeutics, Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA.

Nehal N. Mehta, Cardiovascular Division and Center for Experimental Therapeutics, Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA.

Mahlet G. Tadesse, Department of Mathematics, Georgetown University, Washington, DC.

Megan L. Wolfe, Cardiovascular Division and Center for Experimental Therapeutics, Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA.

Thomas Rhodes, The Department of Epidemiology, Merck Research Laboratories, West Point, PA.

Cynthia Girman, The Department of Epidemiology, Merck Research Laboratories, West Point, PA.

Muredach P Reilly, Cardiovascular Division and Center for Experimental Therapeutics, Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA.

References

1. Mohamed-Ali V, Pinkney JH, Coppack SW. Adipose tissue as an endocrine and paracrine organ. Int J Obes Relat Metab Disord. 1998;22:1145–1158. [PubMed]

2. Koerner A, Kratzsch J, Kiess W. Adipocytokines: leptin--the classical, resistin--the controversical, adiponectin--the promising, and more to come. Best Pract Res Clin Endocrinol Metab. 2005;19:525–546. [PubMed]

3. Takemura Y, Ouchi N, Shibata R, et al. Adiponectin modulates inflammatory reactions via calreticulin receptor-dependent clearance of early apoptotic bodies. J Clin Invest. 2007;117:375–386. [PMC free article] [PubMed]

4. Behre CJ. Adiponectin, obesity and atherosclerosis. Scand J Clin Lab Invest. 2007;67:449–458. [PubMed]

5. Kumada M, Kihara S, Sumitsuji S, et al. Association of hypoadiponectinemia with coronary artery disease in men. Arterioscler Thromb Vasc Biol. 2003;23:85–89. [PubMed]

6. Pilz S, Maerz W, Weihrauch G, et al. Adiponectin serum concentrations in men with coronary artery disease: the LUdwigshafen RIsk and Cardiovascular Health (LURIC) study. Clin Chim Acta. 2006;364:251–255. [PubMed]

7. Schulze MB, Shai I, Rimm EB, Li T, Rifai N, Hu FB. Adiponectin and future coronary heart disease events among men with type 2 diabetes. Diabetes. 2005;54:534–539. [PubMed]

8. Pischon T, Girman CJ, Hotamisligil GS, Rifai N, Hu FB, Rimm EB. Plasma adiponectin levels and risk of myocardial infarction in men. Jama. 2004;291:1730–1737. [PubMed]

9. Lawlor DA, Davey Smith G, Ebrahim S, Thompson C, Sattar N. Plasma adiponectin levels are associated with insulin resistance, but do not predict future risk of coronary heart disease in women. J Clin Endocrinol Metab. 2005;90:5677–5683. [PubMed]

10. Lindsay RS, Resnick HE, Zhu J, et al. Adiponectin and coronary heart disease: the Strong Heart Study. Arterioscler Thromb Vasc Biol. 2005;25:e15–e16. [PubMed]

11. Hamann A, Matthaei S. Regulation of energy balance by leptin. Exp Clin Endocrinol Diabetes. 1996;104:293–300. [PubMed]

12. Segal KR, Landt M, Klein S. Relationship between insulin sensitivity and plasma leptin concentration in lean and obese men. Diabetes. 1996;45:988–991. [PubMed]

13. Chu NF, Spiegelman D, Rifai N, Hotamisligil GS, Rimm EB. Glycemic status and soluble tumor necrosis factor receptor levels in relation to plasma leptin concentrations among normal weight and overweight US men. Int J Obes Relat Metab Disord. 2000;24:1085–1092. [PubMed]

14. Haynes WG. Role of leptin in obesity-related hypertension. Exp Physiol. 2005;90:683–688. [PubMed]

15. Wallace AM, McMahon AD, Packard CJ, et al. Plasma leptin and the risk of cardiovascular disease in the west of Scotland coronary prevention study (WOSCOPS) Circulation. 2001;104:3052–3056. [PubMed]

16. Reilly MP, Iqbal N, Schutta M, et al. Plasma leptin levels are associated with coronary atherosclerosis in type 2 diabetes. J Clin Endocrinol Metab. 2004;89:3872–3878. [PubMed]

17. Soderberg S, Ahren B, Jansson JH, et al. Leptin is associated with increased risk of myocardial infarction. J Intern Med. 1999;246:409–418. [PubMed]

18. Couillard C, Lamarche B, Mauriege P, et al. Leptinemia is not a risk factor for ischemic heart disease in men. Prospective results from the Quebec Cardiovascular Study. Diabetes Care. 1998;21:782–786. [PubMed]

19. Reilly MP, Lehrke M, Wolfe ML, Rohatgi A, Lazar MA, Rader DJ. Resistin is an inflammatory marker of atherosclerosis in humans. Circulation. 2005;111:932–939. [PubMed]

20. Reilly MP, Rohatgi A, McMahon K, et al. Plasma cytokines, metabolic syndrome, and atherosclerosis in humans. J Investig Med. 2007;55:26–35. [PubMed]

21. Reilly MP, Wolfe ML, Localio AR, Rader DJ. C-reactive protein and coronary artery calcification: The Study of Inherited Risk of Coronary Atherosclerosis (SIRCA) Arterioscler Thromb Vasc Biol. 2003;23:1851–1856. [PubMed]

22. Reilly MP, Wolfe ML, Rhodes T, Girman C, Mehta N, Rader DJ. Measures of insulin resistance add incremental value to the clinical diagnosis of metabolic syndrome in association with coronary atherosclerosis. Circulation. 2004;110:803–809. [PubMed]

