Haploinsufficiency of C2GnT-I glycosyltransferase renders T lymphoma cells resistant to cell death

Galectin network in osteoarthritis: galectin-4 programs a pathogenic signature of gene and effector expression in human chondrocytes in vitro

Galectin-4 (Gal-4) is a member of the galectin family, which have been identified as galactose-binding proteins. Gal-4 possesses two tandem repeat carbohydrate recognition domains and acts as a cross-linking bridge in sulfatide-dependent glycoprotein routing. We herein document its upregulation in osteoarthritis (OA) in correlation with the extent of cartilage degradation in vivo. Primary human OA chondrocytes in vitro respond to carbohydrate-inhibitable Gal-4 binding with the upregulation of pro-degradative/-inflammatory proteins such as interleukin-1β (IL-1β) and matrix metalloproteinase-13 (MMP-13), as documented by RT-qPCR-based mRNA profiling and transcriptome data processing. Activation of p65 by phosphorylation of Ser536 within the NF-κB pathway and the effect of three p65 inhibitors on Gal-4 activity support downstream involvement of such signaling. In 3D (pellet) cultures, Gal-4 presence causes morphological and biochemical signs of degradation. Taken together, our findings strongly support the concept of galectins acting as a network in OA pathogenesis and suggest that blocking their activity in disease progression may become clinically relevant in the future.

Spatiotemporal regulation of galectin-1-induced T-cell death in lamina propria from Crohn's disease and ulcerative colitis patients

Inflammatory bowel disease (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), is characterized by chronic, relapsing intestinal inflammation. Galectin-1 (Gal-1) is an endogenous lectin with key pro-resolving roles, including induction of T-cell apoptosis and secretion of immunosuppressive cytokines. Despite considerable progress, the relevance of Gal-1-induced T-cell death in inflamed tissue from human IBD patients has not been ascertained. Intestinal biopsies and surgical specimens from control patients (n = 52) and patients with active or inactive IBD (n = 97) were studied. Gal-1 expression was studied by RT-qPCR, immunoblotting, ELISA and immunohistochemistry. Gal-1-specific ligands and Gal-1-induced apoptosis of lamina propria (LP) T-cells were determined by TUNEL and flow cytometry. We found a transient expression of asialo core 1-O-glycans in LP T-cells from inflamed areas (p < 0.05) as revealed by flow cytometry using peanut agglutinin (PNA) binding and assessing dysregulation of the core-2 β 1-6-N-acetylglucosaminyltransferase 1 (C2GNT1), an enzyme responsible for elongation of core 2 O-glycans. Consequently, Gal-1 binding was attenuated in CD3⁺CD4⁺ and CD3⁺CD8⁺ LP T-cells isolated from inflamed sites (p < 0.05). Incubation with recombinant Gal-1 induced apoptosis of LP CD3⁺ T-cells isolated from control subjects and non-inflamed areas of IBD patients (p < 0.05), but not from inflamed areas. In conclusion, our findings showed that transient regulation of the O-glycan profile during inflammation modulates Gal-1 binding and LP T-cell survival in IBD patients.

How galectins have become multifunctional proteins

Having identified glycans of cellular glycoconjugates as versatile molecular messages, their recognition by sugar receptors (lectins) is a fundamental mechanism within the flow of biological information. This type of molecular interplay is increasingly revealed to be involved in a wide range of (patho)physiological processes. To do so, it is a vital prerequisite that a lectin (and its expression) can develop more than a single skill, that is the general ability to bind glycans. By studying the example of vertebrate galectins as a model, a total of five relevant characteristics is disclosed: i) access to intra- and extracellular sites, ii) fine-tuned gene regulation (with evidence for co-regulation of counterreceptors) including the existence of variants due to alternative splicing or single nucleotide polymorphisms, iii) specificity to distinct glycans from the glycome with different molecular meaning, iv) binding capacity also to peptide motifs at different sites on the protein and v) diversity of modular architecture. They combine to endow these lectins with the capacity to serve as multi-purpose tools. Underscoring the arising broad-scale significance of tissue lectins, their numbers in terms of known families and group members have steadily grown by respective research that therefore unveiled a well-stocked toolbox. The generation of a network of (ga)lectins by evolutionary diversification affords the opportunity for additive/synergistic or antagonistic interplay in situ, an emerging aspect of (ga)lectin functionality. It warrants close scrutiny. The realization of the enormous potential of combinatorial permutations using the five listed features gives further efforts to understand the rules of functional glycomics/lectinomics a clear direction.

