Mitochondrial activity is modulated by TNFα and IL-1β in normal human chondrocyte cells

Objective: Pro-inflammatory cytokines play an important role in osteoarthritis (OA). In osteoarthritic cartilage, chondrocytes exhibit an alteration in mitochondrial activity. This study analyzes the effect of tumor necrosis factor-alpha (TNF alpha) and interleukin-1 beta (IL-1 beta) on the mitochondrial activity of normal human chondrocytes. Materials and methods: Mitochondrial function was evaluated by analyzing the activities of respiratory. chain enzyme complexes and citrate synthase, as well as by mitochondrial membrane potential (Delta psi m) and adenosine triphosphate (ATP) synthesis. Bcl-2 family mRNA expression and protein synthesis were analyzed by RNase protection assay (RPA) and Western-blot, respectively. Cell viability was analyzed by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) and apoptosis by 4', 6-diamidino-2-phenylindole dihydrochloride (DAPI) stain. Glycosaminoglycans were quantified in supernatant by a dimethyl-methylene blue binding assay. Results: Compared to basal cells, stimulation with TNF alpha (10 ng/ml) and IL-1 beta (5 ng/ml) for 48 h significantly decreased the activity of complex I (TNF alpha=35% and IL-1 beta=35%) and the production of ATP (TNF alpha=18% and IL-1 beta=19%). Both TNF alpha and IL-1 beta caused a definitive time-dependent decrease in the red/green fluorescence ratio in chondrocytes, indicating depolarization of the mitochondria. Both cytokines induced mRNA expression and protein synthesis of the Bcl-2 family. Rotenone, an inhibitor of complex I, caused a significant reduction of the red/green ratio, but it did not reduce the viability of the chondrocytes. Rotenone also increased Bcl-2 mRNA expression and protein synthesis. Finally, rotenone as well as TNF alpha and IL-1 beta, reduced the content of proteoglycans in the extracellular matrix of normal cartilage. Conclusion. These results show that both TNF alpha and IL-1 beta regulate mitochondrial function in human articular chondrocytes. Furthermore; the inhibition of complex I by both cytokines could play a key role in cartilage degradation induced by TNF alpha and IL-1 beta. These data could be important for understanding of the OA pathogenesis. (C) 2006 OsteoArthritis Research Society International. Published by Elsevier Ltd. All rights reserved.