Fu Tong Liu

Galectins as modulators of tumour progression

Galectins are a family of animal lectins with diverse biological activities. They function both extracellularly, by interacting with cell-surface and extracellular matrix glycoproteins and glycolipids, and intracellularly, by interacting with cytoplasmic and nuclear proteins to modulate signalling pathways. Current research indicates that galectins have important roles in cancer; they contribute to neoplastic transformation, tumour cell survival, angiogenesis and tumour metastasis. They can modulate the immune and inflammatory responses and might have a key role helping tumours to escape immune surveillance. How do the different members of the Galectin family contribute to these diverse aspects of tumour biology?

Intracellular functions of galectins

Many galectin family members are detected primarily intracellularly in most of the systems studied, although certain members can be found both inside and outside of cells. Specific functions that are consistent with their intracellular localization have now been documented for some of the galectins. Galectin-1 and -3 have been identified as redundant pre-mRNA splicing factors. Galectin-3, -7, and -12 have been shown to regulate cell growth and apoptosis, being either anti-apoptotic or pro-apoptotic. Galectin-3 and -12 have been shown to regulate the cell cycle. In some cases, the mechanisms by which galectins exert their functions have been partially delineated in relation to known intracellular pathways associated with these processes. In addition, a number of intracellular proteins involved in these processes have been identified as the interacting ligands of certain galectins. This review summarizes the intracellular activities displayed by several galectins and discusses the possible underlying mechanisms.

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.

Galectins: structure, function and therapeutic potential

Galectins are a family of animal lectins that bind beta-galactosides. Outside the cell, galectins bind to cell-surface and extracellular matrix glycans and thereby affect a variety of cellular processes. However, galectins are also detectable in the cytosol and nucleus, and may influence cellular functions such as intracellular signalling pathways through protein-protein interactions with other cytoplasmic and nuclear proteins. Current research indicates that galectins play important roles in diverse physiological and pathological processes, including immune and inflammatory responses, tumour development and progression, neural degeneration, atherosclerosis, diabetes, and wound repair. Some of these have been discovered or confirmed by using genetically engineered mice deficient in a particular galectin. Thus, galectins may be a therapeutic target or employed as therapeutic agents for inflammatory diseases, cancers and several other diseases.

Human Galectin-3 Is a Novel Chemoattractant for Monocytes and Macrophages

Galectin-3 is a β-galactoside-binding protein implicated in diverse biological processes. We found that galectin-3 induced human monocyte migration in vitro in a dose-dependent manner, and it was chemotactic at high concentrations (1.0 μM) but chemokinetic at low concentrations (10–100 nM). Galectin-3-induced monocyte migration was inhibited by its specific mAb and was blocked by lactose and a C-terminal domain fragment of the protein, indicating that both the N-terminal and C-terminal domains of galectin-3 are involved in this activity. Pertussis toxin (PTX) almost completely blocked monocyte migration induced by high concentrations of galectin-3. Galectin-3 caused a Ca2+ influx in monocytes at high, but not low, concentrations, and both lactose and PTX inhibited this response. There was no cross-desensitization between galectin-3 and any of the monocyte-reactive chemokines examined, including monocyte chemotactic protein-1, macrophage inflammatory protein-1α, and stromal cell-derived factor-1α. Cultured human macrophages and alveolar macrophages also migrated toward galectin-3, but not monocyte chemotactic protein-1. Finally, galectin-3 was found to cause monocyte accumulation in vivo in mouse air pouches. These results indicate that galectin-3 is a novel chemoattractant for monocytes and macrophages and suggest that the effect is mediated at least in part through a PTX-sensitive (G protein-coupled) pathway.

Targeted disruption of the Galectin-3 gene results in attenuated peritoneal inflammatory responses

