mTOR Ser-2481 autophosphorylation monitors mTORC-specific catalytic activity and clarifies rapamycin mechanism of action

Body weight control via protein kinase CK2: diet-induced obesity counteracted by pharmacological targeting

BACKGROUND

Protein kinase CK2 is a highly conserved enzyme implicated in the pathogenesis of various human illnesses including obesity. Despite compelling evidence for the involvement of this kinase in the pathophysiology of obesity, the molecular mechanisms by which CK2 might regulate fat metabolism are still poorly understood.

METHODS AND RESULTS

In this study, we aimed to elucidate the role of CK2 on lipid metabolism by employing both in vitro and in vivo approaches using mouse pre-adipocytes and a mouse model of diet-induced obesity. We show that pharmacological inhibition of CK2 by CX-4945 results in premature upregulation of p27KIP1 preventing the progression of cells into mature adipocytes by arresting their development at the intermediate phase of adipogenic differentiation. Consistent with this, we show that in vivo, CK2 regulates the expression levels and ERK-mediated phosphorylation of C/EBPβ, which is one of the earliest transcription factors responsive to adipogenic stimuli. Furthermore, we demonstrate the functional implication of CK2 in the expression of late markers of adipogenesis and factors regulating lipogenesis in liver and white adipose tissue. Finally, we show that while mice subjected to high-fat diet increased their body weight, those additionally treated with CX-4945 gained considerably less weight. NMR-based body composition analysis revealed that this is linked to significant differences in body fat mass.

CONCLUSIONS

Taken together, our study provides novel insights into the role of CK2 in fat metabolism in response to chronic lipid overload and confirms CK2 pharmacological targeting as a potentially powerful strategy for body weight control and/or the treatment of obesity and related metabolic disorders.

The cGAS-STING pathway activates transcription factor TFEB to stimulate lysosome biogenesis and pathogen clearance

Induction of autophagy is an ancient function of the cyclic GMP-AMP (cGAMP) synthase (cGAS)-stimulator of interferon genes (STING) pathway through which autophagic cargoes are delivered to lysosomes for degradation. However, whether lysosome function is also modulated by the cGAS-STING pathway remains unknown. Here, we discovered that the cGAS-STING pathway upregulated lysosomal activity by stimulating lysosome biogenesis independently of the downstream protein kinase TANK-binding kinase 1 (TBK1). STING activation enhanced lysosome biogenesis through inducing the nuclear translocation of transcription factor EB (TFEB) as well as its paralogs transcription factor E3 (TFE3) and microphthalmia-associated transcription factor (MITF). STING-induced lipidation of GABA type A receptor-associated protein (GABARAP), an autophagy-related protein, on STING vesicles was responsible for TFEB activation. Membrane-bound GABARAP sequestered the GTPase-activating protein folliculin (FLCN) and FLCN-interacting protein (FNIP) complex to block its function toward the Rag GTPases Ras-related GTP-binding C and D (RagC and RagD), abolishing mechanistic target of rapamycin (mTOR) complex 1 (mTORC1)-dependent phosphorylation and inactivation of TFEB. Functionally, STING-induced lysosome biogenesis within cells facilitated the clearance of cytoplasmic DNA and invading pathogens. Thus, our findings reveal that induction of lysosome biogenesis is another important function of the cGAS-STING pathway.

mTOR potentiates senescent phenotypes and primary cilia formation after cisplatin-induced G2 arrest in retinal pigment epithelial cells

Cisplatin, a platinum-based anticancer drug, is used to treat several types of cancer. Despite its effectiveness, cisplatin-induced side effects have often been reported. Although cisplatin-induced toxicities, such as apoptosis and/or necrosis, have been well studied, the fate of cells after exposure to sublethal doses of cisplatin needs further elucidation. Treatment with a sublethal dose of cisplatin induced cell cycle arrest at the G2 phase in retinal pigment epithelial cells. Following cisplatin withdrawal, the cells irreversibly exited the cell cycle and became senescent. Notably, the progression from the G2 to the G1 phase occurred without mitotic entry, a phenomenon referred to as mitotic bypass, resulting in the accumulation of cells containing 4N DNA content. Cisplatin-exposed cells exhibited morphological changes associated with senescence, including an enlarged size of cell and nucleus and increased granularity. In addition, the senescent cells possessed primary cilia and persistent DNA lesions. Senescence induced by transient exposure to cisplatin involves mTOR activation. Although transient co-exposure with an mTORC1 inhibitor rapamycin did not prevent mitotic bypass and entry into senescence, it delayed the progression of senescence and attenuated senescent phenotypes, resulting in shorter primary cilia formation. Conclusively, cisplatin induces senescence in retinal pigment epithelial cells by promoting mTOR activation.

mTORC2: A neglected player in aging regulation

Mammalian target of rapamycin (mTOR) is a serine/threonine kinase that plays a pivotal role in various biological processes, through integrating external and internal signals, facilitating gene transcription and protein translation, as well as by regulating mitochondria and autophagy functions. mTOR kinase operates within two distinct protein complexes known as mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), which engage separate downstream signaling pathways impacting diverse cellular processes. Although mTORC1 has been extensively studied as a pro-proliferative factor and a pro-aging hub if activated aberrantly, mTORC2 received less attention, particularly regarding its implication in aging regulation. However, recent studies brought increasing evidence or clues for us, which implies the associations of mTORC2 with aging, as the genetic elimination of unique subunits of mTORC2, such as RICTOR, has been shown to alleviate aging progression in comparison to mTORC1 inhibition. In this review, we first summarized the basic characteristics of mTORC2, including its protein architecture and signaling network. We then focused on reviewing the molecular signaling regulation of mTORC2 in cellular senescence and organismal aging, and proposed the multifaceted regulatory characteristics under senescent and nonsenescent contexts. Next, we outlined the research progress of mTOR inhibitors in the field of antiaging and discussed future prospects and challenges. It is our pleasure if this review article could provide meaningful information for our readers and call forth more investigations working on this topic.

mTOR in metabolic homeostasis and disease

The ability to maintain cellular metabolic homeostasis is critical to life, in which mTOR plays an important role. This kinase integrates upstream nutrient signals and performs essential functions in physiology and metabolism by increasing metabolism and suppressing autophagy. Thus, dysregulation of mTOR activity leads to diseases, especially metabolic diseases such as cancer, type 2 diabetes and neurological disorders. Therefore, inhibition of overactivated mTOR becomes a rational approach to treat a variety of metabolic diseases. In this review, we discuss how mTOR responds to upstream signals and how mTOR regulates metabolic processes, including protein, nucleic acid, and lipid metabolism. Furthermore, we discuss the possible causes and consequences of dysregulated mTOR signaling activity, and summarize relevant applications, such as inhibition of mTOR activity to treat these diseases. This review will advance our comprehensive knowledge of the association between mTOR and metabolic homeostasis, which has significant ramifications for human health.

Docosahexanoic Acid Attenuates Palmitate-Induced Apoptosis by Autophagy Upregulation via GPR120/mTOR Axis in Insulin-Secreting Cells

BACKGRUOUND

Polyunsaturated fatty acids (PUFAs) reportedly have protective effects on pancreatic β-cells; however, the underlying mechanisms are unknown.

METHODS

To investigate the cellular mechanism of PUFA-induced cell protection, mouse insulinoma 6 (MIN6) cells were cultured with palmitic acid (PA) and/or docosahexaenoic acid (DHA), and alterations in cellular signaling and apoptosis were examined.

RESULTS

DHA treatment remarkably repressed caspase-3 cleavage and terminal deoxynucleotidyl transferase-mediated UTP nick end labeling (TUNEL)-positive red dot signals in PA-treated MIN6 cells, with upregulation of autophagy, an increase in microtubule- associated protein 1-light chain 3 (LC3)-II, autophagy-related 5 (Atg5), and decreased p62. Upstream factors involved in autophagy regulation (Beclin-1, unc51 like autophagy activating kinase 1 [ULK1], phosphorylated mammalian target of rapamycin [mTOR], and protein kinase B) were also altered by DHA treatment. DHA specifically induced phosphorylation on S2448 in mTOR; however, phosphorylation on S2481 decreased. The role of G protein-coupled receptor 120 (GPR120) in the effect of DHA was demonstrated using a GPR120 agonist and antagonist. Additional treatment with AH7614, a GPR120 antagonist, significantly attenuated DHA-induced autophagy and protection. Taken together, DHA-induced autophagy activation with protection against PA-induced apoptosis mediated by the GPR120/mTOR axis.

CONCLUSION

These findings indicate that DHA has therapeutic effects on PA-induced pancreatic β-cells, and that the cellular mechanism of β-cell protection by DHA may be a new research target with potential pharmacotherapeutic implications in β-cell protection.

Effects of chronic alcohol intoxication on aerobic exercise-induced adaptations in female mice

Chronic alcohol intoxication decreases muscle strength/function and causes mitochondrial dysfunction. Aerobic exercise training improves mitochondrial oxidative capacity and increases muscle mass and strength. Presently, the impact of chronic alcohol on aerobic exercise-induced adaptations was investigated. Female C57BL/6Hsd mice were randomly assigned to one of four groups: control sedentary (CON SED; n = 26), alcohol sedentary (ETOH SED; n = 27), control exercise (CON EX; n = 28), and alcohol exercise (ETOH EX; n = 25). Exercise mice had running wheel access for 2 h a day, 7 days a week. All mice were fed either control or an alcohol-containing liquid diet. Grip strength testing and EchoMRI were performed before and after the interventions. After 6 wk, hindlimb muscles were collected for molecular analyses. A subset of mice performed a treadmill run to fatigue (RTF), then abstained from alcohol for 2 wk and repeated the RTF. Alcohol decreased lean mass and forelimb grip strength compared with control-fed mice. Alcohol blunted the exercise-induced increase in muscle mass (plantaris and soleus), type IIa fiber percentage in the plantaris, and run time to fatigue. Mitochondrial markers (Citrate synthase activity and Complex I-IV, COXIV and Cytochrome C protein expression) were increased with exercise regardless of ETOH in the gastrocnemius but not tibialis anterior muscle. Two weeks of alcohol abstinence improved RTF time in ETOH EX but not in ETOH SED. These data suggest that alcohol impairs some exercise-induced adaptations in skeletal muscle, but not all were negatively affected, indicating that exercise may be a beneficial behavior even while consuming alcohol.NEW & NOTEWORTHY Alcohol consumption during an aerobic exercise training period prevented training-induced increases in run to fatigue time and grip strength. Cessation of alcohol allowed for recovery of endurance performance within 2 wk. The worsened exercise performance after alcohol was unrelated to impairments in markers of mitochondrial health. Therefore, some adaptations to exercise training are impaired with alcohol use (endurance performance, muscle growth, and strength), while others remain mostly unaffected (mitochondrial health).

