Abstract P3-01-15: Targeting small G proteins in triple negative breast cancer

Abstract Breast Cancer (BC) is the most common cancer in women and second leading cause of cancer-associated mortality in women world-wide. Treatment for women with BC is complicated by the molecular heterogeneity of the disease which can be classified by expression of three key receptors: estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2). Those BCs expressing none of these receptors are known as triple negative (TNBC). Importantly, TNBC patients have the highest mortality among BC subtypes. This inequity is due in large part to the lack of targeted treatment options for TNBC patients. While women with ER-positive or HER2-positive BC benefit from therapeutic advances that target these receptors, treatment options for women with TNBC have changed little since the 1960’s with toxic chemotherapy as the primary systemic treatment option. Thus, there is an urgent need to identify new molecular targets and innovative treatment strategies to improve outcome for women with TNBC. RalA and RalB are small GTPases implicated in tumor proliferation, survival, and metastasis in a variety of cancers. However, little is known of their roles in breast cancer. Utilizing publicly available BC patient gene expression datasets, we identified RALA as a potential prognostic biomarker and therapeutic target in TNBC. RALA expression is significantly elevated in TNBC relative to normal mammary tissue or other BC subtypes. Furthermore, RALA expression is highly prognostic of overall and distant metastasis-free survival in the greater BC patient population and specifically in patients with TNBC. Importantly, RALA remains an independent prognostic factor in TNBC when other clinicopathological variables are considered. BC patient tissue microarrays revealed RalA immunohistochemical (IHC) staining is also prognostic of worse overall survival in the ER-negative and TNBC populations but not in BC patients with ER-positive disease. Surprisingly, expression of RALB, which is highly homologous to RALA and has also been implicated as pro-tumorigenic/pro-metastatic in a number of solid tumor types, is decreased in TNBC relative to normal mammary tissue and is not prognostic of TNBC outcome. These data suggest RalA may have a unique and important role in the pathogenesis of TNBC. To determine the necessity of RalA in TNBC growth and metastasis, RalA was depleted in MVT1 cells, a murine TNBC cell line. Knockdown of RalA inhibited MVT1 orthotopic primary tumor growth and metastasis in immunocompetent mice. Similarly, knockdown of RalA in the human TNBC cell line MDA-MB-231 decreased orthotopic primary tumor growth and growth of spontaneous or experimental lung metastases in NOD scid gamma mice. Conversely, stable knockdown of RalB in MDA-MB-231 cells did not impair their tumor growth or metastatic capability in vivo. Recently, BQU57, an experimental small molecule inhibitor of both Ral isoforms was reported to slow growth of lung cancer cell lines in vitro and in vivo. We report BQU57 inhibits RalA and RalB GTP-binding and anchorage independent growth of MDA-MB-231 cells in vitro.In vivo, BQU57 treatment of mice bearing palpable MDA-MB-231 tumors significantly decreased both primary orthotopic tumor growth and spontaneous lung metastasis. BQU57 also slowed the growth of a PDX model derived from a TNBC lung metastasis. Combined, these results demonstrate an important role for RalA in the pathogenesis of TNBC that is not redundant with RalB and warrant further investigation of RalA as a target for the precise treatment of advanced TNBC. Citation Format: Rachel E Schafer, Katie A. Thies, Reena Shakya, Sue Knoblaugh, Gina M Sizemore, Steven T Sizemore. Targeting small G proteins in triple negative breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-01-15.