Oxidants downstream from superoxide inhibit nitric oxide production by vascular endothelium--a key role for selenium-dependent enzymes in vascular health

Impairment in selenocysteine synthesis as a candidate mechanism of inducible coagulopathy in COVID-19 patients

Coagulopathy has recently been recognized as a recurring complication of COVID-19, most typically associated with critical illness. There are epidemiological, mechanistic and transcriptomic evidence that link Selenium with SARS-CoV-2’s intracellular latency. Taking into consideration the vital role of selenoproteins in maintaining an adequate immune response, endothelial homeostasis and a non-prothrombotic platelet activation status, we propose that impairment in selenocysteine synthesis, via perturbations in the aforementioned physiological functions, potentially constitutes a mechanism of coagulopathy in COVID 19 patients other than those developed in critical illness.

Adverse systemic arterial function in patients with selenium deficiency

BACKGROUND

Experimental studies have shown that selenium is involved in the synthesis of selenoproteins which might contribute to cardiovascular protection. However, the relationship between selenium deficiency and vascular function in clinical context remains unknown.

OBJECTIVE

To investigate for any relationship between selenium deficiency and systemic arterial function in patients with high risk of vascular events.

DESIGN

Cross-sectional study.

SETTING AND PARTICIPANTS

306 consecutive patients with high risk for cardiovascular events (coronary artery disease 35%, acute/ recurrent ischemic stroke 40%, diabetes mellitus 54%) followed up at internal medicine outpatient clinics.

MEASUREMENTS

Non-invasive brachial-ankle pulse wave velocity (PWV) was determined using vascular profiling system (VP-2000). Long-term intake of selenium was determined by a validated food frequency questionnaire.

RESULTS

Mean daily selenium intake was 59.5 ± 52.1 mcg/day, and mean PWV was 1782.4 ± 418.4 cm/s. Patients with selenium intake <10th percentile had significantly higher PWV as compared to patients with intake ≥ 10th percentile (1968.2 ± 648.9 cm/s versus 1762.2 ± 381.6 cm/s, P=0.010). After adjusting for potential confounders including age, gender, history of hypertension, hyperlipidemia, diabetes and cardiovascular disease, smoking status, use of cardiovascular medications, waist-hip ratio, education/ financial status, physical activity, calorie intake and intake of antioxidant vitamins, deficient selenium intake <10th percentile remained independently predictive of increased PWV by +363.4 cm/s [95% CI: 68.1 to 658.6, P=0.016, relative increase 21%].

CONCLUSIONS

Selenium deficiency is associated with adverse arterial function in patients with high risk for vascular events.

Review

Production of superoxide anion O2*- by the membrane-bound enzyme NADPH oxidase of phagocytes is a long-known phenomenon; it is generally assumed that O2*-helps phagocytes kill bacterial intruders. The details and the chemistry of the killing process have, however, remained a mystery. Isoforms of NADPH oxidase exist in membranes of nearly every cell, suggesting that reactive oxygen species (ROS) participate in intra- and intercellular signaling processes. What the nature of the signal is exactly, how it is transmitted, and what structural characteristics a receptor of a "radical message" must have, have not been addressed convincingly. This review discusses how the action of messengers is in agreement with radical-specific behavior. In search for the smallest common denominator of cellular free radical activity we hypothesize that O2*- and its conjugate acid, HO2*, may have evolved under primordial conditions as regulators of membrane mechanics and that isoprostanes, widely used markers of "oxidative stress", may be an adventitious correlate of this biologic activity of O2*-/HO2*. An overall picture is presented that suggests that O2*-/HO2* radicals, by modifying cell membranes, help other agents gain access to the hydrophobic region of phospholipid bilayers and hence contribute to lipid-dependent signaling cascades. With this, O2*-/HO2* are proposed as indispensable adjuvants for the generation of cellular signals, for membrane transport, channel gating and hence, in a global sense, for cell viability and growth. We also suggest that many of the allegedly O2*- dependent bacterial pathologies and carcinogenic derailments are due to membrane-modifying activity rather than other chemical reactions of O2*-/HO2*. A consequence of this picture is the potential evolution of the "radical theory of ageing" to a "lipid theory of aging".

