Phosphorylation regulates nucleophosmin targeting to the centrosome during mitosis as detected by cross-reactive phosphorylation-specific MKK1/MKK2 antibodies

Proteomic analysis reveals USP7 as a novel regulator of palmitic acid-induced hepatocellular carcinoma cell death

Nutrient surplus and consequent free fatty acid accumulation in the liver cause hepatosteatosis. The exposure of free fatty acids to cultured hepatocyte and hepatocellular carcinoma cell lines induces cellular stress, organelle adaptation, and subsequent cell death. Despite many studies, the mechanism associated with lipotoxicity and subsequent cell death still remains poorly understood. Here, we have used the proteomics approach to circumvent the mechanism for lipotoxicity using hepatocellular carcinoma cells as a model. Our quantitative proteomics data revealed that ectopic lipids accumulation in cells severely affects the ubiquitin-proteasomal system. The palmitic acid (PA) partially lowered the expression of deubiquitinating enzyme USP7 which subsequently destabilizes p53 and promotes mitotic entry of cells. Our global phosphoproteomics analysis also provides strong evidence of an altered cell cycle checkpoint proteins' expression that abrogates early G2/M checkpoints recovery with damaged DNA and induced mitotic catastrophe leading to hepatocyte death. We observe that palmitic acid prefers apoptosis-inducing factor (AIF) mediated cell death by depolarizing mitochondria and translocating AIF to the nucleus. In summary, the present study provides evidence of PA-induced hepatocellular death mediated by deubiquitinase USP7 downregulation and subsequent mitotic catastrophe.

Novel phosphorylation states of the yeast spindle pole body

Phosphorylation regulates yeast spindle pole body (SPB) duplication and separation and likely regulates microtubule nucleation. We report a phosphoproteomic analysis using tandem mass spectrometry of enriched Saccharomyces cerevisiae SPBs for two cell cycle arrests, G1/S and the mitotic checkpoint, expanding on previously reported phosphoproteomic data sets. We present a novel phosphoproteomic state of SPBs arrested in G1/S by a cdc4-1 temperature-sensitive mutation, with particular focus on phosphorylation events on the γ-tubulin small complex (γ-TuSC). The cdc4-1 arrest is the earliest arrest at which microtubule nucleation has occurred at the newly duplicated SPB. Several novel phosphorylation sites were identified in G1/S and during mitosis on the microtubule nucleating γ-TuSC. These sites were analyzed in vivo by fluorescence microscopy and were shown to be required for proper regulation of spindle length. Additionally, in vivo analysis of two mitotic sites in Spc97 found that phosphorylation of at least one of these sites is required for progression through the cell cycle. This phosphoproteomic data set not only broadens the scope of the phosphoproteome of SPBs, it also identifies several γ-TuSC phosphorylation sites that influence microtubule formation.

Summary: A phosphoproteome of yeast spindle pole bodies in G1/S or M phase identifies phosphorylation sites involved in spindle length control and provides direction for future phosphorylation analyses of spindle pole components.

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.

Studying mechanisms of cAMP and cyclic nucleotide phosphodiesterase signaling in Leydig cell function with phosphoproteomics

Many cellular processes are modulated by cyclic AMP and nucleotide phosphodiesterases (PDEs) regulate this second messenger by catalyzing its breakdown. The major unique function of testicular Leydig cells is to produce testosterone in response to luteinizing hormone (LH). Treatment of Leydig cells with PDE inhibitors increases cAMP levels and the activity of its downstream effector, cAMP-dependent protein kinase (PKA), leading to a series of kinase-dependent signaling and transcription events that ultimately increase testosterone release. We have recently shown that PDE4B and PDE4C as well as PDE8A and PDE8B are expressed in rodent Leydig cells and that combined inhibition of PDE4 and PDE8 leads to dramatically increased steroid biosynthesis. Here we investigated the effect of PDE4 and PDE8 inhibition on the molecular mechanisms of cAMP actions in a mouse MA10 Leydig cell line model with SILAC mass spectrometry-based phosphoproteomics. We treated MA10 cells either with PDE4 family specific inhibitor (Rolipram) and PDE8 family specific inhibitor (PF-04957325) alone or in combination and quantified the resulting phosphorylation changes at five different time points between 0 and 180min. We identified 28,336 phosphosites from 4837 proteins and observed significant regulation of 749 sites in response to PDE4 and PDE8 inhibitor treatment. Of these, 132 phosphosites were consensus PKA sites. Our data strongly suggest that PDE4 and PDE8 inhibitors synergistically regulate phosphorylation of proteins required for many different cellular processes, including cell cycle progression, lipid and glucose metabolism, transcription, endocytosis and vesicle transport. Our data suggests that cAMP, PDE4 and PDE8 coordinate steroidogenesis by acting on not one rate-limiting step but rather multiple pathways. Moreover, the pools of cAMP controlled by these PDEs also coordinate many other metabolic processes that may be regulated to assure timely and sufficient testosterone secretion in response to LH.

Cell cycle-dependent phosphorylation of nucleophosmin and its potential regulation by peptidyl-prolyl cis/trans isomerase

Nucleophosmin (NPM) is a ubiquitously expressed phosphoprotein involved in many cellular processes. Phosphorylation is considered the major regulatory mechanism of the NPM protein, associated with diverse cellular events. In this study, we characterized the phosphorylation status of several physiological phosphorylation sites of NPM, especially the newly confirmed in vivo site threonine 95 (Thr95). NPM-Thr95 exhibits a transient and cell cycle-dependent phosphorylation state compared to several other in vivo phosphorylation sites examined, including Ser4, Thr199 and Thr234/Thr237. In addition, we characterized a functional interaction between NPM and the peptidyl-prolyl isomerase Pin1, which specifically bind to each other during mitosis. The demonstration of this binding represents a novel post-phosphorylation regulatory mechanism for NPM that has not been investigated before. Mutated Pin1 putative binding sites result in defected cell division and reduced number of mitotic cells, suggesting that post-phosphorylation is important for NPM in regulating cell cycle progression.

