Evidence for direct activation of mTORC2 kinase activity by phosphatidylinositol 3,4,5-trisphosphate

Phosphorylation Sites in Protein Kinases and Phosphatases Regulated by Formyl Peptide Receptor 2 Signaling

FPR1, FPR2, and FPR3 are members of Formyl Peptides Receptors (FPRs) family belonging to the GPCR superfamily. FPR2 is a low affinity receptor for formyl peptides and it is considered the most promiscuous member of this family. Intracellular signaling cascades triggered by FPRs include the activation of different protein kinases and phosphatase, as well as tyrosine kinase receptors transactivation. Protein kinases and phosphatases act coordinately and any impairment of their activation or regulation represents one of the most common causes of several human diseases. Several phospho-sites has been identified in protein kinases and phosphatases, whose role may be to expand the repertoire of molecular mechanisms of regulation or may be necessary for fine-tuning of switch properties. We previously performed a phospho-proteomic analysis in FPR2-stimulated cells that revealed, among other things, not yet identified phospho-sites on six protein kinases and one protein phosphatase. Herein, we discuss on the selective phosphorylation of Serine/Threonine-protein kinase N2, Serine/Threonine-protein kinase PRP4 homolog, Serine/Threonine-protein kinase MARK2, Serine/Threonine-protein kinase PAK4, Serine/Threonine-protein kinase 10, Dual specificity mitogen-activated protein kinase kinase 2, and Protein phosphatase 1 regulatory subunit 14A, triggered by FPR2 stimulation. We also describe the putative FPR2-dependent signaling cascades upstream to these specific phospho-sites.

mTOR at the nexus of nutrition, growth, ageing and disease

The mTOR pathway integrates a diverse set of environmental cues, such as growth factor signals and nutritional status, to direct eukaryotic cell growth. Over the past two and a half decades, mapping of the mTOR signalling landscape has revealed that mTOR controls biomass accumulation and metabolism by modulating key cellular processes, including protein synthesis and autophagy. Given the pathway's central role in maintaining cellular and physiological homeostasis, dysregulation of mTOR signalling has been implicated in metabolic disorders, neurodegeneration, cancer and ageing. In this Review, we highlight recent advances in our understanding of the complex regulation of the mTOR pathway and discuss its function in the context of physiology, human disease and pharmacological intervention.

Signaling networks in immunometabolism

Adaptive immunity is essential for pathogen and tumor eradication, but may also trigger uncontrolled or pathological inflammation. T cell receptor, co-stimulatory and cytokine signals coordinately dictate specific signaling networks that trigger the activation and functional programming of T cells. In addition, cellular metabolism promotes T cell responses and is dynamically regulated through the interplay of serine/threonine kinases, immunological cues and nutrient signaling networks. In this review, we summarize the upstream regulators and signaling effectors of key serine/threonine kinase-mediated signaling networks, including PI3K–AGC kinases, mTOR and LKB1–AMPK pathways that regulate metabolism, especially in T cells. We also provide our perspectives about the pending questions and clinical applicability of immunometabolic signaling. Understanding the regulators and effectors of immunometabolic signaling networks may uncover therapeutic targets to modulate metabolic programming and T cell responses in human disease.

Crosstalk between the mTOR and Nrf2/ARE signaling pathways as a target in the improvement of long-term potentiation

In recent years, a significant progress was made in understanding molecular mechanisms of long-term memory. Long-term memory formation requires strengthening of neuronal connections (LTP, long-term potentiation) associated with structural rearrangement of neurons. The key role in the synthesis of proteins essential for these rearrangements belong to mTOR (mammalian target of rapamycin) complexes and signaling pathways involved in mTOR regulation. Suppression of mTOR activity may impair synaptic plasticity and long-term memory, while mTOR activation inhibits autophagy, thereby potentiating amyloidosis and development of Alzheimer's disease (AD) accompanied by irreversible memory loss. Because of this, suppression/inhibition of mTOR might have unpredictable consequences on memory. The Nrf2/ARE signaling pathway affects almost all mitochondrial processes. The activation of this pathway improves memory and exhibits therapeutic effect in AD. In this review, we discuss the crosstalk between the Nrf2/ARE signaling and mTOR in the maintenance of synaptic plasticity. Nrf2 pathway can be activated by pharmacological agents and by changes in mitochondria functioning accompanying various neuronal dysfunctions.

Diiodothyronines regulate metabolic homeostasis in primary human hepatocytes by modulating mTORC1 and mTORC2 activity

Until three decades, ago 3,5-diiodothyronine (3,5-T₂) and 3,3'-diiodothyronine (3,3'-T₂) were considered products of thyroid hormone catabolism without biological activity. Some metabolic effects have been described in rodents, but the physiological relevance in humans and the mechanisms of action are unknown. Aim of this work was to investigate the role and the mechanisms of action of 3,5-T₂ and 3,3'-T₂ in the regulation of metabolic homeostasis in human liver. We used primary human hepatocytes freshly isolated from donors and grown on Matrigel as the golden standard in vitro model to study human hepatic metabolism. Results show that diiodothyronines in the range of plasma physiological concentrations reduced hepatic lipid accumulation, by modulating the activity of the mTORC1/Raptor complex through an AMPK-mediated mechanism, and stimulated the mTORC2/Rictor complex-activated pathway, leading to the down regulation of the expression of key gluconeogenic genes. Hence, we propose that diiodothyronines act as key regulators of hepatic metabolic homeostasis in humans.

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.

Direct physical interaction of active Ras with mSIN1 regulates mTORC2 signaling

Background

The mechanistic (or mammalian) target of rapamycin (mTOR), a Ser/Thr kinase, associates with different subunits forming two functionally distinct complexes, mTORC1 and mTORC2, regulating a diverse set of cellular functions in response to growth factors, cellular energy levels, and nutrients. The mechanisms regulating mTORC1 activity are well characterized; regulation of mTORC2 activity, however, remains obscure. While studies conducted in Dictyostelium suggest a possible role of Ras protein as a potential upstream regulator of mTORC2, definitive studies delineating the underlying molecular mechanisms, particularly in mammalian cells, are still lacking.

Methods

Protein levels were measured by Western blotting and kinase activity of mTORC2 was analyzed by in vitro kinase assay. In situ Proximity ligation assay (PLA) and co-immunoprecipitation assay was performed to detect protein-protein interaction. Protein localization was investigated by immunofluorescence and subcellular fractionation while cellular function of mTORC2 was assessed by assaying extent of cell migration and invasion.

Results

Here, we present experimental evidence in support of the role of Ras activation as an upstream regulatory switch governing mTORC2 signaling in mammalian cancer cells. We report that active Ras through its interaction with mSIN1 accounts for mTORC2 activation, while disruption of this interaction by genetic means or via peptide-based competitive hindrance, impedes mTORC2 signaling.

Conclusions

Our study defines the regulatory role played by Ras during mTORC2 signaling in mammalian cells and highlights the importance of Ras-mSIN1 interaction in the assembly of functionally intact mTORC2.

Targeting mTOR and Metabolism in Cancer: Lessons and Innovations

Cancer cells support their growth and proliferation by reprogramming their metabolism in order to gain access to nutrients. Despite the heterogeneity in genetic mutations that lead to tumorigenesis, a common alteration in tumors occurs in pathways that upregulate nutrient acquisition. A central signaling pathway that controls metabolic processes is the mTOR pathway. The elucidation of the regulation and functions of mTOR can be traced to the discovery of the natural compound, rapamycin. Studies using rapamycin have unraveled the role of mTOR in the control of cell growth and metabolism. By sensing the intracellular nutrient status, mTOR orchestrates metabolic reprogramming by controlling nutrient uptake and flux through various metabolic pathways. The central role of mTOR in metabolic rewiring makes it a promising target for cancer therapy. Numerous clinical trials are ongoing to evaluate the efficacy of mTOR inhibition for cancer treatment. Rapamycin analogs have been approved to treat specific types of cancer. Since rapamycin does not fully inhibit mTOR activity, new compounds have been engineered to inhibit the catalytic activity of mTOR to more potently block its functions. Despite highly promising pre-clinical studies, early clinical trial results of these second generation mTOR inhibitors revealed increased toxicity and modest antitumor activity. The plasticity of metabolic processes and seemingly enormous capacity of malignant cells to salvage nutrients through various mechanisms make cancer therapy extremely challenging. Therefore, identifying metabolic vulnerabilities in different types of tumors would present opportunities for rational therapeutic strategies. Understanding how the different sources of nutrients are metabolized not just by the growing tumor but also by other cells from the microenvironment, in particular, immune cells, will also facilitate the design of more sophisticated and effective therapeutic regimen. In this review, we discuss the functions of mTOR in cancer metabolism that have been illuminated from pre-clinical studies. We then review key findings from clinical trials that target mTOR and the lessons we have learned from both pre-clinical and clinical studies that could provide insights on innovative therapeutic strategies, including immunotherapy to target mTOR signaling and the metabolic network in cancer.

