Leukotrienes: biosynthesis, transport, inactivation, and analysis

Mesenchymal Stem Cell-Derived Extracellular Vesicles Decrease Lung Injury in Mice

Human mesenchymal stem cell (MSC) extracellular vesicles (EV) can reduce the severity of bacterial pneumonia, but little is known about the mechanisms underlying their antimicrobial activity. In the current study, we found that bacterial clearance induced by MSC EV in Escherichia coli pneumonia in C57BL/6 mice was associated with high levels of leukotriene (LT) B₄ in the injured alveolus. More importantly, the antimicrobial effect of MSC EV was abrogated by cotreatment with a LTB₄ BLT1 antagonist. To determine the role of MSC EV on LT metabolism, we measured the effect of MSC EV on a known ATP-binding cassette transporter, multidrug resistance-associated protein 1 (MRP1), and found that MSC EV suppressed MRP1 mRNA, protein, and pump function in LPS-stimulated Raw264.7 cells in vitro. The synthesis of LTB₄ and LTC₄ from LTA₄ are competitive, and MRP1 is the efflux pump for LTC₄ Inhibition of MRP1 will increase LTB₄ production. In addition, administration of a nonspecific MRP1 inhibitor (MK-571) reduced LTC₄ and subsequently increased LTB₄ levels in C57BL/6 mice with acute lung injury, increasing overall antimicrobial activity. We previously found that the biological effects of MSC EV were through the transfer of its content, such as mRNA, microRNA, and proteins, to target cells. In the current study, miR-145 knockdown abolished the effect of MSC EV on the inhibition of MRP1 in vitro and the antimicrobial effect in vivo. In summary, MSC EV suppressed MRP1 activity through transfer of miR-145, thereby resulting in enhanced LTB₄ production and antimicrobial activity through LTB₄/BLT1 signaling.

Arachidonic acid metabolites and endothelial dysfunction of portal hypertension

Increased resistance to portal flow and increased portal inflow due to mesenteric vasodilatation represent the main factors causing portal hypertension in cirrhosis. Endothelial cell dysfunction, defined as an imbalance between the synthesis, release, and effect of endothelial mediators of vascular tone, inflammation, thrombosis, and angiogenesis, plays a major role in the increase of resistance in portal circulation, in the decrease in the mesenteric one, in the development of collateral circulation. Reduced response to vasodilators in liver sinusoids and increased response in the mesenteric arterioles, and, viceversa, increased response to vasoconstrictors in the portal-sinusoidal circulation and decreased response in the mesenteric arterioles are also relevant to the pathophysiology of portal hypertension. Arachidonic acid (AA) metabolites through the three pathways, cyclooxygenase (COX), lipoxygenase, and cytochrome P450 monooxygenase and epoxygenase, are involved in endothelial dysfunction of portal hypertension. Increased thromboxane-A2 production by liver sinusoidal endothelial cells (LSECs) via increased COX-1 activity/expression, increased leukotriens, increased epoxyeicosatrienoic acids (EETs) (dilators of the peripheral arterial circulation, but vasoconstrictors of the portal-sinusoidal circulation), represent a major component in the increased portal resistance, in the decreased portal response to vasodilators and in the hyper-response to vasoconstrictors. Increased prostacyclin (PGI2) via COX-1 and COX-2 overexpression, and increased EETs/heme-oxygenase-1/K channels/gap junctions (endothelial derived hyperpolarizing factor system) play a major role in mesenteric vasodilatation, hyporeactivity to vasoconstrictors, and hyper-response to vasodilators. EETs, mediators of liver regeneration after hepatectomy and of angiogenesis, may play a role in the development of regenerative nodules and collateral circulation, through stimulation of vascular endothelial growth factor (VEGF) inside the liver and in the portal circulation. Pharmacological manipulation of AA metabolites may be beneficial for cirrhotic portal hypertension.

A review on leukotrienes and their receptors with reference to asthma

OBJECTIVE AND METHODS

Leukotrienes (LTs) including cysteinyl leukotrienes (CysLTs) and LTB4 are the most potent inflammatory lipid mediators and play a central role in the pathophysiology of asthma and other inflammatory diseases. These biological molecules mediate a plethora of contractile and inflammatory responses through specific interaction with distinct G protein-coupled receptors (GPCRs). The main objective of this review is to present an overview of the biological effects of CysLTs and their receptors, along with the current knowledge of mechanisms and role of LTs in the pathogenesis of asthma.

RESULTS

CysLTs including LTC4, LTD4 and LTE4 are ligands for CysLT1 and CysLT2 receptors, and LTB4 is the agonist for BLT1 and BLT2 receptors. The role of CysLT1 receptor is well established, and most of the pathophysiological effects of CysLTs in asthma are mediated by CysLT1 receptor. Several CysLT1 antagonists have been developed to date and are currently in clinical practice. Most common among them are classical CysLT1 receptor antagonists such as montelukast, zafirlukast, pranlukast, pobilukast, iralukast, cinalukast and MK571. The pharmacological role of CysLT2 receptor, however, is less defined and there is no specific antagonist available so far. The recent demonstration that mice lacking both known CysLT receptors exhibit full/augmented response to CysLT points to the existence of additional subtypes of CysLT receptors. LTB4, on the other hand, is another potent inflammatory leukotriene, which acts as a strong chemoattractant for neutrophils, but weaker for eosinophils. LTB4 is known to play an important role in the development of airway hyper-responsiveness in severe asthma. However there is no LTB4 antagonist available in clinic to date.

CONCLUSION

This review gives a recent update on the LTs including their biosynthesis, biological effects and the role of anti-LTs in the treatment of asthma. It also discusses about the possible existence of additional subtypes of CysLT receptors.

Leukotrienes as modifiers of preclinical atherosclerosis?

Preclinical atherosclerosis represents a crucial period associated with several pathophysiological reactions in the vascular wall. Failure to diagnose preclinical atherosclerosis at this stage misses a major opportunity to prevent the long-term consequences of this disease. Surrogate biological and structural vascular markers are available to determine the presence and the extension of preclinical vascular injury in the general population. Examples of surrogate markers are carotid intima media thickness and biomarkers including high-sensitivity C-reactive protein, cell adhesion molecules and matrix metalloproteinases, and leukotrienes. Recently, leukotrienes have been implicated as mediators, biomarkers, and possible therapeutic targets in the context of subclinical atherosclerosis. The aim of this short paper is to focus on the relation between preclinical atherosclerosis and leukotrienes, with particular attention to the recent development on the use of leukotriene modifiers in the treatment of atherosclerosis.

