Rag GTPases and AMPK/TSC2/Rheb mediate the differential regulation of mTORC1 signaling in response to alcohol and leucine

Alcohol induces p53-mediated apoptosis in neural crest by stimulating an AMPK-mediated suppression of TORC1, p70/S6K, and ribosomal biogenesis

Prenatal alcohol exposure is a leading cause of permanent neurodevelopmental disability and can feature distinctive craniofacial deficits that partly originate from the apoptotic deletion of craniofacial progenitors, a stem cell lineage called the neural crest (NC). We recently demonstrated that alcohol causes nucleolar stress in NC through its suppression of ribosome biogenesis (RBG) and this suppression is causative in their p53/MDM2-mediated apoptosis. Here, we show that this nucleolar stress originates from alcohol's activation of AMPK, which suppresses TORC1 and the p70/S6K-mediated stimulation of RBG. Alcohol-exposed cells of the pluripotent, primary cranial NC line O9-1 were evaluated with respect to their p70/S6K, TORC1, and AMPK activity. The functional impact of these signals with respect to RBG, p53, and apoptosis were assessed using gain-of-function constructs and small molecule mediators. Alcohol rapidly (<2hr) increased pAMPK, pTSC2, pRaptor, p-mTOR(S2446), and reduced both total and p-p70/S6K in NC cells. These changes persisted for at least 12hr to 18hr following alcohol exposure. Attenuation of these signals via gain- or loss-of-function approaches that targeted AMPK, p70/S6K, or TORC1 prevented alcohol's suppression of rRNA synthesis and the induction of p53-stimulated apoptosis. We conclude that alcohol induces ribosome dysbiogenesis and activates their p53/MDM2-mediated apoptosis via its activation of pAMPK, which in turn activates TSC2 and Raptor to suppress the TORC1-p70/S6K-mediated promotion of ribosome biogenesis. This represents a novel mechanism underlying alcohol's neurotoxicity and is consistent with findings that TORC1-p70/S6K networks are critical for cranial NC survival.

Research progress in the role and mechanism of Leucine in regulating animal growth and development

Leucine, a branched-chain amino acid, is essential in regulating animal growth and development. Recent research has uncovered the mechanisms underlying Leucine's anabolic effects on muscle and other tissues, including its ability to stimulate protein synthesis by activating the mTORC1 signaling pathway. The co-ingestion of carbohydrates and essential amino acids enhances Leucine's anabolic effects. Moreover, Leucine has been shown to benefit lipid metabolism, and insulin sensitivity, making it a promising strategy for preventing and treating metabolic diseases, including type 2 diabetes and obesity. While emerging evidence indicates that epigenetic mechanisms may mediate Leucine's effects on growth and development, more research is needed to elucidate its mechanisms of action fully. Specific studies have demonstrated that Leucine promotes muscle growth and metabolic health in animals and humans, making it a promising therapeutic agent. However, it is essential to note that Leucine supplementation may cause digestive issues or interact with certain medications, and More study is required to determine definitively optimal dosages. Therefore, it is important to understand how Leucine interacts with other nutrients, dietary factors, and lifestyle habits to maximize its benefits. Overall, Leucine's importance in human nutrition is far-reaching, and its potential to prevent muscle loss and enhance athletic performance warrants further investigation.

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.

Review: Nutritional regulation of muscle growth in neonatal swine

Despite advances in the nutritional support of low birth weight and early-weaned piglets, most experience reduced extrauterine growth performance. To further optimize nutritional support and develop targeted intervention strategies, the mechanisms that regulate the anabolic response to nutrition must be fully understood. Knowledge gained in these studies represents a valuable intersection of agriculture and biomedical research, as low birth weight and early-weaned piglets face many of the same morbidities as preterm and low birth weight infants, including extrauterine growth faltering and reduced lean growth. While the reasons for poor growth performance are multifaceted, recent studies have increased our understanding of the role of nutrition in the regulation of skeletal muscle growth in the piglet. The purpose of this review is to summarize the published literature surrounding advances in the current understanding of the anabolic signaling that occurs after a meal and how this response is developmentally regulated in the neonatal pig. It will focus on the regulation of protein synthesis, and especially the upstream and downstream effectors surrounding the master protein kinase, mechanistic target of rapamycin complex 1 (mTORC1) that controls translation initiation. It also will examine the regulatory pathways associated with the postprandial anabolic agents, insulin and specific amino acids, that are upstream of mTORC1 and lead to its activation. Lastly, the integration of upstream signaling cascades by mTORC1 leading to the activation of translation initiation factors that regulate protein synthesis will be discussed. This review concludes that anabolic signaling cascades are stimulated by both insulin and amino acids, especially leucine, through separate pathways upstream of mTORC1, and that these stimulatory pathways result in mTORC1 activation and subsequent activation of downstream effectors that regulate translation initiation Additionally, it is concluded that this anabolic response is unique to the skeletal muscle of the neonate, resulting from increased sensitivity to the rise in both insulin and amino acid after a meal. However, this response is dampened in skeletal muscle of the low birth weight pig, indicative of anabolic resistance. Elucidation of the pathways and regulatory mechanisms surrounding protein synthesis and lean growth allow for the development of potential targeted therapeutics and intervention strategies both in livestock production and neonatal care.

