title: Protein phosphatase 2A and DNA-dependent protein kinase are involved in mediating rapamycin-induced Akt phosphorylation.
abstract: BACKGROUND
  The mechanisms underlying rapamycin-induced Akt phosphorylation have not been fully elucidated.
  
  
  RESULTS
  Inhibition of PP2A or DNA-PK attenuates or abrogates rapamycin-induced Akt phosphorylation and co-inhibition of mTOR and DNA-PK enhances anticancer activity.
  
  
  CONCLUSIONS
  PP2A-dependent and DNA-PK-mediated mechanism is involved in rapamycin-induced Akt phosphorylation.
  
  
  CONCLUSIONS
  A previously unknown mechanism underlying rapamycin-induced Akt phosphorylation and a novel strategy to enhance mTOR-targeted cancer therapy may be suggested. Inhibition of mammalian target of rapamycin complex 1 (mTORC1), for example with rapamycin, increases Akt phosphorylation while inhibiting mTORC1 signaling. However, the underlying mechanisms have not been fully elucidated. The current study has uncovered a previously unknown mechanism underlying rapamycin-induced Akt phosphorylation involving protein phosphatase 2A (PP2A)-dependent DNA protein kinase (DNA-PK) activation. In several cancer cell lines, inhibition of PP2A with okadaic acid, fostriecin, small T antigen, or PP2A knockdown abrogated rapamycin-induced Akt phosphorylation, and rapamycin increased PP2A activity. Chemical inhibition of DNA-PK, knockdown or deficiency of DNA-PK catalytic subunit (DNA-PKcs), or knock-out of the DNA-PK component Ku86 inhibited rapamycin-induced Akt phosphorylation. Exposure of cancer cells to rapamycin increased DNA-PK activity, and gene silencing-mediated PP2A inhibition attenuated rapamycin-induced DNA-PK activity. Collectively these results suggest that rapamycin induces PP2A-dependent and DNA-PK-mediated Akt phosphorylation. Accordingly, simultaneous inhibition of mTOR and DNA-PK did not stimulate Akt activity and synergistically inhibited the growth of cancer cells both in vitro and in vivo. Thus, our findings also suggest a novel strategy to enhance mTOR-targeted cancer therapy by co-targeting DNA-PK.
authors.affiliation: Departments of Hematology and Medical Oncology, Emory University School of Medicine and Winship Cancer Institute, Atlanta, Georgia 30322, USA.
authors.forename: Yikun
authors.initials: Y
authors.lastname: Li
authors.forename: Xuerong
authors.initials: X
authors.lastname: Wang
authors.forename: Ping
authors.initials: P
authors.lastname: Yue
authors.forename: Hui
authors.initials: H
authors.lastname: Tao
authors.forename: Suresh S
authors.initials: SS
authors.lastname: Ramalingam
authors.forename: Taofeek K
authors.initials: TK
authors.lastname: Owonikoko
authors.forename: Xingming
authors.initials: X
authors.lastname: Deng
authors.forename: Ya
authors.initials: Y
authors.lastname: Wang
authors.forename: Haian
authors.initials: H
authors.lastname: Fu
authors.forename: Fadlo R
authors.initials: FR
authors.lastname: Khuri
authors.forename: Shi-Yong
authors.initials: SY
authors.lastname: Sun
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism; D000818:Animals; D000903:Antibiotics, Antineoplastic / Q000494:pharmacology*; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000187:drug effects / Q000201:enzymology; D049109:Cell Proliferation / Q000187:drug effects; D002867:Chromones / Q000494:pharmacology; D003593:Cytoplasm / Q000187:drug effects / Q000201:enzymology; D051747:DNA-Activated Protein Kinase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004357:Drug Synergism; D055785:Gene Knockdown Techniques; D057809:HEK293 Cells; D006801:Humans; D000076222:Mechanistic Target of Rapamycin Complex 1; D051379:Mice; D008819:Mice, Nude; D009025:Morpholines / Q000494:pharmacology; D046912:Multiprotein Complexes; D019319:Okadaic Acid / Q000494:pharmacology; D010766:Phosphorylation; D054648:Protein Phosphatase 2 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011499:Protein Processing, Post-Translational / Q000187:drug effects*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000235:genetics; D000076223:Regulatory-Associated Protein of mTOR; D020123:Sirolimus / Q000494:pharmacology*; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D023041:Xenograft Model Antitumor Assays
chemical_list: C479235:2-(morpholin-4-yl)benzo(h)chromen-4-one; C499693:8-dibenzothiophen-4-yl-2-morpholin-4-yl-chromen-4-one; D048868:Adaptor Proteins, Signal Transducing; D000903:Antibiotics, Antineoplastic; D002867:Chromones; D009025:Morpholines; D046912:Multiprotein Complexes; D034741:RNA, Small Interfering; C463659:RPTOR protein, human; D000076223:Regulatory-Associated Protein of mTOR; D019319:Okadaic Acid; D051747:DNA-Activated Protein Kinase; D000076222:Mechanistic Target of Rapamycin Complex 1; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; C517189:PPP2CA protein, human; D054648:Protein Phosphatase 2; D020123:Sirolimus
keywords: Akt; Cancer Biology; Cancer Therapy; DNA-PK; PP2A; Rapamycin; mTOR
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