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Asymmetric dimethylarginine (ADMA) and
cardiovascular disease: insights from prospective clinical trials
Rainer H Böger
To cite this version:
Rainer H Böger. Asymmetric dimethylarginine (ADMA) and cardiovascular disease: insights from prospective clinical trials. Vascular Medicine, SAGE Publications, 2005, 10 (2), pp.S19-S25.
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Introduction
Evidence has accumulated in recent years that the endothelial production and release of nitric oxide (NO) plays a crucial role in the maintenance of physiological vascular tone and structure. Asym- metric dimethylarginine (ADMA) is an endogenous
competitive inhibitor of NO synthase. It inhibits vascular NO production at concentrations found in pathophysiological conditions, and also causes local vasoconstriction when infused intra-arterially. Thus, elevated ADMA levels may explain the ‘L-arginine paradox’, that is, the observation that supplementation with exogenous L-arginine improves NO-mediated vascular functions in vivo, although its baseline plasma concentration is about 25-fold higher than the Kmof the isolated, purified endothelial NO synthase in vitro.
Dysfunction of the endothelial L-arginine/NO pathway is a common mechanism by which several cardiovascular risk factors mediate their deleterious effects on the vascular wall. Among them are hypercholesterolemia,1hypertension,2smoking,3 dia- betes mellitus,4hyperhomocysteinemia,5and vascular inflammation.6
Asymmetric dimethylarginine (ADMA) and cardiovascular disease: insights from prospective clinical trials
Rainer H Böger
Abstract: Evidence has accumulated that asymmetric dimethylarginine (ADMA) is an endogenous competitive inhibitor of nitric oxide (NO) synthase. ADMA inhibits vascular NO production at concentrations found in pathophysiological conditions; it also causes local vasoconstriction when infused intra-arterially. ADMA is increased in the plasma of humans with hypercholesterolemia, atherosclerosis, hypertension, chronic renal failure, chronic heart failure, and other clinical conditions. Increased ADMA levels are associated with reduced NO synthesis as assessed by impaired endothelium-dependent vasodilation or reduced NO metabolite levels. In several prospective and cross-sectional studies, ADMA has evolved as a marker of cardio- vascular risk. Moreover, prospective clinical studies have suggested that it may play a role as a novel cardiovascular risk factor. Zoccali and coworkers were the first to show that elevated ADMA is associated with a three-fold increased risk of future severe cardiovascular events and mortality in patients undergoing hemodialysis.
Valkonen and coworkers demonstrated in a nested case-control study that elevated ADMA was associated with a four-fold increased risk for acute coronary events in clinically healthy, nonsmoking men. In patients with stable angina pectoris, pre- interventional ADMA indicates the risk of developing restenosis or severe clinical events after coronary intervention. Furthermore, in humans with no underlying cardiovascular disease who are undergoing intensive care unit treatment, ADMA is a marker of the mortality risk. A number of additional prospective clinical trials are currently under way in diverse patient populations, among them individuals with congestive heart failure, cardiac transplantation patients, and patients with pulmonary hypertension.
In summary, an increasing number of prospective clinical trials have shown that the association between elevated ADMA levels and major cardiovascular events and total mortality is robust and extends to diverse patient populations. However, we need to define more clearly in the future who will profit from ADMA determination, in order to use this novel risk marker as a more specific diagnostic tool.
Key words: atherosclerosis; coronary heart disease; endothelium; nitric oxide;
oxidative stress; prognosis
Clinical Pharmacology Unit, University Hospital Hamburg- Eppendorf, Germany
Address for correspondence: Prof. Dr med. Rainer H Böger, Arbeitsgruppe Klinische Pharmakologie, Institut für Experimentelle und Klinische Pharmakologie, Universität- sklinikum Hamburg-Eppendorf, Martinistr. 52, D-20246 Hamburg, Germany. Tel:⫹4940 42803 9759; Fax:⫹4940 42803 9757;
E-mail: boeger@uke.uni-hamburg.de
Vallance and coworkers first described ADMA as an endogenous inhibitor of NO synthase in 1992.7Since their initial observation, the circulating concentration of ADMA has been shown to be increased in humans with hypercholesterolemia, atherosclerosis, hyperten- sion, chronic renal failure, congestive heart failure, diabetes mellitus, and other cardiovascular and meta- bolic diseases. Increased ADMA levels are associated with reduced NO synthesis and may therefore be a link between many other known and evolving cardio- vascular risk factors and the progression of vascular disease. This review focuses on the evidence that has been gathered from clinical studies relating ADMA concentration to disease outcome.