ATP-dependent transport of leukotrienes B4 and C4 by the multidrug resistance protein ABCC4 (MRP4)

The proinflammatory mediators leukotriene (LT) B(4) and LTC(4) must be transported out of cells before they can interact with LT receptors. Previously, we identified the multidrug resistance protein ABCC1 (MRP1) as an efflux pump for LTC(4). However, the molecular basis for the efflux of LTB(4) was unknown. Here, we demonstrate that human ABCC4 mediates the ATP-dependent efflux of LTB(4) in the presence of reduced glutathione (GSH), whereby the latter can be replaced by S-methyl GSH. Transport studies were performed with inside-out membrane vesicles from V79 fibroblasts and Sf9 insect cells that contained recombinant ABCC4, with vesicles from human platelets and myelomonocytic U937 cells, which were rich in endogenous ABCC4, but ABCC1 was below detectability. Moreover, human polymorphonuclear leukocytes contained ABCC4. K(m) values for LTB(4) were 5.2 muM with vesicles from fibroblasts and 5.6 muM with vesicles from platelets. ABCC4, with its broad substrate specificity, also functioned as an ATP-dependent efflux pump for LTC(4) with a K(m) of 0.13 muM in vesicles from fibroblasts and 0.32 muM in vesicles from platelets. However, GSH was not required for the transport of this glutathionylated leukotriene. The transport of LTC(4) by ABCC4 explains its release from platelets during transcellular synthesis. ATP-dependent transport of LTB(4) and LTC(4) by ABCC4 was inhibited by several organic anions, including S-decyl GSH, sulindac sulfide, and by the LTD(4) receptor antagonists montelukast and 3-(((3-(2-(7-chloro-2-quinolinyl)ethenyl)phenyl)-((3-dimethyl-amino-3-oxopropyl)-thio)-methyl)thio)propanoic acid (MK571). Thus, as an efflux pump for the proinflammatory mediators LTB(4) and LTC(4), ABCC4 may represent a novel target for anti-inflammatory therapies.

Cysteinyl-leukotrienes and their receptors in asthma and other inflammatory diseases: critical update and emerging trends

Cysteinyl-leukotrienes (cysteinyl-LTs), that is, LTC4, LTD4, and LTE4, trigger contractile and inflammatory responses through the specific interaction with G protein-coupled receptors (GPCRs) belonging to the purine receptor cluster of the rhodopsin family, and identified as CysLT receptors (CysLTRs). Cysteinyl-LTs have a clear role in pathophysiological conditions such as asthma and allergic rhinitis (AR), and have been implicated in other inflammatory conditions including cardiovascular diseases, cancer, atopic dermatitis, and urticaria. Molecular cloning of human CysLT1R and CysLT2R subtypes has confirmed most of the previous pharmacological characterization and identified distinct expression patterns only partially overlapping. Interestingly, recent data provide evidence for the immunomodulation of CysLTR expression, the existence of additional receptor subtypes, and of an intracellular pool of CysLTRs that may have roles different from those of plasma membrane receptors. Furthermore, genetic variants have been identified for the CysLTRs that may interact to confer risk for atopy. Finally, a crosstalk between the cysteinyl-LT and the purine systems is being delineated. This review will summarize and attempt to integrate recent data derived from studies on the molecular pharmacology and pharmacogenetics of CysLTRs, and will consider the therapeutic opportunities arising from the new roles suggested for cysteinyl-LTs and their receptors.

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.

Inhibition of 5-lipoxygenase and leukotriene C4 synthase in human blood cells by thymoquinone

Black cumin seed, Nigella sativa L., and its oils have traditionally been used for the treatment of asthma and other inflammatory diseases. Thymoquinone (TQ) has been proposed to be one of the major active components of the drug. Since leukotrienes (LTs) are important mediators in asthma and inflammatory processes, the effects of TQ on leukotriene formation were studied in human blood cells. TQ provoked a significant concentration-dependent inhibition of both LTC4 and LTB4 formation from endogenous substrate in human granulocyte suspensions with IC50 values of 1.8 and 2.3 microM, respectively, at 15 min. Major inhibitory effect was on the 5-lipoxygenase activity (IC50 3 microM) as evidenced by suppressed conversion of exogenous arachidonic acid into 5-hydroxy eicosatetraenoic acid (5HETE) in sonicated polymorphonuclear cell suspensions. In addition, TQ induced a significant inhibition of LTC4 synthase activity, with an IC50 of 10 microM, as judged by suppressed transformation of exogenous LTA4 into LTC4. In contrast, the drug was without any inhibitory effect on LTA4 hydrolase activity. When exogenous LTA4 was added to intact or sonicated platelet suspensions preincubated with TQ, a similar inhibition of LTC4 synthase activity was observed as in human granulocyte suspensions. The unselective protein kinase inhibitor, staurosporine failed to prevent inhibition of LTC4 synthase activity induced by TQ. The findings demonstrate that TQ potently inhibits the formation of leukotrienes in human blood cells. The inhibitory effect was dose- and time-dependent and was exerted on both 5-lipoxygenase and LTC4 synthase activity.

Cysteinyl leukotrienes are secretagogues in atrophic coeliac and in normal duodenal mucosa of children

OBJECTIVE

The pathophysiology of intestinal inflammation and diarrhoea is complex and involves the arachidonic acid cascade. Prostaglandins induce chloride secretion in healthy subjects and in patients with coeliac disease. Leukotrienes (LTs) are also known inflammatory mediators which have been shown to stimulate ion secretion in ileum and colon of rats and rabbits. The aim of this study was to determine the effects of leukotrienes C(4) (LTC(4)) and D(4) (LTD(4)) in normal and atrophic intestinal mucosa in children.

MATERIAL AND METHODS

Routine paediatric intestinal biopsies were obtained from 109 children. Sixty-seven control biopsies and 42 biopsies from children with different stages of coeliac disease were mounted in a modified Ussing chamber. Potential difference (Pd) was measured continuously and tissue resistance (R(t)) and the generated current (I(m)) were calculated.

RESULTS

In morphologically normal mucosa of duodenum, LTC(4) and LTD(4) increased Pd and I(m) in a dose-dependent manner. The increase was more pronounced in the distal than in the proximal part, similar to the response to prostaglandin E(2). The induced current was chloride-mediated, since replacement of Cl(-) with SO(4)(2-) in the bathing solution eliminated the response to the LTs. The LTC(4)-induced secretion was significantly decreased in atrophic mucosa, but the response was partially restored after preincubation with the cyclooxygenase inhibitor indomethacin.

CONCLUSIONS

The results showed that LTC(4) and LTD(4) are secretagogues in the duodenal mucosa from healthy children by inducing a net chloride secretion. Restoration of the response in coeliac disease by cyclooxygenase inhibition suggests interactions between the different pathways of the arachidonic cascade in the intestinal mucosa.

Role of leukotrienes on hepatic ischemia/reperfusion injury in rats

BACKGROUND

Leukotrienes (LT), composed of cysteinyl LT (cLT; LTC(4), LTD(4), and LTE(4)) and LTB(4), are potent lipid mediators enhancing the vascular permeability and recruitment of neutrophils, which are common features of hepatic ischemia/reperfusion (I/R) injury. The aim of this study was to investigate whether LT can mediate the liver and lung injuries following hepatic I/R.

MATERIALS AND METHODS

Sprague-Dawley rats were subjected to 90 min of partial hepatic ischemia followed by 3, 12, and 24 h of reperfusion. In the hepatic and pulmonary tissues, LT content and the mRNA expression of LT-synthesis enzymes, 5-lypoxygenase (5-LO), LTC(4) synthase (LTC(4)-S), and LTA(4) hydrolase (LTA(4)-H) were measured. Tissue injuries were assessed by plasma ALT, histological examination, and wet-to-dry tissue weight ratios.

