Cytometric detection of chromatin relaxation, an early reporter of DNA damage response

Oxidative stress-mediated alterations in histone post-translational modifications

Epigenetic regulation of gene expression provides a finely tuned response capacity for cells when undergoing environmental changes. However, in the context of human physiology or disease, any cellular imbalance that modulates homeostasis has the potential to trigger molecular changes that result either in physiological adaptation to a new situation or pathological conditions. These effects are partly due to alterations in the functionality of epigenetic regulators, which cause long-term and often heritable changes in cell lineages. As such, free radicals resulting from unbalanced/extended oxidative stress have been proved to act as modulators of epigenetic agents, resulting in alterations of the epigenetic landscape. In the present review we will focus on the particular effect that oxidative stress and free radicals produce in histone post-translational modifications that contribute to altering the histone code and, consequently, gene expression. The pathological consequences of the changes in this epigenetic layer of regulation of gene expression are thoroughly evidenced by data gathered in many physiological adaptive processes and in human diseases that range from age-related neurodegenerative pathologies to cancer, and that include respiratory syndromes, infertility, and systemic inflammatory conditions like sepsis.

Oxidative, Reductive, and Nitrosative Stress Effects on Epigenetics and on Posttranslational Modification of Enzymes in Cardiometabolic Diseases

Oxidative (OS), reductive (RS), and nitrosative (NSS) stresses produce carbonylation, glycation, glutathionylation, sulfhydration, nitration, and nitrosylation reactions. OS, RS, and NSS are interrelated since RS results from an overactivation of antioxidant systems and NSS is the result of the overactivation of the oxidation of nitric oxide (NO). Here, we discuss the general characteristics of the three types of stress and the way by which the reactions they induce (a) damage the DNA structure causing strand breaks or inducing the formation of 8-oxo-d guanosine; (b) modify histones; (c) modify the activities of the enzymes that determine the establishment of epigenetic cues such as DNA methyl transferases, histone methyl transferases, acetyltransferases, and deacetylases; (d) alter DNA reparation enzymes by posttranslational mechanisms; and (e) regulate the activities of intracellular enzymes participating in metabolic reactions and in signaling pathways through posttranslational modifications. Furthermore, the three types of stress may establish new epigenetic marks through these reactions. The development of cardiometabolic disorders in adult life may be programed since early stages of development by epigenetic cues which may be established or modified by OS, RS, and NSS. Therefore, the three types of stress participate importantly in mediating the impact of the early life environment on later health and heritability. Here, we discuss their impact on cardiometabolic diseases. The epigenetic modifications induced by these stresses depend on union and release of chemical residues on a DNA sequence and/or on amino acid residues in proteins, and therefore, they are reversible and potentially treatable.

Job Opening for Nucleosome Mechanic: Flexibility Required

The nucleus has been studied for well over 100 years, and chromatin has been the intense focus of experiments for decades. In this review, we focus on an understudied aspect of chromatin biology, namely the chromatin fiber polymer’s mechanical properties. In recent years, innovative work deploying interdisciplinary approaches including computational modeling, in vitro manipulations of purified and native chromatin have resulted in deep mechanistic insights into how the mechanics of chromatin might contribute to its function. The picture that emerges is one of a nucleus that is shaped as much by external forces pressing down upon it, as internal forces pushing outwards from the chromatin. These properties may have evolved to afford the cell a dynamic and reversible force-induced communication highway which allows rapid coordination between external cues and internal genomic function.

p53 modulates the effect of ribosomal protein S6 kinase1 (S6K1) on cisplatin toxicity in chronic myeloid leukemia cells