Short-term cellular memory tunes the signaling responses of the chemokine receptor CXCR4

The chemokine receptor CXCR4 regulates fundamental processes in development, normal physiology, and diseases, including cancer. Small subpopulations of CXCR4-positive cells drive the local invasion and dissemination of malignant cells during metastasis, emphasizing the need to understand the mechanisms controlling responses at the single-cell level to receptor activation by the chemokine ligand CXCL12. Using single-cell imaging, we discovered that short-term cellular memory of changes in environmental conditions tuned CXCR4 signaling to Akt and ERK, two kinases activated by this receptor. Conditioning cells with growth stimuli before CXCL12 exposure increased the number of cells that initiated CXCR4 signaling and the amplitude of Akt and ERK activation. Data-driven, single-cell computational modeling revealed that growth factor conditioning modulated CXCR4-dependent activation of Akt and ERK by decreasing extrinsic noise (preexisting cell-to-cell differences in kinase activity) in PI3K and mTORC1. Modeling established mTORC1 as critical for tuning single-cell responses to CXCL12-CXCR4 signaling. Our single-cell model predicted how combinations of extrinsic noise in PI3K, Ras, and mTORC1 superimposed on different driver mutations in the ERK and/or Akt pathways to bias CXCR4 signaling. Computational experiments correctly predicted that selected kinase inhibitors used for cancer therapy shifted subsets of cells to states that were more permissive to CXCR4 activation, suggesting that such drugs may inadvertently potentiate pro-metastatic CXCR4 signaling. Our work establishes how changing environmental inputs modulate CXCR4 signaling in single cells and provides a framework to optimize the development and use of drugs targeting this signaling pathway.

Adrenoceptor regulation of the mechanistic target of rapamycin in muscle and adipose tissue

A vital role of adrenoceptors in metabolism and energy balance has been well documented in the heart, skeletal muscle, and adipose tissue. It has been only recently demonstrated, however, that activation of the mechanistic target of rapamycin (mTOR) makes a significant contribution to various metabolic and physiological responses to adrenoceptor agonists. mTOR exists as two distinct complexes named mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) and has been shown to play a critical role in protein synthesis, cell proliferation, hypertrophy, mitochondrial function, and glucose uptake. This review will describe the physiological significance of mTORC1 and 2 as a novel paradigm of adrenoceptor signalling in the heart, skeletal muscle, and adipose tissue. Understanding the detailed signalling cascades of adrenoceptors and how they regulate physiological responses is important for identifying new therapeutic targets and identifying novel therapeutic interventions. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

The "other" mTOR complex: New insights into mTORC2 immunobiology and their implications

A central role of the mechanistic target of rapamycin (mTOR) in regulation of fundamental cell processes is well recognized. mTOR functions in two distinct complexes: rapamycin-sensitive mTOR complex (C) 1 and rapamycin-insensitive mTORC2. While the role of mTORC1 in shaping immune responses, including transplant rejection, and the influence of its antagonism in promoting allograft tolerance have been studied extensively using rapamycin, lack of selective small molecule inhibitors has limited understanding of mTORC2 biology. Within the past few years, however, intracellular localization of mTORC2, its contribution to mitochondrial fitness, cell metabolism, cytoskeletal modeling and cell migration, and its role in differentiation and function of immune cells have been described. Studies in mTORC2 knockdown/knockout mouse models and a new class of dual mTORC1/2 inhibitors, have shed light on the immune regulatory functions of mTORC2. These include regulation of antigen-presenting cell, NK cell, T cell subset, and B cell differentiation and function. mTORC2 has been implicated in regulation of ischemia/reperfusion injury and graft rejection. Potential therapeutic benefits of antagonizing mTORC2 to inhibit chronic rejection have also been described, while selective in vivo targeting strategies using nanotechnology have been developed. We briefly review and discuss these developments and their implications.

Inhibition of the IGF-1-PI3K-Akt-mTORC2 pathway in lipid rafts increases neuronal vulnerability in a genetic lysosomal glycosphingolipidosis

Glycosphingolipid (GSL) accumulation is implicated in the neuropathology of several lysosomal conditions, such as Krabbe disease, and may also contribute to neuronal and glial dysfunction in adult-onset conditions such as Parkinson's disease, Alzheimer's disease and multiple sclerosis. GSLs accumulate in cellular membranes and disrupt their structure; however, how membrane disruption leads to cellular dysfunction remains unknown. Using authentic cellular and animal models for Krabbe disease, we provide a mechanism explaining the inactivation of lipid raft (LR)-associated IGF-1-PI3K-Akt-mTORC2, a pathway of crucial importance for neuronal function and survival. We show that psychosine, the GSL that accumulates in Krabbe disease, leads to a dose-dependent LR-mediated inhibition of this pathway by uncoupling IGF-1 receptor phosphorylation from downstream Akt activation. This occurs by interfering with the recruitment of PI3K and mTORC2 to LRs. Akt inhibition can be reversed by sustained IGF-1 stimulation, but only during a time window before psychosine accumulation reaches a threshold level. Our study shows a previously unknown connection between LR-dependent regulation of mTORC2 activity at the cell surface and a genetic neurodegenerative disease. Our results show that LR disruption by psychosine desensitizes cells to extracellular growth factors by inhibiting signal transmission from the plasma membrane to intracellular compartments. This mechanism serves also as a mechanistic model to understand how alterations of the membrane architecture by the progressive accumulation of lipids undermines cell function, with potential implications in other genetic sphingolipidoses and adult neurodegenerative conditions. This article has an associated First Person interview with the first author of the paper.

Metabolism in Immune Cell Differentiation and Function

The immune system is a central determinant of organismal health. Functional immune responses require quiescent immune cells to rapidly grow, proliferate, and acquire effector functions when they sense infectious agents or other insults. Specialized metabolic programs are critical regulators of immune responses, and alterations in immune metabolism can cause immunological disorders. There has thus been growing interest in understanding how metabolic processes control immune cell functions under normal and pathophysiological conditions. In this chapter, we summarize how metabolic programs are tuned and what the physiological consequences of metabolic reprogramming are as they relate to immune cell homeostasis, differentiation, and function.

The AKT kinase signaling network is rewired by PTEN to control proximal BCR signaling in germinal center B cells

Compared to naïve B cells (NBCs), both B cell antigen receptor (BCR) and CD40 signaling are rewired in germinal center (GC) B cells (GCBCs) to optimize selection for high-affinity B cells. The mechanism for BCR reprogramming in GCBCs remains unknown. We describe a GC-specific, AKT kinase-driven negative feedback loop that attenuates BCR signaling. A mass spectrometry proteomic approach revealed that AKT activity was retargeted in GCBCs compared to NBCs. Retargeting was linked to differential AKT T308 and S473 phosphorylation, in turn due to GC-specific upregulation of phosphoinositide-dependent protein kinase PDK1 and the phosphatase PTEN, which retuned phosphatidylinositol-3-OH kinase (PI3K) signals. In GCBCs, AKT preferentially targeted CSK, SHP-1 and HPK1, which are negative regulators of BCR signaling. Phosphorylation results in markedly increased enzymatic activity of these proteins, creating a negative-feedback loop that dampens upstream BCR signaling. Inhibiting AKT substantially enhanced activation of BCR proximal kinase LYN as well as downstream BCR signaling molecules in GCBCs, establishing the relevance of this pathway.

