Abstract 2042: TOB1 promotes cell survival in estrogen-independent estrogen receptor-positive breast cancer.

Abstract

Abstract Antiestrogens are the most widely administered endocrine agents for the treatment of estrogen receptor (ER)-expressing breast cancers, such as tamoxifen, fulvestrant and aromatase inhibitor. However, most initially responsive breast tumors acquire resistance and become estrogen-independent. To find potential therapeutic target(s) against drug resistance, we performed siRNA library screening studies to identify cell survival determinants in estrogen independent cell lines from 631 ER network related genes. LCC1 and LCC9 cells, derivatives of MCF-7, are estrogen-independent. LCC1 cells are sensitive to fulvestrant and tamoxifen. LCC9 cells are resistant to fulvestrant and cross-resistant to tamoxifen. A number of genes selectively support the survival of estrogen-independent tumor cells, including TOB1 (Transducer of ErbB2). TOB1 gene knockdown significantly inhibited (>50%) estrogen-independent cell growth in LCC1 and LCC9 cells, but had much less effect on the survival of MCF-7 cells (10%). These cell lines have similar levels of Tob1 protein. TOB1 knockdown did not cause apoptosis or autophagy. DNA replication was reduced and G1 to S phase transition was blocked by TOB1 knockdown in the estrogen-independent cell lines. Next, we studied cell-signaling pathways that regulate cell cycle transition. Cyclin D1 was upregulated and activated by TOB1 knockdown in estrogen-independent cells, but not in MCF-7 cells. Therefore, cyclin D1 might be involved in G1/S transition blockage induced by Tob1 inhibition. TOB1 knockdown also activated cell cycle checkpoint kinase 2 (Chk2) and induced DNA damage. Our data demonstrated that Tob1 promotes cell survival after ER+ breast cancer cells acquire estrogen-independence, which might be mediated by cyclin D1. Citation Format: Yong-Wei Zhang, Rochelle Nasto, Robert Clarke, Louis M. Weiner. TOB1 promotes cell survival in estrogen-independent estrogen receptor-positive breast cancer. [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 2042. doi:10.1158/1538-7445.AM2013-2042