Protection of low density lipoprotein oxidation at chemical and cellular level by the antioxidant drug dipyridamole

Disrupted homeostasis in sickle cells: Expanding the comprehension of metabolism adaptation and related therapeutic strategies

Sickle cell disease (SCD) is a hereditary hemolytic anemia associated with the alteration of the membrane composition of the sickle erythrocytes, the loss of glycolysis, dysregulation of the pyruvate phosphatase pathway, and changes in nucleotide metabolism of the sickle red blood cell (RBC). This review provides a comprehensive overview of the impact of the presence of Hb S, which leads to the disruption of the normal RBC metabolism. The intricate interplay between the redox and energetic balance in erythrocytic cells, where the glycolysis, pentose phosphate pathway, and methemoglobin reductase pathways are all altered in sickle RBC, is a key focus. Moreover, this review summarizes the current knowledge about the disease-modifying agents and their action mechanisms based on the sickle RBC alterations previously mentioned (i.e., their association with beneficial effects on the sickle cells' membrane, to their RBCs' energy metabolism, and to their oxidative status). Therefore, providing a comprehensive understanding of how sickle cells cope with the disruption of metabolic homeostasis and the most promising therapeutic agents able to ameliorate the various consequences of abnormal sickle RBC alterations.

Potential Therapeutic Benefits of Dipyridamole in COVID-19 Patients

BACKGROUND

COVID-19 pandemic is caused by coronavirus also known as severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). The viral infection continues to impact the globe with no vaccine to prevent the infection or highly effective therapeutics to treat the millions of infected people around the world. The disease starts as a respiratory infection, yet it may also be associated with a hypercoagulable state, severe inflammation owing to excessive cytokines production, and a potentially significant oxidative stress. The disease may progress to multiorgan failure and eventually death.

OBJECTIVE

In this article, we summarize the potential of dipyridamole as an adjunct therapy for COVID-19.

METHODS

We reviewed the literature describing the biological activities of dipyridamole in various settings of testing. Data were retrieved from PubMed, SciFinder-CAS, and Web of Science. The review concisely covered relevant studies starting from 1977.

RESULTS

Dipyridamole is an approved antiplatelet drug, that has been used to prevent stroke, among other indications. Besides its antithrombotic activity, the literature indicates that dipyridamole also promotes a host of other biological activities including antiviral, anti-inflammatory, and antioxidant ones.

CONCLUSION

Dipyridamole may substantially help improve the clinical outcomes of COVID-19 treatment. The pharmacokinetics profile of the drug is well established which makes it easier to design an appropriate therapeutic course. The drug is also generally safe, affordable, and available worldwide. Initial clinical trials have shown a substantial promise for dipyridamole in treating critically ill COVID-19 patients, yet larger randomized and controlled trials are needed to confirm this promise.

A novel flow cytometry assay using dihydroethidium as redox-sensitive probe reveals NADPH oxidase-dependent generation of superoxide anion in human platelets exposed to amyloid peptide β

Reactive oxygen species (ROS) generation is critical in the regulation of platelets, which has important implications in the modulation of hemostasis and thrombosis. Nonetheless, despite several assays have been described and successfully utilized in the past, the analysis of ROS generation in human platelets remains challenging. Here we show that dihydroethidium (DHE) allows the characterization of redox responses upon platelet activation by physiological and pathological stimuli. In particular, the flow cytometry assay that we describe here allowed us to confirm that thrombin, collagen-related peptide (CRP) and arachidonic acid but not adenosine diphosphate (ADP) stimulate superoxide anion formation in a concentration-dependent manner. 0.1unit/ml thrombin, 3 μg/ml CRP and 30 μM arachidonic acid are commonly used to stimulate platelets in vitro and here were shown to stimulate a significant increase in superoxide anion formation. The ROS scavenger N-acetylcysteine (NAC) abolished superoxide anion generation in response to all tested stimuli, but the pan-NADPH oxidase (NOX) inhibitor VAS2870 only inhibited superoxide anion formation in response to thrombin and CRP. The involvement of NOXs in thrombin and CRP-dependent responses was confirmed by the inhibition of platelet aggregation induced by these stimuli by VAS2870, while platelet aggregation in response to arachidonic acid was insensitive to this inhibitor. In addition, the pathological platelet stimulus amyloid β (Aβ) 1-42 peptide induced superoxide anion formation in a concentration-dependent manner. Aβ peptide stimulated superoxide anion formation in a NOX-dependent manner, as proved by the use of VAS2870. Aβ 1-42 peptide displayed only moderate activity as an aggregation stimulus, but was able to significantly potentiate platelet aggregation in response to submaximal agonists concentrations, such as 0.03 unit/ml thrombin and 10 μM arachidonic acid. The inhibition of NOXs by 10 μM VAS2870 abolished Aβ-dependent potentiation of platelet aggregation in response to 10 μM arachidonic acid, suggesting that the pro-thrombotic activity of Aβ peptides depends on NOX activity. Similar experiments could not be performed with thrombin or collagen, as NOXs are required for the signaling induced by these stimuli. These findings shed some new light on the pro-thrombotic activity of Aβ peptides. In summary, here we describe a novel and reliable assay for the detection of superoxide anion in human platelets. This is particularly important for the investigation of the pathophysiological role of redox stress in platelets, a field of research of increasing importance, but hindered by the absence of a reliable and easily accessible ROS detection methodology applicable to platelets.

