Increased microparticle production and impaired microvascular endothelial function in aldosterone-salt-treated rats: protective effects of polyphenols

Endothelial microparticles: A mechanosensitive regulator of vascular homeostasis and injury under shear stress

Hemodynamic shear stress (SS), a frictional force generated by blood flow, regulates vascular homeostasis. High and steady SS maintains physiological function of endothelial cells while low and disturbed SS promotes disturbance of vascular homeostasis and the development of atherosclerosis. Endothelial microparticle (EMP), a vesicular structure shed from endothelial cells, has emerged as a surrogate biomarker of endothelial injury and dysfunction. EMP release is triggered by disturbed SS in addition to multiple inflammatory cytokines. This review systematically summarizes the impact of SS on EMPs and the role of EMPs under SS in modulating vascular homeostasis and injury, including endothelial survival, vasodilation, inflammatory response, vascular permeability, and coagulation system.

Exosome-Containing Extracellular Vesicles Contribute to the Transport of Resveratrol Metabolites in the Bloodstream: A Human Pharmacokinetic Study

Exosomes are extracellular vesicles (EVs) that regulate intercellular signaling by transferring small RNAs, proteins, nucleic acids, lipids, and other metabolites to local or distant organs, including the brain, by crossing the blood-brain barrier. However, the transport of (poly)phenols in human EVs has not yet been described. Therefore, we aimed here to explore (i) whether resveratrol and (or) its derived metabolites are found in the cargo of human plasma exosome-containing EVs (E-EVs), (ii) when this incorporation occurs, and (iii) whether resveratrol intake stimulates the release of E-EVs. Thus, in a pharmacokinetic study, healthy volunteers (n = 16) consumed 1 capsule (420 mg resveratrol) in the evening before attending the clinic and one more capsule on the day of the pharmacokinetics. The plasma and the isolated E-EVs were analyzed using UPLC-ESI-QTOF-MS. Of 17 metabolites in the plasma, 9 were identified in the E-EVs, but not free resveratrol. The kinetic profiles of resveratrol metabolites were similar in the plasma and the E-EVs, a higher metabolite concentration being detected in the plasma than in the E-EVs. However, the plasma/E-EVs ratio decreased in the gut microbial metabolites, suggesting their better encapsulation efficiency in E-EVs. In addition, glucuronide conjugates of resveratrol, dihydroresveratrol, and lunularin were incorporated into the E-EVs more efficiently than their corresponding sulfates despite glucuronides reaching lower plasma concentrations. Notably, more E-EVs were detected 10 h after resveratrol consumption. This exploratory study provides the first evidence that (i) resveratrol metabolites are transported by E-EVs, with a preference for glucuronide vs. sulfates, (ii) the gut microbial metabolites concentration and kinetic profiles are closely similar in E-EVs and plasma, and (iii) resveratrol intake elicits E-EVs secretion. Overall, these results open new research avenues on the possible role of E-EVs in (poly)phenol health effects.

Malnutrition and Biomarkers: A Journey through Extracellular Vesicles

Extracellular vesicles (EVs) have been identified as active components in cellular communication, which are easily altered both morphologically and chemically by the cellular environment and metabolic state of the body. Due to this sensitivity to the conditions of the cellular microenvironment, EVs have been found to be associated with disease conditions, including those associated with obesity and undernutrition. The sensitivity that EVs show to changes in the cellular microenvironment could be a reflection of early cellular alterations related to conditions of malnutrition, which could eventually be used in the routine monitoring and control of diseases or complications associated with it. However, little is known about the influence of malnutrition alone; that is, without the influence of additional diseases on the heterogeneity and specific content of EVs. To date, studies in “apparently healthy” obese patients show that there are changes in the size, quantity, and content of EVs, as well as correlations with some metabolic parameters (glucose, insulin, and serum lipids) in comparison with non-obese individuals. In light of these changes, a direct participation of EVs in the development of metabolic and cardiovascular complications in obese subjects is thought to exist. However, the mechanisms through which this process might occur are not yet fully understood. The evidence on EVs in conditions of undernutrition is limited, but it suggests that EVs play a role in the maintenance of homeostasis and muscle repair. A better understanding of how EVs participate in or promote cellular signaling in malnutrition conditions could help in the development of new strategies to treat them and their comorbidities.

