Abstract B46: Rac1 inhibition as a therapeutic strategy in breast cancer

Abstract

Abstract Over the past few decades, targeted therapies have accounted for most of the therapeutic advances in breast cancer. The purpose of this approach is to discover new, actionable therapeutic strategies for breast cancer. We previously used a forward genomic screening approach to identify novel genes capable of cooperating with HER2, and found that loss of HACE1 was prevalent in breast cancer, identifying it as a tumor suppressor gene. HACE1 had significantly lower expression in breast cancer patients compared to normals. HACE1 is an E3 ubiquitin ligase that ubiquitinates Rac1, marking it for degradation. Rac1, a Rho GTPase, is a central component of key signaling pathways in breast cancer. While we identified HACE1 in a HER2 cooperating screen, the observation that HACE1 levels were consistently low across different subtypes of breast cancer led us to suggest that Rac1 is commonly hyperactivated in breast cancer and therefore has potential as a therapeutic target. We tested a Rac1 inhibitor, EHT 1864, on 8 different breast cancer cell lines, starting with an initial soft agar assay screen for sensitivity versus resistance and found that 4 cells lines were sensitive to the Rac1 inhibitor in soft agar cloning assays (MCF7, T47D, BT549, and BT474), while 4 other cell lines were resistant (MDA231, MDA436, MDA468, HS578T). These results were confirmed in 2D proliferation assays. The majority of the cell lines that were resistant to Rac1 inhibition overexpressed EGFR, suggesting that overexpression of EGFR confers resistance to Rac1 inhibition via the MAP Kinase pathway. Isogenic cell lines (MCF7) that express multiple activating proteins of the MAPK signaling pathway including EGFR, HER2, and MEK were treated with EHT1864 to determine if MAPK signaling was capable of overcoming Rac1 inhibition. Interestingly, the expression of EGFR conferred resistance to the Rac1 inhibitor. We further showed that molecular knockdown of EGFR in HS578T cells (EGFR-overexpressing, Rac1 inhibitor-resistant cells) resensitizes this cell line to Rac1 inhibition. Pharmalogical inhibition of EGFR using Gefitinib, an EGFR tyrosine kinase inhibitor, demonstrated the same resensitization to the Rac1 inhibitor. Our results suggest that Rac1 inhibition may be a therapeutic strategy in the treatment of breast cancer, as a single agent alone or in the case of EGFR overexpressing (triple-negative) breast cancers, in combination with EGFR targeted therapies. Citation Format: Dana Senderoff, Georges Azzi, Erik Goka, Marc Lippman. Rac1 inhibition as a therapeutic strategy in breast cancer. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr B46.