Regulation of secretion and surface expression of Mac-2, a galactoside-binding protein of macrophages

Galectin-3: presurgical marker of thyroid follicular epithelial cell-derived carcinomas

Preoperative follicular lesion characterisation represents an unsolved diagnostic problem in thyroid nodular disease. Although fine-needle aspiration biopsy is the most reliable preoperative diagnostic procedure, it shows inherent limitations in differentiating adenoma from follicular carcinoma and, sometimes, follicular variants of papillary carcinoma. Galectin-3 cytoplasmic neoexpression has been proposed as a peculiar feature of thyroid malignant cells, easily detectable in cytological and histological samples. The aim of this study was to re-evaluate the galectin-3 expression in a large sample of thyroid lesions using an immunohistocytochemical biotin-free detection system and a specific anti-human-galectin-3 monoclonal antibody in order to avoid the interference of technical factors, a cause of conflicting results recently reported by some authors. We analysed galectin-3 expression of 39 follicular carcinomas, 26 papillary carcinomas, and 105 adenomas in both cell-block samples and their histological counterparts. All cell-block and histological papillary carcinoma samples showed high levels of galectin-3 immunoreactivity. Thirty-four follicular carcinomas were positive, whereas 5 were negative in cell-blocks but positive in their histological counterparts. Twelve out of 105 adenomas expressed galectin-3 in cell-blocks and histological samples. The diagnostic accuracy of preoperative galectin-3 evaluation in adenomas vs follicular carcinomas was 90.0%. Galectin-3 expression was also investigated in 22 minimally-invasive follicular carcinomas. All of them showed galectin-3 immunoreactivity in both cytological and histological specimens with the exception of two cases, where galectin-3 positivity was observed only in the surgical material. The routine correct use of galectin-3, by increasing the diagnostic accuracy of conventional cytology, improves the management of thyroid nodules and can lead to a sensitive reduction of useless thyroid surgeries.

Galectin-3: differential expression between small-cell and non-small-cell lung cancer

AIMS

To compare the histological expression of galectin-3 in different lung cancers, including small-cell lung cancer (SCLC) and non-small-cell lung cancer (NSCLC). Lung cancer is the leading cause of cancer deaths in the UK. Galectin-3 is a beta-galactoside binding protein with a controversial role in malignant transformation. SCLC metastasizes early and is initially chemosensitive; NSCLC metastasizes later, offering the chance of surgical cure, but is much less chemosensitive. Mixed tumours present a diagnostic and therapeutic problem, with a poorer response to therapy. Insight into the cellular mechanisms that govern metastasis and chemoresistance will profoundly influence the future management of this disease.

METHODS AND RESULTS

In this study the histological expression of galectin-3 was assessed in a panel of lung tumour specimens, using the indirect streptavidin-biotin method. A striking difference in galectin-3 expression was observed between tumours, with high expression in NSCLC (42/47 samples) and low expression in SCLC (negative in 13/18, weak in 5/18).

CONCLUSION

This differential expression of galectin-3 between histological types of lung carcinoma suggests that galectin-3 may have an important influence on tumour cell adhesion, apoptosis and the response of tumours to chemotherapy.

Stimulation of proliferation of rat hepatic stellate cells by galectin-1 and galectin-3 through different intracellular signaling pathways

We found that the expression of galectin-1 and galectin-3 was significantly up-regulated in hepatic stellate cells (HSCs) both in the course of their transdifferentiation into myofibroblasts, a process of "self-activation," and in the fibrosis of liver tissues. Recombinant galectin-1 and galectin-3 stimulated the proliferation of cultured HSCs via the MEK1/2-ERK1/2 signaling pathway. However, galectin-3 utilized protein kinases C and A to induce this process, whereas galectin-1 did not. We also found that thiodigalactoside, a potent inhibitor of beta-galactoside binding, attenuated the effects of both galectins. In addition, galectin-1, but not galectin-3, promoted the migration of HSCs. Thus, it appears that galectin-1 and galectin-3, generated by activated HSCs, could participate in beta-galactoside binding and induce different intracellular signaling pathways leading to the proliferation of HSCs.

Expression of galectin-3 in skeletal tissues is controlled by Runx2

The beta-galatoside-specific lectin galectin-3 is expressed in vivo in osteoblasts as well as in epiphyseal cartilage. Here we show that in vitro, galectin-3 expression is up-regulated in the preosteoblastic cell line MC3T3-E1 during the matrix maturation stage of the osteoblast developmental sequence. Expression persists into late differentiation stages when the mature osteoblastic phenotype is established. The skeletal expression pattern of galectin-3 overlaps at many sites with that of the transcription factor Runx2. Runx2 is a key regulator of osteoblast development and necessary for chondrocyte differentiation in the growth plate. Both human and mouse galectin-3 promoters contain putative Runx-binding sites. The constitutive or inducible forced expression of Runx2 is sufficient for the onset of galectin-3 transcription in the mesenchymal precursor cell line C3H10T1/2. Moreover, Runx2 is able to bind to at least two sites in the galectin-3 promoter region. The crucial role of Runx2 was confirmed in Runx2-deficient mice, which are devoid of galectin-3 expression in skeletal cells. The overlapping expression pattern of galectin-3 with the other two members of the Runt family of transcription factors (Runx1 and Runx3) points to a potential regulation of the galectin-3 gene (LGALS3) by these factors in hematopoietic, skin, and dorsal root ganglial cells.

Specific recognition and cleavage of galectin-3 by Leishmania major through species-specific polygalactose epitope

Lipophosphoglycan is a major surface molecule of Leishmania, protozoa parasites, which are the causative agents of leishmaniasis, a disease that annually afflicts millions of people worldwide. The oligosaccharide structures of lipophosphoglycan varies among species, and epitopes of these species-specific oligosaccharides are suggested to be implicated in the interaction of Leishmania with macrophages as well as species-specific tissue tropism observed in leishmaniasis. The recognition of the species-specific variation of oligosaccharides is likely to be mediated by host carbohydrate-binding proteins, lectins, but the identities of the lectins remain elusive. Galectin-3 is a mammalian soluble beta-galactoside-binding lectin and is expressed in macrophages, dendritic cells, and keratinocytes, as well as fibroblasts, all of which are present in the site of Leishmania infection. In this paper, we found that galectin-3 binds to lipophosphoglycan of Leishmania major but not to those of Leishmania donovani through L. major-specific polygalactose epitopes. Association of galectin-3 with L. major led to the cleavage of galectin-3, resulting in truncated galectin-3 containing the C-terminal lectin domain but lacking the N-terminal domain implicated in lectin oligomerization. This cleavage was inhibited by the galectin-3 antagonist lactose, as well as 1,10-ortho-phenanthroline, suggesting that galectin-3 is cleaved by zinc metalloproteases after its binding to lipophosphoglycans. The modulation of various innate immunity reactions by galectin-3 is affected by its oligomerization; therefore, we propose the L. major-specific truncation of galectin-3 may contribute to the species-specific immune responses induced by Leishmania.

