Control of PHAS-I by insulin in 3T3-L1 adipocytes. Synthesis, degradation, and phosphorylation by a rapamycin-sensitive and mitogen-activated protein kinase-independent pathway

Activation of ERK and p38 Reduces AZD8055-Mediated Inhibition of Protein Synthesis in Hepatocellular Carcinoma HepG2 Cell Line

Protein synthesis is important for maintaining cellular homeostasis under various stress responses. In this study, we screened an anticancer drug library to select compounds with translational repression functions. AZD8055, an ATP-competitive mechanistic target of rapamycin complex 1/2 (mTORC1/2) inhibitor, was selected as a translational suppressor. AZD8055 inhibited protein synthesis in mouse embryonic fibroblasts and hepatocellular carcinoma HepG2 cells. Extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) were activated during the early phase of mTORC1/2 inhibition by AZD8055 treatment. Combined treatment of AZD8055 with the MAPK kinase1/2 (MEK1/2) inhibitor refametinib or the p38 inhibitor SB203580 markedly decreased translation in HepG2 cells. Thus, the inhibition of ERK1/2 or p38 may enhance the efficacy of AZD8055-mediated inhibition of protein synthesis. In addition, AZD8055 down-regulated the phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), and AZD8055-induced phosphorylation of ERK1/2 and p38 had no effect on phosphorylation status of 4E-BP1. Interestingly, AZD8055 modulated the 4E-BP1 mRNA pool by up-regulating ERK1/2 and p38 pathways. Together, these results suggest that AZD8055-induced activation of MAPKs interferes with inhibition of protein synthesis at an early stage of mTORC1/2 inhibition, and that it may contribute to the development of resistance to mTORC1/2 inhibitors.

Culprits or consequences: Understanding the metabolic dysregulation of muscle in diabetes

The prevalence of type 2 diabetes (T2D) continues to rise despite the amount of research dedicated to finding the culprits of this debilitating disease. Skeletal muscle is arguably the most important contributor to glucose disposal making it a clear target in insulin resistance and T2D research. Within skeletal muscle there is a clear link to metabolic dysregulation during the progression of T2D but the determination of culprits vs consequences of the disease has been elusive. Emerging evidence in skeletal muscle implicates influential cross talk between a key anabolic regulatory protein, the mammalian target of rapamycin (mTOR) and its associated complexes (mTORC1 and mTORC2), and the well-described canonical signaling for insulin-stimulated glucose uptake. This new understanding of cellular signaling crosstalk has blurred the lines of what is a culprit and what is a consequence with regard to insulin resistance. Here, we briefly review the most recent understanding of insulin signaling in skeletal muscle, and how anabolic responses favoring anabolism directly impact cellular glucose disposal. This review highlights key cross-over interactions between protein and glucose regulatory pathways and the implications this may have for the design of new therapeutic targets for the control of glucoregulatory function in skeletal muscle.

Molecular mechanisms of cancer cachexia‑induced muscle atrophy (Review)

Muscle atrophy is a severe clinical problem involving the loss of muscle mass and strength that frequently accompanies the development of numerous types of cancer, including pancreatic, lung and gastric cancers. Cancer cachexia is a multifactorial syndrome characterized by a continuous decline in skeletal muscle mass that cannot be reversed by conventional nutritional therapy. The pathophysiological characteristic of cancer cachexia is a negative protein and energy balance caused by a combination of factors, including reduced food intake and metabolic abnormalities. Numerous necessary cellular processes are disrupted by the presence of abnormal metabolites, which mediate several intracellular signaling pathways and result in the net loss of cytoplasm and organelles in atrophic skeletal muscle during various states of cancer cachexia. Currently, the clinical morbidity and mortality rates of patients with cancer cachexia are high. Once a patient enters the cachexia phase, the consequences are difficult to reverse and the treatment methods for cancer cachexia are very limited. The present review aimed to summarize the recent discoveries regarding the pathogenesis of cancer cachexia-induced muscle atrophy and provided novel ideas for the comprehensive treatment to improve the prognosis of affected patients.

From Gene to Protein-How Bacterial Virulence Factors Manipulate Host Gene Expression During Infection

Bacteria evolved many strategies to survive and persist within host cells. Secretion of bacterial effectors enables bacteria not only to enter the host cell but also to manipulate host gene expression to circumvent clearance by the host immune response. Some effectors were also shown to evade the nucleus to manipulate epigenetic processes as well as transcription and mRNA procession and are therefore classified as nucleomodulins. Others were shown to interfere downstream with gene expression at the level of mRNA stability, favoring either mRNA stabilization or mRNA degradation, translation or protein stability, including mechanisms of protein activation and degradation. Finally, manipulation of innate immune signaling and nutrient supply creates a replicative niche that enables bacterial intracellular persistence and survival. In this review, we want to highlight the divergent strategies applied by intracellular bacteria to evade host immune responses through subversion of host gene expression via bacterial effectors. Since these virulence proteins mimic host cell enzymes or own novel enzymatic functions, characterizing their properties could help to understand the complex interactions between host and pathogen during infections. Additionally, these insights could propose potential targets for medical therapy.

Protein malnutrition prevents heat conservation induced by amino acid infusion during general anesthesia in rats

The intravenous administration of an amino acid (AA) mixture during general anesthesia reduces anesthesia-induced hypothermia. AA-induced skeletal muscle protein synthesis and thermogenesis play important roles in the antihypothermic effects of AAs. We hypothesized that a preanesthetic dietary protein deficiency impairs the antihypothermic effects of AAs during general anesthesia due to a reduction in thermogenesis caused by a decrease in muscle protein synthesis. Sprague-Dawley rats were divided into 4 groups: fed a control diet plus saline (CON-SAL) or the AA mixture (CON-AA), and fed a protein-free diet plus saline (PF-SAL) or the AA mixture (PF-AA). SAL solution or AA mixture solution was infused for 180 minutes during sevoflurane anesthesia, and rectal temperatures were measured. Rectal temperatures were significantly higher in the CON-AA group than in the PF-AA group 90 to 180 minutes after initiating the intravenous infusion of the test solutions. There was no significant difference between the PF-SAL and PF-AA groups. Plasma insulin concentrations were significantly higher in the CON-AA group than in the PF-AA group (P < .05). The phosphorylation states of protein kinase B, mammalian target of rapamycin, and eukaryotic initiation factor 4E-binding protein 1 were significantly greater in the CON-AA group than in the PF-AA group (P < .05, P < .05, and P < .01, respectively). Our results indicated that a dietary protein deficiency before general anesthesia impaired the antihypothermic effects of an AA mixture infusion during general anesthesia by decreasing muscle protein synthesis through the insulin-stimulated phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin complex 1 signaling pathway followed by metabolic heat production.

