The two endocytic pathways mediated by the carbohydrate recognition domain and regulated by the collagen-like domain of galectin-3 in vascular endothelial cells

A homogalacturonan-rich pectic polysaccharide isolated from Lonicera japonica Thunb. modulates galectin-4-mediated bioactivity and anti-hepatocellular carcinoma activity

L. japonica is a traditional Chinese medicine with dual-use properties. Herein, a HG-rich pectic polysaccharide, WLJP-03A, was purified from the dried flowers of L. japonica, which composed of Rha (5 %), GalA (60 %), Gal (5 %), and Ara (30 %), with a molecular weight of 28.1 kDa. WLJP-03A could be defined as an HG backbone with α-(1 → 3,5)-linked and α-(1 → 5)-linked arabinan, β-(1 → 3,6)-linked and β-(1 → 3)-linked galactan, and type II arabinogalactan side chains. Its interaction with two truncated structural domain proteins of galectin-4 (Gal-4) revealed stronger binding of WLJP-03A to Gal-4C (MIC = 15 μg/mL) than to Gal-4N (MIC = 62 μg/mL), indicating that WLJP-03A mainly interacted with the C-terminal CRD to inhibit the biological activity of Gal-4. Furthermore, in vitro antitumor assays showed that WLJP-03A could inhibit the cellular proliferation and migration of HCCLM3 cells induced by Gal-4. These results provide new insights into the structure-activity relationship between L. japonica polysaccharide and Gal-4.

Oligosaccharides from Stellaria dichotoma L. var. lanceolate bind to galectin-3 and ameliorate effects of colitis

Even though Stellaria dichotoma L. var. lanceolate (S. dichotoma) is a well-known medicinal plant in the family Caryophyllaceae, its oligosaccharides remain unexplored in terms of their potential as bioactive agents. Here, we isolated a mixture of oligosaccharides from S. dichotoma (Yield: 12 % w/w), that are primarily non-classical raffinose family oligosaccharides (RFOs). Nine major oligosaccharides were purified and identified from the mixture, including sucrose, raffinose, 1-planteose, lychnose, stellariose, along with four new non-classical RFOs. Two of the four new oligosaccharides are linear hexose pentamers with α-galactosyl extensions on their lychnose moieties, and the other two are branched hexose hexamers with α-galactosyl extensions on their stellariose groups. Their interactions with galectin-3 (Gal-3) revealed significant binding, with the terminal galactose providing enhanced affinity for the lectin. Notably, Gal-3 residues Arg144, His158, Asn160, Arg162, Asn174, Trp181, Glu184 and Arg186 coordinate with the lychnose. In vivo studies using the dextran sulfate sodium (DSS) mouse model for colitis demonstrated the ability of these carbohydrates in mitigating ulcerative colitis (UC). Overall, our study has provided structural information and potential applications of S. dichotoma oligosaccharides, also offers new approaches for the development of medicinal oligosaccharides.

Breaking barriers: How modified citrus pectin inhibits galectin-8

Inhibition of galectin-3-mediated interactions by modified citrus pectin (MCP) could affect several rate-limiting steps in cancer metastasis, but the ability of MCP to antagonize galectin-8 function remains unknown. We hypothesized that MCP could bind to galectin-8 in addition to galectin-3. In this study, a combination of gradual ethanol precipitation and DEAE-Sepharose Fast Flow chromatography was used to isolate several fractions from MCP. The ability of these fractions to antagonize galectin-8 function was studied as well as the primary structure and initial structure-function relationship of the major active component MCP-30-3. The results showed that MCP-30-3 (168 kDa) was composed of Gal (13.8%), GalA (63.1%), GlcA (13.0%), and Glc (10.1%). MCP-30-3 could specifically bind to galectin-8, with an MIC value of 0.04 mg mL-1. After MCP-30-3 was hydrolyzed by β-galactosidase or pectinase, its binding activity was significantly reduced. These results provide new insights into the interaction between MCP structure and galectin function, as well as the potential utility in the development of functional foods.

Isomerization of proline-46 in the N-terminal tail of galectin-3 enhances T cell apoptosis via the ROS-ERK pathway

Galectin-3 (Gal-3) is unique in the galectin family, due to the presence of a long N-terminal tail (NT) arising from its conserved carbohydrate recognition domain (CRD). Although functional significance of the NT has remained elusive, our previous studies demonstrated the importance of NT prolines to Gal-3 function. Here, we show that during the time Gal-3 stands in solution for three or more days, Gal-3 NT undergoes a slow, intra-molecular, time-dependent conformational/dynamical change associated with proline cis-trans isomerization. From initial dissolution of Gal-3 in buffer to three days in solution, Gal-3-mediated T cell apoptosis is enhanced from 23 % to 37 %. Western blotting and flow cytometry show that the enhancement occurs via the ROS-ERK pathway, and not by the PKC-ERK pathway. To assess which proline(s) is (are) responsible for this effect, we individually mutated all 14 NT prolines within the first 68 residues to alanines, and assessed their effect on ROS production. Our study shows that isomerization of P46 alone is responsible for the upregulation of ROS and T cell apoptosis. NMR studies show that this unique effect is mediated by a change in dynamic interactions between the NT and CRD F-face, which in turn leads to this change in Gal-3 function.

