MAPK8 and rheumatoid arthritis: The effect of OPN on inflammatory cell migration is mediated through MCP-1 and macrophage inflammatory protein-1β (MIP-1β) via the NF-κB and MAPK signalling pathways, which are involved in the activation of inhibitor of NF-κB kinase-β (IKKβ), p38, and c-Jun N-terminal kinase (JNK) in RA CD14+ monocytes, and the OPN effect can be blocked with an anti-OPN Ab [34].