Label-free global phosphoproteomics experiment using PI3K-AKT-mTOR-S6K and RAF-MEK-ERK-RSK pathway activation and inhibition
ABSTRACT: The signaling network of skeletal muscle cells is controlled by a variety of protein kinases. Although many kinases are known players, their downstream targets are still largely unexplored. To gain further knowledge about the PI3K-AKT-mTOR-S6K and the RAF-MEK-ERK-RSK signaling networks in myotubes, we analyzed changes in protein phosphorylation levels upon pathway activation and direct kinase inhibition on a global scale. Based on the phosphoproteomics data, we further examined target relationships of the basophilic kinases AKT, RSK and S6K, which share the substrate recognition motif RxRxxp[ST].
Project description:Various protein kinases are regulating the intracellular signaling network of skeletal muscle cells. Despite that many of the involved kinases are known, their downstream targets have remained largely unexplored. To deepen our understanding of the PI3K-AKT-mTOR-S6K and the RAF-MEK-ERK-RSK signaling network in myotubes, we globally analyzed changes in protein phosphorylation levels upon kinase inhibition within these pathways. The phosphoproteomics data were used to define potential targets of the kinases AKT, RSK and S6K, which share the substrate recognition motif RxRxxp[ST].
Project description:Based on the global SILAC and label-free phosphoproteomics experiments, we generated an inclusion list for validation of known and novel AKT, RSK and S6K targets containing the RxRxxp[ST] motif. Samples were analyzed by targeted MS using PRM.
Project description:Hormones and growth factors induce the activation of a number of protein kinases that belong to the AGC subfamily, including isoforms of PKA, protein kinase B (also known as Akt), PKC, S6K p70 (ribosomal S6 kinase), RSK (p90 ribosomal S6 kinase) and MSK (mitogen- and stress-activated protein kinase), which then mediate many of the physiological processes that are regulated by these extracellular agonists. It can be difficult to assess the individual functions of each AGC kinase because their substrate specificities are similar. Here we describe the small molecule BI-D1870, which inhibits RSK1, RSK2, RSK3 and RSK4 in vitro with an IC(50) of 10-30 nM, but does not signi-ficantly inhibit ten other AGC kinase members and over 40 other protein kinases tested at 100-fold higher concentrations. BI-D1870 is cell permeant and prevents the RSK-mediated phorbol ester- and EGF (epidermal growth factor)-induced phosphoryl-ation of glycogen synthase kinase-3beta and LKB1 in human embry-onic kidney 293 cells and Rat-2 cells. In contrast, BI-D1870 does not affect the agonist-triggered phosphorylation of substrates for six other AGC kinases. Moreover, BI-D1870 does not suppress the phorbol ester- or EGF-induced phosphorylation of CREB (cAMP-response-element-binding protein), consistent with the genetic evidence indicating that MSK, and not RSK, isoforms mediate the mitogen-induced phosphorylation of this transcription factor.
Project description: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.
Project description:Crosstalk between inflammatory signalling pathways and receptor tyrosine kinases has been revealed as an indicator of cancer malignant progression. In the present study, we focus on EphA2 receptor tyrosine kinase, which is overexpressed in many human cancers. It has been reported that ligand-independent phosphorylation of EphA2 at Ser-897 is induced by Akt. We show that inflammatory cytokines promote RSK-, not Akt-, dependent phosphorylation of EphA2 at Ser-897. In addition, the RSK-EphA2 signalling pathway controls cell migration and invasion of metastatic breast cancer cells. Moreover, Ser-897-phosphorylated EphA2 co-localizes with phosphorylated active form of RSK in various human tumour specimens, and this double positivity is related to poor survival in lung cancer patients, especially those with a smoking history. Taken together, these results indicate that the phosphorylation of EphA2 at Ser-897 is controlled by RSK and the RSK-EphA2 axis might contribute to cell motility and promote tumour malignant progression.
