On the role of galectins in signal transduction

Calpain Small Subunit Mediated Secretion of Galectin-3 Regulates Traction Stress

The complex regulation of traction forces (TF) produced during cellular migration remains poorly understood. We have previously found that calpain 4 (Capn4), the small non-catalytic subunit of the calpain 1 and 2 proteases, regulates the production of TF independent of the proteolytic activity of the larger subunits. Capn4 was later found to facilitate tyrosine phosphorylation and secretion of the lectin-binding protein galectin-3 (Gal3). In this study, recombinant Gal3 (rGal3) was added to the media-enhanced TF generated by capn4-/- mouse embryonic fibroblasts (MEFs). Extracellular Gal3 also rescued defects in the distribution, morphology, and adhesive strength of focal adhesions present in capn4-/- MEF cells. Surprisingly, extracellular Gal3 does not influence mechanosensing. c-Abl kinase was found to affect Gal3 secretion and the production of TF through phosphorylation of Y107 on Gal3. Our study also suggests that Gal3-mediated regulation of TF occurs through signaling pathways triggered by β1 integrin but not by focal adhesion kinase (FAK) Y397 autophosphorylation. Our findings provide insights into the signaling mechanism by which Capn4 and secreted Gal3 regulate cell migration through the modulation of TF distinctly independent from a mechanosensing mechanism.

Proximity labeling technologies to illuminate glycan-protein interactions

Glycosylation is a ubiquitous post-translational modification read by glycan-binding proteins (GBP) to encode important functions, but a robust understanding of these interactions and their consequences can be challenging to uncover. Glycan-GBP interactions are transient and weak, making them difficult to capture, and glycosylation is dynamic and heterogenous, necessitating study in native cellular environments to identify endogenous ligands. Proximity labeling, an experimental innovation that labels biomolecules close to a protein of interest, has recently emerged as a powerful strategy to overcome these limitations, allowing interactors to be tagged in cells for subsequent enrichment and identification by mass spectrometry-based proteomics. We will describe this nascent technique and discuss its applications in the last five years with different GBP classes, including Siglecs, galectins, and non-human lectins.

Characterization of Galectin Fusion Proteins with Glycoprotein Affinity Columns and Binding Assays

Galectins are β-galactosyl-binding proteins that fulfill essential physiological functions. In the biotechnological field, galectins are versatile tools, such as in the development of biomaterial coatings or the early-stage diagnosis of cancer diseases. Recently, we introduced galectin-1 (Gal-1) and galectin-3 (Gal-3) as fusion proteins of a His₆-tag, a SNAP-tag, and a fluorescent protein. We characterized their binding in ELISA-type assays and their application in cell-surface binding. In the present study, we have constructed further fusion proteins of galectins with fluorescent protein color code. The fusion proteins of Gal-1, Gal-3, and Gal-8 were purified by affinity chromatography. For this, we have prepared glycoprotein affinity resins based on asialofetuin (ASF) and fetuin and combined this in a two-step purification with Immobilized Metal Affinity chromatography (IMAC) to get pure and active galectins. Purified galectin fractions were analyzed by size-exclusion chromatography. The binding characteristics to ASF of solely His₆-tagged galectins and galectin fusion proteins were compared. As an example, we demonstrate a 1.6-3-fold increase in binding efficiency for HSYGal-3 (His₆-SNAP-yellow fluorescent protein-Gal-3) compared to the HGal-3 (His₆-Gal-3). Our results reveal an apparent higher binding efficiency for galectin SNAP-tag fusion proteins compared to His₆-tagged galectins, which are independent of the purification mode. This is also demonstrated by the binding of galectin fusion proteins to extracellular glycoconjugates laminin, fibronectin, and collagen IV. Our results indicate the probable involvement of the SNAP-tag in apparently higher binding signals, which we discuss in this study.

Gal-3BP in Viral Infections: An Emerging Role in Severe Acute Respiratory Syndrome Coronavirus 2

Galectin-3 binding protein (Gal-3BP) is a multifunctional glycoprotein involved in cell–cell and cell–matrix interactions known to be upregulated in cancer and various viral infections, including HIV-1, HCV, and SARS-CoV-2, with a key role in regulating the antiviral immune response. Studies have identified a direct correlation between circulating levels of Gal-3BP and the severity of disease and/or disease progression for some viral infections, including SARS-CoV-2, suggesting a role of Gal-3BP in these processes. Due to Gal-3BP’s complex biology, the molecular mechanisms underlying its role in viral diseases have been only partially clarified. Gal-3BP induces the expression of interferons (IFNs) and proinflammatory cytokines, including interleukin-6 (IL-6), mainly interacting with galectin-3, targeting the TNF receptor-associated factors (TRAF-6 and TRAF-3) complex, thus having a putative role in the modulation of TGF-β signaling. In addition, an antiviral activity of Gal-3BP has been ascribed to a direct interaction of the protein with virus components. In this review, we explored the role of Gal-3BP in viral infections and the relationship between Gal-3BP upregulation and disease severity and progression, mainly focusing on SARS-CoV-2. Augmented knowledge of Gal-3BP’s role in virus infections can be useful to evaluate its possible use as a prognostic biomarker and as a putative target to block or attenuate severe disease.

Examining Galectin Binding Specificity Using Glycan Microarrays

Glycan binding proteins (GBPs) possess the unique ability to regulate a wide variety of biological processes through interactions with highly modifiable cell surface glycans. While many studies demonstrate the impact of glycan modification on GBP recognition and activity, the relative contribution of subtle changes in glycan structure on GBP binding can be difficult to define. To overcome limitations in the analysis of GBP-glycan interactions, recent studies utilized glycan microarray platforms containing hundreds of structurally defined glycans. These studies not only provided important information regarding GBP-glycan interactions in general but have also resulted in significant insight into binding specificity and biological activity of the galectin family. We will describe the methods used when employing glycan microarray platforms to examine galectin-glycan binding specificity and function.

