Purification and characterization of an ATPase from human liver which catalyzes ATP hydrolysis in the presence of the conjugates of bilirubin bile acids and glutathione

RALBP1 in Oxidative Stress and Mitochondrial Dysfunction in Alzheimer's Disease

The purpose of our study is to understand the role of the RALBP1 gene in oxidative stress (OS), mitochondrial dysfunction and cognition in Alzheimer's disease (AD) pathogenesis. The RALPB1 gene encodes the 76 kDa protein RLIP76 (Rlip). Rlip functions as a stress-responsive/protective transporter of glutathione conjugates (GS-E) and xenobiotic toxins. We hypothesized that Rlip may play an important role in maintaining cognitive function. The aim of this study is to determine whether Rlip deficiency in mice is associated with AD-like cognitive and mitochondrial dysfunction. Brain tissue obtained from cohorts of wildtype (WT) and Rlip+/- mice were analyzed for OS markers, expression of genes that regulate mitochondrial fission/fusion, and synaptic integrity. We also examined mitochondrial ultrastructure in brains obtained from these mice and further analyzed the impact of Rlip deficiency on gene networks of AD, aging, stress response, mitochondrial function, and CREB signaling. Our studies revealed a significant increase in the levels of OS markers and alterations in the expression of genes and proteins involved in mitochondrial biogenesis, dynamics and synapses in brain tissues from these mice. Furthermore, we compared the cognitive function of WT and Rlip+/- mice. Behavioral, basic motor and sensory function tests in Rlip+/- mice revealed cognitive decline, similar to AD. Gene network analysis indicated dysregulation of stress-activated gene expression, mitochondrial function and CREB signaling genes in the Rlip+/- mouse brain. Our results suggest that Rlip deficiency-associated increases in OS and mitochondrial dysfunction could contribute to the development or progression of OS-related AD processes.

RLIP76 increases apoptosis through Akt/mTOR signaling pathway in gastric cancer

RLIP76 is a stress-responsive multifunctional protein and is usually overexpressed in malignant carcinomas. It plays a significant role in multiple cellular biological behaviors, including cell growth, motility, division and apoptosis, in many types of malignant cells. However, functions of RLIP76 in gastric cancer (GC) remain unknown. In the present study, RLIP76 was overexpressed in GC tissues by immunohistochemistry. RLIP76-targeted shRNA-containing lentivirus (KD) and the scrambled shRNA (NC) were used to explore the knockout of RLIP76 on cellular functions of human GC SGC-7901 and MGC-803 cells. Quantitative RT-PCR and western blotting were used to confirm that the RLIP76 was suppressed both on mRNA and protein levels after transfection. The mRNA level in SGC-7901 and MGC-803 after transfection of RLIP76-targeted shRNA was 0.245722±0.021077 (p<0.05) and 0.225389±0.00974 (p<0.05), respectively. Our results showed that the konckdown of RLIP76 downregulated cell growth after 24 h in Cell Counting Kit-8 (CCK-8) assay, reduced migration from 486.7±128.8 to 219.7±43.6 in SGC-7901 (p<0.05) and from 630±95 to 333.7±46.5 in MGC-803 (p<0.05), decreased invasion from 306±33.5 to 97.7±24.3 in SGC-7901 (p<0.05) and from 350±50.9 to 163.3±87.5 in MGC-803 (p<0.05). Length of vascular endothelial growth factor (VEGF)-induced tube formation also decreased from 202.8±83.3 to 44.5±3.69 in SGC-7901 and from 193±3.5 to 71.8±8.83 in MGC-803 (p<0.05). Phosphorylation level of Akt declined from 138.45±13.8 to 69.9±29.7% in SGC-7901, and from 115.5±26.6 to 49.07±27% in MGC-803 (p<0.05) and phosphorylation level of mTOR also significantly decreased (p<0.05). While apoptosis of GC cells increased which we verified with apoptosis proteins and staining analysis. Our data showed that RLIP76 plays a significant oncogenic role in GC and it maybe a potential target in GC treatment.

