Lin28B, LET-7, and the Molecular Pathogenesis of Colon Cancer
Lin28b is an RNA-binding protein that inhibits biogenesis of tumor-suppressive microRNAs of the let-7 family, and is involved in induction of pluripotency. Although LIN28B has been implicated in cancer, a specific role in colon tumorigenesis has not been elucidated. We have determined that colon tumors exhibit decreased levels of mature let-7 isoforms, and that constitutive let-7 expression inhibits migration and invasion of colon cancer cells in vitro. Importantly, down-regulation of let-7a and let-7b in colon tumors correlates with increased expression of LIN28B, suggesting tumor-promoting properties of this let-7 inhibitor. In order to determine the role of LIN28B in colon cancer, we constitutively expressed LIN28B in immortalized human colonic epithelial cells and colon cancer cell lines via retroviral transduction. We found that constitutive LIN28B expression promotes migration, invasion, and soft-agar colony formation in vitro, as well as differentiated and metastatic phenotypes in vivo. Additionally, constitutive LIN28B expression modulates levels of several mRNA transcripts including the established let-7 targets IGF2BP1 and HMGA2. The intestinal stem cell related genes LGR5 and PROM1, which are not predicted let-7 targets, are also up-regulated with constitutive LIN28B expression – an effect that is not ameliorated by co-expression of let-7, thereby suggesting possible let-7 independent functions of LIN28B . These findings are corroborated by correlation of high LIN28B expression in colon tumors with poor patient prognosis. In summary, this work demonstrates tumor-promoting properties of LIN28B in the colon, and suggests that functions of LIN28B may occur via let-7 independent mechanisms. Potentially, LIN28B may evolve as a novel diagnostic marker or therapeutic target in colon cancer.