Adenoviral expression of 15-lipoxygenase-1 in rabbit aortic endothelium: role in arachidonic acid-induced relaxation

Exercise-Induced Changes in Bioactive Lipids Might Serve as Potential Predictors of Post-Exercise Hypotension. A Pilot Study in Healthy Volunteers

Post-exercise hypotension (PEH) is the phenomenon of lowered blood pressure after a single bout of exercise. Only a fraction of people develops PEH but its occurrence correlates well with long-term effects of sports on blood pressure. Therefore, PEH has been suggested as a suitable predictor for the effectivity of exercise as therapy in hypertension. Local vascular bioactive lipids might play a potential role in this context. We performed a cross-over clinical pilot study with 18 healthy volunteers to investigate the occurrence of PEH after a single short-term endurance exercise. Furthermore, we investigated the plasma lipid profile with focus on arachidonic acid (AA)-derived metabolites as potential biomarkers of PEH. A single bout of ergometer cycling induced a significant PEH in healthy volunteers with the expected high inter-individual variability. Targeted lipid spectrum analysis revealed significant upregulation of several lipids in the direct post-exercise phase. Among these changes, only 15- hydroxyeicosatetranoic acid (HETE) correlated significantly with the extent of PEH but in an AA-independent manner, suggesting that 15-HETE might act as specific PEH-marker. Our data indicate that specific lipid modulation might facilitate the identification of patients who will benefit from exercise activity in hypertension therapy. However, larger trials including hypertonic patients are necessary to verify the clinical value of this hypothesis.

Mammalian lipoxygenases and their biological relevance

Lipoxygenases (LOXs) form a heterogeneous class of lipid peroxidizing enzymes, which have been implicated not only in cell proliferation and differentiation but also in the pathogenesis of various diseases with major public health relevance. As other fatty acid dioxygenases LOXs oxidize polyunsaturated fatty acids to their corresponding hydroperoxy derivatives, which are further transformed to bioactive lipid mediators (eicosanoids and related substances). On the other hand, lipoxygenases are key players in the regulation of the cellular redox homeostasis, which is an important element in gene expression regulation. Although the first mammalian lipoxygenases were discovered 40 years ago and although the enzymes have been well characterized with respect to their structural and functional properties the biological roles of the different lipoxygenase isoforms are not completely understood. This review is aimed at summarizing the current knowledge on the physiological roles of different mammalian LOX-isoforms and their patho-physiological function in inflammatory, metabolic, hyperproliferative, neurodegenerative and infectious disorders. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

Inducible endothelium-derived hyperpolarizing factor: role of the 15-lipoxygenase-EDHF pathway

Endothelium-derived hyperpolarizing factors (EDHFs) regulate vascular tone by contributing to the vasorelaxations to shear stress and endothelial agonists such as bradykinin and acetylcholine. 15(S)-Hydroxy-11,12-epoxyeicosatrienoic acid (15-H-11,12-EETA) and 11(R),12(S),15(S)-trihydroxyeicosatrienoic acid (11,12,15-THETA) are endothelial metabolites of the 15-lipoxygenase (15-LO) pathway of arachidonic acid metabolism and are EDHFs. 11,12,15-THETA activates small conductance, calcium-activated potassium channels on smooth muscle cells causing membrane hyperpolarization, and relaxation. Expression levels of 15-LO in the endothelium regulate the activity of the 15-LO/15-H-11,12-EETA/11,12,15-THETA pathway and its contribution to vascular tone. Regulation of its expression is by transcriptional, translational, and epigenetic mechanisms. Hypoxia, hypercholesterolemia, atherosclerosis, anemia, estrogen, interleukins, and possibly other hormones increase 15-LO expression. An increase in 15-LO results in increased synthesis of 15-H-11,12-EETA and 11,12,15-THETA, increased membrane hyperpolarization, and enhanced contribution to relaxation by endothelial agonists. Thus, the 15-LO pathway represents the first example of an inducible EDHF. In addition to 15-LO metabolites, a number of chemicals have been identified as EDHFs and their contributions to vascular tone vary with species and vascular bed. The reason for multiple EDHFs has evaded explanation. However, EDHF functioning as constitutive EDHFs or inducible EDHFs may explain the need for chemically and biochemically distinct pathways for EDHF activity and the variation in EDHFs between species and vascular beds. This new EDHF classification provides a framework for understanding EDHF activity in physiological and pathological conditions.

