Glycosyldisulfides from dynamic combinatorial libraries as O-glycoside mimetics for plant and endogenous lectins: Their reactivities in solid-phase and cell assays and conformational analysis by molecular dynamics simulations

Inhibitory potential of chemical substitutions at bioinspired sites of β-D-galactopyranose on neoglycoprotein/cell surface binding of two classes of medically relevant lectins

Galactose is the key contact site for plant AB-toxins and the human adhesion/growth-regulatory galectins. Natural anomeric extensions and 3'-substitutions enhance its reactivity, thus prompting us to test the potential of respective chemical substitutions of galactose in the quest to develop potent inhibitors. Biochemical screening of a respective glycoside library with 60 substances in a solid-phase assay was followed by examining the compounds' activity to protect cells from lectin binding. By testing 32 anomeric extensions, 18 compounds with additional 3'-substitution, three lactosides and two Lewis-type trisaccharides rather mild effects compared to the common haptenic inhibitor lactose were detected in both assays. When using trivalent glycoclusters marked enhancements with 6- to 8-fold increases were revealed for the toxin and three of four tested galectins. Since the most potent compound and also 3'-substituted thiogalactosides reduced cell growth of a human tumor line at millimolar concentrations, biocompatible substitutions and scaffolds will be required for further developments. The synthesis of suitable glycoclusters, presenting headgroups which exploit differences in ligand selection in interlectin comparison to reduce cross-reactivity, and the documented strategic combination of initial biochemical screening with cell assays are considered instrumental to advance inhibitor design.

Towards functional glycomics by lectin histochemistry: strategic probe selection to monitor core and branch-end substitutions and detection of cell-type and regional selectivity in adult mouse testis and epididymis

The emerging insights into glycan functionality direct increasing attention to monitor core modifications of N-glycans and branch-end structures. To address this issue in histochemistry, a panel of lectins with respective specificities was devised. The selection of probes with overlapping specificities facilitated to relate staining profiles to likely target structures. The experiments on fixed sections of adult murine testis and epididymis were carried out at non-saturating lectin concentrations to visualize high-affinity sites with optimal signal-to-background ratio. They revealed selectivity in lectin reactivity for distinct cell types and segment-dependent staining in the epididymis. Leydig cells, for instance, were reactive with the Sambucus nigra agglutinin and human siglec-2 (CD22), two lectins also separating principal from basal and apical cells in the caput segments I-III of the epididymis. Apical cells were reactive with the Maackia amurensis agglutinin-I, and basal cells with the erythroagglutinin of Phaseolus vulgaris. The reported differences support the concept of lectin staining as cell marker. They thus intimate to study glycogene (genes for glycosyltransferases and lectins) expression and cellular reactivity with tissue lectins. These investigations will be instrumental to assign a role as biochemical signals to the detected staining properties.

A regulatory network of two galectins mediates the earliest steps of avian limb skeletal morphogenesis

BACKGROUND

The skeletal elements of vertebrate embryonic limbs are prefigured by rod- and spot-like condensations of precartilage mesenchymal cells. The formation of these condensations depends on cell-matrix and cell-cell interactions, but how they are initiated and patterned is as yet unresolved.

RESULTS

Here we provide evidence that galectins, β-galactoside-binding lectins with β-sandwich folding, play fundamental roles in these processes. We show that among the five chicken galectin (CG) genes, two, CG-1A, and CG-8, are markedly elevated in expression at prospective sites of condensation in vitro and in vivo, with their protein products appearing earlier in development than any previously described marker. The two molecules enhance one another's gene expression but have opposite effects on condensation formation and cartilage development in vivo and in vitro: CG-1A, a non-covalent homodimer, promotes this process, while the tandem-repeat-type CG-8 antagonizes it. Correspondingly, knockdown of CG-1A inhibits the formation of skeletal elements while knockdown of CG-8 enhances it. The apparent paradox of mutual activation at the gene expression level coupled with antagonistic roles in skeletogenesis is resolved by analysis of the direct effect of the proteins on precartilage cells. Specifically, CG-1A causes their aggregation, whereas CG-8, which has no adhesive function of its own, blocks this effect. The developmental appearance and regulation of the unknown cell surface moieties ("ligands") to which CG-1A and CG-8 bind were indicative of specific cognate- and cross-regulatory interactions.

