Both ERK1 and ERK2 are required for enterovirus 71 (EV71) efficient replication

Vemurafenib Inhibits Enterovirus A71 Genome Replication and Virus Assembly

Enterovirus A71 (EV-A71) infection is a major cause of hand, foot, and mouth disease (HFMD), which may be occasionally associated with severe neurological complications. There is currently a lack of treatment options for EV-A71 infection. The Raf-MEK-ERK signaling pathway, in addition to its critical importance in the regulation of cell growth, differentiation, and survival, has been shown to be essential for virus replication. In this study, we investigated the anti-EV-A71 activity of vemurafenib, a clinically approved B-Raf inhibitor used in the treatment of late-stage melanoma. Vemurafenib exhibits potent anti-EV-A71 effect in cytopathic effect inhibition and viral load reduction assays, with half maximal effective concentration (EC₅₀) at nanomolar concentrations. Mechanistically, vemurafenib interrupts both EV-A71 genome replication and assembly. These findings expand the list of potential antiviral candidates of anti-EV-A71 therapeutics.

Phosphorylation of ERK-Dependent NF-κB Triggers NLRP3 Inflammasome Mediated by Vimentin in EV71-Infected Glioblastoma Cells

Enterovirus 71 (EV71) is a dominant pathogenic agent that may cause severe central nervous system (CNS) diseases among infants and young children in the Asia-pacific. The inflammasome is closely implicated in EV71-induced CNS injuries through a series of signaling pathways. However, the activation pathway of NLRP3 inflammasome involved in EV71-mediated CNS injuries remains poorly defined. In the studies, EV71 infection, ERK1/2 phosphorylation, and activation of NLRP3 are abolished in glioblastoma cells with low vimentin expression by CRISPR/Cas9-mediated knockdown. PD098059, an inhibitor of p-ERK, remarkably blocks the vimentin-mediated ERK1/2 phosphorylation in EV71-infected cells. Nuclear translocation of NF-κB p65 is dependent on p-ERK in a time-dependent manner. Moreover, NLRP3 activation and caspase-1 production are limited in EV71-infected cells upon the caffeic acid phenethyl ester (CAPE) administration, an inhibitor of NF-κB, which contributes to the inflammasome regulation. In conclusion, these results suggest that EV71-mediated NLRP3 inflammasome could be activated via the VIM-ERK-NF-κB pathway, and the treatment of the dephosphorylation of ERK and NF-κB inhibitors is beneficial to host defense in EV71-infected CNS.

Pharmacological perturbation of CXCL1 signaling alleviates neuropathogenesis in a model of HEVA71 infection

Hand, foot and mouth disease (HFMD) caused by Human Enterovirus A71 (HEVA71) infection is typically a benign infection. However, in minority of cases, children can develop severe neuropathology that culminate in fatality. Approximately 36.9% of HEVA71-related hospitalizations develop neurological complications, of which 10.5% are fatal. Yet, the mechanism by which HEVA71 induces these neurological deficits remain unclear. Here, we show that HEVA71-infected astrocytes release CXCL1 which supports viral replication in neurons by activating the CXCR2 receptor-associated ERK1/2 signaling pathway. Elevated CXCL1 levels correlates with disease severity in a HEVA71-infected mice model. In humans infected with HEVA71, high CXCL1 levels are only present in patients presenting neurological complications. CXCL1 release is specifically triggered by VP4 synthesis in HEVA71-infected astrocytes, which then acts via its receptor CXCR2 to enhance viral replication in neurons. Perturbing CXCL1 signaling or VP4 myristylation strongly attenuates viral replication. Treatment with AZD5069, a CXCL1-specific competitor, improves survival and lessens disease severity in infected animals. Collectively, these results highlight the CXCL1-CXCR2 signaling pathway as a potential target against HFMD neuropathogenesis.

