Mitotic regulation of ribosomal S6 kinase 1 involves Ser/Thr, Pro phosphorylation of consensus and non-consensus sites by Cdc2

Targeting CDK1 in cancer: mechanisms and implications

Cyclin dependent kinases (CDKs) are serine/threonine kinases that are proposed as promising candidate targets for cancer treatment. These proteins complexed with cyclins play a critical role in cell cycle progression. Most CDKs demonstrate substantially higher expression in cancer tissues compared with normal tissues and, according to the TCGA database, correlate with survival rate in multiple cancer types. Deregulation of CDK1 has been shown to be closely associated with tumorigenesis. CDK1 activation plays a critical role in a wide range of cancer types; and CDK1 phosphorylation of its many substrates greatly influences their function in tumorigenesis. Enrichment of CDK1 interacting proteins with Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis was conducted to demonstrate that the associated proteins participate in multiple oncogenic pathways. This abundance of evidence clearly supports CDK1 as a promising target for cancer therapy. A number of small molecules targeting CDK1 or multiple CDKs have been developed and evaluated in preclinical studies. Notably, some of these small molecules have also been subjected to human clinical trials. This review evaluates the mechanisms and implications of targeting CDK1 in tumorigenesis and cancer therapy.

Tceal7 Regulates Skeletal Muscle Development through Its Interaction with Cdk1

We have previously reported Tceal7 as a muscle-specific gene that represses myoblast proliferation and promotes myogenic differentiation. The regulatory mechanism of Tceal7 gene expression has been well clarified recently. However, the underlying mechanism of Tceal7 function in skeletal muscle development remains to be elucidated. In the present study, we have generated an MCK 6.5 kb-HA-Tceal7 transgenic model. The transgenic mice are born normally, while they have displayed defects in the growth of body weight and skeletal muscle myofiber during postnatal development. Although four RxL motifs have been identified in the Tceal7 protein sequence, we have not detected any direct protein-protein interaction between Tceal7 and Cyclin A2, Cyclin B1, Cylin D1, or Cyclin E1. Further analysis has revealed the interaction between Tceal7 and Cdk1 instead of Cdk2, Cdk4, or Cdk6. Transgenic overexpression of Tceal7 reduces phosphorylation of 4E-BP1 Ser65, p70S6K1 Thr389, and Cdk substrates in skeletal muscle. In summary, these studies have revealed a novel mechanism of Tceal7 in skeletal muscle development.

The nutrient-responsive CDK Pho85 primes the Sch9 kinase for its activation by TORC1

Yeast cells maintain an intricate network of nutrient signaling pathways enabling them to integrate information on the availability of different nutrients and adjust their metabolism and growth accordingly. Cells that are no longer capable of integrating this information, or that are unable to make the necessary adaptations, will cease growth and eventually die. Here, we studied the molecular basis underlying the synthetic lethality caused by loss of the protein kinase Sch9, a key player in amino acid signaling and proximal effector of the conserved growth-regulatory TORC1 complex, when combined with either loss of the cyclin-dependent kinase (CDK) Pho85 or loss of its inhibitor Pho81, which both have pivotal roles in phosphate sensing and cell cycle regulation. We demonstrate that it is specifically the CDK-cyclin pair Pho85-Pho80 or the partially redundant CDK-cyclin pairs Pho85-Pcl6/Pcl7 that become essential for growth when Sch9 is absent. Interestingly, the respective three CDK-cyclin pairs regulate the activity and distribution of the phosphatidylinositol-3 phosphate 5-kinase Fab1 on endosomes and vacuoles, where it generates phosphatidylinositol-3,5 bisphosphate that serves to recruit both TORC1 and its substrate Sch9. In addition, Pho85-Pho80 directly phosphorylates Sch9 at Ser726, and to a lesser extent at Thr723, thereby priming Sch9 for its subsequent phosphorylation and activation by TORC1. The TORC1-Sch9 signaling branch therefore integrates Pho85-mediated information at different levels. In this context, we also discovered that loss of the transcription factor Pho4 rescued the synthetic lethality caused by loss of Pho85 and Sch9, indicating that both signaling pathways also converge on Pho4, which appears to be wired to a feedback loop involving the high-affinity phosphate transporter Pho84 that fine-tunes Sch9-mediated responses.

Beyond Protein Synthesis; The Multifaceted Roles of Tuberin in Cell Cycle Regulation

The ability of cells to sense diverse environmental signals, including nutrient availability and conditions of stress, is critical for both prokaryotes and eukaryotes to mount an appropriate physiological response. While there is a great deal known about the different biochemical pathways that can detect and relay information from the environment, how these signals are integrated to control progression through the cell cycle is still an expanding area of research. Over the past three decades the proteins Tuberin, Hamartin and TBC1D7 have emerged as a large protein complex called the Tuberous Sclerosis Complex. This complex can integrate a wide variety of environmental signals to control a host of cell biology events including protein synthesis, cell cycle, protein transport, cell adhesion, autophagy, and cell growth. Worldwide efforts have revealed many molecular pathways which alter Tuberin post-translationally to convey messages to these important pathways, with most of the focus being on the regulation over protein synthesis. Herein we review the literature supporting that the Tuberous Sclerosis Complex plays a critical role in integrating environmental signals with the core cell cycle machinery.

Trichodysplasia spinulosa polyomavirus small T antigen synergistically modulates S6 protein translation and DNA damage response pathways to shape host cell environment

TSPyV is a viral agent linked to Trichodysplasia spinulosa, a disfiguring human skin disease which presents with hyperkeratotic spicule eruption in immunocompromised hosts. This proliferative disease state requires extensive modulation of the host cell environment. While the small T (sT) antigen of TSPyV has been postulated to cause widespread cellular perturbation, its specific substrates and their mechanistic connection are unclear. To identify the cellular substrates and pathways perturbed by TSPyV sT and propose a nuanced model that reconciles the multiple arms of TSPyV pathogenesis, changes in expression of several proteins and phospho-proteins in TSPyV sT expressing and TSPyV sT deletion mutant-expressing cell lysates were interrogated using Western blot assays. TSPyV sT expression exploits the DNA damage response pathway, by inducing hyperphosphorylation of ATM and 53BP1 and upregulation of BMI-1. Concurrently, sT dysregulates the S6 protein translation pathway via hyperphosphorylation of CDC2, p70 S6 kinase, S6, and PP1α. The S6^S244/247 and p-PP1α^T320 phospho-forms are points of overlap between the DDR and S6 networks. We propose a mechanistic rationale for previous reports positioning sT antigen as the key driver of TSPyV pathogenesis. We illuminate novel targets in the S6 and DDR pathways and recognize a potential synergy between these pathways. TSPyV may sensitize the cell to both unrestricted translation and genomic instability. This multi-pronged infection model may inform future therapeutic modalities against TSPyV and possibly other viruses with overlapping host substrates.

CDK1, the Other 'Master Regulator' of Autophagy

Autophagy and cap-dependent mRNA translation are tightly regulated by the mechanistic target of rapamycin complex 1 (mTORC1) signalling complex in response to nutrient availability. However, the regulation of these processes, and mTORC1 itself, is different during mitosis, and this has remained an area of significant controversy; for example, studies have argued that autophagy is either repressed or highly active during mitosis. Recent studies have shown that autophagy initiation is repressed, and cap-dependent mRNA translation is maintained during mitosis despite mTORC1 activity being repressed. This is achieved in large part by a switch from mTORC1- to cyclin-dependent kinase 1 (CDK1)-mediated regulation. Here, we review the history and recent advances and seek to present a unifying model to inform the future study of autophagy and mTORC1 during mitosis.

