Modulation of the Protein Kinase Activity of mTOR

Protein kinase A controls yeast growth in visible light

Background

A wide variety of photosynthetic and non-photosynthetic species sense and respond to light, having developed protective mechanisms to adapt to damaging effects on DNA and proteins. While the biology of UV light-induced damage has been well studied, cellular responses to stress from visible light (400–700 nm) remain poorly understood despite being a regular part of the life cycle of many organisms. Here, we developed a high-throughput method for measuring growth under visible light stress and used it to screen for light sensitivity in the yeast gene deletion collection.

Results

We found genes involved in HOG pathway signaling, RNA polymerase II transcription, translation, diphthamide modifications of the translational elongation factor eEF2, and the oxidative stress response to be required for light resistance. Reduced nuclear localization of the transcription factor Msn2 and lower glycogen accumulation indicated higher protein kinase A (cAMP-dependent protein kinase, PKA) activity in many light-sensitive gene deletion strains. We therefore used an ectopic fluorescent PKA reporter and mutants with constitutively altered PKA activity to show that repression of PKA is essential for resistance to visible light.

Conclusion

We conclude that yeast photobiology is multifaceted and that protein kinase A plays a key role in the ability of cells to grow upon visible light exposure. We propose that visible light impacts on the biology and evolution of many non-photosynthetic organisms and have practical implications for how organisms are studied in the laboratory, with or without illumination.

Metabotropic glutamate receptor I (mGluR1) antagonism impairs cocaine-induced conditioned place preference via inhibition of protein synthesis

Antagonism of group I metabotropic glutamate receptors (mGluR1 and mGluR5) reduces behavioral effects of drugs of abuse, including cocaine. However, the underlying mechanisms remain poorly understood. Activation of mGluR5 increases protein synthesis at synapses. Although mGluR5-induced excessive protein synthesis has been implicated in the pathology of fragile X syndrome, it remains unknown whether group I mGluR-mediated protein synthesis is involved in any behavioral effects of drugs of abuse. We report that group I mGluR agonist DHPG induced more pronounced initial depression of inhibitory postsynaptic currents (IPSCs) followed by modest long-term depression (I-LTD) in dopamine neurons of rat ventral tegmental area (VTA) through the activation of mGluR1. The early component of DHPG-induced depression of IPSCs was mediated by the cannabinoid CB1 receptors, while DHPG-induced I-LTD was dependent on protein synthesis. Western blotting analysis indicates that mGluR1 was coupled to extracellular signal-regulated kinase (ERK) and mammalian target of rapamycin (mTOR) signaling pathways to increase translation. We also show that cocaine conditioning activated translation machinery in the VTA via an mGluR1-dependent mechanism. Furthermore, intra-VTA microinjections of mGluR1 antagonist JNJ16259685 and protein synthesis inhibitor cycloheximide significantly attenuated or blocked the acquisition of cocaine-induced conditioned place preference (CPP) and activation of translation elongation factors. Taken together, these results suggest that mGluR1 antagonism inhibits de novo protein synthesis; this effect may block the formation of cocaine-cue associations and thus provide a mechanism for the reduction in CPP to cocaine.

Global analysis of protein phosphorylation networks in insulin signaling by sequential enrichment of phosphoproteins and phosphopeptides

Although the important role of protein phosphorylation in insulin signaling networks is well recognized, its analysis in vivo has not been pursued in a systematic fashion through proteome-wide studies. Here we undertake a global analysis of insulin-induced changes in the rat liver cytoplasmic and endosomal phosphoproteome by sequential enrichment of phosphoproteins and phosphopeptides. After subcellular fractionation proteins were denatured and loaded onto iminodiacetic acid-modified Sepharose with immobilized Al³⁺ ions (IMAC-Al resin). Retained phosphoproteins were eluted with 50 mM phosphate and proteolytically digested. The digest was then loaded onto an IMAC-Al resin and phosphopeptides were eluted with 50 mM phosphate, and resolved by 2-dimensional liquid chromatography, which combined offline weak anion exchange and online reverse phase separations. The peptides were identified by tandem mass spectrometry, which also detected the phosphorylation sites. Non-phosphorylated peptides found in the flow-through of the IMAC-Al columns were also analyzed providing complementary information for protein identification. In this study we enriched phosphopeptides to ~85% purity and identified 1456 phosphopeptides from 604 liver phosphoproteins. Eighty-nine phosphosites including 45 novel ones in 83 proteins involved in vesicular transport, metabolism, cell motility and structure, gene expression and various signaling pathways were changed in response to insulin treatment. Together these findings could provide potential new markers for evaluating insulin action and resistance in obesity and diabetes.

