Phosphorylation sites in the autoinhibitory domain participate in p70(s6k) activation loop phosphorylation

Capturing Autoinhibited PDK1 Reveals the Linker's Regulatory Role, Informing Innovative Inhibitor Design

PDK1 is crucial for PI3K/AKT/mTOR and Ras/MAPK cancer signaling. It phosphorylates AKT in a PIP3-dependent but S6K, SGK, and RSK kinases in a PIP3-independent manner. Unlike its substrates, its autoinhibited monomeric state has been unclear, likely due to its low population time, and phosphorylation in the absence of PIP3 has been puzzling too. Here, guided by experimental data, we constructed models and performed all-atom molecular dynamics simulations. In the autoinhibited PDK1 conformation that resembles autoinhibited AKT, binding of the linker between the kinase and PH domains to the PIF-binding pocket promotes the formation of the Glu130-Lys111 salt bridge and weakens the association of the kinase domain with the PH domain, shifting the population from the autoinhibited state to states accessible to the membrane and its kinase substrates. The interaction of the substrates' hydrophobic motif and the PDK1 PIF-binding pocket facilitates the release of the autoinhibition even in the absence of PIP3. Phosphorylation of the serine-rich motif within the linker further attenuates the association of the PH domain with the kinase domain. These suggest that while the monomeric autoinhibited state is relatively stable, it can readily shift to its active, catalysis-prone state to phosphorylate its diverse substrates. Our findings reveal the PDK1 activation mechanism and discover the regulatory role of PDK1's linker, which lead to two innovative linker-based inhibitor strategies: (i) locking the autoinhibited PDK1 through optimization of the interactions of AKT inhibitors with the PH domain of PDK1 and (ii) analogs (small molecules or peptidomimetics) that mimic the linker interactions with the PIF-binding pocket.

Pharmacologic inhibition of LAT1 predominantly suppresses transport of large neutral amino acids and downregulates global translation in cancer cells

L‐type amino acid transporter 1 (LAT1; SLC7A5), which preferentially transports large neutral amino acids, is highly upregulated in various cancers. LAT1 supplies cancer cells with amino acids as substrates for enhanced biosynthetic and bioenergetic reactions and stimulates signalling networks involved in the regulation of survival, growth and proliferation. LAT1 inhibitors show anti‐cancer effects and a representative compound, JPH203, is under clinical evaluation. However, pharmacological impacts of LAT1 inhibition on the cellular amino acid transport and the translational activity in cancer cells that are conceptually pivotal for its anti‐proliferative effect have not been elucidated yet. Here, we demonstrated that JPH203 drastically inhibits the transport of all the large neutral amino acids in pancreatic ductal adenocarcinoma cells. The inhibitory effects of JPH203 were observed even in competition with high concentrations of amino acids in a cell culture medium. The analyses of the nutrient‐sensing mTORC1 and GAAC pathways and the protein synthesis activity revealed that JPH203 downregulates the global translation. This study demonstrates a predominant contribution of LAT1 to the transport of large neutral amino acids in cancer cells and the suppression of protein synthesis by JPH203 supposed to underly its broad anti‐proliferative effects across various types of cancer cells.

S6 kinase 1 at the central node of cell size and ageing

Genetic evidence in living organisms from yeast to plants and animals, including humans, unquestionably identifies the Target Of Rapamycin kinase (TOR or mTOR for mammalian/mechanistic) signal transduction pathway as a master regulator of growth through the control of cell size and cell number. Among the mTOR targets, the activation of p70 S6 kinase 1 (S6K1) is exquisitely sensitive to nutrient availability and rapamycin inhibition. Of note, in vivo analysis of mutant flies and mice reveals that S6K1 predominantly regulates cell size versus cell proliferation. Here we review the putative mechanisms of S6K1 action on cell size by considering the main functional categories of S6K1 targets: substrates involved in nucleic acid and protein synthesis, fat mass accumulation, retrograde control of insulin action, senescence program and cytoskeleton organization. We discuss how S6K1 may be involved in the observed interconnection between cell size, regenerative and ageing responses.

Repurposing Tranexamic Acid as an Anticancer Agent

Tranexamic Acid (TA) is a clinically used antifibrinolytic agent that acts as a Lys mimetic to block binding of Plasminogen with Plasminogen activators, preventing conversion of Plasminogen to its proteolytically activated form, Plasmin. Previous studies suggested that TA may exhibit anticancer activity by blockade of extracellular Plasmin formation. Plasmin-mediated cleavage of the CDCP1 protein may increase its oncogenic functions through several downstream pathways. Results presented herein demonstrate that TA blocks Plasmin-mediated excision of the extracellular domain of the oncoprotein CDCP1. In vitro studies indicate that TA reduces the viability of a broad array of human and murine cancer cell lines, and breast tumor growth studies demonstrate that TA reduces cancer growth in vivo. Based on the ability of TA to mimic Lys and Arg, we hypothesized that TA may perturb multiple processes that involve Lys/Arg-rich protein sequences, and that TA may alter intracellular signaling pathways in addition to blocking extracellular Plasmin production. Indeed, TA-mediated suppression of tumor cell viability is associated with multiple biochemical actions, including inhibition of protein synthesis, reduced activating phosphorylation of STAT3 and S6K1, decreased expression of the MYC oncoprotein, and suppression of Lys acetylation. Further, TA inhibited uptake of Lys and Arg by cancer cells. These findings suggest that TA or TA analogs may serve as lead compounds and inspire the production of new classes of anticancer agents that function by mimicking Lys and Arg.

Succinylsulfathiazole modulates the mTOR signaling pathway in the liver of c57BL/6 mice via a folate independent mechanism

Researchers studying the effect of folate restriction on rodents have resorted to the use of the antibiotic succinylsulfathiazole (SST) in the folate depleted diet to induce a folate deficient status. SST has been used extensively in rodent studies since the 1940s. Its localized effect on the gut bacteria as well as its effectiveness in reducing folate producing species is well documented. The possible overlap between the pathways affected by folate depletion and SST could potentially produce a confounding variable in such studies. In our novel study, we analyzed the effect of SST on folate levels in c57Bl/6 male mice fed folate supplemented and deficient diets. We did not observe any significant difference on growth and weight gain at 21 weeks. SST did not significantly affect folate levels in the plasma, liver and colon tissues; however, it did alter energy metabolism and expression of key genes in the mTOR signaling pathway in the liver. This research sheds light on a possible confounding element when using SST to study folate depletion due to the potential overlap with multiple critical pathways such as mTOR. SUMMARY: The antibiotic succinylsulfathiazole (SST) is used to reduce folate producing bacteria in rodent folate depletion studies. SST can modulate critical energy and nutrient sensing pathways converging onto mTOR signaling, and potentially confounding cancer studies.

Crosstalk between beta-adrenergic and insulin signaling mediates mechanistic target of rapamycin hyperactivation in liver of high-fat diet-fed male mice

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease. While increased nutrient intake and sympathetic activity have been associated with the disease, the pathogenesis of NAFLD remains incompletely understood. We investigated the impact of the interaction of high dietary fat and sugar intake with increased beta-adrenergic receptor (β-AR) signaling on the activity of nutrient-sensing pathways and fuel storage in the liver. C57BL/6J mice were fed a standard rodent diet (STD), a high-fat diet (HFD), a high-fat/high-sugar Western diet (WD), a high-sugar diet with mixed carbohydrates (HCD), or a high-sucrose diet (HSD). After 6 week on diets, mice were treated with isoproterenol (ISO) and the activity of liver mTOR complex 1 (mTORC1)-related signaling analyzed by immunoblotting and correlated with tissue triglyceride and glycogen contents. ISO-stimulated AKT- and ERK-mediated activation of mTORC1 in STD-fed mice. Consumption of all four high-calorie diets exacerbated downstream activation of ribosomal protein S6 kinase beta-1 (S6K1) in response to ISO. S6K1 activity was greater with the fat-enriched HFD and WD and correlated with the presence of metabolic syndrome and a stronger activation of AKT and ERK1/2 pathways. Fat-enriched diets also increased triglyceride accumulation and inhibited glycogen mobilization under β-AR stimulation. In conclusion, crosstalk between β-AR and insulin signaling may contribute to HFD-induced hepatic steatosis through ERK1/2- and AKT-mediated hyperactivation of the mTORC1/S6K1 axis. The findings provide further rationale for the development of therapies aimed at targeting augmented β-AR signaling in the pathogenesis of NAFLD.

mTOR regulates GPVI-mediated platelet activation

Background

Due to mTOR (mammalian/mechanistic target of rapamycin) gene-loss mice die during embryonic development, the role of mTOR in platelets has not been evaluated using gene knockout technology.

