MAPK8 and rheumatoid arthritis: In human synovial fibroblasts with rheumatoid arthritis, Tpl2 also induced ERK activation in the presence of IL-1β [76], whereas IL-1β stimulated the activation of ERK but also p38 MAPK signaling and JNK signaling in HeLa epithelial cell line or rat INS-1E β-cells, respectively [23,76].