Analysis of selected blood and immune cell responses to carbohydrate-dependent surface binding of proto- and chimera-type galectins

Heterologous Interactions with Galectins and Chemokines and Their Functional Consequences

Extra- and intra-cellular activity occurs under the direction of numerous inter-molecular interactions, and in any tissue or cell, molecules are densely packed, thus promoting those molecular interactions. Galectins and chemokines, the focus of this review, are small, protein effector molecules that mediate various cellular functions-in particular, cell adhesion and migration-as well as cell signaling/activation. In the past, researchers have reported that combinations of these (and other) effector molecules act separately, yet sometimes in concert, but nevertheless physically apart and via their individual cell receptors. This view that each effector molecule functions independently of the other limits our thinking about functional versatility and cooperation, and, in turn, ignores the prospect of physiologically important inter-molecular interactions, especially when both molecules are present or co-expressed in the same cellular environment. This review is focused on such protein-protein interactions with chemokines and galectins, the homo- and hetero-oligomeric structures that they can form, and the functional consequences of those paired interactions.

O-GlcNAc-Mediated Regulation of Galectin Expression and Secretion in Human Promyelocytic HL-60 Cells Undergoing Neutrophilic Differentiation

In this study, we have tested the hypothesis that the expression and secretion of galectins are driven through mechanisms globally impacted by homeostatic regulation involving the post-translational modification of intracellular proteins with O-linked N-acetylglucosamine (O-GlcNAc). We showed that neutrophilic differentiation of HL-60 cells induced by all-trans retinoic acid (ATRA) and 6-diazo-5-oxo-L-norleucine (DON) was associated with a significant drop of cellular O-GlcNAc levels in serum-contained and serum-free cell culture media. Galectin gene and protein expression profiles in HL-60 cells were specifically modified by ATRA and by inhibitors of O-GlcNAc cycle enzymes, however overall trends for each drug were similar between cells growing in the presence or absence of serum except for LGALS9 and LGALS12. The secretion of four galectins (-1, -3, -9, and -10) by HL-60 cells in a serum-free medium was stimulated by O-GlcNAc-reducing ATRA and DON while O-GlcNAc-elevating thiamet G (O-GlcNAcase inhibitor) failed to change the basal levels of extracellular galectins. Taken together, these results demonstrate that O-GlcNAc homeostasis is essential not only for regulation of galectin expression in cells but also for the secretion of multiple members of this protein family, which can be an important novel aspect of unconventional secretion mechanisms.

The Emerging Role of Galectins and O-GlcNAc Homeostasis in Processes of Cellular Differentiation

Galectins are a family of soluble β-galactoside-binding proteins with diverse glycan-dependent and glycan-independent functions outside and inside the cell. Human cells express twelve out of sixteen recognized mammalian galectin genes and their expression profiles are very different between cell types and tissues. In this review, we summarize the current knowledge on the changes in the expression of individual galectins at mRNA and protein levels in different types of differentiating cells and the effects of recombinant galectins on cellular differentiation. A new model of galectin regulation is proposed considering the change in O-GlcNAc homeostasis between progenitor/stem cells and mature differentiated cells. The recognition of galectins as regulatory factors controlling cell differentiation and self-renewal is essential for developmental and cancer biology to develop innovative strategies for prevention and targeted treatment of proliferative diseases, tissue regeneration, and stem-cell therapy.

The association between increasing levels of O-GlcNAc and galectins in the liver tissue of hibernating thirteen-lined ground squirrels (Ictidomys tridecemlineatus)

