Endoplasmic reticulum stress mediates nitric oxide-induced chondrocyte apoptosis

Nitric oxide (NO) is one of the most important mediators of chondrocyte apoptosis, which is a notable feature of cartilage degeneration. While apoptosis of chondrocytes is induced by p53, NO can also induce endoplasmic reticulum (ER) stress, which may be involved in the process of NO-induced chondrocyte apoptosis. The aims of this study were to determine whether NO-induced ER stress (ERS) leads to apoptosis of chondrocytes and to investigate the temporal relationship between the expression of C/EBP-homologous protein (CHOP), an ERS-associated apoptotic molecule, and the expression of p53 during apoptosis in NO-stimulated chondrocytes. Rat chondrocytes were stimulated by sodium nitroprusside (SNP), a NO donor. Real-time polymerase chain reaction (PCR) was performed to analyze the mRNA expression of CHOP, glucose-regulated protein (GRP78) and p53. Apoptosis of chondrocytes was quantified using an enzyme-linked immunosorbent assay (ELISA). SNP-treated chondrocytes showed an increase in CHOP and GRP78 mRNA expression and underwent apoptosis. Sodium 4-phenylbutyrate (PBA), an ERS inhibitor, reduced CHOP and GRP78, as well as SNP-stimulated apoptosis of chondrocytes, without affecting the SNP-dependent generation of NO. In addition, the blockade of CHOP following siRNA transfection reduced SNP-induced apoptosis of chondrocytes. The CHOP expression increased after apoptosis was detected in the SNP-treated chondrocytes, whereas the p53 expression increased prior to apoptosis. These data demonstrated that NO-induced ERS leads chondrocytes to apoptosis, although this effect appears to be limited to persistent impairment of NO stimulation. These findings may provide insight into the pathology of cartilage degeneration.