CKLF induces microglial activation via triggering defective mitophagy and mitochondrial dysfunction

Although microglial activation is induced by an increase in chemokines, the role of mitophagy in this process remains unclear. This study aimed to elucidate the role of microglial mitophagy in CKLF/CKLF1 (chemokine-like factor 1)-induced microglial activation and neuroinflammation, as well as the underlying molecular mechanisms following CKLF treatment. This study determined that CKLF, an inducible chemokine in the brain, leads to an increase in mitophagy markers, such as DNM1L, PINK1 (PTEN induced putative kinase 1), PRKN, and OPTN, along with a simultaneous increase in autophagosome formation, as evidenced by elevated levels of BECN1 and MAP1LC3B (microtubule-associated protein 1 light chain 3 beta)-II. However, SQSTM1, a substrate of autophagy, was also accumulated by CKLF treatment, suggesting that mitophagy flux was reduced and mitophagosomes accumulated. These findings were confirmed by transmission electron microscopy and confocal microscopy. The defective mitophagy observed in our study was caused by impaired lysosomal function, including mitophagosome-lysosome fusion, lysosome generation, and acidification, resulting in the accumulation of damaged mitochondria in microglial cells. Further analysis revealed that pharmacological blocking or gene-silencing of mitophagy inhibited CKLF-mediated microglial activation, as evidenced by the expression of the microglial marker AIF1 (allograft inflammatory factor 1) and the mRNA of proinflammatory cytokines (Tnf and Il6). Ultimately, defective mitophagy induced by CKLF results in microglial activation, as observed in the brains of adult mice. In summary, CKLF induces defective mitophagy, microglial activation, and inflammation, providing a potential approach for treating neuroinflammatory diseases.Abbreviation: 3-MA: 3-methyladenine; AIF1: allograft inflammatory factor 1; ANOVA: analysis of variance; BAF: bafilomycin A1; BSA: bovine serum albumin; CCCP: carbonyl cyanide m-chlorophenyl hydrazone; cGAMP: cyclic GMP-AMP; CGAS: cyclic GMP-AMP synthase; CKLF/CKLF1: chemokine-like factor 1; CNS: central nervous system; DMEM: Dulbecco's Modified Eagle Medium; DNM1L: dynamin 1 like; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GFP: green fluorescence protein; IRF3: interferon regulatory factor 3; IgG: immunoglobulin G; LAMP1: lysosomal-associated membrane protein 1; LAPTM4A: lysosomal-associated protein transmembrane 4A; MAP1LC3B: microtubule-associated protein 1 light chain 3 beta; Mdivi-1: mitochondrial division inhibitor 1; mRFP: monomeric red fluorescent protein; mtDNA: mitochondrial DNA; MTORC1: mechanistic target of rapamycin kinase complex 1; OPTN: optineurin; PBS: phosphate-buffered saline; PCR: polymerase chain reaction; PINK1: PTEN induced putative kinase 1; PLL: poly-L-lysine; PRKN: parkin RBR E3 ubiquitin protein ligase; qPCR: quantitative polymerase chain reaction; ROS: reactive oxygen species; SQSTM1: sequestosome 1; TBK1: TANK-binding kinase 1; TFEB: transcription factor EB; VDAC: voltage-dependent anion channel.

The mTORC2 signaling network: targets and cross-talks

The mechanistic target of rapamycin, mTOR, controls cell metabolism in response to growth signals and stress stimuli. The cellular functions of mTOR are mediated by two distinct protein complexes, mTOR complex 1 (mTORC1) and mTORC2. Rapamycin and its analogs are currently used in the clinic to treat a variety of diseases and have been instrumental in delineating the functions of its direct target, mTORC1. Despite the lack of a specific mTORC2 inhibitor, genetic studies that disrupt mTORC2 expression unravel the functions of this more elusive mTOR complex. Like mTORC1 which responds to growth signals, mTORC2 is also activated by anabolic signals but is additionally triggered by stress. mTORC2 mediates signals from growth factor receptors and G-protein coupled receptors. How stress conditions such as nutrient limitation modulate mTORC2 activation to allow metabolic reprogramming and ensure cell survival remains poorly understood. A variety of downstream effectors of mTORC2 have been identified but the most well-characterized mTORC2 substrates include Akt, PKC, and SGK, which are members of the AGC protein kinase family. Here, we review how mTORC2 is regulated by cellular stimuli including how compartmentalization and modulation of complex components affect mTORC2 signaling. We elaborate on how phosphorylation of its substrates, particularly the AGC kinases, mediates its diverse functions in growth, proliferation, survival, and differentiation. We discuss other signaling and metabolic components that cross-talk with mTORC2 and the cellular output of these signals. Lastly, we consider how to more effectively target the mTORC2 pathway to treat diseases that have deregulated mTOR signaling.

Alkaline intracellular pH (pHi) increases PI3K activity to promote mTORC1 and mTORC2 signaling and function during growth factor limitation

The conserved protein kinase mTOR (mechanistic target of rapamycin) responds to diverse environmental cues to control cell metabolism and promote cell growth, proliferation, and survival as part of two multiprotein complexes, mTOR complex 1 (mTORC1) and mTORC2. Our prior work demonstrated that an alkaline intracellular pH (pHi) increases mTORC2 activity and cell survival in complete media in part by activating AMP-activated protein kinase, a kinase best known to sense energetic stress. It is important to note that an alkaline pHi represents an underappreciated hallmark of cancer cells that promotes their oncogenic behaviors. In addition, mechanisms that control mTORC1 and mTORC2 signaling and function remain incompletely defined, particularly in response to stress conditions. Here, we demonstrate that an alkaline pHi increases phosphatidylinositide 3-kinase (PI3K) activity to promote mTORC1 and mTORC2 signaling in the absence of serum growth factors. Alkaline pHi increases mTORC1 activity through PI3K-Akt signaling, which mediates inhibitory phosphorylation of the upstream proteins tuberous sclerosis complex 2 and proline-rich Akt substrate of 40 kDa and dissociates tuberous sclerosis complex from lysosomal membranes, thus enabling Rheb-mediated activation of mTORC1. Thus, alkaline pHi mimics growth factor-PI3K signaling. Functionally, we also demonstrate that an alkaline pHi increases cap-dependent protein synthesis through inhibitory phosphorylation of eIF4E binding protein 1 and suppresses apoptosis in a PI3K- and mTOR-dependent manner. We speculate that an alkaline pHi promotes a low basal level of cell metabolism (e.g., protein synthesis) that enables cancer cells within growing tumors to proliferate and survive despite limiting growth factors and nutrients, in part through elevated PI3K-mTORC1 and/or PI3K-mTORC2 signaling.

KAT7-mediated CANX (calnexin) crotonylation regulates leucine-stimulated MTORC1 activity

Amino acids play crucial roles in the MTOR (mechanistic target of rapamycin kinase) complex 1 (MTORC1) pathway. However, the underlying mechanisms are not fully understood. Here, we establish a cell-free system to mimic the activation of MTORC1, by which we identify CANX (calnexin) as an essential regulator for leucine-stimulated MTORC1 pathway. CANX translocates to lysosomes after leucine deprivation, and its loss of function renders either the MTORC1 activity or the lysosomal translocation of MTOR insensitive to leucine deprivation. We further find that CANX binds to LAMP2 (lysosomal associated membrane protein 2), and LAMP2 is required for leucine deprivation-induced CANX interaction with the Ragulator to inhibit Ragulator activity toward RRAG GTPases. Moreover, leucine deprivation promotes the lysine (K) 525 crotonylation of CANX, which is another essential condition for the lysosomal translocation of CANX. Finally, we find that KAT7 (lysine acetyltransferase 7) mediates the K525 crotonylation of CANX. Loss of KAT7 renders the MTORC1 insensitivity to leucine deprivation. Our findings provide new insights for the regulatory mechanism of the leucine-stimulated MTORC1 pathway.Abbreviations: CALR: calreticulin; CANX: calnexin; CLF: crude lysosome fraction; EIF4EBP1: eukaryotic translation initiation factor 4E binding protein 1; ER: endoplasmic reticulum; GST: glutathione S-transferase; HA: hemagglutinin; HEK293T: human embryonic kidney-293T; KAT7: lysine acetyltransferase 7; Kcr; lysine crotonylation; KO: knockout; LAMP2: lysosomal associated membrane protein 2; LAMTOR/Ragulator: late endosomal/lysosomal adaptor: MAPK and MTOR activator; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MTOR: mechanistic target of rapamycin kinase; PDI: protein disulfide isomerase; PTM: post-translational modification; RPS6KB1/p70S6 kinase 1: ribosomal protein S6 kinase B1; RPTOR: regulatory associated protein of MTOR complex 1; SESN2: sestrin 2; TMEM192: transmembrane protein 192; ULK1: unc-51 like autophagy activating kinase 1.

Simultaneous loss of TSC1 and DEPDC5 in skeletal and cardiac muscles produces early-onset myopathy and cardiac dysfunction associated with oxidative damage and SQSTM1/p62 accumulation

By promoting anabolism, MTORC1 is critical for muscle growth and maintenance. However, genetic MTORC1 upregulation promotes muscle aging and produces age-associated myopathy. Whether MTORC1 activation is sufficient to produce myopathy or indirectly promotes it by accelerating tissue aging is elusive. Here we examined the effects of muscular MTORC1 hyperactivation, produced by simultaneous depletion of TSC1 and DEPDC5 (CKM-TD). CKM-TD mice produced myopathy, associated with loss of skeletal muscle mass and force, as well as cardiac failure and bradypnea. These pathologies were manifested at eight weeks of age, leading to a highly penetrant fatality at around twelve weeks of age. Transcriptome analysis indicated that genes mediating proteasomal and macroautophagic/autophagic pathways were highly upregulated in CKM-TD skeletal muscle, in addition to inflammation, oxidative stress, and DNA damage signaling pathways. In CKM-TD muscle, autophagosome levels were increased, and the AMPK and ULK1 pathways were activated; in addition, autophagy induction was not completely blocked in CKM-TD myotubes. Despite the upregulation of autolysosomal markers, CKM-TD myofibers exhibited accumulation of autophagy substrates, such as SQSTM1/p62 and ubiquitinated proteins, suggesting that the autophagic activities were insufficient. Administration of a superoxide scavenger, tempol, normalized most of these molecular pathologies and subsequently restored muscle histology and force generation. However, CKM-TD autophagy alterations were not normalized by rapamycin or tempol, suggesting that they may involve non-canonical targets other than MTORC1. These results collectively indicate that the concomitant muscle deficiency of TSC1 and DEPDC5 can produce early-onset myopathy through accumulation of oxidative stress, which dysregulates myocellular homeostasis.Abbreviations: AMPK: AMP-activated protein kinase; CKM: creatine kinase, M-type; COX: cytochrome oxidase; DEPDC5: DEP domain containing 5, GATOR1 subcomplex subunit; DHE: dihydroethidium; EDL: extensor digitorum longus; EIF4EBP1: eukaryotic translation initiation factor 4E binding protein 1; GAP: GTPase-activating protein; GTN: gastrocnemius; MTORC1: mechanistic target of rapamycin kinase complex 1; PLA: plantaris; QUAD: quadriceps; RPS6KB/S6K: ribosomal protein S6 kinase beta; SDH: succinate dehydrogenase; SOL: soleus; SQSTM1: sequestosome 1; TA: tibialis anterior; TSC1: TSC complex subunit 1; ULK1: unc-51 like autophagy activating kinase 1.

