ATM-dependent dissociation of B55 regulatory subunit from nuclear PP2A in response to ionizing radiation

Protein Phosphatase 2ACα Regulates ATR-Mediated Endogenous DNA Damage Response Against Microcephaly

Protein phosphatase 2A (PP2A) is an abundant heterotrimeric holoenzyme in eukaryotic cells coordinating with specific kinases to regulate spatial-temporal protein dephosphorylation in various biological processes. However, the function of PP2A in cortical neurogenesis remains largely unknown. Here, we report that neuronal-specific deletion of Pp2acα in mice displayed microcephaly, with significantly smaller brains and defective learning and memory ability. Mechanistically, neuronal Pp2acα deficiency resulted in elevated endogenous DNA damage and activation of ATR/CHK1 signaling. It was further induced by the loss of direct interaction between PP2AC and ATR as well as the function of PP2AC to dephosphorylate ATR. Importantly, ATR/CHK1 signaling dysregulation altered both the expression and activity of several critical downstream factors including P53, P21, Bcl2, and Bax, which led to decreased proliferation of cortical progenitor cells and increased apoptosis in developing cortical neurons. Taken together, our results indicate an essential function of PP2ACα in endogenous DNA damage response-mediated ATR signaling during neurogenesis, and defective PP2ACα in neurons contributes to microcephaly.

PME-1 sensitizes glioblastoma cells to oxidative stress-induced cell death by attenuating PP2A-B55α-mediated inactivation of MAPKAPK2-RIPK1 signaling

Glioblastoma (GBM) is the most common primary brain tumor in adults. Current standard therapy is surgery followed by radiotherapy, with concurrent and adjuvant temozolomide chemotherapy. GBM is characterized by almost uniformly fatal outcomes, highlighting the unmet clinical need for more efficient, biomarker-guided treatments. Protein phosphatase methylesterase-1 (PME-1), a regulator of the tumor suppressive phosphatase PP2A, promotes PP2A demethylation and inactivation, and is overexpressed in 44% of GBM, associated with increased tumor grade and cellular proliferation. Here, we aimed to investigate how reactive oxygen species (ROS), a frequent by-product of radiotherapy and temozolomide chemotherapy, regulate PP2A function via its methylesterase PME-1, and how PME-1 overexpression impacts the response of GBM cells to oxidative stress. We found that in two glioblastoma cell lines, U87MG and U251MG, expression of PME-1 is positively correlated with the sensitivity of the cells to H₂O₂ or t-BHP-induced oxidative stress. Experiments using the irreversible pharmacologic PME-1 inhibitor, AMZ30, and different PME-1 mutants, revealed that the methylesterase function, the PP2A binding capacity, and the nuclear localization of PME-1 are all important for the sensitizing effect of PME-1 expression. Furthermore, we identified increased nuclear localization of the PP2A-B55α subunit, increased binding of PP2A-B55α to PME-1, and increased B55α-bound PP2A-C demethylation upon oxidative stress. Lastly, we uncovered increased stress-induced phosphorylation and activity of MAPKAPK2 and RIPK1 in PME-1 overexpressing U87MG cells, which caused the observed sensitization to t-BHP treatment. Our data reveal a novel role for PME-1 in oxidative stress-induced GBM cell death, regulating nuclear PP2A-B55α activity and MAPKAPK2-RIPK1 signaling. Patients with GBM tumors overexpressing PME-1, although having a worse prognosis due to increased cellular proliferation of the tumor, could actually be more responsive to oxidative stress-inducing therapies.

Protein phosphatase 2A has an essential role in promoting thymocyte survival during selection

The development of thymocytes to mature T cells in the thymus is tightly controlled by cellular selection, in which only a small fraction of thymocytes equipped with proper quality of TCRs progress to maturation. It is pivotal to protect the survival of the few T cells, which pass the selection. However, the signaling events, which safeguard the cell survival in thymus, are not totally understood. In this study, protein Ser/Thr phosphorylation in thymocytes undergoing positive selection is profiled by mass spectrometry. The results revealed large numbers of dephosphorylation changes upon T cell receptor (TCR) activation during positive selection. Subsequent substrate analysis pinpointed protein phosphatase 2A (PP2A) as the enzyme responsible for the dephosphorylation changes in developing thymocytes. PP2A catalytic subunit α (Ppp2ca) deletion in the T cell lineage in Ppp2ca ^flox/flox-Lck-Cre mice (PP2A cKO) displayed dysregulated dephosphorylation of apoptosis-related proteins in double-positive (DP) cells and caused substantially decreased numbers of DP CD4⁺ CD8⁺ cells. Increased levels of apoptosis in PP2A cKO DP cells were found to underlie aberrant thymocyte development. Finally, the defective thymocyte development in PP2A cKO mice could be rescued by either Bcl2 transgene expression or by p53 knockout. In summary, our work reveals an essential role of PP2A in promoting thymocyte development through the regulation of cell survival.

Chlamydia trachomatis Inhibits Homologous Recombination Repair of DNA Breaks by Interfering with PP2A Signaling

Cervical and ovarian cancers exhibit characteristic mutational signatures that are reminiscent of mutational processes, including defective homologous recombination (HR) repair. How these mutational processes are initiated during carcinogenesis is largely unclear. Chlamydia trachomatis infections are epidemiologically associated with cervical and ovarian cancers. Previously, we showed that C. trachomatis induces DNA double-strand breaks (DSBs) but suppresses Ataxia-telangiectasia mutated (ATM) activation and cell cycle checkpoints. The mechanisms by which ATM regulation is modulated and its consequences for the repair pathway in C. trachomatis-infected cells remain unknown. Here, we found that Chlamydia bacteria interfere with the usual response of PP2A to DSBs. As a result, PP2A activity remains high, as the level of inhibitory phosphorylation at Y307 remains unchanged following C. trachomatis-induced DSBs. Protein-protein interaction analysis revealed that C. trachomatis facilitates persistent interactions of PP2A with ATM, thus suppressing ATM activation. This correlated with a remarkable lack of homologous recombination (HR) repair in C. trachomatis-infected cells. Chemical inhibition of PP2A activity in infected cells released ATM from PP2A, resulting in ATM phosphorylation. Activated ATM was then recruited to DSBs and initiated downstream signaling, including phosphorylation of MRE11 and NBS1 and checkpoint kinase 2 (Chk2)-mediated activation of the G2/M cell cycle checkpoint in C. trachomatis-infected cells. Further, PP2A inhibition led to the restoration of C. trachomatis-suppressed HR DNA repair function. Taking the data together, this study revealed that C. trachomatis modulates PP2A signaling to suppress ATM activation to prevent cell cycle arrest, thus contributing to a deficient high-fidelity HR pathway and a conducive environment for mutagenesis.IMPORTANCEChlamydia trachomatis induces DNA double-strand breaks in host cells but simultaneously inhibits proper DNA damage response and repair mechanisms. This may render host cells prone to loss of genetic integrity and transformation. Here we show that C. trachomatis prevents activation of the key DNA damage response mediator ATM by preventing the release from PP2A, leading to a complete absence of homologous recombination repair in host cells.

