Eicosapentaenoic acid improves endothelial function and nitric oxide bioavailability in a manner that is enhanced in combination with a statin

The endothelium exerts many vasoprotective effects that are largely mediated by release of nitric oxide (NO). Endothelial dysfunction represents an early but reversible step in atherosclerosis and is characterized by a reduction in the bioavailability of NO. Previous studies have shown that eicosapentaenoic acid (EPA), an omega-3 fatty acid (O3FA), and statins individually improve endothelial cell function, but their effects in combination have not been tested. Through a series of in vitro experiments, this study evaluated the effects of a combined treatment of EPA and the active metabolite of atorvastatin (ATM) on endothelial cell function under conditions of oxidative stress. Specifically, the comparative and time-dependent effects of these agents on endothelial dysfunction were examined by measuring the levels of NO and peroxynitrite (ONOO^-) released from human umbilical vein endothelial cells (HUVECs). The data suggest that combined treatment with EPA and ATM is beneficial to endothelial function and was unique to EPA and ATM since similar improvements could not be recapitulated by substituting another O3FA docosahexaenoic acid (DHA) or other TG-lowering agents such as fenofibrate, niacin, or gemfibrozil. Comparable beneficial effects were observed when HUVECs were pretreated with EPA and ATM before exposure to oxidative stress. Interestingly, the kinetics of EPA-based protection of endothelial function in response to oxidation were found to be significantly different than those of DHA. Lastly, the beneficial effects on endothelial function generated by combined treatment of EPA and ATM were reproduced when this study was expanded to an ex vivo model utilizing rat glomerular endothelial cells. Taken together, these findings suggest that a combined treatment of EPA and ATM can inhibit endothelial dysfunction that occurs in response to conditions such as hyperglycemia, oxidative stress, and dyslipidemia.

Atorvastatin enhanced nitric oxide release and reduced blood pressure, nitroxidative stress and rantes levels in hypertensive rats with diabetes

Clinical trials have shown that atorvastatin benefits patients with diabetes even with normal baseline LDL levels. We hypothesized that atorvastatin improves endothelial cell (EC) function and reduces inflammation in hypertensive rats with diabetes. Non-diabetic and streptozotocin-induced type 2 diabetic male spontaneously hypertensive rats (SHR) were treated with atorvastatin at 20 mg/kg/day. After five weeks, nitric oxide (NO) and peroxynitrite (ONOO(-)) were measured in aortic and glomerular endothelial cells. A tandem of nanosensors was used to simultaneously measure NO and ONOO(-) concentration and their ratio [NO]/[ONOO(-)] was monitored with a time resolution better than 10 μs and detection limit 1 nM. [NO]/[ONOO(-)] was applied as a marker of endothelial NO synthase (eNOS) uncoupling, endothelial dysfunction and nitroxidative stress. Glucose, cholesterol, blood pressure (BP), and the cytokine RANTES were also measured. Diabetic SHR rats had elevated glucose (355 ± 38 mg/dL), mean BP (172 ± 15 mmHg), and plasma RANTES (38.4 ± 2.7 ng/mL), low endothelial NO bioavailability and high ONOO(-). Maximal NO release measured 267 ± 29 nM in aortic endothelium of SHR rats and 214 ± 20 nM for diabetic SHR rats; [NO]/[ONOO(-)] was 0.88 ± 12 and 0.61 ± 0.08, respectively. [NO]/[ONOO(-)] ratios below one indicate a high uncoupling of eNOS, endothelial dysfunction and high nitroxidative stress. Atorvastatin treatment partially restored endothelial function by increasing NO level by 98%, reducing ONOO(-) by 40% and favorably elevating [NO]/[ONOO(-)] to 1.1 ± 0.2 for diabetic SHR rats and 1.6 ± 0.3 for SHR rats. The effects of atorvastatin were similar in glomerular endothelial cells and were partially reproduced by modulators of eNOS or NADPH oxidase. Atorvastatin had no significant effect on fasting glucose or total cholesterol levels but reduced mean BP by 21% and 11% in diabetic and non-diabetic animals, respectively. Atorvastatin also reduced RANTES levels by 50%. Atorvastatin favorably increased the [NO]/[ONOO(-)] balance, enhanced endothelial cytoprotective NO, decreased cytotoxic ONOO(-) and reduced BP, inflammation and RANTES levels in diabetic, hypertensive rats without altering cholesterol levels. These findings provide insights into mechanisms of restoration of endothelial function and vascular protection by atorvastatin in diabetes and hypertension.

Eicosapentaenoic acid inhibits glucose-induced membrane cholesterol crystalline domain formation through a potent antioxidant mechanism

Lipid oxidation leads to endothelial dysfunction, inflammation, and foam cell formation during atherogenesis. Glucose also contributes to lipid oxidation and promotes pathologic changes in membrane structural organization, including the development of cholesterol crystalline domains. In this study, we tested the comparative effects of eicosapentaenoic acid (EPA), an omega-3 fatty acid indicated for the treatment of very high triglyceride (TG) levels, and other TG-lowering agents (fenofibrate, niacin, and gemfibrozil) on lipid oxidation in human low-density lipoprotein (LDL) as well as membrane lipid vesicles prepared in the presence of glucose (200 mg/dL). We also examined the antioxidant effects of EPA in combination with atorvastatin o-hydroxy (active) metabolite (ATM). Glucose-induced changes in membrane structural organization were measured using small angle x-ray scattering approaches and correlated with changes in lipid hydroperoxide (LOOH) levels. EPA was found to inhibit LDL oxidation in a dose-dependent manner (1.0-10.0 µM) and was distinguished from the other TG-lowering agents, which had no significant effect as compared to vehicle treatment alone. Similar effects were observed in membrane lipid vesicles exposed to hyperglycemic conditions. The antioxidant activity of EPA, as observed in glucose-treated vesicles, was significantly enhanced in combination with ATM. Glucose treatment produced highly-ordered, membrane-restricted, cholesterol crystalline domains, which correlated with increased LOOH levels. Of the agents tested in this study, only EPA inhibited glucose-induced cholesterol domain formation. These data demonstrate that EPA, at pharmacologic levels, inhibits hyperglycemia-induced changes in membrane lipid structural organization through a potent antioxidant mechanism associated with its distinct, physicochemical interactions with the membrane bilayer.

Abstract 648: Nebivolol and Valsartan Increase Nitric Oxide Release from Human Endothelial Cells in a Synergistic Fashion

Introduction: Nebivolol is a β1-adrenergic receptor antagonist that stimulates endothelial nitric oxide (NO) release through β3-adrenergic receptor activation, ATP-mediated stimulation of purinergic P2Y receptors, and inhibition of membrane lipid oxidation. Valsartan is an angiotensin II receptor blocker (ARB) that selectively inhibits angiotensin II type 1 (AT1) receptors, thereby exerting no direct effect at type 2 (AT2) receptors, which have been shown to stimulate NO synthase activity through a bradykinin-mediated pathway. As NO is a key regulator of blood pressure and these two antihypertensive agents promote NO release through distinct mechanisms, we compared their individual and combined effects on NO release from human endothelial cells.

