Construction and characterization of a conditionally active version of the serine/threonine kinase Akt

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.

Glucose transporter 4: Insulin response mastermind, glycolysis catalyst and treatment direction for cancer progression

The glucose transporter family (GLUT) consists of fourteen members. It is responsible for glucose homeostasis and glucose transport from the extracellular space to the cell cytoplasm to further cascade catalysis. GLUT proteins are encoded by the solute carrier family 2 (SLC2) genes and are members of the major facilitator superfamily of membrane transporters. Moreover, different GLUTs also have their transporter kinetics and distribution, so each GLUT member has its uniqueness and importance to play essential roles in human physiology. Evidence from many studies in the field of diabetes showed that GLUT4 travels between the plasma membrane and intracellular vesicles (GLUT4-storage vesicles, GSVs) and that the PI3K/Akt pathway regulates this activity in an insulin-dependent manner or by the AMPK pathway in response to muscle contraction. Moreover, some published results also pointed out that GLUT4 mediates insulin-dependent glucose uptake. Thus, dysfunction of GLUT4 can induce insulin resistance, metabolic reprogramming in diverse chronic diseases, inflammation, and cancer. In addition to the relationship between GLUT4 and insulin response, recent studies also referred to the potential upstream transcription factors that can bind to the promoter region of GLUT4 to regulating downstream signals. Combined all of the evidence, we conclude that GLUT4 has shown valuable unknown functions and is of clinical significance in cancers, which deserves our in-depth discussion and design compounds by structure basis to achieve therapeutic effects. Thus, we intend to write up a most updated review manuscript to include the most recent and critical research findings elucidating how and why GLUT4 plays an essential role in carcinogenesis, which may have broad interests and impacts on this field.

Proto-oncoprotein Zbtb7c and SIRT1 repression: implications in high-fat diet-induced and age-dependent obesity

Zbtb7c is a proto-oncoprotein that controls the cell cycle and glucose, glutamate, and lipid metabolism. Zbtb7c expression is increased in the liver and white adipose tissues of aging or high-fat diet-fed mice. Knockout or knockdown of Zbtb7c gene expression inhibits the adipocyte differentiation of 3T3-L1 cells and decreases adipose tissue mass in aging mice. We found that Zbtb7c was a potent transcriptional repressor of SIRT1 and that SIRT1 was derepressed in various tissues of Zbtb7c-KO mice. Mechanistically, Zbtb7c interacted with p53 and bound to the proximal promoter p53RE1 and p53RE2 to repress the SIRT1 gene, in which p53RE2 was particularly critical. Zbtb7c induced p53 to interact with the corepressor mSin3A-HADC1 complex at p53RE. By repressing the SIRT1 gene, Zbtb7c increased the acetylation of Pgc-1α and Pparγ, which resulted in repression or activation of Pgc-1α or Pparγ target genes involved in lipid metabolism. Our study provides a molecular target that can overexpress SIRT1 protein in the liver, pancreas, and adipose tissues, which can be beneficial in the treatment of diabetes, obesity, longevity, etc.

Targeting a regulatory DNA sequence linked to the repression of a critical enzyme during metabolic diseases could prove valuable for future therapies. The SIRT1 enzyme is involved in metabolic processes and stress resistance, and its dysregulation is linked to obesity and diabetes development. SIRT1 expression also decreases with aging and stress, but the precise regulation mechanisms are unclear. In experiments on aging mice and mice fed a high-fat diet, Man-Wook Hur at Yonsei University in Seoul, South Korea, and co-workers demonstrated that SIRT1 expression is repressed by a protein called Zbtb7c, which is highly expressed in fat and liver tissues. Aging mice without the Zbtb7c-encoding gene had less fatty tissue than controls. Zbtb7c represses the SIRT1 gene by interacting with protein p53. A sequence critical to this repression mechanism may provide a therapeutic target.

Tmod3 Phosphorylation Mediates AMPK-Dependent GLUT4 Plasma Membrane Insertion in Myoblasts

Insulin and muscle contractions mediate glucose transporter 4 (GLUT4) translocation and insertion into the plasma membrane (PM) for glucose uptake in skeletal muscles. Muscle contraction results in AMPK activation, which promotes GLUT4 translocation and PM insertion. However, little is known regarding AMPK effectors that directly regulate GLUT4 translocation. We aim to identify novel AMPK effectors in the regulation of GLUT4 translocation. We performed biochemical, molecular biology and fluorescent microscopy imaging experiments using gain- and loss-of-function mutants of tropomodulin 3 (Tmod3). Here we report Tmod3, an actin filament capping protein, as a novel AMPK substrate and an essential mediator of AMPK-dependent GLUT4 translocation and glucose uptake in myoblasts. Furthermore, Tmod3 plays a key role in AMPK-induced F-actin remodeling and GLUT4 insertion into the PM. Our study defines Tmod3 as a key AMPK effector in the regulation of GLUT4 insertion into the PM and glucose uptake in muscle cells, and offers new mechanistic insights into the regulation of glucose homeostasis.

RASSF1A Suppresses Estrogen-Dependent Breast Cancer Cell Growth through Inhibition of the Yes-Associated Protein 1 (YAP1), Inhibition of the Forkhead Box Protein M1 (FOXM1), and Activation of Forkhead Box Transcription Factor 3A (FOXO3A)

The estrogen receptor alpha (ERα) is expressed by the majority of breast cancers and plays an important role in breast cancer development and tumor outgrowth. Although ERα is well known to be a specific and efficient therapeutic target, the molecular mechanisms that are responsible for the control of ERα expression and function in the context of breast cancer initiation and progression are complex and not completely elucidated. In previous work, we have demonstrated that the tumor suppressor RASSF1A inhibits ERα expression and function in ERα-positive breast cancer cells through an AKT-dependent mechanism. Transcriptional activators such as forkhead box protein M1 (FOXM1) and forkhead transcription factor 3A (FOXO3A) and signaling pathways such as the Hippo pathway are also known to modulate ERα expression and activity. Here we report that RASSF1A acts as an inhibitor of ERα-driven breast cancer cell growth through a complex, hierarchically organized network that initially involves suppression of the Hippo effector Yes-associated protein 1 (YAP1), which is followed by inhibition of AKT1 activity, increased FOXO3A activity as well as a blockade of FOXM1 and ERα expression. Together our findings provide important new mechanistic insights into how the loss of RASSF1A contributes to ERα+ breast cancer initiation and progression.

Azoramide improves mitochondrial dysfunction in palmitate-induced insulin resistant H9c2 cells

Azoramide is identified as a new compound with the dual properties for the improvement of ER-folding capacity in various cells as well as for the treatment of T2DM. Although the effect of azoramide in glucose-homeostasis in mammalians is not known very well, a limited number of experimental studies showed that it could improve the insulin sensitivity in genetically obese mice. Therefore, here, we aimed to investigate the direct effect of azoramide on insulin signaling in insulin-resistant (IR) cardiomyocytes using IR-modelled ventricular cardiomyocytes. This model was established in H9c2 cells using palmitic acid incubation (50-μM for 24-h). The development of IR in cells was verified by monitoring the cellular 2-DG6P uptake assays in these treated cells. The 2-DG6P uptake was 50% less in the IR-cells compared to the control cells, while azoramide treatment (20-μM for 48-h) could prevent fully that decrease. In addition, azoramide treatment markedly preserved the IR-induced less ATP production and high-ROS production in these IR-cells. Furthermore, this treatment prevented the functional changes in mitochondria characterized by depolarized mitochondrial membrane potential and mitochondrial fusion or fusion-related protein levels as well as cellular ATP level. Moreover, this treatment provided marked protection against IR-associated changes in the insulin signaling pathway in cells, including recovery in the phosphorylation of IRS1 and Akt as well as the protein level of GLUT4 and Akt. Our present results, for the first time, demonstrated that azoramide plays an important protective role in IR-cardiomyocytes, at most, protective action on mitochondria. Therefore, one can suggest that azoramide, as a novel regulator, can provide direct cardioprotection in the IR-heart, at most, via affecting mitochondria and can be a good candidate as a new drug for the treatment of IR-associated cardiovascular disorders in mammalians with systemic IR.