Downregulation of RLIP76 is associated with vincristine resistance in human colorectal cancer HCT-8/VCR cells

RLIP76 is an anti-apoptotic transporter, participating in the multi-specific drug transport and resistance. In the absence of chemotherapy drugs, the knockout or inhibition of RLIP76 leads to pronounced tumor regression. RLIP76 transports anthracycline and vinca alkaloid drugs and mediates the resistance to these drugs. However, functions of RLIP76 in drug resistance colorectal cancer remain unclear. HCT-8 and the vincristine (VCR)-resistant colorectal cancer cell line HCT-8/VCR (HCT-8/V) were used in the present study. The effects of RLIP76 knockdown by the lentivirus were examined in cultured cells, including growth, apoptosis, invasion, and signaling pathways by qRT-PCR, western blot analysis and transwell assay. The relative level of RLIP76 in HCT-8 and HCT-8/V was assessed by western blot analysis, finding RLIP76 was overexpressed in HCT-8/V. Then, HCT-8/V cancer cells were transfected with lentivirus encoding RLIP76-specific shRNA (KD) and the control (NC), and no significant difference of RLIP76 level between the NC cells and cells without transfection was found, but the relative mRNA level decreased to 0.277±0.016 and protein level also reduced in KD cells. Cell functions changed after RLIP76 knockdown in HCT-8/V. The IC50 of VCR decreased from 164.4±1.734 to 13.95±2.008 (µg/ml) (p<0.05) in cell culture. The cell number reduced from 329.67±20.23 to 176.33±2.52 (p<0.05) in migration assay and from 294.67±30.07 to 153±22.11 (p<0.05) in invasion assay. Moreover, apoptotic proteins, including cleaved-caspase-8, cleaved-caspase-9, cleaved-Parp and Bax increased. The phosphorylation level of Erk also reduced significantly. The present study showed that RLIP76 is a key effector of cancer cell survival, invasion, and migration and possibly an important target to improve drug resistance and tumor treatment.

RLIP76 regulates HIF-1 activity, VEGF expression and secretion in tumor cells, and secretome transactivation of endothelial cells

This study was undertaken to reveal the mechanisms by which RLIP76 regulates endothelial cell angiogenic responses. RLIP76 is an effector of the angiogenic modulator, R-Ras. RLIP76 is overexpressed in many tumors, required for tumor angiogenesis, and blockade of RLIP76 results in tumor regression in multiple models. We report here that RLIP76 was required for expression and secretion of vascular endothelial growth factor (VEGF) in carcinoma and melanoma cells. Conditioned medium derived from RLIP76-depleted tumor cells, but not control knockdown cells, could not stimulate proliferation, migration, or Matrigel cord formation in endothelial cell cultures, which indicates that RLIP76 regulates angiogenic components of the tumor cell secretome. Recombinant VEGF added to conditioned medium from RLIP76-knockdown tumor cells restored these endothelial cell functions. Transcriptional activity of hypoxia-inducible factor 1 (HIF-1), which drives VEGF expression, was blocked in RLIP76-depleted tumor cells. RLIP76 was required for PI3-kinase activation, known to regulate HIF-1, in these cells. However, HIF-1α expression and nuclear localization were unaffected by RLIP76 knockdown, which suggests that RLIP76 regulates HIF-1 at the functional level. Thus, RLIP76 regulates tumor cell transactivation of endothelial cells via control of VEGF expression and secretion, providing a new important link in the mechanism of tumor cell induction of angiogenesis.

Emerging treatments in lung cancer - targeting the RLIP76 molecular transporter

Multidrug resistance in lung cancer cells is a significant obstacle in the treatment of lung cancer. Resistance to chemotherapeutic agents is often the result of efflux of the drugs from cancer cells, mediated by adenosine triphosphate (ATP)-dependent drug transport across the plasma membrane. Thus, identifying molecular targets in the cancer cell transport machinery could be a key factor in successful combinatorial therapy, along with chemotherapeutic drugs. The transport protein Ral-interacting protein of 76 kDa (RLIP76), also known as Ral-binding protein 1 (RalBP1), is a highly promising target for lung cancer treatment. RLIP76 is an ATP-dependent non-ATP-binding cassette (ABC) transporter, responsible for the major transport function in many cells, including many cancer cell lines, causing efflux of glutathione-electrophile conjugates of both endogenous metabolites and environmental toxins. RLIP76 is expressed in most human tissues, and is overexpressed in non-small-cell lung cancer cell lines and in many tumor types. The blockade of RLIP76 by various approaches has been shown to increase the sensitivity to radiation and chemotherapeutic drugs, and leads to apoptosis in cells. In xenograft tumor models in mice, RLIP76 blockade or depletion results in complete and sustained regression across many cancer cell types, including lung cancer cells. In addition to its transport function, RLIP76 has many other cellular and physiological functions based on its domain structure, which includes a unique Ral-binding domain and a Rho GTPase activating protein (RhoGAP)-catalytic domain as well as docking sites for multiple signaling proteins. As a Ral effector, RhoGAP, and adapter protein, RLIP76 has been shown to play important roles in endocytosis, mitochondrial fission, cell spreading and migration, actin dynamics during gastrulation, and Ras-induced tumorigenesis. Additionally, RLIP76 is also important for stromal cell function in tumors, as it was recently shown to be required for efficient endothelial cell function and angiogenesis in solid tumors. However, RLIP76 knockout mice are viable, and blockade effects appear to be selective for implanted tumors in mice, suggesting the possibility that RLIP76-targeting drugs may be successful in clinical trials. In this review, we outline the many cellular and physiological functions of RLIP76 in normal and cancer cells, and discuss the potential for RLIP76-based therapeutics in lung cancer treatment.

