Transcriptional Regulation of Expression of Carbohydrate Ligands for Cell Adhesion Molecules in the Selectin Family

Structure and Role of O-Linked Glycans in Viral Envelope Proteins

N- and O-glycans are both important constituents of viral envelope glycoproteins. O-linked glycosylation can be initiated by any of 20 different human polypeptide O-acetylgalactosaminyl transferases, resulting in an important functional O-glycan heterogeneity. O-glycans are organized as solitary glycans or in clusters of multiple glycans forming mucin-like domains. They are functional both in the viral life cycle and in viral colonization of their host. Negatively charged O-glycans are crucial for the interactions between glycosaminoglycan-binding viruses and their host. A novel mechanism, based on controlled electrostatic repulsion, explains how such viruses solve the conflict between optimized viral attachment to target cells and efficient egress of progeny virus. Conserved solitary O-glycans appear important for viral uptake in target cells by contributing to viral envelope fusion. Dual roles of viral O-glycans in the host B cell immune response, either epitope blocking or epitope promoting, may be exploitable for vaccine development. Finally, specific virus-induced O-glycans may be involved in viremic spread.

NFκB-mediated activation of the cellular FUT3, 5 and 6 gene cluster by herpes simplex virus type 1

Herpes simplex virus type 1 has the ability to induce expression of a human gene cluster located on chromosome 19 upon infection. This gene cluster contains three fucosyltransferases (encoded by FUT3, FUT5 and FUT6) with the ability to add a fucose to an N-acetylglucosamine residue. Little is known regarding the transcriptional activation of these three genes in human cells. Intriguingly, herpes simplex virus type 1 activates all three genes simultaneously during infection, a situation not observed in uninfected tissue, pointing towards a virus specific mechanism for transcriptional activation. The aim of this study was to define the underlying mechanism for the herpes simplex virus type 1 activation of FUT3, FUT5 and FUT6 transcription. The transcriptional activation of the FUT-gene cluster on chromosome 19 in fibroblasts was specific, not involving adjacent genes. Moreover, inhibition of NFκB signaling through panepoxydone treatment significantly decreased the induction of FUT3, FUT5 and FUT6 transcriptional activation, as did siRNA targeting of p65, in herpes simplex virus type 1 infected fibroblasts. NFκB and p65 signaling appears to play an important role in the regulation of FUT3, FUT5 and FUT6 transcriptional activation by herpes simplex virus type 1 although additional, unidentified, viral factors might account for part of the mechanism as direct interferon mediated stimulation of NFκB was not sufficient to induce the fucosyltransferase encoding gene cluster in uninfected cells.

Human multipotent adult progenitor cells transcriptionally regulate fucosyltransferase VII

BACKGROUND AIMS

Targeted recruitment of leukocytes to sites of inflammation is a crucial event in normal host defense against pathogens, and attachment to and rolling on activated endothelial cells is a prerequisite first step for eventual leukocyte extravasation into sites of inflammation. These key events are mediated by interactions between glycosylated ligands expressed on leukocytes and selectins expressed on activated endothelium. Cell surface expression of selectin ligands on leukocytes is regulated by the rate-limiting enzyme fucosyltransferase VII (Fut7), and in its absence extravasation of leukocytes is severely inhibited. Multipotent adult progenitor cells (MAPCs) are an adherent cell population isolated from adult bone marrow. Intravenous administration of MAPCs provided functional improvement in multiple pre-clinical models of injury or disease, but the mechanisms by which these outcomes were achieved remain poorly understood.

METHODS

In vitro cell analysis studies including fluorescence-activated cell sorting, messenger RNA analysis, T-cell proliferation assays and endothelial cell binding assays were performed.

RESULTS

The in vitro cell analysis studies characterized the ability of MAPCs to secrete factors that transcriptionally attenuate expression of Fut7 in T cells, blocking the terminal fucosylation event in the biosynthesis of selectin ligands and reducing T-cell binding to endothelial cells.

CONCLUSIONS

This study presents the first example of a distinct regulatory mechanism involving transcriptional down-regulation of Fut7 by MAPCs that could modulate the trafficking behavior of T cells in vivo.

