Late effects of postnatal administration of monosodium glutamate on insulin action in adult rats

Early postnatal administration of monosodium glutamate (MSG) to rats induces obesity, hyperinsulinemia and hyperglycemia in adulthood, thus suggesting the presence of insulin resistance. We therefore investigated the effects of insulin on glucose transport and lipogenesis in adipocytes as well as insulin binding to specific receptors in the liver, skeletal muscle and fat tissues. An increase of plasma insulin, glucose and leptin levels was found in 3-month-old rats treated with MSG during the postnatal period. The attenuation of insulin stimulatory effect on glucose transport was observed in MSG-treated rats. Despite the lower basal and insulin-stimulated glucose uptake, the incorporation of glucose into lipids was significantly higher in MSG-treated rats, suggesting a shift in glucose metabolism towards lipid synthesis in fat tissue. Insulin binding to plasma membranes from the liver, skeletal muscle and adipocytes was decreased in MSG-treated rats. This is in agreement with the lower insulin effect on glucose transport in these animals. Furthermore, a decreased amount of GLUT4 protein was found in adipocytes from MSG-treated obese rats. The results demonstrated an attenuation of insulin effect on glucose transport due to a lower insulin binding and lower content of GLUT4 protein in MSG-treated rats. However, the effect of insulin on lipogenesis was not changed. Our results indicated that early postnatal administration of MSG exerts an important effect on glucose metabolism and insulin action in adipocytes of adult animals.     

Acute bioenergetic insulin sensitivity of skeletal muscle cells: ATP-demand-provoked glycolysis contributes to stimulation of ATP supply

Skeletal muscle takes up glucose in an insulin-sensitive manner and is thus important for the maintenance of blood glucose homeostasis. Insulin resistance during development of type 2 diabetes is associated with decreased ATP synthesis, but the causality of this association is controversial. In this paper, we report real-time oxygen uptake and medium acidification data that we use to quantify acute insulin effects on intracellular ATP supply and ATP demand in rat and human skeletal muscle cells. We demonstrate that insulin increases overall cellular ATP supply by stimulating the rate of glycolytic ATP synthesis. Stimulation is immediate and achieved directly by increased glycolytic capacity, and indirectly by elevated ATP demand from protein synthesis. Raised glycolytic capacity does not result from augmented glucose uptake. Notably, insulin-sensitive glucose uptake is increased synergistically by nitrite. While nitrite has a similar stimulatory effect on glycolytic ATP supply as insulin, it does not amplify insulin stimulation. These data highlight the multifarious nature of acute bioenergetic insulin sensitivity of skeletal muscle cells, and are thus important for the interpretation of changes in energy metabolism that are seen in insulin-resistant muscle.     

Up-regulation of phosphatidylinositol 3-kinase and glucose transporter 4 in muscle of rats subjected to maternal undernutrition

Early postnatal nutrition has been associated with the long-term effects on glucose homeostasis in adulthood. To elucidate the molecular mechanisms by which undernutrition during early life leads to changes in insulin sensitivity, we investigated the insulin signaling in skeletal muscle of rats during development. Offspring of dams fed with either protein-free or normal diets during the first 10 days of lactation were studied from lactation period until adulthood. Early maternal undernutrition impaired secretion of insulin but maintained normal blood glucose levels until adulthood. Insulin receptor (IR) activation after insulin stimulation was decreased during the period of protein restriction. In addition, glucose uptake, insulin receptor substrate 1 (IRS-1) phosphorylation and glucose transporter 4 (GLUT-4) translocation in muscle were reduced in response to insulin during suckling. In contrast, non- or insulin-stimulated glucose uptake and GLUT-4 translocation were found significantly increased in muscle of adult offspring. Finally, basal association of phosphatidylinositol 3-kinase (PI3-kinase) with IRS-1 was increased and was highly stimulated by insulin in muscle from adult rats. Our findings suggest that early postnatal undernutrition increases insulin sensitivity in adulthood, which appears to be directly related to changes in critical steps required for glucose metabolism.     

