Regulatory roles of glutathione-S-transferases and 4-hydroxynonenal in stress-mediated signaling and toxicity

Glutathione-S-Transferases (GSTs) have primarily been thought to be xenobiotic metabolizing enzymes that protect cells from toxic drugs and environmental electrophiles. However, in last three decades, these enzymes have emerged as the regulators of oxidative stress-induced signaling and toxicity. 4-Hydroxy-trans 2-nonenal (HNE) an end-product of lipid peroxidation, has been shown to be a major determinant of oxidative stress-induced signaling and toxicity. HNE is involved in signaling pathways, including apoptosis, proliferation, modulation of gene expression, activation of transcription factors/repressors, cell cycle arrest, and differentiation. In this article, available evidence for a major role of GSTs in the regulation of HNE-mediated cell signaling processes through modulation of the intracellular levels of HNE is discussed.

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.

Lens specific RLIP76 transgenic mice show a phenotype similar to microphthalmia

RALBP1/RLIP76 is a ubiquitously expressed protein, involved in promotion and regulation of functions initiated by Ral and R-Ras small GTPases. Presence of multiple domains in its structure enables RLIP76 to be involved in a number of physiological processes such as endocytosis, exocytosis, mitochondrial fission, actin cytoskeleton remodeling, and transport of exogenous and endogenous toxicants. Previously, we have established that RLIP76 provides protection to ocular tissues against oxidative stress by transporting the glutathione-conjugates of the toxic, electrophilic products of lipid peroxidation generated during oxidative stress. Therefore, we developed lens specific RLIP76 transgenic mice (lensRLIP76 Tg) to elucidate the role of RLIP76 in protection against oxidative stress, but these transgenic mice showed impaired lens development and a phenotype with small eyes similar to that observed in microphthalmia. These findings prompted us to investigate the mechanisms via which RLIP76 affects lens and eye development. In the present study, we report engineering of lensRLIP76 Tg mice, characterization of the associated phenotype, and the possible molecular mechanisms that lead to the impaired development of eye and lens in these mice. The results of microarray array analysis indicate that the genes involved in pathways for G-Protein signaling, actin cytoskeleton reorganization, endocytosis, and apoptosis are affected in these transgenic mice. The expression of transcription factors, Pax6, Hsf1, and Hsf4b known to be involved in lens development is down regulated in the lens of these Tg mice. However, the expression of heat shock proteins (Hsps), the downstream targets of Hsfs, is differentially affected in the lens showing down regulation of Hsp27, Hsp40, up regulation of Hsp60, and no effect on Hsp70 and Hsp90 expression. The disruption in the organization of actin cytoskeleton of these Tg mice was associated with the inhibition of the activation of Cdc42 and down regulation of cofilin phosphorylation. These mice may provide useful animal model for elucidating the mechanisms of lens development, and etiology of microphthalmia.

4-Hydroxynonenal induces G2/M phase cell cycle arrest by activation of the ataxia telangiectasia mutated and Rad3-related protein (ATR)/checkpoint kinase 1 (Chk1) signaling pathway

4-Hydroxynonenal (HNE) has been widely implicated in the mechanisms of oxidant-induced toxicity, but the detrimental effects of HNE associated with DNA damage or cell cycle arrest have not been thoroughly studied. Here we demonstrate for the first time that HNE caused G2/M cell cycle arrest of hepatocellular carcinoma HepG2 (p53 wild type) and Hep3B (p53 null) cells that was accompanied with decreased expression of CDK1 and cyclin B1 and activation of p21 in a p53-independent manner. HNE treatment suppressed the Cdc25C level, which led to inactivation of CDK1. HNE-induced phosphorylation of Cdc25C at Ser-216 resulted in its translocation from nucleus to cytoplasm, thereby facilitating its degradation via the ubiquitin-mediated proteasomal pathway. This phosphorylation of Cdc25C was regulated by activation of the ataxia telangiectasia and Rad3-related protein (ATR)/checkpoint kinase 1 (Chk1) pathway. The role of HNE in the DNA double strand break was strongly suggested by a remarkable increase in comet tail formation and H2A.X phosphorylation in HNE-treated cells in vitro. This was supported by increased in vivo phosphorylation of H2A.X in mGsta4 null mice that have impaired HNE metabolism and increased HNE levels in tissues. HNE-mediated ATR/Chk1 signaling was inhibited by ATR kinase inhibitor (caffeine). Additionally, most of the signaling effects of HNE on cell cycle arrest were attenuated in hGSTA4 transfected cells, thereby indicating the involvement of HNE in these events. A novel role of GSTA4-4 in the maintenance of genomic integrity is also suggested.

Role of SMC1 in overcoming drug resistance in triple negative breast cancer

Triple-negative breast cancer (TNBC) is one of the hardest subtypes of breast cancer to treat due to the heterogeneity of the disease and absence of well-defined molecular targets. Emerging evidence has shown the role of cohesin in the formation and progression of various cancers including colon and lung cancer but the role of cohesin in breast cancer remains elusive. Our data showed that structural maintenance of chromosome 1 (SMC1), a subunit of the cohesin protein complex, is differentially overexpressed both at RNA and protein level in a panel of TNBC cell lines as compared to normal epithelial or luminal breast cancer cells, suggesting that the amplified product of this normal gene may play role in tumorigenesis in TNBC. In addition, our results show that induced overexpression of SMC1 through transient transfection enhanced cell migration and anchorage independent growth while its suppression with targeted small interfering RNA (siRNA) reduced the migration ability of TNBC cells. Increased expression of SMC1 also lead to increase in the mesenchymal marker vimentin and decrease in the normal epithelial marker, E-cadherin. Immunocytochemical studies along with flow cytometry and cell fractionation showed the localization of SMC1 in the nucleus, cytoplasm and also in the plasma membrane. The knockdown of SMC1 by siRNA sensitized the TNBC cells towards a PARP inhibitor (ABT-888) and IC₅₀ was approximately three fold less than ABT-888 alone. The cytotoxic effect of combination of SMC1 suppression and ABT-888 was also confirmed by the colony propagation assay. Taken together, these studies report for the first time that SMC1 is overexpressed in TNBC cells where it plays a role in cell migration and drug sensitivity, and thus provides a potential therapeutic target for this highly invasive breast cancer subtype.

