Inhibition of Mammalian Target of Rapamycin Activates Apoptosis Signal-regulating Kinase 1 Signaling by Suppressing Protein Phosphatase 5 Activity

• Published in: The Journal of biological chemistry Vol. 279; no. 35; pp. 36490 - 36496
• Main Authors: Huang, Shile, Shu, Lili, Easton, John, Harwood, Franklin C, Germain, Glen S, Ichijo, Hidenori, Houghton, Peter J
• Format: Journal Article
• Language: English
• Published: United States American Society for Biochemistry and Molecular Biology 27-08-2004
• Subjects: Adaptor Proteins, Signal Transducing; Apoptosis; Blotting, Western; Carrier Proteins - metabolism; Catalytic Domain; Cell Cycle Proteins; Cell Death; Cell Division; Cell Line, Tumor; Cell Separation; Culture Media, Serum-Free - pharmacology; Down-Regulation; Flow Cytometry; Humans; JNK Mitogen-Activated Protein Kinases; MAP Kinase Kinase Kinase 5; MAP Kinase Kinase Kinases - metabolism; Mitogen-Activated Protein Kinases - metabolism; Mutation; Nuclear Proteins - antagonists & inhibitors; Phosphoprotein Phosphatases - antagonists & inhibitors; Phosphoproteins - metabolism; Phosphoric Monoester Hydrolases - metabolism; Phosphorylation; Precipitin Tests; Protein Kinase Inhibitors; Protein Kinases; Rhabdomyosarcoma - metabolism; Signal Transduction; Sirolimus - pharmacology; Time Factors; TOR Serine-Threonine Kinases; Tumor Suppressor Protein p53 - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M401208200

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.