What Molecular Recognition Systems Do Mesenchymal Stem Cells/Medicinal Signaling Cells (MSC) Use to Facilitate Cell-Cell and Cell Matrix Interactions? A Review of Evidence and Options

Mesenchymal stem cells, also called medicinal signaling cells (MSC), have been studied regarding their potential to facilitate tissue repair for >30 years. Such cells, derived from multiple tissues and species, are capable of differentiation to a number of lineages (chondrocytes, adipocytes, bone cells). However, MSC are believed to be quite heterogeneous with regard to several characteristics, and the large number of studies performed thus far have met with limited or restricted success. Thus, there is more to understand about these cells, including the molecular recognition systems that are used by these cells to perform their functions, to enhance the realization of their potential to effect tissue repair. This perspective article reviews what is known regarding the recognition systems available to MSC, the possible systems that could be looked for, and alternatives to enhance their localization to specific injury sites and increase their subsequent facilitation of tissue repair. MSC are reported to express recognition molecules of the integrin family. However, there are a number of other recognition molecules that also could be involved such as lectins, inducible lectins, or even a MSC-specific family of molecules unique to these cells. Finally, it may be possible to engineer expression of recognition molecules on the surface of MSC to enhance their function in vivo artificially. Thus, improved understanding of recognition molecules on MSC could further their success in fostering tissue repair.

Relationship of Galectin-3 Expression in Canine Cutaneous Squamous Cell Carcinomas with Histopathological Grading and Proliferation Indices

Squamous cell carcinoma (SCC) is a frequent malignant neoplasm of the skin that usually arises from areas of solar dermatosis. It is characterized by local invasiveness and regional lymph node metastasis, mainly in poorly differentiated tumours. Galectin-3 (Gal-3) is a lectin that is expressed in the nucleus or cytoplasm and has been identified as a prognostic tool for human neoplasms. The purpose of this study was to characterize Gal-3 expression in canine cutaneous SCCs and to investigate its relationship with tumour differentiation and cell proliferation indices. Immunohistochemical analysis of 50 SCCs for Gal-3 revealed no correlation between the localization or intensity of immunolabelling, or number of immunopositive cells, with histological grade of tumour or proliferative activity. The results suggest that Gal-3 expression is not a reliable prognostic marker for cutaneous SCC in dogs.

LGALS1 acts as a pro-survival molecule in AML

The galectin LGALS1 is a glycan binding protein that regulates intracellular (e.g. signal transduction) and extracellular processes (e.g. immunity, leukocyte mobilization) that support cell survival. The protein is best known for its role in RAS signaling. LGALS1 is important in acute lymphoblastic leukemia but its role in acute myeloid leukemia is not well defined. We previously found suppression of LGALS1 in AML cell lines OCI-AML3 and THP-1 sensitized both cell lines to BCL2 inhibitor ABT-737. In this study, we used an in vivo murine OCI-AML3 xenograft model to test whether reduction expression of LGALS1 affects survival. Mice bearing the OCI-AML3 cells with LGALS1 shRNA survived significantly longer than mice with control OCI-AML3 cells. Gene expression profiling using RNASeq was performed using the control and LGALS1 shRNA of p53 WT OCI-AML3 and p53 mutant THP-1 cells. The data reveal distinct differences between the two cell lines in number of genes affected, in pathways associated with these genes, in expression of oncogenes, and in the transcription factors involved. The p53 pathway is prominent in OCI-AML3 cells. An examination of LGALS1 mRNA in an AML patient population reveals elevated LGALS1 mRNA is associated with shorter disease free survival and increased blasts in the BM. This data with the xenograft model data presented suggest LGALS1 may be important in the AML microenvironment. In summary, the data presented here suggest that a strategy targeting LGALS1 may benefit AML patients.

Serum levels of advanced glycation end products and their receptors sRAGE and Galectin-3 in chronic pancreatitis

BACKGROUND

/Objectives: AGE and their receptors like RAGE and Galectin-3 can activate inflammatory pathways and have been associated with chronic inflammatory diseases. Several studies investigated the role of AGE, Galectin-3 and sRAGE in pancreatic diseases, whereas no comprehensive data for chronic pancreatitis (CP) are available.

METHODS

Serum samples from CP patients without an active inflammatory process (85 ACP; 26 NACP patients) and 40 healthy controls were collected. Levels of AGE, sRAGE and Galectin-3 were measured by ELISA. To exclude potential influences of previously described RAGE SNPs on detected serum levels, we analyzed variants rs207128, rs207060, rs1800625, and rs1800624 by melting curve technique in 378 CP patients and 338 controls.

RESULTS

AGE and Galectin-3 serum levels were significantly elevated in both ACP and NACP patients compared to controls (AGE: 56.61 ± 3.043 vs. 31.71 ± 2.308 ng/mL; p < 0.001; Galectin-3: 16.63 ± 0.6297 vs. 10.81 ± 0.4835 ng/mL; p < 0.001). In contrast, mean serum sRAGE levels were significantly reduced in CP patients compared to controls (sRAGE: 829.7 ± 37.10 vs. 1135 ± 55.74 ng/mL; p < 0.001). All results were consistent after correction for gender, age and diabetes mellitus. No genetic association with CP was found.

CONCLUSIONS

Our extensive analysis demonstrated the importance of aging related pathways in the pathogenesis of CP. As the results were consistent in ACP and NACP, both entities most likely share common pathomechanisms. Most probably the involved pathways are a general hallmark of an inflammatory state in CP that is even present in symptom-free intervals.

