Relationship between Serum Gamma-glutamyl Transferase Levels with Ascending Aortic Dilatation.

OBJECTIVE
Increased serum gamma-glutamyl transferase levels (GGT) have been shown to directly promote oxidative stress. Previous studies have shown the relationship between the dilatation of the ascending aorta and oxidative stress. This study was designed to examine the relationship between serum GGT concentrations with dilatation of the ascending aorta.


MATERIALS AND METHODS
Ninety patients with ascending aortic dilatation and 90 age-sex-matched patients without aortic dilatation were included in the study. The patients were evaluated by a complete transthoracic echocardiographic examination including measurement of the aortic dimensions, where a diameter of 3.7 cm and above was accepted as ascending aortic dilatation. Serum GGT concentration was measured in all patients.


RESULTS
In the group with aortic dilatation, HT frequency, serum uric acid, hs-CRP and GGT levels, the LV mass index, and the left atrial volume index were found to be higher than the control group. The logistic regression analysis showed that only HT frequency (OR:1.23, 95% CI 1.11-1.35, p value: 0.02), the LA volume index (OR: 1.34, 95% CI 1.21±1.4, p:0.005) and serum GGT levels (OR: 1.12, 95% CI 1.01±1.20, p:0.03) were found to be independent predictors. There was a significant correlation between serum GGT levels and ascending aortic diameter (r: 0.268, p<0.001). In the ROC curve analysis, AUC was 0.659 (0.580±0.738) for a 23.5 serum GGT cut-off value (64% sensitivity and 53% specificity).


CONCLUSION
We found that serum GGT concentration was significantly associated with ascending aortic dilatation. Large epidemiological studies are required to correlate the findings from this study with clinical outcome.


Introduction
Ascending aortic dilatation is not a rare condition, and several mechanisms have been shown to cause the disease, such as hemodynamic force, destructive remodeling of the extracellular matrix, familial predisposition and transmural inflammation [1][2][3][4][5][6]. Recent studies show that oxidative stress plays an important role in the development of aortic dilatation [1,7,8]. There is much evidence that shows the role of oxidative stress in the development of aortic dilatation. Inflammatory cells, lipid mediators, smooth muscle cells, growth factors and mechanical stretch produce reactive oxygen species, especially by the NADPH pathway, within the aortic local environment [1,9] These factors are responsible for the regulation of matrix metalloproteins and for smooth muscle apoptosis, and changes in this regulation cause the balance between the generation and destruction of the aortic wall to shift [1].
Gamma Glutamyl Transferase (GGT) enzyme is commonly used as predictor for liver diseases such as hepatitis, fatty liver disease and alcohol consumption [1,7,9,10]. Recent studies have shown that elevated serum GGT is significantly associated with increased risk of cardiovascular diseases (and with cardiovascular mortality) such as myocardial infarction, stroke, and complexity of coronary and carotid atherosclerosis and heart failure [2][3][4][5].
Previous studies showed that an increase in serum GGT concentration can be used as a marker for increased oxidative stress [2,3]. GGT plays a role as a protein catalyst in the degradation of glutathione at the physiological serum level, and glutathione is the major thiol antioxidant in the human body [2]. However, the relationship between serum GGT levels and ascending aortic dilatation has not been studied previously. This study was designed to examine the relationship between serum GGT concentrations and dilatation of the ascending aorta.

Materials and Methods
The ethics committee approved the study from the ethics committee of Ataturk Unıversty date 02.09.2013 and number B.30.2.ATA.0.01.00/173, and written consent was obtained from all patients. Ninety consecutive patients who were referred for transthoracic echocardiography in our institution to evaluate chest pain, cardiac murmur, or shortness of breath and had aortic dilatation (>3.7 cm) were included study, along with age-sex matched 90 patient without aortic dilatation (≤3.7 cm).
Hypertension (HT) was defined as the use of antihypertensive agents or elevated blood pressure measurement of ≥140/90 mm Hg on at least three separate clinical visits.
Diabetes mellitus (DM) was defined as having at least two fasting blood sugar measurements >126 mg/dL or using antidiabetic drugs. Body mass index values were calculated based on the height and weight of each patient. The exclusion criteria were alcohol consumption, cirrhosis, congestive heart failure, pericardial disease, renal failure (creatinine ≥1.5 mg/dL), liver disease (alanine aminotransferase, aspartate aminotransferase more than or equal to three times the upper limit normal), hemolytic disorders, concomitant inflammatory diseases such as infections and autoimmune disorders, osteoporosis, rheumatic heart disease, previous stroke, previous myocardial infarction, rheumatoid arthritis or other connective tissue diseases, neoplastic disease and documented coronary artery disease.
All participants underwent comprehensive two-dimensional, M-mode and Doppler echocardiographic examination performed by one experienced research echocardiographer, who was blinded to the biochemical data and used a commercially available echocardiograph equipped with a 2.5and 3.5-MHz transducer (Philips ie33). 2-D and M-mode measurements of left ventricle (LV) dimensions were performed. The LV mass index and the LA volume index were calculated according to the American Society of Echocardiography guidelines.
Aortic valve morphology was examined in parasternal long-and short-axis views. The ascending aortic diameter was measured perpendicular to the long axis of the aorta with the use of the leading edge method in the parasternal long-axis view showing the largest aortic diameter, in accordance with the European Society of Echocardiography guidelines [2]. All measurements were repeated up to five times and averaged. Ascending aortic dilatation was defined as an aortic diameter >3.7 cm. Only patients with ascending aortic dilatation were included in the study; patients with aortic root dilatation were excluded.

