Abstract
Objective Gamma-glutamyltransferase (GGT) is a novel cardiovascular disease (CVD) risk factor, but its use as an independent factor for general CVD risk prediction remains unclear in general population. This study examined the association between serum GGT concentration and 10-year CVD risk in Koreans.
Methods This cross-sectional study was performed on 27,270 Koreans. Besides individual components of 10-year CVD risk, body mass index, fasting blood glucose, liver enzymes, lipid profile, uric acid and high sensitive C-reactive protein data were used. The study subjects were grouped into quartiles according to the levels of GGT. Analyses relating GGT to 10-year CVD risk greater than 20% used multiple confounders-adjusted logistic regression.
Results Positive correlations were established between log-transformed GGT concentration and 10-year CVD risk (Spearman’s rho = 0.51; P < 0.001). Compared to the lowest baseline GGT category, unadjusted odds ratios for 10-year CVD risk greater than 20% were significantly increased from the lowest to the highest GGT quartiles; these results remained significant after adjustments for multiple confounders.
Conclusion Increased GGT concentration is associated with the increase in 10-year CVD risk. Serum GGT may be helpful to predict the future risk of general CVD.
Cardiovascular disease (CVD) events such as coronary heart disease, cerebrovascular disease, peripheral vascular disease, and heart failure are among the leading causes of death in the world.1Gamma-glutamyltransferase (GGT) is located on plasma membranes of several cells and tissues with a predominance of hepatocytes.2A study showed that increased serum GGT activity could be used as a marker for increased oxidative stress in humans.3Oxidative stress is thought to play an important role in the progression of atherosclerosis.4,5An accumulating body of data indicates that GGT as an enzyme responsible for the extracellular catabolism of antioxidant glutathione may explicitly participate in atherogenesis.6,7Several population-based studies have documented that higher serum GGT is highly correlated with development of certain CVD risk factors, irrespective of alcohol consumption.8–12Therefore, although GGT has been widely used as a marker for alcohol consumption or liver disease,2,13robust epidemiological evidence points out that serum GGT may emerge as a potential biochemical risk indicator of both cardiovascular morbidity and mortality. Despite the clinical impact these increased GGT levels have in many studies, however, little is known of the effect of GGT levels in the prediction of the future cardiovascular events after controlling for multiple potential confounding factors in general population. Recently, a sex-specific general CVD risk assessment tool has been developed that can be conveniently used to assess general CVD risk and risk of individual CVD events.14
The present study was grounded on the hypothesis that increasing serum GGT is associated with the risk of CVD, calculated using general CVD risk assessment tool, and should be considered as a factor associated with general CVD risk prediction. Similarly, we have reported in a prior study a significant positive correlation between serum GGT and coronary heart disease risk calculated using the Framingham Risk Score.15In the present study, we extended our investigation to evaluate the relationship between serum GGT and general CVD risk in Koreans.
MATERIALS AND METHODS
Study Data
We used data from a previous large-scale study.15From January 2007 to May 2009, data of 40,606 Koreans aged 31 to 86 years who visited the Health Promotion Center, Ajou University Hospital, Suwon, Gyeonggi-do, Republic of Korea, were reviewed for inclusion in the study. Medical history, demographics, anthropometric, and laboratory data were collected. Data on cigarette smoking and alcohol consumption were collected by a self-reported questionnaire. Subjects who, at the time of the survey, had smoked cigarettes regularly within the prior year were considered to be current smokers, and weekly alcohol intake was calculated and then converted to weekly alcohol consumption (grams of ethanol per week) by graduated frequency method.16Of the initial 40,606 subjects, we excluded 13,336 subjects who met one of the following conditions: 9221 subjects had missing data values for any component of general CVD risk assessment tool or serum GGT or alcohol history; 821 subjects had positive test results for antibody to hepatitis B surface antigen, antibody to hepatitis B virus core antigen or anti-hepatitis C virus; 1314 subjects had a medical history of chronic liver disease or liver cirrhosis, or were taking drugs influencing liver function (hepatotonic and herbal medication); 312 subjects were on medication (statins or other lipid-lowering drugs) for hyperlipidemia; 745 subjects had a diagnosis of CVD diseases (coronary, cerebrovascular, and peripheral artery disease and heart failure); 923 subjects with high-sensitivity CRP (hs-CRP) 1.0 mg/dL or more to preclude any possible occult inflammatory or infectious disease. Thus, a total of 27,270 healthy Korean (12,238 women and 15,032 men) were included in the final analyses. All participants agreed to the use of their health checkup results. The institutional review board of Ajou University Hospital approved this study.
