Expression of recombinant glutathione S-transferase pi, Ya, or Yb1 confers resistance to alkylating agents

Determination of plasma levels of the active thiol form of the direct-acting PrC-210 ROS-scavenger using a fluorescence-based assay

PrC-210 is a direct-acting ROS-scavenger. It's active when administered orally, IV, or topically; it has none of the nausea/emesis nor hypotension side effects that have precluded human amifostine use. PrC-210 confers 100% survival to mice and rats that received an otherwise 100% lethal radiation dose and 36% reduction of ischemia-reperfusion-induced mouse myocardial infarct damage, and thus is a viable candidate to prevent human ROS-induced ischemia-reperfusion and ionizing radiation toxicities. We report the first assay for the pharmacologically active PrC-210 thiol in blood. PrC-210 has no double-bonds nor light absorption, so derivatizing the thiol with a UV-absorbing fluorochrome enables quantification. This assay: i) is done on the benchtop; it's read with a fluorescence plate reader, ii) provides linear product formation through 60 min, iii) quantifies μM to low mM rodent blood levels of PrC-210 that confer complete radioprotection, iv) accurately reflects PrC-210 thiol formation of mixed disulfides with other thiols in blood, and v) shows excellent between-day assay outcome with very low standard deviation and coefficient of variation. A fluorescence assay quantifying formation of a PrC-210 thiol-bimane adduct enables measurement of blood PrC-210 thiol. A blood assay will help in the development of PrC-210 for use in the human clinical setting.

Simultaneous defeat of MCF7 and MDA-MB-231 resistances by a hypericin PDT-tamoxifen hybrid therapy

Currently the greatest challenge in oncology is the lack of homogeneity of the lesions where different cell components respond differently to treatment. There is growing consensus that monotherapies are insufficient to eradicate the disease and there is an unmet need for more potent combinatorial treatments. We have previously shown that hypericin photodynamic therapy (HYP-PDT) triggers electron transport chain (ETC) inhibition in cell mitochondria. We have also shown that tamoxifen (TAM) enhances cytotoxicity in cells with high respiration, when combined with ETC inhibitors. Herein we introduce a synergistic treatment based on TAM chemotherapy and HYP-PDT. We tested this novel combinatorial treatment (HYPERTAM) in two metabolically different breast cancer cell lines, the triple-negative MDA-MB-231 and the estrogen-receptor-positive MCF7, the former being quite sensitive to HYP-PDT while the latter very responsive to TAM treatment. In addition, we investigated the mode of death, effect of lipid peroxidation, and the effect on cell metabolism. The results were quite astounding. HYPERTAM exhibited over 90% cytotoxicity in both cell lines. This cytotoxicity was in the form of both necrosis and autophagy, while high levels of lipid peroxidation were observed in both cell lines. We, consequently, translated our research to an in vivo pilot study encompassing the MDA-MB-231 and MCF7 tumor models in NOD SCID-γ immunocompromised mice. Both treatment cohorts responded very positively to HYPERTRAM, which significantly prolonged mice survival. HYPERTAM is a potent, synergistic modality, which may lay the foundations for a novel, composite anticancer treatment, effective in diverse tumor types.

The diverse roles of glutathione-associated cell resistance against hypericin photodynamic therapy

The diverse responses of different cancers to treatments such as photodynamic therapy of cancer (PDT) have fueled a growing need for reliable predictive markers for treatment outcome. In the present work we have studied the differential response of two phenotypically and genotypically different breast adenocarcinoma cell lines, MCF7 and MDA-MB-231, to hypericin PDT (HYP-PDT). MDA-MB-231 cells were 70% more sensitive to HYP PDT than MCF7 cells at LD50. MCF7 were found to express a substantially higher level of glutathione peroxidase (GPX4) than MDA-MB-231, while MDA-MB-231 differentially expressed glutathione-S-transferase (GSTP1), mainly used for xenobiotic detoxification. Eighty % reduction of intracellular glutathione (GSH) by buthionine sulfoximine (BSO), largely enhanced the sensitivity of the GSTP1 expressing MDA-MB-231 cells to HYP-PDT, but not in MCF7 cells. Further inhibition of the GSH reduction however by carmustine (BCNU) resulted in an enhanced sensitivity of MCF7 to HYP-PDT. HYP loading studies suggested that HYP can be a substrate of GSTP for GSH conjugation as BSO enhanced the cellular HYP accumulation by 20% in MDA-MB-231 cells, but not in MCF7 cells. Studies in solutions showed that L-cysteine can bind the GSTP substrate CDNB in the absence of GSTP. This means that the GSTP-lacking MCF7 may use L-cysteine for xenobiotic detoxification, especially during GSH synthesis inhibition, which leads to L-cysteine build-up. This was confirmed by the lowered accumulation of HYP in both cell lines in the presence of BSO and the L-cysteine source NAC. NAC reduced the sensitivity of MCF7, but not MDA-MB-231, cells to HYP PDT which is in accordance with the antioxidant effects of L-cysteine and its potential as a GSTP substrate. As a conclusion we have herein shown that the different GSH based cell defense mechanisms can be utilized as predictive markers for the outcome of PDT and as a guide for selecting optimal combination strategies.

Non-Hodgkin's B-cell lymphoma: advances in molecular strategies targeting drug resistance

Non-Hodgkin's lymphoma (NHL) is a heterogeneous class of cancers displaying a diverse range of biological phenotypes, clinical behaviours and prognoses. Standard treatments for B-cell NHL are anthracycline-based combinatorial chemotherapy regimens composed of cyclophosphamide, doxorubicin, vincristine and prednisolone. Even though complete response rates of 40-50% with chemotherapy can be attained, a substantial proportion of patients relapse, resulting in 3-year overall survival rates of about 30%. Relapsed lymphomas are refractory to subsequent treatments with the initial chemotherapy regimen and can exhibit cross-resistance to a wide variety of anticancer drugs. The emergence of acquired chemoresistance thus poses a challenge in the clinic preventing the successful treatment and cure of disseminated B-cell lymphomas. Gene-expression analyses have increased our understanding of the molecular basis of chemotherapy resistance and identified rational targets for drug interventions to prevent and treat relapsed/refractory diffuse large B-cell lymphoma. Acquisition of drug resistance in lymphoma is in part driven by the inherent genetic heterogeneity and instability of the tumour cells. Due to the genetic heterogeneity of B-cell NHL, many different pathways leading to drug resistance have been identified. Successful treatment of chemoresistant NHL will thus require the rational design of combinatorial drugs targeting multiple pathways specific to different subtypes of B-cell NHL as well as the development of personalized approaches to address patient-to-patient genetic heterogeneity. This review highlights the new insights into the molecular basis of chemorefractory B-cell NHL that are facilitating the rational design of novel strategies to overcome drug resistance.

Enhanced sensitivity of A549 cells to the cytotoxic action of anticancer drugs via suppression of Nrf2 by procyanidins from Cinnamomi Cortex extract

Nuclear factor-E2-related factor 2 (Nrf2) is an important cytoprotective transcription factor because Nrf2-regulated enzymes play a key role in antioxidant and detoxification processes. Recent studies have reported that lung cancer cells overexpressing Nrf2 exhibit increased resistance to chemotherapy. Suppression of overexpressed Nrf2 is needed for a new therapeutic approach against lung cancers. In the present study, we found that Cinnamomi Cortex extract (CCE) has an ability to suppress Nrf2-regulated enzyme activity and Nrf2 expression in human lung cancer A549 cells with high Nrf2 activity. Moreover, we demonstrated that CCE significantly enhances sensitivity of A549 cells to the cytotoxic action of doxorubicin and etoposide as well as increasing the intracellular accumulation of both drugs. These results suggest that CCE might be an effective concomitant agent to reduce anticancer drug resistance derived from Nrf2 overexpression. Bioactivity-guided fractionation revealed that procyanidin tetramers and pentamers contained in CCE were active components in suppressing Nrf2.

