Okadaic acid inhibits a protein phosphatase activity involved in formation of the mitotic spindle of GH4 rat pituitary cells

Effects of marine phycotoxin dinophysistoxin-1 on the growth and cell cycle of Isochrysis galbana

The phycotoxin dinophysistoxins are widely distributed in the global marine environments and potentially threaten marine organisms and human health. The mechanism of the dinophysistoxin toxicity in inhibiting the growth of microalgae is less well understood. In this study, effects of the dissolved dinophysistoxin-1 (DTX1) on the growth, pigment contents, PSII photosynthetic efficiency, oxidative stress response and cell cycle of the marine microalga Isochrysis galbana were investigated. Growth of I. galbana was significantly inhibited by DTX1 with 0.6-1.5 μmol L-1 in a 96-h batch culture, corresponding the 96 h-EC50 of DTX1 at 0.835 μmol L-1. The maximum quantum yield of PSII (Fv/Fm), and light utilization efficiency (α) were obviously reduced by DTX1 at 1.5 μmol L-1 during 96-h exposure. Contents of most of pigments were generally reduced by DTX1 with a dose-depend pattern in microalgal cells except for diatoxanthin. The ROS levels were increased by DTX1 with 0.6-1.5 μmol L-1 after 72-h exposure, while the contents or activities of MDA, GSH, SOD and CAT were significantly increased by DTX1 at 1.5 μmol L-1 at 96 h. The inhibitory effect of DTX1 on the growth of I. galbana was mainly caused by the production of ROS in the cells. Cell cycle analysis showed that the I. galbana cell cycle was arrested by DTX1 at G2/M phase. This study enhances the understanding of the chemical ecology effects of DTX1 on marine microalgae and also provides fundamental data for deriving water quality criteria of DSTs for marine organisms.

Bridging Integrator-1 protein loss in Alzheimer's disease promotes synaptic tau accumulation and disrupts tau release

Polymorphisms associated with BIN1 (bridging integrator 1) confer the second greatest risk for developing late-onset Alzheimer’s disease. The biological consequences of this genetic variation are not fully understood; however, BIN1 is a binding partner for tau. Tau is normally a highly soluble cytoplasmic protein, but in Alzheimer’s disease, tau is abnormally phosphorylated and accumulates at synapses to exert synaptotoxicity. The purpose of this study was to determine whether alterations in BIN1 and tau in Alzheimer’s disease promote the damaging redistribution of tau to synapses, as a mechanism by which BIN1 polymorphisms may increase the risk of developing Alzheimer’s disease. We show that BIN1 is lost from the cytoplasmic fraction of Alzheimer’s disease cortex, and this is accompanied by the progressive mislocalization of phosphorylated tau to synapses. We confirmed proline 216 in tau as critical for tau interaction with the BIN1-SH3 domain and showed that the phosphorylation of tau disrupts this binding, suggesting that tau phosphorylation in Alzheimer’s disease disrupts tau–BIN1 associations. Moreover, we show that BIN1 knockdown in rat primary neurons to mimic BIN1 loss in Alzheimer’s disease brain causes the damaging accumulation of phosphorylated tau at synapses and alterations in dendritic spine morphology. We also observed reduced release of tau from neurons upon BIN1 silencing, suggesting that BIN1 loss disrupts the function of extracellular tau. Together, these data indicate that polymorphisms associated with BIN1 that reduce BIN1 protein levels in the brain likely act synergistically with increased tau phosphorylation to increase the risk of Alzheimer’s disease by disrupting cytoplasmic tau–BIN1 interactions, promoting the damaging mis-sorting of phosphorylated tau to synapses to alter synapse structure and reducing the release of physiological forms of tau to disrupt tau function.

The Mouse Bioassay for Diarrhetic Shellfish Poisoning: A Gross Misuse of Laboratory Animals and of Scientific Methodology

The UK shellfish industry has recently been affected by the statutory closure of several cockle beds, following the detection of samples causing rapid and severe reactions in the regulatory approved test for diarrhetic shellfish poisoning (DSP) toxins, the mouse bioassay (MBA). It is contended that these so-called atypical results are due to procedural artefacts of the MBA; so far, several studies have failed to identify their cause. This paper critically assesses the development, regulatory use and methodological deficiencies of the MBA. It also discusses how testing for DSP toxins could and should have been improved and made more humane by applying the Three Rs concept of Reduction, Refinement and Replacement, and by the proper validation of the test method used. It is concluded that the MBA should not have been developed for the routine screening of shellfish samples, as it has a substantially severe endpoint and is not used as part of a tiered-testing strategy with non-animal methods. Moreover, during the UK monitoring programme for DSP toxins, the assay has been used without an optimised and universal protocol, and apparently without due regard to the principles of basic scientific methodology. In view of this, the atypical results obtained for cockle samples cannot be relied on as evidence of a human health hazard. It is recommended that the use of the MBA should be discontinued as soon as possible, in favour of other methods, especially those involving non-animal techniques. In the short-term, these methods should be based on analytical chemical detection systems and the essential availability of the relevant pure toxin standards. The lack of any known toxins in samples should be taken as evidence of lack of contamination. The suitability of the existing non-animal methods needs to be assessed as a matter of urgency. It is crucial that all new methods should be properly validated, and that their acceptability for their stated purposes should be endorsed by recognised criteria and validation centres, before being recommended to, or required by, regulatory agencies. In this way, the possibility that scientifically unsuitable methods will once again be used for monitoring for the contamination of shellfish with toxins can be avoided. This gross misuse of laboratory animals and ill-judged application of science should never be allowed to occur again.

