Extracellular signal-regulated kinase 1/2 activity is not required in mammalian cells during late G2 for timely entry into or exit from mitosis

The role of RHAMM in cancer: Exposing novel therapeutic vulnerabilities

Receptor for hyaluronic acid-mediated motility (RHAMM) is a cell surface receptor for hyaluronic acid that is critical for cell migration and a cell cycle protein involved in microtubule assembly and stability. These functions of RHAMM are required for cellular stress responses and cell cycle progression but are also exploited by tumor cells for malignant progression and metastasis. RHAMM is often overexpressed in tumors and is an independent adverse prognostic factor for a number of cancers such as breast and prostate. Interestingly, pharmacological or genetic inhibition of RHAMM in vitro and in vivo ablates tumor invasiveness and metastatic spread, implicating RHAMM as a potential therapeutic target to restrict tumor growth and improve patient survival. However, RHAMM’s pro-tumor activity is dependent on its subcellular distribution, which complicates the design of RHAMM-directed therapies. An alternative approach is to identify downstream signaling pathways that mediate RHAMM-promoted tumor aggressiveness. Herein, we discuss the pro-tumoral roles of RHAMM and elucidate the corresponding regulators and signaling pathways mediating RHAMM downstream events, with a specific focus on strategies to target the RHAMM signaling network in cancer cells.

DUSP7 regulates the activity of ERK2 to promote proper chromosome alignment during cell division

Human cell division is a highly regulated process that relies on the accurate capture and movement of chromosomes to the metaphase plate. Errors in the fidelity of chromosome congression and alignment can lead to improper chromosome segregation, which is correlated with aneuploidy and tumorigenesis. These processes are known to be regulated by extracellular signal-regulated kinase 2 (ERK2) in other species, but the role of ERK2 in mitosis in mammals remains unclear. Here, we have identified the dual-specificity phosphatase 7 (DUSP7), known to display selectivity for ERK2, as important in regulating chromosome alignment. During mitosis, DUSP7 bound to ERK2 and regulated the abundance of active phospho-ERK2 through its phosphatase activity. Overexpression of DUSP7, but not catalytically inactive mutants, led to a decrease in the levels of phospho-ERK2 and mitotic chromosome misalignment, while knockdown of DUSP7 also led to defective chromosome congression that resulted in a prolonged mitosis. Consistently, knockdown or chemical inhibition of ERK2 or chemical inhibition of the MEK kinase that phosphorylates ERK2 led to chromosome alignment defects. Our results support a model wherein MEK-mediated phosphorylation and DUSP7-mediated dephosphorylation regulate the levels of active phospho-ERK2 to promote proper cell division.

DUSP3/VHR: A Druggable Dual Phosphatase for Human Diseases

Protein tyrosine kinases (PTK), discovered in the 1970s, have been considered master regulators of biological processes with high clinical significance as targets for human diseases. Their actions are countered by protein tyrosine phosphatases (PTP), enzymes yet underrepresented as drug targets because of the high homology of their catalytic domains and high charge of their catalytic pocket. This scenario is still worse for some PTP subclasses, for example, for the atypical dual-specificity phosphatases (ADUSPs), whose biological functions are not even completely known. In this sense, the present work focuses on the dual-specificity phosphatase 3 (DUSP3), also known as VH1-related phosphatase (VHR), an uncommon regulator of mitogen-activated protein kinase (MAPK) phosphorylation. DUSP3 expression and activities are suggestive of a tumor suppressor or tumor-promoting enzyme in different types of human cancers. Furthermore, DUSP3 has other biological functions involving immune response mediation, thrombosis, hemostasis, angiogenesis, and genomic stability that occur through either MAPK-dependent or MAPK-independent mechanisms. This broad spectrum of actions is likely due to the large substrate diversity and molecular mechanisms that are still under scrutiny. The growing advances in characterizing new DUSP3 substrates will allow the development of pharmacological inhibitors relevant for possible future clinical trials. This review covers all aspects of DUSP3, since its gene cloning and crystallographic structure resolution, in addition to its classical and novel substrates and the biological processes involved, followed by an update of what is currently known about the DUSP3/VHR-inhibiting compounds that might be considered potential drugs to treat human diseases.