S 47445 counteracts the behavioral manifestations and hippocampal neuroplasticity changes in bulbectomized mice

S 47445 is a positive allosteric modulator of glutamate AMPA-type receptors that possesses procognitive, neurotrophic and enhancing synaptic plasticity properties. Its chronic administration promotes antidepressant- and anxiolytic-like effects in different rodent models of depression. We have evaluated the behavioral effects of S 47445 in the bilateral olfactory bulbectomy mice model (OB) and the adaptive changes in those proteins associated to brain neuroplasticity (BDNF and mTOR pathway). Following OB surgery, adult C57BL/6J male mice were chronically administered S 47445 (1, 3 and 10 mg/kg/day; i.p.) and fluoxetine (18 mg/kg/day; i.p.), and then behaviorally tested in the open field test. Afterwards, the expression levels of BDNF, mTOR, phospho-mTOR, 4EBP1 and phospho-4EBP1 were evaluated in hippocampus and prefrontal cortex. Both drugs reduced the OB-induced locomotor activity, a predictive outcome of antidepressant efficacy, with a similar temporal pattern of action. S 47445, but not fluoxetine, showed an anxiolytic effect as reflected by an increased central activity. Chronic administration of S 47445 reversed OB-induced changes in BDNF and phopho-mTOR expression in hippocampus but not in prefrontal cortex. The chronic administration of S 47445 induced antidepressant- and anxiolytic-like effects at low-medium doses (1 and 3 mg/kg/day, i.p.) associated with the reversal of OB-induced changes in hippocampal BDNF and mTOR signaling pathways.

The Parkinson's disease-linked Leucine-rich repeat kinase 2 (LRRK2) is required for insulin-stimulated translocation of GLUT4

Mutations within Leucine-rich repeat kinase 2 (LRRK2) are associated with late-onset Parkinson's disease. The physiological function of LRRK2 and molecular mechanism underlying the pathogenic role of LRRK2 mutations remain uncertain. Here, we investigated the role of LRRK2 in intracellular signal transduction. We find that deficiency of Lrrk2 in rodents affects insulin-dependent translocation of glucose transporter type 4 (GLUT4). This deficit is restored during aging by prolonged insulin-dependent activation of protein kinase B (PKB, Akt) and Akt substrate of 160 kDa (AS160), and is compensated by elevated basal expression of GLUT4 on the cell surface. Furthermore, we find a crucial role of Rab10 phosphorylation by LRRK2 for efficient insulin signal transduction. Translating our findings into human cell lines, we find comparable molecular alterations in fibroblasts from Parkinson's patients with the known pathogenic G2019S LRRK2 mutation. Our results highlight the role of LRRK2 in insulin-dependent signalling with potential therapeutic implications.

Gut microbiome catabolites as novel modulators of muscle cell glucose metabolism

The gut microbiome supplies essential metabolites such as short-chain fatty acids to skeletal muscle mitochondria, and the composition and activity of the microbiota is in turn affected by muscle fitness. To further our understanding of the complex interactions between the gut microbiome and muscle, we examined the effect of microbiota-derived phenolic metabolites on the ability of human muscle cells to take up and metabolize glucose. As a model, we used the differentiated human skeletal muscle myoblast line, LHCN-M2, which expresses typical muscle phenotypic markers. We initially tested a selected panel of parent phenolic compounds and microbial metabolites, and their respective phenolic conjugates, as found in blood. Several of the tested compounds increased glucose uptake and metabolism, notably in high glucose- and insulin-treated myotubes. One of the most effective was isovanillic acid 3 -O-sulfate (IVAS), a metabolite from the microbiome found in the blood, primarily derived from consumed cyanidin 3 -O-glucoside, a major compound in berry fruits. IVAS stimulated a dose-dependent increase in glucose transport through glucose transporter GLUT4- and PI3K-dependent mechanisms. IVAS also up-regulated GLUT1, GLUT4, and PI3K p85α protein, and increased phosphorylation of Akt. The stimulation of glucose uptake and metabolism by a unique microbiome metabolite provides a novel link among diet, gut microbiota, and skeletal muscle energy source utilization.-Houghton, M. J., Kerimi, A., Mouly, V., Tumova, S., Williamson, G. Gut microbiome catabolites as novel modulators of muscle cell glucose metabolism.

Molecular dissection of effector mechanisms of RAS-mediated resistance to anti-EGFR antibody therapy

Monoclonal antibodies targeting the epidermal growth factor receptor (EGFR), cetuximab and panitumumab, are a mainstay of metastatic colorectal cancer (mCRC) treatment. However, a significant number of patients suffer from primary or acquired resistance. RAS mutations are negative predictors of clinical efficacy of anti-EGFR antibodies in patients with mCRC. Oncogenic RAS activates the MAPK and PI3K/AKT pathways, which are considered the main effectors of resistance. However, the relative impact of these pathways in RAS-mutant CRC is less defined. A better mechanistic understanding of RAS-mediated resistance may guide development of rational intervention strategies. To this end we developed cancer models for functional dissection of resistance to anti-EGFR therapy in vitro and in vivo. To selectively activate MAPK- or AKT-signaling we expressed conditionally activatable RAF-1 and AKT in cancer cells. We found that either pathway independently protected sensitive cancer models against anti-EGFR antibody treatment in vitro and in vivo. RAF-1- and AKT-mediated resistance was associated with increased expression of anti-apoptotic BCL-2 proteins. Biomarkers of MAPK and PI3K/AKT pathway activation correlated with inferior outcome in a cohort of mCRC patients receiving cetuximab-based therapy. Dual pharmacologic inhibition of PI3K and MEK successfully sensitized primary resistant CRC models to anti-EGFR therapy. In conclusion, combined targeting of MAPK and PI3K/AKT signaling, but not single pathways, may be required to enhance the efficacy of anti-EGFR antibody therapy in patients with RAS-mutated CRC as well as in RAS wild type tumors with clinical resistance.

Critical role for arginase 2 in obesity-associated pancreatic cancer

Obesity is an established risk factor for pancreatic ductal adenocarcinoma (PDA). Despite recent identification of metabolic alterations in this lethal malignancy, the metabolic dependencies of obesity-associated PDA remain unknown. Here we show that obesity-driven PDA exhibits accelerated growth and a striking transcriptional enrichment for pathways regulating nitrogen metabolism. We find that the mitochondrial form of arginase (ARG2), which hydrolyzes arginine into ornithine and urea, is induced upon obesity, and silencing or loss of ARG2 markedly suppresses PDA. In vivo infusion of 15N-glutamine in obese mouse models of PDA demonstrates enhanced nitrogen flux into the urea cycle and infusion of 15N-arginine shows that Arg2 loss causes significant ammonia accumulation that results from the shunting of arginine catabolism into alternative nitrogen repositories. Furthermore, analysis of PDA patient tumors indicates that ARG2 levels correlate with body mass index (BMI). The specific dependency of PDA on ARG2 rather than the principal hepatic enzyme ARG1 opens a therapeutic window for obesity-associated pancreatic cancer.Obesity is an established risk factor for pancreatic ductal adenocarcinoma (PDA). Here the authors show that obesity induces the expression of the mitochondrial form of arginase ARG2 in PDA and that ARG2 silencing or loss results in ammonia accumulation and suppression of obesity-driven PDA tumor growth.

The Role of Heparin Cofactor Ⅱ in the Regulation of Insulin Sensitivity and Maintenance of Glucose Homeostasis in Humans and Mice

Aim: Accelerated thrombin action is associated with insulin resistance. It is known that upon activation by binding to dermatan sulfate proteoglycans, heparin cofactor II (HCII) inactivates thrombin in tissues. Because HCII may be involved in glucose metabolism, we investigated the relationship between plasma HCII activity and insulin resistance.

Methods and Results: In a clinical study, statistical analysis was performed to examine the relationships between plasma HCII activity, glycosylated hemoglobin (HbA1c), fasting plasma glucose (FPG), and homeostasis model assessment-insulin resistance (HOMA-IR) in elderly Japanese individuals with lifestyle-related diseases. Multiple regression analysis showed significant inverse relationships between plasma HCII activity and HbA1c (p = 0.014), FPG (p = 0.007), and HOMA-IR (p = 0.041) in elderly Japanese subjects. In an animal study, HCII^+/+ mice and HCII^+/− mice were fed with a normal diet or high-fat diet (HFD) until 25 weeks of age. HFD-fed HCII^+/− mice exhibited larger adipocyte size, higher FPG level, hyperinsulinemia, compared to HFD-fed HCII^+/+ mice. In addition, HFD-fed HCII^+/− mice exhibited augmented expression of monocyte chemoattractant protein-1 and tumor necrosis factor, and impaired phosphorylation of the serine/threonine kinase Akt and AMP-activated protein kinase in adipose tissue compared to HFD-fed HCII^+/+ mice. The expression of phosphoenolpyruvate carboxykinase and glucose-6-phosphatase was also enhanced in the hepatic tissues of HFD-fed HCII^+/− mice.

