Metabolism of leukotriene B4 by cultured human keratinocytes. Formation of glutathione conjugates and dihydro metabolites

Synthesis and Significance of Arachidonic Acid, a Substrate for Cyclooxygenases, Lipoxygenases, and Cytochrome P450 Pathways in the Tumorigenesis of Glioblastoma Multiforme, Including a Pan-Cancer Comparative Analysis

Glioblastoma multiforme (GBM) is one of the most aggressive gliomas. New and more effective therapeutic approaches are being sought based on studies of the various mechanisms of GBM tumorigenesis, including the synthesis and metabolism of arachidonic acid (ARA), an omega-6 polyunsaturated fatty acid (PUFA). PubMed, GEPIA, and the transcriptomics analysis carried out by Seifert et al. were used in writing this paper. In this paper, we discuss in detail the biosynthesis of this acid in GBM tumors, with a special focus on certain enzymes: fatty acid desaturase (FADS)1, FADS2, and elongation of long-chain fatty acids family member 5 (ELOVL5). We also discuss ARA metabolism, particularly its release from cell membrane phospholipids by phospholipase A₂ (cPLA₂, iPLA₂, and sPLA₂) and its processing by cyclooxygenases (COX-1 and COX-2), lipoxygenases (5-LOX, 12-LOX, 15-LOX-1, and 15-LOX-2), and cytochrome P450. Next, we discuss the significance of lipid mediators synthesized from ARA in GBM cancer processes, including prostaglandins (PGE₂, PGD₂, and 15-deoxy-Δ^12,14-PGJ₂ (15d-PGJ₂)), thromboxane A₂ (TxA₂), oxo-eicosatetraenoic acids, leukotrienes (LTB₄, LTC₄, LTD₄, and LTE₄), lipoxins, and many others. These lipid mediators can increase the proliferation of GBM cancer cells, cause angiogenesis, inhibit the anti-tumor response of the immune system, and be responsible for resistance to treatment.

Serum polyunsaturated fatty acid metabolites as useful tool for screening potential biomarker of colorectal cancer

The biomarker identification of cancer is benefit for early detection and less invasion. Polyunsaturated fatty acid (PUFA) metabolite as inflammatory mediators can affect progression and treatment of cancer. In this work, the serum was collected from colorectal cancer patients and healthy volunteers, and then we tested the change of serum PUFA metabolites in both of them by ultra-high performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). Of the 158 PUFA and their metabolites, we found that abnormal change of 2, 3-dinor-8-iso-PGF2α, 19-HETE and 12-keto-LTB4 from arachidonic acid were observed in colorectal cancer patients. Meanwhile, 9-HODE and 13-HODE from linoleic acid were significant lower in colorectal cancer patients. Our data suggested that some PUFA metabolites might be used as a potential biomarker of colorectal cancer, which might provide assistance in clinical diagnosis and treatment.

Assessing the issue of instability due to Michael adduct formation in novel chemical entities possessing a carbon-carbon double bond during early drug development--applicability of common laboratory analytical protocols

The discovery of small-molecule novel chemical entities (NCEs) is often a complex play between appropriate structural requirements and optimization of the desired efficacy, safety and pharmacokinetic properties. One of the typical structural variants such as having an active carbon-carbon double bond (alpha, beta-unsaturated carbonyl group) in xenobiotics may lead to stability issues. Such functionalities are extremely reactive, paving way to nucleophilic attack by endogenously occurring and ubiquitous nucleophiles like thiols. While it is easy to make a unilateral decision to not pursue the development of xenobiotics with such functionalities, we question the wisdom of such a decision. In this report, we present in vitro methodologies with appropriate examples to illustrate the ease of assessing the reactivity of the xenobiotics containing double bonds with a known nucleophile. The protocols involve simple reaction procedures followed by measurements using standard laboratory equipments (UV spectrophotometer, HPLC and LC-MS). Our data suggests that not all xenobiotics with carbon-carbon double bonds readily form a Michael's adduct product with glutathione. Hence, the criterion for dropping discovery compounds because of alpha,beta-unsaturated double bonds needs to be reconsidered.

