Alteration by transforming growth factor-beta 1 of asparagine-linked sugar chains in glucose transporter protein in Swiss 3T3 cells

The action of TNFα and TGFβ include specific alterations of the glycosylation of bovine and human chondrocytes

Joint destruction in arthritis is often associated with high levels of inflammatory cytokines. Previous work has shown that inflammatory conditions can alter the activities of glycosyltransferases that synthesize the glycan chains of glycoproteins, and that these changes in turn can influence the functions of glycoproteins. We therefore examined glycosyltransferases involved in glycoprotein biosynthesis in primary cultures of bovine articular chondrocytes and human chondrocytes isolated from knee cartilage of osteoarthritis patients. Bovine chondrocytes exhibited enzyme activities involved in the synthesis of bi-antennary complex Asn-linked N-glycans, as well as the enzymes involved in the synthesis of GalNAc-Ser/Thr-linked O-glycans with the core 1 structure. Human chondrocytes, in addition, were able to synthesize more complex O-glycans with core 2 structures. TNFalpha was found to induce apoptosis in chondrocytes, and this process was associated with significant changes in lectin binding to chondrocyte cell surface glycans. TGFbeta increased cell proliferation, and had significant effects on cell surface glycosylation in bovine but not in human cells. These cytokine-specific effects were partially correlated with changes in glycosyltransferase activities. Thus, chondrocytes have many of the enzymes necessary for the synthesis of N- and O-glycan chains of glycoproteins. The O-glycosylation pathways and the effects of TNFalpha and TGFbeta on glycosylation differed between bovine and human chondrocytes. These alterations are of potential importance for the regulation of the functions of cell surface receptors on chondrocytes, and for an understanding of the pathophysiology of arthritis.

Glycoproteome profiling of transforming growth factor-β (TGFβ) signaling: Nonglycosylated cell death-inducing DFF-like effector A inhibits TGFβ1-dependent apoptosis

Transforming growth factor-beta (TGFbeta) is a potent regulator of cell growth, differentiation, and apoptosis. TGFbeta binds to specific serine/threonine kinase receptors, which leads to activation of Smad-dependent and Smad-independent signaling pathways. O-Glycosylation is a dynamic PTM which has been observed in many regulatory proteins, but has not been studied in the context of TGFbeta signaling. To explore the effect of TGFbeta1 on protein O-glycosylation in human breast epithelial cells, we performed analyses of proteins which were affinity purified with Helix pomatia agglutinin (HPA). HPA lectin allowed enrichment of proteins containing GalNAc and GlcNAc linked to serine and threonine residues. Using 2-DE and MALDI-TOF-MS, we identified 21 HPA-precipitated proteins, which were affected by treatment of cells with TGFbeta1. Among these proteins, regulators of cell survival, apoptosis, trafficking, and RNA processing were identified. We found that TGFbeta1 inhibited the appearance of cell death-inducing DFF-like effector A (CIDE-A) in 2-D gels with HPA-precipitated proteins. CIDE-A is a cell death activator which promotes DNA fragmentation. We observed that TGFbeta1 did not affect expression of CIDE-A, but inhibited its glycosylation. We found that deglycosylation of CIDE-A correlated with enhanced nuclear export of the protein, and that high level of nonglycosylated CIDE-A inhibited TGFbeta1-dependent cell death. Thus, inhibition of the glycosylation of CIDE-A may be a mechanism to protect cells from apoptosis.

The effect of TNF-alpha on glycosylation pathways in bovine synoviocytes

Synoviocytes are fibroblastic cells that line joint cavities. These cells synthesize numerous cell-surface and extracellular-matrix glycoproteins that are required for maintenance of the joint. Joint inflammation, such as occurs in arthritis, has been shown to have major effects on synoviocyte proliferation and on the biosynthesis of glycoproteins. The structures of the carbohydrate moieties of glycoproteins, however, and the enzymes involved in their synthesis have not yet been described for synoviocytes. Therefore, to characterize the cell-surface glycoconjugates, synoviocytes were isolated from bovine ankles, and the cells were grown in primary cultures. Lectin-binding assays were used to identify exposed N- and O-glycan carbohydrate determinants on synoviocytes, and specific enzyme assays were used to identify some of the glycosyltransferases involved in the synthesis of the glycan chains. A number of the enzymes that synthesize N- and O-linked oligosaccharides were found to be active in cell-free extracts of synoviocytes, including those that synthesize core-1-based O-glycans and the more complex bi-antennary N-glycans. To understand the molecular events underlying the inflammatory response in the synovium of arthritis patients, we examined the effect of the inflammatory cytokine tumour necrosis factor alpha (TNF-alpha) on synoviocytes and on glycosylation profiles. TNF-alpha treatment, which induces apoptosis in synoviocytes, was accompanied by changes in lectin-binding patterns, indicating alterations in the expression of cell-surface oligosaccharides. Concurrently, changes in specific enzyme activities were observed in treated cells. Two enzymes potentially important to the inflammatory process, core 2 beta6-GlcNAc-transferase and beta4-Gal-transferase, increased after TNF-alpha treatment. This is the first study of glycoprotein biosynthesis in synoviocytes, and it shows that synoviocytes have a characteristic glycosylation phenotype that is altered in the presence of inflammatory cytokines.

