Abstract 1312: The estrogen receptor β agonists, Liquiritigenin and S-equol, inhibit breast cancer cell proliferation through the activation of tumor-suppressor and other pathways.

Estrogen and estrogen receptors (ERα and ERβ) play an important role in breast cancer. Currently, antiestrogen (AE) targeted to ERα and aromatase inhibitors (AI), which block local and systemic estrogen production, are used in the treatment of breast cancer. Both treatments improve outcomes for about 50% of patients with early or advanced ERα positive breast cancer. Unfortunately, in women with advanced breast cancer, essentially all breast cancers develop resistance to these two classes of agents. Moreover, the side effects associated with currently used AI limit the long-term utility of them as chemopreventative agents. Therefore there is a critical need to develop effective alternate strategies to prevent or delay the development of resistance to endocrine therapy and the resulting tumor progression. Recent studies have identified several natural compounds that have the potential to function as ERβ agonists. Plant-derived ERβ-agonists, Liquiritigenin (LIQ), from the plant Glycyrrhizae uralensis and an active component of MF101, and S-equol, from the soy isoflavone daidzein, are currently being tested to treat vasomotor hot flashes associated with menopause. In addition, S-equol is also being tested as a treatment for BPH in men. However, the exact mechanism of action of these compounds is not clear. In this study we sought to determine the mechanism by which ERβ acts as a tumor suppressor, and whether these ERβ agonists could resensitize Letrozole-resistant breast cancer cells. MCF7aro and LTLT-ca cells were used as models of hormone-responsive and hormone-resistant breast cancer respectively. MTT assay was used to determine the half maximal inhibitory concentrations of the ERβ agonists. Upon treatment, our results show changes in the levels of p53, NFkB, cyclin D1, KLF5 and others using qRT-PCR and western blot analysis. The effects of ERβ agonists on the activation of these genes were confirmed using specific gene promoter-luciferase reporter gene constructs. Specificity of ERβ-mediated actions was confirmed using a siRNA approach in addition to other methods and knockdown efficiency was confirmed through RT-PCR. Interestingly, our results show that the plant-derived ERβ-agonists, LIQ and S-equol, not only affect the growth of breast cancer cells and tumors but treatment increases the expression of p53 and others, and upon ERβ knockdown this effect is ablated. This study suggests that ERβ acts as a tumor suppressor by regulating the expression of p53 and others involved in cell proliferation. In conclusion, this study demonstrates the potential role for ERβ agonists to improve adjuvant endocrine therapy to treat both hormone-responsive and hormone-resistant breast cancers, as well as provides rationale for new preventive approaches to decrease the incidence of breast cancer. Citation Format: Cathy Samayoa, Anil Kotha, Naveen K. Krishnegowda, Ratna K. Vadlamudi, Rajeshwar Rao Tekmal. The estrogen receptor β agonists, Liquiritigenin and S-equol, inhibit breast cancer cell proliferation through the activation of tumor-suppressor and other pathways. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1312. doi:10.1158/1538-7445.AM2013-1312