Modulation of human DNA topoisomerase IIalpha function by interaction with 14-3-3epsilon

14-3-3ε: a protein with complex physiology function but promising therapeutic potential in cancer

Over the past decade, the role of the 14-3-3 protein has received increasing interest. Seven subtypes of 14-3-3 proteins exhibit high homology; however, each subtype maintains its specificity. The 14-3-3ε protein is involved in various physiological processes, including signal transduction, cell proliferation, apoptosis, autophagy, cell cycle regulation, repolarization of cardiac action, cardiac development, intracellular electrolyte homeostasis, neurodevelopment, and innate immunity. It also plays a significant role in the development and progression of various diseases, such as cardiovascular diseases, inflammatory diseases, neurodegenerative disorders, and cancer. These immense and various involvements of 14-3-3ε in diverse processes makes it a promising target for drug development. Although extensive research has been conducted on 14-3-3 dimers, studies on 14-3-3 monomers are limited. This review aimed to provide an overview of recent reports on the molecular mechanisms involved in the regulation of binding partners by 14-3-3ε, focusing on issues that could help advance the frontiers of this field. Video Abstract.

Topoisomerase II β Gene Specific siRNA Delivery by Nanoparticles Prepared with c-ter Apotransferrin and its Effect on HIV-1 Replication

Topoisomerase II beta (Topo IIβ) is one of the two isoforms of type II topoisomerases present in higher eukaryotes. This 180 kDa nuclear protein involves in different cellular processes like transcription, recombination, etc., apart from its normal topological functions. Previously, we have reported the association of this isoform along with the other isoform topoisomerase II alpha (Topo IIα) with HIV-1 reverse transcription complex and the downregulation of Topo IIβ expression resulted in incomplete reverse transcription. In this study, we have tested the Topo IIβ specific siRNA delivery using protein nanoparticles prepared with c-terminal domine of transferrin (c-ter) for the first time. Results show that, c-ter nanoparticles resemble apotransferrin nanoparticles in drug holding capability and drug delivery but with small in size. Topo IIβ specific siRNA delivered in the form of c-ter nanoformulation resulted in knockdown of Topo IIβ expression for the prolonged periods and which intern resulted in decreased viral replication of HIV-1.

Copper Complexes as Anticancer Agents Targeting Topoisomerases I and II

Organometallics, such as copper compounds, are cancer chemotherapeutics used alone or in combination with other drugs. One small group of copper complexes exerts an effective inhibitory action on topoisomerases, which participate in the regulation of DNA topology. Copper complexes inhibitors of topoisomerases 1 and 2 work by different molecular mechanisms, analyzed herein. They allow genesis of DNA breaks after the formation of a ternary complex, or act in a catalytic mode, often display DNA intercalative properties and ROS production, and sometimes display dual effects. These amplified actions have repercussions on the cell cycle checkpoints and death effectors. Copper complexes of topoisomerase inhibitors are analyzed in a broader synthetic view and in the context of cancer cell mutations. Finally, new emerging treatment aspects are depicted to encourage the expansion of this family of highly active anticancer drugs and to expend their use in clinical trials and future cancer therapy.

Cell Cycle-Dependent Control and Roles of DNA Topoisomerase II

Type II topoisomerases are ubiquitous enzymes in all branches of life that can alter DNA superhelicity and unlink double-stranded DNA segments during processes such as replication and transcription. In cells, type II topoisomerases are particularly useful for their ability to disentangle newly-replicated sister chromosomes. Growing lines of evidence indicate that eukaryotic topoisomerase II (topo II) activity is monitored and regulated throughout the cell cycle. Here, we discuss the various roles of topo II throughout the cell cycle, as well as mechanisms that have been found to govern and/or respond to topo II function and dysfunction. Knowledge of how topo II activity is controlled during cell cycle progression is important for understanding how its misregulation can contribute to genetic instability and how modulatory pathways may be exploited to advance chemotherapeutic development.

