Alcohol impairs insulin and IGF-I stimulation of S6K1 but not 4E-BP1 in skeletal muscle

The Influence of Stress and Binge-Patterned Alcohol Drinking on Mouse Skeletal Muscle Protein Synthesis and Degradation Pathways

Adverse experiences (e.g., acute stress) and alcohol misuse can both impair skeletal muscle homeostasis, resulting in reduced protein synthesis and greater protein breakdown. Exposure to acute stress is a significant risk factor for engaging in alcohol misuse. However, little is known about how these factors together might further affect skeletal muscle health. To that end, this study investigated the effects of acute stress exposure followed by a period of binge-patterned alcohol drinking on signaling factors along mouse skeletal muscle protein synthesis (MPS) and degradation (MPD) pathways. Young adult male C57BL/6J mice participated in the Drinking in the Dark paradigm, where they received 2-4 h of access to 20% ethanol (alcohol group) or water (control group) for four days to establish baseline drinking levels. Three days later, half of the mice in each group were either exposed to a single episode of uncontrollable tail shocks (acute stress) or remained undisturbed in their home cages (no stress). Three days after stress exposure, mice received 4 h of access to 20% ethanol (alcohol) to model binge-patterned alcohol drinking or water for ten consecutive days. Immediately following the final episode of alcohol access, mouse gastrocnemius muscle was extracted to measure changes in relative protein levels along the Akt-mTOR MPS, as well as the ubiquitin-proteasome pathway (UPP) and autophagy MPD pathways via Western blotting. A single exposure to acute stress impaired Akt singling and reduced rates of MPS, independent of alcohol access. This observation was concurrent with a potent increase in heat shock protein seventy expression in the muscle of stressed mice. Alcohol drinking did not exacerbate stress-induced alterations in the MPS and MPD signaling pathways. Instead, changes in the MPS and MPD signaling factors due to alcohol access were primarily observed in non-stressed mice. Taken together, these data suggest that exposure to a stressor of sufficient intensity may cause prolonged disruptions to signaling factors that impact skeletal muscle health and function beyond what could be further induced by periods of alcohol misuse.

Alcohol and Skeletal Muscle in Health and Disease

PURPOSE

Alcohol-related myopathy is one of the earliest alcohol-associated pathological tissue changes that is progressively exacerbated by cumulative long-term alcohol misuse. Acute and chronic alcohol use leads to changes in skeletal muscle mass and function. As discussed in this evidence-based review, alcohol-mediated mechanisms are multifactorial with effects on anabolic and catabolic signaling, mitochondrial bioenergetics, extracellular matrix remodeling, and epigenomic alterations. However, systematic studies are limited, especially regarding the acute effects of alcohol on skeletal muscle.

SEARCH METHODS

This review focuses on peer-reviewed manuscripts published between January 2012 and November 2022 using the search terms "alcohol" or "ethanol" and "skeletal muscle" in MEDLINE, PubMed, and Web of Science using EndNote reference management software.

SEARCH RESULTS

Eligible manuscripts included full-length research papers that discussed acute and chronic effects of alcohol on skeletal muscle mass and function in both clinical and preclinical studies. The review also includes alcohol-mediated skeletal muscle effects in the context of comorbidities. The three databases together yielded 708 manuscripts. Of these, the authors excluded from this review 548 papers that did not have "alcohol" or "muscle" in the title and 64 papers that were duplicates or did not discuss skeletal muscle. Thus, of all the manuscripts considered for this review, 96 are included and 612 are excluded. Additionally, relevant papers published earlier than 2012 are included to provide context to the review.

DISCUSSION AND CONCLUSIONS

Both acute and chronic alcohol use decrease protein synthesis and increase protein degradation. Alcohol also impairs mitochondrial function and extracellular matrix remodeling. However, there is a gap in the literature on the known alcohol-mediated mechanisms, including senescence, role of immune activation, and interorgan communication, on the development of alcohol-related myopathy. With increased life expectancy, changing alcohol use patterns, and increasing frequency of alcohol use among females, current observational studies are needed on the prevalence of alcohol-related myopathy. Additionally, the compounding effects of acute and chronic alcohol use on skeletal muscle with aging or exercise, in response to injury or disuse, and in the context of comorbidities including diabetes and human immunodeficiency virus (HIV), call for further investigation. Though evidence suggests that abstinence or reducing alcohol use can improve muscle mass and function, they are not restored to normal levels. Hence, understanding the pathophysiological mechanisms can help in the design of therapeutic strategies to improve skeletal muscle health.

Mealtime alcohol consumption suppresses skeletal muscle mTORC1 signaling in female mice

OBJECTIVE

To determine whether alcohol consumed within the meal influences the feeding induced increase in mTORC1 signaling.

METHODS

Alcohol provided in the liquid diet was consumed by alcohol naïve, fasted, C57BL/6Hsd female mice and gastrocnemius was collected 1hr after the refeeding. Subsequent experiments determined the extent to which changes in mTORC1 signaling persisted across the day.

RESULTS

Compared with control mice, protein synthesis, mTORC1 (Ser2448), 4EBP1 (Ser65), S6K1 (Thr389), rpS6 (Ser240/244), Akt (Thr308), and ULK1 (Ser757) were lower in EtOH. Similar suppressive patterns were observed in the hours following consumption of alcohol containing food throughout the dark cycle. Higher peak blood alcohol concentrations induced by intraperitoneal injection of alcohol extended the time and magnitude of mTORC1 pathway suppression.

CONCLUSION

Alcohol administered as part of the meal results in lower skeletal muscle mTORC1 signaling while subsequent models show that alcohol may influence this pathway across the day.

The effects of voluntary binge-patterned ethanol ingestion and daily wheel running on signaling of muscle protein synthesis and degradation in female mice

Excessive ethanol ingestion can reduce skeletal muscle protein synthesis (MPS) through the disruption of signaling along the Akt-mTOR pathway and increase muscle protein degradation (MPD) through the Ubiquitin Proteasome Pathway (UPP) and autophagy. Identification of interventions that curb the disrupting effects of alcohol misuse on MPS and MPD are of central importance for the prevention of chronic health complications that arise from muscle loss. Physical activity is one potential strategy to combat the deleterious effects of alcohol on skeletal muscle. Therefore, the purpose of this study was to investigate the interaction between daily wheel running and binge-patterned ethanol consumption, through episodes of voluntary binge-patterned ethanol drinking, on signaling factors along the Akt-mTOR, Ubiquitin-Proteasome, and autophagy pathways. Adult female C57BL/6J mice received daily access to cages with or without running wheels for 2.5 h/day for five weeks. During the final five days of the study, mice received 2-4 h of daily access to sipper tubes containing water (n = 14 sedentary; n = 15 running) or 20% ethanol (n = 14 sedentary; n = 16 running) 30 min after running wheel access, using the "Drinking in the Dark" (DID) model of binge-patterned ethanol consumption. Immediately after the final episode of DID, gastrocnemius muscle was extracted. Western blotting was performed to measure proteins along Akt-mTOR, Ubiquitin-Proteasome, and autophagy pathways, and PCR was used to assess mRNA expression of atrogenes. Ethanol access increased expression of MAFbx by 82% (p = 0.048), but did not robustly influence Akt-mTOR or UPP signaling. Daily wheel access did not prevent alcohol-induced MAFbx expression; however, ethanol access decreased the phosphorylation of p70S6K by 45% in running mice (p = 0.020). These results suggest that physical activity may be insufficient to prevent alcohol-induced changes to signaling factors along pathways involved in muscle loss. Instead, binge-patterned ethanol ingestion may impair the benefits of physical activity on factors involved in MPS.

Alcohol Acutely Antagonizes Refeeding-Induced Alterations in the Rag GTPase-Ragulator Complex in Skeletal Muscle

The Ragulator protein complex is critical for directing the Rag GTPase proteins and mTORC1 to the lysosome membrane mediating amino acid-stimulated protein synthesis. As there is a lack of evidence on alcohol's effect on the Rag-Ragulator complex as a possible mechanism for the development of alcoholic skeletal muscle wasting, the aim of our study was to examine alterations in various protein-protein complexes in the Rag-Ragulator pathway produced acutely by feeding and how these are altered by alcohol under in vivo conditions. Mice (C57Bl/6; adult males) were fasted, and then provided rodent chow for 30 min ("refed") or remained food-deprived ("fasted"). Mice subsequently received ethanol (3 g/kg ethanol) or saline intraperitoneally, and hindlimb muscles were collected 1 h thereafter for analysis. Refeeding-induced increases in myofibrillar and sarcoplasmic protein synthesis, and mTOR and S6K1 phosphorylation, were prevented by alcohol. This inhibition was not associated with a differential rise in the intracellular leucine concentration or plasma leucine or insulin levels. Alcohol increased the amount of the Sestrin1•GATOR2 complex in the fasted state and prevented the refeeding-induced decrease in Sestrin1•GATOR2 seen in control mice. Alcohol antagonized the increase in the RagA/C•Raptor complex formation seen in the refed state. Alcohol antagonized the increase in Raptor with immunoprecipitated LAMPTOR1 (part of the Ragulator complex) after refeeding and decreased the association of RagC with LAMPTOR1. Finally, alcohol increased the association of the V1 domain of v-ATPase with LAMPTOR1 and prevented the refeeding-induced decrease in v-ATPase V1 with LAMPTOR1. Overall, these data demonstrate that acute alcohol intake disrupts multiple protein-protein complexes within the Rag-Ragulator complex, which are associated with and consistent with the concomitant decline in nutrient-stimulated muscle protein synthesis under in vivo conditions.

The distribution of eukaryotic initiation factor 4E after bouts of resistance exercise is altered by shortening of recovery periods

Insufficient duration of recovery between resistance exercise bouts reduces the effects of exercise training, but the influence on muscle anabolic responses is not fully understood. Here, we investigated the changes in the distribution of eukaryotic initiation factor (eIF) 4E, a key regulator of translation initiation, and related factors in mouse skeletal muscle after three successive bouts of resistance exercise with three durations of recovery periods (72 h: conventional, 24 h: shorter, and 8 h: excessively shorter). Bouts of resistance exercise dissociated eIF4E from eIF4E binding protein 1, with the magnitude increasing with shorter recovery. Whereas bouts of resistance exercise with 72 h recovery increased the association of eIF4E and eIF4G, those with shorter recovery did not. Similar results were observed in muscle protein synthesis. These results suggest that insufficient recovery inhibited the association of eIF4E and eIF4G, which might cause attenuation of protein synthesis activation after bouts of resistance exercise.

