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The extrinsic caspase pathway modulates endotoxin-induced diaphragm contractile dysfunction

Department of Medicine, Medical College of Georgia, Augusta, Georgia

Submitted 29 March 2006 ; accepted in final form 3 December 2006

The mechanisms by which infections induce diaphragm dysfunction remain poorly understood. The purpose of this study was to determine which caspase pathways (i.e., the extrinsic, death receptor-linked caspase-8 pathway, and/or the intrinsic, mitochondrial-related caspase-9 pathway) are responsible for endotoxin-induced diaphragm contractile dysfunction. We determined 1) whether endotoxin administration (12 mg/kg IP) to mice induces caspase-8 or -9 activation in the diaphragm; 2) whether administration of a caspase-8 inhibitor (N-acetyl-Ile-Glu-Thr-Asp-CHO, 3 mg/kg iv) or a caspase-9 inhibitor (N-acetyl-Leu-Glu-His-Asp-CHO, 3 mg/kg iv) blocks endotoxin-induced diaphragmatic weakness and caspase-3 activation; 3) whether TNF receptor 1-deficient mice have reduced caspase activation and diaphragm dysfunction following endotoxin; and 4) whether cytokines (TNF- or cytomix, a mixture of TNF-, interleukin-1, interferon-, and endotoxin) evoke caspase activation in C₂C₁₂ myotubes. Endotoxin markedly reduced diaphragm force generation (P < 0.001) and induced increases in caspase-3 and caspase-8 activity (P < 0.03), but failed to increase caspase-9. Inhibitors of caspase-8, but not of caspase-9, prevented endotoxin-induced reductions in diaphragm force and caspase-3 activation (P < 0.01). Mice deficient in TNF receptor 1 also had reduced caspase-8 activation (P < 0.001) and less contractile dysfunction (P < 0.01) after endotoxin. Furthermore, incubation of C₂C₁₂ cells with either TNF- or cytomix elicited significant caspase-8 activation. The caspase-8 pathway is strongly activated in the diaphragm following endotoxin and is responsible for caspase-3 activation and diaphragm weakness.

skeletal muscle; sepsis; weakness; tumor necrosis factor-; tumor necrosis factor receptor 1

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