Abstract 715: Regulation of the ABCG2 drug efflux transporter in breast cancer cells

Abstract

Abstract The ABC transporters, including ABCG2 or breast cancer resistance protein (BCRP) are drug efflux transporters. Their overexpression in cancers including breast cancer has been associated with the development of multidrug resistance (MDR) due to ABC transporter-mediated export from tumour cells of therapeutic drugs such as chemotherapeutic (eg. mitoxantrone, doxorubicin) and targeted agents (eg. tyrosine kinase inhibitors). PCR microarray analysis identified that ABCG2 expression was downregulated in the breast cancer cell lines, MCF-7 and T-47D following treatment of cells with the androgen, 5α-dihydrotestosterone (DHT), the Hedgehog pathway inhibitor, cyclopamine, or a combination of 10-8M DHT and 2μM cyclopamine. RT-qPCR analysis confirmed that DHT and cyclopamine treatments of MCF-7 cells rapidly downregulated ABCG2 mRNA levels by 24 hours. ABCG2 protein levels were also progressively decreased over 8 days of treatment, with earlier reductions in ABCG2 levels observed in cells co-treated with DHT and cyclopamine. As the ABC transporters are primarily active on membrane surfaces as drug exporters, the intracellular localisation of ABCG2 was investigated by immunofluorescence microscopy. In MCF-7 cells, DHT and/or cyclopamine decreased ABCG2 accumulation in cell-to-cell membrane junction complexes and interestingly, accumulation of ABCG2 into cytoplasmic vesicles which resembled aggresomes was observed in cells exposed to cyclopamine. Consistent with the reduced levels of membrane-bound ABCG2, export of the fluorescent ABCG2-specific substrate, mitoxantrone from MCF-7 cells was delayed following treatment of cultures with DHT and cyclopamine, thereby indicating diminished ABCG2 efflux activities. Moreover, these DHT- and cyclopamine-treated cells were also more responsive to the cytotoxic effects of mitoxantrone as the IC50 for mitoxantrone was decreased by ∼80% in treated cells compared to controls. Resistance to drug therapies, which is often facilitated by the ABC transporters, is a major cause of treatment failure and recurrence of breast cancer. As such, development and implementation of adjuvant compounds such as androgens and Hedgehog inhibitors, which inhibit expression and function of ABCG2 and other efflux transporters in breast cancer cells, may improve treatment outcomes and prolong recurrence-free survival of patients. Citation Format: Vivian YL Chua, Jennet Harvey, Jacqueline Bentel. Regulation of the ABCG2 drug efflux transporter in breast cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 715. doi:10.1158/1538-7445.AM2015-715