Gamma glutamyl transpeptidase (GGT) is a membrane-bound enzyme that is essential for the synthesis of glutathione (GSH), a key antioxidant ¹ . In clinical practice elevated serum GGT is generally used as an indicator of liver disease, such as biliary obstruction, alcohol consumption, and exposure to certain medical drugs ¹ . Recently, several epidemiological studies have shown that a higher serum GGT level, even within the normal range, is associated with cardiovascular risk factors such as hypertension, hypertriglyceridemia, obesity, type 2 diabetes mellitus and stroke, as well as certain types of cancer ^{2– 10} . In contrast to these studies, we observed that after surgery for ruptured abdominal aortic aneurysm ¹¹ or after liver resection ¹² , GGT is transiently increased in patients who had a good outcome. In these short-term observational studies GGT level was inversely related to other liver laboratory parameters such as aspartate aminotransferase (ALT), alanine aminotransferase (AST) as well as total bilirubin (TBI) ^{11, 12} . We observe that in the early postoperative period after a liver transplantation (LT) a transient gradual increase in GGT also occurs. How early and late postoperative changes in serum GGT are related to survival is not known.

Here we present a study in which we assessed the relationship between early (postoperative day seven) elevated GGT levels with early and late survival (i.e. survival within the 90 days and five years post-LT, respectively). We also evaluated the relationship between late (six months postoperatively) elevated GGT levels with late survival.

In addition, we studied the early and late post-LT kinetics of GGT, AST, ALT, and TBI in patients who survived more than 90 days and in patients who did not. Likewise, these kinetics were compared with long-term survival.

In this study, we evaluated the changes in GGT over time following liver transplantation and the clinical relevance of these changes for early and late survival. We found that a transiently elevated GGT early after LT was associated with increased survival rates within the first 90 days. In contrast, late elevation of GGT was associated with decreased five-year survival following LT. Although the early GGT elevations was also associated with five-year survival, this difference mainly developed during the first 90 days post-LT.

This peculiar effect was not observed for TBI, AST, and ALT since higher levels for these parameters at POD 7 and six months were associated with increased mortality at both 90 days and five years after LT.

To our knowledge, this is the first study showing the short and long term kinetics of GGT and the clinical relevance of an early elevated serum GGT in LT recipients. Previously, we have reported improved outcome in patients with significantly increased levels of GGT in the early post-operative period following liver resection ¹² and surgical repair of a ruptured abdominal aortic aneurysm ¹¹ . However, those studies were not designed to address changes in GGT progression over time.

While we acknowledge that association does not necessarily indicate causation, these data support the hypothesis that high GGT in an early post-LT setting may be a marker of some protective process.

Although the precise mechanism responsible for an elevated serum GGT early after LT is yet to be determined, experimental studies have demonstrated that cellular GGT modulates crucial redox-sensitive functions, such as antioxidant and antitoxic defenses, cellular proliferation, and apoptotic balance ¹³ . Cellular GGT is a key enzyme in the gamma-glutamyl cycle resulting in production of intracellular GSH ^{14– 16} , an important antioxidant agent that protects the cells against reactive oxygen species (ROS) ^{17– 19} . GSH has been shown to protect the liver against ischemia reperfusion injury in animal models ^{16, 20, 21} . Hepatic ischemia can cause elevation of serum GGT with peak blood levels within 20 and 30 hours after restoration of hepatic arterial blood flow ^{18, 22} . Reperfusion is associated with a surge of ROS, which may overwhelm host natural antioxidant defenses ²¹ . The oxidative stress from the ROS formed after reperfusion may lead to increased cellular death by damaging membrane lipids through peroxidation, disrupting normal enzymatic activities, and diminishing mitochondrial oxidative metabolism ²³ . Cardin and colleagues ²⁴ studied oxidative stress in patients with chronic hepatitis C virus infection. Surprisingly, the authors observed an association between GGT and 8-hydroxydeoxyguanosine (8-OHdG), a marker of oxidative DNA damage. Patients who had a high level of 8-OHdG had significantly higher GGT levels but normal ALT levels ²⁴ .