Autophagy Is Essential for Neural Stem Cell Proliferation Promoted by Hypoxia

Hypoxia as a microenvironment or niche stimulates proliferation of neural stem cells (NSCs). However, the underlying mechanisms remain elusive. Autophagy is a protective mechanism by which recycled cellular components and energy are rapidly supplied to the cell under stress. Whether autophagy mediates the proliferation of NSCs under hypoxia and how hypoxia induces autophagy remain unclear. Here, we report that hypoxia facilitates embryonic NSC proliferation through HIF-1/mTORC1 signaling pathway-mediated autophagy. Initially, we found that hypoxia greatly induced autophagy in NSCs, while inhibition of autophagy severely impeded the proliferation of NSCs in hypoxia conditions. Next, we demonstrated that the hypoxia core regulator HIF-1 was necessary and sufficient for autophagy induction in NSCs. Considering that mTORC1 is a key switch that suppresses autophagy, we subsequently analyzed the effect of HIF-1 on mTORC1 activity. Our results showed that the mTORC1 activity was negatively regulated by HIF-1. Finally, we provided evidence that HIF-1 regulated mTORC1 activity via its downstream target gene BNIP3. The increased expression of BNIP3 under hypoxia enhanced autophagy activity and proliferation of NSCs, which was mediated by repressing the activity of mTORC1. We further illustrated that BNIP3 can interact with Rheb, a canonical activator of mTORC1. Thus, we suppose that the interaction of BNIP3 with Rheb reduces the regulation of Rheb toward mTORC1 activity, which relieves the suppression of mTORC1 on autophagy, thereby promoting the rapid proliferation of NSCs. Altogether, this study identified a new HIF-1/BNIP3-Rheb/mTORC1 signaling axis, which regulates the NSC proliferation under hypoxia through induction of autophagy.

Alcohol, Resistance Exercise, and mTOR Pathway Signaling: An Evidence-Based Narrative Review

Skeletal muscle mass is determined by the balance between muscle protein synthesis (MPS) and degradation. Several intracellular signaling pathways control this balance, including mammalian/mechanistic target of rapamycin (mTOR) complex 1 (C1). Activation of this pathway in skeletal muscle is controlled, in part, by nutrition (e.g., amino acids and alcohol) and exercise (e.g., resistance exercise (RE)). Acute and chronic alcohol use can result in myopathy, and evidence points to altered mTORC1 signaling as a contributing factor. Moreover, individuals who regularly perform RE or vigorous aerobic exercise are more likely to use alcohol frequently and in larger quantities. Therefore, alcohol may antagonize beneficial exercise-induced increases in mTORC1 pathway signaling. The purpose of this review is to synthesize up-to-date evidence regarding mTORC1 pathway signaling and the independent and combined effects of acute alcohol and RE on activation of the mTORC1 pathway. Overall, acute alcohol impairs and RE activates mTORC1 pathway signaling; however, effects vary by model, sex, feeding, training status, quantity, etc., such that anabolic stimuli may partially rescue the alcohol-mediated pathway inhibition. Likewise, the impact of alcohol on RE-induced mTORC1 pathway signaling appears dependent on several factors including nutrition and sex, although many questions remain unanswered. Accordingly, we identify gaps in the literature that remain to be elucidated to fully understand the independent and combined impacts of alcohol and RE on mTORC1 pathway signaling.

Pathophysiological Consequences of At-Risk Alcohol Use; Implications for Comorbidity Risk in Persons Living With Human Immunodeficiency Virus

At-risk alcohol use is a significant risk factor associated with multisystemic pathophysiological effects leading to multiorgan injury and contributing to 5.3% of all deaths worldwide. The alcohol-mediated cellular and molecular alterations are particularly salient in vulnerable populations, such as people living with HIV (PLWH), diminishing their physiological reserve, and accelerating the aging process. This review presents salient alcohol-associated mechanisms involved in exacerbation of cardiometabolic and neuropathological comorbidities and their implications in the context of HIV disease. The review integrates consideration of environmental factors, such as consumption of a Western diet and its interactions with alcohol-induced metabolic and neurocognitive dyshomeostasis. Major alcohol-mediated mechanisms that contribute to cardiometabolic comorbidity include impaired substrate utilization and storage, endothelial dysfunction, dysregulation of the renin-angiotensin-aldosterone system, and hypertension. Neuroinflammation and loss of neurotrophic support in vulnerable brain regions significantly contribute to alcohol-associated development of neurological deficits and alcohol use disorder risk. Collectively, evidence suggests that at-risk alcohol use exacerbates cardiometabolic and neurocognitive pathologies and accelerates biological aging leading to the development of geriatric comorbidities manifested as frailty in PLWH.