Inflammation resolved by retinoid X receptor-mediated inactivation of leukotriene signaling pathways

Leukotrienes are implicated in the pathogenesis of diverse, inflammation-driven diseases. Metabolic inactivation of leukotriene signaling is an innate response to resolve inflammation, yet little is known of mechanisms regulating disposition of leukotrienes in peripheral tissues afflicted in common inflammatory diseases. We studied leukotriene hydroxylases (CYP4F gene products) in human skin, a common target of inflammation and adverse drug reactions. Epidermal keratinocytes express at least six CYP4F enzymes; the most highly expressed and highly regulated is CYP4F3A-the main neutrophil leukotriene hydroxylase. Differentiation-specific factors and retinoids are positive CYP4F regulators in vitro, effecting increased leukotriene B4 hydroxylation (inactivation). CYP4F expression is up-regulated in situ in hyperproliferative dermatoses-an innate mechanism to repair and restore epidermal barrier competency-and after retinoid therapy. Enhanced CYP4F-mediated inactivation of leukotriene signaling is a previously unrecognized antiinflammatory property of therapeutic retinoids mediated by preferential interactions between retinoid X receptors and CYP4F promoter elements in epidermal cells.

Biosynthesis and metabolism of leukotrienes

Leukotrienes are metabolites of arachidonic acid derived from the action of 5-LO (5-lipoxygenase). The immediate product of 5-LO is LTA4 (leukotriene A4), which is enzymatically converted into either LTB4 (leukotriene B4) by LTA4 hydrolase or LTC4 (leukotriene C4) by LTC4 synthase. The regulation of leukotriene production occurs at various levels, including expression of 5-LO, translocation of 5-LO to the perinuclear region and phosphorylation to either enhance or inhibit the activity of 5-LO. Several other proteins, including cPLA2α (cytosolic phospholipase A2α) and FLAP (5-LO-activating protein) also assemble at the perinuclear region before production of LTA4. LTC4 synthase is an integral membrane protein that is present at the nuclear envelope; however, LTA4 hydrolase remains cytosolic. Biologically active LTB4 is metabolized by ω-oxidation carried out by specific cytochrome P450s (CYP4F) followed by β-oxidation from the ω-carboxy position and after CoA ester formation. Other specific pathways of leukotriene metabolism include the 12-hydroxydehydrogenase/15-oxo-prostaglandin-13-reductase that forms a series of conjugated diene metabolites that have been observed to be excreted into human urine. Metabolism of LTC4 occurs by sequential peptide cleavage reactions involving a γ-glutamyl transpeptidase that forms LTD4 (leukotriene D4) and a membrane-bound dipeptidase that converts LTD4 into LTE4 (leukotriene E4) before ω-oxidation. These metabolic transformations of the primary leukotrienes are critical for termination of their biological activity, and defects in expression of participating enzymes may be involved in specific genetic disease.

Free radical oxidation of plasmalogen glycerophosphocholine containing esterified docosahexaenoic acid: structure determination by mass spectrometry

Plasmalogen phospholipids have a vinyl ether substituent at the sn-1 position that is susceptible to oxidative reactions that occur at cell membranes. However, the mechanism by which this oxidation occurs and the effect of the polyunsaturated fatty acid at the sn-2 position have not been established. To gain insight into these mechanisms, the oxidized phospholipid products resulting from the exposure of 1-O-hexadec-1'-enyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (16:0p/22:6-GPCho) to the free radical initiator 2, 2'-azobis (2- amidinopropane) hydrochloride were examined. Electrospray ionization tandem mass spectrometry, UV spectroscopy, and electron ionization-gas chromatography/mass spectrometry were used to structurally characterize the oxidized glycerophosphocholine (GPCho) products. The radical-induced peroxidation of 16:0p/22:6-GPCho revealed two major classes of oxidized phospholipids. The first class of products was formed by oxidation at the sn-1 position and included 1-lyso-2-docosahexaenoyl-GPCho and 1-formyl-2-docosahexaenoyl-GPCho. Additionally, the second class of oxidized products where oxidation occurred at the sn-2 position was classified into three categories that included chain-shortened omega-aldehydes, terminal gamma-hydroxy-alpha,beta-unsaturated aldehydes, and the addition of one or two oxygen atoms onto the sn-2 position of 16:0p/22:6-GPCho. These results clearly indicate that free radical-induced oxidation of plasmalogen phospholipids with esterified docosahexaenoic acid at the sn-2 position underwent oxidation at both the sn-1 and sn-2 positions.