FGFR4 polymorphic alleles modulate mitochondrial respiration: A novel target for somatostatin analog action in pituitary tumors

We reported that a single nucleotide polymorphism (SNP) at codon 388 of the fibroblast growth factor receptor 4 (FGFR4-Gly388Arg) can result in distinct proteins that alter pituitary cell growth and function. Here, we examined the differential properties of the available therapeutic somatostatin analogs, octreotide and pasireotide, in pituitary tumor cells expressing the different FGFR4 isoforms. Consistent with their enhanced growth properties, FGFR4-R388-expressing cells show higher mitochondrial STAT3 serine phosphorylation driving basal and maximal oxygen consumption rate (OCR) than pituitary cells expressing the more common FGFR4-G388 isoform. While both somatostatin analogs reduce the OCR in FGFR4-G388 cells, pasireotide was more effective in decreasing OCR in cells expressing the variant FGFR4-R388 isoform. Down-regulation of somatostatin receptor 5 (SSTR5) abrogated the effect of pasireotide, demonstrating its involvement in mediating this action. The effects on OCR were recapitulated by introducing a constitutively active serine STAT3 but not by a tyrosine-active mutant. Moreover, pharmacologic inhibition demonstrated the role for the phosphatase PP2A in mediating the dephosphorylation of STAT3-S727 by pasireotide. Our data indicate that FGFR4 polymorphic isoforms mediate signaling that yields mitochondrial therapeutic targets of relevance to the actions of different somatostatin analogs.

Microcystin-LR induces mitotic spindle assembly disorders in Vicia faba by protein phosphatase inhibition and not reactive oxygen species induction

We aimed to reveal the mechanisms of mitotic spindle anomalies induced by microcystin-LR (MCY-LR), a cyanobacterial toxin in Vicia faba, a well-known model in plant cell and molecular biology. MCY-LR inhibits type 1 and 2A phosphoserine/threonine specific protein phosphatases (PP1 and PP2A) and induces reactive oxygen species (ROS) formation. The cytoskeleton is one of the main targets of the cyanotoxin during cytopathogenesis. Histochemical-immunohistochemical and biochemical methods were used. A significant number of MCY-LR induced spindle alterations are described for the first time. Disrupted, multipolar spindles and missing kinetochore fibers were detected both in metaphase and anaphase cells. Additional polar microtubule (MT) bundles, hyperbundling of spindle MTs, monopolar spindles, C-S- shaped, additional and asymmetric spindles were detected in metaphase, while midplane kinetochore fibers were detected in anaphase cells only. Several spindle anomalies induced mitotic disorders, i.e. they occurred concomitantly with altered sister chromatid separation. Alterations were dependent on the MCY-LR dose and exposure time. Under long-term (2 and mainly 6 days') exposure they were detected in the concentration range of 0.1-20μgmL(-1) MCY-LR that inhibited PP1 and PP2A significantly without significant ROS induction. Elevated peroxidase/catalase activities indicated that MCY-LR treated V. faba plants showed efficient defense against oxidative stress. Thus, although the elevation of ROS is known to induce cytoskeletal aberrations in general, this study shows that long-term protein phosphatase inhibition is the primary cause of MCY-LR induced spindle disorders.

Okadaic acid meet and greet: an insight into detection methods, response strategies and genotoxic effects in marine invertebrates

Harmful Algal Blooms (HABs) constitute one of the most important sources of contamination in the oceans, producing high concentrations of potentially harmful biotoxins that are accumulated across the food chains. One such biotoxin, Okadaic Acid (OA), is produced by marine dinoflagellates and subsequently accumulated within the tissues of filtering marine organisms feeding on HABs, rapidly spreading to their predators in the food chain and eventually reaching human consumers causing Diarrhetic Shellfish Poisoning (DSP) syndrome. While numerous studies have thoroughly evaluated the effects of OA in mammals, the attention drawn to marine organisms in this regard has been scarce, even though they constitute primary targets for this biotoxin. With this in mind, the present work aimed to provide a timely and comprehensive insight into the current literature on the effect of OA in marine invertebrates, along with the strategies developed by these organisms to respond to its toxic effect together with the most important methods and techniques used for OA detection and evaluation.

The CHROMEVALOA database: a resource for the evaluation of Okadaic Acid contamination in the marine environment based on the chromatin-associated transcriptome of the mussel Mytilus galloprovincialis

Okadaic Acid (OA) constitutes the main active principle in Diarrhetic Shellfish Poisoning (DSP) toxins produced during Harmful Algal Blooms (HABs), representing a serious threat for human consumers of edible shellfish. Furthermore, OA conveys critical deleterious effects for marine organisms due to its genotoxic potential. Many efforts have been dedicated to OA biomonitoring during the last three decades. However, it is only now with the current availability of detailed molecular information on DNA organization and the mechanisms involved in the maintenance of genome integrity, that a new arena starts opening up for the study of OA contamination. In the present work we address the links between OA genotoxicity and chromatin by combining Next Generation Sequencing (NGS) technologies and bioinformatics. To this end, we introduce CHROMEVALOAdb, a public database containing the chromatin-associated transcriptome of the mussel Mytilus galloprovincialis (a sentinel model organism) in response to OA exposure. This resource constitutes a leap forward for the development of chromatin-based biomarkers, paving the road towards the generation of powerful and sensitive tests for the detection and evaluation of the genotoxic effects of OA in coastal areas.

