Selective role for Mek1 but not Mek2 in the induction of epidermal neoplasia

Differential contribution for ERK1 and ERK2 kinases in BRAFV600E-triggered phenotypes in adult mouse models

The BRAF gene is mutated in a plethora of human cancers. The majority of such molecular lesions result in the expression of a constitutively active BRAF variant (BRAFV600E) which continuously bolsters cell proliferation. Although we recently addressed the early effects triggered by BRAFV600E-activation, the specific contribution of ERK1 and ERK2 in BRAFV600E-driven responses in vivo has never been explored. Here we describe the first murine model suitable for genetically dissecting the ERK1/ERK2 impact in multiple phenotypes induced by ubiquitous BRAFV600E-expression. We unveil that ERK1 is dispensable for BRAFV600E-dependent lifespan shortening and for BRAFV600E-driven tumor growth. We show that BRAFV600E-expression provokes an ERK1-independent lymphocyte depletion which does not rely on p21CIP1-induced cell cycle arrest and is unresponsive to ERK-chemical inhibition. Moreover, we also reveal that ERK1 is dispensable for BRAFV600E-triggered cytotoxicity in lungs and that ERK-chemical inhibition abrogates some of these detrimental effects, such as DNA damage, in Club cells but not in pulmonary lymphocytes. Our data suggest that ERK1/ERK2 contribution to BRAFV600E-driven phenotypes is dynamic and varies dependently on cell type, the biological function, and the level of ERK-pathway activation. Our findings also provide useful insights into the comprehension of BRAFV600E-driven malignancies pathophysiology as well as the consequences in vivo of novel ERK pathway-targeted anti-cancer therapies.

Caspase 3-specific cleavage of MEK1 suppresses ERK signaling and sensitizes cells to stress-induced apoptosis

Proper regulation of apoptotic cell death is crucial for normal development and homeostasis in multicellular organisms and is achieved by the balance between pro-apoptotic processes, such as caspase activation, and pro-survival signaling, such as extracellular signal-regulated kinase (ERK) activation. However, the functional interplay between these opposing signaling pathways remains incompletely understood. Here, we identified MAPK/ERK kinase (MEK) 1, a central component of the ERK pathway, as a specific substrate for the executioner caspase-3. During apoptosis, MEK1 is cleaved at an evolutionarily conserved Asp282 residue in the kinase domain, thereby losing its catalytic activity. Gene knockout experiments showed that MEK1 cleavage was mediated by caspase-3, but not by the other executioner caspases, caspase-6 or -7. Following exposure of cells to osmotic stress, elevated ERK activity gradually decreased, and this was accompanied by increased cleavage of MEK1. In contrast, the expression of a caspase-uncleavable MEK1(D282N) mutant in cells maintained stress-induced ERK activity and thereby attenuated apoptotic cell death. Thus, caspase-3-mediated, proteolytic inhibition of MEK1 sensitizes cells to apoptosis by suppressing pro-survival ERK signaling. Furthermore, we found that a RASopathy-associated MEK1(Y130C) mutation prevented this caspase-3-mediated proteolytic inactivation of MEK1 and efficiently protected cells from stress-induced apoptosis. Our data reveal the functional crosstalk between ERK-mediated cell survival and caspase-mediated cell death pathways and suggest that its dysregulation by a disease-associated MEK1 mutation is at least partly involved in the pathophysiology of congenital RASopathies.

Ferrous iron–activatable drug conjugate achieves potent MAPK blockade in KRAS-driven tumors

KRAS mutations drive a quarter of cancer mortality, and most are undruggable. Several inhibitors of the MAPK pathway are FDA approved but poorly tolerated at the doses needed to adequately extinguish RAS/RAF/MAPK signaling in the tumor cell. We found that oncogenic KRAS signaling induced ferrous iron (Fe²⁺) accumulation early in and throughout mutant KRAS-mediated transformation. We converted an FDA-approved MEK inhibitor into a ferrous iron–activatable drug conjugate (FeADC) and achieved potent MAPK blockade in tumor cells while sparing normal tissues. This innovation allowed sustainable, effective treatment of tumor-bearing animals, with tumor-selective drug activation, producing superior systemic tolerability. Ferrous iron accumulation is an exploitable feature of KRAS transformation, and FeADCs hold promise for improving the treatment of KRAS-driven solid tumors.

Context-Dependent Immunomodulatory Effects of MEK Inhibition are Enhanced with T-cell Agonist Therapy

MEK inhibition (MEKi) is proposed to enhance antitumor immunity but has demonstrated mixed results as an immunomodulatory strategy in human clinical trials. MEKi exerts direct immunomodulatory effects on tumor cells and tumor-infiltrating lymphocytes, but these effects have not been independently investigated. Here we modeled tumor-specific MEKi through CRISPR/Cas-mediated genome editing of tumor cells (MEK1 KO) and pharmacologic MEKi with cobimetinib in a RAS-driven model of colorectal cancer. This approach allowed us to distinguish tumor-mediated and tumor-independent mechanisms of MEKi immunomodulation. MEK1 KO tumors demonstrated upregulation of JAK/STAT signaling; enhanced MHCI expression, CD8+ T-cell infiltration and T-cell activation; and impaired tumor growth that is immune-dependent. Pharmacologic MEKi recapitulated tumor-intrinsic effects but simultaneously impaired T-cell activation in the tumor microenvironment. We confirmed a reduction in human peripheral lymphocyte activation from a clinical trial of anti-PD-L1 (atezolizumab) with or without cobimetinib in biliary tract cancers. Impaired activation of tumor-infiltrating lymphocytes treated with pharmacologic MEKi was reversible and was rescued with the addition of a 41BB agonist. Collectively, these data underscore the ability of MEKi to induce context-dependent immunomodulatory effects and suggest that T cell-agonist therapy maximizes the beneficial effects of MEKi on the antitumor immune response.

