2,3-Dinor-5,6-dihydro-15-F(2t)-isoprostane: a bioactive prostanoid metabolite

Association of Thromboxane Generation With Survival in Aspirin Users and Nonusers

BACKGROUND

Persistent systemic thromboxane generation, predominantly from nonplatelet sources, in aspirin (ASA) users with cardiovascular disease (CVD) is a mortality risk factor.

OBJECTIVES

This study sought to determine the mortality risk associated with systemic thromboxane generation in an unselected population irrespective of ASA use.

METHODS

Stable thromboxane B₂ metabolites (TXB₂-M) were measured by enzyme-linked immunosorbent assay in banked urine from 3,044 participants (mean age 66 ± 9 years, 53.8% women) in the Framingham Heart Study. The association of TXB₂-M to survival over a median observation period of 11.9 years (IQR: 10.6-12.7 years) was determined by multivariable modeling.

RESULTS

In 1,363 (44.8%) participants taking ASA at the index examination, median TXB₂-M were lower than in ASA nonusers (1,147 pg/mg creatinine vs 4,179 pg/mg creatinine; P < 0.0001). TXB₂-M were significantly associated with all-cause and cardiovascular mortality irrespective of ASA use (HR: 1.96 and 2.41, respectively; P < 0.0001 for both) for TXB₂-M in the highest quartile based on ASA use compared with lower quartiles, and remained significant after adjustment for mortality risk factors for similarly aged individuals (HR: 1.49 and 1.82, respectively; P ≤ 0.005 for both). In 2,353 participants without CVD, TXB₂-M were associated with cardiovascular mortality in ASA nonusers (adjusted HR: 3.04; 95% CI: 1.29-7.16) but not in ASA users, while ASA use was associated with all-cause mortality in those with low (adjusted HR: 1.46; 95% CI: 1.14-1.87) but not elevated TXB₂-M.

CONCLUSIONS

Systemic thromboxane generation is an independent risk factor for all-cause and cardiovascular mortality irrespective of ASA use, and its measurement may be useful for therapy modification, particularly in those without CVD.

The effects of nano-sized PbO on biomarkers of membrane disruption and DNA damage in a sub-chronic inhalation study on mice

Although the production of engineered nanoparticles increases our knowledge of toxicity and mechanisms of bioactivity during relevant exposures is lacking. In the present study mice were exposed to PbO nanoparticles (PbONP; 192.5 µg/m³; 1.93 × 10⁶ particles/cm³) for 2, 5 and 13 weeks through continuous inhalation. The analyses addressed Pb and PbONP distribution in organs (lung, liver, kidney, brain) using electrothermal atomic absorption spectrometry and transmission electron microscopy, as well as histopathology and analyses of oxidative stress biomarkers. New LC-MS/MS methods were validated for biomarkers of lipid damage F2-isoprostanes (8-iso-prostaglandins F_2-alpha and E₂) and hydroxylated deoxoguanosine (8-OHdG, marker of DNA oxidation). Commonly studied malondialdehyde was also measured as TBARS by HPLC-DAD. The study revealed fast blood transport and distribution of Pb from the lung to the kidney and liver. A different Pb accumulation trend was observed in the brain, suggesting transfer of NP along the nasal nerve to the olfactory bulbs. Long-term inhalation of PbONP caused lipid peroxidation in animal brains (increased levels of TBARS and both isoprostanes). Membrane lipid damage was also detected in the kidney after shorter exposures, but not in the liver or lung. On the contrary, longer exposures to PbONP increased levels of 8-OHdG in the lung and temporarily increased lung weight after 2 and 5 weeks of exposure. The histopathological changes observed mainly in the lung and liver indicated inflammation and general toxicity responses. The present long-term inhalation study indicates risks of PbONP to both human health and the environment.

