ERK nuclear translocation is dimerization-independent but controlled by the rate of phosphorylation

Exportin 1 governs the immunosuppressive functions of myeloid-derived suppressor cells in tumors through ERK1/2 nuclear export

Myeloid-derived suppressor cells (MDSCs) are a main driver of immunosuppression in tumors. Understanding the mechanisms that determine the development and immunosuppressive function of these cells could provide new therapeutic targets to improve antitumor immunity. Here, using preclinical murine models, we discovered that exportin 1 (XPO1) expression is upregulated in tumor MDSCs and that this upregulation is induced by IL-6-induced STAT3 activation during MDSC differentiation. XPO1 blockade transforms MDSCs into T-cell-activating neutrophil-like cells, enhancing the antitumor immune response and restraining tumor growth. Mechanistically, XPO1 inhibition leads to the nuclear entrapment of ERK1/2, resulting in the prevention of ERK1/2 phosphorylation following the IL-6-mediated activation of the MAPK signaling pathway. Similarly, XPO1 blockade in human MDSCs induces the formation of neutrophil-like cells with immunostimulatory functions. Therefore, our findings revealed a critical role for XPO1 in MDSC differentiation and suppressive functions; exploiting these new discoveries revealed new targets for reprogramming immunosuppressive MDSCs to improve cancer therapeutic responses.

MEK inhibitors: a promising targeted therapy for cardiovascular disease

Cardiovascular disease (CVD) represents the leading cause of mortality and disability all over the world. Identifying new targeted therapeutic approaches has become a priority of biomedical research to improve patient outcomes and quality of life. The RAS-RAF-MEK (mitogen-activated protein kinase kinase)-ERK (extracellular signal-regulated kinase) pathway is gaining growing interest as a potential signaling cascade implicated in the pathogenesis of CVD. This pathway is pivotal in regulating cellular processes like proliferation, growth, migration, differentiation, and survival, which are vital in maintaining cardiovascular homeostasis. In addition, ERK signaling is involved in controlling angiogenesis, vascular tone, myocardial contractility, and oxidative stress. Dysregulation of this signaling cascade has been linked to cell dysfunction and vascular and cardiac pathological remodeling, which contribute to the onset and progression of CVD. Recent and ongoing research has provided insights into potential therapeutic interventions targeting the RAS-RAF-MEK-ERK pathway to improve cardiovascular pathologies. Preclinical studies have demonstrated the efficacy of targeted therapy with MEK inhibitors (MEKI) in attenuating ERK activation and mitigating CVD progression in animal models. In this article, we first describe how ERK signaling contributes to preserving cardiovascular health. We then summarize current knowledge of the roles played by ERK in the development and progression of cardiac and vascular disorders, including atherosclerosis, myocardial infarction, cardiac hypertrophy, heart failure, and aortic aneurysm. We finally report novel therapeutic strategies for these CVDs encompassing MEKI and discuss advantages, challenges, and future developments for MEKI therapeutics.

Targeted therapy, immunotherapy, and small molecules and peptidomimetics as emerging immunoregulatory agents for melanoma

Primary cutaneous melanoma is the most lethal of all skin neoplasms and its incidence is increasing. Clinical management of advanced melanoma in the last decade has been revolutionised by the availability of immunotherapies and targeted therapies, used alone and in combination. This article summarizes advances in the treatment of late-stage melanoma including use of protein kinase inhibitors, antibody-based immune checkpoint inhibitors, adoptive immunotherapy, vaccines and more recently, small molecules and peptidomimetics as emerging immunoregulatory agents.

Quantitative live imaging reveals a direct interaction between CD44v6 and MET in membrane domains upon activation with both MET ligands, HGF and internalin B

Deregulation of the receptor tyrosine kinase MET/hepatocyte growth factor (HGF) pathway results in several pathological processes involved in tumor progression and metastasis. In a different context, MET can serve as an entry point for the bacterium Listeria monocytogenes, when activated by the internalin B (InlB) protein during infection of non-phagocytic cells. We have previously demonstrated that MET requires CD44v6 for its ligand-induced activation. However, the stoichiometry and the steps required for the formation of this complex, are still unknown. In this work, we studied the dynamics of the ligand-induced interaction of CD44v6 with MET at the plasma membrane. Using Förster resonance energy transfer-based fluorescence lifetime imaging microscopy in T-47D cells, we evidenced a direct interaction between MET and CD44v6 promoted by HGF and InlB in live cells. In the absence of MET, fluorescence correlation spectroscopy experiments further showed the dimerization of CD44v6 and the increase of its diffusion induced by HGF and InlB. In the presence of MET, stimulation of the cells by HGF or InlB significantly decreased the diffusion of CD44v6, in line with the formation of a ternary complex of MET with CD44v6 and HGF/InlB. Finally, similarly to HGF/InlB, disruption of liquid-ordered domains (Lo) by methyl-β-cyclodextrin increased CD44v6 mobility suggesting that these factors induce the exit of CD44v6 from the Lo domains. Our data led us to propose a model for MET activation, where CD44v6 dimerizes and diffuses rapidly out of Lo domains to form an oligomeric MET/ligand/CD44v6 complex that is instrumental for MET activation.

Grifola frondosa Extract Containing Bioactive Components Blocks Skin Fibroblastic Inflammation and Cytotoxicity Caused by Endocrine Disrupting Chemical, Bisphenol A

Grifola frondosa (GF), a species of Basidiomycotina, is widely distributed across Asia and has been used as an immunomodulatory, anti-bacterial, and anti-cancer agent. In the present study, the pharmacological activity of the GF extract against an ecotoxicological industrial chemical, bisphenol A (BPA) in normal human dermal fibroblasts (NHDFs), was investigated. GF extract containing naringin, hesperidin, chlorogenic acid, and kaempferol showed an inhibitory effect on cell death and inflammation induced by BPA in the NHDFs. For the cell death caused by BPA, GF extract inhibited the production of reactive oxygen species responsible for the unique activation of the extracellular signal-regulated kinase. In addition, GF extract attenuated the expression of apoptosis-related proteins (Bax, Bcl-2, and cleaved caspase-3) and the pro-inflammatory cytokine IL-1β by the suppression of the redox-sensitive transcription factor, nuclear factor-kappa B (NF-κB) in BPA-treated NHDFs. For the inflammation triggered by BPA, GF extract blocked the inflammasome-mediated caspase-1 activation that leads to the secretion of IL-1β protein. These results indicate that the GF extract is a functional antioxidant that prevents skin fibroblastic pyroptosis induced by BPA.

Inhibiting ERK dimerization ameliorates BRAF-driven anaplastic thyroid cancer

Background

RAS-to-ERK signaling is crucial for the onset and progression of advanced thyroid carcinoma, and blocking ERK dimerization provides a therapeutic benefit in several human carcinomas. Here we analyzed the effects of DEL-22379, a relatively specific ERK dimerization inhibitor, on the activation of the RAS-to-ERK signaling cascade and on tumor-related processes in vitro and in vivo.

Methods

We used a panel of four human anaplastic thyroid carcinoma (ATC) cell lines harboring BRAF or RAS mutations to analyze ERK dynamics and tumor-specific characteristics. We also assessed the impact of DEL-22379 on the transcriptional landscape of ATC cell lines using RNA-sequencing and evaluated its therapeutic efficacy in an orthotopic mouse model of ATC.

Results

DEL-22379 impaired upstream ERK activation in BRAF- but not RAS-mutant cells. Cell viability and metastasis-related processes were attenuated by DEL-22379 treatment, but mostly in BRAF-mutant cells, whereas in vivo tumor growth and dissemination were strongly reduced for BRAF-mutant cells and mildly reduced for RAS-mutant cells. Transcriptomics analyses indicated that DEL-22379 modulated the transcriptional landscape of BRAF- and RAS-mutant cells in opposite directions.

Conclusions

Our findings establish that BRAF- and RAS-mutant thyroid cells respond differentially to DEL-22379, which cannot be explained by the previously described mechanism of action of the inhibitor. Nonetheless, DEL-22379 demonstrated significant anti-tumor effects against BRAF-mutant cells in vivo with an apparent lack of toxicity, making it an interesting candidate for the development of combinatorial treatments. Our data underscore the differences elicited by the specific driver mutation for thyroid cancer onset and progression, which should be considered for experimental and clinical approaches.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00018-022-04530-9.

