Peptides derived from human galectin-3 N-terminal tail interact with its carbohydrate recognition domain in a phosphorylation-dependent manner

Pectin: Health-promoting properties as a natural galectin-3 inhibitor

Galectin-3 has a variety of important pathophysiological significance in the human body. Much evidence shows that the abnormal expression of galectin-3 is related to the formation and development of many diseases. Pectin is mostly obtained from processed citrus fruits and apples and is a known natural inhibitor of galactin-3. A large number of peels produced each year are discarded, and it is necessary to recycle some of the economically valuable active compounds in these by-products to reduce resource waste and environmental pollution. By binding with galectin-3, pectin can directly reduce the expression level of galectin-3 on the one hand, and regulate the expression level of cytokines by regulating certain signaling pathways on the other hand, to achieve the effect of treating diseases. This paper begins by presenting an overview of the basic structure of pectin, subsequently followed by a description of the structure of galectin-3 and its detrimental impact on human health when expressed abnormally. The health effects of pectin as a galectin-3 inhibitor were then summarized from the perspectives of anticancer, anti-inflammatory, ameliorating fibrotic diseases, and anti-diabetes. Finally, the challenges and prospects of future research on pectin are presented, which provide important references for expanding the application of pectin in the pharmaceutical industry or developing functional dietary supplements.

Structure of Galectin-3 bound to a model membrane containing ganglioside GM1

Galectin-3 (Gal-3) is a β-galactosidase-binding protein involved in various biological processes, including neuronal growth and adhesion. The pairing of Gal-3 with ganglioside GM1's pentasaccharide chain at the outer leaflet of the plasma membrane, which triggers downstream cell-signaling cascades, seems to be involved in these processes. A crucial feature of Gal-3 is its ability to form oligomers and supramolecular assemblies that connect various carbohydrate-decorated molecules. Although we know the atomistic structure of Gal-3 bound to small carbohydrate ligands, it remains unclear how Gal-3 binds GM1 in a membrane. Furthermore, the influence of this interaction on Gal-3's structure and oligomeric assembly has to be elucidated. In this study, we used X-ray reflectivity (XR) from a model membrane to determine the structure and surface coverage of Gal-3 bound to a membrane containing GM1. We observed that the carbohydrate recognition domain interacts with GM1's pentasaccharide, while the N-terminal domain is pointed away from the membrane, likely to facilitate protein-protein interactions. In a membrane containing 20 mol % GM1, Gal-3 covered ∼50% of the membrane surface with one Gal-3 molecule bound per 2130 Å2. We used molecular dynamics simulations and Voronoi tessellation algorithms to build an atomistic model of membrane-bound Gal-3, which is supported by the XR results. Overall, this work provides structural information describing how Gal-3 can bind GM1's pentasaccharide chain, a prerequisite for triggering regulatory processes in neuronal growth and adhesion.

Liquid-liquid phase separation: Galectin-3 in nuclear speckles and ribonucleoprotein complexes

Nuclear speckles are subcellular structures originally characterized by punctate immunofluorescence staining of the monoclonal antibody SC35, which recognizes an epitope on SRRM2 (serine/arginine repetitive matrix protein 2) and Sfrs2, a member of the SR (serine/arginine-rich) family of splicing factors. Galectin-3 co-localizes with SC35 in nuclear speckles, which represent one group of nuclear bodies that include the nucleolus, Cajal bodies and gems, paraspeckles, etc. Although they appear to have well-delineated physical boundaries, these nuclear bodies are not membrane-bound structures but represent macromolecular assemblies arising from a phenomenon called liquid-liquid phase separation. There has been much recent interest in liquid phase condensation as a newly recognized mechanism by which a cell can organize and compartmentalize subcellular structures with distinct composition. The punctate/speckled staining of galectin-3 with SC3 demonstrates their co-localization in a phase-separated body in vivo, under conditions endogenous to the cell. The purpose of the present review is to summarize the studies that document three key features of galectin-3 for its localization in liquid phase condensates: (a) an intrinsically disordered domain; (b) oligomer formation for multivalent binding; and (c) association with RNA and ribonucleoprotein complexes.

