Regulation of protein turnover by recombinant human insulin-like growth factor-I in L6 myotube cultures

Amphioxus IGF-like peptide induces mouse muscle cell development via binding to IGF receptors and activating MAPK and PI3K/Akt signaling pathways

Insulin-like growth factors (IGFs) are identified in all vertebrates. An insulin/IGF hybrid polypeptide has also been identified in protochordate amphioxus. However, whether this hybrid polypeptide functions as vertebrate IGFs remains unknown. Here we established a primary culture system of mouse muscle satellite cells as an in vitro model to investigate the effects of amphioxus IGF-like molecule on muscle cell development. Like human IGF, recombinant IGF-like molecule was able to stimulate the proliferation of mouse muscle cells. Besides, it was able to bind to the cells and the partially purified IGF receptors from mouse muscle cells. Moreover, recombinant IGF-like molecule was capable of activating MAPK and PI3K/Akt pathways by stimulating phosphorylation of MAPK and Akt. This interaction of amphioxus IGF-like molecule with mammalian (mouse) IGF receptors and its induction of similar downstream signaling pathways add substantially to the hypothesis of the presence of IGF signaling in the common ancestor of protochordate/vertebrates.

Insulin-like growth factors and their binding proteins in domestic animals

Insulin-like growth factors (IGFs) and their binding proteins play an essential role in regulating animal growth and metabolism. The initial portion of the current review focuses on the physiological effects of the IGFs and delineates their role as regulators of animal growth and metabolism. The role of IGFs as mediators of growth hormone effects, as insulin-like metabolic regulators and as foetal growth regulators is discussed. The remainder of the review is devoted to the IGF binding proteins, their modulation of IGF action and their role in foetal and postnatal regulation of growth.

Muscle-specific overexpression of the type 1 IGF receptor results in myoblast-independent muscle hypertrophy via PI3K, and not calcineurin, signaling

The insulin-like growth factors (IGF-I and IGF-II), working through the type 1 IGF receptor (IGF-1R), are key mediators of skeletal muscle fiber growth and hypertrophy. These processes are largely dependent on stimulation of proliferation and differentiation of muscle precursor cells, termed myoblasts. It has not been rigorously determined whether the IGFs can also mediate skeletal muscle hypertrophy in a myoblast-independent fashion. Similarly, although the phosphatidylinositol 3-kinase (PI3K) and calcineurin signaling pathways have been implicated in skeletal muscle hypertrophy, these pathways are also involved in skeletal myoblast differentiation. To determine whether the IGFs can stimulate skeletal muscle hypertrophy in a myoblast-independent fashion, we developed and validated a retroviral expression vector that mediated overexpression of the human IGF-1R in rat L6 skeletal myotubes (immature muscle fibers), but not in myoblasts. L6 myotubes transduced with this vector accumulated significantly higher amounts of myofibrillar proteins, in a ligand- and receptor-dependent manner, than controls and demonstrated significantly increased rates of protein synthesis. Stimulation of myotube hypertrophy was independent of myoblast contributions, inasmuch as these cultures did not exhibit increased levels of myoblast proliferation or differentiation. Experiments with PI3K and calcineurin inhibitors indicated that myoblast-independent myotube hypertrophy was mediated by PI3K, but not calcineurin, signaling. This study demonstrates that IGF can mediate skeletal muscle hypertrophy in a myoblast-independent fashion and suggests that muscle-specific overexpression of the IGF-1R or stimulation of its signaling pathways could be used to develop strategies to ameliorate muscle wasting without stimulating proliferative pathways leading to carcinogenesis or other pathological sequelae.

Isolation and validation of human prepubertal skeletal muscle cells: maturation and metabolic effects of IGF-I, IGFBP-3 and TNFalpha

We have developed a primary skeletal muscle cell culture model derived from normal prepubertal children to investigate the effects of insulin-like growth factor-I (IGF-I), insulin-like growth factor binding protein-3 (IGFBP-3) and tumour necrosis factor alpha (TNFalpha) on growth, differentiation and metabolism. Cells of myoblast lineage were characterized morphologically by desmin staining and differentiated successfully into multinucleated myotubes. Differentiation was confirmed biochemically by an increase in creatine kinase (CK) activity and IGFBP-3 secretion over time. IGF-I promoted whilst TNFalpha inhibited myoblast proliferation, differentiation and IGFBP-3 secretion. IGF-I partially rescued the cells from the inhibiting effects of TNFalpha. Compared to adult myoblast cultures, children's skeletal muscle cells demonstrated higher basal and day 7 CK activities, increased levels of IGFBP-3 secretion, diminished IGF-I/TNFalpha action and absence of the inhibitory effect of exogenous IGFBP-3 on differentiation. Additional studies demonstrated that TNFalpha increased basal glucose transport via GLUT1, nitric oxide synthase and p38MAPK-dependent mechanisms. These studies provide baseline data to study the interactivity effects of growth factors and cytokines on differentiation and metabolism in muscle in relation to important metabolic disorders such as obesity, type II diabetes or chronic wasting diseases.