KIT Mutations Correlate with Higher Galectin Levels and Brain Metastasis in Breast and Non-Small Cell Lung Cancer

To investigate a potential role for galectins as biomarkers that enable diagnosis or prognostication of breast or non-small cell lung cancer, the serum levels of galectins -1, -3, -7, -8, and -9 of cancer patients determined by ELISA assays were compared to the mutation status of 50 known cancer-critical genes, which were determined using multiplex PCR in tumors of the same patients. Mutations in the KIT proto-oncogene, which codes for the c-Kit protein, a receptor tyrosine kinase, correlated with higher levels of galectins -1, -3, -8, and -9 in breast cancer patients and galectin-1 in non-small cell lung cancer patients. Mutations in the KIT gene were more likely found in brain metastases from both of these primary cancers. The most common KIT mutation in our panel was p.M541L, a missense mutation in the transmembrane domain of the c-Kit protein. These results demonstrate an association between KIT oncogenic signaling and elevated serum galectins in patients with metastatic disease. Changes in protein trafficking and the glycocalyx composition of cancer cells may explain the observed alterations in galectin expression. This study can be useful for the targeted selection of receptor tyrosine kinase and galectin inhibitor anti-cancer treatments.

Combination of genetically engineered T cells and immune checkpoint blockade for the treatment of cancer

Immune checkpoint (IC) blockade using monoclonal antibodies is currently one of the most successful immunotherapeutic interventions to treat cancer. By reinvigorating antitumor exhausted T cells, this approach can lead to durable clinical responses. However, the majority of patients either does not respond or present a short-lived response to IC blockade, in part due to a scarcity of tumor-specific T cells within the tumor microenvironment. Adoptive transfer of T cells genetically engineered to express chimeric antigen receptors (CARs) or engineered T cell receptors (TCRs) provide the necessary tumor-specific immune cell population to target cancer cells. However, this therapy has been considerably ineffective against solid tumors in part due to IC-mediated immunosuppressive effects within tumor microenvironment. These limitations could be overcome by associating adoptive cell transfer of genetically engineered T cells and IC blockade. In this comprehensive review, we highlight the strategies and outcomes of preclinical and clinical attempts to disrupt IC signaling in adoptive T cell transfer against cancer. These strategies include combined administration of genetically engineered T cells and IC inhibitors, engineered T cells with intrinsic modifications to disrupt IC signaling and the design of CARs against IC molecules. The current landscape indicates that the synergy of the fast-paced refinements of gene-editing technologies and synthetic biology and the increased comprehension of IC signaling will certainly translate into novel and more effective immunotherapeutic approaches to treat patients with cancer.

Purification, chemical analysis and inhibitory effects on galectin-3 of enzymatic pH-modified citrus pectin

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EMCP is fractionated by ion-exchange and gel permeation chromatographies.

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EMCP fractions contain glucan backbone and different saccharides as side chains.

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RG-II domain may weaken the binding strength between EMCP fractions and Gal-3.

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EMCP-3p and EMCP-2p exhibit strong cytotoxicity against MCF-7 and A549 cell lines.

Modified citrus pectin (MCP), a commercially available dietary supplement prepared from citrus pectin, contains several different polysaccharide domains, but its primary chemical structure and the binding epitopes that antagonize galectin-3 function remain unclear. In this study, five fractions were isolated from MCP after endo-polygalacturonase degradation (EMCP) and a combination of DEAE-cellulose and Sepharose CL-6B or Sephadex G-75 chromatography. Their primary structures, abilities to inhibit galectin-3-mediated hemagglutination, and antiproliferation activities on MCF-7 and A549 cell lines were studied. Results showed that EMCP-3p, one of the five fractions, was composed of Glc (89.8%), Gal (3.8%), Ara (3.1%), GalA (1.1%), Man (0.9%), and Rha (1.3%) with an average molecular weight of 88.4 KDa, which had the most substantial degree of galectin-3 inhibition with an MIC of 31.25 μg/mL, and it exhibited remarkable cytotoxicity against MCF-7 (36.7%) and A549 (57.4%) cell lines. These results provide new insight into the structure–function relationships of EMCP-derived polysaccharides.

Unraveling How Tumor-Derived Galectins Contribute to Anti-Cancer Immunity Failure

Simple Summary

This review compiles our current knowledge of one of the main pathways activated by tumors to escape immune attack. Indeed, it integrates the current understanding of how tumor-derived circulating galectins affect the elicitation of effective anti-tumor immunity. It focuses on several relevant topics: which are the main galectins produced by tumors, how soluble galectins circulate throughout biological liquids (taking a body-settled gradient concentration into account), the conditions required for the galectins’ functions to be accomplished at the tumor and tumor-distant sites, and how the physicochemical properties of the microenvironment in each tissue determine their functions. These are no mere semantic definitions as they define which functions can be performed in said tissues instead. Finally, we discuss the promising future of galectins as targets in cancer immunotherapy and some outstanding questions in the field.

Abstract

Current data indicates that anti-tumor T cell-mediated immunity correlates with a better prognosis in cancer patients. However, it has widely been demonstrated that tumor cells negatively manage immune attack by activating several immune-suppressive mechanisms. It is, therefore, essential to fully understand how lymphocytes are activated in a tumor microenvironment and, above all, how to prevent these cells from becoming dysfunctional. Tumors produce galectins-1, -3, -7, -8, and -9 as one of the major molecular mechanisms to evade immune control of tumor development. These galectins impact different steps in the establishment of the anti-tumor immune responses. Here, we carry out a critical dissection on the mechanisms through which tumor-derived galectins can influence the production and the functionality of anti-tumor T lymphocytes. This knowledge may help us design more effective immunotherapies to treat human cancers.

Galectin-3 as a modifier of anti-microbial immunity: Unraveling the unknowns

Galectins play diverse roles in pathophysiology of infectious diseases and cancers. Galectin-3 is one of the most studied family member and the only chimeric type lectin. Many aspects of its biogenesis, range of activities, and the disease-modifying potential particularly during microbial infections are yet to be known. We review our current understanding of these issues and also highlight gaps in better defining the immune modulatory potential of galectin-3 during different stages of host responsiveness when an infection sets in. Additionally, we discuss commonly used strategies to disrupt galectin-3 functions both extracellulalry and intracellularly. Existing and improved novel strategies could help fine-tune immune responses to achieve better prognosis of infectious diseases.

