Platelet-neutrophil interactions. (12S)-hydroxyeicosatetraen-1,20-dioic acid: a new eicosanoid synthesized by unstimulated neutrophils from (12S)-20-dihydroxyeicosatetraenoic acid

Changes in arachidonic acid (AA)- and linoleic acid (LA)-derived hydroxy metabolites and their interplay with inflammatory biomarkers in response to drastic changes in air pollution exposure

BACKGROUND

Untargeted metabolomics analyses have indicated that fatty acids and their hydroxy derivatives may be important metabolites in the mechanism through which air pollution potentiates diseases. This study aimed to use targeted analysis to investigate how metabolites in arachidonic acid (AA) and linoleic acid (LA) pathways respond to short-term changes in air pollution exposure. We further explored how they might interact with markers of antioxidant enzymes and systemic inflammation.

METHODS

This study included a subset of participants (n = 53) from the Beijing Olympics Air Pollution (BoaP) study in which blood samples were collected before, during, and after the Beijing Olympics. Hydroxy fatty acids were measured by liquid chromatography/mass spectrometry (LC/MS). Native total fatty acids were measured as fatty acid methyl esters (FAMEs) using gas chromatography. A set of chemokines were measured by ELISA-based chemiluminescent assay and antioxidant enzyme activities were analyzed by kinetic enzyme assays. Changes in levels of metabolites over the three time points were examined using linear mixed-effects models, adjusting for age, sex, body mass index (BMI), and smoking status. Pearson correlation and repeated measures correlation coefficients were calculated to explore the relationships of metabolites with levels of serum chemokines and antioxidant enzymes.

RESULTS

12-hydroxyeicosatetraenoic acid (12-HETE) decreased by 50.5% (95% CI: -66.5, -34.5; p < 0.0001) when air pollution dropped during the Olympics and increased by 119.4% (95% CI: 36.4, 202.3; p < 0.0001) when air pollution returned to high levels after the Olympics. In contrast, 13-hydroxyoctadecadienoic acid (13-HODE) elevated significantly (p = 0.023) during the Olympics and decreased nonsignificantly after the games (p = 0.104). Interleukin 8 (IL-8) correlated with 12-HETE (r = 0.399, BH-adjusted p = 0.004) and 13-HODE (r = 0.342, BH-adjusted p = 0.014) over the three points; it presented a positive and moderate correlation with 12-HETE during the Olympics (r = 0.583, BH-adjusted p = 0.002) and with 13-HODE before the Olympics (r = 0.543, BH-adjusted p = 0.008).

CONCLUSION

AA- and LA-derived hydroxy metabolites are associated with air pollution and might interact with systemic inflammation in response to air pollution exposure.

Innate immune receptors in platelets and platelet-leukocyte interactions

Platelets are chief cells in hemostasis. Apart from their hemostatic roles, platelets are major inflammatory effector cells that can influence both innate and adaptive immune responses. Activated platelets have thromboinflammatory functions linking hemostatic and immune responses in several physiological and pathological conditions. Among many ways in which platelets exert these functions, platelet expression of pattern recognition receptors (PRRs), including TLR, Nod-like receptor, and C-type lectin receptor families, plays major roles in sensing and responding to pathogen-associated or damage-associated molecular patterns (PAMPs and DAMPs, respectively). In this review, an increasing body of evidence is compiled showing the participation of platelet innate immune receptors, including PRRs, in infectious diseases, sterile inflammation, and cancer. How platelet recognition of endogenous DAMPs participates in sterile inflammatory diseases and thrombosis is discussed. In addition, platelet recognition of both PAMPs and DAMPs initiates platelet-mediated inflammation and vascular thrombosis in infectious diseases, including viral, bacterial, and parasite infections. The study also focuses on the involvement of innate immune receptors in platelet activation during cancer, and their contribution to tumor microenvironment development and metastasis. Finally, how innate immune receptors participate in platelet communication with leukocytes, modulating leukocyte-mediated inflammation and immune functions, is highlighted. These cell communication processes, including platelet-induced release of neutrophil extracellular traps, platelet Ag presentation to T-cells and platelet modulation of monocyte cytokine secretion are discussed in the context of infectious and sterile diseases of major concern in human health, including cardiovascular diseases, dengue, HIV infection, sepsis, and cancer.

Platelet lipidome: Dismantling the "Trojan horse" in the bloodstream

The platelet-lipid chapter in the story of atherothrombosis is an old one, recapitulated and revised in many contexts. For decades several stimulating facets have been added to it, both unraveling and increasing the perplexity of platelet-lipid interplay and its pathophysiological consequences. The recent paradigm shift in our perspective has evolved with lipidomic analysis of the intraplatelet compartment and platelet releasate. These investigations have disclosed that platelets are in constant interaction with circulatory lipids, often reflected in their lipid repertoire. In addition, they offer a shielded intracellular space for oxidative lipid metabolism generating "toxic" metabolites that escape degradation by plasma lipases and antioxidant defense, circulate undetected by conventional plasma lipid profile, and deposited at atherosclerotic lesions or thrombus. Lipidomics divulges this silent invader in platelet vehicles, thereby providing potential biomarkers of pathologic manifestations and therapeutic targets to be exploited, which is surmised in this review.