Thus, a transient increase in GGT level post-LT may reflect the host compensatory mechanism against oxidative stress and toxic metabolites generated by hypoxia, reperfusion, and surgical stress ²¹ . Therefore, an increased GGT early after LT may reflect the ability of the host to initiate an appropriate systemic response.

Elevated serum GGT in the early post-LT period might also be a marker of better liver regeneration. Eisenbach and colleagues ²⁵ , showed that an early increase in serum GGT after LT was associated with a better outcome and the authors reasoned that this rise could be due to liver regeneration. Although the liver might regenerate to some extent after LT, there is no conclusive evidence to support this hypothesis. As we mentioned earlier, we observed a transient increase in serum GGT levels in patients who survived a surgical repair for a ruptured abdominal aortic aneurysm ¹¹ . In the latter group, it is less likely that significant liver regeneration occurs.

Furthermore, elevated serum GGT early post-LT may also be an indicator of better early secretory function of the liver. GGT in part comes from the surface of the bile ducts and is released from the anchor, which attaches this enzyme to the cell surface, by bile acids in bile. Bile acid/phospholipid ratio in bile has shown to be elevated after LT ²⁶ . Thus elevated serum GGT immediately after transplantation may be an indicator or an active bile flow. Hence, it may protect hepatocytes from cytotoxic bile acids.

Contrary to early elevated GGT, but in line with published literature ^{2– 10} , we observed that a late (i.e. six months post-LT) elevated GGT was significantly associated with decreased survival within the 5-years following LT. Although the finding that normal GGT levels in the late post-operative period is predictive of good outcomes is obvious and intuitive, the contrasting influences of GGT levels between early and late post-LT periods on survival may be compatible with the physiologic function of intracellular GGT. Notably, at three months, six months, and one year post-LT, the relative increase in serum GGT was two to four times higher in patients who did not survive for more than five years compared to those who did survive. This proportion was much higher than that of AST, ALT and TBI, which might imply that an elevated serum GGT is not only a marker of harm to the liver but it could be seen as a systemic response to harmful environmental factors. Indeed, in two studies, Lee and colleagues ^{27, 28} postulated that serum GGT in the general population might be a marker of increased exposure to environmental stress, internal xenobiotics, or other unknown factors that cause oxidative stress in the long run.

To avoid possible bias we performed our analysis after excluding obstructive mechanisms such as NAS, cholestatic disorders, and acute graft rejection early postoperatively. Exclusion of these cases did not change our results significantly, suggesting that the elevation in serum GGT early post-LT is independent of obstructive disorders.

A practical clinical consequence of our findings may be that care providers in hospitals should realize that an abnormally high GGT early post-LT is not a cause for alarm or specific diagnostic procedures. In fact, a GGT activity four to five times above the normal range during the second post-operative week might even be considered beneficial.

We acknowledge some considerable limitations in this study. First, due to the retrospective design of the study we can only identify association rather than causation. This could only be established by specific (intervention) studies that measure the interaction between serum GGT and ROS markers post-LT. However, there is a strong correlation between GGT and oxidative DNA damage in cirrhotic patients ²⁴ . Next, we cannot entirely exclude that liver regeneration plays a role in the early post-LT rise of GGT levels since GSH and to some extent GGT are mainly produced by the liver ¹ . Hence a high serum GGT in patients who survived more than 90 days can also be a reflection of a well-functioning graft in LT patients. It will be important to understand the relationship of serum GGT and cellular GGT in the period immediately after surgery. Besides, cumulative evidence suggests that there is a relationship between the induction and release of ROS and ischemia reperfusion injury after other types of abdominal surgery ^{18, 21, 22} .

In conclusion, an elevated GGT level early after LT was associated with a better short-term outcome. However, chronically elevated GGT was associated with poor long-term outcome in the outpatient setting after LT. This peculiar switch in the prognostic meaning of GGT may result from the superposition of several mechanisms. Apparently, higher expression of GGT reflects some protective process in the acute phase but reflects chronic damage over the long term.

The data referenced by this article are under copyright with the following copyright statement: Copyright: � 2014 Alkozai EM et al.

Data associated with the article are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).

figshare: Serum gamma glutamyl transpeptidase levels post-liver transplantation and survival data, http://dx.doi.org/10.6084/m9.figshare.900343 ²⁹