Branching mode in complex-type triantennary N-glycans as regulatory element of their ligand properties

Artificial Glycoproteins as a Scaffold for Targeted Drug Therapy

Akin to a cellular "fingerprint," the glycocalyx is a glycan-enriched cellular coating that plays a crucial role in mediating cell-to-cell interactions. To gain a better understanding of the factors that govern in vivo recognition, artificial glycoproteins were initially created to probe changes made to the accumulation and biodistribution of specific glycan assemblies through biomimicry. As a result, the organ-specific accumulation for a variety of glycoproteins decorated with simple and/or complex glycans was identified. Additionally, binding trends with regard to cancer cell selectivity were also investigated. To exploit the knowledge gained from these studies, numerous groups thus became engaged in developing targeted drug methodologies based on the use of artificial glycoproteins. This has either been done through adopting the glycoprotein scaffold as a drug carrier, or to directly glycosylate therapeutic proteins/enzymes to localize their biological activity. The principle aim of this Review is to present the foundational research that has driven artificial glycoprotein-based targeting and subsequent adaptations with potential therapeutic applications.

Exploring the glycan interaction in vivo: Future prospects of neo-glycoproteins for diagnostics

Herein the biodistributions and in vivo kinetics of chemically prepared neoglycoproteins are reviewed. Chemical methods can be used to conjugate various mono- and oligosaccharides onto a protein surface. The kinetics and organ-specific accumulation profiles of these glycoconjugates, which are introduced through intravenous injections, have been analyzed using conventional dissection studies as well as noninvasive methods such as single photon emission computed tomography, positron emission tomography and fluorescence imaging. These studies suggest that glycan-dependent protein distribution kinetics may be useful for pharmacological and diagnostic applications.

Chemically synthesized glycoconjugates on proteins: effects of multivalency and glycoform in vivo

The biodistributions and in vivo kinetics of chemically prepared glycoconjugates on proteins are reviewed.

Bi- to tetravalent glycoclusters presenting GlcNAc/GalNAc as inhibitors: from plant agglutinins to human macrophage galactose-type lectin (CD301) and galectins

Emerging insights into the functional spectrum of tissue lectins leads to identification of new targets for the custom-made design of potent inhibitors, providing a challenge for synthetic chemistry. The affinity and selectivity of a carbohydrate ligand for a lectin may immensely be increased by a number of approaches, which includes varying geometrical or topological features. This perspective leads to the design and synthesis of glycoclusters and their testing using assays of physiological relevance. Herein, hydroquinone, resorcinol, benzene-1,3,5-triol and tetra(4-hydroxyphenyl)ethene have been employed as scaffolds and propargyl derivatives obtained. The triazole-containing linker to the α/β-O/S-glycosides of GlcNAc/GalNAc presented on these scaffolds was generated by copper-catalysed azide-alkyne cycloaddition. This strategy was used to give a panel of nine glycoclusters with bi-, tri- and tetravalency. Maintained activity for lectin binding after conjugation was ascertained for both sugars in solid-phase assays with the plant agglutinins WGA (GlcNAc) and DBA (GalNAc). Absence of cross-reactivity excluded any carbohydrate-independent reactivity of the bivalent compounds, allowing us to proceed to further testing with a biomedically relevant lectin specific for GalNAc. Macrophage galactose(-binding C)-type lectin, involved in immune defence by dendritic cells and in virus uptake, was produced as a soluble protein without/with its α-helical coiled-coil stalk region. Binding to ligands presented on a matrix and on cell surfaces was highly susceptible to the presence of the tetravalent inhibitor derived from the tetraphenylethene-containing scaffold, and presentation of GalNAc with an α-thioglycosidic linkage proved favorable. Cross-reactivity of this glycocluster to human galectins-3 and -4, which interact with Tn-antigen-presenting mucins, was rather small. Evidently, the valency and spatial display of α-GalNAc residues is a key factor to design potent and selective inhibitors for this lectin.