Methods

A mouse model with megakaryocyte/platelet-specific deletion of mTOR was established, and be used to evaluate the role of mTOR in platelet activation and thrombus formation.

Results

mTOR^−/− platelets were deficient in thrombus formation when grown on low-concentration collagen-coated surfaces; however, no deficiency in thrombus formation was observed when mTOR^−/− platelets were perfused on higher concentration collagen-coated surfaces. In FeCl₃-induced mouse mesenteric arteriole thrombosis models, wild-type (WT) and mTOR^−/− mice displayed significantly different responses to low-extent injury with respect to the ratio of occluded mice, especially within the first 40 min. Additionally, mTOR^−/− platelets displayed reduced aggregation and dense granule secretion (ATP release) in response to low doses of the glycoprotein VI (GPVI) agonist collagen related peptide (CRP) and the protease-activated receptor-4 (PAR4) agonist GYPGKF-NH₂; these deficiencies were overcame by stimulation with higher concentration agonists, suggesting dose dependence of the response. At low doses of GPVI or PAR agonist, the activation of α_IIbβ₃ in mTOR^−/− platelets was reduced. Moreover, stimulation of mTOR^−/− platelets with low-dose CRP attenuated the phosphorylation of S6K1, S6 and Akt Ser473, and increased the phosphorylation of PKCδ Thr505 and PKCε Ser729. Using isoform-specific inhibitors of PKCs (δ, ɛ, and α/β), we established that PKCδ/ɛ, and especially PKCδ but not PKCα/β or PKCθ, may be involved in low-dose GPVI-mediated/mTOR-dependent signaling.

Conclusion

These observations indicate that mTOR plays an important role in GPVI-dependent platelet activation and thrombus formation.

Synergistic Allostery in Multiligand-Protein Interactions

Amide hydrogen-deuterium exchange mass spectrometry is powerful for describing combinatorial coupling effects of a cooperative ligand pair binding at noncontiguous sites: adenosine at the ATP-pocket and a docking peptide (PIFtide) at the PIF-pocket, on a model protein kinase PDK1. Binding of two ligands to PDK1 reveal multiple hotspots of synergistic allostery with cumulative effects greater than the sum of individual effects mediated by each ligand. We quantified this synergism and ranked these hotspots using a difference in deuteration-based approach, which showed that the strongest synergistic effects were observed at three of the critical catalytic loci of kinases: the αB-αC helices, and HRD-motif loop, and DFG-motif. Additionally, we observed weaker synergistic effects at a distal GHI-subdomain locus. Synergistic changes in deuterium exchange observed at a distal site but not at the intermediate sites of the large lobe of the kinase reveals allosteric propagation in proteins to operate through two modes. Direct electrostatic interactions between polar and charged amino acids that mediate targeted relay of allosteric signals, and diffused relay of allosteric signals through soft matter-like hydrophobic core amino acids. Furthermore, we provide evidence that the conserved β-3 strand lysine of protein kinases (Lys111 of PDK1) functions as an integrator node to coordinate allosteric coupling of the two ligand-binding sites. It maintains indirect interactions with the ATP-pocket and mediates a critical salt bridge with a glutamate (Glu130) of αC helix, which is conserved across all kinases. In summary, allosteric propagation in cooperative, dual-liganded enzyme targets is bidirectional and synergistic and offers a strategy for combinatorial drug development.

The activation mechanism of plant S6 kinase (S6K), a substrate of TOR kinase, is different from that of mammalian S6K

The S6 kinases (S6Ks) are known to be activated by the target of rapamycin through phosphorylation of their hydrophobic motif (HM). However, our previous research showed that the HM site of plant S6Ks is not phosphorylated and is not essential for their activity in yeast cells lacking Ypk3, an ortholog of mammalian S6K. Here, we demonstrate that the HM site of mammalian S6Ks is phosphorylated and is indispensable for their activity in yeast ypk3∆ cells. Furthermore, pseudo-phosphorylation at the HM site of plant S6Ks results in regaining of activity that is lost due to mutation in the conserved phosphorylation sites, namely the T-loop and Turn motif. These results indicate the activation mechanism of plant S6Ks is different from that of mammals.

Multisite Phosphorylation of S6K1 Directs a Kinase Phospho-code that Determines Substrate Selection

Multisite phosphorylation of kinases can induce on-off or graded regulation of catalytic activity; however, its influence on substrate specificity remains unclear. Here, we show that multisite phosphorylation of ribosomal protein S6 kinase 1 (S6K1) alters target selection. Agonist-inducible phosphorylation of glutamyl-prolyl tRNA synthetase (EPRS) by S6K1 in monocytes and adipocytes requires not only canonical phosphorylation at Thr389 by mTORC1 but also phosphorylation at Ser424 and Ser429 in the C terminus by cyclin-dependent kinase 5 (Cdk5). S6K1 phosphorylation at these additional sites induces a conformational switch and is essential for high-affinity binding and phosphorylation of EPRS, but not canonical S6K1 targets, e.g., ribosomal protein S6. Unbiased proteomic analysis identified additional targets phosphorylated by multisite phosphorylated S6K1 in insulin-stimulated adipocytes-namely, coenzyme A synthase, lipocalin 2, and cortactin. Thus, embedded within S6K1 is a target-selective kinase phospho-code that integrates signals from mTORC1 and Cdk5 to direct an insulin-stimulated, post-translational metabolon determining adipocyte lipid metabolism.

A Drosophila genetic screen for suppressors of S6kinase-dependent growth identifies the F-box subunit Archipelago/FBXW7

This study was designed to identify novel negative regulators of the Drosophila S6kinase (dS6K). S6K is a downstream effector of the growth-regulatory complex mTORC1 (mechanistic-Target-of-Rapamycin complex 1). Nutrients activate mTORC1, which in turn induces the phosphorylation of S6K to promote cell growth, whereas fasting represses mTORC1 activity. Here, we screened 11,000 RNA-interfering (RNAi) lines and retained those that enhanced a dS6K-dependent growth phenotype. Since RNAi induces gene knockdown, enhanced tissue growth supports the idea that the targeted gene acts as a growth suppressor. To validate the resulting candidate genes, we monitored dS6K phosphorylation and protein levels in double-stranded RNAi-treated S2 cells. We identified novel dS6K negative regulators, including gene products implicated in basal cellular functions, suggesting that feedback inputs modulate mTORC1/dS6K signaling. We also identified Archipelago (Ago), the Drosophila homologue of FBXW7, which is an E3-ubiquitin-ligase subunit that loads ubiquitin units onto target substrates for proteasome-mediated degradation. Despite a previous report showing an interaction between Ago/FBXW7 and dS6K in a yeast two-hybrid assay and the presence of an Ago/FBXW7-consensus motif in the dS6K polypeptide, we could not see a direct interaction in immunoprecipitation assay. Nevertheless, we observed that loss-of-ago/fbxw7 in larvae resulted in an increase in dS6K protein levels, but no change in the levels of phosphorylated dS6K or dS6K transcripts, suggesting that Ago/FBXW7 indirectly controls dS6K translation or stability. Through the identification of novel negative regulators of the downstream target, dS6K, our study may help deciphering the underlying mechanisms driving deregulations of mTORC1, which underlies several human diseases.

