Mitotic arrest and enhanced nuclear protein phosphorylation in human leukemia K562 cells by okadaic acid, a potent protein phosphatase inhibitor and tumor promoter

Polyoma small T antigen triggers cell death via mitotic catastrophe

Polyoma small T antigen (PyST), an early gene product of the polyoma virus, has been shown to cause cell death in a number of mammalian cells in a protein phosphatase 2A (PP2A)-dependent manner. In the current study, using a cell line featuring regulated expression of PyST, we found that PyST arrests cells in mitosis. Live-cell and immunofluorescence studies showed that the majority of the PyST expressing cells were arrested in prometaphase with almost no cells progressing beyond metaphase. These cells exhibited defects in chromosomal congression, sister chromatid cohesion and spindle positioning, thereby resulting in the activation of the spindle assembly checkpoint. Prolonged mitotic arrest then led to cell death via mitotic catastrophe. Cell cycle inhibitors that block cells in G1/S prevented PyST-induced death. PyST-induced cell death that occurs during M is not dependent on p53 status. These data suggested, and our results confirmed, that PP2A inhibition could be used to preferentially kill cancer cells with p53 mutations that proliferate normally in the presence of cell cycle inhibitors.

The suppression of hematopoiesis function in Balb/c mice induced by prolonged exposure of microcystin-LR

Microcystins (MCs) cause normocytic anemia in patients in a hemodialysis unit in Caruaru, Brazil in 1996, but the underlying mechanisms are still unclear. In the present study, Balb/c mice were intraperitoneally injected with microcystin-LR (MC-LR) at the doses of 0.5, 2 and 8 μg/kg body weight (bw) every 48 h for 30 d. After the prolonged exposure of MC-LR, significant decreases of red blood cell count (RBC), hemoglobin (Hb) and hematocrit (Ht) were observed in 2 and 8 μg/kg bw groups, but erythrocyte mean corpuscular volume (MCV) showed no significant changes. Significantly elevated micronucleus frequency was observed in bone marrow cells (BMCs) in all MC-LR treatments. The proliferation of BMCs significantly declined in both 2 and 8 μg/kg bw groups. Serum levels of some hematopoietic growth factors significantly changed in 8 μg/kg bw group, mainly including granulocyte-macrophage (GM-CSF), erythropoietin (EPO), interleukin-3 (IL-3) and TNF-α. The transcriptional levels of these 4 genes in BMCs were also significantly changed in 8 μg/kg bw group. MC-LR exposure significantly increased the apoptosis rates in all MC-LR treatments. The present study indicates prolonged exposure of MC-LR induces normocytic anemia, and the disturbed hematopoietic growth factors and BMCs apoptosis are responsible for this normocytic anemia.

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.

Effect of IPP5, a novel inhibitor of PP1, on apoptosis and the underlying mechanisms involved

Genes encoding apoptosis-inducing proteins are postulated to be candidate tumour suppressors. The identification of such proteins may benefit the early diagnosis and therapy of tumours. In the present study, we characterized the function of a novel human BMSC (bone marrow stromal cell)-derived protein {IPP5 [inhibitor-5 of PP1 (protein phosphatase 1)]} by large-scale random sequencing of a human BMSC cDNA library. hIPP5 (human IPP5) cDNA encodes a protein of 116 amino acid residues, which shares high homology with human PPI-1 (inhibitor-1 of PP1). The effect of IPP5 on apoptosis and the underlying molecular mechanisms were investigated by overexpression of IPP5 in HeLa cells, a human cervical carcinoma cell line. Our results showed that overexpression of active mutant IPP5 inhibited anchorage-dependent growth and induced apoptosis in HeLa cells, which may be attributed to the up-regulation of p21(waf/cip1) (a 21 kDa cell-cycle regulatory protein), p53 and Bcl-2-antagonist/killer, and down-regulation of Bcl-2 and Bcl-X(L). We also showed that the expression of active mutant IPP5 in HeLa cells was further enhanced on TNF (tumour necrosis factor) treatment and overexpression of active mutant IPP5 sensitized HeLa cells to TNF-induced JNK (c-Jun N-terminal kinase) and p38 activation as well as TNF-mediated apoptosis. Thus overexpression of active mutant IPP5 may increase cell susceptibility to TNF-induced apoptosis by the activation of p38 and JNK pathways. In addition, IPP5 active mutant could interact with PP1alpha as demonstrated by the co-precipitation assay.

A tumor suppressor role for PP2A-B56alpha through negative regulation of c-Myc and other key oncoproteins

Loss or inhibition of the serine/threonine protein phosphatase 2A (PP2A) has revealed a critical tumor suppressor function for PP2A. However, PP2A has also been shown to have important roles in cell cycle progression and survival. Therefore, PP2A is not a typical tumor suppressor. This is most likely due to the fact that PP2A represents a large number of different holoenzymes. Further understanding of PP2A function(s), and especially its tumor suppressor activity, will depend largely on our ability to determine specific targets for these different PP2A holoenzymes and to gain an understanding of how these targets confer tumor suppressor activity or contribute to cell cycle progression and cell survival. Recent work has identified c-Myc as a target of the PP2A holoenzyme, PP2A-B56alpha. This holoenzyme also negatively regulates beta-catenin expression and modulates the anti-apoptotic activity of Bcl2, thus characterizing PP2A-B56alpha as a tumor suppressor PP2A holoenzyme. This review will focus on the role of PP2A-B56alpha in regulating c-Myc and will place this tumor suppressor activity of PP2A within the context of its other tumor suppressor functions. Finally, the mechanism(s) through which PP2A-B56alpha tumor suppressor activity may be lost in cancer will be discussed.

