Dysregulation of the TSC-mTOR pathway in human disease

The effects of proliferation and DNA damage on hematopoietic stem cell function determine aging

In most of the mammalian tissues, homeostasis as well as injury repair depend upon a small number of resident adult stem cells. The decline in tissue/organ function in aged organisms has been directly linked with poorly functioning stem cells. Altered function of hematopoietic stem cells (HSCs) is at the center of an aging hematopoietic system, a tissue with high cellular turnover. Poorly engrafting, myeloid-biased HSCs with higher levels of DNA damage accumulation are the hallmark features of an aged hematopoietic system. These cells show a higher proliferation rate than their younger counterparts. It was proposed that quiescence of these cells over long period of time leads to accumulation of DNA damage, eventually resulting in poor function/pathological conditions in hematopoietic system. However, various mouse models with premature aging phenotype also show highly proliferative HSCs. This review examines the evidence that links proliferation of HSCs with aging, which leads to functional changes in the hematopoietic system. Developmental Dynamics 245:739-750, 2016. © 2016 Wiley Periodicals, Inc.

Structural Basis of the Interaction between Tuberous Sclerosis Complex 1 (TSC1) and Tre2-Bub2-Cdc16 Domain Family Member 7 (TBC1D7)

Mutations in TSC1 or TSC2 cause tuberous sclerosis complex (TSC), an autosomal dominant disorder characterized by the occurrence of benign tumors in various vital organs and tissues. TSC1 and TSC2, the TSC1 and TSC2 gene products, form the TSC protein complex that senses specific cellular growth conditions to control mTORC1 signaling. TBC1D7 is the third subunit of the TSC complex, and helps to stabilize the TSC1-TSC2 complex through its direct interaction with TSC1. Homozygous inactivation of TBC1D7 causes intellectual disability and megaencephaly. Here we report the crystal structure of a TSC1-TBC1D7 complex and biochemical characterization of the TSC1-TBC1D7 interaction. TBC1D7 interacts with the C-terminal region of the predicted coiled-coil domain of TSC1. The TSC1-TBC1D7 interface is largely hydrophobic, involving the α4 helix of TBC1D7. Each TBC1D7 molecule interacts simultaneously with two parallel TSC1 helices from two TSC1 molecules, suggesting that TBC1D7 may stabilize the TSC complex by tethering the C-terminal ends of two TSC1 coiled-coils.

Mutations in TBCK, Encoding TBC1-Domain-Containing Kinase, Lead to a Recognizable Syndrome of Intellectual Disability and Hypotonia

Through an international multi-center collaboration, 13 individuals from nine unrelated families and affected by likely pathogenic biallelic variants in TBC1-domain-containing kinase (TBCK) were identified through whole-exome sequencing. All affected individuals were found to share a core phenotype of intellectual disability and hypotonia, and many had seizures and showed brain atrophy and white-matter changes on neuroimaging. Minor non-specific facial dysmorphism was also noted in some individuals, including multiple older children who developed coarse features similar to those of storage disorders. TBCK has been shown to regulate the mammalian target of rapamycin (mTOR) signaling pathway, which is also stimulated by exogenous leucine supplementation. TBCK was absent in cells from affected individuals, and decreased phosphorylation of phospho-ribosomal protein S6 was also observed, a finding suggestive of downregulation of mTOR signaling. Lastly, we demonstrated that activation of the mTOR pathway in response to L-leucine supplementation was retained, suggesting a possible avenue for directed therapies for this condition.

Autophagy Proteins ATG5 and ATG7 Are Essential for the Maintenance of Human CD34(+) Hematopoietic Stem-Progenitor Cells

Autophagy is a highly regulated catabolic process that involves sequestration and lysosomal degradation of cytosolic components such as damaged organelles and misfolded proteins. While autophagy can be considered to be a general cellular housekeeping process, it has become clear that it may also play cell type-dependent functional roles. In this study, we analyzed the functional importance of autophagy in human hematopoietic stem/progenitor cells (HSPCs), and how this is regulated during differentiation. Western blot-based analysis of LC3-II and p62 levels, as well as flow cytometry-based autophagic vesicle quantification, demonstrated that umbilical cord blood-derived CD34(+) /CD38(-) immature hematopoietic progenitors show a higher autophagic flux than CD34(+) /CD38(+) progenitors and more differentiated myeloid and erythroid cells. This high autophagic flux was critical for maintaining stem and progenitor function since knockdown of autophagy genes ATG5 or ATG7 resulted in reduced HSPC frequencies in vitro as well as in vivo. The reduction in HSPCs was not due to impaired differentiation, but at least in part due to reduced cell cycle progression and increased apoptosis. This is accompanied by increased expression of p53, proapoptotic genes BAX and PUMA, and the cell cycle inhibitor p21, as well as increased levels of cleaved caspase-3 and reactive oxygen species. Taken together, our data demonstrate that autophagy is an important regulatory mechanism for human HSCs and their progeny, reducing cellular stress and promoting survival. Stem Cells 2016;34:1651-1663.

Coexistence of systemic lupus erythematosus, tuberous sclerosis and aggressive natural killer-cell leukaemia: coincidence or correlated?

Among patients with systemic lupus erythematosus (SLE) there is an increased risk of haematological malignancies, especially non-Hodgkin lymphoma. However, the association of SLE with aggressive CD3 negative natural killer (NK)-cell leukaemia has not been reported so far. We present a case of a 39-year-old woman with SLE, aggressive NK-cell leukaemia and tuberous sclerosis complex. The prior probability of developing the combination of these three rare diseases by coincidence is extremely low (<10(-13)). Possible underlying immunological, genetic and toxic/environmental pathways are discussed.

Resveratrol Potentiates Growth Inhibitory Effects of Rapamycin in PTEN-deficient Lipoma Cells by Suppressing p70S6 Kinase Activity

Patients with phosphatase and tensin homolog (PTEN) hamartoma tumor syndrome and germline mutations in PTEN frequently develop lipomatosis, for which there is no standard treatment. Rapamycin was shown to reduce the growth of lipoma cells with heterozygous PTEN deficiency in vitro, but concomitantly induced an upregulation of AKT phosphorylation. Since it was shown that resveratrol stabilizes PTEN, we asked whether co-incubation with resveratrol could suppress the rapamycin-induced AKT phosphorylation in PTEN-deficient lipoma cells. Resveratrol incubation resulted in decreased lipoma cell viability by inducing G1-phase cell cycle arrest and apoptosis. PTEN expression and AKT phosphorylation were not significantly changed, whereas p70S6 kinase (p70S6K) phosphorylation was reduced in PTEN-deficient lipoma cells after resveratrol incubation. Rapamycin/resveratrol co-incubation significantly decreased viability further at lower doses of resveratrol and resulted in decreased p70S6K phosphorylation compared to rapamycin incubation alone, suggesting that resveratrol potentiated the growth inhibitory effects of rapamycin by reducing p70S6K activation. Both viability and p70S6K phosphorylation of primary PTEN wild-type preadipocytes were less affected compared to PTEN-deficient lipoma cells by equimolar concentrations of resveratrol. These results support the concept of combining chemopreventive natural compounds with mammalian target of rapamycin (mTOR) inhibitors to increase the efficacy of chemotherapeutic drugs for patients suffering from overgrowth syndromes.

