Impaired glycolysis and protein catabolism induced by acid in L6 rat muscle cells

Low pH up‐regulates interleukin‐6 mRNA in L6‐G8C5 rat skeletal muscle cells independent of pH sensing by SNAT2(SLC38A2) transporters

Exercise is known to create a transient, but potent increase in skeletal muscle expression of potentially anti‐inflammatory myokine interleukin‐6 (IL‐6). This effect may be clinically important in managing chronic inflammatory states. It has previously been proposed that lactic acidosis following exercise promotes this IL‐6 up‐regulation, but the mechanism of this acidosis effect is unknown. Rat skeletal muscle cell line L6‐G8C5 has been used previously to model metabolic effects of acidosis, sensing low pH through the resulting inhibition of amino acid transporter SNAT2(SLC38A2). Use of ionophore ionomycin to model the rise in intracellular Ca²⁺ concentration occurring in contracting muscle strongly up‐regulates IL‐6 mRNA in L6‐G8C5 myotubes. This study used this model to test the hypothesis that low extracellular pH (7.1) enhances ionomycin‐induced IL‐6 mRNA up‐regulation by inhibiting SNAT2. Incubation of L6‐G8C5 myotubes for 6 h with 0.5 µM ionomycin at control pH (7.4) resulted in a 15‐fold increase in IL‐6 mRNA which was further enhanced (1.74‐fold) at pH 7.1. In contrast low pH had no significant effect on IL‐6 mRNA without ionomycin, nor on the IL‐6 mRNA increase that was induced by cyclic stretch. Even though pH 7.1 halved the transport activity of SNAT2, alternative methods of SNAT2 inhibition (JNK inhibitor SP600125; SNAT2 antagonist MeAIB; or SNAT2 silencing with siRNA) did not mimic the enhancing effect of low pH on IL‐6 mRNA. On the contrary, JNK inhibition blunted the effect of pH 7.1 with ionomycin, but had no effect at pH 7.4. It is concluded that low pH promotes Ca²⁺/ionomycin–induced up‐regulation of IL‐6 mRNA through a novel SNAT2‐independent JNK‐dependent pH‐sensing pathway not previously described in this skeletal muscle model.

Dexamethasone acutely suppresses the anabolic SNAT2/SLC38A2 amino acid transporter protein in L6-G8C5 rat skeletal muscle cells

Chronic metabolic acidosis plays a role in cachexia by enhancing total proteolysis in skeletal muscle. Glucocorticoid also triggers proteolysis and plays a permissive role in the effect of acidosis. The System A amino acid transporter SNAT2/SLC38A2 is ubiquitously expressed in mammalian cells including muscle, performing Na⁺‐dependent active import of neutral amino acids, and is strongly inhibited by low pH. Exposure of rat skeletal muscle cell line L6‐G8C5 to low pH rapidly inhibits SNAT2 transport activity and enhances total proteolysis rate. Pharmacological inhibition or silencing of SNAT2 also enhances proteolysis. This study tests the hypothesis that the glucocorticoid dexamethasone (DEX), like low pH, inhibits SNAT2 activity in L6‐G8C5 myotubes, thus contributing to total proteolysis. Incubation with 500 nM DEX for 4 h reduced the System A amino acid transport rate to half the rate in control cultures. This inhibition depended on glucocorticoid receptor‐mediated gene transcription, but SNAT2 mRNA levels were unaffected by DEX. In contrast, the SNAT2 protein assessed by immunoblotting was significantly depleted. The co‐inhibitory effects of DEX and low pH on System A transport activity were additive in stimulating total proteolysis. In keeping with this mechanism, DEX’s inhibitory effect on SNAT2 transport activity was significantly blunted by the proteasome inhibitor MG132. Proof of principle was achieved in similar experiments using recombinant expression of a GFP‐tagged SNAT2 fusion protein in HEK293A cells. It is concluded that DEX acutely depletes the SNAT2 transporter protein, at least partly through proteasome‐dependent degradation of this functionally important transporter.

Mitochondrial DNA deletion and sarcopenia

PURPOSE

Accumulation of mitochondrial DNA deletions and the resultant impaired oxidative phosphorylation may play a pathogenic role in the mediation of age-related sarcopenia.

METHODS

Twenty four participants of the New Mexico Aging Process Study were classified as normal lean (n = 15) or sarcopenic (n = 9) based on body composition determined by Dual Energy x-ray Absorptiometry. Complex I and Complex IV activities were measured in the skeletal muscle samples obtained from gastrocnemius muscle. A two-stage nested polymerase chain reaction strategy was used to identify the mitochondrial DNA deletions in the entire mitochondrial genome in the skeletal muscle samples.