Garcimultiflorone K inhibits angiogenesis through Akt/eNOS- and mTOR-dependent pathways in human endothelial progenitor cells

Background Garcimultiflorone K is a novel polyprenylated polycyclic acylphloroglucinol isolated from the stems of Garcinia multiflora that exhibits promising anti-angiogenic activity in human endothelial progenitor cells (EPCs). Purpose This study sought to determine the underlying anti-angiogenic mechanisms and pharmacological properties of garcimultiflorone K. Methods We examined the anti-angiogenic effects of garcimultiflorone K and its mechanisms of action using in vitro EPC models and in vivo zebrafish embryos. Results EPCs proliferation, migration, differentiation and capillary-like tube formation were effectively and concentration-dependently inhibited by garcimultiflorone K without any signs of cytotoxicity. Our investigations revealed that garcimultiflorone K suppressed EPCs angiogenesis through Akt, mTOR, p70S6K, and eNOS signaling cascades. Notably, garcimultiflorone K dose-dependently impeded angiogenesis in zebrafish embryos. Conclusion Our data demonstrate the anti-angiogneic effects of garcimultiflorone K in both in vitro and in vivo models. Garcimultiflorone K appears to have potential in the treatment of angiogenesis-related diseases.

Mechanisms Underlying Muscle Protein Imbalance Induced by Alcohol

Both acute intoxication and longer-term cumulative ingestion of alcohol negatively impact the metabolic phenotype of both skeletal and cardiac muscle, independent of overt protein calorie malnutrition, resulting in loss of skeletal muscle strength and cardiac contractility. In large part, these alcohol-induced changes are mediated by a decrease in protein synthesis that in turn is governed by impaired activity of a protein kinase, the mechanistic target of rapamycin (mTOR). Herein, we summarize recent advances in understanding mTOR signal transduction, similarities and differences between the effects of alcohol on this central metabolic controller in skeletal muscle and in the heart, and the effects of acute versus chronic alcohol intake. While alcohol-induced alterations in global proteolysis via activation of the ubiquitin-proteasome pathway are equivocal, emerging data suggest alcohol increases autophagy in muscle. Further studies are necessary to define the relative contributions of these bidirectional changes in protein synthesis and autophagy in the etiology of alcoholic myopathy in skeletal muscle and the heart.

Ethanol acutely antagonizes the refeeding-induced increase in mTOR-dependent protein synthesis and decrease in autophagy in skeletal muscle

The purpose of this study was to determine the impact of acute ethanol administration on the major signal transduction pathways in skeletal muscle responsible for regulating the protein synthetic and degradative response to refeeding. Adult male C57Bl/6 mice were fasted overnight; mice were then either refed normal rodent chow for 30 min or a separate group of mice remained food deprived (i.e., fasted). Thereafter, mice were administered either 3 g/kg ethanol or saline. Gastrocnemius/plantaris was collected 1 h later and analyzed. Acute ethanol decreased basal and prevented the refeeding-induced increase in muscle protein synthesis. While ethanol prevented a nutrient-stimulated increase in S6K1 phosphorylation, it did not alter the increase in 4E-BP1 phosphorylation. Downstream of S6K1, ethanol also attenuated the refeeding-induced increase in S6 and eIF4B phosphorylation, as well as the decrease in eEF2 phosphorylation. Although ethanol decreased ERK and p90 RSK phosphorylation, activation of this signaling pathway was not altered by refeeding in either control or ethanol-treated mice. Related to protein degradation, in vitro-determined proteasome activity and the content of total ubiquitinated proteins were not altered by ethanol and/or refeeding. Control mice appeared to exhibit a refeeding-induced decrease in autophagy as suggested by the increased FoxO3 and ULK1 phosphorylation and total p62 protein as well as decreased LC3B-II; however, ethanol blunted these refeeding-induced changes. These data suggest that ethanol can acutely prevent the normally observed mTOR-dependent increase in protein synthesis and reduction in autophagy in response to nutrient stimulation, but does not appear to acutely alter proteasome activity.

The metabolic and molecular mechanisms of hyperammonaemia- and hyperethanolaemia-induced protein catabolism in skeletal muscle cells

Hyperammonaemia and hyperethanolaemia are thought to be driving factors behind skeletal muscle myopathy in liver disease, that is, cirrhosis. Despite this, the singular and combined impacts of ethanol‐ and ammonia‐induced protein catabolism are poorly defined. As such, we aimed to dissect out the effects of ammonia and ethanol on muscle catabolism. Murine C2C12 myotubes were treated with ammonium acetate (10 mM) and ethanol (100 mM) either alone or in combination for 4 hr and/or 24 hr. Myotube diameter, muscle protein synthesis and anabolic and catabolic signalling pathways were assessed. In separate experiments, cells were cotreated with selected inhibitors of protein breakdown to assess the importance of proteolytic pathways in protein loss with ammonia and ethanol. Ammonia and ethanol in combination resulted in a reduction in myotube width and total protein content, which was greater than the reduction observed with ammonia alone. Both ammonia and ethanol caused reductions in protein synthesis, as assessed by puromycin incorporation. There was also evidence of impairments in regulation of protein translation, and increased protein expression of markers of muscle protein breakdown. Myotube protein loss with ammonia plus ethanol was not affected by autophagy inhibition, but was completely prevented by proteasome inhibition. Thus, combined ammonia and ethanol incubation of C2C12 myotubes exacerbated myotube atrophy and dysregulation of anabolic and catabolic signalling pathways associated with either component individually. Ubiquitin proteasome‐mediated protein breakdown appears to play an important role in myotube protein loss with ethanol and ammonia.

Cellular and molecular mechanisms of alcohol-induced osteopenia

Alcoholic beverages are widely consumed, resulting in a staggering economic cost in different social and cultural settings. Types of alcohol consumption vary from light occasional to heavy, binge drinking, and chronic alcohol abuse at all ages. In general, heavy alcohol consumption is widely recognized as a major epidemiological risk factor for chronic diseases and is detrimental to many organs and tissues, including bones. Indeed, recent findings demonstrate that alcohol has a dose-dependent toxic effect in promoting imbalanced bone remodeling. This imbalance eventually results in osteopenia, an established risk factor for osteoporosis. Decreased bone mass and strength are major hallmarks of osteopenia, which is predominantly attributed not only to inhibition of bone synthesis but also to increased bone resorption through direct and indirect pathways. In this review, we present knowledge to elucidate the epidemiology, potential pathogenesis, and major molecular mechanisms and cellular effects that underlie alcoholism-induced bone loss in osteopenia. Novel therapeutic targets for correcting alcohol-induced osteopenia are also reviewed, such as modulation of proinflammatory cytokines and Wnt and mTOR signaling and the application of new drugs.

Effect of Chronic Alcohol Abuse on Anabolic and Catabolic Signaling Pathways in Human Skeletal Muscle

BACKGROUND

Animal studies showed that alcoholic myopathy is characterized by the reduction in myofiber cross-sectional area (CSA) and by impaired anabolic signaling. The goal of this study was to compare changes in CSA and fiber type composition with modifications in anabolic and catabolic signaling pathways at the early stages of alcohol misuse in humans.

METHODS

Skeletal muscle samples from 7 male patients with chronic alcohol abuse (AL; 47.7 ± 2.0 years old; alcohol misuse duration 7.7 ± 0.6 years) were compared with muscle from a control group of 7 healthy men (C; 39.7 ± 5.0 years old). Biopsies from vastus lateralis muscles were taken and analyzed for the changes in fiber type composition, fiber CSA, and for the alterations in anabolic and catabolic signaling pathways.

RESULTS

AL patients did not have detectable clinical myopathy symptoms or muscle fiber atrophy, but the relative proportion of fast fibers was increased. There was a significant decrease in IGF-1 in plasma and IRS-1 protein content in muscle of AL group. Levels of total and phosphorylated p70S6K1, GSK3β, and p90RSK1 were not different between AL and C groups. Muscle of AL patients had increased mRNA expression of HSP70 and HSP90. A marker of anabolic pathway p-4E-BP1 was decreased, while catabolic markers (MuRF-1, MAFbx, ubiquitinated proteins) were increased in AL patients when compared with C group.

CONCLUSIONS

At the early stages of alcohol misuse in humans, changes in the regulation of anabolic and catabolic signaling pathways precede the development of skeletal muscle atrophy and manifestation of clinical symptoms of alcoholic myopathy.

Effect of Acute Alcohol Ingestion on Resistance Exercise-Induced mTORC1 Signaling in Human Muscle

Duplanty, AA, Budnar, RG, Luk, HY, Levitt, DE, Hill, DW, McFarlin, BK, Huggett, DB, and Vingren, JL. Effect of acute alcohol ingestion on resistance exercise-induced mTORC1 signaling in human muscle. J Strength Cond Res 31(1): 54-61, 2017-The purpose of this project was to further elucidate the effects postexercise alcohol ingestion. This project had many novel aspects including using a resistance exercise (RE) only exercise design and the inclusion of women. Ten resistance-trained males and 9 resistance-trained females completed 2 identical acute heavy RE trials (6 sets of Smith machine squats) followed by ingestion of either alcohol or placebo. All participants completed both conditions. Before exercise (PRE) and 3 (+3 hours) and 5 (+5 hours) hours postexercise, muscle tissue samples were obtained from the vastus lateralis by biopsies. Muscle samples were analyzed for phosphorylated mTOR, S6K1, and 4E-BP1. For men, there was a significant interaction effect for mTOR and S6K1 phosphorylation. At +3 hours, mTOR and S6K1 phosphorylation was higher for placebo than for alcohol. For women, there was a significant main effect for time. mTOR phosphorylation was higher at +3 hours than at PRE and at +5 hours. There were no significant effects found for 4E-BP1 phosphorylation in men or women. The major findings of this study was that although RE elicited similar mTORC1 signaling both in men and in women, alcohol ingestion seemed to only attenuate RE-induced phosphorylation of the mTORC1 signaling pathway in men. This study provides evidence that alcohol should not be ingested after RE as this ingestion could potentially hamper the desired muscular adaptations to RE by reducing anabolic signaling, at least in men.