Chromatin specialization in bivalve molluscs: a leap forward for the evaluation of Okadaic Acid genotoxicity in the marine environment

Marine biotoxins synthesized by Harmful Algal Blooms (HABs) represent one of the most important sources of contamination in marine environments as well as a serious threat to fisheries and aquaculture-based industries in coastal areas. Among these biotoxins Okadaic Acid (OA) is of critical interest as it represents the most predominant Diarrhetic Shellfish Poisoning biotoxin in the European coasts. Furthermore, OA is a potent tumor promoter with aneugenic and clastogenic effects on the hereditary material, most notably DNA breaks and alterations in DNA repair mechanisms. Therefore, a great effort has been devoted to the biomonitoring of OA in the marine environment during the last two decades, mainly based on physicochemical and physiological parameters using mussels as sentinel organisms. However, the molecular genotoxic effects of this biotoxin make chromatin structure a good candidate for an alternative strategy for toxicity assessment with faster and more sensitive evaluation. To date, the development of chromatin-based studies to this purpose has been hampered by the complete lack of information on chromatin of invertebrate marine organisms, especially in bivalve molluscs. Our preliminary results have revealed the presence of histone variants involved in DNA repair and chromatin specialization in mussels and clams. In this work we use this information to put forward a proposal focused on the development of chromatin-based tests for OA genotoxicity in the marine environment. The implementation of such tests in natural populations has the potential to provide an important leap in the biomonitoring of this biotoxin. The outcome of such monitoring may have critical implications for the evaluation of DNA damage in these marine organisms. They will provide as well important tools for the optimization of their harvesting and for the elaboration of additional tests designed to evaluate the safety of their consumption and potential implications for consumer's health.

Identification of differentially expressed genes in SHSY5Y cells exposed to okadaic acid by suppression subtractive hybridization

Background

Okadaic acid (OA), a toxin produced by several dinoflagellate species is responsible for frequent food poisonings associated to shellfish consumption. Although several studies have documented the OA effects on different processes such as cell transformation, apoptosis, DNA repair or embryogenesis, the molecular mechanistic basis for these and other effects is not completely understood and the number of controversial data on OA is increasing in the literature.

Results

In this study, we used suppression subtractive hybridization in SHSY5Y cells to identify genes that are differentially expressed after OA exposure for different times (3, 24 and 48 h). A total of 247 subtracted clones which shared high homology with known genes were isolated. Among these, 5 specific genes associated with cytoskeleton and neurotransmission processes (NEFM, TUBB, SEPT7, SYT4 and NPY) were selected to confirm their expression levels by real-time PCR. Significant down-regulation of these genes was obtained at the short term (3 and 24 h OA exposure), excepting for NEFM, but their expression was similar to the controls at 48 h.

Conclusions

From all the obtained genes, 114 genes were up-regulated and 133 were down-regulated. Based on the NCBI GenBank and Gene Ontology databases, most of these genes are involved in relevant cell functions such as metabolism, transport, translation, signal transduction and cell cycle. After quantitative PCR analysis, the observed underexpression of the selected genes could underlie the previously reported OA-induced cytoskeleton disruption, neurotransmission alterations and in vivo neurotoxic effects. The basal expression levels obtained at 48 h suggested that surviving cells were able to recover from OA-caused gene expression alterations.

Okadaic acid induces morphological changes, apoptosis and cell cycle alterations in different human cell types

Okadaic acid (OA) is a marine toxin produced by dinoflagellate species which is frequently accumulated in molluscs usual in the human diet. The exact action mechanism of OA has not been described yet and the results of most reported studies are often conflicting. The aim of this work was to evaluate the OA effects on morphology, cell cycle and apoptosis induction by means of light microscopy and flow cytometry, in three different types of human cells (leukocytes, HepG2 cells and SHSY5Y cells). Cells were treated with a range of OA concentrations in the presence and absence of S9 fraction. OA induced morphological changes in all the cell types studied, and cell cycle disruption only in leukocytes and neuronal cells. SHSY5Y cells were the most sensitive to OA assault. Results obtained in the presence and absence of metabolic activation were similar, suggesting that OA acts both directly and indirectly. Furthermore, OA was found to increase the subG(1) region in the flow cytometry cell cycle analysis, suggesting induction of apoptosis. These results were confirmed by the employment of specific methodologies for studying apoptosis such as caspase 3 activation and annexin V staining. Increases in the apoptosis rate were obtained in all the cells treated in the absence of S9 fraction, accompanied by increases in caspase 3 activation, suggesting that apoptosis induced by OA is a caspase 3-dependent process. Nevertheless, in the presence of S9 fraction no apoptosis was detected, indicating a metabolic detoxifying activity, although necrosis was observed in neuroblastoma cells.

Evaluation of genotoxicity in gills and hemolymph of clam Ruditapes decussatus fed with the toxic dinoflagellate Prorocentrum lima

Diarrheic shellfish poisoning (DSP) is a gastrointestinal (GIT) disease that appears a few hours after ingesting okadaic acid (OA)-contaminated mollusks; okadaic acid is present in dinoflagellates of the genera Dinophysis and Prorocentrum. Toxic manifestations occur all year round at a higher or lesser intensity, and as a consequence, extractive production factories need to be closed during these periods which affects the economy of aquaculture industries. Although the concentration of harmful algae is usually found at high levels in clam digestive gland, bivalve mortality was not increased. In this study, the genotoxic effects produced by OA in clam Ruditapes decussatus were determined using the comet assay. In vitro (exposing hemocytes to different concentrations of OA) and in vivo (feeding clams with toxic dinoflagellate Prorocentrum lima) experiments were conducted in order to determine the genotoxic effects of OA on bivalve cells. Hemocytes and gill cells were analyzed by in vivo and in vitro approaches. While the in vitro study showed a rapid effect of OA on hemocytes, data obtained in the in vivo experiment reflected contradictory results dependent upon the concentration of OA and cell type evaluated. An increase in DNA damage was observed at the lower concentration and only in gill tissue. The results obtained may contribute to a better understanding of the mechanisms underlying genotoxic effects induced by OA on bivalves.