Galectin-3 translocates to the perinuclear membranes and inhibits cytochrome c release from the mitochondria. A role for synexin in galectin-3 translocation

Galectin-3 is a multifunctional oncogenic protein found in the nucleus and cytoplasm and also the extracellular milieu. Although recent studies demonstrated an anti-apoptotic activity of galectin-3, neither the functional site nor the mechanism of how galectin-3 regulates apoptosis is known. In this study, we examined the subcellular localization of galectin-3 during apoptosis and investigated its anti-apoptotic actions. We report that galectin-3 translocates to the perinuclear membrane following a variety of apoptotic stimuli. Confocal microscopy and biochemical analysis revealed that galectin-3 is enriched in the mitochondria and prevents mitochondrial damage and cytochrome c release. Using a yeast two-hybrid system, we screened for galectin-3-interacting proteins that regulate galectin-3 localization and anti-apoptotic activity. Synexin, a Ca(2+)- and phospholipid-binding protein, was one of the proteins identified. We confirmed direct interaction between galectin-3 and synexin by glutathione S-transferase pull-down assay in vitro. We showed that galectin-3 failed to translocate to the perinuclear membranes when expression of synexin was down-regulated using an oligodeoxyribonucleotide complementary to the synexin mRNA, suggesting a role for synexin in galectin-3 trafficking. Furthermore, synexin down-regulation abolished anti-apoptotic activity of galectin-3. Taken together, these results suggest that synexin mediates galectin-3 translocation to the perinuclear mitochondrial membranes, where it regulates mitochondrial integrity critical for apoptosis regulation.

Association of a macrophage galactoside-binding protein with Mycobacterium-containing phagosomes

Mycobacteria reside intracellularly in a vacuole that allows it to circumvent the antimicrobial environment of the host macrophage. Although the mycobacterial phagosome exhibits selective fusion with vesicles of the endosomal system, identification of host and bacterial factors associated with phagosome bio-genesis is limited. To identify these potential factors, mAbs were generated to a membrane preparation of mycobacterial phagosomes isolated from M. tuberculosis-infected macrophages. A mAb recognizing a 32-35 kDa macrophage protein associated with the phagosomal membrane of Mycobacterium was identified. N-terminal sequence analysis identified this protein as Mac-2 or galectin-3, a galactoside-binding protein of macrophages. Galectin-3 (gal-3) was shown to accumulate in Mycobacterium-containing phagosomes during the course of infection. This accumulation was specific for phagosomes containing live mycobacteria and occurred primarily at the cytosolic face of the phagosome membrane. In addition, bind-ing of gal-3 to mycobacterial phosphatidylinositol mannosides (PIMs) demonstrated a novel interaction between host carbohydrate-binding proteins and released mycobacterial glycolipids. Infection of macrophages from gal-3-deficient mice indicated that the protein did not play a role in infection in vitro. In contrast, infection of gal-3-deficient mice revealed a reduced capacity to clear late but not early infection.

Role of galectin-3 as an adhesion molecule for neutrophil extravasation during streptococcal pneumonia

Recruitment of neutrophils from blood vessels to sites of infection represents one of the most important elements of innate immunity. Movement of neutrophils across blood vessel walls to the site of infection first requires that the migrating cells firmly attach to the endothelial wall. Generally, neutrophil extravasation is mediated at least in part by two classes of adhesion molecules, beta(2) integrins and selectins. However, in the case of streptococcal pneumonia, recent studies have revealed that a significant proportion of neutrophil diapedesis is not mediated by the beta(2) integrin/selectin paradigm. Galectin-3 is a beta-galactoside-binding lectin implicated in inflammatory responses as well as in cell adhesion. Using an in vivo streptococcal pneumonia mouse model, we found that accumulation of galectin-3 in the alveolar space of streptococcus-infected lungs correlates closely with the onset of neutrophil extravasation. Furthermore, immunohistological analysis of infected lung tissue revealed the presence of galectin-3 in the lung tissue areas composed of epithelial and endothelial cell layers as well as of interstitial spaces. In vitro, galectin-3 was able to promote neutrophil adhesion to endothelial cells. Promotion of neutrophil adhesion by galectin-3 appeared to result from direct cross-linking of neutrophils to the endothelium and was dependent on galectin-3 oligomerization. Together, these results suggest that galectin-3 acts as an adhesion molecule that can mediate neutrophil adhesion to endothelial cells. However, accumulation of galectin-3 in lung was not observed during neutrophil emigration into alveoli induced by Escherichia coli infection, where the majority of neutrophil emigration is known to be beta(2) integrin dependent. Thus, based on our results, we propose that galectin-3 plays a role in beta(2) integrin-independent neutrophil extravasation, which occurs during alveolar infection with Streptococcus pneumoniae.

Heteromeric amino acid transporters: biochemistry, genetics, and physiology

The heteromeric amino acid transporters (HATs) are composed of two polypeptides: a heavy subunit (HSHAT) and a light subunit (LSHAT) linked by a disulfide bridge. HSHATs are N-glycosylated type II membrane glycoproteins, whereas LSHATs are nonglycosylated polytopic membrane proteins. The HSHATs have been known since 1992, and the LSHATs have been described in the last three years. HATs represent several of the classic mammalian amino acid transport systems (e.g., L isoforms, y(+)L isoforms, asc, x(c)(-), and b(0,+)). Members of the HAT family are the molecular bases of inherited primary aminoacidurias cystinuria and lysinuric protein intolerance. In addition to the role in amino acid transport, one HSHAT [the heavy subunit of the cell-surface antigen 4F2 (also named CD98)] is involved in other cell functions that might be related to integrin activation. This review covers the biochemistry, human genetics, and cell physiology of HATs, including the multifunctional character of CD98.

Regulated expression and effect of galectin-1 on Trypanosoma cruzi-infected macrophages: modulation of microbicidal activity and survival

Galectin-1 is a beta-galactoside-binding protein with potent anti-inflammatory and immunoregulatory effects. However, its expression and function have not been assessed in the context of an infectious disease. The present study documents, for the first time, the regulated expression of galectin-1 in the context of an infectious process and its influence in the modulation of macrophage microbicidal activity and survival. A biphasic modulation in parasite replication and cell viability was observed when macrophages isolated from Trypanosoma cruzi-infected mice were exposed to increasing concentrations of galectin-1. While low concentrations of this protein increased parasite replication and did not affect macrophage survival, higher inflammatory doses of galectin-1 were able to commit cells to apoptosis and inhibited parasite replication. Furthermore, galectin-1 at its lowest concentration was able to down-regulate critical mediators for parasite killing, such as interleukin 12 (IL-12) and nitric oxide, while it did not affect IL-10 secretion. Finally, endogenous galectin-1 was found to be up-regulated and secreted by the J774 macrophage cell line cultured in the presence of trypomastigotes. This result was extended in vivo by Western blot analysis, flow cytometry, and reverse transcription-PCR using macrophages isolated from T. cruzi-infected mice. This study documents the first association between galectin-1's immunoregulatory properties and its role in infection and provides new clues to the understanding of the mechanisms implicated in host-parasite interactions during Chagas' disease and other parasite infections.

