A Non-canonical PDK1-RSK Signal Diminishes Pro-caspase-8-Mediated Necroptosis Blockade.

Necroptosis induction in vitro often requires caspase-8 (Casp8) inhibition by zVAD because pro-Casp8 cleaves RIP1 to disintegrate the necrosome. It has been unclear how the Casp8 blockade of necroptosis is eliminated naturally. Here, we show that pro-Casp8 within the necrosome can be inactivated by phosphorylation at Thr265 (pC8T265). pC8T265 occurs in vitro in various necroptotic cells and in the cecum of TNF-treated mice. p90 RSK is the kinase of pro-Casp8. It is activated by a mechanism that does not need ERK but PDK1, which is recruited to the RIP1-RIP3-MLKL-containing necrosome. Phosphorylation of pro-Casp8 at Thr265 can substitute for zVAD to permit necroptosis in vitro. pC8T265 mimic T265E knockin mice are embryonic lethal due to unconstrained necroptosis, and the pharmaceutical inhibition of RSK-mediated pC8T265 diminishes TNF-induced cecum damage and lethality in mice by halting necroptosis. Thus, phosphorylation of pro-Casp8 at Thr265 by RSK is an intrinsic mechanism for passing the Casp8 checkpoint of necroptosis.

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.

Dual targeting of brain region-specific kinases potentiates neurological rescue in Spinocerebellar ataxia type 1.

A critical question in neurodegeneration is why the accumulation of disease-driving proteins causes selective neuronal loss despite their brain-wide expression. In Spinocerebellar ataxia type 1 (SCA1), accumulation of polyglutamine-expanded Ataxin-1 (ATXN1) causes selective degeneration of cerebellar and brainstem neurons. Previous studies revealed that inhibiting Msk1 reduces phosphorylation of ATXN1 at S776 as well as its levels leading to improved cerebellar function. However, there are no regulators that modulate ATXN1 in the brainstem-the brain region whose pathology is most closely linked to premature death. To identify new regulators of ATXN1, we performed genetic screens and identified a transcription factor-kinase axis (ZBTB7B-RSK3) that regulates ATXN1 levels. Unlike MSK1, RSK3 is highly expressed in the human and mouse brainstems where it regulates Atxn1 by phosphorylating S776. Reducing Rsk3 rescues brainstem-associated pathologies and deficits, and lowering Rsk3 and Msk1 together improves cerebellar and brainstem function in an SCA1 mouse model. Our results demonstrate that selective vulnerability of brain regions in SCA1 is governed by region-specific regulators of ATXN1, and targeting multiple regulators could rescue multiple degenerating brain areas.

The ribosomal S6 kinase 2 (RSK2)-SPRED2 complex regulates the phosphorylation of RSK substrates and MAPK signaling.

Sprouty-related EVH-1 domain-containing (SPRED) proteins are a family of proteins that negatively regulate the RAS-Mitogen-Activated Protein Kinase (MAPK) pathway, which is involved in the regulation of the mitogenic response and cell proliferation. However, the mechanism by which these proteins affect RAS-MAPK signaling has not been elucidated. Patients with mutations in SPRED give rise to unique disease phenotypes; thus, we hypothesized that distinct interactions across SPRED proteins may account for alternative nodes of regulation. To characterize the SPRED interactome and evaluate how members of the SPRED family function through unique binding partners, we performed affinity purification mass spectrometry. We identified 90-kDa ribosomal S6 kinase 2 (RSK2) as a specific interactor of SPRED2 but not SPRED1 or SPRED3. We identified that the N-terminal kinase domain of RSK2 mediates the interaction between amino acids 123 to 201 of SPRED2. Using X-ray crystallography, we determined the structure of the SPRED2-RSK2 complex and identified the SPRED2 motif, F145A, as critical for interaction. We found that the formation of this interaction is regulated by MAPK signaling events. We also find that this interaction between SPRED2 and RSK2 has functional consequences, whereby the knockdown of SPRED2 resulted in increased phosphorylation of RSK substrates, YB1 and CREB. Furthermore, SPRED2 knockdown hindered phospho-RSK membrane and nuclear subcellular localization. We report that disruption of the SPRED2-RSK complex has effects on RAS-MAPK signaling dynamics. Our analysis reveals that members of the SPRED family have unique protein binding partners and describes the molecular and functional determinants of SPRED2-RSK2 complex dynamics.