Evaluation of okadaic acid-induced genotoxicity in human cells using the micronucleus test and γH2AX analysis

Marine algal blooms have become a public health concern due to increasing frequency in the environment and severity of exposure consequences. Human intoxications produced by phycotoxins occur globally through consumption of marine fish products containing bioaccumulated toxins. Okadaic acid (OA) is the main representative of diarrheic shellfish poisoning (DSP) toxin. OA was found to inhibit protein phosphatases and to produce oxidative damage, as well as to disturb different cellular functions including cell cycle, gene expression, and DNA repair mechanisms. The aim of this study was to determine whether OA induced genotoxicity by using a micronucleus (MN) test and γH2AX analysis, and to elucidate the underlying mechanisms. Human peripheral blood leukocytes, neuroblastoma cells (SHSY5Y), and hepatoma cells (HepG2) were treated with a range of OA concentrations in the presence and absence of S9 fraction. MN induction was observed in leukocytes at all concentrations tested, and in SHSY5Y and HepG2 cells only at the highest concentration (1000 nM). In contrast, γH2AX analysis was only positive for HepG2 cells. Taking together these data, in addition to the comet assay results obtained in a previous study in this issue, OA was found to exert a either a clastogenic or aneugenic effect dependent upon the cell types examined.

Assessment of okadaic acid effects on cytotoxicity, DNA damage and DNA repair in human cells

Okadaic acid (OA) is a phycotoxin produced by several types of dinoflagellates causing diarrheic shellfish poisoning (DSP) in humans. Symptoms induced by DSP toxins are mainly gastrointestinal, but the intoxication does not appear to be fatal. Despite this, this toxin presents a potential threat to human health even at concentrations too low to induce acute toxicity, since previous animal studies have shown that OA has very potent tumour promoting activity. However, its concrete action mechanism has not been described yet and the results reported with regard to OA cytotoxicity and genotoxicity are often contradictory. In the present study, the genotoxic and cytotoxic effects of OA on three different types of human cells (peripheral blood leukocytes, HepG2 hepatoma cells, and SHSY5Y neuroblastoma cells) were evaluated. Cells were treated with a range of OA concentrations in the presence and absence of S9 fraction, and MTT test and Comet assay were performed in order to evaluate cytotoxicity and genotoxicity, respectively. The possible effects of OA on DNA repair were also studied by means of the DNA repair competence assay, using bleomycin as DNA damage inductor. Treatment with OA in absence of S9 fraction induced not statistically significant decrease in cell viability and significant increase in DNA damage in all cell types at the highest concentrations investigated. However, only SHSY5Y cells showed OA induced genotoxic and cytotoxic effects in presence of S9 fraction. Furthermore, we found that OA can induce modulations in DNA repair processes when exposure was performed prior to BLM treatment, in co-exposure, or during the subsequent DNA repair process.

Shugoshin collaborates with protein phosphatase 2A to protect cohesin

Sister chromatid cohesion, mediated by a complex called cohesin, is crucial--particularly at centromeres--for proper chromosome segregation in mitosis and meiosis. In animal mitotic cells, phosphorylation of cohesin promotes its dissociation from chromosomes, but centromeric cohesin is protected by shugoshin until kinetochores are properly captured by the spindle microtubules. However, the mechanism of shugoshin-dependent protection of cohesin is unknown. Here we find a specific subtype of serine/threonine protein phosphatase 2A (PP2A) associating with human shugoshin. PP2A colocalizes with shugoshin at centromeres and is required for centromeric protection. Purified shugoshin complex has an ability to reverse the phosphorylation of cohesin in vitro, suggesting that dephosphorylation of cohesin is the mechanism of protection at centromeres. Meiotic shugoshin of fission yeast also associates with PP2A, with both proteins collaboratively protecting Rec8-containing cohesin at centromeres. Thus, we have revealed a conserved mechanism of centromeric protection of eukaryotic chromosomes in mitosis and meiosis.