Convergence of signaling pathways in mediating actions of leucine and IGF-1 on mTORC1 in L6 myoblasts

Leucine and insulin-like growth factor-1 (IGF-1) are important regulators of protein synthesis in skeletal muscle. The mechanistic target of rapamycin complex 1 (mTORC1) is of particular importance in their mechanism of action. In the present study, pathways through which leucine and IGF-1 converge to mediate activation of mTORC1 were examined in L6 myoblasts that were deprived of leucine and serum followed by readdition of either leucine or IGF-1. Compared with leucine- and serum-deprived myoblasts, IGF-1, but not leucine, promoted phosphorylation of protein kinase B (AKT), tuberous sclerosis complex 2 (TSC2), and the autophosphorylation site on mTOR (S2481) and also stimulated mTOR kinase activity in mTOR immunoprecipitated samples. Both leucine and IGF-1 promoted phosphorylation of mTOR on S2448. The association of mTOR with the regulatory-associated protein of mTOR (Raptor) was altered by IGF-1 treatment and trended (P = 0.065) to be altered by leucine treatment. Alterations in the association of mTOR with Raptor were proportional to changes in phosphorylation of the mTOR substrates, eIF4E-binding protein 1 (4E-BP1), and ribosomal protein S6 Kinase-β1 (p70S6K1). Surprisingly, leucine, but not IGF-1, stimulated protein synthesis suggesting a unique role for nutrients in regulating protein synthesis. Overall, the results are consistent with a model whereby IGF-1 stimulates phosphorylation of 4E-BP1 and p70S6K1 in L6 myoblasts through an AKT-TSC2-mTORC1 signaling pathway that also involves changes in the interaction between mTOR and Raptor. In contrast, leucine signaling to mTOR results in alterations in certain mTOR phosphorylation sites and the interaction between mTOR and Raptor and stimulates protein synthesis.

mTORC1 and mTORC2 Complexes Regulate the Untargeted Metabolomics and Amino Acid Metabolites Profile through Mitochondrial Bioenergetic Functions in Pancreatic Beta Cells

Background: Pancreatic beta cells regulate bioenergetics efficiency and secret insulin in response to glucose and nutrient availability. The mechanistic Target of Rapamycin (mTOR) network orchestrates pancreatic progenitor cell growth and metabolism by nucleating two complexes, mTORC1 and mTORC2. Objective: To determine the impact of mTORC1/mTORC2 inhibition on amino acid metabolism in mouse pancreatic beta cells (Beta-TC-6 cells, ATCC-CRL-11506) using high-resolution metabolomics (HRM) and live-mitochondrial functions. Methods: Pancreatic beta TC-6 cells were incubated for 24 h with either: RapaLink-1 (RL); Torin-2 (T); rapamycin (R); metformin (M); a combination of RapaLink-1 and metformin (RLM); Torin-2 and metformin (TM); compared to the control. We applied high-resolution mass spectrometry (HRMS) LC-MS/MS untargeted metabolomics to compare the twenty natural amino acid profiles to the control. In addition, we quantified the bioenergetics dynamics and cellular metabolism by live-cell imaging and the MitoStress Test XF24 (Agilent, Seahorse). The real-time, live-cell approach simultaneously measures the oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) to determine cellular respiration and metabolism. Statistical significance was assessed using ANOVA on Ranks and post-hoc Welch t-Tests. Results: RapaLink-1, Torin-2, and rapamycin decreased L-aspartate levels compared to the control (p = 0.006). Metformin alone did not affect L-aspartate levels. However, L-asparagine levels decreased with all treatment groups compared to the control (p = 0.03). On the contrary, L-glutamate and glycine levels were reduced only by mTORC1/mTORC2 inhibitors RapaLink-1 and Torin-2, but not by rapamycin or metformin. The metabolic activity network model predicted that L-aspartate and AMP interact within the same activity network. Live-cell bioenergetics revealed that ATP production was significantly reduced in RapaLink-1 (122.23 ± 33.19), Torin-2 (72.37 ± 17.33) treated cells, compared to rapamycin (250.45 ± 9.41) and the vehicle control (274.23 ± 38.17), p < 0.01. However, non-mitochondrial oxygen consumption was not statistically different between RapaLink-1 (67.17 ± 3.52), Torin-2 (55.93 ± 8.76), or rapamycin (80.01 ± 4.36, p = 0.006). Conclusions: Dual mTORC1/mTORC2 inhibition by RapaLink-1 and Torin-2 differentially altered the amino acid profile and decreased mitochondrial respiration compared to rapamycin treatment which only blocks the FRB domain on mTOR. Third-generation mTOR inhibitors may alter the mitochondrial dynamics and reveal a bioenergetics profile that could be targeted to reduce mitochondrial stress.

mTOR substrate phosphorylation in growth control

The target of rapamycin (TOR), discovered 30 years ago, is a highly conserved serine/threonine protein kinase that plays a central role in regulating cell growth and metabolism. It is activated by nutrients, growth factors, and cellular energy. TOR forms two structurally and functionally distinct complexes, TORC1 and TORC2. TOR signaling activates cell growth, defined as an increase in biomass, by stimulating anabolic metabolism while inhibiting catabolic processes. With emphasis on mammalian TOR (mTOR), we comprehensively reviewed the literature and identified all reported direct substrates. In the context of recent structural information, we discuss how mTORC1 and mTORC2, despite having a common catalytic subunit, phosphorylate distinct substrates. We conclude that the two complexes recruit different substrates to phosphorylate a common, minimal motif.

Insulin Receptor Genetic Variants Causal Association with Type 2 Diabetes: A Mendelian Randomization Study

Background

Type 2 diabetes (T2D) is a prevalent chronic disease associated with several comorbidities.

Objectives

This study investigated whether the risk of T2D varied with genetically predicted insulin (INS), insulin receptor (INS-R), or insulin-like growth factor 1 receptor (IGF-1R) using genetic variants in a Mendelian randomization (MR) study.

Methods

A 2-sample MR study was conducted using summary statistics from 2 genome-wide association studies (GWASs). Genetic predictors of the exposures (INS, INS-R, and IGF-1R) were obtained from a publicly available proteomics GWAS of the INTERVAL randomized controlled trial of blood donation in the United Kingdom. For T2D, the study leveraged the DIAbetes Meta-ANalysis of Trans-Ethnic association studies (DIAMANTE) consortium. The estimated associations of INS, INS-R, and IGF-1R proteins with T2D were based on independent single nucleotide polymorphisms (SNPs) strongly (P < 5 × 10-6) predicting each exposure. These SNPs were applied to publicly available genetic associations with T2D from the DIAMANTE case (n = 74,124) and control (n = 824,006) study of people of European descent. SNP-specific Wald estimates were meta-analyzed using inverse variance weighting with multiplicative random effects. Sensitivity analysis was conducted using the weighted median (WM) and MR-Egger.

Results

INS-R (based on 13 SNPs) was associated with a lower risk of T2D (OR: 0.95 per effect size; 95% CI: 0.92, 0.98; P = 0.001), with similar estimates from the WM and MR-Egger. Insulin (8 SNPs) and IGF-1R (10 SNPs) were not associated with T2D. However, 1 of the SNPs for INS-R was from the ABO blood group gene.

Conclusions

This study is consistent with a causally protective association of the INS-R with T2D. INS-R in RBCs regulates glycolysis and thus may affect their functionality and integrity. However, a pleiotropic effect via the blood group ABO gene cannot be excluded. The INS-R may be a target for intervention by repurposing existing therapeutics or otherwise to reduce the risk of T2D.

Prophylactic efficacy of ketamine, but not the low-trapping NMDA receptor antagonist AZD6765, against stress-induced maladaptive behavior and 4E-BP1-related synaptic protein synthesis impairment

Ketamine enhances the resilience against stress-induced depressive-like behavior, but its prophylactic efficacy in anxiety-related behaviors remains to be elucidated. Moreover, there is a need for developing novel preventive strategies against depressive- and anxiety-like behavior. AZD6765, a low-trapping NMDA receptor antagonist, shares with ketamine common molecular targets and produces rapid-onset antidepressant effects, suggesting that it could be a prophylactic agent. Therefore, this study investigated the prophylactic effect of ketamine against the depressive- and anxiety-like behavior induced by chronic restraint stress (2 h/day, for 10 days) in mice. We also investigated if AZD6765 exerts a resilience-enhancing response against these maladaptive behaviors. The contribution of 4E-BP1-related synaptic proteins synthesis (PSD-95/GluA1) in the possible pro-resilience efficacy of ketamine and AZD6765 was investigated. A single administration of ketamine (5 mg/kg, i.p.), but not AZD6765 (1 or 5 mg/kg, i.p.), given 1 week before the stress protocol, was effective in preventing stress-induced depressive-like behavior in the tail suspension test and splash test. Ketamine administered at 1 and 5 mg/kg (i.p.), but not AZD6765 (1 or 5 mg/kg, i.p.), prevented stress-induced anxiety-related self-grooming alterations. Stress-induced reduction on 4E-BP1 phosphorylation and PSD-95 and GluA1 immunocontent in the prefrontal cortex was prevented by ketamine (5 mg/kg, i.p.), but not AZD6765 (1 or 5 mg/kg, i.p.). The results indicate that ketamine, but not AZD6765, exerts a pro-resilience response against stress-induced maladaptive behavior, reinforcing that it could be a prophylactic agent to manage individuals at-risk to develop MDD and anxiety.