Association of ADMA with cardiovascular disease: evidence from epidemiological studies
Numerous studies have been published that have demonstrated a relationship between high ADMA concentration and cardiovascular disease. Elevated ADMA has a high prevalence in hypercholes- terolemia,1hyperhomocysteinemia,8diabetes mellitus,9 peripheral arterial occlusive disease,10 hyperten- sion,11,12 chronic heart failure,13 coronary artery disease,14and other diseases (Table 1). All of the stud- ies cited above were designed as case-control studies, thereby not allowing a conclusion about the possible cause–effect relationship between ADMA and cardio- vascular diseases. However, the observation that ADMA levels increased early during the development of atherosclerosis suggested that ADMA had the potential to be not only a marker but also a mediator of vascular lesions. Data have recently accumulated from a series of cross-sectional clinical studies that confirm this hypothesis.
Miyazaki and coworkers23 performed a study on 116 clinically healthy humans in whom intima-media thickness in the common carotid artery was deter- mined. This had previously been shown to be a surrogate marker for atherosclerosis progression. In stepwise regression analysis including multiple cardiovascular risk markers they found that ADMA concentration was significantly associated with carotid intima-media thickness.
In another study a similar approach was used to assess intima-media thickness as determined by high-resolution ultrasound analysis in 90 patients with end-stage renal disease who were undergoing long- term hemodialysis.24 Carotid intima-media thickness was related to various potential risk factors in a multi- ple regression analysis. Confirming the data of Miyazaki and coworkers,23 it was found that ADMA was highly significantly correlated with intima-media thickness in this high-risk population. Furthermore, in an extension of previous data, the progression of inti- mal thickening during 1 year of follow-up was best pre- dicted by initial ADMA and C-reactive protein levels.
Valkonen and coworkers14performed a nested case- control study including 150 middle aged, nonsmoking men from eastern Finland. They found that those whose ADMA concentration was within the highest quartile of the distribution in this population (i.e.⬎0.62mol/l) had a 3.9-fold elevation of the risk for acute coronary events compared with the other quartiles.
In a multicenter case-control study including a total of 816 patients with objectively proven coronary artery disease and matched controls, we recently found that ADMA was the marker with the best discriminative power to differentiate between cases and controls.25
A close relationship has also been established between insulin resistance and ADMA by the work of Stühlinger et al.21These investigators assessed insulin
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Vascular Medicine 2005; 10: S19–25
Table 1 Pathophysiological conditions in which ADMA has been reported to be elevated.