CONCLUSION

Our findings indicate that CG-1A and CG-8 constitute a multiscale network that is a major mediator, earlier-acting than any previously described, of the formation and patterning of precartilage mesenchymal condensations in the developing limb. This network functions autonomously of limb bud signaling centers or other limb bud positional cues.

Galectin-3 regulates MUC1 and EGFR cellular distribution and EGFR downstream pathways in pancreatic cancer cells

MUC1 is a transmembrane glycoprotein which is typically expressed at the apical membrane of normal epithelial cells. In cancer cells, the over-expression of MUC1 and its aberrant localization around the cell membrane and in the cytoplasm favours its interaction with different protein partners such as epidermal growth factor receptor (EGFR) and can promote tumour proliferation through the activation of oncogenic signalling pathways. Our aims were to study the mechanisms inducing MUC1 cytoplasmic localization in pancreatic cancer cells, and to decipher their impact on EGFR cellular localization and activation. Our results showed that galectin-3, an endogenous lectin, is co-expressed with MUC1 in human pancreatic ductal adenocarcinoma, and that it favours the endocytosis of MUC1 and EGFR. Depletion of galectin-3 by RNA interference increased the interaction between MUC1 and EGFR, EGFR and ERK-1,2 phosphorylation, and translocation of EGFR to the nucleus. On the contrary, silencing of galectin-3 led to a decrease of cyclin-D1 levels and of cell proliferation. The galectin-3-dependent regulation of MUC1/EGFR functions may represent an interesting mechanism modulating the EGFR-stimulated cell growth of pancreatic cancer cells.

Nitric oxide changes distinct aspects of the glycophenotype of human neuroblastoma NB69 cells

It is an open question whether the presence of nitric oxide (NO) affects the cell glycophenotype. A panel of six plant lectins was used in this study to monitor distinct aspects of cell surface glycosylation under nitrosative stress. We determined that treating human neuroblastoma NB69 cells with the long-lived NO donor 2,2'-(hydroxynitrosohydrazono)bis-ethanimine (DETA/NO) and monitoring the non-apoptotic adherent cell population significantly increases the presentation of N-glycans as detected by concanavalin A. Examining fine-structural features, bisected N-glycans and branch-end tailoring including α2,6-sialylation were found to be enhanced. Confocal fluorescence microscopy and cell permeabilization experiments pointed to a major effect of NO on the extent of cell surface N-glycan presentation. We also show that NO increases the level of protein O-GlcNAcylation, a multifunctional post-translational modification. Our results thus establish the first evidence for NO as modulator of distinct aspects of cell glycosylation.

Galectin-1 sensitizes carcinoma cells to anoikis via the fibronectin receptor α5β1-integrin

Anoikis resistance is a hallmark of transformed epithelial cells. Here, we show that treatment of anoikis-resistant carcinoma cell lines with the endogenous lectin galectin-1 (Gal-1) promoted apoptosis via interaction with the unligated fibronectin receptor α(5)β(1)-integrin. Gal-1 efficiency correlated with expression of α(5)β(1)-integrin, and transfection of the α(5)-subunit into deficient cell lines conferred Gal-1 binding and anoikis stimulation. Furthermore, Gal-1 and the α(5)- and β(1)-integrin subunits co-precipitated in Gal-1-stimulated cells undergoing anoikis. Other members of the galectin family failed to be active. The functional interaction between Gal-1 and α(5)β(1)-integrin was glycan dependent with α2,6-sialylation representing a switch-off signal. Desialylation of cell surface glycans resulted in increased electrophoretic mobility of α(5)β(1)-integrin and facilitated Gal-1 binding and anoikis stimulation. On the level of signaling, Gal-1-stimulated anoikis was prevented by filipin, which impaired the internalization of α(5)β(1)-integrin via cholesterol-enriched microdomains, and by pretreatment with a caspase-8 inhibitor. We propose that Gal-1/α(5)β(1)-integrin interaction participates in the control of epithelial integrity and integrin sialylation may enable carcinoma cells to evade this Gal-1-dependent control mechanism.