Pharmacogenomics of cisplatin-induced ototoxicity

Cisplatin ototoxicity affects different individuals in a widely variable manner. These variations are likely to be explained by genetic differences among those affected. It would be highly advantageous to identify genetic variants that predispose to cisplatin ototoxicity in order to minimize the risk to susceptible subgroups. Although this area of research is very important, only a few studies have rigorously examined the genetic basis for cisplatin-induced susceptibility to hearing loss. This article addresses recent progress in clarifying the incidence of cisplatin ototoxicity and the risk factors and controversies regarding the identification of genetic variants associated with cisplatin-induced hearing loss.

Novel amiodarone-doxorubicin cocktail liposomes enhance doxorubicin retention and cytotoxicity in DU145 human prostate carcinoma cells

We have developed novel cocktail liposomes bearing doxorubicin in their hydrophilic cores, and amiodarone, a potent multidrug resistance inhibitor, in their lipid bilayers. The efficacy of these liposomes was studied in DU145 human prostate carcinoma cells. Intracellular calcein retention, which is inversely proportional to multidrug resistance activity, significantly increased following cell incubation with amiodarone loaded liposomes. Fluorescence confocal microscopy on cells incubated with the cocktail liposomes revealed enhanced intranuclear doxorubicin accumulation. Two liposomal drug concentration combinations were employed to assess the differential cytotoxicity of the cocktail liposomes, doxorubicin (1.4 microM)-amiodarone (15 microM) and doxorubicin 3 (microM)-amiodarone (45 microM), and two incubation times, 5 and 19 h. Cell toxicity was determined by XTT assays at 24, 48, and 72 h following incubation and was significantly enhanced for incubation with the cocktail liposomes. On the whole, we believe that these liposomes will greatly contribute to the cancer chemotherapy arena.

Germline glutathione S-transferase variants in breast cancer: relation to diagnosis and cutaneous long-term adverse effects after two fractionation patterns of radiotherapy

PURPOSE

To explore whether certain glutathione S-transferase (GST) polymorphisms are associated with an increased risk of breast cancer or the level of radiation-induced adverse effects after two fractionation patterns of adjuvant radiotherapy.

METHODS AND MATERIALS

The prevalence of germline polymorphic variants in GSTM1, GSTP1, and GSTT1 was determined in 272 breast cancer patients and compared with that in a control group of 270 women from the general population with no known history of breast cancer. The genetic variants were determined using multiplex polymerase chain reaction followed by restriction enzyme fragment analysis. In 253 of the patients surveyed for radiotherapy-induced side effects after a median observation time of 13.7 years (range, 7-22.8 years), the genotypes were related to the long-term effects observed after two fractionation patterns (treatment A, 4.3 Gy in 10 fractions for 156 patients; and treatment B, 2.5 Gy in 20 fractions for 97; both administered within a 5-week period).

RESULTS

None of the GST polymorphisms conferred an increased risk of breast cancer, either alone or in combination. Compared with treatment B, treatment A was followed by an increased level of moderate to severe radiation-induced side effects for all the endpoints studied (i.e., degree of telangiectasia, subcutaneous fibrosis and atrophy, lung fibrosis, costal fractures, and pleural thickening; p <0.001 for all endpoints). A significant association was found between the level of pleural thickening and the GSTP1 Ile105Val variant.

CONCLUSION

The results of this study have illustrated the impact of hypofractionation on the level of adverse effects and indicated that the specific alleles of GSTP1, M1, and T1 studied here may be significant in determining the level of adverse effects after radiotherapy.

Overexpression of GSTA2 protects against cell cycle arrest and apoptosis induced by the DNA inter-strand crosslinking nitrogen mustard, mechlorethamine

The effectiveness of bifunctional alkylating nitrogen mustard compounds in chemotherapy is related to their ability to form DNA inter-strand crosslinks. Patients exposed to DNA inter-strand crosslinking (ICL) agents subsequently experience an elevated incidence of myelodysplastic syndromes (MDS) and MDS related acute myeloid leukemia. Fanconi's anemia (FA) patients are deficient in the repair of crosslink DNA damage and they experience a high incidence of MDS. These observations indicate that hematopoietic cells are specific target for the transforming effects of DNA crosslinking damage. Changes in transcript levels were characterized in human hematopoietic cells occurring in response to the nitrogen mustard, mechlorethamine (HN2), but not in response to monofunctional analogs. Only modest changes in a few gene transcripts were detected in HL60 cells exposed to levels of HN2 tittered to maximal dose that caused growth suppression with minimal cell death and allowed eventual resumption of normal cell growth. Under conditions of transient growth suppression, a subset of glutathione-S-transferase (GST) isoenzyme genes was consistently upregulated three to fourfold by HN2, but not by monofunctional analogs. Subsequent efforts to confirm the changes detected by microarray analyses revealed an unexpected dependence on treatment conditions. The GST alpha class A2 subfamily member transcripts were upregulated 24 h after a 1 h exposure to HN2 that caused an extensive, but transient block in late S/G2 cell cycle phase, but were minimally altered with continuous exposure. The 1-h exposure to HN2 caused a transient late S/G2 cell cycle arrest in both the HL-60 cell line and the Colo 320HSR human colon cancer cell line. Overexpression of GSTA2 by transient transfection protected Colo 320HSR cells against both cycle arrest and apoptosis following exposure to HN2. Overexpression of GSTA2 in Colo 320HSR cells induced after exposure to HN2 did not alter cycle arrest or apoptosis. The results indicate that human GSTA2 facilitates the protection of cells from HN2 damage and not repair. Our results are consistent with the possibility that GSTA2 polymorphisms, variable isoenzyme expression, and variable induced expression may be factors in the pathogenesis of MDS.

Reversal of multidrug resistance by small interfering double-stranded RNAs in ovarian cancer cells

OBJECTIVE

Cisplatin (DDP) resistance is a major barrier to overcome before chemotherapy can become curative for most patients presenting with ovarian cancer. In this study, we investigated the effect of siRNAs on expression of p-gp, GST-pi mRNA and protein in cisplatin-resistant human ovarian cancer cells in order to restore sensitivity to DDP.

METHODS

Small interfering double-stranded RNAs (siRNA) were designed to target p-glycoprotein (p-gp) and glutathione S-transferases (GST) mRNA as a strategy to inhibit both resistant gene expression at the mRNA level. Using Real-Time PCR and western blotting assay the changes of the RNA and protein levels of both drug resistant genes were studied.

RESULTS

Transfection of MDR-1 and GST siRNAs into human multi-drug resistance (MDR) ovarian cancer cell lines, COC1/DDP and SKOV3/DDP, resulted in a time-dependent inhibition of both gene expressions with the decline of the IC(50) values but had no effect on the expression of a-Tubulin. Inhibition of P-gp and GST expression by siRNA enhanced the intracellular accumulation of and restored sensitivity to DDP.

CONCLUSIONS

These studies suggest that p-gp and GST siRNAs are effective inhibitors of MDR gene expression and reverse the resistance of ovarian carcinomas. Our studies may provide a new insight to develop siRNAs as a novel therapeutic tool for the treatment of ovarian carcinomas.

