Thesis (M.Sc.)--Chulalongkorn University, 2014

Tamoxifen is the first line adjuvant treatment for ER-positive breast cancer in post-menopausal patients. However, approximately 50% of advance stage ER-positive breast cancer patients developed tamoxifen resistance. Resistance to tamoxifen leads to more invasive cancer phenotypes, tumor recurrence, and distant metastasis. Salinomycin is carboxylic polyether ionophore that has been widely used as an antibiotic in poultry diseases. Recent studies demonstrated that salinomycin was able to inhibit various types of cancer cells and cancer stem cells. Therefore, the present study is to investigate the effects of salinomycin in inhibiting cell viability, migration, invasion, and increasing tamoxifen sensitivity and determine molecular mechanism of salinomycin to inhibit cancer cell invasion through matrix metalloproteinase 9 (MMP9) expression and increase tamoxifen sensitivity through altering the expression of ER-target genes and genes involved in tamoxifen resistance in anti-estrogen resistant breast cancer cells. The anti-cancer activity of salinomycin was investigated by MTT cell viability assay. Salinomycin demonstrated anti-cancer effects on the anti-estrogen resistant breast cancer cells after 24, 48 and 72 hours of exposure. This drug was further evaluated for the synergistic effects with tamoxifen on the inhibition of cell viability by tamoxifen response assay. Salinomycin can reverse tamoxifen resistance in the anti-estrogen resistant breast cancer cells and increases the efficacy of tamoxifen on the inhibition of wild-type ER-positive breast cancer cells. Moreover, salinomycin inhibited cell migration and invasion in the anti-estrogen resistant breast cancer cells in scratch assay and matrigel invasion assay. The effect of salinomycin on the mRNA expression of MMP9, ER-target genes, and genes involved in tamoxifen resistance was investigated by RT-PCR. Salinomycin significantly down-regulated the expression of MMP9, which is the main enzymes that promote cancer migration and invasion. In addition, salinomycin down-regulated NCOA3 which functions as a co-activator in ER-target genes transcription. Furthermore, salinomycin profoundly down-regulated cyclin D1 and c-myc which play an important role as positive regulators in cell cycle and cell proliferation. In addition, salinomycin up-regulated the expression of p21 which functions as a negative regulator in cell cycle. These findings suggested that salinomycin is potential to be a promising agent for novel treatment or can be used as a novel combination therapy with tamoxifen for increasing the efficacy of treatment in anti-estrogen resistant breast cancer patients.