Toxoplasma gondii stimulates the release of 13- and 9-hydroxyoctadecadienoic acids by human platelets

Oxygenation of polyunsaturated fatty acids and oxidative stress within blood platelets

The oxygenation metabolism of arachidonic acid (ArA) has been early described in blood platelets, in particular with its conversion into the potent labile thromboxane A₂ that induces platelet aggregation and vascular smooth muscle cells contraction. In addition, the primary prostaglandins D₂ and E₂ have been mainly reported as inhibitors of platelet function. The platelet 12-lipoxygenase (12-LOX) product, i.e. the hydroperoxide 12-HpETE, appears to stimulate platelet ArA metabolism at the level of its release from membrane phospholipids through phospholipase A₂ (cPLA₂) and cyclooxygenase (COX-1) activities, the first enzymes in prostanoid production cascade. Also, 12-HpETE may regulate the oxygenation of other polyunsaturated fatty acids (PUFA) by platelets, especially that of eicosapentaenoic acid (EPA). On the other hand, the reduced product of 12-HpETE, 12-HETE, is able to antagonize TxA₂ action. This is even more obvious for the 12-LOX end-products from docosahexaenoic acid (DHA), 11- and 14-HDoHE. In addition, 12-HpETE plays a key role in platelet oxidative stress as observed in pathophysiological conditions, but may be regulated by DHA with a bimodal way according to its concentration. Other oxygenated products of PUFA, especially omega-3 PUFA, produced outside platelets may affect platelet functions as well.

Microbial adhesion of Cryptosporidium parvum sporozoites: purification of an inhibitory lipid from bovine mucosa

Cryptosporidium parvum is a protozoan pathogen of humans and livestock worldwide. Its ability to infect a wide range of species raises questions as to the involvement of a specific host cell receptor for parasite-host recognition. To investigate the mechanism of parasite-host cell recognition, we have developed an in vitro cell suspension binding assay to investigate adhesion of C. parvum sporozoites to host cells. Morphologic features of binding events observed with this assay were identical to those described in natural infections. Glycoconjugates, Madin Darby bovine kidney (MDBK) cell fractions, and plasma membrane vesicles (PMVs) were screened for their ability to block binding of sporozoites to MDBK cells. Mucins, MDBK cell fractions, and PMVs exhibited dose-dependent inhibition of sporozoite binding. The major inhibitory fraction from MDBK cells was found to be insoluble in aqueous medium, nonsaponifiable, and lacking carbohydrate moieties, nitrogen, and phosphorus. Its inhibitory effect was resistant to heat, protease digestion, and glycosidase treatment, suggesting that the inhibitory activity is a lipid or a lipid-like component. The inhibitory activity was purified from MDBK cells, and in larger amounts from bovine small intestinal mucosa, by organic solvent extraction, semipreparative high-pressure liquid chromatography, and preparative high-performance thin-layer chromatography. Biochemical analyses, thin-layer chromatography staining techniques, mass spectrometry, and elemental analysis were used to partially characterize the purified lipid. These results indicate that a host intestinal lipid(s) or a lipid-like component(s) may play an important role in the early stages of host cell invasion by C. parvum.

Production of eicosanoids and other oxylipins by pathogenic eukaryotic microbes

Oxylipins are oxygenated metabolites of fatty acids. Eicosanoids are a subset of oxylipins and include the prostaglandins and leukotrienes, which are potent regulators of host immune responses. Host cells are one source of eicosanoids and oxylipins during infection; however, another potential source of eicosanoids is the pathogen itself. A broad range of pathogenic fungi, protozoa, and helminths produce eicosanoids and other oxylipins by novel synthesis pathways. Why do these organisms produce oxylipins? Accumulating data suggest that phase change and differentiation in these organisms are controlled by oxylipins, including prostaglandins and lipoxygenase products. The precise role of pathogen-derived eicosanoids in pathogenesis remains to be determined, but the potential link between pathogen eicosanoids and the development of TH2 responses in the host is intriguing. Mammalian prostaglandins and leukotrienes have been studied extensively, and these molecules can modulate Th1 versus Th2 immune responses, chemokine production, phagocytosis, lymphocyte proliferation, and leukocyte chemotaxis. Thus, eicosanoids and oxylipins (host or microbe) may be mediators of a direct host-pathogen "cross-talk" that promotes chronic infection and hypersensitivity disease, common features of infection by eukaryotic pathogens.

