Low concentration N-methyl-N'-nitro-N-nitrosoguanidine activates DNA polymerase-beta expression via cyclic-AMP-protein kinase A-cAMP response element binding protein pathway

The role of aging and brain-derived neurotrophic factor signaling in expression of base excision repair genes in the human brain

DNA damage is a central contributor to the aging process. In the brain, a major threat to the DNA is the considerable amount of reactive oxygen species produced, which can inflict oxidative DNA damage. This type of damage is removed by the base excision repair (BER) pathway, an essential DNA repair mechanism, which contributes to genome stability in the brain. Despite the crucial role of the BER pathway, insights into how this pathway is affected by aging in the human brain and the underlying regulatory mechanisms are very limited. By microarray analysis of four cortical brain regions from humans aged 20-99 years (n = 57), we show that the expression of core BER genes is largely downregulated during aging across brain regions. Moreover, we find that expression of many BER genes correlates positively with the expression of the neurotrophin brain-derived neurotrophic factor (BDNF) in the human brain. In line with this, we identify binding sites for the BDNF-activated transcription factor, cyclic-AMP response element-binding protein (CREB), in the promoter of most BER genes and confirm the ability of BDNF to regulate several BER genes by BDNF treatment of mouse primary hippocampal neurons. Together, these findings uncover the transcriptional landscape of BER genes during aging of the brain and suggest BDNF as an important regulator of BER in the human brain.

STAT3 activates MSK1-mediated histone H3 phosphorylation to promote NFAT signaling in gastric carcinogenesis

Epigenetic abnormalities contribute significantly to the development and progression of gastric cancer. However, the underlying regulatory networks from oncogenic signaling pathway to epigenetic dysregulation remain largely unclear. Here we showed that STAT3 signaling, one of the critical links between inflammation and cancer, acted as a control pathway in gastric carcinogenesis. STAT3 aberrantly transactivates the epigenetic kinase mitogen- and stress-activated protein kinase 1 (MSK1), thereby phosphorylating histone H3 serine10 (H3S10) and STAT3 itself during carcinogen-induced gastric tumorigenesis. We further identified the calcium pathway transcription factor NFATc2 as a novel downstream target of the STAT3-MSK1 positive-regulating loop. STAT3 forms a functional complex with MSK1 at the promoter of NFATc2 to promote its transcription in a H3S10 phosphorylation-dependent way, thus affecting NFATc2-related inflammatory pathways in gastric carcinogenesis. Inhibiting the STAT3/MSK1/NFATc2 signaling axis significantly suppressed gastric cancer cell proliferation and xenograft tumor growth, which provides a potential novel approach for gastric carcinogenesis intervention by regulating aberrant epigenetic and transcriptional mechanisms.

Geraniol Inhibits Endometrial Carcinoma via Downregulating Oncogenes and Upregulating Tumour Suppressor Genes

Endometrial carcinoma is the fourth most abundant cancer worldwide in women. Female Wistar rats were segregated into five groups: group I-control, group II-MNNG (N-methyl-N'-nitro-N-nitrosoguanidine-150 mg/kg) administered through intravaginal detention of cotton absorbent, group III-geraniol (GOH) only, group IV-GOH-pretreated (7 days before the start of MNNG administration); and group V-Co-administration of geraniol with MNNG. In this study, reverse transcriptase- PCR of K-ras, MAPK, PI3K, Wnt/β-catenin genes, TGF-β and expressions of PCNA, PTEN, progesterone receptor and E-cadherin by Western blotting were performed from endometrial cancer tissue and control tissues. The mRNA expressions of K-ras, MAPK, PI3K, Wnt/β-catenin and TGF-β were amplified in MNNG induced group. Oral administration of GOH (both pre and co-administration) reversed the mRNA expression towards normal. The reversibility is more predominant in pretreatment groups (p < 0.05). The expression of PCNA was upregulated and downregulation of PTEN, progesterone receptor and E-cadherin was noticed in MNNG induced rats. Pre and co-administration of GOH significantly reversed the expression pattern of proteins. GOH treatment is more effective in pretreatment groups (p < 0.05). These results provide powerful evidences that GOH could influence modulation of MAPK pathways and Wnt signalling pathways in the prevention of endometrial carcinoma in rats.