Impact of vascular thromboxane prostanoid receptor activation on hemostasis, thrombosis, oxidative stress, and inflammation

The activation of thromboxane prostanoid (TP) receptor on platelets, monocytes/macrophages, endothelial cells, and vascular smooth muscle cells (SMC) plays important roles in regulating platelet activation and vascular tone and in the pathogenesis of thrombosis and vascular inflammation. Oxidative stress and vascular inflammation increase the formation of TP receptor agonists, which promote initiation and progression of atherogenesis and thrombosis. Furthermore, TP receptor activation promotes angiogenesis and vessel wall constriction. Besides thromboxane A₂ and its endoperoxide precursors, prostaglandin G₂ and H₂, isoprostanes, and 20-hydroxyeicosatetraenoic acid also activate TP receptor as autocrine or paracrine ligands. These additional TP activators play a role in pathological conditions such as diabetes, obesity, and hypertension, and their biosynthesis is not inhibited by aspirin, at variance with that of thromboxane A₂. The understanding of TP receptor function increased our current knowledge of the pathogenesis of atherosclerosis and thrombosis, highlighting the great impact that this receptor has in cardiovascular disorders.

Systematic review on the association between F2-isoprostanes and cardiovascular disease

Background

Oxidative stress may play an aetiological role in the development and progression of cardiovascular disease (CVD). However, evidence on its biochemical markers has been controversial. This article aimed to assess the role of F2-isoprostanes, a marker for measuring in vivo lipid oxidation, as a biomarker for CVD, including coronary artery disease, stroke and peripheral artery disease.

Methods

A literature search was performed using PubMed and EMBASE (from 1966 to February 2012). Studies that investigated the association between F2-isoprostanes and CVD were eligible.

Results

Of the 22 eligible studies retrieved, 20 studies showed a significant association between F2-isoprostanes and CVD. However, to date, there have been only four population-based studies, with one study reporting null association. Although data from prospective studies are ideal to examine a role of such biomarkers in predicting future CVD events, only two studies were prospective. In addition, differences in population characteristics, sample handling/storage and assays, coupled with a lack of confounding adjustment, may all contribute to the enormous variation in previous studies.

Conclusions

High levels of F2-isoprostanes in urine or blood may be a non-specific indicator of CVD. However, further population-based studies are needed. In addition, multivariable analyses are required for future studies to control confounding and improve classification accuracy.

Effects of aspirin responsiveness and platelet reactivity on early vein graft thrombosis after coronary artery bypass graft surgery

OBJECTIVES

The purpose of this study was to determine if an incomplete response to or inadequate antiplatelet effect of aspirin, or both, contribute to saphenous vein graft (SVG) occlusion after coronary artery bypass graft (CABG) surgery.

BACKGROUND

Thrombosis is the predominant cause of early SVG occlusion. Aspirin, which inhibits cyclooxygenase-1 activity and thromboxane generation in platelets, reduces early SVG occlusion by one-half.

METHODS

Aspirin responsiveness and platelet reactivity were characterized 3 days and 6 months after coronary artery bypass graft surgery in 229 subjects receiving aspirin monotherapy by platelet aggregation to arachidonic acid, adenosine diphosphate, collagen and epinephrine, Platelet Function Analyzer-100 (Siemens Healthcare Diagnostics, Newark, Delaware) closure time (CT) using collagen/epinephrine agonist cartridge and collagen/adenosine diphosphate (CADP) agonist cartridge, VerifyNow Aspirin assay (Accumetrics, Inc., San Diego, California), and urine levels of 11-dehydro-thromboxane B(2) (UTXB(2)). SVG patency was determined 6 months after surgery by computed tomography coronary angiography.

RESULTS

Inhibited arachidonic acid-induced platelet aggregation, indicative of aspirin-mediated cyclooxygenase-1 suppression, occurred in 95% and >99% of subjects 3 days and 6 months after surgery, respectively. Despite this, 73% and 31% of subjects at these times had elevated UTXB(2). Among tested parameters, only UTXB(2) and CADP CT measured 6 months after surgery correlated with outcome. By multivariate analysis, CADP CT of ≤88 s (odds ratio: 2.85, p = 0.006), target vessel diameter of ≤1.5 mm (odds ratio: 2.38, p = 0.01), and UTXB(2) of ≥450 pg/mg creatinine (odds ratio: 2.59, p = 0.015) correlated with SVG occlusion. CADP CT and UTXB(2) in combination further identified subjects at particularly high and low risk for SVG occlusion.