Increased glutathione S-transferase P1-1 expression by mRNA stabilization in hemin-induced differentiation of K562 cells

GSTP1-1 gene expression mechanisms were investigated in hemin-induced erythroid differentiation of K562 cells. Hemoglobin production during differentiation was followed by a significant increase in GSTP1-1 mRNA (1.7-fold, P < 0.01) and protein (1.2-fold, P < 0.01) after 4 days of induction. This increase in mRNA production was not due to transcriptional up-regulation by GATA-1 previously shown to regulate GSTP1-1 during erythroid and megakaryocytic differentiation. Moreover, a drastic decrease in differentiation-specific GATA-1 mRNA expression was correlated to a reduction in GATA-1 promoter binding activity. Neither AP-1 nor NF-kappaB transcription factor binding activities could provide an explanation to the GSTP1-1 mRNA overexpression in hemin-treated cells. GSTP1-1 mRNA stability analysis using actinomycin D as an inhibitor of mRNA neosynthesis showed that mRNA half-life was doubled in hemin-induced erythroid differentiation of K562 cells. These results allow us to add stabilization of GSTP1-1 mRNA as a novel regulatory mechanism during hemin-mediated differentiation of K562 cells.

Effect of chemopreventive agents on glutathione S-transferase P1-1 gene expression mechanisms via activating protein 1 and nuclear factor kappaB inhibition

Glutathione S-transferase P1-1 (GSTP1-1) is a phase II drug metabolism enzyme implicated in carcinogenesis and development of resistance to anti-cancer drugs. It was previously shown that both activating protein 1 (AP-1) and nuclear factor kappaB (NF-kappaB) are involved in its regulation. In the present study we examined the inhibitory effect of several chemopreventive agents on the tumor necrosis factor (TNF) alpha- or 12-O-tetradecanoylphorbol 13 acetate (TPA)-induced promoter activity of GSTP1-1, as demonstrated by transient transfection experiments in K562 and U937 leukemia cells. Our results provide evidence for a differential effect of chemopreventive agents such as beta-lapachone, emodin, sanguinarine and capsaicin, which significantly inhibit reporter gene expression as well as TNFalpha- and TPA-induced binding of AP-1 and NF-kappaB, whereas trans-anethole and silymarin do not produce any inhibitory effect. Our results demonstrate the ability of selected chemopreventive agents to decrease GSTP1-1 gene expression mechanisms and could thus contribute to reduce the incidence of glutathione related drug resistance in human leukemia.

Human osteosarcoma xenografts and their sensitivity to chemotherapy

Despite the increased survival rates of osteosarcoma patients attributed to adjuvant chemotherapy, at least one third of the patients still die due to their disease. Further improvements in the management of osteosarcoma may rely on a more individualised treatment strategy, as well as on the introduction of new drugs. To aid in the preclinical evaluation of new candidate substances against osteosarcoma, we have established 11 human osteosarcoma xenograft lines and characterised them with regard to response to five different reference drugs. Doxorubicin, cisplatin methotrexate, ifosfamide and lomustine were effective in 3/11, 3/11, 1/10, 5/11 and 4/11 of the xenografts, respectively. Five xenografts were resistant to all compounds tested. We also assessed the mRNA expression levels of the xenografts for the O(6)-Methylguanine DNA Methyltransferase (MGMT), DNA topoisomerase II- (Topo II)-alpha, Gluthathione-S-transferase (GST)-pi, Multidrug-resistance related protein (MRP) 1 and Multidrug-resistance (MDR) 1 genes. There was an inverse correlation between the transcript levels of GST-pi and doxorubicin growth inhibition (r=-0.66; p<0.05), and between the transcript levels of MGMT and the effect of lomustine (r=-0.72; p<0.01), whereas the expression of MRP1 and cisplatin growth inhibition was positively correlated (r=0.82; p<0.005). This panel of xenografts should constitute a good tool for pharmacological and molecular studies in osteosarcoma.

Induction of apoptosis by curcumin: mediation by glutathione S-transferase P1-1 inhibition

Expression of glutathione S-transferase P1-1 (GSTP1-1) is correlated to carcinogenesis and resistance of cancer cells against chemotherapeutic agents. Curcumin, a natural compound extracted from Curcuma longa, has shown strong antioxidant and anticancer properties and also the ability to regulate a wide variety of genes that require activating protein 1 and nuclear factor kappaB (NF-kappaB) activation. In the present study, we examined the inhibitory effect of curcumin on the expression of GSTP1-1 mRNA as well as protein, and we correlated this inhibition with the apoptotic effect of curcumin on K562 leukemia cells. Curcumin efficiently inhibited the tumour necrosis factor alpha- and phorbol ester-induced binding of AP-1 and NF-kappaB transcription factors to sites located on the GSTP1-1 gene promoter. TNFalpha-induced GSTP1-1 promoter activity was also inhibited by curcumin as shown by reporter gene assay. In parallel, curcumin induced pro-caspases 8 and 9 as well as poly ADP ribose polymerase cleavage and thus leading to apoptosis in K562 cells. Our results overall add a novel role for curcumin as this chemoprotective compound could contribute to induce apoptosis by its ability to inhibit the GSTP1-1 expression at the level of transcription.

Expression of glutathione S-transferase P1-1 in differentiating K562: role of GATA-1

Glutathione S-transferase P1-1 (GSTP1-1) conjugates glutathione to electrophilic compounds and its expression is correlated to chemotherapeutic drug resistance. Results show that GSTP1-1 mRNA as well as protein expressions are increased during Aclarubicin (Acla)- and Doxorubicin (Dox)-induced erythroid differentiation of human K562 cells. In contrast, during megakaryocytic differentiation by 12-O-tetradecanoyl phorbol 13-acetate (TPA), GSTP1-1 expression decreased at both mRNA and protein levels. In order to clarify the molecular mechanisms leading to these variations, we identified a GATA sequence located at -1208 relative to the transcriptional start site of the GSTP1-1 promoter. By gel shift, competition, and supershift analyses we show here the specificity of the GATA-1 binding regulated by both anthracyclines and TPA. Altogether, these results demonstrate for the first time the implication of GATA-1 in differentiation-specific variations of GSTP1-1 expression.

Increased resistance to nitrogen mustards and antifolates following in vitro selection of murine fibroblasts and primary hematopoietic cells transduced with a bicistronic retroviral vector expressing the rat glutathione S-transferase A3 and a mutant dihydrofolate reductase

We have constructed a retroviral bicistronic vector, MFG/GID, that transduces the expression of both the A3 isoform of the rat glutathione S-transferase (GST A3), and the tyr-22 variant of the human dihydrofolate reductase (DHFR(L22Y)). Transduction of murine 3T3 fibroblasts with this vector increased their in vitro resistance to chlorambucil (1.8-fold) and trimetrexate (TMTX) (748-fold). TMTX selection of a mixed population of 20% GID-transduced NIH 3T3 cells and 80% control cells resulted in a marked increase in the GST peroxidase activity associated with the GST A3 isoform (17.7-fold). MFG/GID-transduced primary clonogenic murine hematopoietic progenitor cells were likewise more resistant to TMTX and chlorambucil than control beta-gal-transduced cells. Selecting GID-transduced hematopoietic cells with a combination of TMTX and a nucleoside transport inhibitor resulted in a marked increase in resistance upon re-exposure to TMTX (99% survival). Similarly, GID-transduced hematopoietic cells selected with TMTX were more resistant to chlorambucil, with 40% survival at a drug concentration that killed practically all control cells. These results suggest that antifolate-mediated selection of MFG/GID-transduced hematopoietic cells could be used as a mean to enrich the population of transduced cells prior to or following transplantation, thus potentially conferring in vivo chemoprotection to nitrogen mustards and antifolates.