Conclusions: The present studies provide evidence to support the idea that HCII plays an important role in the maintenance of glucose homeostasis by regulating insulin sensitivity in both humans and mice. Stimulators of HCII production may serve as novel therapeutic tools for the treatment of type 2 diabetes.

β-Adrenergic Receptor and Insulin Resistance in the Heart

Insulin resistance is characterized by the reduced ability of insulin to stimulate tissue uptake and disposal of glucose including cardiac muscle. These conditions accelerate the progression of heart failure and increase cardiovascular morbidity and mortality in patients with cardiovascular diseases. It is noteworthy that some conditions of insulin resistance are characterized by up-regulation of the sympathetic nervous system, resulting in enhanced stimulation of β-adrenergic receptor (βAR). Overstimulation of βARs leads to the development of heart failure and is associated with the pathogenesis of insulin resistance in the heart. However, pathological consequences of the cross-talk between the βAR and the insulin sensitivity and the mechanism by which βAR overstimulation promotes insulin resistance remain unclear. This review article examines the hypothesis that βARs overstimulation leads to induction of insulin resistance in the heart.

Twist1-mediated 4E-BP1 regulation through mTOR in non-small cell lung cancer

Twist1 overexpression corresponds with poor survival in non-small cell lung cancer (NSCLC), but the underlining mechanism is not clear. The objective of the present study was to investigate the tumorigenic role of Twist1 and its related molecular mechanisms in NSCLC. Twist1 was overexpressed in 34.7% of NSCLC patients. The survival rate was significantly lower in patients with high Twist1 expression than low expression (P < 0.05). Twist1 expression levels were higher in H1650 cells, but relatively lower in H1975 cells. H1650 with stable Twist1 knockdown, H1650shTw, demonstrated a significantly slower rate of wound closure; however, H1975 with stable Twist1 overexpression, H1975Over, had an increased motility velocity. A significant decrease in colony number and size was observed in H1650shTw, but a significant increase in colony number was found in H1975Over (P < 0.05). Tumor growth significantly decreased in mice implanted with H1650shTw compared to H1650 (P < 0.05). 4E-BP1 and p53 gene expressions were increased, but p-4E-BP1 and p-mTOR protein expressions were decreased in H1650shTw. However, 4E-BP1 gene expression was decreased, while p-4E-BP1 and p-mTOR protein expressions were increased in H1975Over. p-4E-BP1 was overexpressed in 24.0% of NSCLC patients. Survival rate was significantly lower in patients with high p-4E-BP1 expression than low p-4E-BP1 (P < 0.01). A significant correlation was found between Twist1 and p-4E-BP1 (P < 0.01). A total of 13 genes in RT-PCR array showed significant changes in H1650shTw. Altogether, Twist1 is correlated with p-4E-BP1 in predicting the prognostic outcome of NSCLC. Inhibition of Twist1 decreases p-4E-BP1 expression possibly through downregulating p-mTOR and increasing p53 expression in NSCLC.

The Timing of Raf/ERK and AKT Activation in Protecting PC12 Cells against Oxidative Stress

Acute brain injuries such as ischemic stroke or traumatic brain injury often cause massive neural death and irreversible brain damage with grave consequences. Previous studies have established that a key participant in the events leading to neural death is the excessive production of reactive oxygen species. Protecting neuronal cells by activating their endogenous defense mechanisms is an attractive treatment strategy for acute brain injuries. In this work, we investigate how the precise timing of the Raf/ERK and the AKT pathway activation affects their protective effects against oxidative stress. For this purpose, we employed optogenetic systems that use light to precisely and reversibly activate either the Raf/ERK or the AKT pathway. We find that preconditioning activation of the Raf/ERK or the AKT pathway immediately before oxidant exposure provides significant protection to cells. Notably, a 15-minute transient activation of the Raf/ERK pathway is able to protect PC12 cells against oxidant strike that is applied 12 hours later, while the transient activation of the AKT pathway fails to protect PC12 cells in such a scenario. On the other hand, if the pathways are activated after the oxidative insult, i.e. postconditioning, the AKT pathway conveys greater protective effect than the Raf/ERK pathway. We find that postconditioning AKT activation has an optimal delay period of 2 hours. When the AKT pathway is activated 30min after the oxidative insult, it exhibits very little protective effect. Therefore, the precise timing of the pathway activation is crucial in determining its protective effect against oxidative injury. The optogenetic platform, with its precise temporal control and its ability to activate specific pathways, is ideal for the mechanistic dissection of intracellular pathways in protection against oxidative stress.

A TORC2-Akt Feed-Forward Topology Underlies HER3 Resiliency in HER2-Amplified Cancers

The requisite role of HER3 in HER2-amplified cancers is beyond what would be expected as a dimerization partner or effector substrate and it exhibits a substantial degree of resiliency that mitigates the effects of HER2-inhibitor therapies. To better understand the roots of this resiliency, we conducted an in-depth chemical-genetic interrogation of the signaling network downstream of HER3. A unique attribute of these tumors is the deregulation of TORC2. The upstream signals that ordinarily maintain TORC2 signaling are lost in these tumors, and instead TORC2 is driven by Akt. We find that in these cancers HER3 functions as a buffering arm of an Akt-TORC2 feed-forward loop that functions as a self-perpetuating module. This network topology alters the role of HER3 from a conditionally engaged ligand-driven upstream physiologic signaling input to an essential component of a concentric signaling throughput highly competent at preservation of homeostasis. The competence of this signaling topology is evident in its response to perturbation at any of its nodes. Thus, a critical pathophysiologic event in the evolution of HER2-amplified cancers is the loss of the input signals that normally drive TORC2 signaling, repositioning it under Akt dependency, and fundamentally altering the role of HER3. This reprogramming of the downstream network topology is a key aspect in the pathogenesis of HER2-amplified cancers and constitutes a formidable barrier in the targeted therapy of these cancers.

Metabolic Competition in the Tumor Microenvironment Is a Driver of Cancer Progression

Failure of T cells to protect against cancer is thought to result from lack of antigen recognition, chronic activation, and/or suppression by other cells. Using a mouse sarcoma model, we show that glucose consumption by tumors metabolically restricts T cells, leading to their dampened mTOR activity, glycolytic capacity, and IFN-γ production, thereby allowing tumor progression. We show that enhancing glycolysis in an antigenic "regressor" tumor is sufficient to override the protective ability of T cells to control tumor growth. We also show that checkpoint blockade antibodies against CTLA-4, PD-1, and PD-L1, which are used clinically, restore glucose in tumor microenvironment, permitting T cell glycolysis and IFN-γ production. Furthermore, we found that blocking PD-L1 directly on tumors dampens glycolysis by inhibiting mTOR activity and decreasing expression of glycolysis enzymes, reflecting a role for PD-L1 in tumor glucose utilization. Our results establish that tumor-imposed metabolic restrictions can mediate T cell hyporesponsiveness during cancer.

Metabolic Competition in the Tumor Microenvironment Is a Driver of Cancer Progression

Failure of T cells to protect against cancer is thought to result from lack of antigen recognition, chronic activation, and/or suppression by other cells. Using a mouse sarcoma model, we show that glucose consumption by tumors metabolically restricts T cells, leading to their dampened mTOR activity, glycolytic capacity, and IFN-γ production, thereby allowing tumor progression. We show that enhancing glycolysis in an antigenic "regressor" tumor is sufficient to override the protective ability of T cells to control tumor growth. We also show that checkpoint blockade antibodies against CTLA-4, PD-1, and PD-L1, which are used clinically, restore glucose in tumor microenvironment, permitting T cell glycolysis and IFN-γ production. Furthermore, we found that blocking PD-L1 directly on tumors dampens glycolysis by inhibiting mTOR activity and decreasing expression of glycolysis enzymes, reflecting a role for PD-L1 in tumor glucose utilization. Our results establish that tumor-imposed metabolic restrictions can mediate T cell hyporesponsiveness during cancer.

