Abstract 2320: Prox1 marks a stem cell population that promotes tumor progression in intestinal adenomas

Colorectal cancer (CRC) initiation and growth is often attributed to stem cells, yet little is known about the regulation of this cell population. We have shown that a subpopulation of cells expressing the Prox1 transcription factor have stem cell activity in intestinal adenomas, but not in the normal intestine. Using in vivo models and 3D ex vivo organoid cultures of mouse adenomas and human CRC, we found that Prox1 deletion reduced the number of stem cells and cell proliferation and decreased intestinal tumor growth in multiple CRC model systems. Loss of Prox1 also decreased autophagy and the survival of hypoxic tumor cells in tumor transplants. Thus, Prox1 is essential for the expansion of the stem cell pool in intestinal adenomas and CRC without being critical for the normal functions of the gut. To further study the adenoma stem cells in relation to the Prox1-positive cell population, we analyzed Apc-mutant intestinal organoids derived from Prox1-CreER; tdTomatoflox/Stop/flox; Apcmin/+ mice. In this lineage tracing model, the induction of Cre activity results in the expression of the red fluorescent protein tdTomato only in the Prox1-positive adenoma cells. FACS sorted tdTomato-positive cell population displayed a highly elevated capability to form new organoids compared to the tdTomato-negative cells, consistent with stem cells enrichment in the Prox1-positive cell population. Gene expression analysis provided new clues about the identity of the Prox1 positive cells.

Citation Format: Zoltan Wiener, Jenny Högström, Ville Hyvönen, Pauliina Kallio, Sarika Heino, Kari Alitalo. Prox1 marks a stem cell population that promotes tumor progression in intestinal adenomas. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2320. doi:10.1158/1538-7445.AM2015-2320

Abstract 1908: The Wnt-target Prox1 promotes colorectal cancer stem cell survival to fuel tumor growth

Colorectal cancer (CRC) is one of the leading causes of cancer mortality in the Western countries. In most of the CRC patients, an initial mutation occurs in the APC or CTNNB1 genes, leading to the ligand-independent activation of the canonical Wnt pathway. Besides having a central role in the development of CRC, the Wnt pathway plays a critical role also in the maintenance of the normal intestine. Attempts at therapeutic inhibition of this pathway could thus lead to serious side effects in CRC patients. We have previously shown that the Prox1 transcription factor is induced in the intestinal epithelium by mutations activating the Wnt pathway and it critically contributes to CRC progression via an unknown mechanism. Here we provide evidence that Prox1 expression is induced in the Lgr5+ adenoma stem cells early after Apc deletion. Our in vivo models and ex vivo organoid experiments suggest that Prox1 silencing or deletion restricts the expansion of the Lgr5+ adenoma stem cell population both in humans and in mice. Interestingly, silencing the phospholipid binding protein Annexin A1 (Anxa1), a gene suppressed by Prox1, is responsible for several of the effects of Prox1 on adenoma cells, such as the re-organization of the actin cytoskeleton, enhanced stem cell activity and tumor growth. Furthermore, Prox1 deletion abnormally increases the mTORC1 pathway activity, which results in decreased survival of the adenoma stem cells. Since Prox1 is expressed at low level only in rare neuroendocrine cells and in some Paneth cells in the wild type intestinal epithelium, furthermore, its genetic deletion in the adult gut does not lead to obvious phenotypes, Prox1 may serve as an attractive therapeutic target for restricting the progression of early intestinal adenomas.

Citation Format: Zoltan Wiener, Ville Hyvönen, Jenny Högström, Tanja Holopainen, Arja Band, Pauliina Kallio, Olli Dufva, Caj Haglund, Olli Kruuna, Guillermo Oliver, Yinon Ben-Neriah, Kari Alitalo. The Wnt-target Prox1 promotes colorectal cancer stem cell survival to fuel tumor growth. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1908. doi:10.1158/1538-7445.AM2014-1908

Abstract B49: Prox1 Transcription factor promotes colorectal cancer growth by expanding the Lgr5-positive stem cell pool

The Wnt pathway that is central in the development of colorectal cancer (CRC) plays a critical role also in the maintenance of the normal gut. Inhibition of this pathway could thus lead to serious side-effects in CRC patients. We have previously shown that the Prox1 transcription factor is expressed only in a few cells of normal intestinal crypts, yet it contributes to CRC progression via an unknown mechanism. In this study, we show that Prox1 deletion in the WT intestinal epithelium has no obvious phenotypic consequence and that Prox1 regulates the number of Lgr5+ CRC stem cells via establishing a proper niche for them. Upon Apc deletion, Prox1 expression is rapidly induced in both the Lgr5+ CRC stem cells and Lgr5- non-stem. Prox1 deletion in 3D organoid cultures derived from mouse intestinal adenomas or human CRC cells leads to a decrease in lumen formation and crypt-like "outpocketings” that are enriched in Lgr5+ CRC stem cells. This results in a decreased number of Lgr5+ stem cells and a shift from the CRC stem cell-like phenotype to a progenitor-like phenotype, which consequently leads to decreased tumor cell growth. Interestingly, we show that silencing of the Prox1-suppressed Wnt-target Annexin A1 (Anxa1) mimics the effects of Prox1, including the establishment of a niche for Lgr5+ CRC stem cells. Anxa1 silencing in human and mouse CRC organoids results in an enhanced formation of outpocketings, an elevated number of Lgr5+ CRC stem cells and increased tumor growth in xenotransplantation assays. We propose a model where the deletion of the Apc gene induces both Anxa1 and Prox1 expression in the intestinal epithelium. Prox1 silencing elevates the expression of Anxa1, which inhibits the formation of crypt-like outpocketings and the Lgr5+ CRC stem cell niche, consequently leading to reduced tumor growth. Based on this study Prox1 is the first molecule which regulates the number of CRC stem cells without affecting the homeostasis of the adult WT intestine and it provides an attractive therapeutic target for drug development in CRC.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B49.

Citation Format: Zoltan Wiener, Jenny Högström, Ville Hyvönen, Tanja Holopainen, Arja Band, Pauliina Kallio, Olli Dufva, Caj Haglund, Olli Kruuna, Yinon Ben-Neriah. Prox1 Transcription factor promotes colorectal cancer growth by expanding the Lgr5-positive stem cell pool. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B49.