title: Drosophila insulin and target of rapamycin (TOR) pathways regulate GSK3 beta activity to control Myc stability and determine Myc expression in vivo.
abstract: BACKGROUND
  Genetic studies in Drosophila melanogaster reveal an important role for Myc in controlling growth. Similar studies have also shown how components of the insulin and target of rapamycin (TOR) pathways are key regulators of growth. Despite a few suggestions that Myc transcriptional activity lies downstream of these pathways, a molecular mechanism linking these signaling pathways to Myc has not been clearly described. Using biochemical and genetic approaches we tried to identify novel mechanisms that control Myc activity upon activation of insulin and TOR signaling pathways.
  
  
  RESULTS
  Our biochemical studies show that insulin induces Myc protein accumulation in Drosophila S2 cells, which correlates with a decrease in the activity of glycogen synthase kinase 3-beta (GSK3β ) a kinase that is responsible for Myc protein degradation. Induction of Myc by insulin is inhibited by the presence of the TOR inhibitor rapamycin, suggesting that insulin-induced Myc protein accumulation depends on the activation of TOR complex 1. Treatment with amino acids that directly activate the TOR pathway results in Myc protein accumulation, which also depends on the ability of S6K kinase to inhibit GSK3β activity. Myc upregulation by insulin and TOR pathways is a mechanism conserved in cells from the wing imaginal disc, where expression of Dp110 and Rheb also induces Myc protein accumulation, while inhibition of insulin and TOR pathways result in the opposite effect. Our functional analysis, aimed at quantifying the relative contribution of Myc to ommatidial growth downstream of insulin and TOR pathways, revealed that Myc activity is necessary to sustain the proliferation of cells from the ommatidia upon Dp110 expression, while its contribution downstream of TOR is significant to control the size of the ommatidia.
  
  
  CONCLUSIONS
  Our study presents novel evidence that Myc activity acts downstream of insulin and TOR pathways to control growth in Drosophila. At the biochemical level we found that both these pathways converge at GSK3β to control Myc protein stability, while our genetic analysis shows that insulin and TOR pathways have different requirements for Myc activity during development of the eye, suggesting that Myc might be differentially induced by these pathways during growth or proliferation of cells that make up the ommatidia.
authors.affiliation: Department of Biology, City College of City University of New York, New York, USA.
authors.forename: Federica
authors.initials: F
authors.lastname: Parisi
authors.forename: Sara
authors.initials: S
authors.lastname: Riccardo
authors.forename: Margaret
authors.initials: M
authors.lastname: Daniel
authors.forename: Mahesh
authors.initials: M
authors.lastname: Saqcena
authors.forename: Nandini
authors.initials: N
authors.lastname: Kundu
authors.forename: Annalisa
authors.initials: A
authors.lastname: Pession
authors.forename: Daniela
authors.initials: D
authors.lastname: Grifoni
authors.forename: Hugo
authors.initials: H
authors.lastname: Stocker
authors.forename: Esteban
authors.initials: E
authors.lastname: Tabak
authors.forename: Paola
authors.initials: P
authors.lastname: Bellosta
mesh_terms: D000818:Animals; D002460:Cell Line; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism*; D004331:Drosophila melanogaster / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D005123:Eye / Q000254:growth & development / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism*; D000071679:Glycogen Synthase Kinase 3 beta; D060227:Imaginal Discs / Q000254:growth & development / Q000378:metabolism; D007328:Insulin / Q000378:metabolism*; D055550:Protein Stability; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics* / Q000378:metabolism; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*; D014921:Wings, Animal / Q000254:growth & development / Q000378:metabolism
chemical_list: D029721:Drosophila Proteins; D007328:Insulin; D016271:Proto-Oncogene Proteins c-myc; D000071679:Glycogen Synthase Kinase 3 beta; D058570:TOR Serine-Threonine Kinases; D038362:Glycogen Synthase Kinase 3