Diazoxide-induced insulin deficiency greatly reduced muscle protein synthesis in rats: involvement of eIF4E

Postprandial leucine deficiency failed to alter muscle protein synthesis in growing and adult rats

OBJECTIVE

This study examined the effect of a specific acute postprandial leucine deficiency on skeletal muscle protein synthesis in growing and adult rats. Because the anabolic action of dietary leucine supplementation is controversial, except during aging, we hypothesized that the maximum leucine effect might be already achieved for a normal postprandial rise of leucine. Preventing this rise during the 1- to 3-h period after feeding may reveal the leucine regulation.

METHODS

On the day of the experiment, rats were fasted (postabsorptive, PA group) or fed for 1 h a control meal (postprandial, control, PP group) or a leucine-poor meal (postprandial, PP-Leu group). Muscle protein synthesis was assessed in vivo, over the 1- to 3-h period after meal distribution, using the flooding dose method (L-1-(13)C phenylalanine).

RESULTS

As expected, the postprandial increase in plasma free leucine was specifically abolished after feeding the leucine-poor meal, whereas all the other plasma free amino acids were roughly at normal postprandial levels. Plasma insulin increased after feeding in young rats but was constant in adult rats. Plasma insulin was similar whatever dietary leucine levels. Rates of muscle protein synthesis were stimulated by feeding in gastrocnemius and soleus muscles from young rats but only in gastrocnemius muscles from adult rats. The PP-Leu group did not differ from the control PP group regarding muscle protein synthesis.

CONCLUSION

The rise in plasma free leucine is not required for the stimulation of muscle protein synthesis during the 1- to 3-h period after feeding young and adult rats, as previously observed in old rats.

Effect of insulin on human skeletal muscle protein synthesis is modulated by insulin-induced changes in muscle blood flow and amino acid availability

Insulin promotes muscle anabolism, but it is still unclear whether it stimulates muscle protein synthesis in humans. We hypothesized that insulin can increase muscle protein synthesis only if it increases muscle amino acid availability. We measured muscle protein and amino acid metabolism using stable-isotope methodologies in 19 young healthy subjects at baseline and during insulin infusion in one leg at low (LD, 0.05), intermediate (ID, 0.15), or high (HD, 0.30 mUxmin(-1)x100 ml(-1)) doses. Insulin was infused locally to induce muscle hyperinsulinemia within the physiological range while minimizing the systemic effects. Protein and amino acid kinetics across the leg were assessed using stable isotopes and muscle biopsies. The LD did not affect phenylalanine delivery to the muscle (-9 +/- 18% change over baseline), muscle protein synthesis (16 +/- 26%), breakdown, or net balance. The ID increased (P < 0.05) phenylalanine delivery (+63 +/- 38%), muscle protein synthesis (+157 +/- 54%), and net protein balance, with no change in breakdown. The HD did not change phenylalanine delivery (+12 +/- 11%) or muscle protein synthesis (+9 +/- 19%), and reduced muscle protein breakdown (-17 +/- 15%), thus improving net muscle protein balance but to a lesser degree than the ID. Changes in muscle protein synthesis were strongly associated with changes in muscle blood flow and phenylalanine delivery and availability. In conclusion, physiological hyperinsulinemia promotes muscle protein synthesis as long as it concomitantly increases muscle blood flow, amino acid delivery and availability.

Differential effects of insulin and dietary amino acids on muscle protein synthesis in adult and old rats

The potential roles of insulin and dietary amino acids in the regulation of skeletal muscle protein synthesis were examined in adult and old rats. Animals were fed over 1 h with either a 25% or a 0% amino acid/protein meal. In each nutritional condition, postprandial insulin secretion was either maintained or blocked with diazoxide injections. Protein synthesis in gastrocnemius and soleus muscles was assessed in vivo using the flooding dose method. Insulin suppression decreased protein synthesis in both muscles irrespective of the nutritional condition and age of the rats. Moreover, reduced insulinaemia was associated with 4E-BP1 dephosphorylation, enhanced assembly of the 4E-BP1-eIF4E inactive complex and hypophosphorylation of eIF4E, p70S6k and protein kinase B, key intermediates in the regulation of translation initiation and protein synthesis. Old rats did not differ from adult rats. The lack of amino acids in the meal of insulin-suppressed rats did not result in any additional decrease in protein synthesis. In the presence of insulin secretion, dietary amino acid suppression significantly decreased gastrocnemius protein synthesis in adult but not in old rats. Amino acid suppression was associated with reduced phosphorylation of 4E-BP1 and p70S6k in adults. Along with protein synthesis, only the inhibition of p70S6k phosphorylation was abolished in old rats. We concluded that insulin is required for the regulation of muscle protein synthesis irrespective of age and that the effect of dietary amino acids is blunted in old rats.