Characteristics of okadaic acid--induced cytotoxic effects in CHO K1 cells

This article reports the results of investigations into the process of cell death induced in the Chinese hamster ovary cell K1 subclone (CHO K1) by okadaic acid (OA), a hydrophobic polyether produced by marine dinoflagellates. The IC50 was about 13 nM OA after 24 h of treatment, as determined using neutral red. With the MTT assay, the IC50 was 25 nM, although in this case 25% of the initial staining was still observed at 100 nM. Hoechst staining showed that mitotic figures accumulated at 12 nM OA after a 24- or 48-h treatment. In experiments limited to a 3-day treatment without changing the medium, CHO K1 cells were engaged in the death process at 50 nM OA after about 20 h and at 10 nM OA after 48 h. In many cells nuclear fragmentation that resulted in the apparent appearance of vesicles correlated with increasing cellular volume. But additional cell fragmentation was not observed with any treatment, and the chromatin material seemed to progressively disappear inside the cells. DNA fragmentation was analyzed by electrophoresis and with the TUNEL technique. With both techniques, the DNA was fragmented by 48 h in both 25 and 50 nM OA. Electrophoresis showed that both adherent and nonadherent cells were affected. Annexin-positive/ propidium iodide (PI)-negative cells were rarely observed after OA treatment. Some were seen under the scanning cytometer after 20 h at 50 nM OA or after 48 h at 10 nM OA, but they were never detected by flow cytometry. Most of the time scanning cytometry showed either unstained cells or PI-positive (annexin-positive or -negative) cells (48 h, 50 nM, or 72 h, 10 nM). Flow cytometry cytograms showed two cell subpopulations: one composed of a majority of smaller cells, the other of larger cells. The larger cells markedly decreased with time and OA treatment (50 and 100 nM). Stained-cell counting showed that all cells that stained were both annexin- and PI positive and that most PI-positive cells were smaller. Ki67 antigen labeling showed the proliferative activity of CHO K1 cultures but also demonstrated the loss of this activity in smaller cells treated with 50 nM OA for 48 h. We concluded that in our culture conditions the main OA target within CHO K1 cultures was dividing cells. Our results suggest that cells with disturbed metaphase-anaphase enter apoptosis, leading to necrotic daughter cells.

Specific inhibition of mouse oocyte nuclear protein phosphatase-1 stimulates germinal vesicle breakdown

Okadaic acid (OA)-induced germinal vesicle breakdown (GVBD) and localization of protein phosphatase-1 (PP1) in oocyte nuclei are suggestive of PP1's role in regulating oocyte GVBD. To explore this possibility, we microinjected protein phosphatase (PP) inhibitors OA, anti-PP1 antibody (anti-PP1), PP1 inhibitor I2, and anti-PP2A antibody (anti-PP2A) into nuclei of roscovitine (ROSC)-arrested mouse oocytes. Oocytes were also injected with recombinant PP1 in the absence of ROSC. Oocytes were assessed for GVBD and metaphase II (MII) development at 2 and 18 hr post-injection. Data were analyzed using Cochran-Mantel-Haenszel Statistics adjusted for time. Microinjection of OA significantly enhanced GVBD in comparison to controls at 2 and 18 hr (P < 0.01), yet had no effect on MII development. Similarly, microinjection of anti-PP1 resulted in significantly higher levels of GVBD compared to controls at 2 and 18 hr (P < 0.01). Interestingly, anti-PP1 microinjection also tended to enhance MII development at 18 hr in comparison to controls (P < 0.09). Microinjection of I2, anti-PP2A, and PP1 had no effect on GVBD or MII development. If reduction of PP1 activity was important for GVBD, one would anticipate an endogenous means of regulating PP1 activity at this developmental stage. In somatic cells, phosphorylation of PP1 at Thr320 causes PP1 inactivation. Germinal vesicle-intact oocytes did not contain phosphorylated PP1, as determined using a specific Thr320-Phospho-PP1 antibody, Western blot analysis, and confocal immunocytochemistry. At or around the time of GVBD, oocyte PP1 became phosphorylated at Thr320, which remained phosphorylated through MII development. These data indicate that inhibition of intra-nuclear PP1, through specific antibody neutralization, mimics OA-stimulated GVBD, providing the first direct evidence that nuclear PP1 is involved in regulation of oocyte nuclear membrane integrity. In addition, phosphorylation of PP1 occurs at/or around GVBD indicating that inactivation of PP1 is an important intracellular event in regulation of nuclear envelope dissolution at GVBD.

Stathmin inhibition enhances okadaic acid-induced mitotic arrest: a potential role for stathmin in mitotic exit

Stathmin is a microtubule-destabilizing phosphoprotein that plays a critical role in the regulation of mitosis. The microtubule-depolymerizing activity of stathmin is lost upon phosphorylation in mitosis. Although the role of phosphorylation of stathmin by p34(cdc2) kinase in the assembly of the mitotic spindle is well established, the role of dephosphorylation of stathmin in mitosis is unknown. In this study, we tested the hypothesis that dephosphorylation of stathmin may be critically important for the depolymerization of the mitotic spindle and the exit from mitosis. We compared the effects of okadaic acid, a specific inhibitor of serine/threonine protein phosphatases, on different parameters of mitotic progression in the presence or absence of stathmin deficiency. Because okadaic acid prevents dephosphorylation of stathmin and results in accumulation of the inactive phosphorylated form, exposure to okadaic acid would be expected to have a more profound effect on mitosis in the presence of relative stathmin deficiency. We found that inhibition of stathmin expression results in increased sensitivity to the antimitotic effects of okadaic acid. This was reflected by increased growth inhibition associated with mitotic arrest. A vast majority of the stathmin-inhibited cells were found to be arrested in late metaphase/anaphase and had severe mitotic spindle abnormalities. Exposure to okadaic acid also resulted in a bigger ratio of polymerized/unpolymerized tubulin in stathmin-inhibited cells relative to control cells. Because the only difference between the control and the stathmin-inhibited cells is the deficiency of stathmin in the latter, the increased susceptibility of the stathmin-inhibited cells to okadaic acid-induced mitotic arrest implies a role for stathmin in the later stages of mitosis.

