Insulin-like Growth factor-I In Skeletal Muscle Research Articles

Effects of Prednisolone on Serum and Tissue Fluid IGF-I Receptor Activation and Post-Receptor Signaling in Humans.

Short-term glucocorticoid exposure increases serum insulinlike growth factor I (IGF-I) concentrations but antagonizes IGF-I tissue signaling. The underlying mechanisms remain unknown. To identify at which levels glucocorticoid inhibits IGF-I signaling. Nineteen healthy males received prednisolone (37.5 mg/d) and placebo for 5 days in a randomized, double-blinded, placebo-controlled crossover study. Serum was collected on days 1, 3, and 5, and abdominal skin suction blister fluid (SBF; ~interstitial fluid) was taken on day 5 (n = 9) together with muscle biopsy specimens (n = 19). The ability of serum and SBF to activate the IGF-I receptor (IGF-IR) (bioactive IGF) and its downstream signaling proteins was assessed using IGF-IR-transfected cells. Prednisolone increased IGF-I concentrations and bioactive IGF in serum (P ≤ 0.001) but not in SBF, which, compared with serum, contained less bioactive IGF (~28%) after prednisolone (P < 0.05). This observation was unexplained by SBF concentrations of IGFs and IGF-binding proteins (IGFBPs) 1 to 4. However, following prednisolone treatment, SBF contained less IGFBP-4 fragments (P < 0.05) generated by pregnancy-associated plasma protein A (PAPP-A). Concomitantly, prednisolone increased SBF levels of stanniocalcin 2 (STC2) (P = 0.02) compared with serum. STC2 blocks PAPP-A from cleaving IGFBP-4. Finally, prednisolone suppressed post-IGF-IR signaling pathways at the level of insulin receptor substrate 1 (P < 0.05) but did not change skeletal muscle IGF-IR, IGF-I, or STC2 messenger RNA. Prednisolone increased IGF-I concentrations and IGF bioactivity in serum but not in tissue fluid. The latter may relate to a STC2-mediated inhibition of PAPP-A in tissue fluids. Furthermore, prednisolone induced post-IGF-IR resistance. Thus, glucocorticoid may exert distinct, compartment-specific effects on IGF action.

IGF-I measurement across blood, interstitial fluid, and muscle biocompartments following explosive, high-power exercise

Insulin-like growth factor-I (IGF-I) resides across different biocompartments [blood, interstitial fluid (ISF), and muscle]. Whether circulating IGF-I responses to exercise reflect local events remains uncertain. We measured the IGF-I response to plyometric exercise across blood, ISF, and muscle biopsy from the vastus lateralis. Twenty volunteers (8 men, 12 women, 22 ± 1 yr) performed 10 sets of 10 plyometric jump repetitions at a 40% 1-repetition maximum. Blood, ISF, and muscle samples were taken pre- and postexercise. Circulating IGF-I increased postexercise: total IGF-I (preexercise = 546 ± 42, midexercise = 585 ± 43, postexercise = 597 ± 45, +30 = 557 ± 42, +60 = 536 ± 40, +120 = 567 ± 42 ng/ml; midexercise, postexercise, and +120 greater than preexercise, P < 0.05); Free IGF-I (preexercise = 0.83 ± 0.09, midexercise = 0.78 ± 0.10, postexercise = 0.79 ± 0.11, +30 = 0.93 ± 0.10, +60 = 0.88 ± 0.10, + 120 = 0.91 ± 0.11 ng/ml; +30 greater than all other preceding time points, P < 0.05). No exercise-induced changes were observed for ISF IGF-I (preexercise = 2.35 ± 0.29, postexercise = 2.46 ± 0.35 ng/ml). No changes were observed for skeletal muscle IGF-I protein, although IGF-I mRNA content increased ∼40% postexercise. The increase in circulating total and free IGF-I was not correlated with increases in ISF IGF-I or muscle IGF-I protein content. Our data indicate that exercise-induced increases in circulating IGF-I are not reflective of local IGF-I signaling.