Ergothioneine-induced neuronal differentiation is mediated through activation of S6K1 and neurotrophin 4/5-TrkB signaling in murine neural stem cells

The promotion of neurogenesis is considered to be an effective therapeutic strategy for neuropsychiatric disorders because impairment of neurogenesis is associated with the onset and progression of these disorders. We have previously demonstrated that orally ingested ergothioneine (ERGO), a naturally occurring antioxidant and hydrophilic amino acid, promotes neurogenesis in the hippocampal dentate gyrus (DG) with its abundant neural stem cells (NSCs) and exerts antidepressant-like effects in mice. Independent of its antioxidant activities, ERGO induces in cultured NSCs this differentiation through induction of the basic helix-loop-helix transcription factor Math1. However, the upstream signaling of Math1 in the mechanisms underlying ERGO-induced neuronal differentiation remains unclear. The purpose of the present study was to elucidate the upstream signaling with the aim of discovering novel targets for the treatment of neuropsychiatric disorders. We focused on neurotrophic factor signaling, as it is important for the promotion of neurogenesis and the induction of antidepressant effects. We also focused on the signaling of the mammalian target of rapamycin (mTOR) complex 1 (mTORC1), a known amino acid sensor, and the members of this signaling pathway, mTOR and p70 ribosomal protein S6 kinase 1 (S6K1). Exposure of cultured NSCs to ERGO significantly increased the expression of phosphorylated S6K1 (p-S6K1) at Thr389 in only 1 h, of phosphorylated mTOR (p-mTOR) in 6 h, and of the gene product of neurotrophin 4/5 (NT5) which activates tropomyosin receptor kinase B (TrkB) in 24 h. ERGO increased the population of βIII-tubulin-positive neurons, and this effect was suppressed by the inhibitors of S6K1 (PF4708671), mTORC1 (rapamycin), and TrkB (GNF5837). Oral administration of ERGO to mice significantly increased in the DG the expression of p-S6K1 at Thr389, the gene product of NT5, and phosphorylated TrkB but not that of p-mTOR. Thus, neuronal differentiation of NSCs induced by ERGO is mediated, at least in part, through phosphorylation of S6K1 at Thr389 and subsequent activation of TrkB signaling through the induction of NT5. Thus, S6K1 and NT5 might be promising target molecules for the treatment of neuropsychiatric disorders.

The Protozoan Parasite Toxoplasma gondii Selectively Reprograms the Host Cell Translatome

The intracellular parasite Toxoplasma gondii promotes infection by targeting multiple host cell processes; however, whether it modulates mRNA translation is currently unknown. Here, we show that infection of primary murine macrophages with type I or II T. gondii strains causes a profound perturbation of the host cell translatome. Notably, translation of transcripts encoding proteins involved in metabolic activity and components of the translation machinery was activated upon infection. In contrast, the translational efficiency of mRNAs related to immune cell activation and cytoskeleton/cytoplasm organization was largely suppressed. Mechanistically, T. gondii bolstered mechanistic target of rapamycin (mTOR) signaling to selectively activate the translation of mTOR-sensitive mRNAs, including those with a 5'-terminal oligopyrimidine (5' TOP) motif and those encoding mitochondrion-related proteins. Consistent with parasite modulation of host mTOR-sensitive translation to promote infection, inhibition of mTOR activity suppressed T. gondii replication. Thus, selective reprogramming of host mRNA translation represents an important subversion strategy during T. gondii infection.

Itraconazole exerts its anti-melanoma effect by suppressing Hedgehog, Wnt, and PI3K/mTOR signaling pathways

Malignant melanoma is the deadliest form of all skin cancers. Itraconazole, a commonly used systemic antifungal drug, has been tested for its anti-tumor effects on basal cell carcinoma, prostate cancer, and non-small cell lung cancer. Whether itraconazole has any specific anti-tumor effect on melanoma remains unknown. However, the goal of this study is to investigate the effect of itraconazole on melanoma and to reveal some details of its underlying mechanism. In the in vivo xenograft mouse model, we find that itraconazole can inhibit melanoma growth and extend the survival of melanoma xenograft mice, compared to non-itraconazole-treated mice. Also, itraconazole can significantly inhibit cell proliferation, as demonstrated by Ki-67 staining in itraconazole-treated tumor tissues. In in vitro, we show that itraconazole inhibits the proliferation and colony formation of both SK-MEL-28 and A375 human melanoma cells. Moreover, we demonstrate that itraconazole significantly down-regulates Gli-1, Gli-2, Wnt3A, β-catenin and cyclin D1, while it up-regulates Gli-3 and Axin-1, indicating potent inhibitory effects of itraconazole on Hedgehog (Hh) and Wnt signaling pathways. Furthermore, itraconazole significantly suppresses the PI3K/mTOR signaling pathway – indicated by the down-regulated phosphorylation of p70S6K, 4E-BP1 and AKT – but has no effect on the phosphorylation of MEK or ERK. Our data suggest that itraconazole inhibits melanoma growth through an interacting regulatory network that includes Hh, Wnt, and PI3K/mTOR signaling pathways. These results suggest that this agent has several potent anti-melanoma features and may be useful in the synergesis of other anti-cancer drugs via blockage of the Hh, Wnt and PI3K/mTOR signaling pathways.

Phenylalanine regulates initiation of digestive enzyme mRNA translation in pancreatic acinar cells and tissue segments in dairy calves

As new nutritional strategies for ruminant are designed to change production efficiency by improving the supply of rumen protect protein, lipid, and even starch, the digestive system must fit to utilize these increased nutrient supplies, especially the pancreas. The objective of this study was to investigate the effects of phenylalanine (Phe) on digestive enzymes synthesis or secretion and cellular signaling in pancreatic acinar (PA) cells of dairy calves. The PA cells isolated from fresh pancreas of dairy calves, and cultured in completed RIPA 1640 medium with no fetal serum but 0, 0.15 and 0.45 mM Phe at 37°C in CO₂ incubator for 120 min. The pancreatic tissue segments (PTS) was cut approximately 2 × 2 mm from the fresh pancreas, and incubated in oxygenated Krebs-Ringer bicarbonate (KRB) buffer containing 0 or 0.35 mM Phe at 39°C for 180 min, and the samples were collected every 60 min after incubation. In PA cells, Phe increased (P < 0.05) the α-amylase secretion and mRNA expression, the phosphorylation of ribosomal protein S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4EBP1). In PTS, the Phe increased (P < 0.05) α-amylase and trypsin synthesis, secretion and mRNA expression, as well as the phosphorylation of S6K1 and 4EBP1. Conclusively, these results suggested that Phe regulates the synthesis or secretion of α-amylase, trypsin and lipase through mRNA translation initiation factors – S6K1 and 4EBP1.

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.

Regulation of mTORC1 by lysosomal calcium and calmodulin

Blockade of lysosomal calcium release due to lysosomal lipid accumulation has been shown to inhibit mTORC1 signaling. However, the mechanism by which lysosomal calcium regulates mTORC1 has remained undefined. Herein we report that proper lysosomal calcium release through the calcium channel TRPML1 is required for mTORC1 activation. TRPML1 depletion inhibits mTORC1 activity, while overexpression or pharmacologic activation of TRPML1 has the opposite effect. Lysosomal calcium activates mTORC1 by inducing association of calmodulin (CaM) with mTOR. Blocking the interaction between mTOR and CaM by antagonists of CaM significantly inhibits mTORC1 activity. Moreover, CaM is capable of stimulating the kinase activity of mTORC1 in a calcium-dependent manner in vitro. These results reveal that mTOR is a new type of CaM-dependent kinase, and TRPML1, lysosomal calcium and CaM play essential regulatory roles in the mTORC1 signaling pathway.

mTORC1 signalling and eIF4E/4E-BP1 translation initiation factor stoichiometry influence recombinant protein productivity from GS-CHOK1 cells