Targeting nucleolin for better survival in diffuse large B-cell lymphoma

Anthracyclines have been a cornerstone in the cure of diffuse large B-cell lymphoma (DLBCL) and other hematological cancers. The ability of anthracyclines to eliminate DLBCL depends on the presence of topoisomerase-II-alpha (TopIIA), a DNA repair enzyme complex. We identified nucleolin as a novel binding partner of TopIIA. Abrogation of nucleolin sensitized DLBCL cells to TopIIA targeting agents (doxorubicin/etoposide). Silencing nucleolin and challenging DLBCL cells with doxorubicin enhanced the phosphorylation of H2AX (γH2AX-marker of DNA damage) and allowed DNA fragmentation. Reconstitution of nucleolin expression in nucleolin-knockdown DLBCL cells prevented TopIIA targeting agent-induced apoptosis. Nucleolin binding to TopIIA was mapped to RNA-binding domain 3 of nucleolin, and this interaction was essential for blocking DNA damage and apoptosis. Nucleolin silencing decreased TopIIA decatenation activity, but enhanced formation of TopIIA-DNA cleavable complexes in the presence of etoposide. Moreover, combining nucleolin inhibitors: aptamer AS1411 or nucant N6L with doxorubicin reduced DLBCL cell survival. These findings are of clinical importance because low nucleolin levels versus high nucleolin levels in DLBCL predicted 90-month estimated survival of 70% versus 12% (P<0.0001) of patients treated with R-CHOP-based therapy.

The C-terminal 20 Amino Acids of Drosophila Topoisomerase 2 Are Required for Binding to a BRCA1 C Terminus (BRCT) Domain-containing Protein, Mus101, and Fidelity of DNA Segregation

Eukaryotic topoisomerase 2 (Top2) and one of its interacting partners, topoisomerase IIβ binding protein 1 (TopBP1) are two proteins performing essential cellular functions. We mapped the interacting domains of these two proteins using co-immunoprecipitation and pulldown experiments with truncated or mutant Drosophila Top2 with various Ser-to-Ala substitutions. We discovered that the last 20 amino acids of Top2 represent the key region for binding with Mus101 (the Drosophila homolog of TopBP1) and that phosphorylation of Ser-1428 and Ser-1443 is important for Top2 to interact with the N terminus of Mus101, which contains the BRCT1/2 domains. The interaction between Mus101 and the Top2 C-terminal regulatory domain is phosphorylation-dependent because treatment with phosphatase abolishes their association in pulldown assays. The binding affinity of N-terminal Mus101 with a synthetic phosphorylated peptide spanning the last 25 amino acids of Top2 (with Ser(P)-1428 and Ser(P)-1443) was determined by surface plasmon resonance with a Kd of 0.57 μm In an in vitro decatenation assay, Mus101 can specifically reduce the decatenation activity of Top2, and dephosphorylation of Top2 attenuates this response. Next, we endeavored to establish a cellular system for testing the biological function of Top2-Mus101 interaction. Top2-silenced S2 cells rescued by Top2Δ20, Top2 with 20 amino acids truncated from the C terminus, developed abnormally high chromosome numbers, which implies that Top2-Mus101 interaction is important for maintaining the fidelity of chromosome segregation during mitosis.

Proteomic identification of 14-3-3ϵ as a linker protein between pERK1/2 inhibition and BIM upregulation in human osteosarcoma cells

Despite advancements in multimodality chemotherapy, conventional cytotoxic treatments still remain ineffective for a subset of patients with aggressive metastatic or multifocal osteosarcoma. It has been shown that pERK1/2 inhibition enhances chemosensitivity to doxorubicin and promotes osteosarcoma cell death in vivo and in vitro. One of the pro-apoptotic mechanisms is upregulation of Bim by pERK1/2 inhibitors. To this end, we examined proteomic changes of 143B human osteosarcoma cells with and without treatment of PD98059, pERK1/2 inhibitor. Specifically, we identified 14-3-3ϵ protein as a potential mediator of Bim expression in response to inhibition of pERK1/2. We hypothesized that 14-3-3ϵ mediates upregulation of Bim expression after pERK1/2 inhibition. We examined the expression of Bim after silencing 14-3-3ϵ using siRNA. The 14-3-3ϵ gene silencing resulted in downregulation of Bim expression after PD98059 treatment. These data indicate that 14-3-3ϵ is required for Bim expression and that it has an anti-cancer effect under pERK1/2 inhibition in 143B cells. By playing an essential role upstream of Bim, 14-3-3ϵ may potentially be a coadjuvant factor synergizing the effect of pERK1/2 inhibitors in addition to conventional cytotoxic agents for more effective osteosarcoma treatments.