Chronic myeloid leukemia (CML) is characterized by the expression of the oncoprotein, BCR-ABL. BCR-ABL inhibitors revolutionized CML chemotherapy while blast crisis (BC) CML patients are less responsive. Since suppression of ribosomal protein S6 kinase1 (S6K1) phosphorylation reverses the resistance to BCR-ABL inhibitor in CML cells and S6K1 inhibitors augment cisplatin toxicity in lung cancer cells, we speculated that combination of S6K1 inhibitor and cisplatin may be beneficial for eliminating BC CML cells. To our surprise, S6K1 inhibition decreased cisplatin-induced DNA damage and cell death only in p53^-/- BC CML cells but not in p53^+/+ BC CML cells. During the progression of CML, p53 expression either decreases or mutates. Moreover, the expression of p53 affects drug response of CML cells. Our results confirmed that S6K1 inhibition reversed cisplatin toxicity is dependent on p53 expression in CML cells. Moreover, p53 attenuated the phosphorylation and localization of S6K1 via attenuating 3-phosphoinositide dependent protein kinase-1 (PDK1) phosphorylation. Furthermore, S6K1 acts via DNA-PKcs to regulate H2AX phosphorylation and PARP cleavage, respectively. Taken together, our results suggest that p53/PDK1/S6K1 is a novel pathway regulating cisplatin toxicity in BC CML cells.

The transcription factor OBP4 controls root growth and promotes callus formation

Plant growth and development require a continuous balance between cell division and differentiation. In root meristems, differentiated cells acquire specialized functions, losing their mitotic potential. Some plant cells, such as pericycle cells, have a remarkable plasticity to regenerate new organs. The molecular mechanisms underlying cell reprogramming are not completely known. In this work, a functional screening of transcription factors identified Arabidopsis OBP4 (OBF Binding Protein 4) as a novel regulator of root growth and cell elongation and differentiation. Overexpression of OBP4 regulates the levels of a large number of transcripts in roots, many involved in hormonal signaling and callus formation. OBP4 controls cell elongation and differentiation in root cells. OBP4 does not induce cell division in the root meristem, but promotes pericycle cell proliferation, forming callus-like structures at the root tip, as shown by the expression of stem cell markers. Callus formation is enhanced by ectopic expression of OBP4 in the wild-type or alf4-1, but is significantly reduced in roots that have lower levels of OBP4. Our data provide molecular insights into how differentiated root cells acquire the potential to generate callus, a pluripotent mass of cells that can regenerate fully functional plant organs.

Subnuclear localization, rates and effectiveness of UVC-induced unscheduled DNA synthesis visualized by fluorescence widefield, confocal and super-resolution microscopy

Unscheduled DNA synthesis (UDS) is the final stage of the process of repair of DNA lesions induced by UVC. We detected UDS using a DNA precursor, 5-ethynyl-2'-deoxyuridine (EdU). Using wide-field, confocal and super-resolution fluorescence microscopy and normal human fibroblasts, derived from healthy subjects, we demonstrate that the sub-nuclear pattern of UDS detected via incorporation of EdU is different from that when BrdU is used as DNA precursor. EdU incorporation occurs evenly throughout chromatin, as opposed to just a few small and large repair foci detected by BrdU. We attribute this difference to the fact that BrdU antibody is of much larger size than EdU, and its accessibility to the incorporated precursor requires the presence of denatured sections of DNA. It appears that under the standard conditions of immunocytochemical detection of BrdU only fragments of DNA of various length are being denatured. We argue that, compared with BrdU, the UDS pattern visualized by EdU constitutes a more faithful representation of sub-nuclear distribution of the final stage of nucleotide excision repair induced by UVC. Using the optimized integrated EdU detection procedure we also measured the relative amount of the DNA precursor incorporated by cells during UDS following exposure to various doses of UVC. Also described is the high degree of heterogeneity in terms of the UVC-induced EdU incorporation per cell, presumably reflecting various DNA repair efficiencies or differences in the level of endogenous dT competing with EdU within a population of normal human fibroblasts.