Many protein-based biotherapeutics are produced in cultured Chinese hamster ovary (CHO) cell lines. Recent reports have demonstrated that translation of recombinant mRNAs and global control of the translation machinery via mammalian target of rapamycin (mTOR) signalling are important determinants of the amount and quality of recombinant protein such cells can produce. mTOR complex 1 (mTORC1) is a master regulator of cell growth/division, ribosome biogenesis and protein synthesis, but the relationship between mTORC1 signalling, cell growth and proliferation and recombinant protein yields from mammalian cells, and whether this master regulating signalling pathway can be manipulated to enhance cell biomass and recombinant protein production (rPP) are not well explored. We have investigated mTORC1 signalling and activity throughout batch culture of a panel of sister recombinant glutamine synthetase-CHO cell lines expressing different amounts of a model monoclonal IgG4, to evaluate the links between mTORC1 signalling and cell proliferation, autophagy, recombinant protein expression, global protein synthesis and mRNA translation initiation. We find that the expression of the mTORC1 substrate 4E-binding protein 1 (4E-BP1) fluctuates throughout the course of cell culture and, as expected, that the 4E-BP1 phosphorylation profiles change across the culture. Importantly, we find that the eIF4E/4E-BP1 stoichiometry positively correlates with cell productivity. Furthermore, eIF4E amounts appear to be co-regulated with 4E-BP1 amounts. This may reflect a sensing of either change at the mRNA level as opposed to the protein level or the fact that the phosphorylation status, as well as the amount of 4E-BP1 present, is important in the co-regulation of eIF4E and 4E-BP1.

Reduction in amino-acid-induced anti-hypothermic effects during general anesthesia in ovariectomized rats with progesterone replacement

PURPOSE

The aim of the present study was to determine whether the ovarian hormones, estrogen and progesterone, had different influences on amino-acid-induced anti-hypothermic effects during general anesthesia.

METHODS

Ovariectomized Sprague-Dawley female rats were divided into four groups: those administered 17β-estradiol plus saline or an amino acid mixture (E2-Sal and E2-AA, respectively) and progesterone plus saline or an amino acid mixture (P-Sal and P-AA, respectively). Five weeks after ovariectomy, rats were given either E2 or P and then administered either Sal or AA solution for 180 min during anesthesia with sevoflurane. Rectal temperatures were measured.

RESULTS

Rectal temperatures were significantly higher in the E2-AA group than in the E2-Sal group 165 and 180 min after initiating the infusion of the test solutions. However, no significant differences were observed between the P-treated groups. The phosphorylation of 4E-BP1 and S6K1 was significantly greater in the E2-AA group than in the E2-Sal group (P < 0.05, P < 0.001, respectively). In contrast, the phosphorylation of 4E-BP1 was significantly lower in the P-AA group than in the P-Sal group (P < 0.001).

CONCLUSIONS

These results suggest that progesterone reduces amino-acid-induced anti-hypothermic effects during general anesthesia.

The extracellular matrix and mitogenic growth factors control G1 phase cyclins and cyclin-dependent kinase inhibitors

Most cell types require both mitogenic growth factors and cell adhesion to the extracellular matrix (ECM) for proliferation. Over the past few years, these growth requirements have received renewed attention and can now be explained by studies showing that signals provided by growth factors and the ECM are jointly required to stimulate the cyclin-dependent kinases (CDKs) that mediate cell-cycle progression through G1 phase. This article summarizes our current understanding of the control of G1 cyclins and CDK inhibitors by growth factors and the ECM. In addition, we have highlighted one or two signal-transduction pathways that presently seem closely linked to regulation of the G1 phase cyclin-CDK system.

Endothelin-1-dependent signaling pathways in the myocardium

Endothelin-1 (ET-1) is a locally acting vasoactive peptide that also has profound effects on the contractile properties and growth of the cardiac myocyte. Binding of ET-1 to its transmembrane heptahelical receptors activates G proteins of the G(q) and G(i) classes. Activation of G(q) stimulates hydrolysis of phosphatidylinositol-4,5-bisphosphate, and the diacylglycerol thus formed stimulates protein kinase C. Subsequently, the protein kinase Raf is activated and this leads to activation of the extracellular signal-regulated protein kinase (ERK) subfamily of mitogen-activated protein kinases. Activation of G(i) counteracts β-adrenoceptor-mediated increases in cAMP concentrations. We have attempted to rationalize the established physiological consequences of ET-1 agonism in the cardiac myocyte (that is, on contraction and growth) in terms of activation of these signaling pathways.

Translational homeostasis via the mRNA cap-binding protein, eIF4E

Translational control of gene expression plays a key role in many biological processes. Consequently, the activity of the translation apparatus is under tight homeostatic control. eIF4E, the mRNA 5' cap-binding protein, facilitates cap-dependent translation and is a major target for translational control. eIF4E activity is controlled by a family of repressor proteins, termed 4E-binding proteins (4E-BPs). Here, we describe the surprising finding that despite the importance of eIF4E for translation, a drastic knockdown of eIF4E caused only minor reduction in translation. This conundrum can be explained by the finding that 4E-BP1 is degraded in eIF4E-knockdown cells. Hypophosphorylated 4E-BP1, which binds to eIF4E, is degraded, whereas hyperphosphorylated 4E-BP1 is refractory to degradation. We identified the KLHL25-CUL3 complex as the E3 ubiquitin ligase, which targets hypophosphorylated 4E-BP1. Thus, the activity of eIF4E is under homeostatic control via the regulation of the levels of its repressor protein 4E-BP1 through ubiquitination.

Leishmania repression of host translation through mTOR cleavage is required for parasite survival and infection

The protozoan parasite Leishmania alters the activity of its host cell, the macrophage. However, little is known about the effect of Leishmania infection on host protein synthesis. Here, we show that the Leishmania protease GP63 cleaves the mammalian/mechanistic target of rapamycin (mTOR), a serine/threonine kinase that regulates the translational repressor 4E-BP1. mTOR cleavage results in the inhibition of mTOR complex 1 (mTORC1) and concomitant activation of 4E-BP1 to promote Leishmania proliferation. Consistent with these results, pharmacological activation of 4E-BPs with rapamycin, results in a dramatic increase in parasite replication. In contrast, genetic deletion of 4E-BP1/2 reduces parasite load in macrophages ex vivo and decreases susceptibility to cutaneous leishmaniasis in vivo. The parasite resistant phenotype of 4E-BP1/2 double-knockout mice involves an enhanced type I IFN response. This study demonstrates that Leishmania evolved a survival mechanism by activating 4E-BPs, which serve as major targets for host translational control.

Reduction in ribosomal protein synthesis is sufficient to explain major effects on ribosome production after short-term TOR inactivation in Saccharomyces cerevisiae

Ribosome synthesis depends on nutrient availability, sensed by the target of rapamycin (TOR) signaling pathway in eukaryotes. TOR inactivation affects ribosome biogenesis at the level of rRNA gene transcription, expression of ribosomal proteins (r-proteins) and biogenesis factors, preribosome processing, and transport. Here, we demonstrate that upon TOR inactivation, levels of newly synthesized ribosomal subunits drop drastically before the integrity of the RNA polymerase I apparatus is severely impaired but in good correlation with a sharp decrease in r-protein production. Inhibition of translation by cycloheximide mimics the rRNA maturation defect observed immediately after TOR inactivation. Both cycloheximide addition and the depletion of individual r-proteins also reproduce TOR-dependent nucleolar entrapment of specific ribosomal precursor complexes. We suggest that shortage of newly synthesized r-proteins after short-term TOR inactivation is sufficient to explain most of the observed effects on ribosome production.