Disruption and inactivation of the PP2A complex promotes the proliferation and angiogenesis of hemangioma endothelial cells through activating AKT and ERK

Hemangioma is a benign vascular neoplasm of unknown etiology. In this study, we generated an endothelial-specific PyMT gene-expressing transgenic mouse model that spontaneously develops hemangioma. Based on this transgenic model, a specific binding between PyMT and the core AC dimer of protein phosphatase 2A (PP2A) was verified in hemangioma vascular endothelial cells. The binding between PyMT and the PP2A AC dimer resulted in dissociation of the B subunit from the PP2A complex and inactivation of PP2A phosphatases, which in turn activated AKT and ERK signaling and promoted cell proliferation, migration and angiogenesis in vitro and tumorigenesis in vivo. Consistent with the in vitro findings, decreased PP2A phosphatase activity and disruption of the PP2A heterotrimeric complex were also observed in both primary transgene-positive TG(+) mouse hemangioma endothelial cells (TG(+) HEC cells) and human proliferating phase hemangioma endothelial (human HEC-P) cells, but not in transgene-negative TG(-) mouse normal vascular endothelial cells (TG(-) NEC cells) and human involuting phase hemangioma endothelial (human HEC-I) cells. Further, it was observed that in human hemangioma cells, endoglin could compete with the PP2A/A, C subunits for binding to the PP2A/B subunit, thereby resulting in dissociation of the B subunit from the PP2A complex. Treatment of Tie2/PyMT transgenic mice with the PP2A activator FTY720 significantly delayed the occurrence of hemangioma. Our data provide evidence of a previously unreported anti-proliferation and anti-angiogenesis effect of PP2A in vascular endothelial cells, and show the therapeutic value of PP2A activators in hemangioma.

Changes in Carboxy Methylation and Tyrosine Phosphorylation of Protein Phosphatase PP2A Are Associated with Epididymal Sperm Maturation and Motility

Mammalian sperm contain the serine/threonine phosphatases PP1γ2 and PP2A. The role of sperm PP1γ2 is relatively well studied. Here we confirm the presence of PP2A in sperm and show that it undergoes marked changes in methylation (leucine 309), tyrosine phosphorylation (tyrosine 307) and catalytic activity during epididymal sperm maturation. Spermatozoa isolated from proximal caput, distal caput and caudal regions of the epididymis contain equal immuno-reactive amounts of PP2A. Using demethyl sensitive antibodies we show that PP2A is methylated at its carboxy terminus in sperm from the distal caput and caudal regions but not in sperm from the proximal caput region of the epididymis. The methylation status of PP2A was confirmed by isolation of PP2A with microcystin agarose followed by alkali treatment, which causes hydrolysis of protein carboxy methyl esters. Tyrosine phosphorylation of sperm PP2A varied inversely with methylation. That is, PP2A was tyrosine phosphorylated when it was demethylated but not when methylated. PP2A demethylation and its reciprocal tyrosine phosphorylation were also affected by treatment of sperm with L-homocysteine and adenosine, which are known to elevate intracellular S-adenosylhomocysteine, a feedback inhibitor of methyltransferases. Catalytic activity of PP2A declined during epididymal sperm maturation. Inhibition of PP2A by okadaic acid or by incubation of caudal epididymal spermatozoa with L-homocysteine and adenosine resulted in increase of sperm motility parameters including percent motility, velocity, and lateral head amplitude. Demethylation or pharmacological inhibition of PP2A also leads to an increase in phosphorylation of glycogen synthase kinase-3 (GSK3). Our results show for the first time that changes in PP2A activity due to methylation and tyrosine phosphorylation occur in sperm and that these changes may play an important role in the regulation of sperm function.

PKR inhibits the DNA damage response, and is associated with poor survival in AML and accelerated leukemia in NHD13 mice

Increased expression of the interferon-inducible double-stranded RNA-activated protein kinase (PKR) has been reported in acute leukemia and solid tumors, but the role of PKR has been unclear. Now, our results indicate that high PKR expression in CD34(+) cells of acute myeloid leukemia (AML) patients correlates with worse survival and shortened remission duration. Significantly, we find that PKR has a novel and previously unrecognized nuclear function to inhibit DNA damage response signaling and double-strand break repair. Nuclear PKR antagonizes ataxia-telangiectasia mutated (ATM) activation by a mechanism dependent on protein phosphatase 2A activity. Thus, inhibition of PKR expression or activity promotes ATM activation, γ-H2AX formation, and phosphorylation of NBS1 following ionizing irradiation. PKR transgenic but not PKR null mice demonstrate a mutator phenotype characterized by radiation-induced and age-associated genomic instability that was partially reversed by short-term pharmacologic PKR inhibition. Furthermore, the age-associated accumulation of somatic mutations that occurs in the Nup98-HOXD13 (NHD13) mouse model of leukemia progression was significantly elevated by co-expression of a PKR transgene, whereas knockout of PKR expression or pharmacologic inhibition of PKR activity reduced the frequency of spontaneous mutations in vivo. Thus, PKR cooperated with the NHD13 transgene to accelerate leukemia progression and shorten survival. Taken together, these results indicate that increased nuclear PKR has an oncogenic function that promotes the accumulation of potentially deleterious mutations. Thus, PKR inhibition may be a therapeutically useful strategy to prevent leukemia progression or relapse, and improve clinical outcomes.

