Modulation of E-cadherin function and dysfunction by N-glycosylation

Abstract

Several mechanisms have been proposed to explain the E-cadherin dysfunction in cancer, including genetic and epigenetic alterations. Nevertheless, a significant number of human carcinomas have been seen that show E-cadherin dysfunction that cannot be explained at the genetic/epigenetic level. A substantial body of evidence has appeared recently that supports the view that other mechanisms operating at the post-translational level may also affect E-cadherin function. The present review addresses molecular aspects related to E-cadherin N-glycosylation and evidence is presented showing that the modification of N-linked glycans on E-cadherin can affect the adhesive function of this adhesion molecule. The role of glycosyltransferases involved in the remodeling of N-glycans on E-cadherin, including N-acetylglucosaminyltransferase III (GnT-III), N-acetylglucosaminyltransferase V (GnT-V), and the α1,6 fucosyltransferase (FUT8) enzyme, is also discussed. Finally, this review discusses an alternative functional regulatory mechanism for E-cadherin operating at the post-translational level, N-glycosylation, that may underlie the E-cadherin dysfunction in some carcinomas.

Fig. 1

Schematic representation of the E-cadherin–catenin complex. The E-cadherin–catenin complex is proposed to interact with F-actin via α-catenin association with actin-binding proteins such as EPLIN [5]. β-Catenin and γ-catenin bind to E-cadherin in a mutually exclusive manner

Fig. 2

E-cadherin post-translational modifications. a The extracellular domain (EC) of human E-cadherin contains four potential N-glycosylation sites, which are located in EC4 and EC5. The phosphorylation of E-cadherin by casein kinase II (CKII) can occur in a short stretch of 30 aa in the cytoplasmic domain (CD), which contains a cluster of 8 Ser residues. Cytoplasmic O-glycosylation (O-GlcNAc addition in Thr/Ser residues) has been reported to regulate E-cadherin. These mechanims can modulate E-cadherin mediated cell–cell adhesion at a post-translational level. b Three-dimensional structure of the extracellular domain (EC1–EC5) of E-cadherin. The crystal structure of human EC1 was used in this representation and EC2–EC5 were modeled based on the crystal structure of C-cadherin. Four N-glycans were modeled with GlyProt (http://www.glycosciences.de/glyprot/), as shown in red

Fig. 3

Biosynthesis of N-linked glycans. Representation of a N-glycan structure with the reactions catalyzed by GnT-III, GnT-V, and FUT8

Fig. 4

Regulatory mechanism of E-cadherin-mediated cell–cell adhesion and GnT-III/GnT-V. GnT-III activity is associated with an increase in bisecting GlcNAc structures in E-cadherin, leading to a concomitant decrease in β1,6 branched structures, due to competition with GnT-V glycosyltransferase. The addition of bisecting GlcNAc residues to E-cadherin down-regulates the tyrosine phosphorylation of β-catenin and thus enhances cell–cell binding to suppress metastasis (lower figure). Conversely, in metastatic cancer cells (upper figure), the addition of β1,6 branched structures by GnT-V to E-cadherin is associated with increased tyrosine phosphorylation of β-catenin through the EGFR and Src signaling pathways, and therefore reduces E-cadherin-mediated cell–cell adhesion thereby contributing to the promotion of cancer metastasis

Fig. 5

The role of N-glycan structures in the carcinogenic process. In normal cells, the GnT-III and GnT-V enzymes are normally underexpressed. The overexpression of GnT-III is associated with increased synthesis of bisecting GlcNAc structures in some important target glycoproteins involved in cell adhesion such as E-cadherin and integrins, the modification of which by bisecting N-glycans is associated with the suppression of metastasis through enhancement of E-cadherin-mediated cell–cell adhesion and a decrease in integrin-mediated cell-extracellular matrix adhesion. Furthermore, GnT-III up-regulation precludes the availability of the substrate for the GnT-V enzyme, which is no longer able to synthesize branched structures. In a metastatic cancer situation, activation of the ras-raf-ets signaling pathway regulates the transcription of the GnT-V gene and the resulting increase in GnT-V leads to increased enzymatic production of β1,6 branched structures that modify glycoproteins involved in the carcinogenic process, including Matriptase; TIMP-1 (Tissue Inhibitor of Metalloproteinase-1, in which β1,6 branching correlates with the invasive and metastatic potential of cancer cells) as well as integrins and E-cadherin, the modification of which contributes to a decrease in cell–cell adhesion, and increase in tumor cell invasion and migration. In addition, other mechanisms also indicate that a secreted type of GnT-V may contribute to tumor angiogenesis