Insulin and amino acids both strongly participate to the regulation of protein metabolism

PURPOSE OF REVIEW

The application of tracer kinetic methods, combined with measurements of the activity of components of the cellular signaling pathways involved in protein synthesis and degradation, affords new insights into the regulation of skeletal muscle protein metabolism in vivo in humans. Feeding is associated with an increase in protein synthesis and a decrease in proteolysis. These changes are mediated by feeding-induced increases in plasma concentrations of both nutrients and hormones.

RECENT FINDINGS

Recent studies definitely demonstrated that insulin and amino acids directly interacted in promoting postprandial anabolism. However, the contribution of amino acids was abolished in old individuals in whom only insulin action persisted. There was a line of evidence that the effect of amino acids originates from leucine, which should not be viewed simply as a building block for protein synthesis, but as a signal in the regulation of cell functions. Although their cellular signaling pathways do not completely overlap, insulin and amino acids both activate the translation initiation of protein synthesis. Insulin presumably inhibits skeletal muscle protein degradation through a decrease in the activity of the ubiquitin proteasome-dependent pathway.

SUMMARY

Whether or not amino acids modify insulin action and have specific effects on proteolysis has not yet been documented. At the molecular level, amino acids such as insulin modulate gene expression. Such studies are needed to gain a better understanding of the interactions between insulin and amino acids in the regulation of skeletal muscle protein anabolism.

Ubiquitin-proteasome-dependent muscle proteolysis responds slowly to insulin release and refeeding in starved rats

The central role of the ubiquitin-proteasome system in the loss of skeletal muscle protein in many wasting conditions has been well established. However, it is unclear what factors are responsible for the suppression of this system during periods of protein gain. Thus, the aim of these studies was to examine the short-term effects of insulin release and nutrients on skeletal muscle protein turnover in young rats starved for 48 h, and then infused intravenously with amino acids (AA), or fed an oral diet. Forty-eight hours of starvation (i.e. prolonged starvation in young rats) decreased muscle protein synthesis and increased proteasome-dependent proteolysis. Four-hour AA infusion and 4 h of refeeding increased plasma insulin release and AA concentrations, and stimulated muscle protein synthesis, but had no effect on either total or proteasome-dependent proteolysis, despite decreased plasma corticosterone concentrations. Both muscle proteasome-dependent proteolysis and the rate of ubiquitination of muscle proteins were not suppressed until 10 h of refeeding. The temporal response of these two measurements correlated with the normalised expression of the 14-kDa E2 (a critical enzyme in substrate ubiquitination in muscle) and the expression of the MSS1 subunit of the 19S regulatory complex of the 26S proteasome. In contrast, the starvation-induced increase in mRNA levels for 20S proteasome subunits was normalised by refeeding within 24 h in muscle, and 6 h in jejunum, respectively. In conclusion, unlike protein synthesis, skeletal muscle proteasome-dependent proteolysis is not acutely responsive in vivo to insulin, AA, and/or nutrient intake in refed starved rats. This suggests that distinct and perhaps independent mechanisms are responsible for the nutrient-dependent regulation of protein synthesis and ubiquitin-proteasome-dependent proteolysis following a prolonged period of catabolism. Furthermore, factors other than the expression of ubiquitin-proteasome pathway components appear to be responsible for the suppression of skeletal muscle proteasome-dependent proteolysis by nutrition.

