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

RLIP: A necessary transporter protein for translating oxidative stress into pro-obesity and pro-carcinogenic signaling

Previously, we showed that knockout mice homozygous for deficiency of the mercapturic acid pathway (MAP) transporter protein, RLIP (RLIP^-/-), are resistant to chemical carcinogenesis, inflammation, and metabolic syndrome (MetS). We also found that RLIP^-/- mice are highly resistant to obesity caused by a high-fat diet (HFD). Interestingly, these studies showed that kinase, cytokine, and adipokine signaling that are characteristics of obesity were blocked despite the presence of increased oxidative stress in RLIP^-/- mice. The deficiencies in obesity-inducing kinase, cytokine, and adipokine signaling were attributable to a lack of clathrin-dependent endocytosis (CDE), a process that is severely deficient in RLIP^-/- mice. Because CDE is also necessary for carcinogenic signaling through EGF, WNT, TGFβ and other cancer-specific peptide hormones, and because RLIP^-/- mice are cancer-resistant, we reasoned that depletion of RLIP by an antisense approach should cause cancer regression in human cancer xenografts. This prediction has been confirmed in studies of xenografts from lung, kidney, prostate, breast, and pancreatic cancers and melanoma. Because these results suggested an essential role for RLIP in carcinogenesis, and because our studies have also revealed a direct interaction between p53 and RLIP, we reasoned that if RLIP played a central role in carcinogenesis, that development of lymphoma in p53^-/- mice, which normally occurs by the time these mice are 6 months old, could be delayed or prevented by depleting RLIP. Recent studies described herein have confirmed this hypothesis, showing complete suppression of lymphomagenesis in p53^-/- mice treated with anti-RLIP antisense until the age of 8 months. All control mice developed lymphoma in the thymus or testis as expected. These findings lead to a novel paradigm predicting that under conditions of increased oxidative stress, the consequent increased flux of metabolites in the MAP causes a proportional increase in the rate of CDE. Because CDE inhibits insulin and TNF signaling but promotes EGF, TGFβ, and Wnt signaling, our model predicts that chronic stress-induced increases in RLIP (and consequently CDE) will induce insulin-resistance and enhance predisposition to cancer. Alternatively, generalized depletion of RLIP would antagonize the growth of malignant cells, and concomitantly exert therapeutic insulin-sensitizing effects. Therefore, this review focuses on how targeted depletion or inhibition of RLIP could provide a novel target for treating both obesity and cancer.

Expression and function of SLC38A5, an amino acid-coupled Na+/H+ exchanger, in triple-negative breast cancer and its relevance to macropinocytosis

Metabolic reprogramming in cancer necessitates increased amino acid uptake, which is accomplished by up-regulation of specific amino acid transporters. However, not all tumors rely on any single amino acid transporter for this purpose. Here, we report on the differential up-regulation of the amino acid transporter SLC38A5 in triple-negative breast cancer (TNBC). The up-regulation is evident in TNBC tumors, conventional and patient-derived xenograft TNBC cell lines, and a mouse model of spontaneous TNBC mammary tumor. The up-regulation is confirmed by functional assays. SLC38A5 is an amino acid-dependent Na+/H+ exchanger which transports Na+ and amino acids into cells coupled with H+ efflux. Since cell-surface Na+/H+ exchanger is an established inducer of macropinocytosis, an endocytic process for cellular uptake of bulk fluid and its components, we examined the impact of SLC38A5 on macropinocytosis in TNBC cells. We found that the transport function of SLC38A5 is coupled to the induction of macropinocytosis. Surprisingly, the transport function of SLC38A5 is inhibited by amilorides, the well-known inhibitors of Na+/H+ exchanger. Down-regulation of SLC38A5 in TNBC cells attenuates serine-induced macropinocytosis and reduces cell proliferation significantly as assessed by multiple methods, but does not induce cell death. The Cancer Genome Atlas database corroborates SLC38A5 up-regulation in TNBC. This represents the first report on the selective expression of SLC38A5 in TNBC and its role as an inducer of macropinocytosis, thus revealing a novel, hitherto unsuspected, function for an amino acid transporter that goes beyond amino acid delivery but is still relevant to cancer cell nutrition and proliferation.

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.