Genetic Understanding of Stroke Treatment: Potential Role for Phosphodiesterase Inhibitors

Phosphodiesterase (PDE) gene family is a large family having at least 21 genes and multiple versions (isoforms) of the phosphodiesterase enzymes. These enzymes catalyze the inactivation of intracellular mediators of signal transduction such as cAMP and cGMP and therefore, play a pivotal role in various cellular functions. PDE inhibitors (PDEI) are drugs that block one or more of the five subtypes of the PDE family and thereby prevent inactivation of the intracellular cAMP and cGMP by the respective PDE-subtypes. The first clinical use of PDEI was reported almost three decades ago. Studies later found the ability of these compounds to increase the levels of ubiquitous secondary messenger molecules that can cause changes in vascular tone, cardiac function and other cellular events and thus these findings paved the way for their use in various medical emergencies. PDEs are found to be distributed in many tissues including brain. Therefore, new therapeutic agents in the form of PDEI are being explored in neurodegenerative diseases including stroke. Although studies have revealed their use in cerebral infarction prevention, their full-fledged application in times of neurological emergency or stroke in specific has been very limited so far. Nevertheless, recent investigations suggest PDE4 and PDE5 inhibitors to play a vital role in mitigating stroke symptoms by modulating signaling mechanisms in PDE pathway. Further, extensive research in terms of their pharmacological properties like dosing, drug specific activities, use of simultaneous medications, ancillary properties of these compounds and studies on adverse drug reactions needs to be carried out to set them as standard drugs of use in stroke.

Molecular hydrogen stabilizes atherosclerotic plaque in low-density lipoprotein receptor-knockout mice

Hydrogen (H(2)) attenuates the development of atherosclerosis in mouse models. We aimed to examine the effects of H(2) on atherosclerotic plaque stability. Low-density lipoprotein receptor-knockout (LDLR(-/-)) mice fed an atherogenic diet were dosed daily with H(2) and/or simvastatin. In vitro studies were carried out in an oxidized-LDL (ox-LDL)-stimulated macrophage-derived foam cell model treated with or without H(2). H(2) or simvastatin significantly enhanced plaque stability by increasing levels of collagen, as well as reducing macrophage and lipid levels in plaques. The decreased numbers of dendritic cells and increased numbers of regulatory T cells in plaques further supported the stabilizing effect of H(2) or simvastatin. Moreover, H(2) treatment decreased serum ox-LDL level and apoptosis in plaques with concomitant inhibition of endoplasmic reticulum stress (ERS) and reduction of reactive oxygen species (ROS) accumulation in the aorta. In vitro, like the ERS inhibitor 4-phenylbutyric acid, H(2) inhibited ox-LDL- or tunicamycin (an ERS inducer)-induced ERS response and cell apoptosis. In addition, like the ROS scavenger N-acetylcysteine, H(2) inhibited ox-LDL- or Cu(2+) (an ROS inducer)-induced reduction in cell viability and increase in cellular ROS. Also, H(2) increased Nrf2 (NF-E2-related factor-2, an important factor in antioxidant signaling) activation and Nrf2 small interfering RNA abolished the protective effect of H(2) on ox-LDL-induced cellular ROS production. The inhibitory effects of H(2) on the apoptosis of macrophage-derived foam cells, which take effect by suppressing the activation of the ERS pathway and by activating the Nrf2 antioxidant pathway, might lead to an improvement in atherosclerotic plaque stability.