Altered Cell Surface N-Glycosylation of Resting and Activated T Cells in Systemic Lupus Erythematosus

Altered cell surface glycosylation in congenital and acquired diseases has been shown to affect cell differentiation and cellular responses to external signals. Hence, it may have an important role in immune regulation; however, T cell surface glycosylation has not been studied in systemic lupus erythematosus (SLE), a prototype of autoimmune diseases. Analysis of the glycosylation of T cells from patients suffering from SLE was performed by lectin-binding assay, flow cytometry, and quantitative real-time PCR. The results showed that resting SLE T cells presented an activated-like phenotype in terms of their glycosylation pattern. Additionally, activated SLE T cells bound significantly less galectin-1 (Gal-1), an important immunoregulatory lectin, while other lectins bound similarly to the controls. Differential lectin binding, specifically Gal-1, to SLE T cells was explained by the increased gene expression ratio of sialyltransferases and neuraminidase 1 (NEU1), particularly by elevated ST6 beta-galactosamide alpha-2,6-sialyltranferase 1 (ST6GAL1)/NEU1 and ST3 beta-galactoside alpha-2,3-sialyltransferase 6 (ST3GAL6)/NEU1 ratios. These findings indicated an increased terminal sialylation. Indeed, neuraminidase treatment of cells resulted in the increase of Gal-1 binding. Altered T cell surface glycosylation may predispose the cells to resistance to the immunoregulatory effects of Gal-1, and may thus contribute to the pathomechanism of SLE.

Galectin-9 binds to O-glycans on protein disulfide isomerase

Changes in the T cell surface redox environment regulate critical cell functions, such as cell migration, viral entry and cytokine production. Cell surface protein disulfide isomerase (PDI) contributes to the regulation of T cell surface redox status. Cell surface PDI can be released into the extracellular milieu or can be internalized by T cells. We have found that galectin-9, a soluble lectin expressed by T cells, endothelial cells and dendritic cells, binds to and retains PDI on the cell surface. While endogenous galectin-9 is not required for basal cell surface PDI expression, exogenous galectin-9 mediated retention of cell surface PDI shifted the disulfide/thiol equilibrium on the T cell surface. O-glycans on PDI are required for galectin-9 binding, and PDI recognition appears to be specific for galectin-9, as galectin-1 and galectin-3 do not bind PDI. Galectin-9 is widely expressed by immune and endothelial cells in inflamed tissues, suggesting that T cells would be exposed to abundant galectin-9, in cis and in trans, in infectious or autoimmune conditions.

Dysfunctional immunoregulation in human liver allograft rejection associated with compromised galectin-1/CD7 pathway function

Regulatory T cells in rejected allograft patients display an inability to control responder T cells. Galectin-1 (Gal1) inhibits responder T cells through binding CD7. We investigated whether the dysfunctional immunoregulation in liver allograft rejection patients results from reduced regulatory T-cell Gal1 expression and/or responder T-cell CD7 expression. Circulating regulatory T cells and responder T cells were profiled from 31 acute rejection transplant patients, 85 transplant patients in remission, and 40 healthy controls. CD7+ and CD7− responder T cells were co-cultured with regulatory T cells to assess regulatory T-cell suppressor function. Gal1-small interfering RNA was used to silence regulatory T-cell Gal1. The CD7+ cell percentage was inversely correlated with AST, ALT, and GGT levels. The proportions of CD7+ responder T cells and Gal1+ regulatory T cells were higher in healthy controls than in transplant patients in remission and lowest in acute rejection transplant patients. Notably, CD7+ responder T-cell susceptibility to Gal1+ regulatory T-cell control was ranked in the same manner. Silencing Gal1 expression in regulatory T cells reduced their ability to suppress CD7+ (but not CD7−) responder T cells. Additionally, the proportions of CD43+ and CD45+ responder T cells were higher in healthy controls than in acute rejection transplant patients. CD43 co-expression (but not CD45 co-expression) on CD7+ responder T cells promoted their apoptosis in a Gal1-dependent manner. In sum, dysfunctional immunoregulation in liver allograft rejection patients can be partly attributed to reduced regulatory T-cell Gal1 expression and reduced responder T-cell CD7 expression. Responder T-cell CD43 downregulation in acute rejection patients may further contribute to reduced responder T-cell responsiveness to regulatory T-cell control.

