Abstract 1143: Prox1 and Notch mark distinct colorectal cancer stem cell populations

Colorectal cancer (CRC) is one of the leading causes of cancer-related morbidity and mortality. Signaling pathways, such as the Wnt and Notch pathways, are essential for the maintenance and differentiation of wild-type intestinal stem cells. We have shown that the Prox1 transcription promotes expansion of the colorectal cancer stem cell pool and that a subpopulation of the Prox1+ CRC cells display stem cell activity. Because of these results we have further analyzed the role of Prox1 in the regulation of CRC stem cell. Here, we report that Prox1 regulates CRC stem-like cells via bidirectional interaction with Notch1 during CRC initiation and progression. Using genetic in vivo models and ex vivo 3D organoid cultures, we show that Notch inhibition decreases the number of Lgr5+ stem cells whereas it increases expression of the Prox1. Although Notch inhibition led to increased proliferation of the Prox1 positive cells, it did not affect their ability to give rise to differentiated Prox1 negative progeny. Ectopic overexpression of the active fragment of Notch1 suppressed Prox1 expression and inhibited stem cell activity in the CRC cells. On the other hand, the PROX1-NuRD complex suppressed the Notch signaling pathway and Prox1 deletion increased Notch target gene expression and promoter activity, indicating reciprocal regulation between Prox1 and Notch1. Thus, although Prox1 and Notch suppress each other in colorectal cancer cells, Prox1+ cells can function as a stem cell population without the need for Notch pathway activity. Citation Format: Jenny Högström, Sarika Heino, Pauliina Kallio, Marianne Lähde, Veli-Matti Leppänen, Seppo Kaijalainen, Diego Balboa, Timo Otonkoski, Sylvie Robine, Zoltan Wiener, Kari Alitalo. Prox1 and Notch mark distinct colorectal cancer stem cell populations [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 1143.