Abstract 1156: Epigenetic re-wiring of breast cancer by pharmacological targeting of C-terminal binding protein

The C-terminal binding protein (CtBP) is a family of dimeric nuclear proteins whose levels are increased in cancers of the colon, ovaries, prostate and breast. Elevated CtBP expression is associated with poor cancer survival and can also distinguish those node negative breast cancer patients who will show worse survival. This implicates CtBP as both a biomarker and a promising candidate for therapeutic intervention. As a dimer, CtBP provides a scaffold that couples multiple different DNA-binding transcriptional regulators with a variety of chromatin modifying protein complexes to alter the epigenetic landscape throughout the nucleus. These properties provide the rationale for pharmacological targeting of CtBP to change epigenetically regulated gene expression in cancer cells. In this study, we employ computer assisted drug design to screen for optimal quantitative structure-activity relationships (QSARs) between small molecules and CtBP to identify 24 potential CtBP inhibitors. Functional screening of these compounds identifies 4 lead compounds with low toxicity and high water solubility. Treatment of breast cancer cells at micro-molar concentrations of these small molecular inhibitors induces significant de-repression of epigenetically silenced pro-epithelial genes in the mesenchymal, triple negative breast cancer cell line, MDA-MB-231. This re-activation is associated with eviction of CtBP from the respective gene promoters, disrupted recruitment of CtBP-chromatin modifying protein complexes, increased deposition of activating epigenetic histone marks, and upregulation of both pro-epithelial gene mRNA and protein expression. In functional assays, CtBP inhibition by these small molecular inhibitors decreases cellular invasion, and improves DNA repair. FRET (Förster resonance energy transfer) analysis demonstrates that CTBP inhibition results in decreased FRET intensity, suggesting that CTBP dimerization is repressed by CTBP inhibition. In addition, pharmacological inhibition of CtBP combines with established epigenetically targeted drugs to synergistically decrease cell migration and potentiate the reactivation of silenced pro-epithelial gene expression in triple negative cancer cells. Finally, CTBP inhibition results in transcriptional repression of MDR1 expression and reduces the population of Doxorubicin resistant cells in the triple negative breast cancer cell lines. These findings implicate the possible use of this class of compounds in strategies fo