Dephosphorylation of γ-H2AX by WIP1: an important homeostatic regulatory event in DNA repair and cell cycle control

Aluminium phthalocyanine-mediated photodynamic therapy induces ATM-related DNA damage response and apoptosis in human oesophageal cancer cells

Introduction

Photodynamic therapy (PDT) is a light-based technique used in the treatment of malignant and non-malignant tissue. Aluminium-phthalocyanine chloride tetra sulfonate (AlPcS4Cl)-mediated PDT has been well investigated on several cancer types, including oesophageal cancer. However, the effects of (AlPcS4Cl)-mediated PDT on DNA damage response and the mechanism of cell death in oesophageal cancer needs further investigation.

Methods

Here, we examined the in vitro effects of AlPcS4Cl-mediated PDT on cell cycle, DNA damage response, oxidative stress, and intrinsic apoptotic cell death pathway in HKESC-1 oesophageal cancer cells. The HKESC-1 cells were exposed to PDT using a semiconductor laser diode (673.2 nm, 5 J/cm2 fluency). Cell viability and cytotoxicity were determined by the ATP cell viability assay and the lactate dehydrogenase (LDH) release assay, respectively. Cell cycle and DNA damage response (DDR) analyses were conducted using the Muse™ cell cycle kit and the Muse® multi-color DNA damage kit, respectively. The mode of cell death was identified using the Annexin V-FITC/PI detection assay and Muse® Autophagy LC3 antibody-based kit. The intrinsic apoptotic pathway was investigated by measuring the cellular reactive oxygen species (ROS) levels, mitochondrial membrane potential (ΔΨm) function, cytochrome c levels and the activity of caspase 3/7 enzymes.

Results

The results show that AlPcS4Cl-based PDT reduced cell viability, induced cytotoxicity, cell cycle arrest at the G0/G1 phase, and DNA double-strand break (DSB) through the upregulation of the ataxia telangiectasia mutated (ATM), a DNA damage sensor. In addition, the findings showed that AlPcS4Cl-based PDT induced cell death via apoptosis, which is observed through increased ROS production, reduced ΔΨm, increased cytochrome c release, and activation of caspase 3/7 enzyme. Finally, no autophagy was observed in the AlPcS4Cl-mediated PDT-treated cells.

Conclusion

Our findings showed that apoptotic cell death is the main cell death mechanism triggered by AlPcS4Cl-mediated PDT in oesophageal cancer cells.

Nano-Architecture of Persistent Focal DNA Damage Regions in the Minipig Epidermis Weeks after Acute γ-Irradiation

Exposure to high acute doses of ionizing radiation (IR) can induce cutaneous radiation syndrome. Weeks after such radiation insults, keratinocyte nuclei of the epidermis exhibit persisting genomic lesions that present as focal accumulations of DNA double-strand break (DSB) damage marker proteins. Knowledge about the nanostructure of these genomic lesions is scarce. Here, we compared the chromatin nano-architecture with respect to DNA damage response (DDR) factors in persistent genomic DNA damage regions and healthy chromatin in epidermis sections of two minipigs 28 days after lumbar irradiation with ~50 Gy γ-rays, using single-molecule localization microscopy (SMLM) combined with geometric and topological mathematical analyses. SMLM analysis of fluorochrome-stained paraffin sections revealed, within keratinocyte nuclei with perisitent DNA damage, the nano-arrangements of pATM, 53BP1 and Mre11 DDR proteins in γ-H2AX-positive focal chromatin areas (termed macro-foci). It was found that persistent macro-foci contained on average ~70% of 53BP1, ~23% of MRE11 and ~25% of pATM single molecule signals of a nucleus. MRE11 and pATM fluorescent tags were organized in focal nanoclusters peaking at about 40 nm diameter, while 53BP1 tags formed nanoclusters that made up super-foci of about 300 nm in size. Relative to undamaged nuclear chromatin, the enrichment of DDR protein signal tags in γ-H2AX macro-foci was on average 8.7-fold (±3) for 53BP1, 3.4-fold (±1.3) for MRE11 and 3.6-fold (±1.8) for pATM. The persistent macro-foci of minipig epidermis displayed a ~2-fold enrichment of DDR proteins, relative to DSB foci of lymphoblastoid control cells 30 min after 0.5 Gy X-ray exposure. A lasting accumulation of damage signaling and sensing molecules such as pATM and 53BP1, as well as the DSB end-processing protein MRE11 in the persistent macro-foci suggests the presence of diverse DNA damages which pose an insurmountable problem for DSB repair.

Phototunable Cell Killing by Photochromic Diarylethene of Thiazoyl and Thienyl Derivatives

We report a unique phototunable cell killing technique using diarylethene molecules as photo-isomerizing-molecular switches. These molecules were delivered to DNA in the cell nucleus due to closed-form generated by UV light, and then blue light triggered cell killing. A UV light irradiation switches the open form, having no DNA intercalation activity, to the closed form to induce intercalation in DNA. This isomer, thus prepared ready for the action, exerts photocytotoxicity upon the subsequent blue light irradiation. Molecular biological analysis clarifies that photocytotoxicity is due to DNA double-strand breaks. Since cell death is observed only when irradiated with light where both the open- and closed-ring isomers have absorption, the possible mechanism of cell death is assumed to be due to the repeated photocyclization and photocycloreversion reactions of the diarylethene molecules, which induce irreparable damage to DNA. This unique photo-controllable action in a cell system can provide the basis of a novel scheme of phototherapy.

The Detection of DNA Damage Response in MCF7 and MDA-MB-231 Breast Cancer Cell Lines after X-ray Exposure

Radiotherapy is one of the main options to cure and control breast cancer. The aim of this study was to investigate the sensitivity of two human breast cancer cell lines, MCF7 and MDA-MD-231, to radiation exposure at timepoints 4 h and 24 h after radiation. MCF7 and MDA-MD-231 were irradiated with different radiation doses using a Gilardoni CHF 320 G X-ray generator (Mandello del Lario, Italy) at 250 kVp, 15 mA [with half-value layer (HVL) = 1.6 mm copper]. The ApoTox-Glo triplex assay combines three assays used to assess viability, cytotoxicity, and apoptosis. The expression of γH2AX and BAX was analyzed by Western blotting. Viability and cytotoxicity did not change 4 h and 24 h after irradiation in either cell line, but we found a significant increase in the expression of cleaved caspase-3/7 at 24 h after irradiation with 8.5 Gy in MDA-MB231. The expression of γH2AX and BAX was low in MCF7, whereas the expression of γH2AX and BAX increased with radiation dose in a dose-dependent manner in MDA-MB231. The results show that the MCF7 cell line is more radioresistant than the MDA-MB 231 cell line at 4 h and 24 h after X-ray irradiation. In contrast, MDA-MB-231 cells were radiosensitive at a high radiation dose of 8.5 Gy at 24 h after irradiation. γH2AX and BAX indicated the radiosensitivity in both cell lines. These results open the possibility of using these cancer cell lines as models for testing new therapeutic strategies to improve radiation therapy.

PPM1D in Solid and Hematologic Malignancies: Friend and Foe?

In the face of constant genomic insults, the DNA damage response (DDR) is initiated to preserve genome integrity; its disruption is a classic hallmark of cancer. Protein phosphatase Mg2+/Mn2+-dependent 1D (PPM1D) is a central negative regulator of the DDR that is mutated or amplified in many solid cancers. PPM1D overexpression is associated with increased proliferative and metastatic behavior in multiple solid tumor types and patients with PPM1D-mutated malignancies have poorer prognoses. Recent findings have sparked an interest in the role of PPM1D in hematologic malignancies. Acquired somatic mutations may provide hematopoietic stem cells with a competitive advantage, leading to a substantial proportion of mutant progeny in the peripheral blood, an age-associated phenomenon termed "clonal hematopoiesis" (CH). Recent large-scale genomic studies have identified PPM1D to be among the most frequently mutated genes found in individuals with CH. While PPM1D mutations are particularly enriched in patients with therapy-related myeloid neoplasms, their role in driving leukemic transformation remains uncertain. Here, we examine the mechanisms through which PPM1D overexpression or mutation may drive malignancy by suppression of DNA repair, cell-cycle arrest, and apoptosis. We also discuss the divergent roles of PPM1D in the oncogenesis of solid versus hematologic cancers with a view to clinical implications and new therapeutic avenues.

