Regulation of both glycogen synthase and PHAS-I by insulin in rat skeletal muscle involves mitogen-activated protein kinase-independent and rapamycin-sensitive pathways

PI3K/AKT/mTOR signaling pathway modulation by circular RNAs in breast cancer progression

The PI3K/Akt/mTOR signaling pathway is responsible for many cellular behaviors, including survival, growth, and proliferation. A newly identified RNA, circular RNA (circRNA), plays a crucial role in the regulation of gene expression. The upregulation of the PI3K/Akt pathway through dysregulated circRNAs promotes breast tumor initiation, growth, and progression. The dysregulation of PI3K/Akt-regulating circRNAs seems to be directly correlated with breast cancer clinical features, including overall survival, tumor size, cancer stage, and lymph node metastasis. In addition, targeting these circRNAs may be a promising option in cancer-targeted therapy. Understanding the molecular pathogenesis of the circRNA-PI3K/AKT axis may give the insight to develop new therapeutic and diagnostic approaches for breast cancer therapy. Here we reviewed the expression and functions of PI3K/AKT-regulating circRNAs, and their correlation with breast cancer clinical features. In addition, the potential of PI3K/AKT-regulating circRNAs as diagnostic/prognostic biomarkers or therapeutic targets was discussed.

mTOR Signaling in Cancer and mTOR Inhibitors in Solid Tumor Targeting Therapy

The mammalian or mechanistic target of rapamycin (mTOR) pathway plays a crucial role in regulation of cell survival, metabolism, growth and protein synthesis in response to upstream signals in both normal physiological and pathological conditions, especially in cancer. Aberrant mTOR signaling resulting from genetic alterations from different levels of the signal cascade is commonly observed in various types of cancers. Upon hyperactivation, mTOR signaling promotes cell proliferation and metabolism that contribute to tumor initiation and progression. In addition, mTOR also negatively regulates autophagy via different ways. We discuss mTOR signaling and its key upstream and downstream factors, the specific genetic changes in the mTOR pathway and the inhibitors of mTOR applied as therapeutic strategies in eight solid tumors. Although monotherapy and combination therapy with mTOR inhibitors have been extensively applied in preclinical and clinical trials in various cancer types, innovative therapies with better efficacy and less drug resistance are still in great need, and new biomarkers and deep sequencing technologies will facilitate these mTOR targeting drugs benefit the cancer patients in personalized therapy.

mTOR Cross-Talk in Cancer and Potential for Combination Therapy

The mammalian Target of Rapamycin (mTOR) pathway plays an essential role in sensing and integrating a variety of exogenous cues to regulate cellular growth and metabolism, in both physiological and pathological conditions. mTOR functions through two functionally and structurally distinct multi-component complexes, mTORC1 and mTORC2, which interact with each other and with several elements of other signaling pathways. In the past few years, many new insights into mTOR function and regulation have been gained and extensive genetic and pharmacological studies in mice have enhanced our understanding of how mTOR dysfunction contributes to several diseases, including cancer. Single-agent mTOR targeting, mostly using rapalogs, has so far met limited clinical success; however, due to the extensive cross-talk between mTOR and other pathways, combined approaches are the most promising avenues to improve clinical efficacy of available therapeutics and overcome drug resistance. This review provides a brief and up-to-date narrative on the regulation of mTOR function, the relative contributions of mTORC1 and mTORC2 complexes to cancer development and progression, and prospects for mTOR inhibition as a therapeutic strategy.

Metabolic crosstalk: molecular links between glycogen and lipid metabolism in obesity

Glycogen and lipids are major storage forms of energy that are tightly regulated by hormones and metabolic signals. We demonstrate that feeding mice a high-fat diet (HFD) increases hepatic glycogen due to increased expression of the glycogenic scaffolding protein PTG/R5. PTG promoter activity was increased and glycogen levels were augmented in mice and cells after activation of the mechanistic target of rapamycin complex 1 (mTORC1) and its downstream target SREBP1. Deletion of the PTG gene in mice prevented HFD-induced hepatic glycogen accumulation. Of note, PTG deletion also blocked hepatic steatosis in HFD-fed mice and reduced the expression of numerous lipogenic genes. Additionally, PTG deletion reduced fasting glucose and insulin levels in obese mice while improving insulin sensitivity, a result of reduced hepatic glucose output. This metabolic crosstalk was due to decreased mTORC1 and SREBP activity in PTG knockout mice or knockdown cells, suggesting a positive feedback loop in which once accumulated, glycogen stimulates the mTORC1/SREBP1 pathway to shift energy storage to lipogenesis. Together, these data reveal a previously unappreciated broad role for glycogen in the control of energy homeostasis.

Glucose dependence of glycogen synthase activity regulation by GSK3 and MEK/ERK inhibitors and angiotensin-(1-7) action on these pathways in cultured human myotubes

Glycogen synthase (GS) is activated by glucose/glycogen depletion in skeletal muscle cells, but the contributing signaling pathways, including the chief GS regulator GSK3, have not been fully defined. The MEK/ERK pathway is known to regulate GSK3 and respond to glucose. The aim of this study was to elucidate the GSK3 and MEK/ERK pathway contribution to GS activation by glucose deprivation in cultured human myotubes. Moreover, we tested the glucose-dependence of GSK3 and MEK/ERK effects on GS and angiotensin (1-7) actions on these pathways. We show that glucose deprivation activated GS, but did not change phospho-GS (Ser640/1), GSK3β activity or activity-activating phosphorylation of ERK1/2. We then treated glucose-replete and -depleted cells with SB415286, U0126, LY294 and rapamycin to inhibit GSK3, MEK1/2, PI3K and mTOR, respectively. SB415286 activated GS and decreased the relative phospho-GS (Ser640/1) level, more in glucose-depleted than -replete cells. U0126 activated GS and reduced the phospho-GS (Ser640/1) content significantly in glucose-depleted cells, while GSK3β activity tended to increase. LY294 inactivated GS in glucose-depleted cells only, without affecting relative phospho-GS (Ser640/1) level. Rapamycin had no effect on GS activation. Angiotensin-(1-7) raised phospho-ERK1/2 but not phospho-GSK3β (Ser9) content, while it inactivated GS and increased GS phosphorylation on Ser640/1, in glucose-replete cells. In glucose-depleted cells, angiotensin-(1-7) effects on ERK1/2 and GS were reverted, while relative phospho-GSK3β (Ser9) content decreased. In conclusion, activation of GS by glucose deprivation is not due to GS Ser640/1 dephosphorylation, GSK3β or ERK1/2 regulation in cultured myotubes. However, glucose depletion enhances GS activation/Ser640/1 dephosphorylation due to both GSK3 and MEK/ERK inhibition. Angiotensin-(1-7) inactivates GS in glucose-replete cells in association with ERK1/2 activation, not with GSK3 regulation, and glucose deprivation reverts both hormone effects. Thus, the ERK1/2 pathway negatively regulates GS activity in myotubes, without involving GSK3 regulation, and as a function of the presence of glucose.