P300 regulates the human RLIP76 promoter activity and gene expression

A 76-kDa Ral-interacting protein (RLIP76) has been implicated in the pathogenesis of cancer and diabetes. It is often over expressed in human malignant cell lines and human tumor samples and has been associated with metastasis and chemoresistance. RLIP76 homozygous knockout mice exhibit increased insulin sensitivity, hypoglycemia, and hypolipidemia, and resist cancer development. Little is known about the mechanism by which the expression of RLIP76 is regulated. In the present study, we functionally characterized the RLIP76 promoter using deletion mapping and mutational analysis to investigate the regulation of RLIP76 transcription. We have identified the promoter regions important for RLIP76 transcription, including a strong cis-activating element in the proximal promoter containing overlapping consensus cMYB and cETS binding sites. Transcription factor cMYB and the coactivator p300 associated with RLIP76 gene promoter as shown by CHIP assay. Knockdown of p300 in HEK293 cells reduced the activity of the promoter fragment containing wild type cMYB/cETS binding site in comparison to that with deleted or mutated cMYB/cETS binding site. Knockdown of p300 also decreased the RLIP76 expression as indicated by immunoblotting, immunocytochemistry and flow cytometry analysis. Thus, we report for the first time that p300 associates with the RLIP76 promoter via an overlapping cMYB and cETS binding site and regulates RLIP76 promoter activity and its expression.

Gsta4 Null Mouse Embryonic Fibroblasts Exhibit Enhanced Sensitivity to Oxidants: Role of 4-Hydroxynonenal in Oxidant Toxicity

The alpha class glutathione s-transferase (GST) isozyme GSTA4-4 (EC2.5.1.18) exhibits high catalytic efficiency to-wards 4-hydroxynon-2-enal (4-HNE), a major end product of oxidative stress induced lipid peroxidation. Exposure of cells and tissues to heat, radiation, and chemicals has been shown to induce oxidative stress resulting in elevated concentrations of 4-HNE that can be detrimental to cell survival. Alternatively, at physiological levels 4-HNE acts as a signaling molecule conveying the occurrence of oxidative events initiating the activation of adaptive pathways. To examine the impact of oxidative/electrophilic stress in a model with impaired 4-HNE metabolizing capability, we disrupted the Gsta4 gene that encodes GSTA4-4 in mice. The effect of electrophile and oxidants on embryonic fibroblasts (MEF) isolated from wild type (WT) and Gsta4 null mice were examined. Results indicate that in the absence of GSTA4-4, oxidant-induced toxicity is potentiated and correlates with elevated accumulation of 4-HNE adducts and DNA damage. Treatment of Gsta4 null MEF with 1,1,4-tris(acetyloxy)-2(E)-nonene [4-HNE(Ac)₃], a pro-drug form of 4-HNE, resulted in the activation and phosphorylation of the c-jun-N-terminal kinase (JNK), extracellular-signal-regulated kinases (ERK 1/2) and p38 mitogen activated protein kinases (p38 MAPK) accompanied by enhanced cleavage of caspase-3. Interestingly, when recombinant mammalian or invertebrate GSTs were delivered to Gsta4 null MEF, activation of stress-related kinases in 4-HNE(Ac)₃ treated Gsta4 null MEF were inversely correlated with the catalytic efficiency of delivered GSTs towards 4-HNE. Our data suggest that GSTA4-4 plays a major role in protecting cells from the toxic effects of oxidant chemicals by attenuating the accumulation of 4-HNE.

Loss of glutathione S-transferase A4 accelerates obstruction-induced tubule damage and renal fibrosis

Glutathione transferase isozyme A4 (GSTA4) exhibits high catalytic efficiency to metabolize 4-hydroxynonenal (4-HNE), a highly reactive lipid peroxidation product that has been implicated in the pathogenesis of various chronic diseases. We investigated the role of 4-HNE in the mechanisms of unilateral ureteral obstruction (UUO)-induced fibrosis and its modulation by GSTA4-4 in a mouse model. Our data indicate that after UUO, accumulation of 4-HNE and its adducts were increased in renal tissues, with a concomitant decrease in the expression of GSTA4-4 in mice. As compared to wild-type (WT) mice, UUO caused an increased expression of fibroblast markers in the interstitium of GSTA4 KO mice. Additionally, increased autophagy and tubular cell damage were more severe in UUO-treated GSTA4 KO mice than in WT mice. Furthermore, GSK-3β phosphorylation and expression of Snail, a regulator of E-cadherin and Occludin, was found to be significantly higher in UUO-inflicted GSTA4 KO mice. GSTA4 over-expression prevented 4-HNE-induced autophagy activation, tubular cell damage and Snail nuclear translocation in vitro. The effects of long-term expression of GSTA4 in restoration of UUO-induced damage in mice with the GSTA4 inducible transposon system indicated that release of obstruction after 3 days of UUO resulted in the attenuation of interstitial SMAα and collagen I expression. This transposon-delivered GSTA4 expression also suppressed UUO-induced loss of tubular cell junction markers and autophagy activation. Together, these results indicate that 4-HNE significantly contributes to the mechanisms of tubule injury and fibrosis and that these effects can be inhibited by the enhanced expression of GSTA4-4.

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.

The expression and function of vascular endothelial growth factor in retinal pigment epithelial (RPE) cells is regulated by 4-hydroxynonenal (HNE) and glutathione S-transferaseA4-4

It is well established that 4-hydroxynonenal (HNE) plays a major role in oxidative stress-induced signaling and the toxicity of oxidants. Surprisingly our recent studies also demonstrate that low levels of HNE generated during oxidative stress promote cell survival mechanisms and proliferation. Since the expression and secretion of VEGF is known to be affected by Oxidative stress, during present studies, we have examined dose dependent effect of HNE on VEGF expression and secretion in a model of retinal pigment epithelial (RPE) cells in culture. Results of these studies showed that while inclusion of 0.1 μM HNE in the medium caused increased secretion of VEGF, its secretion and expression was significantly suppressed in the presence of >5 μM HNE in the media. These concentration dependent hormetic effects of HNE on VEGF secretion could be blocked by the over expression of GSTA4-4 indicating that these effects were specifically attributed to HNE and regulated by GSTA4-4. VEGF secreted into the media showed angiogenic properties as indicated by increased migration and tube formation of HUVEC in matrigel when grown in media from RPE cells treated with 1 μM HNE. The corresponding media from GSTA4-4 over expressing RPE cells had no effect on migration and tube formation of HUVEC in matrigel. These results are consistent with earlier studies showing that at low concentrations, HNE promotes proliferative mechanisms and suggest that HNE induces VEGF secretion from RPE cells that acts in a paracrine fashion to induce angiogenic signaling mechanism in the endothelial cells. These findings may suggest a role of HNE and GSTA4-4 in oxidative stress induced proliferative retinopathies.