Human galectin-3 (Mac-2 antigen): defining molecular switches of affinity to natural glycoproteins, structural and dynamic aspects of glycan binding by flexible ligand docking and putative regulatory sequences in the proximal promoter region

BACKGROUND

Human galectin-3 (Mac-2 antigen) is a cell-type-specific multifunctional effector owing to selective binding of distinct cell-surface glycoconjugates harboring β-galactosides. The structural basis underlying the apparent preferences for distinct glycoproteins and for expression is so far unknown.

METHODS

We strategically combined solid-phase assays on 43 natural glycoproteins with a new statistical approach to fully flexible computational docking and also processed the proximal promoter region in silico.

RESULTS

The degree of branching in N-glycans and clustering of core 1 O-glycans are positive modulators for avidity. Sialylation of N-glycans in α2-6 linkage and of core 1 O-glycans in α2-3 linkage along with core 2 branching was an unfavorable factor, despite the presence of suited glycans in the vicinity. The lectin-ligand contact profile was scrutinized for six natural di- and tetrasaccharides enabling a statistical grading by analyzing flexible docking trajectories. The computational analysis of the proximal promoter region delineated putative sites for Lmo2/c-Ets-1 binding and new sites with potential for RUNX binding.

GENERAL SIGNIFICANCE

These results identify new features of glycan selectivity and ligand contact by combining solid-phase assays with in silico work as well as of reactivity potential of the promoter.

Selenoglycosides in silico: ab initio-derived reparameterization of MM4, conformational analysis using histo-blood group ABH antigens and lectin docking as indication for potential of bioactivity

The identification of glycan epitopes such as the histo-blood group ABH determinants as docking sites for bacterial/viral infections and signals in growth regulation fuels the interest to develop non-hydrolysable mimetics for therapeutic applications. Inevitably, the required substitution of the linkage oxygen atom will alter the derivative's topology. Our study addresses the question of the impact of substitution of oxygen by selenium. In order to characterize spatial parameters and flexibility of selenoglycosides, we first performed ab initio calculations on model compounds to refine the MM4 force field. The following application of the resulting MM4R version appears to reduce the difference to ab initio data when compared to using the MM4 estimator. Systematic conformational searches on the derivatives of histo-blood group ABH antigens revealed increased flexibility with acquisition of additional low-energy conformer(s), akin to the behavior of S-glycosides. Docking analysis using the Glide program for eight test cases indicated potential for bioactivity, giving further experimental investigation a clear direction to testing Se-glycosides as lectin ligands.

Hydrodynamic properties of human adhesion/growth-regulatory galectins studied by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy is applied on homologous human lectins (i.e., adhesion/growth-regulatory galectins) to detect influence of ligand binding and presence of the linker peptide in tandem-repeat-type proteins on hydrodynamic properties. Among five tested proteins, lactose binding increased the diffusion constant only in the cases of homodimeric galectin-1 and the linkerless variant of tandem-repeat-type galectin-4. To our knowledge, the close structural similarity among galectins does not translate into identical response to ligand binding. Kinetic measurements show association and dissociation rate constants in the order of 1 to 10(3) M(-1) s(-1) and 10(-4) s(-1), respectively. Presence of the linker peptide in tandem-repeat-type protein leads to anomalous scaling with molecular mass. These results provide what we believe to be new insights into lectin responses to glycan binding, detectable so far only by small angle neutron scattering, and the structural relevance of the linker peptide. Methodologically, fluorescence correlation spectroscopy is shown to be a rather simple technical tool to characterize hydrodynamic properties of these proteins at a high level of sensitivity.

Tumor suppressor p16 INK4a: Downregulation of galectin-3, an endogenous competitor of the pro-anoikis effector galectin-1, in a pancreatic carcinoma model

The tumor suppressor p16(INK4a) has functions beyond cell-cycle control via cyclin-dependent kinases. A coordinated remodeling of N- and O-glycosylation, and an increase in the presentation of the endogenous lectin galectin-1 sensing these changes on the surface of p16(INK4a)-expressing pancreatic carcinoma cells (Capan-1), lead to potent pro-anoikis signals. We show that the p16(INK4a)-dependent impact on growth-regulatory lectins is not limited to galectin-1, but also concerns galectin-3. By monitoring its expression in relation to p16(INK4a) status, as well as running anoikis assays with galectin-3 and cell transfectants with up- or downregulated lectin expression, a negative correlation between anoikis and the presence of this lectin was established. Nuclear run-off and northern blotting experiments revealed an effect of the presence of p16(INK4a) on steady-state levels of galectin-3-specific mRNA that differed from decreasing the transcriptional rate. On the cell surface, galectin-3 interferes with galectin-1, which initiates signaling toward its pro-anoikis activity via caspase-8 activation. The detected opposite effects of p16(INK4a) at the levels of growth-regulatory galectins-1 and -3 shift the status markedly towards the galectin-1-dependent pro-anoikis activity. A previously undescribed orchestrated fine-tuning of this effector system by a tumor suppressor is discovered.

