Ligand interactions with E-selectin. Identification of a new binding site for recognition of N-acyl aromatic glucosamine substituents of sialyl Lewis X

Selectins in Biology and Human Disease: Opportunity in E-selectin Antagonism

Selectins are cell adhesion proteins discovered in the 1980s. As C-type lectins, selectins contain an essential calcium ion in the ligand-binding pocket and recognize the isomeric tetrasaccharides sialyl Lewisx (sLex) and sialyl Lewisa (sLea). Three selectins, E-selectin, P-selectin, and L-selectin, play distinct, complementary roles in inflammation, hematopoiesis, and tumor biology. They have been implicated in the pathology of diverse inflammatory disorders, and several selectin antagonists have been tested clinically. E-selectin plays a unique role in leukocyte activation, making it an attractive target for intervention, for example, in sickle cell disease (SCD). This review summarizes selectin biology and pathology, structure and ligand binding, and selectin antagonists that have reached clinical testing with an emphasis on E-selectin.

Adhesion molecules before and after propylthiouracil in patients with subclinical hyperthyroidism

OBJECTIVE

This study aimed to investigate the effect of propylthiouracil treatment on adhesion molecules in patients with subclinical hyperthyroidism.

METHODS

In this study, a total of 168 patients diagnosed with subclinical hyperthyroidism were treated with propylthiouracil for one year. The levels of adhesion molecules, consisting of sICAM-1, sVCAM-1, and sE-Selectin, before and after the treatment were measured and compared. These results were compared with the levels of 148 healthy controls who received a placebo.

RESULTS

sICAM-1 levels were significantly higher in subclinical hyperthyroidism patients than in healthy controls (*pa=0.000). sICAM-1 levels were significantly decreased after the treatment (**pb=0.000). Despite this decrease in patients with subclinical hyperthyroidism, it did not decrease to the level of the control group. sVCAM-1 did not change before and after propylthiouracil treatment. The level of sE-selectin was similar to that of the pretreatment control group, but it did not have statistical significance, although it increased after the treatment (**pb=0.004).

CONCLUSION

The sICAM level was significantly higher than the pretreatment values and decreased after the propylthiouracil treatment. However, further studies are needed to reduce the risk of atherosclerosis and cancer in patients with subclinical hyperthyroidism.

Expression of intercellular adhesion molecule-1 and e-selectin in gastric cancer and their clinical significance

Purpose

Among cell adhesion molecules, serum levels of intercellular adhesion molecule-1 and E-selectin are known to be correlated with the metastatic potential of gastric cancer. In the present study, the authors investigated the expression of intercellular adhesion molecule-1 and E-selectin in gastric cancer tissues and cultured gastric cancer cells, and examined their clinical value in gastric cancer.

Materials and Methods

The protein was extracted from gastric cancer tissues and cultured gastric cancer cells (MKN-28 and Kato-III) and the expression of intercellular adhesion molecule-1 and E-selectin was examined by western blotting. The clinical significance of intercellular adhesion molecule-1 and E-selectin was explored, using immunohistochemical staining of specimens from 157 gastric cancer patients.

Results

In western blot analysis, the expressions of intercellular adhesion molecule-1 in gastric cancer tissues and cultured gastric cancer cells were increased, however, E-selectin in gastric cancer tissues and cells were not increased. Among 157 gastric cancer patients, 79 patients (50%) were intercellular adhesion molecule-1 positive and had larger tumor size, an increased depth of tumor invasion, lymph node metastasis and perineural invasion. The intercellular adhesion molecule-1 positive group showed a higher incidence of tumor recurrence (40.5%), and a poorer 3-year survival than the negative group (54.9 vs. 85.9%, respectively).

Conclusions

Intercellular adhesion molecule-1 is overexpressed in gastric cancer tissues and cultured gastric cancer cells, whereas E-selectin is not overexpressed. Increased expression of intercellular adhesion molecule-1 in gastric cancer could be related to the aggressive nature of the tumor, and has a poor prognostic effect on gastric cancer.