Role of Cyclin-Dependent Kinase 1 in Translational Regulation in the M-Phase

Cyclin dependent kinase 1 (CDK1) has been primarily identified as a key cell cycle regulator in both mitosis and meiosis. Recently, an extramitotic function of CDK1 emerged when evidence was found that CDK1 is involved in many cellular events that are essential for cell proliferation and survival. In this review we summarize the involvement of CDK1 in the initiation and elongation steps of protein synthesis in the cell. During its activation, CDK1 influences the initiation of protein synthesis, promotes the activity of specific translational initiation factors and affects the functioning of a subset of elongation factors. Our review provides insights into gene expression regulation during the transcriptionally silent M-phase and describes quantitative and qualitative translational changes based on the extramitotic role of the cell cycle master regulator CDK1 to optimize temporal synthesis of proteins to sustain the division-related processes: mitosis and cytokinesis.

CDK1 couples proliferation with protein synthesis

Cell proliferation exerts a high demand on protein synthesis, yet the mechanisms coupling the two processes are not fully understood. A kinase and phosphatase screen for activators of translation, based on the formation of stress granules in human cells, revealed cell cycle-associated kinases as major candidates. CDK1 was identified as a positive regulator of global translation, and cell synchronization experiments showed that this is an extramitotic function of CDK1. Different pathways including eIF2α, 4EBP, and S6K1 signaling contribute to controlling global translation downstream of CDK1. Moreover, Ribo-Seq analysis uncovered that CDK1 exerts a particularly strong effect on the translation of 5'TOP mRNAs, which includes mRNAs encoding ribosomal proteins and several translation factors. This effect requires the 5'TOP mRNA-binding protein LARP1, concurrent to our finding that LARP1 phosphorylation is strongly dependent on CDK1. Thus, CDK1 provides a direct means to couple cell proliferation with biosynthesis of the translation machinery and the rate of protein synthesis.

An mTORC1-to-CDK1 Switch Maintains Autophagy Suppression during Mitosis

Since nuclear envelope breakdown occurs during mitosis in metazoan cells, it has been proposed that macroautophagy must be inhibited to maintain genome integrity. However, repression of macroautophagy during mitosis remains controversial and mechanistic detail limited to the suggestion that CDK1 phosphorylates VPS34. Here, we show that initiation of macroautophagy, measured by the translocation of the ULK complex to autophagic puncta, is repressed during mitosis, even when mTORC1 is inhibited. Indeed, mTORC1 is inactive during mitosis, reflecting its failure to localize to lysosomes due to CDK1-dependent RAPTOR phosphorylation. While mTORC1 normally represses autophagy via phosphorylation of ULK1, ATG13, ATG14, and TFEB, we show that the mitotic phosphorylation of these autophagy regulators, including at known repressive sites, is dependent on CDK1 but independent of mTOR. Thus, CDK1 substitutes for inhibited mTORC1 as the master regulator of macroautophagy during mitosis, uncoupling autophagy regulation from nutrient status to ensure repression of macroautophagy during mitosis.

Mitosis-related phosphorylation of the eukaryotic translation suppressor 4E-BP1 and its interaction with eukaryotic translation initiation factor 4E (eIF4E)

Eukaryotic translation initiation factor 4E (eIF4E)–binding protein 1 (4E-BP1) inhibits cap-dependent translation in eukaryotes by competing with eIF4G for an interaction with eIF4E. Phosphorylation at Ser-83 of 4E-BP1 occurs during mitosis through the activity of cyclin-dependent kinase 1 (CDK1)/cyclin B rather than through canonical mTOR kinase activity. Here, we investigated the interaction of eIF4E with 4E-BP1 or eIF4G during interphase and mitosis. We observed that 4E-BP1 and eIF4G bind eIF4E at similar levels during interphase and mitosis. The most highly phosphorylated mitotic 4E-BP1 isoform (δ) did not interact with eIF4E, whereas a distinct 4E-BP1 phospho-isoform, EB-γ, phosphorylated at Thr-70, Ser-83, and Ser-101, bound to eIF4E during mitosis. Two-dimensional gel electrophoretic analysis corroborated the identity of the phosphorylation marks on the eIF4E-bound 4E-BP1 isoforms and uncovered a population of phosphorylated 4E-BP1 molecules lacking Thr-37/Thr-46–priming phosphorylation. Moreover, proximity ligation assays for phospho-4E-BP1 and eIF4E revealed different in situ interactions during interphase and mitosis. The eIF4E:eIF4G interaction was not inhibited but rather increased in mitotic cells, consistent with active translation initiation during mitosis. Phosphodefective substitution of 4E-BP1 at Ser-83 did not change global translation or individual mRNA translation profiles as measured by single-cell nascent protein synthesis and eIF4G RNA immunoprecipitation sequencing. Mitotic 5′-terminal oligopyrimidine RNA translation was active and, unlike interphase translation, resistant to mTOR inhibition. Our findings reveal the phosphorylation profiles of 4E-BP1 isoforms and their interactions with eIF4E throughout the cell cycle and indicate that 4E-BP1 does not specifically inhibit translation initiation during mitosis.

Mammalian cell growth dynamics in mitosis

The extent and dynamics of animal cell biomass accumulation during mitosis are unknown, primarily because growth has not been quantified with sufficient precision and temporal resolution. Using the suspended microchannel resonator and protein synthesis assays, we quantify mass accumulation and translation rates between mitotic stages on a single-cell level. For various animal cell types, growth rates in prophase are commensurate with or higher than interphase growth rates. Growth is only stopped as cells approach metaphase-to-anaphase transition and growth resumes in late cytokinesis. Mitotic arrests stop growth independently of arresting mechanism. For mouse lymphoblast cells, growth in prophase is promoted by CDK1 through increased phosphorylation of 4E-BP1 and cap-dependent protein synthesis. Inhibition of CDK1-driven mitotic translation reduces daughter cell growth. Overall, our measurements counter the traditional dogma that growth during mitosis is negligible and provide insight into antimitotic cancer chemotherapies.

Multisite Phosphorylation of S6K1 Directs a Kinase Phospho-code that Determines Substrate Selection

Multisite phosphorylation of kinases can induce on-off or graded regulation of catalytic activity; however, its influence on substrate specificity remains unclear. Here, we show that multisite phosphorylation of ribosomal protein S6 kinase 1 (S6K1) alters target selection. Agonist-inducible phosphorylation of glutamyl-prolyl tRNA synthetase (EPRS) by S6K1 in monocytes and adipocytes requires not only canonical phosphorylation at Thr389 by mTORC1 but also phosphorylation at Ser424 and Ser429 in the C terminus by cyclin-dependent kinase 5 (Cdk5). S6K1 phosphorylation at these additional sites induces a conformational switch and is essential for high-affinity binding and phosphorylation of EPRS, but not canonical S6K1 targets, e.g., ribosomal protein S6. Unbiased proteomic analysis identified additional targets phosphorylated by multisite phosphorylated S6K1 in insulin-stimulated adipocytes-namely, coenzyme A synthase, lipocalin 2, and cortactin. Thus, embedded within S6K1 is a target-selective kinase phospho-code that integrates signals from mTORC1 and Cdk5 to direct an insulin-stimulated, post-translational metabolon determining adipocyte lipid metabolism.

