The repair of gamma-radiation-induced DNA damage is inhibited by microcystin-LR, the PP1 and PP2A phosphatase inhibitor

Risk assessment and identification of factors influencing the historical concentrations of microcystin in Lake Taihu, China

Understanding the history of microcystins (MCs) pollution in large lakes can help inform future lake management. We collected sediment cores from Lake Taihu to: investigate the long-term record of MCs (MC-LR, MC-YR, and MC-RR), explore the main environmental drivers of MCs, and assess their public health and ecological risks. Results showed that MCs content in all cores increased over time. The core from north Taihu had the highest MC concentrations, with an average total MCs (sum of MC-LR, MC-YR, and MC-RR = TMCs) content of (74.31±328.55) ng/g. The core from eastern Taihu showed the lowest average TMCs content of (2.91±3.95) ng/g. PCA showed that sediment MCs at the three sites were positively correlated with sediment chlorophyll-a. MC-LR and MC-YR in northern and western Taihu negatively correlated with both the sediment total organic carbon/sediment total nitrogen ratio (STOC/STN) and water nitrate (NO₃^--N) concentration, but three MC congeners at eastern Taihu showed positive correlations with water orthophosphate (PO₄^3--P), NO₃^--N, and STOC/STN. Generalized additive model analysis at each site revealed that NO₃^--N was the main TMCs driver in northern and western Taihu where phytoplankton dominated, whereas PO₄^3--P was the main TMCs driver in eastern Taihu where macrophytes dominated. At the whole lake scale, total phosphorus (TP) and PO₄^3--P were the most important environmental drivers influencing MCs; TP explained 47.4%, 44.2%, and 47.6% while orthophosphate explained 34.8%, 31.2%, and 34.7% of the deviance on TMCs, MC-LR, and MC-YR, respectively. NO₃^--N also showed a strong effect on MCs variation, especially on MC-YR. Risk assessment showed that both ecological and public health risk has increased in recent years. We conclude that while control of phosphorus and nitrogen input should be a major focus for future lake management, lake zone-specific management strategies may also be important.

Molecular Mechanisms of Specific Cellular DNA Damage Response and Repair Induced by the Mixed Radiation Field During Boron Neutron Capture Therapy

The impact of a mixed neutron-gamma beam on the activation of DNA damage response (DDR) proteins and non-coding RNAs (ncRNAs) is poorly understood. Ionizing radiation is characterized by its biological effectiveness and is related to linear energy transfer (LET). Neutron-gamma mixed beam used in boron neutron capture therapy (BNCT) can induce another type of DNA damage such as clustered DNA or multiple damaged sites, as indicated for high LET particles, such as alpha particles, carbon ions, and protons. We speculate that after exposure to a mixed radiation field, the repair capacity might reduce, leading to unrepaired complex DNA damage for a long period and may promote genome instability and cell death. This review will focus on the poorly studied impact of neutron-gamma mixed beams with an emphasis on DNA damage and molecular mechanisms of repair. In case of BNCT, it is not clear which repair pathway is involved, and recent experimental work will be presented. Further understanding of BNCT-induced DDR mechanisms may lead to improved therapeutic efficiency against different tumors.

Vitamin C Protects Porcine Oocytes From Microcystin-LR Toxicity During Maturation

Microcystin-leucine arginine (MC-LR) is the most toxic cyanotoxin found in water bodies. Microcystins are produced as secondary products of cyanobacteria metabolism. They have a stable structure, and can bioaccumulate in living organisms. Humans and livestock who drink fresh water containing MC-LR can be poisoned. However, few studies have reported the effects of MC-LR exposure on livestock or human reproduction. In this study, we used porcine oocytes as a model to explore the effects of MC-LR on oocyte maturation, and studied the impact of vitamin C (VC) administration on MC-LR-induced meiosis defects. Exposure to MC-LR significantly restricted cumulus cell expansion and decreased first polar body extrusion. Further studies showed that MC-LR exposure led to meiosis arrest by disturbing cytoskeleton dynamics with MC-LR exposed oocytes displaying aberrant spindle organization, low levels of acetylate α-tubulin, and disturbed actin polymerization. Additionally, MC-LR exposure impaired cytoplasmic maturation by inducing mitochondria dysfunction. Moreover, MC-LR also produced abnormal epigenetic modifications, and induced high levels of oxidative stress, caused DNA damage and early apoptosis. The administration of VC provided partial protection from all of the defects observed in oocytes exposed to MC-LR. These results demonstrate that MC-LR has a toxic effect on oocyte meiosis through mitochondrial dysfunction-induced ROS, DNA damage and early apoptosis. Supplementation of VC is able to protect against MC-LR-induced oocyte damage and represents a potential therapeutic strategy to improve the quality of MC-LR-exposed oocytes.