Methods: Human umbilical vein endothelial cells (HUVECs) were incubated for 1 hr with vehicle, nebivolol or valsartan alone (each at 1.0 μM), or nebivolol (1.0 μM) and valsartan (0.5-5.0 μM) combined. The comparative effects of these agents on maximal NO release were measured in individual cells using porphyrinic nanosensors following stimulation with calcium ionophore (1.0 μM).

Results: Nebivolol treatment increased HUVEC NO release by 49% (509 ± 18 nM, mean ± SD) as compared to vehicle treatment alone (342 ± 26 nM; p<0.001). Valsartan had a more modest effect, increasing NO release by 13% (385 ± 19 nM) as compared to vehicle-treated controls (p<0.01). Treatment with both agents at 1.0 μM increased HUVEC NO release by 91% (655 ± 19 nM) as compared to vehicle alone (p<0.001) and was 29% (p<0.001) and 70% (p<0.001) greater than the separate effects observed for nebivolol and valsartan, respectively. The additive effect of valsartan was dose-dependent and was also observed at 0.5 and 5.0 μM in combination with nebivolol.

Conclusions: These data suggest that nebivolol and valsartan, when applied in combination, increase the ability of endothelial cells to release NO in a synergistic manner. The exact mechanism of this process remains unclear but, considering the importance of NO in regulating blood pressure, merits further study.

Amlodipine increased endothelial nitric oxide and decreased nitroxidative stress disproportionately to blood pressure changes

BACKGROUND

Clinical trials have shown that amlodipine reduces cardiovascular events at a rate that is not predicted by changes in brachial arterial pressure alone. These findings may be explained, in part, by the pleiotropic effects of amlodipine on endothelial cell (EC) function. In this study, we elucidated the effect of amlodipine on nitric oxide (NO) bioavailability and cytotoxic peroxynitrite (ONOO(-)) and blood pressure (BP).

METHODS

Spontaneously hypertensive rats (SHRs) were treated with vehicle or amlodipine (5 mg/kg/day) for 8 weeks and compared with untreated, baseline rats. NO and ONOO(-) release from aortic and glomerular ECs were measured ex vivo using amperometric nanosensors following maximal stimulation with calcium ionophore. BP was measured using the tail-cuff method.

RESULTS

As compared with baseline, vehicle treatment had reduced aortic endothelial NO release from 157 ± 11 nM to 55 ± 6 nM and increased ONOO(-) from 69 ± 7 nM to 156 ± 19 nM. The NO/ONOO(-) ratio, a comprehensive measurement of eNOS function, decreased from 2.3 ± 0.3 to 0.3 ± 0.1. Compared with vehicle, amlodipine treatment restored NO to 101 ± 3 nM, decreased ONOO(-) to 50 ± 4 nM, and increased the NO/ONOO(-) ratio to 2.0 ± 0.2, a level similar to baseline. Similar changes were observed for glomerular ECs. Mean arterial blood pressure increased from 149 ± 3 mm Hg (baseline) to 174 ± 1 mm Hg (vehicle). Amlodipine slightly, but significantly, decreased mean arterial blood pressure to 167 ± 3 mm Hg vs. vehicle treatment.

CONCLUSIONS

Amlodipine increased NO bioavailability and decreased nitroxidative stress in SHRs with EC dysfunction disproportionately to BP changes. These direct, vascular effects of amlodipine on EC function may contribute to reduced risk for atherothrombotic events as observed in clinical trials.

1,2-naphthoquinone stimulates lipid peroxidation and cholesterol domain formation in model membranes

PURPOSE

Naphthalene induces cataract formation through the accumulation of its reactive metabolite, 1,2-naphthoquinone (1,2-NQ), in the ocular lens. 1,2-NQ increases lens protein oxidation and disrupts fiber cell membrane function; however, the association of these effects with changes in membrane structure is not understood. The goal of this study was to determine the direct effects of 1,2-NQ on membrane lipid oxidation and structural organization.

METHODS

Iodometric approaches were used to measure the effects of naphthalene and 1,2-NQ on lipid hydroperoxide (LOOH) formation in model membranes composed of cholesterol and dilinoleoylphosphatidylcholine. Membrane samples were prepared at various cholesterol-to-phospholipid mole ratios and subjected to autoxidation at 37°C for 48 hours in the absence or presence of either agent alone (0.1-5.0 μM) or in combination with vitamin E. Small-angle x-ray diffraction was used to measure the effects of naphthalene and 1,2-NQ on membrane structure before and after exposure to oxidative stress.

RESULTS

1,2-NQ increased LOOH formation by 250% (P < 0.001) and 350% (P < 0.001) at 1.0 and 5.0 μM, respectively, whereas naphthalene decreased LOOH levels by 25% (P < 0.01) and 10% (NS). The pro-oxidant effect of 1,2-NQ was inversely affected by membrane cholesterol enrichment and completely blocked by vitamin E. 1,2-NQ also increased cholesterol domain formation by 360% in membranes exposed to oxidative stress; however, no significant changes in membrane lipid organization were observed with naphthalene under the same conditions.

CONCLUSIONS

These data suggest a novel mechanism for naphthalene-induced cataract, facilitated by the direct effects of 1,2-NQ on lipid peroxidation and cholesterol domain formation.

The favorable kinetics and balance of nebivolol-stimulated nitric oxide and peroxynitrite release in human endothelial cells

Background

Nebivolol is a third-generation beta-blocker used to treat hypertension. The vasodilation properties of nebivolol have been attributed to nitric oxide (NO) release. However, the kinetics and mechanism of nebivolol-stimulated bioavailable NO are not fully understood.

Methods

Using amperometric NO and peroxynitrite (ONOO^-) nanosensors, β₃-receptor (agonist: L-755,507; antagonists: SR59230A and L-748,337), ATP efflux (the mechanosensitive ATP channel blocker, gadolinium) and P2Y-receptor (agonists: ATP and 2-MeSATP; antagonist: suramin) modulators, superoxide dismutase and a NADPH oxidase inhibitor (VAS2870), we evaluated the kinetics and balance of NO and ONOO^- stimulated by nebivolol in human umbilical vein endothelial cells (HUVECs). NO and ONOO^- were measured with nanosensors (diameter ~ 300 nm) placed 5 ± 2 μm from the cell membrane and ATP levels were determined with a bioluminescent method. The kinetics and balance of nebivolol-stimulated NO and ONOO^- were compared with those of ATP, 2-MeSATP, and L-755,507.

Results

Nebivolol stimulates endothelial NO release through β₃-receptor and ATP-dependent, P2Y-receptor activation with relatively slow kinetics (75 ± 5 nM/s) as compared to the kinetics of ATP (194 ± 10 nM/s), L-755,507 (108 ± 6 nM/s), and 2-MeSATP (105 ± 5 nM/s). The balance between cytoprotective NO and cytotoxic ONOO^- was expressed as the ratio of [NO]/[ONOO^-] concentrations. This ratio for nebivolol was 1.80 ± 0.10 and significantly higher than that for ATP (0.80 ± 0.08), L-755,507 (1.08 ± 0.08), and 2-MeSATP (1.09 ± 0.09). Nebivolol induced ATP release in a concentration-dependent manner.

