title: Akt is efficiently activated by PIF-pocket- and PtdIns(3,4,5)P3-dependent mechanisms leading to resistance to PDK1 inhibitors.
abstract: Mutations leading to inappropriate activation of Akt isoforms contribute to proliferation and survival of a significant proportion of human cancers. Akt is activated by phosphorylation of its T-loop residue (Thr(308)) by PDK1 (3-phosphoinositide-dependent kinase-1) and its C-terminal hydrophobic motif (Ser(473)) by mTORC2 [mTOR (mammalian target of rapamycin) complex 2]. Potent PDK1 inhibitors such as GSK2334470 have recently been elaborated as potential anti-cancer agents. However, these compounds were surprisingly ineffective at suppressing Akt activation. In the present study we demonstrate that resistance to PDK1 inhibitors results from Akt being efficiently recruited to PDK1 via two alternative mechanisms. The first involves ability of Akt and PDK1 to mutually interact with the PI3K (phosphoinositide 3-kinase) second messenger PtdIns(3,4,5)P3. The second entails recruitment of PDK1 to Akt after its phosphorylation at Ser(473) by mTORC2, via a substrate-docking motif termed the PIF-pocket. We find that disruption of either the PtdIns(3,4,5)P3 or the Ser(473) phosphorylation/PIF-pocket mechanism only moderately impacts on Akt activation, but induces marked sensitization to PDK1 inhibitors. These findings suggest that suppression of Ser(473) phosphorylation by using mTOR inhibitors would disrupt the PIF-pocket mechanism and thereby sensitize Akt to PDK1 inhibitors. Consistent with this, we find combing PDK1 and mTOR inhibitors reduced Akt activation to below basal levels and markedly inhibited proliferation of all of the cell lines tested. Our results suggest further work is warranted to explore the utility of combining PDK1 and mTOR inhibitors as a therapeutic strategy for treatment of cancers that harbour mutations elevating Akt activity.
authors.affiliation: MRC Protein Phosphorylation Unit, College of Life Sciences, University of Dundee, Dow Street, Dundee DD1 5EH, Scotland, UK. a.najafov@dundee.ac.uk
authors.forename: Ayaz
authors.initials: A
authors.lastname: Najafov
authors.forename: Natalia
authors.initials: N
authors.lastname: Shpiro
authors.forename: Dario R
authors.initials: DR
authors.lastname: Alessi
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D019008:Drug Resistance, Neoplasm; D004789:Enzyme Activation; D057809:HEK293 Cells; D006801:Humans; D007191:Indazoles / Q000494:pharmacology; D000076225:Mechanistic Target of Rapamycin Complex 2; D051379:Mice; D008969:Molecular Sequence Data; D046912:Multiprotein Complexes / Q000172:deficiency / Q000235:genetics; D009154:Mutation; D009369:Neoplasms / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism; D018129:Phosphatidylinositol Phosphates / Q000378:metabolism*; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors*; D051057:Proto-Oncogene Proteins c-akt / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011743:Pyrimidines / Q000494:pharmacology; D000081382:Pyruvate Dehydrogenase Acetyl-Transferring Kinase; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D012694:Serine / Q000737:chemistry; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000172:deficiency / Q000235:genetics
chemical_list: C555257:GSK 2334470; D007191:Indazoles; D046912:Multiprotein Complexes; C000631429:PDK1 protein, human; C000631433:Pdk1 protein, mouse; D018129:Phosphatidylinositol Phosphates; D047428:Protein Kinase Inhibitors; D011743:Pyrimidines; D000081382:Pyruvate Dehydrogenase Acetyl-Transferring Kinase; D011994:Recombinant Proteins; C060974:phosphatidylinositol 3,4,5-triphosphate; D012694:Serine; C546842:MTOR protein, human; C494918:AKT1 protein, human; D000076225:Mechanistic Target of Rapamycin Complex 2; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases