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

Drug transport by red blood cells

This review focuses on the role of human red blood cells (RBCs) as drug carriers. First, a general introduction about RBC physiology is provided, followed by the presentation of several cases in which RBCs act as natural carriers of drugs. This is due to the presence of several binding sites within the same RBCs and is regulated by the diffusion of selected compounds through the RBC membrane and by the presence of influx and efflux transporters. The balance between the influx/efflux and the affinity for these binding sites will finally affect drug partitioning. Thereafter, a brief mention of the pharmacokinetic profile of drugs with such a partitioning is given. Finally, some examples in which these natural features of human RBCs can be further exploited to engineer RBCs by the encapsulation of drugs, metabolites, or target proteins are reported. For instance, metabolic pathways can be powered by increasing key metabolites (i.e., 2,3-bisphosphoglycerate) that affect oxygen release potentially useful in transfusion medicine. On the other hand, the RBC pre-loading of recombinant immunophilins permits increasing the binding and transport of immunosuppressive drugs. In conclusion, RBCs are natural carriers for different kinds of metabolites and several drugs. However, they can be opportunely further modified to optimize and improve their ability to perform as drug vehicles.

Anticancer Activity of Ω-6 Fatty Acids through Increased 4-HNE in Breast Cancer Cells

Simple Summary

Epidemiological evidence suggests that breast cancer risk is lowered by Ω-3 and increased by Ω-6 polyunsaturated fatty acids (PUFAs). Paradoxically, the Ω-6 PUFA metabolite 4-hydroxynonenal (4-HNE) inhibits cancer cell growth. This duality prompted us to study whether arachidonic acid (AA) would enhance doxorubicin (dox) cytotoxicity towards breast cancer cells. We found that supplementing AA or inhibiting 4-HNE metabolism potentiated doxorubicin (dox) toxicity toward Her2-dependent breast cancer but spared myocardial cells. Our results suggest that Ω-6 PUFAs could improve outcomes of dox chemotherapy in Her2-overexpressing breast cancer.

Abstract

Her2-amplified breast cancers resistant to available Her2-targeted therapeutics continue to be a challenge in breast cancer therapy. Dox is the mainstay of chemotherapy of all types of breast cancer, but its usefulness is limited by cumulative cardiotoxicity. Because oxidative stress caused by dox generates the pro-apoptotic Ω-6 PUFA metabolite 4-hydroxynonenal (4-HNE), we surmised that Ω-6 PUFAs would increase the effectiveness of dox chemotherapy. Since the mercapturic acid pathway enzyme RALBP1 (also known as RLIP76 or Rlip) that limits cellular accumulation of 4-HNE also mediates dox resistance, the combination of Ω-6 PUFAs and Rlip depletion could synergistically improve the efficacy of dox. Thus, we studied the effects of the Ω-6 PUFA arachidonic acid (AA) and Rlip knockdown on the antineoplastic activity of dox towards Her2-amplified breast cancer cell lines SK-BR-3, which is sensitive to Her2 inhibitors, and AU565, which is resistant. AA increased lipid peroxidation, 4-HNE generation, apoptosis, cellular dox concentration and dox cytotoxicity in both cell lines while sparing cultured immortalized cardiomyocyte cells. The known functions of Rlip including clathrin-dependent endocytosis and dox efflux were inhibited by AA. Our results support a model in which 4-HNE generated by AA overwhelms the capacity of Rlip to defend against apoptosis caused by dox or 4-HNE. We propose that Ω-6 PUFA supplementation could improve the efficacy of dox or Rlip inhibitors for treating Her2-amplified breast cancer.

Parallel Rap1>RalGEF>Ral and Ras signals sculpt the C. elegans nervous system

Ras is the most commonly mutated oncogene in humans and uses three oncogenic effectors: Raf, PI3K, and RalGEF activation of Ral. Understanding the importance of RalGEF>Ral signaling in cancer is hampered by the paucity of knowledge about their function in animal development, particularly in cell movements. We found that mutations that disrupt function of RalGEF or Ral enhance migration phenotypes of mutants for genes with established roles in cell migration. We used as a model the migration of the canal associated neurons (CANs), and validated our results in HSN cell migration, neurite guidance, and general animal locomotion. These functions of RalGEF and Ral are specific to their control of Ral signaling output rather than other published functions of these proteins. In this capacity Ral functions cell autonomously as a permissive developmental signal. In contrast, we observed Ras, the canonical activator of RalGEF>Ral signaling in cancer, to function as an instructive signal. Furthermore, we unexpectedly identified a function for the close Ras relative, Rap1, consistent with activation of RalGEF>Ral. These studies define functions of RalGEF>Ral, Rap1 and Ras signaling in morphogenetic processes that fashion the nervous system. We have also defined a model for studying how small GTPases partner with downstream effectors. Taken together, this analysis defines novel molecules and relationships in signaling networks that control cell movements during development of the nervous system.