Conclusion

The two major pathways (ATP efflux/P2Y receptors and β₃ receptors) and several steps of nebivolol-induced NO and ONOO^- stimulation are mainly responsible for the slow kinetics of NO release and low ONOO^-. The net effect of this slow kinetics of NO is reflected by a favorable high ratio of [NO]/[ONOO^-] which may explain the beneficial effects of nebivolol in the treatment of endothelial dysfunction, hypertension, heart failure, and angiogenesis.

Effects of angiotensin receptor blockers on endothelial nitric oxide release: the role of eNOS variants

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Angiotensin II receptor blockers improve endothelial cell-dependent vasodilation in patients with hypertension through suppression of angiotensin II type 1 receptors but may have additional and differential effects on endothelial nitric oxide synthase (eNOS) function.

WHAT THIS STUDY ADDS

• The key finding from this study is that angiotensin II receptor blockers (ARBs) differentially enhanced nitric oxide (NO) release in a manner influenced by certain genetic variants of eNOS. This finding provides new insights into the effects of ARBs on endothelial cell-dependent vasodilation and eNOS function that are of high importance in vascular medicine and clinical pharmacology. AIM Angiotensin II receptor blockers (ARBs) improve endothelial cell (EC)-dependent vasodilation in patients with hypertension through suppression of angiotensin II type 1 receptors but may have additional and differential effects on endothelial nitric oxide (NO) synthase (eNOS) function. To investigate this question, we tested the effects of various ARBs on NO release in ECs from multiple donors, including those with eNOS genetic variants linked to higher cardiovascular risk.

METHODS

The effects of ARBs (losartan, olmesartan, telmisartan, valsartan), at 1 µm, on NO release were measured with nanosensors in human umbilical vein ECs obtained from 18 donors. NO release was stimulated with calcium ionophore (1 µm) and its maximal concentration was correlated with eNOS variants. The eNOS variants were determined by a single nucleotide polymorphism in the promoter region (T-786C) and in the exon 7 (G894T), linked to changes in NO metabolism. RESULTS All of the ARBs caused an increase in NO release as compared with untreated samples (P < 0.01, n= 4-5 in all eNOS variants). However, maximal NO production was differentially influenced by eNOS genotype. Olmesartan increased maximal NO release by 30%, which was significantly greater (P < 0.01, n= 4-5 in all eNOS variants) than increases observed with other ARBs.

CONCLUSIONS

The ARBs differentially enhanced NO release in ECs in a manner influenced by eNOS single nucleotide polymorphisms. These findings provide new insights into the effects of ARBs on EC-dependent vasodilation and eNOS function.

Alterations in membrane caveolae and BKCa channel activity in skin fibroblasts in Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is an inherited disorder of cholesterol synthesis caused by mutations in DHCR7 which encodes the final enzyme in the cholesterol synthesis pathway. The immediate precursor to cholesterol synthesis, 7-dehydrocholesterol (7-DHC) accumulates in the plasma and cells of SLOS patients which has led to the idea that the accumulation of abnormal sterols and/or reduction in cholesterol underlies the phenotypic abnormalities of SLOS. We tested the hypothesis that 7-DHC accumulates in membrane caveolae where it disturbs caveolar bilayer structure-function. Membrane caveolae from skin fibroblasts obtained from SLOS patients were isolated and found to accumulate 7-DHC. In caveolar-like model membranes containing 7-DHC, subtle, but complex alterations in intermolecular packing, lipid order and membrane width were observed. In addition, the BK(Ca) K(+) channel, which co-migrates with caveolin-1 in a membrane fraction enriched with cholesterol, was impaired in SLOS cells as reflected by reduced single channel conductance and a 50 mV rightward shift in the channel activation voltage. In addition, a marked decrease in BK(Ca) protein but not mRNA expression levels was seen suggesting post-translational alterations. Accompanying these changes was a reduction in caveolin-1 protein and mRNA levels, but membrane caveolar structure was not altered. These results are consistent with the hypothesis that 7-DHC accumulation in the caveolar membrane results in defective caveolar signaling. However, additional cellular alterations beyond mere changes associated with abnormal sterols in the membrane likely contribute to the pathogenesis of SLOS.

Effect of enhanced glycemic control with saxagliptin on endothelial nitric oxide release and CD40 levels in obese rats

AIM

Endothelial cell (EC) dysfunction contributes to insulin resistance in diabetes and is characterized by reduced nitric oxide (NO) release, increased nitroxidative stress and enhanced inflammation. The purpose of this study was to test the effect of improved postprandial glucose control on EC function in insulin-resistant rats as compared to fasting glucose (FG) changes.

METHODS

Obese Zucker rats were treated with 10 mg/kg/day saxagliptin, a dipeptidyl peptidase-4 (DPP4) inhibitor, for 4 or 8 weeks and compared to lean rats. NO and peroxynitrite (ONOO(-)) release from aortic and glomerular ECs was measured ex vivo using amperometric approaches and correlated with FG, postprandial glucose, insulin, soluble CD40 (sCD40) and L-citrulline levels.

RESULTS

Saxagliptin treatment improved NO production and reduced ONOO(-) release prior to any observed changes in FG levels. In untreated obese animals, NO release from aortic and glomerular ECs decreased by 22% and 31%, respectively, while ONOO(-) release increased by 26% and 40%. Saxagliptin increased aortic and glomerular NO release by 18% and 31%, respectively, with comparable reductions in ONOO(-) levels; the NO/ONOO(-) ratio, an indicator of NO synthase coupling, increased by >40%. Improved glycemic control was further associated with a reduction in sCD40 levels by more than ten-fold (from 300 ± 206 to 22 ± 22 pg/mL, p < 0.001).

CONCLUSION

These findings indicate that enhanced glycemic control with DPP4 inhibition improved NO release and reduced inflammation in a manner not predicted by FG changes alone.

Advances in pharmacologic modulation of nitric oxide in hypertension

A number of structural and functional mechanisms have been identified in the pathogenesis of hypertensive vascular disease, each of which requires effective therapy to reduce global cardiovascular risk. Hypertension, together with other cardiovascular risk factors, promotes endothelial dysfunction as evidenced by decreased nitric oxide (NO) release and reduced vascular responsiveness to normal vasodilatory stimuli. In addition, the mechanical forces inherent in hypertension activate neurohormonal mechanisms, including the renin-angiotensin system, which modulate vessel wall structure and function. Antihypertensive drugs may have class-specific hemodynamic and physiologic effects that attenuate these vascular disease processes. Pharmacologic approaches that enhance endothelial NO bioavailability have been shown to restore vasodilation while reducing clinical events. These agents improve NO bioavailability by increasing endogenous production through enzymatic mechanisms or by promoting the direct release of NO by its redox congeners in a spontaneous fashion. In this article, we review the basic mechanisms of endothelial dysfunction along with the use and comparative therapeutic benefits of various pharmacologic interventions, with particular emphasis on antihypertensive agents.

Novel astaxanthin prodrug (CDX-085) attenuates thrombosis in a mouse model

BACKGROUND

Cardiovascular disease remains the leading cause of morbidity and premature mortality in most industrialized countries as well as in developing nations. A pro-oxidative state appears to promote and/or exacerbate vascular disease complications. Furthermore, a state of low-grade chronic inflammation can promote increased oxidative stress and lead to endothelial cell and platelet dysfunction ultimately contributing to thrombogenesis.