Cisplatin nephrotoxicity: molecular mechanisms

Cisplatin is one of the most widely used chemotherapeutic agents for the treatment of several human malignancies. The efficacy of cisplatin is dose dependent, but the significant risk of nephrotoxicity frequently hinders the use of higher doses to maximize its antineoplastic effects. Several advances in our understanding of the biochemical and molecular mechanisms underlying cisplatin nephrotoxicity have recently emerged, and are reviewed in this article. Evidence is presented for distinct mechanisms of cisplatin toxicity in actively dividing tumor cells versus the normally quiescent renal proximal tubular epithelial cells. The unexpected role of gamma-glutamyl transpeptidase in cisplatin nephrotoxicity is elucidated. Recent studies demonstrating the ability of proximal tubular cells to metabolize cisplatin to a nephrotoxin are reviewed. The evidence for apoptosis as a major mechanism underlying cisplatin-induced renal cell injury is presented, along with the data exploring the role of specific intracellular pathways that may mediate the programmed cell death. The information gleaned from this review may provide critical clues to novel therapeutic interventions aimed at minimizing cisplatin-induced nephrotoxicity while enhancing its antineoplastic efficacy.

Metabolism of Cisplatin to a nephrotoxin in proximal tubule cells

Cisplatin, a commonly used chemotherapeutic agent, is nephrotoxic. The mechanism by which cisplatin selectively kills the proximal tubule cells was heretofore unknown. Recent studies in mice and rats have shown that the nephrotoxicity of cisplatin can be blocked by acivicin or (aminooxy)acetic acid, the same enzyme inhibitors that block the metabolic activation of a series of nephrotoxic halogenated alkenes. In this study, it was hypothesized that cisplatin is activated in the kidney to a toxic metabolite through the same pathway that has been shown to activate the halogenated alkenes. This activation begins with the formation of a glutathione-conjugate that is metabolized to a cysteinyl-glycine-conjugate, to a cysteine-conjugate, and finally to a reactive thiol. In this study, a protocol was developed in which confluent monolayers of LLC-PK(1) cells were exposed to clinically relevant concentrations of cisplatin or cisplatin-conjugate for 3 h. Cell viability was assayed at 72 h. The role of gamma-glutamyl transpeptidase (GGT) and cysteine-S-conjugate beta-lyase in the metabolism of each of the cisplatin-conjugates was investigated. Pre-incubation of cisplatin with glutathione, cysteinyl-glycine, or N-acetyl-cysteine to allow for the spontaneous formation of cisplatin-conjugates increased the toxicity of cisplatin toward LLC-PK(1) cells. Inhibition of GGT activity showed that GGT was necessary only for the toxicity of the cisplatin-glutathione-conjugate. Inhibition of cysteine-S-conjugate beta-lyase reduced the toxicity of each of the cisplatin-conjugates. These data demonstrate that metabolism of cisplatin in proximal tubule cells is required for its nephrotoxicity. The elucidation of this pathway provides new targets for the inhibition of cisplatin nephrotoxicity.

Significance of nuclear glutathione S-transferase pi in resistance to anti-cancer drugs

Recent study has shown that nuclear glutathione S-transferase (GST) pi accumulates in cancer cells resistant to doxorubicin hydrochloride (DOX) and may function to prevent nuclear DNA damage caused by DOX (Goto et al., FASEB J., 15, 2702 - 2714 (2001)). It is not clear if the amount of nuclear GSTpi increases in response to other anti-cancer drugs and if so, what is the physiological significance of the nuclear transfer of GSTpi in the acquisition of drug-resistance in cancer cells. In the present study, we employed three cancer cell lines, HCT8 human colonic cancer cells, A549 human lung adenocarcinoma cells, and T98G human glioblastoma cells. We estimated the nuclear transfer of GSTpi induced by the anti-cancer drugs cisplatin (CDDP), irinotecan hydrochloride (CPT-11), etoposide (VP-16) and 5-fluorouracil (5-FU). It was found that: (1) Nuclear GSTpi accumulated in these cancer cells in response to CDDP, DOX, CPT-11, VP-16 and 5-FU. (2) An inhibitor of the nuclear transport of GSTpi, edible mushroom lectin (Agaricus bisporus lectin, ABL), increased the sensitivity of the cancer cells to DOX and CDDP, and partially to CPT-11. Treatment with ABL had no apparent effect on the cytotoxicity of VP-16 and 5-FU. These results suggest that inhibitors of the nuclear transfer of GSTpi have practical value in producing an increase of sensitivity to DOX, CDDP and CPT-11.

Expression of rat liver glutathione-S-transferase GSTA5 in cell lines provides increased resistance to alkylating agents and toxic aldehydes

The glutathione-S-transferases (GST) are a major contributor to the eukaryotic cell's defences against chemical and oxidative stress. However, the role of individual GST isoenzymes in conferring resistance to xenobiotics has not been fully determined. We have examined the effect of the rat GSTA5 isoenzyme in the detoxication of alkylating agents and aldehydes by constructing a cell line in which it is stably expressed. The hamster fibroblast cell line V79 was transfected with a construct expressing GSTA5 from the CMV promoter. A stable clone (V79-GSTA5) was isolated after selecting for the neomycin phosphotransferase gene present on the introduced DNA. The cell line showed significantly increased levels of resistance towards the alkylating agents chorambucil and melphalan. Levels of resistance were 4-6-fold greater in V79-GSTA5 cells than in control cells. Increased levels of resistance were also observed towards the lipid peroxidation product acrolein (IC(50)=80 microM compared with 17 microM in control cells). The V79-GSTA5 cells also showed a 4-fold increase in resistance to trans, trans muconaldehyde (IC(50)=4 micro compared with l microM for control cells). GSTA5 did not protect against 4-hydroxynonenal, but it did provide greater levels of protection to hydrogen peroxide, with an IC(50) of 380 microM in V79-GSTA5 compared with 180 microM in control cells. In contrast, V79-GSTA5 cells were more sensitive to methyl glyoxal, suggesting that a methyl glyoxal-glutathione conjugate is more toxic that the parental compound. These data contribute towards the evaluation of the role of GSTA5 in the detoxication of these compounds.

Doxorubicin-induced DNA intercalation and scavenging by nuclear glutathione S-transferase pi

Glutathione S-transferase (GST) functions in xenobiotic biotransformation and drug metabolism. Increased expression of GSTpi, an isozyme of GST, has been found in cancer cells resistant to doxorubicin hydrochloride (DOX) or cis-diamminedichloroplatinum (II) (CDDP), and this increase was believed to be correlated with drug resistance of cancer cells. GST is mainly expressed in the cytoplasm; GSTpi in the nucleus has been reported in cancer cells, but the meaning of this result is not known. Here, we studied changes in the amount of nuclear GSTpi after exposure of cancer cells to anticancer drugs, and role of the nuclear GSTpi in drug resistance. We found nuclear GSTpi in cancer cells resistant to DOX, and the amount of nuclear GSTpi was enhanced by treatment of the cancer cells with DOX or CDDP. We also found that a mushroom lectin, an inhibitor of nuclear transport, inhibited the nuclear transfer of GSTpi, suggesting the existence of a specific transport system for the nuclear transfer of GSTpi. Nuclear GSTpi protected DNA against damage by anticancer drugs. These results suggest a possible role of GSTpi in the acquisition of resistance to anticancer drugs by cancer cells.

Glutathione S-transferases as emerging therapeutic targets

Glutathione S-transferases (GST) represent a large family of Phase II detoxification enzymes widely expressed in animals and plants. These enzymes catalyse the conjugation of glutathione with some endogenous molecules and a broad range of exogenous substrates including various anticancer drugs. Due to high expression of GSTs in tumours when compared to normal tissues and their high level in plasma from cancer patients, these enzymes are considered to be cancer markers. Their involvement in resistance to anticancer drugs and an inverse correlation between expression and prognosis in many tumours provided a rationale for the design of inhibitors and prodrugs to enhance therapeutic index. The first generation of GST inhibitors included ethacrynic acid and showed promising potentiating activity in vitro but lack of isoenzyme specificity and diuretic side effects restricted clinical use. Novel GST inhibitors include glutathione analogues and demonstrate better specificities with fewer limiting toxicities. One lead compound is a potent inhibitor of the GSTP1-1 isoform in both cell lines and animal models. A GSTP1-1 activated prodrug has also been developed. Testing of the preclinical and clinical efficacy of these agents is presently in progress. Their rational design provides a promising new approach to targeting tumour-specific characteristics in a manner consistent with improving therapeutic index.

