Abstract 2449: Targeting an “undruggable” RNA-binding protein: Discovery of small molecule inhibitors of HuR for novel breast cancer therapy
Authors: Wu Xiaoqing , Lan Lan , Smith Amber , Marquez Rebecca , Wilson David , Rogers Steven , Gao Philip , Lovell Scott , Karanicolas John , Dixon Dan , Aubé Jeffrey , Xu Liang 
Published: August 2015

Abstract
Post-transcriptional gene regulation is essential for normal development, but when dysregulated, has many implications in disease conditions, including cancer. The RNA-binding proteins (RBPs) are critical trans factors that associate with specific cis elements present in mRNAs, thereby regulating the fate of target mRNAs. Due to the lack of well-defined binding pocket, RBPs have been considered “undruggable targets”. The RBP Hu antigen R (HuR) is overexpressed in many types of cancer, including breast cancer. Elevated cytoplasmic HuR level correlates with high-grade malignancy and serves as a prognostic factor of poor clinical outcome in breast cancer. HuR promotes tumorigenesis by interacting with cancer-associated mRNAs, those mRNAs encode proteins that are implicated in different tumor processes including cell proliferation, cell survival, angiogenesis, invasion, and metastasis. HuR also modulates the sensitivity of breast cancer cells to chemotherapy. Our hypothesis is that small molecule compounds that disrupt the HuR-mRNA interaction will block HuR function, leading to the decay and reduced translation of mRNAs of the target genes critical for breast cancer cell growth and progression. High throughput screening (HTS) was carried out in several chemical libraries (∼23,000 compounds) using fluorescence polarization (FP) assay and identified a series of initial hits with sub-micromolar inhibitory constants (Ki). Those potential disruptors were then validated by ALPHA assay (Amplified Luminescent Proximity Homogeneous Assay), confirmed by Surface Plasmon Resonance (SPR). In cell-based assays, top hit ST-3 and its optimized analogs, specifically shortened HuR target mRNAs’ half-life and decreased the level of the encoded proteins (Bcl-2, XIAP and Msi1/2). Moreover, those compounds selectively inhibited breast cancer cell proliferation but not normal cells. Knocking down HuR in breast cancer cells attenuated the activity of those HuR-mRNA disruptors. More cell-based assays are carrying out to delineate the mechanism of action. We are also testing the antitumor efficacy of those HuR inhibitors in mouse xenograft models. In conclusion, we identified potential small molecule disrupters of HuR-mRNA interaction for potential novel cancer therapy that inhibited breast cancer with HuR overexpression.
Citation Format: Xiaoqing Wu, Lan Lan, Amber Smith, Rebecca Marquez, David Wilson, Steven Rogers, Philip Gao, Scott Lovell, John Karanicolas, Dan Dixon, Jeffrey Aubé, Liang Xu. Targeting an “undruggable” RNA-binding protein: Discovery of small molecule inhibitors of HuR for novel breast cancer therapy. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2449. doi:10.1158/1538-7445.AM2015-2449