Sequence of lethal events in HeLa cells exposed to the G2 blocking cytolethal distending toxin

The bacterial cytolethal distending toxin (CDT) was previously shown to block the cell cycle of several cell lines at stage G2 through inactivation of the cyclin-dependent kinase Cdkl and without induction of DNA strand breaks. In the present study, we have analyzed, using various methods of analytical cytometry, the progressive transformation and delayed lethal events in the tumor-derived HeLa cell line temporarily exposed to CDT. The cell proliferation arrest induced by CDT was irreversible but, starting about two days after exposure, the G2 block released partially, concomitantly with a decline in the level of Cdkl phosphorylation. This partial release resulted in endoreduplication, leading to the emergence of a significant subpopulation of cells with a 8C DNA content, and by multipolar abortive mitosis which accounted for the mortality recorded 2 and 3 days after exposure. The other major lethal event was a micronucleation process which started to be significant about 3 days after exposure and amplified later on. Both multipolar abortive mitosis and micronucleation appeared topologically related to centrosomal amplification.

Serine/threonine protein phosphatase type 1gamma1 is required for the completion of cytokinesis in human A549 lung carcinoma cells

In lower eukaryotic organisms, the loss of serine/threonine protein phosphatase type 1 (PP1) results in growth arrest after the onset of mitosis. In humans, four highly homologous isoforms of PP1 (PP1alpha, PP1delta, PP1gamma1, and PP1gamma2) have been identified. Determining the roles of these phosphatases, however, has proven difficult due to the lack of subtype-specific inhibitors. In this study, we developed chimeric antisense 2'-O-(2-methoxy)ethylphosphothioate oligonucleotides targeting human PP1gamma1 that specifically inhibit PP1gamma1 gene expression. Two potent antisense oligonucleotides (ISIS 14435 and 14439; IC(50) approximately 50 nM) were then employed to elucidate the cellular functions of PP1gamma1 during cell cycle progression. In A549 cells, the inhibition of PP1gamma1 expression resulted in a dose-dependent inhibition of cellular proliferation, with growth arrest occurring after approximately 36-48 h, when PP1gamma1 mRNA expression was inhibited by >85%. Fluorescence-activated cell sorter analysis revealed that ISIS 14435/14439-induced growth arrest was associated with an increase in the number of cells containing 4N DNA. Immunostaining of treated cells revealed that the inhibition of PP1gamma1 expression had no apparent effect on the formation of mitotic spindles. However, decreased expression was associated with the failure of cell division in a late stage of cytokinesis and the formation of dikaryons.

Apoptosis and mitotic arrest are two independent effects of the protein phosphatases inhibitor okadaic acid in K562 leukemia cells

Treatment of human myeloid leukemia K562 cells with the serine/threonine protein phosphatases inhibitor okadaic acid induced mitotic arrest followed by apoptosis in a synchronized manner. The effect was observed at drug concentrations that inhibited the protein phosphatase type 2A but not type 1. We investigated whether apoptosis was a consequence of the preceding mitosis arrest or was induced independently by okadaic acid. We found that (1) apoptosis, but not mitotic arrest, was inhibited in cells with constitutive expression of Bcl-2; (2) pretreatment of cells with the DNA synthesis inhibitor hydroxyurea blocked the mitotic arrest but not the apoptosis mediated by okadaic acid; (3) down-regulation of c-myc gene was associated with apoptosis, but not with mitotic arrest; and (4) inhibition of protein synthesis abrogated mitotic arrest, but not apoptosis. The results suggest that inhibition of protein phosphatase 2A by okadaic acid provokes mitotic arrest and apoptosis of leukemia cells by independent mechanisms.

Characterization of protein phosphatases in mouse oocytes

Okadaic acid (OA) enhances the resumption of meiosis in mouse oocytes, indicating that serine/threonine protein phosphatase-1 (PP1) and/or PP2A is involved. However, specific identification of PP1 and/or PP2A in mouse oocytes has not been reported. Here we demonstrate that fully grown germinal vesicle-intact (GVI) mouse oocytes contain mRNA corresponding to two isotypes of PP1, PP1alpha and PP1gamma. In addition, the transcript for PP2A was also present. At the protein level only PP1alpha and PP2A were recognized in fully grown GVI oocytes by Western blot analysis. Neither of the PP1gamma spliced variant proteins, PP1gamma1 and PP1gamma2, was detectable. Immunohistochemical analysis of ovarian tissue from gonadotropin-stimulated adult mice resulted in subcellular localization of both PP1alpha and PP2A, but not PP1gamma, in oocytes from all stages of folliculogenesis. In primordial oocytes, PP1alpha and PP2A were present in the cytoplasm. In more advanced stages of oogenesis, PP1alpha, although still present in the cytoplasm, was highly concentrated in the nucleus, whereas PP2A was predominantly cytoplasmic with a distinct reduction in the nuclear area. Both PP1alpha and PP2A were immunodetectable in oocytes during the prepubertal period. Eleven-day-old mouse oocytes, considered OA-insensitive and germinal vesicle breakdown (GVB)-incompetent, displayed both PP1alpha and PP2A predominantly in the cytoplasm. By 15 days of age mouse oocytes, which are beginning to acquire OA sensitivity and GVB competence, showed a relocation of PP1alpha into the nucleoplasm while PP2A remained predominantly cytoplasmic. This is the first specific identification of PP1alpha and PP2A in mouse oocytes. The differential localization of PP1alpha and PP2A, in addition to the relocation of PP1alpha during the acquisition of meiotic competence, suggests that these PPs have distinct regulatory roles during the resumption of meiosis.

