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Abstract 4513: Notch inhibitors relieve growth suppression by TGF-β and promote growth and metastasis of triple-negative breast cancers

The paradoxical behavior of Notch1 and transforming growth factor-beta (TGF-β) is puzzling as they were shown to have both tumor-suppressive and tumor-promoting activities. We addressed the interaction between these two signaling pathways in cancer development and progression in a systematic way. Towards this goal, we utilized primary human mammary epithelial cells (HMEC) and various triple-negative breast cancer (TNBC) cells and examined the molecular crosstalk between TGF-β1 and Notch signaling pathways using biochemical, molecular and cell biology techniques. First, we observed lower expression of Notch1 in most of the TNBC cells compared to normal cells. Most of these TNBC cells also overexpressed TMEPAI (transmembrane prostate androgen induced), which we previously showed switches TGF-β from tumor-suppressive to a tumor-promoting role in breast cancer. Surprisingly, treatment of TNBC with Notch inhibitors increased their growth rate both in the absence and presence of TGF-β, while Notch inhibitors relieved normal cells from TGF-β mediated growth suppression. Both normal and TNBC cells showed increased growth in the presence of Notch inhibitors with decreased TGF-β signaling activity. In fact, normal cells that express very low levels of TMEPAI both at mRNA and protein levels, overexpressed TMEPAI in presence of various Notch inhibitors, suggesting that Notch1 negatively regulates TMEPAI expression. Correspondingly, knockdown of TMEPAI in TNBC cell lines showed increased expression of Notch1 and Jagged 1, suggesting a negative regulation of Notch1 by TMEPAI. In fact, TMEPAI, which binds to E3 ligases, promoted increased turnover of Notch1. In contrast to notch inhibitors, overexpression of active Notch1 in MDA-MB-231 cells inhibited tumor cell growth both in vitro and in vivo. Our results clearly suggest that a crosstalk between TGF-β and Notch1 signaling exists through TMEPAI. Importantly, these studies should be taken into consideration while contemplating the use of Notch inhibitors in triple-negative breast cancer therapy. Citation Format: Pothana Saikumar, Prajjal K. Singha, Srilakshmi Pandeswara, Manjeri A. Venkatachalam. Notch inhibitors relieve growth suppression by TGF-β and promote growth and metastasis of triple-negative breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4513.