Development of small molecule extracellular signal-regulated kinases (ERKs) inhibitors for cancer therapy

The mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway is widely activated by a variety of extracellular stimuli, and its dysregulation is associated with the proliferation, invasion, and migration of cancer cells. ERK1/2 is located at the distal end of this pathway and rarely undergoes mutations, making it an attractive target for anticancer drug development. Currently, an increasing number of ERK1/2 inhibitors have been designed and synthesized for antitumor therapy, among which representative compounds have entered clinical trials. When ERK1/2 signal transduction is eliminated, ERK5 may provide a bypass route to rescue proliferation, and weaken the potency of ERK1/2 inhibitors. Therefore, drug research targeting ERK5 or based on the compensatory mechanism of ERK5 for ERK1/2 opens up a new way for oncotherapy. This review provides an overview of the physiological and biological functions of ERKs, focuses on the structure-activity relationships of small molecule inhibitors targeting ERKs, with a view to providing guidance for future drug design and optimization, and discusses the potential therapeutic strategies to overcome drug resistance.

Sphingosine 1-Phosphate Receptor 5 (S1P5) Deficiency Promotes Proliferation and Immortalization of Mouse Embryonic Fibroblasts

Simple Summary

Sphingosine 1-phosphate (S1P) is a lipid metabolite involved in cell proliferation, survival or migration. S1P is a ligand for five high-affinity G protein-coupled receptors (S1P1-5), which differ in their tissue distribution, and the specific effects of S1P depend on the suite of S1P receptor subtypes expressed. To date, information regarding the role of S1P5 in cell proliferation is limited and ambiguous. Our results suggest that, unlike other S1P receptors, the S1P5 receptor has an anti-proliferative function. We found that S1P5 deficiency promotes cell immortalization and proliferation by controlling the spatial activation of ERK.

Abstract

Sphingosine 1-phosphate (S1P), a bioactive lipid, interacts with five widely expressed G protein-coupled receptors (S1P1-5), regulating a variety of downstream signaling pathways with overlapping but also opposing functions. To date, data regarding the role of S1P5 in cell proliferation are ambiguous, and its role in controlling the growth of untransformed cells remains to be fully elucidated. In this study, we examined the effects of S1P5 deficiency on mouse embryonic fibroblasts (MEFs). Our results indicate that lack of S1P5 expression profoundly affects cell morphology and proliferation. First, S1P5 deficiency reduces cellular senescence and promotes MEF immortalization. Second, it decreases cell size and leads to cell elongation, which is accompanied by decreased cell spreading and migration. Third, it increases proliferation rate, a phenotype rescued by the reintroduction of exogenous S1P5. Mechanistically, S1P5 promotes the activation of FAK, controlling cell spreading and adhesion while the anti-proliferative function of the S1P/S1P5 signaling is associated with reduced nuclear accumulation of activated ERK. Our results suggest that S1P5 opposes the growth-promoting function of S1P1-3 through spatial control of ERK activation and provides new insights into the anti-proliferative function of S1P5.

ERK signaling for cell migration and invasion

The RAS - Extracellular signal-regulated kinase (RAS-ERK) pathway plays a conserved role in promoting cell migration and invasion. Growth factors, adhesion, and oncogenes activate ERK. While historically studied with respect to its control of cell proliferation and differentiation, the signaling pattern and effectors specific for cell migration are now coming to light. New advances in pathway probes have revealed how steady-state ERK activity fluctuates within individual cells and propagates to neighboring cells. We review new findings on the different modes of ERK pathway stimulation and how an increased baseline level of activity promotes single cell and collective migration and invasion. We discuss how ERK drives actin polymerization and adhesion turnover for edge protrusion and how cell contraction stimulates cell movement and ERK activity waves in epithelial sheets. With the steady development of new biosensors for monitoring spatial and temporal ERK activity, determining how cells individually interpret the multiple in vivo signals to ERK is within reach.

Signalling dynamics in embryonic development

In multicellular organisms, cellular behaviour is tightly regulated to allow proper embryonic development and maintenance of adult tissue. A critical component in this control is the communication between cells via signalling pathways, as errors in intercellular communication can induce developmental defects or diseases such as cancer. It has become clear over the last years that signalling is not static but varies in activity over time. Feedback mechanisms present in every signalling pathway lead to diverse dynamic phenotypes, such as transient activation, signal ramping or oscillations, occurring in a cell type- and stage-dependent manner. In cells, such dynamics can exert various functions that allow organisms to develop in a robust and reproducible way. Here, we focus on Erk, Wnt and Notch signalling pathways, which are dynamic in several tissue types and organisms, including the periodic segmentation of vertebrate embryos, and are often dysregulated in cancer. We will discuss how biochemical processes influence their dynamics and how these impact on cellular behaviour within multicellular systems.

Photobiomodulation With Blue Laser Inhibits Bladder Cancer Progression

Blue lasers are becoming more widely used in the diagnosis and treatment of bladder cancer; however, their photobiomodulation effects on bladder cancer cells remains unclear. The purpose of the current study was to explore the photobiomodulation effect of blue laser irradiation on bladder cancer progression and the associated mechanisms. The human uroepithelial cell line SV-HUC-1 and human bladder cancer cell lines T24 and EJ were exposed to blue laser irradiation (450 nm) at various energy densities, and cell proliferation, migration, invasion, epithelial-mesenchymal transition (EMT), and the levels of the proteins associated with the MAPK pathway proteins were determined. A significant decrease in cell viability was observed in a density-dependent manner after blue laser irradiation at > 4 J/cm² in both bladder cancer cell lines. However, the blue laser did not reduce cell viability in SV-HUC-1 cells until the energy density exceeded 16 J/cm². Meanwhile, Ki67 levels, reflecting cell proliferation and senescence, were also significantly decreased after blue laser irradiation at 4 J/cm² and 8 J/cm² in the absence of cell cycle arrest. Moreover, blue laser irradiation at 4 J/cm² and 8 J/cm² caused a reduction in cell migration and invasion and also reduced the expression levels of MMP-2, MMP-9, Snail, N-cadherin, phospho-MEK and phospho-ERK, and elevated the expression levels of E-cadherin. Meanwhile ERK activator(tBHQ) significantly reversed the irradiation-induced suppression of proliferation, migration and invasion in T24 and EJ cell lines. The present study showed that blue laser irradiation inhibited bladder cancer proliferation in a density-dependent manner and inhibited bladder cancer progression by suppressing migration, invasion, and the EMT process in T24 and EJ cell lines. This inhibition was possibly mediated via suppression of the MAPK/MEK/ERK pathway. Thus, the use of a low-energy blue laser in the diagnosis and treatment of bladder cancer is possibly safe and may have an anti-tumor effect.

Dengue virus is sensitive to inhibition prior to productive replication

Uncovering vulnerable steps in the life cycle of viruses supports the rational design of antiviral treatments. However, information on viral replication dynamics obtained from traditional bulk assays with host cell populations is inherently limited as the data represent averages over a multitude of unsynchronized replication cycles. Here, we use time-lapse imaging of virus replication in thousands of single cells, combined with computational inference, to identify rate-limiting steps for dengue virus (DENV), a widespread human pathogen. Comparing wild-type DENV with a vaccine candidate mutant, we show that the viral spread in the mutant is greatly attenuated by delayed onset of productive replication, whereas wild-type and mutant virus have identical replication rates. Single-cell analysis done after applying the broad-spectrum antiviral drug, ribavirin, at clinically relevant concentrations revealed the same mechanism of attenuating viral spread. We conclude that the initial steps of infection, rather than the rate of established replication, are quantitatively limiting DENV spread.

Astaxanthin Inhibits Autophagic Cell Death Induced by Bisphenol A in Human Dermal Fibroblasts

Astaxanthin, a natural antioxidant carotenoid, is a nutrient with diverse health benefits, given that it decreases the risk of oxidative stress-related diseases. In the present study, we investigate the functional role of astaxanthin during autophagic cell death induced by the estrogenic endocrine-disrupting chemical bisphenol A (BPA) in normal human dermal fibroblasts (NHDF). BPA significantly induced apoptotic cell death and autophagy in NHDF. Autophagic cell death evoked by BPA was significantly restored upon a treatment with astaxanthin (10 μM) via the inhibition of intracellular reactive oxygen species (ROS) production. Astaxanthin inhibited the phosphorylation of extracellular signal-regulated kinases (ERK) stimulated by ROS production, but it did not influence the activation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) in BPA-treated NHDF. Astaxanthin abrogated the ERK-mediated activation of nuclear factor-kappa B (NF-κB), which is responsible for the mRNA expression of LC3-II, Beclin-1, Atg12, and Atg14 during apoptotic cell death induced by BPA. These results indicate that astaxanthin is a pharmacological and nutritional agent that blocks the skin fibroblastic autophagic cell death induced by BPA in human dermal fibroblasts.