Knockdown of PAK1 Inhibits the Proliferation and Invasion of Non-Small Cell Lung Cancer Cells Through the ERK Pathway

The p21-activated kinase (PAK) family of serine/threonine kinases plays a pivotal role in various human tumors, as supported by our previous report on the overexpressed PAK isoforms in non-small cell lung cancer (NSCLC). To better understand the role of PAKs in tumorigenesis, the authors examined PAK1 expression patterns and its significance in NSCLC. It was demonstrated by immunohistochemical staining that PAK1 was increased and localized in the cytoplasm in 151 of 207 cases. High levels of PAK1 expression correlated with a histologic type of tumor (squamous cell carcinoma), tumor node metastasis stage, and lymph nodal status. We also examined the biological role of PAK1 in lung cancer cell lines transfected with PAK1-small interfering RNA. Decreased expression of PAK1 inhibited lung cancer cell proliferation and invasion, which is the major cause of lung cancer malignancy. Downregulated expression of PAK1 hampered rapidly accelerated fibrosarcoma/mitogen-activated extracellular signal-regulated kinase/extracellular signal-regulated kinase pathway activity but did not affect Wnt/β-catenin signaling. Our findings suggest that PAK1 is an important oncogene in NSCLC, as decreased expression of PAK1 inhibited the proliferation and invasion of NSCLC cells by blocking the ERK pathway. These results provide evidence for using PAK1 inhibition as potential anticancer therapy.

Integrin α3β1 Is a Key Regulator of Several Protumorigenic Pathways during Skin Carcinogenesis

Integrin α3β1 plays a crucial role in tumor formation in the two-stage chemical carcinogenesis model (DMBA and TPA treatment). However, the mechanisms whereby the expression of α3β1 influences key oncogenic drivers of this established model are not known yet. Using an in vivo mouse model with epidermal deletion of α3β1 and in vitro Matrigel cultures of transformed keratinocytes, we demonstrate the central role of α3β1 in promoting the activation of several protumorigenic signaling pathways during the initiation of DMBA/TPA‒driven tumorigenesis. In transformed keratinocytes, α3β1-mediated focal adhesion kinase/Src activation leads to in vitro growth of spheroids and to strong Akt and STAT 3 activation when the α3β1-binding partner tetraspanin CD151 is present to stabilize cell‒cell adhesion and promote Smad2 phosphorylation. Remarkably, α3β1 and CD151 can support Akt and STAT 3 activity independently of α3β1 ligation by laminin-332 and as such control the essential survival signals required for suprabasal keratin-10 expression during keratinocyte differentiation. These data demonstrate that α3β1 together with CD151 regulate the signaling pathways that control the survival of differentiating keratinocytes and provide a mechanistic understanding of the essential role of α3β1 in early stages of skin cancer development.

RAS-mediated suppression of PAR3 and its effects on SCC initiation and tissue architecture occur independently of hyperplasia

Proper epithelial development and homeostasis depends on strict control of oriented cell division. Current evidence shows that this process is regulated by intrinsic polarity factors and external spatial cues. Owing to the lack of an appropriate model system that can recapitulate the architecture of the skin, deregulation of spindle orientation in human epithelial carcinoma has never been investigated. Here, using an inducible model of human squamous cell carcinoma (SCC), we demonstrate that RAS-dependent suppression of PAR3 (encoded by PARD3) accelerates epithelial disorganization during early tumorigenesis. Diminished PAR3 led to loss of E-cadherin-mediated cell adhesion, which in turn contributed to misoriented cell division. Pharmacological inhibition of the MAPK pathway downstream of RAS activation reversed the defects in PAR3 expression, E-cadherin-mediated cell adhesion and mitotic spindle orientation. Thus, temporal analysis of human neoplasia provides a powerful approach to study cellular and molecular transformations during early oncogenesis, which allowed identification of PAR3 as a critical regulator of tissue architecture during initial human SCC development.

Growth Inhibitory Signaling of the Raf/MEK/ERK Pathway

In response to extracellular stimuli, the Raf/MEK/extracellular signal-regulated kinase (ERK) pathway regulates diverse cellular processes. While mainly known as a mitogenic signaling pathway, the Raf/MEK/ERK pathway can mediate not only cell proliferation and survival but also cell cycle arrest and death in different cell types. Growing evidence suggests that the cell fate toward these paradoxical physiological outputs may be determined not only at downstream effector levels but also at the pathway level, which involves the magnitude of pathway activity, spatial-temporal regulation, and non-canonical functions of the molecular switches in this pathway. This review discusses recent updates on the molecular mechanisms underlying the pathway-mediated growth inhibitory signaling, with a major focus on the regulation mediated at the pathway level.

Targeting the MAPK Pathway in KRAS-Driven Tumors

KRAS mutations occur in a quarter of all of human cancers, yet no selective drug has been approved to treat these tumors. Despite the recent development of drugs that block KRAS^G12C, the majority of KRAS oncoproteins remain undruggable. Here, we review recent efforts to validate individual components of the mitogen-activated protein kinase (MAPK) pathway as targets to treat KRAS-mutant cancers by comparing genetic information derived from experimental mouse models of KRAS-driven lung and pancreatic tumors with the outcome of selective MAPK inhibitors in clinical trials. We also review the potential of RAF1 as a key target to block KRAS-mutant cancers.

Toosendanin, a natural product, inhibited TGF-β1-induced epithelial-mesenchymal transition through ERK/Snail pathway

Epithelial-mesenchymal transition (EMT) plays important roles in the metastasis of solid tumors. In this study, the effect of toosendanin (TSN), a natural insecticide extracted from Melia toosendan Sieb et Zucc, on transforming growth factor-β1 (TGF-β1)-induced EMT was investigated. EMT was induced by TGF-β1 in A549 and H1975 lung cancer cells. The morphological alterations were observed with a microscopy. The protein expression and localization of EMT biomarkers were determined by Western blotting and immunofluorescence. The migration, invasion, and adhesion were determined by wound-healing, transwell, and adhesion assays. TGF-β1 treatment induced spindle-shaped alterations of cells, upregulation of N-cadherin, Vimentin, p-ERK1/2, and downregulation of E-cadherin. The abilities of migration, invasion, and adhesion were also enhanced. These effects were significantly reversed by TSN at very low concentration (<10 nM). Furthermore, silence Snail significantly reversed TGF-β1-induced EMT biomarkers. In addition, TGF-β1-induced phosphorylation of ERK1/2 without affecting p38 mitogen-activated protein kinases and Jun N-terminal kinase. PD98059 and U0126, inhibitors of ERK1/2, showed similar inhibitory effect to that of TSN. In summary, TSN significantly inhibited TGF-β1-induced EMT and migration, invasion, and adhesion through ERK/Snail pathway in lung cancer cells. This study provides novel anticancer effects and molecular mechanisms for TSN.

