Conversion of leukotriene B4 to dihydro and 19-hydroxy metabolites by rat polymorphonuclear leukocytes

The medicinal chemistry of stable synthetic leukotriene B3 and B4 analogues

Leukotriene B3 and B4 are part of an important class of signaling molecules – the leukotrienes, implicated in the inflammation process. Their pro-inflammatory effects have been widely recognized for almost three decades but it is only recently that their benefit in host defense has begun to be acknowledged. Their use as therapeutic agents is, unfortunately, limited by rapid metabolism. However, over the past 25 years, a number of stable leukotriene B3 and B4 analogues have been produced. In this review, we examine their medicinal chemistry and biological evaluation.

Human neutrophils convert the sebum-derived polyunsaturated fatty acid Sebaleic acid to a potent granulocyte chemoattractant

Sebaleic acid (5,8-octadecadienoic acid) is the major polyunsaturated fatty acid in human sebum and skin surface lipids. The objective of the present study was to investigate the metabolism of this fatty acid by human neutrophils and to determine whether its metabolites are biologically active. Neutrophils converted sebaleic acid to four major products, which were identified by their chromatographic properties, UV absorbance, and mass spectra as 5-hydroxy-(6E,8Z)-octadecadienoic acid (5-HODE), 5-oxo-(6E,8Z)-octadecadienoic acid (5-oxo-ODE), 5S,18-dihydroxy-(6E,8Z)-octadecadienoic acid, and 5-oxo-18-hydroxy-(6E,8Z)-octadecadienoic acid. The identities of these metabolites were confirmed by comparison of their properties with those of authentic chemically synthesized standards. Both neutrophils and human keratinocytes converted 5-HODE to 5-oxo-ODE. This reaction was stimulated in neutrophils by phorbol myristate acetate and in keratinocytes by oxidative stress (t-butyl-hydroperoxide). Both treatments dramatically elevated intracellular levels of NADP(+), the cofactor required by 5-hydroxyeicosanoid dehydrogenase. In keratinocytes, this was accompanied by a rapid increase in intracellular GSSG levels, consistent with the involvement of glutathione peroxidase. 5-Oxo-ODE stimulated calcium mobilization in human neutrophils and induced desensitization to 5-oxo-6,8,11,14-eicosatetraenoic acid but not leukotriene B(4), indicating that this effect was mediated by the OXE receptor. 5-Oxo-ODE and its 8-trans isomer were equipotent with 5-oxo-6,8,11,14-eicosatetraenoic acid in stimulating actin polymerization and chemotaxis in human neutrophils, whereas 5-HODE, 5-oxo-18-hydroxy-(6E,8Z)-octadecadienoic acid, and 5S,18-dihydroxy-(6E,8Z)-octadecadienoic acid were much less active. We conclude that neutrophil 5-lipoxygenase converts sebaleic acid to 5-HODE, which can be further metabolized to 5-oxo-ODE by 5-hydroxyeicosanoid dehydrogenase in neutrophils and keratinocytes. Because of its chemoattractant properties, sebum-derived 5-oxo-ODE could be involved in neutrophil infiltration in inflammatory skin diseases.

Synthesis of 10,11-dihydroleukotriene B(4) metabolites via a nickel-catalyzed coupling reaction of cis-bromides and trans-alkenyl borates

Synthesis of 10,11-dihydro-, 10,11,14,15-tetrahydro-, and 10, 11-dihydro-12-oxoleukotriene B(4) compounds (2, 4, 5) was accomplished stereoselectively by using the nickel-catalyzed coupling reaction illustrated in Scheme 1. The C(1)-C(7) fragments, TBS ether 10a for 2 and 4 and ethyoxyethyl (EE) ether 10b for 5, were prepared in enantiomerically pure forms (>99% ee) by a modified literature procedure (ref 11a). On the other hand, boronate esters 11a and 11b, which correspond to the C(8)-C(20) parts of 2 and 4, respectively, were synthesized from (R)-epichlorohydrin (18) of 99% ee. Briefly, 18 was converted into acetylenes 24 and 32 through epoxide ring-opening with LiC triple bond CC(5)H(11)/BF(3).OEt(2) or C(7)H(15)MgBr/CuCN. Hydroboration of these acetylenes with (+)-(Ipc)(2)BH followed by reaction with MeCHO afforded the corresponding diethyl boronates, which upon ligand exchange with Me(2)C(CH(2)OH)(2) furnished boronate esters 11a and 11b in 75% and 77% yields, respectively. In a similar manner, racemic boronate ester rac-11a, an intermediate for synthesis of 5, was prepared from racemic epichlorohydrin. For synthesis of 2, borate 25 was generated from 11a (1.5 equiv) and MeLi (1.6 equiv). Without isolation, 25 was submitted to reaction with 10a (1 equiv) in the presence of a Ni(0) species at room temperature overnight to afford 26, which upon treatment with TBAF furnished 2 in 64% yield from 10a. Similarly, 11b and 10a furnished 4 in good yield. To synthesize 5, rac-11a and EE ether 10b were joined by the coupling reaction to produce 39, which was transformed into 40 by desilylation with TBAF. After hydrolysis of 40, oxidation with PDC followed by deprotection of the EE group furnished 5 in 36% yield from 40. In addition, 2 was converted into amide 3 in 92% yield.