Differential and targeted vesiculation: pathologic cellular responses to elevated arterial pressure

Extracellular vesicles are small membrane-enclosed particles released during cell activation or injury. They have been investigated for several decades and found to be secreted in various diseases. Their pathogenic role is further supported by the presence of several important molecules among their cargo, including proteins, lipids, and nucleic acids. Many studies have reported enhanced and targeted extracellular vesicle biogenesis in diseases that involve chronic or transient elevation of arterial pressure resulting in endothelial dysfunction, within either the general circulatory system or specific local vascular beds. In addition, several associated pathologic processes have been studied and reported. However, the role of elevated pressure as a common pathogenic trigger across vascular domains and disease chronicity has not been previously described. This review will therefore summarize our current knowledge of the differential and targeted biogenesis of extracellular vesicles in major diseases that are characterized by elevated arterial pressure leading to endothelial dysfunction and propose a unified theory of pressure-induced extracellular vesicle-mediated pathogenesis.

Dietary modulation of large extracellular vesicles: the good and the bad for human health

Extracellular vesicles (EVs) encompassing nanovesicles derived from the endosome system and generated by plasmatic membrane shedding are of increasing interest in view of their ability to sustain cell-to-cell communication and the possibility that they could be used as surrogate biomarkers of healthy and unhealthy trajectories. Nutritional strategies have been developed to preserve health, and the impact of these strategies on circulating EVs is arousing growing interest. Data available from published studies are now sufficient for a first integration to better understand the role of EVs in the relationship between diet and health. Thus, this review focuses on human intervention studies investigating the impact of diet or its components on circulating EVs. Because of analytical bias, only large EVs have been assessed so far. The analysis highlights that poor-quality diets with elevated fat and sugar content increase levels of circulating large EVs, and these can be partly counteracted by healthy food or some food micronutrients and bioactive compounds. However, knowledge of the content and the biological functions of these diet-induced EVs is still missing. It is important to address these aspects in new research in order to state if EVs are mediators of the effects of diet on health.

Extracellular Vesicles-The Next Frontier in Endocrinology

Extracellular vesicles (EVs), including exosomes, are emerging as important carriers of signals in normal and pathological physiology. As EVs are a long-range communication or signaling modality-just like hormones are-the field of endocrinology is uniquely poised to offer insight into their functional biology and regulation. EVs are membrane-bound particles secreted by many different cell types and can have local or systemic effects, being transported in body fluids. They express transmembrane proteins, some of which are shared between EVs and some being specific to the tissue of origin, that can interact with target cells directly (much like hormones can). They also contain cargo within them that includes DNA, RNA, miRNA, and various metabolites. They can fuse with target cells to empty their cargo and alter their target cell physiology in this way also. Similar to the endocrine system, the EV system is likely to be under homeostatic control, making the regulation of their biogenesis and secretion important aspects to study. In this review, we briefly highlight select examples of how EVs are implicated in normal physiology and disease states. We also discuss what is known about their biogenesis and regulation of secretion. We hope that this paper inspires the endocrinology field to use our collective expertise to explore these new multimodal "hormones."

Extracellular Vesicles in Essential Hypertension: Hidden Messengers

PURPOSE OF REVIEW

Hypertension affects about half of all Americans, yet in the vast majority of cases, the factors causing the hypertension cannot be clearly delineated. Developing a more precise understanding of the molecular pathogenesis of HTN and its various phenotypes is therefore a pressing priority. Circulating and urinary extracellular vesicles (EVs) are potential novel candidates as biomarkers and bioactivators in HTN. EVs are a heterogeneous population of small membrane fragments shed from various cell types into various body fluids. As EVs carry protein, RNA, and lipids, they also play a role as effectors and novel cell-to-cell communicators. In this review, we discuss the diagnostic, functional, and regenerative role of EVs in essential HTN and focus on EV protein and RNA cargo as the most extensively studied EV cargo.

RECENT FINDINGS

The field of EVs in HTN is still a young one and earlier studies have not used the novel EV detection tools currently available. More rigor and transparency in EV research are needed. Current data suggest that EVs represent potential novel biomarkers in HTN. EVs correlate with HTN severity and possibly end-organ damage. However, it has yet to be discerned which specific subtype(s) of EV reflects best HTN pathophysiology. Evolving studies are also showing that EVs might be novel regulators in vascular and renal tubular function and also be therapeutic. RNA in EVs has been studied in the context of hypertension, largely in the form of studies of miRNA, which are reviewed herein. Beyond miRNAs, mRNA in urinary EVs changed in response to sodium loading in humans. EVs represent promising novel biomarkers and bioactivators in essential HTN. Novel tools are being developed to apply more rigor in EV research including more in vivo models and translation to humans.