Helicobacter pylori-Induced HB-EGF Upregulates Gastrin Expression via the EGF Receptor, C-Raf, Mek1, and Erk2 in the MAPK Pathway

Helicobacter pylori is associated with hypergastrinemia, which has been linked to the development of gastric diseases. Although the molecular mechanism is not fully understood, H. pylori is known to modulate the Erk pathway for induction of gastrin expression. Herein we found that an epidermal growth factor (EGF) receptor kinase inhibitor significantly blocked H. pylori-induced gastrin promoter activity, suggesting involvement of EGF receptor ligands. Indeed, H. pylori induced mRNA expression of EGF family members such as amphiregulin, EGF, heparin-binding EGF-like growth factor (HB-EGF), and transforming growth factor-α. Of these, specific siRNA targeting of HB-EGF significantly blocked H. pylori-induced gastrin expression. Moreover, H. pylori induced HB-EGF ectodomain shedding, which we found to be a critical process for H. pylori-induced gastrin expression. Thus, we demonstrate a novel role for human mature HB-EGF in stimulating gastrin promoter activity during H. pylori infection. Further investigation using specific siRNAs targeting each isoform of Raf, Mek, and Erk elucidated that the mechanism underlying H. pylori-induced gastrin expression can be delineated as the sequential activation of HB-EGF, the EGF receptor, C-Raf, Mek1, and the Erk2 molecules in the MAPK pathway. Surprisingly, whereas Erk2 acts as a potent activator of gastrin expression, siRNA knockdown of Erk1 induced gastrin promoter activity, suggesting that Erk1 typically acts as a repressor of gastrin expression. Elucidation of the mechanism of gastrin modulation by HB-EGF-mediated EGF receptor transactivation should facilitate the development of therapeutic strategies against H. pylori-related hypergastrinemia and consequently gastric disease development, including gastric cancers.

Lgr4 is crucial for skin carcinogenesis by regulating MEK/ERK and Wnt/β-catenin signaling pathways

Lgr4 is a member of the leucine-rich, G protein-coupled receptor family of proteins, and has recently been shown to augment Wnt/β-catenin signaling via binding to Wnt agonists R-spondins. It plays an important role in skin development, but its involvement in skin tumorigenesis is unclear. Here, we report that mice deficient for Lgr4 are resistant to 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced keratinocyte proliferation and papilloma formation. We show that TPA treatment activates MEK1, ERK1/2 and downstream effector AP-1 in wild-type (WT) epidermal cells and mice, but not in cells or mice where Lgr4 is depleted. Wnt/β-catenin signaling is also dramatically activated by TPA treatment, and this activation is abolished when Lgr4 is deleted. We provide evidences that blocking both MEK1/ERK1/2 and Wnt/β-catenin pathways prevents TPA-induced increase in the expression of Ccnd1 (cyclin D1), a known Wnt/β-catenin target gene, and that the activation of MEK1/ERK1/2 pathway lies upstream of Wnt/β-catenin signal pathway. Collectively, our findings identify Lgr4 as a critical positive factor for skin tumorigenesis by mediating the activation of MEK1/ERK1/2 and Wnt/β-catenin pathways.

Mitogen-activated Protein Kinase Kinase Activity Maintains Acinar-to-Ductal Metaplasia and Is Required for Organ Regeneration in Pancreatitis

BACKGROUND & AIMS

Mitogen-activated protein kinase (MAPK) signaling in the exocrine pancreas has been extensively studied in the context of pancreatic cancer, where its potential as a therapeutic target is limited by acquired drug resistance. However, its role in pancreatitis is less understood. We investigated the role of mitogen-activated protein kinase kinase (MEK)-initiated MAPK signaling in pancreatitis to determine the potential for MEK inhibition in treating pancreatitis patients.

METHODS

To examine the role of MEK signaling in pancreatitis, we used both genetic and pharmacologic approaches to inhibit the MAPK signaling pathway in a murine model of cerulein-induced pancreatitis. We generated mice harboring inducible short hairpins targeting the MEK isoforms Map2k1 and/or Map2k2 specifically in the pancreatic epithelium. We also used the MEK inhibitor trametinib to determine the efficacy of systemic inhibition in mice with pancreatitis.

RESULTS

We demonstrated an essential role for MEK signaling in the initiation of pancreatitis. We showed that both systemic and parenchyma-specific MEK inhibition in established pancreatitis induces epithelial differentiation and stromal remodeling. However, systemic MEK inhibition also leads to a loss of the proliferative capacity of the pancreas, preventing the restoration of organ mass.

CONCLUSIONS

MEK activity is required for the initiation and maintenance of pancreatitis. MEK inhibition may be useful in the treatment of chronic pancreatitis to interrupt the vicious cycle of destruction and repair but at the expense of organ regeneration.

MAP Kinase Pathways: Molecular Roads to Primary Acral Lentiginous Melanoma

The etiology and pathogenesis of lentiginous acral melanomas are poorly understood. Recent studies have postulated that DNA repair mechanisms and cell growth pathways are involved in the development of melanoma, particularly changes in the MAPK pathways (RAS, BRAF, MEK 1/2, and ERK 1/2). The aim of this study is to assess the status of the MAP kinase pathways in the pathogenesis of acral melanomas. The authors examined the components of the RAS–RAF–MEK–ERK cascades by immunohistochemistry in a series of 16 primary acral melanomas by tissue microarray. The expression of MAP kinase cascade proteins changed in most cases. The authors observed that 57.14% of cases were BRAF positive and that 61.53%, 71.42%, and 71.42% of cases were positive for MEK2, ERK1, and ERK2, respectively; RAS was not expressed in 92.31%, and all cases were negative for MEK1. The absence of RAS and positivity for MEK2, ERK1, and ERK2 were most seen in invasive cases with high thickness. These aspects of the MAPK pathway require further examination in acral melanomas between different populations. Nevertheless, the results highlight significant alterations in the MAP kinase cascades that are related to histological indicators of prognosis in primary acral melanomas.

Epidermal RAF prevents allergic skin disease

The RAS pathway is central to epidermal homeostasis, and its activation in tumors or in Rasopathies correlates with hyperproliferation. Downstream of RAS, RAF kinases are actionable targets regulating keratinocyte turnover; however, chemical RAF inhibitors paradoxically activate the pathway, promoting epidermal proliferation. We generated mice with compound epidermis-restricted BRAF/RAF1 ablation. In these animals, transient barrier defects and production of chemokines and Th2-type cytokines by keratinocytes cause a disease akin to human atopic dermatitis, characterized by IgE responses and local and systemic inflammation. Mechanistically, BRAF and RAF1 operate independently to balance MAPK signaling: BRAF promotes ERK activation, while RAF1 dims stress kinase activation. In vivo, JNK inhibition prevents disease onset, while MEK/ERK inhibition in mice lacking epidermal RAF1 phenocopies it. These results support a primary role of keratinocytes in the pathogenesis of atopic dermatitis, and the animals lacking BRAF and RAF1 in the epidermis represent a useful model for this disease.