In Vitro Mutagenic and Genotoxic Assessment of a Mixture of the Cyanotoxins Microcystin-LR and Cylindrospermopsin

The co-occurrence of various cyanobacterial toxins can potentially induce toxic effects different than those observed for single cyanotoxins, as interaction phenomena cannot be discarded. Moreover, mixtures are a more probable exposure scenario. However, toxicological information on the topic is still scarce. Taking into account the important role of mutagenicity and genotoxicity in the risk evaluation framework, the objective of this study was to assess the mutagenic and genotoxic potential of mixtures of two of the most relevant cyanotoxins, Microcystin-LR (MC-LR) and Cylindrospermopsin (CYN), using the battery of in vitro tests recommended by the European Food Safety Authority (EFSA) for food contaminants. Mixtures of 1:10 CYN/MC-LR (CYN concentration in the range 0.04-2.5 µg/mL) were used to perform the bacterial reverse-mutation assay (Ames test) in Salmonella typhimurium, the mammalian cell micronucleus (MN) test and the mouse lymphoma thymidine-kinase assay (MLA) on L5178YTk± cells, while Caco-2 cells were used for the standard and enzyme-modified comet assays. The exposure periods ranged between 4 and 72 h depending on the assay. The genotoxicity of the mixture was observed only in the MN test with S9 metabolic fraction, similar to the results previously reported for CYN individually. These results indicate that cyanobacterial mixtures require a specific (geno)toxicity evaluation as their effects cannot be extrapolated from those of the individual cyanotoxins.

Radioprotective efficacy of dieckol against gamma radiation-induced cellular damage in hepatocyte cells

Naturally occurring antioxidants prevent or delay the harmful effect of free radical formation and radioprotection. The present study aimed to investigate the radioprotective effect of dieckol, a naturally occurring marine bioactive phenolic compound on lipid peroxidation and antioxidant status, DNA damage, and inflammation in gamma-radiation-induced rat primary hepatocytes. Isolated hepatocyte cells exposed to gamma-radiation showed an increased level of lipid peroxidation markers (thiobarbituric acid reactive substances and lipid hydroperoxides) accompanied with the decrease in the activities of enzymatic (SOD, CAT, and GPx) and non-enzymatic (vitamin C, vitamin E, and GSH) antioxidants associated with increased DNA damage coupled with upregulation of inflammatory proteins (NF-κB and COX-2) compared to control. Treatment of dieckol (5, 10, 20 μM) reduces the γ-radiation-induced toxicity and the associated pro-oxidant and antioxidant imbalance as well as decreasing the DNA damage (tail length, tail moment, %DNA in a tail and olive tail moment) and inflammation in hepatocyte cells. These findings indicate that treatment of dieckol offers protection against γ-radiation-induced cellular damage in the liver cells.

Protein Phosphatases-A Touchy Enemy in the Battle Against Glioblastomas: A Review

Glioblastoma (GBM) is the most common malignant tumor arising from brain parenchyma. Although many efforts have been made to develop therapies for GBM, the prognosis still remains poor, mainly because of the difficulty in total resection of the tumor mass from brain tissue and the resistance of the residual tumor against standard chemoradiotherapy. Therefore, novel adjuvant therapies are urgently needed. Recent genome-wide analyses of GBM cases have clarified molecular signaling mechanisms underlying GBM biology. However, results of clinical trials targeting phosphorylation-mediated signaling have been unsatisfactory to date. Protein phosphatases are enzymes that antagonize phosphorylation signaling by dephosphorylating phosphorylated signaling molecules. Recently, the critical roles of phosphatases in the regulation of oncogenic signaling in malignant tumor cells have been reported, and tumorigenic roles of deregulated phosphatases have been demonstrated in GBM. However, a detailed mechanism underlying phosphatase-mediated signaling transduction in the regulation of GBM has not been elucidated, and such information is necessary to apply phosphatases as a therapeutic target for GBM. This review highlights and summarizes the phosphatases that have crucial roles in the regulation of oncogenic signaling in GBM cells.

Brönsted Catalyzed Hydrolysis of Microcystin-LR by Siderite

Six naturally occurring minerals were employed to catalyze the hydrolysis of microcystin-LR (MC-LR) in water. After preliminary screening experiments, siderite stood out among these minerals due to its higher activity and selectivity. In comparison with kaolinite, which is known to act as a Lewis acid catalyst, siderite was found to act primarily as a Brönsted acid catalyst in the hydrolysis of MC-LR. More interestingly, we found that the presence of humic acid significantly inhibited catalytic efficiency of kaolinite, while the efficiency of siderite remained high (∼98%). Reaction intermediates detected by LC-ESI/MS were used to indicate cleavage points in the macrocyclic ring of MC-LR, and XPS was used to characterize siderite interaction with MC-LR. Detailed analysis of the in situ ATR-FTIR absorption spectra of MC-LR indicated hydrogen bonding at the siderite-water-MC-LR interface. A metastable ring, involving hydrogen bonding, between surface bicarbonate of siderite and an amide of MC-LR was proposed to explain the higher activity and selectivity toward MC-LR. Furthermore, siderite was found to reduce the toxicity of MC-LR to mice by hydrolyzing MC-LR peptide bonds. The study demonstrates the potential of siderite, an earth-abundant and biocompatible mineral, for removing MC-LR from water.