Urinary metabolites of leukotriene B4 in the human subject

Leukotriene B4 (LTB4) is a potent chemoattractant for neutrophils and is thought to play a role in a variety of inflammatory responses in humans. The metabolism of LTB4 in vitro is complex with several competing pathways of biotransformation, but metabolism in vivo, especially for normal human subjects, is poorly understood. As part of a Phase I Clinical Trial of human tolerance to LTB4, four human subjects were injected with 150 nmol/kg LTB4 with one additional subject as placebo control. The urine of the subjects was collected in two separate pools (0-6 and 7-24 h), and aliquots from these urine collections were analyzed using high performance liquid chromatography, UV spectroscopy, and negative ion electrospray ionization tandem mass spectrometry for metabolites of LTB4. In the current investigation, 11 different metabolites of LTB4 were identified in the urine from those subjects injected with LTB4, and none were present in the urine from the placebo-injected subject. The unconjugated LTB4 metabolites found in urine were structurally characterized as 18-carboxy-LTB4, 10,11-dihydro-18-carboxy-LTB4, 20-carboxy-LTB4, and 10,11-dihydro-20-carboxy-LTB4. Several glucuronide-conjugated metabolites of LTB4 were characterized including 17-, 18-, 19-, and 20-hydroxy-LTB4, 10-hydroxy-4,6,12-octadecatrienoic acid, LTB4, and 10,11-dihydro-LTB4. The amount of LTB4 glucuronide (16.7-29.4 pmol/ml) and 20-carboxy-LTB4 (18.9-30.6 pmol/ml) present in the urine of subjects injected with LTB4 was determined using an isotope dilution mass spectrometric assay before and after treatment of the urine samples with beta-glucuronidase. The urinary metabolites of LTB4 identified in this investigation were excreted in low amounts, yet it is possible that one or more of these metabolites could be used to assess LTB4 biosynthesis following activation of the 5-lipoxygenase pathway in vivo.

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.

Occurrence of oxidized metabolites of arachidonic acid esterified to phospholipids in murine lung tissue

Isolation and characterization of murine pulmonary phospholipids revealed the normal occurrence of 10 isobaric eicosanoids corresponding to the incorporation of one oxygen atom into the arachidonate esterified to glycerophospholipids. Lungs from mice were removed and lipids were extracted and then separated into free carboxylic acid and phospholipids. Phospholipids were hydrolyzed to yield the free carboxylic acids prior to analysis. Reverse-phase HPLC and electrospray tandem mass spectrometry were used to identify and quantitate six monohydroxyeicosatetraenoic (HETE) and four epoxyeicosatetraenoic (EET) acid regioisomers using d8-HETE as internal standard. HETEs esterified to phospholipids were found to increase following intratracheal administration of tBuOOH (36 mg/kg), but not the levels of esterified EETs. Chiral analysis of esterified 15-HETE revealed an R/S ratio of 0.96, suggesting operation of a free radical mechanism responsible for generation of this monohydroxy arachidonate phospholipid, and this enantiomeric ratio was 1.10 following treatment of the mouse lung with tBuOOH. These results are consistent with a free-radical-based mechanism of oxidation of pulmonary glycerophospholipids containing arachidonate.

