5-Aminoimidazole-4-carboxamide-1-β-4-ribofuranoside (AICAR) enhances the efficacy of rapamycin in human cancer cells

mTOR – the mammalian/mechanistic target of rapamycin – has been implicated as a key signaling node for promoting survival of cancer cells. However, clinical trials that have targeted mTOR with rapamycin or rapamycin analogs have had minimal impact. In spite of the high specificity of rapamycin for mTOR, the doses needed to suppress key mTOR substrates have proved toxic. We report here that rapamycin when combined with AICAR – a compound that activates AMP-activated protein kinase makes rapamycin cytotoxic rather than cytostatic at doses that are tolerated clinically. AICAR by itself is able to suppress mTOR complex 1 (mTORC1), but also stimulates a feedback activation of mTORC2, which activates the survival kinase Akt. However, AICAR also suppresses production of phosphatidic acid (PA), which interacts with mTOR in a manner that is competitive with rapamycin. The reduced level of PA sensitizes mTORC2 to rapamycin at tolerable nano-molar doses leading reduced Akt phosphorylation and apoptosis. This study reveals how the use of AICAR enhances the efficacy of rapamycin such that rapamycin at low nano-molar doses can suppress mTORC2 and induce apoptosis in human cancer cells at doses that are clinically tolerable.     

Active-Site Inhibitors of mTOR Target Rapamycin-Resistant Outputs of mTORC1 and mTORC2

The mammalian target of rapamycin (mTOR) regulates cell growth and survival by integrating nutrient and hormonal signals. These signaling functions are distributed between at least two distinct mTOR protein complexes: mTORC1 and mTORC2. mTORC1 is sensitive to the selective inhibitor rapamycin and activated by growth factor stimulation via the canonical phosphoinositide 3-kinase (PI3K)→Akt→mTOR pathway. Activated mTORC1 kinase up-regulates protein synthesis by phosphorylating key regulators of mRNA translation. By contrast, mTORC2 is resistant to rapamycin. Genetic studies have suggested that mTORC2 may phosphorylate Akt at S473, one of two phosphorylation sites required for Akt activation; this has been controversial, in part because RNA interference and gene knockouts produce distinct Akt phospho-isoforms. The central role of mTOR in controlling key cellular growth and survival pathways has sparked interest in discovering mTOR inhibitors that bind to the ATP site and therefore target both mTORC2 and mTORC1. We investigated mTOR signaling in cells and animals with two novel and specific mTOR kinase domain inhibitors (TORKinibs). Unlike rapamycin, these TORKinibs (PP242 and PP30) inhibit mTORC2, and we use them to show that pharmacological inhibition of mTOR blocks the phosphorylation of Akt at S473 and prevents its full activation. Furthermore, we show that TORKinibs inhibit proliferation of primary cells more completely than rapamycin. Surprisingly, we find that mTORC2 is not the basis for this enhanced activity, and we show that the TORKinib PP242 is a more effective mTORC1 inhibitor than rapamycin. Importantly, at the molecular level, PP242 inhibits cap-dependent translation under conditions in which rapamycin has no effect. Our findings identify new functional features of mTORC1 that are resistant to rapamycin but are effectively targeted by TORKinibs. These potent new pharmacological agents complement rapamycin in the study of mTOR and its role in normal physiology and human disease.     

Leucine stimulates protein synthesis in skeletal muscle of neonatal pigs by enhancing mTORC1 activation

