A genome-wide RNAi screen identifies multiple RSK-dependent regulators of cell migration

Microfluidic devices for precise measurements of cell directionality reveal a role for glutamine during cell migration

Cancer cells that migrate from tumors into surrounding tissues are responsible for cancer dissemination through the body. Microfluidic devices have been instrumental in discovering unexpected features of cancer cell migration, including the migration in self-generated gradients and the contributions of cell-cell contact during collective migration. Here, we design microfluidic channels with five successive bifurcations to characterize the directionality of cancer cell migration with high precision. We uncover an unexpected role for glutamine in epithelial cancer cell orientation, which could be replaced by alfa-keto glutarate but not glucose.

A double-edged sword: DLG5 in diseases

Discs large homolog 5 (DLG5), a key member of the membrane-associated guanylate kinase (MAGUKs) family, is a scaffold molecule for signal transduction complexes and is responsible for assembling receptors and adapters. This scaffold protein stabilizes adhesion and tight bonding complexes in many organs and tissues, and is involved of maintaining epithelial polarity. Although DLG5 plays a role in normal development in mice, it has also been linked to the onset and development of several diseases, particularly Crohn's disease and various malignancies. DLG5 has been shown to impact the progression of cancer through direct or indirect interactions with H-catenin, E-cadherin, Vimentin, p53, P21, Cyclin D1, TGF-β1, AKT, Hippo, and classic G protein signaling pathways. DLG5 and DLG5 variants has been found to have a dual role in human diseases. Although it is overexpressed in pancreatic adenocarcinoma, its expression is reduced in lung, liver, breast, prostate, and bladder cancers. However, two independent studies on glioblastoma (GBM) have shown the opposite effects of DLG5. Our study evaluates the existing literature on the role of DLG5 and DLG5 variants in disease processes, and summarizes the available data on the role of DLG5 in disease based on cell experiments, clinical samples, and animal models, while highlighting its future potential in disease treatment.

Microfluidic Devices for Precise Measurements of Cell Directionality Reveal a Role for Glutamine during Cell Migration

Cancer cells that migrate from tumors into surrounding tissues are responsible for cancer dissemination through the body. Microfluidic devices have been instrumental in discovering unexpected features of cancer cell migration, including the migration in self-generated gradients and the contributions of cell-cell contact during collective migration. Here, we design microfluidic channels with five successive bifurcations to characterize the directionality of cancer cell migration with high precision. We find that the directional decisions of cancer cells moving through bifurcating channels in response to self-generated epidermal growth factor (EGF) gradients require the presence of glutamine in the culture media. A biophysical model helps quantify the contribution of glucose and glutamine to cancer cell orientation during migration in self-generated gradients. Our study uncovers an unexpected interplay between cancer cell metabolism and cancer cell migration studies and may eventually lead to new ways to delay cancer cell invasion.

ERK2 signaling regulates cell-cell adhesion of epithelial cells and enhances growth factor-induced cell scattering

The ERK signaling pathway, consisting of core protein kinases Raf, MEK and effector kinases ERK1/2, regulates various biological outcomes such as cell proliferation, differentiation, apoptosis, or cell migration. Signal transduction through the ERK signaling pathway is tightly controlled at all levels of the pathway. However, it is not well understood whether ERK pathway signaling can be modulated by the abundance of ERK pathway core kinases. In this study, we investigated the effects of low-level overexpression of the ERK2 isoform on the phenotype and scattering of cuboidal MDCK epithelial cells growing in discrete multicellular clusters. We show that ERK2 overexpression reduced the vertical size of lateral membranes that contain cell-cell adhesion complexes. Consequently, ERK2 overexpressing cells were unable to develop cuboidal shape, remained flat with increased spread area and intercellular adhesive contacts were present only on the basal side. Interestingly, ERK2 overexpression was not sufficient to increase phosphorylation of multiple downstream targets including transcription factors and induce global changes in gene expression, namely to increase the expression of pro-migratory transcription factor Fra1. However, ERK2 overexpression enhanced HGF/SF-induced cell scattering as these cells scattered more rapidly and to a greater extent than parental cells. Our results suggest that an increase in ERK2 expression primarily reduces cell-cell cohesion and that weakened intercellular adhesion synergizes with upstream signaling in the conversion of the multicellular epithelium into single migrating cells. This mechanism may be clinically relevant as the analysis of clinical data revealed that in one type of cancer, pancreatic adenocarcinoma, ERK2 overexpression correlates with a worse prognosis.

RSK-Mediated Non-canonical Activation of EphA2 by Tamoxifen

The long-term administration of tamoxifen to estrogen receptor α (ERα)-positive breast cancer patients is an established treatment that reduces mortality and recurrence. However, resistance to tamoxifen and an increased risk of endometrial cancer may occur; therefore, the mechanisms by which tamoxifen causes these adverse effects warrant further study. Tamoxifen has been shown to activate mitogen-activated protein kinase (MAPK) in an ERα-independent manner; therefore, we investigated its effects on the MAPK-mediated non-canonical activation of EphA2, a critical event regulating cell migration. Tamoxifen at slightly higher concentrations induced the rapid phosphorylation of EphA2 at Ser-897 via the MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK)-ERK-ribosomal S6 kinases (RSK) pathway in HeLa cells. In addition, tamoxifen significantly enhanced the migration ability of ERα-negative MDA-MB-231 breast cancer cells in RSK- and EphA2-dependent manners. Phosphorylated EphA2 was internalized and re-localized to the plasma membrane, including lamellipodia, in an RSK-dependent manner. Collectively, the present results provide novel insights into the tumor-promoting activity of tamoxifen.

Systematic analysis of migration factors by MigExpress identifies essential cell migration control genes in non-small cell lung cancer

Cell migration is an essential process in health and in disease, including cancer metastasis. A comprehensive inventory of migration factors is nonetheless lacking-in part due to the difficulty in assessing migration using high-throughput technologies. Hence, there are currently very few screens that systematically reveal factors controlling cell migration. Here, we introduce MigExpress as a platform for the 'identification of Migration control genes by differential Expression'. MigExpress exploits the combination of in-depth molecular profiling and the robust quantitative analysis of migration capacity in a broad panel of samples and identifies migration-associated genes by their differential expression in slow- versus fast-migrating cells. We applied MigExpress to investigate non-small cell lung cancer (NSCLC), which is the most frequent cause of cancer mortality mainly due to metastasis. In 54 NSCLC cell lines, we comprehensively determined mRNA and protein expression. Correlating the transcriptome and proteome profiles with the quantified migration properties led to the discovery and validation of FLNC, DSE, CPA4, TUBB6, and BICC1 as migration control factors in NSCLC cells, which were also negatively correlated with patient survival. Notably, FLNC was the least expressed filamin in NSCLC, but the only one controlling cell migration and correlating with patient survival and metastatic disease stage. In our study, we present MigExpress as a new method for the systematic analysis of migration factors and provide a comprehensive resource of transcriptomic and proteomic data of NSCLC cell lines related to cell migration.

Decreased expression of Dlg5 is associated with a poor prognosis and epithelial-mesenchymal transition in squamous cell lung cancer

Background

Discs large homolog 5 (Dlg5) is a newly discovered member of the membrane-associated guanylate kinase superfamily (MAGUK) that is involved in several important processes, including the maintenance of epithelial cell polarity, cell proliferation control, and cell migration and invasion. Decreased expression of Dlg5 has been reported in malignancies arising from different organs. In the present study, we analyzed Dlg5 expression and its prognostic value in squamous cell lung cancer (SqCLC).

Methods

Tumor tissue and adjacent normal tissue sections were collected from 98 patients with SqCLC. The expression levels of Dlg5 and epithelial-mesenchymal transition (EMT) biomarkers in the tissue sections were examined by immunohistochemistry and western blot.

