Abstract
Objectives
High plasma sphingomyelin level has been associated with subclinical atherosclerosis, coronary artery disease and worse prognosis in subjects with acute coronary syndromes. We assess the predictive value of plasma sphingomyelin levels for incident CHD events in the Multi Ethnic Study of atherosclerosis (MESA).
Method and Results
Plasma sphingomyelin was measured in 6809 out of 6814 subjects with mean age 62.2 ± 10.2 years, participating in the MESA study, a population based cohort study of adults free of clinical CVD at baseline recruited at six clinic sites in USA. The subjects consisted of 52.8% females, 38.5% Caucasian, 11.8% Chinese, 27.8% African Americans and 21.9% Hispanics. Cox proportional hazard analysis was used to examine the association between plasma sphingomyelin and five years of adjudicated incident CHD events including MI, resuscitated cardiac arrest, angina, CHD death and revascularization (CABG or PTCA). Mean (SD) plasma sphingomyelin level was 48 mg/dl (16.0). One hundred and eighty-nine subjects had an adjudicated CHD event during the five years of follow up. In the Kaplan meier analysis, subjects with plasma sphingomyelin level above the sex specific median had similar event free survival rate compared with subjects with plasma sphingomyelin level below or equal to the sex specific median (97.16% vs 97.0%, log rank p= 0.713). In the univariate Cox proportional hazard analysis, plasma sphingomyelin was not a predictor of incident CHD event [hazard ratio 0.992(0.982 – 1.004), p=0.09]. In our multistage multivariable Cox models, higher plasma sphingomyelin had modest negative association with incident CHD events when total cholesterol, HDL and triglycerides were included in the model [hazard ratio 0.985 (0.973 – 0.996), p=0.008] and also in our full model after adjusting for age, gender, total cholesterol, HDL, triglycerides, diabetes, cigarette smoking, systolic BP, diastolic BP, BP medication use, HMG CoA use [hazard ratio 0.984 (0.973 – 0.996), p=0.002]. In other models, plasma sphingomyelin was not associated with incident CHD events.
Conclusion
High plasma sphingomyelin level is not associated with increased risk of incident coronary heart disease in population based adults free of clinical cardiovascular disease at baseline.
Keywords: Plasma sphingomyelin, prognosis, coronary heart disease events, epidemiology
Introduction
The role of sphingolipids in the pathogenesis and progression of atherosclerosis is an area of active research (1, 2). Experimental data from cell biology and animals have suggested an association between both sphingomyelin and ceramide; a metabolite of sphingomyelin and the development and progression of atherosclerosis (3). Sphingomyelin and ceramide has been isolated from atherosclerotic plaques in both humans and animals (4-6). Myriocin, a serine palmitoyl transferase inhibitor which reduces plasma and tissue levels of several sphingolipids, including sphingomyelin, ceramide, sphingosine-1-phosphate, and glycosphingolipids, has been shown to inhibit and even cause regression of atherosclerotic plaques in animal studies (7, 8). Although most of the sphingolipids identified in atherosclerotic plaques appears to be synthesized denovo, current research has shown that some of these sphingolipids originates from plasma (9).
Despite the numerous data associating sphingomyelin level and atherosclerosis in animals (1-5), limited data exist in humans. Plasma sphingolipids are elevated in various primary hyperlipoproteinemic states (10). Cross-sectional studies have shown an association between high plasma sphingomyelin levels with subclinical atherosclerosis (11) and clinical coronary artery disease (12). High plasma sphingomyelin levels have also been associated with worse outcomes in acute coronary syndromes in a small selected cohort (13). Prospective data on the association between plasma sphingomyelin levels and incident coronary heart disease events are lacking.
We therefore assessed the association between plasma sphingomyelin levels and incident coronary heart disease in participants of the Multi Ethnic Study of Atherosclerosis (MESA).
Methods
Study Population and Data Collection
The study design for MESA has been published elsewhere (14). In brief, MESA is a prospective cohort study that began in July 2000 to investigate the prevalence, correlates and progression of subclinical CVD in individuals without known CVD at baseline. The cohort includes 6814 women and men aged 45-84 years old recruited from 6 US communities (Baltimore, Md; Chicago, Ill.; Forsyth County, N.C.; Los Angeles County, Calif.; northern Manhattan, N.Y.; and St. Paul, Minn.). MESA cohort participants were 38% white (n=2624), 28% black (n=1895), 22% Hispanic (n=1492), and 12% Chinese (n=803). Individuals with a history of physician–diagnosed myocardial infarction, angina, heart failure, stroke, or transient ischemic attack, or who had undergone an invasive procedure for CVD (coronary artery bypass graft, angioplasty, valve replacement, pacemaker placement or other vascular surgeries) were excluded from participation. This study was approved by the Institutional Review Boards of each study site and written informed consent was obtained from all participants.
