Galectin-3

Galectin-3...

• is the only member of the chimeric subfamily in mammals
• is a very well-studied glycan-binding protein (GBP)
• has a known crystal structure (C-terminal glycan-binding domain)
• has unique functions intra- and extra-cellularly, due to an unusual N-terminal domain that can participate in protein-protein interactions
• has a unique mode of multimerization
• is the only known anti-apoptotic galectin, and acts through intracellular action
• null mice have distinct phenotypes, including alterations in inflammatory and wound-healing responses, and cyst formation in disease
• has unique functions in innate immune response to microbial pathogens
• has been administered in animal models of disease to assess therapeutic potential
• binds distinct cell surface glycoprotein ligands in lymphocytes compared to galectin-1
• expression is involved in growth modulation

Galectin-3 is the only member of the galectin family with an extended N-terminal region composed of tandem repeats of short amino-acid segments (a total of approximately 120 amino acids) connected to a C-terminal CRD. Like other galectins, galectin-3 lacks a signal sequence required for secretion through the classical secretory pathway, but the protein is released into the extracellular space.

Galectin-3 can oligomerize in the presence of multivalent carbohydrate ligands and is capable of crosslinking glycans on the cell surface, thereby initiating transmembrane signaling events and affecting various cellular functions (reviewed in ). This ability to self-associate is dependent on the N-terminal region of the protein.

Compared to other galectins, intracellular functions of galectin-3 have been more extensively documented (reviewed in ). In some cases, intracellular proteins with which the protein interacts and which possibly mediate these functions have been identified. Galectin-3 can be phosphorylated at serines 6 & 12, and tyrosines 79 & 118 by c-Abl.

CFG Participating Investigators contributing to the understanding of this paradigm

CFG Participating Investigators (PIs) contributing to the understanding of Galectin-3 include: Linda Baum, Susan Bellis, Roger Chammas, Richard Cummings, James Dennis, Margaret, Huflejt, Fu-Tong Liu, Joshiah Ochieng, Noorjahan Panjawani, Mauro Perretti, Avram Raz, James Rini, Maria Roque-Barreira, Sachiko Sato, Tariq Sethi, Irma van Die, Gerardo Vasta, John Wang, Paul Winyard

Progress toward understanding this GBP paradigm

Biological roles of GBP-ligand interaction

Regulation of cellular responses.
Galectin-3 induces various kinds of biological responses in a variety cell types in vitro by engaging glycoproteins or glycolipids on the cell surfaces (reviewed in ).

Galectin-3 can form lattices with selected cell surface glycans, in which galectin-3 oligomers bind to glycans on different glycoproteins displayed on the cell surface. Through this mechanism, galectin-3 modulates the properties and responses of the glycoproteins, such as their lateral mobility on the cell surface, rate of endocytosis, and transmission of signals at the cell surface (reviewed in ).

Endogenous galectin-3 regulates cellular responses by functioning inside the cells, including pre-mRNA splicing, where galectin-3 functions as a component of spliceosomes, and regulation of expression of certain genes, including those for cyclin D1, thyroid-specific TTF-1 transcription factor, MUC2 mucin, and c-Jun N-terminal kinase (reviewed in ).

Endogenous Galectin-3 inhibits apoptosis in various cell types by functioning inside the cells (reviewed in ).

Endogenous Galectin-3 controls intracellular trafficking of glycoproteins, which may be linked to its ability to translocate into the lumen of transport vesicles. Intracellular galectin-3 is associated with centrosomes in epithelial cells transiently during the process of epithelial polarization and may thus regulate epithelial polarization in enterocytes. Galectin-3 contributes to maintenance of the barrier function of ocular surface epithelial cells.

Immunity and inflammation.

Functions demonstrated in vitro.
T and B cells
Endogenous galectin-3

• regulates differentiation of B cells into plasma cells and memory B cells
• is anti-apoptotic in B cell lines
  

In T cells, purified galectin-3

• induces IL-2 production and calcium influx in Jurkat T cells
• induces apoptosis in human T leukemic cell lines, human peripheral blood mononuclear cells, and mouse activated T cells, normal human T cells, and a human tumor infiltrating T cell line. In some T cell lines, such as MOLT-4 cells, galectin-3 induces phosphatidylserine exposure, an early event in apoptosis, but not cell death
• induces apoptosis in both Th1 and Th2 cells
• induces apoptosis in CD4-CD8- human thymocytes
• attenuates interaction of thymocytes with thymic nurse cells
  

Endogenous Galectin-3 has anti-apoptotic activity in the human T cell line Jurkat.