Modulation of IGF2 Expression in the Murine Thymus and Thymic Epithelial Cells Following Coxsackievirus-B4 Infection

Coxsackievirus B4 (CV-B4) can infect human and murine thymic epithelial cells (TECs). In a murine TEC cell line, CV-B4 can downregulate the transcription of the insulin-like growth factor 2 (Igf2) gene coding for the self-peptide of the insulin family. In this study, we show that CV-B4 infections of a murine TEC cell line decreased Igf2 P3 promoter activity by targeting a region near the transcription start site; however, the stability of Igf2 transcripts remained unchanged, indicating a regulation of Igf2 transcription. Furthermore, CV-B4 infections decreased STAT3 phosphorylation in vitro. We also showed that mice infected with CV-B4 had an altered expression of Igf2 isoforms as detected in TECs, followed by a decrease in the pro-IGF2 precursor in the thymus. Our study sheds new light on the intrathymic regulation of Igf2 transcription during CV-B4 infections and supports the hypothesis that a viral infection can disrupt central self-tolerance to insulin by decreasing Igf2 transcription in the thymic epithelium.

Metabolic Reprogramming of Host Cells in Response to Enteroviral Infection

Enterovirus 71 (EV71) infection is an endemic disease in Southeast Asia and China. We have previously shown that EV71 virus causes functional changes in mitochondria. It is speculative whether EV71 virus alters the host cell metabolism to its own benefit. Using a metabolomics approach, we demonstrate that EV71-infected Vero cells had significant changes in metabolism. Glutathione and its related metabolites, and several amino acids, such as glutamate and aspartate, changed significantly with the infectious dose of virus. Other pathways, including glycolysis and tricarboxylic acid cycle, were also altered. A change in glutamine/glutamate metabolism is critical to the viral infection. The presence of glutamine in culture medium was associated with an increase in viral replication. Dimethyl α-ketoglutarate treatment partially mimicked the effect of glutamine supplementation. In addition, the immunoblot analysis revealed that the expression of glutamate dehydrogenase (GDH) and trifunctional carbamoyl-phosphate synthetase 2, aspartate transcarbamylase, and dihydroorotase (CAD) increased during infection. Knockdown of expression of glutaminase (GLS), GDH and CAD drastically reduced the cytopathic effect (CPE) and viral replication. Furthermore, we found that CAD bound VP1 to promote the de novo pyrimidine synthesis. Our findings suggest that virus may induce metabolic reprogramming of host cells to promote its replication through interactions between viral and host cell proteins.

Synthesis and biological evaluation of berberine derivatives as a new class of broad-spectrum antiviral agents against Coxsackievirus B

A series of novel berberine (BBR) analogues were prepared and tested for their antiviral potencies against six different genotype Coxsackievirus B (CVB1-6) strains, taking BBR core for structural modification. Structure-activity relationship (SAR) research revealed that introduction of a primary amine through a linker at position 3 might be beneficial for both antiviral activity and safety. Compound 14c displayed most promising inhibitory potency with IC₅₀ values of 3.08-9.94 µM against tested CVBs 2-6 strains and satisfactory SI value of 34.3 on CVB3, better than that of BBR. Also, 14c could inhibit CVB3 replication through down-regulating the expression of VP1 protein and VP1 RNA. The mechanism revealed that 14c could suppress host components JNK-MAPK, ERK-MAPK and p38-MAPK activation. Therefore, BBR derivatives were considered to be a new class of anti-CVB agents with an advantage of broad-spectrum anti-CVB potency.

Cellular Caspase-3 Contributes to EV-A71 2Apro-Mediated Down-Regulation of IFNAR1 at the Translation Level