Results

There were 80 males and 18 females in the study cohort. Patients at pathological stages I and IIIA accounted for 64.3% and 35.7% of the cohort, respectively. Western blot showed that Dlg5 expression differed between SqCLC and healthy tissues. Western blot also revealed low Dlg5 expression to be associated with low E-cadherin expression and high vimentin expression, which was consistent with the findings of immunohistochemical staining. Dlg5 expression was significantly correlated with lymph node (LN) metastasis (P=0.001) and disease recurrence (P<0.001), as well as with E-cadherin and vimentin expression (P=0.025 and P=0.001, respectively). Univariate analysis showed that overall survival was significantly correlated with the tumor-node-metastasis (P<0.001) and T (P=0.001) stages, LN metastasis (P<0.001), Dig5 expression (P<0.001), β-catenin expression (P=0.004), and vimentin expression (P=0.002). Patients with overexpression of Dlg5 and β-catenin had a more favorable prognosis than those without. Multivariate analysis revealed that tumor-node-metastasis stage [hazard ratio (HR) =2.124; 95% confidence interval (CI), 1.195–3.777; P=0.010], Dlg5 expression (HR =0.548; 95% CI, 0.313–0.959; P=0.035), β-catenin expression (HR =0.545; 95% CI, 0.312–0.953; P=0.033), and vimentin expression (HR =1.850; 95% CI, 1.050–3.258; P=0.033) could all independently predict the overall survival of patients with SqCLC.

Conclusions

Dlg5 is an important player in EMT which may have potential predictive value for SqCLC prognosis after surgery.

Overexpression of RSK4 reverses doxorubicin resistance in human breast cancer cells via PI3K/AKT signalling pathway

Doxorubicin (DOX) is one of the most effective chemotherapy drugs for the treatment of metastatic breast cancer (BC), but drug resistance becomes an obstacle to treatment. This study aims to investigate the role of Ribosomal S6 protein kinase 4 (RSK4) in regulating BC resistance to DOX. We first used Kaplan-Meier Plotter to identify the prognostic roles of RSK4 in BC. DOX-resistant BC cells (MCF-7/DOX) were constructed and the expression of RSK4 was determined by reverse transcript polymerase chain reaction and western blot. Subsequently, we overexpressed the RSK4 in MCF-7/DOX cells, and measured drug resistance, colony formation, cell migration, invasion ability and cell apoptosis after transfection. In addition, western blot was used to explore the expression of apoptosis-related proteins and BC-resistance protein. Effects of RSK4 on activation of the PI3K/AKT signalling pathway were also tested. Furthermore, tumour xenograft in nude mice was constructed to observe the effect of RSK4 overexpression on tumour growth in vivo. In conclusion, RSK4 was positively correlated with survival rate in BC patients, which is lowly expressed in MCF-7/DOX. Meanwhile, the overexpression of RSK4 may inhibit drug resistance, cell migration, invasion, apoptosis and tumour growth. RSK4 may effectively attenuate DOX resistance in BC by inhibiting the PI3K/AKT signalling pathway.

Silence of lncRNA MIAT-mediated inhibition of DLG3 promoter methylation suppresses breast cancer progression via the Hippo signaling pathway

As the foremost common female malignancy, breast cancer (BC) poses a significant public health stumbling block. Although treatment protocols have improved over the years, the overall prognosis of BC remains unsatisfactory. Extensive investigations have taken place into long non coding RNAs (lncRNAs) pertaining to their involvement in carcinogenesis. The current study in connection with bioinformatics tools aimed to identify the myocardial infarction associated transcript (MIAT) as a BC-related differentially expressed lncRNA in an attempt to elucidate the effect of MIAT in BC cells. MIAT was initially overexpressed while DLG3 was down-regulated in BC. BC cells were subsequently treated with si-MIAT or/and si-DLG3, after which the expressions of DLG3 and the Hippo signaling pathway-related proteins were evaluated to analyze their regulatory mechanism in BC, which indicated that MIAT inhibition up-regulated DLG3 and activated the Hippo signaling pathway to suppress proliferation and promote apoptosis of BC cells. MS-PCR and RIP assays demonstrated that MIAT bound to the methylation proteins DNMT1, DNMT3A and DNMT3B, promoted the methylation of CpG islands in DLG3 promoter and inhibited the DLG3 expression. Moreover, our data suggested that DLG3 could bind to MST2 and regulate LAST1, which prevented the nuclear translocation of YAP. The in vitro results were further verified via the in vivo findings. Taken together, the central findings of our study demonstrate that MIAT silencing inhibits BC progression by means of up-regulating DLG3 via activation of the Hippo signaling pathway, highlighting a novel potential therapeutic target for the treatment of the BC.

High Throughput strategies Aimed at Closing the GAP in Our Knowledge of Rho GTPase Signaling

Since their discovery, Rho GTPases have emerged as key regulators of cytoskeletal dynamics. In humans, there are 20 Rho GTPases and more than 150 regulators that belong to the RhoGEF, RhoGAP, and RhoGDI families. Throughout development, Rho GTPases choregraph a plethora of cellular processes essential for cellular migration, cell–cell junctions, and cell polarity assembly. Rho GTPases are also significant mediators of cancer cell invasion. Nevertheless, to date only a few molecules from these intricate signaling networks have been studied in depth, which has prevented appreciation for the full scope of Rho GTPases’ biological functions. Given the large complexity involved, system level studies are required to fully grasp the extent of their biological roles and regulation. Recently, several groups have tackled this challenge by using proteomic approaches to map the full repertoire of Rho GTPases and Rho regulators protein interactions. These studies have provided in-depth understanding of Rho regulators specificity and have contributed to expand Rho GTPases’ effector portfolio. Additionally, new roles for understudied family members were unraveled using high throughput screening strategies using cell culture models and mouse embryos. In this review, we highlight theses latest large-scale efforts, and we discuss the emerging opportunities that may lead to the next wave of discoveries.

Regioselective Synthesis of a C-4'' Carbamate,C-6'' n-Pr Substituted Cyclitol Analogue of SL0101

An asymmetric synthesis of two analogues of SL0101 (1) has been achieved. The effort is aimed at the discovery of inhibitors of the p90 ribosomal S6 kinase (RSK) with improved bioavailability. The route relies upon the use of the Taylor catalyst to regioselectively install C-3″ acetyl or carbamate functionality. This study led to the identification of a third-generation analogue of SL0101 with a C-4″ n-Pr-carbamate and a C-3″ acetate with improved RSK inhibitory activity.

The polarity protein Dlg5 regulates collective cell migration during Drosophila oogenesis

Collective migration plays critical roles in animal development, physiological events, and cancer metastasis. However, the molecular mechanisms of collective cell migration are not well understood. Drosophila border cells represent an excellent in vivo genetic model to study collective cell migration and identify novel regulatory genes for cell migration. Using the Mosaic Analysis with a Repressible Cell Marker (MARCM) system, we screened 240 P-element insertion lines to identify essential genes for border cell migration. Two genes were uncovered, including dlg5 (discs large 5) and CG31689. Further analysis showed that Dlg5 regulates the apical-basal polarity and cluster integrity in border cell clusters. Dlg5 is enriched in lateral surfaces between border cells and central polar cells but also shows punctate localization between border cells. We found that the distribution of Dlg5 in border cell clusters is regulated by Armadillo. Structure-function analysis revealed that the N-terminal Coiled-coil domain and the C-terminal PDZ3-PDZ4-SH3-GUK domains but not the PDZ1-PDZ2 domains of Dlg5 are required for BC migration. The Coiled-coil domain and the PDZ4-SH3-GUK domains are critical for Dlg5’s cell surface localization in border cell clusters.