Oxidative stress signaling to chromatin in health and disease

Oxidative stress has a significant impact on the development and progression of common human pathologies, including cancer, diabetes, hypertension and neurodegenerative diseases. Increasing evidence suggests that oxidative stress globally influences chromatin structure, DNA methylation, enzymatic and non-enzymatic post-translational modifications of histones and DNA-binding proteins. The effects of oxidative stress on these chromatin alterations mediate a number of cellular changes, including modulation of gene expression, cell death, cell survival and mutagenesis, which are disease-driving mechanisms in human pathologies. Targeting oxidative stress-dependent pathways is thus a promising strategy for the prevention and treatment of these diseases. We summarize recent research developments connecting oxidative stress and chromatin regulation.

Activation of new replication foci under conditions of replication stress

DNA damage, binding of drugs to DNA or a shortage of nucleotides can decrease the rate or completely halt the progress of replication forks. Although the global rate of replication decreases, mammalian cells can respond to replication stress by activating new replication origins. We demonstrate that a moderate level of stress induced by inhibitors of topoisomerase I, commencing in early, mid or late S-phase, induces activation of new sites of replication located within or in the immediate vicinity of the original replication factories; only in early S some of these new sites are also activated at a distance greater than 300 nm. Under high stress levels very few new replication sites are activated; such sites are located within the original replication regions. There is a large variation in cellular response to stress - while in some cells the number of replication sites increases even threefold, it decreases almost twofold in other cells. Replication stress results in a loss of PCNA from replication factories and a twofold increase in nuclear volume. These observations suggest that activation of new replication origins from the pool of dormant origins within replication cluster under conditions of mild stress is generally restricted to the original replication clusters (factories) active at a time of stress initiation, while activation of distant origins and new replication factories is suppressed.

Senescence Meets Dedifferentiation

Senescence represents the final stage of leaf development but is often induced prematurely following exposure to biotic and abiotic stresses. Leaf senescence is manifested by color change from green to yellow (due to chlorophyll degradation) or to red (due to de novo synthesis of anthocyanins coupled with chlorophyll degradation) and frequently culminates in programmed death of leaves. However, the breakdown of chlorophyll and macromolecules such as proteins and RNAs that occurs during leaf senescence does not necessarily represent a one-way road to death but rather a reversible process whereby senescing leaves can, under certain conditions, re-green and regain their photosynthetic capacity. This phenomenon essentially distinguishes senescence from programmed cell death, leading researchers to hypothesize that changes occurring during senescence might represent a process of trans-differentiation, that is the conversion of one cell type to another. In this review, we highlight attributes common to senescence and dedifferentiation including chromatin structure and activation of transposable elements and provide further support to the notion that senescence is not merely a deterioration process leading to death but rather a unique developmental state resembling dedifferentiation.

Nucleotide excision repair efficiency in quiescent human fibroblasts is modulated by circadian clock

The efficiency of Nucleotide Excision Repair (NER)process is crucial for maintaining genomic integrity because in many organisms, including humans, it represents the only system able to repair a wide range of DNA damage. The aim of the work was to investigate whether the efficiency of the repair of photoproducts induced by UV-light is affected by the circadian phase at which irradiation occurred. NER activity has been analyzed in human quiescent fibroblasts (in the absence of the cell cycle effect), in which circadian rhythmicity has been synchronized with a pulse of dexamethasone. Our results demonstrate that both DNA damage induction and repair efficiency are strictly dependent on the phase of the circadian rhythm at which the cells are UV-exposed. Furthermore, the differences observed between fibroblasts irradiated at different circadian times (CTs) are abolished when the clock is obliterated. In addition, we observe that chromatin structure is regulated by circadian rhythmicity. Maximal chromatin relaxation occurred at the same CT when photoproduct formation and removal were highest. Our data suggest that the circadian clock regulates both the DNA sensitivity to UV damage and the efficiency of NER by controlling chromatin condensation mainly through histone acetylation.

Stress cycles in stem cells/iPSCs development: implications for tissue repair

Stem cells have become a major topic, both publicly and scientifically, owing to their potential to cure diseases and repair damaged tissues. Particular attention has been given to the so-called "induced pluripotent stem cells" (iPSCs) in which somatic cells are induced by the expression of transcription factor encoding transgenes-a methodology first established by Takahashi and Yamanaka (Cell 126:663-676, 2006)-to acquire pluripotent state. This methodology has captured researchers' imagination as a potential procedure to obtain patient-specific therapies while also solving both the problem of transplant rejection and the ethical concerns often raised regarding the use of embryonic stem cells in regenerative medicine. The study of the biology of stem cells/iPSCs, in recent years, has uncovered some fundamental weaknesses that undermine their potential use in transplantation therapies.

