Structure of baby hamster kidney carbohydrate-binding protein CBP30, an S-type animal lectin

Immunocytochemical evidence for a modulation of galectin 3 (Mac-2), a carbohydrate binding protein, in pulmonary fibrosis

Galectin 3 is endogenous mammalian carbohydrate-binding protein with affinity for terminal beta-galactose residues, polylactosamine glycans, and ABH-blood group carbohydrate epitopes. To determine the distribution and regulation of galectin 3 during pulmonary injury, which is known to be accompanied by profound changes in the carbohydrate moieties of cell surface glycoproteins of alveolar cells, a rat model of irradiation-induced lung inflammation and repair was used. Immunocytochemistry showed that in normal rat lungs, galectin 3 was localized to alveolar macrophages, with weaker staining of bronchial epithelial cells. Shortly after irradiation-induced lung injury, when there is active proliferation of type II alveolar epithelial cells and re-epithelialization of alveolar basement membranes by type I cells, the total galectin concentration in the lung increased dramatically. This increase was due in part to an increased population of galectin 3-positive interstitial and alveolar macrophages. In addition, galectin 3 was expressed prominently at the surface of the newly formed type I alveolar epithelium and to lesser extent at the apical surface of type II cells. These findings suggest that the increased synthesis and secretion of galectin 3 during irradiation-induced lung injury, together with ligation of secreted lectin at the surface of alveolar epithelial cells, may play roles in pulmonary alveolar epithelial expansion and differentiation during injury and repair.

Modulation of functional properties of galectin-3 by monoclonal antibodies binding to the non-lectin domains

Galectin-3 is a member of a newly defined family of animal lectins, which is composed of three domains: a small amino-terminal domain, a domain containing repeating elements, and a carboxyl-terminal domain containing the carbohydrate-recognition site. Various functions have been described or proposed for this lectin, and it appears that galectin-3 has diverse roles. Murine monoclonal antibodies (MAbs) have been generated from mice hyperimmunized with recombinant human galectin-3 or galectin-3C (the carboxyl-terminal domain), and seven MAbs have been characterized in detail. All MAbs generated against the intact galectin-3 recognize the amino-terminal region of the molecule, as demonstrated by ELISA and immunoblotting using recombinant galectin-3C and galectin-3NR, which contains the amino-terminal domain and all the repeating elements. Their epitopes were all found to be within the first 45 amino acids of galectin-3, as determined by using galectin-3 mutants with a truncated amino-terminal region. However, these MAbs were found to profoundly modulate the lectin activities of galectin-3. The MAb B2C10 inhibited (i) the binding of 125I-labeled galectin-3 to IgE coated on microtiter plates; (ii) the galectin-3's hemagglutination activity; and (iii) galectin-3-induced superoxide production by human neutrophils. Other MAbs, especially A3A12, caused marked potentiation of these activities. The results support our model that the lectin function of galectin-3 is influenced by protein homodimerization resulting from self-association of the amino-terminal region of the molecule. The potentiating activities of some MAbs are probably due to facilitation of dimerization galectin-3, and the inhibitory activity of MAb B2C10 is probably the result of its disruption of the self-association process.

Recognition at the cell surface: recent structural insights

In the past few years, structural biology has begun to reveal details of some of the receptors and associated interactions responsible for protein-mediated recognition events at the cell surface. Recent data span the fields of cytokine-receptor interactions, cell adhesion molecules and viral infection.

Evidence for IgG autoantibodies to galectin-3, a beta-galactoside-binding lectin (Mac-2, epsilon binding protein, or carbohydrate binding protein 35) in human serum

