PRAS40: target or modulator of mTORC1 signalling and insulin action?

The complex interplay between redox dysregulation and mTOR signaling pathway in cancer: A rationale for cancer treatment

The mechanistic target of rapamycin (mTOR) is a highly conserved serine/threonine kinase that plays a critical role in regulating cellular processes such as growth, proliferation, and metabolism in healthy cells. Dysregulation of mTOR signaling and oxidative stress have been implicated in various diseases including cancer. This review aims to provide an overview of the current understanding of mTOR and its involvement in cell survival and the regulation of cancer cell metabolism as well as its complex interplay with reactive oxygen species (ROS). On the one hand, ROS can inhibit or activate mTOR pathway in cancer cells through various mechanisms. Conversely, mTOR signaling can induce oxidative stress in tumor cells notably due to the inhibition in the expression of antioxidant enzyme genes. Since mTOR is often activated and plays crucial role in cancer cell survival, the use of mTOR inhibitors, which often induce ROS accumulation, could be an interesting approach for cancer treatment. This review will address the advantages, disadvantages, combination strategies, and limitations associated with therapeutic modulation of mTOR signaling pathway in cancer treatment.

High-density lipoprotein protects normotensive and hypertensive rats against ischemia-reperfusion injury through differential regulation of mTORC1 and mTORC2 signaling

Background

High-density lipoprotein (HDL) protects against myocardial ischemia-reperfusion (I/R) injury. Mammalian target of rapamycin complexes 1 and 2 (mTORC1 and mTORC2) play opposing roles in protecting against I/R injury, whereby mTORC1 appears to be detrimental while mTORC2 is protective. However, the role of HDL and mTORC signaling in protecting against I/R in hypertensive rodents is not clearly understood. In this study, we investigated the involvement of mTORC1 and mTORC2 in HDL-mediated protection against myocardial I/R injury in normotensive Wistar Kyoto (WKY) rats and spontaneously hypertensive rats (SHR).

Methods

Hearts from WKY and SHR were subjected to I/R injury using a modified Langendorff system. Hemodynamics data were collected, and infarct size was measured. Rapamycin and JR-AB2-011 were used to test the role of mTORC1 and mTORC2, respectively. MK-2206 was used to test the role of Akt in HDL-mediated cardiac protection. The expression levels and the activation states of mediators of mTORC1 and mTORC2 signaling and myocardial apoptosis were measured by immunoblotting and/or enzyme-linked immunosorbent assay (ELISA).

Results

HDL protected hearts from WKY and SHR against I/R injury as indicated by significant improvements in cardiac hemodynamics and reduction in infarct size. HDL induced greater protection in WKY compared to SHR. HDL treatment attenuated mTORC1 signaling in WKY by reducing the phosphorylation of P70S6K (mTORC1 substrate). In SHR however, HDL attenuated mTORC1 signaling by reducing the levels of phospho-mTORC1, Rag C (mTORC1 activator), and phospho-PRAS40 (mTORC1 inhibitor). HDL increased the phosphorylation of mTORC2 substrate Akt, specifically the Akt2 isoform in SHR and to a greater extent in WKY. HDL-induced protection was abolished in the presence of Akt antagonist and involved attenuation of GSK, caspases 7 and 8 activation, and cytochrome C release.

Conclusion

HDL mediates cardiac protection via attenuation of mTORC1, activation of mTORC2-Akt2, and inhibition of myocardial apoptosis. HDL regulates mTORC1 and mTORC2 signaling via distinct mechanisms in normotensive and hypertensive rats. HDL attenuation of mTORC1 and activation of mTORC2-Akt2 signaling could be a mechanism by which HDL protects against myocardial I/R injury in hypertension.

IL-36α and Lipopolysaccharide Cooperatively Induce Autophagy by Triggering Pro-Autophagic Biased Signaling

Autophagy is an intracellular catabolic process that controls infections both directly and indirectly via its multifaceted effects on the innate and adaptive immune responses. It has been reported that LPS stimulates this cellular process, whereas the effect of IL-36α on autophagy remains largely unknown. We therefore investigated how IL-36α modulates the endogenous and LPS-induced autophagy in THP-1 cells. The levels of LC3B-II and autophagic flux were determined by Western blotting. The intracellular localization of LC3B was measured by immunofluorescence assay. The activation levels of signaling pathways implicated in autophagy regulation were evaluated by using a phosphokinase array. Our results showed that combined IL-36α and LPS treatment cooperatively increased the levels of LC3B-II and Beclin-1, stimulated the autophagic flux, facilitated intracellular redistribution of LC3B, and increased the average number of autophagosomes per cell. The IL36α/LPS combined treatment increased phosphorylation of STAT5a/b, had minimal effect on the Akt/PRAS40/mTOR pathway, and reduced the levels of phospho-Yes, phospho-FAK, and phospho-WNK1. Thus, this cytokine/PAMP combination triggers pro-autophagic biased signaling by several mechanisms and thus cooperatively stimulates the autophagic cascade. An increased autophagic activity of innate immune cells simultaneously exposed to IL-36α and LPS may play an important role in the pathogenesis of Gram-negative bacterial infections.

