Metabolism of lipoxins A4 and B4 and of their all-trans isomers by human leukocytes and rat liver microsomes

Catalytic characterization and cytokine mediated regulation of cytochrome P450 4Fs in rat hepatocytes

Cytochrome P450 (CYP) 4F mediated leukotriene B(4) (LTB(4)) metabolism modulates inflammation during injury and infection. Here we show that in addition to LTB(4), the recombinant rat CYP4Fs catalyze omega-hydroxylations of lipoxin A(4), and hydroxyeicosatetraeonic acids. CYP4F gene regulation studies in primary hepatocytes reveal that pro-inflammatory cytokines interleukin (IL) -1beta, IL-6 and tumor necrosis factor (TNF) -alpha produce a general inductive response whereas IL-10, an anti-inflammatory cytokine, suppresses CYP4F expression. The molecular mechanism behind IL-6 related induction of CYP4F4 and 4F5 is partially signal transducer and activator of transcription 3 (STAT3) dependent. When hepatocytes are subjected to high concentrations of LTB(4) or prostaglandin E(2), lipid mediators of inflammation, only an increase in CYP4F5 mRNA expression is observed. Collectively, the results from isozyme activity and substrate driven CYP4F induction do not support the notion that an autoregulatory pathway could control the excessive concentrations of LTB(4) during an inflammatory challenge to hepatocytes.

Cytochrome P450 4F subfamily: at the crossroads of eicosanoid and drug metabolism

The cytochrome P450 4F (CYP4F) subfamily has over the last few years come to be recognized for its dual role in modulating the concentrations of eicosanoids during inflammation as well as in the metabolism of clinically significant drugs. The first CYP4F was identified because it catalyzed the hydroxylation of leukotriene B(4) (LTB(4)) and since then many additional members of this subfamily have been documented for their distinct catalytic roles and functional significance. Recent evidence emerging in relation to the temporal change of CYP4F expression in response to injury and infection supports an important function for these isozymes in curtailing inflammation. Their tissue-dependent expression, isoform-based catalytic competence and unique response to the external stimuli imply a critical role for them to regulate organ-specific functions. From this standpoint variations in relative CYP4F levels in humans may have direct influence on the metabolic outcome through their ability to generate and/or degrade bioactive eicosanoids or therapeutic agents. This review covers the enzymatic characteristics and regulatory properties of human and rodent CYP4F isoforms and their physiological relevance to major pathways in eicosanoid and drug metabolism.

Purification and characterization of recombinant rat hepatic CYP4F1

CYP4F1 was discovered by Chen and Hardwick (Arch. Biochem. Biophys. 300, 18-23, 1993) as a new CYP4 cytochrome P450 (P450) preferentially expressed in rat hepatomas. However, the catalytic function of this P450 remained poorly defined. We have purified recombinant CYP4F1 protein to a specific content of 12 nmol of P450/mg of protein from transfected yeast cells by chromatography of solubilized microsomes on an amino-n-hexyl Sepharose 4B column, followed by sequential HPLC on a DEAE column and two hydroxylapatite columns. The purified P450 was homogeneous as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an apparent molecular weight of 53 kDa. The enzyme catalyzed the omega-hydroxylation of leukotriene B(4) with a K(m) of 134 microM and a V(max) of 6.5 nmol/min/nmol of P450 in the presence of rabbit hepatic NADPH-P450 reductase and cytochrome b(5). In addition, 6-trans-LTB(4), lipoxin A(4), prostaglandin A(1), and several hydroxyeicosatetraenoic acids (HETEs) were also omega-hydroxylated. Of several eicosanoids examined, 8-HETE was the most efficient substrate, with a K(m) of 18.6 microM and a V(max) of 15.8 nmol/min/nmol of P450. In contrast, no activity was detected toward lipoxin B(4), laurate, palmitate, arachidonate, and benzphetamine. The results suggest that CYP4F1 participates in the hepatic inactivation of several bioactive eicosanoids.