In vitro effects of soy phytoestrogens on rat L6 skeletal muscle cells

Soy isoflavones display estrogenic activity in humans and animals, and thus are referred to as phytoestrogens. This study was performed to observe the effects of the soy isoflavones genistein, daidzein, and glycitein on cell cultures of rat skeletal muscles. [3H]Thymidine incorporation was used to determine cell proliferation, while protein synthesis and degradation were determined by tracking radiolabeled leucine. For the proliferation studies, insulin, estradiol, genistein, daidzein, or glycitein was supplemented at 0, 0.04, 0.08, 0.16, 0.31, 0.63, 1.25, 2.5, 5, 10, or 20 microM, respectively, or in combinations with final concentrations of 0, 0.1, 1, or 10 microM. Genistein reacted most similarly to estradiol, inhibiting proliferation at > or = 1 microM (P < .001). A combination of phytoestrogens resulted in significant inhibition of cell proliferation, but not to the extent observed with genistein alone. For the protein synthesis and degradation experiments, treatments of 0.1 microM dexamethasone or 1 microM concentrations of insulin, genistein, daidzein, or glycitein were used. Phytoestrogens did not inhibit or stimulate protein degradation or synthesis (P > .05). A one-tailed univariate analysis of variance revealed a trend (P < or = .1) in protein stimulation with genistein and glycitein treatments. These results suggest that the tyrosine kinase inhibiting activity of genistein may be affecting phosphorylation of the mitosis-promoting factor, preventing the advancement of the mitotic cell cycle. In addition, at higher total combined concentrations, daidzein and glycitein may be able to outcompete genistein for receptor sites. These results suggest that soy isoflavones in the diet may potentially modulate normal growth and development in humans and animals that ingest soy-based products.

Basic principles of muscle development and growth in meat-producing mammals as affected by the insulin-like growth factor (IGF) system

This presentation aims to describe how the basic events in prenatal muscle development and postnatal muscle growth are controlled by the insulin-like growth factor system (IGF). The prenatal events (myogenesis) cover the rate of proliferation, the rate and extent of fusion, and the differentiation of three myoblast populations, giving rise to primary fibers, secondary fibers, and a satellite cell population, respectively. The number of muscle fibers, a key determinant of the postnatal growth rate, is fixed late in gestation. The postnatal events contributing to myofiber hypertrophy comprise satellite cell proliferation and differentiation, and protein turnover. Muscle cell cultures produce IGFs and IGF binding proteins (IGFBPs) in various degrees depending on the origin (species, muscle type) and state of development of these cells, suggesting an autocrine/paracrine mode of action of IGF-related factors. In vivo studies and results based on cell lines or primary cell cultures show that IGF-I and IGF-II stimulate both proliferation and differentiation of myoblasts and satellite cells in a time and concentration-dependent way, via interaction with type I IGF receptors. However, IGF binding proteins (IGFBP) may either inhibit or potentiate the stimulating effects of IGFs on proliferation or differentiation. During postnatal growth in vivo or in fully differentiated muscle cells in culture, IGF-I stimulates the rate of protein synthesis and inhibits the rate of protein degradation, thereby enhancing myofiber hypertrophy. The possible roles and actions of the IGF system in regulating and determining muscle growth as affected by developmental stage and age, muscle type, feeding levels, treatment with growth hormone and selection for growth performance are discussed.

Insulin-Like Growth Factor-I Blocks Dexamethasone-Induced Protein Degradation in Cultured Myotubes by Inhibiting Multiple Proteolytic Pathways

In previous studies, insulin-like growth factor-I (IGF-I) inhibited glucocorticoid-induced muscle protein breakdown, but the intracellular mechanisms of this effect of IGF-I are not well understood. The purpose of the present study was to test the hypothesis that IGF-I inhibits multiple proteolytic pathways in dexamethasone-treated cultured L6 myotubes. Myotubes were treated with 1 microM dexamethasone for 6 hours in the absence or presence of 0.1 microg/ml of IGF-I. Protein degradation was determined by measuring the release of trichloroacetic acid-soluble radioactivity from proteins prelabeled with 3H-tyrosine. The contribution of lysosomal, proteasomal-dependent, and calpain-dependent proteolysis to the inhibitory effect of IGF-I on protein degradation was assessed by using inhibitors of the individual proteolytic pathways (methylamine, beta-lactone, and E64, respectively). In addition, the influence of IGF-I on cathepsin B, proteasome, and calpain activities was determined. Treatment of L6 myotubes with dexamethasone resulted in an approximately 20% increase in protein degradation. This effect of dexamethasone was completely blocked by IGF-I. When the different protease inhibitors were used, results showed that IGF-I inhibited lysosomal, proteasomal-dependent, and calpain-dependent proteolysis by 70, 44, and 41%, respectively. Additionally, IGF-I blocked the dexamethasone-induced increase in cathepsin B, proteasome, and calpain activities. The present results suggest that IGF-I inhibits glucocorticoid-induced muscle proteolysis by blocking multiple proteolytic pathways.