Negative regulation of neuroblastoma cell growth by carbohydrate-dependent surface binding of galectin-1 and functional divergence from galectin-3

The cell density-dependent growth inhibition of human SK-N-MC neuroblastoma cells is initiated by increased ganglioside sialidase activity leading to elevated cell surface presentation of ganglioside GM1, a ligand of galectin-1. We herein show that the extent of the cell surface expression of the galectin coincides with marked increases of the sialidase activity. Reverse transcriptase-polymerase chain reaction analysis excludes a regulation at the transcriptional level. Exposure of cells to purified galectin-1 reveals its carbohydrate-dependent activity to reduce cell proliferation. Assays to detect DNA fragmentation biochemically and cytometrically and to block caspases render it unlikely that galectin-1 acts as a classical proapoptotic factor on these cells. Because the chimeric galectin-3 shares binding sites and binding parameters with galectin-1 for these cells, we tested whether this galectin will elicit the same response as the homodimeric cross-linking galectin-1. Evidently, galectin-3 fails to affect cell growth by itself but interferes with galectin-1 upon coincubation. Its proteolytically truncated variant, the C-terminal lectin domain with impaired capacity to form aggregates when surface bound, has only weak binding properties. Thus, the way in which the galectin-1 interacts topologically with an apparently common set of ligands relative to galectin-3 is crucial for eliciting post-binding events. We conclude that galectin-1 is a probable effector in the sialidase-dependent growth control in this system. Moreover, the experiments with galectin-3 reveal functional divergence, most probably based on different topologies of presentation of homologous carbohydrate-binding sites.

Galectins as modulators of cell adhesion

The galectins are a family of carbohydrate-binding proteins that are distributed widely in metazoan organisms. Each galectin exhibits a specific pattern of expression in various cells and tissues, and expression is often closely regulated during development. Although these proteins are found mainly in the cell cytoplasm, some are secreted from cells and interact with appropriately glycosylated proteins at the cell surface or within the extracellular matrix. These receptors include cell-adhesion molecules such as integrins, and matrix glycoproteins such as laminin and fibronectin isoforms. Recent studies have increased understanding of the roles of the galectins in regulating cell-cell and cell-matrix adhesion. These interactions are critically involved in modulation of normal cellular motility and polarity and during tissue formation, and loss of adhesive function is implicated in several disease states including tumour progression, inflammation and cystic development in branching epithelia such as kidney tubules. This review discusses recent progress in defining the specificities and mechanisms of action of secreted galectins as multifunctional cell regulators.

Myelin antigen-specific CD8+ T cells are encephalitogenic and produce severe disease in C57BL/6 mice

Encephalitogenic T cells that mediate experimental autoimmune encephalomyelitis (EAE) are commonly assumed to be exclusively CD4+, but formal proof is still lacking. In this study, we report that synthetic peptides 35-55 from myelin oligodendrocyte glycoprotein (pMOG(35-55)) consistently activate a high proportion of CD8+ alphabetaTCR+ T cells that are encephalitogenic in C57BL/6 (B6) mice. The encephalitogenic potential of CD8+ MOG-specific T cells was established by adoptive transfer of CD8-enriched MOG-specific T cells. These cells induced a much more severe and permanent disease than disease actively induced by immunization with pMOG(35-55). CNS lesions in pMOG(35-55) CD8+ T cell-induced EAE were progressive and more destructive. The CD8+ T cells were strongly pathogenic in syngeneic B6 and RAG-1(-/-) mice, but not in isogeneic beta2-microglobulin-deficient mice. MOG-specific CD8+ T cells could be repeatedly reisolated for up to 287 days from recipient B6 or RAG-1(-/-) mice in which disease was induced adoptively with <1 x 10(6) T cells sensitized to pMOG(35-55). It is postulated that MOG induces a relapsing and/or progressive pattern of EAE by eliciting a T cell response dominated by CD8+ autoreactive T cells. Such cells appear to have an enhanced tissue-damaging effect and persist in the animal for long periods.

The role of galectin-3 in endocytosis of advanced glycation end products and modified low density lipoproteins

Galectin-3, a member of beta-galactoside-binding lectin family, is suggested to be an AGE-receptor. To examine this possibility, we prepared CHO cells overexpressing human galectin-3 (galectin-3-CHO cells). Galectin-3-CHO cells showed a specific and saturable binding to (125)I-AGE-BSA with Kd of 3.1 microg/ml. (125)I-AGE-BSA was endocytosed by galectin-3-CHO cells and underwent lysosomal degradation. The endocytosis of (125)I-AGE-BSA was inhibited not only by unlabeled AGE-BSA but also by acetylated LDL and oxidized LDL, ligands for the scavenger receptor family. Furthermore, (125)I-oxidized LDL and (125)I-acetylated LDL were actively endocytosed by galectin-3-CHO cells and the incubation with acetyl-LDL led to intracellular accumulation of cholesteryl esters, indicating the role of galectin-3 in endocytosis of AGE-proteins and modified LDLs. Since galectin-3 was localized and up-regulated in foam cells at human atherosclerotic lesions, the present results suggest that galectin-3 plays an important role in formation of atherosclerotic lesions in vivo, by modulating endocytic uptake of AGE-proteins and modified LDLs.

Lipopolysaccharide-induced gelatinase granule mobilization primes neutrophils for activation by galectin-3 and formylmethionyl-Leu-Phe

We have earlier shown that galectin-3, a lactose-binding mammalian lectin that is secreted from activated macrophages, basophils, and mast cells, induces activation of the NADPH oxidase in exudated but not in peripheral blood neutrophils (A. Karlsson, P. Follin, H. Leffler, and C. Dahlgren, Blood 91:3430-3438, 1998). The alteration in responsiveness occurring during extravasation correlated with mobilization of the gelatinase and/or specific granules to the cell surface, indicating a role for mobilizable galectin-3 receptors. In this study we have investigated galectin-3-induced NADPH oxidase activation, measured as superoxide production, in lipopolysaccharide (LPS)-primed neutrophils. Upon galectin-3 challenge, the LPS-primed cells produced superoxide, both extracellularly and intracellularly. A primed extracellular response to formylmethionyl-Leu-Phe (fMLF) was also achieved. The exposure of complement receptors 1 and 3 as well as the formyl peptide receptor on the cell surface was markedly increased after LPS treatment, indicating that granule fusion with the plasma membrane had occurred. Further assessment of specific markers for neutrophil granules showed that the LPS treatment had mobilized the gelatinase granules but only a minor fraction of the specific granules. We thus suggest that the mechanism behind LPS priming lies at the level of granule (receptor) mobilization for galectin-3 as well as for fMLF.