Pine nut antioxidant peptides ameliorate the memory impairment in a scopolamine-induced mouse model via SIRT3-induced synaptic plasticity

This study aimed to investigate the effects of a pine nut albumin hydrolysate (fraction <3 kDa) and of its short peptide derivative, Trp-Tyr-Pro-Gly-Lys (WYPGK), on synaptic plasticity and memory function in scopolamine-induced memory-impaired mice, as well as the potential underlying mechanism in PC12 cells. In the scopolamine-induced mouse model, the results revealed that the fraction <3 kDa and WYPGK enhanced synaptic plasticity and improved learning and memory function. H&E and Nissl staining analysis showed that the damage in hippocampal neurons was decreased. Golgi staining and transmission electron microscopy further revealed that the enhanced synaptic plasticity was associated with increased dendritic spine abundance and synaptic density. In an H₂O₂-induced PC12 cell model, treatment with mitochondrial sirtuin 3 (SIRT3) inhibitor and inducer molecules confirmed that the <3 kDa fraction and WYPGK activated SIRT3, leading to the decrease in Ace-SOD2 acetylation and increasing the expression of SYP, SYN-1, SNAP25, and PSD95, thus enhancing synaptic plasticity. The <3 kDa fraction and WYPGK also activated the ERK/CREB pathway and upregulated the expression of brain-derived neurotrophic factor. Our results show that fraction <3 kDa and WYPGK improve learning and memory ability through SIRT3-induced synaptic plasticity in vitro and in vivo.

Structure-activity relationship study of DEL-22379: ERK dimerization inhibitors with increased safety

Aberrant activation of ERK signaling pathway usually leads to oncogenesis, and small molecular agents targeting this pathway are impeded by the emergence of drug resistance due to reactivation of ERK signaling. Compound DEL-22379 has been reported to inhibit ERK dimerization which was unaffected by drug-resistant mechanism reactivating the ERK signaling. Here, we discussed a structure-activity relationship study of DEL-22379. Forty-seven analogues were designed and synthesized. Each synthesized compound was biologically evaluated for their inhibitory rates on several tumor cell lines and compounds with high inhibitory rates were further evaluated for IC50 values. The structure-activity relationship of idolin-2-one scaffold and the impact of Z/E configuration on potency were discussed. Potential safety of two synthesized analogues was investigated and in silico docking study of five compounds was performed to understand the structural basis of ERK dimerization inhibition.

Metabotropic glutamate 2,3 receptor stimulation desensitizes agonist activation of G-protein signaling and alters transcription regulators in mesocorticolimbic brain regions

Metabotropic glutamate (mGlu) receptors are regulators of glutamate release and targets for development of therapies for hyperactive glutamatergic signaling. However, the effects of long-term stimulation of mGlu receptors on cellular signaling in the brain have not been described. This study investigated the effects of 2-day and 14-day osmotic mini-pump administration of the mGlu2,3 agonist LY379268 (3.0 mg kg-1  day-1 ) to rats on receptor-mediated G-protein activation and signaling in mesocorticolimbic regions in rat brain sections. A significant reduction in LY379268-stimulated [35 S]GTPγS binding was observed in the 14-day group in some cortical regions, prefrontal cortex, nucleus accumbens, and ventral pallidum. The 14-day LY379268 treatment group exhibited mGlu2 mRNA levels significantly lower in hippocampus, nucleus accumbens, caudate, and ventral pallidum. In both 2-day and 14-day treatment groups immunodetectable phosphorylated cAMP Response Element-Binding protein (CREB) was significantly reduced across all brain regions. In the 2-day group, we observed significantly lower immunodetectable CREB protein across all brain regions, which was subsequently increased in the 14-day group but failed to achieve control values. Neither immunodetectable extracellular signal-regulated kinase (ERK) protein nor phosphorylated ERK from 2-day or 14-day treatment groups differed significantly from control across all brain regions. However, the ratio of phosphorylated ERK to total ERK protein was significantly greater in the 14-day treatment group compared with the control. These results identify compensatory changes to mGlu2,3 signal transduction in rat brains after chronic systemic administration of agonist, which could be predictive of the mechanism of action in human pharmacotherapies.

Single-Cell Analysis of Signaling Proteins Provides Insights into Proapoptotic Properties of Anticancer Drugs

Single-cell DNA analysis technology has provided unprecedented insights into many physiological and pathological processes. In contrast, technologies that allow protein analysis in single cells have lagged behind. Herein, a method called single-cell Plasmonic ImmunoSandwich Assay (scPISA) that is capable of measuring signaling proteins and protein complexes in single living cells is described. scPISA is straightforward, comprising specific in-cell extraction and ultrasensitive plasmonic detection. It is applied to evaluate the efficacy and kinetics of cytotoxic drugs. It reveals that different drugs exhibit distinct proapoptotic properties at the single-cell level. A set of new parameters is thus proposed for comprehensive and quantitative evaluation of the efficacy of anticancer drugs. It discloses that metformin can dramatically enhance the overall anticancer efficacy when combined with actinomycin D, although it itself is significantly less effective. Furthermore, scPISA reveals that survivin interacts with cytochrome C and caspase-3 in a dynamic fashion in single cells during continuous drug treatment. As compared with conventional assays, scPISA exhibits several significant advantages, such as ultrahigh sensitivity, single-cell resolution, fast speed, and so on. Therefore, this approach may provide a powerful tool for wide, important applications from basic research to clinical applications, particularly precision medicine.

Avoiding or Co-Opting ATP Inhibition: Overview of Type III, IV, V, and VI Kinase Inhibitors

As described in the previous chapter, most kinase inhibitors that have been developed for use in the clinic act by blocking ATP binding; however, there is growing interest in identifying compounds that target kinase activities and functions without interfering with the conserved features of the ATP-binding site. This chapter will highlight alternative approaches that exploit unique kinase structural features that are being targeted to identify more selective and potent inhibitors. The figure below, adapted from (Sammons et al., Molecular Carcinogenesis 58:1551–1570, 2019), provides a graphical description of the various approaches to manipulate kinase activity. In addition to the type I and II inhibitors, type III kinase inhibitors have been identified to target sites adjacent to the ATP-binding site in the catalytic domain. New information on kinase structure and substrate-binding sites has enabled the identification of type IV kinase inhibitor compounds that target regions outside the catalytic domain. The combination of targeting unique allosteric sites outside the catalytic domain with ATP-targeted compounds has yielded a number of novel bivalent type V kinase inhibitors. Finally, emerging interest in the development of irreversible compounds that form selective covalent interactions with key amino acids involved in kinase functions comprise the class of type VI kinase inhibitors.

Nanospheres Loaded with Curcumin Improve the Bioactivity of Umbilical Cord Blood-Mesenchymal Stem Cells via c-Src Activation During the Skin Wound Healing Process

Curcumin, a hydrophobic polyphenol derived from turmeric, has been used a food additive and as a herbal medicine for the treatment of various diseases, but the clinical application of curcumin is restricted by its poor aqueous solubility and its low permeability and bioavailability levels. In the present study, we investigate the functional role of a nanosphere loaded with curcumin (CN) in the promotion of the motility of human mesenchymal stem cells (MSCs) during the skin wound healing process. CN significantly increased the motility of umbilical cord blood (UCB)-MSCs and showed 10,000-fold greater migration efficacy than curcumin. CN stimulated the phosphorylation of c-Src and protein kinase C which are responsible for the distinctive activation of the MAPKs. Interestingly, CN significantly induced the expression levels of α-actinin-1, profilin-1 and filamentous-actin, as regulated by the phosphorylation of nuclear factor-kappa B during its promotion of cell migration. In a mouse skin excisional wound model, we found that transplantation of UCB-MSCs pre-treated with CN enhanced wound closure, granulation, and re-epithelialization at mouse skin wound sites. These results indicate that CN is a functional agent that promotes the mobilization of UCB-MSCs for cutaneous wound repair.