P44/42 MAPK signal pathway-mediated hyperphosphorylation of paxillin and redistribution of E-cadherin was involved in microcystin-LR-reduced cellular adhesion in a human liver cell line

Microcystin-LR (MC-LR) is the most common and toxic variant of microcystins. We hypothesize that p44/42 MAPK (ERK1/2) signal pathway is involved in MC-LR-induced cell adhesion alteration in a human liver cell line-HL7702. We identified that MC-LR constantly activated MEK1/2-ERK1/2 signal pathway for 24 h, 48 h and 72 h in vitro. MC-LR reduced hepatocytes adhesion efficiency. Furthermore, as the focal adhesion biomarker, hyperphosphorylation of paxillin (ser83) was induced by MC-LR, which can be blocked by ERK1/2 pathway inhibitor (U0126) and was enhanced after hepatocytes transfected with pCMV6-MAPK plasmid. E-cadherin, as a biomarker which reflects the dynamic of cell-cell adhesion, its redistribution in hepatocytes was induced by MC-LR, and these redistribution and colocalization can be attenuated by U0126. Furthermore, MC-LR increased the co-localization efficiency of p-ERK1/2 with E-cadherin and paxillin. Finally, MC-LR-induced adhesive alteration of hepatocytes can be blocked by ERK1/2 signal pathway inhibitor. These data suggest ERK1/2-phospho-paxillin (ser83)/E-cadherin axis is involved in MC-LR toxic mechanism, which probably provides adaptive protection against MC-LR-induced hepatocytes adhesion changes.

In Vitro Toxicological Assessment of Cylindrospermopsin: A Review

Cylindrospermopsin (CYN) is a cyanobacterial toxin that is gaining importance, owing to its increasing expansion worldwide and the increased frequency of its blooms. CYN mainly targets the liver, but also involves other organs. Various mechanisms have been associated with its toxicity, such as protein synthesis inhibition, oxidative stress, etc. However, its toxic effects are not yet fully elucidated and additional data for hazard characterization purposes are required. In this regard, in vitro methods can play an important role, owing to their advantages in comparison to in vivo trials. The aim of this work was to compile and evaluate the in vitro data dealing with CYN available in the scientific literature, focusing on its toxicokinetics and its main toxicity mechanisms. This analysis would be useful to identify research needs and data gaps in order to complete knowledge about the toxicity profile of CYN. For example, it has been shown that research on various aspects, such as new emerging toxicity effects, the toxicity of analogs, or the potential interaction of CYN with other cyanotoxins, among others, is still very scarce. New in vitro studies are therefore welcome.

Genotoxic potential of the binary mixture of cyanotoxins microcystin-LR and cylindrospermopsin

Increased eutrophication of water bodies promotes cyanobacterial blooming that is hazardous due to the production of various bioactive compounds. Microcystin-LR (MCLR) is among the most widespread cyanotoxins classified as possible human carcinogen, while cylindrospermopsin (CYN) has only recently been recognized as health concern. Both cyanotoxins are genotoxic; however, the mechanisms of their action differ. They are ubiquitously present in water environment and are often detected together. Therefore, we studied genotoxic potential of the binary mixture of these cyanotoxins. Human hepatoma cells (HepG2) were exposed to a single dose of MCLR (1 μg/mL), graded doses of CYN (0.01-0.5 μg/mL), and their combinations. Comet and Cytokinesis block micronucleus assays were used to detect induction of DNA strand breaks (sb) and genomic instability, respectively, along with the transcriptional analyses of the expression of selected genes involved in xenobiotic metabolism, immediate/early cell response and DNA-damage response. MCLR induced DNA sb that were only transiently present after 4 h exposure, whereas CYN, after 24 h exposure, induced DNA sb and genomic instability. The MCLR/CYN mixture induced DNA sb after 24 h exposure, but to lesser extent as CYN alone. On the other hand, induction of genomic instability by the MCLR/CYN mixture was comparable to that induced by CYN alone. In addition, patterns of changes in the expression of selected genes induced by the MCLR/CYN mixture were not significantly different from those induced by CYN alone. Our results indicate that CYN exerts higher genotoxic potential than MCLR and that genotoxic potential of the MCLR/CYN mixture is comparable to that of CYN alone.

Cathepsin L silencing increases As2O3 toxicity in malignantly transformed pilocytic astrocytoma MPA58 cells by activating caspases 3/7

Low-grade, pilocytic astrocytomas are treated by resection, but additional therapy is necessary for those tumors with anaplastic features. Arsenic trioxide (As₂O₃) is emerging as an effective chemotherapeutic agent for treatment of malignant glioblastoma multiforme, where Cathepsin L silencing enables lower, less harmful As₂O₃ concentrations to achieve the desired cytotoxic effect. Here, we evaluated the effects of As₂O₃ combined with stable Cathepsin L shRNA silencing on cell viability/metabolic activity, and apoptosis in primary cultures of recurrent malignantly transformed pilocytic astrocytoma (MPA). These cells expressed high Cathepsin L levels, and when grown as monolayers and spheroids, they were more resistant to As₂O₃ than the U87MG glioblastoma cell line. Caspases 3/7 activity in MPA58 spheroids was not significantly affected by As₂O₃, possibly due to higher chemoresistance of primary biopsy tissue of less malignant astrocytoma versus the malignant U87MG cell line. However, As₂O₃ treatment was cytotoxic to MPA spheroids after silencing of Cathepsin L expression. While Cathepsin L silencing only slightly decreased the live/dead cell ratio in As₂O₃-treated MPA-si spheroids under our experimental conditions, there was an increase in As₂O₃-mediated apoptosis in MPA-si spheroids, as indicated by elevated caspases 3/7 activity. Therefore, Cathepsin L silencing by gene manipulation can be applied when a more aggressive approach is needed in treatment of pilocytic astrocytomas with anaplastic features.