Establishment of in vitro Calibration Curve for 60Co-γ-rays Induced Phospho-53BP1 Foci, Rapid Biodosimetry and Initial Triage, and Comparative Evaluations With γH2AX and Cytogenetic Assays

A rapid and reliable method for biodosimetry of populations exposed to ionizing radiation in the event of an incident or accident is crucial for initial triage and medical attention. DNA-double strand breaks (DSBs) are indicative of radiation exposure, and DSB-repair proteins (53BP1, γH2AX, ATM, etc.) are considered sensitive markers of DSB quantification. Phospho-53BP1 and γH2AX immunofluorescence technique serves as a sensitive, reliable, and reproducible tool for the detection and quantification of DSB-repair proteins, which can be used for biological dose estimations. In this study, dose-response curves were generated for ⁶⁰Co-γ-rays induced phospho-53 Binding Protein 1 (phospho-53BP1) foci at 1, 2, 4, 8, 16, and 24 h, post-irradiation for a dose range of 0.05–4 Gy using fluorescence microscopy. Following ISO recommendations, minimum detection limits (MDLs) were estimated to be 16, 18, 25, 40, 50, and 75 mGy for dose-response curves generated at 1, 2, 4, 8, 16, and 24 h post-irradiation. Colocalization and correlation of phospho-53BP1 and γH2AX were also measured in irradiated peripheral blood lymphocytes (PBLs) to gain dual confirmation. Comparative evaluation of the established curve was made by γH2AX-immunofluorescence, dicentric chromosome assay (DCA), and reciprocal translocation (RT) assays by reconstructing the dose of 6 dose-blinded samples. Coefficients of respective in-house established dose-response curves were employed to reconstruct the blind doses. Estimated doses were within the variation of 4.124%. For lower doses (0.052 Gy), phospho-53BP1 and γH2AX assays gave closer estimates with the variation of −4.1 to + 9% in comparison to cytogenetic assays, where variations were −8.5 to 24%. For higher doses (3 and 4 Gy), both the cytogenetic and immunofluorescence (phospho-53BP1 and γH2AX), assays gave comparable close estimates, with −11.3 to + 14.3% and −10.3 to −13.7%, variations, respectively.

MS: Wip1 suppresses angiogenesis through the STAT3-VEGF signalling pathway in serous ovarian cancer

Multifaceted functions of the so-called “oncogene” Wip1 have been reported in a previous study, while its actual role remains to be explored in serous ovarian cancer (SOC). In this study, by performing bioinformatic analysis with a public database and immunohistochemical staining of Wip1 in tumour tissue from SOC, we concluded that decreased expression of Wip1 was associated with a higher rate of tumour metastasis and platinum-based therapy resistance and increased ascites volume, which led to poorer prognosis in SOC patients. We also found that overexpression of Wip1 in SKOV3 cells decreased the levels of several cytokines, including VEGF, by secretome profiling analysis, and Wip1 overexpression suppressed angiogenesis both in vitro and in vivo. Mechanistic studies indicated that overexpression of Wip1 decreased the expression of VEGF at both the protein and mRNA levels and that the inhibitory effect was mediated by dephosphorylation of STAT3 at Ser727. Our study uncovered the role of Wip1 in SOC and provides a novel therapeutic strategy for suppressing angiogenesis.

Co-targeting WIP1 and PARP induces synthetic lethality in hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is one of the most fatal cancers. Due to limited strategies for effective treatments, patients with advanced HCC have a very poor prognosis. This study aims to identify new insights in HCC to develop novel strategies for HCC management.

Methods

The role of WIP1 (wild type p53 induced protein phosphatase1) in HCC was analyzed in HCC cells, xenograft model, DEN (Diethylnitrosamine) induced mice liver cancer model with WIP1 knockout mice, and TCGA database. DNA damage was evaluated by Gene Set Enrichment Analysis, western blotting, comet assay, and Immunofluorescence.

Results

High expression of WIP1 is associated with the poor prognosis of patients with HCC. Genetically and chemically suppression of WIP1 drastically reduced HCC cell proliferation. Besides, WIP1 knockout retarded DEN induced mice hepato-carcinogenesis. Mechanically, WIP1 inhibition induced DNA damage by increasing H2AX phosphorylation (γH2AX). Therefore, suppression of WIP1 and PARP induced synthetic lethality in HCC in vitro and in vivo by augmenting DNA damage.

Conclusion

WIP1 plays an oncogenic effect in HCC development, and targeting WIP1-dependent DNA damage repair alone or in combination with PARP inhibition might be a reasonable strategy for HCC management.

Biomarkers of DNA Damage Response Enable Flow Cytometry-Based Diagnostic to Identify Inborn DNA Repair Defects in Primary Immunodeficiencies

DNA damage is a constant event in every cell caused by exogenous factors such as ultraviolet and ionizing radiation (UVR/IR) and intercalating drugs, or endogenous metabolic and replicative stress. Proteins of the DNA damage response (DDR) network sense DNA lesions and induce cell cycle arrest, DNA repair, and apoptosis. Genetic defects of DDR or DNA repair proteins can be associated with immunodeficiency, bone marrow failure syndromes, and cancer susceptibility. Although various diagnostic tools are available to evaluate DNA damage, their quality to identify DNA repair deficiencies differs enormously and depends on affected pathways. In this study, we investigated the DDR biomarkers γH2AX (Ser139), p-ATM (Ser1981), and p-CHK2 (Thr68) using flow cytometry on peripheral blood cells obtained from patients with combined immunodeficiencies due to non-homologous end-joining (NHEJ) defects and ataxia telangiectasia (AT) in response to low-dose IR. Significantly reduced induction of all three markers was observed in AT patients compared to controls. However, delayed downregulation of γH2AX was found in patients with NHEJ defects. In contrast to previous reports of DDR in cellular models, these biomarkers were not sensitive enough to identify ARTEMIS deficiency with sufficient reliability. In summary, DDR biomarkers are suitable for diagnosing NHEJ defects and AT, which can be useful in neonates with abnormal TREC levels (T cell receptor excision circles) identified by newborn screening. We conclude that DDR biomarkers have benefits and some limitations depending on the underlying DNA repair deficiency.

The evolving complexity of DNA damage foci: RNA, condensates and chromatin in DNA double-strand break repair

Formation of biomolecular condensates is increasingly recognized as a mechanism employed by cells to deal with stress and to optimize enzymatic reactions. Recent studies have characterized several DNA repair foci as phase-separated condensates, behaving like liquid droplets. Concomitantly, the apparent importance of long non-coding RNAs and RNA-binding proteins for the repair of double-strand breaks has raised many questions about their exact contribution to the repair process. Here we discuss how RNA molecules can participate in condensate formation and how RNA-binding proteins can act as molecular scaffolds. We furthermore summarize our current knowledge about how properties of condensates can influence the choice of repair pathway (homologous recombination or non-homologous end joining) and identify the open questions in this field of emerging importance.

Phosphatase magnesium-dependent 1 δ (PPM1D), serine/threonine protein phosphatase and novel pharmacological target in cancer

Aberrations in DNA damage response genes are recognized mediators of tumorigenesis and resistance to chemo- and radiotherapy. While protein phosphatase magnesium-dependent 1 δ (PPM1D), located on the long arm of chromosome 17 at 17q22-23, is a key regulator of cellular responses to DNA damage, amplification, overexpression, or mutation of this gene is important in a wide range of pathologic processes. In this review, we describe the physiologic function of PPM1D, as well as its role in diverse processes, including fertility, development, stemness, immunity, tumorigenesis, and treatment responsiveness. We highlight both the advances and limitations of current approaches to targeting malignant processes mediated by pathogenic alterations in PPM1D with the goal of providing rationale for continued research and development of clinically viable treatment approaches for PPM1D-associated diseases.