Profiling Cell Signaling Networks at Single-cell Resolution

Signaling networks process intra- and extracellular information to modulate the functions of a cell. Deregulation of signaling networks results in abnormal cellular physiological states and often drives diseases. Network responses to a stimulus or a drug treatment can be highly heterogeneous across cells in a tissue because of many sources of cellular genetic and non-genetic variance. Signaling network heterogeneity is the key to many biological processes, such as cell differentiation and drug resistance. Only recently, the emergence of multiplexed single-cell measurement technologies has made it possible to evaluate this heterogeneity. In this review, we categorize currently established single-cell signaling network profiling approaches by their methodology, coverage, and application, and we discuss the advantages and limitations of each type of technology. We also describe the available computational tools for network characterization using single-cell data and discuss potential confounding factors that need to be considered in single-cell signaling network analyses.

The EGFR-ZNF263 signaling axis silences SIX3 in glioblastoma epigenetically

The homeotic protein SIX3 is a transcription factor vital for neurogenesis and has a bivalent promoter. We previously showed that SIX3 can be transcriptionally silenced by DNA hypermethylation, functions as a tumor suppressor gene, and inhibits human glioblastoma transcriptionally. Here, we show that the activation of epidermal growth factor (EGFR) induces DNA methylation of SIX3 promoter through the MAPK pathway. ERK, when activated, binds with ZNF263, consequently abrogating the ubiquitination of ZNF263 and leading to its stabilization. ZNF263 binds to the core promoter region of SIX3 and recruits the KAP1/HATS/DNMT corepressor complex to induce transcriptional silencing of SIX3 through H3K27me3 and methylation of SIX3 promoter. Activation of the EGFR-ZNF263 signaling axis in phenotypically normal astrocytes or glioblastoma cells triggers or enhances tumorigenic activities, while elevated expression of the EGFR-ZNF263 signaling components in glioblastoma tissues is associated with poor prognosis of the patients. Together, our findings demonstrate that epigenetic silencing of SIX3 is controlled by a sophisticated and highly ordered oncogenic signaling pathway and therefore provide new insights into initiation and progression of glioblastoma.

Myocardin and myocardin-related transcription factor-A synergistically mediate actin cytoskeletal-dependent inhibition of liver fibrogenesis

Activation of hepatic stellate cells (HSCs), characterized by development of a robust actin cytoskeleton and expression of abundant extracellular matrix (ECM) proteins, such as type 1 collagen (COL.1), is a central cellular and molecular event in liver fibrosis. It has been demonstrated that HSCs express both myocardin and myocardin-related transcription factor-A (MRTF-A). However, the biological effects of myocardin and MRTF-A on HSC activation and liver fibrosis, as well as the molecular mechanism under the process, remain unclear. Here, we report that myocardin and MRTF-A's expression and nuclear accumulation are prominently increased during the HSC activation process, accompanied by robust activation of actin cytoskeleton dynamics. Targeting myocardin and MRTF-A binding and function with a novel small molecule, CCG-203971, led to dose-dependent inhibition of HSC actin cytoskeleton dynamics and abrogated multiple functional features of HSC activation (i.e., HSC contraction, migration and proliferation) and decreased COL.1 expression in vitro and liver fibrosis in vivo. Mechanistically, blocking the myocardin and MRTF-A nuclear translocation pathway with CCG-203971 directly inhibited myocardin/MRTF-A-mediated serum response factor (SRF), and Smad2/3 activation in the COL.1α2 promoter and indirectly abrogated actin cytoskeleton-dependent regulation of Smad2/3 and Erk1/2 phosphorylation and their nuclear accumulation. Finally, there was no effect of CCG-203971 on markers of inflammation, suggesting a direct effect of the compound on HSCs and liver fibrosis. These data reveal that myocardin and MRTF-A are two important cotranscriptional factors in HSCs and represent entirely novel therapeutic pathways that might be targeted to treat liver fibrosis.NEW & NOTEWORTHY Myocardin and myocardin-related transcription factor-A (MRTF-A) are upregulated in activated hepatic stellate cells (HSCs) in vitro and in vivo, closely associated with robustly increased actin cytoskeleton remodeling. Targeting myocardin and MRTF-A by CCG-203971 leads to actin cytoskeleton-dependent inhibition of HSC activation, reduced cell contractility, impeded cell migration and proliferation, and decreased COL.1 expression in vitro and in vivo. Dual expression of myocardin and MRTF-A in HSCs may represent novel therapeutic targets in liver fibrosis.

Kaposi's sarcoma-associated herpesvirus viral protein kinase phosphorylates extracellular signal-regulated kinase and activates MAPK/ERK signaling pathway

Open reading frame 36 (ORF36) of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a serine/threonine-type viral protein kinase (vPK). Previous studies have examined the functions of KSHV vPK; however, its role in the activation of extracellular signal-regulated kinase (ERK1/2) has not yet been described to date. Using HEK 293 cell lines, we performed a human phospho-kinase array analysis to screen for MAPK signaling pathways kinases that are activated by KSHV vPK. In addition, we investigated the regulator protein phosphorylation of up/downstream ERK1/2 pathway; nuclear translocation of phosphorylated ERK1/2; and regulation of transcription factor, inflammatory cytokine, and pro-/anti-apoptotic factor by KSHV vPK transfection. Here, we demonstrated that KSHV vPK activates ERK1/2 signaling pathway and plays an important role in the activation of MAPK/ERK signaling pathway.

The Upstream Pathway of mTOR-Mediated Autophagy in Liver Diseases

Autophagy, originally found in liver experiments, is a cellular process that degrades damaged organelle or protein aggregation. This process frees cells from various stress states is a cell survival mechanism under stress stimulation. It is now known that dysregulation of autophagy can cause many liver diseases. Therefore, how to properly regulate autophagy is the key to the treatment of liver injury. mechanistic target of rapamycin (mTOR)is the core hub regulating autophagy, which is subject to different upstream signaling pathways to regulate autophagy. This review summarizes three upstream pathways of mTOR: the phosphoinositide 3-kinase (PI3K)/protein kinase (AKT) signaling pathway, the adenosine monophosphate-activated protein kinase (AMPK) signaling pathway, and the rat sarcoma (Ras)/rapidly accelerated fibrosarcoma (Raf)/mitogen-extracellular activated protein kinase kinase (MEK)/ extracellular-signal-regulated kinase (ERK) signaling pathway, specifically explored their role in liver fibrosis, hepatitis B, non-alcoholic fatty liver, liver cancer, hepatic ischemia reperfusion and other liver diseases through the regulation of mTOR-mediated autophagy. Moreover, we also analyzed the crosstalk between these three pathways, aiming to find new targets for the treatment of human liver disease based on autophagy.

Signaling pathways in cytoskeletal responses to plasma membrane depolarization in corneal endothelial cells

In previous work, we reported that plasma membrane potential depolarization (PMPD) provokes cortical F-actin remodeling in bovine corneal endothelial (BCE) cells in culture, which eventually leads to the appearance of intercellular gaps. In kidney epithelial cells it has been shown that PMPD determines an extracellular-signal-regulated kinase (ERK)/Rho-dependent increase in diphosphorylated myosin light chain (ppMLC). The present study investigated the signaling pathways involved in the response of BCE cells to PMPD. Differently to renal epithelial cells, we observed that PMPD leads to a decrease in monophosphorylated MLC (pMLC) without affecting diphosphorylated MLC. Also, that the pMLC reduction is a consequence of cyclic adenosine 3',5'-monophosphate (cAMP)/protein kinase A (PKA) activation. In addition, we found evidence that the cAMP increase mostly depends on soluble adenylyl cyclase activity. Inhibition of this enzyme reduces the effect of PMPD on the cAMP rise, F-actin remodeling, and pMLC decrease. No changes in phosho-ERK were observed, although we could determine that RhoA undergoes activation. Our results suggested that active RhoA is not involved in the intercellular gap formation. Overall, the findings of this study support the view that, differently to renal epithelial cells, in BCE cells PMPD determines cytoskeletal reorganization via activation of the cAMP/PKA pathway.