Proton-coupled electron-transfer mechanism for the radical scavenging activity of cardiovascular drug dipyridamole

Dipyridamole (DIP) is a well-known pharmaceutical drug used as a coronary vasodilator and anti-platelet agent in clinics for treating several cardiovascular diseases. Primarily, the therapeutic effects of the drug are attributed to its antioxidant potency. In this research, we aim to declare the unknown antioxidant mechanism of DIP as well as its potent chain-breaking antioxidant activity in polar aqueous medium inside the cells, using different experimental methods and theoretical quantum calculations. Data demonstrated the higher antioxidant capacity of DIP against ROS and free radicals in polar cell's interior. DIP is capable of generating long living and noninvasive DIP^• radicals in oxidant condition that leads to an effective “chain-breaking antioxidant” activity. Quantum computational data indicated that DIP antioxidant has more favorable ionization potential than trolox which means DIP has higher antioxidant activity. Also, data showed that the direct hydrogen-transfer is not a favorable process to construct DIP^• because of high barrier energy, though electron-transfer process can more easily to produce DIP^•+ with the lowest barrier energy. Altogether, the electron donating potency of DIP to reduce ferric ion, having the low anodic oxidation peak potential, producing long lived stable DIP^• radicals and protecting myoblast cells from oxidation, proposed the excellent “chain-breaking antioxidant” potency via electron-transfer mechanism of this vasodilator DIP drug in polar aqueous medium.

Endothelial dysfunction and diabetes: effects on angiogenesis, vascular remodeling, and wound healing

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by inappropriate hyperglycemia due to lack of or resistance to insulin. Patients with DM are frequently afflicted with ischemic vascular disease or wound healing defect. It is well known that type 2 DM causes amplification of the atherosclerotic process, endothelial cell dysfunction, glycosylation of extracellular matrix proteins, and vascular denervation. These complications ultimately lead to impairment of neovascularization and diabetic wound healing. Therapeutic angiogenesis remains an attractive treatment modality for chronic ischemic disorders including PAD and/or diabetic wound healing. Many experimental studies have identified better approaches for diabetic cardiovascular complications, however, successful clinical translation has been limited possibly due to the narrow therapeutic targets of these agents or the lack of rigorous evaluation of pathology and therapeutic mechanisms in experimental models of disease. This paper discusses the current body of evidence identifying endothelial dysfunction and impaired angiogenesis during diabetes.

Anti-platelet therapy: phosphodiesterase inhibitors

Inhibition of platelet aggregation can be achieved either by the blockade of membrane receptors or by interaction with intracellular signalling pathways. Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are two critical intracellular second messengers provided with strong inhibitory activity on fundamental platelet functions. Phosphodiesterases (PDEs), by catalysing the hydrolysis of cAMP and cGMP, limit the intracellular levels of cyclic nucleotides, thus regulating platelet function. The inhibition of PDEs may therefore exert a strong platelet inhibitory effect. Platelets possess three PDE isoforms (PDE2, PDE3 and PDE5), with different selectivity for cAMP and cGMP. Several nonselective or isoenzyme-selective PDE inhibitors have been developed, and some of them have entered clinical use as antiplatelet agents. This review focuses on the effect of PDE2, PDE3 and PDE5 inhibitors on platelet function and on the evidence for an antithrombotic action of some of them, and in particular of dipyridamole and cilostazol.

Multifunctional actions of approved and candidate stroke drugs

Ischemic stroke causes brain damage by multiple pathways. Previous stroke trials have demonstrated that drugs targeting one or only a few of these pathways fail to improve clinical outcome after stroke. Drugs with multimodal actions have been suggested to overcome this challenge. In this review, we describe the mechanisms of action of agents approved for secondary prevention of ischemic stroke, such as antiplatelet, antihypertensive, and lipid-lowering drugs. These drugs exhibit considerable properties beyond their classical mechanisms, including neuroprotective and neuroregenerative properties. In addition, candidate stroke drugs currently studied in clinical phase III trials are described. Among these, albumin, hematopoietic growth factors, and citicoline have been identified as promising agents with multiple mechanisms. These drugs offer hope that additional treatment options for the acute phase after a stroke will become available in the near future.