Fibrocyte CXCR4 regulation as a therapeutic target in pulmonary fibrosis

Fibrotic interstitial lung diseases are characterized by progressive decline in lung function and premature death from respiratory failure. Fibrocytes are circulating bone marrow-derived progenitor cells that traffic to the lungs and contribute to fibrosis and may represent novel therapeutic targets in these diseases. We have previously found the recruitment of fibrocytes to the lung to be dependent on the chemokine ligand CXCL12. Given that the expression of the CXCL12 receptor, CXCR4, can be modulated pharmacologically in other cell types, we tested the hypotheses that the regulation of CXCR4 expression on fibrocytes mediates their influx to the lung in the context of pulmonary fibrosis and that pharmacologic inhibition of this process results in attenuated disease severity. CXCR4 was the predominant chemokine receptor on human fibrocytes, and its expression on fibrocytes was enhanced by hypoxia and by growth factors including platelet-derived growth factor. Both hypoxia-induced and growth factor-induced CXCR4 expressions were attenuated by specific inhibition of PI3-kinase and mTOR. Finally, in the mouse model of bleomycin-induced pulmonary fibrosis, treatment with the mTOR inhibitor rapamycin resulted in reduced numbers of CXCR4-expressing fibrocytes in the peripheral blood and lung as well as reduced lung collagen deposition. Taken together, these experiments support the notion that pharmacologic inhibition of the CXCR4/CXCL12 biological axis is achievable in human fibrocytes and reduces the magnitude of pulmonary fibrosis in an animal model. This approach may hold promise in human fibrotic lung diseases.

Hypercholesterolemia is associated with hyperactive cardiac mTORC1 and mTORC2 signaling

Nutritional excess and hyperlipidemia increase the heart's susceptibility to ischemic injury. Mammalian target of rapamycin (mTOR) controls the cellular response to nutritional status and may play a role in ischemic injury. To explore the effect of hypercholesterolemia on cardiac mTOR signaling, we assessed mTOR signaling in hypercholesterolemic swine (HC) that are also susceptible to increased cardiac ischemia-reperfusion injury. Yucatan pigs were fed a high-fat/high-cholesterol diet for 4 weeks to induce hypercholesterolemia, and mTOR signaling was measured by immunoblotting and immunofluorescence in the non-ischemic left ventricular area. Total myocardial mTOR and raptor levels were markedly increased in the HC group compared to the normocholesterolemic group, and directly correlated with serum cholesterol levels. mTOR exhibited intense perinuclear staining in myocytes only in the HC group. Hypercholesterolemia was associated with hyperactive signaling upstream and downstream of both mTOR complexes, including myocardial Akt, S6K1, 4EBP1, S6 and PKC-alpha, increased levels of cardiac hypertrophy markers, and a trend toward lower levels of myocardial autophagy. Hypercholesterolemia can now be added to the growing list of conditions associated with aberrant mTOR signaling. Hypercholesterolemia produces a unique profile of alterations in cardiac mTOR signaling, which is a potential target in cardiac diseases associated with hypercholesterolemia and nutritional excess.

A tale of two tails: activation of DNA damage checkpoint kinase Mec1/ATR by the 9-1-1 clamp and by Dpb11/TopBP1

The DNA damage and replication checkpoint kinase Mec1/ATR is a member of the PI3-kinase related kinases that function in response to various genotoxic stresses. The checkpoint clamp 9-1-1 (Rad9-Rad1-Hus1 in S. pombe and mammals; Ddc1-Rad17-Mec3 in S. cerevisiae) executes two distinct checkpoint functions. In S. cerevisiae, DNA-bound 9-1-1 directly activates Mec1 kinase activity, a function that has not been demonstrated in other organisms. A second, conserved activity of 9-1-1 is that of TopBP1/Cut5/Dpb11 recruitment to stalled replication sites; subsequent activation of Mec1/ATR is carried out by TopBP1/Cut5/Dpb11. Biochemical studies indicate that the mode of Mec1/ATR activation by S. cerevisiae 9-1-1 is analogous to activation by S. cerevisiae Dpb11 or by vertebrate TopBP1: activation is mediated by the intrinsically disordered C-terminal tail of each activator. The relative contributions made by multiple activators of Mec1/ATR are discussed.

Yeast DNA replication protein Dpb11 activates the Mec1/ATR checkpoint kinase

The Saccharomyces cerevisiae Mec1-Ddc2 protein kinase (human ATR-ATRIP) initiates a signal transduction pathway in response to DNA damage and replication stress to mediate cell cycle arrest. The yeast DNA damage checkpoint clamp Ddc1-Mec3-Rad17 (human Rad9-Hus1-Rad1: 9-1-1) is loaded around effector DNA and thereby activates Mec1 kinase. Dpb11 (Schizosaccharomyces pombe Cut5/Rad4 or human TopBP1) is an essential protein required for the initiation of DNA replication and has a role in checkpoint activation. In this study, we demonstrate that Dpb11 directly activates the Mec1 kinase in phosphorylating the downstream effector kinase Rad53 (human Chk1/2) and DNA bound RPA. However, DNA was not required for Dpb11 to function as an activator. Dpb11 and yeast 9-1-1 independently activate Mec1, but substantial synergism in activation was observed when both activators were present. Our studies suggest that Dpb11 and 9-1-1 may partially compensate for each other during yeast checkpoint function.