Sirt1 Regulates Microtubule Dynamics Through Negative Regulation of Plk1 in Mitosis

Although loss of Sirt1 leads to chromosome aneuploidy, which accounts for higher tumor susceptibility, the molecular mechanisms remain unclear. Herein, we demonstrate that Sirt1 directly regulates Plk1, of which activity is critical for mitotic progression and spindle dynamics. Depletion or inhibition of Sirt1 significantly perturbs the formation of the mitotic spindle, leading to defective chromosome segregation. Elevated depolymerization of the mitotic spindle following loss of Sirt1 was associated with the deregulation of Plk1 activity. Thus, we conclude that Sirt1 may contribute to a mitotic regulator that controls spindle dynamics through Plk1 activity, resulting in fine-tuning of Plk1 dependent microtubule dynamics.

Cytoplasmic nucleophosmin has elevated T199 phosphorylation upon which G2/M phase progression is dependent

The cytoplasmic mutant of nucleophosmin (NPMc) is found approximately in one-third of acute myeloid leukemia (AML) cases and is highly associated with normal karyotype. Whereas previous studies have focused on wtNPM in centrosome duplication, we further elucidate the role of NPM in the cell cycle by utilizing the increased cytoplasmic load of NPMc. Overexpression of NPMc causes increased phosphorylation of NPM on T199 and, to a lesser degree, S4. T199 phosphorylation is dependent on cdk2 but activators of cdk2 were not elevated. Upon inhibition of cdk2, NPMc-overexpressing cells demonstrate a greater G2/M phase arrest than wtNPM or GFP counterparts. However, the number of cells with 2 centrosomes did not increase concordantly. This suggests that the arrest was caused by a delay in centrosome duplication, most likely due to the inhibition of centrosome duplication caused by unphosphorylated NPMc. Overall, these results suggest that the phosphorylation of T199 is important in the mitotic progression of NPMc-expressing cells. This further supports the hypothesis that NPMc is associated with normal karyotypes in AML because the higher cytoplasmic load of NPM can better suppress centrosome overduplication which would otherwise result in unequal segregation of chromosomes during mitosis, leading to aneuploidy and other genomic instabilities.

Centrosome dysfunction contributes to chromosome instability, chromoanagenesis, and genome reprograming in cancer

The unique ability of centrosomes to nucleate and organize microtubules makes them unrivaled conductors of important interphase processes, such as intracellular payload traffic, cell polarity, cell locomotion, and organization of the immunologic synapse. But it is in mitosis that centrosomes loom large, for they orchestrate, with clockmaker's precision, the assembly and functioning of the mitotic spindle, ensuring the equal partitioning of the replicated genome into daughter cells. Centrosome dysfunction is inextricably linked to aneuploidy and chromosome instability, both hallmarks of cancer cells. Several aspects of centrosome function in normal and cancer cells have been molecularly characterized during the last two decades, greatly enhancing our mechanistic understanding of this tiny organelle. Whether centrosome defects alone can cause cancer, remains unanswered. Until recently, the aggregate of the evidence had suggested that centrosome dysfunction, by deregulating the fidelity of chromosome segregation, promotes and accelerates the characteristic Darwinian evolution of the cancer genome enabled by increased mutational load and/or decreased DNA repair. Very recent experimental work has shown that missegregated chromosomes resulting from centrosome dysfunction may experience extensive DNA damage, suggesting additional dimensions to the role of centrosomes in cancer. Centrosome dysfunction is particularly prevalent in tumors in which the genome has undergone extensive structural rearrangements and chromosome domain reshuffling. Ongoing gene reshuffling reprograms the genome for continuous growth, survival, and evasion of the immune system. Manipulation of molecular networks controlling centrosome function may soon become a viable target for specific therapeutic intervention in cancer, particularly since normal cells, which lack centrosome alterations, may be spared the toxicity of such therapies.

Nucleophosmin, a critical Bax cofactor in ischemia-induced cell death

We hypothesized that nucleophosmin (NPM), a nucleolar phosphoprotein, is critical for Bax-mediated cell death. To test this hypothesis, Bax activation was induced by metabolic stress. During stress, nucleolar NPM translocated into the cytosol, NPM-Bax complexes formed, and both NPM and Bax accumulated in mitochondria. Expression of a cytosol-restricted NPM mutant (NPM-ΔNLS), but not a nucleus-restricted NPM mutant, increased NPM-Bax complex formation, mitochondrial NPM and Bax accumulation, mitochondrial membrane injury, caspase 3 activation, and ischemia-induced cell death. Coexpression of NPM-ΔNLS with constitutively active Bax mutants caused nearly universal cell death in the absence of metabolic stress, whereas expression of active Bax or NPM-ΔNLS alone did not. A Bax peptide that disrupts NPM-Bax interaction significantly reduced cell death caused by exposure to metabolic inhibitors in vitro and preserved kidney function after ischemia in vivo. Thus, NPM-Bax interaction enhances mitochondrial Bax accumulation, organelle injury, and cell death. NPM-Bax complex formation is a novel target for preventing ischemic tissue injury.

Nucleoplasmic/nucleolar translocation and identification of a nuclear localization signal (NLS) in Dictyostelium BAF60a/SMARCD1 homologue Snf12

Dictyostelium is a model eukaryote for the study of several cellular processes; however, comparatively little is known about its nucleolus. Identification of nucleolar proteins is key to understanding this nuclear subcompartment, but only four have been identified in Dictyostelium. As discussed in this article, a potential relationship between nucleolar NumA1 and BAF60a/SMARCD1 suggested BAF60a may also reside in the nucleolus. Here, we identify BAF60a homologue Snf12 as the fifth nucleolar protein in Dictyostelium. Immunolocalization experiments demonstrate that Snf12 is nucleoplasmic, but translocates to nucleoli upon actinomycin-D-induced transcription inhibition (0.05 mg/mL, 4 h). Translocation was accompanied by a microtubule-independent protrusion of nucleolar Snf12 regions from the nucleus followed by detection of Snf12 in cytoplasmic circles for at least 48 h. Residues (372)KRKR(375) are both necessary and sufficient for nucleoplasmic localization of Snf12 and represent a functional nuclear localization signal (NLS), similar to recently identified NLSs in other Dictyostelium proteins. Since nucleolar and nucleoplasmic proteins redistribute during mitosis, we investigated Snf12 dynamics during this time. Dictyostelium undergoes closed mitosis, meaning its nuclear envelope remains intact. Despite this, during metaphase and anaphase Snf12 redistributed throughout the cytoplasm before reaccumulating in the nucleus during telophase, unlike the previously reported nucleoplasmic redistribution of nucleolar NumA1. The nuclear exit of Snf12 was independent of its putative nuclear export signal and not inhibited by exportin inhibition, suggesting that the redistribution of nuclear proteins during mitosis in Dictyostelium is mediated by other mechanisms. Snf12 is the second Dictyostelium nucleolar protein for which its dynamics during mitosis have been investigated.

