Section I: RAS manipulation B. Pathophysiologic mechanisms at the vascular wall Pathophysiology of vascular disease Content Points: Atherosclerosis is a life-long disease in which the process of development of an initial lesion to an advanced raised lesion can take decades. The slide summarizes current thinking on the pathobiology of atherosclerosis.3 It is currently believed that risk factors lead to an environment in which the three principal oxidative systems in the vascular wall are activated: xanthine oxidase, NADH/NAD(P)H, and uncoupled eNOS. Excessive production of reactive oxygen species overwhelms endogenous antioxidant mechanisms, leading to oxidation of lipoproteins, nucleic acids, carbohydrates, and proteins. The principal target of this oxidative stress is the vascular endothelium, although data suggest that there may be other targets such as mononuclear (stem) cells. Among the functional alterations induced by reactive oxygen species are impairment of endothelium-dependent vasorelaxation (following a reduction in nitric oxide bioavailability), increase in inflammatory mediators, and development of a procoagulant vascular surface.4 Ultimately structural alterations occur, including plaque growth, vascular wall remodeling, decreased fibrinolysis, vascular smooth muscle cell proliferation and migration, and other structural alterations.5 They can lead to the clinical sequelae of death, MI, stroke, ischemia, and congestive heart failure (CHF).

Vicious Cycle of Ang II production Content Points: Dzau proposes that activation of vascular angiotensin-converting enzyme (ACE) creates a positive feedback mechanism for vascular/local ACE formation with subsequent induction of oxidative stress and inflammation.5 Activation of vascular ACE promotes the release of cytokines and growth factors that increase vessel wall inflammation. Inflammatory cells, in turn, can release enzymes that generate angiotensin II (Ang II), including ACE from monocytes/macrophages, cathepsin G from neutrophils, and chymase from mast cells. Moreover, as the macrophage becomes activated with modified low-density-lipoprotein cholesterol (LDL-C), its ACE expression increases. This cycle creates a positive feedback mechanism for local Ang II formation.

Proinflammatory effects of Ang II Content Points: Ang II is an inflammatory mediator and it can be considered an “honorary” proinflammatory cytokine.6 In addition to its well-known vasoconstrictor properties, Ang II activates inflammatory functions of vascular wall cells. Ang II stimulates leukocyte chemoattractants, such as monocyte chemoattractant protein-1 (MCP-1) elaboration by human smooth muscle cells. Ang II also augments expression of such leukocyte adhesion molecules as VCAM-1. Ang II can increase interleukin-6 (IL-6), instigator of the acute-phase response, provoking elaboration of C-reactive protein, serum amyloid A, and fibrinogen from hepatocytes. Administration of ACE inhibitors can reduce cytokine levels and indexes of nuclear factor (NF)-kB. Activation of the renin-angiotensin system increases production of reactive oxygen species from vascular cells, promoting oxidative stress, which is a potent proinflammatory stimulus. These actions suggest that ACE inhibition might act as anti-inflammatory therapy.

Endothelial dysfunction and increased oxidative stress predict CV events in CAD patients Content Points: In this study, Heitzer and colleagues demonstrated that endothelial dysfunction and increased oxidative stress predict the risk of cardiovascular events in patients with coronary artery disease.7 Endothelium-dependent and -independent vasodilation was determined in 281 patients with documented coronary artery disease by measuring forearm blood flow responses to acetylcholine and sodium nitroprusside. The effect of the coadministration of vitamin C (24 mg/min) was assessed in a subgroup of 179 patients. Cardiovascular events, including death from cardiovascular causes, MI, ischemic stroke, coronary angioplasty, and coronary or peripheral bypass procedures, were studied during a mean follow-up period of 4.5 years. Patients experiencing cardiovascular events (n = 91) had lower vasodilator responses to acetylcholine and sodium nitroprusside, but greater benefit from vitamin C. The Cox proportional regression analysis for conventional risk factors demonstrated that blunted acetylcholine-induced vasodilation (P = 0.001), the effect of vitamin C (P = 0.001), and age (P = 0.016) were independent predictors of cardiovascular events. Clinical implications: Endothelial dysfunction and increased vascular oxidative stress predict the risk of cardiovascular events in patients with coronary artery disease. These data support the concept that oxidative stress may contribute not only to endothelial dysfunction but also to coronary artery disease activity.