Interactions between mitochondrial reactive oxygen species and cellular glucose metabolism

Mitochondrial reactive oxygen species (ROS) production and detoxification are tightly balanced. Shifting this balance enables ROS to activate intracellular signaling and/or induce cellular damage and cell death. Increased mitochondrial ROS production is observed in a number of pathological conditions characterized by mitochondrial dysfunction. One important hallmark of these diseases is enhanced glycolytic activity and low or impaired oxidative phosphorylation. This suggests that ROS is involved in glycolysis (dys)regulation and vice versa. Here we focus on the bidirectional link between ROS and the regulation of glucose metabolism. To this end, we provide a basic introduction into mitochondrial energy metabolism, ROS generation and redox homeostasis. Next, we discuss the interactions between cellular glucose metabolism and ROS. ROS-stimulated cellular glucose uptake can stimulate both ROS production and scavenging. When glucose-stimulated ROS production, leading to further glucose uptake, is not adequately counterbalanced by (glucose-stimulated) ROS scavenging systems, a toxic cycle is triggered, ultimately leading to cell death. Here we inventoried the various cellular regulatory mechanisms and negative feedback loops that prevent this cycle from occurring. It is concluded that more insight in these processes is required to understand why they are (un)able to prevent excessive ROS production during various pathological conditions in humans.

Akt-mediated phosphorylation controls the activity of the Y-box protein MSY3 in skeletal muscle

BACKGROUND

The Y-box protein MSY3/Csda represses myogenin transcription in skeletal muscle by binding a highly conserved cis-acting DNA element located just upstream of the myogenin minimal promoter (myogHCE). It is not known how this MSY3 activity is controlled in skeletal muscle. In this study, we provide multiple lines of evidence showing that the post-translational phosphorylation of MSY3 by Akt kinase modulates the MSY3 repression of myogenin.

METHODS

Skeletal muscle and myogenic C2C12 cells were used to study the effects of MSY3 phosphorylation in vivo and in vitro on its sub-cellular localization and activity, by blocking the IGF1/PI3K/Akt pathway, by Akt depletion and over-expression, and by mutating potential MSY3 phosphorylation sites.

RESULTS

We observed that, as skeletal muscle progressed from perinatal to postnatal and adult developmental stages, MSY3 protein became gradually dephosphorylated and accumulated in the nucleus. This correlated well with the reduction of phosphorylated active Akt. In C2C12 myogenic cells, blocking the IGF1/PI3K/Akt pathway using LY294002 inhibitor reduced MSY3 phosphorylation levels resulting in its accumulation in the nuclei. Knocking down Akt expression increased the amount of dephosphorylated MSY3 and reduced myogenin expression and muscle differentiation. MSY3 phosphorylation by Akt in vitro impaired its binding at the MyogHCE element, while blocking Akt increased MSY3 binding activity. While Akt over-expression rescued myogenin expression in MSY3 overexpressing myogenic cells, ablation of the Akt substrate, (Ser126 located in the MSY3 cold shock domain) promoted MSY3 accumulation in the nucleus and abolished this rescue. Furthermore, forced expression of Akt in adult skeletal muscle induced MSY3 phosphorylation and myogenin derepression.

CONCLUSIONS

These results support the hypothesis that MSY3 phosphorylation by Akt interferes with MSY3 repression of myogenin circuit activity during muscle development. This study highlights a previously undescribed Akt-mediated signaling pathway involved in the repression of myogenin expression in myogenic cells and in mature muscle. Given the significance of myogenin regulation in adult muscle, the Akt/MSY3/myogenin regulatory circuit is a potential therapeutic target to counteract muscle degenerative disease.

General aspects of muscle glucose uptake

Glucose uptake in peripheral tissues is dependent on the translocation of GLUT4 glucose transporters to the plasma membrane. Studies have shown the existence of two major signaling pathways that lead to the translocation of GLUT4. The first, and widely investigated, is the insulin activated signaling pathway through insulin receptor substrate-1 and phosphatidylinositol 3-kinase. The second is the insulin-independent signaling pathway, which is activated by contractions. Individuals with type 2 diabetes mellitus have reduced insulin-stimulated glucose uptake in skeletal muscle due to the phenomenon of insulin resistance. However, those individuals have normal glucose uptake during exercise. In this context, physical exercise is one of the most important interventions that stimulates glucose uptake by insulin-independent pathways, and the main molecules involved are adenosine monophosphate-activated protein kinase, nitric oxide, bradykinin, AKT, reactive oxygen species and calcium. In this review, our main aims were to highlight the different glucose uptake pathways and to report the effects of physical exercise, diet and drugs on their functioning. Lastly, with the better understanding of these pathways, it would be possible to assess, exactly and molecularly, the importance of physical exercise and diet on glucose homeostasis. Furthermore, it would be possible to assess the action of drugs that might optimize glucose uptake and consequently be an important step in controlling the blood glucose levels in diabetic patients, in addition to being important to clarify some pathways that justify the development of drugs capable of mimicking the contraction pathway.

Akt1 regulates vascular smooth muscle cell apoptosis through FoxO3a and Apaf1 and protects against arterial remodeling and atherosclerosis

OBJECTIVE

Vascular smooth muscle cell (VSMC) apoptosis occurs at low levels in atherosclerotic plaques and in vessel remodeling; however, the consequences and mediators of these levels are not known. Akt1 protects against VSMC apoptosis largely through inactivating target proteins such as forkhead class O transcription factor 3a (FoxO3a), but Akt1 signaling is reduced and FoxO3a activity is increased in human atherosclerosis. We therefore sought to determine whether inhibition of VSMC apoptosis via Akt1 activation regulates vessel remodeling and atherogenesis and to identify FoxO3a target proteins that mediate VSMC apoptosis.

APPROACH AND RESULTS

We generated mice that express an Akt1 protein that can be activated specifically in arterial VSMCs. Akt1 activation did not affect normal arteries, but inhibited VSMC apoptosis and negative remodeling after carotid ligation, indicating that VSMC apoptosis is a major determinant of vessel caliber after changes in flow. Akt1 activation inhibited VSMC apoptosis during atherogenesis and increased relative fibrous cap area in plaques. Microarray studies identified multiple FoxO3a-regulated genes involved in VSMC apoptosis, including apoptotic protease activating factor 1 as a novel target. Apoptotic protease activating factor 1 mediated the proapoptotic activity of FoxO3a, was increased in human atherosclerosis, but reduced by Akt1 activity in vivo.

CONCLUSIONS

Akt1 is a major regulator of VSMC survival in vivo during vessel remodeling and atherogenesis, mediated in large part through inhibition of FoxO3a and its downstream genes, including apoptotic protease activating factor 1. Our data suggest that even the low-level VSMC apoptosis seen during changes in flow determines vessel wall structure and promotes fibrous cap thinning during atherogenesis.

PI3K/Akt1 signalling specifies foregut precursors by generating regionalized extra-cellular matrix

During embryonic development signalling pathways act repeatedly in different contexts to pattern the emerging germ layers. Understanding how these different responses are regulated is a central question for developmental biology. In this study, we used mouse embryonic stem cell (mESC) differentiation to uncover a new mechanism for PI3K signalling that is required for endoderm specification. We found that PI3K signalling promotes the transition from naïve endoderm precursors into committed anterior endoderm. PI3K promoted commitment via an atypical activity that delimited epithelial-to-mesenchymal transition (EMT). Akt1 transduced this activity via modifications to the extracellular matrix (ECM) and appropriate ECM could itself induce anterior endodermal identity in the absence of PI3K signalling. PI3K/Akt1-modified ECM contained low levels of Fibronectin (Fn1) and we found that Fn1 dose was key to specifying anterior endodermal identity in vivo and in vitro. Thus, localized PI3K activity affects ECM composition and ECM in turn patterns the endoderm.

DOI:http://dx.doi.org/10.7554/eLife.00806.001

From conception to birth, a single fertilised egg will multiply into trillions of cells, with each cell becoming one of the 200 or so different types of cell that are found in the human body. The development of an embryo is complex and dynamic, with cells giving up their ability to become any cell type and committing to becoming a specific cell type within a given tissue. At the same time, different groups of cells migrate to the appropriate locations within the developing embryo. Although it is challenging to decipher the roles of the individual signalling pathways that control an embryo’s development, several important components have been found.

Fibroblast growth factor (FGF) is a protein that regulates the formation of the endoderm: this is the innermost of the three layers of cells that form in the early embryo, and it gives rise to internal organs such as the gut, liver and pancreas. As well as ‘telling’ cells to become the front part, or anterior, of the endoderm, FGF also controls the migration of these cells within the embryo. However, uncoupling these two roles has been a major challenge, and the molecular mechanisms behind them are unclear.