Induction of terminal differentiation in epithelial cells requires polymerization of hensin by galectin 3

During terminal differentiation, epithelia become columnar and develop specialized apical membrane structures (microvilli) and functions (regulated endocytosis and exocytosis). Using a clonal intercalated epithelial cell line, we found that high seeding density induced these characteristics, whereas low density seeding maintained a protoepithelial state. When cells were plated at low density, but on the extracellular matrix of high density cells, they converted to the more differentiated phenotype. The extracellular matrix (ECM) protein responsible for this activity was purified and found to be a large 230-kD protein, which we termed hensin. High density seeding caused hensin to be polymerized and deposited in the extracellular matrix, and only this form of hensin was able to induce terminal differentiation. Antibodies to hensin blocked the change in phenotype. However, its purification to homogeneity resulted in loss of activity, suggesting that an additional protein might be necessary for induction of terminal differentiation. Here, we found that a 29-kD protein specifically associates with hensin in the ECM. Addition of purified p29 restored the activity of homogenously purified hensin. Mass fingerprinting identified p29 as galectin 3. Purified recombinant galectin 3 was able to bind to hensin and to polymerize it in vitro. Seeding cells at high density induced secretion of galectin 3 into the ECM where it bundled hensin. Hence, the high density state causes a secretion of a protein that acts on another ECM protein to allow the new complex to signal the cell to change its phenotype. This is a new mechanism of inside-out signaling.

Human galectin-3 is a novel chemoattractant for monocytes and macrophages

Galectin-3 is a beta-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 microM) 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-1alpha, and stromal cell-derived factor-1alpha. 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.

Alteration of the cytoplasmic/nuclear expression pattern of galectin-3 correlates with prostate carcinoma progression

Galectin-3, a member of the beta-galactoside-binding lectin family, is involved in a variety of biological events including interactions with galactose-containing glycoconjugates, cell proliferation, differentiation and apoptosis. Galectin-3 appears to intervene during tumor progression and altered expression patterns have been reported in a variety of malignancies. In our study, we have examined the expression of galectin-3 in a population of 145 prostate carcinoma samples using immunohistochemistry. We found that most of the non-tumoral prostatic glands exhibited moderate immunostaining for galectin-3 localized in both nucleus and cytoplasm. In prostatic cancer cells, galectin-3 was usually not expressed or decreased compared with the normal glands. Interestingly, when galectin-3 was detected in the cancer cells, it was consistently excluded from the nucleus and only present in the cytoplasmic compartment. The latter observation was also made for prostatic intraepithelial neoplasia (PIN) cells. Furthermore, we found that the levels of galectin-3 expression in the cancer cells were significantly associated with prostate-specific antigen (PSA) relapse in univariate analysis (p = 0.044). Cytoplasmic expression of galectin-3 in the carcinoma cells was an independent predictor of disease progression in multivariate analysis, after the pathological stage and the Gleason score. Our data demonstrate that galectin-3 is generally down-regulated in human prostate carcinoma cells, and consistently excluded from the nucleus. Interestingly, specific cytoplasmic expression of galectin-3 in a subset of lesions is associated with disease progression. These results suggest that galectin-3 might play anti-tumor activities when present in the nucleus, whereas it could favor tumor progression when expressed in the cytoplasm. Further studies should determine the exact role and mechanisms by which galectin-3 differentially affects cell behavior in the different locations where it is expressed.

Galectin-3 expression in various thyroid neoplasms and its possible role in metastasis formation

Galectin-3 is a member of the beta-galactoside-binding protein family that plays an important role in cell-cell adhesion and in cell-matrix interaction. We have examined the expression of galectin-3 in normal, adenomatous, and malignant thyroid tissues and also in metastatic lesions. Galectin-3 was rarely expressed in normal thyroid tissue but was abundant in the cytoplasm of the neoplastic lesions. Among neoplastic lesions, galectin-3 was expressed to a greater extent in follicular carcinomas than in follicular adenomas and was present in greater amounts in papillary carcinomas than in follicular adenomas or carcinomas. Primary lesions of papillary carcinoma with metastasis contained significantly higher concentrations of galectin-3 than tumors of this type without metastases. However, the expression of galectin-3 was significantly decreased in metastatic lesions in the lymph nodes compared with their primary lesions. From these results, we assumed that galectin-3 works in different ways at different stages of thyroid neoplasm proliferation. Among primary tumors, galectin-3 expression is significantly different in 3 histological types. However, the continuity of progression among these tumors is not yet proven. In later stages, decreased expression of galectin-3 may aid the release of cancer cells from the primary lesions for invasion and metastasis.

Interaction between galectin-3 and FcgammaRII induces down-regulation of IL-5 gene: implication of the promoter sequence IL-5REIII

Our previous work demonstrated the capacity of galectin-3 (a beta-galactoside binding animal lectin) to inhibit IL-5 gene expression in different cell types, but the interaction of lectin with the cells and the pathways for the inhibition process are unknown. One of the purposes of this work was to study the cellular ligand for galectin-3. We have demonstrated that galectin-3 can bind to the low affinity IgG receptor (FcgammaRII or CD32) by using different experimental approaches, such as flow cytometry, fusion protein GST technology, and with a model of FcgammaRII-deficient mice. To further analyze the interaction between FcgammaRII and galectin-3, and its implication in IL-5 gene down-regulation we used FcgammaRII-deficient mice. When PBMC from these mice were incubated with galectin-3, the expression of the IL-5 gene was unchanged. However, when PBMC from wild type mice and FcgammaRIII-deficient mice were incubated with galectin-3, IL-5 gene expression was down-regulated. Finally, we studied the implication of the negative regulatory sequence in the IL-5 gene promoter. In the presence of galectin-3, a DNA-protein complex was formed with the IL-5REIII region. This complex was not observed when unrelated oligonucleotide was used. So, galectin-3 induces a pathway, which activates a transcription factor that binds to IL-5REIII. This interaction is capable of inhibiting IL-5 gene transcription.

Secretion of the galectin family of mammalian carbohydrate-binding proteins

Galectins are cytosolic proteins that lack any signal sequence for transport into the endoplasmic reticulum and are not glycosylated, although several galectins contain consensus sites for N-glycosylation, indicating that these proteins do not traverse the ER-Golgi network. However, there is abundant evidence for the extracellular localisation of some galectins at cell surfaces, in the extracellular matrix and in cell secretions consistent with other evidence for extracellular roles of galectins as modulators of cell adhesion and signalling. How then are galectins secreted if not through the classical secretory pathway? Do all galectins share the same secretory pathway? Can a particular galectin utilise more than one secretory pathway? If galectins play important extracellular roles how is their secretion regulated in relation to function? These are still largely unanswered questions but recent studies are beginning to give glimpses into some novel aspects of the secretion of these intriguing proteins.