Phosphorylation at distinct subcellular locations underlies specificity in mTORC2-mediated activation of SGK1 and Akt

mTORC2 lies at the intersection of signaling pathways that control metabolism and ion transport through phosphorylation of the AGC-family kinases, the Akt and SGK1 proteins. How mTORC2 targets these functionally distinct downstream effectors in a context-specific manner is not known. Here, we show that the salt- and blood pressure-regulatory hormone, angiotensin II (AngII) stimulates selective mTORC2-dependent phosphorylation of SGK1 (S422) but not Akt (S473 and equivalent sites). Conventional PKC (cPKC), a critical mediator of the angiotensin type I receptor (AT₁R, also known as AGTR1) signaling, regulates the subcellular localization of SIN1 (also known as MAPKAP1) and SGK1. Inhibition of cPKC catalytic activity disturbs SIN1 and SGK1 subcellular localization, re-localizing them from the nucleus and a perinuclear compartment to the plasma membrane in advance of hormonal stimulation. Surprisingly, pre-targeting of SIN1 and SGK1 to the plasma membrane prevents SGK1 S422 but not Akt S473 phosphorylation. Additionally, we identify three sites on SIN1 (S128, S315 and S356) that are phosphorylated in response to cPKC activation. Collectively, these data demonstrate that SGK1 activation occurs at a distinct subcellular compartment from that of Akt and suggests a mechanism for the selective activation of these functionally distinct mTORC2 targets through subcellular partitioning of mTORC2 activity.

Amino Acid Metabolism in Dairy Cows and their Regulation in Milk Synthesis

Background:

Reducing dietary Crude Protein (CP) and supplementing with certain Amino Acids (AAs) has been known as a potential solution to improve Nitrogen (N) efficiency in dairy production. Thus understanding how AAs are utilized in various sites along the gut is critical.

Objective:

AA flow from the intestine to Portal-drained Viscera (PDV) and liver then to the mammary gland was elaborated in this article. Recoveries in individual AA in PDV and liver seem to share similar AA pattern with input: output ratio in mammary gland, which subdivides essential AA (EAA) into two groups, Lysine (Lys) and Branchedchain AA (BCAA) in group 1, input: output ratio > 1; Methionine (Met), Histidine (His), Phenylalanine (Phe) etc. in group 2, input: output ratio close to 1. AAs in the mammary gland are either utilized for milk protein synthesis or retained as body tissue, or catabolized. The fractional removal of AAs and the number and activity of AA transporters together contribute to the ability of AAs going through mammary cells. Mammalian Target of Rapamycin (mTOR) pathway is closely related to milk protein synthesis and provides alternatives for AA regulation of milk protein synthesis, which connects AA with lactose synthesis via α-lactalbumin (gene: LALBA) and links with milk fat synthesis via Sterol Regulatory Element-binding Transcription Protein 1 (SREBP1) and Peroxisome Proliferatoractivated Receptor (PPAR).

Conclusion:

Overall, AA flow across various tissues reveals AA metabolism and utilization in dairy cows on one hand. While the function of AA in the biosynthesis of milk protein, fat and lactose at both transcriptional and posttranscriptional level from another angle provides the possibility for us to regulate them for higher efficiency.

T cells transduce T-cell receptor signal strength by generating different phosphatidylinositols

T-cell receptor (TCR) signaling strength is a dominant factor regulating T-cell differentiation, thymic development, and cytokine signaling. The molecular mechanisms by which TCR signal strength is transduced to downstream signaling networks remains ill-defined. Using computational modeling, biochemical assays, and imaging flow cytometry, we found here that TCR signal strength differentially generates phosphatidylinositol species. Weak TCR signals generated elevated phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and reduced phosphatidylinositol (3,4,5)-trisphosphate (PIP3) levels, whereas strong TCR signals reduced PI(4,5)P2 and elevated PIP3 levels. A proteomics screen revealed that focal adhesion kinase bound PI(4,5)P2, biochemical assays disclosed that focal adhesion kinase is preferentially activated by weak TCR signals and is required for optimal Treg induction, and further biochemical experiments revealed how TCR signaling strength regulates AKT activation. Low PIP3 levels generated by weak TCR signals were sufficient to activate phosphoinositide-dependent kinase-1 to phosphorylate AKT on Thr-308 but insufficient to activate mTOR complex 2 (mTORC2), whereas elevated PIP3 levels generated by a strong TCR signal were required to activate mTORC2 to phosphorylate Ser-473 on AKT. Our results provide support for a model that links TCR signaling to mTORC2 activation via phosphoinositide 3-kinase signaling. Together, the findings in this work establish that T cells measure TCR signal strength by generating different levels of phosphatidylinositol species that engage alternate signaling networks to control cell fate decisions.

Identification of MΦ specific POTEE expression: Its role in mTORC2 activation via protein-protein interaction in TAMs

POTE is known as cancer antigen, expressed in many cancers, along with very few normal tissues like prostate, ovary, testes and embryo. Till date, POTEE identified as majorly expressed POTE paralog. Functionally, POTEF regulates TLR signaling which play important role in innate immunity provided clue about expression of POTE in immune cells. We have chosen three Thp1monocytes, Jurkat T1 and MΦ cells as a model. Here, first time we report expression of POTEE in immune cells specifically only in MΦ but not in monocytes or T-cells. In addition, expression level remains unaltered in MΦ subtypes M1 and M2 and MΦ subjected to various stresses, except MΦs treated with Hyp-CM where MΦs acquires properties of TAMs. In TAMs, POTEE was involved differential protein-protein interaction with mTOR, RICTOR, and Rad51 indicating its biological role in cell invasion through mTORC2 activation. siRNA mediated knockdown of POTEE suggests its importance in cell survival of MΦs as well as TAMs.

Immunomodulatory role of branched-chain amino acids

Branched-chain amino acids (BCAAs) have been associated with immunomodulation since the mid-1970s and 1980s and have been used in the nutritional therapy of critically ill patients. Evidence shows that BCAAs can directly contribute to immune cell function, aiding recovery of an impaired immune system, as well as improving the nutritional status in cancer and liver diseases. Branched-chain amino acids may also play a role in treatment of patients with sepsis or trauma, contributing to improved clinical outcomes and survival. Branched-chain amino acids, especially leucine, are activators of the mammalian target of rapamycin (mTOR), which, in turn, interacts with several signaling pathways involved in biological mechanisms of insulin action, protein synthesis, mitochondrial biogenesis, inflammation, and lipid metabolism. Although many in vitro and human and animal model studies have provided evidence for the biological activity of BCAAs, findings have been conflicting, and the mechanisms of action of these amino acids are still poorly understood. This review addresses several aspects related to BCAAs, including their transport, oxidation, and mechanisms of action, as well as their role in nutritional therapy and immunomodulation.

HIV-1 Nef-POTEE; A novel interaction modulates macrophage dissemination via mTORC2 signaling pathway

AIMS

Human immunodeficiency virus -1 [HIV-1] Nef, localizes in different cellular compartments and modulates several cellular pathways. Nef promotes virus pathogenicity through alteration in cell surface receptor expression, apoptosis, protein trafficking etc. Nef regulates viral pathogenesis through interaction with different host proteins. Thus, molecular mechanisms of pathogenesis could be deciphered by identifying novel Nef interacting proteins.

MAIN METHODS

HIV-1 Nef interacting proteins were identified by pull down assay and MALDI-TOF analysis. The interaction was further validated through mammalian two hybrid assay. Functional role of this interaction was identified by immunoprecipitation assay, cell invasion and cell migration studies. Fold Change in mRNA levels of CD163, CD206, CCL17 and CCL18 was analyzed using qPCR.

KEY FINDINGS

In current study, C. elegans protein ACT4C and its human homolog POTEE was identified to be interacting with Nef. This interaction activates mTORC2 complex, which in-turn activates AKT and PKC-α. The activation of mTORC2 complex was found to be initiated by the interaction of Nef, mTORC2, Rictor to POTEE. The cellular phenotype and functions affected by Nef-POTEE interaction resulted in significant increase in cell invasion and migration of macrophages (MΦ).