Map4k4 negatively regulates peroxisome proliferator-activated receptor (PPAR) gamma protein translation by suppressing the mammalian target of rapamycin (mTOR) signaling pathway in cultured adipocytes

The receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is considered a master regulator of adipocyte differentiation and promotes glucose and lipid metabolism in mature adipocytes. We recently identified the yeast Sterile 20 (Ste20) protein kinase ortholog, Map4k4, in an RNA interference-based screen as an inhibitor of PPARgamma expression in cultured adipocytes. Here, we show that RNA interference-mediated silencing of Map4k4 elevates the levels of both PPARgamma1 and PPARgamma2 proteins in 3T3-L1 adipocytes without affecting PPARgamma mRNA levels, suggesting that Map4k4 regulates PPARgamma at a post-transcriptional step. PPARgamma degradation rates are remarkably rapid as measured in the presence of cycloheximide (t(1/2) = 2 h), but silencing Map4k4 had no effect on PPARgamma degradation. However, depletion of Map4k4 significantly enhances [(35)S]methionine/cysteine incorporation into proteins, suggesting that Map4k4 signaling decreases protein translation. We show a function of Map4k4 is to inhibit rapamycin-sensitive mammalian target of rapamycin (mTOR) activity, decreasing 4E-BP1 phosphorylation. In addition, our results show mTOR and 4E-BP1 are required for the increased PPARgamma protein expression upon Map4k4 knockdown. Consistent with this concept, adenovirus-mediated expression of Map4k4 decreased PPARgamma protein levels and mTOR phosphorylation. These data show that Map4k4 negatively regulates PPARgamma post-transcriptionally, by attenuating mTOR signaling and a 4E-BP1-dependent mechanism.

Cholesterol trafficking is required for mTOR activation in endothelial cells

Mammalian target of rapamycin (mTOR) constitutes a nodal point of a signaling network that regulates cell growth and proliferation in response to various environmental cues ranging from growth factor stimulation to nutrients to stress. Whether mTOR is also affected by cholesterol homeostasis, however, has remained unknown. We report that blockade of cholesterol trafficking through lysosome by a newly identified inhibitor of angiogenesis, itraconazole, leads to inhibition of mTOR activity in endothelial cells. Inhibition of mTOR by itraconazole but not rapamycin can be partially restored by extracellular cholesterol delivered by cyclodextrin. Moreover, other known inhibitors of endosomal/lysosomal cholesterol trafficking as well as siRNA knockdown of Niemann-Pick disease type C (NPC) 1 and NPC2 also cause inhibition of mTOR in endothelial cells. In addition, both the accumulation of cholesterol in the lysosome and inhibition of mTOR caused by itraconazole can be reversed by thapsigarin. These observations suggest that mTOR is likely to be involved in sensing membrane sterol concentrations in endothelial cells, and the cholesterol trafficking pathway is a promising target for the discovery of inhibitors of angiogenesis.

Regulation of cap-dependent translation initiation in the early stage porcine parthenotes

The binding of mRNAs to ribosomes is mediated by the protein complex eIF4F in conjunction with eIF4B (eukaryotic initiation factor 4F and 4B). EIF4F is a three subunit complex consisting of eIF4A (RNA helicase), eIF4E (mRNA cap binding protein), and eIF4G (bridging protein). The crucial role is played by eIF4E, which directly binds the 5'-cap structure of the mRNA and facilitates the recruitment to the mRNA of other translation factors and the 40S ribosomal subunit. EIF4E binding to mRNA and to other initiation factors is regulated on several levels, including its phosphorylation on Ser-209, and association with its regulatory protein 4E-binding protein (4E-BP1). In this study we document that both the translation initiation factor eIF4E and its regulator 4E-BP1 become dephosphorylated in the early stage porcine zygotes already 8 hr post-activation. Similarly, the activities of ERK1/2 MAP and Mnk1 kinases, which are both involved in eIF4E phosphorylation, gradually decrease during this period with the timing similar to that of eIF4E dephosphorylation. The formation of an active eIF4F complex is also diminished after 9-15 hr post-activation, although substantial amounts of this complex have been detected also 24 hr post-activation (2-cell stage). The overall protein synthesis in the parthenotes decreases gradually from 12 hr post-activation reaching a minimum after 48 hr (4-cell stage). Although the translation is gradually decreasing during early preimplantation development, the eIF4F complex, which is temporarily formed, might be a premise for the translation of a small subset of mRNAs at this period of development.

Activation of p53 stimulates proteasome-dependent truncation of eIF4E-binding protein 1 (4E-BP1)

BACKGROUND INFORMATION

The translational inhibitor protein 4E-BP1 [eIF4E (eukaryotic initiation factor 4E)-binding protein 1] regulates the availability of polypeptide chain initiation factor eIF4E for protein synthesis. Initiation factor eIF4E binds the 5' cap structure present on all cellular mRNAs. Its ability to associate with initiation factors eIF4G and eIF4A, forming the eIF4F complex, brings the mRNA to the 43S complex during the initiation of translation. Binding of eIF4E to eIF4G is inhibited in a competitive manner by 4E-BP1. Phosphorylation of 4E-BP1 decreases the affinity of this protein for eIF4E, thus favouring the binding of eIF4G and enhancing translation. We have previously shown that induction or activation of the tumour suppressor protein p53 rapidly leads to 4E-BP1 dephosphorylation, resulting in sequestration of eIF4E, decreased formation of the eIF4F complex and inhibition of protein synthesis.

RESULTS

We now report that activation of p53 also results in modification of 4E-BP1 to a truncated form. Unlike full-length 4E-BP1, which is reversibly phosphorylated at multiple sites, the truncated protein is almost completely unphosphorylated. Moreover, the latter interacts with eIF4E in preference to full-length 4E-BP1. Inhibitor studies indicate that the p53-induced cleavage of 4E-BP1 is mediated by the proteasome and is blocked by conditions that inhibit the dephosphorylation of full-length 4E-BP1. Measurements of the turnover of 4E-BP1 indicate that the truncated form is much more stable than the full-length protein.

CONCLUSIONS

The results suggest a model in which proteasome activity gives rise to a stable, hypophosphorylated and truncated form of 4E-BP1, which may exert a long-term inhibitory effect on the availability of eIF4E, thus contributing to the inhibition of protein synthesis and the growth-inhibitory and pro-apoptotic effects of p53.

The mammalian target of rapamycin complex 1 regulates leptin biosynthesis in adipocytes at the level of translation: the role of the 5'-untranslated region in the expression of leptin messenger ribonucleic acid

Leptin production by adipose cells in vivo is increased after feeding and decreased by food deprivation. However, molecular mechanisms that control leptin expression in response to food intake remain unknown. Here, we test the hypothesis that leptin expression in adipose cells is regulated by nutrient- and insulin-sensitive mammalian target of rapamycin complex 1 (mTORC1)-mediated pathway. The activity of mTORC1 in 3T3-L1 adipocytes was up-regulated by stable expression of either constitutively active Rheb or dominant-negative AMP-activated protein kinase. In both cases, expression of endogenous leptin was significantly elevated at the level of translation. To investigate the role of leptin 5'-untranslated region (UTR) in the regulation of protein expression, we created bicistronic reporter constructs with and without the 5'-UTR. We found that the presence of leptin 5'-UTR renders mRNA resistant to regulation by mTORC1. It appears, therefore, that mTORC1 controls translation of leptin mRNA via a novel mechanism that does not require the presence of either the 5'-terminal oligopyrimidine tract or the 5'-UTR.

