Curcumin inhibits IL-1β activity, enabling growth factor induced chondrogenesis in MSCs. A: Fourteen days high-density culture. Treatment of MSC cultures with IL-1β alone (a), IL-1β and the chondrogenic induction medium (b) or a four-hour pre-stimulation with curcumin followed by IL-1β incubation (c) resulted in apoptosis or necrosis of the cells. In contrast, a four-hour pre-stimulation of MSCs with curcumin followed by incubation with IL-1β and the chondrogenic induction medium (d) or followed by incubation with IL-1β, the chondrogenic induction medium and curcumin (e) resulted in chondrogenesis. Magnification, 6,000×; bar, 1 μm; C, chondrocytes; F, fibroblast-like cells; M, ECM; GF, chondrogenic induction medium. B: Apoptotic cells were counted as an indicator of MSC culture degradation. In cultures stimulated with IL-1β alone, with IL-1β and the chondrogenic induction medium, or with IL-1β and curcumin the number of apoptotic cells was increased. The number of apoptotic cells remained significantly lower in MSC cultures stimulated with IL-1β, curcumin and the chondrogenic induction medium. Significant values are marked with (*). C-D: Immunoblots of whole cell lysates were probed using antibodies that recognize CSPGs, collagen type II, β1-integrin, Shc, activated-ERK1/2, Sox-9, activated-caspase-3 and COX-2. Each experiment was performed in triplicate. Expression of the housekeeping gene β-actin was not affected. GF, chondrogenic induction medium.