[An endogenous dithiol with antioxidant properties: alpha-lipoic acid, potential uses in cardiovascular diseases]

Alpha-Lipoic acid (ALA) is a natural compound, chemically named 1,2-dithiolane-3-pentanoic acid, also referred to as thioctic acid. In humans, ALA is synthetized by the liver and other tissues with high metabolic activity: heart, kidney. ALA is both water and fat soluble and therefore, is widely distributed in both cellular membranes and cytosol. Recently, a greater deal of attention has been given to antioxidant function for ALA and its reduced formed: dihydrolipoic acid (DHLA). ALA scavenges hydroxyl radicals, hypochlorous acid and singlet oxygen. It may also exert antioxidant effects in biological systems through transitional metal chelation. Dihydrolipoic acid has been shown to have antioxidant but also pro-oxidant properties in systems in which hydroxyl radical was generated. ALA/DHLA ratio has the capacity to recycle endogenous antioxidants such as vitamin E. A number of experimental as well as clinical studies point to the usefulness of ALA as a therapeutic agent for such diverse conditions as diabetes, atherosclerosis, insulin resistance, neuropathy, neurodegenerative diseases and ischemia-reperfusion injury. ALA represents a potential agent on the vascular endothelium, recording to ALA/DHLA redox couple is one of the most powerful biological antioxidant systems.

In atherogenesis, the apoptosis of endothelial cell itself could directly induce over-proliferation of smooth muscle cells

Over-proliferation of SMC (smooth muscle cell) is one characteristics of atherosclerosis. One well accepted mechanism is that the decrease of ECs (endothelial cells) induced by over apoptosis leads to endothelial dysfunction, which in turn results in over-proliferation of SMC. Obviously, the mechanism works after endothelial apoptosis. Compared with necrosis, apoptosis is time and energy consuming. The question is why the cell ends in the form of apoptosis instead of necrosis. From the evolutionary standpoint, apoptosis has some useful functions other than removing the damaged or unwanted cells. Recent studies showed that cells nearby the apoptotic ones began to proliferate and differentiate before apoptosis and the apoptotic signals could induce the near cells to proliferate without the death of cells. Apparently, some mechanism in apoptosis results in the proliferation of cells. So, we hypotheses that endothelial apoptosis can directly induce the over-proliferation of SMC.

Blood substitutes and redox responses in the microcirculation

Transfusion of hemoglobin-based blood substitutes, designed for their plasma expansion and oxygen transport capabilities, has resulted in some major problems, such as organ dysfunction, during clinical trials. Experimental evidence demonstrates that these hemoglobins damage tissue by producing highly reactive oxygen species. Although cell-free hemoglobin may present a low risk to people with normal redox status, patients who are sick and have a poor antioxidant status may be at risk. Oxidative damage is particularly dangerous in the microcirculation because excess leakage of plasma components into the interstitium will disturb the fluid balance between blood and tissue and alter the kinetics of delivery of intravascularly injected drugs, and endogenous enzymes and hormones, to various tissues. In this review, the redox chemistry of hemoglobin-based blood substitutes is briefly described, and their effects on cultured endothelial cells, and on the exchange properties of the microvasculature, are discussed. Taking into account the possible mechanisms by which oxidative damage can occur, various methods to reduce the deleterious effects of blood substitutes in vivo are evaluated. Finally, several possible cell signaling pathways that are triggered in endothelial cells, in response to modified hemoglobins, are considered in terms of protecting microvascular function.

Acetylcholine-Induced Relaxation of Rabbit Basilar Artery In Vitro Is Rapidly Reduced by Reactive Oxygen Species in Acute Hyperglycemia

We examined the effects of acute hyperglycemia on the function of rabbit cerebral arteries in vitro. It was hypothesized that increased formation of reactive oxygen species (ROS) could occur, which could explain how hyperglycemia aggravates certain pathologic situations such as cerebral ischemia. Three-millimeter basilar artery segments were incubated in either normoglycemic (NG, 5.5 mM D-glucose) or hyperglycemic (HG, 25 mM D-glucose) solution containing 3.10(-6) M indomethacin. After 90 minutes equilibration, a test (=T1) of relaxation to acetylcholine (Ach) at three concentrations was performed on histamine-precontracted segments. Three further identical tests were performed (T2-T4), after 30-minute rest periods. Ach responses in NG solution were stable, whereas those in HG solution, although greater at T1, fell progressively from one test to the next (P < 0.0001 versus NG), whereas nitroprusside responses did not change. In separate experiments, this time-dependent fall in Ach responses was significantly prevented by superoxide dismutase (SOD) plus catalase (P = 0.0003), but not by SOD alone. It was also significantly prevented by the NAD(P)H oxidase inhibitors diphenyleneiodonium (P = 0.020) and apocynin (P = 0.0179), but not by allopurinol (xanthine oxidase inhibitor). Control experiments with l-glucose ruled out a hyperosmotic or non-specific glucose effect. We conclude that, in HG solution in vitro, rapidly increasing ROS production largely derived from NAD(P)H oxidase reduced relaxation to acetylcholine. The rapidity of this effect suggests that the function of these arteries may be affected during brief periods of hyperglycemia in vivo.