Protein coingestion with alcohol following strenuous exercise attenuates alcohol-induced intramyocellular apoptosis and inhibition of autophagy

Alcohol ingestion decreases postexercise rates of muscle protein synthesis, but the mechanism(s) (e.g., increased protein breakdown) underlying this observation is unknown. Autophagy is an intracellular "recycling" system required for homeostatic substrate and organelle turnover; its dysregulation may provoke apoptosis and lead to muscle atrophy. We investigated the acute effects of alcohol ingestion on autophagic cell signaling responses to a bout of concurrent (combined resistance- and endurance-based) exercise. In a randomized crossover design, eight physically active males completed three experimental trials of concurrent exercise with either postexercise ingestion of alcohol and carbohydrate (12 ± 2 standard drinks; ALC-CHO), energy-matched alcohol and protein (ALC-PRO), or protein (PRO) only. Muscle biopsies were taken at rest and 2 and 8 h postexercise. Select autophagy-related gene (Atg) proteins decreased compared with rest with ALC-CHO (P < 0.05) but not ALC-PRO. There were parallel increases (P < 0.05) in p62 and PINK1 commensurate with a reduction in BNIP3 content, indicating a diminished capacity for mitochondria-specific autophagy (mitophagy) when alcohol and carbohydrate were coingested. DNA fragmentation increased in both alcohol conditions (P < 0.05); however, nuclear AIF accumulation preceded this apoptotic response with ALC-CHO only (P < 0.05). In contrast, increases in the nuclear content of p53, TFEB, and PGC-1α in ALC-PRO were accompanied by markers of mitochondrial biogenesis at the transcriptional (Tfam, SCO2, and NRF-1) and translational (COX-IV, ATPAF1, and VDAC1) level (P < 0.05). We conclude that alcohol ingestion following exercise triggers apoptosis, whereas the anabolic properties of protein coingestion may stimulate mitochondrial biogenesis to protect cellular homeostasis.

Nutrition and Alcoholic Liver Disease: Effects of Alcoholism on Nutrition, Effects of Nutrition on Alcoholic Liver Disease, and Nutritional Therapies for Alcoholic Liver Disease

Malnutrition is the most frequent and nearly universal consequence in alcoholic liver disease (ALD) that adversely affects clinical outcomes. Sarcopenia or skeletal muscle loss is the major component of malnutrition in liver disease. There are no effective therapies to prevent or reverse sarcopenia in ALD because the mechanisms are not well understood. Consequences of liver disease including hyperammonemia, hormonal perturbations, endotoxemia and cytokine abnormalities as well as the direct effects of alcohol and its metabolites contribute to sarcopenia in ALD. This article focuses on the prevalence, methods to quantify malnutrition, specifically sarcopenia and potential therapies including novel molecular targeted treatments.

Dysregulation of skeletal muscle protein metabolism by alcohol

Alcohol abuse, either by acute intoxication or prolonged excessive consumption, leads to pathological changes in many organs and tissues including skeletal muscle. As muscle protein serves not only a contractile function but also as a metabolic reserve for amino acids, which are used to support the energy needs of other tissues, its content is tightly regulated and dynamic. This review focuses on the etiology by which alcohol perturbs skeletal muscle protein balance and thereby over time produces muscle wasting and weakness. The preponderance of data suggest that alcohol primarily impairs global protein synthesis, under basal conditions as well as in response to several anabolic stimuli including growth factors, nutrients, and muscle contraction. This inhibitory effect of alcohol is mediated, at least in part, by a reduction in mTOR kinase activity via a mechanism that remains poorly defined but likely involves altered protein-protein interactions within mTOR complex 1. Furthermore, alcohol can exacerbate the decrement in mTOR and/or muscle protein synthesis present in other catabolic states. In contrast, alcohol-induced changes in muscle protein degradation, either global or via specific modulation of the ubiquitin-proteasome or autophagy pathways, are relatively inconsistent and may be model dependent. Herein, changes produced by acute intoxication versus chronic ingestion are contrasted in relation to skeletal muscle metabolism, and limitations as well as opportunities for future research are discussed. As the proportion of more economically developed countries ages and chronic illness becomes more prevalent, a better understanding of the etiology of biomedical consequences of alcohol use disorders is warranted.

Moderate alcohol consumption does not impair overload-induced muscle hypertrophy and protein synthesis

Chronic alcohol consumption leads to muscle weakness and atrophy in part by suppressing protein synthesis and mTORC1-mediated signaling. However, it is unknown whether moderate alcohol consumption also prevents overload-induced muscle growth and related anabolic signaling. Hypertrophy of the plantaris muscle was induced by removal of a section of the gastrocnemius and soleus muscles from one leg of C57BL/6 adult male mice while the contralateral leg remained intact as the sham control. A nutritionally complete alcohol-containing liquid diet (EtOH) or isocaloric, alcohol-free liquid diet (Con) was provided for 14 days post-surgery. EtOH intake was increased progressively (day 1-5) before being maintained at ~20 g/day/kg BW. The plantaris muscle from the sham and OL leg was removed after 14 days at which time there was no difference in body weight between Con and EtOH-fed mice. OL increased muscle weight (90%) and protein synthesis (125%) in both Con and EtOH mice. The overload-induced increase in mTOR (Ser2448), 4E-BP1 (Thr37/46), S6K1 (Thr389), rpS6 (Ser240/244), and eEF2 (Thr56) were comparable in muscle from Con and EtOH mice. Modulation of signaling upstream of mTORC1 including REDD1 protein expression, Akt (Thr308), PRAS40 (Thr246), and ERK (Thr202/Tyr204) also did not differ between Con and EtOH mice. Markers of autophagy (ULK1, p62, and LC3) suggested inhibition of autophagy with overload and activation with alcohol feeding. These data show that moderate alcohol consumption does not impair muscle growth, and therefore imply that resistance exercise may be an effective therapeutic modality for alcoholic-related muscle disease.

Alcohol intoxication following muscle contraction in mice decreases muscle protein synthesis but not mTOR signal transduction

BACKGROUND

Alcohol (ethanol [EtOH]) intoxication antagonizes stimulation of muscle protein synthesis and mammalian target of rapamycin (mTOR) signaling. However, whether the anabolic response can be reversed when alcohol is consumed after the stimulus is unknown.

METHODS

A single bout of electrically stimulated muscle contractions (10 sets of 6 contractions) was induced in fasted male C57BL/6 mice 2 hours prior to alcohol intoxication. EtOH was injected intraperitoneally (3 g/kg), and the gastrocnemius/plantaris muscle complex was collected 2 hours later from the stimulated and contralateral unstimulated control leg.

RESULTS

Muscle contraction increased protein synthesis 28% in control mice, while EtOH abolished this stimulation-induced increase. Further, EtOH suppressed the rate of synthesis ~75% compared to control muscle irrespective of stimulation. This decrease was associated with impaired protein elongation as EtOH increased the phosphorylation of eEF2 Thr(56) . In contrast, stimulation-induced increases in mTOR protein complex-1 (mTORC1) (S6K1 Thr(421) /Ser(424) , S6K1 Thr(389) , rpS6 Ser(240/244) , and 4E-BP1 Thr(37/46) ) and mitogen-activated protein kinase (MAPK) (JNK Thr(183) /Tyr(185) , p38 Thr(180) /Tyr(182) , and rpS6S(235/236) ) signaling were not reversed by acute EtOH.

CONCLUSIONS

These data suggest that EtOH-induced decreases in protein synthesis in fasted mice may be independent of mTORC1 and MAPK signaling following muscle contraction and instead due to the antagonistic actions of EtOH on mRNA translation elongation. Therefore, EtOH suppresses the contraction-induced increase in protein synthesis, and over time has the potential to prevent skeletal muscle hypertrophy induced by repeated muscle contraction.

Alcohol impairs skeletal muscle protein synthesis and mTOR signaling in a time-dependent manner following electrically stimulated muscle contraction

Alcohol (EtOH) decreases protein synthesis and mammalian target of rapamycin (mTOR)-mediated signaling and blunts the anabolic response to growth factors in skeletal muscle. The purpose of the current investigation was to determine whether acute EtOH intoxication antagonizes the contraction-induced increase in protein synthesis and mTOR signaling in skeletal muscle. Fasted male mice were injected intraperitoneally with 3 g/kg EtOH or saline (control), and the right hindlimb was electrically stimulated (10 sets of 6 contractions). The gastrocnemius muscle complex was collected 30 min, 4 h, or 12 h after stimulation. EtOH decreased in vivo basal protein synthesis (PS) in the nonstimulated muscle compared with time-matched Controls at 30 min, 4 h, and 12 h. In Control, but not EtOH, PS was decreased 15% after 30 min. In contrast, PS was increased in Control 4 h poststimulation but remained unchanged in EtOH. Last, stimulation increased PS 10% in Control and EtOH at 12 h, even though the absolute rate remained reduced by EtOH. The stimulation-induced increase in the phosphorylation of S6K1 Thr(421)/Ser(424) (20-52%), S6K1 Thr(389) (45-57%), and its substrate rpS6 Ser(240/244) (37-72%) was blunted by EtOH at 30 min, 4 h, and 12 h. Phosphorylation of 4E-BP1 Ser(65) was also attenuated by EtOH (61%) at 4 h. Conversely, phosphorylation of extracellular signal-regulated kinase Thr(202)/Tyr(204) was increased by stimulation in Control and EtOH mice at 30 min but only in Control at 4 h. Our data indicate that acute EtOH intoxication suppresses muscle protein synthesis for at least 12 h and greatly impairs contraction-induced changes in synthesis and mTOR signaling.

