We have studied the DNA binding profiles of activator protein-1 (AP-1) involved in synovial overgrowth and osteoporosis in rheumatoid arthritis (RA) in relation to the molecular chaperon heat shock protein 90 (HSP90). The AP-1 binding activity of the nuclear extracts of rheumatoid synovial cells was basically increased as compared with osteoarthritic synovial cells. Upon stimulation with inflammatory cytokines IL-1beta or TNFalpha, the AP-1 binding activity was further increased in rheumatoid synovial cells, and increased AP-1 protein was composed as heterodimers of Fos and JunD which was not known before as a major component of AP-1 in rheumatoid synovial cells. The increase of AP-1 binding activity as induced by inflammatory cytokines was specifically inhibited by geldanamycin, radicicol or herbimycin A, specific inhibitors of HSP90, while AP-1 protein was not decreased by geldanamycin. Further, HSP90 protein was not decreased by the inhibitors. The findings indicate that HSP90 is required for increased AP-1 binding activity of rheumatoid synovial cells under inflammatory stimuli and that AP-1 binding activity is inhibited by functionally inactivating HSP90 with the inhibitors.