Serum galectin-3, but not galectin-1, levels are elevated in schizophrenia: implications for the role of inflammation

BACKGROUND

Previous studies have reported that galectin-3 is involved in inflammatory processes in the central nervous system and that neuroinflammation may play a role in the pathogenesis of schizophrenia. However, the link between schizophrenia and various galectins is unclear.

OBJECTIVE

The objective of the present study is to determine whether galectin, a well-known lectin protein that binds to μ-galactoside, is associated with chronic schizophrenia.

METHODS

Thirty-six patients with schizophrenia and 36 healthy controls participated in this study. Schizophrenia symptoms were assessed using the Brief Psychiatry Rating Scale (BPRS). Serum galectin-1 and galectin-3 levels were evaluated using ELISA and compared between the participant groups. Correlation analyses were also performed to examine the relationship between BPRS scores and each galectin level.

RESULTS

Serum galectin-3 levels were significantly higher in patients with schizophrenia than they were in controls (p = 0.009, d = 0.640); however, serum galectin-1 levels were not significantly different between the groups (p = 0.513). No significant correlation was identified between serum galectin-3 level and the total BPRS score; however, a significant positive correlation was found between the serum galectin-3 level and the positive symptom score of the BPRS (ρ = 0.355; p = 0.033). Additionally, a significant negative correlation was identified between serum galectin-3 levels and the negative symptom score of the BPRS (ρ = -0.387; p = 0.020).

CONCLUSIONS

Given the high serum levels of galectin-3 found in patients with schizophrenia compared with that in controls, these findings may support the inflammation hypothesis of schizophrenia.

Is the LysM domain of L. monocytogenes p60 protein suitable for engineering a protein with high peptidoglycan binding affinity?

Lysin motif (LysM) is a highly conserved carbohydrate binding module that is widely present in proteins from both prokaryotes and eukaryotes. LysM domains from many LysM-containing proteins can be taken out of their natural context and retain their ability to bind peptidoglycan. Therefore, LysM has enormous potential for applications in both industry and medicine. This potential has stimulated an intensive search for LysM modules with different evolutionary origins. The p60 protein (Lm-p60) is an NlpC/P60-containing peptidoglycan hydrolase secreted by Listeria monocytogenes. The N-terminus of Lm-p60 contains 2 LysM modules separated by an SH3 module. Our recent study of Lm-p60 demonstrates that the N-terminal half of Lm-p60, comprised of 2 LysM and 1 SH3 module, is able to recognize and bind peptidoglycan. The LysM domain of Lm-p60 contains only 2 LysM modules, which is the minimum number of LysM modules in most NlpC/P60-containing proteins, but it shows strong affinity for peptidoglycan. Moreover, these 2 LysM modules have only 38.64% similarity to each other. These data allowed us to conclude that the 2 LysM modules from Lm-p60 have different evolutionary origins, suggesting that they are suitable candidate peptidoglycan-binding modules for protein engineering in order to create a protein with a high binding affinity to peptidoglycan.

Examining galectin binding specificity using glycan microarrays

Glycan binding proteins (GBPs) possess the unique ability to regulate a wide variety of biological processes through interactions with highly modifiable cell surface glycans. While many studies demonstrate the impact of glycan modification on GBP recognition and activity, the relative contribution of subtle changes in glycan structure on GBP binding can be difficult to define. To overcome limitations in the analysis of GBP-glycan interactions, recent studies utilized glycan microarray platforms containing hundreds of structurally defined glycans. These studies not only provided important information regarding GBP-glycan interactions, but have also resulted in significant insight into the binding specificity and biological activity of the galectin family. We will describe the methods used when employing glycan microarray platforms to examine galectin-glycan binding specificity and function.

Molecular basis for bacterial peptidoglycan recognition by LysM domains

Carbohydrate recognition is essential for growth, cell adhesion and signalling in all living organisms. A highly conserved carbohydrate binding module, LysM, is found in proteins from viruses, bacteria, fungi, plants and mammals. LysM modules recognize polysaccharides containing N-acetylglucosamine (GlcNAc) residues including peptidoglycan, an essential component of the bacterial cell wall. However, the molecular mechanism underpinning LysM-peptidoglycan interactions remains unclear. Here we describe the molecular basis for peptidoglycan recognition by a multimodular LysM domain from AtlA, an autolysin involved in cell division in the opportunistic bacterial pathogen Enterococcus faecalis. We explore the contribution of individual modules to the binding, identify the peptidoglycan motif recognized, determine the structures of free and bound modules and reveal the residues involved in binding. Our results suggest that peptide stems modulate LysM binding to peptidoglycan. Using these results, we reveal how the LysM module recognizes the GlcNAc-X-GlcNAc motif present in polysaccharides across kingdoms.

Galectin-3 drives glycosphingolipid-dependent biogenesis of clathrin-independent carriers

Several cell surface molecules including signalling receptors are internalized by clathrin-independent endocytosis. How this process is initiated, how cargo proteins are sorted and membranes are bent remains unknown. Here, we found that a carbohydrate-binding protein, galectin-3 (Gal3), triggered the glycosphingolipid (GSL)-dependent biogenesis of a morphologically distinct class of endocytic structures, termed clathrin-independent carriers (CLICs). Super-resolution and reconstitution studies showed that Gal3 required GSLs for clustering and membrane bending. Gal3 interacted with a defined set of cargo proteins. Cellular uptake of the CLIC cargo CD44 was dependent on Gal3, GSLs and branched N-glycosylation. Endocytosis of β1-integrin was also reliant on Gal3. Analysis of different galectins revealed a distinct profile of cargoes and uptake structures, suggesting the existence of different CLIC populations. We conclude that Gal3 functionally integrates carbohydrate specificity on cargo proteins with the capacity of GSLs to drive clathrin-independent plasma membrane bending as a first step of CLIC biogenesis.