Key regulators of galectin-glycan interactions

Protein-ligand interactions serve as fundamental regulators of numerous biological processes. Among protein-ligand pairs, glycan binding proteins (GBPs) and the glycans they recognize represent unique and highly complex interactions implicated in a broad range of regulatory activities. With few exceptions, cell surface receptors and secreted proteins are heavily glycosylated. As these glycans often represent highly regulatable post-translational modifications, alterations in glycosylation can fundamentally impact GBP recognition. Among GBPs, galectins in particular appear to engage a diverse set of glycan determinants to impact a broad range of biological processes. In this review, we will explore factors that impact galectin activity, including the effect of glycan modification on galectin-glycan interactions.

Differentiation-related glycan epitopes identify discrete domains of the muscle glycocalyx

The neuromuscular junction (NMJ) is enriched with glycoproteins modified with N-acetylgalactosamine (GalNAc) residues, and four nominally GalNAc-specific plant lectins have historically been used to identify the NMJ and the utrophin-glycoprotein complex. However, little is known about the specific glycan epitopes on skeletal muscle that are bound by these lectins, the glycoproteins that bear these epitopes or how creation of these glycan epitopes is regulated. Here, we profile changes in cell surface glycosylation during muscle cell differentiation and identify distinct differences in the binding preferences of GalNAc-specific lectins, Wisteria floribunda agglutinin (WFA), Vicia villosa agglutinin (VVA), soybean agglutinin (SBA) and Dolichos biflorus agglutinin (DBA). While we find that all four GalNAc binding lectins specifically label the NMJ, each of the four lectins binds distinct sets of muscle glycoproteins; furthermore, none of the major adhesion complexes are required for binding of any of the four GalNAc-specific lectins. Analysis of glycosylation-related transcripts identified target glycosyltransferases and glycosidases that could potentially create GalNAc-containing epitopes; reducing expression of these transcripts by siRNA highlighted differences in lectin binding specificities. In addition, we found that complex N-glycans are required for binding of WFA and SBA to murine C2C12 myotubes and for WFA binding to wild-type skeletal muscle, but not for binding of VVA or DBA. These results demonstrate that muscle cell surface glycosylation is finely regulated during muscle differentiation in a domain- and acceptor-substrate-specific manner, suggesting that temporal- and site-specific glycosylation are important for skeletal muscle cell function.

Galectin-1 regulates tissue exit of specific dendritic cell populations

During inflammation, dendritic cells emigrate from inflamed tissue across the lymphatic endothelium into the lymphatic vasculature and travel to regional lymph nodes to initiate immune responses. However, the processes that regulate dendritic cell tissue egress and migration across the lymphatic endothelium are not well defined. The mammalian lectin galectin-1 is highly expressed by vascular endothelial cells in inflamed tissue and has been shown to regulate immune cell tissue entry into inflamed tissue. Here, we show that galectin-1 is also highly expressed by human lymphatic endothelial cells, and deposition of galectin-1 in extracellular matrix selectively regulates migration of specific human dendritic cell subsets. The presence of galectin-1 inhibits migration of immunogenic dendritic cells through the extracellular matrix and across lymphatic endothelial cells, but it has no effect on migration of tolerogenic dendritic cells. The major galectin-1 counter-receptor on both dendritic cell populations is the cell surface mucin CD43; differential core 2 O-glycosylation of CD43 between immunogenic dendritic cells and tolerogenic dendritic cells appears to contribute to the differential effect of galectin-1 on migration. Binding of galectin-1 to immunogenic dendritic cells reduces phosphorylation and activity of the protein-tyrosine kinase Pyk2, an effect that may also contribute to reduced migration of this subset. In a murine lymphedema model, galectin-1(-/-) animals had increased numbers of migratory dendritic cells in draining lymph nodes, specifically dendritic cells with an immunogenic phenotype. These findings define a novel role for galectin-1 in inhibiting tissue emigration of immunogenic, but not tolerogenic, dendritic cells, providing an additional mechanism by which galectin-1 can dampen immune responses.