SIGNIFICANCE

MΦ is primary target of HIV-1 infection where HIV-1 replicates and polarizes immunosuppressive M2 phenotype. Combine effect of M2 phenotype and Viral-host protein interactions compromise the MΦ associated physiological functions. Infected MΦ dissemination into other system also leads to HIV-1 induced malignancies. Therefore, targeting POTEE-Nef interaction can lead to formulating better therapeutic strategy against HIV-1.

Vasculogenesis and angiogenesis initiation under normoxic conditions through Wnt/β-catenin pathway in gliomas

The canonical Wnt/β-catenin pathway is up-regulated in gliomas and involved in proliferation, invasion, apoptosis, vasculogenesis and angiogenesis. Nuclear β-catenin accumulation correlates with malignancy. Hypoxia activates hypoxia-inducible factor (HIF)-1α by inhibiting HIF-1α prolyl hydroxylation, which promotes glycolytic energy metabolism, vasculogenesis and angiogenesis, whereas HIF-1α is degraded by the HIF prolyl hydroxylase under normoxic conditions. We focus this review on the links between the activated Wnt/β-catenin pathway and the mechanisms underlying vasculogenesis and angiogenesis through HIF-1α under normoxic conditions in gliomas. Wnt-induced epidermal growth factor receptor/phosphatidylinositol 3-kinase (PI3K)/Akt signaling, Wnt-induced signal transducers and activators of transcription 3 (STAT3) signaling, and Wnt/β-catenin target gene transduction (c-Myc) can activate HIF-1α in a hypoxia-independent manner. The PI3K/Akt/mammalian target of rapamycin pathway activates HIF-1α through eukaryotic translation initiation factor 4E-binding protein 1 and STAT3. The β-catenin/T-cell factor 4 complex directly binds to STAT3 and activates HIF-1α, which up-regulates the Wnt/β-catenin target genes cyclin D1 and c-Myc in a positive feedback loop. Phosphorylated STAT3 by interleukin-6 or leukemia inhibitory factor activates HIF-1α even under normoxic conditions. The activation of the Wnt/β-catenin pathway induces, via the Wnt target genes c-Myc and cyclin D1 or via HIF-1α, gene transactivation encoding aerobic glycolysis enzymes, such as glucose transporter, hexokinase 2, pyruvate kinase M2, pyruvate dehydrogenase kinase 1 and lactate dehydrogenase-A, leading to lactate production, as the primary alternative of ATP, at all oxygen levels, even in normoxic conditions. Lactate released by glioma cells via the monocarboxylate lactate transporter-1 up-regulated by HIF-1α and lactate anion activates HIF-1α in normoxic endothelial cells by inhibiting HIF-1α prolyl hydroxylation and preventing HIF labeling by the von Hippel-Lindau protein. Increased lactate with acid environment and HIF-1α overexpression induce the vascular endothelial growth factor (VEGF) pathway of vasculogenesis and angiogenesis under normoxic conditions. Hypoxia and acidic pH have no synergistic effect on VEGF transcription.

The Role of PI3K in Met Driven Cancer: A Recap

The Receptor Tyrosine Kinase (RTK) Met, overexpressed or mutated in cancer, plays a major role in cancer progression and represents an attractive target for cancer therapy. However RTK inhibitors can lead to drug resistance, explaining the necessity to develop therapies that target downstream signaling. Phosphatidylinositide 3-kinase (PI3K) is one of the most deregulated pathways in cancer and implicated in various types of cancer. PI3K signaling is also a major signaling pathway downstream of RTK, including Met. PI3K major effectors include Akt and "mechanistic Target of Rapamycin" (mTOR), which each play key roles in numerous and various cell functions. Advancements made due to the development of molecular and pharmaceutical tools now allow us to delve into the roles of each independently. In this review, we summarize the current understanding we possess of the activation and role of PI3K/Akt/mTOR, downstream of Met, in cancer.

Reduced menin expression impairs rapamycin effects as evidenced by an increase in mTORC2 signaling and cell migration

Background

Mammalian target of rapamycin (mTOR) is a master regulator of various cellular responses by forming two functional complexes, mTORC1 and mTORC2. mTOR signaling is frequently dysregulated in pancreatic neuroendocrine tumors (PNETs). mTOR inhibitors have been used in attempts to treat these lesions, and prolonged progression free survival has been recorded. If this holds true also for the multiple endocrine neoplasia type 1 (MEN1) associated PNETs is yet unclear. We investigated the relationship between expression of the MEN1 protein menin and mTOR signaling in the presence or absence of the mTOR inhibitor rapamycin.

Methods

In addition to use of menin wild type and menin-null mouse embryonic fibroblasts (MEFs), menin was silenced by siRNA in pancreatic neuroendocrine tumor cell line BON-1. Panels of protein phosphorylation, as activation markers downstream of PI3k-mTOR-Akt pathways, as well as menin expression were evaluated by immunoblotting. The impact of menin expression in the presence and absence of rapamycin was determinate upon Wound healing, migration and proliferation in MEFs and BON1 cells.

Results

PDGF-BB markedly increased phosphorylation of mTORC2 substrate Akt, at serine 473 (S473) and threonine 450 (T450) in menin^−/− MEFs but did not alter phosphorylation of mTORC1 substrates ribosomal protein S6 or eIF4B. Acute rapamycin treatment by mTORC1-S6 inhibition caused a greater enhancement of Akt phosphorylation on S473 in menin^−/− cells as compared to menin^+/+ MEFs (116% vs 38%). Chronic rapamycin treatment, which inhibits both mTORC1and 2, reduced Akt phosphorylation of S473 to a lesser extent in menin^−/− MEFs than menin^+/+ MEFs (25% vs 75%). Silencing of menin expression in human PNET cell line (BON1) also enhanced Akt phosphorylation at S473, but not activation of mTORC1. Interestingly, silencing menin in BON1 cells elevated S473 phosphorylation of Akt in both acute and chronic treatments with rapamycin. Finally, we show that the inhibitory effect of rapamycin on serum mediated wound healing and cell migration is impaired in menin^−/− MEFs, as well as in menin-silenced BON1 cells.

Conclusions

Menin is involved in regulatory mechanism between the two mTOR complexes, and its reduced expression is accompanied with increased mTORC2-Akt signaling, which consequently impairs anti-migratory effect of rapamycin.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0278-2) contains supplementary material, which is available to authorized users.

Knockdown of Yin Yang 1 enhances anticancer effects of cisplatin through protein phosphatase 2A-mediated T308 dephosphorylation of AKT

Cisplatin is still one of the first-line drugs for chemotherapy of head and neck squamous cell carcinoma (HNSCC) and shows a survival advantage for HNSCC. However, a substantial proportion of HNSCC eventually becomes resistance to cisplatin and the underlying mechanisms remain to be fully understood. Yin Yang 1 (YY1) is a multifunctional protein regulating both gene transcription and protein modifications and also plays a role in chemotherapy resistance. Here, we reported that knockdown of YY1 by lentivirus-mediated short hairpin RNA or tetracycline-inducible short hairpin RNA enhanced cisplatin-induced apoptosis and inhibition of cell proliferation, migration and invasion in the HNSCC cell lines, and inhibition of the xenograft tumor growth. The underlying mechanisms were revealed that knockdown of YY1 downregulated both S473 and T308 phosphorylation of AKT (protein kinase B), which was mainly responsible for cisplatin resistance, whereas overexpression of YY1 upregulated both S473 and T308 phosphorylation. Cisplatin upregulated YY1 mRNA and protein expression and both S473 and T308 phosphorylation of AKT. In the presence of cisplatin, knockdown of YY1 not only blocked cisplatin-induced increase in S473 and T308 phosphorylation of AKT, but still downregulated T308 phosphorylation. Moreover, protein phosphatase 2A (PP2A) antagonist, okadaic acid, upregulated T308, but not S473, phosphorylation, and simultaneously abolished YY1 knockdown-mediated enhancement of cisplatin-induced inhibition of cell proliferation. In addition, knockdown of YY1 promoted PP2A activity through upregulating mRNA and protein expressions of PP2A catalytic subunit alpha (PPP2CA) through the binding of YY1 in the promoter of PPP2CA. Conversely, activating PP2A by forskolin also promoted YY1 degradation and subsequently inhibited T308 phosphorylation. These results suggested that knockdown of YY1 enhanced anticancer effects of cisplatin through PP2A mediating T308 dephosphorylation of AKT, and that targeting YY1 or PP2A would enhance the efficiency of cisplatin chemotherapy in treatment of HNSCC.