Post-translational glycosylation of proteins with O-linked β-N-acetylglucosamine (O-GlcNAcylation) and changes of galectin expression profiles are essential in many cellular stress responses. We examine this regulation in the liver tissue of hibernating thirteen-lined ground squirrels (Ictidomys tridecemlineatus) representing a biological model of hypometabolism and physiological stress resistance. The tissue levels of O-GlcNAcylated proteins as well as galectin-1 and galectin-3 proteins detected by immunodot blot assay were significantly lower by 4.6-5.4-, 2.2-2.3- and 2.5-2.9-fold, respectively, in the non-hibernating summer squirrels compared with those in winter, whether hibernating or aroused. However, there were no differences in the expression of genes encoding enzymes involved in O-GlcNAc cycle (O-GlcNAc transferase and O-GlcNAcase) and such galectins as LGALS1, LGALS2, LGALS3, LGALS4 and LGALS9. Only the expression of LGALS8 gene in the liver tissue was significantly decreased by 37.6 ± 0.1% in hibernating ground squirrels relative to summer animals. Considering that the expression of a proven genetic biomarker ELOVL6 encoding ELOVL fatty acid elongase 6 was readily upregulated in non-hibernating animals by 11.3-32.9-fold, marginal differential changes in the expression of galectin genes cannot be classified as biomarkers of hibernation. Thus, this study provides evidence that hibernation in Ictidomys tridecemlineatus is associated with increasing O-GlcNAcylation of liver proteins and suggests that the contribution of galectins deserves further studies at the protein level.

Redirecting Pore Assembly of Staphylococcal α-Hemolysin by Protein Engineering

α-Hemolysin (αHL), a β-barrel pore-forming toxin (βPFT), is secreted as a water-soluble monomer by Staphylococcus aureus. Upon binding to receptors on target cell membranes, αHL assembles to form heptameric membrane-spanning pores. We have previously engineered αHL to create a protease-activatable toxin that is activated by site-specific proteolysis including by tumor proteases. In this study, we redesigned αHL so that it requires 2-fold activation on target cells through (i) binding to specific receptors, and (ii) extracellular proteolytic cleavage. To assess our strategy, we constructed a fusion protein of αHL with galectin-1 (αHLG1, αHL-Galectin-1 chimera). αHLG1 was cytolytic toward cells that lack a receptor for wild-type αHL. We then constructed protease-activatable mutants of αHLG1 (PAMαHLG1s). PAMαHLG1s were activated by matrix metalloproteinase 2 (MMP-2) and had approximately 50-fold higher cytolytic activity toward MMP-2 overexpressing cells (HT-1080 cells) than toward non-overexpressing cells (HL-60 cells). Our approach provides a novel strategy for tailoring pore-forming toxins for therapeutic applications.

Towards molecular mechanisms regulating the expression of galectins in cancer cells under microenvironmental stress conditions

Galectins, a family of soluble β-galactoside-binding proteins, serve as mediators of fundamental biological processes, such as cell growth, differentiation, adhesion, migration, survival, and death. The purpose of this review is to summarize the current knowledge regarding the ways in which the expression of individual galectins differs in normal and transformed human cells exposed to various stimuli mimicking physiological and pathological microenvironmental stress conditions. A conceptual point is being made and grounded that the modulation of galectin expression profiles is a key aspect of cellular stress responses. Moreover, this modulation might be precisely regulated at transcriptional and post-transcriptional levels in the context of non-overlapping transcription factors and miRNAs specific to galectins.

Isolation of galectin-1 from human platelets: its interaction with actin

Galectins are a family of animal lectins defined by their β-galactoside-binding specificity and a consensus sequence in their carbohydrate-recognition domain. Galectin-1 (Gal-1) is expressed as a non-covalently linked homodimer present in a variety of tissues. Here we describe its isolation from human platelets by a procedure involving ionic exchange chromatography and affinity chromatography on lactose-agarose. Platelet Gal-1 co-purifies with actin, forming an actin-Gal-1 complex which does no dissociate even after treatment with sodium dodecyl sulfate. The presence of both proteins was confirmed by Western blot and by trypsin digestion followed by mass spectrometry identification. By hemagglutination assays we studied the response of recombinant Gal-1/actin, mixed and pre-incubated in different proportions, and then tested against neuraminidase treated rabbit red blood cells. The complex formation was confirmed by confocal microscopy, showing that both proteins co-localised in resting platelets as well as in thrombin-activated ones. These results suggest that endogenous Gal-1 forms an intracellular complex with monomeric actin and that, after platelet activation, Gal-1 could play a role in the polymerization-depolymerization process of actin, which concludes in platelet aggregation.