Alcohol ingestion impairs maximal post-exercise rates of myofibrillar protein synthesis following a single bout of concurrent training

Introduction

The culture in many team sports involves consumption of large amounts of alcohol after training/competition. The effect of such a practice on recovery processes underlying protein turnover in human skeletal muscle are unknown. We determined the effect of alcohol intake on rates of myofibrillar protein synthesis (MPS) following strenuous exercise with carbohydrate (CHO) or protein ingestion.

Methods

In a randomized cross-over design, 8 physically active males completed three experimental trials comprising resistance exercise (8×5 reps leg extension, 80% 1 repetition maximum) followed by continuous (30 min, 63% peak power output (PPO)) and high intensity interval (10×30 s, 110% PPO) cycling. Immediately, and 4 h post-exercise, subjects consumed either 500 mL of whey protein (25 g; PRO), alcohol (1.5 g·kg body mass⁻¹, 12±2 standard drinks) co-ingested with protein (ALC-PRO), or an energy-matched quantity of carbohydrate also with alcohol (25 g maltodextrin; ALC-CHO). Subjects also consumed a CHO meal (1.5 g CHO·kg body mass⁻¹) 2 h post-exercise. Muscle biopsies were taken at rest, 2 and 8 h post-exercise.

Results

Blood alcohol concentration was elevated above baseline with ALC-CHO and ALC-PRO throughout recovery (P<0.05). Phosphorylation of mTOR^Ser2448 2 h after exercise was higher with PRO compared to ALC-PRO and ALC-CHO (P<0.05), while p70S6K phosphorylation was higher 2 h post-exercise with ALC-PRO and PRO compared to ALC-CHO (P<0.05). Rates of MPS increased above rest for all conditions (∼29–109%, P<0.05). However, compared to PRO, there was a hierarchical reduction in MPS with ALC-PRO (24%, P<0.05) and with ALC-CHO (37%, P<0.05).

Conclusion

We provide novel data demonstrating that alcohol consumption reduces rates of MPS following a bout of concurrent exercise, even when co-ingested with protein. We conclude that alcohol ingestion suppresses the anabolic response in skeletal muscle and may therefore impair recovery and adaptation to training and/or subsequent performance.

Strain-dependent differences for suppression of insulin-stimulated glucose uptake in skeletal and cardiac muscle by ethanol

BACKGROUND

Chronic ethanol (EtOH) consumption impairs the ability of insulin to suppress hepatic glucose production in a strain-dependent manner, with hepatic insulin resistance being greater in Long-Evans (LE) than Sprague-Dawley (SD) rats. We assessed whether strain differences exist for whole-body and tissue glucose uptake under basal and insulin-stimulated conditions and whether they were associated with coordinate strain-dependent elevations in muscle cytokines.

METHODS

Male rats (160 g) were provided the Lieber-DeCarli EtOH-containing (36% total energy) diet or pair-fed a control diet for 8 weeks. Rats were studied in the basal state or during a euglycemic hyperinsulinemic clamp, and whole-body glucose flux assessed using (3) H-glucose and in vivo tissue glucose uptake by (14) C-2-deoxyglucose.

RESULTS

EtOH impaired whole-body insulin-mediated glucose uptake (IMGU) more in SD than LE rats. This difference was due to impaired IMGU by gastrocnemius and heart in EtOH-fed SD versus LE rats. However, decreased IMGU in adipose tissue (epididymal and perirenal) produced by EtOH was comparable between strains. EtOH-induced insulin resistance in muscle from SD rats was associated with reduced AKT and AS160 phosphorylation and plasma membrane-localized GLUT4 protein as well as enhanced phosphorylation of c-Jun N-terminal kinase (JNK) and IRS-1 (S307), changes which were absent in muscle from LE rats. EtOH increased tumor necrosis factor alpha (TNFα) mRNA in gastrocnemius and fat under basal conditions in both SD and LE rats; however, hyperinsulinemia decreased TNFα in skeletal muscle from LE, but not SD rats. Interleukin (IL)-6 mRNA in gastrocnemius was increased under basal conditions and increased further in response to insulin in SD rats, but no EtOH- or insulin-induced change was detected in muscle IL-6 of LE rats.

CONCLUSIONS

These data indicate strain-dependent differences in EtOH-induced IMGU in skeletal and cardiac muscle, but not fat, associated with sustained increases in TNFα and IL-6 mRNA and JNK activation and decreased plasma membrane GLUT4 in response to insulin.

Direct central nervous system effect of alcohol alters synthesis and degradation of skeletal muscle protein

AIMS

Alcohol can directly impair protein synthesis in cultured myocytes as well as in in situ perfused skeletal muscle. However, alcohol in the general circulation diffuses rapidly into the central nervous system (CNS). Therefore, this study determined whether localized elevation of alcohol within the CNS is capable of decreasing muscle protein synthesis.

METHODS

Conscious unstrained male rats received a continuous intracerebroventricular (ICV) infusion of ethanol and skeletal muscle protein synthesis and degradation were assessed.

RESULTS

ICV alcohol decreased protein synthesis in the gastrocnemius after 6 and 24 h, compared with the time-matched controls. The reduction was equivalent for both sarcoplasmic and myofibrillar proteins and was reversible. The inhibitory effect of alcohol was not prevented by the catalase inhibitor 3-amino-1,2,4-triazole and was mimicked by ICV-administered t-butanol. The alcohol-induced decrease in muscle protein synthesis was associated with a concomitant reduction in phosphorylation of 4E-binding protein and ribosomal S6 kinase-1, suggesting impaired mammalian target of rapamycin kinase activity. ICV alcohol also impaired the ability of leucine to stimulate protein synthesis. Conversely, ICV alcohol increased muscle proteasome activity and muscle RING-finger protein-1 mRNA content. Altered muscle protein metabolism was not associated with changes in muscle mRNA content for tumor necrosis factor α, interleukin-6 or insulin-like growth factor (IGF)-I or circulating insulin or IGF-I.

CONCLUSION

Selective elevation of alcohol within the CNS is capable of decreasing protein synthesis and increasing protein degradation in muscle in the absence of alcohol in the general circulation, thus revealing a previously unrecognized central neural mechanism, which may account for part of the inhibitory effect of ingested alcohol on muscle protein homeostasis.

Mechanisms mediating the effects of alcohol and HIV anti-retroviral agents on mTORC1, mTORC2 and protein synthesis in myocytes

Alcoholism and acquired immune deficiency syndrome are associated with severe muscle wasting. This impairment in nitrogen balance arises from increased protein degradation and a decreased rate of protein synthesis. The regulation of protein synthesis is a complex process involving alterations in the phosphorylation state and protein-protein interaction of various components of the translation machinery and mammalian target of rapamycin (mTOR) complexes. This review describes mechanisms that regulate protein synthesis in cultured C2C12 myocytes following exposure to either alcohol or human immunodeficiency virus antiretroviral drugs. Particular attention is given to the upstream regulators of mTOR complexes and the downstream targets which play an important role in translation. Gaining a better understanding of these molecular mechanisms could have important implications for preventing changes in lean body mass in patients with catabolic conditions or illnesses.

Multifaceted role of insulin-like growth factors and mammalian target of rapamycin in skeletal muscle

This review describes the current literature on the interaction between insulin-like growth factors, endocrine hormones, and branched-chain amino acids on muscle physiology in healthy young individuals and during select pathologic conditions. Emphasis is placed on the mechanism by which physical and hormonal signals are transduced at the cellular level to either grow or atrophy skeletal muscle. The key role of the mammalian target of rapamycin and its ability to respond to hypertrophic and atrophic signals informs our understanding how a combination of physical, nutritional, and pharmacologic therapies may be used in tandem to prevent or ameliorate reductions in muscle mass.

Procysteine increases alcohol-depleted glutathione stores in rat plantaris following a period of abstinence

Aims: To assess the effectiveness of procysteine (PRO) supplementation provided during a period of abstinence (ABS) on alcohol-induced skeletal muscle atrophy and oxidant stress. Methods: Age- and gender-matched Sprague–Dawley rats were fed the Lieber–DeCarli liquid diet containing either alcohol or an isocaloric substitution (control diet) for 12 week. Next, subgroups of alcohol-fed rats were fed the control diet for 2 week (ABS) supplemented with either PRO (0.35%, w/v) or vehicle. Plantaris morphology was assessed by hematoxylin and eosin staining. Total, reduced and oxidized glutathione (GSH) levels and total antioxidant potential were determined by commercially available assay kits. Antibody arrays were used to determine cytokine levels. Real-time polymerase chain reaction was used to determine gene expressions of two E3 ubiquitin ligases, atrogin-1 and muscle ring finger protein-1 (MuRF-1). Results: Plantaris muscles from alcohol-fed rats displayed extensive atrophy, as well as decreased GSH levels, a trend for decreased total antioxidant potential and elevated atrogin-1 and MuRF-1 mRNA levels. GSH levels and total antioxidant potential continued to decrease during 2 weeks of ABS from alcohol, which were normalized in abstinent rats provided PRO. Gene levels of both E3 ligases returned to baseline during ABS. In parallel, plantaris cross-sectional area increased in both groups during ABS. Conclusions: PRO supplementation during ABS significantly attenuated alcohol-induced redox stress compared with untreated abstinent rats. Thus, our data may suggest that GSH restoration therapy may provide therapeutic benefits to the overall antioxidant state of skeletal muscle when prescribed in conjunction with an established detoxification program for recovering alcoholics.