Enterovirus A71 (EV-A71) is the major pathogen responsible for the severe hand, foot and mouth disease worldwide, for which few effective antiviral drugs are presently available. Interferon-α (IFN-α) has been used in antiviral therapy for decades; it has been reported that EV-A71 antagonizes the antiviral activity of IFN-α based on viral 2A^pro-mediated reduction of the interferon-alpha receptor 1 (IFNAR1); however, the mechanism remains unknown. Here, we showed a significant increase in IFNAR1 protein induced by IFN-α in RD cells, whereas EV-A71 infection caused obvious down-regulation of the IFNAR1 protein and blockage of IFN-α signaling. Subsequently, we observed that EV-A71 2A^pro inhibited IFNAR1 translation by cleavage of the eukaryotic initiation factor 4GI (eIF4GI), without affecting IFNAR1 mRNA levels induced by IFN-α. The inhibition of IFNAR1 translation also occurred in puromycin-induced apoptotic cells when caspase-3 cleaved eIF4GI. Importantly, we verified that 2A^pro could activate cellular caspase-3, which was subsequently involved in eIF4GI cleavage mediated by 2A^pro. Furthermore, inhibition of caspase-3 activation resulted in the partial restoration of IFNAR1 in cells transfected with 2A or infected with EV-A71, suggesting the pivotal role of both viral 2A^pro and caspase-3 activation in the disturbance of IFN-α signaling. Collectively, we elucidate a novel mechanism by which cellular caspase-3 contributes to viral 2A^pro-mediated down-regulation of IFNAR1 at the translation level during EV-A71 infection, indicating that caspase-3 inhibition could be a potential complementary strategy to improve clinical anti-EV-A71 therapy with IFN-α.

Autophagy induced by enterovirus 71 regulates the production of IL-6 through the p38MAPK and ERK signaling pathways

Enterovirus 71 (EV71) is the main causative agent of hand, foot, and mouth disease (HFMD), which has high morbidity and mortality. It mainly threatens children under six years of age. Because of a poor understanding of its pathogenesis, there are no effective drugs to control EV71 infection. Previous studies showed that EV71 infection induced autophagy and the production of cytokine IL-6. However, the underlying mechanisms between autophagy and the production of IL-6 induced by EV71 remain unclear. This study aimed to reveal the regulatory mechanisms between autophagy and the expression of IL-6 induced by EV71 infection. Our results showed that the proliferation of human gastric epithelial (GES-1) cells was inhibited by EV71 in a time- and dose-dependent manner. In addition, EV71 induced autophagy in GES-1 cells. EV71 infection promoted the expression and the release of IL-6 to the extracellular space, although the expression and release were inhibited by autophagy inhibitors 3-methyladenine (3-MA) and chloroquine (CQ) in GES-1 cells. The phosphorylated levels of p38MAPK and ERK proteins in GES-1 cells also increased after infection with EV71, and these changes were also reversed by 3-MA and CQ treatment. Our findings suggested that EV71-induced autophagy regulated the production of IL-6 through the p38MAPK and ERK signaling pathways.

Substituted 3-benzylcoumarins 13 and 14 suppress enterovirus A71 replication by impairing viral 2Apro dependent IRES-driven translation

Activation of the ERK signaling cascade in host cells has been demonstrated to be essential for enterovirus A71 (EV-A71) replication. Our previous study showed that MEK kinase, which specially activated downstream ERK kinase, is an important and potential target against EV-A71. Furthermore, we reported that a series of substituted 3-benzylcoumarins designed and synthesized as well as verified for inhibiting the MEK-ERK cascade were found to be effective on anti-EV-A71. In this study, we further demonstrated that two substituted 3-benzylcoumarins designated as 13 and 14 were more effective anti-MEK/ERK activity, less cytotoxicity and stronger antiviral effect represented by inhibition of viral-induced CPE, the expression of viral proteins and the replication of the viral genome, as well as the production of progeny virions, compared to those of U0126, an available MEK inhibitor, and sorafenib, a multiple-targeted kinase inhibitor in clinical use. Moreover, we explored that the likely mechanism of action of these two test compounds were to block EV-A71 2A dependent IRES-driven activity essential for successful viral replication. Hence, our results suggest that two substituted 3-benzylcoumarins 13 and 14 could be candidates as potential anti-EV-A71 agents.

