Our study shows statistical significantly increased concentration of GGT and hsCRP in serum in patient with type 2 DM with poor glycemic control compared to healthy persons as well as subjects having type 2 DM with good glycemic control. Also we found a significant positive linear relationship between GGT and hsCRP concentration as well as both with HbA1C, FBS, and PP2BS. These findings suggest a link between oxidative stress (indicated by increased serum GGT concentration), inflammation (raised hsCRP concentration) and glycemic control in patients with type 2 DM and related complications. Also at levels of GGT and hsCRP considered well within the normal range, there was a substantial and significant increased concentration in patients with type 2 DM with good glycemic control compared to healthy subjects. This suggests a role of oxidative stress and chronic low grade inflammation in pathogenesis of type 2 diabetes patients.
Several possible mechanisms which explain increased serum GGT activity and hsCRP level in patients with type 2 DM with good and poor control and its correlation with glycemic control.
Elevation of serum GGT could be the expression of an excess deposition of fat in the liver, termed non-alcoholic fatty liver disease. Fatty liver is thought to cause hepatic insulin resistance and to contribute to the development of systemic insulin resistance and hyperinsulinemia. Thus, GGT could serve as a marker of the insulin resistance syndrome in the pathogenesis of diabetes[8, 13].
There is now growing evidence to suggest that GGT is not only a marker of fatty liver but also a marker of oxidative stress. Experimental studies have reported that GGT has a central role in the maintenance of intracellular antioxidant defences through its mediation of extracellular glutathione transport into most types of cells. It is an ectoenzyme normally present at the outer side of the cell membrane that has the primary function of maintaining intracellular concentrations of glutathione (GSH), a critical antioxidant defence for the cell. Increases in GGT activity can be a response to oxidative stress, facilitating increased transport of GSH precursors into cells. In addition, GGT is leaked into the serum possibly as a result of normal cell turnover and cellular stresses. Several mechanisms for GGT leakage are possible and include increases in oxidative stress, proteolysis, glycosylation, GGT synthesis and endothelial cell damage. Thus, increased serum concentrations of GGT could identify people with a low but persistent increase of oxidative and other cellular stresses.
Oxidative stress is currently suggested as mechanism underlying diabetes and diabetic complications. In recent years, much attention has been focused on the role of oxidative stress, and it has been reported that oxidative stress may constitute the key and common event in the pathogenesis of secondary diabetic complications. Implication of oxidative stress in the pathogenesis of diabetes is suggested, not only by oxygen free-radical generation, but also due to nonenzymatic protein glycosylation, auto-oxidation of glucose, impaired glutathione metabolism, alteration in antioxidant enzymes, lipid peroxides formation and decreased ascorbic acid levels. In addition to GSH, there are other defence mechanisms against free radicals like enzymes superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT) whose activities contribute to eliminate superoxide, hydrogen peroxide and hydroxyl radicals. Raised GGT concentrations could be a marker of oxidative stress, which might also play a role in the cause and development of diabetes and its complications[5, 9].
Various studies suggested that elevated serum GGT could be the expression of subclinical inflammation which also contributes to the development of type 2 DM[16, 17]. As serum GGT is highly associated with WBC count and some features of low-graded inflammation, so elevated GGT could be the expression of subclinical inflammation or an insulin-resistant state, which would represent the underlying mechanism[17, 18]. It is therefore suggested that measurements of other inflammatory markers including C-reactive protein by a validated high-sensitivity assay be added in an attempt to substantiate this hypothesis.
There are various studies which support our results. R Sharma et al. shows rise in levels of hsCRP and GGT in diabetic subjects and their significant association which might be a result of inflammation and oxidative stress in diabetes mellitus. Dilshad Ahmed Khan et al. studied diabetic patients had significantly elevated median of HbA1c, hsCRP, total cholesterol, nitrate and GGT as compared to controls. HbA1c showed a positive correlation with hsCRP, total cholesterol, nitrate and GGT. Oxidative stress and inflammatory markers should be used in addition to HbA1c for assessment of increased cardiac risk in uncontrolled diabetic patients because of accelerated atherosclerosis due to free radical injury. Sarinnapakorn V et al. found that hsCRP levels correlated with HbA1c levels. Mean HbA1c levels were significantly higher in patients who had hsCRP levels of 1 mg/L or more. Other factors such as age, blood pressure, BMI, LDL cholesterol, serum creatinine were not correlated with hsCRP level. Also Bahceci M et al. compare serum hsCRP levels in type 2 diabetic men without coronary heart diseases (CHD), non-diabetic CHD patients and type 2 DM patients with CHD and shows type 2 DM men without CHD had similar CRP levels with non-diabetic CHD patients, whereas CRP levels of type 2 DM men with CHD were higher than non-diabetic men with CHD. Because of a positive correlation between serum hsCRP and HbA1c, fasting insulin and HOMA-IR, inflammation, insulin resistance and hyperglycemia jointly contribute to the cardiovascular risk in type 2 DM men.
