**5.1 Principle function**

Galectins can act pro- or antiadhesive for different cell types. They can either facilitate or reduce adhesion to other cells depending on different factors. Cell adhesion is enhanced if galectins crosslink glycosylated structures on one cell with glycans on other cells or the extracellular matrix. In contrast the adhesion is reduced if soluble galectins block available receptors for other binding interactions. This depends on one hand on galectin concentration. At high concentrations galectins may block all available receptors without interaction with each other which is necessary for crosslinking and therefore for adhesion (Elola et al., 2007). It is for example discussed that galectin-3 outbursts can lead to detachment of cells from the extracellular matrix as galectin-3 blocks integrin binding to ECM glycoproteins (Ochieng et al., 1998b). On the other hand it is important which receptors are available on the specific cell type used in the experiment and if those receptors interact more easily with the soluble galectins or with receptors on the surface the cell attaches to (Elola et al., 2007). Additional the oligomerisation state of the galectins plays an important role as they can either block receptors or crosslink molecules depending on their valency (Hughes, 2001). The oligomerisation is in case of galectin-1 depending on galectin concentration while galectin-3 stays monomeric in solution without ligand binding and builds pentamers after the binding reaction (Ahmad et al., 2004a; Cho & Cummings, 1995; Cho & Cummings, 1997; Morris et al., 2004; Nieminen et al., 2008). Moreover effects of single galectins can hardly be determined as most cell types co-express different galectins which might at least partially result in overlapping or opposite effects (Cooper & Barondes, 1999; Liu & Rabinovich, 2005).

In addition to direct binding of galectins to glycan structures on either membrane-bound receptors or ECM-glycoproteins, regulation of integrin amount, availability and affinity by galectin binding also contributes to adhesion events. Galectin-3 for example is able to increase amount and/or affinity of β2-integrins on the cell surface on neutrophils, thereby regulating the binding to ECM glycoproteins recognised by integrins (Hughes, 2001; Kuwabara & Liu, 1996). Overexpression of galectin-3 correlates with enhanced surface expression of α4β7 integrins resulting in increased cell adhesion (Matarrese et al., 2000). In contrast binding of galectin-3 leads to internalisation of β1-integrins in breast carcinoma cells (Furtak et al., 2001). Moreover the clustering and residence time of other receptors on the cell surface is regulated by the formation of glycan-galectin lattices thereby regulating different signalling processes (Garner & Baum, 2008; Lau & Dennis, 2008; Rabinovich et al., 2007).

sialylated ligands are presented on this subunit (Diskin et al., 2009). Beside the β1-sunbunit galectin-8 N-CRD also binds α5 and some other integrin-subunits, but literature does not give a clear picture about the exact integrin binding partners. For example *N*-glycans on the α4-subunit are once mentioned as main binding partner while other authors do not report binding to this subunit. Similar discrepancies were noticed for other subunits (Cárcamo et al., 2006; Diskin et al., 2009; Hadari et al., 2000; Nishi et al., 2003; Yamamoto et al., 2008). This might be explained by tissue- or cell-specific glycosylation patterns of the single subunits. In contrast to most interactions which are performed by the N-terminal galectin-8 CRD binding to the αM-subunit is performed by the C-terminal CRD (Nishi et al., 2003).

Galectins can act pro- or antiadhesive for different cell types. They can either facilitate or reduce adhesion to other cells depending on different factors. Cell adhesion is enhanced if galectins crosslink glycosylated structures on one cell with glycans on other cells or the extracellular matrix. In contrast the adhesion is reduced if soluble galectins block available receptors for other binding interactions. This depends on one hand on galectin concentration. At high concentrations galectins may block all available receptors without interaction with each other which is necessary for crosslinking and therefore for adhesion (Elola et al., 2007). It is for example discussed that galectin-3 outbursts can lead to detachment of cells from the extracellular matrix as galectin-3 blocks integrin binding to ECM glycoproteins (Ochieng et al., 1998b). On the other hand it is important which receptors are available on the specific cell type used in the experiment and if those receptors interact more easily with the soluble galectins or with receptors on the surface the cell attaches to (Elola et al., 2007). Additional the oligomerisation state of the galectins plays an important role as they can either block receptors or crosslink molecules depending on their valency (Hughes, 2001). The oligomerisation is in case of galectin-1 depending on galectin concentration while galectin-3 stays monomeric in solution without ligand binding and builds pentamers after the binding reaction (Ahmad et al., 2004a; Cho & Cummings, 1995; Cho & Cummings, 1997; Morris et al., 2004; Nieminen et al., 2008). Moreover effects of single galectins can hardly be determined as most cell types co-express different galectins which might at least partially result in overlapping or opposite effects (Cooper & Barondes, 1999;