Hypoxia-Inducible Factors (HIFs) and Phosphorylation: Impact on Stability, Localization, and Transactivity

The hypoxia-inducible factor α-subunits (HIFα) are key transcription factors in the mammalian response to oxygen deficiency. The HIFα regulation in response to hypoxia occurs primarily on the level of protein stability due to posttranslational hydroxylation and proteasomal degradation. However, HIF α-subunits also respond to various growth factors, hormones, or cytokines under normoxia indicating involvement of different kinase pathways in their regulation. Because these proteins participate in angiogenesis, glycolysis, programmed cell death, cancer, and ischemia, HIFα regulating kinases are attractive therapeutic targets. Although numerous kinases were reported to regulate HIFα indirectly, direct phosphorylation of HIFα affects HIFα stability, nuclear localization, and transactivity. Herein, we review the role of phosphorylation-dependent HIFα regulation with emphasis on protein stability, subcellular localization, and transactivation.

Hypoxia-Inducible Factor-1: A Critical Player in the Survival Strategy of Stressed Cells

HIF-1 activation has been well known as an adaptive strategy to hypoxia. Recently it became clear that hypoxia was often accompanied by insufficient supply of glucose or amino acids as a common result of poor circulation that frequently occurs in solid tumors and ischemic lesions, creating a mixed nutrient insufficiency. In response to nutrient insufficiency, stressed cells elicit survival strategies including activation of AMPK and HIF-1 to cope with the stress. Particularly, in solid tumors, HIF-1 promotes cell survival and migration, stimulates angiogenesis, and induces resistance to radiation and chemotherapy. Interestingly, radiation and some chemotherapeutics are reported to trigger the activation of AMPK. Here we discuss the recent advances that may potentially link the stress responsive mechanisms including AMPK activation, ATF4 activation and the enhancement of Hsp70/Hsp90 function to HIF-1 activation. Potential implication and application of the stress-facilitated HIF-1 activation in solid tumors and ischemic disorders will be discussed. A better understanding of HIF-1 activation in cells exposed to stresses is expected to facilitate the design of therapeutic approaches that specifically modulate cell survival strategy.

Polymerase delta-interacting protein 2 regulates collagen accumulation via activation of the Akt/mTOR pathway in vascular smooth muscle cells

OBJECTIVES

Polymerase delta interacting protein 2 (Poldip2) has previously been implicated in migration, proliferation and extracellular matrix (ECM) production in vascular smooth muscle cells. To better understand the role of Poldip2 in ECM regulation, we investigated the mechanism responsible for collagen I accumulation in Poldip2(+/-) mouse aortic smooth muscle cells (MASMs).

APPROACH AND RESULTS

Protein degradation and protein synthesis pathways were investigated. Depletion of Poldip2 had no effect on proteasome activity, but caused a partial reduction in autophagic flux. However, the rate of collagen I degradation was increased in Poldip2(+/-) vs. Poldip2(+/+) MASMs. Conversely, activation of the PI3K/Akt/mTOR signaling pathway, involved in regulation of protein synthesis, was significantly elevated in Poldip2(+/-) MASMs as was β1-integrin expression. Suppressing mTOR signaling using Akt inhibitor or rapamycin and reducing β1-integrin expression using siRNA prevented the increase in collagen I production. While collagen I and fibronectin were increased in Poldip2(+/-) MASMs, overall protein synthesis was not different from that in Poldip2(+/)(+)MASMs, suggesting selectivity of Poldip2 for ECM proteins.

CONCLUSIONS

Poldip2(+/-) MASMs exhibit higher β1-integrin expression and activity of the PI3K/Akt/mTOR signaling pathway, leading to increased ECM protein synthesis. These findings have important implications for vascular diseases in which ECM accumulation plays a role.

Transport to Rhebpress activity

The small GTPases from the rat sarcoma (Ras) superfamily are a heterogeneous group of proteins of about 21 kDa that act as molecular switches, modulating cell signaling pathways and controlling diverse cellular processes. They are active when bound to guanosine triphosphate (GTP) and inactive when bound to guanosine diphosphate (GDP). Ras homolog enriched in brain (Rheb) is a member of the Ras GTPase superfamily and a key activator of the mammalian/mechanistic target of rapamycin complex 1 (mTORC1). We recently determined that microspherule protein 1 (MCRS1) maintains Rheb at lysosomal surfaces in an amino acid-dependent manner. MCRS1 depletion promotes the formation of the GDP-bound form of Rheb, which is then delocalized from the lysosomal platform and transported to endocytic recycling vesicles, leading to mTORC1 inactivation. During this delocalization process, Rheb-GDP remains farnesylated and associated with cellular endomembranes. These findings provide new insights into the regulation of small GTPases, whose activity depends on both their GTP/GDP switch state and their capacity to move between different cellular membrane-bound compartments. Dynamic spatial transport between compartments makes it possible to alter the proximity of small GTPases to their activatory sites depending on the prevailing physiological and cellular conditions.

A Case of a Central Conducting Lymphatic Anomaly Responsive to Sirolimus

The study of vascular anomalies is a rapidly progressing field in medicine. The development of new knowledge in the pathology and management of these disease processes are exemplified in the treatment of hemangiomas with propranolol and generalized lymphatic malformations with sirolimus. Central conducting lymphatic anomalies have traditionally been refractory to medical and surgical interventions. We report a case of a central conducting lymphatic anomaly that was responsive to sirolimus. A 14-year-old boy presented with chylothorax and chyluria with a lymphangiogram demonstrating abnormal lymphatic flow and reflux along the entire course of the central channels. Traditionally, medical management has been limited to somatostatin and low-fat diet with poor response and surgical interventions that are palliative. Sirolimus allows a new medical option that could improve management of this unresponsive population.

Cowden Syndrome and Concomitant Pulmonary Neuroendocrine Tumor: A Presentation of Two Cases

Cowden Syndrome is a rare autosomal dominantly inherited disorder. Patients with Cowden Syndrome are at increased risk of various benign and malignant neoplasms in breast, endometrium, thyroid, gastrointestinal tract, and genitourinary system. Neuroendocrine tumors are ubiquitous neoplasms that may occur anywhere in the human body. Bronchopulmonary neuroendocrine tumors include four different histological subtypes, among these, typical and atypical pulmonary carcinoids. No association between Cowden Syndrome and neuroendocrine tumors has previously been described. We present two cases of Cowden Syndrome that were diagnosed with pulmonary carcinoids.

The tuberous sclerosis complex model Eker (TSC2+/-) rat exhibits hyperglycemia and hyperketonemia due to decreased glycolysis in the liver

Tuberous sclerosis complex (TSC) presents as benign tumors that affect the brain, kidneys, lungs and skin. The inactivation of TSC2 gene, through loss of heterozygosity is responsible for tumor development in TSC. Since TSC patients are carriers of heterozygous a TSC2; mutation, to reveal the risk factors which these patients carry prior to tumor development is important. In this experiment, Eker rat which carry a mutation in this TSC2 gene were analyzed for their metabolic changes. Wild-type (TSC2+/+) and heterozygous mutant TSC2 (TSC2+/-) Eker rats were raised for 100 days. As a result, the Eker rats were found to exhibit hyperglycemia and hyperketonemia. However the high ketone body production in the liver was observed without accompanying increased levels of plasma free fatty acids or insulin. Further, production of the ketone body β-hydroxybutyrate was inhibited due to the low NADH/NAD(+) ratio resulting from the restraint on glycolysis, which was followed by inhibition of the malate-aspartate shuttle and TCA cycle. Therefore, we conclude that glycolysis is restrained in the livers of TSC2 heterozygous mutant rats, and these defects lead to abnormal production of acetoacetate.

