Metformin reverses tamoxifen resistance through the lncRNA GAS5-medicated mTOR pathway in breast cancer

BACKGROUND: Metformin (Met) has antitumor effects on various cancers, but it is still unclear whether it exerts a reversible effect on endocrine resistance in breast cancer (BC) patients. In the present survey, metformin’s effects on tamoxifen-resistant MCF cells (The cell line was established at the Michigan Cancer Foundation, hence the name MCF cell) were evaluated and the molecular mechanism was explored. METHODS: We constructed a tamoxifen-resistant BC cell line MCF-7R, then applied Cell Counting Kit-8 (CCK-8), flow cytometry, and EdU assessments to determine the growth and apoptosis of MCF-7R cells. Quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot were used to evaluate the expression level of lncRNA GAS5 and the mTOR-associated proteins. Overexpression and downregulation of lncRNA GAS5 were carried out to explore the molecular mechanism of Met. RESULTS: The combined effect of Met and OHT (4-hydroxytamoxifen, main active metabolite of tamoxifen) was to remarkably reduce the growth and increase the apoptosis of MCF-7R cells compared with OHT alone. The levels of the mTOR, p-mTOR and p-P70S6K proteins in MCF-7R cells decreased after Met treatment, but the p-AMPK2 and PTEN proteins significantly increased. The overexpression of lncRNA GAS5 in MCF-7R cells inhibited cell growth with decreased mTOR, p-mTOR, PCNA, and Bcl-2 proteins. Metformin had no significant effect on MCF-7R cells with lncRNA GAS5 knockdown but contrarily activated p-mTOR. CONCLUSIONS: Metformin can inhibit overactivation of the mTOR signaling pathway through upregulating lncRNA GAS5 expression, thereby inhibiting the growth and inducing the apoptosis of BC cells, providing a new clinical treatment for BC.