Repair of O(6)-alkylguanine by alkyltransferases

Immunohistochemical analysis of O(6)-methylguanine-DNA methyltransferase in human melanoma in comparison with skin squamous cell carcinoma

Alkylating agents, often used for chemotherapy in patients with melanoma, can produce O(6)-alkylguanine (O(6)AG) which is related to tumor cell killing after treatment with alkylating agents. O(6)AG is effectively eliminated by O(6)-methylguanine-DNA methyltransferase (O(6)MGMT) and its level is correlative to the resistance to alkylating agents. However, little is known about the relationship of O(6)MGMT to the characteristics of melanoma. This study investigated the expression of O(6)MGMT in 12 melanomas and compared it with that in 11 skin squamous cell cancers (SCCs) immunohistochemically to evaluate the O(6)MGMT activity in melanoma and its clinical significance. All of the SCC samples had high O(6)MGMT expression, while the expression of O(6)MGMT in melanoma was diverse and 4 out of 12 samples had no or extremely low O(6)MGMT activity. Out of 6 lesions obtained from metastasis, 4 had a high O(6)MGMT activity. Two out of 3 cases with a low O(6)MGMT activity in each primary lesion did not show any evidence of metastasis or local recurrence. The evaluation of O(6)MGMT activity in melanoma may, therefore, be useful to determine the characteristics of tumor in each melanoma case. In addition, the present study implies the possibility of selective cancer chemotherapy for melanoma in the near future.

Hepatocarcinogenesis driven by GSNOR deficiency is prevented by iNOS inhibition

Hepatocellular carcinoma (HCC) is one of the most common and deadly human cancers and it remains poorly managed. Human HCC development is often associated both with elevated expression of inducible nitric oxide synthase (iNOS) and with genetic deletion of the major denitrosylase S-nitrosoglutathione reductase (GSNOR/ADH5). However, their causal involvement in human HCC is not established. In mice, GSNOR deficiency causes S-nitrosylation and depletion of the DNA repair protein O6-alkylguanine-DNA-alkyltransferase (AGT) and increases rates of both spontaneous and DEN carcinogen-induced HCC. Here, we report that administration of 1400W, a potent and highly selective inhibitor of iNOS, blocked AGT depletion and rescued the repair of mutagenic O6-ethyldeoxyguanosines following DEN challenge in livers of GSNOR-deficient (GSNOR(-/-)) mice. Notably, short-term iNOS inhibition following DEN treatment had little effect on carcinogenesis in wild-type mice, but was sufficient to reduce HCC multiplicity, maximal size, and burden in GSNOR(-/-) mice to levels comparable with wild-type controls. Furthermore, increased HCC susceptibility in GSNOR(-/-) mice was not associated with an increase in interleukin 6, tumor necrosis factor-α, oxidative stress, or hepatocellular proliferation. These results suggested that GSNOR deficiency linked to defective DNA damage repair likely acts at the tumor initiation stage to promote HCC carcinogenesis. Together, our findings provide the first proof of principle that HCC development in the context of uncontrolled nitrosative stress can be blocked by pharmacologic inhibition of iNOS, possibly providing an effective therapy for patients with HCC.

O6-Methylguanine DNA lesions induce an intra-S-phase arrest from which cells exit into apoptosis governed by early and late multi-pathway signaling network activation

The O(6)-methylguanine (O(6)MeG) DNA lesion is well known for its mutagenic, carcinogenic, and cytotoxic properties, and understanding how a cell processes such damage is of critical importance for improving current cancer therapy. Here we use human cells differing only in their O(6)MeG DNA methyltransferase (MGMT) or mismatch repair (MMR) status to explore the O(6)MeG/MMR-dependent molecular and cellular responses to treatment with the methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). We find that O(6)MeG triggers MMR-dependent cell cycle perturbations in both the first and second cell cycle post treatment. At lower levels of damage, we show that a transient arrest in the second S-phase precedes survival and progression into subsequent cell cycles. However, at higher levels of damage, arrest in the second S-phase coincides with a cessation of DNA replication followed by initiation of apoptotic cell death. Further, we show that entry into apoptotic cell death is specifically from S-phase of the second cell cycle. Finally, we demonstrate the key role of an O(6)MeG/MMR-dependent multi-pathway, multi-time-scale signaling network activation, led by early ATM, H2AX, CHK1, and p53 phosphorylation and followed by greatly amplified late phosphorylation of the early pathway nodes along with activation of the CHK2 kinase and the stress-activated JNK kinase.

S-nitrosoglutathione reductase deficiency increases mutagenesis from alkylation in mouse liver

In human hepatocellular carcinoma (HCC) and many other cancers, somatic point mutations are highly prevalent, yet the mechanisms critical in their generation remain poorly understood. S-nitrosoglutathione reductase (GSNOR), a key regulator of protein S-nitrosylation, is frequently deficient in human HCC. Targeted deletion of the GSNOR gene in mice can reduce the activity of the DNA repair protein O (6)-alkylguanine-DNA alkyltransferase (AGT) and promote both carcinogen-induced and spontaneous HCC. In this study, we report that following exposure to the environmental carcinogen diethylnitrosamine, the mutation frequency of a transgenic reporter in the liver of GSNOR-deficient mice (GSNOR(-/-)) is significantly higher than that in wild-type control. In wild-type mice, diethylnitrosamine treatment does not significantly increase the frequency of the transition from G:C to A:T, a mutation deriving from diethylnitrosamine-induced O (6)-ethylguanines that are normally repaired by AGT. In contrast, the frequency of this transition from diethylnitrosamine is increased ~20 times in GSNOR(-/-) mice. GSNOR deficiency also significantly increases the frequency of the transversion from A:T to T:A, a mutation not affected by AGT. GSNOR deficiency in our experiments does not significantly affect either the frequencies of the other diethylnitrosamine-induced point mutations or hepatocyte proliferation. Thus, GSNOR deficiency, through both AGT-dependent and AGT-independent pathways, significantly raises the rates of specific types of DNA mutations. Our results demonstrate a critical role for GSNOR in maintaining genomic integrity in mice and support the hypothesis that GSNOR deficiency is an important cause of the widespread mutations in human HCC.

Preparation of covalently linked complexes between DNA and O(6)-alkylguanine-DNA alkyltransferase using interstrand cross-linked DNA

O(6)-alkylguanine-DNA alkyltransferases (AGT) are responsible for the removal of alkylation at both the O(6) atom of guanine and O(4) atom of thymine. AGT homologues show vast substrate differences with respect to the size of the adduct and which alkylated atoms they can restore. The human AGT (hAGT) has poor capabilities for removal of methylation at the O(4) atom of thymidine, which is not the case in most homologues. No structural data are available to explain this poor hAGT repair. We prepared and characterized O(6)G-butylene-O(4)T (XLGT4) and O(6)G-heptylene-O(4)T (XLGT7) interstrand cross-linked (ICL) DNA as probes for hAGT and the Escherichia coli homologues, OGT and Ada-C, for the formation of DNA-AGT covalent complexes. XLGT7 reacted only with hAGT and did so with a cross-linking efficiency of 25%, while XLGT4 was inert to all AGT tested. The hAGT mediated repair of XLGT7 occurred slowly, on the order of hours as opposed to the repair of O(6)-methyl-2'-deoxyguanosine which requires seconds. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the repair reaction revealed the formation of a covalent complex with an observed migration in accordance with a DNA-AGT complex. The identity of this covalent complex, as determined by mass spectrometry, was composed of a heptamethylene bridge between the O(4) atom of thymidine (in an 11-mer DNA strand) to residue Cys145 of hAGT. This procedure can be applied to produce well-defined covalent complexes between AGT with DNA.