Microvesicles and exosomes in metabolic diseases and inflammation

Metabolic diseases are based on a dysregulated crosstalk between various cells such as adipocytes, hepatocytes and immune cells. Generally, hormones and metabolites mediate this crosstalk that becomes alterated in metabolic syndrome including obesity and diabetes. Recently, Extracellular Vesicles (EVs) are emerging as a novel way of cell-to-cell communication and represent an attractive strategy to transfer fundamental informations between the cells through the transport of proteins and nucleic acids. EVs, released in the extracellular space, circulate via the various body fluids and modulate the cellular responses following their interaction with the near and far target cells. Clinical and experimental data support their role as biomarkers and bioeffectors in several diseases includimg also the metabolic syndrome. Despite numerous studies on the role of macrophages in the development of metabolic diseases, to date, there are little informations about the influence of metabolic stress on the EVs produced by macrophages and about the role of the released vesicles in the organism. Here, we review current understanding about the role of EVs in metabolic diseases, mainly in inflammation status burst. This knowledge may play a relevant role in health monitoring, medical diagnosis and personalized medicine.

Polyphenols: Modulators of Platelet Function and Platelet Microparticle Generation?

Platelets and platelet microparticles (PMPs) play a key role in the pathophysiology of vascular disorders such as coronary artery disease and stroke. In atherosclerosis, for example, the disruption of the plaque exposes endogenous agonists such as collagen, which activates platelets. Platelet hyper-activation and the high levels of PMPs generated in such situations pose a thrombotic risk that can lead to strokes or myocardial infarctions. Interestingly, dietary polyphenols are gaining much attention due to their potential to mimic the antiplatelet activity of treatment drugs such as aspirin and clopidogrel that target the glycoprotein VI (GPVI)-collagen and cyclooxygenease-1 (COX-1)-thromboxane platelet activation pathways respectively. Platelet function tests such as aggregometry and flow cytometry used to monitor the efficacy of antiplatelet drugs can also be used to assess the antiplatelet potential of dietary polyphenols. Despite the low bioavailability of polyphenols, several in vitro and dietary intervention studies have reported antiplatelet effects of polyphenols. This review presents a summary of platelet function in terms of aggregation, secretion, activation marker expression, and PMP release. Furthermore, the review will critically evaluate studies demonstrating the impact of polyphenols on aggregation and PMP release.

Endothelial Dysfunction in Primary Aldosteronism

Primary aldosteronism (PA) is characterized by excess production of aldosterone from the adrenal glands and is the most common and treatable cause of secondary hypertension. Aldosterone is a mineralocorticoid hormone that participates in the regulation of electrolyte balance, blood pressure, and tissue remodeling. The excess of aldosterone caused by PA results in an increase in cardiovascular and cerebrovascular complications, including coronary artery disease, myocardial infarction, stroke, transient ischemic attack, and even arrhythmia and heart failure. Endothelial dysfunction is a well-established fundamental cause of cardiovascular diseases and also a predictor of worse clinical outcomes. Accumulating evidence indicates that aldosterone plays an important role in the initiation and progression of endothelial dysfunction. Several mechanisms have been shown to contribute to aldosterone-induced endothelial dysfunction, including aldosterone-mediated vascular tone dysfunction, aldosterone- and endothelium-mediated vascular inflammation, aldosterone-related atherosclerosis, and vascular remodeling. These mechanisms are activated by aldosterone through genomic and nongenomic pathways in mineralocorticoid receptor-dependent and independent manners. In addition, other cells have also been shown to participate in these mechanisms. The complex interactions among endothelium, inflammatory cells, vascular smooth muscle cells and fibroblasts are crucial for aldosterone-mediated endothelial dysregulation. In this review, we discuss the association between aldosterone and endothelial function and the complex mechanisms from a molecular aspect. Furthermore, we also review current clinical research of endothelial dysfunction in patients with PA.

Larger ascending aorta in primary aldosteronism: a 3-year prospective evaluation of adrenalectomy vs. medical treatment

OBJECTIVE

Primary aldosteronism is associated with higher cardiovascular morbidity as compared with essential hypertension. Vascular complications encompass myocardial infarction and cerebrovascular events. Aortic damage in primary aldosteronism has never been explored, although a few cases of ascending aorta aneurisms have been reported.