Alcohol-induced IGF-I resistance is ameliorated in mice deficient for mitochondrial branched-chain aminotransferase

Acute alcohol intoxication decreases skeletal muscle protein synthesis by impairing mammalian target of rapamycin (mTOR). In 2 studies, we determined whether inhibition of branched-chain amino acid (BCAA) catabolism ameliorates the inhibitory effect of alcohol on muscle protein synthesis by raising the plasma BCAA concentrations and/or by improving the anabolic response to insulin-like growth factor (IGF)-I. In the first study, 4 groups of mice were used: wild-type (WT) and mitochondrial branched-chain aminotransferase (BCATm) knockout (KO) mice orally administered saline or alcohol (5 g/kg, 1 h). Protein synthesis was greater in KO mice compared with WT controls and was associated with greater phosphorylation of eukaryotic initiation factor (eIF)-4E binding protein-1 (4EBP1), eIF4E-eIF4G binding, and 4EBP1-regulatory associated protein of mTOR (raptor) binding, but not mTOR-raptor binding. Alcohol decreased protein synthesis in WT mice, a change associated with less 4EBP1 phosphorylation, eIF4E-eIF4G binding, and raptor-4EBP1 binding, but greater mTOR-raptor complex formation. Comparable alcohol effects on protein synthesis and signal transduction were detected in BCATm KO mice. The second study used the same 4 groups, but all mice were injected with IGF-I (25 microg/mouse, 30 min). Alcohol impaired the ability of IGF-I to increase muscle protein synthesis, 4EBP1 and 70-kilodalton ribosomal protein S6 kinase-1 phosphorylation, eIF4E-eIF4G binding, and 4EBP1-raptor binding in WT mice. However, in alcohol-treated BCATm KO mice, this IGF-I resistance was not manifested. These data suggest that whereas the sustained elevation in plasma BCAA is not sufficient to ameliorate the catabolic effect of acute alcohol intoxication on muscle protein synthesis, it does improve the anabolic effect of IGF-I.

Castration differentially alters basal and leucine-stimulated tissue protein synthesis in skeletal muscle and adipose tissue

Reduced testosterone as a result of catabolic illness or aging is associated with loss of muscle and increased adiposity. We hypothesized that these changes in body composition occur because of altered rates of protein synthesis under basal and nutrient-stimulated conditions that are tissue specific. The present study investigated such mechanisms in castrated male rats (75% reduction in testosterone) with demonstrated glucose intolerance. Over 9 wk, castration impaired body weight gain, which resulted from a reduced lean body mass and preferential sparing of adipose tissue. Castration decreased gastrocnemius weight, but this atrophy was not associated with reduced basal muscle protein synthesis or differences in plasma IGF-I, insulin, or individual amino acids. However, oral leucine failed to normally stimulate muscle protein synthesis in castrated rats. In addition, castration-induced atrophy was associated with increased 3-methylhistidine excretion and in vitro-determined ubiquitin proteasome activity in skeletal muscle, changes that were associated with decreased atrogin-1 or MuRF1 mRNA expression. Castration decreased heart and kidney weight without reducing protein synthesis and did not alter either cardiac output or glomerular filtration. In contradistinction, the weight of the retroperitoneal fat depot was increased in castrated rats. This increase was associated with an elevated rate of basal protein synthesis, which was unresponsive to leucine stimulation. Castration also decreased whole body fat oxidation. Castration increased TNFα, IL-1α, IL-6, and NOS2 mRNA in fat but not muscle. In summary, the castration-induced muscle wasting results from an increased muscle protein breakdown and the inability of leucine to stimulate protein synthesis, whereas the expansion of the retroperitoneal fat depot appears mediated in part by an increased basal rate of protein synthesis-associated increased inflammatory cytokine expression.

Procysteine stimulates expression of key anabolic factors and reduces plantaris atrophy in alcohol-fed rats

BACKGROUND

Long-term alcohol ingestion may produce severe oxidant stress and lead to skeletal muscle dysfunction. Emerging evidence has suggested that members of the interleukin-6 (IL-6) family of cytokines play diverse roles in the regulation of skeletal muscle mass. Thus, our goals were (i) to minimize the degree of oxidant stress and attenuate atrophy by supplementing the diets of alcohol-fed rats with the glutathione precursor, procysteine, and (ii) to identify the roles of IL-6 family members in alcoholic myopathy.

METHODS

Age- and gender-matched Sprague-Dawley rats were fed the Lieber-DeCarli liquid diet containing either alcohol or an isocaloric substitution (control diet) for 35 weeks. Subgroups of alcohol-fed rats received procysteine (0.35%, w/v) for the final 12 weeks. Plantaris morphology was assessed by hematoxylin and eosin staining. Major components of glutathione metabolism were determined using assay kits. Real-time PCR was used to determine expression levels of several genes.

RESULTS

Plantaris muscles from alcohol-fed rats displayed extensive atrophy, as well as decreased glutathione levels, decreased activities of glutathione reductase and glutathione peroxidase, decreased superoxide dismutase (SOD)-2 (Mn-SOD2), and increased NADPH oxidase-1 gene expression-each indicative of significant oxidant stress. Alcohol also induced gene expression of catabolic factors including IL-6, oncostatin M, atrogin-1, muscle ring finger protein-1, and IGFBP-1. Procysteine treatment attenuated plantaris atrophy, restored glutathione levels, and increased catalase, Cu/Zn-SOD1, and Mn-SOD2 mRNA expression, but did not reduce other markers of oxidant stress or levels of these catabolic factors. Instead, procysteine stimulated gene expression of anabolic factors such as insulin-like growth factor-1, ciliary neurotrophic factor, and cardiotrophin-1.

CONCLUSIONS

Procysteine significantly attenuated, but did not completely abrogate, alcohol-induced oxidant stress or catabolic factors. Rather, procysteine minimized the extent of plantaris atrophy by inducing components of several anabolic pathways. Therefore, anti-oxidant treatments such as procysteine supplementation may benefit individuals with alcoholic myopathy.

Alcohol-induced decrease in muscle protein synthesis associated with increased binding of mTOR and raptor: Comparable effects in young and mature rats

BACKGROUND

Acute alcohol (EtOH) intoxication decreases muscle protein synthesis via inhibition of mTOR-dependent translation initiation. However, these studies have been performed in relatively young rapidly growing rats in which muscle protein accretion is more sensitive to growth factor and nutrient stimulation. Furthermore, some in vivo-produced effects of EtOH vary in an age-dependent manner. The hypothesis tested in the present study was that young rats will show a more pronounced decrement in muscle protein synthesis than older mature rats in response to acute EtOH intoxication.

METHODS

Male F344 rats were studied at approximately 3 (young) or 12 (mature) months of age. Young rats were injected intraperitoneally with 75 mmol/kg of EtOH, and mature rats injected with either 75 or 90 mmol/kg EtOH. Time-matched saline-injected control rats were included for both age groups. Gastrocnemius protein synthesis and the activity of the mTOR pathway were assessed 2.5 h after EtOH using [³H]-labeled phenylalanine and the phosphorylation of various protein factors known to regulate peptide-chain initiation.

RESULTS

Blood alcohol levels (BALs) were lower in mature rats compared to young rats after administration of 75 mmol/kg EtOH (154 ± 23 vs 265 ± 24 mg/dL). However, injection of 90 mmol/kg EtOH in mature rats produced BALs comparable to that of young rats (281 ± 33 mg/dL). EtOH decreased muscle protein synthesis similarly in both young and high-dose EtOH-treated mature rats. The EtOH-induced changes in both groups were associated with a concomitant reduction in 4E-BP1 phosphorylation, and redistribution of eIF4E between the active eIF4E.eIF4G and inactive eIF4E.4EBP1 complex. Moreover, EtOH increased the binding of mTOR with raptor in a manner which appeared to be AMPK- and TSC-independent. In contrast, although muscle protein synthesis was unchanged in mature rats given low-dose EtOH, compared to control values, the phosphorylation of rpS6 and eIF4G was decreased.

CONCLUSION

These data indicate that muscle protein synthesis is equally sensitive to the inhibitory effects of EtOH in young rapidly growing rats and older mature rats which are growing more slowly, but that mature rats must be given a relatively larger dose of EtOH to achieve the same BAL. Based on the differential response in mature rats to low- and high-dose EtOH, the decreased protein synthesis was associated with a reduction in mTOR activity which was selectively mediated via a reduction in 4E-BP1 phosphorylation and an increase in mTOR.raptor formation.

Acute alcohol intoxication increases REDD1 in skeletal muscle

BACKGROUND

The mechanism by which acute alcohol (EtOH) intoxication decreases basal muscle protein synthesis via inhibition of the Ser/Thr kinase mammalian target of rapamycin (mTOR) is poorly defined. In this regard, mTOR activity is impaired after over expression of the regulatory protein REDD1. Hence, the present study assessed the ability of REDD1 as a potential mediator of the EtOH-induced decrease in muscle protein synthesis.

METHODS

The effect of acute EtOH intoxication on REDD1 mRNA and protein was determined in striated muscle of rats and mouse myocytes using an RNase protection assay and Western blotting, respectively. Other components of the mTOR signaling pathway were also assessed by immunoblotting. For comparison, REDD1 mRNA/protein was also determined in the muscle of rats chronically fed an alcohol-containing diet for 14 weeks.

RESULTS

Intraperitoneal (IP) injection of EtOH increased gastrocnemius REDD1 mRNA in a dose- and time-dependent manner, and these changes were associated with reciprocal decreases in the phosphorylation of 4E-BP1, which is a surrogate marker for mTOR activity and protein synthesis. No change in REDD1 mRNA was detected in the slow-twitch soleus muscle or heart. Acute EtOH produced comparable increases in muscle REDD1 protein. The EtOH-induced increase in gastrocnemius REDD1 was independent of the route of EtOH administration (oral vs. IP), the nutritional state (fed vs. fasted), gender, and age of the rat. The nonmetabolizable alcohol tert-butanol increased REDD1 and the EtOH-induced increase in REDD1 was not prevented by pretreatment with the alcohol dehydrogenase inhibitor 4-methylpyrazole. In contrast, REDD1 mRNA and protein were not increased in the isolated hindlimb perfused with EtOH or in C2C12 myocytes incubated with EtOH, under conditions previously reported to decrease protein synthesis. Pretreatment with the glucocorticoid receptor antagonist RU486 failed to prevent the EtOH-induced increase in REDD1. Finally, the EtOH-induced increase in REDD1 was not associated with altered formation of the TSC1*TSC2 complex or the phosphorylation of TSC2 which is down stream in the REDD1 stress response pathway. In contradistinction to the changes observed with acute EtOH intoxication, REDD1 mRNA/protein was not changed in gastrocnemius from chronic alcohol-fed rats despite the reduction in 4E-BP1 phosphorylation.

