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Dr. Daniel J. Rader, 11-125 Translational Research Center, 3400 Civic Center Boulevard, Building 421, Philadelphia, Pennsylvania 19104-5158
- coronary heart disease
- high-density lipoprotein cholesterol
- high-density lipoprotein particles
Limitations in our ability to predict cardiovascular risk have fueled efforts to identify novel risk markers and to refine the measurement of traditional risk factors, such as low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C). Standard assays to evaluate LDL-C and HDL-C quantify the cholesterol content within the respective lipoprotein fraction. However, both LDL and HDL particles vary in their content of cholesterol, and thus determining the concentration of lipoprotein particles themselves may be superior to counting cholesterol cargo in assessing cardiovascular risk. For example, there is 1 molecule of apolipoprotein B (apo B) per low-density lipoprotein-particle (LDL-P) and, thus, plasma concentrations of apo B more closely reflect LDL-P concentrations than levels of LDL-C. Some (1,2), but not all (3), prospective population studies indicate a stronger association between apo B and cardiovascular events than between LDL-C and cardiovascular events.
Another approach to quantitation of lipoprotein particle concentrations utilizes nuclear magnetic resonance (NMR) spectroscopy. This method takes advantage of 2 important principles to permit rapid quantification of lipoprotein concentrations without requiring physical separation (4). First, each lipoprotein subclass emits a distinct signal when subjected to electromagnetic pulses in a magnetic field; second, the signal amplitudes generated are directly proportional to the concentration of the particles emitting the signal. Using a library of known lipids, sample signals can be deconvoluted to determine concentrations of individual lipoprotein subclasses. There is now abundant literature on the quantitation of LDL-P number by NMR spectroscopy and association with cardiovascular events, and in most cases LDL-P outperforms LDL-C in predicting cardiovascular risk. Moreover, limited data comparing LDL-C and LDL-P in the setting of discordance—when the 2 measures differ substantially—indicate a more robust association of the latter with cardiovascular events and surrogate imaging markers. In an analysis of the Framingham cohort, event-free survival tracked LDL-P and not LDL-C when the 2 were discordant (5). Similarly, in a previous examination of the MESA (Multi-Ethnic Study of Atherosclerosis), LDL-P proved a more reliable predictor of carotid intima-media thickness (cIMT) than LDL-C when the 2 values differed (6).
The report by Mackey et al. (7) in this issue of the Journal extends the lipoprotein cholesterol content versus lipoprotein particle number debate to HDL. The authors suggest that NMR-derived HDL particle (HDL-P) number may serve as a better method than HDL-C to assess cardiovascular risk. Leveraging the diverse and well-characterized MESA cohort, Mackey et al. (7) studied 5,598 men and women without known coronary heart disease (CHD) or lipid-lowering therapy reported at baseline. The authors evaluated the associations of baseline HDL-C and HDL-P number determined by NMR spectroscopy with maximal cIMT, a validated surrogate marker of atherothrombotic risk, and coronary events (myocardial infarction, CHD death, angina) after a mean follow-up of 6 years. After adjusting for age, sex, ethnicity, hypertension, and smoking, Mackey et al. (7) calculated, per 1-SD increase in HDL-C or HDL-P, a decrease in cIMT of 0.026 mm (95% confidence interval [CI]: 0.017 to 0.035 mm) and 0.030 mm (95% CI: 0.021 to 0.039 mm), and a reduction in CHD risk of 26% (95% CI: 12% to 37%) and 30% (95% CI 18% to 41%), respectively. Although associations between HDL-C and HDL-P and cardiovascular endpoints were similar in standard multivariable-adjusted models, the novel and important contribution of this report was found after adjusting each HDL metric for the other and for LDL-P. Joint analysis revealed that the inverse relationship between HDL-P and cardiovascular risk persisted after controlling for HDL-C and LDL-P, with a 1-SD increase in HDL-P associated with a decrease in cIMT of 0.022 mm (95% CI: 0.011 to 0.034 mm) and a reduction in CHD risk of 25% (95% CI: 7% to 39%). In contrast, HDL-C was neither associated with cIMT nor CHD risk after incorporating HDL-P and LDL-P into the model.