Role of mTOR Signaling in Female Reproduction

Mammalian target of rapamycin (mTOR) is a conserved serine/threonine kinase of the phosphatidylinositol kinase-related kinase family that regulates cell growth, metabolism, and autophagy. Extensive research has linked mTOR to several human diseases including cancer, neurodegenerative disorders, and aging. In this review, recent publications regarding the mechanisms underlying the role of mTOR in female reproduction under physiological and pathological conditions are summarized. Moreover, we assess whether strategies to improve or suppress mTOR expression could have therapeutic potential for reproductive diseases like premature ovarian failure, polycystic ovarian syndrome, and endometriosis.

TBK1 Provides Context-Selective Support of the Activated AKT/mTOR Pathway in Lung Cancer

Emerging observations link dysregulation of TANK-binding kinase 1 (TBK1) to developmental disorders, inflammatory disease, and cancer. Biochemical mechanisms accounting for direct participation of TBK1 in host defense signaling have been well described. However, the molecular underpinnings of the selective participation of TBK1 in a myriad of additional cell biological systems in normal and pathophysiologic contexts remain poorly understood. To elucidate the context-selective role of TBK1 in cancer cell survival, we employed a combination of broad-scale chemogenomic and interactome discovery strategies to generate data-driven mechanism-of-action hypotheses. This approach uncovered evidence that TBK1 supports AKT/mTORC1 pathway activation and function through direct modulation of multiple pathway components acting both upstream and downstream of the mTOR kinase itself. Furthermore, we identified distinct molecular features in which mesenchymal, Ras-mutant lung cancer is acutely dependent on TBK1-mediated support of AKT/mTORC1 pathway activation for survival. Cancer Res; 77(18); 5077-94. ©2017 AACR.

Isoelectric point-based fractionation by HiRIEF coupled to LC-MS allows for in-depth quantitative analysis of the phosphoproteome

Protein phosphorylation is involved in the regulation of most eukaryotic cells functions and mass spectrometry-based analysis has made major contributions to our understanding of this regulation. However, low abundance of phosphorylated species presents a major challenge in achieving comprehensive phosphoproteome coverage and robust quantification. In this study, we developed a workflow employing titanium dioxide phospho-enrichment coupled with isobaric labeling by Tandem Mass Tags (TMT) and high-resolution isoelectric focusing (HiRIEF) fractionation to perform in-depth quantitative phosphoproteomics starting with a low sample quantity. To benchmark the workflow, we analyzed HeLa cells upon pervanadate treatment or cell cycle arrest in mitosis. Analyzing 300 µg of peptides per sample, we identified 22,712 phosphorylation sites, of which 19,075 were localized with high confidence and 1,203 are phosphorylated tyrosine residues, representing 6.3% of all detected phospho-sites. HiRIEF fractions with the most acidic isoelectric points are enriched in multiply phosphorylated peptides, which represent 18% of all the phospho-peptides detected in the pH range 2.5–3.7. Cross-referencing with the PhosphoSitePlus database reveals 1,264 phosphorylation sites that have not been previously reported and kinase association analysis suggests that a subset of these may be functional during the mitotic phase.

PLK1 (polo like kinase 1) inhibits MTOR complex 1 and promotes autophagy

Mechanistic target of rapamycin complex 1 (MTORC1) and polo like kinase 1 (PLK1) are major drivers of cancer cell growth and proliferation, and inhibitors of both protein kinases are currently being investigated in clinical studies. To date, MTORC1's and PLK1's functions are mostly studied separately, and reports on their mutual crosstalk are scarce. Here, we identify PLK1 as a physical MTORC1 interactor in human cancer cells. PLK1 inhibition enhances MTORC1 activity under nutrient sufficiency and in starved cells, and PLK1 directly phosphorylates the MTORC1 component RPTOR/RAPTOR in vitro. PLK1 and MTORC1 reside together at lysosomes, the subcellular site where MTORC1 is active. Consistent with an inhibitory role of PLK1 toward MTORC1, PLK1 overexpression inhibits lysosomal association of the PLK1-MTORC1 complex, whereas PLK1 inhibition promotes lysosomal localization of MTOR. PLK1-MTORC1 binding is enhanced by amino acid starvation, a condition known to increase autophagy. MTORC1 inhibition is an important step in autophagy activation. Consistently, PLK1 inhibition mitigates autophagy in cancer cells both under nutrient starvation and sufficiency, and a role of PLK1 in autophagy is also observed in the invertebrate model organism Caenorhabditis elegans. In summary, PLK1 inhibits MTORC1 and thereby positively contributes to autophagy. Since autophagy is increasingly recognized to contribute to tumor cell survival and growth, we propose that cautious monitoring of MTORC1 and autophagy readouts in clinical trials with PLK1 inhibitors is needed to develop strategies for optimized (combinatorial) cancer therapies targeting MTORC1, PLK1, and autophagy.

Cyclin-dependent Kinase 5 (Cdk5)-dependent Phosphorylation of p70 Ribosomal S6 Kinase 1 (S6K) Is Required for Dendritic Spine Morphogenesis

The maturation and maintenance of dendritic spines depends on neuronal activity and protein synthesis. One potential mechanism involves mammalian target of rapamycin, which promotes protein synthesis through phosphorylation of eIF4E-binding protein and p70 ribosomal S6 kinase 1 (S6K). Upon extracellular stimulation, mammalian target of rapamycin phosphorylates S6K at Thr-389. S6K also undergoes phosphorylation at other sites, including four serine residues in the autoinhibitory domain. Despite extensive biochemical studies, the importance of phosphorylation in the autoinhibitory domain in S6K function remains unresolved, and its role has not been explored in the cellular context. Here we demonstrated that S6K in neuron was phosphorylated at Ser-411 within the autoinhibitory domain by cyclin-dependent kinase 5. Ser-411 phosphorylation was regulated by neuronal activity and brain-derived neurotrophic factor (BDNF). Knockdown of S6K in hippocampal neurons by RNAi led to loss of dendritic spines, an effect that mimics neuronal activity blockade by tetrodotoxin. Notably, coexpression of wild type S6K, but not the phospho-deficient S411A mutant, could rescue the spine defects. These findings reveal the importance of cyclin-dependent kinase 5-mediated phosphorylation of S6K at Ser-411 in spine morphogenesis driven by BDNF and neuronal activity.

Temporal and spatial regulation of translation in the mammalian oocyte via the mTOR-eIF4F pathway

The fully grown mammalian oocyte is transcriptionally quiescent and utilizes only transcripts synthesized and stored during early development. However, we find that an abundant RNA population is retained in the oocyte nucleus and contains specific mRNAs important for meiotic progression. Here we show that during the first meiotic division, shortly after nuclear envelope breakdown, translational hotspots develop in the chromosomal area and in a region that was previously surrounded the nucleus. These distinct translational hotspots are separated by endoplasmic reticulum and Lamin, and disappear following polar body extrusion. Chromosomal translational hotspots are controlled by the activity of the mTOR–eIF4F pathway. Here we reveal a mechanism that—following the resumption of meiosis—controls the temporal and spatial translation of a specific set of transcripts required for normal spindle assembly, chromosome alignment and segregation.

Meiotic maturation of oocytes and early development of mammalian embryos is largely dependent on the translation of mRNAs stored in the oocyte. Here the authors uncover a population of mRNA retained in the oocyte nucleus whose translation is spatially and temporally regulated by the mTOR–eIF4F pathway during meiosis.