Quadruply Stranded Metallo-Supramolecular Helicate [Pd2(hextrz)4]4+ Acts as a Molecular Mimic of Cytolytic Peptides

[Pd₂(hextrz)₄]⁴⁺ is a quadruply stranded helicate, a novel bioinorganic complex designed to mimic the structure and function of proteins due to its high stability and supramolecular size. We have previously reported that [Pd₂(hextrz)₄]⁴⁺ exhibited cytotoxicity toward a range of cell lines, with IC₅₀ values ranging from 3 to 10 μM. Here we demonstrate that [Pd₂(hextrz)₄]⁴⁺ kills cells by forming pores within the cell membrane, a mechanism of cell death analogous to the naturally occurring cytolytic peptides. [Pd₂(hextrz)₄]⁴⁺ induced cell death is characterized by an initial influx of Ca²⁺, followed by nuclear condensation and mitochondrial swelling. This is accompanied by progressive cell membrane damage that results in the formation of large blebs at the cell surface. This allows the efflux of molecules from the cell leading to loss of cell viability. These data suggest that it may be possible to design metallo-supramolecular complexes to mimic the cytotoxic action of pore forming proteins and peptides and so provide a new class of drug to treat cancer, autoimmune disorders, and microbial infection.

RLIP controls receptor-ligand signaling by regulating clathrin-dependent endocytosis

RLIP (Ral-interacting protein) is a multifunctional protein that couples ATP hydrolysis with the movement of substances. Its primary function appears to be in the plasma membrane, where it catalyzes the ATP-dependent efflux of glutathione-conjugates (GS-Es), as well as un-metabolized drugs and toxins. In the plasma membrane, its interaction with the clathrin adaptor protein AP2 localizes it to endocytic vesicle, where its GS-E-stimulated ATPase and transport activity are required for clathrin-dependent endocytosis (CDE). CDE is an essential mechanism for internalizing ligand-receptor complexes that signal proliferation (EGF, insulin, IGF1), apoptosis (TNFα, TRAIL, Fas-L), and differentiation and morphogenesis (TGFβ, WNT, Notch, SHH). Aberrant functioning of these pathways appears crucial for most cancer cells to evade apoptosis, invade surrounding tissues, and metastasize. Internalization of receptor-ligand complexes by CDE begins a sequence of events that can terminate, initiate, or modulate downstream signaling; the consequences of signaling through these downstream pathways may be inherently different in cancer and normal cells, a view supported by numerous basic and clinical observations. In this review, we will discuss the GS-E transport activity of RLIP, which determines the rate of ligand endocytosis, and how the inhibition and/or depletion of RLIP globally disrupts in ligand-receptor signaling.

FSCN1 is an effective marker of poor prognosis and a potential therapeutic target in human tongue squamous cell carcinoma

To estimate the value of FSCN1 in evaluating the prognosis and guiding the targeted therapy for patients with tongue squamous cell carcinoma (TSCC). Using the Oncomine database, we found some genes especially FSCN1 differentially expressed between TSCC samples and tongue normal samples. So we compared FSCN1 expression between TSCC and normal cell lines and knocked down FSCN1 in TSCC cells to observe its influence on the viability and trans-migration in vitro and tumor growth in vivo. Then we measured FSCN1 expression in human cancer tissues and adjacent non-carcinoma tissues (ANT) and explored the relationship between FSCN1 expression and clinical pathological factors and prognosis in TSCC patients. We found that FSCN1 is expressed higher in TSCC cells than in normal cells. Knockdown of FSCN1 reduced TSCC cell viability and trans-migration in vitro and impaired tumor growth in vivo. FSCN1 also expressed higher in human TSCC than in ANT. In addition, FSCN1 expression was related to N classification, clinical stage and relapse. TSCC patients with over-expression of FSCN1 had worse prognosis. In conclusion, over-expression of FSCN1 indicates worse prognosis for patients with TSCC and FSCN1 may be a potential prognostic biomarker and therapeutic target in TSCC.

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.