GH receptor blocker administration and muscle–tendon collagen synthesis in humans

ContextThe growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis stimulates collagen synthesis in tendon and skeletal muscle, but no studies have investigated the effect of reducing IGF-I on collagen synthesis in healthy humans. ObjectiveWe hypothesised, that a GH blockade would decrease IGF-I and collagen synthesis in the connective tissue of skeletal muscle and tendon. DesignThe study was randomised and double blinded. Participants20 healthy young males completed the study. InterventionThe participants were randomised to 2weeks of GH receptor blocker supplementation (pegvisomant, 5mg/day, n=9) or placebo (n=11). Main outcome measuresSerum levels of GH, IGF-I and IGF-binding protein 3 (IGFBP-3) were measured before and after pegvisomant/placebo supplementation. Fractional synthesis rates (FSR) for collagen and myofibrillar protein were determined with stable isotopes in tendon and muscle, and mRNA for collagen (COL1A1 and COL3A1) as well as IGF-I isoforms (Ea and Ec) were measured in skeletal muscle. ResultsPegvisomant decreased serum IGF-I by 20% (p<0.01) and serum IGFBP-3 by 10% (p<0.05). Pegvisomant supplementation had no effect on collagen synthesis in tendon and skeletal muscle, nor was muscle myofibrillar protein synthesis affected. Similarly, pegvisomant supplementation had no effect on mRNA expression of IGF-I and collagen in skeletal muscle. ConclusionGH receptor blocker administration in healthy humans resulted in a moderate decrease in serum IGF-I. Collagen synthesis in tendon and skeletal muscle, as well as skeletal muscle IGF-I and collagen mRNA expression, was unaffected by GH receptor blocker supplementation.

Chronic supplementation of beta-hydroxy-beta methylbutyrate (HMβ) increases the activity of the GH/IGF-I axis and induces hyperinsulinemia in rats

Objective Beta-hydroxy-beta-methylbutyrate (HMβ) is a metabolite of leucine widely used for improving sports performance. Although HMβ is recognized to promote anabolic or anti-catabolic effects on protein metabolism, the impact of its long-term use on skeletal muscle and/or genes that control the skeletal protein balance is not fully known. This study aimed to investigate whether chronic HMβ treatment affects the activity of GH/IGF-I axis and skeletal muscle IGF-I and myostatin mRNA expression. Design Rats were treated with HMβ (320 mg/kg BW) or vehicle, by gavage, for 4 weeks, and killed by decapitation. Blood was collected for evaluation of serum insulin, glucose and IGF-I concentrations. Samples of pituitary, liver, extensor digitorum longus (EDL) and soleus muscles were collected for total RNA or protein extraction to evaluate the expression of pituitary growth hormone (GH) gene (mRNA and protein), hepatic insulin-like growth factor I (IGF-I) mRNA, skeletal muscle IGF-I and myostatin mRNA by Northern blotting/real time-PCR, or Western blotting. Results Chronic HMβ treatment increased the content of pituitary GH mRNA and GH, hepatic IGF-I mRNA and serum IGF-I concentration. No changes were detected on skeletal muscle IGF-I and myostatin mRNA expression. However, the HMβ-treated rats although normoglycemic, exhibited hyperinsulinemia. Conclusions The data presented herein extend the body of evidence on the potential role of HMβ-treatment in stimulating GH/IGF-I axis activity. In spite of this effect, HMβ supplementation also induces an apparent insulin resistance state which might limit the beneficial aspects of the former results, at least in rats under normal nutritional status and health conditions.

