Sialosyl Le(a)-carrying gangliosides present on the surface of colon carcinoma cells are not directly involved in adhesion to E-selectin

The biology of E-selectin ligands in leukemogenesis

Both the cascade whereby a blood-borne cell enters a tissue and the anchoring of hematopoietic stem/progenitor cells (HSPCs) within bone marrow critically pivots on cell-cell interactions mediated by E-selectin binding to its canonical carbohydrate ligand, the tetrasaccharide termed "sialylated Lewis X" (sLeX). E-selectin, a member of the selectin class of adhesion molecules that is exclusively expressed by vascular endothelium, engages sLeX-bearing glycoconjugates that adorn mature leukocytes and HSPCs, as well as malignant cells, thereby permitting these cells to extravasate into various tissues. E-selectin expression is induced on microvascular endothelial cells within inflammatory loci at all tissues. However, conspicuously, E-selectin is constitutively expressed within microvessels in skin and marrow and, additionally, is inducibly expressed at these sites. Within the marrow, E-selectin receptor/ligand interactions promote lodgment of HSPCs and their malignant counterparts within hematopoietic growth-promoting microenvironments, collectively known as "vascular niches". Indeed, E-selectin receptor/ligand interactions have been reported to regulate both hematopoietic stem, and leukemic, cell proliferative dynamics. As such, signaling induced via engagement of E-selectin ligands is gaining interest as a critical mediator of homeostatic and malignant hematopoiesis, and this review will present current perspectives on the glycoconjugates mediating E-selectin receptor/ligand interactions and their currently defined role(s) in leukemogenesis.

Selectin Ligands Sialyl-Lewis a and Sialyl-Lewis x in Gastrointestinal Cancers

The tetrasaccharide structures Siaα2,3Galβ1,3(Fucα1,4)GlcNAc and Siaα2,3Galβ1,4(Fucα1,3)GlcNAc constitute the epitopes of the carbohydrate antigens sialyl-Lewis a (sLe^a) and sialyl-Lewis x (sLe^x), respectively, and are the minimal requirement for selectin binding to their counter-receptors. Interaction of sLe^x expressed on the cell surface of leucocytes with E-selectin on endothelial cells allows their arrest and promotes their extravasation. Similarly, the rolling of cancer cells ectopically expressing the selectin ligands on endothelial cells is potentially a crucial step favoring the metastatic process. In this review, we focus on the biosynthetic steps giving rise to selectin ligand expression in cell lines and native tissues of gastrointestinal origin, trying to understand whether and how they are deregulated in cancer. We also discuss the use of such molecules in the diagnosis of gastrointestinal cancers, particularly in light of recent data questioning the ability of colon cancers to express sLe^a and the possible use of circulating sLe^x in the early detection of pancreatic cancer. Finally, we reviewed the data dealing with the mechanisms that link selectin ligand expression in gastrointestinal cells to cancer malignancy. This promising research field seems to require additional data on native patient tissues to reach more definitive conclusions.

Analysis of glycoprotein E-selectin ligands on human and mouse marrow cells enriched for hematopoietic stem/progenitor cells

Although well recognized that expression of E-selectin on marrow microvessels mediates osteotropism of hematopoietic stem/progenitor cells (HSPCs), our knowledge regarding the cognate E-selectin ligand(s) on HSPCs is incomplete. Flow cytometry using E-selectin-Ig chimera (E-Ig) shows that human marrow cells enriched for HSPCs (CD34(+) cells) display greater E-selectin binding than those obtained from mouse (lin(-)/Sca-1(+)/c-kit(+) [LSK] cells). To define the relevant glycoprotein E-selectin ligands, lysates from human CD34(+) and KG1a cells and from mouse LSK cells were immunoprecipitated using E-Ig and resolved by Western blot using E-Ig. In both human and mouse cells, E-selectin ligand reactivity was observed at ~ 120- to 130-kDa region, which contained two E-selectin ligands, the P-selectin glycoprotein ligand-1 glycoform "CLA," and CD43. Human, but not mouse, cells displayed a prominent ~ 100-kDa band, exclusively comprising the CD44 glycoform "HCELL." E-Ig reactivity was most prominent on CLA in mouse cells and on HCELL in human cells. To further assess HCELL's contribution to E-selectin adherence, complementary studies were performed to silence (via CD44 siRNA) or enforce its expression (via exoglycosylation). Under physiologic shear conditions, CD44/HCELL-silenced human cells showed striking decreases (> 50%) in E-selectin binding. Conversely, enforced HCELL expression of LSK cells profoundly increased E-selectin adherence, yielding > 3-fold more marrow homing in vivo. These data define the key glycoprotein E-selectin ligands of human and mouse HSPCs, unveiling critical species-intrinsic differences in both the identity and activity of these structures.