Increased RLIP76 expression in IDH1 wild‑type glioblastoma multiforme is associated with worse prognosis

Mutation of the isocitrate dehydrogenase (IDH) gene is regarded a novel indicator for the prognosis of patients with glioma. However, the role of the IDH1 gene mutations in carcinogenesis and the mechanisms underlying their function in glioblastoma multiforme (GBM) remain unknown. The present study aimed to determine whether the association of RLIP76 with the different IDH1 mutational status could serve as a putative biomarker for improving disease prognosis. Quantitative PCR, western blotting and immunohistochemical staining assays were used to investigate the expression levels of RLIP76 in 124 patients with GBM with different IDH1 mutational status. In addition, the association between RLIP76 expression, IDH1 mutational status and clinicopathological characteristics was investigated. The effects of RLIP76 expression and IDH1 mutational status on cell proliferation, cell apoptosis, and cell signaling were examined by Cell Counting Kit-8, flow cytometry and western blot assays, respectively. The data demonstrated that IDH1 wild-type (IDH1^Wt) patients with low RLIP76 expression exhibited improved overall and progression-free survival. This effect was not observed in patients with IDH1 mutant (IDH1^Mut) GBM. In vitro assays demonstrated that knockdown of IDH1 or overexpression of the IDH1 R132H mutation suppressed cell proliferation and promoted cell apoptosis in U87 glioma cells. Mechanistic studies further indicated that although the IDH1 R132H mutant phenotype exhibited similar antitumor effects on GBM cells as those observed with the IDH1 knockdown, it acted via a different mechanism with regard to the regulation of the apoptosis signaling pathway. IDH1 R132H mutant cells promoted p53-induced apoptosis, while the IDH1 knockdown inhibited the RLIP76-dependent apoptotic pathway in glioma cells. The findings of the present study provided insight to the contribution of IDH1 mutation in the development of GBM and indicated that RLIP76 may be considered as a prognostic biomarker of IDH1^Wt GBM.

Glutathione: subcellular distribution and membrane transport 1

Glutathione (γ-l-glutamyl-l-cysteinylglycine) is a small tripeptide found at millimolar concentrations in nearly all eukaryotes as well as many prokaryotic cells. Glutathione synthesis is restricted to the cytosol in animals and fungi and to the cytosol and plastids in plants. Nonetheless, glutathione is found in virtually all subcellular compartments. This implies that transporters must exist that facilitate glutathione transport into and out of the various subcellular compartments. Glutathione may also be exported and imported across the plasma membrane in many cells. However, in most cases, the molecular identity of these transporters remains unclear. Whilst glutathione transport is essential for the supply and replenishment of subcellular glutathione pools, recent evidence supports a more active role for glutathione transport in the regulation of subcellular glutathione redox homeostasis. However, our knowledge of glutathione redox homeostasis at the level of specific subcellular compartments remains remarkably limited and the role of glutathione transport remains largely unclear. In this review, we discuss how new tools and techniques have begun to yield insights into subcellular glutathione distribution and glutathione redox homeostasis. In particular, we discuss the known and putative glutathione transporters and examine their contribution to the regulation of subcellular glutathione redox homeostasis.

ERCC1-deficient cells and mice are hypersensitive to lipid peroxidation

Lipid peroxidation (LPO) products are relatively stable and abundant metabolites, which accumulate in tissues of mammals with aging, being able to modify all cellular nucleophiles, creating protein and DNA adducts including crosslinks. Here, we used cells and mice deficient in the ERCC1-XPF endonuclease required for nucleotide excision repair and the repair of DNA interstrand crosslinks to ask if specifically LPO-induced DNA damage contributes to loss of cell and tissue homeostasis. Ercc1-/- mouse embryonic fibroblasts were more sensitive than wild-type (WT) cells to the LPO products: 4-hydroxy-2-nonenal (HNE), crotonaldehyde and malondialdehyde. ERCC1-XPF hypomorphic mice were hypersensitive to CCl4 and a diet rich in polyunsaturated fatty acids, two potent inducers of endogenous LPO. To gain insight into the mechanism of how LPO influences DNA repair-deficient cells, we measured the impact of the major endogenous LPO product, HNE, on WT and Ercc1-/- cells. HNE inhibited proliferation, stimulated ROS and LPO formation, induced DNA base damage, strand breaks, error-prone translesion DNA synthesis and cellular senescence much more potently in Ercc1-/- cells than in DNA repair-competent control cells. HNE also deregulated base excision repair and energy production pathways. Our observations that ERCC1-deficient cells and mice are hypersensitive to LPO implicates LPO-induced DNA damage in contributing to cellular demise and tissue degeneration, notably even when the source of LPO is dietary polyunsaturated fats.

Microarray and Co-expression Network Analysis of Genes Associated with Acute Doxorubicin Cardiomyopathy in Mice

Clinical use of doxorubicin (DOX) in cancer therapy is limited by its dose-dependent cardiotoxicity. But molecular mechanisms underlying this phenomenon have not been well defined. This study was to investigate the effect of DOX on the changes of global genomics in hearts. Acute cardiotoxicity was induced by giving C57BL/6J mice a single intraperitoneal injection of DOX (15 mg/kg). Cardiac function and apoptosis were monitored using echocardiography and TUNEL assay at days 1, 3 and 5. Myocardial glucose and ATP levels were measured. Microarray assays were used to screen gene expression profiles in the hearts at day 5, and the results were confirmed with qPCR analysis. DOX administration caused decreased cardiac function, increased cardiomyocyte apoptosis and decreased glucose and ATP levels. Microarrays showed 747 up-regulated genes and 438 down-regulated genes involved in seven main functional categories. Among them, metabolic pathway was the most affected by DOX. Several key genes, including 2,3-bisphosphoglycerate mutase (Bpgm), hexokinase 2, pyruvate dehydrogenase kinase, isoenzyme 4 and fructose-2,6-bisphosphate 2-phosphatase, are closely related to glucose metabolism. Gene co-expression networks suggested the core role of Bpgm in DOX cardiomyopathy. These results obtained in mice were further confirmed in cultured cardiomyocytes. In conclusion, genes involved in glucose metabolism, especially Bpgm, may play a central role in the pathogenesis of DOX-induced cardiotoxicity.

