Effect of interleukin 1β on histone H3 lysine 4 methylation at microsomal prostaglandin E synthase 1 promoter. Osteoarthritis (OA) chondrocytes were treated with 100 pg/ml interleukin 1β (IL-1β) for the indicated time periods, and chromatin immunoprecipitation (ChIP) assays were performed using antibodies specific to mono-, di- and trimethylated histone H3 lysine 4 (H3K4). The results are expressed as fold changes relative to control (that is unstimulated cells), and the data are the mean ± SD of three independent experiments using cells from four different donors. For each ChIP assay, the immunoprecipitated DNA was quantitated in triplicate on two separate occasions. *P < 0.05 compared with unstimulated cells. me1, Monomethylation; me2, Dimethylation; me3, Trimethylation.