title: Inhibition of mammalian target of rapamycin activates apoptosis signal-regulating kinase 1 signaling by suppressing protein phosphatase 5 activity.
abstract: Under serum-free conditions, rapamycin, an inhibitor of mammalian target of rapamycin (mTOR), induces a cellular stress response characterized by rapid and sustained activation of the apoptosis signal-regulating kinase 1 (ASK1) signaling pathway and selective apoptosis of cells lacking functional p53. Here we have investigated how mTOR regulates ASK1 signaling using p53-mutant rhabdomyosarcoma cells. In Rh30 cells, ASK1 was found to physically interact with protein phosphatase 5 (PP5), previously identified as a negative regulator of ASK1. Rapamycin did not affect either protein level of PP5 or association of PP5 with ASK1. Instead, rapamycin caused rapid dissociation of the PP2A-B" regulatory subunit (PR72) from the PP5-ASK1 complex, which was associated with reduced phosphatase activity of PP5. This effect was dependent on expression of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1). Down-regulation of PP5 activity by rapamycin coordinately activated ASK1, leading to elevated phosphorylation of c-Jun. Amino acid deprivation, which like rapamycin inhibits mTOR signaling, also inhibited PP5 activity, caused rapid dissociation of PR72, and activated ASK1 signaling. Overexpression of PP5, but not the PP2A catalytic subunit, blocked rapamycin-induced phosphorylation of c-Jun, and protected cells from rapamycin-induced apoptosis. The results suggest that PP5 is downstream of mTOR, and positively regulated by the mTOR pathway. The findings suggest that in the absence of serum factors, mTOR signaling suppresses apoptosis through positive regulation of PP5 activity and suppression of cellular stress.
authors.affiliation: Department of Molecular Pharmacology, St. Jude Children's Research Hospital, 332 Lauderdale, Memphis, TN 38105-2794, USA.
authors.forename: Shile
authors.initials: S
authors.lastname: Huang
authors.forename: Lili
authors.initials: L
authors.lastname: Shu
authors.forename: John
authors.initials: J
authors.lastname: Easton
authors.forename: Franklin C
authors.initials: FC
authors.lastname: Harwood
authors.forename: Glen S
authors.initials: GS
authors.lastname: Germain
authors.forename: Hidenori
authors.initials: H
authors.lastname: Ichijo
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Houghton
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D017209:Apoptosis; D015153:Blotting, Western; D002352:Carrier Proteins / Q000378:metabolism; D020134:Catalytic Domain; D018797:Cell Cycle Proteins; D016923:Cell Death; D002455:Cell Division; D045744:Cell Line, Tumor; D002469:Cell Separation; D016895:Culture Media, Serum-Free / Q000494:pharmacology; D015536:Down-Regulation; D005434:Flow Cytometry; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases; D048848:MAP Kinase Kinase Kinase 5; D020930:MAP Kinase Kinase Kinases / Q000378:metabolism*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009154:Mutation; D009687:Nuclear Proteins / Q000037:antagonists & inhibitors*; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D010750:Phosphoproteins / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D010766:Phosphorylation; D011233:Precipitin Tests; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases; D012208:Rhabdomyosarcoma / Q000378:metabolism; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D013997:Time Factors; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D016895:Culture Media, Serum-Free; C087000:EIF4EBP1 protein, human; D009687:Nuclear Proteins; D010750:Phosphoproteins; D047428:Protein Kinase Inhibitors; D016159:Tumor Suppressor Protein p53; D011494:Protein Kinases; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases; D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D048848:MAP Kinase Kinase Kinase 5; D020930:MAP Kinase Kinase Kinases; C482858:MAP3K5 protein, human; D010749:Phosphoprotein Phosphatases; C089641:protein phosphatase 5; D010744:Phosphoric Monoester Hydrolases; D020123:Sirolimus