A sialoglycoprotein from human leukocytes functions as a ligand for P-selectin

ICAM-1 and P-selectin expression in a model of NSAID-induced gastropathy

A growing body of experimental evidence suggests that neutrophilic polymorphonuclear leukocyte (PMN)-endothelial cell interactions play a critical role in the pathophysiology of nonsteroidal anti-inflammatory drug (NSAID)-induced gastropathy. The objective of this study was to directly determine whether the expression of endothelial cell adhesion molecules is enhanced in a model of NSAID-induced gastropathy. Gastropathy was induced in male Sprague-Dawley rats via oral administration of indomethacin (Indo, 20 mg/kg). Lesion scores, blood-to-lumen clearance of 51Cr-EDTA (mucosal permeability), and histological analysis (epithelial necrosis) were used as indexes of gastric mucosal injury. Gastric mucosal vascular expression of intercellular adhesion molecule 1 (ICAM-1) or P-selectin were determined at 1 and 3 h after Indo administration using the dual radiolabeled monoclonal antibody (MAb) technique. For some experiments, a blocking MAb directed at either ICAM-1 (1A29) or P-selectin (RMP-1) or their isotype-matched controls was injected intravenously 10 min before Indo administration. We found that P-selectin expression was significantly increased at 1 h but not 3 h after Indo administration, whereas ICAM-1 expression was significantly increased at both 1 and 3 h after Indo treatment. The blocking ICAM-1 and P-selectin MAbs both inhibited Indo-induced increases in lesion score, mucosal permeability, and epithelial cell necrosis. However, the Indo-induced gastropathy was not associated with significant PMN infiltration into the gastric mucosal interstitium, nor did Indo reduce gastric mucosal blood flow. We propose that NSAID-induced gastric mucosal injury may be related to the expression of P-selectin and ICAM-1; however, this mucosal injury does not appear to be dependent on the extravasation of inflammatory cells or mucosal ischemia.

Modulation of P-selectin expression in the postischemic intestinal microvasculature

The dual radiolabeled monoclonal antibody technique was used to 1) define the magnitude and kinetics of P-selectin expression in murine small intestine exposed to ischemia-reperfusion (I/R), and 2) determine the factor(s) responsible for initiating this response. Within 10 min after release of a 20-min arterial occlusion, intestinal P-selectin expression increased two- to threefold compared with control values. Peak (4-fold) expression of P-selectin was noted at 5 h after reperfusion, returning to the control value at 24 h. The early (10-30 min) I/R-induced upregulation of P-selectin appears to reflect mobilization of a performed pool of the adhesion molecule, whereas the later (5 h) rise appears to be transcription dependent. The early increase in P-selectin expression was not inhibited by pretreatment with either oxypurinol (inhibits xanthine oxidase), diphenhydramine (H1-receptor antagonist), or MK-571 (leukotriene C4/D4 antagonist), nor was it blunted in transgenic mice expressing three times the normal level of copper-zinc superoxide dismutase or in mast cell-deficient mice. However, significant inhibition was noted after treatment with either MK-886 (5-lipoxygenase inhibitor) or a nitric oxide (NO) donor (diethylenetriamine/NO). These findings indicate that the early I/R-induced increase in intestinal P-selectin expression is mediated by a 5-lipoxygenase-dependent NO-inhibitable mechanism.

Endothelial cell adhesion molecule expression in gene-targeted mice

Gene-targeted mice are now routinely employed as tools for defining the contribution of different leukocyte and endothelial cell adhesion molecules to the leukocyte recruitment and tissue injury associated with acute and chronic inflammation. The objective of this study was to determine whether gene-targeted mice that are deficient in CD11/CD18, intracellular adhesion molecule-1 (ICAM-1), or P-selectin exhibit an altered constitutive or induced expression of the endothelial cell adhesion molecules E- and P-selectin. The gene-targeted mice were all developed in the 129Sv mouse strain and backcrossed into C57B1/6J mice. The number of backcrosses ranged between 8 (P-selectin) and 10 (CD18 and ICAM-1) generations. The dual-radiolabeled monoclonal antibody technique was used to quantify E- and P-selectin expression in different vascular beds. In the unstimulated state, E-selectin expression was significantly elevated (relative to wild-type mice) in the stomach, large intestine, and brain of mutants deficient in ICAM-1. In general, constitutive expression of P-selectin did not differ between wild-type, ICAM-1-deficient, and CD11/CD18-deficient mutants. In CD11/CD18-deficient mice, tumor necrosis factor-alpha (TNF-alpha) administration elicited a more profound upregulation of P-selectin in several vascular beds, compared with wild-type and ICAM-1-deficient mice. E-selectin expression in brain of TNF-alpha-stimulated, ICAM-1-deficient, and P-selectin-deficient mice was attenuated compared with wild-type mice. These findings indicate that chronic deficiency of some of the adhesion glycoproteins that mediate leukocyte recruitment alters basal and induced surface expression of other adhesion molecules on endothelial cells.