Characterization of human DNA polymerase κ promoter in response to benzo[a]pyrene diol epoxide

DNA polymerase κ (Pol κ), a member of Y-family DNA polymerases, can synthesize DNA with moderate fidelity on undamaged DNAs and replicate accurately in vitro thymine glycol, 8-oxo-G and aromatic adducts such as benzo[a]pyrene diol epoxide (BPDE). However, few studies have been done on the transcriptional regulation of Pol κ. In this study, we predicted and cloned the promoter region of the human POLK gene. Through the analysis of deletion constructs of the POLK promoter, we demonstrated that the region -336/-141 contained repressing elements and the region -141/+226 contained positive regulatory elements for transcription of human Pol κ. Furthermore, quantitative RT-PCR showed that human POLK mRNA expression was dysregulated in FL cells treated by BPDE. The transcriptional activities of the POLK promoter regions -336/+437 and +20/+437 were significantly reduced by BPDE treatment, indicating that transcription factors in this two regions, such as HSF1, may regulate the transcription of human POLK gene in response to BPDE.

Quantitative and subcellular localization analysis of the nuclear isoform dUTP pyrophosphatase in alkylating agent-induced cell responses

Our previous proteome analysis showed that the nuclear isoform of dUTP pyrophosphatase (DUT-N) was identified in the culture medium of human amnion FL cells after exposure to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). These results suggest that DUT-N may be a potential early biomarker to assess the risk of alkylating agents exposure. DUT-N is one of the two isoforms of deoxyuridine triphosphate nucleotidohydrolase (dUTPase). Our current knowledge of DUT-N expression in human cells is very limited. In the current study, we first investigated the appearance of DUT-N in the culture medium of different human cell lines in response to a low concentration of MNNG exposure. We verified that the MNNG-induced appearance of DUT-N in the extracellular environment is cell-specific. Western blot analysis confirmed that the intracellular DUT-N changes responded to MNNG in a concentration-dependent and cell-specific manner. Furthermore, subcellular fraction experiments showed that 0.25μM MNNG treatment dramatically increased the DUT-N expression levels in the cytoplasmic extracts prepared from both FL and HepG2 cells, increased DUT-N levels in nuclear extracts prepared from HepG2 cells, and decreased DUT-N levels in nuclear extracts from FL cells. Morphological studies using immunofluorescence showed that a low concentration of MNNG could alter the distribution of DUT-N in FL and HepG2 cells in different ways. Taken together, these studies indicate a role of DUT-N in alkylating agent-induced cell responses.

Response of human DNA polymerase ι promoter to N-methyl-N'-nitro-N-nitrosoguanidine

Human Pol ι is a highly distributed, low-fidelity DNA polymerase lacking intrinsic exonuclease proofreading activity, thus its effects are strictly regulated. We predicted and cloned the promoter region of the human POLI gene. Successively, by transfection of deletion constructs of the POLI promoter, we demonstrated that the regions -848/-408 and -30/+215 contained positive regulatory elements, and the region +215/+335 had proximal promoter activity. Overexpression of Sp1 significantly increased the transcriptional activity of the promoter, and mutation of the Sp1 site reversed Sp1-induced promoter transactivation. Quantitative RT-PCR showed that POLI mRNA expression was up-regulated in human amnion FL cells treated by the carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Reporter gene assays demonstrated that MNNG also significantly increased the transcriptional activity of the predicted promoter (-848/+335) and the proximal promoter (+215/+335). However, the promoter with the Sp1 site mutation had no response to MNNG treatment, suggesting that Sp1 plays an important role in the transcriptional regulation of the POLI gene stimulated by MNNG. Our data suggest that abnormal regulation of Pol ι may be involved in the mutagenesis and carcinogenesis induced by environmental chemicals.