Role of Platelets in Leukocyte Recruitment and Resolution of Inflammation

Platelets are most often recognized for their crucial role in the control of acute hemorrhage. However, current research has greatly expanded the appreciation of platelets beyond their contribution to primary hemostasis, indicating that platelets also actively participate in leukocyte recruitment and the regulation of the host defense in response to exogenous pathogens and sterile injury. Early recruitment of leukocytes, especially neutrophils, is the evolutionary stronghold of the innate immune response to successfully control exogenous infections. Platelets have been shown to physically interact with different leukocyte subsets during inflammatory processes. This interaction holds far-reaching implications for the leukocyte recruitment into peripheral tissues as well as the regulation of leukocyte cell autonomous functions, including the formation and liberation of neutrophil extracellular traps. These functions critically depend on the interaction of platelets with leukocytes. The host immune response and leukocyte recruitment must be tightly regulated to avoid excessive tissue and organ damage and to avoid chronification of inflammation. Thus, platelet-leukocyte interactions and the resulting leukocyte activation and recruitment also underlies tight regulation by several inherited feedback mechanisms to limit the extend of vascular inflammation and to protect the host from collateral damage caused by overshooting immune system activation. After the acute inflammatory phase has been overcome the host defense response must eventually be terminated to allow for resolution from inflammation and restoration of tissue and organ function. Besides their essential role for leukocyte recruitment and the initiation and propagation of vascular inflammation, platelets have lately also been implicated in the resolution process. Here, their contribution to phagocyte clearance, T cell recruitment and macrophage reprogramming is also of outmost importance. This review will focus on the role of platelets in leukocyte recruitment during the initiation of the host defense and we will also discuss the participation of platelets in the resolution process after acute inflammation.

Platelet-neutrophil interactions as drivers of inflammatory and thrombotic disease

Neutrophils are well known for their role in infection and inflammatory disease and are first responders at sites of infection or injury. Platelets have an established role in hemostasis and thrombosis and are first responders at sites of vascular damage. However, neutrophils are increasingly recognized for their role in thrombosis, while the immunemodulatory properties of platelets are being increasingly studied. Platelets and neutrophils interact during infection, inflammation and thrombosis and modulate each other’s functions. This review will discuss the consequences of platelet–neutrophil interactions in infection, thrombosis, atherosclerosis and tissue injury and repair.

Role of platelets in neutrophil extracellular trap (NET) production and tissue injury

In addition to their well-known role as the cellular mediator of thrombosis, numerous studies have identified key roles for platelets during various disease processes. Importantly, platelets play a critical role in the host immune response, directly interacting with, and eliminating pathogens, from the blood stream. In addition to pathogen clearance, platelets also contribute to leukocyte recruitment at sites of infection and inflammation, and modulate leukocyte activity. Platelet interaction with activated neutrophils is a potent inducer of neutrophil extracellular trap (NET). NETs consist of a diffuse, sticky web of extracellular DNA, nuclear and granular proteins, and serve to ensnare and kill pathogens. In addition to catching pathogens, the cytotoxic molecules and proteases on NETs have the potential to inflict significant tissue damage. Additionally, NET components have been suggested to be key activators of infection-induced coagulopathy. These critical roles, at the interface between hemostasis and immunity, highlight the need for balance in the platelet response; too little platelet activity results in bleeding and immune deficit, too much leads to tissue pathogenesis. In this review, we highlight recent advances in our understanding of the role platelets play in inflammation, the link between platelets and NETs and the role platelets play in disease pathogenesis.

Directed transport of neutrophil-derived extracellular vesicles enables platelet-mediated innate immune response

The innate immune response to bacterial infections requires the interaction of neutrophils and platelets. Here, we show that a multistep reciprocal crosstalk exists between these two cell types, ultimately facilitating neutrophil influx into the lung to eliminate infections. Activated platelets adhere to intravascular neutrophils through P-selectin/P-selectin glycoprotein ligand-1 (PSGL-1)-mediated binding, a primary interaction that allows platelets glycoprotein Ibα (GPIbα)-induced generation of neutrophil-derived extracellular vesicles (EV). EV production is directed by exocytosis and allows shuttling of arachidonic acid into platelets. EVs are then specifically internalized into platelets in a Mac1-dependent fashion, and relocated into intracellular compartments enriched in cyclooxygenase1 (Cox1), an enzyme processing arachidonic acid to synthesize thromboxane A₂ (TxA₂). Finally, platelet-derived-TxA₂ elicits a full neutrophil response by inducing the endothelial expression of ICAM-1, intravascular crawling, and extravasation. We conclude that critical substrate–enzyme pairs are compartmentalized in neutrophils and platelets during steady state limiting non-specific inflammation, but bacterial infection triggers regulated EV shuttling resulting in robust inflammation and pathogen clearance.

Interaction between platelets and neutrophils promotes neutrophil activation. Here the authors show that neutrophils initiate the cross-talk with platelets by shuttling arachidonic acid via extracellular vesicles, which platelets convert to thromboxane A₂ that then elicits neutrophil activation.

Platelets and allergic inflammation

Irrefutable clinical evidence demonstrates the activation of platelets in allergic diseases, including asthma, allergic rhinitis, and eczema. Indeed, experimental models of allergic disease have now shown that platelets play a fundamental role in the tissue recruitment of leucocytes following exposure to allergens. Furthermore, the extravascular presence of platelets in lungs of patients with asthma, and in animal models of allergic lung inflammation suggests that platelets may also contribute directly to allergic inflammation, through alterations in lung function, or by modulating processes involved in airway wall remodelling. Despite significant platelet activation in patients with allergic diseases, it is of note that these patients have been described as having a mild haemostastic defect, rather than an increased incidence of thrombosis. This suggests a dichotomy exists in platelet activation during inflammation compared to haemostasis, and that hitherto undiscovered platelet activation pathways might be exploited to create novel anti-inflammatory therapies without affecting the critical function of platelets in haemostasis.