Phosphorylation of the nuclear receptor corepressor 1 by protein kinase B switches its corepressor targets in the liver in mice

Nuclear receptor corepressor 1 (NCoR1) is a transcriptional coregulator that has wide-ranging effects on gene expression patterns. In the liver, NCoR1 represses lipid synthesis in the fasting state, whereas it inhibits activation of peroxisome proliferator-activated receptor alpha (PPARα) upon feeding, thereby blunting ketogenesis. Here, we show that insulin by activation of protein kinase B induces phosphorylation of NCoR1 on serine 1460, which selectively favors its interaction with PPARα and estrogen-related receptor alpha (ERRα) over liver X receptor alpha (LXRα). Phosphorylation of NCoR1 on S1460 selectively derepresses LXRα target genes, resulting in increased lipogenesis, whereas, at the same time, it inhibits PPARα and ERRα targets, thereby attenuating oxidative metabolism in the liver. Phosphorylation-gated differential recruitment of NCoR1 to different nuclear receptors explains the apparent paradox that liver-specific deletion of NCoR1 concurrently induces both lipogenesis and oxidative metabolism owing to a global derepression of LXRα, PPARα, and ERRα activity.

CONCLUSION

Phosphorylation-mediated recruitment switch of NCoR1 between nuclear receptor subsets provides a mechanism by which corepressors can selectively modulate liver energy metabolism during the fasting-feeding transition.

mTOR in Brain Physiology and Pathologies

TOR (target of rapamycin) and its mammalian ortholog mTOR have been discovered in an effort to understand the mechanisms of action of the immunosuppressant drug rapamycin extracted from a bacterium of the Easter Island (Rapa Nui) soil. mTOR is a serine/threonine kinase found in two functionally distinct complexes, mTORC1 and mTORC2, which are differentially regulated by a great number of nutrients such as glucose and amino acids, energy (oxygen and ATP/AMP content), growth factors, hormones, and neurotransmitters. mTOR controls many basic cellular functions such as protein synthesis, energy metabolism, cell size, lipid metabolism, autophagy, mitochondria, and lysosome biogenesis. In addition, mTOR-controlled signaling pathways regulate many integrated physiological functions of the nervous system including neuronal development, synaptic plasticity, memory storage, and cognition. Thus it is not surprising that deregulation of mTOR signaling is associated with many neurological and psychiatric disorders. Preclinical and preliminary clinical studies indicate that inhibition of mTORC1 can be beneficial for some pathological conditions such as epilepsy, cognitive impairment, and brain tumors, whereas stimulation of mTORC1 (direct or indirect) can be beneficial for other pathologies such as depression or axonal growth and regeneration.

Prospects for mTOR-mediated functional repair after central nervous system trauma

Recent research has suggested that the growth of central nervous system (CNS) axons during development is mediated through the PI3K/Akt/mammalian target of rapamycin (mTOR) intracellular signalling axis and that suppression of activity in this pathway occurs during maturity as levels of the phosphatase and tensin homologue (PTEN) rise and inhibit PI3K activation of mTOR, accounting for the failure of axon regeneration in the injured adult CNS. This hypothesis is supported by findings confirming that suppression of PTEN in experimental adult animals promotes impressive axon regeneration in the injured visual and corticospinal motor systems. This review focuses on these recent developments, discussing the therapeutic potential of a mTOR-based treatment aimed at promoting functional recovery in CNS trauma patients, recognising that to fulfil this ambition, the new therapy should aim to promote not only axon regeneration but also remyelination of regenerated axons, neuronal survival and re-innervation of denervated targets through accurate axonal guidance and synaptogenesis, all with minimal adverse effects. The translational challenges presented by the implementation of this new axogenic therapy are also discussed.

Genetic Landscape and Biomarkers of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) has emerged as a major cause of cancer-related death. Its mortality has increased in Western populations, with a minority of patients diagnosed at early stages, when curative treatments are feasible. Only the multikinase inhibitor sorafenib is available for the management of advanced cases. During the last 10 years, there has been a clear delineation of the landscape of genetic alterations in HCC, including high-level DNA amplifications in chromosome 6p21 (VEGFA) and 11q13 (FGF19/CNND1), as well as homozygous deletions in chromosome 9 (CDKN2A). The most frequent mutations affect TERT promoter (60%), associated with an increased telomerase expression. TERT promoter can also be affected by copy number variations and hepatitis B DNA insertions, and it can be found mutated in preneoplastic lesions. TP53 and CTNNB1 are the next most prevalent mutations, affecting 25%-30% of HCC patients, that, in addition to low-frequency mutated genes (eg, AXIN1, ARID2, ARID1A, TSC1/TSC2, RPS6KA3, KEAP1, MLL2), help define some of the core deregulated pathways in HCC. Conceptually, some of these changes behave as prototypic oncogenic addiction loops, being ideal biomarkers for specific therapeutic approaches. Data from genomic profiling enabled a proposal of HCC in 2 major molecular clusters (proliferation and nonproliferation), with differential enrichment in prognostic signatures, pathway activation and tumor phenotype. Translation of these discoveries into specific therapeutic decisions is an unmet medical need in this field.

The changing face of a rare disease: lymphangioleiomyomatosis

Lymphangioleiomyomatosis is a rare disease characterised by cystic destruction of the lung, lymphatic abnormalities and abdominal tumours. It affects almost exclusively females and can occur sporadically or in patients with tuberous sclerosis complex. In the past decade remarkable progress has been made in understanding of the pathogenesis of this disease leading to a new therapeutic approach. This review summarises recent advances regarding pathogenic mechanisms and clinical manifestations, and highlights the current and the most promising future therapeutic strategies.

N-Methyl-N-nitrosourea as a mammary carcinogenic agent

The administration of chemical carcinogens is one of the most commonly used methods to induce tumors in several organs in laboratory animals in order to study oncologic diseases of humans. The carcinogen agent N-methyl-N-nitrosourea (MNU) is the oldest member of the nitroso compounds that has the ability to alkylate DNA. MNU is classified as a complete, potent, and direct alkylating compound. Depending on the animals' species and strain, dose, route, and age at the administration, MNU may induce tumors' development in several organs. The aim of this manuscript was to review MNU as a carcinogenic agent, taking into account that this carcinogen agent has been frequently used in experimental protocols to study the carcinogenesis in several tissues, namely breast, ovary, uterus, prostate, liver, spleen, kidney, stomach, small intestine, colon, hematopoietic system, lung, skin, retina, and urinary bladder. In this paper, we also reviewed the experimental conditions to the chemical induction of tumors in different organs with this carcinogen agent, with a special emphasis in the mammary carcinogenesis.

