Differential Regulation of <i>LET-7</i> by LIN28B Isoform–Specific Functions

The RNA-binding protein LIN28B plays an important role in development, stem cell biology, and tumorigenesis. LIN28B has two isoforms: the LIN28B-long and -short isoforms. Although studies have revealed the functions of the LIN28B-long isoform in tumorigenesis, the role of the LIN28B-short isoform remains unclear and represents a major gap in the field. The LIN28B-long and -short isoforms are expressed in a subset of human colorectal cancers and adjacent normal colonic mucosa, respectively. To elucidate the functional and mechanistic aspects of these isoforms, colorectal cancer cells (Caco-2 and LoVo) were generated to either express no LIN28B or the -short or -long isoform. Interestingly, the long isoform suppressed <i>LET-7</i> expression and activated canonical RAS/ERK signaling, whereas the short isoform did not. The LIN28B-long isoform-expressing cells demonstrated increased drug resistance to 5-fluorouracil and cisplatin through the upregulation of ERCC1, a DNA repair gene, in a <i>LET-7</i>-dependent manner. The LIN28B-short isoform preserved its ability to bind pre-<i>let-7</i>, without inhibiting the maturation of <i>LET-7</i>, and competed with the LIN28B-long isoform for binding to pre-<i>let-7</i> Coexpression of the short isoform in the LIN28B-long isoform-expressing cells rescued the phenotypes induced by the LIN28B-long isoform.<b>Implications:</b> This study demonstrates the differential antagonistic functions of the LIN28B-short isoform against the LIN28B-long isoform through an inability to degrade <i>LET-7</i>, which leads to the novel premise that the short isoform may serve to counterbalance the long isoform during normal colonic epithelial homeostasis, but its downregulation during colonic carcinogenesis may reveal the protumorigenic effects of the long isoform. <i>Mol Cancer Res; 16(3); 403-16. ©2018 AACR</i>.