Biosynthetic Machinery Involved in Aberrant Glycosylation: Promising Targets for Developing of Drugs Against Cancer

Cancer cells depend on altered metabolism and nutrient uptake to generate and keep the malignant phenotype. The hexosamine biosynthetic pathway is a branch of glucose metabolism that produces UDP-GlcNAc and its derivatives, UDP-GalNAc and CMP-Neu5Ac and donor substrates used in the production of glycoproteins and glycolipids. Growing evidence demonstrates that alteration of the pool of activated substrates might lead to different glycosylation and cell signaling. It is already well established that aberrant glycosylation can modulate tumor growth and malignant transformation in different cancer types. Therefore, biosynthetic machinery involved in the assembly of aberrant glycans are becoming prominent targets for anti-tumor drugs. This review describes three classes of glycosylation, O-GlcNAcylation, N-linked, and mucin type O-linked glycosylation, involved in tumor progression, their biosynthesis and highlights the available inhibitors as potential anti-tumor drugs.

Galectin-3 binds to CD45 on diffuse large B-cell lymphoma cells to regulate susceptibility to cell death

Diffuse large B-cell lymphoma (DLBCL) is the most common non-Hodgkin lymphoma and an aggressive malignancy. Galectin-3 (gal-3), the only antiapoptotic member of the galectin family, is overexpressed in DLBCL. While gal-3 can localize to intracellular sites, gal-3 is secreted by DLBCL cells and binds back to the cell surface in a carbohydrate-dependent manner. The major counterreceptor for gal-3 on DLBCL cells was identified as the transmembrane tyrosine phosphatase CD45. Removal of cell-surface gal-3 from CD45 with the polyvalent glycan inhibitor GCS-100 rendered DLBCL cells susceptible to chemotherapeutic agents. Binding of gal-3 to CD45 modulated tyrosine phosphatase activity; removal of endogenous cell-surface gal-3 from CD45 with GCS-100 increased phosphatase activity, while addition of exogenous gal-3 reduced phosphatase activity. Moreover, the increased susceptibility of DLBCL cells to chemotherapeutic agents after removal of gal-3 by GCS-100 required CD45 phosphatase activity. Gal-3 binding to a subset of highly glycosylated CD45 glycoforms was regulated by the C2GnT-1 glycosyltransferase, indicating that specific glycosylation of CD45 is important for regulation of gal-3-mediated signaling. These data identify a novel role for cell-surface gal-3 and CD45 in DLBCL survival and suggest novel therapeutic targets to sensitize DLBCL cells to death.

T cells modulate glycans on CD43 and CD45 during development and activation, signal regulation, and survival

Glycosylation affects many essential T cell processes and is intrinsically controlled throughout the lifetime of a T cell. CD43 and CD45 are the two most abundant glycoproteins on the T cell surface and are decorated with O- and N-glycans. Global T cell glycosylation and specific glycosylation of CD43 and CD45 are modulated during thymocyte development and T cell activation; T cells control the type and abundance of glycans decorating CD43 and CD45 by regulating expression of glycosyltransferases and glycosidases. Additionally, T cells regulate glycosylation of CD45 by expressing alternatively spliced isoforms of CD45 that have different glycan attachment sites. The glycophenotype of CD43 and CD45 on T cells influences how T cells interact with the extracellular environment, including how T cells interact with endogenous lectins. This review focuses on changes in glycosylation of CD43 and CD45 occurring throughout T cell development and activation and the role that glycosylation plays in regulating T cell processes, such as migration, T cell receptor signaling, and apoptosis.