The results of Mackey et al. (7) are consistent with previous findings from the EPIC (European Prospective Investigation into Cancer and Nutrition)-Norfolk population-based prospective study (6). A case-control analysis demonstrated a consistent association between HDL-P and coronary artery disease (CAD) risk after adjusting for traditional, metabolic, and inflammatory risk factors as well as mean HDL size assessed by NMR (top vs. bottom quartile, odds ratio [OR] 0.50; 95% CI: 0.37 to 0.66, p < 0.001). Although adjustment for HDL-C was not reported, the close correlation between HDL size and HDL-C (Pearson correlation coefficient 0.76, p < 0.001) suggested that an independent association between HDL-P and CAD risk might remain even after adjustment for HDL-C. In contrast, the MESA findings disagreed with previous results of the Women's Health Study (WHS) (8). In the trial involving 27,673 healthy women followed over an 11-year period, HDL-P was not associated with incident cardiovascular disease after adjustment for age, randomized treatment assignment, smoking status, menopausal status, postmenopausal hormone use, blood pressure, diabetes mellitus, and body mass index (top vs. bottom quintile, hazard ratio [HR]: 0.91; 95% CI: 0.71 to 1.12, p < 0.34). WHS differed from MESA in its exclusive focus on women, who comprised 53% of MESA participants, and in the randomized design, which evaluated low-dose aspirin and vitamin E. Importantly, levels of HDL-P subfractions were substantially different between the 2 studies, which might explain the conflicting results.
The consistent association observed in the MESA analysis between HDL-P and cardiovascular events even after controlling for HDL-C raises several important questions.
1. For which patients might HDL-P measurement be reasonable to refine cardiovascular risk? Establishing the clinical utility of HDL-P requires more than an independent association with cIMT or cardiovascular outcomes. Calibration, discrimination, and reclassification analyses are needed to further clarify the role of HDL-P above and beyond traditional risk factors. Highlighting the incremental prognostic value of HDL-P in the setting of discordance between HDL-P and HDL-C—higher (or lower) HDL-P values than expected for given HDL-C levels— might better focus such examination. The prevalence of and clinical phenotypes associated with discrepant HDL-P and HDL-C also need to be clarified.
2. What is the relationship between HDL-P and apolipoprotein A-I (apo A-I)? Apo A-I is the primary protein constituent of HDL, accounting for 70% of total HDL protein. In numerous epidemiological studies, levels of apo A-I demonstrated an inverse relationship to cardiovascular risk, sometimes greater than that for HDL-C (1,9,10). Importantly, apo A-I is not synonymous with the concentration of HDL-P. Large, spherical HDL-P carry 4 to 5 apo A-I molecules, whereas small discoidal HDL bear 2 to 3 molecules (11). As a result, the correlation between apo A-I and HDL-P number is imperfect, with coefficients of 0.54 and 0.69 observed in the EPIC-Norfolk cohort and the WHS, respectively (6,8). Given the inconsistent relationship between apo A-I and HDL-P, it is surprising that previous analyses of 2 different cohorts yielded nearly identical results to the MESA study using apo A-I instead of HDL-P. In a case-control study of the EPIC-Norfolk cohort (12), HDL-C was no longer predictive of CAD risk after adjustment for apo A-I and apo B (per 1-SD increase, OR: 1.07, 95% CI: 0.89 to 1.28, p < 0.49). However, apo A-I remained significantly associated with CAD risk even after adjusting for HDL-C and apo B (per 1-SD increase, OR: 0.74, 95% CI: 0.62 to 0.88, p < 0.001) or HDL size and apo B (per 1-SD increase, OR: 0.69, 95% CI: 0.61 to 0.79; p < 0.0001). Similar results were found in a post hoc analysis of the IDEAL (Incremental Decrease in End Points through Aggressive Lipid Lowering) trial, a randomized trial comparing atorvastatin 80 and simvastatin 20 mg/day (12). After adjustment for apo A-I and apo B, HDL-C became positively associated with CAD (per 1-SD increase, risk ratio [RR]: 1.21, 95% CI: 1.01 to 1.46, p < 0.04). In contrast, apo A-I remained inversely associated with CAD even after adjustment for HDL-C and apo B (per 1-SD increase, RR: 0.74, 95% CI: 0.61 to 0.90, p < 0.002).
Unfortunately, apo A-I (and apo B) data were unavailable for the MESA cohort, and only 1 large prospective study examined both apo A-I and HDL-P levels (8). In the WHS, unlike HDL-P, HDL-C and apo A-I were inversely associated with incident cardiovascular disease after multivariable adjustment (HDL-C quintile 5 vs. quintile 1, HR: 0.53, 95% CI: 0.42 to 0.64, p < 0.001; apo A-I quintile 5 vs. quintile 1, HR: 0.63, 95% CI: 0.52 to 0.77, p < 0.001). Future research would benefit from simultaneously acquired apo A-I and HDL-P data to compare their relationship with cardiovascular risk, including after adjustment for HDL-C.
