Both rapamycin-sensitive and -insensitive pathways are involved in the phosphorylation of the initiation factor-4E-binding protein (4E-BP1) in response to insulin in rat epididymal fat-cells

Modifying Dietary Protein Impacts mTOR Signaling and Brain Deposition of Amyloid β in a Knock-In Mouse Model of Alzheimer Disease

BACKGROUND

Alzheimer disease (AD) is a neurodegenerative condition defined by the build-up of amyloid plaques in the brain and intraneuronal tangles of the protein tau. Autophagy is a cellular cleaning process involved in the degradation of proteins, including proteins directly responsible for amyloid plaques, but its activity is compromised in AD. The mechanistic target of rapamycin complex (mTORC) 1 inhibits autophagy when activated by amino acids.

OBJECTIVES

We hypothesized that reducing amino acid intake by decreasing dietary protein could promote autophagy, which in turn could prevent amyloid plaque deposition in AD mice.

METHODS

Homozygote (2-mo-old) and heterozygote (4-mo-old) amyloid precursor protein NL-G-F mice, a model of brain amyloid deposition, were used in this study to test this hypothesis. Male and female mice were fed with isocaloric low-protein, control, or high-protein diets for 4 mo and killed for analysis. Locomotor performance was measured using the inverted screen test, and body composition was measured using EchoMRI. Samples were analyzed using western blotting, enzyme-linked immunosorbent assay, mass spectrometry, and immunohistochemical staining.

RESULTS

mTORC1 activity in the cerebral cortex was inversely covaried with protein consumption in both homozygote and heterozygote mice. Low-protein diet improved metabolic parameters and restored locomotor performance only in male homozygous mice. Dietary protein adjustment did not affect amyloid deposition in homozygous mice. However, in the heterozygous amyloid precursor protein NL-G-F mice, amyloid plaque was lower in male mice consuming the low protein compared with that in mice fed with the control diet.

CONCLUSIONS

This study showed that reducing protein intake reduces mTORC1 activity and may prevent amyloid accumulation, at least in male mice. Moreover, dietary protein is a tool that can be used to change mTORC1 activity and amyloid deposition in the mouse brain, and the murine brain's response to dietary protein is sex specific.

Insulin action on protein synthesis and its association with eIF5A expression and hypusination

The hormone insulin plays a central role in the metabolism of carbohydrates, lipids, and proteins. In relation to protein metabolism, insulin stimulates amino acid uptake and activates protein synthesis in responsive cells by modulation of signal transduction pathways, such as associated to Akt/PkB, mTOR, S6Ks, 4E-BP1, and several translation initiation/elongation factors. In this context, there is no information on direct cellular treatment with insulin and effects on eukaryotic translation initiation factor 5A (eIF5A) regulation. The eIF5A protein contains an exclusive amino acid residue denominated hypusine, which is essential for its activity and synthesized by posttranslational modification of a specific lysine residue using spermidine as substrate. The eIF5A protein is involved in cellular proliferation and differentiation processes, as observed for satellite cells derived from rat muscles, revealing that eIF5A has an important role in muscle regeneration. The aim of this study was to determine whether eIF5A expression and hypusination are influenced by direct treatment of insulin on L6 myoblast cells. We observed that insulin increased the content of eIF5A transcripts. This effect occurred in cells treated or depleted of fetal bovine serum, revealing a positive insulin effect independent of other serum components. In addition, it was observed that hypusination follows the maintenance of eIF5A protein content in the serum depleted cells and treated with insulin. These results demonstrate that eIF5A is modulated by insulin, contributing the protein synthesis machinery control, as observed by puromycin incorporation in nascent proteins.

Diabetes alters vascular mechanotransduction data: Pressure-induced regulation of mTor and associated signaling in the rat inferior vena cava

Diabetes is a multifaceted disease with various etiologies. The complexity of this pathology creates a myriad of factors that must be considered when addressing surgical outcomes and prognosis. Of vital importance to cardiovascular surgery is the viability of homographic vein grafts. Due to the fact, diabetic patients have a higher rate of vein graph failure, a greater understanding of the effect diabetes has on vascular mechano-transductive response is critical to improving patient prognosis. This article represents data regarding a study published in Cardiovascular Diabetology (Rice et al., 2006) [1] and Open Journal of Endocrine and Metabolic Diseases (Rice et al., 2015) [2] with the purpose of evaluating the effect of pressurization on rat inferior venae cavae (IVC). Here we provide the information about the method and processing of raw data related to our prior publish work and Data in Brief articles (Rice et al., Submitted for publication) [3,4]. The data contained in this article evaluates the contribution of mTor signaling and associated proteins. IVC from lean and obese animals were exposed to a 30 min perfusion of 120 mm Hg pressure and evaluated for changes in expression and phosphorylation of mTor, p70s6k, GSK3β, and 4EBP-1.

Gerosuppression by pan-mTOR inhibitors

Rapamycin slows organismal aging and delays age-related diseases, extending lifespan in numerous species. In cells, rapamycin and other rapalogs such as everolimus suppress geroconversion from quiescence to senescence. Rapamycin inhibits some, but not all, activities of mTOR. Recently we and others demonstrated that pan-mTOR inhibitors, known also as dual mTORC1/C2 inhibitors, suppress senescent phenotype. As a continuation of these studies, here we investigated in detail a panel of pan-mTOR inhibitors, to determine their optimal gerosuppressive concentrations. During geroconversion, cells become hypertrophic and flat, accumulate lysosomes (SA-beta-Gal staining) and lipids (Oil Red staining) and lose their re-proliferative potential (RPP). We determined optimal gerosuppressive concentrations: Torin1 (30 nM), Torin 2 (30 nM), AZD8055 (100 nM), PP242 (300 nM), both KU-006379 and GSK1059615 (1000 nM). These agents decreased senescence-associated hypertrophy with IC50s: 20, 18, 15, 200 and 400 nM, respectively. Preservation of RPP by pan-mTOR inhibitors was associated with inhibition of the pS6K/pS6 axis. Inhibition of rapamycin-insensitive functions of mTOR further contributed to anti-hypertrophic and cytostatic effects. Torin 1 and PP242 were more "rapamycin-like" than Torin 2 and AZD8055. Pan-mTOR inhibitors were superior to rapamycin in suppressing hypertrophy, senescent morphology, Oil Red O staining and in increasing so-called "chronological life span (CLS)". We suggest that, at doses lower than anti-cancer concentrations, pan-mTOR inhibitors can be developed as anti-aging drugs.

Separation and purification of the bovine milk fat globule membrane protein and its effect on improvement of C2C12mouse skeletal muscle cell proliferation

A novel method to improve the proliferation activity of C₂C₁₂cells by the bovine milk fat globule membrane (MFGM) protein was established in this study.

Dual mTORC1/C2 inhibitors suppress cellular geroconversion (a senescence program)

In proliferating cells, mTOR is active and promotes cell growth. When the cell cycle is arrested, then mTOR converts reversible arrest to senescence (geroconversion). Rapamycin and other rapalogs suppress geroconversion, maintaining quiescence instead. Here we showed that ATP-competitive kinase inhibitors (Torin1 and PP242), which inhibit both mTORC1 and TORC2, also suppressed geroconversion. Despite inhibition of proliferation (in proliferating cells), mTOR inhibitors preserved re-proliferative potential (RP) in arrested cells. In p21-arrested cells, Torin 1 and PP242 detectably suppressed geroconversion at concentrations as low as 1-3 nM and 10-30 nM, reaching maximal gerosuppression at 30 nM and 300 nM, respectively. Near-maximal gerosuppression coincided with inhibition of p-S6K(T389) and p-S6(S235/236). Dual mTOR inhibitors prevented senescent morphology and hypertrophy. Our study warrants investigation into whether low doses of dual mTOR inhibitors will prolong animal life span and delay age-related diseases. A new class of potential anti-aging drugs can be envisioned.

