ATP-Dependent human erythrocyte glutathione-conjugate transporter. I. Purification, photoaffinity labeling, and kinetic characteristics of ATPase activity

Haploinsufficiency Interactions between RALBP1 and p53 in ERBB2 and PyVT Models of Mouse Mammary Carcinogenesis

Simple Summary

Rlip knockout has been reported to prevent cancer in highly cancer-susceptible mice lacking p53, and Rlip knockdown kills many types of cancer cells. In humans, breast cancer shows diverse characteristics, including HER2-driven subtypes and viral-driven subtypes. HER2 can be targeted; however, escape of the cancer from targeted therapies remains a problem. In this work we evaluated the capacity of Rlip knockout to prevent breast cancer in genetically engineered mouse models of HER2-driven breast cancer (Erbb2 model) and polyomavirus-driven breast cancer (PyVT model). We found that in Erbb2 mice, Rlip knockout significantly delayed oncogenesis and reduced the expression of genes associated with poor prognosis in patients. In PyVT mice, Rlip knockout did not delay oncogenesis or tumor growth, but Rlip knockdown reduced tumor metastasis to the lung. We conclude that Rlip inhibitors may significantly improve survival in HER2-positive patients, but are unlikely to offer benefits to patients with polyomavirus-associated tumors.

Abstract

We recently reported that loss of one or both alleles of Ralbp1, which encodes the stress-protective protein RLIP76 (Rlip), exerts a strong dominant negative effect on both the inherent cancer susceptibility and the chemically inducible cancer susceptibility of mice lacking one or both alleles of the tumor suppressor p53. In this paper, we examined whether congenital Rlip deficiency could prevent genetically-driven breast cancer in two transgenic mouse models: the MMTV-PyVT model, which expresses the polyomavirus middle T antigen (PyVT) under control of the mouse mammary tumor virus promoter (MMTV) and the MMTV-Erbb2 model which expresses MMTV-driven erythroblastic leukemia viral oncogene homolog 2 (Erbb2, HER2/Neu) and frequently acquires p53 mutations. We found that loss of either one or two Rlip alleles had a suppressive effect on carcinogenesis in Erbb2 over-expressing mice. Interestingly, Rlip deficiency did not affect tumor growth but significantly reduced the lung metastatic burden of breast cancer in the viral PyVT model, which does not depend on either Ras or loss of p53. Furthermore, spontaneous tumors of MMTV-PyVT/Rlip+/+ mice showed no regression following Rlip knockdown. Finally, mice lacking one or both Rlip alleles differentially expressed markers for apoptotic signaling, proliferation, angiogenesis, and cell cycling in PyVT and Erbb2 breast tumors. Our results support the efficacy of Rlip depletion in suppressing p53 inactivated cancers, and our findings may yield novel methods for prevention or treatment of cancer in patients with HER2 mutations or tumor HER2 expression.

Rlip Depletion Suppresses Growth of Breast Cancer

RLIP76 (RAL-binding protein-1, Rlip) is a stress-protective mercapturic-acid-pathway transporter protein that also plays a key role in regulating clathrin-dependent endocytosis as a Ral effector. Targeted inhibition or depletion of Rlip causes regression of xenografts of many cancers and is capable of abrogating tumor formation in p53-null mice. This is associated with the reversion of the abnormal methylomic profile of p53-null mice to wild-type. In a query of The Cancer Genome Atlas (TCGA) databases, we found that Rlip expression was associated with poor survival and with significant differences in the frequencies of PIK3CA mutation, MYC amplification, and CDKN2A/B deletion, which were the most commonly mutated, amplified, and deleted genes, respectively, among TCGA breast cancer patients. We conducted the present study to further examine the effects of Rlip inhibition and to evaluate the in vitro and in vivo efficacy in breast cancer. Using immunogold electron microscopy, we found that plasma-membrane Rlip was accessible to cell-surface antibodies in the MCF7 (ER+) breast cancer cell line. Rlip depletion resulted in decreased survival of MCF7 and MDA-MB-231 cells and increased terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) positivity and DNA laddering, indicating apoptotic cell death. Additionally, in vitro knockdown of Rlip inhibited EGF endocytosis and WNT/MAPK signaling. Xenograft studies in nude mice showed regression of breast cancer via antisense-mediated depletion of Rlip mRNA as well as by anti-Rlip antibody. Finally, knockdown of Rlip by antisense locked nucleic acid oligonucleotides increased markers for apoptotic signaling and decreased markers for proliferation, angiogenesis, and cell cycling in MCF7 and MDA-MB-231luc xenografts. Our findings validate Rlip as an attractive target in breast cancer.