Galectins in Endothelial Cell Biology and Angiogenesis: The Basics

Angiogenesis, the growth of new blood vessels out of existing vessels, is a complex and tightly regulated process. It is executed by the cells that cover the inner surface of the vasculature, i.e., the endothelial cells. During angiogenesis, these cells adopt different phenotypes, which allows them to proliferate and migrate, and to form tube-like structures that eventually result in the generation of a functional neovasculature. Multiple internal and external cues control these processes and the galectin protein family was found to be indispensable for proper execution of angiogenesis. Over the last three decades, several members of this glycan-binding protein family have been linked to endothelial cell functioning and to different steps of the angiogenesis cascade. This review provides a basic overview of our current knowledge regarding galectins in angiogenesis. It covers the main findings with regard to the endothelial expression of galectins and highlights their role in endothelial cell function and biology.

Targeting galectin-3 with a high-affinity antibody for inhibition of high-grade serous ovarian cancer and other MUC16/CA-125-expressing malignancies

The lectin, galectin-3 (Gal3), has been implicated in a variety of inflammatory and oncogenic processes, including tumor growth, invasion, and metastasis. The interactions of Gal3 and MUC16 represent a potential targetable pathway for the treatment of MUC16-expressing malignancies. We found that the silencing of Gal3 in MUC16-expressing breast and ovarian cancer cells in vitro inhibited tumor cell invasion and led to attenuated tumor growth in murine models. We therefore developed an inhibitory murine monoclonal anti-Gal3 carbohydrate-binding domain antibody, 14D11, which bound human and mouse Gal3 but did not bind human Galectins-1, -7, -8 or -9. Competition studies and a docking model suggest that the 14D11 antibody competes with lactose for the carbohydrate binding pocket of Gal3. In MUC16-expressing cancer cells, 14D11 treatment blocked AKT and ERK1/2 phosphorylation, and led to inhibition of cancer cell Matrigel invasion. Finally, in experimental animal tumor models, 14D11 treatment led to prolongation of overall survival in animals bearing flank tumors, and retarded lung specific metastatic growth by MUC16 expressing breast cancer cells. Our results provide evidence that antibody based Gal3 blockade may be a viable therapeutic strategy in patients with MUC16-expressing tumors, supporting further development of human blocking antibodies against Gal3 as potential cancer therapeutics.

Targeting Galectins With Glycomimetics

Among glycan-binding proteins, galectins, β-galactoside-binding lectins, exhibit relevant biological roles and are implicated in many diseases, such as cancer and inflammation. Their involvement in crucial pathologies makes them interesting targets for drug discovery. In this review, we gather the last approaches toward the specific design of glycomimetics as potential drugs against galectins. Different approaches, either using specific glycomimetic molecules decorated with key functional groups or employing multivalent presentations of lactose and N-acetyl lactosamine analogs, have provided promising results for binding and modulating different galectins. The review highlights the results obtained with these approximations, from the employment of S-glycosyl compounds to peptidomimetics and multivalent glycopolymers, mostly employed to recognize and/or detect hGal-1 and hGal-3.

A chimeric, multivalent assembly of galectin-1 and galectin-3 with enhanced extracellular activity

Galectins are attractive therapeutic candidates to control aberrant immune system activation because they can alter the phenotype and function of various innate and adaptive immune cells. However, use of exogenous galectin-1 ("G1") and galectin-3 ("G3") as immunomodulators is challenged by their high dosing requirements and dynamic quaternary structures. Here we report a chimeric assembly of G1 and G3 with enhanced extracellular activity ("G1/G3 Zipper"), which was created by recombinant fusion of G1 and G3 via a peptide linker that forms a two-stranded α-helical coiled-coil. G1/G3 Zipper had higher apparent binding affinity for immobilized lactose and a lower concentration threshold for inducing soluble glycoprotein crosslinking than G1, a recombinant fusion of G1 and G3 with a flexible peptide linker ("G1/G3"), or a recently reported stable G1 dimer crosslinked by poly(ethylene glycol) diacrylate ("G1-PEG-G1"). As a result, G1/G3 Zipper was more effective at inducing Jurkat T cell apoptosis in media containing serum, and was the only variant that could induce apoptosis at low concentrations under serum-free conditions. The monomeric G1/G3 fusion protein lacked extracellular activity under all conditions tested, suggesting that the enhanced activity of G1/G3 Zipper was due to its quaternary structure and increased carbohydrate-recognition domain valency. Thus, combining G1 and G3 into a non-native chimeric assembly provides a new candidate therapeutic with greater immunomodulatory potency than the wild-type proteins and previously reported engineered variants.

