PD01-03: Src Is a Potential Therapeutic Target in Endocrine Resistant Breast Cancer Exhibiting Low Estrogen Receptor (ER)-Mediated Transactivation.

Abstract

Abstract Aim: Identification of the mechanisms governing sensitivity to dasatinib in endocrine resistant breast cancer. Background: Despite the effectiveness of endocrine therapies over 40% of women relapse. To identify the molecular mechanisms associated with resistance to estrogen-deprivation on an aromatase inhibitor, we previously assessed changes in gene expression during adaptation to long-term culture of MCF7 human breast cancer (BC) cells in the absence of estradiol (E2) (LTED). Analyses of canonical signaling pathways highlighted FAK as one of the major pathways up regulated at the point of resistance and src as a dominant gene in this pathway. Src phosphorylates a plethora of proteins including ER. Surprisingly dasatinib a pan src inhibitor enhanced proliferation of MCF7 cells in the absence of E2 and had limited anti-proliferative effect in the LTED model. In contrast tamoxifen resistant MCF7 cells (TAMR) were exquisitely sensitive to dasantinib [1]. Methods: Proliferation was assessed using Titre Glo™. ER-mediated transcription was measured with an estrogen-response element linked luciferase reporter construct. Protein expression was determined by immunoblotting. Confocal microscopy was used to determine cellular localization of ER. Results: Dasatinib decreased proliferation of the TAMR (IC50 0.05nM) and resensitized them to tamoxifen but had no effect on the wild-type (wt)-MCF7 or LTED cells. Previously a relationship between BC subtype and sensitivity to dasatinib was reported with basal and post-EMT BC cell lines showing the highest sensitivity, which associated with high caveolin (CAV) expression [2]. In ovarian cancer expression of CAV and urinokinase plasminogen activator (uPA) also predicted sensitivity to dasatinib [3]. TAMR cells had a 2-fold increase in CAV-1 and 7-fold increase in uPA compared to wt cells. Comparison of LTED and TAMR showed that the TAMR whilst continuing to express high levels of ER, had a 10-fold lower level of ER-transactivation compared to wt-MCF7 and did not express PGR. In contrast the LTED showed a 7-fold increase in ER-transactivation compared to wt cells in the absence of E2. Strong evidence suggests ER in the TAMR cells acts via a non-genomic mechanism with ER expressed throughout the cell rather than being restricted to the nucleus. This implies the TAMR have an impeded luminal phenotype in relation to ER-genomic function. Dasatinib reduced the expression of ER in the TAMR and resulted in nuclear shuttling of ER whilst having no effect on the wt. BT474 (ER+/HER2 amplified) showed a similar distribution of ER to the TAMR cells but were far less sensitive to the antiproliferative effect of dasatinib. Of note the TAMR have increased phosphorylation of HER2 but no amplification. Conclusion: These data suggest src has differential effects in these endocrine resistant cell lines. The anti-proliferative effect of dasatinib was more pronounced in the endocrine resistant cell line with low ER genomic activity. Src inhibition warrants further clinical evaluation as a therapeutic target in endocrine resistant breast cancer. 1. S.K. Guest, Ann Oncol 2011, 22 (Supp 2):ii29–ii36. 2. Finn RS, Br Can Res Treat. 2007, 105(3):319–26. 3. Konecny GE, BJC. 2009, 101(10):1699–70. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD01-03.