bims-pideca Biomed News
on Class IA PI3K signalling in development and cancer
Issue of 2020‒03‒01
sixteen papers selected by
Ralitsa Radostinova Madsen
University College London Cancer Institute


  1. Cell Rep. 2020 Feb 25. pii: S2211-1247(20)30105-4. [Epub ahead of print]30(8): 2729-2742.e4
    Michalopoulou E, Auciello FR, Bulusu V, Strachan D, Campbell AD, Tait-Mulder J, Karim SA, Morton JP, Sansom OJ, Kamphorst JJ.
      Pancreatic ductal adenocarcinoma (PDAC) features a near-universal mutation in KRAS. Additionally, the tumor suppressor PTEN is lost in ∼10% of patients, and in mouse models, this dramatically accelerates tumor progression. While oncogenic KRAS and phosphatidylinositol 3-kinase (PI3K) cause divergent metabolic phenotypes individually, how they synergize to promote tumor metabolic alterations and dependencies remains unknown. We show that in KRAS-driven murine PDAC cells, loss of Pten strongly enhances both mTOR signaling and macropinocytosis. Protein scavenging alleviates sensitivity to mTOR inhibition by rescuing AKT phosphorylation at serine 473 and consequently cell proliferation. Combined inhibition of mTOR and lysosomal processing of internalized protein eliminates the macropinocytosis-mediated resistance. Our results indicate that mTORC2, rather than mTORC1, is an important regulator of protein scavenging and that protein-mediated resistance could explain the lack of effectiveness of mTOR inhibitors in certain genetic backgrounds. Concurrent inhibition of mTOR and protein scavenging might be a valuable therapeutic approach.
    Keywords:  AKT; cancer metabolism; mTORC2; macropinocytosis; metabolic scavenging; pancreatic ductal adenocarcinoma
    DOI:  https://doi.org/10.1016/j.celrep.2020.01.080
  2. Nat Commun. 2020 Feb 28. 11(1): 1121
    Jayashankar V, Edinger AL.
      Macropinocytic cancer cells scavenge amino acids from extracellular proteins. Here, we show that consuming necrotic cell debris via macropinocytosis (necrocytosis) offers additional anabolic benefits. A click chemistry-based flux assay reveals that necrocytosis provides not only amino acids, but sugars, fatty acids and nucleotides for biosynthesis, conferring resistance to therapies targeting anabolic pathways. Indeed, necrotic cell debris allow macropinocytic breast and prostate cancer cells to proliferate, despite fatty acid synthase inhibition. Standard therapies such as gemcitabine, 5-fluorouracil (5-FU), doxorubicin and gamma-irradiation directly or indirectly target nucleotide biosynthesis, creating stress that is relieved by scavenged nucleotides. Strikingly, necrotic debris also render macropinocytic, but not non-macropinocytic, pancreas and breast cancer cells resistant to these treatments. Selective, genetic inhibition of macropinocytosis confirms that necrocytosis both supports tumor growth and limits the effectiveness of 5-FU in vivo. Therefore, this study establishes necrocytosis as a mechanism for drug resistance.
    DOI:  https://doi.org/10.1038/s41467-020-14928-3
  3. Sci Rep. 2020 Feb 25. 10(1): 3418
    Bettedi L, Yan A, Schuster E, Alic N, Foukas LC.
      The Insulin/IGF-1 signalling (IIS) pathway plays an essential role in the regulation of glucose and lipid homeostasis. At the same time, a reduction in the IIS pathway activity can extend lifespan and healthspan in various model organisms. Amongst a number of body organs that sense and respond to insulin/IGF-1, the adipose tissue has a central role in both the metabolic and lifespan effects of IIS at the organismal level. Genetic inactivation of IIS components specifically in the adipose tissue has been shown before to improve metabolic profile and extend lifespan in various model organisms. We sought to identify conserved molecular mechanisms that may underlie the beneficial effects of IIS inhibition in the adipose tissue, specifically at the level of phosphoinositide 3-kinase (PI3K), a key IIS effector molecule. To this end, we inactivated PI3K by genetic means in the fly fat body and by pharmacological inhibition in mammalian adipocytes. Gene expression studies revealed changes to metabolism and upregulation of mitochondrial activity in mouse adipocytes and fly fat bodies with downregulated PI3K, which were confirmed by biochemical assays in mammalian adipocytes. These data suggest that PI3K inactivation has a conserved effect of upregulating mitochondrial metabolism in both fly and mammalian adipose tissue, which likely contributes to the health- and life-span extending effect of IIS pathway downregulation.