Demographics, medical history, anthropometric and laboratory data for the present study were taken from the first examination of the MESA cohort (July 2000-August 2002). Current smoking was defined as having smoked a cigarette in the last 30 days. Diabetes mellitus was defined as fasting glucose ≥ 126 mg/dl or the use of hypoglycemic medications. Use of antihypertensive and other medications was based on review of prescribed medication containers. Resting blood pressure was measured 3 times in the seated position, and the average of the second and third readings was recorded. Hypertension was defined as a systolic blood pressure ≥ 140 mmHg, diastolic blood pressure ≥ 90 mmHg, or use of medication prescribed for hypertension. Body mass index was calculated as weight (kg)/height (m2).
Laboratory measurements
Blood specimens from fasting participants were processed within 30 minutes of phlebotomy and frozen at −70°C. Total cholesterol was analyzed by use of a cholesterol oxidase method (Roche Diagnostics, Indianapolis, Indiana) (laboratory coefficient of variation, 1.6 percent). High density lipoprotein cholesterol was measured in ethylenediaminetetraacetic acid/plasma by use of the cholesterol oxidase method (Roche Diagnostics) after precipitation of non-high density lipoprotein cholesterol with magnesium/dextran (laboratory coefficient of variation, 2.9 percent). The triglyceride level was measured in ethylenediaminetetraacetic acid/plasma by use of triglyceride GB reagent (Roche Diagnostics) on the Roche COBAS FARA centrifugal analyzer (laboratory coefficient of variation, 4.0 percent). Low density lipoprotein cholesterol was calculated in plasma specimens having a triglyceride value of less than 400 mg/dl with the use of the formula of Friedewald et al. (15). Serum glucose was measured by rate reflectance spectrophotometry on the Vitros analyzer (Johnson & Johnson Clinical Diagnostics, Inc., Rochester, New York) (laboratory coefficient of variation, 1.1 percent). Enzymatic measurement of plasma sphingomyelin levels was carried out at Columbia University using a novel four-step procedure (12). In the first step, bacterial sphingomyelinase hydrolyzed sphingomyelin to phosphorylcholine and N-acylsphingosine. Thereafter, the addition of alkaline phosphatase generated choline from phosphorylcholine. The newly formed choline was used to generate hydrogen peroxide in a reaction catalyzed by choline oxidase. Finally, with peroxidase as a catalyst, hydrogen peroxide was used together with phenol and 4-aminoantipyrine to generate a red quinone pigment, with an optimal absorption at 505 nm. The plasma sphingomyelin levels were measured in a blinded fashion, and the interassay coefficient of variation ranged from 2.5 to 3.1 percent.
Ascertainment of Cardiovascular Events
At intervals of 9 to 12 months, an interviewer contacted each participant by telephone to inquire about all interim hospital admissions, cardiovascular outpatient diagnoses, and deaths. To verify self-reported diagnoses, study personnel requested copies of all death certificates and medical records for all hospitalizations and outpatient cardiovascular diagnoses. Next-of-kin interviews were done for out-of-hospital cardiovascular deaths. Hospital records were obtained for an estimated 98% of hospitalized cardiovascular events, and some information was available for 95% of outpatient diagnostic encounters.
Hospital records that suggested possible cardiovascular events were abstracted by study personnel. The MESA coordinating center collated the abstracted or original endpoint records and sent them to 2 paired cardiologists, cardiovascular epidemiologists or neurologists for independent endpoint classification and assignment of incidence dates. If, after review and adjudication, disagreements persisted, a full Mortality and Morbidity Review Committee made the final classification.
