Effects of Serum Deprivation on Myofibrillar Proteolysis in Chick Myotube Cultures

Leucine suppresses myofibrillar proteolysis by down-regulating ubiquitin-proteasome pathway in chick skeletal muscles

In skeletal muscle, amino acids, together with hormones, are key regulators of protein metabolism. Leucine, in particular, has inhibitory effects of protein degradation in skeletal muscles, but the mechanisms are poorly understood. The present study addressed the role of leucine as a regulator of myofibrillar proteolysis in cultured chick myotubes and chick skeletal muscles, and aimed to determine which cellular responses regulate the process. In chick myotubes, leucine suppressed myofibrillar proteolysis (as measured by N(tau)-methylhistidine release), while also decreasing ubiquitin and proteasome C2 subunit mRNA. Oral administration of leucine also suppressed myofibrillar proteolysis (as measured by plasma N(tau)-methylhistidine concentration), while also decreasing proteasome C2 subunit mRNA in chick skeletal muscle. Leucine activated the phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) (but not the mammalian target of rapamycin) inhibition of these pathways and increased myofibrillar proteolysis, ubiquitin and proteasome C2 subunit mRNA. Thus, an important component of muscle proteolysis inhibition by leucine, through the PI3K and PKC, is its ability to suppress transcription of the ubiquitin and proteasome C2 subunit, and degradation of myofibrillar protein.

Effects of serum deprivation on expression of proteolytic-related genes in chick myotube cultures

We previously reported that serum deprivation stimulates myofibrillar proteolysis in chick myotubes. In the present study, we examined the effect of serum deprivation on expression of the proteolytic-related genes (ubiquitin, proteasome, calpains, and cathepsin B) by real-time PCR of cDNA in chick myotubes. Myotubes were incubated with serum-free medium for 24 h. Ubiquitin and proteasome subunits (C1 and C2) and calpains (m-, mu-, and p94/calpain-3) but not cathepsin B mRNA expression were increased by serum deprivation. These results indicate that serum deprivation stimulates ubiquitin-proteasome and calpain proteolytic pathways, resulting in an increase in myofibrillar proteolysis in chick myotubes.