Inhibition of mercapturic acid pathway-mediated disposal of 4-hydroxynonenal causes complete and sustained remission of human cancer xenografts in nude mice

Environmental electrophilic chemical carcinogens are detoxified via mercapturic acid pathway to be excreted as mercapturic acid derivatives. Mercapturic acid pathway is also involved in the metabolism of pro-apoptotic and toxic endogenous electrophiles such as 4-hydroxynonenal (HNE). HNE is a common denominator in stress induced signaling and is a pro-apoptotic second messenger that affects cell cycle signaling in a concentration dependent manner. It can regulate signaling for apoptosis, differentiation, and gene expression by interacting with the transcriptional factors, transcriptional repressors, membrane receptors and other proteins. First two rate limiting enzymes of the mercapturic acid pathway, GSTs that conjugate HNE to glutathione (GSH), and RLIP76 that excludes GHS-HNE conjugate from cells, regulate the intracellular concentration of HNE. Thus GSTs and RLIP76 can have a profound effect on cell cycle signaling. Our studies have established that increased HNE levels in cells promote apoptotic signaling while at decreased levels below its basal constituted levels HNE promote proliferation. A major outcome of these findings is that by blocking the mercapturic acid pathway mediated detoxification of HNE through the inhibition of RLIP76 catalyzed transport of GS-HNE, a complete remission of many human cancer xenografts in mice can be achieved.

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.

Role of 4-hydroxynonenal in epidermal growth factor receptor-mediated signaling in retinal pigment epithelial cells

Lipid peroxidation (LPO) end-product 4-hydroxynonenal (4-HNE) has been implicated in the mechanism of retinopathy. Lately it has been shown that besides being cytotoxic, 4-HNE plays an important role in oxidative stress-induced signaling. In this study, we have investigated the effect of 4-HNE on epidermal growth factor receptor (EGFR)-mediated signaling, its potential functional consequences, and the regulatory role of the 4-HNE metabolizing isozymes, glutathione S-transferase A4-4 (GSTA4-4) on this signaling in retinal pigment epithelial (RPE) cells. Our results showed that consistent with its known toxicity at relatively higher concentrations, 4-HNE induced cell death in RPE. However, at lower concentrations (as low as 0.1 μM) 4-HNE triggered phosphorylation of EGFR and activation of its down stream signaling components ERK1/2 and Akt that are known to be involved in cell proliferation. These effects of 4-HNE on EGFR could be attenuated by the over expression of GSTA4-4 that reduces intracellular levels of 4-HNE. Our results also indicated that 4-HNE-induced activation of EGFR is a protective mechanism against oxidative stress because EGFR, MEK, and PI3K inhibitors potentiated the toxicity of 4-HNE and also inhibited wound healing in a RPE cell model. These studies suggest that as an initial response to oxidative stress, 4-HNE induces protective mechanism(s) in RPE cells through EGFR-mediated signaling.

Evidence that Gsta4 modifies susceptibility to skin tumor development in mice and humans

BACKGROUND

The incidence of nonmelanoma skin cancer (NMSC) is equivalent to that of all other cancers combined. Previously, we mapped the 12-O-tetradecanoylphorbol-13-acetate (TPA) skin tumor promotion susceptibility locus, Psl1, to distal chromosome 9 in crosses of sensitive DBA/2 mice with relatively resistant C57BL/6 mice. Here, we used the mouse two-stage skin carcinogenesis model to identify the gene(s) responsible for the effects of Psl1.

METHODS

Interval-specific congenic mouse strains (n ≥ 59 mice per strain) were used to more precisely map the Psl1 locus. Having identified glutathione S-transferase α4 (Gsta4) as a candidate tumor promotion susceptibility gene that mapped within the delimited region, we analyzed Gsta4-deficient mice (n = 62) for susceptibility to skin tumor promotion by TPA. We used quantitative polymerase chain reaction, western blotting, and immunohistochemistry to verify induction of Gsta4 in mouse epidermis following TPA treatment and biochemical assays to associate Gsta4 activity with tumor promotion susceptibility. In addition, single-nucleotide polymorphisms (SNPs) in GSTA4 were analyzed in a case-control study of 414 NMSC patients and 450 control subjects to examine their association with human NMSC. Statistical analyses of tumor studies in mice were one-sided, whereas all other statistical analyses were two-sided.

RESULTS

Analyses of congenic mice indicated that at least two loci, Psl1.1 and Psl1.2, map to distal chromosome 9 and confer susceptibility to skin tumor promotion by TPA. Gsta4 maps to Psl1.2 and was highly induced (mRNA and protein) in the epidermis of resistant C57BL/6 mice compared with that of sensitive DBA/2 mice following treatment with TPA. Gsta4 activity levels were also higher in the epidermis of C57BL/6 mice following treatment with TPA. Gsta4-deficient mice (C57BL/6.Gsta4(-/-) mice) were more sensitive to TPA skin tumor promotion (0.8 tumors per mouse vs 0.4 tumors per mouse in wild-type controls; difference = 0.4 tumors per mouse; 95% confidence interval = 0.1 to 0.7, P = .007). Furthermore, inheritance of polymorphisms in GSTA4 was associated with risk of human NMSC. Three SNPs were found to be independent predictors of NMSC risk. Two of these were associated with increased risk of NMSC (odds ratios [ORs] = 1.60 to 3.42), while the third was associated with decreased risk of NMSC (OR = 0.63). In addition, a fourth SNP was associated with decreased risk of basal cell carcinoma only (OR = 0.44).

CONCLUSIONS

Gsta4/GSTA4 is a novel susceptibility gene for NMSC that affects risk in both mice and humans.

Mechanisms of 4-hydroxy-2-nonenal induced pro- and anti-apoptotic signaling

In recent years, 4-hydroxy-2-nonenal (4-HNE) has emerged as an important second messenger in cell cycle signaling. Here, we demonstrate that 4-HNE induces signaling for apoptosis via both the Fas-mediated extrinsic and the p53-mediated intrinsic pathways in HepG2 cells. 4-HNE induces a Fas-mediated DISC independent apoptosis pathway by activating ASK1, JNK, and caspase-3. Parallel treatment of 4-HNE to HepG2 cells also induces apoptosis by the p53 pathway through activation of Bax, p21, JNK, and caspase-3. Exposure of HepG2 cells to 4-HNE leads to the activation of both Fas and Daxx, promotes the export of Daxx from the nucleus to cytoplasm, and facilitates Fas-Daxx binding. Depletion of Daxx by siRNA results in the potentiation of apoptosis, indicating that Fas-Daxx binding in fact is inhibitory to Fas-mediated apoptosis in cells. 4-HNE-induced translocation of Daxx is also accompanied by the activation and nuclear accumulation of HSF1 and up-regulation of heat shock protein Hsp70. All these effects of 4-HNE in cells can be attenuated by ectopic expression of hGSTA4-4, the isozyme of glutathione S-transferase with high activity for 4-HNE. Through immunoprecipitation and liquid chromatography-tandem mass spectrometry, we have demonstrated the covalent binding of 4-HNE to Daxx. We also demonstrate that 4-HNE modification induces phosphorylation of Daxx at Ser668 and Ser671 to facilitate its cytoplasmic export. These results indicate that while 4-HNE exhibits toxicity through several mechanisms, in parallel it evokes signaling for defense mechanisms to self-regulate its toxicity and can simultaneously affect multiple signaling pathways through its interactions with membrane receptors and transcription factors/repressors.