RLIP: An existential requirement for breast carcinogenesis

Breast cancer (BC) is the most common cancer among women worldwide. Due to its complexity in nature, effective BC treatment can encounter many challenges. The human RALBP1 gene encodes a 76-kDa splice variant protein, RLIP (ral-binding protein1, RalBP1), a stress-protective mercapturic acid pathway (MAP) transporter protein, that also plays a key role in regulating clathrin-dependent endocytosis (CDE) as a Ral effector. Growing evidence shows that targeting RLIP may be an effective strategy in cancer therapy, as RLIP is over-expressed in multiple cancers and is known to induce resistance to apoptosis and chemotherapeutic drugs. Recent studies demonstrated that RLIP is expressed in human BC tissues, as well as BC cell lines. Knockdown of RLIP resulted in apoptotic death of BC cells in vitro, and targeted inhibition and depletion of RLIP resulted in regression of BC in xenograft studies of nude mice. Signaling studies showed that RLIP depletion inhibited endocytosis and differentially regulated signaling to Akt, Myc, and ERK1/2. However, the proliferation and multi-specific transport mechanisms that promote RLIP-mediated cell death in BC are not well understood. In this review, we will discuss a missing but an essentially determining and connecting piece of the puzzle on the understanding of proliferation and transport mechanisms by focused analyses of the apoptotic, drug- and radiation-sensitivity regulated by RLIP, a stress-responsive non-ATP-binding cassette (ABC), high capacity MAP transporter, in breast cancer.

Anticancer activity of 2'-hydroxyflavanone towards lung cancer

In previous studies, we found that 2'-hydroxyflavonone (2HF), a citrus flavonoid, inhibits the growth of renal cell carcinoma in a VHL-dependent manner. This was associated with the inhibition of glutathione S-transferases (GSTs), the first step enzyme of the mercapturic acid pathway that catalyzes formation of glutathione-electrophile conjugates (GS-E). We studied 2HF in small cell (SCLC) and non-small cell (NSCLC) lung cancer cell lines for sensitivity to 2HF antineoplastic activity and to determine the role of the GS-E transporter Rlip (Ral-interacting protein; RLIP76; RALBP1) in the mechanism of action of 2HF. Our results show that 2HF induced apoptosis in both histological types of lung cancer and inhibited proliferation and growth through suppression of CDK4, CCNB1, PIK3CA, AKT and RPS6KB1 (P70S6K) signaling. Increased E-cadherin and reduced fibronectin and vimentin indicated inhibition of epithelial-mesenchymal transition. Additionally, 2HF inhibited efflux of doxorubicin and increased its accumulation in the cells, but did not add to the transport inhibitory effect of anti-Rlip antibodies alone. Binding of Rlip to 2HF was evident from successful purification of Rlip by 2HF affinity chromatography. Consistent with increased drug accumulation, combined treatment with 1-chloro-2, 4-dinitrobenzene, reduced the GI50 of 2HF by an order of magnitude. Results of in-vivo nude mouse xenograft studies of SCLC and NSCLC, which showed that orally administered 2HF inhibited growth of both histological types of lung cancer, confirmed in-vitro study results. Our result suggest that Rlip inhibition is likely a mechanism of action. Our findings are basis of proposing 2HF as therapeutic or preventative drug for lung cancer.

