Elsevier

Blood Reviews

Volume 29, Issue 4, July 2015, Pages 269-279
Blood Reviews

REVIEW
The cancer glycome: Carbohydrates as mediators of metastasis

https://doi.org/10.1016/j.blre.2015.01.003Get rights and content

Abstract

Glycosylation is a frequent post-translational modification which results in the addition of carbohydrate determinants, “glycans”, to cell surface proteins and lipids. These glycan structures form the “glycome” and play an integral role in cell–cell and cell–matrix interactions through modulation of adhesion and cell trafficking.

Glycosylation is increasingly recognized as a modulator of the malignant phenotype of cancer cells, where the interaction between cells and the tumor micro-environment is altered to facilitate processes such as drug resistance and metastasis. Changes in glycosylation of cell surface adhesion molecules such as selectin ligands, integrins and mucins have been implicated in the pathogenesis of several solid and hematological malignancies, often with prognostic implications. In this review we focus on the functional significance of alterations in cancer cell glycosylation, in terms of cell adhesion, trafficking and the metastatic cascade and provide insights into the prognostic and therapeutic implications of recent findings in this fast-evolving niche.

Introduction

Glycosylation is a post-translational modification that occurs in the endoplasmic reticulum (ER) and results in the addition of carbohydrate motifs, “glycans”, to proteins and lipids that are, in most cases, destined for the cell surface. The resultant “glycoprotein” or “glycolipid” structures at the cell surface form a carbohydrate rich layer which plays an integral role in the interaction of the cell with its surrounding environment. Of the more than 200 different types of protein PTMs, glycosylation occurs frequently and results in the addition of functional carbohydrate motifs to protein structures [1], [2]. Glycans interact with carbohydrate binding proteins known as “lectins” that are specific for glycan moieties and are commonly used in purified form to study glycosylation in-vitro. One of the main functions of lectins in mammalian cells is to mediate cell–cell interactions and therefore interactions of glycans with their respective lectins have major implications for cell trafficking.

Glycosylation of a given protein is achieved through a complex series of post-translational enzymatic steps that lead to the formation of protein-bound glycans with specific and diverse biological functions. These carbohydrate side chains are capable of modulating the interaction of the protein with its environment influencing key factors such as protein half-life, solubility, binding activity and specificity. Proteins with the same amino acid sequence can possess different glycan structures, producing different glycoforms of the same protein. These glycoforms can differ in key properties such as stability, folding, localization and ligand specificity [3] with consequent implications for physiological processes, including protein folding and trafficking, cell–cell and cell–matrix interactions, cellular differentiation and the immune response [4], [5], [6]. Therefore, the glycosylation status of a protein can be used to differentiate protein glycoforms and molecular changes in glycosylation of proteins have been used to distinguish normal from disease states in humans [7], [8]. Furthermore, as cell communication, adhesion, and signaling also play a major role in cancer, changes in glycosylation of surface proteins on malignant cells can alter interactions between cancer cells and their surrounding environment [6], [9], [10], [11].

Glycosyltransferases are enzymes that regulate the process of glycosylation in humans where their action is dependent on the availability of precursor monosaccharide molecules and other parameters [12], [13]. Glycosyltransferases, along with glycosidases, work to add and subtract monosaccharides to and from glycan structures, examples of these enzymes include sialyltransferases and fucosyltransferases, which are responsible for the addition of sialic acid and fucose moieties, respectively. The intracellular sites of action of these enzymes include the ER, golgi apparatus, cytosol and nucleus.

Two major types of glycosylation occur on proteins; 1) O-linked glycosylation refers to the addition of N-acetyl-galactosamine to serine or threonine residues by the enzyme UDP-N-acetyl-d-galactosamine transferase, this is then followed by the addition of other carbohydrates such as galactose, N-acetyl-d-glucosamine or sialic acid (Fig. 1).; 2) N-linked glycosylation occurs in the ER and refers to the process by which an oligosaccharide chain is enzymatically attached to the amide group of an asparagine in the consensus sequence Asn-X-Ser/Thr where X represents any residue except proline (Fig. 1). This sequence can be used to identify potential N-glycosylation sites in peptide sequences.

