The DING protein: an autocrine growth-stimulatory protein related to the human synovial stimulatory protein

A synovial stimulating protein (SSP) has previously been isolated from rheumatoid arthritis synovial fluid and from the culture fluid of rheumatoid arthritis synovial fibroblasts. We have previously isolated, from skin fibroblast cultures, a 40 kDa hirudin-binding protein, which had amino acid sequence homology with the SSP. We sought to clarify the relationship, if any, between the SSP and the hirudin-binding protein. We show that the hirudin-binding protein is immunologically cross-reactive with a protein identical with, or very similar to, the SSP. This hirudin-binding protein is produced by normal and rheumatoid arthritis fibroblasts in culture, and also by cervical carcinoma cells. Traces of an SSP-like protein, and of proteins intermediate in size between the SSP and the hirudin-binding protein, suggest that the hirudin-binding protein may be proteolytically derived from the SSP. An SSP-like protein of about 200 kDa is present in all synovial fluid samples, arthritic and normal, indicating that its presence is not a primary cause of rheumatoid arthritis. There is no evidence for the existence of smaller fragments of the SSP-like protein in synovial fluid. A cDNA sequence, coding for part of the 40 kDa protein, has been obtained. The derived amino acid sequence indicates that a domain, previously identified in the dishevelled gene from Drosophila melanogaster, is present in this protein. Peptides predicted from the cDNA sequence were used to raise antisera, which recognise both the 40 kDa protein and the SSP-like protein. One of the antibody preparations is a good inhibitor of fibroblast proliferation, which confirms the autocrine growth-stimulatory role originally proposed for these proteins.