The nitrosated bile acid DNA lesion O6-carboxymethylguanine is a substrate for the human DNA repair protein O6-methylguanine-DNA methyltransferase

The consumption of red meat is a risk factor in human colorectal cancer (CRC). One hypothesis is that red meat facilitates the nitrosation of bile acid conjugates and amino acids, which rapidly convert to DNA-damaging carcinogens. Indeed, the toxic and mutagenic DNA adduct O⁶-carboxymethylguanine (O⁶-CMG) is frequently present in human DNA, increases in abundance in people with high levels of dietary red meat and may therefore be a causative factor in CRC. Previous reports suggested that O⁶-CMG is not a substrate for the human version of the DNA damage reversal protein O⁶-methylguanine-DNA methyltransferase (MGMT), which protects against the genotoxic effects of other O⁶-alkylguanine lesions by removing alkyl groups from the O⁶-position. We now show that synthetic oligodeoxyribonucleotides containing the known MGMT substrate O⁶-methylguanine (O⁶-MeG) or O⁶-CMG effectively inactivate MGMT in vitro (IC₅₀ 0.93 and 1.8 nM, respectively). Inactivation involves the removal of the O⁶-alkyl group and its transfer to the active-site cysteine residue of MGMT. O⁶-CMG is therefore an MGMT substrate, and hence MGMT is likely to be a protective factor in CRC under conditions where O⁶-CMG is a potential causative agent.

Chloroethylating and methylating dual function antineoplastic agents display superior cytotoxicity against repair proficient tumor cells

Two new agents based upon the structure of the clinically active prodrug laromustine were synthesized. These agents, 2-(2-chloroethyl)-N-methyl-1,2-bis(methylsulfonyl)-N-nitrosohydrazinecarboxamide (1) and N-(2-chloroethyl)-2-methyl-1,2-bis(methylsulfonyl)-N-nitrosohydrazinecarboxamide (2), were designed to retain the potent chloroethylating and DNA cross-linking functions of laromustine, and gain the ability to methylate DNA at the O-6 position of guanine, while lacking the carbamoylating activity of laromustine. The methylating arm was introduced with the intent of depleting the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT). Compound 1 is markedly more cytotoxic than laromustine in both AGT minus EMT6 mouse mammary carcinoma cells and high AGT expressing DU145 human prostate carcinoma cells. DNA cross-linking studies indicated that its cross-linking efficiency is nearly identical to its predicted active decomposition product, 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (90CE), which is also produced by laromustine. AGT ablation studies in DU145 cells demonstrated that 1 can efficiently deplete AGT. Studies assaying methanol and 2-chloroethanol production as a consequence of the methylation and chloroethylation of water by 1 and 2 confirmed their ability to function as methylating and chloroethylating agents and provided insights into the superior activity of 1.

Achaete-scute complex homolog-1 promotes DNA repair in the lung carcinogenesis through matrix metalloproteinase-7 and O(6)-methylguanine-DNA methyltransferase

Lung cancer is the leading cause of cancer-related deaths in the world. Achaete-scute complex homolog-1 (Ascl1) is a member of the basic helix-loop-helix (bHLH) transcription factor family that has multiple functions in the normal and neoplastic lung such as the regulation of neuroendocrine differentiation, prevention of apoptosis and promotion of tumor-initiating cells. We now show that Ascl1 directly regulates matrix metalloproteinase-7 (MMP-7) and O(6)-methylguanine-DNA methyltransferase (MGMT). Loss- and gain-of-function experiments in human bronchial epithelial and lung carcinoma cell lines revealed that Ascl1, MMP-7 and MGMT are able to protect cells from the tobacco-specific nitrosamine NNK-induced DNA damage and the alkylating agent cisplatin-induced apoptosis. We also examined the role of Ascl1 in NNK-induced lung tumorigenesis in vivo. Using transgenic mice which constitutively expressed human Ascl1 in airway lining cells, we found that there was a delay in lung tumorigenesis. We conclude that Ascl1 potentially enhances DNA repair through activation of MMP-7 and MGMT which may impact lung carcinogenesis and chemoresistance. The study has uncovered a novel and unexpected function of Ascl1 which will contribute to better understanding of lung carcinogenesis and the broad implications of transcription factors in tobacco-related carcinogenesis.

Hsp27 (HSPB1): a possible surrogate molecular marker for loss of heterozygosity (LOH) of chromosome 1p in oligodendrogliomas but not in astrocytomas

In oligodendrogliomas, 1p loss of heterozygosity (LOH) is a predictor of good prognosis and treatment response. In contrast, in uveal melanomas, LOH of chromosome 3 has been linked to poor prognosis and downregulation of Hsp27. In the present study, we have analyzed the expression of heat-shock proteins (Hsps) to characterize subtypes of gliomas and their histopathologic features and to correlate with other molecular markers including LOH of 1p. Biopsies from patients with primary gliomas (n = 65) were analyzed by immunohistochemistry, chromogenic in situ hybridization and fluorescent in situ hybridization and methylation-specific PCR (MSP). Elevated Hsp27 and total Hsp70 expression levels were associated with high-grade astrocytomas (p = 0.0001 and p = 0.01, respectively). In grade III oligodendrogliomas, the Hsp27 levels were significantly higher (p = 0.03). Low O6-methylguanine-DNA methyltransferase (MGMT) expression was associated with grade II astrocytomas. Elevated β-catenin expression was associated with grade III/IV astrocytomas (p = 0.003); p53 (+) tumors were more frequently found in grade III/IV astrocytomas (p = 0,001). LOH on 1p was associated with oligodendroglial tumours. In addition, a higher Hsp27 expression correlated with LOH of 1p (p = 0.017); this was also tested in two glioma cell lines. MSP was successful in only six samples. No significant correlations were found for the other markers. In conclusion, in oligodendroglial tumors, Hsp27 appeared as a surrogate marker of LOH of 1p which could also help to predict the disease prognosis. In gliomas, p53, Hsp27, Hsp70, MGMT, and β-catenin correlated with histopathological characteristics, suggesting that these markers could predict the disease outcome and the response to treatments.

Tumor-associated mutations in O⁶ -methylguanine DNA-methyltransferase (MGMT) reduce DNA repair functionality

MGMT is the primary vehicle for cellular removal of alkyl lesions from the O-6 position of guanine and the O-4 position of thymine. While key to the maintenance of genomic integrity, MGMT also removes damage induced by alkylating chemotherapies, inhibiting the efficacy of cancer treatment. Germline variants of human MGMT are well-characterized, but somatic variants found in tumors were, prior to this work, uncharacterized. We found that MGMT G132R, from a human esophageal tumor, and MGMT G156C, from a human colorectal cancer cell line, are unable to rescue methyltransferase-deficient Escherichia coli as well as wild type (WT) human MGMT after treatment with a methylating agent. Using pre-steady state kinetics, we biochemically characterized these variants as having a reduced rate constant. G132R binds DNA containing an O⁶ -methylguanine lesion half as tightly as WT MGMT, while G156C has a 40-fold decrease in binding affinity for the same damaged DNA versus WT. Mammalian cells expressing either G132R or G156C are more sensitive to methylating agents than mammalian cells expressing WT MGMT. G132R is slightly resistant to O⁶ -benzylguanine, an inhibitor of MGMT in clinical trials, while G156C is almost completely resistant to this inhibitor. The impared functionality of expressed variants G132R and G156C suggests that the presence of somatic variants of MGMT in a tumor could impact chemotherapeutic outcomes.