Relationship between DNA damage response, initiated by camptothecin or oxidative stress, and DNA replication, analyzed by quantitative 3D image analysis

A method of quantitative analysis of spatial (3D) relationship between discrete nuclear events detected by confocal microscopy is described and applied in analysis of a dependence between sites of DNA damage signaling (γH2AX foci) and DNA replication (EdU incorporation) in cells subjected to treatments with camptothecin (Cpt) or hydrogen peroxide (H2O2). Cpt induces γH2AX foci, likely reporting formation of DNA double-strand breaks (DSBs), almost exclusively at sites of DNA replication. This finding is consistent with the known mechanism of induction of DSBs by DNA topoisomerase I (topo1) inhibitors at the sites of collisions of the moving replication forks with topo1-DNA "cleavable complexes" stabilized by Cpt. Whereas an increased level of H2AX histone phosphorylation is seen in S-phase of cells subjected to H2O2, only a minor proportion of γH2AX foci coincide with DNA replication sites. Thus, the increased level of H2AX phosphorylation induced by H2O2 is not a direct consequence of formation of DNA lesions at the sites of moving DNA replication forks. These data suggest that oxidative stress induced by H2O2 and formation of the primary H2O2-induced lesions (8-oxo-7,8-dihydroguanosine) inhibits replication globally and triggers formation of γH2AX at various distances from replication forks. Quantitative analysis of a frequency of DNA replication sites and γH2AX foci suggests also that stalling of replicating forks by Cpt leads to activation of new DNA replication origins. © 2013 International Society for Advancement of Cytometry.

DNA damage signaling assessed in individual cells in relation to the cell cycle phase and induction of apoptosis

Reviewed are the phosphorylation events reporting activation of protein kinases and the key substrates critical for the DNA damage signaling (DDS). These DDS events are detected immunocytochemically using phospho-specific Abs; flow cytometry or image-assisted cytometry provide the means to quantitatively assess them on a cell by cell basis. The multiparameter analysis of the data is used to correlate these events with each other and relate to the cell cycle phase, DNA replication and induction of apoptosis. Expression of γH2AX as a possible marker of induction of DNA double strand breaks is the most widely studied event of DDS. Reviewed are applications of this multiparameter approach to investigate constitutive DDS reporting DNA damage by endogenous oxidants byproducts of oxidative phosphorylation. Also reviewed are its applications to detect and explore mechanisms of DDS induced by variety of exogenous agents targeting DNA such as exogenous oxidants, ionizing radiation, radiomimetic drugs, UV light, DNA topoisomerase I and II inhibitors, DNA crosslinking drugs and variety of environmental genotoxins. Analysis of DDS induced by these agents provides often a wealth of information about mechanism of induction and the type of DNA damage (lesion) and is reviewed in the context of cell cycle phase specificity, DNA replication, and induction of apoptosis or cell senescence. Critically assessed is interpretation of the data as to whether the observed DDS events report induction of a particular type of DNA lesion.

Nuclear remodeling as a mechanism for genomic instability in cancer

This chapter focuses on the three-dimensional organization of the nucleus in normal, early genomically unstable, and tumor cells. A cause-consequence relationship is discussed between nuclear alterations and the resulting genomic rearrangements. Examples are presented from studies on conditional Myc deregulation, experimental tumorigenesis in mouse plasmacytoma, nuclear remodeling in Hodgkin's lymphoma, and in adult glioblastoma. A model of nuclear remodeling is proposed for cancer progression in multiple myeloma. Current models of nuclear remodeling are described, including our model of altered nuclear architecture and the onset of genomic instability.

