Extracellular signal-regulated kinase 1/extracellular signal-regulated kinase 2 mitogen-activated protein kinase signaling and activation of activator protein 1 and nuclear factor kappa B transcription factors play central roles in interleukin-8 expression stimulated by monosodium urate monohydrateand calcium pyrophosphate crystals in monocytic cells

Objective. Monosodium urate monohydrate (MSU) and calcium pyrophosphate dih ydrate (CPPD) crystals cause acute gout and pseudogout, respectively. Becau se acute gout and pseudogout appear to be dependent on interleukin-8 (IL-8) -induced neutrophil ingress, this study was undertaken to define and compar e how MSU and CPPD crystals stimulate IL-S messenger RNA (mRNA) expression in mononuclear phagocytes. Methods. MSU and CPPD crystal-induced mitogen-activated protein kinase (MAP K) signal transduction and IL-8 transcriptional activation were studied in human monocytic cells, using the THP-1 cell line. Results. MSU and CPPD crystals (0.5 mg/ml) induced activation of c-Jun N-te rminal kinase, extracellular signal-regulated kinase 1 (ERK-1)/ERK3, and p3 8 MAPK pathways in THP-1 cells. Activation of the ERK-1/ERK-2 pathway was e ssential for MSU and CPPD crystal-induced IL-8 mRNA expression, whereas the p38 pathway played a greater role in IL-8 mRNA expression in response to C PPD crystals in comparison with MSU crystals. Both crystals induced the bin ding of nuclear factor KB (NF-KB), including the NF-KB complex c-Rel/RelA, and activator protein 1 (AP-1, including N-terminal phosphorylated c-Jun) t o the IL-8 promoter. Both crystals induced transcriptional activation of th e IL-8 promoter, which was dependent on activation of c-Rel/RelA and AP-1, Activation of the NF-IL-6 transcription factor played a lesser role. Finall y, crystal-induced IL-8 promoter activation was mediated by activation of t he ERK-1/ERK-2 pathway, as demonstrated by transfection of dominant-negativ e raf-l, Conclusion. These results indicate that ERK-1/ERK-2 signaling and transcrip tional activation through AP-1 and NF-kappa B are essential for the inducti on of IL-8 expression in mononuclear phagocytes in response to CPPD and MSU crystals.