mTORC2 Activity Disrupts Lysosome Acidification in Systemic Lupus Erythematosus by Impairing Caspase-1 Cleavage of Rab39a

Lysosomes maintain immune homeostasis through the degradation of phagocytosed apoptotic debris; however, the signaling events regulating lysosomal maturation remain undefined. In this study, we show that lysosome acidification, key to the maturation process, relies on mTOR complex 2 (mTORC2), activation of caspase-1, and cleavage of Rab39a. Mechanistically, the localization of cofilin to the phagosome recruits caspase-11, which results in the localized activation of caspase-1. Caspase-1 subsequently cleaves Rab39a on the phagosomal membrane, promoting lysosome acidification. Although caspase-1 is critical for lysosome acidification, its activation is independent of inflammasomes and cell death mediated by apoptosis-associated speck-like protein containing a caspase recruitment domain, revealing a role beyond pyroptosis. In lupus-prone murine macrophages, chronic mTORC2 activity decouples the signaling pathway, leaving Rab39a intact. As a result, the lysosome does not acidify, and degradation is impaired, thereby heightening the burden of immune complexes that activate FcγRI and sustain mTORC2 activity. This feedforward loop promotes chronic immune activation, leading to multiple lupus-associated pathologies. In summary, these findings identify the key molecules in a previously unappreciated signaling pathway that promote lysosome acidification. It also shows that this pathway is disrupted in systemic lupus erythematosus.

Differential T-cell receptor signals for T helper cell programming

Upon encounter with their cognate antigen, naive CD4 T cells become activated and are induced to differentiate into several possible T helper (Th) cell subsets. This differentiation depends on a number of factors including antigen-presenting cells, cytokines and co-stimulatory molecules. The strength of the T-cell receptor (TCR) signal, related to the affinity of TCR for antigen and antigen dose, has emerged as a dominant factor in determining Th cell fate. Recent studies have revealed that TCR signals of high or low strength do not simply induce quantitatively different signals in the T cells, but rather qualitatively distinct pathways can be induced based on TCR signal strength. This review examines the recent literature in this area and highlights important new developments in our understanding of Th cell differentiation and TCR signal strength.

Who does TORC2 talk to?

The target of rapamycin (TOR) is a protein kinase that, by forming complexes with partner proteins, governs diverse cellular signalling networks to regulate a wide range of processes. TOR thus plays central roles in maintaining normal cellular functions and, when dysregulated, in diverse diseases. TOR forms two distinct types of multiprotein complexes (TOR complexes 1 and 2, TORC1 and TORC2). TORC1 and TORC2 differ in their composition, their control and their substrates, so that they play quite distinct roles in cellular physiology. Much effort has been focused on deciphering the detailed regulatory links within the TOR pathways and the structure and control of TOR complexes. In this review, we summarize recent advances in understanding mammalian (m) TORC2, its structure, its regulation, and its substrates, which link TORC2 signalling to the control of cell functions. It is now clear that TORC2 regulates several aspects of cell metabolism, including lipogenesis and glucose transport. It also regulates gene transcription, the cytoskeleton, and the activity of a subset of other protein kinases.

M2 polarization of macrophages by Oncostatin M in hypoxic tumor microenvironment is mediated by mTORC2 and promotes tumor growth and metastasis

Oncostatin M (OSM), an inflammatory cytokine belonging to the interleukin-6 (IL-6) superfamily, plays a vital role in multitude of physiological and pathological processes. Its role in breast tumor progression and metastasis to distant organs is well documented. Recent reports implicate OSM in macrophage M2 polarization, a key pro-tumoral phenomenon. M2 polarization of macrophages is believed to promote tumor progression by potentiating metastasis and angiogenesis. In the current study, we delineated the mechanism underlying OSM induced macrophage M2 polarization. The findings revealed that OSM skews macrophages towards an M2 polarized phenotype via mTOR signaling complex 2 (mTORC2). mTORC2 relays signals through two effector kinases i.e. PKC-α and Akt. Our results indicated that mTORC2 mediated M2 polarization of macrophages is not dependent on PKC-α and is primarily affected via Akt, particularly Akt1. In vivo studies conducted on 4T1/BALB/c mouse orthotropic model of breast cancer further corroborated these observations wherein i.v. reintroduction of mTORC2 abrogated monocytes into orthotropic mouse model resulted in diminished acquisition of M2 specific attributes by tumor associated macrophages. Metastasis to distant organs like lung, liver and bone was reduced as evident by decrease in formation of focal metastatic lesions in mTORC2 abrogated monocytes mice. Our study pinpoints key role of mTORC2-Akt1 axis in OSM induced macrophage polarization and suggests for possible usage of Oncostatin-M blockade and/or selective mTORC2 inhibition as a potential anti-cancer strategy particularly with reference to metastasis of breast cancer to distant organs such as lung, liver and bone.

Mechanism of activation of SGK3 by growth factors via the Class 1 and Class 3 PI3Ks

Derailment of the PI3K-AGC protein kinase signalling network contributes to many human diseases including cancer. Recent work has revealed that the poorly studied AGC kinase family member, SGK3, promotes resistance to cancer therapies that target the Class 1 PI3K pathway, by substituting for loss of Akt kinase activity. SGK3 is recruited and activated at endosomes, by virtue of its phox homology domain binding to PtdIns(3)P. Here, we demonstrate that endogenous SGK3 is rapidly activated by growth factors such as IGF1, through pathways involving both Class 1 and Class 3 PI3Ks. We provide evidence that IGF1 enhances endosomal PtdIns(3)P levels via a pathway involving the UV-RAG complex of hVPS34 Class 3 PI3K. Our data point towards IGF1-induced activation of Class 1 PI3K stimulating SGK3 through enhanced production of PtdIns(3)P resulting from the dephosphorylation of PtdIns(3,4,5)P₃ Our findings are also consistent with activation of Class 1 PI3K promoting mTORC2 phosphorylation of SGK3 and with oncogenic Ras-activating SGK3 solely through the Class 1 PI3K pathway. Our results highlight the versatility of upstream pathways that activate SGK3 and help explain how SGK3 substitutes for Akt following inhibition of Class 1 PI3K/Akt pathways. They also illustrate robustness of SGK3 activity that can remain active and counteract physiological conditions or stresses where either Class 1 or Class 3 PI3K pathways are inhibited.

Regulatory T Cell Migration Is Dependent on Glucokinase-Mediated Glycolysis

Migration of activated regulatory T (Treg) cells to inflamed tissue is crucial for their immune-modulatory function. While metabolic reprogramming during Treg cell differentiation has been extensively studied, the bioenergetics of Treg cell trafficking remains undefined. We have investigated the metabolic demands of migrating Treg cells in vitro and in vivo. We show that glycolysis was instrumental for their migration and was initiated by pro-migratory stimuli via a PI3K-mTORC2-mediated pathway culminating in induction of the enzyme glucokinase (GCK). Subsequently, GCK promoted cytoskeletal rearrangements by associating with actin. Treg cells lacking this pathway were functionally suppressive but failed to migrate to skin allografts and inhibit rejection. Similarly, human carriers of a loss-of-function GCK regulatory protein gene-leading to increased GCK activity-had reduced numbers of circulating Treg cells. These cells displayed enhanced migratory activity but similar suppressive function, while conventional T cells were unaffected. Thus, GCK-dependent glycolysis regulates Treg cell migration.