Skeletal muscle in the neonate grows at a rapid rate due in part to an enhanced sensitivity to the postprandial rise in amino acids, particularly leucine. To elucidate the molecular mechanism by which leucine stimulates protein synthesis in neonatal muscle, overnight-fasted 7-day-old piglets were treated with rapamycin [an inhibitor of mammalian target of rapamycin (mTOR) complex (mTORC)1] for 1 h and then infused with leucine for 1 h. Fractional rates of protein synthesis and activation of signaling components that lead to mRNA translation were determined in skeletal muscle. Rapamycin completely blocked leucine-induced muscle protein synthesis. Rapamycin markedly reduced raptor-mTOR association, an indicator of mTORC1 activation. Rapamycin blocked the leucine-induced phosphorylation of mTOR, S6 kinase 1 (S6K1), and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1) and formation of the eIF4E.eIF4G complex and increased eIF4E.4E-BP1 complex abundance. Rapamycin had no effect on the association of mTOR with rictor, a crucial component for mTORC2 activation, or G protein beta-subunit-like protein (GbetaL), a component of mTORC1 and mTORC2. Neither leucine nor rapamycin affected the phosphorylation of AMP-activated protein kinase (AMPK), PKB, or tuberous sclerosis complex (TSC)2, signaling components that reside upstream of mTOR. Eukaryotic elongation factor (eEF)2 phosphorylation was not affected by leucine or rapamycin, although current dogma indicates that eEF2 phosphorylation is mTOR dependent. Together, these in vivo data suggest that leucine stimulates muscle protein synthesis in neonates by enhancing mTORC1 activation and its downstream effectors.     

Protor-1 is required for efficient mTORC2-mediated activation of SGK1 in the kidney

The mTOR (mammalian target of rapamycin) protein kinase is an important regulator of cell growth and is a key target for therapeutic intervention in cancer. Two complexes of mTOR have been identified: complex 1 (mTORC1), consisting of mTOR, Raptor (regulatory associated protein of mTOR) and mLST8 (mammalian lethal with SEC13 protein 8) and complex 2 (mTORC2) consisting of mTOR, Rictor (rapamycin-insensitive companion of mTOR), Sin1 (stress-activated protein kinase-interacting protein 1), mLST8 and Protor-1 or Protor-2. Both complexes phosphorylate the hydrophobic motifs of AGC kinase family members: mTORC1 phosphorylates S6K (S6 kinase), whereas mTORC2 regulates phosphorylation of Akt, PKCα (protein kinase Cα) and SGK1 (serum- and glucocorticoid-induced protein kinase 1). To investigate the roles of the Protor isoforms, we generated single as well as double Protor-1- and Protor-2-knockout mice and studied how activation of known mTORC2 substrates was affected. We observed that loss of Protor-1 and/or Protor-2 did not affect the expression of the other mTORC2 components, nor their ability to assemble into an active complex. Moreover, Protor knockout mice display no defects in the phosphorylation of Akt and PKCα at their hydrophobic or turn motifs. Strikingly, we observed that Protor-1 knockout mice displayed markedly reduced hydrophobic motif phosphorylation of SGK1 and its physiological substrate NDRG1 (N-Myc downregulated gene 1) in the kidney. Taken together, these results suggest that Protor-1 may play a role in enabling mTORC2 to efficiently activate SGK1, at least in the kidney.     

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.     

mTORC1 serves ER stress-triggered apoptosis via selective activation of the IRE1-JNK pathway

Mammalian target of rapamycin (mTOR) has a key role in the regulation of an array of cellular function. We found that rapamycin, an inhibitor of mTOR complex 1 (mTORC1), attenuated endoplasmic reticulum (ER) stress-induced apoptosis. Among three major branches of the unfolded protein response, rapamycin selectively suppressed the IRE1-JNK signaling without affecting PERK and ATF6 pathways. ER stress rapidly induced activation of mTORC1, which was responsible for induction of the IRE1-JNK pathway and apoptosis. Activation of mTORC1 reduced Akt phosphorylation, which was an event upstream of IRE-JNK signaling and consequent apoptosis. In vivo, administration with rapamycin significantly suppressed renal tubular injury and apoptosis in tunicamycin-treated mice. It was associated with enhanced phosphorylation of Akt and suppression of JNK activity in the kidney. These results disclosed that, under ER stress conditions, mTORC1 causes apoptosis through suppression of Akt and consequent induction of the IRE1-JNK pathway.     