The RLIP76 N-terminus binds ARNO to regulate PI 3-kinase, Arf6 and Rac signaling, cell spreading and migration

RLIP76 is a multifunctional protein involved in tumor growth and angiogenesis, and a promising therapeutic target in many cancers. RLIP76 harbors docking sites for many proteins, and we have found that it interacts with ARNO, a guanine nucleotide exchange factor for Arf6, and that RLIP76 regulates activation of Rac1 via Arf6, and regulates cell spreading and migration in an ARNO and Arf6-dependent manner. Here we show that ARNO interacts with the RLIP76 N-terminal domain, and this domain was required for RLIP76-dependent cell spreading and migration. We identified two sites in the RLIP76 N-terminus with differential effects on ARNO binding and downstream signaling: Ser29/Ser30 and Ser62. Ser29/30 mutation to Alanine inhibited ARNO interaction and was sufficient to block RLIP76-dependent cell spreading and migration, as well as RLIP76-dependent Arf6 activation. In contrast, RLIP76(S62A) interacted with ARNO and supported Arf6 activation. However, both sets of mutations blocked Rac1 activation. RLIP76-mediated Rac and Arf6 activation required PI3K activity. S29/30A mutations inhibited RLIP76-dependent PI3K activation, but S62A mutation did not. Together these results show that ARNO interaction with the RLIP76 N-terminus regulates cell spreading and motility via PI3K and Arf6, independent of RLIP76 control of Rac.

Pharmacokinetics and pharmacogenomics of daunorubicin in children: a report from the Children's Oncology Group

PURPOSE

We explored the impact of obesity, body composition, and genetic polymorphisms on the pharmacokinetics (PK) of daunorubicin in children with cancer.

PATIENTS AND METHODS

Patients ≤21 years receiving daunorubicin as an infusion of any duration <24 h for any type of cancer were eligible. Plasma drug concentrations were measured by high-performance liquid chromatography. Body composition was measured by dual-energy X-ray absorptiometry. Obesity was defined as a BMI >95% for age or as body fat >30%. NONMEM was used to perform PK model fitting. The Affymetrix DMET chip was used for genotyping. The impact of genetic polymorphisms was investigated using SNP/haplotype association analysis with estimated individual PK parameters.

RESULTS

A total of 107 subjects were enrolled, 98 patients had PK sampling, and 50 patients underwent DNA analysis. Population estimates for daunorubicin clearance and volume of distribution were 116 L/m(2)/h ± 14% and 68.1 L/m(2) ± 24%, respectively. Apparent daunorubicinol clearance and volume of distribution were 26.8 L/m(2)/h ± 5.6% and 232 L/m(2) ± 10%, respectively. No effect of body composition or obesity was observed on PK. Forty-four genes with variant haplotypes were tested for association with PK. FMO3-H1/H3 genotype was associated with lower daunorubicin clearance than FMO3-H1/H1, p = 0.00829. GSTP1*B/*B genotype was also associated with lower daunorubicin clearance compared to GSTP1*A/*A, p = 0.0347. However, neither of these associations was significant after adjusting for multiple testing by either Bonferroni or false discovery rate correction.

CONCLUSIONS

We did not detect an effect of body composition or obesity on daunorubicin PK. We found suggestive associations between FMO3 and GSTP1 haplotypes with daunorubicin PK that could potentially affect efficacy and toxicity.

Mechanisms tagging senescent red blood cells for clearance in healthy humans

This review focuses on the analysis and evaluation of the diverse senescence markers suggested to prime red blood cells (RBC) for clearance in humans. These tags develop in the course of biochemical and structural alterations accompanying RBC aging, as the decrease of activities of multiple enzymes, the gradual accumulation of oxidative damage, the loss of membrane in form of microvesicles, the redistribution of ions and alterations in cell volume, density, and deformability. The actual tags represent the penultimate galactosyl residues, revealed by desialylation of glycophorins, or the aggregates of the anion exchanger (band 3 protein) to which anti-galactose antibodies bind in the first and anti-band 3 naturally occurring antibodies (NAbs) in the second case. While anti-band 3 NAbs bind to the carbohydrate-free portion of band 3 aggregates in healthy humans, induced anti-lactoferrin antibodies bind to the carbohydrate-containing portion of band 3 and along with anti-band 3 NAbs may accelerated clearance of senescent RBC in patients with anti-neutrophil cytoplasmic antibodies (ANCA). Exoplasmically accessible phosphatidylserine (PS) and the alterations in the interplay between CD47 on RBC and its receptor on macrophages, signal regulatory protein alpha (SIRPalpha protein), were also reported to induce erythrocyte clearance. We discuss the relevance of each mechanism and analyze the strength of the data.

Role of 4-hydroxynonenal and its metabolites in signaling

Available evidence from a multitude of studies on the effects of 4-hydroxynonenal (HNE) on cellular processes seem to converge on some common themes: (i) concentration-dependent opposing effects of HNE on key signaling components (e.g. protein kinase C, adenylate cyclase) predict that certain constitutive levels of HNE may be needed for normal cell functions - lowering of this constitutive HNE level in cells promotes proliferative machinery while an increase in this level promotes apoptotic signaling; (ii) HNE is a common denominator in stress-induced apoptosis caused by H(2)O(2), superoxide, UV, heat or oxidant chemicals such as doxorubicin; and (iii) HNE can modulate ligand-independent signaling by membrane receptors such as EGFR or Fas (CD95) and may act as a sensor of external stimuli for eliciting stress-response. Against a backdrop of various reported effects of HNE, in vitro and in vivo, we have critically evaluated the above mentioned hypotheses suggesting a key role of HNE in signaling.

