Comparison of leukotriene A4 metabolism into leukotriene C4 by human platelets and endothelial cells

New analogues of 13-hydroxyocatdecadienoic acid and 12-hydroxyeicosatetraenoic acid block human blood platelet aggregation and cyclooxygenase-1 activity

Background

Thromboxane A₂ is derived from arachidonic acid through the action of cyclooxygenases and thromboxane synthase. It is mainly formed in blood platelets upon activation and plays an important role in aggregation. Aspirin is effective in reducing the incidence of complications following acute coronary syndrome and stroke. The anti-thrombotic effect of aspirin is obtained through the irreversible inhibition of cyclooxygenases. Analogues of 12-hydroxyeicosatetraenoic acid and 13-hydroxyocatdecadienoic acid were shown previously to modulate platelet activation and to block thromboxane receptors.

Results and discussion

We synthesized 10 compounds based on the structures of analogues of 12-hydroxyeicosatetraenoic acid and 13-hydroxyocatdecadienoic acid and evaluated their effect on platelet aggregation triggered by arachidonic acid. The structure activity relationship was evaluated. Five compounds showed a significant inhibition of platelet aggregation and highlighted the importance of the lipidic hydrophobic hydrocarbon chain and the phenol group. Their IC₅₀ ranged from 7.5 ± 0.8 to 14.2 ± 5.7 μM (Mean ± S.E.M.). All five compounds decreased platelet aggregation and thromboxane synthesis in response to collagen whereas no modification of platelet aggregation in response to thromboxane receptor agonist, U46619, was observed. Using COS-7 cells overexpressing human cyclooxygenase-1, we showed that these compounds are specific inhibitors of cyclooxygenase-1 with IC₅₀ ranging from 1.3 to 12 μM. Docking observation of human recombinant cyclooxygenase-1 supported a role of the phenol group in the fitting of cyclooxygenase-1, most likely related to hydrogen bonding with the Tyr 355 of cyclooxygenase-1.

Conclusions

In conclusion, the compounds we synthesized at first based on the structures of analogues of 12 lipoxygenase metabolites showed a role of the phenol group in the anti-platelet and anti-cyclooxygenase-1 activities. These compounds mediate their effects via blockade of cyclooxygenase-1.

Eicosanoid Transcellular Biosynthesis: From Cell-Cell Interactions to in Vivo Tissue Responses

The biosynthesis of the biologically active metabolites of arachidonic acid involves a number of enzymes that are differentially expressed in cells. Prostaglandins and thromboxanes are derived from the chemically unstable prostaglandin (PG) H(2) intermediate synthesized by PGH synthases (cyclooxygenase-1/2) and leukotrienes from chemically unstable leukotriene A(4) by 5-lipoxygenase. Additional enzymes transform these reactive intermediates to a variety of chemical structures known collectively as the lipid mediators. Although some cells have the complete cassette of enzymes required for the production of biologically active prostaglandins and leukotrienes, the actual biosynthetic events often are a result of cell-cell interaction and a transfer of these chemically reactive intermediates, PGH(2) and leukotriene A(4), between cells. This process has come to be known as transcellular biosynthesis of eicosanoids and requires a donor cell to synthesize and release one component of the biosynthetic cascade and a second, accessory cell to take up that intermediate and process each into the final biologically active product. This review focuses on the evidence for transcellular biosynthetic events for prostaglandins, leukotrienes, and lipoxins occurring during cell-cell interactions. Evidence for arachidonic acid serving as a transcellular biosynthetic intermediate is presented. Experiments for transcellular events taking place in vivo that reveal the true complexity of eicosanoid biosynthesis within tissues are also reviewed.