    DOI:  https://doi.org/10.1038/s41598-020-60210-3
  4. Elife. 2020 Feb 25. pii: e49917. [Epub ahead of print]9
    Martinez BA, Reis Rodrigues P, Nuñez Medina RM, Mondal P, Harrison NJ, Lone MA, Webster A, Gurkar AU, Grill B, Gill MS.
      In the nematode C. elegans, insulin signaling regulates development and aging in response to the secretion of numerous insulin peptides. Here, we describe a novel, non-signaling isoform of the nematode insulin receptor (IR), DAF-2B, that modulates insulin signaling by sequestration of insulin peptides. DAF-2B arises via alternative splicing and retains the extracellular ligand binding domain but lacks the intracellular signaling domain. A daf-2b splicing reporter revealed active regulation of this transcript through development, particularly in the dauer larva, a diapause stage associated with longevity. CRISPR knock-in of mScarlet into the daf-2b genomic locus confirmed that DAF-2B is expressed in vivo and is likely secreted. Genetic studies indicate that DAF-2B influences dauer entry, dauer recovery and adult lifespan by altering insulin sensitivity according to the prevailing insulin milieu. Thus, in C. elegans alternative splicing at the daf-2 locus generates a truncated IR that fine-tunes insulin signaling in response to the environment.
    Keywords:  C. elegans; DAF-2; aging; alternative splicing; dauer formation; developmental biology; genetics; genomics; insulin sensitivity
    DOI:  https://doi.org/10.7554/eLife.49917
  5. Cancers (Basel). 2020 Feb 21. pii: E501. [Epub ahead of print]12(2):
    Sims D, Maranyane HM, Damerell V, Govender D, Isaacs AW, Peres J, Prince S.
      Rhabdomyosarcoma is a highly aggressive malignant cancer that arises from skeletal muscle progenitor cells and is the third most common solid tumour in children. Despite significant advances, rhabdomyosarcoma still presents a therapeutic challenge, and while targeted therapy has shown promise, there are limited options because the molecular drivers of rhabdomyosarcoma are poorly understood. We previously reported that the T-box transcription factor 3 (TBX3), which has been identified as a druggable target in many cancers, is overexpressed in rhabdomyosarcoma patient samples and cell lines. To identify new molecular therapeutic targets to treat rhabdomyosarcoma, this study investigates the potential oncogenic role(s) for TBX3 and the factors responsible for upregulating it in this cancer. To this end, rhabdomyosarcoma cell culture models in which TBX3 was either stably knocked down or overexpressed were established and the impact on key hallmarks of cancer were examined using growth curves, soft agar and scratch motility assays, as well as tumour-forming ability in nude mice. Our data show that TBX3 promotes substrate-dependent and -independent proliferation, migration and tumour formation. We further reveal that TBX3 is upregulated by c-Myc transcriptionally and AKT1 post-translationally. This study identifies c-Myc/AKT1/TBX3 as an important axis that could be targeted for the treatment of rhabdomyosarcoma.
    Keywords:  AKT1; T-box transcription factor-3 (TBX3); c-Myc; childhood cancer; embryonal rhabdomyosarcoma (ERMS); oncogene; rhabdomyosarcoma (RMS)
    DOI:  https://doi.org/10.3390/cancers12020501
  6. Cell. 2020 Feb 25. pii: S0092-8674(20)30156-2. [Epub ahead of print]
    Xiao H, Jedrychowski MP, Schweppe DK, Huttlin EL, Yu Q, Heppner DE, Li J, Long J, Mills EL, Szpyt J, He Z, Du G, Garrity R, Reddy A, Vaites LP, Paulo JA, Zhang T, Gray NS, Gygi SP, Chouchani ET.
      Mammalian tissues engage in specialized physiology that is regulated through reversible modification of protein cysteine residues by reactive oxygen species (ROS). ROS regulate a myriad of biological processes, but the protein targets of ROS modification that drive tissue-specific physiology in vivo are largely unknown. Here, we develop Oximouse, a comprehensive and quantitative mapping of the mouse cysteine redox proteome in vivo. We use Oximouse to establish several paradigms of physiological redox signaling. We define and validate cysteine redox networks within each tissue that are tissue selective and underlie tissue-specific biology. We describe a common mechanism for encoding cysteine redox sensitivity by electrostatic gating. Moreover, we comprehensively identify redox-modified disease networks that remodel in aged mice, establishing a systemic molecular basis for the long-standing proposed links between redox dysregulation and tissue aging. We provide the Oximouse compendium as a framework for understanding mechanisms of redox regulation in physiology and aging.
