Identification of RSK substrates using an analog-sensitive kinase approach.

The p90 ribosomal S6 kinases (RSK) family of serine/threonine kinases comprises four isoforms (RSK1-4) that lie downstream of the ERK1/2 mitogen-activated protein kinase pathway. RSKs are implicated in fine tuning of cellular processes such as translation, transcription, proliferation, and motility. Previous work showed that pathogens such as Cardioviruses could hijack any of the four RSK isoforms to inhibit PKR activation or to disrupt cellular nucleocytoplasmic trafficking. In contrast, some reports suggest nonredundant functions for distinct RSK isoforms, whereas Coffin-Lowry syndrome has only been associated with mutations in the gene encoding RSK2. In this work, we used the analog-sensitive kinase strategy to ask whether the cellular substrates of distinct RSK isoforms differ. We compared the substrates of two of the most distant RSK isoforms: RSK1 and RSK4. We identified a series of potential substrates for both RSKs in cells and validated RanBP3, PDCD4, IRS2, and ZC3H11A as substrates of both RSK1 and RSK4, and SORBS2 as an RSK1 substrate. In addition, using mutagenesis and inhibitors, we confirmed analog-sensitive kinase data showing that endogenous RSKs phosphorylate TRIM33 at S1119. Our data thus identify a series of potential RSK substrates and suggest that the substrates of RSK1 and RSK4 largely overlap and that the specificity of the various RSK isoforms likely depends on their cell- or tissue-specific expression pattern.

SOCS1 links cytokine signaling to p53 and senescence.

SOCS1 is lost in many human tumors, but its tumor suppression activities are not well understood. We report that SOCS1 is required for transcriptional activity, DNA binding, and serine 15 phosphorylation of p53 in the context of STAT5 signaling. In agreement, inactivation of SOCS1 disabled p53-dependent senescence in response to oncogenic STAT5A and radiation-induced apoptosis in T cells. In addition, SOCS1 was sufficient to induce p53-dependent senescence in fibroblasts. The mechanism of activation of p53 by SOCS1 involved a direct interaction between the SH2 domain of SOCS1 and the N-terminal transactivation domain of p53, while the C-terminal domain of SOCS1 containing the SOCS Box mediated interaction with the DNA damage-regulated kinases ATM/ATR. Also, SOCS1 colocalized with ATM at DNA damage foci induced by oncogenic STAT5A. Collectively, these results add another component to the p53 and DNA damage networks and reveal a mechanism by which SOCS1 functions as a tumor suppressor.

Phosphoproteomic analysis identifies the tumor suppressor PDCD4 as a RSK substrate negatively regulated by 14-3-3.

The Ras/MAPK signaling cascade regulates various biological functions, including cell growth and proliferation. As such, this pathway is frequently deregulated in several types of cancer, including most cases of melanoma. RSK (p90 ribosomal S6 kinase) is a MAPK-activated protein kinase required for melanoma growth and proliferation, but relatively little is known about its exact function and the nature of its substrates. Herein, we used a quantitative phosphoproteomics approach to define the signaling networks regulated by RSK in melanoma. To more accurately predict direct phosphorylation substrates, we defined the RSK consensus phosphorylation motif and found significant overlap with the binding consensus of 14-3-3 proteins. We thus characterized the phospho-dependent 14-3-3 interactome in melanoma cells and found that a large proportion of 14-3-3 binding proteins are also potential RSK substrates. Our results show that RSK phosphorylates the tumor suppressor PDCD4 (programmed cell death protein 4) on two serine residues (Ser76 and Ser457) that regulate its subcellular localization and interaction with 14-3-3 proteins. We found that 14-3-3 binding promotes PDCD4 degradation, suggesting an important role for RSK in the inactivation of PDCD4 in melanoma. In addition to this tumor suppressor, our results suggest the involvement of RSK in a vast array of unexplored biological functions with relevance in oncogenesis.

Phosphorylation of SOCS1 Inhibits the SOCS1-p53 Tumor Suppressor Axis.

Expression of the suppressor of cytokine signaling-1 (SOCS1) is inactivated in hematopoietic and solid cancers by promoter methylation, miRNA-mediated silencing, and mutations. Paradoxically, SOCS1 is also overexpressed in many human cancers. We report here that the ability of SOCS1 to interact with p53 and regulate cellular senescence depends on a structural motif that includes tyrosine (Y)80 in the SH2 domain of SOCS1. Mutations in this motif are found at low frequency in some human cancers, and substitution of Y80 by a phosphomimetic residue inhibits p53-SOCS1 interaction and its functional consequences, including stimulation of p53 transcriptional activity, growth arrest, and cellular senescence. Mass spectrometry confirmed SOCS1 Y80 phosphorylation in cells, and a new mAb was generated to detect its presence in tissues by IHC. A tyrosine kinase library screen identified the SRC family as Y80-SOCS1 kinases. SRC family kinase inhibitors potentiated the SOCS1-p53 pathway and reinforced SOCS1-induced senescence. Samples from human lymphomas that often overexpress SOCS1 also displayed SRC family kinase activation, constitutive phosphorylation of SOCS1 on Y80, and SOCS1 cytoplasmic localization. Collectively, these results reveal a mechanism that inactivates the SOCS1-p53 senescence pathway and suggest that inhibition of SRC family kinases as personalized treatment in patients with lymphomas may be successful. SIGNIFICANCE: These findings show that SOCS1 phosphorylation by the SRC family inhibits its tumor-suppressive activity, indicating that patients with increased SOCS1 phosphorylation may benefit from SRC family kinase inhibitors.

CdGAP/ARHGAP31 is regulated by RSK phosphorylation and binding to 14-3-3ß adaptor protein.

Cdc42 GTPase-activating protein (CdGAP, also named ARHGAP31) is a negative regulator of the GTPases Rac1 and Cdc42. Associated with the rare developmental disorder Adams-Oliver Syndrome (AOS), CdGAP is critical for embryonic vascular development and VEGF-mediated angiogenesis. Moreover, CdGAP is an essential component in the synergistic interaction between TGFß and ErbB-2 signaling pathways during breast cancer cell migration and invasion, and is a novel E-cadherin transcriptional co-repressor with Zeb2 in breast cancer. CdGAP is highly phosphorylated on serine and threonine residues in response to growth factors and is a substrate of ERK1/2 and GSK-3. Here, we identified Ser1093 and Ser1163 in the C-terminal region of CdGAP, which are phosphorylated by RSK in response to phorbol ester. These phospho-residues create docking sites for binding to 14-3-3 adaptor proteins. The interaction between CdGAP and 14-3-3 proteins inhibits the GAP activity of CdGAP and sequesters CdGAP into the cytoplasm. Consequently, the nucleocytoplasmic shuttling of CdGAP is inhibited and CdGAP-induced cell rounding is abolished. In addition, 14-3-3ß inhibits the ability of CdGAP to repress the E-cadherin promoter and to induce cell migration. Finally, we show that 14-3-3ß is unable to regulate the activity and subcellular localization of the AOS-related mutant proteins lacking these phospho-residues. Altogether, we provide a novel mechanism of regulation of CdGAP activity and localization, which impacts directly on a better understanding of the role of CdGAP as a promoter of breast cancer and in the molecular causes of AOS.

SOCS1, a novel interaction partner of p53 controlling oncogene-induced senescence.

Members of the signal transducers and activators of transcription (STATs) family of proteins, which connect cytokine signaling to activation of transcription, are frequently activated in human cancers. Suppressors of cytokine signaling (SOCS) are transcriptional targets of activated STAT proteins that negatively control STAT signaling. SOCS1 expression is silenced in multiple human cancers suggesting a tumor suppressor role for this protein. However, SOCS1 not only regulates STAT signaling but can also localize to the nucleus and directly interact with the p53 tumor suppressor through its central SH2 domain. Furthermore, SOCS1 contributes to p53 activation and phosphorylation on serine 15 by forming a ternary complex with ATM or ATR. Through this mechanism SOCS1 regulates the process of oncogene-induced senescence, which is a very important tumor suppressor response. A mutant SOCS1 lacking the SOCS box cannot interact with ATM/ATR, stimulate p53 or induce the senescence phenotype, suggesting that the SOCS box recruits DNA damage activated kinases to its interaction partners bound to its SH2 domain. Proteomic analysis of SOCS1 interaction partners revealed other potential targets of SOCS1 in the DNA damage response. These newly discovered functions of SOCS1 help to explain the increased susceptibility of Socs1 null mice to develop cancer as well as their propensity to develop autoimmune diseases. Consistently, we found that mice lacking SOCS1 displayed defects in the regulation of p53 target genes including Mdm2, Pmp22, PUMA and Gadd45a. The involvement of SOCS1 in p53 activation and the DNA damage response defines a novel tumor suppressor pathway and intervention point for future cancer therapeutics.

Transcriptome analysis and tumor suppressor requirements of STAT5-induced senescence.

Although it is acknowledged that senescent cells accumulate with age, the molecular mechanisms leading to cell senescence as a function of age remain to be identified. In cell culture models, it has been clearly shown that senescence involves the activation of a DNA damage response secondary to short telomeres or oncogene expression. Oncogenes are altered versions of genes coding for proteins that mediate signals from extracellular factors such as cytokines, growth factors, and hormones. In particular, we show here that constitutive activation of the JAK/STAT5 signaling pathway induces senescence in both mouse and human normal cells. The process involves activation of the p53 and Rb tumor suppressor pathways and mitochondrial dysfunction. Gene expression analysis of STAT5-induced senescence revealed changes in the expression of genes coding for cytokines, proteins in cytokine signaling pathways, and several metabolic enzymes. We discuss a model called senescence-induced senescence, in which cytokines secreted by senescent cells can propagate the process as a function of age.