Neutrophil and platelet complexes and their relevance to neutrophil recruitment and activation

The manifestation of platelet 'satallitism' around neutrophils in whole blood is a long acknowledged phenomenon [1]. Circulating platelet-neutrophil complexes (PNC) occur in a diverse range of inflammatory disorders and infections that affect numerous organs of the body. Animal models have revealed that the formation of PNC is required for the recruitment of neutrophils to inflamed tissue, since platelets 'prime' neutrophils for efficient adhesion to vascular endothelium via the up-regulation of integrins and enhanced responsiveness to chemokines (Fig. 1). Perhaps surprisingly, the surface contact between platelets and neutrophils additionally enhances other neutrophil functions, such as chemotaxis that is required for migration into tissues, trans-cellular production of eicosanoids, phagocytosis and trapping of pathogens, increased respiratory burst leading to the production of reactive oxygen species (ROS), and modulation of neutrophil apoptosis (Fig. 1). Platelet P-selectin appears to have a particular role in enhancing the majority of these activities, and the influence of platelet P-selectin is not therefore confined to the initial rolling events in the process of neutrophil extravasation.

Pharmacological modulation of the inflammatory actions of platelets

Patients with inflammatory diseases often exhibit a change in platelet function, with these alterations being clearly distinct from the well-characterized role of platelets in haemostasis and thrombosis. It has recently been revealed that platelets can behave as innate inflammatory cells in immune responses with roles in leukocyte recruitment, migration into tissues, release of cytotoxic mediators, and in tissue remodelling following injury.Platelets exhibit a wide range of receptors for mediators involved in the inflammatory pathway and the immune response (Fig. 1). These include purinergic receptors, selectins, integrins, toll-like receptors, immunoglobulins, and chemokine receptors, but the precise role platelets play in the inflammatory process is still under investigation. Nevertheless, given that many of these receptors are distinct from those involved in thrombosis and haemostasis, this raises the real possibility of targeting these receptors to regulate inflammatory diseases without compromising haemostasis.

Interference of neutrophil-platelet interaction by YC-1: a cGMP-dependent manner on heterotypic cell-cell interaction

N-Formyl-Met-Leu-Phe (fMLP) activated neutrophils and then induced neutrophil-platelet complex formation in co-incubation condition. In addition, fMLP induce intracellular calcium mobilization in platelets, only when it is incubated along with neutrophils. This data established that fMLP-stimulated neutrophils activate platelets. 9E1, a monoclonal antibody of P-selectin, significantly blocks the formation of neutrophil-platelet complex induced by fMLP, indicating the involvement of P-selectin in the neutrophil-platelet complex formation. 3-(5'-hydroxymethyl-2'-furyl-1-benzylindazole (YC-1), an unique nitric oxide-independent activator of soluble guanylate cyclase, was evaluated for its effect on neutrophil-platelet complex. YC-1 inhibits fMLP-induced neutrophil-platelet complex formation in a concentration-dependent manner with an IC50 value of 15.3+/-3.5 microM. However, this effect of YC-1 is partially reversed by pre-treatment of 1H-(1,2,4)oxadiazolo[4,3-a]quinozalin-1-one (ODQ; 10 microM), which is a soluble guanylate cyclase inhibitor. Pre-treatment of either neutrophils or platelets with YC-1 (50 microM) prevent the fMLP-induced neutrophil-platelet complex formation, indicating that YC-1 could potentially exert its effects individually on either neutrophils or platelets alone. Cathepsin G released from fMLP-stimulated neutrophil activates the nearby platelets. YC-1 was also shown to inhibit this release of cathepsin G in a concentration-dependent manner. The IC50 value was 6.2+/-0.2 microM. This inhibitory effect of YC-1 on cathepsin G release is reversed by ODQ (10 microM) and a protein kinase G inhibitor [1-oxo-9.12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-l][1,6]benzodiazocine-10-carbooxylic acid methyl ester (KT5835); 1 microM]. YC-1 inhibits cathepsin G-induced P-selectin expression on human platelet at the IC50 value of 32.5+/-2.6 microM. A further study showed that YC-1 inhibits fMLP-induced neutrophil-platelet complex formation in whole blood at the IC50 value of 35.8+/-8.1 microM in a concentration-dependent manner. According to these data, it was hypothesized that fMLP stimulates neutrophils to release cathepsin G, which subsequently activates the nearby platelets, creating neutrophil-platelet complexes. YC-1 inhibits fMLP-induced neutrophil from releasing cathepsin G via a cGMP-dependent pathway. This inhibitory effect of YC-1 on cathepsin G release is a major mechanism for affecting fMLP-induced neutrophil-platelet complex. YC-1's inhibition P-selectin expression on platelet may potentiate its effects. These inhibitory effects may contribute to the inhibition of neutrophil-platelet complex formation in whole blood.