Condition x-fold increase Reference
Chronic renal failure 2–7-fold Zoccali et al 200115
Hypertension 2-fold Surdacki et al 199912
Childhood hypertension 2.3-fold Goonasekera et al 199711
Pregnancy-related hypertension 2-fold Chen et al 199716
Pre-eclampsia 2-fold Pettersson et al 199817
Pulmonary hypertension 2–3-fold Gorenflo et al 200118
Stable coronary artery disease 2-fold Valkonen et al 200114 Peripheral arterial occlusive disease 2–3-fold Böger et al 199710
Stroke 2-fold Yoo et al 200119
Hypercholesterolemia 2-fold Böger et al 19981
Hyperhomocysteinemia 2-fold Sydow et al 20038
Chronic heart failure 2–3-fold Usui et al 199813
Hyperthyroidism 2-fold Hermenegildo et al 200220
Diabetes mellitus type 2 2–3-fold Abbasi et al 20019
Insulin resistance 2-fold Stühlinger et al 200221
Liver failure 2-fold Tsikas et al 200322
sensitivity in a group of 64 nondiabetic people using the insulin-glucose clamp technique. They found that a tight linear relationship existed between steady state plasma glucose (a measure of insulin resistance) and plasma ADMA. These data indicate that elevation of ADMA occurs before the manifestation of overt dia- betes mellitus, which is also associated with elevated plasma ADMA.9
ADMA is a novel cardiovascular risk factor:
evidence from prospective clinical trials
In the first prospective trial studying ADMA as a potential new cardiovascular risk factor, at the University Hospital Hamburg-Eppendorf, we meas- ured plasma ADMA as well as plasma levels of conventional and emerging cardiovascular risk factors in 225 patients on hemodialysis. After a median 33.4 months of follow-up, 120 major cardiovascular events and a total of 83 deaths (53 vascular deaths) had occurred. In a Cox’s proportional hazards model ADMA and age were the strongest predictors of cardiovascular events and total mortality.15Although the median plasma ADMA concentration in this population (2.52mol/l) was higher than in healthy normal adults (usually below 1mol/l), patients with an ADMA concentration above the 75th percentile within this group had a three-fold higher risk of death from any cause than those with ADMA levels below the median.
In another prospective study, Lu and coworkers26 addressed the question of whether ADMA may predict outcome after percutaneous coronary intervention in patients with stable angina pectoris. A total of 153 consecutive patients were followed for a median of 16 months. There were 51 major cardiovascular events during follow-up. There was a clear increase in risk with rising ADMA levels, which was independent of other potential confounding factors in a multifactorial Cox’s regression analysis (including age, smoking, hypercholesterolemia, the use of stents, and others).
The remarkable outcome of this study is that ADMA levels were within a range that many would consider as ‘normal’; patients in the lowest tertile of ADMA had a median circulating concentration of 0.42mol/l compared with 0.75mol/l in the highest tertile.
Nijveldt and coworkers analysed potential predictors of death in patients undergoing treatment on a multi- disciplinary intensive care unit. They included patients with multiple organ failure and found that ADMA was the strongest predictor of death, with a 17-fold excess in mortality for patients in the highest ADMA quartile compared with those in the lowest quartile.27 ADMA was significantly associated with hepatic failure, and with lactic acid and bilirubin concentrations, suggest- ing that hepatic function is an important determinant of circulating ADMA concentration.
The currently available data on ADMA from prospective clinical studies are summarized in Table 2.
Additional prospective clinical studies have been initiated in patients with congestive heart failure, acute coronary syndrome, stable coronary heart dis- ease, heart transplantation, and pulmonary hyperten- sion. These data will enable us more precisely to define the role of ADMA as an emerging cardiovascu- lar risk factor.
The pathophysiological link between ADMA and cardiovascular disease: evidence from experimental studies in humans
Experimental studies performed in vitro or in animal models have recently been summarized in several reviews.28–30 This paper will therefore focus on data generated in humans.
In-vivo pathophysiological studies in humans have been performed by several groups of investigators.
These data shed more light on the relationship between ADMA and vascular disease.
Intra-arterial infusion of ADMA into the brachial artery significantly reduced forearm blood flow.31 However, ADMA concentrations reached in the local circulation after intra-arterial infusion by far outstrip those that can be measured in patients. Thus, two studies in which ADMA was administered intra- venously added data to the dose–response relationship of ADMA in humans. In one study, ADMA was infused to reach a systemic plasma concentration of about 2.6mol/l,32which caused a significant eleva- tion of arterial blood pressure and systemic vascular resistance, reduced cardiac output and heart rate, and decreased the vascular response to exercise. In another
Table 2 Results from published prospective studies of ADMA as a cardiovascular risk factor.