Role for protein phosphatases in the cell-cycle-regulated phosphorylation of stathmin

Stathmin is a major cytosolic phosphoprotein that regulates microtubule dynamics during the assembly of the mitotic spindle. The activity of stathmin itself is regulated by changes in its state of phosphorylation during the transition from interphase to metaphase. For a better understanding of the regulation of stathmin activity during the cell cycle, we explored the mechanism(s) responsible for the decrease in the level of phosphorylation of stathmin as cells complete mitosis and enter a new G1 phase. We show that stathmin mRNA and protein are expressed constitutively throughout the different phases of the cell cycle. This suggests that the non-phosphorylated stathmin that predominates during G1 is not generated by degradation of phosphorylated stathmin in mitosis and synthesis of new unphosphorylated stathmin as cells enter a new G1 phase. This suggested that protein phosphatases might be responsible for dephosphorylating stathmin as cells enter a new cell cycle. Okadaic acid-mediated inhibition of protein phosphatases in vivo showed a major increase in the level of phosphorylation of stathmin. Dephosphorylation studies in vitro showed differential patterns of site-specific dephosphorylaton of stathmin to protein phosphatase type 1, protein phosphatase type 2A and protein phosphatase type 2B. Thus stathmin might be a target for okadaic acid-sensitive protein phosphatase(s), and its activity in eukaryotic cells might be modulated by the sequential activity of specific protein kinases and phosphatases.

Distinct roles of PP1 and PP2A-like phosphatases in control of microtubule dynamics during mitosis

Assembly of a mitotic spindle requires the accurate regulation of microtubule dynamics which is accomplished, at least in part, by phosphorylation-dephosphorylation reactions. Here we have investigated the role of serine-threonine phosphatases in the control of microtubule dynamics using specific inhibitors in Xenopus egg extracts. Type 2A phosphatases are required to maintain the short steady-state length of microtubules in mitosis by regulating the level of microtubule catastrophes, in part by controlling the the microtubule-destabilizing activity and phosphorylation of Op18/stathmin. Type 1 phosphatases are only required for control of microtubule dynamics during the transitions into and out of mitosis. Thus, although both type 2A and type 1 phosphatases are involved in the regulation of microtubule dynamics, they have distinct, non-overlapping roles.

Epidermal growth factor and okadaic acid stimulate Sp1 proteolysis

Sp1 nuclear levels have been shown to directly correlate with the proliferative state of the cell. We therefore studied changes in the abundance of Sp1 in a rat pituitary cell line GH4 whose growth rate is regulated by epidermal growth factor (EGF). Nuclear extracts from GH4 cells treated with 10 nM EGF for at least 16 h showed a 50% decrease in Sp1 binding to a GC-rich element present in the gastrin promoter. The decrease in binding correlated with a decrease in cell proliferation, a loss of nuclear Sp1 protein and a 50-60% decrease in Sp1-mediated transactivation through an Sp1 enhancer element in transfection assays. Okadaic acid, a phosphatase inhibitor, was synergistic with the effect of EGF on Sp1 protein levels suggesting that the loss of Sp1 was mediated by phosphorylation events. This result was confirmed by showing a 2-fold increase in orthophosphate-labeled Sp1 with EGF and okadaic acid. Cycloheximide prevented the expected loss of Sp1 mediated by EGF and okadaic acid suggesting that the synthesis of a protease may mediate these events. This hypothesis was tested directly by showing that the cysteine protease inhibitor leupeptin prevented Sp1 degradation. Using the PEST-FIND computer program, the computed PEST score for human and rat Sp1 is 10.4 and 13.7, respectively, indicating that Sp1 has a domain with a high concentration of proline, glutamic acid, serine, and threonine residues as reported for a number of proteins with inducible rates of degradation. Collectively, these results indicate that sustained stimulation of GH4 cells by EGF initiates a cascade of phosphorylation events that promotes Sp1 proteolysis, decreased Sp1 nuclear levels and decreased cellular proliferation.

Differential inhibition and posttranslational modification of protein phosphatase 1 and 2A in MCF7 cells treated with calyculin-A, okadaic acid, and tautomycin

Calyculin-A (CA), okadaic acid (OA), and tautomycin (TAU) are potent inhibitors of protein phosphatases 1 (PP1) and 2A (PP2A) and are widely used on cells in culture. Despite their well characterized selectivity in vitro, their exact intracellular effects on PP1 and PP2A cannot be directly deduced from their extracellular concentration because their cell permeation properties are not known. Here we demonstrate that, due to the tight binding of the inhibitors to PP1 and/or PP2A, their cell penetration could be monitored by measuring PP1 and PP2A activities in cell-free extracts. Treatment of MCF7 cells with 10 nM CA for 2 h simultaneously inhibited PP1 and PP2A activities by more than 50%. A concentration of 1 microM OA was required to obtain a similar time course of PP2A inhibition in MCF7 cells to that observed with 10 nM CA, whereas PP1 activity was unaffected. PP1 was predominantly inhibited in MCF7 cells treated with TAU but even at 10 microM TAU PP1 inhibition was much slower than that observed with 10 nM CA. Furthermore, binding of inhibitors to PP2Ac and/or PP1c in MCF7 cells led to differential posttranslational modifications of the carboxyl termini of the proteins as demonstrated by Western blotting. OA and CA, in contrast to TAU, induced demethylation of the carboxyl-terminal Leu309 residue of PP2Ac. On the other hand, CA and TAU, in contrast to OA, elicited a marked decrease in immunoreactivity of the carboxyl terminus of the alpha-isoform of PP1c, probably reflecting proteolysis of the protein. These results suggest that in MCF7 cells OA selectively inhibits PP2A and TAU predominantly affects PP1, a conclusion supported by their differential effects on cytokeratins in this cell line.