Lack of glutathione conjugation to adriamycin in human breast cancer MCF-7/DOX cells. Inhibition of glutathione S-transferase p1-1 by glutathione conjugates from anthracyclines

One of the proposed mechanisms for multidrug resistance relies on the ability of resistant tumor cells to efficiently promote glutathione S-transferase (GST)-catalyzed GSH conjugation of the antitumor drug. This type of conjugation, observed in several families of drugs, has never been documented satisfactorily for anthracyclines. Adriamycin-resistant human breast cancer MCF-7/DOX cells, presenting a comparable GSH concentration, but a 14-fold increase of the GST P1-1 activity relative to the sensitive MCF-7 cells, have been treated with adriamycin in the presence of verapamil, an inhibitor of the 170 P-glycoprotein (P-gp) drug transport protein, and scrutinized for any production of GSH-adriamycin conjugates. HPLC analysis of cell content and culture broths have shown unequivocally that no GSH conjugates are present either inside the cell or in the culture broth. The only anthracycline present inside the cells after 24 hr of incubation was > 98% pure adriamycin. Confocal laser scanning microscopic observation showed that in MCF-7/DOX cells adriamycin was localized mostly in the Golgi apparatus rather than in the nucleus, the preferred site of accumulation for sensitive MCF-7 cells. These findings rule out GSH conjugation or any other significant biochemical transformation as the basis for resistance to adriamycin and as a ground for the anomalous localization of the drug in the cell. Adriamycin, daunomycin, and menogaril did not undergo meaningful conjugation to GSH in the presence of GST P1-1 at pH 7.2. Indeed, their synthetic C(7)-aglycon-GSH conjugates exerted a strong inhibitory effect on GST P1-1, with K(i) at 25 degrees in the 1-2 microM range, scarcely dependent on their stereochemistry at C(7).

Activity of allelic variants of Pi class human glutathione S-transferase toward chlorambucil

Clinical efficacy of alkylating anticancer drugs, such as chlorambucil, is often limited by the emergence of drug resistant tumor cells. Increased glutathione (GSH) conjugation (inactivation) of alkylating anticancer drugs or their activated metabolites due to overexpression of the Pi class GSH S-transferase (hGSTP1-1) is believed to be an important mechanism in tumor cell resistance to alkylating agents. Interestingly, the hGSTP1 locus is polymorphic in human populations and involves amino acid residues in positions 104 (isoleucine or valine) and/or 113 (alanine or valine). Here, we report that the allelic variants of hGSTP1-1 significantly differ in their efficiency in catalyzing the GSH conjugation of chlorambucil. Catalytic efficiency of the hGSTP1-1(I104,A113) isoform toward chlorambucil was approximately 2.5-, 7.5- and 15-fold higher compared with I104,V113, V104,A113 and V104,V113 variants of hGSTP1-1, respectively. The results of the present study suggest that hGSTP1-1 polymorphism may be an important factor in GST-mediated tumor cell resistance to some alkylating agents.

Probing subunit interactions in alpha class rat liver glutathione S-transferase with the photoaffinity label glutathionyl S-[4-(succinimidyl)benzophenone]

Glutathionyl S-[4-(succinimidyl)benzophenone] (GS-Succ-BP), an analogue of the product of glutathione and electrophilic substrate, acts as a photoaffinity label of dimeric rat liver glutathione S-transferase (GST), isoenzyme 1-1. A time-dependent loss of enzyme activity is observed upon irradiation of the enzyme with long wavelength UV light in the presence of the reagent. The initial rate of inactivation exhibits nonlinear dependence on the concentration of the reagent, characterized by an apparent dissociation constant of the enzyme-reagent complex (K(R)) of 99 +/- 2 microM and k(max) of 0.082 +/- 0.005 min(-1). Protection against this inactivation is provided by the electrophilic substrate (ethacrynic acid), electrophilic substrate analogue (dinitrophenol), and product analogues (S-hexylglutathione and p-nitrobenzylglutathione) but not by steroids (Delta(5)-androstene-3,17-dione and 17beta-estradiol-3, 17-disulfate). These results suggest that GS-Succ-BP binds and reacts with the enzyme within the xenobiotic substrate binding site, and this reaction site is distinct from the substrate and nonsubstrate steroid binding sites of the enzyme. About 1 mol of reagent is incorporated into 1 mol of enzyme dimer when the enzyme is completely inactivated. Met-208 is the only amino acid target of the reagent, and modification of this residue in one enzyme subunit of the GST 1-1 dimer completely abolishes the enzyme activity of both subunits. In order to evaluate the role of subunit interactions in the Alpha class glutathione S-transferases, inactive GS-Succ-BP-modified GST 1-1 was mixed with unlabeled, active GST 2-2. The enzyme subunits were dissociated in dilute trifluoroacetic acid and then renatured at pH 7.8 and separated by chromatofocusing into GST 1-1, 1-2, and 2-2. The specific activities of the heterodimer toward several substrates indicate that the loss of catalytic activity in the unmodified subunit of the modified GST 1-1 is the indirect result of the interaction between the two enzyme subunits and that this subunit interaction is absent in the heterodimer GST 1-2.

Characterization of a chlorambucil-resistant human ovarian carcinoma cell line overexpressing glutathione S-transferase mu

Ovarian carcinoma cells 10-fold resistant to the alkylating agent chlorambucil (CBL) were isolated after repeated exposure of the parent cells to gradually escalating concentrations of the drug. The resistant variant, A2780(100), was highly cross-resistant (9-fold) to melphalan and showed lower-level resistance to other cross-linking agents. The resistant A2780(100) cells had almost 5-fold higher glutathione S-transferase (GST) activity than the parental A2780 cells with 1-chloro-2,4-dinitrobenzene (CDNB) as substrate. The pi-class GST(s) was the major isoform(s) in both cell lines. However, the resistant A2780(100) cells had at least 11-fold higher GST mu as compared with the parental cells, in which this isoform was barely detectable. A significant induction of GST mu was observed in A2780 cells, but not in the resistant cells, 18 hr after a single exposure to 100 microM CBL. The induction of GST mu by CBL was both time- and concentration-dependent. Assays of the conjugation of CBL with GSH showed that the human mu-class GST had 3.6- and 5.2-fold higher catalytic efficiency relative to the pi- and alpha-class GSTs, respectively. This difference was reflected in the relatively higher (about 6-fold) efficiency of CBL conjugation in A2780(100) cells as compared with the parental cells. These results have demonstrated for the first time a near-linear correlation between CBL resistance and overexpression of mu-class GSTs and suggest that this overexpression maybe responsible, at least in part, for the acquired resistance of ovarian carcinoma cells to CBL, and possibly the other bifunctional alkylating agents. Consistent with this hypothesis, we found evidence for decreased formation of DNA lesions in A2780(100) compared with the drug-sensitive A2780 cells after exposure to CBL.