CONCLUSIONS

Aspirin-insensitive thromboxane generation measured by UTXB(2) and shear-dependent platelet hyper-reactivity measured by Platelet Function Analyzer-100 CADP CT are novel independent risk factors for early SVG thrombosis after coronary artery bypass graft surgery.

Measurement of 8-iso-prostaglandin F2alpha in biological fluids as a measure of lipid peroxidation

Several lines of evidence suggest that reactive oxygen species are implicated in human disease, including atherosclerosis, hypertension, and restenosis after angioplasty. The measurement of F(2)-isoprostanes (F(2)-iPs), formed nonenzymatically through free radical catalyzed attack on esterified arachidonate, provides a reliable tool for identifying populations with enhanced rates of lipid peroxidation. Among F(2)-isoPs, 8-iso-PGF(2alpha) (also referred to IPF(2alpha)-III) and IPF(2alpha)-VI are the most frequently measured in biological fluids. A variety of methods have been proposed to measure F(2)-isoprostanes in urine and plasma. Mass spectrometry has been developed for the measurement of both F(2)-isoprostanes but its use is limited as it is time-consuming and highly expensive. We have developed validated enzyme immunoassay (EIA) and radioimmunoassay (RIA) techniques using highly specific antisera for the measurement of 8-iso-PGF(2alpha). In contrast, the commercially available immunoassay kits are limited for their poor specificity. The measurement of specific isoprostanes, such as 8-iso-PGF(2alpha), in urine is a reliable, noninvasive index of lipid peroxidation that is of valuable help in dose-finding studies of natural and synthetic antioxidant agents.

Biologically active oxidized lipids (phytoprostanes) in the plant diet and parenteral lipid nutrition

Phytoprostanes (PP) are autoxidation products of alpha-linolenate that are present in all plant tissues. Several classes of PP with a prostaglandin (PG) F1-, E1-, A1- and B1-like structure were identified and quantified by gas chromatography-mass spectrometry in vegetable oils and parenteral nutrition (intralipid). High levels of PP (0.09 up to 99 mg/l) were found even in apparently fresh vegetable oils. After oral consumption of olive or soybean oil, PPF1 were absorbed, found to circulate in plasma in conjugated form and excreted in free form into urine. Evidence is emerging that certain PP, such as the PPE1, may modulate the function of immune cells in a PG-like fashion. Here, we show that PPA1- and deoxy-PPJ1 display potent anti-inflammatory and apoptosis inducing activities similar to PGA1 and deoxy-PGJ2. Results of this study indicate that PP are novel, biologically active lipids in plant nutrition.

F(2)-isoprostanes as novel biomarkers for type 2 diabetes: a review

Oxidative stress (OS) has been implicated as one of the major underlying mechanisms behind many acute and chronic diseases. However, the measurement of free radicals or their end products is complicated. Isoprostanes, derived from the non-enzymatic peroxidation of arachidonic acid are now considered to be reliable biomarkers of oxidant stress in the human body. Isoprostanes are involved in many of the human diseases such as type 2 diabetes. In type 2 diabetes elevated levels of F₂-Isoprostanes (F₂-IsoPs) have been observed. The measurement of bioactive F₂-IsoPs levels offers a unique noninvasive analytical tool to study the role of free radicals in physiology, oxidative stress-related diseases, and acute or chronic inflammatory conditions. Measurement of oxidative stress by various other methods lacks specificity and sensitivity. This review aims to shed light on the implemention of F₂-IsoPs measurement as a gold-standard biomarker of oxidative stress in type 2 diabetics.

Vitamin C in plasma is inversely related to blood pressure and change in blood pressure during the previous year in young Black and White women

Background

The prevalence of hypertension and its contribution to cardiovascular disease risk makes it imperative to identify factors that may help prevent this disorder. Extensive biological and biochemical data suggest that plasma ascorbic acid may be such a factor. In this study we examined the association between plasma ascorbic acid concentration and blood pressure (BP) in young-adult women.