Experimental identification of cancer driver alterations in the era of pan-cancer genomics

Rapidly accumulating data from large-scale cancer genomics studies have been generating important information about genes and their somatic alterations underlying cell transformation, cancer onset and tumor progression. However, these events are usually defined by using computational techniques, whereas the understanding of their actual functional roles and impact typically warrants validation by experimental means. Critical information has been obtained from targeted genetic perturbation (gene knockout) studies conducted in animals, yet these investigations are cost-prohibitive and time-consuming. In addition, the 3R principles (replacement, reduction, refinement) have been set in place to reduce animal use burden and are increasingly observed in many areas of biomedical research. Consequently, the focus has shifted to new designs of innovative cell-based experimental models of cell immortalization and transformation in which the critical cancer driver events can be introduced by mutagenic insult and studied functionally, at the level of critical phenotypic readouts. From these efforts, primary cell-based selective barrier-bypass models of cell immortalization have emerged as an attractive system that allows studies of the functional relevance of acquired mutations as well as their role as candidate cancer driver events. In this review, we provide an overview of various experimental systems linking carcinogen exposure-driven cell transformation with the study of cancer driver events. We further describe the advantages and disadvantages of the currently available cell-based models while outlining future directions for in vitro modeling and functional testing of cancer driver events.

p90 ribosomal S6 kinase (RSK) phosphorylates myosin phosphatase and thereby controls edge dynamics during cell migration

Cell migration is essential to embryonic development, wound healing, and cancer cell dissemination. Cells move via leading-edge protrusion, substrate adhesion, and retraction of the cell's rear. The molecular mechanisms by which extracellular cues signal to the actomyosin cytoskeleton to control these motility mechanics are poorly understood. The growth factor-responsive and oncogenically activated protein extracellular signal-regulated kinase (ERK) promotes motility by signaling in actin polymerization-mediated edge protrusion. Using a combination of immunoblotting, co-immunoprecipitation, and myosin-binding experiments and cell migration assays, we show here that ERK also signals to the contractile machinery through its substrate, p90 ribosomal S6 kinase (RSK). We probed the signaling and migration dynamics of multiple mammalian cell lines and found that RSK phosphorylates myosin phosphatase–targeting subunit 1 (MYPT1) at Ser-507, which promotes an interaction of Rho kinase (ROCK) with MYPT1 and inhibits myosin targeting. We find that by inhibiting the myosin phosphatase, ERK and RSK promote myosin II–mediated tension for lamella expansion and optimal edge dynamics for cell migration. These findings suggest that ERK activity can coordinately amplify both protrusive and contractile forces for optimal cell motility.

Hacking the Cancer Genome: Profiling Therapeutically Actionable Long Non-coding RNAs Using CRISPR-Cas9 Screening

Long non-coding RNAs (lncRNAs) represent a huge reservoir of potential cancer targets. Such "onco-lncRNAs" have resisted traditional RNAi methods, but CRISPR-Cas9 genome editing now promises functional screens at high throughput and low cost. The unique biology of lncRNAs demands screening strategies distinct from protein-coding genes. The first such screens have identified hundreds of onco-lncRNAs promoting cell proliferation and drug resistance. Ongoing developments will further improve screen performance and translational relevance. This Review aims to highlight the potential of CRISPR screening technology for discovering new onco-lncRNAs, and to guide molecular oncologists wishing to apply it to their cancer of interest.

DLG5 suppresses breast cancer stem cell-like characteristics to restore tamoxifen sensitivity by inhibiting TAZ expression

Tamoxifen (TAM) is a primary drug for treatment of estrogen receptor positive breast cancer. However, TAM resistance remains a serious threat to breast cancer patients and may be attributed to increased stemness of breast cancer. Here, we show that discs large homolog 5 (DLG5) expression is down‐regulated in TAM‐resistant breast cancer and cells. DLG5 silencing decreased the sensitivity to TAM and increased the frequency and stemness of CD44⁺/CD24⁻ breast cancer stem cells (BCSCs) and TAZ, a transducer of the Hippo pathway, expression in MCF7 cells while DLG5 overexpression had opposite effects. TAZ silencing restored the sensitivity to TAM and reduced the frequency and stemness in TAM‐resistant breast cancer cells. Taken together, our data indicate that down‐regulated DLG5 expression increases the stemness of breast cancer cells by enhancing TAZ expression, contributing to TAM resistance in breast cancer.

c-MYC Drives Breast Cancer Metastasis to the Brain, but Promotes Synthetic Lethality with TRAIL

Brain metastasis in breast cancer is particularly deadly, but effective treatments remain out of reach due to insufficient information about the mechanisms underlying brain metastasis and the potential vulnerabilities of brain-metastatic breast cancer cells. Here, human breast cancer cells and their brain-metastatic derivatives (BrMs) were used to investigate synthetic lethal interactions in BrMs. First, it was demonstrated that c-MYC activity is increased in BrMs and is required for their brain-metastatic ability in a mouse xenograft model. Specifically, c-MYC enhanced brain metastasis by facilitating the following processes within the brain microenvironment: (i) invasive growth of BrMs, (ii) macrophage infiltration, and (iii) GAP junction formation between BrMs and astrocytes by upregulating connexin 43 (GJA1/Cx43). Furthermore, RNA-sequencing (RNA-seq) analysis uncovered a set of c-MYC-regulated genes whose expression is associated with higher risk for brain metastasis in breast cancer patients. Paradoxically, however, increased c-MYC activity in BrMs rendered them more susceptible to TRAIL (TNF-related apoptosis-inducing ligand)-induced apoptosis. In summary, these data not only reveal the brain metastasis-promoting role of c-MYC and a subsequent synthetic lethality with TRAIL, but also delineate the underlying mechanism. This suggests TRAIL-based approaches as potential therapeutic options for brain-metastatic breast cancer. IMPLICATIONS: This study discovers a paradoxical role of c-MYC in promoting metastasis to the brain and in rendering brain-metastatic cells more susceptible to TRAIL, which suggests the existence of an Achilles' heel, thus providing a new therapeutic opportunity for breast cancer patients.

Novel oncogene COPS3 interacts with Beclin1 and Raf-1 to regulate metastasis of osteosarcoma through autophagy

Background

Expression of COP9 signalosome subunit 3 (COPS3), an oncogene overexpressed in osteosarcoma, has been demonstrated to be significantly correlated with tumor metastasis. However, the underlying mechanism by which COPS3 promotes metastasis of osteosarcoma and its role in autophagy remain unknown.

Methods

The expression of COPS3 was detected in primary osteosarcoma tissues and matching lung metastasis tissues by immunohistochemistry (IHC). The effect of COPS3 on the metastasis of osteosarcoma cells was investigated by transwell, wound healing assays and animal studies. Indicated proteins was analyzed by western blotting when COPS3 was knockdown or overexpressed. The COPS3 Interacting protein was determined by immunoprecipitation assay. The relationship between COPS3 and autophagy was detected by western blotting and immunofluorescence.

Results

We found that knockdown of COPS3 significantly reduced the lung metastasis of osteosarcoma cells in a mouse model, coinciding with downregulation of mitogen-activated protein kinase (MEK) and extracellular signal-regulated kinase (ERK) signaling. The silencing of COPS3 also inhibited the epithelial–mesenchymal transition (EMT) through the 90 kDa ribosomal S6 kinases (RSK), a family of signal transduction proteins downstream of MEK/ERK. Reciprocal immunoprecipitation assays revealed that COPS3 directly interacts with Raf-1, an upstream regulator of MEK/ERK. Surprisingly, Beclin1, an important autophagic protein, appeared in the COPS3-immunoprecipitates, along with the autophagic markers LC3-I and LC3-II. Loss of COPS3 completely inhibited H₂O₂-induced autophagic flux and reduced Beclin1 expression. Additionally, autophagy inhibitor or silencing of Beclin1 both decreased cell metastasis.