Effect of human 15-lipoxygenase-1 metabolites on vascular function in mouse mesenteric arteries and hearts

Lipoxygenases regulate vascular function by metabolizing arachidonic acid (AA) to dilator eicosanoids. Previously, we showed that endothelium-targeted adenoviral vector-mediated gene transfer of the human 15-lipoxygenase-1 (h15-LO-1) enhances arterial relaxation through the production of vasodilatory hydroxyepoxyeicosatrienoic acid (HEETA) and trihydroxyeicosatrienoic acid (THETA) metabolites. To further define this function, a transgenic (Tg) mouse line that overexpresses h15-LO-1 was studied. Western blot, immunohistochemistry and RT-PCR results confirmed expression of 15-LO-1 transgene in tissues, especially high quantity in coronary arterial wall, of Tg mice. Reverse-phase HPLC analysis of [(14)C]-AA metabolites in heart tissues revealed enhanced 15-HETE synthesis in Tg vs. WT mice. Among the 15-LO-1 metabolites, 15-HETE, erythro-13-H-14,15-EETA, and 11(R),12(S),15(S)-THETA relaxed the mouse mesenteric arteries to the greatest extent. The presence of h15-LO-1 increased acetylcholine- and AA-mediated relaxation in mesenteric arteries of Tg mice compared to WT mice. 15-LO-1 was most abundant in the heart; therefore, we used the Langendorff heart model to test the hypothesis that elevated 15-LO-1 levels would increase coronary flow following a short ischemia episode. Both peak flow and excess flow of reperfused hearts were significantly elevated in hearts from Tg compared to WT mice being 2.03 and 3.22 times greater, respectively. These results indicate that h15-LO-1-derived metabolites are highly vasoactive and may play a critical role in regulating coronary blood flow.

Platelet-derived growth factor (PDGF) induces pulmonary vascular remodeling through 15-LO/15-HETE pathway under hypoxic condition

15-lipoxygenase (15-LO) is known to play an important role in chronic pulmonary hypertension. Accumulating evidence for its down-stream participants in the vasoconstriction and remodeling processes of pulmonary arteries, while how hypoxia regulates 15-LO/15-hydroxyeicosatetraenoic acid (15-HETE) to mediate hypoxic pulmonary hypertension is still unknown. Platelet-derived growth factor (PDGF) is an important vascular regulator whose concentration increases under hypoxic condition in the lungs of both humans and mice with pulmonary hypertension. The present study was carried out to determine whether hypoxia advances the pulmonary vascular remodeling through the PDGF/15-LO/15-HETE pathway. We found that pulmonary arterial medial thickening caused by hypoxia was alleviated after a treatment of the hypoxic rats with imatinib, which was associated with down-regulations of 15-LO-2 expression and 15-HETE production. Moreover, the increases in cell proliferation and endogenous 15-HETE content by hypoxia were attenuated by the inhibitors of PDGF-β receptor in pulmonary artery smooth muscle cells (PASMCs). The effects of PDGF-BB on cell proliferation and survival were weakened after the administration of 15-LO inhibitors or 15-LO RNA interference. These results suggest that hypoxia promotes PASMCs proliferation and survival, contributing to pulmonary vascular medial hypertrophy, which is likely to be mediated via the PDGF-BB/15-LO-2/15-HETE pathway.

Endothelial nitric oxide and 15-lipoxygenase-1 metabolites independently mediate relaxation of the rabbit aorta