CONCLUSIONS

These data indicate that in fast-twitch skeletal muscle (i) REDD1 mRNA/protein is increased in vivo by acute EtOH intoxication but not in response to chronic alcohol feeding, (ii) elevated REDD1 in response to acute EtOH appears due to the production of an unknown secondary mediator which is not corticosterone, and (iii) the EtOH-induced decrease in protein synthesis can be dissociated from a change in REDD1 suggesting that the induction of this protein is not responsible for the rapid decrease in protein synthesis after acute EtOH administration or for the development of alcoholic myopathy in rats fed an alcohol-containing diet.

Molecular and cellular events in alcohol-induced muscle disease

Alcohol consumption induces a dose-dependent noxious effect on skeletal muscle, leading to progressive functional and structural damage of myocytes, with concomitant reductions in lean body mass. Nearly half of high-dose chronic alcohol consumers develop alcoholic skeletal myopathy. The pathogenic mechanisms that lie between alcohol intake and loss of muscle tissue involve multiple pathways, making the elucidation of the disease somewhat difficult. This review discusses the recent advances in basic and clinical research on the molecular and cellular events involved in the development of alcohol-induced muscle disease. The main areas of recent research interest on this field are as follows: (i) molecular mechanisms in alcohol exposed muscle in the rat model; (ii) gene expression changes in alcohol exposed muscle; (iii) the role of trace elements and oxidative stress in alcoholic myopathy; and (iv) the role of apoptosis and preapoptotic pathways in alcoholic myopathy. These aforementioned areas are crucial in understanding the pathogenesis of this disease. For example, there is overwhelming evidence that both chronic alcohol ingestion and acute alcohol intoxication impair the rate of protein synthesis of myofibrillar proteins, in particular, under both postabsorptive and postprandial conditions. Perturbations in gene expression are contributory factors to the development of alcoholic myopathy, as ethanol-induced alterations are detected in over 400 genes and the protein profile (i.e., the proteome) of muscle is also affected. There is supportive evidence that oxidative damage is involved in the pathogenesis of alcoholic myopathy. Increased lipid peroxidation is related to muscle fibre atrophy, and reduced serum levels of some antioxidants may be related to loss of muscle mass and muscle strength. Finally, ethanol induces skeletal muscle apoptosis and increases both pro- and antiapoptotic regulatory mechanisms.

Oxidant-induced atrogin-1 and transforming growth factor-beta1 precede alcohol-related myopathy in rats

Alcohol-related chronic myopathy is characterized by severe biochemical and structural changes to skeletal muscle. Our goals were to: (1) identify early regulatory elements that precede the overt manifestation of plantaris atrophy; and (2) circumvent these derangements by supplementing alcohol-fed rats with the glutathione precursor, procysteine. After 6 weeks of daily ingestion, before the development of overt atrophy of the plantaris muscle, alcohol increased several markers of oxidative stress and increased gene expressions of atrogin-1 and transforming growth factor-beta1 (TGF-beta1) by approximately 60- and approximately 65-fold, respectively, which were attenuated by procysteine supplementation. Interestingly, after 28 weeks of alcohol ingestion, when overt plantaris atrophy had developed, atrogin-1 and TGF-beta1 gene expression had returned to baseline levels. Together, these findings suggest that alcohol-induced, redox-sensitive alterations drive pro-atrophy signaling pathways that precede muscle atrophy. Therefore, targeted anti-oxidant treatments such as procysteine supplementation may benefit individuals with chronic alcohol abuse, particularly if given prior to the development of clinically significant myopathy.

Alcohol accelerates loss of muscle and impairs recovery of muscle mass resulting from disuse atrophy

BACKGROUND

Muscle disuse atrophy is observed in patients recovering from trauma and there is an increased risk and severity of injury in patients abusing alcohol (EtOH). However, the interaction of EtOH and disuse on muscle protein balance has not been examined. Therefore, the present study addressed the hypothesis that EtOH accelerates the disuse atrophy and/or impairs the accretion of muscle protein during muscle recovery.

METHODS

To address this aim, disuse atrophy was induced in rats by 3 days of unilateral hindlimb immobilization (casting), using the contralateral leg as control, with EtOH or saline being orally gavaged twice, each day during this period. In a separate study, EtOH-treated rats received Velcade to inhibit proteasomal degradation. Finally, in the last study, rats had 1 limb casted for 5 days, the cast removed, and EtOH or saline gavaged twice daily during a 5-day recovery period. Muscle protein metabolism was assessed using surrogate markers of protein synthesis [i.e., phosphorylation of 4E-binding protein 1 (BP1) and S6 kinase 1 (S6K1)] and protein degradation (i.e., mRNA content of the ubiquitin E3 ligases atrogin-1 and MuRF1).

RESULTS

Ethanol alone did not decrease muscle weight in the uncasted muscle. However, the loss of mass of immobilized muscle from EtOH-gavaged rats was 80% greater than in the animals not receiving EtOH. This atrophic response was not associated with a change in Akt, 4E-BP1 or S6K1 phosphorylation among groups. In contrast, immobilization alone increased both atrogin-1 and MuRF1 mRNA, and EtOH further increased their expression in immobilized muscle. The proteasome inhibitor Velcade attenuated atrophy produced by EtOH + disuse. When administered during the recovery period, EtOH prevented the normal accretion of muscle mass. This EtOH effect was associated with increased atrogin-1 mRNA, a reduction in 4E-BP1 and S6 phosphorylation, and an increased AMP-activated kinase phosphorylation.

CONCLUSIONS

Based on the changes in these surrogate markers, our data suggest that EtOH accelerates disuse atrophy by stimulating ubiquitin-mediated proteolysis, and blunts repletion of muscle protein during recovery from disuse by increasing proteolysis and decreasing protein synthesis.

Alcohol-induced disruption of endocrine signaling

This article contains the proceedings of a symposium at the 2006 ISBRA meeting in Sydney Australia, organized and cochaired by Martin J. Ronis and Thomas M. Badger. The presentations were (1) Effect of long-term ethanol consumption on liver injury and repair, by Jack R. Wands; (2) Alcohol-induced insulin resistance in liver: potential roles in regulation of ADH expression, ethanol clearance, and alcoholic liver disease, by Thomas M. Badger; (3) Chronic gestational exposure to ethanol causes brain insulin and insulin-like growth factor resistance, by Suzanne M de la Monte; (4) Disruption of IGF-1 signaling in muscle: a mechanism underlying alcoholic myopathy, by Charles H. Lang; (5) The role of reduced plasma estradiol and impaired estrogen signaling in alcohol-induced bone loss, by Martin J. Ronis; and (6) Short-term influence of alcohol on appetite-regulating hormones in man, by Jan Calissendorff.

Functional genomics identifies TOR-regulated genes that control growth and division

BACKGROUND

The TOR (target of rapamycin) ser/thr protein kinase is the central component of a eukaryotic signaling pathway that regulates growth and is the direct target of the clinically useful drug rapamycin. Recent efforts have identified at least two multiprotein complexes that contain TOR, but little is known in higher eukaryotes about the genes downstream of TOR that control growth.

RESULTS

By combining the use of a small molecule inhibitor (rapamycin), transcriptional profiling, and RNA interference in Drosophila tissue culture cells, we identified genes whose expression responds to Drosophila TOR (dTOR) inhibition and that regulate cell size. Several of the dTOR-regulated genes that function in cell size control have additional roles in cell division. Most of these genes are conserved in mammals and several are linked to human disease. This set of genes is highly enriched for regulators of ribosome biogenesis, which emphasizes the importance of TOR-dependent transcription in building the protein synthesis machinery in higher eukaryotes. In addition, we identify two dTOR-regulated genes, CG3071 and CG6677, whose human orthologs, SAW and ASH2L, are also under TOR-dependent transcriptional control and encode proteins with conserved functional roles in growth.

CONCLUSIONS

We conclude that combining RNA interference with genomic analysis approaches, such as transcriptional profiling, is an effective way to identify genes functioning in a particular biological process. Moreover, this strategy, if applied in model systems with simpler genomes, can identify genes with conserved functions in mammals.

Acetaldehyde promotes rapamycin-dependent activation of p70(S6K) and glucose uptake despite inhibition of Akt and mTOR in dopaminergic SH-SY5Y human neuroblastoma cells

Alcohol intake is one of the important lifestyle factors for the risk of insulin resistance and type 2 diabetes. Acetaldehyde, the major ethanol metabolite which is far more reactive than ethanol, has been postulated to participate in alcohol-induced tissue injury although its direct impact on insulin signaling is unclear. This study was designed to examine the effect of acetaldehyde on glucose uptake and insulin signaling in human dopaminergic SH-SY5Y cells. Akt, mammalian target of rapamycin (mTOR), ribosomal-S6 kinase (p70(S6K)), the eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) and insulin receptor substrate (IRS)-2 were evaluated by Western blot analysis. Glucose uptake and apoptosis were measured using [(3)H]-2-deoxyglucose uptake and caspase-3 assay, respectively. Short-term exposure (12 h) of acetaldehyde (150 muM) facilitated glucose uptake in a rapamycin-dependent manner without affecting apoptosis, IRS-2 expression and insulin-stimulated glucose uptake in SH-SY5Y cells. Acetaldehyde suppressed basal and insulin-stimulated Akt phosphorylation without affecting total Akt expression. Acetaldehyde inhibited mTOR phosphorylation without affecting total mTOR and insulin-elicited response on mTOR phosphorylation. Rapamycin, which inhibits mTOR leading to inactivation of p70(S6K), did not affect acetaldehyde-induced inhibition on phosphorylation of Akt and mTOR. Interestingly, acetaldehyde enhanced p70(S6K) activation and depressed 4E-BP1 phosphorylation, the effect of which was blunted and exaggerated, respectively, by rapamycin. Collectively, these data suggested that acetaldehyde did not adversely affect glucose uptake despite inhibition of insulin signaling cascade at the levels of Akt and mTOR, possibly due to presence of certain mechanism(s) responsible for enhanced p70(S6K) phosphorylation.