Heterologous cell-cell interactions: thromboregulation, cerebroprotection and cardioprotection by CD39 (NTPDase-1)

Blood platelets maintain vascular integrity and promote primary and secondary hemostasis following interruption of vessel continuity. Biochemical or physical damage to the coronary, carotid or peripheral arteries is followed by excessive platelet activation and recruitment culminating in vascular occlusion and tissue ischemia. Currently inadequate therapeutic approaches to stroke and coronary artery disease are a public health issue. Following our demonstration of neutrophil leukotriene production from arachidonate released from activated aspirin-treated platelets, we studied interactions between platelets and other blood cells, leading to concepts of transcellular metabolism and thromboregulation. Thrombosis has a proinflammatory component whereby biologically active substances are synthesized by interactions between different cell types that could not individually synthesize the product(s). Endothelial cells control platelet reactivity via three biochemical systems-autacoids leading to production of prostacyclin and nitric oxide, and endothelial ecto-ADPase/CD39/NTPDase-1. The autacoids are fluid-phase reactants, not produced by tissues in the basal state. They are only synthesized intracellularly and released upon interactions of cells with an agonist. When released, autacoids exert fleeting actions in the immediate milieu, and are rapidly inactivated. CD39 is an integral component of the endothelial cell surface and is substrate-activated. It maintains vascular fluidity in the complete absence of prostacyclin and nitric oxide, indicating that they are ancillary components of hemostasis. Therapeutic implications for the autacoids have not been compelling because of their transient, local and fleeting action, and limited potency. Conversely, CD39, acting solely on the platelet releasate, is efficacious in three different animal models. It metabolically neutralizes a prothrombotic platelet releasate via deletion of ADP--the major recruiting agent responsible for formation of an occlusive thrombus. In addition, solCD39 reduced ATP- and ischemia-induced norepinephrine release in the heart. This reduction can prevent fatal arrhythmia. Moreover, solCD39 ameliorated the sequelae of stroke in CD39 null mice. CD39 represents the next generation of cardioprotective and cerebroprotective molecules.

Metabolic control of excessive extracellular nucleotide accumulation by CD39/ecto-nucleotidase-1: implications for ischemic vascular diseases

Platelets are responsible for maintaining vascular integrity. In thrombocytopenic states, vascular permeability and fragility increase, presumably due to the absence of this platelet function. Chemical or physical injury to a blood vessel induces platelet activation and platelet recruitment. This is beneficial for the arrest of bleeding (hemostasis), but when an atherosclerotic plaque is ulcerated or fissured, it becomes an agonist for vascular occlusion (thrombosis). Experiments in the late 1980s cumulatively indicated that endothelial cell CD39-an ecto-ADPase-reduced platelet reactivity to most agonists, even in the absence of prostacyclin or nitric oxide. As discussed herein, CD39 rapidly and preferentially metabolizes ATP and ADP released from activated platelets to AMP, thereby drastically reducing or even abolishing platelet aggregation and recruitment. Since ADP is the final common agonist for platelet recruitment and thrombus formation, this finding highlights the significance of CD39. A recombinant, soluble form of human CD39, solCD39, has enzymatic and biological properties identical to the full-length form of the molecule and strongly inhibits human platelet aggregation induced by ADP, collagen, arachidonate, or TRAP (thrombin receptor agonist peptide). In sympathetic nerve endings isolated from guinea pig hearts, where neuronal ATP enhances norepinephrine exocytosis, solCD39 markedly attenuated norepinephrine release. This suggests that NTPDase (nucleoside triphosphate diphosphohydrolase) could exert a cardioprotective action by reducing ATP-mediated norepinephrine release, thereby offering a novel therapeutic approach to myocardial ischemia and its consequences. In a murine model of stroke, driven by excessive platelet recruitment, solCD39 reduced the sequelae of stroke, without an increase in intracerebral hemorrhage. CD39 null mice, generated by deletion of apyrase-conserved regions 2 to 4, exhibited a decrease in postischemic perfusion and an increase in cerebral infarct volume when compared with controls. "Reconstitution" of CD39 null mice with solCD39 reversed these changes. We hypothesize that solCD39 has potential as a novel therapeutic agent for thrombotic diatheses.

Effects of oxidized low density lipoprotein, lipid mediators and statins on vascular cell interactions

The integrin heterodimer CDllb/CD18 (alphaMbeta2, Mac-1, CR3) expressed on monocytes or polymorphonuclear leukocytes (PMN) is a receptor for iC3b, fibrinogen, heparin, and for intercellular adhesion molecule (ICAM)-1 on endothelium, crucially contributing to vascular cell interactions in inflammation and atherosclerosis. In this report, we summarize our findings on the effects of lipid mediators and lipid-lowering drugs. Exposure of endothelial cells to oxidized low density lipoprotein (oxLDL) induces upregulation of ICAM-1 and increases adhesion of monocytic cells expressing Mac-1. Inhibition experiments show that monocytes use distinct ligands, i.e. ICAM-1 and heparan sulfate proteoglycans for adhesion to oxLDL-treated endothelium. An albumin-transferable oxLDL activity is inhibited by the antioxidant pyrrolidine dithiocarbamate (PDTC), while 8-epi-prostaglandin F2alpha (8-epi-PGF2alpha) or lysophosphatidylcholine had no effect, implicating yet unidentified radicals. Sequential adhesive and signaling events lead to the firm adhesion of rolling PMN on activated and adherent platelets, which may occupy areas of endothelial denudation. Shear-resistant arrest of PMN on thrombin-stimulated platelets in flow conditions requires distinct regions of Mac-1, involving its interactions with fibrinogen bound to platelet alphallbbeta3, and with other platelet ligands. Both arrest and adhesion strengthening under flow are stimulated by platelet-activating factor and leukotriene B4, but not by the chemokine receptor CXCR2. We tested whether Mac-1-dependent monocyte adhesiveness is affected by inhibitors of hydroxy-methylglutaryl-Coenzyme A reductase (statins) which improve morbidity and survival of patients with coronary heart disease. As compared to controls, adhesion of isolated monocytes to endothelium ex vivo was increased in patients with hypercholesterolemia. Treatment with statins decreased total and low density lipoprotein (LDL) cholesterol plasma levels, surface expression of Mac-1, and resulted in a dramatic reduction of Mac-1-mediated monocyte adhesion to endothelium. The inhibition of monocyte adhesion was reversed by mevalonate but not LDL in vitro, indicating that isoprenoid precursors are crucial for adhesiveness of Mac-1. Such effects may crucially contribute to the clinical benefit of statins, independent of cholesterol-lowering, and may represent a paradigm for novel, anti-inflammatory mechanisms of action by this class of drugs.