Why structurally different cyclic peptides can be glycomimetics of the HNK-1 carbohydrate antigen

The cyclic peptides c-(LSETTl) and c-(RTLPFS) are of potential clinical interest--they stimulate neurite outgrowth in a way that is similar to the effects of the HNK-1 (human natural killer cell-1) antigenic carbohydrate chains, which are terminated by 3'-sulfated glucuronic acid attached to an N-acetyllactosamine unit. To investigate the structure-activity relationships of the ability of the cyclic peptides to mimic HNK-1 carbohydrates, conformational analysis and examination of hydrophobic and hydrophilic patterns were performed and compared with the characteristics of a synthetic HNK-1 trisaccharide derivative. Data obtained demonstrate that both the trisaccharide and the glycomimetic peptide c-(LSETTl) exhibit a similar relationship between their hydrophobic moieties and their negatively charged sites. However, the second cyclic glycomimetic peptide investigated here, c-(RTLPFS), has a positively charged group as a potential contact point due to its Arg residue. Therefore, we studied the amino acid composition of all known receptor structures in the Protein Data Bank that are in contact with uronic acid and/or sulfated glycans. Interactions of the HNK-1 trisaccharide, c-(LSETTl), and c-(RTLPFS) with a laminin fragment involved in HNK-1 carbohydrate binding (i.e., the 21mer peptide: KGVSSRSYVGCIKNLEISRST) were also analyzed. Because the structure of the HNK-1-binding laminin domain is not available in the Protein Data Bank, we used the HNK-1-binding 21mer peptide fragment of laminin for the construction of a model receptor that enabled us to compare the molecular interplay of the HNK-1 trisaccharide and the two cyclopeptides c-(LSETTl) and c-(RTLPFS) with a reliable receptor structure in considerable detail.

Phosphorylation of adhesion- and growth-regulatory human galectin-3 leads to the induction of axonal branching by local membrane L1 and ERM redistribution

Serine phosphorylation of the beta-galactoside-binding protein galectin-3 (Gal-3) impacts nuclear localization but has unknown consequences for extracellular activities. Herein, we reveal that the phosphorylated form of galectin-3 (pGal-3), adsorbed to substratum surfaces or to heparan sulphate proteoglycans, is instrumental in promoting axon branching in cultured hippocampal neurons by local actin destabilization. pGal-3 interacts with neural cell adhesion molecule L1, and enhances L1 association with Thy-1-rich membrane microdomains. Concomitantly, membrane-actin linker proteins ezrin-radixin-moesin (ERM) are recruited to the same membrane site via interaction with the intracellular domain of L1. We propose that the local regulation of the L1-ERM-actin pathway, at the level of the plasma membrane, underlies pGal-3-induced axon branching, and that galectin phosphorylation in situ could act as a molecular switch for the axon response to Gal-3.

Alpha-O-linked glycopeptide mimetics: synthesis, conformation analysis, and interactions with viscumin, a galactoside-binding model lectin

Efficient cycloaddition of a silylidene-protected galactal with a suitable heterodiene yielded the basis for a facile diastereoselective route to a glycopeptide-mimetic scaffold. Its carbohydrate part was further extended by beta1-3-linked galactosylation. The pyranose rings retain their (4)C(1) chair conformation, as shown by molecular modeling and NMR spectroscopy, and the typical exo-anomeric geometry was observed for the disaccharide. The expected bioactivity was ascertained by saturation-transfer-difference NMR spectroscopy by using the galactoside-specific plant toxin viscumin as a model lectin. The experimental part was complemented by molecular docking. The described synthetic route and the strategic combination of computational and experimental techniques to reveal conformational properties and bioactivity establish the prepared alpha-O-linked glycopeptide mimetics as promising candidates for further exploitation of this scaffold to give O-glycans for lectin blocking and vaccination.