Sub-optimal taurine status may promote platelet hyperaggregability in vegetarians

Although vegan diets typically have a very favorable effect on a range of vascular risk factors, several independent groups have reported that the platelets of vegetarians are more sensitive to pro-aggregatory agonists than are those of omnivores. In light of clear and convincing evidence that platelet function has an important impact on risk for thromboembolic events, it is important to clarify the basis of platelet hyperaggregability in vegetarians. A dietary deficit of long-chain omega-3 fatty acids is not likely to explain this phenomenon, since most omnivore diets do not include enough of these fats to discernibly influence platelet function. A more plausible possibility is that relatively poor taurine status--a function of the facts that plants are devoid of taurine and the human capacity for taurine synthesis is limited - is responsible. Plasma taurine levels are lower, and urinary taurine excretion is substantially lower, in vegetarians than in omnivores. Platelets are rich in taurine, which functions physiologically to dampen the calcium influx evoked by aggregating agonists--thereby down-regulating platelet aggregation. Supplemental intakes of taurine as low as 400 mg daily have been reported to markedly decrease the sensitivity of platelets to aggregating agonists ex vivo. Although the average daily intake of taurine from omnivore diets may be only about 150 mg, it is credible to speculate that a supplemental intake of this magnitude could normalize the platelet function of vegetarians in the long term; in any case, this thesis is readily testable clinically. Taurine is just one of a number of nutrients found almost solely in animal products--"carninutrients"--which are rational candidates for supplementation in vegans.

Nitrosative Injury and Antioxidant Therapy in the Management of Diabetic Neuropathy

Strong evidence implicates oxidative stress as a mediator of diabetes-induced microvascular complications, including distal symmetric polyneuropathy. Dorsal root ganglia neurons are particularly susceptible to glucose-mediated oxidative stress and die by apoptotic mechanisms in animal and cell culture models of diabetes. Key mediators of glucose-induced oxidative injury are superoxide anions and nitric oxide (NO). Superoxides are believed to underlie many of the oxidative changes in hyperglycemic conditions, including increases in aldose reductase and protein kinase C activity. Superoxides can also react with NO, forming peroxynitrite (ONOO-), which rapidly causes protein nitration or nitrosylation, lipid peroxidation, deoxyribonucleic acid (DNA) damage, and cell death. ONOO- formation is dependent on both superoxide and NO concentrations; therefore, cells that constitutively express NO synthase, such as endothelial cells and neurons, may be more vulnerable to ONOO–induced cell death in conditions favoring the production of superoxides. Although NO and ONOO- can cause endothelial and neuronal cell death in vitro, in animal models of diabetes, reductions in endothelial NO production can inhibit vasodilatation and cause nerve ischemia. Therefore, ideal therapeutic approaches should limit the formation of superoxides and ONOO while preventing reductions in vascular NO. Despite strong evidence that oxidative stress is associated with complications of diabetes, including neuropathy, the results of clinical trials of antioxidants have shown some promise but not established therapeutic efficacy. Clinical studies of several antioxidants, including α-lipoic acid, vitamins C and E, aldose reductase inhibitors, and growth factors, in diabetic neuropathy are discussed.