Measurements
Before collection of blood, each subject was asked to fast for 10 hours or more. After overnight fasting, a venous blood sample was obtained between 08:00 and 10:00 AM. Fasting blood specimens were used for measuring lipids, glucose, liver enzymes, hs-CRP, and uric acid. Serum GGT was assayed by the standard method recommended by the International Federation for Clinical Chemistry using L-γ-glutamyl-3-carboxy-4-nitroanilide as substrate with a Toshiba 200FR autoanalyzer. Fasting blood glucose, liver enzymes, lipid levels, and uric acid were assayed by using a Toshiba-200FR automatic analyzer (Toshiba Medical Systems, Tokyo, Japan). High-sensitivity CRP was measured by a high-sensitivity nephelometric method (Dade Behring Marburg GMBH, Marburg, Germany). Blood pressure (BP) was measured using a standard mercury manometer, with the participant in a sitting position for 5 minutes before measurement; the average of 2 measurements was recorded. Hypertension was defined as a systolic BP of 140 mm Hg or greater or a diastolic BP 90 mm Hg or greater, or the use of antihypertensive medication. Diabetes was defined by a fasting blood glucose of 126 mg/dL or greater or the use of oral hypoglycemic agents or insulin. Body mass index (BMI) was calculated as weight (kg) divided by height squared (m2).
General CVD Risk Assessment Tool
The general CVD risk prediction score was calculated from Cox proportional hazards regression to evaluate the risk of developing a first CVD event in 8491 Framingham study participants, based on 6 CVD risk factors: age, total and high-density lipoprotein cholesterol, systolic BP, smoking, and diabetes status.14Among these factors, age and cholesterol levels were categorized according to their values, and systolic BP was categorized according to their values separated by treatment presence or absence for hypertension. Smoking status was classified as either “current smoker” or “nonsmoker” and diabetes status was classified as either “yes” or “no” as defined earlier. Finally, the corresponding point was given to each man and women, and then the total score was used as the individual’s CVD risk level. The general CVD risk assessment tool has been used to predict the 10-year risk of CVD events (coronary, cerebrovascular, and peripheral arterial disease and heart failure).
Statistical Analyses
The distribution of GGT values and alcohol consumption were right skewed; therefore, a natural log transformation was applied. To assess the relationship between biomarkers and the individual components of the general CVD risk assessment tool, Spearman rank correlation coefficients relating individual risk factor scores and the total of 10-year CVD risk to log-transformed GGT levels were analyzed. In addition, for scatter plot, the relationship between the 10-year CVD risk and serum GGT concentration was evaluated, and for linear regression, the relationship between the 10-year CVD risk and log-transformed GGT levels was evaluated. The study subjects were grouped into quartiles according to the circulating levels of GGT. The participants were divided into 2 groups14: low and intermediate risk (⩽20% risk of developing a CVD event over the next 10 years), and high risk (>20% risk). We used the logistic regression analysis to evaluate whether the higher GGT quartile had increased odds ratio (OR) of high risk for CVD (10-year risk >20%) more than the lower GGT quartile. For analyses relating GGT to high risk for CVD, we constructed adjusted logistic regression analyses that considered alcohol consumption + BMI, and (alcohol consumption + BMI) + novel and conventional risk factors including hs-CRP, low-density lipoprotein (LDL) cholesterol and uric acid. Results of group data are expressed as mean ± SD. All statistical analyses were performed using SPSS 13.0 software (SPSS, Chicago, IL). P < 0.05 was considered statistically significant.
RESULTS
Subjects with higher total CVD point score and 10-year CVD risk were men because they had more general CVD risk factors such as diabetes mellitus, hypertension, dyslipidemia, alcohol consumption, and current smoking (Table 1). There was a significant positive correlation of log-transformed GGT levels with 10-year CVD risk (Spearman rho = 0.51, P < 0.001; Table 2), which was of consistent magnitude in men (Spearman rho = 0.21, P < 0.001; data not shown) and women (Spearman rho = 0.35, P < 0.001; data not shown). Log-transformed GGT also was well correlated with individual risk factors. Table 3 shows the relative contribution of log-transformed GGT to the 10-year CVD risk by linear regression. A simple correlation analysis showed a significant positive correlation of log-transformed GGT with the 10-year CVD risk (β = 0.36, P < 0.001). This association remained significant and independent in a multiple linear regression analysis adjusted for other variables. Figure 1 shows the scatter plot of the relation between the 10-year CVD risk and serum GGT concentration. The median (range) of the first-to-fourth quartiles of GGT values was 10.69 >(4–13), 17.14 (14–21), 28.37 (22–27), and 79.78 (≥38) IU/L, respectively. In the highest quartile, 87.2% of men and 93.9% of women had GGT values within the reference range (eg, men, ⩽66 IU/L; women, ⩽39 IU/L). In multivariable logistic models adjusted for log-transformed weekly alcohol consumption and BMI, higher GGT was associated with greater risk for 10-year CVD risk greater than 20% (high risk for general CVD) in the top quartile relative to the lowest (Table 4). In an additional model that included hs-CRP, LDL cholesterol, and uric acid, compared to the lowest baseline GGT category, OR for 10-year CVD risk greater than 20% were 5.4 (confidence interval [CI], 2.3–12.7), 7.2 (CI, 3.1–16.6), and 9.2 (CI, 3.9–21.3) (P < 0.001 for trend) in the other 3 GGT categories.