OBJECTIVES

In this study, the effect of a proprietary astaxanthin prodrug (CDX-085) on thrombus formation was investigated using a mouse model of arterial thrombosis. The influence of free astaxanthin, the active drug of CDX-085, on human endothelial cells and rat platelets was evaluated to investigate potential mechanisms of action.

METHODS AND RESULTS

Oral administration of CDX-085 (0.4% in chow, approximately 500 mg/kg/day) to 6-8 week old C57BL/6 male mice for 14 days resulted in significant levels of free astaxanthin in the plasma, liver, heart and platelets. When compared to control mice, the CDX-085 fed group exhibited significant increases in basal arterial blood flow and significant delays in occlusive thrombus formation following the onset of vascular endothelial injury. Primary human umbilical vein endothelial cells (HUVECs) and platelets isolated from Wistar-Kyoto rats treated with free astaxanthin demonstrated significantly increased levels of released nitric oxide (NO) and significantly decreased peroxynitrite (ONOO-) levels.

CONCLUSION

Observations of increased NO and decreased ONOO- levels in endothelial cells and platelets support a potential mechanism of action for astaxanthin (CDX-085 active drug). These studies support the potential of CDX-085 and its metabolite astaxanthin in the treatment or prevention of thrombotic cardiovascular complications.

Glucose promotes membrane cholesterol crystalline domain formation by lipid peroxidation

Oxidative damage to vascular cell membrane phospholipids causes physicochemical changes in membrane structure and lipid organization, contributing to atherogenesis. Oxidative stress combined with hyperglycemia has been shown to further increase the risk of vascular and metabolic diseases. In this study, the effects of glucose on oxidative stress-induced cholesterol domain formation were tested in model membranes containing polyunsaturated fatty acids and physiologic levels of cholesterol. Membrane structural changes, including cholesterol domain formation, were characterized by small angle X-ray scattering (SAXS) analysis and correlated with spectrophotometrically-determined lipid hydroperoxide levels. Glucose treatment resulted in a concentration-dependent increase in lipid hydroperoxide formation, which correlated with the formation of highly-ordered cholesterol crystalline domains (unit cell periodicity of 34 A) as well as a decrease in overall membrane bilayer width. The effect of glucose on lipid peroxidation was further enhanced by increased levels of cholesterol. Treatment with free radical-scavenging agents inhibited the biochemical and structural effects of glucose, even at elevated cholesterol levels. These data demonstrate that glucose promotes changes in membrane organization, including cholesterol crystal formation, through lipid peroxidation.

Biologic activity of carotenoids related to distinct membrane physicochemical interactions

Carotenoids are naturally occurring organic pigments that are believed to have therapeutic benefit in treating cardiovascular disease (CVD) because of their antioxidant properties. However, prospective randomized trials have failed to demonstrate a consistent benefit for the carotenoid beta-carotene in patients at risk for CVD. The basis for this apparent paradox is not well understood but may be attributed to the distinct antioxidant properties of various carotenoids resulting from their structure-dependent physicochemical interactions with biologic membranes. To test this hypothesis, we measured the effects of astaxanthin, zeaxanthin, lutein, beta-carotene, and lycopene on lipid peroxidation using model membranes enriched with polyunsaturated fatty acids. The correlative effects of these compounds on membrane structure were determined using small-angle x-ray diffraction approaches. The nonpolar carotenoids, lycopene and beta-carotene, disordered the membrane bilayer and stimulated membrane lipid peroxidation (>85% increase in lipid hydroperoxide levels), whereas astaxanthin (a polar carotenoid) preserved membrane structure and exhibited significant antioxidant activity (>40% decrease in lipid hydroperoxide levels). These results suggest that the antioxidant potential of carotenoids is dependent on their distinct membrane lipid interactions. This relation of structure and function may explain the differences in biologic activity reported for various carotenoids, with important therapeutic implications.

Circulating lipid hydroperoxides predict cardiovascular events in patients with stable coronary artery disease: the PREVENT study

OBJECTIVES

This study was designed to determine the predictive value of lipid hydroperoxide (LOOH) levels for adverse cardiovascular outcomes in patients with stable coronary artery disease (CAD).

BACKGROUND

Oxidative modification of circulating lipids contributes to inflammation and endothelial dysfunction, which are hallmark features of atherosclerosis. A serum biomarker of oxidation is LOOH, which is a primary product of fatty acid peroxidation.

METHODS

Serum LOOH levels were measured and correlated with clinical events over a 3-year period in 634 patients with angiographic evidence of CAD.

RESULTS

Baseline LOOH levels in the highest quartile were associated with hazard ratios of 3.24 (95% confidence interval [CI] 1.86 to 5.65; p = 0.0001) for nonfatal vascular events (n = 149), 1.80 (95% CI 1.13 to 2.88; p = 0.014) for major vascular procedures (n = 139), and 2.23 (95% CI 1.44 to 3.44; p = 0.0003) for all vascular events and procedures. Baseline LOOH levels correlated with serum levels of soluble intercellular adhesion molecule-1 (p = 0.001) and thiobarbituric acid reactive substances (p = 0.001) as well as the mean percent change in stenosis for large segments >50% stenosed (p = 0.048). A multivariate proportional hazards model, adjusted for traditional risk factors and inflammatory markers, showed an independent effect of LOOH on nonfatal vascular events, vascular procedures, and all events or procedures. Amlodipine treatment was associated with reduced cardiovascular events and changes in LOOH levels compared with placebo.

CONCLUSIONS

Elevated LOOH levels were predictive of nonfatal vascular events and procedures in patients with stable CAD, independent of traditional risk factors and inflammatory markers.

Synergistic effect of amlodipine and atorvastatin in reversing LDL-induced endothelial dysfunction

PURPOSE

Statins and certain calcium channel blockers may improve nitric oxide (NO) release and endothelial function through various mechanisms, but their combined effects are not well understood.

METHODS

The separate versus combined effects of amlodipine (AML) and atorvastatin (AT) on NO and peroxynitrite (ONOO-) were measured in human umbilical vein endothelial cells (HUVEC) in the presence and absence of low-density lipoprotein (LDL) using electrochemical nanosensors.

RESULTS

The combination of AML (5 micromol/l) and AT (3-6 micromol/l) directly stimulated NO release that was about twofold greater than the sum of their separate effects (p < 0.05). This synergistic activity is attributed to enhanced endothelial NO synthase (eNOS) function and decreased cytotoxic ONOO-. LDL (100 mg/dl) caused a dysfunction of HUVEC manifested by a 60% reduction in NO and an almost twofold increase in ONOO-. Treatment with AML/AT partially reversed the effects of LDL on endothelial function, including a 90% increase in NO and 50% reduction in ONOO-. Small-angle X-ray diffraction analysis indicates that AML and AT are lipophilic and share an overlapping molecular location in the cell membrane that could facilitate electron transfer for antioxidant mechanisms.

CONCLUSION

These findings indicate a synergistic effect of AML and AT on an increase in NO concentration, reduction of nitroxidative stress. Also, AML/AT partially restored the NO level of LDL-induced dysfunctional endothelium. Their combined effects may be enhanced by antioxidant properties related to their intermolecular actions in the cell membrane and an increase in the expression and coupling of endothelial nitric oxide synthase.