A guide into glycosciences: How chemistry, biochemistry and biology cooperate to crack the sugar code

BACKGROUND

The most demanding challenge in research on molecular aspects within the flow of biological information is posed by the complex carbohydrates (glycan part of cellular glycoconjugates). How the 'message' encoded in carbohydrate 'letters' is 'read' and 'translated' can only be unraveled by interdisciplinary efforts.

SCOPE OF REVIEW

This review provides a didactic step-by-step survey of the concept of the sugar code and the way strategic combination of experimental approaches characterizes structure-function relationships, with resources for teaching.

MAJOR CONCLUSIONS

The unsurpassed coding capacity of glycans is an ideal platform for generating a broad range of molecular 'messages'. Structural and functional analyses of complex carbohydrates have been made possible by advances in chemical synthesis, rendering production of oligosaccharides, glycoclusters and neoglycoconjugates possible. This availability facilitates to test the glycans as ligands for natural sugar receptors (lectins). Their interaction is a means to turn sugar-encoded information into cellular effects. Glycan/lectin structures and their spatial modes of presentation underlie the exquisite specificity of the endogenous lectins in counterreceptor selection, that is, to home in on certain cellular glycoproteins or glycolipids.

GENERAL SIGNIFICANCE

Understanding how sugar-encoded 'messages' are 'read' and 'translated' by lectins provides insights into fundamental mechanisms of life, with potential for medical applications.

Chemical Approach to a Whole Body Imaging of Sialo-N-Linked Glycans

PET and noninvasive fluorescence imaging of the sialo-N-linked glycan derivatives are described. To establish the efficient labeling protocol for N-glycans and/or glycoconjugates, new labeling probes of fluorescence and ⁶⁸Ga-DOTA, as the positron emission nucleus for PET, through rapid 6π-azaelectrocyclization were designed and synthesized, (E)-ester aldehydes. The high reactivity of these probes enabled the labeling of lysine residues in peptides, proteins, and even amino groups on the cell surfaces at very low concentrations of the target molecules (~10⁻⁸ M) within a short reaction time (~5 min) to result in "selective" and "non-destructive" labeling of the more accessible amines. The first MicroPET of glycoproteins, ⁶⁸Ga-DOTA-orosomucoid and asialoorosomucoid, successfully visualized the differences in the circulatory residence of glycoproteins, in the presence or absence of sialic acids. In vivo dynamics of the new N-glycoclusters, prepared by the "self-activating" Huisgen cycloaddition reaction, could also be affected significantly by their partial structures at the non-reducing end, i.e., the presence or absence of sialic acids, and/or sialoside linkages to galactose. Azaelectrocyclization chemistry is also applicable to the engineering of the proteins and/or the cell surfaces by the oligosaccharides; lymphocytes chemically engineered by sialo-N-glycan successfully target the tumor implanted in BALB/C nude mice, detected by noninvasive fluorescence imaging.

The third dimension of reading the sugar code by lectins: design of glycoclusters with cyclic scaffolds as tools with the aim to define correlations between spatial presentation and activity

Coding of biological information is not confined to nucleic acids and proteins. Endowed with the highest level of structural versatility among biomolecules, the glycan chains of cellular glycoconjugates are well-suited to generate molecular messages/signals in a minimum of space. The sequence and shape of oligosaccharides as well as spatial aspects of multivalent presentation are assumed to underlie the natural specificity/selectivity that cellular glycans have for endogenous lectins. In order to eventually unravel structure-activity profiles cyclic scaffolds have been used as platforms to produce glycoclusters and afford valuable tools. Using adhesion/growth-regulatory galectins and the pan-galectin ligand lactose as a model, emerging insights into the potential of cyclodextrins, cyclic peptides, calixarenes and glycophanes for this purpose are presented herein. The systematic testing of lectin panels with spatially defined ligand presentations can be considered as a biomimetic means to help clarify the mechanisms, which lead to the exquisite accuracy at which endogenous lectins select their physiological counterreceptors from the complexity of the cellular glycome.