Activation of extracellular regulated kinase and mechanistic target of rapamycin pathway in focal cortical dysplasia

Neuropathology of resected brain tissue has revealed an association of focal cortical dysplasia (FCD) with drug-resistant epilepsy (DRE). Recent studies have shown that the mechanistic target of rapamycin (mTOR) pathway is hyperactivated in FCD as evidenced by increased phosphorylation of the ribosomal protein S6 (S6) at serine 240/244 (S(240/244) ), a downstream target of mTOR. Moreover, extracellular regulated kinase (ERK) has been shown to phosphorylate S6 at serine 235/236 (S(235/236) ) and tuberous sclerosis complex 2 (TSC2) at serine 664 (S(664) ) leading to hyperactive mTOR signaling. We evaluated ERK phosphorylation of S6 and TSC2 in two types of FCD (FCD I and FCD II) as a candidate mechanism contributing to mTOR pathway dysregulation. Tissue samples from patients with tuberous sclerosis (TS) served as a positive control. Immunostaining for phospho-S6 (pS6(240/244) and pS6(235/236) ), phospho-ERK (pERK), and phospho-TSC2 (pTSC2) was performed on resected brain tissue with FCD and TS. We found increased pS6(240/244) and pS6(235/236) staining in FCD I, FCD II and TS compared to normal-appearing tissue, while pERK and pTSC2 staining was increased only in FCD IIb and TS tissue. Our results suggest that both the ERK and mTOR pathways are dysregulated in FCD and TS; however, the signaling alterations are different for FCD I as compared to FCD II and TS.

Vertical Targeting of AKT and mTOR as Well as Dual Targeting of AKT and MEK Signaling Is Synergistic in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the sixth most common cancer, and the third most common cause of cancer related death worldwide. The multi-kinase inhibitor Sorafenib represents the only systemic treatment option until today, and results from clinical trials with allosteric mTOR inhibitors were sobering. Since the PI3K/AKT/mTOR and RAF/MEK/ERK signaling pathways are frequently upregulated in HCC, we have analyzed the effects of AKT inhibitor MK-2206, MEK inhibitor AZD6244 (ARRY 142886) and mTOR kinase inhibitor AZD8055, given as single drugs or in combination, on proliferation and apoptosis of three HCC cell lines in vitro. We show that all three inhibitor combinations synergistically inhibit proliferation of the three HCC cell lines, with the strongest synergistic effect observed after vertical inhibition of AKT and mTORC1/2. We demonstrate that AKT kinase activity is restored 24h after blockade of mTORC1/2 by increased phosphorylation of T308, providing a rationale for combined targeting of AKT and mTOR inhibition in HCC. Our data suggest that a combination of inhibitors targeting those respective pathways may be a viable approach for future application in patients with hepatocellular carcinoma.

Palmitate activates mTOR/p70S6K through AMPK inhibition and hypophosphorylation of raptor in skeletal muscle cells: Reversal by oleate is similar to metformin

Excessive saturated free fatty acids (SFFAs; e.g. palmitate) in blood are a pathogenic factor in diabetes, obesity, cardiovascular disease and liver failure. In contrast, monounsaturated free fatty acids (e.g. oleate) prevent the toxic effect of SFFAs in various types of cells. The mechanism is poorly understood and involvement of the mTOR complex is untested. In the present study, we demonstrate that oleate preconditioning, as well as coincubation, completely prevented palmitate-induced markers of inflammatory signaling, insulin resistance and cytotoxicity in C2C12 myotubes. We then examined the effect of palmitate and/or oleate on the mammalian target of rapamycin (mTOR) signal path and whether their link is mediated by AMP-activated protein kinase (AMPK). Palmitate decreased the phosphorylation of raptor and 4E-BP1 while increasing the phosphorylation of p70S6K. Palmitate also inhibited phosphorylation of AMPK, but did not change the phosphorylated levels of mTOR or rictor. Oleate completely prevented the palmitate-induced dysregulation of mTOR components and restored pAMPK whereas alone it produced no signaling changes. To understand this more, we show activation of AMPK by metformin also prevented palmitate-induced changes in the phosphorylations of raptor and p70S6K, confirming that the mTORC1/p70S6K signaling pathway is responsive to AMPK activity. By contrast, inhibition of AMPK phosphorylation by Compound C worsened palmitate-induced changes and correspondingly blocked the protective effect of oleate. Finally, metformin modestly attenuated palmitate-induced insulin resistance and cytotoxicity, as did oleate. Our findings indicate that palmitate activates mTORC1/p70S6K signaling by AMPK inhibition and phosphorylation of raptor. Oleate reverses these effects through a metformin-like facilitation of AMPK.

Dally Proteoglycan Mediates the Autonomous and Nonautonomous Effects on Tissue Growth Caused by Activation of the PI3K and TOR Pathways

How cells acquiring mutations in tumor suppressor genes outcompete neighboring wild-type cells is poorly understood. The phosphatidylinositol 3-kinase (PI3K)–phosphatase with tensin homology (PTEN) and tuberous sclerosis complex (TSC)-target of rapamycin (TOR) pathways are frequently activated in human cancer, and this activation is often causative of tumorigenesis. We utilized the Gal4-UAS system in Drosophila imaginal primordia, highly proliferative and growing tissues, to analyze the impact of restricted activation of these pathways on neighboring wild-type cell populations. Activation of these pathways leads to an autonomous induction of tissue overgrowth and to a remarkable nonautonomous reduction in growth and proliferation rates of adjacent cell populations. This nonautonomous response occurs independently of where these pathways are activated, is functional all throughout development, takes place across compartments, and is distinct from cell competition. The observed autonomous and nonautonomous effects on tissue growth rely on the up-regulation of the proteoglycan Dally, a major element involved in modulating the spreading, stability, and activity of the growth promoting Decapentaplegic (Dpp)/transforming growth factor β(TGF-β) signaling molecule. Our findings indicate that a reduction in the amount of available growth factors contributes to the outcompetition of wild-type cells by overgrowing cell populations. During normal development, the PI3K/PTEN and TSC/TOR pathways play a major role in sensing nutrient availability and modulating the final size of any developing organ. We present evidence that Dally also contributes to integrating nutrient sensing and organ scaling, the fitting of pattern to size.

The loss of tumor suppressor genes induces a nonautonomous reduction of growth and proliferation rates in adjacent cell populations by competing for available growth factors; the proteoglycan Dally helps to mediate this effect.

The final size of a developing organ is finely modulated by nutrient conditions through the activity of nutrient sensing pathways, and deregulation of these pathways is often causative of tumorigenesis. Besides the well-known roles of these pathways in inducing tissue and cell growth, here we identify a nonautonomous effect of activation of these pathways on growth and proliferation rates and on the final size of neighboring cell populations. We reveal that the observed autonomous and nonautonomous effects on tissue growth and proliferation rates rely on the up-regulation of the proteoglycan Dally, a major factor involved in modulating the spreading, stability, and activity of the growth promoting Decapentaplegic (Dpp)/transforming growth factor β(TGF-β) signaling molecule. Our data indicate that a reduction in the amount of available growth factors contributes to the outcompetition of wild-type cells by overgrowing cell populations. Whereas nutrient-sensing pathways modulate the final size of the adult structure according to nutrient availability to the feeding animal, Dpp plays an organ-intrinsic role in the coordination of growth and patterning. We identify the proteoglycan Dally as the rate-limiting factor that contributes to the tissue-autonomous and nonautonomous effects on growth caused by targeted activation of the nutrient-sensing pathways. Thus, our results unravel a role of Dally as a molecular bridge between the organ-intrinsic and organ-extrinsic mechanisms that regulate organ size.