12-hydroxyeicosatetraenoic acid is a long-lived substance in the rabbit circulation

12-Hydroxyeicosatetraenoic acid (12-HETE) is one of the major metabolites formed from arachidonic acid in platelets. We have recently shown that the in vitro metabolism of 12-HETE by human leukocytes, with and without stimulation, is effectively inhibited by the addition of physiological concentrations of albumin, probably by sequestration of the compound. In the present paper, we have studied the in vivo metabolism of 12-HETE in the rabbit, using either [1-14C]- or [14C(U)]12-HETE. Distribution of radioactivity was followed in urine, plasma, and bile, as well as in a number of tissues. In most of the tissues examined, the hydrophilic radioactivity constituted more than 50% of the total radioactivity after 20 min. When the lipophilic fraction was analyzed, around 15% of the radioactivity was shown to be unesterified 12-HETE, and only a very minor part could be detected as metabolites. The dominating lipophilic compound in the circulation after i.v. administration of radiolabeled 12-HETE was at all time points (1-60 min.) the parent compound, as analyzed by HPTLC and HPLC. A comparison of the plasma metabolite profiles obtained when [1-14C]- and [14C(U)]12-HETE were used displayed almost identical patterns, thus indicating that beta-oxidized metabolites either were not formed or were rapidly removed from the circulation. The appearance of large amounts of water-soluble radioactivity with time supported the latter conclusion. Several minor metabolites were seen that chromatographed in the dihydroxy acid region as judged by HPLC and TLC. The major one of these compounds represented about 10% of the lipophilic plasma radioactivity after 60 min., while unmetabolized 12-HETE at this stage still represented about 30%. The metabolite had a polarity similar to 12,20-dihydroxyeicosatetraenoic acid; however, when chromatographed together, these two compounds separated, indicating a different structure of the metabolite. Our findings are in agreement with in vitro data concerning the protective effect of albumin on the metabolism of 12-HETE and is the first extensive metabolic study of 12-HETE in vivo covering all metabolic possibilities involving the carbon skeleton.

Oxidation of fatty acids by kidney microsomes of musk shrew (Suncus murinus)

Substrate specificity and other properties of a fatty acid monooxygenase system in kidney microsomes of the Japanese house musk shrew (Suncus murinus) were examined. The suncus kidney microsomes catalyzed the hydroxylation of various saturated and unsaturated fatty acids to the omega- and (omega-1)-hydroxy derivatives. Laurate was most effectively hydroxylated among saturated and unsaturated fatty acids. The specific activity (53.79 +/- 5.59 [mean +/- SD, n = 6] nmol/nmol cytochrome P450/min) of laurate in suncus kidney microsomes was very high compared with that in liver and kidney microsomes of other species. C18 unsaturated fatty acids were converted to epoxides by a cytochrome P450-dependent fatty acid monooxygenase system in suncus kidney microsomes, in addition to omega- and (omega-1)-hydroxylation products. The monooxygenase system metabolized arachidonic acid only to omega- and (omega-1)-hydroxylation products, not to epoxidation products.

Remodeling and neointimal formation in the carotid artery of normal and P-selectin-deficient mice

BACKGROUND

Inflammatory reactions such as leukocyte activation with platelet adherence and release of inflammatory mediators occur after percutaneous transluminal coronary angioplasty and may play a role in restenosis. Vascular remodeling with neointimal formation was studied in normal C57Bl/J6 and P-selectin-deficient mice.

METHODS AND RESULTS

The left common carotid artery was ligated just proximal to the carotid bifurcation. Four weeks later, left carotids and contralateral controls were snap-frozen. Computer-aided morphometry was performed to measure ratios of neointimal to medial area (NI/M) in 10 sections per animal as a measure of the thickness of the neointimal lesion. For normal mice, NI/M was 1.13+/-0.2 (n=20), whereas NI/M was reduced by 76% to 0.27+/-0.1 (n= 19) in P-selectin knockout mice. Vascular constriction (as measured by the length of external elastic lamina) was the same in both groups, but the circumference of the lumen in knockout mice was 26% larger. Also, normal and P-selectin-deficient mice were killed at 3 and 7 days after ligation (n=6 for each group per time point). Histological staining and immunostaining for CD45 showed no inflammatory cell presence in P-selectin knockout mice. However, in normal mice, leukocyte infiltration was observed in the adventitia, media, and developing neointima. Also, P-selectin immunostaining was observed in media and developing neointima of normal mice.

CONCLUSIONS

These data suggest that P-selectin is involved in processes leading to cell migration and proliferation associated with vascular remodeling, presumably by mediating leukocyte recruitment and the interaction between platelets and leukocytes.