Synthesis of oligodeoxyribonucleotides containing a conformationally-locked anti analogue of O6-methyl-2'-deoxyguanosine and their recognition by MGMT and Atl1

We show that DNA containing a conformationally-locked anti analogue of O(6)-alkylguanine is a poor substrate for human O(6)-methylguanine-DNA methyltransferase (MGMT) and the alkyltransferase-like protein, Atl1. This highlights the requirement for the syn conformation and rationalises why certain O(6)-alkylguanines are poor MGMT substrates.

The identification of a novel gene, MAPO2, that is involved in the induction of apoptosis triggered by O⁶-methylguanine

O⁶-Methylguanine, one of alkylated DNA bases, is especially mutagenic. Cells containing this lesion are eliminated by induction of apoptosis, associated with the function of mismatch repair (MMR) proteins. A retrovirus-mediated gene-trap mutagenesis was used to isolate new genes related to the induction of apoptosis, triggered by the treatment with an alkylating agent, N-methyl-N-nitrosourea (MNU). This report describes the identification of a novel gene, MAPO2 (O⁶-methylguanine-induced apoptosis 2), which is originally annotated as C1orf201. The MAPO2 gene is conserved among a wide variety of multicellular organisms and encodes a protein containing characteristic PxPxxY repeats. To elucidate the function of the gene product in the apoptosis pathway, a human cell line derived from HeLa MR cells, in which the MAPO2 gene was stably knocked down by expressing specific miRNA, was constructed. The knockdown cells grew at the same rate as HeLa MR, thus indicating that MAPO2 played no role in the cellular growth. After exposure to MNU, HeLa MR cells and the knockdown cells underwent cell cycle arrest at G₂/M phase, however, the production of the sub-G₁ population in the knockdown cells was significantly suppressed in comparison to that in HeLa MR cells. Moreover, the activation of BAK and caspase-3, and depolarization of mitochondrial membrane, hallmarks for the induction of apoptosis, were also suppressed in the knockdown cells. These results suggest that the MAPO2 gene product might positively contribute to the induction of apoptosis triggered by O⁶-methylguanine.

Combining stereotactic radiosurgery and systemic therapy for brain metastases: a potential role for temozolomide

Brain metastases are unfortunately very common in the natural history of many solid tumors and remain a life-threatening condition, associated with a dismal prognosis, despite many clinical trials aimed at improving outcomes. Radiation therapy options for brain metastases include whole brain radiotherapy (WBRT) and stereotactic radiosurgery (SRS). SRS avoids the potential toxicities of WBRT and is associated with excellent local control (LC) rates. However, distant intracranial failure following SRS remains a problem, suggesting that untreated intracranial micrometastatic disease is responsible for failure of treatment. The oral alkylating agent temozolomide (TMZ), which has demonstrated efficacy in primary malignant central nervous system tumors such as glioblastoma, has been used in early phase trials in the treatment of established brain metastases. Although results of these studies in established, macroscopic metastatic disease have been modest at best, there is clinical and preclinical data to suggest that TMZ is more efficacious at treating and controlling clinically undetectable intracranial micrometastatic disease. We review the available data for the primary management of brain metastases with SRS, as well as the use of TMZ in treating established brain metastases and undetectable micrometastatic disease, and suggest the role for a clinical trial with the aims of treating macroscopically visible brain metastases with SRS combined with TMZ to address microscopic, undetectable disease.

O(6)-methylguanine-DNA methyltransferase in glioma therapy: promise and problems

Gliomas are the most frequent adult primary brain tumor, and are invariably fatal. The most common diagnosis glioblastoma multiforme (GBM) afflicts 12,500 new patients in the U.S. annually, and has a median survival of approximately one year when treated with the current standard of care. Alkylating agents have long been central in the chemotherapy of GBM and other gliomas. The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT), the principal human activity that removes cytotoxic O(6)-alkylguanine adducts from DNA, promotes resistance to anti-glioma alkylators, including temozolomide and BCNU, in GBM cell lines and xenografts. Moreover, MGMT expression assessed by immunohistochemistry, biochemical activity or promoter CpG methylation status is associated with the response of GBM to alkylator-based therapies, providing evidence that MGMT promotes clinical resistance to alkylating agents. These observations suggest a role for MGMT in directing adjuvant therapy of GBM and other gliomas. Promoter methylation status is the most clinically tractable measure of MGMT, and there is considerable enthusiasm for exploring its utility as a marker to assign therapy to individual patients. Here, we provide an overview of the biochemical, genetic and biological characteristics of MGMT as they relate to glioma therapy. We consider current methods to assess MGMT expression and discuss their utility as predictors of treatment response. Particular emphasis is given to promoter methylation status and the methodological and conceptual impediments that limit its use to direct treatment. We conclude by considering approaches that may improve the utility of MGMT methylation status in planning optimal therapies tailored to individual patients.

O(6)-methylguanine-DNA methyltransferase (MGMT): impact on cancer risk in response to tobacco smoke

Tobacco, smoked, snuffed and chewed, contains powerful mutagens and carcinogens. At least three of them, N-dimethylnitrosamine, N'-nitrosonornicotine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, attack DNA at the O(6)-position of guanine. The resulting O(6)-alkylguanine adducts are repaired by the suicide enzyme O(6)-methylguanine-DNA methyltransferase (MGMT), which is known to protect against the mutagenic, genotoxic and carcinogenic effects of monofunctional alkylating agents. While in rat liver MGMT was shown to be subject to regulation by genotoxic stress leading to adaptive changes in its activity, in humans evidence of adaptive modulation of MGMT levels is still lacking. Several polymorphisms are known, which are suspected to impact on the risk of developing cancer. In this review we focus on three questions: (a) Has tobacco consumption by smoking or chewing an impact on MGMT expression and MGMT promoter methylation in normal and tumor tissue? (b) Is there an association between MGMT polymorphisms and cancer risk and is this risk related to smoking? (c) Does MGMT protect against tobacco-associated cancer? There are several lines of evidence for an increase of MGMT activity in the normal tissue of smokers compared to non-smokers. Furthermore, in tumors developed in smokers a tendency towards an increase of MGMT expression was found. The data points to the possibility that agents in tobacco smoke are able to trigger upregulation of MGMT in normal and tumor tissue. For MGMT promoter methylation data is conflicting. There is some evidence for an association between MGMT polymorphisms and smoking-induced cancer risk. The key question whether or not MGMT protects against tobacco smoke-induced cancer is difficult to answer since prospective studies on smokers versus non-smokers are lacking and appropriate animal studies with MGMT transgenic mice exposed to the complex mixture of tobacco smoke have not been performed, which indicates the need for further explorations.

DNA methylation based biomarkers: practical considerations and applications

A biomarker is a molecular target analyzed in a qualitative or quantitative manner to detect and diagnose the presence of a disease, to predict the outcome and the response to a specific treatment allowing personalized tailoring of patient management. Biomarkers can belong to different types of biochemical molecules such as proteins, DNA, RNA or lipids, whereby protein biomarkers have been the most extensively studied and used, notably in blood-based protein quantification tests or immunohistochemistry. The rise of interest in epigenetic mechanisms has allowed the identification of a new type of biomarker, DNA methylation, which is of great potential for many applications. This stable and heritable covalent modification mostly affects cytosines in the context of a CpG dinucleotide in humans. It can be detected and quantified by a number of technologies including genome-wide screening methods as well as locus- or gene-specific high-resolution analysis in different types of samples such as frozen tissues and FFPE samples, but also in body fluids such as urine, plasma, and serum obtained through non-invasive procedures. In some cases, DNA methylation based biomarkers have proven to be more specific and sensitive than commonly used protein biomarkers, which could clearly justify their use in clinics. However, very few of them are at the moment used in clinics and even less commercial tests are currently available. The objective of this review is to discuss the advantages of DNA methylation as a biomarker, the practical considerations for their development, and their use in disease detection, prediction of outcome or treatment response, through multiple examples mainly focusing on cancer, but also to evoke their potential for complex diseases and prenatal diagnostics.

