The role of cancer stem cell phenotype in the emergence of epirubicin resistance in triple-negative breast cancer

Author: Lashika Weerakoon

Weerakoon, Lashika, 2021 The role of cancer stem cell phenotype in the emergence of epirubicin resistance in triple-negative breast cancer, Flinders University, College of Medicine and Public Health

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Despite the advances in treatment strategies, cancers remain fatal to most patients mainly due to tumour cell’s remarkable ability to develop resistance to therapy. Epirubicin is an anthracycline, extensively utilised in the chemotherapeutic regimens for treating early as well as advanced breast cancer. Chemoresistance to epirubicin is a common occurrence and a major factor limiting the efficacy of epirubicin-incorporated regimens, which leads to treatment failure, disease progression, relapse, and mortality.

This study focuses on triple-negative breast cancer (TNBC), a biologically aggressive breast cancer subtype with few therapeutic options in the form of targeted therapies. TNBC may show primary or acquired resistance to epirubicin, and the development of collateral resistance to next in line therapy is an added complexity in the clinical management of TNBC. Elucidation of cellular mechanisms contributing to drug resistance is crucial to predict, prevent and overcome drug resistance in clinical settings.

The cancer stem cell (CSC) phenotype is a critical element of the multi-dimensional architecture of drug resistance. Resembling normal stem cells, CSCs are often slow-cycling and can survive cytotoxic treatments that target fast proliferating cancer cells. They can subsequently expand due to their inherent self-renewal capacity and cause tumours to re-emerge with a heightened resistance driven by enhanced plasticity. These secondary tumours are often more aggressive and multi-drug resistant. Targeting the CSC population and stemness pathways that promote drug resistance may reduce cancer recurrence.

This study aimed to investigate the role of BCSCs in the emergence of resistance to the chemotherapeutic agent epirubicin in TNBC. In the first Aim, quantitative gene expression analysis data indicated an upregulation of key stemness markers ALDH1, OCT4, and SOX2 along with the drug transporter ABCB1, upon a short term (48 hours) epirubicin treatment of MDA-MB-231 TNBC cells. These adaptive expression responses to epirubicin exposure were partly sustained after a period of drug withdrawal, and notably, ALDH1 expression showed further upregulation following the drug withdrawal. Despite showing sustained expression of stemness markers, the cells did not show enhanced self-renewal capacity when tested using a mammosphere formation assay. However, this finding appeared to relate to the diminished proliferative capacity of epirubicin treated cells. In the second Aim, an MDA-MB-231 sub-line was generated by long-term epirubicin selection and assessed for enhanced drug resistance and expression of stemness and drug resistance markers. While consistent IC50 values for epirubicin could not be defined, the drug-selected subline appeared less sensitive to high doses of epirubicin. However, it did not show enhanced expression of stemness markers or the drug efflux transporter.

The findings of this study lend support for the multifactorial nature of chemoresistance against epirubicin involving pathways that control drug exposure (such as drug efflux) and stem cell-associated survival pathways. Specifically, the results show that induction of these pathways may serve as a short-term adaptive response to epirubicin exposure; however, they may be dispensable in developing long-term stable drug resistance.

Keywords: Triple-negative breast cancer, Drug resistance, Epirubicin, Cancer Stem Cells

Subject: Biotechnology thesis

Thesis type: Masters
Completed: 2021
School: College of Medicine and Public Health
Supervisor: Robyn Meech