title: Loss of Tsc1/Tsc2 activates mTOR and disrupts PI3K-Akt signaling through downregulation of PDGFR.
abstract: Tuberous sclerosis (TSC) is a familial tumor syndrome due to mutations in TSC1 or TSC2, in which progression to malignancy is rare. Primary Tsc2(-/-) murine embryo fibroblast cultures display early senescence with overexpression of p21CIP1/WAF1 that is rescued by loss of TP53. Tsc2(-/-)TP53(-/-) cells, as well as tumors from Tsc2(+/-) mice, display an mTOR-activation signature with constitutive activation of S6K, which is reverted by treatment with rapamycin. Rapamycin also reverts a growth advantage of Tsc2(-/-)TP53(-/-) cells. Tsc1/Tsc2 does not bind directly to mTOR, however, nor does it directly influence mTOR kinase activity or cellular phosphatase activity. There is a marked reduction in Akt activation in Tsc2(-/-)TP53(-/-) and Tsc1(-/-) cells in response to serum and PDGF, along with a reduction in cell ruffling. PDGFRalpha and PDGFRbeta expression is markedly reduced in both the cell lines and Tsc mouse renal cystadenomas, and ectopic expression of PDGFRbeta in Tsc2-null cells restores Akt phosphorylation in response to serum, PDGF, EGF, and insulin. This activation of mTOR along with downregulation of PDGFR PI3K-Akt signaling in cells lacking Tsc1 or Tsc2 may explain why these genes are rarely involved in human cancer. This is in contrast to PTEN, which is a negative upstream regulator of this pathway.
authors.affiliation: Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Hongbing
authors.initials: H
authors.lastname: Zhang
authors.forename: Gregor
authors.initials: G
authors.lastname: Cicchetti
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Onda
authors.forename: Henry B
authors.initials: HB
authors.lastname: Koon
authors.forename: Kirsten
authors.initials: K
authors.lastname: Asrican
authors.forename: Natalia
authors.initials: N
authors.lastname: Bajraszewski
authors.forename: Francisca
authors.initials: F
authors.lastname: Vazquez
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Carpenter
authors.forename: David J
authors.initials: DJ
authors.lastname: Kwiatkowski
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D016922:Cellular Senescence; D015536:Down-Regulation; D008297:Male; D051379:Mice; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology*; D011494:Protein Kinases / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases; D011506:Proteins / Q000502:physiology*; D011518:Proto-Oncogene Proteins / Q000502:physiology*; D051057:Proto-Oncogene Proteins c-akt; D017479:Receptors, Platelet-Derived Growth Factor / Q000032:analysis*; D012097:Repressor Proteins / Q000502:physiology*; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D058570:TOR Serine-Threonine Kinases; D000077004:Tuberous Sclerosis Complex 1 Protein; D000077005:Tuberous Sclerosis Complex 2 Protein; D016159:Tumor Suppressor Protein p53 / Q000032:analysis; D025521:Tumor Suppressor Proteins
chemical_list: D019869:Phosphatidylinositol 3-Kinases; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D011506:Proteins; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D017479:Receptors, Platelet-Derived Growth Factor; D012097:Repressor Proteins; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D000077004:Tuberous Sclerosis Complex 1 Protein; D000077005:Tuberous Sclerosis Complex 2 Protein; D016159:Tumor Suppressor Protein p53; D025521:Tumor Suppressor Proteins; C000624650:TSC1 protein, human; C000624653:TSC2 protein, human; C000624651:Tsc1 protein, mouse; C000624654:Tsc2 protein, mouse; C546842:MTOR protein, human; C546843:mTOR protein, mouse