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.

Energy deficiency caused by CTPS downregulation in decidua may contribute to pre-eclampsia by impairing decidualization

Pre-eclampsia (PE) is a pregnancy-related disorder that occurs after 20 weeks of gestation. It seriously affects the health of maternity and the fetus. However, the pathogenesis of PE is still unknown. Decidualization deficiency is considered a contributing factor to the development of PE. CTP synthetase (CTPS) which is the rate-limiting enzyme in the CTP de novo biosynthesis, is essential for nucleic acid synthesis and cellular energy metabolism, and often appears as cytoophidium in many cell types. Here, we found that the expression of CTPS was significantly downregulated in decidual tissues of patients with severe PE compared with healthy pregnant women. During in vitro decidualization, changes in CTPS were accompanied by opposite fluctuation of the AMPK signaling pathway. Moreover, the downregulation of CTPS by glutamine analogs or CTPS small interfering RNA inhibited the decidualization process and the AMPK signaling pathway. Investigating the underlying mechanism of action by co-immunoprecipitation coupled with mass spectrometry showed that CTPS interacted with ATP synthase (ATPS) and maintained the content of ATP on Day 3 of decidualization. However, when combined with mitochondrial stress protein STRESS-70 instead of ATPS, the concentration of ATP on Day 6 of induction was reduced. Corresponding to this, CTPS was mainly distributes in the cytoplasm on Day 3 of induction, while it appeared both in the cytoplasm and the nucleus on Day 6 in decidualized cells, which was similar to that in cells before induction. In summary, we believe that CTPS plays an important role in decidualization by participating in energy metabolism. Abnormal expression of CTPS in decidualization would lead to abnormal decidualization and consequently result in the occurrence of PE.

Differential regulation of mTORC1 activation by leucine and β-hydroxy-β-methylbutyrate in skeletal muscle of neonatal pigs

Leucine (Leu) and its metabolite β-hydroxy-β-methylbutyrate (HMB) stimulate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1)-dependent protein synthesis in the skeletal muscle of neonatal pigs. This study aimed to determine whether HMB and Leu utilize common nutrient-sensing mechanisms to activate mTORC1. In study 1, neonatal pigs were fed one of five diets for 24 h: low protein (LP), high protein (HP), or LP supplemented with 4 (LP+HMB4), 40 (LP+HMB40), or 80 (LP+HMB80) μmol HMB·kg body wt^-1·day^-1. In study 2, neonatal pigs were fed for 24 h: LP, LP supplemented with Leu (LP+Leu), or HP diets delivering 9, 18, and 18 mmol Leu·kg body wt^-1·day^-1, respectively. The upstream signaling molecules that regulate mTORC1 activity were analyzed. mTOR phosphorylation on Ser2448 and Ser2481 was greater in LP+HMB40, LP+HMB80, and LP+Leu than in LP and greater in HP than in HMB-supplemented groups (P < 0.05), whereas HP and LP+Leu were similar. Rheb-mTOR complex formation was lower in LP than in HP (P < 0.05), with no enhancement by HMB or Leu supplementation. The Sestrin2-GATOR2 complex was more abundant in LP than in HP and was reduced by Leu (P < 0.05) but not HMB supplementation. RagA-mTOR and RagC-mTOR complexes were higher in LP+Leu and HP than in LP and HMB groups (P < 0.05). There were no treatment differences in RagB-SH3BP4, Vps34-LRS, and RagD-LRS complex abundances. Phosphorylation of Erk1/2 and TSC2, but not AMPK, was lower in LP than HP (P < 0.05) and unaffected by HMB or Leu supplementation. Our results demonstrate that HMB stimulates mTORC1 activation in neonatal muscle independent of the leucine-sensing pathway mediated by Sestrin2 and the Rag proteins.NEW & NOTEWORTHY Dietary supplementation with either leucine or its metabolite β-hydroxy-β-methylbutyrate (HMB) stimulates protein synthesis in skeletal muscle of the neonatal pig. Our results demonstrate that both leucine and HMB stimulate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) phosphorylation in neonatal muscle. This leucine-stimulated process involves dissociation of the Sestrin2-GATOR2 complex and increased binding of Rag A/C to mTOR. However, HMB's activation of mTORC1 is independent of this leucine-sensing pathway.

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.

