Eugenia Gkaliagkousi

Platelet-Derived Nitric Oxide Signaling and Regulation

Nitric oxide (NO) exerts important vasodilatory, antiplatelet, antioxidant, antiadhesive, and antiproliferative effects. Although endothelium derived NO has been shown to be of prime importance in cardio- and vasculoprotection, until recently little was known about the role of platelet-derived NO. New evidence suggests that NO synthesized by platelets regulates platelet functions, in particular suppressing platelet activation and intravascular thrombosis. Moreover, platelet NO biosynthesis may be decreased in patients with cardiovascular risk factors or with coronary heart disease, and this may contribute to arterial thrombotic disease in these patients. Here, we review the current state of knowledge as regards the role of platelet-derived NO, both in normal physiology and in cardiovascular disease states, and compare platelet NO signaling and regulation with that in endothelial cells.

Nitric oxide signalling in the regulation of cardiovascular and platelet function

Nitric oxide (NO) exerts important protective actions on the cardiovascular system. Generated from L-arginine by the action of endothelial (or type 3) nitric oxide synthase (NOS3), NO regulates vascular tone in humans and causes endothelium-dependent vasodilation. Additionally endothelium-derived NO exerts antioxidant, antiproliferative and anti-inflammatory properties, thus playing an important role in inhibiting the atherosclerotic process. With regard to effects on platelet function, NO produced by both endothelial cells and platelets has important antithrombotic effects by decreasing platelet activation, a phenomenon which contributes importantly to the thrombotic tendency which accompanies a variety of cardiovascular disease states. Additionally, by inhibiting platelet activation, NO prevents heterotypic aggregation between platelets and monocytes, thereby reducing monocyte-platelet aggregates in the circulation which are believed to play an important pathophysiological role in the initiation and progression of atherosclerosis. New therapeutic interventions aimed at improving NO availability have been investigated in animal as well as in vitro studies and show considerable promise, but it remains to be seen whether such therapies will be equally efficacious in humans clinically.

Cardiovascular risk in rheumatoid arthritis: pathogenesis, diagnosis, and management.

Abstract Rheumatoid arthritis is characterized by early and accelerated atherosclerosis leading to increased cardiovascular morbidity and mortality. Beyond traditional cardiovascular risk factors, several pathogenetic mechanisms have been proposed, including emerging inflammatory and autoimmune mechanisms. Inflammatory stimuli are now believed to cause vascular damage, which can be estimated by well-established noninvasive techniques. Carotid intima-media thickness, pulse-wave velocity and flow-mediated dilatation, markers of subclinical atherosclerosis, arterial stiffness, and endothelial function, respectively, have been recently used to detect vascular dysfunction in the wide spectrum of autoimmune diseases. The role of anti–tumor necrosis factor &agr; and novel biologic agents remains unclear, although early control of the inflammatory process seems crucial for reducing cardiovascular risk. Considering the importance of cardiovascular risk management, further well-designed studies are warranted to clarify the potential benefits and harms of anti-inflammatory treatment.

Nitric oxide dysfunction in vascular endothelium and platelets: role in essential hypertension.

Both endothelial and platelet-derived nitric oxide have important vasculoprotective properties. Increasing evidence suggests a dysfunctional platelet nitric oxide synthase type 3 (NOS3) pathway in essential hypertension, whereas for endothelial-derived nitric oxide the picture is more complicated, with many studies suggesting an impairment of endothelial nitric oxide generation, whilst others have suggested that the endothelial nitric oxide pathway is preserved. Controversy also exists as to whether any observed reduction in endothelial or platelet-derived nitric oxide exerts a pathogenetic role or is simply the result of the raised blood pressure. In this review, we examine the evidence that endothelial and/or platelet-derived nitric oxide are disturbed in essential hypertension, and whether such disturbances are cause or effect.