RESULTS

Although Complex I activity was not significantly different (5.5 +/- 0.9 vs. 4.6 +/- 0.7 mU/mg protein, P > 0.05), Complex IV activity was higher in sarcopenic subjects (1.4 +/- 0.3 vs. 1.0 +/- 0.1 mU/mg protein, P < 0.05). Mitochondrial DNA deletions were mostly located in the region of Complex I and spanned from nicotinamide adenine dinucleotide dehydrogenase 1 to nicotinamide adenine dinucleotide dehydrogenase 6. Deletions in the 8,577-10,407 bp and 10,233-11,249 bp regions were associated with a significant decrease in Complex I activity (P < 0.05 and P = 0.02, respectively). Total cumulative deletion, defined as the sum of individual length of deletions in a subject, was comparable in subjects with and without sarcopenia (1760 +/- 726 vs. 1782 +/- 888 bp, P > 0.05). The magnitude of mitochondrial DNA deletion, however, correlated positively with lean body mass (r = 0.43, P < 0.05).

CONCLUSION

Thus, mitochondrial DNA deletions are common in elderly subjects and are negatively related to Complex I activity. The positive association between mitochondrial DNA deletions and lean body mass needs to be confirmed by studies in a larger study population.

Inhibition of SNAT2 by metabolic acidosis enhances proteolysis in skeletal muscle

Insulin resistance is a major cause of muscle wasting in patients with ESRD. Uremic metabolic acidosis impairs insulin signaling, which normally suppresses proteolysis. The low pH may inhibit the SNAT2 l-Glutamine (L-Gln) transporter, which controls protein synthesis via amino acid-dependent insulin signaling through mammalian target of rapamycin (mTOR). Whether SNAT2 also regulates signaling to pathways that control proteolysis is unknown. In this study, inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate or metabolic acidosis (pH 7.1) depleted intracellular L-Gln and stimulated proteolysis in cultured L6 myotubes. At pH 7.1, inhibition of the proteasome led to greater depletion of L-Gln, indicating that amino acids liberated by proteolysis sustain L-Gln levels when SNAT2 is inhibited by acidosis. Acidosis shifted the dose-response curve for suppression of proteolysis by insulin to the right, confirming that acid increases proteolysis by inducing insulin resistance. Blocking mTOR or phosphatidylinositol-3-kinase (PI3K) increased proteolysis, indicating that both signaling pathways are involved in its regulation. When both mTOR and PI3K were inhibited, methylaminoisobutyrate or acidosis did not stimulate proteolysis further. Moreover, partial silencing of SNAT2 expression in myotubes and myoblasts with small interfering RNA stimulated proteolysis and impaired insulin signaling through PI3K. In conclusion, SNAT2 not only regulates mTOR but also regulates proteolysis through PI3K and provides a link among acidosis, insulin resistance, and protein wasting in skeletal muscle cells.

Acidosis-sensing glutamine pump SNAT2 determines amino acid levels and mammalian target of rapamycin signalling to protein synthesis in L6 muscle cells

Wasting of lean tissue as a consequence of metabolic acidosis is a serious problem in patients with chronic renal failure. A possible contributor is inhibition by low pH of the System A (SNAT2) transporter, which carries the amino acid L-glutamine (L-Gln) into muscle cells. The aim of this study was to determine the effect of selective SNAT2 inhibition on intracellular amino acid profiles and amino acid-dependent signaling through mammalian target of rapamycin in L6 skeletal muscle cells. Inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate, metabolic acidosis (pH 7.1), or silencing SNAT2 expression with small interfering RNA all depleted intracellular L-Gln. SNAT2 inhibition also indirectly depleted other amino acids whose intracellular concentrations are maintained by the L-Gln gradient across the plasma membrane, notably the anabolic amino acid L-leucine. Consequently, SNAT2 inhibition strongly impaired signaling through mammalian target of rapamycin to ribosomal protein S6 kinase, ribosomal protein S6, and 4E-BP1, leading to impairment of protein synthesis comparable with that induced by rapamycin. It is concluded that even though SNAT2 is only one of several L-Gln transporters in muscle, it may determine intracellular anabolic amino acid levels, regulating the amino acid signaling that affects protein mass, nucleotide/nucleic acid metabolism, and cell growth.

Effect of insulin upon protein degradation in cultured human myocytes

BACKGROUND

The anabolic effects of insulin are well recognized but the mechanism by which insulin decreases muscle protein degradation in human is unclear. However, in a variety of catabolic conditions it is believed to be changes in the activity of the ATP-dependent ubiquitin proteolytic pathway that are responsible for changes in protein degradation in skeletal muscle. The aim of this study was to test the hypothesis that insulin regulates the ATP-dependent ubiquitin proteolytic pathway in human muscle.

MATERIAL AND METHODS

The effects of insulin and acidosis on protein degradation were measured in human myocytes using L-[14C]phenylalanine. The effect of insulin on the activity of the ATP-dependent ubiquitin pathway was assessed from the mRNA expression of ubiquitin and the ubiquitin-conjugating enzyme E214k in human myocytes.

RESULTS AND CONCLUSIONS

Coincubation of human myocytes with 100 nM of insulin was associated with a significant reduction in protein degradation. Metabolic acidosis is known to increase skeletal muscle protein degradation rates, and in our experiments protein degradation at a pH of 7.0 was significantly higher than pH 7.35. Eight-hour exposure to 100 nM of insulin resulted in a significant reduction in the expression of E214k but no change in the expression of ubiquitin.

