Blanco 2013

Imaging mRNA decay dynamics in colorectal cancer

Fernando F. Blanco and Dan A. Dixon

Department of Cancer Biology, The University of Kansas Medical Center, Kansas City, KS 66160

Messenger RNA (mRNA) decay is a tightly regulated mechanism that serves as a fundamental means to control gene expression. In the intestinal epithelium, normal cell growth is controlled by rapid decay of growth-related mRNAs. This class of mRNAs is targeted for degradation through AU-rich element (ARE) RNA motifs present in the mRNA 3’ untranslated region (3’UTR). A unique aspect of this post-transcriptional regulation is that it can be visualized through the presence of cytoplasmic processing (P-) bodies that contain various components of mRNA degradation machinery. Here, we demonstrate in non-transformed intestinal epithelial cells that the mRNA decay factor tristetraprolin (TTP) binds and delivers ARE-containing mRNAs to P-bodies where the mRNA is degraded. Co-localization studies and 3D reconstruction analysis revealed an association between TTP and various P-body components such as the mRNA decapping protein Dcp1a upon TTP-driven delivery of ARE-mRNAs to P-bodies. However, P-body formation was impaired in colorectal cancer cells and oncogenic Ras-transformed intestinal epithelial cells. More importantly, P-body levels were significantly reduced in colorectal adenomas and adenocarcinomas compared to normal colonic epithelium. Consistent with loss of P-body formation, transformed cells displayed concurrent loss of TTP expression, and expression profiling revealed TTP to be lost in >75% of colorectal adenocarcinomas. The functional consequence of TTP loss is observed with a 3-to 4- fold enrichment in ARE-containing gene expression in colorectal adenocarcinomas. In agreement with these observations, colonic epithelium from TTP-deficient mice show diminished P-body numbers and elevated pro-inflammatory gene expression.  Furthermore, restoring TTP expression in cancer cells resulted in rescue of P-body assembly accompanied by TTP-dependent growth-inhibition. These findings implicate TTP as a central physiological driver of ARE-mediated mRNA decay by virtue of its ability to sequester ARE-mRNAs and nucleate cytoplasmic P-bodies, and loss of this mechanism contributes to colorectal cancer progression.