RALBP1/RLIP76 depletion in mice suppresses tumor growth by inhibiting tumor neovascularization

RalBP1/RLIP76 is a widely expressed multifunctional protein that binds the Ral and R-Ras small GTPases. In the mouse, RLIP76 is nonessential but its depletion or blockade promotes tumorigenesis and heightens the sensitivity of normal and tumor cells to radiation and cytotoxic drugs. However, its pathobiologic functions, which support tumorigenesis, are not well understood. Here, we show that RLIP76 is required for angiogenesis and for efficient neovascularization of primary solid tumors. Tumor growth from implanted melanoma or carcinoma cells was blunted in RLIP76(-/-) mice. An X-ray microcomputed tomography-based method to model tumor vascular structures revealed defects in both the extent and form of tumor angiogenesis in RLIP76(-/-) mice. Specifically, tumor vascular volumes were diminished and vessels were fewer in number, shorter, and narrower in RLIP76(-/-) mice than in wild-type mice. Moreover, we found that angiogenesis was blunted in mutant mice in the absence of tumor cells, with endothelial cells isolated from these animals exhibiting defects in migration, proliferation, and cord formation in vitro. Taken together, our results establish that RLIP76 is required for efficient endothelial cell function and angiogenesis in solid tumors.

RLIP76 and Cancer

RLIP76 is a multifunctional membrane protein that transports glutathione conjugates of electrophilic compounds and other xenobiotics including chemotherapy agents out of cells. The protein is overexpressed in lung carcinomas, ovarian carcinomas, and melanomas. The protein also binds Ral and participates in mitotic spindle function, clathrin-dependent endocytosis, and triggers GTPase-activating protein activity. It is found throughout the cell, in membrane, cytosol, and the nucleus, and is known to shift between these compartments in response to stress. Loss of RLIP76 by antibody or antisense therapy is associated with increased sensitivity to radiation and chemotherapy. Conversely, liposomally delivered RLIP may treat poisoning and wounds.

Linking stress-signaling, glutathione metabolism, signaling pathways and xenobiotic transporters

Multi-specific drug-transport mechanisms are intricately involved in mediating a pleiotropic drug-resistance in cancer cells by mediating drug-accumulation defects in cells in which they are over-expressed. The existence and over-expression in drug-resistant neoplasms of transporter proteins belonging to ATP-binding cassette (ABC) family indicate that these myriad transporters contribute to the multidrug-resistance phenomena by removing or sequestering of toxins and metabolites. Another prominent mechanism of multispecific drug-resistance involves glutathione and glutathione linked enzymes, particularly those of the mercapturic acid pathway, which are involved in metabolism and excretion of both endogenous and exogenous electrophilic toxins. A key step in the mercapturic acid pathway, efflux of the glutathione-electrophile conjugate has recently been shown to be catalyzed largely by the stress-responsive protein RLIP76, a splice variant peptide endowed by the human gene RALBP1. The known involvement of RLIP76 in membrane signaling pathways and endocytosis has resulted in a new paradigm for transport and metabolism related drug-resistance in which RLIP76 plays a central role. Our recent studies demonstrating a key anti-apoptotic and stress-responsive role of RLIP76, and the demonstration of dramatic response in malignancies to RLIP76 depletion indicate that targeting this mercapturic acid pathway transporter may be a highly effective and multifaceted antineoplastic strategy.