Cell cycle regulation by the nutrient-sensing mammalian target of rapamycin (mTOR) pathway

Cell division involves a series of ordered and controlled events that lead to cell proliferation. Cell cycle progression implies not only demanding amounts of cell mass, protein, lipid, and nucleic acid content but also a favorable energy state. The mammalian target of rapamycin (mTOR), in response to the energy state, nutrient status, and growth factor stimulation of cells, plays a pivotal role in the coordination of cell growth and the cell cycle. Here, we review how the nutrient-sensing mTOR-signaling cascade molecularly integrates nutritional and mitogenic/anti-apoptotic cues to accurately coordinate cell growth and cell cycle. First, we briefly outline the structure, functions, and regulation of the mTOR complexes (mTORC1 and mTORC2). Second, we concisely evaluate the best known ability of mTOR to control G1-phase progression. Third, we discuss in detail the recent evidence that indicates a new genome stability caretaker function of mTOR based on the specific ability of phosphorylated forms of several mTOR-signaling components (AMPK, raptor, TSC, mTOR, and S6K1), which spatially and temporally associate with essential mitotic regulators at the mitotic spindle and at the cytokinetic cleavage furrow.

Cell-cycle control of developmentally regulated transcription factors accounts for heterogeneity in human pluripotent cells

Heterogeneity within pluripotent stem cell (PSC) populations is indicative of dynamic changes that occur when cells drift between different states. Although the role of metastability in PSCs is unclear, it appears to reflect heterogeneity in cell signaling. Using the Fucci cell-cycle indicator system, we show that elevated expression of developmental regulators in G1 is a major determinant of heterogeneity in human embryonic stem cells. Although signaling pathways remain active throughout the cell cycle, their contribution to heterogeneous gene expression is restricted to G1. Surprisingly, we identify dramatic changes in the levels of global 5-hydroxymethylcytosine, an unanticipated source of epigenetic heterogeneity that is tightly linked to cell-cycle progression and the expression of developmental regulators. When we evaluated gene expression in differentiating cells, we found that cell-cycle regulation of developmental regulators was maintained during lineage specification. Cell-cycle regulation of developmentally regulated transcription factors is therefore an inherent feature of the mechanisms underpinning differentiation.

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Embryonic stem cells are lineage primed in G1

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Transcription of developmentally regulated genes is cell-cycle regulated

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5hmC is cell-cycle regulated

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Stem cells initiate differentiation from G1

Mitotic raptor promotes mTORC1 activity, G(2)/M cell cycle progression, and internal ribosome entry site-mediated mRNA translation

The mTOR signaling complex integrates signals from growth factors and nutrient availability to control cell growth and proliferation, in part through effects on the protein-synthetic machinery. Protein synthesis rates fluctuate throughout the cell cycle but diminish significantly during the G(2)/M transition. The fate of the mTOR complex and its role in coordinating cell growth and proliferation signals with protein synthesis during mitosis remain unknown. Here we demonstrate that the mTOR complex 1 (mTORC1) pathway, which stimulates protein synthesis, is actually hyperactive during mitosis despite decreased protein synthesis and reduced activity of mTORC1 upstream activators. We describe previously unknown G(2)/M-specific phosphorylation of a component of mTORC1, the protein raptor, and demonstrate that mitotic raptor phosphorylation alters mTORC1 function during mitosis. Phosphopeptide mapping and mutational analysis demonstrate that mitotic phosphorylation of raptor facilitates cell cycle transit through G(2)/M. Phosphorylation-deficient mutants of raptor cause cells to delay in G(2)/M, whereas depletion of raptor causes cells to accumulate in G(1). We identify cyclin-dependent kinase 1 (cdk1 [cdc2]) and glycogen synthase kinase 3 (GSK3) pathways as two probable mitosis-regulated protein kinase pathways involved in mitosis-specific raptor phosphorylation and altered mTORC1 activity. In addition, mitotic raptor promotes translation by internal ribosome entry sites (IRES) on mRNA during mitosis and is demonstrated to be associated with rapamycin resistance. These data suggest that this pathway may play a role in increased IRES-dependent mRNA translation during mitosis and in rapamycin insensitivity.

Novel Ser/Thr protein phosphatase 5 (PP5) regulated targets during DNA damage identified by proteomics analysis

The DNA damage response likely includes a global phosphorylation signaling cascade process for sensing the damaged DNA condition and coordinating responses to cope with and repair the perturbed cellular state. We utilized a label-free liquid chromatography-mass spectrometry approach to evaluate changes in protein phosphorylation associated with PP5 activity during the DNA damage response. Biological replicate analyses of bleomycin-treated HeLa cells expressing either WT-PP5 or mutant inactive PP5 lead to the identification of six potential target proteins of PP5 action. Four of these putative targets have been previously reported to be involved in DNA damage responses. Using phospho-site specific antibodies, we confirmed that phosphorylation of one target, ribosomal protein S6, was selectively decreased in cells overexpressing catalytically inactive PP5. Our findings also suggest that PP5 may play a role in controlling translation and in regulating substrates for proline-directed kinases, such as MAP kinases and cyclin-dependent protein kinases that are involved in response to DNA damage.

Raptor is phosphorylated by cdc2 during mitosis

Background

The appropriate control of mitotic entry and exit is reliant on a series of interlocking signaling events that coordinately drive the biological processes required for accurate cell division. Overlaid onto these signals that promote orchestrated cell division are checkpoints that ensure appropriate mitotic spindle formation, a lack of DNA damage, kinetochore attachment, and that each daughter cell has the appropriate complement of DNA. We recently discovered that AMP-activated protein kinase (AMPK) modulates the G2/M phase of cell cycle progression in part through its suppression of mammalian target of rapamycin (mTOR) signaling. AMPK directly phosphorylates the critical mTOR binding partner raptor inhibiting mTORC1 (mTOR-raptor rapamycin sensitive mTOR kinase complex 1). As mTOR has been previously tied to mitotic control, we examined further how raptor may contribute to this process.

Methodology/Principal Findings

We have discovered that raptor becomes highly phosphorylated in cells in mitosis. Utilizing tandem mass spectrometry, we identified a number of novel phosphorylation sites in raptor, and using phospho-specific antibodies demonstrated that raptor becomes phosphorylated on phospho-serine/threonine-proline sites in mitosis. A combination of site-directed mutagenesis in a tagged raptor cDNA and analysis with a series of new phospho-specific antibodies generated against different sites in raptor revealed that Serine 696 and Threonine 706 represent two key sites in raptor phosphorylated in mitosis. We demonstrate that the mitotic cyclin-dependent kinase cdc2/CDK1 is the kinase responsible for phosphorylating these sites, and its mitotic partner Cyclin B efficiently coimmunoprecipitates with raptor in mitotic cells.

Conclusions/Significance

This study demonstrates that the key mTOR binding partner raptor is directly phosphorylated during mitosis by cdc2. This reinforces previous studies suggesting that mTOR activity is highly regulated and important for mitotic progression, and points to a direct modulation of the mTORC1 complex during mitosis.

Quantitative phosphoproteomics reveals widespread full phosphorylation site occupancy during mitosis

Eukaryotic cells replicate by a complex series of evolutionarily conserved events that are tightly regulated at defined stages of the cell division cycle. Progression through this cycle involves a large number of dedicated protein complexes and signaling pathways, and deregulation of this process is implicated in tumorigenesis. We applied high-resolution mass spectrometry-based proteomics to investigate the proteome and phosphoproteome of the human cell cycle on a global scale and quantified 6027 proteins and 20,443 unique phosphorylation sites and their dynamics. Co-regulated proteins and phosphorylation sites were grouped according to their cell cycle kinetics and compared to publicly available messenger RNA microarray data. Most detected phosphorylation sites and more than 20% of all quantified proteins showed substantial regulation, mainly in mitotic cells. Kinase-motif analysis revealed global activation during S phase of the DNA damage response network, which was mediated by phosphorylation by ATM or ATR or DNA-dependent protein kinases. We determined site-specific stoichiometry of more than 5000 sites and found that most of the up-regulated sites phosphorylated by cyclin-dependent kinase 1 (CDK1) or CDK2 were almost fully phosphorylated in mitotic cells. In particular, nuclear proteins and proteins involved in regulating metabolic processes have high phosphorylation site occupancy in mitosis. This suggests that these proteins may be inactivated by phosphorylation in mitotic cells.