Endothelial 15-lipoxygenase-1 (15-LO-1) metabolites of arachidonic acid (AA), 11,12,15-trihydroxyeicosatrienoic acid (THETA) and 15-hydroxy-11,12-epoxyeicosatrienoic acid (HEETA) and nitric oxide (NO) mediate relaxations to acetylcholine (ACH). However, interactions between NO and the 15-LO-1 pathway have not been explored. Therefore, the effect of physiological and pharmacological concentrations of NO on 15-LO activity and relaxation was studied in rabbit aorta. In indomethacin-treated aortic rings, maximal ACH relaxations of 91.3±4.0%, decreased to 54.5±3.0% by the NO synthase inhibitor, nitro-l-arginine (LNA), to 49.8±3% by the guanylate cyclase (GC) inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, to 63.7±4.9% by the lipoxygenase (LO) inhibitor, nordihydroguaiaretic acid (NDGA) and were completely inhibited by the combination of LNA and NDGA. AA relaxations were not affected by GC inhibition but were reduced by LO inhibition. The NO donor, dipropylenetriamine-NONOate (DPTA) caused concentration-related relaxations (EC(50)=4.7×10(-6)M). Aortic metabolism of (14)C-AA to THETA and HEETA was not altered by EC(50) concentrations of DPTA but were reduced 10-fold by 10(-3)M DPTA. In LNA-treated aorta, DPTA (3×10(-6)M) caused relaxations of 38.2.5±4%. Maximum relaxations to ACH did not differ in the presence and absence 3×10(-6)M DPTA (49.5±5% and 44.2±4%, respectively). These results indicate that NO and 15-LO-1 act in parallel to mediate ACH relaxations and NO does not alter 15-LO-1 activity.

Functional and pathological roles of the 12- and 15-lipoxygenases

The 12/15-lipoxygenase enzymes react with fatty acids producing active lipid metabolites that are involved in a number of significant disease states. The latter include type 1 and type 2 diabetes (and associated complications), cardiovascular disease, hypertension, renal disease, and the neurological conditions Alzheimer's disease and Parkinson's disease. A number of elegant studies over the last thirty years have contributed to unraveling the role that lipoxygenases play in chronic inflammation. The development of animal models with targeted gene deletions has led to a better understanding of the role that lipoxygenases play in various conditions. Selective inhibitors of the different lipoxygenase isoforms are an active area of investigation, and will be both an important research tool and a promising therapeutic target for treating a wide spectrum of human diseases.

Intravitreal adenoviral 15-lipoxygenase-1 gene transfer prevents vascular endothelial growth factor A-induced neovascularization in rabbit eyes

Excessive angiogenesis mediated by vascular endothelial growth factor (VEGF) plays an important role in angioproliferative ocular diseases. We have previously developed a large animal model for these diseases by intravitreal adenoviral gene transfer of VEGF-A(165). 15-Lipoxygenase-1 (15-LO-1), an oxidizing enzyme producing reactive lipid hydroperoxides, has been shown to induce aberrant angiogenesis in cancer models of transgenic mice overexpressing human 15-LO-1. Our purpose was to study the effects of 15-LO-1 on VEGF-A(165)-induced angiogenesis in New Zealand White rabbit eyes, using intravitreal adenovirus-mediated gene transfers. AdCMV and Adh15-LO-1 alone served as controls. As determined by immunohistochemistry, VEGF-A(165) significantly increased the number and size of the capillaries in various compartments of the eyes. 15-LO-1 efficiently inhibited VEGF-A(165)-induced neovascularization and pathological changes by reducing VEGF-A(165) mRNA and protein expression, determined by RT-PCR, ELISA, and immunohistochemistry. 15-LO-1, which produces endogenous ligands for peroxisome proliferator-activated receptor-gamma (PPARgamma), also prevented VEGF-A(165)-induced expression of PPARgamma and VEGF receptor-2, as measured by quantitative RT-PCR. In conclusion, our findings show that 15-LO-1 prevents VEGF-A(165)-induced angiogenesis and consequent pathology in the eyes, suggesting that intravitreal 15-LO-1 gene transfer could be a potential new strategy for the treatment of neovascular complications in the eyes.

Role of arachidonic acid lipoxygenase metabolites in the regulation of vascular tone