Other lines of evidence support a relationship between elevated serum GGT and poor glycemic control and metabolic syndrome are also found. Higher GGT levels are accompanied by more insulin resistance and greater risk for developing type 2 DM and poor glycemic control[23–27]. The strong association of serum GGT activity with some diabetes related metabolic disorders, such as atherogenic dyslipidemia and poor glycemic control, may be explained by underlying, not mutually exclusive, biological mechanisms such as fatty liver, insulin resistance, and enhanced oxidative stress[23, 28–32]. It is possible that the occurrence of GGT-mediated redox reactions plays a direct role in the pathogenesis of atherogenic dyslipidemia and poor glycemic control, independently of the presence of fatty liver, possibly through the induction of chronic inflammation and insulin resistance. Supporting a role of serum GGT in the inflammation and oxidative stress, serum GGT level predicted future levels of inflammation and oxidative stress markers, such as fibrinogen, uric acid, CRP, and F2-isoprostanes, in a dose–response manner. Several studies demonstrate that hsCRP remained a significant predictor of diabetes risk even after adjusting with body mass index, family history of diabetes mellitus, smoking and other factors.
In general, serum GGT concentration is closely related with other enzymes more specific to the liver, serum ALT or AST concentration, so we did parallel analyses with ALT and AST to further explore the possible role of liver damage in the association of GGT with diabetes. Within their normal ranges, ALT and AST showed significant increase in type 2 diabetes patients with poor control (p value <0.001). Moreover, markers of hepatic fat content, such as serum GGT activity and other liver enzymes, have been shown in large prospective studies to predict the incidence of type 2 diabetes, insulin resistance, and cardiovascular disease independently of obesity[23, 28, 35, 36].
Our results also suggest that liver enzymes are closely associated with the risk of metabolic syndrome and type 2 diabetes and that among these enzymes serum GGT is the most powerful risk indicator for developing the metabolic syndrome and type 2 diabetes. Another possible pathophysiological mechanism is that elevated liver enzymes may reflect inflammation, which impairs insulin signalling both in the liver and systemically[27, 37, 38]. In this population, mean WBC count increased with an increase in serum GGT. WBCs, a major component of the inflammatory process, are activated by cytokines, especially interleukin-6 and interleukin-8. Elevated GGT could reflect subclinical inflammation, which would represent the underlying mechanism. Serum GGT level may be a simple and reliable marker of visceral and hepatic fat and, by inference, of hepatic insulin resistance. These findings suggest that a raised serum GGT level is an independent risk factor for NIDDM. The elevated GGT concentration should be added to the list of risk factors that are common to NIDDM and cardiovascular disease.
Our study had several limitations. First, it is cross sectional study with no causality effect to report. Also serum GGT and hsCRP during follow-up was not included in the analysis. Further, we could not include several confounding variables in this study, such as fasting insulin concentration. Therefore, fasting insulin concentration should be included in future studies.
Despite these potential limitations, our findings, which were obtained from a cross sectional study shows that serum GGT activity and hsCRP level is significantly increased in patients with type 2 diabetes mellitus compared to healthy control. Both are further increased in diabetic patients with complications and poor glycemic control. Also there is a significant positive correlation between serum GGT activity and hsCRP. Both are also independently positively correlated with HbA1c, FBS and PP2BS (short and long term glycemic control). So far, the underlying pathophysiological mechanisms are not entirely clear. It seems that insulin resistance, oxidative stress and chronic low grade systemic inflammation may be involved. All these finding suggesting a link between oxidative stress, inflammation and glycemic control in patient with type 2 diabetes mellitus. Further studies are needed to investigate the biological mechanisms underlying this association.