Lipoxin A4 and B4 are potent stimuli for human monocyte migration and adhesion: selective inactivation by dehydrogenation and reduction

Monocyte recruitment and adherence are important events in inflammatory and vascular diseases. Here, we evaluated the actions of lipoxin A4 (LXA4) and LXB4, a series of lipoxygenase products from arachidonic acid generated by cell-cell interactions, on human monocytes. LXA4 and LXB4 (10(-7) M) each increased monocyte migration in chamber chemotaxis assays and, in migration under agarose, exhibited chemotactic indices similar to those of the chemotactic peptide formyl-methionyl-leucyl-phenylalanine at 10(-10)-10(-8) M and to the chemokine macrophage inflammatory protein-1 alpha (MIP-1 alpha) at 10(-8)-10(-7) M with a rank order of potency: Monocyte chemotactic protein-1 alpha > LXA4 approximately LXB4 approximately MIP-1 alpha. Lipoxins also stimulated monocyte adherence to laminin. In addition, human monocytes rapidly transformed LXA4 and LXB4 to several metabolites. LXB4 (> 80%) was converted within 30 s to new products, in a trend similar to that of LXA4. The novel monocyte-derived LXB4 products were identified as 5-oxo-6,7-dihydro-LXB4 and 6,7-dihydro-LXB4, indicating a role for site-selective dehydrogenation and reduction. Unlike monocytes, intact polymorphonuclear leukocytes (PMN) did not metabolize LXA4 in significant quantities, and only approximately 12% of exogenous LXB4 was omega-oxidized to 20-OH-LXB4 and 20-COOH-LXB4 by PMN. To determine if lipoxin conversion altered bioactivity, we evaluated the actions of these metabolites on monocytes. Each of the novel products of LXA4 and LXB4 from monocytes, namely oxo- and dihydrolipoxins, were essentially inactive in stimulating monocyte adherence. In contrast, the omega-oxidation products of LXB4 isolated from PMN were equipotent with LXB4 for monocyte adherence. Dehydrogenation of LXA4 in monocytes appears to be carried out by a 15-hydroxyprostaglandin dehydrogenase, which is present in human monocytes as determined by reverse transcription PCR and Western blots. Together, these results provide the first evidence that LXA4 and LXB4 are both potent stimulants for migration and adherence of human monocytes. Moreover, they underscore the importance of the major route of lipoxin metabolism in leukocytes, namely, the rapid dehydrogenation and inactivation carried out by monocytes.

omega-Oxidation of lipoxin B4 by rat liver. Identification of an omega-carboxy metabolite of lipoxin B4

Lipoxin B4 (LXB4) is metabolized to 20-hydroxy-LXB4 by rat liver microsomes. The omega-hydroxylation requires both molecular oxygen and NADPH, and is inhibited by carbon monoxide, indicating involvement of a cytochrome P-450 (P-450). This is supported by inhibition of the reaction by antibodies raised against NADPH-P-450 reductase. The P-450 appears to be the one responsible for leukotriene B4 omega-hydroxylation, because leukotriene B4 inhibits the formation of 20-hydroxy-LXB4 and LXB4 blocks the leukotriene B4 omega-hydroxylase activity in microsomes. Incubation of 20-hydroxy-LXB4 with both rat liver cytosol and NAD+ leads to formation of a more polar metabolite on high-performance liquid chromatography. The metabolite is identified as 20-carboxy-LXB4, a novel metabolite of LXB4, based on analyses by ultraviolet spectrometry and by gas chromatography/mass spectrometry. The 20-carboxy-LXB4-forming activity is localized in cytosol, with an optimal pH of 8.5. The activity is dependent on NAD+, but NADP+ can not replace NAD+. The reaction is inhibited by pyrazole and 4-methylpyrazole, inhibitors of alcohol dehydrogenase, and by substrates of the enzyme such as ethanol and 20-hydroxy-leukotriene B4. Disulfiram, an inhibitor of aldehyde dehydrogenase, also blocks the 20-carboxy-LXB4 formation. These observations suggest that both alcohol dehydrogenase and aldehyde dehydrogenase participate in the oxidation of 20-hydroxy-LXB4 to 20-carboxy-LXB4.

