LIN28B fosters colon cancer migration, invasion and transformation through let-7-dependent and -independent mechanisms

Lin28b is an RNA-binding protein that inhibits biogenesis of let-7 microRNAs. LIN28B is overexpressed in diverse cancers, yet a specific role in the molecular pathogenesis of colon cancer has to be elucidated. We have determined that human colon tumors exhibit decreased levels of mature let-7 isoforms and increased expression of LIN28B. To determine LIN28B's mechanistic role in colon cancer, we expressed LIN28B in immortalized colonic epithelial cells and human colon cancer cell lines. We found that LIN28B promotes cell migration, invasion and transforms immortalized colonic epithelial cells. In addition, constitutive LIN28B expression increases expression of intestinal stem cell markers LGR5 and PROM1 in the presence of let-7 restoration. This may occur as a result of Lin28b protein binding LGR5 and PROM1 mRNA, suggesting that a subset of LIN28B functions is independent of its ability to repress let-7. Our findings establish a new role for LIN28B in human colon cancer pathogenesis, and suggest LIN28B post-transcriptionally regulates LGR5 and PROM1 through a let-7-independent mechanism.