Interaction between Galectin-3 and Integrins Mediates Cell-Matrix Adhesion in Endothelial Cells and Mesenchymal Stem Cells

Galectin-3 (Gal-3) is a β-galactoside-binding protein that influences various cell functions, including cell adhesion. We focused on the role of Gal-3 as an extracellular ligand mediating cell-matrix adhesion. We used human adipose tissue-derived stem cells and human umbilical vein endothelial cells that are promising for vascular tissue engineering. We found that these cells naturally contained Gal-3 on their surface and inside the cells. Moreover, they were able to associate with exogenous Gal-3 added to the culture medium. This association was reduced with a β-galactoside LacdiNAc (GalNAcβ1,4GlcNAc), a selective ligand of Gal-3, which binds to the carbohydrate recognition domain (CRD) in the Gal-3 molecule. This ligand was also able to detach Gal-3 newly associated with cells but not Gal-3 naturally present on cells. In addition, Gal-3 preadsorbed on plastic surfaces acted as an adhesion ligand for both cell types, and the cell adhesion was resistant to blocking with LacdiNAc. This result suggests that the adhesion was mediated by a binding site different from the CRD. The blocking of integrin adhesion receptors on cells with specific antibodies revealed that the cell adhesion to the preadsorbed Gal-3 was mediated, at least partially, by β1 and αV integrins-namely α5β1, αVβ3, and αVβ1 integrins.

Galectin-3 N-terminal tail prolines modulate cell activity and glycan-mediated oligomerization/phase separation

Galectin-3 (Gal-3) has a long, aperiodic, and dynamic proline-rich N-terminal tail (NT). The functional role of the NT with its numerous prolines has remained enigmatic since its discovery. To provide some resolution to this puzzle, we individually mutated all 14 NT prolines over the first 68 residues and assessed their effects on various Gal-3-mediated functions. Our findings show that mutation of any single proline (especially P37A, P55A, P60A, P64A/H, and P67A) dramatically and differentially inhibits Gal-3-mediated cellular activities (i.e., cell migration, activation, endocytosis, and hemagglutination). For mechanistic insight, we investigated the role of prolines in mediating Gal-3 oligomerization, a fundamental process required for these cell activities. We showed that Gal-3 oligomerization triggered by binding to glycoproteins is a dynamic process analogous to liquid-liquid phase separation (LLPS). The composition of these heterooligomers is dependent on the concentration of Gal-3 as well as on the concentration and type of glycoprotein. LLPS-like Gal-3 oligomerization/condensation was also observed on the plasma membrane and disrupted endomembranes. Molecular- and cell-based assays indicate that glycan binding-triggered Gal-3 LLPS (or LLPS-like) is driven mainly by dynamic intermolecular interactions between the Gal-3 NT and the carbohydrate recognition domain (CRD) F-face, although NT-NT interactions appear to contribute to a lesser extent. Mutation of each proline within the NT differentially controls NT-CRD interactions, consequently affecting glycan binding, LLPS, and cellular activities. Our results unveil the role of proline polymorphisms (e.g., at P64) associated with many diseases and suggest that the function of glycosylated cell surface receptors is dynamically regulated by Gal-3.

Self-Assembled Folic Acid-Targeted Pectin-Multi-Arm Polyethylene Glycol Nanoparticles for Tumor Intracellular Chemotherapy

Ursolic acid is widely used as an effective anticancer drug for the treatment of various cancers. However, its poor water solubility, short circulation time in vivo, and lack of targeting have made it a burden for clinical applications. We report a self-assembled folate-modified pectin nanoparticle for loading ursolic acid (HCPT@F-Pt-PU NPs) and embed the anticancer drug hydroxycamptothecin to achieve synergistic treatment with ursolic acid. In addition, the galactose residue of the pectin molecule can be recognized by the asialoglycoprotein receptor on the surface of the liver cancer cell, promoting the rapid penetration and release of HCPT@F-Pt-PU NPs intracellularly. In particular, the introduction of multiarm polyethylene glycol can improve the uniformity (106 nm) and concealment of the nanoparticles and avoid the early release of the drug or the toxicity to normal cells. HCPT@F-Pt-PU NPs have a high drug loading (7.27 wt %) and embedding efficiency (19.84 wt %) and continuous circulation up to 80 h, leading to more apoptosis (91.61%). HCPT@F-Pt-PU NP intracellular drug delivery will be a promising strategy.

Mapping glycan-mediated galectin-3 interactions by live cell proximity labeling

Galectin-3 is a glycan-binding protein (GBP) that binds β-galactoside glycan structures to orchestrate a variety of important biological events, including the activation of hepatic stellate cells and regulation of immune responses. While the requisite glycan epitopes needed to bind galectin-3 have long been elucidated, the cellular glycoproteins that bear these glycan signatures remain unknown. Given the importance of the three-dimensional (3D) arrangement of glycans in dictating GBP interactions, strategies that allow the identification of GBP receptors in live cells, where the native glycan presentation and glycoprotein expression are preserved, have significant advantages over static and artificial systems. Here we describe the integration of a proximity labeling method and quantitative mass spectrometry to map the glycan and glycoprotein interactors for galectin-3 in live human hepatic stellate cells and peripheral blood mononuclear cells. Understanding the identity of the glycoproteins and defining the structures of the glycans will empower efforts to design and develop selective therapeutics to mitigate galectin-3-mediated biological events.