DNA damage signaling guards against perturbation of cyclin D1 expression triggered by low-dose long-term fractionated radiation

Cyclin D1 expression is precisely controlled during cell-cycle progression. However, repeated exposure to low-dose fractionated radiation (FR) abrogates cell cycle-dependent cyclin D1 degradation by constitutive activation of AKT survival signaling in normal human fibroblasts. The resulting abnormal nuclear cyclin D1 accumulation induces defects in DNA replication and resulting DNA double-strand breaks, and is associated with induction of genomic instability in low-dose irradiated cells. Here, we investigated the role of DNA damage signaling against such perturbed cell-cycle control of cyclin D1 expression. Nuclear cyclin D1 accumulation was induced within 7 days after low-dose FR (0.01 Gy or 0.05 Gy per fraction) in ATM-deficient cells (AT5BIVA), but appeared later in AT5BIVA cells harboring human ATM cDNA. Thus, ATM prevents abnormal nuclear cyclin D1 accumulation at early time points after low-dose FR. We further demonstrated that ATM-mediated downregulation of protein phosphatase 2A activity caused activation of the AKT/cyclin D1 pathway after long-term FR. Perturbation of cyclin D1 expression induced Rad51 foci that indicate homologous recombination repair (HRR) in control cells, while ATM- and NBS1-deficient cells (GM7166) failed to induce Rad51 foci after long-term low-dose FR. After 21 days of FR, NBS1- and ATM-deficient cells showed a decrease in nuclear cyclin D1-positive cells, and an increase in apoptotic cells. Similarly, inhibition of ATM with KU55933 abrogated nuclear cyclin D1 accumulation by induction of apoptosis in ATM-complemented cells exposed to low-dose FR. In conclusion, we here demonstrate that ATM is involved in controlling cyclin D1 levels after low-dose FR. DNA damage signaling mitigates the harmful effects of low-dose long-term FR by suppression of cell death induced by perturbation of cyclin D1 expression.

Greatwall-phosphorylated Endosulfine is both an inhibitor and a substrate of PP2A-B55 heterotrimers

During M phase, Endosulfine (Endos) family proteins are phosphorylated by Greatwall kinase (Gwl), and the resultant pEndos inhibits the phosphatase PP2A-B55, which would otherwise prematurely reverse many CDK-driven phosphorylations. We show here that PP2A-B55 is the enzyme responsible for dephosphorylating pEndos during M phase exit. The kinetic parameters for PP2A-B55's action on pEndos are orders of magnitude lower than those for CDK-phosphorylated substrates, suggesting a simple model for PP2A-B55 regulation that we call inhibition by unfair competition. As the name suggests, during M phase PP2A-B55's attention is diverted to pEndos, which binds much more avidly and is dephosphorylated more slowly than other substrates. When Gwl is inactivated during the M phase-to-interphase transition, the dynamic balance changes: pEndos dephosphorylated by PP2A-B55 cannot be replaced, so the phosphatase can refocus its attention on CDK-phosphorylated substrates. This mechanism explains simultaneously how PP2A-B55 and Gwl together regulate pEndos, and how pEndos controls PP2A-B55. DOI: http://dx.doi.org/10.7554/eLife.01695.001.

Over expression of PPP2R2C inhibits human glioma cells growth through the suppression of mTOR pathway

PPP2R2C encodes a gamma isoform of the subunit B55 subfamily, which is a regulatory subunit of Protein phosphatase type 2A (PP2A). Our study shows that PPP2R2C is downregulated in glioma cells and human brain cancer patient samples. Overexpression of PPP2R2C inhibited cancer cell proliferation both in vitro and in vivo through the suppression of the activity of S6K in the mTOR pathway. Moreover, exogenous expression of PPP2R2C promoted the formation of a complex with the PP2A-C subunit to further enhance the binding of PP2A-C with S6K. Our results suggest that PPP2R2C is a potential tumor suppressor gene in human brain cancers. This study will provide novel insight into the development of therapeutic strategies in the treatment of human brain tumors.

PPP2R2C loss promotes castration-resistance and is associated with increased prostate cancer-specific mortality

Metastatic prostate cancers generally rely on androgen receptor (AR) signaling for growth and survival, even following systemic androgen-deprivation therapy (ADT). However, recent evidence suggests that some advanced prostate cancers escape ADT by using signaling programs and growth factors that bypass canonical AR ligand-mediated mechanisms. We used an in vitro high-throughput RNA interference (RNAi) screen to identify pathways in androgen-dependent prostate cancer cell lines whose loss-of-function promotes androgen ligand-independent growth. We identified 40 genes where knockdown promoted proliferation of both LNCaP and VCaP prostate cancer cells in the absence of androgen. Of these, 14 were downregulated in primary and metastatic prostate cancer, including two subunits of the protein phosphatase 2 (PP2A) holoenzyme complex: PPP2R1A, a structural subunit with known tumor-suppressor properties in several tumor types; and PPP2R2C, a PP2A substrate-binding regulatory subunit that has not been previously identified as a tumor suppressor. We show that loss of PPP2R2C promotes androgen ligand depletion-resistant prostate cancer growth without altering AR expression or canonical AR-regulated gene expression. Furthermore, cell proliferation induced by PPP2R2C loss was not inhibited by the AR antagonist MDV3100, indicating that PPP2R2C loss may promote growth independently of known AR-mediated transcriptional programs. Immunohistochemical analysis of PPP2R2C protein levels in primary prostate tumors determined that low PPP2R2C expression significantly associated with an increased likelihood of cancer recurrence and cancer-specific mortality. These findings provide insights into mechanisms by which prostate cancers resist AR-pathway suppression and support inhibiting PPP2R2C complexes or the growth pathway(s) activated by PPP2R2C as a therapeutic strategy.

Response to DNA damage: why do we need to focus on protein phosphatases?