Control of eIF4E cellular localization by eIF4E-binding proteins, 4E-BPs

Eukaryotic initiation factor (eIF) 4E, the mRNA 5'-cap-binding protein, mediates the association of eIF4F with the mRNA 5'-cap structure to stimulate cap-dependent translation initiation in the cytoplasm. The assembly of eIF4E into the eIF4F complex is negatively regulated through a family of repressor proteins, called the eIF4E-binding proteins (4E-BPs). eIF4E is also present in the nucleus, where it is thought to stimulate nuclear-cytoplasmic transport of certain mRNAs. eIF4E is transported to the nucleus via its interaction with 4E-T (4E-transporter), but it is unclear how it is retained in the nucleus. Here we show that a sizable fraction (approximately 30%) of 4E-BP1 is localized to the nucleus, where it binds eIF4E. In mouse embryo fibroblasts (MEFs) subjected to serum starvation and/or rapamycin treatment, nuclear 4E-BPs sequester eIF4E in the nucleus. A dramatic loss of nuclear 4E-BP1 occurs in c-Ha-Ras-expressing MEFs, which fail to show starvation-induced nuclear accumulation of eIF4E. Therefore, 4E-BP1 is a regulator of eIF4E cellular localization.

Regulation of proline-rich Akt substrate of 40 kDa (PRAS40) function by mammalian target of rapamycin complex 1 (mTORC1)-mediated phosphorylation

The rapamycin-sensitive mammalian target of rapamycin (mTOR) complex 1 (mTORC1) contains mTOR, raptor, mLST8, and PRAS40 (proline-rich Akt substrate of 40 kDa). PRAS40 functions as a negative regulator when bound to mTORC1, and it dissociates from mTORC1 in response to insulin. PRAS40 has been demonstrated to be a substrate of mTORC1, and one phosphorylation site, Ser-183, has been identified. In this study, we used two-dimensional phosphopeptide mapping in conjunction with mutational analysis to show that in addition to Ser-183, mTORC1 also phosphorylates Ser-212 and Ser-221 in PRAS40 when assayed in vitro. Mutation of all three residues to Ala markedly reduces mTORC1-mediated phosphorylation of PRAS40 in vitro. All three sites were confirmed to be phosphorylated in vivo by [(32)P]orthophosphate labeling and peptide mapping. Phosphorylation of Ser-221 and Ser-183 but not Ser-212 is sensitive to rapamycin treatment. Furthermore, we demonstrate that mutation of Ser-221 to Ala reduces the interaction with 14-3-3 to the same extent as mutation of Thr-246, the Akt/protein kinase B-phosphorylated site. We also find that mutation of Ser-221 to Ala increases the inhibitory activity of PRAS40 toward mTORC1. We propose that after mTORC1 kinase activation by upstream regulators, PRAS40 is phosphorylated directly by mTOR, thus contributing to the relief of PRAS40-mediated substrate competition.

Photic regulation of the mTOR signaling pathway in the suprachiasmatic circadian clock

Here we analyzed the light-responsiveness of the mammalian target of rapamycin (mTOR) cascade, a key regulator of inducible translation, in the suprachiasmatic nuclei (SCN), the locus of the master circadian clock. Brief light exposure during the subjective night, but not during the subjective day, triggered rapid phosphorylation (a marker of catalytic activity) of the mTOR translation effectors p70 S6K, ribosomal S6 protein (S6) and 4E-BP1. In the absence of photic stimulation, marked S6 and 4E-BP1 phosphorylation was detected, indicating tonic mTOR activity in the SCN. Light stimulated the colocalized activation of p70 S6K and extracellular signal-regulated protein kinase (ERK), and pharmacological disruption of ERK signaling abolished light-induced mTOR activity, revealing that the MAPK cascade is an essential intermediate that couples light to mTOR. Together these data identify a light-responsive mTOR cascade in the SCN, and thus, raise the possibility that inducible translation contributes to the clock entrainment process.

Biochemical approaches for nutritional support of skeletal muscle protein metabolism during sepsis

Sepsis initiates a unique series of modifications in the homeostasis of N metabolism and profoundly alters the integration of inter-organ cooperatively in the overall N and energy economy of the host. The net effect of these alterations is an overall N catabolic state, which seriously compromises recovery and is semi-refractory to treatment with current therapies. These alterations lead to a functional redistribution of N (amino acids and proteins) and substrate metabolism among injured tissues and major body organs. The redistribution of amino acids and proteins results in a quantitative reordering of the usual pathways of C and N flow within and among regions of the body with a resultant depletion of the required substrates and cofactors in important organs. The metabolic response to sepsis is a highly integrated, complex series of reactions. To understand the regulation of the response to sepsis, a comprehensive, integrated analysis of the fundamental physiological relationships of key metabolic pathways and mechanisms in sepsis is essential. The catabolism of skeletal muscles, which is manifested by an increase in protein degradation and a decrease in synthesis, persists despite state-of-the-art nutritional care. Much effort has focused on the modulation of the overall amount of nutrients given to septic patients in a hope to improve efficiencies in utilisation and N economies, rather than the support of specific end-organ targets. The present review examines current understanding of the processes affected by sepsis and testable means to circumvent the sepsis-induced defects in protein synthesis in skeletal muscle through increasing provision of amino acids (leucine, glutamine, or arginine) that in turn act as nutrient signals to regulate a number of cellular processes.

Mannan and peptidoglycan induce COX-2 protein in human PMN via the mammalian target of rapamycin

The induction of cyclooxygenase-2 (COX-2) protein expression was assessed in human polymorphonuclear leukocytes (PMN) stimulated via receptors of the innate immune system. Peptidoglycan (PGN) and mannan, and at a lower extent the bacterial lipoprotein mimic palmitoyl-3-cysteine-serine-lysine-4, induced COX-2 protein expression. In contrast, lipoteichoic acid and muramyldipeptide were irrelevant stimuli. The mRNA encoding COX-2 was present in resting PMN at an extent quite similar to that detected in stimulated PMN, whereas the expression of COX-2 protein was undetectable. Treatment with the phosphatidylinositol 3-kinase inhibitor (PI3K) wortmaninn, the mammalian target of rapamycin (mTOR) inhibitor rapamycin, and the translation inhibitor cycloheximide blocked the induction of COX-2 protein in response to mannan and PGN, whereas the transcriptional inhibitor actinomycin D did not show a significant effect. These results disclose a capability of pathogen-associated molecular patterns to induce the oxidative metabolism of arachidonic acid more robust than that of PMN archetypal chemoattractants, since mannan and PGN make it coincidental the release of arachidonic acid with a rapid induction of COX-2 protein regulated by a signaling cascade involving PI3K, mTOR, and the translation machinery. This mechanism of COX-2 protein induction expression in PMN is substantially different from that operative in mononuclear phagocytes, which is highly dependent on transcriptional regulation.