RLIP76 Gene Variants are not Associated with Drug Response in Turkish Epilepsy Patients

Approximately 30% of epileptic patients remain untreated, in spite of trials with maximum tolerable doses of more than one drug. The RalA binding protein 1 (RALBP1/RLIP76), a multifunctional, anti-apoptot-ic, multidrug transporter protein, has been proposed as being responsible for the drug resistance mechanism in epilepsy. We have investigated polymorphic differences in the coding regions and exonintron boundaries of the RLIP76 gene, between 146 refractory and 155 non refractory epileptic patients in Turkey, using denaturing high performance liquid chromatography (HPLC) and sequencing analysis techniques. We have detected the following sequence variants: c.160-4G>A, c.187C>G, c.1562-38G>A, c.1670+107G>A, c.1670+93G>A, c.1670+96G>A, c.1670+100C>T, c.1670+130C>T, c.1670+131G>C, c.1670+140 G>C, and found no statistically significant correlation between allele frequencies and drug response status. We conclude that sequence variants of this gene are not involved in drug resistance in epilepsy.

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.

Role of 4-hydroxynonenal in chemopreventive activities of sulforaphane

Chemoprevention of cancer via herbal and dietary supplements is a logical approach to combating cancer and currently it is an attractive area of research investigation. Over the years, isothiocyanates, such as sulforaphane (SFN) found in cruciferous vegetables, have been advocated as chemopreventive agents, and their efficacy has been demonstrated in cell lines and animal models. In vivo studies with SFN suggest that in addition to protecting normal healthy cells from environmental carcinogens, it also exhibits cytotoxicity and apoptotic effects against various cancer cell types. Among several mechanisms for the chemopreventive activity of SFN against chemical carcinogenesis, its effect on drug-metabolizing enzymes that cause activation/neutralization of carcinogenic metabolites is well established. Recent studies suggest that SFN exerts its selective cytotoxicity to cancer cells via reactive oxygen species-mediated generation of lipid peroxidation products, particularly 4-hydroxynonenal (HNE). Against the background of the known biochemical effects of SFN on normal and cancer cells, in this article we review the underlying molecular mechanisms responsible for the overall chemopreventive effects of SFN, focusing on the role of HNE in these mechanisms, which may also contribute to its selective cytotoxicity to cancer cells.

Gender disparity in susceptibility to oxidative stress and apoptosis induced by autoantibodies specific to RLIP76 in vascular cells

AIM

Ral-binding protein 1 (RLIP76) is a cell surface protein that catalyzes the extrusion from the cell of reduced glutathione (GSH) conjugates. We recently demonstrated the presence of serum antibodies to RLIP76 (aaRLIP76) in patients with immune-mediated diseases characterized by vascular dysfunction. The aim of this work was to analyze the possible implication of gender in this issue, investigating the effects of aaRLIP76 in rat vascular smooth muscle cells and human endothelial cells from males and females.

RESULTS

We observed that, after aaRLIP76 treatment, vascular cells from females showed a significantly higher susceptibility to the disturbance of intracellular redox balance, in terms of H(2)O(2) and O(2)(*) production, 4-hydroxy-t-2,3-nonenal and GSH levels, C-Jun NH2 kinase signaling activation, and apoptosis in comparison with cells from males. Interestingly, under mild oxidative stress (H(2)O(2) 30 μm for 30 min), these sex-associated differences became significantly more pronounced. Experiments carried out in the presence of sex hormones in the culture medium clearly suggested that estrogens could significantly increase the susceptibility of cells from females to the effects of aaRLIP76, whereas cells from males appeared unaffected.

INNOVATION

These results open a new perspective in the gender-dependent pathogenic mechanisms of autoimmune diseases characterized by vascular dysfunction.

CONCLUSIONS

Altogether these results suggest that the impairment of RLIP76 by aaRLIP76 can play a role in the damage of vascular cells from females, contributing to the gender-associated pathogenesis of immune-mediated vascular diseases.

Targeting p53-null neuroblastomas through RLIP76

The search for p53-independent mechanism of cancer cell killing is highly relevant to pediatric neuroblastomas, where successful therapy is limited by its transformation into p53-mutant and a highly drug-resistant neoplasm. Our studies on the drug-resistant p53-mutant as compared with drug-resistant p53 wild-type neuroblastoma revealed a novel mechanism for resistance to apoptosis: a direct role of p53 in regulating the cellular concentration of proapoptotic alkenals by functioning as a specific and saturable allosteric inhibitor of the alkenal-glutathione conjugate transporter, RLIP76. The RLIP76-p53 complex was showed by both immunoprecipitation analyses of purified proteins and immunofluorescence analysis. Drug transport studies revealed that p53 inhibited both basal and PKCα-stimulated transport of glutathione conjugates of 4HNE (GSHNE) and doxorubicin. Drug resistance was significantly greater for p53-mutant as compared with p53 wild-type neuroblastoma cell lines, but both were susceptible to depletion of RLIP76 by antisense alone. In addition, inhibition of RLIP76 significantly enhanced the cytotoxicity of cisplatin. Taken together, these studies provide powerful evidence for a novel mechanism for drug and apoptosis resistance in p53-mutant neuroblastoma, based on a model of regulation of p53-induced apoptosis by RLIP76, where p53 is a saturable and specific allosteric inhibitor of RLIP76, and p53 loss results in overexpression of RLIP76; thus, in the absence of p53, the drug and glutathione-conjugate transport activities of RLIP76 are enhanced. Most importantly, our findings strongly indicate RLIP76 as a novel target for therapy of drug-resistant and p53-mutant neuroblastoma.

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.