Mucin-type O-glycosylation controls pluripotency in mouse embryonic stem cells via Wnt receptor endocytosis

Mouse embryonic stem cells (ESCs) can differentiate into a range of cell types during development, and this pluripotency is regulated by various extrinsic and intrinsic factors. Mucin-type O-glycosylation has been suggested to be a potential factor in the control of ESC pluripotency, and is characterized by the addition of N-acetylgalactosamine (GalNAc) to serine or threonine residues of membrane-anchored proteins and secreted proteins. To date, the relationship between mucin-type O-glycosylation and signaling in ESCs remains undefined. Here, we identify the elongation pathway via C1GalT1 that synthesizes T antigen (Galβ1-3GalNAc) as the most prominent among mucin-type O-glycosylation modifications in ESCs. Moreover, we show that mucin-type O-glycosylation on the Wnt signaling receptor frizzled-5 (Fzd5) regulates its endocytosis via galectin-3 binding to T antigen, and that reduction of T antigen results in the exit of the ESCs from pluripotency via canonical Wnt signaling activation. Our findings reveal a novel regulatory mechanism that modulates Wnt signaling and, consequently, ESC pluripotency.This article has an associated First Person interview with the first author of the paper.

NMR-based insight into galectin-3 binding to endothelial cell adhesion molecule CD146: Evidence for noncanonical interactions with the lectin's CRD β-sandwich F-face

Galectin-3 (Gal-3) binds to cell adhesion glycoprotein CD146 to promote cytokine secretion and mediate endothelial cell migration. Here, we used Nuclear Magnetic Resonance (NMR) 15N-Heteronuclear Single Quantum Coherence (HSQC) spectroscopy to investigate binding between 15N-labeled Gal-3 and the extracellular domain (eFL) of purified CD146 (five Ig-like ectodomains D1-D5) and a shorter, D5-deleted version of CD146 (D1-D4). Binding of Gal-3 and its carbohydrate recognition domain (CRD) to CD146 D1-D4 is greatly reduced vis-à-vis CD146 eFL, supporting the proposal of a larger number of glycosylation sites on D5. Even though the canonical sugar-binding β-sheet S-face (β-strands 1, 10, 3, 4, 5, 6) of the Gal-3 β-sandwich is involved in interactions with CD146 (e.g. N-linked glycosylation sites), equivalent HSQC spectral perturbations at residues on the opposing Gal-3 F-face β-sheet (β-strands 11, 2, 7, 8, 9) indicate involvement of the Gal-3 F-face in binding CD146. This is supported by the observation that addition of lactose, while significantly attenuating Gal-3 binding (primarily with the S-face) to CD146 eFL, does not abolish it. Bio-Layer Interferometry studies with Gal-3 F-face mutants yield KD values to demonstrate a significant decrease (L203A) or increase (V204A, L218A, T243A) in net binding to CD146 eFL compared to wild type Gal-3. However, HSQC lactose titrations show no highly significant effects on sugar binding to the Gal-3 CRD S-face. Overall, our findings indicate that Gal-3 binding to CD146 is more involved than simple interactions with β-galactoside epitopes on the cell receptor, and that there is a direct role for the lectin's CRD F-face in the CD146 binding process.

Galectin-3 binds selectively to the terminal, non-reducing end of β(1→4)-galactans, with overall affinity increasing with chain length

Galactans are linear polysaccharides of β(1→4)-linked galactose residues. Although they can antagonize galectin function, the nature of their binding to galectins needs to be better defined to develop them as drugs. Here, we investigated interactions between galectin-3 (Gal-3) and a series of galactans ranging in weight average molecular weight from 670 to 7550 Da. 15N-1H HSQC NMR studies with 15N-labeled Gal-3 carbohydrate recognition domain (CRD) indicate that each of these galactans interacts primarily with residues in β-strands 4, 5 and 6 on the canonical, β-galactoside sugar binding S-face. Although these galactans also bind to full length Gal-3 (CRD plus N-terminal tail) to the same extent, it appears that binding to the S-face attenuates interactions between the CRD F-face and N-terminal tail, making interpretation of site-specific binding unclear. Following assignment of galactan 13C and 1H resonances using HSQC, HMBC and TOCSY experiments, we used 13C-1H HSQC data to demonstrate that the Gal-3 CRD binds to the terminal, non-reducing end of these galactans, regardless of their size, but with binding affinity increasing as the galactan chain length increases. Overall, our findings increase understanding as to how galactans interact with Gal-3 at the non-reducing, terminal end of galactose-containing polysaccharides as found on the cell surface.

Crystallization of a human galectin-3 variant with two ordered segments in the shortened N-terminal tail

Among members of the family of adhesion/growth-regulatory galectins, galectin-3 (Gal-3) bears a unique modular architecture. A N-terminal tail (NT) consisting of the N-terminal segment (NTS) and nine collagen-like repeats is linked to the canonical lectin domain. In contrast to bivalent proto- and tandem-repeat-type galectins, Gal-3 is monomeric in solution, capable to self-associate in the presence of bi- to multivalent ligands, and the NTS is involved in cellular compartmentalization. Since no crystallographic information on Gal-3 beyond the lectin domain is available, we used a shortened variant with NTS and repeats VII-IX. This protein crystallized as tetramers with contacts between the lectin domains. The region from Tyr101 (in repeat IX) to Leu114 (in the CRD) formed a hairpin. The NTS extends the canonical β-sheet of F1-F5 strands with two new β-strands on the F face. Together, crystallographic and SAXS data reveal a mode of intramolecular structure building involving the highly flexible Gal-3’s NT.

Novel polysaccharide binding to the N-terminal tail of galectin-3 is likely modulated by proline isomerization

Interactions between galectins and polysaccharides are crucial to many biological processes, and yet these are some of the least understood, usually being limited to studies with small saccharides and short oligosaccharides. The present study is focused on human galectin-3 (Gal-3) interactions with a 60 kDa rhamnogalacturonan RG-I-4 that we use as a model to garner information as to how galectins interact with large polysaccharides, as well as to develop this agent as a therapeutic against human disease. Gal-3 is unique among galectins, because as the only chimera-type, it has a long N-terminal tail (NT) that has long puzzled investigators due to its dynamic, disordered nature and presence of numerous prolines. Here, we use 15N-1H heteronuclear single quantum coherence NMR spectroscopy to demonstrate that multiple sites on RG-I-4 provide epitopes for binding to three sites on 15N-labeled Gal-3, two within its carbohydrate recognition domain (CRD) and one at a novel site within the NT encompassing the first 40 residues that are highly conserved among all species of Gal-3. Moreover, strong binding of RG-I-4 to the Gal-3 NT occurs on a very slow time scale, suggesting that it may be mediated by cis-trans proline isomerization, a well-recognized modulator of many biological activities. The NT binding epitope within RG-I-4 appears to reside primarily in the side chains of the polysaccharide, some of which are galactans. Our results provide new insight into the role of the NT in Gal-3 function.