Characterization of a novel adhesion function blocking monoclonal antibody to rat/mouse P-selectin generated in the P-selectin-deficient mouse

P-selectin is an important adhesion molecule involved in leukocyte migration. However, to date, no monoclonal antibodies (MAb) generated against rat P-selectin have been identified which block P-selectin mediated leukocyte adhesion. Most studies in the rat have utilized crossreacting antibodies generated against P-selectin in higher species. In a P-selectin deficient mouse we generated an anti-rat/mouse P-selectin MAb, designated RMP-1, by immunization with activated rat platelets. This IgG2a MAb immunoprecipitates a 140 kDa protein under reducing conditions from rat platelet lysate. By ELISA and immunofluorescence flow cytometry, MAb RMP-1 reacts with thrombin-activated but not unactivated rat platelets. In addition, by ELISA MAb RMP-1 binds to activated mouse platelets and recombinant rat and mouse P-selectin. MAb RMP-1 inhibited adhesion of HL-60 myeloid cells to immobilized mouse P-selectin by 97% and to activated rat and mouse platelets by 100% under static conditions, confirming the adhesion function blocking activity of MAb RMP-1. This novel MAb should be useful for studying P-selectin function in vitro and in vivo in both rat and mouse inflammation models.

Expression of a P-selectin ligand in zona pellucida of porcine oocytes and P-selectin on acrosomal membrane of porcine sperm cells. Potential implications for their involvement in sperm-egg interactions

The selectin family of cell adhesion molecules mediates initial leukocyte adhesion to vascular endothelial cells at sites of inflammation. O-glycan structural similarities between oligosaccharides from human leukocyte P-selectin glycoprotein ligand-1 (PSGL-1) and from zona pellucida glycoproteins of porcine oocytes indicate the possible existence of a P-selectin ligand in the zona pellucida. Here, using biochemical as well as morphological approaches, we demonstrate that a P-selectin ligand is expressed in the porcine zona pellucida. In addition, a search for a specific receptor for this ligand leads to the identification of P-selectin on the acrosomal membrane of porcine sperm cells. In vitro binding of porcine acrosome-reacted sperm cells to oocytes was found to be Ca2+ dependent and inhibitable with either P-selectin, P-selectin receptor-globulin, or leukocyte adhesion blocking antibodies against P-selectin and PSGL-1. Moreover, porcine sperm cells were found to be capable of binding to human promyeloid cell line HL-60. Taken together, our findings implicate a potential role for the oocyte P-selectin ligand and the sperm P-selectin in porcine sperm-egg interactions.

Molecular determinants of oxidized low-density lipoprotein-induced leukocyte adhesion and microvascular dysfunction

Oxidized low-density lipoproteins (oxLDL) have been implicated in the leukocyte recruitment and microvascular dysfunction associated with atherosclerosis. The objectives of this study were to define the adhesion molecules that mediate oxLDL-induced leukocyte-endothelial cell adhesion and to determine whether leukocyte-endothelial cell adhesion contributes to the endothelial barrier dysfunction elicited by oxLDL. Leukocyte-endothelial cell adhesion and emigration, albumin extravasation, and mast cell degranulation were monitored in rat mesentery in response to native LDL (nLDL) or copper-oxidized LDL (oxLDL). Intra-arterial infusion of oxLDL but not nLDL elicited increases in leukocyte adherence and emigration, mast cell degranulation, and albumin leakage. The oxLDL-induced leukocyte adherence/emigration was attenuated by pretreatment with monoclonal antibodies directed against CD11/CD18, intercellular adhesion molecule-1, P-selectin, and L-selectin but not by pretreatment with a nonbinding monoclonal antibody. The albumin leakage and mast cell degranulation responses were attenuated by all of the same monoclonal antibodies except L-selectin. In addition, a peptide previously shown to inhibit leukocyte-endothelial cell adhesion in vitro also attenuated leukocyte adherence and mast cell degranulation in this model. These findings implicate CD11/ CD18, L-selectin, intercellular adhesion molecule-1, and P-selectin in the leukocyte recruitment elicited by oxLDL and invoke a role for adherent leukocytes in the accompanying increase in mast cell degranulation and albumin leakage.