Stimulation of vascular endothelial cells with agonists such as acetylcholine (ACh) or bradykinin or with shear stress activates phospholipases and releases arachidonic acid (AA). AA is metabolized by cyclooxygenases, cytochrome P-450s, and lipoxygenases (LOs) to vasoactive products. In some arteries, a substantial component of the vasodilator response is dependent on LO metabolites of AA. Nitric oxide (NO)- and prostaglandin (PG)-independent vasodilatory responses to ACh and AA are reduced by inhibitors of LO and by antisense oligonucleotides specifically against 15-LO-1. Vasoactive 15-LO metabolites derived from the vascular endothelium include 15-hydroxy-11,12-epoxyeicosatrienoic acid (15-H-11,12-HEETA) that is hydrolyzed by soluble epoxide hydrolase to 11,12,15-trihydroxyeicosatrienoic acid (11,12,15-THETA). HEETA and THETA are endothelium-derived hyperpolarizing factors that induce vascular relaxations by activation of smooth muscle apamin-sensitive, calcium-activated, small-conductance K(+) channels causing hyperpolarization. In other arteries, the 12-LO metabolite 12-hydroxyeicosatetraenoic acid is synthesized by the vascular endothelium and relaxes smooth muscle by large-conductance, calcium-activated K(+) channel activation. Thus formation of vasodilator eicosanoids derived from LO pathways contributes to the regulation of vascular tone, local blood flow, and blood pressure.

Chronic hypoxia enhances 15-lipoxygenase-mediated vasorelaxation in rabbit arteries

15-Lipoxygenase (15-LO-1) metabolizes arachidonic acid (AA) to 11,12,15-trihydroxyeicosatrienoic acids (THETAs) and 15-hydroxy-11,12-epoxyeicosatrienoic acids (HEETA) that dilate rabbit arteries. Increased endothelial 15-LO-1 expression enhances arterial relaxations to agonists. We tested the effect of hypoxia on 15-LO-1 expression, THETA and HEETA synthesis, and relaxations in rabbit arteries. The incubation of rabbit aortic endothelial cells and isolated aortas in 0.7% O(2) increased 15-LO-1 expression. Rabbits were housed in a hypoxic atmosphere of 12% O(2) for 5 days. 15-LO-1 expression increased in the endothelium of the arteries of rabbits in 12% O(2) compared with room air. THETA and HEETA synthesis was also enhanced in aortas and mesenteric arteries. AA hyperpolarized the smooth muscle cells in indomethacin- and phenylephrine-treated mesenteric arteries of hypoxic rabbits from -29.4 +/- 1 to -50.1 +/- 3 mV. The hyperpolarization to AA was less in arteries of normoxic rabbits (from -26.0 +/- 2 to -37 +/- 2 mV). This AA-induced hyperpolarization was inhibited by the 15-LO inhibitor BW-755C. Nitric oxide and prostaglandin-independent maximum relaxations to acetylcholine (79.7 +/- 2%) and AA (38.3 +/- 4%) were enhanced in mesenteric arteries from hypoxic rabbits compared with the normoxic rabbits (49.7 +/- 6% and 19.9 +/- 2%, respectively). These relaxations were inhibited by BW-755C and nordihydroguaiaretic acid. Therefore, hypoxia increased the relaxations to agonists in the rabbit mesenteric arteries by enhancing endothelial 15-LO-1 expression and synthesis of the hyperpolarizing factors THETA and HEETA.

15-Lipoxygenase metabolites contribute to age-related reduction in acetylcholine-induced hypotension in rabbits

Arachidonic acid (AA) metabolites from the 15-lipoxygenase-1 (15-LO-1) pathway, trihydroxyeicosatrienoic acids (THETAs) and hydroxy-epoxyeicosatrienoic acids (HEETAs), are endothelium-derived hyperpolarizing factors (EDHFs) and relax rabbit arteries. Rabbit vascular 15-LO-1 expression, THETA and HEETA synthesis, and nitric oxide and prostaglandin-independent relaxations to acetylcholine (ACh) and AA decreased with age (neonates to 16-wk-old). We characterized age-dependent ACh-hypotensive responses in vivo in 1-, 4-, 8-, and 16-wk-old rabbits and the contribution of THETAs and HEETAs to these responses. In anesthetized rabbits, blood pressure responses to ACh (4-4,000 ng/kg) were determined in the presence of vehicle or various inhibitors. ACh responses decreased with age (P > 0.001). In the absence or presence of N(omega)-nitro-l-arginine methyl ester (l-NAME) and indomethacin (Indo), maximum responses in 1 (-54.7 +/- 7.4 and -37.9 +/- 3.9%)- and 4 (-48.8 +/- 2.4 and -35.5 +/- 7.8%)-wk-old rabbits were higher than 8 (-30.0 +/- 2.8 and -26.6 +/- 4.4%)- and 16 (-36.7 +/- 3.5 and -27.3 +/- 10%)-wk-old rabbits. A lipoxygenase inhibitor, BW755C, reduced THETA and HEETA synthesis in mesenteric arteries. In the presence of Indo and N(omega)-nitro-l-arginine, ACh relaxations were reduced by BW755C to a greater extent in the mesenteric arteries from the younger rabbits. In 4-wk-old rabbits treated with l-NAME and Indo, the maximum ACh hypotension was reduced by the potassium channel inhibitors apamin and charybdotoxin to -6.9 +/- 0.9%, by apamin alone to -19.5 +/- 1.4%, and by BW755C to -18.8 +/- 3.5%. The present study indicates that the age-related decrease in ACh-induced hypotension is mediated by the decreased synthesis of the 15-LO-1 metabolites THETAs and HEETAs.