    Keywords:  ROS; aging; cysteine; proteomics; reactive oxygen species
    DOI:  https://doi.org/10.1016/j.cell.2020.02.012
  7. Curr Opin Cell Biol. 2020 Feb 22. pii: S0955-0674(20)30020-X. [Epub ahead of print]63 144-153
    Liu JY, Wellen KE.
      Recent years have seen a great expansion in our knowledge of the roles that metabolites play in cellular signaling. Structural data have provided crucial insights into mechanisms through which amino acids are sensed. New nutrient-coupled protein and RNA modifications have been identified and characterized. A growing list of functions has been ascribed to metabolic regulation of modifications such as acetylation, methylation, and glycosylation. A current challenge lies in developing an integrated understanding of the roles that metabolic signaling mechanisms play in physiology and disease, which will inform the design of strategies to target such mechanisms. In this brief article, we review recent advances in metabolic signaling through post-translational modification during cancer progression, to provide a framework for understanding signaling roles of metabolites in the context of cancer biology and illuminate areas for future investigation.
    Keywords:  Cancer; Metabolic signaling; Metabolism
    DOI:  https://doi.org/10.1016/j.ceb.2020.01.013
  8. Cancers (Basel). 2020 Feb 25. pii: E532. [Epub ahead of print]12(3):
    Gallyas F, Sumegi B, Szabo C.
      Poly(ADP-ribose) polymerase (PARP) inhibitors have recently been introduced in the therapy of several types of cancers not responding to conventional treatments. However, de novo and acquired PARP inhibitor resistance is a significant limiting factor in the clinical therapy, and the underlying mechanisms are not fully understood. Activity of the cytoprotective phosphatidylinositol-3 kinase (PI3K)-Akt pathway is often increased in human cancer that could result from mutation, expressional change, or amplification of upstream growth-related factor signaling elements or elements of the Akt pathway itself. However, PARP-inhibitor-induced activation of the cytoprotective PI3K-Akt pathway is overlooked, although it likely contributes to the development of PARP inhibitor resistance. Here, we briefly summarize the biological role of the PI3K-Akt pathway. Next, we overview the significance of the PARP-Akt interplay in shock, inflammation, cardiac and cerebral reperfusion, and cancer. We also discuss a recently discovered molecular mechanism that explains how PARP inhibition induces Akt activation and may account for apoptosis resistance and mitochondrial protection in oxidative stress and in cancer.
    Keywords:  PARP-Akt interplay; PI3K; apoptosis resistance; cytoprotection; mTOR; mitochondrial protection; oxidative stress
    DOI:  https://doi.org/10.3390/cancers12030532
  9. J Hematol Oncol. 2020 Feb 22. 13(1): 13
    Coussy F, El Botty R, Lavigne M, Gu C, Fuhrmann L, Briaux A, de Koning L, Dahmani A, Montaudon E, Morisset L, Huguet L, Sourd L, Painsec P, Chateau-Joubert S, Larcher T, Vacher S, Melaabi S, Salomon AV, Marangoni E, Bieche I.
      BACKGROUND: Metaplastic breast cancer (MBC) is a rare form of breast cancer characterized by an aggressive clinical presentation, with a poor response to standard chemotherapy. MBCs are typically triple-negative breast cancers (TNBCs), frequently with alterations to genes of the PI3K-AKT-mTOR and RTK-MAPK signaling pathways. The objective of this study was to determine the response to PI3K and MAPK pathway inhibitors in patient-derived xenografts (PDXs) of MBCs with targetable alterations.METHODS: We compared survival between triple-negative MBCs and other histological subtypes, in a clinical cohort of 323 TNBC patients. PDX models were established from primary breast tumors classified as MBC. PI3K-AKT-mTOR and RTK-MAPK pathway alterations were detected by targeted next-generation sequencing (NGS) and analyses of copy number alterations. Activation of the PI3K-AKT-mTOR and RTK-MAPK signaling pathways was analyzed with reverse-phase protein arrays (RPPA). PDXs carrying an activating mutation of PIK3CA and genomic changes to the RTK-MAPK signaling pathways were treated with a combination consisting of a PI3K inhibitor and a MEK inhibitor.