Galectin-3 is a member of a growing family of beta-galactoside-binding animal lectins. Previous studies have demonstrated a variety of biological activities for this protein in vitro, including activation of cells, modulation of cell adhesion, induction of pre-mRNA splicing, and regulation of apoptosis. To assist in fully elucidating the physiological and pathological functions of this protein, we have generated galectin-3-deficient (gal3(-/-)) mice by targeted interruption of the galectin-3 gene. Gal3(-/-) mice consistently developed fewer inflammatory cell infiltrations in the peritoneal cavities than the wild-type (gal3(+/+)) mice in response to thioglycollate broth treatment, mainly due to lower numbers of macrophages. Also, when compared to cells from gal3(+/+) mice, thioglycollate-elicited inflammatory cells from gal3(-/-) mice exhibited significantly lower levels of NF-kappaB response. In addition, dramatically different cell-spreading phenotypes were observed in cultured macrophages from the two genotypes. Whereas macrophages from gal3(+/+) mice exhibited well spread out morphology, those from gal3(-/-) mice were often spindle-shaped. Finally, we found that peritoneal macrophages from gal3(-/-) mice were more prone to undergo apoptosis than those from gal3(+/+) mice when treated with apoptotic stimuli, suggesting that expression of galectin-3 in inflammatory cells may lead to longer cell survival, thus prolonging inflammation. These results strongly support galectin-3 as a positive regulator of inflammatory responses in the peritoneal cavity.

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.

Galectin-3 expression and secretion links macrophages to the promotion of renal fibrosis.

Macrophages have been proposed as a key cell type in the pathogenesis of renal fibrosis; however, the mechanism by which macrophages drive fibrosis is still unclear. We show that expression of galectin-3, a beta-galactoside-binding lectin, is up-regulated in a mouse model of progressive renal fibrosis (unilateral ureteric obstruction, UUO), and absence of galectin-3 protects against renal myofibroblast accumulation/activation and fibrosis. Furthermore, specific depletion of macrophages using CD11b-DTR mice reduces fibrosis severity after UUO demonstrating that macrophages are key cells in the pathogenesis of renal fibrosis. Disruption of the galectin-3 gene does not affect macrophage recruitment after UUO, or macrophage proinflammatory cytokine profiles in response to interferon-gamma/lipopolysaccharide. In addition, absence of galectin-3 does not affect transforming growth factor-beta expression or Smad 2/3 phosphorylation in obstructed kidneys. Adoptive transfer of wild-type but not galectin-3(-/-) macrophages did, however, restore the fibrotic phenotype in galectin-3(-/-) mice. Cross-over experiments using wild-type and galectin-3(-/-) macrophage supernatants and renal fibroblasts confirmed that secretion of galectin-3 by macrophages is critical in the activation of renal fibroblasts to a profibrotic phenotype. Therefore, we demonstrate for the first time that galectin-3 expression and secretion by macrophages is a major mechanism linking macrophages to the promotion of renal fibrosis.

Regulation of Mast Cell Survival by IgE

Mast cells play critical roles in hypersensitivity and in defense against certain parasites. We provide evidence that mouse mast cell survival and growth are promoted by monomeric IgE binding to its high-affinity receptor, Fc epsilon RI. Monomeric IgE does not promote DNA synthesis but suppresses the apoptosis induced by growth factor deprivation. This antiapoptotic effect occurs in parallel with IgE-induced increases in Fc epsilon RI surface expression but requires the continuous presence of IgE. This process does not involve the FasL/Fas death pathway or several Bcl-2 family proteins and induces a distinctly different signal than Fc epsilon RI cross-linking. The ability of IgE to enhance mast cell survival and Fc epsilon RI expression may contribute to amplified allergic reactions.

Critical role of galectin-3 in phagocytosis by macrophages

Galectin-3 is a member of a large family of animal lectins. This protein is expressed abundantly by macrophages, but its function in this cell type is not well understood. We have studied the effect of galectin-3 gene targeting on phagocytosis, a major function of macrophages. Compared with wild-type macrophages, galectin-3-deficient (gal3-/-) cells exhibited reduced phagocytosis of IgG-opsonized erythrocytes and apoptotic thymocytes in vitro. In addition, gal3-/- mice showed attenuated phagocytic clearance of apoptotic thymocytes by peritoneal macrophages in vivo. These mice also exhibited reduced IgG-mediated phagocytosis of erythrocytes by Kupffer cells in a murine model of autoimmune hemolytic anemia. Additional experiments indicate that extracellular galectin-3 does not contribute appreciably to the phagocytosis-promoting function of this protein. Confocal microscopic analysis of macrophages containing phagocytosed erythrocytes revealed localization of galectin-3 in phagocytic cups and phagosomes. Furthermore, gal3-/- macrophages exhibited a lower degree of actin rearrangement upon Fcgamma receptor crosslinkage. These results indicate that galectin-3 contributes to macrophage phagocytosis through an intracellular mechanism. Thus, galectin-3 may play an important role in both innate and adaptive immunity by contributing to phagocytic clearance of microorganisms and apoptotic cells.