Reviewers assigned a diagnosis of myocardial infarction based on combinations of symptoms, electrocardiographic findings, and cardiac biomarker levels. Death from CHD was classified as definite, probable or absent based on hospital records, death certificates and conversations with families. Definite fatal CHD required a myocardial infarction within 28 days of death, chest pain within 72 hours before death, or history of CHD, and the absence of a known nonatherosclerotic or noncardiac cause of death. If the definite fatal CHD criteria were not met, probable fatal CHD could be assigned with an underlying cause of death consistent with fatal CHD; this required the absence of a known nonatherosclerotic or noncardiac cause of death. The definition of angina was adapted from the Women's Health Initiative criteria and was classified by reviewers as definite, probable or absent. Definite or probable angina required clinical symptoms to be considered a MESA event, with definite angina requiring objective evidence of coronary atherosclerosis.
Definition of the Primary Outcome
For the purposes of this study a coronary heart disease event was defined as an incident myocardial infarction, definite angina, coronary revascularization (coronary artery bypass grafting and percutaneous coronary intervention), resuscitated cardiac arrest or CHD death as defined by the MESA protocol.
Statistical Analysis
Descriptive data are presented as mean ±SD for continuous variables and the frequencies of subjects in each category for categorical variables. Linear regression analysis was used to evaluate the association of plasma sphingomyelin levels and traditional cardiovascular risk factors. Kaplan-Meier analysis and log-rank tests were used to compare the event-free survival rates for incident coronary heart disease events among those above vs. those below or equal to the sex-specific median plasma sphingomyelin. Cox proportional hazard models were used to evaluate the association between plasma sphingomyelin levels treated as a continuous variable and event-free survival after adjusting for potential confounding variables. The covariates were selected based on prior evidence of an association with CHD events from previous studies and statistical evidence of a univariate association with the primary outcome in the current study (a priori p ≤ 0.20). The covariates included: age, gender, diabetes mellitus, systolic and diastolic blood pressure, total and HDL cholesterol, triglycerides, smoking, use of HMG-CoA reductase inhibitors, blood pressure medication use. The extended Cox model, using the time-dependent variable approach, was used to test for the proportionality assumption. The predictive value of plasma sphingomyelin levels for the various components of the composite outcome was also explored.
A two-tailed value of p<0.05 was considered significant. Statistical analysis was performed using SAS version 9.1 (SAS institute, Cary, N.C.).
Results
All of the 6809 (out of the 6814 MESA participants) who had plasma sphingomyelin measured at baseline were included in the analysis. One hundred and eighty-nine (2.8%) subjects had an adjudicated CHD event during the five years of follow up. The mean age of the cohort at baseline was 62.2 years, 52.8% were females, 38.5% Caucasian Americans, 11.8% Chinese Americans, 27.8% African Americans and 21.9% Hispanic Americans. The mean and standard deviation of plasma sphingomyelin levels for the MESA cohort was 48.0 ±16.0 mg/dl (range: 17 – 309 mg/dl). Table 1 shows the baseline characteristics of subjects with plasma sphingomyelin levels above and below/equal to the sex specific median. There was no significant difference between the mean plasma sphingomyelin levels of subjects who had an event and those who did not have an event after five years of follow up [mean ± std: 46.03 ± 14.4 mg/dl(n= 189) vs 48.06 ± 15.7 mg/dl(n= 6620), p=0.10 respectively].
Table 1.
Demographic characteristics of subjects with plasma sphingomyelin levels above and below/equal to the sex specific median.