Targeting PRAS40: a novel therapeutic strategy for human diseases

Proline-rich Akt substrate of 40 kD (PRAS40) is not only the substrate of protein kinase B (PKB/Akt), but also the binding protein of 14-3-3 protein. PRAS40 is expressed in a variety of tissues in vivo and has multiple phosphorylation sites, which its activity is closely related to phosphorylation. Studies have shown that PRAS40 is involved in regulating cell growth, cell apoptosis, oxidative stress, autophagy and angiogenesis, as well as various of signalling pathways such as mammalian target of mammalian target rapamycin (mTOR), protein kinase B (PKB/Akt), nuclear factor kappa-B(NF-κB), proto-oncogene serine/threonine-protein kinase PIM-1(PIM1) and pyruvate kinase M2 (PKM2). The interactive roles between PRAS40 and these signal proteins were analysed by bioinformatics in this paper. Moreover, it is of great necessity for analyse the important roles of PRAS40 in some human diseases including cardiovascular disease, ischaemia-reperfusion injury, neurodegenerative disease, cancer, diabetes and other metabolic diseases. Finally, the effects of miRNA on the regulation of PRAS40 function and the occurrence and development of PRAS40-related diseases are also discussed. Overall, PRAS40 is expected to be a drug target and provide a new treatment strategy for human diseases.

Targeting protein kinase-b3 (akt3) signaling in melanoma

INTRODUCTION

Deregulated Akt activity leading to apoptosis inhibition, enhanced proliferation and drug resistance has been shown to be responsible for 35-70% of advanced metastatic melanomas. Of the three isoforms, the majority of melanomas have elevated Akt3 expression and activity. Hence, potent inhibitors targeting Akt are urgently required, which is possible only if (a) the factors responsible for the failure of Akt inhibitors in clinical trials is known; and (b) the information pertaining to synergistically acting targeted therapeutics is available. Areas covered: This review provides a brief introduction of the PI3K-Akt signaling pathway and its role in melanoma development. In addition, the functional role of key Akt pathway members such as PRAS40, GSK3 kinases, WEE1 kinase in melanoma development are discussed together with strategies to modulate these targets. Efficacy and safety of Akt inhibitors is also discussed. Finally, the mechanism(s) through which Akt leads to drug resistance is discussed in this expert opinion review. Expert opinion: Even though Akt play key roles in melanoma tumor progression, cell survival and drug resistance, many gaps still exist that require further understanding of Akt functions, especially in the (a) metastatic spread; (b) circulating melanoma cells survival; and

Phosphorylation of interleukin (IL)-24 is required for mediating its anti-cancer activity

Interleukin (IL)-24 is a tumor suppressor/cytokine gene that undergoes post-translational modifications (PTMs). Glycosylation and ubiquitination are important for IL-24 protein stabilization and degradation respectively. Little is known about IL-24 protein phosphorylation and its role in IL-24-mediated anti-tumor activities. In this study we conducted molecular studies to determine whether IL-24 phosphorylation is important for IL-24-mediated anti-cancer activity.

Human H1299 lung tumor cell line that was stably transfected with a doxycycline (DOX)-inducible (Tet-on) plasmid vector carrying the cDNA of IL-24-wild-type (IL-24^wt) or IL-24 with all five phosphorylation sites replaced (IL-24^mt) was used in the present study. Inhibition of tumor cell proliferation, cell migration and invasion, and induction of G2/M cell cycle arrest was observed in DOX-induced IL-24^wt-expressing cells but not in IL-24^mt-expressing cells. Secretion of IL-24^mt protein was greatly reduced compared to IL-24^wt protein. Further, IL-24^wt and IL-24^mt proteins markedly differed in their subcellular organelle localization. IL-24^wt but not IL-24^mt inhibited the AKT/mTOR signaling pathway. SiRNA-mediated AKT knockdown and overexpression of myristolyated AKT protein confirmed that IL-24^wt but not IL-24^mt mediated its anti-cancer activity by inhibiting the AKT signaling pathway.

Our results demonstrate that IL-24 phosphorylation is required for inhibiting the AKT/mTOR signaling pathway and exerting its anti-cancer activities.

Hypoxia upregulates Rab11-family interacting protein 4 through HIF-1α to promote the metastasis of hepatocellular carcinoma

