Production of MMP-1, −3, −13, and TIMP-1 in cultured supernatant. Production of MMP-1, −3, −13, and TIMP-1 was analyzed by enzyme-linked immunosorbent assay (ELISA). Data were expressed as means ± standard deviation (SD). *P < 0.05, **P < 0.01, ***P < 0.001, compared with cells stimulated with IL-1β alone; #P < 0.01, compared with normal chondrocytes. (a), (b), (c), (d) represented as the production of MMP-1,MMP-3,MMP-13 and TIMP-1 respectively.