Rlip depletion prevents spontaneous neoplasia in TP53 null mice

Mice that have homozygous deletion of the p53 tumor suppressor protein universally die of malignancy, generally before 6 months of age. We show that hemizygous deficiency of RALBP1 (RLIP76 or Rlip) confers a degree of protection from spontaneous malignancy that has never previously been observed. This discovery introduces a paradigm for p53 function, in which Rlip plays a central role as an effector that appears necessary for the cancer susceptibility of p53 null mice. Because p53 loss has a powerful effect on genomic instability that contributes to the initiation and promotion of cancers and to drug and radiation resistance in humans, our findings provide a method for prevention and therapy of p53-deficient cancer.

TP53 (p53) is a tumor suppressor whose functions are lost or altered in most malignancies. p53 homozygous knockout (p53^−/−) mice uniformly die of spontaneous malignancy, typically T-cell lymphoma. RALBP1 (RLIP76, Rlip) is a stress-protective, mercapturic acid pathway transporter protein that also functions as a Ral effector involved in clathrin-dependent endocytosis. In stark contrast to p53^−/− mice, Rlip^−/− mice are highly resistant to carcinogenesis. We report here that partial Rlip deficiency induced by weekly administration of an Rlip-specific phosphorothioate antisense oligonucleotide, R508, strongly inhibited spontaneous as well as benzo(a)pyrene-induced carcinogenesis in p53^−/− mice. This treatment effectively prevented large-scale methylomic and transcriptomic abnormalities suggestive of inflammation found in cancer-bearing p53^−/− mice. The remarkable efficiency with which Rlip deficiency suppresses spontaneous malignancy in p53^−/− mice has not been observed with any previously reported pharmacologic or genetic intervention. These findings are supported by cross-breeding experiments demonstrating that hemizygous Rlip deficiency also reduces the spontaneous malignancy phenotype of p53^+/− mice. Rlip is found on the cell surface, and antibodies directed against Rlip were found to inhibit growth and promote apoptosis of cell lines as effectively as Rlip siRNA. The work presented here investigates several features, including oxidative DNA damage of the Rlip–p53 association in malignant transformation, and offers a paradigm for the mechanisms of tumor suppression by p53 and the prospects of suppressing spontaneous malignancy in hereditary cancer syndromes such as Li-Fraumeni.

Overexpression of RLIP76 Required for Proliferation in Meningioma Is Associated with Recurrence

The GTPase-activating protein RLIP76 is overexpressed in and correlates with the pathological grade of many malignant tumor cells. But the potential correlation between RLIP76 and clinical outcomes in patients with meningioma remains unknown. In this study, we examined the expression of RLIP76 in meningioma and correlated the RLIP76 expression to the patient outcome. RLIP76 expression in tumor tissues was examined with immunohistochemistry, quantitative reverse-transcription polymerase chain reaction(RT-PCR) and Western-blot. Immunohistochemistry showed an increased RLIP76 immunostaining score in anaplastic and atypical meningiomas versus classical meningiomas. Statistical analyses revealed that RLIP76 immunostaining positively correlated with immunostaining for Ki-67, a nuclear protein highly expressed in proliferating cells(r=0.29, p=0.034 by Spearman's correlation coefficient). Clinicopathological evaluation suggested that RLIP76 expression be associated with tumor grade and recurrence(P<0.05). Univariate and Cox analysis indicated that RLIP76 was an independent prognostic factor for tumor recurrence. Furthermore, the human malignant meningioma cell lines IOMM-Lee and CH157-MN stably transfected with short hairpin RNA (siRNA) targeting RLIP76 were then examined by in vitro growth assays, and apoptosis assays. RLIP76 knockdown in IOMM-Lee and CH157-MN cells inhibited cell proliferation and induced apoptosis. Western blot analysis revealed that cells underexpressing RLIP76 exhibited decreased B-cell lymphoma-2(Bcl-2) expression but increased apoptosis effector caspase-3 expression. These findings demonstrate that high RLIP76 expression is associated with a poor outcome of meningioma and may provide a new gene therapy approach for patients with malignant meningiomas.