Metal-dependent Ser/Thr protein phosphatase PPM family: Evolution, structures, diseases and inhibitors

Protein phosphatases and kinases control multiple cellular events including proliferation, differentiation, and stress responses through regulating reversible protein phosphorylation, the most important post-translational modification. Members of metal-dependent protein phosphatase (PPM) family, also known as PP2C phosphatases, are Ser/Thr phosphatases that bind manganese/magnesium ions (Mn²⁺/Mg²⁺) in their active center and function as single subunit enzymes. In mammals, there are 20 isoforms of PPM phosphatases: PPM1A, PPM1B, PPM1D, PPM1E, PPM1F, PPM1G, PPM1H, PPM1J, PPM1K, PPM1L, PPM1M, PPM1N, ILKAP, PDP1, PDP2, PHLPP1, PHLPP2, PP2D1, PPTC7, and TAB1, whereas there are only 8 in yeast. Phylogenetic analysis of the DNA sequences of vertebrate PPM isoforms revealed that they can be divided into 12 different classes: PPM1A/PPM1B/PPM1N, PPM1D, PPM1E/PPM1F, PPM1G, PPM1H/PPM1J/PPM1M, PPM1K, PPM1L, ILKAP, PDP1/PDP2, PP2D1/PHLPP1/PHLPP2, TAB1, and PPTC7. PPM-family members have a conserved catalytic core region, which contains the metal-chelating residues. The different isoforms also have isoform specific regions within their catalytic core domain and terminal domains, and these regions may be involved in substrate recognition and/or functional regulation of the phosphatases. The twenty mammalian PPM phosphatases are involved in regulating diverse cellular functions, such as cell cycle control, cell differentiation, immune responses, and cell metabolism. Mutation, overexpression, or deletion of the PPM phosphatase gene results in abnormal cellular responses, which lead to various human diseases. This review focuses on the structures and biological functions of the PPM-phosphatase family and their associated diseases. The development of specific inhibitors against the PPM phosphatase family as a therapeutic strategy will also be discussed.

WIP1 dephosphorylation of p27Kip1 Serine 140 destabilizes p27Kip1 and reverses anti-proliferative effects of ATM phosphorylation

The phosphoinositide-3-kinase like kinases (PIKK) such as ATM and ATR play a key role in initiating the cellular DNA damage response (DDR). One key ATM target is the cyclin-dependent kinase inhibitor p27^Kip1 that promotes G1 arrest. ATM activates p27^Kip1-induced arrest in part through phosphorylation of p27^Kip1 at Serine 140. Here we show that this site is dephosphorylated by the type 2C serine/threonine phosphatase, WIP1 (Wildtype p53-Induced Phosphatase-1), encoded by the PPM1D gene. WIP1 has been shown to dephosphorylate numerous ATM target sites in DDR proteins, and its overexpression and/or mutation has often been associated with oncogenesis. We demonstrate that wildtype, but not phosphatase-dead WIP1, efficiently dephosphorylates p27^Kip1 Ser140 both in vitro and in cells and that this dephosphorylation is sensitive to the WIP1-specific inhibitor GSK 2830371. Increased expression of wildtype WIP1 reduces stability of p27^Kip1 while increased expression of similar amounts of phosphatase-dead WIP1 has no effect on p27^Kip1 protein stability. Overexpression of wildtype p27^Kip1 reduces cell proliferation and colony forming capability relative to the S140A (constitutively non-phosphorylated) form of p27. Thus, WIP1 plays a significant role in homeostatic modulation of p27^Kip1 activity following activation by ATM.

A Drosophila cell-free system that senses DNA breaks and triggers phosphorylation signalling

Preblastoderm Drosophila embryo development is characterized by fast cycles of nuclear divisions. Extracts from these embryos can be used to reconstitute complex chromatin with high efficiency. We now discovered that this chromatin assembly system contains activities that recognize unprotected DNA ends and signal DNA damage through phosphorylation. DNA ends are initially bound by Ku and MRN complexes. Within minutes, the phosphorylation of H2A.V (homologous to γH2A.X) initiates from DNA breaks and spreads over tens of thousands DNA base pairs. The γH2A.V phosphorylation remains tightly associated with the damaged DNA and does not spread to undamaged DNA in the same reaction. This first observation of long-range γH2A.X spreading along damaged chromatin in an in vitro system provides a unique opportunity for mechanistic dissection. Upon further incubation, DNA ends are rendered single-stranded and bound by the RPA complex. Phosphoproteome analyses reveal damage-dependent phosphorylation of numerous DNA-end-associated proteins including Ku70, RPA2, CHRAC16, the exonuclease Rrp1 and the telomer capping complex. Phosphorylation of spindle assembly checkpoint components and of microtubule-associated proteins required for centrosome integrity suggests this cell-free system recapitulates processes involved in the regulated elimination of fatally damaged syncytial nuclei.

Reconstitution of the Ataxia-Telangiectasia Cellular Phenotype With Lentiviral Vectors

Ataxia-telangiectasia (A-T) is a complex disease arising from mutations in the ATM gene (Ataxia-Telangiectasia Mutated), which plays crucial roles in repairing double-strand DNA breaks (DSBs). Heterogeneous immunodeficiency, extreme radiosensitivity, frequent appearance of tumors and neurological degeneration are hallmarks of the disease, which carries high morbidity and mortality because only palliative treatments are currently available. Gene therapy was effective in animal models of the disease, but the large size of the ATM cDNA required the use of HSV-1 or HSV/AAV hybrid amplicon vectors, whose characteristics make them unlikely tools for treating A-T patients. Due to recent advances in vector packaging, production and biosafety, we developed a lentiviral vector containing the ATM cDNA and tested whether or not it could rescue cellular defects of A-T human mutant fibroblasts. Although the cargo capacity of lentiviral vectors is an inherent limitation in their use, and despite the large size of the transgene, we successfully transduced around 20% of ATM-mutant cells. ATM expression and phosphorylation assays indicated that the neoprotein was functional in transduced cells, further reinforced by their restored capacity to phosphorylate direct ATM substrates such as p53 and their capability to repair radiation-induced DSBs. In addition, transduced cells also restored cellular radiosensitivity and cell cycle abnormalities. Our results demonstrate that lentiviral vectors can be used to rescue the intrinsic cellular defects of ATM-mutant cells, which represent, in spite of their limitations, a proof-of-concept for A-T gene therapy.

Chemical Features Important for Activity in a Class of Inhibitors Targeting the Wip1 Flap Subdomain

The wild-type p53 induced phosphatase 1, Wip1 (PP2Cδ), is a protein phosphatase 2C (PP2C) family serine/threonine phosphatase that negatively regulates the function of the tumor suppressor p53 and several of its positive regulators such as ATM, Chk1, Chk2, Mdm2, and p38 MAPK. Wip1 dephosphorylates and inactivates its protein targets, which are critical for cellular stress responses. Additionally, Wip1 is frequently amplified and overexpressed in several human cancer types. Because of its negative role in regulating the function of tumor suppressor proteins, Wip1 has been identified as a potential therapeutic target in various types of cancers. Based on a recently reported Wip1 inhibitor (G-1), we performed an extensive structure-activity relationship (SAR) analysis. This led us to interesting findings in SAR trends and to the discovery of new chemical analogues with good specificity and bioavailability.

ATR kinase regulates its attenuation via PPM1D phosphatase recruitment to chromatin during recovery from DNA replication stress signalling

In eukaryotes, in response to replication stress, DNA damage response kinase, ATR is activated, whose signalling abrogation leads to cell lethality due to aberrant fork remodelling and excessive origin firing. Here we report that inhibition of ATR kinase activity specifically during replication stress recovery results in persistent ATR signalling, evidenced by the presence of ATR-dependent phosphorylation marks (gamma H2AX, pChk1 and pRad17) and delayed cell cycle re-entry. Further, such disruption of ATR signalling attenuation leads to double-strand breaks, fork collapse and thereby 'replication catastrophe'. PPM1D phosphatase, a nucleolar localized protein, relocates to chromatin during replication stress and reverts back to nucleolus following stress recovery, under the control of ATR kinase action. Inhibition of ATR kinase activity, specifically during post replication stress, triggers dislodging of the chromatin-bound PPM1D from nucleus to cytoplasm followed by its degradation, thereby leading to persistence of activated ATR marks in the nuclei. Chemical inhibition of PPM1D activity or SiRNA mediated depletion of the protein during post replication stress recovery 'phenocopies' ATR kinase inhibition by failing to attenuate ATR signalling. Collectively, our observations suggest a novel role of ATR kinase in mediating its own signal attenuation via PPM1D recruitment to chromatin as an essential mechanism for restarting the stalled forks, cell-cycle re-entry and cellular recovery from replication stress.

