Effects of serum and insulin-like growth factor I on protein degradation and protease gene expression in rat L8 myotubes

We examined the effects of horse and fetal bovine sera and insulin-like growth factor I (IGF-I) on proteolysis and protease gene expression in rat L8 skeletal myotube cultures. Protein degradation was measured as release of radioactive trichloroacetic acid (TCA)-soluble materials from intracellular proteins prelabeled with [3H]tyrosine. Horse serum and fetal bovine serum inhibited (P < .05) protein degradation by 19.7 and 8.1%, respectively. The IGF-I at 200 ng/mL inhibited protein degradation by 14% (P < .01) over a 6-h measurement period. To study the regulation of proteolysis by IGF-I, we evaluated its effects on protease mRNA and alpha-tubulin mRNA concentrations by Northern blot analysis. Proteases under investigation included cathepsins B and D, proteasome C2 subunit, and m-calpain. The IGF-I had no effect (P > .05) on cathepsin B and D gene expression but slightly increased (P < .05) m-calpain and alpha-tubulin mRNA concentrations. Proteasome mRNA concentration was reduced (P < .05) by IGF-I treatment. The changes in proteasome mRNA levels paralleled the IGF-I-dependent alterations in proteolysis. These observations suggest that effects of IGF-I on muscle protein degradation may be mediated by the specific down-regulation of proteasomal subunit mRNAs.