Nucleolar proteomics and viral infection

Recent advances in proteomics have been combined with traditional methods for isolation of nucleoli from mammalian and plant cells. This approach has confirmed the growing body of data showing a wide role for the nucleolus in eukaryotic cell biology beyond ribosome generation into many areas of cell function from regulation of the cell cycle, modulation of the cell stress response to innate immune responses. This has been reflected in the growing body of evidence that viruses specifically target the nucleolus by sequestering cellular nucleolar proteins or by targeting viral proteins to the nucleolus in order to maximise viral replication. This review covers those key areas and looks at the latest approaches using high‐throughput quantitative proteomics of the nucleolus in virus infected cells to gain an insight into the role of this fascinating compartment in viral infection.

Nucleolar localization and identification of nuclear/nucleolar localization signals of the calmodulin-binding protein nucleomorphin during growth and mitosis in Dictyostelium

The calmodulin-binding protein nucleomorphin isoform NumA1 is a nuclear number regulator in Dictyostelium that localizes to intra-nuclear patches adjacent to the nuclear envelope and to a lesser extent the nucleoplasm. Earlier studies have shown similar patches to be nucleoli but only three nucleolar proteins have been identified in Dictyostelium. Here, actinomycin-D treatment caused the loss of NumA1 localization, while calcium and calmodulin antagonists had no effect. In keeping with a nucleolar function, NumA1 moved out of the presumptive nucleoli during mitosis redistributing to areas within the nucleus, the spindle fibers, and centrosomal region before re-accumulating in the presumptive nucleoli at telophase. Together, these data verify NumA1 as a true nucleolar protein. Prior to this study, the dynamics of specific nucleolar proteins had not been determined during mitosis in Dictyostelium. FITC-conjugated peptides equivalent to presumptive nuclear localization signals within NumA1 localized to nucleoli indicating that they also act as nucleolar localization signals. To our knowledge, these represent the first precisely defined nucleolar localization signals as well as the first nuclear/nucleolar localization signals identified in Dictyostelium. Together, these results reveal that NumA1 is a true nucleolar protein and the only nucleolar calmodulin-binding protein identified in Dictyostelium. The possible use of nuclear/nucleolar localization signal-mediated drug targeting to nucleoli is discussed.

Extensive crosstalk between O-GlcNAcylation and phosphorylation regulates cytokinesis

Like phosphorylation, the addition of O-linked beta-N-acetylglucosamine (O-GlcNAcylation) is a ubiquitous, reversible process that modifies serine and threonine residues on nuclear and cytoplasmic proteins. Overexpression of the enzyme that adds O-GlcNAc to target proteins, O-GlcNAc transferase (OGT), perturbs cytokinesis and promotes polyploidy, but the molecular targets of OGT that are important for its cell cycle functions are unknown. Here, we identify 141 previously unknown O-GlcNAc sites on proteins that function in spindle assembly and cytokinesis. Many of these O-GlcNAcylation sites are either identical to known phosphorylation sites or in close proximity to them. Furthermore, we found that O-GlcNAcylation altered the phosphorylation of key proteins associated with the mitotic spindle and midbody. Forced overexpression of OGT increased the inhibitory phosphorylation of cyclin-dependent kinase 1 (CDK1) and reduced the phosphorylation of CDK1 target proteins. The increased phosphorylation of CDK1 is explained by increased activation of its upstream kinase, MYT1, and by a concomitant reduction in the transcript for the CDK1 phosphatase, CDC25C. OGT overexpression also caused a reduction in both messenger RNA expression and protein abundance of Polo-like kinase 1, which is upstream of both MYT1 and CDC25C. The data not only illustrate the crosstalk between O-GlcNAcylation and phosphorylation of proteins that are regulators of crucial signaling pathways but also uncover a mechanism for the role of O-GlcNAcylation in regulation of cell division.

Over Expression of Nucleophosmin and Nucleolin Contributes to the Suboptimal Activation of a G2/M Checkpoint in Ataxia Telangiectasia Fibroblasts

Ataxia Telangiectasia (AT) cells exhibit suboptimal activation of radiation-induced cell cycle checkpoints despite having a wild type p53 genotype. Reducing or eliminating this delay could restore p53 function and reinstate normal cellular response to genotoxic stress. Here we show that the levels of Nuclephosmin (NPM), NPM phosphorylated at Serine 125, p53, p53 phosphorylated at Serine 15 and Serine 392 and the levels of Nucleolin (NCL) are high in AT fibroblasts compared to normal cells. Transfection of a functional ATM into AT fibroblasts reduced p53, phospo-p53, phospho-NPM and NCL levels to wild type fibroblasts levels. Our data indicate that ATM regulates phospho-NPM and NCL indirectly through the Protein Phosphatase 1 (PP1). Both, NPM and NCL interact with p53 and hinder its phosphorylation at Serine 15 in response to bleomycin. Moreover, NPM and NCL are phosphorylated by several of the same kinases targeting p53 and could potentially compete with p53 for phosphorylation in AT cells. In addition, our data indicate that down regulation of NCL and to a lesser extent NPM increase the number of AT cells arrested in G2/M in response to bleomycin. Together this data indicate that the lack of PP1 activation in AT cells result in increased NPM and NCL protein levels which prevents p53 phosphorylation in response to bleomycin and contributes to a defective G2/M checkpoint.

