Synovial fluid fibronectin fragments: no evidence for a mitogenic effect on fibroblasts

Fragments of bovine plasma fibronectin produced by cathepsin D digestion are reportedly mitogenic for hamster fibroblasts. Rheumatoid arthritis synovial fluid contains many fibronectin fragments, which may contribute to the proliferation of synovial cells. We have therefore investigated the potential of fibronectin fragments to stimulate proliferation of synovial fibroblast-like cells using human material. Affinity-purified human plasma and synovial fluid fibronectin was digested with cathepsin D at pH 3.5 for 0-18 h and proteolysis stopped with pepstatin. A variety of fragments were produced ranging from 50 to 200 kDa when analysed by SDS-PAGE. The proliferative activity of various test preparations was studied using quiescent human skin and synovial fibroblasts. Tests were applied for 24 h to 10(4) cells and DNA synthesis measured by tritiated thymidine incorporation. Both undigested and peptides of fibronectin consistently failed to stimulate DNA synthesis in fibroblasts at all concentrations tested, compared with a phosphate-buffered saline control. This was in marked contrast to human synovial fluid from either rheumatoid arthritis or osteoarthritis patients, which stimulated DNA synthesis in the same system (P less than 0.01). Therefore, our data do not confirm the findings of previous studies in which animal materials were used. We can find no evidence that fibronectin fragments play a role in stimulating synovial proliferation in inflammatory arthritis.