Cooperation of Nutlin-3a and a Wip1 inhibitor to induce p53 activity

Targeting the Mdm2 oncoprotein by drugs has the potential of re-establishing p53 function and tumor suppression. However, Mdm2-antagonizing drug candidates, e. g. Nutlin-3a, often fail to abolish cancer cell growth sustainably. To overcome these limitations, we inhibited Mdm2 and simultaneously a second negative regulator of p53, the phosphatase Wip1/PPM1D. When combining Nutlin-3a with the Wip1 inhibitor GSK2830371 in the treatment of p53-proficient but not p53-deficient cells, we observed enhanced phosphorylation (Ser 15) and acetylation (Lys 382) of p53, increased expression of p53 target gene products, and synergistic inhibition of cell proliferation. Surprisingly, when testing the two compounds individually, largely distinct sets of genes were induced, as revealed by deep sequencing analysis of RNA. In contrast, the combination of both drugs led to an expression signature that largely comprised that of Nutlin-3a alone. Moreover, the combination of drugs, or the combination of Nutlin-3a with Wip1-depletion by siRNA, activated p53-responsive genes to a greater extent than either of the compounds alone. Simultaneous inhibition of Mdm2 and Wip1 enhanced cell senescence and G2/M accumulation. Taken together, the inhibition of Wip1 might fortify p53-mediated tumor suppression by Mdm2 antagonists.

Wip1 suppresses ovarian cancer metastasis through the ATM/AKT/Snail mediated signaling

Inactivation of p53 greatly contributes to serous ovarian cancer, while the role of the wild-type p53 induced phosphatase 1 (Wip1) is quite unclear. In this study, by silencing or overexpression of Wip1, we found that Wip1 suppressed ovarian cancer cell invasion, migration, epithelial to mesenchymal transition (EMT), and ovarian cancer metastasis in xenograft animal models. Mechanistic studies showed that Wip1 may block ovarian cancer metastasis through inhibition of Snail and p-Akt expression because silencing or overexpression of Wip1 either upregulated or downregulated the expression of Snail and p-Akt (Ser 473), while further knockdown of Snail by shRNA or inhibition of p-Akt by a chemical compound attenuated cell invasion, migration and EMT in Wip1 silencing cells. We also found that the phosphorylation of Akt at Ser 473 might be mediated through p-ATM (Ser 1981). Thus, Wip1 may suppress ovarian cancer metastasis through negative regulation of p-ATM, p-Akt, and Snail, which was also evidenced in the limited clinical specimens. Therefore, our data may provide a novel therapeutic indication for serous ovarian cancer based on the uncovered mechanism associated with the precise function of Wip1 independent of p53.

Phosphatase Wip1 in Immunity: An Overview and Update

Wild-type p53-induced phosphatase 1 (Wip1) is a newly identified serine/threonine phosphatase, which belongs to the PP2C family. Due to its involvement in stress-induced networks and overexpression in human tumors, primary studies have mainly focused on the role of Wip1 in tumorigenesis. It now has also been implicated in regulating several other physiological processes such as organism aging and neurogenesis. Recent evidence highlights a new role of Wip1 in controlling immune response through regulating immune cell development and function, as well as through the interplay with inflammatory signaling pathways such NF-κB and p38 mitogen-activated protein kinase. In this short review, we will give an overview of Wip1 in immunity to better understand this important phosphatase.

Ultra-short laser-accelerated proton pulses have similar DNA-damaging effectiveness but produce less immediate nitroxidative stress than conventional proton beams

Ultra-short proton pulses originating from laser-plasma accelerators can provide instantaneous dose rates at least 10(7)-fold in excess of conventional, continuous proton beams. The impact of such extremely high proton dose rates on A549 human lung cancer cells was compared with conventionally accelerated protons and 90 keV X-rays. Between 0.2 and 2 Gy, the yield of DNA double strand breaks (foci of phosphorylated histone H2AX) was not significantly different between the two proton sources or proton irradiation and X-rays. Protein nitroxidation after 1 h judged by 3-nitrotyrosine generation was 2.5 and 5-fold higher in response to conventionally accelerated protons compared to laser-driven protons and X-rays, respectively. This difference was significant (p < 0.01) between 0.25 and 1 Gy. In conclusion, ultra-short proton pulses originating from laser-plasma accelerators have a similar DNA damaging potential as conventional proton beams, while inducing less immediate nitroxidative stress, which probably entails a distinct therapeutic potential.

Wee1 inhibition potentiates Wip1-dependent p53-negative tumor cell death during chemotherapy

Inactivation of p53 found in more than half of human cancers is often associated with increased tumor resistance to anti-cancer therapy. We have previously shown that overexpression of the phosphatase Wip1 in p53-negative tumors sensitizes them to chemotherapeutic agents, while protecting normal tissues from the side effects of anti-cancer treatment. In this study, we decided to search for kinases that prevent Wip1-mediated sensitization of cancer cells, thereby interfering with efficacy of genotoxic anti-cancer drugs. To this end, we performed a flow cytometry-based screening in order to identify kinases that regulated the levels of γH2AX, which were used as readout. Another criterion of the screen was increased sensitivity of p53-negative tumor cells to cisplatin (CDDP) in a Wip1-dependent manner. We have found that a treatment with a low dose (75 nM) of MK-1775, a recently described specific chemical inhibitor of Wee1, decreases CDDP-induced H2AX phosphorylation in p53-negative cells and enhances the Wip1-sensitization of p53-negative tumors. We were able to reduce CDDP effective concentration by 40% with a combination of Wip1 overexpression and Wee1 kinase inhibition. We have observed that Wee1 inhibition potentiates Wip1-dependent tumor sensitization effect by reducing levels of Hipk2 kinase, a negative regulator of Wip1 pathway. In addition, during CDDP treatment, the combination of Wee1 inhibition and Wip1 overexpression has a mild but significant protective effect in normal cells and tissues. Our results indicate that inhibition of the negative regulators of Wip1 pathway, Wee1 and Hipk2, in p53-negative tumors could potentiate efficiency of chemotherapeutic agents without concomitant increase of cytotoxicity in normal tissues. The development and clinical use of Wee1 and Hipk1 kinase chemical inhibitors might be a promising strategy to improve anti-cancer therapy.

Regulators in the DNA damage response

Maintenance of genome integrity is essential for the proper function of all cells and organisms. In response to both endogenous and exogenous DNA damaging agents, mammalian cells have evolved a delicate system to sense DNA damage, stop cell cycle progression, modulate cell metabolism, repair damaged DNA, and induce programmed cell death if the damage is too severe. This coordinated global signaling network, namely the DNA damage response (DDR), ensures the genome stability under DNA damaging stress. A variety of regulators have been shown to modulate the activity and levels of key proteins in the DDR, including kinases, phosphatases, ubiquitin ligases, deubiquitinases, and other protein modifying enzymes. Epigenetic regulators, particularly microRNAs and long noncoding RNAs, have been emerging as an important payer of regulation in addition to canonical DNA damage signaling proteins. In this review, we will discuss the functional interaction between the regulators and their targets in the DDR.

ATM kinase: Much more than a DNA damage responsive protein

ATM, mutation of which causes Ataxia telangiectasia, has emerged as a cardinal multifunctional protein kinase during past two decades as evidenced by various studies from around the globe. Further to its well established and predominant role in DNA damage response, ATM has also been understood to help in maintaining overall functional integrity of cells; since its mutation, inactivation or deficiency results in a variety of pathological manifestations besides DNA damage. These include oxidative stress, metabolic syndrome, mitochondrial dysfunction as well as neurodegeneration. Recently, high throughput screening using proteomics, metabolomics and transcriptomic studies revealed several proteins which might be acting as substrates of ATM. Studies that can help in identifying effective regulatory controls within the ATM-mediated pathways/mechanisms can help in developing better therapeutics. In fact, more in-depth understanding of ATM-dependent cellular signals could also help in the treatment of variety of other disease conditions since these pathways seem to control many critical cellular functions. In this review, we have attempted to put together a detailed yet lucid picture of the present-day understanding of ATM's role in various pathophysiological conditions involving DNA damage and beyond.