Methoxyamine sensitizes the resistant glioblastoma T98G cell line to the alkylating agent temozolomide

Chemoresistance represents a major obstacle to successful treatment for malignant glioma with temozolomide. N (7)-methyl-G and N (3)-methyl-A adducts comprise more than 80 % of DNA lesions induced by temozolomide and are processed by the base excision repair, suggesting that the cellular resistance could be caused, in part, by this efficient repair pathway, although few studies have focused on this subject. The aim of this study was to evaluate the cellular responses to temozolomide treatment associated with methoxyamine (blocker of base excision repair) in glioblastoma cell lines, in order to test the hypothesis that the blockage of base excision repair pathway might sensitize glioblastoma cells to temozolomide. For all the tested cell lines, only T98G showed significant differences between temozolomide and temozolomide plus methoxyamine treatment, observed by reduced survival rates, enhanced the levels of DNA damage, and induced an arrest at G2-phase. In addition, ~10 % of apoptotic cells (sub-G1 fraction) were observed at 48 h. Western blot analysis demonstrated that APE1 and FEN1 presented a slightly reduced expression levels under the combined treatment, probably due to AP sites blockade by methoxyamine, thus causing a minor requirement of base excision repair pathway downstream to the AP removal by APE1. On the other hand, PCNA expression in temozolomide plus methoxyamine-treated cells does not rule out the possibility that such alteration might be related to the blockage of cell cycle (G2-phase), as observed at 24 h of recovery time. The results obtained in the present study demonstrated the efficiency of methoxyamine to overcome glioblastoma resistance to temozolomide treatment.

Lesion-specific DNA-binding and repair activities of human O⁶-alkylguanine DNA alkyltransferase

Binding experiments with alkyl-transfer-active and -inactive mutants of human O⁶-alkylguanine DNA alkyltransferase (AGT) show that it forms an O⁶-methylguanine (6mG)-specific complex on duplex DNA that is distinct from non-specific assemblies previously studied. Specific complexes with duplex DNA have a 2:1 stoichiometry that is formed without accumulation of a 1:1 intermediate. This establishes a role for cooperative interactions in lesion binding. Similar specific complexes could not be detected with single-stranded DNA. The small difference between specific and non-specific binding affinities strongly limits the roles that specific binding can play in the lesion search process. Alkyl-transfer kinetics with a single-stranded substrate indicate that two or more AGT monomers participate in the rate-limiting step, showing for the first time a functional link between cooperative binding and the repair reaction. Alkyl-transfer kinetics with a duplex substrate suggest that two pathways contribute to the formation of the specific 6mG-complex; one at least first order in AGT, we interpret as direct lesion binding. The second, independent of [AGT], is likely to include AGT transfer from distal sites to the lesion in a relatively slow unimolecular step. We propose that transfer between distal and lesion sites is a critical step in the repair process.

Proteomic analysis of dimethoate-responsive proteins in the oyster (Saccostrea cucullata) gonad

INTRODUCTION

The organophosphorus pesticide dimethoate (DM) has been widely used in agriculture, and its extensive use could still have left many environmental problems.

METHODS

In the present study, the oyster (Saccostrea cucullata) was subjected to acute DM toxicity (2 mg/L), and gas chromatographic analysis revealed and quantified residues of DM in the oyster gonad.

RESULTS

Two-dimensional gel electrophoresis showed 12 differentially expressed proteins in the DM-exposed oyster gonad in comparison to the control. Among these 12 protein spots, nine were down-regulated, and three were up-regulated. Both matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry and database searching were utilized to identify these differential proteins, and revealed five proteins previously described as being related to DM toxicity. In addition, the levels of mRNA expression corresponding to these differential proteins were further proved in part by real-time PCR. The functions of these proteins were summarized as: carrying out energy metabolism, DNA repair, DNA transcriptional regulation, and oxidative protection. The remaining seven protein spots were of particular interest in terms of their responses to DM, which have seldom been reported.

CONCLUSION

These data might point to a number of novel and significant biomarkers for evaluating the contamination levels of DM and provide useful insight into the mechanisms of DM toxicity in vivo.

Fluorescence resonance energy transfer-based technologies in the study of protein-protein interactions at the cell surface

Understanding of the molecular mechanisms of protein-protein interactions (PPIs) at the cell surface of living cells is fundamental to comprehend the functional meaning of a large number of cellular processes. Here we discuss how new methodological strategies derived from non-invasive fluorescence-based approaches (i.e. fluorescence resonance energy transfer, FRET) have been successfully developed to characterize plasma membrane PPIs. Importantly, these technologies alone - or in concert with complementary methods (i.e. SNAP-tag/TR-FRET, TIRF/FRET) - can become extremely powerful approaches for visualizing cell surface PPIs, even between more than two proteins and also in native tissues. Interestingly, these methods would also be relevant in drug discovery in order to develop new high-throughput screening approaches or to identify new therapeutic targets. Accordingly, herein we provide a thorough assessment on all biotechnological aspects, including strengths and weaknesses, of these fluorescence-based methodologies when applied in the study of PPIs occurring at the cell surface of living cells.

Analysis of protein turnover by quantitative SNAP-based pulse-chase imaging

Assessment of protein dynamics in living cells is crucial for understanding their biological properties and functions. The SNAP-tag, a self labeling suicide enzyme, presents a tool with unique features that can be adopted for determining protein dynamics in living cells. Here we present detailed protocols for the use of SNAP in fluorescent pulse-chase and quench-chase-pulse experiments. These time-slicing methods provide powerful tools to assay and quantify the fate and turnover rate of proteins of different ages. We cover advantages and pitfalls of SNAP-tagging in fixed- and live-cell studies and evaluate the recently developed fast-acting SNAPf variant. In addition, to facilitate the analysis of protein turnover datasets, we present an automated algorithm for spot recognition and quantification.

Protein oriented ligation on nanoparticles exploiting O6-alkylguanine-DNA transferase (SNAP) genetically encoded fusion

A bimodular genetic fusion comprising a delivery module (scFv) and a capture module (SNAP) is proposed as a novel strategy for the site-specific covalent conjugation of targeting peptides to nanoparticles. An scFv mutant selective for HER2 tumor antigen is chosen as the targeting ligand. SNAP-scFv is immobilized on magnetofluorescent nanoparticles and its targeting efficiency against HER2-positive cells is assessed by flow cytometry and immunofluorescence.

[Cytotoxicity of lysomustine and its isomers, and their potential use for selection of cells]

N epsilon-Nitroso-N epsilon- [N'-(2-chloroethyl)carbamoyl]-L-lysine (I) and N epsilon- [N'-(2-chloroethyl)-N'-nitrosocarbamoyl]-L-lysine (II), the isomers being the constituents of antitumor agent Lysomustine, were obtained by RFHPLC. The study of cytotoxicity of the above compounds against K562 cells showed that the lesions induced by isomer (II) produce a significant cytotoxic effect but can be efficiently repaired by the action of MGMT (O6-methylaguanine DNA methyltransferase). Under similar conditions, the lesions induced by isomer (I) produce substantially smaller effect but are weakly if at all repairable by MGMT. The effects of a clinically approved agent Lysomustine, which is the mixture of isomers (I) and (II), are similar to those of isomer (II). The results obtained point to a different chemical nature of DNA lesions induced by two Lysomustine isomers. Our data indicate that Lysomustine and its isomer (II) can be used for in vitro selection of cells expressing MGMT.