Methods

Participants were 242 Black and White women aged 18–21 yr from the Richmond, CA, cohort of the National Heart, Lung and Blood Institute Growth and Health Study. We examined the associations of plasma ascorbic acid with BP at follow-up year 10, and with change in BP during the previous year.

Results

In cross-sectional analysis, plasma ascorbic acid at year 10 was inversely associated with systolic BP and diastolic BP after adjusting for race, body mass index, education, and dietary intake of fat and sodium. Persons in the highest one-fourth of the plasma ascorbic acid distribution had 4.66 mmHg lower systolic BP (95% CI 1.10 to 8.22 mmHg, p = 0.005) and 6.04 mmHg lower diastolic BP (95% CI 2.70 to 9.38 mmHg, p = 0.0002) than those in the lowest one-fourth of the distribution. In analysis of the change in BP, plasma ascorbic acid was also inversely associated with change in systolic BP and diastolic BP during the previous year. While diastolic blood pressure among persons in the lowest quartile of plasma ascorbic acid increased by 5.97 mmHg (95% CI 3.82 to 8.13 mmHg) from year 9 to year 10, those in the highest quartile of plasma vitamin C increased by only 0.23 mmHg (95% CI -1.90 to +2.36 mmHg) (test for linear trend: p < 0.0001). A similar effect was seen for change in systolic BP, p = 0.005.

Conclusion

Plasma ascorbic acid was found to be inversely associated with BP and change in BP during the prior year. The findings suggest the possibility that vitamin C may influence BP in healthy young adults. Since lower BP in young adulthood may lead to lower BP and decreased incidence of age-associated vascular events in older adults, further investigation of treatment effects of vitamin C on BP regulation in young adults is warranted.

Beyond prostaglandins--chemistry and biology of cyclic oxygenated metabolites formed by free-radical pathways from polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFAs) are important constituents in all organisms. They fulfil many functions, ranging from modulating the structure of membranes to acting as precursors of physiologically important molecules, such as the prostaglandins, which for a long time were the most prominent cyclic PUFA metabolites. However, since the beginning of the 1990s a large variety of cyclic metabolites have been discovered that form under autoxidative conditions in vivo to a much larger extent than do prostaglandins. These compounds--isoprostanes, neuroprostanes, phytoprostanes, and isofurans--proved subsequently to be ubiquitous in nature. They display a wide range of biological activities, and isoprostanes have become the currently most reliable indicators of oxidative stress in humans. In a relatively short time, the structural variety, properties, and applications of the autoxidatively formed cyclic PUFA derivatives have been uncovered.

F2-isoprostanes in human health and diseases: from molecular mechanisms to clinical implications

Oxidative stress is implicated as one of the major underlying mechanisms behind many acute and chronic diseases, and involved in normal aging. However, the measurement of free radicals or their end products is complicated. Thus, proof of association of free radicals in pathologic conditions has been absent. Isoprostanes are prostaglandin-like bioactive compounds that are biosynthesized in vivo independent of cyclooxygenases, principally through free-radical catalyzation of arachidonic acid. Isoprostanes are now considered to be reliable biomarkers of oxidative stress, as evidenced by an autonomous study organized recently by the National Institutes of Health (NIH) in the United States. A number of these compounds have potent biologic activities such as vasoconstrictive and certain inflammatory properties. Isoprostanes are involved in many human diseases. Additionally, elevated levels of F(2)-isoprostanes have been seen in normal human pregnancy and after intake of some fatty acids, but their physiologic assignments have not yet been distinctive. This evidence indicates that measurement of bioactive F(2)-isoprostanes in body fluids offers a unique noninvasive analytic utensil to study the role of free radicals in physiology, oxidative stress-related diseases, experimental acute or chronic inflammatory conditions, and also in the assessment of various antioxidants, radical scavengers, and drugs.

trans-Arachidonic acids induce a heme oxygenase-dependent vasorelaxation of cerebral microvasculature

