Caseinolytic activity. Supernatants of cartilage monocultures (n = 5, with two replicates each) and co-cultures with synovial fibroblasts (SFB) (n = 5, with two replicates each) with/without stimulation with TNF-α, IL-1β or TNF-α/IL-1β (14 days) were used. In order to assess the total caseinolytic activity (lower panel), the bands for both the active and the latent forms were used for quantification. Means +/- standard error of the mean are plotted. § p ≤ 0.05 Mann-Whitney U Test compared with non-stimulated control; + p ≤ 0.05 Mann-Whitney U Test compared with cartilage monoculture. Parallel analysis of the supernatants by western blot revealed that bovine/human matrix metalloproteases (MMP) 1 and MMP-3 (upper and middle panel) are responsible for the caseinolytic enzyme activity in culture supernatants.