O-linked glycosylation also contributes to the production of proteoglycans by the addition of glycosaminoglycan (GAG) chains to a core protein. GAGs consist of repeating disaccharide units composed of an N-acetylated or N-sulfated hexosamine and either a uronic acid (glucuronic acid or iduronic acid) or a galactose. Examples of GAGs include hyaluronan, dermatan sulfate, keratan sulfate, chondroitin sulfate, heparin, and heparan sulfate. Heparan and chondroitin sulfate are linked to serine residues of core proteins by xylose and this process is mediated by a xylosyltransferase. Proteoglycans and their associated GAGs form essential components of the extracellular matrix where they function in cell adhesion via interactions between the complex carbohydrate motifs [14].

It is clear that alterations in gene expression and protein expression are not the sole factors responsible for phenotype determination in cancer cells, where not only the cell itself is affected, but also the microenvironmental components such as the extracellular matrix (ECM). The impact of post-translational modifications (PTMs) on proteins and lipids has identified a layer of complexity, beyond the amino acid sequence, which has the consequence of greatly altering the function and even the purpose of that protein in a given context. Although the protein sequence is governed by the relevant genomic code, many properties of functional cell surface proteins, and circulating glycoproteins, are governed by the modification of glycans and therefore consideration must be given to the glycosylation status of a protein when considering its activity within a biological system.

This rapidly developing field has provided new cancer biomarkers and potential targets recently in a variety of solid and hematological cancers [15], [16], [17]. In this review we focus on the enzymes involved in this process and the cell surface proteins that become modified as a result of their action, with an overall focus on the implications for cell trafficking and metastasis of cancer cells.

Section snippets

Glycosylation and cancer

The normal process of glycosylation is disrupted during malignant transformation of cells [18], [19]. These changes result in alterations in tumor cell surface glycans and therefore interactions with endogenous lectins are impacted, which influences the metastatic potential of the tumor cells. Complex carbohydrate structures that can be found attached to proteins and lipids on the surface of cancer cells have a major influence on their phenotype and the interactions that they have with the

Selectins

As previously mentioned, selectins are vascular cell adhesion molecules which mediate adhesion of leukocytes and platelets with the endothelium. There are three members of the selectin family: P-, E-, and L-selectins. P-selectin is present in the storage granules of platelets (α-granules) and endothelial cells (Weibel–Palade bodies), and rapidly translocates to the cell surface upon activation [26]. L-selectin is expressed on the surface of almost all leukocytes. The physiological functions of

Glycosyltransferases: mediators of carbohydrate modifications in cancer

Glycosyltransferases are a large and diverse family of enzymes that are responsible for the assembly of monosaccharide moieties into linear and branched glycan chains. These enzymes tend to act sequentially so that the product of one enzyme prepares its acceptor as the substrate of the next enzyme in the process. Glycosyltransferases are specific for the type of linkage (α or β), and the linkage position of the glycoside bond formed [e.g. α(1  3) or β(1  4)]. Glycosyltransferases were initially

Mechanisms of regulation of glycosylation changes in cancer

Although glycans have been shown to be extensively altered in cancer, the mechanisms of regulation that govern the expression of the implicated genes are not well understood. It is likely that the genetic landscape of glycomics is not regulated by any one process but instead is an interplay of many factors, made more complex in the malignant state. However progress in this area is being made and evidence is accumulating that these genes may be altered by hypoxic conditions in the local

Clinical significance — diagnostics, prognostics and therapy

Given the large body of evidence that has accumulated to definitively implicate changes in glycosylation in the development and progression of certain cancers, there has been a focus on clinically applicable glycan targeting for diagnostic and prognostic purposes [145]. To date, this has taken the form of development of tumor-associated glycan markers as diagnostic and prognostic tools alongside a large focus on the development of vaccines in this area; however increasing attention is being

Summary

With recent advances in glyco-analytical technologies a greater understanding of the functional significance of seemingly minor changes in carbohydrate linkages on cell surface proteins and lipids has come to light. There has been renewed interest in glycosylation as a dynamic process that can evolve quickly and transiently to accommodate changes in the local microenvironment of the cell and facilitate adhesive and migratory interactions. Our understanding of changes in glycan determinants on

Research agenda

  • The use of glycosylation inhibition in cancer, including inhibitors of sialyltransferases.

  • The detection of specific glycoforms of adhesion molecules such as integrins in cancer that can serve as targets.

Conflict of interest

The authors declare no conflicts of interest for this work.

Acknowledgments

LJ acknowledges the Science Foundation Ireland [Alimentary Glycoscience Research Cluster, grant number 08/SRC/B1393 and Stokes Professor for Glycosciences, grant number 07/SK/B1250] and EU-FP7 GlycoHIT (grant number 260600) for their support. MO'D acknowledges the support of the Health Research Board, grant number CSA/2012/10.

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