Prognostic value of three different methods of MGMT promoter methylation analysis in a prospective trial on newly diagnosed glioblastoma

Hypermethylation in the promoter region of the MGMT gene encoding the DNA repair protein O⁶-methylguanine-DNA methyltransferase is among the most important prognostic factors for patients with glioblastoma and predicts response to treatment with alkylating agents like temozolomide. Hence, the MGMT status is widely determined in most clinical trials and frequently requested in routine diagnostics of glioblastoma. Since various different techniques are available for MGMT promoter methylation analysis, a generally accepted consensus as to the most suitable diagnostic method remains an unmet need. Here, we assessed methylation-specific polymerase chain reaction (MSP) as a qualitative and semi-quantitative method, pyrosequencing (PSQ) as a quantitative method, and methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) as a semi-quantitative method in a series of 35 formalin-fixed, paraffin-embedded glioblastoma tissues derived from patients treated in a prospective clinical phase II trial that tested up-front chemoradiotherapy with dose-intensified temozolomide (UKT-05). Our goal was to determine which of these three diagnostic methods provides the most accurate prediction of progression-free survival (PFS). The MGMT promoter methylation status was assessable by each method in almost all cases (n = 33/35 for MSP; n = 35/35 for PSQ; n = 34/35 for MS-MLPA). We were able to calculate significant cut-points for the continuous methylation signals at each CpG site analysed by PSQ (range, 11.5 to 44.9%) and at one CpG site assessed by MS-MLPA (3.6%) indicating that a dichotomisation of continuous methylation data as a prerequisite for comparative survival analyses is feasible. Our results show that, unlike MS-MLPA, MSP and PSQ provide a significant improvement of predicting PFS compared with established clinical prognostic factors alone (likelihood ratio tests: p<0.001). Conclusively, taking into consideration prognostic value, cost effectiveness and ease of use, we recommend pyrosequencing for analyses of MGMT promoter methylation in high-throughput settings and MSP for clinical routine diagnostics with low sample numbers.

Human O6 -methylguanine-DNA methyltransferase containing C145A does not prevent hepatocellular carcinoma in C3HeB/FeJ transgenic mice

The prevalence of hepatocellular carcinoma (HCC) was diminished from 60% to 18% at 15 months of age in C3HeB/FeJ male transgenic mice expressing hMGMT in our previous studies. To directly test if the methyltransferase activity is required for diminished tumor prevalence, two separate lines of transgenic mice bearing an enzymatically inactive form of hMGMT were used. In these lines, cysteine 145 was substituted with alanine (C145A). Expression of the hMGMT C145A transgene in liver was demonstrated by Northern blots and Western blots. Immunohistochemistry revealed predominantly nuclear localization of the hMGMT C145A protein. hMGMT C145A transgenic mice were crossed with lacI transgenic mice to assess mutant frequencies in the presence of the mutant protein. Mutant frequencies were similar among livers of lacI × hMGMT C145A bi-transgenic mice and lacI × wild-type (WT) mice. DNA sequence analysis of recovered lacI mutants revealed similar mutation spectra for hMGMT C145A and WT mice. The prevalence of HCC was also similar for the two tested lines of hMGMT C145A mice, 45% and 48% prevalence with median tumor sizes of 11 and 8 mm, and WT mice, 40% prevalence and median tumor size of 10 mm. These results provide evidence that residue C145 in hMGMT is required to reduce the prevalence of HCC in C3HeB/FeJ mice transgenic for hMGMT.

Cigarette smoke-induced failure of apoptosis resulting in enhanced neoplastic transformation in human bronchial epithelial cells

The lack of apoptotic pathways may lead to undesirable cell survival and proliferation, which are recognized hallmarks of cancer. It is well known that exposure to cigarette smoke induces DNA lesions in pulmonary cells. At present, it is not fully elucidated whether these lesions are repaired to restore normal functions or induce apoptosis. In order to examine the role of apoptosis in smoking-induced effects, immortalized human bronchial epithelial cells (BEAS-2B) were exposed to cigarette smoke and examined for parameters associated with apoptosis and neoplastic transformation. Our results indicated a significant reduction in apoptosis and enhanced neoplastic transformation and decreased mitochondrial membrane potential Δψm of mitochondria compared to control cells. Time-course experiments revealed increased aberrant methylation of CpG islands of RAS-associated domain family protein 1A (RASSF1A) and O (6)-methylguanine-DNA-methyltransferase (MGMT). The activities were downregulated and repair of DNA adducts was inhibited. Our observations suggested that although cigarette smoke-induced damage in BEAS-2B cells after chronic exposure is not necessarily lethal, as evidenced by cell viability, the protein expression levels of caspase-3 showed a decrease in the S20 passage (metaphase) but subsequently increased from S30 to S40 (anaphase). Survivin expression was significantly changed in S5 cells, and this rise was maintained until S40. Our data suggest that the potency of cigarettes as carcinogens may be due to their ability to induce aberrant gene expression and failure to trigger apoptosis leads to subsequent neoplastic transformation.

Alkylpurine-DNA-N-glycosylase confers resistance to temozolomide in xenograft models of glioblastoma multiforme and is associated with poor survival in patients

Glioblastoma multiforme (GBM) is the most common and lethal of all gliomas. The current standard of care includes surgery followed by concomitant radiation and chemotherapy with the DNA alkylating agent temozolomide (TMZ). O⁶-methylguanine-DNA methyltransferase (MGMT) repairs the most cytotoxic of lesions generated by TMZ, O⁶-methylguanine. Methylation of the MGMT promoter in GBM correlates with increased therapeutic sensitivity to alkylating agent therapy. However, several aspects of TMZ sensitivity are not explained by MGMT promoter methylation. Here, we investigated our hypothesis that the base excision repair enzyme alkylpurine-DNA-N-glycosylase (APNG), which repairs the cytotoxic lesions N³-methyladenine and N⁷-methylguanine, may contribute to TMZ resistance. Silencing of APNG in established and primary TMZ-resistant GBM cell lines endogenously expressing MGMT and APNG attenuated repair of TMZ-induced DNA damage and enhanced apoptosis. Reintroducing expression of APNG in TMZ-sensitive GBM lines conferred resistance to TMZ in vitro and in orthotopic xenograft mouse models. In addition, resistance was enhanced with coexpression of MGMT. Evaluation of APNG protein levels in several clinical datasets demonstrated that in patients, high nuclear APNG expression correlated with poorer overall survival compared with patients lacking APNG expression. Loss of APNG expression in a subset of patients was also associated with increased APNG promoter methylation. Collectively, our data demonstrate that APNG contributes to TMZ resistance in GBM and may be useful in the diagnosis and treatment of the disease.

