Isoleukotrienes are biologically active free radical products of lipid peroxidation

The isoprostanes: a perspective

The isoprostanes are a new class of natural products produced in vivo by a non-enzymatic free-radical-induced peroxidation of polyunsaturated fatty acid. In the case of arachidonic acid, for example, four classes of isoprostanes can be produced. Because of the specific structural features distinguishing them from other free-radical-generated products, e.g., HETEs, etc., the isoprostanes can provide an exclusive and selective index for the oxidant component of several inflammatory and degenerative diseases. The possible mechanisms of formation of the individual isoprostanes is discussed in detail. Class III products, such as 8-iso-PGF2 alpha and 8-iso-PGE2 have been shown to be vasoconstrictors and modulate platelet function. Several synthetic representatives from the four classes of arachidonic-acid-derived isoprostanes have already been prepared by total synthesis. These synthetic standards have been used for the identification and quantitation of these isoprostanes in biological fluids using gas chromatography/mass spectrometry methodology.

Increased formation of the isoprostanes IPF2alpha-I and 8-epi-prostaglandin F2alpha in acute coronary angioplasty: evidence for oxidant stress during coronary reperfusion in humans

BACKGROUND

The role of oxidant stress in cardiac ischemia/reperfusion injury in humans remains controversial. This is due, in part, to the limitations of available indices of oxidant stress in vivo. Isoprostanes are stable, free radical-catalyzed products of arachidonic acid. We assessed their formation in patients undergoing coronary reperfusion via percutaneous transluminal coronary angioplasty (PTCA).

METHODS AND RESULTS

We developed specific, mass spectrometry assays for two structurally distinct F2 isoprostanes, 8-epi-PGF2alpha and IPF2alpha-I. Urine samples for isoprostane determination were collected in patients undergoing coronary arteriography (n=11), elective PTCA (n=15), and angiography after thrombolysis for acute myocardial infarction (MI) (n=10). Urinary levels (pmol/mmol creatinine) of both isoprostanes were markedly increased from baseline in the first 6 hours after PTCA for acute MI (105+/-17.8 versus 230+/-66 for 8-epi-PGF2alpha [P=.009] and 466+/-91 versus 833+/-153 for IPF2alpha-I [P=.001]) and returned toward preprocedural values by 24 hours (122+/-18 for 8-epi-PGF2alpha and 457+/-102 for IPF2alpha-I). There was a slight increase in urinary 8-epi-PGF2alpha levels (64.7+/-9.5 versus 84.9+/-10.6; P=.02) after diagnostic coronary arteriography and elective PTCA (88.7+/-7.5 versus 114.3+/-16.1; P=.01). A striking correlation was observed (r=.68, P<.0001; n=33) between urinary 8-epi-PGF2alpha and IPF2alpha-I levels in patients receiving thrombolytic agents for acute MI.

CONCLUSIONS

Urinary F2 isoprostane levels are elevated in patients after treatments resulting in reperfusion for acute MI. These findings provide evidence consistent with increased oxidant stress in vivo in this setting. Measurement of urinary isoprostanes may offer a noninvasive approach to the assessment of oxidant stress and the efficacy of antioxidant therapies in these syndromes.

Evidence for the formation of F3-isoprostanes during peroxidation of eicosapentaenoic acid

8-Epi PGF2alpha, a potent vasocontrictor, is a specific product of non-enzymatic peroxidation of arachidonic acid. It seems likely that similar products could arise from other polyunsaturated fatty acids (PUFAs) and might be useful biomarkers of their peroxidation in vivo. This was investigated using eicosapentaenoic acid (EPA). EPA liposomes (1 mg/ml PBS) were exposed at 37 degrees C to either 2,2'-azobis-(2-amidinopropane) dichloride (AAPH) or copper ions at final concentrations of 1 mM and 10 microM, respectively. Sample processing involved solid-phase extraction on a C18-followed by an NH2 cartridge. After conversion to pentafluorobenzyl ester/trimethylsilyl derivatives, F3-isoprostanes were analysed by negative ion-chemical ionisation mass spectrometry (GC-MS/NICI) using tetradeuterated PGF2alpha (PGF2-d4) as the internal standard. Quantitative analysis was carried out by selected ion monitoring of the carboxylated anion [M-180] at m/z 567 and 573 for the PGF3-like compounds and PGF2-d4, respectively. EPA oxidised by AAPH or by copper ions gave rise to a family of F3-isoprostanes with 8-epi PGF3alpha as a minor product. Formation of F3-isoprostanes correlated well with other indices of lipid peroxidation (hydroperoxides and thiobarbituric acid reactive substances). The possibility of analysing specific lipid peroxidation products from individual fatty acids should facilitate nutritional and biomedical studies.