Nitrative stress is an important regulator of vascular tone. We have recently described that trans-arachidonic acids (TAA) are major products of NO(2)(.)-mediated isomerization of arachidonic acid in cell membranes and that nitrative stress increases TAA levels leading to neural microvascular degeneration. In the present study, we explored whether TAA exert acute effects on neuromicrovascular tone and investigated potential mechanisms thereof. TAA induced an endothelium-dependent vasorelaxation of rat brain pial microvasculature. This vasorelaxation was independent of nitric oxide, prostanoids, lipoxygenase products, and CYP(450) metabolite trans-hydroxyeicosatetraenoic acids. However, inhibition of heme oxygenase (using zinc protoporphyrin IX) and of dependent soluble guanylate cyclase (sGC; using ODQ) significantly diminished (by approximately 70%) the TAA-induced vasorelaxation. Consistent with these findings, TAA stimulated heme oxygenase (HO)-2-dependent bilirubin (using siRNA HO-2) and cGMP formation, and the HO product carbon monoxide (using CO-releasing CORM-2) reproduced the sGC-dependent cGMP formation and vasorelaxation. Further exploration revealed that TAA-induced vasorelaxation and bilirubin formation (HO activation) were nearly abrogated by large-conductance calcium-dependent potassium channels (BK(Ca)) (using TEA and iberiotoxin). Opening of BK(Ca) with the selective activator NS1619 induced a concentration-dependent vasorelaxation, which was inhibited by HO and sGC inhibitors. Coimmunoprecipitation suggested a molecular complex interaction between BK(Ca) and HO-2 (but not HO-1). Collectively, these findings identify new properties of TAA, specifically cerebral vasorelaxation through interactive activation of BK(Ca) with HO-2 and, in turn, sGC. Our findings provide new insights into the characterization of nitrative stress-derived TAA products, by showing they can act as acute mediators of nitrative stress on neurovascular tone.

Pharmacological actions of isoprostane metabolites and phytoprostanes in human and bovine pulmonary smooth muscles

We examined the responses to various isoprostane derivatives in bovine/human airway and pulmonary arteries. All biological activity of 15-F(2t)-IsoP was lost in its two major metabolites (15-keto-15-F(2t)-IsoP and 13,14-dihydro-15-keto-15-F(2t)-IsoP). We also examined the effects of several metabolites of 15-F(2t)-IsoP synthesized within our own laboratory-both epimers of 2,3-dinor-15-F(2t)-IsoP and of 2,3-dinor-5,6-dihydro-15-F(2t)-IsoP, as well as 20-carboxy-2,3,4,5-tetranor-15 oxo-5,6,13,14-tetrahydro-15-F(2t)-isoP)-finding none of these to have any substantial excitatory effect. Finally, several plant-derived isoprostanes ("phytoprostanes") synthesized within our laboratory elicited little or no excitatory response in these three pulmonary smooth muscle preparations. We conclude that, although isoprostane exhibit powerful constrictor effects on airway and pulmonary vascular smooth muscles, metabolic processing of those isoprostanes essentially abolishes those biological actions; also, the phytoprostanes lack any appreciable pharmacological activity on those smooth muscle preparations.

Cardiovascular pharmacology and physiology of the isoprostanes

F(2)-isoprostanes are a complex family of compounds produced from arachidonic acid via a free radical-catalyzed mechanism. Their quantification as a pathophysiological biomarker provides a unique opportunity to investigate lipid peroxidation in vascular diseases. Their measurement also provides an interesting biomarker for the rational dose selection of antioxidants in vascular diseases where oxidative stress might be involved. In addition to their use as biomarkers, some isoprostanes possess a biological activity. The 15-series F(2)- and E(2)-isoprostanes mediate vasoconstriction in different vascular beds and species. In addition, 15-F(2t)-IsoP induces smooth muscle cells mitogenesis and monocyte adhesion to endothelial cells. The data available supports but does not prove the hypothesis that isoprostanes are involved in vascular physiology and pathogenesis.