Didymin: an orally active citrus flavonoid for targeting neuroblastoma

Neuroblastoma, a rapidly growing yet treatment responsive cancer, is the third most common cancer of children and the most common solid tumor in infants. Unfortunately, neuroblastoma that has lost p53 function often has a highly treatment-resistant phenotype leading to tragic outcomes. In the context of neuroblastoma, the functions of p53 and MYCN (which is amplified in ~25% of neuroblastomas) are integrally linked because they are mutually transcriptionally regulated, and because they together regulate the catalytic activity of RNA polymerases. Didymin is a citrus-derived natural compound that kills p53 wild-type as well as drug-resistant p53-mutant neuroblastoma cells in culture. In addition, orally administered didymin causes regression of neuroblastoma xenografts in mouse models, without toxicity to non-malignant cells, neural tissues, or neural stem cells. RKIP is a Raf-inhibitory protein that regulates MYCN activation, is transcriptionally upregulated by didymin, and appears to play a key role in the anti-neuroblastoma actions of didymin. In this review, we discuss how didymin overcomes drug-resistance in p53-mutant neuroblastoma through RKIP-mediated inhibition of MYCN and its effects on GRK2, PKCs, Let-7 micro-RNA, and clathrin-dependent endocytosis by Raf-dependent and -independent mechanisms. In addition, we will discuss studies supporting potential clinical impact and translation of didymin as a low cost, safe, and effective oral agent that could change the current treatment paradigm for refractory neuroblastoma.

Non-covalent anchoring of oligonucleotides on single-walled carbon nanotubes via short bioreducible linker

This paper describes a simple approach to obtain hybrids of single-walled carbon nanotubes with therapeutically relevant oligonucleotides that are able to be released upon glutathione treatment at physiological concentrations.

Antioxidant role of glutathione S-transferases: 4-Hydroxynonenal, a key molecule in stress-mediated signaling

4-Hydroxy-2-trans-nonenal (4HNE), one of the major end products of lipid peroxidation (LPO), has been shown to induce apoptosis in a variety of cell lines. It appears to modulate signaling processes in more than one way because it has been suggested to have a role in signaling for differentiation and proliferation. 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 LPO end-products such as 4HNE. Available evidence from earlier studies together with results of recent studies in our laboratories strongly suggests that LPO products, particularly hydroperoxides and 4HNE, 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 4HNE. We demonstrate that 4HNE induced apoptosis in various cell lines is accompanied with c-Jun-N-terminal kinase (JNK) and caspase-3 activation. Cells exposed to mild, transient heat or oxidative stress acquire the capacity to exclude intracellular 4HNE at a faster rate by inducing GSTA4-4 which conjugates 4HNE to glutathione (GSH), and RLIP76 which mediates the ATP-dependent transport of the GSH-conjugate of 4HNE (GS-HNE). The balance between formation and exclusion promotes different cellular processes - higher concentrations of 4HNE promote apoptosis; whereas, lower concentrations promote proliferation. In this article, we provide a brief summary of the cellular effects of 4HNE, followed by a review of its GST-catalyzed detoxification, with an emphasis on the structural attributes that play an important role in the interactions with alpha-class GSTA4-4. Taken together, 4HNE is a key signaling molecule and that GSTs being determinants of its intracellular concentrations, can regulate stress-mediated signaling, are reviewed in this article.

Targeting xCT, a cystine-glutamate transporter induces apoptosis and tumor regression for KSHV/HIV-associated lymphoma

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the etiological agent of primary effusion lymphoma (PEL), which represents a rapidly progressing malignancy arising in HIV-infected patients. Conventional chemotherapy for PEL treatment induces unwanted toxicity and is ineffective — PEL continues to portend nearly 100% mortality within a period of months, which requires novel therapeutic strategies. The amino acid transporter, xCT, is essential for the uptake of cystine required for intracellular glutathione (GSH) synthesis and for maintaining the intracellular redox balance. Inhibition of xCT induces growth arrest in a variety of cancer cells, although its role in virus-associated malignancies including PEL remains unclear. In the current study, we identify that xCT is expressed on the surface of patient-derived KSHV⁺ PEL cells, and targeting xCT induces caspase-dependent cell apoptosis. Further experiments demonstrate the underlying mechanisms including host and viral factors: reducing intracellular GSH while increasing reactive oxygen species (ROS), repressing cell-proliferation-related signaling, and inducing viral lytic genes. Using an immune-deficient xenograft model, we demonstrate that an xCT selective inhibitor, Sulfasalazine (SASP), prevents PEL tumor progression in vivo. Together, our data provide innovative and mechanistic insights into the role of xCT in PEL pathogenesis, and the framework for xCT-focused therapies for AIDS-related lymphoma in future.