Galectins in epithelial functions

Galectins are a family of animal lectins comprising 15 members in vertebrates. These proteins are involved in many biological processes including epithelial homeostasis and tumor progression by displaying intracellular and extracellular activities. Hence Galectins can be found either in the cytoplasm or the nucleus, associated with membranes or in the extracellular matrix. Current studies aim at understanding the roles of Galectins in cell-cell and cell-matrix adhesion, cellular polarity and motility. This review discusses recent progress in defining the specificities and mechanisms of action of Galectins as cell regulators in epithelial cells. Physiological, cellular and molecular aspects of Galectin specificities will be treated successively.

Galectin-7 is important for normal uterine repair following menstruation

Menstruation involves the shedding of the functional layer of the endometrium in the absence of pregnancy. At sites where tissue shedding is complete, re-epithelialization of the tissue is essential for repair and termination of bleeding. The complement of growth factors that mediate post-menstrual endometrial repair are yet to be completely elucidated. Galectins regulate many cell functions important for post-menstrual repair, such as cell adhesion and migration. Galectin-7 has a well characterized role in re-epithelialization and wound healing. We hypothesized that galectin-7 would be important in re-epithelialization during post-menstrual repair. We aimed to identify endometrial expression of galectin-7 in women undergoing normal endometrial repair and in women with amenorrhoea who do not experience endometrial breakdown and repair, and to determine whether galectin-7 enhances endometrial re-epithelialization in vitro. Galectin-7 immunolocalized to the endometrial luminal and glandular epithelium during the late secretory and menstrual phases, and to decidualized stroma in regions exhibiting tissue breakdown. Immunostaining intensity was significantly reduced in the endometrium of women with amenorrhoea compared with normally cycling woman. ELISA identified galectin-7 in menstrual fluid at significantly elevated levels compared with matched peripheral plasma. Exogenous galectin-7 (2.5 µg/ml) significantly enhanced endometrial epithelial wound repair in vitro; this was abrogated by inhibition of integrin binding. Galectin-7 elevated epithelial expression of extracellular matrix-related molecules likely involved in repair including β-catenin, contactin and TGF-β1. In conclusion, galectin-7 is produced by the premenstrual and menstrual endometrium, where it accumulates in menstrual fluid and likely acts as a paracrine factor to facilitate post-menstrual endometrial re-epithelialization.

Dendritic cells: a spot on sialic Acid

Glycans decorating cell surface and secreted proteins and lipids occupy the juncture where critical host–host and host-pathogen interactions occur. The role of glycan epitopes in cell–cell and cell-pathogen adhesive events is already well-established, and cell surface glycan structures change rapidly in response to stimulus and inflammatory cues. Despite the wide acceptance that glycans are centrally implicated in immunity, exactly how glycans and their changes contribute to the overall immune response remains poorly defined. Sialic acids are unique sugars that usually occupy the terminal position of the glycan chains and may be modified by external factors, such as pathogens, or upon specific physiological cellular events. At cell surface, sialic acid-modified structures form the key fundamental determinants for a number of receptors with known involvement in cellular adhesiveness and cell trafficking, such as the Selectins and the Siglec families of carbohydrate recognizing receptors. Dendritic cells (DCs) preside over the transition from innate to the adaptive immune repertoires, and no other cell has such relevant role in antigen screening, uptake, and its presentation to lymphocytes, ultimately triggering the adaptive immune response. Interestingly, sialic acid-modified structures are involved in all DC functions, such as antigen uptake, DC migration, and capacity to prime T cell responses. Sialic acid content changes along DC differentiation and activation and, while, not yet fully understood, these changes have important implications in DC functions. This review focuses on the developmental regulation of DC surface sialic acids and how manipulation of DC surface sialic acids can affect immune-critical DC functions by altering antigen endocytosis, pathogen and tumor cell recognition, cell recruitment, and capacity for T cell priming. The existing evidence points to a potential of DC surface sialylation as a therapeutic target to improve and diversify DC-based therapies.

Galectin-3 deficiency protects pancreatic islet cells from cytokine-triggered apoptosis in vitro

Beta cell apoptosis is a hallmark of diabetes. Since we have previously shown that galectin-3 deficient (LGALS3(-/-) ) mice are relatively resistant to diabetes induction, the aim of this study was to examine whether beta cell apoptosis depends on the presence of galectin-3 and to delineate the underlying mechanism. Deficiency of galectin-3, either hereditary or induced through application of chemical inhibitors, β-lactose or TD139, supported survival and function of islet beta cells compromised by TNF-α + IFN-γ + IL-1β stimulus. Similarly, inhibition of galectin-3 by β-lactose or TD139 reduced cytokine-triggered apoptosis of beta cells, leading to conclusion that endogenous galectin-3 propagates beta apoptosis in the presence of an inflammatory milieu. Exploring apoptosis-related molecules expression in primary islet cells before and after treatment with cytokines we found that galectin-3 ablation affected the expression of major components of mitochondrial apoptotic pathway, such as BAX, caspase-9, Apaf, SMAC, caspase-3, and AIF. In contrast, anti-apoptotic molecules Bcl-2 and Bcl-XL were up-regulated in LGALS3(-/-) islet cells when compared to wild-type (WT) counterparts (C57BL/6), resulting in increased ratio of anti-apoptotic versus pro-apoptotic molecules. However, Fas-triggered apoptotic pathway as well as extracellular signal-regulated kinase 1/2 (ERK1/2) was not influenced by LGALS-3 deletion. All together, these results point to an important role of endogenous galectin-3 in beta cell apoptosis in the inflammatory milieu that occurs during diabetes pathogenesis and implicates impairment of mitochondrial apoptotic pathway as a key event in protection from beta cell apoptosis in the absence of galectin-3.