Influenza virus differentially activates mTORC1 and mTORC2 signaling to maximize late stage replication

Influenza A virus usurps host signaling factors to regulate its replication. One example is mTOR, a cellular regulator of protein synthesis, growth and motility. While the role of mTORC1 in viral infection has been studied, the mechanisms that induce mTORC1 activation and the substrates regulated by mTORC1 during influenza virus infection have not been established. In addition, the role of mTORC2 during influenza virus infection remains unknown. Here we show that mTORC2 and PDPK1 differentially phosphorylate AKT upon influenza virus infection. PDPK1-mediated phoshorylation of AKT at a distinct site is required for mTORC1 activation by influenza virus. On the other hand, the viral NS1 protein promotes phosphorylation of AKT at a different site via mTORC2, which is an activity dispensable for mTORC1 stimulation but known to regulate apoptosis. Influenza virus HA protein and down-regulation of the mTORC1 inhibitor REDD1 by the virus M2 protein promote mTORC1 activity. Systematic phosphoproteomics analysis performed in cells lacking the mTORC2 component Rictor in the absence or presence of Torin, an inhibitor of both mTORC1 and mTORC2, revealed mTORC1-dependent substrates regulated during infection. Members of pathways that regulate mTORC1 or are regulated by mTORC1 were identified, including constituents of the translation machinery that once activated can promote translation. mTORC1 activation supports viral protein expression and replication. As mTORC1 activation is optimal midway through the virus life cycle, the observed effects on viral protein expression likely support the late stages of influenza virus replication when infected cells undergo significant stress.

Drug-resistant influenza viruses commonly arise due to frequent genetic changes and current antiviral drugs are not highly efficient. These underscore the need for new antiviral therapies effective against influenza viruses. Understanding how influenza virus uses cellular proteins for infection can potentially identify novel targets for pharmacological intervention. Influenza virus modulates cellular pathways to promote its replication and avoid immune restriction. Here we reveal the interplay between the cellular protein mTOR, which functions in two distinct protein complexes, and influenza virus infection. mTOR complex 1 (mTORC1) is activated during influenza virus infection through a cascade of specific modifications, or phosphorylation events, and by reducing the levels of another cellular protein termed REDD1, which is an mTORC1 inhibitor. Activation of mTORC1 results in additional phosphorylation events that together promote viral protein expression and replication. On the other hand, mTOR complex 2 (mTORC2) phosphorylates AKT at a specific site during infection, which is a process mediated by the viral NS1 protein that is known to regulate viral-mediated cell death. Since these effects occur midway through the virus life cycle in the infected cell, mTORC1 and mTORC2 activation are likely important to regulate the cellular environment in order to facilitate the late stages of viral infection.

PI3K-mTOR-S6K Signaling Mediates Neuronal Viability via Collapsin Response Mediator Protein-2 Expression

Collapsin response mediator protein (CRMP)-2 and the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway are associated with common physiological functions such as neuronal polarity, axonal outgrowth and synaptic strength, as well as various brain disorders including epilepsy. But, their regulatory and functional links are unclear. Alterations in CRMP-2 expression that lead to its functional changes are implicated in brain disorders such as epilepsy. Here, we investigate whether changes in CRMP-2 expression, possibly regulated by mTOR-related signaling, correlates with neuronal growth and viability. Inhibition of mTOR and/or phosphoinositol-3-kinase (PI3K) led to deceased p-S6K, and p-S6 signals also reduced CRMP-2 expression. These changes corresponded to inhibition of neuronal viability and proliferation in cultured hippocampal HT-22 cells under both basal serum-free and serum- or insulin-induced mTOR pathway-activated conditions. CRMP-2 expression tended to be increased by mTOR activation, indicated by an increase in p-S6/S6 level, in pentylentetrazole (PTZ)-induced epileptic rat hippocampal tissues was also significantly reduced by mTOR inhibition. Knockdown of CRMP-2 by si-RNA reduced the neuronal viability without changes in mTOR signaling, and overexpression of CRMP-2 recovered the glutamate-induced neurotoxicity and decrease of mTOR signaling in HT-22 cells. In conclusion, CRMP-2 protein expression controlled by the PI3K-mTOR-S6K signaling axis exerts its important functional roles in neuronal growth and survival.

Molecular signaling cascades involved in nonmelanoma skin carcinogenesis

Nonmelanoma skin cancer (NMSC) is the most common cancer worldwide and the incidence continues to rise, in part due to increasing numbers in high-risk groups such as organ transplant recipients and those taking photosensitizing medications. The most significant risk factor for NMSC is ultraviolet radiation (UVR) from sunlight, specifically UVB, which is the leading cause of DNA damage, photoaging, and malignant transformation in the skin. Activation of apoptosis following UVR exposure allows the elimination of irreversibly damaged cells that may harbor oncogenic mutations. However, UVR also activates signaling cascades that promote the survival of these potentially cancerous cells, resulting in tumor initiation. Thus, the UVR-induced stress response in the skin is multifaceted and requires coordinated activation of numerous pathways controlling DNA damage repair, inflammation, and kinase-mediated signal transduction that lead to either cell survival or cell death. This review focuses on the central signaling mechanisms that respond to UVR and the subsequent cellular changes. Given the prevalence of NMSC and the resulting health care burden, many of these pathways provide promising targets for continued study aimed at both chemoprevention and chemotherapy.

TCR Signal Strength Regulates Akt Substrate Specificity To Induce Alternate Murine Th and T Regulatory Cell Differentiation Programs

The Akt/mTOR pathway is a key driver of murine CD4⁺ T cell differentiation, and induction of regulatory T (Treg) cells results from low TCR signal strength and low Akt/mTOR signaling. However, strong TCR signals induce high Akt activity that promotes Th cell induction. Yet, it is unclear how Akt controls alternate T cell fate decisions. We find that the strength of the TCR signal results in differential Akt enzymatic activity. Surprisingly, the Akt substrate networks associated with T cell fate decisions are qualitatively different. Proteomic profiling of Akt signaling networks during Treg versus Th induction demonstrates that Akt differentially regulates RNA processing and splicing factors to drive T cell differentiation. Interestingly, heterogeneous nuclear ribonucleoprotein (hnRNP) L or hnRNP A1 are Akt substrates during Treg induction and have known roles in regulating the stability and splicing of key mRNAs that code for proteins in the canonical TCR signaling pathway, including CD3ζ and CD45. Functionally, inhibition of Akt enzymatic activity results in the dysregulation of splicing during T cell differentiation, and knockdown of hnRNP L or hnRNP A1 results in the lower induction of Treg cells. Together, this work suggests that a switch in substrate specificity coupled to the phosphorylation status of Akt may lead to alternative cell fates and demonstrates that proteins involved with alternative splicing are important factors in T cell fate decisions.

PI3K signaling in cancer: beyond AKT

The phosphoinositide 3-kinase (PI3K) signaling pathway is one of the most frequently altered pathways in human cancer and has a critical role in driving tumor initiation and progression. Although PI3K and its lipid product phosphatidylinositol-3,4,5-trisphosphate (PIP₃) have been shown to activate multiple downstream signaling proteins, the vast majority of studies have focused on the protein kinase AKT as the dominant effector of PI3K signaling. However, recent studies have demonstrated many contexts under which other PIP₃-dependent signaling proteins critically contribute to cancer progression, illustrating the importance of understanding AKT-independent signaling downstream of PI3K. Here, we highlight three PI3K-dependent, but AKT-independent, signaling branches that have recently been shown to have important roles in promoting phenotypes associated with malignancy. First, the PDK1-mTORC2-SGK axis can substitute for AKT in survival, migration, and growth signaling and has emerged as a major mechanism of resistance to PI3K and AKT inhibitors. Second, Rac signaling mediates the reorganization of the actin cytoskeleton to regulate cancer cell migration, invasion, and metabolism. Finally, the TEC family kinase BTK has a critical role in B cell function and malignancy and represents a recent example of an effective therapeutic target in cancer. These mechanisms highlight how understanding PI3K-dependent, but AKT-independent, signaling mechanisms that drive cancer progression will be crucial for the development of novel and more effective approaches for targeting the PI3K pathway for therapeutic benefit in cancer.