PtdIns(3,4,5)P3-Dependent Activation of the mTORC2 Kinase Complex

mTOR serves as a central regulator of cell growth and metabolism by forming two distinct complexes, mTORC1 and mTORC2. Although mechanisms of mTORC1 activation by growth factors and amino acids have been extensively studied, the upstream regulatory mechanisms leading to mTORC2 activation remain largely elusive. Here, we report that the pleckstrin homology (PH) domain of SIN1, an essential and unique component of mTORC2, interacts with the mTOR kinase domain to suppress mTOR activity. More importantly, PtdIns(3,4,5)P3, but not other PtdInsPn species, interacts with SIN1-PH to release its inhibition on the mTOR kinase domain, thereby triggering mTORC2 activation. Mutating critical SIN1 residues that mediate PtdIns(3,4,5)P3 interaction inactivates mTORC2, whereas mTORC2 activity is pathologically increased by patient-derived mutations in the SIN1-PH domain, promoting cell growth and tumor formation. Together, our study unravels a PI3K-dependent mechanism for mTORC2 activation, allowing mTORC2 to activate AKT in a manner that is regulated temporally and spatially by PtdIns(3,4,5)P3.

SIGNIFICANCE

The SIN1-PH domain interacts with the mTOR kinase domain to suppress mTOR activity, and PtdIns(3,4,5)P3 binds the SIN1-PH domain to release its inhibition on the mTOR kinase domain, leading to mTORC2 activation. Cancer patient-derived SIN1-PH domain mutations gain oncogenicity by loss of suppressing mTOR activity as a means to facilitate tumorigenesis.

CD8+ T-cell senescence: no role for mTOR

Aging is accompanied by immune decline leading to increased incidence of infections and malignancies, given the demographic shift of humans towards an older age the identification of strategies for the manipulation of immunity is an important goal. Evidence implicates mammalian target of rapamycin (mTOR) to be a key modulator of aging and the use of mTOR inhibitors has been shown to ameliorate much age-related pathology; however, recent data suggest that senescent CD8(+) T-cells function independently of mTOR. This review article will challenge the perceived dogma that mTOR universally controls CD8(+) T-cell function.

Intestinal mTOR regulates GLP-1 production in mouse L cells

AIMS/HYPOTHESIS

Glucagon-like peptide (GLP-1), an intestinal incretin produced in L cells through proglucagon processing, is released in response to meal intake. The intracellular mechanism by which L cells sense the organism energy level to coordinate the production of GLP-1 remains unclear. Mechanistic target of rapamycin (mTOR) is an intracellular fuel sensor critical for energy homeostasis. In this study, we investigated whether intestinal mTOR regulates GLP-1 production in L cells.

METHODS

The effects of mTOR on GLP-1 production were examined in lean- or high-fat diet (HFD) induced diabetic C57/BL6, db/db, Neurog3-Tsc1(-/-) mice, and STC-1 cells. GLP-1 expression was investigated by real-time PCR and western blotting. Plasma GLP-1 and insulin were detected by enzyme immunoassay and radioimmunoassay, respectively.

RESULTS

Fasting downregulated mTOR activity, which was associated with a decrement of intestinal proglucagon and circulating GLP-1. Upon re-feeding, these alterations returned to the levels of fed animals. In HFD induced diabetic mice, ileal mTOR signalling, proglucagon and circulating GLP-1 were significantly decreased. Inhibition of mTOR signalling by rapamycin decreased levels of intestinal and plasma GLP-1 in both normal and diabetic mice. Activation of the intestinal mTOR signalling by L-leucine or Tsc1 gene deletion increased levels of intestinal proglucagon and plasma GLP-1. Overexpression of mTOR stimulated proglucagon promoter activity and GLP-1 production, whereas inhibition of mTOR activity by overexpression of tuberous sclerosis 1 (TSC1) or TSC2 decreased proglucagon promoter activity and GLP-1 production in STC-1 cells.

CONCLUSIONS/INTERPRETATION

mTOR may link energy supply with the production of GLP-1 in L cells.

Missense mutation in the PTEN promoter of a patient with hemifacial hyperplasia

The cellular mechanisms involved in the asymmetric facial overgrowth syndrome, hemifacial hyperplasia (HFH), are not well understood. This study was conducted to compare primary cell cultures from hyperplastic and normal HFH bone for cellular and molecular differences. Primary cultures developed from biopsies of a patient with isolated HFH showed a twofold difference in cell size and cell number between hyperplastic and normal bone. Microarray data suggested a 40% suppression of PTEN (phosphatase-tensin homolog) transcripts. Sequencing of the PTEN gene and promoter identified novel C/G missense mutation (position -1053) in the regulatory region of the PTEN promoter. Western blots of downstream pathway components showed an increase in PKBa/Akt1 phosphorylation and TOR (target of rapamcyin) signal. Sirolimus, an inhibitor of TOR, when added to overgrowth cells reversed the cell size, cell number and total protein differences between hyperplastic and normal cells. In cases of facial overgrowth, which involve PTEN/Akt/TOR dysregulation, sirolimus could be used for limiting cell overgrowth.

Regulation of Hematopoiesis and Methionine Homeostasis by mTORC1 Inhibitor NPRL2

Nitrogen permease regulator-like 2 (NPRL2) is a component of a conserved complex that inhibits mTORC1 (mammalian Target Of Rapamycin Complex 1) in response to amino acid insufficiency. Here, we show that NPRL2 is required for mouse viability and that its absence significantly compromises fetal liver hematopoiesis in developing embryos. Moreover, NPRL2 KO embryos have significantly reduced methionine levels and exhibit phenotypes reminiscent of cobalamin (vitamin B12) deficiency. Consistent with this idea, NPRL2 KO liver and mouse embryonic fibroblasts (MEFs) show defective processing of the cobalamin-transport protein transcobalamin 2, along with impaired lysosomal acidification and lysosomal gene expression. NPRL2 KO MEFs exhibit a significant defect in the cobalamin-dependent synthesis of methionine from homocysteine, which can be rescued by supplementation with cyanocobalamin. Taken together, these findings demonstrate a role for NPRL2 and mTORC1 in the regulation of lysosomal-dependent cobalamin processing, methionine synthesis, and maintenance of cellular re-methylation potential, which are important during hematopoiesis.

Huaier Extract Induces Autophagic Cell Death by Inhibiting the mTOR/S6K Pathway in Breast Cancer Cells

Huaier extract is attracting increased attention due to its biological activities, including antitumor, anti-parasite and immunomodulatory effects. Here, we investigated the role of autophagy in Huaier-induced cytotoxicity in MDA-MB-231, MDA-MB-468 and MCF7 breast cancer cells. Huaier treatment inhibited cell viability in all three cell lines and induced various large membranous vacuoles in the cytoplasm. In addition, electron microscopy, MDC staining, accumulated expression of autophagy markers and flow cytometry revealed that Huaier extract triggered autophagy. Inhibition of autophagy attenuated Huaier-induced cell death. Furthermore, Huaier extract inhibited the mammalian target of the rapamycin (mTOR)/S6K pathway in breast cancer cells. After implanting MDA-MB-231 cells subcutaneously into the right flank of BALB/c nu/nu mice, Huaier extract induced autophagy and effectively inhibited xenograft tumor growth. This study is the first to show that Huaier-induced cytotoxicity is partially mediated through autophagic cell death in breast cancer cells through suppression of the mTOR/S6K pathway.

