Intracellular functions of galectins. (41/367)

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.  (+info)

Galectin-1 is a stromal cell ligand of the pre-B cell receptor (BCR) implicated in synapse formation between pre-B and stromal cells and in pre-BCR triggering. (42/367)

Although preB cell-receptor (pre-BCR) formation and cell-surface expression is essential for B cell development, pre-BCR generation of signal transduction remains elusive. Here, we report that recombinant pre-BCRs and the surrogate light chain bind specifically to the bone marrow stromal cell galectin-1 (GAL1), an S-type lectin. The surrogate light chain/GAL1 association is a direct protein-protein interaction (K(a) = 2 x 10(6) M(-1)), and the NH(2) extra loop of lambda-like is the major binding element. Pre-BCR binding to stromal cells depends upon GAL1 anchoring to glycosylated counter-receptors, and these complexes completely relocalize to form a synapse at the contact zone between preB and stromal cells. This immune developmental synapse is accompanied by the initiation of intracellular tyrosine kinase activity and signal transduction from the pre-BCR.  (+info)

Regulated expression and ultrastructural localization of galectin-1, a proapoptotic beta-galactoside-binding lectin, during spermatogenesis in rat testis. (43/367)

Galectin-1, a highly conserved beta-galactoside-binding protein, induces apoptosis of activated T cells and suppresses the development of autoimmunity and chronic inflammation. To gain insight regarding the potential role of galectin-1 as a novel mechanism of immune privilege, we investigated expression and ultrastructural localization of galectin-1 in rat testis. Galectin-1 expression was assessed by Western blot analysis and immunocytochemical localization in testes obtained from rats aged from 9 to 60 days. Expression of this carbohydrate-binding protein was developmentally regulated, and its immunolabeling exhibited a stage-specific pattern throughout the spermatogenic process. Immunogold staining using the anti-galectin-1 antibody revealed the typical Sertoli cell profile in the seminiferous epithelium, mainly at stages X-II. During spermiation (stages VI-VIII), a strong labeling was observed at the luminal pole of seminiferous epithelium, localized on apical stalks of Sertoli cells, on heads of mature spermatids, and on bodies of residual cytoplasm. Moreover, spermatozoa released into the lumen showed a strong immunostaining. Following spermiation (stage VIII), galectin-1 expression was restored at the basal portion of Sertoli cells and progressively spread out through the whole cells as differentiation of germinal cells proceeded. Immunoelectron microscopy confirmed distribution of galectin-1 in nuclei and cytoplasmic projections of Sertoli cells and on heads and tails of late spermatids and residual bodies. Surface localization of galectin-1 was evidenced in spermatozoa from caput epididymis. Thus, the regulated expression of galectin-1 during the spermatogenic cycle suggests a novel role for this immunosuppressive lectin in reproductive biology.  (+info)

The ST6Gal I sialyltransferase selectively modifies N-glycans on CD45 to negatively regulate galectin-1-induced CD45 clustering, phosphatase modulation, and T cell death. (44/367)

The addition of sialic acid to T cell surface glycoproteins influences essential T cell functions such as selection in the thymus and homing in the peripheral circulation. Sialylation of glycoproteins can be regulated by expression of specific sialyltransferases that transfer sialic acid in a specific linkage to defined saccharide acceptor substrates and by expression of particular glycoproteins bearing saccharide acceptors preferentially recognized by different sialyltransferases. Addition of alpha2,6-linked sialic acid to the Galbeta1,4GlcNAc sequence, the preferred ligand for galectin-1, inhibits recognition of this saccharide ligand by galectin-1. SAalpha2,6Gal sequences, created by the ST6Gal I enzyme, are present on medullary thymocytes resistant to galectin-1-induced death but not on galectin-1-susceptible cortical thymocytes. To determine whether addition of alpha2,6-linked sialic acid to lactosamine sequences on T cell glycoproteins inhibits galectin-1 death, we expressed the ST6Gal I enzyme in a galectin-1-sensitive murine T cell line. ST6Gal I expression reduced galectin-1 binding to the cells and reduced susceptibility of the cells to galectin-1-induced cell death. Because the ST6Gal I preferentially utilizes N-glycans as acceptor substrates, we determined that N-glycans are essential for galectin-1-induced T cell death. Expression of the ST6Gal I specifically resulted in increased sialylation of N-glycans on CD45, a receptor tyrosine phosphatase that is a T cell receptor for galectin-1. ST6Gal I expression abrogated the reduction in CD45 tyrosine phosphatase activity that results from galectin-1 binding. Sialylation of CD45 by the ST6Gal I also prevented galectin-1-induced clustering of CD45 on the T cell surface, an initial step in galectin-1 cell death. Thus, regulation of glycoprotein sialylation may control susceptibility to cell death at specific points during T cell development and peripheral activation.  (+info)

Identification of galectin I and thioredoxin peroxidase II as two arsenic-binding proteins in Chinese hamster ovary cells. (45/367)