Mutational tuning of galectin-3 specificity and biological function

Galectins are defined by a conserved β-galactoside binding site that has been linked to many of their important functions in e.g. cell adhesion, signaling, and intracellular trafficking. Weak adjacent sites may enhance or decrease affinity for natural β-galactoside-containing glycoconjugates, but little is known about the biological role of this modulation of affinity (fine specificity). We have now produced 10 mutants of human galectin-3, with changes in these adjacent sites that have altered carbohydrate-binding fine specificity but that retain the basic β-galactoside binding activity as shown by glycan-array binding and a solution-based fluorescence anisotropy assay. Each mutant was also tested in two biological assays to provide a correlation between fine specificity and function. Galectin-3 R186S, which has selectively lost affinity for LacNAc, a disaccharide moiety commonly found on glycoprotein glycans, has lost the ability to activate neutrophil leukocytes and intracellular targeting into vesicles. K176L has increased affinity for β-galactosides substituted with GlcNAcβ1–3, as found in poly-N-acetyllactosaminoglycans, and increased potency to activate neutrophil leukocytes even though it has lost other aspects of galectin-3 fine specificity. G182A has altered carbohydrate-binding fine specificity and altered intracellular targeting into vesicles, a possible link to the intracellular galectin-3-mediated anti-apoptotic effect known to be lost by this mutant. Finally, the mutants have helped to define the differences in fine specificity shown by Xenopus, mouse, and human galectin-3 and, as such, the evidence for adaptive change during evolution.

Multiple functional targets of the immunoregulatory activity of galectin-1: Control of immune cell trafficking, dendritic cell physiology, and T-cell fate

In the postgenomic era, the study of the glycome-the whole repertoire of saccharides in cells and tissues-has enabled the association of unique glycan structures with specific physiological and pathological processes. The responsibility for deciphering this biological information belongs to endogenous glycan-binding proteins or lectins. Galectin-1, a prototypic member of a family of structurally related proteins, has demonstrated selective antiinflammatory and immunoregulatory effects either by controlling immune cell trafficking, "fine-tuning" dendritic cell physiology and regulating T-cell fate. These regulatory functions mediated by an endogenous glycan-binding protein may contribute to fulfill the needs for immune cell homeostasis, including preservation of fetomaternal tolerance and prevention of collateral damage as a result of microbial invasion or autoimmune pathology. We will discuss here the conceptual framework which led to the study of galectin-glycan lattices as a novel paradigm of immune cell communication in physiological and pathological processes and will highlight selected methods and experimental strategies which have contributed to the study of the immunoregulatory activities of this multifaceted glycan-binding protein both in in vitro and in vivo biological settings.

N- and O-glycans modulate galectin-1 binding, CD45 signaling, and T cell death

Galectin-1, a beta-galactoside-binding protein highly expressed in the thymus, induces apoptosis of specific thymocyte subsets and activated T cells. Galectin-1 binds to N- and O-glycans on several glycoprotein receptors, including CD7, CD43, and CD45. Here we show that galectin-1 signaling through CD45, which carries both N- and O-glycans, is regulated by CD45 isoform expression, core 2 O-glycan formation and the balance of N-glycan sialylation. Regulation of galectin-1 T cell death by O-glycans is mediated through CD45 phosphatase activity. While galectin-1 signaling in cells expressing low molecular weight isoforms of CD45 requires expression of core 2 O-glycans (high affinity ligands for galectin-1), galectin-1 signaling in cells expressing a high molecular weight isoform of CD45 does not require core 2 O-glycans, suggesting that a larger amount of core 1 O-glycans (low affinity ligands for galectin-1) is sufficient to overcome lack of core 2 O-glycans. Furthermore, regulation of galectin-1 signaling by alpha2,6-sialylation of N-glycans is not solely dependent on CD45 phosphatase activity and can be modulated by the relative expression of enzymes that attach sialic acid in an alpha2,6- or alpha2,3-linkage. Thus, N- and O-glycans modulate galectin-1 T cell death by distinct mechanisms, and different glycosylation events can render thymocytes susceptible or resistant to galectin-1.