Expression of E-selectin, integrin β1 and immunoglobulin superfamily member in human gastric carcinoma cells and its clinicopathologic significance

AIM: To study the expression levels of E- selectin, integrin β₁ and immunoglobulin supperfamily member-intercellular adhesion molecule-1 (ICAM-1) in human gastric carcinoma cells, and to explore the relationship between these three kinds of cell adhesion molecules and gastric carcinoma.

METHODS: The serum contents of E-selectin, integrin β₁ and ICAM-1 were detected by enzyme-linked immunosorbent assay (ELISA), in 47 healthy individuals (control group) and in 57 patients with gastric carcinoma (gastric carcinoma group) respectively prior to operation and 7 d after operation.

RESULTS: The serum E-selectin, ECAM-1 and integrin β₁ were found to be expressed in both control and gastric carcinoma groups. However, they were highly expressed in patients with gastric carcinoma patients before operation or with unresectable tumours. The expression levels of ICAM-1 and integrin β₁ were significantly higher in gastric carcinoma patients than in controls (P < 0.01). A comparison of the E-selectin levels between the two groups showed statistically insignificant difference (P = 0.64). In addition, the expression levels were all decreased substantially in the postoperative patients subjected to radical resection of the tumours, indicating that the high level expressions of these compounds might be the important factor for predicting the prognosis of these patients.

CONCLUSION: Serum E-selectin, ICAM-1 and integrin β₁ expression levels are probably related to the metastasis and relapse of gastric cancer.

Clinical significance of serum levels of E-selectin, intercellular adhesion molecule-1, and vascular cell adhesion molecule-1 in gastric cancer patients

OBJECTIVES

This study evaluated serum concentrations of soluble cell adhesion molecules in patients with gastric cancer and in healthy control subjects. Our objectives were to correlate these levels with clinicopathological features, established tumor markers, and patient survival, and to assess changes in serum levels of cell adhesion molecules after tumor surgery.

METHODS

The serum concentrations of the adhesion molecules E-selectin, intercellular adhesion molecule-1 (ICAM-1), and vascular cell adhesion molecule-1 (VCAM-1) were investigated by ELISA in 57 gastric cancer patients, both before and 7 days after surgery, and in 47 healthy control subjects.

RESULTS

Preoperative serum concentrations of ICAM-1 and VCAM-1 in gastric cancer patients were significantly higher when compared with those of healthy controls, whereas there were no differences regarding serum E-selectin levels. Serum levels of E-selectin, ICAM-1, and VCAM-1 correlated significantly with each other. There was a significant association between preoperative levels of all three adhesion molecules and disease stage, gastric wall invasion, lymph node involvement, and presence of distant metastases. Their concentrations decreased significantly after radical resection of the tumor, whereas they remained almost unchanged in patients with unresectable disease. Elevated preoperative serum levels of E-selectin, ICAM-1, and VCAM-1 levels were found in 24.6%, 33.3%, and 28.1% of patients, respectively. Elevated levels of all three molecules were significant prognostic factors for patient survival but not independent of disease stage.

CONCLUSIONS

These findings suggest that serum concentrations of E-selectin, ICAM-1, and VCAM-1 may reflect tumor progression and metastasis, and may be clinically useful.

Synthesis of a potent antagonist of E-selectin

A synthesis of an antagonist of E-selectin previously reported by a group at Novartis Pharma in Basel is described. An important feature involves the formation of an ether linkage based on a Rh(II)-catalyzed reaction. Stereocontrolled glycosylations rely on the anomeric activation of 2-pyridylthio carbonate as leaving group for the attachment of beta-D-galactopyranosyl and alpha-L-fucopyranosyl units on a common 1,5-anhydro D-glucitol scaffold.

