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.