Conformational analysis of thioglycoside derivatives of histo-blood group ABH antigens using an ab initio-derived reparameterization of MM4: implications for design of non-hydrolysable mimetics

Histo-blood group ABH antigens serve as recognition sites for infectious microorganisms and tissue lectins in intercellular communication, e.g. in tumor progression. Thus, they are of interest as a starting point for drug design. In this respect, potent non-hydrolysable derivatives such as thioglycosides are of special interest. As prerequisite to enable estimations of ligand properties relative to their natural counterparts, conformational properties of the thioglycosidic derivatives of ABH trisaccharides and their disaccharide units were calculated using systematic and filtered systematic searches with the MM4 force field. Parameters for the glycosidic torsions of thioglycosides were independently derived from ab initio calculations. The resulting energy deviations required a reparameterization of MM4 to a new parameter set called MM4R. The data sets obtained using MM4R reveal that the thioglycosides have somewhat increased levels of flexibility about the major low-energy conformations shared with the corresponding O-glycosides. In the trisaccharides, the thiosubstitution of the Gal[NAc]alpha1-3Gal linkage leads to a preference for a conformation which is the secondary minimum of the natural counterparts. This conformation also generates contacts between the N-acetyl group and the fucose moiety in the blood group A derivative. Calculations further indicate that thiosubstitution of only the Fucalpha1-2Gal linkage does not affect the conformational preferences compared to the natural trisaccharide. Thiosubstitution of both linkages in the trisaccharide results in increased flexibility but the favored conformation of the natural trisaccharides is preferred. The study suggests that thioglycoside derivatives of ABH antigens could have pharmaceutical interest as ligands of lectins and other carbohydrate-binding proteins.

Phenotypic characterization of human keratinocytes in coculture reveals differential effects of fibroblasts from benign fibrous histiocytoma (dermatofibroma) as compared to cells from its malignant form and to normal fibroblasts

BACKGROUND

Benign and malignant fibrous histiocytoma present with a considerable difference concerning cellular organization in their vicinity.

OBJECTIVE

Normally appearing epithelium covers the malignant form in contrast to hyperplastic epidermis for benign tumors. It is an open question as to whether the tumor-associated fibroblasts are capable to affect phenotypic features of normal keratinocytes, prompting this comparative analysis.

METHODS

Fibroblasts were isolated from benign and malignant fibrous histiocytomas, respectively, and also from normal dermis. The resulting cell populations were thoroughly characterized immunocytochemically using a large panel of antibodies. The three fibroblast preparations were cocultured with normal interfollicular keratinocytes. Their phenotype was characterized for distinct properties including differentiation and proliferation.

RESULTS

Fibroblasts prepared from both tumor types were phenotypically practically identical with normal dermal fibroblasts. Their activities on keratinocytes were different. Cells prepared from benign fibrous histiocytoma were capable to effect strong expression of keratin 19 and production of a galectin-1-rich extracellular matrix. Fibroblasts isolated from malignant fibrous histiocytoma led to a phenotype very similar to that when keratinocytes were cocultured with normal dermal fibroblasts.

CONCLUSION

Fibroblasts prepared from benign fibrous histiocytoma were biologically active on keratinocytes in a particular manner. Our results on fibroblast activity are suggested to be relevant for morphologic differences observed in vivo between normal epidermis and epidermis adjacent to the studied tumor types.

Cross-linking of GM1 ganglioside by galectin-1 mediates regulatory T cell activity involving TRPC5 channel activation: possible role in suppressing experimental autoimmune encephalomyelitis