ANABOLIC AGENTS FOR IMPROVING MUSCLE REGENERATION AND FUNCTION AFTER INJURY

1. In the present review, we describe how muscles can be injured by external factors, internal factors or during the performance of some actions during sports. In addition, we describe the injury to a muscle that occurs when its blood supply is interrupted, an occurrence common in clinical settings. An overview of muscle regeneration is presented, as well as a discussion of some of the potential complications that can compromise successful muscle repair and lead to impaired function and permanent disability. 2. Improving muscle regeneration is important for hastening muscle repair and restoring muscle function and the present review describes ways in which this can be achieved. We describe recent advances in tissue engineering that offer considerable promise for treating muscle damage, but highlight the fact that these techniques require rigorous evaluation before they can become mainstream clinical treatments. 3. Growth-promoting agents are purported to increase the size of existing and newly regenerating muscle fibres and, therefore, could be used to improve muscle function if administered at appropriate times during the repair process. The present review provides an update on the efficacy of some growth-promoting agents, including anabolic steroids, insulin-like growth factor-I (IGF-I) and beta(2)-adrenoceptor agonists, to improve muscle function after injury. Although these approaches have clinical merit, a better understanding of the androgenic, IGF-I and b-adrenoceptor signalling pathways in skeletal muscle is important if we are to devise safe and effective therapies to enhance muscle regeneration and function after injury.

Skeletal muscle IGF-I isoform expression in healthy women after isometric exercise

Context Little is known about the expression of skeletal muscle-specific isoforms of the insulin-like growth factor-I (IGF-I) gene in response to exercise in humans. Data for women are sparse and the influences of age and physical training status are unknown. Here we aimed to describe the expression of mRNA for the IGF-I isoforms in the quadriceps muscles of women at rest and after isometric exercise. Objective To categorise mRNA expression of the IGF-I splice variants IGF-IEa and IGF-IEc (MGF) in healthy women, determine whether isometric exercise stimulates expression, and to determine the duration of the response. Design Observational study. Setting Clinical research facility. Participants Six healthy women (median age 38 years, range 25–43 years). Intervention Single bout of maximal isometric knee extension undertaken as 20 sets of 6 contractions. Muscle samples from the lateral mass of the quadriceps were obtained by needle biopsy at baseline, 2.5 h and 50 h after exercise. Main outcome measures Quantification of mRNAs for IGF-IEa and MGF. Results mRNA for the IGF-IEa transcript was significantly elevated by 50 h post-exercise compared to baseline ( p = 0.005) and mRNA for MGF was significantly elevated by 2.5 h ( p = 0.026). There were no statistically significant differences between measurements at 2.5 h and 50 h for either transcript. Conclusions We have shown that the upregulation of the muscle specific IGF-IEa and MGF isoforms occurs within 2.5 h of a single bout of isometric exercise in women. The upregulation persists for at least 2 days after exercise. We have also demonstrated a potentially safe and effective way of studying the responsiveness of these isoforms to resistance exercise in future studies of older and/or frail individuals.

The many flavors of IGF-I

In the commercial world, there is the Pepsi taste test to see if blinded individuals prefer Pepsi over Coke. Although not a perfect analogy, in the scientific world, insulin-like growth factor I (IGF-I) exists in multiple “flavors” of peptide sequences, and the question addressed by Barton's

Age Differences In IGF-I System Responses To Resistance Loading In Human Skeletal Muscle

Skeletal muscle insulin-like growth factor-I (IGF-I) is an autocrine/paracrine factor that is mitogenic to satellite cells and increases protein synthesis in the mature myofiber. We have previously shown in young adults that resistance loading (RL) acutely increases IGF-I and suppresses IGF binding protein-4 (BP4) mRNA levels, presumably leading to greater amounts of bioavailable tissue IGF-I. PURPOSE We tested the hypothesis that age and gender would affect acute RL responses of local IGF-I system components including IGF-I, IGFBP4, and the type 1 IGF receptor (IGFR1) in young (Y) and older (O) males (M) and females (F). METHODS Twenty Y (20–35 yr, 10 YF, 10 YM) and 18 O (60–75 yr, 9 OF, 9 OM) consented to vastus lateralis biopsy before and 24h after a bout of RL (3 sets × 8–12 repetitions to volitional fatigue of squat, leg press, knee extension). Gene expression levels were determined by relative RT-PCR with 18S as an internal standard and analyzed by age × gender × load repeated measures ANOVA. RESULTS A main load effect was found for IGF-I (+34%, p < 0.01), and post hoc testing within groups revealed that the increase was mainly driven by O (+48%, p < 0.05) and M (+43%, p < 0.05). For IGFR1, an age × load interaction (p < 0.05) resulted from a 16% increase in O after RL with no change in Y. A main age effect (p < 0.01) was found for IGFBP4, as higher levels were noted in O (17%). CONCLUSIONS While acute RL increased IGF-I and IGFR1 mRNAs in O, higher levels of IGFBP4 in O may inhibit IGF-I mediated actions and may play a role in attenuating myofiber hypertrophy in older vs. younger adults. Supported by NIH Grant R01 AGI7896 and GCRC M01 RR00032