Elevated plasma free fatty acids decrease basal protein synthesis, but not the anabolic effect of leucine, in skeletal muscle

Elevations in free fatty acids (FFAs) impair glucose uptake in skeletal muscle. However, there is no information pertaining to the effect of elevated circulating lipids on either basal protein synthesis or the anabolic effects of leucine and insulin-like growth factor I (IGF-I). In chronically catheterized conscious rats, the short-term elevation of plasma FFAs by the 5-h infusion of heparin plus Intralipid decreased muscle protein synthesis by approximately 25% under basal conditions. Lipid infusion was associated with a redistribution of eukaryotic initiation factor (eIF)4E from the active eIF4E.eIF4G complex to the inactive eIF4E.4E-BP1 complex. This shift was associated with a decreased phosphorylation of eIF4G but not 4E-BP1. Lipid infusion did not significantly alter either the total amount or phosphorylation state of mTOR, TSC2, S6K1, or the ribosomal protein S6 under basal conditions. In control rats, oral leucine increased muscle protein synthesis. This anabolic response was not impaired by lipid infusion, and no defects in signal transduction pathways regulating translation initiation were detected. In separate rats that received a bolus injection of IGF-I, lipid infusion attenuated the normal redistribution of eIF4E from the active to inactive complex and largely prevented the increased phosphorylation of 4E-BP1, eIF4G, S6K1, and S6. This IGF-I resistance was associated with enhanced Ser(307) phosphorylation of insulin receptor substrate-1 (IRS-1). These data indicate that the short-term elevation of plasma FFAs impairs basal protein synthesis in muscle by altering eIF4E availability, and this defect may be related to impaired phosphorylation of eIF4G, not 4E-BP1. Moreover, hyperlipidemia impairs IGF-I action but does not produce leucine resistance in skeletal muscle.

Molecular mechanisms responsible for alcohol-induced myopathy in skeletal muscle and heart

Chronic alcohol abuse has the potential to modulate striated muscle physiology and function. The skeletal muscle alcoholic myopathy is characterized by muscle weakness and difficulties in gait and locomotion, while chronic alcohol consumption ultimately leads to a decrease in cardiac contractility and output. In both tissues a loss of protein mass results in part from a decreased protein synthesis that initially manifests as a defect in translational efficiency. This review focuses on recent developments in understanding the cellular and molecular mechanisms by which alcohol impairs mRNA translation in skeletal and cardiac muscle, including identification of the signaling pathways and biochemical sites negatively impacted. Defective signaling potentially results from resistance to the normal stimulating effects of anabolic hormones (insulin and insulin-like growth factor-I) and nutrients (leucine) as well as increased production of several negative regulators of muscle mass. Overall, the biochemical mechanisms contributing to the pathogenesis of loss of skeletal and cardiac muscle are reviewed.

Indinavir alters regulators of protein anabolism and catabolism in skeletal muscle

The HIV protease inhibitor indinavir adversely impairs carbohydrate and lipid metabolism, whereas its influence on protein metabolism under in vivo conditions remains unknown. The present study tested the hypothesis that indinavir also decreases basal protein synthesis and impairs the anabolic response to insulin in skeletal muscle. Indinavir was infused intravenously for 4 h into conscious rats, at which time the homeostasis model assessment of insulin resistance was increased. Indinavir decreased muscle protein synthesis by 30%, and this reduction was due to impaired translational efficiency. To identify potential mechanisms responsible for regulating mRNA translation, several eukaryotic initiation factors (eIFs) were examined. Under basal fasted conditions, there was a redistribution of eIF4E from the active eIF4E.eIF4G complex to the inactive eIF4E.4E-BP1 complex, and this change was associated with a marked decrease in the phosphorylation of 4E-BP1 in muscle. Likewise, indinavir decreased constitutive phosphorylation of eIF4G and mTOR in muscle, but not S6K1 or the ribosomal protein S6. In contrast, the ability of a maximally stimulating dose of insulin to increase the phosphorylation of PKB, 4E-BP1, S6K1, or mTOR was not altered 20 min after intravenous injection. Indinavir increased mRNA expression of the ubiquitin ligase MuRF1, but the plasma concentration of 3-methylhistidine remained unaltered. These indinavir-induced changes were associated with a marked reduction in the plasma testosterone concentration but were independent of changes in plasma levels of IGF-I, corticosterone, TNF-alpha, or IL-6. In conclusion, indinavir acutely impairs basal protein synthesis and translation initiation in skeletal muscle but, in contrast to muscle glucose uptake, does not impair insulin-stimulated signaling of protein synthetic pathways.

TNFalpha mediates sepsis-induced impairment of basal and leucine-stimulated signaling via S6K1 and eIF4E in cardiac muscle

Decreased translation initiation adversely impacts protein synthesis and contributes to the myocardial dysfunction produced by sepsis. Therefore, the purpose of the present study was to identify sepsis-induced changes in signal transduction pathways known to regulate translation initiation in cardiac muscle and to determine whether the stimulatory effects of leucine can reverse the observed defects. To address this aim, sepsis was produced by cecal ligation and puncture (CLP) in anesthetized rats and the animals studied in the fasted condition 24 h later. Separate groups of septic and time-matched control rats also received an oral gavage of leucine. To identify potential mechanisms responsible for regulating cap-dependent mRNA translation in cardiac muscle, several eukaryotic initiation factors (eIFs) were examined. Under basal conditions, hearts from septic rats demonstrated a redistribution of the rate-limiting factor eIF4E due to increased binding of the translational repressor 4E-BP1 with eIF4E. However, this change was independent of an alteration in the phosphorylation state of 4E-BP1. The phosphorylation of mTOR, S6K1, the ribosomal protein (rp) S6, and eIF4G was not altered in hearts from septic rats under basal conditions. In control rats, leucine failed to alter eIF4E distribution but increased the phosphorylation of S6K1 and S6. In contrast, in hearts from septic rats leucine acutely reversed the alterations in eIF4E distribution. However, the ability of leucine to increase S6K1 and rpS6 phosphorylation in septic hearts was blunted. Sepsis increased the content of tumor necrosis factor (TNF)-alpha in heart and pre-treatment of rats with a TNF antagonist prevented the above-mentioned sepsis-induced changes. These data indicate that oral administration of leucine acutely reverses sepsis-induced alterations eIF4E distribution observed under basal conditions but the anabolic actions of this amino acid on S6K1 and rpS6 phosphorylation remain blunted, providing evidence for a leucine resistance. Finally, TNFalpha, either directly or indirectly, appears to mediate the sepsis-induced defects in myocardial translation initiation.

Endotoxin disrupts the leucine-signaling pathway involving phosphorylation of mTOR, 4E-BP1, and S6K1 in skeletal muscle

Endotoxin (i.e., lipopolysaccharide, LPS) impairs skeletal muscle protein synthesis. Although this impairment is not acutely associated with a decreased plasma concentration of total amino acids, LPS may blunt the anabolic response to amino acids. To examine this hypothesis, rats were injected intraperitoneally with LPS or saline (Sal) and 4 h thereafter were orally administered either leucine (Leu) or Sal. The gastrocnemius was removed 20 min later to assess signaling components important in the translational control of protein synthesis. In the Sal-Leu group phosphorylation of 4E-BP1 in muscle was markedly increased, compared to values from time-matched saline-treated control rats. This change was associated with a redistribution of eukaryotic initiation factor (eIF) 4E from the inactive eIF4E x 4E-BP1 complex to the active eIF4E x eIF4G complex. In LPS-treated rats, the Leu-induced phosphorylation of 4E-BP1 and changes in eIF4E distribution were partially or completely abrogated. LPS also antagonized the Leu-induced increase in phosphorylation of S6K1, ribosomal protein S6 and mTOR. Neither LPS nor leu altered the total amount or phosphorylation of TSC2 in muscle. The ability of LPS to blunt the anabolic effects of Leu could not be attributed to differences in the plasma concentrations of insulin or Leu between groups. Furthermore, the replacement of plasma insulin-like growth factor (IGF)-I in LPS-treated rats to basal levels also did not ameliorate the defect in leucine-induced phosphorylation of S6K1 or S6, although it did reverse the LPS-induced decrease in the constitutive phosphorylation of mTOR, S6 and 4E-BP1. Pretreatment with the glucocorticoid receptor antagonist RU486 was unable to prevent the LPS-induced leucine resistance. In contrast, to the abovementioned results with leucine, LPS did not prevent the ability of pharmacological levels of IGF-I to phosphorylate 4E-BP1, S6K1, mTOR or alter the availability of eIF4E. Hence, LPS working via a glucocorticoid-independent mechanism produces a leucine resistance in skeletal muscle that might be expected to impair the ability of this amino acid to stimulate translation initiation and protein synthesis.

Alcohol Intoxication Impairs Phosphorylation of S6K1 and S6 in Skeletal Muscle Independently of Ethanol Metabolism

BACKGROUND

The purpose of this study was to characterize the ability of alcohol to suppress insulin-like growth factor (IGF)-I stimulation of ribosomal S6 kinase 1 (S6K1) and 4E-BP1 phosphorylation, which are central elements in the signal transduction pathway used to coordinate the protein synthetic response and may contribute to the development of alcoholic myopathy.

METHODS

In vivo studies examined the dose and time dependency of the ability of alcohol to impair signal transduction under basal and IGF-I-stimulated conditions. Additional studies examined the effect of gender, nutritional state, and route of alcohol administration. A separate study determined the direct effects of alcohol on muscle metabolism by using the isolated perfused hindlimb preparation.

RESULTS

The phosphorylation of S6K1 and S6 in muscle was increased after injection of IGF-I in control rats. In contrast, IGF-I failed to stimulate S6K1 or S6 phosphorylation 2.5 hr after intraperitoneal administration of alcohol when the blood alcohol concentration was increased between approximately 165 and 300 mg/dl. With a maximal suppressive dose of alcohol, the inhibitory effect on S6K1/S6 phosphorylation was observed as early as 1 hr and for up to 8 hr. The ability of alcohol to impair phosphorylation of S6K1 and S6 was independent of gender (male versus female), nutritional status (fed versus fasted), and route of alcohol administration (intraperitoneal versus oral). Furthermore, the suppressive effect of alcohol was still observed in rats pretreated with 4-methylpyrazole, suggesting that the response was independent of the oxidative metabolism of ethanol. The direct effect of alcohol on IGF-stimulated S6K1/S6 phosphorylation was also present when the isolated hindlimb was perfused in situ with buffer containing alcohol. In contrast to S6K1, acute alcohol intoxication did not consistently impair the ability of IGF-I to stimulate 4E-BP1 phosphorylation under any of the experimental conditions.