Salicylate, a catalytic inhibitor of topoisomerase II, inhibits DNA cleavage and is selective for the α isoform

Topoisomerase II (topo II) is a ubiquitous enzyme that is essential for cell survival through its role in regulating DNA topology and chromatid separation. Topo II can be poisoned by common chemotherapeutics (such as doxorubicin and etoposide), leading to the accumulation of cytotoxic enzyme-linked DNA double-stranded breaks. In contrast, nonbreak-inducing topo II catalytic inhibitors have also been described and have more limited use in clinical chemotherapy. These agents, however, may alter the efficacy of regimens incorporating topo II poisons. We previously identified salicylate, the primary metabolite of aspirin, as a novel catalytic inhibitor of topo II. We have now determined the mechanism by which salicylate inhibits topo II. As catalytic inhibitors can act at a number of steps in the topo II catalytic cycle, we used multiple independent, biochemical approaches to interrogate the catalytic cycle. Furthermore, as mammalian cells express two isoforms of topo II (α and β), we examined whether salicylate was isoform selective. Our results demonstrate that salicylate is unable to intercalate DNA, and does not prevent enzyme-DNA interaction, nor does it promote stabilization of topo IIα in closed clamps on DNA. Although salicylate decreased topo IIα ATPase activity in a dose-dependent noncompetitive manner, this was secondary to salicylate-mediated inhibition of DNA cleavage. Surprisingly, comparison of salicylate's effects using purified human topo IIα and topo IIβ revealed that salicylate selectively inhibits the α isoform. These findings provide a definitive mechanism for salicylate-mediated inhibition of topo IIα and provide support for further studies determining the basis for its isoform selectivity.

Therapeutic intervention by the simultaneous inhibition of DNA repair and type I or type II DNA topoisomerases: one strategy, many outcomes

Many anticancer drugs reduce the integrity of DNA, forming strand breaks. This can cause mutations and cancer or cell death if the lesions are not repaired. Interestingly, DNA repair-deficient cancer cells (e.g., those with BRCA1/2 mutations) have been shown to exhibit increased sensitivity to chemotherapy. Based on this observation, a new therapeutic approach termed 'synthetic lethality' has been developed, in which radiation therapy or cytotoxic anticancer agents are employed in conjunction with selective inhibitors of poly(ADP-ribose)polymerase-1 (PARP-1). Such combinations can cause severe genomic instability in transformed cells resulting in cell death. The synergistic effects of combining PARP-1 inhibition with anticancer drugs have been demonstrated. However, the outcome of this therapeutic strategy varies significantly between cancer types, suggesting that synthetic lethality may be influenced by additional cellular factors. This review focuses on the outcomes of the combined action of PARP-1 inhibitors and agents that affect the activity of DNA topoisomerases.

Ku70 and Rad51 vary in their importance for the repair of doxorubicin- versus etoposide-induced DNA damage

For DNA targeting anticancer drugs, cellular DNA repair mechanisms may cause resistance and hamper the therapeutic outcome. DNA damage induced by topoisomerase IIα inhibitors like etoposide and anthracyclines, which are a mainstay of cancer therapy, is also repaired in many cell types, but the impact and precise mechanisms of this repair are still obscure. To investigate the DNA damage response of human adenocarcinoma HT29-cells to doxorubicin and to compare the involvement of Ku70 and Rad51 in the repair of doxorubicin- versus etoposide-induced DNA damage, we assessed cell cycle distribution and cell death, DNA damage, proteins relevant for repair by homologous recombination and non-homologous end-joining, and clonogenicity following exposure to doxorubicin at clinically achievable concentrations. Also, we assessed changes in the repair kinetics after siRNA-mediated attenuation of Ku70 or Rad51 expression. We found that exposure to doxorubicin for 24 h induced a substantial amount of DNA damage that was largely repaired when doxorubicin was removed and the cells were maintained in drug-free medium. Nevertheless, a pronounced G(2)/M arrest occurred at times when repair was maximal. This was followed by a distinct increase in cell death and loss of clonogenicity. In this regard, responses to doxorubicin and etoposide were similar. However, distinct differences in the repair process following doxorubicin versus etoposide were seen in concentration dependency, time-course and requirement of Ku70 and Rad51 proteins. In spite of the shared molecular target of doxorubicin and etoposide, DNA lesions induced by these compounds are repaired differently.