RLIP76: a versatile transporter and an emerging target for cancer therapy

In the last few years, extensive research has been made to elucidate the functional significance of RLIP76. The resulting novel breakthroughs have helped us understand its transport and signaling functions. RLIP76 is a ubiquitously expressed, key stress-defensive, anti-apoptotic, multi-functional protein that transports glutathione-conjugates of electrophilic compounds, thus controlling the intracellular concentration of pro-apoptotic oxidized lipid byproducts and other xenobiotics such as chemotherapeutic agents. These properties place RLIP76 at a very important position in the hierarchy of the stress defense mechanism adopted by the cell. Selective over-expression of RLIP76 in malignant cells of diverse origin is one of the possible mechanisms by which these cells overcome chemotherapy and radiation induced oxidative damage. RLIP76 has also been shown to be an effective transporter of many conventional chemotherapeutic drugs. Such transport, if inhibited, can lead to increased cellular accumulation of drugs which in turn translates to enhanced drug sensitivity. Recent studies have shown that inhibition and/or depletion of RLIP76 by antibodies, siRNA, or antisense can lead to drastic and sustained regression of lung, kidney, melanoma, colon, and prostate cancer xenografts with no observed recurrence of tumors. All these findings converge on the fact that such inhibition/depletion of RLIP76 can be used clinically to terminate cancer growth and progression. In the present review, we will discuss the role of RLIP76 as a multi-drug transporter, its involvement in cancer, and the prospects of using RLIP76 inhibition as an emerging treatment for cancer.

Role of RLIP76 in doxorubicin resistance in lung cancer

Lung cancer is still a major cause of cancer deaths in spite of considerable efforts in its systemic therapy. Chemotherapy, along with local irradiation is frequently employed but as a palliative therapy. Inherent and acquired resistance in NSCLC and SCLC towards chemotherapeutic agents further makes chemotherapy an incommodious problem. The resistance mechanisms responsible for inherent DOX-resistance of NSCLC and acquired DOX-resistance in SCLC have been the subject of numerous investigations. This review will focus on the recent studies done for understanding the mechanism(s) of inherent and acquired resistance in NSCLC and SCLC and how these can be exploited for the future development of more effective novel biologic agents for the treatment of lung cancer.

Physiological and pharmacological significance of glutathione-conjugate transport

Transport of the glutathione conjugates (GS-E) of electrophilic compounds generated during biotransformation of drugs and environmental pollutants is central to the mechanisms of defense against oxidative/electrophilic stress. In recent years emphasis has been placed on ATP-binding cassette (ABC) transport proteins in the transport of GS-E and their involvement in the detoxification mechanisms, including drug resistance. Recent studies, however, suggested that the majority of GS-E transport in human and rodent cells is mediated by a non-ABC, multifunctional stress-response protein, RLIP76 or RalBP1 (ral-binding GTPase activating protein 1), which also functions as an effector in the Ral-Ras-Rho signaling pathway. In this review, after briefly describing the major discoveries in the field of glutathione (GSH)-conjugate transport, recent findings are presented on the role of RLIP76 in ATP-dependent transport of GS-E, and the relevance of this transport process to the mechanisms of toxicity of xenobiotics, radiation, and endogenous electrophilic toxicants is described. Furthermore, recent studies suggesting a link between RLIP76 mediated GS-E transport and cell cycle signaling are presented.

RLIP76: a target for kidney cancer therapy

RLIP76 is a multifunctional transporter protein that serves as an energy-dependent efflux mechanism for endogenously generated toxic metabolites as well as exogenous toxins, including chemotherapy drugs. Our recent studies in cultured cells, syngeneic animal tumor model, and in xenograft model have shown that RLIP76 serves a major cancer-specific antiapoptotic role in a wide variety of histologic types of cancer, including leukemia, melanoma, colon, lung, prostate, and ovarian cancer. Results of present studies in cell culture and xenograft model of Caki-2 cells show that RLIP76 is an important anticancer for kidney cancer because inhibition of RLIP76 function by antibody or its depletion by small interfering RNA or antisense DNA caused marked and sustained regression of established human kidney xenografts of Caki-2 cells in nude mouse.