Genetic parameter estimates for serum insulin-like growth factor-I concentration and ultrasound measurements of backfat thickness and longissimus muscle area in Angus beef cattle

A divergent selection experiment for serum IGF-I concentration began at the Eastern Ohio Resource Development Center in 1989 using 100 spring-calving (50 high line and 50 low line) and 100 fall-calving (50 high line and 50 low line) purebred Angus cows. Following weaning, bull and heifer calves were fed in drylot for a 140-d period. Real-time ultrasound measurements of backfat thickness and longissimus muscle area were taken on d 56 and 140 of the postweaning test. Only ultrasound data from calves born from fall 1995 through spring 1999 were included in the analysis. At the time of this study, IGF-I measurements were available for 1,521 bull and heifer calves, and ultrasound data were available for 636 bull and heifer calves. Data were analyzed by multiple-trait, derivative-free, restricted maximum likelihood methods. Estimates of direct heritability for IGF-I concentration at d 28, 42, and 56 of the postweaning period, and for mean IGF-I concentration were 0.26 +/- 0.07, 0.32 +/- 0.08, 0.26 +/- 0.07, and 0.32 +/- 0.08, respectively. Direct heritabilities for ultrasound estimates of backfat thickness ranged from 0.17 +/- 0.11 to 0.28 +/- 0.12, whereas direct heritabilities for longissimus muscle area ranged from 0.20 +/- 0.10 to 0.36 +/- 0.12, depending on the time of measurement and the covariate used for adjustment (age vs. weight). Direct genetic correlations of IGF-I concentrations with backfat thickness at d 56 and 140 and with longissiumus muscle area at d 56 and 140 averaged 0.02, 0.20, -0.08, and 0.23, respectively, when age was used as the covariate for both IGF-I and ultrasound measurements. Corresponding genetic correlations when age was used as the covariate for IGF-I and weight was used as the covariate for ultrasound measurements were 0.05, -0.07, -0.22, and -0.04, respectively. Therefore, the positive associations of serum IGF-I concentration with backfat thickness and longissimus muscle area at d 140 seem to have been partially mediated by weight. Results of this study do not indicate strong associations of serum IGF-I concentration with fat thickness or muscling of bulls and heifers during the postweaning feedlot period.

Overexpression of interleukin-15 induces skeletal muscle hypertrophy in vitro: implications for treatment of muscle wasting disorders

Interleukin-15 (IL-15) is a novel anabolic factor for skeletal muscle which inhibits muscle wasting associated with cancer (cachexia) in a rat model. To develop a cell culture system in which the mechanism of the anabolic action of IL-15 on skeletal muscle could be examined, the mouse C2 skeletal myogenic cell line was transduced with a retroviral expression vector for IL-15 and compared to sister cells transduced with a control vector. Overexpression of IL-15 induced fivefold higher levels of sarcomeric myosin heavy chain and alpha-actin accumulation in differentiated myotubes. Secreted factors from IL-15-overexpressing myogenic cells, but not from control cells, induced increased myofibrillar protein accumulation in cocultured control myotubes. IL-15 overexpression induced a hypertrophic myotube morphology similar to that described for cultured myotubes which overexpressed the well-characterized anabolic factor insulin-like growth factor-I (IGF-I). However, in contrast to IGF-I, the hypertrophic action of IL-15 on skeletal myogenic cells did not involve stimulation of skeletal myoblast proliferation or differentiation. IL-15 induced myotube hypertrophy at both low and high IGF-I concentrations. Furthermore, in contrast to IGF-I, which stimulated only protein synthesis under these culture conditions, IL-15 both stimulated protein synthesis and inhibited protein degradation in cultured skeletal myotubes. These findings indicate that IL-15 action on skeletal myogenic cells is distinct from that of IGF-I. Due to the ability of IGF-I to stimulate cell division and its association with several forms of cancer, controversy exists concerning the advisability of treating cachexia or age-associated muscle wasting with IGF-I. Administration of IL-15 or modulation of the IL-15 signaling pathway may represent an alternative strategy for maintaining skeletal muscle mass under these conditions.

Muscle protein synthesis rate is altered in response to a single injection of insulin-like growth factor-I in seven-day-old Leghorn chicks

To determine if a single injection of insulin-like growth factor-I (IGF-I) can affect muscle protein synthesis in chickens, 7-d-old male Single Comb White Leghorn chicks were injected s.c. with physiological saline (control) or 35 microg of recombinant human IGF-I. After 2 h 30 min, or 6, 12, or 24 h the chicks were injected with 3H-phenylalanine and killed, and the fractional synthesis rate (Ks) of breast muscle protein was measured. The Ks of IGF-I-treated birds were lower (P = 0.03) than controls at 2 h 30 min post-injection, higher (P = 0.07) than controls at 6 h post-injection, but not different from controls at later times. A second experiment examined serum changes during the 6 h after chicks were injected with IGF-I or saline as in the first experiment. Serum IGF-I concentration increased relative to almost undetectable levels (1 ng/mL) of controls to 216 +/- 59 ng/mL at 20 min after IGF-I injection (P < 0.001) and decreased to 12 +/- 6 ng/ mL by 6 h. Serum glucose and nonprotein nitrogen concentrations were significantly decreased for all or most of the 3 h after IGF-I injection, respectively, but only glucose concentration was the same as controls by 6 h. Low serum glucose and nonprotein nitrogen during the first few hours after IGF-I injection may contribute to the inhibition of Ks at 2.5 h, but the mechanisms behind the increased Ks at 6 h are not clear. These results support a role for IGF-I in the posthatching muscle development of chicks.

Insulin-like growth factor-I inhibits lysosomal and proteasome-dependent proteolysis in skeletal muscle after burn injury

Previous studies suggest that insulin-like growth factor-I (IGF-I) inhibits burn-induced muscle wasting mainly by reducing muscle protein degradation. The intracellular mechanisms of this effect of IGF-I are not known. In the present study, we examined the influence of IGF-I on individual proteolytic pathways in muscles from burned rats. Extensor digitorum longus muscles from burned rats were incubated with specific blockers of lysosomal, calcium-calpain-dependent, and ubiquitin-proteasome-dependent proteolytic pathways in the absence or presence of IGF-I. In addition, cathepsin B and L activities and 20S proteasome activity were determined. IGF-I inhibited lysosomal and ubiquitin-proteasome-dependent protein breakdown in skeletal muscle from burned rats by 70 and 90%, respectively, but did not influence calcium-calpain-dependent protein breakdown. The hormone blocked the burn-induced increase in cathepsin B and L activities but did not reduce 20S proteasome activity. Results are important because they provide novel information about intracellular mechanisms by which IGF-I inhibits the catabolic response to burn injury in skeletal muscle.