PDK4 Protein Promotes Tumorigenesis through Activation of cAMP-response Element-binding Protein (CREB)-Ras Homolog Enriched in Brain (RHEB)-mTORC1 Signaling Cascade

Mechanistic target of rapamycin (mTOR) integrates multiple extracellular and intracellular signals to regulate cell growth and survival. Hyperactivation of mTOR has been observed in various cancers. Regulation of mTOR activity is thus of importance in physiological processes and tumor development. Here, we present pyruvate dehydrogenase kinase 4 (PDK4) as a novel regulator of mTORC1 signaling. mTORC1 activity was augmented with PDK4 overexpression and reduced by PDK4 suppression in various cell lines. Furthermore, PDK4 bound to cAMP-response element-binding protein (CREB) and prevented its degradation. The enhanced CREB consequently transactivated the expression of Ras homolog enriched in brain (RHEB), a direct key activator of mTORC1, independent of AMP-activated protein kinase or tuberous sclerosis complex protein 2. PDK4 potentiated the mTORC1 effectors hypoxia-inducible factor 1α and pyruvate kinase isozymes M2 and promoted aerobic glycolysis (Warburg effect). Knockdown of PDK4 suppressed the tumor development of cancer cells with activated mTORC1. The abundance of PDK4 dictated the responsiveness of cells to the mTOR inhibitor, rapamycin. Combinatory suppression of mTOR and PDK4 exerted synergistic inhibition on cancer cell proliferation. Therefore, PDK4 promotes tumorigenesis through activation of the CREB-RHEB-mTORC1 signaling cascade.     

mTORC1 inhibition increases neurotensin secretion and gene expression through activation of the MEK/ERK/c-Jun pathway in the human endocrine cell line BON

The mammalian target of rapamycin (mTOR) signaling exists in two complexes: mTORC1 and mTORC2. Neurotensin (NT), an intestinal hormone secreted by enteroendocrine (N) cells in the small bowel, has important physiological effects in the gastrointestinal tract. The human endocrine cell line BON abundantly expresses the NT gene and synthesizes and secretes NT in a manner analogous to that of N cells. Here, we demonstrate that the inhibition of mTORC1 by rapamycin (mTORC1 inhibitor), torin1 (both mTORC1 and mTORC2 inhibitor) or short hairpin RNA-mediated knockdown of mTOR, regulatory associated protein of mTOR (RAPTOR), and p70 S6 kinase (p70S6K) increased basal NT release via upregulating NT gene expression in BON cells. c-Jun activity was increased by rapamycin or torin1 or p70S6K knockdown. c-Jun overexpression dramatically increased NT promoter activity, which was blocked by PD98059, an mitogen-activated protein kinase kinase (MEK) inhibitor. Furthermore, overexpression of MEK1 or extracellular signal-regulated kinase 1 (ERK1) increased c-Jun expression and NT promoter activity. More importantly, PD98059 blocked rapamycin- or torin1-enhanced NT secretion. Consistently, rapamycin and torin1 also increased NT gene expression in Hep3B cells, a human hepatoma cell line that, similar to BON, expresses high levels of NT. Phosphorylation of c-Jun and ERK1/2 was also increased by rapamycin and torin1 in Hep3B cells. Finally, we showed activation of mTOR in BON cells treated with amino acids, high glucose, or serum and, concurrently, the attenuation of ERK1/2 and c-Jun phosphorylation and NT secretion. Together, mTORC1, as a nutrient sensor, negatively regulates NT secretion via the MEK/ERK/c-Jun signaling pathway. Our results identify a physiological link between mTORC1 and MEK/ERK signaling in controlling intestinal hormone gene expression and secretion.     