Invited Review: Role of insulin in translational control of protein synthesis in skeletal muscle by amino acids or exercise

Protein synthesis in skeletal muscle is modulated in response to a variety of stimuli. Two stimuli receiving a great deal of recent attention are increased amino acid availability and exercise. Both of these effectors stimulate protein synthesis in part through activation of translation initiation. However, the full response of translation initiation and protein synthesis to either effector is not observed in the absence of a minimal concentration of insulin. The combination of insulin and either increased amino acid availability or endurance exercise stimulates translation initiation and protein synthesis in part through activation of the ribosomal protein S6 protein kinase S6K1 as well as through enhanced association of eukaryotic initiation factor eIF4G with eIF4E, an event that promotes binding of mRNA to the ribosome. In contrast, insulin in combination with resistance exercise stimulates translation initiation and protein synthesis through enhanced activity of a guanine nucleotide exchange protein referred to as eIF2B. In both cases, the amount of insulin required for the effects is low, and a concentration of the hormone that approximates that observed in fasting animals is sufficient for maximal stimulation. This review summarizes the results of a number of recent studies that have helped to establish our present understanding of the interactions of insulin, amino acids, and exercise in the regulation of protein synthesis in skeletal muscle.

Amino acids and insulin are both required to regulate assembly of the eIF4E. eIF4G complex in rat skeletal muscle

The respective roles of insulin and amino acids in regulation of skeletal muscle protein synthesis and degradation after feeding were examined in rats fasted for 17 h and refed over 1 h with either a 25 or a 0% amino acid/protein meal. In each nutritional condition, postprandial insulin secretion was either maintained (control groups: C(25) and C(0)) or blocked with diazoxide injections (diazoxide groups: DZ(25) and DZ(0)). Muscle protein metabolism was examined in vitro in epitrochlearis muscles. Only feeding the 25% amino acid/protein meal in the presence of increased plasma insulin concentration (C(25) group) stimulated protein synthesis and inhibited proteolysis in skeletal muscle compared with the postabsorptive state. The stimulation of protein synthesis was associated with increased phosphorylation of eukaryotic initiation factor (eIF)4E binding protein-1 (4E-BP1), reduced binding of eIF4E to 4E-BP1, and increased assembly of the active eIF4E. eIF4G complex. The p70 S6 kinase (p70(S6k)) was also hyperphosphorylated in response to the 25% amino acid/protein meal. Acute postprandial insulin deficiency induced by diazoxide injections totally abolished these effects. Feeding the 0% amino acid/protein meal with or without postprandial insulin deficiency did not stimulate muscle protein synthesis, reduce proteolysis, or regulate initiation factors and p70(S6k) compared with fasted rats. Taken together, our results suggest that both insulin and amino acids are required to stimulate protein synthesis, inhibit protein degradation, and regulate the interactions between eIF4E and 4E-BP1 or eIF4G in response to feeding.

Eukaryotic initiation factors and protein synthesis after resistance exercise in rats

Translational control of protein synthesis depends on numerous eukaryotic initiation factors (eIFs) and we have previously shown (Am. J. Physiol. Endocrinol. Metab. 276: E721-E727, 1999) that increases in one factor, eIF2B, are associated with increases in rates of protein synthesis after resistance exercise in rats. In the present study we investigated whether the eIF4E family of initiation factors is also involved with an anabolic response to exercise. Male Sprague-Dawley rats either remained sedentary (n = 6) or performed acute resistance exercise (n = 6), and rates of protein synthesis were assessed in vivo 16 h after the last session of resistance exercise. eIF4E complexed to eIF4G (eIF4E x eIF4G), eIF4E binding protein 1 (4E-BP1) complexed to eIF4E, and phosphorylation state of eIF4E and 4E-BP1 (gamma-form) were assessed in gastrocnemius. Rates of protein synthesis were higher in exercised rats compared with sedentary rats [205 +/- 8 (SE) vs. 164 +/- 5.5 nmol phenylalanine incorporated x g muscle(-1) x h(-1), respectively; P < 0.05]. Arterial plasma insulin concentrations were not different between the two groups. A trend (P = 0.09) for an increase in eIF4E x eIF4G with exercise was noted; however, no statistically significant differences were observed in any of the components of the eIF4E family in response to resistance exercise. These new data, along with our previous report on eIF2B, suggest that the regulation of peptide chain initiation after exercise is more dependent on eIF2B than on the eIF4E system.