CONCLUSIONS

In human muscle we have demonstrated regulation by insulin of the ATP-dependent ubiquitin pathway at the level of ubiquitin conjugation.

Amino acid transporters: roles in amino acid sensing and signalling in animal cells

Amino acid availability regulates cellular physiology by modulating gene expression and signal transduction pathways. However, although the signalling intermediates between nutrient availability and altered gene expression have become increasingly well documented, how eukaryotic cells sense the presence of either a nutritionally rich or deprived medium is still uncertain. From recent studies it appears that the intracellular amino acid pool size is particularly important in regulating translational effectors, thus, regulated transport of amino acids across the plasma membrane represents a means by which the cellular response to amino acids could be controlled. Furthermore, evidence from studies with transportable amino acid analogues has demonstrated that flux through amino acid transporters may act as an initiator of nutritional signalling. This evidence, coupled with the substrate selectivity and sensitivity to nutrient availability classically associated with amino acid transporters, plus the recent discovery of transporter-associated signalling proteins, demonstrates a potential role for nutrient transporters as initiators of cellular nutrient signalling. Here, we review the evidence supporting the idea that distinct amino acid "receptors" function to detect and transmit certain nutrient stimuli in higher eukaryotes. In particular, we focus on the role that amino acid transporters may play in the sensing of amino acid levels, both directly as initiators of nutrient signalling and indirectly as regulators of external amino acid access to intracellular receptor/signalling mechanisms.

Impaired system A amino acid transport mimics the catabolic effects of acid in L6 cells

BACKGROUND

Metabolic acidaemia stimulates protein catabolism in skeletal muscle cells, leading to muscle wasting. As this occurs without decreasing cytosolic pH, the initial signal is unclear. A possible explanation is that extracellular pH acts on solute transporters at the cell surface, inhibiting nutrient influx.

DESIGN

Influx through glucose and Pi transporters and System A amino acid transporters into L6 skeletal muscle cells was assessed using 3H-2-deoxyglucose (2-DG), 33Pi and 14C-methylaminoisobutyrate (MeAIB), respectively. Protein degradation (PD) was assessed from 14C efflux from cells prelabelled with 14C-Phe. Branched-chain amino acids and Phe were assayed by selective fluorimetric assays.

RESULTS

While acid (pH 7.1) had little immediate effect on 2-DG or 33Pi influx, exposure to pH 7.1 rapidly inhibited MeAIB influx. To determine whether System A inhibition was sufficient to trigger PD, it was blocked at pH 7.5 by a saturating dose (10 mmol L(-1)) of nonmetabolisable substrate (MeAIB). Like acid, this increased PD and decreased total protein. It also mimicked the decreases in protein synthesis, DNA synthesis, glucose transport and glycolysis, and depletion of branched-chain amino acids and Phe, which are induced in L6 by acid. The onset of inhibition of PD by an extracellular Gln load was retarded at pH 7.1, and stimulation of PD by acid was negligible if PD had already been stimulated by Gln depletion. The stimulatory effect of MeAIB on PD was selectively blunted by an excess of Gln, whereas the inhibitory effect of Gln on PD was blocked by excess MeAIB.

CONCLUSIONS

The similarity of changes in response to MeAIB and acid implies that these share a common intracellular signalling pathway triggered by inhibition of System A. Even though System A is only a minor contributor to total Gln influx in L6 cells, it is suggested that blockade of System A with acid or MeAIB induces a catabolic state by denying Gln access to a key intracellular regulatory site.

Leucine suppresses acid-induced protein wasting in L6 rat muscle cells

BACKGROUND

Metabolic acidosis induces protein wasting in skeletal muscle cells, accompanied by decreased glycolysis and compensatory increased consumption of other metabolic fuels, implying that protein wasting arises from fuel starvation and might be rectified by fuel supplements. Design To test this hypothesis, total protein and protein degradation (release of 14C-phenylalanine) were measured in L6 skeletal muscle cells cultured in Eagle's Minimum Essential Medium at pH 7.1-7.5 for 3 days with metabolic inhibitors or metabolic fuel supplements.

RESULTS

Inducing metabolic fuel starvation with inhibitors (1 mmol L(-1) 2-deoxyglucose or 0.1 mmol L(-1) KCN [potassium cyanide]) failed to stimulate protein degradation or net protein wasting under nonacidaemic conditions (pH 7.5). Conversely metabolic fuel supplements (1 mmol L(-1) octanoate, pyruvate or alanine) failed to increase the protein content of the cultures at any pH tested, in spite of significant consumption of the fuels by the cells. Only leucine (1-3 mmol L(-1)) increased protein content and suppressed protein degradation in opposition to the catabolic effect of acidaemia (pH 7.1). Conclusion Leucine exerts a beneficial anabolic effect on cultured skeletal muscle cells in the face of metabolic acidaemia. The failure of other metabolic fuels to do this, and of the metabolic inhibitors to exert a catabolic effect, suggests that leucine acts as a specific modulator of protein turnover and not as a nonspecific source of carbon for oxidation as a fuel.