Abstract 2962: Loss of the E3 ubiquitin ligase HACE1 results in enhanced Rac1 signaling contributing to breast cancer progression

Abstract

Abstract The transition from ductal carcinoma in situ (DCIS) to invasive disease is a crucial step in breast cancer progression. The specific alterations that govern this transition have not been elucidated. HER2/neu overexpression is frequently seen (50-60%) at the DCIS stage but is significantly less common in invasive breast cancer (20-30%) suggesting that additional alterations cooperate with HER2/neu to fully transform mammary epithelial cells. We identified novel genes that are capable of cooperating with HER2/neu allowing full malignant transformation, we used a transgenic mouse model that expresses wild-type neu and performed an insertional mutagenesis screen to identify genes whose alterations induce anchorage-independent growth on primary mouse mammary epithelial cells, an in vitro tumor formation assay. We identified the HECT and ankyrin domain containing E3 ligase 1 (HACE1) as a breast cancer tumor suppressor gene whose loss contributes to the transformation process. Loss of HACE1 expression is commonly seen in breast cancer patients and established breast cancer cell lines supporting the role of HACE1 as a breast cancer tumor suppressor gene. HACE1 ubiquitylates the activated GTP-bound form of the Rho family GTPase Rac1, resulting in ubiquitin mediated degradation of Rac1 via the 26S proteasome. The loss of HACE1 results in the accumulation of activated Rac1 leading to enhanced clonogenicity, migration, and invasion of human mammary epithelial cells (HMECs). In addition, the overexpression of HER2 results in activation of Rac1 levels which is further enhanced upon HACE1 loss in HMECs that further enhance colony formation in soft agar, migration, invasion and allows tumor formation in mice. Importantly, knocking down Rac1 using RNA interference or targeting RAC1 using the small molecule inhibitor EHT 1864 can rescue the effects of HACE1 loss mediated transformation. The results establish HACE1 as a breast cancer tumor suppressor gene by attenuating Rac1 signaling. In addition, our work further supports the role of Rac1 as a critical signaling node in breast cancer and that loss of HACE1 leads to enhanced Rac1 signaling that may contribute to breast cancer progression. These results suggest that HACE1 loss may identify precancerous lesions capable of becoming malignant as well as identify cancers susceptible to RAC targeted therapies. Citation Format: Erik T. Goka, Marc E. Lippman. Loss of the E3 ubiquitin ligase HACE1 results in enhanced Rac1 signaling contributing to breast cancer progression. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2962. doi:10.1158/1538-7445.AM2014-2962