Nitric oxide regulates mitochondrial respiration and functions of articular chondrocytes

Biologic effects of nitric oxide (NO) have been shown to increase under hypoxic conditions. Because the oxygen tension in joint cavities of patients with arthritis is fairly low, biologic effects of NO would be expected to be significantly large in these compartments. This study was undertaken to investigate the effects of NO on the energy metabolism and functions of articular chondrocytes under different oxygen tension conditions.

Articular chondrocytes from rabbits were cultured under various oxygen concentrations in the presence or absence of NO and NOC18, an NO donor. Cellular respiration was measured using a Clark-type oxygen electrode. Levels of ATP in the cells were determined according to the luciferin-luciferase method. Cellular synthesis of proteoglycans was determined by measuring the incorporation of radioactivity (derived from 35S-labeled SO4) into glycosaminoglycans. Expression of stress-related proteins was evaluated by Western blotting analysis using specific antibodies.

Respiration and ATP synthesis of cultured chondrocytes were inhibited by NO, particularly under low oxygen concentrations. The presence of either NO or specific inhibitors of mitochondrial electron transport suppressed the synthesis of proteoglycans without affecting cell viability. When exposed to NO, cellular levels of heat-shock protein 70 (hsp70) and heme oxygenase 1 (HO-1) increased markedly. The presence of inhibitors of mitochondrial electron transport also increased cellular levels of hsp70 and HO-1.

These results suggest that NO generated in the joint might inhibit energy metabolism and the synthesis of proteoglycans of chondrocytes, thereby modulating pathophysiologic processes occurring in patients with arthritis.