Mitochondrial dysfunction activates cyclooxygenase 2 expression in cultured normal human chondrocytes.

OBJECTIVE: Mitochondrial alterations play a key role in the pathogenesis of osteoarthritis (OA). This study evaluated a potential role of mitochondrial respiratory chain (MRC) dysfunction in the inflammatory response of normal human chondrocytes. METHODS: Commonly used inhibitors of the MRC were utilized to induce mitochondrial dysfunction in normal human chondrocytes. Levels of prostaglandin E(2) (PGE(2)) protein and expression of cyclooxygenase 2 (COX-2) and COX-1 messenger RNA (mRNA) and protein were analyzed. To identify the underlying mechanisms responsible for PGE(2) liberation, reactive oxygen species (ROS) were measured. Inhibitors of ROS, including vitamin E, and inhibitors of mitochondrial Ca(2+) and NF-kappaB were used to test their effects on the MRC. RESULTS: Antimycin A and oligomycin (inhibitors of mitochondrial complexes III and V, respectively) significantly increased the levels of PGE(2) (mean +/- SEM 505 +/- 132 pg/50,000 cells and 288 +/- 104 pg/50,000 cells, respectively, at 24 hours versus a basal level of 29 +/- 9 pg/50,000 cells; P < 0.05) and increased the expression of COX-2 at both the mRNA and protein levels. Expression of COX-1 did not show any modulation with either inhibitor. Further experiments revealed that antimycin A and oligomycin induced a marked increase in the levels of ROS. Production of PGE(2) and expression of COX-2 protein were inhibited by antioxidants, vitamin E, and mitochondrial Ca(2+) and NF-kappaB inhibitors. The response to blockers of mitochondrial Ca(2+) movement showed that ROS production was dependent on mitochondrial Ca(2+) accumulation. CONCLUSION: These results strongly suggest that, in human chondrocytes, the inhibition of complexes III and V of the MRC induces an inflammatory response, which could be especially relevant in relation to PGE(2) production via mitochondrial Ca(2+) exchange, ROS production, and NF-kappaB activation. These data may prove valuable for a better understanding of the participation of mitochondria in the pathogenesis of OA.

[Evidence of inflammatory mechanisms in osteoarthritis]. / Evidencia de mecanismos inflamatorios en la osteoartritis.

Classically, osteoarthritis (OA) is not considered an inflammatory arthropathy, because of the presence of a small number of neutrophils in the synovial fluid and the absence of systemic manifestations of inflammation. Besides, the characteristics of articular cartilage (avascular, alymphatic and aneural) do disable to fulfill with the classical signs of inflammation (redness, swelling, heat, pain). However, thanks to development of molecular and cellular biology, there are multiple studies which shown that different proinflammatory mediators, such as the cytokines IL-1ß and TNFα, could be important in the development of this disease. Therefore, the stimulation of chondrocytes, the only cell type living in the cartilage matrix and for this reason the principal responsible of integrity of cartilage matrix extracellular, with these proinflamatory cytokines increases the production of metalloproteinases, keys molecules in the irreversible degradation of normal architecture of cartilage. As well, inhibits the synthesis of cartilage proteoglycans and type II collagen, stimulates the production of reactive oxygen species such as nitric oxide, and increases the production of prostaglandin E(2). Likewise, the effects of synovial inflammation expected contribute to deregulation of chondrocyte function in a similar fashion, favouring the lost of equilibrium between the catabolic and anabolic activities of the chondrocyte necessary for maintaining the extracellular cartilage matrix.