Bi- to tetravalent glycoclusters: synthesis, structure-activity profiles as lectin inhibitors and impact of combining both valency and headgroup tailoring on selectivity

The emerging functional versatility of cellular glycans makes research on the design of synthetic inhibitors a timely topic. In detail, the combination of ligand (or headgroup or contact site) structure with spatial parameters that depend on topological and geometrical factors underlies the physiological selectivity of glycan-protein (lectin) recognition. We herein tested a panel of bi-, tri- and tetravalent compounds against two plant agglutinins and adhesion/growth-regulatory lectins (galectins). In addition, we examined the impact of headgroup tailoring (converting lactose to 2'-fucosyllactose) in combination with valency increase in two assay types of increasing biorelevance (from solid-phase binding to cell binding). Compounds were prepared using copper-catalysed azide alkyne cycloaddition from peracetylated lactosyl or 2'-fucosyllactosyl azides. Significant inhibition was achieved for the plant toxin with a tetravalent compound. Different levels of sensitivity were noted for the three groups of the galectin family. The headgroup extension to 2'-fucosyllactose led to a selectivity gain, especially for the chimera-type galectin-3. Valency increase established discrimination against the homodimeric proteins, whereas the combination of valency with the headgroup extension led to discrimination against the tandem-repeat-type galectin-8 for chicken galectins but not human galectins-3 and -4. Thus, detailed structure-activity profiling of glycoclusters combined with suitably modifying the contact site for the targeted lectin will help minimize cross-reactivity among this class of closely related proteins.

Colonic carcinogenesis along different genetic routes: glycophenotyping of tumor cases separated by microsatellite instability/stability

Different genetic routes account for colonic carcinogenesis. However, when analyzing colon cancer specimens, separation into different groups based on genetic alterations is commonly not performed. Thus, we here initiate the comparative phenotyping considering microsatellite instability/stability for clinical specimens. The focus is given to glycan epitopes, expression of which is known to be modulated by signal-transducing proteins that act as key regulators of normal colon epithelial growth and differentiation. In addition to six plant lectins used as sensors, the presence of two adhesion/growth-regulatory galectins is studied. Overall, a considerable level of intra- and interindividual heterogeneity is revealed. Alterations in the proportion of stained cells between tumor-adjacent and malignant epithelia concerned plant lectins, which bind substituted N-glycan cores, α2,6-sialylated branch ends, core 1 O-glycans and N-acetylgalactosamine. A tendency for changes was noted between microsatellite-unstable and microsatellite-stable cases for core substitution (bisected N-glycan, presence of β1,6-branching) and status of α2,6-sialylation. Statistical significance was reached for presence of galectin-3, found to be elevated in microsatellite-stable compared to microsatellite-unstable tumors. These results emphasize the potential of distinct signaling pathways to regulate certain aspects of the glycophenotype in vivo and thus delineate a perspective to discern functionally relevant deviations in expression of endogenous lectins and their counter-receptors.

From lectin structure to functional glycomics: principles of the sugar code

Lectins are carbohydrate-binding proteins which lack enzymatic activity on their ligand and are distinct from antibodies and free mono- and oligosaccharide sensor/transport proteins. Emerging insights into the functional dimension of lectin binding to cellular glycans have strongly contributed to the shaping of the 'sugar code'. Fittingly, over a dozen folds and a broad spectrum of binding site architecture, ranging from shallow grooves to deep pockets, have developed sugar-binding capacity. A central question is how the exquisite target specificity of endogenous lectins for certain cellular glycans can be explained. In this regard, affinity regulation is first systematically dissected into six levels. Experimentally, the strategic combination of methods to monitor distinct aspects of the lectin-glycan interplay offers a promising perspective to answer this question.

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.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

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.

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.

Synthesis of multiantennary complex type N-glycans by use of modular building blocks

A modular set of oligosaccharide building blocks was developed for the synthesis of multiantennary N-glycans of the complex type, which are commonly found on glycoproteins. The donor building blocks were laid out for the elongation of a core trisaccharide acceptor (beta-mannosyl chitobiose) conveniently protected with a single benzylidene moiety at the beta-mannoside. Through two consecutive regio- and stereoselective couplings the donors gave N-glycans with three to five antennae in high yields. Due to the consistent protection group pattern of the donors the deprotection of the final products can be performed by using a general reaction sequence.

