Transcellular metabolism of arachidonic acid in platelets and polymorphonuclear leukocytes activated by physiological agonists: enhancement of leukotriene B4 synthesis

N-Docosahexaenoylethanolamine (synaptamide): Carbon-14 radiolabeling and metabolic studies

N-Docosahexaenoylethanolamine (synaptamide) is structurally similar to the endocannabinoid N-arachidonoylethanolamine (anandamide), but incorporates the omega-3 22:6 fatty acid docosahexaenoic acid (DHA) in place of the omega-6 20:4 fatty acid arachidonic acid (AA). Some brain membrane lipid effects may be mediated via synaptamide. In competition experiments with mouse brain homogenate in vitro, we found that synaptamide was an order-of-magnitude poorer inhibitor of radioactive anandamide hydrolysis than was anandamide itself. Also, enzyme-mediated hydrolysis of synaptamide was observed to occur at a slower rate than for anandamide. We have synthesized synaptamide radiolabeled with carbon-14 in both the ethanolamine ([α,β-¹⁴C₂]synaptamide) and in the DHA ([1-¹⁴C]synaptamide) moieties. The brain penetration, distribution, and metabolism of radiolabeled synaptamide were studied in mice in vivo relative to anandamide, DHA, and AA. Brain uptake of labeled synaptamide was greater than for labeled DHA, consistent with previous studies of labeled anandamide and AA in our laboratory. After administering either isotopomer of radiolabeled synaptamide, radiolabeled phospholipids were found in mouse brain. Pretreatment of mice with PF3845, a potent, specific inhibitor of fatty acid amide hydrolase (FAAH), eliminated formation of labeled phospholipids measured after 15min, suggesting that synaptamide is hydrolyzed nearly exclusively by FAAH, though it is a poorer substrate for FAAH than anandamide.

Platelet-leukocyte crosstalk: Linking proinflammatory responses to procoagulant state

Platelet activation is known to be associated with the release of a vast array of chemokines and proinflammatory lipids which induce pleiotropic effects on a wide variety of tissues and cells, including leukocytes. During thrombosis, the recruitment of leukocytes to activated platelets is considered an important step which not only links thrombosis to inflammatory responses but may also enhance procoagulant state. This phenomenon is highly regulated and influenced by precise mutual interactions between the cells at site of vascular injury and thrombi formation. Platelet-leukocyte interaction involves a variety of mediators including adhesion molecules, chemokines and chemoattractant molecules, shed proteins, various proinflammatory lipids and other materials. The current review addresses the detailed mechanisms underlying platelet-leukocyte crosstalk. This includes their adhesive interactions, transcellular metabolisms, induced tissue factor activity and neutrophil extracellular traps formation as well as the impacts of these phenomena in modulation of the proinflammatory and procoagulant functions in a reciprocal manner that enhances the physiological responses.

Mechanisms underlying the cerebral microvascular responses to angiotensin II-induced hypertension

Angiotensin II (AngII) and AngII type-1 receptors (AT1r) have been implicated in the pathogenesis of hypertension and ischemic stroke. The objectives of this study was to determine if/how chronic AngII administration affects blood-brain barrier (BBB) function and blood cell adhesion in the cerebral microvasculature. AngII-loaded osmotic pumps were implanted in wild type (WT) and mutant mice. Leukocyte and platelet adhesion were monitored in cerebral venules by intravital microscopy and BBB permeability detected by Evans blue leakage. AngII (two week) infusion increased blood pressure in WT mice. This was accompanied by an increased BBB permeability and a high density of adherent leukocytes and platelets. AT1r (on the vessel wall, but not on blood cells) was largely responsible for the microvascular responses to AngII. Immunodeficient (Rag-1(-/-) ) mice exhibited blunted blood cell recruitment responses without a change in BBB permeability. A similar protection pattern was noted in RANTES(-/-) and P-selectin(-/-) mice, with bone marrow chimeras (blood cell deficiency only) yielding responses comparable to the respective knockouts. These findings implicate AT1r in the microvascular dysfunction associated with AngII-induced hypertension and suggest that immune cells and blood cell-associated RANTES and P-selectin contribute to the blood cell recruitment, but not the BBB failure, elicited by AngII.

Inflammatory bowel disease: a paradigm for the link between coagulation and inflammation

Inflammatory bowel diseases (IBDs) are associated with platelet activation and an increased risk for thromboembolism. While the mechanisms that underlie the altered platelet function and hypercoagulable state in IBD remain poorly understood, emerging evidence indicates that inflammation and coagulation are interdependent processes that can initiate a vicious cycle wherein each process propagates and intensifies the other. This review addresses the mechanisms that may account for the mutual activation of coagulation and inflammation during inflammation and summarizes evidence that implicates a role for platelets and the coagulation system in the pathogenesis of human and experimental IBD. The proposed link between inflammation and coagulation raises the possibility of targeting the inflammation-coagulation interface to reduce the morbidity and mortality associated with IBD.

