Abstract A67: Impact of estrogen on the response of ovarian tumor cells to cisplatin.

Introduction: Estrogen is a risk factor for the development of ovarian cancer. The estrogen receptor is found in 60% of epithelial ovarian cancers. However, its function remains unknown. After debulking surgery, patients are in an estrogen-free state. After initial chemotherapy, many will recur and eventually develop chemoresistant disease. Previous clinical trials indicate that estrogen replacement therapy after debulking surgery does not have a negative effect on progression-free or overall survival. We sought to determine the effects of estrogen on ovarian cancer cells during and after treatment with chemotherapy. We hypothesize that menopausal estrogen therapy after ovarian cancer debulking is not harmful and in platinum-resistant disease, may increase tumor sensitivity to chemotherapy. Methods: PEO1_Zs is known ERα positive and 2008_Zs is a known ERα negative cell line. To study chemoresistance, we developed cisplatin resistant cell lines using PEO1_Zs, and 2008_Zs as parent cell lines, the resistant cell lines are named PEO1_Zs_Resistant and 2008_Zs_Resistant. Cell counts were done with the above cell lines. Cells were plated in RPMI 1640 phenol red-free media with 10% charcoal stripped fetal bovine serum and treated with various concentrations of 17-β-estradiol. Control wells were treated with absolute ethanol (1%, vehicle). Cells were cultured in the presence of estrogen for 72hrs then treated with cisplatin for 72h. Viability was assessed with trypan blue exclusion and cells were counted. All treatment groups were done in triplicate wells and repeated three times. Student9s t test was used for the difference between control and estrogen treated groups. Apoptosis was measured by cleaved caspase-3 activity. PEO1_Zs_Resistant cells and sensitive cells were treated with 100 nM 17-β-estradiol or vehicle for 72 h. Cells were then additionally treated for 72 h with and without 6μM cisplatin. Cells were fixed and immunofluorescence done for cleaved caspase-3. Caspase-3 activity was quantified as mean percent of cell population positive for caspase 3 activity ± SEM. Student9s t test was used for the difference between control and estrogen treated groups. Results: No difference was found among treatment groups of cisplatin when PEO1_Zs (ERα positive) cells were treated with or without 100 nM estrogen. When PEO1_Zs_Resistant cell line was exposed to 100 nM estrogen and treated with 6 μM cisplatin, we observed a significant reduction in cell counts as compared to the non-estrogen treated PEO1_Zs_Resistant line (p value We looked at caspase-3 activity to determine if apoptosis was involved in reducing the number of tumor cells treated with estrogen-cisplatin. The caspase-3 activity was similar in all treatment groups of PEO1_Zs cells. The platinum-resistant cell line, PEO1_Zs_Resistant, showed 90% apoptotic activity as compared to 30% in cisplatin only (p value Conclusions: These results support clinical observations that estrogen is not harmful in treating menopausal symptoms in ovarian cancer. Furthermore, our in vitro studies suggest it is not harmful during ovarian cancer platinum therapy and may reverse platinum-resistance through inducing apoptosis. Our observations may lead to novel ovarian cancer chemotherapy regimens and therapeutic strategies for treating and alleviating menopausal symptoms during ovarian cancer. Citation Format: Georgina Cheng, Carly Galimanis, Heidi Baumgartner Wilson, Kian Behbakht. Impact of estrogen on the response of ovarian tumor cells to cisplatin. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr A67.