Hypoxic microenvironment is a powerful driving force for the invasion and metastasis of hepatocellular carcinoma (HCC). Hypoxia-inducible factor 1α (HIF-1α), as a crucial regulator of transcriptional responses to hypoxia, induces the expression of multiple target genes involved in different steps of HCC metastatic process. It is critical to find target genes associated with metastasis under hypoxia for shedding new light on molecular mechanism of HCC metastasis. In this study, we uncovered that hypoxia could induce the upregulation of Rab11-family interacting protein 4 (Rab11-FIP4) and activation of Rab11-FIP4 promoter by HIF-1α. The overexpression of Rab11-FIP4 significantly enhanced the mobility and invasiveness of HCC cells in vitro, also contributed to distant lung metastasis in vivo, whereas silencing of Rab11-FIP4 decreased the ability of migration and invasion in HCC cells in vitro and suppressed lung metastasis in vivo. Rab11-FIP4 facilitated HCC metastasis through the phosphorylation of PRAS40, which was regulated by mTOR. Furthermore, the expression level of Rab11-FIP4 was significantly increased in HCC tissues and high expression of Rab11-FIP4 was closely correlated with vascular invasion and poor prognosis in HCC patients. A markedly positive correlation between the expression of Rab11-FIP4 and HIF-1α was observed in HCC tissues and combination of Rab11-FIP4 and HIF-1α was a more valuable predictor of poor prognosis for HCC patients. In conclusion, Rab11-FIP4 is a target gene of HIF-1α and has a pro-metastatic role in HCC, suggesting that Rab11-FIP4 may be a promising candidate target for HCC treatment.

Quantitative phosphoproteomics reveals novel phosphorylation events in insulin signaling regulated by protein phosphatase 1 regulatory subunit 12A

Serine/threonine protein phosphatase 1 regulatory subunit 12A (PPP1R12A) modulates the activity and specificity of the catalytic subunit of protein phosphatase 1, regulating various cellular processes via dephosphorylation. Nonetheless, little is known about phosphorylation events controlled by PPP1R12A in skeletal muscle insulin signaling. Here, we used quantitative phosphoproteomics to generate a global picture of phosphorylation events regulated by PPP1R12A in a L6 skeletal muscle cell line, which were engineered for inducible PPP1R12A knockdown. Phosphoproteomics revealed 3876 phosphorylation sites (620 were novel) in these cells. Furthermore, PPP1R12A knockdown resulted in increased overall phosphorylation in L6 cells at the basal condition, and changed phosphorylation levels for 698 sites (assigned to 295 phosphoproteins) at the basal and/or insulin-stimulated conditions. Pathway analysis on the 295 phosphoproteins revealed multiple significantly enriched pathways related to insulin signaling, such as mTOR signaling and RhoA signaling. Moreover, phosphorylation levels for numerous regulatory sites in these pathways were significantly changed due to PPP1R12A knockdown. These results indicate that PPP1R12A indeed plays a role in skeletal muscle insulin signaling, providing novel insights into the biology of insulin action. This new information may facilitate the design of experiments to better understand mechanisms underlying skeletal muscle insulin resistance and type 2 diabetes.

BIOLOGICAL SIGNIFICANCE

These results identify a large number of potential new substrates of serine/threonine protein phosphatase 1 and suggest that serine/threonine protein phosphatase 1 regulatory subunit 12A indeed plays a regulatory role in multiple pathways related to insulin action, providing novel insights into the biology of skeletal muscle insulin signaling. This information may facilitate the design of experiments to better understand the molecular mechanism responsible for skeletal muscle insulin resistance and associated diseases, such as type 2 diabetes and cardiovascular diseases.

Nuclear PRAS40 couples the Akt/mTORC1 signaling axis to the RPL11-HDM2-p53 nucleolar stress response pathway

The Ribosomal Protein (RP)-HDM2-p53 pathway has been shown to play key roles in oncogene-induced apoptosis and senescence, but the mechanism regulating this pathway remains elusive. The Proline-Rich Akt Substrate of 40 kDA (PRAS40) has recently been identified as a binding partner and inhibitor of the mechanistic Target of Rapamycin Complex 1 (mTORC1). Although other inhibitors of mTORC1 are known tumor suppressors, PRAS40 promotes cell survival and tumorigenesis. Here we demonstrate that Akt- and mTORC1-mediated phosphorylation of PRAS40 at T246 and S221, respectively, promotes nuclear-specific association of PRAS40 with Ribosomal Protein L11 (RPL11). Importantly, silencing of PRAS40 induces upregulation of p53 in a manner dependent upon RPL11. This effect is rescued by wild type PRAS40, but not by the RPL11 binding-null PRAS40 T246A mutant. We find that PRAS40 negatively regulates the RPL11-HDM2-p53 nucleolar stress response pathway and suppresses induction of p53-mediated cellular senescence. This work identifies nuclear PRAS40 as a dual-input signaling checkpoint that links cell growth and proliferation to inhibition of cellular senescence. These findings may help to explain the pro-tumorigenic effect of PRAS40 and identify the PRAS40-RPL11 complex as a promising target for p53-restorative anti-cancer drug discovery.