Nystatin interferes with the effects of N-methyl-N'-nitro-N-nitrosoguanidine on sphingolipid metabolism in human FL cells

Previously we have shown that an alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) can induce receptor clustering and the activation of a downstream signal molecule NF-kappaB, and that the receptor clustering is associated with changes in sphingolipids metabolism. On the other hand, the polyene antibiotic nystatin can block MNNG-induced receptor clustering. In this study, using a lipidomic approach, we further evaluated whether nystatin influenced the effects of MNNG on sphingolipids metabolism. It was found that nystatin itself induced changes in the sphingolipids profile in human amnion FL cells to a certain extent, including an increase or decrease of some sphingolipid species. Interestingly, nystatin can block, at least partially, the changes of sphingolipids-induced by MNNG. In addition, nystatin can also partially inhibit the activation of NF-kappaB induced by MNNG. Neither MNNG nor nystatin affects the mRNA levels of serine palmitoyltransferase, acid sphingomyelinase (ASM), and sphingomyelin synthase, key enzymes in the sphingolipids biosynthesis pathway. However, MNNG can activate ASM and neutral sphingomyelinase, while nystatin preincubation inhibits the activation. Taken together, these data suggested that nystatin interferes with the effects of MNNG, and might elicit its function through altered sphingolipids metabolism.

Proteomic analysis of different temporal expression patterns induced by N-methyl-N'-nitro-N-nitrosoguanidine treatment

We have previously shown that N-methyl- N'-nitro- N-nitrosoguanidine (MNNG), a well-known DNA alkylating agent and carcinogen, can induce multiple cellular responses with dynamic characteristics, including such responses as nontargeted mutations (NTM) at undamaged bases in DNA, up-regulation of low fidelity DNA polymerases, clustering of epidermal growth factor receptor (EGFR) and interference with its downstream signaling pathway. A dose-related analysis also revealed that different concentrations of MNNG can trigger diverse proteome changes associated with different cytotoxic effects. To further understand the dynamic cellular responses and hazardous effects caused by environmental carcinogen, a proteomic time-course study of whole cellular proteins from human amniotic epithelial cells after MNNG treatment was performed. Analysis at three different time points (3, 12 and 24 h after exposure) revealed that the major changes were taking place around 3 and 12 h after exposure. Using MALDI-TOF MS coupled with a micro solid-phase extraction (SPE) device, 90% ( n = 70) differentially expressed proteins were identified. Functional assignment revealed that many important pathways were affected, including the protein biosynthesis pathway and Ran GTPase system. We also carried out a network analysis of these proteins and the data suggest a central role for some key regulators in different pathways.

dUTP Pyrophosphatase, its appearance in extracellular compartment may serve as a potential biomarker forN-methyl-N'-nitro-N-nitrosoguanidine exposure in mammalian cells

The monofunctional alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) is a model chemical widely used for studying the molecular events induced by the widespread environmental N-nitroso alkylating carcinogen. Many studies have focused on understanding MNNG-induced mutagenesis and carcinogenesis. However, the search for specific indicators of MNNG exposure is still underway. In this study, we analyzed the proteins in culture medium of human amnion epithelial cells (FL cells) exposed to MNNG by 2-DE followed by MALDI-TOF MS, in the hope of finding a specific protein marker suitable for MNNG risk assessment. Image visualization and statistical analysis indicated that 12 spots appeared and 4 spots up-regulated after MNNG exposure. Most of them were identified by MS. These proteins include nuclear isoform of dUTP pyrophosphatase (DUT-N), phosphoglycerate mutase 1, heparan sulfate proteoglycan perlecan, etc., which are involved in multiple cellular functions. Interestingly, 2-DE and MS analyses of cell lysate exposed to MNNG revealed that DUT-N was down-regulated. The appearance of DUT-N in culture medium and its down-regulation in cell lysate was confirmed by Western blot. These data suggest that these proteins, especially DUT-N, could be used as candidate biomarkers for monitoring MNNG exposure.

