Mechanistic Target of Rapamycin (mTOR) in the Cancer Setting

This special issue on mammalian target of rapamycin (mTOR) explores the importance of mTOR in cell growth control and cancer. Cancer cells often exploit mTOR as a mechanism to enhance their capacity to grow. While protein synthesis is by far the best-characterized mTOR-driven process, this special issue also describes a wider array of mTOR-driven biological processes that cancer cells benefit from, including autophagy, cell cycle control, metabolic transformation, angiogenic signaling, and anabolic processes such as nucleotide biosynthesis and ribosomal biogenesis. Other areas of mTOR signaling covered in these reviews delve into cell migration, inflammation, and regulation of transcription factors linked to cancer progression

Exploiting cancer vulnerabilities: mTOR, autophagy, and homeostatic imbalance

Mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) at lysosomes plays a pivotal role in cell growth control where an array of large multiprotein complexes relay nutrient, energy, and growth signal inputs through mTORC1. In cancer cells, such regulation often becomes disconnected, leading to uncontrolled cell growth and an elevation in cellular stress. Consequently, cancer cells often lose homeostatic balance as they grow in unfavorable conditions, i.e. when nutrients and energy are limited yet mTORC1 is still aberrantly activated. Cancer cells lose signaling flexibility because of hyperactive mTORC1 that leads to heightened cellular stress and loss of nutrient and energy homeostasis, all of which are potential avenues for cancer therapy. Cancer cells often enhance mTORC1 to drive cell growth and proliferation, while also maintaining their survival. Autophagy regulation by mTORC1 is critically involved in nutrient and energy homeostasis, cell growth control, and survival. Studying mTORC1 and autophagy as a potential therapeutic target for cancer treatment has been the focus of a wide range of research over the past few decades. This review will explore the signaling pathways central to mTORC1 and autophagy regulation, and cancer vulnerabilities while considering anticancer therapies

Tuberous Sclerosis Complex Gene Products, Tuberin and Hamartin, Control mTOR Signaling by Acting as a GTPase-Activating Protein Complex toward Rheb

AbstractBackground: Tuberous Sclerosis Complex (TSC) is a genetic disorder that occurs through the loss of heterozygosity of either TSC1 or TSC2, which encode Hamartin or Tuberin, respectively. Tuberin and Hamartin form a tumor suppressor heterodimer that inhibits the mammalian target of rapamycin (mTOR) nutrient signaling input, but how this occurs is unclear.Results: We show that the small G protein Rheb (Ras homolog enriched in brain) is a molecular target of TSC1/TSC2 that regulates mTOR signaling. Overexpression of Rheb activates 40S ribosomal protein S6 kinase 1 (S6K1) but not p90 ribosomal S6 kinase 1 (RSK1) or Akt. Furthermore, Rheb induces phosphorylation of eukaryotic initiation factor 4E binding protein 1 (4E-BP1) and causes 4E-BP1 to dissociate from eIF4E. This dissociation is completely sensitive to rapamycin (an mTOR inhibitor) but not wortmannin (a phosphoinositide 3-kinase [PI3K] inhibitor). Rheb also activates S6K1 during amino acid insufficiency via a rapamycin-sensitive mechanism, suggesting that Rheb participates in nutrient signaling through mTOR. Moreover, Rheb does not activate a S6K1 mutant that is unresponsive to mTOR-mediated signals, confirming that Rheb functions upstream of mTOR. Overexpression of the Tuberin-Hamartin heterodimer inhibits Rheb-mediated S6K1 activation, suggesting that Tuberin functions as a Rheb GTPase activating protein (GAP). Supporting this notion, TSC patient-derived Tuberin GAP domain mutants were unable to inactivate Rheb in vivo. Moreover, in vitro studies reveal that Tuberin, when associated with Hamartin, acts as a Rheb GTPase-activating protein. Finally, we show that membrane localization of Rheb is important for its biological activity because a farnesylation-defective mutant of Rheb stimulated S6K1 activation less efficiently.Conclusions: We show that Rheb acts as a novel mediator of the nutrient signaling input to mTOR and is the molecular target of TSC1 and TSC2 within mammalian cells