CD146 interacts with galectin-3 to mediate endothelial cell migration

Here, we investigated the role of the cell membrane protein CD146 in galectin-3-mediated endothelial cell migration at the molecular level. Our results show that knocking down CD146 significantly attenuates galectin-3-mediated cell migration. Pull-down assays, gel filtration, and biolayer interferometry further demonstrate that galectin-3 binds to the CD146 ectodomain (eFL) with a K_D of ~1.1 μm. To identify the galectin-3-binding site, we used mass spectrometry to show that CD146 eFL has four N-glycosites, with PNGase F treatment indicating that N-glycans define the binding epitope. Galectin-3 likely interacts with Domain 5 on CD146 eFL, because it contains poly-N-acetyllactosamine sites, and deletion of this domain significantly reduces binding. Overall, our findings provide a better understanding of how galectin-3 interacts with cell membrane receptors to mediate endothelial cell migration.

The N-terminal tail coordinates with carbohydrate recognition domain to mediate galectin-3 induced apoptosis in T cells

Galectin-3 is a galectin with a unique flexible N-terminal tail (NT) connected to the conserved carbohydrate recognition domain (CRD). Galectin-3 is associated with tumor immune tolerance and exhibits an ability to induce T cell apoptosis. We used Jurkat, Jurkat E6-1 and CEM T-cell lines and human peripheral blood mononuclear cells (PBMCs) to investigate the specific roles of the CRD and NT in inducing T cell apoptosis. Galectin-3 triggered sustained extracellular signal-regulated kinase (ERK) phosphorylation that induced apoptosis. ERK was situated upstream of caspase-9 and was independently activated by reactive oxygen species (ROS) and protein kinase C (PKC). The first twelve NT residues had no role in the apoptosis. Residues 13-68 were essential for activating ROS, but did not activate PKC. However, residues 69-110 were required for activation of PKC. An NT fragment and a NT-specific antibody antagonized the apoptosis triggered by full-length galectin-3 further supporting our findings. These findings indicate the CRD and NT play important roles during induction of T cell apoptosis, which suggests their potential as therapeutic targets for reversing cancer immune tolerance.

Cleavage and phosphorylation: important post-translational modifications of galectin-3

As the unique chimeric member of the β-galactoside-binding protein family, galectin-3 is a multivalent and multifunctional oncogenic protein involved in multiple physiological and pathological processes, including cell growth, cell differentiation, cell adhesion, RNA splicing, cell apoptosis, and malignant transformation. Post-translational modifications can effectively increase a protein's functional diversity, either by degradation or adding chemical modifications, thus regulating activity, localization, and ligand interaction. In order to clearly understand the functional mechanisms of galectin-3 involved in normal cell biology and pathogenesis, here, we have summarized the previously reported post-translational modifications of galectin-3, including cleavage and phosphorylation. Cleavage of galectin-3 by MMPs, PSA, and proteases from parasites generated intact carbohydrate-recognition domain and N-terminal peptides of varying lengths that retained lectin binding activity but lost multivalence. Serine and tyrosine phosphorylation of galectin-3 by c-Abl, CKI, and GSK-3β could regulate its localization and associated signal transduction. Accordingly, cleavage and phosphorylation play an important role in regulating galectin-3 function via altering its multivalence, localization, and ligand interaction.

Critical Role of HAX-1 in Promoting Avian Influenza Virus Replication in Lung Epithelial Cells

The PB1-F2 protein of influenza A virus has been considered a virulence factor, but its function in inducing apoptosis may be of disadvantage to viral replication. Host mechanisms to regulate PB1-F2-induced apoptosis remain unknown. We generated a PB1-F2-deficient avian influenza virus (AIV) H9N2 and found that the mutant virus replicated less efficiently in human lung epithelial cells. The PB1-F2-deficient virus produced less apoptotic cells, indicating that PB1-F2 of the H9N2 virus promotes apoptosis, occurring at the early stage of infection, in the lung epithelial cells. To understand how host cells regulate PB1-F2-induced apoptosis, we explored to identify cellular proteins interacting with PB1-F2 and found that HCLS1-associated protein X-1 (HAX-1), located mainly in the mitochondria as an apoptotic inhibitor, interacted with PB1-F2. Increased procaspase-9 activations, induced by PB1-F2, could be suppressed by HAX-1. In HAX-1 knockdown A549 cells, the replication of AIV H9N2 was suppressed in parallel to the activation of caspase-3 activation, which increased at the early stage of infection. We hypothesize that HAX-1 promotes AIV replication by interacting with PB1-F2, resulting in the suppression of apoptosis, prolonged cell survival, and enhancement of viral replication. Our data suggest that HAX-1 may be a promoting factor for AIV H9N2 replication through desensitizing PB1-F2 from its apoptotic induction in human lung epithelial cells.