Threshold levels of fluid shear promote leukocyte adhesion through selectins (CD62L,P,E)

Leukocyte adhesion through L-selectin to peripheral node addressin (PNAd, also known as MECA-79 antigen), an L-selectin ligand expressed on high endothelial venules, has been shown to require a minimum level of fluid shear stress to sustain rolling interactions (Finger, E.B., K.D. Puri, R. Alon, M.B. Lawrence, V.H. von Andrian, and T.A. Springer. 1996. Nature (Lond.). 379:266-269). Here, we show that fluid shear above a threshold of 0.5 dyn/cm2 wall shear stress significantly enhances HL-60 myelocyte rolling on P- and E-selectin at site densities of 200/microm2 and below. In addition, gravitational force is sufficient to detach HL-60 cells from P- and E-selectin substrates in the absence, but not in the presence, of flow. It appears that fluid shear-induced torque is critical for the maintenance of leukocyte rolling. K562 cells transfected with P-selectin glycoprotein ligand-1, a ligand for P-selectin, showed a similar reduction in rolling on P-selectin as the wall shear stress was lowered below 0.5 dyn/cm2. Similarly, 300.19 cells transfected with L-selectin failed to roll on PNAd below this level of wall shear stress, indicating that the requirement for minimum levels of shear force is not cell type specific. Rolling of leukocytes mediated by the selectins could be reinitiated within seconds by increasing the level of wall shear stress, suggesting that fluid shear did not modulate receptor avidity. Intravital microscopy of cremaster muscle venules indicated that the leukocyte rolling flux fraction was reduced at blood centerline velocities less than 1 mm/s in a model in which rolling is mediated by L- and P-selectin. Similar observations were made in L-selectin-deficient mice in which leukocyte rolling is entirely P-selectin dependent. Leukocyte adhesion through all three selectins appears to be significantly enhanced by a threshold level of fluid shear stress.

Effects of chronic arterial hypertension on constitutive and induced intercellular adhesion molecule-1 expression in vivo

Recent reports indicate that bacterial endotoxin (lipopolysaccharide) and cytokines elicit a more profound increase in the surface expression of intercellular adhesion molecule-1 (ICAM-1) in cultured endothelial cells derived from spontaneously hypertensive (SHR) versus normotensive Wistar-Kyoto rats (WKY). Our objective in this study was to characterize and compare in vivo ICAM-1 expression in SHR and WKY under basal conditions and after 5 hours of endothelial cell activation with either lipopolysaccharide (5 mg/kg i.p.) or tumor necrosis factor-alpha (TNF-alpha; 1, 5, and 10 micrograms/kg i.p.). ICAM-1 expression was quantified in different tissues by the double-radiolabeled monoclonal antibody technique. When constitutive (baseline) ICAM-1 expression was corrected for endothelial cell surface area, significantly higher values were noted in SHR than WKY but only in splanchnic organs. Lipopolysaccharide and TNF-alpha elicited significant increases in ICAM-1 expression in all tissues of both WKY and SHR. However, the magnitude of the lipopolysaccharide-induced ICAM-1 upregulation in heart, stomach, skeletal muscle, and brain was significantly lower in SHR than WKY. A similar blunted ICAM-1 upregulation was noted in the stomach of SHR after administration of 5 micrograms/kg TNF-alpha. The differences in induced ICAM-1 expression between SHR and WKY do not appear to be due to differences in endothelial cell surface area or plasma glucocorticoid levels. These results suggest that chronic arterial hypertension results in altered ICAM-1 expression on the endothelium, which may contribute to the abnormal inflammatory responses associated with this disease.

