title: mTOR inhibition and levels of the DNA repair protein MGMT in T98G glioblastoma cells.
abstract: BACKGROUND
  Glioblastoma multiforme (GBM), the most common and most aggressive type of primary adult brain tumour, responds poorly to conventional treatment. Temozolomide (TMZ) chemotherapy remains the most commonly used treatment, despite a large proportion of tumours displaying TMZ resistance. 60% of GBM tumours have unmethylated MGMT promoter regions, resulting in an overexpression of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT), which is responsible for tumour resistance to TMZ chemotherapy. Tumours also often exhibit hyperactive PI3-kinase/mTOR signalling, which enables them to resynthesise proteins quickly. Since MGMT is a suicide protein that is degraded upon binding to and repairing TMZ-induced O6-methylguanine adducts, it has been hypothesized that inhibition of translation via the mTOR signalling pathway could generate a tumour-specific reduction in MGMT protein and increase TMZ sensitivity.
  
  
  METHODS
  MGMT was monitored at the post-transcriptional, translational and protein levels, to determine what effect mTOR inhibition was having on MGMT protein expression in vitro.
  
  
  RESULTS
  We show that inhibiting mTOR signalling is indeed associated with acute inhibition of protein synthesis. Western blots show that despite this, relative to loading control proteins, steady state levels of MGMT protein increased and MGMT mRNA was retained in heavy polysomes. Whilst TMZ treatment resulted in maintained MGMT protein levels, concomitant treatment of T98G cells with TMZ and KU0063794 resulted in increased MGMT protein levels without changes in total mRNA levels.
  
  
  CONCLUSIONS
  These in vitro data suggest that, counterintuitively, mTOR inhibition may not be a useful adjunct to TMZ therapy and that more investigation is needed before applying mTOR inhibitors in a clinical setting.
authors.affiliation: Department of Biochemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QG, UK. s.smalley@bsms.ac.uk.
authors.forename: Sarah
authors.initials: S
authors.lastname: Smalley
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Chalmers
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Morley
mesh_terms: D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology*; D045744:Cell Line, Tumor; D003513:Cycloheximide / Q000494:pharmacology; D015254:DNA Modification Methylases / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D004260:DNA Repair / Q000187:drug effects* / Q000235:genetics; D045643:DNA Repair Enzymes / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D004273:DNA, Neoplasm / Q000037:antagonists & inhibitors* / Q000378:metabolism; D003606:Dacarbazine / Q000031:analogs & derivatives* / Q000037:antagonists & inhibitors / Q000494:pharmacology; D015870:Gene Expression; D006801:Humans; D009025:Morpholines / Q000494:pharmacology*; D009154:Mutation; D009457:Neuroglia / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D014176:Protein Biosynthesis / Q000187:drug effects; D011743:Pyrimidines / Q000494:pharmacology*; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D000077204:Temozolomide; D025521:Tumor Suppressor Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism
chemical_list: D018906:Antineoplastic Agents, Alkylating; D004273:DNA, Neoplasm; D009025:Morpholines; D011743:Pyrimidines; D025521:Tumor Suppressor Proteins; D003606:Dacarbazine; C541932:Ku 0063794; D003513:Cycloheximide; D015254:DNA Modification Methylases; C515491:MGMT protein, human; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases; D045643:DNA Repair Enzymes; D000077204:Temozolomide