The death effector domain-containing DEDD forms a complex with Akt and Hsp90, and supports their stability

Insulin secretion and glucose transport are the major mechanisms to balance glucose homeostasis. Recently, we found that the death effector domain-containing DEDD inhibits cyclin-dependent kinase-1 (Cdk1) function, thereby preventing Cdk1-dependent inhibitory phosphorylation of S6 kinase-1 (S6K1), downstream of phosphatidylinositol 3-kinase (PI3K), which overall results in maintenance of S6K1 activity. Here we newly show that DEDD forms a complex with Akt and heat-shock protein 90 (Hsp90), and supports the stability of both proteins. Hence, in DEDD(-/-) mice, Akt protein levels are diminished in skeletal muscles and adipose tissues, which interferes with the translocation of glucose-transporter 4 (GLUT4) upon insulin stimulation, leading to inefficient incorporation of glucose in these organs. Interestingly, as for the activation of S6K1, suppression of Cdk1 is involved in the stabilization of Akt protein by DEDD, since diminishment of Cdk1 in DEDD(-/-) cells via siRNA expression or treatment with a Cdk1-inhibitor, increases both Akt and Hsp90 protein levels. Such multifaceted involvement of DEDD in glucose homeostasis by supporting both insulin secretion (via maintenance of S6K1 activity) and glucose uptake (via stabilizing Akt protein), may suggest an association of DEDD-deficiency with the pathogenesis of type 2 diabetes mellitus.

Translational control of eukaryotic gene expression

Translational control mechanisms are, besides transcriptional control and mRNA stability, the most determining for final protein levels. A large number of accessory factors that assist the ribosome during initiation, elongation, and termination of translation are required for protein synthesis. Cap-dependent translational control occurs mainly during the initiation step, involving eukaryotic initiation factors (eIFs) and accessory proteins. Initiation is affected by various stimuli that influence the phosphorylation status of both eIF4E and eIF2 and through binding of 4E-binding proteins to eIF4E, which finally inhibits cap- dependent translation. Under conditions where cap-dependent translation is hampered, translation of transcripts containing an internal ribosome entry site can still be supported in a cap-independent manner. An interesting example of translational control is the switch between cap-independent and cap-dependent translation during the eukaryotic cell cycle. At the G1-to-S transition, translation occurs predominantly in a cap-dependent manner, while during the G2-to-M transition, cap-dependent translation is inhibited and transcripts are predominantly translated through a cap-independent mechanism.

The death effector domain-containing DEDD supports S6K1 activity via preventing Cdk1-dependent inhibitory phosphorylation

Cell cycle regulation and biochemical responses upon nutrients and growth factors are the major regulatory mechanisms for cell sizing in mammals. Recently, we identified that the death effector domain-containing DEDD impedes mitotic progression by inhibiting Cdk1 (cyclin-dependent kinase 1) and thus maintains an increase of cell size during the mitotic phase. Here we found that DEDD also associates with S6 kinase 1 (S6K1), downstream of phosphatidylinositol 3-kinase, and supports its activity by preventing inhibitory phosphorylation of S6K1 brought about by Cdk1 during the mitotic phase. DEDD(-/-) cells showed reduced S6K1 activity, consistently demonstrating decreased levels in activating phosphorylation at the Thr-389 site. In addition, levels of Cdk1-dependent inhibitory phosphorylation at the C terminus of S6K1 were enhanced in DEDD(-/-) cells and tissues. Consequently, as in S6K1(-/-) mice, the insulin mass within pancreatic islets was reduced in DEDD(-/-) mice, resulting in glucose intolerance. These findings suggest a novel cell sizing mechanism achieved by DEDD through the maintenance of S6K1 activity prior to cell division. Our results also suggest that DEDD may harbor important roles in glucose homeostasis and that its deficiency might be involved in the pathogenesis of type 2 diabetes mellitus.

Microtubule interfering agents and KSP inhibitors induce the phosphorylation of the nuclear protein p54(nrb), an event linked to G2/M arrest

Microtubule interfering agents (MIAs) are anti-tumor drugs that inhibit microtubule dynamics, while kinesin spindle protein (KSP) inhibitors are substances that block the formation of the bipolar spindle during mitosis. All these compounds cause G2/M arrest and cell death. Using 2D-PAGE followed by Nano-LC-ESI-Q-ToF analysis, we found that MIAs such as vincristine (Oncovin) or paclitaxel (Taxol) and KSP inhibitors such as S-tritil-l-cysteine induce the phosphorylation of the nuclear protein p54(nrb) in HeLa cells. Furthermore, we demonstrate that cisplatin (Platinol), an anti-tumor drug that does not cause M arrest, does not induce this modification. We show that the G2/M arrest induced by MIAs is required for p54(nrb) phosphorylation. Finally, we demonstrate that CDK activity is required for MIA-induced phosphorylation of p54(nrb).

Substrate specificity of the herpes simplex virus type 2 UL13 protein kinase

The UL13 protein kinase is conserved among many herpesviruses but HSV-2 UL13 specificity is not known. Here, we found that HSV-2 UL13 is a phosphoprotein that autophosphorylates, and that serines within ERK and Cdc2 motifs were important for autophosphorylation but not for UL13 phosphorylation of exogenous substrates. HSV-2 UL13 phosphorylated a peptide also recognized by ERK and Cdc2. However, mutation of substrate residues critical for Cdc2 or Erk phosphorylation did not alter HSV-2 UL13 phosphorylation of the peptide, and HSV-2 UL13 did not phosphorylate standard Cdc2 or Erk peptide substrates. Mutation of prolines surrounding the peptide phosphoacceptor site reduced phosphorylation by HSV-2 UL13, and a peptide containing serine-proline amid alanines and glycines was phosphorylated. Thus, HSV-2 UL13 does not mimic ERK or Cdc2 substrate recognition and its minimal recognition motif can be serine-proline. This motif's simplicity indicates that distal sequence or protein structure contributes to HSV-2 UL13 substrate specificity.

Loss of Lkb1 provokes highly invasive endometrial adenocarcinomas

Mutations in the LKB1 tumor suppressor gene result in the Peutz-Jeghers syndrome, an autosomal dominant condition characterized by hamartomatous polyps of the gastrointestinal tract and a dramatically increased risk of epithelial malignancies at other sites, including the female reproductive tract. Here we show that female mice heterozygous for a null Lkb1 allele spontaneously develop highly invasive endometrial adenocarcinomas. To prove that these lesions were indeed due to Lkb1 inactivation, we introduced an adenoviral Cre vector into the uterine lumen of mice harboring a conditional allele of Lkb1. This endometrial-specific deletion of the Lkb1 gene provoked highly invasive and sometimes metastatic endometrial adenocarcinomas closely resembling those observed in Lkb1 heterozygotes. Tumors were extremely well differentiated and histopathologically distinctive and exhibited alterations in AMP-dependent kinase signaling. Although Lkb1 has been implicated in the establishment of cell polarity, and loss of polarity defines most endometrial cancers, Lkb1-driven endometrial cancers paradoxically exhibit (given their highly invasive phenotype) normal cell polarity and apical differentiation. In human endometrial cancers, Lkb1 expression was inversely correlated with tumor grade and stage, arguing that Lkb1 inactivation or down-regulation also contributes to endometrial cancer progression in women. This study shows that Lkb1 plays an important role in the malignant transformation of endometrium and that Lkb1 loss promotes a highly invasive phenotype.