Application of gas chromatography-mass spectrometry and gas chromatography-tandem mass spectrometry to assess in vivo synthesis of prostaglandins, thromboxane, leukotrienes, isoprostanes and related compounds in humans

Prostaglandins, thromboxane, leukotrienes, isoprostanes and other arachidonic acid metabolites are structurally closely related, potent, biologically active compounds. One of the most challenging tasks in eicosanoids research has been to define the role of the various eicosanoids in human health and disease, and to monitor the effects of drugs on the in vivo synthesis of these lipid mediators in man. Great advances in instrumentation and ionization techniques, in particular the development of tandem mass spectrometry and negative-ion chemical ionization (NICI), in gas chromatography and also advances in methodologies for solid-phase extraction and sample purification by thin-layer chromatography and high-performance liquid chromatography have been made. Now gas chromatography-mass spectrometry (GC-MS) and GC-tandem MS in the NICI mode are currently indispensable analytical tools for reliable routine quantitation of eicosanoid formation in vivo in humans. In this article analytical methods for eicosanoids based on GC-MS and GC-tandem MS are reviewed emphasizing the quantitative measurement of specific index metabolites in human urine and its importance in clinical studies in man. Aspects of method validation and quality control are also discussed.

12(R)-methyl-leukotriene B3: a stable leukotriene B analogue toward the reductase metabolism

Biological potencies of 12(R)-methyl-LTB3 [12(R)-Me-LTB3] and 12(S)-Me-LTB3 and their stability toward reductase metabolism are described. 12(R)- and 12(S)-Me-LTB3 of more than 95% chemical purity were synthesized highly stereoselectively via the palladium catalyzed coupling reaction of the vinylborane derived from the enzyme 1 and Sia2BH with the iodide 2 of R and S configuration. To assess biological activity of 12-Me-LTB3, cytosolic free calcium ([Ca2+]i) rise in rat PMNLs and binding affinity to the LTB4 receptors were compared with those of natural LTB4. The potency of 12(R)-Me-LTB3 was found to be almost equal to LTB4, while, by complete contrast, 12(S) isomer showed very low activity of 1/200-1/400. These results indicate that the C(12) hydroxyl group of R configuration is essential to elicit the biological activity and that [Ca2+]i rise elicited by 12-Me-LTB3 is mediated through interaction with the LTB4 receptors. Stability of 12(R)-Me-LTB3 toward the reductase metabolism was evaluated by using rat PMNLs. Thus, rat PMNLs were incubated at 37 degrees C with 12(R)-Me-LTB3 and LTB4, respectively. The amount of 12(R)-Me-LTB3 was almost unchanged up to 30 min under these conditions, though LTB4 was rapidly consumed in a time-dependent manner, thus strongly indicating that 12(R)-Me-LTB3 is not metabolized via the reductase pathway.

Analysis of leukotriene B4 metabolism in human promyelocytic HL-60 cells

We previously reported that human alveolar macrophages rapidly metabolize the chemotactic active lipid mediator leukotriene B4 (LTB4) into the dihydro-LTB4 by reduction of one of the conjugated double bonds. We herein report that human HL-60 cells (a myeloid precursor which can be differentiated into granulocyte- as well as monocyte-like cells by dimethyl sulphoxide or phorbol myristate acetate) express a highly active LTB4 reductase in the undifferentiated state. Differentiation by dimethyl sulphoxide (1.3%) along the granulocyte lineage, as confirmed by light microscopy, conversion of NitroBlue Tetrazolium into formazan, failed to induce a substantial capacity for omega-oxidation of LTB4; this reaction is exclusively found in mature granulocytes. Studies with the cell homogenate of undifferentiated HL-60 cells indicated that the activity of the enzyme depends on the presence of NADPH, Ca2+ and Mg2+, with a pH optimum of 7.5 at 37 degrees C. The enzyme was not released into the supernatant after stimulation of HL-60 cells with phorbol myristate acetate (100 ng) or Ca2+ ionophore (7.5 microM). Subcellular fractionation revealed evidence that the LTB4 reductase is located within the membrane fraction. Purification of the enzyme by gel filtration and gel electrophoresis suggests an apparent molecular mass of 40 kDa.