Platelet Activation in Essential Hypertension: Implications for Antiplatelet Treatment

Essential hypertension is associated with increased risk of arterial thrombotic disease. Among other factors, enhanced platelet activity contributes significantly to this phenomenon. An increased level of circulating monocyte-platelet aggregates (MPAs) represents one of the most robust markers of platelet activation; furthermore, these aggregates are also believed to contribute to the pathophysiology of atherothrombotic disease. Putative mechanisms that contribute to platelet activation in essential hypertension include endothelial dysfunction, neurohumoral (sympathetic and renin-angiotensin systems) overactivity, decreased platelet nitric oxide (NO) biosynthesis, and platelet degranulation secondary to increased shear. Current recommendations are that hypertensive patients receive aspirin therapy only if their calculated cardiovascular risk is high and their blood pressure (BP) is adequately controlled. By contrast, the use of antiplatelet treatment in low-risk hypertensive patients is not established and merits further investigation. Moreover, the place of alternative antiplatelet agents other than aspirin, such as clopidogrel, is unclear at present. Some experimental evidence suggests that clopidogrel may confer an additive protective effect over and above aspirin in hypertensive patients, by virtue of effects on the evolution of the atherosclerotic process. This now needs to be investigated in long-term clinical outcome studies.

β-Actin regulates platelet nitric oxide synthase 3 activity through interaction with heat shock protein 90

Cytoskeletal proteins are crucial in maintaining cellular structure and, in certain cell types, also play an essential role in motility and shape change. Nitric oxide (NO) is an important paracrine mediator of vascular and platelet function and is produced in the vasculature by the enzyme NO synthase type 3 (NOS-3). Here, we demonstrate in human platelets that the polymerization state of β-actin crucially regulates the activation state of NOS-3, and hence NO formation, through altering its binding of heat shock protein 90 (Hsp90). We found that NOS-3 binds to the globular, but not the filamentous, form of β-actin, and the affinity of NOS-3 for globular β-actin is, in turn, increased by Hsp90. Formation of this ternary complex among NOS-3, globular β-actin, and Hsp90, in turn, results in an increase in both NOS activity and cyclic guanosine-3′,5′-monophosphate, an index of bioactive NO, as well as an increased rate of Hsp90 degradation, thus limiting the duration for which NOS-3 remains activated. These observations suggest that β-actin plays a critical role in regulating NO formation and signaling in platelets.

Clinical Significance of Endothelial Dysfunction in Essential Hypertension

The endothelium is recognized as a major determinant of vascular physiology and pathophysiology. Over the last few decades, a plethora of studies have implicated endothelial dysfunction in the progression of atherosclerosis and the subclinical target organ damage observed in essential hypertension. However, the clinical significance of diagnosing endothelial dysfunction in patients with essential hypertension remains under investigation. Although a number of vascular and non-vascular markers of endothelial dysfunction have been proposed, there is an ongoing quest for a marker in the clinical setting that is optimal, inexpensive, and reproducible. In addition, endothelial dysfunction emerges as a promising therapeutic target of agents that are readily available in clinical practice. In this context, a better understanding of its role in essential hypertension becomes of great importance. Here, we aim to investigate the clinical significance of endothelial dysfunction in essential hypertension by accumulating novel data on (a) early diagnosis using robust markers with prognostic value in cardiovascular risk prediction, (b) the association of endothelial dysfunction with subclinical vascular organ damage, and (c) potential therapeutic targets.

Association between retinal vessel caliber and arterial stiffness in a population comprised of normotensive to early-stage hypertensive individuals.

BACKGROUND Although impairment of the micro- and macrocirculation is considered inherent to sustained hypertension, there is a substantial lack of studies investigating whether an association exists between micro- and macrovascular damage, especially in early-stage hypertension. METHODS We studied a meticulously selected population, free of diabetes and cardiovascular disease, of 223 individuals: 137 never-treated, newly diagnosed patients with recent onset of hypertension and 86 normotensive individuals. Nonmydriatic retinal photography was used to assess retinal microvascular diameters, including central retinal arteriolar (CRAE) and venular equivalent and arteriovenous ratio (AVR). Arterial stiffness was evaluated by measurement of pulse wave velocity (PWV) and aortic augmentation index (AIx). RESULTS Compared with normotensive subjects, hypertensive patients exhibited significantly increased PWV (8.1 vs. 7.1 m/sec; P < 0.001) and AIx (23.86% vs. 18.8%; P = 0.01) and decreased CRAE (86.47 vs. 91.44 μm; P = 0.001) and AVR (0.74 vs. 0.78; P = 0.007). A significant inverse association was demonstrated between PWV and CRAE (r = -0.205; P = 0.002), which remained significant after multivariable analysis. Likewise, CRAE (P = 0.04) and AVR (P = 0.02) were independent predictors of AIx. CONCLUSIONS This study shows for the first time an association between quantitatively assessed retinal abnormalities and increased arterial stiffness in a sample of early-stage hypertensive and normotensive individuals, suggesting that micro- and macrocirculation impairment in hypertension is a dynamic, mutual, interdependent process present from its very early stages. Given the predictive value of both retinal arteriolar narrowing and arterial stiffness in terms of cardiovascular mortality and morbidity, identification of combined micro- and macrovascular damage might be helpful in cardiovascular risk stratification of hypertensive patients.