Insulin action on skeletal muscle protein metabolism during catabolic states

Insulin plays a major role in the regulation of skeletal muscle protein turnover but its mechanism of action is not fully understood, especially in vivo during catabolic states. These aspects are presently reviewed. Insulin inhibits the ATP-ubiquitin proteasome proteolytic pathway which is presumably the predominant pathway involved in the breakdown of muscle protein. Evidence of the ability of insulin to stimulate muscle protein synthesis in vivo was also presented. Many catabolic states in rats, e.g. streptozotocin diabetes, glucocorticoid excess or sepsis-induced cytokines, resulted in a decrease in insulin action on protein synthesis or degradation. The effect of catabolic factors would therefore be facilitated. In contrast, the antiproteolytic action of insulin was improved during hyperthyroidism in man and early lactation in goats. Excessive muscle protein breakdown should therefore be prevented. In other words, the anabolic hormone insulin partly controlled the 'catabolic drive'. Advances in the understanding of insulin signalling pathways and targets should provide information on the interactions between insulin action, muscle protein turnover and catabolic factors.     

Prior exercise increases basal and insulin-induced p38 mitogen-activated protein kinase phosphorylation in human skeletal muscle.

We have examined the effects of insulin on p38 mitogen-activated protein kinase (MAPK) phosphorylation in human skeletal muscle and the effects of prior exercise hereon. Seven men performed 1-h one-legged knee extensor exercise 3 h before the initiation of a 100-min euglycemic-hyperinsulinemic (600 pmol/l) clamp. Glucose uptake across the legs was measured with the leg balance technique, and muscle biopsies were obtained from the rested and exercised vastus lateralis before and during insulin infusion. Net glucose uptake during the clamp was approximately 50% higher (P < 0.05) in the exercised leg than in the rested leg. Insulin induced a modest sustained 1.2- and 1.3-fold increase (P < 0.05) in p38 MAPK phosphorylation in the rested and exercised legs, respectively. However, p38 phosphorylation was approximately 50% higher (P < 0.05) in the exercised compared with the rested leg before and during insulin infusion. We conclude that a physiological concentration of insulin causes modest but sustained activation of the p38 MAPK pathway in human skeletal muscle. Furthermore, the stimulatory effect of exercise on p38 phosphorylation is persistent for at least 3 h after exercise and remains evident during subsequent insulin stimulation. Because p38 MAPK has been suggested to play a necessary role in activation of GLUT-4 at the cell surface, the present data may suggest a putative role of p38 MAPK in the increased insulin sensitivity of skeletal muscle after exercise.     

Acute supplementation of amino acids increases net protein accretion in IUGR fetal sheep

Placental insufficiency decreases fetal amino acid uptake from the placenta, plasma insulin concentrations, and protein accretion, thus compromising normal fetal growth trajectory. We tested whether acute supplementation of amino acids or insulin into the fetus with intrauterine growth restriction (IUGR) would increase net fetal protein accretion rates. Late-gestation IUGR and control (CON) fetal sheep received acute, 3-h infusions of amino acids (with euinsulinemia), insulin (with euglycemia and euaminoacidemia), or saline. Fetal leucine metabolism was measured under steady-state conditions followed by a fetal muscle biopsy to quantify insulin signaling. In CON, increasing amino acid delivery rates to the fetus by 100% increased leucine oxidation rates by 100%. In IUGR, amino acid infusion completely suppressed fetal protein breakdown rates but increased leucine oxidation rate by only 25%, resulting in increased protein accretion rates by 150%. Acute insulin infusion, however, had very little effect on amino acid delivery rates, fetal leucine disposal rates, or fetal protein accretion rates in CON or IUGR fetuses despite robust signaling of the fetal skeletal muscle insulin-signaling cascade. These results indicate that, when amino acids are given directly into the fetal circulation independently of changes in insulin concentrations, IUGR fetal sheep have suppressed protein breakdown rates, thus increasing net fetal protein accretion.     

A novel fetal insulin-like growth factor (IGF) I receptor. Mechanism for increased IGF I- and insulin-stimulated tyrosine kinase activity in fetal muscle