Galectin-3 has also been documented in the following T cell functions:

• binds to Mgat5-modified T cell receptor (TCR) and suppresses T cell activation induced by TCR engagement; this is associated with a decrease in lateral mobility of TCR
• attenuates association of CD8 and TCR on CD8+ tumor-infiltrating lymphocytes, thus causing anergy
• negatively regulates TCR-mediated CD4+ T cell activation at the immunological synapse, by intracellular action

Dendritic cells
Endogenous Galectin-3

• suppresses the production of IL-12 by dendritic cells and may suppress Th1 responses
• promotes Th2 polarization in the setting of antigen presentation to T cells by dendritic cells. Another study suggests that galectin-3 suppresses the antigen-presenting function of dendritic cells)
• promotes dendritic cell trafficking by functioning intracellularly

Galectin-3 promotes adhesion of mouse dendritic cells.

Neutrophils
Galectin-3 acts on these cells in the following manner:

• induces oxidative burst and L-selectin shedding as well as IL-8 production
• promotes neutrophil adhesion to the extracellular protein laminin and endothelial cells
• induces phosphatidylserine exposure in the absence of cell death, and induces apoptosis

Endogenous galectin-3 protects neutrophils from apoptosis.

Macrophages
Endogenous galectin-3

• is anti-apoptotic in macrophages treated with LPS and IFN-γ. It plays a critical role in the phagocytic function of macrophages in ingesting opsonized sheep red blood cells and apoptotic thymocytes.
  
• plays a critical role in alternative macrophage activation

Recombinant galectin-3

• triggers human peripheral blood monocytes to produce superoxide anion and potentiates LPS-induced IL-1 production
• functions as a chemoattractant for monocytes and macrophages
• is an opsonin and enhances the macrophage clearance of apoptotic neutrophils
• activates microglia (tissue macrophages of the central nervous system) to phagocytose degenerated myelin mediated by complement receptor-3 and scavenger receptor
  
• binds to a major xenoantigen, α-Gal [Galα(1,3)Galβ(1,4)GlcNAc], expressed on porcine endothelial cells and mediates adhesion of human monocytes to porcine endothelial cells
• suppresses LPS-induced production of inflammatory cytokines by macrophages, including IL-6, IL-12, and TNF-α
  

Mast cells
Galectin-3 induces mediator release from both IgE-sensitized and nonsensitized mast cells, but apoptosis following prolonged treatment (18-44 h). Endogenous Galectin-3 is a positive regulator of mast cell mediator release and cytokine production.

Eosinophils
Recombinant galectin-3

• suppresses IL-5 production by human eosinophils
• mediates rolling and adhesion of eosinophils on immobilized VCAM-1 under conditions of flow.
  
  

Functions demonstrated in vivo.

A number of biological functions have been identified by using Lgals3-/- mice. With respect to acute inflammation and allergic inflammation galectin-3:

1. has a proinflammatory role in acute inflammation induced by intraperitoneal injection of thioglycollate broth, in terms of the neutrophil response, and macrophage response
2. promotes allergic airway inflammation, airway hyperresponsiveness, and a Th2 response in a mouse model of asthma in which mice are sensitized with ovalbumin systemically and challenged with the same antigen through the airways
3. promotes allergic skin inflammation and a systemic Th2 response in a model of atopic dermatitis, in which mice are repeatedly sensitized with ovalbumin epicutaneously
4. promotes allergic contact hypersensitivity, in which mice are sensitized with the hapten oxazalone, and then challenged with the same hapten at another skin site (38)

However, rats and mice treated by intranasal delivery of cDNA encoding Galectin-3 showed reduced eosinophil infiltration following airway antigen challenge.

With regard to autoimmunity, galectin-3

• contributes to the disease severity in a mouse model of autoimmune encephalomyelitis (EAE) induced by immunization with a myelin oligodendrocyte glycoprotein peptide
• suppresses the development of glomerulopathy in mice rendered diabetic with streptozotocin, associated with lower accumulation of advanced glycation end products (AGE) in the kidneys
• may serve as an AGE receptor and protects from AGE-induced tissue injury and age-dependent changes
• contributes to development of diabetes induced by multiple low doses of streptozotocin; this may be related to its upregulation of TNF-&alpha: and nitric oxide production by macrophages
• contributes to ischemia and neovascularization in retina in a mouse model of oxygen-induced proliferative retinopathy after perfusion of preformed AGEs
• is expressed in foam cells and macrophages in atherosclerotic lesions and contributes to the development of atherosclerosis in apolipoprotein (Apo)E-deficient mice
  
  

Infectious processes.