Selected In-Season Nutritional Strategies to Enhance Recovery for Team Sport Athletes: A Practical Overview

Team sport athletes face a variety of nutritional challenges related to recovery during the competitive season. The purpose of this article is to review nutrition strategies related to muscle regeneration, glycogen restoration, fatigue, physical and immune health, and preparation for subsequent training bouts and competitions. Given the limited opportunities to recover between training bouts and games throughout the competitive season, athletes must be deliberate in their recovery strategy. Foundational components of recovery related to protein, carbohydrates, and fluid have been extensively reviewed and accepted. Micronutrients and supplements that may be efficacious for promoting recovery include vitamin D, omega-3 polyunsaturated fatty acids, creatine, collagen/vitamin C, and antioxidants. Curcumin and bromelain may also provide a recovery benefit during the competitive season but future research is warranted prior to incorporating supplemental dosages into the athlete's diet. Air travel poses nutritional challenges related to nutrient timing and quality. Incorporating strategies to consume efficacious micronutrients and ingredients is necessary to support athlete recovery in season.

Amino Acid- and Insulin-Induced Activation of mTORC1 in Neonatal Piglet Skeletal Muscle Involves Sestin2-GATOR2, Rag A/C-mTOR, and RHEB-mTOR Complex Formation

Background

Feeding stimulates protein synthesis in skeletal muscle of neonates and this response is regulated through activation of mechanistic target of rapamycin complex 1 (mTORC1). The identity of signaling components that regulate mTORC1 activation in neonatal muscle has not been fully elucidated.

Objective

We investigated the independent effects of the rise in amino acids (AAs) and insulin after a meal on the abundance and activation of potential regulators of mTORC1 in muscle and whether the responses are modified by development.

Methods

Overnight-fasted 6- and 26-d-old pigs were infused for 2 h with saline (control group) or with a balanced AA mixture (AA group) or insulin (INS group) to achieve fed levels while insulin or AAs, respectively, and glucose were maintained at fasting levels. Muscles were analyzed for potential mTORC1 regulatory mechanisms and results were analyzed by 2-factor ANOVA followed by Tukey's post hoc test.

Results

The abundances of DEP domain-containing mTOR-interacting protein (DEPTOR), growth factor receptor bound protein 10 (GRB10), and regulated in development and DNA damage response 2 (REDD2) were lower (65%, 73%, and 53%, respectively; P < 0.05) and late endosomal/lysosomal adaptor, MAPK and mTOR activator 1/2 (LAMTOR1/2), vacuolar H+-ATPase (V-ATPase), and Sestrin2 were higher (94%, 141%, 145%, and 127%, respectively; P < 0.05) in 6- than in 26-d-old pigs. Both AA and INS groups increased phosphorylation of GRB10 (P < 0.05) compared with control in 26- but not in 6-d-old pigs. Formation of Ras-related GTP-binding protein A (RagA)-mTOR, RagC-mTOR, and Ras homolog enriched in brain (RHEB)-mTOR complexes was increased (P < 0.05) and Sestrin2-GTPase activating protein activity towards Rags 2 (GATOR2) complex was decreased (P < 0.05) by both AA and INS groups and these responses were greater (P < 0.05) in 6- than in 26-d-old pigs.

Conclusion

The results suggest that formation of RagA-mTOR, RagC-mTOR, RHEB-mTOR, and Sestrin2-GATOR2 complexes may be involved in the AA- and INS-induced activation of mTORC1 in skeletal muscle of neonates after a meal and that enhanced activation of the mTORC1 signaling pathway in neonatal muscle is in part due to regulation by DEPTOR, GRB10, REDD2, LAMTOR1/2, V-ATPase, and Sestrin2.

Nutrition and Muscle in Cirrhosis

As the cirrhosis progresses, development of complication like ascites, hepatic encephalopathy, variceal bleeding, kidney dysfunction, and hepatocellular carcinoma signify increasing risk of short term mortality. Malnutrition and muscle wasting (sarcopenia) is yet other complications that negatively impact survival, quality of life, and response to stressors, such as infection and surgery in patients with cirrhosis. Conventionally, these are not routinely looked for, because nutritional assessment can be a difficult especially if there is associated fluid retention and/or obesity. Patients with cirrhosis may have a combination of loss of skeletal muscle and gain of adipose tissue, culminating in the condition of "sarcopenic obesity." Sarcopenia in cirrhotic patients has been associated with increased mortality, sepsis complications, hyperammonemia, overt hepatic encephalopathy, and increased length of stay after liver transplantation. Assessment of muscles with cross-sectional imaging studies has become an attractive index of nutritional status evaluation in cirrhosis, as sarcopenia, the major component of malnutrition, is primarily responsible for the adverse clinical consequences seen in patients with liver disease. Cirrhosis is a state of accelerated starvation, with increased gluconeogenesis that requires amino acid diversion from other metabolic functions. Protein homeostasis is disturbed in cirrhosis due to several factors such as hyperammonemia, hormonal, and cytokine abnormalities, physical inactivity and direct effects of ethanol and its metabolites. New approaches to manage sarcopenia are being evolved. Branched chain amino acid supplementation, Myostatin inhibitors, and mitochondrial protective agents are currently in various stages of evaluation in preclinical studies to prevent and reverse sarcopenia, in cirrhosis.