| Variables | Above sex - specific median (mean ± std) N = 3245 |
Below or equal to sex -specific median (mean ± std) N = 3564 |
P value |
|---|---|---|---|
| Age | 63.5 ± 10.2 | 60.9 ± 10.1 | <0.001 |
| Female (%) | 1737 (53.5) | 1864 (52.3) | 0.463 |
| Race (%) | <0.001 | ||
| Caucasian | 1125 (34.7) | 1497 (42.0) | |
| Chinese | 385 (11.9) | 418 (11.7) | |
| African American | 962 (29.6) | 931 (26.1) | |
| Hispanics | 775 (23.9) | 721 (20.2) | |
| Body mass index (Kg/m2) | 28.4 ± 5.4 | 28.2 ± 5.5 | 0.039 |
| Systolic BP (mm Hg) | 127.9 ± 22.0 | 125.4 ± 21.0 | <0.001 |
| Diastolic BP (mmHg) | 71.7 ± 10.3 | 72.1 ±10.3 | 0.254 |
| Cholesterol (mg/dl) | |||
| Total | 199.7 ± 36.9 | 189.0 ± 33.9 | <0.001 |
| LDL | 119.8 ± 32.1 | 114.9 ± 30.7 | <0.001 |
| HDL | 50.9 ± 15.5 | 51.0 ± 14.2 | 0.522 |
| Triglycerides | 149.1 ± 112.6 | 115.6 ± 64.4 | <0.001 |
| Cigarette smoking (%) | <0.001 | ||
| Never | 1689 (52.2) | 1729 (48.6) | |
| Former | 1186 (36.7) | 1301 (36.6) | |
| Current | 361 (11.1) | 526 (14.8) | |
| Plasma Sphingomyelin (mg/dl) | 60.0 ± 14.3 | 36.8 ± 6.2 | <0.001 |
| Diabetes mellitus (%) | 396 (12.2) | 377 (13.7) | 0.004 |
| HMG CoA inhibitor use (%) | 522 (16.1) | 487 (13.7) | 0.005 |
| BP medication use (%) | 1178 (36.3) | 1089 (30.6) | <0.001 |
| Framingham risk score (%) | 9.8± 9.1 | 8.3 ± 8.1 | <0.001 |
Associations of Plasma Sphingomyelin with Traditional CV risk Factors
Age, BMI, systolic blood pressure, total cholesterol and triglycerides were positively associated with plasma sphingomyelin level. Diastolic blood pressure and cigarette smoking were negatively associated with plasma sphingomyelin. Diabetes mellitus was not associated with plasma sphingomyelin level. (Table 2)
Table 2.
Association of plasma sphingomyelin levels with traditional cardiovascular risk factors in linear regression models.
| Variable | Beta Coefficient ± standard error | P value |
|---|---|---|
| Age (years) | 0.155 ± 0.02 | <0.001 |
| BMI (Kg/m2) | 0.124 ± 0.03 | 0.001 |
| Systolic BP (mmHg) | 0.053 ± 0.01 | <0.001 |
| Diastolic BP (mmHg) | -0.061 ± 0.02 | 0.001 |
| HDL cholesterol (mg/dl) | 0.031 ± 0.01 | 0.018 |
| Total cholesterol (mg/dl) | 0.056 ± 0.01 | <0.001 |
| Triglycerides (mg/dl) | 0.075 ± 0.00 | <0.001 |
| Diabetes mellitus | 0.326 ± 0.49 | 0.503 |
| Cigarette smoking | -1.55 ± 0.28 | <0.001 |
Univariate Analysis
In the Kaplan meier analysis, subjects with plasma sphingomyelin levels above the sex specific median had similar event free survival rates compared with subjects with plasma sphingomyelin levels below or equal to the sex specific median (97.16% vs 97.0% log rank p= 0.713) (Figure 1). In the univariate Cox proportional hazard analysis age, male gender, HDL cholesterol, triglycerides levels, diabetes mellitus, cigarette smoking, systolic and diastolic blood pressure, HMG CoA inhibitor use and blood pressure medication use were significant predictors of incident CHD event (Table 3). Plasma sphingomyelin levels was not a significant predictor incident CHD event [hazard ratio 0.992(0.982 – 1.004), p=0.09].
Figure 1.
Kaplan Meier curves of event-free survival of subjects with plasma sphingomyelin (sm) levels above vs less than or equal to the sex- specific median (curves truncated at 1700 days due to few subjects at risk).
Table 3.
Association of variables with incident coronary heart disease events in the univariate and full multivariable Cox proportional hazard models.