Essential role of D1R in the regulation of mTOR complex1 signaling induced by cocaine

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that is involved in neuronal adaptions that underlie cocaine-induced sensitization and reward. mTOR exists in two functionally distinct multi-component complexes known as mTORC1 and mTORC2. In this study, we show that increased mTORC1 activity induced by cocaine is mediated by the dopamine D1 receptor (D1R). Specifically, cocaine treatment increased the phosphorylation on residues Thr2446 and Ser2481 but not on Ser2448 in the nucleus accumbens (NAc) and that this increase in phosphorylated mTOR levels was also apparent when complexed with its binding partner Raptor. Furthermore, the increase in phosphorylated mTOR levels, as well as phosphorylated 4E-BP1 and S6K, downstream targets of mTORC1 were blocked with SCH23390 treatment. Similar results were also observed in the dopamine-transporter knockout mice as the increase in phosphorylated mTOR Thr2446 and Ser2481 was blocked by SCH23390 but not with raclopride. To further validate D1R role in mTORC1 signaling, decrease in phosphorylated mTOR levels were observed in D1R knockout mice, whereas administration of SKF81297 elevated phosphorylated mTOR in the NAc. Lastly deletion of mTOR or Raptor in D1R expressing neurons reduced cocaine-induced locomotor activity. Together, our data supports a mechanism whereby mTORC1 signaling is activated by cocaine administration through the stimulation of D1R.

The ever-evolving role of mTOR in translation

Control of translation allows for the production of stoichiometric levels of each protein in the cell. Attaining such a level of fine-tuned regulation of protein production requires the coordinated temporal and spatial control of numerous cellular signalling cascades impinging on the various components of the translational machinery. Foremost among these is the mTOR signalling pathway. The mTOR pathway regulates both the initiation and elongation steps of protein synthesis through the phosphorylation of numerous translation factors, while simultaneously ensuring adequate folding of nascent polypeptides through co-translational degradation of misfolded proteins. Perhaps most remarkably, mTOR is also a key regulator of the synthesis of ribosomal proteins and translation factors themselves. Two seminal studies have recently shown in translatome analysis that the mTOR pathway preferentially regulates the translation of mRNAs encoding ribosomal proteins and translation factors. Therefore, the role of the mTOR pathway in the control of protein synthesis extends far beyond immediate translational control. By controlling ribosome production (and ultimately ribosome availability), mTOR is a master long-term controller of protein synthesis. Herein, we review the literature spanning the early discoveries of mTOR on translation to the latest advances in our understanding of how the mTOR pathway controls the synthesis of ribosomal proteins.

Translation factors and ribosomal proteins control tumor onset and progression: how?

Gene expression is shaped by translational control. The modalities and the extent by which translation factors modify gene expression have revealed therapeutic scenarios. For instance, eukaryotic initiation factor (eIF)4E activity is controlled by the signaling cascade of growth factors, and drives tumorigenesis by favoring the translation of specific mRNAs. Highly specific drugs target the activity of eIF4E. Indeed, the antitumor action of mTOR complex 1 (mTORc1) blockers like rapamycin relies on their capability to inhibit eIF4E assembly into functional eIF4F complexes. eIF4E biology, from its inception to recent pharmacological targeting, is proof-of-principle that translational control is druggable. The case for eIF4E is not isolated. The translational machinery is involved in the biology of cancer through many other mechanisms. First, untranslated sequences on mRNAs as well as noncoding RNAs regulate the translational efficiency of mRNAs that are central for tumor progression. Second, other initiation factors like eIF6 show a tumorigenic potential by acting downstream of oncogenic pathways. Third, genetic alterations in components of the translational apparatus underlie an entire class of inherited syndromes known as 'ribosomopathies' that are associated with increased cancer risk. Taken together, data suggest that in spite of their evolutionary conservation and ubiquitous nature, variations in the activity and levels of ribosomal proteins and translation factors generate highly specific effects. Beside, as the structures and biochemical activities of several noncoding RNAs and initiation factors are known, these factors may be amenable to rational pharmacological targeting. The future is to design highly specific drugs targeting the translational apparatus.

The Parkinson's disease associated LRRK2 exhibits weaker in vitro phosphorylation of 4E-BP compared to autophosphorylation

Mutations in the gene encoding Leucine-rich repeat kinase 2 (LRRK2) are the most common cause of inherited Parkinson's disease (PD). LRRK2 is a multi-domain protein kinase containing a central catalytic core and a number of protein-protein interaction domains. An important step forward in the understanding of both the biology and the pathology of LRRK2 would be achieved by identification of its authentic physiological substrates. In the present study we examined phosphorylation of 4E-BP (eukaryotic initiation factor 4E (eIF4E)-binding protein), a recently proposed substrate for LRRKs. We found that LRRK2 is capable of phosphorylating 4E-BP in vitro. The PD related LRRK2-G2019S mutant was ∼2 fold more active than wild type protein. However, LRRK2 autophosphorylation was stronger than 4E-BP phosphorylation under conditions of molar excess of 4E-BP to LRRK2. We also tested three other kinases (STK3, MAPK14/p38α and DAPK2) and found that MAPK14/p38α could efficiently phosphorylate 4E-BP at the same site as LRRK2 in vitro. Finally, we did not see changes in 4E-BP phosphorylation levels using inducible expression of LRRK2 in HEK cell lines. We also found that MAPK14/p38α phosphorylates 4E-BP in transient overexpression experiments whereas LRRK2 did not. We suggest that increased 4E-BP phosphorylation reported in some systems may be related to p38-mediated cell stress rather than direct LRRK2 activity. Overall, our results suggest that 4E-BP is a relatively poor direct substrate for LRRK2.

Regulation of protein synthesis by ionizing radiation

Ionizing radiation (IR) is a physiologically important stress to which cells respond by the activation of multiple signaling pathways. Using a panel of immortalized and transformed breast epithelial cell lines, we demonstrate that IR regulation of protein synthesis occurs in nontransformed cells and is lost with transformation. In nontransformed cells, IR rapidly activates the MAP kinases ERK1/2, resulting in an early transient increase in cap-dependent mRNA translation that involves mTOR and is radioprotective, enhancing the translation of a subset of mRNAs encoding proteins involved in DNA repair and cell survival. Following a transient increase in translation, IR-sensitive (nontransformed) cells inhibit cap-dependent protein synthesis through a mechanism that involves activation of p53, induction of Sestrin 1 and 2 genes, and stimulation of AMP kinase, inhibiting mTOR and hypophosphorylating 4E-BP1. IR is shown to block proteasome-mediated decay of 4E-BP1, increasing its abundance and the sequestration of eIF4E. The IR signal that impairs mTOR-dependent protein synthesis at late times is assembly of the DNA damage response machinery, consisting of Mre11, Rad50, and NBS1 (MRN); activation of the MRN complex kinase ATM; and p53. These results link genotoxic signaling from the DNA damage response complex to the control of protein synthesis.

Leptin stimulates protein synthesis-activating translation machinery in human trophoblastic cells

Leptin was originally considered as an adipocyte-derived signaling molecule for the central control of metabolism. However, pleiotropic effects of leptin have been identified in reproduction and pregnancy, particularly in placenta, where it may work as an autocrine hormone, mediating angiogenesis, growth, and immunomodulation. Leptin receptor (LEPR, also known as Ob-R) shows sequence homology to members of the class I cytokine receptor (gp130) superfamily. In fact, leptin may function as a proinflammatory cytokine. We have previously found that leptin is a trophic and mitogenic factor for trophoblastic cells. In order to further investigate the mechanism by which leptin stimulates cell growth in JEG-3 cells and trophoblastic cells, we studied the phosphorylation state of different proteins of the initiation stage of translation and the total protein synthesis by [(3)H]leucine incorporation in JEG-3 cells. We have found that leptin dose-dependently stimulates the phosphorylation and activation of the translation initiation factor EIF4E as well as the phosphorylation of the EIF4E binding protein EIF4EBP1 (PHAS-I), which releases EIF4E to form active complexes. Moreover, leptin dose-dependently stimulates protein synthesis, and this effect can be partially prevented by blocking mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3 kinase (PIK3) pathways. In conclusion, leptin stimulates protein synthesis, at least in part activating the translation machinery, via the activation of MAPK and PIK3 pathways.