Role of lipid peroxidation in cellular responses to D,L-sulforaphane, a promising cancer chemopreventive agent

D,L-sulforaphane (SFN), a synthetic analogue of the broccoli-derived l-isomer, is a highly promising cancer chemopreventive agent substantiated by inhibition of chemically induced cancer in rodents and prevention of cancer development and distant site metastasis in transgenic mouse models of cancer. SFN is also known to inhibit growth of human cancer cells in association with cell cycle arrest and reactive oxygen species-dependent apoptosis, but the mechanism of these cellular responses to SFN exposure is not fully understood. Because 4-hydroxynonenal (4-HNE), a product of lipid peroxidation (LPO), the formation of which is regulated by hGSTA1-1, assumes a pivotal role in oxidative stress-induced signal transduction, we investigated its contribution in growth arrest and apoptosis induction by SFN using HL60 and K562 human leukemic cell lines as a model. The SFN-induced formation of 4-HNE was suppressed in hGSTA1-1-overexpressing cells, which also acquired resistance to SFN-induced cytotoxicity, cell cycle arrest, and apoptosis. While resistance to SFN-induced cell cycle arrest by ectopic expression of hGSTA1-1 was associated with changes in levels of G2/M regulatory proteins, resistance to apoptosis correlated with an increased Bcl-xL/Bax ratio, inhibition of nuclear translocation of AIF, and attenuated cytochrome c release in cytosol. The hGSTA1-1-overexpressing cells exhibited enhanced cytoplasmic export of Daxx, nuclear accumulation of transcription factors Nrf2 and HSF1, and upregulation of their respective client proteins, gamma-GCS and HSP70. These findings not only reveal a central role of 4-HNE in cellular responses to SFN but also reaffirm that 4-HNE contributes to oxidative stress-mediated signaling.

A central role of RLIP76 in regulation of glycemic control

OBJECTIVE

Pathology associated with oxidative stress frequently results in insulin resistance. Glutathione (GSH) and GSH-linked metabolism is a primary defense against oxidative stress. Electrophilic lipid alkenals, such as 4-hydroxy-t-2-nonenal (4HNE), generated during oxidative stress are metabolized primarily to glutathione electrophile (GS-E) conjugates. Recent studies show that RLIP76 is the primary GS-E conjugate transporter in cells, and a regulator of oxidative-stress response. Because RLIP76(-/-) mice are hypoglycemic, we studied the role of RLIP76 in insulin resistance.

RESEARCH DESIGN AND METHODS

Blood glucose, insulin, lipid measurements, and hyperinsulinemic-euglycemic and hyperglycemic clamp experiments were performed in RLIP76(+/+) and RLIP76(-/-) C57B mice, using Institutional Animal Care and Use Committee-approved protocols. Time-resolved three-dimensional confocal fluorescence microscopy was used to study insulin endocytosis.

RESULTS

The plasma insulin/glucose ratio was ordered RLIP76(-/-) < RLIP76(+/-) < RLIP76(+/+); administration of purified RLIP76 in proteoliposomes to RLIP76(+/+) animals further increased this ratio. RLIP76 was induced by oxidative or hyperglycemic stress; the concomitant increase in insulin endocytosis was completely abrogated by inhibiting the transport activity of RLIP76. Hydrocortisone could transiently correct hypoglycemia in RLIP76(-/-) animals, despite inhibited activity of key glucocorticoid-regulated hepatic gluconeogenic enzymes, phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and fructose 1,6-bisphosphatase, in RLIP76(-/-).

CONCLUSIONS

The GS-E conjugate transport activity of RLIP76 mediates insulin resistance by enhancing the rate of clathrin-dependent endocytosis of insulin. Because RLIP76 is induced by oxidative stress, it could play a role in insulin resistance seen in pathological conditions characterized by increased oxidative stress.

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.

4-Hydroxynonenal induces p53-mediated apoptosis in retinal pigment epithelial cells

4-Hydroxynonenal (4-HNE) has been suggested to be involved in stress-induced signaling for apoptosis. In present studies, we have examined the effects of 4-HNE on the intrinsic apoptotic pathway associated with p53 in human retinal pigment epithelial (RPE and ARPE-19) cells. Our results show that 4-HNE causes induction, phosphorylation, and nuclear accumulation of p53 which is accompanied with down regulation of MDM2, activation of the pro-apoptotic p53 target genes viz. p21 and Bax, JNK, caspase3, and onset of apoptosis in treated RPE cells. Reduced expression of p53 by an efficient silencing of the p53 gene resulted in a significant resistance of these cells to 4-HNE-induced cell death. The effects of 4-HNE on the expression and functions of p53 are blocked in GSTA4-4 over expressing cells indicating that 4-HNE-induced, p53-mediated signaling for apoptosis is regulated by GSTs. Our results also show that the induction of p53 in tissues of mGsta4 (-/-) mice correlate with elevated levels of 4-HNE due to its impaired metabolism. Together, these studies suggest that 4-HNE is involved in p53-mediated signaling in in vitro cell cultures as well as in vivo that can be regulated by GSTs.

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.

Role of the electrophilic lipid peroxidation product 4-hydroxynonenal in the development and maintenance of obesity in mice

The lipid peroxidation product 4-hydroxynonenal (4-HNE) is a signaling mediator with wide-ranging biological effects. In this paper, we report that disruption of mGsta4, a gene encoding the 4-HNE-conjugating enzyme mGSTA4-4, causes increased 4-HNE tissue levels and is accompanied by age-dependent development of obesity which precedes the onset of insulin resistance in 129/sv mice. In contrast, mGsta4 null animals in the C57BL/6 genetic background have normal 4-HNE levels and remain lean, indicating a role of 4-HNE in triggering or maintaining obesity. In mGsta4 null 129/sv mice, the expression of the acetyl-CoA carboxylase (ACC) transcript is enhanced several-fold with a concomitant increase in the tissue level of malonyl-CoA. Also, mitochondrial aconitase is partially inhibited, and tissue citrate levels are increased. Accumulation of citrate could lead to allosteric activation of ACC, further augmenting malonyl-CoA levels. Aconitase may be inhibited by 4-HNE or by peroxynitrite generated by macrophages which are enriched in white adipose tissue of middle-aged mGsta4 null 129/sv mice and, upon lipopolysaccharide stimulation, produce more reactive oxygen species and nitric oxide than macrophages from wild-type mice. Excessive malonyl-CoA synthesized by the more abundant and/or allosterically activated ACC in mGsta4 null mice leads to fat accumulation by the well-known mechanisms of promoting fatty acid synthesis and inhibiting fatty acid beta-oxidation. Our findings complement the recent report that obesity causes both a loss of mGSTA4-4 and an increase in the level of 4-HNE [Grimsrud, P. A., et al. (2007) Mol. Cell. Proteomics 6, 624-637]. The two reciprocal processes are likely to establish a positive feedback loop that would promote and perpetuate the obese state.