Eukaryotic cells are continuously threatened by unavoidable errors during normal DNA replication or various sources of genotoxic stresses that cause DNA damage or stalled replication. To maintain genomic integrity, cells have developed a coordinated signaling network, known as the DNA damage response (DDR). Following DNA damage, sensor molecules detect the presence of DNA damage and transmit signals to downstream transducer molecules. This in turn conveys the signals to numerous effectors, which initiate a large number of specific biological responses, including transient cell cycle arrest mediated by checkpoints, DNA repair, and apoptosis. It is recently becoming clear that dephosphorylation events are involved in keeping DDR factors inactive during normal cell growth. Moreover, dephosphorylation is required to shut off checkpoint arrest following DNA damage and has been implicated in the activation of the DDR. Spatial and temporal regulation of phosphorylation events is essential for the DDR, and fine-tuning of phosphorylation is partly mediated by protein phosphatases. While the role of kinases in the DDR has been well documented, the complex roles of protein dephosphorylation have only recently begun to be investigated. Therefore, it is important to focus on the role of phosphatases and to determine how their activity is regulated upon DNA damage. In this work, we summarize current knowledge on the involvement of serine/threonine phosphatases, especially the protein phosphatase 1, protein phosphatase 2A, and protein phosphatase Mg²⁺/Mn²⁺-dependent families, in the DDR.

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.

Dynamics of p53 and NF-κB regulation in response to DNA damage and identification of target proteins suitable for therapeutic intervention

Background

The genome is continuously attacked by a variety of agents that cause DNA damage. Recognition of DNA lesions activates the cellular DNA damage response (DDR), which comprises a network of signal transduction pathways to maintain genome integrity. In response to severe DNA damage, cells undergo apoptosis to avoid transformation into tumour cells, or alternatively, the cells enter permanent cell cycle arrest, called senescence. Most tumour cells have defects in pathways leading to DNA repair or apoptosis. In addition, apoptosis could be counteracted by nuclear factor kappa B (NF-κB), the main anti-apoptotic transcription factor in the DDR. Despite the high clinical relevance, the interplay of the DDR pathways is poorly understood. For therapeutic purposes DNA damage signalling processes are induced to induce apoptosis in tumour cells. However, the efficiency of radio- and chemotherapy is strongly hampered by cell survival pathways in tumour cells. In this study logical modelling was performed to facilitate understanding of the complexity of the signal transduction networks in the DDR and to provide cancer treatment options.

Results

Our comprehensive discrete logical model provided new insights into the dynamics of the DDR in human epithelial tumours. We identified new mechanisms by which the cell regulates the dynamics of the activation of the tumour suppressor p53 and NF-κB. Simulating therapeutic intervention by agents causing DNA single-strand breaks (SSBs) or DNA double-strand breaks (DSBs) we identified candidate target proteins for sensitization of carcinomas to therapeutic intervention. Further, we enlightened the DDR in different genetic diseases, and by failure mode analysis we defined molecular defects putatively contributing to carcinogenesis.

Conclusion

By logic modelling we identified candidate target proteins that could be suitable for radio- and chemotherapy, and contributes to the design of more effective therapies.

PIM1-activated PRAS40 regulates radioresistance in non-small cell lung cancer cells through interplay with FOXO3a, 14-3-3 and protein phosphatases

Resistance of cancer cells to ionizing radiation plays an important role in the clinical setting of lung cancer treatment. To date, however, the exact molecular mechanism of radiosensitivity has not been well explained. In this study, we compared radioresistance in two types of non-small cell lung cancer (NSCLC) cells, NCI-H460 and A549, and investigated the signaling pathways that confer radioresistance. In radioresistant cells, exposure to radiation led to overexpression of PIM1 and reduction of protein phosphatases (PP2A and PP5), which induced translocation of PIM1 into the nucleus. Increased nuclear PIM1 phosphorylated PRAS40. Consequently, pPRAS40 made a trimeric complex with 14-3-3 and AKT-activated pFOXO3a, which then moved rapidly to the cytoplasm. Cytoplasmic retention of FOXO3a was associated with downregulation of proapoptotic genes and possibly radioresistance. On the other hand, no suppressive effect of radiation on protein phosphatases was detected and, concomitantly, protein phosphatases downregulated PIM1 in radiosensitive cells. In this setting, PIM1-activated pPRAS40, AKT-activated pFOXO3a, and their complex formation with 14-3-3 could be key regulators of the radiation-induced radioresistance in NSCLC cells.

Phosphatases in the cellular response to DNA damage

In the last fifteen years, rapid progress has been made in delineating the cellular response to DNA damage. The DNA damage response network is composed of a large number of proteins with different functions that detect and signal the presence of DNA damage in order to coordinate DNA repair with a variety of cellular processes, notably cell cycle progression. This signal, which radiates from the chromatin template, is driven primarily by phosphorylation events, mainly on serine and threonine residues. While we have accumulated detailed information about kinases and their substrates our understanding of the role of phosphatases in the DNA damage response is still preliminary. Identifying the phosphatases and their regulation will be instrumental to obtain a complete picture of the dynamics of the DNA damage response. Here we give an overview of the DNA damage response in mammalian cells and then review the data on the role of different phosphatases and discuss their biological relevance.

Ataxia telangiectasia mutated nuclear localization in head and neck cancer cells is PPP2R2B-dependent

Background: Protein phosphatase 2A (PP2A) has been implicated in radiation-induced activation of cellular responses, likely by its ability to regulate the autophosphorylation of the ataxia telangiectasia mutated (ATM) protein, a key molecule involved in the DNA damage response initiated by double-stranded DNA breaks. Interestingly, a hereditary defect in the PPP2R2B gene, which encodes the beta isoform of PP2A regulatory subunit B, causes autosomal dominant spinocerebellar ataxia 12, a clinical condition resembling that of ataxia telangiectasia patients. Moreover, PPP2R2B is significantly down-regulated in many human cancers, including head and neck squamous cell carcinomas (HNSCCs). Objective: Examine whether PPP2R2B regulates ATM function, thereby contributing to tumor progression due to the resulting defective DNA repair. Methods: The roles of PPP2R2B were evaluated in irradiated HNSCC cell lines, siRNAPPP2R2B cells and okadaic acid treated cells. Expression of PPP2R2B was measured by microarray, Western blot analysis and real time quantitative rtPCR. ATM quantity and localization, ATM phosphorylation and γ-H2AX were determined by Western blot analysis and/or immunofluorescence assay. Clonogenic cell survival assay was performed to determine ionizing radiation sensitivity. Results: PPP2R2B expression is reduced in multiple tumor types, including HNSCCs. Indeed, HNSCC cell lines that have lower PPP2R2B mRNA expression and siRNAPPP2R2B cells lower basal and radiation-induced levels of phosphorylated ATM and the consequent reduction in the levels of phosphorylation of the downstream ATM target, γ-H2AX. Depletion of PPP2R2B and inhibition of PP2A with okadaic acid resulted in limited ATM nuclear localization. Finally, siRNAPPP2R2B cells displayed enhanced sensitivity to death after radiation. Conclusion: In HNSCCs, ATM nuclear localization is PPP2R2B dependent, and decreased PPP2R2B expression may result in limited ATM activation by preventing its nuclear accumulation and ATM-chromatin interaction. Therefore, decreased PPP2R2B expression in HNSCCs may contribute to genomic instability, cancer development and radiation sensitivity by limiting ATM functions.

