title: MGMT inhibition restores ERα functional sensitivity to antiestrogen therapy.
abstract: Antiestrogen therapy resistance remains a huge stumbling block in the treatment of breast cancer. We have found significant elevation of O(6) methylguanine DNA methyl transferase (MGMT) expression in a small sample of consecutive patients who have failed tamoxifen treatment. Here, we show that tamoxifen resistance is accompanied by upregulation of MGMT. Further we show that administration of the MGMT inhibitor, O(6)-benzylguanine (BG), at nontoxic doses, leads to restoration of a favorable estrogen receptor alpha (ERα) phosphorylation phenotype (high p-ERα Ser167/low p-ERα Ser118), which has been reported to correlate with sensitivity to endocrine therapy and improved survival. We also show BG to be a dual inhibitor of MGMT and ERα. In tamoxifen-resistant breast cancer cells, BG alone or in combination with antiestrogen (tamoxifen [TAM]/ICI 182,780 [fulvestrant, Faslodex]) therapy enhances p53 upregulated modulator of apoptosis (PUMA) expression, cytochrome C release and poly (ADP-ribose) polymerase (PARP) cleavage, all indicative of apoptosis. In addition, BG increases the expression of p21(cip1/waf1). We also show that BG, alone or in combination therapy, curtails the growth of tamoxifen-resistant breast cancer in vitro and in vivo. In tamoxifen-resistant MCF7 breast cancer xenografts, BG alone or in combination treatment causes significant delay in tumor growth. Immunohistochemistry confirms that BG increases p21(cip1/waf1) and p-ERα Ser167 expression and inhibits MGMT, ERα, p-ERα Ser118 and ki-67 expression. Collectively, our results suggest that MGMT inhibition leads to growth inhibition of tamoxifen-resistant breast cancer in vitro and in vivo and resensitizes tamoxifen-resistant breast cancer cells to antiestrogen therapy. These findings suggest that MGMT inhibition may provide a novel therapeutic strategy for overcoming antiestrogen resistance.
authors.affiliation: MD Anderson Cancer Center Orlando, Orlando, Florida 32827, United States of America. george.bobustuc@aurora.org
authors.forename: George C
authors.initials: GC
authors.lastname: Bobustuc
authors.forename: Joshua S
authors.initials: JS
authors.lastname: Smith
authors.forename: Sreeram
authors.initials: S
authors.lastname: Maddipatla
authors.forename: Sheila
authors.initials: S
authors.lastname: Jeudy
authors.forename: Arati
authors.initials: A
authors.lastname: Limaye
authors.forename: Beth
authors.initials: B
authors.lastname: Isley
authors.forename: Maria-Lourdes M
authors.initials: ML
authors.lastname: Caparas
authors.forename: Susan M
authors.initials: SM
authors.lastname: Constantino
authors.forename: Nikita
authors.initials: N
authors.lastname: Shah
authors.forename: Cheryl H
authors.initials: CH
authors.lastname: Baker
authors.forename: Kalkunte S
authors.initials: KS
authors.lastname: Srivenugopal
authors.forename: Said
authors.initials: S
authors.lastname: Baidas
authors.forename: Santhi D
authors.initials: SD
authors.lastname: Konduri
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology / Q000627:therapeutic use; D017209:Apoptosis / Q000187:drug effects / Q000235:genetics; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000201:enzymology* / Q000235:genetics / Q000473:pathology; D059447:Cell Cycle Checkpoints / Q000187:drug effects / Q000235:genetics; D045744:Cell Line, Tumor; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics / Q000378:metabolism; D015254:DNA Modification Methylases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D045643:DNA Repair Enzymes / Q000037:antagonists & inhibitors* / Q000378:metabolism; D019008:Drug Resistance, Neoplasm / Q000187:drug effects / Q000235:genetics; D004791:Enzyme Inhibitors / Q000494:pharmacology / Q000627:therapeutic use; D004965:Estrogen Antagonists / Q000494:pharmacology / Q000627:therapeutic use*; D047628:Estrogen Receptor alpha / Q000378:metabolism*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006147:Guanine / Q000031:analogs & derivatives / Q000494:pharmacology / Q000627:therapeutic use; D006801:Humans; D007150:Immunohistochemistry; D051379:Mice; D008819:Mice, Nude; D010766:Phosphorylation / Q000187:drug effects; D010768:Phosphoserine / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding / Q000187:drug effects / Q000235:genetics; D013629:Tamoxifen / Q000627:therapeutic use; D014158:Transcription, Genetic / Q000187:drug effects; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D025521:Tumor Suppressor Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism
chemical_list: D000970:Antineoplastic Agents; D050759:Cyclin-Dependent Kinase Inhibitor p21; C506487:ESR1 protein, human; D004791:Enzyme Inhibitors; D004965:Estrogen Antagonists; D047628:Estrogen Receptor alpha; D016159:Tumor Suppressor Protein p53; D025521:Tumor Suppressor Proteins; C064976:O(6)-benzylguanine; D013629:Tamoxifen; D010768:Phosphoserine; D006147:Guanine; D015254:DNA Modification Methylases; C515491:MGMT protein, human; D045643:DNA Repair Enzymes