The Functional Role of LGR5 in Colon Cancer

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Abstract Colorectal cancer is the second leading cause of cancer mortality in the US, in part due to the lack of effective therapies for advanced disease. Thus, there is an urgent need to identify molecules/pathways involved in oncogenic transformation and progression for cancer treatment. LGR5 has been discovered as a proliferating adult stem cell marker in the small intestine. It has been reported that, after binding R-spondins, LGR5 forms a physical complex with the Wnt receptor Frizzled and its co-receptor LRP6 to amplify Wnt canonical signaling. In addition, LGR5 expression has been shown to be upregulated in colon cancer as compared to normal tissues. Therefore, LGR5 has been proposed to be a tumor promoter in the intestine and colon. However, our studies of patient samples indicate that although expression of LGR5 is higher in stage I/II adenocarcinomas than in normal crypts (* P < 0.05), its expression decreases in stage III/IV tumors as compared to stage I/II tumors (** P < 0.01). These results suggest that loss of LGR5 is associated with advanced stage cancer, which argues against a promoting role, especially at later stages. We showed that LGR5 inhibits clonogenecity and survival in colon cancer cells and that knockdown of LGR5 expression increases their metastatic potential in an orthotopic model. Mechanistically, LGR5 activates TGFβ signaling, which occurs even in the absence of TGFβ RII, and the presence of RII further enhances the activation by LGR5. In addition, LGR5 inhibits Wnt signaling in a Smad4-dependent manner. Our results point to a suppressive role of LGR5 in colon cancer progression/metastasis. In this proposal, we will determine the mechanism(s) by which LGR5 activates TGFβ signaling and inhibits Wnt activation. We will also determine the functional role of LGR5 in colon cancer development/progression using genetic mouse models, investigate whether restoration of LGR5 expression elicits metastasis regression and whether LGR5 functions through the activation of TGFβsignaling and/or inhibition of Wnt signaling in an orthotopic mouse model and in patient specimens. The completion of these studies will identify RSPO/LGR5 /RI as a novel TGFβ transduceome and a colon cancer metastasis suppressor and substantially advance our understanding of the molecular mechanisms of LGR5, an intestinal stem cell marker, in suppressing colon cancer progression and metastasis.