Phosphoproteomic Analysis of KSHV-Infected Cells Reveals Roles of ORF45-Activated RSK during Lytic Replication

Kaposi’s Sarcoma-Associated Herpesvirus (KSHV) is an oncogenic virus which has adapted unique mechanisms to modulate the cellular microenvironment of its human host. The pathogenesis of KSHV is intimately linked to its manipulation of cellular signaling pathways, including the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) pathway. We have previously shown that KSHV ORF45 contributes to the sustained activation of both ERK and p90 ribosomal S6 kinase (RSK, a major functional mediator of ERK/MAPK signaling) during KSHV lytic replication. ORF45-activated RSK is required for optimal KSHV lytic gene expression and progeny virion production, though the underlying mechanisms downstream of this activation are still unclear. We hypothesized that the activation of RSK by ORF45 causes differential phosphorylation of cellular and viral substrates, affecting biological processes essential for efficient KSHV lytic replication. Accordingly, we observed widespread and significant differences in protein phosphorylation upon induction of lytic replication. Mass-spectrometry-based phosphoproteomic screening identified putative substrates of ORF45-activated RSK in KSHV-infected cells. Bioinformatic analyses revealed that nuclear proteins, including several transcriptional regulators, were overrepresented among these candidates. We validated the ORF45/RSK-dependent phosphorylation of several putative substrates by employing KSHV BAC mutagenesis, kinase inhibitor treatments, and/or CRISPR-mediated knockout of RSK in KSHV-infected cells. Furthermore, we assessed the consequences of knocking out these substrates on ORF45/RSK-dependent regulation of gene expression and KSHV progeny virion production. Finally, we show data to support that ORF45 regulates the translational efficiency of a subset of viral/cellular genes with complex secondary structure in their 5’ UTR. Altogether, these data shed light on the mechanisms by which KSHV ORF45 manipulates components of the host cell machinery via modulation of RSK activity. Thus, this study has important implications for the pathobiology of KSHV and other diseases in which RSK activity is dysregulated.

Kaposi’s sarcoma-associated herpesvirus (KSHV) is a human tumor virus which hijacks the host signaling pathways in order to maintain persistent infection. We previously discovered that the KSHV protein ORF45 binds to and activates the cellular kinase RSK (p90 ribosomal S6 kinase), and that this activation is vital for optimal KSHV gene expression and virion production. Here, we performed a phosphoproteomic analysis of KSHV-infected cells to further characterize the specific substrates of ORF45-activated RSK. Bioinformatic analyses provided insights into the functional roles of these substrates. We verified the ORF45/RSK-dependent phosphorylation of a subset of these substrates by various means. Finally, we used genome editing to knock out RSK, as well as several cellular substrates identified by our screening, and characterized the consequent effect(s) on regulation of gene expression and virion production. Thus, this work further elucidates one of the key signaling nodes modulated by KSHV, and implicates ORF45-mediated activation of RSK in the regulation of viral and host gene expression during KSHV lytic replication.

Posttranslational modifications of FOXO1 regulate epidermal growth factor receptor tyrosine kinase inhibitor resistance for non-small cell lung cancer cells

Epidermal growth factor receptor tyrosine kinase inhibitors (TKIs) are effective clinical therapies for advanced non-small cell lung cancer (NSCLC) patients, while resistance to TKIs remains a serious problem in clinical practice. Recently, it has been proposed that targeting mTOR could overcome TKI resistance in NSCLC cells. Forkhead box class O1 (FOXO1) has emerged as an important rheostat that modulates the activity of Akt and mTOR signaling pathway. However, the role of FOXO1 and related regulatory mechanism in TKI resistance in NSCLC remain largely unknown. Here, we find that mTOR-AKT-FOXO1 signaling cascade is deregulated in TKI-resistant NSCLC cells and that FOXO1 was highly phosphorylated and lowly acetylated upon erlotinib treatment. Combination of mTOR or PI3K inhibitor and erlotinib overcomes TKI resistance to inhibit cell growth and induce apoptosis in TKI-resistant NSCLC cells. Furthermore, the phosphorylation and acetylation of FOXO1 are reversely modulated by mTORC2-AKT signaling pathway. FOXO1 mutation analyses reveal that FOXO1 acetylation inhibits cell proliferation and promotes NSCLC cell apoptosis, while the phosphorylation of FOXO1 plays opposite roles in NSCLC cells. Importantly, increasing FOXO1 acetylation by a HDAC inhibitor, depsipeptide, overcomes TKI resistance to effectively induce TKI-resistant NSCLC cell apoptosis. Together, FOXO1 plays dual roles in TKI resistance through posttranslational modifications in NSCLC and this study provides a possible strategy for treatment of TKI-resistant NSCLC patients.

Mathematical modelling of metabolic regulation in aging

The underlying cellular mechanisms that characterize aging are complex and multifaceted. However, it is emerging that aging could be regulated by two distinct metabolic hubs. These hubs are the pathway defined by the mammalian target of rapamycin (mTOR) and that defined by the NAD+-dependent deacetylase enzyme, SIRT1. Recent experimental evidence suggests that there is crosstalk between these two important pathways; however, the mechanisms underpinning their interaction(s) remains poorly understood. In this review, we propose using computational modelling in tandem with experimentation to delineate the mechanism(s). We briefly discuss the main modelling frameworks that could be used to disentangle this relationship and present a reduced reaction pathway that could be modelled. We conclude by outlining the limitations of computational modelling and by discussing opportunities for future progress in this area.

Caenorhabditis elegans as a model for cancer research

The term cancer describes a group of multifaceted diseases characterized by an intricate pathophysiology. Despite significant advances in the fight against cancer, it remains a key public health concern and burden on societies worldwide. Elucidation of key molecular and cellular mechanisms of oncogenic diseases will facilitate the development of better intervention strategies to counter or prevent tumor development. In vivo and in vitro models have long been used to delineate distinct biological processes involved in cancer such as apoptosis, proliferation, angiogenesis, invasion, metastasis, genome instability, and metabolism. In this review, we introduce Caenorhabditis elegans as an emerging animal model for systematic dissection of the molecular basis of tumorigenesis, focusing on the well-established processes of apoptosis and autophagy. Additionally, we propose that C. elegans can be used to advance our understanding of cancer progression, such as deregulation of energy metabolism, stem cell reprogramming, and host-microflora interactions.

Influence of Rictor and Raptor Expression of mTOR Signaling on Long-Term Outcomes of Patients with Hepatocellular Carcinoma

BACKGROUND

Aberrant signaling mediated by the mammalian target of rapamycin (mTOR) occurs at high frequency in hepatocellular carcinoma (HCC), indicating that mTOR is a candidate for targeted therapy. mTOR forms two complexes called mTORC1 (mTOR complexed with raptor) and mTORC2 (mTOR complexed with rictor). There are minor studies of the expression kinetics of mTORC1 and mTORC2 in HCC.

METHODS

We studied 62 patients with HCC who underwent curative resection. We used univariate and multivariate analyses to identify factors that potentially influence disease and overall survival after hepatectomy. The mRNA and protein levels of mTOR, rictor and raptor in cancer and non-cancer tissues were analyzed using quantitative RT-PCR, immunohistochemistry and Western blotting.