RLIP76: A novel glutathione-conjugate and multi-drug transporter

RLIP76, a stress-responsive, multi-functional protein with multi-specific transport activity towards glutathione-conjugates (GS-E) and chemotherapeutic agents, is frequently over-expressed in malignant cells. Our recent studies suggest that it plays a prominent anti-apoptotic role selectively in cancer cells. We have previously shown that RLIP76 accounts for up to 80% of the transport of GS-E and blocking the RLIP76-mediated transport of GS-E in cells results in the accumulation of pro-apoptotic endogenous electrophiles and on-set of apoptosis. Here we demonstrate that when RLIP76 mediate transport of GS-E is abrogated either by anti-RLIP76 IgG or accumulation of 4-hydroxynonenal (4-HNE) and its GSH-conjugate (GS-HNE) occurs and a massive apoptosis is observed in cells, indicate that the inhibition of RLIP76 transport activity at the cell surface is sufficient for observed anti-tumor activity. RLIP76 is linked with certain cellular functions including membrane plasticity and movement (as a primary 'effector' in the Ral pathway, perhaps functioning as a GTPase activating protein, or GAP), and as a component of clathrin-coated pit-mediated receptor-ligand endocytosis-a process that mediates movement of membrane vesicles.

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.

Mass spectroscopic phosphoprotein mapping of Ral binding protein 1 (RalBP1/Rip1/RLIP76)

RalBP1, a multifunctional protein implicated in cancer cell proliferation, radiation and chemoresistance, and ligand dependent receptor internalization, is upregulated in bladder cancer and is a downstream effector of RalB, a GTPase associated with metastasis. RalBP1 can be regulated by phosphorylation by protein kinase C (PKC). No studies have comprehensively mapped RalBP1 phosphorylation sites or whether RalB affects these. We identified 14 phosphorylation sites of RalBP1 in human bladder carcinoma UMUC-3 and embryonic kidney derived 293T cells. The phosphorylated residues are concentrated at the N-terminus. Ten of the first 100 amino acids of the primary structure were phosphorylated. Nine were serine residues, and one a threonine. We evaluated the effect of RalB overexpression on RalBP1 phosphorylation and found the largest change in phosphorylation status at S463 and S645. Further characterization of these sites will provide novel insights on RalBP1 biology, its functional relationship to RalB and possible avenues for therapeutic intervention.

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.

POB1 over-expression inhibits RLIP76-mediated transport of glutathione-conjugates, drugs and promotes apoptosis

RLIP76 (RALBP1) is a Ral-binding nucleotidase which functions as an energy-dependent transporter for glutathione (GSH)-conjugates as well as structurally unrelated xenobiotics. Partner of RALBP1 (POB1), also referred to as REPS2, was identified as the human RLIP76-binding protein, which contains a coiled-coil C-terminal region that binds with the RLIP76. Recent studies show that over-expression of POB1 in prostate cancer cells induces apoptosis. In present studies, we have purified POB1 and one of its deletion mutants POB1(1-512) (lacking the RLIP76-binding domain), and examined their effect on the transport activity of RLIP76. Both doxorubicin and a model GSH-conjugate, dinitrophenyl-S-glutathione (DNP-SG), transport were inhibited by POB1 in a concentration-dependent manner but not by POB1(1-512), lacking RLIP76-binding site. Liposomal delivery of recombinant POB1 to H358 (NSCLC) cancer cells caused apoptosis in a concentration-dependent manner, whereas the POB1 mutant deficient in RLIP76-binding site did not exert this effect. Augmentation of cellular POB1 resulted in increased intracellular DOX-accumulation as well as decreased rate of efflux from cells. These results show for the first time that POB1 can regulate the transport function of RLIP76 and are consistent with our previous studies showing that inhibition of RLIP76 induces apoptosis in cancer cells through the accumulation of endogenously formed GSH-conjugates.