Myocardial ischaemia inhibits mitochondrial metabolism of 4-hydroxy-trans-2-nonenal

Myocardial ischaemia is associated with the generation of lipid peroxidation products such as HNE (4-hydroxy-trans-2-nonenal); however, the processes that predispose the ischaemic heart to toxicity by HNE and related species are not well understood. In the present study, we examined HNE metabolism in isolated aerobic and ischaemic rat hearts. In aerobic hearts, the reagent [(3)H]HNE was glutathiolated, oxidized to [(3)H]4-hydroxynonenoic acid, and reduced to [(3)H]1,4-dihydroxynonene. In ischaemic hearts, [(3)H]4-hydroxynonenoic acid formation was inhibited and higher levels of [(3)H]1,4-dihydroxynonene and [(3)H]GS-HNE (glutathione conjugate of HNE) were generated. Metabolism of [(3)H]HNE to [(3)H]4-hydroxynonenoic acid was restored upon reperfusion. Reperfused hearts were more efficient at metabolizing HNE than non-ischaemic hearts. Ischaemia increased the myocardial levels of endogenous HNE and 1,4-dihydroxynonene, but not 4-hydroxynonenoic acid. Isolated cardiac mitochondria metabolized [(3)H]HNE primarily to [(3)H]4-hydroxynonenoic acid and minimally to [(3)H]1,4-dihydroxynonene and [(3)H]GS-HNE. Moreover, [(3)H]4-hydroxynonenoic acid was extruded from mitochondria, whereas other [(3)H]HNE metabolites were retained in the matrix. Mitochondria isolated from ischaemic hearts were found to contain 2-fold higher levels of protein-bound HNE than the cytosol, as well as increased [(3)H]GS-HNE and [(3)H]1,4-dihydroxynonene, but not [(3)H]4-hydroxynonenoic acid. Mitochondrial HNE oxidation was inhibited at an NAD(+)/NADH ratio of 0.4 (equivalent to the ischaemic heart) and restored at an NAD(+)/NADH ratio of 8.6 (equivalent to the reperfused heart). These results suggest that HNE metabolism is inhibited during myocardial ischaemia owing to NAD(+) depletion. This decrease in mitochondrial metabolism of lipid peroxidation products and the inability of the mitochondria to extrude HNE metabolites could contribute to myocardial ischaemia/reperfusion injury.

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.

The determination of glutathione-4-hydroxynonenal (GSHNE), E-4-hydroxynonenal (HNE), and E-1-hydroxynon-2-en-4-one (HNO) in mouse liver tissue by LC-ESI-MS

Glutathione (GSH) conjugation of 4-hydroxy-2(E)-nonenal (HNE) is an efficient means of cellular detoxification. HNE is a byproduct of lipid peroxidation which has shown toxicity but also signaling roles. E-1-hydroxynon-2-en-4-one (HNO) is another byproduct of lipid peroxidation which has the same molecular weight as HNE. This study presents the LC-MS detection of GS-HNE, HNE, and HNO in tissue samples without derivatization and with minimal sample preparation. Tissue samples were taken from wild-type mice and knock-out mice, which have been bred without the RLIP76 transfer protein. Extraction procedures were developed to determine GS-HNE and HNE levels in the mouse liver tissue. A gradient elution LC-MS method was developed for GS-HNE analysis using electrospray ionization and selected ion monitoring (SIM). The HNE/HNO method involves isocratic elution due to instability issues. Higher levels of GSHNE, HNE, and HNO were found in the knock-out animals, due to the absence of the RLIP76 transport mechanism.

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.

Autoantibodies to the C-terminal subunit of RLIP76 induce oxidative stress and endothelial cell apoptosis in immune-mediated vascular diseases and atherosclerosis

Although detection of autoantibodies in the peripheral blood from patients with immune-mediated endothelial dysfunctions has so far failed to provide tools of diagnostic or pathogenetic value, putative bioindicators include anti-endothelial cell antibodies, a heterogeneous family of antibodies that react with autoantigens expressed by endothelial cells. In this study, to identify endothelial autoantigens involved in the autoimmune processes causing endothelial damage, we screened a human microvascular endothelial cell cDNA library with sera from patients with Behçet's disease. We identified antibodies to the C-terminus of Ral binding protein1 (RLIP76), a protein that catalyzes the ATP-dependent transport of glutathione (GSH) conjugates including GSH-4-hydroxy-t-2,3-nonenal, in the serum of a significant percentage of patients with various diseases characterized by immune-mediated endothelial dysfunction, including Behçet disease, systemic sclerosis, systemic lupus erythematosus and carotid atherosclerosis. These autoantibodies increased intracellular levels of 4-hydroxy-t-2,3-nonenal, decreased levels of GSH and activated C-Jun NH2 Kinase signaling (JNK), thus inducing oxidative stress-mediated endothelial cell apoptosis. The dietary antioxidant alpha-tocopherol counteracted endothelial cell demise. These findings suggest that autoantibodies to RLIP76 play a pathogenetic role in immune-mediated vascular diseases and represent a valuable peripheral blood bioindicator of atherosclerosis and immune-mediated vascular diseases.