Involvement of viral glycoprotein gC-1 in expression of the selectin ligand sialyl-Lewis X induced after infection with herpes simplex virus type 1

Several herpesviruses induce expression of the selectin receptor sialyl-Lewis X (sLe(x) ) by activating transcription of one or more of silent host FUT genes, each one encoding a fucosyltransferase that catalyses the rate-limiting step of sLe(x) synthesis. The aim here was to identify the identity of the glycoconjugate associated with sLe(x) glycoepitope in herpes simplex virus type 1 (HSV-1) infected human diploid fibroblasts, using immunofluorescence confocal microscopy. Cells infected with all tested HSV-1 strains analysed demonstrated bright sLe(x) fluorescence, except for two mutant viruses that were unable to induce proper expression of viral glycoprotein gC-1: One gC-1 null mutant and another mutant expressing gC-1 devoid of its major O-glycan-containing region (aa 33-116). The sLe(x) reactivity of HSV-1 infected cells was abolished by mild alkali treatment. Altogether the results indicated that the detectable sLe(x) was associated with O-linked glycans, situated in the mucin region of gC-1. No evidence for sLe(x) (i) in other HSV-1 glycoproteins with mucin domains such as gI-1 or (ii) in host cell glycoproteins/glycolipids was found. Thus, the mucin domain of HSV-1 gC-1 may support expression of selectin ligands such as sLe(x) and other larger O-linked glycans in cell types lacking endogenous mucin domain-containing glycoproteins, optimized for O-glycan expression, provided that the adequate host glycosyltransferase genes are activated.

Induction of sialyl-Lex expression by herpes simplex virus type 1 is dependent on viral immediate early RNA-activated transcription of host fucosyltransferase genes

We have previously shown that varicella-zoster virus (VZV) and cytomegalovirus (CMV) infection of diploid human fibroblasts (HEL) results in neo-expression of Lewis antigens sialyl Lewis x (sLe(x)) and Lewis y (Le(y)), respectively, after transcriptional activation of different combinations of dormant human fucosyltransferase genes (FUT1, FUT3, FUT5, and FUT6), whose gene products are responsible for the synthesis of Le antigens. Here, we show that herpes simplex virus type 1 (HSV-1) also induces sLe(x) expression dependent on induction of FUT3, FUT5, and FUT6 transcription in infected cells. HSV-1 induction of FUT5 was subsequently used as a model system for analyzing the mechanism of viral activation of dormant fucosyltransferase genes. We show that this is a rapid process, which gives rise to elevated FUT5 RNA levels already at 90 min postinfection. Augmented FUT5 transcription was found to be dependent on transcription of viral genes, but not dependent on the immediate early proteins ICP0 and ICP4, as demonstrated by experiments with HSV-1 mutants defective in expression of these genes. Augmented FUT5 transcription takes place in cycloheximide-treated HSV-1-infected cells, suggesting a more direct role for IE viral RNA during activation of cellular FUT5.

Activation of host antiviral RNA-sensing factors necessary for herpes simplex virus type 1-activated transcription of host cell fucosyltransferase genes FUT3, FUT5, and FUT6 and subsequent expression of sLe(x) in virus-infected cells

Herpes simplex virus type 1 (HSV-1) induces expression of a selectin receptor, the carbohydrate epitope sialyl Lewis X (sLe(x)), at the surface of infected cells. The molecular background to this phenomenon is that a viral immediate early RNA interacts with as yet unidentified host factors, eventually resulting in transcription of three dormant host fucosyltransferase genes (FUT3, FUT5, and FUT6), whose gene products are rate-limiting for synthesis of sLe(x). The aim of the present study was to define the immediate targets for the viral RNA in this process. We found that the Protein Kinase R (PKR) inhibitors 2-aminopurine (2-AP) and C16 inhibited FUT3, FUT5, and FUT6 expression as well as HSV-1-induced expression of sLe(x), indicating a primary role of PKR as a viral RNA target. The PKR-dependent activation of the FUT genes seemed neither to involve PKR effects on translation nor to involve NF-kappaB- or JNK-dependent activation. IMD-0354, known as an inhibitor of the NF-kappaB-activating factor IKK-2, induced FUT transcription via a novel IKK-2-independent mechanism, irrespective of whether the cells were virus-infected or not. Altogether, the results suggested that PKR is the primary target for HSV-1 early RNA during induction of FUT3, FUT5, and FUT6, and that the subsequent steps in the transcriptional activation of these host genes involve a hitherto unknown IMD-0354, yet IKK-2-independent, pathway.