The ruthenium compound KP1339 potentiates the anticancer activity of sorafenib in vitro and in vivo

KP1339 is a promising ruthenium-based anticancer compound in early clinical development. This study aimed to test the effects of KP1339 on the in vitro and in vivo activity of the multi-kinase inhibitor sorafenib, the current standard first-line therapy for advanced hepatoma. Anticancer activity of the parental compounds as compared to the drug combination was tested against a panel of cancer cell lines with a focus on hepatoma. Combination of KP1339 with sorafenib induced in the majority of all cases distinctly synergistic effects, comprising both sorafenib-resistant as well as sorafenib-responsive cell models. Several mechanisms were found to underlie these multifaceted synergistic activities. Firstly, co-exposure induced significantly enhanced accumulation levels of both drugs resulting in enhanced apoptosis induction. Secondly, sorafenib blocked KP1339-mediated activation of P38 signalling representing a protective response against the ruthenium drug. In addition, sorafenib treatment also abrogated KP1339-induced G2/M arrest but resulted in check point-independent DNA-synthesis block and a complete loss of the mitotic cell populations. The activity of the KP1339/sorafenib combination was evaluated in the Hep3B hepatoma xenograft. KP1339 monotherapy led to a 2.4-fold increase in life span and, thus, was superior to sorafenib, which induced a 1.9-fold prolonged survival. The combined therapy further enhanced the mean survival by 3.9-fold. Synergistic activity was also observed in the VM-1 melanoma xenograft harbouring an activating braf mutation. Together, our data indicate that the combination of KP1339 with sorafenib displays promising activity in vitro and in vivo especially against human hepatoma models.

1,3-Bis(3,5-dichlorophenyl) urea compound 'COH-SR4' inhibits proliferation and activates apoptosis in melanoma

The current clinical interventions in malignant melanomas are met with poor response to therapy due to dynamic regulation of multiple melanoma signaling pathways consequent to administration of single target agents. In this context of limited response to single target agents, novel candidate molecules capable of effectively inducing tumor inhibition along with targeting multiple critical nodes of melanoma signaling assume translational significance. In this regard, we investigated the anti-cancer effects of a novel dichlorophenyl urea compound called COH-SR4 in melanoma. The SR4 treatment decreased the survival and inhibited the clonogenic potential of melanomas along with inducing apoptosis in vitro cultures. SR4 treatments lead to inhibition of GST activity along with causing G2/M phase cell cycle arrest. Oral administration of 4 mg/kg SR4 leads to effective inhibition of tumor burdens in both syngeneic and nude mouse models of melanoma. The SR4 treatment was well tolerated and no overt toxicity was observed. The histopathological examination of resected tumor sections revealed decreased blood vessels, decrease in the levels of angiogenesis marker, CD31, and proliferation marker, Ki67, along with an increase in pAMPK levels. Western blot analyses of resected tumor lysates revealed increased PARP cleavage, Bim, pAMPK along with decreased pAkt, vimentin, fibronectin, CDK4 and cyclin B1. Thus, SR4 represents a novel candidate for the further development of mono and combinatorial therapies to effectively target aggressive and therapeutically refractory melanomas.

RLIP76 regulates PI3K/Akt signaling and chemo-radiotherapy resistance in pancreatic cancer

Purpose

Pancreatic cancer is an aggressive malignancy with characteristic metastatic course of disease and resistance to conventional chemo-radiotherapy. RLIP76 is a multi-functional cell membrane protein that functions as a major mercapturic acid pathway transporter as well as key regulator of receptor-ligand complexes. In this regard, we investigated the significance of targeting RLIP76 on PI3K/Akt pathway and mechanisms regulating response to chemo-radiotherapy.

Research Design and Methods

Cell survival was assessed by MTT and colony forming assays. Cellular levels of proteins and phosphorylation was determined by Western blot analyses. The impact on apoptosis was determined by TUNEL assay. The anti-cancer effects of RLIP76 targeted interventions in vivo were determined using mice xenograft model of the pancreatic cancer. The regulation of doxorubicin transport and radiation sensitivity were determined by transport studies and colony forming assays, respectively.