Identification and biological activity of dihydroleukotriene B4: a prominent metabolite of leukotriene B4 in the human lung

Exogenous [3H]leukotriene B4 (LTB4) was converted into several polar and non-polar metabolites in the chopped human lung. One of the major metabolites was identified as 5(S),12-dihydroxy-6,8,14-eicosatrienoic acid (10,11-dihydro-LTB4) by means of co-chromatography with authentic standards, ultraviolet spectrometry and gas chromatography-mass spectrometry. Analysis of chiral straight phase HPLC revealed the presence of both the 12(S) and 12(R) epimers of dihydro-LTB4. Dihydro-LTB4 was also formed from endogenously generated LTB4 in ionophore A23187 stimulated incubations. The dihydro metabolites were approximately 100 times less potent than LTB4 in causing guinea pig lung strip contraction and leukocyte-dependent inflammation in the hamster cheek pouch in vivo.

Characteristics of formation and further metabolism of leukotrienes in the chopped human lung

The bronchoconstrictive leukotrienes (LTs) LTC4, LTD4 and LTE4 (cysteinyl-LTs) and the chemoattractant LTB4 were formed in chopped human lung stimulated by the calcium ionophore A23187, or supplied with the precursor LTA4. In contrast, challenge with anti-IgE exclusively induced release of cysteinyl-LTs, indicating that LTB4 is not released as a primary consequence of IgE-mediated reactions in the human lung. Furthermore, several differences were observed with respect to formation and further conversion of LTB4 and LTC4 in the chopped lung preparation. Thus, exogenous [1-14C]arachidonic acid was dose-dependently converted to radioactive LTB4, whereas the cysteinyl-LTs released were not radiolabeled and the amounts of LTC4, D4 and E4 were not influenced by addition of increasing concentrations of arachidonic acid. LTC4 was rapidly and completely converted into LTD4 and LTE4, with no further catabolism of LTE4 within 90 min. The metabolism of LTB4 was much slower than that of LTC4. Thus, following a 60 min incubation approx. 25% of the material remained as LTB4, whereas 35% was omega-oxidized and 40% eluted on RP-HPLC as two unidentified peaks.

Conversion of stereoisomers of leukotriene B4 to dihydro and tetrahydro metabolites by porcine leukocytes

We have previously shown that porcine leukocytes convert leukotriene B4 (LTB4) to two major products, 10,11-dihydro-LTB4 and 10,11-dihydro-12-oxo-LTB4. Although we did not detect these products after incubation of LTB4 with human polymorphonuclear leukocytes, these cells converted 12-epi-6-trans-LTB4 to the corresponding 6,11-dihydro metabolite (i.e., there appeared to be a shift in the positions of the remaining double bonds). The objective of the present investigation was to determine whether 6-trans isomers of LTB4 are metabolized by porcine leukocytes by a pathway similar to LTB4, or whether they are metabolized by a pathway analogous to that in human leukocytes. We found that 6-trans-LTB4 and 12-epi-6-trans-LTB4 are metabolized more much extensively than LTB4 by porcine leukocytes. 6-trans-LTB4 appears to be converted by two different reductase pathways to two dihydro products differing in the positions of the two remaining double bonds between carbons 5 and 12. Dihydro-12-oxo and dihydro-5-oxo metabolites are also formed from this substrate. Porcine leukocytes also convert 6-trans-LTB4, presumably by a combination of the above two pathways, to tetrahydro, tetrahydro-12-oxo and tetrahydro-5-oxo metabolites, none of which possesses any conjugated double bonds. 12-epi-6-trans-LTB4 is also converted to tetrahydro metabolites by these cells. Experiments with deuterium-labeled 6-trans-LTB4 indicated that the deuterium in the 5-position was almost completely lost during the formation of tetrahydro-6-trans-LTB4, whereas about 80-85% of the deuterium in the 12-position was lost, suggesting a requirement for a 5-oxo intermediate. As with LTB4, 12-epi-8-cis-6-trans-LTB4, the product of the combined actions of 5-lipoxygenase and 12-lipoxygenase, was converted principally to dihydro and dihydro-12-oxo metabolites. Only a relatively small amount of the tetrahydro metabolite was detected.