Translational therapeutics of dipyridamole

Dipyridamole (DP) is a phosphodiesterase inhibitor that increases the intracellular levels of cyclic adenosine monophosphate (cAMP) and cyclic guanine monophosphate (cGMP) by preventing their conversion to AMP and GMP, respectively. By increasing cAMP and cGMP levels in platelets, DP reversibly inhibits platelet aggregation and platelet-mediated thrombotic disease. In addition, DP may potentiate some of the vascular protective effects of endothelium-derived nitric oxide (NO), which increases cGMP by stimulating soluble guanylyl cyclase. Endothelium-derived NO is an important regulator of vascular tone, blood flow, and tissue perfusion. Indeed, endothelial NO synthase-deficient (eNOS-/-) mice exhibit elevated systemic blood pressure and have larger myocardial and cerebral infarct size after ischemic injury. Other NO/cGMP-dependent effects that may be potentiated by DP include inhibition of vascular smooth muscle proliferation and prevention of endothelial-leukocyte interaction. In addition, DP increases local concentrations of adenosine and prostacyclin, which could affect vascular tone and inflammation. Finally, DP has antioxidant properties, which could stabilize platelet and vascular membranes as well as prevent the oxidation of low-density lipoprotein. These platelet and nonplatelet actions of DP may contribute to some of its therapeutic benefits in vascular disease.

Progress in clinical neurosciences: The 'antiplatelet' agents and the role of the endothelium

The antiplatelet drugs, commonly used in the prevention and treatment of cerebrovascular disease, possess a number of effects that are independent of direct antiplatelet actions. Beneficial and detrimental effects both occur. The endothelium is an important mediator of these non-antiplatelet effects. We performed a literature search to locate articles related to acetylsalicylic acid (aspirin), clopidogrel, ticlopidine, and dipyridamole and the interactions of these medications with the endothelium. The role of each of the above medications is explored in relation to vasodilation, inflammation, oxidation, platelet-leukocyte interactions, and thrombogenic tendency via platelet-vessel wall interactions.

The effect of dipyridamole on vascular cell-derived reactive oxygen species

Platelet and vascular stimulation leads to release of reactive oxygen species (ROS) that are known to influence vascular reactivity and thrombosis. Dipyridamole is a vasodilator and platelet inhibitor that has previously been shown to have direct antioxidant properties. The antioxidant effects of dipyridamole on vascular cell-derived ROS are not known; therefore, dipyridamole was incubated with endothelial cells and platelets and cellular redox status and release of endogenous ROS were assessed. Dipyridamole decreased intracellular basal ROS generation from endothelial cells as measured by DCFDA (2',7'-dichlorodihydrofluorescein diacetate) oxidation. Incubation of endothelial cells with dipyridamole also attenuated t-butylhydroperoxide-induced oxidative stress. Using a redox-sensitive fluorescent dye, dipyridamole improved cellular activity after treatment with t-butylhydroperoxide. Incubation with dipyridamole did not alter platelet release of nitric oxide or hydrogen peroxide but significantly attenuated superoxide release. Using flow cytometry and confocal microscopy, dipyridamole decreased platelet ROS generation. Dipyridamole also suppressed platelet-soluble CD40 ligand release. In summary, at therapeutically relevant concentrations, dipyridamole suppresses the formation of ROS in platelets and endothelial cells and improves cellular redox status. These data suggest that the redox-dependent properties of dipyridamole have a direct effect on vascular cells.

Role of oxidatively modified low density lipoproteins and anti-oxidants in atherothrombosis

