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      Acute Acidosis Attenuates Leucine Stimulated Signal Transduction and Protein Synthesis in Rat Skeletal Muscle

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          Background: Critical illnesses are often complicated by acute metabolic acidosis, which if persistent, adversely affects outcome. Among the harmful effects that it might cause are impaired utilization of nutrients, increased proteolysis and depressed protein synthesis, leading to muscle wasting. As the amino acid leucine stimulates protein synthesis by activating mTOR signaling, we explored whether in acidosis, impaired leucine-stimulated signaling might be a contributor to the depressed protein synthesis. Methods: Male pair-fed rats were gavaged with NH<sub>4</sub>Cl (acidosis) or NaCl (control) for 2 days and then gavaged once with leucine and sacrificed 45 min later. Extensor digitorum longus muscles were isolated, incubated with or without leucine and protein synthesis measured. The anterior tibial muscle signaling was analysed by Western immunobloting. Results: Despite pair-feeding, acidotic rats lost body and muscle weight vs. controls. Moreover, leucine-induced protein synthesis in isolated muscle from acidotic rats was impaired. In-vivo, 45 min after an oral leucine load, anterior tibial muscle mTOR and 4E-BP1 phosphorylation increased significantly and comparably in control and acidotic rats. In contrast, leucine-stimulated phosphorylation of S6K1, a regulator of translation initiation and protein synthesis, was attenuated to approximately 56% of the control value (p < 0.05). Conclusion: This study reveals that an acute metabolic acidosis impairs leucine-stimulated protein synthesis and activation of signaling downstream of mTOR at the level of S6K1. We propose that this S6K1abnormality may account in part, for the resistance to leucine-stimulated muscle protein synthesis, and may thereby contribute to the impaired nutrient utilization and ultimately the muscle wasting that develops in acidosis.

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          Most cited references 33

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          Bidirectional transport of amino acids regulates mTOR and autophagy.

          Amino acids are required for activation of the mammalian target of rapamycin (mTOR) kinase which regulates protein translation, cell growth, and autophagy. Cell surface transporters that allow amino acids to enter the cell and signal to mTOR are unknown. We show that cellular uptake of L-glutamine and its subsequent rapid efflux in the presence of essential amino acids (EAA) is the rate-limiting step that activates mTOR. L-glutamine uptake is regulated by SLC1A5 and loss of SLC1A5 function inhibits cell growth and activates autophagy. The molecular basis for L-glutamine sensitivity is due to SLC7A5/SLC3A2, a bidirectional transporter that regulates the simultaneous efflux of L-glutamine out of cells and transport of L-leucine/EAA into cells. Certain tumor cell lines with high basal cellular levels of L-glutamine bypass the need for L-glutamine uptake and are primed for mTOR activation. Thus, L-glutamine flux regulates mTOR, translation and autophagy to coordinate cell growth and proliferation.
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            Skeletal muscle protein anabolic response to resistance exercise and essential amino acids is delayed with aging.

            Skeletal muscle loss during aging leads to an increased risk of falls, fractures, and eventually loss of independence. Resistance exercise is a useful intervention to prevent sarcopenia; however, the muscle protein synthesis (MPS) response to resistance exercise is less in elderly compared with young subjects. On the other hand, essential amino acids (EAA) increase MPS equally in both young and old subjects when sufficient EAA is ingested. We hypothesized that EAA ingestion following a bout of resistance exercise would stimulate anabolic signaling and MPS similarly between young and old men. Each subject ingested 20 g of EAA 1 h following leg resistance exercise. Muscle biopsies were obtained before and 1, 3, and 6 h after exercise to measure the rate of MPS and signaling pathways that regulate translation initiation. MPS increased early in young (1-3 h postexercise) and later in old (3-6 h postexercise). At 1 h postexercise, ERK1/2 MNK1 phosphorylation increased and eIF2alpha phosphorylation decreased only in the young. mTOR signaling (mTOR, S6K1, 4E-BP1, eEF2) was similar between groups at all time points, but MNK1 phosphorylation was lower at 3 h and AMP-activated protein kinase-alpha (AMPKalpha) phosphorylation was higher in old 1-3 h postexercise. We conclude that the acute MPS response after resistance exercise and EAA ingestion is similar between young and old men; however, the response is delayed with aging. Unresponsive ERK1/2 signaling and AMPK activation in old muscle may be playing a role in the delayed activation of MPS. Notwithstanding, the combination of resistance exercise and EAA ingestion should be a useful strategy to combat sarcopenia.
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              Atrophy of S6K1(-/-) skeletal muscle cells reveals distinct mTOR effectors for cell cycle and size control.

              The mammalian target of rapamycin (mTOR) and Akt proteins regulate various steps of muscle development and growth, but the physiological relevance and the downstream effectors are under investigation. Here we show that S6 kinase 1 (S6K1), a protein kinase activated by nutrients and insulin-like growth factors (IGFs), is essential for the control of muscle cytoplasmic volume by Akt and mTOR. Deletion of S6K1 does not affect myoblast cell proliferation but reduces myoblast size to the same extent as that observed with mTOR inhibition by rapamycin. In the differentiated state, S6K1(-/-) myotubes have a normal number of nuclei but are smaller, and their hypertrophic response to IGF1, nutrients and membrane-targeted Akt is blunted. These growth defects reveal that mTOR requires distinct effectors for the control of muscle cell cycle and size, potentially opening new avenues of therapeutic intervention against neoplasia or muscle atrophy.

                Author and article information

                Am J Nephrol
                American Journal of Nephrology
                S. Karger AG
                November 2014
                29 October 2014
                : 40
                : 4
                : 362-370
                aResearch Service, Veterans Affairs Health Care Palo Alto, Palo Alto, Calif., and bMedicine Department Nephrology Division, Stanford University, Stanford, Calif., USA
                Author notes
                *Ralph Rabkin, MD, VAPAHCS (111R), 3801 Miranda Avenue, Palo Alto, CA 94304 (USA), E-Mail rabkin@stanford.edu
                366524 Am J Nephrol 2014;40:362-370
                © 2014 S. Karger AG, Basel

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                Page count
                Figures: 4, Tables: 3, Pages: 9
                Original Report: Laboratory Investigation


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