Plant peroxisomes as a source of signalling molecules

Peroxisomes are pleiomorphic, metabolically plastic organelles. Their essentially oxidative function led to the adoption of the name 'peroxisome'. The dynamic and diverse nature of peroxisome metabolism has led to the realisation that peroxisomes are an important source of signalling molecules that can function to integrate cellular activity and multicellular development. In plants defence against predators and a hostile environment is of necessity a metabolic and developmental response--a plant has no place to hide. Mutant screens are implicating peroxisomes in disease resistance and signalling in response to light. Characterisation of mutants disrupted in peroxisomal beta-oxidation has led to a growing appreciation of the importance of this pathway in the production of jasmonic acid, conversion of indole butyric acid to indole acetic acid and possibly in the production of other signalling molecules. Likewise the role of peroxisomes in the production and detoxification of reactive oxygen, and possibly reactive nitrogen species and changes in redox status, suggests considerable scope for peroxisomes to contribute to perception and response to a wide range of biotic and abiotic stresses. Whereas the peroxisome is the sole site of beta-oxidation in plants, the production and detoxification of ROS in many cell compartments makes the specific contribution of the peroxisome much more difficult to establish. However progress in identifying peroxisome specific isoforms of enzymes associated with ROS metabolism should allow a more definitive assessment of these contributions in the future.

Quantification of dinor, dihydro metabolites of F2-isoprostanes in urine by liquid chromatography/tandem mass spectrometry

The F2-isoprostanes (F2-IsoP) are a series of prostaglandin (PG)-F2-like compounds that are produced by free-radical-mediated oxidation of arachidonic acid. One F2-IsoP with potent biological activity is 15-F2t-IsoP and increased levels of 15-F(2t)-IsoP have been measured in several diseases. The major urinary metabolite of 15-F2t-IsoP (8-iso-PGF(2alpha)) is 2,3-dinor-5,6-dihydro-15-F2t-IsoP (15-F2t-IsoP-M). Previously, we developed a stable isotope dilution gas chromatography/negative chemical ionization/mass spectrometry (MS) assay for 15-F2t-IsoP-M, which, while highly sensitive, required time-consuming derivatization and thin-layer chromatography purification. We now report the development of a more rapid high-performance liquid chromatography method coupled to electrospray ionization-tandem mass spectrometry (LC/MS/MS) to analyze all of the dinor,dihydro metabolites of the F2-IsoP isomers (F2-IsoP-M). The precision of this assay was +/-5.0% and the accuracy 80%. The assay remained linear over a range of 1-100 ng injected onto the LC column. Levels of F2-IsoP-M determined by the LC/MS/MS assay method significantly correlated with levels of 15-F2t-IsoP-M determined by the GC/MS assay (R = 0.77y = 67.2x-0.5). The levels of F2-IsoP-M detected in spot urines from 40 normal subjects were 38.1+/-19.1 ng/mg creatinine (mean+/-SD). This method provides an accurate and rapid assay to assess oxidative status in vivo.

Isoprostanes: markers and mediators of oxidative stress

Some years ago it was discovered that prostaglandin F2-like compounds are formed in vivo by nonenzymatic free radical-catalyzed peroxidation of arachidonic acid. Because these compounds are a series of isomers that contain the prostane ring of prostaglandins, they were termed F2-isoprostanes. Intermediates in the isoprostane pathway are prostaglandin H2-like compounds that become reduced to form F2-isoprostanes but also undergo rearrangement in vivo to form E2-, D2-, A2-, J2-isoprostanes, isothromboxanes, and highly reactive gamma-ketoaldehydes, termed isoketals. Analogous compounds have also been shown to be formed from free radical mediated oxidation of docosoahexaenoic acid. Because docosahexaenoic acid is highly enriched in neurons, these compounds have been termed neuroprostanes and neuroketals. An important aspect of the discovery of isoprostanes is that measurement of F2-isoprostanes has emerged as one of the most reliable approaches to assess oxidative stress status in vivo, providing an important tool to explore the role of oxidative stress in the pathogenesis of human disease. Measurement of F4-neuroprostanes has also proved of value in exploring the role of oxidative stress in neurodegenerative diseases. Products of the isoprostane pathway have been found to exert potent biological actions and therefore may participate as physiological mediators of disease.