Mass spectrometry based approach to study the kinetics of O6-alkylguanine DNA alkyltransferase-mediated repair of O6-pyridyloxobutyl-2'-deoxyguanosine adducts in DNA

O(6)-POB-dG (O(6)-[4-oxo-4-(3-pyridyl)but-1-yl]deoxyguanosine) are promutagenic nucleobase adducts that arise from DNA alkylation by metabolically activated tobacco-specific nitrosamines such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosonicotine (NNN). If not repaired, O(6)-POB-dG adducts cause mispairing during DNA replication, leading to G → A and G → T mutations. A specialized DNA repair protein, O(6)-alkylguanine-DNA-alkyltransferase (AGT), transfers the POB group from O(6)-POB-dG in DNA to a cysteine residue within the protein (Cys145), thus restoring normal guanine and preventing mutagenesis. The rates of AGT-mediated repair of O(6)-POB-dG may be affected by local DNA sequence context, potentially leading to adduct accumulation and increased mutagenesis at specific sites within the genome. In the present work, isotope dilution high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ESI(+)-MS/MS)-based methodology was developed to investigate the influence of DNA sequence on the kinetics of AGT-mediated repair of O(6)-POB-dG adducts. In our approach, synthetic DNA duplexes containing O(6)-POB-dG at a specified site are incubated with recombinant human AGT protein for defined periods of time. Following spiking with D(4)-O(6)-POB-dG internal standard and mild acid hydrolysis to release O(6)-POB-guanine (O(6)-POB-G) and D(4)-O(6)-POB-guanine (D(4)-O(6)-POB-G), samples are purified by solid phase extraction (SPE), and O(6)-POB-G adducts remaining in DNA are quantified by capillary HPLC-ESI(+)-MS/MS. The new method was validated by analyzing mixtures containing known amounts of O(6)-POB-G-containig DNA and the corresponding unmodified DNA duplexes and by examining the kinetics of alkyl transfer in the presence of increasing amounts of AGT protein. The disappearance of O(6)-POB-dG from DNA was accompanied by pyridyloxobutylation of AGT Cys-145 as determined by HPLC-ESI(+)-MS/MS of tryptic peptides. The applicability of the new approach was shown by determining the second order kinetics of AGT-mediated repair of O(6)-POB-dG adducts placed within a DNA duplex representing modified rat H-ras sequence (5'-AATAGTATCT[O(6)-POB-G]GAGCC-3') opposite either C or T. Faster rates of alkyl transfer were observed when O(6)-POB-dG was paired with T rather than with C (k = 1.74 × 10(6) M(-1) s(-1) vs 1.17 × 10(6) M(-1) s(-1)).

Predictive value of MGMT, hMLH1, hMSH2 and BRCA1 protein expression for pathological complete response to neoadjuvant chemotherapy in basal-like breast cancer patients

PURPOSE

To evaluate the importance of biological markers to predict pathologic complete response (pCR) to neoadjuvant chemotherapy (NACT) in patients with locally advanced basal-like breast cancers (BLBCs).

PATIENTS AND METHODS

Thirty-two BLBC patients receiving NACT with an anthracycline-based regimen plus taxane were included in this study. The immunoreactivities of MGMT, MLH1, MSH2 and BRCA1 before and after NACT were evaluated.

RESULTS

A pCR was obtained in 10 of 32 cases (31%). Cancer-related (P = 0.013) and disease-free (P = 0.023) survival rates were significantly higher in the pCR group than in the non-pCR group. In biopsy samples before NACT, attenuated expression of MGMT, MLH1, MSH2 and BRCA1 was observed in 12/32 (38%), 0/32 (0%), 5/32 (16%) and 28/32 (88%) cases, respectively. On evaluation of pCR, patients' characteristics (patients' age, menopausal status, or clinical and pathological stages) and immunohistochemical patterns, attenuated expression of MGMT was only found to be significantly predictive of a pCR (P = 0.018). Paired biopsy sample before NACT and a surgical tumor material after NACT were available for 19 cases of non-pCR. In these cases, decrease in expression during NACT were more frequently observed for MGMT as compared to MLH1, MSH2 or BRCA1 (P = 0.021).

CONCLUSIONS

MGMT status is a predictive factor for pCR with neoadjuvant anthracycline-based plus taxane combination chemotherapy, which may be helpful in the selection of appropriate NACT for Japanese patients with BLBC.

Protein-based tumor molecular imaging probes

Molecular imaging is an emerging discipline which plays critical roles in diagnosis and therapeutics. It visualizes and quantifies markers that are aberrantly expressed during the disease origin and development. Protein molecules remain to be one major class of imaging probes, and the option has been widely diversified due to the recent advances in protein engineering techniques. Antibodies are part of the immunosystem which interact with target antigens with high specificity and affinity. They have long been investigated as imaging probes and were coupled with imaging motifs such as radioisotopes for that purpose. However, the relatively large size of antibodies leads to a half-life that is too long for common imaging purposes. Besides, it may also cause a poor tissue penetration rate and thus compromise some medical applications. It is under this context that various engineered protein probes, essentially antibody fragments, protein scaffolds, and natural ligands have been developed. Compared to intact antibodies, they possess more compact size, shorter clearance time, and better tumor penetration. One major challenge of using protein probes in molecular imaging is the affected biological activity resulted from random labeling. Site-specific modification, however, allows conjugation happening in a stoichiometric fashion with little perturbation of protein activity. The present review will discuss protein-based probes with focus on their application and related site-specific conjugation strategies in tumor imaging.

6-Carboxyfluorescein and structurally similar molecules inhibit DNA binding and repair by O⁶-alkylguanine DNA alkyltransferase

Human O⁶-alkylguanine-DNA alkyltransferase (AGT) repairs mutagenic O⁶-alkylguanine and O⁴-alkylthymine adducts in single-stranded and duplex DNAs. These activities protect normal cells and tumor cells against drugs that alkylate DNA; drugs that inactivate AGT are under test as chemotherapeutic enhancers. In studies using 6-carboxyfluorescein (FAM)-labeled DNAs, AGT reduced the fluorescence intensity by ∼40% at binding saturation, whether the FAM was located at the 5' or the 3' end of the DNA. AGT protected residual fluorescence from quenching, indicating a solute-inaccessible binding site for FAM. Sedimentation equilibrium analyses showed that saturating AGT-stoichiometries were higher with FAM-labeled DNAs than with unlabeled DNAs, suggesting that the FAM provides a protein binding site that is not present in unlabeled DNAs. Additional fluorescence and sedimentation measurements showed that AGT forms a 1:1 complex with free FAM. Active site benzylation experiments and docking calculations support models in which the primary binding site is located in or near the active site of the enzyme. Electrophoretic analyses show that FAM inhibits DNA binding (IC₅₀∼76μM) and repair of DNA containing an O⁶-methylguanine residue (IC₅₀∼63μM). Similar results were obtained with other polycyclic aromatic compounds. These observations demonstrate the existence of a new class of non-covalent AGT-inhibitors. After optimization for binding-affinity, members of this class might be useful in cancer chemotherapy.

Prognostic significance of IDH-1 and MGMT in patients with glioblastoma: one step forward, and one step back?

A group of 160 patients with primary glioblastoma treated with radiotherapy and temozolomide was analyzed for the impact of O6-methly-guanly-methyl-transferase (MGMT)-promoter methylation as well as isocitrate dehydrogenase (IDH)1-mutational status. Unexpectedly, overall survival or progression-free survival were not longer in the group with methylated MGMT-promoter as compared to patients without that methylation. IDH-1 mutations were significantly associated with increased overall survival.

Inhibition of DNA repair with MGMT pseudosubstrates: phase I study of lomeguatrib in combination with dacarbazine in patients with advanced melanoma and other solid tumours

Background:

The DNA repair protein O⁶-methylguanine-DNA methyltransferase (MGMT) reverses the O⁶-methylguanine (O⁶-meG) lesion induced by dacarbazine. Depletion of MGMT can be achieved using O⁶-meG pseudosubstrates. Herein, we report the first phase I experience of the novel O⁶-meG pseudosubstrate lomeguatrib, combined with dacarbazine.

Methods:

This is a phase I dose-escalation study to determine the maximum tolerated dose and recommended phase II dose (RP2D) of lomeguatrib combined with a single dose of dacarbazine on a 21-day schedule.

Results:

The vast majority of the 41 patients enrolled had metastatic melanoma (36/41) and most had no previous chemotherapy (30/41). The most frequent non-hematological adverse events (AEs) were nausea (52%), and fatigue (42%). The most frequent AEs of grade 3–4 severity were neutropaenia (42%), leukopaenia (17%), and thrombocytopaenia (12%). Only 1 patient had a partial response and 10 patients had stable disease.