Genomic cloning and characterization of the rat glutathione S-transferase-A3-subunit gene

The rat glutathione S-transferase-A3-subunit (GSTA3) gene is a member of the class Alpha GSTs, which we have previously reported to be overexpressed in anti-cancer-drug-resistant cells. In this study, we report the isolation and characterization of the entire rat GSTA3 (rGST Yc1) subunit gene. The rat GSTA3 subunit gene is approximately 15 kb in length and consists of seven exons interrupted by introns of different lengths. Exon 1, with a length of 219 bp, contains only the 5'-untranslated region of the gene. Each exon-intron splicing junction exhibited the consensus sequence for a mammalian splice site. The transcription start site and exon 1 of rat GSTA3 were characterized by a combination of primer extension and rapid amplification of the cDNA ends. Position +1 was identified 219 bp upstream of the first exon-intron splicing junction. The proximal promoter region of the rat GSTA3 subunit gene does not contain typical TATA or CAAT boxes. A computer-based search for potential transcription-factor binding sites revealed the existence of a number of motifs such as anti-oxidant-responsive element, ras-response element, activator protein-1, nuclear factor-kappaB, cAMP-response-element-binding protein, Barbie box and E box. The functional activity of the regulatory region of the rat GSTA3 subunit gene was shown by its ability to drive the expression of a chloramphenicol acetyltransferase reporter gene in rat mammary carcinoma cells, and its activity was greater in melphalan-resistant cells known to have transcriptional activation of this gene by previous studies. The structure of the gene, with a large intron upstream of the translation-initiation site, may explain why the isolation of this promoter has been so elusive. This information will provide the opportunity to examine the involvement of the rat GSTA3 subunit gene in drug resistance and carcinogenesis.

Overexpression of the regulatory subunit of gamma-glutamylcysteine synthetase in HeLa cells increases gamma-glutamylcysteine synthetase activity and confers drug resistance

gamma-Glutamylcysteine synthetase (GCS) is reported to catalyse the rate-limiting step in glutathione biosynthesis, and is a heterodimer composed of a catalytic subunit [heavy subunit (GCSh) of Mr 73000] and a regulatory subunit [light subunit (GCSl) of Mr 31000]. In the present study, we have demonstrated for the first time a potential role for GCSl in resistance towards doxorubicin and cadmium chloride. Addition of recombinant GCSl to HeLa cell extracts in vitro was found to result in an increase in GCS activity of between 2- and 3-fold. Transient transfections of COS-1 cells with the GCSl cDNA cause an increase in GCS activity of approx. 2-fold, and a small but significant (P=0.008) increase in glutathione levels from 126.9+/-4. 2 nmol/mg protein to 178.8+/-19.1 nmol/mg protein. We proceeded to make a HeLa cell line (LN73), which stably overexpresses GCSl. These cells overexpress GCSl approx. 20-fold above basal levels. LN73 was found to have a 2-fold increase in GCS activity (437.3+/-85.2 pmol/min per mg) relative to the control cell line, HL9 (213.4+/-71. 8 pmol/min per mg). In contrast with the transient transfections in COS-1 cells, stable overexpression of GCSl was found not to be associated with an increase in glutathione content. However, when the LN73 and HL9 cells were treated with the glutathione-depleting agent, diethylmaleate, the LN73 cells were found to have an enhanced ability to regenerate glutathione, compared with HL9 cells. The cell lines were treated with various anti-cancer drugs, and their cytotoxicity was examined. No obvious differences in toxicity were observed between the different cell lines following treatment with cisplatin and melphalan. The redox-cycling agent doxorubicin, however, was found to be more toxic (approx. 2-fold) to the HL9 cells than the LN73 cells. When the cells were treated with the carcinogenic transition-metal compound, cadmium chloride, LN73 cells were found to be approx. 3-fold more resistant than HL9 cells.

Mutations of Gly to Ala in human glutathione transferase P1-1 affect helix 2 (G-site) and induce positive cooperativity in the binding of glutathione

Previous kinetic studies on human glutathione transferase P1-1 have indicated that the motions of an irregular alpha-helix (helix 2) lining the glutathione (GSH) binding site are viscosity dependent and may modulate the affinity of GSH binding. The effect of single amino acid residue substitutions (Gly to Ala) in this region is investigated here by site-directed mutagenesis. Three mutants (Gly41Ala, Gly50Ala and Gly41Ala/Gly50Ala) were overexpressed in Escherichia coli, purified, and characterized by kinetic, structural, and spectroscopic studies. All these mutant enzymes show kcat values similar to that of the wild-type enzyme, while the [S]0.5 for GSH increases about eight-fold in the Gly41Ala mutant and more than 100-fold in the Gly41Ala/Gly50Ala double mutant. This change in affinity towards GSH is accompanied by an induced positive cooperativity as reflected by Hill coefficients of 1.4 (Gly41Ala) and 1.7 (Gly41Ala/Gly50Ala) upon substrate binding. Taken together, these data suggest that the region around helix 2 is markedly altered leading to the observed intersubunit communication. Molecular modeling of the Gly41Ala/Gly50Ala mutant and of the inactive oxidized form of the native enzyme provides a structural explanation of our results.

A 47-amino-acid fragment of SV40 T antigen represses transcription from human GSTalpha promoters

SV40 T antigen downregulates the expression of an important detoxification enzyme, glutathione S-transferase alpha (GSTalpha). We show here that the target of this repression is a 14-bp element common to the human GSTA1 and GSTA2 promoters. This element, which we have named TAGR, is also critical for high-level, constitutive expression from these promoters. The TAGR element does not appear to contain a binding site for any transcription factor known to be present in fibroblasts, although the TAGR element does resemble the binding site for the Ikaros transcription factor found in hematopoietic cells. We also have identified a 47-amino-acid fragment of T antigen that includes amino acids 83-100 and 119-147, which is sufficient to repress transcription from the GSTalpha promoter in transient transcription assays. Thus, GSTalpha repression does not require binding of T antigen to pRb, p300, or p53, since the domains of T antigen required for binding these cellular proteins are missing from this T antigen fragment. We show, however, that this fragment does bind to three cellular proteins with approximate molecular weights of 54, 59, and 94 kDa.

Coordinated action of glutathione S-transferases (GSTs) and multidrug resistance protein 1 (MRP1) in antineoplastic drug detoxification. Mechanism of GST A1-1- and MRP1-associated resistance to chlorambucil in MCF7 breast carcinoma cells

To examine the role of multidrug resistance protein 1 (MRP1) and glutathione S-transferases (GSTs) in cellular resistance to antineoplastic drugs, derivatives of MCF7 breast carcinoma cells were developed that express MRP1 in combination with one of three human cytosolic isozymes of GST. Expression of MRP1 alone confers resistance to several drugs representing the multidrug resistance phenotype, drugs including doxorubicin, vincristine, etoposide, and mitoxantrone. However, co-expression with MRP1 of any of the human GST isozymes A1-1, M1-1, or P1-1 failed to augment MRP1-associated resistance to these drugs. In contrast, combined expression of MRP1 and GST A1-1 conferred approximately 4-fold resistance to the anticancer drug chlorambucil. Expression of MRP1 alone failed to confer resistance to chlorambucil, showing that the observed protection from chlorambucil cytotoxicity was absolutely dependent upon GST A1-1 protein. Moreover, using inhibitors of GST (dicumarol) or MRP1 (sulfinpyrazone), it was shown that in MCF7 cells resistance to chlorambucil requires both intact MRP1-dependent efflux pump activity and, for full protection, GST A1-1 catalytic activity. These results are the first demonstration that GST A1-1 and MRP1 can act in synergy to protect cells from the cytotoxicity of a nitrogen mustard, chlorambucil.