In vivo genotoxic potential of microcystin-LR: a cyanobacterial toxin, investigated both by the unscheduled DNA synthesis (UDS) and the comet assays after intravenous administration

Microcystin-LR (MC-LR) is a toxin produced by freshwater cyanobacteria and is a potential threat to human health. MC-LR has been shown to be both a specific inhibitor of serine/threonine protein phosphatases PP1 and PP2A and a potent tumor promoter in rat liver. However, the genotoxic potential of MCs remains unclear. In this article, we investigated the ability of MC-LR to induce DNA damage on rat hepatocytes following intravenous (iv) administration by using two in vivo genotoxicity assays: the unscheduled DNA synthesis (UDS) and the comet assays. The UDS assay measures DNA synthesis induced from the excision repair of DNA damaged regions and the comet assay is a very sensitive technique for detecting various forms of DNA damage. After an exposure time of 2-4 h or 12-16 h and a dose ranging from 12.5 to 50 microg/kg bw, no DNA damage could be observed in both assays on rat hepatocytes following iv administration. These findings have been discussed and compared with recently published genotoxic results obtained in other organs from mice after oral and intraperitoneal treatments to better understand the mechanism of action of this toxin in relation with its cancerogenicity potential.

PP2A regulates ionizing radiation-induced apoptosis through Ser46 phosphorylation of p53

In response to ionizing radiation, p53 plays a critical role in regulating DNA repair and apoptosis. Among multiple phosphorylation sites, evidence suggests that Ser46 promotes apoptotic cell death through mitochondrial outer membrane permeabilization (MOMP) and subsequent activation of the caspase 7-PARP pathway. Therefore, we investigated which phosphatase regulates Ser46 after ionizing radiation, reasoning that the responsible phosphatase should be a target for radiosensitization. We determined that both inhibition of PP2A by the cell-permeable inhibitor calyculin A and knockdown of PP2A by RNAi (a) enhanced Ser46 phosphorylation in p53 and (b) induced coincident caspase 7 and PARP cleavage in response to ionizing radiation. Furthermore, mutation of p53 Ser46 to Ala attenuated ionizing radiation-induced apoptotic signaling. Consequently, we concluded that PP2A regulates ionizing radiation-induced apoptotic signaling through dephosphorylation of p53 Ser46.

PKR regulates B56(alpha)-mediated BCL2 phosphatase activity in acute lymphoblastic leukemia-derived REH cells

Protein phosphatase 2A (PP2A) is a heterotrimer comprising catalytic, scaffold, and regulatory (B) subunits. There are at least 21 B subunit family members. Thus PP2A is actually a family of enzymes defined by which B subunit is used. The B56 family member B56alpha is a phosphoprotein that regulates dephosphorylation of BCL2. The stress kinase PKR has been shown to phosphorylate B56alpha at serine 28 in vitro, but it has been unclear how PKR might regulate the BCL2 phosphatase. In the present study, PKR regulation of B56alpha in REH cells was examined, because these cells exhibit robust BCL2 phosphatase activity. PKR was found to be basally active in REH cells as would be predicted if the kinase supports B56alpha-mediated dephosphorylation of BCL2. Suppression of PKR promoted BCL2 phosphorylation with concomitant loss of B56alpha phosphorylation at serine 28 and inhibition of mitochondrial PP2A activity. PKR supports stress signaling in REH cells, as suppression of PKR promoted chemoresistance to etoposide. Suppression of PKR promoted B56alpha proteolysis, which could be blocked by a proteasome inhibitor. However, the mechanism by which PKR supports B56alpha protein does not involve PKR-mediated phosphorylation of the B subunit at serine 28 but may involve eIF2alpha activation of AKT. Phosphorylation of serine 28 by PKR promotes mitochondrial localization of B56alpha, because wild-type but not mutant S28A B56alpha promoted mitochondrial PP2A activity. Cells expressing wild-type B56alpha but not S28A B56alpha were sensitized to etoposide. These results suggest that PKR regulates B56alpha-mediated PP2A signaling in REH cells.

Early events in the mammalian response to DNA double-strand breaks

Physical and chemical agents that induce DNA double-strand breaks (DSBs) are among the most potent mutagens. The mammalian cell response to DSB comprises a highly co-ordinated, yet complex network of proteins that have been categorized as sensors, signal transducers, mediators and effectors of damage and repair. While this provides an accessible classification system, review of the literature indicates that many proteins satisfy the criteria of more than one category, pointing towards a series of highly co-operative pathways with overlapping function. In summary, the MRE11-NBS1-RAD50 complex is necessary for achieving optimal activation of ataxia-telangiectasia-mutated (ATM) kinase, which catalyses a phosphorylation-mediated signal transduction cascade. Among the subset of proteins phosphorylated by ATM are histone H2AX (H2AX), mediator of damage checkpoint protein 1, nibrin (NBS1), P53-binding protein 1 and breast cancer protein 1, all of which subsequently redistribute into DSB-containing sub-nuclear compartments. Post-translational modification of DSB responding proteins achieves a rapid and reversible change in protein behaviour and mediates damage-specific interactions, hence imparting a high degree of vigilance to the cell. This review highlights events fundamental in maintaining genetic integrity with emphasis on early stages of the DSB response.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.