HCELL is the major E- and L-selectin ligand expressed on LS174T colon carcinoma cells

Engagement of vascular E-selectin and leukocyte L-selectin with relevant counter-receptors expressed on tumor cells contributes to the hematogenous spread of colon carcinoma. We recently demonstrated that the LS174T colon carcinoma cell line expresses the CD44 glycoform known as hematopoietic cell E-/L-selectin ligand (HCELL), which functions as a high affinity E- and L-selectin ligand on these cells. To define the contribution of HCELL to selectin-mediated adhesion on intact tumor cells, we measured the binding of LS174T cells transduced with CD44 short interfering RNA (siRNA) or with vector alone to 6-h interleukin-1beta-stimulated human umbilical vein endothelial cells (HUVEC) and to human peripheral blood mononuclear cells (PBMC) under physiological flow conditions. LS174T cell attachment to HUVEC was entirely E-selectin-dependent, and PBMC tethering to tumor cell monolayers was completely L-selectin-dependent. At physiological shear stress, CD44 siRNA transduction led to an approximately 50% decrease in the number of LS174T cells binding to stimulated HUVEC relative to vector alone-transduced cells. CD44 siRNA-transduced cells also rolled significantly faster than vector-transduced cells on HUVEC, indicating prominent HCELL participation in stabilizing tumor cell-endothelial adhesive interactions against fluid shear. Furthermore, HCELL was identified as the principal L-selectin ligand on LS174T cells, as PBMC binding to CD44 siRNA-transduced tumor cells was reduced approximately 80% relative to vector-transduced cells. These data indicate that expression of HCELL confers robust and predominant tumor cell binding to E- and L-selectin, highlighting a central role for HCELL in promoting shear-resistant adhesive interactions essential for hematogenous cancer dissemination.

Sialyl Lewis antigens: association with MUC5AC protein and correlation with post-operative recurrence of non-small cell lung cancer

Sialyl Lewis antigens (sLe(x/a)) are cancer-associated carbohydrate determinants, serve as ligands of the selectin family and are associated with hematogenous metastasis of cancer. So far, the clinicopathologic values of sialyl Lewis x (sLe(x)) and sialyl Lewis a (sLe(a)) in lung cancer have remained controversial. Using immunohistochemistry, the expressions of sLe(x) and sLe(a) antigens, and an airway mucin (MUC5AC) protein, which was supposed to be the major carrying protein of sialyl Lewis moieties, were studied in surgically resected tumor tissues of 61 patients with stages I or II NSCLC. Thirty-two (52.5%) of the 61 studied subjects were found to be positive for expression of sLe(a), 40 (65.6%) were positive for expression of sLe(x), and 16 (26.2%) were positive for MUC5AC protein. Both the expression of sLe(x) and MUC5AC were associated with adenocarcinoma subtype. Patients bearing tumors with MUC5AC and/or sLe(x) expression had a higher probability of post-operative distant metastasis. Survival analysis demonstrated that patients bearing tumors with expression of sLe(x) antigen or MUC5AC had shorter overall survival. The multivariate logistic regression showed that age >65 years old (OR = 0.207, 95% CI = 0.075-0.569, P = 0.002), nodal status (OR = 6.575, 95% CI = 2.459-17.583, P < 0.001), and MUC5AC (OR = 5.545, 95% CI = 1.998-15.386, P = 0.001) were independent factors affecting survival. We concluded that the expression of sLe(x) was related to MUC5AC protein, while patients with tumors co-expressing both MUC5AC and sLe(x) antigen had the worst survival.

Colon carcinoma cell glycolipids, integrins, and other glycoproteins mediate adhesion to HUVECs under flow

This study was undertaken to investigate the molecular constituents mediating LS174T colon adenocarcinoma cell adhesion to 4-h TNF-alpha-stimulated human umbilical vein endothelial cells (HUVECs) under flow. At 1 dyn/cm(2), approximately 57% of cells rolled and then became firmly adherent, whereas others continuously rolled on endothelium. Initial cell binding was primarily mediated by endothelial E-selectin. By using neuraminidase, glycolipid biosynthesis inhibitor d,l-threo-1-phenyl-2-hexadecanoylamino-3-pyrrolidino-1-propanol. HCl, trypsin, and flow cytometry, LS174T cells were shown to express sialyl Lewis(x) (sLe(x))- and di-sLe(x)-decorated, but not sLe(a)-decorated, glycolipid and glycoprotein ligands for E-selectin. The cells preferentially employed sialylated glycoproteins over glycolipids in adhesion as measured by conversion of rolling to firm adhesion, resistance to detachment by increased shear stress, and rolling velocity. However, a nonsialylated E-selectin counterreceptor also exists. Furthermore, LS174T alpha(2), alpha(6), and beta(1) integrins support a minor pathway in adhesion to HUVECs. Finally, tumor cell attachment specifically increases HUVEC endocytosis of E-selectin. Altogether, the data indicate the complexity of carcinoma cell-endothelium adhesion via sialylated glycoconjugates, integrins, and their respective counterreceptors.