Arachidonic acid metabolism among human mononuclear leukocytes. Lipoxygenase-related pathways

Ethanol-enhanced transmembrane penetration of arachidonic acid and activation of the 5-lipoxygenase pathway in human leukocytes

Enhanced penetration by ethanol of exogenous arachidonic acid into human leukocyte preparations results in the production of large amounts of eicosanoids including 5-, 12- and 15-hydroxyeicosatetraenoic acids as well as the leukotrienes C4 delta 6-trans-leukotriene B4, 12-epi-delta 6-trans-leukotriene B4, leukotriene B4 and 5(S), 12(S)-dihydroxyeicosatetraenoic acid. The production of these compounds is affected by the concentrations of both ethanol and arachidonic acid independently in a complex manner with stimulation at lower concentrations and later relative inhibition. It was shown that the resulting leukotriene B4 exhibited the same specific activity as exogenous arachidonic acid when labelled substrate was used.

Human peritoneal eosinophils and formation of arachidonate cyclooxygenase products

Human peritoneal eosinophils were obtained from the waste dialysis bags of patients undergoing continuous ambulatory peritoneal dialysis. The number of eosinophils obtained from each bag varied from 3 X 10(7) to 288 X 10(7). The cells were incubated for 1 h in tissue culture medium and prostaglandin E2 (PGE2), 6-keto-prostaglandin F1 (6-keto-PGF1), and thromboxane B2 (TXB2) were determined by radioimmunoassay of the supernatant. The basal release as well as the stimulated release from the purified eosinophils of TXB2 were five times greater than the release of PGE2 and thirty times greater than the release of 6-keto-PGF1. A dose-response curve was achieved for all three cyclooxygenase products with the calcium ionophore A23187. The release of TXB2 was inhibited in a dose-dependent manner by the specific thromboxane A2 (TXA2) synthase inhibitor OKY-1581 and a corresponding increase in PGE2 and 6-keto-PGF1 was obtained. Indomethacin (5.6 X 10(-6) M) inhibited the cyclooxygenase products to almost undetectable levels.

Arachidonic acid inhibits 5-lipoxygenase in human T cells

The data on whether T cells produce leukotrienes or other 5-lipoxygenase metabolites of arachidonic acid is conflicting. We report that exogenous arachidonic acid added to phytohemagglutin-stimulated human T cells profoundly inhibits leukotriene B4 production, with 90% inhibition caused by 10(-6) M arachidonic acid. The 12- and 15-lipoxygenase pathways were also inhibited by arachidonic acid. Recent reports that human T cells produce no 5-lipoxygenase metabolites of arachidonic acid might be explained by the fact that the studies used greater than or equal to 10(-5)M arachidonic acid in the incubation media.

Hydroxyeicosatetraenoic acids and other unsaturated fatty acids inhibit endotoxin-induced thromboplastin activity in human monocytes

Human peripheral blood monocytes possessed increased thromboplastin activity when exposed to 10 micrograms/ml of bacterial endotoxin for 2 h. The effects of endotoxin were strongly inhibited by 10-20 microM of several unsaturated fatty acids: arachidonic, dihomo-gamma-linolenic, linoleic, linolenic and oleic acids. Two saturated fatty acids, arachidic and stearic acids, produced minimal inhibition at 20 microM. Three lipoxygenase-derived hydroxyfatty acids were also inhibitory, with IC50 values of 10 microM (5- and 12-hydroxyeicosatetraenoic acids) and 20 microM (15-hydroxyeicosatetraenoic acid). Leukotriene B4, 1 nM to 1 microM, was inactive. None of the fatty acids affected the ability of standard thromboplastin to shorten the clotting time of normal plasma or affected the activity of endotoxin as measured by a Limulus clotting assay.

A novel eicosanoid is the major arachidonic acid metabolite of cultured human monocytes

Human peripheral blood monocyte-macrophages (M phi) generate a novel eicosanoid during in vitro culture. The metabolite is generated during incubation of the cells with 14C - arachidonic acid (AA). Lack of prior recognition of this metabolite probably results from the facts that: 1) on thin-layer chromatography (TLC) in two standard solvent systems, the novel metabolite co-chromatographed with either prostaglandin D2 or thromboxane B2, and 2) its generation, under the conditions studied, does not occur until between 90 and 180 minutes after culture initiation which is a time period beyond that used for most leukocyte studies. The generation of the metabolite is inhibited by nordihydroguaiaretic acid (NDGA) but not by indomethacin. Base hydrolysis did not alter its migration on TLC. On both reversed phase and straight phase high pressure liquid chromatography (HPLC), the novel peak isolated by TLC elutes as a single major peak of radioactivity with a retention time different from the known leukotrienes, hydroxy acids, or their metabolites. Furthermore, the peak isolated on HPLC has a single ultraviolet absorption maximum at 270 nm. M phi cultured for 1 week prior to a 24 hour incubation with 14C-AA generated proportionally less of the novel eicosanoid (roughly 68% of total radiolabeled product) than did M phi cultured for 3 weeks prior to a similar incubation with 14C-AA (roughly 86% of total radiolabeled product). Under the conditions studied, the novel eicosanoid is the major AA metabolite generated from exogenous AA by cultured M phi and it appears to be generated in increasing quantity as the M phi differentiate.

Polymorphonuclear leukocyte 5-lipoxygenase activity in psoriasis

5-lipoxygenase-derived products of arachidonic acid are implicated in the pathophysiology of psoriasis, a common hyperproliferative and inflammatory skin disease. We therefore examined whether there is an activation of this enzymatic pathway in extracutaneous tissues. For this purpose, we measured the conversion of 14C-arachidonic acid by polymorphonuclear leukocytes from psoriatic patients and controls. No significant difference in the generation of leukotriene B4 and 5-hydroxyeicosatetraenoic acid by polymorphonuclear leukocytes was noted between the two groups. We conclude that in psoriasis there is no enhanced activity of the 5-lipoxygenase pathway in circulating polymorphonuclear leukocytes.

Leukotrienes C4 and D4 in psoriatic skin lesions

Chemoattractant arachidonate lipoxygenase products have been recovered from the skin lesions of psoriasis, and may play a role in eliciting the intra-epidermal neutrophil infiltrate that characterises this disease. In view of evidence for lipoxygenase activity in psoriasis, the characteristic vasodilation in psoriatic lesions, and the vasodilator properties of leukotriene (LT) C4 and D4 in human skin, the presence of these LTs in psoriatic lesions has been investigated. Skin chamber fluid from abraded psoriatic lesions contained significantly greater amounts of immunoreactive material than that from clinically normal skin, as determined by a double antibody radioimmunoassay (RIA) that uses antiserum cross-reacting with both LTC4 and LTD4. Purification of lesional chamber fluid and scale extracts by high performance liquid chromatography (HPLC) and RIA of fractions showed immunoreactivity which co-eluted with standard LTC4 and LTD4. These findings suggest that LTC4 and LTD4 may play a role in mediating the vasodilation and increased blood flow that characterise psoriatic skin lesions.