The impact of DAPK1 and mTORC1 signaling association on autophagy in cancer

BACKGROUND

The autophagy pathway is used by eukaryotic cells to maintain metabolic homeostasis. Autophagy has two functions in cancerous cells which could inhibit tumorigenesis or lead to cancer progression by increasing cell survival and proliferation.

METHODS AND RESULTS

In this review article, Web of Science, PubMed, Scopus,  and Google Scholar were searched and summarized published studies to explore the relationship between DAPK1 and mTORC1 signaling association on autophagy in cancer. Autophagy is managed through various proteins including the mTOR, which is two separated structural and functional complexes known as mTORC1 and mTORC2. MTORC1 is an important component of the regulatory pathway affecting numerous cellular functions including proliferation, migration, invasion, and survival. This protein plays a key role in human cancers. The activity level of mTORC1 is regulated by the death-associated protein kinases (DAPks) family, especially DAPK1. In many cancers, DAPK1 acts as a tumor suppressor which can be attributed to its ability to suppress cellular transformation and to inhibit metastasis.

CONCLUSIONS

A deep investigation not only will reveal more about the function of DAPK1 but also might provide insights into novel therapies aimed to modulate the autophagy pathway in cancer and to achieve better cancer therapy.

Exercise increases phosphorylation of the putative mTORC2 activity readout NDRG1 in human skeletal muscle

In mice, exercise is suggested to activate the mechanistic target of rapamycin complex 2 (mTORC2) in skeletal muscle, and mTORC2 is required for normal muscle glucose uptake during exercise. Whether this translates to human skeletal muscle and what signaling pathways facilitate the exercise-induced mTORC2 activation is unknown. We herein tested the hypothesis that exercise increases mTORC2 activity in human skeletal muscle and investigated if β₂-adrenergic receptor (AR) activation mediates exercise-induced mTORC2 activation. We examined several mTORC2 activity readouts (p-NDRG1 Thr346, p-Akt Ser473, p-mTOR S2481, and p-Akt Thr450) in human skeletal muscle biopsies after uphill walking or cycling exercise. In mouse muscles, we assessed mTORC2 activity readouts following acute activation of muscle β₂-adrenergic or G_S signaling and during in vivo and ex vivo muscle contractions. Exercise increased phosphorylation of NDRG1 Thr346 in human soleus, gastrocnemius, and vastus lateralis muscle, without changing p-Akt Ser473, p-Akt Thr450, and p-mTOR Ser2481. In mouse muscle, stimulation of β₂-adrenergic or G_S signaling and ex vivo contractions failed to increase p-NDRG1 Thr346, whereas in vivo contractions were sufficient to induce p-NDRG1 Thr346. In conclusion, the mTORC2 activity readout p-NDRG1 Thr346 is a novel exercise-responsive signaling protein in human skeletal muscle. Notably, contraction-induced p-NDRG1 Thr346 appears to require a systemic factor. Unlike exercise, and in contrast to published data obtained in cultured muscles cells, stimulation of β₂-adrenergic signaling is not sufficient to trigger NDRG1 phosphorylation in mature mouse skeletal muscle.NEW & NOTEWORTHY The mTORC2 readout p-NDRG Thr346 is a novel exercise-responsive protein in human skeletal muscle. β2-AR and G_S signaling are not sufficient to induce mTORC2 signaling in adult muscle. In vivo, but not ex vivo, contraction induced p-NDRG Thr346, which indicates requirement of a systemic factor for exercise-induced mTORC2 activation.

Heat shock factor 1 suppression induces spindle abnormalities and sensitizes cells to antimitotic drugs

BACKGROUND

Heat shock factor 1 (HSF1) is the master regulator of the heat shock response and supports malignant cell transformation. Recent work has shown that HSF1 can access the promoters of heat shock proteins (HSPs) and allow HSP expression during mitosis. It also acts as a mitotic regulator, controlling chromosome segregation. In this study, we investigated whether the transactivation activity of HSF1 is required for the assembly of mitotic spindles.

RESULTS

Our results showed that phosphorylation of HSF1 at serine 326 (S326) and its transactivation activity were increased during mitosis. Inhibition of the transactivation activity of HSF1 by KRIBB11 or CCT251263 during mitosis significantly increased the proportion of mitotic cells with abnormal spindles. It also hampered the reassembly of spindle microtubules after nocodazole treatment and washout by impeding the formation of chromosomal microtubule asters. Depletion of HSF1 led to defects in mitotic spindle assembly, subsequently attenuating cell proliferation and anchorage-independent cell growth (AIG). These HSF1 depletion-induced effects could be rescued by ectopically expressing wild-type HSF1 or a constitutively active mutant (∆202-316, caHSF1) but not the S326A or dominant negative (∆361-529, dnHSF1) mutants. In addition, overexpression of HSP70 partially reduced HSF1 depletion-induced spindle abnormalities. These results indicate that HSF1 may support cell proliferation and AIG by maintaining spindle integrity through its transactivation activity. Furthermore, inhibition of HSF1 transactivation activity by KRIBB11 or CCT251236 can enhance diverse anti-mitosis drug-induced spindle defects and cell death.

CONCLUSIONS

The increased transactivation activity of HSF1 during mitosis appears to be required for accurate assembly of mitotic spindles, thereby supporting cell viability and probably AIG. In addition, inhibition of the transactivation activity of HSF1 may enhance the mitotic errors and cell death induced by anti-mitosis drugs.

Reserpine improves Enterobacteriaceae resistance in chicken intestine via neuro-immunometabolic signaling and MEK1/2 activation

Salmonella enterica persist in the chicken gut by suppressing inflammatory responses via expansion of intestinal regulatory T cells (Tregs). In humans, T cell activation is controlled by neurochemical signaling in Tregs; however, whether similar neuroimmunological signaling occurs in chickens is currently unknown. In this study, we explore the role of the neuroimmunological axis in intestinal Salmonella resistance using the drug reserpine, which disrupts intracellular storage of catecholamines like norepinephrine. Following reserpine treatment, norepinephrine release was increased in both ceca explant media and Tregs. Similarly, Salmonella killing was greater in reserpine-treated explants, and oral reserpine treatment reduced the level of intestinal Salmonella Typhimurium and other Enterobacteriaceae in vivo. These antimicrobial responses were linked to an increase in antimicrobial peptide and IL-2 gene expression as well as a decrease in CTLA-4 gene expression. Globally, reserpine treatment led to phosphorylative changes in epidermal growth factor receptor (EGFR), mammalian target of rapamycin (mTOR), and the mitogen-associated protein kinase 2(MEK2). Exogenous norepinephrine treatment alone increased Salmonella resistance, and reserpine-induced antimicrobial responses were blocked using beta-adrenergic receptor inhibitors, suggesting norepinephrine signaling is crucial in this mechanism. Furthermore, EGF treatment reversed reserpine-induced antimicrobial responses, whereas mTOR inhibition increased antimicrobial activities, confirming the roles of metabolic signaling in these responses. Finally, MEK1/2 inhibition suppressed reserpine, norepinephrine, and mTOR-induced antimicrobial responses. Overall, this study demonstrates a central role for MEK1/2 activity in reserpine induced neuro-immunometabolic signaling and subsequent antimicrobial responses in the chicken intestine, providing a means of reducing bacterial colonization in chickens to improve food safety.

Cancer-related cognitive impairment is associated with perturbations in inflammatory pathways

Cancer-related cognitive impairment (CRCI) is a significant problem for patients receiving chemotherapy. While a growing amount of pre-clinical and clinical evidence suggests that inflammatory mechanisms underlie CRCI, no clinical studies have evaluated for associations between CRCI and changes in gene expression. Therefore, the purpose of this study was to evaluate for differentially expressed genes and perturbed inflammatory pathways across two independent samples of patients with cancer who did and did not report CRCI. The Attentional Function Index (AFI) was the self-report measure used to assess CRCI. AFI scores of <5 and of >7.5 indicate low versus high levels of cognitive function, respectively. Of the 185 patients in Sample 1, 49.2% had an AFI score of <5 and 50.8% had an AFI score of >7.5. Of the 158 patients in Sample 2, 50.6% had an AFI score of <5 and 49.4% had an AFI score of >7.5. Data from 182 patients in Sample 1 were analyzed using RNA-seq. Data from 158 patients in Sample 2 were analyzed using microarray. Twelve KEGG signaling pathways were significantly perturbed between the AFI groups, five of which were signaling pathways related to inflammatory mechanisms (e.g., cytokine-cytokine receptor interaction, tumor necrosis factor signaling). This study is the first to describe perturbations in inflammatory pathways associated with CRCI. Findings highlight the role of cytokines both in terms of cytokine-specific pathways, as well as pathways involved in cytokine production and cytokine activation. These findings have the potential to identify new targets for therapeutics and lead to the development of interventions to improve cognition in patients with cancer.

Signaling defects associated with insulin resistance in nondiabetic and diabetic individuals and modification by sex

Insulin resistance is present in one-quarter of the general population, predisposing these people to a wide range of diseases. Our aim was to identify cell-intrinsic determinants of insulin resistance in this population using induced pluripotent stem cell-derived (iPSC-derived) myoblasts (iMyos). We found that these cells exhibited a large network of altered protein phosphorylation in vitro. Integrating these data with data from type 2 diabetic iMyos revealed critical sites of conserved altered phosphorylation in IRS-1, AKT, mTOR, and TBC1D1 in addition to changes in protein phosphorylation involved in Rho/Rac signaling, chromatin organization, and RNA processing. There were also striking differences in the phosphoproteome in cells from men versus women. These sex-specific and insulin-resistance defects were linked to functional differences in downstream actions. Thus, there are cell-autonomous signaling alterations associated with insulin resistance within the general population and important differences between men and women, many of which also occur in diabetes, that contribute to differences in physiology and disease.

Intermittent Bolus Compared With Continuous Feeding Enhances Insulin and Amino Acid Signaling to Translation Initiation in Skeletal Muscle of Neonatal Pigs

BACKGROUND

Nutrition administered as intermittent bolus feeds rather than continuously promotes greater protein synthesis rates in skeletal muscle and enhances lean growth in a neonatal piglet model. The molecular mechanisms responsible remain unclear.

OBJECTIVES

We aimed to identify the insulin- and/or amino acid-signaling components involved in the enhanced stimulation of skeletal muscle by intermittent bolus compared to continuous feeding in neonatal pigs born at term.

METHODS

Term piglets (2-3 days old) were fed equal amounts of sow milk replacer [12.8 g protein and 155 kcal/(kg body weight · d)] by orogastric tube as intermittent bolus meals every 4 hours (INT) or by continuous infusion (CTS). After 21 days, gastrocnemius muscle samples were collected from CTS, INT-0 (before a meal), and INT-60 (60 minutes after a meal) groups (n = 6/group). Insulin- and amino acid-signaling components relevant to mechanistic target of rapamycin complex (mTORC) 1 activation and protein translation were measured.