Reduced levels of Dusp3/Vhr phosphatase impair normal spindle bipolarity in an Erk1/2 activity-dependent manner

Dual specificity phosphatase-3 (Dusp3/Vhr) regulates cell cycle progression by counteracting the effects of mitogen-activated protein kinases (Mapk) Erk1/2 and Jnk. Despite the known upregulation of Dusp3 at M phase in mammalian cells, its mitotic functions are poorly characterized. Here, we report that loss of Dusp3 by RNAi leads to the formation of multipolar spindles in human mitotic cancer cells in an Erk1/2-dependent manner. In the phosphatase-silenced cells, the normal bipolar spindle structure was restored by chemical inhibition of Erk1/2 and ectopic overexpression of Dusp3. We propose that at M phase Dusp3 keeps Erk1/2 activity in check to facilitate normal mitosis.

Germline variants in the SEMA4A gene predispose to familial colorectal cancer type X

Familial colorectal cancer type X (FCCTX) is characterized by clinical features of hereditary non-polyposis colorectal cancer with a yet undefined genetic background. Here we identify the SEMA4A p.Val78Met germline mutation in an Austrian kindred with FCCTX, using an integrative genomics strategy. Compared with wild-type protein, SEMA4A(V78M) demonstrates significantly increased MAPK/Erk and PI3K/Akt signalling as well as cell cycle progression of SEMA4A-deficient HCT-116 colorectal cancer cells. In a cohort of 53 patients with FCCTX, we depict two further SEMA4A mutations, p.Gly484Ala and p.Ser326Phe and the single-nucleotide polymorphism (SNP) p.Pro682Ser. This SNP is highly associated with the FCCTX phenotype exhibiting increased risk for colorectal cancer (OR 6.79, 95% CI 2.63 to 17.52). Our study shows previously unidentified germline variants in SEMA4A predisposing to FCCTX, which has implications for surveillance strategies of patients and their families.

Mitogen-activated protein kinase (MAPK) pathway regulates branching by remodeling epithelial cell adhesion

Although the growth factor (GF) signaling guiding renal branching is well characterized, the intracellular cascades mediating GF functions are poorly understood. We studied mitogen-activated protein kinase (MAPK) pathway specifically in the branching epithelia of developing kidney by genetically abrogating the pathway activity in mice lacking simultaneously dual-specificity protein kinases Mek1 and Mek2. Our data show that MAPK pathway is heterogeneously activated in the subset of G1- and S-phase epithelial cells, and its tissue-specific deletion results in severe renal hypodysplasia. Consequently to the deletion of Mek1/2, the activation of ERK1/2 in the epithelium is lost and normal branching pattern in mutant kidneys is substituted with elongation-only phenotype, in which the epithelium is largely unable to form novel branches and complex three-dimensional patterns, but able to grow without primary defects in mitosis. Cellular characterization of double mutant epithelium showed increased E-cadherin at the cell surfaces with its particular accumulation at baso-lateral locations. This indicates changes in cellular adhesion, which were revealed by electron microscopic analysis demonstrating intercellular gaps and increased extracellular space in double mutant epithelium. When challenged to form monolayer cultures, the mutant epithelial cells were impaired in spreading and displayed strong focal adhesions in addition to spiky E-cadherin. Inhibition of MAPK activity reduced paxillin phosphorylation on serine 83 while remnants of phospho-paxillin, together with another focal adhesion (FA) protein vinculin, were augmented at cell surface contacts. We show that MAPK activity is required for branching morphogenesis, and propose that it promotes cell cycle progression and higher cellular motility through remodeling of cellular adhesions.

Epstein-Barr virus-encoded latent membrane protein 1 impairs G2 checkpoint in human nasopharyngeal epithelial cells through defective Chk1 activation

Nasopharyngeal carcinoma (NPC) is a common cancer in Southeast Asia, particularly in southern regions of China. EBV infection is closely associated with NPC and has long been postulated to play an etiological role in the development of NPC. However, the role of EBV in malignant transformation of nasopharyngeal epithelial cells remains enigmatic. The current hypothesis of NPC development is that premalignant nasopharyngeal epithelial cells harboring genetic alterations support EBV infection and expression of EBV genes induces further genomic instability to facilitate the development of NPC. The latent membrane protein 1 (LMP1) is a well-documented EBV-encoded oncogene. The involvement of LMP1 in human epithelial malignancies has been implicated, but the mechanisms of oncogenic actions of LMP1, particularly in nasopharyngeal cells, are unclear. Here we observed that LMP1 expression in nasopharyngeal epithelial cells impaired G2 checkpoint, leading to formation of unrepaired chromatid breaks in metaphases after γ-ray irradiation. We further found that defective Chk1 activation was involved in the induction of G2 checkpoint defect in LMP1-expressing nasopharyngeal epithelial cells. Impairment of G2 checkpoint could result in loss of the acentrically broken chromatids and propagation of broken centric chromatids in daughter cells exiting mitosis, which facilitates chromosome instability. Our findings suggest that LMP1 expression facilitates genomic instability in cells under genotoxic stress. Elucidation of the mechanisms involved in LMP1-induced genomic instability in nasopharyngeal epithelial cells will shed lights on the understanding of role of EBV infection in NPC development.