Conclusions

Taken together, these data reveal a novel function of COPS3 in the regulation of autophagy and highlight the relationship between autophagy and metastasis in osteosarcoma cells.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0791-6) contains supplementary material, which is available to authorized users.

An RNAi screen of Rho signalling networks identifies RhoH as a regulator of Rac1 in prostate cancer cell migration

BACKGROUND

Cell migration is essential for development and tissue repair, but it also contributes to disease. Rho GTPases regulate cell migration, but a comprehensive analysis of how each Rho signalling component affects migration has not been carried out.

RESULTS

Through an RNA interference screen, and using a prostate cancer cell line, we find that approximately 25% of Rho network components alter migration. Some genes enhance migration while others decrease basal and/or hepatocyte growth factor-stimulated migration. Surprisingly, we identify RhoH as a screen hit. RhoH expression is normally restricted to haematopoietic cells, but we find it is expressed in multiple epithelial cancer cell lines. High RhoH expression in samples from prostate cancer patients correlates with earlier relapse. RhoH depletion reduces cell speed and persistence and decreases migratory polarity. Rac1 activity normally localizes to the front of migrating cells at areas of dynamic membrane movement, but in RhoH-depleted cells active Rac1 is localised around the whole cell periphery and associated with membrane regions that are not extending or retracting. RhoH interacts with Rac1 and with several p21-activated kinases (PAKs), which are Rac effectors. Similar to RhoH depletion, PAK2 depletion increases cell spread area and reduces cell migration. In addition, RhoH depletion reduces lamellipodium extension induced by PAK2 overexpression.

CONCLUSIONS

We describe a novel role for RhoH in prostate cancer cell migration. We propose that RhoH promotes cell migration by coupling Rac1 activity and PAK2 to membrane protrusion. Our results also suggest that RhoH expression levels correlate with prostate cancer progression.

ERα-Mediated Nuclear Sequestration of RSK2 Is Required for ER+ Breast Cancer Tumorigenesis

Although ribosomal protein S6 kinase A3 (RSK2) activation status positively correlates with patient responses to antiestrogen hormonal therapies, the mechanistic basis for these observations is unknown. Using multiple in vitro and in vivo models of estrogen receptor-positive (ER⁺) breast cancer, we report that ERα sequesters active RSK2 into the nucleus to promote neoplastic transformation and facilitate metastatic tumor growth. RSK2 physically interacted with ERα through its N terminus to activate a proneoplastic transcriptional network critical to the ER⁺ lineage in the mammary gland, thereby providing a gene signature that effectively stratified patient tumors according to ERα status. ER⁺ tumor growth was strongly dependent on nuclear RSK2, and transgenic mice engineered to stably express nuclear RSK2 in the mammary gland developed high-grade ductal carcinoma in situ Mammary cells isolated from the transgenic model and introduced systemically successfully disseminated and established metastatic lesions. Antiestrogens disrupted the interaction between RSK2 and ERα, driving RSK2 into the cytoplasm and impairing tumor formation. These findings establish RSK2 as an obligate participant of ERα-mediated transcriptional programs, tumorigenesis, and divergent patient responses to antiestrogen therapies.Significance: Nuclear accumulation of active RSK drives a protumorigenic transcriptional program and renders ER⁺ breast cancer susceptible to endocrine-based therapies. Cancer Res; 78(8); 2014-25. ©2018 AACR.

RSK-mediated down-regulation of PDCD4 is required for proliferation, survival, and migration in a model of triple-negative breast cancer

The p90 ribosomal S6 kinase (RSK) is a family of MAPK-activated serine/threonine kinases (RSK1-4) whose expression and/or activity are deregulated in several cancers, including breast cancer. Up-regulation of RSKs promotes cellular processes that drive tumorigenesis in Triple Negative Breast Cancer (TNBC) cells. Although RSKs regulate protein synthesis in certain cell types, the role of RSK-mediated translational control in oncogenic progression has yet to be evaluated. We demonstrate that proliferation and migration of TNBC MDA-MB-231 cells, unlike ER/PR-positive MCF7 cells, rely on RSK activity. We show that RSKs regulate the activities of the translation initiation factor eIF4B and the translational repressor PDCD4 in TNBC cells with up-regulated MAPK pathway, but not in breast cancer cells with hyperactivated PI3K/Akt/mTORC1 pathway. These results identify PDCD4 as a novel RSK substrate. We demonstrate that RSK-mediated phosphorylation of PDCD4 at S76 promotes PDCD4 degradation. Low PDCD4 levels reduce PDCD4 inhibitory effect on the translation initiation factor eIF4A, which increases translation of "eIF4A sensitive" mRNAs encoding factors involved in cell cycle progression, survival, and migration. Consequently, low levels of PDCD4 favor proliferation and migration of MDA-MB-231 cells. These results support the therapeutic use of RSK inhibitors for treatment of TNBC with deregulated MAPK/RSK pathway.

Discs large homolog 5 decreases formation and function of invadopodia in human hepatocellular carcinoma via Girdin and Tks5

Invadopodium formation is a crucial early event of invasion and metastasis of hepatocellular carcinoma (HCC). However, the molecular mechanisms underlying regulation of invadopodia remain elusive. This study aimed to investigate the potential role of discs large homolog 5 (Dlg5) in invadopodium formation and function in HCC. We found that Dlg5 expression was significantly lower in human HCC tissues and cell lines than adjacent nontumor tissues and liver cells. Lower Dlg5 expression was associated with advanced stages of HCC, and poor overall and disease-free survival of HCC patients. Dlg5-silencing promoted epithelial-mesenchymal transition, invadopodium formation, gelatin degradation function, and invadopodium-associated invasion of HepG2 cells. In contrast, Dlg5 overexpression inhibited epithelial-mesenchymal transition, functional invadopodium formation, and invasion of SK-Hep1 cells. Both Girdin and Tks5, but not the Tks5 nonphosphorylatable mutant, were responsible for the enhanced invadopodium formation and invasion of Dlg5-silenced HepG2 cells. Furthermore, Dlg5 interacted with Girdin and interfered with the interaction of Girdin and Tks5. Dlg5 silencing promoted Girdin and Tks5 phosphorylation, which was abrogated by Girdin silencing and rescued by inducing shRNA-resistant Girdin expression. Moreover, Dlg5 overexpression significantly inhibited HCC intrahepatic and lung metastasis in vivo. Taken together, our data indicate that Dlg5 acts as a novel regulator of invadopodium-associated invasion via Girdin and by interfering with the interaction between Girdin and Tks5, which might be important for Tks5 phosphorylation in HCC cells. Conceivably, Dlg5 may act as a new biomarker for prognosis of HCC patients.

Defining the role of the RSK isoforms in cancer

The 90kDa ribosomal S6 kinase (RSK) family is a group of Ser/Thr protein kinases (RSK1-4) that function downstream of the Ras/mitogen-activated protein kinase (MAPK) signalling pathway. RSK regulates many substrates involved in cell survival, growth, and proliferation, and as such, deregulated RSK activity has been associated with multiple cancer types. RSK expression and activity are dysregulated in several malignancies, including breast, prostate, and lung cancer, and available evidence suggests that RSK may be a promising cancer therapeutic target. Current limitations include the lack of RSK inhibitors with suitable pharmacokinetics and selectivity toward particular isoforms. This review briefly describes the current knowledge on RSK activation and function, with a particular emphasis on RSK-dependent mechanisms associated with tumorigenesis and pharmacological inhibition.

