Time-Series Analysis on the Process of Acquiring Resistance for Estrogen Receptor Antagonist in Breast Cancer Cells

Endocrine resistance remains a major clinical challenge in the therapy for estrogen-receptor (ER) positive breast cancers though its multiple mechanisms have beenfound to be associated with the resistance. The process of acquiring drug resistance in cancer cells can be understood as a state transition from non-resistant stage to resistant via pre-resistant, and it isaccompanied with rewiring of molecular network triggered by the change of extracellular environment. Although detecting early signature of the resistance contribute to anti-resistance strategies, few studies are focused on pre-resistant stage.We tried to elucidate the molecular mechanismscontributing to the process of acquiring drug resistance by measuring the gene expression pattern in the tamoxifen (TAM: estrogen receptor antagonist)-treated cells. ER positive human breast cancer MCF-7 cells were cultured under a continuous exposure of TAM for 12 weeks. TAM inhibited the growth of MCF-7 at 3 weeks of exposure, and this inhibiting effect was partially and gradually canceled from 5 to 8 weeks. RNA-sequencing of the TAM-treated cells at each weekrevealed that some genes regulated by TGFβ, NOTCH, HIF-1,andas well assome cell adhesion molecules may contribute to the TAM resistance, because those expression levels were increased when the cell growth rate was recovered. In addition, the expression levels of FOXO signaling and autophagy related genes were elevated only before the cell growth rate wasrecovered. This impliesthat these pathways may play key roles in the acquisitionof drug resistance and be biomarkers for pre-resistant state. To estimate the molecular network which becomes unstable before and after acquisition of resistance, we are analyzing cell-to-cell variability of these molecules by using imaging cytometer. In this presentation, we discuss about the latest results and thefuture research planning.