Does acute glutamine depletion enhance the response of glutamine synthesis to fasting in muscle in adult and old rats?

Glutamine metabolism in advanced age

Glutamine, reviewed extensively in the last century, is a key substrate for the splanchnic bed in the whole body and is a nutrient of particular interest in gastrointestinal research. A marked decrease in the plasma glutamine concentration has recently been observed in neonates and adults during acute illness and stress. Although some studies in newborns have shown parenteral and enteral supplementation with glutamine to be of benefit (by decreasing proteolysis and activating the immune system), clinical trials have not demonstrated prolonged advantages such as reductions in mortality or risk of infections in adults. In addition, glutamine is not able to combat the muscle wasting associated with disease or age-related sarcopenia. Oral glutamine supplementation initiated before advanced age in rats increases gut mass and improves the villus height of mucosa, thereby preventing the gut atrophy encountered in advanced age. Enterocytes from very old rats continuously metabolize glutamine into citrulline, which allowed, for the first time, the use of citrulline as a noninvasive marker of intestinal atrophy induced by advanced age.

FOXO1 activates glutamine synthetase gene in mouse skeletal muscles through a region downstream of 3'-UTR: possible contribution to ammonia detoxification

Skeletal muscle is a reservoir of energy in the form of protein, which is degraded under catabolic conditions, resulting in the formation of amino acids and ammonia as a byproduct. The expression of FOXO1, a forkhead-type transcription factor, increases during starvation and exercise. In agreement, transgenic FOXO1-Tg mice that overexpress FOXO1 in skeletal muscle exhibit muscle atrophy. The aim of this study was to examine the role of FOXO1 in amino acid metabolism. The mRNA and protein expressions of glutamine synthetase (GS) were increased in skeletal muscle of FOXO1-Tg mice. Fasting induced FOXO1 and GS expression in wild-type mice but hardly increased GS expression in muscle-specific FOXO1 knockout (FOXO1-KO) mice. Activation of FOXO1 also increased GS mRNA and protein expression in C2C12 myoblasts. Using a transient transfection reporter assay, we observed that FOXO1 activated the GS reporter construct. Mutation of a putative FOXO1-binding consensus sequence in the downstream genomic region of GS decreased basal and FOXO1-dependent reporter activity significantly. A chromatin immunoprecipitation assay showed that FOXO1 was recruited to the 3' region of GS in C2C12 myoblasts. These results suggest that FOXO1 directly upregulates GS expression. GS is considered to mediate ammonia clearance in skeletal muscle. In agreement, an intravenous ammonia challenge increased blood ammonia concentrations to a twofold higher level in FOXO1-KO than in wild-type mice, demonstrating that the capacity for ammonia disposal correlated inversely with the expression of GS in muscle. These data indicate that FOXO1 plays a role in amino acid metabolism during protein degradation in skeletal muscle.

Consequences of age-related splanchnic sequestration of leucine on interorgan glutamine metabolism in old rats

Dietary leucine (Leu) serves as a nitrogen donor for de novo glutamine (Gln) synthesis in muscle. However, aging is characterized by an increase in the splanchnic extraction of Leu (SPELeu), i.e., splanchnic sequestration (SSLeu), which may affect muscle Gln metabolism and its subsequent homeostasis at the whole-body level. The aim of the work was to assess the effect of age-related SSLeu on Gln metabolism in the muscle, gut, liver, kidney, and Gln exchanges among these organs during fed conditions. Young-adult (3-mo-old) or aged (24-mo-old), male Sprague-Dawley rats were studied during fed condition [infusion of amino acids (AA) into the duodenum from time 0 min (T0) to T60] under anesthesia. L-[5-15N]Gln and L[1-13C]Leu were infused into the jugular vein and L-[5,5,5-2H3]Leu into the duodenum. At T60, blood samples were taken from carotid artery, portal vein, hepatic vein, renal vein, and inferior vena cava for tracer-tracee ratio and AA level measurements. SSLeu was observed in old rats and was negatively correlated with muscle Gln production ( r = −0.501, P < 0.01). In addition, reduced Gln muscle release in old rats was accompanied by reduced Gln uptake by the gut and kidney. However, net Gln balance across organs was not different between young adult and old rats. During fed conditions in old rats, muscle Gln production and release are reduced in relation to the observed, increased SPELeu and reduced renal and intestinal Gln uptake to maintain whole-body Gln homeostasis. Our results demonstrate the existence of an age-related change of interorgan Gln metabolism, which may be, in part, driven by SSLeu.

