Abstract P6-04-16: The role of RIP140 and FOXA1 in breast cancer endocrine sensitivity and resistance

Background: The estrogen receptor-α (ERα) is implicated in the initiation and progression of breast cancer. Endocrine therapeutics, including tamoxifen (OHT) and fulvestrant (ICI182780; ICI) improve disease-free survival. Despite positive effects, de novo or acquired resistance to endocrine therapies frequently occurs. It has recently been published that nuclear receptor–interacting protein 1 , NRIP1 , also called RIP140 , encodes an ER corepressor RIP140 and has a role in the regulation of breast cancer cell growth in response to anti-estrogens. FOXA1 has previously been shown to function as a ‘pioneer factor’ and dictate ER dependent gene expression in both normal and malignant breast epithelial cells. In this study we investigated the expression of ERα cofactors RIP140 and FOXA1 and their roles in hormonal therapy and endocrine resistance. Methods: Tamoxifen sensitive or resistant breast cancer cell lines were analysed for protein and mRNA expression by western blotting and quantitative real-time PCR with or without the inhibition of ERα with tamoxifen or ICI. To determine the functional role of RIP140 and FOXA1 as well as their interaction in breast cancer, tamoxifen sensitive breast cancer cells (eg. MCF-7 and ZR-75-1) were transfected with either RIP140 or FOXA1 expression vector and examined for the expression of ER-target genes by Western blot and RT-qPCR analysis. The consequence of depletion of RIP140 and FOXA1 by RNA interference (RNAi) was also studied in these breast cancer cells. The expression of RIP140 and FOXA1 and potential target genes were also validated in breast cancer patient samples by Immunohistochemistry. Results: We first examined the basal expression levels of RIP140 and FOXA1 in tamoxifen sensitive as well as resistant breast cancer cell lines. The expression levels of RIP140 were low in the resistance cells compared with their parental sensitive counterparts, while that of FOXA1 had no significant difference between the cell lines. The degradation of ERα by ICI treatment resulted in the down-regulation of RIP140 and FOXA1 expression in the endocrine sensitive. Our experiments also demonstrated the functional interaction between RIP140 and FOXA1 and showed that depletion of RIP140 by RNA interference (RNAi) in sensitive cells resulted in reduction of FOXA1 expression. We also indentified from this work, a number of ER-regulated genes which are activated by FOXA1 and repressed by RIP140 expression. The regulation of these target genes by ERα, FOXA1 and RIP140 were also confirmed in the breast cancer patient samples. Conclusion: The results from this study demonstrated that RIP140 and FOXA1 play a pivotal role in dictating hormonal sensitivity and resistance in breast cancer. Moreover, this work also identified a number of ERα-regulated genes which are downstream targets of FOXA1 and RIP140, and as such, they may be reliable prognostic markers for endocrine sensitivity in breast cancer. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-04-16.