Status of caveolin-1 in various membrane domains of the bovine lens

Recent studies of the distribution and relative concentration of caveolin-1 in fractions of bovine lens epithelial and fiber cells have led to the novel concept that caveolin-1 may largely exist as a peripheral membrane protein in some cells. Caveolin-1 is typically viewed as a scaffolding protein for caveolae in plasma membrane. In this study, membrane from cultured bovine lens epithelial cells and bovine lens fiber cells were divided into urea soluble and insoluble fractions. Cytosolic lipid vesicles were also recovered from the lens epithelial cells. Lipid-raft domains were recovered from fiber cells following treatment with detergents and examined for caveolin and lipid content. Aliquots of all fractions were Western blotted for caveolin-1. Fluorescence microscopy and double immunofluorescence labeling were used to examine the distribution of caveolin-1 in cultured epithelial cells. Electron micrographs revealed an abundance of caveolae in plasma membrane of cultured lens epithelial cells. About 60% of the caveolin-1 in the epithelial-crude membrane was soluble in urea, a characteristic of peripheral membrane proteins. About 30% of the total was urea-insoluble membrane protein that likely supports the structure of caveolae. The remaining caveolin was part of cytosolic lipid vesicles. By contrast, most caveolin in the bovine lens fiber cell membrane was identified as intrinsic protein, being present at relatively low concentrations in caveolae-free lipid raft domains enriched in cholesterol and sphingomyelin. We estimate that these domains occupied 25-30% of the fiber cell membrane surface. Thus, the status of caveolin-1 in lens epithelial cells appears markedly different from that in fiber cells.

Regulation of the gating of BKCa channel by lipid bilayer thickness

Transmembrane segments of ion channels tend to match the hydrophobic thickness of lipid bilayers to minimize mismatch energy and to maintain their proper organization and function. To probe how ion channels respond to mismatch with lipid bilayers of different thicknesses, we examined the single channel activities of BK(Ca) (hSlo alpha-subunit) channels in planar bilayers of binary mixtures of DOPE (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine) with phosphatidylcholines (PCs) of varying chain lengths, including PC 14:1, PC 18:1, PC 22:1, PC 24:1, and with porcine brain sphingomyelin. Bilayer thickness and structure was measured with small angle x-ray diffraction and atomic force microscopy. The open probability (P(o)) of the BK(Ca) channel was finely tuned by bilayer thickness, first decreasing with increases in bilayer thickness from PC 14:1 to PC 22:1 and then increasing from PC 22:1 to PC 24:1 and to porcine brain sphingomyelin. Single channel kinetic analyses revealed that the mean open time of the channel increased monotonically with bilayer thickness and, therefore, could not account for the biphasic changes in P(o). The mean closed time increased with bilayer thickness from PC 14:1 up to PC 22:1 and then decreased with further increases in bilayer thickness to PC 24:1 and sphingomyelin, correlating with changes in P(o). This is consistent with the proposition that bilayer thickness affects channel activity mainly through altering the stability of the closed state. We suggest a simple mechanical model that combines forces of lateral stress within the lipid bilayer with local hydrophobic mismatch between lipids and the protein to account for the biphasic modulation of BK(Ca) gating.

A biological rationale for the cardiotoxic effects of rofecoxib: comparative analysis with other COX-2 selective agents and NSAids

Clinical investigations have demonstrated a relationship between the extended use of rofecoxib and increased risk for atherothrombotic events. This has led to the removal of rofecoxib from the market and explicit cardiovascular safety warnings for other COX-2 selective and non-selective agents that remain on the market. Early explanations for the cardiotoxicity of rofecoxib, such as the relative cardioprotective effect of comparator agents (naproxen) or an "imbalance" between thromboxane and prostacyclin biosynthesis due to an absence of concomitant aspirin use, have not been substantiated by the evidence. New experimental findings indicate that the cardiotoxicity of rofecoxib is not a general class effect but may be due to its intrinsic chemical structure and unique primary metabolism. Specifically, rofecoxib has been shown to increase the susceptibility of human LDL and cell membrane lipids to oxidative modification, a hallmark feature of atherosclerosis. Rofecoxib was also found to promote the non-enzymatic formation of isoprostanes from biological lipids, which act as important mediators of inflammation in the atherosclerotic plaque. The explanation for such cardiotoxicity is that rofecoxib forms a reactive maleic anhydride in the presence of oxygen due to its chemical structure and primary metabolism (cytoplasmic reductase). By contrast, adverse effects on rates of LDL and membrane lipid oxidation were not observed with other chemically distinct (sulfonamide) COX-2 inhibitors under identical conditions. These findings provide a compelling rationale for distinguishing the differences in cardiovascular risk among COX-selective inhibitors on the basis of their intrinsic physico-chemical properties.

Differential effects of carotenoids on lipid peroxidation due to membrane interactions: X-ray diffraction analysis

The biological benefits of certain carotenoids may be due to their potent antioxidant properties attributed to specific physico-chemical interactions with membranes. To test this hypothesis, we measured the effects of various carotenoids on rates of lipid peroxidation and correlated these findings with their membrane interactions, as determined by small angle X-ray diffraction approaches. The effects of the homochiral carotenoids (astaxanthin, zeaxanthin, lutein, beta-carotene, lycopene) on lipid hydroperoxide (LOOH) generation were evaluated in membranes enriched with polyunsaturated fatty acids. Apolar carotenoids, such as lycopene and beta-carotene, disordered the membrane bilayer and showed a potent pro-oxidant effect (>85% increase in LOOH levels) while astaxanthin preserved membrane structure and exhibited significant antioxidant activity (40% decrease in LOOH levels). These findings indicate distinct effects of carotenoids on lipid peroxidation due to membrane structure changes. These contrasting effects of carotenoids on lipid peroxidation may explain differences in their biological activity.

Effect of nebivolol on endothelial nitric oxide and peroxynitrite release in hypertensive animals: Role of antioxidant activity

We tested the activity of nebivolol, a beta1-selective blocker with respect to nitric oxide (NO) and peroxynitrite (ONOO) generation in the endothelium of normotensive Wistar Kyoto (WKY rats) and spontaneously hypertensive rats (SHR). The endothelial effects of nebivolol and its 2 optical enantiomers were correlated with its antioxidant activity and compared to another beta-blocker, atenolol, and 2 agonists of nitric oxide synthase (eNOS), calcium ionophore (CI) and acetylcholine (ACh). The effects of nebivolol on the bioavailability of NO and ONOO, indicators of endothelial function and dysfunction, respectively, were measured in vitro using nanosensors placed in mesenteric arteries. Compared with WKY rats, treatment of SHR vessels either with ACh (1 micromol/L) or CI (1 micromol/L) showed marked deficiencies (>40%, P < 0.01) in bioavailable NO concomitant with increased ONOO levels (>50%, P < 0.01). The [NO]/[ONOO] ratio measured after stimulation with CI was 2.77 +/- 0.05 in WKY rats and much lower (1.14 +/- 0.11) in SHR indicating significant eNOS uncoupling and endothelial dysfunction in hypertensive animals. Treatment with nebivolol (10 micromol/L) inhibited eNOS uncoupling and reduced endothelial dysfunction in SHR, as evidenced by an increase in the [NO]/[ONOO] ratio to 3.09 +/- 0.04. The basis for nebivolol activity is attributed to its unique membrane interactions as determined by small-angle x-ray diffraction, as well as its antioxidant activity at nanomolar to micromolar levels. The antioxidant effects of nebivolol and its enantiomers were not reproduced by atenolol. These results demonstrate that nebivolol inhibits endothelial dysfunction through a potent antioxidant mechanism attributed to its physicochemical interactions with the membrane, independent of beta1-blockade activity.