Galectin-3 is a beta-galactoside-binding animal lectin formerly called epsilon protein, Mac-2, carbohydrate binding protein 35, CBH 30, L-29, or L34. The possible occurrence of autoantibodies to galectin-3 was investigated because crosslinking of galectins bound to IgE or Fc epsilon RI might produce mediator release from mast cells or basophils. Unexpectedly, a control serum from an individual free of current allergic symptoms was found to have a significantly elevated level of IgG anti-galectin-3 by ELISA employing galectin-3-coated wells incubated with test serum followed by HRPO-conjugated goat anti-human IgG. The reaction was not inhibitable by lactose, suggesting that it is not a result of binding of IgG by galectin-3 through lectin-carbohydrate interactions. The antibody activity was specifically adsorbed by galectin-3 and protein A-conjugated Sepharose and was associated primarily with subclass IgG1. The presence of the antibodies was confirmed by immunoblotting showing binding of IgG to the 30-kD galectin-3 band. The relevant epitopes were in the galectin-3 N-terminal domain. The propositus was subsequently found to have adenocarcinoma of the colon, and titers of IgG anti-galectin-3 were found to be sharply elevated after hemicolectomy. Similar antibody titers have not been found in family members, but small numbers of normal persons and patients with malignant neoplasms have been found to have evidence of IgG anti-galectin-3 antibodies at lower titers than the propositus. The pathogenesis of this autoimmune reaction is unclear, though there is a trend for it to occur in older persons.

Galectin-3 expression and effects on cyst enlargement and tubulogenesis in kidney epithelial MDCK cells cultured in three-dimensional matrices in vitro

Galectin-3 is a member of a closely related family of beta-galactoside-binding soluble proteins found in many vertebrate epithelial and myeloid cell types. The developmentally regulated presence of galectin-3 in tissues, for example kidney, and an affinity for many cell-surface and matrix glycoproteins indicate its importance in extracellular biological processes. Since a polarised expression and secretion of galectin-3 was observed in monolayer-cultured MDCK cells, an understanding of the secretion and distribution of this lectin in a three-dimensional in vitro model would help to uncover its role(s) in the interplay between cell-surface adhesion molecules and extracellular matrix components occurring during cell aggregation and polarisation in tissue formation. In this study, the cellular distribution and secretion of galectin-3 were examined in MDCK cells cultured within a gel matrix. MDCK cells were cultured within type I collagen or Matrigel to obtain multicellular cysts, and tubule formation was induced in collagen gels with hepatocyte growth factor. Immunofluorescent staining of these structures using antibodies against galectin-3 and other cell-surface domain markers was carried out either in situ or on cryosections and was visualised by confocal and conventional epifluorescence microscopy. Our results show that MDCK cells suspended in hydrated collagen gels or Matrigel exhibit differential and polarised galectin-3 expression on the baso-lateral surface domains of cells lining the cysts. The lectin is colocalised with laminin on the basal surface. In tubule-forming cysts, galectin-3 is excluded from the initial spikes and the progressing tips of the tubules although its basolateral expression on the cyst body remains. Galectin-3 added exogenously to cultures, as well as antibodies against laminin subunits and integrin beta 1 subunit, exerted an inhibitory effect on cyst enlargement of MDCK cells in 3-D Matrigel while galectin-3-specific antibodies could promote this process. The results suggest that galectin-3 exerts its effect on MDCK cells in a three-dimensional environment through modulation of both cell-cell and cell-substratum adhesions, and the interplay between these adhesions is important in the growth of multicellular aggregates and extensions occurring during normal kidney tubulogenesis.

Cross-linking of galectin 3, a galactose-binding protein of mammalian cells, by tissue-type transglutaminase

The 30 kDa beta-galactoside-binding protein of baby hamster kidney (BHK) cells [Mehul et al. (1994), J. Biol. Chem. 269, 18250-18258] homologous to galectin 3, a widely distributed mammalian lectin, has been found to be a substrate for tissue type transglutaminase, as shown by the incorporation in a calcium- and time-dependent manner of 5-(biotinamido) pentylamine in the presence of guinea pig liver transglutaminase. The amino-terminal domain of hamster galectin 3, which is a repetitive sequence rich in glutamine, tyrosine, glycine and proline, is also an excellent substrate. A single lysine residue in the N-terminal domain is an essential requirement for transglutaminase-mediated oligomerization, and two equivalent glutamine residues present in identical sequence repeats within this domain appear to be involved as amine acceptors in cross-linking reactions. Transglutaminase-mediated cross-linking of galectin 3 to itself or to matrix components may be one mechanism for stabilisation of a multivalent binding form of the lectin in cell secretions or in extracellular matrices.