Studies on selectin blocker. 3. Investigation of the carbohydrate ligand sialyl Lewis X recognition site of P-selectin

Selectins-The Two Dr. Jekyll and Mr. Hyde Faces of Adhesion Molecules-A Review

Selectins belong to a group of adhesion molecules that fulfill an essential role in immune and inflammatory responses and tissue healing. Selectins are glycoproteins that decode the information carried by glycan structures, and non-covalent interactions of selectins with these glycan structures mediate biological processes. The sialylated and fucosylated tetrasaccharide sLe^x is an essential glycan recognized by selectins. Several glycosyltransferases are responsible for the biosynthesis of the sLe^x tetrasaccharide. Selectins are involved in a sequence of interactions of circulated leukocytes with endothelial cells in the blood called the adhesion cascade. Recently, it has become evident that cancer cells utilize a similar adhesion cascade to promote metastases. However, like Dr. Jekyll and Mr. Hyde’s two faces, selectins also contribute to tissue destruction during some infections and inflammatory diseases. The most prominent function of selectins is associated with the initial stage of the leukocyte adhesion cascade, in which selectin binding enables tethering and rolling. The first adhesive event occurs through specific non-covalent interactions between selectins and their ligands, with glycans functioning as an interface between leukocytes or cancer cells and the endothelium. Targeting these interactions remains a principal strategy aimed at developing new therapies for the treatment of immune and inflammatory disorders and cancer. In this review, we will survey the significant contributions to and the current status of the understanding of the structure of selectins and the role of selectins in various biological processes. The potential of selectins and their ligands as therapeutic targets in chronic and acute inflammatory diseases and cancer will also be discussed. We will emphasize the structural characteristic of selectins and the catalytic mechanisms of glycosyltransferases involved in the biosynthesis of glycan recognition determinants. Furthermore, recent achievements in the synthesis of selectin inhibitors will be reviewed with a focus on the various strategies used for the development of glycosyltransferase inhibitors, including substrate analog inhibitors and transition state analog inhibitors, which are based on knowledge of the catalytic mechanism.

Affinity of low molecular weight fucoidan for P-selectin triggers its binding to activated human platelets

BACKGROUND

P-selectin is an adhesion receptor expressed on activated platelets and endothelial cells. Its natural ligand, P-selectin glycoprotein ligand-1, is expressed on leucocytes and the P-selectin/PSGL-1 interaction is involved in leukocyte rolling. We have compared the interaction of P-selectin with several low molecular weight polysaccharides: fucoidan, heparin and dextran sulfate.

METHODS

Binding assays were obtained from the interaction of the polysaccharides with Sialyl Lewis X and PSGL-1 based constructs onto microtiter plates coated with P-selectin. SELDI TOF mass spectrometry was performed with anionic chips arrays coated with P-selectin in the absence or in the presence of polysaccharides. Kd were obtained from surface plasmon resonance experiments with immobilized P-selectin constructs, polysaccharides being injected in the mobile phase. Human whole blood flow cytometry experiments were performed with fluorescein isothiocyanate labelled polysaccharides with or without platelets activators.

RESULTS

The fucoidan prevented P-selectin binding to Sialyl Lewis X with an IC(50) of 20 nM as compared to 400 nM for heparin and <25000 nM for dextran sulfate. It exhibited the highest affinity for immobilized P-selectin with a KD of 1.2 nM, two orders of magnitude greater than the K(D) of the other polysaccharides. Mass spectrometry evidenced the formation of a complex between P-selectin and fucoidan. The intensity of the fucoidan binding to platelets was dependent on the level of platelet activation. Competition between fucoidan and an anti P-selectin antibody demonstrated the specificity of the interaction.

GENERAL SIGNIFICANCE

Low molecular weight fucoidan is a promising therapeutic agent of natural origin for biomedical applications.

A new macromolecular paramagnetic MR contrast agent binds to activated human platelets

A new functionalized macromolecular magnetic resonance (MR) contrast agent has been developed from a carboxymethyldextran-Gd(DOTA) devoid of biospecificity. The functionalized contrast agent was synthesized in order to mimic PSGL-1, the main ligand of P-selectin, a glycoprotein mainly expressed on the surface of activated platelets. The starting compound, CM1, was first carboxymethylated by monochloroacetic acid leading to a series of 10 derivatives varying in their carboxymethyl content. CM8 derivative, with a degree of substitution in carboxymethyl of 0.84, was chosen for subsequent fluorolabeling and sulfation to give CM8FS. CM8FS has an average number molecular weight of 27 000 +/- 500 g/mol, a hydrodynamic radius of 5.7 +/- 0.2 nm and a high relaxivity (r(1) = 11.2/mM (Gd)/s at 60 MHz). Flow cytometry experiments on whole human blood or on isolated platelets evidenced in vitro a preferential binding of CM8FS on TRAP-activated human platelets. Interestingly, CM8FS did not bind to other blood cells or to resting platelets. Pellets of TRAP-activated human platelets have also been imaged in tubes with a 1.5 T MR imager. A MR signal was observed for activated platelets incubated with CM8FS. Altogether, these in vitro results evidenced the recognition of activated human platelets by a fluorescent paramagnetic contrast agent grafted with carboxyl and sulfate groups. This biomimetic approach associated with the versatile macromolecular platform appears promising for the development of new contrast agents for molecular imaging of activated platelets in cardiovascular diseases such as atherosclerosis and aneurysms.

Fluorescent indolylboronic acids that are useful reporters for the synthesis of boronolectins

Lectins are known to regulate a wide variety of biological processes. Therefore, small molecule mimics of lectins have the potential to be used as novel diagnostic and therapeutic agents. In our combinatorial search for lectin mimics, we are in need a large number of boronic acids that change fluorescent properties upon carbohydrate binding. Along this line, a series of indolylboronic acids have been found to show significant fluorescent property changes upon binding with carbohydrates in 0.1 M phosphate buffer at physiological pH. These boronic acids will be very useful for the synthesis of lectin mimics for biological applications.