Development and review of radioimmunoassay of 12-S-hydroxyheptadecatrienoic acid

For more than 25 years 12-S-hydroxyheptadecatrienoic acid (HHT) has been known to be a product of thromboxanesynthase (TX-Syn) when synthesized with thromboxane A2 (TXA2). Although there are some hints that HHT has anti-aggregatory effects, to date, it has neither been shown to have any specific pathological relevance nor is there much information about its physiological role. This review presents a summary of the physicochemical properties of HHT, its chemical synthesis, the impact of various biological systems on its enzymatic and non-enzymatic production and its physiological function and metabolization, as well as a survey of the most important methods for analyzing this unsaturated hydroxy-fatty acid. Due to the low antibody-raising potency expected in HHT, no immunological system for HHT quantification has been developed so far. In our report we present the development and validation of a sensitive and reliable, competitive radioimmunoassay (RIA) suitable for the quantitative determination of HHT. HHT was produced by an enhanced enzymatic method using platelet-rich plasma (PRP). With an effective and modified liquid-liquid and solid-phase extraction method we were able to produce highly purified HHT (97% purity by GC/MS) in sub-milligram ranges. These fractions were used for the synthesis of BSA-antigen-conjugates and for immunization of rabbits. The tritiated tracer was synthesized using prostaglandin H synthase for the production of prostaglandin H2 (PGH2) followed by an aqueous reaction with Fe(2+)-solution to rear-range PGH2 to HHT. The dynamic range of the assay was from 30-400 pg/tube, with a sensitivity of approximately 40 pg/tube. The evaluation of the assay was performed by a HPLC-RIA method as well as by correlation with a quantitative HPLC method and correlation with TXB2 concentrations in a blood coagulation study. The assay may be useful for the quantification of HHT in several tissues and body fluids under various physiological conditions and may also help to understand the possible physiological role of HHT in biological processes.

Toxoplasma gondii alters eicosanoid release by human mononuclear phagocytes: role of leukotrienes in interferon gamma-induced antitoxoplasma activity

Toxoplasma gondii tachyzoites markedly alter the profile of eicosanoids released by human mononuclear phagocytes. Freshly isolated, 2-h adherent human monocytes release both cyclooxygenase (e.g., thromboxane [TX] B2, prostaglandin [PG] E2) and 5-lipoxygenase (e.g., leukotriene [LT] B4, LTC4) products of arachidonic acid metabolism after stimulation by the calcium ionophore A23187 or ingestion of opsonized zymosan particles or heat-killed T. gondii. However, after incubation with viable T. gondii, normal and chronic granulomatous disease monocytes release only the cyclooxygenase products TXB2 and PGE2 and fail to form LTB4, LTC4, or other 5-lipoxygenase products. Monocytes maintained in culture for 5 d lose this capacity to release TXB2 and PGE2 after incubation with T. gondii. T. gondii significantly inhibit calcium ionophore A23187-induced LTB4 release by monocyte-derived macrophages; heat-killed organisms do not affect this calcium ionophore A23187-induced release of LTB4. T. gondii-induced inhibition of LTB4 release by calcium ionophore A23187-stimulated monocyte-derived macrophage is reversed by interferon (IFN)-gamma treatment of the monolayers. LTB4 induced extensive damage to the cellular membranes and cytoplasmic contents of the organisms as observed by transmission electron microscopy. Exogenous LTB4 (10(-6) M) induced intracellular killing of ingested T. gondii by non-IFN-gamma-treated monocyte-derived macrophages. IFN-gamma-induced antitoxoplasma activity in monocyte-derived macrophages was inhibited by the selective 5-lipoxygenase inhibitor zileuton but not by the cyclooxygenase inhibitor indomethacin. These findings suggest a novel role for 5-lipoxygenase arachidonic acid products in human macrophage IFN-gamma-induced antitoxoplasma activity.

Cytotoxic activity of 13-hydroxyoctadecadienoic acid against Toxoplasma gondii

Recent data indicate that platelets may play an important role in the host defence against Toxoplasma gondii infections. T. gondii-stimulated human platelets release thromboxane A2 (TXA2) and 12-hydroxyeicosatetraenoic acid (12-HETE) from arachidonic acid and 13-hydroxyoctadecadienoic acid (13-HODE) from linoleic acid (Yong et al. 1991; Henderson et al. 1992). We have previously demonstrated that the eicosanoid TXA2 has potent cytotoxic activity against T. gondii trophozoites (Yong et al. 1991). In this study, we examined whether 12-HETE, 13-HODE, and linoleic acid also have toxoplasmacidal activity. 13-HODE at concentrations > or = 10(-8) M rapidly induced cytotoxic changes in T. gondii. Ultrastructural changes induced by 13-HODE in T. gondii included an initial leakage of cytoplasmic contents into a space between the inner and outer parasite bilayer membrane units which was followed by intracellular vacuolation and loss of cytoplasmic contents. In contrast, linoleic acid and 12-HETE lacked toxoplasmacidal activity at 10(-10)-10(-6) M concentrations. These data indicate that 13-HODE, a product of linoleic acid metabolism, has potent cytotoxic activity against T. gondii; this toxoplasmacidal activity may be important in the inflammatory response to this pathogen.