DISCUSSION
The present data demonstrate that serum GGT levels positively correlate with the risk of CVD calculated using the general CVD risk assessment tool. The association was not confounded by alcohol or the other known CVD risk factors (LDL cholesterol, BMI, hs-CRP, and uric acid).
Although best known as a reliable diagnostic tool for hepatobiliary disorders and alcohol abuse in clinical practice, a large number of studies suggest that serum GGT is not only a marker for oxidative stress17,18but also a relative factor of CVD.8–12,19,20Increased oxidative stress and systemic inflammation are considered key factors for the progression of atherosclerosis and CVD.4,21,22Thus, it was hypothesized that GGT could be a marker for general CVD risk prediction regardless of alcohol consumption. Our data show that compared to the lowest baseline GGT category, 10-year CVD risk greater than 20% increased significantly with increased serum GGT levels. After adjustment for alcohol consumption and other known CVD risk factors, the relationship between GGT and 10-year CVD risk greater than 20% remained significant. The observations that the OR of high-risk for CVD by GGT quartile was significantly increased within the reference range of GGT levels suggest that GGT could be regarded as a risk stratification marker of 10-year CVD risk greater than 20%, despite the reference range. The present results are in accordance with previous studies, which reported to predict CVD events and mortality from CVD disease even when within its reference range.23,24
There are several possible explanations for the observed link between serum GGT levels and general CVD risk prediction. First, although the mechanisms that explain the contribution of GGT to CVD remain largely enigmatic, the pathogenesis involves a GGT-linked pro-oxidant effect of glutathione catabolism within the plaque.6,7Indeed, the potential role of GGT in promoting plaque formation and evolution has been confirmed by the identification of catalytically active GGT in coronary, cerebral, and carotid plaques colocalized with oxidized LDL and CD68+ foam cells.25These results suggest that GGT derives partially from atheromatous plaques, which would be more common and diffuse in patients with adverse CVD risk profiles, or serum GGT is related to the risk factors even before the plaques are entirely developed. Second, GGT activities may also modulate the redox status of protein thiols at the cell surface, leading to the production of free radicals and reactive oxygen species, and LDL oxidation.26,27Thus, GGT is considered a factor that contributes to oxidative stress pathways in various organ systems, localizes to atheromatous plaques containing oxidized lipoprotein, and is proinflammatory, further implicating this protein in atherogenesis.28,29Indeed, data exist that fibrate therapy has a favorable efficacy on diminishing serum GGT concentrations among hypertriglyceridemic patients as well as improving lipid profile.30This is consistent with a role for fatty liver in generating increased serum GGT and being associated with an increase in CVD risk, and it provides a practical means of risk reduction.31These studies support the existence of preventive effects of decreased serum GGT against CVD.
This study has several strengths and limitations. One of the strengths is the large scale of the study, with subjects of both sex and all age groups. Therefore, the results can be generalized into the entire Korean population. Second, we used the categories of the estimated 10-year CVD risk calculated by using the standardized criteria for general CVD incidence. However, this study is suggesting a mathematical surrogate for the presence of general CVD. Therefore, further longitudinal cohort studies are needed to evaluate promising biohumoral predictors for the increased risk of general CVD. The present study was cross-sectional and we did not investigate oxidative stress markers and thus could not examine any association of oxidative stress with serum GGT levels.
In conclusion, the present study demonstrates that serum GGT levels are associated with the estimated 10-year CVD risk calculated by using the general CVD risk assessment tool in Korean. These significant associations in serum GGT levels even within the reference range seem to have an additional benefit in the prediction of future development of general CVD. Thus, we suggest that serum GGT could be an additional surrogate marker of a 10-year risk of a CVD event and aid detection of individuals at high risk for future cardiovascular events.