Age Differences In IGF-I System Responses To Resistance Loading In Human Skeletal Muscle

Skeletal muscle insulin-like growth factor-I (IGF-I) is an autocrine/paracrine factor that is mitogenic to satellite cells and increases protein synthesis in the mature myofiber. We have previously shown in young adults that resistance loading (RL) acutely increases IGF-I and suppresses IGF binding protein-4 (BP4) mRNA levels, presumably leading to greater amounts of bioavailable tissue IGF-I. PURPOSE We tested the hypothesis that age and gender would affect acute RL responses of local IGF-I system components including IGF-I, IGFBP4, and the type 1 IGF receptor (IGFR1) in young (Y) and older (O) males (M) and females (F). METHODS Twenty Y (20–35 yr, 10 YF, 10 YM) and 18 O (60–75 yr, 9 OF, 9 OM) consented to vastus lateralis biopsy before and 24h after a bout of RL (3 sets × 8–12 repetitions to volitional fatigue of squat, leg press, knee extension). Gene expression levels were determined by relative RT-PCR with 18S as an internal standard and analyzed by age × gender × load repeated measures ANOVA. RESULTS A main load effect was found for IGF-I (+34%, p < 0.01), and post hoc testing within groups revealed that the increase was mainly driven by O (+48%, p < 0.05) and M (+43%, p < 0.05). For IGFR1, an age × load interaction (p < 0.05) resulted from a 16% increase in O after RL with no change in Y. A main age effect (p < 0.01) was found for IGFBP4, as higher levels were noted in O (17%). CONCLUSIONS While acute RL increased IGF-I and IGFR1 mRNAs in O, higher levels of IGFBP4 in O may inhibit IGF-I mediated actions and may play a role in attenuating myofiber hypertrophy in older vs. younger adults. Supported by NIH Grant R01 AGI7896 and GCRC M01 RR00032

Clenbuterol induces muscle-specific attenuation of atrophy through effects on the ubiquitin-proteasome pathway

The ubiquitin-proteasome pathway is primarily responsible for myofibrillar protein degradation during hindlimb unweighting (HU). Beta-adrenergic agonists such as clenbuterol (CB) induce muscle hypertrophy and attenuate muscle atrophy due to disuse or inactivity. However, the molecular mechanism by which CB exerts these effects remains poorly understood. The aims of this study were to investigate whether CB attenuates HU-induced muscle atrophy through an inhibition of the ubiquitin-proteasome pathway and whether insulin-like growth factor I (IGF-I) mediates this inhibition. Rats were randomized to the following groups: weight-bearing control, 14-day CB-treated, 14-day HU, and CB + HU. HU-induced atrophy was associated with increased proteolysis and upregulation of components of the ubiquitin-proteasome pathway (ubiquitin conjugates, ubiquitin conjugating enzyme E2-14 kDa, and 20S proteasome activity). Upregulation of the ubiquitin proteasome occurred in all muscles tested but was more pronounced in muscles composed primarily of slow-twitch fibers (soleus) than in fast-twitch muscles (plantaris and tibialis anterior). Although CB induced hypertrophy in all muscles, CB attenuated the HU-induced atrophy and reduced ubiquitin conjugates only in the fast plantaris and tibialis anterior and not in the slow soleus muscle. CB did not elevate IGF-I protein content in either of the muscles examined. These results suggest that CB induces hypertrophy and alleviates HU-induced atrophy, particularly in the fast muscles, at least in part through a muscle-specific inhibition of the ubiquitin-proteasome pathway and that these effects are not mediated by the local production of IGF-I in skeletal muscle.

Effect of voluntary exercise on the expression of IGF-I and androgen receptor in three rat skeletal muscles and on serum IGF-I and testosterone levels.