Cdk2 and Cdk4 regulate the centrosome cycle and are critical mediators of centrosome amplification in p53-null cells

The two mitotic centrosomes direct spindle bipolarity to maintain euploidy. Centrosome amplification-the acquisition of > or =3 centrosomes-generates multipolar mitoses, aneuploidy, and chromosome instability to promote cancer biogenesis. While much evidence suggests that Cdk2 is the major conductor of the centrosome cycle and that it mediates centrosome amplification induced by various altered tumor suppressors, the role played by Cdk4 in a normal or deregulated centrosome cycle is unknown. Using a gene knockout approach, we report that Cdk2 and Cdk4 are critical to the centrosome cycle, since centrosome separation and duplication are premature in Cdk2(-)(/)(-) mouse embryonic fibroblasts (MEFs) and are compromised in Cdk4(-)(/)(-) MEFs. Additionally, ablation of Cdk4 or Cdk2 abrogates centrosome amplification and chromosome instability in p53-null MEFs. Absence of Cdk2 or Cdk4 prevents centrosome amplification by abrogating excessive centriole duplication. Furthermore, hyperactive Cdk2 and Cdk4 deregulate the licensing of the centrosome duplication cycle in p53-null cells by hyperphosphorylating nucleophosmin (NPM) at Thr199, as evidenced by observations that ablation of Cdk2, Cdk4, or both Cdk2 and Cdk4 abrogates that excessive phosphorylation. Since a mutant form of NPM lacking the G(1) Cdk phosphorylation site (NPM(T199A)) prevents centrosome amplification to the same extent as ablation of Cdk2 or Cdk4, we conclude that the Cdk2/Cdk4/NPM pathway is a major guardian of centrosome dysfunction and genomic integrity.

Specific phosphorylation and activation of ERK1c by MEK1b: a unique route in the ERK cascade

Extracellular signal-regulated kinases (ERKs) are key signaling molecules that regulate a large number of cellular processes, including mitosis. We showed previously that ERK1c, an alternatively spliced form of ERK1, facilitates mitotic Golgi fragmentation without the involvement of ERK1 and ERK2. Here we demonstrate that activation of ERK1c is mainly mediated by mitogen-activated protein kinase (MAPK)/ERK kinase 1b (MEK1b), which is an alternatively spliced form of MEK1 that was previously considered an inactive kinase. MEK1b phosphorylation and activity are preferentially stimulated by nocodazole, to induce its specific activity toward ERK1c. MEK1/2, on the other hand, preferentially target ERK1/2 in response to growth factors, such as EGF. As previously demonstrated for ERK1c, also MEK1b expression and activity are elevated during mitosis, and thereby enhance Golgi fragmentation and mitotic rate. MEK1 activity is also increased during mitosis, but this isoform facilitates mitotic progression without affecting the Golgi architecture. These results illustrate that the ERK cascade is divided into two routes: the classic MEK1/2-ERK1/2 and the splice-variant MEK1b-ERK1c, each of which regulates distinct cellular processes and thus extends the cascade specificity.

Proteomic analysis of the differential protein expression reveals nuclear GAPDH in activated T lymphocytes

Despite the important role of T cell activation in the adaptive immunity, very little is known about the functions of proteins that are differentially expressed in the activated T cells. In this study, we have employed proteomic approach to study the differentially expressed proteins in activated T cells. A total of 25 proteins was characterized that displayed a decreased expression, while a total of 20 proteins was characterized that displayed an increased expression in the activated T cells. Among them, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was identified unexpectedly as one of the up-regulated proteins. Western blot analysis of proteins separated by 2-dimensional gel electrophoresis had identified several modified GAPDHs which were detectable only in the activated T cells, but not in resting T cells. These modified GAPDHs had higher molecular mass and more basic PI, and were present in the nucleus of activated T cells. Promoter occupancy studies by chromatin immunoprecipitation assay revealed that nuclear GAPDH could be detected in the promoter of genes that were up-regulated during T cell activation, but not in the promoter of genes that were not unaffected or down-regulated. Our results suggest that nuclear GAPDH may function as transcriptional regulator in activated T cells.

Nucleophosmin protein expression level, but not threonine 198 phosphorylation, is essential in growth and proliferation

Nucleophosmin (NPM), an oligomeric phosphoprotein and nucleolar target of the ARF tumor suppressor, contributes to several critical cellular processes. Previous studies have shown that the human NPM's phosphorylation by cyclin E-cyclin-dependent kinase 2 (cdk2) on threonine (Thr) 199 regulates its translocation from the centrosome during cell cycle progression. Given our previous finding that ARF directly binds NPM, impeding its transit to the cytoplasm and arresting cells before S-phase entry, we hypothesized that ARF might also inhibit NPM phosphorylation. However, ARF induction did not impair phosphorylation of the cdk2 target residue in murine NPM, Thr198. Furthermore, phosphorylation of Thr198 occurred throughout the cell cycle and was concomitant with increases in overall NPM expression. To investigate the cell's presumed requirement for NPM-Thr198 phosphorylation in promoting the processes of growth and proliferation, we examined the effects of a non-phosphorylatable NPM mutant, T198A, in a clean cell system in which endogenous NPM had been removed by RNA interference. Here, we show that the T198A mutant is fully capable of executing NPM's described roles in nucleocytoplasmic shuttling, ribosome export and cell cycle progression. Moreover, the proliferative defects observed with stable NPM knockdown were restored by mutant NPM-T198A expression. Thus, we demonstrate that the reduction in NPM protein expression blocks cellular growth and proliferation, whereas phosphorylation of NPM-Thr198 is not essential for NPM's capacity to drive cell cycle progression and proliferation.

Budding yeast centrosome duplication requires stabilization of Spc29 via Mps1-mediated phosphorylation

Protein phosphorylation plays an important role in the regulation of centrosome duplication. In budding yeast, numerous lines of evidence suggest a requirement for multiple phosphorylation events on individual components of the centrosome to ensure their proper assembly and function. Here, we report the first example of a single phosphorylation event on a component of the yeast centrosome, or spindle pole body (SPB), that is required for SPB duplication and cell viability. This phosphorylation event is on the essential SPB component Spc29 at a conserved Thr residue, Thr(240). Mutation of Thr(240) to Ala is lethal at normal gene dosage, but an increased copy number of this mutant allele results in a conditional phenotype. Phosphorylation of Thr(240) was found to promote the stability of the protein in vivo and is catalyzed in vitro by the Mps1 kinase. Furthermore, the stability of newly synthesized Spc29 is reduced in a mutant strain with reduced Mps1 kinase activity. These results demonstrate the first evidence for a single phosphorylation event on an SPB component that is absolutely required for SPB duplication and suggest that the Mps1 kinase is responsible for this protein-stabilizing phosphorylation.

Antibody specificity profiling on functional protein microarrays

Antibodies represent the end product of an exquisitely complex biological process including recombination, somatic hypermutation, affinity maturation, and self-tolerance, culminating in binding reagents directed against a vast repertoire of antigens. The resultant high affinity and diversity of specificity of these biomolecules has been exploited through the development of immunoassays and biotherapeutics that inaugurated a new era in experimental molecular biology and pharmaceutical drug development. Despite the utility of antibodies for research applications and in disease treatment, they must be employed in the context of an accurate understanding of their binding profile. High-content microarrays comprised of thousands of native, full length human proteins are an important tool in the assessment of antibody specificity.