Blood Sample Collection
Blood samples were obtained after an overnight fasting period and were withdrawn from the antecubital vein. Serum GGT levels were measured by the enzymatic calorimetric test (Roche/Hitachi analyzer, Mannheim, Germany), and the normal range of GGT activity was recognized as 7-49 U/l.

Statistical Analysis
All analyses were conducted using Statistical Package for the Social Sciences (SPSS) 11.5 (IBM SPSS, Chicago, IL, USA). Continuous variables were expressed as mean+s.d., and categorical variables were expressed as percentages.
To compare the means of continuous variables between the groups categorized according to ascending aorta size, Student's t test or the Mann-Whitney U test was utilized. x 2 analysis was used to compare the distribution of categorical variables. The Pearson correlation test was used to examine correlations between ascending aorta size and serum GGT concentration. To assess the influence of several baseline factors on the aortic diameter, multiple logistic regression analysis was performed. These variables were hypertension, uric acid, hs-CRP (high sensitivity C-reactive protein), GGT, the left ventricular (LV) mass index and the left atrial (LA) volume index with p<0.05 in the univariate analysis. Significance was defined at p<0.05.

Results
Ninety patients with ascending aortic dilatation and age-sex matched 90 patients without aortic dilatation were included in this study. The clinical, biochemical and echocardiographic characteristics are summarized in Table 1.

Discussion
This is the first study to investigate the relationship between GGT activity and ascending aortic dilatation. In the ascending aortic dilatation group, hypertension, the LA  volume index and serum GGT were found to be significantly higher than in the control group. Our results showed that increased GGT concentrations were independently associated with an increased risk of ascending aortic dilatation. Oxidative stress is a form of tissue damage occurring after the increased production and/or decreased destruction of reactive oxygen species. The balance between reactive oxygen species production and destruction depends on the levels of endogenous cellular antioxidants and antioxidant enzymes. The NADPH oxidase and xanthine oxidase pathways are the two major systems responsible from vascular oxidative stress [2]. In the etiology of aortic dilatation, matrix metalloproteinases and reactive oxygen species play important roles [2]. Reactive oxygen and nitrogen species have been shown to regulate matrix metalloproteinase transcription and activation [2]. Matrix metalloproteinases are the most frequently detected proteolytic species in the development and establishment of aortic dilatation [2]. Oxidative stress also has been implicated in the pathogenesis of diseases ranging from hypertension to Parkinson's disease [4].
The mechanism of increasing GGT activity in cardiovascular disease is currently unclear. However, oxidative stress and glutathione metabolism are thought to be potential mechanisms for GGT activity [4]. Additionally, serum GGT activity is negatively associated with several antioxidants such as beta carotene and vitamin C and positively associated with oxidative stress parameters. Therefore, higher concentrations of GGT may reflect increased oxidative stress. Ascending aortic dilatation involves several mechanisms, such as hemodynamic force, destructive remodeling of the extracellular matrix, familial predisposition and transmural inflammation [2,6]. Recent studies show that oxidative stress plays an important role in the development of thoracic and abdominal aortic dilatation as well as progression [2,8]. Similarly, Esen et al [1] showed that there was a positive correlation between an increase in oxidative stress and ascending aortic dilatation. They measured total antioxidant capacity and serum uric acid levels and showed significant correlation of uric acid concentration and antioxidant capacity with aortic dilatation. In our study, we showed a positive correlation between GGT, which is associated with an increase in oxidative stress, and aortic dilatation. Additionally, HT and the LA volume index were associated with aortic dilatation. In contrast to Esen et al. [1] in our study, the association of uric acid with aortic dilatation was significant in univariate analysis but not significant in multivariate analysis.
This study was the first to show that GGT levels are associated with aortic dilatation. There is much evidence regarding the relationship between GGT levels and oxidative stress. Furthermore, it has been shown that the development and progression of aortic dilatation are associated with oxidative stress. Therefore, the relationship between serum GGT levels and aortic dilatation is important, as shown in our study. However, the relationship between GGT levels and the progression of aortic dilatation could not be shown in this study because long-term monitoring of serum GGT levels and aortic diameter was not possible.
There are a couple of limitations to our study. First, the study population is small and narrowly defined. Second, aortic dilatation was detected and quantified by transthoracic echocardiography, and the data should be confirmed by CT imaging.
In conclusion our findings suggest that ascending aortic dilatation is associated with higher serum GGT levels, which may indicate the role of increased oxidative stress in ascending aortic dilatation. Large-scale prospective, randomized studies are necessary to test the clinical value of this association in patients with ascending aortic dilatation and with high GGT levels.