Cellular stress induces non-canonical activation of the receptor tyrosine kinase EphA2 through the p38-MK2-RSK signaling pathway.

The receptor tyrosine kinase ephrin type-A receptor 2 (EphA2) is overexpressed in malignant tumors. We previously reported that non-canonical EphA2 phosphorylation at Ser-897 was catalyzed by p90 ribosomal S6 kinase (RSK) via the MEK-ERK pathway in ligand- and tyrosine kinase-independent manners. Non-canonical EphA2 activation plays a key role in tumor progression; however, its activation mechanism remains unclear. In the present study, we focused on cellular stress signaling as a novel inducer of non-canonical EphA2 activation. p38, instead of ERK in the case of epidermal growth factor signaling, activated RSK-EphA2 under cellular stress conditions, including anisomycin, cisplatin, and high osmotic stress. Notably, p38 activated the RSK-EphA2 axis via downstream MAPK-activated protein kinase 2 (MK2). Furthermore, MK2 directly phosphorylated both RSK1 Ser-380 and RSK2 Ser-386, critical residues for the activation of their N-terminal kinases, which is consistent with the result showing that the C-terminal kinase domain of RSK1 was dispensable for MK2-mediated EphA2 phosphorylation. Moreover, the p38-MK2-RSK-EphA2 axis promoted glioblastoma cell migration induced by temozolomide, a chemotherapeutic agent for the treatment of glioblastoma patients. Collectively, the present results reveal a novel molecular mechanism for non-canonical EphA2 activation under stress conditions in the tumor microenvironment.

Ribosomal S6 kinase (RSK) plays a critical role in DNA damage response via the phosphorylation of histone lysine demethylase KDM4B.

BACKGROUND: Epigenetic dysregulation affecting oncogenic transcription and DNA damage response is a hallmark of cancer. The histone demethylase KDM4B, a factor regulating these processes, plays important roles in estrogen receptor-mediated transcription and DNA repair in breast cancer. However, how oncogenic phospho-signal transduction affects epigenetic regulation is not fully understood. Here we found that KDM4B phosphorylation by ribosomal S6 kinase (RSK), a downstream effector of the Ras/MAPK pathway, is critical for the function of KDM4B in response to DNA damage. METHODS: KDM4B-knockout breast cancer cell lines were generated via CRISPR/Cas9-mediated gene editing. Re-expression of wild-type or phospho-site mutated KDM4B in knockout cells was performed by lentivirus-mediated gene transfer. Gene knockdown was achieved by RNA interference. DNA double-strand breaks (DSBs) were induced by ionizing radiation or laser-microirradiation. Protein accumulation at DSB sites was analyzed by immunofluorescence. KDM4B phosphorylation by RSK was assessed by in vitro and in vivo kinase assays. Gene and protein expression levels were analyzed by RT‒PCR and western blotting. The sensitivity of cells to ionizing radiation was examined by a clonogenic survival assay. RESULTS: RSK phosphorylated KDM4B at Ser666, and inhibition of the phosphorylation by RSK depletion or RSK inhibitors abrogated KDM4B accumulation at the sites of DNA double-strand breaks (DSBs). DSB repair was significantly delayed in KDM4B-knockout cells or cells treated with RSK inhibitors. The replacement of endogenous KDM4B with the phosphomimetic mutant S666D restored KDM4B accumulation and DSB repair that had been inhibited by RSK inhibitors, suggesting a critical role for RSK at the specific serine residue of KDM4B in the effect of RSK inhibitors on DSB repair. As a consequence of these aberrant responses, inhibition of KDM4B phosphorylation increased the sensitivity of the cells to ionizing radiation. CONCLUSIONS: Overall, the present study uncovered a novel function of RSK on the DNA damage response, which provides an additional role of its inhibitor in cancer therapy.