| Variable | Univariate Hazard Ratio (95% CI) | P value | Multivariable Hazard Ratio (95% CI) | P value |
|---|---|---|---|---|
| Plasma sphingomyelin* | 0.992 (0.982- 1.004) | 0.09 | 0.984(0.973-0.996) | 0.002 |
| Age* | 1.061 (1.045 – 1.077) | <0.0001 | 1.054(1.035-1.073) | <0.0001 |
| Male gender | 2.941 (2.140 – 4.040) | <0.0001 | 3.149(2.146-4.622) | <0.0001 |
| Caucasian | 0.999 (0.886 – 1.125) | 0.983 | - | |
| Body mass index* | 0.998 (0.973 – 1.025) | 0.895 | - | |
| Total cholesterol* | 1.004 (1.000 – 1.007) | 0.056 | 1.011(1.006- 1.014) | <0.0001 |
| LDL cholesterol* | 1.004 (1.000 – 1.009) | 0.076 | - | |
| HDL cholesterol* | 0.976 (0.965 – 0.987) | <0.0001 | 1.001(0.987-1.015) | 0.909 |
| Triglycerides* | 1.001 (1.001 – 1.002) | 0.001 | 1.005(1.003-1.007) | <0.0001 |
| Diabetes mellitus | 1.441 (1.288 – 1.613) | <0.0001 | 1.448(1.234-1.700) | <0.0001 |
| Cigarette smoking | 1.370 (1.130 – 1.661) | 0.001 | 1.358(1.096-1.683) | 0.005 |
| Systolic blood pressure* | 1.018 (1.012 – 1.024) | <0.0001 | 1.013(1.003-1.022) | 0.008 |
| Diastolic blood pressure* | 1.020 (1.008 – 1.034) | 0.005 | 0.989(0.969-1.009) | 0.287 |
| HMG CoA inhibitor use | 1.880 (1.349 – 2.120) | 0.0002 | 1.583(1.110-2.256) | 0.011 |
| BP medication use | 2.050 (1.541 – 2.727) | <0.0001 | 1.590(1.161-2.178) | 0.004 |
Footnote: - indicates variable was not included in the final multivariable model.
The units for hazard ratios for each of these variables is per 1 unit increase. eg. The hazard ratio for plasma sphingomyelin in the univariate model is 0.992 per 1mg/dl increase.
Multivariable Analysis
Higher plasma sphingomyelin level was modestly associated with lower risk of incident CHD events in the multistage multivariable Cox proportional hazard analysis when adjusted for age [hazard ratio 0.977(0.977 – 0.997), p=0.013]; age, gender, total cholesterol, triglycerides and HDL cholesterol [hazard ratio 0.985(0.973 – 0.998), p=0.008] and in our full model after adjusting for age, gender, total cholesterol, HDL, triglycerides, diabetes mellitus, cigarette smoking, systolic and diastolic blood pressure, HMG CoA inhibitor use and blood pressure medication use [hazard ratio 0.984(0.973 – 0.996), p=0.002] (Tables 3 and 4). Forcing race/ethnicity into our final model did not change the direction or the estimates of the hazard ratios. In other models, plasma sphingomyelin was not a significant predictor of incident CHD events after adjusting for; age and gender [hazard ratio 0.994(0.984 – 1.004), p=0.245]; age, gender and race [hazard ratio 0.993(0.983 – 1.004), p=0.201] among others (Table 4). No significant interactions were observed in all our models.
Table 4.
Association of plasma sphingomyelin levels with incident coronary heart disease events in a multi stage multivariable Cox proportional hazard models.
| Models | Hazard Ratio (95%CI) | P value |
|---|---|---|
| Plasma Sphingomyelin(sm) | 0.992(0.982 – 1.002) | 0.09 |
| Sm + age | 0.987(0.977 – 0.997) | 0.013 |
| Sm + age, gender | 0.994(0.984 – 1.004) | 0.245 |
| Sm + age, gender, race | 0.993(0.983 – 1.004) | 0.201 |
| Sm + age, gender, race, BMI | 0.993 (0.983 – 1.004) | 0.199 |
| Sm + age, gender, HDL, total chol,triglycerides | 0.985 (0.973 – 0.996) | 0.008 |
| Sm + age, gender, diabetes, cigarette smoking, systolic BP, diastolic BP, HMG CoA use, BP medication use | 0.994(0.984 – 1.004) | 0.225 |
| Full model: Sm + age, gender, total chol, HDL, triglycerides, diabetes, cigarette smoking, systolic BP, diastolic BP, BP medication use, HMG CoA use | 0.984 (0.973 – 0.996) | 0.002 |
Footnote: all hazard ratios are expressed as per 1mg/dl increase in plasma sphingomyelin. Forcing race/ethnicity into the final model yielded similar result.
Higher sphingomyelin level showed modest negative associations with the components of the composite outcome such as myocardial infarction, angina and hard coronary heart disease events (Table 5). Higher sphingomyelin level was not associated with increased incident coronary heart disease events when the full multivariable Cox model was stratified by race (Table 6).
Table 5.