RESULTS

Phosphorylation of the insulin receptor, IRS-1, PDK1, mTORC2, pan-Akt, Akt1, Akt2, and TSC2 was 106% to 273% higher in the skeletal muscle of INT-60 piglets than in INT-0 and CTS piglets (P  < 0.05), but phosphorylation of PTEN, PP2A, Akt3, ERK1/2, and AMPK did not differ among groups, nor did abundances of PHLPP, SHIP2, and Ubl4A. The association of GATOR2 with Sestrin1/2, but not CASTOR1, was 51% to 52% lower in INT-60 piglets than in INT-0 and CTS piglets (P  < 0.05), but the abundances of SLC7A5/LAT1, SLC38A2/SNAT2, SLC38A9, Lamtor1/2, and V-ATPase did not differ. Associations of mTOR with RagA, RagC, and Rheb and phosphorylation of S6K1 and 4EBP1, but not eIF2α and eEF2, were 101% to 176% higher in INT-60 piglets than in INT-0 and CTS piglets (P < 0.05).

CONCLUSIONS

The enhanced rates of muscle protein synthesis and growth with intermittent bolus compared to continuous feeding in a neonatal piglet model can be explained by enhanced activation of both the insulin- and amino acid-signaling pathways that regulate translation initiation.

The innate immune kinase TBK1 directly increases mTORC2 activity and downstream signaling to Akt

TBK1 responds to microbes to initiate cellular responses critical for host innate immune defense. We found previously that TBK1 phosphorylates mTOR (mechanistic target of rapamycin) on S2159 to increase mTOR complex 1 (mTORC1) signaling in response to the growth factor EGF and the viral dsRNA mimetic poly(I:C). mTORC1 and the less well studied mTORC2 respond to diverse cues to control cellular metabolism, proliferation, and survival. Although TBK1 has been linked to Akt phosphorylation, a direct relationship between TBK1 and mTORC2, an Akt kinase, has not been described. By studying MEFs lacking TBK1, as well as MEFs, macrophages, and mice bearing an Mtor S2159A knock-in allele (MtorA/A) using in vitro kinase assays and cell-based approaches, we demonstrate here that TBK1 activates mTOR complex 2 (mTORC2) directly to increase Akt phosphorylation. We find that TBK1 and mTOR S2159 phosphorylation promotes mTOR-dependent phosphorylation of Akt in response to several growth factors and poly(I:C). Mechanistically, TBK1 coimmunoprecipitates with mTORC2 and phosphorylates mTOR S2159 within mTORC2 in cells. Kinase assays demonstrate that TBK1 and mTOR S2159 phosphorylation increase mTORC2 intrinsic catalytic activity. Growth factors failed to activate TBK1 or increase mTOR S2159 phosphorylation in MEFs. Thus, basal TBK1 activity cooperates with growth factors in parallel to increase mTORC2 (and mTORC1) signaling. Collectively, these results reveal cross talk between TBK1 and mTOR, key regulatory nodes within two major signaling networks. As TBK1 and mTOR contribute to tumorigenesis and metabolic disorders, these kinases may work together in a direct manner in a variety of physiological and pathological settings.

ArfGAP1 inhibits mTORC1 lysosomal localization and activation

The mammalian target of rapamycin complex 1 (mTORC1) integrates nutrients, growth factors, stress, and energy status to regulate cell growth and metabolism. Amino acids promote mTORC1 lysosomal localization and subsequent activation. However, the subcellular location or interacting proteins of mTORC1 under amino acid-deficient conditions is not completely understood. Here, we identify ADP-ribosylation factor GTPase-activating protein 1 (ArfGAP1) as a crucial regulator of mTORC1. ArfGAP1 interacts with mTORC1 in the absence of amino acids and inhibits mTORC1 lysosomal localization and activation. Mechanistically, the membrane curvature-sensing amphipathic lipid packing sensor (ALPS) motifs that bind to vesicle membranes are crucial for ArfGAP1 to interact with and regulate mTORC1 activity. Importantly, ArfGAP1 represses cell growth through mTORC1 and is an independent prognostic factor for the overall survival of pancreatic cancer patients. Our study identifies ArfGAP1 as a critical regulator of mTORC1 that functions by preventing the lysosomal transport and activation of mTORC1, with potential for cancer therapeutics.

MTOR Signaling and Metabolism in Early T Cell Development

The mechanistic target of rapamycin (mTOR) controls cell fate and responses via its functions in regulating metabolism. Its role in controlling immunity was unraveled by early studies on the immunosuppressive properties of rapamycin. Recent studies have provided insights on how metabolic reprogramming and mTOR signaling impact peripheral T cell activation and fate. The contribution of mTOR and metabolism during early T-cell development in the thymus is also emerging and is the subject of this review. Two major T lineages with distinct immune functions and peripheral homing organs diverge during early thymic development; the αβ- and γδ-T cells, which are defined by their respective TCR subunits. Thymic T-regulatory cells, which have immunosuppressive functions, also develop in the thymus from positively selected αβ-T cells. Here, we review recent findings on how the two mTOR protein complexes, mTORC1 and mTORC2, and the signaling molecules involved in the mTOR pathway are involved in thymocyte differentiation. We discuss emerging views on how metabolic remodeling impacts early T cell development and how this can be mediated via mTOR signaling.

Regulation and metabolic functions of mTORC1 and mTORC2

Cells metabolize nutrients for biosynthetic and bioenergetic needs to fuel growth and proliferation. The uptake of nutrients from the environment and their intracellular metabolism is a highly controlled process that involves cross talk between growth signaling and metabolic pathways. Despite constant fluctuations in nutrient availability and environmental signals, normal cells restore metabolic homeostasis to maintain cellular functions and prevent disease. A central signaling molecule that integrates growth with metabolism is the mechanistic target of rapamycin (mTOR). mTOR is a protein kinase that responds to levels of nutrients and growth signals. mTOR forms two protein complexes, mTORC1, which is sensitive to rapamycin, and mTORC2, which is not directly inhibited by this drug. Rapamycin has facilitated the discovery of the various functions of mTORC1 in metabolism. Genetic models that disrupt either mTORC1 or mTORC2 have expanded our knowledge of their cellular, tissue, as well as systemic functions in metabolism. Nevertheless, our knowledge of the regulation and functions of mTORC2, particularly in metabolism, has lagged behind. Since mTOR is an important target for cancer, aging, and other metabolism-related pathologies, understanding the distinct and overlapping regulation and functions of the two mTOR complexes is vital for the development of more effective therapeutic strategies. This review discusses the key discoveries and recent findings on the regulation and metabolic functions of the mTOR complexes. We highlight findings from cancer models but also discuss other examples of the mTOR-mediated metabolic reprogramming occurring in stem and immune cells, type 2 diabetes/obesity, neurodegenerative disorders, and aging.

The Roles of Post-Translational Modifications on mTOR Signaling

The mechanistic target of rapamycin (mTOR) is a master regulator of cell growth, proliferation, and metabolism by integrating various environmental inputs including growth factors, nutrients, and energy, among others. mTOR signaling has been demonstrated to control almost all fundamental cellular processes, such as nucleotide, protein and lipid synthesis, autophagy, and apoptosis. Over the past fifteen years, mapping the network of the mTOR pathway has dramatically advanced our understanding of its upstream and downstream signaling. Dysregulation of the mTOR pathway is frequently associated with a variety of human diseases, such as cancers, metabolic diseases, and cardiovascular and neurodegenerative disorders. Besides genetic alterations, aberrancies in post-translational modifications (PTMs) of the mTOR components are the major causes of the aberrant mTOR signaling in a number of pathologies. In this review, we summarize current understanding of PTMs-mediated regulation of mTOR signaling, and also update the progress on targeting the mTOR pathway and PTM-related enzymes for treatment of human diseases.

Improved Glucose Intolerance through a Distinct Mouse Olfactory Receptor 23-Induced Signaling Pathway Mediating Glucose Uptake in Myotubes and Adipocytes

SCOPE

It is aimed to determine the role of mouse olfactory receptor 23 (MOR23) in regulation of glucose uptake in myotubes and adipocytes and investigate whether administration of a possible MOR23 ligand, α-cedrene, attenuates the high fat diet (HFD)-induced glucose intolerance by enhancing the OR-mediated signaling pathway in mice.

METHODS AND RESULTS

MOR23 is genetically inactivated by specific small interfering RNA in C2C12 myotubes and 3T3-L1 adipocytes and stimulated with α-cedrene under both basal and insulin-stimulated conditions. In addition, Male C57BL/6N mice are fed a normal diet, HFD, or HFD supplemented with 0.2% α-cedrene. In C2C12 myotubes and 3T3-L1 adipocytes, genetic inactivation of MOR23 significantly decrease glucose uptake and MOR23 downstream signaling under both basal and insulin-stimulated conditions. On the other hand, α-cedrene-mediated MOR23 stimulation results in increased glucose uptake and upregulation of MOR23 signaling molecules, absent in MOR23-depleted myotubes and adipocytes. Moreover, in mice, α-cedrene administration ameliorates HFD-induced glucose intolerance. Activation of MOR23 signaling cascade is also confirmed in basal and insulin stimulated skeletal muscles and adipose tissues of α-cedrene-treated mice.

CONCLUSIONS

These findings suggest that MOR23 is a novel factor for the regulation of glucose uptake and whole-body glucose homeostasis and has therapeutic potential for diabetes treatment.

