Interactions between human OA chondrocytes and HNE-modified Col II induced COX-2 and MMP-13 expression. Cells were incubated as described in Legend 1 and culture media was collected. PGE2 (a) and MMP-13 (d) release was assessed by enzyme immunoassay and ELISA kits. Levels of COX-2 protein (b) as well as COX-2 mRNA (c) and MMP-13 mRNA (d) were analyzed in cellular extracts by Western blot and real-time RT-PCR, respectively. ANOVA tests and Bonferroni's multiple comparison were performed to compare each condition. The data are means ± SEM of n = 6. *P < 0.05, **P < 0.01, ***P < 0.001.