Intracellular protein degradation in cultures of dystrophic muscle cells and fibroblasts

Effects of a beta-adrenergic agonist on protein turnover in muscle cells in culture

beta-Adrenergic agents powerfully stimulate muscle growth in animals. Whether the mechanism of action involves a direct effect on muscle cell beta-receptors or is secondarily due to a beta-induced alteration in the hormonal environment is not known. To assess whether direct beta-receptor activation results in muscle protein accretion, we examined the effect of the beta-agonist zinterol on several anabolic processes in L8 muscle cells in culture. In vivo feeding of zinterol (26.5 ppm) to rats significantly increased muscle weight by 15%. In vitro, zinterol stimulated lactate release from L8 cells whereas propranolol inhibited this process, demonstrating that these cells have functional beta-receptors both before and after fusion. We measured several anabolic processes, in both serum-stimulated and quiescent cells, over a wide range of zinterol concentrations. Zinterol had no effect on protein or DNA synthesis, protein degradation, or rates of amino acid uptake. These data suggest that the in vivo muscle growth stimulation is either indirect or some in vivo requirements (e.g. tension and nerve interactions) are necessary for expression of the effect.

Regulation of protein synthesis and degradation in L8 myotubes. Effects of serum, insulin and insulin-like growth factors

We have examined the regulation of protein turnover in rat skeletal myotubes from the L8 cell line. We measured protein synthesis by the rates of incorporation of radiolabelled tyrosine into protein in the presence of a flooding dose of non-radioactive tyrosine. We monitored degradation of proteins labelled with radioactive tyrosine by the release of acid-soluble radioactivity into medium containing excess nonradioactive tyrosine. Extracellular tyrosine pools and intracellular tyrosyl-tRNA equilibrate rapidly during measurements of protein synthesis, and very little reutilization of the radiolabelled tyrosine occurs during degradation measurements. Measured rates of protein synthesis and degradation are constant for several hours, and changes in myotube protein content can be accurately predicted by the measured rates of protein synthesis and degradation. Most of the myotube proteins labelled with radioactive tyrosine for 2 days are degraded, with half-lives (t1/2) of approx. 50 h. A small proportion (less than 2.5%) of the radiolabelled proteins are degraded more rapidly (t1/2 less than 10 h), and, at most, a small proportion (less than 15%) are degraded more slowly (t1/2 greater than 50 h). A variety of agents commonly added to primary muscle cell cultures or to myoblast cell lines (18% Medium 199, 1% chick-embryo extract, antibiotics and antifungal agents) had no effect on rates of protein synthesis or degradation. Horse serum, fetal bovine serum and insulin stimulate protein synthesis and inhibit the degradation of long-lived proteins without affecting the degradation of short-lived proteins. Insulin-like growth factors (IGF)-1 and -2 also stimulate protein synthesis and inhibit protein degradation. The stimulation of protein synthesis and the inhibition of protein degradation are of similar magnitude (a maximum of approx. 2-fold) and display similar sensitivities to a particular anabolic agent. Insulin stimulates protein synthesis and inhibits protein degradation only at supraphysiological doses, whereas IGF-1 and -2 are effective at physiological concentrations. These and other findings suggest that IGFs may be important regulators of skeletal muscle growth during the fetal and early neonatal periods.

Histidine and proline are important sites of free radical damage to proteins

Our hypothesis that proline and histidine are major sites of damage during radical attack upon proteins, becoming respectively glutamate and aspartate, was investigated using proteins biosynthetically labelled with radioactive proline or histidine as targets. Free radicals were generated by copper and H2O2, or by gamma radiolysis. Protein-bound histidine was substantially converted into aspartate. Much proline was modified during radical attack, but it was not converted into glutamate. We conclude that histidine and proline are important sites of protein attack by radicals; protein cleavage may result from these reactions.