Kaposi's Sarcoma-Associated Herpesvirus Immediate Early Proteins Trigger FOXQ1 Expression in Oral Epithelial Cells, Engaging in a Novel Lytic Cycle-Sustaining Positive Feedback Loop.

Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic gammaherpesvirus that can replicate in oral epithelial cells to promote viral transmission via saliva. To identify novel regulators of KSHV oral infection, we performed a transcriptome analysis of KSHV-infected primary human gingival epithelial (HGEP) cells, which identified the gene coding for the host transcription factor FOXQ1 as the top induced host gene. FOXQ1 is nearly undetectable in uninfected HGEP and telomerase-immortalized gingival keratinocytes (TIGK) cells but is highly expressed within hours of KSHV infection. We found that while the FOXQ1 promoter lacks activating histone acetylation marks in uninfected oral epithelial cells, these marks accumulate in the FOXQ1 promoter in infected cells, revealing a rapid epigenetic reprogramming event. To evaluate FOXQ1 function, we depleted FOXQ1 in KSHV-infected TIGK cells, which resulted in reduced accumulation of KSHV lytic proteins and viral DNA over the course of 4 days of infection, uncovering a novel lytic cycle-sustaining role of FOXQ1. A screen of KSHV lytic proteins demonstrated that the immediate early proteins ORF45 and replication and transcription activator (RTA) were both sufficient for FOXQ1 induction in oral epithelial cells, indicating active involvement of incoming and rapidly expressed factors in altering host gene expression. ORF45 is known to sustain extracellular signal-regulated kinase (ERK) p90 ribosomal s6 kinase (RSK) pathway activity to promote lytic infection. We found that an ORF45 mutant lacking RSK activation function failed to induce FOXQ1 in TIGK cells, revealing that ORF45 uses a shared mechanism to rapidly induce both host and viral genes to sustain lytic infection in oral epithelial cells. IMPORTANCE The oral cavity is a primary site of initial contact and entry for many viruses. Viral replication in the oral epithelium promotes viral shedding in saliva, allowing interpersonal transmission, as well as spread to other cell types, where chronic infection can be established. Understanding the regulation of KSHV infection in the oral epithelium would allow for the design of universal strategies to target the first stage of viral infection, thereby halting systemic viral pathogenesis. Overall, we uncover a novel positive feedback loop in which immediate early KSHV factors drive rapid host reprogramming of oral epithelial cells to sustain the lytic cycle in the oral cavity.

p90RSK pathway inhibition synergizes with cisplatin in TMEM16A overexpressing head and neck cancer.

Head and neck squamous cell carcinoma (HNSCC) constitutes one of the most common types of human cancers and often metastasizes to lymph nodes. Platinum-based chemotherapeutic drugs are commonly used for treatment of a wide range of cancers, including HNSCC. Its mode of action relies on its ability to impede DNA repair mechanisms, inducing apoptosis in cancer cells. However, due to acquired resistance and toxic side-effects, researchers have been focusing on developing novel combinational therapeutic strategies to overcome cisplatin resistance. In the current study, we identified p90RSK, an ERK1/2 downstream target, as a key mediator and a targetable signaling node against cisplatin resistance. Our results strongly support the role of p90RSK in cisplatin resistance and identify the combination of p90RSK inhibitor, BI-D1870, with cisplatin as a novel therapeutic strategy to overcome cisplatin resistance. In addition, we have identified TMEM16A expression as a potential upstream regulator of p90RSK through the ERK pathway and a biomarker of response to p90RSK targeted therapy in the context of cisplatin resistance.