The role of mTORC1 in regulating protein synthesis and skeletal muscle mass in response to various mechanical stimuli

Skeletal muscle plays a fundamental role in mobility, disease prevention, and quality of life. Skeletal muscle mass is, in part, determined by the rates of protein synthesis, and mechanical loading is a major regulator of protein synthesis and skeletal muscle mass. The mammalian/mechanistic target of rapamycin (mTOR), found in the multi-protein complex, mTORC1, is proposed to play an essential role in the regulation of protein synthesis and skeletal muscle mass. The purpose of this review is to examine the function of mTORC1 in relation to protein synthesis and cell growth, the current evidence from rodent and human studies for the activation of mTORC1 signaling by different types of mechanical stimuli, whether mTORC1 signaling is necessary for changes in protein synthesis and skeletal muscle mass that occur in response to different types of mechanical stimuli, and the proposed molecular signaling mechanisms that may be responsible for the mechanical activation of mTORC1 signaling.

The mTOR signalling pathway in human cancer

The conserved serine/threonine kinase mTOR (the mammalian target of rapamycin), a downstream effector of the PI3K/AKT pathway, forms two distinct multiprotein complexes: mTORC1 and mTORC2. mTORC1 is sensitive to rapamycin, activates S6K1 and 4EBP1, which are involved in mRNA translation. It is activated by diverse stimuli, such as growth factors, nutrients, energy and stress signals, and essential signalling pathways, such as PI3K, MAPK and AMPK, in order to control cell growth, proliferation and survival. mTORC2 is considered resistant to rapamycin and is generally insensitive to nutrients and energy signals. It activates PKC-α and AKT and regulates the actin cytoskeleton. Deregulation of multiple elements of the mTOR pathway (PI3K amplification/mutation, PTEN loss of function, AKT overexpression, and S6K1, 4EBP1 and eIF4E overexpression) has been reported in many types of cancers, particularly in melanoma, where alterations in major components of the mTOR pathway were reported to have significant effects on tumour progression. Therefore, mTOR is an appealing therapeutic target and mTOR inhibitors, including the rapamycin analogues deforolimus, everolimus and temsirolimus, are submitted to clinical trials for treating multiple cancers, alone or in combination with inhibitors of other pathways. Importantly, temsirolimus and everolimus were recently approved by the FDA for the treatment of renal cell carcinoma, PNET and giant cell astrocytoma. Small molecules that inhibit mTOR kinase activity and dual PI3K-mTOR inhibitors are also being developed. In this review, we aim to survey relevant research, the molecular mechanisms of signalling, including upstream activation and downstream effectors, and the role of mTOR in cancer, mainly in melanoma.

The complexes of mammalian target of rapamycin

The mammalian target of rapamycin (mTOR) has attracted substantial attention because of its involvement in a variety of diseases, such as cancer, cardiac hypertrophy, diabetes and obesity. Current knowledge indicates that mTOR functions as two distinct multiprotein complexes, mTORC1 and mTORC2. mTORC1 phosphorylates p70 S6 kinase (S6K1) and eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1), and regulates cell growth, proliferation, and survival by integrating hormones, growth factors, nutrients, stressors and energy signals. In contrast, mTORC2 is insensitive to nutrients or energy conditions. However, in response to hormones or growth factors, mTORC2 phosphorylates Akt, and regulates actin cytoskeleton and cell survival. These findings not only reveal the crucial role of mTOR in physiology and pathology, but also reflect the complexity of the mTOR signaling network. In this review, we discuss the advances in studies of the mTOR complexes, including the interacting proteins, the upstream regulators and the downstream effectors of mTOR complexes, as well as their implication in certain human diseases.

Inhibition of glycogen biosynthesis via mTORC1 suppression as an adjunct therapy for Pompe disease

Pompe disease, also known as glycogen storage disease (GSD) type II, is caused by deficiency of lysosomal acid alpha-glucosidase (GAA). The resulting glycogen accumulation causes a spectrum of disease severity ranging from a rapidly progressive course that is typically fatal by 1-2years of age to a more slowly progressive course that causes significant morbidity and early mortality in children and adults. Recombinant human GAA (rhGAA) improves clinical outcomes with variable results. Adjunct therapy that increases the effectiveness of rhGAA may benefit some Pompe patients. Co-administration of the mTORC1 inhibitor rapamycin with rhGAA in a GAA knockout mouse reduced muscle glycogen content more than rhGAA or rapamycin alone. These results suggest mTORC1 inhibition may benefit GSDs that involve glycogen accumulation in muscle.

Role of mTOR in physiology and pathology of the nervous system

Mammalian target of rapamycin (mTOR) is a serine-threonine protein kinase that regulates several intracellular processes in response to extracellular signals, nutrient availability, energy status of the cell and stress. mTOR regulates survival, differentiation and development of neurons. Axon growth and navigation, dendritic arborization, as well as synaptogenesis, depend on proper mTOR activity. In adult brain mTOR is crucial for synaptic plasticity, learning and memory formation, and brain control of food uptake. Recent studies reveal that mTOR activity is modified in various pathologic states of the nervous system, including brain tumors, tuberous sclerosis, cortical displasia and neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases. This review presents current knowledge about the role of mTOR in the physiology and pathology of the nervous system, with special focus on molecular targets acting downstream of mTOR that potentially contribute to neuronal development, plasticity and neuropathology.

The growing role of mTOR in neuronal development and plasticity

Neuronal development and synaptic plasticity are highly regulated processes in which protein kinases play a key role. Recently, increasing attention has been paid to a serine/threonine protein kinase called mammalian target of rapamycin (mTOR) that has well-known functions in cell proliferation and growth. In neuronal cells, mTOR is implicated in multiple processes, including transcription, ubiquitin-dependent proteolysis, and microtubule and actin dynamics, all of which are crucial for neuronal development and long-term modification of synaptic strength. The aim of this article is to present our current understanding of mTOR functions in axon guidance, dendritic tree development, formation of dendritic spines, and in several forms of long-term synaptic plasticity. We also aim to present explanation for the mTOR effects on neurons at the level of mTORregulated genes and proteins.

Rapamycin inhibits the growth and muscle-sparing effects of clenbuterol

Clenbuterol and other β2-adrenergic agonists are effective at inducing muscle growth and attenuating muscle atrophy through unknown mechanisms. This study tested the hypothesis that clenbuterol-induced growth and muscle sparing is mediated through the activation of Akt and mammalian target of rapamycin (mTOR) signaling pathways. Clenbuterol was administered to normal weight-bearing adult rats to examine the growth-inducing effects and to adult rats undergoing muscle atrophy as the result of hindlimb suspension or denervation to examine the muscle-sparing effects. The pharmacological inhibitor rapamycin was administered in combination with clenbuterol in vivo to determine whether activation of mTOR was involved in mediating the effects of clenbuterol. Clenbuterol administration increased the phosphorylation status of PKB/Akt, S6 kinase 1/p70s6k, and eukaryotic initiation factor 4E binding protein 1/PHAS-1. Clenbuterol treatment induced growth by 27–41% in normal rats and attenuated muscle loss during hindlimb suspension by 10–20%. Rapamycin treatment resulted in a 37–97% suppression of clenbuterol-induced growth and a 100% reduction of the muscle-sparing effect. In contrast, rapamycin was unable to block the muscle-sparing effects of clenbuterol after denervation. Clenbuterol was also shown to suppress the expression of the MuRF1 and MAFbx transcripts in muscles from normal, denervated, and hindlimb-suspended rats. These results demonstrate that the effects of clenbuterol are mediated, in part, through the activation of Akt and mTOR signaling pathways.

