Abstract P4-02-10: Significance of ER-Coregulator-Src Axis in Hormonal Resistance
Authors: Vallabhaneni S , Nair BC , Cortez V , Chakravarty D , Challa R , Tekmal RR , Vadlamudi RK. 
Published: April 2011

Background: The estrogen receptor (ER) is implicated in the progression of breast cancer. Endocrine therapy using Tamoxifen, a selective estrogen receptor modulator (SERM), improves disease-free survival in postmenopausal women. Despite positive effects, initial or acquired resistance to endocrine therapies frequently occurs. Recent studies suggested the presence of alternative signaling pathways (including HER2 and c-Src kinase) that contribute to ER activation in the absence of estrogen. ER-coregulators play an essential role in hormonal responsiveness and coregulators such as PELP1 couples Src axis to ER thus representing a new pathway for targeted drug therapy. In this study, we examined the therapeutic efficacy of targeting the PELP-Src axis in blocking therapy resistance.

Methods: Three breast cancer model cell lines with resistance to Tamoxifen were used in this study: (1) MCF7-PELP1 (2) MCF7-HER2/neu and (3) MCF7-Tam. To establish the significance of ER-PELP1 coregulator-Src axis, we have generated additional model cells that stably express Src-ShRNA (MCF7-PELP1 -Src-shRNA and MCF7-HER2-SrcshRNA). Model cells were tested for proliferation using CellTiter-Glo assays, anchorage dependence and ER-extranuclear signaling by Western analysis using phospho-antibodies. Orally soluble Src inhibitor Dasatinib (SPRYCEL)was used to test whether it (a) blocks of ER-coactivator-Src signaling and (b) sensitizes the resistant breast cancer cells to endocrine therapy. Pre-clinical nude mice xenograft based studies were performed to validate the effect of Dasatinib alone or in combination with tamoxifen in vivo. IHC analysis of the tumors was performed to examine the effect of Dasatinib on ER signaling components.

Results: Src knock down or Dasatinib (100 nM) treatment substantially inhibited the growth of MCF7-PELP1, MCF7-HER2/neu and MCF7- Tam model cells in proliferation assays. Depletion of Src using siRNA substantially reduced E2 mediated activation of Src, MAPK and AKT in resistant model cells. In post-menopausal xenograft based studies, treatment with Dasatinib significantly inhibited the growth of therapy resistant cells. IHC analysis revealed that the tumors are ER positive, and Dasatinib (15 mg/kg/d) treated tumors exhibited alterations in Src and AKT pathways. Combinatorial therapy of Tamoxifen (100 μg/mouse/day) with Dasatinib showed better therapeutic effect compared to single agent therapy on the growth of therapy resistant tumors.

Conclusions: The results from this study showed that ER-PELP1 coregulator-Src axis play an important role in promoting hormonal resistance and blocking this axis prevents the development of hormonal independence in vivo. ER coregulator PELP1 and Src kinase are commonly deregulated in breast cancers therefore combination therapies using both endocrine agents and Dasatinib may have better therapeutic effect by delaying the development of hormonal resistance This study is supported by DOD grant W81XWH-08-1-0604.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-02-10.