Design and synthesis of sialyl Lewis(x) mimics as E- and P-selectin inhibitors

The selectins are a family of cell-adhesion proteins that mediate the rolling of leukocytes on activated endothelial cells through the recognition of the carbohydrate epitope sialyl Lewis(x) (sLe(x)). Control of the leukocyte-endothelial cell adhesion process may prove useful in cases where excess recruitment of leukocytes can contribute to acute diseases such as stroke and reperfusion injury and chronic diseases such as psoriasis and rheumatoid arthritis. The development of molecules that block the interactions between sLe(x) and the selectins has become an active area of research. In this review, we will highlight the various approaches taken toward the development of sLe(x) mimetics as antagonists of E- and P-selectin, including the use of structural information about the selectins and their interactions with sLe(x) that have been revealed through the use of NMR, protein crystallography and molecular modeling.

First successful molecular design of an artificial Lewis oligosaccharide binding system utilizing positive homotropic allosterism

We have designed phenylboronic acid group appended Ce(IV) bis(porphyrinate) double decker 1 and meso-meso linked porphyrin 2, useful for the allosteric binding of biologically important saccharides, Lewis oligosaccharides. Compound 1 binds Lewis oligosaccharides in aqueous media because of the boronic acid-diol interaction, but the complexation event can occur only above the critical concentrations because of the sigmoidal [oligosaccharide] versus [complex] isotherm. Compound 1 has a sufficiently high affinity with Lewis oligosaccharides (K = 10(5)-10(6) M(-2)) with Hill coefficients n of 1.8-2.0, and Lewis(X) series and Lewis(a) series give opposite, symmetrical CD spectra. This is the first example of efficient binding of Lewis oligosaccharides to the artificial receptor, which has become possible by positive homotropic allosterism.

Studies on selectin blocker. 9. SARs of non-sugar selectin blocker against E-, P-, L-selectin bindings

As a part of study of selectin blockers, we have already reported that a non-sugar selectin antagonist (3) was successfully discovered using a computational screening (Hiramatsu, Y.; Tsukida, T.; Nakai, Y.; Inoue, Y.; Kondo, H. J. Med. Chem. 2000, 43, 1476). To investigate the SARs of compound 3 against E-, P-, and L-selectins, we synthesized the derivatives of compound 3 and evaluated their inhibitory activities toward selectin bindings. The structural diversity of compound 3 contained the following: (1) a modification of the spacer unit (4--7), (2) a modification of the tail unit (8--11), (3) a modification of the head unit (12--18). As a result, it was found that a non-sugar based selectin blocker (3) could be a potential lead compound for E-, P-, and L-selectin blockers and some of the derivatives showed broad and/or selective inhibitory activities toward the E-, P-, and L-selectins. In addition, it was found that the experimental evidence well supported that the computational screening using 3D-pharmacophore model could be useful methodology to find out a new lead for the several type of selectin blockers, which included a broad and/or a selective inhibitor.

Study on selectin blocker. 8. Lead discovery of a non-sugar antagonist using a 3D-pharmacophore model

We have developed a pharmacophore model of a ligand/E-selectin complex to screen drug candidates for selectin blockers. In a series of sugar mimetic studies of the E-selectin ligand, sialyl Lewis X (sLe(x)), we have already found a potent compound, a sulfated Le(x) analogue (1), and also have proposed how compound 1 binds to E-selectin (Tsujishita, H.; Hiramatsu, Y.; Kondo, N.; Ohmoto, H.; Kondo, H.; Kiso, M.; Hasegawa, A. J. Med. Chem. 1997, 40, 362-369). To find drug candidates that fit into the binding pocket of E-selectin, we constructed an original 3D-pharmacophore model from structural information of a compound 1/E-selectin complex model and screened lead compounds for selectin blockers using a commercially available database ACD-3D. As a result, we discovered a lead compound (2) containing good selectin inhibitory activity, and in addition, we succeeded to preliminarily optimize it to a more active lead compound (3) with micromolar IC(50) values, based on the 3D-pharmacophore model investigation. This methodology using the 3D-pharmacophore model could be applicable as a pre-screen system for selectin blockers.

Aryl C-glycosides: physiologically stable glycomimetics of sialyl Lewis X

In the course of the search for physiologically stable, structurally simple, and low molecular weight sLeX mimetics, aryl C-glycosides with carboxylic acid functionality 2 were found to be extremely potent inhibitors against L- and P-selectins with IC50 in the low microM range.

The C-type lectin superfamily in the immune system

Protein-carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar-binding proteins) mediate both pathogen recognition and cell-cell interactions using structurally related Ca(2+)-dependent carbohydrate-recognition domains (C-type CRDs). Pathogen recognition by soluble collections such as serum mannose-binding protein and pulmonary surfactant proteins, and also the macrophage cell-surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide-binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non-self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C-type lectin-like domains (CTLDs) that are evolutionarily divergent from the C-type lectins and which would be predicted to function through different mechanisms.

Oligosaccharide structures: theory versus experiment

Recently, the interdependency of theoretical and experimental approaches in the structure determination of oligosaccharides has been confirmed. More accurate simulations are possible because of the advances in software and computers. Meanwhile, improvements in NMR techniques permit the measurement of numerous structural and dynamical parameters, either for the free state or for carbohydrate ligands bound to receptors. Several crystal structures of isolated or protein-complexed oligosaccharides give new clues for modeling the intermolecular forces that drive the interactions.