Serum gamma-glutamyl transpeptidase: A prognostic marker in cardiovascular diseases

BioFactors 17 (2003) 199–205 IOS Press

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Serum gamma-glutamyl transpeptidase: A prognostic marker in cardiovascular diseases

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Michele Emdina, Claudio Passinoa, Alfonso Pompellab and Aldo Paolicchib a CNR

Institute of Clinical Physiology, Via G. Moruzzi 1, Pisa, Italy of Experimental Pathology, University of Pisa, Italy

b Department

1. Introduction Are arterial atherosclerosis and the haemodynamic consequences of a tight vessel stenosis sufficient to provoke the clinical ischemic syndrome, as always thought? Or something else occurs, triggering the most serious and fatal sequelae (unstable angina, acute myocardial infarction, stroke), towards irreversible organ damage and subsequent failure or sudden death [1]? The concept of instability of the plaque (vulnerable plaque), has been introduced not so long ago, to underline the behavioral difference among stenotic lesions with a similar lumen diameter reduction, in terms of provoking the clinical event. In other words, the “culprit” lesion might be not the “tightest” but the most “active” one. A variety of factors has been claimed, both clinical and humoral, with assessed prognostic significance, likely contributing to the evolution of the atherosclerotic lesion (by facilitating thrombosis, or plaque thickening and rupture); among others, inflammation has been pointed out as having a major role [2]. Plaques at risk for disruption tend to demonstrate outward vessel remodeling, to contain a large lipid core, thinned out fibrous cap, reduced collagen content, and increased inflammatory cell infiltration. To explain the beginning and the progression of the flogistic process, even a role for some “environmental” factors, such as viral or bacterial agents has been hypothesized [3], among many other possible triggers of proinflammatory cellular responses (e.g. cytokines, angiotensin II, hypertension, hyperglycemia, smoking, oxidative stress and modified LDL) [4]. Low density lipoproteins (LDL) may per se induce inflammation whenever their peroxidation and oxidation/glycosylation of their protein moiety occur within the matrix of media layer, representing a stimulus for the endothelial cells to release chemokines, to promote monocyte adhesion and migration through the vessel wall into the plaque. The activated monocytes phagocytose the modified LDL, thus becoming “foam cells” which after death release lipids enlarging the plaque core. Proteolytic processes and oxygen related species destroy matrix molecules and damage muscle cells, which will not be able 1 This article also appeared in Thiol Metabolism and Redox Regulation of Cellular Functions, A. Pompella, G. B´anhegyi and M. Wellman-Rousseau, eds, IOS Press, Amsterdam.

0951-6433/03/$8.00  2003 – IOS Press. All rights reserved

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M. Emdin et al. / Serum gamma-glutamyl transpeptidase: A prognostic marker in cardiovascular diseases

Fig. 1. Gamma-glutamyl transpeptidase (γ-GT) metabolism of glutathione (GSH). Outside the cell, the CysGly dipeptide, is a powerful reductant for Fe3+ , able to generate at the same time Fe2+ and a free thiyl radical. Thereafter, oxygen reactive species, by the same reaction, contribute to a net prooxidant effect.

to repair the cap. The damage of the cap will start the thrombotic process. Oxidized LDL are actually lethal for various kinds of cells within the atheroma (myocytes, fibroblasts, macrophages, endothelial cells). Among the mechanisms able to induce in vitro LDL oxidation (mieloperoxidase, lipoxygenase, oxygen reactive species, Cu and Fe ions), the metabolism of thiols, and in particular of glutathione (GSH, gammaglutamyl-cisteinyl-glycine, the main intracellular antioxidant agent) has been invoked as a possible trigger through the hydrolysis of its γ -glutamyl bond between glutamate and cysteine [5], through an extracellular reaction catalyzed by the gamma-glutamyltranspeptidase (γ -GT). Through the action of membrane dipeptidases, this reaction provides cysteine and glycine to the intracellular milieu, as precursors for the GSH synthesis. Outside the cell, however, the cys-gly dipeptide is a powerful reductant for Fe3+ , able to generate at the same time Fe 2+ and a free thiyl radical (Fig. 1). Furthermore, in the presence of GSH e Fe 3+ , γ -GT is able to catalyze the LDL lipoprotein oxidation, at an enzyme activity similar to that currently found in human sera. While it is unlikely that this reaction takes place in plasma, due to the high levels of antioxidant agents, this is not the case of the plaque milieu, where free iron is present, and where Paolicchi et al. immuno-histochemically demonstrated the activity of γ -GT, both in coronary and cerebrovascular human atheromas, colocalized with oxidized lipids and with inflammatory infiltration [5]. Serum GGT is partially absorbed onto LDL lipoproteins, which might carry γ -GT activity inside the plaque. Another line of evidence points to iron metabolism as possibly linked with the evolution of the atherosclerotic process, as indicated by the association of increased iron body tissue reserve with an increased risk of myocardial infarction. Tissue iron content might be a relevant cofactor in influencing the predictive value of γ -GT, thus suggesting adequate prospective studies [6]. 2. GT role in cardiovascular diseases, an emerging perspective The serum levels of γ -GT activity are currently considered as an index of hepato-biliary dysfunction and alcohol abuse [7]. Nevertheless, within its normal range, γ -GT has many other, even stronger