Identification of CD66a and CD66b as the Major Galectin-3 Receptor Candidates in Human Neutrophils

The mammalian lectin galectin-3 is a potent stimulus of human neutrophils, provided that the receptor(s) for the lectin has been mobilized to the cell surface before activation. We have recently shown that the receptors for galectin-3 are stored in intracellular mobilizable granules. Here we show supportive evidence for this in that DMSO-differentiated (neutrophil-like) HL-60 cells, which lack gelatinase and specific granules, are nonresponsive when exposed to galectin-3. Neutrophil granules were subsequently used for isolation of galectin-3 receptors by affinity chromatography. Proteins eluted from a galectin-3-Sepharose column by lactose were analyzed on SDS-polyacrylamide gels and showed two major bands of 100 and 160 kDa and a minor band of 120 kDa. By immunoblotting, these proteins were shown to correspond to CD66a (160 kDa), CD66b (100 kDa), and lysosome-associated membrane glycoprotein-1 and -2 (Lamp-1 and -2; 120 kDa). The unresponsive HL-60 cells lacked the CD66 Ags but contained the Lamps, implying that neutrophil CD66a and/or CD66b may be the functional galectin-3 receptors. This conclusion was supported by the subcellular localization of the CD66 proteins to the gelatinase and specific granules in resting neutrophils.

Determinants in the N-terminal domains of galectin-3 for secretion by a novel pathway circumventing the endoplasmic reticulum-Golgi complex

Galectin-3 is a beta-galactoside-binding protein that is secreted from many cells although the protein lacks a signal sequence for transfer into the endoplasmic reticulum and Golgi compartments and entry into classical secretory pathways. Previously it was shown that attachment of the first 120 amino acid residues of the N-terminal sequence of hamster galectin-3 to the cytoplasmic protein chloramphenicol acetyltransferase (CAT) supported the rapid secretion of the fusion protein from transiently transfected Cos cells under conditions in which CAT protein was not secreted. Here we report that progressive N-terminal truncation gradually reduced secretion of the fusion proteins, eventually to very low levels compared with the starting product, but did not totally eliminate secretion until a significant majority of the sequence was removed. Mutant CAT fusion proteins containing internal deletions in residues 97-120 of the galectin-3 N-terminal sequence were also secreted to a similar extent to the starting product, but further deletion of residues 89-96 abolished detectable secretion. Proline to alanine mutagenesis of the sequence YP(90)SAP(93)GAY in two secretion-competent CAT fusion proteins greatly reduced or abolished their secretion, whereas similar mutagenesis of proline pairings present elsewhere in the galectin-3 N-terminal segments of these proteins had no effect. The results indicate that this sequence is one essential determinant for secretion of galectin-3-CAT fusion proteins and by inference galectin-3, at least from transfected Cos cells. However, the short sequence of residues 89-96 by itself is insufficient to direct secretion of CAT fusion proteins and appears to be active only in the context of a larger portion of the galectin-3 N-terminal sequence.

Galectins: a key intersection between glycobiology and immunology

Galectins are a family of evolutionarily conserved animal lectins, widely distributed from lower invertebrates to mammals. They share sequence and structure similarities in the carbohydrate recognition domain and specificity for polylactosamine-enriched glycoconjugates. In the last few years significant experimental data have been accumulated concerning their participation in different biological processes requiring carbohydrate recognition such as cell adhesion, cell growth regulation, inflammation, immunomodulation, apoptosis and metastasis. In the present review we will discuss some exciting questions and advances in galectin research, highlighting the significance of these proteins in immunological processes and their implications in biomedical research, disease diagnosis and clinical intervention. Designing novel therapeutic strategies based on carbohydrate recognition will provide answers for the treatment of autoimmune disorders, inflammatory processes, allergic reactions and tumor spreading.

Cyclophilin C-associated protein: a normal secreted glycoprotein that down-modulates endotoxin and proinflammatory responses in vivo

Mouse cyclophilin C-associated protein (CyCAP) is a member of the scavenger-receptor cysteine-rich domain superfamily and is 69% identical to the human Mac-2 binding protein. Here, we show that CyCAP is a widely expressed secreted glycoprotein that modulates the host response to endotoxin. Gene-targeted CyCAP-deficient mice are more sensitive to the lethal effects of endotoxin. In response to endotoxin, CyCAP-deficient mice overproduced interleukin 12 and interferon-gamma systemically and tumor necrosis factor alpha locally; these are proinflammatory molecules that also promote T helper 1 responses. Furthermore, macrophages stimulated in vitro with endotoxin in serum deficient in CyCAP secreted more tumor necrosis factor alpha, supporting the proposal that CyCAP specifically down-modulates endotoxin signaling.

Galectin-3 modulates rat mesangial cell proliferation and matrix synthesis during experimental glomerulonephritis induced by anti-Thy1.1 antibodies

Galectin-3 is a beta-galactoside-binding protein synthesized by macrophages and other inflammatory cells and expressed in various branching epithelia, including the developing kidney. The expression of galectin-3 has been studied in a rat model of acute mesangial proliferative glomerulonephritis in which a single injection of anti-Thy1.1 antibodies leads to destruction of mesangial cells expressing a Thy1.1 epitope on their surface. The glomerular lesion is characterized by expansion of the mesangial matrix, especially laminin and collagen type IV, and mesangial hypercellularity. Galectin-3 expression, which is sparse in mature rat kidney and confined to the apical face of some distal tubules, is increased within 1-3 days following antibody administration, with the recruitment of glomerular macrophages and pronounced neo-expression in the cytoplasm and at the basal face of distal tubules. At later times, galectin-3 is detected immunohistochemically in the repopulating mesangial cell mass, preceding the extensive mesangial deposition of laminin and collagen type IV. Mesangial cells in culture do not produce appreciable amounts of galectin-3 but do bind and endocytose exogenously added lectin. Addition of galectin-3 to primary cultures of mesangial cells prepared from normal rats induces a 1.5-fold increase in the synthesis of collagen type IV and it also acts in synergy with a quantitatively similar stimulatory effect of transforming growth factor beta (TGF-beta) on matrix synthesis. Exogenous galectin-3 prolongs the survival of mesangial cells in serum-free cultures and also protects these cells against cytotoxic effects of TGF-beta. The data support the notion that the increased expression and secretion of galectin-3 in infiltrating macrophages and in distal tubular epithelia, together with up-regulation of IL-1beta and TGF-beta genes, play a role in mesangial hypercellularity in the progression of one model of inflammatory renal disease.