4-Hydroxynonenal self-limits fas-mediated DISC-independent apoptosis by promoting export of Daxx from the nucleus to the cytosol and its binding to Fas

Previously, we have shown that 4-hydroxynonenal (4-HNE) induces Fas-mediated apoptosis in HLE B-3 cells through a pathway which is independent of FasL, FADD, procaspase 8, and DISC (Li, J., et al. (2006) Biochemistry 45, 12253-12264). The involvement of Daxx has also been suggested in this pathway, but its role is not clear. Here, we report that Daxx plays an important regulatory role during 4-HNE-induced, Fas-mediated apoptosis in Jurkat cells. 4-HNE induces Fas-dependent apoptosis in procaspase 8-deficient Jurkat cells via the activation of ASK1, JNK, and caspase 3, and the apoptosis can be inhibited by masking Fas with the antagonistic anti-Fas antibodies. We demonstrate that 4-HNE exposure to Jurkat cells leads to the induction of both Fas and Daxx. 4-HNE binds to both Fas and Daxx and promotes the export of Daxx from the nucleus to the cytosol, where it binds to Fas and inhibits apoptosis. Depletion of Daxx results in an increase in the activation of ASK1, JNK, and caspase 3 along with exacerbation of 4-HNE-induced apoptosis, suggesting that Daxx inhibits apoptosis by binding to Fas. 4-HNE-induced translocation of Daxx is also accompanied by the activation of the transcription factor HSF1. The results of these studies are consistent with a model in which, by interacting with Fas, 4-HNE promotes proapoptotic signaling via ASK1, JNK, and caspase 3. In parallel, 4-HNE induces Daxx and promotes its export from the nucleus to the cytosol, where it interacts with Fas to self-limit the extent of apoptosis by inhibiting the downstream proapoptotic signaling. Cytoplasmic translocation of Daxx also results in up-regulation of HSF1-associated stress-responsive genes.

Glutathione level regulates HNE-induced genotoxicity in human erythroleukemia cells

4-hydroxy-trans-2-nonenal (HNE) is one of the most abundant and toxic lipid aldehydes formed during lipid peroxidation by reactive oxygen species. We have investigated the genotoxic effects of HNE and its regulation by cellular glutathione (GSH) levels in human erythroleukemia (K562) cells. Incubation of K562 cells with HNE (5-10 microM) significantly elicited a 3- to 5-fold increased DNA damage in a time- and dose-dependent manner as measured by comet assay. Depletion of GSH in cells by L-buthionine-[S,R]-sulfoximine (BSO) significantly increased HNE-induced DNA damage, whereas supplementation of GSH by incubating the cells with GSH-ethyl ester significantly decreased HNE-induced genotoxicity. Further, overexpression of mGSTA4-4, a HNE-detoxifying GST isozyme, significantly prevented HNE-induced DNA damage in cells, and ablation of GSTA4-4 and aldose reductase with respective siRNAs further augmented HNE-induced DNA damage. These results suggest that the genotoxicity of HNE is highly dependent on cellular GSH/GST/AR levels and favorable modulation of the aldehyde detoxification system may help in controlling the oxidative stress-induced complications.

The non-ABC drug transporter RLIP76 (RALBP-1) plays a major role in the mechanisms of drug resistance

RLIP76 or Ral binding protein (RalBP-1) was initially cloned as a Ral-effector that was proposed as a link between Ral and Ras pathways. This protein is encoded in humans on chromosome 18p11.3 by a gene with 11 exons and 9 introns and is found ubiquitously from drosophila to humans. RLIP76 displays inhibitory GTPase activity toward Rho/Rac class G-protein cdc42 which is involved in regulation of cytoskeletal organization, lamellipodia, cell migration and apoptosis via Ras. We have recently shown that RLIP76 is also a multispecific transporter of chemotherapeutic agents and glutathione conjugates (GS-E). In human cells RLIP76 accounts for more than two third of the transport activity for GS-E and drugs as opposed to the ABC-transporters including MRP1, which account for less than one third of this activity. Evidence is mounting that RLIP76 is a stress-responsive multi-specific, non-ABC transporter which represents an entirely novel link between stress-inducible G-protein signaling, receptor tyrosine-kinase signaling, endocytosis, heat-shock and stress defense pathways, and transport mediated drug-resistance. The expression of RLIP76 is significantly greater in human cancer cells of diverse origin as compared to the non-malignant cells. Inhibition of RLIP76, using antibodies towards a cell surface epitope, or depletion of RLIP76 using either siRNA or anti-sense phosphorothioate oligonucleotides preferentially causes apoptosis in malignant cells. Administration of RLIP76 antibodies, siRNA, or anti-sense oligonucleotides to mice bearing syngeneic B16 mouse melanoma tumors causes rapid and complete regression of tumors. Studies summarized in this review strongly suggest that RLIP76 is a logical target for clinical intervention of not only multi-drug resistance but also for diseases resulting from oxidative stress.

Regression of Lung and Colon Cancer Xenografts by Depleting or Inhibiting RLIP76 (Ral-Binding Protein 1)

Ral-binding protein 1 (RALBP1) is a stress-responsive and stress-protective multispecific transporter of glutathione conjugates (GS-E) and xenobiotic toxins. It is frequently overexpressed in malignant cells and plays a prominent antiapoptotic role selectively in cancer cells through its ability to control cellular concentration of proapoptotic oxidized lipid byproducts. In the absence of chemotherapy, depletion or inhibition of RALBP1 causes regression of syngeneic mouse B16 melanoma. Because RALBP1 transports anthracycline and Vinca alkaloid drugs, as well as GS-E, and because it confers resistance to these drugs, we proposed that depletion or inhibition of RALBP1 should cause regression of human solid tumors that overexpress RALBP1 and augment chemotherapy efficacy. Non-small-cell lung cancer (NSCLC) H358 and H520 and colon SW480 cell lines were used. Cytotoxic synergy between anti-RALBP1 immunoglobulin G (IgG), cis-diammine-dichloroplatinum (II) [CDDP], and vinorelbine was examined in cell culture and xenografts of NSCLC cells. Effects of RALBP1 depletion by antisense were examined in xenografts of NSCLC H358, NSCLC H520, and colon SW480 cells. RALBP1 depletion by phosphorothioate antisense was confirmed and was associated with rapid, complete, and sustained remissions in established s.c. human lung and colon xenografts. RALBP1 inhibition by anti-RALBP1 IgG was equally as effective as antisense and enhanced CDDP-vinorelbine in lung cancer xenografts. These studies show that RALBP1 is a transporter that serves as a key effector function in cancer cell survival and is a valid target for cancer therapy, and confirm that inhibitory modulation of RALBP1 transport activity at the cell surface is sufficient for antitumor effects.