CONCLUSIONS

These data indicate that acute alcohol intoxication selectively impairs IGF-I signaling via S6K1, but not 4E-BP1, and that this defect is independent of gender, nutritional state, route of administration, and alcohol metabolism. The IGF-I resistance may represent a participating mechanism by which alcohol directly limits the translation of selected messenger RNAs and, ultimately, protein synthesis in skeletal muscle.

Differential effect of sepsis on ability of leucine and IGF-I to stimulate muscle translation initiation

Polymicrobial sepsis impairs skeletal muscle protein synthesis, which results from impairment in translation initiation under basal conditions. The purpose of the present study was to test the hypothesis that sepsis also impairs the anabolic response to amino acids, specifically leucine (Leu). Sepsis was induced by cecal ligation and puncture, and 24 h later, Leu or saline (Sal) was orally administered to septic and time-matched nonseptic rats. The gastrocnemius was removed 20 min later for assessment of protein synthesis and signaling components important in peptide-chain initiation. Oral Leu increased muscle protein synthesis in nonseptic rats. Leu was unable to increase protein synthesis in muscle from septic rats, and synthetic rates remained below those observed in nonseptic + Sal rats. In nonseptic + Leu rats, phosphorylation of eukaryotic initiation factor (eIF)4E-binding protein 1 (4E-BP1) in muscle was markedly increased compared with values from time-matched Sal-treated nonseptic rats. This change was associated with redistribution of eIF4E from the inactive eIF4E.4E-BP1 to the active eIF4E.eIF4G complex. In septic rats, Leu-induced phosphorylation of 4E-BP1 and changes in eIF4E distribution were completely abrogated. Sepsis also antagonized the Leu-induced increase in phosphorylation of S6 kinase 1 and ribosomal protein S6. Sepsis attenuated Leu-induced phosphorylation of mammalian target of rapamycin and eIF4G. The ability of sepsis to inhibit anabolic effects of Leu could not be attributed to differences in plasma concentrations of insulin, insulin-like growth factor I, or Leu between groups. In contrast, the ability of exogenous insulin-like growth factor I to stimulate the same signaling components pertaining to translation initiation was not impaired by sepsis. Hence, sepsis produces a relatively specific Leu resistance in skeletal muscle that impairs the ability of this amino acid to stimulate translation initiation and protein synthesis.

Indinavir impairs protein synthesis and phosphorylations of MAPKs in mouse C2C12 myocytes

Anti-retroviral therapy promotes clinical, immunologic, and virologic improvement in human immunodeficiency virus-infected patients. Whereas this therapy adversely affects carbohydrate and lipid metabolism, the effects of anti-retroviral drugs on muscle protein synthesis and degradation have not been reported. To examine these processes, we treated C2C12 myocytes with increasing concentrations of the protease inhibitor indinavir for 1 or 2 days. Treatment of myocytes with a therapeutic concentration of indinavir (20 microM) for 24 h decreased basal protein synthesis by 18%, whereas a 42% decline was observed after 48 h. A similar decrement, albeit quantitatively smaller, was detected with other protease inhibitors. Indinavir did not alter the rate of proteolysis. Likewise, indinavir did not impair the anabolic effect of insulin-like growth factor-I on protein synthesis. Mechanistically, indinavir decreased the phosphorylation of the S6 ribosomal protein (rpS6), and this reduction was associated with a decreased phosphorylation of p70S6 kinase and p90rsk as well as the upstream regulators ERK1/2 and MEK1/2. Indinavir also decreased the phosphorylation of Mnk1 and its upstream effectors, p38 MAPK and ERK1/2. Indinavir did not affect the phosphorylation of mTOR or 4E-BP1, but it did decrease the amount of the active eukaryotic initiation factor eIF4G-eIF4E complex. In conclusion, indinavir decreased protein synthesis in myocytes. This decrease was associated with the disruption of the ERK1/2 and p38 MAPK pathways and a reduction in both the level of functional eIF4F complex and rpS6 phosphorylation.

IGF-I/IGFBP-3 ameliorates alterations in protein synthesis, eIF4E availability, and myostatin in alcohol-fed rats

Chronic alcohol consumption decreases the concentration of the anabolic hormone IGF-I, and this change is associated with impaired muscle protein synthesis. The present study evaluated the ability of IGF-I complexed with IGF-binding protein (IGFBP)-3 to modulate the alcohol-induced inhibition of muscle protein synthesis in gastrocnemius. After 16 wk on an alcohol-containing diet, either the IGF-I/IGFBP-3 binary complex (BC) or saline was injected two times daily for three consecutive days. After the final injection of BC (3 h), plasma IGF-I concentrations were elevated in alcohol-fed rats to values not different from those of similarly treated control animals. Alcohol feeding decreased the basal rate of muscle protein synthesis by limiting translational efficiency. BC treatment of alcohol-fed rats increased protein synthesis back to basal control values, but the rate remained lower than that of BC-injected control rats. The BC partially reversed the alcohol-induced decrease in the binding of eukaryotic initiation factor (eIF)4E with eIF4G. This change was associated with reversal of the alcohol-induced dephosphorylation of eIF4G but was independent of changes in the phosphorylation of either 4E-BP1 or eIF4E. However, BC reversed the alcohol-induced increase in IGFBP-1 and muscle myostatin, known negative regulators of IGF-I action and muscle mass. Hence, exogenous IGF-I, administered as part of a BC to increase its circulating half-life, can in part reverse the decreased protein synthesis observed in muscle from chronic alcohol-fed rats by stimulating selected components of translation initiation. The data support the role of IGF-I as a mediator of chronic alcohol myopathy in rats.

Alcohol impairs leucine-mediated phosphorylation of 4E-BP1, S6K1, eIF4G, and mTOR in skeletal muscle

Acute alcohol (EtOH) intoxication impairs skeletal muscle protein synthesis. Although this impairment is not associated with a decrease in the total plasma amino acid concentration, EtOH may blunt the anabolic response to amino acids. To examine this hypothesis, rats were administered EtOH or saline (Sal) and 2.5 h thereafter were orally administered either leucine (Leu) or Sal. The gastrocnemius was removed 20 min later to assess protein synthesis and signaling components important in translational control of protein synthesis. Oral Leu increased muscle protein synthesis by the same magnitude in Sal- and EtOH-treated rats. However, the increase in the latter group was insufficient to overcome the suppressive effect of EtOH, and the rate of synthesis remained lower than that observed in rats from the Sal-Sal group. Leu markedly increased phosphorylation of Thr residues 36, 47, and 70 on 4E-binding protein (BP)1 in muscle from rats not receiving EtOH, and this response was associated with a redistribution of eukaryotic initiation factor (eIF) 4E from the inactive eIF4E. 4E-BP1 to the active eIF4E. eIF4G complex. In EtOH-treated rats, the Leu-induced phosphorylation of 4E-BP1 and changes in eIF4E availability were partially abrogated. EtOH also prevented the Leu-induced increase in phosphorylation of eIF4G, the serine/threonine protein kinase S6K1, and the ribosomal protein S6. Moreover, EtOH attenuated the Leu-induced phosphorylation of the mammalian target of rapamycin (mTOR). The ability of EtOH to blunt the anabolic effects of Leu could not be attributed to differences in the plasma concentrations of insulin, insulin-like growth factor I, or Leu. Finally, although EtOH increased the plasma corticosterone concentration, inhibition of glucocorticoid action by RU-486 was unable to prevent EtOH-induced defects in the ability of Leu to stimulate 4E-BP1, S6K1, and mTOR phosphorylation. Hence, ethanol produces a leucine resistance in skeletal muscle, as evidenced by the impaired phosphorylation of 4E-BP1, eIF4G, S6K1, and mTOR, that is independent of elevations in endogenous glucocorticoids.

Acute in vivo elevation of insulin-like growth factor (IGF) binding protein-1 decreases plasma free IGF-I and muscle protein synthesis

This study examined whether the acute elevation of IGF-binding protein-1 (IGFBP-1) decreases the plasma free IGF-I concentration and alters in vivo rates of muscle protein synthesis and glucose uptake. The plasma concentration of human IGFBP-1 was increased to approximately 95 ng/ml in conscious catheterized rats infused iv with human IGFBP-1 for 4 h. Infusion of IGFBP-1 also increased the concentration of endogenous (e.g. rat) IGFBP-1 in the blood, and this response was associated with a 2- to 3-fold elevation of IGFBP-1 mRNA in liver and kidney. IGFBP-1 did not significantly alter the plasma concentration of total IGF-I, but decreased circulating free IGF-I levels by about 50%. IGFBP-1 decreased protein synthesis in the predominantly fast-twitch gastrocnemius muscle (20%), and this change resulted from a decreased translational efficiency that was associated with a decreased phosphorylation of S6K1, but not 4E-BP1. Complementary studies demonstrated that IGFBP-1 also decreased the rates of protein synthesis under basal conditions and in response to stimulation by IGF-I when added in vitro to the fast-twitch epitrochlearis muscle. In contrast, IGFBP-1 did not alter in vivo-determined rates of protein synthesis in the slow-twitch soleus muscle, heart, liver, or kidney. The infusion of IGFBP-1 did not significantly alter the plasma glucose or lactate concentration or the whole body rate of glucose production or disposal. The above-mentioned changes were not mediated indirectly by changes in the plasma insulin or corticosterone concentrations, decreased high energy phosphate content in muscle, or hepatoxicity produced by the infused IGFBP-1. These results demonstrate that acute in vivo elevation in IGFBP-1, of the magnitude observed in various catabolic conditions, is capable of selectively decreasing protein synthesis in fast-twitch skeletal muscle and up-regulating the hepatic and renal syntheses of IGFBP-1 per se. Hence, elevations in circulating and tissue levels of IGFBP-1 may be an important mediator for the muscle catabolism observed in various stress conditions.