A Role for the Twins Protein Phosphatase (PP2A-B55) in the Maintenance of Drosophila Genome Integrity

The protein phosphatase 2A (PP2A) is a conserved heterotrimeric enzyme that regulates several cellular processes including the DNA damage response and mitosis. Consistent with these functions, PP2A is mutated in many types of cancer and acts as a tumor suppressor. In mammalian cells, PP2A inhibition results in DNA double strand breaks (DSBs) and chromosome aberrations (CABs). However, the mechanisms through which PP2A prevents DNA damage are still unclear. Here, we focus on the role of the Drosophila twins (tws) gene in the maintenance of chromosome integrity; tws encodes the B regulatory subunit (B/B55) of PP2A. Mutations in tws cause high frequencies of CABs (0.5 CABs/cell) in Drosophila larval brain cells and lead to an abnormal persistence of γ-H2Av repair foci. However, mutations that disrupt the PP4 phosphatase activity impair foci dissolution but do not cause CABs, suggesting that a delayed foci regression is not clastogenic. We also show that Tws is required for activation of the G2/M DNA damage checkpoint while PP4 is required for checkpoint recovery, a result that points to a conserved function of these phosphatases from flies to humans. Mutations in the ATM-coding gene tefu are strictly epistatic to tws mutations for the CAB phenotype, suggesting that failure to dephosphorylate an ATM substrate(s) impairs DNA DSBs repair. In addition, mutations in the Ku70 gene, which do not cause CABs, completely suppress CAB formation in tws Ku70 double mutants. These results suggest the hypothesis that an improperly phosphorylated Ku70 protein can lead to DNA damage and CABs.

Gene signature of the post-Chernobyl papillary thyroid cancer

Purpose

Following the nuclear accidents in Chernobyl and later in Fukushima, the nuclear community has been faced with important issues concerning how to search for and diagnose biological consequences of low-dose internal radiation contamination. Although after the Chernobyl accident an increase in childhood papillary thyroid cancer (PTC) was observed, it is still not clear whether the molecular biology of PTCs associated with low-dose radiation exposure differs from that of sporadic PTC.

Methods

We investigated tissue samples from 65 children/young adults with PTC using DNA microarray (Affymetrix, Human Genome U133 2.0 Plus) with the aim of identifying molecular differences between radiation-induced (exposed to Chernobyl radiation, ECR) and sporadic PTC. All participants were resident in the same region so that confounding factors related to genetics or environment were minimized.

Results

There were small but significant differences in the gene expression profiles between ECR and non-ECR PTC (global test, p < 0.01), with 300 differently expressed probe sets (p < 0.001) corresponding to 239 genes. Multifactorial analysis of variance showed that besides radiation exposure history, the BRAF mutation exhibited independent effects on the PTC expression profile; the histological subset and patient age at diagnosis had negligible effects. Ten genes (PPME1, HDAC11, SOCS7, CIC, THRA, ERBB2, PPP1R9A, HDGF, RAD51AP1, and CDK1) from the 19 investigated with quantitative RT-PCR were confirmed as being associated with radiation exposure in an independent, validation set of samples.

Conclusion

Significant, but subtle, differences in gene expression in the post-Chernobyl PTC are associated with previous low-dose radiation exposure.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-015-3303-3) contains supplementary material, which is available to authorized users.

Inhibition of protein phosphatase 2A with a small molecule LB100 radiosensitizes nasopharyngeal carcinoma xenografts by inducing mitotic catastrophe and blocking DNA damage repair

Nasopharyngeal carcinoma (NPC), while uncommon worldwide, is a major health problem in China. Although local radiation and surgery provide good control of NPC, better treatments that permit reductions in radiation dosing are needed. Inhibition of protein phosphatase 2A (PP2A), a ubiquitous multifunctional enzyme with critical roles in cell cycle regulation and DNA-damage response, reportedly sensitizes cancer cells to radiation and chemotherapy. We studied PP2A inhibition with LB100, a small molecule currently in a Phase I clinical trial, on radiosensitization of two human nasopharyngeal cell lines: CNE1, which is reportedly radioresistant, and CNE2. In both cell lines, LB100 exposure increased intracellular p-Plk1, TCTP, and Cdk1 and decreased p53, changes associated with cell cycle arrest, mitotic catastrophe and radio-inhibition of cell proliferation. Mice bearing subcutaneous xenografts of either cell line were administered 1.5 mg/kg LB100 daily for three days and a single dose of 20 Gy radiation (day 3), which produced marked and prolonged tumor mass regression (dose enhancement factors of 2.98 and 2.27 for CNE1 and CNE2 xenografts, respectively). Treatment with either LB100 or radiation alone only transiently inhibited xenograft growth. Our results support further exploration of PP2A inhibition as part of radiotherapy regimens for NPC and potentially other solid tumors.