Proline-rich Akt substrate of 40-kDa contains a nuclear export signal

The proline-rich Akt substrate of 40-kDa (PRAS40) has been linked to the regulation of the activity of the mammalian target of rapamycin complex 1 as well as insulin action. Despite these cytosolic functions, PRAS40 was originally identified as nuclear phosphoprotein in Hela cells. This study aimed to detail mechanisms and consequences of the nucleocytosolic trafficking of PRAS40. Sequence analysis identified a potential leucine-rich nuclear export signal (NES) within PRAS40. Incubation of A14 fibroblasts overexpressing human PRAS40 (hPRAS40) resulted in nuclear accumulation of the protein. Furthermore, mutation of the NES mimicked the effects of leptomycin B, a specific inhibitor of nuclear export, on the subcellular localization of hPRAS40. Finally, A14 cells expressing the NES-mutant showed impaired activation of components of the Akt-pathway as well as of the mTORC1 substrate p70 S6 kinase after insulin stimulation. This impaired insulin signaling could be ascribed to reduced protein levels of insulin receptor substrate 1 in cells expressing mutant NES. In conclusion, PRAS40 contains a functional nuclear export signal. Furthermore, enforced nuclear accumulation of PRAS40 impairs insulin action, thereby substantiating the function of this protein in the regulation of insulin sensitivity.

PIM1 kinase inhibitors induce radiosensitization in non-small cell lung cancer cells

Radiotherapy plays a critical role in the treatment of non-small cell lung cancer (NSCLC). However, radioresistance is a major barrier against increasing the efficiency of radiotherapy for NSCLC. To understand the mechanisms underlying NSCLC radioresistance, we previously focused on the potential involvement of PIM1, PRAS40, FOXO3a, 14-3-3, and protein phosphatases. Among these proteins, PIM1 functioned as an oncogene and was found to act as a crucial mediator in radioresistant NSCLC cells. Therefore, we investigated the use of PIM1-specific inhibitors as novel therapeutic drugs to regulate radiosensitivity in NSCLC. After structure-based drug selection, SGI-1776, ETP-45299, and tryptanthrin were selected as candidates of PIM1 inhibitors that act as radiosensitizers. With irradiation, these drugs inhibited only PIM1 kinase activity without affecting PIM1 mRNA/protein levels or cellular localization. When PIM1 kinase activity was suppressed by these inhibitors, PRAS40 was not phosphorylated. Consequently, unphosphorylated PRAS40 did not form trimeric complexes with 14-3-3 and FOXO3a, leading to increased nuclear localization of FOXO3a. Nuclear FOXO3a promoted the expression of pro-apoptotic proteins such as Bim and FasL, resulting in a radiosensitizing effect on radioresistant NSCLC cells. Moreover, an in vivo xenograft mouse model confirmed this radiosensitizing effect induced by PIM1 inhibitors. In these model systems, tumor volume was significantly reduced by a combinational treatment with irradiation and PIM1 inhibitors compared to irradiation alone. Taken together, our findings provided evidence that PIM1-specific inhibitors, SGI-1776, ETP-45299, and tryptanthrin, can act as novel radiosensitizers to enhance the efficacy of radiotherapy by inhibiting irradiation-induced signaling pathway associated with radioresistance.

Role of PRAS40 in Akt and mTOR signaling in health and disease

The proline-rich Akt substrate of 40 kDa (PRAS40) acts at the intersection of the Akt- and mammalian target of rapamycin (mTOR)-mediated signaling pathways. The protein kinase mTOR is the catalytic subunit of two distinct signaling complexes, mTOR complex 1 (mTORC1) and mTORC2, that link energy and nutrients to the regulation of cellular growth and energy metabolism. Activation of mTOR in response to nutrients and growth factors results in the phosphorylation of numerous substrates, including the phosphorylations of S6 kinase by mTORC1 and Akt by mTORC2. Alterations in Akt and mTOR activity have been linked to the progression of multiple diseases such as cancer and type 2 diabetes. Although PRAS40 was first reported as substrate for Akt, investigations toward mTOR-binding partners subsequently identified PRAS40 as both component and substrate of mTORC1. Phosphorylation of PRAS40 by Akt and by mTORC1 itself results in dissociation of PRAS40 from mTORC1 and may relieve an inhibitory constraint on mTORC1 activity. Adding to the complexity is that gene silencing studies indicate that PRAS40 is also necessary for the activity of the mTORC1 complex. This review summarizes the regulation and potential function(s) of PRAS40 in the complex Akt- and mTOR-signaling network in health and disease.

Epidermal growth factor-induced vacuolar (H+)-atpase assembly: a role in signaling via mTORC1 activation

Using proteomics and immunofluorescence, we demonstrated epidermal growth factor (EGF) induced recruitment of extrinsic V(1) subunits of the vacuolar (H(+))-ATPase (V-ATPase) to rat liver endosomes. This was accompanied by reduced vacuolar pH. Bafilomycin, an inhibitor of V-ATPase, inhibited EGF-stimulated DNA synthesis and mammalian target of rapamycin complex 1 (mTORC1) activation as indicated by a decrease in eukaryotic initiation factor 4E-binding 1 (4E-BP1) phosphorylation and p70 ribosomal S6 protein kinase (p70S6K) phosphorylation and kinase activity. There was no corresponding inhibition of EGF-induced Akt and extracellular signal-regulated kinase (Erk) activation. Chloroquine, a neutralizer of vacuolar pH, mimicked bafilomycin effects. Bafilomycin did not inhibit the association of mTORC1 with Raptor nor did it affect AMP-activated protein kinase activity. Rather, the intracellular concentrations of essential but not non-essential amino acids were decreased by bafilomycin in EGF-treated primary rat hepatocytes. Cycloheximide, a translation elongation inhibitor known to augment intracellular amino acid levels, prevented the effect of bafilomycin on amino acids levels and completely reversed its inhibition of EGF-induced mTORC1 activation. In vivo administration of EGF stimulated the recruitment of Ras homologue enriched in brain (Rheb) but not mammalian target of rapamycin (mTOR) to endosomes and lysosomes. This was inhibited by chloroquine treatment. Our results suggest a role for vacuolar acidification in EGF signaling to mTORC1.