Role of beta-oxidation in jasmonate biosynthesis and systemic wound signaling in tomato

Jasmonic acid (JA) is a lipid-derived signal that regulates plant defense responses to biotic stress. Here, we report the characterization of a JA-deficient mutant of tomato (Lycopersicon esculentum) that lacks local and systemic expression of defensive proteinase inhibitors (PIs) in response to wounding. Map-based cloning studies demonstrated that this phenotype results from loss of function of an acyl-CoA oxidase (ACX1A) that catalyzes the first step in the peroxisomal beta-oxidation stage of JA biosynthesis. Recombinant ACX1A exhibited a preference for C12 and C14 straight-chain acyl-CoAs and also was active in the metabolism of C18 cyclopentanoid-CoA precursors of JA. The overall growth, development, and reproduction of acx1 plants were similar to wild-type plants. However, the mutant was compromised in its defense against tobacco hornworm (Manduca sexta) attack. Grafting experiments showed that loss of ACX1A function disrupts the production of the transmissible signal for wound-induced PI expression but does not affect the recognition of this signal in undamaged responding leaves. We conclude that ACX1A is essential for the beta-oxidation stage of JA biosynthesis and that JA or its derivatives is required both for antiherbivore resistance and the production of the systemic wound signal. These findings support a role for peroxisomes in the production of lipid-based signaling molecules that promote systemic defense responses.

Isoprostanes: more than just mere markers

Isoprostanes are members of a family of prostaglandin isomers that are produced by free radical-catalysed mechanisms. They have become well-recognized indicators of oxidant-induced cell damage in a variety of pathophysiological conditions. Several isoprostanes have been shown to possess biological activity in whole-animal, isolated tissue and cell-based systems. Their actions include vasoconstriction, platelet aggregation and cardiac hypertrophy. Current evidence suggests that these effects are mediated by prostanoid receptors through a complex set of interactions that involve agonism, partial agonism, desensitization and co-operative behaviors. It is likely that other mechanisms of action are waiting to be discovered. Based on a consideration of these biological effects, we argue that isoprostanes are more than mere markers and may serve as active participants in promoting and exaggerating pathophysiological changes. To tease out their roles requires considerable more work and a willingness to suspend disbelief based on limited evidence.

The isoprostanoid pathway in plants

Higher plants are generally unable to synthesize arachidonic acid, and thus, do neither form prostaglandins nor C20-isoprostanes. Instead, plants utilize linolenic acid for the synthesis of prostaglandin-like compounds of the jasmonate type via the lipoxygenase/allene oxide synthase pathway and C18-isoprostanoids, termed phytoprostanes, via a nonenzymatic, free radical catalyzed pathway analogous to the isoprostane pathway in animals. Both pathways are constitutively present in many if not all plants. Formation of jasmonates can be triggered by specific stimuli interacting with membrane receptors while phytoprostane synthesis can be induced by ROS and heavy metals. Jasmonates are established plant signal compounds that induce defense responses including accumulation of antimicrobial secondary metabolites (phytoalexins). Preliminary data indicates that phytoprostanes also induce phytoalexins in a variety of plant species suggesting a possible function of phytoprostanes as mediators of defense reactions in response to oxidative stress in plants.

Isoprostanes: an emerging role in vascular physiology and disease?

Among the isoprostanes, the 15-series F2-isoprostanes and 15-E2t-IsoP mediate vasoconstriction in different vascular beds and species. In addition, 15-F2t-IsoP induces smooth muscle cells mitogenesis and monocyte adhesion to endothelial cells. In clinical studies, 15-F2t-IsoP levels are increased in some vascular disorders involving atherosclerosis, ischemia-reperfusion and inflammation. Whether the same effects observed in vitro are observed consistently in vivo at physiological concentrations and whether these effects contribute to pathological states in vivo is still debated.