Lipoxin A4 metabolism by differentiated HL-60 cells and human monocytes: conversion to novel 15-oxo and dihydro products

Lipoxins are tetraene-containing eicosanoids that possess biological activity in several organ systems. To determine their route of further metabolism, [11,12-3H]lipoxin A4 was prepared and incubated with human neutrophils, promyelocytic leukemia (HL-60) cells, and adherent monocytes. Intact neutrophils and undifferentiated HL-60 cells did not significantly metabolize [11,12-3H]LXA4, while HL-60 cells differentiated with PMA to monocyte/macrophage lineage rapidly (< 15 s) transformed this eicosanoid. The major radiolabeled LXA4-derived metabolites were characterized by physical methods and were shown to be 15-oxo-LXA4, 13,14-dihydro-15-oxo-LXA4, and 13,14-dihydro-LXA4. Substrate competition with cell-free supernatants from differentiated HL-60 cells suggests that lipoxins compete for 15-hydroxyprostaglandin dehydrogenase activity or an equivalent enzyme system. In addition, adherent monocytes exposed to [11,12-3H]LXA4 rapidly metabolized (> 60% within 30 s) the label to its oxo and dihydro derivatives. These results indicate that, unlike leukotrienes, LXA4 is subject to dehydrogenation and reduction of its conjugated tetraene to form triene-containing products. Moreover, they suggest that monocytes participate in lipoxin metabolism in their local milieu.

Formation of a novel 20-hydroxylated metabolite of lipoxin A4 by human neutrophil microsomes

Lipoxin A4 (LXA4) is a biologically active compound produced from arachidonic acid via interactions of lipoxygenases. Incubation of LXA4 either with human neutrophils or with the neutrophil microsomes leads to formation of a polar compound on a reverse-phase high-performance liquid chromatography. We have identified the metabolite as 20-hydroxy-LXA4, a novel metabolite of arachidonic acid, on the basis of ultraviolet spectrometry and gas chromatography-mass spectrometry. The LXA4 omega-hydroxylation requires both molecular oxygen and NADPH, and is inhibited by carbon monoxide, by antibodies raised against NADPH-cytochrome P-450 reductase, or competitively by leukotriene B4 (LTB4) and LTB5, substrates of LTB4 omega-hydroxylase. These findings indicate that the formation of 20-hydroxy-LXA4 is catalyzed by a neutrophil cytochrome P-450, the LTB4 omega-hydroxylase.

Conversion of 5,6-dihydroxyeicosatetraenoic acids. A novel pathway for lipoxin formation by human platelets

Leukotriene A4 may be metabolized to 5(S),6(R)- and 5(S),6(S)-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acids by enzymatic or non-enzymatic hydrolysis. Incubation of human platelet suspensions with these dihydroxy acids led to the formation of lipoxin A4 and 6(S)-lipoxin A4 via lipoxygenation at C-15. Furthermore, human platelets converted the two 5(R),6(S)- and 5(R),6(R)-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acids to tetraene-containing trihydroxyeicosatetraenoic acids. In contrast, leukotrienes C4, D4 and E4 were not transformed to cysteinyl-lipoxins. Time-course studies of leukotriene A4 metabolism in human platelet suspensions indicated lipoxin formation via two pathways: (i) direct conversion of leukotriene A4, leading to formation of the lipoxin intermediate 15-hydroxy-leukotriene A4; and (ii) 15-lipoxygenation of the 5(S),6(R)- and 5(S),6(S)-dihydroxyeicosatetraenoic acids. The results demonstrate that lipoxygenation at C-15 of 5,6-dihydroxy-7,9,11,14-eicosatetraenoic acids may be an alternative novel pathway for platelet-dependent lipoxin formation.