Leukotriene B4 metabolism by hepatic cytochrome P-450

Importance of the leukotriene B4-BLT1 and LTB4-BLT2 pathways in asthma

For several decades, the leukotriene pathways have been implicated as playing a central role in the pathophysiology of asthma. The presence and elevation of numerous metabolites in the blood, sputum, and bronchoalveolar lavage fluid from asthmatics or experimental animals adds support to this notion. However, targeting of the leukotriene pathways has had, in general, limited success. The single exception in asthma therapy has been targeting of the cysteinyl leukotriene receptor 1, which clinically has proven effective but only in certain clinical situations. Interference with 5-lipoxygenase has had limited success, in part due to adverse drug effects. The importance of the LTB4-BLT1 pathway in asthma pathogenesis has extensive experimental support and findings, albeit limited, from clinical samples. The LTB4-BLT1 pathway was shown to be important as a neutrophil chemoattractant. Despite observations made more than two decades ago, the LTB4-BLT1 pathway has only recently been shown to exhibit important activities on subsets of T lymphocytes, both as a chemoattractant and on lymphocyte activation, as well as on dendritic cells, the major antigen presenting cell in the lung. The role of BLT2 in asthma remains unclear. Targeting of components of the LTB4-BLT1 pathway offers innovative therapeutic opportunities especially in patients with asthma that remain uncontrolled despite intensive corticosteroid treatment.

Role of the LTB4/BLT1 pathway in allergen-induced airway hyperresponsiveness and inflammation

LTB4, a proinflammatory lipid mediator generated from arachidonic acid through the action of 5-lipoxygenase, has been known for over two decades and is implicated in a wide variety of inflammatory disorders. BLT1, a G-protein-coupled receptor, has recently been identified as a high affinity receptor specific for LTB4. Recent studies in allergen-induced airway hyperresponsiveness and inflammation using mice lacking BLT1 have shown crucial new roles for leukotriene B4 and BLT1 in Th2 cytokine IL-13 production from lung T cells and recruitment of antigen-specific effector CD8+ T cells, suggesting novel mechanisms for their actions. The leukotriene B4-BLT1 pathway is an important target for the treatment of bronchial asthma.

Mediators, cytokines, and growth factors in liver-lung interactions

Multiple mediators have been implicated in the interactions between the liver and the lungs in various disease states. The best characterized mediator of liver-lung interaction is alpha 1-antitrypsin. Several cytokines and mediators may be involved in the pathogenesis of the hepatopulmonary syndrome and in the cytokine cascades that are activated in systemic inflammatory states such as acute respiratory distress syndrome. Hepatocyte growth factor or scatter factor is a recently described peptide with a broad range of biologic effects that may mediate lung-liver interactions.

Novel 3-hydroxylated leukotriene b4 metabolites from ethanol-treated rat hepatocytes

Coincubations of radiolabeled leukotriene B4 (LTB4) and ethanol with isolated rat hepatocytes led to formation of one dihydroxylated and two novel β-oxidized metabolites of LTB4. The major radioactive peaks from reverse-phase-high performance liquid chromatography (RP-HPLC) eluted with material absorbing UV light maximally at 270 nm, with shoulders at 260 and 280 nm, indicating retention of the conjugated triene structure of the parent molecule in each metabolite structure. Following purification, catalytic reduction, and derivatization, mass spectrometric analysis revealed that all three metabolites were hydroxylated at the C-3 carbon atom based on characteristic ions at m/z 201 and 175 in the electron ionization mass spectra of the metabolites. Negative-ion electron capture mass spectrometry of the metabolites as pentafluorobenzyl (PFB) ester, trimethylsilyl ether derivatives aided structural characterizations while revealing interesting fragmentations. A ketene-containing ion appeared to result from the loss of both PFB groups (one as PFB alcohol), while a lactone alkoxide ion appeared to result following loss of PFB and bis (trimethylsilyl) ether. From these data three novel LTB4 metabolites were suggested to be 3,20-dihydroxy-LTB4 (3,20-diOH-LTB4), 3-hydroxy-18-carboxy-LTB4 (3-OH-18-COOH-LTB4), and 3-hydroxy-16-carboxy-LTB3 (3-OH-16-COOH-LTB3). The significance of the almost exclusive formation of these 3-hydroxylated LTB4 metabolites in the presence of ethanol is currently unknown, but may result from interrupted β-oxidation from the C-1 carboxyl moiety.

