Regulation of Ca2+-dependent protein turnover in skeletal muscle by thyroxine

Hispaglabridin B, a constituent of liquorice identified by a bioinformatics and machine learning approach, relieves protein-energy wasting by inhibiting forkhead box O1

BACKGROUND AND PURPOSE

Liquorice is the root of Glycyrrhiza glabra, which is a popular food in Europe and China that has previously shown benefits for skeletal fatigue and nutrient metabolism. However, the mechanism and active ingredients remain largely unclear. The aim of this study was to investigate the active ingredients of liquorice for muscle wasting and elucidate the underlying mechanisms.

EXPERIMENTAL APPROACH

RNA-Seq and bioinformatics analysis were applied to predict the main target of liquorice. A machine learning model and a docking tool were used to predict active ingredients. Isotope labelling experiments, immunostaining, Western blots, qRT-PCR, ChIP-PCR and luciferase reporters were utilized to test the pharmacological effects in vitro and in vivo. The reverse effects were verified through recombination-based overexpression.

KEY RESULTS

The liposoluble constituents of liquorice improved muscle wasting by inhibiting protein catabolism and fibre atrophy. We further identified FoxO1 as the target of liposoluble constituents of liquorice. In addition, hispaglabridin B (HB) was predicted as an inhibitor of FoxO1. Further studies determined that HB improved muscle wasting by inhibiting catabolism in vivo and in vitro. HB also markedly suppressed the transcriptional activity of FoxO1, with decreased expression of the muscle-specific E3 ubiquitin ligases MuRF1 and Atrogin-1.

CONCLUSIONS AND IMPLICATIONS

HB can serve as a novel natural food extract for preventing muscle wasting in chronic kidney disease and possibly other catabolic conditions.

Rapamycin nanoparticles target defective autophagy in muscular dystrophy to enhance both strength and cardiac function

Duchenne muscular dystrophy in boys progresses rapidly to severe impairment of muscle function and death in the second or third decade of life. Current supportive therapy with corticosteroids results in a modest increase in strength as a consequence of a general reduction in inflammation, albeit with potential untoward long-term side effects and ultimate failure of the agent to maintain strength. Here, we demonstrate that alternative approaches that rescue defective autophagy in mdx mice, a model of Duchenne muscular dystrophy, with the use of rapamycin-loaded nanoparticles induce a reproducible increase in both skeletal muscle strength and cardiac contractile performance that is not achievable with conventional oral rapamycin, even in pharmacological doses. This increase in physical performance occurs in both young and adult mice, and, surprisingly, even in aged wild-type mice, which sets the stage for consideration of systemic therapies to facilitate improved cell function by autophagic disposal of toxic byproducts of cell death and regeneration.

Metabolic control and future opportunities for growth regulation

The last 10 years have seen a significant expansion in the scope of attempts to manipulate the growth of animals (Buttery, Lindsay and Haynes, 1986). The expansion of interest has been driven by a number of factors, both economic and theoretical. At the economic level the need to develop energetically and economically efficient strategies of animal production has been coupled with a renewed awareness of the implications for human health of excessive intakes of saturated fats. Emphasis then has switched from the maximization of weight gain as an end in itself towards a need to promote protein deposition at any given intake and, at the same time, to reduce the fat content of meat and meat products. These twin objectives might be achieved by one of three strategies: the promotion of protein deposition alone, because at any given rate of weight gain this will tend to minimize the rate of fat deposition (the so-called repartitioning effect); the reduction of fat gain (an approach that has received particularly close attention by those concerned primarily with human obesity); or ideally the simultaneous promotion of protein accretion and depression of that of fat.

The hormonal control of protein metabolism

While all the hormones described have regulatory effects on the rates of protein synthesis and breakdown there is a complex interaction between them in this control process. Insulin, GH and IGF-I play a dominant role in the day-to-day regulation of protein metabolism. In humans insulin appears to act primarily to inhibit proteolysis while GH stimulates protein synthesis. In the post-absorptive state IGF-I has acute insulin-like effects on proteolysis but in the fed state, or when substrate is provided for protein synthesis in the form of an amino acid infusion, IGF-I has been shown to stimulate protein synthesis. Growth hormone and testosterone have an important role during growth but continue to be required to maintain body protein during adulthood. Thyroid hormones are also required for normal growth and development. The hormones glucagon, glucocorticoids and adrenaline are all increased in catabolic states and may work in concert to increase protein breakdown in muscle tissue and to increase amino acid uptake in liver for gluconeogenesis. While increased glucocorticoids result in reduced muscle mass the effects of glucagon may be predominantly in the liver resulting in increased uptake of amino acids. In contrast to the catabolic effect of adrenaline on glucose and lipid metabolism, studies to date suggest that adrenaline may have an anti-catabolic effect on protein metabolism. Despite this adrenaline increases the production of the gluconeogenic amino acid alanine by muscle and its uptake by the splanchnic bed. There is considerable interest in the use of anabolic hormones, either alone or in combination, in the treatment of catabolic states. GH combined with insulin has been shown to improve whole-body and skeletal muscle kinetics while GH combined with IGF-I has a greater positive effect on protein metabolism in catabolic states than either hormone alone. If catabolic states are to be treated successfully a greater understanding of the role of the catabolic hormones in these states and the possible treatment of these states with anabolic hormones is required.