Phosphorylation of Raptor by p38beta participates in arsenite-induced mammalian target of rapamycin complex 1 (mTORC1) activation

Cell growth is influenced by environmental stress. Mammalian target of rapamycin (mTOR), the central regulator of cell growth, can be positively or negatively regulated by various stresses through different mechanisms. The p38 MAP kinase pathway is essential in cellular stress responses. Activation of MK2, a downstream kinase of p38α, enhances mTOR complex 1 (mTORC1) activity by preventing TSC2 from inhibiting mTOR activation. The p38β-PRAK cascade targets Rheb to inhibit mTORC1 activity upon glucose depletion. Here we show the activation of p38β participates in activation of mTOR complex 1 (mTORC1) induced by arsenite but not insulin, nutrients, anisomycin, or H(2)O(2). Arsenite treatment of cells activates p38β and induces interaction between p38β and Raptor, a regulatory component of mTORC1, resulting in phosphorylation of Raptor on Ser(863) and Ser(771). The phosphorylation of Raptor on these sites enhances mTORC1 activity, and contributes largely to arsenite-induced mTORC1 activation. Our results shown here and in previous work demonstrate that the p38 pathway can regulate different components of the mTORC1 pathway, and that p38β can target different substrates to either positively or negatively regulate mTORC1 activation when a cell encounters different environmental stresses.     

mTOR complex 2 (mTORC2) controls hydrophobic motif phosphorylation and activation of serum- and glucocorticoid-induced protein kinase 1 (SGK1)

SGK1 (serum- and glucocorticoid-induced protein kinase 1) is a member of the AGC (protein kinase A/protein kinase G/protein kinase C) family of protein kinases and is activated by agonists including growth factors. SGK1 regulates diverse effects of extracellular agonists by phosphorylating regulatory proteins that control cellular processes such as ion transport and growth. Like other AGC family kinases, activation of SGK1 is triggered by phosphorylation of a threonine residue within the T-loop of the kinase domain and a serine residue lying within the C-terminal hydrophobic motif (Ser(422) in SGK1). PDK1 (phosphoinositide-dependent kinase 1) phosphorylates the T-loop of SGK1. The identity of the hydrophobic motif kinase is unclear. Recent work has established that mTORC1 [mTOR (mammalian target of rapamycin) complex 1] phosphorylates the hydrophobic motif of S6K (S6 kinase), whereas mTORC2 (mTOR complex 2) phosphorylates the hydrophobic motif of Akt (also known as protein kinase B). In the present study we demonstrate that SGK1 hydrophobic motif phosphorylation and activity is ablated in knockout fibroblasts possessing mTORC1 activity, but lacking the mTORC2 subunits rictor (rapamycin-insensitive companion of mTOR), Sin1 (stress-activated-protein-kinase-interacting protein 1) or mLST8 (mammalian lethal with SEC13 protein 8). Furthermore, phosphorylation of NDRG1 (N-myc downstream regulated gene 1), a physiological substrate of SGK1, was also abolished in rictor-, Sin1- or mLST8-deficient fibroblasts. mTORC2 immunoprecipitated from wild-type, but not from mLST8- or rictor-knockout cells, phosphorylated SGK1 at Ser(422). Consistent with mTORC1 not regulating SGK1, immunoprecipitated mTORC1 failed to phosphorylate SGK1 at Ser(422), under conditions which it phosphorylated the hydrophobic motif of S6K. Moreover, rapamycin treatment of HEK (human embryonic kidney)-293, MCF-7 or HeLa cells suppressed phosphorylation of S6K, without affecting SGK1 phosphorylation or activation. The findings of the present study indicate that mTORC2, but not mTORC1, plays a vital role in controlling the hydrophobic motif phosphorylation and activity of SGK1. Our findings may explain why in previous studies phosphorylation of substrates, such as FOXO (forkhead box O), that could be regulated by SGK, are reduced in mTORC2-deficient cells. The results of the present study indicate that NDRG1 phosphorylation represents an excellent biomarker for mTORC2 activity.     