The tumor-associated antigen 90K/Mac-2-binding protein secreted by human colon carcinoma cells enhances extracellular levels of promatrilysin and is a novel substrate of matrix metalloproteinases-2, -7 (matrilysin) and -9: Implications of proteolytic cleavage

BACKGROUND

The tumor-associated antigen 90K (TAA90K)/Mac-2-binding protein is expressed at elevated level in cancerous tissues and associated with poor prognosis. Since TAA90K has been implicated in the restructuring of the extracellular matrix, we examined the functional relationship between colon cancer cell-derived TAA90K and the matrix metalloproteinase (MMP) promatrilysin (proMMP-7), and also tested whether TAA90K is a novel substrate for MMPs-2, -7 and -9.

METHODS

The effect of TAA90K on proMMP-7 levels in HT-29 conditioned media was quantified by enzyme-linked immunosorbent assays. Binding of TAA90K to MMPs, extracellular matrix proteins and galectin-3 was measured by solid-phase binding assays. Proteolytic cleavage of TAA90K by MMPs was documented by SDS-PAGE and protein sequencing analysis.

RESULTS

TAA90K enhanced extracellular levels of proMMP-7 in HT-29 cells. In addition, TAA90K was cleaved by MMPs-2, -7 and -9. MMP-7-mediated cleavage of TAA90K did not affect its binding to MMP-7, laminin-1, collagen IV and galectin-3 but reduced its interaction with fibronectin and laminin-10, and lowered the levels of proMMP-7 in the HT-29 medium.

CONCLUSION

TAA90K is a novel substrate for MMPs-2, -7 and -9 and modulates proMMP-7 levels in colon cancer cells.

GENERAL SIGNIFICANCE

Proteolytic cleavage of TAA90K may have functional implications in colon cancer.

Glycans: bioactive signals decoded by lectins

The glycan part of cellular glycoconjugates affords a versatile means to build biochemical signals. These oligosaccharides have an exceptional talent in this respect. They surpass any other class of biomolecule in coding capacity within an oligomer (code word). Four structural factors account for this property: the potential for variability of linkage points, anomeric position and ring size as well as the aptitude for branching (first and second dimensions of the sugar code). Specific intermolecular recognition is favoured by abundant potential for hydrogen/co-ordination bonds and for C-H/pi-interactions. Fittingly, an array of protein folds has developed in evolution with the ability to select certain glycans from the natural diversity. The thermodynamics of this reaction profits from the occurrence of these ligands in only a few energetically favoured conformers, comparing favourably with highly flexible peptides (third dimension of the sugar code). Sequence, shape and local aspects of glycan presentation (e.g. multivalency) are key factors to regulate the avidity of lectin binding. At the level of cells, distinct glycan determinants, a result of enzymatic synthesis and dynamic remodelling, are being defined as biomarkers. Their presence gains a functional perspective by co-regulation of the cognate lectin as effector, for example in growth regulation. The way to tie sugar signal and lectin together is illustrated herein for two tumour model systems. In this sense, orchestration of glycan and lectin expression is an efficient means, with far-reaching relevance, to exploit the coding potential of oligosaccharides physiologically and medically.

Duodenal phytohaemagglutinin (red kidney bean lectin) stimulates gallbladder contraction in humans

AIM

Lectins, carbohydrate-specific proteins without enzymatic activity on the ligand, are daily ingested plant proteins which survive the passage through the gastrointestinal tract in a biologically active form. Their binding to glycan determinants of natural glycoconjugates can trigger biological effects. The lectin phytohaemagglutinin (PHA) is abundantly present in red kidney beans and induces cholecystokinin (CCK) release in rats. The aim of the study was to investigate the effect of intraduodenal administration of PHA on plasma CCK levels and gallbladder contraction in humans and to elucidate potential mechanisms of action.

METHODS

Five healthy volunteers underwent four studies. After a basal intraduodenal saline infusion for 30 min, PHA or heat-inactivated PHA was infused in increasing doses: 150 microg, 1.5 mg and 15 mg for 30 min each. Intravenous saline, CCK(1) receptor antagonist dexloxiglumide or atropine were administered in random order. Gallbladder volumes were measured by ultrasonography and plasma CCK levels by radioimmunoassay.

