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.
Hong
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Department of Animal Science, Oregon State University, Corvallis 97331-6702.
Forsberg
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NE
D000818:Animals; D001483:Base Sequence; D001790:Blood Physiological Phenomena; D015152:Blotting, Northern; D002417:Cattle; D002460:Cell Line; D010450:Endopeptidases / Q000235:genetics*; D005312:Fetal Blood / Q000502:physiology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D006736:Horses; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D008969:Molecular Sequence Data; D009124:Muscle Proteins / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000201:enzymology*; D015345:Oligonucleotide Probes / Q000737:chemistry; D012333:RNA, Messenger / Q000096:biosynthesis; D012339:RNA, Ribosomal, 28S / Q000096:biosynthesis; D051381:Rats; D014404:Tubulin / Q000235:genetics
D009124:Muscle Proteins; D015345:Oligonucleotide Probes; D012333:RNA, Messenger; D012339:RNA, Ribosomal, 28S; D014404:Tubulin; D007334:Insulin-Like Growth Factor I; D010450:Endopeptidases