The rapid activation of protein synthesis by growth hormone requires signaling through mTOR

An important function of growth hormone (GH) is to promote cell and tissue growth, and a key component of these effects is the stimulation of protein synthesis. In this study, we demonstrate that, in H4IIE hepatoma cells, GH acutely activated protein synthesis through signaling via the mammalian target of rapamycin (mTOR) and specifically through the rapamycin-sensitive mTOR complex 1 (mTORC1). GH treatment enhanced the phosphorylation of two targets of mTOR signaling, 4E-BP1 and ribosomal protein S6. Phosphorylation of S6 and 4E-BP1 was maximal at 30-45 min and 10-20 min after GH stimulation, respectively. Both proteins modulate components of the translational machinery. The GH-induced phosphorylation of 4E-BP1 led to its dissociation from eIF4E and increased binding of eIF4E to eIF4G to form (active) eIF4F complexes. The ability of GH to stimulate the phosphorylation of S6 and 4E-BP1 was blocked by rapamycin. GH also led to the dephosphorylation of a third translational component linked to mTORC1, the elongation factor eEF2. Its regulation followed complex biphasic kinetics, both phases of which required mTOR signaling. GH rapidly activated both the MAP kinase (ERK) and PI 3-kinase pathways. Signaling through PI 3-kinase alone was, however, sufficient to activate the downstream mTORC1 pathway. Consistent with this, GH increased the phosphorylation of TSC2, an upstream regulator of mTORC1, at sites that are targets for Akt/PKB. Finally, the activation of overall protein synthesis by GH in H4IIE cells was essentially completely inhibited by wortmannin or rapamycin. These results demonstrate for the first time that mTORC1 plays a major role in the rapid activation of protein synthesis by GH.

Regulation of protein synthesis by insulin

Insulin rapidly activates protein synthesis by activating components of the translational machinery including eIFs (eukaryotic initiation factors) and eEFs (eukaryotic elongation factors). In the long term, insulin also increases the cellular content of ribosomes to augment the capacity for protein synthesis. The rapid activation of protein synthesis by insulin is mediated primarily through phosphoinositide 3-kinase. This involves the activation of PKB (protein kinase B). In one case, PKB acts to phosphorylate and inactivate glycogen synthase kinase 3, which in turn phosphorylates and inhibits eIF2B. Insulin elicits the dephosphorylation and activation of eIF2B. Since eIF2B is required for recycling of eIF2, a factor required for all cytoplasmic translation initiation events, this will contribute to overall activation of protein synthesis. PKB also phosphorylates the TSC1 (tuberous sclerosis complex 1)–TSC2 complex to relieve its inhibitory action on the mTOR (mammalian target of rapamycin). Inhibition of mTOR by rapamycin markedly impairs insulin-activated protein synthesis. mTOR controls translation initiation and elongation. The cap-binding factor eIF4E can be sequestered in inactive complexes by 4E-BP1 (eIF4E-binding protein 1). Insulin elicits phosphorylation of 4E-BP1 and its release from eIF4E, allowing eIF4E to form initiation factor complexes. Insulin induces dephosphorylation and activation of eEF2 to accelerate elongation. Both effects are blocked by rapamycin. Insulin inactivates eEF2 kinase by increasing its phosphorylation at several mTOR-regulated sites. Insulin also stimulates synthesis of ribosomal proteins by promoting recruitment of their mRNAs into polyribosomes. This is inhibited by rapamycin. Several key questions remain about, for example, the mechanisms by which mTOR controls 4E-BP1 and eEF2 kinase and the control of ribosomal protein translation.

Effect of severe short-term malnutrition on diaphragm muscle signal transduction pathways influencing protein turnover

The aim of this study was to evaluate the effect of nutritional deprivation (ND) on signal transduction pathways influencing the translational apparatus in the diaphragm muscle. Male rats were divided into two groups: 1) 20% of usual food intake for 4 days (ND) with water provided at libitum and 2) free-eating control (Ctl). Total protein and RNA were extracted from the diaphragm. Insulin-like growth factor I mRNA was analyzed by RT-PCR. Protein analyses of key cytoplasmic proteins for three signaling pathways deemed important in influencing protein turnover [phosphatidylinositol 3-kinase- Akt-mammalian target of rapamycin, P13K/Akt/glycogen synthase kinase (GSK)-3, and MAPK-ERK] were performed by Western blot. Body weight decreased 30% in ND and increased 17% in Ctl animals. Diaphragm mass decreased 29% in ND animals. Muscle insulin-like growth factor I mRNA abundance was reduced 63% in ND animals. ND resulted in a 55% reduction in phosphorylated (Ser473) Akt. Phosphorylation of mammalian target of rapamycin at Ser2448 was reduced by 85% in ND animals. Downstream effectors important in translation initiation were also affected by ND. Phosphorylated (Thr389) 70-kDa ribosomal protein S6 kinase was significantly reduced (35%) by ND. ND also resulted in significant dephosphorylation of the translational repressor initiation factor 4E-binding protein 1. Phosphorylation of GSK-3alpha (Ser21) and GSK-3beta (Ser9) was increased 55 and 45%, respectively, with ND. Phosphorylation of ERK1 (Thr202) and ERK2 (Tyr204), p44 and p42, respectively, was reduced 64 and 55%, respectively, with ND. Total protein concentration for all signaling intermediates of the three pathways was preserved. We conclude that short-term ND altered the phosphorylation states of key proteins of several pathways involved in protein turnover. This forms the framework for future studies aimed at identifying therapeutic targets in the management of short-term nutritionally induced cachectic states.

Activation of protein synthesis in cardiomyocytes by the hypertrophic agent phenylephrine requires the activation of ERK and involves phosphorylation of tuberous sclerosis complex 2 (TSC2)

The hypertrophic Gq-protein-coupled receptor agonist PE (phenylephrine) activates protein synthesis. We showed previously that activation of protein synthesis by PE requires MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase] and mTOR (mammalian target of rapamycin). However, it remained unclear whether ERK activation was required and which downstream components were involved in activating mTOR and protein synthesis. Using an adenovirus encoding the MKP3 (MAPK phosphatase 3) to inhibit ERK activity, we demonstrate that ERK is essential for the activation of protein synthesis by PE. Activation and phosphorylation of S6K1 (ribosomal protein S6 kinase 1) and phosphorylation of eIF4E (eukaryotic initiation factor 4E)-binding protein (both are mTOR targets) were also inhibited by MKP3, suggesting that ERK is also required for the activation of mTOR signalling. PE stimulation of cardiomyocytes induced the phosphorylation of TSC2 (tuberous sclerosis complex 2), a negative regulator of mTOR activity. TSC2 was phosphorylated only weakly at Thr1462, but phosphorylated at additional sites within the sequence RXRXX(S/T). This differs from the phosphorylation induced by insulin, indicating that MEK/ERK signalling targets distinct sites in TSC2. This phosphorylation may be mediated by p90RSK (90 kDa ribosomal protein S6K), which is activated by ERK, and appears to involve phosphorylation at Ser1798. Activation of protein synthesis by PE is partially insensitive to the mTOR inhibitor rapamycin. Inhibition of the MAPK-interacting kinases by CGP57380 decreases the phosphorylation of eIF4E and PE-induced protein synthesis. Moreover, CGP57380+rapamycin inhibited protein synthesis to the same extent as blocking ERK activation, suggesting that MAPK-interacting kinases and regulation of mTOR each contribute to the activation of protein synthesis by PE in cardiomyocytes.

Mechanism of feedback regulation of insulin receptor substrate-1 phosphorylation in primary adipocytes

Serine and threonine phosphorylation of IRS-1 (insulin receptor substrate-1) has been reported to decrease its ability to be tyrosine-phosphorylated by the insulin receptor. Insulin itself may negatively regulate tyrosine phosphorylation of IRS-1 through a PI3K (phosphoinositide 3-kinase)-dependent feedback pathway. In the present study, we examined the regulation and role of IRS-1 serine phosphorylation in the modulation of IRS-1 tyrosine phosphorylation in physiologically relevant cells, namely freshly isolated primary adipocytes. We show that insulin-stimulated phosphorylation of Ser312 and Ser616 in IRS-1 was relatively slow, with maximal phosphorylation achieved after 20 and 5 min respectively. The effect of insulin on phosphorylation of both these sites required the activation of PI3K and the MAPKs (mitogen-activated protein kinases) ERK1/2 (extracellular-signal-regulated kinase 1 and 2), but not the activation of mTOR (mammalian target of rapamycin)/p70S6 kinase, JNK (c-Jun N-terminal kinase) or p38MAPK. Although inhibition of PI3K and ERK1/2 both substantially decreased insulin-stimulated phosphorylation of Ser312 and Ser616, only wortmannin enhanced insulin-stimulated tyrosine phosphorylation of IRS-1. Furthermore, inhibition of mTOR/p70S6 kinase, JNK or p38MAPK had no effect on insulin-stimulated IRS-1 tyrosine phosphorylation. The differential effect of inhibition of ERK1/2 on insulin-stimulated IRS-1 phosphorylation of Ser312/Ser616 and tyrosine indicates that these events are independent of each other and that phosphorylation of Ser312/Ser616 is not responsible for the negative regulation of IRS-1 tyrosine phosphorylation mediated by PI3K in primary adipocytes.