Now, Villegas et al. have discovered that FGF activates a signalling cascade involving two enzymes called PI3K and Akt1. In lab-grown embryonic stem cells—cells that can be coaxed to become any of the cell types formed during development—this signalling cascade is essential for FGF to trigger differentiation of the cell types found in the anterior endoderm. The PI3K/Akt1 signalling cascade achieves this by reducing the level of a protein called fibronectin in the ‘extracellular matrix’ that surrounds the cells. This low level of fibronectin will in turn induce cells to stick together in an organized layer; and this rearrangement of cell-cell and cell-matrix interactions appears linked to triggering the differentiation of anterior endoderm cell types.

Villegas et al. showed that the PI3K/Akt1 pathway was also essential for endoderm formation in living mouse embryos. As a normal embryo develops, the anterior endoderm cells move into a ‘groove’ at the front the embryo, where the level of fibronectin is lower than it is at the posterior end of the embryo.

These findings highlight the importance of the extracellular matrix in the regulation of embryonic development, and should assist in the effort to turn lab-grown stem cells into the useful cell types found in internal organs.

DOI:http://dx.doi.org/10.7554/eLife.00806.002

MicroRNA-based discovery of barriers to dedifferentiation of fibroblasts to pluripotent stem cells

Individual microRNAs (miRNAs) can target hundreds of messenger RNAs forming networks of presumably cooperating genes. To test this presumption, we functionally screened miRNAs and their targets in the context of de-differentiation of mouse fibroblasts to induced pluripotent stem cells (iPSCs). Along with the miR-302/miR-294 family, the miR-181 family arose as a novel enhancer of the initiation phase of reprogramming. Endogenous miR-181 miRNAs were transiently elevated with introduction of Oct4, Sox2, and Klf4 (OSK), and their inhibition diminished iPSC colony formation. We tested the functional contribution of 114 individual targets of the two families, revealing twenty-five genes that normally suppress initiation. Co-inhibition of targets cooperatively promoted both the frequency and kinetics of OSK reprogramming. These data establish two of the largest functionally defined networks of miRNA-mRNA interactions, elucidating novel relationships among genes that act together to suppress early stages of reprogramming.

The Cdk inhibitor flavopiridol enhances temozolomide-induced cytotoxicity in human glioma cells

The recent progress in chemotherapy for malignant gliomas is attributable to the introduction of the DNA-methylating agent temozolomide (TMZ); however, drug resistance remains a major issue. Previous studies have shown that TMZ induces prolonged arrest of human glioma cells in the G2/M phase of the cell cycle followed by a senescence-like phenomenon or mitotic catastrophe. These findings suggest that the G2 checkpoint is linked to DNA repair mechanisms. We investigated the effect of a cyclin-dependent kinase (Cdk) inhibitor flavopiridol (FP) that inhibits the action of Cdc2, a key protein in the G2 checkpoint pathway, on TMZ-treated glioma cells. Colony formation efficiency revealed that FP potentiated the cytotoxicity of TMZ in glioma cells in a p53-independent manner. This effect was clearly associated with the suppression of key proteins at the G2-M transition, accumulation of the cells exclusively at the G2 phase, and increase in a double-stranded DNA break marker (seen on performing immunoblotting). TMZ-resistant clones showed activation of the G2 checkpoint in response to TMZ, while FP treatment resensitized these clones to TMZ. FP also enhanced the cytotoxicity of TMZ in U87MG-AktER cells. Moreover, administration of TMZ and/or FP to nude mice with xenografted U87MG cells revealed that FP sensitized xenografted U87MG cells to TMZ in these mice. Our findings suggest that TMZ resistance could be promoted by enhanced DNA repair activity in the G2-M transition and that a Cdk inhibitor could suppress this activity, leading to potentiation of TMZ action on glioma cells.

Interleukin-10 inhibits lipopolysaccharide induced miR-155 precursor stability and maturation

The anti-inflammatory cytokine interleukin-10 (IL-10) is essential for attenuating the inflammatory response, which includes reducing the expression of pro-inflammatory microRNA-155 (miR-155) in lipopolysaccharide (LPS) activated macrophages. miR-155 enhances the expression of pro-inflammatory cytokines such as TNFα and suppresses expression of anti-inflammatory molecules such as SOCS1. Therefore, we examined the mechanism by which IL-10 inhibits miR-155. We found that IL-10 treatment did not affect the transcription of the miR-155 host gene nor the nuclear export of pre-miR-155, but rather destabilized both pri-miR-155 and pre-miR-155 transcripts, as well as interfered with the final maturation of miR-155. This inhibitory effect of IL-10 on miR-155 expression involved the contribution of both the STAT3 transcription factor and the phosphoinositol phosphatase SHIP1. This is the first report showing evidence that IL-10 regulates miRNA expression post-transcriptionally.

4E-BP1 regulates the differentiation of white adipose tissue

4E Binding protein 1 (4E-BP1) suppresses translation initiation. The absence of 4E-BP1 drastically reduces the amount of adipose tissue in mice. To address the role of 4E-BP1 in adipocyte differentiation, we characterized 4E-BP1(-/-) mice in this study. The lack of 4E-BP1 decreased the amount of white adipose tissue and increased the amount of brown adipose tissue. In 4E-BP1(-/-) MEF cells, PPARγ coactivator 1 alpha (PGC-1α) expression increased and exogenous 4E-BP1 expression suppressed PGC-1α expression. The level of 4E-BP1 expression was higher in white adipocytes than in brown adipocytes and showed significantly greater up-regulation in white adipocytes than in brown adipocytes during preadipocyte differentiation into mature adipocytes. The amount of PGC-1α was consistently higher in HB cells (a brown preadipocyte cell line) than in HW cells (a white preadipocyte cell line) during differentiation. Moreover, the ectopic over-expression of 4E-BP1 suppressed PGC-1α expression in white adipocytes, but not in brown adipocytes. Thus, the results of our study indicate that 4E-BP1 may suppress brown adipocyte differentiation and PGC-1α expression in white adipose tissues.

Leptin, resistin and visfatin: the missing link between endocrine metabolic disorders and immunity

Adipose tissue is still regarded as a principle site for lipid storage and mobilizing tissue with an important role in the control of energy homeostasis. Additionally, adipose tissue-secreted hormones such as leptin, visfatin, resistin, apelin, omentin, sex steroids, and various growth factors are now regarded as a functional part of the endocrine system. These hormones also play an important role in the immune system. Several in vitro and in vivo studies have suggested the complex role of adipocyte-derived hormones in immune system and inflammation. Adipokines mediate beneficial and detrimental effects in immunity and inflammation. Many of these adipocytokines have a physiological role in metabolism. The uncontrolled secretions of several adipocytokines were associated with the stimulation of inflammatory processes leading to metabolic disorders including obesity, atherosclerosis, insulin resistance and type 2 diabetes. Obesity leads to the dysfunction of adipocytes andcorrelated with the imbalance of adipokines levels. In obese and diabetic conditions, leptin deficiency inhibited the Jak/Stat3/PI3K and insulin pathways. In this review, ample evidence exists to support the recognition of the adipocyte's role in various tissues and pathologies. New integral insights may add dimensions to translate any potential agents into the future clinical armamentarium of chronic endocrine metabolic and inflammatory diseases. Functional balance of both adipocytes and immune cells is important to exert their effects on endocrine metabolic disorders; furthermore, adipose tissue should be renamed not only as a functional part of the endocrine system but also as a new part of the immune system.

Bax exists in a dynamic equilibrium between the cytosol and mitochondria to control apoptotic priming

The proapoptotic Bcl-2 protein Bax is predominantly found in the cytosol of nonapoptotic cells and is commonly thought to translocate to mitochondria following an apoptotic stimulus. The current model for Bax activation is that BH3 proteins bind to cytosolic Bax, initiating mitochondrial targeting and outer-membrane permeabilization. Here, we challenge this and show that Bax is constitutively targeted to mitochondria but in nonapoptotic cells is constantly translocated back to the cytosol. Using live-cell spinning-disk confocal imaging with a combination of FLIP, FRAP, and photoactivatable GFP-Bax, we demonstrate that disrupting adhesion-dependent survival signals slows the rate of Bax’s dissociation from mitochondria, leading to its accumulation on the outer mitochondrial membrane. The overall accumulation of mitochondrial Bax following loss of survival signaling sensitizes cells to proapoptotic BH3 proteins. Our findings show that Bax is normally in a dynamic equilibrium between cytosol and mitochondria, enabling fluctuations in survival signals to finely adjust apoptotic sensitivity.