Activity of TSC2 is inhibited by AKT-mediated phosphorylation and membrane partitioning

Loss of tuberin, the product of TSC2 gene, increases mammalian target of rapamycin (mTOR) signaling, promoting cell growth and tumor development. However, in cells expressing tuberin, it is not known how repression of mTOR signaling is relieved to activate this pathway in response to growth factors and how hamartin participates in this process. We show that hamartin colocalizes with hypophosphorylated tuberin at the membrane, where tuberin exerts its GTPase-activating protein (GAP) activity to repress Rheb signaling. In response to growth signals, tuberin is phosphorylated by AKT and translocates to the cytosol, relieving Rheb repression. Phosphorylation of tuberin at serines 939 and 981 does not alter its intrinsic GAP activity toward Rheb but partitions tuberin to the cytosol, where it is bound by 14-3-3 proteins. Thus, tuberin bound by 14-3-3 in response to AKT phosphorylation is sequestered away from its membrane-bound activation partner (hamartin) and its target GTPase (Rheb) to relieve the growth inhibitory effects of this tumor suppressor

The effectiveness of schemes that refine referrals between primary and secondary care - the UK experience with glaucoma referrals: the Health Innovation & Education Cluster (HIEC) Glaucoma Pathways Project

Objectives: A comparison of glaucoma referral refinement schemes (GRRS) in the UK during a time period of considerable change in national policy and guidance. Design: Retrospective multisite review. Setting: The outcomes of clinical examinations by optometrists with a specialist interest in glaucoma (OSIs) were compared with optometrists with no specialist interest in glaucoma (non-OSIs). Data from Huntingdon and Nottingham assessed non-OSI findings, while Manchester and Gloucestershire reviewed OSI findings. Participants: 1086 patients. 434 patients were from Huntingdon, 179 from Manchester, 204 from Gloucestershire and 269 from Nottingham. Results: The first-visit discharge rate (FVDR) for all time periods for OSIs was 14.1% compared with 36.1% from non-OSIs (difference 22%, CI 16.9% to 26.7%; p<0.001). The FVDR increased after the April 2009 National Institute for Health and Clinical Excellence (NICE) glaucoma guidelines compared with pre-NICE, which was particularly evident when pre-NICE was compared with the current practice time period (OSIs 6.2–17.2%, difference 11%, CI −24.7% to 4.3%; p=0.18, non-OSIs 29.2–43.9%, difference 14.7%, CI −27.8% to −0.30%; p=0.03). Elevated intraocular pressure (IOP) was the commonest reason for referral for OSIs and non-OSIs, 28.7% and 36.1%, respectively, of total referrals. The proportion of referrals for elevated IOP increased from 10.9% pre-NICE to 28.0% post-NICE for OSIs, and from 19% to 45.1% for non-OSIs. Conclusions: In terms of ‘demand management’, OSIs can reduce FVDR of patients reviewed in secondary care; however, in terms of ‘patient safety’ this study also shows that overemphasis on IOP as a criterion for referral is having an adverse effect on both the non-OSIs and indeed the OSIs ability to detect glaucomatous optic nerve features. It is recommended that referral letters from non-OSIs be stratified for risk, directing high-risk patients straight to secondary care, and low-risk patients to OSIs

Defective Leukocyte Adhesion and Chemotaxis Contributes to Combined Immunodeficiency in Humans with Autosomal Recessive MST1 Deficiency.