RESULTS

The cLT content in the hepatic tissue after 12 and 24 h reperfusion was increased 4- to 5-fold compared to controls and this was accompanied by the enhancement of hepatic edema and plasma ALT elevation. There were no significant changes in the mRNA expression of LT-synthesis enzymes in both tissues. LTB(4) levels were not increased despite a significant neutrophil infiltration in both tissues.

CONCLUSIONS

These data suggest that cLT are generated in the liver during the reperfusion period and may contribute to the development of hepatic edema and exert cytotoxicity. Factors other than LTB(4) may contribute to neutrophil infiltration.

Hepatobiliary excretion of fluconazole and its interaction with cyclosporin A in rat blood and bile using microdialysis

In order to investigate the hepatobiliary excretion of Fluconazole, we develop a rapid and sensitive method using high-performance liquid chromatography coupled with microdialysis for the simultaneous determination of unbound fluconazole in rat blood and bile. Microdialysis probes were inserted into both the jugular vein toward the right atrium and bile duct of male Sprague-Dawley rats for biological fluid sampling after administration of fluconazole at 10 mg/kg through the femoral vein. Fluconazole and dialysates were separated using a Zorbax phenyl column maintained at ambient temperature. The detection limit of fluconazole was 50 ng/ml. Biological fluid sampling thereby allowed the simultaneous determination of fluconazole levels in blood and bile. The disposition of fluconazole in the blood and bile fluid suggests that there was rapid exchange and equilibration between the blood and hepatobiliary system. In addition, to investigate the mechanism of P-glycoprotein related hepatobiliary excretion of fluconazole, we examined the drug-drug interaction of fluconazole and cyclosporin A in the aspect of pharmacokinetics. These results indicate that the plasma level of fluconazole was no different than that in bile, and that fluconazole undergoes hepatobiliary excretion, maybe unrelated to the P-glycoprotein transported system.

Pharmacological differences among CysLT(1) receptor antagonists with respect to LTC(4) and LTD(4) in human lung parenchyma

We have previously reported, by means of equilibrium binding studies, the existence of two distinct binding sites with receptor characteristics for LTC(4) and LTD(4) in human lung parenchyma (HLP) membranes using S-decyl-glutathione (S-decyl-GSH) to inhibit LTC(4) binding to a number of non-receptor sites. Recently, we have been able to avoid the use of S-decyl-GSH in kinetic experiments and to characterize a distinctive pharmacological profile for the LTC(4) high affinity binding sites which do not correlates with the ability of both LTD(4) and LTC(4) to contract isolated HLP strips through the CysLT(1) receptor. Here, we report that the most advanced CysLT(1) receptor antagonists, some of which are already in clinical use, displayed a different behavior toward LTC(4) and LTD(4) in HLP. Equilibrium and kinetic binding studies demonstrated the following rank order of potency for (3)H-LTD(4) receptor (CysLT(1)): zafirlukast = montelukast > LM-1507 = LM-1484 = pranlukast. In addition, LM-1507, LM-1484, pranlukast and montelukast but not zafirlukast are able to interact also with the high affinity site for (3)H-LTC(4) (LM-1507 = LM-1484 > pranlukast; montelukast not detectable in the presence of S-decyl-GSH). In this respect, the behavior of the LM antagonists closely resembles that of pranlukast although LM-1507 and LM-1484 display a higher affinity for (3)H-LTC(4) sites. Montelukast has an intermediate behavior, inasmuch as its interaction with (3)H-LTC(4) sites can be revealed only in kinetic studies, while zafirlukast is totally unable to inhibit (3)H-LTC(4) binding. It might be, therefore, most relevant for a complete understanding of the clinical efficacy, besides their nominal potency, of the most advanced CysLT(1) receptor antagonists to consider their pharmacological differences with respect not only to LTD(4)/LTE(4), but also to LTC(4).

Transport of leukotriene C4 and structurally related conjugates

Transport proteins control the release of the endogenous glutathione conjugate leukotriene C4 (LTC4) from leukotriene-synthesizing cells as well as its hepatobiliary and renal elimination. The photolabile conjugated triene structure of LTC4 has enabled direct photoaffinity labeling of the multidrug resistance protein 1 (MRP1, symbol ABC C1) in membranes from mastocytoma cells, leading to the identification of the function of this protein as an ATP-dependent export pump for LTC4 and structurally related conjugates. MRP1 is assigned to the C branch of the superfamily of ATP-binding cassette (ABC) transporters and was originally identified by virtue of its association with drug resistance in tumor cells. Besides LTC4, which is a high-affinity substrate, a variety of conjugates of hydrophobic endogenous or xenobiotic substances with glutathione, glucuronate, or sulfate are transported by MRP1. In addition, hydrophobic compounds may undergo cotransport with glutathione. Effective inhibitors of MRP1-mediated transport include structural analogs of LTC4 and of other cysteinyl leukotrienes. The ATP-dependent transport system which transports cysteinyl leukotrienes across the hepatocyte canalicular membrane into bile was cloned and characterized as the second isoform or paralog of the mammalian MRP family, MRP2 (ABC C2). MRP2 is localized to the apical membrane of polarized cells. The overall substrate specificities of MRP1 and MRP2 are similar, despite an amino acid identity of only 48%. The transport proteins mediating the uptake of LTC4 into hepatocytes across the basolateral membrane are members of the organic anion transporter (OATP) branch of the solute carrier (SLC) superfamily and are thus distinct from the ATP-dependent export pumps of the MRP family.

5-lipoxygenase inhibition reduces intrahepatic vascular resistance of cirrhotic rat livers: a possible role of cysteinyl-leukotrienes

BACKGROUND & AIMS

Cysteinyl-leukotrienes (Cys-LTs) increase intrahepatic vascular resistance in normal rat livers. CCl4 cirrhotic rat livers have increased Cys-LT production and 5-lipoxygenase messenger RNA (mRNA) expression. The aim of this study was to investigate the role of 5-lipoxygenase-derived eicosanoids regulating intrahepatic vascular tone in control and CCl4-induced cirrhotic rat livers.

METHODS

In different groups of portally perfused control and cirrhotic rat livers, the following were analyzed: a portal perfusion pressure (PP) dose-response curve to LTD4; the effects on PP caused by either vehicle, the selective 5-lipoxygenase inhibitor AA-861, the selective Cys-LT1 receptor antagonist MK-571, or the dual Cys-LT1 and Cys-LT2 receptor antagonist BAY u9773; and immunohistochemistry for 5-lipoxygenase in liver sections of cirrhotic and control livers.

RESULTS

Cirrhotic livers have a hyperesponse to LTD4. In control livers, AA-861 and MK-571 produced a moderate and similar reduction in PP. In cirrhotic livers, 5-lipoxygenase inhibition produced a marked and significantly greater reduction in PP than in controls. However, no effect on PP was observed after MK-571 or BAY u9773. 5-Lipoxygenase-positive cells were markedly increased in cirrhotic livers.

CONCLUSIONS

Our results suggest that 5-lipoxygenase-derived eicosanoids may contribute to the increased intrahepatic vascular resistance of cirrhotic rat livers and therefore the pathogenesis of portal hypertension.