Nutrient signaling components controlling protein synthesis in striated muscle

Accretion of muscle mass is dependent upon faster rates of protein synthesis than degradation. When an animal is deprived of dietary protein, loss of body weight and negative nitrogen balance ensue. Likewise, refeeding accelerates protein synthesis and results in resumption of positive nitrogen balance. Amino acids and anabolic hormones both interact to maximally enhance rates of protein synthesis acutely during refeeding through an acceleration of the messenger RNA (mRNA) translation initiation. The review will illuminate the molecular mechanisms responsible for increasing mRNA translation initiation in striated muscle. The hastening of mRNA translation initiation most likely results from a stimulation of mammalian target of rapamycin (mTOR) acting through its downstream effector proteins eukaryotic initiation factors (eIF)4E binding protein1 and possibly eIF4G to enhance assembly of eIF4G with eIF4E and 70-kDa ribosomal S6 kinase1. Amino acids and leucine in particular are as effective as a complete meal in stimulating mRNA translation initiation by targeting these specific signal transduction systems. The physiologic importance lies in the potential ability of amino acids as specific nutrients designed to counteract the accelerated host protein wasting associated with a number of disease entities, including cancer, HIV infection, sepsis, and diabetes, and to improve nutrition to maintain muscle mass in aging populations and ensure muscle growth in neonatal populations.

Effects of protein phosphorylation on ubiquitination and stability of the translational inhibitor protein 4E-BP1

The availability of the eukaryotic polypeptide chain initiation factor 4E (eIF4E) for protein synthesis is regulated by the 4E-binding proteins (4E-BPs), which act as inhibitors of cap-dependent mRNA translation. The ability of the 4E-BPs to sequester eIF4E is regulated by reversible phosphorylation at multiple sites. We show here that, in addition, 4E-BP1 is a substrate for polyubiquitination and that some forms of 4E-BP1 are simultaneously polyubiquitinated and phosphorylated. In Jurkat cells inhibition of proteasomal activity by MG132 enhances the level of hypophosphorylated, unmodified 4E-BP1 but only modestly increases the accumulation of high-molecular-weight, phosphorylated forms of 4E-BP1. In contrast, inhibition of protein phosphatase activity with calyculin A reduces the level of unmodified 4E-BP1 but strongly enhances the amount of phosphorylated, high-molecular-weight 4E-BP1. Turnover measurements in the presence of cycloheximide show that, whereas 4E-BP1 is normally a very stable protein, calyculin A decreases the apparent half-life of the normal-sized protein. Affinity chromatography on m(7)GTP-Sepharose indicates that the larger forms of 4E-BP1 bind very poorly to eIF4E. We suggest that the phosphorylation of 4E-BP1 may play a dual role in the regulation of protein synthesis, both reducing the affinity of 4E-BP1 for eIF4E and promoting the conversion of 4E-BP1 to alternative, polyubiquitinated forms.

PRAS40 Regulates mTORC1 Kinase Activity by Functioning as a Direct Inhibitor of Substrate Binding

Mammalian target of rapamycin (mTOR) functions in two distinct signaling complexes, mTORC1 and mTORC2. In response to insulin and nutrients, mTORC1, consisting of mTOR, raptor (regulatory-associated protein of mTOR), and mLST8, is activated and phosphorylates eukaryotic initiation factor 4E-binding protein (4EBP) and p70 S6 kinase to promote protein synthesis and cell size. Previously we found that activation of mTOR kinase in response to insulin was associated with increased 4EBP1 binding to raptor. Here we identify prolinerich Akt substrate 40 (PRAS40) as a binding partner for mTORC1. A putative TOR signaling motif, FVMDE, is identified in PRAS40 and shown to be required for interaction with raptor. Insulin stimulation markedly decreases the level of PRAS40 bound by mTORC1. Recombinant PRAS40 inhibits mTORC1 kinase activity in vivo and in vitro, and this inhibition depends on PRAS40 association with raptor. Furthermore, decreasing PRAS40 expression by short hairpin RNA enhances 4E-BP1 binding to raptor, and recombinant PRAS40 competes with 4E-BP1 binding to raptor. We, therefore, propose that PRAS40 regulates mTORC1 kinase activity by functioning as a direct inhibitor of substrate binding.

Expression of RUNX2 isoforms: involvement of cap-dependent and cap-independent mechanisms of translation

RUNX2, a major regulator of skeletogenesis, is expressed as type-I and type-II isoforms. Whereas most eukaryotic mRNAs are translated by the cap-dependent scanning mechanism, translation of many mRNAs including type-I and type-II RUNX2 mRNAs has been reported to be initiated by a cap independent internal ribosomal entry site (IRES). Since the dicistronic plasmid assay used to demonstrate IRES has been questioned, we investigated the presence of IRES in RUNX2 mRNAs using dicistronic plasmid and mRNA assays. Our results show that the dicistronic plasmid assay cannot be used to demonstrate IRES in RUNX2 mRNAs because the intercistronic region of dicistronic plasmids containing the 5'-UTRs of both RUNX2 mRNAs operates as a cryptic promoter. In dicistronic mRNA transfection studies the 5'-UTRs of both RUNX2 mRNAs exhibited no IRES activity. When transfected into osteoblastic cells, monocistronic reporter mRNA preceded by the 5'-UTR of type-II RUNX2 (Type-II-FLuc-A100) was translated to a high degree only in the presence of a functional cap (m(7)GpppG); in contrast, luciferase mRNA preceded by the 5'-UTR of type-I RUNX2 mRNA (Type-I-FLuc-A100) was translated poorly in the presence of either m(7)GpppG or a nonfunctional cap (ApppG). Notably, in transfected cells inhibitors of cap-dependent translation suppressed the translation of m(7)GpppG-capped Type-II-FLuc-A100, but not ApppG-capped reporter mRNA preceded by the IRES-containing hepatitis C virus (HCV) 5'-UTR. Our study demonstrates that type-II RUNX2 mRNA is translated by the cap-dependent mechanism. Although efficient translation of type-I RUNX2 mRNA appears to require a process other than cap-dependent, the mechanism of type-I RUNX2 mRNA translation remains to be resolved.

Sex-dependent differences in the regulation of myocardial protein synthesis following long-term ethanol consumption

Chronic heavy alcohol consumption alters cardiac structure and function. Controversies remain as to whether hearts from females respond to the chronic ethanol intake in a manner analogous to males. In particular, sex differences in the myocardial response to chronic alcohol consumption remain unresolved at the molecular level. The purpose of the present set of experiments was to determine whether alterations in cardiac structure and protein metabolism show sexual dimorphism following chronic alcohol consumption for 26 wk. In control animals, hearts from female rats showed lowered heart weights and had thinner ventricular walls compared with males. The smaller heart size was associated with a lower protein content that occurred in part from a reduced rate of protein synthesis. Chronic alcohol consumption in males, but not in females, caused a thinning of the ventricular wall and intraventricular septum, as assessed by echocardiography, correlating with the loss of heart mass. The alterations in cardiac size occurred, in part, through a lowering of the protein content secondary to a diminished rate of protein synthesis. The decreased rate of protein synthesis appeared related to a reduced assembly of active eukaryotic initiation factor (eIF)4G.eIF4E complex secondary to both a diminished phosphorylation of eIF4G and increased formation of inactive 4Ebinding protein (4EBP1).eIF4E complex. The latter effects occurred as a result of decreased phosphorylation of 4EBP1. None of these ethanol-induced alterations in hearts from males were observed in hearts from females. These data suggest that chronic alcohol-induced impairments in myocardial protein synthesis results, in part, from marked decreases in eIF4E.eIF4G complex formation in males. The failure of female rats consuming ethanol to show structural changes appears related to the inability of ethanol to affect the regulation protein synthesis to the same extent as their male counterparts.

