title: Pharmacological and genetic evaluation of proposed roles of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK), extracellular signal-regulated kinase (ERK), and p90(RSK) in the control of mTORC1 protein signaling by phorbol esters.
abstract: The mammalian target of rapamycin complex 1 (mTORC1) links the control of mRNA translation, cell growth, and metabolism to diverse stimuli. Inappropriate activation of mTORC1 can lead to cancer. Phorbol esters are naturally occurring products that act as potent tumor promoters. They activate isoforms of protein kinase C (PKCs) and stimulate the oncogenic MEK/ERK signaling cascade. They also activate mTORC1 signaling. Previous work indicated that mTORC1 activation by the phorbol ester PMA (phorbol 12-myristate 13-acetate) depends upon PKCs and may involve MEK. However, the precise mechanism(s) through which they activate mTORC1 remains unclear. Recent studies have implicated both the ERKs and the ERK-activated 90-kDa ribosomal S6 kinases (p90(RSK)) in activating mTORC1 signaling via phosphorylation of TSC2 (a regulator of mTORC1) and/or the mTORC1 component raptor. However, the relative importance of each of these kinases and phosphorylation events for the activation of mTORC1 signaling is unknown. The recent availability of MEK (PD184352) and p90(RSK) (BI-D1870) inhibitors of improved specificity allowed us to address the roles of these protein kinases in controlling mTORC1 in a variety of human and rodent cell types. In parallel, we used specific shRNAs against p90(RSK1) and p90(RSK2) to further test their roles in regulating mTORC1 signaling. Our data indicate that p90(RSKs) are dispensable for the activation of mTORC1 signaling by phorbol esters in all cell types tested. Our data also reveal striking diversity in the requirements for MEK/ERK in the control of mTORC1 between different cell types, pointing to additional signaling connections between phorbol esters and mTORC1, which do not involve MEK/ERK. This study provides important information for the design of efficient strategies to combat the hyperactivation of mTORC1 signaling by oncogenic pathways.
authors.affiliation: Division of Molecular Physiology, College of Life Sciences, University of Dundee, Dundee DD1 5EH, United Kingdom.
authors.forename: Bruno D
authors.initials: BD
authors.lastname: Fonseca
authors.forename: Tommy
authors.initials: T
authors.lastname: Alain
authors.forename: Leona K
authors.initials: LK
authors.lastname: Finestone
authors.forename: Brandon P H
authors.initials: BP
authors.lastname: Huang
authors.forename: Mark
authors.initials: M
authors.lastname: Rolfe
authors.forename: Tian
authors.initials: T
authors.lastname: Jiang
authors.forename: Zhong
authors.initials: Z
authors.lastname: Yao
authors.forename: Greco
authors.initials: G
authors.lastname: Hernandez
authors.forename: Christopher F
authors.initials: CF
authors.lastname: Bennett
authors.forename: Christopher G
authors.initials: CG
authors.lastname: Proud
mesh_terms: D000818:Animals; D002460:Cell Line; D006801:Humans; D051379:Mice; D041681:NIH 3T3 Cells; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011494:Protein Kinases / Q000187:drug effects / Q000378:metabolism*; D011621:Pteridines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000187:drug effects*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D014157:Transcription Factors / Q000378:metabolism*
chemical_list: C516541:BI D1870; D047428:Protein Kinase Inhibitors; D011621:Pteridines; D014157:Transcription Factors; D011494:Protein Kinases; D013755:Tetradecanoylphorbol Acetate