G protein-dependent signaling triggers a β-arrestin-scaffolded p70S6K/ rpS6 module that controls 5'TOP mRNA translation

Many interaction partners of β-arrestins intervene in the control of mRNA translation. However, how β-arrestins regulate this cellular process has been poorly explored. In this study, we show that β-arrestins constitutively assemble a p70S6K/ribosomal protein S6 (rpS6) complex in HEK293 cells and in primary Sertoli cells of the testis. We demonstrate that this interaction is direct, and experimentally validate the interaction interface between β-arrestin 1 and p70S6K predicted by our docking algorithm. Like most GPCRs, the biological function of follicle-stimulating hormone receptor (FSHR) is transduced by G proteins and β-arrestins. Upon follicle-stimulating hormone (FSH) stimulation, activation of G protein-dependent signaling enhances p70S6K activity within the β-arrestin/p70S6K/rpS6 preassembled complex, which is not recruited to the FSHR. In agreement, FSH-induced rpS6 phosphorylation within the β-arrestin scaffold was decreased in cells depleted of Gα_s. Integration of the cooperative action of β-arrestin and G proteins led to the translation of 5' oligopyrimidine track mRNA with high efficacy within minutes of FSH input. Hence, this work highlights new relationships between G proteins and β-arrestins when acting cooperatively on a common signaling pathway, contrasting with their previously shown parallel action on the ERK MAP kinase pathway. In addition, this study provides insights into how GPCR can exert trophic effects in the cell.-Tréfier, A., Musnier, A., Landomiel, F., Bourquard, T., Boulo, T., Ayoub, M. A., León, K., Bruneau, G., Chevalier, M., Durand, G., Blache, M.-C., Inoue, A., Fontaine, J., Gauthier, C., Tesseraud, S., Reiter, E., Poupon, A., Crépieux, P. G protein-dependent signaling triggers a β-arrestin-scaffolded p70S6K/ rpS6 module that controls 5'TOP mRNA translation.

Activation of the Stress Response Kinase JNK (c-Jun N-terminal Kinase) Attenuates Insulin Action in Retina through a p70S6K1-dependent Mechanism

Despite recent advances in therapeutics, diabetic retinopathy remains a leading cause of vision impairment. Improvement in the treatment of diabetic retinopathy requires a better understanding of the molecular mechanisms that cause neurovascular complications, particularly in type 2 diabetes. Recent studies demonstrate that rodents fed a high fat diet exhibit retinal dysfunction concomitant with attenuated Akt phosphorylation. The purpose of the present study was to evaluate the impact of a high fat/high sucrose diet on retinal insulin signaling and evaluate the mechanism(s) responsible for the changes. Mice fed a high fat/sucrose diet exhibited attenuated Akt phosphorylation in the retina as compared with mice fed normal chow. Retinas of mice fed a high fat/sucrose diet also exhibited elevated levels of activated JNK as well as enhanced p70S6K1 autoinhibitory domain phosphorylation. In cells, JNK activation enhanced p70S6K1 phosphorylation and mTORC1-dependent activation of the kinase, as evidenced by enhanced phosphorylation of key substrates. Rictor phosphorylation by p70S6K1 was specifically enhanced by the addition of phosphomimetic mutations in the autoinhibitory domain and was more sensitive to inhibition of the kinase as compared with rpS6. Notably, rictor and IRS-1 phosphorylation by p70S6K1 attenuate insulin action through a negative feedback pathway. Indeed, p70S6K1 inhibition prevented the repressive effect of JNK activation on insulin action in retinas. Overall, the results identify the JNK/S6K1 axis as a key molecular mechanism whereby a high fat/sucrose diet impairs insulin action in retina.

mTORC1 and SIRT1 Cooperate to Foster Expansion of Gut Adult Stem Cells during Calorie Restriction

Longevity-promoting caloric restriction is thought to trigger downregulation of mammalian target of rapamycin complex 1 (mTORC1) signaling and upregulation of SIRT1 activity with associated health benefits. Here, we show that mTORC1 signaling in intestinal stem cells (ISCs) is instead upregulated during calorie restriction (CR). SIRT1 deacetylates S6K1, thereby enhancing its phosphorylation by mTORC1, which leads to an increase in protein synthesis and an increase in ISC number. Paneth cells in the ISC niche secrete cyclic ADP ribose that triggers SIRT1 activity and mTORC1 signaling in neighboring ISCs. Notably, the mTOR inhibitor rapamycin, previously reported to mimic effects of CR, abolishes this expansion of ISCs. We suggest that Paneth cell signaling overrides any direct nutrient sensing in ISCs to sculpt the observed response to CR. Moreover, drugs that modulate pathways important in CR may exert opposing effects on different cell types.

The role of AMPK in cardiomyocyte health and survival

Cellular energy homeostasis is a fundamental process that governs the overall health of the cell and is paramount to cell survival. Central to this is the control of ATP generation and utilization, which is regulated by a complex myriad of enzymatic reactions controlling cellular metabolism. In the cardiomyocyte, ATP generated from substrate catabolism is used for numerous cellular processes including maintaining ionic homeostasis, cell repair, protein synthesis and turnover, organelle turnover, and contractile function. In many instances, cardiovascular disease is associated with impaired cardiac energetics and thus the signalling that regulates pathways involved in cardiomyocyte metabolism may be potential targets for pharmacotherapy designed to help treat cardiovascular disease. An important regulator of cardiomyocyte energy homeostasis is adenosine monophosphate-activated protein kinase (AMPK). AMPK is a serine-threonine kinase that functions primarily as a metabolic sensor to coordinate anabolic and catabolic activities in the cell via the phosphorylation of multiple proteins involved in metabolic pathways. In addition to the direct role that AMPK plays in the regulation of cardiomyocyte metabolism, AMPK can also either directly or indirectly influence other cellular processes such as regulating mitochondrial function, post-translation acetylation, autophagy, mitophagy, endoplasmic reticulum stress, and apoptosis. Thus, AMPK is implicated in the control of a wide variety of cellular processes that can influence cardiomyocyte health and survival. In this review, we will discuss the important role that AMPK plays in regulating cardiac metabolism, as well as the additional cellular processes that may contribute to cardiomyocyte function and survival in the healthy and the diseased heart. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan. F.C. Glatz.

S6K-STING interaction regulates cytosolic DNA-mediated activation of the transcription factor IRF3

Cytosolic DNA-mediated activation of the transcription factor IRF3 is a key event in host antiviral responses. Here we found that infection with DNA viruses induced interaction of the metabolic checkpoint kinase mTOR downstream effector and kinase S6K1 and the signaling adaptor STING in a manner dependent on the DNA sensor cGAS. We further demonstrated that the kinase domain, but not the kinase function, of S6K1 was required for the S6K1-STING interaction and that the TBK1 critically promoted this process. The formation of a tripartite S6K1-STING-TBK1 complex was necessary for the activation of IRF3, and disruption of this signaling axis impaired the early-phase expression of IRF3 target genes and the induction of T cell responses and mucosal antiviral immunity. Thus, our results have uncovered a fundamental regulatory mechanism for the activation of IRF3 in the cytosolic DNA pathway.

Ribosomal Protein S6 Phosphorylation in the Nervous System: From Regulation to Function

Since the discovery of the phosphorylation of the 40S ribosomal protein S6 (rpS6) about four decades ago, much effort has been made to uncover the molecular mechanisms underlying the regulation of this post-translational modification. In the field of neuroscience, rpS6 phosphorylation is commonly used as a readout of the mammalian target of rapamycin complex 1 signaling activation or as a marker for neuronal activity. Nevertheless, its biological role in neurons still remains puzzling. Here we review the pharmacological and physiological stimuli regulating this modification in the nervous system as well as the pathways that transduce these signals into rpS6 phosphorylation. Altered rpS6 phosphorylation observed in various genetic and pathophysiological mouse models is also discussed. Finally, we examine the current state of knowledge on the physiological role of this post-translational modification and highlight the questions that remain to be addressed.

Molecular Targeting of ERKs/RSK2 Signaling Axis in Cancer Prevention

RSK2 is a downstream signaling protein of ERK1 and ERK2 and plays a key role in physiological homeostasis. For this reason, RSK2 is a highly conserved protein among the p90RSK family members. In its location in the signaling pathway, RSK2 is a kinase just upstream of transcription and epigenetic factors, and a few kinases involved in cell cycle regulation and protein synthesis. Moreover, activation of RSK2 by growth factors is directly involved in cell proliferation, anchorage-independent cell transformation and cancer development. Direct evidences regarding the etiological roles of RSK2 in cancer development in humans have been published by our research group illustrating that elevated total- and phospho-RSK2 protein levels mediated by ERK1 and ERK2 are higher in skin cancer tissues compared to normal skin tissues. Notably, it has been shown that RSK2 ectopic expression in JB6 Cl41 cells induces cell proliferation and anchorage- independent cell transformation. Importantly, knockdown of RSK2 suppresses Ras-mediated foci formation and anchorage-independent colony growth of cancer cells. Kaempferol is a one of the natural compounds showing selectivity in inhibiting RSK2 activity in epidermal growth factor-induced G1/S cell cycle transition and cell transformation. Thus, ERKs/RSK2 signaling axis is an important target signaling molecule in chemoprevention.