The role of PP2A-associated proteins and signal pathways in microcystin-LR toxicity

Microcystins are a family of monocyclic heptapeptides produced by cyanobacteria during water blooms. Microcystin-LR (MC-LR) is the most common member of this family. Microcystins induce a variety of toxic cellular effects, including oxidative damage, apoptosis, cytoskeletal destabilization, and cancer cell invasion. Recent studies have examined the molecular mechanism of their toxicity. Protein phosphatase 2A (PP2A) is emerging as a critical regulator of the microcystin-induced molecular network. Furthermore, it has been shown that several molecules or signal pathways associated with PP2A play important roles in microcystin-induced toxic effects. This review summarizes the recent research progress of the molecular mechanism and focuses on the role of PP2A in MC-LR toxicity, which will contribute to a better understanding of the mechanism of microcystin toxicity, and will provide biomarkers for toxicity assessment and control.

Role of nitric oxide in the genotoxic response to chronic microcystin-LR exposure in human-hamster hybrid cells

Microcystin-LR (MC-LR) is the most abundant and toxic microcystin congener and has been classified as a potential human carcinogen (Group 2B) by the International Agency for Research on Cancer. However, the mechanisms underlying the genotoxic effects of MC-LR during chronic exposure are still poorly understood. In the present study, human-hamster hybrid (AL) cells were exposed to MC-LR for varying lengths of time to investigate the role of nitrogen radicals in MC-LR-induced genotoxicity. The mutagenic potential at the CD59 locus was more than 2-fold higher (p<0.01) in AL cells exposed to a cytotoxic concentration (1 μmol/L) of MC-LR for 30 days than in untreated control cells, which was consistent with the formation of micronucleus. MC-LR caused a dose-dependent increase in nitric oxide (NO) production in treated cells. Moreover, this was blocked by concurrent treatment with the NO synthase inhibitor NG-methyl-L-arginine (L-NMMA), which suppressed MC-LR-induced mutations as well. The survival of mitochondrial DNA-depleted (ρ0) AL cells was markedly decreased by MC-LR treatment compared to that in AL cells, while the CD59 mutant fraction was unaltered. These results provided clear evidence that the genotoxicity associated with chronic MC-LR exposure in mammalian cells was mediated by NO and might be considered as a basis for the development of therapeutics that prevent carcinogenesis.

Multifunctional role of ATM/Tel1 kinase in genome stability: from the DNA damage response to telomere maintenance

The mammalian protein kinase ataxia telangiectasia mutated (ATM) is a key regulator of the DNA double-strand-break response and belongs to the evolutionary conserved phosphatidylinositol-3-kinase-related protein kinases. ATM deficiency causes ataxia telangiectasia (AT), a genetic disorder that is characterized by premature aging, cerebellar neuropathy, immunodeficiency, and predisposition to cancer. AT cells show defects in the DNA damage-response pathway, cell-cycle control, and telomere maintenance and length regulation. Likewise, in Saccharomyces cerevisiae, haploid strains defective in the TEL1 gene, the ATM ortholog, show chromosomal aberrations and short telomeres. In this review, we outline the complex role of ATM/Tel1 in maintaining genomic stability through its control of numerous aspects of cellular survival. In particular, we describe how ATM/Tel1 participates in the signal transduction pathways elicited by DNA damage and in telomere homeostasis and its importance as a barrier to cancer development.

Genotoxicity in earthworm after combined treatment of ionising radiation and mercury

This study was performed to investigate the acute genotoxic effects of mercury and radiation on earthworms (Eisenia fetida). The levels of DNA damage and the repair kinetics in the coelomocytes of E. fetida treated with mercuric chloride (HgCl₂) and ionising radiation (gamma rays) were analysed by means of the comet assay. For detection of DNA damage and repair, E. fetida was exposed to HgCl₂ (0-160 mg kg(-1)) and irradiated with gamma rays (0-50 Gy) in vivo. The increase in DNA damage depended on the concentration of mercury or dose of radiation. The results showed that the more the oxidative stress induced by mercury and radiation the longer the repair time that was required. When a combination of HgCl₂ and gamma rays was applied, the cell damage was much higher than those treated with HgCl₂ or radiation alone, which indicated that the genotoxic effects were increased after the combined treatment of mercury and radiation.

