MAPK8 and rheumatoid arthritis: In vitro studies have demonstrated that myostatin promotes interleukin-1β expression by synovial fibroblasts through extracellular signal-regulated kinase-1 (ERK), c-Jun N-terminal kinase (JNK), and AP-1 signalling pathways that inhibit miR-21-5p [37]; likewise, myostatin induces tumour necrosis factor-α expression in synovial fibroblasts of RA through the phosphatidylinositol 3-kinase (PI3K)–Akt–AP-1 signalling pathway [38].