Differential regulation of the expression of fast-type sarcoplasmic-reticulum Ca(2+)-ATPase by thyroid hormone and insulin-like growth factor-I in the L6 muscle cell line

The aim of this study was to investigate the mechanism(s) underlying the thyroid-hormone (L-tri-iodothyronine, T3)-induced elevation of fast-type sarcoplasmic-reticulum Ca(2+)-ATPase (SERCA1) levels in L6 myotubes and the potentiating effect of insulin-like growth factor-I (IGF-I) [Muller, van Hardeveld, Simonides and van Rijn (1991) Biochem. J. 275, 35-40]. T3 increased the SERCA1 protein level (per microgram of DNA) by 160%. The concomitant increase in the SERCA1 mRNA level was somewhat higher (240%). IGF-I also increased SERCA1 protein (110%) and mRNA levels (50%), whereas IGF-I + T3 increased SERCA1 protein and mRNA levels by 410% and 380% respectively. These SERCA1 mRNA analyses show that the more-than-additive action of T3 and IGF-I on SERCA1 expression is, at least in part, pre-translational in nature. Further studies showed that the half-life of SERCA1 protein in L6 cells (17.5 h) was not altered by T3. In contrast, IGF-I prolonged the half-life of SERCA1 protein 1.5-1.9-fold, which may contribute to the disproportional increase in SERCA1 protein content compared with mRNA by IGF-I. Measurements of SERCA1 mRNA half-life (as determined by actinomycin D chase) showed no difference from the control values (15.5 h) in the presence of T3 or IGF-I alone. When T3 and IGF-I were both present, the SERCA1 mRNA half-life was prolonged 2-fold. No significant effects of T3 and IGF-I were observed on the half-life of total protein (37.4 h) and total RNA (37.0 h). The absence of an effect of T3 on SERCA1 protein and mRNA stability, when it was present alone, suggested transcriptional regulation, which was confirmed by nuclear run-on experiments, showing a 3-fold increase in transcription frequency of the SERCA1 gene by T3. We conclude that the synergistic stimulating effects of T3 and IGF-I on SERCA1 expression are the result of both transcriptional and post-transcriptional regulation. T3 acts primarily at the transcriptional level by increasing the transcription frequency of the SERCA1 gene, whereas IGF-I seems to act predominantly at post-transcriptional levels by enhancing SERCA1 protein and mRNA stability, the latter, however, only in the presence of T3.

Clenbuterol, a beta 2-agonist, retards wasting and loss of contractility in irradiated dystrophic mdx muscle

Treatment with the adrenergic beta 2-receptor agonist clenbuterol prevented, in dystrophic muscle from mdx mice, a pronounced loss of contractile strength that is observed after blockade of muscle regeneration with gamma irradiation. In addition, muscle mass and myosin content were greater (62-109%) in irradiated hindlimbs from clenbuterol-treated mdx mice, whereas the effects of the beta 2-agonist were relatively smaller (12-21%) in the nonirradiated hindlimbs. Together, these results suggest that beta 2-agonists can antagonize degenerative processes occurring in muscle fibers lacking dystrophin.

Cell calcium, oncogenes, and hypertrophy

The cellular mechanisms of cardiac hypertrophy remain unclear despite tantalizing clues gleaned from a variety of experimental approaches. Here we examine the hypothesis that an increase in cytosolic free Ca2+ concentration ([Ca2+]i) triggers the expression of proto-oncogenes, which in turn direct the characteristic increase in protein synthesis. New results from perfused ferret hearts are presented demonstrating that [Ca2+]i increases as a direct consequence of an elevation in perfusion pressure. It therefore seems plausible that [Ca2+]i constitutes the crucial link between the initial stimulus for hypertensive hypertrophy (elevated perfusion pressure) and the secondary alterations in gene expression. Nevertheless, further investigation will be required to establish whether changes in [Ca2+]i are necessary or sufficient to stimulate myocardial cell growth.