A chemical-genetic interaction map of small molecules using high-throughput imaging in cancer cells

Small molecules often affect multiple targets, elicit off-target effects, and induce genotype-specific responses. Chemical genetics, the mapping of the genotype dependence of a small molecule's effects across a broad spectrum of phenotypes can identify novel mechanisms of action. It can also reveal unanticipated effects and could thereby reduce high attrition rates of small molecule development pipelines. Here, we used high-content screening and image analysis to measure effects of 1,280 pharmacologically active compounds on complex phenotypes in isogenic cancer cell lines which harbor activating or inactivating mutations in key oncogenic signaling pathways. Using multiparametric chemical-genetic interaction analysis, we observed phenotypic gene-drug interactions for more than 193 compounds, with many affecting phenotypes other than cell growth. We created a resource termed the Pharmacogenetic Phenome Compendium (PGPC), which enables exploration of drug mode of action, detection of potential off-target effects, and the generation of hypotheses on drug combinations and synergism. For example, we demonstrate that MEK inhibitors amplify the viability effect of the clinically used anti-alcoholism drug disulfiram and show that the EGFR inhibitor tyrphostin AG555 has off-target activity on the proteasome. Taken together, this study demonstrates how combining multiparametric phenotyping in different genetic backgrounds can be used to predict additional mechanisms of action and to reposition clinically used drugs.

Activation of MEK2 is sufficient to induce skin papilloma formation in transgenic zebrafish

Background

Mutations in mitogen-activated protein kinase (MAPK) kinase 1 (MEK1) that occur during cell proliferation and tumor formation are well described. Information on the roles of MEK2 in these effects is still limited. We established a constitutive MEK2 transgenic zebrafish, Tg(krt14:MEK2S219D-GFP), to elucidate the role of MEK2 in skin tumor formation.

Results

We found that both constitutive MEK2 and MEK1 are able to phosphorylate the extracellular signal-regulated kinase 1 (ERK1) protein. Transient expression of constitutive MEK2 and MEK1 in the zebrafish epidermis induced papillary formation at 48 h post-fertilization, but no effects were observed due to the expression of MEK1, MEK2, or the dominant negative form of MEK2. The transgenic zebrafish, Tg(krt14:MEK2S219D-GFP), developed skin papillomas in the epidermis within 6 days post-fertilization (dpf). The phospho-ERK signal was detected in section of skin papillomas in an immunohistochemical experiment. Treatment with 50 μM of the MEK inhibitor, U0126, had significantly decreased the skin papilloma formation in Tg(krt14:MEK2S219D-GFP) zebrafish by 6 dpf. In vitro and in vivo proliferation assay in COS-1 cells and in Tg(krt14:MEK2S219D-GFP) transgenic fish show significantly increased cell number and Ki-67 signaling.

Conclusion

Our data indicate that MEK2 is sufficient to induce epidermal papilloma formation through MAPK signaling in zebrafish, and this transgenic model can be used as a new platform for drug screening.

MEK1 dependent and independent ERK activation regulates IL-10 and IL-12 production in bone marrow derived macrophages

The mitogen activated protein kinases ERK1/2 play an important role in response to toll like receptor (TLR) activation and cytokine production, including IL-10 and IL-12. Here, we examined the role of MEK1 in ERK1/2 activation in response to TLR4 agonist by using bone marrow-derived macrophages (BMDMs) from wild type (WT) and Mek1(d/d)Sox2(Cre) mice. Our data demonstrates that MEK1 is essential for ERK1/2 activation in response to LPS. Furthermore, stimulation of the TLR4 receptor of BMDMs derived from Mek1(d/d)Sox2(Cre) mice showed enhanced STAT4 phosphorylation and increased IL-12 secretion, but exhibited a significantly lower IL-10 production as compared to WT macrophages. Most interestingly, TLR ligation in the presence of recombinant IL-10 (rIL-10) or retinoic acid (RA) led to ERK1/2 activation independent of MEK1 in BMDMs derived from Mek1(d/d)Sox2(Cre) mice and led to inhibition of STAT4 and decreased IL-12 levels. Collectively, these data suggest that MEK1 is required for TLR4 mediated ERK activation and in turn regulates the production of IL-10 and IL-12. It also indicates that ERK1/2 can be activated independent of MEK1 in the presence of IL-10 and RA and this activation negatively regulates IL-12, but positively regulates IL-10 production. These findings may have significant implications for the development of drugs that modulate MEK1 activity in the treatment of inflammatory, autoimmune and proliferative diseases such as cancer.

ERK2 Mediates Metabolic Stress Response to Regulate Cell Fate

Insufficient nutrients disrupt physiological homeostasis, resulting in diseases and even death. Considering the physiological and pathological consequences of this metabolic stress, the adaptive responses that cells utilize under this condition are of great interest. We show that under low-glucose conditions, cells initiate adaptation followed by apoptosis responses using PERK/Akt and MEK1/ERK2 signaling, respectively. For adaptation, cells engage the ER stress-induced unfolded protein response, which results in PERK/Akt activation and cell survival. Sustained and extreme energetic stress promotes a switch to isoform-specific MEK1/ERK2 signaling, induction of GCN2/eIF2α phosphorylation, and ATF4 expression, which overrides PERK/Akt-mediated adaptation and induces apoptosis through ATF4-dependent expression of pro-apoptotic factors including Bid and Trb3. ERK2 activation during metabolic stress contributes to changes in TCA cycle and amino acid metabolism, and cell death, which is suppressed by glutamate and α-ketoglutarate supplementation. Taken together, our results reveal promising targets to protect cells or tissues from metabolic stress.