DESIGN AND METHODS

We consecutively enrolled patients affected by primary aldosteronism (n = 45) and compared them with patients affected by essential hypertension (n = 47), on an outpatient setting. Echocardiographic data of patients with primary aldosteronism were collected during a mean follow-up of 3 years, in subjects who underwent adrenal surgery (n = 12) and those on mineralocorticoid receptor antagonists (n = 33).

RESULTS AND CONCLUSION

We found that patients with primary aldosteronism had larger ascending aorta diameters than those with essential hypertension before starting any specific treatment. Patients with primary aldosteronism did not show significant changes in the size of ascending aorta during a mean of 3 years of follow-up, irrespective of the type of treatment (medical vs. surgical treatment). A longer follow-up will better clarify if worsening of the aortic damage may be better prevented by surgery rather than by mineralocorticoid receptor antagonists.

Vésicules extracellulaires, biomarqueurs et bioeffecteurs du syndrome métabolique

Les vésicules extracellulaires (VE) suscitent un intérêt croissant lié à leur capacité à transférer du contenu biologique entre cellules. Les VE, émises dans l’espace extracellulaire, circulent via les différents fluides de l’organisme et modulent localement ou à distance les réponses des cellules avec lesquelles elles ont interagi. Des données cliniques et expérimentales étayent leur rôle dans les maladies liées au syndrome métabolique. Les VE bousculent la vision traditionnelle de la communication intercellulaire et représentent ainsi un mode de communication alternatif et versatile, qui ouvre la porte à de nouveaux concepts et opportunités tant biologiques que thérapeutiques.

Extracellular vesicles and microvascular pathology: Decoding the active dialogue

Extracellular vesicles (EV) are a heterogeneous collection of membrane-surrounded structures released from all studied cells, under both physiological and pathological conditions. These nano-size vesicles carry complex cargoes including different classes of proteins, lipids and nucleic acids and are known to act as a communication and signalling vesicles in various cellular process. In addition to their role in development and progression of pathological disorders which make them potentially great biomarkers, EV have beneficial effects, as they take part in homeostasis. In this review we have analysed the evidence for the role of microvesicles and exosomes secreted from other cells on microvascular endothelium (EV uptake) as well as the role of endothelial-derived vesicles on their neighbouring and distant cells (EV release).

The Impact of Vascular Disease Treatment on Platelet-Derived Microvesicles

Platelet-derived microvesicles (pMVs) are small, heterogeneous vesicles released from platelet membranes as a result of activation. These microvesicles possess a wide range of properties, including prothrombotic, proatherogenic, proinflammatory, immunomodulatory, and even anticoagulant activity. The elevated release of these microvesicles has been observed in various metabolic, inflammatory, thrombotic, and vascular diseases, including ischemic heart disease, stroke, hypertension, diabetes, and connective tissue disease. Modulation of both pMV generation and the expression of their surface molecules may have beneficial clinical implications and could become a novel therapeutic target. However, mechanisms by which pharmacological agents can modify pMV formation are elusive. The purpose of this review is to discuss the effects of drugs routinely used in primary and secondary prevention of vascular disease on the release of pMV and expression of their surface procoagulant and proinflammatory molecules.

Potential mechanisms underlying cardiovascular protection by polyphenols: Role of the endothelium

Epidemiological studies have indicated that regular intake of polyphenol-rich diets such as red wine and tea, are associated with a reduced risk of cardiovascular diseases. The beneficial effect of polyphenol-rich products has been attributable, at least in part, to their direct action on the endothelial function. Indeed, polyphenols from tea, grapes, cacao, berries, and plants have been shown to activate endothelial cells to increase the formation of potent vasoprotective factors including nitric oxide (NO) and to delay endothelial ageing. Moreover, intake of such polyphenol-rich products has been associated with the prevention and/or the improvement of an established endothelial dysfunction in several experimental models of cardiovascular diseases and in Humans with cardiovascular diseases. This review will discuss both experimental and clinical evidences indicating that polyphenols are able to promote endothelial and vascular health, as well as the underlying mechanisms.