Punicalagin as a novel selective aryl hydrocarbon receptor (AhR) modulator upregulates AhR expression through the PDK1/p90RSK/AP-1 pathway to promote the anti-inflammatory response and bactericidal activity of macrophages.

Aryl hydrocarbon receptor (AhR) plays an important role in inflammation and immunity as a new therapeutic target for infectious disease and sepsis. Punicalagin (PUN) is a Chinese herbal monomer extract of pomegranate peel that has beneficial anti-inflammatory, antioxidant and anti-infective effects. However, whether PUN is a ligand of AhR, its effect on AhR expression, and its signaling pathway remain poorly understood. In this study, we found that PUN was a unique polyphenolic compound that upregulated AhR expression at the transcriptional level, and regulated the AhR nongenomic pathway. AhR expression in lipopolysaccharide-induced macrophages was upregulated by PUN in vitro and in vivo in a time- and dose-dependent manner. Using specific inhibitors and siRNA, induction of AhR by PUN depended on sequential phosphorylation of 90-kDa ribosomal S6 kinase (p90RSK), which was activated by the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) and phosphoinositide-dependent protein kinase (PDK)1 pathways. PUN promoted p90RSK-mediated activator protein-1 (AP-1) activation. AhR knockout or inhibitors reversed suppression of interleukin (IL)-6 and IL-1ß expression by PUN. PUN decreased Listeria load and increased macrophage survival via AhR upregulation. In conclusion, we identified PUN as a novel selective AhR modulator involved in AhR expression via the MEK/ERK and PDK1 pathways targeting p90RSK/AP-1 in inflammatory macrophages, which inhibited macrophage inflammation and promoted bactericidal activity.

Designing a novel drug-drug conjugate as a prodrug for breast cancer therapy: in silico insights.

Breast cancer (BC) poses a significant global health threat, necessitating innovative therapeutic approaches. The ribosomal s6 kinase 2 (RSK2) has emerged as a promising target due to its roles in cell proliferation and survival. This study proposes a drug-drug conjugate prodrug comprising Methotrexate (hydrophobic) and Capecitabine (hydrophilic) for BC treatment. In silico approaches, including Molecular Docking, Molecular Dynamics Simulations, MM-PBSA, ADME, and DFT calculations were employed to evaluate the prodrug's potential. The designed MET-CAP ligand exhibits a robust docking score (-8.980 kcal/mol), superior binding affinity (-53.16 kcal/mol), and stable dynamic behavior (0.62 nm) compared to native ligands. The DFT results reveal intramolecular charge transfer in MET-CAP (HLG = 0.09 eV), indicating its potential as a BC inhibitor. ADME analysis suggests satisfactory pharmaceutically relevant properties. The results indicate that the conjugated MET-CAP ligand exhibits favorable binding characteristics, stability, and pharmaceutically relevant properties, making it a potential RSK2 inhibitor for BC therapy. The multifaceted approach provides insights into binding interactions, stability, and pharmacokinetic properties, laying the foundation for further experimental validation and potential clinical development.

Oncogenic KRAS engages an RSK1/NF1 pathway to inhibit wild-type RAS signaling in pancreatic cancer.