Unraveling the regulation of mTORC2 using logical modeling

Background

The mammalian target of rapamycin (mTOR) is a regulator of cell proliferation, cell growth and apoptosis working through two distinct complexes: mTORC1 and mTORC2. Although much is known about the activation and inactivation of mTORC1, the processes controlling mTORC2 remain poorly characterized. Experimental and modeling studies have attempted to explain the regulation of mTORC2 but have yielded several conflicting hypotheses. More specifically, the Phosphoinositide 3-kinase (PI3K) pathway was shown to be involved in this process, but the identity of the kinase interacting with and regulating mTORC2 remains to be determined (Cybulski and Hall, Trends Biochem Sci 34:620-7, 2009).

Method

We performed a literature search and identified 5 published hypotheses describing mTORC2 regulation. Based on these hypotheses, we built logical models, not only for each single hypothesis but also for all combinations and possible mechanisms among them. Based on data provided by the original studies, a systematic analysis of all models was performed.

Results

We were able to find models that account for experimental observations from every original study, but do not require all 5 hypotheses to be implemented. Surprisingly, all hypotheses were in agreement with all tested data gathered from the different studies and PI3K was identified as an essential regulator of mTORC2.

Conclusion

The results and additional data suggest that more than one regulator is necessary to explain the behavior of mTORC2. Finally, this study proposes a new experiment to validate mTORC1 as second essential regulator.

Gene expression alterations in the medial prefrontal cortex and blood cells in a mouse model of depression during menopause

Aims

The prevalence of major depressive disorder (MDD) is higher in women than in men, and this may be due to the decline in estrogen levels that occurs during the menopausal transition. We studied the biological alterations in the medial prefrontal cortex (mPFC), which is a region that is highly implicated in the neurobiology of MDD, and the blood cells (BCs) of ovariectomized (OVX) mice subjected to chronic mild stress (CMS), which represents a mouse model of depression during menopause.

Main methods

The mPFC and the BCs were obtained from the same individuals. Gene expression levels were analyzed by microarray. The data were used for the Ingenuity Pathway Analysis and the Gene Ontology analysis.

Key findings

The gene expression alterations (GEAs) induced by OVX were mainly associated with ribosomal and mitochondrial functions in both the mPFC and the BCs. Rapamycin-insensitive companion of mTOR (RICTOR) was identified as a possible upstream regulator of the OVX-induced GEAs in both tissues. The CMS-induced GEAs were associated with retinoic acid receptor signaling, inflammatory cytokines and post-synaptic density in the mPFC, but not in the BCs.

Significance

OVX and CMS independently affect biological pathways in the mPFC, which is involved in the development of the depression-like phenotype. Because a subset of the OVX-induced GEAs in the mPFC also occurred in the BCs, the GEAs in the BCs might be a useful probe to predict biological pathways in the corresponding brain tissue under specific conditions such as OVX in females.

mTORC2 activation is regulated by the urokinase receptor (uPAR) in bladder cancer

Mammalian target of rapamycin complex 2 (mTORC2) has been identified as a major regulator of bladder cancer cell migration and invasion. Upstream pathways that mediate mTORC2 activation remain poorly defined. Urokinase-type plasminogen activator receptor (uPAR) is a GPI-anchored membrane protein and known activator of cell-signaling. We identified increased uPAR expression in 94% of invasive human bladder cancers and in 54-71% of non-invasive bladder cancers, depending on grade. Normal urothelium was uPAR-immunonegative. Analysis of publicly available datasets identified uPAR gene amplification or mRNA upregulation in a subset of bladder cancer patients with reduced overall survival. Using biochemical approaches, we showed that uPAR activates mTORC2 in bladder cancer cells. Highly invasive bladder cancer cell lines, including T24, J82 and UM-UC-3 cells, showed increased uPAR mRNA expression and protein levels compared with the less aggressive cell lines, UROtsa and RT4. uPAR gene-silencing significantly reduced phosphorylation of Serine-473 in Akt, an mTORC2 target. uPAR gene-silencing also reduced bladder cancer cell migration and Matrigel invasion. S473 phosphorylation was observed by immunohistochemistry in human bladder cancers only when the tumors expressed high levels of uPAR. S473 phosphorylation was not controlled by uPAR in bladder cancer cell lines that are PTEN-negative; however, this result probably did not reflect altered mTORC2 regulation. Instead, PTEN deficiency de-repressed alternative kinases that phosphorylate S473. Our results suggest that uPAR and mTORC2 are components of a single cell-signaling pathway. Targeting uPAR or mTORC2 may be beneficial in patients with bladder cancer.

PDK1-SGK1 Signaling Sustains AKT-Independent mTORC1 Activation and Confers Resistance to PI3Kα Inhibition

PIK3CA, which encodes the p110α subunit of PI3K, is frequently mutated and oncogenic in breast cancer. PI3Kα inhibitors are in clinical development and despite promising early clinical activity, intrinsic resistance is frequent among patients. We have previously reported that residual downstream mTORC1 activity upon treatment with PI3Kα inhibitors drives resistance to these agents. However, the mechanism underlying this phenotype is not fully understood. Here we show that in cancer cells resistant to PI3Kα inhibition, PDK1 blockade restores sensitivity to these therapies. SGK1, which is activated by PDK1, contributes to the maintenance of residual mTORC1 activity through direct phosphorylation and inhibition of TSC2. Targeting either PDK1 or SGK1 prevents mTORC1 activation, restoring the antitumoral effects of PI3Kα inhibition in resistant cells.

The rise and fall of insulin signaling in Alzheimer's disease

The prevalence of both diabetes and Alzheimer's disease (AD) are reaching epidemic proportions worldwide. Alarmingly, diabetes is also a risk factor for Alzheimer's disease. The AD brain is characterised by the accumulation of peptides called Aβ as plaques in the neuropil and hyperphosphorylated tau protein in the form of neurofibrillary tangles within neurons. How diabetes confers risk is unknown but a simple linear relationship has been proposed whereby the hyperinsulinemia associated with type 2 diabetes leads to decreased insulin signaling in the brain, with downregulation of the PI3K/AKT signalling pathway and its inhibition of the major tau kinase, glycogen synthase kinase 3β. The earliest studies of post mortem AD brain tissue largely confirmed this cascade of events but subsequent studies have generally found either an upregulation of AKT activity, or that the relationship between insulin signaling and AD is independent of glycogen synthase kinase 3β altogether. Given the lack of success of beta-amyloid-reducing therapies in clinical trials, there is intense interest in finding alternative or adjunctive therapeutic targets for AD. Insulin signaling is a neuroprotective pathway and represents an attractive therapeutic option. However, this incredibly complex signaling pathway is not fully understood in the human brain and particularly in the context of AD. Here, we review the ups and downs of the research efforts aimed at understanding how diabetes modifies AD risk.

TORC2 Structure and Function

The target of rapamycin (TOR) kinase functions in two multiprotein complexes, TORC1 and TORC2. Although both complexes are evolutionarily conserved, only TORC1 is acutely inhibited by rapamycin. Consequently, only TORC1 signaling is relatively well understood; and, at present, only mammalian TORC1 is a validated drug target, pursued in immunosuppression and oncology. However, the knowledge void surrounding TORC2 is dissipating. Acute inhibition of TORC2 with small molecules is now possible and structural studies of both TORC1 and TORC2 have recently been reported. Here we review these recent advances as well as observations made from tissue-specific mTORC2 knockout mice. Together these studies help define TORC2 structure-function relationships and suggest that mammalian TORC2 may one day also become a bona fide clinical target.

mTORC2 mediates CXCL12-induced angiogenesis

The chemokine CXCL12, through its receptor CXCR4, positively regulates angiogenesis by promoting endothelial cell (EC) migration and tube formation. However, the relevant downstream signaling pathways in EC have not been defined. Similarly, the upstream activators of mTORC2 signaling in EC are also poorly defined. Here, we demonstrate for the first time that CXCL12 regulation of angiogenesis requires mTORC2 but not mTORC1. We find that CXCR4 signaling activates mTORC2 as indicated by phosphorylation of serine 473 on Akt and does so through a G-protein- and PI3K-dependent pathway. Significantly, independent disruption of the mTOR complexes by drugs or multiple independent siRNAs reveals that mTORC2, but not mTORC1, is required for microvascular sprouting in a 3D in vitro angiogenesis model. Importantly, in a mouse model, both tumor angiogenesis and tumor volume are significantly reduced only when mTORC2 is inhibited. Finally, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3), which is a key regulator of glycolytic flux, is required for microvascular sprouting in vitro, and its expression is reduced in vivo when mTORC2 is targeted. Taken together, these findings identify mTORC2 as a critical signaling nexus downstream of CXCL12/CXCR4 that represents a potential link between mTORC2, metabolic regulation, and angiogenesis.