Extracellular Vesicles in Metabolic Syndrome

Metabolic syndrome defines a cluster of interrelated risk factors for cardiovascular disease and diabetes mellitus. These factors include metabolic abnormalities, such as hyperglycemia, elevated triglyceride levels, low high-density lipoprotein cholesterol levels, high blood pressure, and obesity, mainly central adiposity. In this context, extracellular vesicles (EVs) may represent novel effectors that might help to elucidate disease-specific pathways in metabolic disease. Indeed, EVs (a terminology that encompasses microparticles, exosomes, and apoptotic bodies) are emerging as a novel mean of cell-to-cell communication in physiology and pathology because they represent a new way to convey fundamental information between cells. These microstructures contain proteins, lipids, and genetic information able to modify the phenotype and function of the target cells. EVs carry specific markers of the cell of origin that make possible monitoring their fluctuations in the circulation as potential biomarkers inasmuch their circulating levels are increased in metabolic syndrome patients. Because of the mixed components of EVs, the content or the number of EVs derived from distinct cells of origin, the mode of cell stimulation, and the ensuing mechanisms for their production, it is difficult to attribute specific functions as drivers or biomarkers of diseases. This review reports recent data of EVs from different origins, including endothelial, smooth muscle cells, macrophages, hepatocytes, adipocytes, skeletal muscle, and finally, those from microbiota as bioeffectors of message, leading to metabolic syndrome. Depicting the complexity of the mechanisms involved in their functions reinforce the hypothesis that EVs are valid biomarkers, and they represent targets that can be harnessed for innovative therapeutic approaches.

Potential of Food and Natural Products to Promote Endothelial and Vascular Health

Endothelial dysfunction is now well established as a pivotal early event in the development of major cardiovascular diseases including hypertension, atherosclerosis, and diabetes. The alteration of the endothelial function is often triggered by an imbalance between the endothelial formation of vasoprotective factors including nitric oxide (NO) and endothelium-dependent hyperpolarization, and an increased level of oxidative stress involving several prooxidant enzymes such as NADPH oxidase and, often also, the appearance of cyclooxygenase-derived vasoconstrictors. Preclinical studies have indicated that polyphenol-rich food and food-derived products such as grape-derived products, black and red berries, green and black teas and cocoa, and omega-3 fatty acids can trigger activating pathways in endothelial cells promoting an increased formation of nitric oxide and endothelium-dependent hyperpolarization. Moreover, intake of such food-derived products has been associated with the prevention and/or the improvement of an established endothelial dysfunction in several experimental models of cardiovascular diseases and in humans with cardiovascular diseases. This review will discuss both experimental and clinical evidences indicating that different types of food and natural products are able to promote endothelial and vascular health, as well as the underlying mechanisms.

Increased monocyte/neutrophil and pro-coagulant microparticle levels and overexpression of aortic endothelial caveolin-1β in dyslipidemic sand rat, Psammomys obesus

AIMS

To compare the effects of a high-energy diet (HED) with those of a low-energy diet (LED) on biochemical parameters, microparticle (MP) subpopulations and endothelial caveolin-1 (cav-1) protein expression in Psammomys obesus (P. obesus).

METHODS

After 12weeks of feeding with either the HED or LED, fasting plasma glucose and lipid parameters were measured using an enzymatic colorimetric kit while serum insulin concentration was determined with radioimmunoassay kits. MP subpopulations and cav-1 protein expression were quantified using flow cytometry and western blot analysis, respectively.

RESULTS

We observed that the HED caused a marked increase in lipid parameters, even in normoglycemic P. obesus. The total number of circulating MPs and the numbers of platelet-, leukocyte-, and erythrocyte-derived MPs were unaltered in the HED group. However, the HED induced increases in the numbers of monocytes/neutrophils and procoagulant MPs and a decrease in the endothelial MP levels. Cav-1β protein expression and reactive oxygen species production were increased in the vascular endothelium of HED-treated P. obesus.

CONCLUSION

From these findings, it is indicated that the HED exerts deleterious effects on the vascular system by increasing the monocyte/neutrophil and procoagulant MP levels, which may lead to cav-1β protein overexpression in dyslipidemic P. obesus.

Extracellular vesicles: Pharmacological modulators of the peripheral and central signals governing obesity

Obesity and its metabolic resultant dysfunctions such as insulin resistance, hyperglycemia, dyslipidemia and hypertension, grouped as the "metabolic syndrome", are chronic inflammatory disorders that represent one of the most severe epidemic health problems. The imbalance between energy intake and expenditure, leading to an excess of body fat and an increase of cardiovascular and diabetes risks, is regulated by the interaction between central nervous system (CNS) and peripheral signals in order to regulate behavior and finally, the metabolism of peripheral organs. At present, pharmacological treatment of obesity comprises actions in both CNS and peripheral organs. In the last decades, the extracellular vesicles have emerged as participants in many pathophysiological regulation processes. Whether used as biomarkers, targets or even tools, extracellular vesicles provided some promising effects in the treatment of a large variety of diseases. Extracellular vesicles are released by cells from the plasma membrane (microvesicles) or from multivesicular bodies (exosomes) and contain lipids, proteins and nucleic acids, such as DNA, protein coding, and non-coding RNAs. Owing to their composition, extracellular vesicles can (i) activate receptors at the target cell and then, the subsequent intracellular pathway associated to the specific receptor; (ii) transfer molecules to the target cells and thereby change their phenotype and (iii) be used as shuttle of drugs and, thus, to carry specific molecules towards specific cells. Herein, we review the impact of extracellular vesicles in modulating the central and peripheral signals governing obesity.