Inhibitory effects of helenalin and related compounds on 5-lipoxygenase and leukotriene C(4) synthase in human blood cells

The sesquiterpene lactone helenalin, which can be isolated from several plant species of the Asteraceae family, is a potent anti-inflammatory and antineoplastic agent. In agreement, alcohol extracts of these plants are used for local external treatment of inflammatory conditions. Since leukotrienes are important mediators in inflammatory processes, the inhibitory effects of helenalin and some derivatives on leukotriene (LT) biosynthesis were studied. Treatment of human platelets with helenalin provoked irreversible inhibition of LTC(4) synthase in a concentration- and time-dependent manner with an IC(50) of 12 microM after a 60 min preincubation. 11alpha,13-Dihydrohelenalin acetate was less potent. Interestingly, individual donors could be divided into two distinct groups with respect to the efficacy of helenalin to suppress platelet LTC(4) synthase. In human granulocytes, helenalin inhibited both the 5-lipoxygenase (IC(50) 9 microM after 60 min preincubation) and LTC(4) synthase in a concentration- and time-dependent fashion. In contrast, the drug was without effect on LTA(4) hydrolase. The GSH-containing adducts (2beta-(S-glutathionyl)-2,3-dihydrohelenalin and 2beta-(S-glutathionyl)-2,3,11alpha,13-tetra hydrohelenalin acetate) did not significantly inhibit LTC(4) synthase. The present results indicate a mechanism for the anti-inflammatory effect of helenalin and related compounds.

Multidrug resistance (MDR) in cancer. Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs

In recent years, there has been an increased understanding of P-glycoprotein (P-GP)-mediated pharmacokinetic interactions. In addition, its role in modifying the bioavailability of orally administered drugs via induction or inhibition has been also been demonstrated in various studies. This overview presents a background on some of the commonly documented mechanisms of multidrug resistance (MDR), reversal using modulators of MDR, followed by a discussion on the functional aspects of P-GP in the context of the pharmacokinetic interactions when multiple agents are coadministered. While adverse pharmacokinetic interactions have been documented with first and second generation MDR modulators, certain newer agents of the third generation class of compounds have been less susceptible in eliciting pharmacokinetic interactions. Although the review focuses on P-GP and the pharmacology of MDR reversal using MDR modulators, relevance of these drug transport proteins in the context of pharmacokinetic implications (drug absorption, distribution, clearance, and interactions) will also be discussed.

Covalent binding of LTA(4) to nucleosides and nucleotides

Leukotriene A(4) (LTA(4)) is a chemically reactive conjugated triene epoxide that is formed by 5-lipoxygenase and is an intermediate in the formation of the biologically active eicosanoids leukotriene B(4) and leukotriene C(4). The present study was undertaken to determine whether or not LTA(4) could serve as an electrophilic species that nucleosides and nucleotides could attack, ultimately resulting in a covalent adduct. Electrospray ionization mass spectrometry and tandem mass spectrometry were used to study the covalent binding of LTA(4) with uridine, cytidine, adenosine, and guanosine. The reaction with guanosine was found to yield five major and at least six minor adduct species. Reversed phase HPLC and mass spectrometric data suggested that the guanosine attacked LTA(4) either at carbon-12 or carbon-6 with opening the epoxide at carbon-5 to yield a series of adducts characterized by the molecular anion [M-H](-) at m/z 600.3. Reactions of LTA(4) with mixtures of nucleosides and nucleotides revealed that guanine-containing nucleosides were the most reactive toward LTA(4). The facility of the reaction of guanine with LTA(4) raises the possibility that this intermediate of leukotriene biosynthesis formed on or near the cellular nuclear envelope may react with nucleosides and nucleotides present in RNA or DNA.

The multidrug resistance protein 1: a functionally important activation marker for murine Th1 cells

Previously, we described the expression of an energy-dependent pump in resting murine Th2 (but not resting Th1) cells which extruded the fluorescent dye Fluo-3. After stimulation with Ag and APCs, Th1 cells also expressed this pump. Furthermore, expression of the murine multidrug resistance protein 1 (mrp1) correlated with the presence of the pump. In this study, we report that Fluo-3 is indeed transported by murine mrp1 or its human ortholog MRP1, as revealed by transfection of HEK 293 cells with mrp1 or MRP1 cDNA. Like antigenic activation, IL-2 dose-dependently enhanced the Fluo-3-extruding activity in murine Th1 cells. Although TNF-alpha and IL-12 by themselves only weakly enhanced Fluo-3 extrusion, each of them did so in strong synergism with IL-2. An Ab directed against mrp1 was used to quantify the expression of mrp1 protein in T cells at the single-cell level. Like the Fluo-3 pump, mrp1 protein expression was enhanced by IL-2. Immunohistochemical studies using confocal laser microscopy indicated that mrp1 is localized mainly at the plasma membrane. In addition, protein expression of mrp1 was induced in Vbeta8+CD4+ T cells 12 h after in vivo application of Staphylococcal enterotoxin B. Finally, mrp1 was functionally relevant during the activation process of Th1 cells, because T cell activation could be suppressed by exposure of cells to the mrp1 inhibitor MK571. Thus, we present mrp1 as a novel, functionally important activation marker for Th1 cells and short-term in vivo activated CD4+ T cells, whereas its expression seems to be constitutive in Th2 cells.

Interleukin-6 production by activated human monocytic cells is enhanced by MK-571, a specific inhibitor of the multi-drug resistance protein-1

1. The intracellular transport of leukotriene C4 (LTC4) in hematopoietic cells such as human monocytes is controlled by an ATP dependent carrier encoded by the multidrug resistance protein1 (MRPI) gene whose function can be blocked by the compound MK-571. Since LTs play a major role in control of cytokine expression in monocytes, we questioned whether blocking of the MRPI mediated function by MK-571 might affect cytokine production. 2. MK-571 strongly enhanced IL-6 expression at mRNA and protein level in lipopolysaccharide (LPS) and interleukin-1 (IL-1) stimulated human monocytes giving rise to 2.0+/-0.4 (x+/-s.d.) and 5.7+/-3.5 fold induction of IL-6 protein secretion. The increase in IL-6 secretion was accompanied by an enhanced phosphorylation of p38 but not of c-Jun-N terminal kinase. 3. The involvement of the kinase signalling pathways was further analysed by using SB203580 and PD98059, specific inhibitors of the p38 and ERK1/2 signalling route. MK-571 mediated upregulation of IL-6 in the presence of IL-1 was partially attenuated by SB203580 and PD98059. Electrophoretic mobility shift assays demonstrated that MK-571 did not affect the IL-1 induced DNA binding activity of Activator Protein-1 and Nuclear Factor-kappaB but rather enhanced the transactivational activity of an IL-6 promoter construct. Finally it was shown that the MK-571 mediated effects on IL-6 secretion could not be inhibited by the LT synthesis inhibitor SB203347 or by the anti-oxidant pyrrolidine dithiocarbamate (PDTC). 4. These results indicate that the membrane transporter MRP1 is involved in the regulation of IL-6 expression in activated human peripheral blood monocytes.