Insulin and insulin-like growth factor (IGF) I receptors from fetal and adult rat skeletal muscle were compared in order to gain insight into the evolving functions of the hormones during development. Basal, insulin-stimulated, and IGF I-stimulated receptor phosphorylation and tyrosine kinase activity are severalfold higher in partially purified receptor preparations from fetal muscle in comparison with equal numbers of receptors from adult muscle. There are distinct insulin and IGF I receptors with Mr 95,000 beta subunits in adult muscle, as evidenced by hormone dose-response curves, immunoprecipitation with specific antibodies, binding to insulin and IGF I affinity columns, and analysis of tryptic phosphopeptides. In addition to these two receptor species, fetal muscle contains a receptor with a Mr 105,000 beta subunit. The fetal receptor is structurally more closely related to the IGF-I receptor than the insulin receptor on the basis of its precipitation with specific antibodies, binding to an IGF I affinity column, and tryptic phosphopeptide map. The fetal receptor does not appear to bind insulin but, unlike the IGF-I receptor, its phosphorylation is stimulated by low physiological concentrations of both insulin and IGF I. This could be explained by the cross-phosphorylation of fetal receptors by activated insulin receptors. Expression of the fetal receptor is highest in the fetus and decreases markedly during the first 2 weeks of postnatal life. The fetal receptor appears to account for the high tyrosine kinase activity of fetal muscle and may be an important mediator of responses to both insulin and IGF I early in development.     

Determination in vitro of the rate of protein synthesis and degradation in human-skeletal-muscle tissue.

The present studies were aimed to evaluate the possibility to use a system for estimation in vitro of the biosynthesis and degradation rates of human skeletal muscle protein. A previously characterized human skeletal muscle preparation was used. Amino acids and insulin stimulated significantly the incorporation rate of leucine into proteins. The effect of amino acids was more pronounced than that of insulin. The stimulatory effect of insulin could be decreased by amino acids. Insulin did not influence the tissue uptake or the oxidation rate of leucine. The release of [14C]leucine deriving from degradation of prelabelled skeletal muscle fibre proteins was linear for at least 2.5 h of incubation and optimal with leucine at concentrations beyond 12.5 mmol/1 or in the presence of puromycin in the incubation medium. The rate of the release of radioactivity was significantly inhibited by amino acids and at borderline significance by insulin but not by puromycin. The specific radioactivity in prelabelled proteins decreased significantly in the presence of puromycin suggesting that leucine derived from protein degradation was reutilized in vitro. This reutilization was found to be 9 +/- 1% of leucine released from degradation of proteins in 30 subjects. A statistically significant positive correlation between the cathepsin D activity in human skeletal muscle tissue and the degradative rate of prelabelled muscle proteins in vitro was observed. The results indicate that biosynthesis and degradation of skeletal muscle proteins in this system in vitro were subjected to control mechanisms. It is suggested that the release of radioactivity from prelabelled muscle fibre proteins during incubation probably only reflects the degradation of some rapidly-turning-over proteins.     

Modulation of muscle protein synthesis by insulin is maintained during neonatal endotoxemia

Sepsis promotes insulin resistance and reduces protein synthesis in skeletal muscle of adults. The effect of sepsis on insulin-stimulated muscle protein synthesis has not been determined in neonates, a highly anabolic population that is uniquely sensitive to insulin. Overnight fasted neonatal pigs were infused for 8 h with endotoxin [lipopolysaccharide (LPS), 0 and 10 mug.kg(-1).h(-1)]. Glucose and amino acids were maintained at fasting levels, insulin was clamped at either fasting or fed (2 or 10 muU/ml) levels, and fractional protein synthesis rates were determined at the end of the infusion. LPS infusion induced a septic-like state, as indicated by a sustained elevation in body temperature, heart rate, and cortisol. At fasting insulin levels, LPS reduced fractional protein synthesis rates in gastrocnemius muscle (-26%) but had no effect on the masseter and heart. By contrast, LPS stimulated liver protein synthesis (+28%). Increasing insulin to fed levels accelerated protein synthesis rates in gastrocnemius (controls by +38%, LPS by +60%), masseter (controls by +50%, LPS by +43%), heart (controls by +34%, LPS by +40%), and diaphragm (controls by +54%, LPS by +29%), and the response to insulin was similar in LPS and controls. Insulin did not alter protein synthesis in liver, kidney, or jejunum in either group. These findings suggest that acute endotoxemia lowers basal fasting muscle protein synthesis in neonates but does not alter the response of protein synthesis to insulin.     