Newcastle Disease Virus V Protein Promotes Viral Replication in HeLa Cells through the Activation of MEK/ERK Signaling

Newcastle disease virus (NDV) can infect a wide range of domestic and wild bird species. The non-structural V protein of NDV plays an important role in antagonizing innate host defenses to facilitate viral replication. However, there is a lack of knowledge related to the mechanisms through which the V protein regulates viral replication. The extracellular signal-regulated kinase (ERK) signaling pathway in the host is involved in a variety of functions and is activated by several stimuli, including viral replication. In this study, we show that both the lentogenic strain, La Sota, and the velogenic strain, F48E9, of NDV activate the mitogen-activated protein kinase (MEK)/ERK signaling pathway. The pharmacological inhibition of ERK1/2 phosphorylation using the highly selective inhibitors U0126 and SCH772984 resulted in the reduced levels of NDV RNA in cells and virus titers in the cell supernatant, which established an important role for the MEK/ERK signaling pathway in NDV replication. Moreover, the overexpression of the V protein in HeLa cells increased the phosphorylation of ERK1/2 and induced the transcriptional changes in the genes downstream of the MEK/ERK signaling pathway. Taken together, our results demonstrate that the V protein is involved in the ERK signaling pathway-mediated promotion of NDV replication and thus, can be investigated as a potential antiviral target.

Synthesis and Evolution of Berberine Derivatives as a New Class of Antiviral Agents against Enterovirus 71 through the MEK/ERK Pathway and Autophagy

Taking berberine (BBR) as the lead, 23 new BBR derivatives were synthesized and examined for their antiviral activities against four different genotype enterovirus 71 (EV71) strains with a cytopathic effect (CPE) assay. Structure-activity relationship (SAR) studies indicated that introduction of a suitable substituent at the 9-position might be beneficial for potency. Among them, compound 2d exhibited most potent activities with IC₅₀ values of 7.12⁻14.8 μM, similar to that of BBR. The effect of 2d was further confirmed in a dose-dependent manner both in RNA and protein level. The mechanism revealed that 2d could inhibit the activation of MEK/ERK signaling pathway. Meanwhile, it could suppress the EV71-induced autophagy by activating AKT and inhibiting the phosphorylation of JNK and PI3KIII proteins. We consider BBR derivatives to be a new family of anti-EV71 agents through targeting host components, with an advantage of broad-spectrum anti-EV71 potency.

Enterovirus A71 Proteins: Structure and Function

Enterovirus A71 (EV-A71) infection has grown to become a serious threat to global public health. It is one of the major causes of hand, foot, and mouth disease (HFMD) in infants and young children. EV-A71 can also infect the central nervous system (CNS) and induce diverse neurological complications, such as brainstem encephalitis, aseptic meningitis, and acute flaccid paralysis, or even death. Viral proteins play a crucial role in EV-A71 infection. Many recent studies have discussed the structure and function of EV-A71 proteins, and the findings reported will definitely aid the development of vaccines and therapeutic approaches. This article reviews the progress in the research on the structure and function of EV-A71 proteins. Available literature can provide a basis for studying the pathogenesis of EV-A71 infection in detail.

Inhibition of EV71 by curcumin in intestinal epithelial cells

EV71 is a positive-sense single-stranded RNA virus that belongs to the Picornaviridae family. EV71 infection may cause various symptoms ranging from hand-foot-and-mouth disease to neurological pathological conditions such as aseptic meningitis, ataxia, and acute transverse myelitis. There is currently no effective treatment or vaccine available. Various compounds have been examined for their ability to restrict EV71 replication. However, most experiments have been performed in rhabdomyosarcoma or Vero cells. Since the gastrointestinal tract is the entry site for this pathogen, we anticipated that orally ingested agents may exert beneficial effects by decreasing virus replication in intestinal epithelial cells. In this study, curcumin (diferuloylmethane, C₂₁H₂₀O₆), an active ingredient of turmeric (Curcuma longa Linn) with anti-cancer properties, was investigated for its anti-enterovirus activity. We demonstrate that curcumin treatment inhibits viral translation and increases host cell viability. Curcumin does not exert its anti-EV71 effects by modulating virus attachment or virus internal ribosome entry site (IRES) activity. Furthermore, curcumin-mediated regulation of mitogen-activated protein kinase (MAPK) signaling pathways is not involved. We found that protein kinase C delta (PKCδ) plays a role in virus translation in EV71-infected intestinal epithelial cells and that curcumin treatment decreases the phosphorylation of this enzyme. In addition, we show evidence that curcumin also limits viral translation in differentiated human intestinal epithelial cells. In summary, our data demonstrate the anti-EV71 properties of curcumin, suggesting that ingestion of this phytochemical may protect against enteroviral infections.