Results

Our current studies reveal an encompassing model for the role of RLIP76 in regulating the levels of fundamental proteins like PI3K, Akt, E-cadherin, CDK4, Bcl2 and PCNA which are of specific importance in the signal transduction from critical upstream signaling cascades that determine the proliferation, apoptosis and differentiation of pancreatic cancer cells. RLIP76 depletion also caused marked and sustained regression of established human BxPC-3 pancreatic cancer tumors in nude mouse xenograft model. RLIP76 turned out to be a major regulator of drug transport along with contributing to the radiation resistance in pancreatic cancer.

Conclusions/Significance

RLIP76 represents a mechanistically significant target for developing effective interventions in aggressive and refractory pancreatic cancers.

Glutathione-conjugate transport by RLIP76 is required for clathrin-dependent endocytosis and chemical carcinogenesis

Targeted depletion of the RALBP1-encoded 76-kDa splice variant, RLIP76, causes marked and sustained regression of human xenografts of lung, colon, prostate, and kidney cancers without toxicity in nude mouse models. We proposed that the remarkable efficacy and broad spectrum of RLIP76-targeted therapy is because its glutathione-conjugate (GS-E) transport activity is required for clathrin-dependent endocytosis (CDE), which regulates all ligand-receptor signaling, and that RLIP76 is required not only for survival of cancer cells but also for their very existence. We studied RLIP76 mutant proteins and the functional consequences of their expression into RLIP76(-/-) MEFs, identified key residues for GS-E binding in RLIP76, established the requirement of RLIP76-mediated GS-E transport for CDE, and showed a direct correlation between GS-E transport activities with CDE. Depletion of RLIP76 nearly completely blocked signaling downstream of EGF in a CDE-dependent manner and Wnt5a signaling in a CDE-independent manner. The seminal prediction of this hypothesis-RLIP76(-/-) mice will be deficient in chemical neoplasia-was confirmed. Benzo[a]pyrene, dimethylbenzanthracene, and phorbol esters are ineffective in causing neoplasia in RLIP76(-/-). PMA-induced skin carcinogenesis in RLIP76(+/+) mouse was suppressed completely by depletion of either PKCα or RLIP76 by siRNA or antisense and could be restored by topical application of RLIP76 protein in RLIP76(-/-) mouse skin. Likewise, chemical pulmonary carcinogenesis was absent in female and nearly absent in male RLIP76(-/-) mice. In RLIP76(-/-) mice, p53, p38, and JNK activation did not occur in response to either carcinogen. Our findings show a fundamental role of RLIP76 in chemical carcinogenesis.

Anti-melanoma activity of the 9.2.27PE immunotoxin in dacarbazine resistant cells

We have earlier shown that the 9.2.27 Pseudomonas Exotoxin A (PE) immunotoxin (IT) efficiently kills melanoma cells through inhibition of protein synthesis followed by some morphologic and biochemical features of apoptosis, a different cell killing mechanism than the one caused by Dacarbazine (DTIC), a chemotherapeutic drug used to treat malignant melanoma. To examine whether induced DTIC resistance also is a determining factor for the effectiveness of 9.2.27PE IT, we developed a DTIC resistant subline, FEMX-200DR, from the DTIC sensitive cell line FEMX. The cell variants were treated with 9.2.27PE, an IT binding to the high molecular weight-melanoma associated antigen (HMW-MAA) expressed on most malignant melanoma cells. The IT was equally effective in killing the FEMX-200DR and the FEMX cells, and the cell death was primarily caused by inhibition of protein synthesis. The DNA repair enzyme and apoptotic marker PARP, a substrate of caspase-3, was inactivated, although we observed only a minor activation of caspase-3 and caspase-8, intracellular proteases involved in apoptosis. In addition to being DTIC resistant, the FEMX-200DR cells were also more resistant to apoptosis than the parent cells as a 3 times higher concentration of the apoptotic inducer Staurosporine was needed to obtain IC50. Furthermore, in early passage malignant melanoma cell lines established from lymph node metastases, the 9.2.27PE caused a time-dependent and dose-dependent decrease in cell viability independent of their DTIC sensitivity. These findings show that the 9.2.27PE IT efficiently can cause cell death in malignant melanoma cells independent of their level of resistance to apoptosis and DTIC.