The effects of anabolic agents and training on skeletal muscle are believed to be mediated by a variety of growth and transcription factors. Among these regulatory proteins, insulin-like growth factor-I (IGF-I) and androgen receptor (AR) play a crucial role. The purpose of this study was to investigate the effects of wheel running on IGF-I and AR mRNA expression in three distinct rat skeletal muscles (i.e., gastrocnemius, vastus lateralis, and soleus), as well as on the serum levels of IGF-I and testosterone. Twenty male Wistar rats were housed in cages with free access to running wheels for 12 weeks, while nine rats served as controls. Analysis of the mRNA expression of IGF-I and AR using real time RT-PCR revealed no significant differences between the trained and untrained rats in any of the muscles studied. Enzyme immunoassay showed significantly lower serum levels of IGF-I and testosterone in the trained compared to the untrained animals. These results suggest that chronic exercise in wheels does not affect IGF-I and AR mRNA levels in rat skeletal muscle, while decreasing the circulating levels of two anabolic factors, i.e., IGF-I and testosterone. It is concluded that IGF-I, AR and testosterone seem to play a marginal role during the adaptation process of rat skeletal muscle to long-term wheel running.

Inhibition of MAP/ERK kinase prevents IGF-I-induced hypertrophy in rat muscles.

Insulin-like growth factor-I (IGF-I) has been shown to stimulate a hypertrophy response in skeletal muscles in vivo. In vitro studies have delineated two primary intracellular pathways that appear to mediate the effects of IGF-I in skeletal muscle: the Ras-ERK pathway and the phosphoinositide-3 kinase pathway. In vitro, the Ras pathway appears to regulate the mitogenic effects of IGF-I signaling, whereas the phosphoinositide-3 kinase pathway is associated with cellular differentiation. On the basis of the results from in vitro studies, we hypothesized that the coinfusion of both IGF-I and an inhibitor of the Ras pathway would result in some increase in muscle protein but an inhibition of cell proliferation. Our results show that 14 days of coinfusion of MAPK/ERK kinase inhibitor PD-098059 (PD) limited the phosphorylation of ERK and prevented IGF-I induced increases in protein (18%, P < 0.05 vs. 7%, not significant) or myofibrillar protein (23%, P < 0.01 vs. 5%, not significant). However, there were similar increases in indicators of cell proliferation (e.g., total DNA, 50 and 52%, P < 0.001) in both the IGF- and IGF+PD-infused muscles. The most notable impact on IGF-I signaling was a significant blunting of IGF-I induced increase in S6K1 phosphorylation by PD-98059 coinfusion ( approximately 5-fold, P < 0.001 vs. 3-fold, P < 0.01). These results suggest that there are interactions between the various pathways down stream of the IGF-I receptor that may behave differently in vivo than in myogenic cell lines in vitro.

Recombinant human growth hormone and recombinant human insulin-like growth factor I diminish the catabolic effects of hypogonadism in man: metabolic and molecular effects.