Arachidonic Acid metabolites in the cardiovascular system: the role of lipoxygenase isoforms in atherogenesis with particular emphasis on vascular remodeling

Vascular remodeling refers to lasting structural alterations in the vessel wall that are initiated in response to external and internal stimuli. These changes are distinct from acute functional responses of blood vessels when challenged by increased blood pressure, altered hemodynamics, or vasoactive mediators. In early atherogenesis, when lesion formation is starting to impact local hemodynamics, the vessel wall responds with outward vascular remodeling to maintain normal blood flow. However, inward remodeling may also occur during the time course of plaque formation, contributing to vascular stenosis. Lipoxygenases form a heterogeneous family of lipid-peroxidizing enzymes, which have been implicated in atherogenesis. Several lines of in vitro and in vivo evidence indicated their involvement in disease development, but the precise function of different lipoxygenase isoforms is still a matter of discussion. Vascular remodeling is an early response during plaque development; therefore, lipoxygenases may be involved in this process. Unfortunately, little is known about the potential role of lipoxygenase isoforms in vascular remodeling. This review will briefly summarize our knowledge of the role of lipoxygenases in vascular biology and will critically review the activities of the 3 most athero-relevant lipoxygenase isoforms in atherogenesis, with particular emphasis on vascular remodeling.

15-Lipoxygenase-1 Prevents Vascular Endothelial Growth Factor A– and Placental Growth Factor–Induced Angiogenic Effects in Rabbit Skeletal Muscles via Reduction in Growth Factor mRNA Levels, NO Bioactivity, and Downregulation of VEGF Receptor 2 Expression

Human 15-lipoxygenase-1 (15-LO-1) is an oxidizing enzyme capable of producing reactive lipid hydroperoxides. 15-LO-1 and its products have been suggested to be involved in many pathological conditions, such as inflammation, atherogenesis, and carcinogenesis. We used adenovirus-mediated gene transfers to study the effects of 15-LO-1 on vascular endothelial growth factor (VEGF)-A 165 – and placental growth factor (PlGF)-induced angiogenesis in rabbit skeletal muscles. 15-LO-1 significantly decreased all angiogenic effects induced by these growth factors, including capillary perfusion, vascular permeability, vasodilatation, and an increase in capillary number. The effects are attributable to the reduction in the amount of VEGF-A 165 and PlGF transcripts by 15-LO-1, resulting in reduced protein expression. The most likely mediator of the VEGF family–induced capillary vasodilatation is nitric oxide (NO), which is produced by NO synthases. Endothelial NO synthase protein expression and NO synthase activity were significantly induced by VEGF-A 165 , and these inductions were reduced by 15-LO-1. VEGF-A 165 induces its angiogenic effects primarily via vascular endothelial growth factor receptor (VEGFR)2, and also PlGF mediates angiogenic signaling via VEGFR2, even though it binds to VEGFR1. VEGFR2 expression is induced by peroxisome proliferator-activating receptor γ. We showed by quantitative RT-PCR and immunohistochemistry that expression of endogenous rabbit peroxisome proliferator-activating receptor γ and VEGFR2 were significantly increased in the growth factor–transduced muscles, but these inductions were efficiently prevented by 15-LO-1. In conclusion, the results suggest that expression of 15-LO-1 has an efficient antiangiogenic effect in vivo via reduction in growth factor mRNA levels, NO bioactivity, and VEGFR2 expression.

