Abstract 499: β-catenin is required for triple-negative breast tumorigenesis.

Triple-negative breast cancer is associated with a poor prognosis and high frequency of recurrence, but because the molecular drivers of these tumors are not well understood, they lack any targeted therapies. Our previous data illustrated that activation of the canonical Wnt signaling pathway, as assessed by nuclear and cytosolic accumulation of β-catenin, was enriched in triple-negative breast cancer and associated with reduced overall survival in all patients. To investigate whether Wnt signaling may be a promising therapeutic target for triple-negative breast cancer, we silenced β-catenin expression stably in MDA-MB-231 and HCC38 triple-negative breast cancer cells using lentiviral delivery of β-catenin-specific small hairpin RNAs (shRNAs). Upon implantation of the cells in the mammary fat pad of immunocompromised mice, we found that β-catenin shRNA HCC38 cells formed markedly smaller tumors than parental or scrambled shRNA control cells, and the tumors had slower growth rates. In vitro, β-catenin silencing significantly reduced the percentage of Aldefluor-positive cells, a read-out of the stem-like cell population, as well as the expression of stem cell-related target genes including Bmi-1 and c-Myc. β-catenin-knockdown cells were also significantly impaired in their ability to migrate in wound-filling assays and form anchorage-independent colonies in soft agar. To address the contribution of the signaling pool of β-catenin to these phenotypes, the parental cells were treated with a Wnt pathway inhibitor, ICG-001, that blocks the interaction of β-catenin with the CBP transcriptional co-activator. In contrast to β-catenin depletion, ICG-001 treatment did not inhibit tumor cell migration but did significantly suppress proliferation, suggesting that at least some of the oncogenic functions of β-catenin may not be due to its transcriptional activity. We next addressed the influence of Wnt signaling on the therapeutic response of triple-negative breast cancer cells and found, surprisingly, that β-catenin-knockdown cells were more resistant to several therapeutic agents, including cisplatin, doxorubicin and paclitaxel. ICG-001 treatment also conferred resistance of HCC38 cells to cisplatin and doxorubicin, supporting the involvement of β-catenin-mediated transcription in this effect. Collectively, these data suggest that β-catenin is required for triple-negative breast cancer development by controlling numerous tumor-associated properties, such as migration, stemness and anchorage-independent growth, by mechanisms that may extend beyond β-catenin-mediated transcription control. These findings also highlight the need to more completely dissect pathways implicated in driving triple-negative breast cancer not only so that they can be exploited as therapeutic targets, but also so their roles in modulating chemotherapeutic responses can be appreciated. Citation Format: Jinhua Xu, Jenifer R. Prosperi, Noura Choudhury, Kathleen H. Goss. β-catenin is required for triple-negative breast tumorigenesis. [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 499. doi:10.1158/1538-7445.AM2013-499