Antioxidant alpha-lipoic acid and protein turnover in insulin-resistant rat muscle

We have shown previously that the antioxidant alpha-lipoic acid (ALA) can stimulate glucose transport and can enhance the stimulation of this process by insulin in skeletal muscle from insulin-resistant obese Zucker rats. As insulin can also acutely activate general protein synthesis and inhibit net protein degradation in skeletal muscle, we hypothesized that ALA could directly affect protein turnover and also increase the effect of insulin on protein turnover in isolated skeletal muscle from developing obese Zucker rats. In epitrochlearis muscles isolated from obese Zucker rats, insulin (2 mU/ml) significantly (p < 0.05) increased in vitro protein synthesis (phenylalanine incorporation into protein) and decreased net protein degradation (tyrosine release), whereas a racemic mixture of ALA (2 mM) had no effect on either process. Interestingly, rates of protein synthesis in muscle from obese Zucker rats were substantially lower compared to those values observed in age-matched insulin-sensitive Wistar rats, whereas rates of protein degradation were comparable. Obese Zucker rats were also treated chronically with either vehicle or ALA (50 mg/kg/d for 10 d). Again, insulin significantly increased net protein synthesis and decreased net protein degradation in epitrochlearis muscles isolated from vehicle-treated obese Zucker rats; however, this stimulatory effect of insulin was not improved by prior in vivo ALA treatment. These results indicate that the previously described effect of the antioxidant ALA to increase insulin-stimulated glucose transport in skeletal muscle of obese, insulin-resistant rats does not apply to another important insulin-regulatable process, protein turnover. These findings imply that the cellular mode of action for ALA is restricted to signaling factors unique to the activation of glucose transport, and does not involve the pathway of stimulation of general protein synthesis and net protein degradation.     

Regulation of protein synthesis and degradation in L8 myotubes. Effects of serum, insulin and insulin-like growth factors.

We have examined the regulation of protein turnover in rat skeletal myotubes from the L8 cell line. We measured protein synthesis by the rates of incorporation of radiolabelled tyrosine into protein in the presence of a flooding dose of non-radioactive tyrosine. We monitored degradation of proteins labelled with radioactive tyrosine by the release of acid-soluble radioactivity into medium containing excess nonradioactive tyrosine. Extracellular tyrosine pools and intracellular tyrosyl-tRNA equilibrate rapidly during measurements of protein synthesis, and very little reutilization of the radiolabelled tyrosine occurs during degradation measurements. Measured rates of protein synthesis and degradation are constant for several hours, and changes in myotube protein content can be accurately predicted by the measured rates of protein synthesis and degradation. Most of the myotube proteins labelled with radioactive tyrosine for 2 days are degraded, with half-lives (t1/2) of approx. 50 h. A small proportion (less than 2.5%) of the radiolabelled proteins are degraded more rapidly (t1/2 less than 10 h), and, at most, a small proportion (less than 15%) are degraded more slowly (t1/2 greater than 50 h). A variety of agents commonly added to primary muscle cell cultures or to myoblast cell lines (18% Medium 199, 1% chick-embryo extract, antibiotics and antifungal agents) had no effect on rates of protein synthesis or degradation. Horse serum, fetal bovine serum and insulin stimulate protein synthesis and inhibit the degradation of long-lived proteins without affecting the degradation of short-lived proteins. Insulin-like growth factors (IGF)-1 and -2 also stimulate protein synthesis and inhibit protein degradation. The stimulation of protein synthesis and the inhibition of protein degradation are of similar magnitude (a maximum of approx. 2-fold) and display similar sensitivities to a particular anabolic agent. Insulin stimulates protein synthesis and inhibits protein degradation only at supraphysiological doses, whereas IGF-1 and -2 are effective at physiological concentrations. These and other findings suggest that IGFs may be important regulators of skeletal muscle growth during the fetal and early neonatal periods.     

Hormone-sensitive lipase knockout mice have increased hepatic insulin sensitivity and are protected from short-term diet-induced insulin resistance in skeletal muscle and heart

Insulin resistance in skeletal muscle and heart plays a major role in the development of type 2 diabetes and diabetic heart failure and may be causally associated with altered lipid metabolism. Hormone-sensitive lipase (HSL) is a rate-determining enzyme in the hydrolysis of triglyceride in adipocytes, and HSL-deficient mice have reduced circulating fatty acids and are resistant to diet-induced obesity. To determine the metabolic role of HSL, we examined the changes in tissue-specific insulin action and glucose metabolism in vivo during hyperinsulinemic euglycemic clamps after 3 wk of high-fat or normal chow diet in awake, HSL-deficient (HSL-KO) mice. On normal diet, HSL-KO mice showed a twofold increase in hepatic insulin action but a 40% decrease in insulin-stimulated cardiac glucose uptake compared with wild-type littermates. High-fat feeding caused a similar increase in whole body fat mass in both groups of mice. Insulin-stimulated glucose uptake was reduced by 50-80% in skeletal muscle and heart of wild-type mice after high-fat feeding. In contrast, HSL-KO mice were protected from diet-induced insulin resistance in skeletal muscle and heart, and these effects were associated with reduced intramuscular triglyceride and fatty acyl-CoA levels in the fat-fed HSL-KO mice. Overall, these findings demonstrate the important role of HSL on skeletal muscle, heart, and liver glucose metabolism.     

