Background: Aberrant regulation of the Wnt signaling pathway has been shown to be associated with the development of aggressive types of breast cancer. We hypothesize that increased co-activator interaction of CBP with nuclear β-catenin is associated with epithelial to mesenchymal transition (EMT) in breast cancer with stem-cell (basal) phenotype, disease progression, drug resistance, relapse and metastasis. We further hypothesize that pharmacologic disruption of this complex leads to forced differentiation of cancer stem cells and re-sensitization to conventional therapies. Material/Methods: We have previously developed the small molecule ICG-001 that specifically disrupts the CBP/β-catenin interaction. Several breast cancer cell lines (MDA-MB-231,-468,-435) were treated with ICG-001 and investigated for changes in proliferation, invasive potential and changes in sensitivity to chemotherapy. RNA sequencing and qRT-PCR were used to evaluate gene-expression and alternative splicing events. CoIP and ChIP were used to demonstrate CBP/β-catenin disruption and differential co-activator usage at specific gene promoters. Protein expression levels were assessed by western blot and FACS analysis. Viability assays were performed to determine drug-sensitization. Several stem cell assays were used to study the effect of ICG-001 on cancer stem cell populations. In a human primary breast cancer xenograft model in NSG mice we investigated the in vivo effect of ICG-001. Results: CoIP demonstrated global disruption of the CBP/β-catenin interaction under treatment with ICG-001. Significant down-regulation of EMT related genes (e.g. S100A4, vimentin, CDH2) and up-regulation of epithelial genes (e.g. CD24), in basal-like breast cancer, resulting in a reversal of the stem cell phenotype (CD44+/CD24- to CD24+). ChIP experiments showed re-expression of CD24 being dependent on the usage of the co-activator p300. Re-expression of epithelial splice factors and alternative gene splicing were observed. Cancer stem cell assays revealed that ICG-001 eradicates cancer stem cell-like populations and causes down-regulation of stemness related genes (e.g. nestin, PROCR). An in-vitro invasion assay demonstrated loss of invasive potential. Treatment with ICG-001 resulted in the down-regulation of cell cycle related genes (e.g. Aurora kinases, Cyclins, CDK's) and anti-apoptotic genes (e.g. survivin). Marked inhibition of cancer cell proliferation was observed in vitro. Treatment with ICG-001 re-sensitized hormone receptor-negative cell lines to tamoxifen. In-vivo studies using primary human xenografts in a NSG mouse model suggested similar effects in-vivo. Conclusion: Antagonizing CBP/β-catenin signaling using a small molecule inhibitor should be an attractive strategy for the treatment of drug-resistant aggressive stem-like breast cancer and the eradication of breast cancer stem cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 323. doi:1538-7445.AM2012-323