Prostaglandin production in myotube cultures. Influence on protein turnover

The production of prostaglandins (PG) E2 and F2 alpha and their possible role in regulation of protein turnover in cultured skeletal-muscle cells were examined. Primary chick myoblasts and myotubes, and L8 myotubes, produced PGE2 and PGF2 alpha from endogenous arachidonic acid. PG production by all three cell types was increased manyfold by the addition of exogenous arachidonic acid. Arachidonate-stimulated PG production was inhibited by the addition of indomethacin (0.1 mM). When L8 and chick myotubes were treated with PGE2, PGF2 alpha, arachidonic acid (0.01 mM) or indomethacin (0.1 mM), no significant alterations in rates of protein synthesis or degradation were observed. Rates of protein synthesis and degradation in these cells were responsive to the addition of 10% fetal-bovine serum under identical experimental conditions. Thus, in contrast with incubated adult skeletal muscle, it appears that the production of prostaglandin metabolites from arachidonic acid is unrelated to regulation of protein turnover in cultured muscle cells.

Abnormal protein metabolism in skin fibroblasts in vitro from patients with Duchenne muscular dystrophy

Rates of protein turnover have been measured on a statistical basis in Duchenne muscular dystrophy and normal skin fibroblasts populations in vitro. At comparable numbers of cumulative population doublings, protein synthesis was significantly reduced by about 24% in DMD fibroblasts as compared to normal fibroblasts (p less than 0.01, N = 12). Degradation of short lived proteins was significantly enhanced by about 60% (p less than 0.05, N = 18), and the degradation of long lived proteins was significantly increased by about 28% (p less than 0.05, N = 18) in DMD fibroblast populations in vitro. The enhanced degradation of long lived proteins in DMD fibroblasts can be reduced to basal levels of degradation by the use of the protease inhibitors leupeptin and Ep475 (p less than 0.05, N = 9).

Tissue culture studies of muscle disorders: Part 2. Biochemical studies, nerve-muscle culture, metabolic myopathies, and animal models

This review continues with studies of protein, lipid, and purine metabolism of Duchenne muscular dystrophy (DMD) cells in vitro and of muscle cells in combined culture with nerve cells. In vitro studies of human metabolic myopathies are tabulated. Results using the hamster, chicken, and mouse (dy25, dy, mdg, and mdx) myopathies are discussed. Interesting findings include suggestions of altered collagen synthesis by DMD cells. Analysis of cell proteins by two-dimensional gel electrophoresis and the use of combined nerve-muscle cultures remain important areas of development. It is disappointing that so few attempts have been made to repeat significant findings in this field, and when a number of laboratories have examined the same phenomenon, the results are often contradictory. It remains to be shown how these various abnormalities found in cells in vitro are related to each other and to those pathologic features of diseased muscle observed in vivo.

Differential degradation of [35S]methionine polypeptides in Duchenne muscular dystrophy skin fibroblasts in vitro

Rates of protein turnover have been measured in three normal and three Duchenne muscular dystrophy (DMD) skin fibroblast cell lines. Cell populations were analyzed at identical states with regard to cell number, state of topoinhibition, and cumulative population doublings (CPD). Net protein synthesis measured by the incorporation of [35S]methionine in an 18-hr pulse was reduced by an average of 34%; degradation of total cellular protein measured after an 18-hr pulse with [35S]methionine and a 24-hr chase was enhanced by an average of 50% in DMD fibroblasts. Two-dimensional gel electrophoresis analyses revealed that the breakdown of the majority of [35S]methionine polypeptides was markedly increased in DMD fibroblasts. Quantitative determinations of the differential degradation rates of 10 selected proteins in the tropomyosin region of two-dimensional gels were undertaken by scintillation counting and computer analyses. In three series of experiments, the degradation of the 10 proteins in DMD fibroblasts was enhanced by individual polypeptides between 12.0% and 151.2% as measured by scintillation counting or between 0.8% and 128% as determined by computer analyses.

Protein degradation in cultured skeletal muscle from Duchenne muscular dystrophy patients

The loss of contractile protein in Duchenne muscular dystrophy could result from low rates of synthesis, abnormally high rates of protein degradation, or a combination of both. We measured overall protein degradation rates in cultured human muscle cells obtained at biopsy from patients with Duchenne dystrophy or various muscle diseases and normal subjects. Measurements were performed on confluent cultures exhibiting no growth and containing a mixed cell population of myoblasts, fibroblasts, and multinucleated myotubes. Using a new double-isotope labeling protocol, we found protein degradation rates in all three groups to be similar (KD = 0.0171-0.0176 hr-1), suggesting no detectable abnormality of overall protein degradation in cells derived from Duchenne dystrophy patients.