DNA damage-induced regulatory interplay between DAXX, p53, ATM kinase and Wip1 phosphatase

Death domain-associated protein 6 (DAXX) is a histone chaperone, putative regulator of apoptosis and transcription, and candidate modulator of p53-mediated gene expression following DNA damage. DAXX becomes phosphorylated upon DNA damage, however regulation of this modification, and its relationship to p53 remain unclear. Here we show that in human cells exposed to ionizing radiation or genotoxic drugs etoposide and neocarzinostatin, DAXX became rapidly phosphorylated in an ATM kinase-dependent manner. Our deletion and site-directed mutagenesis experiments identified Serine 564 (S564) as the dominant ATM-targeted site of DAXX, and immunofluorescence experiments revealed localization of S564-phosphorylated DAXX to PML nuclear bodies. Furthermore, using a panel of human cell types, we identified the p53-regulated Wip1 protein phosphatase as a key negative regulator of DAXX phosphorylation at S564, both in vitro and in cells. Consistent with the emerging oncogenic role of Wip1, its DAXX-dephosphorylating impact was most apparent in cancer cell lines harboring gain-of-function mutant and/or overexpressed Wip1. Unexpectedly, while Wip1 depletion increased DAXX phosphorylation both before and after DNA damage and increased p53 stability and transcriptional activity, knock-down of DAXX impacted neither p53 stabilization nor p53-mediated expression of Gadd45a, Noxa, Mdm2, p21, Puma, Sesn2, Tigar or Wip1. Consistently, analyses of cells with genetic, TALEN-mediated DAXX deletion corroborated the notion that neither phosphorylated nor non-phosphorylated DAXX is required for p53-mediated gene expression upon DNA damage. Overall, we identify ATM kinase and Wip1 phosphatase as opposing regulators of DAXX-S564 phosphorylation, and propose that the role of DAXX phosphorylation and DAXX itself are independent of p53-mediated gene expression.

Absolute quantification of acetylation and phosphorylation of the histone variant H2AX upon ionizing radiation reveals distinct cellular responses in two cancer cell lines

Histone modifications change upon the cellular response to ionizing radiation, and their cellular amounts could reflect the DNA damage response activity. We previously reported a sensitive and reliable method for the absolute quantification of γH2AX within cells, using liquid chromatography-tandem mass spectrometry (LC/MS/MS). The technique has broad adaptability to a variety of biological systems and can quantitate different modifications of histones. In this study, we applied it to quantitate the levels of γH2AX and K5-acetylated H2AX, and to compare the radiation responses between two cancer cell lines: HeLa and U-2 OS. The two cell lines have distinct properties in terms of their H2AX modifications. HeLa cells have relatively high γH2AX (3.1 %) against the total H2AX even in un-irradiated cells, while U-2 OS cells have an essentially undetectable level (nearly 0 %) of γH2AX. In contrast, the amounts of acetylated histones are lower in HeLa cells (9.3 %) and higher in U-2 OS cells (24.2 %) under un-irradiated conditions. Furthermore, after ionizing radiation exposure, the time-dependent increases and decreases in the amounts of histone modifications differed between the two cell lines, especially at the early time points. These results suggest that each biological system has distinct kinase/phosphatase and/or acetylase/deacetylase activities. In conclusion, for the first time, we have succeeded in simultaneously monitoring the absolute amounts of phosphorylated and acetylated cellular H2AX after ionizing radiation exposure. This multi-criteria assessment enables precise comparisons of the effects of radiation between any biological systems.

Residual γH2AX foci after ex vivo irradiation of patient samples with known tumour-type specific differences in radio-responsiveness

PURPOSE

To apply our previously published residual ex vivo γH2AX foci method to patient-derived tumour specimens covering a spectrum of tumour-types with known differences in radiation response. In addition, the data were used to simulate different experimental scenarios to simplify the method.

MATERIALS AND METHODS

Evaluation of residual γH2AX foci in well-oxygenated tumour areas of ex vivo irradiated patient-derived tumour specimens with graded single doses was performed. Immediately after surgical resection, the samples were cultivated for 24h in culture medium prior to irradiation and fixed 24h post-irradiation for γH2AX foci evaluation. Specimens from a total of 25 patients (including 7 previously published) with 10 different tumour types were included.

RESULTS

Linear dose response of residual γH2AX foci was observed in all specimens with highly variable slopes among different tumour types ranging from 0.69 (95% CI: 1.14-0.24) to 3.26 (95% CI: 4.13-2.62) for chondrosarcomas (radioresistant) and classical seminomas (radiosensitive) respectively. Simulations suggest that omitting dose levels might simplify the assay without compromising robustness.

CONCLUSION

Here we confirm clinical feasibility of the assay. The slopes of the residual foci number are well in line with the expected differences in radio-responsiveness of different tumour types implying that intrinsic radiation sensitivity contributes to tumour radiation response. Thus, this assay has a promising potential for individualized radiation therapy and prospective validation is warranted.

Downregulation of MDC1 and 53BP1 by short hairpin RNA enhances radiosensitivity in laryngeal carcinoma cells

DNA double-strand breaks (DSBs) induced by ionizing radiation (IR) are among the most cytotoxic types of DNA damage. The DNA damage response (DDR) may be a reason for the cancer cell resistance to radiotherapy using IR. Identified as critical upstream mediators of the phosphorylation of ataxia telangiectasia-mutated (ATM) pathway, mediator of DNA damage checkpoint 1 (MDC1) and p53-binding proteins 1 (53BP1) may affect the radiosensitivity of tumor cells. In the present study, we generated two HEP-2 cell lines with a stable knockdown of MDC1 or 53BP1 with short hairpin RNA (shRNA), respectively, and investigated the effect of MDC1 and 53BP1 on cell radiosensitivity, cell cycle distribution and the formation of cell foci. Downregulation of the two proteins reduced the number of clonogenic cells that treated with IR. Accumulation of G2/M phase cells was detected after the MDC1 and 53BP1 downregulation. These results indicated that the expression of MDC1 or 53BP1 limited tumor cell sensitivity to radiotherapy and may play an important role in the DNA repair progression. Furthermore, the MDC1 foci was identified and presented in the 53BP1-inhibited cells. By contrast, the 53BP1 foci was absent from the MDC1-inhibited cells. The results confirmed that the recruitment of 53BP1 into the foci occurred in an MDC1-dependent manner.

γH2AX assay in ex vivo irradiated tumour specimens: A novel method to determine tumour radiation sensitivity in patient-derived material

PURPOSE

To establish a clinically applicable protocol for quantification of residual γH2AX foci in ex vivo irradiated tumour samples and to apply this method in a proof-of-concept feasibility study to patient-derived tumour specimens.

MATERIAL AND METHODS

Evaluation of γH2AX foci formation and disappearance in excised FaDu tumour specimens after (a) different incubation times in culture medium, 4Gy irradiation and fixation after 24h (cell recovery), (b) 10h medium incubation, 4Gy irradiation and fixation after various time points (double strand break repair kinetics), and (c) 10h medium incubation, irradiation with graded single radiation doses and fixation after 24h (dose-response). The optimised protocol was applied to patient-derived samples of seminoma, prostate cancer and glioblastoma multiforme.

RESULTS

Post excision or biopsy, tumour tissues showed stable radiation-induced γH2AX foci values in oxic cells after >6h of recovery in medium. Kinetics of foci disappearance indicated a plateau of residual foci after >12h following ex vivo irradiation. Fitting the dose-response of residual γH2AX foci yielded slopes comparable with in situ irradiation of FaDu tumours. Significant differences in the slopes of ex vivo irradiated patient-derived tumour samples were found.

CONCLUSION

A novel clinically applicable method to quantify residual γH2AX foci in ex vivo irradiated tumour samples was established. The first clinical results suggest that this method allows to distinguish between radiosensitive and radioresistant tumour types. These findings support further translational evaluation of this assay to individualise radiation therapy.