Severe gonadal androgen deficiency can have profound catabolic effects in man. Hypogonadal men develop a loss of lean body mass, increased adiposity, and decreased muscle strength despite normal GH and insulin-like growth factor I (IGF-I) concentrations. We designed these studies to investigate whether GH or IGF-I administration to male subjects with profound hypogonadism can diminish or abolish the catabolic effects of testosterone deficiency. Moreover, we also examined the nature of the interactions among GH, IGF-I, and androgens in specific genes of the im system. A group of 13 healthy subjects (mean age, 22 +/- 1 yr) was studied at baseline (D1) and 10 weeks after being made hypogonadal using a GnRH analog (GnRHa; D2). At 6 weeks from baseline they were started on either recombinant human (rh) IGF-I (60 microg/kg, sc, twice daily) or rhGH (12.5 microg/kg, sc, daily) for 4 weeks. On each study day subjects had infusions of L-[(13)C]leucine; indirect calorimetry; isokinetic dynamometry of the knee extensors; determination of body composition (dual energy x-ray absortiometry) and hormone and growth factor concentrations, as well as percutaneous muscle biopsies. Their data were compared with those of previously studied male subjects who received only GNRHA: Administration of rhIGF-I and rhGH to the hypogonadal men had similar effects on whole body metabolism, with maintenance of protein synthesis rates, fat oxidation rates, and fat-free mass compared with the eugonadal state, preventing the decline observed with hypogonadism alone. This was further amplified by the molecular assessment of important genes in muscle function. During rhIGF-I treatment, im expression of IGF-I declined, and IGF-binding protein-4 increased, similar to the changes during GnRHa alone. However, rhGH administration was associated with a marked increase in IGF-I and androgen receptor messenger ribonucleic acid concentrations in skeletal muscle with a reciprocal decline in IGF-binding protein-4 expression in the hypogonadal men. The gene expression for myostatin did not change. These effects were accompanied by a much greater increase in plasma IGF-I concentrations after rhIGF-I (225 +/- 32 vs. 768 +/- 117 microg/L) compared with the concentrations achieved during rhGH (217 +/- 20 vs. 450 +/- 19 microg/L). We conclude that 1) rhGH and rhIGF-I both may be beneficial in preserving lean body mass and sustaining rates of protein synthesis during states of severe androgen deficiency in man; 2) GH may affect the im IGF system via an a paracrine, local production of IGF-I; 3) androgens may be necessary for the full anabolic effect of GH/IGF-I in man. These hormones, particularly GH, may play a role in the treatment of hypogonadal men rendered hypogonadal pharmacologically or those unable to take full testosterone replacement. The latter requires further study.

Growth Hormone Promotes Somatic and Skeletal Muscle Growth Recovery in Rats Following Chronic Protein-Energy Malnutrition

The efficacy of recombinant human growth hormone (GH) and/or a diet enriched in protein and energy to improve growth recovery following prolonged malnutrition was examined in male rats food-restricted from birth until 120 d of age. At d 121, restricted rats were randomly assigned to recovery groups receiving either a control or enriched diet with or without daily subcutaneous injections of GH. Rats were killed after 16 or 47 d of recovery. At d 16, GH treatment stimulated liver, heart, plantaris, soleus, carcass and body weight gain and inhibited fat gain when compared to recovery controls. Rats receiving GH also exhibited the highest serum insulin-like growth factor-I (IGF-I) concentrations and total muscle protein. At d 47, GH effects on body and muscle recovery were minimal, and differences among recovery groups in serum IGF-I concentration and total muscle protein were no longer present. Consumption of an enriched diet increased fat pad and liver mass, but did not promote muscle recovery. There were no differences among treatment groups in skeletal muscle IGF-I mRNA levels at d 16 or 47. In summary, GH had positive effects on somatic and skeletal muscle growth early in the recovery process, possibly via endocrine IGF-I-stimulated protein accretion. In contrast, the enriched diet promoted fat deposition with no impact on skeletal muscle growth recovery.

Overexpression of IGF-I in skeletal muscle of transgenic mice does not prevent unloading-induced atrophy.

This study examined the association between local insulin-like growth factor I (IGF-I) overexpression and atrophy in skeletal muscle. We hypothesized that endogenous skeletal muscle IGF-I mRNA expression would decrease with hindlimb unloading (HU) in mice, and that transgenic mice overexpressing human IGF-I (hIGF-I) specifically in skeletal muscle would exhibit less atrophy after HU. Male transgenic mice and nontransgenic mice from the parent strain (FVB) were divided into four groups (n = 10/group): 1) transgenic, weight-bearing (IGF-I/WB); 2) transgenic, hindlimb unloaded (IGF-I/HU); 3) nontransgenic, weight-bearing (FVB/WB); and 4) nontransgenic, hindlimb unloaded (FVB/HU). HU groups were hindlimb unloaded for 14 days. Body mass was reduced (P < 0.05) after HU in both IGF-I (-9%) and FVB mice (-13%). Contrary to our hypothesis, we found that the relative abundance of mRNA for the endogenous rodent IGF-I (rIGF-I) was unaltered by HU in the gastrocnemius (GAST) muscle of wild-type FVB mice. High-level expression of hIGF-I peptide and mRNA was confirmed in the GAST and tibialis anterior (TA) muscles of the transgenic mice. Nevertheless, masses of the GAST and TA muscles were reduced (P < 0.05) in both FVB/HU and IGF-I/HU groups compared with FVB/WB and IGF-I/WB groups, respectively, and the percent atrophy in mass of these muscles did not differ between FVB and IGF-I mice. Therefore, skeletal muscle atrophy may not be associated with a reduction of endogenous rIGF-I mRNA level in 14-day HU mice. We conclude that high local expression of hIGF-I mRNA and peptide in skeletal muscle alone cannot attenuate unloading-induced atrophy of fast-twitch muscle in mice.

