Effect of dexamethasone, ammonium ions, and serum-deprivation on intracellular proteolysis in cultured muscle cells

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.

Hormone-responsive alkaline proteinase in rat skeletal muscle is not a mast cell-derived enzyme

Proteinase activity was determined in myofibrils from intact rat skeletal muscle and from skeletal muscle myocytes grown in culture. In vivo administration of the mast cell degranulator compound 48/80 abolished the alkaline proteinase activity in myofibrils obtained from normal or streptozotocin-diabetic rats. Exposure of myocytes to compound 48/80 in cell cultures had no effect on their myofibrillar proteinase activity, nor did it affect the rate of overall protein degradation in these cells. Co-incubation of cultured mast cells (line P815Y) with myocytes followed by sonication of the cell mixture resulted in a marked reduction of the proteinase activity in the pellet fraction, suggesting that the mast cells contain inhibitor(s) of myofibrillar proteinase activity. It is suggested that the myofibril-bound alkaline proteinase activity is not a mast cell-derived enzyme but a genuine component of muscle cells. The in vivo 48/80-induced reduction of muscle myofibrillar proteinase activity appears to be due to release of a soluble inhibitory activity rather than removal of mast cell proteinase from the tissue by degranulation.

Hormone-responsive myofibrillar protease activity in cultured rat myoblasts

The effect of exposure to dexamethasone and serum-deprivation on myofibrillar protease activity was determined by following cleavage of [14C]globin by isolated myofibrils obtained from rat skeletal muscle in culture. Dexamethasone [10(-7) M] produced a 46% increase in protease activity, and serum-deprivation caused a 50% increase in activity over that of the enzyme in control cultures. The increases in proteolysis occurred concurrently with increased rate of overall protein degradation in these cells and were not associated with changes in cell viability. In cultured rat cardiac muscle cells dexamethasone failed to enhance myofibrillar protease activity, while serum-deprivation produced a 52% increase in the enzyme activity. Addition of insulin (50 mU/ml) to the cultures did not affect proteolysis or myofibrillar protease activity, but completely prevented the dexamethasone-induced increase of these activities. This effect of insulin suggests that the increase of muscle proteolysis in insulin-deficient diabetic animals reflects an enhanced response of the muscle to circulating glucocorticoids rather than a direct effect of insulin-deprivation on muscle proteolysis. Taken together, the present observations indicate that muscle cells in culture retain the ability to respond to catabolic stimuli by adaptive changes in the myofibrillar protease activity in a manner analogous to that of their parent tissue in the intact animal.