Intestinal cell kinase (ICK) promotes activation of mTOR complex 1 (mTORC1) through phosphorylation of Raptor Thr-908

Intestinal cell kinase (ICK), named after its cloning origin, the intestine, is actually a ubiquitously expressed and highly conserved serine/threonine protein kinase. Recently we reported that ICK supports cell proliferation and G(1) cell cycle progression. ICK deficiency significantly disrupted the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) signaling events. However, the biological substrates that mediate the downstream signaling effects of ICK in proliferation and the molecular mechanisms by which ICK interacts with mTORC1 are not well defined. Our prior studies also provided biochemical evidence that ICK interacts with the mTOR/Raptor complex in cells and phosphorylates Raptor in vitro. In this report, we investigated whether and how ICK targets Raptor to regulate the activity of mTORC1. Using the ICK substrate consensus sequence [R-P-X-S/T-P/A/T/S], we identified a putative phosphorylation site, RPGT908T, for ICK in human Raptor. By mass spectrometry and a phospho-specific antibody, we showed that Raptor Thr-908 is a novel in vivo phosphorylation site. ICK is able to phosphorylate Raptor Thr-908 both in vitro and in vivo and when Raptor exists in protein complexes with or without mTOR. Although expression of the Raptor T908A mutant did not affect the mTORC1 integrity, it markedly impaired the mTORC1 activation by insulin or by overexpression of the small GTP-binding protein RheB under nutrient starvation. Our findings demonstrate an important role for ICK in modulating the activity of mTORC1 through phosphorylation of Raptor Thr-908 and thus implicate a potential signaling mechanism by which ICK regulates cell proliferation and division.

PRAS40 is a functionally critical target for EWS repression in Ewing sarcoma

Ewing sarcoma family tumors (ESFT) are highly aggressive and highly metastatic tumors caused by a chromosomal fusion between the Ewing sarcoma protein (EWS) with the transcription factor FLI-1. However, expression of the EWS/FLI-1 chimeric oncogene by itself is insufficient for carcinogenesis, suggesting that additional events are required. Here, we report the identification of the Akt substrate PRAS40 as an EWS target gene. EWS negatively regulates PRAS40 expression by binding the 3' untranslated region in PRAS40 mRNA. ESFT cell proliferation was suppressed by treatment with an Akt inhibitor, and ESFT cell proliferation and metastatic growth were suppressed by siRNA-mediated PRAS40 knockdown. Furthermore, PRAS40 knockdown was sufficient to reverse an increased cell proliferation elicited by EWS knockdown. In support of a pathologic role for PRAS40 elevation in EFST, we documented inverse protein levels of EWS and PRAS40 in ESFT cells. Together, our findings suggest that PRAS40 promotes the development of ESFT and might therefore represent a novel therapeutic target in this aggressive disease.

Tuberin and PRAS40 are anti-apoptotic gatekeepers during early human amniotic fluid stem-cell differentiation

Embryoid bodies (EBs) are three-dimensional multicellular aggregates allowing the in vitro investigation of stem-cell differentiation processes mimicking early embryogenesis. Human amniotic fluid stem (AFS) cells harbor high proliferation potential, do not raise the ethical issues of embryonic stem cells, have a lower risk for tumor development, do not need exogenic induction of pluripotency and are chromosomal stable. Starting from a single human AFS cell, EBs can be formed accompanied by the differentiation into cells of all three embryonic germ layers. Here, we report that siRNA-mediated knockdown of the endogenous tuberous sclerosis complex-2 (TSC2) gene product tuberin or of proline-rich Akt substrate of 40 kDa (PRAS40), the two major negative regulators of mammalian target of rapamycin (mTOR), leads to massive apoptotic cell death during EB development of human AFS cells without affecting the endodermal, mesodermal and ectodermal cell differentiation spectrum. Co-knockdown of endogenous mTOR demonstrated these effects to be mTOR-dependent. Our findings prove this enzyme cascade to be an essential anti-apoptotic gatekeeper of stem-cell differentiation during EB formation. These data allow new insights into the regulation of early stem-cell maintenance and differentiation and identify a new role of the tumor suppressor tuberin and the oncogenic protein PRAS40 with the relevance for a more detailed understanding of the pathogenesis of diseases associated with altered activities of these gene products.