Dysregulation of mRNA translation and energy metabolism in cancer

Dysregulated mRNA translation and aberrant energy metabolism are frequent in cancer. Considering that mRNA translation is an energy demanding process, cancer cells must produce sufficient ATP to meet energy demand of hyperactive translational machinery. In recent years, the mammalian/mechanistic target of rapamycin (mTOR) emerged as a central regulatory node which coordinates energy consumption by the translation apparatus and ATP production in mitochondria. Aberrant mTOR signaling underpins the vast majority of cancers whereby increased mTOR activity is thought to be a major determinant of both malignant translatomes and metabolomes. Nonetheless, the role of mTOR and other related signaling nodes (e.g. AMPK) in orchestrating protein synthesis and cancer energetics is only recently being unraveled. In this review, we discuss recent findings that provide insights into the molecular underpinnings of coordination of translational and metabolic programs of cancer cells, and potential strategies to translate these findings into clinical treatments.

Sarcopenia in Alcoholic Liver Disease: Clinical and Molecular Advances

Despite advances in treatment of alcohol use disorders that focus on increasing abstinence and reducing recidivism, alcoholic liver disease (ALD) is projected to be the major cause of cirrhosis and its complications. Malnutrition is recognized as the most frequent complication in ALD, and despite the high clinical significance, there are no effective therapies to reverse malnutrition in ALD. Malnutrition is a relatively imprecise term, and sarcopenia or skeletal muscle loss, the major component of malnutrition, is primarily responsible for the adverse clinical consequences in patients with liver disease. It is, therefore, critical to define the specific abnormality (sarcopenia) rather than malnutrition in ALD, so that therapies targeting sarcopenia can be developed. Skeletal muscle mass is maintained by a balance between protein synthesis and proteolysis. Both direct effects of ethanol (EtOH) and its metabolites on the skeletal muscle and the consequences of liver disease result in disturbed proteostasis (protein homeostasis) and consequent sarcopenia. Once cirrhosis develops in patients with ALD, abstinence is unlikely to be effective in completely reversing sarcopenia, as other contributors including hyperammonemia, hormonal, and cytokine abnormalities aggravate sarcopenia and maintain a state of anabolic resistance initiated by EtOH. Cirrhosis is also a state of accelerated starvation, with increased gluconeogenesis that requires amino acid diversion from signaling and substrate functions. Novel therapeutic options are being recognized that are likely to supplant the current "deficiency replacement" approach and instead focus on specific molecular perturbations, given the increasing availability of small molecules that can target specific signaling components. Myostatin antagonists, leucine supplementation, and mitochondrial protective agents are currently in various stages of evaluation in preclinical studies to prevent and reverse sarcopenia, in cirrhosis in general, and ALD, specifically. Translation of these data to human studies and clinical application requires priority for allocation of resources.

FoxO1-AMPK-ULK1 Regulates Ethanol-Induced Autophagy in Muscle by Enhanced ATG14 Association with the BECN1-PIK3C3 Complex

BACKGROUND

Excessive alcohol (EtOH) consumption causes an imbalance in protein metabolism. EtOH impairs protein synthesis in C2C12 myoblasts via a FoxO1-AMPK-TSC2-mTORC1 pathway and also induces protein degradation. As the underlying regulatory signaling cascades for these processes are currently poorly defined, we tested the hypothesis that alcohol-induced autophagy is mediated via activation of the PIK3C3 complex that is regulated by FoxO1-AMPK.

METHODS

C2C12 myoblasts were incubated with EtOH for various periods of time, and autophagy pathway-related proteins were assessed by Western blotting and immunoprecipitation. Expression of targeted genes was suppressed using electroporation of specific siRNAs and chemical inhibitors.

RESULTS

Incubation of C2C12 myoblasts with 100 mM EtOH increased the autophagy markers LC3B-II and ATG7, whereas levels of SQSTM1/p62 decreased. The lysosomal inhibitor bafilomycin A1 caused a similar response, although there was no additive effect when combined with EtOH. EtOH altered ULK1 S555 and S757 phosphorylation in a time- and AMPK-dependent manner. The activation of AMPK and ULK1 was associated with increased BECN1 (S93, S14) and PIK3C3/VPS34 (S164) phosphorylation as well as increased total ATG14 and PIK3C3. These changes promoted formation of the ATG14-AMBRA1-BECN1-PIK3C3 proautophagy complex that is important in autophagosome formation. EtOH-induced changes were not associated with increased production of PtdIns3P, which may be due to enhanced PIK3C3 complex binding with 14-3-3θ. Reduction of AMPK using siRNA suppressed the stimulatory effect of EtOH on BECN1 S93, BECN1 S14, and PIK3C3 S164 phosphorylation in a time-dependent manner. Likewise, knockdown of AMPK or chemical inhibition of FoxO1 attenuated phosphorylation of ULK1 at both residues. Knockdown of ULK1 or BECN1 antagonized the effect of EtOH on LC3B-II, SQSTM1, and ATG7 protein expression.