Extracellular-Signal Regulated Kinase: A Central Molecule Driving Epithelial-Mesenchymal Transition in Cancer

Epithelial-mesenchymal transition (EMT) is a reversible cellular process, characterized by changes in gene expression and activation of proteins, favoring the trans-differentiation of the epithelial phenotype to a mesenchymal phenotype. This process increases cell migration and invasion of tumor cells, progression of the cell cycle, and resistance to apoptosis and chemotherapy, all of which support tumor progression. One of the signaling pathways involved in tumor progression is the MAPK pathway. Within this family, the ERK subfamily of proteins is known for its contributions to EMT. The ERK subfamily is divided into typical (ERK 1/2/5), and atypical (ERK 3/4/7/8) members. These kinases are overexpressed and hyperactive in various types of cancer. They regulate diverse cellular processes such as proliferation, migration, metastasis, resistance to chemotherapy, and EMT. In this context, in vitro and in vivo assays, as well as studies in human patients, have shown that ERK favors the expression, function, and subcellular relocalization of various proteins that regulate EMT, thus promoting tumor progression. In this review, we discuss the mechanistic roles of the ERK subfamily members in EMT and tumor progression in diverse biological systems.

VEGFC acts as a double-edged sword in renal cell carcinoma aggressiveness

Hypoxic zones are common features of metastatic tumors. Due to inactivation of the von Hippel-Lindau gene (VHL), renal cell carcinomas (RCC) show constitutive stabilization of the alpha subunit of the hypoxia-inducible factor (HIF). Thus, RCC represents a model of chronic hypoxia. Development of the lymphatic network is dependent on vascular endothelial growth factor C (VEGFC) and lies at the front line of metastatic spreading. Here, we addressed the role of VEGFC in RCC aggressiveness and the regulation of its expression in hypoxia. Methods: Transcriptional and post transcriptional regulation of VEGFC expression was evaluated by qPCR and with reporter genes. The involvement of HIF was evaluated using a siRNA approach. Experimental RCC were performed with immuno-competent/deficient mice using human and mouse cells knocked-out for the VEGFC gene by a CRISPR/Cas9 method. The VEGFC axis was analyzed with an online available data base (TCGA) and using an independent cohort of patients. Results: Hypoxia induced VEGFC protein expression but down-regulated VEGFC gene transcription and mRNA stability. Increased proliferation, migration, over-activation of the AKT signaling pathway and enhanced expression of mesenchymal markers characterized VEGFC-/- cells. VEGFC-/- cells did not form tumors in immuno-deficient mice but developed aggressive tumors in immuno-competent mice. These tumors showed down-regulation of markers of activated lymphocytes and M1 macrophages, and up-regulation of M2 macrophages markers and programmed death ligand 1 (PDL1). Over-expression of lymphangiogenic genes including VEGFC was linked to increased disease-free and overall survival in patients with non-metastatic tumors, whereas its over-expression correlated with decreased progression-free and overall survival of metastatic patients. Conclusion: Our study revisited the admitted dogma linking VEGFC to tumor aggressiveness. We conclude that targeting VEGFC for therapy must be considered with caution.

The Nuclear Translocation of Mitogen-Activated Protein Kinases: Molecular Mechanisms and Use as Novel Therapeutic Target

The mitogen-activated protein kinase (MAPK) cascades are central signaling pathways that play a central role in the regulation of most stimulated cellular processes including proliferation, differentiation, stress response and apoptosis. Currently 4 such cascades are known, each termed by its downstream MAPK components: the extracellular signal-regulated kinase 1/2 (ERK1/2), cJun-N-terminal kinase (JNK), p38 and ERK5. One of the hallmarks of these cascades is the stimulated nuclear translocation of their MAPK components using distinct mechanisms. ERK1/2 are shuttled into the nucleus by importin7, JNK and p38 by a dimer of importin3 with either importin9 or importin7, and ERK5 by importin-α/β. Dysregulation of these cascades often results in diseases, including cancer and inflammation, as well as developmental and neurological disorders. Much effort has been invested over the years in developing inhibitors to the MAPK cascades to combat these diseases. Although some inhibitors are already in clinical use or clinical trials, their effects are hampered by development of resistance or adverse side-effects. Recently, our group developed 2 myristoylated peptides: EPE peptide, which inhibits the interaction of ERK1/2 with importin7, and PERY peptide, which prevents JNK/p38 interaction with either importin7 or importin9. These peptides block the nuclear translocation of their corresponding kinases, resulting in prevention of several cancers, while the PERY peptide also inhibits inflammation-induced diseases. These peptides provide a proof of concept for the use of the nuclear translocation of MAPKs as therapeutic targets for cancer and/or inflammation.

Structure-based mechanism of action of a viral poly(ADP-ribose) polymerase 1-interacting protein facilitating virus replication

Poly(ADP-ribose) polymerase 1 (PARP-1), an enzyme that modifies nuclear proteins by poly(ADP-ribosyl)ation, regulates various cellular activities and restricts the lytic replication of oncogenic gammaherpesviruses by inhibiting the function of replication and transcription activator (RTA), a key switch molecule of the viral life cycle. A viral PARP-1-interacting protein (vPIP) encoded by murine gammaherpesvirus 68 (MHV-68) orf49 facilitates lytic replication by disrupting interactions between PARP-1 and RTA. Here, the structure of MHV-68 vPIP was determined at 2.2 Å resolution. The structure consists of 12 α-helices with characteristic N-terminal β-strands (Nβ) and forms a V-shaped-twist dimer in the asymmetric unit. Structure-based mutagenesis revealed that Nβ and the α1 helix (residues 2-26) are essential for the nuclear localization and function of vPIP; three residues were then identified (Phe5, Ser12 and Thr16) that were critical for the function of vPIP and its interaction with PARP-1. A recombinant MHV-68 harboring mutations of these three residues showed severely attenuated viral replication both in vitro and in vivo. Moreover, ORF49 of Kaposi's sarcoma-associated herpesvirus also directly interacted with PARP-1, indicating a conserved mechanism of action of vPIPs. The results elucidate the novel molecular mechanisms by which oncogenic gammaherpesviruses overcome repression by PARP-1 using vPIPs.

ERK1/2 inhibitors: New weapons to inhibit the RAS-regulated RAF-MEK1/2-ERK1/2 pathway

The RAS-regulated RAF-MEK1/2-ERK1/2 signalling pathway is de-regulated in a variety of cancers due to mutations in receptor tyrosine kinases (RTKs), negative regulators of RAS (such as NF1) and core pathway components themselves (RAS, BRAF, CRAF, MEK1 or MEK2). This has driven the development of a variety of pharmaceutical agents to inhibit RAF-MEK1/2-ERK1/2 signalling in cancer and both RAF and MEK inhibitors are now approved and used in the clinic. There is now much interest in targeting at the level of ERK1/2 for a variety of reasons. First, since the pathway is linear from RAF-to-MEK-to-ERK then ERK1/2 are validated as targets per se. Second, innate resistance to RAF or MEK inhibitors involves relief of negative feedback and pathway re-activation with all signalling going through ERK1/2, validating the use of ERK inhibitors with RAF or MEK inhibitors as an up-front combination. Third, long-term acquired resistance to RAF or MEK inhibitors involves a variety of mechanisms (KRAS or BRAF amplification, MEK mutation, etc.) which re-instate ERK activity, validating the use of ERK inhibitors to forestall acquired resistance to RAF or MEK inhibitors. The first potent highly selective ERK1/2 inhibitors have now been developed and are entering clinical trials. They have one of three discrete mechanisms of action - catalytic, "dual mechanism" or covalent - which could have profound consequences for how cells respond and adapt. In this review we describe the validation of ERK1/2 as anti-cancer drug targets, consider the mechanism of action of new ERK1/2 inhibitors and how this may impact on their efficacy, anticipate factors that will determine how tumour cells respond and adapt to ERK1/2 inhibitors and consider ERK1/2 inhibitor drug combinations.

PTPRS Regulates Colorectal Cancer RAS Pathway Activity by Inactivating Erk and Preventing Its Nuclear Translocation

Colorectal cancer (CRC) growth and progression is frequently driven by RAS pathway activation through upstream growth factor receptor activation or through mutational activation of KRAS or BRAF. Here we describe an additional mechanism by which the RAS pathway may be modulated in CRC. PTPRS, a receptor-type protein tyrosine phosphatase, appears to regulate RAS pathway activation through ERK. PTPRS modulates ERK phosphorylation and subsequent translocation to the nucleus. Native mutations in PTPRS, present in ~10% of CRC, may reduce its phosphatase activity while increasing ERK activation and downstream transcriptional signaling.