Cyclin-dependent Kinase 5 (Cdk5)-dependent Phosphorylation of p70 Ribosomal S6 Kinase 1 (S6K) Is Required for Dendritic Spine Morphogenesis

The maturation and maintenance of dendritic spines depends on neuronal activity and protein synthesis. One potential mechanism involves mammalian target of rapamycin, which promotes protein synthesis through phosphorylation of eIF4E-binding protein and p70 ribosomal S6 kinase 1 (S6K). Upon extracellular stimulation, mammalian target of rapamycin phosphorylates S6K at Thr-389. S6K also undergoes phosphorylation at other sites, including four serine residues in the autoinhibitory domain. Despite extensive biochemical studies, the importance of phosphorylation in the autoinhibitory domain in S6K function remains unresolved, and its role has not been explored in the cellular context. Here we demonstrated that S6K in neuron was phosphorylated at Ser-411 within the autoinhibitory domain by cyclin-dependent kinase 5. Ser-411 phosphorylation was regulated by neuronal activity and brain-derived neurotrophic factor (BDNF). Knockdown of S6K in hippocampal neurons by RNAi led to loss of dendritic spines, an effect that mimics neuronal activity blockade by tetrodotoxin. Notably, coexpression of wild type S6K, but not the phospho-deficient S411A mutant, could rescue the spine defects. These findings reveal the importance of cyclin-dependent kinase 5-mediated phosphorylation of S6K at Ser-411 in spine morphogenesis driven by BDNF and neuronal activity.

The discovery of potent ribosomal S6 kinase inhibitors by high-throughput screening and structure-guided drug design

The ribosomal P70 S6 kinases play a crucial role in PI3K/mTOR regulated signalling pathways and are therefore potential targets for the treatment of a variety of diseases including diabetes and cancer. In this study we describe the identification of three series of chemically distinct S6K1 inhibitors. In addition, we report a novel PKA-S6K1 chimeric protein with five mutations in or near its ATP-binding site, which was used to determine the binding mode of two of the three inhibitor series, and provided a robust system to aid the optimisation of the oxadiazole-substituted benzimidazole inhibitor series. We show that the resulting oxadiazole-substituted aza-benzimidazole is a potent and ligand efficient S6 kinase inhibitor, which blocks the phosphorylation of RPS6 at Ser235/236 in TSC negative HCV29 human bladder cancer cells by inhibiting S6 kinase activity and thus provides a useful tool compound to investigate the function of S6 kinases.

Cross-talk between sirtuin and mammalian target of rapamycin complex 1 (mTORC1) signaling in the regulation of S6 kinase 1 (S6K1) phosphorylation

p70 ribosomal S6 kinase (S6K1), a major substrate of the mammalian target of rapamycin (mTOR) kinase, regulates diverse cellular processes including protein synthesis, cell growth, and survival. Although it is well known that the activity of S6K1 is tightly coupled to its phosphorylation status, the regulation of S6K1 activity by other post-translational modifications such as acetylation has not been well understood. Here we show that the acetylation of the C-terminal region (CTR) of S6K1 blocks mTORC1-dependent Thr-389 phosphorylation, an essential phosphorylation site for S6K1 activity. The acetylation of the CTR of S6K1 is inhibited by the class III histone deacetylases, SIRT1 and SIRT2. An S6K1 mutant lacking acetylation sites in its CTR shows enhanced Thr-389 phosphorylation and kinase activity, whereas the acetylation-mimetic S6K1 mutant exhibits decreased Thr-389 phosphorylation and kinase activity. Interestingly, relative to the acetylation-mimetic S6K1 mutant, the acetylation-defective mutant displays higher affinity toward Raptor, an essential scaffolding component of mTORC1 that recruits mTORC1 substrates. These observations indicate that sirtuin-mediated regulation of S6K1 acetylation is an additional important regulatory modification that impinges on the mechanisms underlying mTORC1-dependent S6K1 activation.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

A novel SOS1 mutation in Costello/CFC syndrome affects signaling in both RAS and PI3K pathways

CONTEXT

Pathological upregulation of the RAS/MAPK pathway causes Costello, Noonan and cardio-facio-cutaneous (CFC) syndrome; however, little is known about PI3K/AKT signal transduction in these syndromes. Previously, we found a novel mutation of the SOS1 gene (T158A) in a patient with Costello/CFC overlapping phenotype.

OBJECTIVE

The aim of this study was to investigate how this mutation affects RAS/MAPK as well as PI3K/AKT pathway signal transduction.

MATERIALS AND METHODS

Wild-type and mutant (T158A) Son of Sevenless 1 (SOS1) were transfected into 293T cells. The levels of phospho- and total ERK1/2, AKT, p70S6K and pS6 were examined under epidermal growth factor (EGF) stimulation.

RESULTS

After EGF stimulation, the ratio of phospho-ERK1/2 to total ERK1/2 was highest at 5 min in mutant (T158A) SOS1 cells, and at 15 min in wild-type SOS1 cells. Phospho-AKT was less abundant at 60 min in mutant than in wild-type SOS1 cells. Phosphorylation at various sites in p70S6K differed between wild-type and mutant cells. Eighteen hours after activation by EGF, the ratio of phospho-ERK1/2 to total ERK1/2 remained significantly higher in mutant than in wild-type SOS1 cells, but that of phospho-AKT to total AKT was unchanged.

DISCUSSION

T158A is located in the histone-like domain, which may have a role in auto-inhibition of RAS exchanger activity of SOS1. T158A may disrupt auto-inhibition and enhance RAS signaling. T158A also affects PI3K/AKT signaling, probably via negative feedback via phospho-p70S6K.

CONCLUSION

The SOS1 T158A mutation altered the phosphorylation of gene products involved in both RAS/MAPK and PI3K/AKT pathways.

Phosphorylation and degradation of S6K1 (p70S6K1) in response to persistent JNK1 Activation

S6K (ribosomal S6 kinase p70, p70S6K) activation requires phosphorylation at two stages. The first phosphorylation is independent of insulin stimulation and mediated by an unknown kinase. The second phosphorylation is mediated by mTOR in insulin dependent manner. In this study, we identified JNK1 (c-Jun N-terminal kinase 1) as a kinase in the first phosphorylation. S6K protein was phosphorylated by JNK1 at S411 and S424 in the carboxyl terminal autoinhibitory domain. The phosphorylation was observed in kinase assay with purified S6K as a substrate, and in cells after JNK1 activation by TNF-α or MEKK1 expression. The phosphorylation was detected in JNK2 null cells, but not in JNK1 null cells after TNF-α treatment. When JNK1 activation was inhibited by MKK7 knockdown, the phosphorylation was blocked in cells. The phosphorylation led to S6K protein degradation in NF-κB deficient cells. The degradation was blocked by inhibition of proteasome activity with MG132. In wide type cells, the phosphorylation did not promote S6K degradation when IKK2 (IKKβ, IκB kinase beta) was activated. Instead, the phosphorylation allowed S6K activation by mTOR, which stabilizes S6K protein. In IKK2 null cells or cells treated by IKK2 inhibitor, the phosphorylation led to S6K degradation. These data suggest that S6K is phosphorylated by JNK1 and the phosphorylation makes S6K protein unstable in the absence of IKK2 activation. This study provides a mechanism for regulation of S6K protein stability.

