The synovial tissue affected by rheumatoid arthritis (RA) is characterized by hyperproliferation of synovial cells. High amounts of epidermal growth factor (EGF) in the synovial fluid of RA patients contribute to the growth of rheumatoid synovial cells. To characterize the receptor for EGF in rheumatoid synovial cells, the expression and function of ErbB family members were examined.
Synovial tissues were obtained from surgical excisions. The expression of ErbB products was examined by immunohistochemistry and immunoblotting by using specific antibodies. Primary cultures were established from the surgical materials. Cell growth was measured using MTT. The levels and phosphorylation state of the ErbB-2 protein were analyzed by immunoprecipitation and immunoblotting.
The expression of ErbB-2, but not other ErbB-related products, was detected in synovium with RA as compared with that with osteoarthritis (OA) and ligament injury. Growth of primary synovial cells with RA was inhibited by genistein, a tyrosine kinase inhibitor, and herceptin, a specific monoclonal antibody against ErbB-2. Herceptin showed a small effect on growth of primary synovial cells with OA. EGF stimulated the phosphorylation of ErbB-2 in primary synovial cells with RA. This EGF-stimulated phosphorylation was completely abrogated by genistein and herceptin.
ErbB-2 is expressed in rheumatoid synovial cells and may function as the receptor for EGF. Our data suggest that mitotic signals from EGF family members are transduced by ErbB-2 in these cells. Inhibition of ErbB-2 may provide a new approach to the effective treatment for RA.