Single or repeated treatments with hydroxynonenal regulate differently prostaglandin E
release and cyclooxygenase-2 expression. Human osteoarthritis chondrocytes were treated for different periods of incubation: (a), (b) with a single addition of 10 μM 4-hydroxynonenal (HNE), or (c), (d) with repeated treatments by adding 10 μM HNE to the cultures at 2-hour intervals up to 14 hours. (a), (c) Prostaglandin E2 (PGE2) concentrations were measured by enzyme immunoassay commercial kit. (b), (d) Cyclooxygenase-2 (COX-2) protein expression was analyzed by western blotting. Data are means ± standard errors of the mean, n = 4, expressed as percentages of untreated cells. Student's unpaired t test: *P < 0.05, **P < 0.01, ***P < 0.001.