In this study, we report the identification of two arsenic-binding proteins from Chinese hamster ovary (CHO) cells. The crude extract derived from CHO and SA7 (arsenic-resistant CHO cells) was applied to a phenylarsine oxide-agarose affinity column, and after extensive washing, the absorbed proteins were eluted with buffers containing 20 mM 2-mercaptoethanol (2-ME) or dithiothreitol (DTT). Three differentially expressed proteins, galectin 1 (Gal-1; in the 2-ME-eluted fraction from CHO cells), glutathione S-transferase P-form (GST-P) and thioredoxin peroxidase II (TPX-II), respectively in the 2-ME- and DTT-eluted fractions from SA7 cells, were identified by partial amino acid sequence analysis after separation by SDS/PAGE. The GST-P protein has been previously shown to facilitate the excretion of sodium arsenite [As(III)] from SA7 cells. TPX II was detected predominately in SA7 cells [routinely cultured in As(III)-containing medium], but not in CHO or SA7N (a revertant of SA7 cells cultured in regular medium) cells. In contrast, Gal-1 was specifically identified in CHO and SA7N cells, but not in SA7 cells. The preferential expression of Gal-1 in CHO cells and TPX-II in SA7 cells was further illustrated by quantitative PCR analysis. The binding of Gal-1 and TPX-II with As(III) was further verified by both co-immunoprecipitation and co-elution of Gal-1 and TPX-II with As(III). It is suggested that Gal-1 and TPX-II are two proteins that serve as high-affinity binding sites for As(III) and thus both may be involved in the biological action of As(III).  (+info)

Galectin 1 inhibits incorporation of vitronectin and chondroitin sulfate B into the extracellular matrix of human vascular smooth muscle cells. (46/367)

Galectin-1, a beta-galactoside-binding dimeric lectin, interacts with the extracellular matrix (ECM) of smooth muscle cells (SMCs) and with particular ECM proteins. Enrichment of the ECM with galectin-1 affects adhesion and proliferation of cultured SMCs. Here we investigated whether galectin-1 (1) interacts with glycosaminoglycan (GAG) chains, (2) cross-links between ligands and facilitates the incorporation of GAGs, vitronectin and plasma fibronectin in the ECM of vascular SMCs. A recombinant galectin-1 fusion protein GalH, used in this study, formed dimers and interacted with ECM proteins. GAG chains inhibited these interactions. Among the studied GAG chains, only chondroitin sulfate B interacted with GalH in beta-galactoside-dependent manner. GalH did not bridge between ECM proteins on solid phase and [125I]-labelled ECM proteins or GAGs in solution. The ECM incorporated less vitronectin in the presence of soluble GalH. GalH-enriched ECM incorporated less vitronectin and chondroitin sulfate B. The ECM partially depleted of endogenous galectins incorporated more chondroitin sulfate B compared to untreated ECM. These results suggest that galectin-1 is likely to be involved in the ECM assembly affecting incorporation of some ECM components important for SMC behaviour.  (+info)

Direct visualization of Ras proteins in spatially distinct cell surface microdomains. (47/367)

Localization of signaling complexes to specific microdomains coordinates signal transduction at the plasma membrane. Using immunogold electron microscopy of plasma membrane sheets coupled with spatial point pattern analysis, we have visualized morphologically featureless microdomains, including lipid rafts, in situ and at high resolution. We find that an inner-plasma membrane lipid raft marker displays cholesterol-dependent clustering in microdomains with a mean diameter of 44 nm that occupy 35% of the cell surface. Cross-linking an outer-leaflet raft protein results in the redistribution of inner leaflet rafts, but they retain their modular structure. Analysis of Ras microlocalization shows that inactive H-ras is distributed between lipid rafts and a cholesterol-independent microdomain. Conversely, activated H-ras and K-ras reside predominantly in nonoverlapping, cholesterol-independent microdomains. Galectin-1 stabilizes the association of activated H-ras with these nonraft microdomains, whereas K-ras clustering is supported by farnesylation, but not geranylgeranylation. These results illustrate that the inner plasma membrane comprises a complex mosaic of discrete microdomains. Differential spatial localization within this framework can likely account for the distinct signal outputs from the highly homologous Ras proteins.  (+info)

The cancer antigen CA125 represents a novel counter receptor for galectin-1. (48/367)

CA125 is an ovarian cancer antigen whose recently elucidated primary structure suggests that CA125 is a giant mucin-like glycoprotein present on the cell surface of tumor cells. Here, we establish a functional link between CA125 and beta-galactoside-binding, cell-surface lectins, which are components of the extracellular matrix implicated in the regulation of cell adhesion, apoptosis, cell proliferation and tumor progression. On the basis of mass spectrometry and immunological analyses, we find that CA125 is a counter receptor for galectin-1, as both soluble and membrane-associated fragments of CA125 derived from HeLa cell lysates are shown to bind specifically to human galectin-1 with high efficiency. This interaction is demonstrated (1) to depend on beta-galactose-terminated, O-linked oligosaccharide chains of CA125, (2) to be preferential for galectin-1 versus galectin-3 and (3) to be regulated by the cellular background in which CA125 is expressed. Despite lacking a conventional signal peptide, a CA125 C-terminal fragment of 1148 amino acids, representing less than 10% of the full-length protein, retains the ability to integrate into secretory membranes such as the endoplasmic reticulum (ER) and the Golgi, and is targeted to the plasma membrane by conventional secretory transport. As demonstrated by a novel assay that reconstitutes non-conventional secretion of galectin-1 based on fluorescence-activated cell sorting (FACS), we find that tumor-derived HeLa cells expressing endogenous CA125 present more than ten times as much galectin-1 on their surface compared with non-tumor-derived, CA125-deficient CHO cells. Intriguingly, both the galectin-1 expression level and the cell-surface binding capacity for galectin-1 are shown to be similar in CHO and HeLa cells, suggesting that CA125 might be a factor involved in the regulation of galectin-1 export to the cell surface.  (+info)