Several animal autoimmune disorders are suppressed by treatment with the GM1 cross-linking units of certain toxins such as B subunit of cholera toxin (CtxB). Due to the recent observation of GM1 being a binding partner for the endogenous lectin galectin-1 (Gal-1), which is known to ameliorate symptoms in certain animal models of autoimmune disorders, we tested the hypothesis that an operative Gal-1/GM1 interplay induces immunosuppression in a manner evidenced by both in vivo and in vitro systems. Our study of murine experimental autoimmune encephalomyelitis (EAE) indicated suppressive effects by both CtxB and Gal-1 and further highlighted the role of GM1 in demonstrating enhanced susceptibility to EAE in mice lacking this ganglioside. At the in vitro level, polyclonal activation of murine regulatory T (Treg) cells caused up-regulation of Gal-1 that was both cell bound and released to the medium. Similar activation of murine CD4(+) and CD8(+) effector T (Teff) cells resulted in significant elevation of GM1 and GD1a, the neuraminidase-reactive precursor to GM1. Activation of Teff cells also up-regulated TRPC5 channels which mediated Ca(2+) influx upon GM1 cross-linking by Gal-1 or CtxB. This involved co-cross-linking of heterodimeric integrin due to close association of these alpha(4)beta(1) and alpha(5)beta(1) glycoproteins with GM1. Short hairpin RNA (shRNA) knockdown of TRPC5 in Teff cells blocked contact-dependent proliferation inhibition by Treg cells as well as Gal-1/CtxB-triggered Ca(2+) influx. Our results thus indicate GM1 in Teff cells to be the primary target of Gal-1 expressed by Treg cells, the resulting co-cross-linking and TRPC5 channel activation contributing importantly to the mechanism of autoimmune suppression.

Compensation of loss of protein function in microsatellite-unstable colon cancer cells (HCT116): a gene-dependent effect on the cell surface glycan profile

Tumors that display a high level of microsatellite instability (MSI-H) accumulate somatic frameshift mutations in several genes. The compensation of this loss of function by transfection represents a suitable approach to tie respective gene deficiency to alterations in cellular characteristics. In view of the emerging significance of cell surface glycans as biochemical signals for presentation/activity of various receptors/integrins and for susceptibility to adhesion/growth-regulatory tissue lectins, we examined the glycophenotype in the MSI-H colon cancer cell line HCT116 for activin type 2 receptor (ACVR2), absent in melanoma 2 (AIM2), and transforming growth factor beta-type 2 receptor (TGFBR2) known to be associated with MSI colorectal carcinogenesis. A panel of probes specific for functional carbohydrate epitopes including human lectins was used to trace changes in cell surface levels, thereby initiating glycan analysis related to MSI. In particular, the presence of core substitutions and branching in N-glycans, the sialylation status of N- and O-glycans, and the presence of Le(a/x)-epitopes were profiled. Transient transfection affected the glycophenotype, depending on the nature of the gene and the probe. The TGFBR2 presence reduced binding of probes specific for a core substitution and increased branch length in N-glycosylation, even reaching a P-value of 0.0016. ACVR2/AIM2 influenced core 1 mucin-type O-glycosylation differentially, upregulation by ACVR2, and downregulation by AIM2. These alterations of cell surface glycosylation by gene products that are not directly associated with the machinery for glycan generation direct attention to pursue analysis of glycosylation in MSI tumor cells on the level of target glycoproteins and open the way for functional studies.

Detection and chromatographic removal of lipopolysaccharide in preparations of multifunctional galectins

The functional spectrum of human galectins is currently explored, with a wide range of activities being described. The role of galectin-3 as adhesin for bacteria is based on its strong binding to lipopolysaccharides (LPSs), which brings the possibility of such a contamination in galectin preparations to awareness. This assumption was verified in three independent functional assay systems using polymyxin B as inhibitor of LPS-dependent effects. Moreover, a commercial LPS quantification kit also revealed LPS in galectin preparations. Chromatography was effective in removing LPS, suggesting that such a technique needs to be applied to prevent assigning cellular responses to galectins rather than LPS.

2,3,4,6-Tetra-O-acetyl-β-d-galacto-pyranosyl 2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl disulfide tetra-hydro-furan solvate

The asymmetric unit of title compound, C(28)H(38)O(18)S(2)·C(4)H(8)O, comprises one disulfide-bridged sugar molecule and one solvent molecule. No significant differences in structural parameters are found between the present structure and the previously determined unsolvated form [Brito, López-Rodríguez, Bényei & Szilagyi (2006 ▶). Carbohydr. Res.341, 2967-2972]. The compounds are characterized by a compact structure with spatial proximity of the two pyranosyl rings. One of the carbonyl atoms is disordered over two sites [site occupancy = 0.69 (7) for major component] and the displacement parameters for the THF species are unsually large.