Contribution of natural inhibitors to the understanding of the PI3K/PDK1/PKB pathway in the insulin-mediated intracellular signaling cascade

The critical initial steps in insulin action include phosphorylation of adapter proteins and activation of phosphatidylinositol 3-kinase (PI3K). One of important components in this process is a protein called Akt/protein kinase B (PKB). The work of numerous different researchers indicates a role of PKB in regulating insulin-stimulated glucose uptake. The crucial role of lipid second messengers in PKB activation has been dissected through the use of the PI3K-specific inhibitors wortmannin and LY294002. Receptor-activated PI3K synthesizes the lipid second messenger PtdIns[3,4,5]-trisphosphate, leading to the recruitment of PKB to the membrane. Membrane attachment of PKB is mediated by its pleckstrin homology domain binding to PtdIns[3,4,5]-trisphosphate or PtdIns[3,4]-bisphosphate with high affinity. Activation of PKB alpha is then achieved at the plasma membrane by phosphorylation of Thr308 in the activation-loop of the kinase domain and Ser473 in the carboxy-terminal regulatory region, respectively. 3-Phosphoinositide-dependent protein kinase-1 (PDK1) is responsible for T308 phosphorylation. The usage of specific inhibitors and natural compound has significantly contributed to investigate the molecular mechanism of PI3K/PDK1/PKB signaling pathway, leading to the putative therapeutics benefits of patients. This review focuses on the contribution of natural inhibitor or compound in our understanding of the mechanism by which insulin induces, especially in PI3K/PDK1/PKB signaling.

Akt- and Foxo1-interacting WD-repeat-FYVE protein promotes adipogenesis

We have previously identified a protein, consisting of seven WD-repeats, forming a putative beta-propeller, and an FYVE domain, ProF, which is highly expressed in 3T3-L1 cells, a cell line that can be differentiated into adipocytes. We recently found ProF to interact with the kinases Akt and protein kinase Czeta. Here we demonstrate that ProF is a positive regulator of adipogenesis. Knockdown of ProF by RNA interference leads to decreased adipocyte differentiation. This is shown by reduced lipid accumulation, decreased expression of the differentiation markers PPARgamma and C/EBPalpha, and reduced glucose uptake in differentiated cells. Furthermore, ProF overexpression leads to increased adipogenesis. ProF binds to the transcription factor Foxo1 (Forkhead box O1), a negative regulator of insulin action and adipogenesis, and facilitates the phosphorylation and thus inactivation of Foxo1 by Akt. Additionally, dominant-negative Foxo1 restores adipogenesis in ProF knockdown cells. Thus, ProF modulates Foxo1 phosphorylation by Akt, promoting adipocyte differentiation. Furthermore, ProF might be involved in metabolic disorders such as diabetes.

PI3 K/Akt/mTOR-mediated translational control regulates proliferation and differentiation of lineage-restricted RoSH stem cell lines

BACKGROUND

We have previously derived highly similar lineage-restricted stem cell lines, RoSH and E-RoSH cell lines from mouse embryos and CD9hi SSEA-1- differentiated mouse embryonic stem cells, respectively. These cell lines are not pluripotent and differentiate readily into endothelial cells in vitro and in vivo.

RESULTS

We investigated the signaling pathway that maintains proliferation of these cells in an undifferentiated state, and demonstrate that PI3 K/Akt/mTOR, but not Raf/MEK/Erk, signaling in these cells was active during proliferation and was downregulated during endothelial differentiation. Inhibition of PI3 K/Akt/mTOR signaling, but not Raf/MEK/Erk, reduced proliferation and induced expression of endothelial specific proteins. During differentiation or inhibition of PI3 K/Akt/mTOR signaling, cyclinD2 transcript abundance in ribosome-enriched RNA but not in total RNA was reduced with a corresponding reduction in protein level. In contrast, transcript abundance of endothelial-specific genes e.g. Kdr, Tek and Pdgfralpha in ribosome-enriched RNA fraction was not reduced and their protein levels were increased. Together these observations suggested that translational control mediated by PI3K/Akt/mTOR signaling was critical in regulating proliferation and endothelial differentiation of lineage-restricted RoSH-like stem cell lines.

CONCLUSION

This study highlights translation regulation as a critical regulatory mechanism during proliferation and differentiation in stem cells.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

Therapeutic potential of natural product signal transduction agents

Modern drug discovery embraces a strategy of targeting cellular signal transduction pathways as a means of finding new therapeutic agents. Historically, natural products derived from microorganisms have played an important role as drug leads and clinical candidates under this paradigm. The future drug potential of natural products as signal transduction agents looks promising, as illustrated by two key examples. First, substantial advances have been made in the development of inhibitors based on immunophilin ligand polyketides, which target the TOR-mediated pathways and can modulate processes including cell proliferation and cell-cycle arrest. Second, the discovery of natural product inhibitors of the ubiquitin-proteasome proteolytic signal transduction pathway represents an emerging field. Given these examples, together with the diversity of as yet undiscovered agents, natural product signal transduction agents offer great potential for future drug discovery efforts.