Mitosis in vertebrates: the G2/M and M/A transitions and their associated checkpoints

In this review, I stress the importance of direct data and accurate terminology when formulating and communicating conclusions on how the G2/M and metaphase/anaphase transitions are regulated. I argue that entry into mitosis (i.e., the G2/M transition) is guarded by several checkpoint control pathways that lose their ability to delay or stop further cell cycle progression once the cell becomes committed to divide, which in vertebrates occurs in the late stages of chromosome condensation. After this commitment, progress through mitosis is then mediated by a single Mad/Bub-based checkpoint that delays chromatid separation, and exit from mitosis (i.e., completion of the cell cycle) in the presence of unattached kinetochores. When cells cannot satisfy the mitotic checkpoint, e.g., when in concentrations of spindle poisons that prohibit the stable attachment of all kinetochores, they are delayed in mitosis for many hours. In normal cells, the duration of this delay depends on the organism and ranges from ∼4 h in rodents to ∼22 h in humans. Recent live cell studies reveal that under this condition, many cancer cells (including HeLa and U2OS) die in mitosis by apoptosis within ∼24 h, which implies that biochemical studies on cancer cell populations harvested in mitosis after a prolonged mitotic arrest are contaminated with dead or dying cells.

Mechanisms of aneuploidy induction by RAS and RAF oncogenes

Most cancers progress with the accumulation of genetic mutations with time and this is frequently associated with the acquisition of genomic instability in the form of whole chromosome changes, chromosomal rearrangements, gene amplifications or smaller changes at the nucleotide level. Whole chromosome instability (W-CIN), characterised by aneuploidy, is a major form of genomic instability observed in human cancers and several lines of evidence now support the argument that W-CIN is a promoter of tumourigenesis rather than being a passenger event. The primary mechanism proposed for evolution of CIN is abnormalities in mitosis/cytokinesis. However, mutations in genes directly involved in controlling mitosis/cytokinesis are rare in human cancers and so the mechanisms underpinning the evolution of CIN in cancers are not currently clear. On the other hand, mutations in RAS or BRAF are frequently found in human cancers, many of which demonstrate CIN, suggesting a possible link between deregulated signaling through the RAS/RAF/MEK/ERK pathway and CIN. In this review, we focus on a potential relationship between deregulated RAS/RAF signaling and CIN, and discuss possible mechanisms connecting the two.

Extracellular-regulated kinase-mitogen-activated protein kinase cascade: unsolved issues

This review point out several aspects regarding the mitogen-activated protein kinase (MAPK)/extracellular-regulated kinase (Erk) network, which are still pending issues in the understanding how this pathway integrate information to drive cell fates. Focusing on the role of Erk during cell cycle, it has to be underlined that Erk downstream effectors, which are required for mitosis progression and contribute to aneuploidy during tumorigenesis, remain to be determined. In addition to the identity of the terminal enzymes or effectors of Erk, it has to be stressed that the dynamic nature of the Erk signal is itself a key factor in cell phenotype decisions. Development of biophotonics strategies for monitoring the Erk network at the spatiotemporal level in living cells, as well as computational and hypothesis-driven approaches, are called to unravel the principles by which signaling networks create biochemical and biological specificities. Finally, Erk dynamics might also be impacted by other post-translational modification than phosphorylation, such as O-GlcNAcylation.

