Pressure Overload and Cardiac Remodelling-A Smoking Gun Points to Uncoupled Nitric Oxide Synthase: Oxidant Stress from Nitric Oxide Synthase-3 Uncoupling Stimulates Cardiac Pathologic Remodeling from Chronic Pressure Load. J Clin Invest 115: 1221-1231, 2005.
Cardiac hypertrophy is a frequent complication of renal malfunction. In essential hypertension left ventricular hypertrophy (LVH) predicts cardiovascular death (1) BP independently. In dialysed uremic patients, Silberberg was the first to show that LVH predicted survival (2) and this has been confirmed in virtually all subsequent studies (3). In patients with renal disease, LV remodelling and diastolic malfunction are seen even before GFR is decreased (4). The prevalence of LVH increases progressively as renal function declines (5,6). At the start of dialysis a normal echocardiogram was found in no more than 16%, concentric hypertrophy in 41%, and LV dilation in 28% (7). Median time to the development of heart failure, with dire prognostic implications (8), was 48 mo in patients with concentric LVH compared with >66 mo in patients with normal echocardiograms (7). Routine echcardiograms may underestimate the functional impact, and the midwall fractional shortening/end systolic stress relation is a more sensitive and common indicator of systolic malfunction in concentric LVH (9). The genesis of cardiac hypertrophy in uremia is certainly multifactorial. Pressure overload (increased peripheral resistance associated with hypertension and vascular remodelling) and impedance (from increased stiffness of central arteries) as well as volume overload (salt-water retention, anemia, arteriovenous shunt) play a role (10). Undoubtedly, however, systemic factors independent of pressure and volume overload must also play a role, presumably sensitizing the heart to the impact of the classic LVH risk factors. We concluded this from the fact that experimentally one finds hypertrophy not only of the left, but also of the right ventricle and that interventions including abrogation of hypertension and volume overload failed to completely prevent LVH (11). Such aggravating systemic factors presumably include among others neurohumoral activation, particularly sympathetic overactivity, hyperparathyroidism, and oxidative stress. Why oxidative stress? In recent years there has been increasing evidence that reactive oxygen species (ROS) play a role in the genesis of cardiac hypertrophy and the progression of heart failure (12,13). Furthermore, interventions such as administration of vitamin E (14) have been shown to ameliorate LVH. Conversely, disruption of the potent antioxidant thioredoxin-1 system by creating a dominant negative mouse mutant caused oxidative stress in the heart and more severe cardiac hypertrophy in response to aortic banding. This was abrogated by administration of an antioxidant (15). It had been known that generation of ROS may be the result of agonists such as catecholamines (16) or angiotensin II (17). It was also known that ROS mediate the hypertrophic response to mechanical stretch (18), induce reexpression of the fetal gene program (19), and trigger cardiac remodelling by activating metalloproteinases (19). What had not been defined, however, was the exact pathway through which ROS were generated in hearts developing hypertrophy. Candidates were the mitochondrial electron transport system (20), NADPH oxidase (21), xanthin oxidase (22), and nitric oxide synthase (NOS) (23). This field has now been carried further by the important study of Takimoto (24), who identified NOS as the crucial pathway for the generation of oxidative stress in the hypertrophying heart and who also found an intervention which might provide novel perspectives for intervention. The authors generated knockout mice lacking the endothelial isoform NOS-3 (previously called eNOS) of NOS. In response to transverse aortic constriction wild-type mice developed progressive dilatory LV remodelling, an increase in cardiomyocyte size and fibrosis; in the NOS-3 knockout mice, aortic banding caused only modest initial nonprogressive concentric hypertrophy with little fibrosis. In wild-type mice, impaired systolic (dP/dt) and diastolic (rate of pressure decline) function was noted, but there was little change in the NOS-3 knockout mice. Striking differences between these groups of animals were also seen with respect to the expression of fetal genes in the heart. The role of ROS was proven by measuring NO production via a luminal chemiluminescence assay and by staining for and measurement of nitrotyrosine, a downstream ROS reaction product. Nitrotyrosine was minimally elevated in the NOS-3 knockout mice, but strikingly so in the wild-type mice. The finding of next to no ROS production in animals deficient in NOS identified this enzyme as the mediator of ROS generation in the model of LV hypertrophy in response to aortic banding. But how does aortic banding increase the activity of NOS-3 in the wild-type mice? NOS is normally present as a dimer that transforms L-arginine into the products NO and L-citrulline. If it is present as a monomer and is uncoupled, however, the enzyme generates superoxide (O2−) instead of NO. This transition from the monomeric to the uncoupled dimeric form is seen when the enzyme is exposed to peroxynitrite (ONOO−) or deprived of the cofactor tetrahydrobiopterin (BH4) or the substrate, i.e., L-arginine, respectively. Therefore the authors studied immuneprecipitated NOS3 by nondenaturing gel electrophoresis. They were able to show that after aortic banding the enzyme was present in the monomeric form. They also found the reason why: They measured the concentration of the cofactor BH4 and it was low. The logical next step was to administer BH4. This blunted the concentric hypertrophic response to aortic banding. If the same is true in humans, administration of BH4 might well provide a novel form of intervention. These findings are of obvious interest against the background of the epidemiology of LVH in renal patients (2–9). There is, however, an added consideration. Uremia is a state of increased oxidative stress (25–27), as reflected by a number of indicators such as plasma aminothiol oxidation (28), lipid peroxidation (29), appearance of advanced oxidation protein products (30), or formation of the DNA damage product 8-hydroxy 2′-deoxyguanosine (31). The relevance of ROS is underlined by observations that advanced oxidation protein products predict cardiovascular events in patients with advanced renal failure (30) and that interventions to reduce oxidative stress such as vitamin E (d,l-α-tocopherol) (32) or N-acetylcysteine (33) reduce cardiac and atherosclerotic abnormalities in experimental uremia. This raises the issue whether the uremic organism might not also be particularly susceptible to the cardiac effects of uncoupled NO synthase and furthermore whether evidence of cardiac hypertrophy in the absence of pressure and volume overload (11) might not be the consequence of uncoupled NO synthase. Finally tantalizingly incomplete evidence had pointed to potential cardiovascular benefit from administration of vitamin E (34) and N-acetycysteine (35) in hemodialysed patients. Controlled trials in this field are obviously needed. A point that should now also be considered, however, is whether BH4 should be added as another candidate. Obviously further experimental and pharmacologic information is needed—but the potential prospect is fascinating.
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