Proteomic Profiling for Cellular Responses to Different Concentrations of N-Methyl-N‘-nitro-N-nitrosoguanidine

Alkylating agent MNNG (N-methyl-N'-nitro-N-nitrosoguanidine) can induce DNA damages which can lead to chromosomal aberrations, mutations, and cell death. Previous reports from our laboratory have found that low concentration of MNNG can induce nontargeted mutations (NTM) at undamaged bases in DNA, clustering of epidermal growth factor receptor (EGFR) and interference of EGFR mediated signaling, as well as activation of endoplasmic reticulum stress. Thus, the cellular responses to MNNG exposure are very complex, and can be triggered by signals originated from different compartments of the exposed cells. To further probe the molecular mechanisms involved in cellular responses to MNNG treatment, and to find potential biomarkers for MNNG induced stress condition, we performed proteomic analysis of whole cellular proteins from human amnion epithelial cells after exposing to MNNG at 3 different doses. More than 80 proteins were affected by MNNG treatment, and 71 proteins among them were identified using matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. These proteins take part in a wide variety of cellular processes including regulation of transcription, metabolism, cytoskeleton organization, cell cycle, cell proliferation, signal transduction, transportation, etc. The significance of these proteins in the genesis of MNNG induced cellular defensive response and hazardous effect remains to be elucidated, the results may also give a clue for biomarker search for monitoring the exposure risk of MNNG.

p38 mitogen-activated protein kinase downregulates endothelial progenitor cells

BACKGROUND

Transplantation of endothelial progenitor cells (EPCs) improves neovascularization after ischemia, but patients with coronary artery disease (CAD) or diabetes mellitus show a reduced number of EPCs and impaired functional activity. Therefore, we investigated the effects of risk factors, such as glucose and TNF-alpha, on the number of EPCs in vitro to elucidate the underlying mechanisms.

METHODS AND RESULTS

EPCs of patients or healthy subjects were isolated from peripheral blood. Incubation with glucose or TNF-alpha dose-dependently reduced the number of EPCs (79.9+/-1.3% and 74.3+/-8.1% of control; P<0.05, respectively). This reduction was not caused by apoptosis. TNF-alpha and glucose induced a dose- and time-dependent activation of the p38 MAP kinase, the downstream kinase mitogen- and stress-activated kinase 1, and the transcription factor cAMP-responsive element-binding protein (CREB), in EPCs. Moreover, EPCs from CAD patients had significantly higher basal p38-phosphorylation levels (1.83+/-0.2-fold increase; P<0.05) compared with healthy subjects. The inhibition of the p38-kinase by SB203580 or infection with a dominant negative p38 kinase adenovirus significantly increased basal number of EPCs (136.7+/-6.3% and 142.9+/-18% versus control, respectively). Likewise, ex vivo cultivation of EPCs from patients with CAD with SB203580 significantly increased the number of EPCs and partially reversed the impaired capacity for neovascularization of EPCs in vivo (relative blood flow: 0.40+/-0.03 versus 0.64+/-0.08, P<0.05). The increased numbers of EPCs by SB203580 were associated with an augmentation of EPC proliferation and a reduction of cells expressing the monocytic marker proteins CD14 and CD64, suggesting that p38 regulates proliferation and differentiation events.

CONCLUSIONS

These results demonstrate that p38 MAP kinase plays a pivotal role in the signal transduction pathways regulating the number of EPCs ex vivo. SB203580 can prevent the negative effects of TNF-alpha and glucose on the number of EPCs and may be useful to improve the number of EPCs for potential cell therapy.