Mechanism of impaired regeneration of fatty liver in mouse partial hepatectomy model

BACKGROUND AND AIM

The mechanism of injury in steatotic liver under pathological conditions been extensively examined. However, the mechanism of an impaired regeneration is still not well understood. The aim of this study was to analyze the mechanism of impaired regeneration of steatotic liver after partial hepatectomy (PH).

METHODS

db/db fatty mice and lean littermates were used for the experiments. Following 70% PH, the survival rate and recovery of liver mass were examined. Liver tissue was histologically examined and analyzed by western blotting and RT-PCR.

RESULTS

Of 35 db/db mice, 25 died within 48 h of PH, while all of the control mice survived. Liver regeneration of surviving db/db mice was largely impaired. In db/db mice, mitosis of hepatocytes after PH was disturbed, even though proliferating cell nuclear antigen (PCNA) expression (G1 to S phase marker) in hepatocytes was equally observed in both mice groups. Interestingly, phosphorylation of Cdc2 in db/db mice was suppressed by reduced expression of Wee1 and Myt1, which phosphorylate Cdc2 in S to G2 phase.

CONCLUSIONS

In steatotic liver, cell-cycle-related proliferative disorders occurred at mid-S phase after PCNA expression. Reduced expression of Wee1 and Myt1 kinases may therefore maintain Cdc2 in an unphosphorylated state and block cell cycle progression in mid-S phase. These kinases may be critical factors involved in the impaired liver regeneration in fatty liver.

Regulation of mTOR and S6K1 activation by the nPKC isoforms, PKCepsilon and PKCdelta, in adult cardiac muscle cells

Activation of both mTOR and its downstream target, S6K1 (p70 S6 kinase) have been implicated to affect cardiac hypertrophy. Our earlier work, in a feline model of 1-48 h pressure overload, demonstrated that mTOR/S6K1 activation occurred primarily through a PKC/c-Raf pathway. To further delineate the role of specific PKC isoforms on mTOR/S6K1 activation, we utilized primary cultures of adult feline cardiomyocytes in vitro and stimulated with endothelin-1 (ET-1), phenylephrine (PE), TPA, or insulin. All agonist treatments resulted in S2248 phosphorylation of mTOR and T389 and S421/T424 phosphorylation of S6K1, however only ET-1 and TPA-stimulated mTOR/S6K1 activation was abolished with infection of a dominant negative adenoviral c-Raf (DN-Raf) construct. Expression of DN-PKC(epsilon) blocked ET-1-stimulated mTOR S2448 and S6K1 S421/T424 and T389 phosphorylation but had no effect on insulin-stimulated S6K1 phosphorylation. Expression of DN-PKC(delta) or pretreatment of cardiomyocytes with rottlerin, a PKC(delta) specific inhibitor, blocked both ET-1 and insulin stimulated mTOR S2448 and S6K1 T389 phosphorylation. However, treatment with Gö6976, a specific classical PKC (cPKC) inhibitor did not affect mTOR/S6K1 activation. These data indicate that: (i) PKC(epsilon) is required for ET-1-stimulated T421/S424 phosphorylation of S6K1, (ii) both PKC(epsilon) and PKC(delta) are required for ET-1-stimulated mTOR S2448 and S6K1 T389 phosphorylation, (iii) PKC(delta) is also required for insulin-stimulated mTOR S2448 and S6K1 T389 phosphorylation. Together, these data delineate both distinct and combinatorial roles of specific PKC isoforms on mTOR and S6K1 activation in adult cardiac myocytes following hypertrophic stimulation.

A Fas-associated death domain protein/caspase-8-signaling axis promotes S-phase entry and maintains S6 kinase activity in T cells responding to IL-2

Fas-associated death domain protein (FADD) constitutes an essential component of TNFR-induced apoptotic signaling. Paradoxically, FADD has also been shown to be crucial for lymphocyte development and activation. In this study, we report that FADD is necessary for long-term maintenance of S6 kinase (S6K) activity. S6 phosphorylation at serines 240 and 244 was only observed after long-term stimulation of wild-type cells, roughly corresponding to the time before S-phase entry, and was poorly induced in T cells expressing a dominantly interfering form of FADD (FADDdd), viral FLIP, or possessing a deficiency in caspase-8. Defects in S6K1 phosphorylation were also observed. However, defective S6K1 phosphorylation was not a consequence of a wholesale defect in mammalian target of rapamycin function, because 4E-BP1 phosphorylation following T cell activation was unaffected by FADDdd expression. Although cyclin D3 up-regulation and retinoblastoma hypophosphorylation occurred normally in FADDdd T cells, cyclin E expression and cyclin-dependent kinase 2 activation were markedly impaired in FADDdd T cells. These results demonstrate that a FADD/caspase-8-signaling axis promotes T cell cycle progression and sustained S6K activity.

Cdk1 phosphorylation sites on Cdc27 are required for correct chromosomal localisation and APC/C function in syncytial Drosophila embryos

Anaphase-promoting complex or cyclosome (APC/C) controls the metaphase-to-anaphase transition and mitosis exit by triggering the degradation of key cell cycle regulators such as securin and B-type cyclins. However, little is known about the functions of individual APC/C subunits and how they might regulate APC/C activity in space and time. Here, we report that two potential Cdk1 kinase phosphorylation sites are required for the chromosomal localisation of GFP::Cdc27 during mitosis. Either or both of the highly conserved proline residues in the Cdk1 phosphorylation consensus sequence motifs were mutated to alanine (Cdc27 P304A or P456A). The singly mutated fusion proteins, GFP::Cdc27P304A and GFP::Cdc27P456A, can still localise to mitotic chromosomes in a manner identical to wild-type GFP::Cdc27 and are functional in that they can rescue the phenotype of the cdc27L7123 mutant in vivo. However, when both of the Cdk1 phosphorylation sequence motifs were mutated, the resulting GFP::Cdc27P304A,P456A construct was not localised to the chromosomes during mitosis and was no longer functional, as it failed to rescue mutant phenotypes of the cdc27L7123 gene. High levels of cyclin B and cyclin A were detected in mutant third instar larvae brain samples compared with its wild-type control. These results show for the first time that the two potential Cdk1 phosphorylation sites on Drosophila Cdc27 are required for its chromosomal localisation during mitosis and imply that these localisations specific to Cdc27 are crucial for APC/C functions.

Phosphorylation of threonine 204 of DEAD-box RNA helicase DDX3 by cyclin B/cdc2 in vitro

DDX3 is a DEAD-box RNA helicase involved in human immunodeficiency virus mRNA export and translation. Previously, we reported that DDX3 is required for cyclin A expression. To examine whether DDX3 is regulated at the post-transcriptional level, we determined the phosphorylation sites of hamster DDX3 in vitro. Threonine 204 (Thr204) is a conserved amino acid residue of DDX3 homologues in yeast, frog, hamster, and human that is located within motif Q of DEAD-box RNA helicases. A Thr204 to Glu204 DDX3 mutant protein lost its function, suggesting that phosphorylation at Thr204 affects DDX3 function. Thr204 was phosphorylated by cyclin B/cdc2. Thr323 in motif Ib was also phosphorylated by cyclin B/cdc2 kinase. We propose a novel function of cyclin B/cdc2 kinase in mitosis, which is to cause a loss of DDX3 function to repress cyclin A expression and to decrease ribosome biogenesis and translation during mitosis.