A newly identified horseshoe crab lectin with specificity for blood group A antigen recognizes specific O-antigens of bacterial lipopolysaccharides

A 14-kDa lectin, named tachylectin-3, was newly identified from hemocytes of the Japanese horseshoe crab, Tachypleus tridentatus. This lectin exhibited hemagglutinating activity against human A-type erythrocytes, but not against the B- and O-types of erythrocytes and animal erythrocytes, including those of sheep, rabbit, horse, and bovine. The hemagglutinating activity of tachylectin-3 was equivalent to that of a previously identified lectin, named tachylectin-2, with affinity for N-acetyl-D-glucosamine or N-acetyl-D-galactosamine. However, the activity of tachylectin-3 was not inhibited by these two N-acetylhexosamines at 100 mM but was inhibited by a blood group A-pentasaccharide at a minimum inhibitory concentration of 0.16 mM. Furthermore, the hemagglutinating activity was strongly inhibited by bacterial S-type lipopolysaccharides (LPSs) from Gram-negative bacteria but not by R-type LPSs lacking O-antigens. One of the most effective S-type LPSs was from Escherichia coli O111:B4, with a minimum inhibitory concentration of 6 ng/ml. These data suggest that tachylectin-3 specifically recognizes Gram-negative bacteria through the unique structural units of O-antigens. Ultracentrifugation analysis revealed that tachylectin-3 is present in dimer in solution. A cDNA coding for tachylectin-3 was isolated from a hemocyte cDNA library. Tachylectin-3 consisted of two repeating sequences, each with a partial sequence similarity to rinderpest virus neuraminidase. Tachylectin-3 and three previously isolated types of tachylectins were all predominantly expressed in hemocytes and released from hemocytes in response to external stimuli. These lectins present at injured sites suggest that they probably serve synergistically to accomplish an effective host defense against invading microbes.

Galectin-3 stimulates cell proliferation

Galectin-3, a beta-galactoside-binding protein, has been shown to be involved in multiple biological processes through interaction with its complementary glycoconjugates. Here we provide the first evidence of galectin-3 as a mitogen. Incubation of quiescent cultures of normal human lung fibroblast IMR-90 cells with recombinant galectin-3 (rgalectin-3) stimulated DNA synthesis as well as cell proliferation in a dose-dependent manner. This mitogenic activity was dependent on the lectin property of galectin-3, as it could be significantly inhibited by lactose, a disaccharide competitive for carbohydrate-binding by galectin-3. Chemical cross-linking and affinity-purification experiments identified binding of rgalectin-3 to cell surface glycoproteins, which were not recognized by antibodies directed against lysosome-associated membrane proteins (LAMPs), putative cellular ligands for galectin-3. Moreover, pulse-chase analysis revealed no secretion of galectin-3 by IMR-90 cells. These results indicate that galectin-3 is a mitogen capable of stimulating fibroblast cell proliferation in a paracrine fashion through interaction with cell surface glycoconjugates different from LAMPs and suggest a possible involvement of galectin-3 in tissue remodeling.

Galectin-3 gene (LGALS3) expression in experimental atherosclerosis and cultured smooth muscle cells

The galectin-3 gene (LGALS3) encodes a beta-galactose binding lectin. LGALS3 expression is associated with neoplastic transformation and with differentiation of monocytes to macrophages. Factors involved in migration, proliferation, adhesion and differentiation of vascular smooth muscle cells (SMC) play a major role during atherosclerosis development. Expression of the galectin-3 gene was not detected in quiescent SMC but was activated in aortas of hypercholesterolemic rabbits, in aortas of rats after balloon injury and in cultured SMC. These results suggest that galectin-3 production is involved in the developmental process of atherogenesis.

Maintenance of granulocyte numbers during acute peritonitis is defective in galectin-3-null mutant mice

Galectin-3, also known as the macrophage marker Mac-2, is a member of a family of structurally related animal lectins that exhibit specificity for beta-galactosides. In order to investigate the role of galectin-3 in acute inflammation, we have compared the number of leucocytes present in the peritoneal cavity of wild type and galectin-3 null mutant mice after intraperitoneal (i.p.) injection of thioglycolate broth. At day 1 after injection, we found no difference in the recruitment of mononuclear phagocytes and granulocytes to the peritoneal cavity. However, 4 days after thioglycolate injection, galectin-3 mutant mice exhibited a significantly reduced number of recoverable granulocytes compared to wild-type animals. As mutant granulocytes did not exhibit an accelerated rate of apoptosis and their uptake by macrophages appeared to be unaffected by the mutation, the phenotype described here suggests that galectin-3 participates in an additional level of control during the resolution of acute inflammation.

Galectin-7, a marker of all types of stratified epithelia

Galectin-7 is a 14-kDA member of the lectin family we have previously cloned in the human. Its expression was found at all stages of differentiation of the human epidermis and was reduced but not suppressed when oral metaplasia of reconstructed epidermis was induced by retinoic acid. This suggested that galectin-7 could be a marker of both keratinized and non-keratinized stratified epithelia. To ascertain this hypothesis, we cloned the rat and the mouse cDNAs and produced a specific antiserum raised against a synthetic peptide. The distribution of galectin-7 mRNAs and protein was studied by in situ hybridization and immunolabelling of various human, rat and mouse epithelia. Galectin-7 was found to be expressed in interfollicular epidermis and in the outer root sheath of the hair follicle, but not in the hair matrix, nor in the sebaceous glands. It was present in esophagus and oral epithelia, cornea, Hassal's corpuscles of the thymus, but not in simple and transitional epithelia. Galectin-7 can thus be considered as a marker of all subtypes of keratinocytes. In that respect it differs from both "basal-specific" keratins K5-K14 and from "suprabasal-specific" markers such as keratins K1-K10 and involucrin. Galectin-7 also differs from some desmosomal proteins, which are present in all types of epithelia and in myocardium. Galectin-7 was absent from cultured carcinoma cell lines and was reduced both in human carcinomas and in murine tumors produced with the two-stage carcinogenesis protocol.

Activated Rat Macrophages Produce a Galectin-1-Like Protein That Induces Apoptosis of T Cells: Biochemical and Functional Characterization

Galectins, a family of closely related β-galactoside-binding proteins, show specific immunomodulatory properties. We have recently identified the presence of a galectin-like protein in rat peritoneal macrophages by means of a cross-reactivity with a polyclonal Ab raised against a galectin purified from adult chicken liver. Galectin expression was up-regulated in inflammatory and activated macrophages, revealing a significant increase in phorbol ester- and formylmethionine oligopeptide-treated cells. In an attempt to further explore its functional significance, rat macrophage galectin was purified from activated macrophages by a single-step affinity chromatography on a lactosyl-Sepharose matrix. The eluted fraction was resolved as a single protein band of ∼15,000 Da by SDS-PAGE that immunoreacted strongly with the anti-chicken galectin serum. Gel filtration studies revealed that the protein behaved like a dimer under native conditions, and saccharides bearing a β-d-galactoside configuration were able to inhibit the hemagglutinating activity displayed by the purified galectin. In agreement with its isoelectric point of ∼4.8, the amino acid analysis showed a definitive acidic pattern. Internal amino acid sequencing of selected peptides obtained by proteolytic cleavage revealed that this carbohydrate-binding protein shares all the absolutely preserved and critical residues found in other members of the mammalian galectin-1 subfamily. Finally, biochemical and ultrastructural evidence, obtained by genomic DNA fragmentation and transmission electron microscopy, are also provided to show its potential implications in the apoptotic program of T cells. This effect was quantified by using the terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end-labeling assay and was found to be associated to the specific carbohydrate-binding properties of galectin.