GLUT1 and TGF-beta: the link between hyperglycaemia and diabetic nephropathy

Recent experimental work implicates transforming growth factor-beta (TGF-beta) as an aetiologic mediator of diabetic nephropathy and the ubiquitous glucose transporter GLUT1 as an important permissive factor for the tissue injury caused by hyperglycaemia. High ambient glucose increases GLUT1 expression and glucose transport activity when compared with physiologic glucose concentrations. Treatment of rat mesangial cells with TGF-beta up-regulates GLUT1 mRNA and protein levels and significantly increases glucose uptake. Addition of neutralizing anti-TGF-beta antibody prevents the stimulatory effects of high glucose on GLUT1 expression. Cultured rat mesangial cells transduced with the human GLUT1 gene and thus overexpressing the GLUT1 protein show marked increase in glucose uptake and the synthesis of extracellular matrix molecules, even when grown in normal ambient glucose concentrations. Thus, TGF-beta and GLUT1, two proteins that are up-regulated in glomerular mesangial cells in a hyperglycaemic milieu, can influence the expression of one another. It is therefore fair to conclude that, with successful interruption of the TGF-beta-GLUT1 axis, the beneficial effects of strict glucose control on the development of diabetic nephropathy could likely be augmented.

Enhanced expression of glucose transporter GLUT3 in tumorigenic HeLa cell hybrids associated with tumor suppressor dysfunction

Previous studies on human cell hybrids between HeLa and normal human fibroblasts have indicated that the tumorigenicy may be controlled by a putative tumor suppressor gene on chromosome 11. We previously demonstrated a twofold increase in glucose uptake with a reduced Km by tumorigenic HeLa cell hybrids which expressed a highly glycosylated GLUT1. In this study, we reported that a tumorigenic cell hybrid, CGL4, also expressed a glucose transporter isoform, GLUT3, that was undetectable in nontumorigenic CGL1 cells. The expression of GLUT3 together with GLUT1 of 70 kDa was also evident in three gamma-ray-induced tumorigenic clones isolated from CGL1 cells, while control nontumorigenic irradiated cells expressed 50 kDa GLUT1 alone. In accordance with this, GLUT3 mRNA was specifically expressed in tumorigenic cell hybrids. To examine the role of GLUT3, clones which stably overexpress GLUT3 were developed from both CGL1 and CGL4 cells. In these transfectants, the affinity for 2-deoxyglucose markedly increased, in parallel with the amount of expressed GLUT3 irrespective of its N-glycosylation state. These results suggest that the enhanced GLUT3 expression in HeLa cell hybrids associated with the tumorigenic phenotypes may account for the increased affinity for 2-deoxyglucose. Possible roles of the putative tumor suppressor in control of gene expression and glucose uptake is discussed.

Altered N-glycosylation of glucose transporter-1 associated with radiation-induced tumorigenesis of human cell hybrids

Studies on human cell hybrids between a cervical carcinoma cell line, HeLa, and normal fibroblasts have indicated that their tumorigenicity is under the control of a putative tumor suppressor on chromosome 11. We have previously demonstrated that a tumorigenic cell hybrid CGL4 expresses a larger glucose transporter, GLUT1, due to altered glycosylation when compared to the nontumorigenic counterpart CGL1. In this study, we demonstrated this glycosylation change in GLUT1 in gamma-ray-induced tumorigenic mutants (GIMs) isolated from CGL1 cells as expressing a tumor-associated surface antigen, intestinal alkaline phosphatase. In contrast, GLUT1 in the gamma-irradiated nontumorigenic control cells (CONs) did not show this alteration. In accordance with this glycosylation change, affinity to 2-deoxyglucose in these GIM clones was increased by about twofold when compared to the nontumorigenic CONs. These results suggest a close correlation between the glycosylation change in GLUT1 with increased affinity to D-glucose and tumorigenicity of these human cell hybrids.

Increased glucose transport in ras-transformed fibroblasts: a possible role for N-glycosylation of GLUT1

2-Deoxyglucose uptake was enhanced in ts371 KiMuSV-NRK cells when growing at the permissive temperature to allow the expression of a transforming p21 ras protein. This change is due to a decrease in the K(m) by approximately 2.5-fold without affecting the V(max) of the transporter. The amount of the GLUT1 glucose transporter dit not increase as deduced from immunoblot experiments on total membranes. Nevertheless, ras-transformed GLUT1 displays a higher molecular mass due to an increased N-glycosylation of the protein. Experiments made in tunicamycin-treated cells indicates that a higher glycosylation is responsible for the increase in 2-deoxyglucose uptake in ras-transformed cells.

A tumor-associated glycosylation change in the glucose transporter GLUT1 controlled by tumor suppressor function in human cell hybrids

Studies of human cell hybrids have provided evidence that the tumorigenicity of a cervical carcinoma (HeLa) is under the control of a putative tumor suppressor on chromosome 11. Using these human cell hybrids, we found a tumor-associated glycosylation change in the glucose transporter GLUT1, which is an N-linked glycoprotein at the plasma membrane. The non-tumorigenic HeLa × fibroblast cell hybrid CGL1 and the normal diploid fibroblast WI38 expressed the 50–55 kDa GLUT1, whereas in a tumorigenic segregant hybrid, CGL4, as well as in parental HeLa cells, GLUT1 glycosylation was altered and its molecular mass was about 70 kDa. However, the altered GLUT1 glycosylation was not observed in SV40-transformed WI38 cells, suggesting a correlation between this glycosylation change and a putative tumor suppressor function. Further investigations using glycosidases, glycosylation inhibitors and lectin-affinity chromatography demonstrated that the tumor-associated glycosylation change in GLUT1 was mainly due to the increase in N-acetyl-lactosamine repeats in the N-linked oligosaccharides. In accordance with the altered glycosylation, affinity for 2-deoxyglucose in the tumorigenic CGL4 cells increased 2-fold, but there was little change in the Vmax. These results suggest there may be a functional role for the modulation by glycosylation of GLUT1 in the tumorigenic behavior of CGL4 and HeLa cells.