WIP1 phosphatase as a potential therapeutic target in neuroblastoma

The wild-type p53-induced phosphatase 1 (WIP1) is a serine/threonine phosphatase that negatively regulates multiple proteins involved in DNA damage response including p53, CHK2, Histone H2AX, and ATM, and it has been shown to be overexpressed or amplified in human cancers including breast and ovarian cancers. We examined WIP1 mRNA levels across multiple tumor types and found the highest levels in breast cancer, leukemia, medulloblastoma and neuroblastoma. Neuroblastoma is an exclusively TP53 wild type tumor at diagnosis and inhibition of p53 is required for tumorigenesis. Neuroblastomas in particular have previously been shown to have 17q amplification, harboring the WIP1 (PPM1D) gene and associated with poor clinical outcome. We therefore sought to determine whether inhibiting WIP1 with a selective antagonist, GSK2830371, can attenuate neuroblastoma cell growth through reactivation of p53 mediated tumor suppression. Neuroblastoma cell lines with wild-type TP53 alleles were highly sensitive to GSK2830371 treatment, while cell lines with mutant TP53 were resistant to GSK2830371. The majority of tested neuroblastoma cell lines with copy number gains of the PPM1D locus were also TP53 wild-type and sensitive to GSK2830371A; in contrast cell lines with no copy gain of PPM1D were mixed in their sensitivity to WIP1 inhibition, with the primary determinant being TP53 mutational status. Since WIP1 is involved in the cellular response to DNA damage and drugs used in neuroblastoma treatment induce apoptosis through DNA damage, we sought to determine whether GSK2830371 could act synergistically with standard of care chemotherapeutics. Treatment of wild-type TP53 neuroblastoma cell lines with both GSK2830371 and either doxorubicin or carboplatin resulted in enhanced cell death, mediated through caspase 3/7 induction, as compared to either agent alone. Our data suggests that WIP1 inhibition represents a novel therapeutic approach to neuroblastoma that could be integrated with current chemotherapeutic approaches.

Overexpression of wip1 is associated with biologic behavior in human clear cell renal cell carcinoma

Wild-type p53-induced phosphatase (Wip1 or PPM1D) has been reported to be aberrantly expressed in various cancers and correlated with the malignant behavior of cancer cells. However, the function of Wip1 in RCC remains unclear. The present study investigated its abnormal expression and dysfunctions in clear cell renal cell carcinoma (ccRCC) in vitro. With the combination of immunohistochemistry, western blotting, immunofluorescence, qRT-PCR, and cell proliferation, migration and invasion assays, we found that levels of Wip1 mRNA and protein were dramatically increased in human ccRCC tissues (P<0.001 for both), and upregulation of Wip1 was significantly associated with depth of invasion (P<0.001), Distant metastasis (P = 0.001), lymph node status (P<0.001) and Fuhrman grade (P<0.001). Wip1 knockdown inhibited the proliferation, migration and invasion of 786-O and RLC-310 cells, whereas Wip1 overexpression promoted the growth and aggressive phenotype of 786-O and RLC-310 cells in vitro. The uni- and multivariate analyses indicated that expression of Wip1 was an independent predictor for survival of ccRCC patients (P = 0.003, P = 0.027 respectively). Wip1- negative patients had a higher tumor-free/overall survival rate than patients with high Wip1 expression (P = 0.001, P = 0.002 respectively). Overexpression of Wip1 is useful in the prediction of survival in ccRCC patients.

Wip1 abrogation decreases intestinal tumor frequency in APC(Min/+) mice irrespective of radiation quality

Low-linear energy transfer (low-LET) γ-ray exposure is a risk factor for colorectal cancer (CRC). Due to their high-LET nature, energetic iron ions found in space are expected to pose greater CRC risks to astronauts undertaking long-duration space missions beyond low Earth orbit. Wild-type p53-induced phosphatase 1 (Wip1) is important for cellular DNA damage response and its abrogation has been shown to inhibit spontaneous intestinal tumorigenesis in APC(Min/+) mice, a well-studied mouse model of human CRC. However, the relationship of Wip1 to radiation-induced intestinal tumorigenesis, especially by energetic iron ions, has not been investigated in APC(Min/+) mice. We have previously reported that there is a greater intestinal tumorigenic potential of iron-ion radiation relative to (137)Cs γ rays, so the purpose of the current study was to investigate whether Wip1 abrogation could influence high-LET dependent intestinal tumorigenesis in APC(Min/+) mice. Intestinal tumor frequency and grade were assessed in APC(Min/+)/Wip1(-/-) mice and results were compared to those in APC(Min/+)/Wip1(+/+) mice after exposure to a mean absorbed dose of 2 Gy from (137)Cs γ rays or 1.6 Gy from 1 GeV/n iron ions. Cellular differentiation and proliferation were also assessed in the intestinal tumors of sham-irradiated and irradiated mice. Decreased tumor frequency and lower tumor grade were observed in APC(Min/+)/Wip1(-/-) relative to APC(Min/+)/Wip1(+/+) mice. Notably, a similar decrease (∼6-fold in both groups) in tumor number was observed in sham-irradiated and γ-irradiated APC(Min/+)/Wip1(-/-) relative to APC(Min/+)/Wip1(+/+) mice. However, tumorigenesis in the energetic iron-ion exposed group was reduced ∼8-fold in APC(Min/+)/Wip1(-/-) relative to APC(Min/+)/Wip1(+/+) mice. A significantly lower proliferation/differentiation index in tumors of iron-ion exposed APC(Min/+)/Wip1(-/-) relative to APC(Min/+)/Wip1(+/+) mice suggests that reduced proliferation and enhanced differentiation as a result of Wip1 abrogation maybe involved. In conclusion, the current study demonstrated that the absence of Wip1 blocked radiation-induced intestinal tumorigenesis irrespective of radiation quality and has implications for developing preventive strategies against the tumorigenic potential of radiation exposure on earth and in outer space.

WIP1 regulates the proliferation and invasion of nasopharyngeal carcinoma in vitro

Wild-type p53-induced phosphatase (WIP1) is overexpressed and functionally altered in multiple human malignancies. The present study investigated its abnormal expression and dysfunctions in nasopharyngeal carcinoma (NPC) in vitro. Here, analysis of WIP1 mRNA and protein in human NPC tissues revealed that both WIP1 messenger RNA (mRNA) and protein were elevated and were correlated with NPC clinical stage and metastasis in patients. In vitro experiments further showed that WIP1 inhibition led to a decrease in the proliferative ability of NPC CNE-2 and 5-8F cells accompanied by cell cycle arrest and increased apoptosis. In addition, WIP1 knockdown inhibited the invasiveness of CNE-2 and 5-8F cells and was associated with the down-regulation of the expression of matrix metallopeptidase 9 (MMP-9) mRNA and protein. Taken together, our data demonstrate that WIP1 regulates the proliferation and invasiveness of NPC cells in vitro, and this may be correlated with its modulation of MMP-9 expression, cell cycle progression and apoptosis. WIP1 functioned as a potential therapeutic target in NPC management.

The DNA double-strand break response is abnormal in myeloblasts from patients with therapy-related acute myeloid leukemia

The complex chromosomal aberrations found in therapy-related acute myeloid leukemia (t-AML) suggest that the DNA double-strand break (DSB) response may be altered. In this study we examined the DNA DSB response of primary bone marrow cells from t-AML patients and performed next-generation sequencing of 37 canonical homologous recombination (HR) and non-homologous end-joining (NHEJ) DNA repair genes, and a subset of DNA damage response genes using tumor and paired normal DNA obtained from t-AML patients. Our results suggest that the majority of t-AML patients (11 of 15) have tumor-cell intrinsic, functional dysregulation of their DSB response. Distinct patterns of abnormal DNA damage response in myeloblasts correlated with acquired genetic alterations in TP53 and the presence of inferred chromothripsis. Furthermore, the presence of trisomy 8 in tumor cells was associated with persistently elevated levels of DSBs. Although tumor-acquired point mutations or small indels in canonical HR and NHEJ genes do not appear to be a dominant means by which t-AML leukemogenesis occurs, our functional studies suggest that an abnormal response to DNA damage is a common finding in t-AML.

Binding of a third metal ion by the human phosphatases PP2Cα and Wip1 is required for phosphatase activity

The PPM phosphatases require millimolar concentrations of Mg²⁺ or Mn²⁺ to activate phosphatase activity in vitro. The human phosphatases PP2Cα (PPM1A) and Wip1 (PPM1D) differ in their physiological function, substrate specificity, and apparent metal affinity. A crystallographic structure of PP2Cα shows only two metal ions in the active site. However, recent structural studies of several bacterial PP2C phosphatases have indicated three metal ions in the active site. Two residues that coordinate the third metal ion are highly conserved, suggesting that human PP2C phosphatases may also bind a third ion. Here, isothermal titration calorimetry analysis of Mg²⁺ binding to PP2Cα distinguished binding of two ions to high affinity sites from the binding of a third ion with a millimolar affinity, similar to the apparent metal affinity required for catalytic activity. Mutational analysis indicated that Asp239 and either Asp146 or Asp243 was required for low-affinity binding of Mg²⁺, but that both Asp146 and Asp239 were required for catalysis. Phosphatase activity assays in the presence of MgCl₂, MnCl₂, or mixtures of the two, demonstrate high phosphatase activity toward a phosphopeptide substrate when Mg²⁺ was bound to the low-affinity site, whether Mg²⁺ or Mn²⁺ ions were bound to the high affinity sites. Mutation of the corresponding putative third metal ion-coordinating residues of Wip1 affected catalytic activity similarly both in vitro and in human cells. These results suggest that phosphatase activity toward phosphopeptide substrates by PP2Cα and Wip1 requires the binding of a Mg²⁺ ion to the low-affinity site.

