Akt-RSK-S6 Kinase Signaling Networks Activated by Oncogenic Receptor Tyrosine Kinases

Cell Signaling Technology Inc., Danvers, MA 01923, USA.
Science Signaling (Impact Factor: 6.28). 08/2010; 3(136):ra64. DOI: 10.1126/scisignal.2000998
Source: PubMed

ABSTRACT Receptor tyrosine kinases (RTKs) activate pathways mediated by serine-threonine kinases, such as the PI3K (phosphatidylinositol 3-kinase)-Akt pathway, the Ras-MAPK (mitogen-activated protein kinase)-RSK (ribosomal S6 kinase) pathway, and the mTOR (mammalian target of rapamycin)-p70 S6 pathway, that control important aspects of cell growth, proliferation, and survival. The Akt, RSK, and p70 S6 family of protein kinases transmits signals by phosphorylating substrates on an RxRxxS/T motif (R, arginine; S, serine; T, threonine; and x, any amino acid). We developed a large-scale proteomic approach to identify more than 300 substrates of this kinase family in cancer cell lines driven by the c-Met, epidermal growth factor receptor (EGFR), or platelet-derived growth factor receptor alpha (PDGFRalpha) RTKs. We identified a subset of proteins with RxRxxS/T sites for which phosphorylation was decreased by RTK inhibitors (RTKIs), as well as by inhibitors of the PI3K, mTOR, and MAPK pathways, and we determined the effects of small interfering RNA directed against these substrates on cell viability. Phosphorylation of the protein chaperone SGTA (small glutamine-rich tetratricopeptide repeat-containing protein alpha) at serine-305 was essential for PDGFRalpha stabilization and cell survival in PDGFRalpha-dependent cancer cells. Our approach provides a new view of RTK and Akt-RSK-S6 kinase signaling, revealing previously unidentified Akt-RSK-S6 kinase substrates that merit further consideration as targets for combination therapy with RTKIs.

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    • "The evolutionary conserved serine/threonine protein kinase mTOR is a member of the phosphatidylinositol 3-kinase (PI3K)-related kinase (PIKK) family [3]. mTOR integrates both extracellular and intracellular signals and acts as a central regulator of cell metabolism, growth, proliferation and survival [4]. "
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    • "S6K in vivo activates MEF2-T20 kinase and may be responsible for promoting expression of the enzymes of lipogenesis and glycogenesis in healthy, well-fed animals. [67], i.e. S6K enhances anabolism and repress catabolism in response to nutrient signals [68]. "
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    • "MS analysis of in vitro phosphorylated MBP-RacGAP1 and FLAG-RacGAP1 immunoprecipitated from 293T cells identified a single predominant PKB/Akt phosphorylation site in RacGAP1 but could not distinguish between T249 (within the sole PKB/Akt consensus within RacGAP1, previously reported in MS analysis; Moritz et al., 2010) and T251 (unpublished data). In vitro phosphorylation using recombinant active PKB/Akt, with either putative phosphorylation site mutated to alanine, revealed T249 to be the predominant PKB/Akt phosphorylation site within RacGAP1 (Fig. 3 A). "
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