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High Molecular Weight Adiponectin Is Not Associated with Incident Coronary Heart Disease in Older Women: A Nested Prospective Case-Control Study

British Heart Foundation Glasgow Cardiovascular Research Centre (N.S., P.W., L.C.), University of Glasgow, Glasgow G12 8TA, Scotland, United Kingdom; Department of Epidemiology and Population Health (S.E.), London School of Hygiene and Tropical Medicine, London WC1E 7HT, England, United Kingdom; and Medical Research Council Centre of Causal Analyses in Translational Epidemiology (G.D.S., D.A.L.), University of Bristol, Bristol BS8 2PR, England, United Kingdom

Address all correspondence and requests for reprints to: Naveed Sattar, Professor of Metabolic Medicine, BHF Glasgow Cardiovascular Research Centre, University of Glasgow, Glasgow G12 8TA, United Kingdom. E-mail: nsattar{at}clinmed.gla.ac.uk.

	Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Context: Adiponectin levels appear weakly linked to incident vascular disease, but the high molecular weight (HMW) fraction may be more relevant.

Objective: Our objective was to test whether HMW adiponectin, the key biologically active fraction, is linked to incident coronary heart disease (CHD) events.

Design, Participants, and Main Outcome Measures: We assessed the association between HMW adiponectin (measured by ELISA) and CHD risk in a prospective (4-yr) case-control study nested within the British Women's Heart and Health Study. All women were postmenopausal.

Setting: Women were seen in a primary care setting.

Results: Among both cases (n = 167) and controls (n = 333), HMW adiponectin positively correlated with age and high-density lipoprotein cholesterol and inversely correlated with waist to hip ratio, fasting insulin, fasting glucose, homeostasis model assessment for insulin resistance scores, C-reactive protein, and triglycerides, in similar fashion to total adiponectin. The age-adjusted relative risk ratio for a doubling of HMW adiponectin was 0.96 (95% confidence interval, 0.78–1.18), and adjustment for any of the potential confounding or mediating variables did not substantively alter this. Additional adjustments for childhood social class, alcohol consumption, hormone replacement therapy use, statin, aspirin, or blood pressure medication did not alter the null association. When we examined the effect of HMW adiponectin by quarters of its distribution, there was no evidence of any associations (P trend = 0.71). There was also no association of the ratio of HMW adiponectin to total adiponectin with CHD risk; age-adjusted relative risk per doubling of the ratio was 1.10 (95% confidence interval, 0.80–1.50).

Conclusions: Despite associations with total adiponectin and insulin resistance, our data go against any apparent association between HMW adiponectin levels and incident CHD events.

	Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
High levels of adiponectin are unequivocally associated with lower risk of incident diabetes (1, 2, 3). High adiponectin was initially reported to be independently associated with lower risk for incident myocardial events (4). Consequently, its role in the common soil hypothesis was proposed. However, subsequent studies, inclusive of the British Regional Heart Study as well as a metaanalysis of seven prospective studies involving a total of 1318 coronary heart disease (CHD) cases, did not confirm a significant association of adiponectin with risk for incident CHD events [odds ratio for top third vs. bottom, 0.84; 95% confidence interval (CI), 0.70–1.01] (5). Nevertheless, because the high molecular weight (HMW) form of adiponectin appears to be 1) better associated with insulin sensitivity than total adiponectin (6), 2) more strongly associated with lower risk for incident diabetes (7), and 3) more strongly linked to protection against endothelial cell apoptosis (8), there is a need to examine whether this fraction is related to risk for vascular events.

Using a nested case-control study of the British Women's Heart and Health Study (BWHHS), we tested the hypothesis that higher levels of the HMW rather than total adiponectin (9) would be associated with lower risk of incident CHD events.

	Subjects and Methods

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The BWHHS is a prospective cohort study of 4286 women randomly selected from 23 British towns between 1999 and 2001 and who were aged 60–79 yr at baseline (10). The prospective case-control study and endpoint ascertainment nested within this cohort and used herein has been previously described (9). Briefly, an incident case was defined as either 1) death with an underlying or contributing cause of CHD (ICD10 codes I20-I25, I51.6) or 2) a myocardial infarction (World Health Organization criteria), first diagnosis of angina, or coronary artery bypass or angioplasty. There were 167 incident cases of CHD (101 nonfatal and 66 fatal). Two controls were randomly selected, within 5-yr age groups of the cases, from women without CHD at the baseline assessment. The BWHHS has local and multicenter research ethics committee approvals, and all participants provided written informed consent for their medical records to be reviewed.