Protein kinase Cε regulates nuclear translocation of extracellular signal-regulated kinase, which contributes to bradykinin-induced cyclooxygenase-2 expression

The proinflammatory mediator bradykinin stimulated cyclooxygenase-2 (COX-2) expression and subsequently prostaglandin E₂ synthesis in dermal fibroblasts. The involvement of B2 receptors and Gαq in the role of bradykinin was suggested by using pharmacological inhibitors. The PKC activator PMA stimulated COX-2 mRNA expression. Bradykinin failed to induce COX-2 mRNA expression in the presence of PKC inhibitors, whereas the effect of bradykinin was observed in the absence of extracellular Ca²⁺. Bradykinin-induced COX-2 mRNA expression was inhibited in cells transfected with PKCε siRNA. These observations suggest that the novel PKCε is concerned with bradykinin-induced COX-2 expression. Bradykinin-induced PKCε phosphorylation and COX-2 mRNA expression were inhibited by an inhibitor of 3-phosphoinositide-dependent protein kinase-1 (PDK-1), and bradykinin-induced PDK-1 phosphorylation was inhibited by phospholipase D (PLD) inhibitors, suggesting that PLD/PDK-1 pathway contributes to bradykinin-induced PKCε activation. Pharmacological and knockdown studies suggest that the extracellular signal-regulated kinase 1 (ERK1) MAPK signaling is involved in bradykinin-induced COX-2 expression. Bradykinin-induced ERK phosphorylation was attenuated in the cells pretreated with PKC inhibitors or transfected with PKCε siRNA. We observed the interaction between PKCε and ERK by co-immunoprecipitation experiments. These observations suggest that PKCε activation contributes to the regulation of ERK1 activation. Bradykinin stimulated the accumulation of phosphorylated ERK in the nuclear fraction, that was inhibited in the cells treated with PKC inhibitors or transfected with PKCε siRNA. Consequently, we concluded that bradykinin activates PKCε via the PLD/PDK-1 pathway, which subsequently induces activation and translocation of ERK1 into the nucleus, and contributes to COX-2 expression for prostaglandin E₂ synthesis in dermal fibroblasts.

Extracellular-Regulated Kinases: Signaling From Ras to ERK Substrates to Control Biological Outcomes

The extracellular-regulated kinases ERK1 and ERK2 are evolutionarily conserved, ubiquitous serine-threonine kinases that are involved in regulating cellular signaling in both normal and pathological conditions. Their expression is critical for development and their hyperactivation is a major factor in cancer development and progression. Since their discovery as one of the major signaling mediators activated by mitogens and Ras mutation, we have learned much about their regulation, including their activation, binding partners and substrates. In this review I will discuss some of what has been discovered about the members of the Ras to ERK pathway, including regulation of their activation by growth factors and cell adhesion pathways. Looking downstream of ERK activation I will also highlight some of the many ERK substrates that have been discovered, including those involved in feedback regulation, cell migration and cell cycle progression through the control of transcription, pre-mRNA splicing and protein synthesis.

Biophysics of Biochemical Signaling in Dendritic Spines: Implications in Synaptic Plasticity

Dendritic spines are mushroom-shaped postsynaptic compartments that host biochemical signal cascades important for synaptic plasticity and, ultimately, learning and memory. Signaling events in spines involve a signaling network composed of hundreds of signaling proteins interacting with each other extensively. Synaptic plasticity is typically induced by Ca²⁺ elevation in spines, which activates a variety of signaling pathways. This leads to changes in the actin cytoskeleton and membrane dynamics, which in turn causes structural and functional changes of the spine. Recent studies have demonstrated that the activities of these proteins have a variety of spatiotemporal patterns, which orchestrate signaling activity in different subcellular compartments at different timescales. The diffusion and the decay kinetics of signaling molecules play important roles in determining the degree of their spatial spreading, and thereby the degree of the spine specificity of the signaling pathway.

Dual-specificity phosphatase 5 controls the localized inhibition, propagation, and transforming potential of ERK signaling

Deregulated extracellular signal-regulated kinase (ERK) signaling drives cancer growth. Normally, ERK activity is self-limiting by the rapid inactivation of upstream kinases and delayed induction of dual-specificity MAP kinase phosphatases (MKPs/DUSPs). However, interactions between these feedback mechanisms are unclear. Here we show that, although the MKP DUSP5 both inactivates and anchors ERK in the nucleus, it paradoxically increases and prolongs cytoplasmic ERK activity. The latter effect is caused, at least in part, by the relief of ERK-mediated RAF inhibition. The importance of this spatiotemporal interaction between these distinct feedback mechanisms is illustrated by the fact that expression of oncogenic BRAF^V600E, a feedback-insensitive mutant RAF kinase, reprograms DUSP5 into a cell-wide ERK inhibitor that facilitates cell proliferation and transformation. In contrast, DUSP5 deletion causes BRAF^V600E-induced ERK hyperactivation and cellular senescence. Thus, feedback interactions within the ERK pathway can regulate cell proliferation and transformation, and suggest oncogene-specific roles for DUSP5 in controlling ERK signaling and cell fate.

JNK Signaling: Regulation and Functions Based on Complex Protein-Protein Partnerships

The c-Jun N-terminal kinases (JNKs), as members of the mitogen-activated protein kinase (MAPK) family, mediate eukaryotic cell responses to a wide range of abiotic and biotic stress insults. JNKs also regulate important physiological processes, including neuronal functions, immunological actions, and embryonic development, via their impact on gene expression, cytoskeletal protein dynamics, and cell death/survival pathways. Although the JNK pathway has been under study for >20 years, its complexity is still perplexing, with multiple protein partners of JNKs underlying the diversity of actions. Here we review the current knowledge of JNK structure and isoforms as well as the partnerships of JNKs with a range of intracellular proteins. Many of these proteins are direct substrates of the JNKs. We analyzed almost 100 of these target proteins in detail within a framework of their classification based on their regulation by JNKs. Examples of these JNK substrates include a diverse assortment of nuclear transcription factors (Jun, ATF2, Myc, Elk1), cytoplasmic proteins involved in cytoskeleton regulation (DCX, Tau, WDR62) or vesicular transport (JIP1, JIP3), cell membrane receptors (BMPR2), and mitochondrial proteins (Mcl1, Bim). In addition, because upstream signaling components impact JNK activity, we critically assessed the involvement of signaling scaffolds and the roles of feedback mechanisms in the JNK pathway. Despite a clarification of many regulatory events in JNK-dependent signaling during the past decade, many other structural and mechanistic insights are just beginning to be revealed. These advances open new opportunities to understand the role of JNK signaling in diverse physiological and pathophysiological states.

Live cell imaging shows hepatocyte growth factor-induced Met dimerization

The canonical model of receptor tyrosine kinase (RTK) activation assumes that ligand-induced dimerization of inactive receptor monomers is a prerequisite for autophosphorylation. For several RTK families, recent results of fluorescence microscopy provided evidence for preformed receptor dimers that may or may not require ligand binding for kinase activity. Here we report, for the first time, the application of advanced quantitative fluorescence microscopy techniques to study changes in the oligomerization state of the RTK Met in response to stimulation by its endogenous ligand hepatocyte growth factor (HGF). We used inducible C-terminal fusions between Met and enhanced green fluorescent protein (EGFP) or red fluorescent protein (RFP) in combination with fluorescence resonance energy transfer (FRET)-based fluorescence-lifetime imaging microscopy (FLIM) and fluorescence correlation spectroscopy (FCS). A small fraction of HGF-independent Met dimers appeared to be present in cells even at low receptor density. At high receptor density, both the fraction of Met dimers and the level of Met autophosphorylation increased in the absence of HGF. Stimulation with HGF at low receptor density significantly increased the fraction of Met dimers on live cells. We found no indications of Met oligomers larger than dimers. Our findings thus confirm a model of Met activation through HGF-induced dimerization and at the same time they support previous reports of Met dimers in unstimulated cells. The tools established in this work will be useful to further characterize the mechanism of Met activation and to define the contribution of co-receptors.

ERK1 and ERK2 Map Kinases: Specific Roles or Functional Redundancy?

The MAP kinase signaling cascade Ras/Raf/MEK/ERK has been involved in a large variety of cellular and physiological processes that are crucial for life. Many pathological situations have been associated to this pathway. More than one isoform has been described at each level of the cascade. In this review we devoted our attention to ERK1 and ERK2, which are the effector kinases of the pathway. Whether ERK1 and ERK2 specify functional differences or are in contrast functionally redundant, constitutes an ongoing debate despite the huge amount of studies performed to date. In this review we compiled data on ERK1 vs. ERK2 gene structures, protein sequences, expression levels, structural and molecular mechanisms of activation and substrate recognition. We have also attempted to perform a rigorous analysis of studies regarding the individual roles of ERK1 and ERK2 by the means of morpholinos, siRNA, and shRNA silencing as well as gene disruption or gene replacement in mice. Finally, we comment on a recent study of gene and protein evolution of ERK isoforms as a distinct approach to address the same question. Our review permits the evaluation of the relevance of published studies in the field especially when measurements of global ERK activation are taken into account. Our analysis favors the hypothesis of ERK1 and ERK2 exhibiting functional redundancy and points to the concept of the global ERK quantity, and not isoform specificity, as being the essential determinant to achieve ERK function.