Arachidonic acid cascade in endothelial pathobiology

Arachidonic acid (AA) and its metabolites (eicosanoids) represent powerful mediators, used by organisms to induce and suppress inflammation as a part of the innate response to disturbances. Several cell types participate in the synthesis and release of AA metabolites, while many cell types represent the targets for eicosanoid action. Endothelial cells (EC), forming a semi-permeable barrier between the interior space of blood vessels and underlying tissues, are of particular importance for the development of inflammation, since endothelium controls such diverse processes as vascular tone, homeostasis, adhesion of platelets and leukocytes to the vascular wall, and permeability of the vascular wall for cells and fluids. Proliferation and migration of endothelial cells contribute significantly to new vessel development (angiogenesis). This review discusses endothelial-specific synthesis and action of arachidonic acid derivatives with a particular focus on the mechanisms of signal transduction and associated intracellular protein targets.

Formation of murine macrophage-derived 5-oxo-7-glutathionyl-8,11,14-eicosatrienoic acid (FOG7) is catalyzed by leukotriene C4 synthase

5-Oxo-7-glutathionyl-8,11,14-eicosatrienoic acid (FOG(7)), a biologically active glutathione (GSH) adduct of the eicosanoid 5-oxo-eicosatrienoic acid (5-oxoETE), is the major metabolite formed within the murine peritoneal macrophage. The conjugation of GSH to electrophilic 5-oxoETE in vitro was found to be catalyzed by both soluble glutathione S-transferase and membrane-bound leukotriene C(4) (LTC(4)) synthase. The cytosolic glutathione S-transferase-catalyzed products were not biologically active; however, the adduct formed from recombinant LTC(4) synthase had identical mass spectrometric properties and biological activity to the macrophage-derived FOG(7). The biosynthesis of FOG(7) in the macrophage was inhibited by MK-886, a known inhibitor of LTC(4) synthase, suggesting that this nuclear membrane-bound enzyme might be responsible for GSH conjugation to 5-oxoETE in the intact cell. Subcellular fractionation revealed that the microsomal fraction from the murine macrophage contained the enzyme responsible for FOG(7) biosynthesis. Western blot analysis confirmed the presence of LTC(4) synthase in the microsomal fraction that did not catalyze conjugation of GSH to 1-chloro-2,4-dinitrobenzene, indicating an absence of microsomal glutathione S- transferase activity. These results suggest that LTC(4) synthase, thought to be specific for the conjugation of GSH to LTA(4), can also recognize 5-oxoETE as an electrophilic substrate.

Role of 5-Lipoxygenase Products in the Local Accumulation of Neutrophils in Dermal Inflammation in the Rabbit

Studies were undertaken to define the role of 5-lipoxygenase (5-LO) products and, in particular, of leukotriene (LT) B4 in the polymorphonuclear leukocyte (PMN) emigration process using a rabbit model of dermal inflammation. Our results show that i.v. administration to rabbits of MK-0591, a compound that inhibits LT biosynthesis in blood and tissues when administered in vivo, significantly reduced 51Cr-labeled PMN accumulation in response to intradermally injected chemotactic agonists, including IL-8, FMLP, C5a, and LTB4 itself. In addition, pretreatment of the labeled PMN with MK-0591 ex vivo before their injection in recipient animals was equally effective in reducing 51Cr-labeled PMN emigration to dermal inflammatory sites. These results support a role for de novo synthesis of 5-LO metabolites by PMN for their chemotactic response to inflammatory mediators. Other studies demonstrated that elevated intravascular concentration of LTB4 interferes with PMN extravasation inasmuch as a continuous i.v. infusion of LTB4, in the range of 5–300 ng/min/kg, dose-dependently inhibited extravascular PMN accumulation to acute inflammatory skin sites elicited by the chemoattractants LTB4, FMLP, C5a, and IL-8 and by TNF-α, IL-1β, and LPS; such phenomena may constitute a natural protective mechanism from massive tissue invasion by activated PMN in specific pathologic conditions such as ischemia (and reperfusion). These studies demonstrate additional functions of 5-LO products in the regulation of PMN trafficking, distinct from the well-characterized chemotactic activity of LTB4 present in the extravascular compartment.