Inhibition of leukotriene omega-oxidation by omega-trifluoro analogs of leukotrienes

omega-Oxidation with subsequent beta-oxidation from the omega-end is the major pathway for inactivation and degradation of leukotrienes. Oxidative degradation of leukotriene E4 (LTE4), N-acetyl-LTE4, and LTB4 was inhibited by the omega-trifluoro analogs of LTE4, omega-trifluoro-LTE4 (omega-F3-LTE4), and (1S,2R)-5-(3-[1-hydroxy-15,15,15-trifluoro-2-(2-1H- tetrazol-5-ylethyl-thio)pentadeca-3(E),5(Z)-dienyl+ ++]phenyl)-1H-tetrazole (LY 245769). The latter substance inhibited the oxidative degradation of LTE4 and N-acetyl-LTE4 in the rat in vivo by 50% at a dose of 7 mumol/kg body weight. In rat hepatocyte cultures both omega-trifluoro analogs interfered with the omega-oxidation of N-acetyl-LTE4 and LTB4 with IC50 values of about 4 microM. Both analogs inhibited the omega-hydroxylation in isolated rat liver microsomes with IC50 values between 16 and 37 microM. This inhibition is apparently competitive. In addition, in liver cytosol, the conversion of the omega-hydroxylated leukotrienes to omega-carboxy-LTE4 and omega-carboxy-LTB4 was inhibited by both compounds. omega-Trifluoro analogs of leukotrienes provide a new tool for interfering with the inactivation of leukotrienes in the omega-oxidation pathway.

Effects of D-galactosamine-induced acute liver injury on mortality and pulmonary responses to Escherichia coli lipopolysaccharide. Modulation by arachidonic acid metabolites

Multiple extrapulmonary organ system failures increase mortality, permeability edema, and alveolar inflammation during gram-negative sepsis because of abnormal regulation of host inflammatory responses. We tested the hypothesis that acute hepatocytic injury induced by the selective hepatotoxin, D-galactosamine (GalN), augments mortality and amplifies pulmonary microvascular permeability to albumin and neutrophilic influx after administering Escherichia coli lipopolysaccharide (LPS) 24 h later by impairing the metabolism of endogenously synthesized products of arachidonic acid. We determined the lung extravascular leak of 125I-human serum albumin measured at multiple time points after LPS and enumerated polymorphonuclear leukocytes (PMNs) in bronchoalveolar lavage fluid (BALF). Because the liver is important in prostaglandin (PG) and leukotriene (LT) metabolism, we measured plasma concentrations of 6-keto-PGF1 alpha and thromboxane B2 (TxB2) in addition to paired plasma BALF concentrations of LTB4 and BALF LTC4 60 min and 24 h after LPS. We further assessed the protective effects of a single 20-mg/kg injection given intraperitoneally (i.p.) of the LTA4 synthetase inhibitor, diethylcarbamazine (DEC). After 400 mg/kg GalN, LPS at 2.5 or 1.25 mg/kg i.p. increased mortality (p less than 0.001), albumin leak 60 and 90 min after LPS (p less than 0.05), plasma 6-keto-PGF1 alpha, TxB2, and LTB4 levels and BALF LTC4 within 60 min (p less than 0.05). LTB4 and LTC4 levels in BALF 24 h later were similarly increased (p less than 0.05) as were bronchoalveolar PMNs (p less than 0.001). DEC improved mortality and albumin leak (p less than 0.001), reduced lung influx of PMNs and peripheral leukocytosis (p less than 0.05), attenuated plasma LTB4 and BALF LTC4 levels 60 min after LPS (p less than 0.05), and decreased BALF LTB4 and LTC4 at 24 h (p less than 0.05), but was associated with higher plasma 6-keto-PGF1 alpha and TxB2 values at 60 min. Changes in eicosanoid levels and modulation of responses by DEC in this model suggest that impaired metabolism of endogenously synthesized leukotriences by the damaged liver underlies these phenomena. We conclude that this mechanism may enhance septic lung injury during acute liver dysfunction.

Hepatic uptake and metabolic disposition of leukotriene B4 in rats

1. In isolated perfused rat liver and in vivo, up to 25% of [3H]leukotriene B4 was eliminated from the circulation via hepatic uptake and biliary excretion within 1 h. Total body recovery of 3H amounted to about 60% of infused [3H]leukotriene B4. 2. Hepatobiliary excretion of leukotriene B4 and its metabolites exceeded renal elimination by about 4-fold and depended, in contrast with excretion of cysteinyl leukotriene E4, upon continuous taurocholate supply. 3. Analyses of bile, liver and recirculated perfusate using h.p.l.c. indicated that the liver metabolized leukotriene B4 extensively to omega-carboxyleukotriene B4 and its beta-oxidized derivatives, and no unmetabolized leukotriene B4 appeared in bile. These results substantiate the important contribution of the hepatobiliary system with respect to the metabolic fate of leukotriene B4.