PURPOSE: To investigate the clinical and functional aspects of MST1 (STK4) deficiency in a profoundly CD4-lymphopenic kindred with a novel homozygous nonsense mutation in STK4. Although recent studies have described the cellular effects of murine Mst1 deficiency, the phenotype of MST1-deficient human lymphocytes has yet to be fully explored. Patient lymphocytes were therefore investigated in the context of current knowledge of murine Mst1 deficiency. METHODS: Genetic etiology was identified by whole exome sequencing of genomic DNA from two siblings, combined with linkage analysis in the wider family. MST1 protein expression was assessed by immunoblotting. The ability of patient lymphocytes to adhere to ICAM-1 under flow conditions was measured, and transwell assays were used to assess chemotaxis. Chemokine receptor expression was examined by flow cytometry and receptor signalling by immunoblotting. RESULTS: A homozygous nonsense mutation in STK4 (c.442C > T, p.Arg148Stop) was found in the patients, leading to a lack of MST1 protein expression. Patient leukocytes exhibited deficient chemotaxis after stimulation with CXCL11, despite preserved expression of CXCR3. Patient lymphocytes were also unable to bind effectively to immobilised ICAM-1 under flow conditions, in keeping with a failure to develop high affinity binding. CONCLUSION: The observed abnormalities of adhesion and migration imply a profound trafficking defect among human MST1-deficient lymphocytes. By analogy with murine Mst1 deficiency and other defects of leucocyte trafficking, this is likely to contribute to immunodeficiency by impairing key aspects of T-cell development and function such as positive selection in the thymus, thymic egress and immune synapse formation in the periphery.This is thepublished version. It first appeared at http://link.springer.com/article/10.1007%2Fs10875-016-0232-2

Estimating the global cost of vision impairment and its major causes: protocol for a systematic review

Introduction: Vision impairment (VI) places a burden on individuals, health systems and society in general. In order to support the case for investing in eye health services, an updated cost of illness study that measures the global impact of VI is necessary. To perform such a study, a systematic review of the literature is needed. Here we outline the protocol for a systematic review to describe and summarise the costs associated with VI and its major causes. Methods and analysis: We will systematically search in Medline (Ovid) and the Centre for Reviews and Dissemination database which includes the National Health Service Economics Evaluation Database. No language or geographical restriction will be applied. Additional literature will be identified by reviewing the references in the included studies and by contacting field experts. Grey literature will be considered. The review will include any study published from 1 January 2000 to November 2019 that provides information about costs of illness, burden of disease and/or loss of well-being in participants with VI due to an unspecified cause or due to one of the seven leading causes globally. Two reviewers will independently screen studies and extract relevant data from included studies. Methodological quality of economic studies will be assessed based on the British Medical Journal checklist for economic submissions adapted to costs of illness studies. This protocol has been prepared following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis protocols and has been published prospectively in Open Science Framework. Ethics and dissemination Formal ethical approval is not required, as primary data will not be collected in this review. The findings of this study will be disseminated through peer-reviewed publications, stakeholder meetings and inclusion in the ongoing Lancet Global Health Commission on Global Eye Health. Registration details https://osf.io/9au3w (DOI 10.17605/OSF.IO/6F8VM)

The zinc finger/RING domain protein Unkempt regulates cognitive flexibility

Correct orchestration of nervous system development is a profound challenge that involves coordination of complex molecular and cellular processes. Mechanistic target of rapamycin (mTOR) signaling is a key regulator of nervous system development and synaptic function. The mTOR kinase is a hub for sensing inputs including growth factor signaling, nutrients and energy levels. Activation of mTOR signaling causes diseases with severe neurological manifestations, such as tuberous sclerosis complex and focal cortical dysplasia. However, the molecular mechanisms by which mTOR signaling regulates nervous system development and function are poorly understood. Unkempt is a conserved zinc finger/RING domain protein that regulates neurogenesis downstream of mTOR signaling in Drosophila. Unkempt also directly interacts with the mTOR complex I component Raptor. Here we describe the generation and characterisation of mice with a conditional knockout of Unkempt (UnkcKO) in the nervous system. Loss of Unkempt reduces Raptor protein levels in the embryonic nervous system but does not affect downstream mTORC1 targets. We also show that nervous system development occurs normally in UnkcKO mice. However, we find that Unkempt is expressed in the adult cerebellum and hippocampus and behavioural analyses show that UnkcKO mice have improved memory formation and cognitive flexibility to re-learn. Further understanding of the role of Unkempt in the nervous system will provide novel mechanistic insight into the role of mTOR signaling in learning and memory