Distinct signaling events downstream of mTOR cooperate to mediate the effects of amino acids and insulin on initiation factor 4E-binding proteins

Signaling through the mammalian target of rapamycin (mTOR) controls cell size and growth as well as other functions, and it is a potential therapeutic target for graft rejection, certain cancers, and disorders characterized by inappropriate cell or tissue growth. mTOR signaling is positively regulated by hormones or growth factors and amino acids. mTOR signaling regulates the phosphorylation of several proteins, the best characterized being ones that control mRNA translation. Eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) undergoes phosphorylation at multiple sites. Here we show that amino acids regulate the N-terminal phosphorylation sites in 4E-BP1 through the RAIP motif in a rapamycin-insensitive manner. Several criteria indicate this reflects a rapamycin-insensitive output from mTOR. In contrast, the insulin-stimulated phosphorylation of the C-terminal site Ser64/65 is generally sensitive to rapamycin, as is phosphorylation of another well-characterized target for mTOR signaling, S6K1. Our data imply that it is unlikely that mTOR directly phosphorylates Thr69/70 in 4E-BP1. Although 4E-BP1 and S6K1 bind the mTOR partner, raptor, our data indicate that the outputs from mTOR to 4E-BP1 and S6K1 are distinct. In cells, efficient phosphorylation of 4E-BP1 requires it to be able to bind to eIF4E, whereas phosphorylation of 4E-BP1 by mTOR in vitro shows no such preference. These data have important implications for understanding signaling downstream of mTOR and the development of new strategies to impair mTOR signaling.

eIF4E binding protein 1 and H-Ras are novel substrates for the protein kinase activity of class-I phosphoinositide 3-kinase

Class-I phosphoinositide 3-kinases (PI 3-kinases) are dual specificity enzymes that possess both lipid and protein kinase activity. While the best characterized property of this protein kinase is as an autokinase activity, there have also been reports it can phosphorylate exogenous substrates including peptides, IRS-1 and PDE-3B. The identification of two novel potential protein substrates of PI 3-kinase is described here. By employing in vitro kinase assays using recombinant proteins as the substrates, it is shown that the translational regulator 4EBP1 becomes phosphorylated by the p110alpha and p110gamma isoforms of class-I PI 3-kinases. The lipid kinase activity of both these isoforms is increased by allosteric binding of H-Ras or betagamma subunits of heterotrimeric G proteins, but we find this is not the case for the protein kinase activity. Surprisingly though, a site on H-Ras is phosphorylated by p110alpha and p110gamma. This raises the possibility that these proteins could serve as physiological substrates for the protein kinase activity of PI 3-kinase and suggests this activity operates in a physiological context by phosphorylating substrates other than the PI 3-kinase itself. This may be particularly important in regulating the interaction of Ras with PI 3-kinase.

Phosphatidylinositol 3-kinase in angiotensin II-induced hypertrophy of vascular smooth muscle cells

Activation of 4E-binding protein 1 (4E-BP1) by growth factors regulates protein synthesis in vascular smooth muscle cells. The interaction between G protein-coupled receptors and activated 4E-BP1 is unclear. We examined phosphadityl inositol (PI) 3-kinase in angiotensin II-induced 4E-BP1 phosphorylation in cultured rat vascular smooth muscle cells. Angiotensin II time and dose dependently stimulated phosphorylation of 4E-BP1 through the angiotensin AT(1) receptor. Pretreatment with wortmannin or 2-(4-Morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002), a PI 3-kinase inhibitor, suppressed angiotensin II-induced phosphorylation, but a mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinases (ERK) kinase-1 (MEK-1) inhibitor, 2'-Amino-3'-methoxyflavone (PD98059), and a p38 MAPK inhibitor, 4-(4-Fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580), had no effect. With regard to the involvement of mammalian target of rapamycin (mTOR) and p70 S6 kinase, angiotensin II-induced phosphorylation was abolished by pretreatment with rapamycin, but not by tosylphenylalanine chloromethyl ketone or tosyllysine chloromethyl ketone. Ca(2+) was involved, since intracellular Ca(2+) chelation inhibited angiotensin II-induced phosphorylation while a Ca(2+) ionophore, A23187, stimulated phosphorylation. Thus, angiotensin II induces the phosphorylation of 4E-BP1 via the PI 3-kinase/mTOR pathway, but not via ERK or p70 S6 kinase.

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.

Reactive oxygen species sensitivity of angiotensin II-dependent translation initiation in vascular smooth muscle cells

Translation initiation, the rate-limiting step in protein synthesis, is a key event in vascular smooth muscle cell growth, a major component of vascular disease. Translation initiation is regulated by interaction between PHAS-I and the eukaryotic initiation factor 4E (eIF4E). Although angiotensin II (Ang II)-induced vascular smooth muscle cell hypertrophy requires the generation of reactive oxygen species (ROS), the ROS sensitivity of these events and their upstream activators remain unclear. Here, we investigated the role of ROS in the regulation of PHAS-I phosphorylation on Thr-70 and Ser-65, an event required for the release of eIF4E from PHAS-I. Ang II-induced Ser-65 phosphorylation was ROS-dependent as assessed by pretreatment with ebselen (3.6 +/- 0.2 versus 1.1 +/- 0.2), diphenylene iodonium (3.6 +/- 0.2 versus 1.0 +/- 0.1), and N-acetyl cysteine (3.6 +/- 0.2 versus 1.2 +/- 0.1), but Ang II-stimulated phosphorylation of Thr-70 was ROS-insensitive. Although phosphatidylinositol 3-kinase pathway inhibition by LY294004 blocked both Ser-65 and Thr-70 phosphorylation (3.8 +/- 0.1 versus 0.8 +/- 0.1 and 3.2 +/- 0.2 versus 1.0 +/- 0.01, respectively), protein phosphatase 2A inhibition by okadaic acid selectively increased (3.3 +/- 0.1 versus 5.2 +/- 0.1) and p38 mitogen-activated protein kinase inhibition by SB203580 selectively decreased (3.8 +/- 0.1 versus 1.4 +/- 0.3) Ser-65 phosphorylation. Dominant negative Akt adenovirus also inhibited only Ser-65 phosphorylation (3.7 +/- 0.1 versus 1.0 +/- 0.03). These results demonstrate a unique differential ROS sensitivity of two separate residues on PHAS-I, which seems to be explained by the selective involvement of distinct signaling pathways in the regulation of these phosphorylation events.

Dynamic visualization of expressed gene networks

Cellular biochemical machineries, what we call pathways, consist of dynamically assembling and disassembling macromolecular complexes. While our models for the organization of biochemical machines are derived largely from in vitro experiments, do they reflect their organization in living cells? We have developed a general experimental strategy that addresses this question by allowing the quantitative probing of molecular interactions in intact living cells. The experimental strategy is based on protein fragment complementation assays (PCA), a method whereby protein interactions are coupled to refolding of enzymes from cognate fragments where reconstitution of enzyme activity acts as the detector of a protein interaction. A biochemical machine or pathway is defined by grouping interacting proteins into those that are perturbed in the same way by common factors (hormones, metabolites, enzyme inhibitors, etc). In this review, we describe how we go from descriptive to quantitative representations of biochemical networks at an individual to whole genome level and how our approach will lead ultimately to better descriptions of the biochemical machineries that underlie living processes.