Inhibition of leukotriene omega-oxidation by isonicotinic acid hydrazide (isoniazid)

Metabolism of leukotrienes via omega-oxidation represents a major degradative and inactivating pathway of these biologically active icosanoids. Isonicotinic acid hydrazide (isoniazid) inhibited this process in rats in vivo, in the isolated perfused rat liver, and in hepatic microsomes. The in vivo catabolism of leukotriene E4 via N-acetyl-leukotriene E4 to its omega-oxidized metabolites was inhibited by 50% or 71% using single intravenous isoniazid doses of 0.6 mmol or 1.0 mmol/kg body mass, respectively. Isoniazid interfered with leukotriene catabolism at the initial omega-oxidation step, resulting in an accumulation of N-acetyl-leukotriene E4. Analogous although weaker inhibition of leukotriene omega-oxidation in vivo was observed by pretreatment with isonicotinic acid 2-isopropylhydrazide and monoacetyl hydrazine. In the isolated perfused liver, isoniazid at concentrations varying over 0.2-10 mM decreased the omega-oxidation of cysteinyl leukotrienes dose-dependently by up to 94%. omega-Oxidation of both leukotriene E4 and leukotriene B4 by rat liver microsomes was inhibited by isoniazid, isonicotinic acid 2-isopropylhydrazide, and monoacetyl hydrazine with half-maximal concentrations in the range of 5-15 mM. Our measurements indicate that the impairment of leukotriene omega-oxidation by isoniazid involves both cytochrome-P450-dependent enzyme systems responsible for omega-oxidation of leukotriene E4 and leukotriene B4. In effect, under isoniazid treatment one can expect a prolongation of the proinflammatory actions of endogenously produced leukotrienes.

Ethanol-induced inhibition of leukotriene degradation by omega-oxidation

omega-Oxidation of leukotrienes is a major pathway in the degradation and inactivation of these proinflammatory mediators. Ethanol inhibited this process in vivo and in vitro. In rat liver in vivo the catabolism of LTE4 to omega-carboxylated leukotrienes was inhibited by 57% by an ethanol dose of 25 mmol/kg body mass administered intragastrically. The site of inhibition was the oxidation of omega-hydroxy-N-acetyl-LTE4 to omega-carboxy-N-acetyl-LTE4 resulting in an accumulation of omega-hydroxy-N-acetyl-LTE4 and of N-acetyl-LTE4. Analogous results were obtained for the oxidative degradation of LTB4 and omega-hydroxy-LTB4 in rat hepatocyte suspensions. Ethanol, at a concentration of 12.5 mmol/l (0.07%; by vol.), caused 68% inhibition of the oxidation of omega-hydroxy-LTB4 by 50% in hepatocyte suspensions. The conversion of omega-hydroxy-LTB4 to omega-carboxy-LTB4 by rat and human liver cytosol was inhibited by ethanol with half maximal concentrations of 100 mumols/l and 300 mumols/l, respectively. Our measurements indicate that direct interference by ethanol of the omega-oxidation of leukotrienes as well as an increased NADH/NAD+ ratio induced by ethanol led to the inhibition of leukotriene degradation. The impairment of leukotriene inactivation in the liver by ethanol may contribute to the development of the inflammatory reaction in acute alcoholic liver disease.

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.

NADPH-dependent microsomal metabolism of 14,15-epoxyeicosatrienoic acid to diepoxides and epoxyalcohols

The arachidonic acid epoxygenase metabolite 14,15-epoxyeicosatrienoic acid is further metabolized by rat liver microsomal fractions to regioisomeric diepoxides and epoxyalcohols. Diepoxides result from epoxidation at the 5,6-, 8,9-, or 11,12-olefins. Hydroxylation leading to epoxyalcohols with a cis, trans-conjugated dienol occurs at carbons 5, 8, 9, or 12. Structural assignments were established by chromatographic and mass spectral comparisons with synthetic standards. The reaction requires NADPH and is inhibited by typical cytochrome P-450 inhibitors. Analysis of the time course of product formation during arachidonic acid oxidation by rat liver microsomal fractions indicated that all four regioisomeric epoxyeicosatrienoic acids can be further metabolized by the enzyme system.

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.

