title: Oncogenic HER2{Delta}16 suppresses miR-15a/16 and deregulates BCL-2 to promote endocrine resistance of breast tumors.
abstract: Tamoxifen is the most commonly prescribed therapy for patients with estrogen receptor (ER)α-positive breast tumors. Tumor resistance to tamoxifen remains a serious clinical problem especially in patients with tumors that also overexpress human epidermal growth factor receptor 2 (HER2). Current preclinical models of HER2 overexpression fail to recapitulate the clinical spectrum of endocrine resistance associated with HER2/ER-positive tumors. Here, we show that ectopic expression of a clinically important oncogenic isoform of HER2, HER2Δ16, which is expressed in >30% of ER-positive breast tumors, promotes tamoxifen resistance and estrogen independence of MCF-7 xenografts. MCF-7/HER2Δ16 cells evade tamoxifen through upregulation of BCL-2, whereas mediated suppression of BCL-2 expression or treatment of MCF-7/HER2Δ16 cells with the BCL-2 family pharmacological inhibitor ABT-737 restores tamoxifen sensitivity. Tamoxifen-resistant MCF-7/HER2Δ16 cells upregulate BCL-2 protein levels in response to suppressed ERα signaling mediated by estrogen withdrawal, tamoxifen treatment or fulvestrant treatment. In addition, HER2Δ16 expression results in suppression of BCL-2-targeting microRNAs miR-15a and miR-16. Reintroduction of miR-15a/16 reduced tamoxifen-induced BCL-2 expression and sensitized MCF-7/HER2Δ16 to tamoxifen. Conversely, inhibition of miR-15a/16 in tamoxifen-sensitive cells activated BCL-2 expression and promoted tamoxifen resistance. Our results suggest that HER2Δ16 expression promotes endocrine-resistant HER2/ERα-positive breast tumors and in contrast to wild-type HER2, preclinical models of HER2Δ16 overexpression recapitulate multiple phenotypes of endocrine-resistant human breast tumors. The mechanism of HER2Δ16 therapeutic evasion, involving tamoxifen-induced upregulation of BCL-2 and suppression of miR-15a/16, provides a template for unique therapeutic interventions combining tamoxifen with modulation of microRNAs and/or ABT-737-mediated BCL-2 inhibition and apoptosis.
authors.affiliation: Department of Pathology, School of Medicine, University of Colorado Denver, Aurora, CO 80045, USA.
authors.forename: Diana M
authors.initials: DM
authors.lastname: Cittelly
authors.forename: Partha M
authors.initials: PM
authors.lastname: Das
authors.forename: Virgilio A
authors.initials: VA
authors.lastname: Salvo
authors.forename: Juan P
authors.initials: JP
authors.lastname: Fonseca
authors.forename: Matthew E
authors.initials: ME
authors.lastname: Burow
authors.forename: Frank E
authors.initials: FE
authors.lastname: Jones
mesh_terms: D000818:Animals; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000473:pathology; D045744:Cell Line, Tumor; D019008:Drug Resistance, Neoplasm; D004965:Estrogen Antagonists / Q000627:therapeutic use*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D019254:Genes, bcl-2; D006801:Humans; D051379:Mice; D008819:Mice, Nude; D035683:MicroRNAs / Q000037:antagonists & inhibitors* / Q000502:physiology; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000502:physiology*; D013629:Tamoxifen / Q000627:therapeutic use*
chemical_list: D004965:Estrogen Antagonists; D035683:MicroRNAs; D018719:Erb-b2 Receptor Tyrosine Kinases; D013629:Tamoxifen; C505361:MIRN15 microRNA, human; C505362:MIRN16 microRNA, human; C508053:ERBB2 protein, human