Gene expression in sexually dimorphic muscles in sheep

Testosterone is known to act differentially on skeletal muscle from different regions of the body. Two genes likely to mediate the testosterone effect are insulin-like growth factor I (IGF-I), an important growth regulator acting in an autocrine and paracrine way, and androgen receptor (AR), because receptor density could account for differential muscle growth. Another muscle-specific gene that may play a role in differential muscle growth is myostatin, a member of the transforming growth factor-beta superfamily, shown to be a negative regulator of skeletal muscle mass. The objective of this study was to quantify and compare the steady state expression of these three genes in two different skeletal muscles in sheep. Eleven Dorset rams were slaughtered after reaching puberty and total RNA was extracted from samples of semitendinosus and splenius muscles. Insulin-like growth factor I mRNA was measured using a competitive reverse-transcription-polymerase chain reaction. Androgen receptor and myostatin mRNA were measured by a ribonuclease protection assay (RPA) with standard curves. The means (attomoles/microg RNA) for splenius and semitendinosus muscles were 1.39 and 1.02 (SE = 0.14), 4.05 and 2.96 (SE = 0.24), and 4.30 and 3.85 (SE = 0.37) for IGF-I, AR, and myostatin, respectively. The difference between the two muscles was significant for IGF-I and AR mRNA levels with higher levels in the splenius but not significant for myostatin. Our results show that locally produced IGF-I and the regulation of AR expression may be important for sexually dimorphic muscle growth patterns.

Role of IGF-I and IGF-binding proteins within diaphragm muscle in modulating the effects of nandrolone

Recent studies suggest that the anabolic effects of testosterone in muscle may be mediated, in part, by the insulin-like growth factor (IGF) system. The aim of this study was to examine the effects of nandrolone (NAN) on both IGF-I and IGF-binding proteins (IGFBPs) in the diaphragm muscle of 1-yr-old female rats. NAN (6.6 mg. kg(-1) x day(-1)) was infused continuously for 17 days using a subcutaneous Silastic implant, whereas controls (CTL) received blank capsules. Muscle fibers were classified immunohistochemically, and fiber cross-sectional areas (CSA) were determined quantitatively. IGF-I levels in both serum and muscle were determined by RIA. Immunoreactivity to an IGF-I antibody was used to localize IGF-I expression within individual muscle fibers. Muscle IGFBPs were determined by SDS-PAGE and Western ligand blotting and measured by scanning densitometry. Body weight was higher in the NAN group compared with CTL (9.4 +/- 4.5% vs. -0.6 +/- 3.1%). There were no changes in the fiber composition of the diaphragm. NAN increased the CSA of type IIa (20%) and type IIx/b (30%) diaphragm fibers. Levels of IGF-I in the diaphragm muscle were significantly higher (50%) in NAN-treated animals. Immunohistochemistry revealed increased localization of IGF-I within type IIx/b diaphragm fibers. In addition, NAN increased IGFBP-3 within the diaphragm (69%), whereas IGFBP-4 decreased (40%). We conclude that NAN-induced diaphragm muscle fiber hypertrophy is mediated, in part, by influences of the IGF system within the muscle, such that coordinated changes in IGFBPs reflect a direction of change that has been associated with an anabolic response in other test systems.

Insulin-like growth factor I reduces ubiquitin and ubiquitin-conjugating enzyme gene expression but does not inhibit muscle proteolysis in septic rats

We examined the effect of insulin-like growth factor I (IGF-I), administered in vivo, on protein turnover rates and gene expression of the ubiquitin-proteasome proteolytic pathway in skeletal muscle of septic rats. Sepsis was induced by cecal ligation and puncture. Other rats were sham-operated. Miniosmotic pumps were implanted sc, and groups of rats received IGF-I (7 mg/kg x 24 h) or saline. Protein synthesis and breakdown rates were determined in incubated extensor digitorum longus muscles. Messenger RNA levels for ubiquitin and the ubiquitin-conjugating enzyme E2(14k) were determined by Northern blot analysis. Sepsis resulted in an approximately 30% reduction of muscle protein synthesis, and this effect of sepsis was blunted in rats treated with IGF-I. In contrast, IGF-I did not prevent the sepsis-induced increase in total and myofibrillar muscle protein breakdown. Ubiquitin and E2(14k) messenger RNA levels were increased several fold in muscle from septic rats, and this effect of sepsis was abolished in IGF-I treated rats. The results suggest that administration of IGF-I may improve sepsis-induced muscle cachexia by stimulating protein synthesis. However, because muscles were resistant to IGF-I, with regard to regulation of protein breakdown, the use of IGF-I to treat muscle cachexia during sepsis remains unclear. An additional important implication of the present study is that changes in messenger RNA levels for ubiquitin and the ubiquitin-conjugating enzyme E2(14k) do not always reflect changes in muscle protein breakdown rates.