Expression and regulation of leukotriene-synthesis enzymes in rat liver cells

The liver plays a major role in metabolism and elimination of leukotrienes (LT). It produces cysteinyl leukotrienes (cLT), and cLT have been implicated in hepatocellular toxicity in several models of lipopolysaccharide (LPS)-associated liver injury. However, the liver cell types responsible for cLT production are poorly defined, and the expression of the LT-synthesis enzymes, 5-lipoxygenase (5-LO) and LTC4 synthase (LTC4-S), in liver cells has never been demonstrated. The aim of the present study was to examine the ability of rat liver cells to produce cLT by determining whether hepatocytes, Kupffer cells, and sinusoidal endothelial cells express mRNA and enzyme activities of the LT-synthesis enzymes and whether expression is altered by LPS. 5-LO mRNA was expressed in whole liver, and expression was enhanced by LPS. Cell fractionation studies demonstrated that expression was present in Kupffer cells and sinusoidal endothelial cells, but not in hepatocytes. LTC4-S mRNA was detected in whole liver, hepatocytes, and sinusoidal endothelial cells, but not in Kupffer cells. Semiquantitative reverse-transcriptase polymerase chain reaction (RT-PCR) showed that LPS increased LTC4-S expression in hepatocytes by a factor of 3 (n = 3; P < .03). LTC4-S enzyme activity in the microsomal fraction of hepatocytes was also increased from 0.52 +/- 0.13 to 1.90 +/- 0.66 nmol . mg protein-1 . 5 min-1 (n = 6; P < .015) after LPS treatment. These results indicate that hepatocytes do not possess the ability for de novo synthesis of cLT from arachidonic acid, but they may actively participate in cLT production by conjugation of LTA4 with glutathione to produce LTC4. LPS enhances LTC4-S expression in hepatocytes. This intrinsic cLT production may contribute to hepatocellular injury during inflammation.

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.

Human mast cells secreting leukotriene C4 express the MRP1 gene-encoded conjugate export pump

Mast cells are known to secrete endogenously synthesized leukotriene C4 (LTC4), but the identity of the responsible export pump in human mast cells was unknown. The multidrug resistance proteins MRP1 and MRP2 have been identified as primary-active ATP-dependent export pumps for various amphiphilic anions including the glutathione conjugate LTC4. We therefore studied the expression at the RNAand protein levels of both MRP1 and MRP2 as well as the ATP-dependent LTC4 transport in the human mast cell line HMC-1. Upon stimulation by 1 microM ionomycin, intact HMC-1 cells generated 26 pmol LTC4/10(8) cells within 20 min. Transport experiments using inside-out HMC-1 membrane vesicles demonstrated an ATP-dependent LTC4 transport amounting to 1.4 pmol x (mg protein)(-1) x min(-1). Reverse transcription PCR indicated that HMC-1 cells express mRNA of MRP1, but not of MRP2 or MRP3. Cloning and sequencing of the amplified PCR fragment confirmed its identity with the human MRP1 sequence. Immunoblots using antibodies against MRP1 and MRP2 demonstrated that HMC-1 cells contain the MRP1 but not the MRP2 protein. Our results indicate that the 190 kDa integral membrane glycoprotein MRP1 mediates the ATP-dependent export of LTC4 from human mast cells to the extracellular space.

Drug export activity of the human canalicular multispecific organic anion transporter in polarized kidney MDCK cells expressing cMOAT (MRP2) cDNA

The canalicular (apical) membrane of the hepatocyte contains an ATP-dependent transport system for organic anions, known as the multispecific organic anion transporter (cMOAT). The deduced amino acid sequence of cMOAT is 49% identical to that of the human multidrug resistance- associated protein (MRP) MRP1, and cMOAT and MRP1 are members of the same sub-family of adenine nucleotide binding cassette transporters. In contrast to MRP1, cMOAT was predominantly found intracellularly in nonpolarized cells, suggesting that cMOAT requires a polarized cell for plasma membrane routing. Therefore, we expressed cMOAT cDNA in polarized kidney epithelial MDCK cell lines. When these cells are grown in a monolayer, cMOAT localizes to the apical plasma membrane. We demonstrate that cMOAT causes transport of the organic anions S-(2,4-dinitrophenyl)-glutathione, the glutathione conjugate of ethacrynic acid, and S-(PGA1)-glutathione, a substrate not shown to be transported by organic anion transporters previously. Transport is inhibited only inefficiently by compounds known to block MRP1. We also show that cMOAT causes transport of the anticancer drug vinblastine to the apical side of a cell monolayer. We conclude that cMOAT is a 5'-adenosine triphosphate binding cassette transporter that potentially might be involved in drug resistance in mammalian cells.

Identification and characterization of two cysteinyl-leukotriene high affinity binding sites with receptor characteristics in human lung parenchyma

We report the characterization of two distinct binding sites with receptor characteristics for leukotriene (LT)D4 and LTC4 in membranes from human lung parenchyma. The use of S-decyl-glutathione allowed us to characterize a previously unidentified high affinity binding site for LTC4. Computerized analysis of binding data revealed that each leukotriene interacts with two distinct classes of binding sites (Kd = 0.015 and 105 nM for LTC4 and 0.023 and 230 nM for LTD4) and that despite cross-reactivity, the two high affinity sites are different entities. LTD4 binding sites displayed features of G protein-coupled receptors, whereas LTC4 binding sites did not show any significant modulation by guanosine-5'-(beta, gamma-imido)triphosphate or stimulation of GTPase activity. The antagonists ICI 198,615 and SKF 104353 were unselective for the high and low affinity states of LTD4 receptor, whereas only SKF 104353 was able to recognize the two [3H]LTC4 binding sites although with different affinities. These data indicate that in human lung parenchyma, LTD4 and LTC4 recognize two different binding sites; these binding sites are different entities; and for LTD4, the two binding sites represent the interconvertible affinity states of a G protein-coupled receptor, whereas for LTC4, the high affinity site is likely to be a specific LTC4 receptor.

Ionophore A23187-induced leukotriene biosynthesis in equine granulocytes-neutrophils, but not eosinophils require exogenous arachidonic acid

Equine granulocyte suspensions, mainly consisting of neutrophils, failed to produce detectable amounts of leukotrienes when stimulated with calcium ionophore A23187 alone, whereas leukotrienes were dose-dependently formed in control incubations with human granulocytes. In contrast, ionophore A23187 initiated synthesis of leukotrienes B4 and C4 in equine granulocytes when added in combination with low concentrations of exogenous arachidonic acid. Similarly, ionophore A23187 provoked leukotriene biosynthesis when added alone to human whole blood, whereas addition of exogenous arachidonic acid was a prerequisite for ionophore A23187-induced leukotriene formation in equine whole blood. Leukotriene biosynthesis was provoked by A23187 alone after addition of homologous platelets to equine granulocyte suspensions. After separation of equine neutrophils and eosinophils, purified eosinophil suspensions produced LTC4 after stimulation with ionophore A23187 alone, whereas exogenous arachidonic acid was required for ionophore-induced LTB4 formation in purified neutrophil suspensions. Leukotriene synthesis in both eosinophils and neutrophils was suppressed by the 5-lipoxygenase activating protein (FLAP) inhibitor, MK-886. Exogenous arachidonic acid was needed for ionophore-induced leukotriene synthesis also in bovine granulocytes, but was not a prerequisite for the production of leukotrienes in porcine granulocytes or in rat and rabbit white blood cell suspensions. The results indicate differences in the mechanisms regulating leukotriene synthesis in equine neutrophils, as compared to human granulocytes or equine eosinophils, and suggest that elevation of intracellular calcium is an insufficient stimulus to provoke utilisation of endogenous arachidonic acid for leukotriene synthesis in equine neutrophils.

