title: Endocrine resistance in breast cancer--An overview and update.
abstract: Tumors that express detectable levels of the product of the ESR1 gene (estrogen receptor-α; ERα) represent the single largest molecular subtype of breast cancer. More women eventually die from ERα+ breast cancer than from either HER2+ disease (almost half of which also express ERα) and/or from triple negative breast cancer (ERα-negative, progesterone receptor-negative, and HER2-negative). Antiestrogens and aromatase inhibitors are largely indistinguishable from each other in their abilities to improve overall survival and almost 50% of ERα+ breast cancers will eventually fail one or more of these endocrine interventions. The precise reasons why these therapies fail in ERα+ breast cancer remain largely unknown. Pharmacogenetic explanations for Tamoxifen resistance are controversial. The role of ERα mutations in endocrine resistance remains unclear. Targeting the growth factors and oncogenes most strongly correlated with endocrine resistance has proven mostly disappointing in their abilities to improve overall survival substantially, particularly in the metastatic setting. Nonetheless, there are new concepts in endocrine resistance that integrate molecular signaling, cellular metabolism, and stress responses including endoplasmic reticulum stress and the unfolded protein response (UPR) that provide novel insights and suggest innovative therapeutic targets. Encouraging evidence that drug combinations with CDK4/CDK6 inhibitors can extend recurrence free survival may yet translate to improvements in overall survival. Whether the improvements seen with immunotherapy in other cancers can be achieved in breast cancer remains to be determined, particularly for ERα+ breast cancers. This review explores the basic mechanisms of resistance to endocrine therapies, concluding with some new insights from systems biology approaches further implicating autophagy and the UPR in detail, and a brief discussion of exciting new avenues and future prospects.
authors.affiliation: Department of Oncology, Georgetown University Medical Center, Washington DC 20057, USA. Electronic address: clarker@georgetown.edu.
authors.forename: Robert
authors.initials: R
authors.lastname: Clarke
authors.affiliation: Department of Biological Sciences, Virginia Polytechnic and State University, Blacksburg, VA 24061, USA.
authors.forename: John J
authors.initials: JJ
authors.lastname: Tyson
authors.affiliation: Edinburgh Breast Unit, Western General Hospital, Edinburgh, UK.
authors.forename: J Michael
authors.initials: JM
authors.lastname: Dixon
mesh_terms: D047072:Aromatase Inhibitors / Q000627:therapeutic use; D001943:Breast Neoplasms / Q000188:drug therapy / Q000235:genetics*; D019008:Drug Resistance, Neoplasm; D059865:Endoplasmic Reticulum Stress; D020847:Estrogen Receptor Modulators / Q000627:therapeutic use; D047628:Estrogen Receptor alpha / Q000235:genetics; D005260:Female; D006801:Humans; D009154:Mutation; D056811:Unfolded Protein Response
chemical_list: D047072:Aromatase Inhibitors; C506487:ESR1 protein, human; D020847:Estrogen Receptor Modulators; D047628:Estrogen Receptor alpha
keywords: Antiestrogens; Autophagy; Breast cancer; Computational biology; Endocrine resistance; Systems biology; Tamoxifen; Unfolded protein response