Immunohistochemical localisation of a galectin from Bufo arenarum ovary

Galectins are a group of soluble animal lectins that exhibit specificity for beta-galactosides and conserve sequence homology in the carbohydrate-recognition domain. The galectin from Bufo arenarum ovary showed a strong cross-reaction with the lectin of 14.5 kDa purified from embryos at early blastula stage. In this paper, we studied the immunohistochemical localisation of the galectin of 14.5 kDa from ovary of the toad B. arenarum in adult ovary sections. We also analysed the immunohistochemical localisation of the embryonic lectin during early development using the antiserum anti-ovary galectin. In the ovary, oocytes in the previtellogenic stage showed strong reactivity in the nucleus and the cortex but not in the cytoplasm. Oocytes in the stage of primary vitellogenesis exhibited a similar pattern in the nuclear and cortical areas but showed immunostaining in the cytoplasm. Intense nuclear staining was detected in oocytes in the stage of late vitellogenesis and in mature oocytes, which also presented strong reactions in the yolk platelets that completely covered the cytoplasm. In blastula embryos the staining was found in the blastomeres, the yolk platelets and the blastocoele. Each lectin localisation is discussed in relation to potential biological roles in the corresponding tissues.

Murine microglial cells express functionally active galectin-3 in vitro

In the present study we have analyzed the expression of galectin-3, a beta-galactoside-specific soluble animal lectin, by microglial cells in vitro. In enriched microglial cell cultures derived from neonatal mouse brain after 2 to 3 weeks in vitro, almost all microglial cells expressed galectin-3 intracellularly and about 90% expressed the molecule on the cell surface. Western blot analyses of lysates from microglial cells using galectin-3-specific antibodies revealed a single band with an apparent molecular weight of 29 kD. The carbohydrate recognition domain of microglia-derived galectin-3 was functional as the molecule could be affinity purified on lactose-agarose. Upon an incubation with lactose-, but not with sucrose-containing buffers the amount of cell surface expressed galectin-3 was strongly reduced, suggesting that the molecule appears to be associated with the plasma membrane via its carbohydrate recognition domain. The total amount as well as the portion of cell surface expressed galectin-3 increased upon treatment with granulocyte-macrophage colony-stimulating factor. Our findings suggest that galectin-3 expression is subject to regulation by growth factors supposed to be involved in the cascade of microglial activation under pathological conditions.

Detection and distribution of the carbohydrate binding protein galectin-3 in human notochord, intervertebral disc and chordoma

Galectin-3 is an endogenous carbohydrate-binding protein which plays a role in cell differentiation, morphogenesis and cancer biology. We investigated the occurrence and distribution of galectin-3 in the embryonic and fetal human notochord, the developing human vertebral column, adult intervertebral discs and in six chordomas, which are tumors thought to originate from notochordal remnants. By means of Western blots, the expression of galectin-3 was confirmed in tissue probes from the vertebral column region beginning with the 8th gestational week. These results were supported by immunohistochemical data which revealed the presence of galectin-3 in the cytoplasm of cells of the notochord also from the 8th gestational week onwards. Notochordal immunostaining became stronger with increasing gestational age. A persisting notochordal remnant in an adult intervertebral disc and various cells of the nucleus pulposus also contained galectin-3. All chordomas showed moderate or strong immunoreactivity irrespective of their cellular composition. Subcellularly, galectin-3 was localized mostly in the cytoplasm, while a subset of tumor cells also showed nuclear distribution. Differences in staining patterns of chordoma cells could not, in general, be correlated to any histological features of these tumors.

Cloning and expression of the mRNA of human galectin-4, an S-type lectin down-regulated in colorectal cancer

We are interested in the characterization of genes whose expressions in the colon are modified during colorectal carcinogenesis. Our approach was to establish the phenotype of a colon tumor by partial sequencing of a large number of transcripts, then to select mRNAs of potential interest by differential screening with complex probes from normal or cancerous colon. In this paper, we report the cloning and sequencing of a mRNA strongly underexpressed in colorectal cancer. It corresponded to a protein comprising 323 amino acids, that appeared to be human galectin-4 on the basis of 76% and 79% amino acid identity to the rat and pig counterparts, respectively. Tissue distribution analysis showed that its expression was restricted to the small intestine, colon and rectum. Galectin-4 expression was compared in tumor and normal adjacent colon of 19 patients. In 18 patients, the mRNA concentration was 1.5-50-times lower in the tumor. No significant correlation was observed between decreased expression of galectin-4 and the degree of differentiation of the tumor or Duke's state. These results suggest that decreased galectin-4 mRNA expression may be an early event in colon carcinogenesis. Among five cell lines derived from colon carcinoma, only two (HT29 and LS174T) expressed galectin-4 mRNA.

Strikingly different localization of galectin-3 and galectin-4 in human colon adenocarcinoma T84 cells. Galectin-4 is localized at sites of cell adhesion

Two beta-galactoside-binding proteins were found to be prominently expressed in the human colon adenocarcinoma T84 cell line. Cloning and sequencing of one, a 36-kDa protein, identified it as the human homolog of galectin-4, a protein containing two carbohydrate binding domains and previously found only in the epithelial cells of the rat and porcine alimentary tract. The other, a 29-kDa protein, is galectin-3, containing a single carbohydrate binding domain, previously found in a number of different cell types including human intestinal epithelium. Despite the marked similarities in the carbohydrate binding domains of these two galectins, their cellular distribution patterns are strikingly different and vary with cellular conditions. In confluent T84 cells, galectin-4 is mostly cytosolic and concentrated at the basal membrane, whereas galectin-3 tends to be concentrated in large granular inclusions mostly at the apical membrane. In subconfluent T84 cells, each galectin is distributed to specific domains of lamellipodia, with galectin-4 concentrated in the leading edge and galectin-3 more proximally. Such different localization of galectins-4 and -3 within T84 cells implies different targeting mechanisms, ligands, and functions. The localization of galectin-4 suggests a role in cell adhesion which is also supported by the ability of immobilized recombinant galectin-4 to stimulate adhesion of T84 cells.