Comprehensive analysis of proteins which interact with the antioxidant responsive element: correlation of ARE-BP-1 with the chemoprotective induction response

Transcriptional activation of the mouse glutathione S-transferase Ya gene by chemoprotective molecules is mediated through the interaction of trans-acting factors with an antioxidant responsive element (ARE) in the promoter region of this gene. In a step toward identifying those factors which bind productively to the GST Ya ARE, all of the discernible, specific ARE-binding proteins (ARE-BP) in nuclear extracts from HepG2 cells were systematically characterized. By gel-mobility-shift analysis, seven specific ARE-BPs, termed ARE-BP-1 through 7 in order of increasing mobility, were observed that did not vary in concentration or migration between induced and uninduced cell extracts. The molecular weights of the individual ARE-BP subunits were determined by a two-dimensional electrophoresis protocol. Ferguson gel analysis of native protein size indicated that several of the ARE-BP-DNA complexes are composed of multiple protein subunits. Wild-type AREs and GST Ya ARE fragments and mutant sequences were evaluated for their ability to mediate induction in a reporter gene system in HepG2 cells. This same panel of sites was tested in an in vitro binding assay for the ability to compete for the ARE-BPs. A binding profile for each ARE-BP was compiled. Correlation between the ARE-BP binding profiles and induction results indicated that: (i) the ARE-BP-1 and ARE-BP-2 complexes formed only with AREs that supported induction, and (ii) the ARE-BP-4 complex formed with all inducible AREs, but it also bound to ARE mutants that failed to support induction. Based on the studies, an early composite regulatory element model for ARE-mediated expression is presented. ARE-BP-1 is proposed to be the mediator of the ARE's unique induction response to chemoprotective agents.

An Nrf2/small Maf heterodimer mediates the induction of phase II detoxifying enzyme genes through antioxidant response elements

The induction of phase II detoxifying enzymes is an important defense mechanism against intake of xenobiotics. While this group of enzymes is believed to be under the transcriptional control of antioxidant response elements (AREs), this contention is experimentally unconfirmed. Since the ARE resembles the binding sequence of erythroid transcription factor NF-E2, we investigated the possibility that the phase II enzyme genes might be regulated by transcription factors that also bind to the NF-E2 sequence. The expression profiles of a number of transcription factors suggest that an Nrf2/small Maf heterodimer is the most likely candidate to fulfill this role in vivo. To directly test these questions, we disrupted the murine nrf2 gene in vivo. While the expression of phase II enzymes (e.g., glutathione S-transferase and NAD(P)H: quinone oxidoreductase) was markedly induced by a phenolic antioxidant in vivo in both wild type and heterozygous mutant mice, the induction was largely eliminated in the liver and intestine of homozygous nrf2-mutant mice. Nrf2 was found to bind to the ARE with high affinity only as a heterodimer with a small Maf protein, suggesting that Nrf2/small Maf activates gene expression directly through the ARE. These results demonstrate that Nrf2 is essential for the transcriptional induction of phase II enzymes and the presence of a coordinate transcriptional regulatory mechanism for phase II enzyme genes. The nrf2-deficient mice may prove to be a very useful model for the in vivo analysis of chemical carcinogenesis and resistance to anti-cancer drugs.

High-level production and purification of biologically active proteins from bacterial and mammalian cells using the tandem pGFLEX expression system

Because of the complexities involved in the regulation of gene expression in Escherichia coli and mammalian cells, it is considered general practice to use different vectors for heterologous expression of recombinant proteins in these host systems. However, we have developed and report a shuttle vector system, pGFLEX, that provides high-level expression of recombinant glutathione S-transferase (GST) fusion proteins in E. coli and mammalian cells. pGFLEX contains the cytomegaloma virus (CMV) immediate-early promoter in tandem with the E. coli lacZpo system. The sequences involved in gene expression have been appropriately modified to enable high-level production of fusion proteins in either cell type. The pGFLEX expression system allows production of target proteins fused to either the N or C terminus of the GST pi protein and provides rapid purification of target proteins as either GST fusions or native proteins after cleavage with thrombin. The utility of this vector in identifying and purifying a component of a multi-protein complex is demonstrated with cyclin A. The pGFLEX expression system provides a singular and widely applicable tool for laboratory or industrial production of biologically active recombinant proteins in E. coli and mammalian cells.

Inhibitors of glutathione S-transferases as therapeutic agents

Glutathione S-transferases (GST) are a family of phase II detoxification enzymes with broad substrate specificities. They catalyze the conjugation of glutathione with many different types of xenobiotics, rendering the compound more water soluble and thus more easily eliminated. Resistance to cancer chemotherapeutic drugs, such as the alkylating agents, has been directly correlated with the overexpression of GSTs. Subsequently, a rationale has been established to utilize agents that inhibit GST in combination with alkylating agents to circumvent this resistance. Two such agents, ethacrynic acid (EA) and Terrapin 199 (TER 199), have been examined for this purpose. EA, an inhibitor of all classes of GST isozymes, has been used clinically in combination with thiotepa. More recently, TER 199, a glutathione analog-based GST inhibitor, has been modeled specifically to inhibit GST pi, an enzyme which is commonly found at high levels in human tumor biopsies. Furthermore, a therapeutic strategy has been designed to take advantage of GST pi activation of a prodrug, TER 286. Recent studies have investigated the molecular mechanisms involved in the cellular response to GST inhibitors and have employed techniques such as differential display to examine altered gene expression as well as to identify novel genes induced by these agents. Overall, this strategy may provide further insight into the action of these agents in the cell as well as prove useful in endeavors to modulate anticancer drug resistance.

SV40 and adenovirus may act as cocarcinogens by downregulating glutathione S-transferase expression

We have discovered a novel function of the SV40 T antigen and the adenovirus E1A proteins: the ability to downregulate the endogenous expression of an important detoxification enzyme, glutathione S-transferase alpha (GST alpha). GST alpha mRNA is much less abundant in rat and human cells that express SV40 T antigen than in the parental cell lines. This GST alpha downregulation does not require expression of SV40 small t antigen or complex formation between large T antigen and p53, p300, or the pRb family of proteins. As might be predicted, cells that express SV40 T antigen are more sensitive than normal cells to alkylating drugs, which GST alpha is known to detoxify. Finally, GST alpha expression is also downregulated in cells that express the adenovirus E1A proteins. We propose that by downregulating GST alpha expression and inactivating p53 function, SV40 and adenovirus may contribute to the initiation of, or the progression toward, malignancy. Thus, in their quest to establish persistent infections, these viruses may inadvertently make the cellular environment more permissive for tumorigenesis.

Transcriptional and post-transcriptional mechanisms can regulate cell-specific expression of the human Pi-class glutathione S-transferase gene

Previous studies from this laboratory have identified transcriptional mechanisms that are utilized to increase expression of the human glutathione S-transferase gene GSTP1 in a multidrug-resistant derivative (VCREMS) of the human mammary carcinoma cell line MCF7 [Moffat, McLaren and Wolf (1994) J. Biol. Chem. 269, 16397-16402]. The data presented here provide strong evidence that post-transcriptional mechanisms can also play an important role in determining cell-specific expression of the GSTP1 gene. GSTP1 mRNA levels were shown to be elevated 3.1-fold in the human bladder carcinoma cell line EJ compared with VCREMS cells. Despite this observation, transient transfection assays revealed a decreased rate of GSTP1 promoter activity in EJ cells. Indeed, GSTP1 transcriptional repressor activity, mediated by a region located between nucleotides -105 and -86 (as we have previously described in MCF7 cells), was observed in EJ cells. However, in contrast with our results in MCF7 cells, the EJ repressor activity did not displace the essential nuclear complex bound to the C1 promoter element (-73 to -54) in vitro. In addition, competition experiments indicated that an AP-1-like protein is an integral component of the C1-bound complex in EJ cells. Interestingly, experiments utilizing actinomycin D to inhibit transcription demonstrated significantly greater stability of GSTP1 mRNA in EJ cells than in VCREMS cells. These findings suggest that cell-specific differences in the rates of GSTP1 mRNA decay provide the predominant mechanism responsible for elevated expression of the GSTP1 gene in EJ cells.