14-3-3epsilon protein increases matrix metalloproteinase-2 gene expression via p38 MAPK signaling in NIH3T3 fibroblast cells

One of the 14-3-3 protein isoforms, 14-3-3epsilon, was previously shown to be increased during skin aging. We suggest here a possible role for the 14-3-3epsilon protein in skin aging by providing evidence that 14-3-3epsilon increases the expression of the matrix-metalloproteinase (MMP)-2 gene in NIH3T3 fibroblast cells. Expression of the 14-3-3epsilon gene in NIH3T3 cells primarily up-regulated the expression of the MMP-2 gene at the transcriptional level by inducing specific DNA binding proteins bound to an upstream region of the MMP-2 promoter from -1,629 to -1,612. Inhibition of endogenous 14-3-3epsilon gene expression by RNA interference also decreased endogenous MMP-2 gene expression. Furthermore, up-regulation of the MMP-2 gene by 14-3-3epsilon was suppressed by expression of a dominant-negative mutant of p38 MAP kinase. These findings strongly suggest that increased expression of 14-3-3epsilon contributes to remodeling of extracellular matrix in skin through increasing MMP-2 gene expression via p38 MAP kinase signaling.

DNA topoisomerase II and its growing repertoire of biological functions

DNA topoisomerases are enzymes that disentangle the topological problems that arise in double-stranded DNA. Many of these can be solved by the generation of either single or double strand breaks. However, where there is a clear requirement to alter DNA topology by introducing transient double strand breaks, only DNA topoisomerase II (TOP2) can carry out this reaction. Extensive biochemical and structural studies have provided detailed models of how TOP2 alters DNA structure, and recent molecular studies have greatly expanded knowledge of the biological contexts in which TOP2 functions, such as DNA replication, transcription and chromosome segregation -- processes that are essential for preventing tumorigenesis.

DNA topoisomerases: harnessing and constraining energy to govern chromosome topology

DNA topoisomerases are a diverse set of essential enzymes responsible for maintaining chromosomes in an appropriate topological state. Although they vary considerably in structure and mechanism, the partnership between topoisomerases and DNA has engendered commonalities in how these enzymes engage nucleic acid substrates and control DNA strand manipulations. All topoisomerases can harness the free energy stored in supercoiled DNA to drive their reactions; some further use the energy of ATP to alter the topology of DNA away from an enzyme-free equilibrium ground state. In the cell, topoisomerases regulate DNA supercoiling and unlink tangled nucleic acid strands to actively maintain chromosomes in a topological state commensurate with particular replicative and transcriptional needs. To carry out these reactions, topoisomerases rely on dynamic macromolecular contacts that alternate between associated and dissociated states throughout the catalytic cycle. In this review, we describe how structural and biochemical studies have furthered our understanding of DNA topoisomerases, with an emphasis on how these complex molecular machines use interfacial interactions to harness and constrain the energy required to manage DNA topology.

Repair of idarubicin-induced DNA damage: A cause of resistance?

Idarubicin, a widely used anticancer drug inhibits topoisomerase (topo) IIalpha and induces DNA double strand breaks. The finding that idarubicin-induced DNA damage is repaired before cell death is initiated encouraged us to examine the role of DNA repair for the cytotoxicity of idarubicin in human promyelocytic HL60 leukaemia cells. We found that DNA double strand breaks induced by a 90 min transient exposure to 0.5 microgml(-1) idarubicin were rapidly repaired throughout the whole population, while topo IIalpha itself was degraded. In spite of DNA repair, the vast majority of cells died within 40 h. Using differential staining of the chromatids and microscopic evaluation of DNA break points, we found evidence for a high number of false ligations of loose DNA strands arising from the inhibition of topo IIalpha action by idarubicin. If mainly actively transcribed genes are affected, this results in a disruption of vital genetic information, of regulatory sequences and, ultimately, in induction of the cell death pathway. Our results confirm the hypothesis that misrepair of DNA damage is a decisive event in idarubicin-induced cell death. They are discussed in the context of topo IIalpha-function and the currently known mechanisms of DNA double strand break repair.

Nuclear interactions of topoisomerase II alpha and beta with phospholipid scramblase 1

DNA topoisomerase (topo) II modulates DNA topology and is essential for cell division. There are two isoforms of topo II (α and β) that have limited functional redundancy, although their catalytic mechanisms appear the same. Using their COOH-terminal domains (CTDs) in yeast two-hybrid analysis, we have identified phospholipid scramblase 1 (PLSCR1) as a binding partner of both topo II α and β. Although predominantly a plasma membrane protein involved in phosphatidylserine externalization, PLSCR1 can also be imported into the nucleus where it may have a tumour suppressor function. The interactions of PLSCR1 and topo II were confirmed by pull-down assays with topo II α and β CTD fusion proteins and endogenous PLSCR1, and by co-immunoprecipitation of endogenous PLSCR1 and topo II α and β from HeLa cell nuclear extracts. PLSCR1 also increased the decatenation activity of human topo IIα. A conserved basic sequence in the CTD of topo IIα was identified as being essential for binding to PLSCR1 and binding of the two proteins could be inhibited by a synthetic peptide corresponding to topo IIα amino acids 1430-1441. These studies reveal for the first time a physical and functional interaction between topo II and PLSCR1.