Ca(2+)-independent protein kinase C activity is required for alpha1-adrenergic-receptor-mediated regulation of ribosomal protein S6 kinases in adult cardiomyocytes

The alpha(1)-adrenergic agonist, phenylephrine (PE), exerts hypertrophic effects in the myocardium and activates protein synthesis. Both Ca(2+)-dependent protein kinase C (PKC, PKCalpha) and Ca(2+)-independent PKC isoforms (PKCdelta and epsilon ) are detectably expressed in adult rat cardiomyocytes. Stimulation of the alpha(1)-adrenergic receptor by PE results in activation of Ca(2+)-independent PKCs, as demonstrated by translocation of the delta and epsilon isoenzymes from cytosol to membrane fractions. PE also induces activation of p70 ribosomal protein S6 kinases (S6K1 and 2) in adult cardiomyocytes. We have studied the role of Ca(2+)-independent PKCs in the regulation of S6K activity by PE. Activation of S6K1/2 by PE was blocked by the broad-spectrum PKC inhibitor bisindolylmaleimide (BIM) I, whereas Gö6976, a compound that only inhibits Ca(2+)-dependent PKCs, did not inhibit S6K activation. Rottlerin, which selectively inhibits PKCdelta, also prevented PE-induced S6K activation. The isoform-specific PKC inhibitors had similar effects on the phosphorylation of eukaryotic initiation factor 4E (eIF4E)-binding protein 1, a translation repressor that, like the S6Ks, lies downstream of the mammalian target of rapamycin (mTOR). Infection of cells with adenoviruses encoding dominant-negative PKCdelta or epsilon inhibited the activation of extracellular-signal-regulated kinase (ERK) by PE, and also inhibited the activation and/or phosphorylation of S6Ks 1 and 2. The PE-induced activation of protein synthesis was abolished by BIM I and markedly attenuated by rottlerin. Our data thus suggest that Ca(2+)-independent PKC isoforms play an important role in coupling the alpha(1)-adrenergic receptor to mTOR signalling and protein synthesis in adult cardiomyocytes.

Regulation of targets of mTOR (mammalian target of rapamycin) signalling by intracellular amino acid availability

In mammalian cells, amino acids affect the phosphorylation state and function of several proteins involved in mRNA translation that are regulated via the rapamycin-sensitive mTOR (mammalian target of rapamycin) pathway. These include ribosomal protein S6 kinase, S6K1, and eukaryotic initiation factor 4E-binding protein, 4E-BP1. Amino acids, especially branched-chain amino acids, such as leucine, promote phosphorylation of 4E-BP1 and S6K1, and permit insulin to further increase their phosphorylation. However, it is not clear whether these effects are exerted by extracellular or intracellular amino acids. Inhibition of protein synthesis is expected to increase the intracellular level of amino acids, whereas inhibiting proteolysis has the opposite effect. We show in the present study that inhibition of protein synthesis by any of several protein synthesis inhibitors tested allows insulin to regulate 4E-BP1 or S6K1 in amino-acid-deprived cells, as does the addition of amino acids to the medium. In particular, insulin activates S6K1 and promotes initiation factor complex assembly in amino-acid-deprived cells treated with protein synthesis inhibitors, but cannot do so in the absence of these compounds. Their effects occur at concentrations commensurate with their inhibition of protein synthesis and are not due to activation of stress-activated kinase cascades. Inhibition of protein breakdown (autophagy) impairs the ability of insulin to regulate 4E-BP1 or S6K1 under such conditions. These and other data presented in the current study are consistent with the idea that it is intracellular amino acid levels that regulate mTOR signalling.

The C terminus of initiation factor 4E-binding protein 1 contains multiple regulatory features that influence its function and phosphorylation

Eukaryotic initiation factor 4E (eIF4E) binds the mRNA cap structure and forms eIF4F complexes that recruit 40S subunits to the mRNA. Formation of eIF4F is blocked by eIF4E-binding proteins such as 4E-BP1, which interacts with eIF4E via a motif in the center of its 118-residue sequence. 4E-BP1 plays key roles in cell proliferation, growth, and survival. Binding of 4E-BP1 to eIF4E is regulated by hierarchical multisite phosphorylation. Here we demonstrate that three different features in the C terminus of 4E-BP1 play distinct roles in regulating its phosphorylation and function. Firstly, we identify a new phosphorylation site in its C terminus (S101). A serine or glutamate at this position is required for efficient phosphorylation at Ser65. A second C-terminal site, S112, directly affects binding of 4E-BP1 to eIF4E without influencing phosphorylation of other sites. Thirdly, a conserved C-terminal motif influences phosphorylation of multiple residues, including rapamycin-insensitive sites. These relatively long-range effects are surprising given the reportedly unstructured nature of 4E-BP1 and may imply that phosphorylation of 4E-BP1 and/or binding to eIF4E induces a more-ordered structure. 4E-BP2 and -3 lack phosphorylatable residues corresponding to both S101 and S112. However, in 4E-BP3, replacement of the alanine at the position corresponding to S112 by serine or glutamate did not confer the ability to be released from eIF4E in response to insulin.

Reciprocal feedback regulation of insulin receptor and insulin receptor substrate tyrosine phosphorylation by phosphoinositide 3-kinase in primary adipocytes

Signalling by the insulin receptor substrate (IRS) proteins is critically dependent on the tyrosine phosphorylation of specific binding sites that recruit Src homology 2 (SH2)-domain-containing proteins, such as the p85 subunit of phosphoinositide 3-kinase (PI 3-kinase), the tyrosine phosphatase SHP-2 and the adapter protein Grb2. Here we show that stimulation by insulin of freshly isolated primary adipocytes resulted in the expected rapid tyrosine phosphorylation of the insulin receptor, IRS-1 and IRS-3. Inhibition of PI 3-kinase enhanced the insulin-stimulated phosphorylation of IRS-1 on (i) Tyr(612) and Tyr(941) (p85 binding sites), concomitant with an increased association of the p85 subunit of PI 3-kinase; (ii) Tyr(896) (a Grb2 binding site); and (iii) Tyr(1229) (an SHP-2 binding site), although little or no binding of SHP-2 to IRS-1 was detectable under any conditions. In contrast, inhibition of PI 3-kinase led to a decrease in insulin-stimulated p85 binding to IRS-3, but had no effect on SHP-2 binding. Furthermore, insulin-induced insulin receptor tyrosine phosphorylation, phosphorylation of Tyr(1158) and insulin receptor tyrosine kinase activity were all reduced by inhibition of PI 3-kinase at later time points (>or=20 min). The results demonstrate that, in primary adipocytes, PI 3-kinase feedback control of signalling by the insulin receptor and IRS proteins is multifaceted and reciprocal, illustrating the complexity of predicting the net flux of the insulin signal(s) through the IRS proteins.

Pancreastatin, a chromogranin A-derived peptide, activates protein synthesis signaling cascade in rat adipocytes

Pancreastatin (PST), a chromogranin A-derived peptide, has been found to modulate glucose, lipid, and protein metabolism in rat adipocytes. PST has an overall counterregulatory effect on insulin action by activating a specific receptor-effector system (Galpha(q/11) protein-PLC-beta-PKC(classical)). However, PST stimulates both basal and insulin-mediated protein synthesis in rat adipocytes. In order to further investigate the mechanisms underlying the effect of PST stimulating protein synthesis, we sought to study the regulation of different components of the core translational machinery by the signaling triggered by PST. Thus, we studied ribosomal p70 S6 kinase, phosphorylation of the cap-binding protein (initiation factor) eIF4E, and phosphorylation of the eIF4E-binding protein 4E-BP1 (PHAS-I). We have found that PST stimulates the S6 kinase activity, as assessed by kinase assay using specific immunoprecipitates and substrate. This effect was checked by Western blot with specific antibodies against the phosphorylated S6 kinase. Thus, PST dose-dependently stimulates Thr421/Ser424 phosphorylation of S6 kinase. Moreover, PST promotes phosphorylation of regulatory sites in 4E-BP1 (PHAS-I) (Thr37, Thr46). The initiation factor eIF4E itself, whose activity is also increased upon phosphorylation, is phosphorylated in Ser209 by PST stimulation. Finally, we have found that these effects of PST on S6 kinase and the translation machinery can be blocked by preventing the activation of PKC. These results indicate that PST stimulates protein synthesis machinery by activating PKC and provides some evidence of the molecular mechanisms involved, i.e., the activation of S6K and the phosphorylation of 4E-BP1 (PHAS-I) and the initiation factor eIF4E.

