TNF and osteoarthritis: Previous studies have reported that MAPK activation is increased within synovial fibroblast cells derived from rheumatoid arthritis patients compared to those derived from osteoarthritis patients, and induced by pro-inflammatory cytokines including tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and interleukin-1 (IL-1) in human synovial cells (Schett et al., 2000).