Hyperphosphorylation of retinoblastoma protein and stimulation of growth by okadaic acid in human pancreatic cancer

Phosphorylation/dephosphorylation of intracellular proteins are important steps in the regulation of cell growth. Okadaic acid, an inhibitor of the serine/threonine protein phosphatases 1 and 2A, is a potent tumor promoter. This effect may be through the inhibition of dephosphorylation (termed "hyperphosphorylation") and subsequent inactivation of tumor-suppressor proteins. We examined whether okadaic acid regulates growth of human pancreatic cancer cells (MIA PaCa-2 and Panc-1) or alters the phosphorylation of the retinoblastoma tumor-suppressor protein. Growth studies, nuclear labeling analyses, and Western blotting for retinoblastoma protein were performed. Okadaic acid stimulated cell growth and induced hyperphosphorylation of the retinoblastoma protein. The growth-stimulatory effect of okadaic acid on these human pancreatic cancer cells may be mediated by inactivation of the growth suppressive effect of the retinoblastoma protein by hyperphosphorylation. These studies suggest that the growth of these human pancreatic cancer cells is still regulated by tumor-suppressor proteins.

The centrosome in animal cells and its functional homologs in plant and yeast cells

The centrosome is the principal microtubule-organizing center in mammalian cells. Until recently, the centrosome could only be studied at the ultrastructural level and defined as a functional entity. However, during the past decade a number of clever experimental strategies have been used to identify numerous molecular components of the centrosome. The identification of biochemical subunits of the centrosome complex has allowed the centrosome to be investigated in much more detail, resulting in important advances being made in our understanding of microtubule nucleation events, spindle formation, the assembly and replication of the centrosome, and the nature of the microtubule-organizing centers in plant cells and lower eukaryotes. The next several years should see additional rapid progress in our understanding of the microtubule cytoskeleton as investigators begin to assign functions to the centrosome proteins that have already been reported and as additional centrosome components are discovered.

Suppression by carotenoids of microcystin-induced morphological changes in mouse hepatocytes

Microcystin-LR is a liver tumor promoter in the okadaic acid class, a group of potent inhibitors of protein phosphatases 1 and 2A. Because of inhibition of protein phosphatases, microcystin-LR induces hyperphosphorylation of cellular proteins, including cytoskeletal proteins--cytokeratins 8 and 18--and causes morphological changes in mouse hepatocytes in primary culture. We studied the effects of carotenoids to antagonize microcystin-LR-induced morphological changes in hepatocytes. beta-carotene (100 nM to 100 microns) suppressed the morphological changes induced by 100 nM microcystin-LR in a dose-dependent manner. Other carotenoids tested exerted similar suppressive effects, although retinoids, such as all-trans retinol, all-trans retinoic acid, and 9-cis retinoic acid, were only weakly suppressive. The relative potency of the suppression correlated significantly with the number of conjugated double bonds in the trans configuration. beta-carotene strongly suppressed the hyperphosphorylation of cellular proteins induced by microcystin-LR without significant changes in the basal phosphorylation level. Other antioxidants, such as alpha-tocopherol, did not protect the cells against microcystin-LR. Taken together, the antagonistic effects of carotenoids against microcystin-LR are difficult to explain by their antioxidant or provitamin A activities. Suppression of the hyperphosphorylation of cellular proteins may be a novel mechanism by which carotenoids inhibit tumor promotion.

Okadaic acid induces dephosphorylation of histone H1 in metaphase-arrested HeLa cells

It is shown here that treatment of metaphase-arrested HeLa cells with okadaic acid (0.15-2.5 microM) leads to dephosphorylation of histone H1. This effect is presumably due to the specific ability of okadaic acid to inhibit protein phosphatases 1 and/or 2A, because okadaic acid tetraacetate, which is not a phosphatase inhibitor, has no effect. Dephosphorylation of H1 does not occur if okadaic acid-treated cells are simultaneously treated with 20 nM calyculin A, or if the okadaic acid concentration is 5.0 microM or greater. The mechanism behind this phenomenon is not known. However, the results suggest that the chain of events leading to histone dephosphorylation may be negatively controlled by a protein phosphatase 2A, while the phosphatase which actually dephosphorylates H1 could be a protein phosphatase 1. It remains to be determined whether the phosphatase involved here is the same enzyme as that which dephosphorylates H1 at the end of normal mitosis.

cAMP-induced morphological changes in an immortalized Schwann cell line: a prelude to differentiation?

Schwann cells (SC), the myelinating cells of the peripheral nervous system, show a remarkable capacity to switch from a differentiated state to a proliferative state both during development and peripheral nerve regeneration. In order to better understand the regulatory mechanisms involved with this change we are studying a Schwann cell line transfected with the SV-40 large T gene (TSC). Serum-free medium combined with elevating intra-cellular cAMP levels produced a slower proliferating TSC whose morphology changed from pleiomorphic to process bearing, reminiscent of primary SC in culture. This change was abrogated by colcemid but was unaltered by cytochalasin D, indicating a major role for microtubules. Ultrastructural studies demonstrated numerous microtubules in the cellular extensions which correlated with strong immunocytochemical staining for tubulin in the processes. Analysis of cytoskeletal fractions from the treated cells revealed a greater proportion of tubulin in the polymerized state compared with untreated cells which closely resembled the distribution in primary SC. The cytoskeletal changes observed in the TSC as a result of elevating the intra-cellular cAMP levels may reflect the earliest cellular changes in the induction of myelination.