Doxorubicin transport by RALBP1 and ABCG2 in lung and breast cancer

RALBP1 (RLIP76) is the major transporter of doxorubicin (DOX) in lung cancer cells, and that the difference in sensitivity of small cell lung cancer (SCLC) cells to DOX is due to differential phosphorylation by PKCalpha. Our recent studies have suggested that RALBP1 present in MCF-7 breast cancer cells has significantly lower specific activity for transport of DOX than wild-type recombinant protein, and its level of expression is significantly lower than that in lung cancer cells. In the present study, we have explored whether or not this is a generalized phenomenon for breast cancer, and have compared the relative contributions of RALBP1 and the ABC-family transporter, ABCG2 to total DOX transport activities in two SCLC (H1417 and H1618), two non-small cell lung cancer (NSCLC) (H358 and H520), and three breast cancer (T-47D, MDA-MB231, and MCF-7) cell lines. Results of these studies show lower protein expression and specific activity of RALBP1 in all three breast cancer cell lines as compared with lung cancer cell lines. Furthermore, we demonstrate that RALBP1 contributes only a minor fraction of DOX transport activity in breast cancer cell lines, suggesting that greater DOX sensitivity of breast cancer may be related to lower RALBP1 transporter activity and that the transport mechanisms involved in multidrug resistance of lung and breast cancer are distinct.

Regulation of CD95 (Fas) Expression and Fas-Mediated Apoptotic Signaling in HLE B-3 Cells by 4-Hydroxynonenal†

The Fas (apo/CD95) receptor which belongs to the TNF-alpha family is a transmembrane protein involved in the signaling for apoptosis through the extrinsic pathway. During this study, we have examined a correlation between intracellular levels of 4-HNE and expression of Fas in human lens epithelial (HLE B-3) cells. Our results show that in HLE B-3 cells, Fas is induced by 4-HNE in a concentration- and time-dependent manner, and it is accompanied by the activation of JNK, caspase 3, and the onset of apoptosis. Fas induction and activation of JNK are also observed in various tissues of mGsta4 null mice which have elevated levels of 4-HNE. Conversely, when 4-HNE is depleted in HLE B-3 cells by a transient transfection with hGSTA4, Fas expression is suppressed. However, upon the cessation of hGSTA4 expression in these transiently transfected cells, Fas and 4-HNE return to their basal levels. Fas-deficient transformed HLE B-3 cells stably transfected with hGSTA4 show remarkable resistance to apoptosis. Also, the wild-type HLE B-3 cells in which Fas is partially depleted by siRNA acquire resistance to 4-HNE-induced apoptosis, suggesting an at least partial role of Fas in 4-HNE-induced apoptosis in HLE B-3 cells. We also demonstrate that during 4-HNE-induced apoptosis of HLE B-3 cells, Daxx is induced and it binds to Fas. Together, these results show an important role of 4-HNE in regulation of the expression and functions of Fas.

Glutathione S-transferases as antioxidant enzymes: Small cell lung cancer (H69) cells transfected with hGSTA1 resist doxorubicin-induced apoptosis

It has been suggested that the alpha-class glutathione S-transferases (GSTs) protect various cell types from oxidative stress and lipid peroxidation (LPO). In order to examine the protective role of alpha-class GST isozyme hGSTA1-1 against doxorubicin (DOX)-induced lipid peroxidation, cytotoxicity, and apoptosis, human small cell lung cancer (SCLC) H69 cells were stably transfected with hGSTA1. Immunological and biochemical characterization of hGSTA1-transfected cells revealed the expression of functionally active hGSTA1-1 localized near the cellular plasma membranes. hGSTA1-transfected cells acquired significantly increased resistance to the DOX-induced cytotoxicity by suppressing lipid peroxidation levels in these cells. Overexpression of hGSTA1-1 in cells inhibited DOX-mediated depletion of GSH and higher GSH levels were found in DOX-treated hGSTA1-transfected cells as compared with empty vector-transfected controls. hGSTA1-1 overexpression also provided protection to cells from DOX-induced apoptosis by inhibiting phosphorylation of c-Jun-N-terminal kinases (JNK), caspase-3 activation, and by preserving the levels of anti-apoptotic protein Bcl-2. These results are consistent with the idea that the alpha-class GSTs provide protection against oxidative stress by attenuating lipid peroxidation and these enzymes can modulate signaling for apoptosis.

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.

Determinants of differential doxorubicin sensitivity between SCLC and NSCLC

Doxorubicin (DOX) transport activity of Ral-interacting protein (RLIP76) in non-small cell lung cancer (NSCLC) is approximately twice that of in small cell lung cancer (SCLC). Since protein-kinase-C (PKC)alpha mediated phosphorylation of RLIP76 causes doubling of the specific activity of RLIP76, and NSCLC cells are known to have greater PKCalpha activity, we examined the contribution of PKC mediated phosphorylation of RLIP76 towards intrinsic DOX-resistance in human NSCLC. Expression of a deletion mutant RLIP76(delPKCalpha-sites) followed by depletion of the wild-type RLIP76 using a siRNA targeted at one of the deleted regions resulted in generation of cells expressing only the mutant protein, which could not be phosphorylated by PKCalpha. DOX-transport activity of the mutant RLIP76 purified from NSCLC and SCLC was similar and comparable to that of RLIP76 purified from the wild-type SCLC. However, this activity was significantly lower than that of RLIP76 purified from the wild-type NSCLC. After siRNA mediated depletion of PKCalpha, DOX-transport activities of RLIP76 purified from SCLC and NSCLC were indistinguishable. Depletion of PKCalpha inhibited the growth of NSCLC more than SCLC cells (70+/-3% vs. 43+/-5%, respectively). PKCalpha-depletion lowered the IC(50) of NSCLC cell lines for DOX to the same level as that observed for SCLC. RLIP76(-/-) mouse embryonic fibroblasts (MEFs) were significantly more sensitive to DOX as compared with RLIP76(+/+) MEFs (IC(50) 25 vs. 125nM, respectively). However, PKCalpha-depletion did not affect DOX-cytotoxicity towards RLIP76(-/-) MEFs, as opposed to RLIP76(+/+) MEFs which were sensitized by 2.2-fold. These results demonstrate that RLIP76 is a primary determinant of DOX-resistance, and that PKCalpha mediated accumulation defect and DOX-resistance in NSCLC is primarily due to differential phosphorylation of RLIP76 in SCLC and NSCLC.