The human glucagon-like peptide-1 analogue liraglutide regulates pancreatic beta-cell proliferation and apoptosis via an AMPK/mTOR/P70S6K signaling pathway

Glucagon-like peptide-1 (GLP-1), an effective therapeutic agent for the treatment of diabetes, has been proven to protect pancreatic beta cells through many pathways. Recent evidence demonstrates that AMP-activated protein kinase (AMPK), as a metabolic regulator, coordinates beta-cell protein synthesis through regulation of the mammalian target of rapamycin (mTOR) signaling pathway. The purpose of the present study was to explore whether liraglutide, a human GLP-1 analogue, protects beta cells via AMPK/mTOR signaling. We evaluated INS-1 beta-cell line proliferation using the Cell Counting Kit-8, and examined the effect of GLP-1 on cellular ATP levels using an ATP assay kit. mTOR pathway protein expression levels were tested by Western blotting and glucolipotoxicity-induced cell apoptosis was evaluated by flow cytometry. Liraglutide increased beta-cell viability at an optimum concentration of 100 nmol/L in the presence of 11.1 or 30 mmol/L glucose. Liraglutide (100 nmol/L) activated mTOR and its downstream effectors, 70-kDa ribosomal protein S6 kinase and eIF4E-binding protein-1, in INS-1 cells. This effect was abated by pathway blockers: the AMPK activator AICAR and the mTOR inhibitor rapamycin. Furthermore, the effect of liraglutide on beta-cell proliferation was inhibited by AICAR and rapamycin. Liraglutide increased cellular ATP levels. In addition, liraglutide protected beta cells from glucolipotoxicity-induced apoptosis. This response was also prevented by rapamycin treatment. These results suggest that the enhancement of beta-cell proliferation by that GLP-1 receptor agonist liraglutide is mediated, at least in part, by AMPK/mTOR signaling. Liraglutide also prevents beta-cell glucolipotoxicity by activating mTOR.     

Fatty acids are novel nutrient factors to regulate mTORC1 lysosomal localization and apoptosis in podocytes

Podocyte apoptosis is a potent mechanism of proteinuria in diabetic nephropathy. More detailed mechanistic insight into podocyte apoptosis is needed to better understand the pathogenesis of diabetic nephropathy. An elevated level of serum free fatty acid (FFA), as well as hyperglycemia, is a clinical characteristic in diabetes, although its causal role in podocyte apoptosis remains unclear. This study examined the effect of three types of FFAs, saturated, monounsaturated and polyunsaturated FFAs, on podocyte apoptosis. Palmitate, a saturated FFA, induced endoplasmic reticulum (ER) stress-dependent apoptosis in podocytes. Oleate, a monounsaturated FFA, and eicosapentaenoic acid (EPA), an ω − 3 polyunsaturated FFA did not induce apoptosis; rather, they antagonized palmitate-induced apoptosis. Palmitate activated mammalian target of rapamycin (mTOR) complex 1 (mTORC1), a nutrient-sensing kinase regulating a wide range of cell biology. Furthermore, inhibition of mTORC1 activity by rapamycin or siRNA for Raptor, a component of mTORC1, ameliorated palmitate-induced ER stress and apoptosis in podocytes. Activity of mTORC1 is regulated by upstream kinases and Rag/Ragulator-dependent recruitment of mTOR onto lysosomal membranes. Palmitate activated mTORC1 by enhancing recruitment of mTOR onto lysosomal membranes, which was inhibited by co-incubation with oleate or EPA. Inhibition of mTOR translocation onto lysosomes by transfection with dominant-negative forms of Rag ameliorated palmitate-induced apoptosis. This study suggests that saturated and unsaturated FFAs have opposite effects on podocyte apoptosis by regulating mTORC1 activity via its translocation onto lysosomal membranes, and the results provide a better understanding of the pathogenesis in diabetic nephropathy and a novel role of mTORC1 in cell apoptosis.     