Treatment of burned rats with insulin-like growth factor I inhibits the catabolic response in skeletal muscle

Thermal injury is associated with a pronounced catabolic response in skeletal muscle, reflecting inhibited protein synthesis and increased protein breakdown, in particular myofibrillar protein breakdown. Administration of insulin-like growth factor I (IGF-I) has a nitrogen-sparing effect after burn injury, but the influence of this treatment on protein turnover rates in skeletal muscle is not known. In the present study, we examined the effect of IGF-I on muscle protein synthesis and breakdown rates following burn injury in rats. After a 30% total body surface area burn injury or sham procedure, rats were treated with a continuous infusion of IGF-I (3. 5 or 7 mg . kg-1 . 24 h-1) for 24 h. Protein synthesis and breakdown rates were determined in incubated extensor digitorum longus muscles. Burn injury resulted in increased total and myofibrillar protein breakdown rates and reduced protein synthesis in muscle. The increase in protein breakdown rates was blocked by both doses of IGF-I and the burn-induced inhibition of muscle protein synthesis was partially reversed by the higher dose of the hormone. IGF-I did not influence muscle protein turnover rates in nonburned rats. The results suggest that the catabolic response to burn injury in skeletal muscle can be inhibited by IGF-I.

IGF-I stimulates protein synthesis but does not inhibit protein breakdown in muscle from septic rats

Sepsis is associated with reduced protein synthesis and increased protein degradation in skeletal muscle. We examined the effects of insulin-like growth factor I (IGF-I) on protein synthesis and breakdown in muscles from nonseptic and septic rats. Sepsis was induced by cecal ligation and puncture; control rats were sham operated. Extensor digitorum longus muscles were incubated in the absence or presence of IGF-I at concentrations ranging from 100 ng/ml to 10 micrograms/ml. Total and myofibrillar protein breakdown rates were measured as net release of tyrosine and 3-methylhistidine, respectively. Protein synthesis was determined by measuring incorporation of [U-14C]phenylalanine into protein. IGF-I stimulated protein synthesis in a dose-dependent fashion in muscles from both sham-operated and septic rats, with a maximal effect seen at a hormone concentration between 500 and 1,000 ng/ml. IGF-I inhibited total and myofibrillar protein breakdown in muscles from sham-operated rats, whereas in muscles from septic rats, IGF-I had no effect on protein breakdown, even at high concentrations. The results suggest that protein breakdown in skeletal muscle becomes resistant to IGF-I during sepsis and that this resistance reflects a postreceptor defect.

Insulin-like growth factor 1 stimulates protein synthesis and inhibits protein breakdown in muscle from burned rats

BACKGROUND

Burn injury is associated with substantial whole-body protein loss, reflecting mainly a catabolic response in skeletal muscle. Recent studies suggest that treatment with insulin-like growth factor 1 (IGF-1) may reverse the catabolic response to burn injury, but the effects of IGF-1 on muscle protein synthesis and breakdown rates after burn injury are not known. We tested the hypothesis that IGF-1 blunts the catabolic response in skeletal muscle after burn injury by stimulating protein synthesis and inhibiting protein breakdown and that this effect of IGF-1 is caused by a direct effect on muscle tissue.

METHODS

Intact extensor digitorum longus muscles from burned, sham-burned, and untreated rats were incubated in the absence or presence of different concentrations of IGF-1. Total and myofibrillar protein breakdown rates were measured as net release of tyrosine and 3-methylhistidine, respectively. Protein synthesis rates were determined by measuring the incorporation of (U-14C)-phenylalanine into protein.

RESULTS

IGF-1 stimulated protein synthesis and inhibited protein breakdown in a dose-dependent fashion in muscles from burned and unburned rats. The maximal effect of IGF-1 on protein synthesis was seen at a hormone concentration of 100 ng/mL, whereas protein breakdown was further inhibited when the hormone concentration was increased to 1 microgram/mL. Ubiquitin messenger RNA (mRNA) levels were reduced by IGF-1 in incubated muscles, suggesting that IGF-1 may inhibit ubiquitin-dependent protein breakdown.

CONCLUSIONS

These results suggest that the anabolic effects of IGF-1 after burn may reflect inhibited protein breakdown and stimulated protein synthesis in skeletal muscle and that this response may be caused by a direct effect of IGF-1 on muscle tissue.

Proliferation precedes differentiation in IGF-I-stimulated myogenesis

The insulin-like growth factors (IGFs) have dramatic and complex effects on the growth of many tissues and have been implicated in both the proliferation and differentiation of skeletal muscle cells. A detailed analysis of gene expression was performed in L6E9 myoblast cultures treated with IGF-I to dissect the early events leading to the stimulation of myogenic differentiation by this growth factor. A time course of transcript accumulation in confluent L6E9 myoblasts treated with defined media containing IGF-I revealed an initial transient decrease in myogenic factors, accompanied by an increase in cell cycle markers and cell proliferation. This pattern was reversed at later time points, when the subsequent activation of myogenic factors resulted in a net increase in structural gene expression and larger myotubes. The data presented here support the hypothesis that IGF-I activates proliferation first, and subsequently stimulates events leading to the expression of muscle-specific genes in myogenic cell cultures.

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.