RHAMM promotes interphase microtubule instability and mitotic spindle integrity through MEK1/ERK1/2 activity

An oncogenic form of RHAMM (receptor for hyaluronan-mediated motility, mouse, amino acids 163-794 termed RHAMM(Delta163)) is a cell surface hyaluronan receptor and mitotic spindle protein that is highly expressed in aggressive human cancers. Its regulation of mitotic spindle integrity is thought to contribute to tumor progression, but the molecular mechanisms underlying this function have not previously been defined. Here, we report that intracellular RHAMM(Delta163) modifies the stability of interphase and mitotic spindle microtubules through ERK1/2 activity. RHAMM(-/-) mouse embryonic fibroblasts exhibit strongly acetylated interphase microtubules, multi-pole mitotic spindles, aberrant chromosome segregation, and inappropriate cytokinesis during mitosis. These defects are rescued by either expression of RHAMM or mutant active MEK1. Mutational analyses show that RHAMM(Delta163) binds to alpha- and beta-tubulin protein via a carboxyl-terminal leucine zipper, but in vitro analyses indicate this interaction does not directly contribute to tubulin polymerization/stability. Co-immunoprecipitation and pulldown assays reveal complexes of RHAMM(Delta163), ERK1/2-MEK1, and alpha- and beta-tubulin and demonstrate direct binding of RHAMM(Delta163) to ERK1 via a D-site motif. In vitro kinase analyses, expression of mutant RHAMM(Delta163) defective in ERK1 binding in mouse embryonic fibroblasts, and blocking MEK1 activity collectively confirm that the effect of RHAMM(Delta163) on interphase and mitotic spindle microtubules is mediated by ERK1/2 activity. Our results suggest a model wherein intracellular RHAMM(Delta163) functions as an adaptor protein to control microtubule polymerization during interphase and mitosis as a result of localizing ERK1/2-MEK1 complexes to their tubulin-associated substrates.

Quantitative site-specific phosphorylation dynamics of human protein kinases during mitotic progression

Reversible protein phosphorylation is a key regulatory mechanism of mitotic progression. Importantly, protein kinases themselves are also regulated by phosphorylation-dephosphorylation processes; hence, phosphorylation dynamics of kinases hold a wealth of information about phosphorylation networks. Here, we investigated the site-specific phosphorylation dynamics of human kinases during mitosis using synchronization of HeLa suspension cells, kinase enrichment, and high resolution mass spectrometry. In biological triplicate analyses, we identified 206 protein kinases and more than 900 protein kinase phosphorylation sites, including 61 phosphorylation sites on activation segments, and quantified their relative abundances across three specific mitotic stages. Around 25% of the kinase phosphorylation site ratios were found to be changed by at least 50% during mitotic progression. Further network analysis of jointly regulated kinase groups suggested that Cyclin-dependent kinase- and mitogen-activated kinase-centered interaction networks are coordinately down- and up-regulated in late mitosis, respectively. Importantly, our data cover most of the already known mitotic kinases and, moreover, identify attractive candidates for future studies of phosphorylation-based mitotic signaling. Thus, the results of this study provide a valuable resource for cell biologists and provide insight into the system properties of the mitotic phosphokinome.

Mitotic division of the mammalian Golgi apparatus

Successful cell reproduction requires faithful duplication and proper segregation of cellular contents, including not only the genome but also intracellular organelles. Since the Golgi apparatus is an essential organelle of the secretory pathway, its accurate inheritance is therefore of importance to sustain cellular function. Regulation of Golgi division and its coordination with cell cycle progression involves a series of sequential events that are subjected to a precise spatiotemporal control. Here, we summarize the current knowledge about the underlying mechanisms, the molecular players and the biological relevance of this process, particularly in mammalian cells, and discuss the unsolved problems and future perspectives opened by the recent studies.

The ability to survive mitosis in the presence of microtubule poisons differs significantly between human nontransformed (RPE-1) and cancer (U2OS, HeLa) cells

We used live cell imaging to compare the fate of human nontransformed (RPE-1) and cancer (HeLa, U2OS) cells as they entered mitosis in nocodazole or taxol. In the same field, and in either drug, a cell in all lines could die in mitosis, exit mitosis and die within 10 h, or exit mitosis and survive > or =10 h. Relative to RPE-1 cells, significantly fewer HeLa or U2OS cells survived mitosis or remained viable after mitosis: in nocodazole concentrations that inhibit spindle microtubule assembly, or in 500 nM taxol, 30% and 27% of RPE-1 cells, respectively, died in or within 10 h of exiting mitosis while 90% and 49% of U2OS and 78% and 81% of HeLa died. This was even true for clinically relevant taxol concentrations (5 nM) which killed 93% and 46%, respectively, of HeLa and U2OS cells in mitosis or within 10 h of escaping mitosis, compared to 1% of RPE-1 cells. Together these data imply that studies using HeLa or U2OS cells, harvested after a prolonged block in mitosis with nocodazole or taxol, are significantly contaminated with dead or dying cells. We also found that the relationship between the duration of mitosis and survival is drug and cell type specific and that lethality is related to the cell type and drug used to prevent satisfaction of the kinetochore attachment checkpoint. Finally, work with a pan-caspase inhibitor suggests that the primary apoptotic pathway triggered by nocodazole during mitosis in RPE-1 cells is not active in U2OS cells. Cell Motil. Cytoskeleton 2008. (c) 2008 Wiley-Liss, Inc.