CONCLUSIONS

EtOH-induced autophagy is mediated through changes in phosphorylation and interaction of various PIK3C3 complex components. This, in turn, is regulated either directly via FoxO1-AMPK or indirectly via the FoxO1-AMPK-ULK1 signaling cascade in a mTORC1-independent or mTORC1-dependent manner.

Physiological processes underlying organ injury in alcohol abuse

This review summarizes the American Physiological Society (APS) Presidential Symposium 1 entitled "Physiological Processes Underlying Organ Injury in Alcohol Abuse" at the 2016 Experimental Biology meeting. The symposium was organized by Dr. Patricia Molina, past president of the APS, was held on April 3 at the Convention Center in San Diego, CA, and was funded by the National Institute on Alcohol Abuse and Alcoholism. The "Physiological Processes Underlying Organ Injury in Alcohol Abuse Symposium" assembled experts and leaders in the field and served as a platform to discuss and share knowledge on the latest developments and scientific advances on the mechanisms underlying organ injury in alcohol abuse. This symposium provided unique, interdisciplinary alcohol research, including several organs, liver, muscle, adipose, and brain, affected by excessive alcohol use.

Pulsatile delivery of a leucine supplement during long-term continuous enteral feeding enhances lean growth in term neonatal pigs

Neonatal pigs are used as a model to study and optimize the clinical treatment of infants who are unable to maintain oral feeding. Using this model, we have shown previously that pulsatile administration of leucine during continuous feeding over 24 h via orogastric tube enhanced protein synthesis in skeletal muscle compared with continuous feeding alone. To determine the long-term effects of leucine pulses, neonatal piglets (n = 11-12/group) were continuously fed formula via orogastric tube for 21 days, with an additional parenteral infusion of either leucine (CON + LEU; 800 μmol·kg^-1·h^-1) or alanine (CON + ALA) for 1 h every 4 h. The results show that body and muscle weights and lean gain were ∼25% greater, and fat gain was 48% lower in CON + LEU than CON + ALA; weights of other tissues were unaffected by treatment. Fractional protein synthesis rates in longissimus dorsi, gastrocnemius, and soleus muscles were ∼30% higher in CON + LEU compared with CON + ALA and were associated with decreased Deptor abundance and increased mTORC1, mTORC2, 4E-BP1, and S6K1 phosphorylation, SNAT2 abundance, and association of eIF4E with eIF4G and RagC with mTOR. There were no treatment effects on PKB, eIF2α, eEF2, or PRAS40 phosphorylation, Rheb, SLC38A9, v-ATPase, LAMTOR1, LAMTOR2, RagA, RagC, and LAT1 abundance, the proportion of polysomes to nonpolysomes, or the proportion of mRNAs encoding rpS4 or rpS8 associated with polysomes. Our results demonstrate that pulsatile delivery of a leucine supplement during 21 days of continuous enteral feeding enhances lean growth by stimulating the mTORC1-dependent translation initiation pathway, leading to protein synthesis in skeletal muscle of neonates.

Adamts1 mediates ethanol-induced alterations in collagen and elastin via a FoxO1-sestrin3-AMPK signaling cascade in myocytes

A variety of stressors including alcohol (EtOH) are known to induce collagen production and fibrotic diseases. Matrix metalloproteinases (MMP) play an important role in regulating fibrosis, but little is known regarding the relationship between EtOH and MMPs. In addition, the signaling cascades involved in this process have not been elucidated. We have identified the MMP Adamts1 as a target of EtOH regulation. To characterize the function of Adamts1, we examined EtOH-induced alterations in collagen I and elastin protein levels in C2C12 myocytes. Incubation of myocytes with 100 mM EtOH decreased elastin and increased collagen content, respectively, and these changes were associated with increased O-GLcNAc modification of Adamts1. Conversely, silencing of Adamts1 by siRNA blocked the adverse effects of EtOH on collagen and elastin levels. Similar results were obtained after treatment with a pharmacological inhibitor of MMP. Changes in collagen were due, at least in part, to a decreased interaction of Adamts1 with its endogenous inhibitor TIMP3. The AMPK inhibitor compound C blocked the EtOH-induced stimulation of collagen and O-GLcNAc Adamts1 protein. Changes in AMPK appear linked to FoxO1, since inhibition of FoxO1 blocked the effects of EtOH on AMPK phosphorylation and O-GLcNAc levels. These FoxO-dependent modifications were associated with an upregulation of the FoxO1 transcription target sestrin 3, as well as increased binding of sestrin 3 with AMPK. Collectively, these data indicate that EtOH regulates the collagen I and elastin content in an Adamts1-dependent manner in myocytes. Furthermore, Adamts1 appears to be controlled by the FoxO1-sestrin 3-AMPK signaling cascade.

