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remains to be fully understood. Muscle tissue stem cells, called satellite cells, play crucial roles in providing myonuclei for postnatal muscle growth as well as in muscle maintenance, repair, regeneration, and hypertrophy in adults ( Dumont et al
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source of muscle growth and regeneration. When myoblasts are quiescent, they are termed satellite cells. After satellite cells are activated to become myoblasts, they enter the proliferation stage and differentiate into myotubes, which finally fuse with
Department of Pediatrics, Texas Children’s Hospital, Baylor College of Medicine, Houston, Texas, USA
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secondary myoblasts and satellite cells. Satellite cells are muscle stem cells that can self-renew and are responsible for the addition of myonuclei to existent myofibers. Thus, if GLC alter these processes, the number of myofibers and/or their hypertrophy
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ABSTRACT
The effects of insulin and insulin-like growth factor-I (IGF-I) on glucose transport were compared in myotubes derived from chicken breast muscle satellite cells in vitro. Myotubes were incubated (for 0·5 or 4 h) with or without glucose in the presence or absence of insulin or IGF-I. Glucose uptake was subsequently measured by the incorporation of 2-[1,2-3H(N)] deoxy-d-glucose ([3H]2DG) in glucose-free medium (10 min at 20 °C). Glucose uptake was almost completely abolished by the addition of cytochalasin B or phloretin. It was increased by a decrease in glucose concentration in the incubation medium. Insulin (5 mg/l) stimulated [3H]2DG uptake to a maximum of 43 ± 10% above basal after 30-min incubation and 101 ± 15% after 4-h incubation. IGF-I and insulin at equimolar concentrations (25 μg/l and 20 μg/l respectively) were almost equipotent after 0·5 h but after 4-h incubation IGF-I was 17-fold more potent, suggesting that this 'late' effect was mediated through the IGF-I receptor. Incubation with cycloheximide suggested that the effect of IGF-I involved increased protein synthesis. The results suggest that chicken myotubes express a glucose transporter which is regulated by IGF-I and glucose concentration. However, they do not appear to express a typical insulin-responsive transport system.
Journal of Endocrinology (1993) 137, 465–472
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ABSTRACT
Methods were developed for the isolation and culture of satellite cells from adult sheep muscle. Differentiated cultures of these cells were used to investigate the effects of four hormones and growth factors on protein synthesis and degradation. Insulin was found to have no effect except at supraphysiological concentrations (100 nmol/l and 1 μmol/l) where it is probably cross-reacting with the insulin-like growth factor (IGF) type-I receptor. IGF-I was found to be anabolic at lower concentrations (1–3 nmol/l). Epidermal growth factor (EGF) had a smaller effect on protein synthesis and degradation than insulin or IGF-I. The specific activity of the muscle-specific enzyme creatine phosphokinase (CPK) was increased by treatment with EGF. When both IGF-I and EGF were present in the test media an additive effect on protein synthesis was observed. However, no additive effect of IGF-I and insulin was noted. No effects of bovine GH were seen.
Journal of Endocrinology (1989) 122, 565–571
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ABSTRACT
Insulin-like growth factors-I and -II (IGF-I and IGF-II) stimulate proliferation, differentiation, nutrient uptake and protein accretion in muscle cells. These effects are thought to be mediated through the type-I IGF receptor although a role for the type-II IGF receptor cannot be ruled out, since it has been found in most cells studied so far. Current evidence suggests that the chicken does not have a type-II IGF receptor and therefore provides a good model to study the function of IGF peptides. We have compared the effects of insulin and insulin-like growth factors on DNA synthesis with the binding of these peptides to receptors in primary chicken muscle satellite cells.
Human IGF-I (hIGF-I), hIGF-II and porcine insulin increased thymidine incorporation into DNA by threefold in muscle satellite cells prepared from neonatal chickens. IGF-I and -II were almost equipotent, with half-maximum effective concentrations of 10 μg/l, and were 1000-fold more potent than insulin. A combination of maximum effective concentrations of all three peptides was not additive, suggesting that their effect was mediated by the same receptor.