ERK implication in cell cycle regulation

The Ras/Raf/MEK/ERK signaling cascade that integrates an extreme variety of extracellular stimuli into key biological responses controlling cell proliferation, differentiation or death is one of the most studied intracellular pathways. Here we present some evidences that have been accumulated over the last 15 years proving the requirement of ERK in the control of cell proliferation. In this review we focus (i) on the spatio-temporal control of ERK signaling, (ii) on the key cellular components linking extracellular signals to the induction and activation of cell cycle events controlling G1 to S-phase transition and (iii) on the role of ERK in the growth factor-independent G2/M phase of the cell cycle. As ERK pathway is often co-activated with the PI3 kinase signaling, we highlight some of the key points of convergence leading to a full activation of mTOR via ERK and AKT synergies. Finally, ERK and AKT targets being constitutively activated in so many human cancers, we briefly touched the cure issue of using more specific drugs in rationally selected cancer patients.

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.

A functional role for p38 MAPK in modulating mitotic transit in the absence of stress

Although p38 MAPK is known to be activated in response to various environmental stresses and to have inhibitory roles in cell proliferation and tumor progression, its role in cell cycle progression in the absence of stress is unknown in most cell types. In the case of G(2)/M cell cycle control, p38 activation has been shown to trigger a rapid G(2)/M cell cycle checkpoint after DNA damage stress and a spindle checkpoint after microtubule disruption. In the course of our studies, we observed that p38 became actively phosphorylated, and its kinase activity increased transiently during G(2)/M cell cycle transition. Using an immunocytochemistry approach, the active form of p38 was found at the centrosome from late G(2) throughout mitosis, which suggests functional relevance for active p38 protein during mitotic entry. A closer examination reveals that p38 inhibition by pharmacologic inhibitors significantly accelerated the timing of mitotic entry. In addition, long term exposure of the inhibitor enhanced Cdc2 activity. These results indicate that p38 activity during G(2)/M may be involved in a mechanism for fine tuning the initiation of mitosis and perhaps transit of mitosis. Consistent with our previous findings, Cdc25B was phosphorylated on serine 309 at the centrosome during G(2)/M when p38 was active at this site; Cdc25B phosphorylation inhibits Cdc25B activity, and this phosphorylation was found to be p38-dependent. Taken together, our findings suggest that p38 regulates the timing of mitotic entry via modulation of Cdc25B activity under normal nonstress conditions.

Nucleophosmin: a versatile molecule associated with hematological malignancies

Nucleophosmin (NPM) is a nucleolar phosphoprotein that plays multiple roles in ribosome assembly and transport, cytoplasmic-nuclear trafficking, centrosome duplication and regulation of p53. In hematological malignancies, the NPM1 gene is frequently involved in chromosomal translocation, mutation and deletion. The NPM1 gene on 5q35 is translocated with the anaplastic lymphoma kinase (ALK) gene in anaplastic large cell lymphoma with t(2;5). The MLF1 and RARA genes are fused with NPM1 in myelodysplastic syndrome and acute myeloid leukemia (AML) with t(3;5) and acute promyelocytic leukemia with t(5;17), respectively. In each fused protein, the N-terminal NPM portion is associated with oligomerization of a partner protein leading to altered signal transduction or transcription. Recently, mutations of exon 12 have been found in a significant proportion of de novo AML, especially in those with a normal karyotype. Mutant NPM is localized aberrantly in the cytoplasm, but the molecular mechanisms for leukemia remain to be studied. Studies of knock-out mice have revealed new aspects regarding NPM1 as a tumor-suppressor gene. This review focuses on the clinical significance of the NPM1 gene in hematological malignancies and newly discovered roles of NPM associated with oncogenesis.

Global functional analysis of nucleophosmin in Taxol response, cancer, chromatin regulation, and ribosomal DNA transcription

Analysis of lung cancer response to chemotherapeutic agents showed the accumulation of a Taxol-induced protein that reacted with an anti-phospho-MEK1/2 antibody. Mass spectroscopy identified the protein as nucleophosmin/B23 (NPM), a multifunctional protein with diverse roles: ribosome biosynthesis, p53 regulation, nuclear-cytoplasmic shuttling, and centrosome duplication. Our work demonstrates that following cellular exposure to mitosis-arresting agents, NPM is phosphorylated and its chromatographic property is altered, suggesting changes in function during mitosis. To determine the functional relevance of NPM, its expression in tumor cells was reduced by siRNA. Cells with reduced NPM were treated with Taxol followed by microarray profiling accompanied by gene/protein pathway analyses. These studies demonstrate several expected and unexpected consequences of NPM depletion. The predominant downstream effectors of NPM are genes involved in cell proliferation, cancer, and the cell cycle. In congruence with its role in cancer, NPM is over-expressed in primary malignant lung cancer tissues. We also demonstrate a role for NPM in the expression of genes encoding SET (TAF1beta) and the histone methylase SET8. Additionally, we show that NPM is required for a previously unobserved G2/M upregulation of TAF1A, which encodes the rDNA transcription factor TAF(I)48. These results demonstrate multi-faceted functions of NPM that can affect cancer cells.

Characterization of ATP-independent ERK inhibitors identified through in silico analysis of the active ERK2 structure

The extracellular signal-regulated kinases (ERK1 and ERK2) are important mediators of cell proliferation. Constitutive activation of the ERK proteins plays a critical role in the proliferation of many human cancers. Taking advantage of recently identified substrate docking domains on ERK2, we have used computer-aided drug design (CADD) to identify novel low molecular weight compounds that interact with ERK2 in an ATP-independent manner and disrupt substrate-specific interactions. In the current study, a CADD screen of the 3D structure of active phosphorylated ERK2 protein was used to identify inhibitory compounds. We tested 13 compounds identified by the CADD screen in ERK-specific phosphorylation, cell proliferation, and binding assays. Of the 13 compounds tested, 4 compounds strongly inhibited ERK-mediated phosphorylation of ribosomal S6 kinase-1 (Rsk-1) and/or the transcription factor Elk-1 and inhibited the proliferation of HeLa cervical carcinoma cells with IC(50) values in the 2-10 microM range. These studies demonstrate that CADD can be used to identify lead compounds for development of novel non-ATP-dependent inhibitors selective for active ERK and its interactions with substrates involved in cancer cell proliferation.