Lectin-induced activation of plasma membrane NADPH oxidase in cholesterol-depleted human neutrophils

The gp91phox subunit of flavocytochrome b(558) is the catalytic core of the phagocyte plasma membrane NADPH oxidase. Its activation occurs within lipid rafts and requires translocation of four subunits to flavocytochrome b(558). gp91phox is the only glycosylated subunit of NADPH oxidase and no data exist about the structure or function of its glycans. Glycans, however, bind to lectins and this can stimulate NADPH oxidase activity. Given this information, we hypothesized that lectin-gp91phox interactions would facilitate the assembly of a functionally active NADPH oxidase in the absence of lipid rafts. To test this, we used lectins with different carbohydrate-binding specificity to examine the effects on H(2)O(2) generation by human neutrophils treated with the lipid raft disrupting agent methyl-β-cyclodextrin (MβCD). MβCD treatment removed membrane cholesterol, caused changes in cell morphology, inhibited lectin-induced cell aggregation, and delayed lectin-induced assembly of the NADPH oxidase complex. More importantly, MβCD treatment either stimulated or inhibited H(2)O(2) production in a lectin-dependent manner. Together, these results show selectivity in lectin binding to gp91phox, and provide evidence for the biochemical structures of the gp91phox glycans. Furthermore, the data also indicate that in the absence of lipid rafts, neutrophil NADPH oxidase activity can be altered by these select lectins.

Red blood cell proteomics

Since its discovery in the 17th century, the red blood cell, recognized in time as the critical cell component for survival, has been the focus of much attention. Its unique role in gas exchange (oxygen/CO(2) transport) and its distinct characteristics (absence of nucleus; biconcave cell shape) together with an - in essence - unlimited supply lead to extensive targeted biochemical, molecular and structural studies. A quick PubMed query with the word "erythrocyte" results in 198 013 scientific articles of which 162 are red blood cell proteomics studies, indicating that this new technique has been only recently applied to the red blood cell and related fields. Standard and comparative proteomics have been widely used to study different blood components. A growing body of proteomics literature has since developed, which deals with the characterization of red blood cells in health and disease. The possibility offered by proteomics to obtain a global snapshot of the whole red blood cell protein make-up, has provided unique insights to many fields including transfusion medicine, anaemia studies, intra-red blood cell parasite biology and translational research. While the contribution of proteomics is beyond doubt, a full red blood cell understanding will ultimately require, in addition to proteomics, lipidomics, glycomics, interactomics and study of post-translational modifications. In this review we will briefly discuss the methodology and limitations of proteomics, the contribution it made to the understanding of the erythrocyte and the advances in red blood cell-related fields brought about by comparative proteomics.

Activity profile of the CA125 antigen towards human red blood cells

Starting from the mucin nature of the CA125 antigen and conditions associated with high serum concentrations, this study is an attempt to gain insight into its activity profile towards human erythrocytes. Carcinomaassociated and pregnancy-associated CA125 antigens were tested in agglutination/aggregation, adhesion and hemolysis assays. The results obtained indicated that CA125 antigens increased agglutination/aggregation and inhibited erythrocyte adhesion, but differed in their effective concentrations. Galectin-1 slightly modulated the effects observed. CA125 antigens had no effect on hemolysis. The activity profile of the CA125 antigen towards erythrocytes may have biomedical consequences in different microenvironments in relevant physiological and pathophysiological conditions.

In-depth analysis of the membrane and cytosolic proteome of red blood cells

In addition to transporting oxygen and carbon dioxide to and from the tissues, a range of other functions are attributed to red blood cells (RBCs) of vertebrates. Diseases compromising RBC performance in any of these functions warrant in-depth study. Furthermore, the human RBC is a vital host cell for the malaria parasite. Much has been learned from classical biochemical approaches about RBC composition and membrane organization. Here, we use mass spectrometry (MS)-based proteomics to characterize the normal RBC protein profile. The aim of this study was to obtain the most complete and informative human RBC proteome possible by combining high-accuracy, high-sensitivity protein identification technology (quadrupole time of flight and Fourier transform MS) with selected biochemical procedures for sample preparation. A total of 340 membrane proteins and 252 soluble proteins were identified, validated, and categorized in terms of subcellular localization, protein family, and function. Splice isoforms of proteins were identified, and polypeptides that migrated with anomalously high or low apparent molecular weights could be grouped into either ubiquitinylated, partially degraded, or ester-linked complexes. Our data reveal unexpected complexity of the RBC proteome, provide a wealth of data on its composition, shed light on several open issues in RBC biology, and form a departure point for comprehensive understanding of RBC functions.

