Expression of Retinoic Acid Receptor (RAR) α Protein in the Synovial Membrane from Patients with Osteoarthritis and Rheumatoid Arthritis.

Retinoic acid receptors (RAR) are expressed in inflammatory cells and, through ligand binding, play an important role in cell proliferation and differentiation, as well as in regulation of cytokine and matrix metalloproteinase (MMP) production. Inflammatory cytokines and MMPs play a significant role in cartilage destruction in osteoarthritis (OA) and in joint destruction in rheumatoid arthritis (RA), the prototype of inflammatory arthritis. To determine if RARα is expressed in the synovial membrane (SM) of patients with OA and compare it with RA, SM biopsy samples were used in this study which were from 31 patients with late OA and 14 patients with late RA. Cryostat sections were studied by immunochemistry using a RARα-specific antibody. All SM samples from OA and RA patients exhibited cellular localization for RARα. Immunoreactivity was present in mononuclear inflammatory cells, endothelial cells, synovial lining cells, and fibroblasts. Inflammatory infiltrates were interstitial and nodular. Roughly one half of mononuclear cells in the inflammatory nodules in OA and RA were positive for RARα. The conclusion is that the presence of RARα in SM of patients with OA and RA suggests that RARs may play a role in the immunomodulation of synovial inflammation and therefore can be a potential target of therapeutic intervention in these arthritides.