Topoisomerase II binds importin alpha isoforms and exportin/CRM1 but does not shuttle between the nucleus and cytoplasm in proliferating cells

Resistance to anticancer drugs that target DNA topoisomerase II (topo II) isoforms alpha and/or beta is associated with decreased nuclear and increased cytoplasmic topo IIalpha. Earlier studies have confirmed that functional nuclear localization and export signal sequences (NLS and NES) are present in both isoforms. In this study, we show that topo II alpha and beta bind and are imported into the nucleus by importin alpha1, alpha3, and alpha5 in conjunction with importin beta. Topo IIalpha also binds exportin/CRM1 in vitro. However, wild-type topo IIalpha has only been observed in the cytoplasm of cells that are entering plateau phase growth. This suggests that topo IIalpha may shuttle between the nucleus and the cytoplasm with the equilibrium towards the nucleus in proliferating cells but towards the cytoplasm in plateau phase cells. The CRM1 inhibitor Leptomycin B increases the nuclear localization of GFP-tagged topo IIalpha with a mutant NLS, suggesting that its export is being inhibited. However, homokaryon shuttling experiments indicate that fluorescence-tagged wild-type topo II alpha and beta proteins do not shuttle in proliferating Cos-1 or HeLa cells. We conclude that topo II alpha and beta nuclear export is inhibited in proliferating cells so that these proteins do not shuttle.

14-3-3 proteins: a historic overview

This chapter includes a historic overview of 14-3-3 proteins with an emphasis on the differences between potentially cancer-relevant isoforms on the genomic, protein and functional level. The focus will therefore be on mammalian 14-3-3s although many important developments in the field have involved Drosophila 14-3-3 proteins for example and the cross-fertilisation from parallel studies on plant 14-3-3 should not be underestimated. In the major part of this review I will attempt to focus on some novel data and aspects of 14-3-3 structure and function, in particular regulation of 14-3-3 isoforms by oncogene-related protein kinase phosphorylation and aspects of 14-3-3 research with which newcomers to the field may be less familiar.

Down-regulation of DNA topoisomerase IIalpha leads to prolonged cell cycle transit in G2 and early M phases and increased survival to microtubule-interacting agents

Microtubule binders are cell cycle-specific agents with preferential cytotoxicity toward mitotic cells. We have characterized vincristine-selected human leukemia cells to establish whether development of vincristine resistance was accompanied by changes in cell cycle kinetics and distribution. Our results indicate that vincristine resistance is accompanied by delayed G2 transit and prolonged early mitosis in both the absence and the presence of the microtubule binder nocodazole. The altered G2/M regulation is accompanied by resistance to short-term (12 h) but not continuous nocodazole exposure in agreement with the transient nature of the observed cell cycle alterations. Western blot analysis indicates that vincristine-selection is accompanied by down-regulation of topoisomerase IIalpha without detectable alterations of the other mitotic regulators studied, including Cdk1, p21, 14-3-3sigma, and 14-3-3epsilon. This was associated with at least 7-fold less chromosome-associated topoisomerase IIalpha, decreased catalytic activity, and cross-resistance to topoisomerase II inhibitors. Characterization of isogenic cell lines expressing different levels of topoisomerase II proteins shows that cellular levels of topoisomerase IIalpha, but not the closely related topoisomerase IIbeta, directly influence the cell cycle kinetics in G2 and early mitosis as well as the resistance to nocodazole. These results underline the importance of topoisomerase IIalpha in late G2 and early M phases and provide evidence for an as-yet-unsuspected interaction between topoisomerase II and microtubule-directed agents.

14-3-3 proteins: a number of functions for a numbered protein

Many signal transduction events are orchestrated by specific interactions of proteins mediated through discrete phosphopeptide-binding motifs. Although several phosphospecific-binding domains are now known, 14-3-3s were the first proteins recognized to specifically bind a discrete phosphoserine or phosphothreonine motif. The 14-3-3 proteins are a family of ubiquitously expressed, exclusively eukaryotic proteins with an astonishingly large number of binding partners. Consequently, 14-3-3s modulate an enormous and diverse group of cellular processes. The effects of 14-3-3 proteins on their targets can be broadly defined using three categories: (i) conformational change; (ii) physical occlusion of sequence-specific or structural protein features; and (iii) scaffolding. This review will describe the current state of knowledge on 14-3-3 proteins, highlighting several important advances, and will attempt to provide a framework by which 14-3-3 functions can be understood.