The capture of phosphoproteins by 14-3-3 proteins mediates actions of insulin

How does signalling via PI3K-PKB (AKT)-mTORC1-p70S6K and ERK-p90RSK mediate wide-ranging physiological responses to insulin? Quantitative proteomics and biochemical experiments are revealing that these signalling pathways induce the phosphorylation of large and overlapping sets of proteins, which are then captured by phosphoprotein-binding proteins named 14-3-3s. The 14-3-3s are dimers that dock onto dual-phosphorylated sites in a configuration with special signalling and mechanical properties. They interact with the Rab GTPase-activating proteins AS160 and TBC1D1 to regulate glucose uptake into target tissues in response to insulin and energy stress. Dynamic patterns in the 14-3-3-binding phosphoproteome are providing new insights into how insulin triggers coherent shifts in metabolism that are integrated with other cellular response systems.

PIM1-activated PRAS40 regulates radioresistance in non-small cell lung cancer cells through interplay with FOXO3a, 14-3-3 and protein phosphatases

Resistance of cancer cells to ionizing radiation plays an important role in the clinical setting of lung cancer treatment. To date, however, the exact molecular mechanism of radiosensitivity has not been well explained. In this study, we compared radioresistance in two types of non-small cell lung cancer (NSCLC) cells, NCI-H460 and A549, and investigated the signaling pathways that confer radioresistance. In radioresistant cells, exposure to radiation led to overexpression of PIM1 and reduction of protein phosphatases (PP2A and PP5), which induced translocation of PIM1 into the nucleus. Increased nuclear PIM1 phosphorylated PRAS40. Consequently, pPRAS40 made a trimeric complex with 14-3-3 and AKT-activated pFOXO3a, which then moved rapidly to the cytoplasm. Cytoplasmic retention of FOXO3a was associated with downregulation of proapoptotic genes and possibly radioresistance. On the other hand, no suppressive effect of radiation on protein phosphatases was detected and, concomitantly, protein phosphatases downregulated PIM1 in radiosensitive cells. In this setting, PIM1-activated pPRAS40, AKT-activated pFOXO3a, and their complex formation with 14-3-3 could be key regulators of the radiation-induced radioresistance in NSCLC cells.

Limited therapeutic effect of N-acetylcysteine on hepatic insulin resistance in an experimental model of alcohol-induced steatohepatitis

BACKGROUND

Alcohol-related steatohepatitis is associated with increased oxidative stress, DNA damage, lipotoxicity, and insulin resistance in liver. As inflammation and oxidative stress can promote insulin resistance, effective treatment with antioxidants, for example, N-acetylcysteine (NAC), may restore ethanol-impaired insulin signaling in the liver.

METHODS

Adult male Sprague-Dawley rats were fed for 130 days with liquid diets containing 0 or 37% ethanol by caloric content, and simultaneously treated with vehicle or NAC. Chow-fed controls were studied in parallel. Liver tissues were used for histopathology, cytokine activation, and insulin/IGF-1 signaling assays.

RESULTS

We observed significant positive trends of increasing severity of steatohepatitis (p = 0.016) with accumulation of neutral lipid (p = 0.0002) and triglycerides (p = 0.0004) from chow to control, to the ethanol diet, irrespective of NAC treatment. In ethanol-fed rats, NAC reduced inflammation, converted the steatosis from a predominantly microvesicular to a mainly macrovesicular histological pattern, reduced pro-inflammatory cytokine gene expression, ceramide load, and acid sphingomyelinase activity, and increased expression of IGF-1 receptor and IGF-2 in liver. However, NAC did not abrogate ethanol-mediated impairments in signaling through insulin/IGF-1 receptors, IRS-1, Akt, GSK-3β, or p70S6K, nor did it significantly reduce pro-ceramide or GM3 ganglioside gene expression in liver.

CONCLUSIONS

Antioxidant treatments reduce the severity of chronic alcohol-related steatohepatitis, possibly because of the decreased expression of inflammatory mediators and ceramide accumulation, but they do not restore insulin/IGF-1 signaling in liver, most likely due to persistent elevation of GM3 synthase expression. Effective treatment of alcohol-related steatohepatitis most likely requires dual targeting of oxidative stress and insulin/IGF resistance.

A herpesvirus kinase that masquerades as Akt: you don't have to look like Akt, to act like it

The cellular protein synthesis machinery is tightly regulated and capable of rapid reaction to a variety of physiological inputs critical in stress-response, cell cycle control, cancer biology, and virus infection. One important strategy for stimulating protein synthesis involves the ser/thr kinase Akt, which subsequently triggers inactivation of the cap-dependent translational repressor 4E-BP1 by an mTOR-containing protein complex (mTORC1). A recent paper demonstrated that herpes simplex virus utilizes a remarkable tactic to activate mTOR in infected cells. Instead of using the cellular Akt, the virus produces a ser / thr kinase called Us3 that doesn't look like Akt, but masquerades as Akt. By making the Akt-like protein unrecognizable, this disguise allows it to bypass the strict limits normally imposed on the real cellular Akt. Importantly, preventing the virus Akt-imposter from triggering mTORC1 inhibited viral growth, suggesting a new way to block herpes simplex virus. This study also raises the possibility that other Akt-impersonators may lurk hidden in our own genomes, possibly contributing to diseases ranging from diabetes to cancer.

Sepsis-induced alterations in protein-protein interactions within mTOR complex 1 and the modulating effect of leucine on muscle protein synthesis

Sepsis-induced muscle atrophy is produced in part by decreased protein synthesis mediated by inhibition of mTOR (mammalian target of rapamycin). The present study tests the hypothesis that alteration of specific protein-protein interactions within the mTORC1 (mTOR complex 1) contributes to the decreased mTOR activity observed after cecal ligation and puncture in rats. Sepsis decreased in vivo translational efficiency in gastrocnemius and reduced the phosphorylation of eukaryotic initiation factor (eIF) 4E-binding protein (BP) 1, S6 kinase (S6K) 1, and mTOR, compared with time-matched pair-fed controls. Sepsis decreased T246-phosphorylated PRAS40 (proline-rich Akt substrate 40) and reciprocally increased S792-phosphorylated raptor (regulatory associated protein of mTOR). Despite these phosphorylation changes, sepsis did not alter PRAS40 binding to raptor. The amount of the mTOR-raptor complex did not differ between groups. In contrast, the binding and retention of both 4E-BP1 and S6K1 to raptor were increased, and, conversely, the binding of raptor with eIF3 was decreased in sepsis. These changes in mTORC1 in the basal state were associated with enhanced 5'-AMP activated kinase activity. Acute in vivo leucine stimulation increased muscle protein synthesis in control, but not septic rats. This muscle leucine resistance was associated with coordinated changes in raptor-eIF3 binding and 4E-BP1 phosphorylation. Overall, our data suggest the sepsis-induced decrease in muscle protein synthesis may be mediated by the inability of 4E-BP1 and S6K1 to be phosphorylated and released from mTORC1 as well as the decreased recruitment of eIF3 necessary for a functional 48S complex. These data provide additional mechanistic insight into the molecular mechanisms by which sepsis impairs both basal protein synthesis and the anabolic response to the nutrient signal leucine in skeletal muscle.

Akt phosphorylation on Thr308 but not on Ser473 correlates with Akt protein kinase activity in human non-small cell lung cancer

Background:

The activity of the protein kinase Akt is frequently dysregulated in cancer and is an important factor in the growth and survival of tumour cells. Akt activation involves the phosphorylation of two residues: threonine 308 (Thr308) in the activation loop and serine 473 (Ser473) in the C-terminal hydrophobic motif. Phosphorylation of Ser473 has been extensively studied in tumour samples as a correlate for Akt activity, yet the phosphorylation of Thr308 or of downstream Akt substrates is rarely assessed.

Methods:

The phosphorylation status of Thr308 and Ser473 was compared with that of three separate Akt substrates – PRAS40, TSC2 and TBC1D4 – in fresh frozen samples of early-stage human non-small cell lung cancer (NSCLC).

Results:

Akt Thr308 phosphorylation correlated with the phosphorylation of each Akt substrate tested, whereas Akt Ser473 phosphorylation did not correlate with the phosphorylation of any of the substrates examined.

Conclusion:

The phosphorylation of Thr308 is a more reliable biomarker for the protein kinase activity of Akt in tumour samples than Ser473. Any evaluation of the link between Akt phosphorylation or activity in tumour samples and the prediction or prognosis of disease should, therefore, focus on measuring the phosphorylation of Akt on Thr308 and/or at least one downstream Akt substrate, rather than Akt Ser473 phosphorylation alone.

Targeting the dysregulated mammalian target of rapamycin pathway in organ transplantation: killing 2 birds with 1 stone

Dysregulation and hyperactivation of the mammalian target of rapamycin (mTOR) pathway define the molecular basis of the hamartoma syndromes, including Cowden syndrome, tuberous sclerosis complex (TSC)/lymphangioleiomyomatosis, and Peutz-Jeghers syndrome. Loss of the tumor suppressors phosphatase and tensin homolog (PTEN), TSC1, TSC2, and LKB1 results in uncontrolled growth of usually benign tumors in various organs that, however, frequently lead to organ failure. Therefore, organ transplantation is a common therapeutic option in distinct patients with hamartoma syndromes, especially those with TSC/lymphangioleiomyomatosis. mTOR inhibitors are currently used in allogeneic transplantation as immunosuppressants and for the treatment of a growing number of cancers with dysregulated mTOR/phosphoinositide 3-kinase pathway. This dual targeting provides the unique opportunity for mTOR inhibitors to affect hamartoma syndromes at the molecular level along with potent immunosuppression in transplanted individuals. Here, we review the molecular mechanisms of hamartoma syndromes and discuss the recent clinical progress in transplant patients with hamartomas. Combining the identification of novel molecular targets of the phosphoinositide 3-kinase/mTOR pathway with insights into the clinical effectiveness of current therapeutic strategies sets the stage for a broader translational potential essential for further progress both in the treatment of cancer and for transplantation.