Synthesis of 10,11-Dihydro-12-oxo-LTB(4), a Key Biochemical Intermediate

The first total synthesis of the 5(S)-hydroxy-10,11-dihydro-12-oxo-6(Z),8(E),14(Z)-eicosatrienoic acid (10,11-dihydro-12-oxo-LTB(4)) (3) is reported. This compound is a key pivotal intermediate in the biotransformation of LTB(4) by the so-called "LTB(4) reductase pathway".

Direct mass spectrometric analysis of ozonides: application to unsaturated glycerophosphocholine lipids

The reaction of ozone with double bonds present in glycerophosphocholine lipids results in formation of ozonides that can be directly analyzed by mass spectrometry as either positive or negative molecular ion species generated by electrospray ionization. Polyunsaturated fatty acyl groups esterified to the phospholipid yielded a mixture of ozonide species with the maximum number of ozone molecules added equal to the total number of double bonds. Ozonide decomposition resulted in omega-aldehyde and omega-carboxylic acid products as revealed by ESI-MS. Collisional activation of the ozone adducts for mono- and polyunsaturated phospholipids gave rise to fragment ions indicative of the position of the double bonds in these molecules. The major decomposition pathway for either positive or negative ozonide ion species involved charge remote fragmentation of the ozonide initiated by homolytic cleavage of the peroxide bridge followed by rearrangement to form the omega-aldehyde and omega-carboxylate acyl species. The reaction of ozone with phospholipids containing polyunsaturated fatty acyl groups is a useful method to probe the position of double bonds by electrospray ionization mass spectrometry.

cDNA cloning, expression, and mutagenesis study of leukotriene B4 12-hydroxydehydrogenase

Leukotriene B4 12-hydroxydehydrogenase catalyzes the conversion of leukotriene B4 into its biologically less active metabolite, 12-oxo-leukotriene B4. This is an initial and key step of metabolic inactivation of leukotriene B4 in various tissues other than leukocytes. Here we report the cDNA cloning for porcine and human enzymes from kidney cDNA libraries. A full-length cDNA of the porcine enzyme contains an open reading frame consisting of 987 base pairs, corresponding to 329 amino acids. The human enzyme showed a 97.1% homology with the porcine enzyme. Northern blotting of human tissues revealed its high expression in the kidney, liver, and intestine but not in leukocytes. The porcine enzyme was expressed as a glutathione S-transferase fusion protein in Escherichia coli, which exhibited similar characteristics with the native enzyme. Because the enzymes have a homology, in part, with NAD(P)(+)-dependent alcohol dehydrogenases, a site-directed mutagenesis study was carried out. We found that three glycines at 152, 155, and 166 have crucial roles in the enzyme activity, possibly by producing an NADP+ binding pocket.

Negative ion electrospray tandem mass spectrometric structural characterization of leukotriene B4 (LTB 4) and LTB 4-derived metabolites

The low energy collision induced dissociation (CID) of the carboxylate anions generated by electrospray ionization of leukotriene B4 (LTB4) and 16 of its metabolites was studied in a tandem quadrupole mass spectrometer. LTB4 is a biologically active lipid mediator whose activity is terminated by metabolism into a wide variety of structural variants. The collision-induced dissociation spectra of the carboxylate anions revealed structurally informative ions whose formation was determined by the position of hydroxyl substituents and double bonds present in the LTB4 metabolite. Major ions resulted from charge remote α-hydroxy fragmentation or charge directed α-hydroxy fragmentation. The conjugated triene moiety present in some metabolites was proposed to undergo cyclization to a 1,3-cyclohexadiene structure prior to charge remote or charge driven a-hydroxy fragmentation. The mechanisms responsible for all major ions observed in the CID spectra were studied using stable isotope labeled analogs of the LTB4 metabolites. In general, the collision-induced decomposition of carboxylate anions produced unique spectra for all LTB4 derived metabolites. The observed decomposition product ions from the carboxylate anion could be useful in developing assays for these molecules in biological fluids.