Regulation of enterovirus 2A protease-associated viral IRES activities by the cell's ERK signaling cascade: Implicating ERK as an efficiently antiviral target

In a previous study the ERK1/2 pathway was found to be crucially involved in positive regulation of the enterovirus A 71(EV-A71) IRES (vIRES), thereby contributing to the efficient replication of an important human enterovirus causing death in young children (<5yrs) worldwide. This study focuses on unraveling more about the detailed mechanism of ERK's involvement in this regulation of vIRES. Through the use of siRNAs and specifically pharmacological inhibitor U0126, the ERK cascade was shown to positively regulate EV-A71-mediated cleavage of eIF4GI that established the cellular conditions which favour vIRES-dependent translation. Site-directed mutagenesis of the viral 2A protease (2A^pro) was undertaken to show that the positive regulation of virus replication by the ERK cascade was mediated through effects on both the cis-cleavage of the viral polyprotein by 2A^pro and its trans-cleavage of cellular eIF4GI. This ERK-2A^pro linked network coordinating vIRES efficiency was also found in other important human enteroviruses. This identification of the ERK cascade as having a key role in maintaining the 2A^pro proteolytic activity required to maximize enterovirus IRES activity, expands current understanding of the diverse functions of the ERK signaling cascade in the regulation of viral translation, therefore providing a potentially comprehensive drug target for anti-enterovirus infection.

Crosstalk between Smad2/3 and specific isoforms of ERK in TGF-β1-induced TIMP-3 expression in rat chondrocytes

This study investigated the roles of ERK1 and ERK2 in transforming growth factor‐β1 (TGF‐β1)‐induced tissue inhibitor of metalloproteinases‐3 (TIMP‐3) expression in rat chondrocytes, and the specific roles of ERK1 and ERK2 in crosstalk with Smad2/3 were investigated to demonstrate the molecular mechanism of ERK1/2 regulation of TGF‐β1 signalling. To examine the interaction of specific isoforms of ERK and the Smad2/3 signalling pathway, chondrocytes were infected with LV expressing either ERK1 or ERK2 siRNA and stimulated with or without TGF‐β1. At indicated time‐points, TIMP‐3 expression was determined by real‐time PCR and Western blotting; p‐Smad3, nuclear p‐Smad3, Smad2/3, p‐ERK1/2 and ERK1/2 levels were assessed. And then, aggrecan, type II collagen and the intensity of matrix were examined. TGF‐β1‐induced TIMP‐3 expression was significantly inhibited by ERK1 knock‐down, and the decrease in TIMP‐3 expression was accompanied by a reduction of p‐Smad3 in ERK1 knock‐down cells. Knock‐down of ERK2 had no effect on neither TGF‐β1‐induced TIMP‐3 expression nor the quantity of p‐Smad3. Moreover, aggrecan, type II collagen expression and the intensity of matrix were significantly suppressed by ERK1 knock‐down instead of ERK2 knock‐down. Taken together, ERK1 and ERK2 have different roles in TGF‐β1‐induced TIMP‐3 expression in rat chondrocytes. ERK1 instead of ERK2 can regulate TGF‐β/Smad signalling, which may be the mechanism through which ERK1 regulates TGF‐β1‐induced TIMP‐3 expression.

Berberine inhibits enterovirus 71 replication by downregulating the MEK/ERK signaling pathway and autophagy

Background

The MEK-ERK signaling pathway and autophagy play an important role for enterovirus71(EV71) replication. Inhibition of MEK-ERK signaling pathway and autophagy is shown to impair EV71 replication. Berberine (BBR), an isoquinoline alkaloid isolated from Berberis vulgaris L., has been reported to have ability to regulate this signaling pathway and autophagy. Herein, we want to determine whether berberine can inhibit EV71 infection by downregulating the MEK/ERK signaling pathway and autophagy.

Methods

The antiviral effect of berberine was determined by cytopathic effect (CPE) assay, western blotting assay and qRT-PCR assay. The mechanism of BBR anti-virus was determined by western blotting assay and immunofluorescence assay.