Decreased Phosphoactive ERKs and JNKs in Malachite-Green-Transformed Syrian Hamster Embryo Fibroblasts Are Associated with Increased Phosphoactive p38 Kinase: Possible Therapeutic Importance

BACKGROUND

Malachite green (MG), consisting of green crystals with a metallic lustre, is highly soluble in water, cytotoxic to various mammalian cells and also acts as a liver tumor promoter. In view of its industrial importance and possible exposure of human beings, MG poses a potential environmental health hazard. We have earlier reported the malignant transformation of Syrian hamster embryo (SHE) cells by MG.

METHODS

Cell transformation assays were carried out as described in the literature. Western blotting and flow cytometry were carried out by standard methods.

RESULTS

In this study, we have studied the role of all three isoforms of mitogen-activated protein (MAP) kinases, i.e. extracellular regulated kinases (ERKs), Jun N-terminal kinases (JNKs) and p38 kinase in the MG-transformed SHE fibroblasts compared to controls. Our results showed that transformed cells were associated with decreased expression of ERKs and JNKs as evidenced by Western blotting studies. However, the p38 MAP kinase was found to be upregulated. Flow cytometric DNA histogram analysis indicated an increase in the expression of S phase cells in the transformed cell line as compared to their control counterparts.

CONCLUSIONS

The present studies indicate that decreased phosphoactive ERKs and JNKs and increased phosphoactive p38 kinase are associated with increased S phase cells during transformation of SHE cells by MG.

The uptake kinetics and immunotoxic effects of microcystin-LR in human and chicken peripheral blood lymphocytes in vitro

Microcystin-LR is a cyanobacterial heptapeptide that presents acute and chronic hazards to animal and human health. We investigated the influence of this toxin on human and chicken immune system modulation in vitro. Peripheral blood lymphocytes were treated with microcystin-LR at environmentally relevant doses of 1, 10 and 25 microg/ml for 12, 24, 48, 72 h (for proliferation assay cells were treated for 72 h). T-cell and B-cell proliferation as well as apoptosis and necrosis were determined in human and chicken samples. IL-2 and IL-6 production by human lymphocytes also was measured. In addition, uptake kinetics of microcystin-LR into human and chicken peripheral blood lymphocytes were calculated by Liquid Chromatography (LS) /Mass Spectrometry (MS) analysis. At the highest dose microcystin-LR decreased T-cell proliferation and all doses of microcystin-LR inhibited B-cell proliferation. The frequency of apoptotic and necrotic cells increased in a dose and time-dependent manner. Human lymphocytes responded to stimulation with microcystin-LR by increased production of IL-6 and decreased production of IL-2. Human lymphocytes were able to uptake from 0.014 to 1.663 microg/ml and chicken lymphocytes from 0.035 to 1.733 microg/ml of the microcystin-LR added to the cultures, depending on the treatment time and dose. In conclusion, microcystin-LR acted as an immunomodulator in cytokine production and down-regulated lymphocyte functions by induction of apoptosis and necrosis. However, further studies dealing with the influence of microcystin-LR on expression cytokine genes and transcription factors are necessary to confirm these hypotheses.

Effect of microcystin-LR and cyanobacterial extract from Polish reservoir of drinking water on cell cycle progression, mitotic spindle, and apoptosis in CHO-K1 cells

Microcystin-LR is a cyanobacterial toxin possessing a potent tumor-promoting activity mediated through inhibition of protein phosphatases PP1 and PP2A. Because these enzymes are involved in fundamental cell processes, we decided to examine the influence of microcystin-LR on cell cycle progression, onset of anaphase, segregation of chromosomes by the mitotic spindle, and apoptosis in Chinese hamster ovary (CHO-K1) cells. Cells were incubated with 25, 50, and 100 microM of pure microcystin-LR and a cyanobacterial extract for 14, 18, and 22 h. Giemsa staining of cells treated with these toxins revealed a dose- and time-dependent increase of mitotic indices, accumulation of abnormal G(2)/M figures with hypercondensed chromosomes, abnormal anaphases with defective chromosome separation, and polyploid cells. Because spindle checkpoint is a fundamental regulatory mechanism that assures the onset of anaphase and subsequent exit from mitosis, we examined the spindle organization in microcystin-treated cells. The majority of the mitotic cells showed monopolar and multipolar mitotic spindles (multiple asters). Microtubule bundles were present in interphase cells. Our results indicate that microcystin-LR induces apoptosis and necrosis in a dose- and time-dependent manner and that the frequency of dead cells cells is positively correlated with the frequency of polyploid cells.