Rofecoxib Increases Susceptibility of Human LDL and Membrane Lipids to Oxidative Damage: A Mechanism of Cardiotoxicity

Clinical investigations have demonstrated a relationship between the extended use of rofecoxib and the increased risk for atherothrombotic events. This has led to the removal of rofecoxib from the market and concern over the cardiovascular safety of other cyclooxygenase (COX)-2 selective agents. Experimental findings from independent laboratories now indicate that the cardiotoxicity of rofecoxib may not be a class effect but because of its intrinsic chemical properties. Specifically, rofecoxib has been shown to increase the susceptibility of human low-density lipoprotein and cellular membrane lipids to oxidative modification, a contributing factor to plaque instability and thrombus formation. Independently of COX-2 inhibition, rofecoxib also promoted the nonenzymatic formation of isoprostanes and reactive aldehydes from biologic lipids. The basis for these observations is that rofecoxib alters lipid structure and readily forms a reactive maleic anhydride in the presence of oxygen. By contrast, other selective (celecoxib, valdecoxib) and nonselective (naproxen, diclofenac) inhibitors did not influence rates of low-density lipoprotein and membrane lipid oxidation. We have now further confirmed these findings by demonstrating that the prooxidant activity of rofecoxib can be blocked by the potent antioxidant astaxanthin in homochiral form (all-trans 3S, 3'S). These findings provide a mechanistic rationale for differences in cardiovascular risk among COX-selective inhibitors because of their intrinsic physicochemical properties.

Active Metabolite of Atorvastatin Inhibits Membrane Cholesterol Domain Formation by an Antioxidant Mechanism

The advanced atherosclerotic lesion is characterized by the formation of microscopic cholesterol crystals that contribute to mechanisms of inflammation and apoptotic cell death. These crystals develop from membrane cholesterol domains, a process that is accelerated under conditions of hyperlipidemia and oxidative stress. In this study, the comparative effects of hydroxymethylglutaryl-CoA (HMG-CoA) reductase inhibitors (statins) on oxidative stress-induced cholesterol domain formation were tested in model membranes containing physiologic levels of cholesterol using small angle x-ray diffraction approaches. In the absence of HMG-CoA reductase, only the atorvastatin active o-hydroxy metabolite (ATM) blocked membrane cholesterol domain formation as a function of oxidative stress. This effect of ATM is attributed to electron donation and proton stabilization mechanisms associated with its phenoxy group located in the membrane hydrocarbon core. ATM inhibited lipid peroxidation in human low density lipoprotein and phospholipid vesicles in a dose-dependent manner, unlike its parent and other statins (pravastatin, rosuvastatin, simvastatin). These findings indicate an atheroprotective effect of ATM on membrane lipid organization through a potent antioxidant mechanism.

Nebivolol reduces nitroxidative stress and restores nitric oxide bioavailability in endothelium of black Americans

BACKGROUND

Alterations in endothelial function may contribute to increased susceptibility of black Americans to cardiovascular disease. The ability to pharmacologically reverse endothelial dysfunction in blacks was tested with nebivolol, a beta1-selective agent with vasodilating and antioxidant properties.

METHODS AND RESULTS

The effects of nebivolol on endothelial nitric oxide (NO), superoxide (O2-), and peroxynitrite concentration (ONOO-) release were studied in human umbilical vein endothelial cells and iliac artery endothelial cells isolated from age-matched black and white donors. Kinetics and concentrations of NO/O2-/ONOO- were measured simultaneously with nanosensors from single cells and shown to have significant interracial differences. The rate of NO release was &5 times slower in blacks than in whites (94 versus 505 nmol . L(-1).s(-1)), whereas the rates of release were faster by &2 times for O2- and &4 times for ONOO- (22.1 versus 9.4 nmol.L(-1).s(-1) for O2- and 810 versus 209 nmol.L(-1).s(-1) for ONOO-). Pretreatment with 1.0 to 5.0 micromol/L nebivolol restored NO bioavailability in endothelial cells from black donors with concurrent reductions in O2- and ONOO- release, similar to levels in the endothelium of whites. The effects of nebivolol were dose-dependent and not observed with atenolol; similar effects were observed with apocynin, an NAD(P)H oxidase inhibitor.

CONCLUSIONS

Reduced endothelial NO bioavailability in American blacks is mainly due to excessive O2- and ONOO- generation by NAD(P)H and uncoupled endothelial NO synthase. Nebivolol decreased O2- and ONOO- concentrations and restored NO bioavailability in blacks to the level recorded in cells from whites, independently of beta1-selective blockade.

Lipid Peroxidation Induces Cholesterol Domain Formation in Model Membranes

Numerous reports have established that lipid peroxidation contributes to cell injury by altering the basic physical properties and structural organization of membrane components. Oxidative modification of polyunsaturated phospholipids has been shown, in particular, to alter the intermolecular packing, thermodynamic, and phase parameters of the membrane bilayer. In this study, the effects of oxidative stress on membrane phospholipid and sterol organization were measured using small angle x-ray diffraction approaches. Model membranes enriched in dilinoleoylphosphatidylcholine were prepared at various concentrations of cholesterol and subjected to lipid peroxidation at physiologic conditions. At cholesterol-to-phospholipid mole ratios (C/P) as low as 0.4, lipid peroxidation induced the formation of discrete, membrane-restricted cholesterol domains having a unit cell periodicity or d-space value of 34 A. The formation of cholesterol domains correlated directly with lipid hydroperoxide levels and was inhibited by treatment with vitamin E. In the absence of oxidative stress, similar cholesterol domains were observed only at C/P ratios of 1.0 or higher. In addition to changes in sterol organization, lipid peroxidation also caused reproducible changes in overall membrane structure, including a 10 A reduction in the width of the surrounding, sterol-poor membrane bilayer. These data provided direct evidence that lipid peroxidation alters the essential organization and structure of membrane lipids in a manner that may contribute to changes in membrane function during aging and oxidative stress-related disorders.

Intermolecular differences of 3-hydroxy-3-methylglutaryl coenzyme a reductase inhibitors contribute to distinct pharmacologic and pleiotropic actions

Statin drugs inhibit 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and share the common mechanism of lowering circulating levels of low-density lipoprotein (LDL) cholesterol, a powerful indicator of risk for cardiovascular disease. Large clinical trials have documented the benefit of hypolipidemic therapy for both primary and secondary prevention of coronary artery disease and stroke. Recent clinical findings, including direct comparator studies, now indicate that certain statins may slow progression of disease at a rate and to an extent that cannot be solely attributed to LDL reduction. The proposed mechanisms for such pleiotropic actions include enhancement of endothelial-dependent nitric oxide bioavailability, anti-inflammatory activity, and inhibition of oxidative stress. To understand the biochemical basis for such differences among statins, this article reviews their physicochemical properties and pharmacology at the molecular level.