Vascular endothelium is the organ chiefly responsible for the catabolism of plasma asymmetric dimethylarginine – an explanation for the elevation of plasma ADMA in disorders characterized by endothelial dysfunction

Plasma levels of asymmetric dimethylarginine (ADMA), an endogenously produced competitive inhibitor of nitric oxide synthase (NOS), have been found to be elevated in a large number of disorders characterized by endothelial dysfunction; this remarkable phenomenon has yet to receive a plausible explanation. ADMA arises by proteolysis of methylated proteins throughout the body; the majority of this ADMA is catabolized by the enzyme dimethylarginine dimethylaminohydrolase (DDAH), found in many tissues, including those that express NOS. Since the production of ADMA can be considered constitutive, and little intact ADMA emerges in the urine, impaired catabolism is most likely responsible for elevations of plasma ADMA. The association of elevated ADMA with endotheliopathy is readily explained if we assume that vascular endothelium is the organ chiefly responsible for the catabolism of plasma ADMA--a view that is credible owing to the privileged access of endothelium to plasma, the capacity of endothelium for active transport of arginine (and methylated arginines), and the ample DDAH activity of healthy endothelial cells--and further assume that endothelial dysfunction is often attended by a loss of DDAH activity and/or an impairment of arginine transport, reducing the efficiency of ADMA catabolism. Indeed, there is recent evidence that DDAH is inhibited by endothelial oxidative stress, a typical feature of endotheliopathy; there is also some reason to suspect that arginine transport may be less efficient in dysfunctional endothelium. From this perspective, increased plasma ADMA is not the primary cause of the endothelial dysfunction in various disorders, but rather its effect--though the rise in ADMA can then exacerbate this dysfunction by inhibiting endothelial NOS. Supplemental arginine should be of some clinical benefit in disorders characterized by elevated ADMA, since it can offset that adverse impact of ADMA on NOS activity, and possibly exert other beneficial effects on endothelium--but it cannot be expected to reverse the primary cause of the endothelial dysfunction. Whether or not ADMA plays an important pathogenic role, it seems likely to emerge as a potent risk factor for adverse vascular events, since it may be viewed as a barometer of endothelial health.

Policosanol safely down-regulates HMG-CoA reductase - potential as a component of the Esselstyn regimen

Many of the wide-ranging health benefits conferred by statin therapy are mediated, not by reductions in LDL cholesterol, but rather by inhibition of isoprenylation reactions essential to the activation of Rho family GTPases; this may be the mechanism primarily responsible for the favorable impact of statins on risk for ischemic stroke, senile dementia, and fractures, as well as the anti-hypertensive and platelet-stabilizing actions of these drugs. Indeed, the extent of these benefits is such as to suggest that most adults would be wise to take statins; however, owing to the significant expense of statin therapy, as well as to the potential for dangerous side effects that mandates regular physician follow-up, this strategy appears impractical. However, policosanol, a mixture of long-chain aliphatic alcohols extractable from sugar cane wax, has shown cholesterol-lowering potency comparable to that of statins, and yet appears to be devoid of toxic risk. Recent evidence indicates that policosanol down-regulates cellular expression of HMG-CoA reductase, and thus has the potential to suppress isoprenylation reactions much like statins do. Consistent with this possibility, the results of certain clinical and animal studies demonstrate that policosanol has many effects analogous to those of statins that are not likely explained by reductions of LDL cholesterol. However, unlike statins, policosanol does not directly inhibit HMG-CoA reductase, and even in high concentrations it fails to down-regulate this enzyme by more than 50% - thus likely accounting for the safety of this nutraceutical. In light of the fact that policosanol is quite inexpensive and is becoming available as a non-prescription dietary supplement, it may represent a practical resource that could enable the general public to enjoy health benefits comparable to those conferred by statins. In a long-term clinical study enrolling patients with significant symptomatic coronary disease, Esselstyn has demonstrated that a low-fat, whole-food vegan diet, coupled with sufficient statin therapy to maintain serum cholesterol below 150 mg/dL, can stop the progression of coronary disease and virtually eliminate further risk for heart attack. A comparable regimen, in which policosanol is used in place of statins, may represent a practical strategy whereby nearly everyone willing to commit to health-protective eating can either prevent coronary disease, or prevent pre-existing coronary disease from progressing to a life-threatening event.

