Interferon-γ (IFN-γ) does not mimic the catabolic effects of tumor necrosis factor-α (TNF-α)

Cachexia is common in chronic inflammatory diseases and is attributed, in part, to elevation of circulating pro-inflammatory cytokines. TNF-α is the prototype in this category. IFN-γ is also thought to play a role, but the evidence supporting this model is primarily indirect. To determine the direct effects of IFN-γ stimulation on muscle cells, we selected key components of the pro-catabolic signaling pathways by which TNF-α stimulates protein loss. We tested two hypotheses: a.) IFN-γ mimics TNF-α signaling by increasing intracellular oxidant activity and activating mitogen activated protein kinases (MAPKs) and NF-κB, and; b.) IFN-γ increases expression of the ubiquitin ligases atrogin1/MAFbx and MuRF1. Results showed that treatment with IFN-γ 60 ng/ml increased Stat1 phosphorylation after 15 min, indicating receptor activation. IFN-γ had no effect on cytosolic oxidant activity as measured by 2’,7’-dichlorofluorescein oxidation. Nor did IFN-γ activate jun N-terminal kinase (JNK), extracellular-regulated kinases 1 and 2 (ERK1/2) or p38 MAPK as assessed by Western blot. Treatment for up to 60 min did not decrease IκBα protein levels as measured by Western blot or DNA binding activity of NF-κB activity as measured by electrophoresis mobility shift assay. After 6 hr, IFN-γ decreased Akt phosphorylation and increased atrogin1/MAFbx and MuRF1 mRNA. Daily treatment for up to 72 hr did not alter adult fast-type myosin heavy chain (MHCf) content or total protein:DNA ratio. These data show that responses of myotubes to IFN-γ and TNF-α differ markedly and provide little evidence for a direct catabolic effect of IFN-γ on muscle.