Human glomerular mesangial cells inactivate leukotriene B4 by reduction into dihydro-leukotriene B4 metabolites

Due to its potent chemotactic properties leukotriene B4 is an important mediator of inflammatory reactions. Cultured human kidney mesangial cells converted exogenously added leukotriene B4 efficiently into three different more lipophilic metabolites, two of them probably representing dihydro-leukotriene B4 isomers. This represents an alternative metabolic pathway, in contrast to leukotriene B4 omega-oxidation found in human polymorphonuclear leukocytes. Both dihydro-leukotriene B4 isomers had nearly completely lost their ability to induce leukocyte chemotaxis as compared to leukotriene B4.

Effects of dietary vitamin E on the biosynthesis of 5-lipoxygenase products by rat polymorphonuclear leukocytes (PMNL)

Activation of polymorphonuclear neutrophils (PMNL) leads to the release of arachidonate from cellular phospholipids via a phospholipase A2, and conversion of products of the 5-lipoxygenase pathway. Evidence to date indicates the dietary vitamin E ((R,R,R)-alpha-tocopherol) can influence both cyclooxygenase and phospholipase A2 activities and that the effect of this vitamin is cell/tissue specific. The present study was undertaken in order to examine the effects of varying dietary tocopherol on PMNL tocopherol content and 5-lipoxygenase product profile using the ionophore A23187 as stimulant in the presence and absence of exogenous arachidonate. Feeding semi-purified diets containing 0, 30 or 3000 ppm of (R,R,R)-alpha-tocopherol acetate to weanling rats for 17 weeks resulted in a dose-related enrichment of PMNL tocopherol. Stimulation of PMNL elicited a significant and rapid loss of tocopherol. When PMNL were stimulated with A23187 alone, the synthesis of 5-HETE, LTB4 and 19-hydroxy-LTB4 was decreased in proportion to increasing dietary tocopherol concentrations. However, when exogenous arachidonate was provided with A23187, intermediate amounts of dietary tocopherol (30 ppm) still suppressed the formation of 5-lipoxygenase products, but high doses (3000 ppm) did not have any additional inhibitory effect. This differential response to high concentrations of vitamin E in the presence and absence of exogenous arachidonate highly suggest that at these concentrations, tocopherol may act principally at the level of substrate release whereas at lower concentrations, 5-lipoxygenase is inhibited. Data from this study demonstrated that attenuation of the formation of 5-lipoxygenase products in PMNL can be achieved by dietary vitamin E enrichment.

Human monocytes convert leukotriene B4 to two dihydro-leukotriene B4-metabolites

Human monocytes metabolize LTB4 by an additional pathway different from omega-oxidation. Reverse-phase high performance liquid chromatography showed four metabolites: 20-COOH-LTB4, 20-OH-LTB4 and two metabolites less polar than LTB4 with an UV maximum at 232 nm. Gas-chromatography mass-spectrometry showed nearly identical mass spectra for both metabolites. The main mass fragments of the two metabolites were increased by two mass units compared to LTB4. Our findings suggest that LTB4 had been reduced to a known and a new dihydro-metabolite of LTB4. Both metabolites together amounted to 85% of total metabolites. The remaining 15% were omega-oxidation products. Thus, the major pathway of LTB4 metabolism by human monocytes is reduction to dihydro-LTB4.

12S,19- and 12S,20-dihydroxyeicosanoids: novel 12S-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid metabolites formed by hydroxylation and reduction in murine lymphocytes

Murine spleen cells and purified B lymphocytes oxidized arachidonic acid via the lipoxygenase pathway. The major metabolite of both the whole spleen and enriched B lymphocytes was 12S-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid. A novel metabolite was observed that did not have an absorbance from 210 to 400 nm, indicating the absence of a conjugated double bond system. The new metabolite was converted to the methyl ester, reduced by platinum oxide, derivatized to the trimethylsilyl ether, and analyzed by gas chromatography-mass spectrometry. A major and a minor component were observed in the analysis of the new compound. The major component had major diagnostic ions indicating the presence of hydroxyl groups at C-12 and C-19. The minor component had major diagnostic ions indicating the presence of hydroxyl groups at C-12 and C-20. The new metabolites are characterized as a mixture of 12S,19- and 12S,20-dihydroxyeicosanoids presumably formed by hydroxylation and reduction of one or more double bonds of 12S-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid. These metabolites were formed predominantly with whole spleen lymphocytes but could be detected at longer incubation times or by using 12S-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid as the starting substrate with highly enriched B lymphocytes.