Age-related decrease in 15-lipoxygenase contributes to reduced vasorelaxation in rabbit aorta

Rabbit 15-lipoxygenase-1 (15-LO-1) oxygenates arachidonic acid (AA) into 15-hydroperoxyeicosatetraenoic acid, which is then converted to the vasodilatory 15-hydroxy-11,12-epoxyeicosatrienoic acid (HEETA) and 11,12,15-trihydroxyeicosatrienoic acid (THETA). We studied the age-dependent expression of the 15-LO-1 in rabbit aorta and its effects on the synthesis of THETA, HEETA, and vasoactivity. Aortas of 1-wk-old rabbits express greater amounts of 15-LO-1 mRNA and protein compared with aortas of 4-, 8-, or 16-wk-old rabbits. The synthesis of THETA and HEETA in the rabbit aorta was also reduced with age. THETA synthesis was maximal in 1-wk-old aortas but decreased in aortas of 4- (42%), 8- (4%), and 16-wk-old (1%) rabbits. Similarly, THETA and HEETA synthesis decreased with age in mesenteric arteries from 1-, 4-, 8-, and 16-wk-old rabbits. The maximum vasorelaxation response to acetylcholine (10−6M) in the presence of indomethacin and nitro-l-arginine decreased in the order of 1 wk (64.5 ± 6.9%), 4 wk (52.6 ± 8.9%), 8 wk (53.0 ± 9.4%), and 16 wk (33.3 ± 6.6%). Similarly, the maximum relaxation to AA (3 × 10−4M) decreased with age in the order of 1 wk (60.4 ± 8.9%), 4 wk (56.3 ± 5.8%), 8 wk (41.8 ± 12.3%), and 16 wk (28.9 ± 1.6%). In contrast, the vasorelaxation to sodium nitroprusside was not significantly altered by age. These data indicate that aortic 15-LO-1 expression and activity are downregulated with aging in rabbits. This decrease is paralleled by the reduced synthesis of vasoactive THETA and HEETA and aortic relaxations to acetylcholine and AA.

Endothelial 15-lipoxygenase-1 overexpression increases acetylcholine-induced hypotension and vasorelaxation in rabbits

Arachidonic acid is metabolized by the 15-lipoxygenase-1 pathway to the vasodilatory eicosanoids hydroxy-epoxyeicosatrienoic acid and trihydroxyeicosatrienoic acid. We determined the in vitro and in vivo effects of the 15-lipoxygenase-1 metabolites in rabbits infected with adenovirus containing cDNA for human 15-lipoxygenase-1 (Ad-15-LO-1). Forty hours after intravenous adenoviral injection, 15-lipoxygenase-1 expression increased in liver and mesenteric arteries 10-fold and 3-fold, respectively. Expression of 15-LO-1 was limited to the endothelium of mesenteric arteries. Overexpression did not occur in tissues from rabbits infected with a beta-galactosidase containing adenovirus. Trihydroxyeicosatrienoic acid and hydroxy-epoxyeicosatrienoic acid synthesis per milligram of tissue increased by 2.1- and 1.5-fold, respectively, in mesenteric arteries from Ad-15-LO-1-infected rabbits compared with normal rabbits. Pretreatment with a 15-lipoxygenase inhibitor BW755C inhibited the synthesis. NO and prostaglandin-independent, maximal relaxations to acetylcholine were greater in mesenteric arteries from Ad-15-LO-1-infected rabbits (98.3+/-1.7%) compared with normal (60.93+/-10.5%) and beta-galactosidase containing adenovirus-infected rabbits (68.3+/-7.7%). Pretreatment with BW755C decreased these relaxations. Mean arterial pressure and heart rate did not differ in Ad-15-LO-1-infected rabbits compared with normal or beta-galactosidase containing adenovirus-infected rabbits. The hypotensive responses to acetylcholine in the presence and absence of inhibition of NO and prostaglandins were greater in Ad-15-LO-1-infected rabbits (-52+/-2% and -47+/-2%) compared with normal (-31+/-3% and -25+/-5%) or beta-galactosidase containing adenovirus-infected rabbits (-38+/-2% and -30+/-3%). Therefore, increased expression of 15-LO-1 increases acetylcholine relaxation in arteries and hypotensive responses in rabbits because of increased hydroxy-epoxyeicosatrienoic acid and trihydroxyeicosatrienoic acid synthesis.