Insulin signaling and glucose transport in insulin resistant human skeletal muscle

Insulin increases glucose uptake and metabolism in skeletal muscle by signal transduction via protein phosphorylation cascades. Insulin action on signal transduction is impaired in skeletal muscle from Type 2 diabetic subjects, underscoring the contribution of molecular defects to the insulin resistant phenotype. This review summarizes recent work to identify downstream intermediates in the insulin signaling pathways governing glucose homeostasis, in an attempt to characterize the molecular mechanism accounting for skeletal muscle insulin resistance in Type 2 diabetes. Furthermore, the effects of pharmaceutical treatment of Type 2 diabetic patients on insulin signaling and glucose uptake are discussed. The identification and characterization of pathways governing insulin action on glucose metabolism will facilitate the development of strategies to improve insulin sensitivity in an effort to prevent and treat Type 2 diabetes mellitus.     

Changes in protein synthesis in rats in response to endurance training

Experiments were conducted to investigate the influence of endurance exercise training on protein synthesis in skeletal muscle, heart, and liver. Training decreased incorporation of [¹⁴C]-leucine into proteins of the stromal fraction of muscle but there was no change in amino acid incorporation into proteins of the sarcoplasmic and myofibrillar fractions. Incorporation of [¹⁴C]-leucine into the protein of heart, liver, and plasma was depressed in trained rats compared to untrained rats. The specific radioactivity of [¹⁴C]-leucine was similar in tissues of trained and untrained rats and thus the depressed amino acid incorporation represents a decrease in the rate of protein synthesis. These observations demonstrate that the adaptation of muscle protein metabolism to endurance training is quite different than the alterations during work-induced hypertrophy of muscle. The difference in adaptation probably relates to the functional differences between the types of exercise. However depression of protein synthesis in trained rats is a general effect in several tissues and not an effect localized in muscle tissue.     

Insulin and contraction directly stimulate UCP2 and UCP3 mRNA expression in rat skeletal muscle in vitro

To study the regulation of the mitochondrial uncoupling protein 2 and 3 (UCP2 and UCP3), we studied the effect of insulin and muscle contraction on UCP mRNA expression in rat skeletal muscle in vitro. Insulin dose-dependently increased skeletal muscle UCP2 and UCP3 mRNA expression in m. extensor digitorum longus (EDL) with maximal stimulation obtained at around 0.6-6 nM. The concentration of insulin giving half-maximal stimulation was 60 pM for the UCP2 and 48 pM for the UCP3 mRNA expression. The effect of insulin was maximal after 2 h and the effect was sustained during the whole study period (6 h). The insulin-induced increase in UCP mRNA was independent of the glucose uptake (as UCP mRNA was stimulated even in incubations without glucose). In addition, electrically induced contractions (in vitro) increased UCP2 and UCP3 mRNA expression 60-120 min after a single bout of contraction (for 10 min). Both the increment of UCP2 and UCP3 mRNA were sustained throughout the study period (4 h) (153 +/- 62 and 216 +/- 71% above basal, P < 0.05 respectively). Finally, 5-aminoimidazole-4-carboxamid-ribosid (AICAR), an activator of the AMP-activated protein kinase (AMPK), that is activated during exercise, was able to mimic the increase in UCP2 and UCP3 mRNA expression. In conclusion, UCP2 and UCP3 mRNA expression in skeletal muscle are stimulated rapidly by insulin and contraction in vitro, thus the stimulation is direct and not caused by changes in other hormones or metabolites. Even a brief bout of contraction induces an increase in UCP2 and UCP3 expression, an effect that could be mimicked by activation of the AMP-activated protein kinase by AICAR.     