GLUT4 exocytosis

GLUT4 is an insulin-regulated glucose transporter that is responsible for insulin-regulated glucose uptake into fat and muscle cells. In the absence of insulin, GLUT4 is mainly found in intracellular vesicles referred to as GLUT4 storage vesicles (GSVs). Here, we summarise evidence for the existence of these specific vesicles, how they are sequestered inside the cell and how they undergo exocytosis in the presence of insulin. In response to insulin stimulation, GSVs fuse with the plasma membrane in a rapid burst and in the continued presence of insulin GLUT4 molecules are internalised and recycled back to the plasma membrane in vesicles that are distinct from GSVs and probably of endosomal origin. In this Commentary we discuss evidence that this delivery process is tightly regulated and involves numerous molecules. Key components include the actin cytoskeleton, myosin motors, several Rab GTPases, the exocyst, SNARE proteins and SNARE regulators. Each step in this process is carefully orchestrated in a sequential and coupled manner and we are beginning to dissect key nodes within this network that determine vesicle-membrane fusion in response to insulin. This regulatory process clearly involves the Ser/Thr kinase AKT and the exquisite manner in which this single metabolic process is regulated makes it a likely target for lesions that might contribute to metabolic disease.

Akt and ERK control the proliferative response of mammary epithelial cells to the growth factors IGF-1 and EGF through the cell cycle inhibitor p57Kip2

Epithelial cells respond to growth factors including epidermal growth factor (EGF), insulin-like growth factor 1 (IGF-1), and insulin. Using high-content immunofluorescence microscopy, we quantitated differences in signaling networks downstream of EGF, which stimulated proliferation of mammary epithelial cells, and insulin or IGF-1, which enhanced the proliferative response to EGF but did not stimulate proliferation independently. We found that the abundance of the cyclin-dependent kinase inhibitors p21Cip1 and p57Kip2 increased in response to IGF-1 or insulin but decreased in response to EGF. Depletion of p57Kip2, but not p21Cip1, rendered IGF-1 or insulin sufficient to induce cellular proliferation in the absence of EGF. Signaling through the PI3K (phosphatidylinositol 3-kinase)-Akt-mTOR (mammalian target of rapamycin) pathway was necessary and sufficient for the increase in p57Kip2, whereas MEK [mitogen-activated or extracellular signal-regulated protein kinase (ERK) kinase]-ERK activity suppressed this increase, forming a regulatory circuit that limited proliferation in response to unaccompanied Akt activity. Knockdown of p57Kip2 enhanced the proliferative phenotype induced by tumor-associated PI3K mutant variants and released mammary epithelial acini from growth arrest during morphogenesis in three-dimensional culture. These results provide a potential explanation for the context-dependent proliferative activities of insulin and IGF-1 and for the finding that the CDKN1C locus encoding p57Kip2 is silenced in many breast cancers, which frequently show hyperactivation of the PI3K pathway. The status of p57Kip2 may thus be an important factor to assess when considering targeted therapy against the ERK or PI3K pathways.

Inhibition of SAPK2/p38 enhances sensitivity to mTORC1 inhibition by blocking IRES-mediated translation initiation in glioblastoma

A variety of mechanisms confer hypersensitivity of tumor cells to the macrolide rapamycin, the prototypic mTORC1 inhibitor. Several studies have shown that the status of the AKT kinase plays a critical role in determining hypersensitivity. Cancer cells in which AKT activity is elevated are exquisitely sensitive to mTORC1 inhibitors while cells in which the kinase is quiescent are relatively resistant. Our previous work has shown that a transcript-specific protein synthesis salvage pathway is operative in cells with quiescent AKT levels, maintaining the translation of crucial mRNAs involved in cell-cycle progression in the face of global eIF-4E-mediated translation inhibition. The activation of this salvage pathway is dependent on SAPK2/p38-mediated activation of IRES-dependent initiation of the cyclin D1 and c-MYC mRNAs, resulting in the maintenance of their protein expression levels. Here, we show that both genetic and pharmacologic inhibition of SAPK2/p38 in glioblastoma multiforme cells significantly reduces rapamycin-induced IRES-mediated translation initiation of cyclin D1 and c-MYC, resulting in increased G(1) arrest in vitro and inhibition of tumor growth in xenografts. Moreover, we observed that the AKT-dependent signaling alterations seen in vitro are also displayed in engrafted tumors cells and were able to show that combined inhibitor treatments markedly reduced the mRNA translational state of cyclin D1 and c-MYC transcripts in tumors isolated from mice. These data support the combined use of SAPK2/p38 and mTORC1 inhibitors to achieve a synergistic antitumor therapeutic response, particularly in rapamycin-resistant quiescent AKT-containing cells.

Proline-rich Akt substrate of 40kDa (PRAS40): a novel downstream target of PI3k/Akt signaling pathway

Modifications in signaling of the proline-rich Akt substrate of 40-kDa (PRAS40) pathway is implicated in type 2 diabetes and melanoma. PRAS40 is known for its ability to regulate the mammalian target of rapamycin complex 1 (mTORC1) kinase activity, possessing a key regulatory role at the cross point of signal transduction pathways activated by growth factor receptors. Recently it has been found that PRAS40 is regulated by its upstream phosphatidylinositol 3-kinase/Akt (PI3K/Akt) which is activated by many tyrosine kinase receptors growth factors including insulin-like growth factor 1. Also, PRAS40 functions downstream of mTORC1 and upstream from its effectors ribosomal protein S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1). Phosphorylation of PRAS40 by Akt and mTORC1 disrupts the binding between mTORC1 and PRAS40, and relieves the inhibitory constraint of PRAS40 on mTORC1 activity. This review summarizes the signaling regulating PRAS40 phosphorylation, as well as the dual function of PRAS40 as substrate and inhibitor of mTORC1 upon growth factor stimulation and under pathophysiological conditions.

Focal adhesion kinase functions as an akt downstream target in migration of colorectal cancer cells

Migration is a complex process that, besides its various physiological functions in embryogenesis and adult tissues, plays a crucial role in cancer cell invasion and metastasis. The focus of this study is the involvement and collaboration of Akt, focal adhesion kinase (FAK), and Src kinases in migration and invasiveness of colorectal cancer cells. We show that all three kinases can be found in one protein complex; nevertheless, the interaction between Akt and Src is indirect and mediated by FAK. Interestingly, induced Akt signaling causes an increase in tyrosine phosphorylation of FAK, but this increase is attenuated by the Src inhibitor SU6656. We also show that active Akt strongly stimulates cell migration, but this phenomenon is fully blocked by FAK knockdown or partly by inhibition of Src kinase. In addition, we found that all three kinases were indispensable for the successful invasion of colorectal cancer cells. Altogether, the presented data bring new insights into the mechanism how the phosphatidylinositol-3-kinase (PI3-K)/Akt pathway can influence migration of colorectal adenocarcinoma cells. Because FAK is indispensable for cell movements and functions downstream of Akt, our results imply FAK kinase as a potential key molecule during progression of tumors with active PI3-K/Akt signaling.

Phosphomimetic substitution of heterogeneous nuclear ribonucleoprotein A1 at serine 199 abolishes AKT-dependent internal ribosome entry site-transacting factor (ITAF) function via effects on strand annealing and results in mammalian target of rapamycin complex 1 (mTORC1) inhibitor sensitivity

The relative activity of the AKT kinase has been demonstrated to be a major determinant of sensitivity of tumor cells to mammalian target of rapamycin (mTOR) complex 1 inhibitors. Our previous studies have shown that the multifunctional RNA-binding protein heterogeneous nuclear ribonucleoprotein (hnRNP) A1 regulates a salvage pathway facilitating internal ribosome entry site (IRES)-dependent mRNA translation of critical cellular determinants in an AKT-dependent manner following mTOR inhibitor exposure. This pathway functions by stimulating IRES-dependent translation in cells with relatively quiescent AKT, resulting in resistance to rapamycin. However, the pathway is repressed in cells with elevated AKT activity, rendering them sensitive to rapamycin-induced G(1) arrest as a result of the inhibition of global eIF-4E-mediated translation. AKT phosphorylation of hnRNP A1 at serine 199 has been demonstrated to inhibit IRES-mediated translation initiation. Here we describe a phosphomimetic mutant of hnRNP A1 (S199E) that is capable of binding both the cyclin D1 and c-MYC IRES RNAs in vitro but lacks nucleic acid annealing activity, resulting in inhibition of IRES function in dicistronic mRNA reporter assays. Utilizing cells in which AKT is conditionally active, we demonstrate that overexpression of this mutant renders quiescent AKT-containing cells sensitive to rapamycin in vitro and in xenografts. We also demonstrate that activated AKT is strongly correlated with elevated Ser(P)(199)-hnRNP A1 levels in a panel of 22 glioblastomas. These data demonstrate that the phosphorylation status of hnRNP A1 serine 199 regulates the AKT-dependent sensitivity of cells to rapamycin and functionally links IRES-transacting factor annealing activity to cellular responses to mTOR complex 1 inhibition.