Design and synthesis of glycoprotein-based multivalent glyco-ligands for influenza hemagglutinin and human galectin-3

We report a facile synthesis of glycoprotein-based glyco-ligands and their binding with influenza hemagglutinin and human galectin-3. Human serum albumin (HSA) was used as the scaffold and an Asn-linked complex type N-glycan prepared from chicken eggs was used as the glycan building block. It was found that Cu(I)-catalyzed alkyne-azide cycloaddition reaction (click chemistry) between the alkyne-labeled glycan and the azide-tagged HSA led to an efficient formation of the glycoconjugates. The density of glycan ligands on the protein scaffold was readily varied by changing the molar ratios of the two reactants. Binding studies indicated that the sialylated and desialylated multivalent glycoligands could selectively bind to influenza hemagglutinin and human galectin-3, respectively, with high affinity. In the two glycan-lectin interactions, a clear multivalent effect was observed. Moreover, a cell-based assay showed that the synthetic multivalent glyco-ligands could efficiently inhibit the attachment of galectin-3 to human prostate cancer and lung cancer cell lines. This study suggests that the synthetic glycoprotein-based glyco-ligands can be useful for different applications, including blocking the function of galectin-3 in cancer metastasis.

A galectin-3-dependent pathway upregulates interleukin-6 in the microenvironment of human neuroblastoma

Interleukin-6 (IL-6) is a pleiotropic cytokine with a broad range of physiologic and pathologic functions. Because in cancer, IL-6 contributes to a microenvironment that promotes tumor cell survival, angiogenesis, and inflammation, understanding the mechanism responsible for its production is important. In neuroblastoma, the second most common solid tumor in children, IL-6 is produced not by tumor cells but by stromal cells such as monocytes and bone marrow mesenchymal stem cells (BMMSC). Here we show that the production of IL-6 in BMMSCs is in part stimulated by galectin-3 binding protein (Gal-3BP) secreted by neuroblastoma cells. We identified a distal region of the IL-6 promoter that contains 3 CCATT/enhancer binding protein (C/EBP) binding domains involved in the transcriptional upregulation of IL-6 by Gal-3BP. Gal-3BP interacted with Galectin-3 (Gal-3) present in BMMSCs, and a Gal-3BP/Gal-3/Ras/MEK/ERK signaling pathway was responsible for the transcriptional upregulation of IL-6 in BMMSCs in which Gal-3 has a necessary function. In support of the role of this pathway in human neuroblastoma tumors, Gal-3BP was found to be present in tumor cells and in the adjacent extracellular matrix of 96% of 78 primary neuroblastoma tumor samples examined by immunohistochemistry. Considering the protumorigenic function of IL-6 in cancer, this tumor cell-stromal cell interactive pathway could be a target for anticancer therapy.

Galectin-3 mediates cross-talk between K-Ras and Let-7c tumor suppressor microRNA

Background

Galectin-3 (Gal-3) and active (GTP-bound) K-Ras contribute to the malignant phenotype of many human tumors by increasing the rate of cell proliferation, survival, and migration. These Gal-3-mediated effects result from a selective binding to K-Ras.GTP, causing increased nanoclustering in the cell membrane and leading to robust Ras signaling. Regulation of the interactions between Gal-3 and active K-Ras is not fully understood.

Methods and Findings

To gain a better understanding of what regulates the critical interactions between these two proteins, we examined the role of Gal-3 in the regulation of K-Ras by using Gal-3-knockout mouse embryonic-fibroblasts (Gal-3^-/- MEFs) and/or Gal-3/Gal-1 double-knockout MEFs. We found that knockout of Gal-3 induced strong downregulation (∼60%) of K-Ras and K-Ras.GTP. The downregulation was somewhat more marked in the double-knockout MEFs, in which we also detected robust inhibition(∼50%) of ERK and Akt activation. These additional effects are probably attributable to inhibition of the weak interactions of K-Ras.GTP with Gal-1. Re-expression of Gal-3 reversed the phenotype of the Gal-3^-/- MEFs and dramatically reduced the disappearance of K-Ras in the presence of cycloheximide to the levels seen in wild-type MEFs. Furthermore, phosphorylation of Gal-3 by casein kinase-1 (CK-1) induced translocation of Gal-3 from the nucleus to the cytoplasm and the plasma membrane, leading to K-Ras stabilization accompanied by downregulation of the tumor suppressor miRNA let-7c, known to negatively control K-Ras transcription.

Conclusions

Our results suggest a novel cross-talk between Gal-3-mediated downregulation of let 7c microRNA (which in turn negatively regulates K-Ras transcription) and elucidates the association among Gal-3 let-7c and K-Ras transcription/translation, cellular compartmentalization and activity.

Antiproliferative effects of galectin-1 from Rana catesbeiana eggs on human leukemia cells and its binding proteins in human cells

Galectin-1 from American bullfrog, RCG1, was isolated to high purity, and its growth inhibitory properties against human cells were examined. The results demonstrated that highly purified RCG1 induced large cell aggregates and revealed cell-type-specific growth inhibition. It significantly inhibited all human leukemia cell lines tested such as HL-60, U937, and K562 cells but did not inhibit human colon cancer cell line, Colo 201, or mouse mammary tumor cell line FM3A cells. Although most of the galectin-induced growth inhibitions are known to be apoptic, RCG1 induced growth arrest and neither apoptosis nor necrosis. RCG1-mediated growth inhibition was specifically suppressed by the corresponding sugar, lactose, but not by sucrose or even the structurally similar sugar, melibiose. Several studies have reported that galectin-mediated biological functions were modulated by charge modification. Since the high purity of RCG1 was demonstrated but a moderate degree of growth inhibition occurred, it is possible protein charge modification was examined by isoelectric focusing, and it was found to be highly heterogeneous in charge. RCG1 binding proteins in human cells were analyzed by lectin blotting using biotinylated RCG1, and lectin blotting revealed that in human cell extracts the specific proteins at molecular weight 37 and 50 kDa possessed the responsive features of RCG1 binding and lactose competition.