Regulatory Roles of Galectins in the Immune Response

Galectins are a family of animal lectins with affinity for beta-galactosides. They are differentially expressed by various immune cells and their expression levels appear to be dependent on cell differentiation and activation. They can interact with cell-surface and extracellular matrix glycoconjugates (glycoproteins and glycolipids), through lectin-carbohydrate interactions. Through this action, they can promote cell growth, affect cell survival, modulate cell adhesions, and induce cell migration. They appear to do so by binding to different glycoconjugates decorated by suitable saccharides, rather than through specific receptors. Galectins do not have a classical signal peptide and are often localized in intracellular compartments, including the nucleus. Intracellularly, they can regulate cell growth and survival by interacting with cytoplasmic and nuclear proteins, through protein-protein interactions, thereby affecting intracellular signaling pathways. Current research indicates that galectins play important roles in the immune response through regulating the homeostasis and functions of the immune cells.

Galectin-3 and soluble fibrinogen act in concert to modulate neutrophil activation and survival: involvement of alternative MAPK pathways

Galectin-3 (Gal-3), a member of a family of highly conserved carbohydrate-binding proteins, has recently emerged as a novel cellular modulator at inflammatory foci. Here we investigated the effects of Gal-3 on central effector functions of human neutrophils, including phagocytosis, exocytosis of secretory granules, and survival. We examined the effects of Gal-3 alone or in combination with soluble fibrinogen (sFbg), an extracellular mediator that plays a key role during the early phase of the inflammatory response through binding to integrin receptors. In addition we evaluated the intracellular signals triggered by these mediators in human neutrophils. Human neutrophils incubated with recombinant Gal-3 alone increased their phagocytic activity and CD66 surface expression. In contrast to the known antiapoptotic effect of Gal-3 on many cellular types, Gal-3 enhanced PMN apoptotic rate. Preincubation with Gal-3 primed neutrophils to the effects of sFbg, resulting in a synergistic action on degranulation. On the other hand, Gal-3 and sFbg had opposite effects on PMN survival, and the simultaneous action of both agonists partially counteracted the proapoptotic effects of Gal-3. In addition, although sFbg induced its effects through the activation of the ERKs, Gal-3 led to p38 phosphorylation. Disruption of this signaling pathway abrogated Gal-3-mediated modulation of neutrophil degranulation, phagocytosis, and apoptosis. Together, our results support the notion that Gal-3 and sFbg are two physiological mediators present at inflammatory sites that activate different components of the MAPK pathway and could be acting in concert to modulate the functionality and life span of neutrophils.

Determination of structural and functional overlap/divergence of five proto-type galectins by analysis of the growth-regulatory interaction with ganglioside GM1in silicoandin vitroon human neuroblastoma cells

The growth-regulatory interplay between ganglioside GM1 on human SK-N-MC neuroblastoma cells and an endogenous lectin provides a telling example for glycan (polysaccharide) functionality. Galectin-1 is the essential link between the sugar signal and the intracellular response. The emerging intrafamily complexity of galectins raises the question on defining extent of their structural and functional overlap/divergence. We address this problem for proto-type galectins in this system: ganglioside GM1 as ligand, neuroblastoma cells as target. Using the way human galectin-1 interacts with this complex natural ligand as template, we first defined equivalent positioning for distinct substitutions in the other tested proto-type galectins, e.g., Lys63 vs. Leu60/Gln72 in galectins-2 and -5. As predicted from our in silico work, the tested proto-type galectins have affinity for the pentasaccharide of ganglioside GM1. In contrast to solid-phase assays, cell surface presentation of the ganglioside did not support binding of galectin-5, revealing the first level of regulation. Next, a monomeric proto-type galectin (CG-14) can impair galectin-1-dependent negative growth control by competitively blocking access to the shared ligand without acting as effector. Thus, the quaternary structure of proto-type galectins is an efficient means to give rise to functional divergence. The identification of this second level of regulation is relevant for diagnostic monitoring. It might be exploited therapeutically by producing galectin variants tailored to interfere with galectin activities associated with the malignant phenotype. Moreover, the given strategy for comparative computational analysis of extended binding sites has implications for the rational design of galectin-type-specific ligands.

