Issam Ben-Sahra, PhD - US grants

SUMMARY The mechanistic target of rapamycin complex 1 (mTORC1) senses and integrates diverse environmental signals to control energy and nutrient-consuming biosynthetic processes, such as protein, lipid, and nucleotide synthesis. mTORC1 stimulates anabolic cell growth through posttranslational and transcriptional mechanisms leading to increased macromolecule synthesis a prerequisite to augment cellular biomass priming cells for growth and division. In many diseases, the prominence of mTORC1 signaling reinforces the importance of considering targeting mTORC1 signaling in several diseases including neurodegenerative disorders, diabetes, tumor syndromes, and aging. However, direct mTORC1 targeted therapies, being conceptually and preclinically a promising target, displayed only limited efficacy in human patients. Therefore, a better understanding of the biology downstream of mTORC1 and the development of more effective and specific therapeutic strategies in the treatment of mTORC1-driven diseases are needed. To achieve the biosynthetic demands accompanying proliferation, cells must increase the transport of nutrients from the environment. Glucose, lactate, and glutamine are the principal nutrients that promote biosynthesis and survival in mammalian cells. An emerging aspect of nutrient utilization in aging and proliferative diseases includes the role of dietary methionine restriction, which was recently explored in the context of obesity, metabolic syndrome, and cancer. Methionine is an essential amino acid that is catabolized and recycled in a sequence of metabolic reactions designated as the methionine cycle. Methionine and ATP are converted into the universal methyl donor S-adenosylmethionine (SAM) via the methionine adenosyltransferase 2 alpha (MAT2A) enzyme. Under this proposal, we propose to study the influence of mTORC1 signaling on S-adenosylmethionine (SAM) synthesis and the subsequent methylation processes supporting anabolic metabolism. We have identified that mTORC1 stimulates SAM synthesis in various cell settings through direct transcriptional control of MAT2A expression by c-MYC. We propose to evaluate the influence of mTORC1 signaling on SAM synthesis in a variety of human cells (Specific Aim1). Will identify the mechanisms by which mTORC1 signaling promotes RNA methylation, particularly the N6- methyladenosine (m6A) mark. We will determine the role of m6A on RNA downstream of mTORC1 in the control of cell growth (Specific Aim2). Furthermore, we will determine the implication of the mTORC1-MAT2A axis on tumor growth and the potential therapeutic strategy derived from this mechanism (Specific Aim3). Thus, the overall goal of this proposal is to decipher the molecular mechanisms by which mTORC1 controls RNA methylation in normal and pathological settings. We anticipate that the proposed studies will yield new insights into how SAM levels alter anabolic metabolism and will uncover therapeutic targets to perturb mTORC1-driven diseases.

PROJECT SUMMARY   Cells  and  organisms  must  coordinate  their  metabolic  activity  with  changes  in  their  nutrient  environment.  This  coordination  is  achieved  via  the  signaling  networks  that  integrate  local  and  systemic  nutrient  inputs  and  relay  nutrient  status  to  the  control  of  cellular  anabolic  and  catabolic  processes.  This  task  can be  carried out by  the  RAS-­RAF-­MEK-­ERK  cascade,  a  signaling  system  that  is  commonly  activated  by  various  growth  factors  and  oncogenic events. In response to a mitogen factor such as the epithelial growth factor (EGF), ERK is activated  and  promotes  cell  proliferation  and  differentiation  by  regulating  activity  of  transcription  factors  involved  in  cell  cycle progression and proliferation. However, much less is understood about how ERK signaling directly controls  metabolic processes. Targeting the kinases RAF, MEK or ERK is currently a strategy employed to treat several  diseases including cancer, type 2 diabetes, metabolic disorders and neurodegeneration, however mechanisms  of  resistance  often  occur.  Therefore,  elucidating  the  downstream  targets  of  ERK  and  more  specifically  the  molecular  mechanisms  by  which  ERK  signaling  drives  metabolism  is  of  great  interest  in  order  to  identify  new  therapeutic  strategies  against  ERK  driven  disease.  Recently  we  discovered  that  the  mechanistic  target  of  rapamycin  complex  1  (mTORC1)  stimulates  synthesis  of  purines  and  pyrimidines  de  novo  through  different  molecular mechanisms. Nucleotides play a central role in metabolism at a fundamental and cellular level. Purine  and pyrimidine bases can be synthesized de novo or recycled through the salvage pathways. Nucleotides carry  packets  of  chemical  energy  (e.g.  ATP,  GTP)  throughout  the  cell  to  the  many  cellular  functions  that  demand  energy, which include: synthesizing nucleic acids, proteins and cell membranes. Under this proposal, we propose  to study the influence of ERK signaling on nucleotide synthesis. We have identified that ERK signaling stimulates  de  novo  purine  synthesis  in  various  settings  through  posttranslational  modification  of  the  enzyme  PFAS  (phosphoformylglycinamidine synthase) which belongs to the de novo purine synthesis pathway. We propose to  dissect  the  molecular  mechanisms  underlying  this  regulation  (Specific  Aim1).  We  will  determine  the  role  of  the  ERK-­PFAS axis in the control of cell growth (Specific Aim 2). Furthermore, we will determine the implication of  this regulation in ERK-­mediated biology and disease (Specific Aim3). Thus, the overall goal of this proposal is to  decipher  the  molecular  mechanisms  by  which  ERK  controls  de  novo  nucleotide  synthesis  in  normal  and  pathological  settings.  We  anticipate  that  the  proposed  studies  will  yield  new  insights  into  how  nucleotide  synthesis is regulated by ERK and will uncover therapeutic targets to perturb ERK-­mediated disease.