Receptor binding studies on satellite cells demonstrated the presence of specific IGF receptors. Human IGF-I inhibited the binding of 125I-labelled hIGF-I with a much higher potency than insulin, as usually observed for a type-I IGF receptor. However, unlabelled hIGF-II exhibited a higher potency than hIGF-I in displacing 125I-labelled hIGF-I. Affinity cross-linking of 125I-labelled hIGF-I and -II, followed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, showed that hIGF-I and -II bound to a receptor with the structural characteristics of a type-I IGF receptor and confirmed the lack of a type-II IGF receptor in these cells. The concentrations of IGF-I, -II and insulin required for biological action and to displace 125I-labelled hIGF-I binding were similar, and support the hypothesis that their effects on proliferation were mediated exclusively through a type-I IGF receptor.
Journal of Endocrinology (1991) 128, 35–42
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fusing to form the muscle fibre – the key cellular activator of this process being the satellite cell ( Wozniak et al. 2005 ). A sequence of transcription factors act on satellite cells – the myogenic regulatory factors (MRFs) –comprising myogenin, MRF4
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dependent on the proliferation and differentiation of the muscle stem cell population, satellite cells (SC), in a process known as myogenesis. SC niches are located between the muscle fibre sarcolemma and the basal lamina, which are normally close to the
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The IGF axis is nutritionally sensitive in vivo and IGFs stimulate myoblast proliferation and differentiation in vitro, while myostatin inhibits these processes in vitro. We hypothesised that underfeeding would reversibly inhibit the myogenic activity of satellite cells in vivo together with decreased IGF-I and increased myostatin in muscle. Satellite cell activity was measured indirectly from the expression of proliferating cell nuclear antigen (PCNA) and the myogenic regulatory factors (MRFs), MyoD, Myf-5 and myogenin. Young sheep were underfed (30% of maintenance) and some killed after 1, 4, 12, 17, 21 and 22 weeks. Remaining underfed animals were then re-fed a control ration of pellets and killed after 2 days, and 1, 6 and 30 weeks. Expression of PCNA and MRFs decreased during the first week of underfeeding. This coincided with reduced IGF-I and myostatin mRNA, and processed myostatin. Subsequently, Myf-5, MyoD, myostatin mRNA and processed myostatin increased, suggesting that satellite cells may have become progressively quiescent. Long-term underfeeding caused muscle necrosis in some animals and IGF-I and MRF expression was increased in these, indicating the activation of satellite cells for muscle repair. Re-feeding initiated rapid muscle growth and increased expression of PCNA, IGF-I and the MRFs concurrently with decreased myostatin proteins. In conclusion, these data indicate that IGF-I and myostatin may work in a coordinated manner to regulate the proliferation, differentiation and quiescence of satellite cells in vivo.
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ABSTRACT
Tumours secreting GH (GH1) or GH plus prolactin (GH3) were induced in young (1-week-old) and mature (17-week-old) female Wistar–Furth rats. Young animals were killed at 11 weeks and mature rats at 30 weeks of age. Induction of tumours increased serum GH concentrations and body and soleus muscle weights when compared with those of control rats. The soleus and extensor digitorum longus (EDL) muscles were examined in transverse section by electron microscopy. The percentages of myofibres with myonuclei and with satellite cell nuclei present in both the soleus and EDL muscles were generally greater in young rats with tumours than in young control rats. The percentage of myofibres in the EDL muscle with myonuclei present was higher in mature rats with GH1 tumours compared with age-matched controls. The presence of tumours did not affect the number of fibres in the soleus muscle of either young or mature rats. It was concluded that increased GH concentrations increased muscle weight by increasing the DNA content of myofibres and by myofibre hypertrophy. The rate of proliferation of satellite cells apparently exceeded the rate of incorporation of daughter nuclei into the fibres of the young tumourbearing rats when compared with that of young controls. Thus, the factors regulating satellite cell proliferation and nuclear incorporation into the myofibre do not appear to be directly coupled.
J. Endocr. (1986) 111, 279–285