Macromolecular assemblies of complex polysaccharides with galectin-3 and their synergistic effects on function

Although pectin-derived polysaccharides can antagonize galectin function in various pathological disorders, the nature of their binding interactions needs to be better defined for developing them as drugs. Moreover, given their relatively large size and complexity, pectin-derived polysaccharides are also useful as model systems to assess inter-polysaccharide and protein-polysaccharide interactions. Here, we investigated interactions between galectin-3 (Gal-3) and pectin-derived polysaccharides: a rhamnogalacturonan (RG) and two homogalacturonans (HGs). BioLayer Interferometry and fluorescence-linked immunosorbent assays indicate that these polysaccharides bind Gal-3 with macroscopic or apparent K_D values of 49 nM, 46 µM, and 138 µM, respectively. ¹⁵N-¹H heteronuclear single quantum coherence (HSQC) NMR studies reveal that these polysaccharides interact primarily with the F-face of the Gal-3 carbohydrate recognition domain. Even though their binding to Gal-3 does not inhibit Gal-3-mediated T-cell apoptosis and only weakly attenuates hemagglutination, their combination in specific proportions increases activity synergistically along with avidity for Gal-3. This suggests that RG and HG polysaccharides act in concert, a proposal supported by polysaccharide particle size measurements and ¹³C-¹H HSQC data. Our model has HG interacting with RG to promote increased avidity of RG for Gal-3, likely by exposing additional lectin-binding sites on the RG. Overall, the present study contributes to our understanding of how complex HG and RG polysaccharides interact with Gal-3.

Multiple approaches to assess pectin binding to galectin-3

Although several approaches have been used to evaluate binding of carbohydrates to lectins, results are not always comparable, especially with larger polysaccharides. Here, we quantitatively assessed and compared binding of pectin-derived polysaccharides to galectin-3 (Gal-3) using five methods: surface plasmon resonance (SPR), bio-layer interferometry (BLI), fluorescence polarization (FP), competitive fluorescence-linked immunosorbance (cFLISA), and the well-known cell-based hemagglutination assay (G3H). Our studies revealed that whereas Gal-3-pectin binding parameters determined by SPR and BLI were comparable and correlated with inhibitory potencies from the G3H assay, results using FP and cFLISA assays were highly variable and depended greatly on the probe and mass of the polysaccharide. In the cFLISA assay, for example, pectins showed no inhibition when using the DTAF-labeled asialofetuin probe, but did when using a DTAF-labeled pectin probe. And the FP approach with the DTAF-lactose probe did not work on polysaccharides and large galactan chains, although it did work well with smaller galactans. Nevertheless, even though results derived from all of these methods are in general agreement, derived KD, IC50, and MIC values do differ. Our results reflect the variability using various techniques and therefore will be useful to investigators who are developing pectin-derived Gal-3 antagonists as anti-cancer agents.

Recycling of galectin-3 in epithelial cells

Galectins, a family of β-galactoside binding proteins, do not possess a signalling sequence to enter the endoplasmic reticulum as a starting point for the classical secretory pathway. They use a so-called unconventional secretion mechanism for translocation across the plasma membrane and/or into the lumen of transport vesicles. The β-galactoside binding protein galectin-3 is highly expressed in a variety of epithelial cell lines. Polarized MDCK cells secrete this lectin predominantly into the apical medium. The lectin re-enters the cell by non-clathrin mediated endocytosis and passages through endosomal organelles. This internalized galectin-3 plays an important role in apical protein trafficking by directing the subcellular targeting of apical glycoproteins via oligomerization into high molecular weight clusters, a process that can be fine-tuned by changes in the environmental pH. Following release at the apical plasma membrane, the lectin can reenter the cell for another round of recycling and apical protein sorting. This review will briefly address galectin-3-functions in epithelia and focus on distinct phases in apical recycling of the lectin.

Building endocytic pits without clathrin

How endocytic pits are built in clathrin- and caveolin-independent endocytosis still remains poorly understood. Recent insight suggests that different forms of clathrin-independent endocytosis might involve the actin-driven focusing of membrane constituents, the lectin-glycosphingolipid-dependent construction of endocytic nanoenvironments, and Bin-Amphiphysin-Rvs (BAR) domain proteins serving as scaffolding modules. We discuss the need for different types of internalization processes in the context of diverse cellular functions, the existence of clathrin-independent mechanisms of cargo recruitment and membrane bending from a biological and physical perspective, and finally propose a generic scheme for the formation of clathrin-independent endocytic pits.

Galectin expression in cancer diagnosis and prognosis: A systematic review

Galectins are a family of proteins that bind to specific glycans thereby deciphering the information captured within the glycome. In the last two decades, several galectin family members have emerged as versatile modulators of tumor progression. This has initiated the development and preclinical assessment of galectin-targeting compounds. With the first compounds now entering clinical trials it is pivotal to gain insight in the diagnostic and prognostic value of galectins in cancer as this will allow a more rational selection of the patients that might benefit most from galectin-targeted therapies. Here, we present a systematic review of galectin expression in human cancer patients. Malignant transformation is frequently associated with altered galectin expression, most notably of galectin-1 and galectin-3. In most cancers, increased galectin-1 expression is associated with poor prognosis while elevated galectin-9 expression is emerging as a marker of favorable disease outcome. The prognostic value of galectin-3 appears to be tumor type dependent and the other galectins require further investigation. Regarding the latter, additional studies using larger patient cohorts are essential to fully unravel the diagnostic and prognostic value of galectin expression. Furthermore, to better compare different findings, consensus should be reached on how to assess galectin expression, not only with regard to localization within the tissue and within cellular compartments but also regarding alternative splicing and genomic variations. Finally, linking galectin expression and function to aberrant glycosylation in cancer cells will improve our understanding of how these versatile proteins can be exploited for diagnostic, prognostic and even therapeutic purposes in cancer patients.