Antioxidant role of glutathione S-transferases: protection against oxidant toxicity and regulation of stress-mediated apoptosis

It has been known that glutathione S-transferases (GSTs) can reduce lipid hydroperoxides through their Se-independent glutathione peroxidase activity and that these enzymes can also detoxify lipid peroxidation end products such as 4-hydroxynonenal (4-HNE). In this article, recent studies suggesting that the Alpha class GSTs provide a formidable defense against oxidative stress are critically evaluated and the role of these enzymes in the regulation of oxidative stress-mediated signaling is reviewed. Available evidence from earlier studies together with results of recent studies in our laboratories strongly suggests that lipid peroxidation products, particularly hydroperoxides and 4-HNE, are involved in the mechanisms of stress-mediated signaling and that it can be modulated by the Alpha class GSTs through the regulation of the intracellular concentrations of 4-HNE.

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.

RLIP76, a novel transporter catalyzing ATP-dependent efflux of xenobiotics

Transport of xenobiotics and their metabolites by ATP-binding cassette (ABC) transporters particularly P-glycoprotein (Pgp) and the multidrug resistance associated protein (MRP1) has been extensively studied during last decade. Our recent studies demonstrate that RLIP76, a previously known GTPase-activating protein catalyzes ATP-dependent, uphill transport of anionic glutathione conjugates as well as of weakly cationic anthracyclines including doxorubicin (Adriamycin), a widely used drug in cancer chemotherapy. RLIP76 has inherent ATPase activity, which is stimulated by doxorubicin and glutathione conjugates. RLIP76 does not meet the criteria for classical ABC proteins such as MRP1 or Pgp, but similar to ABC proteins, it has two ATP-binding sequences, (69)GKKKGK(74) and (418)GGIKDLSK(425). Mutations in these sequences abrogate its ATP-binding, ATPase activity, and transport function. Purified RLIP76 when reconstituted in proteoliposomes mediates ATP-dependent saturable transport of doxorubicin and glutathione conjugates. Transfection of K562 cells with RLIP76 confers these cells resistance to doxorubicin and 4-hydroxynonenal. Cells enriched with RLIP76 also acquire resistance to radiation toxicity. RLIP76 also catalyzes the transport of physiologic ligands such as leukotrienes (LTC4) and the conjugate of 4-hydroxynonenal and glutathione. In some cells (e.g., erythrocytes and lung cancer cells), the majority of transport activity for Adriamycin and glutathione conjugates including LTC4 is accounted for by RLIP76. These studies strongly suggest that RLIP76-mediated transport of organic ions has physiological and toxicological relevance and that it may play an important role in the mechanism of drug resistance.

Accelerated metabolism and exclusion of 4-hydroxynonenal through induction of RLIP76 and hGST5.8 is an early adaptive response of cells to heat and oxidative stress

To explore the role of lipid peroxidation (LPO) products in the initial phase of stress mediated signaling, we studied the effect of mild, transient oxidative or heat stress on parameters that regulate the cellular concentration of 4-hydroxynonenal (4-HNE). When K562 cells were exposed to mild heat shock (42 degrees C, 30 min) or oxidative stress (50 microM H2O2, 20 min) and allowed to recover for 2 h, there was a severalfold induction of hGST5.8, which catalyzes the formation of glutathione-4-HNE conjugate (GS-HNE), and RLIP76, which mediates the transport of GS-HNE from cells (Awasthi, S., Cheng, J., Singhal, S. S., Saini, M. K., Pandya, U., Pikula, S., Bandorowicz-Pikula, J., Singh, S. V., Zimniak, P., and Awasthi, Y. C. (2000) Biochemistry 39, 9327-9334). Enhanced LPO was observed in stressed cells, but the major antioxidant enzymes and HSP70 remained unaffected. The stressed cells showed higher GS-HNE-conjugating activity and increased efflux of GS-HNE. Stress-pre-conditioned cells with induced hGST5.8 and RLIP76 acquired resistance to 4-HNE and H2O2-mediated apoptosis by suppressing a sustained activation of c-Jun N-terminal kinase and caspase 3. The protective effect of stress pre-conditioning against apoptosis was abrogated by coating the cells with anti-RLIP76 IgG, which inhibited the efflux of GS-HNE from cells, indicating that the cells acquired resistance to apoptosis by metabolizing and excluding 4-HNE at a higher rate. Induction of hGST5.8 and RLIP76 by mild, transient stress and the resulting resistance of stress-pre-conditioned cells to apoptosis appears to be a general phenomenon since it was not limited to K562 cells but was also evident in lung cancer cells, H-69, H-226, human leukemia cells, HL-60, and human retinal pigmented epithelial cells. These results strongly suggest a role of LPO products, particularly 4-HNE, in the initial phase of stress mediated signaling.

