- Extracellular signal-regulated kinase 1/extracellular signal-regulated kinase 2 mitogen-activated protein kinase signaling and activation of activator protein 1 and nuclear factor kappaB transcription factors play central roles in interleukin-8 expression stimulated by monosodium urate monohydrate and calcium pyrophosphate crystals in monocytic cells. Liu, R., O'Connell, M., Johnson, K., Pritzker, K., Mackman, N., Terkeltaub, R. Arthritis Rheum. (2000)