Development of potent non-carbohydrate imidazole-based small molecule selectin inhibitors with antiinflammatory activity

A novel series of non-carbohydrate imidazole-based selectin inhibitors has been discovered via high-throughput screening using a P-selectin ELISA-based assay system. The initial lead 1 had an IC(50) of 17 microM in the P-selectin ELISA; this potency was significantly improved via an extensive SAR exploration. One of the current lead compounds (29) has an IC(50) of 300 nM in a P-selectin ELISA; it also has good activity in P- and E-selectin cell adhesion assays and shows efficacy in vivo. These compounds represent a novel series of sLe(X) mimetics with antiinflammatory activity. Their unique profile supports our interest in their further evaluation as drug candidates for the treatment of inflammation. Herein we describe the syntheses, optimization, and SAR of this series of novel potent selectin antagonists.

A structure-function study of ligand recognition by CD22beta

B-cell-specific CD22 is a member of a group of cell adhesion molecules within the immunoglobulin superfamily that display binding to glycans with terminal sialic acid residues. Binding of endogenous ligands to CD22 triggers B-cell activation and proliferation. It is therefore conceivable that high affinity ligands for CD22 may be of value as inhibitors of B-cell activation in allergy and chronic inflammation. In this study, we aimed to delineate the structural requirements for ligand binding to CD22. A library of 20 mono-, di-, and trisaccharide analogs of the basic binding motif Neu5Ac(alpha2,6)Lac was synthesized and screened for affinity for CD22beta. In general, CD22 ligand recognition appeared to be rather tolerant with respect to structural modifications of the anomeric sugar on a mono-, di-, and trisaccharide level, although affinity was increased by the presence of a nitro aromatic group at C-2. The most potent monovalent ligand, Neu5Ac-4-nitrobenzoyl-Glc, was selected to generate multivalent ligands based on either a glutamate or Tris cluster core. All multivalent ligands displayed at least a 10-fold increased affinity for CD22 compared with the corresponding monovalent glycoside. Interestingly, a maximal gain in affinity was already obtained for bivalent ligands, regardless of the terminal glycoside. A trivalent Tris-based cluster of Neu5Ac-4-nitrobenzoyl-Glc displayed a 300-fold higher affinity compared with the basic binding motif, which makes it, to our knowledge, the most potent antagonist for CD22 yet synthesized. As our in vitro fluorescence-activated cell sorting studies demonstrated efficient cellular uptake of a CD22 substrate, the most potent ligand in this study may hold promise as a homing device for immunomodulatory compounds and cytostatics.

Synthesis and biological evaluation of a sialyl Lewis X mimic with significantly improved E-selectin inhibition

The synthesis of the highly potent E-selectin inhibitor 5 is described. Sialyl Lewis X mimic 5 was rationally designed by combining two previously disclosed beneficial sLe(x) modifications in a single molecule. The compound was found to be 30-fold more potent than sLe(x) in a static, cell-free equilibrium assay. Furthermore, compound 5 was highly active (IC50 = 10 microM) in a dynamic non-equilibrium assay in which sLe(x) did not inhibit neutrophil rolling at up to 1000 microM.

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

The design and synthesis of novel beta-C-mannosides that inhibit the binding of sialyl Lewis x to E-selectin are described. Compounds that contained a phenyl substituent at the C-6 position were found to have increased potency.

Design and synthesis of a multivalent homing device for targeting to murine CD22

CD22 is a cell-surface glycoprotein uniquely located on mature B-cells and B-cell derived tumour cells. Current evidence suggests that binding of endogenous ligands to CD22 leads to modulation of B-cell activation by antigen. Incidentally, however, B-cell activation may derail. and lead to an undesired immune response, for example in cases of allergy, rheumatoid arthritis and Crohn's disease. In this situation, synthetic high-affinity ligands for CD22 may be of therapeutic value as inhibitors of B-cell activation. Recent studies have revealed that natural ligands for CD22 contain the trisaccharide NeuAc alpha-2,6-Lac as the basic binding motif. In addition, it has been demonstrated that binding to CD22 is strongly enhanced by multivalent presentation of the basic binding motif (cluster effect). In this paper. the stepwise development of a novel multivalent high-affinity ligand for CD22 is described. In the first stage, a series of monovalent NeuAc alpha-2,6-Glc(Y)X type binding motifs was prepared, and their affinity for murine CD22 was monitored, to obtain more insight into the effect of separate structure elements on ligand recognition. In the second stage, we prepared a trivalent cluster, based on the monovalent motif that displayed the highest affinity for CD22, NeuAc alpha-2,6-GlcNBzNO2OMe (7). This cluster, TRIS(NeuAc alpha-2,6-GlcNBzNO2)3 (52), displayed a more than 58-fold higher affinity for CD22 than the reference structure NeuAc alpha-2,6-LacOMe (10). To our knowledge, the cluster 52 is one of the most potent antagonists for CD22 yet synthesised.