23. Reilly MP, Wolfe ML, Localio AR, Rader DJ. Coronary artery calcification and cardiovascular risk factors: impact of the analytic approach. Atherosclerosis. 2004;173:69–78. [PubMed]

24. Wilson PW, D'Agostino RB, Levy D, Belanger AM, Silbershatz H, Kannel WB. Prediction of coronary heart disease using risk factor categories. Circulation. 1998;97:1837–1847. [PubMed]

25. Grundy SM, Cleeman JI, Daniels SR, et al. Diagnosis and management of the metabolic syndrome: an American Heart Association/National Heart, Lung, and Blood Institute Scientific Statement. Circulation. 2005;112:2735–2752. [PubMed]

26. Matthews DR, Hosker JP, Rudenski AS, Naylor BA, Treacher DF, Turner RC. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia. 1985;28:412–419. [PubMed]

27. Agatston AS, Janowitz WR, Hildner FJ, Zusmer NR, Viamonte M, Jr, Detrano R. Quantification of coronary artery calcium using ultrafast computed tomography. J Am Coll Cardiol. 1990;15:827–832. [PubMed]

28. Tobin J. Estimation of Relationships for Limited Dependent Variables. Econometrica. 1958;26:24–36.

29. Enriori PJ, Evans AE, Sinnayah P, Cowley MA. Leptin resistance and obesity. Obesity (Silver Spring) 2006;14 Suppl 5:254S–258S. [PubMed]

30. Parhami F, Tintut Y, Ballard A, Fogelman AM, Demer LL. Leptin enhances the calcification of vascular cells: artery wall as a target of leptin. Circ Res. 2001;88:954–960. [PubMed]

31. Stangl K, Cascorbi I, Laule M, et al. Elevated serum leptin in patients with coronary artery disease: no association with the Trp64Arg polymorphism of the beta3-adrenergic receptor. Int J Obes Relat Metab Disord. 2000;24:369–375. [PubMed]

32. Wolk R, Berger P, Lennon RJ, Brilakis ES, Johnson BD, Somers VK. Plasma leptin and prognosis in patients with established coronary atherosclerosis. J Am Coll Cardiol. 2004;44:1819–1824. [PubMed]

33. van den Beld AW, Bots ML, Janssen JA, Pols HA, Lamberts SW, Grobbee DE. Endogenous hormones and carotid atherosclerosis in elderly men. Am J Epidemiol. 2003;157:25–31. [PubMed]

34. Ciccone M, Vettor R, Pannacciulli N, et al. Plasma leptin is independently associated with the intima-media thickness of the common carotid artery. Int J Obes Relat Metab Disord. 2001;25:805–810. [PubMed]

35. Iribarren C, Husson G, Go AS, et al. Plasma leptin levels and coronary artery calcification in older adults. J Clin Endocrinol Metab. 2007;92:729–732. [PubMed]

36. Stefan N, Stumvoll M. Adiponectin--its role in metabolism and beyond. Horm Metab Res. 2002;34:469–474. [PubMed]

37. Ouchi N, Kihara S, Arita Y, et al. Adiponectin, an adipocyte-derived plasma protein, inhibits endothelial NF-kappaB signaling through a cAMP-dependent pathway. Circulation. 2000;102:1296–1301. [PubMed]

38. Ouchi N, Kihara S, Arita Y, et al. Adipocyte-derived plasma protein, adiponectin, suppresses lipid accumulation and class A scavenger receptor expression in human monocyte-derived macrophages. Circulation. 2001;103:1057–1063. [PubMed]

39. Liang KW, Sheu WH, Lee WL, et al. Decreased circulating protective adiponectin level is associated with angiographic coronary disease progression in patients with angina pectoris. Int J Cardiol. 2007 [PubMed]

40. Maahs DM, Ogden LG, Kinney GL, et al. Low plasma adiponectin levels predict progression of coronary artery calcification. Circulation. 2005;111:747–753. [PubMed]

41. Sattar N, Wannamethee G, Sarwar N, et al. Adiponectin and coronary heart disease: a prospective study and meta-analysis. Circulation. 2006;114:623–629. [PubMed]

42. Steffes MW, Gross MD, Lee DH, Schreiner PJ, Jacobs DR., Jr Adiponectin, visceral fat, oxidative stress, and early macrovascular disease: the Coronary Artery Risk Development in Young Adults Study. Obesity (Silver Spring) 2006;14:319–326. [PubMed]

43. Pajvani UB, Du X, Combs TP, et al. Structure-function studies of the adipocyte-secreted hormone Acrp30/adiponectin. Implications fpr metabolic regulation and bioactivity. J Biol Chem. 2003;278:9073–9085. [PubMed]

44. Yamauchi T, Kamon J, Waki H, et al. Globular adiponectin protected ob/ob mice from diabetes and ApoE-deficient mice from atherosclerosis. J Biol Chem. 2003;278:2461–2468. [PubMed]

45. Bertoni AG, Wong ND, Shea S, et al. Insulin resistance, metabolic syndrome, and subclinical atherosclerosis: the Multi-Ethnic Study of Atherosclerosis (MESA) Diabetes Care. 2007;30:2951–2956. [PubMed]

46. Mautner GC, Mautner SL, Froehlich J, et al. Coronary artery calcification: assessment with electron beam CT and histomorphometric correlation. Radiology. 1994;192:619–623. [PubMed]

47. Kondos GT, Hoff JA, Sevrukov A, et al. Electron-beam tomography coronary artery calcium and cardiac events: a 37-month follow-up of 5635 initially asymptomatic low- to intermediate-risk adults. Circulation. 2003;107:2571–2576. [PubMed]