Current development of mTOR inhibitors as anticancer agents

Mammalian target of rapamycin (mTOR) is a kinase that functions as a master switch between catabolic and anabolic metabolism and as such is a target for the design of anticancer agents. The most established mTOR inhibitors--rapamycin and its derivatives--showed long-lasting objective tumour responses in clinical trials, with CCI-779 being a first-in-class mTOR inhibitor that improved the survival of patients with advanced renal cell carcinoma. This heralded the beginning of extensive clinical programmes to further evaluate mTOR inhibitors in several tumour types. Here we review the clinical development of this drug class and look at future prospects for incorporating these agents into multitarget or multimodality strategies against cancer.

Activation of mammalian target of rapamycin (mTOR) by insulin is associated with stimulation of 4EBP1 binding to dimeric mTOR complex 1

Insulin stimulates protein synthesis by promoting phosphorylation of the eIF4E-binding protein, 4EBP1. This effect is rapamycin-sensitive and mediated by mammalian target of rapamycin (mTOR) complex 1 (mTORC1), a signaling complex containing mTOR, raptor, and mLST8. Here we demonstrate that insulin produces a stable increase in the kinase activity of mTORC1 in 3T3-L1 adipocytes. The response was associated with a marked increase in 4EBP1 binding to raptor in mTORC1, and it was abolished by disrupting the TOR signaling motif in 4EBP1. The stimulatory effects of insulin on both 4EBP1 kinase activity and binding occurred rapidly and at physiological concentrations of insulin, and both effects required an intact mTORC1. Results of experiments involving size exclusion chromatography and coimmunoprecipitation of epitope-tagged subunits provide evidence that the major insulin-responsive form is dimeric mTORC1, a structure containing two heterotrimers of mTOR, raptor, and mLST8.

mTOR-dependent stimulation of the association of eIF4G and eIF3 by insulin

Insulin stimulates protein synthesis by increasing translation initiation. This response is mediated by mTOR and is believed to result from 4EBP1 phosphorylation, which allows eIF4E to bind eIF4G. Here, we present evidence that mTOR interacts directly with eIF3 and that mTOR controls the association of eIF3 and eIF4G. Activating mTOR signaling with insulin increased by as much as five-fold the amount of eIF4G bound to eIF3. This novel effect was blocked by rapamycin and other inhibitors of mTOR, and it required neither eIF4E binding to eIF4G nor eIF3 binding to the 40S ribosomal subunit. The increase in eIF4G associated with eIF3 occurred rapidly and at physiological concentrations of insulin. Moreover, the magnitude of the response was similar to the increase in eIF4E binding to eIF4G produced by insulin. Thus, increasing eIF4G association with eIF3 represents a potentially important mechanism by which insulin, as well as amino acids and growth factors that activate mTOR, stimulate translation.

Zinc as an insulin replacement in hybridoma cultures

There are many advantages to the use of protein-free media for biologics production, including a reduced risk of viral contamination from animal-derived proteins and simplification of downstream purification. In the course of developing protein-free media for hybridoma and myeloma cells, zinc was found to be an effective replacement for insulin, with no negative impact on viable cell density and antibody production. Transcript profiling using DNA microarrays indicated no major change in the global expression profile between the insulin and zinc-supplemented cultures, which is consistent with their similar growth and metabolic characteristics. Both DNA microarray and quantitative RT-PCR analysis showed increase in insulin receptor substrate 1 (Irs1) expression in zinc-supplemented cultures, while several key genes downstream of Irs1 in the insulin-signaling pathway, such as protein kinase B (PKB/Akt) and 3-phosphoinositide dependent protein kinase 1 (Pdpk1) did not show significant differences at the transcript level. Comparison of transcript profiles from cultures with low versus optimal zinc supplementation implicated the involvement of the insulin-related genes Pax6 and Phas1. Subtle differences were also observed between insulin and zinc in the serine-473 phosphorylation of Akt. Zinc increased serine-473 phosphorylation of Akt, but to a lesser extent than insulin. The phosphoinositide 3-kinase (PI3K) inhibitor, wortmannin, totally blocked the effect of both zinc and insulin on Akt activation, indicating the involvement of PI3K in the activation of Akt by zinc, rather than zinc acting on Akt directly. Our results highlight the impact of trace metal supplementation as protein-free media formulations move towards greater chemical definition.

Rapamycin: an anti-cancer immunosuppressant?

Rapamycin and its derivatives are promising therapeutic agents with both immunosuppressant and anti-tumor properties. These rapamycin actions are mediated through the specific inhibition of the mTOR protein kinase. mTOR serves as part of an evolutionarily conserved signaling pathway that controls the cell cycle in response to changing nutrient levels. The mTOR signaling network contains a number of tumor suppressor genes including PTEN, LKB1, TSC1, and TSC2, and a number of proto-oncogenes including PI3K, Akt, and eIF4E, and mTOR signaling is constitutively activated in many tumor types. These observations point to mTOR as an ideal target for anti-cancer agents and suggest that rapamycin is such an agent. In fact, early preclinical and clinical studies indicate that rapamycin derivatives have efficacy as anti-tumor agents both alone, and when combined with other modes of therapy. Rapamycin appears to inhibit tumor growth by halting tumor cell proliferation, inducing tumor cell apoptosis, and suppressing tumor angiogenesis. Rapamycin immunosuppressant actions result from the inhibition of T and B cell proliferation through the same mechanisms that rapamycin blocks cancer cell proliferation. Therefore, one might think that rapamycin-induced immunosuppression would be detrimental to the use of rapamycin as an anti-cancer agent. To the contrary, rapamycin decreases the frequency of tumor formation that occurs in organ transplant experiments when combined with the widely used immunosuppressant cyclosporine compared with the tumor incidence observed when cyclosporine is used alone. The available evidence indicates that with respect to tumor growth, rapamycin anti-cancer activities are dominant over rapamycin immunosuppressant effects.

Elevated glucose activates protein synthesis in cultured cardiac myocytes

Diabetes mellitus results in chronic hyperglycemia, a serious metabolic disorder associated with a markedly increased risk of cardiovascular disease. However, the effects of high glucose (HG) on cardiac myocyte growth have not been fully clarified. In this study, the effect of glucose on cardiac myocyte growth was examined using leucine incorporation as an index of protein synthesis. High glucose (HG, 25 mmol/L) increased leucine incorporation (167% +/- 0.2% over normal glucose, n=4, P<.01) compared with a physiological glucose concentration (5.5 mmol/L, normal glucose). The HG-induced increase in leucine incorporation was time- and dose-dependent and was not due to osmotic changes because 25 mmol/L mannitol did not change leucine incorporation. High glucose also significantly reduced elongation factor 2 phosphorylation, an effect known to result in increased protein synthesis at the elongation step. Western blot analysis showed that HG-activated protein kinase B (PKB), also called Akt (PKB/Akt), at 18 hours. High glucose-induced leucine incorporation was attenuated with phosphatidylinositol 3-kinase (PI3K) inhibition using wortmannin and LY294002 and by rapamycin, a mammalian target of rapamycin (mTOR) inhibitor, 72%, 64%, and 65% (P<.05), respectively. High glucose also activated extracellular signal-regulated kinase 1/2 activity with peak stimulation at 5 minutes. In addition, PD98059, an inhibitor of mitogen-activated protein kinase kinase, attenuated HG-induced leucine incorporation. These data show for the first time that elevated glucose increases protein synthesis in cardiac myocytes. The increase appears to be mediated by activation of PI3K-PKB/Akt and/or PI3K-mTOR as well as extracellular signal-regulated kinase 1/2. These results provide new evidence for a direct effect of glucose independent of insulin on cardiac myocyte growth.

