Nitric oxide stress in sporadic inclusion body myositis muscle fibres: inhibition of inducible nitric oxide synthase prevents interleukin-1β-induced accumulation of β-amyloid and cell death.

Sporadic inclusion body myositis is a severely disabling myopathy. The design of effective treatment strategies is hampered by insufficient understanding of the complex disease pathology. Particularly, the nature of interrelationships between inflammatory and degenerative pathomechanisms in sporadic inclusion body myositis has remained elusive. In Alzheimer's dementia, accumulation of β-amyloid has been shown to be associated with upregulation of nitric oxide. Using quantitative polymerase chain reaction, an overexpression of inducible nitric oxide synthase was observed in five out of ten patients with sporadic inclusion body myositis, two of eleven with dermatomyositis, three of eight with polymyositis, two of nine with muscular dystrophy and two of ten non-myopathic controls. Immunohistochemistry confirmed protein expression of inducible nitric oxide synthase and demonstrated intracellular nitration of tyrosine, an indicator for intra-fibre production of nitric oxide, in sporadic inclusion body myositis muscle samples, but much less in dermatomyositis or polymyositis, hardly in dystrophic muscle and not in non-myopathic controls. Using fluorescent double-labelling immunohistochemistry, a significant co-localization was observed in sporadic inclusion body myositis muscle between β-amyloid, thioflavine-S and nitrotyrosine. In primary cultures of human myotubes and in myoblasts, exposure to interleukin-1β in combination with interferon-γ induced a robust upregulation of inducible nitric oxide synthase messenger RNA. Using fluorescent detectors of reactive oxygen species and nitric oxide, dichlorofluorescein and diaminofluorescein, respectively, flow cytometry revealed that interleukin-1β combined with interferon-γ induced intracellular production of nitric oxide, which was associated with necrotic cell death in muscle cells. Intracellular nitration of tyrosine was noted, which partly co-localized with amyloid precursor protein, but not with desmin. Pharmacological inhibition of inducible nitric oxide synthase by 1400W reduced intracellular production of nitric oxide and prevented accumulation of β-amyloid, nitration of tyrosine as well as cell death inflicted by interleukin-1β combined with interferon-γ. Collectively, these data suggest that, in skeletal muscle, inducible nitric oxide synthase is a central component of interactions between interleukin-1β and β-amyloid, two of the most relevant molecules in sporadic inclusion body myositis. The data further our understanding of the pathology of sporadic inclusion body myositis and may point to novel treatment strategies.