Mechanisms of HIV-1 Nef function and intracellular signaling

Advances in the last several years have enhanced mechanistic understanding of Nef-induced CD4 and MHCI downregulation and have suggested a new paradigm for analyzing Nef function. In both of these cases, Nef acts by forming ternary complexes with significant contributions to stability imparted by non-canonical interactions. The mutational analyses and binding assays that have led to these conclusions are discussed. The recent progress has been dependent on conservative mutations and multi-protein binding assays. The poorly understood Nef functions of p21 activated protein kinase (PAK2) activation, enhancement of virion infectivity, and inhibition of immunoglobulin class switching are also likely to involve ternary complexes and non-canonical interactions. Hence, investigation of these latter Nef functions should benefit from a similar approach. Six historically used alanine substitutions for determining structure-function relationships of Nef are discussed. These are M20A, E62A/E63A/E64A/E65A (AAAA), P72A/P75A (AXXA), R106A, L164A/L165A, and D174A/D175A. Investigations of less-disruptive mutations in place of AAAA and AXXA have led to different interpretations of mechanism. Two recent examples of this alternate approach, F191I for studying PAK2 activation and D123E for the critical residue D123 are discussed. The implications of the new findings and the resulting new paradigm for Nef structure-function are discussed with respect to creating a map of Nef functions on the protein surface. We report the results of a PPI-Pred analysis for protein-protein interfaces. There are three predicted patches produced by the analysis which describe regions consistent with the currently known mutational analyses of Nef function.

Genetic ablation of S6-kinase does not prevent processing of SREBP1

The SREBP family of transcription factors regulates the expression of genes involved in fatty acid and cholesterol biosynthesis. The activation of SREBP transcription factors requires proteolytic cleavage of the inactive precursor and nuclear translocation of the mature form of the protein. It has been shown that nuclear accumulation of the mature form of SREBP1 is induced in response to activation of the serine/threonine kinase Akt, an important effector of the Ras/PI3-kinase signalling pathway. Activation of SREBP by Akt depends on the mammalian target of rapamycin complex 1 (mTORC1) but the exact mechanism of this activation remains unclear. We have investigated whether ablation of different signalling molecules downstream of mTORC1 affects expression of SREBP targets genes. We could show that inhibition of S6-kinases 1 and 2 expression using RNA interference did not block induction of expression of fatty acid synthase (FASN) or ATP-citrate lyase (ACLY) following activation of Akt in human retinal pigment epithelial cells. Furthermore, accumulation of mature SREBP1 was not inhibited after combined silencing of S6-kinases 1 and 2. Genetic ablation of both kinases also did not prevent the formation of mature SREBP1 in mouse embryonic fibroblasts. Taken together, these results suggest that S6-kinases 1 and 2 are dispensable for the induction of SREBP processing in the experimental systems used here.

Rac controls PIP5K localisation and PtdIns(4,5)P2 synthesis, which modulates vinculin localisation and neurite dynamics

In N1E-115 cells, neurite retraction induced by neurite remodelling factors such as lysophosphatidic acid, sphingosine 1-phosphate and semaphorin 3A require the activity of phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks). PIP5Ks synthesise the phosphoinositide lipid second messenger phosphatidylinositol(4,5)bisphosphate [PtdIns(4,5)P2], and overexpression of active PIP5K is sufficient to induce neurite retraction in both N1E-115 cells and cerebellar granule neurones. However, how PIP5Ks are regulated or how they induce neurite retraction is not well defined. Here, we show that neurite retraction induced by PIP5Kβ is dependent on its interaction with the low molecular weight G protein Rac. We identified the interaction site between PIP5Kβ and Rac1 and generated a point mutant of PIP5Kβ that no longer interacts with endogenous Rac. Using this mutant, we show that Rac controls the plasma membrane localisation of PIP5Kβ and thereby the localised synthesis of PtdIns(4,5)P2 required to induce neurite retraction. Mutation of this residue in other PIP5K isoforms also attenuates their ability to induce neurite retraction and to localise at the membrane. To clarify how increased levels of PtdIns(4,5)P2 induce neurite retraction, we show that mutants of vinculin that are unable to interact with PtdIns(4,5)P2, attenuate PIP5K- and LPA-induced neurite retraction. Our findings support a role for PtdIns(4,5)P2 synthesis in the regulation of vinculin localisation at focal complexes and ultimately in the regulation of neurite dynamics.

Resiliency and vulnerability in the HER2-HER3 tumorigenic driver

About 25% of breast cancers harbor the amplified oncogene human epidermal growth factor receptor 2 (HER2) and are dependent on HER2 kinase function, identifying HER2 as a vulnerable target for therapy. However, HER2-HER3 signaling is buffered so that it is protected against a nearly two-log inhibition of HER2 catalytic activity; this buffering is driven by the negative regulation of HER3 by Akt. We have now further characterized HER2-HER3 signaling activity and have shown that the compensatory buffering prevents apoptotic tumor cell death from occurring as a result of the combined loss of mitogen-activated protein kinase (MAPK) and Akt signaling. To overcome the cancer cells' compensatory mechanisms, we coadministered a phosphoinositide 3-kinase-mammalian target of rapamycin inhibitor and a HER2 tyrosine kinase inhibitor (TKI). This treatment strategy proved equivocal because it induced both TKI-sensitizing and TKI-desensitizing effects and robust cross-compensation of MAPK and Akt signaling pathways. Noting that HER2-HER3 activity was completely inhibited by higher, fully inactivating doses of TKI, we then attempted to overcome the cells' compensatory buffering with this higher dose. This treatment crippled all downstream signaling and induced tumor apoptosis. Although such high doses of TKI are toxic in vivo when given continuously, we found that intermittent doses of TKI administered to mice produced sequential cycles of tumor apoptosis and ultimately complete tumor regression in mouse models, with little toxicity. This strategy for inactivation of HER2-HER3 tumorigenic activity is proposed for clinical testing.

The Akt-SREBP nexus: cell signaling meets lipid metabolism

Phosphatidylinositol 3'-kinase (PI3K) and Akt are signaling kinases involved in cell survival and proliferation. Recent evidence suggests that PI3K/Akt activates the sterol-regulatory element-binding proteins (SREBPs), master transcriptional regulators of lipid metabolism. The precise molecular mechanisms are controversial and differ between SREBP isoforms; proposed mechanisms include increased trafficking and processing of SREBP, reduced degradation, and involvement of the downstream signaling hub, mammalian target of rapamycin complex 1 (mTORC1). In this report, we explore the various mechanistic links between Akt and SREBP. We consider this relationship in diseases where Akt and lipids play crucial roles, including diabetes, viral infections and cancer, suggesting that this Akt-SREBP link provides fresh insights into human health and disease.

Hypoxia converts the myogenic action of insulin-like growth factors into mitogenic action by differentially regulating multiple signaling pathways

Insulin-like growth factors (IGFs) stimulate myoblast proliferation and differentiation. It remains elusive how these mutually exclusive cellular responses are elicited by the same growth factor. Here we report that whereas IGF promotes myoblast differentiation under normoxia, it stimulates proliferation under hypoxia. Hypoxia activates the HIF-1 transcriptional program and knockdown of HIF-1alpha changes the mitogenic action of IGF into myogenic action under hypoxia. Conversely, overexpression of HIF-1alpha abolishes the myogenic effect of IGF under normoxia. Under normoxia, IGF activates the Akt-mTOR, p38, and Erk1/2 MAPK pathways. Hypoxia suppresses basal and IGF-induced Akt-mTOR and p38 activity, whereas it enhances and prolongs IGF-induced Erk1/2 activation in a HIF-1-dependent fashion. Activation of Akt-mTOR and p38 promotes myogenesis, and p38 also inhibits proliferation. Activation of Erk stimulates myoblast proliferation but inhibits differentiation. These results suggest that hypoxia converts the myogenic action of IGFs into mitogenic action by differentially regulating multiple signaling pathways via HIF-1-dependent mechanisms. Our findings provide a mechanistic explanation for the paradoxical actions of IGFs during myogenesis and reveal a novel mechanism by which cells sense and integrate growth factor signals and oxygen availability in their microenvironments.