Condition x-fold risk excess ADMA level Reference
Hemodialysis 3-fold Highest quartile (⬎3.85mol/l) Zoccali et al 200115 Stable coronary artery disease 3.9-fold Highest quartile (⬎0.62mol/l) Valkonen et al 200114 Intensive care unit treatment 17.2-fold Highest quartile (⬎1.05mol/l) Nijveldt et al 200327 Stable coronary artery disease 5.3-fold Highest tertile (⬎0.62mol/l) Lu et al 200326
study it was shown that ADMA decreased plasma cyclic guanosine monophosphate concentrations at pathophysiologically relevant concentrations, whereas renal plasma flow and glomerular filtration rate were reduced only at very high concentrations.33
Chan and coworkers studied the adhesion of periph- eral blood monocytes to cultured human endothelial cells. They found that the adhesiveness index was related to the circulating ADMA level of the patient from whom the monocytes were isolated. Moreover, when they manipulated the ratio of L-arginine over ADMA by giving supplemental L-arginine, they found that monocyte adhesion normalized during 12 weeks of treatment.34
Taken together, the above mentioned studies pro- vide a plethora of evidence that ADMA modulates endothelial NO synthase activity within the concentra- tion range found in patients with vascular and meta- bolic diseases. Moreover, this evidence also suggests that even small modifications of ADMA significantly change vascular NO production, vascular tone, and systemic vascular resistance.
One important question in interpreting and understanding these findings and their relationship to ADMA plasma or serum levels measured in prospec- tive clinical trials is the following: why do such small differences in circulating ADMA concentration as reported from prospective trials indicate such dra- matic increases in cardiovascular event rate if such high levels of ADMA need to be reached during its systemic administration? The current interpretation shared by many experienced investigators in the field is that circulating levels of endogenous ADMA are the result of a ‘spillover’ from ADMA released within the cytosol during protein degradation. As diemthylargi- nine dimethylaminohydrolase (DDAH) enzymatic activity is present in many cells, and as ADMA is rap- idly cleared from plasma, small changes in circulating levels may indicate large changes in local and/or intra- cellular ADMA concentration that clearly affect NO production and function. However, when ADMA is administered exogenously, it enters the circulation first and has to be transported into cells by CAT (the cationic amino acid transporter system y⫹) before it can affect cellular function. Much higher increases in plasma ADMA must therefore be attained to induce pathophysiological changes when exogenous ADMA is administered.
Measurement of ADMA in serum and plasma: methodological considerations and clinical relevance as a diagnostic tool
Research into ADMA was an exotic, purely experi- mental topic during the 1990s. However, as the above data show, it is now coming of age as an emerging
cardiovascular risk factor. Currently, effort is being made to define the normal range of ADMA concentra- tion in healthy humans of different age groups and of both sexes. Moreover, an international research initia- tive has undertaken a correlation of analytical methods, in order to allow the correlation of data measured in different laboratories around the world. Finally, besides elaborate analytical methods such as high- performance liquid chromatography and mass spec- trometry, more widely accessible and less time consuming (and therefore cheaper) ways of measuring ADMA have become available with the development and validation of a novel competitive enzyme immunoassay that allows the sensitive and specific assessment of ADMA levels.35,36
As the scene is set for a wider application of ADMA measurement in clinical routine, the question is: who should be tested? ADMA is elevated in association with almost all traditional cardiovascular risk factors and in patients with established cardiovascular or metabolic disorders. However, its concentration shows significant variability even within these patient populations. Moreover, multivariate regression analyses that have taken into consideration all of the traditional and some other novel risk factors have shown that ADMA remains independently associated with cardiovascular risk or total mortality even after adjustment for the other risk factors has been performed.
The steadily increasing number of prospective clinical trials show that ADMA not only helps to discriminate between health and cardiovascular disease (for which there are other established diagnos- tic criteria), but these data point out that, even within a group of patients who have previously been regar- ded as homogeneous for their similar risk factor pro- file, ADMA is a tool that can be used to identify individuals with a comparatively high probability of experiencing a major event. There is therefore the potential that cardiovascular risk assessment, which currently is achieved by risk factor scores such as the Framingham risk score,37 the PROCAM risk score,38or the risk charts developed by the European Society of Cardiology,39 can be strengthened by adding ADMA as another risk indicator. This may hold especially true for patients who do not clearly fall into the high-risk (i.e. greater than 20% risk for a major event during the subsequent 10 years) or low- risk (i.e. below 10% risk) categories, but who fall within the ‘grey zone’ of intermediate risk.