Leukotriene B4 fails to induce penetration of polymorphonuclear leukocytes into psoriatic lesions

The intraepidermal accumulation of polymorphonuclear leukocytes following the epicutaneous application of leukotriene B4 (LTB4) was studied in lesional and clinically uninvolved skin of five patients with chronic stable plaque psoriasis. The lesions were found to be wholly unresponsive to LTB4, doses of 100 ng failing to produce either micropustules or exocytosis. This phenomenon was sharply localized; the response immediately adjacent to the lesion being identical to that in more distant uninvolved skin. We speculate that both the reduced response to LTB4 in the psoriatic patient and also the tolerance to LTB4 seen after repeated applications, result from the induction of a P450-linked hydroxylase.

Characterization of leukotriene B4-omega-hydroxylase activity within human polymorphonuclear granulocytes

Human polymorphonuclear granulocytes (PMN) metabolize exogenous [3H]leukotriene B4 (LTB4) into 20-hydroxy- and 20-carboxy-[3H]LTB4. The conversion was enhanced at acidic pH values (pH 6.0-7.0). Sonication of purified PMN and subcellular fractionation by differential centrifugation showed that major LTB4-hydroxylase activity was associated with the microsomal fraction (105,000 g pellet). In contrast to intact cells, LTB4-hydroxylase activity within the microsomal fraction revealed optimal activity at neutral pH and was inhibited by a wide range of divalent cations. There was a strict requirement for the presence of suitable electron donors such as NADPH. Heterocyclic nitrogenous bases, such as imidazole and pyridine, inhibited the LTB4 conversion induced by intact PMN as well as by their microsomes. These observations combined with the spectrophotometric analysis (carbon monoxide dithionite-reduced difference spectrum) supported the assumption that LTB4-hydroxylase resembled a cytochrome P-450 enzyme. The LTB4-hydroxylase within human PMN was not identical with the cytochrome P-450 of rat liver; hepatic microsomes only showed minute conversion of LTB4.

Oxygen concentration-dependent metabolism of leukotriene B4 by hepatocyte monolayers

Leukotriene B4 was found to be metabolized by rat hepatocyte monolayers at a rate that was linear with increasing substrate concentration from 74 to 740 nM leukotriene B4. The rates of metabolism were dependent on the O2 concentration and were 315, 213, 80, and 36 pmol leukotriene B4 per min per nmol cytochrome P-450 at 20% (212 microM), 4% (42.5 microM), 2% (21.2 microM), and 1% (10.6 microM) O2, respectively. The metabolic rate was not linear with respect to O2 concentration; however, half maximal rate occurred at 4% O2, and O2 concentration found in the pericentral region of normally oxygenated liver. These results suggest that in vivo conditions of hypoxia or ischemia that lead to blood O2 concentrations less than 4% may drastically decrease hepatic clearance of leukotriene B4.

Human hepatocytes metabolizing ethanol generate a non-polar chemotactic factor for human neutrophils

When human hepatocytes were incubated with low concentrations of ethanol they general chemotactic activity for human neutrophils. Generation of chemotactic activity was dependent upon duration of incubation and concentration of ethanol used. Production of chemotactic activity by ethanol-treated hepatocytes was inhibited completely in the presence of the alcohol dehydrogenase inhibitor 4-methylpyrazole. PMN isolated from rats, in contrast, do not respond chemotactically to the factor released by homologous cells. Preliminary studies indicated that the chemotactic factor is non-polar in nature (perhaps related to leukotriene B4). These results indicate that human hepatocytes, when exposed to ethanol, generate chemotactic factor(s) for human PMN. The occurrence of this phenomenon may explain, in part, the PMN infiltrates observed in human liver during the course of acute alcoholic hepatitis.

Leukotriene B4 omega-hydroxylase in human polymorphonuclear leukocytes. Partial purification and identification as a cytochrome P-450

Human polymorphonuclear leukocytes (PMN) not only synthesize and respond to leukotriene B4 (LTB4), but also catabolize this mediator of inflammation rapidly and specifically by omega-oxidation. To characterize the enzyme(s) responsible for omega-oxidation of LTB4, human PMN were disrupted by sonication and subjected to differential centrifugation to yield membrane, granule, and cytosol fractions (identified by biochemical markers). LTB4 omega-hydroxylase activity was concentrated (together with NADPH cytochrome c reductase activity) only in the membrane fraction (specific activity increased 10-fold as compared to whole sonicates, 41% recovery). Negligible activity was detected in granule or cytosol fractions. LTB4 omega-hydroxylase activity in isolated PMN membranes was linear with respect to duration of incubation and protein concentration, was maximal at pH 7.4, had a Km for LTB4 of 0.6 microM, and was dependent on oxygen and on reduced pyridine nucleotides (apparent Km for NADPH = 0.5 microM; apparent Km for NADH = 223 microM). The LTB4 omega-hydroxylase was inhibited significantly by carbon monoxide, ferricytochrome c, SKF-525A, and Triton X-100, but was not affected by alpha-naphthoflavone, azide, cyanide, catalase, and superoxide dismutase. Finally, isolated PMN membranes exhibited a carbon monoxide difference spectrum with a peak at 452 nm. Thus, we have partially purified the LTB4 omega-hydroxylase in human PMN and identified the enzyme as a membrane-associated, NADPH-dependent cytochrome P-450.