The course of CCl4 induced hepatotoxicity is altered in mGSTA4-4 null (-/-) mice

Glutathione S-transferases (GSTs) play a key role in cellular detoxification of environmental toxicants through their conjugation to glutathione (GSH). Recent studies have shown that the alpha-class GSTs also provide protection against oxidative stress and lipid peroxidation (LPO). GSTA4-4 is a member of a sub group of the alpha-class GSTs. It has been shown to metabolize 4-hydroxynonenal (4-HNE) with high catalytic efficiency through its conjugation to glutathione (GSH) and has been suggested to be a major component of cellular defense against toxic electrophiles such as 4-HNE generated during LPO. Since the hepatotoxicity of carbon tetrachloride (CCl(4)) has been suggested to be due to the generation of free radicals leading to membrane LPO, the present studies were designed to compare hepatotoxicity of CCl(4) in GSTA4-4 null (-/-) and wild type (+/+) mice. The results show that administration of a single dose of CCl(4) (1 ml/kg i.p.) resulted in time dependent hepatotoxicity in both -/- and +/+ mice; the extent of cellular damage by serum enzymes suggests that progression was more rapid in -/- mice, although injury was similar by 24 h. Histopathologic examination showed similar degrees of centrilobular necrosis by 24 h but much greater surrounding degenerative change, including cellular swelling, disarray, and vacuolization, in the liver of -/- mice. As expected -/- mice did not show any expression of mGSTA4-4; after CCl(4) a compensatory increase in the activities of total GST activity was noted at 24 h. Major alterations in other antioxidant enzymes was not observed. 4-HNE levels in the liver of -/- mice were about four-fold higher than in +/+ mice, suggesting a positive correlation between 4-HNE levels and the altered course of CCl(4) hepatotoxicity. These studies suggest that GSTA4-4 is an important component during the early stages (1-6 h) of cellular defense against oxidative stress and LPO although, it is not effective in protecting against the ultimate degree of overall cell injury.

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.

Depletion of 4-hydroxynonenal in hGSTA4-transfected HLE B-3 cells results in profound changes in gene expression

Previously, we have shown that overexpression of 4-hydroxy-2-nonenal (HNE)-detoxifying enzyme glutathione S-transferase A4-4 (hGSTA4-4) in human lens epithelial cells (HLE B-3) leads to pro-carcinogenic phenotypic transformation of these cells [R. Sharma, et al. Eur. J. Biochem. 271 (2004) 1960-1701]. We now demonstrate that hGSTA4-transfection also causes a profound change in the expression of genes involved in cell adhesion, cell cycle control, proliferation, cell growth, and apoptosis, which is consistent with phenotypic changes of the transformed cells. The expression of p53, p21, p16, fibronectin 1, laminin gamma1, connexin 43, Fas, integrin alpha6, TGFalpha, and c-jun was down-regulated, while the expression of protein kinase C beta II (PKCbetaII), c-myc, cyclin-dependent kinase 2 (CDK2), and TGFbeta was up-regulated in transfected cells. These results demonstrate that HNE serves as a crucial signaling molecule and, by modulating the expression of genes, can influence cellular functions.

Depletion of RLIP76 sensitizes lung cancer cells to doxorubicin

Ral-interacting protein (RLIP76) (RALBP1) is an anti-apoptotic non-ABC glutathione (GSH)-conjugate transporter involved in receptor-ligand endocytosis, as well as in multispecific drug transport and resistance. Partial inhibition of RLIP76 using antibodies in the absence of chemotherapy drug causes apoptosis in multiple small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC) cell lines and in the presence of doxorubicin (DOX), marked synergy is observed. These findings indicated that RLIP76 should be a good target for cancer cell killing; its down-regulation would promote apoptosis through both drug-dependent and drug-independent effects. To examine the effect of complete and specific RLIP76 depletion on apoptosis, we tested the effects of RLIP76 siRNA in a number of lung cancer cell lines. Growth inhibition and apoptosis was observed in all cases upon RLIP76 depletion. Consistent with these findings, augmenting cellular RLIP76 through transfection or liposomal protein delivery conferred resistance to apoptosis mediated by either DOX or 4-hydroxynonenal (4-HNE). Taken together, our results show that RLIP76 is rational and promising new target for lung cancer therapy.

The role of PKCα and RLIP76 in transport-mediated doxorubicin-resistance in lung cancer

In deletion mutant analyses of potential phosphorylation sites in RLIP76, we identified T297 and S509 as targets for phosphorylation by PKCalpha. Phosphorylation at T297 increased doxorubicin (DOX)-transport activity approximately 2-fold for RLIP76 purified from recombinant source, or from three small (H69, H1417, H1618) and three non-small cell, one each derived from H226 (squamous), H358 (bronchio alveolar), and H1395 (adenocarcinoma) lung cancer cell lines. T297 phosphorylation conferred sensitivity to tryptic digestion at R293. The specific activity for DOX-transport by RLIP76 purified from non-small cell, which was primarily in the phosphorylated form, was approximately twice that in small cell lung cancer cell lines. These finding offer a novel explanation for the observed intrinsic differences in sensitivity to DOX between non-small cell and small cell lung cancer cell lines.

Toxicity of oxidized β-carotene to cultured human cells

Carotenoids are effective antioxidants in vitro, but they are also susceptible to autoxidation, which generates volatile and biologically active aldehydes and ketones. In a previous study, we showed that autoxidized beta-carotene inhibits Na+-K+-ATPase activity more effectively than aldehydic products derived from lipid peroxidation, such as 4-hydroxynonenal. In this study, we compared mitochondrial dysfunction in cultured human K562 erythroleukaemic and 28 SV4 retinal pigment epithelium (RPE) cells in response to the degradation products of beta-carotene autoxidation using the MTT assay. We found that oxidized beta-carotene is cytotoxic and that mitochondrial function is decreased in both K562 and RPE cells. In addition, the RPE cells were more resistant to this form of oxidative stress, suggesting that its cytotoxicity may depend on cellular antioxidant capacity.

RLIP76 Is a Major Determinant of Radiation Sensitivity

RLIP76 (RALBP1) is a glutathione-conjugate transporter that is a critical component of clathrin-coated pit-mediated endocytosis, as well as in stress responses. In cultured cells, it provides protection from stressors including heat, oxidant chemicals, chemotherapeutic agents, UV irradiation, and X-irradiation. Here, we show marked reduction in glutathione conjugate transport capacity and stepwise increase in radiation sensitivity associated with heterozygous or homozygous loss of the RLIP76 gene in mice. Survival after radiation in homozygous knockout animals was significantly shorter than either the heterozygous knockouts or the wild type. Delivery of recombinant RLIP76 to mice lacking RLIP76 via a liposomal delivery system rescued radiation sensitivity. Furthermore, treatment of wild-type mice with RLIP76-containing liposomes conferred resistance to radiation. These findings suggest that inhibiting RLIP76 could be used for sensitization to radiation during cancer therapy and that RLIP76 liposomes could be radioprotective agents useful for treatment of iatrogenic or catastrophic radiation poisoning.