Antioxidant effect of dipyridamole (DIP) and its derivative RA 25 upon lipid peroxidation and hemolysis in red blood cells

The antioxidant effects of dipyridamol (DIP), a coronary vasodilator, and its derivative RA-25 were compared in intact red blood cells (RBC) and in isolated ghost membranes. Both compounds are quite effective antioxidants in cumene hydroperoxide-induced lipid peroxidation of RBC, showing a much smaller effect for hydrogen peroxide oxidation. The antioxidant effect of DIP was considerably higher than that of RA25. For isolated ghost membranes, the apparent IC50 (the drug concentration that produces 50% inhibition of lipid peroxidation) in cumene hydroperoxide-induced peroxidation was 25 microM, while the maximum protective effect of RA-25 was around 30% in the drug concentration range of 50-100 microM. The drugs can protect the oxidative hemolysis induced by cumene hydroperoxide with a lower effect when the hemolysis is induced by H2O2. The significant antioxidant effect against damages induced by cumene hydroperoxide suggests that DIP, due to its lipophilic character, can interact with RBC membranes, and the protective effect is associated with the binding of the drug to the membrane. On the other hand, RA-25 is more hydrophilic than DIP, binds to the membrane to a smaller extent, and, for this reason, has a lower antioxidant effect.

Sulfone COX-2 inhibitors increase susceptibility of human LDL and plasma to oxidative modification: comparison to sulfonamide COX-2 inhibitors and NSAIDs

Clinical investigations have demonstrated a link between use of the sulfone cyclooxygenase-2 (COX-2) inhibitor, rofecoxib, and increased risk for atherothrombotic events. This increased risk was not observed for a sulfonamide COX-2 inhibitor (celecoxib), indicating a potential non-enzymatic mechanism for rofexocib. To test this hypothesis, we compared the independent effects of COX-2 inhibitors on human LDL oxidation, an important contributor to atherosclerotic cardiovascular disease. The results showed that rofecoxib (100 nM) significantly decreased (>40%, p<0.001) the lag time for LDL conjugated diene formation and increased levels of thiobarbituric-acid-reactive-substances (TBARS) in vitro. The pro-oxidant activity of rofecoxib was dose-dependent and attenuated by 70% (p<0.001) with the antioxidant, Trolox. Rofecoxib and etoricoxib (100 nM) also caused a marked increase (>35%, p<0.001) in non-enzymatic generation of isoprostanes, as measured by mass spectroscopy. Addition of rofecoxib to fresh human plasma reduced the oxygen radical antioxidant capacity (ORAC) by 34% (p<0.0001). By contrast, other selective (celecoxib, valdecoxib, meloxicam) and non-selective COX inhibitors (ibuprofen, naproxen, diclofenac) had no significant effect on LDL oxidation rates or plasma ORAC values, even at suprapharmacologic levels. X-ray diffraction analysis showed that sulfone COX-2 inhibitors interact differently with membrane phospholipids, suggesting a physico-chemical basis for the pro-oxidant activity. These results demonstrate that sulfone COX-2 inhibitors increase the susceptibility of biological lipids to oxidative modification through a non-enzymatic process. These findings may provide mechanistic insight into reported differences in cardiovascular risk for COX-2 inhibitors.

Serum levels of thiobarbituric acid reactive substances predict cardiovascular events in patients with stable coronary artery disease

OBJECTIVES

The objective of this study was to test the predictive value of an oxidative stress biomarker in 634 patients from the Prospective Randomized Evaluation of the Vascular Effects of Norvasc Trial (PREVENT).

BACKGROUND

Oxidative stress contributes to mechanisms of atherosclerosis and plaque instability. Biomarkers of oxidation, such as malondialdehyde (MDA), may represent independent indicators of risk for patients with stable coronary artery disease (CAD).

METHODS

Serum MDA levels were measured as thiobarbituric acid reactive substances (TBARS) in 634 patients with documented CAD using reverse-phase high-performance liquid chromatography and spectrophotometric approaches.

RESULTS

During the three-year study, there were 51 major vascular events such as fatal/nonfatal myocardial infarction, 149 hospitalizations for nonfatal vascular events, and 139 patients underwent a major vascular procedure. At baseline, patients with TBARS levels in the highest quartile had a relative risk (RR) of 3.30 (95% confidence interval [CI] 1.47 to 7.42; p = 0.038) for major vascular events, RR of 4.10 (95% CI 2.55 to 6.60; p < 0.0001) for nonfatal vascular events, and RR of 3.84 (95% CI 2.56 to 5.76; p < 0.0001) for major vascular procedures. The effect of TBARS on events and procedures was also seen in a multivariate model adjusted for inflammatory markers (C-reactive protein, soluble intercellular adhesion molecule-1, interleukin-6), and other risk factors (age, low-density lipoprotein, high-density lipoprotein, total cholesterol, triglycerides, body mass index, and blood pressure). This analysis showed an independent effect of TBARS on major vascular events (p = 0.0149), nonfatal vascular events (p < 0.0001), major vascular procedures (p < 0.001), and all vascular events and procedures (p < 0.0001).

CONCLUSIONS

Serum levels of TBARS were strongly predictive of cardiovascular events in patients with stable CAD, independently of traditional risk factors and inflammatory markers.

The antioxidant effects of statins

Oxidative stress contributes to the initiation and the development of atherosclerotic plaques and adversely influences myocardial integrity. Statins interfere with oxidation in several ways that may contribute to reducing the atherogenic process. In addition to direct antioxidant effects, statins reduce circulating oxidized low-density lipoproteins (oxLDL) and inhibit their uptake by macrophages. They also reduce circulating markers of oxidation such as F2-isoprostane and nitrotyrosine. Statins inhibit oxidant enzymes activity such as that of reduced nicotinamide adenine dinucleotide phosphate (NAD[P]H) oxidase and myeloperoxidase and up-regulate the activity of antioxidant enzymes such as catalase and paraoxonase. They reduce endothelial dysfunction mainly by their ability to enhance endothelial nitric oxide bioavailability, which is achieved by several mechanisms. The antioxidant properties of statins extend to organ protection especially the myocardium and the lungs.

Effects of HMG-CoA reductase inhibitors on endothelial function: role of microdomains and oxidative stress

Certain pleiotropic activities reported for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are related to reductions in cellular cholesterol biosynthesis and isoprenoid levels. In endothelial cells, these metabolic changes contribute to favorable effects on nitric oxide (NO) bioavailability. Given the essential role of NO in preserving vascular structure and function, this effect of statins is of considerable therapeutic importance. Statins have been demonstrated to restore endothelial NO production by several mechanisms, including upregulating endothelial NO synthase (eNOS) protein expression and blocking formation of reactive oxygen species. In this article, we will discuss additional ways in which statins restore endothelial NO production and improve endothelial function. (1) Statins modulate membrane microdomain formation, resulting in reduced expression of proteins that specifically inhibit eNOS activation. (2) Statins reduce sterol biosynthesis, thus interfering with the formation of pathologic microdomains, including cholesterol crystalline structures. This observation has important implications for plaque stabilization, as these microdomains contribute to cholesterol crystal formation and endothelial apoptosis. Finally, (3) statins improve endothelial function by interfering with oxidative stress pathways through both enzymatic and nonenzymatic mechanisms. The relationships between membrane microdomains, cholesterol biosynthesis, and endothelial function will be discussed.