Relationship of DNA damage signaling to DNA replication following treatment with DNA topoisomerase inhibitors camptothecin/topotecan, mitoxantrone, or etoposide

DNA topoisomerase I (Top1) and topoisomerase II (Top2) inhibitors are widely used to treat a variety of cancers. Their mechanism of action involves stabilization of otherwise transient ("cleavable") complexes between Top1 or Top2 and DNA; collisions of DNA replication forks with such stabilized complexes lead to formation of DNA double-strand breaks (DSBs). In this study, using 5-ethynyl-2'deoxyuridine (EdU) as a DNA precursor, we directly assessed the relationship between DNA replication and induction of DSBs revealed as γH2AX foci in A549 cells treated with Top1 inhibitors topotecan (Tpt) or camptothecin (Cpt) and Top2 inhibitors mitoxantrone (Mxt) and etoposide (Etp). Analysis of cells by multiparameter laser scanning cytometry following treatment with Tpt or Cpt revealed that only DNA replicating cells showed induction of γH2AX and a strong correlation between DNA replication and formation of DSBs (r = 0.86). In cells treated with Mxt or Etp, the correlation was weaker (r = 0.52 and 0.64). In addition, both Mtx and Etp caused induction of γH2AX in cells not replicating DNA. Confocal imaging of nuclei of cells treated with Tpt revealed the presence of γH2AX foci predominantly in DNA replicating cells and close association and co-localization of γH2AX foci with DNA replication sites. In cells treated with Mxt or Etp, the γH2AX foci were induced in DNA replicating as well as non-replicating cells but the close association between a large proportion of γH2AX foci and DNA replication sites was also apparent. The data are consistent with the view that collision of DNA replication forks with cleavable Top1-DNA complexes stabilized by Tpt/Cpt is the sole cause of induction of DSBs. Additional mechanisms such as involvement of transcription and/or generation of oxidative stress may contribute to DSBs induction by Mxt and Etp. The confocal analysis of the association between DNA replication sites and the sites of DSBs (γH2AX foci) opens a new approach for mechanistic studies of the involvement of DNA replication in induction of DNA damage.

Oncolytic viruses in radiation oncology

Oncolytic viruses are investigational cancer treatments. They are currently being assessed as single agents or in combination with standard therapies such as external beam radiotherapy - a DNA damaging agent that is a standard of care for many tumour types. Preclinical data indicate that combinations of oncolytic viruses and radiation therapy are promising, showing additional or synergistic antitumour effects in in vitro and in vivo studies. This interaction has the potential to be multifaceted: viruses may act as radiosensitizing agents, but radiation may also enhance viral oncolysis by increasing viral uptake, replication, gene expression and cell death (apoptosis, autophagy or necrosis) in irradiated cells. Phase I and II clinical trials investigating combinations of viruses and radiation therapy have been completed, paving the way for ongoing phase III studies. The aim of this review is to focus on the therapeutic potential of these combinations and to highlight their mechanistic bases, with particular emphasis on the role of the DNA damage response.

Analysis of individual molecular events of DNA damage response by flow- and image-assisted cytometry

This chapter describes molecular mechanisms of DNA damage response (DDR) and presents flow- and image-assisted cytometric approaches to assess these mechanisms and measure the extent of DDR in individual cells. DNA damage was induced by cell treatment with oxidizing agents, UV light, DNA topoisomerase I or II inhibitors, cisplatin, tobacco smoke, and by exogenous and endogenous oxidants. Chromatin relaxation (decondensation) is an early event of DDR chromatin that involves modification of high mobility group proteins (HMGs) and histone H1 and was detected by cytometry by analysis of the susceptibility of DNA in situ to denaturation using the metachromatic fluorochrome acridine orange. Translocation of the MRN complex consisting of Meiotic Recombination 11 Homolog A (Mre11), Rad50 homolog, and Nijmegen Breakage Syndrome 1 (NMR1) into DNA damage sites was assessed by laser scanning cytometry as the increase in the intensity of maximal pixel as well as integral value of Mre11 immunofluorescence. Examples of cytometric detection of activation of Ataxia telangiectasia mutated (ATM), and Check 2 (Chk2) protein kinases using phospho-specific Abs targeting Ser1981 and Thr68 of these proteins, respectively are also presented. We also discuss approaches to correlate activation of ATM and Chk2 with phosphorylation of p53 on Ser15 and histone H2AX on Ser139 as well as with cell cycle position and DNA replication. The capability of laser scanning cytometry to quantify individual foci of phosphorylated H2AX and/or ATM that provides more dependable assessment of the presence of DNA double-strand breaks is outlined. The new microfluidic Lab-on-a-Chip platforms for interrogation of individual cells offer a novel approach for DDR cytometric analysis.