M. Emdin et al. / Serum gamma-glutamyl transpeptidase: A prognostic marker in cardiovascular diseases

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determinants than alcohol consumption. In sex-specific, multiple regression analyses [8,9], performed over large unselected populations on a total of 12511 men, and 12494 females, aged 12–62 years, screened in two different health survey programs, γ -GT showed a strong positive association with body mass index, alcohol use, total serum cholesterol and a somewhat weaker positive association with serum triglycerides, high density lipoprotein cholesterol, heart rate, blood pressure, use of analgesics, time since last meal. Strong negative associations were found for coffee consumption, hour of the day at which the examination was performed and, in males, physical activity. In females, use of oral contraceptives and menopause were positively associated with serum γ -GT, whereas pregnant females had lower values. Fewer than 3.8–5.5% of the males and 0.81.5% of the females had values exceeding 50 units/liter, as reported by these two distinct studies from Norway [8,9]. 3. Prognostic role of γ -GT in ischemic heart diseases An increasing number of population studies have evaluated the relation between serum γ -GT activity and mortality, since the observation of Conigrave [10], indicating that γ -GT does have a predictive value for mortality irrespective of hepatic disease or alcohol consumption. More recently, Jousilahti et al. analyzed the association of two widely recognized markers of alcohol consumption – carbohydratedeficient transferrin (CDT) and γ -GT – and self-reported alcohol consumption, with prevalent ischaemic heart disease in a random sample of 3666 Finnish men aged 25 to 74 years participating in a risk factor survey in 1997: finally, the CDT levels were inversely and γ -GT levels positively correlated with CHD risk. In a composite risk assessment, men with normal CDT levels (
20 U/L) and elevated γ -GT levels (>80 U/L) had nearly 8-fold adjusted risk of ischaemic heart disease, as compared with the men with normal γ -GT levels and elevated CDT levels [11]. Self-reported alcohol consumption had an inverse association with ischemic heart disease risk, which disappeared after adjustment for the other risk factors. Thereafter, Wannamethee et al. [12] reported in a large unselected population of middle-aged men that γ -GT has a negative prognostic value, as concerns both overall and cardiac mortality – namely mortality in patients with a previous history of ischemic heart disease – thus suggesting a linkage with underlying atherosclerotic coronary artery disease. In a prospective study of 7613 middle-aged British men followed for 11 years, γ -GT levels were strongly associated with all-cause mortality, largely due to a significant increase in deaths from ischemic heart disease in the top quintile of the γ -GT distribution. Serum γ -GT levels were positively associated to preexisting ischemic heart disease, diabetes mellitus, antihypertensive medication, systolic and diastolic blood pressure, total and high density lipoprotein cholesterol, heart rate, and blood glucose, and negatively associated with physical activity and lung function. After adjustment for these variables, elevated γ -GT (highest quintile,  24 unit/liter, vs. the rest) was still associated with a significant increase in mortality from all causes and from ischemic heart disease. The increased risk of ischemic heart disease mortality was more marked in those with evidence of ischemic heart disease at screening, particularly in those with previous myocardial infarction. Another recent study was aimed at evaluating the long-term prognosis among 714 patients with a very small or unconfirmed acute myocardial infarction (AMI) aged