New insights into estrogenic regulation of O~6-methylguanine DNA-methyltransferase(MGMT) in human breast cancer cells:Co-degradation of ER-α and MGMT proteins by fulvestrant or O~6-benzylguanine indicates fresh avenues for therapy

作者：Ameya; Paranjpe; Nathan; I.Bailey; San...estrogensignalingmgmtdnarepairpathwaybreastcancer

摘要：Endocrine therapy using estrogen receptor-α(ER-α) antagonists for attenuating horm2one-driven cell proliferation is a major treatment modality for breast cancers.To exploit any DNA repair deficiencies associated with endocrine therapy,we investigated the functional and physical interactions of ER-α with O~6-methylguanine DNA methyltransferase(MGMT),a unique DNA repair protein that confers tumor resistance to various anticancer alkylating agents.The ER-α-positive breast cancer cell lines(MCF-7,T47D) and ER- negative cell lines(MDAMB-468,MDAMB-231),and established inhibitors of ER-α and MGMT,namely,ICI-182,780(Faslodex) and O~6-benzylguanine,respectively,were used to study MGMT- ER interactions.The MGMT gene promoter was found to harbor one full and two half estrogen-responsive elements(EREs) and two antioxidant-responsive elements(AREs).MGMT expression was upregulated by estrogen,downregulated by tamoxifen in Western blot and promoter-linked reporter assays.Similarly,both transient and stable transfections of Nrf-2(nuclear factor-erythroid 2-related factor-2)increased the levels of MGMT protein and activity 3 to 4-fold reflecting novel regulatory nodes for this drugresistance determinant.Of the different ER-α antagonists tested,the pure anti-estrogen fulvestrant was most potent in inhibiting the MGMT activity in a dose,time and ER-α dependent manner,similar to O~6-benzylguanine.Interestingly,fulvestrant exposure led to a degradation of both ER-α and MGMT proteins and O~6-benzylguanine also induced a specific loss of ER-α and MGMT proteins in MCF-7 and T47 D breast cancer cells with similar kinetics.Immunoprecipitation revealed a specific association of ER-α and MGMT proteins in breast cancer cells.Furthermore,silencing of MGMT gene expression triggered a decrease in the levels of both MGMT and ER-α proteins.The involvement of proteasome in the drug-induced degradation of both proteins was also demonstrated.Fulvestrant enhanced the cytotoxicity of MGMT-targeted alkylating agents,namely,temozolomide and B

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