Glutamine directly downregulates glutamine synthetase protein levels in mouse C2C12 skeletal muscle myotubes

This study examined the regulation of glutamine synthetase protein levels, in response to changes in external glutamine concentration, in mouse C2C12 skeletal muscle cells. Glutamine, at concentrations as low as 0.25 mmol/L, downregulated endogenous and exogenous (plasmid encoded) glutamine synthetase with maximal effect at 2 mmol/L. Glutamine appears to act by changing the stability of the glutamine synthetase protein, and the effect was partially blocked by the proteasome inhibitor MG132. The addition of the glutamine structural analog and glutaminase inhibitor, 6-diazo-5-oxo-L-norleucine, in the presence or absence of glutamine, also resulted in low glutamine synthetase protein levels. Otherwise, the effect was specific for glutamine, and the only compounds able to mimic the effect of glutamine were amino acids, glutamate, alanine, and ornithine, which can be converted to glutamine. Other amino acids, analogs, and products of glutamine metabolism were without effect. Methionine sulfoximine, an inhibitor of glutamine synthetase, stabilized the protein and prevented the glutamine effect. Thus, in mouse C2C12 skeletal muscle cells, glutamine synthetase protein expression is regulated by glutamine through changes in the rate of degradation of the protein. The effect is specific to glutamine, which acts directly without requiring prior metabolism.

Glutamine, insulin and glucocorticoids regulate glutamine synthetase expression in C2C12 myotubes, Hep G2 hepatoma cells and 3T3 L1 adipocytes

The cell-specific regulation of glutamine synthetase expression was studied in three cell lines. In C2C12 myotubes, glucocorticoids increased the abundance of both glutamine synthetase protein and mRNA. Culture in the absence of glutamine also resulted in very high glutamine synthetase protein abundance but mRNA levels were unchanged. Glucocorticoids also increased the abundance of glutamine synthetase mRNA in Hep G2 hepatoma cells but this was not reflected in changes in protein abundance. Culture of Hep G2 cells without glutamine resulted in very high levels of protein, again with no change in mRNA abundance. Insulin was without effect in both C2C12 and Hep G2 cells. In 3T3 L1 adipocytes glucocorticoids increased the abundance of both glutamine synthetase mRNA and protein, insulin added alone had no effect but in the presence of glucocorticoids resulted in lower mRNA levels than seen with glucocorticoids alone, although protein levels remained high under such conditions. In contrast to the other cell lines glutamine synthetase protein levels were relatively unchanged by culture in the absence of glutamine. The results support the hypothesis that in myocytes, and hepatomas, but not in adipocytes, glutamine acts to moderate glutamine synthetase induction by glucocorticoids.

Alterations in glutamine synthetase activity in rat skeletal muscle are associated with advanced age

OBJECTIVE

Glutamine synthetase (GS), a key enzyme in the production of glutamine, is preserved in skeletal muscle during early aging (<24 mo). Because the effects of advanced age on GS are unknown, we investigated the effect of advanced age (>24 mo) on GS activity in skeletal muscle. We hypothesized that advanced age would enhance muscle GS activity.

METHODS

Muscle GS activities were assessed in adult (8 mo), mature adult (15 mo), aged (20-22 mo), advanced age (25-27 mo), or very advanced age (29-32 mo) female Wistar rats. Male Wistar (6-27 mo) were used to investigate the effect of gender on this activity.

RESULTS

Glutamine synthetase activity remained low and unaltered in rats from 8 to 22 mo of age, as previously demonstrated. In contrast, GS activity was high ( approximately 75% of individual values were higher than the low value mean) in 25-mo to 27-mo-old rats. In very-old-aged rats (29-32 mo), approximately 55% of GS activity data points exhibited low values. Changes in GS protein content paralleled those in GS activities. In male rats, GS activity was also high ( approximately 80% of individual values were higher than the mean value of 6-mo to 19-mo-old rats) at the upper limit of life expectancy (27 mo).

CONCLUSION

There is enhanced GS activity in old female and male rats suggesting a greater need for glutamine. In some very old rats, low GS activity may be associated with longevity or reflect a limitation in glutamine production due to extremely advanced age per se.