Regulation of s6 kinase 1 activation by phosphorylation at ser-411

Ribosomal S6 kinase 1 (S6K1), as a key regulator of mRNA translation, plays an important role in cell cycle progression through the G(1) phase of proliferating cells and in the synaptic plasticity of terminally differentiated neurons. Activation of S6K1 involves the phosphorylation of its multiple Ser/Thr residues, including the proline-directed sites (Ser-411, Ser-418, Thr-421, and Ser-424) in the autoinhibitory domain near the C terminus. Phosphorylation at Thr-389 is also a crucial event in S6K1 activation. Here, we report that S6K1 phosphorylation at Ser-411 is required for the rapamycin-sensitive phosphorylation of Thr-389 and the subsequent activation of S6K1. Mutation of Ser-411 to Ala ablated insulin-induced Thr-389 phosphorylation and S6K1 activation, whereas mutation mimicking Ser-411 phosphorylation did not show any effect. Furthermore, phosphomimetic mutation of Thr-389 overcame the inhibitory effect of the mutation S411A. Thus, Ser-411 phosphorylation regulates S6K1 activation via the control of Thr-389 phosphorylation. In nervous system neurons, Cdk5-p35 kinase associates with S6K1 via the direct interaction between p35 and S6K1 and catalyzes S6K1 phosphorylation specifically at Ser-411. Inhibition of the Cdk5 activity or suppression of Cdk5 expression blocked S6K1 phosphorylation at Ser-411 and Thr-389, resulting in S6K1 inactivation. Similar results were obtained by treating asynchronous populations of proliferating cells with the CDK inhibitor compound roscovitine. Altogether, our findings suggest a novel mechanism by which the CDK-mediated phosphorylation regulates the activation of S6K1.

Immunolocalization of phospho-S6 kinases: a new way to detect mitosis in tissue sections and in cell culture

During a study on the mTor pathway in the rat kidney we observed a striking increase of the phosphorylation of the S6 kinase in mitosis. In cryostat sections of perfusion-fixed tissue mitotic cells appeared as bright spots in immunofluorescence using an antibody specific for the phosphorylation site Thr421/Ser424. They were easily spotted in overviews with the objective 4× and 10×. Immunofluorescence was weak during the interphase. During the prophase it increased in both the nucleus and the cytoplasm and it remained bright during the subsequent phases of mitosis. All mitotic cells which were found in tubules and in the interstitium of the kidney using a chromatin stain displayed the bright immunofluorescence for phospho-S6 kinase. The same phenomenon was observed in rat liver and in mouse kidney as well as in a human cell line. Provided a rapid fixation, mitotic cells could be identified with the immunoperoxidase technique in paraffin sections of immersion-fixed tissue. This is the first report of phosphorylation of S6 kinase during mitosis in vivo. Thus, immunohistochemistry with anti-phospho-S6 kinase (Thr421/Ser424) appears to provide a convenient way to detect mitotic cells at low magnification.

Hydrogen peroxide impairs GRK2 translation via a calpain-dependent and cdk1-mediated pathway

Oxidative mechanisms of injury are involved in many neurodegenerative diseases such as stroke, ischemia-reperfusion injury and multiple sclerosis. G protein-coupled receptor kinase 2 (GRK2) plays a key role in G protein-coupled receptor (GPCR) signaling modulation, and its expression levels are decreased after brain hypoxia/ischemia and reperfusion as well as in several inflammatory conditions. We report here that hydrogen peroxide downregulates GRK2 expression in C6 rat glioma cells. The hydrogen peroxide-induced decrease in GRK2 is prevented by a calpain protease inhibitor, but does not involve increased GRK2 degradation or changes in GRK2 mRNA level. Instead we show that hydrogen peroxide treatment impairs GRK2 translation in a process that requires Cdk1 activation and involves the mTOR pathway. This novel mechanism for the control of GRK2 expression in glial cells upon oxidative stress challenge may contribute to the modulation of GPCR signaling in different pathological conditions.

Role of amino acids in translational mechanisms governing milk protein synthesis in murine and ruminant mammary epithelial cells

The role of amino acids (AA) on translational regulation in mammary epithelial cells cultured under lactogenic conditions was studied. The rates of total protein synthesis and beta-lactoglobulin (BLG) synthesis in mouse CID-9 cells were 2.1- or 3.1-fold higher, respectively, than in their bovine L-1 counterparts. Total AA deprivation or selective deprivation of Leu had a negative protein-specific effect on BLG synthesis that was more pronounced in bovine cells than in murine cells. Dephosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and S6 kinase (S6K1) on Thr(389) but not on Ser(411) was also more prominent in bovine cells. Noteably, deprivation of Leu had a less marked effect on BLG synthesis and 4E-BP1 or S6K1 phosphorylation than deprivation of all AA. In AA-deprived CID-9 cells, Leu specifically restored BLG synthesis from pre-existing mRNA whereas AA also restored total protein synthesis. This restoration was associated with a more pronounced effect on 4E-BP1 and S6K1 phosphorylation in bovine versus murine cells. Rapamycin specifically reduced Leu- and AA-stimulated BLG translation initiation in a dose-dependent manner. A further reduction was observed for Leu-treated cells in the presence of LY294002, a PI3K (phosphatidylinositol 3-kinase) inhibitor, which also reduced total protein synthesis. These findings suggest that direct signaling from AA to the translational machinery is involved in determining the rates of milk protein synthesis in mammary epithelial cells.

Expression and immunohistochemical localization of Cdc2 and P70S6K in different stages of mouse germ cells

In order to determine the function and possible relationship between Cdc2 and P(70)S6K, Western blot analysis and immunohistochemistry analysis were used to study the expression and kinase activity of Cdc2 and P(70)S6K in male mouse germ cells. With the maturation of germ cells in the testis, the expression of Cdc2 and P(70)S6K was relatively constant. However, the kinase activity of P(70)S6K was increased and the phosphorylation of Tyr15 residue of Cdc2 was enhanced, which suggests that the kinase activity of Cdc2 is decreasing. Immunohistochemistry analysis also showed that there was a P(70)S6K transfer from nucleus to cytoplasm during spermatogenesis. During spermatogenesis, cell division of the germ cell in male mouse is decelerated; nevertheless, cell growth is enhanced. Cdc2 and P(70)S6K are involved in these two processes. It could be an alternative mechanism to prepare for future fertilization that Cdc2 is able to maintain a subtle balance between the production and growth of male germ cells by regulating P(70)S6K.