Extracellular growth factors and mitogens cooperate to drive mitochondrial biogenesis

Cells generate new organelles when stimulated by extracellular factors to grow and divide; however, little is known about how growth and mitogenic signalling pathways regulate organelle biogenesis. Using mitochondria as a model organelle, we have investigated this problem in primary Schwann cells, for which distinct factors act solely as mitogens (neuregulin) or as promoters of cell growth (insulin-like growth factor 1; IGF1). We find that neuregulin and IGF1 act synergistically to increase mitochondrial biogenesis and mitochondrial DNA replication, resulting in increased mitochondrial density in these cells. Moreover, constitutive oncogenic Ras signalling results in a further increase in mitochondrial density. This synergistic effect is seen at the global transcriptional level, requires both the ERK and phosphoinositide 3-kinase (PI3K) signalling pathways and is mediated by the transcription factor ERRalpha. Interestingly, the effect is independent of Akt-TOR signalling, a major regulator of cell growth in these cells. This separation of the pathways that drive mitochondrial biogenesis and cell growth provides a mechanism for the modulation of mitochondrial density according to the metabolic requirements of the cell.

PI3-kinase-dependent activation of apoptotic machinery occurs on commitment of epidermal keratinocytes to terminal differentiation

We have investigated the earliest events in commitment of human epidermal keratinocytes to terminal differentiation. Phosphorylated Akt and caspase activation were detected in cells exiting the basal layer of the epidermis. Activation of Akt by retroviral transduction of primary cultures of human keratinocytes resulted in an increase in abortive clones founded by transit amplifying cells, while inhibition of the upstream kinase, PI3-kinase, inhibited suspension-induced terminal differentiation. Caspase inhibition also blocked differentiation, the primary mediator being caspase 8. Caspase activation was initiated by 2 h in suspension, preceding the onset of expression of the terminal differentiation marker involucrin by several hours. Incubation of suspended cells with fibronectin or inhibition of PI3-kinase prevented caspase induction. At 2 h in suspension, keratinocytes that had become committed to terminal differentiation had increased side scatter, were 7-aminoactinomycin D (7-AAD) positive and annexin V negative; they exhibited loss of mitochondrial membrane potential and increased cardiolipin oxidation, but with no increase in reactive oxygen species. These properties indicate that the onset of terminal differentiation, while regulated by PI3-kinase and caspases, is not a classical apoptotic process.

Effects of Akt signaling on nuclear reprogramming

Reprogramming of the epigenetic state from differentiated to pluripotent cells can be attained by cell fusion of differentiated somatic cells with embryonic stem (ES) cells or transfer of the nucleus of a differentiated cell into an enucleated oocyte. Activation of Akt signaling is sufficient to maintain pluripotency of ES cells and promotes derivation of embryonic germ (EG) cells from primordial germ cells (PGCs). Here we analyzed the effects of Akt signaling on somatic cell nuclear reprogramming after cell fusion and nuclear transfer. We found that forced activation of Akt signaling stimulated reprogramming after cell fusion of ES cells with thymocytes or mouse embryonic fibroblasts. These hybrid cells showed ES cell characteristics, including in vitro and in vivo differentiation capacity. In contrast, Akt signaling significantly reduced the efficiency of reprogramming with nuclear transfer. Our results demonstrate that Akt signaling plays important roles on the nuclear reprogramming of somatic cells.

Glycogen synthase kinase 3 in MLL leukaemia maintenance and targeted therapy

Glycogen synthase kinase 3 (GSK3) is a multifunctional serine/threonine kinase that participates in numerous signalling pathways involved in diverse physiological processes. Several of these pathways are implicated in disease pathogenesis, which has prompted efforts to develop GSK3-specific inhibitors for therapeutic applications. However, before now, there has been no strong rationale for targeting GSK3 in malignancies. Here we report pharmacological, physiological and genetic studies that demonstrate an oncogenic requirement for GSK3 in the maintenance of a specific subtype of poor prognosis human leukaemia, genetically defined by mutations of the MLL proto-oncogene. In contrast to its previously characterized roles in suppression of neoplasia-associated signalling pathways, GSK3 paradoxically supports MLL leukaemia cell proliferation and transformation by a mechanism that ultimately involves destabilization of the cyclin-dependent kinase inhibitor p27(Kip1). Inhibition of GSK3 in a preclinical murine model of MLL leukaemia provides promising evidence of efficacy and earmarks GSK3 as a candidate cancer drug target.

IGFBP-5 regulates muscle cell differentiation by binding to IGF-II and switching on the IGF-II auto-regulation loop

IGF-II stimulates both mitogenesis and myogenesis through its binding and activation of the IGF-I receptor (IGF-IR). How this growth factor pathway promotes these two opposite cellular responses is not well understood. We investigate whether local IGF binding protein-5 (IGFBP-5) promotes the myogenic action of IGF-II. IGFBP-5 is induced before the elevation of IGF-II expression during myogenesis. Knockdown of IGFBP-5 impairs myogenesis and suppresses IGF-II gene expression. IGF-II up-regulates its own gene expression via the PI3K-Akt signaling pathway. Adding IGF-II or constitutively activating Akt rescues the IGFBP-5 knockdown-caused defects. However, an IGF analogue that binds to the IGF-IR but not IGFBP has only a limited effect. When added with low concentrations of IGF-II, IGFBP-5 restores IGF-II expression and myogenic differentiation, whereas an IGF binding–deficient IGFBP-5 mutant has no effect. These findings suggest that IGFBP-5 promotes muscle cell differentiation by binding to and switching on the IGF-II auto-regulation loop.

Beta1 integrin inhibition dramatically enhances radiotherapy efficacy in human breast cancer xenografts

Beta(1) integrin signaling has been shown to mediate cellular resistance to apoptosis after exposure to ionizing radiation (IR). Other signaling molecules that increase resistance include Akt, which promotes cell survival downstream of beta(1) integrin signaling. We previously showed that beta(1) integrin inhibitory antibodies (e.g., AIIB2) enhance apoptosis and decrease growth in human breast cancer cells in three-dimensional laminin-rich extracellular matrix (lrECM) cultures and in vivo. Here, we asked whether AIIB2 could synergize with IR to modify Akt-mediated IR resistance. We used three-dimensional lrECM cultures to test the optimal combination of AIIB2 with IR treatment of two breast cancer cell lines, MCF-7 and HMT3522-T4-2, as well as T4-2 myr-Akt breast cancer colonies or HMT3522-S-1, which form normal organotypic structures in three-dimensional lrECM. Colonies were assayed for apoptosis and beta(1) integrin/Akt signaling pathways were evaluated using Western blot. In addition, mice bearing MCF-7 xenografts were used to validate the findings in three-dimensional lrECM. We report that AIIB2 increased apoptosis optimally post-IR by down-regulating Akt in breast cancer colonies in three-dimensional lrECM. In vivo, addition of AIIB2 after IR significantly enhanced tumor growth inhibition and apoptosis compared with either treatment alone. Remarkably, the degree of tumor growth inhibition using AIIB2 plus 2 Gy radiation was similar to that of 8 Gy alone. We previously showed that AIIB2 had no discernible toxicity in mice; here, its addition allowed for a significant reduction in the IR dose that was necessary to achieve comparable growth inhibition and apoptosis in breast cancer xenografts in vivo.

SLP-65 regulates immunoglobulin light chain gene recombination through the PI(3)K-PKB-Foxo pathway

Although the essential role of the adaptor protein SLP-65 in pre-B cell differentiation is established, the molecular mechanism underlying its function is poorly understood. In this study, we uncover a link between SLP-65-dependent signaling and the phosphoinositide-3-OH kinase (PI(3)K)-protein kinase B (PKB)-Foxo pathway. We show that the forkhead box transcription factor Foxo3a promotes light chain rearrangement in pre-B cells. Our data suggest that PKB suppresses light chain recombination by phosphorylating Foxo proteins, whereas reconstitution of SLP-65 function counteracts PKB activation and promotes Foxo3a and Foxo1 activity in pre-B cells. Together, these data illuminate a molecular function of SLP-65 and identify a key role for Foxo proteins in the regulation of light chain recombination, receptor editing and B cell selection.