Isoprostanes and the liver

The liver has been central to our understanding of the physiology and biology of the F2-isoprostanes. The discovery of F2-IsoPs and the initial demonstration that they could be used to localize oxidative stress was first demonstrated in a rat model of oxidative liver injury (carbon tetrachloride), and the first demonstration that plasma concentrations are increased in a human disease was in patients with liver failure and the hepatorenal syndrome [J. Clin. Invest. 90 (6) (1992b) 2502; J. Lipid Mediat. 6 (1/3) (1993) 417]. This article will cover the measurement of F2-IsoPs as markers of lipid peroxidation in vivo in liver disease, and review their biological activity as mediators of disease.

The biochemistry of the isoprostane, neuroprostane, and isofuran pathways of lipid peroxidation

F2-isoprostanes are prostaglandin F2-like compounds that are formed nonenzymatically by free radical mediated peroxidation of arachidonic acid. Intermediate in the pathway of the formation of isoprostanes are labile prostaglandin H2-like bicyclic endoperoxides (H2-isoprostanes), which are reduced to F2-isoprostanes and also undergo rearrangement in vivo to form E-ring and D-ring isoprostanes, isothromboxanes, and highly reactive acyclic gamma-ketoaldehdyes (isoketals). Docosahexaenoic acid (C22:6omega3) is highly enriched in neurons in the brain and is highly susceptible to oxidation. Free radical mediated oxidation of docosahexaenoic acid results in the formation of isoprostane-like compounds (neuroprostanes). F4- and E4/D4-neuroprostanes as well as neuroketals have been shown to be produced in vivo. Finally, we recently discovered a new pathway of lipid peroxidation that forms compounds with a substituted tetrahydrofuran ring (isofurans). Oxygen concentrations differentially modulate the formation of isoprostanes and isofurans; at elevated oxygen concentrations, the formation of isofurans is favored whereas the formation of isoprostanes is disfavored.

Modulation of JB6 mouse epidermal cell transformation response by the prostaglandin F2alpha receptor

Prostaglandin F(2alpha) (PGF(2alpha)) modulates clonal selection processes in the mouse skin model of carcinogenesis. In this study we investigated whether JB6 mouse epidermal cells expressed a functional PGF(2alpha) receptor (FP) coupled with a cell-transformation response. Treatment of JB6 cells with an FP agonist (fluprostenol) potently (pM-nM) increased anchorage-dependent and anchorage-independent growth. Inositol phospholipid accumulation and extracellular signal-regulated kinase (Erk) activity were increased in cells treated with FP agonists, consistent with established FP-related signal transduction. FP mRNA was detected by reverse transcription-polymerase chain reaction, and the average specific [(3)H]PGF(2alpha) binding was 8.25 +/- 0.95 fmol/mg protein. Erk activity and colony size were increased by cotreatment of JB6 cells with epidermal growth factor (EGF) and fluprostenol to a greater extent than with either treatment alone, whereas the cotreatment effect on colony number appeared to be simply additive. Collectively, our data indicated that JB6 cells expressed a functional FP coupled with transformation-related signal transduction and the regulation of clonal selection processes. Erk activity appears to be a convergence point in the EGF and FP pathways. The data raise the possibility that the FP contributes to clonal selection processes but probably plays a more important role as a response modifier.

Isoprostanes as a biomarker of lipid peroxidation in humans: physiology, pharmacology and clinical implications

Isoprostanes are a complex family of compounds produced from arachidonic acid via a free-radical-catalyzed mechanism. They can be quantified as reliable markers of lipid peroxidation. Among the isoprostanes, 15-F(2t)-IsoP and 15-E(2t)-IsoP are biologically active and mediate vasoconstriction and bronchoconstriction and augment nociception. These effects are thought to be mediated via the activation of prostanoid TP receptors, with isoprostanes acting as full or partial agonists. A strong link between lipid peroxidation and diseases associated with ischaemia-reperfusion, atherosclerosis and inflammation has been suggested by elevated levels of F(2)-isoprostanes observed in such diseases. Thus, quantification of F(2)-isoprostanes as a pathophysiological marker provides a unique opportunity to investigate lipid peroxidation in human diseases and provides an interesting biomarker for rational dose selection of antioxidants in diseases where oxidative stress might be involved.