The impact of the mTOR inhibitor sirolimus on the proliferation and function of pancreatic islets and ductal cells

AIMS/HYPOTHESIS

The Edmonton Protocol for islet transplantation has provided hope for type 1 diabetic patients. However, this protocol requires lifelong immunosuppression, specifically sirolimus, a cellular antiproliferate. The effect of sirolimus on human pancreatic ductal cells (HDCs) is not known. This may be important since HDCs are believed to be islet precursors. Since neonatal porcine islets (NPIs), which contain many ductal precursor cells, could be a potential clinical source of islets, we also tested the effects of sirolimus on this tissue.

METHODS

HDCs (n=4), NPIs (n=9) and human islets (n=5) were cultured with and without sirolimus (20 ng/ml) for 6 days.

RESULTS

HDCs and NPIs cultured with sirolimus showed a 50 and 28% decrease, respectively, in cell number relative to control (p<0.05). Control cultures expanded 1.65- and 2.44-fold relative to time 0. Decreases in cell number of sirolimus-treated HDCs were not due to apoptosis as measured by TUNEL staining. No functional effects on human islets or NPIs were observed following static incubation with high glucose. Treatment of syngeneically transplanted and naïve BALC/c mice with sirolimus resulted in altered OGTT profiles with prolonged elevation of hyperglycaemia and weight gain. There was no difference in graft and organ insulin content between treatment groups.

CONCLUSIONS/INTERPRETATION

Our results indicate that sirolimus decreases ductal cell numbers in culture and alters glucose-stimulated insulin secretion in vivo. The administration of sirolimus to islet transplant recipients is likely to impair graft function as a result of decreasing ductal neogenesis and induction of insulin resistance.

Gene expression profiling of mice with genetically modified muscle glycogen content

Glycogen, a branched polymer of glucose, forms an energy re-serve in numerous organisms. In mammals, the two largest glyco-gen stores are in skeletal muscle and liver, which express tissue-specific glycogen synthase isoforms. MGSKO mice, in which mGys1 (mouse glycogen synthase) is disrupted, are devoid of muscle glycogen [Pederson, Chen, Schroeder, Shou, DePaoli-Roach and Roach (2004) Mol. Cell. Biol. 24, 7179-7187]. The GSL30 mouse line hyper-accumulates glycogen in muscle [Manchester, Skurat, Roach, Hauschka and Lawrence (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 10707-10711]. We performed a microarray analysis of mRNA from the anterior tibialis, medial gastrocnemius and liver of MGSKO mice, and from the gastroc-nemius of GSL30 mice. In MGSKO mice, transcripts of 79 genes varied in their expression in the same direction in both the anterior tibialis and gastrocnemius. These included several genes encoding proteins proximally involved in glycogen metabolism. The Ppp1r1a [protein phosphatase 1 regulatory (inhibitor) sub-unit 1A] gene underwent the greatest amount of downregulation. In muscle, the downregulation of Pfkfb1 and Pfkfb3, encoding isoforms of 6-phosphofructo-2-kinase/fructose-2,6-bisphospha-tase, is consistent with decreased glycolysis. Pathways for branched-chain amino acid, and ketone body utilization appear to be downregulated, as is the capacity to form the gluconeogenic precursors alanine, lactate and glutamine. Expression changes among several members of the Wnt signalling pathway were identified, suggesting an as yet unexplained role in glycogen meta-bolism. In liver, the upregulation of Pfkfb1 and Pfkfb3 expression is consistent with increased glycolysis, perhaps as an adaptation to altered muscle metabolism. By comparing changes in muscle expression between MGSKO and GSL30 mice, we found a subset of 44 genes, the expression of which varied as a function of muscle glycogen content. These genes are candidates for regulation by glycogen levels. Particularly interesting is the observation that 11 of these genes encode cardiac or slow-twitch isoforms of muscle contractile proteins, and are upregulated in muscle that has a greater oxidative capacity in MGSKO mice.

Signaling by target of rapamycin proteins in cell growth control

Target of rapamycin (TOR) proteins are members of the phosphatidylinositol kinase-related kinase (PIKK) family and are highly conserved from yeast to mammals. TOR proteins integrate signals from growth factors, nutrients, stress, and cellular energy levels to control cell growth. The ribosomal S6 kinase 1 (S6K) and eukaryotic initiation factor 4E binding protein 1(4EBP1) are two cellular targets of TOR kinase activity and are known to mediate TOR function in translational control in mammalian cells. However, the precise molecular mechanism of TOR regulation is not completely understood. One of the recent breakthrough studies in TOR signaling resulted in the identification of the tuberous sclerosis complex gene products, TSC1 and TSC2, as negative regulators for TOR signaling. Furthermore, the discovery that the small GTPase Rheb is a direct downstream target of TSC1-TSC2 and a positive regulator of the TOR function has significantly advanced our understanding of the molecular mechanism of TOR activation. Here we review the current understanding of the regulation of TOR signaling and discuss its function as a signaling nexus to control cell growth during normal development and tumorigenesis.

Oral leucine administration stimulates protein synthesis in rat skeletal muscle

Oral administration of a single bolus of leucine in an amount equivalent to the daily intake (1.35 g/kg body wt) enhances skeletal muscle protein synthesis in food-deprived rats. To elucidate whether smaller amounts of leucine can also stimulate protein synthesis, rats were administered the amino acid at concentrations ranging from 0.068 to 1.35 g/kg body wt by oral gavage. Thirty minutes following the administration of doses of leucine as low as 0.135 g/kg body wt, skeletal muscle protein synthesis was significantly greater than control values. The increase in protein synthesis was associated with changes in the regulation of biomarkers of mRNA translation initiation as evidenced by upregulated phosphorylation of the translational repressor, eukaryotic initiation factor (eIF)4E-binding protein 1 (4E-BP1), the association of eIF4G with the mRNA cap binding protein eIF4E, and the phosphorylation of the 70-kDa ribosomal protein S6 kinase. Alterations in the phosphorylation of eIF4G, as well as the association of 4E-BP1 with eIF4E, were observed following leucine administration; however, these changes appeared to be biphasic with maximal changes occurring when circulating insulin concentrations were elevated. Thus it appears that leucine administration affects mRNA translation and skeletal muscle protein synthesis through modulation of multiple biomarkers of mRNA translation. The ability of small doses of leucine to stimulate skeletal muscle protein synthesis suggests that future research on the regulation of skeletal muscle protein synthesis by orally administered leucine will be feasible in humans.