RLIP76 is the major ATP-dependent transporter of glutathione-conjugates and doxorubicin in human erythrocytes

We have recently demonstrated that RLIP76, a Ral-binding GTPase activating protein mediates ATP-dependent transport of glutathione (GSH) conjugates of electrophiles (GS-E) as well as doxorubicin (DOX), and that it is identical with DNP-SG ATPase, a GS-E transporter previously characterized by us in erythrocyte membranes (Awasthi et al. Biochemistry 39, 9327-9334). Multidrug resistance-associated protein (MRP1) belonging to the family of the ABC-transporters has also been suggested to be a GS-E transporter in human erythrocytes. Using immunological approaches, the present studies were designed to elucidate the relative contributions of RLIP76, MRP1, and P-glycoprotein (Pgp), in the ATP-dependent transport of GS-E and DOX in human erythrocytes. In Western blot analyses using antibodies against RLIP76, a strong expression of RLIP76 was observed in erythrocytes. Immunohistochemical studies using a fluorescent probe showed association of RLIP76 with erythrocyte membrane, which was consistent with its transport function. Neither MRP1 nor Pgp were detected in erythrocytes when the antibodies against MRP1 or Pgp were used. In erythrocyte inside-out vesicles (IOVs) coated with antibodies against RLIP76, a dose-dependent inhibition of the ATP-dependent transport of DOX and GS-E, including S-(dinitrophenyl)glutathione (DNP-SG), leukotriene C(4), and the GSH conjugate of 4-hydroxynonenal, was observed with a maximal inhibition of about 70%. On the contrary, in the IOVs coated with the antibodies against MRP1 or Pgp no significant inhibition of the ATP-dependent transport of these compounds was observed. These findings suggest that RLIP76 is the major ATP-dependent transporter of GS-E and DOX in human erythrocytes.

Role of glutathione S-transferases in protection against lipid peroxidation. Overexpression of hGSTA2-2 in K562 cells protects against hydrogen peroxide-induced apoptosis and inhibits JNK and caspase 3 activation

The physiological significance of the selenium-independent glutathione peroxidase (GPx) activity of glutathione S-transferases (GSTs), associated with the major Alpha class isoenzymes hGSTA1-1 and hGSTA2-2, is not known. In the present studies we demonstrate that these isoenzymes show high GPx activity toward phospholipid hydroperoxides (PL-OOH) and they can catalyze GSH-dependent reduction of PL-OOH in situ in biological membranes. A major portion of GPx activity of human liver and testis toward phosphatidylcholine hydroperoxide (PC-OOH) is contributed by the Alpha class GSTs. Overexpression of hGSTA2-2 in K562 cells attenuates lipid peroxidation under normal conditions as well as during the oxidative stress and confers about 1.5-fold resistance to these cells from H(2)O(2) cytotoxicity. Treatment with 30 microm H(2)O(2) for 48 h or 40 microm PC-OOH for 8 h causes apoptosis in control cells, whereas hGSTA2-2-overexpressing cells are protected from apoptosis under these conditions. In control cells, H(2)O(2) treatment causes an early (within 2 h), robust, and persistent (at least 24 h) activation of JNK, whereas in hGSTA2-2-overexpressing cells, only a slight activation of JNK activity is observed at 6 h which declines to basal levels within 24 h. Caspase 3-mediated poly(ADP-ribose) polymerase cleavage is also inhibited in cells overexpressing hGSTA2-2. hGSTA2 transfection does not affect the function of antioxidant enzymes including GPx activity toward H(2)O(2) suggesting that the Alpha class GSTs play an important role in regulation of the intracellular concentrations of the lipid peroxidation products that may be involved in the signaling mechanisms of apoptosis.