Sialic acids in molecular and cellular interactions

Sialic acids (Sias) are terminal components of many glycoproteins and glycolipids especially of higher animals. In this exposed position they contribute significantly to the structural properties of these molecules, both in solution and on cell surfaces. Therefore, it is not surprising that Sias are important regulators of cellular and molecular interactions, in which they play a dual role. They can either mask recognition sites or serve as recognition determinants. Whereas the role of Sias in masking and in binding of pathogens to host cells has been documented over many years, their role in nonpathological cellular interaction has only been shown recently. The aim of this chapter is to summarize our knowledge about Sias in masking, for example, galactose residues, and to review the progress made during the past few years with respect to Sias as recognition determinants in the adhesion of pathogenic viruses, bacteria, and protozoa, and particularly as binding sites for endogenous cellular interaction molecules. Finally, perspectives for future research on these topics are discussed.

Heterogeneity of expression of E- and P-selectins in vivo

A novel technique involving radiolabeled monoclonal antibodies was used to characterize and compare the expression of E- and P-selectin on unstimulated, histamine-challenged, and endotoxin-challenged endothelial cells in various tissues of the mouse. Under unstimulated conditions, E-selectin was absent in all organs, but significant expression of P-selectin was observed in several organs. Histamine induced a rapid time-dependent upregulation of P-selectin, with the largest responses observed in mesentery and lung. Significant fold elevations in P-selectin expression occurred as early as 5 minutes after the histamine injection and remained elevated up to 1 hour. Histamine-induced P-selectin upregulation was inhibited by the H1 receptor antagonist diphenhydramine, whereas the H2 receptor antagonist cimetidine had no effect. Endotoxin (lipopolysaccharide [LPS]) also induced a time-dependent expression of P-selectin that reached a maximum between 4 and 8 hours after endotoxin administration. LPS-induced upregulation of P-selectin was greatest in heart and stomach, which exhibited insignificant constitutive expression of P-selectin. LPS also induced a time-dependent upregulation of E-selectin, with maximal expression occurring 3 to 5 hours after intraperitoneal administration. The lung and small intestine exhibited the largest responses to LPS challenge. Histamine administration did not affect E-selectin expression in any tissue. E- and P-selectin-deficient mice were used to test the specificity of monoclonal antibody binding in unstimulated, histamine-challenged, and LPS-stimulated tissues. Vascular binding of the radiolabeled E-selectin and P-selectin monoclonal antibodies was not observed in the respective deficient mice. These findings suggest that P-selectin is constitutively expressed on vascular endothelium in some tissues of the mouse and that there are significant regional differences in the magnitude and time course of histamine- and endotoxin-induced P-selectin expression. In contrast, E-selectin appears to be absent on unstimulated vascular endothelium but is upregulated within 3 hours after the administration of endotoxin in most tissues.

Visualization of P-selectin glycoprotein ligand-1 as a highly extended molecule and mapping of protein epitopes for monoclonal antibodies

P-selectin glycoprotein ligand-1 (PSGL-1), a sialomucin on human leukocytes, mediates rolling of leukocytes on P-selectin expressed by activated platelets or endothelial cells under shear forces. PSGL-1 requires both tyrosine sulfate and O-linked glycans to bind P-selectin. Electron microscopy of rotary-shadowed PSGL-1 purified from human neutrophils indicated that it is a highly extended molecule with an extracellular domain that is -50 nm long. Both individual PSGL-1 molecules and rosettes composed of several molecules presumably attached at their transmembrane segments were observed. The extracellular domain of PSGL-1 has 318 residues, including a signal peptide from residues 1-18 and a propeptide from residues 19-41. Using bacterially expressed fusion proteins and synthetic peptides derived from the extracellular domain, we mapped the epitopes for two IgG anti-PSGL-1 monoclonal antibodies, PL1 and PL2. PL2 bound to a region within residues 188-235 that is located in a series of decameric consensus repeats. PL1, which blocks binding of PSGL-1 to P-selectin, recognized an epitope spanning residues 49-62. This sequence overlaps the tyrosine sulfation sites at residues 46, 48, and 51 that have been implicated in binding of PSGL-1 to P-selectin. Our results demonstrate that PSGL-1 is a long, extended molecule and suggest that the P-selectin binding site is located near the N terminus, well above the membrane. This location may facilitate interactions of PSGL-1 with P-selectin under shear stress.