Caspase inhibition shifts neuroepithelioma cell response to okadaic acid from apoptosis to an apoptotic-like form of death

We have previously shown that the protein phosphatase inhibitor okadaic acid (OA) induces caspase-3 activation and apoptosis in CHP-100 human neuroepithelioma cells. Herein we provide a more general picture of the effects brought about by OA in this system, also investigating whether caspase activation is necessary for apoptosis induction. We report that incubation for 24 h with 10 nM OA induced a large fraction of the cell population to undergo premature chromosome condensation (PCC) or mitotic arrest, but not apoptosis. The former two effects were also observed after cell treatment with 20 nM OA; however, at this concentration, typical apoptotic cells were also detected, characterized by pycnotic and fragmented nuclei. Occurrence of the above-mentioned apoptotic figures turned extensive at 100 nM OA. The pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD.fmk, 100 microM) fully prevented apoptosis induced by 20 nM OA, increasing PCC incidence. Conversely, 100 nM OA induced an apoptotic-like phenotype, even in the presence of Z-VAD.fmk: in this case, however, nuclei, albeit pycnotic, displayed morphological characteristics distinct from those of typical apoptotic cells; moreover, as assessed by flow cytometry, they were largely unfragmented. The reported OA effects occurred in a setting in which neither p53 nor p21(Cip1/Waf1) was upregulated, thus ruling out a role for these proteins in apoptosis induction. On the other hand, apoptotic doses of OA induced a shift of the retinoblastoma gene product to the hypophosphorylated state and its downregulation by a caspase-dependent mechanism.

Protein phosphatase 1alpha-mediated stimulation of apoptosis is associated with dephosphorylation of the retinoblastoma protein

Protein phosphatase 1 (PP1) plays important roles in many different aspects of cellular activities including cell cycle control. One important function of PP1 is to activate the retinoblastoma protein pRB. Here we show that pRB is one of PP1's downstream targets during apoptosis. When HL-60 cells synchronized at the G1/S boundary were treated with pro-apoptotic cytosine arabinoside (araC), PP1alpha protein increased twofold and PP1 activity about 30% within 1 h. This was followed by pRB dephosphorylation, pRB cleavage by caspases, DNA fragmentation, the appearance of cells with <2n DNA content and finally, dying and dead cells. In vitro, pRB was protected from caspase-3 digestion by prior Cdk-mediated phosphorylation, whereas PP1alpha converted phospho-pRB into an efficient substrate for caspase-3. Introduction of active PP1alpha into HL-60 cells by electroporation was sufficient to induce characteristics of apoptosis. Similarly, araC-resistant cells, normally unable to die in response to araC, initiated apoptosis when electroporated with active PP1alpha. This was also accompanied by pRB cleavage. In contrast, introduction of inhibitor-2 delayed the onset of araC-induced apoptosis, whereas concomitant introduction of PP1alpha and inhibitor-2 completely prevented PP1alpha-induced apoptosis. These results suggest that dephosphorylation of key proteins by PP1alpha may be crucial for the initiation of apoptosis and further support the concept of PP1 serving as a potential target for anti-cancer therapy.

Protein phosphatase 2A interacts with the Src kinase substrate p130(CAS)

In this study, we report that the Src substrate Cas (p130 Crk-associated substrate) associates with protein phosphatase 2A (PP2A), a serine/threonine phosphatase. We investigated this interaction in cells expressing a temperature-sensitive mutant form of v-Src. v-Src activation (by shifting cells from the nonpermissive to the permissive temperature) led to an increase in the tyrosine phosphorylation of v-Src and Cas, as well as in the association between v-Src and Cas. v-Src has previously been shown to bind to PP2A and to phosphorylate the catalytic subunit of PP2A, resulting in inhibition of phosphatase activity. We found that the association between v-Src and PP2A decreased as cells were shifted to the permissive temperature. In contrast, the levels of PP2A that co-immunoprecipitated with Cas increased when v-Src was activated. We obtained similar results in pull-down experiments with immobilized Microcystin, a PP2A inhibitor. Serine/threonine phosphorylation of Cas has previously been shown to occur in a cell cycle regulated matter. Treatment of NIH3T3 cells with okadaic acid, a PP2A inhibitor, augments the serine/threonine phosphorylation of Cas that occurs at mitosis. Furthermore, PP2A dephosphorylates serine residues on Cas in vitro. Taken together, our results suggest that PP2A may be involved in the cell cycle-specific dephosphorylation of Cas.

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.

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.

Airway nerves and protein phosphatases

Much attention has focused on the important role played by phosphatases in the control of gene transcription, cell differentiation and memory regulation. It is also clear that phosphatases may regulate a number of biochemical pathways which can modulate cellular function. Of particular interest is the role of phosphatases in the control of neuronal function. Alterations in neuronal function may contributed to the heightened airways responsiveness observed in asthma to a number of physiological stimuli including distilled water, sulfur dioxide, metabisulfite, hypertonic saline, exercise, allergens, viruses and cold air. An understanding of the mechanisms which regulate the function of sensory nerves could have important clinical implications. In this review we will highlight a number of studies that have investigated the role of phosphatases in the regulation of airway nerve function.