Genotoxicity of microcystin-LR in in vitro and in vivo experimental models

Microcystin-LR (MCLR) is a cyanobacterial toxin known for its acute hepatotoxicity. Despite being recognized as tumour promoter, its genotoxicity is far from being completely clarified, particularly in organs other than liver. In this work, we used the comet and/or the micronucleus (MN) assays to study the genotoxicity of MCLR in kidney- (Vero-E6) and liver-derived (HepG2) cell lines and in blood cells from MCLR-exposed mice. MCLR treatment (5 and 20 μM) caused a significant induction in the MN frequency in both cell lines and, interestingly, a similar positive effect was observed in mouse reticulocytes (37.5 μg MCLR/kg, i.p. route). Moreover, the FISH-based analysis of the MN content (HepG2 cells) suggested that MCLR induces both chromosome breaks and loss. On the other hand, the comet assay results were negative in Vero-E6 cells and in mouse leukocytes, with the exception of a transient increase in the level of DNA damage 30 minutes after mice exposure. Overall, the present findings contributed to increase the weight of evidence in favour of MCLR genotoxicity, based on its capacity to induce permanent genetic damage either in vitro or in vivo. Moreover, they suggest a clastogenic and aneugenic mode of action that might underlie a carcinogenic effect.

Radiation sensitivity of human and murine peripheral blood lymphocytes, stem and progenitor cells

Immunodeficiency is a severe side effect of radiation therapy, notably at high radiation doses. It may also impact healthy individuals exposed to environmental ionizing radiation. Although it is believed to result from cytotoxicity of bone marrow cells and of immunocompetent cells in the peripheral blood, the response of distinct bone marrow and blood cell subpopulations following exposure to ionizing radiation is not yet fully explored. In this review, we aim to compile the knowledge on radiation sensitivity of immunocompetent cells and to summarize data from bone marrow and peripheral blood cells derived from mouse and human origin. In addition, we address the radiation response of blood stem and progenitor cells. The data indicate that stem cells, T helper cells, cytotoxic T cells, monocytes, neutrophils and, at a high degree, B cells display a radiation sensitive phenotype while regulatory T cells, macrophages, dendritic cells and natural killer cells appear to be more radioresistant. No conclusive data are available for basophil and eosinophil granulocytes. Erythrocytes and thrombocytes, but not their precursors, seem to be highly radioresistant. Overall, the data indicate considerable differences in radiosensitivity of bone marrow and blood normal and malignant cell populations, which are discussed in the light of differential radiation responses resulting in hematotoxicity and related clinical implications.

Promotion of melanoma cell invasion and tumor metastasis by microcystin-LR via phosphatidylinositol 3-kinase/AKT pathway

Recently, we have indicated that microcystin-LR, a cyanobacterial toxin produced in eutrophic lakes or reservoirs, can increase invasive ability of melanoma MDA-MB-435 cells; however, the stimulatory effect needs identification by in vivo experiment and the related molecular mechanism is poorly understood. In this study, in vitro and in vivo experiments were conducted to investigate the effect of microcystin-LR on invasion and metastasis of human melanoma cells, and the underlying molecular mechanism was also explored. MDA-MB-435 xenograft model assay showed that oral administration of nude mice with microcystin-LR at 0.001-0.1 mg/kg/d posed no significant effect on tumor weight. Histological examination demonstrated that microcystin-LR could promote lung metastasis, which is confirmed by Matrigel chamber assay suggesting that microcystin-LR treatment at 25 nM can increase the invasiveness of MDA-MB-435 cells. In vitro and in vivo experiments consistently showed that microcystin-LR exposure increased mRNA and protein levels of matrix metalloproteinases (MMP-2/-9) by activating phosphatidylinositol 3-kinase (PI3-K)/AKT. Additionally, microcystin-LR treatment at low doses (≤25 nM) decreased lipid phosphatase PTEN expression, and the microcystin-induced invasiveness enhancement and MMP-2/-9 overexpression were reversed by the PI3-K/AKT chemical inhibitor LY294002 and AKT siRNA, indicating that microcystin-LR promotes invasion and metastasis of MDA-MB-435 cells via the PI3-K/AKT pathway.

Dissolved microcystins in surface and ground waters in regions with high cancer incidence in the Huai River Basin of China

Microcystins (MCs) are potent hepatotoxins and have also implicated in liver tumor promotion. The present study investigates the temporal and spatial variations of MCs in different water bodies in the Huai River Basin in China. Water samples including rivers, ponds and wells were collected every quarter during December 2008 and December 2009. MCs were determined by high pressure liquid chromatography after solid phase extraction. MCs concentrations in river samples were 0.741±0.623μgL(-1) with maximum of 1.846μgL(-1). MCs in pond were 0.597±0.960μgL(-1) with maximum of 2.298μgL(-1). MCs were also detected in 51.7% of the groundwater samples, MCs in groundwater were 0.060±0.085μgL(-1) with maximum of 0.446μgL(-1). MCs concentrations in groundwater did not differ significantly among different depths or towns (Wilcoxon test, p>0.05). The average MCs in groundwater in each sampling period were 0.068μgL(-1), 0.118μgL(-1), 0.052μgL(-1), 0.059μgL(-1) and 0.020μgL(-1). Through multi linear regression, the best fit model was built on MCs in groundwater with River B (R(2)=0.13, p<0.05), rather than with pond water. The results suggested that MCs contamination in groundwater originated from rivers, causing potential health risk on population who drink groundwater directly.