A schiff base with mildly oxidized carbohydrate ligands stabilizes L-selectin and not P-selectin or E-selectin rolling adhesions in shear flow

Selectins are a family of lectins, that mediate tethering and rolling of leukocytes on endothelium in vascular shear flow. Mild periodate oxidation of the L-selectin ligand CD34, or L-selectin ligands on leukocytes, enhanced resistance to detachment in shear and decreased rolling velocity equivalent to an 8-fold increase in ligand density, yet had little effect on the rate of tethering. Enhanced interactions were also seen with mildly oxidized sialyl Lewisa and sialyl Lewisx glycolipids. Enhancement was completely reversed by borohydride reduction, yielding a strength of interaction equivalent to that with the native ligands. No effect on the strength of P-selectin and E-selectin interactions was seen after mild oxidation of their ligands. Completeness of modification of sialic acid by mild periodate was verified with monoclonal antibody to sialyl Lewisx-related structures and resistance to neuraminidase. The addition of cyanoborohydride to leukocytes rolling through L-selectin on mildly oxidized but not native CD34 caused arrest of rolling cells and formation of EDTA-resistant bonds to the substrate, suggesting that a Schiff base was reduced. Cyanoborohydride reduction of mildly oxidized cells rolling on P-selectin and E-selectin also caused arrest and formation of EDTA-resistant bonds but with slower kinetics. These data suggest that interactions with a sialic acid aldehyde group on mildly oxidized ligands that include interconversion to a Schiff base can occur with three selectins yet only stabilize binding through the selectin with the fastest koff, L-selectin.

PSGL-1 recognition of P-selectin is controlled by a tyrosine sulfation consensus at the PSGL-1 amino terminus

P-selectin binding to neutrophils requires a specific protein, P-selectin glycoprotein ligand 1 (PSGL-1), as well as sialyl-Lewis X (sLex) glycan determinants. We have found that a short segment near the amino terminus of PSGL-1 that contains a tyrosine sulfation consensus is essential for P-selectin adhesion and that addition of the amino-terminal segment to some but not all mucin-like molecules confers on those molecules the ability to bind P-selectin. PSGL-1 synthesized in the presence of sulfation inhibitors binds P-selectin weakly, and within the amino-terminal 20 residues, mutation of the tyrosines to phenylalanine abolishes binding. Rolling of HL-60 cells on P-selectin-coated coverslips is strongly attenuated by treatment of cells with an inhibitor of sulfation.

The P-selectin glycoprotein ligand functions as a common human leukocyte ligand for P- and E-selectins

P- and E-selectins belong to a family of Ca(2+)-dependent lectins and function as receptors for myeloid leukocytes. We have described a panel of monoclonal antibodies which recognize a sialoglycoprotein from human neutrophils and HL-60 promyelocytic cells and inhibit adhesion of these cells to P-selectin. In this study, we show that the E-selectin receptor-globulin (E-selectin Rg) affinity chromatography can isolate specifically only one glycoprotein from [3H]glucosamine-labeled HL-60 cells in a Ca(2+)-dependent manner. This protein has a molecular mass of approximately 120 kDa under reducing conditions, which appears to be identical with the previously characterized glycoprotein ligand for P-selectin. The molecule can be cross-depleted by and cross-bound to the E- and P-selectin columns. The chromatographic profile of desialylated O-linked carbohydrates from molecules purified by P- and E-selectin affinity chromatography are identical. Both have five structures at 12.8, 9.8, 6.3, 3.5, and 2.5 glucose units. PL5 monoclonal antibody to the P-selectin sialoglycoprotein ligand, E-selectin Rg, and antiserum to P-selectin glycoprotein ligand-1 (PSGL-1) all recognize the purified P-selectin ligand on ligand blots and immunoblots. Furthermore, PL5 monoclonal antibody blocks adhesion of HL-60 cells and human neutrophils to E-selectin Rg. Taken together, our results demonstrate that the P- and E-selectin ligand defined in this study is PSGL-1 and suggest that this molecule is an important leukocyte ligand for both P- and E-selectins.