Sphingosine-1-Phosphate as a Regulator of Hypoxia-Induced Factor-1α in Thyroid Follicular Carcinoma Cells

Sphingosine-1-phosphate (S1P) is a bioactive lipid, which regulates several cancer-related processes including migration and angiogenesis. We have previously shown S1P to induce migration of follicular ML-1 thyroid cancer cells. Hypoxia-induced factor-1 (HIF-1) is an oxygen-sensitive transcription factor, which adapts cells to hypoxic conditions through increased survival, motility and angiogenesis. Due to these properties and its increased expression in response to intratumoral hypoxia, HIF-1 is considered a significant regulator of tumor biology. We found S1P to increase expression of the regulatory HIF-1α subunit in normoxic ML-1 cells. S1P also increased HIF-1 activity and expression of HIF-1 target genes. Importantly, inhibition or knockdown of HIF-1α attenuated the S1P-induced migration of ML-1 cells. S1P-induced HIF-1α expression was mediated by S1P receptor 3 (S1P₃), G_i proteins and their downstream effectors MEK, PI3K, mTOR and PKCβI. Half-life measurements with cycloheximide indicated that S1P treatment stabilized the HIF-1α protein. On the other hand, S1P activated translational regulators eIF-4E and p70S6K, which are known to control HIF-1α synthesis. In conclusion, we have identified S1P as a non-hypoxic regulator of HIF-1 activity in thyroid cancer cells, studied the signaling involved in S1P-induced HIF-1α expression and shown S1P-induced migration to be mediated by HIF-1.

AGC protein kinases: from structural mechanism of regulation to allosteric drug development for the treatment of human diseases

The group of AGC protein kinases includes more than 60 protein kinases in the human genome, classified into 14 families: PDK1, AKT/PKB, SGK, PKA, PKG, PKC, PKN/PRK, RSK, NDR, MAST, YANK, DMPK, GRK and SGK494. This group is also widely represented in other eukaryotes, including causative organisms of human infectious diseases. AGC kinases are involved in diverse cellular functions and are potential targets for the treatment of human diseases such as cancer, diabetes, obesity, neurological disorders, inflammation and viral infections. Small molecule inhibitors of AGC kinases may also have potential as novel therapeutic approaches against infectious organisms. Fundamental in the regulation of many AGC kinases is a regulatory site termed the "PIF-pocket" that serves as a docking site for substrates of PDK1. This site is also essential to the mechanism of activation of AGC kinases by phosphorylation and is involved in the allosteric regulation of N-terminal domains of several AGC kinases, such as PKN/PRKs and atypical PKCs. In addition, the C-terminal tail and its interaction with the PIF-pocket are involved in the dimerization of the DMPK family of kinases and may explain the molecular mechanism of allosteric activation of GRKs by GPCR substrates. In this review, we briefly introduce the AGC kinases and their known roles in physiology and disease and the discovery of the PIF-pocket as a regulatory site in AGC kinases. Finally, we summarize the current status and future therapeutic potential of small molecules directed to the PIF-pocket; these molecules can allosterically activate or inhibit the kinase as well as act as substrate-selective inhibitors. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Exploiting p70 S6 kinase as a target for ovarian cancer

INTRODUCTION

The p70 S6 kinase (p70(S6K)) is frequently active in ovarian and a wide range of cancer types, and it has a crucial role in several processes considered hallmarks of cancer. Therefore, blocking p70(S6K) expression or activity may present a promising strategy for anticancer treatment.

AREAS COVERED

The current understanding of the molecular mechanisms that govern p70(S6K) regulation as well as its tumorigenic effects, which are involved in the initiation and progression in ovarian cancer, in particular the emerging new role of p70(S6K) in cell migration, which is a prerequisite of tumor metastasis. The p70(S6K) cellular substrates and/or interacting proteins. The current state of drugs that target this kinase, either alone or in combination with other targeted agents.

EXPERT OPINION

Targeting p70(S6K) through the use of small-molecule inhibitors, microRNAs and natural compounds may represent a beneficial new avenue for cancer therapy and opens new areas of investigation in p70(S6K) biology.

Rapamycin inhibition of baculovirus recombinant (BVr) ribosomal protein S6 kinase (S6K1) is mediated by an event other than phosphorylation

Background

Ribosomal protein S6 kinase 1(S6K1) is an evolutionary conserved kinase that is activated in response to growth factors and viral stimuli to influence cellular growth and proliferation. This downstream effector of target of rapamycin (TOR) signaling cascade is known to be directly activated by TOR- kinase mediated hydrophobic motif (HM) phosphorylation at Threonine 412 (T412). Selective loss of this phosphorylation by inactivation of TOR kinase or activation/recruitment of a phosphatase has accordingly been implicated in mediating inhibition by rapamycin.

Findings

We present evidence that baculovirus driven expression of S6K1 in insect cells (Sf9) fails to activate the enzyme and instead renders it modestly active representing 4-6 folds less activity than its fully active mammalian counterpart. Contrary to the contention that viral infection activates TOR signaling pathway, we report that BVr enzyme fails to exhibit putative TOR dependent phosphorylation at the HM and the resultant phosphorylation at the activation loop (AL) of the enzyme, correlating with the level of activity observed. Surprisingly, the BVr enzyme continued to exhibit sensitivity to rapamycin that remained unaffected by mutations compromised for TOR phosphorylation (T412A) or deletions compromised for TOR binding (ΔNH _2-46/ΔCT₁₀₄).

Conclusions

These data together with the ability of the BVr enzyme to resist inactivation by phosphatases indicate that inhibition by rapamycin is not mediated by any phosphorylation event in general and TOR dependent phosphorylation in particular.

Regulation and function of ribosomal protein S6 kinase (S6K) within mTOR signalling networks

The ribosomal protein S6K (S6 kinase) represents an extensively studied effector of the TORC1 [TOR (target of rapamycin) complex 1], which possesses important yet incompletely defined roles in cellular and organismal physiology. TORC1 functions as an environmental sensor by integrating signals derived from diverse environmental cues to promote anabolic and inhibit catabolic cellular functions. mTORC1 (mammalian TORC1) phosphorylates and activates S6K1 and S6K2, whose first identified substrate was rpS6 (ribosomal protein S6), a component of the 40S ribosome. Studies over the past decade have uncovered a number of additional S6K1 substrates, revealing multiple levels at which the mTORC1-S6K1 axis regulates cell physiology. The results thus far indicate that the mTORC1-S6K1 axis controls fundamental cellular processes, including transcription, translation, protein and lipid synthesis, cell growth/size and cell metabolism. In the present review we summarize the regulation of S6Ks, their cellular substrates and functions, and their integration within rapidly expanding mTOR (mammalian TOR) signalling networks. Although our understanding of the role of mTORC1-S6K1 signalling in physiology remains in its infancy, evidence indicates that this signalling axis controls, at least in part, glucose homoeostasis, insulin sensitivity, adipocyte metabolism, body mass and energy balance, tissue and organ size, learning, memory and aging. As dysregulation of this signalling axis contributes to diverse disease states, improved understanding of S6K regulation and function within mTOR signalling networks may enable the development of novel therapeutics.

Regulators of protein metabolism are affected by cyclical nutritional treatments with diets varying in protein and energy content

There is evidence that the E3 ubiquitin ligases muscle ring finger-1 (MuRF1) and atrogin-1, which mediate the ubiquitination of certain proteins and thereby their proteolysis, are regulated by cyclical nutritional treatments varying in lysine content. In order to explore further the regulatory mechanisms involved in metabolic adaptation to dietary changes, we investigated the effects of daily variations in energy [2800 (E-) followed by 3200 kcal/kg (E+)], protein [230 (P+) followed by 150g/kg (P-)] or both [E-P+ followed by E+P-] on muscle protein metabolism in 2-week-old male broiler chickens. Growth performance was similar for all treatments. Expression of atrogin-1 and MuRF1 was changed by alternation of diets varying in protein (higher expression with P- vs. P+) and energy content (higher expression with E- vs. E+). The expression of atrogin-1 was regulated with mixed diets (increase in E+P- vs. E-P+) but not that of MuRF1. Such regulation may involve the mammalian target of rapamycin (mTOR), which was more phosphorylated with P+ than with P-. Eukaryotic initiation factor 4E binding protein, p70S6 kinase and ribosomal protein S6, which are mTOR targets known to control protein synthesis, were highly activated by increased protein content (P+ vs. P-). The mechanisms coordinating the protein synthesis/proteolysis balance remain to be characterized.