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.

Galectin-3 Activation and Inhibition in Heart Failure and Cardiovascular Disease: An Update

Galectin-3 is a versatile protein orchestrating several physiological and pathophysiological processes in the human body. In the last decade, considerable interest in galectin-3 has emerged because of its potential role as a biotarget. Galectin-3 is differentially expressed depending on the tissue type, however its expression can be induced under conditions of tissue injury or stress. Galectin-3 overexpression and secretion is associated with several diseases and is extensively studied in the context of fibrosis, heart failure, atherosclerosis and diabetes mellitus. Monomeric (extracellular) galectin-3 usually undergoes further "activation" which significantly broadens the spectrum of biological activity mainly by modifying its carbohydrate-binding properties. Self-interactions of this protein appear to play a crucial role in regulating the extracellular activities of this protein, however there is limited and controversial data on the mechanisms involved. We therefore summarize (recent) literature in this area and describe galectin-3 from a binding perspective providing novel insights into mechanisms by which galectin-3 is known to be "activated" and how such activation may be regulated in pathophysiological scenarios.

Galectin-3 alters the lateral mobility and clustering of β1-integrin receptors

Glycoprotein receptors are influenced by myriad intermolecular interactions at the cell surface. Specific glycan structures may interact with endogenous lectins that enforce or disrupt receptor-receptor interactions. Glycoproteins bound by multivalent lectins may form extended oligomers or lattices, altering the lateral mobility of the receptor and influencing its function through endocytosis or changes in activation. In this study, we have examined the interaction of Galectin-3 (Gal-3), a human lectin, with adhesion receptors. We measured the effect of recombinant Gal-3 added exogenously on the lateral mobility of the α5β1 integrin on HeLa cells. Using single-particle tracking (SPT) we detected increased lateral mobility of the integrin in the presence of Gal-3, while its truncated C-terminal domain (Gal-3C) showed only minor reductions in lateral mobility. Treatment of cells with Gal-3 increased β1-integrin mediated migration with no apparent changes in viability. In contrast, Gal-3C decreased both cell migration and viability. Fluorescence microscopy allowed us to confirm that exogenous Gal-3 resulted in reorganization of the integrin into larger clusters. We used a proteomics analysis to confirm that cells expressed endogenous Gal-3, and found that addition of competitive oligosaccharide ligands for the lectin altered the lateral mobility of the integrin. Together, our results are consistent with a Gal-3-integrin lattice model of binding and confirm that the lateral mobility of integrins is natively regulated, in part, by galectins.

The intrinsically disordered N-terminal domain of galectin-3 dynamically mediates multisite self-association of the protein through fuzzy interactions

Galectins are a family of lectins that bind β-galactosides through their conserved carbohydrate recognition domain (CRD) and can induce aggregation with glycoproteins or glycolipids on the cell surface and thereby regulate cell activation, migration, adhesion, and signaling. Galectin-3 has an intrinsically disordered N-terminal domain and a canonical CRD. Unlike the other 14 known galectins in mammalian cells, which have dimeric or tandem-repeated CRDs enabling multivalency for various functions, galectin-3 is monomeric, and its functional multivalency therefore is somewhat of a mystery. Here, we used NMR spectroscopy, mutagenesis, small-angle X-ray scattering, and computational modeling to study the self-association-related multivalency of galectin-3 at the residue-specific level. We show that the disordered N-terminal domain (residues ∼20-100) interacts with itself and with a part of the CRD not involved in carbohydrate recognition (β-strands 7-9; residues ∼200-220), forming a fuzzy complex via inter- and intramolecular interactions, mainly through hydrophobicity. These fuzzy interactions are characteristic of intrinsically disordered proteins to achieve liquid-liquid phase separation, and we demonstrated that galectin-3 can also undergo liquid-liquid phase separation. We propose that galectin-3 may achieve multivalency through this multisite self-association mechanism facilitated by fuzzy interactions.

