Studies of Arginine–Arene Interactions through Synthesis and Evaluation of a Series of Galectin-Binding Aromatic Lactose Esters

Galectins in innate immunity: dual functions of host soluble beta-galactoside-binding lectins as damage-associated molecular patterns (DAMPs) and as receptors for pathogen-associated molecular patterns (PAMPs)

The glycocalyx is a glycan layer found on the surfaces of host cells as well as microorganisms and enveloped virus. Its thickness may easily exceed 50 nm. The glycocalyx does not only serve as a physical protective barrier but also contains various structurally different glycans, which provide cell- or microorganism-specific 'glycoinformation'. This information is decoded by host glycan-binding proteins, lectins. The roles of lectins in innate immunity are well established, as exemplified by collectins, dectin-1, and dendritic cell (DC)-specific intracellular adhesion molecule-3-grabbing non-integrin (DC-SIGN). These mammalian lectins are synthesized in the secretory pathway and presented on the cell surface to bind to specific glycan 'epitopes'. As they recognize non-self glycans presented by microorganisms, they can be considered as receptors for pathogen-associated molecular patterns (PAMPs), i.e. pattern recognition receptors (PRRs). One notable exception is the galectin family. Galectins are synthesized and stored in the cytoplasm, but upon infection-initiated tissue damage and/or following prolonged infection, cytosolic galectins are either passively released by dying cells or actively secreted by inflammatory activated cells through a non-classical pathway, the 'leaderless' secretory pathway. Once exported, galectins act as PRR, as well as immunomodulators (or cytokine-like modulators) in the innate response to some infectious diseases. As galectins are dominantly found in the lesions where pathogen-initiated tissue damage signals appear, this lectin family is also considered as potential damage-associated molecular pattern (DAMP) candidates that orchestrate innate immune responses alongside the PAMP system.

Galectin-inhibitory thiodigalactoside ester derivatives have antimigratory effects in cultured lung and prostate cancer cells

Aromatic 3,3'-diesters of thiodigalactoside were synthesized in a rapid three-step sequence from commercially available thiodigalactoside and evaluated as inhibitors of cancer- and immunity-related galectins. For each of galectins-1, -3, -7, and -9N-terminal domain, aromatic 3,3'-diesters of thiodigalactoside were found to have affinities in the low micromolar range, which represents a 7-70 fold enhancement over thiodigalactoside itself. No significant improvement was found for galectin-8 N-terminal domain. Two of the compounds were selected for testing in cell culture and were shown to have potent antimigratory effects on human PC-3 prostate and human A549 nonsmall-cell lung cancer cells.

Galectin-3 functions as an opsonin and enhances the macrophage clearance of apoptotic neutrophils

Galectin-3, a beta-galactoside binding, endogenous lectin, takes part in various inflammatory events and is produced in substantial amounts at inflammatory foci. We investigated whether extracellular galectin-3 could participate in the phagocytic clearance of apoptotic neutrophils by macrophages, a process of crucial importance for termination of acute inflammation. Using human leukocytes, we show that exogenously added galectin-3 increased the uptake of apoptotic neutrophils by monocyte-derived macrophages (MDM). Both the proportion of MDM that engulfed apoptotic prey and the number of apoptotic neutrophils that each MDM engulfed were enhanced in the presence of galectin-3. The effect was lactose-inhibitable and required galectin-3 affinity for N-acetyllactosamine, a saccharide typically found on cell surface glycoproteins, since a mutant lacking this activity was without effect. The enhanced uptake relied on the presence of galectin-3 during the cellular interaction and was paralleled by lectin binding to apoptotic cells as well as MDM in a lactose-dependent manner. These findings suggest that galectin-3 functions as a bridging molecule between phagocyte and apoptotic prey, acting as an opsonin. The process of clearance, whereby apoptotic neutrophils are removed by macrophages, is crucial for the resolution of acute inflammation and our data imply that the increased levels of galectin-3 often found at inflammatory sites could potently affect this process.

Protein subtype-targeting through ligand epimerization: talose-selectivity of galectin-4 and galectin-8

A series of O2 and O3-derivatized methyl beta-d-talopyranosides were synthesized and evaluated in vitro as inhibitors of the galactose-binding galectin-1, -2, -3, -4 (N- and C-terminal domains), 8 (N-terminal domain), and 9 (N-terminal domain). Galectin-4C and 8N were found to prefer the d-talopyranose configuration to the natural ligand d-galactopyranose configuration. Derivatization at talose O2 and/or O3 provided selective submillimolar inhibitors for these two galectins.

Different affinity of galectins for human serum glycoproteins: galectin-3 binds many protease inhibitors and acute phase proteins

Here we report the first survey of galectins binding to glycoproteins of human serum. Serum was subjected to affinity chromatography using immobilized galectins, and the bound glycoproteins were analyzed by electrophoresis, Western blotting, and mass spectrometry. Galectins-3, -8, and -9 bound a much broader range of ligands in serum than previously known, galectin-1 bound less, and galectins-2, -4, and -7 bound only traces or no serum ligands. Galectin-3 bound most major glycoproteins, including alpha-2-macroglobulin and acute phase proteins such as haptoglobin. It bound only a selected minor fraction of transferrin, and bound none or little of IgG. Galectins-8 and -9 bound a similar range of glycoproteins as galectin-3, but in lower amounts, and galectin-8 had a relative preference for IgA. Galectin-1 bound mainly a fraction of alpha-2-macroglobulin and only traces of other glycoproteins. The binding of galectin-3 to serum glycoproteins requires affinity for LacNAc, since a mutant (R186S), which has lost this affinity, did not bind any serum glycoproteins. The average affinity of galectin-3 for serum glycoproteins was estimated to correspond to K(d) approximately 1-5 muM by modeling of the affinity chromatography and a fluorescence anisotropy assay. Since galectins are expressed on endothelial cells and other cells exposed to serum components, this report gives new insight into function of galectins and the role of their different fine specificity giving differential binding to the serum glycoproteins.