Repression of interleukin (IL)-1β-inducible matrix metalloproteinase (MMP)-3 and MMP-13 RNA expression by mithramycin. Quiescent human chondrosarcoma (a), primary human chondrocytes (b) or bovine chondrocytes (c) were pretreated with different concentrations of mithramycin for 30 min, followed by additional treatment with IL-1β for 24 hours. The MMP-3, MMP-13 and 28S RNA levels were measured by Northern hybridization, and MMP-13 protein levels were measured by Western blot analysis. For protein gels, 3 μg (a) or 4 μg (b, c) of protein was applied to each lane. The resulting autoradiograms indicating the respective gene products are shown.