Chronic heart failure (CHF) is a highly prevalent syndrome all over the industrialized world and is associated with significant morbidity and mortality. Several types of biomarkers reflecting neurohumoral activation, systemic inflammation, oxidative stress, metabolism and renal dysfunction, as well as anemia, have been shown to be associated with disease severity and progression.1 In addition to brain natriuretic peptide, its derivatives and C-reactive protein, particular attention in CHF prognosis has been paid to 2 inexpensive and easily accessible, highly sensitive laboratory tests, namely, uric acid (UA) and gamma-glutamyl transferase (GGT) determination in plasma. Although elevated plasma levels of UA and GGT are significantly associated with disease severity, their prognostic significance is still controversial in CHF.2,3 Recent work by Poelzl et al4 have indicated a mutual relationship between these biomarkers, since GGT levels were also associated with higher levels of UA and C-reactive protein.4 Nevertheless, the mechanism underlying the association between UA, GGT and CHF progression and prognosis remains largely unknown.

Both GGT and the enzyme xanthine oxidase, one of the putative sources of elevated UA in CHF, are involved in free radical production, followed by enhanced oxidation of biological macromolecules. Free radicals and oxidative stress byproducts are implicated in the key pathophysiological events in the course of CHF progression–endothelial dysfunction and remodeling. Thus, free radicals produced by xanthine oxidase and in GGT-mediated reactions may contribute to sequestration of nitric oxide and the resulting endothelial dysfunction in CHF. Endothelial function, as determined by the dilation of the brachial artery following transient occlusion (flow-mediated vasodilation [FMD]), is inversely correlated with serum UA levels in persons participantswith asymptomatic hyperuricemia associated with essential hypertension,5 as well as in patients with chronic kidney disease.6 Conversely, reduction of UA with a xanthine oxidase inhibitor improves endothelial function in persons participantswith asymptomatic hyperuricemia, as well as in patients with CHF.7,8 Although endothelial dysfunction has been documented in peripheral and coronary arteries in CHF patients,9 the relationship between UA and endothelial function has been investigated in only 1 study, while data on the association between GGT activity and endothelial dysfunction are lacking. In addition, a growing body of evidence suggests an important role of increased oxidative stress in adverse left ventricular remodeling after myocardial infarction.10,11 Our previous study and the others12,13 have shown that the level of the oxidative stress byproduct, malondialdehyde, correlates with the degree of ventricular remodeling in CHF secondary to myocardial infarction and represents an independent predictor of death in these patients. Moreover, recent evidence shows that hyperuricemia contributes to the pathogenesis of myocardial remodeling in experimental heart failure,14–16 Nevertheless, there are no data on the role of UA or GGT in cardiac dysfunction in the clinical settings. We hypothesized that elevated UA and upregulated GGT activity correlate with endothelial dysfunction and ventricular remodeling. Additionally, we hypothesized that potential association of these 2 laboratory markers with endothelial or ventricular dysfunction may be mediated by oxidative stress.

In this translational study, we addressed the association of plasma GGT activity and UA level with FMD and echocardiographic indices of cardiac dysfunction in 120 patients with ischemic CHF and investigated whether these effects are mediated by enhanced oxidative stress.

Although endothelial dysfunction has been documented in peripheral and coronary arteries in CHF patients,9 only 1 study in 38 mostly male participants with NYHA class II and III CHF has found an association between UA and endothelial function. Because the strict inclusion criteria (FMD < 8%) in this study could impair the generalization of the results to a less severe CHF population, we hypothesized that, in the early stages of CHF, increased serum UA levels, as an indicator of xanthine oxidase activation, could be used as a risk marker while, in the later phases, UA could act as an antioxidant, having a protective action on extracellular antioxidant enzymes and improving endothelial function. However, the results of the current study, which included 3 times more participants with various stages of CHF, have shown an inverse correlation between UA and FMD, while the activity of the antioxidant enzyme glutathione peroxidase was inversely correlated with UA. These results are biologically plausible, given that the main source of elevated UA in CHF is xanthine oxidase, the enzyme that produces superoxide or hydrogen peroxide as byproducts of the terminal steps of purine metabolism in the presence of hypoxia. Under these conditions, an increase in superoxide may inactivate nitric oxide,18 suggesting an important underlying mechanism in the development of vascular endothelial dysfunction in CHF, which contributes to systemic vasoconstriction and increased cardiac loading.