Preferential esterification of endogenously formed 5-hydroxyeicosatetraenoic acid to phospholipids in activated polymorphonuclear leukocytes

The esterification of endogenously formed 5-hydroxyeicosatetraenoic acid (5-HETE) to cellular lipids in rat polymorphonuclear leukocytes (PMNL) was studied quantitatively by a fluorometric method using HPLC. Rapid and maximal production of free 5-HETE was observed after a 5-min stimulation of PMNL with A23187. The amount of free 5-HETE then declined rapidly, while that of 5-HETE esterified to phospholipids and triacylglycerol concomitantly increased in a time-dependent manner. Stimulation by A23187 yielded approximately 100 ng/10(7) cells esterified 5-HETE in 60 min, which corresponded to the decrease in the amount of free 5-HETE from 5 min to 60 min and indicated that free 5-HETE, which was formed endogenously, was metabolized predominantly by esterification to cellular lipids. The esterification profile of exogenous 5-HETE was different from that of endogenous 5-HETE. 5-[3H]HETE, which was added exogenously to the culture medium, was rapidly incorporated into PMNL and almost 80% of the total radioactivity was located in triacylglycerol. A quantitative study revealed that endogenous 5-HETE was esterified equally to phospholipids and triacylglycerol. Like PMNL, peritoneal macrophages treated with A23187 released significant amounts of 5-HETE. However, less 5-HETE was esterified to cellular lipids than in PMNL. Negligible amounts of 12-HETE, produced by activated peritoneal macrophages or activated platelets after a challenge with A23187, were esterified during the entire incubation. Exogenous 5-HETE was rapidly taken up by PMNL, but was incorporated into macrophages much more slowly than into PMNL. No uptake of 12-HETE into macrophages was observed. The rapid uptake of exogenous 5-HETE was strongly inhibited by the suppression of acylation of 5-HETE by triacsin C. These results suggest that esterification might be one of the factors that regulate the rate of incorporation of 5-HETE.

Platelet-activating factor: a candidate human immunodeficiency virus type 1-induced neurotoxin

The pathogenesis of central nervous system disease during human immunodeficiency virus type 1 (HIV-1) infection revolves around productive viral infection of brain macrophages and microglia. Neuronal losses in the cortex and subcortical gray matter accompany macrophage infection. The question of how viral infection of brain macrophages ultimately leads to central nervous system (CNS) pathology remains unanswered. Our previous work demonstrated high-level production of tumor necrosis factor alpha, interleukin 1 beta, arachidonic acid metabolites, and platelet-activating factor (PAF) from HIV-infected monocytes and astroglia (H. E. Gendelman, P. Genis, M. Jett, and H. S. L. M. Nottet, in E. Major, ed., Technical Advances in AIDS Research in the Nervous System, in press; P. Genis, M. Jett, E. W. Bernton, H. A. Gelbard, K. Dzenko, R. Keane, L. Resnick, D. J. Volsky, L. G. Epstein, and H. E. Gendelman, J. Exp. Med. 176:1703-1718, 1992). These factors, together, were neurotoxic. The relative role(s) of each of these candidate neurotoxins in HIV-1-related CNS dysfunction was not unraveled by these initial experiments. We now report that PAF is produced during HIV-1-infected monocyte-astroglia interactions. PAF was detected at high levels in CSF of HIV-1-infected patients with immunosuppression and signs of CNS dysfunction. The biologic significance of the results for neurological disease was determined by addition of PAF to cultures of primary human fetal cortical or rat postnatal retinal ganglion neurons. Here, PAF at concentrations of > or = 300 pg/ml produced neuronal death. The N-methyl-D-aspartate receptor antagonist MK-801 or memantine partially blocked the neurotoxic effects of PAF. The identification of PAF as an HIV-1-induced neurotoxin provides new insights into how HIV-1 causes neurological impairment and how it may ultimately be ameliorated.

Oxygenation of unsaturated fatty acids by hepatic microsomes of musk shrew (Suncus murinus)

The cytochrome P-450-dependent monooxygenase in musk shrew (suncus; Suncus murinus) liver microsomes metabolized unsaturated fatty acids (oleic, linoleic, alpha-linolenic and arachidonic acids) to a variety of oxygenated products. alpha-Linolenic acid was the most active substrate. The oxygenation activity increased with an increase in the number of cis double-bonds in the C18 fatty acids. This suggests that the introduction of cis double-bonds in C18 fatty acids is important for the binding of cytochrome P-450 in suncus liver microsomes. Regioselectivity of arachidonic acid oxygenation was observed in suncus liver microsomes; rat liver microsomal cytochrome P-450 generated epoxyeicosatrienoic acids and dihydroxyeicosatrienoic acid as major products while the cytochrome P-450-dependent monooxygenase in suncus liver microsomes yielded omega- and (omega-1)-hydroxyarachidonic acids as major reaction products.

Adhesion between neutrophils and platelets: a modulator of thrombotic and inflammatory events?

Recent studies have demonstrated that neutrophilic granulocytes and platelets have the capability to interact with each other. Either type of cell may release soluble factors, which either activate the other or increase its response to separate stimulating agents. Direct contact between the two types of cell generally increases the response, and it may be predicted that neutrophil-platelet adhesion will augment the potential for biochemical interaction in vivo. Adhesion may also promote immobilisation of neutrophils or platelets in thrombotic or inflamed tissue, while circulation aggregates could cause vascular blockage. Adhesion between neutrophils and platelets is dependent on P-selectin on activated platelets and sialyated carbohydrate moieties on the neutrophil, although other molecular mechanisms could stabilise the interaction. The adhesive interaction is strong enough to allow flowing neutrophils to adhere to immobilised platelets, and induce formation of aggregates in vivo. Thus, inhibition of neutrophil-platelet adhesion and modulation of their biochemical interactions could be a novel approach to treatment of thrombotic, ischaemic or inflammatory disorders.

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.