A RNA interference screen identifies the protein phosphatase 2A subunit PR55gamma as a stress-sensitive inhibitor of c-SRC

Protein Phosphatase type 2A (PP2A) represents a family of holoenzyme complexes with diverse biological activities. Specific holoenzyme complexes are thought to be deregulated during oncogenic transformation and oncogene-induced signaling. Since most studies on the role of this phosphatase family have relied on the use of generic PP2A inhibitors, the contribution of individual PP2A holoenzyme complexes in PP2A-controlled signaling pathways is largely unclear. To gain insight into this, we have constructed a set of shRNA vectors targeting the individual PP2A regulatory subunits for suppression by RNA interference. Here, we identify PR55γ and PR55δ as inhibitors of c-Jun NH₂-terminal kinase (JNK) activation by UV irradiation. We show that PR55γ binds c-SRC and modulates the phosphorylation of serine 12 of c-SRC, a residue we demonstrate to be required for JNK activation by c-SRC. We also find that the physical interaction between PR55γ and c-SRC is sensitive to UV irradiation. Our data reveal a novel mechanism of c-SRC regulation whereby in response to stress c-SRC activity is regulated, at least in part, through loss of the interaction with its inhibitor, PR55γ.

Protein Phosphatase type 2A (PP2A) represent a family of holoenzyme complexes involved in wide range of activities such as growth, differentiation, and cell death. The PP2A holoenzyme complex is made up of a catalytic, a structural, and one of various “B” subunits. These “B” subunits are thought to provide the substrate specificity required for PP2A activity. Previous work on PP2A has mostly been derived by inhibiting the catalytic subunit through chemical inhibition, as such inhibiting all of the pathways associated with PP2A. To identify individual “B” subunits involved in specific cellular processes we have generated a “B” subunit gene knockdown library, which allows us to inhibit each of the known “B” subunits individually. One of the many pathways regulated by PP2A is the c-Jun NH₂-terminal kinase (JNK) kinase pathway, which, depending on stimulus, can affect either cell survival or cell proliferation. Here we report that the “B” subunit PR55γ acts as a negative regulator of JNK activity and cell death. We show that PR55γ influences JNK activity by inhibiting one of its upstream regulators, the proto-oncogene c-SRC, through dephosphorylation at one of the key residues on c-SRC, a site we show to be critical for c-SRC activation following cell stress. Overall our work describes the novel function of a specific PP2A subunit involved in cell survival and identifies a novel mechanism of c-SRC regulation.

PP2A holoenzyme assembly: in cauda venenum (the sting is in the tail)

Protein phosphatase 2A (PP2A), a major phospho-serine/threonine phosphatase, is conserved throughout eukaryotes. It dephosphorylates a plethora of cellular proteins, including kinases and other signaling molecules involved in cell division, gene regulation, protein synthesis and cytoskeleton organization. PP2A enzymes typically exist as heterotrimers comprising catalytic C-, structural A- and regulatory B-type subunits. The B-type subunits function as targeting and substrate-specificity factors; hence, holoenzyme assembly with the appropriate B-type subunit is crucial for PP2A specificity and regulation. Recently, several biochemical and structural determinants have been described that affect PP2A holoenzyme assembly. Moreover, the effects of specific post-translational modifications of the C-terminal tail of the catalytic subunit indicate that a 'code' might regulate dynamic exchange of regulatory B-type subunits, thus affecting the specificity of PP2A.

Mutations in String/CDC25 inhibit cell cycle re-entry and neurodegeneration in a Drosophila model of Ataxia telangiectasia

Mutations in ATM (Ataxia telangiectasia mutated) result in Ataxia telangiectasia (A-T), a disorder characterized by progressive neurodegeneration. Despite advances in understanding how ATM signals cell cycle arrest, DNA repair, and apoptosis in response to DNA damage, it remains unclear why loss of ATM causes degeneration of post-mitotic neurons and why the neurological phenotype of ATM-null individuals varies in severity. To address these issues, we generated a Drosophila model of A-T. RNAi knockdown of ATM in the eye caused progressive degeneration of adult neurons in the absence of exogenously induced DNA damage. Heterozygous mutations in select genes modified the neurodegeneration phenotype, suggesting that genetic background underlies variable neurodegeneration in A-T. The neuroprotective activity of ATM may be negatively regulated by deacetylation since mutations in a protein deacetylase gene, RPD3, suppressed neurodegeneration, and a human homolog of RPD3, histone deacetylase 2, bound ATM and abrogated ATM activation in cell culture. Moreover, knockdown of ATM in post-mitotic neurons caused cell cycle re-entry, and heterozygous mutations in the cell cycle activator gene String/CDC25 inhibited cell cycle re-entry and neurodegeneration. Thus, we hypothesize that ATM performs a cell cycle checkpoint function to protect post-mitotic neurons from degeneration and that cell cycle re-entry causes neurodegeneration in A-T.

Activation and regulation of ATM kinase activity in response to DNA double-strand breaks

The ataxia-telangiectasia-mutated (ATM) protein kinase is rapidly and specifically activated in response to DNA double-strand breaks in eukaryotic cells. In this review, we summarize recent insights into the mechanism of ATM activation, focusing on the role of the Mre11/Rad50/Nbs1 (MRN) complex in this process. We also compare observations of the ATM activation process in different biological systems and highlight potential candidates for cellular factors that may participate in regulating ATM activity in human cells.

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.