Direct inhibition of cyclooxygenase-1 and -2 by the kinase inhibitors SB 203580 and PD 98059. SB 203580 also inhibits thromboxane synthase

The kinase inhibitors SB 203580 and PD 98059 have been reported to be specific inhibitors of the 38- and 42/44-kDa mitogen-activated protein kinase (MAPK) pathways, respectively. In this study, the two inhibitors were found to decrease platelet aggregation induced by low concentrations of arachidonic acid, suggesting that they also interfere with the metabolism of arachidonic acid to thromboxane A2. In support of this, SB 203580 and PD 98059 inhibited the conversion of exogenous [3H]arachidonic acid to [3H]thromboxane in intact platelets. Measurement of platelet cyclooxygenase-1 activity following immunoprecipitation revealed that SB 203580 and PD 98059 are direct inhibitors of this enzyme. Both compounds were shown to inhibit purified cyclooxygenase-1 and -2 by a reversible mechanism. In addition, SB 203580 (but not PD 98059) inhibited platelet aggregation induced by prostaglandin H2 and the conversion of prostaglandin H2 to thromboxane A2 in intact platelets. SB 203580 also inhibited this pathway in platelet microsome preparations, suggesting a direct inhibitory effect on thromboxane synthase. These results demonstrate that direct effects of the two kinase inhibitors on active arachidonic acid metabolites have to be excluded before using these compounds for the investigation of MAPKs in signal transduction pathways. This is of particular relevance to studies on the regulation of cytosolic phospholipase A2 as these two MAPKs are capable of phosphorylating cytosolic phospholipase A2, thereby increasing its intrinsic activity.

New concepts in coagulation

The process of blood coagulation is a complex and incompletely understood process. In the last decade or so critical steps have been taken toward better understanding this process. It is now widely agreed that the principal initiating pathway of coagulation is the so-called extrinsic pathway due to the action of tissue factor and Factor VII. Concomitant with this appreciation has come an understanding and elucidation of the roles of tissue factor pathway inhibitor. Although the "intrinsic pathway" is no longer felt to be the initiator of coagulation, recent evidence suggests that Factor XIa may constitute an important amplification pathway of the coagulation system in vivo. Refinement of flow cytometry has enabled the detection of novel platelet antigens on activated platelet surfaces. It is hoped that detection and characterization of these antigens, including adhesion molecules such as P-selectin, will enable further understanding of the platelet's role in pathological coagulation and inflammation. The endothelium is also intricately involved and recent work has determined the importance of endothelial produced factors such as endothelium-derived relaxation factor, endothelin, and thrombomodulin. Finally, with the meteoric rise in molecular genetic technology, specific genetic abnormalities in a number of plasma proteins has been elucidated, with marked implications on the understanding of the coagulation process. For example, the mutation on the gene for Factor V, leading to Arg506 replacement with Gln, produces activated protein C resistance with a concomitant increased risk of venous thrombosis. Thus, significant advances in knowledge of the endothelium, platelets, and plasma factors involved in coagulation have been made and now the challenge of the future is to better elucidate the interactions of these components.

Production of leukotriene C4 in different human tissues is attributable to distinct membrane bound biosynthetic enzymes