Abnormal cardiac development in the absence of heart glycogen

Glycogen serves as a repository of glucose in many mammalian tissues. Mice lacking this glucose reserve in muscle, heart, and several other tissues were generated by disruption of the GYS1 gene, which encodes an isoform of glycogen synthase. Crossing mice heterozygous for the GYS1 disruption resulted in a significant underrepresentation of GYS1-null mice in the offspring. Timed matings established that Mendelian inheritance was followed for up to 18.5 days postcoitum (dpc) and that approximately 90% of GYS1-null animals died soon after birth due to impaired cardiac function. Defects in cardiac development began between 11.5 and 14.5 dpc. At 18.5 dpc, the hearts were significantly smaller, with reduced ventricular chamber size and enlarged atria. Consistent with impaired cardiac function, edema, pooling of blood, and hemorrhagic liver were seen. Glycogen synthase and glycogen were undetectable in cardiac muscle and skeletal muscle from the surviving null mice, and the hearts showed normal morphology and function. Congenital heart disease is one of the most common birth defects in humans, at up to 1 in 50 live births. The results provide the first direct evidence that the ability to synthesize glycogen in cardiac muscle is critical for normal heart development and hence that its impairment could be a significant contributor to congenital heart defects.

Thr2446 Is a Novel Mammalian Target of Rapamycin (mTOR) Phosphorylation Site Regulated by Nutrient Status

The mammalian target of rapamycin (mTOR) is a key regulator of protein translation. Signaling via mTOR is increased by growth factors but decreased during nutrient deprivation. Previous studies have identified Ser2448 as a nutrient-regulated phosphorylation site located in the mTOR catalytic domain, insulin stimulates Ser2448 phosphorylation via protein kinase B (PKB), while Ser2448 phosphorylation is attenuated with amino acid starvation. Here we have identified Thr2446 as a novel nutrient-regulated phosphorylation site on mTOR. Thr2446 becomes phosphorylated when CHO-IR cells are nutrient-deprived, but phosphorylation is reduced by insulin stimulation. Nutrient deprivation activates AMP-activated protein kinase (AMPK). To test whether this could be involved in regulating phoshorylation of mTOR, we treated cultured murine myotubes with 5'-aminoimidazole-4-carboxamide ribonucleoside (AICAR) or dinitrophenol (DNP). Both treatments activated AMPK and also caused a concomitant increase in phosphorylation of Thr2446 and a parallel decrease in insulin's ability to phosphorylate p70 S6 kinase. In vitro kinase assays using peptides based on the sequence in amino acids 2440-2551 of mTOR found that PKB and AMPK are capable of phosphorylating sites in this region. However, phosphorylation by PKB is restricted when Thr2446 is mutated to an acidic residue mimicking phosphorylation. Conversely, AMP-kinase-induced phosphorylation is reduced when Ser2448 is phosphorylated. These data suggest differential phosphorylation Thr2446 and Ser2448 could act as a switch mechanism to integrate signals from nutrient status and growth factors to control the regulation of protein translation.

Inhibition of MAP/ERK kinase prevents IGF-I-induced hypertrophy in rat muscles

Insulin-like growth factor-I (IGF-I) has been shown to stimulate a hypertrophy response in skeletal muscles in vivo. In vitro studies have delineated two primary intracellular pathways that appear to mediate the effects of IGF-I in skeletal muscle: the Ras-ERK pathway and the phosphoinositide-3 kinase pathway. In vitro, the Ras pathway appears to regulate the mitogenic effects of IGF-I signaling, whereas the phosphoinositide-3 kinase pathway is associated with cellular differentiation. On the basis of the results from in vitro studies, we hypothesized that the coinfusion of both IGF-I and an inhibitor of the Ras pathway would result in some increase in muscle protein but an inhibition of cell proliferation. Our results show that 14 days of coinfusion of MAPK/ERK kinase inhibitor PD-098059 (PD) limited the phosphorylation of ERK and prevented IGF-I induced increases in protein (18%, P < 0.05 vs. 7%, not significant) or myofibrillar protein (23%, P < 0.01 vs. 5%, not significant). However, there were similar increases in indicators of cell proliferation (e.g., total DNA, 50 and 52%, P < 0.001) in both the IGF- and IGF+PD-infused muscles. The most notable impact on IGF-I signaling was a significant blunting of IGF-I induced increase in S6K1 phosphorylation by PD-98059 coinfusion ( approximately 5-fold, P < 0.001 vs. 3-fold, P < 0.01). These results suggest that there are interactions between the various pathways down stream of the IGF-I receptor that may behave differently in vivo than in myogenic cell lines in vitro.

Kinase activities associated with mTOR

Although mTOR is a member of the PI-kinase-related kinase family, mTOR possesses serine-threonine protein kinase activities, which phosphorylate itself and exogenous substrates. mTOR autophosphorylates in vitro and is phosphorylated in vivo on serine residues. Ser2481, which is located in a His-Ser-Phe motif near the conserved carboxyl-terminal mTOR tail, has been reported as an autophosphorylation site in vivo and in vitro. The significance of the autophosphorylation remains unclear. Another phosphorylation site on mTOR in vivo is Ser2448. This site appears not to be an autophosphorylation site but a site potentially phosphorylated by protein kinase B (PKB). mTOR immunopurified from culture cells or tissues phosphorylates in vitro p70 S6 kinase (p70) alpha and p70beta, mainly on Thr412 or Thr401, respectively, located in a Phe-Thr-Tyr motif. Another exogenous substrate phosphorylated by immunopurified mTOR in vitro is eIF4E-binding protein 1 (4E-BP1) at sites corresponding to those phosphorylated in vivo during insulin stimulation in a Ser/Thr-Pro motif. Recently, raptor, a 150-kDa TOR-binding protein that contains a carboxyl-terminal WD-repeat domain, was discovered as a scaffold for the mTOR-catalyzed phosphorylation of 4E-BP1 and for the mTOR-mediated phosphorylation and activation of p70alpha. Other potential substrates phosphorylated by mTOR are nPKCdelta, nPKCepsilon, STAT3, and p53. The requirement of raptor for binding to and phosphorylation by mTOR of these potential substrates would clarify their physiological importance in the mTOR signaling pathway.

TOR signaling

The mammalian target of rapamycin, mTOR, is a protein Ser-Thr kinase that functions as a central element in a signaling pathway involved in the control of cell growth and proliferation. The activity of mTOR is controlled not only by amino acids, but also by hormones and growth factors that activate the protein kinase Akt. The signaling pathway downstream of Akt leading to mTOR involves the protein products of the genes mutated in tuberous sclerosis, TSC1 and TSC2, and the small guanosine triphosphatase, Rheb. In cells, mTOR is found in a complex with two other proteins, raptor and mLST8. In this review, we describe recent progress in understanding the control of the mTOR signaling pathway and the role of mTOR-interacting proteins.