Exogenous pathogen and plant 15-lipoxygenase initiate endogenous lipoxin A4 biosynthesis

Lipoxin A4 (LXA4) is a potent endogenous lipoxygenase-derived eicosanoid with antiinflammatory and proresolving properties. Supraphysiological levels of LXA4 are generated during infection by Toxoplasma gondii, which in turn reduces interleukin (IL) 12 production by dendritic cells, thus dampening Th1-type cell-mediated immune responses and host immunopathology. In the present work, we sought evidence for the structural basis of T. gondii's ability to activate LXA4 biosynthesis. Proteomic analysis of T. gondii extract (soluble tachyzoite antigen [STAg]), which preserves the immunosuppressive and antiinflammatory activity of the parasite, yielded several peptide matches to known plant lipoxygenases. Hence, we incubated STAg itself with arachidonic acid and found using LC-UV-MS-MS-based lipidomics that STAg produced both 15-HETE and 5,15-diHETE, indicating that T. gondii carries 15-lipoxygenase activity. In addition, T. gondii tachyzoites (the rapidly multiplying and invasive stage of the parasite) generated LXA4 when provided with arachidonic acid. Local administration of a plant (soybean) lipoxygenase itself reduced neutrophilic infiltration in murine peritonitis, demonstrating that 15-lipoxygenase possesses antiinflammatory properties. Administration of plant 15-lipoxygenase generated endogenous LXA4 and mimicked the suppression of IL-12 production by splenic dendritic cells observed after T. gondii infection or STAg administration. Together, these results indicate that 15-lipoxygenase expressed by a pathogen as well as exogenously administered 15-lipoxygenase can interact with host biosynthetic circuits for endogenous "stop signals" that divert the host immune response and limit acute inflammation.

Resolvins: a family of bioactive products of omega-3 fatty acid transformation circuits initiated by aspirin treatment that counter proinflammation signals

Aspirin (ASA) is unique among current therapies because it acetylates cyclooxygenase (COX)-2 enabling the biosynthesis of R-containing precursors of endogenous antiinflammatory mediators. Here, we report that lipidomic analysis of exudates obtained in the resolution phase from mice treated with ASA and docosahexaenoic acid (DHA) (C22:6) produce a novel family of bioactive 17R-hydroxy-containing di- and tri-hydroxy-docosanoids termed resolvins. Murine brain treated with aspirin produced endogenous 17R-hydroxydocosahexaenoic acid as did human microglial cells. Human COX-2 converted DHA to 13-hydroxy-DHA that switched with ASA to 17R-HDHA that also proved a major route in hypoxic endothelial cells. Human neutrophils transformed COX-2-ASA-derived 17R-hydroxy-DHA into two sets of novel di- and trihydroxy products; one initiated via oxygenation at carbon 7 and the other at carbon 4. These compounds inhibited (IC(50) approximately 50 pM) microglial cell cytokine expression and in vivo dermal inflammation and peritonitis at ng doses, reducing 40-80% leukocytic exudates. These results indicate that exudates, vascular, leukocytes and neural cells treated with aspirin convert DHA to novel 17R-hydroxy series of docosanoids that are potent regulators. These biosynthetic pathways utilize omega-3 DHA and EPA during multicellular events in resolution to produce a family of protective compounds, i.e., resolvins, that enhance proresolution status.

Licofelone, a dual lipoxygenase-cyclooxygenase inhibitor, downregulates polymorphonuclear leukocyte and platelet function

Polymorphonuclear leukocytes are strongly implicated in the pathogenesis of inflammatory disease. Polymorphonuclear leukocyte recruitment at sites of inflammation, mainly sustained by the beta2-integrins, is followed by the synthesis and release of inflammatory mediators, such as leukotrienes, proteolytic enzymes and reactive oxygen species. Functional and metabolic interactions between polymorphonuclear leukocytes and platelets can contribute to and exacerbate the process. The effects of the dual 5-lipoxygenase and cyclooxygenase inhibitor licofelone ([2,2-dimethyl-6-(4-chlorophenyl)-7-phenyl-2,3-dihydro-1H-pyrrolizine-5-yl]-acetic acid) were studied on arachidonic acid transcellular metabolism occurring between polymorphonuclear leukocytes and platelets. The formation of leukotriene C(4), a leukotriene A(4)-derived metabolite, by mixed polymorphonuclear leukocyte/platelet suspensions stimulated with 10 microM A23187 was inhibited by licofelone with an IC(50) of 3.8 +/- 0.07 microM. The formation of 5,12-di-hydroxy-eicosatetraenoic acid (HETE) was abolished at concentrations > or = 10 microM. Licofelone also inhibited the generation of reactive oxygen species by polymorphonuclear leukocytes stimulated with 1 microM n-formyl-methionyl-leucyl-phenylalanine (fMLP), 10 nM complement fraction 5a (C5a) and 1 microM platelet activating factor (PAF) with IC(50)s of 24.4 +/- 0.6, 11.0 +/- 1.5 and 11.7 +/-1.2 microM; elastase release induced by the three agonists was inhibited with IC(50)s of 12.2 +/- 2.2, 23.5 +/- 8 and 2.6 +/- 1 microM, respectively. Homotypic polymorphonuclear leukocyte aggregation induced by fMLP, C5A and PAF was inhibited by licofelone with IC(50)s of 23.7 +/- 4.8, 15.6 +/- 3.4 and 15.4 +/- 4 microM, respectively. The present study extends the anti-lipoxygenase and anti-cyclooxygenase activities of licofelone to the production of arachidonic acid metabolites generated as a consequence of polymorphonuclear leukocyte-platelet transcellular metabolism and to polymorphonuclear leukocyte responses relevant to the pathogenesis of inflammation. The coexistence within the same molecule of a wide spectrum of anti-inflammatory properties is of interest.