Altered physiology and action of insulin-like growth factor 1 in skeletal muscle in chronic renal failure

Chronic renal failure is associated with abnormalities of skeletal muscle that include reduction in size, fibrosis, protein depletion, and functional disorders. These disorders may be caused by several factors, including protein-calorie malnutrition, acidemia, and superimposed illnesses. However, animal research suggests that these abnormalities may occur with chronic renal failure per se in the absence of the above complications. Insulin-like growth factor 1 (IGF-1) is an anabolic hormone that, in skeletal muscle, stimulates intracellular amino acid and glucose transport and protein synthesis, suppresses protein degradation, and causes hypertrophy. In chronic renal failure, there is evidence for inhibition of the actions of IGF-1. Studies in humans with chronic renal failure given a subcutaneous injection of recombinant human IGF-1 (rhIGF-1) indicate that the rhIGF-1 induced acute suppression of plasma amino acids, insulin, and C peptide is impaired. In skeletal muscle of rats with chronic renal failure, we observed reduced IGF-1 and IGF-1 mRNA levels, resistance to the rhIGF-1-induced suppression of protein degradation, and stimulation of protein synthesis, increased IGF-1 receptor mRNA and IGF-1 receptor number and impaired receptor tyrosine kinase activity. These findings suggest that in skeletal muscle in chronic renal failure there are several abnormalities in the physiology of IGF-1 and the sensitivity to IGF-1. It is possible that these alterations contribute to the disorders of skeletal muscle structure and function in chronic renal failure. Notwithstanding these abnormalities, repeated subcutaneous injections of rhIGF-1 in malnourished patients undergoing continuous ambulatory peritoneal dialysis led to strongly positive nitrogen balance that was sustained for the 20 days of study.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of prednisone on protein metabolism in Duchenne dystrophy

Prednisone improves strength in Duchenne dystrophy and changes the natural history of the disease. We studied the in vivo effects of prednisone (0.75 mg.kg-1.day-1) on muscle and whole body protein metabolism in six patients with Duchenne dystrophy and three patients with Becker dystrophy. Patients were admitted to the Clinical Research Center for study and consumed a constant flesh-free diet. Strength was measured by manual and quantitative muscle testing. Fractional muscle protein breakdown was estimated by the ratio of 3-methylhistidine to creatinine excretion determined in three consecutive 24-h urine collections. Whole body protein kinetics were studied in the postabsorptive state using a primed continuous infusion of L-[1-13C]leucine. Fractional muscle protein synthesis was determined from tracer incorporation into noncollagen muscle protein obtained by needle biopsy. After 6-8 wk of prednisone treatment, average muscle strength increased by 15% (P < 0.04), and 24-h creatinine excretion (an index of muscle mass) increased by 21% (P = 0.002). 3-Methylhistidine excretion decreased by 10%, but the change was not statistically significant. The ratio of 3-methylhistidine to creatinine excretion decreased by 26% (P < 0.04). Fractional muscle protein synthesis and whole body protein synthesis and breakdown did not change significantly. We conclude that the beneficial effect of prednisone on strength in Duchenne dystrophy appears to be associated with an increase in muscle mass, which may be mediated by inhibition of muscle proteolysis rather than stimulation of muscle protein synthesis.

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.

Effects of insulin-like growth factor-I (IGF-I), insulin and combined IGF-I-insulin infusions on protein metabolism in dogs

The effect of infusions of recombinant insulin-like growth factor-I (IGF-I) (34, 103 or 688 pmol min-1 kg-1), insulin (3.4, 10.3 or 68.8 pmol min-1 kg-1) or combined infusions (34 pmol IGF-I + 3.4 pmol min-1 kg-1 insulin or 103 pmol IGF-I + 3.4 pmol min-1 kg-1 insulin) on protein metabolism, using an infusion of [1-14C]leucine was investigated in anaesthetized fasted dogs. Leucine concentration, production rate (measure of protein degradation), oxidation rate and non-oxidative disappearance rate (measure of protein synthesis) were decreased in a similar dose dependent manner by the IGF-I and insulin infusions (P < 0.01). The decrease in these measurements of leucine metabolism were greater following 34 pmol IGF-I + 3.4 pmol insulin than with either component infused alone (P < 0.05). Free fatty acid concentrations were decreased by all insulin doses (P < 0.01) but only by 103 and 688 pmol min-1 kg-1 insulin-like growth factor (P < 0.05, P < 0.01). These data demonstrate that IGF-I, like insulin, has a dose dependent effect on protein metabolism and that combined insulin and IGF-I infusions have additive effects on protein metabolism.

Regulation of amino acid transport and protein metabolism in myotubes derived from chicken muscle satellite cells by insulin-like growth factor-I

The effects of insulin and insulin-like growth factor-I (IGF-I) on amino acid transport and protein metabolism were compared in myotubes derived from chicken breast muscle satellite cells. Protein synthesis was assessed by continuous labelling with [3H]-tyrosine. Protein degradation was estimated by the release of trichloroacetic acid (TCA) soluble radioactivity by cells which had been previously labelled with [3H]-tyrosine for 3 days. Amino acid transport was measured in myotubes incubated in Dulbecco's modified Eagle's medium (DMEM) 0.5% bovine serum albumin (BSA) with or without insulin or IGF-I. Subsequent [3H]-aminoisobutyric acid (AIB) uptake was then measured in amino acid-free medium. IGF-I was more efficient than insulin at equimolar concentration (3.2 nmol/l) in stimulating protein synthesis (127 and 113% of basal, respectively) and inhibiting protein degradation (32% and 13% inhibition of protein degradation following 4 h incubation). Half maximal effective concentrations for stimulation of AIB uptake were 0.27 +/- 0.03 nmol/l and 34.8 +/- 3.1 nmol/l for IGF-I and insulin respectively, with maximal stimulation of about 340% of basal. Cycloheximide (3.6 mumol/l) diminished IGF-I-stimulated AIB uptake by 55%. Chicken growth hormone had no effect on basal AIB uptake in these cells and neither glucagon nor dexamethasone had an effect on basal or IGF-I-stimulated AIB uptake. This study demonstrates an anabolic effect for IGF-I in myotubes derived from primary chicken satellite cells which is mediated by the type I IGF receptor, since the cation-independent mannose 6-phosphate receptor does not bind IGF-II in chicken cells.