Loss of DLG5 promotes breast cancer malignancy by inhibiting the Hippo signaling pathway

Discs Large Homolog 5 (DLG5) plays an important role in the maintenance of epithelial cell polarity. Recent research showed that DLG5 is decreased in Yes-associated protein (YAP)-overexpressing cells. However, the exact relationship between DLG5 and YAP is not clear. In this study, we showed that loss of DLG5 promoted breast cancer cell proliferation by inhibiting the Hippo signaling pathway and increasing nuclear YAP expression. Furthermore, depletion of DLG5 induced epithelial-mesenchymal transition (EMT) and disrupted epithelial cell polarity, which was associated with altered expression of Scribble, ZO1, E-cadherin and N-cadherin and their mislocalization. Interestingly, we first reported that loss of DLG5 inhibited the interaction of Mst1 and Lats1 with Scribble, which was crucial for YAP activation and the transcription of TEA domain (TEAD) family members. In summary, loss of DLG5 expression promoted breast cancer malignancy by inactivating the Hippo signaling pathway and increasing nuclear YAP.

Peritoneal and hematogenous metastases of ovarian cancer cells are both controlled by the p90RSK through a self-reinforcing cell autonomous mechanism

The molecular mechanisms orchestrating peritoneal and hematogenous metastases of ovarian cancer cells are assumed to be distinct. We studied the p90RSK family of serine/threonine kinases that lie downstream the RAS-ERK/MAPK pathway and modulate a variety of cellular processes including cell proliferation, survival, motility and invasiveness. We found the RSK1 and RSK2 isoforms expressed in a number of human ovarian cancer cell lines, where they played redundant roles in sustaining in vitro motility and invasiveness. In vivo, silencing of both RSK1 and RSK2 almost abrogated short-term and long-term metastatic engraftment of ovarian cancer cells in the peritoneum. In addition, RSK1/RSK2 silenced cells failed to colonize the lungs after intravenous injection and to form hematogenous metastasis from subcutaneous xenografts. RSK1/RSK2 suppression resulted in lessened ovarian cancer cell spreading on endogenous fibronectin (FN). Mechanistically, RSK1/RSK2 knockdown diminished FN transcription, α5β1 integrin activation and TGF-β1 translation. Reduced endogenous FN deposition and TGF-β1 secretion depended on the lack of activating phosphorylation of the transcription/translation factor YB-1 by p90RSK. Altogether data show how p90RSK activates a self-reinforcing cell autonomous pro-adhesive circuit necessary for metastatic seeding of ovarian cancer cells. Thus, p90RSK inhibitors might hinder both the hematogenous and the peritoneal metastatic spread of human ovarian cancer.

Development of a RSK Inhibitor as a Novel Therapy for Triple-Negative Breast Cancer

Metastatic breast cancer is an incurable disease and identification of novel therapeutic opportunities is vital. Triple-negative breast cancer (TNBC) frequently metastasizes and high levels of activated p90RSK (RSK), a downstream MEK-ERK1/2 effector, are found in TNBC. We demonstrate, using direct pharmacologic and genetic inhibition of RSK1/2, that these kinases contribute to the TNBC metastatic process in vivo Kinase profiling showed that RSK1 and RSK2 are the predominant kinases targeted by the new inhibitor, which is based on the natural product SL0101. Further evidence for selectivity was provided by the observations that silencing RSK1 and RSK2 eliminated the ability of the analogue to further inhibit survival or proliferation of a TNBC cell line. In vivo, the new derivative was as effective as the FDA-approved MEK inhibitor trametinib in reducing the establishment of metastatic foci. Importantly, inhibition of RSK1/2 did not result in activation of AKT, which is known to limit the efficacy of MEK inhibitors in the clinic. Our results demonstrate that RSK is a major contributor to the TNBC metastatic program and provide preclinical proof-of-concept for the efficacy of the novel SL0101 analogue in vivo Mol Cancer Ther; 15(11); 2598-608. ©2016 AACR.

Repositioning "old" drugs for new causes: identifying new inhibitors of prostate cancer cell migration and invasion

The majority of prostate cancer (PCa) deaths occur due to the metastatic spread of tumor cells to distant organs. Currently, there is a lack of effective therapies once tumor cells have spread outside the prostate. It is therefore imperative to rapidly develop therapeutics to inhibit the metastatic spread of tumor cells. Gain of cell motility and invasive properties is the first step of metastasis and by inhibiting motility one can potentially inhibit metastasis. Using the drug repositioning strategy, we developed a cell-based multi-parameter primary screening assay to identify drugs that inhibit the migratory and invasive properties of metastatic PC-3 PCa cells. Following the completion of the primary screening assay, 33 drugs were identified from an FDA approved drug library that either inhibited migration or were cytotoxic to the PC-3 cells. Based on the data obtained from the subsequent validation studies, mitoxantrone hydrochloride, simvastatin, fluvastatin and vandetanib were identified as strong candidates that can inhibit both the migration and invasion of PC-3 cells without significantly affecting cell viability. By employing the drug repositioning strategy instead of a de novo drug discovery and development strategy, the identified drug candidates have the potential to be rapidly translated into the clinic for the management of men with aggressive forms of PCa.

Leucine-enkephalin promotes wound repair through the regulation of hemidesmosome dynamics and matrix metalloprotease

The skin responds to environmental stressors by coordinated actions of neuropeptides and their receptors. An endogenous peptide for δ-opioid receptor (DOPr), Leu-enkephalin (L-ENK), is expressed in the skin and its expression is altered in pathological conditions. Although the importance of DOPr is rapidly gaining recognition, the molecular mechanisms underlying its effects on wound healing are largely undefined. We show here that L-ENK induced activation of Erk, P90(RSK), and Elk-1 and promoted the disruption of hemidesmosomes and the expression of matrix metalloprotease (MMP)-2 and MMP-9, important processes for wound healing. Treatment with Erk inhibitor blocked activation of P90(RSK) and Elk-1 and significantly blunted wound repair. Therefore, our results suggest that activation of Erk and its downstream effectors, P90(RSK) and Elk-1, are critical for DOPr-mediated skin homeostasis.

Expression of polarity genes in human cancer

Polarity protein complexes are crucial for epithelial apical–basal polarity and directed cell migration. Since alterations of these processes are common in cancer, polarity proteins have been proposed to function as tumor suppressors or oncogenic promoters. Here, we review the current understanding of polarity protein functions in epithelial homeostasis, as well as tumor formation and progression. As most previous studies focused on the function of single polarity proteins in simplified model systems, we used a genomics approach to systematically examine and identify the expression profiles of polarity genes in human cancer. The expression profiles of polarity genes were distinct in different human tissues and classified cancer types. Additionally, polarity expression profiles correlated with disease progression and aggressiveness, as well as with identified cancer types, where specific polarity genes were commonly altered. In the case of Scribble, gene expression analysis indicated its common amplification and upregulation in human cancer, suggesting a tumor promoting function.

MAPK7 Regulates EMT Features and Modulates the Generation of CTCs

Epithelial-to-mesenchymal transition (EMT) has been implicated in models of tumor cell migration, invasion, and metastasis. In a search for candidate therapeutic targets to reverse this process, nontumorigenic MCF10A breast epithelial cells were infected with an arrayed lentiviral kinome shRNA library and screened for either suppression or enhancement of a 26-gene EMT RNA signature. No individual kinase gene knockdown was sufficient to induce EMT. In contrast, grouped epithelial markers were induced by knockdown of multiple kinases, including mitogen activated protein kinase 7 (MAPK7). In breast cancer cells, suppression of MAPK7 increased E-cadherin (CDH1) expression and inhibited cell migration. In an orthotopic mouse model, MAPK7 suppression reduced the generation of circulating tumor cells and the appearance of lung metastases. Together, these observations raise the possibility that targeting kinases that maintain mesenchymal cell properties in cancer cells, such as MAPK7, may lessen tumor invasiveness.