Changes in 4E-BP1 and p70S6K phosphorylation in skeletal muscle of the ovine fetus after prolonged maternal fasting: effects of insulin and IGF-I

This study was conducted to investigate fasting-induced alterations in insulin signaling to the regulatory components of the translation machinery. Insulin (890 mIU/h) and IGF-I (40 nM/h) were infused into a chronically catheterized ovine fetus (0.85 gestation) for 7 h following a 5-d maternal fast. Amino acid and glucose concentrations were clamped to minimize the effects of alterations in circulating substrate concentrations. The IGF-I induced increase in 4E-BP1 phosphorylation (percentage in the gamma form) increased from 28% in control to 44% (NS). The insulin-induced increase in 4E-BP1 phosphorylation was more pronounced, and the gamma percentage was 56% on average in the insulin group. The insulin-induced increase in 4E-BP1 phosphorylation was lower than in fed animals and did not result in significant changes in eIF4E.4E-BP1 binding or eIF4E.eIF4G binding. Insulin increased PKB/Akt phosphorylation and p70S6K phosphorylation to a similar extent as in fed animals. We conclude that maternal fasting resulted in reduced insulin sensitivity of 4E-BP1 phosphorylation and eIF4F formation. This reduced insulin-induced 4E-BP1 phosphorylation was not due to a global defect in insulin signaling; the defects underlying the reduced basal phosphorylation and insulin-responsiveness of 4E-BP1 in fasted animals may be in signaling components other than, or downstream of, PKB/Akt. Selective inhibition of downstream components of insulin signaling allows fetuses to adapt to nutritional stress by decreasing the anabolic response to insulin and other growth factors, so that more amino acids can be used as oxidative substrate to compensate for shortage of energy due to reduced glucose supply.

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.

Signal transduction pathways leading to increased eIF4E phosphorylation caused by oxidative stress

Phosphorylation of eIF4E is associated with increased activity of the translational machinery. Oxidative stress of resident vascular cells and macrophages potently enhances eIF4E phosphorylation. Oxidative stress activates numerous intracellular signaling pathways, including MAP-family kinase pathways and pathways leading to S6 kinase activation. The activation of MAP-family kinase pathways leads to the activation of Mnk and hence eIF4E phosphorylation, whereas the S6 kinase pathway is not involved, based on insensitivity to its inhibitors rapamycin and wortmannin. Ca-dependent pathways have been implicated in eIF4E phosphorylation, but the oxidative stress response pathway targeting eIF4E does not appear to require their participation. The results suggest that the potent activation of ERK and p38 protein kinases is sufficient to account for the enhanced eIF4E phosphorylation. Either is independently sufficient to effect the change, as neither PD098059 (Erk pathway inhibitor) nor SB202190 (p38 pathway inhibitor) alone can block the response, but when combined the response is almost completely abrogated. Mnk activation by oxidative stress leading to enhanced eIF4E phosphorylation may play a role in promoting stress-induced hyperproliferative diseases, such as smooth muscle cell proliferation and hypertrophy in cardiovascular disease, as the synthesis of several key regulators of cell growth has been shown to be held in check by moderation of eIF4E activity.

Acute alcohol intoxication enhances myocardial eIF4G phosphorylation despite reducing mTOR signaling

Acute alcohol intoxication impairs myocardial protein synthesis in rats, secondary to a diminished mRNA translational efficiency. Decreased mRNA translational efficiency occurs through altered regulation of peptide chain initiation. The purpose of the present set of experiments was to determine whether acute alcohol intoxication alters the phosphorylation state of eukaryotic initiation factor (eIF) 4G, eIF4G·eIF4E complex formation, and the mammalian target of rapamycin (mTOR) signaling pathway in the heart. Acute alcohol intoxication was induced by injection of alcohol (75 mmol/kg body wt ip). Control animals received an equal volume of saline. Alcohol administration enhanced phosphorylation of eIF4G (Ser1108) approximately threefold. Alcohol administration lowered formation of the active eIF4G·eIF4E complex by >90%, whereas it increased the abundance of the inactive 4E-binding protein 1 (4E-BP1)·eIF4E complex by ∼160%. Phosphorylation of mTOR on Ser2448and Ser2481was decreased by 50%. Reduced mTOR phosphorylation did not result from decreased phosphorylation of PKB. Phosphorylation of 4E-BP1 and S6 kinase 1 (Thr389), downstream targets of mTOR, were also reduced after acute alcohol administration. These data suggest that acute alcohol-induced impairments in myocardial mRNA translation initiation result, in part, from marked decreases in eIF4G·eIF4E complex formation, which appear to be independent of changes in phosphorylation of eIF4G but dependent on mTOR.

Nutrient sensing and metabolic decisions

Cells have several sensory systems that detect energy and metabolic status and adjust flux through metabolic pathways accordingly. Many of these sensors and signaling pathways are conserved from yeast to mammals. In this review, we bring together information about five different nutrient-sensing pathways (AMP kinase, mTOR, PAS kinase, hexosamine biosynthesis and Sir2), highlighting their similarities, differences and roles in disease.

Dissociation of raptor from mTOR is a mechanism of rapamycin-induced inhibition of mTOR function

The mammalian target of rapamycin (mTOR) is a Ser/Thr protein kinase that plays a crucial role in a nutrient-sensitive signalling pathway that regulates cell growth. TOR signalling is potently inhibited by rapamycin, through the direct binding of a FK506-binding protein 12 (FKBP12)/rapamycin complex to the TOR FRB domain, a segment amino terminal to the kinase catalytic domain. The molecular basis for the inhibitory action of FKBP12/rapamycin remains uncertain. Raptor (regulatory associated protein of mTOR) is a recently identified mTOR binding partner that is essential for mTOR signalling in vivo, and whose binding to mTOR is critical for mTOR-catalysed substrate phosphorylation in vitro. Here we investigated the stability of endogenous mTOR/raptor complex in response to rapamycin in vivo, and to the direct addition of a FKBP12/rapamycin complex in vitro. Rapamycin diminished the recovery of endogenous raptor with endogenous or recombinant mTOR in vivo; this inhibition required the ability of mTOR to bind the FKBP12/rapamycin complex, but was independent of mTOR kinase activity. Rapamycin, in the presence of FKBP12, inhibited the association of raptor with mTOR directly in vitro, and concomitantly reduced the mTOR-catalysed phosphorylation of raptor-dependent, but not raptor-independent substrates; mTOR autophosphorylation was unaltered. These observations indicate that rapamycin inhibits mTOR function, at least in part, by inhibiting the interaction of raptor with mTOR; this action uncouples mTOR from its substrates, and inhibits mTOR signalling without altering mTOR's intrinsic catalytic activity.