Metabolic and molecular responses to leucine-enriched branched chain amino acid supplementation in the skeletal muscle of alcoholic cirrhosis

Skeletal muscle loss (sarcopenia) is a major clinical complication in alcoholic cirrhosis with no effective therapy. Skeletal muscle autophagic proteolysis and myostatin expression (inhibitor of protein synthesis) are increased in cirrhosis and believed to contribute to anabolic resistance. A prospective study was performed to determine the mechanisms of sarcopenia in alcoholic cirrhosis and potential reversal by leucine. In six well-compensated, stable, alcoholic patients with cirrhosis and eight controls, serial vastus lateralis muscle biopsies were obtained before and 7 hours after a single oral branched chain amino acid mixture enriched with leucine (BCAA/LEU). Primed-constant infusion of l-[ring-(2) H5 ]-phenylalanine was used to quantify whole-body protein breakdown and muscle protein fractional synthesis rate using liquid chromatography/mass spectrometry. Muscle expression of myostatin, mammalian target of rapamycin (mTOR) targets, autophagy markers, protein ubiquitination, and the intracellular amino acid deficiency sensor general control of nutrition 2 were quantified by immunoblots and the leucine exchanger (SLC7A5) and glutamine transporter (SLC38A2), by real-time polymerase chain reaction. Following oral administration, plasma BCAA concentrations showed a similar increase in patients with cirrhosis and controls. Skeletal muscle fractional synthesis rate was 9.63 ± 0.36%/hour in controls and 9.05 ± 0.68%/hour in patients with cirrhosis (P = 0.54). Elevated whole-body protein breakdown in patients with cirrhosis was reduced with BCAA/LEU (P = 0.01). Fasting skeletal muscle molecular markers showed increased myostatin expression, impaired mTOR signaling, and increased autophagy in patients with cirrhosis compared to controls (P < 0.01). The BCAA/LEU supplement did not alter myostatin expression, but mTOR signaling, autophagy measures, and general control of nutrition 2 activation were consistently reversed in cirrhotic muscle (P < 0.01). Expression of SLC7A5 was higher in the basal state in patients with cirrhosis than controls (P < 0.05) but increased with BCAA/LEU only in controls (P < 0.001).

CONCLUSIONS

Impaired mTOR1 signaling and increased autophagy in skeletal muscle of patients with alcoholic cirrhosis is acutely reversed by BCAA/LEU.

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.

Pdgfra protects against ethanol-induced craniofacial defects in a zebrafish model of FASD

Human birth defects are highly variable and this phenotypic variability can be influenced by both the environment and genetics. However, the synergistic interactions between these two variables are not well understood. Fetal alcohol spectrum disorders (FASD) is the umbrella term used to describe the wide range of deleterious outcomes following prenatal alcohol exposure. Although FASD are caused by prenatal ethanol exposure, FASD are thought to be genetically modulated, although the genes regulating sensitivity to ethanol teratogenesis are largely unknown. To identify potential ethanol-sensitive genes, we tested five known craniofacial mutants for ethanol sensitivity: cyp26b1, gata3, pdgfra, smad5 and smoothened. We found that only platelet-derived growth factor receptor alpha (pdgfra) interacted with ethanol during zebrafish craniofacial development. Analysis of the PDGF family in a human FASD genome-wide dataset links PDGFRA to craniofacial phenotypes in FASD, prompting a mechanistic understanding of this interaction. In zebrafish, untreated pdgfra mutants have cleft palate due to defective neural crest cell migration, whereas pdgfra heterozygotes develop normally. Ethanol-exposed pdgfra mutants have profound craniofacial defects that include the loss of the palatal skeleton and hypoplasia of the pharyngeal skeleton. Furthermore, ethanol treatment revealed latent haploinsufficiency, causing palatal defects in ∼62% of pdgfra heterozygotes. Neural crest apoptosis partially underlies these ethanol-induced defects in pdgfra mutants, demonstrating a protective role for Pdgfra. This protective role is mediated by the PI3K/mTOR pathway. Collectively, our results suggest a model where combined genetic and environmental inhibition of PI3K/mTOR signaling leads to variability within FASD.

Leucine pulses enhance skeletal muscle protein synthesis during continuous feeding in neonatal pigs

Infants unable to maintain oral feeding can be nourished by orogastric tube. We have shown that orogastric continuous feeding restricts muscle protein synthesis compared with intermittent bolus feeding in neonatal pigs. To determine whether leucine infusion can be used to enhance protein synthesis during continuous feeding, neonatal piglets received the same amount of formula enterally by orogastric tube for 25.25 h continuously (CON) with or without LEU or intermittently by bolus every 4 h (BOL). For the CON+LEU group, leucine pulses were administered parenterally (800 μmol·kg(-1)·h(-1)) every 4 h. Insulin and glucose concentrations increased after the BOL meal and were unchanged in groups fed continuously. LEU infusion during CON feeding increased plasma leucine after the leucine pulse and decreased essential amino acids compared with CON feeding. Protein synthesis in longissimus dorsi (LD), gastrocnemius, and soleus muscles, but not liver or heart, were greater in CON+LEU and BOL than in the CON group. BOL feeding increased protein synthesis in the small intestine. Muscle S6K1 and 4E-BP1 phosphorylation and active eIF4E·eIF4G complex formation were higher in CON+LEU and BOL than in CON but AMPKα, eIF2α, and eEF2 phosphorylation were unchanged. LC3-II-to-total LC3 ratio was lower in CON+LEU and BOL than in CON, but there were no differences in atrogin-1 and MuRF-1 abundance and FoxO3 phosphorylation. In conclusion, administration of leucine pulses during continuous orogastric feeding in neonates increases muscle protein synthesis by stimulating translation initiation and may reduce protein degradation via the autophagy-lysosome, but not the ubiquitin-proteasome pathway.