S6 kinase localizes to the presynaptic active zone and functions with PDK1 to control synapse development

S6 kinase localizes to the active zone in a Brp-dependent manner and collaborates with presynaptic PDK1 to modulate neuronal cell size, bouton size, active zone number, and neurotransmitter release.

The dimensions of neuronal dendrites, axons, and synaptic terminals are reproducibly specified for each neuron type, yet it remains unknown how these structures acquire their precise dimensions of length and diameter. Similarly, it remains unknown how active zone number and synaptic strength are specified relative the precise dimensions of presynaptic boutons. In this paper, we demonstrate that S6 kinase (S6K) localizes to the presynaptic active zone. Specifically, S6K colocalizes with the presynaptic protein Bruchpilot (Brp) and requires Brp for active zone localization. We then provide evidence that S6K functions downstream of presynaptic PDK1 to control synaptic bouton size, active zone number, and synaptic function without influencing presynaptic bouton number. We further demonstrate that PDK1 is also a presynaptic protein, though it is distributed more broadly. We present a model in which synaptic S6K responds to local extracellular nutrient and growth factor signaling at the synapse to modulate developmental size specification, including cell size, bouton size, active zone number, and neurotransmitter release.

The role of skeletal muscle mTOR in the regulation of mechanical load-induced growth

Chronic mechanical loading (CML) of skeletal muscle induces compensatory growth and the drug rapamycin has been reported to block this effect. Since rapamycin is considered to be a highly specific inhibitor of the mammalian target of rapamycin (mTOR), many have concluded that mTOR plays a key role in CML-induced growth regulatory events. However, rapamycin can exert mTOR-independent actions and systemic administration of rapamycin will inhibit mTOR signalling in all cells throughout the body. Thus, it is not clear if the growth inhibitory effects of rapamycin are actually due to the inhibition of mTOR signalling, and more specifically, the inhibition of mTOR signalling in skeletal muscle cells. To address this issue, transgenic mice with muscle specific expression of various rapamycin-resistant mutants of mTOR were employed. These mice enabled us to demonstrate that mTOR, within skeletal muscle cells, is the rapamycin-sensitive element that confers CML-induced hypertrophy, and mTOR kinase activity is necessary for this event. Surprisingly, CML also induced hyperplasia, but this occurred through a rapamycin-insensitive mechanism. Furthermore, CML was found to induce an increase in FoxO1 expression and PKB phosphorylation through a mechanism that was at least partially regulated by an mTOR kinase-dependent mechanism. Finally, CML stimulated ribosomal RNA accumulation and rapamycin partially inhibited this effect; however, the effect of rapamycin was exerted through a mechanism that was independent of mTOR in skeletal muscle cells. Overall, these results demonstrate that CML activates several growth regulatory events, but only a few (e.g. hypertrophy) are fully dependent on mTOR signalling within the skeletal muscle cells.

PKCδ regulates hepatic insulin sensitivity and hepatosteatosis in mice and humans

C57BL/6J and 129S6/Sv (B6 and 129) mice differ dramatically in their susceptibility to developing diabetes in response to diet- or genetically induced insulin resistance. A major locus contributing to this difference has been mapped to a region on mouse chromosome 14 that contains the gene encoding PKCδ. Here, we found that PKCδ expression in liver was 2-fold higher in B6 versus 129 mice from birth and was further increased in B6 but not 129 mice in response to a high-fat diet. PRKCD gene expression was also elevated in obese humans and was positively correlated with fasting glucose and circulating triglycerides. Mice with global or liver-specific inactivation of the Prkcd gene displayed increased hepatic insulin signaling and reduced expression of gluconeogenic and lipogenic enzymes. This resulted in increased insulin-induced suppression of hepatic gluconeogenesis, improved glucose tolerance, and reduced hepatosteatosis with aging. Conversely, mice with liver-specific overexpression of PKCδ developed hepatic insulin resistance characterized by decreased insulin signaling, enhanced lipogenic gene expression, and hepatosteatosis. Therefore, changes in the expression and regulation of PKCδ between strains of mice and in obese humans play an important role in the genetic risk of hepatic insulin resistance, glucose intolerance, and hepatosteatosis; and thus PKCδ may be a potential target in the treatment of metabolic syndrome.

A limited role for PI(3,4,5)P3 regulation in controlling skeletal muscle mass in response to resistance exercise

Background

Since activation of the PI3K/(protein kinase B; PKB/akt) pathway has been shown to alter muscle mass and growth, the aim of this study was to determine whether resistance exercise increased insulin like growth factor (IGF) I/phosphoinositide 3-kinase (PI3K) signalling and whether altering PI(3,4,5)P₃ metabolism genetically would increase load induced muscle growth.

Methodology/Principal Findings

Acute and chronic resistance exercise in wild type and muscle specific PTEN knockout mice were used to address the role of PI(3,4,5)P₃ regulation in the development of skeletal muscle hypertrophy. Acute resistance exercise did not increase either IGF-1 receptor phosphorylation or IRS1/2 associated p85. Since insulin/IGF signalling to PI3K was unchanged, we next sought to determine whether inactivation of PTEN played a role in load-induced muscle growth. Muscle specific knockout of PTEN resulted in small but significant increases in heart (PTEN^+/+  = 5.00±0.02 mg/g, PTEN^−/−  = 5.50±0.09 mg/g), and TA (PTEN^+/+  = 1.74±0.04 mg/g, PTEN^−/−  = 1.89 ±0.03) muscle mass, while the GTN, SOL, EDL and PLN remain unchanged. Following ablation, hypertrophy of the PLN, SOL or EDL muscles was similar between PTEN^−/− and PTEN^+/+ animals. Even though there were some changes in overload-induced PKB and S6K1 phosphorylation, 1 hr following acute resistance exercise there was no difference in the phosphorylation state of S6K1 Thr389 between genotypes.

Conclusions/Significance

These data suggest that physiological loading does not lead to the enhanced activation of the PI3K/PKB/mTORC1 axis and that neither PI3K activation nor PTEN, and by extension PI(3,4,5)P₃ levels, play a significant role in adult skeletal muscle growth.

Effects of heat exposure on Akt/S6K1 signaling and expression of genes related to protein and energy metabolism in chicken (Gallus gallus) pectoralis major muscle

In order to improve understanding of the heat-induced changes in muscle growth, we determined the expression of genes related to protein and energy metabolism in the pectoralis major muscle of chickens. We also explored the protein kinase B (PKB also called Akt)/p70 S6 kinase (S6K1)/S6 pathway that mediates anabolic signals thereby regulating metabolism and hypertrophic/atrophic balance. Four-week-old chickens were exposed to 32 or 22 degrees C for 1 week. Chickens from both groups were then fasted for 16 h or left fed, and submitted to an oral administration of glucose-arginine to induce an anabolic response (30-min treatment) or left untreated. High ambient temperature and the associated decrease in feed intake modified the expression of certain energy-related genes (e.g. -40% for PGC-1alpha) and protein metabolism (e.g. about +80% for atrogin-1), but the expression of several muscle metabolism-related genes considered here was unchanged. The capacity for muscle protein synthesis, i.e. RNA/protein ratio, was reduced in warm conditions (approximately -20%). Slightly lower activation of S6 induced by glucose-arginine treatment was found at 32 degrees C compared to 22 degrees C, which might indicate somewhat lower efficiency of mRNA translation. Analysis of glucose/insulin balance suggested changes in glucose metabolism under heat exposure. However, this remains to be characterized.

Dynamic switch of negative feedback regulation in Drosophila Akt-TOR signaling

Akt represents a nodal point between the Insulin receptor and TOR signaling, and its activation by phosphorylation controls cell proliferation, cell size, and metabolism. The activity of Akt must be carefully balanced, as increased Akt signaling is frequently associated with cancer and as insufficient Akt signaling is linked to metabolic disease and diabetes mellitus. Using a genome-wide RNAi screen in Drosophila cells in culture, and in vivo analyses in the third instar wing imaginal disc, we studied the regulatory circuitries that define dAkt activation. We provide evidence that negative feedback regulation of dAkt occurs during normal Drosophila development in vivo. Whereas in cell culture dAkt is regulated by S6 Kinase (S6K)–dependent negative feedback, this feedback inhibition only plays a minor role in vivo. In contrast, dAkt activation under wild-type conditions is defined by feedback inhibition that depends on TOR Complex 1 (TORC1), but is S6K–independent. This feedback inhibition is switched from TORC1 to S6K only in the context of enhanced TORC1 activity, as triggered by mutations in tsc2. These results illustrate how the Akt–TOR pathway dynamically adapts the routing of negative feedback in response to the activity load of its signaling circuit in vivo.