N-methyl-N'-nitro-N-nitrosoguanidine sensitivity, mutator phenotype and sequence specificity of spontaneous mutagenesis in FEN-1-deficient cells

Intact pZ189 DNA was allowed to replicate in FL-FEN-1(-) cell line that was established in this laboratory in which the expression of FEN-1 gene was blocked by dexamethasone-inducible expression of antisense RNA to FEN-1. E. coli MBM7070 was transfected with the replicated plasmid, and those with mutations in the supF gene were identified. The frequency of mutants that did not contain recognizable changes in the electrophoretic mobility of the plasmid DNA was scored. The frequency of such mutants was 19.1 x 10(-4) (34/17781), significantly higher than those of 2.9 x 10(-4) (4/13668) and 3.0 x 10(-4) (3/9857) in the corresponding controls, respectively. Sequence analysis of the supF genes of these mutants showed that all (37/37) the base substitutions occurred at C:G base pairs; 68% (23/37) of the base substitutions were base transversions, while 32% (12/37) were transitions. Approximately 76% (23/37) of these base substitutions occurred frequently at nine positions; two of these sites contain triple pyrimidine (T or C) repeat upstream to the mutated base; four of these sites consist of 5'-TTN1N2 and mutations occurred at N1 site sequence; another two sites have the characteristics of triple A flanked at both 5' and 3' side by TCT, with the base substitution occurring at C in the context sequence. These data suggested that these sites are the hot spot of mutagenesis in plasmid replicated in FEN-1-deficient cells. Besides the mutator phenotype of the FEN-1-deficient cell, it was also demonstrated that FEN-1-deficient cell exhibited an increased N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) sensitive phenotype.

What turns CREB on?

The transactivation domain of the cAMP response element-binding protein (CREB) consists of two major domains. The glutamine-rich Q2 domain, which interacts with the general transcription factor TAFII130/135, is sufficient for the recruitment of a functional RNA polymerase II complex and allows basal transcriptional activity. The kinase-inducible domain, however, mediates signal-induced activation of CREB-mediated transcription. It is generally believed that recruitment of the coactivators CREB-binding protein (CBP) and p300 after signal-induced phosphorylation of this domain at serine-133 strongly enhances CREB-dependent transcription. Transcriptional activity of CREB can also be potentiated by phosphoserine-133-independent mechanisms, and not all stimuli that provoke phosphorylation of serine-133 stimulate CREB-dependent transcription. This review presents an overview of the diversity of stimuli that induce CREB phosphorylation at Ser-133, focuses on phosphoserine-133-dependent and -independent mechanisms that affect CREB-mediated transcription, and discusses different models that may explain the discrepancy between CREB Ser-133 phosphorylation and activation of CREB-mediated transcription.

Response of REV3 promoter to N-methyl-N'-nitro-N-nitrosoguanidine

Previously, we have shown that low concentration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) led to the upregulation of REV3 gene at transcriptional level in cultured human amnion FL cells. In this study, using bioinformatic analysis the putative binding sites for different transcription factors were found to exist in REV3 gene promoter region. A 2570-bp fragment of the 5' flanking region of REV3 gene was amplified by PCR from PAC clone RP3-415N12 and inserted into the pGL3-Basic reporter vector. Dual-luciferase reporter assay demonstrated that the reconstructed plasmid did respond to MNNG exposure in transfected FL cells. Several variants of the reporter plasmids with different deletions of the REV3 promoter region were also constructed and their promoter strength was analyzed. It was found that the MNNG response element might locate at the REV3 gene promoter region -404 to -102 between two Sma1 sites. The shortest responsive fragment containing the putative binding sites for transcription factors CREBP, AP-2, NF-kappaB, and SP1 was also identified.

Proteomic analysis of cellular responses to low concentration N-methyl-N'-nitro-N-nitrosoguanidine in human amnion FL cells

We have shown previously that exposure to a low concentration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) induces comprehensive changes in the protein expression profile of human amnion FL cells, including the induction, suppression, upregulation, and downregulation of various proteins. In addition, by proteomic analysis combining two-dimensional gel electrophoresis (2-DE) and mass spectrometry, some of the induced and suppressed proteins were identified. In this study, we identified an additional 18 proteins among those that were either up- or downregulated by MNNG treatment. The proteins identified were a heterogeneous group that included several zinc finger proteins, proteins involved in signal transduction, cytoskeletal proteins, cell-cycle regulation proteins, and proteins with unknown functions. The involvement of these proteins in the cellular responses to alkylating agents has not been reported before and their physiological relevance is not clear. Therefore, our findings may help better understand the global cellular stress responses to chemical carcinogens, and may lead to new studies on the functions of these MNNG-responsive proteins. Furthermore, some of these proteins may serve as biomarkers for detecting exposure of human populations to environmental carcinogens.