Pancreatic ductal adenocarcinoma (PDAC) is a lethal malignancy with limited treatment options. Although activating mutations of the KRAS GTPase are the predominant dependency present in >90% of PDAC patients, targeting KRAS mutants directly has been challenging in PDAC. Similarly, strategies targeting known KRAS downstream effectors have had limited clinical success due to feedback mechanisms, alternate pathways, and dose-limiting toxicities in normal tissues. Therefore, identifying additional functionally relevant KRAS interactions in PDAC may allow for a better understanding of feedback mechanisms and unveil potential therapeutic targets. Here, we used proximity labeling to identify protein interactors of active KRAS in PDAC cells. We expressed fusions of wild-type (WT) (BirA-KRAS4B), mutant (BirA-KRAS4BG12D), and nontransforming cytosolic double mutant (BirA-KRAS4BG12D/C185S) KRAS with the BirA biotin ligase in murine PDAC cells. Mass spectrometry analysis revealed that RSK1 selectively interacts with membrane-bound KRASG12D, and we demonstrate that this interaction requires NF1 and SPRED2. We find that membrane RSK1 mediates negative feedback on WT RAS signaling and impedes the proliferation of pancreatic cancer cells upon the ablation of mutant KRAS. Our findings link NF1 to the membrane-localized functions of RSK1 and highlight a role for WT RAS signaling in promoting adaptive resistance to mutant KRAS-specific inhibitors in PDAC.

Triple targeting of RSK, AKT, and S6K as pivotal downstream effectors of PDPK1 by TAS0612 in B-cell lymphomas.

B-cell lymphomas (BCLs) are the most common disease entity among hematological malignancies and have various genetically and molecularly distinct subtypes. In this study, we revealed that the blockade of phosphoinositide-dependent kinase-1 (PDPK1), the master kinase of AGC kinases, induces a growth inhibition via cell cycle arrest and the induction of apoptosis in all eight BCL-derived cell lines examined, including those from activated B-cell-like diffuse large B-cell lymphoma (DLBCL), double expressor DLBCL, Burkitt lymphoma, and follicular lymphoma. We also demonstrated that, in these cell lines, RSK2, AKT, and S6K, but not PLK1, SGK, or PKC, are the major downstream therapeutic target molecules of PDPK1 and that RSK2 plays a central role and AKT and S6K play subsidiary functional roles as the downstream effectors of PDPK1 in cell survival and proliferation. Following these results, we confirmed the antilymphoma efficacy of TAS0612, a triple inhibitor for total RSK, including RSK2, AKT, and S6K, not only in these cell lines, regardless of disease subtypes, but also in all 25 patient-derived B lymphoma cells of various disease subtypes. At the molecular level, TAS0612 caused significant downregulation of MYC and mTOR target genes while inducing the tumor suppressor TP53INP1 protein in these cell lines. These results prove that the simultaneous blockade of RSK2, AKT, and S6K, which are the pivotal downstream substrates of PDPK1, is a novel therapeutic target for the various disease subtypes of BCLs and line up TAS0612 as an attractive candidate agent for BCLs for future clinical development.

RSK2 inactivation cooperates with AXIN1 inactivation or ß-catenin activation to promote hepatocarcinogenesis.

BACKGROUND & AIMS: Recurrent somatic mutations of the RPS6KA3 gene encoding for the serine/threonine kinase RSK2 were identified in hepatocellular carcinomas (HCCs), suggesting its tumour-suppressive function. Our goal was to demonstrate the tumour suppressor role of RSK2 in the liver and investigate the functional consequences of its inactivation. METHODS: We analysed a series of 1,151 human HCCs for RSK2 mutations and 20 other driver genetic alterations. We then modelled RSK2 inactivation in mice in various mutational contexts recapitulating or not those naturally found in human HCC, using transgenic mice and liver-specific carcinogens. These models were monitored for liver tumour appearance and subjected to phenotypic and transcriptomic analyses. Functional consequences of RSK2 rescue were also investigated in a human RSK2-deficient HCC cell line. RESULTS: RSK2-inactivating mutations are specific to human HCC and frequently co-occur with AXIN1-inactivating or ß-catenin-activating mutations. Modelling of these co-occurrences in mice showed a cooperative effect in promoting liver tumours with transcriptomic profiles recapitulating those of human HCCs. By contrast, there was no cooperation in liver tumour induction between RSK2 loss and BRAF-activating mutations chemically induced by diethylnitrosamine. In human liver cancer cells, we also showed that RSK2 inactivation confers some dependency to the activation of RAS/MAPK signalling that can be targeted by MEK inhibitors. CONCLUSIONS: Our study demonstrates the tumour suppressor role of RSK2 and its specific synergistic effect in hepatocarcinogenesis when its loss of function is specifically combined with AXIN1 inactivation or ß-catenin activation. Furthermore, we identified the RAS/MAPK pathway as a potential therapeutic target for RSK2-inactivated liver tumours. IMPACT AND IMPLICATIONS: This study demonstrated the tumour suppressor role of RSK2 in the liver and showed that its inactivation specifically synergises with AXIN1 inactivation or ß-catenin activation to promote the development of HCC with similar transcriptomic profiles as found in humans. Furthermore, this study highlights that activation of the RAS/MAPK pathway is one of the key signalling pathways mediating the oncogenic effect of RSK2 inactivation that can be targeted with already available anti-MEK therapies.