The development of multi-cellular organisms depends on the precise choreography of a diverse array of signal transduction pathways. This requires balanced regulation by activating as well as repressing signals. Negative feedback, defined as a signaling response counteracting the stimulus, is a frequently used mechanism to dampen signaling pathway activity. Accordingly, loss of negative feedback is often observed during progression of cancer, while constitutive engagement of negative feedback contributes to chronic loss-of-function phenotypes. Ectopic activation of the Akt–TOR pathway is frequently associated with tumor susceptibility and cancer and contributes to obesity-induced metabolic disease and type II diabetes. Using Drosophila cell culture and the developing fly, we dissect the regulatory circuitry defining negative feedback regulation of dAkt. Our work shows that dAkt activity is regulated by two qualitatively different negative feedback mechanisms and that the activity level of the dAkt pathway dictates which feedback mechanism is utilized. Under normal physiological activity conditions, we observe a feedback mechanism that is dependent on TOR complex 1, but independent of S6K. Under conditions of pathological high pathway activity, we observe an S6K–dependent negative feedback mechanism. Our identification of a quantitative-to-qualitative switch in dAkt–TOR negative feedback signaling might have important implications in the biology of cancer and metabolic diseases.

Luteinizing hormone stimulates mammalian target of rapamycin signaling in bovine luteal cells via pathways independent of AKT and mitogen-activated protein kinase: modulation of glycogen synthase kinase 3 and AMP-activated protein kinase

LH stimulates the production of cAMP in luteal cells, which leads to the production of progesterone, a hormone critical for the maintenance of pregnancy. The mammalian target of rapamycin (MTOR) signaling cascade has recently been examined in ovarian follicles where it regulates granulosa cell proliferation and differentiation. This study examined the actions of LH on the regulation and possible role of the MTOR signaling pathway in primary cultures of bovine corpus luteum cells. Herein, we demonstrate that activation of the LH receptor stimulates the phosphorylation of the MTOR substrates ribosomal protein S6 kinase 1 (S6K1) and eukaryotic translation initiation factor 4E binding protein 1. The actions of LH were mimicked by forskolin and 8-bromo-cAMP. LH did not increase AKT or MAPK1/3 phosphorylation. Studies with pathway-specific inhibitors demonstrated that the MAPK kinase 1 (MAP2K1)/MAPK or phosphatidylinositol 3-kinase/AKT signaling pathways were not required for LH-stimulated MTOR/S6K1 activity. However, LH decreased the activity of glycogen synthase kinase 3Beta (GSK3B) and AMP-activated protein kinase (AMPK). The actions of LH on MTOR/S6K1 were mimicked by agents that modulated GSK3B and AMPK activity. The ability of LH to stimulate progesterone secretion was not prevented by rapamycin, a MTOR inhibitor. In contrast, activation of AMPK inhibited LH-stimulated MTOR/S6K1 signaling and progesterone secretion. In summary, the LH receptor stimulates a unique series of intracellular signals to activate MTOR/S6K1 signaling. Furthermore, LH-directed changes in AMPK and GSK3B phosphorylation appear to exert a greater impact on progesterone synthesis in the corpus luteum than rapamycin-sensitive MTOR-mediated events.

cAMP-dependent activation of mammalian target of rapamycin (mTOR) in thyroid cells. Implication in mitogenesis and activation of CDK4

How cAMP-dependent protein kinases [protein kinase A (PKA)] transduce the mitogenic stimulus elicited by TSH in thyroid cells to late activation of cyclin D3-cyclin-dependent kinase 4 (CDK4) remains enigmatic. Here we show in PC Cl3 rat thyroid cells that TSH/cAMP, like insulin, activates the mammalian target of rapamycin (mTOR)-raptor complex (mTORC1) leading to phosphorylation of S6K1 and 4E-BP1. mTORC1-dependent S6K1 phosphorylation in response to both insulin and cAMP required amino acids, whereas inhibition of AMP-activated protein kinase and glycogen synthase kinase 3 enhanced insulin but not cAMP effects. Unlike insulin, TSH/cAMP did not activate protein kinase B or induce tuberous sclerosis complex 2 phosphorylation at T1462 and Y1571. However, like insulin, TSH/cAMP produced a stable increase in mTORC1 kinase activity that was associated with augmented 4E-BP1 binding to raptor. This could be caused in part by T246 phosphorylation of PRAS40, which was found as an in vitro substrate of PKA. Both in PC Cl3 cells and primary dog thyrocytes, rapamycin inhibited DNA synthesis and retinoblastoma protein phosphorylation induced by TSH and insulin. Although rapamycin reduced cyclin D3 accumulation, the abundance of cyclin D3-CDK4 complexes was not affected. However, rapamycin inhibited the activity of these complexes by decreasing the TSH and insulin-mediated stimulation of activating T172 phosphorylation of CDK4. We propose that mTORC1 activation by TSH, at least in part through PKA-dependent phosphorylation of PRAS40, crucially contributes to mediate cAMP-dependent mitogenesis by regulating CDK4 T172-phosphorylation.

Discovery of PDK1 kinase inhibitors with a novel mechanism of action by ultrahigh throughput screening

The phosphoinositide 3-kinase/AKT signaling pathway plays a key role in cancer cell growth, survival, and angiogenesis. Phosphoinositide-dependent protein kinase-1 (PDK1) acts at a focal point in this pathway immediately downstream of phosphoinositide 3-kinase and PTEN, where it phosphorylates numerous AGC kinases. The PDK1 kinase domain has at least three ligand-binding sites: the ATP-binding pocket, the peptide substrate-binding site, and a groove in the N-terminal lobe that binds the C-terminal hydrophobic motif of its kinase substrates. Based on the unique PDK1 substrate recognition system, ultrahigh throughput TR-FRET and Alphascreen screening assays were developed using a biotinylated version of the PDK1-tide substrate containing the activation loop of AKT fused to a pseudo-activated hydrophobic motif peptide. Using full-length PDK1, K(m) values were determined as 5.6 mum for ATP and 40 nm for the fusion peptide, revealing 50-fold higher affinity compared with the classical AKT(Thr-308)-tide. Kinetic and biophysical studies confirmed the PDK1 catalytic mechanism as a rapid equilibrium random bireactant reaction. Following an ultrahigh throughput screen of a large library, 2,000 compounds were selected from the reconfirmed hits by computational analysis with a focus on novel scaffolds. ATP-competitive hits were deconvoluted by dose-response studies at 1x and 10x K(m) concentrations of ATP, and specificity of binding was assessed in thermal shift assay. Inhibition studies using fusion PDK1-tide1 substrate versus AKT(Thr-308)-tide and kinase selectivity profiling revealed a novel selective alkaloid scaffold that evidently binds to the PDK1-interacting fragment pocket. Molecular modeling suggests a structural paradigm for the design of inhibitory versus activating allosteric ligands of PDK1.