Association between plasma sphingomyelin levels with components of the composite outcome in the final multivariable Cox proportional hazard model.
| Outcome | # Events | Hazard Ratio (95% CI) | P value |
|---|---|---|---|
| Composite | 189(2.8%) | 0.984 (0.973 – 0.996) | 0.002 |
| Hard CHD | 106 (1.6%) | 0.978 (0.964 – 0.993) | 0.004 |
| MI | 84 (1.2%) | 0.976 (0.960 – 0.991) | 0.005 |
| Angina | 136 (2.0%) | 0.985 (0.973 – 0.998) | 0.025 |
Hard CHD: myocardial Infarction, Resuscitated cardiac arrest or CHD death.
All hazard ratios are expressed as per 1mg/dl increase in plasma sphingomyelin.
Table 6.
Association between plasma sphingomyelin levels with incident coronary heart disease events stratified by race in the final multivariable Cox proportional hazard model
| # Events | Hazard Ratio (95%CI) | P value | |
|---|---|---|---|
| Caucasian | 78 (3.0%) | 0.982(0.963 – 1.001) | 0.070 |
| Chinese | 18 (2.2%) | 1.002(0.957 – 1.048) | 0.946 |
| African American | 48 (2.5%) | 0.992(0.971 – 1.014) | 0.467 |
| Hispanic | 45 (3.0%) | 0.979(0.956 – 1.002) | 0.080 |
All hazard ratios are expressed as per 1mg/dl increase in plasma sphingomyelin.
Discussion
The goal of this large prospective multiethnic study was to evaluate the hypothesis that higher plasma sphingomyelin levels is associated with high incident coronary heart disease events in population based adults. We found no association between high plasma sphingomyelin levels and high incident CHD events. In our multivariable models, higher plasma sphingomyelin was associated with a modest reduction in incident CHD outcomes contrary to prior publication in humans (13).
Current data from cell culture and animal studies and from human population studies supports the hypothesis that plasma sphingomyelin participates in the development and progression of atherosclerosis. Numerous animals' studies have associated high sphingomyelin levels with the progression of atherosclerosis (1, 3). Cross-sectional data from MESA showed that high plasma sphingomyelin levels are associated with increased carotid intima media thickness, coronary calcium score and ankle brachial index (11). Jiang et al showed in a case control study that high plasma sphingomyelin level is associated with angiographically defined coronary artery disease (12). In a subsequent publication, high plasma sphingomyelin levels were associated with coronary artery disease and worse outcomes in subjects with acute coronary syndromes (13). The finding of the present study is consistent with no association between plasma sphingomyelin levels and the progression of atherosclerosis to clinical CHD events.
Reverse causality, a hypothesis that was evoked by some authors to explain the association between plasma homocysteine level and cardiovascular disease (16, 17), may help explain the findings of prior studies on the association of plasma sphingomyelin level and cardiovascular disease (11-13) and the result of the present study. In both case control studies by Jiang et al and Schlitt et al, subjects with coronary artery disease (high atherosclerotic burden) had high plasma sphingomyelin level compared to controls (low atherosclerotic burden). In the study by Nelson et al, increased measures of subclinical atherosclerosis was associated with high plasma sphingomyelin levels consistent with the findings of the case control studies (12, 13). The present prospective study however examines the association between plasma sphingomyelin levels and incident CHD events in a population without clinical cardiovascular disease at baseline. Even though at baseline (11) the extent/degree of subclinical atherosclerosis was associated with plasma sphingomyelin levels in the MESA cohort, and subclinical measures of atherosclerosis is strongly associated with clinical CHD events (18), plasma sphingomyelin level has no association with clinical CHD events. Thus plasma sphingomyelin levels have no influence on the development or progression of atherosclerosis but rather the burden of atherosclerosis influences plasma levels of sphingomyelin and hence plasma sphingomyelin levels have no effect on clinical CHD events.
Current data also suggests that much of the sphingomyelin in human and animal atherosclerotic plaques are synthesized denovo. Almost all the current data associating sphingomyelin with atherosclerosis (clinical or subclinical) in humans used plasma sphingomyelin levels (11-13). Much of the promising data showing regression of atherosclerotic plaques in animals used inhibitors of sphingomyelin synthases. These sphingomyelin synthase inhibitors may affect both the denovo synthesis of sphingomyelin in atherosclerotic plaques and those in plasma (19, 20). The relationship between plasma sphingomyelin and the sphingomyelin in atherosclerotic plaques is not clearly defined. Thus the beneficial effect of these sphingomyelin synthase inhibitors in animals may have been due to the reduction of sphingomyelin synthesis in the denovo pathway and not necessarily in plasma. Thus despite the null association between plasma sphingomyelin and incident CHD events in the present study, the denovo synthesized sphingomyelin levels in the arterial wall may be associated with incident CHD events. Studies evaluating the association between plasma sphingomyelin and sphingomyelin levels in atherosclerotic plaques are needed.