The Chk1-mediated S-phase checkpoint targets initiation factor Cdc45 via a Cdc25A/Cdk2-independent mechanism

DNA damage induced by the carcinogen benzo[a]pyrene dihydrodiol epoxide (BPDE) induces a Chk1-dependent S-phase checkpoint. Here, we have investigated the molecular basis of BPDE-induced S-phase arrest. Chk1-dependent inhibition of DNA synthesis in BPDE-treated cells occurred without detectable changes in Cdc25A levels, Cdk2 activity, or Cdc7/Dbf4 interaction. Overexpression studies showed that Cdc25A, cyclin A/Cdk2, and Cdc7/Dbf4 were not rate-limiting for DNA synthesis when the BPDE-induced S-phase checkpoint was active. To investigate other potential targets of the S-phase checkpoint, we tested the effects of BPDE on the chromatin association of DNA replication factors. The levels of chromatin-associated Cdc45 (but not soluble Cdc45) were reduced concomitantly with BPDE-induced Chk1 activation and inhibition of DNA synthesis. The chromatin association of Mcm7, Mcm10, and proliferating cell nuclear antigen was unaffected by BPDE treatment. However, the association between Mcm7 and Cdc45 in the chromatin fraction was inhibited in BPDE-treated cells. Chromatin immunoprecipitation analyses demonstrated reduced association of Cdc45 with the beta-globin origin of replication in BPDE-treated cells. The inhibitory effects of BPDE on DNA synthesis, Cdc45/Mcm7 associations, and interactions between Cdc45 and the beta-globin locus were abrogated by the Chk1 inhibitor UCN-01. Taken together, our results show that the association between Cdc45 and Mcm7 at origins of replication is negatively regulated by Chk1 in a Cdk2-independent manner. Therefore, Cdc45 is likely to be an important target of the Chk1-mediated S-phase checkpoint.

Protein phosphatase 2A antagonizes ATM and ATR in a Cdk2- and Cdc7-independent DNA damage checkpoint

We previously used a soluble cell-free system derived from Xenopus eggs to investigate the role of protein phosphatase 2A (PP2A) in chromosomal DNA replication. We found that immunodepletion of PP2A or inhibition of PP2A by okadaic acid (OA) inhibits initiation of DNA replication by preventing loading of the initiation factor Cdc45 onto prereplication complexes. Evidence was provided that PP2A counteracts an inhibitory protein kinase that phosphorylates and inactivates a crucial Cdc45 loading factor. Here, we report that the inhibitory effect of OA is abolished by caffeine, an inhibitor of the checkpoint kinases ataxia-telangiectasia mutated protein (ATM) and ataxia-telangiectasia related protein (ATR) but not by depletion of ATM or ATR from the extract. Furthermore, we demonstrate that double-strand DNA breaks (DSBs) cause inhibition of Cdc45 loading and initiation of DNA replication and that caffeine, as well as immunodepletion of either ATM or ATR, abolishes this inhibition. Importantly, the DSB-induced inhibition of Cdc45 loading is prevented by addition of the catalytic subunit of PP2A to the extract. These data suggest that DSBs and OA prevent Cdc45 loading through different pathways, both of which involve PP2A, but only the DSB-induced checkpoint implicates ATM and ATR. The inhibitory effect of DSBs on Cdc45 loading does not result from downregulation of cyclin-dependent kinase 2 (Cdk2) or Cdc7 activity and is independent of Chk2. However, it is partially dependent on Chk1, which becomes phosphorylated in response to DSBs. These data suggest that PP2A counteracts ATM and ATR in a DNA damage checkpoint in Xenopus egg extracts.

Downregulation of protein phosphatase 2A activity in HeLa cells at the G2-mitosis transition and unscheduled reactivation induced by 12-O-tetradecanoyl phorbol-13-acetate (TPA)

In the cell cycle the transition from G2 phase to cell division (M) is strictly controlled by protein phosphorylation-dephosphorylation reactions effected by several protein kinases and phosphatases. Although much indirect and direct evidence point to a key role of protein phosphatase 2A (PP2A) at the G2/M transition, the control of the enzyme activity prior to and after the transition are not fully clarified. Using synchronized HeLa cells we determined the PP2A activity (i.e. the increment sensitive to inhibition by 2nM okadaic acid) in immunoprecipitates obtained with antibodies raised against a conserved peptide sequence (residues 169-182, Ab(169/182)) of the PP2A catalytic subunit (PP2A C). Two different substrates were offered: the phospho-peptide KR(p)TIRR and histone H1 phosphorylated by means of the cyclin-dependent protein kinase p34(cdc2). The results indicate that in HeLa cells the specific activity of PP2A towards both substrates goes through a minimum in late G2 phase and stays low until metaphase. Treatment of G2 cells with TPA (10(-7) M) caused a reactivation of the downregulated PP2A activity within 20 min, i.e. the same time frame within which TPA was shown earlier to block HeLa cells at the transition from G2 to mitosis [Kinzel et al., 1988. Cancer Res. 48, 1759-1762]. Activation of PP2A was also induced by TPA in mitotic cells. The low activity of PP2A in mitotic cells was accompanied by a strong reaction of mitotic PP2A C with anti-P-Tyr antibodies in Western blots, which was reversed by treatment of mitotic cells with TPA. The results suggest that the activity of cellular PP2A requires downregulation for the transition from G2 phase to mitosis. Unscheduled reactivation of PP2A induced by TPA in late G2 phase appears to inhibit the progress into mitosis.

PP2A: the expected tumor suppressor

PP2A is one of the few serine/threonine-specific phosphatases in the cell, and its complex structure and regulation guarantees its many different functions. Some viruses have chosen to target this enzyme system in order to manage the host cell machinery for their own profit and to program cells into a malignant state. Suppression of PR61/B'gamma, a specific third regulatory subunit of PP2A, can substitute for the viral SV40 protein small t antigen in causing tumorigenic transformation of several human cell lines -- provided that telomerase, SV40 large T antigen and oncogenic Ras are also present. Accumulation of c-Myc seems to be the common denominator.

Autophosphorylation of ataxia-telangiectasia mutated is regulated by protein phosphatase 2A

Ionizing radiation induces autophosphorylation of the ataxia-telangiectasia mutated (ATM) protein kinase on serine 1981; however, the precise mechanisms that regulate ATM activation are not fully understood. Here, we show that the protein phosphatase inhibitor okadaic acid (OA) induces autophosphorylation of ATM on serine 1981 in unirradiated cells at concentrations that inhibit protein phosphatase 2A-like activity in vitro. OA did not induce gamma-H2AX foci, suggesting that it induces ATM autophosphorylation by inactivation of a protein phosphatase rather than by inducing DNA double-strand breaks. In support of this, we show that ATM interacts with the scaffolding (A) subunit of protein phosphatase 2A (PP2A), that the scaffolding and catalytic (C) subunits of PP2A interact with ATM in undamaged cells and that immunoprecipitates of ATM from undamaged cells contain PP2A-like protein phosphatase activity. Moreover, we show that IR induces phosphorylation-dependent dissociation of PP2A from ATM and loss of the associated protein phosphatase activity. We propose that PP2A plays an important role in the regulation of ATM autophosphorylation and activity in vivo.