Wip1 suppresses apoptotic cell death through direct dephosphorylation of BAX in response to γ-radiation

Wild-type p53-induced phosphatase 1 (Wip1) is a p53-inducible serine/threonine phosphatase that switches off DNA damage checkpoint responses by the dephosphorylation of certain proteins (i.e. p38 mitogen-activated protein kinase, p53, checkpoint kinase 1, checkpoint kinase 2, and uracil DNA glycosylase) involved in DNA repair and the cell cycle checkpoint. Emerging data indicate that Wip1 is amplified or overexpressed in various human tumors, and its detection implies a poor prognosis. In this study, we show that Wip1 interacts with and dephosphorylates BAX to suppress BAX-mediated apoptosis in response to γ-irradiation in prostate cancer cells. Radiation-resistant LNCaP cells showed dramatic increases in Wip1 levels and impaired BAX movement to the mitochondria after γ-irradiation, and these effects were reverted by a Wip1 inhibitor. These results show that Wip1 directly interacts with and dephosphorylates BAX. Dephosphorylation occurs at threonines 172, 174 and 186, and BAX proteins with mutations at these sites fail to translocate efficiently to the mitochondria following cellular γ-irradiation. Overexpression of Wip1 and BAX, but not phosphatase-dead Wip1, in BAX-deficient cells strongly reduces apoptosis. Our results suggest that BAX dephosphorylation of Wip1 phosphatase is an important regulator of resistance to anticancer therapy. This study is the first to report the downregulation of BAX activity by a protein phosphatase.

p53 and the PWWP domain containing effector proteins in chromatin damage repair

In eukaryotic cells, DNA damage repair occurs on a template DNA that is organized with histones to form nucleosomes and chromatin structures. As such, chromatin plays an important role in DNA damage repair. In this review, we will use "chromatin damage repair" as a framework and highlight recent progress in understanding the role of chromatin, chromatin modifiers, chromatin binding effectors (e.g., the PWWP domain proteins), and the p53 tumor suppressor. We view chromatin as an active participant during DNA damage repair.

DNA damage signaling induced by the G-quadruplex ligand 12459 is modulated by PPM1D/WIP1 phosphatase

The triazine derivative 12459 is a potent G-quadruplex ligand that triggers apoptosis or delayed growth arrest, telomere shortening and G-overhang degradation, as a function of its concentration and time exposure to the cells. We have investigated here the DNA damage response induced by 12459 in A549 cells. Submicromolar concentrations of 12459 triggers a delayed Chk1-ATR–mediated DNA damage response associated with a telomeric dysfunction and a G2/M arrest. Surprisingly, increasing concentrations of 12459 leading to cell apoptosis induced a mechanism that bypasses the DNA damage signaling and leads to the dephosphorylation of Chk1 and γ-H2AX. We identified the phosphatase Protein Phosphatase Magnesium dependent 1D/Wild-type P53-Induced Phosphatase (PPM1D/WIP1) as a factor responsible for this dephosphorylation. SiRNA-mediated depletion of PPM1D/WIP1 reactivates the DNA damage signaling by 12459. In addition, PPM1D/WIP1 is activated by reactive oxygen species (ROS) induced by 12459. ROS generated by 12459 are sufficient to trigger an early DNA damage in A549 cells when PPM1D/WIP1 is depleted. However, ROS inactivation by N-acetyl cysteine (NAC) treatment does not change the apoptotic response induced by 12459. Because PPM1D expression was recently reported to modulate the recruitment of DNA repair molecules, our data would suggest a cycle of futile protection against 12459, thus leading to a delayed mechanism of cell death.

Molecular determinants of ovarian cancer chemoresistance: new insights into an old conundrum

Ovarian cancer is the most lethal gynecological malignancy. Cisplatin and its derivatives are first-line chemotherapeutics, and their resistance is a major hurdle in successful ovarian cancer treatment. Understanding the molecular dysregulation underlying chemoresistance is important for enhancing therapeutic outcome. Here, we review two established pathways in cancer chemoresistance. p53 is a major tumor suppressor regulating proliferation and apoptosis, and its mutation is a frequent event in human malignancies. The PI3K/Akt axis is a key oncogenic pathway regulating survival and tumorigenesis by controlling several tumor suppressors, including p53. The interplay between these pathways is well established, although the oncogenic phosphatase PPM1D adds a new layer to this intricate relationship and provides new insights into the processes determining cell fate. Inhibition of the PI3K/Akt pathway by functional food compounds as an adjunct to chemotherapeutics may tip the balance in favor of apoptosis rather than survival, enhancing therapeutic efficacy, and reducing side effects.

Persistent DNA damage after high dose in vivo gamma exposure of minipig skin

Background

Exposure to high doses of ionizing radiation (IR) can lead to localized radiation injury of the skin and exposed cells suffer dsDNA breaks that may elicit cell death or stochastic changes. Little is known about the DNA damage response after high-dose exposure of the skin. Here, we investigate the cellular and DNA damage response in acutely irradiated minipig skin.

Methods and Findings

IR-induced DNA damage, repair and cellular survival were studied in 15 cm² of minipig skin exposed in vivo to ∼50 Co-60 γ rays. Skin biopsies of control and 4 h up to 96 days post exposure were investigated for radiation-induced foci (RIF) formation using γ-H2AX, 53BP1, and active ATM-p immunofluorescence. High-dose IR induced massive γ-H2AX phosphorylation and high 53BP1 RIF numbers 4 h, 20 h after IR. As time progressed RIF numbers dropped to a low of <1% of keratinocytes at 28–70 days. The latter contained large RIFs that included ATM-p, indicating the accumulation of complex DNA damage. At 96 days most of the cells with RIFs had disappeared. The frequency of active-caspase-3-positive apoptotic cells was 17-fold increased 3 days after IR and remained >3-fold elevated at all subsequent time points. Replicating basal cells (Ki67+) were reduced 3 days post IR followed by increased proliferation and recovery of epidermal cellularity after 28 days.

Conclusions

Acute high dose irradiation of minipig epidermis impaired stem cell replication and induced elevated apoptosis from 3 days onward. DNA repair cleared the high numbers of DBSs in skin cells, while RIFs that persisted in <1% cells marked complex and potentially lethal DNA damage up to several weeks after exposure. An elevated frequency of keratinocytes with persistent RIFs may thus serve as indicator of previous acute radiation exposure, which may be useful in the follow up of nuclear or radiological accident scenarios.

Wip1 sensitizes p53-negative tumors to apoptosis by regulating the Bax/Bcl-xL ratio

Wip1 is a stress-response phosphatase that negatively regulates several tumor suppressors, including p53. In a sizeable fraction of tumors, overexpression or amplification of Wip1 compromises p53 functions; inhibition of Wip1 activity is an attractive strategy for improving treatment of these tumors. However, over half of human tumors contain mutations in the p53 gene or have lost both alleles. Recently, we observed that in cancer cells lacking wild type p53, reduction of Wip1 expression was ineffective, whereas, surprisingly, overexpression of Wip1 increased anticancer drug sensitivity. The increased sensitivity resulted from activation of the intrinsic pathway of apoptosis through increased levels of the pro-apoptotic protein Bax and decreased levels of the anti-apoptotic protein Bcl-xL. We showed that interaction of Wip1 and the transcription factor RUNX2, specifically through dephosphorylation of RUNX2 phospho-S432, resulted in increased expression of Bax. Interestingly, overexpression of Wip1 increased drug sensitivity only in the p53-negative tumor cells while protecting the wild type p53-containing normal cells from drug-induced collateral injury. Here, we provide evidence that Wip1 overexpression decreases expression of Bcl-xL through negative regulation of NFκB activity. Thus, Wip1 overexpression increases the sensitivity of p53-negative cancer cells to anticancer drugs by separately affecting Bax and Bcl-xL protein levels.

