Abstract P3-03-11: Notch inhibition promotes triple negative breast cancer growth

Abstract

Abstract Notch1 and Transforming Growth Factor-beta (TGF-β) signaling pathways in cancer have a paradoxical roles as they were shown to have both tumor suppressive and tumor promoting activities. The interaction between these two signaling pathways in cancer progression was addressed in a systematic way. In these studies, we utilized primary human mammary epithelial cells (HMEC) and various triple negative breast cancer (TNBC) cells and examined the molecular cross-talk between TGF-β and Notch signaling pathways using biochemical, molecular and cell biology techniques. First, we noticed lower expression of Notch1 in most of the TNBC cells compared to normal cells. Second, most of these TNBC cells overexpressed TMEPAI (transmembrane prostate androgen induced) protein, which was shown to switch TGF-β from a tumor suppressor to a tumor promoter role in breast cancer. Surprisingly, treatment of TNBC cells with Notch inhibitors increased their growth rate both in the absence or presence of TGF-β. Moreover, Notch inhibition relieved the growth suppression by TGF-β in normal cells. Both normal and TNBC cells showed increased growth in the presence of Notch inhibitors with decreased TGF-β signaling activity. Normal cells that express very low levels of TMEPAI both at mRNA and protein levels, overexpressed TMEPAI protein in the presence of various Notch inhibitors, suggesting that Notch 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. Consistent with these observations, TMEPAI, a known binding partner of E3 ligases, causes increased turnover of Notch1 protein. In fact, over expression 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 exists between TGF-β and Notch1 signaling through TMEPAI. Therefore, our studies clearly indicate that use of Notch inhibitors is contraindicated in triple negative breast cancer patients. Citation Format: Pothana Saikumar, Prajjal K Singha, Manjeri A Venkatachalam. Notch inhibition promotes triple negative breast cancer growth [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-03-11.