1. A cell culture system of C2C12 myotubes was established as a model of the muscle. With the aid of this model, the half-lives of intracellular proteins as well as the activities and mRNA levels of proteasomes (26S and 20S) and cathepsins (B, L, and H) were examined in the presence of various amounts of cytokines.

2. It was found that 100 units/ml recombinant human interleukin-6 somewhat shortened the half-life of long-lived proteins to 23.79 ± 1.55 h (control: 25.60 ± 1.87 h). When 1% fetal bovine serum contained in the culture medium was replaced by 0.5 mg/ml bovine serum albumin, interleukin-6 was more effective since 10 units/ml of interleukin-6 shortened the half-life to 19.09 ± 2.87 h (control: 22.26 ± 321 h). Interleukin-6 (100 units/ml) increased the activity of 26S proteasome by 31.5%, of cathepsin B by 53.5% and of cathepsin B + L by 21.3%. These increases occurred in association with an increase in their transcription.

3. On the other hand, 1000 units/ml of recombinant human tumour necrosis factor α prolonged the half-life of long-lived proteins while reducing the protease activities of 20S proteasome (−27.1%), cathepsins B (−64.6%) and B + L (−54.9%).

4. These results suggest that interleukin-6 induces degradation of long-lived intracellular proteins by activating both the non-lysosomal (proteasomes) and lysosomal (cathepsins) proteolytic pathways. It is therefore concluded that interleukin-6 is a candidate for a proteolysis-inducing factor in myotubes and may play an important role in the progression of muscle degradation in systemic inflammatory responses induced by sepsis or severe injury.

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