Inter-domain tagging implicates caveolin-1 in insulin receptor trafficking and Erk signaling bias in pancreatic beta-cells

OBJECTIVE

The role and mechanisms of insulin receptor internalization remain incompletely understood. Previous trafficking studies of insulin receptors involved fluorescent protein tagging at their termini, manipulations that may be expected to result in dysfunctional receptors. Our objective was to determine the trafficking route and molecular mechanisms of functional tagged insulin receptors and endogenous insulin receptors in pancreatic beta-cells.

METHODS

We generated functional insulin receptors tagged with pH-resistant fluorescent proteins between domains. Confocal, TIRF and STED imaging revealed a trafficking pattern of inter-domain tagged insulin receptors and endogenous insulin receptors detected with antibodies.

RESULTS

Surprisingly, interdomain-tagged and endogenous insulin receptors in beta-cells bypassed classical Rab5a- or Rab7-mediated endocytic routes. Instead, we found that removal of insulin receptors from the plasma membrane involved tyrosine-phosphorylated caveolin-1, prior to trafficking within flotillin-1-positive structures to lysosomes. Multiple methods of inhibiting caveolin-1 significantly reduced Erk activation in vitro or in vivo, while leaving Akt signaling mostly intact.

CONCLUSIONS

We conclude that phosphorylated caveolin-1 plays a role in insulin receptor internalization towards lysosomes through flotillin-1-positive structures and that caveolin-1 helps bias physiological beta-cell insulin signaling towards Erk activation.

A PARP1-Erk2 synergism is required for stimulation-induced expression of immediate early genes

A PARP1-Erk2 synergism was required to generate synaptic long-term potentiation in the CA3-CA1 hippocampal connections. This molecular mechanism was associated with the recently identified pivotal role of polyADP-ribosylation in learning. High frequency electrical stimulation of cortical and hippocampal neurons induced binding of phosphorylated Erk2 (transported into the nucleus) to the nuclear protein PARP1. PARP1-Erk2 binding induced PARP1 activation and polyADP-ribosylation of its prominent substrate, linker histone H1. A facilitated access of PARP1-bound phosphorylated Erk2 to its substrates, transcription factors Elk1 and CREB was attributed to the release of polyADP-ribosylated H1 from the DNA, causing local DNA relaxation. Erk-induced phosphorylation of transcription factors activating the HAT activity of CBP (CREB binding protein), recruited acetylated histone H4 to the promoters of immediate early genes (IEG) cfos, zif268 and arc, which are implicated in synaptic plasticity. In accordance, their induced expression was suppressed after PARP1 genetic deletion in PARP1-KO mice, or after PARP1 inhibition or silencing. Moreover, under these conditions, long-term synaptic potentiation (LTP) (indicating synaptic plasticity) was not generation in the hippocampal CA3-CA1 connections, and learning abilities were impaired. Furthermore, both IEG expression and LTP generation failed when cerebral neurons accumulated single strand DNA breaks, due to a predominant binding of PARP1 to nicked DNA, occluding its Erk binding sites. Thus, a declined synaptic plasticity is anticipated when aged cerebral neurons accumulate DNA single-strand breaks during life span.

Chemopreventive effect of resveratrol and apocynin on pancreatic carcinogenesis via modulation of nuclear phosphorylated GSK3β and ERK1/2

Despite progress in clinical cancer medicine in multiple fields, the prognosis of pancreatic cancer has remained dismal. Recently, chemopreventive strategies using phytochemicals have gained considerable attention as an alternative in the management of cancer. The present study aimed to evaluate the chemopreventive effects of resveratrol (RV) and apocynin (AC) in N-Nitrosobis(2-oxopropyl)amine-induced pancreatic carcinogenesis in hamster. RV- and AC-treated hamsters showed significant reduction in the incidence of pancreatic cancer with a decrease in Ki-67 labeling index in dysplastic lesions. RV and AC suppressed cell proliferation of human and hamster pancreatic cancer cells by inhibiting the G1 phase of the cell cycle with cyclin D1 downregulation and inactivation of AKT-GSK3β and ERK1/2 signaling. Further, decreased levels of GSK3β(Ser9) and ERK1/2 phosphorylation and cyclin D1 expression in the nuclear fraction were observed in cells treated with RV or AC. Nuclear expression of phosphorylated GSK3β(Ser9) was also decreased in dysplastic lesions and adenocarcinomas of hamsters treated with RV or AC in vivo. These results suggest that RV and AC reduce phosphorylated GSK3β(Ser9) and ERK1/2 in the nucleus, resulting in inhibition of the AKT-GSK3β and ERK1/2 signaling pathways and cell cycle arrest in vitro and in vivo. Taken together, the present study indicates that RV and AC have potential as chemopreventive agents for pancreatic cancer.

Novel Reporter for Faithful Monitoring of ERK2 Dynamics in Living Cells and Model Organisms

Uncoupling of ERK1/2 phosphorylation from subcellular localization is essential towards the understanding of molecular mechanisms that control ERK1/2-mediated cell-fate decision. ERK1/2 non-catalytic functions and discoveries of new specific anchors responsible of the subcellular compartmentalization of ERK1/2 signaling pathway have been proposed as regulation mechanisms for which dynamic monitoring of ERK1/2 localization is necessary. However, studying the spatiotemporal features of ERK2, for instance, in different cellular processes in living cells and tissues requires a tool that can faithfully report on its subcellular distribution. We developed a novel molecular tool, ERK2-LOC, based on the T2A-mediated coexpression of strictly equimolar levels of eGFP-ERK2 and MEK1, to faithfully visualize ERK2 localization patterns. MEK1 and eGFP-ERK2 were expressed reliably and functionally both in vitro and in single living cells. We then assessed the subcellular distribution and mobility of ERK2-LOC using fluorescence microscopy in non-stimulated conditions and after activation/inhibition of the MAPK/ERK1/2 signaling pathway. Finally, we used our coexpression system in Xenopus laevis embryos during the early stages of development. This is the first report on MEK1/ERK2 T2A-mediated coexpression in living embryos, and we show that there is a strong correlation between the spatiotemporal subcellular distribution of ERK2-LOC and the phosphorylation patterns of ERK1/2. Our approach can be used to study the spatiotemporal localization of ERK2 and its dynamics in a variety of processes in living cells and embryonic tissues.

Kavain Inhibition of LPS-Induced TNF-α via ERK/LITAF

Kavain, an extract from the shrub Piper Methysticum, was recently reported to modulate TNF-α expression in both human and mouse cells via regulation of LPS-Induced TNF-Alpha Factor (LITAF). The purpose of the present study was to define the molecular pathway(s) associated with Kavain effects on TNF modulation. In vitro studies using WT mouse primary macrophages showed that Kavain significantly reduced E.coli LPS-induced TNF-α production but this effect was almost abrogated in LITAF^-/- and ERK2^-/- cells. Therefore we reintroduced the ERK2 gene in ERK2^-/- cells and partially restored E.coli LPS-induced LITAF-mediated TNF-α production. The translocation of LITAF into to nucleus was found to be dependent on ERK2 S206 residue. Kavain inhibits LITAF/TNF-α expression via dephosphorylation of ERK2 in response to E.coli LPS. Finally, in vivo, Kavain had a significant anti-inflammatory effect on wild type mice that developed Collagen Antibody Induced Arthritis (CAIA), but only a minor effect in ERK2^-/- mice also affected by CAIA. Based on these findings, we concluded that ERK2 may be the kinase upstream of LITAF with its Serine residue 206 being crucial for the regulation of LPS-induced TNF-α.

