Despite a large body of evidence identifying aberrant glycosylation pathways and the resulting glycan structures that are found in cancers, relatively little is known about the mechanisms of how these structures confer a selective advantage to the cancer cell. Perhaps the best characterised mechanism to date involves the interaction of sialylated Lewis structures with members of the selectin family of C- type lectins. The selectin group of C-type lectins are membrane receptors that bind to sialylated Lewisx structures (Steeber et al., 1995; Vestweber and Blanks, 1999). There are three members of the selectin family: P-selectin is found on activated platelets and Weibel-Palade bodies in vascular endothelial cells, E-selectin is found on activated endothelial cells and L-selectin is found on the surface of leukocytes (Nelson and Bevilacqua, 1993). Interactions between selectins and their specific glycoprotein ligands, which contain sialylated Lewis structures, are important temporary adhesion events between leukocytes or platelets and endothelial cells (Smith and Cummings, 1992). The selectin-dependent tethering and rolling of leukocytes to endothelial cell surfaces is important in their migration into sites of inflammation (McEver, 1994).
T h r S e r/ NH2 C OHO C1Ga T-1l S e r/ T h r NH2 COOH S e r/ T h r NH2 OH CO ST GalNAc-I6 ST3Gal-I h/T r S e r NH2 O CO H T r S e r/ h NH2 OH CO C2GnT Ser/Thr NH2 COOH Core 2 T a c S 6G lNA -II STn Tn T SialylT DisialylT
Figure 1.15 Examples of O-linked glycan truncation pathways. The action of ST6 GalNAc-I
on the Tn antigen produces the STn antigen, which cannot be further elongated. Similarly, the action of ST3-Gal I on the T antigen produces the sialyl-T antigen, which is not a substrate for C2GnTs, but can be further sialylated
The mechanisms of leukocyte rolling are thought to be exploited by cancerous cells that have successfully migrated from a primary tumour into the vasculature (Gorelik, 2001; Kannagi, 1997; Kim and Varki, 1997). Cancer cells projecting of a large array of sialylated Lewis structures on their surface can interact with selectins on endothelial cells, platelets and leukocytes. The adhesion to selectins on endothelial cells can aid the successful migration of the cancer cell into a secondary tissue. Alternatively, interactions of cancer cells with platelets and leukocytes can produce cell aggregates which protect the cancer cell from immune defence molecules and can form clots, slowing the flow rate of the bloodstream and thereby assisting the adhesion of cancer cells to the endothelial cell wall (Kannagi et al., 2004; Kim and Varki, 1997).
The role of sialylated Lewis antigens in cancer cell metastasis is supported experimentally by the direct contribution of sialyl Lewisa and sialyl Lewisx in the adhesion of cancer cells to the vascular endothelium (Takada et al., 1993) and clinically by the frequently observed correlation between the expression of sialyl Lewisx and the metastatic potential of a variety of cancers (Kannagi et al., 2004). The role of selectins in this process is supported by the reduction of cancer cell metastases in mice deficient in P-selectin or L-selectin (Borsig et al., 2002). Interactions between platelets and colon cancer cells are also reduced in P-selectin- deficient mice (Kim et al., 1998). Overexpression of E-selectin in the liver of transgenic mice was found to redirect the metastasis of a cancer cell line that normally colonises the lung to the liver (Biancone et al., 1996).
Expression of both Tn and STn antigens on mucins correlate with increase in metastasis and a poor prognosis in cancer patients (Itzkowitz et al., 1990). Mucins are thought to aid metastatic potential of the cell through anti-adhesive functions, disrupting normal cell-cell interactions due to their large size and negative charge (Wesseling et al., 1996). The increase in truncated, sialylated O-glycans is thought to aid the anti-adhesive functions and increase cell mobility (Julien et al., 2005). Interactions between the mucin protein MUC1, the expression of which is frequently increased in cancer cells of breast epithelial origin, and the macrophage receptor sialoadhesion, a member of the siglec family of sialic-acid binding lectins, has been demonstrated (Nath et al., 1999). Interactions between cancer mucins and sialic-acid binding proteins on immune cell molecules may serve to provide protection to the
cancer cell whilst in the bloodstream or may modulate innate immune responses toward the cancer cell.
The T antigen carried on MUC1 serves as a ligand for galectin-3, a member of the galectin family of galactose-binding proteins (Glinsky et al., 2000). Galectins are expressed intracellularly and extracellularly and are frequently found in increased concentrations in the sera of patients with cancer (Liu and Rabinovich, 2005). Galectin-3 is a monomeric protein containing a lectin domain and an N-terminal chimeric region. Binding of galectin-3 monomers to dense arrays of glycans such as those found on MUC1 can lead to oligomerisation through interactions in the chimeric domains, forming large aggregates (Takenaka et al., 2004). Binding of galectin-3 to MUC1 has been proposed to mediate homotypic aggregation of a breast cancer cell line and also to promote the adhesion of the breast cancer cells to the endothelium, by reversing the anti-adhesive properties of MUC1 (Yu et al., 2007).
The increase in β1-6 branching is thought to aid cancer cell migration through several mechanisms (Dennis et al., 1999). The increase in N-glycan size caused by β1-6 branches, the effects of which may be exasperated by the differing conformation of the β1-6 branch, has been proposed to modulate interactions of cell-surface molecules involved in adhesion. The increased expression of the enzyme GlcNAcT-V was shown to inhibit the clustering of integrin subunits increasing the motility of cells (Guo et al., 2002). Poly-N-acetyllactosamine extensions are preferentially formed on the β1-6 branches of N-glycans (see Section 1.7.2) and an increase in the level of extended branches may represent better scaffolds for the projection of Lewis-type structures. Poly-N-acetyllactosamine branches projected by both N- and O-linked glycans are high affinity ligands for galectins, in particular galectin-3 (Song et al., 2009). The binding of galectin-3 to poly-N-acetyllactosamine chains on N-linked glycans has been implicated in the successful metastasis of a melanoma cell line (Srinivasan et al., 2009). Interaction between galectins and poly-N-acetyllactosamine chains may therefore mediate homotypic and heterotypic adhesion of cancer cells in a manner similar to those reported for MUC1 and the T antigen.