MicroRNA-451 regulates AMPK/mTORC1 signaling and fascin1 expression in HT-29 colorectal cancer

The earlier studies have shown that Fascin1 (FSCN1), the actin bundling protein, is over-expressed in colorectal cancers, and is associated with cancer cell progression. Here, we aimed to understand the molecular mechanisms regulating FSCN1 expression by focusing on mammalian target of rapamycin (mTOR) signaling and its regulator microRNA-451. We found that microRNA-451 was over-expressed in multiple colorectal cancer tissues, and its expression was correlated with mTOR complex 1 (mTORC1) activity and FSCN1 expression. In cultured colorectal cancer HT-29 cells, knockdown of FSCN1 by RNAi inhibited cell migration and proliferation. Activation of mTORC1 was required for FSCN1 expression, HT-29 cell migration and proliferation, as RAD001 and rapamycin, two mTORC1 inhibitors, suppressed FSCN1 expression, HT-29 cell migration and proliferation. Meanwhile, forced activation of AMP-activated protein kinase (AMPK), the negative regulator of mTORC1, by its activators or by the genetic mutation, inhibited mTORC1 activation, FSCN1 expression, cell migration and proliferation. In HT-29 cells, we found that over-expression of microRNA-451 inhibited AMPK activation, causing mTORC1 over-activation and FSCN1 up-regulation, cells were with high migration ability and proliferation rate. Significantly, these effects by microRNA-451 were largely inhibited by mTORC1 inhibitors or the AMPK activator AICAR. On the other hand, knockdown of miRNA-451 by the treatment of HT-29 cells with miRNA-451 antagomir inhibited mTORC1 activation and FSCN1 expression. The proliferation and migration of HT-29 cells after miRNA-45 knockdown were also inhibited. Our results suggested that the over-expressed microRNA-451 in colon cancer cells might inhibit AMPK to activate mTORC1, which mediates FSCN1 expression and cancer cell progression.     

Phosphatidic acid activates mammalian target of rapamycin complex 1 (mTORC1) kinase by displacing FK506 binding protein 38 (FKBP38) and exerting an allosteric effect

Phosphatidic acid (PA) is a critical mediator of mitogenic activation of mammalian target of rapamycin complex 1 (mTORC1) signaling, a master regulator of mammalian cell growth and proliferation. The mechanism by which PA activates mTORC1 signaling has remained unknown. Here, we report that PA selectively stimulates mTORC1 but not mTORC2 kinase activity in cells and in vitro. Furthermore, we show that PA competes with the mTORC1 inhibitor, FK506 binding protein 38 (FKBP38), for mTOR binding at a site encompassing the rapamycin-FKBP12 binding domain. This leads to PA antagonizing FKBP38 inhibition of mTORC1 kinase activity in vitro and rescuing mTORC1 signaling from FKBP38 in cells. Phospholipase D 1, a PA-generating enzyme that is an established upstream regulator of mTORC1, is found to negatively affect mTOR-FKBP38 interaction, confirming the role of endogenous PA in this regulation. Interestingly, removal of FKBP38 alone is insufficient to activate mTORC1 kinase and signaling, which require PA even when the FKBP38 level is drastically reduced by RNAi. In conclusion, we propose a dual mechanism for PA activation of mTORC1: PA displaces FKBP38 from mTOR and allosterically stimulates the catalytic activity of mTORC1.     

Insulin receptor substrate-2 proteasomal degradation mediated by a mammalian target of rapamycin (mTOR)-induced negative feedback down-regulates protein kinase B-mediated signaling pathway in beta-cells