Inhibitory effect of selenomethionine on the growth of three selected human tumor cell lines

Selenium supplementation has been shown for many years to work as an anticarcinogenic agent both in epidemiology and in in vitro studies. Selenium supplementation has recently been shown to decrease total cancer incidence. However, the mechanism of action of selenium as an anticarcinogenic agent has yet to be elucidated. Selenomethionine was the predominant form of selenium in the dietary supplement in the study by Clark et al. (Clark, L.C., Combs, G.F., Turnbull, W.B., Slate, E.H., Chalker, D.K., Chow, J., Davis, L.S., Glover, R.A., Graham, G.F., Gross, E.G., Krongrad, A., Lesher, J.L., Park, H.K., Sanders, B.B., Smith, C.L., Taylor, J.R. and The Nutritional Prevention of Cancer Study Group (1996) Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin: a randomized controlled trial. J. Am. Med. Assoc., 276 (24), 1957-1963) and therefore we evaluated the growth inhibitory effects of selenomethionine against human tumor cells. Selenomethionine was tested against each of three human tumor cell lines (MCF-7/S breast carcinoma, DU-145 prostate cancer cells and UACC-375 melanoma) and against normal human diploid fibroblasts. All cell lines demonstrated a dose-dependent manner of growth inhibition by selenomethionine. Selenomethionine inhibited the growth of all of the human tumor cell lines in the micromolar (microM) range (ranging from 45 to 130 microM) while growth inhibition of normal diploid fibroblasts required 1 mM selenomethionine, approximately 1000-fold higher than for the cancer cell lines. In short, normal diploid fibroblasts were less sensitive than the cancer cell lines to the growth inhibitory effects of selenomethionine. Furthermore, we show that selenomethionine administration to these cancer cell lines results in apoptotic cell death and aberrant mitoses. These results demonstrate the differential sensitivity of tumor cells and normal cells to selenomethionine.

Atypical protein kinase C iota protects human leukemia cells against drug-induced apoptosis

Protein kinase C (PKC) isozymes play distinct roles in cellular function. In human K562 leukemia cells, PKC alpha is important for cellular differentiation and PKC betaII is required for proliferation. In this report, we assess the role of the atypical PKC isoform PKC iota in K562 leukemia cell physiology. K562 cells were stably transfected with expression plasmids containing the cDNA for human PKC iota in sense or antisense orientation to increase or decrease cellular PKC iota levels, respectively. Overexpression or inhibition of expression of PKC iota had no significant effect on the proliferative capacity of K562 cells nor their sensitivity to phorbol myristate acetate-induced cytostasis and megakaryocytic differentiation, suggesting that PKC iota does not play a critical role in these processes. Rather, PKC iota serves to protect K562 cells against drug-induced apoptosis. K562 cells, which are resistant to most apoptotic agents, undergo apoptosis when treated with the protein phosphatase inhibitor okadaic acid (OA). Overexpression of PKC iota leads to increased resistance to OA-induced apoptosis whereas inhibition of PKC iota expression sensitizes cells to OA-induced apoptosis. Overexpression of the related atypical PKC zeta has no protective effect, demonstrating that the effect is isotype-specific. PKC iota also protects K562 cells against taxol-induced apoptosis, indicating that it plays a general protective role against apoptotic stimuli. These data support a role for PKC iota in leukemia cell survival.

Low concentrations of the phosphatase inhibitor okadaic acid stop tumor cell locomotion

The phosphatase inhibitor okadaic acid exerted a biphasic effect on the shape of spontaneously polarized Walker carcinosarcoma cells. At lower concentrations, the drug suppressed cell polarity (IC50 = 0.14 microM) and the cells reverted to a spherical shape. At higher concentrations (> 0.25 microM), cells developed large blebs (IC50 = 0.4 microM). Furthermore, 0.2 microM okadaic acid completely suppressed spontaneous cell locomotion. Two specific inhibitors of protein kinase C did not prevent the actions of okadaic acid on cell shape, showing that this enzyme is very likely not involved. Another phosphatase inhibitor, calyculin A, also suppressed polarity (IC50 = 60 nM) and produced blebbing cells (IC50 = 70 nM). 1 microM okadaic acid induced a 40- to 70-fold increase in phosphorylation of the intermediate filament protein vimentin in intact cells. Increased phosphorylation of this major phosphoprotein correlated with the generation of blebbing cells, rather than with inhibition of polarity and may thus be involved in generating the marked shape changes. We conclude that constitutive phosphatase activity is required for motility and control of shape in Walker carcinosarcoma cells.