Therapeutic resistance in lung cancer

Despite considerable progress over the last 25 years in the systemic therapy of lung cancer, intrinsic and acquired resistance to chemotherapeutic agents and radiation remains a vexing problem. The number of mechanisms of therapeutic resistance in lung cancer has expanded considerably over the past three decades, and the crucial role of stress resistance pathways is increasingly recognised as a cause of intrinsic and acquired chemo- and radiotherapy resistance. This paper reviews recent evidence for stress defence proteins, particularly RALBP1/RLIP76, in mediating intrinsic and acquired chemotherapy and radiation resistance in human lung cancer.

Transport of glutathione conjugates and chemotherapeutic drugs by RLIP76 (RALBP1): a novel link between G-protein and tyrosine kinase signaling and drug resistance

Our studies have shown that RLIP76 (RALBP1), a 76 kDa Ral-binding, Rho/Rac-GAP and Ral effector protein, is a novel multispecific transporter of xenobiotics as well as GS-Es. Like previously characterized ABC transporters, it mediates ATP-dependent transport of structurally unrelated amphiphilic xenobiotics and displays inherent ATPase activity, which is stimulated by its substrate allocrites. It does not have significant sequence homology with ABC transporters and differs from the ABC transporters in several other important aspects, including (i) lack of any close homologs in humans, (ii) lack of a classical Walker domain, (iii) integral membrane association without clearly defined transmembrane domains and (iv) its role as a direct link to Ras/Ral/Rho and EGF-R signaling through its multifunctional nature, including GAP activity, regulation of exocytosis as well as clathrin-coated pit-mediated receptor endocytosis. Its multifunctional nature derives from the presence of multiple motifs, including a Rho/Rac GAP domain, a Ral effector domain binding motif, 2 distinct ATP-binding domains, a H(+)-ATPase domain, PKC and tyrosine kinase phosphorylation sites and the ability to undergo fragmentation into multiple smaller peptides which participate as components of macromolecular functional complexes. One of the physiologic functions of RLIP76 is regulation of intracellular concentration of the electrophilic intermediates of oxidative lipid metabolism by mediating efflux of GS-E formed from oxidative degradation of arachidonic acid, including leukotrienes and the 4HNE-GSH conjugate. RLIP76-mediated transport of amphiphilic chemotherapeutic agents such as anthracyclines and vinca alkaloids as well as GS-E produced during oxidative metabolism places this multifunctional protein in a central role as a resistance mechanism for preventing apoptosis caused by chemotherapeutic agents and a variety of external/internal stressors, including oxidative stress, heat shock and radiation.

ATP-Dependent colchicine transport by human erythrocyte glutathione conjugate transporter

We have recently demonstrated mutually inhibitory ATP-dependent transport of dinitrophenyl-S-glutathione (DNP-SG) and doxorubicin by DNP-SG ATPase purified from human erythrocyte membranes (S. Awasthi et al., 1998a,b). Our previous studies indicate a broad substrate specificity for this transport mechanism, including some P-glycoprotein substrates. Present studies were carried out to determine whether colchicine (COL), a classical P-glycoprotein substrate, could be transported by purified human erythrocyte DNP-SG ATPase reconstituted in artificial liposomes. We also investigated whether leukotriene C4 (LTC4), an endogenous proinflammatory glutathione-conjugate derived from arachidonic acid, would inhibit colchicine transport. Uptake of COL was compared in proteoliposomes reconstituted with the purified DNP-SG ATPase as well as control liposomes in the presence or absence of ATP. Increased colchicine uptake was observed upon addition of ATP to proteoliposomes, but not control liposomes. Uptake was linear with respect to the amount of vesicle protein used. Sensitivity to osmolarity was consistent with intravesicular COL accumulation. The ATP-dependent colchicine uptake was sensitive to temperature in a manner consistent with a protein-mediated transport process with activation energy of 7.3 kcal/mol. Time-dependent COL uptake by proteoliposomes in the presence of ATP was consistent with a single compartment model with an apparent rate constant of 0.21 +/- 0.02 min-1. Kinetic studies indicated a saturable behavior with respect to ATP (Km 2.3 +/- 0.7 mM) and colchicine (Km 4.3 +/- 0.2 microM). LTC4 was found to be a competitive inhibitor of COL transport (Kis 16.4 microM). Since DNP-SG ATPase is present in many tissues, it may play an important role in determining colchicine accumulation in cells. Increased LTC4 would tend to increase cellular COL accumulation.