Cardiac remodeling is an unfavorable prognostic factor associated with myocardial hypertrophy, fibrosis, and ventricular dysfunction after myocardial infarction.19 The proposed mechanism causing a decrease in myocardial contractility is the cell damage produced by oxygen-free radicals, leading to peroxidation of membrane phospholipids, which can result in an increase in membrane fluidity, increasing permeability and loss of membrane integrity.20,21 In this study, we have shown that the UA level correlates with the degree of myocardial dysfunction, based on a linear regression analysis of the association between UA level and the echocardiographic indices of left ventricular remodeling. This effect of UA on remodeling seems to be at least partially mediated by enhanced lipid peroxidation, since UA correlated significantly with the lipid peroxidation byproduct malondialdehyde and antioxidant glutathione peroxidase activity. Xanthine oxidase expression and activity were found to be markedly increased in the tissue 2mm surrounding the infarct area22 in a mice model of myocardial infarction. In the heart, xanthine oxidase is localized solely in the capillary endothelium.23 Therefore, the UA generated in hypoxic states originates from capillary endothelial cells, rather than from the myocardium,24 and hyperuricemia in heart failure may reflect the metabolic effects of hypoxia on the microvasculature. The correlation between UA and left ventricular dysfunction found in the current study confirms the results of in vitro studies that have shown that reactive oxygen species production by xanthine oxidase leads to a depression of the excitation-contraction coupling mechanism in cardiac muscle.25,26 These effects would induce a decrease in cardiac contractility and in the rate of cardiac muscle relaxation. In addition to xanthine oxidase, hyperuricemia itself can also influence free radical production and myocardial remodeling. Chen et al16 have shown that hyperuricemia induced by oxonic acid stimulates myocardial superoxide production, resulting in enhanced endothelin-1-induced ventricular remodeling in infarcted rats. Furthermore, treatment of hyperuricemic rats with allopurinol and benzbromarone, UA-lowering agents, attenuated remodeling after myocardial infarction. However, Cicoira et al27, who evaluated the effects of elevated UA levels on cardiac function in 150 CHF patients resulting from dilated cardiomyopathy of diverse etiology, found a significant correlation with diastolic dysfunction, but not with markers of systolic function or left ventricular volumes. The discrepancies between these findings and our own may be a consequence of differences in the study cohort, which was homogenous in our study with respect to the cause of CHF. We showed a positive correlation between UA levels and both malondialdehyde and echocardiographic indices of remodeling. Although these findings do not support a causal relationship between UA and remodeling, they do suggest that UA could contribute to the increased oxidative stress present in CHF. Further experimental trials should be conducted to clarify the real impact of UA on the physiology of cardiovascular disease.

Several population-based studies have consistently shown that serum GGT activity, mostly within normal ranges, are strongly associated with most cardiovascular risk factors. Although the mechanism underlying this association remains largely unknown, several explanations for this phenomenon have been proposed, including hepatic congestion, increased free radical production and inflammation.4 In this study, we addressed the potential involvement of GGT in the pathogenesis of endothelial dysfunction and left ventricular remodeling in heart failure, since both of these events are related to oxidative stress. Specifically, membrane bound GGT is involved in degradation of the antioxidant glutathione, which ultimately results in the amino acids cysteine and glycine.28 The reactive thiol of cysteinyl-glycine can generate superoxide anion radicals and hydrogen peroxide through its interaction with free iron.29 These GGT-mediated reactions have been shown to catalyze the oxidation of low-density lipoproteins, which may contribute to oxidative events influencing plaque evolution and rupture.30 Indeed, a recent study has shown an association of GGT with coronary atherosclerosis progression in patients with ischemic heart disease on statin treatment.31 Coronary artery disease and myocardial infarction were the causes of CHF in this study. Cysteine and glycine constitute the precursors of intracellular glutathione. Hence, GGT also provides a supply for uptake and reutilization in intracellular glutathione synthesis. In this way, GGT serves as a rescue enzyme for cellular glutathione synthesis and thus plays an important role in antioxidant defense systems. Accordingly, it has been suggested that an increase in serum GGT activity could be used as a marker for increased oxidative stress in humans.32,33 Although the results of our study have indicated a relationship between cardiac dysfunction and GGT activity, it seems that it is not related to either increased lipid oxidation or impairment of antioxidant activity. Thus, in contrast to UA, serum GGT activity was not associated with the biomarkers of oxidative stress in CHF. Therefore, it may be speculated that elevated GGT in heart failure is only a part of the cholestatic profile of laboratory elevations seen in these patients, secondary to hepatic congestion. Nevertheless, the question of the mechanisms involved in the association between GGT and cardiac dysfunction should be addressed in future in vitro and animal studies.

The last decade has seen a significant advance in the pathophysiology of heart failure. As a result, many different diagnostic and prognostic markers have been proposed and few have shown clear clinical use.34,35 Among those, serum UA seems to fulfill these criteria. First, accurate, repeated measurements of UA are available to the clinician at a reasonable cost and with short turnaround times; second, the determination of serum UA provides information that is not already available from a careful clinical assessment; and finally, knowing the measured UA concentration should aid in medical decision-making. The current study provides a moderate correlation between levels of UA and indices of remodeling. On the other hand, UA and other currently assessable markers of CHF lack cardiac specificity, and their levels can be influenced by both systemic inflammatory and infective processes, which occur frequently in heart failure patients. Until now, several new biomarkers with a plausible biological link with heart failure pathophysiology have been identified. Among them, ST-2 and galectin share the ability to define the severity of the ongoing ventricular remodeling process.34,36 However, their clinical use should be confirmed in large-scale studies.

In this study, we show that serum levels of UA and GGT activity are associated with echocardiographic indices of left ventricular remodeling in patients with CHF secondary to ischemic coronary disease. Regarding endothelial function, only serum UA showed an inverse correlation with endothelium-dependent vasodilation in CHF. The effects of UA on endothelial function and left ventricular dysfunction may be at least partially explained by its relationship with oxidative stress markers such as malondialdehyde and glutathione peroxidase activity.

This work was supported by the grant 175052 from the Serbian Ministry of Education, Science and Technological Developmental.

None declared.