CD15 expression in human myeloid cell differentiation is regulated by sialidase activity

The glycan determinant CD15 (also known as Lewis x, or Le(x)) is a distinguishing marker for human myeloid cells and mediates neutrophil adhesion to dendritic cells. Despite broad interest in this structure, the mechanisms underlying CD15 expression remain relatively uncharacterized. Accordingly, we investigated the molecular basis of increasing CD15 expression associated with human myeloid cell differentiation. Flow cytometric analysis of differentiating cells together with biochemical studies using inhibitors of glycan synthesis and of sialidases showed that increased CD15 expression is not due to de novo biosynthesis of CD15, but results predominantly from induction of alpha(2-3)-sialidase activity, which yields CD15 from cell-surface sialyl-CD15 (also known as sialyl-Lewis x, sLe(x) or CD15s). This differentiation-associated conversion of surface CD15s to CD15 occurs mainly on glycoproteins. Until now, modulation of post-translational glycan modifications has been attributed solely to dynamic variations in glycosyltransferase expression. Our results unveil a new paradigm by demonstrating a critical role for post-Golgi membrane glycosidase activity in the 'biosynthesis' of a key glycan determinant.

Tumor-microenvironment interactions: dangerous liaisons

The interaction between microenvironmental components and tumor cells is bidirectional. Tumor cells and their products are capable of regulating and altering gene expression in nontumor cells residing in or infiltrating into the microenvironment and exert selective pressures on such cells, thereby shaping their phenotype. Conversely, microenvironmental components regulate gene expression in tumor cells thereby directing the tumor into one or several possible molecular evolution pathways, some of which may lead to metastasis. This review summarizes six instances in which the tumor liaises with different components of its microenvironment. These liaisons result, in most cases, in enhanced tumor progression. In these cases (responses of tumor and nontumor cells to microenvironmental stress, the interaction of the tumor with fibroblasts, endothelial cells and macrophages, the formation of the metastatic niche, and the interaction of the tumor with immunoglobulins) the tumor, directly or indirectly, alters the phenotype of its interaction partners thereby enlisting them to promote its progression. Does the tumor need all these pathways to form metastasis? Is there a hierarchy of interactions with respect to impact on tumor progression? These questions remain open. They may be answered by approaches employed in the analysis of hypercomplex systems.

Analysis of glycosyltransferase expression in metastatic prostate cancer cells capable of rolling activity on microvascular endothelial (E)-selectin

Prostate cancer (PCa) cell tethering and rolling on microvascular endothelium has been proposed to promote the extravasation of PCa cells. We have shown that these adhesive events are mediated through binding interactions between endothelial (E)-selectin and Lewis carbohydrates on PCa cells. Prior data indicate that E-selectin-mediated rolling of bone-metastatic PCa MDA PCa 2b (MDA) cells is dependent on sialyl Lewis X (sLe(X))-bearing glycoproteins. To explore the molecular basis of sLe(X) synthesis and E-selectin ligand (ESL) activity on PCa cells, we compared and contrasted the expression level of glycosyltransferases, characteristically involved in sLe(X) and ESL synthesis, in ESL(+) MDA cells among other ESL(-) metastatic PCa cell lines. We also created and examined ESL(hi) and ESL(lo) variants of MDA cells to provide a direct comparison of the glycosyltransferase expression level. We found that normal prostate tissue and all metastatic PCa cell lines expressed glycosyltransferases required for sialo-lactosamine synthesis, including N-acetylglucosaminyl-, galactosyl-, and sialyltransferases. However, compared with expression in normal prostate tissue, ESL(+) MDA cells expressed a 31- and 10-fold higher level of alpha1,3 fucosyltransferases (FT) 3 and 6, respectively. Moreover, FT3 and FT6 were expressed at 2- to 354-fold lower levels in ESL(-) PCa cell lines. Consistent with these findings, ESL(hi) MDA cells expressed a 131- and 51-fold higher level of FT3 and FT6, respectively, compared with expression in ESL(lo) MDA cells. We also noted that alpha1,3 FT7 was expressed at a 5-fold greater level in ESL(hi) MDA cells. Furthermore, ESL(lo) MDA cells did not display sLe(X) on glycoproteins capable of bearing sLe(X), notably P-selectin glycoprotein ligand-1. These results implicate the importance of alpha1,3 FT3, FT6, and/or FT7 in sLe(X) and ESL synthesis on metastatic PCa cells.