Affinity of anti-insulin-like growth factor Ι receptor antibody binding to the receptor altered by plant lectins

The binding ability of anti-insulin-like growth factor Ι receptor (IGF-ΙR) single-chain variable fragments (scFvs) to IGF-IR was measured in the presence of plant lectins. Combinations of concanavalin A (Con A), wheat germ agglutinin (WGA), or peanut agglutinin (PNA) and 1H7 or 3B7 anti-IGF-ΙR scFv/phage antibodies that were previously produced and characterized were used. WGA inhibited binding of both scFvs proteins to the receptor. PNA slightly enhanced the binding of 1H7 scFv and phage antibody to the receptor. Con A led to enhancement of 3B7 scFv-binding but had no effect in a test of phage antibodies and determination of kinetic parameters. The effect of lectins differed for scFvs and phage antibodies, implying that affinity altered by lectins is dependent upon the molecular structure of the antibodies. Results indicated that animal lectins may affect the affinity of therapeutic antibodies targeting cell membrane receptors in vivo.

Expression of galectin-9 by IFN-γ stimulated human nasal polyp fibroblasts through MAPK, PI3K, and JAK/STAT signaling pathways

Galectin-9 exhibited potent and selective eosinophil chemoattractant activity and attracted eosinophils in vitro and in vivo. Nasal polyposis is a chronic inflammatory disease of the upper airway characterized by the marked presence of inflammatory cells, particularly eosinophils. Thus, galectin-9 may be implicated in the pathogenesis of nasal polyposis. The study was designed to investigate whether interferon-gamma (IFN-γ) can induce the augmentation of galectin-9 expression and induce the expression of galectin-9 in nasal polyps. We examined the correlation between galectin-9 expression and eosinophil infiltration in nasal polyps. In addition, we identified the signaling pathways involved in the elevation of galectin-9 expression in response to IFN-γ. Our data demonstrate that the involvement of mitogen-activated protein kinases (MAPKs), phosphatidylinositol 3 phosphate kinase (PI3K), and Janus kinase/signal transducer and activator of transcription (JAK/STAT) may play important roles in the selective recruitment of eosinophils in nasal polyp tissues through the production of galectin-9. These findings suggest that galectin-9 expression is associated with eosinophil infiltration in polyps of patients with nasal polyposis.

Galectin-3 secretion and tyrosine phosphorylation is dependent on the calpain small subunit, Calpain 4

Cell adhesion and migration are important events that occur during embryonic development, immune surveillance, wound healing and in tumor metastasis. It is a multi-step process that involves both mechanical and biochemical signaling that results in cell protrusion, adhesion, contraction and retraction. Each of these events generates mechanical forces into the environment measured as traction forces. We have previously found that the calpain small subunit, Calpain 4, is required for normal traction forces, and that this mechanism is independent of the catalytic activities of the holoenzymes that are formed between Calpain 4 and each of the proteolytic heavy chains of Calpain 1 and 2. To define a potential mechanism for the Calpain 4 regulation of traction force, we have evaluated the levels of tyrosine phosphorylation, a hallmark of force dependent signaling within focal adhesions. Using 2D gel electrophoresis we compared tyrosine phosphorylation profiles of Calpain 4 deficient mouse embryonic fibroblasts (MEFs) to the levels in wildtype MEFs and MEF's deficient in the large catalytic subunits, Capn1 and Capn2. Of particular interest, was the identification of Galectin-3, a galactose binding protein known to interact with integrins. Galectin-3 has previously been shown to regulate cell adhesion and migration in both normal and tumor cells; however its full mechanism remains elusive. We have found that Calpain 4 is essential for the tyrosine phosphorylation of galectin-3, and its ultimate secretion from the cell, and speculate that its secretion interferes with the production of traction forces.

Emerging role of alpha2,6-sialic acid as a negative regulator of galectin binding and function

Galectins are β-galactoside-binding lectins that regulate diverse cell behaviors, including adhesion, migration, proliferation, and apoptosis. Galectins can be expressed both intracellularly and extracellularly, and extracellular galectins mediate their effects by associating with cell-surface oligosaccharides. Despite intensive current interest in galectins, strikingly few studies have focused on a key enzyme that acts to inhibit galectin signaling, namely β-galactoside α2,6-sialyltransferase (ST6Gal-I). ST6Gal-I adds an α2,6-linked sialic acid to the terminal galactose of N-linked glycans, and this modification blocks galectin binding to β-galactosides. This minireview summarizes the evidence suggesting that ST6Gal-I activity serves as an "off switch" for galectin function.

Mitochondrial proteomic approach reveals galectin-7 as a novel BCL-2 binding protein in human cells

Our results reveal a network of new potential Bcl-2 partners identified through the Bcl-2 immunocapture and mass spectrometry approach and analyzed by gene ontology mining. Importantly, we report for the first time the identification of galectin-7, a member of a family of β-galactoside-binding lectins, as a new mitochondrial Bcl-2 interacting partner.