Metabolism of 12(R)-hydroxyeicosatetraenoic acid by rat liver microsomes

The in vitro metabolism of 12(R)-hydroxyeicosatetraenoic acid was studied using freshly isolated rat liver microsomes. Ten metabolites were isolated and identified by a combination of ultraviolet spectroscopy and gas chromatography/mass spectrometry. The two major metabolites were dihydroxyeicosatetraenoic acids generated by omega/omega-1 hydroxylation. Oxidation at C-5 resulted in the formation of four leukotriene-like compounds, two of which differed from leukotriene B4 in double-bond geometry alone. The other two differed from leukotriene B4 in olefin geometry and C-5 configuration. Epoxidation at the 14,15-olefin resulted in the formation of two diastereomeric epoxy alcohols, while C-16 hydroxylation gave two diastereomeric dihydroxyeicosatetraenoic acids.

Beta-oxidation of 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid by MOLT-4 lymphocytes

MOLT-4 lymphocytes metabolize 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12(S)-HETE via beta-oxidation with retention of the hydroxyl group at the omega 9 carbon atom. The isolation of 6-hydroxy-4,8-tetradecadienoic acid documents that these cells have the capacity to catabolize the conjugated diene system. 12(S)-HETE was also metabolized to 3,12-dihydroxy-8,10,14-eicosatrienoic acid and 1,9-dihydroxy-5,7,11-heptadecatriene as well as to 17- and 19-carbon aldehydes. When MOLT-4 cells were incubated with the beta-oxidation product, 10-hydroxy-6,8,12-octadecatrienoic acid, it was in part further catabolized but in addition it served as an anabolic precursor as defined by the accumulation 3,12-dihydroxy-8,10,14-eicosatrienoic acid as well as 1,11-dihydroxy-7,9,13-nonadecatriene. Neither 10-hydroxy-6,8,12-octadecatrienoic acid nor 13-hydroxy-5,8,11-octadecatrienic acid was as potent in inhibiting phytohemagglutin-induced lymphocyte mitogenesis as were their parent compounds--i.e., 12(S)- and 15(S)-HETE. These findings argue against the hypothesis that beta-oxidation products of 12(S)- and 15(S)-HETE are the potential modulators of lymphocyte function. However, neither the pathway for synthesis, nor the role of odd chain aldehydes and diols as potential lipid mediators was determined in this study.

Effects of platelet activating factor on rat platelets in vivo

Platelet activating factor (PAF) is not able to aggregate rat platelets in vitro. Due to the agonist effects of PAF on multiple cells, a possible role of PAF in the activation of rat platelets in vivo where different cells may influence each other was investigated. The pulmonary microembolization of 51Cr-labelled activated platelets was used as in vivo model. This model allowed us to monitor platelet behaviour by means of non-invasive methods. In contrast to results obtained in vitro, PAF activated rat platelets in vivo. The pulmonary microembolization of the platelets was dose-dependent and rapidly reversible. About 0.5 microgram/kg PAF caused a half-maximal rise of platelet-bound radioactivity in the thorax. Activation of the platelets by PAF was followed by extreme desensitization, so that a second injection of PAF did not provoke a significant response of the platelets. Platelet function was, however, not completely impaired because they still accumulated in the thorax after the application of ADP (50 micrograms/kg). That pulmonary entrapment had taken place was shown by a 3-fold increase in lung specific radioactivity. This was accompanied by a short-lasting thrombocytopenia. The PAF antagonist, WEB 2170 (30 micrograms/kg), significantly inhibited the microembolization of the platelets induced by PAF. Under in vitro conditions leukocytes purified from rat blood and activated by PAF were able to induce platelet aggregation. These results demonstrate that a PAF-specific activation of rat platelets is achievable in vivo which is probably mediated by other cells.

Platelet release products modulate some aspects of polymorphonuclear leukocyte activation

The aim of this research was to evaluate in vitro interactions between platelets and polymorphonuclear leukocytes. The effects of supernatant from thrombin-activated platelets and two platelet release products (adenosine triphosphate and beta-thromboglobulin) were tested on the following features of polymorphonuclear leukocytes activation: opsonized zymosan and phorbol myristate acetate stimulated chemiluminescence, release of membrane bound calcium, NADPH-oxidase activity, and membrane fluidity (fluorescent polarization). The results showed that the addition of platelet supernatant to polymorphonuclear leukocytes induces a significant activation of cells. On the other hand, after three hours of preincubation of polymorphonuclear leukocytes with platelet supernatant, a decreased response of polymorphonuclear leukocytes to stimulation with phorbol myristate acetate, a significant decrease in NADPH-oxidase activity, and a lowered membrane fluidity were observed. Adenosine triphosphate modulated only opsonized zymosan stimulated chemiluminescence, with and without preincubation with polymorphonuclear leukocytes. Beta-thromboglobulin caused a decrease of the chemiluminescent response of polymorphonuclear leukocytes, using both agonists, with and without preincubation with polymorphonuclear leukocytes. Moreover beta-thromboglobulin only caused a decrease of the polymorphonuclear leukocytes membrane fluidity without preincubation with the cells. These results support the thesis that platelets have a "time-related" modulating activity on polymorphonuclear leukocytes.