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.

Erk1/2 MAP kinases are required for epidermal G2/M progression

Erk1/2 mitogen-activated protein kinases (MAPKs) are often hyperactivated in human cancers, where they affect multiple processes, including proliferation. However, the effects of Erk1/2 loss in normal epithelial tissue, the setting of most extracellular signal-regulated kinase (Erk)-associated neoplasms, are unknown. In epidermis, loss of Erk1 or Erk2 individually has no effect, whereas simultaneous Erk1/2 depletion inhibits cell division, demonstrating that these MAPKs are necessary for normal tissue self-renewal. Growth inhibition caused by Erk1/2 loss is rescued by reintroducing Erk2, but not by activating Erk effectors that promote G1 cell cycle progression. Unlike fibroblasts, in which Erk1/2 loss decreases cyclin D1 expression and induces G1/S arrest, Erk1/2 loss in epithelial cells reduces cyclin B1 and c-Fos expression and induces G2/M arrest while disrupting a gene regulatory network centered on cyclin B1-Cdc2. Thus, the cell cycle stages at which Erk1/2 activity is required vary by cell type, with Erk1/2 functioning in epithelial cells to enable progression through G2/M.

Mitogen-activated protein (MAP) kinase/MAP kinase phosphatase regulation: roles in cell growth, death, and cancer

Mitogen-activated protein kinase dual-specificity phosphatase-1 (also called MKP-1, DUSP1, ERP, CL100, HVH1, PTPN10, and 3CH134) is a member of the threonine-tyrosine dual-specificity phosphatases, one of more than 100 protein tyrosine phosphatases. It was first identified approximately 20 years ago, and since that time extensive investigations into both mkp-1 mRNA and protein regulation and function in different cells, tissues, and organs have been conducted. However, no general review on the topic of MKP-1 exists. As the subject matter pertaining to MKP-1 encompasses many branches of the biomedical field, we focus on the role of this protein in cancer development and progression, highlighting the potential role of the mitogen-activated protein kinase (MAPK) family. Section II of this article elucidates the MAPK family cross-talk. Section III reviews the structure of the mkp-1 encoding gene, and the known mechanisms regulating the expression and activity of the protein. Section IV is an overview of the MAPK-specific dual-specificity phosphatases and their role in cancer. In sections V and VI, mkp-1 mRNA and protein are examined in relation to cancer biology, therapeutics, and clinical studies, including a discussion of the potential role of the MAPK family. We conclude by proposing an integrated scheme for MKP-1 and MAPK in cancer.

Increased susceptibility of breast cancer cells to stress mediated inhibition of protein synthesis

Protein synthesis is a tightly controlled process, and its deregulation plays an important role in tumorigenesis. Protein synthesis remains poorly understood with very few well-identified validated targets for therapeutic purposes. In this study, we use nitric oxide (NO), which suppresses protein synthesis by inactivating eukaryotic initiation factor 2-alpha (eIF2-alpha), to examine the mechanism by which low and high oxidative stress inhibits protein synthesis. In breast cancer cells, low NO stress induced heme-regulated inhibitor (HRI) activation, which facilitated gradual decline in short half-life proteins. High NO stress induced HRI and protein kinase R (PKR) activation, leading to a sharp decline in protein synthesis as accessed by a decline in short and long half-life proteins and dramatic morphologic changes. In contrast, human mammary epithelial (HME) and Ras transfected untransformed HME (MCF-10A1 neo N) cells were less susceptible to NO-induced inhibition of protein synthesis and cytostasis. Our results suggest that NO-induced cytostasis in breast cancer cells was due to PKR activation and increased phosphorylation of eIF2-alpha, whereas the reduced susceptibility of normal mammary epithelial cells to NO could be due to the inaccessibility of PKR, which is bound to inhibitor p58.