Regulation of mammalian translation factors by nutrients

Protein synthesis requires both amino acids, as precursors, and a substantial amount of metabolic energy. It is well established that starvation or lack of nutrients impairs protein synthesis in mammalian cells and tissues. Branched chain amino acids are particularly effective in promoting protein synthesis. Recent work has revealed important new information about the mechanisms involved in these effects. A number of components of the translational machinery are regulated through signalling events that require the mammalian target of rapamycin, mTOR. These include translational repressor proteins (eukaryotic initiation factor 4E-binding proteins, 4E-BPs) and protein kinases that act upon the small ribosomal subunit (S6 kinases). Amino acids, especially leucine, positively regulate mTOR signalling thereby relieving inhibition of translation by 4E-BPs and activating the S6 kinases, which can also regulate translation elongation. However, the molecular mechanisms by which amino acids modulate mTOR signalling remain unclear. Protein synthesis requires a high proportion of the cell's metabolic energy, and recent work has revealed that metabolic energy, or fuels such as glucose, also regulate targets of the mTOR pathway. Amino acids and glucose modulate a further important regulatory step in translation initiation, the activity of the guanine nucleotide-exchange factor eIF2B. eIF2B controls the recruitment of the initiator methionyl-tRNA to the ribosome and is activated by insulin. However, in the absence of glucose or amino acids, insulin no longer activates eIF2B. Since control of eIF2B is independent of mTOR, these data indicate the operation of additional, and so far unknown, regulatory mechanisms that control eIF2B activity.

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.

The identification of ATP-citrate lyase as a protein kinase B (Akt) substrate in primary adipocytes

Protein kinase B (Akt) plays a central role in cellular regulation, although many of the physiologically relevant substrates for the kinase remain to be identified. In this study, we have isolated a protein from primary epididymal adipocytes with an apparent molecular weight of 125,000. This protein exhibited immunoreactivity, in an insulin-dependent manner, with a phosphospecific antibody raised against the protein kinase B substrate consensus sequence RXRXX(pS/pT) as well as a phosphospecific antibody that recognizes serine 21/9 of GSK-3alpha/beta. MALDI-TOF mass spectrometry revealed the protein to be ATP-citrate lyase, suggesting that the two phosphospecific antibodies recognize phosphoserine 454, a previously reported insulin- and isoproterenol-stimulated ATP-citrate lyase phosphorylation site. Indeed, both insulin and isoproterenol stimulated the phosphorylation of this protein on the site recognized by the phosphospecific antibodies in a wortmannin-sensitive and -insensitive manner, respectively. In addition, transient expression of a constitutively active protein kinase B in primary adipocytes mimicked the effect of insulin on ATP-citrate lyase phosphorylation. Furthermore, ATP-citrate lyase was phosphorylated in vitro by recombinant protein kinase B on the same site. Taken together, these results demonstrate that serine 454 of ATP-citrate lyase is a novel and major in vivo substrate for protein kinase B.

Cellular stresses profoundly inhibit protein synthesis and modulate the states of phosphorylation of multiple translation factors

We have examined the effects of widely used stress-inducing agents on protein synthesis and on regulatory components of the translational machinery. The three stresses chosen, arsenite, hydrogen peroxide and sorbitol, exert their effects in quite different ways. Nonetheless, all three rapidly ( approximately 30 min) caused a profound inhibition of protein synthesis. In each case this was accompanied by dephosphorylation of the eukaryotic initiation factor (eIF) 4E-binding protein 1 (4E-BP1) and increased binding of this repressor protein to eIF4E. Binding of 4E-BP1 to eIF4E correlated with loss of eIF4F complexes. Sorbitol and hydrogen peroxide each caused inhibition of the 70-kDa ribosomal protein S6 kinase, while arsenite activated it. The effects of stresses on the phosphorylation of eukaryotic elongation factor 2 also differed: oxidative stress elicited a marked increase in eEF2 phosphorylation, which is expected to contribute to inhibition of translation, while the other stresses did not have this effect. Although all three proteins (4E-BP1, p70 S6 kinase and eEF2) can be regulated through the mammalian target of rapamycin (mTOR), our data imply that stresses do not interfere with mTOR function but act in different ways on these three proteins. All three stresses activate the p38 MAP kinase pathway but we were able to exclude a role for this in their effects on 4E-BP1. Our data reveal that these stress-inducing agents, which are widely used to study stress-signalling in mammalian cells, exert multiple and complex inhibitory effects on the translational machinery.

The extracellular signal-regulated kinase pathway regulates the phosphorylation of 4E-BP1 at multiple sites

The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), a potent stimulator of Erk, leads to the phosphorylation of 4E-BP1 and its dissociation from eIF4E. In contrast to agonists such as insulin, this occurs independently of PKB activation. In this report, we investigate the mechanism by which TPA regulates 4E-BP1 phosphorylation. Treatment of HEK293 cells with TPA was found to result in the phosphorylation of 4E-BP1 at Ser(64), Thr(69), and Thr(36/45). The TPA-stimulated phosphorylation of all these sites is sensitive to inhibitors of MEK and to the inhibitor of mTOR, rapamycin, indicating that inputs from both mTOR and MEK are required for the regulation of 4E-BP1 phosphorylation by TPA. Indeed, evidence is presented that mTOR may initially be required for the phosphorylation of Thr(45) in a priming step, which is necessary for the subsequent phosphorylation of Ser(64) and Thr(69) through an Erk-dependent pathway. Overexpression of constitutively active MEK in HEK293 cells resulted both in the phosphorylation of 4E-BP1 at Ser(64) and Thr(36/45) and its release from eIF4E. In this case, the phosphorylation of these sites was also blocked by inhibitors of MEK or by rapamycin. In conclusion, the Erk pathway, via mechanisms also requiring mTOR, regulates the phosphorylation of multiple sites in 4E-BP1 in vivo and this is sufficient for the release of 4E-BP1 from eIF4E.

Translational regulation of renal proximal tubular epithelial cell transforming growth factor-beta1 generation by insulin

We have previously demonstrated that the proximal tubular cell may contribute to the pathogenesis of renal interstitial fibrosis in diabetes. Transforming growth factor (TGF)-beta1 is one of a group of pro-fibrotic cytokines and growth factors, which have been associated with the development of interstitial fibrosis. The aim of the current study was to examine the effect of insulin on the generation of TGF-beta1 by proximal tubular cells. HK-2 cells were grown to confluence in the absence of insulin, and serum deprived for 48 hours before all experimental manipulations. Addition of insulin (5 microg/ml) to the culture medium led to a time-dependent increase in TGF-beta1 concentration in the cell culture supernatant, and increased incorporation of radiolabeled amino acids into TGF-beta1 suggestive of de novo TGF-beta1 protein synthesis. Addition of insulin did not alter TGF-beta1 mRNA expression as assessed by reverse transcriptase-polymerase chain reaction or Northern analysis. Insulin-induced increase in TGF-beta1 concentration was not abrogated by actinomycin D, however, stimulation by insulin, in the presence of cycloheximide led to a dose-dependent decrease in TGF-beta1 production. Addition of insulin had no effect on TGF-beta1 mRNA stability as assessed by actinomycin D chase, but led to increased binding of a cytoplasmic protein to a putative stem loop structure in the 5'-UTR of TGF-beta1 mRNA, previously implicated in the posttranscriptional control of TGF-beta1 synthesis. To address the functional significance of insulin-induced alteration in TGF-beta1 synthesis, we examined its effect on matrix turnover. Insulin stimulated type IV collagen gene expression and an increase in the concentrations of the type IV collagen laid down in the extracellular matrix. This increase in type IV collagen was abrogated when cells were stimulated by insulin in the presence of an anti-TGF-beta1-blocking antibody. In conclusion the data demonstrate that insulin may directly alter the production of TGF-beta1 by renal proximal tubular cells by a posttranscriptional mechanism, and that this may have implications for the increase in extracellular matrix that accompanies diabetic nephropathy.

Endotoxin (LPS) stimulates 4E-BP1/PHAS-I phosphorylation in macrophages

INTRODUCTION

Translational control of cytokine production in endotoxin (LPS)-stimulated macrophages is poorly characterized but likely important. An early step in protein translation is engagement of mRNA by eukaryotic initiation factor 4E (eIF-4E). Translation initiation can be prevented by small 4E-binding proteins (4E-BP1 or PHAS-I) which must be phosphorylated in order to disengage eIF-4E. We examined whether LPS alters 4E-BP1 phosphorylation in macrophages.