RESULTS

Intraduodenal PHA induced gallbladder contraction in a dose-dependent fashion starting with the lowest dose. The highest dose reduced the gallbladder volume to 65.3 +/- 9.4% of basal volume (P < 0.001) whereas heat-inactivated PHA did not have any effect. Blocking CCK(1) or muscarinic receptors completely abolished PHA-stimulated gallbladder contraction (dexloxiglumide 208.7 +/- 23.7%; atropine 104 +/- 7.0% of basal volume) while none of the treatments affected CCK levels.

CONCLUSION

Duodenal administration of PHA potently stimulates gallbladder contraction in humans. This contraction is mediated via cholinergic pathway.

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.

Tumor suppressor p16INK4a--modulator of glycomic profile and galectin-1 expression to increase susceptibility to carbohydrate-dependent induction of anoikis in pancreatic carcinoma cells

Expression of the tumor suppressor p16(INK4a) after stable transfection can restore the susceptibility of epithelial tumor cells to anoikis. This property is linked to increases in the expression and cell-surface presence of the fibronectin receptor. Considering its glycan chains as pivotal signals, we assumed an effect of p16(INK4a) on glycosylation. To test this hypothesis for human Capan-1 pancreatic carcinoma cells, we combined microarray for selected glycosyltransferase genes with 2D chromatographic glycan profiling and plant lectin binding. Major differences between p16-positive and control cells were detected. They concerned expression of beta1,4-galactosyltransferases (down-regulation of beta1,4-galactosyltransferases-I/V and up-regulation of beta1,4-galactosyltransferase-IV) as well as decreased alpha2,3-sialylation of O-glycans and alpha2,6-sialylation of N-glycans. The changes are compatible with increased beta(1)-integrin maturation, subunit assembly and binding activity of the alpha(5)beta(1)-integrin. Of further functional relevance in line with our hypothesis, we revealed differential reactivity towards endogenous lectins, especially galectin-1. As a result of reduced sialylation, the cells' capacity to bind galectin-1 was enhanced. In parallel, the level of transcription of the galectin-1 gene increased conspicuously in p16(INK4a)-positive cells, and even figured prominently in a microarray on 1996 tumor-associated genes and in proteomic analysis. The cells therefore gain optimal responsiveness. The correlation between genetically modulated galectin-1 levels and anoikis rates in engineered transfectants inferred functional significance. To connect these findings to the fibronectin receptor, galectin-1 was shown to be co-immunoprecipitated. We conclude that p16(INK4a) orchestrates distinct aspects of glycosylation that are relevant for integrin maturation and reactivity to an endogenous effector as well as the effector's expression. This mechanism establishes a new aspect of p16(INK4a) functionality.

Stromal fibroblasts from basal cell carcinoma affect phenotype of normal keratinocytes

BACKGROUND

Epithelial-mesenchymal interactions are important not only to direct the course of prenatal development of skin and its appendages but also to influence the behaviour of transformed epithelial cells.

OBJECTIVES

Evaluation of the role of stromal fibroblasts on the phenotype of epithelial cells of basal cell carcinoma (BCC).

METHODS

The phenotype of human BCC was compared with the in vitro model where the growth and phenotypic pattern of normal human keratinocytes were monitored in co-culture with fibroblasts prepared from stroma of BCC (BCCFs), with normal dermal fibroblasts or with two established fibroblast lines. We visualized the expression of a panel of keratins, three types of endogenous lectin [galectin (Gal)-1, Gal-3 and Gal-7], binding sites for Gal-1 and Gal-3, a proliferation marker Ki67, nucleolar protein nucleostemin (NuclS) and membrane protein Ber-EP4. A phenotype and karyotype of BCCFs were also monitored. BCCFs were also grafted to NOD/LtSz-Rag1(null) mice to evaluate their malignant potential.