Platelet autoantibodies in dementia and schizophrenia. Possible implication for mental disorders

Platelets isolated from blood of demented and schizophrenic patients were found to bear surface antibodies at a considerably higher titer than those found on platelets from normal age-matched groups or patients with affective disorders. The platelet count in demented and schizophrenic patients correlated inversely with the level of the platelet associated antibodies (PAA) which suggested an autoimmune route of opsonization. In most individual cases of dementia or schizophrenia PAA and platelet count were found to oscillate with time between high PAA-low platelet number and low PAA-high platelet number in approximately inverse correlation. PAA isolated from demented patients were found to cross-react with platelets from normals and with brain tissue from rats. Furthermore, molecular weights of specific brain antigens were identified by binding to PAA. These observations support the possibility that PAA might be implicated in the etiology of some mental dysfunctions associated with dementia and schizophrenia.

Interference of anti-inflammatory and anti-asthmatic drugs with neutrophil-mediated platelet activation: singularity of azelastine

1. The capacity of various drugs (acetylsalicylic acid (ASA), ketoprofen, diclofenac, piroxicam, BW 755C, BW A4C, nedocromil sodium and azelastine) to inhibit human polymorphonuclear neutrophil (PMN)-mediated platelet activation was investigated. In this model, stimulated PMN release cathepsin G (Cat G), a serine proteinase which, in turn, induces platelet activation. 2. Among the different tested drugs, azelastine (100 microM for 1 min) was the only one able to prevent platelet aggregation. The cyclo-oxygenase inhibitors were all inactive, although used at effective concentrations as judged by inhibition of thromboxane B2 (TxB2) formation. Inhibition of platelet aggregation by azelastine was concentration-dependent, the range of active concentrations being of 20-70 microM. Release from platelets of 5-hydroxytryptamine was also inhibited at 30 microM and above, but never reached 100%. 3. The inhibition by azelastine is due to an effect on both cells. Indeed, beta-glucuronidase release from activated PMN and platelet activation by purified Cat G were both affected. 4. However, used at high concentrations (greater than 100 microM) azelastine was toxic since it released significant amounts of lactate dehydrogenase (LDH) from PMN and platelets. 5. These results show the capacity of azelastine, an anti-allergic and anti-asthmatic compound, to inhibit the cell-to-cell communication between PMN and platelets, an effect which may be relevant for its therapeutic efficacy or for a new application in diseases in which PMN and platelets are involved.

Antiplatelet therapy for atherosclerotic disorders

Recent studies have revealed the important roles of platelets in atherogenesis via vascular injury. Our in vivo and in vitro studies clearly demonstrate that activated platelets directly inflict injury to vascular endothelial cells, which is associated with a decrease in intracellular cyclic AMP levels in vascular tissues. Antiplatelet therapy is clinically important not only for the prevention of thrombotic episodes but also for the prevention of vascular injury and atherosclerosis. A small dose of aspirin (80 mg) induces clinically hypoaggregativeness of platelets with concomitantly decreased levels of thromboxane A2 in plasma. Our clinical study involving more than 3 years of treatment with small doses of aspirin demonstrated favorable therapeutic effects characterized by hypoaggregation of platelets and increased levels of cAMP and 6-keto PGF1 alpha in plasma which will aid in the prevention of atherosclerosis.

Formation of 20-oxoleukotriene B4 by an alcohol dehydrogenase isolated from human neutrophils

When 20-hydroxyleukotriene B4 (20-OH-LTB4) is incubated at pH 10.5 in the presence of NAD+ with an alcohol dehydrogenase isolated from human neutrophils, a polar product is formed as detected on reverse-phase high-performance liquid chromatography (RP-HPLC). The product is identified as 20-oxo-LTB4 (20-CHO-LTB4) on the basis of its co-elution with the authentic compound on HPLC, ultraviolet spectrometry and gas chromatography-mass spectrometry. The 20-CHO-LTB4-forming activity requires NAD+, but NADP+ scarcely replaces NAD+. The apparent Km for 20-OH-LTB4 is 83 microM and the Vmax is 2.04 mumol/min per mg of protein. The activity is inhibited by omega-hydroxy fatty acids such as 12-hydroxylauric acid, 16-hydroxypalmitic acid and 12(S), 20-dihydroxyeicosatetraenoic acid, but not by 4-methylpyrazole. At pH 7.0 with NADH, the purified dehydrogenase catalyzes the reverse reaction, the reduction of 20-CHO-LTB4 to 20-OH-LTB4.

Platelet-dependent induction and amplification of polymorphonuclear leucocyte lysosomal enzyme release

Degranulation of human polymorphonuclear leucocytes induced by opsonized zymosan (OpZ) was studied in the presence or absence of platelets, either resting or stimulated by thrombin. Lysozyme secretion from polymorphonuclear leucocytes stimulated with OpZ increased in the presence of platelets. A further increase was observed when platelets had been stimulated with thrombin. The effect was dependent on platelet concentration (10-80 platelets/polymorphonuclear leucocytes). Resting polymorphonuclear leucocytes could be activated by platelets that had been stimulated with thrombin with an optimal effect observed at 0.1 U/ml of thrombin. Substitution of platelets with platelet-released products only resulted in significant stimulation of polymorphonuclear leucocytes at concentrations above those used in the coincubations experiments. Zymosan coated with various opsonins showed a gradient in the relative intensity of polymorphonuclear leucocytes degranulation. Platelet-dependent enhancement of polymorphonuclear leucocytes degranulation was higher with opsonins that were poorer at inducing lysosomal secretion. The role of platelets as helper cells in the activation of polymorphonuclear leucocytes appears to be dominant when polymorphonuclear leucocytes are challenged in suboptimal conditions. Positive interactions between platelets and polymorphonuclear leucocytes may be relevant in vivo at the site of inflammation where platelets could enhance effector functions of polymorphonuclear leucocytes.