Radiation inactivation suggests that human multidrug resistance-associated protein 1 occurs as a dimer in the human erythrocyte membrane

Molecular masses of functional units of two components of 2, 4-dinitrophenyl-S-glutathione (DNP-SG) transport across the erythrocyte membrane determined by radiation inactivation were 437 +/- 69 kDa for the high-affinity component and 466 +/- 67 kDa for the low-affinity component. These results confirm that the multidrug resistance-associated protein (MRP) 1 is responsible for the high-affinity DNP-SG transport across the erythrocyte membrane and suggest that MRP1 exists in the membrane as a dimer. The molecular size of the low-affinity transporter is similar if not identical to that of MRP1. Moreover, while the molecular mass of the DNP-SG-ATPase activity of the erythrocyte membrane corresponds also to that of MRP (375 +/- 36 kDa), the molecular mass of the functional unit of dinitrophenol-stimulated ATPase is significantly lower (232 +/- 26 kDa), which suggests that thisactivity is linked to a different protein, perhapsaminophospholipid translocase.

A nucleotide-binding domain of porcine liver annexin VI. Proteolysis of annexin VI labelled with 8-azido-ATP, purification by affinity chromatography on ATP-agarose, and fluorescence studies

Porcine liver annexin VI (AnxVI) of Mr 68.000 is an ATP-binding protein as evidenced by specific and saturable UV-dependent labelling with 8-azido-[gamma-32P]ATP or the fluorescent analog of ATP, 2'-(or 3')-O-(2,4,6-trinitrophenyl)adenosine triphosphate and by binding of AnxVI to ATP-agarose. These characteristics of purified AnxVI were used to identify and characterize preliminary nucleotide-binding domain of the protein. AnxVI labelled with 8-azido-ATP was subjected to limited proteolysis and the proteolytic fragments of AnxVI that retained the covalently-bound nucleotide were separated by means of gel electrophoresis and visualized by exposure of the gel to a phosphor storage screen. It was found that the AnxVI proteolytic fragments of Mr 34-36.000 and smaller retained the nucleotide. In a reciprocal experiment, AnxVI was digested with proteolytic enzymes and in an ATP eluate from an ATP-agarose column protein fragments of similar Mr to these labelled with 8-azido-ATP were identified. The extent of AnxVI labelling with 8-azido-ATP and the distribution of proteolytic fragments varied upon calcium concentration. These results lead to the conclusion that there is a nucleotide-binding domain within the AnxVI molecule that is functionally similar to the nucleotide-binding domains of other nucleotide-binding proteins. The nucleotide-binding domain is located close to the tryptophan residue 343 of AnxVI and in close vicinity to the Ca2+- and phospholipid-binding sites of the protein. This is confirmed by the observation that the tryptophan fluorescence intensity of AnxVI decreases in the presence of a fluorescence analog of ATP in a calcium-dependent manner, due to the quenching properties of the nucleotide and/or fluorescence energy transfer from AnxVI tryptophan to fluorophore. Both processes were modulated by the presence of phospholipid molecules.

ATP-dependent transport of the linear renin-inhibiting peptide EMD 51921 by canalicular plasma membrane vesicles of rat liver: evidence of drug-stimulatable ATP-hydrolysis

Certain peptide drugs, such as the linear hydrophobic renin-inhibitor EMD 51921, are rapidly eliminated via the bile. At the sinosoidal membrane of liver cells EMD 51921 is taken up via a sodium-independent carrier-mediated mechanism, competing for the uptake of bile acids. Until now, the mechanisms of biliary excretion of EMD 51921 were unknown. In this study we describe an ATP-dependent transport system for the enzymatically and metabolically stable hydrophobic linear renin-inhibiting peptide EMD 51921. The ATP-dependent uptake into the osmotic reactive intravesicular space is saturable (Km 12 microM, Vmax 663 pmol/min per mg protein), temperature dependent and specifically requires ATP. Transport is inhibited by vanadate but not by ouabain, EGTA or NaN3, and does not function in basolateral plasma membrane vesicles. Transport is not altered in canalicular membrane vesicles isolated from Tr- rats lacking the canalicular ATP-dependent transport of cysteinyl leukotrienes and related anions. Transport is inhibited by taurocholate, a typical substrate of the canalicular ATP-dependent bile acid transporter, but also by vincristine and daunomycin, substrates of P-glycoproteins. EMD 51921, however, only inhibits the uptake of taurocholate, whereas the transport of daunomycin is not influenced. Taurocholate and EMD 51921 are mutually non- or un-competitive transport inhibitors. Incubation of rat liver canalicular membranes with micromolar concentrations of EMD 51921 resulted in a 1.8-2.5-fold increase in the rate of ATP-hydrolysis. In contrast, ATP-hydrolysis was not affected by fragments of the peptide that are not transported in an ATP-dependent manner. The apparent Km value (EMD) for ATP-hydrolysis is 68 microM. Vmax is 0.032 U/mg protein. ATPase activity is pH dependent. Stimulation of ATP-hydrolysis is inhibited by vanadate, NEM, hydroxymercuribenzoate and ascorbate, but is not affected by ouabain, EGTA or NaN3. EMD 51921 does not stimulate the ATPase activity of the Na+/K(+)-ATPase isolated from kidney medulla. The EMD-stimulatable ATPase seems to be distinct from the glutathione-S-conjugate stimulatable ATPase and the mdr 1a/b gene products and differs in its characteristics from that of the canalicular ecto-ATPase.