Enhanced MAPK1 Function Causes a Neurodevelopmental Disorder within the RASopathy Clinical Spectrum.

Signal transduction through the RAF-MEK-ERK pathway, the first described mitogen-associated protein kinase (MAPK) cascade, mediates multiple cellular processes and participates in early and late developmental programs. Aberrant signaling through this cascade contributes to oncogenesis and underlies the RASopathies, a family of cancer-prone disorders. Here, we report that de novo missense variants in MAPK1, encoding the mitogen-activated protein kinase 1 (i.e., extracellular signal-regulated protein kinase 2, ERK2), cause a neurodevelopmental disease within the RASopathy phenotypic spectrum, reminiscent of Noonan syndrome in some subjects. Pathogenic variants promote increased phosphorylation of the kinase, which enhances translocation to the nucleus and boosts MAPK signaling in vitro and in vivo. Two variant classes are identified, one of which directly disrupts binding to MKP3, a dual-specificity protein phosphatase negatively regulating ERK function. Importantly, signal dysregulation driven by pathogenic MAPK1 variants is stimulus reliant and retains dependence on MEK activity. Our data support a model in which the identified pathogenic variants operate with counteracting effects on MAPK1 function by differentially impacting the ability of the kinase to interact with regulators and substrates, which likely explains the minor role of these variants as driver events contributing to oncogenesis. After nearly 20 years from the discovery of the first gene implicated in Noonan syndrome, PTPN11, the last tier of the MAPK cascade joins the group of genes mutated in RASopathies.

RSK4 confers paclitaxel resistance to ovarian cancer cells, which is resensitized by its inhibitor BI-D1870.

Many ovarian cancers initially respond well to chemotherapy, but often become drug-resistant after several years. Therefore, analysis of drug resistance mechanisms and overcoming resistance are urgently needed. Paclitaxel is one of the first-choice and widely-used drugs for ovarian cancer, but like most drugs, drug resistance is observed in subsequent use. RSK4 is known as a tumor-suppressor, however, it has increasingly been reported to lead to drug resistance. Here, we found that RSK4 expression was elevated in paclitaxel-resistant ovarian cancer cells using DNA microarray, quantitative real-time PCR, and western blotting analysis. We examined the contribution of RSK4 to paclitaxel resistance and found that paclitaxel sensitivity was restored by RSK inhibitor co-treatment. We analyzed the mechanism by which resistance is developed when RSK4 level is elevated, and accelerated phosphorylation of the downstream translation factor eIF4B was discovered. In the Kaplan-Meier plot, the overall survival time was longer with RSK4 high, supporting its role as a tumor suppressor, as in previous findings, but the tendency was reversed when focusing on paclitaxel treatment. In addition, RSK4 levels were higher in non-responders than in responders in the ROC plotter. Finally, external expression of RSK4 in ovarian cancer cells increased the cell viability under paclitaxel treatment. These findings suggest that RSK4 may contribute to paclitaxel resistance, and that co-treatment with RSK4 inhibitors is effective treatment of paclitaxel-resistant ovarian cancer in which RSK4 is elevated.