Capillary Isoelectric Focusing of Akt Isoforms Identifies Highly Dynamic Phosphorylation in Neuronal Cells and Brain Tissue

The PI3K/PTEN/Akt pathway has been established as a core signaling pathway that is crucial for the integration of neurons into neuronal circuits and the maintenance of the architecture and function of neurons in the adult brain. Akt1–3 kinases are specifically activated by two phosphorylation events on residues Thr³⁰⁸ and Ser⁴⁷³ upon growth factor signaling, which subsequently phosphorylate a vast cohort of downstream targets. However, we still lack a clear understanding of the complexity and regulation of isoform specificity within the PI3K/PTEN/Akt pathway. We utilized a capillary-based isoelectric focusing method to study dynamics of Akt phosphorylation in neuronal cells and the developing brain and identify previously undescribed features of Akt phosphorylation and activation. First, we show that the accumulation of multiple phosphorylation events on Akt forms occur concurrently with Ser⁴⁷³ and Thr³⁰⁸ phosphorylation upon acute PI3K activation and provide evidence for uncoupling of Ser⁴⁷³ and Thr³⁰⁸ phosphorylation, as well as differential sensitivities of Akt1 forms upon PI3K inhibition. Second, we detect a transient shift in Akt isoform phosphorylation and activation pattern during early postnatal brain development, at stages corresponding to synapse development and maturation. Third, we show differential sensitivities of Ser⁴⁷³-Akt species to PTEN deletion in mature neurons, which suggests inherent differences in the Akt pools that are accessible to growth factors as compared with the pools that are controlled by PTEN. Our study demonstrates the presence of complex phosphorylation events of Akt in a time- and signal-dependent manner in neurons.

Effect of mTORC1/mTORC2 inhibition on T cell function: potential role in graft-versus-host disease control

The mechanistic target of rapamycin (mTOR) pathway is crucial for the activation and function of T cells, which play an essential role in the development of graft-versus-host disease (GvHD). Despite its partial ability to block mTOR pathway, the mTORC1 inhibitor rapamycin has shown encouraging results in the control of GvHD. Therefore, we considered that simultaneous targeting of both mTORC1 and mTORC2 complexes could exert a more potent inhibition of T cell activation and, thus, could have utility in GvHD control. To assess this assumption, we have used the dual mTORC1/mTORC2 inhibitors CC214-1 and CC214-2. In vitro studies confirmed the superior ability of CC214-1 versus rapamycin to block mTORC1 and mTORC2 activity and to reduce T cell proliferation. Both drugs induced a similar decrease in Th1/Th2 cytokine secretion, but CC214-1 was more efficient in inhibiting naïve T cell activation and the expression of T-cell activation markers. In addition, CC214-1 induced specific tolerance against alloantigens, while preserving anti-cytomegalovirus response. Finally, in a mouse model of GvHD, the administration of CC214-2 significantly improved mice survival and decreased GvHD-induced damages. In conclusion, the current study shows, for the first time, the immunosuppressive ability of CC214-1 on T lymphocytes and illustrates the role of CC214-2 in the allogeneic transplantation setting as a possible GvHD prophylaxis agent.

Regulation of PtdIns(3,4,5)P3/Akt signalling by inositol polyphosphate 5-phosphatases

The phosphoinositide 3-kinase (PI3K) generated lipid signals, PtdIns(3,4,5)P3 and PtdIns(3,4)P2, are both required for the maximal activation of the serine/threonine kinase proto-oncogene Akt. The inositol polyphosphate 5-phosphatases (5-phosphatases) hydrolyse the 5-position phosphate from the inositol head group of PtdIns(3,4,5)P3 to yield PtdIns(3,4)P2. Extensive work has revealed several 5-phosphatases inhibit PI3K-driven Akt signalling, by decreasing PtdIns(3,4,5)P3 despite increasing cellular levels of PtdIns(3,4)P2. The roles that 5-phosphatases play in suppressing cell proliferation and transformation are slow to emerge; however, the 5-phosphatase PIPP [proline-rich inositol polyphosphate 5-phosphatase; inositol polyphosphate 5-phosphatase (INPP5J)] has recently been identified as a putative tumour suppressor in melanoma and breast cancer and SHIP1 [SH2 (Src homology 2)-containing inositol phosphatase 1] inhibits haematopoietic cell proliferation. INPP5E regulates cilia stability and INPP5E mutations have been implicated ciliopathy syndromes. This review will examine 5-phosphatase regulation of PI3K/Akt signalling, focussing on the role PtdIns(3,4,5)P3 5-phosphatases play in developmental diseases and cancer.

Phosphatidylinositol 3-Kinase p110δ Isoform Regulates CD8+ T Cell Responses during Acute Viral and Intracellular Bacterial Infections

The p110δ isoform of PI3K is known to play an important role in immunity, yet its contribution to CTL responses has not been fully elucidated. Using murine p110δ-deficient CD8(+) T cells, we demonstrated a critical role for the p110δ subunit in the generation of optimal primary and memory CD8(+) T cell responses. This was demonstrated in both acute viral and intracellular bacterial infections in mice. We show that p110δ signaling is required for CD8(+) T cell activation, proliferation and effector cytokine production. We provide evidence that the effects of p110δ signaling are mediated via Akt activation and through the regulation of TCR-activated oxidative phosphorylation and aerobic glycolysis. In light of recent clinical trials that employ drugs targeting p110δ in certain cancers and other diseases, our study suggests caution in using these drugs in patients, as they could potentially increase susceptibility to infectious diseases. These studies therefore reveal a novel and direct role for p110δ signaling in in vivo CD8(+) T cell immunity to microbial pathogens.

Hydrophobic motif site-phosphorylated protein kinase CβII between mTORC2 and Akt regulates high glucose-induced mesangial cell hypertrophy

PKCβII controls the pathologic features of diabetic nephropathy, including glomerular mesangial cell hypertrophy. PKCβII contains the COOH-terminal hydrophobic motif site Ser-660. Whether this hydrophobic motif phosphorylation contributes to high glucose-induced mesangial cell hypertrophy has not been determined. Here we show that, in mesangial cells, high glucose increased phosphorylation of PKCβII at Ser-660 in a phosphatidylinositol 3-kinase (PI3-kinase)-dependent manner. Using siRNAs to downregulate PKCβII, dominant negative PKCβII, and PKCβII hydrophobic motif phosphorylation-deficient mutant, we found that PKCβII regulates activation of mechanistic target of rapamycin complex 1 (mTORC1) and mesangial cell hypertrophy by high glucose. PKCβII via its phosphorylation at Ser-660 regulated phosphorylation of Akt at both catalytic loop and hydrophobic motif sites, resulting in phosphorylation and inactivation of its substrate PRAS40. Specific inhibition of mTORC2 increased mTORC1 activity and induced mesangial cell hypertrophy. In contrast, inhibition of mTORC2 decreased the phosphorylation of PKCβII and Akt, leading to inhibition of PRAS40 phosphorylation and mTORC1 activity and prevented mesangial cell hypertrophy in response to high glucose; expression of constitutively active Akt or mTORC1 restored mesangial cell hypertrophy. Moreover, constitutively active PKCβII reversed the inhibition of high glucose-stimulated Akt phosphorylation and mesangial cell hypertrophy induced by suppression of mTORC2. Finally, using renal cortexes from type 1 diabetic mice, we found that increased phosphorylation of PKCβII at Ser-660 was associated with enhanced Akt phosphorylation and mTORC1 activation. Collectively, our findings identify a signaling route connecting PI3-kinase to mTORC2 to phosphorylate PKCβII at the hydrophobic motif site necessary for Akt phosphorylation and mTORC1 activation, leading to mesangial cell hypertrophy.