Retrospective studies have demonstrated an association between coronary artery disease (CAD) and increased plasma levels of oxidised low density lipoproteins (LDL). A very recent prospective study in heart transplant patients has demonstrated that oxidised LDL is an independent risk factor for transplant CAD, thus further supporting the hypothesis that oxidised LDL is actively involved in the development of CAD. The increase of circulating oxidised LDL is most probably caused by back-diffusion from the atherosclerotic arterial wall in the blood, independent of plaque rupture. Indeed, plasma levels of oxidised LDL were very similar in patients with stable CAD and in patients with acute coronary syndromes. These were, however, associated with increased release of malondialdehyde (MDA)-modified LDL. Oxidised LDL may be generated by radical-mediated or by lipoxygenase or phospholipase catalysed lipid oxidation, and by myeloperoxidase catalysed protein and lipid oxidation. Prostaglandin synthesis by endothelial cells under oxidative stress and platelet activation are associated with the release of aldehydes; these induce the oxidative modification of the apolipoprotein B-100 moiety of LDL in the absence of lipid peroxidation, and thus generate MDA-modified LDL. Efficient prevention of in vivo oxidation may involve efficient cholesterol lowering, improving the anti-oxidative status of LDL by increasing the anti-oxidant content and increasing the oleate content of LDL, and by shifting the LDL away from phenotype B (characterised by small dense LDL particles). Anti-oxidative and anti-inflammatory enzymes associated with HDL may inhibit the oxidation of LDL or reverse the atherothrombotic effects of LDL.

The oxidant stress hypothesis of atherogenesis

Atherosclerosis is the commonest lesion of blood vessels and is responsible for life-threatening events such as myocardial infarction and stroke. In the last two decades a series of excellent studies unraveled biochemical mechanisms that provided the background for a theory of atherogenesis. This theory is centered on foam cells and on free radical-mediated modification of low density lipoprotein (LDL). Foam cells are the main cell type of atherosclerotic lesions and originate from monocytes migrated from blood and from smooth muscle cells of the arterial wall. Foam cells are engulfed of lipids taken from LDL. Paradoxically, accumulation of LDL in developing foam cells does not occur via the classic LDL receptor. Incubation of macrophages with even very high concentrations of LDL does not appreciably increase cholesterol content. Chemically modified LDL easily enter the cells of atherosclerotic plaque via an unregulated receptor, the scavenger receptor. The most studied chemical modification of LDL is that induced by free radicals.

In vitro antioxidant and photo-oxidant properties of dipyridamole

The in vitro antioxidant and photo-oxidant activity of dipyridamole was studied by its effect on superoxide- and singlet oxygen-mediated photohemolysis and viability of neutrophils. Dipyridamole was found to be phototoxic when examined by the photohemolysis on human erythrocytes and on linoleic acid as lipid peroxidation model at concentrations above 3.0 x 10(-5) M. On the contrary, when lower concentrations (1.0 x 10(-5) to 1.0 x 10(-6) M) were used, dipyridamole showed a protector action against singlet oxygen-mediated photohemolysis by other phototoxic compounds like triamterene. This antioxidant property is proposed to result from quenching of triamterene mediated by fluorescence energy transfer. Auto-oxidation and fluorescence-energy transfer is clearly an important mechanism for protection for this drug.

Dipyridamole inhibits sickling-induced cation fluxes in sickle red blood cells

Sickling-induced cation fluxes contribute to cellular dehydration of sickle red blood cells (SS RBCs), which in turn potentiates sickling. This study examined the inhibition by dipyridamole of the sickling-induced fluxes of Na(+), K(+), and Ca(++) in vitro. At 2% hematocrit, 10 microM dipyridamole inhibited 65% of the increase in net fluxes of Na(+) and K(+) produced by deoxygenation of SS RBCs. Sickle-induced Ca(++) influx, assayed as (45)Ca(++) uptake in quin-2-loaded SS RBCs, was also partially blocked by dipyridamole, with a dose response similar to that of Na(+) and K(+) fluxes. In addition, dipyridamole inhibited the Ca(++)-activated K(+) flux (via the Gardos pathway) in SS RBCs, measured as net K(+) efflux in oxygenated cells exposed to ionophore A23187 in the presence of external Ca(++), but this effect resulted from reduced anion conductance, rather than from a direct effect on the K(+) channel. The degree of inhibition of sickling-induced fluxes was dependent on hematocrit, and up to 30% of dipyridamole was bound to RBC membranes at 2% hematocrit. RBC membrane content of dipyridamole was measured fluorometrically and correlated with sickling-induced flux inhibition at various concentrations of drug. Membrane drug content in patients taking dipyridamole for other clinical indications was similar to that producing inhibition of sickling-induced fluxes in vitro. These data suggest that dipyridamole might inhibit sickling-induced fluxes of Na(+), K(+), and Ca(++) in vivo and therefore have potential as a pharmacological agent to reduce SS RBC dehydration. (Blood. 2001;97:3976-3983)

Antioxidant effect of oral dipyridamole during cerebral hypoperfusion with human carotid endarterectomy