Hyposomatomedinemia in quadriplegic men

Many studies have shown that vigorous exercise acutely stimulates growth hormone (GH) release but the relative contribution of daily physical activity to maintaining the GH/somatomedin C (SmC) axis is not known. It has been reported that basal and post-exercise plasma SmC values are higher in physically conditioned young men than in sedentary men of similar age. To assess the effect of severe inactivity on the plasma SmC level, basal concentrations of this hormone were measured in patients with quadriplegia (QP) resulting from spinal cord injury (SCI). Venous blood samples were obtained after overnight fast in 41 QP men, ages 24-66, and compared with 119 healthy men of similar ages. Nonparametric analysis of variance showed SmC to be significantly lower in QP than in healthy men (p < .007). Plasma SmC below 0.35 U/ml in adults usually indicates little or no GH secretion by the pituitary gland. In QP, 46% of plasma SmC values were < 0.35 U/ml compared to 24% in the healthy group (p < .02). In both groups, an inverse relationship of SmC and increasing age was observed (p < .01). The data suggest that severe inactivity or SCI tend to cause hyposomatomedinemia. The latter endocrine alteration may contribute to the decrease in lean body mass and muscle atrophy of QP patients, and add further functional impairment to the original neurologic deficit. In addition, hyposomatomedinemia could increase the tendency for pressure sore formation and osteoporosis in SCI patients.

Effects of recombinant human growth hormone in patients with severe sepsis

The objective of this study was to evaluate the safety and the effect of recombinant exogenous growth hormone (GH) on nitrogen production in patients with severe sepsis. It was designed as a prospective, randomized, placebo-controlled trial, and performed in the medical intensive care unit of a university hospital. Twenty patients admitted with septic shock and receiving standard parenteral nutrition served as subjects. Treatment consisted of GH 0.1 mg/kg/day or placebo administered as continuous intravenous infusion on the second, third, and fourth days after admission. The study period was eight days. During GH administration, nitrogen production decreased significantly in the GH group and increased in controls (p < 0.01). Nitrogen balance became slightly positive in the GH group during treatment: 1.2 +/- 6.4 versus controls -3.7 +/- 3.8 g/day (day 3) (p < 0.05). Within 24 hours after cessation of treatment, differences between GH and controls disappeared. 3-Methylhistidine excretion as a measure of absolute muscle breakdown declined during the study period, but did not differ between groups. The levels of insulin, insulinlike growth factor 1, glycerol, free fatty acids, and beta-hydroxybutyrate increased during treatment. Despite continuous intravenous administration, GH levels gradually declined during the 3 treatment days, indicating increased metabolic clearance. Side effects other than insulin resistance were not observed. Growth hormone administration reduces nitrogen production and improves nitrogen balance in patients with severe sepsis. These effects are not sustained after cessation of treatment.

Effects of recombinant IGF-I on protein and glucose metabolism in rTNF-infused lambs

Achieving nitrogen accretion in patients with critical surgical illness or cancer cachexia is often not possible by the simple provision of calories and nitrogen. Cachexia may result from the metabolic derangements caused by release of inflammatory mediators such as tumor necrosis factor (TNF). We wished to determine whether recombinant human insulin-like growth factor I (rhIGF-I) preserves its protein-sparing effects in the face of high plasma TNF concentrations. Primed constant infusions of [15N]urea and [6-3H]glucose tracers were used to measure protein and glucose kinetics in fasted lambs. The lambs were divided into four groups: two groups received normal saline infusions of 480 min, and two groups received recombinant TNF (rTNF) infusions of 1 microgram.kg-1.h-1. During the last 300 min, one of the normal saline and one of the rTNF-infused groups were infused with rhIGF-I at a dose of 50 micrograms.kg-1.h-1. rTNF infusion resulted in the lambs becoming febrile and significantly increased plasma cortisol, glucagon, and insulin levels. rhIGF-I infusion in the control animals reduced the rate of loss of protein by 15% (P less than 0.01) and increased the rate of peripheral glucose clearance by 55% (P less than 0.01). rhIGF-I infusion in the rTNF-treated animals reduced the rate of net protein loss by 15% (P less than 0.01) and caused similar changes in glucose kinetics, as were observed in the control animals. We conclude that as rhIGF-I preserves its protein anabolic action in the face of high rTNF levels, further investigation into a possible clinical role for rhIGF-I in severe surgical illness is warranted.