    RESULTS: In our clinical cohort, the patients with MBC had a worse prognosis than those with other histological subtypes. We established nine metaplastic TNBC PDXs. Three had a pathogenic mutation of PIK3CA and additional alterations to genes associated with RTK-MAPK signaling. The MBC PDXs expressed typical EMT and stem cell genes and were of the mesenchymal or mesenchymal stem-like TNBC subtypes. On histological analysis, MBC PDXs presented squamous or chondroid differentiation. RPPA analysis showed activation of the PI3K-AKT-mTOR and RTK-MAPK signaling pathways. In vivo, the combination of PI3K and MAPK inhibitors displayed marked antitumor activity in PDXs carrying genomic alterations of PIK3CA, AKT1, BRAF, and FGFR4.
    CONCLUSION: The treatment of metaplastic breast cancer PDXs by activation of the PI3K-AKT-mTOR and RTK-MAPK pathways at the genomic and protein levels with a combination of PI3K and MEK inhibitors resulted in tumor regression in mutated models and may therefore be of interest for therapeutic purposes.
    Keywords:  Combination of targeted therapies; MEK inhibitor; Metaplastic breast cancer; PI3K inhibitor
    DOI:  https://doi.org/10.1186/s13045-020-0846-y
  10. Nat Rev Cancer. 2020 Feb 24.
    Hausser J, Alon U.
      Tumours vary in gene expression programmes and genetic alterations. Understanding this diversity and its biological meaning requires a theoretical framework, which could in turn guide the development of more accurate prognosis and therapy. Here, we review the theory of multi-task evolution of cancer, which is based upon the premise that tumours evolve in the host and face selection trade-offs between multiple biological functions. This theory can help identify the major biological tasks that cancer cells perform and the trade-offs between these tasks. It introduces the concept of specialist tumours, which focus on one task, and generalist tumours, which perform several tasks. Specialist tumours are suggested to be sensitive to therapy targeting their main task. Driver mutations tune gene expression towards specific tasks in a tissue-dependent manner and thus help to determine whether a tumour is specialist or generalist. We discuss potential applications of the theory of multi-task evolution to interpret the spatial organization of tumours and intratumour heterogeneity.
    DOI:  https://doi.org/10.1038/s41568-020-0241-6
  11. J Cell Sci. 2020 Feb 24. pii: jcs.239277. [Epub ahead of print]
    Zheng D, Sussman JH, Jeon MP, Parrish ST, MacMullan MA, Delfarah A, Graham NA.
      Oncogenes can create metabolic vulnerabilities in cancer cells. We tested how AKT and MYC affect the ability of cells to shift between respiration and glycolysis. Using immortalized mammary epithelial cells, we discovered that constitutively active AKT but not MYC induced cell death in galactose culture, where cells rely on oxidative phosphorylation for energy generation. However, the negative effects of AKT were temporary, and AKT-expressing cells recommenced growth after ∼15 days in galactose. To identify the mechanisms regulating AKT-mediated cell death, we used metabolomics and found that AKT cells dying in galactose upregulated glutathione metabolism. Proteomic profiling revealed that AKT cells dying in galactose also upregulated nonsense-mediated mRNA decay, a marker of sensitivity to oxidative stress. We therefore measured levels of reactive oxygen species (ROS) and discovered that galactose induced ROS exclusively in cells expressing AKT. Furthermore, ROS were required for galactose-induced death of AKT-expressing cells. We then confirmed that galactose induced ROS-mediated cell death in breast cancer cells with upregulated AKT signaling. These results demonstrate that AKT but not MYC restricts the flexibility of cancer cells to use oxidative phosphorylation.
    Keywords:  MYC; Metabolomics; Oncogene; PI3K/AKT signaling; Proteomics; Reactive Oxygen Species
    DOI:  https://doi.org/10.1242/jcs.239277
  12. Curr Opin Cell Biol. 2020 Feb 20. pii: S0955-0674(20)30017-X. [Epub ahead of print]63 125-134
    Stefan CJ.