Okadaic acid treatment induces DNA adduct formation in BHK21 C13 fibroblasts and HESV keratinocytes

Okadaic acid (OA), a toxin involved in diarrhetic shellfish poisoning (DSP), has been shown to be a potent tumor promoter in mouse skin and glandular stomach. However, more recent studies tended to show that OA can also act as a genotoxic. In this study, using the 32P-postlabelling method, DNA adduct formation was obtained in two cell lines (BHK21 C13 fibroblasts and HESV keratinocytes) after treatment by OA for 24 h with a dose range between 0.01 and 5 nM. Nineteen adducts were observed with BHK21 C13 cells and 15 with HESV ones. Low doses did not show adduct formation. Intermediate doses have given the most important number of adducts and with higher doses, the number of adducts decreased dose dependently. Ten adducts were similar in the two strains while 9 were specific of BHK21 C13 cell line and 5 of HESV one. The highest total DNA adduct level from origin parts was estimated at 95.6 adducts/10(9) nucleotides for BHK21 C13 fibroblasts (1 nM OA treatment) and 31.1 adducts/10(9) nucleotides for HESV keratinocytes (0.5 nM OA treatment). In this case, the major adduct (number 3) represented 20% for the fibroblastic cell line and 30% for the keratinocytic strain. The genotoxic effect of OA showed in this study should lead to a more careful survey of DSP outbreaks.

Inhibition of thymidylate synthase and thymidine kinase by okadaic acid in regenerating rat liver after partial hepatectomy

The effects of okadaic acid, a potent and specific inhibitor of protein phosphatases 1 and 2A, on liver regeneration after partial (70%) hepatectomy were investigated. The injection of okadaic acid (25 micrograms/kg body weight) inhibited the increases in the activities of thymidylate synthase and thymidine kinase in regenerating rat liver at 24 hr after partial hepatectomy, with a concomitant reduction in DNA content. Northern blot analysis showed that the suppression of thymidylate synthase and thymidine kinase activities was caused by comparable decreases in their mRNA levels. The protein levels of thymidylate synthase and thymidine kinase were confirmed by immunoblotting assay to be proportional to the activity and mRNA levels. These findings suggest that okadaic acid-sensitive protein phosphatases are involved in transcriptional control of the dTMP-synthesizing enzymes during liver regeneration.

Alteration of cell cycle-dependent histone phosphorylations by okadaic acid. Induction of mitosis-specific H3 phosphorylation and chromatin condensation in mammalian interphase cells

Effects of okadaic acid (OA), a protein phosphatase inhibitor, on chromatin structure and phosphorylation of histones were examined using HeLa and N18 cells. The chromatin condensation in HeLa cells was mild and resemble prometaphase nuclei, while the condensation in N18 cells was extensive and chromatin became a compact body. H2A in HeLa cells was extensively and consistently phosphorylated at the same site throughout the cell cycle, and H3 was demonstrated to be phosphorylated at the mitosis-specific site Ser10. In contrast, H1 phosphorylation was rapidly decreased in most sites within 3 h. The reduction of H1 phosphorylation was accompanied by a quantitative change in the set of H1 phosphopeptides. During the early phase of the OA treatment, H1 phosphorylation was transiently elevated in tandem, whereas H3 phosphorylation reached a maximum somewhat later. The results suggest that mitosis-specific events (cdc2/H1 kinase activation, H1 superphosphorylation, mitosis-specific H3 phosphorylation and chromatin condensation) induced by OA are sequentially associated. The changes appear to reflect a molecular mechanism similar to that operating in normal mitosis.

Induction of apoptosis in cultured retinoblastoma cells by the protein phosphatase inhibitor, okadaic acid

The induction of apoptosis in cultured retinoblastoma cells by diverse drugs was examined by analyzing DNA fragmentation, a hallmark of apoptosis. First, the ability of six retinoblastoma cell lines to undergo apoptosis was surveyed using etoposide (30 micrograms/ml, 20 h exposure). The NCC-RbC-60 cell line, established in this laboratory showed DNA fragmentation clearly, whereas the other cell lines tested, including the representative retinoblastoma cell line, Y-79, did not show distinct DNA fragmentation. Biochemical modulators, such as A23187, forskolin, retinoic acid, phorbol 12-myristate 13-acetate and okadaic acid, were examined to ascertain whether they could induce apoptosis in NCC-RbC-60 and Y-79 cells after exposure for 20 h. Only okadaic acid induced DNA fragmentation in all the retinoblastoma cell lines tested and it induced DNA fragmentation in Y-79 cells in a time- and concentration-dependent manner. Flow-cytometric analysis and microscopic examination revealed that Y-79 cells treated with okadaic acid for 24-48 h accumulated at the G2/M, especially M, phases, before undergoing DNA fragmentation. Other mitotic poisons, nocodazole, colcemid and taxol, also induced apoptosis in Y-79 cells. In the K1034 cell line, established from non-malignant retinal pigmented epithelium, okadaic acid failed to induce both G2/M arrest and DNA fragmentation. These findings suggest that okadaic-acid-induced apoptosis occurs as a result of metaphase arrest.