Metabolism of leukotriene B4 in hepatic microsomes

Leukotriene B4 was metabolized in rat hepatic microsomes to two products. Mass spectral analysis of these two metabolites indicated that the major metabolite was the 20-hydroxy metabolite while the minor metabolite was the 19-hydroxy metabolite. The formation of these metabolites required NADPH and was linear with time (20 min) and protein (1.6 mg/ml). The Km apparent and Vmax for omega hydroxylation of LTB4 was 14 uM and 0.138 nmol/min/mg protein. In contrast, the km and Vmax for omega minus one hydroxylation was 54 uM and 0.093 nmol/min/mg protein. These results suggest that omega and omega minus one hydroxylations of LTB4 may be mediated by different isozymes of hepatic P-450.

A cell-free preparation of human neutrophils catalyzing NADPH-dependent conversion of leukotriene B4

The sonicate of human neutrophils converted leukotriene B4 to a polar product in aerobic condition in the presence of NADPH at a rate comparable to that of the intact cells. NADH could scarcely replace NADPH. The conversion was not observed in anaerobic conditions and was inhibited by carbon monoxide (CO/O2 = 4/1) or by 1 mM p-chlormercuribenzoate, while it was not affected by 1 mM KCN, 5 mM NaN3, 200 micrograms/ml catalase, 100 mM mannitol, and 10 micrograms/ml superoxide dismutase. These observations suggest that the myeloperoxidase-H2O2-halide system and active oxygen species are not involved in the reaction. The activity was observed in the 100,000xg supernatant from the homogenate, in which cytochrome P-450 was not detected.

Carbon monoxide inhibits omega-oxidation of leukotriene B4 by human polymorphonuclear leukocytes: evidence that catabolism of leukotriene B4 is mediated by a cytochrome P-450 enzyme

Carbon monoxide significantly inhibits omega-oxidation of exogenous leukotriene B4 to 20-OH-leukotriene B4 and 20-COOH-leukotriene B4 by unstimulated polymorphonuclear leukocytes as well as omega-oxidation of leukotriene B4 that is generated when cells are stimulated with the calcium ionophore, A23187. Inhibition of omega-oxidation by carbon monoxide is concentration-dependent, completely reversible, and specific. Carbon monoxide does not affect synthesis of leukotriene B4 by stimulated polymorphonuclear leukocytes or other cell functions (i.e., degranulation, superoxide anion generation). These findings suggest that a cytochrome P-450 enzyme in human polymorphonuclear leukocytes is responsible for catabolizing leukotriene B4 by omega-oxidation.

The Ah receptor: binding specificity only for foreign chemicals?

The murine Ah locus controls the induction of at least four drug-metabolizing enzymes: cytochromes P1-450, P2-450, and P3-450, and UDP-glucuronosyltransferase. The Ah gene codes for a cytosolic receptor. It is known that the induction response includes: (i) high-affinity binding of specific foreign chemicals to the Ah receptor; (ii) temperature-dependent translocation of the "activated" inducer-receptor complex into the nucleus; (iii) binding of the complex presumably to chromatin components; (iv) transcriptional activation of specific genes; (v) maximal increases in intranuclear high-molecular-weight precursor mRNA (pre-mRNA) that precede by several hours the maximal increases in cytoplasmic mRNA; (vi) translation of the mRNA principally on membrane-bound polysomes; and (vii) increases in the specific membrane-bound proteins (including architectural arrangement with other membrane-bound moieties) that reflect enhanced specific drug-metabolizing activities. It is not known how many of the other drug metabolism induction responses are also governed by receptors. The Ah locus studies have been chiefly unraveled in the mouse, due to several inbred strains having a receptor defect. In addition to "classical" pharmacologic methods (such as structure-activity studies) and standard biochemical techniques, the newer methods of recombinant DNA technology and somatic-cell genetics in culture are shown to be important in understanding the Ah receptor and its induction response. It is possible that this receptor is required for endogenous functions critical to life processes, as well as its function in the induction of drug metabolism by certain polycyclic aromatic compounds.