      The endoplasmic reticulum (ER) forms an extensive network of membrane contact sites with intra-cellular organelles and the plasma membrane (PM). Interorganelle contacts have vital roles in membrane lipid and ion dynamics. In particular, ER-PM contacts are integral to numerous inter-cellular and intra-cellular signaling pathways including phosphoinositide lipid and calcium signaling, mechanotransduction, metabolic regulation, and cell stress responses. Accordingly, ER-PM contacts serve important signaling functions in excitable cells including neurons and muscle and endocrine cells. This review highlights recent advances in our understanding of the vital roles for ER-PM contacts in phosphoinositide and calcium signaling and how signaling pathways in turn regulate proteins that form and function at ER-PM contacts.
    Keywords:  Calcium; Cell signaling; Endoplasmic reticulum; Membrane contact site; Phosphoinositide; Plasma membrane
    DOI:  https://doi.org/10.1016/j.ceb.2020.01.010
  13. Hum Pathol. 2020 Feb 20. pii: S0046-8177(20)30020-4. [Epub ahead of print]
    Guillet C, Rechsteiner M, Bellini E, Choschzick M, Moskovszky L, Dedes K, Papassotiropoulos B, Varga Z.
      Juvenile papillomatosis (JP), the so-called Swiss cheese disease is a rare benign breast disease of young adults. An association (up to 28 %) with breast-cancer within the family of affected patients has been reported. A multinodular cystic breast-mass lesion and calcifications characterizes JP in imaging studies. The histological picture is diverse and comprises multiple intraductal-papillomas, usual ductal hyperplasia (UDH), ductectasias, perifocal sclerosing adenosis and calcification. Patients with complete excision of JP lesions have an excellent follow-up; breast cancer develops only on a very low subset of patients. Molecular background of JP has not been investigated until now. In this study, we addressed mutational analysis of JP cases and correlated these results with follow-up and family-history in context with a comprehensive review of JP literature. We identified 13 cases fulfilling the criteria of JP. All patients were female with a median-age of 38 years (26 to 50 years). Follow-up information was available in 11 of 13 patients. Sufficient paraffin embedded tissue and good DNA quality for next generation sequencing (NGS) was available in 10 patients. Paraffin blocks were microdissected in the area of intraductal proliferative disease, the tissue cores underwent NGS analysis using Oncomine Comprehensive Panel. In 5 of 10 patients, we found PIK3CA mutations, in 2 of 10 patients AKT1 mutations in known hotspot regions. Further mutations in MET, FGFR3, PTEN, ATM, NF1 and GNAS genes were detected in individual patients. Some of these mutations were present at high allelic frequencies suggesting germ line mutations. 2 of 3 patients with positive family history had PIK3CA mutation; one patient with positive family history had an AKT1 mutation. One patient who subsequently developed invasive ductal carcinoma in the contralateral breast possibly had a germ line ATM mutation. Our results confirm hotspot mutations in PIK3CA and AKT1 genes in JP associated with positive family history for breast cancer, although these mutations are not specific for JP. The genetic link between JP, positive family history and subsequent risk of breast cancer, needs further studies.
    Keywords:  Mutations; breast cancer; juvenile papillomatosis; positive family history
    DOI:  https://doi.org/10.1016/j.humpath.2020.02.002
  14. Curr Top Med Chem. 2020 Feb 23.
    Uko NE, Güner OF, Matesic DF, Bowen JP.
      Cancer is a devastating disease that has plagued humans from ancient times to this day. After decades of slow research progress, promising drug development, and the identification of new targets, the war on cancer was launched in 1972. The P13K/Akt pathway is a growth-regulating cellular signaling pathway, which in many human cancers is over-activated. Studies have demonstrated that a decrease in Akt phosphorylation by Akt inhibitors is associated with a reduction in tumor cell proliferation. There have been several promising drug candidates that have been studied, including but not limited to ipatasertib (RG7440), 1; afuresertib (GSK2110183), 2; uprosertib (GSK2141795), 3; capivasertib (AZD5363), 4; which reportedly bind to the ATP active site and inhibit Akt activity, thus exerting cytotoxic and antiproliferative activities against human cancer cells. For most of the Akt inhibitors listed, preclinical data reveals robust mechanistic rationales evaluating their effects in various cancer types with hyperactivated Akt signaling. Allosteric inhibitors are also known to alter the activity of kinases. Perifosine (KRX-0401), 5, an alkylphospholipid, is known as the first allosteric Akt inhibitor to enter clinical development and is mechanistically characterized as a PH-domain dependent inhibitor, non-competitive with ATP. This results in a reduction in Akt enzymatic and cellular activities. Other small molecule (MK-2206, 6, PHT-427, Akti-1/2) inhibitors with a similar mechanism of action, alter Akt activity through the suppression of cell growth mediated by inhibition of Akt membrane localization and subsequent activation. The natural product solenopsin has been identified as a downstream inhibitor of Akt. A few promising solenopsin derivatives have emerged through pharmacophore modeling, energy-based calculations, and property predictions.