IMPLICATIONS

Suppression of MAPK7 induces epithelial markers, reduces generation of circulating tumor cells and appearance of lung metastases.

Tumor-secreted Hsp90 subverts polycomb function to drive prostate tumor growth and invasion

Prostate cancer remains the second highest contributor to male cancer-related lethality. The transition of a subset of tumors from indolent to invasive disease is associated with a poor clinical outcome. Activation of the epithelial to mesenchymal transition (EMT) genetic program is a major risk factor for cancer progression. We recently reported that secreted extracellular Hsp90 (eHsp90) initiates EMT in prostate cancer cells, coincident with its enhanced expression in mesenchymal models. Our current work substantially extended these findings in defining a pathway linking eHsp90 signaling to EZH2 function, a methyltransferase of the Polycomb repressor complex. EZH2 is also implicated in EMT activation, and its up-regulation represents one of the most frequent epigenetic alterations during prostate cancer progression. We have now highlighted a novel epigenetic function for eHsp90 via its modulation of EZH2 expression and activity. Mechanistically, eHsp90 initiated sustained activation of MEK/ERK, a signal critical for facilitating EZH2 transcriptional up-regulation and recruitment to the E-cadherin promoter. We further demonstrated that an eHsp90-EZH2 pathway orchestrates an expanded repertoire of EMT-related events including Snail and Twist expression, tumor cell motility, and anoikis resistance. To evaluate the role of eHsp90 in vivo, eHsp90 secretion was stably enforced in a prostate cancer cell line resembling indolent disease. Remarkably, eHsp90 was sufficient to induce tumor growth, suppress E-cadherin, and initiate localized invasion, events that are exquisitely dependent upon EZH2 function. In summary, our findings illuminate a hitherto unknown epigenetic function for eHsp90 and support a model wherein tumor eHsp90 functions as a rheostat for EZH2 expression and activity to orchestrate mesenchymal properties and coincident aggressive behavior.

Identification of kinases and regulatory proteins required for cell migration using a transfected cell-microarray system

Background

Cell migration plays a major role in a variety of normal biological processes, and a detailed understanding of the associated mechanisms should lead to advances in the medical sciences in areas such as cancer therapy. Previously, we developed a simple chip, based on transfected-cell microarray (TCM) technology, for the identification of genes related to cell migration. In the present study, we used the TCM chip for high-throughput screening (HTS) of a kinome siRNA library to identify genes involved in the motility of highly invasive NBT-L2b cells.

Results

We performed HTS using TCM coupled with a programmed image tracer to capture time-lapse fluorescence images of siRNA-transfected cells and calculated speeds of cell movement. This first screening allowed us to identify 52 genes. After quantitative PCR (qPCR) and a second screening by a conventional transfection method, we confirmed that 32 of these genes were associated with the migration of NBT-L2b cells. We investigated the subcellular localization of proteins and levels of expression of these 32 genes, and then we used our results and databases of protein-protein interactions (PPIs) to construct a hypothetic but comprehensive signal network for cell migration.

Conclusions

The genes that we identified belonged to several functional categories, and our pathway analysis suggested that some of the encoded proteins functioned as the hubs of networks required for cell migration. Our signal pathways suggest that epidermal growth factor receptor (EGFR) is an upstream regulator in the network, while Src and GRB2 seem to represent nodes for control of respective the downstream proteins that are required to coordinate the many cellular events that are involved in migration. Our microarray appears to be a useful tool for the analysis of protein networks and signal pathways related to cancer metastasis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0170-7) contains supplementary material, which is available to authorized users.

RSK1 activation promotes invasion in nodular melanoma

The two major melanoma histologic subtypes, superficial spreading and nodular melanomas, differ in their speed of dermal invasion but converge biologically once they invade and metastasize. Herein, we tested the hypothesis that distinct molecular alterations arising in primary melanoma cells might persist as these tumors progress to invasion and metastasis. Ribosomal protein S6 kinase, 90 kDa, polypeptide 1 (RSK1; official name RPS6KA1) was significantly hyperactivated in human melanoma lines and metastatic tissues derived from nodular compared with superficial spreading melanoma. RSK1 was constitutively phosphorylated at Ser-380 in nodular but not superficial spreading melanoma and did not directly correlate with BRAF or MEK activation. Nodular melanoma cells were more sensitive to RSK1 inhibition using siRNA and the pharmacological inhibitor BI-D1870 compared with superficial spreading cells. Gene expression microarray analyses revealed that RSK1 orchestrated a program of gene expression that promoted cell motility and invasion. Differential overexpression of the prometastatic matrix metalloproteinase 8 and tissue inhibitor of metalloproteinases 1 in metastatic nodular compared with metastatic superficial spreading melanoma was observed. Finally, using an in vivo zebrafish model, constitutive RSK1 activation increased melanoma invasion. Together, these data reveal a novel role for activated RSK1 in the progression of nodular melanoma and suggest that melanoma originating from different histologic subtypes may be biologically distinct and that these differences are maintained as the tumors invade and metastasize.

Collective and individual migration following the epithelial-mesenchymal transition

During cancer progression, malignant cells in the tumour invade surrounding tissues. This transformation of adherent cells to a motile phenotype has been associated with the epithelial-mesenchymal transition (EMT). Here, we show that EMT-activated cells migrate through micropillar arrays as a collectively advancing front that scatters individual cells. Individual cells with few neighbours dispersed with fast, straight trajectories, whereas cells that encountered many neighbours migrated collectively with epithelial biomarkers. We modelled these emergent dynamics using a physical analogy to phase transitions during binary-mixture solidification, and validated it using drug perturbations, which revealed that individually migrating cells exhibit diminished chemosensitivity. Our measurements also indicate a degree of phenotypic plasticity as cells interconvert between individual and collective migration. The study of multicellular behaviours with single-cell resolution should enable further quantitative insights into heterogeneous tumour invasion.

RNAi-based functional selection identifies novel cell migration determinants dependent on PI3K and AKT pathways

Lentiviral short hairpin RNA (shRNA)-mediated genetic screening is a powerful tool for identifying loss-of-function phenotype in mammalian cells. Here, we report the identification of 91 cell migration-regulating genes using unbiased genome-wide functional genetic selection. Individual knockdown or cDNA overexpression of a set of 10 candidates reveals that most of these cell migration determinants are strongly dependent on the PI3K/PTEN/AKT pathway and on their downstream signals, such as FOXO1 and p70S6K1. ALK, one of the cell migration promoting genes, uniquely uses p55γ regulatory subunit of PI3K, rather than more common p85 subunit, to trigger the activation of the PI3K-AKT pathway. Our method enables the rapid and cost-effective genome-wide selection of cell migration regulators. Our results emphasize the importance of the PI3K/PTEN/AKT pathway as a point of convergence for multiple regulators of cell migration.