Novel wiring of the AKT-RSK2 signaling pathway plays an essential role in cancer cell proliferation via a G1/S cell cycle transition.

p90 Ribosomal S6 kinase 2 (RSK2), a member of mitogen-activated protein kinase regulating cell proliferation and transformation induced by tumor promoters, such as epidermal growth factor, plays a vital role as a signaling hub to modulate cell proliferation, transformation, cell cycle transition, and chromatin remodeling by tumor promoter stimulation such as epidermal growth factor. On the other hand, the RSK2-mediated signaling networks that regulate cancer cell proliferation are unclear. In this study, SKOV3, an ovarian cancer cell that exhibits chemoresistant properties, and TOV-112D cells showed different sensitivities to colony growth in soft agar. Based on the protein profile shown in a previous report, RSK2 knockdown preferentially and significantly suppressed cell proliferation and colony growth. Moreover, RSK2 interacted with AKTs (AKT 1-3) via the N-terminal kinase domain (NTKD) of RSK2, resulting in the phosphorylation of RSK2. The AKT-mediated phosphorylation consensus sequence, RxRxxS/T, on RSK2 NTKD (Thr115) was well conserved in different species. In particular, an in vitro kinase assay showed that NTKD deleted and Thr115Ala mutants of RSK2 abolished AKT1-mediated phosphorylation. In the physiological assay of RSK2 phosphorylation at Thr115 on cell proliferation, AKT1-mediated RSK2 phosphorylation at Thr115 played an essential role in cell proliferation. The re-introduction of RSK2-T115A to RSK2-/- MEF attenuated the EGF-induced G1/S cell cycle transition compared to RSK2-wt introducing RSK2-/- MEFs. This attenuation was observed by EGF stimulations and insulin-like growth factor-1. Overall, these results show that novel wiring of the AKT/RSKs signaling axis plays an important role in cancer cell proliferation by modulating the G1/S cell cycle transition.

cPKCγ Inhibits Caspase-9-Initiated Neuronal Apoptosis in an Ischemia Reperfusion Model In Vitro Through p38 MAPK-p90RSK-Bad Pathway.

Strokes are one of the leading causes of death and disability in the world. Previously we have found that conventional protein kinase Cγ (cPKCγ) plays neuroprotective role in ischemic strokes. Further, we found that cPKCγ knockdown increased the level of cleaved (cl)-Caspase-3. However, the precise mechanisms underlying cPKCγ-mediated neuronal death remain unclear. To this end, a model incorporating 1 h oxygen-glucose deprivation/24 h reoxygenation (1 h OGD/24 h R) was established in cortical neurons. We found that cPKCγ knockdown remarkably increased neuronal death after OGD. We also found that cPKCγ knockdown increased the level of cl-Caspase-3 through the upstream initiators Capsases-9 (not Caspase-8/12) in OGD-treated neurons. Overexpression of cPKCγ could decrease neuronal death and cl-Caspase-3 and -9 levels. Moreover, cPKCγ knockdown further reduced the phosphorylation levels of p38 MAPK, p90RSK, and Bad. In addition, the protein levels of Bcl-2 and Bcl-xl were decreased after cPKCγ knockdown, whereas that of Bax was increased. In conclusion, our results suggest that cPKCγ partly alleviates ischemic injury through activating the p38 MAPK-p90RSK-Bad pathway and inhibiting Caspase-9 initiated apoptosis. This may have potential as a therapeutic target for ischemic stroke.

Ribosomal S6 kinase 4 (RSK4) tumor suppressor gene promoter methylation status in ovarian cancer.