Activation of AMPK/TSC2/PLD by alcohol regulates mTORC1 and mTORC2 assembly in C2C12 myocytes

BACKGROUND

Ethanol (EtOH) decreases muscle protein synthesis, and this is associated with reduced mammalian target of rapamycin complex (mTORC)1 and increased mTORC2 activities. In contrast, phospholipase D (PLD) and its metabolite phosphatidic acid (PA) positively regulate mTORC1 signaling, whereas their role in mTORC2 function is less well defined. Herein, we examine the role that PLD and PA play in EtOH-mediated mTOR signaling.

METHODS

C2C12 myoblasts were incubated with EtOH for 18 to 24 hours. For PA experiments, cells were pretreated with the drug for 25 minutes followed by 50-minute incubation with PA in the presence or absence of EtOH. The phosphorylation state of various proteins was assessed by immunoblotting. Protein-protein interactions were determined by immunoprecipitation and immunoblotting. PLD activity was measured using the Amplex Red PLD assay kit. PA concentrations were determined with a total PA assay kit.

RESULTS

PA levels and PLD activity increased in C2C12 myocytes exposed to EtOH (100 mM). Increased PLD activity was blocked by inhibitors of AMP-activated protein kinase (AMPK) (compound C) and phosphoinositide 3-kinase (PI3K) (wortmannin). Likewise, suppression of PLD activity with CAY10594 prevented EtOH-induced Akt (S473) phosphorylation. PLD inhibition also enhanced the binding of Rictor to mSin1 and the negative regulatory proteins Deptor and 14-3-3. Addition of PA to myocytes decreased Akt phosphorylation, but changes in mTORC2 activity were not associated with altered binding of complex members and 14-3-3. PA increased S6K1 phosphorylation, with the associated increase in mTORC1 activity being regulated by reduced phosphorylation of AMPKα (T172) and its target tuberous sclerosis protein complex (TSC)2 (S1387). This resulted in increased Rheb and RagA/RagC GTPase interactions with mTOR, as well as suppression of mTORC2.

CONCLUSIONS

EtOH-induced increases in PLD activity and PA may partially counterbalance the adverse effects of this agent. EtOH and PA regulate mTORC1 via a PI3K/AMPK/TSC2/PLD signaling cascade. PA stimulates mTORC1 function and suppresses activation of mTORC2 as part of an mTORC1/2 feedback loop.

Aging accentuates alcohol-induced decrease in protein synthesis in gastrocnemius

The present study sought to determine whether the protein catabolic response in skeletal muscle produced by chronic alcohol feeding was exaggerated in aged rats. Adult (3 mo) and aged (18 mo) female F344 rats were fed a nutritionally complete liquid diet containing alcohol (36% of total calories) or an isocaloric isonitrogenous control diet for 20 wk. Muscle (gastrocnemius) protein synthesis, as well as mTOR and proteasome activity did not differ between control-fed adult and aged rats, despite the increased TNF-α and IL-6 mRNA and decreased IGF-I mRNA in muscle of aged rats. Compared with alcohol-fed adult rats, aged rats demonstrated an exaggerated alcohol-induced reduction in lean body mass and protein synthesis (both sarcoplasmic and myofibrillar) in gastrocnemius. Alcohol-fed aged rats had enhanced dephosphorylation of 4E-BP1, as well as enhanced binding of raptor with both mTOR and Deptor, and a decreased binding of raptor with 4E-BP1. Alcohol feeding of both adult and aged rats reduced RagA binding to raptor. The LKB1-AMPK-REDD1 pathway was upregulated in gastrocnemius from alcohol-fed aged rats. These exaggerated alcohol-induced effects in aged rats were associated with a greater decrease in muscle but not circulating IGF-I, but no further increase in inflammatory mediators. In contrast, alcohol did not exaggerate the age-induced increase in atrogin-1 and MuRF1 mRNA or the increased proteasome activity. Our results demonstrate that, compared with adult rats, the gastrocnemius from aged rats is more sensitive to the catabolic effects of alcohol on protein synthesis, but not protein degradation, and this exaggerated response may be AMPK-dependent.

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.