Leukotriene B(4) promotes reactive oxidant generation and leukocyte adherence during acute hypoxia

Acute systemic hypoxia produces rapid leukocyte adherence in the rat mesenteric microcirculation, although the underlying mechanisms are not fully known. Hypoxia is known to increase reactive oxygen species (ROS) generation, which could result in formation of the lipid inflammatory mediator leukotriene B(4) (LTB(4)). The goal of this study was to examine the role of LTB(4) in hypoxia-induced microvascular alterations. Using intravital microscopy, we determined the effect of the LTB(4) antagonist, LTB(4)-dimethyl amide (LTB(4)-DMA), on ROS generation and leukocyte adherence in mesenteric venules during hypoxia. Exogenous LTB(4) increased ROS generation to 144 +/- 8% compared with control values and also promoted leukocyte adherence. These responses to LTB(4) were blocked by pretreating the mesentery with LTB(4)-DMA. Leukopenia did not significantly attenuate the LTB(4)-induced increase in ROS generation (142 +/- 12.1%). LTB(4)-DMA substantially, though not completely, reduced hypoxia-induced ROS generation from 66 +/- 18% to 11 +/- 4% above control values. Hypoxia-induced leukocyte adherence was significantly attenuated by LTB(4)-DMA. Our results support a role for LTB(4) in the mechanism of hypoxia-induced ROS generation and leukocyte adherence in the rat mesenteric microcirculation.

Posttreatment with eicosatetraynoic acid decreases lung edema in guinea pigs exposed to phosgene: the role of leukotrienes

Acetylenic acids such as 5,8,11,14-eicosatetraynoic acid (ETYA), have been shown to be effective in preventing pulmonary edema formation (PEF). In phosgene-exposed guinea pigs, we examined the effects of ETYA on PEF, measured as real time lung weight gain (lwg). Pulmonary artery pressure (Ppa), airway pressure (Paw), perfusate leukotrienes (LT) C4/D4/E4/B4, and lung tissue lipid peroxidation (TBARS) were measured using the isolated, buffer-perfused lung model. Guinea pigs were challenged to 175 mg/m3 (44 ppm) phosgene for 10 minutes giving a concentration x time product of 1750 mg.min/m3 (437 ppm.min). Five minutes after removal from the exposure chamber, guinea pigs were treated, i.p., with 200 microL of 100 microM ETYA. 200 microL of 50 microM ETYA was added to the perfusate every 40 minutes, beginning at 60 minutes after start of exposure (t = 0). There were four groups in this study: air-treated, phosgene-exposed, ETYA-posttreated + phosgene, and ETYA-posttreated + air ETYA-posttreated + phosgene guinea pigs had significantly lower Ppa (P = .006), Paw (P = .009), and lwg (P = .016) compared with phosgene-exposed animals. Phosgene exposure reduced LTB4 compared with air-treated controls (P = .09). ETYA-posttreatment + phosgene had significantly increased perfusate LTB4 (P = .0006) compared with phosgene exposure only group. Total perfusate, LTC4 + LTD4 + LTE4, was not different between phosgene-exposed, air-treated or ETYA-posttreatment + phosgene over time. Posttreatment with ETYA significantly lowered TBARS formation, 206 +/- 13 versus 285 +/- 23 nmol/mg protein (P = .016), compared with phosgene-exposed lungs. Paradoxically, ETYA posttreatment decreased PEF and lipid peroxidation, but increased sulfidopeptide LT release from the lung during perfusion. We conclude that LTC4/D4/E4, and B4, may play different roles than previously thought for PEF in the isolated perfused lung model.

Eicosanoids, fatty acids and neutrophils: their relevance to the pathophysiology of disease

PUFA and their eicosanoid metabolites are potent biological modifiers. They have beneficial effects in a number of diseases, which may result in part from their direct actions on neutrophils as well as from their ability to modulate eicosanoid biosynthesis. A consideration of their interactions with other cell types, e.g. lymphocytes and macrophages, is beyond the scope of this review. Small alterations in structure can result in large changes in the neutrophil response. This will have important implications for the further development and use of fatty acids for therapeutic purposes.