title: The importance of HER2 signaling in the tumor-initiating cell population in aromatase inhibitor-resistant breast cancer.
abstract: Aromatase inhibitors (AIs) are an effective therapy in treating estrogen receptor-positive breast cancer. Nonetheless, a significant percentage of patients either do not respond or become resistant to AIs. Decreased dependence on ER-signaling and increased dependence on growth factor receptor signaling pathways, particularly human epidermal growth factor receptor 2 (EGFR2/HER2), have been implicated in AI resistance. However, the role of growth factor signaling remains unclear. This current study investigates the possibility that signaling either through HER2 alone or through interplay between epidermal growth factor receptor 1 (EGFR/HER1) and HER2 mediates AI resistance by increasing the tumor initiating cell (TIC) subpopulation in AI-resistant cells via regulation of stem cell markers, such as breast cancer resistance protein (BCRP). TICs and BCRP are both known to be involved in drug resistance. Results from in vitro analyses of AI-resistant versus AI-sensitive cells and HER2-versus HER2+ cells, as well as from in vivo xenograft tumors, indicate that (1) AI-resistant cells overexpress both HER2 and BCRP and exhibit increased TIC characteristics compared to AI-sensitive cells; (2) inhibition of HER2 and/or BCRP decrease TIC characteristics in letrozole-resistant cells; and (3) HER2 and its dimerization partner EGFR/HER1 are involved in the regulation of BCRP. Overall, these results suggest that reducing or eliminating the TIC subpopulation with agents that target BCRP, HER2, EGFR/HER1, and/or their downstream kinase pathways could be effective in preventing and/or treating acquired AI resistance.
authors.affiliation: Department of Pharmacology and Experiment Therapeutics, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
authors.forename: Rabia A
authors.initials: RA
authors.lastname: Gilani
authors.forename: Armina A
authors.initials: AA
authors.lastname: Kazi
authors.forename: Preeti
authors.initials: P
authors.lastname: Shah
authors.forename: Amanda J
authors.initials: AJ
authors.lastname: Schech
authors.forename: Saranya
authors.initials: S
authors.lastname: Chumsri
authors.forename: Gauri
authors.initials: G
authors.lastname: Sabnis
authors.forename: Anil K
authors.initials: AK
authors.lastname: Jaiswal
authors.forename: Angela H
authors.initials: AH
authors.lastname: Brodie
mesh_terms: D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism; D000818:Animals; D047072:Aromatase Inhibitors / Q000494:pharmacology* / Q000627:therapeutic use; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology*; D045744:Cell Line, Tumor; D019008:Drug Resistance, Neoplasm; D066246:ErbB Receptors / Q000378:metabolism; D005260:Female; D006801:Humans; D000077289:Letrozole; D061986:MCF-7 Cells; D051379:Mice; D008819:Mice, Nude; D009363:Neoplasm Proteins / Q000378:metabolism; D014411:Neoplastic Stem Cells / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D009570:Nitriles / Q000494:pharmacology*; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects; D014230:Triazoles / Q000494:pharmacology*; D023041:Xenograft Model Antitumor Assays
chemical_list: D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D047072:Aromatase Inhibitors; D066246:ErbB Receptors; D000077289:Letrozole; D009363:Neoplasm Proteins; D009570:Nitriles; D018719:Erb-b2 Receptor Tyrosine Kinases; D014230:Triazoles; C116614:ABCG2 protein, human; C512478:EGFR protein, human; C508053:ERBB2 protein, human