The effect of melittin on proliferation and death of thymocytes

The effect of melittin, an activator of phospholipase A2, on proliferation and death of rat thymocytes in a broad concentration range was studied. Cell proliferation was estimated by the accumulation of colchicin metaphases, necrotic death was determined from lysis and staining of cells with trypan blue, and apoptosis was assessed from the type of DNA fragmentation, the amount of fragmented DNA, and the percentage of cells with subdiploid DNA. It was shown that low melittin concentrations (below 5 microg/ml) stimulate thymocyte proliferation. At high melittin concentrations, thymocytes die by the primary necrosis type. Throughout the concentration range studied, melittin does not produce apoptosis in thymocytes. Conversely, high melittin concentrations even inhibit thymocyte apoptosis in the control and after irradiation. An inhibitor of RNA synthesis actinomycin D does not affect thymocyte death in the presence of melittin. It is concluded that the activation of phospholipase A2 can induce necrosis but not apoptosis and thus is not a necessary step in the signaling cascade that initiates apoptosis in thymocytes.

Leukotriene D4 activates a chloride conductance in hepatocytes from lipopolysaccharide-treated rats

Endotoxin (LPS) can cause hepatocellular injury under several circumstances, and leukotrienes have been implicated as a contributing factor. Since ion channel activation has been associated with cytotoxicity, the aim of this study was to determine the circumstances under which LPS and/or leukotrienes activate ionic conductances in hepatocytes. LPS treatment of rats increased Cl- conductance in hepatocytes from 232+/-42 to 1236+/-134 pS/pF. Voltage dependence and inhibitor specificity of this conductance were similar to that of a swelling-activated Cl- conductance, and internal dialysis with nucleoside analogues suggested control by an inhibitory G protein. The lipoxygenase inhibitor nordihydroguaiaretic acid, the specific leukotriene D4 (LTD4) receptor antagonist MK-571, and the 5-lipoxygenase activating protein inhibitor MK-886 all significantly inhibited the conductance. Intracellular dialysis with LTD4 (1.5 microM) elevated intracellular Ca2+ from 143+/-6.5 to 388+/-114 nM within 6 min and stimulated an outwardly rectifying conductance from 642+/-159 to 1669+/-224 pS/pF (n = 9, P < 0.001). In hepatocytes prepared from untreated rats, this concentration of intracellular LTD4 neither raised intracellular Ca2+ nor activated the conductance. The LTD4 response could be induced in normal hepatocytes by culture with either conditioned medium from LPS-treated macrophages or purified TNF-alpha. In conclusion, intracellular LTD4 activates a chloride conductance in hepatocytes isolated from rats treated with LPS or primed in vitro with TNF-alpha. Changes in the hepatocellular accumulation of leukotrienes therefore mediate channel activation and may contribute to liver injury during sepsis and other inflammatory conditions.

Increased biliary secretion of cysteinyl-leukotrienes in human bile duct obstruction

BACKGROUND/AIMS

The pathophysiological role of leukotrienes in liver disease is not well understood. Redistribution or enhanced formation in cholestatic states may result in increased hepatic concentrations that are expected to contribute to liver injury. To disclose the potential role of cysteinyl-leukotrienes in chronic liver diseases, we studied biliary and urinary secretion in the model situation of relief of bile duct obstruction.

METHODS

Concentrations of cysteinyl-leukotrienes were determined in bile and urine of patients with extrahepatic biliary obstruction in the course of therapeutic decompression by endoscopic or transhepatic techniques. Leukotrienes were measured by radioimmunoassay after HPLC separation. Concentrations of bile acids in bile and serum were measured for comparison.

RESULTS

Bile collected 2 h after decompression contained high concentrations of leukotrienes (57.5 +/- 22 microM). Biliary secretion decreased over 24 h reaching equilibrium values after 48-72 h (2.8 +/- 1.7 mM and 6.4 +/- 6.6 microM, respectively). Total bile acid concentration in serum followed a similar time course. In contrast, biliary bile acid concentration showed high interindividual variations. Bile contained all leukotriene C4, D4, E4 and NAc-LTE4, but LTC4 was predominant. Urinary leukotriene secretion in cholestasis (199.7 pmol/mmol creatinine) was less than 7% of maximal biliary secretion. It further decreased to 116.4 pmol/mmol creatinine within 72 h. Urine also contained all species of cysteinyl-leukotrienes, but the relative amounts of LTE4 and NAc-LTE4 were higher than in bile.

CONCLUSIONS

Formation of cysteinyl-leukotrienes is increased in obstructive jaundice resulting in increased urinary excretion before and both biliary and urinary excretion after relief of the obstruction. Predominance of LTC4 suggests that the secreted leukotrienes are newly formed. Increased synthesis and retention of hepatic cysteinyl-leukotrienes may contribute to hepatic and extrahepatic consequences of cholestasis.

Bile ductular proliferation and altered leukotriene elimination in thioacetamide-induced fibrosis of rat liver

BACKGROUND/AIMS

Liver fibrosis is accompanied by both bile ductular proliferation and inflammation under various conditions. The functional consequences and the interrelationships between these changes are unknown. Altered biliary elimination and retention of cholephilic mediators may be a factor in fibrogenesis. Therefore, the relationship between fibrosis, ductular proliferation and functional changes in biliary elimination was studied.

METHODS

Micronodular liver fibrosis was induced by thioacetamide in rats. The relative amount of bile ductular epithelial cells was determined by microscopic morphometry. The functional changes in bile secretion and metabolism of leukotriene C4 were assessed in isolated perfused livers of treated rats.

RESULTS

Pretreatment with thioacetamide in vivo resulted in enhanced bile fluid formation in subsequently isolated and perfused livers. Infusion of isoproterenol into the portal vein stimulated bile flow. Both unstimulated and isoproterenol-stimulated bile flows were increased in fibrotic livers and were correlated with liver content of bile ductular epithelia. In contrast, biliary secretion of infused leukotriene C4 was lowered in correlation with that of taurocholate. Enhanced metabolism resulted in a shift of the major fraction in bile from leukotriene C4 to leukotriene D4.

CONCLUSIONS

Thioacetamide-induced liver fibrosis is associated with an increased number of functionally intact bile ductules that are responsive to isoproterenol stimulating bile fluid formation. In contrast, biliary secretion of cysteinyl-leukotrienes and taurocholate is inhibited and the relative amount of leukotriene D4 is increased. Bile ductular proliferation as well as retention and altered metabolism of leukotrienes are factors associated with the development of liver fibrosis.

PGE2 and LTB4 inhibit cytokine-stimulated nitric oxide synthase type 2 expression in isolated rat hepatocytes

Prostaglandins have been shown to have a wide range of effects on nitric oxide synthesis when studied in different cell populations. The proximity of hepatocytes to eicosanoid-producing endothelial cells and Kupffer cells prompted us to determine the effects of PGE2 and LTB4 on hepatocyte NO production by the inducible nitric oxide synthase (iNOS, NOS-2) in vitro. PGE2 decreased hepatocyte NO synthesis in a concentration-dependent manner when the cells were stimulated with a combination of cytokines or IL-1 alone. LTB4 had a similar effect. PGE2 had to be present at the time of cytokine exposure to produce maximal inhibition of NO synthesis. Reduced synthesis of NO2- was associated with reduced NOS-2 mRNA levels suggesting that the induction of NOS-2 was inhibited. These findings demonstrate that eicosanoids can regulate hepatocyte NO synthesis in vitro.

