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

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.

Effects of Leucine Ingestion and Contraction on the Sestrin/GATOR2 Pathway and mTORC1 Activation in Rat Fast-Twitch muscle

BACKGROUND

Leucine activates the mechanistic/mammalian target of rapamycin complex 1 (mTORC1) in mammalian skeletal muscle. Recent studies have shown that Sestrin, a leucine sensor, might play a role in this process. However, it remains unknown whether Sestrin dissociates from GATOR2 in a dose- and time-dependent manner and whether an acute bout of muscle contraction augments this dissociation.

OBJECTIVE

This study aimed to examine the effects of leucine ingestion and muscle contraction on the interaction between Sestrin1/2 and GATOR2 and on mTORC1 activation.

METHODS

Male Wistar rats were randomly assigned to control (C), leucine 3 (L3), or leucine 10 (L10) groups. Intact gastrocnemius muscles were subjected to 30 repetitive unilateral contractions. The L3 and L10 groups were then orally administered 3 and 10 mmol/kg body weight of L-leucine 2 h after the end of the contractions, respectively. Blood and muscle samples were collected 30, 60, or 120 min after the administration.

RESULTS

The blood and muscle leucine concentrations increased in a dose-dependent manner. The ratio of phosphorylated ribosomal protein S6 kinase (S6K) to total S6K (which indicates mTORC1 signaling activation) was markedly increased by muscle contraction and increased in a dose-dependent manner only in rested muscle. Leucine ingestion but not muscle contraction increased Sestrin1 dissociation from GATOR2 and Sestrin2 association with GATOR2. A negative relationship was observed between the blood and muscle leucine concentrations and the Sestrin1 association with GATOR2.

CONCLUSIONS

The results suggest that Sestrin1, but not Sestrin2, regulates leucine-related mTORC1 activation via its dissociation from GATOR2 and that acute exercise-induced mTORC1 activation involves pathways other than the leucine-related Sestrin1/GATOR2 pathway.

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.

Enhanced glycolysis in granulosa cells promotes the activation of primordial follicles through mTOR signaling

In mammals, nonrenewable primordial follicles are activated in an orderly manner to maintain the longevity of reproductive life. Mammalian target of rapamycin (mTOR)-KIT ligand (KITL) signaling in pre-granulosa cells and phosphatidylinositol 3-kinase (PI3K)-protein kinase B (Akt)-forkhead Box O3a (FOXO3a) signaling in oocytes are important for primordial follicle activation. The activation process is accompanied by the enhancement of energy metabolism, but the causal relationship is unclear. In the present study, the levels of glycolysis-related proteins GLUT4, HK1, PFKL, and PKM2 were significantly increased in granulosa cells but were decreased in oocytes during the mouse primordial-to-primary follicle transition. Both short-term pyruvate deprivation in vitro and acute fasting in vivo increased the glycolysis-related gene and protein levels, decreased AMPK activity, and increased mTOR activity in mouse ovaries. The downstream pathways Akt and FOXO3a were phosphorylated, resulting in mouse primordial follicle activation. The blockade of glycolysis by 2-deoxyglucose (2-DG), but not the blockade of the communication network between pre-granulosa cells and oocyte by KIT inhibitor ISCK03, decreased short-term pyruvate deprivation-promoted mTOR activity. Glycolysis was also increased in human granulosa cells during the primordial-to-primary follicle transition, and short-term pyruvate deprivation promoted the activation of human primordial follicles by increasing the glycolysis-related protein levels and mTOR activity in ovarian tissues. Taken together, the enhanced glycolysis in granulosa cells promotes the activation of primordial follicles through mTOR signaling. These findings provide new insight into the relationship between glycolytic disorders and POI/PCOS.