8-epi PGF2 alpha generation during coronary reperfusion. A potential quantitative marker of oxidant stress in vivo

BACKGROUND

Myocardial reperfusion is believed to be associated with free radical injury. However, indexes of oxidative stress in vivo have been limited by their poor specificity and sensitivity. Isoprostanes are stable products of arachidonic acid formed in a nonenzymatic, free radical-catalyzed manner. We have developed a sensitive and specific assay for one of these compounds, 8-epi prostaglandin (PG) F2 alpha.

METHODS AND RESULTS

To address its utility as an index of oxidative stress during coronary reperfusion, we measured urinary levels by gas chromatography/mass spectrometry in a canine model of coronary thrombolysis, in patients with acute myocardial infarction treated with thrombolytic therapy, and in patients after elective coronary artery bypass surgery. Urinary 8-epi PGF2 alpha was unchanged after circumflex artery occlusion in a canine model of coronary thrombolysis (n = 13; 437.2 +/- 56.4 versus 432.7 +/- 55.2 pmol/mmol creatinine) but increased significantly (P < .05) immediately after reperfusion (553.8 +/- 64.7 pmol/mmol). Urinary levels were increased (P < .001) in patients (n = 12) with acute myocardial infarction given lytic therapy (265.8 +/- 40.8 pmol/mmol) compared with age-matched control subjects (n = 20; 91.5 +/- 11.8 pmol/mmol) and patients with stable coronary disease (n = 20; 95.7 +/- 6.3 pmol/mmol). Preoperative levels rose from 113.2 +/- 11.8 to 248.2 +/- 86.3 pmol/mmol at 30 minutes into revascularization to 332.2 +/- 82.6 pmol/mmol by 15 minutes after global myocardial reperfusion (P < .05) and dropped to 181.2 +/- 50.4 pmol/mmol at 30 minutes and 120.2 +/- 9.9 pmol/mmol at 24 hours after bypass surgery (n = 5). Corresponding changes in spin adduct formation, found with electron paramagnetic resonance, were noted in 2 patients.

CONCLUSIONS

These data support the hypothesis that free radical generation occurs during myocardial reperfusion. Measurement of isoprostane production may serve as a noninvasive index of oxidative stress.

Modulation of oxidant stress in vivo in chronic cigarette smokers

BACKGROUND

Free radical-induced oxidative damage is thought to be involved in the pathogenesis of diseases associated with cigarette smoking. We examined the production of 8-epi-prostaglandin (PG) F2 alpha, a stable product of lipid peroxidation in vivo, and its modulation by aspirin and antioxidant vitamins in chronic cigarette smokers.

METHODS AND RESULTS

We performed the following studies: (1) a cross-sectional comparison of smokers and control subjects, (2) an examination of the dose-response relationship, (3) an exploration of the effect of smoking cessation (3 weeks) and nicotine patch supplementation, (4) the effect of aspirin consumption, and (5) the effects of 5 days' dosing with vitamin E (100 and 800 U), vitamin C (2 g), and their combination. 8-epi-PGF2 alpha excretion (in pmol/mmol, mean +/- SEM) was 176.5+/-30.6 in heavy smokers, 92.7+/-4.8 (P<.05) in moderate smokers, and 54.1+/-2.7 (P<.005) in nonsmokers. Urinary levels fell from 145.5+/-24.9 to 114.6+/-27.1 (week 2, P<.05) and 112.6+/-24.9 (week 3, P<.05) on cessation of smoking. Aspirin treatment failed to suppress urinary levels of 8-epi-PGF2 alpha despite a significant reduction in urinary 11-dehydro-TxB2 production and suppression of 8-epi-PGF2 alpha and TxB2 in serum. Vitamin C (pre, 194.6+/-40.9; post, 137.2+/-34.1; P<.05) and a combination of vitamin C and E (pre, 171.0+/-39.8; post, 133.5+/-29.6 P<.05) suppressed urinary 8-epi-PGF2 alpha, whereas vitamin E alone had no effect.