ERK1/2 can feedback-regulate cellular MEK1/2 levels

Signal transduction of the Raf/MEK/ERK pathway is regulated by various feedback mechanisms. Given the greater molar ratio between Raf-MEK than between MEK-ERK in cells, it may be possible that MEK1/2 levels are regulated to modulate Raf/MEK/ERK activity upon pathway stimulation. Nevertheless, it has not been reported whether MEK1/2 expression can be subject to a feedback regulation. Here, we report that the Raf/MEK/ERK pathway can feedback-regulate cellular MEK1 and MEK2 levels. In different cell types, ΔRaf-1:ER- or B-Raf(V600E)-mediated MEK/ERK activation increased MEK1 but decreased MEK2 levels. These regulations were abrogated by ERK1/2 knockdown mediated by RNA interference, suggesting the presence of a feedback mechanism that regulates MEK1/2 levels. Subsequently, analyses using qPCR and luciferase reporters of the DNA promoter and 3' untranslated region revealed that the feedback MEK1 upregulation was in part attributed to increased transcription. However, the feedback MEK2 downregulation was only observed at protein levels, which was blocked by the proteasome inhibitors, MG132 and bortezomib, suggesting that the MEK2 regulation is mediated at a post-translational level. These results suggest that the Raf/MEK/ERK pathway can feedback-regulate cellular levels of MEK1 and MEK2, wherein MEK1 levels are upregulated at transcriptional level whereas MEK2 levels are downregulated at posttranslational level.

In vivo gene manipulation reveals the impact of stress-responsive MAPK pathways on tumor progression

It has been widely accepted that tumor cells and normal stromal cells in the host environment coordinately modulate tumor progression. Mitogen-activated protein kinase pathways are the representative stress-responsive cascades that exert proper cellular responses to divergent environmental stimuli. Genetically engineered mouse models and chemically induced tumorigenesis models have revealed that components of the MAPK pathway not only regulate the behavior of tumor cells themselves but also that of surrounding normal stromal cells in the host environment during cancer pathogenesis. The individual functions of MAPK pathway components in tumor initiation and progression vary depending on the stimuli and the stromal cell types involved in tumor progression, in addition to the molecular isoforms of the components and the origins of the tumor. Recent studies have indicated that MAPK pathway components synergize with environmental factors (e.g. tobacco smoke and diet) to affect tumor initiation and progression. Moreover, some components play distinct roles in the course of tumor progression, such as before and after the establishment of tumors. Hence, a comprehensive understanding of the multifaceted functions of MAPK pathway components in tumor initiation and progression is essential for the improvement of cancer therapy. In this review, we focus on the reports that utilized knockout, conditional knockout, and transgenic mice of MAPK pathway components to investigate the effects of MAPK pathway components on tumor initiation and progression in the host environment.

Modeling cutaneous squamous carcinoma development in the mouse

Cutaneous squamous cell carcinoma (SCC) is one of the most common cancers in Caucasian populations and is associated with a significant risk of morbidity and mortality. The classic mouse model for studying SCC involves two-stage chemical carcinogenesis, which has been instrumental in the evolution of the concept of multistage carcinogenesis, as widely applied to both human and mouse cancers. Much is now known about the sequence of biological and genetic events that occur in this skin carcinogenesis model and the factors that can influence the course of tumor development, such as perturbations in the oncogene/tumor-suppressor signaling pathways involved, the nature of the target cell that acquires the first genetic hit, and the role of inflammation. Increasingly, studies of tumor-initiating cells, malignant progression, and metastasis in mouse skin cancer models will have the potential to inform future approaches to treatment and chemoprevention of human squamous malignancies.

MEK1/2 overactivation can promote growth arrest by mediating ERK1/2-dependent phosphorylation of p70S6K

The extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein (MAP) kinase pathway has been involved in the positive and negative regulation of cell proliferation. Upon mitogen stimulation, ERK1/ERK2 activation is necessary for G1- to S-phase progression whereas when hyperactived, this pathway could elicit cell cycle arrest. The mechanisms involved are not fully elucidated but a kinase-independent function of ERK1/2 has been evidenced in the MAPK-induced growth arrest. Here, we show that p70S6K, a central regulator of protein biosynthesis, is essential for the cell cycle arrest induced by overactivation of ERK1/2. Indeed, whereas MEK1 silencing inhibits cell cycle progression, we demonstrate that active mutant form of MEK1 or MEK2 triggers a G1 phase arrest by stimulating an activation of p70S6K by ERK1/2 kinases. Silencing of ERK1/2 activity by shRNA efficiently suppresses p70S6K phosphorylation on Thr421/Ser424 and S6 phosphorylation on Ser240/244 as well as p21 expression, but these effects can be partially reversed by the expression of kinase-dead mutant form of ERK1 or ERK2. In addition, we demonstrate that the kinase p70S6K modulates neither the p21 gene transcription nor the stability of the protein but enhances the translation of the p21 mRNA. In conclusion, our data emphasizes the importance of the translational regulation of p21 by the MEK1/2-ERK1/2-p70S6K pathway to negatively control the cell cycle progression.

Growth arrest signaling of the Raf/MEK/ERK pathway in cancer

The Raf/MEK/extracellular signal-regulated kinase (ERK) pathway has a pivotal role in facilitating cell proliferation, and its deregulated activation is a central signature of many epithelial cancers. However paradoxically, sustained activity of Raf/MEK/ERK can also result in growth arrest in many different cell types. This anti-proliferative Raf/MEK/ERK signaling also has physiological significance, as exemplified by its potential as a tumor suppressive mechanism. Therefore, significant questions include in which cell types and by what mechanisms this pathway can mediate such an opposing context of signaling. Particularly, our understating of the role of ERK1 and ERK2, the focal points of pathway signaling, in growth arrest signaling is still limited. This review discusses these aspects of Raf/MEK/ERK-mediated growth arrest signaling.

MEK1 and MEK2 differentially control the duration and amplitude of the ERK cascade response

The Raf/MEK/ERK cascade is one of the most studied and important signal transduction pathways. However, existing models largely ignore the existence of isoforms of the constituent kinases and their interactions. Here, we propose a model of the ERK cascade that includes heretofore neglected differences between isoforms of MEK. In particular, MEK1 is subject to a negative feedback from activated ERK, which is further conferred to MEK2 via hetero-dimerization. Specifically, ERK phosphorylates MEK1 at the residue Thr292, hypothetically creating an additional phosphatase binding site, accelerating MEK1 and MEK2 dephosphorylation. We incorporated these recently discovered interactions into a mathematical model of the ERK cascade that reproduces the experimental results of Catalanotti et al (2009 Nature Struct. Mol. Biol. 16 294-303) and Kamioka et al (2010 J. Biol. Chem. 285 33540-8). Furthermore, the model allows for predictions regarding the differences in the catalytic activity and function of the MEK isoforms. We propose that the MEK1/MEK2 ratio regulates the duration of the response, which increases with the level of MEK2 and decreases with the level of MEK1. In turn, the amplitude of the response is controlled by the total amount of the two isoforms. We confirm the proposed model structure performing a random parameter sampling, which led us to the conclusion that the sampled parameters, selected to properly reproduce wild-type (WT) cell behavior, to allow for qualitative reproduction of differences in behavior WT cells and cell mutants studied experimentally.