We sought to test the hypothesis of an antioxidant effect of dipyridamole in vivo in a model of cerebral hypoperfusion. Twenty-one patients (65+/-10 years, 11 men) undergoing carotid endarterectomy were allocated in two groups (group 1, 10 with dipyridamole, 200 mg p.o., 3-4 h before surgery; group II, 11 with placebo) in a double-blind placebo-controlled randomized design. Blood was sampled from ipsilateral jugular bulb, and plasma vitamin E content was assayed before, after 15 and 30 min of clamp, and 2 and 10 min after declamping. In 12 of them, lipoperoxides were assayed. Vitamin E plasma content decreased significantly in group II (rest, 3.71+/-0.22 mmol/mol of cholesterol, 100%) after clamp (91.5% of rest, p < 0.01) and remained unchanged during declamping (90.9% of rest, p < 0.01), but did not change in group I (rest, 3.5+/-0.44 mmol/mol of cholesterol, 100%) during clamping (99.9% of rest; p = NS) and after declamping (97.6% of rest; p = NS). Lipoperoxide concentration did not change in group I (rest, 302+/-8 a.u.; clamp, 296+/-13 a.u.; p = NS vs. rest; declamp, 304+/-8 a.u.; p = NS vs. rest), and increased significantly in group II (rest, 313+/-5 a.u.; clamp, 352+/-9 a.u.; p < 0.01 vs. rest; declamp, 343+/-6 a.u.; p < 0.05 vs. rest). Cerebral oxidative stress associated with human carotid endarterectomy can be attenuated by pretreatment with oral dipyridamole.

Dipyridamole and its derivatives modify the kinetics of the electron transport in reaction centers from Rhodobacter sphaeroides

A well known vasodilator dipyridamole (DIP), 2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido[5,4-d]pyrim idine, and its derivatives have recently been shown as potential co-activators (modulators) in the phenomenon of multidrug resistance (MDR) in cancer therapy. They inhibit the specific function of a transmembrane P-glycoprotein responsible for the ex-flux of anti-cancer drugs from tumor cells. To clarify molecular mechanisms of the anti-MDR activity of DIP and its two derivatives, RA25 and RA47, we have studied their effects on electron transport in reaction centers (RC) from purple photosynthetic bacteria Rb. sphaeroides, using RC as a model system. Increasing concentrations of DIP and RA47 progressively accelerate the back electron transfer from the primary quinone acceptor QA to the bacteriochlorophyll dimer Bchl2 (Bchl2+ -QA- recombination). In the absence of o-phenantroline, when both quinone acceptors QA and QB are involved in the electron transport, RA47 is more effective than DIP. DIP stabilizes the electron on the secondary quinone acceptor QB, the effect manifested as the retardation of Bchl2+ -QB- recombination. Effects of RA25 are negligible in all cases. The drugs are proposed to change the electron transport affecting the RC structural dynamics and the stabilization of the electron on quinone acceptors through modification of H-bonds in the system.

Fluorescence quenching of dipyridamole associated to peroxyl radical scavenging: a versatile probe to measure the chain breaking antioxidant activity of biomolecules

Dypiridamole is a highly efficient chain breaking antioxidant (Iuliano et al., Free Radic. Biol. Med. 18 (1995) 239-247) with an aromatic ring system responsible for an intense absorption band in the 400-480-nm region and for an intense fluorescence. Dipyridamole fluorescence is quantitatively quenched upon reaction with peroxyl radicals. In the presence of a flux of peroxyl radicals generated by thermal dissociation of azo-initiators, dipyridamole fluorescence decays linearly, showing a first-order reaction with respect to peroxyl radicals, and zero-order with respect to dipyridamole. The pH optimum for the fluorescence quenching is in the 7-8 range, from pH 7 to 6, the decay of fluorescence rapidly decreases to became negligible below pH 5.5. Dipyridamole consumption is blocked in the presence of an added chain breaking antioxidant for a time that is proportional to the antioxidant concentration. This effect is shown for ascorbic acid, trolox, vitamin E, uric acid, and N, N'-diphenyl-p-phenylenediamine. The slope of the linear correlation relative to trolox allows calculation of the bimolecular rate constant for a given molecule and peroxyl radicals. Comparison of data obtained by the dipyridamole consumption are comparable to values obtained by the oxygen consumption method.