5-Oxo-eicosanoids and hematopoietic cytokines cooperate in stimulating neutrophil function and the mitogen-activated protein kinase pathway

The newly defined eicosatetraenoates (ETEs), 5-oxoETE and 5-oxo-15(OH)-ETE, share structural motifs, synthetic origins, and bioactions with leukotriene B4 (LTB4). All three eicosanoids stimulate Ca2+ transients and chemotaxis in human neutrophils (PMN). However, unlike LTB4, 5-oxoETE and 5-oxo-15(OH)-ETE alone cause little degranulation and no superoxide anion production. However, we show herein that, in PMN pretreated with granulocyte-macrophage or granulocyte colony-stimulating factor (GM-CSF or G-CSF), the oxoETEs become potent activators of the last responses. The oxoETEs also induce translocation of secretory vesicles from the cytosol to the plasmalemma, an effect not requiring cytokine priming. To study the mechanism of PMN activation in response to the eicosanoids, we examined the activation of mitogen-activated protein kinase (MAPK) and cytosolic phospholipase A2 (cPLA2). PMN expressed three proteins (40, 42, and 44 kDa) that reacted with anti-MAPK antibodies. The oxoETEs, LTB4, GM-CSF, and G-CSF all stimulated PMN to activate the MAPKs and cPLA2, as defined by shifts in these proteins' electrophoretic mobility and tyrosine phosphorylation of the MAPKs. However, the speed and duration of the MAPK response varied markedly depending on the stimulus. 5-OxoETE caused a very rapid and transient activation of MAPK. In contrast, the response to the cytokines was rather slow and persistent. PMN pretreated with GM-CSF demonstrated a dramatic increase in the extent of MAPK tyrosine phosphorylation and electrophoretic mobility shift in response to 5-oxoETE. Similarly, 5-oxoETE induced PMN to release some preincorporated [14C]arachidonic acid, while GM-CSF greatly enhanced the extent of this release. Thus, the synergism exhibited by these agents is prominent at the level of MAPK stimulation and phospholipid deacylation. Pertussis toxin, but not Ca2+ depletion, inhibited MAPK responses to 5-oxoETE and LTB4, indicating that responses to both agents are coupled through G proteins but not dependent upon Ca2+ transients. 15-OxoETE and 15(OH)-ETE were inactive while 5-oxo-15(OH)-ETE and 5(OH)-ETE had 3- and 10-fold less potency than 5-oxoETE, indicating a rather strict structural specificity for the 5-keto group. LY 255283, a LTB4 antagonist, blocked the responses to LTB4 but not to 5-oxoETE. Therefore, the oxoETEs do not appear to operate through the LTB4 receptor. In summary, the oxoETEs are potent activators of PMN that share some but not all activities with LTB4. The response to the oxoETEs is greatly enhanced by pretreatment with cytokines, indicating that combinations of these mediators may be very important in the pathogenesis of inflammation.

Four distinct membrane-bound dipeptidase RNAs are differentially expressed and show discordant regulation with gamma-glutamyl transpeptidase

Membrane-bound dipeptidase (MBD) participates in the degradation of glutathione by cleaving the cysteinyl-glycine bond of cystinyl bisglycine (oxidized cysteinyl-glycine) following removal of a gamma-glutamyl group by gamma-glutamyl transpeptidase (GGT). In the mouse, MBD RNA is most abundant in small intestine, kidney, and lung and is represented by four distinct RNA species. These are generated by transcription from two promoters located 6 kilobases apart in the 5' flanking region of the gene and by the use of two different poly(A) addition sites. Promoter I is used primarily in small intestine and kidney, whereas promoter II is most active in lung and kidney. We found a discordance in the expected co-expression of MBD and GGT; as expected, MBD and GGT are both expressed at high levels in the kidney and small intestine. However, in the lung, MBD is expressed at high levels, whereas GGT is almost undetectable. The reverse is true in the seminal vesicles and fetal liver. Thus, although both enzymes may function in concert to metabolize glutathione in kidney and small intestine, in other tissues they appear to act independently, suggesting that they have independent roles in other biological processes.

The human multidrug resistance-associated protein functionally complements the yeast cadmium resistance factor 1

A Saccharomyces cerevisiae strain with a disrupted yeast cadmium resistance factor (YCF1) gene (DTY168) is hypersensitive to cadmium. YCF1 resembles the human multidrug resistance-associated protein MRP (63% amino acid similarity), which confers resistance to various cytotoxic drugs by lowering the intracellular drug concentration. Whereas the mechanism of action of YCF1 is not known, MRP was recently found to transport glutathione S-conjugates across membranes. Here we show that expression of the human MRP cDNA in yeast mutant DTY168 cells restores cadmium resistance to the wild-type level. Transport of S-(2,4-dinitrobenzene)-glutathione into isolated yeast microsomal vesicles is strongly reduced in the DTY168 mutant and this transport is restored to wild-type level in mutant cells expressing MRP cDNA. We find in cell fractionation experiments that YCF1 is mainly localized in the vacuolar membrane in yeast, whereas MRP is associated both with the vacuolar membrane and with other internal membranes in the transformed yeast cells. Our results indicate that yeast YCF1 is a glutathione S-conjugate pump, like MRP, and they raise the possibility that the cadmium resistance in yeast involves cotransport of cadmium with glutathione derivatives.

Multidrug resistance protein (MRP)-mediated transport of leukotriene C4 and chemotherapeutic agents in membrane vesicles. Demonstration of glutathione-dependent vincristine transport

The 190-kDa multidrug resistance protein (MRP) has recently been associated with the transport of cysteinyl leukotrienes and several glutathione (GSH) S-conjugates. In the present study, we have examined the transport of leukotriene C4 (LTC4) in membrane vesicles from MRP-transfected HeLa cells (T14), as well as drug-selected H69AR lung cancer cells which express high levels of MRP. V(max) and K(m) values for LTC4 transport by membrane vesicles from T14 cells were 529 +/- 176 pmol mg(-1) min(-1) and 105 +/- 31 nM, respectively. At 50 nM LTC4, the K(m) (ATP) was 70 micron. Transport in T14 vesicles was osmotically-sensitive and was supported by various nucleoside triphosphates but not by non- or slowly-hydrolyzable ATP analogs. LTC4 transport rates in membrane vesicles derived from H69AR cells and their parental and revertant variants were consistent with their relative levels of MRP expression. A 190-kDa protein in T14 membrane vesicles was photolabeled by [3H]LTC4 and immunoprecipitation with MRP-specific monoclonal antibodies (mAbs) confirmed that this protein was MRP. LTC4 transport was inhibited by an MRP-specific mAb (QCRL-3) directed against an intracellular conformational epitope of MRP, but not by a mAb (QCRL-1) which recognizes a linear epitope. Photolabeling with [3H]LTC4 was also inhibitable by mAb QCRL-3 but not mAb QCRL-1. GSH did not inhibit LTC4 transport. However, the ability of alkylated GSH derivatives to inhibit transport increased markedly with the length of the alkyl group. S-Decylglutathione was a potent competitive inhibitor of [3H]LTC4 transport (K(i(app)) 116 nM), suggesting that the two compounds bind to the same, or closely related, site(s) on MRP. Chemotherapeutic agents including colchicine, doxorubicin, and daunorubicin were poor inhibitors of [3H]LTC4 transport. Taxol, VP-16, vincristine, and vinblastine were also poor inhibitors of LTC4 transport but inhibition by these compounds was enhanced by GSH. Uptake of [3H]vincristine into T14 membrane vesicles in the absence of GSH was low and not dependent on ATP. However, in the presence of GSH, ATP-dependent vincristine transport was observed. Levels of transport increased with concentrations of GSH up to 5 mM. The identification of an MRP-specific mAb that inhibits LTC4 transport and prevents photolabeling of MRP by LTC4, provides conclusive evidence of the ability of MRP to transport cysteinyl leukotrienes. Our studies also demonstrate that MRP is capable of mediating ATP-dependent transport of vincristine and that transport is GSH-dependent.