Loss of PPP2R2A inhibits homologous recombination DNA repair and predicts tumor sensitivity to PARP inhibition

Reversible phosphorylation plays a critical role in DNA repair. Here, we report the results of a loss-of-function screen that identifies the PP2A heterotrimeric serine/threonine phosphatases PPP2R2A, PPP2R2D, PPP2R5A, and PPP2R3C in double-strand break (DSB) repair. In particular, we found that PPP2R2A-containing complexes directly dephosphorylated ATM at S367, S1893, and S1981 to regulate its retention at DSB sites. Increased ATM phosphorylation triggered by PPP2R2A attenuation dramatically upregulated the activity of the downstream effector kinase CHK2, resulting in G(1) to S-phase cell-cycle arrest and downregulation of BRCA1 and RAD51. In tumor cells, blocking PPP2R2A thereby impaired the high-fidelity homologous recombination repair pathway and sensitized cells to small-molecule inhibitors of PARP. We found that PPP2R2A was commonly downregulated in non-small cell lung carcinomas, suggesting that PPP2R2A status may serve as a marker to predict therapeutic efficacy to PARP inhibition. In summary, our results deepen understanding of the role of PP2A family phosphatases in DNA repair and suggest PPP2R2A as a marker for PARP inhibitor responses in clinic.

Microcystin-LR promotes melanoma cell invasion and enhances matrix metalloproteinase-2/-9 expression mediated by NF-κB activation

This study aimed to explore the molecular mechanisms behind the stimulation effects of microcystin-LR (a well-known cyanobacterial toxin produced in eutrophic lakes or reservoirs) on cancer cell invasion and matrix metalloproteinases (MMPs) expression. Boyden chamber assay showed that microcystin-LR exposure (>12.5 nM) evidently enhanced the invasion ability of the melanoma cells (MDA-MB-435). Tumor Metastasis PCR Array demonstrated that 24 h microcystin-LR treatment (25 nM) caused overexpression of eight genes involved in tumor metastasis, including MMP-2, MMP-9, and MMP-13. Quantitative real-time PCR, Western blotting and gelatin zymography consistently demonstrated that mRNA and protein levels of MMP-2/-9 were increased in the cells after microcystin-LR exposure (P < 0.05 each). Immunofluorescence assay and electrophoretic mobility shift assay revealed that microcystin-LR could activate nuclear factor kappaB (NF-κB) by accelerating NF-κB translocation into the nucleus and enhancing NF-κB binding ability. Furthermore, addition of NF-κB inhibitor in culture medium could suppress the invasiveness enhancement and MMP-2/-9 overexpression. This study indicates that microcystin-LR can act as a NF-κB activator to promote MMP-2/-9 expression and melanoma cell invasion, which deserves more environmental health concerns.

Microcystin-LR induced DNA damage in human peripheral blood lymphocytes

Human exposure to microcystins, which are produced by freshwater cyanobacterial species, is of growing concern due to increasing appearance of cyanobacterial blooms as a consequence of global warming and increasing water eutrophication. Although microcystins are considered to be liver-specific, there is evidence that they may also affect other tissues. These substances have been shown to induce DNA damage in vitro and in vivo, but the mechanisms of their genotoxic activity remain unclear. In human peripheral blood lymphocytes (HPBLs) exposure to non-cytotoxic concentrations (0, 0.1, 1 and 10μg/ml) of microcystin-LR (MCLR) induced a dose- and time-dependent increase in DNA damage, as measured with the comet assay. Digestion of DNA from MCLR-treated HPBLs with purified formamidopyrimidine-DNA glycosylase (Fpg) displayed a greater number of DNA strand-breaks than non-digested DNA, confirming the evidence that MCLR induces oxidative DNA damage. With the cytokinesis-block micronucleus assay no statistically significant induction of micronuclei, nucleoplasmic bridges and nuclear buds was observed after a 24-h exposure to MCLR. At the molecular level, no changes in the expression of selected genes involved in the cellular response to DNA damage and oxidative stress were observed after a 4-h exposure to MCLR (1μg/ml). After 24h, DNA damage-responsive genes (p53, mdm2, gadd45a, cdkn1a), a gene involved in apoptosis (bax) and oxidative stress-responsive genes (cat, gpx1, sod1, gsr, gclc) were up-regulated. These results provide strong support that MCLR is an indirectly genotoxic agent, acting via induction of oxidative stress, and that lymphocytes are also the target of microcystin-induced toxicity.

Delayed effects of exposure to a moderate radiation dose on transcription profiles in human primary fibroblasts

Ionizing radiation (IR) is used in a wide variety of medical and nonmedical applications and poses a potential threat to human health. Knowledge of changes in gene expression in irradiated cells may be helpful for the establishment of effective paradigms for radiation protection. IR-induced DNA damage triggers a complex cascade of signal transduction. Recently, genome-wide approaches have allowed the detection of alterations in gene expression across a wide range of radiation doses. However, the delayed or long-term biological effects of mild-doses of IR remain largely unknown. The main objective of the present study was to investigate the effects of a moderate dose of gamma-rays (50 cGy) on gene expression 6 days post-irradiation. Gene expression using cDNA microarrays revealed statistically significant changes in the expression of 59 genes (FDR < 0.07), whose functions are related to cell-cycle control, protein trafficking, ubiquitin cycle, Rho-GTPAse pathway, protein phosphatase signalization, oxidoreductase control, and stress response. A set of 464 genes was also selected by a less stringent approach, and we demonstrate that this broader set of genes can efficiently distinguish the irradiated samples from the unirradiated, defining a long-term IR signature in human primary fibroblasts. Our findings support the existence of persistent responses to mild doses of IR detectable by changes in gene expression profiles. These results provide insight into delayed effects observed in human primary cells as well as the role of long-term response in neoplastic transformation. Environ.

