IDENTIFICATION OF THE RNA BINDING PROTEIN RBM3 AS A NOVEL EFFECTOR OF β-CATENIN SIGNALING AND COLON CANCER STEM CELLS

Abstract

The intestinal epithelium is one of the fastest renewing tissues within the adult. This renewal is primarily driven by the intestinal epithelial stem cell compartment and homeostasis of this compartment needs to be strictly maintained. Loss of regulation can lead to hyperplasia and subsequent malignant transformation. Moreover, cancers are also thought to contain a stem cell population which maintains long term tumor viability and recurrence following therapy. Therefore, a thorough understanding of the molecular machinery that governs stem cell homeostasis can further our understanding of colon cancer initiation and progression. The RNA binding protein RBM3 is upregulated in many solid tumors including colon, prostate and breast. It also serves as a proto-oncogene inducing the malignant transformation of normal cells when overexpressed. To further characterize the mechanism, we overexpressed RBM3 in DLD-1 and HCT 116 colon cancer cell lines. We show that RBM3 overexpression is capable of increasing the cancer stem cell phenotype as measured by side population assay, spheroid formation and expression of the putative stem cell markers DCLK1, LGR5 and CD44. Interestingly, RBM3 also increases chemoresistance through increased multidrug efflux and quiescence, both characteristics commonly attributed to cancer stem cells. Moreover, RBM3 overexpression appears to increase activity of the β-catenin signaling cascade, a pathway critical for the maintenance of stem cell self-renewal and implicated in the pathogenesis of colon cancer. We also show that RBM3 overexpression decreases the activity of GSK3β, a member of the destruction complex known to suppress β-catenin levels within the cell. Consistent with this, pharmacologic inhibition of GSK3β similarly phenocopies RBM3 induced β-catenin transcriptional activity. Taken together, we conclude that RBM3 overexpression is capable of increasing the cancer stem cell population. We also show a novel role for RBM3 in increasing β-catenin signaling activity. Based on these data, we infer that the overexpression of RBM3 observed within solid tumors may be critical for maintaining self-renewal within the cancer stem cells and necessary for long term viability of tumor formation.