MAPK signaling in cisplatin-induced death: predominant role of ERK1 over ERK2 in human hepatocellular carcinoma cells

Hepatocellular carcinoma treatment by arterial infusion of cis-diamminedichloroplatinum-II (cisplatin) exhibits certain therapeutic efficacy. However, optimizations are required and the mechanisms underlying cisplatin proapoptotic effect remain unclear. The mitogen-activated protein kinase (MAPK) pathway plays a key role in cell response to cisplatin and the functional specificity of the isoform MAPK/ERK kinase 1 and 2 (MEK1/2) and ERK1/2 could influence this response. The individual contribution of each kinase on cisplatin-induced death was thus analyzed after a transient or stable specific inhibition by RNA interference in the human hepatocellular carcinoma cells Huh-7 or in knockout mice. We demonstrated here that ERK1 played a predominant role over ERK2 in cisplatin-induced death, whereas MEK1 and MEK2 acted in a redundant manner. Indeed, at clinically relevant concentrations of cisplatin, ERK1 silencing alone was sufficient to protect cells from cisplatin-induced death both in vitro, in Huh-7 cells and ERK1(-/-) hepatocytes, and in vivo, in ERK1-deficient mice. Moreover, we showed that ERK1 activity correlated with the induction level of the proapoptotic BH3-only protein Noxa, a critical mediator of cisplatin toxicity. On the contrary, ERK2 inhibition upregulated ERK1 activity, favored Noxa induction and sensitized hepatocarcinoma cells to cisplatin. Our results point to a crucial role of ERK1 in cisplatin-induced proapoptotic signal and lead us to propose that ERK2-specific targeting could improve the efficacy of cisplatin therapy by increasing ERK1 prodeath functions.

p21-Activated kinase 1 is required for efficient tumor formation and progression in a Ras-mediated skin cancer model

The RAS genes are the most commonly mutated oncogenes in human cancer and present a particular therapeutic dilemma, as direct targeting of Ras proteins by small molecules has proved difficult. Signaling pathways downstream of Ras, in particular Raf/Mek/Erk and PI3K/Akt/mTOR, are dominated by lipid and protein kinases that provide attractive alternate targets in Ras-driven tumors. As p21-activated kinase 1 (Pak1) has been shown to regulate both these signaling pathways and is itself upregulated in many human cancers, we assessed the role of Pak1 in Ras-driven skin cancer. In human squamous cell carcinoma (SCC), we found a strong positive correlation between advanced stage and grade and PAK1 expression. Using a mouse model of Kras-driven SCC, we showed that deletion of the mouse Pak1 gene led to markedly decreased tumorigenesis and progression, accompanied by near total loss of Erk and Akt activity. Treatment of Kras(G12D) mice with either of two distinct small molecule Pak inhibitors (PF3758309 and FRAX597) caused tumor regression and loss of Erk and Akt activity. Tumor regression was also seen in mice treated with a specific Mek inhibitor, but not with an Akt inhibitor. These findings establish Pak1 as a new target in KRAS-driven tumors and suggest a mechanism of action through the Erk, but not the Akt, signaling pathway.

MEK genomics in development and disease

The mitogen-activated protein kinase kinases (the MAPK/ERK kinases; MKKs or MEKs) and their downstream substrates, the extracellular-regulated kinases have been intensively studied for their roles in development and disease. Until recently, it had been assumed any mutation affecting their function would have lethal consequences. However, the identification of MEK1 and MEK2 mutations in developmental syndromes as well as chemotherapy-resistant tumors, and the discovery of genomic variants in MEK1 and MEK2 have led to the realization the extent of genomic variation associated with MEKs is much greater than had been appreciated. In this review, we will discuss these recent advances, relating them to what is currently understood about the structure and function of MEKs, and describe how they change our understanding of the role of MEKs in development and disease.

Essential, non-redundant roles of B-Raf and Raf-1 in Ras-driven skin tumorigenesis

Ras-driven tumorigenesis is assumed to depend on Raf for ERK activation and proliferation; yet, an in vivo requirement for Raf as MEK/ERK activator in this setting has not been demonstrated to date. Here, we show that epidermis-restricted B-Raf ablation restrains the onset and stops the progression of established Ras-driven tumors by limiting MEK/ERK activation and proliferation. Concomitant elimination of B-Raf and Raf-1 enforces the abrupt regression of established tumors owing to the decrease in ERK activation and proliferation caused by B-Raf ablation combined with the ERK-independent increase in Rho-dependent kinase (Rok) signaling and differentiation triggered by Raf-1 inactivation. Thus, B-Raf and Raf-1 have non-redundant functions in Ras-driven tumorigenesis. Of note, Raf kinase inhibitors achieve impressive results in melanomas harboring oncogenic BRAF, but are ineffective against Ras-driven tumors; moreover, therapy-related skin tumors driven by a paradox ERK activation as well as primary and acquired resistance have been reported. Our results suggest that therapies targeting both Raf kinase-dependent and -independent pathways may be effective against a broader range of malignancies and reduce the risks of adverse effects and/or resistance.

JWA deficiency suppresses dimethylbenz[a]anthracene-phorbol ester induced skin papillomas via inactivation of MAPK pathway in mice

Our previous studies indicated that JWA plays an important role in DNA damage repair, cell migration, and regulation of MAPKs. In this study, we investigated the role of JWA in chemical carcinogenesis using conditional JWA knockout (JWA^Δ2/Δ2) mice and two-stage model of skin carcinogenesis. Our results indicated that JWA^Δ2/Δ2 mice were resistant to the development of skin papillomas initiated by 7, 12-dimethylbenz(a)anthracene (DMBA) followed by promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA). In JWA^Δ2/Δ2 mice, the induction of papilloma was delayed, and the tumor number and size were reduced. In primary keratinocytes from JWA^Δ2/Δ2 mice, DMBA exposure induced more intensive DNA damage, while TPA-promoted cell proliferation was reduced. The further mechanistic studies showed that JWA deficiency blocked TPA-induced activation of MAPKs and its downstream transcription factor Elk1 both in vitro and in vivo. JWA^Δ2/Δ2 mice are resistance to tumorigenesis induced by DMBA/TPA probably through inhibition of transcription factor Elk1 via MAPKs. These results highlight the importance of JWA in skin homeostasis and in the process of skin tumor development.