Although the anti-apoptotic activity of Bcl-2 has been extensively studied, its mode of action remains incompletely understood. Deciphering the network of Bcl-2 interacting factors is necessary to better understand the key function of Bcl-2 in apoptosis initiation. To identify novel Bcl-2 mitochondrial partners, we have combined a Bcl-2 immunocapture with a mass spectrometry analysis using highly pure mitochondrial fractions isolated from human cancer cells. We identified at high confidence 127 potential Bcl-2–interacting proteins. Gene ontology mining reveals enrichment for mitochondrial proteins, endoplasmic reticulum–associated proteins, and cytoskeleton-associated proteins. Importantly, we report the identification of galectin-7 (Gal7), a member of a family of β-galactoside–binding lectins that was already known to exhibit a pro-apoptotic function, as a new mitochondrial Bcl-2 interacting partner. Our data further show that endogenous Bcl-2 coimmunoprecipitates with Gal7 and that recombinant Gal7 directly interacts with recombinant Bcl-2. A fraction of Gal7 is constitutively localized at mitochondria in a Bcl-2–dependent manner and sensitizes the mitochondria to the apoptotic signal. In addition, we show that the Bcl-2/Gal7 interaction is abolished following genotoxic stress. Taken together, our findings suggest that the binding of Gal7 to Bcl-2 may constitute a new target for enhancing the intrinsic apoptosis pathway.

Galectin-3 expression is associated with bladder cancer progression and clinical outcome

Galectin-3 belongs to a family of carbohydrate-binding proteins whose function is not fully characterized. However, it is believed to play a role in adhesion, proliferation and apoptosis in solid tumors. We aimed at investigating galectin-3 expression in bladder cancer. Galectin-3 expression was assessed by transcript profiling (U133A arrays) in a series or frozen bladder tumors (n = 105). Immunohistochemistry was performed on tissue arrays containing bladder tumors (n = 389) to evaluate associations of protein expression patterns of galectin-3 with proliferation (Ki67), apoptosis (apopdetek), bcl-2, and clinicopathologic variables. Galectin-3 protein levels were then quantified in 160 urinary specimens of bladder cancer patients and controls by enzymeimmunoanalysis. Galectin-3 gene expression levels increased in invasive tumours as compared with non-muscle invasive lesions (p = 0.001) and were associated with poor survival in patients with advanced disease (p = 0.03). Protein expression patterns also correlated galectin-3 with tumor stage (p < 0.001), grade (p = 0.03), Ki67 and apopdetek (p < 0.001), and overall survival in patients with T1G3 tumors (p < 0.001). Furthermore, galectin-3 urinary levels segregated bladder cancer patients from controls with high diagnostic accuracy (AUC = 0.7). Independent series of bladder tumors showed that transcript and protein levels of galectin-3 were differentially expressed along bladder cancer progression. Urinary protein levels served to identify bladder cancer patients. These observations suggest a role for galectin-3 as a biomarker for bladder cancer diagnostics, staging, and outcome prognosis.

Galectin-3 is critical for the development of the allergic inflammatory response in a mouse model of atopic dermatitis

Galectin-3 belongs to a family of beta-galactoside-binding animal lectins expressed in several cell types, including epithelial and immune cells. To establish the role of galectin-3 in the development of allergic skin inflammation, we compared inflammatory skin responses of galectin-3-deficient (gal3(-/-)) and wild-type (gal3(+/+)) mice to epicutaneous sensitization with ovalbumin (OVA). OVA-treated gal3(-/-) mice exhibited markedly reduced epidermal thickening, lower eosinophil infiltration, and lower serum IgE levels compared with gal3(+/+) mice. The former evoked lower interleukin-4, but higher interferon-gamma, mRNA expression at OVA-treated skin sites. Moreover, gal3(-/-) splenocytes from OVA-sensitized mice secreted more interleukin-12 compared with gal3(+/+) splenocytes. In addition, antigen presentation by gal3(-/-) dendritic cells to T cells in vitro were T helper cell (Th1)-polarized relative to presentation by gal3(+/+) dendritic cells. When exposed to OVA, recipients engrafted with T cells from gal3(-/-) OVA-specific T cell receptor transgenic mice developed significantly reduced dermatitis and a markedly lower Th2 response compared with recipients of comparable gal3(+/+) T cells. We conclude that galectin-3 is critical for the development of inflammatory Th2 responses to epicutaneously administered antigens; in its absence, mice develop a Th1-polarized response. This regulatory effect of galectin-3 on Th development is exerted at both the dendritic cell and T cell levels. Our studies suggest that galectin-3 may play an important role in the acute phase of human atopic dermatitis.

Glycobiology in the cytosol: the bitter side of a sweet world

Progress in glycobiology has undergone explosive growth over the past decade with more of the researchers now realizing the importance of glycan chains in various inter- and intracellular processes. However, there is still an area of glycobiology awaiting exploration. This is especially the case for the field of "glycobiology in the cytosol" which remains rather poorly understood. Yet evidence is accumulating to demonstrate that the glycoconjugates and their recognition molecules (i.e. lectins) are often present in this subcellular compartment.

Expression of galectin-3 involved in prognosis of patients with hepatocellular carcinoma

AIMS

Galectins are multifunctional lectins binding to the beta-galactoside of glycoproteins that affect diverse physiological and pathophysiological processes such as development, inflammation and tumor growth. In hepatocellular carcinoma (HCC), the over-expression of galectin-1, 3, and 4 has been reported, although their function and correlation with tumor progression remain unknown. Thus, we aimed to assess the role of galectin-3 during HCC progression.

METHODS

Specimens were obtained during curative operations and used for immunohistochemical analysis of galectin-3 (n = 52), and statistically assessed for correlations with the clinical profiles and the prognoses of the patients. The serum galectin-3 levels from the patients with liver diseases including HCC were assessed by ELISA.