3. What is the biological explanation for the more robust association between HDL-P and cardiovascular risk? Identifying which aspects of HDL functionality are uniquely captured by HDL-P and not HDL-C represents an important next step. Recent publications shed light on promising assays that measure reverse cholesterol transport and anti-inflammatory activity. One study utilized a validated ex vivo system to quantify cholesterol efflux capacity using incubation of macrophages with apo B-depleted serum (13). Healthy participants exhibited an inverse relationship between efflux capacity and cIMT before and after adjustment for HDL-C. Among subjects who underwent coronary angiography for clinically suspected CHD, efflux capacity remained a strong inverse predictor of coronary disease status after adjustment for traditional risk factors as well as HDL-C (adjusted OR for CAD per 1-SD increase in efflux capacity: 0.75; 95% CI: 0.63 to 0.90) and apo A-I (OR: 0.74; 95% CI: 0.61 to 0.89). A second study demonstrated an association between coronary disease status and the HDL inflammatory index (HII), the latter quantified as the ratio of in vitro LDL oxidation of a fluorescein substrate incubated with and without participant HDL (14). Among 193 symptomatic patients who underwent angiography, HII was significantly higher (less antioxidant capacity) among those with acute coronary syndrome (ACS) than those without CAD (1.57 vs. 1.17, p < 0.005) or with stable CAD (1.57 vs. 1.11, p < 0.006). Association with ACS remained significant after adjusting for traditional risk factors (OR 3.8, p < 0.003). Studies incorporating both HDL-P assessment and functional assays such as cholesterol efflux or anti-inflammatory activity are needed to better understand the physiological basis of the more reliable association between HDL-P and cardiovascular outcomes.
4. Can HDL-P serve as a more reliable surrogate marker of antiatherogenic potential for HDL-directed therapies in development? In the setting of HDL-directed therapies, a consistent inverse relationship between HDL-C and cardiovascular risk can no longer be assumed (15). Reliable surrogate markers are needed to facilitate the development of novel HDL therapies. As previously discussed, assays that measure specific dimensions of HDL functionality are being evaluated. Physicochemical measures of HDL-P, such as quantity, size, and composition, represent a second type of surrogate marker that may serve as proxies for function and, in turn, antiatherogenic potential (16). Assays to determine these intrinsic HDL characteristics generally benefit from greater ease and precision than methods to interrogate HDL function, facilitating use in large population studies and clinical trials. Although the study of Mackey et al. (7) was not an intervention study—participants on lipid-lowering therapy were excluded—the independent association of HDL-P with cardiovascular events suggests that HDL-P may serve as a useful tool to assess HDL-directed pharmacotherapies. Moreover, the findings suggest that increasing HDL-P levels may be more desirable than augmenting HDL-C levels. To date, few studies have examined the effect of existing and emerging HDL-targeted drugs on HDL-P. In a post hoc analysis of the VA-HIT (Veterans Affairs High-Density Lipoprotein Intervention Trial), a randomized, placebo-controlled trial of gemfibrozil among men with CHD, on-treatment HDL-P was strongly associated with CHD events (OR: 0.71, 95% CI: 0.61 to 0.81, p < 0.0001), whereas HDL-C was not (OR: 0.95, 95% CI: 0.83 to 1.08, p < 0.42). Extended-release niacin 2000 mg/day increased both HDL-C and total HDL-P by 23% after 12 weeks of use (17). The relationship between niacin, HDL-P, and cardiovascular endpoints has not been elucidated; changes in HDL-P in the recently reported AIM-HIGH (Atherothrombosis Intervention in Metabolic syndrome with low HDL/high triglycerides: Impact on Global Health outcomes) trial (18) that failed to show benefit of niacin in reducing cardiovascular events would be of interest. Changes in total HDL-P with administration of cholesteryl ester transfer protein inhibitors have not been reported in the published literature. Further studies are needed to assess changes in HDL-P resulting from HDL-directed therapies and to determine whether the inverse relationship between HDL-P and cardiovascular outcomes persists in the context of these interventions.
In summary, the study of Mackey et al. (7) demonstrated a more consistent inverse association between cardiovascular endpoints and NMR-derived HDL-P compared with HDL-C. These findings suggest that direct quantification of the concentration of HDL-P may be useful to refine cardiovascular risk and to evaluate novel HDL-directed therapies. Further studies are needed to clarify the role of HDL-P in clinical practice.
Dr. deGoma has reported that he has no relationships relevant to the contents of this paper to disclose. Dr. Rader has relationships with LipoScience and Vascular Strategies.
↵⁎ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
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- Pischon T.,
- Girman C.J.,
- Sacks F.M.,
- Rifai N.,
- Stampfer M.J.,
- Rimm E.B.
- Benn M.,
- Nordestgaard B.G.,
- Jensen G.B.,
- Tybjaerg-Hansen A.
- Mackey R.H.,
- Greenland P.,
- Goff D.C. Jr.,
- Lloyd-Jones D.,
- Sibley C.T.,
- Mora S.
- Lamarche B.,
- Moorjani S.,
- Lupien P.J.,
- et al.
- Kontush A.,
- Chapman M.J.
- van der Steeg W.A.,
- Holme I.,
- Boekholdt S.M.,
- et al.
- deGoma E.M.,
- deGoma R.L.,
- Rader D.J.
- Rosenson R.S.,
- Brewer H.B. Jr..,
- Chapman M.J.,
- et al.