Functional antioxidant responsive elements

Exposure of human and rodent cells to a wide variety of chemoprotective compounds confers resistance against a broad set of carcinogens. For a subset of the chemoprotective compounds, protection is generated by an increase in the abundance of protective enzymes like glutathione S-transferases (GST). Antioxidant responsive elements (AREs) mediate the transcriptional induction of a battery of genes which comprise much of this chemoprotective response system. Past studies identified a necessary ARE "core" sequence of RTGACnnnGC, but this sequence alone is insufficient to mediate induction. In this study, the additional sequences necessary to define a sufficient, functional ARE are identified through systematic mutational analysis of the murine GST Ya ARE. Introduction of the newly identified necessary nucleotides into the regions flanking a nonresponsive, ARE-like, GST-Mu promoter sequence produced an inducible element. A screen of the GenBank database with the newly identified ARE consensus identified 16 genes which contained the functional ARE consensus sequence in their promoters. Included within this group was an ARE sequence from the murine ferritin-L promoter that mediated induction when tested. In an electrophoretic mobility-shift assay, the ferritin-L ARE was bound by ARE-binding protein 1, a protein previously identified as the likely mediator of the chemoprotective response. A three-level ARE classification system is presented to account for the distinct induction strengths observed in our mutagenesis studies. A model of the ARE as a composite regulatory site, where multiple transcription factors interact, is presented to account for the complex characteristics of ARE-mediated chemoprotective gene expression.

Establishment by adriamycin exposure of multidrug-resistant rat ascites hepatoma AH130 cells showing low DT-diaphorase activity and high cross resistance to mitomycins

A resistant subline (AH130/5A) selected from rat hepatoma AH130 cells after exposure to adriamycin (ADM) showed remarkable resistance to multiple antitumor drugs, including mitomycin C (MMC) and porfiromycin (PFM). PFM, vinblastine (VLB), and ADM accumulated in AH130/5A far less than in the parent AH130 (AH130/P) cells. AH130/5A cells showed overexpression of P-glycoprotein (PGP), an increase in glutathione S-transferase activity, and a decrease in DT-diaphorase and glutathione peroxidase activity. The resistance to MMC and VLB of AH130/5A cells was partly reversed by H-87, an inhibitor of PGP. Buthionine sulfoximine, an inhibitor of glutathione synthase, did not affect the action of MMC. tert-Butylhydroquinone induced DT-diaphorase activity, increased PFM uptake, and enhanced the growth-inhibitory action of MMC in AH130/5A cells. Dicumarol, an inhibitor of DT-diaphorase, decreased PFM uptake and reduced the growth-inhibitory action of MMC in AH130/P cells. These results indicated that the adriamycin treatment of hepatoma cells caused multifactorial multidrug resistance involving a decrease in DT-diaphorase activity.

Functional characterization of the transcription silencer element located within the human Pi class glutathione S-transferase promoter

We have previously demonstrated enhanced transcriptional activity of the human Pi class glutathione S-transferase (GSTP1) promoter in a multidrug-resistant derivative (VCREMS) of the human mammary carcinoma cell line, MCF7 (Moffat, G. J., McLaren, A. W., and Wolf, C. R. (1994) J. Biol. Chem. 269, 16397-16402). Furthermore, we have identified an essential sequence (C1; -70 to -59) within the GSTP1 promoter that bound a Jun-Fos heterodimer in VCREMS but not in MCF7 cells. These present studies have examined the negative regulatory element (-105 to -86), which acted to suppress GSTP1 transcription in MCF7 cells. Mutational analysis of this silencer element further defined the repressor binding site to be located between nucleotides -97 and -90. In vitro DNA binding assays suggested that the repressor exerted its action by causing displacement of the essential non-AP-1-like MCF7 C1 complex. However, the addition of MCF7 nuclear extract did not disrupt binding of the VCREMS Jun-Fos C1 complex to the GSTP1 promoter. Furthermore, upstream insertion of the GSTP1 silencer element failed to inhibit activity of a heterologous promoter in MCF7 cells. These results highlighted the cell and promoter specificity of the GSTP1 transcriptional repressor and implicated a functional requirement for contact between the repressor and C1 complex. In this regard, the introduction of half-helical turns between the silencer and the C1 element abrogated repressor activity, thus leading to the hypothesis that a direct interaction between the repressor and C1 complex was required to suppress GSTP1 transcription. Moreover, these findings suggest that cell-specific differences in the composition of the C1 nuclear complex may dictate repressor activity.

Retrovirus-mediated gene transfer of rat glutathione S-transferase Yc confers in vitro resistance to alkylating agents in human leukemia cells and in clonogenic mouse hematopoietic progenitor cells

Recently, we have reported that N2Yc, a Moloney-based retrovirus vector expressing the Yc isoform of rat glutathione S-transferase (GST-Yc), conferred resistance to alkylating agents in mouse NIH-3T3 fibroblasts. In this report, we address the feasibility of using rat GST-Yc somatic gene transfer to confer chemoprotection to the hematopoietic system. Human chronic myelogenous leukemia K-562 cells were efficiently transduced with the N2Yc retrovirus vector and showed a significant increase in the 50% inhibitory concentration of chlorambucil (3.2- to 3.3-fold), mechlorethamine (4.7- to 5.3-fold), and melphalan (2.1- to 2.2-fold). In addition, primary murine clonogenic hematopoietic progenitor cells transduced with the N2Yc vector were significantly more resistant to alkylating agents in vitro than cells transduced with the antisense N2revYc vector. The survival of Yc-transduced hematopoietic colonies at 400 nM mechlorethamine and 4 mu M chlorambucil was 39.4% and 42.6%, respectively, compared to 27.2% and 30.4% for N2revYc-transduced cells. Future experiments will determine the level of chemoprotection achievable in vivo, following transplantation of N2Yc-transduced hematopoietic cells in mice.

Sp1-mediated transcriptional activation of the human Pi class glutathione S-transferase promoter

Previous studies in this laboratory have identified an essential AP-1 recognition sequence (C1 region; -69 to -63) in th human Pi class glutathione s-transferase (GSTP1) promoter and a negatively acting regulatory element (-105 to -86) that acts to suppress GSTP1 transcription in the human mammary carcinoma cell line, MCF7 (1). The data presented here further delineate the functional characteristics of the GSTP1 promoter by examining the significance of two potential binding sites for the transcription factor, Sp1 (-57 to -49 and -47 to -39). The introduction of mutations within these Sp1-like elements and the use of Sp1 antisera in electrophoretic mobility shift assays demonstrated that Sp1 was bound to this region of the GSTP1 promoter in three different cell lines, MCF7, VCREMS, and EJ. Moreover, these in vitro studies indicated that only one of the two putative Sp1 response elements was utilized. Transient transfection assays using GSTP1 promoter constructs that incorporated mutations of the Sp1 elements clearly demonstrated that binding of Sp1 to the GSTP1 promoter was absolutely required for optimal levels of GSTP1 transcription. In particular, disruption of the distal Sp1 recognition motif (-57 to -49) markedly reduced GSTP1 promoter activity in each cell line, thus indicating preferential binding of Sp1 to the distal site. However, insertion of the repressor binding site (-105 to -86) into these constructs suggested that Sp1 was not involved in mediating the suppressive effects of the GSTP1 transcriptional repressor in MCF7 cells, because inhibition of Sp1 binding did not alleviate repressor activity. Therefore, these studies provide strong evidence that Sp1 plays a central role in regulating basal levels of GSTP1 transcription.