Selective induction of cell cycle regulatory genes cdk1 (p34cdc2), cyclins A/B, and the tumor suppressor gene Rb in transformed cells by okadaic acid

Genes encoding cdk1 (p34cdc2), cyclin A, cyclin B, and the tumor suppressor gene Rb are fundamental regulators of cell cycle progression which associate as a complex with the transcription factor E2F. Expression of many of these proteins has previously been shown to be repressed by okadaic acid, a specific inhibitor of protein phosphatases 1/2A (PP1/PP2A), resulting in growth arrest in nontransformed but immortalized cells. We have investigated levels of mRNA encoding cdk1 (p34cdc2), cyclin A, cyclin B, Rb, GAPDH, c-myc, and histone H4 genes for sensitivity to okadaic acid in HeLa cells to determine if transformation altered their regulation. Serum starvation slowed growth and diminished mRNA levels for all genes tested except c-myc and GAPDH. When starved cells were subsequently exposed to 19 nM okadaic acid or refed 10% serum, mRNA levels of cyclin A, cyclin B, cdk1, and Rb dramatically increased while mRNA levels for c-myc and GAPDH were largely unaffected. Histone H4 mRNA levels and the rate of DNA synthesis were greatly enhanced by serum addition but not affected appreciably by okadaic acid. Okadaic acid was also effective in blocking proliferation of exponentially growing HeLa cells at G2/M and S phase. Despite the cell cycle phase-specific block, elevated mRNA levels for cdk1, cyclin A, cyclin B, Rb, and suppression of H4 mRNA levels were detected and persisted for at least 12 hr following okadaic acid removal. The results demonstrate that cell cycle progression is blocked and several cell cycle regulatory genes, encoding transcription factor E2F-associated proteins, experience elevation of mRNA levels through mechanisms sensitive to okadaic acid likely through a PP1/PP2A-sensitive mechanism. Data from transformed cells contrast with data from immortalized but nontransformed cells in which okadaic acid also blocks cell cycle progression during G2/M phase but suppresses expression of these genes. Such contrasts may be correlated with reduced growth factor dependence and transformation.

Okadaic acid increased annexin I and induced differentiation of human promyelocytic leukemia cells

The differentiation of a cell line of human promyelocytic leukemia, HL-60 cells, triggered by 12-O-tetradecanoyl 13-phorbol acetate (TPA), depends on the phosphorylation of some proteins, such as 17, 27, and 34 kDa proteins, by protein kinase C. For elucidation of the mechanism of ligand-induced differentiation of HL-60 cells, the effects of okadaic acid (OA), a phosphatase inhibitor, on cell differentiation and protein phosphorylation were studied. After treatment with OA, HL-60 cells differentiated into macrophage-like cells; within 16 h, 70% or more of the treated cells adhered to plastic dishes. The adherent cells did not undergo mitosis but began activities such as phagocytosis. OA increased the phosphorylation of 17, 23, 27, and 34 kDa proteins, as did TPA. The amount of annexin I (39 kDa protein) in HL-60 cells caused to differentiate with OA was 7.5-fold that without such treatment. Kinetic analysis showed that increased transcription of annexin I mRNA caused the increase in annexin I in the differentiated cells. Thus, OA and TPA increased cellular levels of annexin I and caused the differentiation of HL-60 cells into macrophage-like cells.

Okadaic acid Co-induces vimentin expression and cell cycle arrest in MPC-11 mouse plasmacytoma cells

The effect of the tumor promoter okadaic acid on cell cycle progression and on vimentin expression in MPC-11 mouse plasmacytoma cells was compared with that of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Cell cycle progression of asynchronously grown MPC-11 cells was inhibited by both agents, but, in contrast to the G1 phase arrest caused by TPA, okadaic acid gave rise to G2/M phase and S phase arrest. This effect of okadaic acid was delayed significantly compared to the TPA-caused arrest. Furthermore, okadaic acid was able to induce vimentin expression to an extent comparable to the TPA response. However, vimentin expression was markedly delayed in okadaic acid-treated relative to TPA-treated cells. Another protein phosphatase inhibitor, calyculin A, also induced cell cycle changes and vimentin expression at concentrations at or above 1 x 10(-9) M. Based on these observations, we suggest an involvement of protein phosphatase 1 (possibly also phosphatase 2A and/or other phosphatases) in both the G2/M cell cycle block and the induction of vimentin expression in MPC-11 cells by okadaic acid.

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.

Okadaic acid, a protein phosphatase inhibitor, induces sister-chromatid exchanges depending on the presence of bromodeoxyuridine

Okadaic acid (OA), a potent tumor promoter and an inhibitor of protein phosphatase 1 and 2A, induced sister-chromatid exchanges (SCEs) in human lymphoblastoid cells and Chinese hamster ovary cells at low concentrations of 2-10 nM, when the cells were grown for two cell cycles in the presence of OA and bromodeoxyuridine (BrdUrd). Prolonged treatment with OA prior to addition of BrdUrd did not induce SCEs, indicating an essential role of BrdUrd. A similar important role of BrdUrd in SCE induction has been reported in the cases of benzamide (BA) (Natarajan et al., 1981) and camptothecin (CPT) (Zhao et al., 1992), which are inhibitors of poly(ADP-ribose)polymerase and DNA topoisomerase I (topo I), respectively. Unlike many DNA-damaging agents, they are required to be present during S phase along with BrdUrd in the medium and/or in the parental DNA as BrdUMP. Thus OA, like BA and CPT, is a new type of SCE inducer. Exposing cells to a combined treatment with OA, BA and CPT, a significantly higher level of SCEs was induced than that expected if the numbers of SCE caused by these three inhibitors were additive, while no such synergistic increase was seen in every combination of two agents. Since both phosphorylation and poly(ADP-ribosyl)ation have been known to modify topo I activity, the results suggest a common involvement of topo I for SCE formation by OA, BA and CPT. In addition to SCE induction, OA resulted in an increase of mitotic cells which were characterized by a marked chromosome condensation. OA also induced chromosome fragmentation/pulverization in human lymphoblastoid cells and fragmented nuclei in Chinese hamster cells.