Deregulation of the centrosome cycle and the origin of chromosomal instability in cancer

Although we have begun to tap into the mechanisms behind Boveri's initial observation that supernumerary centrosomes cause chromosome missegregation in sea urchin eggs, there is still much left to discover with regard to chromosomal instability in cancer. Many of the molecular players involved in regulation of the centrosome and cell cycles, and the coupling of the two cycles to produce a bipolar mitotic spindle have been identified. One theme that has become apparent is that cross talk and interrelatedness of the pathways serve to provide redundant mechanisms to maintain genomic integrity. In spite of this, cells occasionally fall prey to insults that initiate and maintain the chromosomal instability that results in viable malignant tumours. Deregulation of centrosome structure is an integral aspect of the origin of chromosomal instability in many cancers. There are numerous routes to centrosome amplification including: environmental insults such as ionising radiation and exposure to estrogen (Li et al., 2005); failure of cytokinesis; and activating mutations in key regulators of centrosome structure and function. There are two models for initiation of centrosome amplification (Figure 2). In the first, centrosome duplication and chromosome replication remain coupled and cells enter G2 with 4N chromosomes and duplicated centrosomes. However, these cells may fail to complete mitosis, and thus reenter G1 as tetraploid cells with amplified centrosomes. In the second, the centrosome cycle is uncoupled from chromosome replication and cells go through one or more rounds of centriole/centrosome duplication in the absence of chromosome replication. If these cells then go through chromosome replication accompanied by another round of centrosome duplication, cells complete G2 with 4N chromosomes and more than 2 centrosomes, and therefore are predisposed to generate multipolar mitotic spindles. Fragmentation of centrosomes due to ionising radiation is a variation of the second model. Once centrosome amplification is present, even in a diploid cell, that cell has the potential to yield viable aneuploid progeny. The telophase cell in Figure 3C illustrates this scenario. In a normal telophase configuration, the total number of chromosomes is 92 (resulting from the segregation of 46 pairs of chromatids), with each daughter nucleus containing 46 individual chromosomes. Based on the number of kinetochore signals present, the lower nucleus in Figure 3C has approximately 28 chromosomes, and the elongate upper nucleus has approximately 60, for a total of 88. Due to superimposition of kinetochores in this maximum projection image, 88 is an underestimate of the actual number of kinetochores and is not significantly different from the expected total of 92. A cell resulting from the lower nucleus with only around 28 chromosomes would probably not be viable, much as Boveri's experiments indicated. However, the upper nucleus with at least 60 chromosomes could be viable. This cell would enter G1 as hypotriploid (69 chromosomes = triploid) with 2 centrosomes. During S and G2, the centrosomes and chromosomes would double, and the following mitosis could be tetrapolar with a 6N chromosome content. When centrosome amplification is accompanied by permissive lapses in cell cycle checkpoints, the potential for malignant growth is present. These lapses could result from specific genetic mutations and amplifications, epigenetic gene silencing, or from massive chromosomal instability caused by the centrosome amplification. Centrosome amplification, therefore, can serve to exacerbate and/or generate genetic instabilities associated with cancers.

The centrosome cycle

Centrosomes are dynamic organelles involved in many aspects of cell function and growth. Centrosomes act as microtubule organizing centers, and provide a site for concerted regulation of cell cycle progression. While there is diversity in microtubule organizing center structure among eukaryotes, many centrosome components, such as centrin, are conserved. Experimental analysis has provided an outline to describe centrosome duplication, and numerous centrosome components have been identified. Even so, more work is needed to provide a detailed understanding of the interactions between centrosome components and their roles in centrosome function and duplication. Precise duplication of centrosomes once during each cell cycle ensures proper mitotic spindle formation and chromosome segregation. Defects in centrosome duplication or function are linked to human diseases including cancer. Here we provide a multifaceted look at centrosomes with a detailed summary of the centrosome cycle.

Inhibition of mixed-lineage kinase (MLK) activity during G2-phase disrupts microtubule formation and mitotic progression in HeLa cells

The mixed-lineage kinases (MLK) are serine/threonine protein kinases that regulate mitogen-activated protein (MAP) kinase signaling pathways in response to extracellular signals. Recent studies indicate that MLK activity may promote neuronal cell death through activation of the c-Jun NH2-terminal kinase (JNK) family of MAP kinases. Thus, inhibitors of MLK activity may be clinically useful for delaying the progression of neurodegenerative diseases, such as Parkinson's. In proliferating non-neuronal cells, MLK may have the opposite effect of promoting cell proliferation. In the current studies we examined the requirement for MLK proteins in regulating cell proliferation by examining MLK function during G2 and M-phase of the cell cycle. The MLK inhibitor CEP-11004 prevented HeLa cell proliferation by delaying mitotic progression. Closer examination revealed that HeLa cells treated with CEP-11004 during G2-phase entered mitosis similar to untreated G2-phase cells. However, CEP-11004 treated cells failed to properly exit mitosis and arrested in a pro-metaphase state. Partial reversal of the CEP-11004 induced mitotic arrest could be achieved by overexpression of exogenous MLK3. The effects of CEP-11004 treatment on mitotic events included the inhibition of histone H3 phosphorylation during prophase and prior to nuclear envelope breakdown and the formation of aberrant mitotic spindles. These data indicate that MLK3 might be a unique target to selectively inhibit transformed cell proliferation by disrupting mitotic spindle formation resulting in mitotic arrest.