Microsomal glutathione S-transferase-II (GST-II) has recently been discovered and characterized as a member of the 5-lipoxygenase-activating protein (FLAP)/5(S)-hydroxy-6(R)-S-glutathionyl-7,9-trans-11, 14-cis-eicosatetraenoic acid (LTC4) synthase gene family, which also includes microsomal glutathione S-transferase-I (GST-I) as a distant member of this gene family. This new enzyme is unique as it is the only member of this family capable of efficiently conjugating reduced glutathione to both 5,6-oxido-7,9,11,14-eicosatetraenoic acid (LTA4) and 1-chloro-2,4-dinitrobenzene. Although microsomal GST-II has been demonstrated to display both general glutathione S-transferase (GST) and specific LTC4 synthase activities, its biological function remains unknown. In this study, we investigated the physiological location of microsomal GST-II as well as the relative importance of this enzyme versus LTC4 synthase for the production of LTC4 in various human tissues and cells that have been previously demonstrated to possess LTC4 synthase activity. As determined by Western blot, microsomal GST-II was predominantly expressed in human liver microsomes, human endothelial cell membranes, and sparsely detected in human lung membranes. In contrast, LTC4 synthase was prevalent in human lung membranes, human platelet homogenates, and human kidney tissue. Concomitant to the formation of LTC4, microsomal GST-II also produces a new metabolite of LTA4, a postulated LTC4 isomer. This isomer was used to distinguish between microsomal GST-II and LTC4 synthase activities involved in the biosynthesis of LTC4. Based on the relative production of LTC4 to the LTC4 isomer, microsomal GST-II was demonstrated to be the principal enzyme responsible for LTC4 production in human liver microsomes and human endothelial cells and played a minor role in the formation of LTC4 in human lung membranes. In comparison, LTC4 synthase was the main enzyme capable of catalyzing the conjugation of reduced glutathione to LTA4 in human lung membranes and human platelet homogenates. Therefore, microsomal GST-II appears to be an integral component in the detoxification of biological systems due to its marked presence in human liver, in accordance with its known GST activity. Microsomal GST-II, however, may also be pivotal for cysteinyl leukotriene formation in endothelial cells, and this could change our current understanding of the regulation of leukotriene biosynthesis in inflammatory disorders such as asthma.

Cyclooxygenase-1 and -2 of endothelial cells utilize exogenous or endogenous arachidonic acid for transcellular production of thromboxane

The presence of prostaglandin (PG) H2 in the supernatant of human umbilical vein endothelial cells (HUVEC) stimulated by thrombin restores the capacity of aspirin-treated platelets to generate thromboxane (TX) B2. Induction of cyclooxygenase-2 (Cox-2) by interleukin (IL)-1alpha or a phorbol ester increases this formation. HUVEC treated with aspirin lost their capacity to generate PGs but recovery occurred after 3- or 6-h induction of Cox-2 with phorbol ester or IL-1alpha. Enzyme activity of the newly synthesized Cox-2 in aspirin-treated cells, evaluated after immunoprecipitation, was similar to untreated cells but after 18 h of cell stimulation only 50-60% recovery of Cox-1 was observed. The use of SC58125, a selective Cox-2 inhibitor, confirmed these findings in intact cells. Cyclooxygenase activity was related to the amount of Cox proteins present in the cells, but after induction of Cox-2, contribution of the latter to PG production was 6-8-fold that of Cox-1. Aspirin-treated or untreated cells were incubated in the absence or presence of SC58125 and stimulated by thrombin, the ionophore A23187, or exogenous arachidonic acid. The production of endogenous (6-keto-PGF1alpha, PGE2, PGF2alpha) versus transcellular (TXB2) metabolites was independent of the inducer, the source of arachidonic acid and the Cox isozyme. However, in acetylsalicylic acid-treated cells, after 6-h stimulation with IL-1alpha, newly synthesized Cox-2 produced less TXB2 than 6-keto-PGF1alpha compared to untreated cells. At later times (>18 h), there was no metabolic difference between the cells. These studies suggest that in HUVEC, Cox compartmentalization occurring after short-term activation may selectively affect transcellular metabolism, but not constitutive production, of PGs.

Receptor-mediated regulation of leukotriene C4 synthase activity in human platelets

Human platelets possess a specific membrane-bound leukotriene (LT) C4 synthase, which catalyzes the conversion of LTA4 to LTC4. Stimulation of the receptors for thrombin, collagen or thromboxane A2 provoked inhibition of this enzyme, as judged by suppressed transformation of exogenous LTA4 to LTC4. Similarly, direct activation of protein kinase (PK) C with nanomolar concentrations of 4 beta-phorbol 12-myristate 13-acetate (PMA) inhibited the production of LTC4. Kinetic studies demonstrated that the inhibition induced by thrombin and PMA was non-competitive. Elevation of intracellular cAMP levels with carbacyclin did not affect basal LTC4 formation, but abolished the attenuation of platelet LTC4 synthase activity induced by the thromboxane receptor agonist U-46619. The unselective protein kinase inhibitor staurosporine prevented both receptor-mediated and PMA-induced suppression of LTC4 formation. In contrast, two selective PKC inhibitors, Ro 31-8220 and GF 109203X, reversed the inhibitory effect provoked by PMA, but failed to prevent thrombin-induced inhibition. Furthermore, the protein tyrosine phosphatase inhibitor, sodium orthovanadate, induced dose-dependent inhibition of LTC4 production in platelet sonicates. In conclusion, receptor-mediated activation of human platelets leads to decreased LTC4 synthase activity via phosphoregulation. Although the present results demonstrate that platelet LTC4 synthase can be regulated via PKC-dependent events, alternative mechanisms appears to be involved in the physiological regulation of this enzyme. The findings suggest the possible importance of protein tyrosine phosphorylations in this process.