The effects of infusion of insulinlike growth factor (IGF) I, IGF-II, and insulin on glucose and protein metabolism in fasted lambs

In vivo effects of 300-min infusions of recombinant insulinlike growth factor I (IGF-I) and IGF-II on glucose and protein metabolism have been investigated in awake, fasted lambs. Two doses of recombinant human (rh) IGF-I were infused: 6.7 nmol/kg.h, which induced hypoglycemia, and 2.0 nmol/kg.h, which did not. The effects were compared with an insulin infusion (0.17 nmol/kg.h) that had the same hypoglycemic potential as the high dose rhIGF-I infusion. rhIGF-II was infused at a rate of 6.7 nmol/kg.h. Primed constant infusions of isotopically labeled glucose, urea and leucine tracers were used to determine glucose and protein kinetics. rhIGF-I lowered blood glucose by increasing the rate of glucose clearance (P less than 0.01), in contrast to insulin, which both increased clearance and reduced glucose production. Net protein loss was reduced after infusion of low and high dose rhIGF-I and insulin by 11% (P less than 0.05), 15% (P less than 0.01), and 12% (P less than 0.05), respectively. rhIGF-II infusion did not alter the rate of net protein loss. In contrast to insulin, high dose rhIGF-I infusion increased the rate of protein synthesis in skeletal (P less than 0.05) and cardiac muscle (P less than 0.01) and in hepatic tissue (P less than 0.05). We conclude that (a) protein metabolism is more sensitive than glucose metabolism to rhIGF-I infusion, as protein loss was reduced by an rhIGF-I infusion that did not alter glucose kinetics; (b) protein synthesis is increased by rhIGF-I infusion but not by insulin infusion; and (c) rhIGF-II is a less effective anabolic agent than rhIGF-I. We speculate that the effects of rhIGF-I on protein metabolism are not mediated by insulin receptors.

Insulin and IGF-I induce pronounced hypertrophy of skeletal myofibers in tissue culture

Skeletal myofibers differentiated from primary avian myoblasts in tissue culture can be maintained in positive nitrogen balance in a defined serum-free medium for at least 6-7 days when embedded in a three-dimensional collagen gel matrix. Incubation of established myofiber cultures for 3-7 days with insulin (1 microM) or insulin-like growth factor I (IGF-I, 32 nM) stimulates both cell hyperplasia and myofiber hypertrophy. Mean myofiber diameter increases 71-98%. Insulin-like growth factor II stimulates cell hyperplasia but not myofiber hypertrophy. Cell growth results from a 42-62% increase in total protein synthesis and a 28-38% decrease in protein degradation. Myosin heavy-chain content increases 183-258% because of a 55% stimulation of myosin synthesis and 33-61% inhibition of degradation. Associated with myofiber hypertrophy is a 87-148% increase in the number of myofiber nuclei per unit myofiber length. The results indicate that insulin and IGF-I, but not IGF-II, can induce rapid myofiber hypertrophy in vitro, most likely by stimulating myoblast proliferation and/or fusion to established myofibers.

Hyposomatomedinemia in men with post-poliomyelitis syndrome

The age of onset of the post-poliomyelitis syndrome (PPS) coincides with the tendency for declining activity of the growth hormone/somatomedin C (GH/SmC) axis. The normal plasma SmC range in men before the age of 40 is 0.50 to 1.50 units/mL. After age 40 about 30% of men have a plasma SmC level below 0.35 units/mL, signifying no detectable spontaneous GH secretory pulses. Because the GH/SmC axis stimulates DNA, RNA, and protein synthesis in muscle cells and increases their size and number, a deficiency of the GH/SmC axis could theoretically contribute as a secondary factor to the occurrence or severity of the PPS. Accordingly, the authors measured the plasma SmC level in 10 men with PPS, ages 35 to 63, and in 94 healthy men of similar age. In the PPS men, 100% of the values were less than or equal to 0.40 units/mL, and 90% were less than or equal to 0.35 units/mL. The corresponding proportions in the healthy men were 40% and 27%. Analysis of variance including age as a factor showed SmC to be significantly lower in the PPS men than in the healthy men. In an additional comparison, totally immobile nursing home men did not have lowered SmC values. In fact their SmC values were slightly higher than those of healthy men of similar age. The data revealed a new biochemical feature of PPS, hyposomatomedinemia, which might play a contributory role in the pathogenesis of the syndrome.

The role of the IGFs in myogenic differentiation

Of the three families of growth factors/hormones (the FGFs, TGF-betas, and IGFs) that have major effects on the differentiation of skeletal muscle cells, only the IGFs stimulate the process; indeed, the IGFs are the only well-defined agents thus far shown to stimulate myogenesis. All of these agents affect the expression of myogenin, one of the recently discovered family of myogenesis controlling genes, and TGF-beta and FGF inhibit the expression of MyoD1 as well. (L6 cells do not express MyoD1, so we have not looked for an effect of IGFs on it.) At least partly as a result of this action, these agents inhibit or stimulate all aspects of myogenic differentiation--fusion, expression of a set of muscle-specific proteins, and attainment of a postmitotic state--in all tested cell lines and primary muscle cell cultures. It is becoming clear that the myogenic controlling genes are capable of regulating expression of genes for the entire family of muscle specific proteins, so the principal question remaining about actions of these growth factors is the mechanism by which they inhibit or induce expression of the myogenin or MyoD1 genes. In spite of the uncertainty about their interactions, the discovery of the myogenesis controlling genes now provides a much sharper focus for studies on the processes involved in terminal differentiation of skeletal muscle cells. The demonstration that expression of these genes is controlled, both positively and negatively, by specific growth factors that are now readily available opens exciting new possibilities in endocrinology and developmental biology.