Increased expression of 14-3-3varepsilon protein in intrinsically aged and photoaged human skin in vivo

Skin aging is a complicated process associated with the passage of time and environmental exposure, especially to UV light. This aging phenomenon is related to alterations in various cellular mechanisms, such as changes in apoptosis, perturbations to cellular signaling, and an increased genetic instability. In this study, we investigated changes of proteins involved in intrinsic aging by the proteomic analysis of human sun-protected (upper inner arm) young and aged dermis. One of the proteins upregulated in aged dermis was identified as 14-3-3epsilon. This protein is an isoform of 14-3-3 protein, which is involved in cellular processes like signal transduction, cell cycle arrest, and apoptosis. 14-3-3epsilon is consistently found to be upregulated in the sun-protected dermis of aged skin, by Western blotting and immunohistochemical staining. In addition, we demonstrate that the expression of 14-3-3epsilon is further upregulated in the sun-exposed (photodamaged) dermis, and that the UV irradiation of young skin significantly upregulates 14-3-3epsilon in vivo. Our results suggest the possibility that the cellular processes related to 14-3-3epsilon protein play an important role in the photoaging and intrinsic aging of human skin.

14-3-3 proteins: a number of functions for a numbered protein

Many signal transduction events are orchestrated by specific interactions of proteins mediated through discrete phosphopeptide-binding motifs. Although several phosphospecific-binding domains are now known, 14-3-3s were the first proteins recognized to specifically bind a discrete phosphoserine or phosphothreonine motif. The 14-3-3 proteins are a family of ubiquitously expressed, exclusively eukaryotic proteins with an astonishingly large number of binding partners. Consequently, 14-3-3s modulate an enormous and diverse group of cellular processes. The effects of 14-3-3 proteins on their targets can be broadly defined using three categories: (i) conformational change; (ii) physical occlusion of sequence-specific or structural protein features; and (iii) scaffolding. This review will describe the current state of knowledge on 14-3-3 proteins, highlighting several important advances, and will attempt to provide a framework by which 14-3-3 functions can be understood.

Isoform-specific differences in rapid nucleocytoplasmic shuttling cause distinct subcellular distributions of 14-3-3 sigma and 14-3-3 zeta

Nucleocytoplasmic transport of proteins plays an important role in the regulation of many cellular processes. Differences in nucleocytoplasmic shuttling can provide a basis for isoform-specific biological functions for members of multigene families, like the 14-3-3 protein family. Many organisms contain multiple 14-3-3 isoforms, which play a role in numerous processes, including signalling, cell cycle control and apoptosis. It is still unclear whether these isoforms have specialised biological functions and whether this specialisation is based on isoform-specific ligand binding, expression regulation or specific localisation. Therefore, we studied the subcellular distribution of 14-3-3 sigma and 14-3-3 zeta in vivo in various mammalian cell types using yellow fluorescent protein fusions and isoform-specific antibodies. 14-3-3 sigma was mainly localised in the cytoplasm and only low levels were present in the nucleus, whereas 14-3-3 zeta was found at relatively higher levels in the nucleus. Fluorescence recovery after photobleaching (FRAP) experiments indicated that the 14-3-3 proteins rapidly shuttle in and out of the nucleus through active transport and that the distinct subcellular distributions of 14-3-3 sigma and 14-3-3 zeta are caused by differences in nuclear export. 14-3-3 sigma had a 1.7x higher nuclear export rate constant than 14-3-3 zeta, while import rate constants were equal. The 14-3-3 proteins are exported from the nucleus at least in part by a Crm1-dependent, leptomycin B-sensitive mechanism. The differences in subcellular distribution of 14-3-3 that we found in this study are likely to reflect a molecular basis for isoform-specific biological specialisation.

