INTERLEUKIN-6 INDUCES PROTEOLYSIS BY ACTIVATING INTRACELLULAR PROTEASES (CATHEPSIN-B AND CATHEPSIN-L, PROTEASOME) IN C2C12 MYOTUBES

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 intracell ular proteins as well as the activities and mRNA levels of proteasomes (26S and 20S) and cathepsins (B, L, and H) were examined in the prese nce of various amounts of cytokines. 2. It was found that 100 units/ml recombinant human interleukin-6 somewhat shortened of long-lived prot eins to 23.79+/-1.55h (control: 25.60+/-1.87h). When 1% fetal bovine s erum contained in the culture medium was replaced by 0.5mg/ml bovine s erum albumin, interleukin-6 was more effective since 10 units/ml of in terleukin-6 shortened the half-life to 19,09+/-2.87h (control: 22.26+/ -321h). Interleukin-6 (100 units/ml) increased the activity of 26S pro teasome 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 t ranscription. 3. On the other hand, 1000 units/ml of recombinant human tumour necrosis factor alpha prolonged the half-life of long-lived pr oteins while reducing the protease activities of 20S proteasome (-27,1 %), cathepsins B (-64,6%) and B+L (-54.9%). 4. These results suggest t hat interleukin-6 induces degradation of long-lived intracellular prot eins by activating both the non-lysosomal (proteasomes) and lysosomal (cathepsins) proteolytic pathways. It is therefore concluded that inte rleukin-6 is a candidate for a proteolysis-inducing factor in myotubes and may play an important role in the progression of muscle degradati on in systemic inflammatory responses induced by sepsis or severe inju ry.