Activation of protein kinase A and clustering of cell surface receptors by N-methyl-N'-nitro-N-nitrosoguanidine are independent of genomic DNA damage

Alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) induces cellular stress leading to chromosomal aberrations, mutations and cell death. Previous reports from our laboratory have shown that low concentration of MNNG induces untargeted mutation (UTM), which occurs on intact DNA in mammalian cells through changes in gene expression profile. It also causes the activation of cAMP-protein kinase A (PKA) and up-regulation of POL-beta, which is demonstrated to play a role in DNA repair system. In order to find out the possible initial signal involved in UTM, we try to investigate whether the activation of PKA-CREB signal pathway is closely related to DNA damage. Our data shows that the treatment of low concentration MNNG (0.2 microM) activates PKA-CREB pathway in a comparable level both in a nuclear and enucleated cell system. And similar to the cell response caused by UV, the clustering of cell surface receptors of epidermal growth factor (EGF) and tumor necrosis factor alpha (TNFalpha) was also observed in cells exposed to MNNG. It was further demonstrated that the clustering of the surface receptors is independent of the genomic DNA damage, as this phenomenon was also observed in enucleated cells. These observations indicate that the initiation of signal cascades induced by low concentration of MNNG might be associated with its interaction with cell surface receptors and/or direct activation of related signal proteins but not its DNA damaging property.

Response of human REV3 gene to gastric cancer inducing carcinogen N-methyl-N’-nitro-N-nitrosoguanidine and its role in mutagenesis

AIM: To understand the response of human REV3 gene to gastric cancer inducing carcinogen N-methyl-N’-nitro-N-nitrosoguanidine (MNNG) and its role in human mutagenesis.

METHODS: The response of the human REV3 gene to MNNG was measured in human 293 cells and FL cells by RT-PCR. By using antisense technology, mutation analysis at HPRT locus (on which lesion-targeted mutation usually occurs) was conducted in human transgenic cell line FL-REV3^- by 8-azaguanine screening, and mutation occurred on undamaged DNA template was detected by using a shuttle plasmid pZ189 as the probe in human transgenic cell lines 293-REV3^- and FL-REV3^-. The blockage effect of REV3 was measured by combination of reverse transcription-polymerase chain reaction to detect the expression of antisense REV3 RNA and Western blotting to detect the REV3 protein level.

RESULTS: The human REV3 gene was significantly activated by MNNG treatment, as indicated by the upregulation of REV3 gene expression at the transcriptional level in MNNG-treated human cells, with significant increase of REV3 expression level by 0.38 fold, 0.33 fold and 0.27 fold respectively at 6 h, 12 h and 24 h in MNNG-treated 293 cells (P < 0.05); and to 0.77 fold and 0.65 fold at 12 h and 24 h respectively in MNNG-treated FL cells (P < 0.05). In transgenic cell line (in which REV3 was blocked by antisense REV3 RNA), high level of antisense REV3 RNA was detected, with a decreased level of REV3 protein. MNNG treatment significantly increased the mutation frequencies on undamaged DNA template (untargeted mutation), and also at HPRT locus (lesion-targeted mutation). However, when REV3 gene was blocked by antisense REV3 RNA, the MNNG-induced mutation frequency on undamaged DNA templates was significantly decreased by 3.8 fold (P < 0.05) and 5.8 fold (P < 0.01) respectively both in MNNG-pretreated transgenic 293 cells and FL cells in which REV3 was blocked by antisense RNA, and almost recovered to their spontaneous mutation levels. The spontaneous HPRT mutation was disappeared in REV3-disrupted cells, and induced mutation frequency at HPRT locus significantly decreased from 8.66 × 10^-6 in FL cells to 0.14 × 10^-6 in transgenic cells as well (P < 0.01).

