Linda G. Baum

Galectins: versatile modulators of cell adhesion, cell proliferation, and cell death.

Abstract Lectins, or carbohydrate binding proteins, recognize specific oligosaccharide structures on glycoproteins and glycolipids. Several families of animal lectins have been identified; for some of these lectins, functions such as leukocyte adhesion and microbial opsonization have been described. The galectins are a family of lectins found in species ranging from sponges and nematodes to humans. Members of the galectin family have been proposed to mediate cell adhesion, to regulate cell growth, and to trigger or inhibit apoptosis. The expression pattern of different galectins changes during development, and this pattern is also altered at sites of inflammation and in breast, colon, prostate, and thyroid carcinomas. In addition, the level of expression of some galectins by tumor cells has been shown to be correlated with metastatic potential. The mechanisms by which galectins exert these diverse effects remain largely unknown. Some glycoprotein counterreceptors recognized by certain galectins have been identified; this is an important first step in understanding the cell-type specific effects of different galectins. This review discusses the way in which the modulation of galectin activity may affect strategies for treatment of a variety of human diseases, including autoimmunity and cancer.

Galectins and their ligands: amplifiers, silencers or tuners of the inflammatory response?

Recent evidence has implicated galectins and their ligands as master regulators of immune cell homeostasis. Whereas some members of this family, such as galectin-3, behave as amplifiers of the inflammatory cascade, others, such as galectin-1, trigger homeostatic signals to shut off T-cell effector functions. These carbohydrate-binding proteins, identified by shared consensus amino acid sequences and affinity for beta-galactoside-containing sugars, participate in the homeostasis of the inflammatory response, either by regulating cell survival and signaling, influencing cell growth and chemotaxis, interfering with cytokine secretion, mediating cell-cell and cell-matrix interactions or influencing tumor progression and metastasis. The current wealth of new information promises a future scenario in which individual members of the galectin family or their ligands will be used as powerful anti-inflammatory mediators and selective modulators of the immune response.

Differential glycosylation of TH1, TH2 and TH-17 effector cells selectively regulates susceptibility to cell death.

Regulated glycosylation controls T cell processes, including activation, differentiation and homing by creating or masking ligands for endogenous lectins. Here we show that stimuli promoting T helper type 1 (TH1), TH2 or interleukin 17–producing T helper (TH-17) differentiation can differentially regulate the glycosylation pattern of T helper cells and modulate their susceptibility to galectin-1, a glycan-binding protein with anti-inflammatory activity. Although TH1- and TH-17–differentiated cells expressed the repertoire of cell surface glycans critical for galectin-1–induced cell death, TH2 cells were protected from galectin-1 through differential sialylation of cell surface glycoproteins. Consistent with those findings, galectin-1–deficient mice developed greater TH1 and TH-17 responses and enhanced susceptibility to autoimmune neuroinflammation. Our findings identify a molecular link among differential glycosylation of T helper cells, susceptibility to cell death and termination of the inflammatory response.

Galectin-1 is essential in tumor angiogenesis and is a target for antiangiogenesis therapy

We describe that galectin-1 (gal-1) is a receptor for the angiogenesis inhibitor anginex, and that the protein is crucial for tumor angiogenesis. gal-1 is overexpressed in endothelial cells of different human tumors. Expression knockdown in cultured endothelial cells inhibits cell proliferation and migration. The importance of gal-1 in angiogenesis is illustrated in the zebrafish model, where expression knockdown results in impaired vascular guidance and growth of dysfunctional vessels. The role of gal-1 in tumor angiogenesis is demonstrated in gal-1-null mice, in which tumor growth is markedly impaired because of insufficient tumor angiogenesis. Furthermore, tumor growth in gal-1-null mice no longer responds to antiangiogenesis treatment by anginex. Thus, gal-1 regulates tumor angiogenesis and is a target for angiostatic cancer therapy.

Influenza virus strains selectively recognize sialyloligosaccharides on human respiratory epithelium; the role of the host cell in selection of hemagglutinin receptor specificity

The complement of sialyloligosaccharides present on the surface of human tracheal epithelium has been implicated as an important factor in the selection of hemagglutinin receptor specificity of human influenza A virus. Human strains of influenza A virus preferentially recognize host cell receptors bearing SA alpha 2,6Gal sequences, a sequence which is found on the surface of ciliated tracheal epithelium. A fluorescently-labelled H3 human virus strain bound avidly to the apical surface of human tracheal epithelium, while a fluorescently-labelled receptor variant strain, which preferentially binds SA alpha 2,3Gal sequences, showed little binding to the epithelial surface and localized primarily to intracellular mucin droplets. Extracts of human bronchial mucin, which is known to contain sialic acid primarily in the SA alpha 2,3Gal linkage, was a potent inhibitor of the binding of the receptor variant strain to trachea sections, while the binding of the parent strain was unaffected by the presence of mucin. Human bronchial mucin also inhibited the binding of the receptor variant strains, but not the parent virus strains, to human erythrocytes derivatized to contain SA alpha 2,6Gal sequences. These results suggest that a combination of selection pressures present in the respiratory tract environment have resulted in the evolution of a hemagglutinin receptor specificity in human influenza A virus strains which optimizes recognition of, binding to and infection of host cells.

Clusters, bundles, arrays and lattices: novel mechanisms for lectin–saccharide-mediated cellular interactions

Multivalent protein-carbohydrate interactions regulate essential cellular events, including cell proliferation, adhesion and death. These multivalent interactions can create homogeneous complexes of lectins, such as the galectins, with their saccharide ligands. Lectin-saccharide complexes can concentrate specific glycoproteins or glycolipids within the lattice, while excluding other cell surface molecules. The formation of lectin-saccharide lattices on the cell surface can thus organize the plasma membrane into specialized domains that perform unique functions.

