Regulation of EMT in Colorectal Cancer: A Culprit in Metastasis

Abstract

Epithelial to mesenchymal transition (EMT) is a process during which cells lose their epithelial characteristics, for instance cell polarity and cell-cell contact, and gain mesenchymal properties, such as increased motility. In colorectal cancer (CRC), EMT is associated with an invasive or metastatic phenotype. In this review, we discuss recent studies exploring novel regulation mechanisms of EMT in CRC, including the identification of new CRC EMT regulators. Upregulation of inducers can promote EMT, leading to increased invasiveness and metastasis in CRC. These inducers can downregulate E-cadherin and upregulate N-cadherin and vimentin (VIM) through modulating EMT-related signaling pathways, for instance WNT/β-catenin and TGF-β, and EMT transcription factors, such as zinc finger E-box binding homeobox 1 (ZEB1) and ZEB2. In addition, several microRNAs (miRNAs), including members of the miR-34 and miR-200 families, are found to target mRNAs of EMT-transcription factors, for example ZEB1, ZEB2, or SNAIL. Downregulation of these miRNAs is associated with distant metastasis and advanced stage tumors. Furthermore, the role of EMT in circulating tumor cells (CTCs) is also discussed. Mesenchymal markers on the surface of EMT CTCs were found to be associated with metastasis and could serve as potential biomarkers for metastasis. Altogether, these studies indicate that EMT is orchestrated by a complicated network, involving regulators of different signaling pathways. Further studies are required to understand the mechanisms underlying EMT in CRC.

Keywords: EMT; colorectal cancer; metastasis.

Figure 1

Crosstalk network of novel EMT inducers. Several inducers of EMT in CRC have been identified. These inducers were found to directly regulate expression of EMT-transcriptional factors or do so through other oncogenic signaling pathways. Myocyte enhancer factor 2D (MEF2D), hypoxia-inducible factor 1 alpha (HIF-1α), and transmembrane protease/serine 4 (TMPRSS4) can promote EMT by inducing expression of ZEB transcriptional factors, whereas nucleotide binding protein-like (NUBPL) can upregulate SNAIL. Other proteins such as serine–threonine kinase receptor-associated protein (STRAP) and TUCS3 can induce the WNT/β-Catenin signaling pathway, which has been found to be involved in EMT in CRC. Additionally, neuropilin-2 (NRP2), growth differentiation factor 15 (GDF15), and formin-like2 (FMNL2) can promote TGF-β signaling and induce EMT. Overexpression of eukaryotic initiation factor 5A2 (EIF5A2) can promote EMT through an unknown mechanism which may involve C-Myc and metastasis-associated protein 1 (MTA-1).