Akt is a molecular target for signal transduction therapy in brain ischemic insult

Growth factors including insulin-like growth factor-1 (IGF-1) promote cell survival in ischemic brain injury. Stimulation of IGF-1 receptor coupled with tyrosine kinase activates phosphatidylinositol 3-kinase and subsequently, protein kinase B (Akt) in hippocampal neurons. Here we introduce a new approach of signal transduction therapy for brain damage occurring in ischemic insult. As has been shown for IGF-1, intracerebroventricular injection of sodium orthovanadate, a protein tyrosine phosphatase inhibitor, prior to ischemic insult blocked delayed neuronal death in the CA1 region. The neuroprotective effects of orthovanadate and IGF-1 were associated with an increased Akt activity in the CA1 region. We discuss here potential targets for Akt relevant to such neuroprotective activity. Our findings lead to the conclusion that Akt activity is a potential target for neuroprotective drugs in brain ischemic insult and other episodes of excitotoxic neuronal apoptosis such as seizure and Huntington's and Parkinson's diseases.

Overexpression of glycogen synthase in mouse muscle results in less branched glycogen

Glycogen, a branched polymer of glucose, serves as an energy reserve in many organisms. The degree of branching likely reflects the balance between the activities of glycogen synthase and branching enzyme. Mice overexpressing constitutively active glycogen synthase in skeletal muscle (GSL30) have elevated muscle glycogen. To test whether excess glycogen synthase activity affected glycogen branching, we examined the glycogen from skeletal muscle of GSL30 mice. The absorption spectrum of muscle glycogen determined in the presence of iodine was shifted to higher wavelengths in the GSL30 animals, consistent with a decrease in the degree of branching. As judged by Western blotting, the levels of glycogenin and the branching enzyme were also elevated. Branching enzyme activity also increased approximately threefold. However, this compared with an increase in glycogen synthase of some 50-fold, so that the increase in branching enzyme in response to overexpression of glycogen synthase was insufficient to synthesize normally branched glycogen.

Beta -oxidation of free fatty acids is required to maintain translational control of protein synthesis in heart

The study described herein investigated the role of free fatty acids (FFAs) in the maintenance of protein synthesis in vivo in rat cardiac and skeletal muscle. Suppression of FFA beta-oxidation by methyl palmoxirate caused a marked reduction in protein synthesis in the heart. The effect on protein synthesis was mediated in part by changes in the function of eukaryotic initiation factors (eIFs) involved in the initiation of mRNA translation. The guanine nucleotide exchange activity of eIF2B was repressed, phosphorylation of the alpha-subunit of eIF2 was enhanced, and phosphorylation of eIF4E-binding protein-1 and ribosomal protein S6 kinase was reduced. Similar changes in protein synthesis and translation initiation were not observed in the gastrocnemius following treatment with methyl palmoxirate. In heart, repressed beta-oxidation of FFA correlated, as demarcated by changes in the ATP/AMP ratio and phosphorylation of AMP-activated kinase, with alterations in the energy status of the tissue. Therefore, the activation state of signal transduction pathways that are responsive to cellular energy stress represents one mechanism whereby translation initiation may be regulated in cardiac muscle.

Regulation of protein synthesis by IGF-I in proximal tubular epithelial cells

Protein synthesis is required for renal hypertrophy, and proximal tubular epithelial cells are an important cell type involved in this process. We examined IGF-I regulation of protein synthesis in murine proximal tubular epithelial (MCT) cells. We focused on initial events in protein translation and the signaling events involved. Translation of capped mRNAs is under the control of eukaryotic initiation factor 4E (eIF4E). In the resting cell, eIF4E is normally kept in an inactive state by binding to 4E-BP1, its binding protein. Phosphorylation of 4E-BP1 results in dissociation of the eIF4E-4E-BP1 complex allowing eIF4E to initiate peptide synthesis. IGF-I stimulated protein synthesis, augmented phosphorylation of 4E-BP1 and promoted the dissociation of eIF4E from 4E-BP1. IGF-I stimulated the activities of phosphatidylinositol (PI) 3-kinase, Akt, and ERK1/2-type MAPK in MCT cells. IGF-I-induced phosphorylation of 4E-BP1, dissociation of the 4E-BP1-eIF4E complex, and increase in protein synthesis required activation of both PI 3-kinase and ERK pathways. Furthermore, ERK activation by IGF-I was also PI 3-kinase dependent. Transfection with the Thr37,46-->Ala37,46 mutant of 4E-BP1 showed that phosphorylation of Thr37,46 residues was required for IGF-I induction of protein synthesis in MCT cells. Our observations reveal the importance of initial events in protein translation in IGF-I-induced protein synthesis in MCT cells and identify the regulatory signaling pathways involved.

Ras/Erk signaling is essential for activation of protein synthesis by Gq protein-coupled receptor agonists in adult cardiomyocytes

The Gq protein-coupled receptor agonists phenylephrine (PE) and endothelin-1 (ET-1) induce cardiac hypertrophy and stimulate protein synthesis in cardiomyocytes. This study aims to investigate how they activate mRNA translation in adult cardiomyocytes. PE and ET-1 do not activate protein kinase B but stimulate Ras and Erk, and their ability to activate protein synthesis was blocked by inhibition of Ras or MEK and by rapamycin, which inhibits mTOR (mammalian target of rapamycin). These agonists activated ribosomal protein S6 kinase 1 (S6K1) and induced phosphorylation of eIF4E-binding protein-1 (4E-BP1) and its release from eIF4E. These effects were blocked by inhibitors of MEK. Furthermore, adenovirus-mediated expression of constitutively-active MEK1 caused activation of S6K1, phosphorylation of 4E-BP1, and activation of protein synthesis in a rapamycin-sensitive manner. Expression of N17Ras inhibited the regulation of S6K1 and protein synthesis by GqPCR agonists. These data point to a signaling pathway involving Ras and MEK that acts, with mTOR, to control regulatory translation factors and activate protein synthesis. This study provides new insights into the mechanisms underlying the stimulation of protein synthesis by hypertrophic agents in heart.

Control of Ser2448 phosphorylation in the mammalian target of rapamycin by insulin and skeletal muscle load

We have investigated the effects of insulin, amino acids, and the degree of muscle loading on the phosphorylation of Ser(2448), a site in the mammalian target of rapamycin (mTOR) phosphorylated by protein kinase B (PKB) in vitro. Phosphorylation was assessed by immunoblotting with a phosphospecific antibody (anti-Ser(P)(2448)) and with mTAb1, an activating antibody whose binding is inhibited by phosphorylation in the region of mTOR that contains Ser(2448). Incubating rat diaphragm muscles with insulin increased Ser(2448) phosphorylation but did not change the total amount of mTOR. Insulin, but not amino acids, activated PKB, as evidenced by increased phosphorylation of both Ser(308) and Thr(473) in the kinase. Ser(2448) phosphorylation was also modulated by muscle-loading. Overloading the rat plantaris muscle by synergist muscle ablation, which promotes hypertrophy of the plantaris muscle, increased Ser(2448) phosphorylation. In contrast, unloading the gastrocnemius muscle by hindlimb suspension, which promotes atrophy of the muscle, decreased Ser(2448) phosphorylation, an effect that was fully reversible. Neither overloading nor hindlimb suspension significantly changed the total amount of mTOR. In summary, our results demonstrate that atrophy and hypertrophy of skeletal muscle are associated with decreases and increases in Ser(2448) phosphorylation, suggesting that modulation of this site may have an important role in the control of protein synthesis.

