MAPK8 and rheumatoid arthritis: JNK and p38 MAPKs play critical roles in regulating synovial invasion, inflammatory cytokine secretion by activated macrophages, and collagenase synthesis by RA FLSs, while ERK MAPK promotes the production of several cytokines and pannus formation (Ganesan et al., 2016; Ross et al., 2016; Yang et al., 2016).