MMP1 and rheumatoid arthritis: miR-17-92 cluster (including miR-18a, 19a, 20a, and 92a) induced by TNF-α stimulation could activate primary rheumatoid arthritis synovial fibroblasts (RASFs) by increasing matrix metalloproteinase 1 (MMP-1), IL-6, IL-8, and monocyte chemoattractant protein 1 (MCP-1) and promoting chronic joint inflammation through positive feedback in NF-κB signaling [1].