Causal association between mTOR-dependent EIF-4E and EIF-4A circulating protein levels and type 2 diabetes: a Mendelian randomization study

The mammalian Target of Rapamycin complex 1 (mTORC1) nutrient-sensing pathway is a central regulator of cell growth and metabolism and is dysregulated in diabetes. The eukaryotic translation initiation factor 4E (EIF-4E) protein, a key regulator of gene translation and protein function, is controlled by mTORC1 and EIF-4E Binding Proteins (EIF4EBPs). Both EIF4EBPs and ribosomal protein S6K kinase (RP-S6K) are downstream effectors regulated by mTORC1 but converge to regulate two independent pathways. We investigated whether the risk of type 2 diabetes varied with genetically predicted EIF-4E, EIF-4A, EIF-4G, EIF4EBP, and RP-S6K circulating levels using Mendelian Randomization. We estimated the causal role of EIF-4F complex, EIF4EBP, and S6K in the circulation on type 2 diabetes, based on independent single nucleotide polymorphisms strongly associated (p = 5 × 10^–6) with EIF-4E (16 SNPs), EIF-4A (11 SNPs), EIF-4G (6 SNPs), EIF4EBP2 (12 SNPs), and RP-S6K (16 SNPs). The exposure data were obtained from the INTERVAL study. We applied these SNPs for each exposure to publically available genetic associations with diabetes from the DIAbetes Genetics Replication And Meta-analysis (DIAGRAM) case (n = 26,676) and control (n = 132,532) study (mean age 57.4 years). We meta-analyzed SNP-specific Wald-estimates using inverse variance weighting with multiplicative random effects and conducted sensitivity analysis. Mendelian Randomization (MR-Base) R package was used in the analysis. The PhenoScanner curated database was used to identify disease associations with SNP gene variants. EIF-4E is associated with a lowered risk of type 2 diabetes with an odds ratio (OR) 0.94, 95% confidence interval (0.88, 0.99, p = 0.03) with similar estimates from the weighted median and MR-Egger. Similarly, EIF-4A was associated with lower risk of type 2 diabetes with odds ratio (OR) 0.90, 95% confidence interval (0.85, 0.97, p = 0.0003). Sensitivity analysis using MR-Egger and weighed median analysis does not indicate that there is a pleiotropic effect. This unbiased Mendelian Randomization estimate is consistent with a protective causal association of EIF-4E and EIF-4A on type 2 diabetes. EIF-4E and EIF-4A may be targeted for intervention by repurposing existing therapeutics to reduce the risk of type 2 diabetes.

Regulation of mTORC2 Signaling

Mammalian target of rapamycin (mTOR), a serine/threonine protein kinase and a master regulator of cell growth and metabolism, forms two structurally and functionally distinct complexes, mTOR complex 1 (mTORC1) and mTORC2. While mTORC1 signaling is well characterized, mTORC2 is relatively poorly understood. mTORC2 appears to exist in functionally distinct pools, but few mTORC2 effectors/substrates have been identified. Here, we review recent advances in our understanding of mTORC2 signaling, with particular emphasis on factors that control mTORC2 activity.

The Synergistic Effect of an ATP-Competitive Inhibitor of mTOR and Metformin on Pancreatic Tumor Growth

BACKGROUND

The mechanistic target of rapamycin complex 1 (mTORC1) is a nutrient-sensing pathway and a key regulator of amino acid and glucose metabolism. Dysregulation of the mTOR pathways is implicated in the pathogenesis of metabolic syndrome, obesity, type 2 diabetes, and pancreatic cancer.

OBJECTIVES

We investigated the impact of inhibition of mTORC1/mTORC2 and synergism with metformin on pancreatic tumor growth and metabolomics.

METHODS

Cell lines derived from pancreatic tumors of the KPC (KrasG12D/+; p53R172H/+; Pdx1-Cre) transgenic mice model were implanted into the pancreas of C57BL/6 albino mice (n = 10/group). Two weeks later, the mice were injected intraperitoneally with daily doses of 1) Torin 2 (mTORC1/mTORC2 inhibitor) at a high concentration (TH), 2) Torin 2 at a low concentration (TL), 3) metformin at a low concentration (ML), 4) a combination of Torin 2 and metformin at low concentrations (TLML), or 5) DMSO vehicle (control) for 12 d. Tissues and blood samples were collected for targeted xenometabolomics analysis, drug concentration, and cell signaling.

RESULTS

Metabolomic analysis of the control and treated plasma samples showed differential metabolite profiles. Phenylalanine was significantly elevated in the TLML group compared with the control (+426%, P = 0.0004), whereas uracil was significantly lower (-38%, P = 0.009). The combination treatment reduced tumor growth in the orthotopic mouse model. TLML significantly decreased pancreatic tumor volume (498 ± 104 mm3; 37%; P < 0.0004) compared with control (1326 ± 134 mm3; 100%), ML (853 ± 67 mm3; 64%), TL (745 ± 167 mm3; 54%), and TH (665 ± 182 mm3; 50%) (ANOVA and post hoc tests). TLML significantly decreased tumor weights (0.66 ± 0.08 g; 52%) compared with the control (1.28 ± 0.19 g; 100%) (P < 0.002).

CONCLUSIONS

The combination of mTOR dual inhibition by Torin 2 and metformin is associated with an altered metabolomic profile and a significant reduction in pancreatic tumor burden compared with single-agent therapy, and it is better tolerated.

Regulation of mTORC1 by Upstream Stimuli

The mammalian target of rapamycin (mTOR) is an evolutionary conserved Ser/Thr protein kinase that senses multiple upstream stimuli to control cell growth, metabolism, and autophagy. mTOR is the catalytic subunit of mTOR complex 1 (mTORC1). A significant amount of research has uncovered the signaling pathways regulated by mTORC1, and the involvement of these signaling cascades in human diseases like cancer, diabetes, and ageing. Here, we review advances in mTORC1 regulation by upstream stimuli. We specifically focus on how growth factors, amino acids, G-protein coupled receptors (GPCRs), phosphorylation, and small GTPases regulate mTORC1 activity and signaling.

mTORC1 and mTORC2 expression in inner retinal neurons and glial cells

The retina is one of the most metabolically active tissues, yet the processes that control retinal metabolism remains poorly understood. The mTOR complex (mTORC) that drives protein and lipid biogenesis and autophagy has been studied extensively in regards to retinal development and responses to optic nerve injury but the processes that regulate homeostasis in the adult retina have not been determined. We previously demonstrated that normal adult retina has high rates of protein synthesis compared to skeletal muscle, associated with high levels of mechanistic target of rapamycin (mTOR), a kinase that forms multi-subunit complexes that sense and integrate diverse environmental cues to control cell and tissue physiology. This study was undertaken to: 1) quantify expression of mTOR complex 1 (mTORC1)- and mTORC2-specific partner proteins in normal adult rat retina, brain and liver; and 2) to localize these components in normal human, rat, and mouse retinas. Immunoblotting and immunoprecipitation studies revealed greater expression of raptor (exclusive to mTORC1) and rictor (exclusive for mTORC2) in normal rat retina relative to liver or brain, as well as the activating mTORC components, pSIN1 and pPRAS40. By contrast, liver exhibits greater amounts of the mTORC inhibitor, DEPTOR. Immunolocalization studies for all three species showed that mTOR, raptor, and rictor, as well as most other known components of mTORC1 and mTORC2, were primarily localized in the inner retina with mTORC1 primarily in retinal ganglion cells (RGCs) and mTORC2 primarily in glial cells. In addition, phosphorylated ribosomal protein S6, a direct target of the mTORC1 substrate ribosomal protein S6 kinase beta-1 (S6K1), was readily detectable in RGCs, indicating active mTORC1 signaling, and was preserved in human donor eyes. Collectively, this study demonstrates that the inner retina expresses high levels of mTORC1 and mTORC2 and possesses active mTORC1 signaling that may provide cell- and tissue-specific regulation of homeostatic activity. These findings help to define the physiology of the inner retina, which is key for understanding the pathophysiology of optic neuropathies, glaucoma and diabetic retinopathy.

A PI3K- and GTPase-independent Rac1-mTOR mechanism mediates MET-driven anchorage-independent cell growth but not migration

Receptor tyrosine kinases (RTKs) are often overexpressed or mutated in cancers and drive tumor growth and metastasis. In the current model of RTK signaling, including that of MET, downstream phosphatidylinositol 3-kinase (PI3K) mediates both cell proliferation and cell migration, whereas the small guanosine triphosphatase (GTPase) Rac1 mediates cell migration. However, in cultured NIH3T3 and glioblastoma cells, we found that class I PI3K mediated oncogenic MET-induced cell migration but not anchorage-independent growth. In contrast, Rac1 regulated both processes in distinct ways. Downstream of PI3K, Rac1 mediated cell migration through its GTPase activity, whereas independently of PI3K, Rac1 mediated anchorage-independent growth in a GTPase-independent manner through an adaptor function. Through its RKR motif, Rac1 formed a complex with the kinase mTOR to promote its translocation to the plasma membrane, where its activity promoted anchorage-independent growth of the cell cultures. Inhibiting mTOR with rapamycin suppressed the growth of subcutaneous MET-mutant cell grafts in mice, including that of MET inhibitor-resistant cells. These findings reveal a GTPase-independent role for Rac1 in mediating a PI3K-independent MET-to-mTOR pathway and suggest alternative or combined strategies that might overcome resistance to RTK inhibitors in patients with cancer.

Guanosine potentiates the antidepressant-like effect of subthreshold doses of ketamine: Possible role of pro-synaptogenic signaling pathway

Background Augmentation therapies may be effective strategies to potentiate the ketamine's actions with lower potential for knock-on effects. Thus, this study investigated the ability of combined administration of guanosine plus ketamine to elicit an antidepressant-like effect associated with mTOR pathway modulation. The ability of this combined administration to exert an antidepressant-like effect in a model of depression was also evaluated. Methods Mice were administered with subthreshold doses of ketamine (0.1 mg/kg, i.p.) and guanosine (0.01 mg/kg, p.o.) and submitted to the tail suspension test, and immunoblotting analyses (p-mTOR, p-p70S6K, PSD-95, GluA1, and synapsin) in the hippocampus and prefrontal cortex. The antidepressant-like effect of ketamine plus guanosine in mice subjected to administration of corticosterone (20 mg/kg, p.o., 21 days) was also evaluated. Results Ketamine plus guanosine treatment elicited an antidepressant-like effect, which was associated with increased mTOR (Ser²⁴⁴⁸) and p70S6K (Thr³⁸⁹) phosphorylation in the hippocampus, but not in the prefrontal cortex. Furthermore, increased PSD-95 and GluA1 immunocontent were observed in the prefrontal cortex, but not in the hippocampus of ketamine plus guanosine-treated mice. Reinforcing the notion that guanosine may potentiate the ketamine's behavioral response, a single administration of subthreshold doses of ketamine plus guanosine counteracted the corticosterone-induced depressive-like behavior. Conclusions Our results indicate that guanosine potentiates the antidepressant-like effect of subthreshold doses of ketamine, an effect likely associated with the stimulation of synaptogenic pathway in the hippocampus and prefrontal cortex, although with a different profile. The augmentation effect of ketamine by guanosine could have therapeutic relevance for patients with treatment-resistant depression.

Partially non-homogeneous dynamic Bayesian networks based on Bayesian regression models with partitioned design matrices

Motivation

Non-homogeneous dynamic Bayesian networks (NH-DBNs) are a popular modelling tool for learning cellular networks from time series data. In systems biology, time series are often measured under different experimental conditions, and not rarely only some network interaction parameters depend on the condition while the other parameters stay constant across conditions. For this situation, we propose a new partially NH-DBN, based on Bayesian hierarchical regression models with partitioned design matrices. With regard to our main application to semi-quantitative (immunoblot) timecourse data from mammalian target of rapamycin complex 1 (mTORC1) signalling, we also propose a Gaussian process-based method to solve the problem of non-equidistant time series measurements.