Transfection of HepG2 cells with hGSTA4 provides protection against 4-hydroxynonenal-mediated oxidative injury

4-Hydroxynonenal (4-HNE) is a mutagenic alpha,beta-unsaturated aldehyde produced during oxidative injury that is conjugated by several glutathione S-transferase (GST) isoforms. The alpha class human GSTA4-4 enzyme (hGSTA4-4) has a particularly high catalytic efficiency toward 4-HNE conjugation. However, hGST4-4 expression is low in most human cells and there are other aldehyde metabolizing enzymes that detoxify 4-HNE. In the current study, we determined the effect of over-expression of hGSTA4 mRNA on the sensitivity of HepG2 cells to 4-HNE injury. HepG2 cells transfected with an hGSTA4 vector construct exhibited high steady-state hGSTA4 mRNA, high GST-4-HNE catalytic activities, but lower basal glutathione (GSH) concentrations relative to insert-free vector (control) cells. Exposure to 4-HNE elicited an increase in GSH concentrations in the control and hGSTA4 cells, although the dose-response of GSH induction differed among the two cell types. Specifically, hGSTA4 cells had significantly higher GSH concentrations when exposed to 5-15 microM 4-HNE, but not at 20 microM 4-HNE, suggesting extensive GSH utilization at high concentrations of 4-HNE. The hGSTA4 cells exhibited a significant growth advantage relative to control cells in the absence of 4-HNE, and a trend towards increased growth at low dose exposures to 4-HNE. However, the hGSTA4 cells did not exhibit a growth advantage relative to control cells at higher 4-HNE exposures associated with increased GSH utilization. As expected, the hGSTA4 cells showed resistance to 4-HNE stimulated lipid peroxidation at all 4-HNE doses. In summary, our data indicates that over-expression of hGSTA4 at levels conferring high GST-4-HNE conjugating activity confers a partial growth advantage to HepG2 cells and protects against 4-HNE oxidative injury. However, the loss of proliferative capacity of hGSTA4 cells challenged with levels of 4-HNE associated with severe oxidative stress indicates a role of other aldehyde metabolizing enzymes, and/or GSH-electrophile transporter proteins, in providing full cellular protection against 4-HNE toxicity.

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.

Mitogenic and drug-resistance mediating effects of PKCalpha require RLIP76

PKCalpha-activation is a key signaling event governing cell growth, stress-resistance, and drug-resistance. Our recent studies demonstrated that DOX-resistance mediating effects of PKCalpha require the presence of RLIP76, and their concerted action is sufficient to explain intrinsic DOX-resistance of NSCLC [S.S. Singhal, D. Wickramarachchi, J. Singhal, S. Yadav, Y.C. Awasthi, et al., Determinants of differential doxorubicin sensitivity between SCLC and NSCLC. FEBS Lett. 580 (2006) 2258-2264]. Present studies were carried out to further explore the suggestion from the previous studies that the mitogenic effects of PKCalpha also require RLIP76. RLIP76-/- MEFs were resistant to PKCalpha-depletion mediated growth inhibition, as well as to the PKCalpha-dependent mitogen, phorbol 12-myristate 13-acetate (PMA). Augmenting cellular levels of RLIP76 using purified recombinant RLIP76 increased growth rate in all cells, and restored the sensitivity of RLIP76-/- MEFs to both inhibition through PKCalpha-depletion and stimulation through PMA. These results show that RLIP76 is a necessary down-stream effector for PKCalpha-mediated mitogenesis.

Mitogenic Responses of Vascular Smooth Muscle Cells to Lipid Peroxidation-derived Aldehyde 4-Hydroxy-trans-2-nonenal (HNE)

Products of lipid peroxidation such as 4-hydroxy-trans-2-nonenal (HNE) trigger multiple signaling cascades that variably affect cell growth, differentiation, and apoptosis. Because glutathiolation is a significant metabolic fate of these aldehydes, we tested the possibility that the bioactivity of HNE depends upon its conjugation with glutathione. Addition of HNE or the cell-permeable esters of glutathionyl-4-hydroxynonenal (GS-HNE) or glutathionyl-1,4-dihydroxynonene (GS-DHN) to cultures of rat aortic smooth muscle cells stimulated protein kinase C, NF-kappaB, and AP-1, and increased cell growth. The mitogenic effects of HNE, but not GS-HNE or GS-DHN, were abolished by glutathione depletion. Pharmacological inhibition or antisense ablation of aldose reductase (which catalyzes the reduction of GS-HNE to GS-DHN) prevented protein kinase C, NF-kappaB, and AP-1 stimulation and the increase in cell growth caused by HNE and GS-HNE, but not GS-DHN. The growth stimulating effect of GS-DHN was enhanced in cells treated with antibodies directed against the glutathione conjugate transporters RLIP76 (Ral-binding protein) or the multidrug resistance protein-2. Overexpression of RLIP76 abolished the mitogenic effects of HNE and its glutathione conjugates, whereas ablation of RLIP76 using RNA interference promoted the mitogenic effects. Collectively, our findings suggest that the mitogenic effects of HNE are mediated by its glutathione conjugate, which has to be reduced by aldose reductase to stimulate cell growth. These results raise the possibility that the glutathione conjugates of lipid peroxidation products are novel mediators of cell signaling and growth.

Regression of melanoma in a murine model by RLIP76 depletion

RLIP76/RALBP1 is a stress-responsive membrane protein implicated in the regulation of multiple cellular signaling pathways. It represents the predominant glutathione-conjugate transporter in cells, and our previous studies have shown that its inhibition by antibodies or depletion by short interfering RNA (siRNA) causes apoptosis in a number of cancer cell types. The present studies were done to explore the potential clinical applicability of our previous observations by comparing the relative expression of RLIP76 in cancer versus normal cell lines and to determine whether depletion of RLIP76 activity can exert cancer-specific apoptosis. RLIP76 expression was found to be significantly greater in malignant cells compared to nonmalignant cells. Inhibition of RLIP76, using antibodies towards a cell surface epitope, or depletion of RLIP76 using either siRNA or antisense phosphorothioate oligonucleotides preferentially caused apoptosis in malignant cells. More importantly, in vivo studies showed that administration of RLIP76 antibodies, siRNA, or antisense oligonucleotides to mice bearing syngeneic B16 mouse melanoma cells caused complete tumor regression within 10 days. These findings strongly suggest that RLIP76 depletion by genetic approaches or inhibition by antibodies may be a clinically viable antineoplastic therapy, particularly for melanoma.

