To investigate the presence, number, and level of expression of tumor necrosis factor receptors (TNF-R) in normal and osteoarthritic (OA) human synovial fibroblasts; to examine which receptor isotype mediates the biological response of these cells to TNF-alpha; and to study homologous regulatory mechanisms of TNF-R by TNF-alpha.
We used radioligand binding assay with [125I]TNF-alpha and flow cytometric analysis with specific antireceptor antibodies to characterize receptor populations, densities, and ligand induced internalization of TNF-R. Inducible cyclooxygenase (COX-2) synthesis, prostaglandin E2 (PGE2) release, and TNF-R shedding (soluble receptors, TNF-sR) were measured after incubation with TNF-alpha the presence or absence of receptor specific blocking antibodies.
Although radioligand binding assays showed no difference in the density or affinity of TNF-R in OA synovial fibroblasts compared with normal cells, flow cytometric analysis revealed that OA cells express a significantly higher level of TNF-R55 (p < 0.04) than normal cells. The TNF-R55 was found to be the major receptor species responsible for ligand binding activity, such as COX-2 induction and PGE2 synthesis, since a specific antireceptor TNF-R55 blocking antibody inhibited about 76% of TNF-alpha binding and TNF-alpha stimulated COX-2 induction and PGE2 production. Further experiments revealed that TNF-R55 was the only receptor type internalized after binding TFN-alpha, whereas TNF-R75 was concomitantly shed. Moreover, reducing the shedding of TNF-sR, particularly the TNF-sR75, with a synthetic inhibitor decreased TNF-alpha induced PGE2 production.
TNF-R55 is the major receptor isoform transducing PGE2 and COX-2 responses to TNF-alpha in OA synovial fibroblasts; soluble receptors could be involved in facilitating the binding of TNF-alpha to its receptor.