Radiolabeled native low-density lipoprotein injected into patients with carotid stenosis accumulates in macrophages of atherosclerotic plaque : effect of vitamin E supplementation

BACKGROUND

Accumulation of LDL within the arterial wall appears to play a crucial role in the initiation and progression of atherosclerotic plaque. The dynamic sequence of this event has not been fully elucidated in humans.

METHODS AND RESULTS

In 7 patients with previous transient ischemic attack or stroke and critical (>70%) carotid stenosis, autologous native [(125)I]-labeled LDL or [(125)I]-labeled human serum albumin were injected 24 to 72 hours before endarterectomy. Carotid specimens obtained at endarterectomy were analyzed by autoradiography and immunohistochemistry. Autoradiographic study showed that LDL was localized prevalently in the foam cells of atherosclerotic plaques, whereas the accumulation in the lipid core was negligible. Immunohistochemistry revealed that foam cells that had accumulated radiolabeled LDL were mostly CD68 positive, whereas a small number were alpha-actin positive. No accumulation of the radiotracer was detected in atherosclerotic plaques after injection of radiolabeled human serum albumin. In 3 patients treated for 4 weeks with vitamin E (900 mg/d), an almost complete suppression of radiolabeled LDL uptake by macrophages was observed.

CONCLUSIONS

This study shows that circulating LDL rapidly accumulates in human atherosclerotic plaque. The prevalent accumulation of LDL by macrophages provides strong support to the hypothesis that these cells play a crucial role in the pathogenesis of atherosclerosis.

Antioxidants in the prevention of renal disease

In view of the role of oxidative processes in inflicting damage that leads to glomerulosclerosis and renal medullary interstitial fibrosis, more attention could be paid to the use of antioxidant food constituents and the usage of drugs with recognized antioxidant potential. In any case atherosclerosis is an important component of chronic renal diseases. There is a wide choice of foods and drugs that could confer benefit. Supplementation with vitamins E and C, use of soy protein diets and drinking green tea could be sufficient to confer remarkable improvements.

Inhibition of Alzheimer's beta-amyloid induced vasoactivity and proinflammatory response in microglia by a cGMP-dependent mechanism

beta-amyloid (Abeta) peptides are the major protein components of senile plaques in Alzheimer's disease (AD) brains. Vascular damage and reactive gliosis are found colocalized with amyloid deposits in AD brains, suggesting that the vasculature may be a clinically significant site of AD pathology. Our results show that freshly solubilized Abeta1-40 enhances the vasoconstriction induced by endothelin-1 (ET-1) and increases resistance to relaxation triggered by nitric oxide (NO), suggesting that Abeta may oppose the NO/cGMP pathway. Using specific inhibitors and activators of the NO/cGMP pathway, we show that Abeta vasoactivity is not due to a modulation of nitric oxide synthase (NOS) or soluble guanylyl cyclase (sGC). However, we find that a selective cGMP phosphodiesterase (cGMP-PDE) inhibitor (dipyridamole) is able to interactively block the enhanced vasoconstriction as well as the opposition to relaxation induced by Abeta, suggesting that Abeta could effect the activity of this enzyme. Cyclic GMP levels, but not cAMP concentrations, are reduced after Abeta treatment of rat aortic rings, further substantiating this hypothesis. Moreover, in examination of this pathway in another cell type pertinent to AD, we find that Abeta induces a proinflammatory response in microglia as evidenced by increased leukotriene B4 release. We show that both dipyridamole and compounds which increase cGMP levels prevent Abeta-induced microglial inflammation. Our results suggest that therapeutic intervention aimed at reduction of microglial-mediated inflammation via inhibition of cGMP-PDE or elevation of cGMP may be beneficial in the treatment of AD.