Role of oxidative stress in stem, cancer, and cancer stem cells

The term ‘‘oxidative stress” refers to a cell’s state characterized by excessive production of reactive oxygen species (ROS) and oxidative stress is one of the most important regulatory mechanisms for stem, cancer, and cancer stem cells. The concept of cancer stem cells arose from observations of similarities between the self-renewal mechanism of stem cells and that of cancer stem cells, but compared to normal stem cells, they are believed to have no control over the cell number. ROS have been implicated in diverse processes in various cancers, and generally the increase of ROS in cancer cells is known to play an important role in the initiation and progression of cancer. Additionally, ROS have been considered as the most significant mutagens in stem cells; when elevated, blocking self-renewal and at the same time, serving as a signal stimulating stem cell differentiation. Several signaling pathways enhanced by oxidative stress are suggested to have important roles in tumorigenesis of cancer or cancer stem cells and the self-renewal ability of stem or cancer stem cells. It is now well established that mitochondria play a prominent role in apoptosis and increasing evidence supports that apoptosis and autophagy are physiological phenomena closely linked with oxidative stress. This review elucidates the effect and the mechanism of the oxidative stress on the regulation of stem, cancer, and cancer stem cells and focuses on the cell signaling cascades stimulated by oxidative stress and their mechanism in cancer stem cell formation, as very little is known about the redox status in cancer stem cells. Moreover, we explain the link between ROS and both of apoptosis and autophagy and the impact on cancer development and treatment. Better understanding of this intricate link may shed light on mechanisms that lead to better modes of cancer treatment.

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.

Targeting cancer cells by ROS-mediated mechanisms: a radical therapeutic approach?

Increased generation of reactive oxygen species (ROS) and an altered redox status have long been observed in cancer cells, and recent studies suggest that this biochemical property of cancer cells can be exploited for therapeutic benefits. Cancer cells in advanced stage tumours frequently exhibit multiple genetic alterations and high oxidative stress, suggesting that it might be possible to preferentially eliminate these cells by pharmacological ROS insults. However, the upregulation of antioxidant capacity in adaptation to intrinsic oxidative stress in cancer cells can confer drug resistance. Abrogation of such drug-resistant mechanisms by redox modulation could have significant therapeutic implications. We argue that modulating the unique redox regulatory mechanisms of cancer cells might be an effective strategy to eliminate these cells.

RLIP76 in defense of radiation poisoning

PURPOSE

To determine the role of RLIP76 in providing protection from radiation and chemotherapy. In the present report, we used RLIP76 to refer to both the mouse (Ralbp1) and the human (RLIP76) 76-kDa splice variant proteins (RLIP76) for convenience and to avoid confusion. In other reports, Ralbp1 refers to the mouse enzyme (encoded by the Ralbp1 gene), which is structurally and functionally homologous to RLIP76, the human protein encoded by the human RALBP1 gene.

METHODS AND MATERIALS

Median lethal dose studies were performed in RLIP76(-/-) and RLIP76(+/+) C57B mice after treatment with a single dose of RLIP76 liposomes 14 h after whole body radiation. The radiosensitivity of the cultured mouse embryonic fibroblasts and the effects of buthionine sulfoximine (BSO), amifostine, c-jun N-terminal kinase (JNK), protein kinase B (Akt), and MAPK/ERK kinase (MEK) were determined by colony-forming assays. Glutathione-linked enzyme activities were measured by spectrophotometric assays, glutathione by dithiobis-2-nitrobenzoic acid (DTNB), lipid hydroperoxides by iodometric titration, and aldehydes and metabolites by thiobarbitauric acid reactive substances and liquid chromatography-mass spectrometry (LCMS).

RESULTS

RLIP76(-/-) mice were significantly more sensitive to radiation than were the wild-type, and RLIP76 liposomes prolonged survival in a dose-dependent manner in both genotypes. The levels of 4-hydroxynonenal and glutathione-conjugate of 4-hydroxynonenal were significantly increased in RLIP76(-/-) tissues compared with RLIP76(+/+). RLIP76(-/-) mouse embryonic fibroblasts were markedly more radiosensitive than RLIP76(+/+) mouse embryonic fibroblasts, despite increased glutathione levels in the former. RLIP76 augmentation had a remarkably greater protective effect compared with amifostine. The magnitude of effects of RLIP76 loss on radiation sensitivity was greater than those caused by perturbations of JNK, MEK, or Akt, and the effects of RLIP76 loss could not be completely compensated for by modulating the levels of these signaling proteins.