Project description:Cells adjust to nutrient fluctuations to restore metabolic homeostasis. The mechanistic target of rapamycin (mTOR) complex 2 responds to nutrient levels and growth signals to phosphorylate protein kinases belonging to the AGC (Protein Kinases A,G,C) family such as Akt and PKC. Phosphorylation of these AGC kinases at their conserved hydrophobic motif (HM) site by mTORC2 enhances their activation and mediates the functions of mTORC2 in cell growth and metabolism. Another AGC kinase family member that is known to undergo increased phosphorylation at the homologous HM site (Ser380) is the p90 ribosomal S6 kinase (RSK). Phosphorylation at Ser380 is facilitated by the activation of the mitogen-activated protein kinase/extracellular signal regulated kinase (MAPK/ERK) in response to growth factor stimulation. Here, we demonstrate that optimal phosphorylation of RSK at this site requires an intact mTORC2. We also found that RSK is robustly phosphorylated at Ser380 upon nutrient withdrawal or inhibition of glycolysis, conditions that increase mTORC2 activation. However, pharmacological inhibition of mTOR did not abolish RSK phosphorylation at Ser380, indicating that mTOR catalytic activity is not required for this phosphorylation. Since RSK and SIN1? colocalize at the membrane during serum restimulation and acute glutamine withdrawal, mTORC2 could act as a scaffold to enhance RSK HM site phosphorylation. Among the known RSK substrates, the CCT? subunit of the chaperonin containing TCP-1 (CCT) complex had defective phosphorylation in the absence of mTORC2. Our findings indicate that the mTORC2-mediated phosphorylation of the RSK HM site could confer RSK substrate specificity and reveal that RSK responds to nutrient fluctuations.
Project description:Mad1, a member of the Myc/Max/Mad family, suppresses Myc-mediated transcriptional activity by competing with Myc for heterodimerization with its obligatory partner, Max. The expression of Mad1 suppresses Myc-mediated cell proliferation and transformation. The levels of Mad1 protein are generally low in many human cancers, and Mad1 protein has a very short half-life. However, the mechanism that regulates the turnover of Mad1 protein is poorly understood. In this study, we showed that Mad1 is a substrate of p90 ribosomal kinase (RSK) and p70 S6 kinase (S6K). Both RSK and S6K phosphorylate serine 145 of Mad1 upon serum or insulin stimulation. Ser-145 phosphorylation of Mad1 accelerates the ubiquitination and degradation of Mad1 through the 26S proteasome pathway, which in turn promotes the transcriptional activity of Myc. Our study provides a direct link between the growth factor signaling pathways regulated by PI3 kinase/Akt and MAP kinases with Myc-mediated transcription.
Project description:Phosphoinositide-dependent kinase l (PDK1) phosphorylates and activates multiple AGC serine kinases, including protein kinase B (PKB), p70Ribosomal S6 kinase (S6K) and p90Ribosomal S6 kinase (RSK). PDK1 is required for thymocyte differentiation and proliferation, and herein, we explore the molecular basis for these essential functions of PDK1 in T lymphocyte development. A key finding is that PDK1 is required for the expression of key nutrient receptors in T cell progenitors: CD71 the transferrin receptor and CD98 a subunit of L-amino acid transporters. PDK1 is also essential for Notch-mediated trophic and proliferative responses in thymocytes. A PDK1 mutant PDK1 L155E, which supports activation of PKB but no other AGC kinases, can restore CD71 and CD98 expression in pre-T cells and restore thymocyte differentiation. However, PDK1 L155E is insufficient for thymocyte proliferation. The role of PDK1 in thymus development thus extends beyond its ability to regulate PKB. In addition, PDK1 phosphorylation of AGC kinases such as S6K and RSK is also necessary for thymocyte development.
Project description:The p90 ribosomal S6 kinase (RSK) family is a group of highly conserved Ser/Thr kinases that promote cell proliferation, growth, motility and survival. As they are almost exclusively activated downstream of extracellular signal-regulated kinases 1 and 2 (ERK1/2), therapeutic intervention by RSK inhibition is less likely to produce such severe side effects as those observed following inhibition of the upstream master regulators Raf, MEK and ERK1/2. Here, we report that BI-D1870, a potent small molecule inhibitor of RSKs, induces apoptosis, although preferentially, in a p21-deficient background. On the other hand, BI-D1870 also induces a strong transcription- and p53-independent accumulation of p21 protein and protects cells from gamma irradiation (γIR)-induced apoptosis, driving them into senescence even in the absence of γIR. Although we identified p21 in in vitro kinase assays as a novel RSK substrate that specifically becomes phosphorylated by RSK1-3 at Ser116 and Ser146, RNA-interference, overexpression and co-immunoprecipitation studies as well as the use of SL0101, another specific RSK inhibitor, revealed that BI-D1870 mediates p21 accumulation via a yet unknown pathway that, besides its off-site targets polo-like kinase-1 and AuroraB, also does also not involve RSKs. Thus, this novel off-target effect of BI-D1870 should be taken into serious consideration in future studies investigating the role of RSKs in cellular signaling and tumorigenesis.