Suppression of Ras/Mapk pathway signaling inhibits Myc-induced lymphomagenesis

Although the Myc transcription factor has been shown necessary for the oncogenic function of Ras, the contribution of Ras pathway signaling to the oncogenic function of Myc remains unresolved. We report the novel findings that Myc alone induced Ras/Mapk pathway signaling, and increased signaling following growth factor stimulation. Deletion of the scaffold protein kinase suppressor of Ras 1 (Ksr1) attenuated signaling through the Ras/Mapk pathway, including activation following Myc induction. B cells that lacked Ksr1 exhibited reduced proliferation and increased cytokine deprivation-induced apoptosis. Overexpression of Myc rescued the proliferation defect of Ksr1-null B cells, but loss of Ksr1 increased sensitivity of B cells to Myc-induced apoptosis. Notably, there was a significant delay in lymphoma development in Ksr1-null mice overexpressing Myc in B cells (Eμ-myc transgenic mice). There was an elevated frequency of p53 inactivation, indicative of increased selective pressure to bypass the p53 tumor suppressor pathway, in Ksr1-null Eμ-myc lymphomas. Therefore, loss of Ksr1 inhibits Ras/Mapk pathway signaling leading to increased Myc-induced B-cell apoptosis, and this results in reduced B-cell transformation and lymphoma development. Our data indicate that suppression of Myc-induced Ras/Mapk pathway signaling significantly impairs Myc oncogenic function. These results fill a significant gap in knowledge about Myc and should open new avenues of therapeutic intervention for Myc-overexpressing malignancies.

Exploiting the origins of Ras mediated squamous cell carcinoma to develop novel therapeutic interventions

The small GTPase Ras is activated in a high proportion of human cancers. Attempts to clinically block Ras activity through pharmacological means has proven largely ineffective thus far. We employed an inducible mouse model of squamous cell carcinoma (SCC) to study the effect of Ras activation and show that hair follicle stem cells (HFSCs) are a cell of origin for SCC, whereas their more restricted progeny cannot serve as cancer cells of origin and are refractory to Ras activation. We propose that by identifying the unique mechanisms by which HFSCs are mobilized to initiate Ras mediated tumorigenesis, the molecular process behind SCC can be more completely elucidated and context dependent activities for Ras more clearly defined. Here, we summarize our recent results and point to future experiments designed to create novel therapeutics by exploiting the differential sensitivities of various cells within the epidermis to Ras activation.

Novel mitogen-activated protein kinase kinase inhibitors

INTRODUCTION

the development of new drugs over the last few decades has targeted specific proteins thought to be a key to the disease state. MAPK kinases 1 and 2 (commonly known as MEK1-2) represent such proteins as they lie downstream of important drug targets for oncology, such as EGFR, RAS and RAF. Several MEK1-2 inhibitors are currently in Phase I and II clinical trials in oncology.

AREAS COVERED

this review of current literature and recent conferences provides a background on the RAS-RAF-MEK-ERK signaling pathway and a discussion of early MEK inhibitors. The potential of MEK1-2 inhibitors for the treatment of inflammation is briefly presented. Preclinical and early clinical results are discussed for MEK inhibitors currently in development. Completed clinical trials of MEK inhibitors in oncology include some disappointments as well as some promising signs of the value of these compounds and we discuss the potential for MEK inhibitors as monotherapy and their use in drug combinations.

EXPERT OPINION

the utility of MEK inhibitors as anticancer agents will depend on careful patient selection based on the presence of mutations in genes such as KRAS and BRAF, the identification of additional predictive biomarkers, and an improved understanding of the benefit of drug combinations utilizing both established and emerging therapeutics.

c-Raf, but not B-Raf, is essential for development of K-Ras oncogene-driven non-small cell lung carcinoma

We have investigated the role of individual members of the Raf/Mek/Erk cascade in the onset of K-Ras oncogene-driven non-small cell lung carcinoma (NSCLC). Ablation of Erk1 or Erk2 in K-Ras oncogene-expressing lung cells had no significant effect due to compensatory activities. Yet, elimination of both Erk kinases completely blocked tumor development. Similar results were obtained with Mek kinases. Ablation of B-Raf had no significant effect on tumor development. However, c-Raf expression was absolutely essential for the onset of NSCLC. Interestingly, concomitant elimination of c-Raf and B-Raf in adult mice had no deleterious consequences for normal homeostasis. These results indicate that c-Raf plays a unique role in mediating K-Ras signaling and makes it a suitable target for therapeutic intervention.

Defining the origins of Ras/p53-mediated squamous cell carcinoma

The precise identity of cancer cells of origin and the molecular events of tumor initiation in epidermal squamous cell carcinoma (SCC) are unknown. Here we show that malignancy potential is related to the developmental capacity of the initiating cancer cell in a genetically defined, intact, and inducible in vivo model. Specifically, these data demonstrate that SCCs can originate from inside the hair follicle stem cell (SC) niche or from immediate progenitors, whereas more developmentally restricted progeny, the transit amplifying (TA) cells, are unable to generate even benign tumors in the same genetic context. Using a temporal model of tumorigenesis in situ, we highlight the phenotypes of cancer progression from the hair follicle SC niche, including hyperplasia, epithelial to mesenchymal transition, and SCC formation. Furthermore, we provide insights into the inability of hair follicle TA cells to respond to tumorigenic stimuli.