Phosphorylated human lectin galectin-3: analysis of ligand binding by histochemical monitoring of normal/malignant squamous epithelia and by isothermal titration calorimetry

The human lectin galectin-3 is a multifunctional effector with special functions in regulation of adhesion and apoptosis. Its unique trimodular organization includes the 12-residue N-terminal sequence, a substrate for protein kinase CK1-dependent phosphorylation. As a step towards elucidating its significance, we prepared phosphorylated galectin-3, labelled it and used it as a tool in histochemistry. We monitored normal and malignant squamous epithelia. Binding was suprabasal with obvious positive correlation to the degree of differentiation and negative correlation to proliferation. The staining pattern resembled that obtained with the unmodified lectin. Basal cell carcinomas were invariably negative. The epidermal positivity profile was akin to distribution of the desmosomal protein desmoglein, as also seen with keratinocytes in vitro. In all cases, binding was inhibitable by the presence of lactose, prompting further investigation of the activity of the lectin site by a sensitive biochemical method, i.e. isothermal titration calorimetry. The overall affinity and the individual enthalpic and entropic contributions were determined. No effect of phosphorylation was revealed. This strategic combination of histo- and biochemical techniques applied to an endogenous effector after its processing by a protein kinase thus enabled a detailed monitoring of the binding properties of the post-translationally modified lectin. It underscores the value of using endogenous lectins as a histochemical tool. The documented approach has merit for applications beyond lectinology.

Cell-type-specific expression of murine multifunctional galectin-3 and its association with follicular atresia/luteolysis in contrast to pro-apoptotic galectins-1 and -7

Galectin-3 is a multifunctional protein with modular design. A distinct expression profile was determined in various murine organs when set into relation to homodimeric galectins-1 and -7. Fittingly, the signature of putative transcription-factor-binding sites in the promoter region of the galectin-3 gene affords a toolbox for a complex combinatorial regulation, distinct from the respective sequence stretches in galectins-1 and -7. A striking example for cell-type specificity was the ovary, where these two lectins were confined to the surface epithelium. Immunohistochemically, galectin-3 was found in macrophages of the cortical interstitium between developing follicles and medullary interstitium, matching the distribution of the F4/80 antigen. With respect to atresia and luteolysis strong signals in granulosa cells of atretic preantral but not antral follicles and increasing positivity in corpora lutea upon regression coincided with DNA fragmentation. Labeled galectin-3 revealed lactose-inhibitable binding to granulosa cells. Also, slender processes of vital granulosa cells which extended into the zona pellucida were positive. This study demonstrates cell-type specificity and cycle-associated regulation for galectin-3 with increased presence in atretic preantral follicles and in late stages of luteolysis.

Human galectins as sensors for apoptosis/necrosis-associated surface changes of granulocytes and lymphocytes

Changes in the glycomic profile can significantly affect the cells' communication with the environment. Plant lectins have so far been used to address the issue as to whether the courses of apoptosis or necrosis are associated with such alterations. We, here, initiate the study of members of the family of functionally pleiotropic human galectins in this respect. Established protocols for the induction of apoptosis/necrosis of blood cells and for flow cytometry using annexin V/propidium iodide were combined with cell surface staining using biotinylated galectins at a nontoxic concentration. The galectin panel covered members from all three subfamilies. Flow cytometry revealed specific binding of galectins to viable control cells and conspicuous staining differences when testing apoptotic or necrotic cells. Onset and especially progression of cell death led to pronounced reactivity with the proto-type galectins-1, -2, and -7 and tandem-repeat-type galectin-4. Extent of staining depended on the nature and stage of cell death, type of dying cell, and type of galectin. Galectins act as sensors for cell-death-associated surface changes. Staining of late-apoptotic polymorphonuclear cells was particularly strong. Examining the functional significance of this result may reveal a new aspect within the surveillance system to protect against autoinflammation.