The present study has the following limitations. Like prior human studies, plasma sphingomyelin level was measured once in the MESA cohort. Intra-subject variability of plasma sphingomyelin level and the determinants of the variability of plasma sphingomyelin level over time have not been well studied. Thus a single measurement of plasma sphingomyelin level may be a poor measure of an individual's true biological levels and that multiple measurements over time may be needed to improve its accuracy. The MESA cohort is also a relatively healthy cohort and as such only 2.8 percent of the cohort had an adjudicated event during the five years of follow up. It is possible that the small number of events may have affected the findings of the present study. Finally, plasma sphingomyelin is a relatively new biomarker. Therefore not all the factors/markers that influences it biologic levels are known. It is unlikely but possible that the findings of the present study may be due to residual confounding.
Conclusion
Plasma sphingomyelin level is not associated with increased risk of incident CHD events in population based adults free of clinical cardiovascular disease at baseline. In our multi variable Cox proportional hazard models, high plasma sphingomyelin was associated with a modest reduction incident CHD event contrary to prior published data. Studies evaluating the denovo synthesis of sphingomyelin in arterial walls/atherosclerotic plaques and its association with clinical and subclinical cardiovascular disease are needed.
Acknowledgments
Funding/Support: This research was supported by contracts N01-HC-95159 through N01-HC-95166, and N01-HC-95169, and grants NHLBI T32 HL-07355, all from the National Heart, Lung and Blood Institute, Bethesda, MD.
Footnotes
Disclosure: All authors have no conflict of interest to disclose.
Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
References
- 1.Tabas I. Sphingolipids and atherosclerosis: a mechanistic connection? A therapeutic opportunity? Circulation. 2004;10:3400–1. doi: 10.1161/01.CIR.0000150861.98087.56. [DOI] [PubMed] [Google Scholar]
- 2.Bismuth J, Lin P, Yao Q, Chen C. Ceramide a common pathway for atherosclerosis. Atherosclerosis. 2008;196:497–504. doi: 10.1016/j.atherosclerosis.2007.09.018. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Alewijnse AE, Peter SL. Sphingolipid signalling in the cardiovascular system: good, bad or both. Eur J Pharmacol. 2008 May 13;585(2-3):292–302. doi: 10.1016/j.ejphar.2008.02.089. [DOI] [PubMed] [Google Scholar]
- 4.Smith EB. Intimal and medial lipids in human aorta. Lancet. 1960;1:799–803. doi: 10.1016/s0140-6736(60)90680-2. [DOI] [PubMed] [Google Scholar]
- 5.Schissel SL, Tweedie-Hardman J, Rapp JH, Graham G, Williams KJ, Tabas I. Rabbit aorta and human atherosclerotic lesions hydrolyze the sphingomyelin of retained low-density lipoprotein: proposed role for arterial-wall sphingomyelinase in subendothelial retention and aggregation of atherogenic lipoproteins. J Clin Invest. 1996;98:1455–1464. doi: 10.1172/JCI118934. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Devlin CM, Leventhal AR, Kuriakose G, Schuchman EH, Williams KJ, Tabas I. Acid sphingomyelinase promotes lipoprotein retention within early atheromata and accelerates lesion progression. Arterioscler Thromb Vasc Biol. 2008 Oct;28(10):1723–30. doi: 10.1161/ATVBAHA.108.173344. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Park TS, Panek RL, Mueller SB, Hanselman JC, Rosebury WS, Robertson AW, Kindt EK, Homan R, Karathanasis SK, Rekhter MD. Inhibition of sphingomyelin synthesis reduces atherogenesis in apolipoprotein E-knockout mice. Circulation. 2004;30(110):3465–71. doi: 10.1161/01.CIR.0000148370.60535.22. [DOI] [PubMed] [Google Scholar]