Insulin and amino acids independently stimulate skeletal muscle protein synthesis in neonatal pigs

Infusion of physiological levels of insulin and/or amino acids reproduces the feeding-induced stimulation of muscle protein synthesis in neonates. To determine whether insulin and amino acids independently stimulate skeletal muscle protein synthesis in neonates, insulin secretion was blocked with somatostatin in fasted 7-day-old pigs (n = 8-12/group) while glucose and glucagon were maintained at fasting levels and insulin was infused to simulate either less than fasting, fasting, intermediate, or fed insulin levels. At each dose of insulin, amino acids were clamped at either the fasting or fed level; at the highest insulin dose, amino acids were also reduced to less than fasting levels. Skeletal muscle protein synthesis was measured using a flooding dose of l-[4-(3)H]phenylalanine. Hyperinsulinemia increased protein synthesis in skeletal muscle during hypoaminoacidemia and euaminoacidemia. Hyperaminoacidemia increased muscle protein synthesis during hypoinsulinemia and euinsulinemia. There was a dose-response effect of both insulin and amino acids on muscle protein synthesis. At each insulin dose, hyperaminoacidemia increased muscle protein synthesis. The effects of insulin and amino acids on muscle protein synthesis were largely additive until maximal rates of protein synthesis were achieved. Amino acids enhanced basal protein synthesis rates but did not enhance the sensitivity or responsiveness of muscle protein synthesis to insulin. The results suggest that insulin and amino acids independently stimulate protein synthesis in skeletal muscle of the neonate.     

Mechanism of insulin's anabolic effect on muscle: measurements of muscle protein synthesis and breakdown using aminoacyl-tRNA and other surrogate measures

Despite being an anabolic hormone in skeletal muscle, insulin's anticatabolic mechanism in humans remains controversial, with contradictory reports showing either stimulation of protein synthesis (PS) or inhibition of protein breakdown (PB) by insulin. Earlier measurements of muscle PS and PB in humans have relied on different surrogate measures of aminoacyl-tRNA and intracellular pools. We report that insulin's effect on muscle protein turnover using aminoacyl-tRNA as the precursor of PS and PB is calculated by mass balance of tracee amino acid (AA). We compared the results calculated from various surrogate measures. To determine the physiological role of insulin on muscle protein metabolism, we infused tracers of leucine and phenylalanine into 18 healthy subjects, and after 3 h, 10 subjects received a 4-h femoral arterial infusion of insulin (0.125 mUxkg(-1)xmin(-1)), while eight subjects continued with saline. Tracer-to-tracee ratios of leucine, phenylalanine, and ketoisocaproate were measured in the arterial and venous plasma, muscle tissue fluid, and AA-tRNA to calculate muscle PB and PS. Insulin infusion, unlike saline, significantly reduced the efflux of leucine and phenylalanine from muscle bed, based on various surrogate measures which agreed with those based on leucyl-tRNA (-28%), indicating a reduction in muscle PB (P < 0.02) without any significant effect on muscle PS. In conclusion, using AA-tRNA as the precursor pool, it is demonstrated that, in healthy humans in the postabsorptive state, insulin does not stimulate muscle protein synthesis and confirmed that insulin achieves muscle protein anabolism by inhibition of muscle protein breakdown.     

Insulin signaling during perinatal liver development in the rat

Insulin has long been assigned a key role in the regulation of growth and metabolism during fetal life. Our prior observations indicated that hepatic insulin signaling is attenuated in the late-gestation fetal rat. Therefore, we studied the perinatal ontogeny of hepatic insulin signaling extending from phosphatidylinositol 3-kinase (PI3K) to the ribosome. Initial studies demonstrated markedly decreased insulin-mediated activation of ribosomal protein S6 kinase 1 (S6K1) in the fetus. We found a similar pattern in the regulation of Akt, a kinase upstream from S6K1. Insulin produced minimal activation of insulin receptor substrate (IRS)-1-associated PI3K activity in fetal liver. A modest IRS-2-associated response was seen in the fetus. However, levels of both IRS-1 and IRS-2 were very low in fetal liver relative to adult liver. IRS-1 content and insulin responsiveness of PI3K, Akt, and S6K1 showed a transition to the adult phenotype during the first several postnatal weeks. Examination of downstream insulin signaling to the translational apparatus showed marked attenuation, relative to the adult, of fetal hepatic insulin-mediated phosphorylation of 4E-BP1, the regulatory protein for the eukaryotic initiation factor eIF4E, and ribosomal protein S6. The mammalian target of rapamycin (mTOR), a key integrator of nutritional and metabolic regulation of translation, was present in low amounts, was hypophosphorylated, and was not insulin sensitive in the fetus. Our results indicate that protein synthesis during late-gestation liver development may be mTOR and insulin independent. Reexamination of the role of insulin in fetal liver physiology may be warranted.