RLIP76, a non-ABC transporter, and drug resistance in epilepsy

Background

Permeability of the blood-brain barrier is one of the factors determining the bioavailability of therapeutic drugs and resistance to chemically different antiepileptic drugs is a consequence of decreased intracerebral accumulation. The ABC transporters, particularly P-glycoprotein, are known to play a role in antiepileptic drug extrusion, but are not by themselves sufficient to fully explain the phenomenon of drug-resistant epilepsy. Proteomic analyses of membrane protein differentially expressed in epileptic foci brain tissue revealed the frequently increased expression of RLIP76/RALBP1, a recently described non-ABC multi-specific transporter. Because of a significant overlap in substrates between P-glycoprotein and RLIP76, present studies were carried out to determine the potential role of RLIP76 in AED transport in the brain.

Results

RLIP76 was expressed in brain tissue, preferentially in the lumenal surface of endothelial cell membranes. The expression was most prominent in blood brain barrier tissue from excised epileptic foci. Saturable, energy-dependent, anti-gradient transport of both phenytoin and carbamazepine were demonstrated using recombinant RLIP76 reconstituted into artificial membrane liposomes. Immunotitration studies of transport activity in crude membrane vesicles prepared from whole-brain tissue endothelium showed that RLIP76 represented the dominant transport mechanism for both drugs. RLIP76^-/- knockout mice exhibited dramatic toxicity upon phenytoin administration due to decreased drug extrusion mechanisms at the blood-brain barrier.

Conclusion

We conclude that RLIP76 is the predominant transporter of AED in the blood brain barrier, and that it may be a transporter involved in mechanisms of drug-resistant epilepsy.

Regulation of 4‐Hydroxynonenal Mediated Signaling By Glutathione S‐Transferases

4-Hydroxy-trans-2-nonenal (HNE) was initially considered to be merely a toxic end product of lipid peroxidation that contributed to oxidative stress-related pathogenesis. However, in recent years its physiological role as an important "signaling molecule" has been established. HNE can modulate various signaling pathways in a concentration-dependent manner. Glutathione S-transferases (GSTs) are major determinants of the intracellular concentration of HNE, because these enzymes account for the metabolism of most cellular HNE through its conjugation to glutathione. Evidence is emerging that GSTs are involved in the regulation of the HNE-mediated signaling processes. Against the backdrop of our current understanding on the formation, metabolism, and role of HNE in signaling processes, the physiological role of GSTs in regulation of HNE-mediated signaling processes is critically evaluated in this chapter. Available evidence strongly suggests that besides their well-established pharmacological role of detoxifying xenobiotics, GSTs also play an important physiological role in the regulation of cellular signaling processes.

RLIP76 (RALBP1)-mediated transport of leukotriene C4 (LTC4) in cancer cells: implications in drug resistance

Increased active transport of LTC(4) observed frequently in multidrug-resistant cancer cells have been attributed to ABC-transporter proteins particularly, MRP1. We have demonstrated recently that a novel non-ABC transporter, RLIP76 (RALBP1) can also mediate ATP-dependent transport of GSH-conjugates (GS-E) as well as doxorubicin (DOX). We demonstrate RLIP76 reconstituted in artificial liposomes can catalyze ATP-dependent transport of LTC(4), which can be modulated by PKC-alpha. The ATPase activity of E. coli expressed homogenous RLIP76 was stimulated in a saturable fashion by LTC(4) with half maximal stimulation at 130 nM. Proteoliposomes reconstituted with RLIP76 catalyzed temperature and osmolar sensitive ATP-dependent transport of LTC(4) with K(m) values of 5.1 mM and 210 nM for ATP and LTC(4), respectively. V(max) for transport was found to be 3.2 nmol/min/mg. Colchicine inhibited LTC(4) transport to 50% at 5.8 microM. PKC-alpha catalyzed phosphorylation of RLIP76 and increased its transport activity by 2-3-fold. Membrane vesicles prepared from the small (SCLC) and non-small (NSCLC) lung cancer cell lines as well as HL-60 (leukemia) and U937 (lymphoma) cell lines exhibited ATP-dependent transport of LTC(4), which was inhibited by anti-RLIP76 antibodies. The rate of transport of LTC(4) in SCLC (H69, H378) was half of that observed in NSCLC cell lines but after transfection with RLIP76, the transport rate of LTC(4) in H69 became comparable to that in NSCLC cell lines. Anti-RLIP76 antibodies inhibited LTC(4) transport by 67-81% in all 8 cell lines examined, whereas N-19 anti-MRP1 antibodies inhibited transport of LTC(4) by only 11-26%. These results suggest that RLIP76 is the major LTC(4) transporter in cancer cells and that its transport activity is regulated by PKC-alpha-mediated phosphorylation.

Regulation of 4-hydroxynonenal-mediated signaling by glutathione S-transferases

4-Hydroxynonenal (HNE), one of the major end products of lipid peroxidation, has been shown to be involved in signal transduction and available evidence suggests that it can affect cell cycle events in a concentration-dependent manner. Glutathione S-transferases (GSTs) can modulate the intracellular concentrations of HNE by affecting its generation during lipid peroxidation by reducing hydroperoxides and also by converting it into a glutathione conjugate. We have recently demonstrated that overexpression of the Alpha class GSTs in cells leads to lower steady-state levels of HNE, and these cells acquire resistance to apoptosis induced by lipid peroxidation-causing agents such as H(2)O(2), UVA, superoxide anion, and pro-oxidant xenobiotics, suggesting that signaling for apoptosis by these agents is transduced through HNE. Cells with the capacity to exclude HNE from the intracellular environment at a faster rate are relatively more resistant to apoptosis caused by H(2)O(2), UVA, superoxide anion, and pro-oxidant xenobiotics as well as by HNE, suggesting that HNE may be a common denominator in mechanisms of apoptosis caused by oxidative stress. We have also shown that transfection of adherent cells with HNE-metabolizing GSTs leads to transformation of these cells due to depletion of HNE. These recent studies from our laboratories, which strongly suggest that HNE is a key signaling molecule and that GSTs, being determinants of its intracellular concentrations, can regulate stress-mediated signaling, are reviewed in this article.