    Keywords:  ATP inhibitors; Afuresertib (GSK2110183); Akt activation; Akt binding site; Akt inhibitors; Akt kinase; Cancer; Capivasertib (AZD5363); Computational chemistry; Computer-assisted drug design; Drug design; Molecular modeling; P13K/Akt pathway; PKB; Perifosine (KRX-0401); Pharmacophore; Phosphorylation; Solenopsin; Solenopsin analogues.; Uprosertib (GSK2141795); to Ipatasertib (RG7440)
    DOI:  https://doi.org/10.2174/1568026620666200224101808
  15. Endocrinology. 2020 Feb 27. pii: bqaa029. [Epub ahead of print]
    Sanwal R, Khosraviani N, Advani SL, Advani A, Lee WL.
      To act on tissues, circulating insulin must perfuse the relevant organ and then leave the blood stream by crossing the endothelium - a process known as insulin delivery. It has been postulated that the continuous endothelium is a rate-limiting barrier to insulin delivery but existing data are contradictory. This conflict is in part due to the limitations of current models, including the inability to maintain a constant blood pressure in animals and the absence of shear stress in cultured cells.We developed a murine cardiac ex vivo perfusion model which delivers insulin to the heart in situ at a constant flow. We hypothesized that if the endothelial barrier were rate-limiting to insulin delivery, increasing endothelial permeability would accelerate insulin action. The kinetics of myocardial insulin action were determined in the presence or absence of agents that increased endothelial permeability. Permeability was measured using Evans Blue which binds with high affinity to albumin.During our experiments, the myocardium remained sensitive to insulin and the vasculature retained barrier integrity. Perfusion with insulin induced Akt phosphorylation in myocytes but not in the endothelium. Infusion of platelet-activating factor or vascular endothelial growth factor significantly increased permeability to albumin without altering insulin action. Amiloride, an inhibitor of fluid-phase uptake, also did not alter insulin action. These data suggest that the endothelial barrier is not rate-limiting to insulin's action in the heart; its passage out of the coronary circulation is consistent with diffusion or convection. Modulation of trans-endothelial transport to overcome insulin resistance is unlikely to be a viable therapeutic strategy.
    DOI:  https://doi.org/10.1210/endocr/bqaa029
  16. Proc Natl Acad Sci U S A. 2020 Feb 24. pii: 201912864. [Epub ahead of print]
    Philippe L, van den Elzen AMG, Watson MJ, Thoreen CC.
      Terminal oligopyrimidine (TOP) motifs are sequences at the 5' ends of mRNAs that link their translation to the mTOR Complex 1 (mTORC1) nutrient-sensing signaling pathway. They are commonly regarded as discrete elements that reside on ∼100 mRNAs that mostly encode translation factors. However, the full spectrum of TOP sequences and their prevalence throughout the transcriptome remain unclear, primarily because of uncertainty over the mechanism that detects them. Here, we globally analyzed translation targets of La-related protein 1 (LARP1), an RNA-binding protein and mTORC1 effector that has been shown to repress TOP mRNA translation in a few specific cases. We establish that LARP1 is the primary translation regulator of mRNAs with classical TOP motifs genome-wide, and also that these motifs are extreme instances of a broader continuum of regulatory sequences. We identify the features of TOP sequences that determine their potency and quantify these as a metric that accurately predicts mTORC1/LARP1 regulation called a TOPscore. Analysis of TOPscores across the transcriptomes of 16 mammalian tissues defines a constitutive "core" set of TOP mRNAs, but also identifies tissue-specific TOP mRNAs produced via alternative transcription initiation sites. These results establish the central role of LARP1 in TOP mRNA regulation on a transcriptome scale and show how it connects mTORC1 to a tunable and dynamic program of gene expression that is tailored to specific biological contexts.
    Keywords:  LARP1; TOP mRNA; mTORC1; translation
    DOI:  https://doi.org/10.1073/pnas.1912864117