Okadaic acid induces premature chromosome condensation reflecting the cell cycle progression in one-cell stage mouse embryos

Haploid parthenogenetic embryos as well as fertilized mouse eggs were treated in vitro with 1-10 microM okadaic acid (OA) at the one-cell stage. Cytogenetic analysis detected that OA induces nuclear envelope breakdown (NEBD) and premature condensation of interphase chromosomes in pronuclei as well as in 2nd polar body (PB) nuclei. G1-, S-, and G2-type prematurely condensed chromosomes (PCC) were found in pronuclei of embryos of different age, which reflects their progression through the first cell cycle. In nuclei from 2nd PBs only G1- and S-type PCC were observed. Using the types of PCC as a criterion of different phases of the cell cycle, it was possible to estimate that in haploid parthenogenetic embryos G1-phase lasts until 5.5 hr post activation (hpa), S-phase takes from 4.5 to 9.5 hpa, and from 8.5 hpa G2-phase had started. Second PBs were found to be in G1-phase until 6.5 hpa and S-phase started in some as early as 5.5 hpa, but in most not before 7.5 hpa. Treatment with OA visualizes G1-chromosomes in pronuclei as well as in 2nd PBs, and it is easy to count the number of these chromosomes and recognize a T6 marker chromosome. The possibility to apply cytogenetic analysis of G1-chromosomes from 2nd PBs for a more accurate detection of maternal meiotic nondisjunction is discussed.

Cobra venom cardiotoxin (cytotoxin) isoforms and neurotoxin: comparative potency of protein kinase C inhibition and cancer cell cytotoxicity and modes of enzyme inhibition

Effects of cobra cardiotoxin (cytotoxin) and its isoforms, and neurotoxin, on protein kinase C (PKC) and cancer cells were investigated. A positive correlation existed between hydrophobicities and activities of the toxins to inhibit PKC activity (assayed using phosphatidylserine vesicle, arachidonate monomer, and Triton/phosphatidylserine mixed micelle systems), phorbol ester binding to PKC, proliferation of several cancer cell lines, and phorbol ester-induced HL60 cell differentiation. Their relative hydrophobicities and activities, in a decreasing order, were cardiotoxin-1 approximately cardiotoxin-3 > cardiotoxin (a mixture of isoforms) > cardiotoxin-4 >> neurotoxin (inactive). Under the mixed micelle assay system (containing 0.3% Triton X-100, 8 mol % phosphatidylserine, 2 mol % diolein, and 200 microM CaCl2), cardiotoxin inhibited PKC competitively with respect to phosphatidylserine (apparent Ki of about 0.06 mol % or 2.5 microM), and in a mixed-type manner with respect to both diolein (apparent Ki of about 0.04 mol % or 1.7 microM) and Ca2+ (apparent Ki of about 2.9 microM). On the basis of findings that IC50 (approximately 0.3 microM) of cardiotoxin for inhibition of HL60 cell proliferation and differentiation was lower than its IC50 (9 microM) for PKC inhibition in vitro in the phosphatidylserine vesicle system and that PKC inhibition was the only known molecular mechanism of cardiotoxin, it was suggested that cardiotoxin might be highly membrane interacting and that the observed cellular effects of cardiotoxin might be mediated, in part, via PKC inhibition.

Comparative effects of protein phosphatase inhibitors (okadaic acid and calyculin A) on human leukemia HL60, HL60/ADR and K562 cells

Inhibitors of protein phosphatases 1/2A (okadaic acid and calyculin A) exhibited differential cytotoxicity toward three human leukemia cell lines, in an increasing order of resistance, HL60 less than HL60/ADR less than K562 cells. Cytotoxicity of the toxins was associated with marked mitotic arrest of the cells, characterized by chromatid scattering/overcondensation and abnormal mitotic spindles. In all cases, calyculin A was more potent than okadaic acid. Protein phosphorylation experiments in intact cells revealed that HL60/ADR, the adriamycin-resistant variant, showed a higher overall phosphorylation of nuclear proteins than the drug-sensitive parental HL60, and that phorbol ester (protein kinase C activator) and calyculin A appeared to more specifically stimulate phosphorylation of p66 and p60, respectively. It was suggested that the toxins might be useful in delineating mechanisms underlying certain properties of cancer cells (such as multidrug resistance, mitosis and differentiation) related to protein phosphorylation/dephosphorylation reactions.

