Apurinic endonuclease activity in adult gliomas and time to tumor progression after alkylating agent-based chemotherapy and after radiotherapy.
ABSTRACT Apurinic/apyrimidinic endonuclease (Ap endo) is a key DNA repair enzyme that cleaves DNA at cytotoxic abasic sites caused by alkylating agents and radiation. We have observed that human glioma cells deficient in Ap endo activity are hypersensitive to clinically used alkylators (Silber et al., Clin Cancer Res 2002;8:3008.). Here we examine the association of glioma Ap endo activity with clinical response after alkylating agent-based chemotherapy or after radiotherapy.
Cox proportional hazards regression models were used to analyze the relationship of Ap endo activity with time to tumor progression (TTP).
In a univariate model with Ap endo activity entered as a continuous variable, the hazard ratio (HR) for progression after alkylator therapy in 30 grade III gliomas increased by a factor of 1.061 for every 0.01 increase in activity (P = 0.013). Adjusting for age, gender, extent of resection, and prior treatment strengthened slightly the association (HR = 1.094; P = 0.003). Similarly, the HR for progression after radiotherapy in 44 grade II and III tumors increased by a factor of 1.069 (P = 0.008). Adjusting for the aforementioned variables had little effect on the association. In contrast, we observed no association between activity and TTP in grade IV gliomas after either alkylator therapy in 34 tumors or radiotherapy in 26 tumors.
Our data suggest that Ap endo activity mediates resistance to alkylating agents and radiation and may be a useful predictor of progression after adjuvant therapy in a subset of gliomas.
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ABSTRACT: Alkylating agents have long played a central role in the adjuvant therapy of glioblastoma (GBM). More recently, inclusion of temozolomide (TMZ), an orally administered methylating agent with low systemic toxicity, during and after radiotherapy has markedly improved survival. Extensive in vitro and in vivo evidence has shown that TMZ-induced O(6)-methylguanine (O(6)-meG) mediates GBM cell killing. Moreover, low or absent expression of O(6)-methylguanine-DNA methyltransferase (MGMT), the sole human repair protein that removes O(6)-meG from DNA, is frequently associated with longer survival in GBMs treated with TMZ, promoting interest in developing inhibitors of MGMT to counter resistance. However, the clinical efficacy of TMZ is unlikely to be due solely to O(6)-meG, as the agent produces approximately a dozen additional DNA adducts, including cytotoxic N3-methyladenine (3-meA) and abasic sites. Repair of 3-meA and abasic sites, both of which are produced in greater abundance than O(6)-meG, is mediated by the base excision repair (BER) pathway, and occurs independently of removal of O(6)-meG. These observations indicate that BER activities are also potential targets for strategies to potentiate TMZ cytotoxicity. Here we review the evidence that 3-meA and abasic sites mediate killing of GBM cells. We also present in vitro and in vivo evidence that alkyladenine-DNA glycosylase, the sole repair activity that excises 3-meA from DNA, and Ape1, the major human abasic site endonuclease, mediate TMZ resistance in GBMs and represent potential anti-resistance targets.Frontiers in Oncology 01/2012; 2:176.
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ABSTRACT: Abstract Background: Radiotherapy is an important treatment for the patients with advanced pancreatic cancer. Emerging studies determined apurinic/apyrimidinic endonuclease 1/redox factor-1 (APE1/Ref-1) might associate with the resistance of human pancreatic cancer cells to radiotherapy. Aims: To investigate whether downregulation of APE1/Ref-1 expression by ribonucleic acid interference would increase the sensitivity of chromic-P32 phosphate to pancreatic cancer cells. Methods: The plasmids containing APE-specific and unspecific short hairpin were transfected into Patu-8898 cells. Stable cell clones were selected by G418. The mRNA expression of APE1/Ref-1 was detected by semiquantitative reverse transcription-polymerase chain reaction and the protein expression of APE1/Ref-1 was detected by Western blot analysis; cell proliferation was studied by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) and colony formation assay; apoptosis was detected by flow cytometry. Results: After 24 hours irradiation, APE1/Ref-1 mRNA and protein expression were upregulated, in a concentration-dependent manner. Suppression of APE1/Ref-1 by siRNA increased the pancreatic cancer cells hypersensitive to (32)P-CP. In the combination of (32)P-CP and siRNA group, MTT assay showed that the cell inhibition increased to (74.33%±9.02%), the surviving fraction in the colony formation assay was only 25.00%, and the apoptosis rate was up to (16.77%±0.98%). Conclusions: Knockdown APE1/Ref-1 gene expression may significantly sensitize the Patu-8988 cells to radiotherapy, which may be a useful target for modifying radiation resistance of pancreatic cancer cells to irradiation.Cancer Biotherapy & Radiopharmaceuticals 12/2012; · 1.44 Impact Factor
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ABSTRACT: Gliomas are the most frequent adult primary brain tumor, and are invariably fatal. The most common diagnosis glioblastoma multiforme (GBM) afflicts 12,500 new patients in the U.S. annually, and has a median survival of approximately one year when treated with the current standard of care. Alkylating agents have long been central in the chemotherapy of GBM and other gliomas. The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT), the principal human activity that removes cytotoxic O(6)-alkylguanine adducts from DNA, promotes resistance to anti-glioma alkylators, including temozolomide and BCNU, in GBM cell lines and xenografts. Moreover, MGMT expression assessed by immunohistochemistry, biochemical activity or promoter CpG methylation status is associated with the response of GBM to alkylator-based therapies, providing evidence that MGMT promotes clinical resistance to alkylating agents. These observations suggest a role for MGMT in directing adjuvant therapy of GBM and other gliomas. Promoter methylation status is the most clinically tractable measure of MGMT, and there is considerable enthusiasm for exploring its utility as a marker to assign therapy to individual patients. Here, we provide an overview of the biochemical, genetic and biological characteristics of MGMT as they relate to glioma therapy. We consider current methods to assess MGMT expression and discuss their utility as predictors of treatment response. Particular emphasis is given to promoter methylation status and the methodological and conceptual impediments that limit its use to direct treatment. We conclude by considering approaches that may improve the utility of MGMT methylation status in planning optimal therapies tailored to individual patients.Biochimica et Biophysica Acta 01/2012; 1826(1):71-82. · 4.66 Impact Factor