Glutathione conjugation and conversion to mercapturic acids can occur as an intrahepatic process

By catalyzing the reaction of electrophilic compounds with the sulfhydryl group of glutathione, the glutathione S-transferases play physiologically important roles in the detoxication of potential alkylating agents. The glutathione S-conjugates thus formed are transported out of cells for further metabolism by gamma-glutamyltransferase and dipeptidases, ectoproteins that catalyze the sequential removal of the glutamyl and glycyl moieties, respectively. These ectoproteins are not found in all cells, but are localized predominantly to the apical surface of epithelial tissues. The resulting cysteine S-conjugates can be reabsorbed by specific cell types, and acetylated on the amino group of the cysteinyl residue by intracellular N-acetyl-transferases, to form the corresponding mercapturic acids (N-acetylcysteine S-conjugates). Mercapturic acids are then released into the circulation and delivered to the kidney for excretion in urine, or they may undergo further metabolism. Mercapturic acid biosynthesis is generally considered to be an interorgan process, with the liver serving as the major site of glutathione conjugation, and the kidney as the primary site for conversion of glutathione conjugates to cysteine conjugates. Cysteine conjugates formed in the kidney appear to be transported back to the liver for acetylation. This interorgan model of mercapturic acid synthesis is based largely on the interorgan distribution of the enzymes involved in their formation, and in particular of the enzyme gamma-glutamyltransferase. Rats have relatively low hepatic and high renal activities of gamma-glutamyltransferase, the only protein known to initiate the breakdown of glutathione S-conjugates. The low gamma-glutamyltransferase activity in rat liver limits the hepatic degradation of glutathione S-conjugates, particularly after large doses of xenobiotic. In contrast, hepatic gamma-glutamyltransferase is significantly higher in species such as rabbit, guinea pig, and dog, and as a consequence, nearly all of the glutathione and glutathione S-conjugates released by liver cells of these species is degraded within the liver. Recent studies demonstrate that glutathione S-conjugates synthesized within hepatocytes are secreted preferentially across the canalicular membrane into bile, and are broken down within biliary spaces to form cysteine S-conjugates. The latter are then reabsorbed by the liver, N-acetylated to form mercapturic acids, and reexcreted into bile, completing an intrahepatic pathway for mercapturic acid biosynthesis. The contribution of this intrahepatic pathway to overall mercapturate formation is dependent on dose of the electrophile, route of exposure, and the physicochemical properties of the glutathione S-conjugate formed, as well as the tissue distribution and activity of gamma-glutamyltransferase.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the ATP-dependent leukotriene C4 export carrier in mastocytoma cells