Results

On synthetic network data and on yeast gene expression data the new model leads to improved network reconstruction accuracies. We then use the new model to reconstruct the topologies of the circadian clock network in Arabidopsis thaliana and the mTORC1 signalling pathway. The inferred network topologies show features that are consistent with the biological literature.

Availability and implementation

All datasets have been made available with earlier publications. Our Matlab code is available upon request.

Supplementary information

Supplementary data are available at Bioinformatics online.

Effects of short-term high-intensity interval and continuous exercise training on body composition and cardiac function in obese sarcopenic rats

AIM

We investigated the effects of high-intensity interval and continuous short-term exercise on body composition and cardiac function after myocardial ischemia-reperfusion injury (IRI) in obese rats.

METHODS

Rats fed with a standard chow diet (SC) or high-fat diet (HFD) for 20 weeks underwent systolic blood pressure (SBP), glycemia and dual-energy X-ray absorptiometry analyses. Then, animals fed with HFD were subdivided into three groups: sedentary (HFD-SED); moderate-intensity continuous training (HFD-MICT); and high-intensity interval training (HFD-HIIT). Exercised groups underwent four isocaloric aerobic exercise sessions, in which HFD-MICT maintained the intensity continuously and HFD-HIIT alternated it. After exercise sessions, all groups underwent global IRI and myocardial infarct size (IS) was determined histologically. Fat and muscle mass were weighted, and protein levels involved in muscle metabolism were assessed in skeletal muscle.

RESULTS

HFD-fed versus SC-fed rats reduced lean body mass by 31% (P < 0.001), while SBP, glycemia and body fat percentage were increased by 10% (P = 0.04), 30% (P = 0.006) and 54% (P < 0.001); respectively. HFD-induced muscle atrophy was restored in exercised groups, as only HFD-SED presented lower gastrocnemius (32%; P = 0.001) and quadriceps mass (62%; P < 0.001) than SC. PGC1-α expression was 2.7-fold higher in HFD-HIIT versus HFD-SED (P = 0.04), whereas HFD-HIIT and HFD-MICT exhibited 1.7-fold increase in p-mTOR^Ser2481 levels compared to HFD-SED (P = 0.04). Although no difference was detected among groups for IS (P = 0.30), only HFD-HIIT preserved left-ventricle developed pressure after IRI (+0.7 mmHg; P = 0.9).

SIGNIFICANCE

Short-term exercise, continuous or HIIT, restored HFD-induced muscle atrophy and increased mTOR expression, but only HIIT maintained myocardial contractility following IRI in obese animals.

Dietary and Pharmacological Interventions That Inhibit Mammalian Target of Rapamycin Activity Alter the Brain Expression Levels of Neurogenic and Glial Markers in an Age-and Treatment-Dependent Manner

Intermittent fasting (IF) and its mimetic, rapamycin extend lifespan and healthspan through mechanisms that are not fully understood. We investigated different short-term durations of IF and rapamycin on cellular and molecular changes in the brains of young (6-10 months) and old (26-31 months) zebrafish. Interestingly, our results showed that IF significantly lowered glucose levels while increasing DCAMKL1 in both young and old animals. This proliferative effect of IF was supported by the upregulation of foxm1 transcript in old animals. Rapamycin did not change glucose levels in young and old animals but had differential effects depending on age. In young zebrafish, proliferating cell nuclear antigen and the LC3-II/LC3-I ratio was decreased, whereas glial fibrillary acidic protein and gephyrin were decreased in old animals. The changes in proliferative markers and a marker of autophagic flux suggest an age-dependent interplay between autophagy and cell proliferation. Additionally, changes in glia and inhibitory tone suggest a suppressive effect on neuroinflammation but may push the brain toward a more excitable state. Mammalian target of rapamycin (mTOR) activity in the brain following the IF and rapamycin treatment was differentially regulated by age. Interestingly, rapamycin inhibited mTOR more potently in young animals than IF. Principal component analysis supported our conclusion that the regulatory effects of IF and rapamycin were age-specific, since we observed different patterns in the expression levels and clustering of young and old animals. Taken together, our results suggest that even a short-term duration of IF and rapamycin have significant effects in the brain at young and old ages, and that these are age and treatment dependent.

Differential roles and activation of mammalian target of rapamycin complexes 1 and 2 during cell migration in prostate cancer cells

BACKGROUND

Mammalian target of rapamycin (mTOR) is a downstream substrate activated by PI3K/AKT pathway and it is essential for cell migration. It exists as two complexes: mTORC1 and mTORC2. mTORC1 is known to be regulated by active AKT, but the activation of mTORC2 is poorly understood. In this study, we investigated the roles and differential activation of the two mTOR complexes during cell migration in prostate cancer cells.

METHODS

We used small interfering RNA to silence the expression of Rac1 and the main components of mTOR complexes (regulatory associated protein of mTOR [RAPTOR] and rapamycin-insensitive companion of mTOR [RICTOR]) in LNCaP, DU145, and PC3 prostate cancer cell lines. We performed transwell migration assay to evaluate the migratory capability of the cells, and Western blot analysis to study the activation levels of mTOR complexes.

RESULTS

Specific knockdown of RAPTOR and RICTOR caused a decrease of cell migration, suggesting their essential role in prostate cancer cell movement. Furthermore, epidermal growth factor (EGF) treatments induced the activation of both the mTOR complexes. Lack of Rac1 activity in prostate cancer cells blocked EGF-induced activation of mTORC2, but had no effect on mTORC1 activation. Furthermore, the overexpression of constitutively active Rac1 resulted in significant increase in cell migration and activation of mTORC2 in PC3 cells, but had no effect on mTORC1 activation. Active Rac1 was localized in the plasma membrane and was found to be in a protein complex, with RICTOR, but not RAPTOR.

CONCLUSION

We suggest that EGF-induced activation of Rac1 causes the activation of mTORC2 via RICTOR. This mechanism plays a critical role in prostate cancer cell migration.

SIRT2 Affects Primary Cilia Formation by Regulating mTOR Signaling in Retinal Pigmented Epithelial Cells

SIRT2, a member of the Class III HDAC family, participates in diverse cellular processes and regulates several pathological conditions. Although a few reports show that SIRT2 regulates the cell cycle, the causes and outcomes of SIRT2-dependent cell proliferation remain unclear. Here, we examined the effects of SIRT2 suppression in human RPE1 cells using siRNA targeting SIRT2, and AK-1, a SIRT2-specific inhibitor. The number of primary cilia in SIRT2-suppressed cells increased under serum-present conditions. Suppressing SIRT2 induced cell cycle arrest at G0/G1 phase by inactivating mammalian target of rapamycin (mTOR) signaling, possibly through mTORC1. Treatment with torin 1, an inhibitor of mTORC1/mTORC2, yielded results similar to those observed after SIRT2 suppression. However, SIRT2 suppression did not affect primary cilia formation or mTOR signaling following serum starvation. This suggests that SIRT2 acts as a critical sensor that links growth factor-dependent signal transduction and primary cilia formation by regulating the cell cycle.

Copper is an essential regulator of the autophagic kinases ULK1/2 to drive lung adenocarcinoma

While the transition metal copper (Cu) is an essential nutrient that is conventionally viewed as a static cofactor within enzyme active sites, a nontraditional role for Cu as a modulator of kinase signaling is emerging. We discovered that Cu is required for the activity of the autophagic kinases ULK1/2 through a direct Cu-ULK1/2 interaction. Genetic loss of the Cu transporter Ctr1 or mutations in ULK1 that disrupt Cu-binding reduced ULK1/2-dependent signaling and autophagosome complex formation. Elevated intracellular Cu levels are associated with starvation induced autophagy and sufficient to enhance ULK1 kinase activity and in turn autophagic flux. The growth and survival of lung tumors driven by KRAS^G12D is diminished in the absence of Ctr1, depends on ULK1 Cu-binding, and is associated with reduced autophagy levels and signaling. These findings suggest a molecular basis for exploiting Cu-chelation therapy to forestall autophagy signaling to limit proliferation and survival in cancer.

A Special Amino-Acid Formula Tailored to Boosting Cell Respiration Prevents Mitochondrial Dysfunction and Oxidative Stress Caused by Doxorubicin in Mouse Cardiomyocytes

Anthracycline anticancer drugs, such as doxorubicin (DOX), can induce cardiotoxicity supposed to be related to mitochondrial damage. We have recently demonstrated that a branched-chain amino acid (BCAA)-enriched mixture (BCAAem), supplemented with drinking water to middle-aged mice, was able to promote mitochondrial biogenesis in cardiac and skeletal muscle. To maximally favor and increase oxidative metabolism and mitochondrial function, here we tested a new original formula, composed of essential amino acids, tricarboxylic acid cycle precursors and co-factors (named α5), in HL-1 cardiomyocytes and mice treated with DOX. We measured mitochondrial biogenesis, oxidative stress, and BCAA catabolic pathway. Moreover, the molecular relevance of endothelial nitric oxide synthase (eNOS) and mechanistic/mammalian target of rapamycin complex 1 (mTORC1) was studied in both cardiac tissue and HL-1 cardiomyocytes. Finally, the role of Krüppel-like factor 15 (KLF15), a critical transcriptional regulator of BCAA oxidation and eNOS-mTORC1 signal, was investigated. Our results demonstrate that the α5 mixture prevents the DOX-dependent mitochondrial damage and oxidative stress better than the previous BCAAem, implying a KLF15/eNOS/mTORC1 signaling axis. These results could be relevant for the prevention of cardiotoxicity in the DOX-treated patients.