Galectin-3 and Galectin-1 Bind Distinct Cell Surface Glycoprotein Receptors to Induce T Cell Death

Galectins are a family of mammalian β-galactoside-binding proteins that positively and negatively regulate T cell death. Extracellular galectin-1 directly induces death of T cells and thymocytes, while intracellular galectin-3 blocks T cell death. In contrast to the antiapoptotic function of intracellular galectin-3, we demonstrate that extracellular galectin-3 directly induces death of human thymocytes and T cells. However, events in galectin-3- and galectin-1-induced cell death differ in a number of ways. Thymocyte subsets demonstrate different susceptibility to the two galectins: whereas galectin-1 kills double-negative and double-positive human thymocytes with equal efficiency, galectin-3 preferentially kills double-negative thymocytes. Galectin-3 binds to a complement of T cell surface glycoprotein receptors distinct from that recognized by galectin-1. Of these glycoprotein receptors, CD45 and CD71, but not CD29 and CD43, appear to be involved in galectin-3-induced T cell death. In addition, CD7 that is required for galectin-1-induced death is not required for death triggered by galectin-3. Following galectin-3 binding, CD45 remains uniformly distributed on the cell surface, in contrast to the CD45 clustering induced by galectin-1. Thus, extracellular galectin-3 and galectin-1 induce death of T cells through distinct cell surface events. However, as galectin-3 and galectin-1 cell death are neither additive nor synergistic, the two death pathways may converge inside the cell.

Cutting Edge: CD7 Delivers a Pro-Apoptotic Signal During Galectin-1-Induced T Cell Death

Galectin-1, an endogenous lectin expressed in lymphoid organs and immune-privileged sites, induces death of human and murine thymocytes and T cells. Galectin-1 binds to several glycoproteins on the T cell surface, including CD7. However, the T cell surface glycoprotein receptors responsible for delivering the galectin-1 death signal have not been identified. We show that CD7 is required for galectin-1-mediated death. This demonstrates a novel function for CD7 as a death trigger and identifies galectin-1/CD7 as a new biologic death signaling pair.

The ST6Gal I sialyltransferase selectively modifies N-glycans on CD45 to negatively regulate galectin-1-induced CD45 clustering, phosphatase modulation, and T cell death.

The addition of sialic acid to T cell surface glycoproteins influences essential T cell functions such as selection in the thymus and homing in the peripheral circulation. Sialylation of glycoproteins can be regulated by expression of specific sialyltransferases that transfer sialic acid in a specific linkage to defined saccharide acceptor substrates and by expression of particular glycoproteins bearing saccharide acceptors preferentially recognized by different sialyltransferases. Addition of α2,6-linked sialic acid to the Galβ1,4GlcNAc sequence, the preferred ligand for galectin-1, inhibits recognition of this saccharide ligand by galectin-1. SAα2,6Gal sequences, created by the ST6Gal I enzyme, are present on medullary thymocytes resistant to galectin-1-induced death but not on galectin-1-susceptible cortical thymocytes. To determine whether addition of α2,6-linked sialic acid to lactosamine sequences on T cell glycoproteins inhibits galectin-1 death, we expressed the ST6Gal I enzyme in a galectin-1-sensitive murine T cell line. ST6Gal I expression reduced galectin-1 binding to the cells and reduced susceptibility of the cells to galectin-1-induced cell death. Because the ST6Gal I preferentially utilizes N-glycans as acceptor substrates, we determined that N-glycans are essential for galectin-1-induced T cell death. Expression of the ST6Gal I specifically resulted in increased sialylation of N-glycans on CD45, a receptor tyrosine phosphatase that is a T cell receptor for galectin-1. ST6Gal I expression abrogated the reduction in CD45 tyrosine phosphatase activity that results from galectin-1 binding. Sialylation of CD45 by the ST6Gal I also prevented galectin-1-induced clustering of CD45 on the T cell surface, an initial step in galectin-1 cell death. Thus, regulation of glycoprotein sialylation may control susceptibility to cell death at specific points during T cell development and peripheral activation.

CD45 Modulates Galectin-1-Induced T Cell Death: Regulation by Expression of Core 2 O-Glycans

Galectin-1 induces death of immature thymocytes and activated T cells. Galectin-1 binds to T cell-surface glycoproteins CD45, CD43, and CD7, although the precise roles of each receptor in cell death are unknown. We have determined that CD45 can positively and negatively regulate galectin-1-induced T cell death, depending on the glycosylation status of the cells. CD45+ BW5147 T cells lacking the core 2 β-1,6-N-acetylglucosaminyltransferase (C2GnT) were resistant to galectin-1 death. The inhibitory effect of CD45 in C2GnT− cells appeared to require the CD45 cytoplasmic domain, because Rev1.1 cells expressing only CD45 transmembrane and extracellular domains were susceptible to galectin-1 death. Moreover, treatment with the phosphotyrosine-phosphatase inhibitor potassium bisperoxo(1,10-phenanthroline)oxovanadate(V) enhanced galectin-1 susceptibility of CD45+ T cell lines, but had no effect on the death of CD45− T cells, indicating that the CD45 inhibitory effect involved the phosphatase domain. Expression of the C2GnT in CD45+ T cell lines rendered the cells susceptible to galectin-1, while expression of the C2GnT in CD45− cells had no effect on galectin-1 susceptibility. When CD45+ T cells bound to galectin-1 on murine thymic stromal cells, only C2GnT+ T cells underwent death. On C2GnT+ cells, CD45 and galectin-1 co-localized in patches on membrane blebs while no segregation of CD45 was seen on C2GnT− T cells, suggesting that oligosaccharide-mediated clustering of CD45 facilitated galectin-1-induced cell death.