Persubstituted Cyclodextrin-Based Glycoclusters as Inhibitors of Protein−Carbohydrate Recognition Using Purified Plant and Mammalian Lectins and Wild-Type and Lectin-Gene-Transfected Tumor Cells as Targets†

Multivalent glycoclusters have the potential to become pharmaceuticals by virtue of their target specificity toward clinically relevant sugar receptors. Their application can also provide fundamental insights into the impact of two spatial factors on binding, i.e., topologies of ligand (branching mode, cluster presentation) and carbohydrate recognition domains in lectins. Persubstituted macrocycles derived from nucleophilic substitution of iodide from heptakis 6-deoxy-6-iodo-beta-cyclodextrin by the unprotected sodium thiolate of 3-(3-thioacetyl propionamido)propyl glycosides (galactose, lactose and N-acetyllactosamine) were prepared. The produced glycoclusters were first tested as competitive inhibitors in solid-phase assays. A plant toxin from mistletoe and an immunoglobulin G fraction from human serum were markedly susceptible. A nearly 400-fold increase in inhibitory potency of each galactose moiety in the heptavalent form relative to free lactose (217-fold relative to free galactose) was detected. Thus, these glycoclusters can efficiently interfere, for example, with xenoantigen-dependent hyperacute rejection. Among the tested galectins selected from this family of adhesion- and growth-regulatory endogenous lectins, the substituted beta-cyclodextrins acted as sensors to delineate topological differences between the two dimeric prototype proteins. The relatively strong reactivity with chimera-type galectin-3, a mediator of tumor metastasis, disclosed selectivity for glycocluster binding among galectins. Equally important, the geometry of ligand display (maxiclusters, bi- or triantennary N-glycans) made its mark on the inhibitory potency. To further determine the sensitivity of a distinct galectin presented on the cell surface and not in solution, we established a stably transfected tumor cell clone. We detected a significant response to presence of the multivalent inhibitor. This type of chemical scaffold with favorable pharmacologic properties might thus be exploited for the design of galectin- and ligand-type-selective glycoclusters.

First demonstration of differential inhibition of lectin binding by synthetic tri- and tetravalent glycoclusters from cross-coupling of rigidified 2-propynyl lactoside

The interplay of mammalian lectins such as galectins with cellular glycoconjugates is intimately involved in crucial reaction pathways including tumor cell adhesion, migration or growth regulation. These clinically relevant functions explain the interest in designing glycoclusters with potent activity to interfere with lectin binding. In view of the perspective for medical applications the following objective arises: to correlate topological factors of ligand display most favorably to reactivity against endogenous lectins. To date, plant agglutinins have commonly been used as models. Properly addressing this issue we first prepared di- to tetravalent clusters from 2-propynyl lactoside under mild oxidative homocoupling conditions and using the Sonogashira palladium-catalyzed cross-coupling reaction with triiodobenzene or pentaerythritol cores. These products were tested for bioactivity in a competitive solid-phase assay using different labeled sugar receptors as probes, i,e. the beta-trefoil mistletoe lectin, the natural lactoside-binding immunoglobulin G fraction from human serum and three mammalian galectins from two subgroups. The lactose headgroups in the derivatives retained ligand properties. Differences in inhibitory capacity were marked between the galectins. In contrast to homodimeric proto-type galectins-1 and -7 significant inhibition of galectin-3 binding with a 7-fold increase in relative potency was observed for the trivalent compound. In comparison, the binding of the beta-trefoil mistletoe agglutinin was reduced best by tetravalent substances The result for galectin-3 was independently confirmed by haemagglutination and cytofluorometric cell binding assays. These data underline the feasibility of galectin-type target selectivity by compound design despite using an identical headgroup (lactose) in synthesis.