Different metabolizing ability of thiol reactants in human and rat blood: biochemical and pharmacological implications

The effect of oxidants, electrophiles, and NO donors in rat or human erythrocytes was analyzed to investigate the influence of protein sulfhydryl groups on the metabolism of these thiol reactants. Oxidant-evoked alterations in thiolic homeostasis were significantly different in the two models; large amounts of glutathione protein mixed disulfides were produced in rat but not in human erythrocytes by treatment with hydroperoxides or diamide. The disappearance of all forms of glutathione (reduced, disulfide, protein mixed disulfide) was induced by menadione only in human erythrocytes. The treatment of rat red blood cells with electrophiles produced glutathione S-conjugates to a much lower extent than in human red blood cells; GSH was only minimally depleted in rat red blood cells. The NO donor S-nitrosocysteine induced a rapid transnitrosation reaction with hemoglobin in rat erythrocytes producing high levels of S-nitrosohemoglobin; this reaction in human red blood cells was negligible. All drugs were cleared more rapidly in rat than in human erythrocytes. Unlike human Hb, rat hemoglobin contains three families of protein SH groups; one of these located at position beta125 is directly implicated in the metabolism of thiol reactants. This is thought to influence significantly the biochemical, pharmacological, and toxicological effects of some drugs.

Glutathione-dependent biotransformation of 1-chloro-2,4-dinitrobenzene in arterial and venous blood of the striped bass (Morone saxitilis)

Glutathione (GSH) mediated metabolism of a model electrophile, 1-chloro-2,4-dintrobenzene (CDNB), by striped bass (Morone saxitilis) whole blood and reconstituted erythrocytes equilibrated to 'arterial' and 'venous' blood gas concentrations was assessed. Reduced GSH was found to be an abundant nucleophile (2-2.4 mM) in striped bass erythrocytes. Incubation of whole blood and intact erythrocytes, reconstituted in phosphate buffer solution (PBS), with CDNB resulted in rapid depletion of >98% of cellular GSH levels and rapid, though not stoichiometric, accumulation of a glutathione S-conjugate, 2,4-dintrophenyl S-glutathione (DNP-SG) within the 60 min exposure period. The GSH depletion half-lives were 11.0+/-2.6 min and 10.3+/-2.0 min, respectively, for 'arterial' and 'venous' trials using whole blood compared to 6.4+/-1.8 min and 7.1+/-0.9 min, respectively, for erythrocytes in PBS. The conjugate was not metabolized further within erythrocytes, but was slowly transported linearly from the cells to the plasma or PBS medium. Reconstitution of erythrocytes in PBS produced significantly greater rates of CDNB biotransformation compared to whole blood, although the total amount of DNP-SG formed per unit of GSH depleted was significantly lower. Differences may be attributed to altered rates of CDNB membrane permeation and/or efflux and changes in the rate of GSH oxidation in the presence of plasma versus PBS. Whole blood and erythrocytes in PBS equilibrated to 'arterial' and 'venous' blood gas concentrations showed no significant difference in rates of GSH depletion, DNP-SG formation, or conjugate efflux. GSH concentration decreased after exposure to CDNB and was not regenerated within the 60 min incubation period. GSH reacted nonenzymatically with CDNB at a very slow and relatively insignificant rate under physiological blood gas concentrations. Enzymatic biotransformation of lipophilic electrophiles by erythrocytic GSH results in rapid depletion of the protective thiol and constitutes the first step in mercapturic acid formation. This investigation provides strong evidence for the presence of significant levels of glutathione S-transferase in striped bass erythrocytes.

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.