Activation of MAP kinase by insulin and vanadate in adipocytes from young and old rats

Vanadate has insulin-like effects in adipocytes without stimulating insulin receptor kinase activity. However, it activates IRS-1 associated PI 3-kinase, suggesting that it mimics insulin effects by stimulating signaling elements downstream of PI 3-kinase. Here we analysed the stimulation of MAPK by insulin and vanadate and observed that both elicit a rapid 3.5-4 fold activation which is abolished by wortmannin and PD98059. Simultaneous addition of insulin and vanadate does not result in an additive effect neither on MAPK nor in MEK. Whereas insulin action is transient, vanadate stimulation lasts up to 20 min. In insulin-resistant adipocytes from old rats, insulin stimulates poorly MAPK, whereas a normal activation is achieved with vanadate. We conclude that: (a) insulin and vanadate use a common signaling pathway from PI 3-kinase to MEK and MAPK; (b) vanadate but not insulin, elicits a sustained activation of both enzymes; (c) this pathway is functional in old rat adipocytes.

Insulin-stimulated phosphorylation of lipin mediated by the mammalian target of rapamycin

The phosphorylation of a previously uncharacterized protein of apparent M(r) approximately 140,000 was found to be increased when rat adipocytes were incubated with insulin. The sequences of peptides generated by digesting the protein with trypsin matched perfectly with sequences in mouse lipin. Lipin is the product of the gene that is mutated in fatty liver dystrophy (fld) mice [Peterfy, M., Phan, J., Xu, P. & Reue, K (2001) Nat. Genet. 27, 121-124], which exhibit several phenotypic abnormalities including hyperlipidemia, defects in adipocyte differentiation, impaired glucose tolerance, and slow growth. When immunoblots were prepared with lipin antibodies, both endogenous adipocyte lipin and recombinant lipin overexpressed in HEK293 cells appeared as bands ranging in apparent M(r) from 120,000 to 140,000. Incubating adipocytes with insulin decreased the electrophoretic mobility and stimulated the phosphorylation of both Ser and Thr residues in lipin. The effects of insulin were abolished by inhibitors of phosphatidylinositol 3-OH kinase, and by rapamycin, a specific inhibitor of the mammalian target of rapamcyin (mTOR). The inhibition by rapamycin was blocked by FK506, which competitively inhibits those effects of rapamycin that are mediated by inhibition of mTOR. Moreover, amino acids, which activate mTOR, mimicked insulin by increasing lipin phosphorylation in a rapamycin-sensitive manner. Thus, lipin represents a target of the mTOR pathway, and potentially links this nutrient-sensing pathway to adipocyte development.

Exogenous amino acids regulate trophectoderm differentiation in the mouse blastocyst through an mTOR-dependent pathway

At the late blastocyst stage, the epithelial trophectoderm cells of the mammalian embryo undergo a phenotypic change that allows them to invade into the uterine stroma and make contact with the maternal circulation. This step can be regulated in vitro by the availability of amino acids. Embryos cultured in defined medium lacking amino acids cannot form trophoblast cell outgrowths on fibronectin, an in vitro model of implantation, but remain viable for up to 3 days in culture and will form outgrowths when transferred into complete medium. The amino acid requirement is a developmentally regulated permissive event that occurs during a 4- to 8-h period at the early blastocyst stage. Amino acids affect spreading competence specifically by regulating the onset of protrusive activity and not the onset of integrin activation. Rapamycin, a specific inhibitor of the kinase mTOR/FRAP/RAFT1, blocks amino acid stimulation of embryo outgrowth, demonstrating that mTOR is required for the initiation of trophectoderm protrusive activity. Inhibition of global protein translation with cycloheximide also inhibits amino acid-dependent signals, suggesting that mTOR regulates the translation of proteins required for trophoblast differentiation. Our data suggest that mTOR activity has a developmental regulatory function in trophectoderm differentiation that may serve to coordinate embryo and uterus at the time of implantation.

Mediation of IGF-1-induced skeletal myotube hypertrophy by PI(3)K/Akt/mTOR and PI(3)K/Akt/GSK3 pathways

Skeletal muscle is composed of multinucleated fibres, formed after the differentiation and fusion of myoblast precursors. Skeletal muscle atrophy and hypertrophy refer to changes in the diameter of these pre-existing muscle fibres. The prevention of atrophy would provide an obvious clinical benefit; insulin-like growth factor 1 (IGF-1) is a promising anti-atrophy agent because of its ability to promote hypertrophy. However, the signalling pathways by which IGF-1 promotes hypertrophy remain unclear, with roles suggested for both the calcineurin/NFAT (nuclear factor of activated T cells) pathway and the PtdIns-3-OH kinase (PI(3)K)/Akt pathway. Here we employ a battery of approaches to examine these pathways during the hypertrophic response of cultured myotubes to IGF-1. We report that Akt promotes hypertrophy by activating downstream signalling pathways previously implicated in activating protein synthesis: the pathways downstream of mammalian target of rapamycin (mTOR) and the pathway activated by phosphorylating and thereby inhibiting glycogen synthase kinase 3 (GSK3). In contrast, in addition to demonstrating that calcineurin does not mediate IGF-1-induced hypertrophy, we show that IGF-1 unexpectedly acts via Akt to antagonize calcineurin signalling during myotube hypertrophy.

Akt/mTOR pathway is a crucial regulator of skeletal muscle hypertrophy and can prevent muscle atrophy in vivo

Skeletal muscles adapt to changes in their workload by regulating fibre size by unknown mechanisms. The roles of two signalling pathways implicated in muscle hypertrophy on the basis of findings in vitro, Akt/mTOR (mammalian target of rapamycin) and calcineurin/NFAT (nuclear factor of activated T cells), were investigated in several models of skeletal muscle hypertrophy and atrophy in vivo. The Akt/mTOR pathway was upregulated during hypertrophy and downregulated during muscle atrophy. Furthermore, rapamycin, a selective blocker of mTOR, blocked hypertrophy in all models tested, without causing atrophy in control muscles. In contrast, the calcineurin pathway was not activated during hypertrophy in vivo, and inhibitors of calcineurin, cyclosporin A and FK506 did not blunt hypertrophy. Finally, genetic activation of the Akt/mTOR pathway was sufficient to cause hypertrophy and prevent atrophy in vivo, whereas genetic blockade of this pathway blocked hypertrophy in vivo. We conclude that the activation of the Akt/mTOR pathway and its downstream targets, p70S6K and PHAS-1/4E-BP1, is requisitely involved in regulating skeletal muscle fibre size, and that activation of the Akt/mTOR pathway can oppose muscle atrophy induced by disuse.

