Abstract LB-094: Systemic RSPO1 delivery leads to regression of intestinal adenomas via activation of the TGFβ/SMAD pathway
Authors: Lähde Marianne , Heino Sarika , Högström Jenny , Leppänen Veli-Matti , Kaijalainen Seppo , Ritvos Olli , Sansom Owen , Alitalo Kari 
Published: August 2019

Abstract
Colorectal cancers (CRCs) most commonly develop from adenomas harboring a mutant APC gene that leads to aberrant activation of the canonical Wnt/ß-catenin pathway. One of the mechanisms of adenoma progression involves the PROX1 transcription factor that promotes tumor dysplasia and progression in CRC. In the normal intestinal epithelial cells, ligands of the R-Spondin (RSPO) family are able to enhance the canonical Wnt pathway activity by binding to receptor proteins of the LGR family, such as LGR5. Tumorigenic RSPO gene fusions have recently been discovered in CRCs, but the role of RSPOs in cancer is incompletely known. We previously reported that TGFß induced apoptosis of APC-mutant organoids, including the LGR5+ stem cells, was mediated by the proapoptotic protein BIM, whereas wild-type intestinal crypts were markedly less sensitive to TGFß. The KRAS oncogene increased apoptosis resistance in the adenomas via activation of the Erk1/2 kinase pathway, which led to Bim down-regulation and increased resistance to TGFß. A subsequent study showed that RSPO/LGR5 can inhibit the in vitro growth of cultured human CRC cell line by activating the TGFß/SMAD signaling pathway.
Because we were interested in analyzing the effect of exogenous Wnt signals on PROX1 expression in intestinal tumorigenesis, we constructed AAV-vectors for systemic expression of a soluble RSPO1 protein in ApcMin/+ mice. We found that the RSPO1-Fc fusion protein suppresses the Wnt/ß-catenin signaling activity in intestinal adenomas and in adenoma-derived intestinal organoids ex vivo, but not in normal intestinal epithelial cells. In the Apc mutant cells, the RSPO1-Fc fusion protein activated the TGFß/SMAD signaling pathway to suppress several Wnt target genes and adenoma growth, which effect was rescued suppressed by the TGFß receptor kinase inhibitor SB-431542. Simultaneously, RSPO1-Fc induced proliferation of the normal intestinal stem cells, giving them a growth advantage over the mutant cells, which enabled the intestinal epithelium to eventually outgrow the adenoma cells. Prolonged systemic expression of AAV-RSPO1-Fc decreased significantly the number of the intestinal adenomas and improved the overall survival of ApcMin/+ mice. Thus RSPO1-Fc provides the normal intestinal epithelial cells a growth advantage when compared to the adenoma cells, which eventually leads to the extrusion of the adenomatous tissue. An attractive idea now is to exploit such differential response of normal vs. cancer cells in cancer therapy.
Citation Format: Marianne Lähde, Sarika Heino, Jenny Högström, Veli-Matti Leppänen, Seppo Kaijalainen, Olli Ritvos, Owen Sansom, Kari Alitalo. Systemic RSPO1 delivery leads to regression of intestinal adenomas via activation of the TGFβ/SMAD pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-094.