Conclusion:

The RP2D of lomeguatrib was 40 mg orally twice daily for 10 days combined with 400 mg m⁻² of dacarbazine IV on day 2. Oral administration of lomeguatrib substantially increases the haematological toxicity of dacarbazine consistent with experience with other O⁶-meG pseudosubstrates.

Sputum-based molecular biomarkers for the early detection of lung cancer: limitations and promise

Lung cancer is the leading cause of cancer deaths, with an overall survival of 15% at five years. Biomarkers that can sensitively and specifically detect lung cancer at early stage are crucial for improving this poor survival rate. Sputum has been the target for the discovery of non-invasive biomarkers for lung cancer because it contains airway epithelial cells, and molecular alterations identified in sputum are most likely to reflect tumor-associated changes or field cancerization caused by smoking in the lung. Sputum-based molecular biomarkers include morphology, allelic imbalance, promoter hypermethylation, gene mutations and, recently, differential miRNA expression. To improve the sensitivity and reproducibility of sputum-based biomarkers, we recommend standardization of processing protocols, bronchial epithelial cell enrichment, and identification of field cancerization biomarkers.

O(6)-Methylguanine-DNA methyltransferase (MGMT) in normal tissues and tumors: enzyme activity, promoter methylation and immunohistochemistry

O(6)-Methylguanine-DNA methyltransferase (MGMT) is a suicide enzyme that repairs the pre-mutagenic, pre-carcinogenic and pre-toxic DNA damage O(6)-methylguanine. It also repairs larger adducts on the O(6)-position of guanine, such as O(6)-[4-oxo-4-(3-pyridyl)butyl]guanine and O(6)-chloroethylguanine. These adducts are formed in response to alkylating environmental pollutants, tobacco-specific carcinogens and methylating (procarbazine, dacarbazine, streptozotocine, and temozolomide) as well as chloroethylating (lomustine, nimustine, carmustine, and fotemustine) anticancer drugs. MGMT is therefore a key node in the defense against commonly found carcinogens, and a marker of resistance of normal and cancer cells exposed to alkylating therapeutics. MGMT also likely protects against therapy-related tumor formation caused by these highly mutagenic drugs. Since the amount of MGMT determines the level of repair of toxic DNA alkylation adducts, the MGMT expression level provides important information as to cancer susceptibility and the success of therapy. In this article, we describe the methods employed for detecting MGMT and review the literature with special focus on MGMT activity in normal and neoplastic tissues. The available data show that the expression of MGMT varies greatly in normal tissues and in some cases this has been related to cancer predisposition. MGMT silencing in tumors is mainly regulated epigenetically and in brain tumors this correlates with a better therapeutic response. Conversely, up-regulation of MGMT during cancer treatment limits the therapeutic response. In malignant melanoma, MGMT is not related to the therapeutic response, which is due to other mechanisms of inherent drug resistance. For most cancers, studies that relate MGMT activity to therapeutic outcome following O(6)-alkylating drugs are still lacking.

Mutagenicity associated with O6-methylguanine-DNA damage and mechanism of nucleotide flipping by AGT during repair

Methylated guanine damage at O6 position (i.e. O6MG) is dangerous due to its mutagenic and carcinogenic character that often gives rise to G:C-A:T mutation. However, the reason for this mutagenicity is not known precisely and has been a matter of controversy. Further, although it is known that O6-alkylguanine-DNA alkyltransferase (AGT) repairs O6MG paired with cytosine in DNA, the complete mechanism of target recognition and repair is not known completely. All these aspects of DNA damage and repair have been addressed here by employing high level density functional theory in gas phase and aqueous medium. It is found that the actual cause of O6MG mediated mutation may arise due to the fact that DNA polymerases incorporate thymine opposite to O6MG, misreading the resulting O6MG:T complex as an A:T base pair due to their analogous binding energies and structural alignments. It is further revealed that AGT mediated nucleotide flipping occurs in two successive steps. The intercalation of the finger residue Arg128 into the DNA double helix and its interaction with the O6MG:C base pair followed by rotation of the O6MG nucleotide are found to be crucial for the damage recognition and nucleotide flipping.

Chemical tags: applications in live cell fluorescence imaging

Technologies to visualize cellular structures and dynamics enable cell biologists to gain insight into complex biological processes. Currently, fluorescent proteins are used routinely to investigate the behavior of proteins in live cells. Chemical biology techniques for selective labeling of proteins with fluorescent labels have become an attractive alternative to fluorescent protein labeling. In the last ten years the progress in the development of chemical tagging methods have been substantial offering a broad palette of applications for live cell fluorescent microscopy. Several methods for protein labeling have been established, using protein tags, peptide tags and enzyme mediated tagging. This review focuses on the different strategies to achieve the attachment of fluorophores to proteins in live cells and cast light on the advantages and disadvantages of each individual method. Selected experiments in which chemical tags have been successfully applied to live cell imaging will be discussed and evaluated.

Regulation of DNA repair by S-nitrosylation

BACKGROUND

Expression of the inducible nitric oxide synthase (iNOS) is commonly induced in inflammation, an important risk factor of cancer. Nitric oxide (NO) and related reactive nitrogen species can directly cause DNA damage to increase DNA mutation. They can also indirectly affect DNA mutation by modulation of DNA repair proteins, in particular through protein S-nitrosylation, a key regulatory mechanism of NO.

SCOPE OF REVIEW

Here we review protein targets, molecular mechanisms, and potential roles of NO in the regulation of DNA repair, with a focus on S-nitrosylation of DNA repair proteins by endogenous NO synthase activity.

MAJOR CONCLUSIONS

Recent studies have identified a number of key DNA repair proteins as targets of S-nitrosylation, including O(6)-alkylguanine-DNA-alkyltransferase (AGT), 8-oxoguanine glycosylase, apurinic-apyrimidinic endonuclease 1, and DNA-dependent protein kinase catalytic subunit. S-nitrosylation has been shown to modulate the activity, stability, and cellular localization of DNA repair proteins. The level of protein S-nitrosylation depends both on NO synthesis by NO synthases and on denitrosylation by a major denitrosylase, S-nitrosoglutathione reductase (GSNOR). Dysregulated S-nitrosylation of AGT due to GSNOR deficiency inactivates AGT-dependent DNA repair and appears to contribute critically to hepatocarcinogenesis.

GENERAL SIGNIFICANCE

Studies on the S-nitrosylation of DNA repair proteins have started to reveal molecular mechanisms for the contribution of inflammation to mutagenesis and carcinogenesis. The modulation of protein S-nitrosylation to affect the activity of DNA repair proteins may provide a therapeutic strategy to prevent DNA damage and mutation frequently associated with chronic inflammation and to sensitize cancer cells to DNA-damaging drugs. This article is part of a Special Issue entitled Regulation of Cellular Processes by S-nitrosylation.

Multifaceted roles of alkyltransferase and related proteins in DNA repair, DNA damage, resistance to chemotherapy, and research tools

O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a widely distributed, unique DNA repair protein that acts as a single agent to directly remove alkyl groups located on the O(6)-position of guanine from DNA restoring the DNA in one step. The protein acts only once, and its alkylated form is degraded rapidly. It is a major factor in counteracting the mutagenic, carcinogenic, and cytotoxic effects of agents that form such adducts including N-nitroso-compounds and a number of cancer chemotherapeutics. This review describes the structure, function, and mechanism of action of AGTs and of a family of related alkyltransferase-like proteins, which do not act alone to repair O(6)-alkylguanines in DNA but link repair to other pathways. The paradoxical ability of AGTs to stimulate the DNA-damaging ability of dihaloalkanes and other bis-electrophiles via the formation of AGT-DNA cross-links is also described. Other important properties of AGTs include the ability to provide resistance to cancer therapeutic alkylating agents, and the availability of AGT inhibitors such as O(6)-benzylguanine that might overcome this resistance is discussed. Finally, the properties of fusion proteins in which AGT sequences are linked to other proteins are outlined. Such proteins occur naturally, and synthetic variants engineered to react specifically with derivatives of O(6)-benzylguanine are the basis of a valuable research technique for tagging proteins with specific reagents.