Metformin decreases IGF1-induced cell proliferation and protein synthesis through AMP-activated protein kinase in cultured bovine granulosa cells

Although its mechanism of action is still unclear, metformin is an anti-diabetic drug effective to restore cyclicity and spontaneous ovulation in women with polycystic ovary syndrome. It may also reduce the risk of cancer. We have recently shown that metformin treatment decreases steroidogenesis through AMP-activated kinase (AMPK) in granulosa cells of various species. Here, we investigated the effects and the molecular mechanisms of metformin in IGF1-induced proliferation and protein synthesis in cultured bovine granulosa cells. Treatment with metformin (10 mM) for 24 h reduced cell proliferation and the levels of cyclin D2 and E, and increased the associations cyclin D2/p21 and cyclin D2/p27 without affecting cell viability in response to IGF1 (10(-8) M). It also decreased IGF1-induced protein synthesis and phosphorylation of P70S6 kinase and ribosomal S6 protein. Interestingly, metformin treatment for 1 h decreased MAPK3/1 (ERK1/2) and P90RSK phosphorylation without affecting AKT phosphorylation in response to IGF1. Adenovirus-mediated expression of dominant-negative AMPK totally abolished the effects of metformin on cell proliferation and phosphorylation of P70S6K in response to IGF1. It also eliminated the inhibitory effects of metformin on MAPK3/1 and P90RSK phosphorylation. Taken together, our results strongly suggest that metformin reduces cell growth, protein synthesis, MAPK3/1, and P90RSK phosphorylation in response to IGF1 through an AMPK-dependent mechanism in cultured bovine granulosa cells.

Developmental regulation of p70 S6 kinase by a G protein-coupled receptor dynamically modelized in primary cells

The mechanisms whereby G protein-coupled receptors (GPCR) activate signalling pathways involved in mRNA translation are ill-defined, in contrast to tyrosine kinase receptors (TKR). We compared a GPCR and a TKR, both endogenously expressed, for their ability to mediate phosphorylation of 70-kDa ribosomal S6 kinase p70S6K in primary rat Sertoli cells at two developmental stages. In proliferating cells stimulated with follicle-stimulating hormone (FSH), active p70S6K was phosphorylated on T389 and T421/S424, through cAMP-dependent kinase (PKA) and phosphatidyl-inositide-3 kinase (PI3K) antagonizing actions. In FSH-stimulated differentiating cells, active p70S6K was phosphorylated solely on T389, PKA and PI3K independently enhancing its activity. At both developmental stages, insulin-induced p70S6K regulation was consistent with reported data. Therefore, TKR and GPCR trigger distinct p70S6K active conformations. p70S6K developmental regulation was formalized in a dynamic mathematical model fitting the data, which led to experimentally inaccessible predictions on p70S6K phosphorylation rate.

GPCR signalling to the translation machinery

G protein-coupled receptors (GPCRs) are involved in most physiological processes, many of them being engaged in fully differentiated cells. These receptors couple to transducers of their own, primarily G proteins and beta-arrestins, which launch intracellular signalling cascades. Some of these signalling events regulate the translational machinery to fine-tune general cell metabolism or to alter protein expression pattern. Though extensively documented for tyrosine kinase receptors, translational regulation by GPCRs is still poorly appreciated. The objective of this review paper is to address the following questions: i) is there a "GPCR signature" impacting on the translational machinery, and ultimately on the type of mRNA translated? ii) are the regulatory networks involved similar as those utilized by tyrosine kinase receptors? In particular, we will discuss the specific features of translational control mediated by GPCRs and highlight the intrinsic properties of GPCRs these mechanisms could rely on.

Structural basis of human p70 ribosomal S6 kinase-1 regulation by activation loop phosphorylation

p70 ribosomal S6 kinase (p70S6K) is a downstream effector of the mTOR signaling pathway involved in cell proliferation, cell growth, cell-cycle progression, and glucose homeostasis. Multiple phosphorylation events within the catalytic, autoinhibitory, and hydrophobic motif domains contribute to the regulation of p70S6K. We report the crystal structures of the kinase domain of p70S6K1 bound to staurosporine in both the unphosphorylated state and in the 3'-phosphoinositide-dependent kinase-1-phosphorylated state in which Thr-252 of the activation loop is phosphorylated. Unphosphorylated p70S6K1 exists in two crystal forms, one in which the p70S6K1 kinase domain exists as a monomer and the other as a domain-swapped dimer. The crystal structure of the partially activated kinase domain that is phosphorylated within the activation loop reveals conformational ordering of the activation loop that is consistent with a role in activation. The structures offer insights into the structural basis of the 3'-phosphoinositide-dependent kinase-1-induced activation of p70S6K and provide a platform for the rational structure-guided design of specific p70S6K inhibitors.

Quantification of the effect of amino acids on an integrated mTOR and insulin signaling pathway

Integration of nutrient and growth factor signaling pathways through mammalian TOR (mTOR) plays a central role in the regulation of cell growth. However, the mechanism of integration of these two signals in mTOR activation is largely unknown. Moreover, the nutritional input involving amino acids is yet to be characterized. Excess amino acid conditions, such as in obesity and protein-rich diets, are known to regulate insulin signaling through mTOR activation resulting in insulin resistance. Here, we develop a dynamic model to identify the regulatory role of amino acids in mTOR activation and to study its effect on insulin signaling in relation to multiple feedback loops present in the insulin signaling pathway. The analysis revealed that amino acids bring about multiple effects in the regulation of mTOR that might be represented by a single mechanism. Insulin signaling was demonstrated to operate between two extreme conditions involving tumor growth and insulin resistance, with multiple feedback loops tightly controlling and maintaining a robust insulin response. The state of insulin resistance was characterized by a decrease in the time lag or an increase in the magnitude of the negative feedback loop facilitated through perturbations such as excess input of amino acids. Such a condition disturbs the delicate balance between positive and negative feedback loops to yield an insulin-resistant state.

Translational signaling responses preceding resistance training-mediated myofiber hypertrophy in young and old humans

While skeletal muscle protein accretion during resistance training (RT)-mediated myofiber hypertrophy is thought to result from upregulated translation initiation signaling, this concept is based on responses to a single bout of unaccustomed resistance exercise (RE) with no measure of hypertrophy across RT. Further, aging appears to affect acute responses to RE, but whether age differences in responsiveness persist during RT leading to impaired RT adaptation is unclear. We therefore tested whether muscle protein fractional synthesis rate (FSR) and Akt/mammalian target of rapamycin (mTOR) signaling in response to unaccustomed RE differed in old vs. young adults, and whether age differences in acute responsiveness were associated with differences in muscle hypertrophy after 16 wk of RT. Fifteen old and 21 young adult subjects completed the 16-wk study. The phosphorylation states of Akt, S6K1, ribosomal protein S6 (RPS6), eukaryotic initiation factor 4E (eIF4E) binding protein (4EBP1), eIF4E, and eIF4G were all elevated (23-199%) 24 h after a bout of unaccustomed RE. A concomitant 62% increase in FSR was found in a subset (6 old, 8 young). Age x time interaction was found only for RPS6 phosphorylation (+335% in old subjects only), while there was an interaction trend (P = 0.084) for FSR (+96% in young subjects only). After 16 wk of RT, gains in muscle mass, type II myofiber size, and voluntary strength were similar in young and old subjects. In conclusion, at the level of translational signaling, we found no evidence of impaired responsiveness among older adults, and for the first time, we show that changes in translational signaling after unaccustomed RE were associated with substantial muscle protein accretion (hypertrophy) during continued RT.

TOR regulation of AGC kinases in yeast and mammals

The TOR (target of rapamycin), an atypical protein kinase, is evolutionarily conserved from yeast to man. Pharmacological studies using rapamycin to inhibit TOR and yeast genetic studies have provided key insights on the function of TOR in growth regulation. One of the first bona fide cellular targets of TOR was the mammalian protein kinase p70 S6K (p70 S6 kinase), a member of a family of kinases called AGC (protein kinase A/protein kinase G/protein kinase C-family) kinases, which include PKA (cAMP-dependent protein kinase A), PKG (cGMP-dependent kinase) and PKC (protein kinase C). AGC kinases are also highly conserved and play a myriad of roles in cellular growth, proliferation and survival. The AGC kinases are regulated by a common scheme that involves phosphorylation of the kinase activation loop by PDK1 (phosphoinositide-dependent kinase 1), and phosphorylation at one or more sites at the C-terminal tail. The identification of two distinct TOR protein complexes, TORC1 (TOR complex 1) and TORC2, with different sensitivities to rapamycin, revealed that TOR, as part of either complex, can mediate phosphorylation at the C-terminal tail for optimal activation of a number of AGC kinases. Together, these studies elucidated that a fundamental function of TOR conserved throughout evolution may be to balance growth versus survival signals by regulating AGC kinases in response to nutrients and environmental conditions. This present review highlights this emerging function of TOR that is conserved from budding and fission yeast to mammals.