Carcinogen-induced squamous papillomas and oncogenic progression in the absence of the SSeCKS/AKAP12 metastasis suppressor correlate with FAK upregulation

The ability of SSeCKS/Gravin/AKAP12 (SSeCKS) to negatively regulate cell cycle progression is thought to relate to its spatiotemporal scaffolding activity for key signaling molecules such as protein kinase A and C, calmodulin and cyclins. SSeCKS is downregulated upon progression to malignancy in many cancer types, including melanoma and nonmelanoma skin cancer. The forced re-expression of SSeCKS is especially potent in suppressing metastasis through the inhibition of VEGF-mediated neovascularization. We have previously shown that SSeCKS-null (KO) mice exhibit hyperplasia and focal dysplasia in the prostate marked by activated Akt. To address whether KO mice exhibit increased skin carcinogenesis, WT and KO C57BL/6 mice were treated topically with 12-O-tetradecanoylphorbol-13-acetate and 7,12-dimethylbenzanthracene. Compared to WT mice, KO mice developed squamous papillomas more rapidly and in greater numbers and also exhibited significantly increased progression to squamous cell carcinoma. Untreated KO epidermal layers were thicker than those in age-matched WT mice and exhibited significantly increased levels of FAK and phospho-ERK1/2, known mediators of carcinogen-induced squamous papilloma progression to carcinoma. Compared to protein levels in WT mouse embryo fibroblasts (MEF), SSeCKS levels were increased in FAK-null cells, whereas FAK levels were increased in SSeCKS-null cells. RNAi studies in WT MEF cells suggest that SSeCKS and FAK attenuate each other's expression. Our study implicates a role for SSeCKS in preventing of skin cancer progression possibly through negatively regulating FAK expression.

ATM regulates a DNA damage response posttranscriptional RNA operon in lymphocytes

Maintenance of genomic stability depends on the DNA damage response, a biologic barrier in early stages of cancer development. Failure of this response results in genomic instability and high predisposition toward lymphoma, as seen in patients with ataxia-telangiectasia mutated (ATM) dysfunction. ATM activates multiple cell-cycle checkpoints and DNA repair after DNA damage, but its influence on posttranscriptional gene expression has not been examined on a global level. We show that ionizing radiation modulates the dynamic association of the RNA-binding protein HuR with target mRNAs in an ATM-dependent manner, potentially coordinating the genotoxic response as an RNA operon. Pharmacologic ATM inhibition and use of ATM-null cells revealed a critical role for ATM in this process. Numerous mRNAs encoding cancer-related proteins were differentially associated with HuR depending on the functional state of ATM, in turn affecting expression of encoded proteins. The findings presented here reveal a previously unidentified role of ATM in controlling gene expression posttranscriptionally. Dysregulation of this DNA damage response RNA operon is probably relevant to lymphoma development in ataxia-telangiectasia persons. These novel RNA regulatory modules and genetic networks provide critical insight into the function of ATM in oncogenesis.

MEK2 is sufficient but not necessary for proliferation and anchorage-independent growth of SK-MEL-28 melanoma cells

Mitogen-activated protein kinase kinases (MKK or MEK) 1 and 2 are usually treated as redundant kinases. However, in assessing their relative contribution towards ERK-mediated biologic response investigators have relied on tests of necessity, not sufficiency. In response we developed a novel experimental model using lethal toxin (LeTx), an anthrax toxin-derived pan-MKK protease, and genetically engineered protease resistant MKK mutants (MKKcr) to test the sufficiency of MEK signaling in melanoma SK-MEL-28 cells. Surprisingly, ERK activity persisted in LeTx-treated cells expressing MEK2cr but not MEK1cr. Microarray analysis revealed non-overlapping downstream transcriptional targets of MEK1 and MEK2, and indicated a substantial rescue effect of MEK2cr on proliferation pathways. Furthermore, LeTx efficiently inhibited the cell proliferation and anchorage-independent growth of SK-MEL-28 cells expressing MKK1cr but not MEK2cr. These results indicate in SK-MEL-28 cells MEK1 and MEK2 signaling pathways are not redundant and interchangeable for cell proliferation. We conclude that in the absence of other MKK, MEK2 is sufficient for SK-MEL-28 cell proliferation. MEK1 conditionally compensates for loss of MEK2 only in the presence of other MKK.

Ras and Raf pathways in epidermis development and carcinogenesis

The epidermis is the outermost layer of the body and protects it from environmental insults. This crucial function is sustained by a continuous process of self-renewal involving the carefully balanced proliferation and differentiation of progenitor cells constantly replacing the mature cells at the surface of the epidermis. Genetic changes in the signalling pathways controlling keratinocyte proliferation and differentiation disrupt this balance and lead to pathological changes including carcinogenesis. This review discusses the role of Ras, an oncogene critically involved in the development of skin neoplasia, and its downstream effector Raf in epidermal homeostasis and tumourigenesis. In particular, we will focus on the recently established role of Raf-1 as the decisive element that, by restraining keratinocyte differentiation, allows the development and maintenance of Ras-driven tumours.

Growth factor signaling pathways as targets for prevention of epithelial carcinogenesis

Growth factor receptor (GFR) signaling controls epithelial cell growth by responding to various endogenous or exogenous stimuli and subsequently activating downstream signaling pathways including Stat3, PI3K/Akt/mTOR, MAPK, and c-Src. Environmental chemical toxicants and UVB irradiation cause enhanced and prolonged activation of GFR signaling and downstream pathways that contributes to epithelial cancer development including skin cancer. Recent studies, especially those with tissue-specific transgenic mouse models, have demonstrated that GFRs and their downstream signaling pathways contribute to all three stages of epithelial carcinogenesis by regulating a wide variety of biological functions including proliferation, apoptosis, angiogenesis, cell adhesion, and migration. Inhibiting these signaling pathways early in the carcinogenic process results in reduced cell proliferation and survival, leading to decreased tumor formation. Collectively, these studies suggest that GFR signaling and subsequent downstream signaling pathways are potential targets for the prevention of epithelial cancers including skin cancer.

MAPK pathway activation delays G2/M progression by destabilizing Cdc25B

Activation of the mitogen-activated protein kinase (MAPK) pathway by growth factors or phorbol esters during G(2) phase delays entry into mitosis; however, the role of the MAPK pathway during G(2)/M progression remains controversial. Here, we demonstrate that activation of the MAPK pathway with either epidermal growth factor or 12-O-tetradecanoylphorbol-13-acetate induces a G(2) phase delay independent of known G(2) phase checkpoint pathways but was specifically dependent on MAPK/extracellular signal-regulated kinase kinase (MEK1). Activation of MAPK signaling also blocked exit from a G(2) phase checkpoint arrest. Both the G(2) phase delay and blocked exit from the G(2) checkpoint arrest were mediated by the MEK1-dependent destabilization of the critical G(2)/M regulator cdc25B. Reintroduction of cdc25B overcame the MEK1-dependent G(2) phase delay. Thus, we have demonstrated a new function for MEK1 that controls G(2)/M progression by regulating the stability of cdc25B. This represents a novel mechanism by which factors that activate MAPK signaling can influence the timing of entry into mitosis, particularly exit from a G(2) phase checkpoint arrest.