Energy expenditure associated with sodium/potassium transport and protein synthesis in skeletal muscle and isolated hepatocytes from hyperthyroid sheep

The object of the present study was to determine the effect of thyroxine (T4) treatment of sheep on protein synthesis and associated energy costs in skeletal muscle and hepatocytes. Protein synthesis, and ouabain-sensitive and cycloheximide-sensitive respiration in isolated intercostal muscle and hepatocytes were determined in sheep after 5 weeks of daily injections of either saline or T4. Plasma T4 and total triiodothyronine (T3) concentrations were doubled and free T3 concentrations were quadrupled by T4 injections. The fractional rates of protein synthesis increased in isolated external intercostal muscle and hepatocytes from hyperthyroid sheep. Fractional rates of protein synthesis in isolated external intercostal muscle and hepatocytes were linearly correlated with plasma free T3 concentrations. Total oxygen consumption of muscle and hepatocytes was unaffected by T4 injections. Ouabain-sensitive respiration increased in hepatocytes and muscle of T4-treated animals. Cycloheximide-sensitive respiration was elevated in hepatocytes from hyperthyroid sheep. Cycloheximide-sensitive respiration in muscle was unaffected by T4 treatment. The present experiment demonstrates that T4 increases protein synthesis in ruminants. The energy expenditure in support of Na+, K(+)-ATPase and protein synthesis in skeletal muscle and hepatocytes may account for 34-60% of total cellular energy expenditure.

Depressed translational activity in the androgen sensitive levator ani muscle of the rat

The androgen-dependent levator ani (LA) muscle of the rat provides a suitable model to explore the molecular mechanism of steroid hormone action in target tissues. The objective of the present series of experiments was to study the effect of gonadectomy (GDX) and androgen replacement therapy on the in vitro protein synthetic capacity of the LA muscle. The incorporation of labeled methionine into the contractile protein fraction of the LA muscle maintained in organ culture decreases in a time-dependent manner following GDX. Translation of total polyadenylated mRNA in the rabbit reticulocyte translation system revealed that the decrease in protein synthetic capacity was not associated with differences in the template activity of the mRNA derived from GDX tissue. However, when polyribosomes were used to direct the same in vitro synthesis system, a significant time-dependent loss of translational activity was observed following GDX. The polyribosomes of the LA muscle of control and GDX rats were shown to contain equivalent amount of rRNA and mRNA of comparable translation efficiency. Collectively the results of these experiments indicate that the decrease in protein synthetic capacity of the LA muscle in androgen deficient rats is due, in part, to a repression of the translation process associated to the functional integrity of polyribosomes.

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.

The effect of thyroxine on the calmodulin-dependent (Ca2+-Mg2+)ATPase activity and protein phosphorylation in rabbit fast skeletal muscle sarcolemma

Enzymatic properties and the protein pattern of sarcolemma fractions isolated from three groups of rabbits: euthyroid, hyperthyroid and hypothyroid, were studied. The amount of phosphorylated intermediate formed by the calmodulin-dependent (Ca2+-Mg2+)ATPase and the activity of this enzyme as well as that of (Na+-K+)ATPase were the highest in membranes isolated at the hyperthyroid state. On the other hand, sarcolemma obtained from the hypothyroid animals exhibited a decreased activity of (Na+-K+)ATPase, while the activity of calmodulin-dependent (Ca2+-Mg2+)ATPase was the same as in the preparations obtained from euthyroid animals. Thyroid hormones also changed the protein pattern of muscle sarcolemma. Membranes isolated from hyperthyroid animals lacked peptides of apparent molecular masses of 41 kDa and 53 kDa, while a peptide of the apparent molecular mass of 63 kDa was enriched in the preparation from hypothyroid animals. Thyroid hormones affected endogenous cAMP-dependent protein phosphorylation. The sarcolemma fraction obtained from hyperthyroid animals exhibited a decreased phosphorylation of peptides of apparent molecular masses of 30 kDa and 47 kDa, while the cAMP-independent phosphorylation of several other peptides was augmented. Moreover, sarcolemma preparations isolated from hyperthyroid animals showed higher activity of cAMP-independent protein kinase(s) and lower activity of cAMP-dependent protein kinase when compared to the euthyroid preparations. It is proposed that thyroxine increases the content of calmodulin-dependent (Ca2+-Mg2+)ATPase protein and affects the activity of cAMP-independent and cAMP-dependent protein kinases bound to sarcolemma.

Regulation of skeletal muscle myofibrillar protein degradation: relationships to fatigue and exercise

1. Exercise results in large alterations in cellular metabolic homeostasis and protein turnovers. Exhaustive exercise (as well as starvation, dystrophy, motor nerve disease) results in myofibrillar degradation and has been associated with the decreased force generating capabilities of muscle at fatigue. 2. Complete protein degradation is accomplished by the combined actions of non-lysosomal and lysosomal proteases and the initial breakdown of myofibrillar protein appears to be non-lysosomal mediated. 3. Current evidence suggests that covalent modification (mixed-function oxidation, formation of mixed disulfides, oxidation of methionine residues and phosphorylation) of proteins may mark them for degradation by rendering them more susceptible to proteolytic attack. 4. The rate of covalent modification can be controlled by the level of stabilizing and destabilizing ligands and by factors affecting the activity of the marking reaction. 5. The activities of individual proteases may be controlled by activators and inhibitors. 6. It is suggested that the large alterations in metabolism (hormonal profiles, energy status, redox status and Ca2+ levels) which accompany exercise serve to activate specific proteases and/or induce covalent modifications which mark specific myofibrillar proteins for subsequent proteolytic attack.