Baseline blood samples were taken after a minimum 6-h fast. HMW adiponectin was measured by an ELISA (ALPCO Diagnostics, Salem, NH) with a protease to digest both the low and medium molecular weight forms, leaving the HMW form intact for measurement (11). This ELISA has been validated against conventional fractionation, with excellent correlations (r = 0.94–0.97) between methods (http://www.alpco.com/pdfs/47/Human%20Multimeric%20Adiponectin%20Details.pdf). The intraassay coefficient of variation was 6%. Because we had previously measured total adiponectin in the same cohort by a different ELISA (R&D Systems, Abingdon, UK) (9), we correlated total adiponectin by the ALPCO and R&D ELISAs in a subset of 80 samples; an excellent correlation of r = 0.89 between methods was noted despite blinded measurements, conducted several months apart with different dilutions. Repeated-sample freeze-thaw did not influence HMW adiponectin values.

Methods for insulin, glucose, and lipids levels and C-reactive protein (CRP) and homeostasis model assessment of insulin resistance (HOMA-IR) have been detailed previously (10). All blood samples were taken between 0800 and 1800 h with the time of sampling (to nearest minute) recorded. Levels of total adiponectin and HMW adiponectin did not vary by time of day of sampling.

Medication history, blood pressure, body mass index, waist and hip circumference, and information on adult and childhood occupational social class, smoking, alcohol consumption, and physical activity were determined as previously described (10).

Statistical analyses

Spearman's rank correlation coefficients were used to assess the associations of continuous covariables with HMW adiponectin and total adiponectin. There was no statistical evidence that any of these associations differed by case-control status, and therefore correlation coefficients are presented for the whole sample (cases and controls combined). Distributions of HMW adiponectin, total adiponectin, and other characteristics are presented for cases and controls. Differences between cases and controls were assessed using an unpaired t test for continuous variables and ² test for categorical variables. Multiple logistic regression was used to assess the association of HMW adiponectin with CHD with adjustment for covariables. Geometric means and their 95% CI were used for positively skewed variables (HMW adiponectin, total adiponectin, CRP, glucose, insulin, HOMA-IR scores, and triglycerides) with logged values used in regression models. HMW adiponectin and total adiponectin were entered as continuous variables (natural logs of their levels) in these regression models. Because regression coefficients for logged exposure variables are difficult to interpret, these effect estimates are expressed as the risk ratio of CHD for a doubling of HMW or total adiponectin, as in other prospective studies of the association of total adiponectin with CHD. Possible nonlinear associations were explored by entering quarters of the HMW adiponectin distribution first as a series of three indicator variables and then as a continuous score and computing a likelihood ratio test comparing these two nested models. In the nested case control study design, the odds ratio derived from logistic regression directly estimates the incidence rate ratio and hence the risk ratio (12, 13). We repeated our analyses using conditional logistic regression and found the same, although less precise, results. All analyses were conducted in Stata version 9.0 (Stata Corp., College Station, TX).

	Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
One control did not have a baseline sample; thus, data were available for 167 cases and 333 controls. HMW adiponectin was correlated with total adiponectin (r = 0.75) (Table 1). Both were inversely associated with waist to hip ratio, fasting insulin, glucose, HOMA-IR, CRP, and triglyceride levels and positively associated with high-density lipoprotein cholesterol (HDL-C) levels. There was no strong statistical evidence of heterogeneity between the associations of either form of adiponectin with any of these traits (all P values > 0.3).

HMW adiponectin levels were lower in women who were in manual, compared with nonmanual, social classes in adulthood, but there were no strong associations of smoking, physical activity, alcohol consumption, or statin, aspirin, or HRT use with HMW adiponectin levels (full data not shown).