MATERIALS AND METHODS

Elicited rat peritoneal macrophages and Raw 264.7 macrophages were treated with signal transduction inhibitors and then LPS. Cells were harvested and equal protein amounts were electrophoresed (SDS-PAGE). Western blots (WB) were developed with 4E-BP1 antibody. Alternatively cell lysates were exposed to 7-methyl GTP Sepharose beads in order to isolate the cap-binding protein eIF-4E. The relative amounts of 4E-BP1 associated with eIF-4E were then determined by WB.

RESULTS

Macrophage 4E-BP1 is phosphorylated upon stimulation by LPS as evidenced by the appearance of a more slowly migrating gamma (hyperphosphorylated) band on gel electrophoresis. Inhibition of both the p42/p44 MAPK pathway (PD 98059) and the p38 MAPK pathway (SB 203580) failed to alter LPS-induced 4E-BP1 phosphorylation. Rapamycin (FRAP/mTOR inhibitor) blocked 4E-BP1 phosphorylation causing a predominance of the alpha (hypophosphorylated) band. This was confirmed further by 7-methyl-GTP Sepharose isolation of eIF-4E with which 4E-BP1 coprecipitates.

CONCLUSION

LPS stimulates 4E-BP1 phosphorylation in macrophages through FRAP/mTOR signaling. This pathway may contribute to the translational control of cytokine gene expression in macrophages.

Branched chain amino acids activate messenger ribonucleic acid translation regulatory proteins in human skeletal muscle, and glucocorticoids blunt this action

Branched chain amino acids (BCAA) are particularly effective anabolic agents. Recent in vitro studies suggest that amino acids, particularly leucine, activate a signaling pathway that enhances messenger ribonucleic acid translation and protein synthesis. The physiological relevance of these findings to normal human physiology is uncertain. We examined the effects of BCAA on the phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (eIF4E-BP1) and ribosomal protein S6 kinase (p70(S6K)) in skeletal muscle of seven healthy volunteers. We simultaneously examined whether BCAA affect urinary nitrogen excretion and forearm skeletal muscle protein turnover and whether the catabolic action of glucocorticoids could be mediated in part by inhibition of the action of BCAA on the protein synthetic apparatus. BCAA infusion decreased urinary nitrogen excretion (P < 0.02), whole body phenylalanine flux (P < 0.02), plasma phenylalanine concentration (P < 0.001), and improved forearm phenylalanine balance (P = 0.03). BCAA also increased the phosphorylation of both eIF4E-BP1 (P < 0.02) and p70(S6K) (P < 0.03), consistent with an action to activate the protein synthetic apparatus. Dexamethasone increased plasma phenylalanine concentration (P < 0.001), prevented the BCAA-induced anabolic shift in forearm protein balance, and inhibited their action on the phosphorylation of p70(S6K). We conclude that in human skeletal muscle BCAA act directly as nutrient signals to activate messenger ribonucleic acid translation and potentiate protein synthesis. Glucocorticoids interfere with this action, and that may be part of the mechanism by which they promote net protein catabolism in muscle.

Rapamycin-insensitive regulation of 4e-BP1 in regenerating rat liver

In cultured cells, growth factor-induced phosphorylation of two translation modulators, p70 S6 kinase and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), is blocked by nanomolar concentrations of the immunosuppressant rapamycin. Rapamycin also attenuates liver regeneration after partial hepatectomy, but it is not known if this growth-suppressive effect is due to dephosphorylation of p70 S6 kinase and/or 4E-BP1. We found that partial hepatectomy induced a transient increase in liver p70 S6 kinase activity and 4E-BP1 phosphorylation as compared with sham-operated rats. The amount of p70 S6 kinase protein in regenerating liver did not increase, but active kinase from partially hepatectomized animals was highly phosphorylated. Phosphorylated 4E-BP1 from regenerating liver was unable to form an inhibitory complex with initiation factor 4E. Rapamycin blocked the activation of p70 S6 kinase in response to partial hepatectomy in a dose-dependent manner, but 4E-BP1 phosphorylation was not inhibited. By contrast, functional phosphorylation of 4E-BP1 induced by injection of cycloheximide or growth factors was partially reversed by the drug. The mammalian target of rapamycin (mTOR) has been proposed to directly phosphorylate 4E-BP1. Western blot analysis using phospho-specific antibodies showed that phosphorylation of Thr-36/45 and Ser-64 increased in response to partial hepatectomy in a rapamycin-resistant manner. Thus, rapamycin inhibits p70 S6 kinase activation and liver regeneration, but not functional phosphorylation of 4E-BP1, in response to partial hepatectomy. These results indicate that the effect of rapamycin on 4E-BP1 function in vivo can be significantly different from its effect in cultured cells.

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.

Participation of ATM in insulin signalling through phosphorylation of eIF-4E-binding protein 1

One of the critical responses to insulin treatment is the stimulation of protein synthesis through induced phosphorylation of the eIF-4E-binding protein 1 (4E-BP1), and the subsequent release of the translation initiation factor, eIF-4E. Here we report that ATM, the protein product of the ATM gene that is mutated in the disease ataxia telangiectasia, phosphorylates 4E-BP1 at Ser 111 in vitro and that insulin treatment induces phosphorylation of 4E-BP1 at Ser 111 in vivo in an ATM-dependent manner. In addition, insulin treatment of cells enhances the specific kinase activity of ATM. Cells lacking ATM kinase activity exhibit a significant decrease in the insulin-induced dissociation of 4E-BP1 from eIF-4E. These results suggest an unexpected role for ATM in an insulin-signalling pathway that controls translation initiation. Through this mechanism, a lack of ATM activity probably contributes to some of the metabolic abnormalities, such as poor growth and insulin resistance, reported in ataxia telangiectasia cells and patients with ataxia telangiectasia.

Assessment of cell-signaling pathways in the regulation of mammalian target of rapamycin (mTOR) by amino acids in rat adipocytes

Enhanced phosphorylation of the ribosomal protein s6 kinase, p70(s6k), and the translational repressor, 4E-BP1, are associated with either insulin-induced or amino acid-induced protein synthesis. Hyperphosphorylation of p70(s6k) and 4E-BP1 in response to insulin or amino acids is mediated through the mammalian target of rapamycin (mTOR). In several cell lines, mTOR or its downstream targets can be regulated by phosphatidylinositol (PI) 3-kinase; protein kinases A, B, and C; heterotrimeric G-proteins; a PD98059-sensitive kinase or calcium; as well as by amino acids. Regulation by amino acids appears to involve detection of levels of charged t-RNA or t-RNA synthetase activity and is sensitive to inhibition by amino acid alcohols. In the present article, however, we show that the rapamycin-sensitive regulation of 4E-BP1 and p70(s6k) in freshly isolated rat adipocytes is not inhibited by either L-leucinol or L-histidinol. This finding is in agreement with other recent studies from our laboratory suggesting that the mechanism by which amino acids regulate mTOR in freshly isolated adipocytes may be different than the mechanism found in a number of cell lines. Therefore we investigated the possible role of growth factor-regulated and G-protein-regulated signaling pathways in the rapamycin-sensitive, amino acid alcohol-insensitive actions of amino acids on 4E-BP1 phosphorylation. We found, in contrast to previously published results using 3T3-L1 adipocytes or other cell lines, that the increase in 4E-BP1 phosphorylation promoted by amino acids was insensitive to agents that regulate protein kinase A, mobilize calcium, or inhibit protein kinase C. Furthermore, amino acid-induced 4E-BP1 phosphorylation was not blocked by pertussis toxin nor was it mimicked by the G-protein agonists fluoroaluminate or MAS-7. However, amino acids failed to activate either PI 3-kinase, protein kinase B, or mitogen-activated protein kinase and failed to promote tyrosine phosphorylation of cellular proteins, similar to observations made using cell lines. In summary, amino acids appear to use an amino acid alcohol-insensitive mechanism to regulate mTOR in freshly isolated adipocytes. This mechanism is independent of cell-signaling pathways implicated in the regulation of mTOR or its downstream targets in other cells. Overall, our study emphasizes the need for caution when extending results obtained using established cell lines to the differentiated nondividing cells found in most tissues.

