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Akt-RSK-S6 Kinase Signaling Networks Activated by Oncogenic Receptor Tyrosine Kinases
Albrecht Moritz, Yu Li, Ailan Guo, Judit Villén, Yi Wang, Joan MacNeill, Jon Kornhauser, Kam Sprott, Jing Zhou, Anthony Possemato, Jian Min Ren, Peter
Hornbeck, Lewis C. Cantley, Steven P. Gygi, John Rush and Michael J. Comb
Receptor tyrosine kinases (RTKs) activate pathways mediated by serine-threonine kinases, such as thePI3K (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 transmit 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 a (PDGFRa) 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 smallinterfering RNA directed against these substrates on cell viability. Phosphorylation of the protein chaperone SGTA (small glutamine-rich tetratricopeptide repeat-containing protein a) at serine-305 was essential for PDGFRa stabilization and cell survival in PDGFR a-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.