Discordance between phosphorylation and recruitment of 53BP1 in response to DNA double-strand breaks

During the DNA damage response (DDR), chromatin modifications contribute to localization of 53BP1 to sites of DNA double-strand breaks (DSBs). 53BP1 is phosphorylated during the DDR, but it is unclear whether phosphorylation is directly coupled to chromatin binding. In this study, we used human diploid fibroblasts and HCT116 tumor cells to study 53BP1 phosphorylation at Serine-25 and Serine-1778 during endogenous and exogenous DSBs (DNA replication and whole-cell or sub-nuclear microbeam irradiation, respectively). In non-stressed conditions, endogenous DSBs in S-phase cells led to accumulation of 53BP1 and γH2AX into discrete nuclear foci. Only the frank collapse of DNA replication forks following hydroxyurea treatment initiated 53BP1(Ser25) and 53BP1(Ser1778) phosphorylation. In response to exogenous DSBs, 53BP1(Ser25) and 53BP1(Ser1778) phosphoforms localized to sites of initial DSBs in a cell cycle-independent manner. 53BP1 phosphoforms also localized to late residual foci and associated with PML-NBs during IR-induced senescence. Using isogenic cell lines and small-molecule inhibitors, we observed that DDR-induced 53BP1 phosphorylation was dependent on ATM and DNA-PKcs kinase activity but independent of MRE11 sensing or RNF168 chromatin remodeling. However, loss of RNF168 blocked recruitment of phosphorylated 53BP1 to sites of DNA damage. Our results uncouple 53BP1 phosphorylation from DSB localization and support parallel pathways for 53BP1 biology during the DDR. As relative 53BP1 expression may be a biomarker of DNA repair capacity in solid tumors, the tracking of 53BP1 phosphoforms in situ may give unique information regarding different cancer phenotypes or response to cancer treatment.

p53-Independent expression of wild-type p53-induced phosphatase 1 (Wip1) in methylmethane sulfonate-treated cancer cell lines and human tumors

Wild-type p53-induced phosphatase 1 (Wip1, PPM1D) is induced by p53 in response to various stressors and dephosphorylates cellular target proteins involved in DNA repair and cell cycle checkpoint pathways. The Wip1 gene is frequently amplified or overexpressed in human cancers, promoting tumor growth by switching off major checkpoint kinases and p53. To explore wild-type p53-independent Wip1 induction, Wip1 promoter activity and its transcript level were evaluated by luciferase assay and real-time PCR, after methylmethane sulfonate (MMS) treatment in breast cancer cell lines and p53-null cell lines. Wip1 promoter activities in response to UV irradiation and various anti-cancer agents were compared between wild-type and a p53-response element (p53RE) mutated construct. Wip1 expression and its effects were examined in primary non-small cell lung cancer (NSCLC) and colon tumor cells by using Wip1-specific siRNA. MMS induced Wip1 promoter activity in Hs578T, MDA-MB-231, and SK-BR-3 cells expressing DNA binding-deficient p53 mutants. A549-E6 and HCT116 (p53(-/-)) cells retained substantial Wip1 induction. Wip1 promoter activity was reduced, but not eliminated, in cells expressing a promoter containing a mutated p53-response element. Wip1 induction was not blocked by SB202190 or SP600125. MMS increased Wip1 expression in primary non-small cell lung cancer cells expressing a p53 R175H mutant. Our data indicate that Wip1 is induced in the absence of functional p53, like p38 MAPK and JNK, as a stress response terminator.

Downregulation of Wip1 phosphatase modulates the cellular threshold of DNA damage signaling in mitosis

Cells are constantly challenged by DNA damage and protect their genome integrity by activation of an evolutionary conserved DNA damage response pathway (DDR). A central core of DDR is composed of a spatiotemporally ordered net of post-translational modifications, among which protein phosphorylation plays a major role. Activation of checkpoint kinases ATM/ATR and Chk1/2 leads to a temporal arrest in cell cycle progression (checkpoint) and allows time for DNA repair. Following DNA repair, cells re-enter the cell cycle by checkpoint recovery. Wip1 phosphatase (also called PPM1D) dephosphorylates multiple proteins involved in DDR and is essential for timely termination of the DDR. Here we have investigated how Wip1 is regulated in the context of the cell cycle. We found that Wip1 activity is downregulated by several mechanisms during mitosis. Wip1 protein abundance increases from G(1) phase to G(2) and declines in mitosis. Decreased abundance of Wip1 during mitosis is caused by proteasomal degradation. In addition, Wip1 is phosphorylated at multiple residues during mitosis, and this leads to inhibition of its enzymatic activity. Importantly, ectopic expression of Wip1 reduced γH2AX staining in mitotic cells and decreased the number of 53BP1 nuclear bodies in G(1) cells. We propose that the combined decrease and inhibition of Wip1 in mitosis decreases the threshold necessary for DDR activation and enables cells to react adequately even to modest levels of DNA damage encountered during unperturbed mitotic progression.

Wip1-dependent signaling pathways in health and diseases

Spatial and temporal regulation of protein phosphorylation is key to the control of different molecular networks. This regulation is achieved in part through dephosphorylation of numerous signaling molecules, and emerging evidence highlights the importance of a new member of the PP2C family of phosphatase, Wild-type p53 induced phosphatase 1 (Wip1), in regulating stress-induced and DNA damage-induced networks. In recent years, analysis of Wip1 has focused primarily on its role in tumorigenesis because of its overexpression in human tumors and a profound tumor-resistant phenotype of Wip1-deficient mice. Recently, Wip1 has also been shown to play an important role in several physiological processes including adult neurogenesis and organismal aging. This review addresses how Wip1 phosphatase regulates different signaling networks in a spatial and temporal manner and how these differences contribute to various biological outcomes in the context of physiological and pathological conditions.

Regulation of the Wip1 phosphatase and its effects on the stress response

Wip1 (PPM1D) is a stress responsive PP2C phosphatase that plays a key role in stress signaling. Although originally identified as a gene induced by p53 after genotoxic stress, we now know that Wip1 expression is additionally regulated by other mechanisms. Wip1 is not only a target of p53, but is also a target of other transcription factors, including Estrogen Receptor-alpha and NF-kappaB. Additionally, Wip1 expression is regulated by post-transcriptional mechanisms such as mRNA stabilization and alternative splicing. Upon induction, Wip1 dampens the stress response by dephosphorylating and inactivating proteins such as p53, p38 MAPK, and ATM, usually as part of a negative feedback loop. As a result, Wip1 functions to abrogate cell cycle checkpoints and inhibit senescence, apoptosis, DNA repair, and the production of inflammatory cytokines. Furthermore, Wip1 is overexpressed in several types of human cancers and has oncogenic functions. The regulation of Wip1, the role of Wip1 in stress signaling, and the cooperation of Wip1 with oncogenes in promoting tumorigenesis will be discussed in this review.

Differential epithelium DNA damage response to ATM and DNA-PK pathway inhibition in human prostate tissue culture

The ability of cells to respond and repair DNA damage is fundamental for the maintenance of genomic integrity. Ex vivo culturing of surgery-derived human tissues has provided a significant advancement to assess DNA damage response (DDR) in the context of normal cytoarchitecture in a non-proliferating tissue. Here, we assess the dependency of prostate epithelium DDR on ATM and DNA-PKcs, the major kinases responsible for damage detection and repair by nonhomologous end-joining (NHEJ), respectively. DNA damage was caused by ionizing radiation (IR) and cytotoxic drugs, cultured tissues were treated with ATM and DNA-PK inhibitors, and DDR was assessed by phosphorylation of ATM and its targets H2AX and KAP1, a heterochromatin binding protein. Phosphorylation of H2AX and KAP1 was fast, transient and fully dependent on ATM, but these responses were moderate in luminal cells. In contrast, DNA-PKcs was phosphorylated in both luminal and basal cells, suggesting that DNA-PK-dependent repair was also activated in the luminal cells despite the diminished H2AX and KAP1 responses. These results indicate that prostate epithelial cell types have constitutively dissimilar responses to DNA damage. We correlate the altered damage response to the differential chromatin state of the cells. These findings are relevant in understanding how the epithelium senses and responds to DNA damage.