Results

We showed that berberine does-dependently reduced EV71 RNA and protein synthesis, which was, at least in part, the result of inhibition of activation of MEK/ERK signaling pathway. Furthermore, we found that berberine suppressed the EV71-induced autophagy by activating AKT protein and inhibiting the phosphorylation of JNK and PI3KIII.

Conclusions

BBR inhibited EV71 replication by downregulating autophagy and MEK/ERK signaling pathway. These findings suggest that BBR may be a potential agent or supplement against EV71 infection.

Mitogen-Activated Protein Kinase Kinase 2, a Novel E2-Interacting Protein, Promotes the Growth of Classical Swine Fever Virus via Attenuation of the JAK-STAT Signaling Pathway

The mitogen-activated protein kinase kinase/extracellular regulated kinase (MEK1/2/ERK1/2) cascade is involved in the replication of several members of the Flaviviridae family, including hepatitis C virus and dengue virus. The effects of the cascade on the replication of classical swine fever virus (CSFV), a fatal pestivirus of pigs, remain unknown. In this study, MEK2 was identified as a novel binding partner of the E2 protein of CSFV using yeast two-hybrid screening. The E2-MEK2 interaction was confirmed by glutathione S-transferase pulldown, coimmunoprecipitation, and laser confocal microscopy assays. The C termini of E2 (amino acids [aa] 890 to 1053) and MEK2 (aa 266 to 400) were mapped to be crucial for the interaction. Overexpression of MEK2 significantly promoted the replication of CSFV, whereas knockdown of MEK2 by lentivirus-mediated small hairpin RNAs dramatically inhibited CSFV replication. In addition, CSFV infection induced a biphasic activation of ERK1/2, the downstream signaling molecules of MEK2. Furthermore, the replication of CSFV was markedly inhibited in PK-15 cells treated with U0126, a specific inhibitor for MEK1/2/ERK1/2, whereas MEK2 did not affect CSFV replication after blocking the interferon-induced Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway by ruxolitinib, a JAK-STAT-specific inhibitor. Taken together, our results indicate that MEK2 positively regulates the replication of CSFV through inhibiting the JAK-STAT signaling pathway.

IMPORTANCE Mitogen-activated protein kinase kinase 2 (MEK2) is a kinase that operates immediately upstream of extracellular regulated kinase 1/2 (ERK1/2) and links to Raf and ERK via phosphorylation. Currently, little is known about the role of MEK2 in the replication of classical swine fever virus (CSFV), a devastating porcine pestivirus. Here, we investigated the roles of MEK2 and the MEK2/ERK1/2 cascade in the growth of CSFV for the first time. We show that MEK2 positively regulates CSFV replication. Notably, we demonstrate that MEK2 promotes CSFV replication through inhibiting the interferon-induced JAK-STAT signaling pathway, a key antiviral pathway involved in innate immunity. Our work reveals a novel role of MEK2 in CSFV infection and sheds light on the molecular basis by which pestiviruses interact with the host cell.

Susceptibility of human tonsillar epithelial cells to enterovirus 71 with normal cytokine response

A recent histopathologic study implicated human tonsillar crypt epithelium as an important site for EV71 replication in EV71-caused fatal cases. This study aimed to confirm the susceptibility of human tonsillar epithelium to EV71. Two human tonsillar epithelial cell lines (UT-SCC-60A and UT-SCC-60B) were susceptive to EV71, and PI3K/AKT, p38, ERK1/2, and JNK1/2 signal pathways were activated. Interferon-α, IL-8, IL-1β, IL-6 and IL-12p40 were induced and regulated by PI3K/AKT, p38, ERK1/2, and JNK1/2 signal pathways. PI3K/AKT pathway activation appeared to suppress the induction of TNF-α, which induced cell survival by inhibiting GSK-3β. The activation of NF-κB was observed but inhibited by these pathways in EV71 infection. Furthermore, ERK1/2 and JNK1/2 were essential for efficient EV71 replication. Human tonsillar epithelial cells support EV71 replication and display innate antiviral immunity in vitro, indicating that human tonsillar epithelial cells may be novel targets for EV71 infection and replication in vivo.

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.