Thus, ADMA levels will be helpful in estimating the risk for patients who deserve intensified treatment for the prevention of cardiac events. L-arginine may be one means of specifically antagonizing the deleterious effects of ADMA on endothelial function via nutritional supplementation in primary and secondary prevention.
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Vascular Medicine 2005; 10: S19–25
When ADMA is elevated: therapeutic consequences?
Clinical and experimental evidence suggests that ele- vation of ADMA causes a relative L-arginine deficiency, even in the presence of ‘normal’ L-arginine plasma concentrations. As ADMA is a competitive inhibitor of NO synthase, its inhibitory action can be overcome by increasing the concentration of this enzyme’s natu- ral substrate, L-arginine.40,41Thus, it can be predicted that nutritional supplementation with L-arginine may help to restore the physiological balance by normaliz- ing the L-arginine/ADMA ratio, whereas it will have little or no effect on humans with no disturbed
L-arginine/ADMA balance. This view is supported by data from experimental studies with L-arginine supplementation.
Azuma and coworkers showed, in an experimental study, that ADMA accumulates in regenerated endothelial cells after balloon angioplasty.42It is well known that regenerated endothelial cells growing at the site of an arterial injury lack full functional capacity, with a major defect in the L-arginine–NO pathway.43This finding may explain why the local or systemic administration of L-arginine improves local arterial function after injury.44
Few patients experience pathologically low L-arginine concentrations. However, clinical and experimental evidence suggests that elevation of ADMA may cause a relative L-arginine deficiency, even in the presence of ‘normal’L-arginine levels. ADMA is a competitive inhibitor of nitric oxide synthase, so its inhibitory action can be overcome by increasing the con- centration of the enzyme’s substrate, L-arginine. The studies cited above have shown that ADMA levels may increase in situations associated with cardiovascular diseases, in which elevated ADMA concentration is one possible explanation for endothe- lial dysfunction and decreased NO production.
The beneficial vascular effects of cholesterol- lowering drugs – statins – have been shown to depend in part on their ability to upregulate endothelial NO synthase gene expression.45However, in clinical stud- ies, statins failed to improve endothelium-dependent vasodilation in about as many studies as there were in favour of this hypothesis. Once more, this discrepancy may be resolved with the help of ADMA. Janatuinen et al46 recently found that pravastatin was able to enhance myocardial blood flow (measured by positron electron resonance tomography) only in patients with a low ADMA level, whereas this drug was ineffective in those with an elevated ADMA. We speculated that ADMA may block NO synthase despite its upregu- lated gene expression after statin treatment, and that this blockade may be overcome by L-arginine supplementation.47Indeed, we found in a randomized, controlled trial that, in patients with an elevated ADMA concentration, simvastatin enhanced
endothelium-dependent vasodilation only when it was combined with supplemental nutritional L-arginine.48
Thus, ADMA may explain the results of clinical tri- als in which L-arginine was shown either to improve endothelial function or fail to do so, a discrepancy that has so far remained unexplained and which has previously been called the ‘L-arginine paradox’.40 Moreover, the ability of L-arginine to counteract the detrimental vascular effects of ADMA and thereby improve endothelial function as well as the symptoms of vascular disease demonstrates that the risk associ- ated with elevated ADMA is modifiable. This is another critical aspect that needs to be addressed before one may call ADMA an ‘emerging cardiovas- cular risk factor’.
The observation of an elevated ADMA concentra- tion in a given patient may therefore justify L-arginine supplementation in order to improve the ability of the endothelium to generate NO, and thereby counteract the adhesion of circulating blood cells, reverse vasoconstriction, and attenuate the production of oxygen-derived free radicals.
Acknowledgments
The work of the author was funded by the Deutsche Forschungsgemeinschaft (DFG Bo 1431/3-1) and by the Else-Kröner-Fresenius-Stiftung. The author owns a patent on an ADMA assay and receives royalties from the license.
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