In addition to direct binding of galectins to glycan structures on either membrane-bound receptors or ECM-glycoproteins, regulation of integrin amount, availability and affinity by galectin binding also contributes to adhesion events. Galectin-3 for example is able to increase amount and/or affinity of β2-integrins on the cell surface on neutrophils, thereby regulating the binding to ECM glycoproteins recognised by integrins (Hughes, 2001; Kuwabara & Liu, 1996). Overexpression of galectin-3 correlates with enhanced surface expression of α4β7 integrins resulting in increased cell adhesion (Matarrese et al., 2000). In contrast binding of galectin-3 leads to internalisation of β1-integrins in breast carcinoma cells (Furtak et al., 2001). Moreover the clustering and residence time of other receptors on the cell surface is regulated by the formation of glycan-galectin lattices thereby regulating different signalling processes (Garner & Baum, 2008; Lau & Dennis, 2008; Rabinovich et al.,

**5. Galectin function in cell adhesion and cell migration** 

**5.1 Principle function** 

Liu & Rabinovich, 2005).

2007).

**5.2 Selected examples of cell adhesion and motility regulated by galectins-1, -3 and -8** 

We here present only few examples of galectin function in cell adhesion and motility processes. The list is by far not complete. Other review articles focus more detailed on cell adhesion events mediated by galectins (Elola et al., 2007; Hughes, 2001).

Galectin-1 is an important factor for the adhesion and proliferation of neural stem cells and neural progenitor cells. The adhesion is mediated by the carbohydrate recognition domain and interaction of this binding domain with integrin β1 subunit (Sakaguchi et al., 2006; Sakaguchi et al., 2010). Moreover galectin-1 can reduce the motility of immune cells which might explain parts of its anti-inflammatory effects (Elola et al., 2007; Liu, 2005; Rabinovich et al., 2002a; Rabinovich et al., 2002b).

One important function of galectin-3 is associated with angiogenesis (Nangia-Makker et al., 2000a; Nangia-Makker et al., 2000b). Galectin-3 increases for example angiogenesis by forming integrin αvβ3 lattices on the cell-surface leading to FAK regulated downstream signalling. Galectin-3 mediated angiogenesis depends on the growth factors VEGF and bFGF (Markowska et al., 2010). Another interesting function of galectin-3 is the chemotattraction of monocytes via a G-protein coupled receptor pathway and the role in eosinophil rolling to sites of inflammation (Rao et al., 2007; Sano et al., 2000). Most of those functions can only be performed by full length galectin-3 showing the importance of glycan binding and oligomerisation of the protein (Markowska et al., 2010; Sano et al., 2000). Different other biological activities are also depending on both N- and C-terminal domain (Nieminen et al., 2005; Ochieng et al., 1998a; Sano et al., 2000; Sato et al., 2002; Yamaoka et al., 1995). This proves the possibility of regulating galectin-3 function by protease cleavage as mentioned in chapter 2.3.3.

Galectin-8 has been assigned to matricellular proteins which are able to promote cell adhesion. CHO-cells on galectin-8 show similar binding kinetics as on fibronectin but differ in their formation of cytoskeleton (Boura-Halfon et al., 2003). Moreover the binding to galectin-8 triggers specific signalling cascades as Ras, MAPK and Erk pathway (Levy et al., 2003). A physiological function in human might be the modulation of neutrophil function. Galectin-8 promotes neutrophil adhesion by binding αM integrin and promatrix metalloproteinase-9. Moreover superoxide production which is essentiell for neutrophil function is triggered by galectin-8 C-terminal CRD (Nishi et al., 2003). Another galectin-8 function might be the promotion of angiogenesis as it was also shown for galectin-3. Galectin-8 increases tube formation *in vitro* and angiogenesis *in vivo* in dependence of its specific carbohydrate affinity at physiological concentrations. This regulatory function is at least partially depending on CD 166 (Delgado et al., 2011).