Response of the isolated guinea pig bladder to exogenous and endogenous leukotrienes

Noninfectious urinary bladder inflammation is a poorly understood phenomenon, and the participation of leukotrienes (LTs) in the pathogenesis of bladder inflammation is unclear. Leukotrienes are synthesized by the bladder, and exogenous LTs induce contraction of isolated bladder segments. LTD4 and LTC4 were more potent contractile agents than LTE4. Leukotriene-induced contractions were blocked by ICI 198,615 (10(-6) M. and 10(-7) M.) a specific LT receptor antagonist. In the presence of indomethacin (5 x 10(-6) M.), bladder contraction in response to LTD4 was increased. Endogenous LT release was studied using an experimental model of cystitis. Antigen (ovalbumin 10(-6) to 10(-2) mg./ml.) challenge of bladder segments isolated from actively sensitized animals induced release of LT, prostaglandin D2 and histamine. A-64077 (Zileuton), a 5-lipoxygenase (5-LO) inhibitor, significantly reduced contraction of sensitized bladder tissue in response to antigen challenge in a concentration-dependent manner and abolished LT release. These data indicate that the guinea pig urinary bladder produces sulfidopeptide-LTs that can be released upon specific stimulation. Furthermore, LTs activate specific receptors promoting bladder contraction. Our findings suggest that specific 5-LO inhibitors or LT-receptor antagonists might be useful in treating or preventing bladder inflammation.

Enzyme immunometric assay for leukotriene C4

An enzyme immunometric assay of LTC4 named SPIE-IA is described. The assay involves different sequential steps: (1) immunocapture of LTC4 by monoclonal anti-LTC4 antibodies coated on 96-well microtiter plates; (2) cross-linking of LTC4 via its amino group to the wells using glutaraldehyde; (3) treatment with HCl; (4) measurement of linked LTC4 using the same monoclonal anti-LTC4 antibodies labeled with acetylcholinesterase. A minimal detectable concentration of 2 pg/ml after 60 min of enzymatic reaction was obtained. Cross-reactivity was less than 15% with LTD4 or LTE4. The coefficient of variation was less than 6% in the 20-1000 pg/ml range. Good correlation was observed between SPIE-IA and a competitive enzyme immunoassay for biological samples. The different sequential steps of the assay are investigated.

Transcellular biosynthesis of arachidonic acid metabolites: from in vitro investigations to in vivo reality

The discovery that arachidonic acid metabolism in a multicellular environment could be different from that expected from the sum of individual cell types has led to the concept of transcellular metabolism. In this process, several cells can contribute to the formation of a novel compound with potent biological action. The study of this mode of synthesis is important in the context of the current appraisal of thrombotic diseases as part of an inflammatory reaction. In this context, blood cell-vessel wall interactions present a regulated expression of adhesive molecules on either type of cell. These complex processes are initiated by signalling molecules such as cytokines that can deeply modify the phenotype of endothelial cells, which may ultimately lead to a change in the vascular tone and to atherosclerotic complications. Such reaction processes are part of the autocrine-endocrine system whereby cells can control and modify their own phenotype through the action of a local network of mediators. In this context, arachidonic acid metabolites may be an important part of unifying signal molecules that participate in these changes. The significance of transcellular biosynthesis where combined cells acquire a different metabolic potential can be viewed as an additional modification of blood cell and vessel cell phenotype in thrombotic diseases.