Activation of the inducible nitric oxide synthase pathway contributes to inflammation-induced osteoporosis by suppressing bone formation and causing osteoblast apoptosis

Objective. Osteoporosis is a major clinical problem in chronic inflammatory diseases such as rheumatoid arthritis. The mechanism of bone loss in this condition remains unclear, but previous studies have indicated that depress ed bone formation plays a causal role. Since cytokine-induced nitric oxide (NO) production has been shown to inhibit osteoblast growth and differentia tion in vitro, this study was undertaken to investigate the role of the ind ucible NO synthase (iNOS) pathway in the pathogenesis of inflammation-media ted osteoporosis (IMO) by studying mice with targeted inactivation of the i NOS gene (iNOS knockout [iNOS KO] mice). Methods. IMO was induced in wild-type (WT) and iNOS KO mice by subcutaneous injections of magnesium silicate. The skeletal response was assessed at th e tibial metaphysis by measurements of bone mineral density, (BMD), using p eripheral quantitative computed tomography, by bone histomorphometry, and b y measurements of bone cell apoptosis. Results. NO production increased 2.5-fold (P < 0.005) in WT mice with IMO, but did not change significantly in iNOS KO mice. Total BMD values decrease d by, a mean +/- SEM of 14.4 +/- 2.0% in WT mice with IMO, compared with a decrease of 8.6 +/- 1.2% in iNOS KO mice with IMO (P < 0.01). Histomorphome tric analysis confirmed that trabecular bone volume was lower in WT mice wi th IMO compared with iNOS KO mice with IMO (16.2 +/- 1.5% versus 23.4 +/- 2 .6%; P < 0.05) and showed that IMO was associated with reduced bone formati on and a 320% increase in osteoblast apoptosis (P < 0.005) in WT mice. In c ontrast, iNOS KO mice with IMO showed less inhibition of bone formation tha n WT mice and showed no significant increase in osteoblast apoptosis. Conclusion. Inducible NOS-mediated osteoblast apoptosis and depressed bone formation play important roles in the pathogenesis of IMO.