RESULTS

In total, galectin-3 expression was found in 34 of 52 tumors (65%) and was statistically correlated with histological differentiation and vascular invasion. Kaplan-Meier's analysis showed that patients with galectin-3 expression tended to relapse in the earlier phase and had worse overall survival. In particular, a higher expression rate of nuclear galectin-3 showed a markedly worse prognosis, and it was independent in the multivariate analysis for overall survival. Serum galectin-3 levels were significantly increased in HCC compared with chronic liver disease. The sensitivity and specificity of galectin-3 were equivalent to alpha-fetoprotein and Vitamin K absence or antagonist II, and the combination of HCC biomarkers with galectin-3 improved the diagnostic performance.

CONCLUSIONS

Galectin-3 expression was involved in the tumor progression and related to the prognosis of HCC. Our observations suggested that galectin-3 could be a novel tumor marker and therapeutic target.

Structural basis for chitotetraose coordination by CGL3, a novel galectin-related protein from Coprinopsis cinerea

Recent advances in genome sequencing efforts have revealed an abundance of novel putative lectins. Among these, many galectin-related proteins, characterized by many conserved residues but intriguingly lacking critical amino acids, have been found in all corners of the eukaryotic superkingdom. Here we present a structural and biochemical analysis of one representative, the galectin-related lectin CGL3 found in the inky cap mushroom Coprinopsis cinerea. This protein contains all but one conserved residues known to be involved in beta-galactoside binding in galectins. A Trp residue strictly conserved among galectins is changed to an Arg in CGL3 (R81). Accordingly, the galectin-related protein is not able to bind lactose. Screening of a glycan array revealed that CGL3 displays preference for oligomers of beta1-4-linked N-acetyl-glucosamines (chitooligosaccharides) and GalNAc beta 1-4GlcNAc (LacdiNAc). Carbohydrate-binding affinity of this novel lectin was quantified using isothermal titration calorimetry, and its mode of chitooligosaccharide coordination not involving any aromatic amino acid residues was studied by X-ray crystallography. Structural information was used to alter the carbohydrate-binding specificity and substrate affinity of CGL3. The importance of residue R81 in determining the carbohydrate-binding specificity was demonstrated by replacing this Arg with a Trp residue (R81W). This single-amino-acid change led to a lectin that failed to bind chitooligosaccharides but gained lactose binding. Our results demonstrate that, similar to the legume lectin fold, the galectin fold represents a conserved structural framework upon which dramatically altered specificities can be grafted by few alterations in the binding site and that, in consequence, many metazoan galectin-related proteins may represent lectins with novel carbohydrate-binding specificities.

Galectin-3 regulates RasGRP4-mediated activation of N-Ras and H-Ras

Galectin-3 (Gal-3) is a pleiotropic beta-galactoside-binding protein expressed at relatively high levels in human neoplasms. Its carbohydrate recognition domain (CRD) contains a hydrophobic pocket that can accommodate the farnesyl moiety of K-Ras. Binding of K-Ras to Gal-3 stabilizes K-Ras in its active (GTP-bound) state. Gal-3, which does not interact with N-Ras, was nevertheless shown to reduce N-Ras-GTP in BT-549 cells by an unknown mechanism that we explored here. First, comparative analysis of various cancer cell lines (glioblastomas, breast cancer cells and ovarian carcinomas) showed a positive correlation between low N-Ras-GTP/high K-Ras-GTP phenotype and Gal-3 expression levels. Next we found that epidermal growth factor-stimulated GTP loading of N-Ras, but not of K-Ras, is blocked in cells expressing high levels of Gal-3. Activation of Ras guanine nucleotide releasing proteins (RasGRPs) by phorbol 12-myristate 13-acetate (PMA) or downregulation of Gal-3 by Gal-3 shRNA increased the levels of N-Ras-GTP in Gal-3 expressing cells. We further show that the N-terminal domain of Gal-3 interacts with and inhibits RasGRP4-mediated GTP loading on N-Ras and H-Ras proteins. Growth of BT-549 cells stably expressing the Gal-3 N-terminal domain was strongly attenuated. Overall, these experiments demonstrate a new control mechanism of Ras activation in cancer cells whereby the Gal-3 N-terminal domain inhibits activation of N-Ras and H-Ras proteins.

Involvement of decreased Galectin-3 expression in the pathogenesis and progression of prostate cancer

BACKGROUND

Altered expression or loss of function of Galectin-3 (Gal-3) was suggested to be involved in the pathogenesis and progression of various human cancer entities. The aim of the present study is to determine the expression of Gal-3 in prostate tissue emerging from a benign to a malignant, in the beginning hormone-sensitive and finally hormone-refractory status to further elucidate the role of this carbohydrate-binding protein for the pathogenesis and/or progression of malignant prostatic disease.

MATERIALS AND METHODS

Five hundred and eighty three tissue samples from malignant, tumor adjacent benign, and histologically benign intra-prostatic areas, retrieved out of 25 whole mounted prostate cancer (CaP) specimens and additional 95 samples of hormone-refractory CaP, were processed to tissue microarrays. Immunohistochemical Gal-3 expression was correlated with clinicopathological parameters among the different tissue entities.

RESULTS

Gal-3 expression was significantly decreased in the hormone-sensitive CaP specimens when compared with the respective benign tissue either localized far distant from the malignant lesion (P < 0.0001, Wilcoxon test) or directly neighboring the primary tumor (P < 0.0001). The staining reaction in the benign tissue areas directly neighboring the primary cancerous lesions differed significantly from the benign glands localized distant from the primary tumors (P < 0.001). A statistically highly significant, almost complete loss of Gal-3 was observed in the hormone-refractory when compared with the hormone-sensitive tumors (P < 0.0001; mean staining score: 27.7% vs. 8.5%).

CONCLUSIONS

The present investigation clearly indicates decreased expression of Gal-3 to be substantially involved in the pathogenesis and further progression of CaP from benign prostate glands to a finally hormone-refractory malignant disease.