RESULTS/CONCLUSION

High ratio of the levels of rictor and raptor mRNAs in tumors was identified as independent prognostic indicators for disease-free survival. Low and high levels of preoperative serum albumin and mTOR mRNA in the tumor, respectively, were identified as independent indicators of overall survival. HCC is likely to recur early after hepatic resection in patients with high levels of mTOR and rictor mRNAs and high rictor/raptor ratios in cancer tissues. We conclude that analysis of mTOR expression in cancer tissues represents an essential strategy to predict HCC recurrence after curative treatment.

A cell-penetrating ester of the neural metabolite lanthionine ketimine stimulates autophagy through the mTORC1 pathway: Evidence for a mechanism of action with pharmacological implications for neurodegenerative pathologies

Autophagy is a fundamental cellular recycling process vulnerable to compromise in neurodegeneration. We now report that a cell-penetrating neurotrophic and neuroprotective derivative of the central nervous system (CNS) metabolite, lanthionine ketimine (LK), stimulates autophagy in RG2 glioma and SH-SY5Y neuroblastoma cells at concentrations within or below pharmacological levels reported in previous mouse studies. Autophagy stimulation was evidenced by increased lipidation of microtubule-associated protein 1 light chain 3 (LC3) both in the absence and presence of bafilomycin-A1 which discriminates between effects on autophagic flux versus blockage of autophagy clearance. LKE treatment caused changes in protein level or phosphorylation state of multiple autophagy pathway proteins including mTOR; p70S6 kinase; unc-51-like-kinase-1 (ULK1); beclin-1 and LC3 in a manner essentially identical to effects observed after rapamycin treatment. The LKE site of action was near mTOR because neither LKE nor the mTOR inhibitor rapamycin affected tuberous sclerosis complex (TSC) phosphorylation status upstream from mTOR. Confocal immunofluorescence imaging revealed that LKE specifically decreased mTOR (but not TSC2) colocalization with LAMP2(+) lysosomes in RG2 cells, a necessary event for mTORC1-mediated autophagy suppression, whereas rapamycin had no effect. Suppression of the LK-binding adaptor protein CRMP2 (collapsin response mediator protein-2) by means of shRNA resulted in diminished autophagy flux, suggesting that the LKE action on mTOR localization may occur through a novel mechanism involving CRMP2-mediated intracellular trafficking. These findings clarify the mechanism-of-action for LKE in preclinical models of CNS disease, while suggesting possible roles for natural lanthionine metabolites in regulating CNS autophagy.

Frameshift mutations in mammalian target of rapamycin pathway genes and their regional heterogeneity in sporadic colorectal cancers

Mammalian target of rapamycin (mTOR) pathway is known to be involved in cancer pathogenesis. The aim of our study was to find whether mTOR-related genes were mutated and expressionally altered in colorectal cancers (CRCs). Through public database searching, we found that PIK3CB, insulin receptor substrate 1/2 (IRS1), RPS6, EIF4B, RPS6KA5, and PRKAA2 that were known as mTOR-related genes possessed mononucleotide repeats in DNA coding sequences that could be mutated in cancers with microsatellite instability (MSI). We analyzed 124 CRCs by single-strand conformation polymorphism analysis and DNA sequencing and found 7 (8.9%), 8 (10.1%), and 3 (3.8%) of 79 CRCs with high MSI that harbored IRS1, EIF4B, and RPS6KA5 frameshift mutations, respectively. These mutations were not identified in stable MSI/low MSI (0/45). In addition, we analyzed intratumoral heterogeneity (ITH) of PIK3CB, IRS1, RPS6, EIF4B, RPS6KA5, and PRKAA2 frameshift mutations in 16 CRCs and found that IRS1, EIF4B, and RPS6KA5 mutations had regional ITH in 2, 2, and 1 CRCs, respectively. We also analyzed IRS1 expression in the CRCs by immunohistochemistry. Loss of IRS1 expression was identified in 31% of the CRCs. The loss of expression was more common in those with IRS1 mutation than those with wild-type IRS1. Our data indicate mTOR-related genes harbored not only somatic mutations but also mutational ITH and loss of expression, which together might play a role in tumorigenesis of CRC, especially with high MSI. Our data also suggest that mutation analysis in multiregional areas is needed for a precise evaluation of mutation status in CRC with MSI-H.

Targeting EGFR-PI3K-AKT-mTOR signaling enhances radiosensitivity in head and neck squamous cell carcinoma

INTRODUCTION

Head and neck squamous cell carcinoma (HNSCC) is frequently characterized by high resistance to radiotherapy, which critically depends on both altered signaling pathways within tumor cells and their dynamic interaction with the tumor microenvironment.

AREAS COVERED

This review covers EGFR-phosphoinositide 3-kinase (PI3K)-protein kinase B (AKT)-mechanistic target of rapamycin (mTOR) signaling in HNSCC. The role of each pathway node in radioresistance is discussed. Preclinical and clinical innovative aspects of targeting EGFR-PI3K-AKT and mTOR are demonstrated. Ongoing clinical trials and future perspectives are presented.

EXPERT OPINION

Different cellular signaling pathways seem to mediate radioresistance in advanced HNSCC and various molecular targeted therapies are currently being investigated to sensitize tumor cells to radiotherapy. Recently, new insights in the mutational landscape of HNSCC unraveled critical alterations in putative oncogenes and tumor suppressor genes and have emphasized the importance of PI3K and the corresponding upstream and downstream signaling pathways in pathogenesis and treatment response. The frequent activation of the EGFR-PI3K-AKT-mTOR pathway in HNSCC and its implication in the context of radiosensitivity make this pathway one of the most promising targets in the therapy of HNSCC patients. Clinical studies targeting EGFR and mTOR in combination with radiotherapy are under investigation.

Tuberous sclerosis complex 1-mechanistic target of rapamycin complex 1 signaling determines brown-to-white adipocyte phenotypic switch

Interconversion of white and brown adipocytes occurs between anabolic and catabolic states. The molecular mechanism regulating this phenotypic switch remains largely unknown. This study explores the role of tuberous sclerosis complex 1 (TSC1)-mechanistic target of rapamycin (mTOR) signaling in the conversion of brown to white adipose tissue (WAT). A colony of Fabp4-Tsc1(-/-) mice, in which the Tsc1 gene was specifically deleted by the fatty acid binding protein 4 (FABP4)-Cre, was established. Western blotting and immunostaining demonstrated the absence of TSC1 and activation of ribosomal protein S6 kinase 1, the downstream target of mTOR complex 1 (mTORC1) signaling, in the brown adipose tissues (BATs) of Fabp4-Tsc1(-/-) mice. Accumulation of lipid droplets in BAT was significantly increased. Levels of brown adipocyte markers were markedly downregulated, while white adipocyte markers were upregulated. Rapamycin reversed the conversion from BAT to WAT in Fabp4-Tsc1(-/-) mice. Deletion of the Tsc1 gene in cultured brown preadipocytes significantly increased the conversion to white adipocytes. FoxC2 mRNA, the transcriptional factor for brown adipocyte determination, was significantly decreased, while mRNAs for retinoblastoma protein, p107 and RIP140, the transcriptional factors for white adipocyte determination, increased in the BAT of Fabp4-Tsc1(-/-) mice. Our study demonstrates that TSC1-mTORC1 signaling contributes to the brown-to-white adipocyte phenotypic switch.