Proteomic characterization of oxidative dysfunction in human umbilical vein endothelial cells (HUVEC) induced by exposure to oxidized LDL

The oxidative modification of low-density lipoprotein (LDL) and subsequent alteration of endothelial cell function are generally accepted as an important early event in the pathogenesis of atherosclerosis. To understand the mechanism by which oxidized LDL (oxLDL) causes dysfunction in endothelial cells, human umbilical vein endothelial cells (HUVEC) were exposed to oxLDL at a concentration that induces cellular dysfunction, and proteomic analysis was carried out, together with the analysis of cellular lipid peroxidation products. Time-dependent accumulation of 7-ketocholesterol and the progression of oxidative modification of peroxiredoxin 2 were observed, together with the suppression of cell proliferation. Proteomic analysis using two-dimensional gel electrophoresis (2-D gel) revealed that nucleophosmin, stathmin, and nucleolin were differentially expressed after exposure to oxLDL. Both 2-D gel and western blot analyses revealed that (1) nucleophosmin was dephosphorylated in a time-dependent manner; (2) stathmin was transiently phosphorylated at 6 h, and the unphosphorylated form was continuously down-regulated; and (3) nucleolin was identified as a 20-kDa fragment and a 76-kDa form, which were down-regulated. These observations suggest that the exposure of HUVEC to oxLDL results in the suppression of cell proliferation, which is ascribed to protein modification and/or altered expression of nucleophosmin, stathmin, and nucleolin under these oxidative stress conditions.

Cdc2-mediated Inhibition of Epidermal Growth Factor Activation of the Extracellular Signal-regulated Kinase Pathway during Mitosis

Inhibition of general transcription and translation occurs during mitosis to preserve the high energy requirements needed for the dynamic structural changes that are occurring at this time of the cell cycle. Although the mitotic kinase Cdc2 appears to directly phosphorylate and inhibit key proteins directly involved in transcription and translation, the role of Cdc2 in regulating up-stream growth factor receptor-mediated signal transduction pathways is limited. In the present study, we examined mechanisms involved in uncoupling receptor-mediated activation of the extracellular signal-regulated (ERK) signaling pathway in mitotic cells. Treatment with epidermal growth factor (EGF) failed to activate the ERK pathway in mitotic cells, although partial activation of ERK could be achieved in mitotic cells treated with phorbol 12-myristate 13-acetate (PMA). The discrepancy between EGF and PMA-mediated ERK activation suggested that multiple events in the ERK pathway were regulated during mitosis. We show that Cdc2 inhibits EGF-mediated ERK activation through direct interaction and phosphorylation of several ERK pathway proteins, including the guanine nucleotide exchange factor, Sos-1, and Raf-1 kinase. Inhibition of Cdc2 activity with roscovitine in mitotic cells restored ERK activation by EGF and PMA. Similarly, mitotic inhibition of ERK activity in cells expressing active mutants of H-Ras and Raf-1 kinase could also be reversed following Cdc2 inhibition. In contrast, ERK activation in cells expressing active MEK1 was not inhibited during mitosis or affected by roscovitine. These data suggest that Cdc2 inhibits growth factor receptor-mediated ERK activation during mitosis by primarily targeting signaling proteins that are upstream of MEK1.

Polyamine depletion induces nucleophosmin modulating stability and transcriptional activity of p53 in intestinal epithelial cells

Our previous studies have shown that polyamines are required for normal intestinal mucosal growth and that decreased levels of polyamines inhibit intestinal epithelial cell (IEC) proliferation by stabilizing p53 and other growth-inhibiting proteins. Nucleophosmin (NPM) is a multifunctional protein that recently has been shown to regulate p53 activity. In the present study, we sought to determine whether polyamine depletion increases NPM modulating the stability and transcriptional activity of p53 in a normal IEC-6 intestinal epithelial cell line. Depletion of cellular polyamines by alpha-difluoromethylornithine, the specific inhibitor of polyamine biosynthesis, stimulated expression of the NPM gene and induced nuclear translocation of NPM protein. Polyamine depletion stimulated NPM expression primarily by increasing NPM gene transcription and its mRNA stability, and it induced NPM nuclear translocation through activation of phosphorylation of mitogen-activated protein kinase kinase. Increased NPM interacted with p53 and formed a NPM/p53 complex in polyamine-deficient cells. Inhibition of NPM expression by small interfering RNA targeting NPM (siNPM) not only destabilized p53 as indicated by a decrease in its protein half-life but also prevented the increased p53-dependent transactivation as shown by suppression of the p21 promoter activity. Decreased expression of NPM by siNPM also promoted cell growth in polyamine-deficient cells. These results indicate that 1) polyamine depletion increases expression of the NPM gene and enhances NPM nuclear translocation and 2) increased NPM interacts with and stabilizes p53, leading to inhibition of IEC-6 cell proliferation.

RPGR ORF15 isoform co-localizes with RPGRIP1 at centrioles and basal bodies and interacts with nucleophosmin

The ORF15 isoform of RPGR (RPGR(ORF15)) and RPGR interacting protein 1 (RPGRIP1) are mutated in a variety of retinal dystrophies but their functions are poorly understood. Here, we show that in cultured mammalian cells both RPGR(ORF15) and RPGRIP1 localize to centrioles. These localizations are resistant to the microtubule destabilizing drug nocodazole and persist throughout the cell cycle. RPGR and RPGRIP1 also co-localize at basal bodies in cells with primary cilia. The C-terminal (C2) domain of RPGR(ORF15) (ORF15(C2)) is highly conserved across 13 mammalian species, suggesting that it is a functionally important domain. Using matrix-assisted laser desorption ionization time-of-flight mass spectrometry, we show that this domain interacts with a 40 kDa shuttling protein nucleophosmin (NPM). The RPGR(ORF15)-NPM interaction was confirmed by (i) yeast two-hybrid analyses; (ii) binding of both recombinant and native HeLa cell NPM to RPGR(ORF15) fusion proteins in vitro; (iii) co-immunoprecipitation of native NPM, RPGR(ORF15) and RPGRIP1 from bovine retinal extracts and of native HeLa cell NPM and transfected RPGR(ORF15) from cultured cells and (iv) co-localization of NPM and RPGR(ORF15) at metaphase centrosomes in cultured cells. NPM is a multifunctional protein chaperone that shuttles between the nucleoli and the cytoplasm and has been associated with licensing of centrosomal division. RPGR and RPGRIP1 join a growing number of centrosomal proteins involved in human disease.