Galectin-3 Determines Tumor Cell Adaptive Strategies in Stressed Tumor Microenvironments

Galectin-3 is a member of the β-galactoside-binding lectin family, whose expression is often dysregulated in cancers. While galectin-3 is usually an intracellular protein found in the nucleus and in the cytoplasm, under certain conditions, galectin-3 can be secreted by an yet unknown mechanism. Under stressing conditions (e.g., hypoxia and nutrient deprivation) galectin-3 is upregulated, through the activity of transcription factors, such as HIF-1α and NF-κB. Here, we review evidence that indicates a positive role for galectin-3 in MAPK family signal transduction, leading to cell proliferation and cell survival. Galectin-3 serves as a scaffold protein, which favors the spatial organization of signaling proteins as K-RAS. Upon secretion, extracellular galectin-3 interacts with a variety of cell surface glycoproteins, such as growth factor receptors, integrins, cadherins, and members of the Notch family, among other glycoproteins, besides different extracellular matrix molecules. Through its ability to oligomerize, galectin-3 forms lectin lattices that act as scaffolds that sustain the spatial organization of signaling receptors on the cell surface, dictating its maintenance on the plasma membrane or their endocytosis. Galectin-3 induces tumor cell, endothelial cell, and leukocyte migration, favoring either the exit of tumor cells from a stressed microenvironment or the entry of endothelial cells and leukocytes, such as monocytes/macrophages into the tumor organoid. Therefore, galectin-3 plays homeostatic roles in tumors, as (i) it favors tumor cell adaptation for survival in stressed conditions; (ii) upon secretion, galectin-3 induces tumor cell detachment and migration; and (iii) it attracts monocyte/macrophage and endothelial cells to the tumor mass, inducing both directly and indirectly the process of angiogenesis. The two latter activities are potentially targetable, and specific interventions may be designed to counteract the protumoral role of extracellular galectin-3.

Intra- and intermolecular interactions of human galectin-3: assessment by full-assignment-based NMR

Galectin-3 is an adhesion/growth-regulatory protein with a modular design comprising an N-terminal tail (NT, residues 1-111) and the conserved carbohydrate recognition domain (CRD, residues 112-250). The chimera-type galectin interacts with both glycan and peptide motifs. Complete (13)C/(15)N-assignment of the human protein makes NMR-based analysis of its structure beyond the CRD possible. Using two synthetic NT polypeptides covering residues 1-50 and 51-107, evidence for transient secondary structure was found with helical conformation from residues 5 to 15 as well as proline-mediated, multi-turn structure from residues 18 to 32 and around PGAYP repeats. Intramolecular interactions occur between the CRD F-face (the 5-stranded β-sheet behind the canonical carbohydrate-binding 6-stranded β-sheet of the S-face) and NT in full-length galectin-3, with the sequence P(23)GAW(26)…P(37)GASYPGAY(45) defining the primary binding epitope within the NT. Work with designed peptides indicates that the PGAX motif is crucial for self-interactions between NT/CRD. Phosphorylation at position Ser6 (and Ser12) (a physiological modification) and the influence of ligand binding have minimal effect on this interaction. Finally, galectin-3 molecules can interact weakly with each other via the F-faces of their CRDs, an interaction that appears to be assisted by their NTs. Overall, our results add insight to defining binding sites on galectin-3 beyond the canonical contact area for β-galactosides.

Binding of polysaccharides to human galectin-3 at a noncanonical site in its carbohydrate recognition domain

Galectin-3 (Gal-3) is a multifunctional lectin, unique to galectins by the presence of a long N-terminal tail (NT) off of its carbohydrate recognition domain (CRD). Many previous studies have investigated binding of small carbohydrates to its CRD. Here, we used nuclear magnetic resonance spectroscopy ((15)N-(1)H heteronuclear single quantum coherence data) to assess binding of (15)N-Gal-3 (and truncated (15)N-Gal-3 CRD) to several, relatively large polysaccharides, including eight varieties of galactomannans (GMs), as well as a β(1 → 4)-polymannan and an α-branched mannan. Overall, we found that these polysaccharides with a larger carbohydrate footprint interact primarily with a noncanonical carbohydrate-binding site on the F-face of the Gal-3 CRD β-sandwich, and to a less extent, if at all, with the canonical carbohydrate-binding site on the S-face. While there is no evidence for interaction with the NT itself, it does appear that the NT somehow mediates stronger interactions between the Gal-3 CRD and the GMs. Significant Gal-3 resonance broadening observed during polysaccharide titrations indicates that interactions occur in the intermediate exchange regime, and analysis of these data allows estimation of affinities and stoichiometries that range from 4 × 10(4) to 12 × 10(4) M(-1) per site and multiple sites per polysaccharide, respectively. We also found that lactose can still bind to the CRD S-face of GM-bound Gal-3, with the binding of one ligand attenuating affinity of the other. These data are compared with previous results on Gal-1, revealing differences and similarities. They also provide research direction to the development of these polysaccharides as galectin-targeting therapeutics in the clinic.