Interaction of GATA-3/T-bet transcription factors regulates expression of sialyl Lewis X homing receptors on Th1/Th2 lymphocytes

Selectin-dependent cell adhesion mediates inflammatory extravasation and routine homing of lymphocytes. Most resting peripheral T lymphocytes lack expression of sialyl Lewis X, the carbohydrate ligand for selectins, and are induced to strongly express it upon activation. T helper 1 (Th1) cells are known to more preferentially express sialyl Lewis X as compared with T helper 2 (Th2) cells upon activation. The molecular basis for this preferential expression, however, has not been elucidated to date. Here we show that the gene for fucosyltransferase VII (FUT7), the rate-limiting enzyme for sialyl Lewis X synthesis, is a unique example of the human genes with binding sites for both GATA-3 and T-bet, two opposing factors for Th1 and Th2 development, and is regulated transcriptionally by a balance of the two interacting transcription factors. T-bet promotes and GATA-3 represses FUT7 transcription. Our results indicated that T-bet interferes with the binding of GATA-3 to its target DNA, and also that GATA-3 significantly interferes with the binding of T-bet to the FUT7 promoter. T-bet has a binding ability to GATA-3, CBP/P300, and Sp1 to form a transcription factor complex, and GATA-3 regulates FUT7 transcription by phosphorylation-dependently recruiting histone deacetylase (HDAC)-3/HDAC-5 and by competing with CBP/P300 in binding to the N terminus of T-bet. Suppression of GATA-3 activity by dominant-negative GATA-3 or repressor of GATA (ROG) was necessary to attain a maximum expression of FUT7 and sialyl Lewis X in human T lymphoid cells. These results indicate that the GATA-3/T-bet transcription factor complex regulates the cell-lineage-specific expression of the lymphocyte homing receptors.

Cell-Cell Interaction-dependent Regulation of N-Acetylglucosaminyltransferase III and the Bisected N-Glycans in GE11 Epithelial Cells

Changes in oligosaccharide structures are associated with numerous physiological and pathological events. In this study, the effects of cell-cell interactions on N-linked oligosaccharides (N-glycans) were investigated in GE11 epithelial cells. N-glycans were purified from whole cell lysates by hydrazinolysis and then detected by high performance liquid chromatography and mass spectrometry. Interestingly, the population of the bisecting GlcNAc-containing N-glycans, the formation of which is catalyzed by N-acetylglucosaminyltransferase III (GnT-III), was substantially increased in cells cultured under dense conditions compared with those cultured under sparse conditions. The expression levels and activities of GnT-III but not other glycosyltransferases, such as GnT-V and alpha1,6-fucosyltransferase, were also consistently increased in these cells. However, this was not observed in mouse embryonic fibroblasts or MDA-MB231 cells, in which E-cadherin is deficient. In contrast, perturbation of E-cadherin-mediated adhesion by treatment with EDTA or a neutralizing anti-E-cadherin antibody abolished the up-regulation of expression of GnT-III. Furthermore, we observed the significant increase in GnT-III activity under dense growth conditions after restoration of the expression of E-cadherin in MDA-MB231 cells. Our data together indicate that a E-cadherin-dependent pathway plays a critical role in regulation of GnT-III expression. Given the importance of GnT-III and the dynamic regulation of cell-cell interaction during tissue development and homeostasis, the changes in GnT-III expression presumably contribute to intracellular signaling transduction during such processes.

Molecular mechanism for cancer-associated induction of sialyl Lewis X and sialyl Lewis A expression-The Warburg effect revisited

Cell adhesion mediated by selectins and their carbohydrate ligands, sialyl Lewis X and sialyl Lewis A, figures heavily in cancer metastasis. Expression of these carbohydrate determinants is markedly enhanced in cancer cells, but the molecular mechanism that leads to cancer-associated expression of sialyl Lewis X/A has not been well understood. Results of recent studies indicated involvement of two principal mechanisms in the accelerated expression of sialyl Lewis X/A in cancers; 'incomplete synthesis' and ' neo synthesis.' As to 'incomplete synthesis,' we have recently found further modified forms of sialyl Lewis X and sialyl Lewis A in non-malignant colonic epithelium, which have additional 6-sulfation or 2 --> 6 sialylation. The impairment of GlcNAc 6-sulfation and 2 --> 6 sialylation upon malignant transformation leads to accumulation of sialyl Lewis X/A in colon cancer cells. Epigenetic changes such as DNA methylation and/or histone deacetylation are suggested to lie behind such incomplete synthesis. As to the mechanism called ' neo synthesis,' recent studies have indicated that cancer-associated alterations in the sugar transportation and intermediate carbohydrate metabolism play important roles. Cancer cells are known to exhibit a metabolic shift from oxidative to elevated anaerobic glycolysis (Warburg effect), which is correlated with the increased gene expression of sugar transporters and glycolytic enzymes induced by common cancer-specific genetic alterations. The increased sialyl Lewis X/A expression in cancer is a link in the chains of these events because our recent results indicated that these events accompany transcriptional induction of a set of genes closely related to its expression.