Induction of prostaglandin E synthase (PGES) activity in chondrocytes stimulated with IL-1β (IL-1β). (a) For estimation of the PGES activity that produces prostaglandin (PG)E2 from PGH2, exogenous PGH2 (2 μg) was added to lysates of chondrocytes that had been incubated with (filled circles) or without (open circles) IL-1β (1 ng/ml) for the indicated durations. The lysates were then incubated at 24°C for 30 s in the presence of reduced glutathione, the reaction was stopped with 100 mM FeCl2, and the PGE2 concentration was measured by enzyme-linked immunosorbent assay. Spontaneous conversion of PGH2 to PGE2 in the absence of cell lysates was subtracted from each result. Data are means ± SD for triplicate cultures; representative results from three patients are shown. *P < 0.01 and †P < 0.05 compared with 0 h. (b) Chondrocytes were incubated with or without IL-1β (1 ng/ml) for 48 h. Then the PGES activity of the cell lysates in the presence or absence of MK-886 (1–100 μM) was measured as described in Materials and methods, and calculated as a percentage of the control value (IL-1β alone). Data are means ± SD for triplicate experiments; representative results from three patients are shown. #P < 0.01, untreated control compared with IL-1β alone; *P < 0.01, IL-1β alone compared with IL-1β plus MK-886.