BACKGROUND: Previously, we reported lower RSK4 mRNA and protein levels in malignant ovarian tumors compared to normal and benign ovarian tissues. Also, we observed a significant inverse correlation between the advanced ovarian cancer stages and RSK4 mRNA levels. We did not investigate the mechanisms involved in RSK4-reduced expression in ovarian cancer. Thus, this study investigates whether RSK4 promoter methylation in ovarian cancer tissues is responsible for its low expression. Additionally, the reactivation of RSK4 expression and its effect was studied in ovarian cancer cell lines. METHODS AND RESULTS: RSK4 promoter methylation percentage in malignant and benign ovarian tumors and normal ovary tissues was determined by combined bisulfite restriction analysis. The reactivation of RSK4 expression by decitabine treatment was studied in OVCAR3, SKOV3, TOV-112D, and TOV-21G cells by Western blotting. Cell proliferation was determined by XTT. A significantly high methylation percentage of the RSK4 promoter was observed among malignant and benign ovarian tumors but not in normal ovarian tissue. RSK4 promoter methylation was not associated with age, histological subtype, or stages of ovarian cancer. RSK4 promoter methylation correlates weakly but not significantly with RSK4 protein expression. No correlation was shown between RSK4 methylation and RSK4 mRNA expression. Decitabine induces RSK4 reactivation in all cell lines. However, cell proliferation was reduced only in TOV-112D cells. CONCLUSION: These data indicate that although RSK4 promoter methylation is increased in malignant ovarian tumors, this mechanism is unlikely to regulate its expression in ovarian cancer. RSK4 reactivation reduced cell proliferation only in the endometroid histological subtype.

Dissecting the mechanism of signaling-triggered nuclear export of newly synthesized influenza virus ribonucleoprotein complexes.

Influenza viruses (IV) exploit a variety of signaling pathways. Previous studies showed that the rapidly accelerated fibrosarcoma/mitogen-activated protein kinase/extracellular signal-regulated kinase (Raf/MEK/ERK) pathway is functionally linked to nuclear export of viral ribonucleoprotein (vRNP) complexes, suggesting that vRNP export is a signaling-induced event. However, the underlying mechanism remained completely enigmatic. Here we have dissected the unknown molecular steps of signaling-driven vRNP export. We identified kinases RSK1/2 as downstream targets of virus-activated ERK signaling. While RSK2 displays an antiviral role, we demonstrate a virus-supportive function of RSK1, migrating to the nucleus to phosphorylate nucleoprotein (NP), the major constituent of vRNPs. This drives association with viral matrix protein 1 (M1) at the chromatin, important for vRNP export. Inhibition or knockdown of MEK, ERK or RSK1 caused impaired vRNP export and reduced progeny virus titers. This work not only expedites the development of anti-influenza strategies, but in addition demonstrates converse actions of different RSK isoforms.

Phosphorylation-Assisted Luciferase Complementation Assay Designed to Monitor Kinase Activity and Kinase-Domain-Mediated Protein-Protein Binding.

Protein kinases are key regulators of cell signaling and have been important therapeutic targets for three decades. ATP-competitive drugs directly inhibit the activity of kinases but these enzymes work as part of complex protein networks in which protein-protein interactions (often referred to as kinase docking) may govern a more complex activation pattern. Kinase docking is indispensable for many signaling disease-relevant Ser/Thr kinases and it is mediated by a dedicated surface groove on the kinase domain which is distinct from the substrate-binding pocket. Thus, interfering with kinase docking provides an alternative strategy to control kinases. We describe activity sensors developed for p90 ribosomal S6 kinase (RSK) and mitogen-activated protein kinases (MAPKs: ERK, p38, and JNK) whose substrate phosphorylation is known to depend on kinase-docking-groove-mediated protein-protein binding. The in vitro assays were based on fragment complementation of the NanoBit luciferase, which is facilitated upon substrate motif phosphorylation. The new phosphorylation-assisted luciferase complementation (PhALC) sensors are highly selective and the PhALC assay is a useful tool for the quantitative analysis of kinase activity or kinase docking, and even for high-throughput screening of academic compound collections.