Islet amyloid polypeptide decreases the effects of insulin-like growth factor-I on glucose transport and glycogen synthesis in skeletal muscle

Previous studies have shown that islet amyloid polypeptide (IAPP) is co-secreted with insulin from the β-cell. IAPP reduces insulin-stimulated rates of glycogen synthesis in skeletal muscle but the mechanisms are unclear. Insulin-like growth factor I (IGF-I) is an important regulator of glucose metabolism in skeletal muscle and acts through its own receptor, which has many structural and functional similarities with the insulin receptor. Despite this, the effects of IGF-I on glucose utilization are not identical to those of insulin. The aim of the study was to determine the effects of IAPP on IGF-I-stimulated rates of glucose transport and metabolism (measured by 3- O-methyl[ 3H]glucose and [U- 14C]glucose, respectively) in rat soleus muscle, and compare them with those stimulated by insulin. IAPP (10 nM) decreased the sensitivity of 3- O-methylglucose transport, the flux of glucose to hexosemonophosphate and the sensitivity of glycogen synthesis to IGF-I. In contrast, IAPP had no effect on IGF-I-stimulated rates of lactate formation (i.e. glycolysis). IAPP decreased the sensitivity of 3- O-methylglucose transport and glycogen synthesis to insulin. It is concluded that IAPP blunts the stimulation of glucose uptake and deposition by IGF-I or insulin in skeletal muscle. These observations expand those made initially for IAPP and insulin and suggest that IAPP affects IGF-I- or insulin-stimulated glucose metabolism in muscle by a mechanism which is common for both hormones. These experiments may serve as a framework for future studies in order to clarify the mechanisms by which IAPP affects glucose metabolism in skeletal muscle.

Cytokines block the effects of insulin-like growth factor-I (IGF-I) on glucose uptake and lactate production in skeletal muscle but do not influence IGF-I-induced changes in protein turnover.

There is evidence that proinflammatory cytokines are involved in the regulation of muscle protein breakdown in various catabolic conditions but the mechanisms are not fully understood. Previous studies suggest that cytokines reduce circulating and tissue levels of insulin-like growth factor-I (IGF-I) and may block the anabolic effects of the hormone in certain cell types and tissues. We tested the hypothesis that a mixture of tumor necrosis factor alpha, interleukin-1 alpha, and interferon-gamma block the anabolic effects of IGF-I in skeletal muscle. Muscles from burned or unburned rats were incubated in the absence or presence of 1 microgram/mL of IGF-I with or without the addition of the cytokines. As expected, IGF-I stimulated protein synthesis and inhibited protein breakdown in incubated muscles. The cytokines did not influence protein turnover rates in muscles incubated with or without IGF-I. In additional experiments, the effects of IGF-I on glucose uptake and lactate production were tested. IGF-I increased glucose uptake approximately 2.5-fold and stimulated lactate production approximately 5-fold. These effects of the hormone were significantly inhibited by the cytokine mixture. The results suggest that cytokines do not induce protein catabolism by directly inhibiting the anabolic effects of IGF-I in muscle tissue. The inhibitory effects of the cytokines on IGF-I-stimulated glucose transport and lactate production suggest that the lack of effect of cytokines on protein metabolism was not due to a metabolic unresponsiveness of the incubated muscles to the cytokines.

