Abstract P5-09-11: BP1, a homeoprotein, regulates estrogen receptor alpha and induces estrogen independence

Abstract

Abstract Introduction: BP1 is a member of the homeobox gene family of transcription factors. Our recent studies have shown that BP1 may play a role in breast cancer cell survival, aggressiveness and metastasis. BP1 protein (pBP1) is expressed in 80% of invasive ductal breast tumors. Moreover, 100% of inflammatory breast tumors are BP1 positive. These data led us to define the mechanism of BP1-related tumorigenesis and aggressiveness in breast cancer. Materials and Methods: MCF-7/O1, O2 and O4 cells overexpressing BP1 and control V1 and V2 cells were tested for growth in estrogen free media, malignant potential and invasiveness using cell viability assays, soft agar assays and matrigel assays, respectively. To determine the influence of BP1 overexpression on tumor characteristics, empty vector cells (V1) and overexpressor cells (O2 and O4) were injected into the fat pads of athymic nude mice. Mice were supplemented with estrogen pellets or were unsupplemented. Chromatin immunoprecipitation assays (ChIP) were used to validate the binding of pBP1 to ERa and EP300. The effects of BP1 expression on ERα and EP300 were investigated using immunoblotting and qRT-PCR. Effects of BP1 overexpression on tamoxifen sensitivity were measured using the MTT assay. Results: Cells overexpressing BP1 showed higher viability (p<0.05) when grown in the absence of serum, greater invasive potential (p<0.05) and formed larger and more rapidly growing colonies (p<0.0001) compared with cells containing the empty vector. Tumors were larger in mice receiving O1, O2 or O4 cells than in mice receiving V1 or V2 cells (p<0.0001). There was also a positive correlation between BP1 mRNA levels and tumor size in patients (p = 0.01). 20% of mice injected with O2 or O4 developed tumors in the absence of estrogen, in contrast to control mice which did not develop tumors. Several mechanisms of estrogen independence related to BP1 were established: (1) an indirect mechanism, in which BP1 stabilizes ERα protein by transcriptional activation of p300, and (2) a direct mechanism, in which BP1 binds to the first intron of ERα and upregulates ERα RNA and protein expression. In addition, breast cancer anti-estrogen resistance 1 (BCAR1) RNA expression was higher in O2 as compared to V1 cells. Consistent with these findings, O2 cells exhibited increased proliferation when treated with tamoxifen, while V1 cells showed growth inhibition. Conclusion: High BP1 levels can lead to estrogen independence in ER positive breast cancer cells and tumors in mice by at least two mechanisms, indirect and direct, and are associated with resistance to tamoxifen. These results suggest that BP1 may be an important therapeutic target in ER positive breast cancer. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-09-11.