Direct evidence for cholesterol crystalline domains in biological membranes: role in human pathobiology

This review will discuss the use of small-angle X-ray diffraction approaches to study the organization of lipids in plasma membranes derived from two distinct mammalian cell types: arterial smooth muscle cells and ocular lens fiber cells. These studies indicate that cholesterol at an elevated concentration can self-associate and form immiscible domains in the plasma membrane, a phenomenon that contributes to both physiologic and pathologic cellular processes, depending on tissue source. In plasma membrane samples isolated from atherosclerotic smooth muscle cells, the formation of sterol-rich domains is associated with loss of normal cell function, including ion transport activity and control of cell replication. Analysis of meridional diffraction patterns from intact and reconstituted plasma membrane samples indicates the presence of an immiscible cholesterol domain with a unit cell periodicity of 34 A, consistent with a cholesterol monohydrate tail-to-tail bilayer, under disease conditions. These cholesterol domains were observed in smooth muscle cells enriched with cholesterol in vitro as well as from cells obtained ex vivo from an animal model of atherosclerosis. By contrast, well-defined cholesterol domains appear to be essential to the normal physiology of fiber cell plasma membranes of the human ocular lens. The organization of cholesterol into separate domains underlies the role of lens fiber cell plasma membranes in maintaining lens transparency. These domains may also interfere with cataractogenic aggregation of soluble lens proteins at the membrane surface. Taken together, these analyses provide examples of both physiologic and pathologic roles that sterol-rich domains may have in mammalian plasma membranes. These findings support a model of the membrane in which cholesterol aggregates into structurally distinct regions that regulate the function of the cell membrane.

Evidence for distinct cholesterol domains in fiber cell membranes from cataractous human lenses

Previous studies in our laboratory have provided direct evidence for the existence of distinct cholesterol domains within the plasma membranes of human ocular lens fiber cells. The fiber cell plasma membrane is unique in that it contains unusually high concentrations of cholesterol, with cholesterol to phospholipid (C/P) mole ratios ranging from 1 to 4. Since membrane cholesterol content is disturbed in the development of cataracts, it was hypothesized that perturbation of cholesterol domain structure occurs in cataracts. In this study, fiber cell plasma membranes were isolated from both normal (control) and cataractous lenses and assayed for cholesterol and phospholipid. Control and cataractous whole lens membranes had C/P mole ratios of 3.1 and 1.7, respectively. Small angle x-ray diffraction approaches were used to directly examine the structural organization of the cataractous lens plasma membrane versus control. Both normal and cataractous oriented membranes yielded meridional diffraction peaks corresponding to a unit cell periodicity of 34.0 A, consistent with the presence of immiscible cholesterol domains. However, comparison of diffraction patterns indicated that cataractous lens membranes contained more pronounced and better defined cholesterol domains than controls, over a broad range of temperature (5-40 degrees C) and relative humidity (52-92%) levels. In addition, diffraction analyses of the sterol-poor regions of cataractous membranes indicated increased membrane rigidity as compared with control membranes. Modification of the membrane lipid environment, such as by oxidative insult, is believed to be one potential mechanism for the formation of highly resolved cholesterol domains despite significantly reduced cholesterol content. The results of this x-ray diffraction study provide evidence for fundamental changes in the lens fiber cell plasma membrane structure in cataracts, including the presence of more prominent and highly ordered, immiscible cholesterol domains.

Antioxidant activity of the monoamine oxidase B inhibitor lazabemide

Free radical-induced damage to lipid and protein constituents of neuronal membranes contributes to the pathophysiology of neurodegenerative diseases, including Alzheimer's disease (AD). The development of an effective inhibitor of oxidative stress represents an important goal for the treatment of AD. In this study, the intrinsic antioxidant activity of lazabemide, a potent and reversible inhibitor of monoamine oxidase B (MAO-B), was tested in a membrane-based model of oxidative stress. Under physiologic-like conditions, lazabemide inhibited lipid peroxidation in a highly concentration-dependent manner. At low, pharmacologic levels of lazabemide (100.0 nM), there was a significant (P < 0.001) and catalytic reduction in lipid peroxide formation, as compared with control samples. The antioxidant activity of lazabemide was significantly more effective than that of either vitamin E or the MAO-B inhibitor, selegiline. The ability of lazabemide to inhibit oxidative damage is attributed to physico-chemical interactions with the membrane lipid bilayer, as determined by small angle x-ray diffraction methods. By partitioning into the membrane hydrocarbon core, lazabemide can inhibit the propagation of free radicals by electron-donating and resonance-stabilization mechanisms. These findings indicate that lazabemide is a potent and concentration-dependent inhibitor of membrane oxy-radical damage as a result of inhibiting membrane lipid peroxidation, independent of MAO-B interactions.

Hierarchy of research design used to categorize the "strength of evidence" in answering clinical dental questions

The purpose of this article is to highlight important features of research design that clinicians can use to determine which articles are useful when attempting to answer clinical questions and determine the best therapy for a particular patient. This article offers a systematic means of categorizing the quality of research reports for clinicians and clinical investigators. A recurring clinical theme of hygiene education is used to exemplify how phrasing the clinical question determines the type of study design that could be used. The article describes the continuum of research reports, and categorizes them by their inherent strengths and weaknesses. The report describes why the research designs in the supreme position of the research hierarchy, are the most valuable to clinicians seeking evidence that defines the best therapy for their patients.

Direct evidence for immiscible cholesterol domains in human ocular lens fiber cell plasma membranes

The molecular structure of human ocular lens fiber cell plasma membranes was examined directly using small angle x-ray diffraction approaches. A distinct biochemical feature of these membranes is their high relative levels of free cholesterol; the mole ratio of cholesterol to phospholipid (C/P) measured in these membranes ranges from 1 to 4. The organization of cholesterol in this membrane system is not well understood, however. In this study, the structure of plasma membrane samples isolated from nuclear (3.3 C/P) and cortical (2.4 C/P) regions of human lenses was evaluated with x-ray diffraction approaches. Meridional diffraction patterns obtained from the oriented membrane samples demonstrated the presence of an immiscible cholesterol domain with a unit cell periodicity of 34.0 A, consistent with a cholesterol monohydrate bilayer. The dimensions of the sterol-rich domains remained constant over a broad range of temperatures (5-20 degrees C) and relative humidity levels (31-97%). In contrast, dimensions of the surrounding sterol-poor phase were significantly affected by experimental conditions. Similar structural features were observed in membranes reconstituted from fiber cell plasma membrane lipid extracts. The results of this study indicate that the lens fiber cell plasma membrane is a complex structure consisting of separate sterol-rich and -poor domains. Maintenance of these separate domains may be required for the normal function of lens fiber cell plasma membrane and may interfere with the cataractogenic aggregation of soluble lens proteins at the membrane surface.