Cell cycle dependent gene expressions and activities of protein phosphatases PP1 and PP2A in mouse NIH3T3 fibroblasts

We determined the mRNA levels and the activities in nuclear and non-nuclear fractions of protein phosphatase type 1 (PP1) and type 2A (PP2A) through the cell cycle in synchronized mouse NIH3T3 fibroblasts. The mRNA level for PP1 alpha was gradually elevated in late G1 phase, began to decrease in M phase, and reached the control level with entering into the next G1 phase. The mRNA level for PP2A was rapidly increased in early G1 phase, kept at the high level, and decreased after S phase. In nuclear fractions of cells, spontaneous activities of both PP1 and PP2A were gradually increased until M phase and rapidly decreased with entering the next G1 phase, while in non-nuclear fraction such dramatic alterations were not observed. Potential activities of PP1 in both fractions revealed by Co(2+)-trypsin treatment showed an oscillaion patterns similar to those of the spontaneous activities. These results strongly suggest that cell cycle dependent gene expressions and activities of PP1 and PP2A play roles in DNA synthesis and mitosis during the cell cycle.

Membrane interactions of mastoparan analogues related to their differential effects on protein kinase C, Na, K-ATPase and HL60 cells

Membrane interactions of tetradecapeptide toxin mastoparan (MP) and analogues (MP-3, MP-X and polistes MP), as indicated by inhibition of various enzymatic and cellular activities, were investigated. MP-3 was found to be the least active in inhibiting protein kinase C (PKC; activated by phosphatidylserine vesicles, synaptosomal membranes or phorbol ester), synaptosomal membrane Na,K-ATPase and proliferation and viability of leukemia HL60 cells. MP-3, however, was as active as others in inhibiting PKC activated by arachidonate monomers and phorbol ester binding. The unique properties of MP-3, the [des-Ile1-Asn2]-analogue of MP, might be related to its low functional amphiphilicity compared to others and useful in further delineating biological activities associated with or regulated by membranes.

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

Okadaic acid, a selective inhibitor of serine/threonine protein phosphatases, was utilized to investigate the requirement for phosphatases in cell cycle progression of GH4 rat pituitary cells. Okadaic acid inhibited GH4 cell proliferation in a concentration-dependent manner with a half-maximal inhibition (IC50) of approximately 5 nM. Treatment of GH4 cells with 10 nM okadaic acid resulted in a 40-60% decrease in phosphatase activity and an increase in the proportion of phosphorylated retinoblastoma (RB) protein. Cell cycle analysis indicated that okadaic acid increased the percentage of cells in G2-M, decreased proportionally the percentage of cells in G1 phase, and had little effect on the percentage of cells in S-phase. The absence of a change in the proportion of S-phase cells indicates that G1-specific phosphatases responsible for dephosphorylation of RB protein were not inhibited by 10 mM okadaic acid. Mitotic index revealed that 10 nM okadaic acid decreased proliferation of GH4 cells specifically by slowing the progression through mitosis. Immunostaining with anti-tubulin demonstrated that 10 nM okadaic acid-treated mitotic cells contained mitotic spindles; however, the spindle apparatus in these cells frequently contained multiple poles. These results suggest that the organization of spindle microtubules during prometaphase requires a protein phosphatase that is sensitive to nanomolar concentrations of okadaic acid. Chromosomes in 10 nM okadaic acid-treated cells appear to be attached to spindle microtubules and the nuclear envelope is absent. The effects of okadaic acid on the spindle differ from those elicited by the calcium channel blocker, nimodipine, indicating that this okadaic acid sensitive phosphatase is not part of the calcium signalling events which participate in mitotic progression.

Induction of differentiation and c-jun expression in human leukemic cells by okadaic acid, an inhibitor of protein phosphatases

Okadaic acid, a protein phosphatase inhibitor, is a strong tumor promoter which activates protein phosphorylation. Because another activator of protein phosphorylation, phorbol esters, stimulates hematopoietic differentiation, we sought to determine whether okadaic acid could also induce the differentiation of the human leukemic cell line U937. Differentiation was assessed by measuring changes in the following: mRNA levels, cell growth, morphology, cell surface markers, and the ability to induce superoxide. We found that okadaic acid treatment of U937 cells induces immediate increases in total cellular levels of both c-jun and c-fos mRNAs. Nuclear run-on experiments demonstrate that initial increases are secondary to increases in transcription, whereas latter changes may be secondary to mRNA stabilization. Like phorbol esters, okadaic acid treatment also activates AP-1 enhancer activity and induces the phosphorylation of c-Jun protein. Approximately 6-12 hours after treatment with okadaic acid, mRNA levels of c-myc, p34cdc2, and p58GTA, two cell cycle regulated protein kinases, decrease. Okadaic acid inhibits the growth of U937 cells, induces changes in nuclear morphology, stimulates increases in Mac-1 and Leu 11 surface antigens, and induces these cells to produce superoxide. These changes, taken together, suggest that U937 cells have been induced by okadaic acid to differentiate towards a more mature cell type.