RESULTS

Prolonged cultivation of BCCFs has led to morphological changes, loss of contact inhibition, loss of fibroblast surface antigens and progressive aneuploidity. However, a fully malignant phenotype did not develop as these cells did not form tumours in immunodeficient mice. Co-culture of BCCFs with normal keratinocytes in vitro led to their phenotypical changes resembling those in BCC, namely, expression of keratin 19. These keratinocytes also strongly express nuclear binding sites for Gal-1 and NuclS. This phenotype was not observed when normal keratinocytes were cultured with nontumour-originated fibroblasts.

CONCLUSIONS

These observations indicate that BCCFs may differ from normal fibroblasts and may play a regulatory role in BCC biology.

Activity–structure correlations in divergent lectin evolution: fine specificity of chicken galectin CG-14 and computational analysis of flexible ligand docking for CG-14 and the closely related CG-16

Gene duplication and sequence divergence are driving forces toward establishing protein families. To examine how sequence changes affect carbohydrate specificity, the two closely related proto-type chicken galectins CG-14 and CG-16 were selected as models. Binding properties were analyzed using a highly sensitive solid-phase assay. We tested 56 free saccharides and 34 well-defined glycoproteins. The two galectins share preference for the II (Galbeta1-4GlcNAc) versus I (Galbeta1-3GlcNAc) version of beta-galactosides. A pronounced difference is found owing to the reactivity of CG-14 with histo-blood group ABH active oligosaccharides and A/B active glycoproteins. These experimental results prompted to determine activity-structure correlations by modeling. Computational analysis included consideration of the flexibility of binding partners and the presence of water molecules. It provided a comparative description of complete carbohydrate recognition domains, which had so far not been characterized in animal galectins. The structural models assigned II, I selectivity to a region downstream of the central Trp moiety. Docking revealed that the tetrasaccharides can be accommodated in their free-state low-energy conformations. CG-14's preference for A versus B epitopes could be attributed to a contact between His124 and the N-acetyl group of GalNAc. Regarding intergalectin comparison, the Ala53/Cys51 exchange affects the interaction potential of His54/His52. Close inspection of simulated dynamic interplay revealed reorientation of His124 at the site of the His124/Glu123 substitution, with potential impact on ligand dissociation. In summary, this study identifies activity differences and provides information on their relation to structural divergence, epitomizing the value of this combined approach beyond galectins.

AB-type lectin (toxin/agglutinin) from mistletoe: differences in affinity of the two galactoside-binding Trp/Tyr-sites and regulation of their functionality by monomer/dimer equilibrium

Viscumin of mistletoe (Viscum album L.) has a concentration-dependent activity profile unique to plant AB-toxins. It starts with lectin-dependent mitogenicity and then covers toxicity and cell agglutination, associated with shifts in the monomer/dimer equilibrium. Each lectin subunit harbors two sections for ligand contact. In the dimer, the B-chain sites in subdomain 2 gamma (designated as the Tyr-sites) appear fully accessible, whereas Trp-sites in subdomain 1 alpha are close to the dimer interface. It is unclear whether both types of sites operate similarly in binding glycoligands in solution. By systematically covering a broad range of lactose/lectin ratio in isothermal titration calorimetry, we obtained evidence for two sites showing dissimilar binding affinity. Intriguingly, the site with higher affinity was only partially occupied. To assign the observed properties to the Trp/Tyr-sites, we next performed chemically induced dynamic nuclear polarization measurements of Trp and Tyr accessibility. A Tyr signal, but not distinct Trp peaks, was recorded when testing the dimer. Lactose-quenchable Trp peaks became visible on the destabilization of the dimer by citraconylation, intimating Trp involvement in ligand contact in the monomer. Fittingly, Tyr acetylation but not mild Trp oxidation reduced the dimer hemagglutination activity and the extent of binding to asialofetuin-Sepharose 4B. Altogether, the results attribute lectin activity in the dimer primarily to Tyr-sites. Full access to Trp-sites is gained on dimer dissociation. Thus, the monomer/dimer equilibrium of viscumin regulates the operativity of these sites. Their structural divergence affords the possibility for differences in ligand selection when comparing monomers (Tyr- and Trp-sites) with dimers (primarily Tyr-sites).