Divergent Retinal Vascular Abnormalities in Normotensive Persons and Patients With Never-Treated, Masked, White Coat Hypertension

BACKGROUND Hypertensive patients with retinal arteriolar abnormalities are at increased risk for cardiovascular events. However, the extent of retinal microvascular changes in naïve, never-treated patients with hypertension of short duration has not been established. In addition to this, the lack of relevant data about other phenotypes of hypertension (masked and white-coat hypertension) determined by ambulatory blood-pressure measurement (ABPM) is notable, despite their relationship to increased cardiovascular risk mediated by underlying target-organ and vascular damage. METHODS We conducted a study in which nonmydriatic retinal photography was used to assess central retinal artery equivalent (CRAE) and central retinal vein equivalent (CRVE) diameters and the retinal arteriovenus ratio (AVR) in a group of 103 individuals with never-treated hypertension of recent (< 1 year) appearance, 28 individuals with masked and 20 with white-coat hypertension, and 50 normotensive individuals, as appropriately classified by ABPM. RESULTS Patients with sustained and masked hypertension had narrower values of CRAE than did normotensive individuals (86.7±10.1 and 87.6±9.2 vs. 94.8±10.6, P < 0.001 and P = 0.02, respectively). The AVR was lower in patients with sustained hypertension (0.736±0.102), masked hypertension (0.716±0.123), and white-coat hypertension (0.739±0.127) than in normotensive subjects (0.820±0.095), P < 0.001, P < 0.001, and P = 0.03, respectively. Both AVR and CRAE were negatively associated with mean systolic and diastolic daytime, nighttime, and 24-hour blood pressures, even after adjustment for other factors. CONCLUSIONS Subtle retinal microvascular signs of pathology are observed in hypertensive patients at early stages of hypertension and in patients with both masked and white coat hypertension. These changes may be indicative or may mediate the differences in cardiovascular mortality in persons with masked and white-coat hypertension, and relevant information about this can be easily accessed with retinal photography.

Endothelial Function and Arterial Compliance are not Impaired in Subjects With Heart Failure of Non-Ischemic Origin

BACKGROUND Patients with heart failure and underlying ischemic heart disease (IHD) exhibit both endothelial dysfunction and increased arterial stiffness. We investigated whether this is also the case in heart failure of nonischemic etiology. METHODS AND RESULTS Eleven patients with heart failure and IHD, 12 patients with heart failure from angiographically verified idiopathic nonischemic dilated cardiomyopathy (DCM), and 16 healthy subjects of similar age and sex were compared. Endothelium-dependent and independent function were evaluated by ultrasonic measurement of flow-mediated dilatation (FMD) and glyceryl trinitrate (GTN)-induced dilatation of the brachial artery respectively. Vascular compliance was assessed by carotid-femoral pulse wave velocity (PWV) and augmentation index (AIx). Heart failure severity was similar in IHD and DCM patients. FMD was impaired in the subjects with IHD as compared with control subjects (4.8 +/- 0.3 vs. 8.0 +/- 3.6 %, P < .01), but not in those with DCM. GTN-induced vasodilatation was not different in patients and controls. PWV was also increased in IHD patients compared with controls (12.1 +/- 3.6 vs. 8.0 +/- 1.6 m/s, P < .01), but not in DCM patients. AIx was similar in patients and controls. CONCLUSION Heart failure of nonischemic etiology is not associated with abnormalities of endothelium-mediated dilatation or of arterial compliance. The findings of our study now need to be confirmed in larger studies.