Conveying glycan information into T-cell homeostatic programs: a challenging role for galectin-1 in inflammatory and tumor microenvironments

The immune system has evolved sophisticated mechanisms composed of several checkpoints and fail-safe processes that enable it to orchestrate innate and adaptive immunity, while at the same time limiting aberrant or unfaithful T-cell function. These multiple regulatory pathways take place during the entire life-span of T cells including T-cell development, homing, activation, and differentiation. Galectin-1, an endogenous glycan-binding protein widely expressed at sites of inflammation and tumor growth, controls a diversity of immune cell processes, acting either extracellularly through specific binding to cell surface glycan structures or intracellularly through modulation of pathways that remain largely unexplored. In this review, we highlight the discoveries that have led to our current understanding of the role of galectin-1 in distinct immune cell process, particularly those associated with T-cell homeostasis. Also, we emphasize findings emerging from the study of experimental models of autoimmunity, chronic inflammation, fetomaternal tolerance, and tumor growth, which have provided fundamental insights into the critical role of galectin-1 and its specific saccharide ligands in immunoregulation. Challenges for the future will embrace the rational manipulation of galectin-1-glycan interactions both towards attenuating immune responses in autoimmune diseases, graft rejection, and recurrent fetal loss, while at the same overcoming immune tolerance in chronic infections and cancer.

Turning 'sweet' on immunity: galectin-glycan interactions in immune tolerance and inflammation

The function of deciphering the biological information encoded by the glycome, which is the entire repertoire of complex sugar structures expressed by cells and tissues, is assigned in part to endogenous glycan-binding proteins or lectins. Galectins, a family of animal lectins that bind N-acetyllactosamine-containing glycans, have many roles in diverse immune cell processes, including those relevant to pathogen recognition, shaping the course of adaptive immune responses and fine-tuning the inflammatory response. How do galectins translate glycan-encoded information into tolerogenic or inflammatory cell programmes? An improved understanding of the mechanisms underlying these functions will provide further opportunities for developing new therapies based on the immunoregulatory properties of this multifaceted protein family.

T cell apoptosis at the maternal-fetal interface in early human pregnancy, involvement of galectin-1

The human fetus is not rejected by the maternal immune system despite expressing paternal antigens. Natural killer cells, the major lymphocyte population of the human decidua (dNKs), express genes with immunomodulatory potential. These include galectin-1 (gal1), a lectin with apoptotic activity on activated CD8(+) T cells, Th1 and Th17 CD4(+) cells. Although many cell types at the maternal-fetal interface also produce gal1, its production by dNKs has been used here to study its function in pregnancy. Media conditioned by dNKs containing gal1 induced apoptosis of activated T cells. This effect was blocked by anti-gal1 antibodies. Decidual T (dT) cells but not peripheral T (pT) cells bound gal1 and presented a distinct glycophenotype compatible with sensitivity to gal1. Annexin V staining, TUNEL, and hypodiploidy showed a substantial proportion of apoptotic dT cells. Immunohistochemistry revealed widespread expression of gal1 as well as periglandular apoptotic dT foci that colocalized with dNKs. Thus, secretion of gal1 by dNKs and other decidual cells contributes to the generation of an immune-privileged environment at the maternal-fetal interface.

Mammalian glycosylation in immunity

Glycosylation produces a diverse and abundant repertoire of glycans, which are collectively known as the glycome. Glycans are one of the four fundamental macromolecular components of all cells, and are highly regulated in the immune system. Their diversity reflects their multiple biological functions that encompass ligands for proteinaceous receptors known as lectins. Since the discovery that selectins and their glycan ligands are important for the regulation of leukocyte trafficking, it has been shown that additional features of the vertebrate immune system are also controlled by endogenous cellular glycosylation. This Review focuses on the emerging immunological roles of the mammalian glycome.

Sialylation of beta1 integrins blocks cell adhesion to galectin-3 and protects cells against galectin-3-induced apoptosis