Extracellular vesicles as new pharmacological targets to treat atherosclerosis

Extracellular vesicles released by most cell types, include apoptotic bodies (ABs), microvesicles (MVs) and exosomes. They play a crucial role in physiology and pathology, contributing to "cell-to-cell" communication by modifying the phenotype and the function of target cells. Thus, extracellular vesicles participate in the key processes of atherosclerosis from endothelial dysfunction, vascular wall inflammation to vascular remodeling. The purpose of this review is to summarize recent findings on extracellular vesicle formation, structure, release and clearance. We focus on the deleterious and beneficial effects of extracellular vesicles in the development of atherosclerosis. The potential role of extracellular vesicles as biomarkers and pharmacological targets, their innate therapeutic capacity, or their use for novel drug delivery devices in atherosclerotic cardiovascular diseases will also be discussed.

Cystic fibrosis transmembrane conductance regulator is involved in polyphenol-induced swelling of the endothelial glycocalyx

Previous studies show that polyphenol-rich compounds can induce a swelling of the endothelial glycocalyx (eGC). Our goal was to reveal the mechanism behind the eGC-swelling. As polyphenols are potent modulators of fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel, the hypothesis was tested whether polyphenol-induced increase in CFTR activity is responsible for the eGC-swelling. The impact of the polyphenols resveratrol, (-)-epicatechin, and quercetin on nanomechanics of living endothelial GM7373 cells was monitored by AFM-nanoindentation. The tested polyphenols lead to eGC-swelling with a simultaneous decrease in cortical stiffness. EGC-swelling, but not the change in cortical stiffness, was prevented by the inhibition of CFTR. Polyphenol-induced eGC-swelling could be mimicked by cytochalasin D, an actin-depolymerizing agent. Thus, in the vascular endothelium, polyphenols induce eGC-swelling by softening cortical actin and activating CFTR. Our findings imply that CFTR plays an important role in the maintenance of vascular homeostasis and may explain the vasoprotective properties of polyphenols.

FROM THE CLINICAL EDITOR

Many vascular problems clinically can be attributed to a dysregulation of endothelial glycocalyx (eGC). The underlying mechanism however remains unclear. In this article, the authors used nanoindentation and showed that polyphenols could swell the endothelial glycocalyx and alter its function. This investigative method can lead to further mechanistic studies of other molecular pathways.

Modulation of nitric oxide by flavonoids

One of the main mechanisms by which dietary flavonoids are thought to influence cardiovascular disease is via protection of the bioactivity of the endothelium-derived nitric oxide (NO). Additionally, flavonoids may also interfere with the signalling cascades of inflammation and prevent overproduction of NO and its deleterious consequences in shock and ischemia-reperfusion injury. In the present paper we review the evidence of the effects of flavonoids on NO. Flavonoids exert complex actions on the synthesis and bioavailability of NO which may result both in enhanced or decreased NO levels: (1) in cell free systems, several flavonoids may scavenge NO via its pro-oxidant properties by increasing superoxide. However, under conditions of oxidative stress, flavonoids may also protect NO from superoxide-driven inactivation. (2) In intact healthy tissues, some flavonoids increase eNOS activity in endothelial cells. Paradoxically this effect involves a pro-oxidant effect which results in Ca(2+)-dependent activation of eNOS. As inhibitors of PI3K, flavonoids may potentially inhibit the PI3K/Akt-dependent activation of eNOS. (3) Under conditions of inflammation and oxidative stress, flavonoids may prevent the inflammatory signalling cascades via inhibition of NFκB and thereby downregulate iNOS. On the other hand, they also prevent the overexpression of ROS generating enzymes, reducing superoxide and peroxynitrite levels, and hence preventing superoxide-induced NO inactivation and eNOS uncoupling. Therefore, the final effect of flavonoids on NO levels will depend on the flavonoid structure and the concentrations used, on the cell type under study and particularly on the presence of inflammatory/oxidative conditions.