Vibrio vulnificus VvpE Stimulates IL-1β Production by the Hypomethylation of the IL-1β Promoter and NF-κB Activation via Lipid Raft–Dependent ANXA2 Recruitment and Reactive Oxygen Species Signaling in Intestinal Epithelial Cells

An inflammatory response is a hallmark of necrosis evoked by bacterial pathogens. Vibrio vulnificus, VvpE, is an elastase that is responsible for tissue necrosis and inflammation; however, the molecular mechanism by which it regulates host cell death has not been characterized. In the present study, we investigate the cellular mechanism of VvpE with regard to host cell death and the inflammatory response of human intestinal epithelial (INT-407) cells. The recombinant protein (r)VvpE (50 pg/ml) caused cytotoxicity mainly via necrosis coupled with IL-1β production. The necrotic cell death induced by rVvpE is highly susceptible to the knockdown of annexin A (ANXA)2 and the sequestration of membrane cholesterol. We found that rVvpE induces the recruitment of NADPH oxidase 2 and neutrophil cytosolic factor 1 into membrane lipid rafts coupled with ANXA2 to facilitate the production of reactive oxygen species (ROS). The bacterial signaling of rVvpE through ROS production is uniquely mediated by the phosphorylation of redox-sensitive transcription factor NF-κB. The silencing of NF-κB inhibited IL-1β production during necrosis. rVvpE induced hypomethylation and region-specific transcriptional occupancy by NF-κB in the IL-1β promoter and has the ability to induce pyroptosis via NOD-, LRR-, and pyrin domain–containing 3 inflammasome. In a mouse model of V. vulnificus infection, the mutation of the vvpE gene from V. vulnificus negated the proinflammatory responses and maintained the physiological levels of the proliferation and migration of enterocytes. These results demonstrate that VvpE induces the hypomethylation of the IL-1β promoter and the transcriptional regulation of NF-κB through lipid raft–dependent ANXA2 recruitment and ROS signaling to promote IL-1β production in intestinal epithelial cells.

The mouse radial spoke protein 3 is a nucleocytoplasmic shuttling protein that promotes neurogenesis

Radial spoke protein 3 (RSP3) was first identified in Chlamydomonas as a component of radial spoke, which is important for flagellar motility. The mammalian homolog of the Chlamydomonas RSP3 protein is found to be a mammalian protein kinase A-anchoring protein that binds ERK1/2. Here we show that mouse RSP3 is a nucleocytoplasmic shuttling protein. The full-length RSP3-EGFP fusion protein is mainly located in the cytoplasm of Chinese hamster ovary cells. However, by using deletion mutants of RSP3, we identified two nuclear localization signals and a nuclear export signal in RSP3. Moreover, using in utero electroporation, we found that overexpression of RSP3 in the developing cerebral cortex promotes neurogenesis. The layer II/III of the neocortex was much thicker in the RSP3-transfected region than that of the untransfected region in the neocortex. We also show that RSP3 is specifically located in the primary cilia of the radial glial cells, where it acts as a signaling mediator that regulates neurogenesis. Thus, our results suggest that RSP3 is a nucleocytoplasmic shuttling protein and plays an essential role in neurogenesis.

Vibrio vulnificus VvhA induces NF-κB-dependent mitochondrial cell death via lipid raft-mediated ROS production in intestinal epithelial cells

The Gram-negative bacterium Vibrio vulnificus produces hemolysin (VvhA), which induces cytotoxicity in mammalian cells. However, our understanding of the cytotoxic mechanism and the modes of action of VvhA are still fragmentary and incomplete. The recombinant protein (r) VvhA (50 pg/ml) significantly induces necrotic cell death and apoptosis in human intestinal epithelial (INT-407) cells. The apoptotic cell death induced by rVvhA is highly susceptible to the sequestration of cholesterol by methyl-β-cyclodextrin, whereas for necrotic cell death, this shows a marginal effect. We found that rVvhA induces the aggregation of lipid raft components coupled with NADPH oxidase enzymes, in which rVvhA increased the interaction of NADPH oxidase 2 (NOX2, gp91^phox) with a cytosolic protein NCF1 (p47^phox) to facilitate the production of reactive oxygen species (ROS). rVvhA uniquely stimulated a conventional PKC isoform PKCα and induced the phosphorylation of both ERK and JNK, which are responsible for the activation of transcription factor NF-κB. rVvhA induced an NF-κB-dependent imbalance of the Bcl-2/Bax ratio, the release of mitochondrial cytochrome c, and caspase-3/-9 activation during its promotion of apoptotic cell death. In addition, rVvhA has the ability to inhibit the expression of cell cycle-related proteins, such as CDK2, CDK4, cyclin D1, and cyclin E. These results demonstrate that rVvhA induces NF-κB-dependent mitochondrial cell death via lipid raft-mediated ROS production by the distinct activation of PKCα and ERK/JNK in intestinal epithelial cells.

Active ERK2 is sufficient to mediate growth arrest and differentiation signaling

Although extracellular signal-regulated kinases (ERK1/2) have been shown to be required in Raf/MEK/ERK pathway signaling, its sufficiency for mediating the pathway signaling has not been firmly established. In an effort to address this, we evaluated previously described ERK2 mutants that exhibit enhanced autophosphorylation of TEY sites in the activation loop in terms of their ability to induce growth arrest and differentiation in LNCaP and PC12 cells. We demonstrate that expression of ERK2-L73P/S151D, containing Lys73Pro and Ser151Asp substitutions that synergistically promote ERK autophosphorylation, is sufficient to induce growth arrest and differentiation, whereas expression of ERK2-I84A and ERK2-R65S/D319N is not as effective. When compared to the constitutively active MEK1-ΔN3/S218E/S222D, expression of ERK2-L73P/S151D only mildly increased ERK kinase activity in cells, as assessed using the ERK substrates p90(RSK) and ETS domain-containing protein (ELK1). However, ERK2-L73P/S151D expression effectively induced down-regulation of androgen receptors, Retinoblastoma (Rb) protein and E2F1 transcription factor, and up-regulation of p16(INK4A) and p21(CIP1), accompanied by cell-cycle arrest and morphological differentiation in LNCaP cells and neurite-like processes in PC12 cells. These effects and the TEY site phosphorylation of ERK2-L73P/S151D were abrogated upon introduction of the active site-disabling Lys52Arg mutation, suggesting that its autoactivation drives this signaling. Moreover, introduction of mutations Asp316/319Ala or Asp319Asn, which impair the common docking site/D-domain-based physical interaction of ERK, did not significantly affect ERK2-L73P/S151D signaling, suggesting that ERK2 mediates growth arrest and differentiation independently of the conventional ERK-target interaction mechanism. Thus, our study presents convincing evidence of ERK sufficiency for Raf/MEK/ERK signaling.

Reactive oxygen species induce MMP12-dependent degradation of collagen 5 and fibronectin to promote the motility of human umbilical cord-derived mesenchymal stem cells

BACKGROUND AND PURPOSE

Reactive oxygen species (ROS) are potent regulators of stem cell behaviour; however, their physiological significance as regards MMP-mediated regulation of the motility of human umbilical cord blood-derived mesenchymal stem cells (UCB-MSCs) has not been characterized. In the present study, we investigated the role of hydrogen peroxide (H2O2 ) and associated signalling pathways in promoting UCB-MSCs motility.

EXPERIMENTAL APPROACH

The regulatory effects of H2O2 on the activation of PKC, MAPKs, NF-κB and β-catenin were determined. The expressions of MMP and extracellular matrix proteins were examined. Pharmacological inhibitors and gene-specific siRNA were used to identify the signalling pathways of H2O2 that affect UCB-MSCs motility. An experimental skin wound-healing model was used to confirm the functional role of UCB-MSCs treated with H2O2 in ICR mice.

KEY RESULTS

H2O2 increased the motility of UCB-MSCs by activating PKCα via a calcium influx mechanism. H2O2 activated ERK and p38 MAPK, which are responsible for the distinct activation of transcription factors NF-κB and β-catenin. UCB-MSCs expressed eight MMP genes, but only MMP12 expression was uniquely regulated by NF-κB and β-catenin activation. H2O2 increased the MMP12-dependent degradation of collagen 5 (COL-5) and fibronectin (FN) associated with UCB-MSCs motility. Finally, topical transplantation of UCB-MSCs treated with H2O2 enhanced skin wound healing in mice.

CONCLUSIONS AND IMPLICATIONS

H2O2 stimulated UCB-MSCs motility by increasing MMP12-dependent degradation of COL-5 and FN through the activation of NF-κB and glycogen synthase kinase-3β/β-catenin, which is critical for providing a suitable microenvironment for MSCs transplantation and re-epithelialization of skin wounds in mice.