Cytokinesis regulator ECT2 changes its conformation through phosphorylation at Thr-341 in G2/M phase

The Rho activator ECT2 functions as a key regulator in cytokinesis. ECT2 is phosphorylated during G2/M phase, but the physiological significance of this event is not well known. In this study, we show that phosphorylation of ECT2 at threonine-341 (T341) affects the autoregulatory mechanism of ECT2. In G2/M phase, ECT2 was phosphorylated at T341 most likely by Cyclin B/Cyclin-dependent kinase 1 (Cdk1), and then dephosphorylated before cytokinesis. Depletion of ECT2 by RNA interference (RNAi) efficiently induced multinucleate cells. Expression of the phospho-deficient mutant of ECT2 at T341 suppressed the multinucleation induced by RNAi to ECT2, indicating that ECT2 is biologically active even when it is not phosphorylated at T341. However, the phospho-mimic mutation at T341 weakly stimulates the catalytic activity of ECT2 as detected by serum response element reporter gene assays. As T341 is located at the hinge region of the N-terminal regulatory domain and C-terminal catalytic domain, phosphorylation of T341 may help accessing downstream signaling molecules to further activate ECT2. We found that the phospho-mimic mutation T341D increases binding with itself or the N-terminal half of ECT2. These results suggest a conformational change of ECT2 upon phosphorylation at T341. Therefore, ECT2 activity might be regulated by the phosphorylation status of T341. We propose that T341 phosphorylation by Cyclin B/Cdk1 could be a trigger for further activation of ECT2.

Translational control during mitosis

Translation is now recognized as an important process in the regulation of gene expression. During the cell cycle, translation is tightly regulated. Protein synthesis is necessary for entry into and progression through mitosis and conversely, modifications of translational activity are observed during the cell cycle. This review focuses on translational control during mitosis (or M-phase) and the role of CDK1/cyclin B, the universal cell cycle regulator implicated in the G2/M transition, in protein synthesis regulation.

Hamartin, the tuberous sclerosis complex 1 gene product, interacts with polo-like kinase 1 in a phosphorylation-dependent manner

Tuberous sclerosis complex (TSC) is a tumor suppressor gene syndrome caused by mutations in TSC1 and TSC2. Hamartin and tuberin, the products of TSC1 and TSC2, respectively, form heterodimers and inhibit the mammalian target of rapamycin. Previously, we have shown that hamartin is phosphorylated by CDC2/cyclin B1 during the G(2)/M phase of the cell cycle. Here, we report that hamartin is localized to the centrosome and that phosphorylated hamartin and phosphorylated tuberin co-immunoprecipitate with the mitotic kinase Plk1. Plk1 interacts with the N-terminus of hamartin (amino acids 1-880), which contains two potential Plk1-binding sites (T310 and S332). Phosphorylated hamartin interacts with Plk1 independent of tuberin with all three proteins present in a complex. A non-phosphorylatable hamartin mutant with an alanine substitution at residue T310 does not interact with Plk1, whereas a non-phosphorylatable hamartin mutant at residue S332 in conjunction with alanine mutations at the other CDC2/cyclin B1 sites (T417, S584 and T1047) does not impact hamartin binding to Plk1. Hamartin negatively regulates the protein levels of Plk1. Finally, Tsc1(-/-) mouse embryonic fibroblasts (MEFs) have increased number of centrosomes and increased DNA content, compared to Tsc1(+/+) cells. Both phenotypes are rescued after pre-treatment with the mTOR inhibitor rapamycin. RNAi inhibition of Plk1 in Tsc1(-/-) MEFs failed to rescue the increased centrosome number phenotype. These data reveal a novel subcellular localization for hamartin and a novel interaction partner for the hamartin/tuberin complex and implicate hamartin and mTOR in the regulation of centrosome duplication.

Rapamycin: an anti-cancer immunosuppressant?

Rapamycin and its derivatives are promising therapeutic agents with both immunosuppressant and anti-tumor properties. These rapamycin actions are mediated through the specific inhibition of the mTOR protein kinase. mTOR serves as part of an evolutionarily conserved signaling pathway that controls the cell cycle in response to changing nutrient levels. The mTOR signaling network contains a number of tumor suppressor genes including PTEN, LKB1, TSC1, and TSC2, and a number of proto-oncogenes including PI3K, Akt, and eIF4E, and mTOR signaling is constitutively activated in many tumor types. These observations point to mTOR as an ideal target for anti-cancer agents and suggest that rapamycin is such an agent. In fact, early preclinical and clinical studies indicate that rapamycin derivatives have efficacy as anti-tumor agents both alone, and when combined with other modes of therapy. Rapamycin appears to inhibit tumor growth by halting tumor cell proliferation, inducing tumor cell apoptosis, and suppressing tumor angiogenesis. Rapamycin immunosuppressant actions result from the inhibition of T and B cell proliferation through the same mechanisms that rapamycin blocks cancer cell proliferation. Therefore, one might think that rapamycin-induced immunosuppression would be detrimental to the use of rapamycin as an anti-cancer agent. To the contrary, rapamycin decreases the frequency of tumor formation that occurs in organ transplant experiments when combined with the widely used immunosuppressant cyclosporine compared with the tumor incidence observed when cyclosporine is used alone. The available evidence indicates that with respect to tumor growth, rapamycin anti-cancer activities are dominant over rapamycin immunosuppressant effects.

Nuclear PTEN-mediated growth suppression is independent of Akt down-regulation

The tumor suppressor gene PTEN is a phosphoinositide phosphatase that is inactivated by deletion and/or mutation in diverse human tumors. Wild-type PTEN is expressed both in the cytoplasm and nucleus in normal cells, with a preferential nuclear localization in differentiated or resting cells. To elucidate the relationship between PTEN's subcellular localization and its biologic activities, we constructed different PTEN mutants that targeted PTEN protein into different subcellular compartments. Our data show that the subcellular localization patterns of a PTEN (deltaPDZB) mutant versus a G129R phosphatase mutant were indistinguishable from those of wild-type PTEN. In contrast, the Myr-PTEN mutant demonstrated an enhanced association with the cell membrane. We found that nuclear PTEN alone is capable of suppressing anchorage-independent growth and facilitating G1 arrest in U251MG cells without inhibiting Akt activity. Nuclear compartment-specific PTEN-induced growth suppression is dependent on possessing a functional lipid phosphatase domain. In addition, the down-regulation of p70S6K could be mediated, at least in part, through activation of AMP-activated protein kinase in an Akt-independent fashion. Introduction of a constitutively active mutant of Akt, Akt-DD, only partially rescues nuclear PTEN-mediated growth suppression. Our collective results provide the first direct evidence that PTEN can contribute to G1 growth arrest through an Akt-independent signaling pathway.

Paclitaxel induces the phosphorylation of the eukaryotic translation initiation factor 4E-binding protein 1 through a Cdk1-dependent mechanism

Initial chemotherapeutic treatment triggers a stress-related response, which can lead to an increase in the expression of survival proteins. In this study we examine whether paclitaxel (PTX) alters the expression and/or phosphorylation of the translation initiation proteins, eukaryotic initiation factor 4E (eIF-4E) and 4E-binding protein (4E-BP1), a suppressor of eIF-4E in the dephosphorylated state. We found that PTX induced the hyperphosphorylation of 4E-BP1 in the breast cancer cell line, MDA MB 231, which reduced its association with eIF-4E, but did not alter the expression and phosphorylation of eIF-4E. The hyperphosphorylation of 4E-BP1 correlated with G2/M accumulation and with an increase in the phosphorylation of cdk1 substrates. Cotreatment with a histone deacetylase inhibitor (an indirect inhibitor of cdk activity), purvalanol A and roscovitine (direct cdk inhibitors), and the reduction of cyclin B expression using RNA interference decreased the hyperphosphorylation of 4E-BP1 in PTX treated cells. The hyperphosphorylation of 4E-BP1 by PTX increased the association of eIF-4E with eIF-4G, whereas cotreatment with purvalanol A inhibited the association of eIF-4E with eIF-4G in PTX treated cells. Taken together, our data suggest that PTX-increases the functional level of eIF-4E by promoting the hyperphosphorylation and release of 4E-BP1 through a cdk1-dependent mechanism.