Abstract 1519: Estrogen receptor beta and p53 signaling crosstalk: implications for ER beta as a potential therapeutic target in triple negative breast cancer

Despite over 20 years of research, there is no current consensus on whether estrogen receptor β (ERβ) is pro- or anti-tumorigenic. With its expression found among all breast cancer subtypes, ERβ is a potentially valuable therapeutic target for breast cancers that lack effective, targeted therapies, especially the triple negative breast cancer (TNBC) subtype. In the published literature, there has been no consensus as to whether ERβ functions as a pro- or anti-tumorigenic protein, although a bifaceted role for ERβ has been proposed. Previous studies from our lab have shown that ERβ can directly bind and interact with p53, resulting in inhibition of its functions. Knockdown of ERβ in a breast cancer cell line with wild-type (wt) p53 (MCF-7) elicits opposite functional effects compared to knockdown of ERβ in a TNBC breast cancer cell line with mutant (mut) p53 (MDA-MB-231). Therefore, our hypothesis is that ERβ functions are dependent on p53 context. Besides having different p53 statuses, these two cell systems also have many other genetic differences that may or may not contribute to the opposing functions of ERβ. Currently, we are using an isogenic cell system that allows for expression of different combinations of ERβ and p53 (wt or mut) proteins. Here, we used CRISPR/Cas9 technology to knockout (KO) the TP53 gene in MDA-MB-231 cells, resulting in a complete absence of the endogenous mut-p53 protein. By transiently transfecting back in wt-p53, we can induce a prototypical wt-p53 response, which causes an increase in p53 target genes accompanied by a decrease in cell growth. Meanwhile, the parental cell line with endogenous mut-53 protein expression shows downregulation of p53 target genes and increases cell growth. This demonstrates that wt-p53 is still functional in the context of a cell that originally expressed mut-p53, and that wt-p53 and mut-p53 have opposing functions. In this system, we demonstrate that ERβ decreases p53 target gene expression and apoptosis in the wt-p53 context. Conversely, in the mut-p53 context, ERβ increases p53 target gene expression, while decreasing cell cycle progression. Taken together, these data support a pro-tumorigenic role of ERβ in the wt-p53 context, and an anti-tumorigenic role of ERβ in the mut-p53 context in our isogenic system. Finally, we explore the ability of ERβ ligands to modulate the ERβ-p53 interaction and subsequently affect p53 functions. We have found that two ERβ antagonists, 4-hydroxytamoxifen and PHTPP, have opposite effects on p53 target genes, suggesting a difference in their ability to modulate the ERβ-p53 interaction. Our data show that p53 status of the tumor should be taken into consideration for therapeutic targeting of ERβ in TNBC. Citation Format: Christina E. Adams, Gokul M. Das. Estrogen receptor beta and p53 signaling crosstalk: implications for ER beta as a potential therapeutic target in triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1519. doi:10.1158/1538-7445.AM2017-1519