Activation of mRNA translation in rat cardiac myocytes by insulin involves multiple rapamycin-sensitive steps

Insulin acutely activates protein synthesis in ventricular cardiomyocytes from adult rats. In this study, we have established the methodology for studying the regulation of the signaling pathways and translation factors that may be involved in this response and have examined the effects of acute insulin treatment on them. Insulin rapidly activated the 70-kDa ribosomal S6 kinase (p70 S6k), and this effect was inhibited both by rapamycin and by inhibitors of phosphatidylinositol 3-kinase. The activation of p70 S6k is mediated by a signaling pathway involving the mammalian target of rapamycin (mTOR), which also modulates other translation factors. These include the eukaryotic initiation factor (eIF) 4E binding proteins (4E-BPs) and eukaryotic elongation factor 2 (eEF2). Insulin caused phosphorylation of 4E-BP1 and induced its dissociation from eIF4E, and these effects were also blocked by rapamycin. Concomitant with this, insulin increased the binding of eIF4E to eIF4G. Insulin also activated protein kinase B (PKB), which may lie upstream of p70 S6k and 4E-BP1, with the activation of the different isoforms being in the order alpha>beta>gamma. Insulin also caused inhibition of glycogen synthase kinase 3, which lies downstream of PKB, and of eEF2 kinase. The phosphorylation of eEF2 itself was also decreased by insulin, and this effect and the inactivation of eEF2 kinase were attenuated by rapamycin. The activation of overall protein synthesis by insulin in cardiomyocytes was substantially inhibited by rapamycin (but not by inhibitors of other specific signaling pathways, e.g., mitogen-activated protein kinase), showing that signaling events linked to mTOR play a major role in the control of translation by insulin in this cell type.

alpha(1A) adrenergic receptor induces eukaryotic initiation factor 4E-binding protein 1 phosphorylation via a Ca(2+)-dependent pathway independent of phosphatidylinositol 3-kinase/Akt

Phosphorylation of the translation repressor eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) is thought to be partly responsible for increased protein synthesis induced by growth factors. This study investigated the effect of a G(q)-coupled receptor on protein synthesis and the phosphorylation state and function of 4E-BP1 in Rat-1 fibroblasts expressing the human alpha(1A) adrenergic receptor. Treatment of cells with phenylephrine (PE), a specific alpha(1) adrenergic receptor agonist, increased protein synthesis and induced the phosphorylation of 4E-BP1 and its release from translation initiation factor 4E. Although the PE-induced phosphorylation of 4E-BP1 was blocked by the phosphatidylinositol 3-kinase inhibitor LY294002, neither phosphatidylinositol 3-kinase nor Akt, its downstream effector, is activated in cells treated with PE (Ballou, L. M., Cross, M. E., Huang, S., McReynolds, E. M., Zhang, B. X., and Lin, R. Z., J. Biol. Chem. 275, 4803-4809). The effect of PE on 4E-BP1 phosphorylation was also abolished in cells depleted of intracellular Ca(2+) and in cells pretreated with calmodulin antagonists. By contrast, phosphorylation of 4E-BP1 still occurred in cells in which the Ca(2+)- and diacylglycerol-dependent isoforms of protein kinase C were down-regulated by prolonged exposure to a phorbol ester. We conclude that activation of the alpha(1A) adrenergic receptor in Rat-1 fibroblasts leads to phosphorylation of 4E-BP1 via a pathway that is Ca(2+)- and calmodulin-dependent. Phosphatidylinositol 3-kinase, Akt, and phorbol ester-sensitive protein kinase C isoforms do not appear to be required in this signaling pathway.

Mammalian target of rapamycin is a direct target for protein kinase B: identification of a convergence point for opposing effects of insulin and amino-acid deficiency on protein translation

Growth factor induced activation of phosphoinositide 3-kinase and protein kinase B (PKB) leads to increased activity of the mammalian target of rapamycin (mTOR). This subsequently leads to increased phosphorylation of eIF4E binding protein-1 (4EBP1) and activation of p70 ribosomal S6 protein kinase (p70(S6K)), both of which are important steps in the stimulation of protein translation. The stimulation of translation is attenuated in cells deprived of amino acids and this is associated with the attenuation of 4EBP1 phosphorylation and p70(S6K) activation. It has been suggested that PKB regulates mTOR function by phosphorylation although direct phosphorylation of mTOR by PKB has not been demonstrated previously. In the present work, we have found that PKB directly phosphorylates mTOR and, using phosphospecific antibodies, we have shown this phosphorylation occurs at Ser(2448). Insulin also induces phosphorylation on Ser(2448) and this effect is blocked by wortmannin but not rapamycin, consistent with the effect being mediated by PKB. Amino-acid starvation rapidly attenuated the reactivity of the Ser(2448) phosphospecific antibody with mTOR and this could not be restored by either insulin stimulation of cells or incubation with PKB in vitro. Our findings demonstrate that mTOR is a direct target for PKB and support the conclusion that regulation of phosphorylation of Ser(2448) is a point of convergence for the counteracting regulatory effects of growth factors and amino acid levels.

Mammalian target of rapamycin is a direct target for protein kinase B: identification of a convergence point for opposing effects of insulin and amino-acid deficiency on protein translation

Growth factor induced activation of phosphoinositide 3-kinase and protein kinase B (PKB) leads to increased activity of the mammalian target of rapamycin (mTOR). This subsequently leads to increased phosphorylation of eIF4E binding protein-1 (4EBP1) and activation of p70 ribosomal S6 protein kinase (p70(S6K)), both of which are important steps in the stimulation of protein translation. The stimulation of translation is attenuated in cells deprived of amino acids and this is associated with the attenuation of 4EBP1 phosphorylation and p70(S6K) activation. It has been suggested that PKB regulates mTOR function by phosphorylation although direct phosphorylation of mTOR by PKB has not been demonstrated previously. In the present work, we have found that PKB directly phosphorylates mTOR and, using phosphospecific antibodies, we have shown this phosphorylation occurs at Ser(2448). Insulin also induces phosphorylation on Ser(2448) and this effect is blocked by wortmannin but not rapamycin, consistent with the effect being mediated by PKB. Amino-acid starvation rapidly attenuated the reactivity of the Ser(2448) phosphospecific antibody with mTOR and this could not be restored by either insulin stimulation of cells or incubation with PKB in vitro. Our findings demonstrate that mTOR is a direct target for PKB and support the conclusion that regulation of phosphorylation of Ser(2448) is a point of convergence for the counteracting regulatory effects of growth factors and amino acid levels.

Nutrients differentially regulate multiple translation factors and their control by insulin

Eukaryotic initiation factor eIF2B and eukaryotic elongation factor eEF2 each mediate regulatory steps important for the overall regulation of mRNA translation in mammalian cells and are activated by insulin. Here, we demonstrate that their activation by insulin requires the presence, in the medium in which the cells are maintained, of both amino acids and glucose: insulin only induced activation of eIF2B and the dephosphorylation of eEF2 when cells were exposed to both types of nutrient. Other translational regulators, e.g. the 70 kDa ribosomal protein S6 kinase (p70 S6 kinase) and the eIF4E binding protein 1, 4E-BP1, are also regulated by insulin but their control does not require glucose, only amino acids. The effects of nutrients on the activation of eIF2B do not reflect changes in the phosphorylation of eIF2 (and, by inference, operation of a kinase analogous to yeast Gcn2p), or a requirement for nutrients for inactivation of glycogen synthase kinase-3 or dephosphorylation of eIF2B. Nutrients did not affect the ability of insulin to activate protein kinase B. These data show that activation by insulin of p70 S6 kinase, which modulates the translation of specific mRNAs, depends on the availability of amino acids whereas regulation of factors involved in overall activation of translation (eIF2B, eEF2) requires both amino acids and glucose. These results add substantially to the emerging evidence that nutrients themselves modulate functions of mammalian cells and indicate that (i) nutrients modulate the activation of eIF2B and eEF2 through as-yet unidentified mechanisms and (ii) regulation of p70 S6 kinase and 4E-BP1 by insulin requires other inputs in addition to protein kinase B.