- 8.Hojjati MR, Li Z, Zhou H, Tang S, Huan C, Ooi E, Lu S, Jiang XC. Effect of myriocin on plasma sphingolipid metabolism and atherosclerosis in apoE-deficient mice. J Biol Chem. 2005;280:10284–9. doi: 10.1074/jbc.M412348200. [DOI] [PubMed] [Google Scholar]
- 9.Mukhin Dmitry N, Chao Fei-Fei, Kruth Howard S. Glycosphingolipid Accumulation in the Aortic Wall Is Another Feature of Human Atherosclerosis. Arterioscler Thromb, and Vascular Biol. 1995;15:1607–1615. doi: 10.1161/01.atv.15.10.1607. [DOI] [PubMed] [Google Scholar]
- 10.Noel C, Marcel YL, Davignon J. Plasma phospholipids in the different types of primary hyperlipoproteinemia. J Lab Clin Med. 1972;79:611–612. [PubMed] [Google Scholar]
- 11.Nelson JC, Jiang XC, Tabas I, Tall A, Shea S. Plasma sphingomyelin and subclinical atherosclerosis: findings from the multi-ethnic study of atherosclerosis. Am J Epidemiol. 2006;163:903–12. doi: 10.1093/aje/kwj140. [DOI] [PubMed] [Google Scholar]
- 12.Jiang XC, Paultre F, Pearson TA, Reed RG, Francis CK, Lin M, Berglund L, Tall AR. Plasma sphingomyelin level as a risk factor for coronary artery disease. Arterioscler Thromb Vasc Biol. 2000;20:2614–18. doi: 10.1161/01.atv.20.12.2614. [DOI] [PubMed] [Google Scholar]
- 13.Schlitt A, Blankenberg S, Yan D, von Gizycki H, Buerke M, Werdan K, Bickel C, Lackner KJ, Meyer J, Rupprecht HJ, Jiang XC. Further evaluation of plasma sphingomyelin levels as a risk factor for coronary artery disease. Nutr Metab (Lond) 2006;5:3–5. doi: 10.1186/1743-7075-3-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Bild DE, Bluemke DA, Burke GL, Detrano R, Diez Roux AV, Folsom AR, Greenland P, Jacob DR, Jr, Kronmal R, Liu K, Nelson JC, O'Leary D, Saad MF, Shea S, Szklo M, Tracy RP. Multi-Ethnic Study of Atherosclerosis: objectives and design. Am J Epidemiol. 2002;156:871–881. doi: 10.1093/aje/kwf113. [DOI] [PubMed] [Google Scholar]
- 15.Friedewald WF, Levy RI, Fredrickson DS. Estimation of the concentration of low density lipoprotein cholesterol in plasma without use of the preparative ultracentrifuge. Clin Chem. 1972;18:499–502. [PubMed] [Google Scholar]
- 16.Boston AG, Selhub J. Homocysteine and atherosclerosis: subclinical and clinical disease association. Circulation. 1999;99:2361–2363. doi: 10.1161/01.cir.99.18.2361. [DOI] [PubMed] [Google Scholar]
- 17.Lonn E. Homocysteine – lowering B vitamin therapy in cardiovascular prevention – wrong again? JAMA. 2008;299:2086–2087. doi: 10.1001/jama.299.17.2086. [DOI] [PubMed] [Google Scholar]
- 18.Libby P, Theroux P. Pathophysiology of coronary artery disease. Circulation. 2005;111(25):3481–3488. doi: 10.1161/CIRCULATIONAHA.105.537878. [DOI] [PubMed] [Google Scholar]
- 19.Liu J, Huan C, Chakraborty M, Zhang H, Lu D, Kuo MS, Cao G, Jiang XC. Macrophage sphingomyelin synthase 2 deficiency decreases atherosclerosis in mice. Circ Res. 2009 Jul 31;105(3):295–303. doi: 10.1161/CIRCRESAHA.109.194613. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20.Liu J, Zhang H, Li Z, Hailemariam TK, Chakraborty M, Jiang K, Qui D, Bui HH, Peake DA, Kuo MS, Wadgaonkar R, Cao G, Jiang XC. Sphingomyelin synthase 2 is one of the determinants for plasma and liver sphingomyelin levels in mice. Arterioscler Thromb Vasc Biol. 2009 Jun;29(6):850–6. doi: 10.1161/ATVBAHA.109.185223. [DOI] [PMC free article] [PubMed] [Google Scholar]