CONCLUSION

The results of our study have shown that RLIP76 plays a central role in radiation resistance.

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.

Self-regulatory role of 4-hydroxynonenal in signaling for stress-induced programmed cell death

Within the last two decades, 4-hydroxynonenal has emerged as an important second messenger involved in the regulation of various cellular processes. Our recent studies suggest that HNE can induce apoptosis in various cells through the death receptor Fas (CD95)-mediated extrinsic pathway as well as through the p53-dependent intrinsic pathway. Interestingly, through its interaction with the nuclear protein Daxx, HNE can self-limit its apoptotic role by translocating Daxx to cytoplasm where it binds to Fas and inhibits Fas-mediated apoptosis. In this paper, after briefly describing recent studies on various biological activities of HNE, based on its interactions with Fas, Daxx, and p53, we speculate on possible mechanisms through which HNE may affect a multitude of cellular processes and draw a parallel between signaling roles of H(2)O(2) and HNE.

Hsf-1 and POB1 induce drug sensitivity and apoptosis by inhibiting Ralbp1

Hsf-1 (heat shock factor-1) is a transcription factor that is known to regulate cellular heat shock response through its binding with the multispecific transporter protein, Ralbp1. Results of present studies demonstrate that Hsf-1 causes specific and saturable inhibition of the transport activity of Ralbp1 and that the combination of Hsf-1 and POB1 causes nearly complete inhibition through specific bindings with Ralbp1. Augmentation of cellular levels of Hsf-1 and POB1 caused dramatic apoptosis in non-small cell lung cancer cell line H358 through Ralbp1 inhibition. These findings indicate a novel model for mutual regulation of Hsf-1 and Ralbp1 through Ralbp1-mediated sequestration of Hsf-1 in the cellular cytoskeleton and Hsf-1-mediated inhibition of the transport activity of membrane-bound Ralbp1.

The x(c)- cystine/glutamate antiporter: a potential target for therapy of cancer and other diseases

The x(c) (-) cystine/glutamate antiporter is a major plasma membrane transporter for the cellular uptake of cystine in exchange for intracellular glutamate. Its main functions in the body are mediation of cellular cystine uptake for synthesis of glutathione essential for cellular protection from oxidative stress and maintenance of a cystine:cysteine redox balance in the extracellular compartment. In the past decade it has become evident that the x(c) (-) transporter plays an important role in various aspects of cancer, including: (i) growth and progression of cancers that have a critical growth requirement for extracellular cystine/cysteine, (ii) glutathione-based drug resistance, (iii) excitotoxicity due to excessive release of glutamate, and (iv) uptake of herpesvirus 8, a causative agent of Kaposi's sarcoma. The x(c) (-) transporter also plays a role in certain CNS and eye diseases. This review focuses on the expression and function of the x(c) (-) transporter in cells and tissues with particular emphasis on its role in disease pathogenesis. The potential use of x(c) (-) inhibitors (e.g., sulfasalazine) for arresting tumor growth and/or sensitizing cancers is discussed.

The central role of glutathione in the pathophysiology of human diseases

Reduced glutathione (L-gamma-glutamyl-L-cysteinyl-glycine, GSH) is the prevalent low-molecular-weight thiol in mammalian cells. It is formed in a two-step enzymatic process including, first, the formation of gamma-glutamylcysteine from glutamate and cysteine, by the activity of the gamma-glutamylcysteine synthetase; and second, the formation of GSH by the activity of GSH synthetase which uses gamma-glutamylcysteine and glycine as substrates. While its synthesis and metabolism occur intracellularly, its catabolism occurs extracellularly by a series of enzymatic and plasma membrane transport steps. Glutathione metabolism and transport participates in many cellular reactions including: antioxidant defense of the cell, drug detoxification and cell signaling (involved in the regulation of gene expression, apoptosis and cell proliferation). Alterations in its concentration have also been demonstrated to be a common feature of many pathological conditions including diabetes, cancer, AIDS, neurodegenerative and liver diseases. Additionally, GSH catabolism has been recently reported to modulate redox-sensitive components of signal transduction cascades. In this manuscript, we review the current state of knowledge on the role of GSH in the pathogenesis of human diseases with the aim to underscore its relevance in translational research for future therapeutic treatment design.