Chemically modified, non-anticoagulant heparin derivatives are potent galectin-3 binding inhibitors and inhibit circulating galectin-3-promoted metastasis

Concentrations of circulating galectin-3, a metastasis promoter, are greatly increased in cancer patients. Here we show that 2- or 6-de-O-sulfated, N-acetylated heparin derivatives are galectin-3 binding inhibitors. These chemically modified heparin derivatives inhibited galectin-3-ligand binding and abolished galectin-3-mediated cancer cell-endothelial adhesion and angiogenesis. Unlike standard heparin, these modified heparin derivatives and their ultra-low molecular weight sub-fractions had neither anticoagulant activity nor effects on E-, L- or P-selectin binding to their ligands nor detectable cytotoxicity. Intravenous injection of such heparin derivatives (with cancer cells pre-treated with galectin-3 followed by 3 subcutaneous injections of the derivatives) abolished the circulating galectin-3-mediated increase in lung metastasis of human melanoma and colon cancer cells in nude mice. Structural analysis using nuclear magnetic resonance and synchrotron radiation circular dichroism spectroscopies showed that the modified heparin derivatives bind to the galectin-3 carbohydrate-recognition domain. Thus, these chemically modified, non-anticoagulant, low-sulfated heparin derivatives are potent galectin-3 binding inhibitors with substantial potential as anti-metastasis/cancer drugs.

Galectin-3 as a regulator of the epithelial junction: Implications to wound repair and cancer

Epithelial cells are closely connected to each other and to the extracellular matrix by a set of adhesive contacts that provide tissues with unique barrier properties and play a prominent role in cell morphology, tissue physiology, and cell signaling. This review highlights advances made in understanding the contributions of galectin-3, a carbohydrate-binding protein with affinity toward β-galactosides, as a modulator of epithelial junction assembly and function. The interactions of galectin-3 within adhesive structures are discussed in relation to wound healing and tumor progression.

Preliminary X-ray crystallographic analysis of an engineered variant of human chimera-type galectin-3 with a shortened N-terminal domain

How lectins translate sugar-encoded information into cellular effects not only depends on glycan recognition. Other domains of the protein can contribute to the functional profile of a lectin. Human galectin-3 (Gal-3), an adhesion/growth-regulatory galectin, is composed of three different domains and is thus called a chimera-type protein. In addition to the carbohydrate-recognition domain, this lectin encompasses an N-terminal domain consisting of a peptide harbouring two phosphorylation sites and nine non-triple-helical collagen-like repeats. This region plays an as yet structurally undefined role in Gal-3 aggregation and ligand recognition. To date, crystallization of full-length Gal-3 has not been achieved. With the aim of providing structural insights into this modular organization, a Gal-3 variant was crystallized maintaining the terminal peptide and three of the nine collagen-like repeats. The crystals belonged to the orthorhombic space group P212121, with unit-cell parameters a = 94.04, b = 97.96, c = 236.20 Å, and diffracted to a resolution of 3.3 Å.

Glycan dependence of Galectin-3 self-association properties

Human Galectin-3 is found in the nucleus, the cytoplasm and at the cell surface. This lectin is constituted of two domains: an unfolded N-terminal domain and a C-terminal Carbohydrate Recognition Domain (CRD). There are still uncertainties about the relationship between the quaternary structure of Galectin-3 and its carbohydrate binding properties. Two types of self-association have been described for this lectin: a C-type self-association and a N-type self-association. Herein, we have analyzed Galectin-3 oligomerization by Dynamic Light Scattering using both the recombinant CRD and the full length lectin. Our results proved that LNnT induces N-type self-association of full length Galectin-3. Moreover, from Nuclear Magnetic Resonance (NMR) and Surface Plasmon Resonance experiments, we observed no significant specificity or affinity variations for carbohydrates related to the presence of the N-terminal domain of Galectin-3. NMR mapping clearly established that the N-terminal domain interacts with the CRD. We propose that LNnT induces a release of the N-terminal domain resulting in the glycan-dependent self-association of Galectin-3 through N-terminal domain interactions.