Abstract 1989: Targeting Sin3-Pf1 complex: Novel site-specific epigenetic therapy for triple negative breast cancer

Abstract

Abstract Introduction: Dearth of clinically validated drug targets is a major challenge in Triple Negative Breast Cancer (TNBC) that limits the treatment options to chemotherapy with intense cytotoxic consequences. We previously identified PAH2 domain of Sin3 protein as potential therapeutic target in TNBC and shown that protein-protein interactions of Sin3 via its PAH-2 domain can be disrupted by decoys (Mad1-SID peptide/small molecule inhibitors) designed based on the Sin3 interaction domain (SID) of transcription factor Mad1. Chromatin regulator Pf1 shown to be overexpressed in breast cancer and interacting with PAH2 domain of Sin3 can also be dissociated from Sin3 complex by Mad1-SID. In this study we identify Pf1 to be a significant contributor to the oncogenic phenotype associated with TNBC. Methods: Pf1 knockdown lines were generated by stably transfecting MDA-MB-231 cells with Pf1-shRNA or scr-shRNA. Cells were cultured in 2D and assayed for cancer stem cell (CSC) markers and ALDH activity by flow cytometry. For colony morphogenesis cells were cultured in 3D Matrigel. qRT-PCR were performed to assay expression of Nanog, Sox2 and Oct4. Fluorescence anisotropy and co-immunoprecipitation assays were used to test Sin3-Mad1 and Sin3-Pf1 interaction in the presence or absence of Mad1-SID and/or Pf1-SID. Results: MDA-MB-231 cells transfected with Pf1-shRNA had over two fold reduced ability to form colonies in 3D Matrigel cultures, with ∼95% of colonies non-invasive in contrast to the invasive star-like colonies formed by cells transfected with Scr-shRNA. Additionally reduction in Pf1 level was accompanied by a ∼1.5-fold reduction (p<0.05) in the tumorsphere forming ability of MDA-MB-231 cells. Pf1 depletion also significantly reduced RNA and protein of Nanog, Oct4 and Sox2. Confocal imaging revealed reduced levels and nuclear accumulation of Nanog, Sox2 and Oct4 in cells transfected with Pf1-shRNA compared to Scr-shRNA. Pf1 knockdown resulted in a 2.5-fold decrease in ALDH1 positive cells (6.55% in Scr-shRNA vs 2.59% in Pf1-shRNA). The CD44low/CD24low/neg population was enriched 3 fold over cells transfected with Scr-shRNA. From a therapeutic perspective we have designed a linear peptide corresponding to the Sin3 interaction domain of Pf1 (Pf1-SID). By fluorescence anisotropy we show that in comparison to Mad1-SID (IC50 = 1.4±0.3 μM), several hundred times higher concentration of Pf1-peptide (IC50 = 826±162 μM) is required to dissociate Sin3-Mad1 interaction. Co-immunoprecipitation assays show that Pf1-SID can specifically dissociate Pf1 from Sin3 without affecting the binding of Mad1 with PAH2 domain of Sin3. Pf1-SID treated MDA-MB-231 cells also have reduced invasive capacity and CSC traits. Conclusion: PAH2 domain of Sin3 and its interaction with Pf1 is potential drug target and Pf-1SID-mediated disruption of Sin3-Pf1 complex as translation relevance for first site-specific epigenetic therapy for TNBC. Citation Format: Nidhi Bansal, Joanna Wexler, Yeon-jin Kwon, Elena C. Gil, Boris Leibovitch, Rajal Sharma, Arthur Zelent, Ming-Ming Zhou, Eduardo Farias, Samuel Waxman. Targeting Sin3-Pf1 complex: Novel site-specific epigenetic therapy for triple negative breast cancer. [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 1989. doi:10.1158/1538-7445.AM2015-1989