Abstract P5-09-02: Cell culture models to study mechanisms for endocrine resistant breast cancer, new treatment options and new biomarkers

Abstract

Abstract Introduction: Estrogen is the main driver of growth of the majority of breast cancers and many patients benefit from endocrine therapy. However, both innate and acquired resistance is a major problem. In order to study resistance mechanisms and to explore targeted treatment options for endocrine resistant breast cancer, we have developed in vitro cell culture models based on the estrogen receptor (ER) positive and estrogen responsive human breast cancer cell lines MCF-7 and T47D. Methods: Panels of MCF-7 and T47D sublines with acquired resistant to tamoxifen, fulvestrant and aromatase inhibitors were established by selection of colonies surviving long term treatment. Long-term estrogen deprived (LTED) MCF-7/S9 cells were developed by step-wise serum reduction until transfer to serum-free medium. The endocrine resistant sublines were analysed with respect to expression and function of the ER and the HER system, with respect to response to 195 kinase inhibitors and to second-line endocrine therapy. Results: ER expression was reduced in tamoxifen and fulvestrant resistant MCF-7 sublines, unchanged in tamoxifen resistant T47D sublines, whereas fulvestrant resistant T47D sublines were ER negative. In general, ER was functional in the resistant MCF-7 cells. Both tamoxifen and fulvestrant resistant MCF-7 sublines expressed increased level of HER1, the activated form of HER3 and activated ERK1/2, whereas HER4 was severely decreased. In fulvestrant resistant T47D sublines, HER1 and HER4 expression were decreased whereas HER2 expression was highly upregulated. The kinase inhibitor screen disclosed HER receptors as common hits in the tamoxifen and fulvestrant resistant MCF-7 sublines. Regarding T47D, c-Src and Aurora were common hits for tamoxifen and fulvestrant resistant sublines. Tamoxifen resistant MCF-7 and T47D sublines were severely growth inhibited by ER knockdown with either siRNA or fulvestrant, but complete growth arrest required combined treatment with e.g ERK1/2 inhibitor. LTED MCF-7 cells were resistant to aromatase inhibitor treatment and sensitive to both tamoxifen and fulvestrant. Conclusions: In our large panels of MCF-7 and T47D derived endocrine resistant sublines, it was a general finding that ER is the main driver of growth of tamoxifen resistant tumor cells, and also of growth of LTED MCF-7 cells. The ER down modulator fulvestrant inhibited growth of tamoxifen resistant sublines but complete growth arrest required combined treatment with e.g. a kinase inhibitor targeting the activated growth promoting pathways in the resistant cells. In fulvestrant resistant sublines, ER played only a minor role in MCF-7 sublines and no role in T47D sublines. The HER system played a major role in fulvestrant resistant MCF-7 cells, whereas Aurora B and c-Src were new candidate targets for fulvestrant resistant T47D cells. These cell culture models reflect clinical findings and will be available for collaborative studies, e.g. to discover new predictive markers to select targeted therapy for resistant tumors. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-09-02.