CONCLUSIONS

Urinary 8-epi-PGF2 alpha may represent a noninvasive, quantitative index of oxidant stress in vivo. Elevated levels of 8-epi-PGF2 alpha in smokers may be modulated by quitting cigarettes and switching to nicotine patches or by antioxidant vitamin therapy.

Local amplification of platelet function by 8-Epi prostaglandin F2alpha is not mediated by thromboxane receptor isoforms

8-epi-Prostaglandin (PG) F2alpha may be formed by cyclooxygenases 1 and 2 or by a free radical catalyzed process as an isoprostane. Concentrations of 8-epi-PGF2alpha in the range 1 nM to 1 microM induce a dose-dependent increase in platelet shape change, in calcium release from intracellular stores [Ca2+]iand in inositol phosphates; it also causes irreversible platelet aggregation, dependent on thromboxane generation, when incubated with subthreshold concentrations of ADP, thrombin, collagen, and arachidonic acid. Much higher concentrations of 8-epi-PGF2alpha (10-20 microM) alone induce weak, reversible aggregation. Although these effects are prevented by pharmacological thromboxane receptor antagonists, they are unlikely to be mediated by thromboxane receptors. Thus, 8-epi-PGF2alpha does not compete for binding at the stably expressed placental or endothelial isoforms of the thromboxane receptor or for binding of thromboxane ligands to human platelets. Furthermore, the response to 8-epi PGF2alpha exhibits structural specificity versus 8-epi PGF3alpha and PGF2alpha. Concentrations in the range that evoke its effects on platelets do not desensitize the aggregation response stimulated by thromboxane or PGH2 analogs. Unlike primary prostaglandins, which are rapidly metabolized to inactive products, 8-epi PGF2alpha circulates in plasma. However, the systemic concentrations found in healthy volunteers (median 48 pmol/liter) and in patients with hepatic cirrhosis (median 147 pmol/liter), a syndrome of oxidant stress in vivo, fall well below those which modulate platelet function. 8-Epi PGF2alpha may amplify the response to platelet agonists in syndromes where oxidant stress and platelet activation coincide. Despite blockade by thromboxane antagonists, 8-epi PGF2alpha does not activate either of the thromboxane receptor isoforms described in platelets. Activation of a distinct receptor would be consistent with the enzymatic formation of 8-epi PGF2alpha by cyclooxygenases. However, incidental activation of such a receptor by systemic concentrations of 8-epi PGF2alpha is unlikely to occur, even in syndromes of excessive free radical generation in vivo.

Metabolism of 6-trans-isomers of leukotriene B4 in cultured hepatoma cells and in human polymorphonuclear leukocytes. Identification of a delta 6-reductase metabolic pathway

The intermediate metabolic events which degrade hydroxy polyunsaturated fatty acids is largely unknown. Such molecules are common products of lipid peroxidation and lipoxygenase catalyzed oxidation of arachidonic acid. Metabolism of two 5,12-dihydroxyeicosatetraenoic acids, 6-trans-LTB4 (leukotriene B4), and 6-trans-12-epi-LTB4 was studied in HepG2 cells (a human-derived hepatoma cell line). Extensive metabolism was observed with a major metabolite identified as 4-hydroxy-6-dodecenoic acid for both epimers. Incubation of 6-trans-LTB4 epimers at shorter times revealed the formation of intermediate metabolites, including 6-hydroxy-4,8-tetradecadienoic acid and 8-hydroxy-4,6,10-hexadecatrienoic acid suggesting beta-oxidation as the major pathway leading to the formation of the common terminal metabolite. Two additional metabolites were structurally elucidated as 5-oxo-6,7-dihydro-LTB4 and 6,7-dihydro-LTB4 which have not been previously described. Formation of 5-oxo-6,7-dihydro-LTB4 and 6,7-dihydro-LTB4 were also observed during metabolism of 6-trans-12-epi-LTB4 in human polymorphonuclear leukocytes. Of particular interest is the metabolism of these compounds by beta-oxidation from the carboxyl terminus, a process which is not observed with leukotriene B4 or leukotriene C4. Identification of these metabolites suggested the operation of the 5-hydroxyeicosanoid dehydrogenase pathway followed by a delta 6-reductase metabolic pathway which has not been previously described. This pathway of beta-oxidation may limit the activity of various 5,12-diHETEs including nonenzymatic hydrolysis products of LTA4 and also the recently described B4-isoleukotrienes.