Differential regulation of mTORC1 activation by leucine and β-hydroxy-β-methylbutyrate in skeletal muscle of neonatal pigs

Leucine (Leu) and its metabolite β-hydroxy-β-methylbutyrate (HMB) stimulate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1)-dependent protein synthesis in the skeletal muscle of neonatal pigs. This study aimed to determine whether HMB and Leu utilize common nutrient-sensing mechanisms to activate mTORC1. In study 1, neonatal pigs were fed one of five diets for 24 h: low protein (LP), high protein (HP), or LP supplemented with 4 (LP+HMB4), 40 (LP+HMB40), or 80 (LP+HMB80) μmol HMB·kg body wt^-1·day^-1. In study 2, neonatal pigs were fed for 24 h: LP, LP supplemented with Leu (LP+Leu), or HP diets delivering 9, 18, and 18 mmol Leu·kg body wt^-1·day^-1, respectively. The upstream signaling molecules that regulate mTORC1 activity were analyzed. mTOR phosphorylation on Ser2448 and Ser2481 was greater in LP+HMB40, LP+HMB80, and LP+Leu than in LP and greater in HP than in HMB-supplemented groups (P < 0.05), whereas HP and LP+Leu were similar. Rheb-mTOR complex formation was lower in LP than in HP (P < 0.05), with no enhancement by HMB or Leu supplementation. The Sestrin2-GATOR2 complex was more abundant in LP than in HP and was reduced by Leu (P < 0.05) but not HMB supplementation. RagA-mTOR and RagC-mTOR complexes were higher in LP+Leu and HP than in LP and HMB groups (P < 0.05). There were no treatment differences in RagB-SH3BP4, Vps34-LRS, and RagD-LRS complex abundances. Phosphorylation of Erk1/2 and TSC2, but not AMPK, was lower in LP than HP (P < 0.05) and unaffected by HMB or Leu supplementation. Our results demonstrate that HMB stimulates mTORC1 activation in neonatal muscle independent of the leucine-sensing pathway mediated by Sestrin2 and the Rag proteins.NEW & NOTEWORTHY Dietary supplementation with either leucine or its metabolite β-hydroxy-β-methylbutyrate (HMB) stimulates protein synthesis in skeletal muscle of the neonatal pig. Our results demonstrate that both leucine and HMB stimulate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) phosphorylation in neonatal muscle. This leucine-stimulated process involves dissociation of the Sestrin2-GATOR2 complex and increased binding of Rag A/C to mTOR. However, HMB's activation of mTORC1 is independent of this leucine-sensing pathway.

Growth Factor-Dependent and -Independent Activation of mTORC2

The target of rapamycin complex 2 (TORC2) was discovered in 2002 in budding yeast. Its mammalian counterpart, mTORC2, was first described in 2004. Soon thereafter it was demonstrated that mTORC2 directly phosphorylates Akt on Ser473, ending a long search for the elusive 'second' insulin-responsive Akt kinase. In this review we discuss key evidence pertaining to the subcellular localization of mTORC2, highlighting a spatial heterogeneity that relates to mTORC2 activation. We summarize current models for how growth factors (GFs), such as insulin, trigger mTORC2 activation, and we provide a comprehensive discussion focusing on a new exciting frontier, the molecular mechanisms underpinning GF-independent activation of mTORC2.

PSEN2 (presenilin 2) mutants linked to familial Alzheimer disease impair autophagy by altering Ca2+ homeostasis

PSEN2 (presenilin 2) is one of the 3 proteins that, when mutated, causes early onset familial Alzheimer disease (FAD) cases. In addition to its well-known role within the γ-secretase complex (the enzyme ultimately responsible for Aβ peptides formation), PSEN2 is endowed with some γ-secretase-independent functions in distinct cell signaling pathways, such as the modulation of intracellular Ca2+ homeostasis. Here, by using different FAD-PSEN2 cell models, we demonstrate that mutated PSEN2 impairs autophagy by causing a block in the degradative flux at the level of the autophagosome-lysosome fusion step. The defect does not depend on an altered lysosomal functionality but rather on a decreased recruitment of the small GTPase RAB7 to autophagosomes, a key event for normal autophagy progression. Importantly, FAD-PSEN2 action on autophagy is unrelated to its γ-secretase activity but depends on its previously reported ability to partially deplete ER Ca2+ content, thus reducing cytosolic Ca2+ response upon IP3-linked cell stimulations. Our data sustain the pivotal role for Ca2+ signaling in autophagy and reveal a novel mechanism by which FAD-linked presenilins alter the degradative process, reinforcing the view of a causative role for a dysfunctional quality control pathway in AD neurodegeneration.Abbreviations: Aβ: amyloid β; AD: Alzheimer disease; ACTB: actin beta; AMPK: AMP-activated protein kinase; APP: amyloid-beta precursor protein; BafA: bafilomycin A1; BAPTA-AM: 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester; CFP: cyan fluorescent protein; EGTA-AM: ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid acetoxymethyl ester; ER: endoplasmic reticulum; EGFP-HDQ74: enhanced GFP-huntingtin exon 1 containing 74 polyglutamine repeats; FAD: familial Alzheimer disease; FCS: fetal calf serum; FRET: fluorescence/Förster resonance energy transfer; GFP: green fluorescent protein; IP3: inositol trisphosphate; KD: knockdown; LAMP1: lysosomal associated membrane protein 1; MAP1LC3-II/LC3-II: lipidated microtubule-associated protein 1 light chain 3; MCU: mitochondrial calcium uniporter; MICU1: mitochondrial calcium uptake 1; MEFs: mouse embryonic fibroblasts; MFN2: mitofusin 2; MTOR: mechanistic target of rapamycin kinase; MTORC1: MTOR complex 1; SQSTM1/p62: sequestosome 1; PSEN1: presenilin 1; PSEN2: presenilin 2; RAB7: RAB7A: member RAS oncogene family; RFP: red fluorescent protein; ATP2A/SERCA: ATPase sarcoplasmic/endoplasmic reticulum Ca2+ transporting; siRNA: small interference RNA; V-ATPase: vacuolar-type H+-ATPase; WT: wild type.

Preclinical efficacy of dual mTORC1/2 inhibitor AZD8055 in renal cell carcinoma harboring a TFE3 gene fusion

Background

Renal cell carcinomas (RCC) harboring a TFE3 gene fusion (TfRCC) represent an aggressive subset of kidney tumors. Key signaling pathways of TfRCC are unknown and preclinical in vivo data are lacking. We investigated Akt/mTOR pathway activation and the preclinical efficacy of dual mTORC1/2 versus selective mTORC1 inhibition in TfRCC.

Methods

Levels of phosphorylated Akt/mTOR pathway proteins were compared by immunoblot in TfRCC and clear cell RCC (ccRCC) cell lines. Effects of the mTORC1 inhibitor, sirolimus, and the dual mTORC1/2 inhibitor, AZD8055, on Akt/mTOR activation, cell cycle progression, cell viability and cytotoxicity were compared in TfRCC cells. TfRCC xenograft tumor growth in mice was evaluated after 3-week treatment with oral AZD8055, intraperitoneal sirolimus and respective vehicle controls.

Results

The Akt/mTOR pathway was activated to a similar or greater degree in TfRCC than ccRCC cell lines and persisted partly during growth factor starvation, suggesting constitutive activation. Dual mTORC1/2 inhibition with AZD8055 potently inhibited TfRCC viability (IC50 = 20-50 nM) due at least in part to cell cycle arrest, while benign renal epithelial cells were relatively resistant (IC50 = 400 nM). Maximal viability reduction was greater with AZD8055 than sirolimus (80–90% versus 30–50%), as was the extent of Akt/mTOR pathway inhibition, based on significantly greater suppression of P-Akt (Ser473), P-4EBP1, P-mTOR and HIF1α. In mouse xenograft models, AZD8055 achieved significantly better tumor growth inhibition and prolonged mouse survival compared to sirolimus or vehicle controls.

Conclusions

Akt/mTOR activation is common in TfRCC and a promising therapeutic target. Dual mTORC1/2 inhibition suppresses Akt/mTOR signaling more effectively than selective mTORC1 inhibition and demonstrates in vivo preclinical efficacy against TFE3-fusion renal cell carcinoma.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-6096-0) contains supplementary material, which is available to authorized users.

Stimulation of mTORC2 by integrin αIIbβ3 is required for PI3Kβ-dependent activation of Akt but is dispensable for platelet spreading on fibrinogen

Phosphatidylinositol 3 kinase (PI3K) is a major player in platelet activation and regulates thrombus formation and stabilization. The β isoform of PI3K is implicated in integrin αIIbβ3 outside-in signaling, is required for the phosphorylation of Akt, and controls efficient platelet spreading upon adhesion to fibrinogen. In this study we found that during integrin αIIbβ3 outside-in signaling PI3Kβ-dependent phosphorylation of Akt on Serine473 is mediated by the mammalian target of rapamycin complex 2 (mTORC2). The activity of mTORC2 is stimulated upon platelet adhesion to fibrinogen, as documented by increased autophosphorylation. However, mTORC2 activation downstream of integrin αIIbβ3 is PI3Kβ-independent. Inhibition of mTORC2, but not mTORC1, also prevents Akt phosphorylation of Threonine308 and affects Akt activity, resulting in the inhibition of GSK3α/β phosphorylation. Nevertheless, mTORC2 or Akt inhibition does not alter PI3Kβ-dependent platelet spreading on fibrinogen. The activation of the small GTPase Rap1b downstream of integrin αIIbβ3 is regulated by PI3Kβ but is not affected upon inhibition of either mTORC2 or Akt. Altogether, these results demonstrate for the first time the activation of mTORC2 and its involvement in Akt phosphorylation and stimulation during integrin αIIbβ3 outside-in signaling. Moreover, the results demonstrate that the mTORC2/Akt pathway is dispensable for PI3Kβ-regulated platelet spreading on fibrinogen.

AMPK directly activates mTORC2 to promote cell survival during acute energetic stress

AMP-activated protein kinase (AMPK) senses energetic stress and, in turn, promotes catabolic and suppresses anabolic metabolism coordinately to restore energy balance. We found that a diverse array of AMPK activators increased mTOR complex 2 (mTORC2) signaling in an AMPK-dependent manner in cultured cells. Activation of AMPK with the type 2 diabetes drug metformin (GlucoPhage) also increased mTORC2 signaling in liver in vivo and in primary hepatocytes in an AMPK-dependent manner. AMPK-mediated activation of mTORC2 did not result from AMPK-mediated suppression of mTORC1 and thus reduced negative feedback on PI3K flux. Rather, AMPK associated with and directly phosphorylated mTORC2 (mTOR in complex with rictor). As determined by two-stage in vitro kinase assay, phosphorylation of mTORC2 by recombinant AMPK was sufficient to increase mTORC2 catalytic activity toward Akt. Hence, AMPK phosphorylated mTORC2 components directly to increase mTORC2 activity and downstream signaling. Functionally, inactivation of AMPK, mTORC2, and Akt increased apoptosis during acute energetic stress. By showing that AMPK activates mTORC2 to increase cell survival, these data provide a potential mechanism for how AMPK paradoxically promotes tumorigenesis in certain contexts despite its tumor-suppressive function through inhibition of growth-promoting mTORC1. Collectively, these data unveil mTORC2 as a target of AMPK and the AMPK-mTORC2 axis as a promoter of cell survival during energetic stress.