Tamoxifen resistance induction results in mitoxantrone resistance in MCF-7 breast cancer cells via upregulation of ABCG2 expression

Abstract

Abstract One of the important barriers in the treatment of breast cancer is the development of tamoxifen resistance. Different mechanisms underlying tamoxifen resistance were identified. In this thesis, we aimed to assess the effect of tamoxifen resistance induction on ABCG2 gene/protein expression level and function in the parent MCF-7 breast cancer cell line and its tamoxifen-resistant (MCF-7/TAMR) cell line. The ABCG2 mRNA and protein expression were compared in MCF-7 and its tamoxifen-resistant derivative MCF-7/TAMR cells using Real-time PCR and western blot methods, respectively. For investigating the effect of tamoxifen-induced resistance on cross-resistance to other chemotherapy drugs such as mitoxantrone (as a well-known ABCG2 substrate), an MTT assay was used. Flow cytometry was applied to compare ABCG2 pump function between two cell lines using mitoxantrone accumulation assay. ABCG2 mRNA expression was also analyzed in tamoxifen-sensitive (TAM-S) (N=9) and tamoxifen-resistant (TAM-R) (N=9) breast tumor tissues. Our results indicated that the levels of ABCG2 mRNA, protein, and activity were significantly higher in MCF-7/TAMR cells compared to tamoxifen sensitive cell line and mitoxantrone was less toxic in the resistant cell line. Data further discovered that tamoxifen-resistant cells with high ABCG2 activation had a poor response when given rescue mitoxantrone chemotherapy. ABCG2 was also upregulated in tissue samples obtained from tamoxifen-resistant ER+ breast cancer patients compared to tamoxifen-sensitive patients. Increased expression of the ABCG2 mRNA and protein is a phenomenon that occurs after the emergence of resistance to tamoxifen and can cause cross-resistance to mitoxantrone in MCF-7 breast cancer cells.