Protein Phosphatase 2A Activity Associated with Golgi Membranes during the G2/M Phase May Regulate Phosphorylation of ERK2

The extracellular signal-regulated kinase (ERK) 1 and 2 proteins are mitogen-activated protein kinase (MAPK) members that regulate cell proliferation and differentiation. ERK proteins are activated exclusively by MAPK kinase 1 and 2 phosphorylation of threonine and tyrosine residues located within the conserved TXY MAPK activation motif. Although dual phosphorylation of Thr and Tyr residues confers full activation of ERK, in vitro studies suggest that a single phosphorylation on either Thr or Tyr may yield partial ERK activity. Previously, we have demonstrated that phosphorylation of the tyrosine residue (Tyr(P) ERK) may be involved in regulating the Golgi complex structure during the G2 and M phases of the cell cycle (Cha, H., and Shapiro, P. (2001) J. Cell Biol. 153, 1355-1368). In the present study, we examined mechanisms for generating Tyr(P) ERK by determining cell cycle-dependent changes in localized phosphatase activity. Using fractionated nuclei-free cell lysates, we find increased serine/threonine phosphatase activity associated with Golgi-enriched membranes in cells synchronized in the late G2/early M phase as compared with G1 phase cells. The addition of phosphatase inhibitors in combination with immunodepletion assays identified this activity to be related to protein phosphatase 2A (PP2A). The increased activity was accounted for by elevated PP2A association with mitotic Golgi membranes as well as increased catalytic activity after normalization of PP2A protein levels in the phosphatase assays. These data indicate that localized changes in PP2A activity may be involved in regulating proteins involved in Golgi disassembly as cells enter mitosis.

Mass spectrometric contributions to the practice of phosphorylation site mapping through 2003: a literature review

Reversible phosphorylation of proteins is among the most important post-translational modifications, and elucidation of sites of phosphorylation is essential to understanding the regulation of key cellular processes such as signal transduction. Unfortunately phosphorylation site mapping is as technically challenging as it is important. Limitations in the traditional method of Edman degradation of (32)P-labeled phosphoproteins have spurred the development of mass spectrometric methods for phosphopeptide identification and sequencing. To assess the practical contributions of the various technologies we conducted a literature search of publications using mass spectrometry to discover previously unknown phosphorylation sites. 1281 such phosphorylation sites were reported in 203 publications between 1992 and 2003. This review examines and catalogs those methods, identifies the trends that have emerged in the past decade, and presents representative examples from among these methods.

Nek2A kinase regulates the localization of numatrin to centrosome in mitosis

Chromosome segregation in mitosis is orchestrated by the kinetochore and spindle microtubules stemming from two centrosomes. Our recent studies demonstrated the importance of Nek2A in faithful chromosome segregation during mitosis. Here, we report that Nek2A regulates the function of numatrin in mitosis. The biochemical interaction between Nek2A and numatrin in mitotic cells was revealed by a set of reciprocal immunoprecipitation experiments using Nek2A and numatrin antibodies, respectively. The interaction is validated by a pull-down assay using recombinant Nek2A and numatrin proteins. Moreover, our immunofluorescence studies demonstrate that numatrin becomes centrosome-associated as the cell enters into mitosis and depart from the centrosome after sister chromatid separation in anaphase. The co-localization of numatrin and Nek2A to the centrosome suggests their interaction with and involvement in centrosome function. Indeed, elimination of Nek2A kinase by siRNA diminished its association with the centrosome. Furthermore, we show that numatrin is phosphorylated by wild type but not kinase-death Nek2A. Our studies suggest that the Nek2A kinase cascade is essential for the localization of numatrin to the centrosome.

B23/nucleophosmin serine 4 phosphorylation mediates mitotic functions of polo-like kinase 1

Phosphoprotein profiling by Kinetworks trade mark analysis of M-phase-arrested HeLa cells by nocodazole treatment revealed that a novel mitosis-specific phosphorylation event occurred in the nucleolar protein B23/nucleophosmin at a conserved Ser-4 residue. Consistent with the resemblance of the Ser-4 phosphorylation site to the Polo-like kinase 1 (Plk1) consensus recognition sequence, inhibition of Plk1 by a kinase-defective mutation (K82M) abrogated B23 Ser-4 phosphorylation, whereas activation of Plk1 by a constitutively active mutation (T210D) enhanced its phosphorylation following in vivo transfection and in vitro phosphorylation assays. Depletion of endogenous Plk1 by RNA interference abolished B23 Ser-4 phosphorylation. The physical interaction of Plk1 and B23 was further demonstrated by their co-immunoprecipitation and glutathione S-transferase fusion protein pull-down assays. Interference of Ser-4 phosphorylation of B23 induced multiple mitotic defects in HeLa cells, including aberrant numbers of centrosomes, elongation and fragmentation of nuclei, and incomplete cytokinesis. The phenotypes of B23 mutants are reminiscent of a subset of those described previously in Plk1 mutants. Our findings provide insights into the biochemical mechanism underlying the role of Plk1 in mitosis regulation through the identification of Ser-4 in B23 as a major physiological substrate of Plk1.

MEK inhibition and phosphorylation of serine 4 on B23 are two coincident events in mitosis

Previous studies have shown that activation of the Raf/MEK/ERK pathway is necessary for G2/M transition. However, as for the activation state of MEK in mitosis the conclusion is not consistent. Here we show that MEK is inhibited in mitosis. In addition, we identify a multifunctional protein named B23 that strongly cross-reacts with a phospho-MEK antibody in mitotic cells. Sequence homology between the N-terminus surrounding Ser 4 of B23 and the Raf phosphorylation site on MEK suggests a mechanism for cross-reaction of the antibody. Thus, mutation of Ser 4 to alanine abolishes cross-reactivity between B23 and the phospho-MEK antibody. Our findings may explain the discrepancy of results obtained with the use of phospho-MEK antibody regarding the activation state of MEK in mitosis.

MEK1 and MEK2, different regulators of the G1/S transition

The ERK cascade is activated by hormones, cytokines, and growth factors that result in either proliferation or growth arrest depending on the duration and intensity of the ERK activation. Here we provide evidence that the MEK1/ERK module preferentially provides proliferative signals, whereas the MEK2/ERK module induces growth arrest at the G1/S boundary. Depletion of either MEK subtype by RNA interference generated a unique phenotype. The MEK1 knock down led to p21cip1 induction and to the appearance of cells with a senescence-like phenotype. Permanent ablation of MEK1 resulted in reduced colony formation potential, indicating the importance of MEK1 for long term proliferation and survival. MEK2 deficiency, in contrast, was accompanied by a massive induction of cyclin D expression and, thus, CDK4/6 activation followed by nucleophosmin hyperphosphorylation and centrosome over-amplification. Our results suggest that the two MEK subtypes have distinct ways to contribute to a regulated ERK activity and cell cycle progression.

The Centrosome in Higher Organisms: Structure, Composition, and Duplication

The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.