The biosynthesis of leukotriene C4 (LTC4) must be followed by an export of this mediator into the extracellular space where it interacts with receptors. Using mastocytoma cells we have demonstrated the existence of a primary-active, ATP-dependent transport mediating this export of LTC4 [Schaub, T., Ishikawa, T. & Keppler, D. (1991) FEBS Lett. 279, 83-86]. The following inhibitors served to characterize further this transport system in plasma membrane vesicles from mastocytoma cells: Probenecid, an inhibitor of organic anion transport, induced half-maximal inhibition of the ATP-dependent LTC4 transport at 71 microM. Cyclosporin A and its non-immunosuppressive analog PSC 833 inhibited the ATP-dependent transport with Ki values of 4.5 microM and 30 microM, respectively. The LTD4 receptor antagonist 3-([(3-(2-[7-chloro-2-quinolinyl]ethenyl)phenyl)-[(3-dimethylamino-3- oxopropyl)-thio]-methyl]thio)propanoic acid (MK 571) was the most potent competitive inhibitor of the export carrier with a Ki value of 0.8 microM. The transport inhibitor MK 571 served as competitor in the photoaffinity labeling of LTC4-binding membrane proteins using [3H]LTC4 as the photolabile ligand. Proteins with molecular masses of about 190 kDa and 35 kDa were predominantly labeled. In addition, a minor [3H]LTC4 labeling was observed in the molecular mass range of 100 kDa. The [3H]LTC4 labeling of the 190-kDa protein was competed for by MK 571. The labeled proteins resisted extraction from the membrane with 2% sodium taurocholate suggesting that they are integral membrane proteins. Treatment of the membrane proteins with peptide N-glycosidase F resulted in the appearance of an additional labeled polypeptide of about 140 kDa suggesting that the 190-kDa protein is a glycoprotein. Photoaffinity labeling with 8-azido[alpha-32P]ATP predominantly labeled the LTC4-binding 35-kDa protein. The [3H]LTC4-labeled 190-kDa protein showed a mean isoelectric point at pH 6.3 with a range of pH 5.8-6.7, while the 35-kDa protein had an isoelectric point at pH 6.8. Specific labeling of a 190-kDa membrane glycoprotein by the glutathione conjugate LTC4, which is competed for by a potent inhibitor of the ATP-dependent LTC4 export carrier, pinpoints its involvement in the ATP-dependent transport of LTC4 and related conjugates.

Organic anions exhibit distinct inhibition patterns on the low-Km and high-Km transport of S-(2,4-dinitrophenyl)glutathione through the human erythrocyte membrane

The low-Km and high-Km components of S-dinitrophenyl-glutathione (DNPSG) uptake by inside-out vesicles of human erythrocytes show different pH profiles and inhibition properties with organic anions. Both components are competitively inhibited by various polyvalent anions, including glutathione conjugates, conjugated steroid hormones and bile salts, and bilirubin ditaurate. A variety of monovalent anions, including glucuronidated and sulphated drugs and taurocholate, inhibit the high-Km system only. Taurocholate is taken up by the erythrocyte vesicles in an ATP-dependent manner. The anionic dyes fluorescein, Indocyanine Green and bromosulphophthalein inhibit the low-Km system competitively and the high-Km system non-competitively. The study shows that interactions between different types of biologically occurring conjugates can occur at the level of the transport step out of erythrocytes. The kinetic properties suggest overlapping substrate specificities for the two systems, in which the low-Km component is physiologically more important for transport of glutathione conjugates and polyvalent organic anions, whereas the high-Km component is of significance for transport of monovalent organic anions. Low- and high-Km transport of DNPSG is also observed in plasma membrane vesicles from rat, pig and bovine erythrocytes.

Functional integrity of hepatocyte canalicular membrane transport of taurocholate and bilirubin diglucuronide in Eisai hyperbilirubinuria rats

Eisai hyperbilirubinuria rats (EHBR) are characterized by conjugated hyperbilirubinemia, and impaired or defective excretion of bilirubin, reduced glutathione and other organic anions from hepatocytes. Hepatocyte canalicular membrane vesicles (CMV) from EHBR and normal SD rats were studied with regard to taurocholate (TC) transport driven by ATP or a membrane potential and bicarbonate-stimulated bilirubin diglucuronide (BDG) transport. ATP-dependent uptake or association of BDG with CMV was also studied in both strains of rats. No significant differences in the uptake of TC and BDG by CMV were observed. This indicates the functional integrity of the canalicular transporters for both organic anions in EHBR. Biliary excretion of taurolithocholic acid sulfate (TLCS) is defective in EHBR. However, TLCS inhibited ATP-dependent TC uptake by SD rat CMV competitively, which may be against the hypothesis that a common organic anion carrier is defective in canalicular membranes of jaundiced rats.

