Abstract 3894: A positive feedback loop prevents differentiation of endocrine resistant luminal progenitor breast cancer cells
Authors: Ffrench Brendan , Charmsaz Sara , Breen Eamon , Hill Arnold , Young Leonie 
Published: July 2017

Abstract
Introduction: Treatment of breast cancer patients has advanced greatly with the stratification of ER+, HER2+ and triple negative cohorts. ER+ breast cancer is the most commonly diagnosed mammary malignancy and is managed with anti-endocrine therapies. Therapy resistant disease emerges in approximately one in three women. Within the ER+ cohort, breast cancer remains a heterogeneous disease, particularly at the cellular level. For example; ER+ breast cancer can range from &lt;10 % to &gt;90 % ER+ cells within a tumor. This study set out to define and target differences in cellular heterogeneity between endocrine sensitive and resistant ER+ breast cancer.
Methods: A flow cytometry based cell surface protein (CSP) screen (37 proteins) identified changes in cellular heterogeneity between endocrine sensitive and resistant ER+ models. FACS isolation and single cell plating characterized the self-renewal and differentiation potential of the identified sub-populations. Conditioned media experiments assayed secretory activity of the endocrine resistant phenotype.
Results: The CSP screen identified FUT4+MUC1- cells as central to the endocrine resistant phenotype. The endocrine sensitive model (MCF7) was found to contain roughly equal proportions of FUT4+MUC1- (31.2 ± 1.6 %), FUT4-MUC1- (38.5 ± 7.1 %) and FUT4-MUC1+ (23.0 ± 6.9 %) cells. While two independent models of resistance were found to be dominated by FUT4+MUC1- cells (LY2: 86.0 ± 5.5 %; TMR: 80.6 ± 6.5 %). Conventional theory suggests that these resistant FUT4+MUC1- cells are the apex cancer stem cells (CSCs). To test this hypothesis, single cell clonogenicity assays were carried out on FUT4+MUC1- cells isolated from the endocrine sensitive model. It was found that these FUT4+MUC1- cells were not apex CSCs, but rather mid-tier progenitor cells, as they could not produce all the cell types of the sensitive model. Paired conditioned media and clonogenicity studies demonstrated that the resistant secretome is sufficient to trap FUT4+MUC1- cells isolated from the sensitive model in a self-renewing, resistant-like state (p-value: 0.011; Chi-Squared test). Moreover, resistant cells which were removed from treatment conditions for &gt;8 weeks maintained the resistant FUT4+MUC1- phenotype. These data indicate co-operation between cell- and treatment-driven mechanisms creating a FUT4+MUC1- resistant phenotype.
Conclusions: Chronic tamoxifen therapy drives heterogeneity towards a FUT4+MUC1- phenotype. Single cell self-renewal and differentiation studies identify this cell type as a mid-tier progenitor cell rather than an apex cancer stem cell. A positive feedback loop supplements the treatment induced change, to maintain the resistant phenotype in the absence of further tamoxifen treatment. Targeting this feedback loop or its downstream effectors may allow for re-sensitization of endocrine resistant disease.
Citation Format: Brendan Ffrench, Sara Charmsaz, Eamon Breen, Arnold Hill, Leonie Young. A positive feedback loop prevents differentiation of endocrine resistant luminal progenitor breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3894. doi:10.1158/1538-7445.AM2017-3894