The muscle-specific protein phosphatase PP1G/R(GL)(G(M))is essential for activation of glycogen synthase by exercise

In skeletal muscle both insulin and contractile activity are physiological stimuli for glycogen synthesis, which is thought to result in part from the dephosphorylation and activation of glycogen synthase (GS). PP1G/R(GL)(G(M)) is a glycogen/sarcoplasmic reticulum-associated type 1 phosphatase that was originally postulated to mediate insulin control of glycogen metabolism. However, we recently showed (Suzuki, Y., Lanner, C., Kim, J.-H., Vilardo, P. G., Zhang, H., Jie Yang, J., Cooper, L. D., Steele, M., Kennedy, A., Bock, C., Scrimgeour, A., Lawrence, J. C. Jr., L., and DePaoli-Roach, A. A. (2001) Mol. Cell. Biol. 21, 2683-2694) that insulin activates GS in muscle of R(GL)(G(M)) knockout (KO) mice similarly to the wild type (WT). To determine whether PP1G is involved in glycogen metabolism during muscle contractions, R(GL) KO and overexpressors (OE) were subjected to two models of contraction, in vivo treadmill running and in situ electrical stimulation. Both procedures resulted in a 2-fold increase in the GS -/+ glucose-6-P activity ratio in WT mice, but this response was completely absent in the KO mice. The KO mice, which also have a reduced GS activity associated with significantly reduced basal glycogen levels, exhibited impaired maximal exercise capacity, but contraction-induced activation of glucose transport was unaffected. The R(GL) OE mice are characterized by enhanced GS activity ratio and an approximately 3-4-fold increase in glycogen content in skeletal muscle. These animals were able to tolerate exercise normally. Stimulation of GS and glucose uptake following muscle contraction was not significantly different as compared with WT littermates. These results indicate that although PP1G/R(GL) is not necessary for activation of GS by insulin, it is essential for regulation of glycogen metabolism under basal conditions and in response to contractile activity, and may explain the reduced muscle glycogen content in the R(GL) KO mice, despite the normal insulin activation of GS.

Recombinant DNA technology in the treatment of diabetes: insulin analogs

After more than half a century of treating diabetics with animal insulins, recombinant DNA technologies and advanced protein chemistry made human insulin preparations available in the early 1980s. As the next step, over the last decade, insulin analogs were constructed by changing the structure of the native protein with the goal of improving the therapeutic properties of it, because the pharmacokinetic characteristics of rapid-, intermediate-, and long-acting preparations of human insulin make it almost impossible to achieve sustained normoglycemia. The first clinically available insulin analog, lispro, confirmed the hopes by showing that improved glycemic control can be achieved without an increase in hypoglycemic events. Two new insulin analogs, insulin glargine and insulin aspart, have recently been approved for clinical use in the United States, and several other analogs are being intensively tested. Thus, it appears that a rapid acceleration of basic and clinical research in this arena will be seen, which will have direct significance to both patients and their physicians. The introduction of new short-acting analogs and the development of the first truly long-acting analogs and the development of analogs with increased stability, less variability, and perhaps selective action, will help to develop more individualized treatment strategies targeted to specific patient characteristics and to achieve further improvements in glycemic control. Data on the currently available and tested analogs, as well as data on those currently being developed, are reviewed.

Impaired muscle glycogen synthase in type 2 diabetes is associated with diminished phosphatidylinositol 3-kinase activation

Insulin signaling pathways potentially involved in regulation of skeletal muscle glycogen synthase were compared in differentiated human muscle cell cultures from nondiabetic and type 2 diabetic patients. Insulin stimulation of glycogen synthase activity as well as phosphorylation of MAPK, p70 S6 kinase, and protein kinase B (Akt) were blocked by the phosphatidylinositol 3-kinase inhibitors wortmannin (50 nM) and LY294002 (10 microM). In contrast to lean and obese nondiabetic subjects, where there were minimal effects (15-20% inhibition), insulin stimulation of glycogen synthase in muscle cultures from diabetic subjects was greatly diminished ( approximately 75%) by low concentrations of wortmannin (25 nM) or LY294002 (2 microM). This increased sensitivity of diabetic muscle to impairment of insulin-stimulated glycogen synthase activity occurs together with diminished insulin-stimulation (by 40%) of IRS-1-associated phosphatidylinositol 3-kinase activity in the same cells. Protein expression of IRS-1, p85, p110, Akt, p70 S6 kinase, and MAPK were normal in diabetic cells, as was insulin-stimulated phosphorylation of Akt, p70 S6 kinase, and MAPK. These studies indicate that, despite prolonged growth and differentiation of diabetic muscle under normal metabolic culture conditions, defects of insulin-stimulated phosphatidylinositol 3-kinase and glycogen synthase activity in diabetic muscle persist, consistent with intrinsic (rather than acquired) defects of insulin action.

Insulin control of glycogen metabolism in knockout mice lacking the muscle-specific protein phosphatase PP1G/RGL

The regulatory-targeting subunit (RGL), also called GM) of the muscle-specific glycogen-associated protein phosphatase PP1G targets the enzyme to glycogen where it modulates the activity of glycogen-metabolizing enzymes. PP1G/RGL has been postulated to play a central role in epinephrine and insulin control of glycogen metabolism via phosphorylation of RGL. To investigate the function of the phosphatase, RGL knockout mice were generated. Animals lacking RGL show no obvious defects. The RGL protein is absent from the skeletal and cardiac muscle of null mutants and present at approximately 50% of the wild-type level in heterozygotes. Both the level and activity of C1 protein are also decreased by approximately 50% in the RGL-deficient mice. In skeletal muscle, the glycogen synthase (GS) activity ratio in the absence and presence of glucose-6-phosphate is reduced from 0.3 in the wild type to 0.1 in the null mutant RGL mice, whereas the phosphorylase activity ratio in the absence and presence of AMP is increased from 0.4 to 0.7. Glycogen accumulation is decreased by approximately 90%. Despite impaired glycogen accumulation in muscle, the animals remain normoglycemic. Glucose tolerance and insulin responsiveness are identical in wild-type and knockout mice, as are basal and insulin-stimulated glucose uptakes in skeletal muscle. Most importantly, insulin activated GS in both wild-type and RGL null mutant mice and stimulated a GS-specific protein phosphatase in both groups. These results demonstrate that RGL is genetically linked to glycogen metabolism, since its loss decreases PP1 and basal GS activities and glycogen accumulation. However, PP1G/RGL is not required for insulin activation of GS in skeletal muscle, and rather another GS-specific phosphatase appears to be involved.

