Nitrosative stress induces osteoblast apoptosis through downregulating MAPK-mediated NFκB/AP-1 activation and subsequent Bcl-X_L expression

During inflammation, a large amount of reactive oxygen species is produced and causes insults to osteoblasts. This study was aimed to evaluate the molecular mechanisms of sodium nitroprusside (SNP)-induced insults to rat osteoblasts. Exposure of osteoblasts, prepared from neonatal rat calvaria to SNP increased the levels of cellular nitric oxide and intracellular reactive oxygen species, and simultaneously induced apoptotic insults in concentration- or time-dependent manners. Exposure of rat osteoblasts to SNP time-dependently decreased antiapoptotic Bcl-XL messenger RNA and protein syntheses. Treatment of rat osteoblasts with SNP decreased the translocation of transcription factors nuclear factor-kappaB (NFκB) and activator protein (AP)-1 from the cytoplasm to nuclei. Sequentially, phosphorylations of the mitogen-activated protein kinases (MAPKs) of ERK1/2, JNK1/2, and p38 MAPK decreased following SNP administration. Application of ERK1 and JNK1 small interference (si)RNAs into rat osteoblasts decreased the translation of these MAPKs and synergistically enlarged SNP-caused alterations in Bcl-XL mRNA expression and cell apoptosis. Therefore, this study shows that the SNP-induced nitrosative stress decreased Bcl-XL expression, and then induced apoptotic insults to rat osteoblasts through downregulating phosphorylation of MAPKs and subsequent activation of NFκB and AP-1.