Nitric Oxide does not mediate Atrogin-1/MAFbx upregulation by inflammatory mediators

Accelerated proteolysis through the ubiquitin-proteasome system has been recognized as a major contributor to muscle wasting, a serious complication frequently associated with a number of inflammatory disorders. Muscle expression of atrogin-1/MAFbx, a rate-limiting ubiquitin ligase for muscle wasting, is upregulated in various inflammatory conditions, and is considered a therapeutic target for muscle wasting. As one of the free radicals whose production is elevated in inflammatory conditions, nitric oxide (NO) is implicated in the pathogenesis of muscle wasting. To understand how inflammatory mediators upregulate atrogin-1/MAFbx expression, we tested the hypothesis that NO mediates the upregulation of atrogin-1/MAFbx expression. C2C12 myotubes were incubated with a cocktail comprised of TNF-α, interferon γ and lipopolysaccharide (LPS), which stimulated NO production and atrogin-1/MAFbx expression. Pre-incubation of the myotubes with nitric oxide synthase (NOS) inhibitor L-NAME or S-ethylisothiourea (SETU) blocked the stimulation of NO production by the cocktail. However, the stimulation of atrogin-1/MAFbx expression was not disrupted. Intraperitoneal administration of LPS to mice resulted in elevated atrogin-1/MAFbx expression in gastrocnemius muscle. But, pretreatment of the mice with L-NAME did not alter LPS stimulation of atrogin-1/MAFbx expression. Therefore, NO does not mediate upregulation of atrogin-1/MAFb expression by inflammatory mediators.