The G2 p38-mediated stress-activated checkpoint pathway becomes attenuated in transformed cells

When human cells are stressed during G2, they are delayed from entering mitosis via a checkpoint mediated by the p38 kinase, and this delay can be modeled by the selective activation of p38 with anisomycin. Here, we report, on the basis of live-cell studies, that 75 nM anisomycin transiently (1 hr) activates p38 which, in turn, rapidly and completely blocks entry into mitosis for at least 4 hr in all primary, telomerase- or spontaneously immortalized (p53+ and pRB+) human cells. However, the same treatment does not delay entry into mitosis in cancer cells, or the delay in entering mitosis is shortened, even though it induces a similar transient and comparable (or stronger) activation of p38. Because the primary substrate of p38, the MK2 kinase, is also transiently (1-2 hr) activated by anisomycin in both normal and cancer cells, checkpoint disruption in transformed cells occurs downstream of MK2. Finally, observations on isogenic lines reveal that the duration of the stress checkpoint is shortened in cells lacking both p53 and pRb and that the constitutive expression of an active H-Ras oncogene in these cells further attenuates the checkpoint via an ERK1/2-dependent manner. Thus, transformation leads to attenuation of the p38-mediated stress checkpoint. This outcome is likely selected for during transformation because it confers the ability to outgrow normal cells under stressful in vitro (culture) or in vivo (tumor) environments. Our data caution against using cancer cells to study how p38 produces a G2 arrest.

Targeting of dystroglycan to the cleavage furrow and midbody in cytokinesis

Dystroglycan is a cell adhesion molecule that interacts with ezrin family proteins and also components of the extracellular signal-regulated kinase pathway. Ezrin and extracellular signal-regulated kinase are both involved in aspects of the cell division cycle. We therefore examined the role of dystroglycan during cytokinesis. Endogenous dystroglycan colocalised with ezrin at the cleavage furrow and midbody during cytokinesis in REF52 cells. Live cell imaging of green fluorescent protein-tagged dystroglycan in Swiss 3T3 and Hela cells revealed a similar localisation. Live cell imaging of a dystroglycan lacking its cytoplasmic domain revealed an even membrane localisation but no cleavage furrow or midbody localisation. Deletion of a previously identified ezrin-binding site in the dystroglycan cytoplasmic domain however only resulted in a slight reduction in cleavage furrow localisation but loss of midbody staining. There was no apparent cytokinetic defect in cells depleted for dystroglycan, however apoptosis levels were considerably higher in dystroglycan knockdown cells. Cell cycle analysis showed a delay in G2/M transition, possibly caused by a more than 50% reduction in extracellular signal-regulated kinase levels in the knockdown cells. Dystroglycan may therefore not only have a role in organising the contractile ring through direct or indirect associations with actin, but can also modulate the cell cycle by affecting extracellular signal-regulated kinase levels.

Erk5 is activated and acts as a survival factor in mitosis

Erk5 is a recently discovered MAPK claimed to be responsible for some of the roles attributed to Erk1/2; here we report that it is activated in mitosis in comparison to G1/S. When Erk5 is inactivated pharmacologically or largely ablated by RNAi, cell survival in mitosis is diminished. We have previously shown Bim, a BH3-only protein of the Bcl-2 family, to be phosphorylated in mitosis, in a MEK-dependent manner (M. Grãos, A. D. Almeida, S. Chatterjee, Biochem. J. 388 (2005) 185). Inactivation of Erk5 in mitosis causes dephosphorylation of Bim. Bim is in the mitochondria in mitosis and when dephosphorylated interacts with Bax, inducing caspase activation. We also show that in mitosis Bim co-immunoprecipitates with Erk5 and Erk5 phosphorylates GST-Bim in in vitro kinase reaction. Taken together, our results identify a new target of the still largely mysterious Erk5 and suggest that Erk5 in mitosis may be a decisive step for the survival of proliferating cells.

The ERK1/2 mitogen-activated protein kinase pathway as a master regulator of the G1- to S-phase transition

The Ras-dependent extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein (MAP) kinase pathway plays a central role in cell proliferation control. In normal cells, sustained activation of ERK1/ERK2 is necessary for G1- to S-phase progression and is associated with induction of positive regulators of the cell cycle and inactivation of antiproliferative genes. In cells expressing activated Ras or Raf mutants, hyperactivation of the ERK1/2 pathway elicits cell cycle arrest by inducing the accumulation of cyclin-dependent kinase inhibitors. In this review, we discuss the mechanisms by which activated ERK1/ERK2 regulate growth and cell cycle progression of mammalian somatic cells. We also highlight the findings obtained from gene disruption studies.