Expression of activin A in inflammatory arthropathies

The findings that bone marrow fibroblastoid stromal cells are important for activin A production prompted our investigation of activin A expression in fibroblast-like synoviocytes in joint capsule in this and previous studies. In the proliferative reactive synovial membrane obtained from rheumatoid arthritis patients, activin A is detected prominently in the fibroblastoid synovial cells, as well as in the smooth muscle and the endothelial layer of the arteries in these vascularized proliferative tissues. The concentration of activin A in the rheumatoid arthritis synovial fluid was 33.6±5.0 ng/ml, much higher than the activin A content of osteoarthritis fluid (10.0±1.1 ng/ml). Furthermore, our previous studies also showed that inflammatory cytokines, such as interleukin (IL)-1, transforming growth factor (TGF)-β, interferon (IFN)-γ, IL-8, and IL-10 markedly enhance the expression of activin A mRNA in synoviocytes. These findings are consistent with our studies in regard to the regulatory control of activin A production in bone marrow stroma and monocytes. In addition, the relationship of activin A to IL-6-induced biological activities in various cell types was also investigated. Although activin A has not been directly associated with inflammatory processes, future studies are needed to investigate its production in response to the accumulated levels of inflammatory cytokines in the synovium of the patients, as well as the quantitative differences in activin A concentrations in many patients with other inflammatory diseases.