title: Mutation of the Rb1 pathway leads to overexpression of mTor, constitutive phosphorylation of Akt on serine 473, resistance to anoikis, and a block in c-Raf activation.
abstract: Atk can be activated by two independent phosphorylation events. Growth factor-dependent phosphorylation of threonine 308 (Akt-308) by phosphatidylinositol 3-kinase-dependent PDK1 leads to activation of mammalian target of rapamycin (mTor) complex 1 (TORC1) and stimulation of protein synthesis. Phosphorylation on serine 473 (Akt-473) is catalyzed by mTor in a second complex (TORC2), and Akt-473 phosphorylates Foxo3a to inhibit apoptosis. Accumulation of both phosphorylated forms of Akt is frequent in cancer, and TORC2 activity is required for progression to prostate cancer with Pten mutation. Here, we link Akt-473 to the Rb1 pathway and show that mTor is overexpressed with loss of the Rb1 family pathway. This leads to constitutive Akt-473 and, in turn, phosphorylation of Foxo3a and resistance to cell adhesion-dependent apoptosis (anoikis). Additionally, Akt-473 accumulation blocks c-Raf activation, thereby preventing downstream Erk activation. This block cannot be overcome by constitutively active Ras, and it also prevents induction of the Arf tumor suppressor by Ras. These studies link inactivation of the Rb1 pathway, a hallmark of cancer, to accumulation of Akt-473, resistance to anoikis, and a block in c-Raf/Erk activation.
authors.affiliation: James Brown Cancer Center, Department of Ophthalmology, University of Louisville Health Sciences Center, 301 E. Muhammad Ali Blvd., Louisville, KY 40202, USA.
authors.forename: Shahenda
authors.initials: S
authors.lastname: El-Naggar
authors.forename: Yongqing
authors.initials: Y
authors.lastname: Liu
authors.forename: Douglas C
authors.initials: DC
authors.lastname: Dean
mesh_terms: D000818:Animals; D023102:Anoikis; D002453:Cell Cycle / Q000235:genetics; D003593:Cytoplasm / Q000201:enzymology; D004622:Embryo, Mammalian / Q000166:cytology; D004789:Enzyme Activation; D005347:Fibroblasts / Q000166:cytology / Q000201:enzymology; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000378:metabolism; D020935:MAP Kinase Signaling System; D051379:Mice; D018345:Mice, Knockout; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3 / Q000378:metabolism; D009154:Mutation / Q000235:genetics*; D010766:Phosphorylation; D010768:Phosphoserine / Q000378:metabolism*; D011494:Protein Kinases / Q000378:metabolism*; D021381:Protein Transport; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000378:metabolism; D016160:Retinoblastoma Protein / Q000235:genetics* / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases; D018631:ras Proteins / Q000378:metabolism
chemical_list: D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; C474033:FoxO3 protein, mouse; D034741:RNA, Small Interfering; D016160:Retinoblastoma Protein; D010768:Phosphoserine; D011494:Protein Kinases; C546843:mTOR protein, mouse; D051057:Proto-Oncogene Proteins c-akt; D019908:Proto-Oncogene Proteins c-raf; D058570:TOR Serine-Threonine Kinases; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D018631:ras Proteins
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