The water network in galectin-3 ligand binding site guides inhibitor design

Galectin-3 (Gal-3) which shows affinity of β-galactosides is a cancer-related protein. Thus, it is important to understand its ligand binding mechanism and then design its specific inhibitor. It was suggested that the positions of water molecules in Gal-3 ligand-binding site could be replaced by appropriate chemical groups of ideal inhibitors. However, the reported structures of Gal-3 carbohydrate recognition domain (CRD) complexed with lactose showed that the number of water molecules are different and the water positions are inconsistent in the ligand-binding site. This study reported four high-resolution (1.24-1.19 Å) structures of Gal-3 CRD complexed with lactose, and accurately located 12 conserved water molecules in the water network of Gal-3 CRD ligand-binding site by merging these structures. These water molecules either directly stabilize the binding of Gal-3 CRD and lactose, or hold the former water molecules at the right place. In particular, water molecule 4 (W4) which only coordinates with water molecule 5 (W5) and water molecule 6 (W6) plays a key role in stabilizing galactose residue. In addition, by three-dimensional alignment of the positions of all residues, 14 flexible parts of Gal-3 CRD were found to dynamically fluctuate in the crystalline environment.

Galectin-3 induces cell migration via a calcium-sensitive MAPK/ERK1/2 pathway

The presence and level of circulating galectin-3 (Gal-3), a member of the galectin family, is associated with diverse diseases ranging from heart failure, immune disorders to cancer metastasis and serves as a biomarker of diagnosis and treatment response. However, the mechanisms by which exogenous Gal-3 affects pathobiology events remain elusive. In the current study, we found that exogenous Gal-3 slightly delays, while prolonging tyrosine phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in HeLa cells through a calcium-sensitive and PKC-dependent signaling pathway. The activation was dependent on the sugar-binding properties of Gal-3, since the antagonist lactose could inhibit it. The sugar-binding motif of Gal-3 was required for the activation of ERK1/2. The activation of ERK1/2 was necessary for the initiation and induction of cell migration associated with the phosphorylation of paxillin. All the results presented in this study suggest a novel calcium-sensitive and PKC-dependent pathway through which circulating Gal-3 promotes cell migration and activating the ERK1/2. Taken together, the data depicted here propose a biological function and a target for the diseases' associated circulating Gal-3.

Galectin-3 in angiogenesis and metastasis

Galectin-3 is a member of the family of β-galactoside-binding lectins characterized by evolutionarily conserved sequences defined by structural similarities in their carbohydrate-recognition domains. Galectin-3 is a unique, chimeric protein consisting of three distinct structural motifs: (i) a short NH2 terminal domain containing a serine phosphorylation site; (ii) a repetitive proline-rich collagen-α-like sequence cleavable by matrix metalloproteases; and (iii) a globular COOH-terminal domain containing a carbohydrate-binding motif and an NWGR anti-death motif. It is ubiquitously expressed and has diverse biological functions depending on its subcellular localization. Galectin-3 is mainly found in the cytoplasm, also seen in the nucleus and can be secreted by non-classical, secretory pathways. In general, secreted galectin-3 mediates cell migration, cell adhesion and cell-cell interactions through the binding with high affinity to galactose-containing glycoproteins on the cell surface. Cytoplasmic galectin-3 exhibits anti-apoptotic activity and regulates several signal transduction pathways, whereas nuclear galectin-3 has been associated with pre-mRNA splicing and gene expression. Its unique chimeric structure enables it to interact with a plethora of ligands and modulate diverse functions such as cell growth, adhesion, migration, invasion, angiogenesis, immune function, apoptosis and endocytosis emphasizing its significance in the process of tumor progression. In this review, we have focused on the role of galectin-3 in tumor metastasis with special emphasis on angiogenesis.

Increased levels of serum galectin-3 in patients with primary Sjögren's syndrome: associated with interstitial lung disease

OBJECTIVES

To explore the potential values of serum galectin-3 (Gal-3) levels in diagnosis of interstitial lung disease (ILD) for patients with primary Sjögren's syndrome (pSS).

METHODS

The concentrations of serum Gal-3 and interleukin (IL)-17 were measured by enzymelinked immunosorbent assay (ELISA) in 87 patients with pSS and 30 healthy controls (HC). The levels of plasma C-reactive protein (CRP), rheumatoid factor (RF), immunoglobulin (Ig)G, complement (C3), albumin (ALB) and Fibrinogen (FIB) and erythrocyte sedimentation rate (ESR) were measured. ILD was identified on high-resolution computed tomography.

RESULTS

The levels of serum Gal-3 and IL-17 were significantly higher in pSS patients than in HC. Stratification analyses indicated significantly higher levels of Gal-3 in pSS patients with ILD and in those with positive ANCA. In comparison with that of pSS patients without ILD, significantly higher levels of ESR, CRP, FIB, IgG, C3 and lower ALB were detected in pSS patients with ILD. The levels of galectin-3 were correlated positively with the values of CRP, FIB, IgG or IL-17 in patients with pSS.

CONCLUSIONS

These findings suggest that higher levels of serum galectin-3 may be associated with the development of pSS, particularly with ILD.

Human chimera-type galectin-3: defining the critical tail length for high-affinity glycoprotein/cell surface binding and functional competition with galectin-1 in neuroblastoma cell growth regulation

Many human proteins have a modular design with receptor and structural domains. Using adhesion/growth-regulatory galectin-3 as model, we describe an interdisciplinary strategy to define the functional significance of its tail established by nine non-triple helical collagen-like repeats (I-IX) and the N-terminal peptide. Genetic engineering with sophisticated mass spectrometric product analysis provided the tools for biotesting, i.e. eight protein variants with different degrees of tail truncation. Evidently,various aspects of galectin-3 activity (cis binding and cell bridging) are affected by tail shortening in a different manner. Thus, this combined approach reveals an unsuspected complexity of structure-function relationship, encouraging further application beyond this chimera-type galectin.

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.