The roles of galectin-3 in a large number of mouse models of infectious disease have been studied in Lgals3-/- mice, as follows:

1. suppresses LPS-induced shock accompanied by lower inflammatory cytokine and nitric oxide production, possibly a result of its ability to bind to this endotoxin. However, it enhances sensitivity to Salmonella infection
2. contributes to recruitment of neutrophils to lungs of mice infected with S. pneumoniae and has a protective role in development of pneumonia after the infection, possibly by augmenting the function of neutrophils
3. contributes to inflammatory responses in intestines, liver, and brain (but not in lungs) and a lower systemic Th1-polarized response in mice infected by Toxoplasma gondii; galectin-3 suppresses parasite burden in the brain
4. promotes development of T and B responses in the spleen, as well formation of liver granulomas, but suppresses the Th1-polarized response in mice infected with Schistosoma mansoni
5. contributes to sensitivity in lethal effects of Rhodococcus equi, a facultative intracellular bacterium of macrophages. It suppresses inflammatory responses, including production of the Th1 cytokines IL-12 and IFN-γ, as well as IL-1β
   
6. promotes resistance of mice to infection by “Paracoccidioides brasiliensis” and favors a Th1-polarized immune response
   

Interestingly, recombinant Galectin-3 is able to induce cell death in the yeast Candida albicans in vitro.

Tumor development/progression.
Galectin-3 expression is altered in a variety of tumors in comparison to normal tissues. The diagnostic utility of Galectin-3 expression in thyroid cancer has been extensively demonstrated (e.g., ). The role of Galectin-3 in tumor growth, progression, and metastasis has been comprehensively documented (reviewed in ). There is evidence that Galectin-3 expression is necessary for the initiation of the transformed phenotype of tumors, possibly related to its ability to interact with oncogenic K-Ras.

The most extensively studied function of Galectin-3 is its inhibition of apoptosis in a range of tumor cell types exposed to diverse apoptotic stimuli (reviewed in (13)). The mechanism by which Galectin-3 inhibits apoptosis in tumor cells has been extensively studied (1, 82). Apoptosis induced by the tumor suppressor p53 involves repression of Galectin-3 (83).

Endogenous galectin-3 promotes tumor cell growth (reviewed in (1)), one mechanism may involve interaction with transcription factors (84), another may be facilitation of the signaling of K-Ras to Raf and PI3 kinase (85). Endogenous galectin-3 also regulates tumor progression by influencing cell cycling; its binds to β-catenin and stimulates the expression of cyclin D and c-Myc (86).

Galectin-3 can affect tumor metastasis by exerting its effect in the tumor microenvironment, including angiogenesis and fibrosis (1). Galectin-3 plays a role in activation of myofibroblasts in the liver and contributes to liver fibrosis induced by carbon tetrachloride (87).

In a human melanoma tumor model in immunodeficient mice, administration of galectin-3 results in suppressing the tumor killing effect of tumor-reactive T cells (28). Tumor-associated galectin-3 may also contribute to tumor immune escape by rendering tumor-infiltrating cytolytic lymphocytes anergic (33).

Galectin-3 affects the motility of tumor cells and influences their invasiveness in vitro. However, both positive and negative effects have been reported (88, 89). Endogenous galectin-3 can also contribute to cell motility and in vitro invasiveness (90, 91, 92). Galectin-3 has angiogenic activity, which may be related to its ability to induce migration of endothelial cells (93).

Studies with animal models have provided evidence for the role of galectins in tumor metastasis in vivo (reviewed in (1)). For example, liver metastases of human adenocarcinoma xenotransplants in SCID mice are inhibitable by anti-galectin-3 antibody. Breast carcinoma cells overexpressing transgenic galectin-3 have higher metastatic potential. In an orthotopic nude mouse model of human breast cancer, tumor metastasis is inhibitable by C-terminal domain fragment of galectin-3 (galectin-3C) (94).

Galectin-3 contributes to chemotherapeutic resistance of thyroid cancer cells in vitro [51], the progression of disease in prostate cancer (95) and development of carcinogen-induced lung tumorigenesis (96) in mouse models. However, the absence of galectin-3 may not affect the evolution of cancers (97). Galectin-3-targeting small molecule inhibitors enhancs apoptosis induced by chemo- and radio-therapy in papillary thyroid cancer in vitro (98). GCS-100, a galectin-3 antagonist, induces myeloma cell death in vitro (99).

CFG resources used in investigations

The best examples of CFG contributions to this paradigm are described below, with links to specific data sets. For a complete list of CFG data and resources relating to this paradigm, see the CFG database search results for Galectin-3.

Knockout mouse lines

Galectin-3 knockout mice were phenotyped by the CFG and continue to be used by investigators to study the biological functions of Galectin-3.

Glycan array

Investigators have used CFG carbohydrate compounds and glycan arrays to study ligand binding specificity of Galectin-3 (for example, click here). To see all glycan array results for Galectin-3, click here.

Related GBPs

Structure unique among galectins in mammals; homologues in vertebrates & invertebrates.

Acknowledgements

The CFG is grateful to the following PIs for their contributions to this wiki page: Linda Baum, Richard Cummings, Michael Demetriou, Fu-Tong Liu