Protection of low density lipoprotein oxidation by the antioxidant agent IRFI005, a new synthetic hydrophilic vitamin E analogue

The oxidative modification of low density lipoprotein (LDL) is thought to be an important factor in the initiation and development of atherosclerosis. Antioxidants have been shown to protect LDL from oxidation and to inhibit atherosclerosis development in animals. Potent synthetic antioxidants are currently being tested, but they are not necessarily safe for human use. We here characterize the antioxidant activity of IRFI005, the active metabolite of Raxofelast (IRFI0016) that is a novel synthetic analog of vitamin E under clinical development, and demonstrate that it prevents oxidative modification of LDL. IFI005 inhibited the oxidative modification of LDL, measured through the generation of MDA, electrophoretic mobility and apo B100 fluorescence. During the oxidation process IRF1005 was consumed with the formation of the benzoquinone oxidation product. The powerful antioxidant activity of IRFI005 is at least in part mediated by a chain breaking mechanism as it is an efficient peroxyl radical scavenger with a rate constant k(IRFI005 + LOO(o)) of 1.8 X 10(6) M(-1)s(-1). 4. IRFI005 substantially preserved LDL-associated antioxidants, alpha-tocopherol and carotenoids, and when co-incubated with physiologic levels of ascorbate provoked a synergistic inhibition of LDL oxidation. Also the co-incubation of IRFI005 with Trolox caused a synergistic effect, and a lag phase in the formation of the trolox-benzoquinone oxidation product. A synergistic inhibition of lipid peroxidation was also demonstrated by co-incubating IRFI005 and alpha-tocopherol incorporated in linoleic acid micelles. These data strongly suggest that IRFI005 can operate by a recycling mechanism similar to the vitamin E/ascorbate sysem.

Medium effects on the antioxidant activity of dipyridamole

Very strong medium effects have been observed when testing the antioxidant activity of dipyridamole (DP) in different media such as benzene, tert-butanol, methanol solutions and egg yolk lecithin unilamellar and multilamellar vesicles. Actually, dipyridamole behaves as a very poor antioxidant in benzene while its ability to inhibit the lipid peroxidation reaction increases with increasing solvent polarity, being the highest in lipid vesicles. This behavior can not be rationalized on the basis of the classical chain breaking mechanism which operates in the case of phenolic and amine antioxidants and involving the transfer of a hydrogen atom to peroxyl radicals. An explanation is instead given in terms of an electron transfer reaction which leads to the oxidation of DP by the chain carrying peroxyl radical to give the dipyridamole cation radical, DP+*, and the peroxyl anion LOO-, and whose rate constant is expected to increase in strongly polar media. EPR and electrochemical data supporting this interpretation have been collected.

IPF2alpha-I: an index of lipid peroxidation in humans

Isoprostanes are prostaglandin isomers produced from arachidonic acid by a free radical-catalyzed mechanism. Urinary excretion of 8-iso-prostaglandin F2alpha, an isomer of the PGG/H synthase (cyclooxygenase or COX) enzyme product, prostaglandin F2alpha (PGF2alpha), has exhibited promise as an index of oxidant stress in vivo. We have developed a quantitative method to measure isoprostane F2alpha-I, (IPF2alpha-I) a class I isomer (8-iso-PGF2alpha is class IV), using gas chromatography/mass spectrometry. IPF2alpha-I is severalfold as abundant in human urine as 8-iso-PGF2alpha, with mean values of 737 +/- 20.6 pg/mg creatinine. Both isoprostanes are formed in a free radical-dependent manner in low density lipoprotein oxidized by copper in vitro. However, IPF2alpha-I, unlike 8-iso-PGF2alpha, is not formed in a COX-dependent manner by platelets activated by thrombin or collagen in vitro. Similarly, COX inhibition in vivo has no effect on IPF2alpha-I. Neither serum IPF2alpha-I, an index of cellular capacity to generate the isoprostane, nor urinary excretion of IPF2alpha-I, an index of actual generation in vivo, is depressed by aspirin or indomethacin. In contrast, both serum thromboxane B2 and urinary excretion of its 11-dehydro metabolite are depressed by the COX inhibitors. Although serum 8-iso-PGF2alpha formation is substantially depressed by COX inhibitors, urinary excretion of the compound is unaffected. Urinary IPF2alpha-I is elevated in cigarette smokers compared with controls (1525 +/- 180 versus 740 +/- 40 pg/mg creatinine; P < 0.01) and is highly correlated with urinary 8-iso-PGF2alpha (r = 0.9; P < 0.001). Urinary IPF2alpha-I is a novel index of lipid peroxidation in vivo, which can be measured with precision and sensitivity. It is an abundant F2-isoprostane formed in a free radical- but not COX-dependent manner. Although 8-iso-PGF2alpha may be formed as a minor product of COX, this pathway contributes trivially, if at all, to levels in urine. Urinary excretion of both isoprostanes is elevated in cigarette smokers.