Abstract P3-10-03: Exploiting p53-dependent functional duality of estrogen receptor-beta to repurpose tamoxifen for triple negative breast cancer therapy

Whether estrogen receptor beta (ERβ/ESR2) is a pro- or anti-oncogenic protein in breast cancer has been controversial. ERβ levels are generally high in triple-negative breast cancer (TNBC). Reports including the Cancer Genome Atlas (TCGA) show that about 80% of TNBC express mutant p53 and it is a major driver of these cancers. We tested the hypothesis that WT versus mutant status of p53 will have an important role in determining the duality of ERβ functions. We showed that ERβ directly binds to TP53 in human breast cancer cells. Using glutathione-S-transferase (GST)-pull down and co-imunoprecipitation assays, we have delineated the DNA binding domain (DBD) along with the hinge domain of ERβ and the C-terminal regulatory domain of p53 to be essential for the interaction. The highly sensitive in situ proximity ligation assay (PLA) showed that ERβ is capable of interacting with both wild type (WT) and mutant p53 in breast cancer cells and tumor tissues. ERβ and p53 antibodies validated for specificity were used for all experiments. Combination of proliferation, cell cycle, and apoptosis assays, RNAi technology, quantitative ChIP (qChIP), and real-time PCR (qRT-PCR) showed that ERβ is pro-proliferative in the context of WTp53, whereas it is anti-proliferative in the context of mutant p53 in multiple breast cancer cell lines. The p53-dependent diametrically opposite functions of ERβ were recapitulated in isogenic MDA-MB-231 TNBC cells (generated by CRISPR) that differ only in the presence of WT versus mutant p53. A major gain-of-function of mutant p53 is its ability to bind and inactivate tumor suppressor p73. We show that ERβ binds and sequesters mutant p53 from mutant p53−p73 complex leading to reactivation of p73. Consistent with these observations, immunohistochemistry (IHC) in TNBC patient tumor tissue microarray (TMA) showed that patients with tumors expressing mutant p53 along with high levels of ERβ were of smaller size and lower stage. Complementing these findings, our analysis of the subgroup of the basal-like/TNBC tumors expressing mutant p53 (but not WTp53) in the METABRIC dataset showed that patients with tumors expressing higher levels of ERβ RNA (ESR2) had significantly better breast cancer-specific survival. The finding that ERβ–mutant p53 combination prognosticates survival in TNBC is important as to date there are no effective prognostic markers for TNBC and suggest the potential for using ERβ−mutant p53 combination in stratification of TNBC into therapeutically actionable subgroups.Furthermore, our findings provide a mechanistic explanation for the functional duality of ERβ and the controversy over its pro-versus anti-tumorigenic role.Surprisingly, Tamoxifen (Tam) increased ERβ-mut-p53 interaction in TNBC cells leading to increased transcription of anti-proliferation genes and knockdown experiments showed that the effect on transcription was dependent on both p73 and ERβ. Importantly, when combined with doxorubicin (Adriamycin) Tam decreased several fold the IC50 of doxorubicin resulting in increased apoptosis. The combination was more effective in inhibiting TNBC xenograft tumor growth in vivo compared to either treatment alone. Significant clinical implications of these findings include the potential for treating patients with doxorubicin at much lower dose than what is currently used in the management of TNBC, thereby reducing its major cumulative dose side effects. Importantly, although at present Tam is not indicated for TNBC, our data suggest the possibility for repurposing Tam therapy alone or in combination with chemotherapy to treat TNBC stratified based on ERβ and p53 status. If validated in a clinical trial, our findings could lead to a therapy that is fundamentally better in terms of effectiveness, cost and time needed to reach the TNBC patients. Citation Format: Gokul M Das, Utpal K Mukhopadhyay, Chetan C Oturkar, Christina Adams, Nadi Wickramasekera, Sanjay Bansal, Rajesh Medisetty, Austin Miller, Wendy M Swetzig, Laxmi Silwal-Pandit, Anne-Lise Borresen-Dale, Creighton J Creighton, Jun Hyoung Park, Santhi D Konduri, Alka Mukhopadhyay, Alexander Caradori, Benny Abraham Kaipparettu. Exploiting p53-dependent functional duality of estrogen receptor-beta to repurpose tamoxifen for triple negative breast cancer therapy [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-10-03.