Metabolism of 6-trans-isomers of leukotriene B4 in cultured hepatoma cells and in human polymorphonuclear leukocytes. Identification of a delta 6-reductase metabolic pathway

The intermediate metabolic events which degrade hydroxy polyunsaturated fatty acids is largely unknown. Such molecules are common products of lipid peroxidation and lipoxygenase catalyzed oxidation of arachidonic acid. Metabolism of two 5,12-dihydroxyeicosatetraenoic acids, 6-trans-LTB4 (leukotriene B4), and 6-trans-12-epi-LTB4 was studied in HepG2 cells (a human-derived hepatoma cell line). Extensive metabolism was observed with a major metabolite identified as 4-hydroxy-6-dodecenoic acid for both epimers. Incubation of 6-trans-LTB4 epimers at shorter times revealed the formation of intermediate metabolites, including 6-hydroxy-4,8-tetradecadienoic acid and 8-hydroxy-4,6,10-hexadecatrienoic acid suggesting beta-oxidation as the major pathway leading to the formation of the common terminal metabolite. Two additional metabolites were structurally elucidated as 5-oxo-6,7-dihydro-LTB4 and 6,7-dihydro-LTB4 which have not been previously described. Formation of 5-oxo-6,7-dihydro-LTB4 and 6,7-dihydro-LTB4 were also observed during metabolism of 6-trans-12-epi-LTB4 in human polymorphonuclear leukocytes. Of particular interest is the metabolism of these compounds by beta-oxidation from the carboxyl terminus, a process which is not observed with leukotriene B4 or leukotriene C4. Identification of these metabolites suggested the operation of the 5-hydroxyeicosanoid dehydrogenase pathway followed by a delta 6-reductase metabolic pathway which has not been previously described. This pathway of beta-oxidation may limit the activity of various 5,12-diHETEs including nonenzymatic hydrolysis products of LTA4 and also the recently described B4-isoleukotrienes.

Potential role of the flavin-containing monooxygenases in the metabolism of endogenous compounds

Several xenobiotics and their corresponding cysteine S-conjugates are metabolized in vivo to cysteine S-conjugate sulfoxides and/or N-acetylcysteine S-conjugate sulfoxides. Homocysteine S-conjugates, such as methionine and ethionine, are also metabolized in vivo to sulfoxides. The enzymatic basis for these metabolic reactions is not known. Recently, the rat liver and kidney S-benzyl-L-cysteine S-oxidase activities were found to be associated with flavin-containing monooxygenases that are structurally and immunochemically related to known FMO1 isoforms. Further evidence for FMO1 being the major FMO isoform involved in S-benzyl-L-cysteine sulfoxidation was obtained from kinetic studies with cDNA-expressed rabbit FMOs. Endogenous cysteine S-conjugates, e.g. cysteinylcatecholamines, cysteinylleukotrienes, lanthionine and djenkolic acid may also be substrates for FMOs, since S-benzyl-L-cysteine can be considered a model for these compounds. Methionine, an endogenous homocysteine S-conjugate, was shown to be a substrate for cDNA-expressed rabbit FMO1, FMO2, and FMO3, however, the methionine sulfoxidation reaction was preferentially catalyzed by FMO3. These results suggest that FMOs may also play a role in the in vivo metabolism of endogenous homocysteine S-conjugates.

Cloning and expression of a novel form of leukotriene B4 omega-hydroxylase from human liver

We have isolated and sequenced a cDNA for human liver LTB4 omega-hydroxylase. The cDNA encoded a protein of 520 amino acids with a molecular weight of 59,853 Da. The cDNA-deduced amino acid sequence showed 87.3% homology to that of human polymorphonuclear leukocytes (PMN) LTB4 omega-hydroxylase (CYP4F3). Northern blot analysis revealed that the mRNA hybridized to the specific cDNA fragment is expressed in human liver, but not in human PMN. The microsomes from yeast cells transfected with the cDNA catalyzed the omega-hydroxylation of LTB4 with a Km of 44.8 microM. These results clearly show that a new form of the CYP4F LTB4 omega-hydroxylase exists in human liver.