Phosphorylation of RSK2 at Tyr529 by FGFR2-p38 enhances human mammary epithelial cells migration

The members of p90 ribosomal S6 kinase (RSK) family of Ser/Thr kinases are downstream effectors of MAPK/ERK pathway that regulate diverse cellular processes including cell growth, proliferation and survival. In carcinogenesis, RSKs are thought to modulate cell motility, invasion and metastasis. Herein, we have studied an involvement of RSKs in FGF2/FGFR2-driven behaviours of mammary epithelial and breast cancer cells. We found that both silencing and inhibiting of FGFR2 attenuated phosphorylation of RSKs, whereas FGFR2 overexpression and/or its stimulation with FGF2 enhanced RSKs activity. Moreover, treatment with ERK, Src and p38 inhibitors revealed that p38 kinase acts as an upstream RSK2 regulator. We demonstrate for the first time that in FGF2/FGFR2 signalling, p38 but not MEK/ERK, indirectly activated RSK2 at Tyr529, which facilitated phosphorylation of its other residues (Thr359/Ser363, Thr573 and Ser380). In contrast to FGF2-triggered signalling, inhibition of p38 in the EGF pathway affected only RSK2-Tyr529, without any impact on the remaining RSK phosphorylation sites. p38-mediated phosphorylation of RSK2-Tyr529 was crucial for the transactivation of residues located at kinase C-terminal domain and linker-region, specifically, in the FGF2/FGFR2 signalling pathway. Furthermore, we show that FGF2 promoted anchorage-independent cell proliferation, formation of focal adhesions and cell migration, which was effectively abolished by treatment with RSKs inhibitor (FMK). These indicate that RSK2 activity is indispensable for FGF2/FGFR2-mediated cellular effects. Our findings identified a new FGF2/FGFR2-p38-RSK2 pathway, which may play a significant role in the pathogenesis and progression of breast cancer and, hence, may present a novel therapeutic target in the treatment of FGFR2-expressing tumours.

TRAP1-dependent regulation of p70S6K is involved in the attenuation of protein synthesis and cell migration: relevance in human colorectal tumors

TNF receptor-associated protein 1 (TRAP1) is an HSP90 chaperone involved in stress protection and apoptosis in mitochondrial and extramitochondrial compartments. Remarkably, aberrant deregulation of TRAP1 function has been observed in several cancer types with potential new opportunities for therapeutic intervention in humans. Although previous studies by our group identified novel roles of TRAP1 in quality control of mitochondria-destined proteins through the attenuation of protein synthesis, molecular mechanisms are still largely unknown. To shed further light on the signaling pathways regulated by TRAP1 in the attenuation of protein synthesis, this study demonstrates that the entire pathway of cap-mediated translation is activated in cells following TRAP1 interference: consistently, expression and consequent phosphorylation of p70S6K and RSK1, two translation activating kinases, are increased upon TRAP1 silencing. Furthermore, we show that these regulatory functions affect the response to translational stress and cell migration in wound healing assays, processes involving both kinases. Notably, the regulatory mechanisms controlled by TRAP1 are conserved in colorectal cancer tissues, since an inverse correlation between TRAP1 and p70S6K expression is found in tumor tissues, thereby supporting the relevant role of TRAP1 translational regulation in vivo. Taken as a whole, these new findings candidate TRAP1 network for new anti-cancer strategies aimed at targeting the translational/quality control machinery of tumor cells.

DLG5 in cell polarity maintenance and cancer development

Failure in establishment and maintenance of epithelial cell polarity contributes to tumorigenesis. Loss of expression and function of cell polarity proteins is directly related to epithelial cell polarity maintenance. The polarity protein discs large homolog 5 (DLG5) belongs to a family of molecular scaffolding proteins called Membrane Associated Guanylate Kinases (MAGUKs). As the other family members, DLG5 contains the multi-PDZ, SH3 and GUK domains. DLG5 has evolved in the same manner as DLG1 and ZO1, two well-studied MAGUKs proteins. Just like DLG1 and ZO1, DLG5 plays a role in cell migration, cell adhesion, precursor cell division, cell proliferation, epithelial cell polarity maintenance, and transmission of extracellular signals to the membrane and cytoskeleton. Since the roles of DLG5 in inflammatory bowel disease (IBD) and Crohn's disease (CD) have been reviewed, here, our review focuses on the roles of DLG5 in epithelial cell polarity maintenance and cancer development.

Luteolin is a novel p90 ribosomal S6 kinase (RSK) inhibitor that suppresses Notch4 signaling by blocking the activation of Y-box binding protein-1 (YB-1)

Triple-negative breast cancers (TNBC) are notoriously difficult to treat because they lack hormone receptors and have limited targeted therapies. Recently, we demonstrated that p90 ribosomal S6 kinase (RSK) is essential for TNBC growth and survival indicating it as a target for therapeutic development. RSK phosphorylates Y-box binding protein-1 (YB-1), an oncogenic transcription/translation factor, highly expressed in TNBC (~70% of cases) and associated with poor prognosis, drug resistance and tumor initiation. YB-1 regulates the tumor-initiating cell markers, CD44 and CD49f however its role in Notch signaling has not been explored. We sought to identify novel chemical entities with RSK inhibitory activity. The Prestwick Chemical Library of 1120 off-patent drugs was screened for RSK inhibitors using both in vitro kinase assays and molecular docking. The lead candidate, luteolin, inhibited RSK1 and RSK2 kinase activity and suppressed growth in TNBC, including TIC-enriched populations. Combining luteolin with paclitaxel increased cell death and unlike chemotherapy alone, did not enrich for CD44⁺ cells. Luteolin’s efficacy against drug-resistant cells was further indicated in the primary x43 cell line, where it suppressed monolayer growth and mammosphere formation. We next endeavored to understand how the inhibition of RSK/YB-1 signaling by luteolin elicited an effect on TIC-enriched populations. ChIP-on-ChIP experiments in SUM149 cells revealed a 12-fold enrichment of YB-1 binding to the Notch4 promoter. We chose to pursue this because there are several reports indicating that Notch4 maintains cells in an undifferentiated, TIC state. Herein we report that silencing YB-1 with siRNA decreased Notch4 mRNA. Conversely, transient expression of Flag:YB-1^WT or the constitutively active mutant Flag:YB-1^D102 increased Notch4 mRNA. The levels of Notch4 transcript and the abundance of the Notch4 intracellular domain (N4ICD) correlated with activation of P-RSK^S221/7 and P-YB-1^S102 in a panel of TNBC cell lines. Silencing YB-1 or RSK reduced Notch4 mRNA and this corresponded with loss of N4ICD. Likewise, the RSK inhibitors, luteolin and BI-D1870, suppressed P-YB-1 ^S102 and thereby reduced Notch4. In conclusion, inhibiting the RSK/YB-1 pathway with luteolin is a novel approach to blocking Notch4 signaling and as such provides a means of inhibiting TICs.

Cell migration in confined environments

We describe a protocol for measuring the speed of human neutrophils migrating through small channels, in conditions of mechanical confinement comparable to those experienced by neutrophils migrating through tissues. In such conditions, we find that neutrophils move persistently, at constant speed for tens of minutes, enabling precise measurements at single cells resolution, for large number of cells. The protocol relies on microfluidic devices with small channels in which a solution of chemoattractant and a suspension of isolated neutrophils are loaded in sequence. The migration of neutrophils can be observed for several hours, starting within minutes after loading the neutrophils in the devices. The protocol is divided into four main steps: the fabrication of the microfluidic devices, the separation of neutrophils from whole blood, the preparation of the assay and cell loading, and the analysis of data. We discuss the practical steps for the implementation of the migration assays in biology labs, the adaptation of the protocols to various cell types, including cancer cells, and the supplementary device features required for precise measurements of directionality and persistence during migration.

Targeting protein kinases with selective and semipromiscuous covalent inhibitors

Protein kinase inhibitors are an important class of therapeutics. In addition, selective kinase inhibitors can often reveal unexpected biological insights, augmenting genetic approaches and playing a decisive role in preclinical target validation studies. Nevertheless, developing protein kinase inhibitors with sufficient selectivity and pharmacodynamic potency presents significant challenges. Targeting noncatalytic cysteines with covalent inhibitors is a powerful approach to address both challenges simultaneously. Here, we describe our efforts to design irreversible and reversible electrophilic inhibitors with varying degrees of kinase selectivity. Highly selective covalent inhibitors have been used to elucidate the roles of p90 ribosomal protein S6 kinases in animal models of atherosclerosis and diabetes. By contrast, semipromiscuous covalent inhibitors have revealed new therapeutic targets in disease-causing parasites and have shown utility as chemoproteomic probes for interrogating kinase occupancy in living cells.