CONCLUSION: The expression of the human REV3 can be upregulated at the transcriptional level in response to MNNG. The human REV3 gene plays a role not only in lesion-targeted DNA mutagenesis, but also in mutagenesis on undamaged DNA templates that is called untargeted mutation.

Hypersensitivity of DNA polymerase beta null mouse fibroblasts reflects accumulation of cytotoxic repair intermediates from site-specific alkyl DNA lesions

Monofunctional alkylating agents react with DNA by S(N)1 or S(N)2 mechanisms resulting in formation of a wide spectrum of cytotoxic base adducts. DNA polymerase beta (beta-pol) is required for efficient base excision repair of N-alkyl adducts, and we make use of the hypersensitivity of beta-pol null mouse fibroblasts to investigate such alkylating agents with a view towards understanding the DNA lesions responsible for the cellular phenotype. The inability of O(6)-benzylguanine to sensitize wild-type or beta-pol null cells to S(N)1-type methylating agents indicates that the observed hypersensitivity is not due to differential repair of cytotoxic O-alkyl adducts. Using a 3-methyladenine-specific agent and an inhibitor of such methylation, we find that inefficient repair of 3-methyladenine is not the reason for the hypersensitivity of beta-pol null cells to methylating agents, and further that 3-methyladenine is not the adduct primarily responsible for methyl methanesulfonate (MMS)- and methyl nitrosourea-induced cytotoxicity in wild-type cells. Relating the expected spectrum of DNA adducts and the relative sensitivity of cells to monofunctional alkylating agents, we propose that the hypersensitivity of beta-pol null cells reflects accumulation of cytotoxic repair intermediates, such as the 5'-deoxyribose phosphate group, following removal of 7-alkylguanine from DNA. In support of this conclusion, beta-pol null cells are also hypersensitive to the thymidine analog 5-hydroxymethyl-2'-deoxyuridine (hmdUrd). This agent is incorporated into cellular DNA and elicits cytotoxicity only when removed by glycosylase-initiated base excision repair. Consistent with the hypothesis that there is a common repair intermediate resulting in cytotoxicity following treatment with both types of agents, both MMS and hmdUrd-initiated cell death are preceded by a similar rapid concentration-dependent suppression of DNA synthesis and a later cell cycle arrest in G(0)/G(1) and G(2)M phases.

Altered expression of zinc finger proteins, ADAMs, and integrin-related proteins following treatment of cultured human cells with a low concentration of N-methyl-N'-nitro-N-nitrosoguanidine

Proteomic analysis is an important approach to characterize the proteome and study protein functions. It is also a powerful screening method for detecting unexpected alterations in protein expression that may be overlooked by conventional biochemical techniques. N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG) is an alkylating agent that can induce nontargeted mutagenesis in treated cells, although the mechanism remains unclear. Using an efficient proteomic method, we identified several cellular proteins that are responsive to low-concentration MNNG treatment in human FL cells. After MNNG treatment, whole cellular proteins were separated using two-dimensional gel electrophoresis and visualized by silver staining; the digitized images then were analyzed with 2D analysis software. More than 60 proteins showed significant changes in MNNG-treated cells compared to control cells (DMSO treatment). Thirty-one proteins only detected in MNNG-treated or control cells were subjected to in-gel digestion with trypsin and identified by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry using peptide mass fingerprinting. Eighteen of theses proteins have been identified, including several zinc finger proteins, two members of the ADAMs (a disintegrin and metalloprotease domain) family, and two integrins. Most of these proteins have unknown functions and their involvement in the cellular responses to alkylating agents have not been reported. Therefore, our findings may offer new insights into the mechanisms of low-concentration MNNG-induced nontargeted mutagenesis and these proteins may serve as new biomarkers for detecting exposure of human populations to environmental carcinogens.