IL-1β-mediated increase in MMP-13 in cultured articular chondrocytes is enhanced in absence of FrzB. Primary chondrocytes were treated for 24 hours with interleukin (IL)-1β (10, 100 or 1,000 pg/ml). Results from the treated chondrocytes were normalized to those from nontreated samples (Ctrl), so that the graphs represent the fold-induction in response to IL-1β. (A) Real-time polymerase chain reaction analysis of Mmp-13 gene expression. IL-1β-induced Mmp-13 gene expression tends to be enhanced in FrzB−/− chondrocytes compared with wild-type (WT) chondrocytes, especially with 1,000 pg/ml (P = 0.05, n = 3). (B) Culture media were analyzed for MMP-13 by western blotting. Quantification of the MMP-13 blot is shown above the blot. IL-1β-induced MMP-13 protein release tends to be enhanced in FrzB−/− chondrocytes compared with WT chondrocytes, especially with 1,000 pg/ml (P = 0.17, n = 4). Bars represent the mean ± standard error of the mean. *P ≤ 0.05 versus Ctrl, #P ≤ 0.10 between FrzB−/− and WT. FrzB, frizzled-related protein B; KO, knockout; MMP, matrix metalloproteinase; ND, not detected.