Dinitrophenyl S-glutathione ATPase purified from human muscle catalyzes ATP hydrolysis in the presence of leukotrienes

Dinitrophenyl S-glutathione (Dnp-SG) ATPase has been purified from human muscle to apparent homogeneity using Dnp-SG affinity chromatography and immunoaffinity chromatography using antibodies raised against human erythrocyte Dnp-SG ATPase. The enzyme purified from human muscle showed a subunit M(r) value of about 38 kDa in denaturing gels. The M(r) value of the native enzyme as determined by Sephadex G-200 gel filtration was found to be about 80 kDa, which indicates that it is a dimer. The N-terminus of the enzyme was blocked. Its immunological and kinetic properties were similar to Dnp-SG ATPase of human erythrocytes. Besides catalyzing the ATP hydrolysis in the presence of Dnp-SG, the muscle enzyme also catalyzed ATP hydrolysis in the presence of various leukotrienes, namely LTC4.LTD4, LTE4, and N-acetyl LTE4. The specific activity of the enzyme toward LTC4 was relatively higher than other GSH-xenobiotic conjugates. The muscle enzyme exhibits a low Km value for all leukotrienes as compared to Dnp-SG, indicating high affinity of the enzyme for leukotrienes as activators. The enzyme also catalyzed ATP hydrolysis in the presence of GSH conjugates of endogenously generated fatty acid epoxides. Our results might suggest that Dnp-SG ATPase is involved in the transport of GSH conjugates, leukotrienes, and other organic anions in muscle, erythrocytes, liver, and probably other tissues.

ATP-dependent bile-salt transport in canalicular rat liver plasma-membrane vesicles

The present study identifies and characterizes a novel ATP-dependent bile-salt transport system in isolated canalicular rat liver plasma-membrane (cLPM) vesicles. ATP (1-5 mM) stimulated taurocholate uptake into cLPM vesicles between 6- and 8-fold above equilibrium uptake values (overshoot) and above values for incubations in the absence of ATP. The ATP-dependent portion of taurocholate uptake was 2-fold higher in the presence of equilibrated KNO3 as compared with potassium gluconate, indicating that the stimulatory effect of ATP was not due to the generation of an intravesicular positive membrane potential. Saturation kinetics revealed a very high affinity (Km approximately 2.1 microM) of the system for taurocholate. The system could only minimally be stimulated by nucleotides other than ATP. Furthermore, it was preferentially inhibited by conjugated univalent bile salts. Further strong inhibitory effects were observed with valinomycin, oligomycin, 4,4'-di-isothiocyano-2,2'-stilbene disulphonate, sulphobromophthalein, leukotriene C4 and N-ethylmaleimide, whereas nigericin, vanadate, GSH, GSSG and daunomycin exerted only weak inhibitory effects or none at all. These results indicate the presence of a high-affinity primary ATP-dependent bile-salt transport system in cLPM vesicles. This transport system might be regulated in vivo by the number of carriers present at the perspective transport site(s), which, in addition to the canalicular membrane, might also include pericanalicular membrane vesicles.

Erythrocyte membrane transport of glutathione conjugates and oxidized glutathione in the Dubin-Johnson syndrome and in rats with hereditary hyperbilirubinemia

The Dubin-Johnson syndrome is manifested by conjugated hyperbilirubinemia and pigment accumulation in hepatocellular lysosomes. The TR-rat model is a phenotypic model of the Dubin-Johnson syndrome and is characterized by defective ATP-dependent transport of a group of nonbile acid organic anions, including glutathione-S-conjugates and oxidized glutathione, across the bile canaliculus. Similar ATP-dependent transport mechanisms have been described in erythrocytes. Intact erythrocytes and inverted erythrocyte membrane vesicles from Dubin-Johnson patients, TR-rats and appropriate controls were studied with regard to ATP-dependent transport of dinitrophenyl glutathione and oxidized glutathione. No significant differences were observed, indicating that the erythrocyte and canalicular ATP-dependent transporters for these substrates are functionally and potentially genetically distinct.

Identification of an anion-transport ATPase that catalyzes glutathione conjugate-dependent ATP hydrolysis in canalicular plasma membranes from normal rats and rats with conjugated hyperbilirubinemia (GY mutant)

Rat liver canalicular plasma membranes were found to contain a 37-kDa protein that is immunologically cross-reactive with the dinitrophenyl glutathione-stimulated ATPase previously identified in human tissues. The protein, which was partially purified by affinity chromatography, exhibited ATPase activity dependent on dinitrophenyl glutathione, bilirubin ditaurate, and other dianionic compounds. The localization of this protein in the canalicular membrane and its measured enzymatic activity indicate that it is involved in the transport of glutathione derivatives and other dianionic organic compounds. A rat mutant in which the above transport activities are impaired contained the protein in amounts similar to those in a normal control.