DNA damage and repair in human peripheral blood lymphocytes following treatment with microcystin-LR

The purpose of this study was to find a possible explanation of the inconsistency of data regarding the genotoxicity of microcystin-LR (MC-LR). We compared the results of the comet assay with the results of the analysis of chromosome aberrations and apoptosis. In order to investigate the influence of MC-LR on DNA damage in human lymphocytes, cells were treated with MC-LR at different concentrations (1, 10 and 25 microg/ml) for 6, 12, 18 and 24 h. Analyses of Olive Tail Moment (OTM) as an indicator of DNA damage showed that MC-LR treatment induced DNA damage in a time-dependent manner, reaching its maximum after 18 h. The lowest values of OTM were observed after 24 h. MC-LR had no effect on the frequency of chromosome aberrations in lymphocytes. Since some data available in the literature indicate that apoptosis may lead to overestimated or false positive results regarding the genotoxicity of mutagens in the comet assay, we measured the frequency of late apoptotic cells by use of the comet assay and the frequency of early apoptotic cells with the TUNEL method. The comet assay results revealed that the highest level of apoptosis was observed after 24 h and the lowest after 18 h. The comparison of the frequency of apoptotic cells determined by the comet assay with DNA damage (OTM) examined by the comet assay revealed a statistically significant, negative correlation. The TUNEL results showed that the frequency of apoptotic cells progressively increased in a dose- and time-dependent manner. The comparison of the frequency of apoptotic cells determined by TUNEL method with DNA damage (OTM) examined by the comet assay showed a significant positive correlation for lymphocytes treated with MC-LR for 6, 12 and 18 h. Therefore, our findings indicate that microcystin-LR-induced DNA damage observed in the comet assay may be related to the early stages of apoptosis due to cytotoxicity but not genotoxicity. In addition, we examined the DNA repair kinetics in lymphocytes following treatment with microcystin-LR and ionizing radiation. Our results indicate that MC-LR has an inhibiting effect on the repair of radiation-induced damage.

Irod/Ian5: an inhibitor of gamma-radiation- and okadaic acid-induced apoptosis

Protein phosphatase-directed toxins such as okadaic acid (OA) are general apoptosis inducers. We show that a protein (inhibitor of radiation- and OA-induced apoptosis, Irod/Ian5), belonging to the family of immune-associated nucleotide binding proteins, protected Jurkat T-cells against OA- and gamma-radiation-induced apoptosis. Unlike previously described antiapoptotic proteins Irod/Ian5 did not protect against anti-Fas, tumor necrosis factor-alpha, staurosporine, UV-light, or a number of chemotherapeutic drugs. Irod antagonized a calmodulin-dependent protein kinase II-dependent step upstream of activation of caspase 3. Irod has predicted GTP-binding, coiled-coil, and membrane binding domains. Irod localized to the centrosomal/Golgi/endoplasmic reticulum compartment. Deletion of either the C-terminal membrane binding domain or the N-terminal GTP-binding domain did not affect the antiapoptotic function of Irod, nor the centrosomal localization. The middle part of Irod, containing the coiled-coil domain, was therefore responsible for centrosomal anchoring and resistance toward death. Being widely expressed and able to protect also nonimmune cells, the function of Irod may not be limited to the immune system. The function and localization of Irod indicate that the centrosome and calmodulin-dependent protein kinase II may have important roles in apoptosis signaling.

Single-stranded DNA induces ataxia telangiectasia mutant (ATM)/p53-dependent DNA damage and apoptotic signals

Single-stranded DNA has been speculated to be the initial signal in the DNA damage signaling pathway. We showed that introduction of single-stranded DNA with diverse sequences into mammalian cells induced DNA damage as well as apoptosis signals. Like DNA damaging agents, single-stranded DNA up-regulated p53 and activated the nuclear kinase ataxia telangiectasia mutant (ATM) as evidenced by phosphorylation of histone 2AX, an endogenous ATM substrate. Single-stranded DNA also triggered apoptosis as evidenced by the formation of caspase-dependent chromosomal DNA strand breaks, cytochrome c release, and increase in reactive oxygen species production. Moreover, single-stranded DNA-induced apoptosis was reduced significantly in p53 null cells and in cells treated with ATM small interfering RNA. These results suggest that single-stranded DNA may act upstream of ATM/p53 in DNA damage signaling.

Ionizing radiation activates nuclear protein phosphatase-1 by ATM-dependent dephosphorylation

Ionizing radiation (IR) is known to activate multiple signaling pathways, resulting in diverse stress responses including apoptosis, cell cycle arrest, and gene induction. IR-activated cell cycle checkpoints are regulated by Ser/Thr phosphorylation, so we tested to see if protein phosphatases were targets of an IR-activated damage-sensing pathway. Jurkat cells were subjected to IR or sham radiation followed by brief (32)P metabolic labeling. Nuclear extracts were subjected to microcystin affinity chromatography to recover phosphatases, and the proteins were analyzed by two-dimensional gel electrophoresis. Protein sequencing revealed that the microcystin-bound proteins with the greatest reduction in (32)P intensity following IR were the alpha and delta isoforms of protein phosphatase 1 (PP1). Both of these PP1 isoforms contain an Arg-Pro-Ile/Val-Thr-Pro-Pro-Arg sequence near the C terminus, a known site of phosphorylation by Cdc/Cdk kinases, and phosphorylation attenuates phosphatase activity. In wild-type Jurkat cells or ataxia telangiectasia (AT) cells that are stably transfected with full-length ATM kinase, IR resulted in net dephosphorylation of this site in PP1 and produced activation of PP1. However, in AT cells that are deficient in ATM, IR failed to induce dephosphorylation or activation of PP1. IR-induced PP1 activation in the nucleus may be a critical component in an ATM-mediated pathway controlling checkpoint activation.