Molecular aspects of microcystin-induced hepatotoxicity and hepatocarcinogenesis

It is known that microcystin (MC) is a cyanotoxin that is a potent environmental inhibitor of eucariotic protein serine/threonine phosphatase 1 and 2A, both in vitro and in vivo. Consequently, these cyanobacterial toxins (MC-IARC group 2B carcinogen, MC extracts-group 3) are potent tumor promoters and there is an indication that they may also act as tumor initiators. The ability of microcystin-LR (MC-LR) to act as a tumor initiator is based on fact that it can induce DNA damage either by direct interaction with DNA or by indirect mechanisms through formation of reactive oxygen species (ROS). Both acute and chronic exposures, to either low or high doses of MC-LR, can activate apoptotic pathways. Chronic exposure to low concentrations of MC-LR contributes to increased risk for cancer development. Epidemiological studies, in certain areas of China, have suggested that MC is one of the risk factors for the high incidence of primary liver cancer (PLC). Recently, we have reported a correlation between PLC and cyanobacterial "blooms" in reservoirs used as a source for drinking water supply in central Serbia. It appears that the combination of acute and chronic exposures to both high and low doses of MC can lead to PLC initiation and promotion. Based on this, we propose that the requirement for the co-factors such as aflatoxin B1 and other mycotoxins, HBV, HCV, alcohol, etc. is not needed for initiation and promotion of PLC by MC-LR as was suggested earlier. The possible mechanisms of the genotoxicity of MC and its role as a hepatocarcinogen are outlined in this review. Furthermore, we show that the exposure of hepatocytes to MC can lead either to malignant proliferation or apoptosis.

Cell signalling and radiation survival: the impact of protein phosphatases

PURPOSE

This review will examine the role of phosphatases in cancer cell signalling and also outline emerging findings regarding the influence of phosphatases on tumor cell survival after ionising radiation.

CONCLUSION

The exposure of tumour cells to clinically relevant doses of ionising radiation causes DNA damage and rapidly activates a series of signaling cascades involved in cell survival (reviewed in (Valerie et al. 2007 )). The role of kinases in this signalling has been extensively studied, but the role of phosphatases is less well defined. There is an abundance of literature implicating phosphatases in cell cycle control, cell growth and survival but there has been much less reported on the involvement of these enzymes as determinants of radiosensitivity. Recent studies, however, suggest that phosphatases may modulate tumor cell radiosensitivity and may be targets for the enhancement of radiotherapy.

In vivo DNA damage induced by the cyanotoxin microcystin-LR: comparison of intra-peritoneal and oral administrations by use of the comet assay

Microcystin-LR (MC-LR), involved in human and animal poisonings by cyanobacteria, has been shown to be both a potent tumour promoter in rat liver and an inhibitor of serine/threonine protein phosphatases, specifically PP1 and PP2A. The research on the genotoxic potential of MC-LR counts only few in vivo studies. In order to determine the target organs for DNA-damage induction by MC-LR, the single-cell gel electrophoresis (SCGE) or comet assay was performed in mice. Following a single oral administration of 2 and 4mg/kg bw of MC-LR, a statistically significant induction of DNA damage in blood cells was obtained after 3h. However, after an intra-peritoneal injection (ip), DNA lesions were mainly induced in the liver, but were also reported in the kidney, the intestine and the colon. The sensitivity of the ip route compared to the oral route suggested a difference in the bio-disponibility of the toxin. In any case, DNA damage was induced by MC-LR irrespective of the administration route. Among the target organs, the DNA damage induced in the intestinal tissues (ileum and colon) may contribute to an increased cancer risk.

Inhibition of nucleotide excision repair (NER) by microcystin-LR in CHO-K1 cells

Microcystin-LR (MC-LR), a potent inhibitor of PP1 and PP2A protein phosphatases, is related to tumor promotion and initiation. Although the genotoxic properties of this toxin have been extensively investigated with a variety of non-mammalian and mammalian test systems, the existing results are contradictory. Based on our previous results regarding the impact of MC-LR on the processes of DNA repair we decided to examine in greater detail its effect on the capacity of nucleotide excision repair (NER). CHO-K1 cells were pre-treated with increasing doses of MC-LR (1, 10 and 20 microg/ml) and then exposed to UV radiation (25 J/m(2)). Apoptosis was analyzed to exclude the possibility of false positive results in the comet assay. The results suggest that MC-LR targets the nucleotide excision repair mechanisms by interference with the incision/excision phase as well as the rejoining phase of NER and leads to an increased level of UV-induced cytogenetic DNA damage in CHO-K1 cells.