Rep68 protein of adeno-associated virus type 2 interacts with 14-3-3 proteins depending on phosphorylation at serine 535

Rep78/68 proteins of adeno-associated virus type 2 (AAV-2) are involved in many aspects of the viral life cycle, including replication, gene expression, and site-specific integration. To understand the molecular mechanisms of the actions of Rep proteins, we searched for Rep68-interacting cellular proteins by utilizing a one-step affinity purification technique and identified two members of 14-3-3 proteins (14-3-3 epsilon and gamma). We found that phosphorylation of 535Ser at the carboxy terminus of Rep68 was critical for its association with 14-3-3. The association of 14-3-3 proteins to Rep68 resulted in reduction of the affinity of Rep68 for DNA. Furthermore, genome DNA replication of a recombinant mutant virus carrying a phosphorylation-deficient Rep68 (Ser535Ala) was more efficient than that of the wild-type virus. These results suggest that phosphorylation of Rep68 and subsequent association with 14-3-3 proteins regulates Rep-mediated functions during the AAV life cycle.

Aberrant lamination in the cerebral cortex of mouse embryos lacking DNA topoisomerase IIbeta

We have examined corticogenesis in mouse embryos lacking DNA topoisomerase IIbeta (IIbeta) in the brain or in all tissues. The absence of IIbeta, a type II DNA topoisomerase normally expressed in postmitotic cells in the developing cortex, severely affects cerebral stratification: no subplate is discernible, and neurons born at later stages of corticogenesis fail to migrate to the superficial layers. This abnormal pattern of neuron positioning in the cerebral cortex is reminiscent of that observed in mouse mutants defective in the reelin-signaling pathway. Significantly, the level of reelin in the neocortex is much reduced when IIbeta is absent. These results implicate a role of IIbeta in brain development. The enzyme may be required in implementing particular genetic programs in postmitotic cells, such as reelin expression in Cajal-Retzius cells, perhaps through its action on nucleoprotein structure of particular chromosomal regions.

14-3-3sigma is a cruciform DNA binding protein and associates in vivo with origins of DNA replication

A human cruciform binding protein (CBP) was previously shown to bind to cruciform DNA in a structure-specific manner and be a member of the 14-3-3 protein family. CBP had been found to contain the 14-3-3 isoforms beta, gamma, epsilon, and zeta. Here, we show by Western blot analysis that the CBP-cruciform DNA complex eluted from band-shift polyacrylamide gels also contains the 14-3-3sigma isoform, which is present in HeLa cell nuclear extracts. An antibody specific for the 14-3-3sigma isoform was able to interfere with the formation of the CBP-cruciform DNA complex. The effect of the same anti-14-3-3sigma antibody in the in vitro replication of p186, a plasmid containing the minimal replication origin of the monkey origin ors8, was also analyzed. Pre-incubation of total HeLa cell extracts with this antibody decreased p186 in vitro replication to approximately 30% of control levels, while non-specific antibodies had no effect. 14-3-3sigma was found to associate in vivo with the monkey origins of DNA replication ors8 and ors12 in a cell cycle-dependent manner, as assayed by a chromatin immunoprecipitation (ChIP) assay that involved formaldehyde cross-linking, followed by immunoprecipitation with anti-14-3-3sigma antibody and quantitative PCR. The association of 14-3-3sigma with the replication origins was maximal at the G(1)/S phase. The results indicate that 14-3-3sigma is an origin binding protein involved in the regulation of DNA replication via cruciform DNA binding.

Culprit and victim -- DNA topoisomerase II

The phylogenetic antiquity of DNA topoisomerases indicates their vital function. Structure and maintenance of genomic DNA depend on the activity of these enzymes, and without them DNA replication and cell division are impossible. Topoisomerase II alpha has therefore become the main target of many antitumour therapy regimens, even though the exact mechanism of cell killing remains elusive. The success of this approach is limited by the development of spontaneous resistance, and drug-induced DNA damage can increase malignancy. Nevertheless, the combined use of topoisomerase-inhibiting drugs with different mechanisms of action promises to improve particular treatment designs. The degree of topoisomerase II expression in tumours may predict the clinical course and responsiveness to therapy.

Catalytic inhibition of human DNA topoisomerase IIalpha by hypericin, a naphthodianthrone from St. John's wort (Hypericum perforatum)