In previous studies, we determined that beta1 integrins from human colon tumors have elevated levels of alpha2-6 sialylation, a modification added by beta-galactosamide alpha-2,6-sialyltranferase I (ST6Gal-I). Intriguingly, the beta1 integrin is thought to be a ligand for galectin-3 (gal-3), a tumor-associated lectin. The effects of gal-3 are complex; intracellular forms typically protect cells against apoptosis through carbohydrate-independent mechanisms, whereas secreted forms bind to cell surface oligosaccharides and induce apoptosis. In the current study, we tested whether alpha2-6 sialylation of the beta1 integrin modulates binding to extracellular gal-3. Herein we report that SW48 colonocytes lacking alpha2-6 sialylation exhibit beta1 integrin-dependent binding to gal-3-coated tissue culture plates; however, binding is attenuated upon forced expression of ST6Gal-I. Removal of alpha2-6 sialic acids from ST6Gal-I expressors by neuraminidase treatment restores gal-3 binding. Additionally, using a blot overlay approach, we determined that gal-3 binds directly and preferentially to unsialylated, as compared with alpha2-6-sialylated, beta1 integrins. To understand the physiologic consequences of gal-3 binding, cells were treated with gal-3 and monitored for apoptosis. Galectin-3 was found to induce apoptosis in parental SW48 colonocytes (unsialylated), whereas ST6Gal-I expressors were protected. Importantly, gal-3-induced apoptosis was inhibited by function blocking antibodies against the beta1 subunit, suggesting that beta1 integrins are critical transducers of gal-3-mediated effects on cell survival. Collectively, our results suggest that the coordinate up-regulation of gal-3 and ST6Gal-I, a feature that is characteristic of colon carcinoma, may confer tumor cells with a selective advantage by providing a mechanism for blockade of the pro-apoptotic effects of secreted gal-3.

Different affinity of galectins for human serum glycoproteins: galectin-3 binds many protease inhibitors and acute phase proteins

Here we report the first survey of galectins binding to glycoproteins of human serum. Serum was subjected to affinity chromatography using immobilized galectins, and the bound glycoproteins were analyzed by electrophoresis, Western blotting, and mass spectrometry. Galectins-3, -8, and -9 bound a much broader range of ligands in serum than previously known, galectin-1 bound less, and galectins-2, -4, and -7 bound only traces or no serum ligands. Galectin-3 bound most major glycoproteins, including alpha-2-macroglobulin and acute phase proteins such as haptoglobin. It bound only a selected minor fraction of transferrin, and bound none or little of IgG. Galectins-8 and -9 bound a similar range of glycoproteins as galectin-3, but in lower amounts, and galectin-8 had a relative preference for IgA. Galectin-1 bound mainly a fraction of alpha-2-macroglobulin and only traces of other glycoproteins. The binding of galectin-3 to serum glycoproteins requires affinity for LacNAc, since a mutant (R186S), which has lost this affinity, did not bind any serum glycoproteins. The average affinity of galectin-3 for serum glycoproteins was estimated to correspond to K(d) approximately 1-5 muM by modeling of the affinity chromatography and a fluorescence anisotropy assay. Since galectins are expressed on endothelial cells and other cells exposed to serum components, this report gives new insight into function of galectins and the role of their different fine specificity giving differential binding to the serum glycoproteins.

Structural features of galectin-9 and galectin-1 that determine distinct T cell death pathways

The galectin family of lectins regulates multiple biologic functions, such as development, inflammation, immunity, and cancer. One common function of several galectins is the ability to trigger T cell death. However, differences among the death pathways triggered by various galectins with regard to glycoprotein receptors, intracellular death pathways, and target cell specificity are not well understood. Specifically, galectin-9 and galectin-1 both kill thymocytes, peripheral T cells, and T cell lines; however, we have found that galectin-9 and galectin-1 require different glycan ligands and glycoprotein receptors to trigger T cell death. The two galectins also utilize different intracellular death pathways, as galectin-9, but not galectin-1, T cell death was blocked by intracellular Bcl-2, whereas galectin-1, but not galectin-9, T cell death was blocked by intracellular galectin-3. Target cell susceptibility also differed between the two galectins, as galectin-9 and galectin-1 killed different subsets of murine thymocytes. To define structural features responsible for distinct activities of the tandem repeat galectin-9 and dimeric galectin-1, we created a series of bivalent constructs with galectin-9 and galectin-1 carbohydrate recognition domains connected by different peptide linkers. We found that the N-terminal carbohydrate recognition domain and linker peptide contributed to the potency of these constructs. However, we found that the C-terminal carbohydrate recognition domain was the primary determinant of receptor recognition, death pathway signaling, and target cell susceptibility. Thus, carbohydrate recognition domain specificity, presentation, and valency make distinct contributions to the specific effects of different galectins in initiating T cell death.