Teaching the basics of autophagy and mitophagy to redox biologists--mechanisms and experimental approaches

Autophagy is a lysosomal mediated degradation activity providing an essential mechanism for recycling cellular constituents, and clearance of excess or damaged lipids, proteins and organelles. Autophagy involves more than 30 proteins and is regulated by nutrient availability, and various stress sensing signaling pathways. This article provides an overview of the mechanisms and regulation of autophagy, its role in health and diseases, and methods for its measurement. Hopefully this teaching review together with the graphic illustrations will be helpful for instructors teaching graduate students who are interested in grasping the concepts and major research areas and introducing recent developments in the field.

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mTOR, Beclin–VPS34, LC3 homologs, and adaptor proteins in autophagy.

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Autophagosomal membranes may derive from multiple sources.

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Autophagosomal–lysosomal fusion contributes to the control of autophagic flux.

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Assess autophagy by autophagosomal and protein turnover, and morphological alterations.

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Autophagy adysfunction in cancer, aging, neurodegeneration and infection.

Glucocorticoids retard skeletal muscle development and myoblast protein synthesis through a mechanistic target of rapamycin (mTOR)-signaling pathway in broilers (Gallus gallus domesticus)

Glucocorticoids exert a well-known catabolic protein action on skeletal muscle. The mechanistic target of rapamycin (mTOR) signaling pathway acts as a central regulator of protein metabolism. Whether glucocorticoids regulate protein synthesis through the mTOR pathway in skeletal muscle of chickens remains unknown. This study was performed to characterize the effect of glucocorticoids on the mTOR pathway in skeletal muscle development in chickens, and on protein synthesis in cultured embryonic myoblasts. Male 29-d-old chickens were given a dexamethasone injection (2 mg/kg) twice per day for 4 d (n = 16). Chicken embryonic myoblasts were exposed to dexamethasone for 24 h (100 µmol/L, n = 4 cultures). The interaction between dexamethasone and leucine was also investigated. ANOVA and Duncan's multiple test were used to analyze the effects of the dexamethasone and leucine treatments. The results showed that dexamethasone decreased body weight gain, body weight, and feed efficiency. Protein synthesis was inhibited by in vitro dexamethasone exposure. Phosphorylation of mTOR and ribosomal protein S6 protein kinase (p70S6K) were inhibited by dexamethasone, suggesting the mTOR pathway may be involved in dexamethasone-regulated muscle protein synthesis. Phosphorylation of AMP-activated protein kinase (AMPK) was not altered in vitro but was reduced in vivo by dexamethasone. These results imply that the mTOR and AMPK pathways are both involved in retarding muscle development and protein synthesis by glucocorticoids, but the mTOR pathway is a critical point linking glucocorticoid and protein synthesis. Leucine, at least partially, inhibited the effects of dexamethasone on protein synthesis via the mTOR pathway.

Renal epithelioid angiomyolipoma with malignant features: Histological evaluation and novel immunohistochemical findings

Renal epithelioid angiomyolipoma (EAML) is a potentially malignant tumor type whose characteristics and biomarkers predictive of malignant behavior have not been elucidated. Here, we report three cases of renal EAML with malignant features but without histories of tuberous sclerosis complex. Case 1 involved a 29-year-old man with a 12-cm solid mass in the right kidney who underwent radical right nephrectomy. Case 2 involved a 22-year-old woman with a retroperitoneal mass who underwent radical right nephrectomy and retroperitoneal tumorectomy. Local recurrence was detected 7 years post-surgery. Case 3 involved a 23-year-old man with a 14-cm solid mass in the left kidney who underwent radical left nephrectomy. Microscopically, the tumors in all cases demonstrated proliferation of epithelioid cells with atypia, mitotic activity, necrosis, hemorrhage, and vascular invasion. Epithelioid cells in all cases were immunohistochemically positive for melanocytic and myoid markers and weakly positive for E-cadherin and β-catenin. Immunohistochemistry revealed activation of the mammalian target of rapamycin pathway. Here, we report the morphological and immunohistochemical features of clinically or histologically malignant renal EAML.

The cardioprotection of the insulin-mediated PI3K/Akt/mTOR signaling pathway

Apoptosis occurs frequently in myocardial infarction, oxidative stress injury, and ischemia/reperfusion injury, and plays a pivotal role in the development of heart diseases. Inhibition of apoptosis alone does not necessarily lead to meaningful rescue in terms of either cardiomyocyte survival or function. Activation of the PI3K/Akt signaling pathway induced by insulin not only inhibits cardiomyocyte apoptosis but also substantially preserves and even improves regional and overall cardiac function. Insulin can protect cardiomyocytes from apoptosis by regulating a number of signaling molecules, such as eNOS, FOXOs, Bad, GSK-3β, mTOR, NDRG2, and Nrf2, through activating PI3K and Akt. This review focuses on the protective mechanisms and targets of insulin identified in the prevention and treatment of myocardial injury.

mTOR Signaling in Protein Translation Regulation: Implications in Cancer Genesis and Therapeutic Interventions

mTOR is a central nutrient sensor that signals a cell to grow and proliferate. Through distinct protein complexes it regulates different levels of available cellular energy substrates required for cell growth. One of the important functions of the complex is to maintain available amino acid pool by regulating protein translation. Dysregulation of mTOR pathway leads to aberrant protein translation which manifests into various pathological states. Our review focuses on the role mTOR signaling plays in protein translation and its physiological role. It also throws some light on available data that show translation dysregulation as a cause of pathological complexities like cancer and the available drugs that target the pathway for cancer treatment.

Dual phosphorylation of Sin1 at T86 and T398 negatively regulates mTORC2 complex integrity and activity

Mammalian target of rapamycin (mTOR) plays essential roles in cell proliferation, survival and metabolism by forming at least two functional distinct multi-protein complexes, mTORC1 and mTORC2. External growth signals can be received and interpreted by mTORC2 and further transduced to mTORC1. On the other hand, mTORC1 can sense inner-cellular physiological cues such as amino acids and energy states and can indirectly suppress mTORC2 activity in part through phosphorylation of its upstream adaptors, IRS-1 or Grb10, under insulin or IGF-1 stimulation conditions. To date, upstream signaling pathways governing mTORC1 activation have been studied extensively, while the mechanisms modulating mTORC2 activity remain largely elusive. We recently reported that Sin1, an essential mTORC2 subunit, was phosphorylated by either Akt or S6K in a cellular context-dependent manner. More importantly, phosphorylation of Sin1 at T86 and T398 led to a dissociation of Sin1 from the functional mTORC2 holo-enzyme, resulting in reduced Akt activity and sensitizing cells to various apoptotic challenges. Notably, an ovarian cancer patient-derived Sin1-R81T mutation abolished Sin1-T86 phosphorylation by disrupting the canonical S6K-phoshorylation motif, thereby bypassing Sin1-phosphorylation-mediated suppression of mTORC2 and leading to sustained Akt signaling to promote tumorigenesis. Our work therefore provided physiological and pathological evidence to reveal the biological significance of Sin1 phosphorylation-mediated suppression of the mTOR/Akt oncogenic signaling, and further suggested that misregulation of this process might contribute to Akt hyper-activation that is frequently observed in human cancers.