Identification of placental form of glutathione S-transferase in ACNU-resistant murine glioma cell lines

The role of glutathione in brain tumor drug resistance

Chemotherapy is central to the current treatment modality for primary human brain tumors, but despite high-dose and intensive treatment regimens there has been little improvement in patient outcome. The development of tumor chemoresistance has been proposed as a major contributor to this lack of response. While there have been some improvements in our understanding of the molecular mechanisms underlying brain tumor drug resistance over the past decade, the contribution of glutathione (GSH) and the GSH-related enzymes to drug resistance in brain tumors have been largely overlooked. GSH constitutes a major antioxidant defense system in the brain and together with the GSH-related enzymes plays an important role in protecting cells against free radical damage and dictating tumor cell response to adjuvant cancer therapies, including irradiation and chemotherapy. Glutamate cysteine ligase (GCL), glutathione synthetase (GS), glutathione peroxidase (GPx), glutathione reductase (GR), glutathione-S-transferases (GST), and GSH complex export transporters (GS-X pumps) are major components of the GSH-dependent enzyme system that function in a dynamic cascade to maintain redox homeostasis. In many tumors, the GSH system is often dysregulated, resulting in a more drug resistant phenotype. This is commonly associated with GST-mediated GSH conjugation of various anticancer agents leading to the formation of less toxic GSH-drug complexes, which can be readily exported from the cell. Advances in our understanding of the mechanisms of drug resistance and patient selection based on biomarker profiles will be crucial to adapt therapeutic strategies and improve outcomes for patients with primary malignant brain tumors.

Anticancer drug resistance in primary human brain tumors

The difficult clinical situation still associated with most types of primary human brain tumors has fostered significant interest in defining novel therapeutic modalities for this heterogeneous group of neoplasms. Beginning in the 1980s chemotherapy has been incorporated into the treatment protocol of a number of intractable brain tumors. However, it has predominantly failed to improve patient outcome. The unsatisfactory results with chemotherapeutic intervention have chiefly been attributed to tumor cell resistance. In recent years, there has been a literal explosion in our understanding about the mechanisms by which cancer cells become chemoresistant. During the course of their evolution (intrinsic resistance) or in response to chemotherapy (acquired resistance) these cells may follow a number of pathways of genetic alterations to possess a common (multidrug) or drug-specific (individual drug) resistant phenotype. Genomic aberrations, deregulation of membrane transporting proteins and cellular enzymes, and an altered susceptibility to commit to apoptosis are among the steps on the way that contribute to the genesis of chemotherapeutic treatment failure. Although, through the years we have come to yield information and inferences as to the roles that different molecular events may have in the resistance phenotype of cancer cells, the actual involvement of single genetic alterations in conferring drug resistance in primary brain tumors remains debatable. This uncertainty and, besides, the lack of proper drug resistance diagnostics, in a vicious circle, hinder the development of effective resistance-modulation strategies. Clinical non-responsiveness to chemotherapy remains a formidable obstacle to the successful treatment of brain tumors and one of the most serious problems to be solved in the therapy of these lesions. Future advances in the chemotherapeutic management of these neoplasms will come with an improved understanding of the significance and interrelationship of the multiple biological systems operative in promoting resistance to this treatment modality. The focus of this review is to summarize current knowledge concerning major drug resistance-related markers, to describe their functional interaction en route to chemoresistance, and to discuss their implication in rendering human brain tumor cells resistant to chemotherapy.

Glutathione S-transferase and drug sensitivity in malignant glioma

Malignant gliomas are largely resistant to current approaches of adjuvant chemotherapy. Gluthatione S-transferases (GST) have been attributed a role in the resistance of cancer cells, e.g., to nitrosoureas. Here, we assessed the expression levels of GST-pi and GST-mu RNA and protein as well as total GST activity in a panel of 12 human glioma cell lines and correlated these data with p53 status, BCL-2 family protein expression and drug sensitivity in these cells. Neither GST protein levels nor GST activity correlated with genetic or functional p53 status or with the expression of various BCL-2 family proteins. No evidence for GST-mediated protection from chemotherapeutic drugs became apparent. In contrast, high levels of GST-pi protein, probably the major source of GST activity in glioma cells, and of total GST activity correlated with enhanced sensitivity to vincristine-induced clonogenic cell death. Expression of GST-pi in human glioblastomas in vivo was confirmed by immunohistochemistry. Neither total, nor cytoplasmic or nuclear, GST-pi immunoreactivity correlated with the response to adjuvant radiotherapy or radiochemotherapy. A comparative analysis of primary and recurrent tumors showed that GST expression was not enhanced by radiochemotherapy in vivo. We conclude that GST does not account for the differential chemosensitivity of glioma cell lines in vitro and does not accumulate in glioma subpopulations that form recurrent tumors after radiochemotherapy in vivo.

Drug resistance and apoptosis in ENU-induced rat brain tumors treated with anti-cancer drugs

To cast light on the mechanisms of drug-resistance, experimental brain tumors were immunohistochemically evaluated for expression of glutathione S-transferase (GST)-alpha, mu, pi, p-glycoprotein and apoptosis-related factors, such as bcl-2 and p53, as well as by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling (TUNEL) method. Rat brain tumors induced by means of prenatal exposure to ethylnitrosourea (ENU) were treated with 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) and/or vincristine. Tumors more than 2 mm in size were considered to be drug resistant. The expression of GST-mu was strongly positive in ACNU-treated brain tumors, while p-glycoprotein was overexpressed in vincristine-treated brain tumors. Neither p53 nor bcl-2 expression directly correlated with apoptosis identified by TUNEL method, but tumors lacking apoptotic cells always demonstrated the expression of either GST-mu or p-glycoprotein. These results indicate that tumors resistant to chemotherapy might not be susceptible to induction of apoptosis, and therefore that mechanisms of drug resistance are related to programmed cell death in brain tumors.

Application of antisense ribonucleic acid complementary to O6-methylguanine-deoxyribonucleic acid methyltransferase messenger ribonucleic acid for therapy of malignant gliomas

OBJECTIVE

A derivative of chloroethylnitrosoureas, 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU), is a drug of choice for the chemotherapy of human malignant brain tumors. However, the cytocidal effect of ACNU is effectively repressed through repair of ACNU-mediated deoxyribonucleic acid lesions by O6-methylguanine-deoxyribonucleic acid methyltransferase (MGMT). Because a variety of human tumors, including brain tumors, contain high levels of MGMT activity, we investigated the effect of antisense ribonucleic acid (RNA) complementary to MGMT messenger RNA on ACNU resistance in tumor cells.

METHODS

We established a stable ACNU-resistant clone, C6AR, from the rat glioma cell line C6 exposed to a stepwise increasing concentration of ACNU. We transfected a plasmid deoxyribonucleic acid-encoding antisense MGMT RNA under the control of the human metallothionein promoter into C6AR cells and determined the effect of the antisense RNA on ACNU resistance of tumor cells by a colony-forming efficiency assay.

RESULTS

C6AR cells expressed abundant MGMT messenger RNA, although the transcription level of the MGMT gene in parental C6 cells was below the lower limits of detection under the same assay conditions. ACNU resistance of C6AR cells was significantly repressed by transfected gene-dependent antisense MGMT RNA expression that resulted in decreased survival of the tumor cells.

CONCLUSION

ACNU resistance resulting from the expression of MGMT in rat glioma cells is significantly overcome by the expression of antisense MGMT RNA. This result suggests that the antisense MGMT RNA system might be a useful strategy for overcoming ACNU resistance in the treatment of intractable malignant gliomas.