Abstract P2-06-21: Endogenous expression of ERβ variants contributes towards chemotherapy-resistance in the triple negative breast cancer cell line HCC-1806

Triple negative breast cancer (TNBC) still remains a challenge to treat in the clinic due to a lack of good targets for treatment. Although TNBC lacks expression of ERα, the expression of ERβ and its variants are detected quite frequently in this cancer type and can represent an avenue for treatment. We show that the variants of ERβ, namely ERβ1, ERβ2, ERβ4, and ERβ5, contributes to aggressiveness of the TNBC cell line HCC1806 by affecting E-Cadherin expression and chemotherapy sensitivity. We have previously shown that the HCC1806 cell line expresses all the ERβ variants. To investigate their function we used the recently developed dCas9-KRAB system with Guide RNA for both ERβ promoters 0N and 0K, using 3 guide RNA's for each promoter and we found that expression of panERβ decreased from a ct. of 28 to 33 indicating loss of total ERβ. Treatment of HCC1806 cells with chemotherapy in combination with an ERβ agonist, LY500307, is more effective in reducing cancer cell viability. Knockdown of total ERβ in HCC1806 cells using CRISPR-Cas9 technology, decreased the proliferation and increased chemo-sensitivity to docetaxel agent in cells with the 0K promoter knock down, while cells with the 0N promoter knock down were less affected. Furthermore, after ERβ knockdown, the cell morphology of HCC-1806 changed to a more epithelial phenotype and there was increase in the expression of the classical epithelial marker, E-cadherin. N-Cadherin was unchanged in the cells with the 0K promoter knock down, while E-Cadherin was slightly decreased in cells with the 0N promoter knock down. We also found that cells with the 0K promoter knock down migrated less than control cells in a migration assay, while cells with the 0N promoter knock down were less affected. These findings are in agreement with reports showing that the 0K promoter is responsible for expression of the variants ERβ2 and ERβ5, while the 0N promoter expresses more ERβ1. Furthermore, we have earlier shown that ERβ2 and ERβ5 increase aggressiveness of TNBC via several oncogenic signaling pathways. In conclusion, knockdown of ERβ variants in HCC-1806 indicated that the variants may be responsible for conferring oncogenic properties to TNBC such as EMT, cancer cell proliferation and chemo-resistance. Therefore, treating TNBC patients with combination therapy including ERβ agonists, may prove to be extremely beneficial for securing better treatment responses and future pre-clinical studies should design appropriate experiments to confirm these findings in vivo. Citation Format: Faria M, Gustafsson J-A, Strom A. Endogenous expression of ERβ variants contributes towards chemotherapy-resistance in the triple negative breast cancer cell line HCC-1806 [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-06-21.