ACE activity is increased in coronary artery specimens from patients with ACS Content Points: Hoshida and colleagues measured the ACE activity of vascular tissue obtained by directional coronary atherectomy in patients with acute coronary syndrome (n = 17) and in patients with stable ischemic heart disease (n = 36), with and without restenosis.8 ACE activity of the culprit coronary lesions was significantly increased in patients with acute coronary syndromes, but not in patients with ischemic heart disease and restenosis when compared with those patients with ischemic heart disease without restenosis (n = 25). This is the first study to report differences between vascular ACE activity of culprit lesions in patients with acute coronary syndromes and stable ischemic heart disease. The increase in ACE activity in blood vessel specimens obtained from patients with acute coronary syndromes (ACS) indicates that a pathophysiological role of increased ACE activity related to the instability of atherosclerotic plaque exists. This is not unexpected, because ACE is a known local mediator of inflammation, and unstable plaques have a strong inflammatory composition.

Ang II induces superoxide production in human vascular tissue Content Points: Berry and colleagues studied the sources of superoxide (O2-) production in human blood vessels to investigate whether, and by what mechanism, Ang II might alter superoxide production.9 They report the first demonstration that Ang II can increase superoxide production in human arteries. The study was performed in human internal mammary artery segments. Arteries were incubated with Ang II at increasing concentrations for 4 hours. Ang II increased concentrations in internal mammary arteries in a dose-related manner, increasing 83% at the lowest dose and by 99% at the highest dose (1 µmol/L). These effects were blocked by incubation of diphenyleneiodonium (DPI), an inhibitor of NAD(P)H oxidase, demonstrating that Ang II-associated increases in oxidative stress are mediated by this enzyme

Inhibition of NAD(P)H oxidase by ACEI blunts superoxide (O2-) production Content Points: This schematic summarizes the relation between ACE action and vascular oxidative stress.10 As discussed on previous slides, emerging evidence suggests that ACE inhibitors have important implications for vascular oxidative stress. In patients, ACE activity is increased in atherosclerotic plaques; NAD(P)H oxidase activity increases as a function of cardiovascular risk factors. Activation of NAD(P)H oxidase-derived superoxide (O2-) has important implications for the progression of atherosclerosis and the development of clinical events. Briefly, superoxide combines with nitric oxide in a rapid action that produces peroxynitrite (ONOO-), thereby “shunting” nitric oxide away from its vascular-health promoting effects such as vasodilation and platelet inhibition. Impaired nitric oxide bioactivity predicts atherosclerotic disease activity. In addition to reducing nitric oxide bioavailability, peroxynitrite formation promotes lipid and protein oxidation in atherosclerotic lesions. Thus excess vascular superoxide has the dual effect of reducing nitric oxide bioactivity and promoting vascular oxidative stress. The action of superoxide dismutase (SOD) on superoxide produces hydrogen peroxide (H2O2) an agent typically associated with lipid and protein oxidation, as well as smooth muscle cell proliferation. As summarized on the slide, emerging evidence implicates the renin-angiotensin system in regulation of oxidative stress. Under conditions of renin-angiotensin system activation, superoxide production is increased. ACE inhibitors counter oxidative stress by blunting Ang II-mediated activation of NAD(P)H oxidase and by increasing bradykinin-mediated synthesis of nitric oxide, thereby avoiding a number of deleterious downstream effects.

ACE inhibition: Continuum of beneficial effects Content Points: ACE inhibition has multiple effects that may enhance its potential to stabilize the atherosclerotic plaque.11 Acting through different mechanisms, these agents interfere with important atherogenic processes, including smooth muscle cell migration and proliferation, inflammatory reactions, mediator expression, and macrophage activation. They enhance the fibrinolytic state through multiple effects including a reduction in plasminogen activator inhibitor (PAI-1) and an increase in tissue plasminogen activator (tPA). They also reduce platelet activation and enhance plaque stabilization. These and other properties may result in improved treatment of cardiovascular disease.