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.

Role of red blood cells in pharmacokinetics of chemotherapeutic agents

After oral or parenteral administration of chemotherapeutic agents, these drugs are transported to the tissues by the blood in different fractions: plasma water, plasma proteins or cells. Erythrocytes may play an important role in the storage, transport and metabolism of chemotherapeutic agents. Anthracyclines, ifosfamide and its metabolites, and topoisomerase I and I/II inhibitors are incorporated in red blood cells. They may be transported to the tumour tissue and mobilised from the erythrocyte by different active or passive transport mechanisms. Erythrocytes may also be used as carriers for drugs such as asparaginase. This leads to a decreased toxicity profile. Finally, it has been shown that red blood cells are important in the transport and metabolism of mercaptopurine. The erythrocyte concentration of mercaptopurine has a prognostic value in the treatment of childhood acute lymphoblastic leukemia. In this review, the role of red blood cells for various anticancer drugs is further discussed.

Cells preconditioned with mild, transient UVA irradiation acquire resistance to oxidative stress and UVA-induced apoptosis: role of 4-hydroxynonenal in UVA-mediated signaling for apoptosis

Because 4-hydroxynonenal (4-HNE) has been suggested to be involved in oxidative stress-mediated apoptosis (Cheng, J. Z., Sharma, R., Yang, Y., Singhal, S. S., Sharma, A., Saini, M. K., Singh, S. V., Zimniak, P., Awasthi, S., and Awasthi, Y. C. (2001) J. Biol. Chem. 276, 41213-41223) and UVA irradiation also causes lipid peroxidation, we have examined the role of 4-HNE in UVA-mediated apoptosis. K562 cells irradiated with UVA (3.0 milliwatts/cm2) for 5, 15, and 30 min showed a time dependent increase in 4-HNE levels. As judged by the activation of caspases, apoptosis was observed only in cells irradiated for 30 min. Within 2 h of recovery in normal medium, 4-HNE levels in 5 and 15 min UVA, irradiated cells returned to the basal or even lower levels but in cells irradiated for 30 min, 4-HNE levels remained consistently higher. The cells irradiated with UVA for 5 min and allowed to recover for 2 h in normal medium (UVA-preconditioned cells) showed a remarkable induction of hGST5.8, which catalyzes conjugation of 4-HNE to glutathione (GSH), and RLIP76 (Ral BP-1), which mediates the transport of the conjugate, GS-HNE. In cells irradiated with UVA for 30 min the induction of RLIP76 or hGST5.8 was not observed. The preconditioned cells transported GS-HNE into the medium at a rate about 2-fold higher than the controls and the transport was inhibited (65%) by coating the cells with anti-RLIP76 IgG. Upon treatment with xanthine/xanthine oxidase (XA/XO), 4-HNE, or prolonged UVA exposure, the control cells showed a sustained activation of c-Jun N-terminal kinase (JNK) and apoptosis. However, in the UVA-preconditioned cells, apoptosis was not observed, and JNK activation was inhibited. This resistance of preconditioned cells to XA/XO-, 4-HNE-, or UVA-induced apoptosis could be abrogated when these cells were coated with anti-RLIP76 IgG to block the efflux of GS-HNE. These studies strongly suggest a role of 4-HNE in UVA-mediated apoptosis.

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.

Ral-GTPases: approaching their 15 minutes of fame

Andy Warhol, the famous pop artist, once claimed that "in the future everyone will be famous for 15 minutes". The same, it seems, can be said of proteins, because at any given time some proteins become more "fashionable" to study than others. But most proteins have been highly conserved throughout millions of years of evolution, which implies that they all have essential roles in cell biology. Thus, each one will no doubt enter the limelight if the right experiment in the right cell type is done. A good example of this is the Ras-like GTPases (Ral-GTPases), which until recently existed in the shadow of their close cousins--the Ras proto-oncogenes. Recent studies have yielded insights into previously unappreciated roles for Ral-GTPases in intensively investigated disciplines such as vesicle trafficking, cell morphology, transcription and possibly even human oncogenesis.

Role of 4-hydroxynonenal in stress-mediated apoptosis signaling

In this mini review we summarize recent studies from our laboratory, which show the involvement of 4-hydroxynonenal (4-HNE) in cell cycle signaling. We demonstrate 4-HNE induced apoptosis in various cell lines is accompanied with c-Jun-N-terminal kinase and caspase-3 activation. Cells exposed to mild, transient, heat or oxidative stress acquire capacity to exclude intracellular 4-HNE at a faster rate by inducing hGST5.8 which conjugate 4-HNE to GSH, and RLIP76 which mediates the ATP-dependent transport of the GSH-conjugate of 4-HNE. The cells preconditioned with mild transient stress acquire resistance to H(2)O(2) and 4-HNE induced apoptosis by excluding intracellular 4-HNE at an accelerated pace. Furthermore, a decrease in intracellular concentration of 4-HNE achieved by transfecting cells with mGSTA4-4 or hGSTA4-4 results in a faster growth rate. These studies strongly suggest a role of 4-HNE in stress mediated signaling.

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.