Kinetics of the UV-induced DNA damage response in relation to cell cycle phase. Correlation with DNA replication

It has been reported that exposure to UV light triggers DNA damage response (DDR) seen as induction of gammaH2AX not only in S- but also in G(1)-phase cells. In the present study, in addition to gammaH2AX, we assessed other markers of DDR, namely phosphorylation of ATM on Ser1981, of ATM/ATR substrate on Ser/Thr at SQ/TQ cluster domains and of the tumor suppressor p53 on Ser15, in human pulmonary carcinoma A549 cells irradiated with 50 J/m(2) of UV-B. Phosphorylation of these proteins detected with phospho-specific Abs and measured by laser scanning cytometry in relation the cell cycle phase was found to be selective to S-phase cells. The kinetics of phosphorylation of ATM was strikingly similar to that of ATM/ATR substrate, peaking at 30 min after UV irradiation and followed by rapid dephosphorylation. The peak of H2AX phosphorylation was seen at 2 h and the peak of p53 phosphorylation at 4 h after exposure to UV light. Local high spatial density of these phospho-proteins reported by intensity of maximal pixel of immunofluorescence in the DDR nuclear foci was distinctly more pronounced in the early compared to late portion of S-phase. Exposure of cells to UV following 1 h pulse-labeling of their DNA with 5-ethynyl-2'deoxyuridine (EdU) made it possible to correlate the extent of DNA replication during the pulse with the extent of the UV-induced H2AX phosphorylation within the same cells. This correlation was very strong (R(2) = 0.98) and the cells that did not incorporate EdU showed no evidence of H2AX phosphorylation. The data are consistent with the mechanism in which stalling of DNA replication forks upon collision with the primary UV-induced DNA lesions and likely formation of double-strand DNA breaks triggers DDR. The prior reports (including our own) on induction of gammaH2AX in G(1) cells by UV may have erroneously identified cells initiating DNA replication following UV exposure as G(1) cells due to the fact that their DNA content did not significantly differ from that of G(1) cells that had not initiated DNA replication.

DNA damage response induced by exposure of human lung adenocarcinoma cells to smoke from tobacco- and nicotine-free cigarettes

Cigarette smoke (CS) is the major cause of lung cancer and contributes to the development of other malignancies. Attempts have been made to construct reduced toxicity cigarettes, presumed to have diminished genotoxic potential. One such product on the market is the tobacco and nicotine free (T&N-free) cigarette type made from lettuce and herbal extracts. We have recently developed a sensitive assay of the genotoxicity of CS based on cytometric analysis of induction of the DNA damage response (DDR) in normal human pulmonary endothelial or A549 pulmonary adenocarcinoma cells. In the present study, we observed that exposure of A549 cells to CS from T/N-free cigarettes induced a smoke-dose dependent DDR as evidenced by phosphorylation (activation) of the Ataxia telangiectasia mutated (ATM) protein kinase and of the histone H2AX (γH2AX). The extent of DDR induced by T&N-free smoke was distinctly greater than that induced by comparable doses of CS from reference cigarettes (2R4F) containing tobacco and nicotine. The pattern of DDR induced by T&N-free smoke was similar to that of 2R4F cigarettes in terms of the cell cycle phase specificity and involvement of reactive oxygen species (ROS). The data also imply that similar to 2R4F exposure of cells to T/N-free smoke leads to formation of double-strand DNA breaks (DSBs) resulting from collapse of replication forks upon collision with the primary ssDNA lesions induced by smoke. Since DSBs are potentially carcinogenic our data indicate that smoking tobacco and nicotine-free cigarettes is at least as hazardous as smoking cigarettes containing tobacco and nicotine.