OVER-EXPRESSION OF IGF-I IN SKELETAL MUSCLE DOES NOT PREVENT UNLOADING-INDUCED ATROPHY 385

This study examined the hypothesis that high level local expression of insulin-like growth factor I (IGF-I) in skeletal muscle would prevent or minimize unloading-induced atrophy. We employed transgenic mice in which the-424 promoter, first intron and 3' untranslated region of the avian skeletal alpha actin gene drives expression of human IGF-I. Expression of the transgene results in -47 fold increase in IGF-I concentration specific to skeletal muscle (J. Biol. Chem. 270: 12109-12116, 1995). Male transgenic mice and non-transgenic mice from the parent strain (FVB) were divided into four groups (n = 10/group): 1) Transgenic, weight-bearing (IGF-I/WB); 2) transgenic, hindlimb unloaded (IGF-I/HU); 3) non-transgenic, weight-bearing(FVB/WB); and 4) non-transgenic, hindlimb unloaded (FVB/HU). HU groups were tail suspended for 14 days. Body mass was reduced (P<0.05) following HU in both IGF-I (-9%) and FVB mice (-13%). Mass of the gastrocnemius (GAST) and tibialis anterior (TA) muscles were reduced (p<0.05) in both FVB/HU and IGF-I/HU groups compared to FVB/WB and IGF-I/WB, respectively. The percent atrophy of mass in the GAST and TA following HU did not differ between IGF-I and FVB mice. Elevated expression of IGF-I mRNA was confirmed in the GAST and TA of the transgenic mice. However, HU resulted in a marked reduction in transgene expression in these muscles (although still much greater than FVB mice). Further, transgene expression was found to be undetectable in the soleus of transgenic mice. Therefore, high level local expression of IGF-I in skeletal muscle does not prevent unloading-induced skeletal muscle atrophy. Also, the avian skeletal alpha actin regulatory sequences used to drive this transgene confer fiber-type specific expression of human IGF-I and are sensitive to loading of the muscles.

Parallel insulin-like growth factor I and insulin resistance in muscles of rats fed a high fat diet.

The possibility of a similarity between insulin-like growth factor-I (IGF-I) and insulin resistance in epitrochlearis muscles of rats fed a high fat diet [20% (wt/wt) fat] vs. a low fat diet [5% (wt/wt) fat] was investigated. Half-maximally and maximally effective concentrations of IGF-I- and insulin-stimulated 3-O-methyl-glucose transport were 2.32 and 25 nM, respectively, for IGF-I, compared with 0.79 and 20 nM for insulin. The high fat diet reduced both responsiveness and sensitivity of IGF-I-stimulated 3-O-methyl-glucose transport in parallel with the impaired insulin-stimulated 3-O-methyl-glucose transport. IGF-I binding at 2.5 nM to incubated epitrochlearis muscle was also decreased by the high fat diet (P < 0.05). Thus, the high fat diet caused simultaneous IGF-I and insulin resistance in rat skeletal muscle. The IGF-I resistance induced by feeding the high fat diet seems at least partially due to a reduction in IGF-I binding.

Myogenic Vector Expression of Insulin-like Growth Factor I Stimulates Muscle Cell Differentiation and Myofiber Hypertrophy in Transgenic Mice

The avian skeletal alpha-actin gene was used as a template for construction of a myogenic expression vector that was utilized to direct expression of a human IGF-I cDNA in cultured muscle cells and in striated muscle of transgenic mice. The proximal promoter region, together with the first intron and 1.8 kilobases of 3'-noncoding flanking sequence of the avian skeletal alpha-actin gene directed high level expression of human insulin-like growth factor I (IGF-I) in stably transfected C2C12 myoblasts and transgenic mice. Expression of the actin/IGF-I hybrid gene in C2C12 muscle cells increased levels of myogenic basic helix-loop-helix factor and contractile protein mRNAs and enhanced myotube formation. Expression of the actin/IGF-I hybrid gene in mice elevated IGF-I concentrations in skeletal muscle 47-fold resulting in myofiber hypertrophy. IGF-I concentrations in serum and body weight were not increased by transgene expression, suggesting that the effects of transgene expression were localized. These results indicate that sustained overexpression of IGF-I in skeletal muscle elicits myofiber hypertrophy and provides the basis for manipulation of muscle physiology utilizing skeletal alpha-actin-based vectors.