The function of the multidrug resistance proteins (MRP and cMRP) in drug conjugate transport and hepatobiliary excretion

The MRP gene encodes a 190-kDa integral membrane glycoprotein which functions as a primary-active ATP-dependent export pump for amphiphilic anions. The MRP gene-encoded conjugate export pump and its canalicular isoform represent the transport activity which has been described earlier as multispecific organic anion transporter, non-bile acid organic anion transporter, glutathione S-conjugate export pump, or leukotriene export pump. Analyses of the substrate specificity of the human MRP pump were performed in plasma membrane vesicles from MRP-overexpressing drug-selected cells (7) and cells transfected with an MRP expression vector (8). Substrates for MRP include thioether-linked conjugates of lipophilic compounds with glutathione, cysteinyl glycine, cysteine, and N-acetyl cysteine, but also glutathione disulfide, and glucuronate conjugates such as etoposide glucuronide. This broad-specificity ATP-dependent export pump is not only overexpressed in several multidrug resistant tumor cells and tissues, but is also present in most normal cells and tissues. The expression of cMRP and MRP in human liver and of cMrp and its homolog Mrp in rat liver was demonstrated by reverse transcription PCR, cDNA sequencing, and immunoblotting (13). The important function of the cMRP gene-encoded broad-specificity conjugate export pump in hepatobiliary excretion is illustrated by the selective absence of this canalicular isoform from the hepatocyte canalicular membrane in transport-deficient mutant rats. This altered lack of cMrp is the basis for the hereditary detect of the hepatobiliary excretion of anionic conjugates in the mutant animals (13). The absence of this canalicular Mrp in the mutants is analogous to the defect in the human Dubin-Johnson syndrome which is characterized by an impaired excretion of conjugated anions across the canalicular membrane.

Expression of the MRP gene-encoded conjugate export pump in liver and its selective absence from the canalicular membrane in transport-deficient mutant hepatocytes

We have previously shown that the multi-drug resistance protein (MRP) mediates the ATP-dependent membrane transport of glutathione S-conjugates and additional amphiphilic organic anions. In the present study we demonstrate the expression of MRP in hepatocytes where it functions in hepatobiliary excretion. Analysis by reverse transcription-PCR of human and normal rat liver mRNA resulted in two expected cDNA fragments of MRP. Four different antibodies against MRP reacted on immunoblots with the glycoprotein of about 190 kD from human canalicular as well as basolateral hepatocyte membrane preparations. A polyclonal antibody directed against the carboxy-terminal sequence of MRP detected the rat homolog of MRP in liver. Double immunofluorescence microscopy and confocal laser scanning microscopy showed the presence of human MRP and rat Mrp in the canalicular as well as in the lateral membrane domains of hepatocytes. The transport function of the mrp gene-encoded conjugate export pump was assayed in plasma membrane vesicles with leukotriene C4 as a high-affinity glutathione S-conjugate substrate. The deficient ATP-dependent conjugate transport in canalicular membranes from TR- mutant rat hepatocytes was associated with a lack of amplification of one of the mrp cDNA fragments and with a selective loss of Mrp on immunoblots of canalicular membranes. Double immunofluorescence microscopy of livers from transport-deficient TR- mutant rats localized Mrp only to the lateral but not to the canalicular membrane. Our results indicate that the absence of Mrp or an isoform of Mrp from the canalicular membrane is the basis for the hereditary defect of the hepatobiliary excretion of anionic conjugates by the transport-deficient hepatocyte.

Metabolism of leukotrienes is impaired in hepatocytes from rats with thioacetamide-induced liver cirrhosis

It is likely that the hepatocellular metabolism of potent mediators of inflammation is impaired in chronic liver injury. Therefore, in this study the degradation of the leukotrienes LTC4, LTE4 and LTB4 was investigated in isolated liver parenchymal cells (LPC) from rats with thioacetamide-induced macronodular liver cirrhosis or after bile duct ligation. The degradation of LTE4 as well as the formation of N-acetyl-LTE4 was significantly delayed in LPC from macronodular cirrhotic rats but not in those from bile duct-ligated rats. LPC from macronodular cirrhotic rats eliminated LTC4 at the same rate as isolated hepatocytes from control animals. The rate of LTB4 degradation was significantly decreased by 35% in LPC from macronodular cirrhotic rats. Furthermore, the rate of LTB4 hydroxylation was significantly lower by 50% in microsomes isolated from hepatocytes of macronodular cirrhotic rats than in those from controls. In summary, one may conclude that the N-acetylation reaction of LTE4 and the hydroxylation reaction of LTB4 is impaired in LPC from rats with thioacetamide-induced macronodular cirrhosis.

Evaluation of tumour metabolism and multidrug resistance in patients with treated malignant lymphomas

The management of patients with treated malignant lymphomas requires functional methods to differentiate a residual soft tissue mass. Patients with treated Hodgkin's lymphoma (HL, n = 20, 68 malignant lesions, three benign lesions) or non-Hodgkin's lymphoma (NHL, n = 26, 46 malignant lesions, one benign lesion) were studied with positron emission tomography (PET) and fluorine-18 deoxyglucose (FDG). Oxygen-15 labelled water was used (n = 14, 25 lesions) in addition to FDG in order to obtain information on the tissue perfusion. Long-term follow-up studies with PET and FDG were performed in nine patients up to 511 days after the initiation of second-line therapy. Fourteen patients underwent single-photon emission tomography (SPET) with technetium-99m sestamibi immediately prior to the first PET examination. PET with FDG displays a high sensitivity for the detection of viable tumour tissue, all the malignant lesions being correctly classified in this study. The possible limitations are inflammatory processes, which may obscure tumour detection due to increased FDG uptake, and malignant lesions with low FDG uptake due to reduced perfusion. Difficulties exist in the prognosis of long-term response, since the change in FDG uptake may be variable. Long-term therapy outcome was correlated with the slope values obtained from the standardized integral uptake (SIU) data, which provides a new approach for the evaluation of PET follow-up studies. 99mTc-sestamibi, which should reflect the multidrug resistance, was evaluated with respect to therapy outcome. A high uptake of 99mTc-sestamibi was observed in patients with stable disease or better. The data support the hypothesis that sestamibi may reflect multidrug resistance.(ABSTRACT TRUNCATED AT 250 WORDS)