Ingrid 't Hart

14 Chapter 1 1 Cell-cell interac�ons provide an important way to transduce signals to slow down cell growth if the cell density gets too high. The tumor suppressor Merlin/NF2 is involved in this contact inhibi�on and reduced expression of Merlin/NF2 has been associated with tumorigenesis. A study on the effect of GSLs on contact inhibi�on revealed that GSLs GD3 and Gb3 inhibited cell growth, possibly through interac�ons with Merlin/NF2. This study provides a possible role of GSLs in cell contact inhibi�on, however its exact role in cancer remains to be elucidated. 50 Another proposed mechanism for GSLs, such as GM3 and Gb4 are carbohydrate-carbohydrate interac�ons (CCIs) between cells. The role of CCI on cell adhesion in cancer remains to be further inves�gated. 51 GSLs also modulate cancer related processes by cis-interac�ons or transmembrane signal transduc�on. Whereas Gb4 and Gb5 have been reported to promote ac�va�on of EGFR, GT1b, GD1a GM3 and GM1 are proposed to inhibit EGFR phosphoryla�on and thereby ac�va�on. 2,23,52 In addi�on, GD3 has been found to interact with various membrane molecules, including neogenin-1. It was found that neogenin-1 colocalizes with GD3 in lipid ra�s and is involved in enhancing cell growth, invasion and migra�on. 53 MSGb5 was found to cluster with different signal transducing proteins, leading to downstream ac�va�on of extra-cellular-signal-regulated-kinase (ERK), resul�ng in a decreased expression of integrins. This pathway could be an underlying mechanism for MSGb5 in cancer cell invasion. 54 Another important detected interac�on is between platelet-derived growth factor receptor α (PDGFRα) and GSLs GD2 and GD3, resul�ng in downstream ac�va�on of signals that increase invasion, cell growth and migra�on. 53 Furthermore, GSLs are suggested to be involved in the epithelial-to-mesenchymal transi�on (EMT). 55 A recent study revealed that loss of globo-series GSLs through CRISPR- Cas9 mediated dele�on of the enzyme A4GalT induces EMT. 56 Different immune cell interac�ons have been reported for GSLs, mainly gangliosides. For instance, ganglioside GD3 binds with high affinity to CD1d, which is a protein that presents lipid an�gens to T cells. GD3 administra�on inhibitedNKT immune cell ac�va�on in vivo . 57 Other gangliosides are known to modulate immune cells by binding sialic-acid binding Ig-like lec�ns (Siglecs). Siglecs are expressed on various immune cells and most of these proteins contain an immunoreceptor tyrosine-based inhibitory mo�f (ITIM). Upon ligand binding, these Siglecs transduce signals that result in the inhibi�on of an immune response. Therefore, if cancer cells express ligands for Siglecs, they can escape recogni�on and removal by immune cells. Siglec-7 -9, -10 and -15 all have been found to be involved in immune regula�on in cancer. 58,59 As an example, gangliosides GD3, GT1b and DSGb5 are all (proposed) ligands for Siglec-7, an inhibitory receptor on NK cells (Fig. 6). 60,61,62 A co-crystal structure of a GT1b analog with Siglec-7, revealed that the terminal α2,8-sialic acid is the major determinant for ligand binding. A key interac�on takes place between the sialic acid carboxylate and Arg-124 in Siglec-7 and removal of either the sialic acid or Arg-124 abolished binding. Furthermore, it was observed that the neutral glycan core interacts with the C-C’ loop of Siglec-7. The GT1b analog used for these studies lacked the ceramide residue. 62 However, another ( in vitro ) study revealed that altered lipid composi�ons of GD3, such as phytosphingosine instead of sphingosine or an α-hydroxyl on the fa�y acid, abolished Siglec-7 binding. 60