Assessing the inhibitory potency of galectin ligands identified from combinatorial (glyco)peptide libraries using surface plasmon resonance spectroscopy

Combinatorial (glyco)peptide libraries offer the possibility to define effective inhibitors of protein (lectin)-glycan interactions. If a (glyco)peptide surpasses the inhibitory potency of the free sugar, then the new peptide-lectin contacts underlying the affinity enhancement may guide further rational drug design. Focusing on the adhesion/growth regulatory human galectins 1 and 3, a screening of three combinatorial solid-phase (glyco)peptide libraries, containing Gal(beta1-O)Thr, Gal(beta1-S)Cys/Gal(beta1-N)Asn, and Lac(beta1-O)Thr, with the fluorescently labeled lectins had led to a series of lead compounds. To define the inhibitory potency of a selection of resynthesized (glyco)peptides systematically, a surface plasmon resonance-based inhibition assay with immobilized asialofetuin was set up. (Glyco)Peptides with up to 66-fold potency relative to free lactose as inhibitor were characterized. The presence of lactose in the most effective glycopeptides indicated the presence of affinity-enhancing peptide-lectin contacts. In addition to drug design, they may be helpful for fine-structural analysis of the binding sites.

Marker profiling of normal keratinocytes identifies the stroma from squamous cell carcinoma of the oral cavity as a modulatory microenvironment in co-culture

PURPOSE

The microenvironment established by stromal cells may or may not influence phenotypic aspects of epithelial cells and may be relevant for tumor and stem cell biology. We address this issue for keratinocytes using tumor-derived stromal cells in a co-culture system.

MATERIALS AND METHODS

We isolated stromal cells from human squamous cell carcinoma tissue and studied their effect on phenotypic characteristics of normal human interfollicular keratinocytes in vitro.

RESULTS

Stromal fibroblasts significantly influence immuno- and lectin cytochemical properties of co-cultured normal keratinocytes. Expression of keratins 8 and 19, the nucleolar protein nucleostemin, parameters related to adhesion/growth-regulatory galectins and the epithelial-mesenchymal transition were altered. This biological activity of tumor-derived stromal cells, which did not require cell contact, appeared to be stable, because it was maintained during passaging of keratinocytes in the absence of cancer cells.

CONCLUSIONS

Tumor-derived stromal fibroblasts acquire distinct properties to shape a microenvironment conducive to altering the phenotypic characteristics of normal epithelial cells in vitro.

Phosphorylated human galectin-3: facile large-scale preparation of active lectin and detection of structural changes by CD spectroscopy

Galectin-3 has a unique modular design. Its short N-terminal stretch can be phosphorylated, relevant for nuclear export and anti-anoikis/apoptosis activity. Enzymatic modification by casein kinase 1 at constant ATP concentration yielded mg quantities of mono- and diphosphorylated derivatives at Ser5/Ser11 in a 2:1 ratio. Their carbohydrate-inhibitable binding to asialofetuin, cell surfaces of three tumor lines, rabbit erythrocytes leading to haemagglutination and cytoplasmic sites in fixed tissue sections was not markedly altered relative to phosphate-free galectin-3. Spectroscopically, phosphorylation induced alterations in the far UV CD, indicative of an increase in ordered structure. This is accompanied by changes in the environment of aromatic amino acids signified by shifts in the near UV CD.

Gene-expression signature of adhesion/growth-regulatory tissue lectins (galectins) in transitional cell cancer and its prognostic relevance

AIMS

Lectins, and especially galectins, appear to be important in malignancy-associated processes. The aim was to analyse comprehensively the presence of galectins in urothelial tumours.

METHODS AND RESULTS

Non-cross-reactive antibodies against seven family members from the three subgroups (prototype: galectin-1, -2 and -7; chimera type: galectin-3; tandem-repeat type: galectin-4, -8 and -9) were used. Gene expression was monitored in specimens of normal urothelium, fresh tumour tissue and cell lines by real-time polymerase chain reaction (PCR). The presence and evidence of tumour-associated up-regulation were shown for galectin-1 and -3. This was less clear-cut for galectin-4 and -8. Galectin-7 was expressed in all cell lines; galectin-2 and -9 were detected at comparatively low levels. Galectin-2, -3 and -8 up-regulation was observed in superficial tumours, but not in muscle-invasive tumours (P < 0.05). Immunoreactivity correlated with tumour grading for galectin-1, -2 and -8, and disease-dependent mortality correlated with galectin-2 and -8 expression. Binding sites were visualized using labelled galectins.

CONCLUSIONS

The results demonstrate a complex expression pattern of the galectin network in urothelial carcinomas. Galectin-1, -2, -3 and -8 are both potential disease markers and also possible targets for bladder cancer therapy.