Regulation of insulin receptor substrate (IRS)-2 expression is critical to beta-cell survival, but the mechanisms that control this are complex and undefined. Here in pancreatic beta-cells (INS-1), chronic exposure (>8 h) to 15 mm glucose and/or 5 nm IGF-1, increased Ser/Thr phosphorylation of IRS-2, which correlated with decreased IRS-2 levels. This glucose/IGF-1-induced decrease in IRS-2 levels was prevented by the proteasomal inhibitor, lactacystin. In addition, the glucose/IGF-1-induced increase in Ser/Thr phosphorylation of IRS-2 and the subsequent decrease in INS-1 cell IRS-2 protein levels was thwarted by the mammalian target of rapamycin(mTOR) inhibitor, rapamycin. Moreover, adenoviral-mediated expression of constitutively active mTOR (mTORDelta) further increased glucose/IGF-1-induced Ser/Thr phosphorylation of IRS-2 and decreased IRS-2 protein levels, whereas adenoviral-mediated expression of "kinase-dead" mTOR (mTOR-KD) conversely reduced Ser/Thr phosphorylation of IRS-2 and maintained IRS-2 protein levels. In adenoviral-infected beta-cells expressing mTORDelta, the decrease in IRS-2 protein levels was also prevented by rapamycin or lactacystin, further indicating a proteasomal mediated degradation of IRS-2 mediated via mTOR-induced Ser/Thr phosphorylation of IRS-2. Finally, we found that chronic activation of mTOR leading to decreased levels of IRS-2 in INS-1 cells led to a significant decrease in PKB activation and consequently increased beta-cell apoptosis. Thus, chronic activation of mTOR by glucose (and/or IGF-1) in beta-cells leads to increased Ser/Thr phosphorylation of IRS-2 that targets it for proteasomal degradation, resulting in decreased IRS-2 expression and increased beta-cell apoptosis. This may be a contributing mechanism as to how beta-cell mass is decreased by chronic hyperglycemia in the pathogenesis of type-2 diabetes.     

Kinase Suppressor of Ras 1 Is Not Required for the Generation of Regulatory and Memory T Cells

The mammalian target of rapamycin (mTOR) kinase is a critical regulator of the differentiation of helper and regulatory CD4+ T cells, as well as memory CD8+ T cells. In this study, we investigated the role of the ERK signaling pathway in regulating mTOR activation in T cells. We showed that activation of ERK following TCR engagement is required for sustained mTOR complex 1 (mTORC1) activation. Absence of kinase suppressor of Ras 1 (KSR1), a scaffold protein of the ERK signaling pathway, or inhibition of ERK resulted in decreased mTORC1 activity following T cell activation. However, KSR1-deficient mice displayed normal regulatory CD4+ T cell development, as well as normal memory CD8+ T cell responses to LCMV and Listeria monocytogenes infection. These data indicate that despite its role in mTORC1 activation, KSR1 is not required in vivo for mTOR-dependent T cell differentiation.     

ERK and Akt signaling pathways function through parallel mechanisms to promote mTORC1 signaling

The mammalian target of rapamycin (mTOR) is a protein kinase that, when present in a complex referred to as mTOR complex 1 (mTORC1), acts as an important regulator of growth and metabolism. The activity of the complex is regulated through multiple upstream signaling pathways, including those involving Akt and the extracellular-regulated kinase (ERK). Previous studies have shown that, in part, Akt and ERK promote mTORC1 signaling through phosphorylation of a GTPase activator protein (GAP), referred to as tuberous sclerosis complex 2 (TSC2), that acts as an upstream inhibitor of mTORC1. In the present study we extend the earlier studies to show that activation of the Akt and ERK pathways acts in a synergistic manner to promote mTORC1 signaling. Moreover, we provide evidence that the Akt and ERK signaling pathways converge on TSC2, and that Akt phosphorylates residues on TSC2 distinct from those phosphorylated by ERK. The results also suggest that leucine-induced stimulation of mTORC1 signaling occurs through a mechanism distinct from TSC2 and the Akt and ERK signaling pathways. Overall, the results are consistent with a model in which Akt and ERK phosphorylate distinct sites on TSC2, leading to greater repression of its GAP activity, and consequently a magnified stimulation of mTORC1 signaling, when compared with either input alone. The results further suggest that leucine acts through a mechanism distinct from TSC2 to stimulate mTORC1 signaling.