Abstract P6-04-18: Evaluation of the molecular mechanisms behind fulvestrant resistant breast cancer

Abstract

Abstract To study the mechanisms behind treatment resistance, we have previously established a large panel of antiestrogen resistant breast cancer cell lines based on the estrogen receptor (ER)-positive breast cancer cell line, MCF-7. It seems to be a general phenomenon that MCF-7 cells switch from ER driven to human epidermal growth factor receptor (HER) driven cell growth upon development of fulvestrant resistance. To obtain a broader knowledge of the resistance mechanisms, we have established four fulvestrant resistant subclones from the ER-positive breast cancer cell line, T47D. The parental T47D cells were confirmed to be estrogen responsive and short-term treatment of the parental T47D cells with fulvestrant severely reduced cell growth as well as protein expression of ER and the estrogen regulated proteins, insulin-like growth factor 1 receptor alpha (IGF-1Rα) and progesterone receptor (PR). All four fulvestrant resistant cell lines had reduced mRNA and protein expression of HER1 and HER4 compared to T47D, while HER2 was highly up regulated in the fulvestrant resistant cell lines and ER was lost. No difference in response to the HER2 inhibitor trastuzumab or the specific HER2 kinase inhibitor AG825 was observed between the fulvestrant resistant cell lines and T47D, indicating that HER2 is not the main driver of fulvestrant resistance in T47D cells. We found no indication of involvement of the two classical signaling pathways downstream of the HER receptors (Ras/Raf/Erk and PI3K/Akt). However, the JNK inhibitor SP600125 preferentially inhibited growth of two tested fulvestrant resistant cell lines. Withdrawal of fulvestrant for several weeks did not result in regain of ER expression, or in change of HER receptor expression, supporting a stable resistant phenotype. In conclusion, loss of ER expression explains the lack of effect of fulvestrant in the T47D fulvestrant resistant cell lines. Noteworthy, the observed over expression of HER2 in the fulvestrant resistant T47D cells does not seem to be the main driver of cell growth, as HER2 targeted therapies like trastuzumab and HER2 kinase inhibitor did not exert a preferential growth inhibition of resistant cells. These breast cancer cells with high endogenous HER2 expression will be used as a model mimicking HER2 positive breast cancer which does not respond to standard HER2 targeted therapies. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-04-18.