Phosphorylation of KIBRA by the extracellular signal-regulated kinase (ERK)-ribosomal S6 kinase (RSK) cascade modulates cell proliferation and migration

In mammals, KIBRA is defined as a memory performance-associated protein. The physiological function and regulation of KIBRA in non-neuronal cells are much less understood. Recent studies have identified KIBRA as a novel regulator of the Hippo signaling pathway, which plays a critical role in tumorigenesis by inhibiting cell proliferation and promoting apoptosis. We recently reported that KIBRA is phosphorylated by the mitotic kinases Aurora and cyclin-dependent kinase 1 during mitosis. In this current study, we show that KIBRA is also phosphorylated by the ERK (extracellular signal-regulated kinases)-RSK (p90 ribosomal S6 kinases) cascade. We demonstrated that ERK1/2 phosphorylate KIBRA at Ser(548) in cells as well as in vitro. Moreover, we found that RSK1/2 specifically phosphorylates KIBRA at two highly conserved sites (Thr(929) and Ser(947)) in vitro and in cells. RSK-mediated phosphorylation is required for KIBRA binding to RSK1, but not RSK2. Surprisingly, KIBRA knockdown impaired cell migration and proliferation in breast cancer cells. By using inducible-expression cell lines, we further show that phospho-regulation of KIBRA by ERK1/2 and RSK1/2 is required for proper cell proliferation and RSK-mediated phosphorylation also modulates KIBRA's migratory activity in MDA-MB-231 breast cancer cells. Our findings uncover unexpected results and a new mechanism through which KIBRA regulates cell migration and proliferation.

RSK isoforms in cancer cell invasion and metastasis

Metastasis, the spreading of cancer cells from a primary tumor to secondary sites throughout the body, is the primary cause of death for patients with cancer. New therapies that prevent invasion and metastasis in combination with current treatments could therefore significantly reduce cancer recurrence and morbidity. Metastasis is driven by altered signaling pathways that induce changes in cell-cell adhesion, the cytoskeleton, integrin function, protease expression, epithelial-to-mesenchymal transition and cell survival. The ribosomal S6 kinase (RSK) family of kinases is a group of extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) effectors that can regulate these steps of metastasis by phosphorylating both nuclear and cytoplasmic targets. However, our understanding of RSK function in metastasis remains incomplete and is complicated by the fact that the four RSK isoforms perform nonredundant, sometimes opposing functions. Although some isoforms promote cell motility and invasion by altering transcription and integrin activity, others impair cell motility and invasion through effects on the actin cytoskeleton. The mechanism of RSK action depends both on the isoform and the cancer type. However, despite the variance in RSK-mediated outcomes, chemical inhibition of this group of kinases has proven effective in blocking invasion and metastasis of several solid tumors in preclinical models. RSKs are therefore a promising drug target for antimetastatic cancer treatments that could supplement and improve current therapeutic approaches. This review highlights contradiction and agreement in the current data on the function of RSK isoforms in metastasis and suggests ways forward in developing RSK inhibitors as new antimetastasis drugs.

ERK and RSK regulate distinct steps of a cellular program that induces transition from multicellular epithelium to single cell phenotype

The ERK (extracellular signal-regulated kinases) cascade has an evolutionarily conserved three tier architecture consisting of protein kinases Raf, MEK (MAPK/ERK kinase) and ERK. Following activation, ERK phosphorylates various cellular elements leading to diverse cellular responses. Downstream of ERK the family of p90 ribosomal S6 kinases (RSKs) has been proven to be an important conveyor of ERK signaling, however, little is known if ERK and RSK coordinate their functions to generate a specific biological response. Here we show that in epithelial cells conditional activation of the ERK pathway causes phenotypic conversion of epithelial cells to autonomously migrating cells. This process involves two sequential steps characterized by loss of apical-basal polarity followed by cell scattering. The activation of ERK, but not RSK, is sufficient for the execution of the first step and it requires calpain mediated remodeling of actin cytoskeleton. Conversely, RSK regulates the successive stage characterized by cell-cell contact weakening and increased cellular migration. Thus, ERK and RSK regulate different cellular subprograms and coordinated execution of these subprograms in time generates a relevant biological response. Our data also suggest that the mechanism by which the ERK pathway controls a cellular response may be distributed between ERK and RSK, rather than being elicited by a single effector kinase.

The p90 RSK family members: common functions and isoform specificity

The p90 ribosomal S6 kinases (RSK) are implicated in various cellular processes, including cell proliferation, survival, migration, and invasion. In cancer, RSKs modulate cell transformation, tumorigenesis, and metastasis. Indeed, changes in the expression of RSK isoforms have been reported in several malignancies, including breast, prostate, and lung cancers. Four RSK isoforms have been identified in humans on the basis of their high degree of sequence homology. Although this similarity suggests some functional redundancy between these proteins, an increasing body of evidence supports the existence of isoform-based specificity among RSKs in mediating particular cellular processes. This review briefly presents the similarities between RSK family members before focusing on the specific function of each of the isoforms and their involvement in cancer progression.

Pathogenesis of RON receptor tyrosine kinase in cancer cells: activation mechanism, functional crosstalk, and signaling addiction

The RON receptor tyrosine kinase, a member of the MET proto-oncogene family, is a pathogenic factor implicated in tumor malignancy. Specifically, aberrations in RON signaling result in increased cancer cell growth, survival, invasion, angiogenesis, and drug resistance. Biochemical events such as ligand binding, receptor overexpression, generation of structure-defected variants, and point mutations in the kinase domain contribute to RON signaling activation. Recently, functional crosstalk between RON and signaling proteins such as MET and EFGR has emerged as an additional mechanism for RON activation, which is critical for tumorigenic development. The RON signaling crosstalk acts either as a regulatory feedback loop that strengthens or enhances tumorigenic phenotype of cancer cells or serves as a signaling compensatory pathway providing a growth/survival advantage for cancer cells to escape targeted therapy. Moreover, viral oncoproteins derived from Friend leukemia or Epstein-Barr viruses interact with RON to drive viral oncogenesis. In cancer cells, RON signaling is integrated into cellular signaling network essential for cancer cell growth and survival. These activities provide the molecular basis of targeting RON for cancer treatment. In this review, we will discuss recent data that uncover the mechanisms of RON activation in cancer cells, review evidence of RON signaling crosstalk relevant to cancer malignancy, and emphasize the significance of the RON signaling addiction by cancer cells for tumor therapy. Understanding aberrant RON signaling will not only provide insight into the mechanisms of tumor pathogenesis, but also lead to the development of novel strategies for molecularly targeted cancer treatment.

MSP-RON signalling in cancer: pathogenesis and therapeutic potential

Since the discovery of MSP (macrophage-stimulating protein; also known as MST1 and hepatocyte growth factor-like (HGFL)) as the ligand for the receptor tyrosine kinase RON (also known as MST1R) in the early 1990s, the roles of this signalling axis in cancer pathogenesis has been extensively studied in various model systems. Both in vitro and in vivo evidence has revealed that MSP-RON signalling is important for the invasive growth of different types of cancers. Currently, small-molecule inhibitors and antibodies blocking RON signalling are under investigation. Substantial responses have been achieved in human tumour xenograft models, laying the foundation for clinical validation. In this Review, we discuss recent advances that demonstrate the importance of MSP-RON signalling in cancer and its potential as a therapeutic target.