Impaired DNA damage response--an Achilles' heel sensitizing cancer to chemotherapy and radiotherapy

Despite the progress in targeting particular molecular abnormalities specific to different cancers (targeted therapy), chemo- and radiotherapies are still the most effective of all anticancer modalities. Induction of DNA damage and inhibition of cell proliferation are the objects of most chemotherapeutic agents and radiation. Their effectiveness was initially thought to be due to the high rate of proliferation of cancer cells. However, normal cell proliferation rate in some tissues often exceeds that of curable tumors. Most tumors have impaired DNA damage response (DDR) and the evidence is forthcoming that this confers sensitivity to chemo- or radiotherapy. DDR is a complex set of events which elicits a plethora of molecular interactions engaging signaling pathways designed to: (a) halt cell cycle progression and division to prevent transfer of DNA damage to progeny cells; (b) increase the accessibility of the damaged sites to the DNA repair machinery; (c) engage DNA repair mechanisms and (d) activate the apoptotic pathway when DNA cannot be successfully repaired. A defective DDR makes cancer cells unable to effectively stop cell cycle progression, engage in DNA repair and/or trigger the apoptotic program when treated with DNA damaging drugs. With continued exposure to the drug, such cells accumulate DNA damage which leads to their reproductive death that may have features of cell senescence. Cancers with nonfunctional BRCA1 and BRCA2 are particularly sensitive to combined treatment with DNA damaging drugs and inhibitors of poly(ADP-ribose) polymerase. Antitumor strategies are being designed to treat cancers having particular defects in their DDR, concurrent with protecting normal cells.

DNA damage response induced by tobacco smoke in normal human bronchial epithelial and A549 pulmonary adenocarcinoma cells assessed by laser scanning cytometry

Cigarette smoke (CS) is a major cause of lung cancer and a contributor to the development of a wide range of other malignancies. There is an acute need to develop a methodology that can rapidly assess the potential carcinogenic properties of the genotoxic agents present in CS. We recently reported that exposure of normal human bronchial epithelial cells (NHBEs) or A549 pulmonary carcinoma cells to CS induces the activation of ATM through its phosphorylation on Ser1981 and phosphorylation of histone H2AX on Ser139 (gammaH2AX) most likely in response to the formation of potentially carcinogenic DNA double-strand breaks (DSBs). To obtain a more complete view of the DNA damage response (DDR) we explored the correlation between ATM activation, H2AX phosphorylation, activation of Chk2 through its phosphorylation on Thr68, and phosphorylation of p53 on Ser15 in NHBE and A549 cell exposed to CS. Multiparameter analysis by laser scanning cytometry made it possible to relate these DDR events, detected immunocytochemically, with cell cycle phase. The CS-dose-dependent induction and increase in the extent of phosphorylation of ATM, Chk2, H2AX, and p53 were seen in both cell types. ATM and Chk2 were phosphorylated approximately 1 h prior to phosphorylation of H2AX and p53. The dephosphorylation of ATM, Chk2, and H2AX was seen after 2 h following CS exposure. The dose-dependency and kinetics of DDR were essentially similar in both cell types, which provide justification for the use of A549 cells in the assessment of genotoxicity of CS in lieu of normal bronchial epithelial cells. The observation that DDR was more pronounced in S-phase cells is consistent with the mechanism of induction of DSBs occurring as a result of collision of replication forks with primary lesions such as DNA adducts that can be caused by CS-generated oxidants. The cytometric assessment of CS-induced DDR provides a means to estimate the genotoxicity of CS and to explore the mechanisms of the response as a function of cell cycle phase and cell type.