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

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

RLIP76 Transports Vinorelbine and Mediates Drug Resistance in Non–Small Cell Lung Cancer

Vinorelbine (Navelbine), an amphiphilic semisynthetic Vinca alkaloid, has displayed superior activity and decreased resistance in the treatment of advanced non–small cell lung cancer (NSCLC) compared with other members of its class. Recently, vinorelbine and cisplatin combination chemotherapy has been shown for the first time to confer a significant survival advantage in early-stage lung cancer after surgical therapy. The biological mechanisms underlying the differential response of NSCLC to cytocidal activity of vinorelbine have yet to be elucidated. Our recent findings indicate a role of RLIP76, a non–ATP binding cassette transport protein, in catalyzing the ATP-dependent efflux of structurally and functionally unrelated chemotherapeutic agents such as doxorubicin and vinblastine in NSCLC. Present studies were conducted to assess whether RLIP76 mediates vinorelbine transport and resistance. Here we show that RLIP76 catalyzes the transport of vinorelbine in a saturable manner with respect to vinorelbine (Km 75 nmol/L) and ATP (Km = 3.4 mmol/L). Three-fold overexpression of RLIP76 in NSCLC and SCLC confers increased resistance to cytotoxicity. RLIP76 overexpression causes a sustained intracellular decrease in vinorelbine concentration because of increased efflux, and anti-RLIP76 antibodies sensitize lung cancer cells to vinorelbine by inhibiting its efflux. These studies for the first time show that RLIP76 mediates vinorelbine transport and is capable of conferring drug accumulation defect and resistance to lung cancer cells.

RLIP76 transports vinorelbine and mediates drug resistance in non-small cell lung cancer

Vinorelbine (Navelbine), an amphiphilic semisynthetic Vinca alkaloid, has displayed superior activity and decreased resistance in the treatment of advanced non-small cell lung cancer (NSCLC) compared with other members of its class. Recently, vinorelbine and cisplatin combination chemotherapy has been shown for the first time to confer a significant survival advantage in early-stage lung cancer after surgical therapy. The biological mechanisms underlying the differential response of NSCLC to cytocidal activity of vinorelbine have yet to be elucidated. Our recent findings indicate a role of RLIP76, a non-ATP binding cassette transport protein, in catalyzing the ATP-dependent efflux of structurally and functionally unrelated chemotherapeutic agents such as doxorubicin and vinblastine in NSCLC. Present studies were conducted to assess whether RLIP76 mediates vinorelbine transport and resistance. Here we show that RLIP76 catalyzes the transport of vinorelbine in a saturable manner with respect to vinorelbine (K(m) 75 nmol/L) and ATP (K(m) = 3.4 mmol/L). Three-fold overexpression of RLIP76 in NSCLC and SCLC confers increased resistance to cytotoxicity. RLIP76 overexpression causes a sustained intracellular decrease in vinorelbine concentration because of increased efflux, and anti-RLIP76 antibodies sensitize lung cancer cells to vinorelbine by inhibiting its efflux. These studies for the first time show that RLIP76 mediates vinorelbine transport and is capable of conferring drug accumulation defect and resistance to lung cancer cells.

Enhanced expression of glutathione-S-transferase A1-1 protects against oxidative stress in human retinal pigment epithelial cells

Glutathione-S-transferases (GSTs) play an important role in protection mechanisms against oxidative stress. We sought to determine whether over-expression of human GSTA1-1 in RPE cells is able to attenuate H(2)O(2)-induced oxidative stress. SV40-transformed human fetal RPE cells were stably transfected with pRC/hGSTA1-1 vector which carries a full-length of human GSTA1-1 cDNA. The control RPE cells were either non-transfected or transfected with control vector pRC. Expression of hGSTA1-1 protein in these cells was confirmed by Western blot and immunocytochemical analyses. The protective effects of hGSTA1-1 on cell viability and mitochondrial DNA (mtDNA) damage caused by H(2)O(2) were examined with MTT assay and quantitative PCR (QPCR), respectively. The hGSTA1-1 transfected RPE cells exhibited a similar morphology and growth rate as control RPE cells. Immunocytochemical analysis showed robust expression hGSTA1-1 in hGSTA1-1 transfected cells versus background staining in control cells. Western blotting of protein extracts from cells transfected with hGSTA1-1 revealed a 26 kDa protein band which corresponds to the size of recombinant mature hGSTA1-1. The active GST present in the hGSTA1-1 transfected cells was approximately three times higher than in control cells. The MTT assay showed a significantly greater viability of hGSTA1-1 cells in response to H(2)O(2) (100 and 200 microm) compared to control cells (p<0.05). QPCR indicated that mtDNA damage was significantly decreased in hGSTA1-1 cells than in control cells (p<0.05). Human GSTA1-1 transfection protect against RPE cell death and mtDNA damage caused by H(2)O(2), suggesting an important role of GST in protection against oxidative stress in RPE cells.

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.

Identification of Membrane-Anchoring Domains of RLIP76 Using Deletion Mutant Analyses†

RLIP76 (RALBP1) is a multifunctional transporter involved in signaling and transmembrane movement of solute allocrites, which include glutathione conjugates and several natural product antineoplastic agents [Awasthi, S., et al. (2000) Biochemistry 39, 9327-9334; (2001) Biochemistry 40, 4159-4168]. Our previous studies suggested that the membrane-anchoring domain resides in the N-terminus of RLIP76, despite the lack of identifiable membrane-spanning domains. Amino acid sequence analysis indicated that this region of RLIP76 contains sequences that are similar to those of vector peptides. We, therefore, have studied the effect of a series of deletion mutant proteins on hydrophobicity and transport activity. RLIP76 or one of its derived deletion mutants was expressed in Escherichia coli, and bacteria were lysed and extracted in buffer without or with the nonionic detergent polidocanol. The ratio of RLIP76 in the detergent/aqueous extracts was found to be 2.5 for the wild-type protein, but decreased to 0.7 in the mutant in which amino acids 154-219 were deleted. Deletion of only one segment of this region (amino acids 171-185) alone resulted in a significant decrease in this ratio to 1.0. For the mutants with deletions within the region from amino acid 154 to 219, loss of hydrophobicity correlated with less incorporation of mutants into artificial liposomes, and decreased transport activity toward doxorubicin and dinitrophenyl-S-glutathione. In contrast, deletion of one of the two ATP-binding sites (at amino acids 65-80 or 415-448) or both sites did not affect hydrophobicity but reduced or abrogated transport activity. NSCLC (H358) stably transfected with del171-185 and del154-219 showed that loss of these regions results in a decrease in the extent of membrane association of RLIP76. Confocal laser immunohistochemistry colocalized amino acids 171-185 with her2/neu on the cell surface. Depletion of wild-type RLIP76 using si-RNA directed to this region in cells transfected with del171-185 resulted in the loss of cell surface expression. These finding demonstrate that amino acids 171-185 constitute a cell surface epitope which is necessary for optimal transport of anthracycline and glutathione conjugates by RLIP76, and that this peptide could be a novel target for antineoplastic therapy.

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.