The inhibitory effects of a rhamnogalacturonan I (RG-I) domain from ginseng pectin on galectin-3 and its structure-activity relationship

Pectin has been shown to inhibit the actions of galectin-3, a β-galactoside-binding protein associated with cancer progression. The structural features of pectin involved in this activity remain unclear. We investigated the effects of different ginseng pectins on galectin-3 action. The rhamnogalacturonan I-rich pectin fragment, RG-I-4, potently inhibited galectin-3-mediated hemagglutination, cancer cell adhesion and homotypic aggregation, and binding of galectin-3 to T-cells. RG-I-4 specifically bound to the carbohydrate recognition domain of galectin-3 with a dissociation constant of 22.2 nm, which was determined by surface plasmon resonance analysis. The structure-activity relationship of RG-I-4 was investigated by modifying the structure through various enzymatic and chemical methods followed by activity tests. The results showed that (a) galactan side chains were essential to the activity of RG-I-4, whereas arabinan side chains positively or negatively regulated the activity depending on their location within the RG-I-4 molecule. (b) The activity of galactan chain was proportional to its length up to 4 Gal residues and largely unchanged thereafter. (c) The majority of galactan side chains in RG-I-4 were short with low activities. (d) The high activity of RG-I-4 resulted from the cooperative action of these side chains. (e) The backbone of the molecule was very important to RG-I-4 activity, possibly by maintaining a structural conformation of the whole molecule. (f) The isolated backbone could bind galectin-3, which was insensitive to lactose treatment. The novel discovery that the side chains and backbone play distinct roles in regulating RG-I-4 activity is valuable for producing highly active pectin-based galectin-3 inhibitors.

Galectin-3 guides intracellular trafficking of some human serotransferrin glycoforms

Transferrin internalization via clathrin-mediated endocytosis and subsequent recycling after iron delivery has been extensively studied. Here we demonstrate a previously unrecognized parameter regulating this recycling, the binding of galectin-3 to particular glycoforms of transferrin. Two fractions of transferrin, separated by affinity chromatography based on their binding or not to galectin-3, are targeted to kinetically different endocytic pathways in HFL-1 cells expressing galectin-3 but not in SKBR3 cells lacking galectin-3; the SKBR3 cells, however, can acquire the ability to target these transferrin glycoforms differently after preloading with exogenously added galectin-3. In all, this study provides the first evidence of a functional role for transferrin glycans, in intracellular trafficking after uptake. Moreover, the galectin-3-bound glycoform increased in cancer, suggesting a pathophysiological regulation. These are novel aspects of transferrin cell biology, which has previously considered only a degree of iron loading, but not other forms of heterogeneity.

Vascular galectins: regulators of tumor progression and targets for cancer therapy

Galectins are a family of carbohydrate binding proteins with a broad range of cytokine and growth factor-like functions in multiple steps of cancer progression. They contribute to tumor cell transformation, promote tumor angiogenesis, hamper the anti-tumor immune response, and facilitate tumor metastasis. Consequently, galectins are considered as multifunctional targets for cancer therapy. Interestingly, many of the functions related to tumor progression can be linked to galectins expressed by endothelial cells in the tumor vascular bed. Since the tumor vasculature is an easily accessible target for cancer therapy, understanding how galectins in the tumor endothelium influence cancer progression is important for the translational development of galectin-targeting therapies.

pH-dependent recycling of galectin-3 at the apical membrane of epithelial cells

The β-galactoside binding protein galectin-3 is highly expressed in a variety of epithelial cell lines. Polarized MDCK cells secrete this lectin predominantly into the apical medium by non-classical secretion. Once within the apical extracellular milieu, galectin-3 can re-enter the cell followed by passage through endosomal organelles and modulate apical protein sorting. Here, we could show that galectin-3 is internalized by non-clathrin mediated endocytosis. Within endosomal organelles this pool associates with newly synthesized neurotrophin receptor in the biosynthetic pathway and assists in its membrane targeting. This recycling process is accompanied by transient interaction of galectin-3 with detergent insoluble membrane microdomains in a lactose- and pH-dependent manner. Moreover, in the presence of lactose, apical sorting of the neurotrophin receptor is affected following endosomal deacidification. Taken together, our results suggest that internalized galectin-3 directs the subcellular targeting of apical glycoproteins by membrane recycling.

Galectin-3- and phospho-caveolin-1-dependent outside-in integrin signaling mediates the EGF motogenic response in mammary cancer cells

Galectin-3 binding to N-glycans promotes EGF receptor signaling to integrin in mammary cancer cells. This leads to phospho-caveolin-1–, Src-, and ILK-dependent activation of RhoA, resulting in actin reorganization in circular dorsal ruffles, cell migration, and fibronectin remodeling.

In murine mammary epithelial cancer cells, galectin-3 binding to β1,6-acetylglucosaminyltransferase V (Mgat5)–modified N-glycans restricts epidermal growth factor (EGF) receptor mobility in the plasma membrane and acts synergistically with phospho-caveolin-1 to promote integrin-dependent matrix remodeling and cell migration. We show that EGF signaling to RhoA is galectin-3 and phospho-caveolin-1 dependent and promotes the formation of transient, actin-rich, circular dorsal ruffles (CDRs), cell migration, and fibronectin fibrillogenesis via Src- and integrin-linked kinase (ILK)–dependent signaling. ILK, Src, and galectin-3 also mediate EGF stimulation of caveolin-1 phosphorylation. Direct activation of integrin with Mn²⁺ induces galectin-3, ILK, and Src-dependent RhoA activation and caveolin-1 phosphorylation. This suggests that in response to EGF, galectin-3 enables outside-in integrin signaling stimulating phospho-caveolin-1–dependent RhoA activation, actin reorganization in CDRs, cell migration, and fibronectin remodeling. Similarly, caveolin-1/galectin-3–dependent EGF signaling induces motility, peripheral actin ruffling, and RhoA activation in MDA-MB-231 human breast carcinoma cells, but not HeLa cells. These studies define a galectin-3/phospho-caveolin-1/RhoA signaling module that mediates integrin signaling downstream of growth factor activation, leading to actin and matrix remodeling and tumor cell migration in metastatic cancer cells.