Leukotrienes. Biology and role in disease

Leukotrienes are a novel group of chemical messengers derived from arachidonic acid. They are produced by several different tissues by processes linked to phospholipid flux in response to specific stimuli. The leukotrienes interact with specific receptors in target cell membranes to initiate a response. Most of these responsive cells are derived from bone marrow, skin, smooth muscle, and vascular endothelium. Leukotrienes are powerful mediators of inflammation and smooth muscle contraction, and there is increasing evidence that they are important factors in immune-mediated disease. Several available effective antiinflammatory drugs may act partially by inhibiting the production of leukotrienes.

Hydroxylation of leukotriene B4 in leukocytes from various species: identification of a metabolite to authentic 5S,12R,19-trihydroxy-6Z,8E,10E,14Z-eicosatetra enoic acid and relative importance of 19- and 20-hydroxylations

The major hydroxylated metabolite of leukotriene B4 in rat PMNL was found identical (UV spectrum and retention times in 3 different HPLC systems) to a synthetic compound of known stereochemistry, 19-hydroxy-LTB4. PMNL from various species exhibited 3 different types of behaviour for LTB4 hydroxylation. Human and monkey PMNL showed a high hydroxylating activity and a high regioselectivity with almost exclusive formation of products from 20-hydroxylation. Rat and mini-pig PMNL exhibited a very different regioselectivity with major formation of 19-OH-LTB4 (3:1 ratio). Finally, pig and beef PMNL were found almost devoid of any hydroxylating activity toward LTB4.

Biological properties of dihydro-leukotriene B4, an alternative leukotriene B4 metabolite

Dihydro-leukotriene B4 (a 5,12-dihydroxy-eicosatrienoic acid) has been shown to be the primary metabolite of leukotriene B4 (LTB4) in a variety of cells other than human polymorphonuclear leukocytes (PMNLs). In this report we show that dihydro-LTB4 is significantly less active than LTB4 in different biological assay systems, i.e. leukocyte chemotaxis, chemokinesis, aggregation, adhesion to endothelium and superoxide anion production. This suggests that primary reduction constitutes a second so far unknown deactivation pathway for LTB4.

The omega-hydroxylation of arachidonic acid by human polymorphonuclear leukocytes

Incubations of [1-14C]arachidonic acid with unstimulated human polymorphonuclear leukocytes resulted in the formation of four new metabolites in a previously described reverse-phase HPLC system. Three of these metabolites were largely suppressed in a CO/O2 (80/20, by vol.) atmosphere indicating a cytochrome-P450-dependent monooxygenase reaction. In agreement with this assumption is their NADPH/O2-dependent formation in the microsomal fraction. One metabolite was identified by gas chromatography/mass spectrometry analysis as omega-hydroxy-arachidonic acid and the two others were secondary products identified as omega-carboxy-arachidonic acid and 5,20-dihydroxy-E,Z,Z,Z-6,8,11,14-eicosatetraenoic acid. Since the affinity for arachidonate of the omega-monooxygenase was quite low and the presence of LTB4 suppressed the omega-hydroxylation of arachidonate, we conclude that the known LTB4 omega-monooxygenase is responsible for the formation of omega-hydroxy-arachidonate. It is unlikely, however, that significant concentrations of these metabolites are formed by activated polymorphonuclear leukocytes in vivo. The fourth metabolite remains tightly associated with the leukocytes but has not been further characterized.

A novel metabolic pathway for leukotriene B4 in different cell types: primary reduction of a double bond

Addition of leukotriene B4 together with trace amounts of tritiated leukotriene B4 to different cell types, such as bone marrow-derived macrophages, T-lymphocytes, mesangial cells or fibroblast tumor cells, led to the formation of several hitherto unknown degradation products within hours. None of them could be identified as 20-hydroxy- or 20-carboxyleukotriene B4, known to be produced by polymorphonuclear leukocytes. The primarily formed transient leukotriene B4 metabolite was less polar than leukotriene B4 and was detectable by measuring its ultraviolet absorbance at 232 nm or its radioactivity. Mass spectral analysis showed very similar fragmentation spectra of leukotriene B4 and its primary metabolite. The most abundant ion and the main fragments of the new metabolite were increased by two mass units compared to leukotriene B4. These observations suggest that, in a variety of cells, leukotriene B4 is first reduced to a 5,12-dihydroxyeicosatrienoic acid, which is further converted to secondary hydrophilic degradation products. This raises the question of the major route of leukotriene B4 metabolism in vivo.