PHAS proteins as mediators of the actions of insulin, growth factors and cAMP on protein synthesis and cell proliferation

PHAS-I and PHAS-II are members of a newly discovered family of proteins that regulate translation initiation. PHAS-I is expressed in a wide variety of cell types, but it is highest in adipocytes, where protein synthesis is markedly increased by insulin. PHAS-II is highest in liver and kidney, where very little PHAS-I is found. PHAS proteins bind to eIF-4E, the mRNA cap-binding protein, and inhibit translation of capped mRNA in vitro and in cells. In rat adipocytes PHAS-I is phosphorylated in at least five sites, all of which conform to the consensus, (Ser/Thr)-Pro. Both PHAS proteins are phosphorylated in response to insulin or growth factors, such as EGF, PDGF and IGF-1. Phosphorylation in the appropriate site(s) promotes dissociation of PHAS/eIF-4E complexes. This allows eIF-4E to bind to eIF-4G (p220), thereby increasing the amount of the eIF-4F complex and the rate of translation initiation. Increasing cAMP promotes PHAS-I dephosphorylation and increases binding to eIF-4E. Unlike PHAS-I, PHAS-II is readily phosphorylated by PKA in vitro, suggesting that regulation of the two proteins differs. However, increasing cAMP in cells also promotes dephosphorylation of PHAS-II. Thus, PHAS proteins appear to be key mediators not only of the stimulatory effects of insulin and growth factors on protein synthesis, but also of the inhibitory effects of cAMP. Moreover, by controlling eIF-4E PHAS proteins may be involved in the control of cell proliferation, as increasing eIF-4E is mitogenic and can even cause malignant transformation of cells. MAP kinase readily phosphorylates both PHAS-I and PHAS-II in vitro, but inhibiting activation of MAP kinase does not attenuate the effects of insulin on increasing phosphorylation of the PHAS proteins in adipocytes or skeletal muscle. MAP kinase phosphorylates neither PHAS-I nor PHAS-II at a significant rate when the proteins are bound to eIF-4E. Therefore, the role of MAP kinase in promoting the dissociation of PHAS/eIF-4E complexes is not clear. Of several protein kinases tested, only casein kinase-II phosphorylated PHAS-I when it was bound eIF-4E. Indeed, the bound form of PHAS-I was phosphorylated more rapidly than the free form. However, it is unlikely that casein kinase II regulates either PHAS protein, as the major site (Ser111) in PHAS-I phosphorylated by casein kinase II in vitro is not phosphorylated in adipocytes, and PHAS-II is not a substrate for casein kinase-II. Pharmacological and genetic evidence indicates that the mTOR/p70S6K pathway is involved in the control of PHAS-I and -II. Thus, PHAS proteins may be mediators of the effects of this pathway on protein synthesis and cell proliferation.

Insulin regulation of protein translation repressor 4E-BP1, an eIF4E-binding protein, in renal epithelial cells

BACKGROUND

Augmented protein translation by insulin involves activation of eukaryotic initiation factor 4E (eIF4E) that follows release of eIF4E from a heterodimeric complex by phosphorylation of its inhibitory binding protein, 4E-BP1. We examined insulin regulation of 4E-BP1 phosphorylation in murine proximal tubular epithelial cells.

METHODS AND RESULTS

Insulin (1 nmol/L) increased de novo protein synthesis by 58 +/- 11% (P < 0.001). Insulin also augmented 4E-BP1 phosphorylation and phosphatidylinositol 3-kinase (PI 3-kinase) activity in antiphosphotyrosine immunoprecipitates. This could be prevented by PI 3-kinase inhibitors, Wortmannin, and LY294002. Insulin also activated Akt that lies downstream of PI 3-kinase. Rapamycin abrogated 4E-BP1 phosphorylation in response to insulin, suggesting involvement of mammalian target of rapamycin (mTOR), a kinase downstream of Akt. Insulin-stimulated phosphorylation of 4E-BP1 was also inhibited by PD098059, implying involvement of Erk-1/-2 mitogen-activated protein (MAP) kinase. An increase in Erk-1/-2 type MAP kinase activity by insulin was directly confirmed in an immunokinase assay and was found to be PI 3-kinase dependent.

CONCLUSIONS

In proximal tubular epithelial cells, insulin augments 4E-BP1 phosphorylation, which is PI 3-kinase and mTOR dependent. The requirement for Erk-1/-2 MAP kinase activation for 4E-BP1 phosphorylation by insulin suggests a cross-talk between PI 3-kinase and Erk-1/-2-type MAP kinase pathways.

Regulation of glycogen synthesis by amino acids in cultured human muscle cells

Insulin and a number of metabolic factors stimulate glycogen synthesis and the enzyme glycogen synthase. Using human muscle cells we find that glycogen synthesis is stimulated by treatment of the cells with lithium ions, which inhibit glycogen synthase kinase 3. Insulin further stimulates glycogen synthesis in the presence of lithium ions, an effect abolished by wortmannin and rapamycin. We report also that amino acids stimulate glycogen synthesis and glycogen synthase, these effects also being blocked by rapamycin and wortmannin. Amino acids stimulate p70(s6k) and transiently inhibit glycogen synthase kinase 3 without effects on the activity of protein kinase B or the mitogen-activated protein kinase pathway. Thus, the work reported here demonstrates that amino acid availability can regulate glycogen synthesis. Furthermore, it demonstrates that glycogen synthase kinase 3 can be inactivated within cells independent of activation of protein kinase B and p90(rsk).

Feeding stimulates protein synthesis in muscle and liver of neonatal pigs through an mTOR-dependent process

Protein synthesis is repressed in both skeletal muscle and liver after a short-term fast and is rapidly stimulated in response to feeding. Previous studies in rats and pigs have shown that the feeding-induced stimulation of protein synthesis is associated with activation of the 70-kDa ribosomal protein S6 kinase (S6K1) as well as enhanced binding of eukaryotic initiation factor eIF4E to eIF4G to form the active eIF4F complex. In cells in culture, hormones and nutrients regulate both of these events through a protein kinase termed the mammalian target of rapamycin (mTOR). In the present study, the involvement of mTOR in the feeding-induced stimulation of protein synthesis in skeletal muscle and liver was examined. Pigs at 7 days of age were fasted for 18 h, and then one-half of the animals were fed. In addition, one-half of the animals in each group were administered rapamycin (0.75 mg/kg) 2 h before feeding. The results reveal that treating 18-h fasted pigs with rapamycin, a specific inhibitor of mTOR, before feeding prevented the activation of S6K1 and the changes in eIF4F complex formation observed in skeletal muscle and liver after feeding. Rapamycin also ablated the feeding-induced stimulation of protein synthesis in liver. In contrast, in skeletal muscle, rapamycin attenuated, but did not prevent, the stimulation of protein synthesis in response to feeding. The results suggest that feeding stimulates hepatic protein synthesis through an mTOR-dependent process involving enhanced eIF4F complex formation and activation of S6K1. However, in skeletal muscle, these two processes may account for only part of the stimulation of protein synthesis, and thus additional steps may be involved in the response.