O-glycosylation regulates LNCaP prostate cancer cell susceptibility to apoptosis induced by galectin-1

Resistance to apoptosis is a critical feature of neoplastic cells. Galectin-1 is an endogenous carbohydrate-binding protein that induces death of leukemia and lymphoma cells, breast cancer cells, and the LNCaP prostate cancer cell line, but not other prostate cancer cell lines. To understand the mechanism of galectin-1 sensitivity of LNCaP cells compared with other prostate cancer cells, we characterized glycan ligands that are important for conferring galectin-1 sensitivity in these cells, and analyzed expression of glycosyltransferase genes in galectin-1-sensitive, prostate-specific antigen-positive (PSA(+)) LNCaP cells compared with a galectin-1-resistant PSA(-) LNCaP subclone. We identified one glycosyltransferase, core 2 N-acetylglucosaminyltransferase, which is down-regulated in galectin-1-resistant PSA(-) LNCaP cells compared with galectin-1-sensitive PSA(+) LNCaP cells. Intriguingly, this is the same glycosyltransferase required for galectin-1 susceptibility of T lymphoma cells, indicating that similar O-glycan ligands on different polypeptide backbones may be common death trigger receptors recognized by galectin-1 on different types of cancer cells. Blocking O-glycan elongation by expressing alpha2,3-sialyltransferase 1 rendered LNCaP cells resistant to galectin-1, showing that specific O-glycans are critical for galectin-1 susceptibility. Loss of galectin-1 susceptibility and synthesis of endogenous galectin-1 has been proposed to promote tumor evasion of immune attack; we found that galectin-1-expressing prostate cancer cells killed bound T cells, whereas LNCaP cells that do not express galectin-1 did not kill T cells. Resistance to galectin-1-induced apoptosis may directly contribute to the survival of prostate cancer cells as well as promote immune evasion by the tumor.

Affinity of galectin-8 and its carbohydrate recognition domains for ligands in solution and at the cell surface

Galectin-8 has two different carbohydrate recognition domains (CRDs), the N-terminal Gal-8N and the C-terminal Gal-8C linked by a peptide, and has various effects on cell adhesion and signaling. To understand the mechanism for these effects further, we compared the binding activities of galectin-8 in solution with its binding and activation of cells. We used glycan array analysis to broaden the specificity profile of the two galectin-8 CRDs, as well as intact galectin-8s (short and long linker), confirming the unique preference for sulfated and sialylated glycans of Gal-8N. Using a fluorescence anisotropy assay, we examined the solution affinities for a subset of these glycans, the highest being 50 nM for NeuAcalpha2,3Lac by Gal-8N. Thus, carbohydrate-protein interactions can be of high affinity without requiring multivalency. More importantly, using fluorescence polarization, we also gained information on how the affinity is built by multiple weak interactions between different fragments of the glycan and its carrier molecule and the galectin CRD subsites (A-E). In intact galectin-8 proteins, the two domains act independently of each other in solution, whereas at a surface they act together. Ligands with moderate or weak affinity for the isolated CRDs on the array are bound strongly by intact galectin-8s. Also galectin-8 binding and signaling at cell surfaces can be explained by combined binding of the two CRDs to low or medium affinity ligands, and their highest affinity ligands, such as sialylated galactosides, are not required.

Dissecting the pathophysiologic role of endogenous lectins: Glycan-binding proteins with cytokine-like activity?

Several families of endogenous glycan-binding proteins have been implicated in a wide variety of immunological functions including first-line defence against pathogens, cell trafficking, and immune regulation. These include, among others, the C-type lectins (collectins, selectins, mannose receptor, and others), S-type lectins (galectins), I-type lectins (siglecs and others), P-type lectins (phosphomannosyl receptors), pentraxins, and tachylectins. This review will concentrate on the immunoregulatory roles of galectins (particularly galectin-1) and collectins (mannose-binding lectins and surfactant proteins) to illustrate the ability of endogenous glycan-binding proteins to act as cytokines, chemokines or growth factors, and thereby modulating innate and adaptive immune responses under physiological or pathological conditions. Understanding the pathophysiologic relevance of endogenous lectins in vivo will reveal novel targets for immunointervention during chronic infection, autoimmunity, transplantation and cancer.