Specificity of antioxidant enzyme inhibition in skeletal muscle to reactive nitrogen species donors

Nitric oxide (*NO) and its by-products modulate many physiological functions of skeletal muscle including blood flow, metabolism, glucose uptake, and contractile function. However, growing evidence suggests that an overproduction of nitric oxide contributes to muscle wasting in a number of pathologies including chronic heart failure, sepsis, COPD, muscular dystrophy, and extreme disuse. Limited data point to the potential of inhibition various enzymes by reactive nitrogen species (RNS), including (.)NO and its downstream products such as peroxynitrite, primarily in purified systems. We hypothesized that exposure of skeletal muscle to RNS donors would reduce or downregulate activities of the crucial antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX). Diaphragm muscle fiber bundles were extracted from 4-month-old Fischer-344 rats and, in a series of experiments, exposed to either (a) 0 (control), 1, or 5 mM diethylamine NONOate (DEANO: *NO donor); (b) 0, 100, 500 microM, or 1 mM sodium nitroprusside (SNP: *NO donor); (c) 0 or 2 mM S-nitroso-acetylpenicillamine (SNAP: *NO donor); or (d) 0 or 500 microM SIN-1 (peroxynitrite donor) for 60 min. DEANO resulted in a 50% reduction in CAT, GPX, and a dose-dependent inhibition of Cu, Zn-SOD. SNP resulted in significantly lower activities for total SOD, Mn-SOD isoform, Cu, Zn-SOD isoform, CAT, and GPX in a dose-dependent fashion. Two millimolar SNAP and 500 microM SIN-1 also resulted in a large and significant inhibition of total SOD and CAT. These data indicate that reactive nitrogen species impair antioxidant enzyme function in an RNS donor-specific and dose-dependent manner and are consistent with the hypothesis that excess RNS production contributes to skeletal muscle oxidative stress and muscle dysfunction.

Nutritional aspects of nitric oxide: human health implications and therapeutic opportunities

Nitric oxide (NO), the most potent natural vasorelaxant known, has close historical ties to cardiovascular physiology, despite NO's rich physiologic chemistry as an ubiquitous, signal-transducing radical. Aspects of NO biology critical to gastrointestinal health and, consequently, nutritional status are increasingly being recognized. Attempts are underway to exploit the gastrointestinal actions of NO for therapeutic gain. Cross-talk between NO and micronutrients within and outside the gastrointestinal system affects the establishment or progression of several diseases with pressing medical needs. These concepts imply that NO biology can influence nutrition and be nutritionally modulated to affect mammalian (patho)physiology. At least four nutritional facets of NO biology are at the forefront of contemporary biomedical research: 1) NO as modulator of feeding behavior and mediator of gastrointestinal homeostasis; 2) NO supplementation as a therapeutic modality for preserving gastrointestinal health; 3) interactions among elemental micronutrients (e.g., zinc), NO, and inflammation as potential contributors to diarrheal disease; and 4) vitamin micronutrients (e.g., vitamins E and C) as protectors of NO-dependent vascular function. Discussion of extant data on these topics prompts speculation that future research will broaden NO's nutritional role as an integrative signaling molecule supporting gastrointestinal and nutritional well-being.

Up-regulation of endothelial nitric oxide activity as a central strategy for prevention of ischemic stroke - just say NO to stroke!

Nitric oxide (NO) produced by the endothelium of cerebral arterioles is an important mediator of endothelium-dependent vasodilation (EDV), and also helps to prevent thrombosis and vascular remodeling. A number of risk factors for ischemic stroke are associated with impaired EDV, and this defect is usually at least partially attributable to a decrease in the production and/or stability of NO. These risk factors include hypertension, high-sodium diets, homocysteine, diabetes, visceral obesity, and aging. Conversely, many measures which may provide protection from ischemic stroke - such as ample dietary intakes of potassium, arginine, fish oil, and selenium - can have a favorable impact on EDV. Protection afforded by exercise training, estrogen replacement, statin drugs, green tea polyphenols, and cruciferous vegetables may reflect increased expression of the endothelial NO synthase. IGF-I activity stimulates endothelial NO production, and conceivably is a mediator of the protection associated with higher-protein diets in Japanese epidemiology and in hypertensive rats. These considerations prompt the conclusion that modulation of NO availability is a crucial determinant of risk for ischemic stroke. Multifactorial strategies for promoting effective cerebrovascular NO activity, complemented by measures that stabilize platelets and moderate blood viscosity, should minimize risk for ischemic stroke and help maintain vigorous cerebral perfusion into ripe old age. The possibility that such measures will also diminish risk for Alzheimer's disease, and slow the normal age-related decline in mental acuity, merits consideration. A limited amount of ecologic epidemiology suggests that both stroke and senile dementia may be extremely rare in cultures still consuming traditional unsalted whole-food diets. Other lines of evidence suggest that promotion of endothelial NO activity may decrease risk for age-related macular degeneration.