St. John's wort (Hypericum perforatum) is the most widely used herbal medicine for the treatment of depression. However, concerns have arisen about the potential of its interaction with other drugs due to the induction of cytochrome P450 isozymes 1A2 and 3A4 by the components hypericin and hyperforin, respectively. Structurally similar natural products are often employed as antitumor agents due to their action as inhibitors of DNA topoisomerases, nuclear enzymes that modify DNA during cellular proliferation. Preliminary findings that hypericin inhibited the DNA relaxation activity of topoisomerase IIalpha (topo II; EC 5.99.1.3) led us to investigate the mechanism of enzyme inhibition. Rather than stabilizing the enzyme in covalent complexes with DNA (cleavage complexes), hypericin inhibited the enzyme prior to DNA cleavage. In vitro assays indicate that hypericin is a potent antagonist of cleavage complex stabilization by the chemotherapeutics etoposide and amsacrine. This antagonism appears to be due to the ability of hypericin to intercalate or distort DNA structure, thereby precluding topo II binding and/or DNA cleavage. Supporting its non-DNA damaging, catalytic inhibition of topo II, hypericin was shown to be equitoxic to both wild-type and amsacrine-resistant HL-60 leukemia cell lines. Moreover, hypericin was incapable of stimulating DNA damage-responsive gene promoters that are activated by etoposide. As with the in vitro topo II assay, antagonism of DNA damage stimulated by 30 microM etoposide was evident in leukemia cells pretreated with 5 microM hypericin. Since many cancer patients experience clinical depression and concomitantly self-medicate with herbal remedies, extracts of St. John's wort should be investigated further for their potential to antagonize topo II-directed chemotherapy regimens.

Recruitment of cdc2 kinase by DNA topoisomerase II is coupled to chromatin remodeling

Although initiation of chromosome condensation during early prophase is linked temporally to the appearance of the mitotic cdc2 kinase in the nucleus, it is not known what targets the kinase to the nucleus and how this is coupled to chromatin remodeling. We now report that cdc2 kinase forms stable molecular complexes with the nuclear enzyme DNA topoisomerase II, which is associated with marked stimulation of both DNA binding and catalytic activity of topoisomerase II, albeit in a phosphorylation-independent manner. The molecular interaction is required for recruitment of cdc2 kinase, as shown by incubation of purified enzymes with chicken erythrocyte nuclei, which have neither endogenous topoisomerase II nor cdc2 kinase. The physical association between the two enzymes alters the DNA/topoisomerase II interaction as shown by pulse-field electrophoresis after incubation of intact nuclei with the specific topoisomerase II inhibitor VM-26. Furthermore, the presence of both enzymes, but not either enzyme alone, is accompanied by extensive chromatin remodeling converting the interphase nuclei into precondensation chromosomes with striking resemblance to early prophase structures. Our results reveal a novel property of cyclin-dependent kinases and demonstrate that the recruitment of cdc2 kinase by topoisomerase II is coupled to chromatin remodeling.

Benzene metabolites antagonize etoposide-stabilized cleavable complexes of DNA topoisomerase IIalpha

Chronic exposure to benzene is associated with hematotoxicity and acute myelogenous leukemia. Inhibition of topoisomerase IIalpha (topo II) has been implicated in the development of benzene-induced cytogenetic aberrations. The purpose of this study was to determine the mechanism of topo II inhibition by benzene metabolites. In a DNA cleavage/relaxation assay, topo II was inhibited by p-benzoquinone and hydroquinone at 10 microM and 10 mM, respectively. On peroxidase activation, inhibition was seen with 4,4'-biphenol, hydroquinone, and catechol at 10 microM, 10 microM, and 30 microM, respectively. But, in no case was cleavable complex stabilization observed and the metabolites appeared to act at an earlier step of the enzyme cycle. In support of this conclusion, several metabolites antagonized etoposide-stabilized cleavable complex formation and inhibited topo II-DNA binding. It is therefore unlikely that benzene-induced acute myelogenous leukemia stems from events invoked for leukemogenic topo II cleavable complex-stabilizing antitumor agents. (Blood. 2001;98:830-833)

Phosphoserine/threonine-binding domains

Phosphorylation of proteins on serine and threonine residues has traditionally been viewed as a means to allosterically regulate catalytic activity. Research within the past five years, however, has revealed that serine/threonine phosphorylation can also directly result in the formation of multimolecular signaling complexes through specific interactions between phosphoserine/threonine (pSer/Thr)-binding modules and phosphorylated sequence motifs. pSer/Thr-binding proteins and domains currently include 14-3-3, WW domains, forkhead-associated domains, and, tentatively, WD40 repeats and leucine-rich regions. It seems likely that additional modules will be found in the future. The amino acid sequences recognized by these pSer/Thr-binding modules show partial overlap with the optimal phosphorylation motifs for different protein kinase subfamilies, allowing the formation of specific signaling complexes to be controlled through combinatorial interactions between particular upstream kinases and a particular binding module. The structural basis for pSer/Thr binding differs dramatically between 14-3-3 proteins, WW domains and forkhead-associated domains, suggesting that their pSer/Thr binding function was acquired through convergent evolution.