Plasma membrane targetting, vesicular budding and release of galectin 3 from the cytoplasm of mammalian cells during secretion

Galectin 3, a 30 kDa galactoside-binding protein distributed widely in epithelial and immune cells, contains no signal sequence and is externalized by a mechanism independent of the endoplasmic reticulum (ER)-Golgi complex. We show here that hamster galectin 3 overexpressed in transfected cos-7 cells is secreted at a very low rate. A chimaera of galectin 3 fused to the N-terminal acylation sequence of protein tyrosine kinase p56(lck), Nt-p56(lck)-galectin 3, which is myristoylated and palmitoylated and rapidly transported to plasma membrane domains, is efficiently released from transfected cells indicating that movement of cytoplasmic galectin 3 to plasma membrane domains is a rate limiting step in lectin secretion. N-terminal acylation is not sufficient for protein secretion since p56(lck) and the chimaera Nt-p56(lck)-CAT are not secreted from transfected cells. The amino-terminal half of galectin 3 is sufficient to direct export of a chimaeric CAT protein indicating that part of the signal for plasma membrane translocation lies in the N-terminal domains of the lectin. Immunofluorescence studies show that Nt-p56(lck)-galectin 3 aggregates underneath the plasma membrane and is released by membrane blebbing. Vesicles of low buoyant density isolated from conditioned medium are enriched in galectin 3. The lectin is initially protected from exogenous collagenase but is later released in soluble protease-sensitive form from the lectin-loaded vesicles. Using murine macrophages, which secrete their endogenous galectin 3 at a moderate rate especially in the presence of Ca2+-ionophores, we were also able to trap a galectin 3-loaded vesicular fraction which was released into the culture supernatant.

Retargeting of cytosolic proteins to the plasma membrane by the Lck protein tyrosine kinase dual acylation motif

Several members of the Src family of protein tyrosine kinases have a N-terminal dual acylation motif which specifies their myristoylation and S-acylation. These lipid modifications are necessary for correct intracellular localisation to the plasma membrane and to detergent-resistant glycolipid-enriched membrane domains (GEMs). Using chimaeras of the Lck dual acylation motif with two normally cytosolic proteins (chloramphenicol acetyl transferase and galectin-3), we show here that this motif is sufficient to encode correct lipid modification and to target these chimaeras to the plasma membrane, as demonstrated by subcellular fractionation and confocal immunofluorescence microscopy of transiently transfected COS cells. In addition, the chimaeras are resistant to extraction with cold non-ionic detergent, indicating targeting to GEM subdomains in the plasma membrane. The dual acylation motif has potential for targeting proteins to specific plasma membrane subdomains involved in signalling.

The genomic structure of the human Charcot-Leyden crystal protein gene is analogous to those of the galectin genes

The Charcot-Leyden crystal (CLC) protein, or eosinophil lysophospholipase, is a characteristic protein of human eosinophils and basophils; recent work has demonstrated that the CLC protein is both structurally and functionally related to the galectin family of beta-galactoside binding proteins. The galectins as a group share a number of features in common, including a linear ligand binding site encoded on a single exon. In this work, we demonstrate that the intron-exon structure of the gene encoding CLC is analogous to those encoding the galectins. The coding sequence of the CLC gene is divided into four exons, with the entire beta-galactoside binding site encoded by exon III. We have isolated CLC beta-galactoside binding sites from both orangutan (Pongo pygmaeus) and murine (Mus musculus) genomic DNAs, both encoded on single exons, and noted conservation of the amino acids shown to interact directly with the beta-galactoside ligand. The most likely interpretation of these results suggest the occurrence of one or more exon duplication and insertion events, resulting in the distribution of this lectin domain to CLC as well as to the multiple galectin genes.

Fungal galectins, sequence and specificity of two isolectins from Coprinus cinereus

Galectins are members of a genetically related family of beta-galactoside-binding lectins. At least eight distinct mammalian galectins have been identified. More distantly related, but still conserving amino acid residues critical for carbohydrate-binding, are galectins in chicken, eel, frog, nematode, and sponge. Here we report that galectins are also expressed in a species of fungus, the inky cap mushroom, Coprinus cinereus. Two dimeric galectins are expressed during fruiting body formation which are 83% identical to each other in amino acid sequence and conserve all key residues shared by members of the galectin family. Unlike most galectins, these have no N-terminal post-translational modification and no cysteine residues. We expressed one of these as a recombinant protein and studied its carbohydrate-binding specificity using a novel nonradioactive assay. Binding specificity has been well studied for a number of other galectins, and like many of these, the recombinant C. cinereus galectin shows particular affinity for blood group A structures. These results demonstrate not only that the galectin gene family is evolutionarily much older than previously realized but also that fine specificity for complex saccharide structures has been conserved. Such conservation implies that galectins evolved to perform very basic cellular functions, presumably by interaction with glycoconjugates bearing complex lactoside carbohydrates resembling blood group A.

Lectin-carbohydrate interaction in the immune system

The immune system consists of various types of cells and molecules that specifically interact with each other to initiate the host defense mechanism. Recent studies have shown that carbohydrates and lectins (carbohydrate-binding proteins) play an essential role in mediating such interactions. Both lectins and carbohydrates are widely distributed in the mammalian tissues as well as in microorganisms. Carbohydrates, due to their chemical nature, can potentially form structures that are more variable than proteins and nucleic acids. Lectins can exist in either soluble or cell-associated form, and although overall structures vary, invariably possess carbohydrate-recognition domains (CRD) with various specificities. The interaction between lectins and carbohydrates have been shown to be involved in such activities as opsonization of microorganisms, phagocytosis, cell adhesion and migration, cell activation and differentiation, and apoptosis. The number of lectins identified in the immune system is increasing at a rapid pace. The development in this area has opened a new aspect in studying the immune system, and at the same time, provided new therapeutic routes for the treatment and prevention of disease.

Galectin-3 stimulates uptake of extracellular Ca2+ in human Jurkat T-cells

Galectin-3, a mammalian galactoside-binding protein, is not expressed in the Jurkat T-lymphoblastoid cell line. However, Jurkat cells express surface glycoprotein receptors for galectin-3, one of which is shown to be the glycosylated heavy chain of CD98 (4F2 antigen), a T-cell activation marker. Addition of galectin-3 to Jurkat cells triggers a sustained influx of extracellular Ca2+ in a concentration dependent manner. The induced increase in cytosolic [Ca2+]i is blocked by sugar hapten inhibitors of galectin-3. The galectin-3-induced effect is insensitive to voltage-gated Ca2+ channel antagonists such as prenylamine, nifedipine and diltiazem and to pertussis toxin but is inhibited by cholera toxin. The results suggest that galectin-3 released by accessory cells such as macrophages may bind in vivo to T-cell activation antigens and also participate in Ca2+ signalling.