mTORC1 signaling: what we still don't know

The mammalian target of rapamycin (mTOR) is a protein kinase that plays key roles in cellular regulation. It forms complexes with additional proteins. The best-understood one is mTOR complex 1 (mTORC1). The regulation and cellular functions of mTORC1 have been the subjects of intense study; despite this, many questions remain to be answered. They include questions about the actual mechanisms by which mTORC1 signaling is stimulated by hormones and growth factors, which involves the small GTPase Rheb, and by amino acids, which involves other GTPase proteins. The control of Rheb and the mechanism by which it activates mTORC1 remain incompletely understood. Although it has been known for many years that rapamycin interferes with some functions of mTORC1, it is not known how it does this, or why only some functions of mTORC1 are affected. mTORC1 regulates diverse cellular functions. Several mTORC1 substrates are now known, although in several cases their physiological roles are poorly or incompletely understood. In the case of several processes, although it is clear that they are regulated by mTORC1, it is not known how mTORC1 does this. Lastly, mTORC1 is implicated in ageing, but again it is unclear what mechanisms account for this. Given the importance of mTORC1 signaling both for cellular functions and in human disease, it is a high priority to gain further insights into the control of mTORC1 signaling and the mechanisms by which it controls cellular functions and animal physiology.

BacMam-enabled LanthaScreen cellular assays for PI3K/Akt pathway compound profiling in disease-relevant cell backgrounds

The authors recently reported the development and application of multiple LanthaScreen cellular assays to interrogate specific steps within the PI3K/Akt pathway. The importance of this signaling cascade in regulating fundamental aspects of cell growth and survival, as well as in the progression of cancer, underscores the need for portable cell-based assays for compound profiling in multiple disease-relevant cell backgrounds. To meet this need, the authors have now expanded their LanthaScreen assay platform across a variety of cell types using a gene delivery technology known as BacMam. Here, they have demonstrated the successful detection of Akt-dependent phosphorylation of PRAS40 at Thr246 in 10 different cell lines harboring mutations known to activate the PI3K/Akt pathway. In addition, they generated inhibitory profiles of 17 known pathway inhibitors in these same cells to validate the approach of using the BacMam-enabled LanthaScreen cellular assay format to rapidly profile compounds in disease-relevant cell types. Importantly, their results provide a broad illustration of how the genetic alterations that affect PI3K/Akt signaling can also influence the inhibitory profile of a given compound.

Phosphorylation of PRAS40 on Thr246 by PKB/AKT facilitates efficient phosphorylation of Ser183 by mTORC1

Type 2 diabetes is associated with alterations in protein kinase B (PKB/Akt) and mammalian target of rapamycin complex 1 (mTORC1) signalling. The proline-rich Akt substrate of 40-kDa (PRAS40) is a component of mTORC1, which has a regulatory function at the intersection of the PKB/Akt and mTORC1 signalling pathway. Phosphorylation of PRAS40-Thr246 by PKB/Akt, and PRAS40-Ser183 and PRAS40-Ser221 by mTORC1 results in dissociation from mTORC1, and its binding to 14-3-3 proteins. Although all phosphorylation sites within PRAS40 have been implicated in 14-3-3 binding, substitution of Thr246 by Ala alone is sufficient to abolish 14-3-3 binding under conditions of intact mTORC1 signalling. This suggests that phosphorylation of PRAS40-Thr246 may facilitate efficient phosphorylation of PRAS40 on its mTORC1-dependent sites. In the present study, we investigated the mechanism of PRAS40-Ser183 phosphorylation in response to insulin. Insulin promoted PRAS40-Ser183 phosphorylation after a euglycaemic-hyperinsulinaemic clamp in human skeletal muscle. The insulin-induced PRAS40-Ser183 phosphorylation was further evidenced in vivo in rat skeletal and cardiac muscle, and in vitro in A14 fibroblasts, 3T3L1 adipocytes and L6 myotubes. Inhibition of mTORC1 by rapamycin or amino acid deprivation partially abrogated insulin-mediated PRAS40-Ser183 phosphorylation in cultured cell lines. However, lowering insulin-induced PRAS40-Thr246 phosphorylation using wortmannin or palmitate in cell lines, or by feeding rats a high-fat diet, completely abolished insulin-mediated PRAS40-Ser183 phosphorylation. In addition, replacement of Thr246 by Ala reduced insulin-mediated PRAS40-Ser183 phosphorylation. We conclude that PRAS40-Ser183 is a component of insulin action, and that efficient phosphorylation of PRAS40-Ser183 by mTORC1 requires the phosphorylation of PRAS40-Thr246 by PKB/Akt.

Molecular mechanisms of metabolic regulation by insulin in Drosophila

The insulin signalling pathway is highly conserved from mammals to Drosophila. Insulin signalling in the fly, as in mammals, regulates a number of physiological functions, including carbohydrate and lipid metabolism, tissue growth and longevity. In the present review, I discuss the molecular mechanisms by which insulin signalling regulates metabolism in Drosophila, comparing and contrasting with the mammalian system. I discuss both the intracellular signalling network, as well as the communication between organs in the fly.