Design, synthesis and biological evaluation of aryl-substituted sialyl Lewis X mimetics prepared via cross-metathesis of C-fucopeptides

Several aryl substituted C-fucopeptides have been developed as sialyl Lewis X mimetics. Although the compounds have a much simpler structure compared to SLe(x), up to 3-times higher binding affinity toward E-selectin and > 1000 times toward P-selectin was observed. Furthermore, a convenient strategy for generating a number of analogues from a SLe(x) mimetic template at a very late stage of the synthesis was introduced, using a ruthenium catalyzed cross olefin metathesis under benchtop conditions.

Synthesis of sialyl Lewis x mimetics as selectin inhibitors by enzymatic aldol condensation reactions

Several D-mannosyl phosphate/phosphonate derivatives have been enzymatically prepared as sialyl Lewis x tetrasaccharide mimics, which showed strong-to-moderate inhibition against E-, P-, and L-selectins. The synthesis of these mimics is very straightforward; mannosyl aldehyde derivatives are condensed with dihydroxyacetone phosphate (DHAP) in the presence of a DHAP-dependent aldolase to provide mannosyl phosphates.

Glycosyltransferase catalyzed assemblage of sialyl-Lewis(a)-saccharopeptides

A series of methyl hexopyranosiduronic acids are coupled to type I disaccharide amines to give 'trisaccharides' which have the natural N-acetyl group of the type I disaccharides replaced by uronic acids (-->saccharopeptides). These saccharopeptides are surprisingly good substrates for alpha-2,3,-sialyltransferase and fucosyltransferase III. The enzymes transfer N-acetylneuraminic acid and fucose, respectively, onto these acceptor substrates, despite the far reaching alterations, regio- and stereospecifically in the expected manner to yield sialyl-Lewis(a)-saccharopeptides.

Exploration of beta-turn scaffolding motifs as components of sialyl Le(X) mimetics and their relevance to P-selectin

Monocyclic and bicyclic lactam units representing beta-turn surrogates were incorporated into a sialyl Le(X) structure by replacement of the natural sugar components. Low micromolar activity was found in a new P-selectin binding assay.

Synthesis of sialyl Lewis X mimetics using the Ugi four-component reaction

Application of the Ugi four-component condensation to rapidly synthesize a library of glycopeptide mimics of the tetrasaccharide SLe(x) as inhibitors of E- and P-selectin, and to study the effect of varied functionality in mimics on the inhibition is described.

Design and synthesis of C-linked fucosides as inhibitors of E-selectin

Two series of C-linked fucosides as mimetics for the tetrasaccharide sialyl Lewis X have been synthesized and tested as inhibitors of E-Selectin. The fucopeptides have been prepared from three key intermediates, including alpha-C-allyl fucose, natural and unnatural amino acids bearing hydroxyl groups and an alpha, omega-diacid moiety for the imitation of the essential three parts of SLex, i.e., the Fuc, Gal, and NeuAc. The nature and distance of the linkage of the fucose moiety to the amino acids as well as the distance between the amino acids and the terminal carboxylic acid group turned out to be crucial for the biological activity. In addition the necessity of both OH groups (4- and 6-OH) in the Gal part could be confirmed. Conformational NMR study of the most active mimetic supports the structure-activity relationship. A second series of mimetics was prepared, where Fuc and Gal moieties were purely C-linked. In the synthesis of beta-C-allyl galactose an intramolecular 1,2-hydride shift led to an interesting side product. However, the substituted glycosidic oxygens led to a substantial loss of conformational constrain, which could not be compensated and resulted in low activity.