In multivariable analyses, there was no association between HMW adiponectin and incident CHD (Table 2). The age-adjusted relative risk ratio for incident CHD for a doubling of HMW adiponectin was 0.96 (95% CI, 0.78–1.18), and adjustment for any of the potential confounding or mediating variables, including glucose, did not substantively alter this (Table 2). The same was true when we removed the 25 women with baseline diabetes (data not shown). Additional adjustments for childhood social class, alcohol consumption, HRT use, statin, aspirin, or blood pressure medication did not alter the null association (data not shown). When we examined the effect of HMW adiponectin by quarters of its distribution, there was no evidence of any linear or nonlinear associations (Fig. 1). There was also no association of the ratio of HMW adiponectin to total adiponectin with CHD risk; age-adjusted relative risk per doubling of the ratio was 1.10 (95% CI, 0.80–1.50).

View larger version (6K): [in this window] [in a new window]   FIG. 1. Proportion (percentage) of women with incident CHD (cases) by quarters of the distribution of HMW adiponectin. The vertical lines represent 95% CI.

	Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

Several aspects of our study merit discussion. First, our method of assessment of HMW adiponectin is relatively new but robust. Although Bluher and colleagues (14) questioned the robustness of this method for measurement of HMW adiponectin, their study had only 60 women spanning glycemia thresholds and reported no significant association of total adiponectin by the ALPCO method with two other methods for total adiponectin (both r = 0.25; P value only described as nonsignificant in paper) and also no association of either total or HMW fractions with fasting insulin (r = –0.09 and r = –0.04, respectively, both nonsignificant). By contrast, in our study of 500 women predominantly without diabetes, not only did the HMW fraction correlate with total adiponectin (r = 0.75; P < 0.001) by a different method, the total adiponectin method by ALPCO correlated with the R&D ELISA (r = 0.89; P < 0.001) in a subset of 80 samples. In addition, the HMW fraction correlated with both fasting insulin (r = –0.35; P < 0.001) and with other metabolic parameters in the expected directions of association. Others have reported similar correlations between total and HMW adiponectin; Halperin et al. (15) reported a correlation of r = 0.72 between percent HMW and total adiponectin using the velocity sedimentation analysis method. Finally, the ALPCO HMW method has been previously validated against the more conventional methods employed for adiponectin fractionation (see Subjects and Methods) and has been shown by others (6, 16) to perform well and correlate to insulin sensitivity and related parameters, at least as well as total adiponectin, in larger sized cross-sectional studies than that published by Bluher et al. (14). Thus, methodological issues relating to the assay are unlikely to explain our null findings.

Interestingly, in line with our data, Halperin et al. noted no evidence of a relationship between total adiponectin or percent HMW and endothelium-dependent vasodilation (15), a vascular risk surrogate, in offspring of diabetic parents. Furthermore, higher adiponectin levels predict a higher, not lower, risk for cardiovascular disease mortality in the general population (17). HMW adiponectin also predicts higher risk of mortality in those with heart failure (18), an observation consistent with a positive correlation between circulating adiponectin and brain natriuretic peptide (19). These findings suggest that total and HMW adiponectin have complex relationships with vascular disease and that in any given population, some individuals have physiological (i.e. beneficial) elevations, whereas others have pathologically driven elevated levels (i.e. reflecting harmful signals). This suggestion provides a working hypothesis to explain paradoxical observations.

Our study has potential limitations. We used different assays for total (R&D) and HMW (ALPCO) adiponectin, but our observations above suggest this was an acceptable approach. Our results cannot be extrapolated necessarily to men or other ethnic groups. We accept that larger studies are needed to provide more reassurance of our null result, although the adjusted result of 1.01 (0.81–1.24) for incident CHD events with a doubling of HMW adiponectin is close to unity. Finally, although the correlation between HMW adiponectin and HOMA-IR score was not greater than between total adiponectin and HOMA-IR score, others have seen broadly similar results (20).

In conclusion, despite evidence to suggest HMW adiponectin may be the biologically active fraction of the adiponectin species, our results go against a strong inverse association between HMW adiponectin and incident CHD events. These findings further support a stronger association of adiponectin (and its fractions) with diabetes than vascular disease.

	Footnotes

 
Disclosure Statement: The authors have nothing to disclose.

First Published Online February 26, 2008

Abbreviations: BWHHS, British Women's Heart and Health Study; CHD, coronary heart disease; CI, confidence interval; CRP, C-reactive protein; HDL-C, high-density lipoprotein-cholesterol; HMW, high molecular weight; HOMA-IR, homeostasis model assessment for insulin resistance; HRT, hormone replacement therapy; LDL, low-density lipoprotein-cholesterol.

Received November 27, 2007.

Accepted February 15, 2008.

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