Temozolomide induced c-Myc-mediated apoptosis via Akt signalling in MGMT expressing glioblastoma cells

PURPOSE

We investigated the molecular mechanisms underlying the cytotoxic effect of Temozolomide (TMZ) in both O(6)-methylguanine-DNA methyl transferase (MGMT) depleted as well as undepleted glioblastoma cell lines. Since TMZ is used in clinics in combination with radiotherapy, we also studied the effects of TMZ in combination with ionising radiation (IR).

METHODS

Cell colony-forming ability was measured using a clonogenic assay. Cell cycle analysis and apoptosis were evaluated by Flow Cytometry (FCM). Proteins involved in cell cycle control were detected by Western blot and co-immunoprecipitation assays.

RESULTS

Our data showed that TMZ, independent of MGMT expression, inhibited glioblastoma cell growth via an irreversible G(2) block in MGMT depleted cells or the induction of apoptosis in MGMT normal expressing cells. When TMZ was administered in combination with IR, apoptosis was greater than observed with either agent separately. This TMZ-induced apoptosis in the MGMT expressing cells occurred through Akt/Glycogen-Synthase-Kinase-3ß (GSK3ß) signalling and was mediated by Myelocytomatosis (c-Myc) oncoprotein. Indeed, TMZ phosphorylated/activated Akt led to phosphorylation/inactivation of GSK3ß which resulted in the stabilisation of c-Myc protein and subsequent modulation of the c-Myc target genes involved in the apoptotic processes.

CONCLUSION

C-Myc expression could be considered a good indicator of TMZ effectiveness.

Targeted deletion of GSNOR in hepatocytes of mice causes nitrosative inactivation of O6-alkylguanine-DNA alkyltransferase and increased sensitivity to genotoxic diethylnitrosamine

S-nitrosoglutathione reductase (GSNOR), a ubiquitously expressed protein central to the control of protein S-nitrosylation, plays critical roles in many biological systems. We showed recently that GSNOR is often deficient in human hepatocellular carcinoma and that germ line deletion of the GSNOR gene in mice causes hepatocellular carcinoma through S-nitrosylation and proteasomal degradation of the key DNA repair protein O⁶-alkylguanine-DNA alkyltransferase (AGT). We report here the generation of mice with targeted deletion of GSNOR in hepatocytes or in cells of the hematopoietic lineage. We found that during inflammatory responses induced by intraperitoneal injection of diethylnitrosamine (DEN) or lipopolysaccharide, the amount of liver AGT was not changed in mice with GSNOR deletion in hematopoietic cells but was almost completely depleted in mice with GSNOR deletion in hepatocytes. In livers of DEN-challenged mice, GSNOR deletion in hepatocytes but not hematopoietic cells resulted in an increase in phosphorylated histone H2AX, a well-established marker of DNA double-strand breaks. Hepatocyte deletion of GSNOR increased DEN-induced mortality, which was abolished in mice deficient in both GSNOR and inducible nitric oxide synthase. Thus, protection of AGT and resistance to nitrosamine-induced genotoxicity critically depends on GSNOR in hepatocytes. In addition, our findings suggest that nitrosative inactivation of AGT from GSNOR deficiency might sensitize cancerous cells to alkylating drugs in cancer treatment.

Protective effect of chokeberry on chemical-induced oxidative stress in rat

Male Wistar rats were treated with chokeberry juice per os, 10 mL/kg/day, for 28 days and a single intraperitoneal (i.p.) dose of N-nitrosodiethylamine (NDEA), 150 mg/kg, or carbon tetrachloride (CCl(4)), 2 ml/kg. The level of hepatic microsomal lipid peroxidation, expressed as thiobarbituric acid reactive substances (TBARS), was increased in animals dosed with NDEA and CCl(4). Juice pretreatment resulted in a significant decrease in TBARS by 53% and 92%, respectively. In rats administered juice alone, 50% decrease in TBARS was noted. The activities of all antioxidant enzymes were decreased in the liver of rats administered either toxicant by 29%-52% as compared to controls. Juice pretreatment resulted in an increase in the activity of catalase, glutathione peroxidase and glutathione reductase by 117%, 56% and 44%, respectively, only in rats challenged with NDEA. Although no response of plasma protein carbonyls to both toxicants was observed, the pretreatment with juice caused a 55% decrease of this parameter in CCl(4)-dosed rats. DNA damage in blood leukocytes induced by either toxicant was slightly reduced, by 24%, in the rats pretreated with juice and administered NDEA. The results of the study showed that pretreatment with chokeberry juice confers some protection against chemical-induced oxidative stress.

Mutations that probe the cooperative assembly of O⁶-alkylguanine-DNA alkyltransferase complexes

O(6)-Alkylguanine-DNA alkyltransferase (AGT) repairs mutagenic O(6)-alkylguanine and O(4)-alkylthymine adducts present in DNA that has been exposed to alkylating agents. AGT binds DNA cooperatively, and models of cooperative complexes predict that residues 1-7 of one protein molecule and residues 163-169 of a neighboring protein are closely juxtaposed. To test these models, we used directed mutagenesis to substitute triplets of alanine for triplets of native residues across these two sequences. Six of eight designed mutants expressed AGT at detectable levels. All mutant AGTs that were expressed were folded compactly, bound DNA with stoichiometries equivalent to that of the wild-type protein, and were able to protect Escherichia coli to varying degrees from the potent alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). All mutations attenuated DNA binding cooperativity, but unexpectedly, they also reduced the affinity of AGT for DNA. This suggests that the protein-protein and protein-DNA interactions of AGT are strongly coupled. When normalized for differences in AGT expression, cells expressing mutants KDC(3-5)-AAA, DCE(4-6)-AAA, and KEW(165-167)-AAA were significantly more susceptible to MNNG than wild-type cells. This is the first evidence, to the best of our knowledge, of a role for residues at the protein-protein interface and, by implication, cooperative protein-protein interactions in the cell-protective mechanisms of AGT.

Determination of O⁶-methylguanine DNA methyltransferase promoter methylation in non-small cell lung cancer

Aberrant methylation of promoter CpG islands is known to be a major inactivation mechanism of the tumor-related genes including DNA repair genes. The objective of this study was to determine the frequency of promoter methylation of the O⁶-methylguanine DNA methyltransferase (MGMT) gene as a DNA repair gene in nonsmall cell lung cancer (NSCLC) and to analyze the correlation with clinicopathological parameters including age, gender, smoking status, histological subtype, and clinical stage. Eighty patients with NSCLC were included in this study. The analysis of DNA methylation was performed on formalin-fixed, paraffin-embedded lung cancer tissues. Following DNA isolation and bisulfite treatment, DNA methylation was analyzed by methylation-specific real-time polymerase chain reaction. MGMT promoter methylation was detected in 51 of 80 (64%) NSCLC patients. There was a significant correlation between MGMT methylation and tumor stage (p = 0.01). The frequencies of the promoter methylation of MGMT gene in smokers and older patients were higher than in their counterparts. In conclusion, the present study provides strong evidence for a higher frequency of promoter methylation of the MGMT gene in NSCLC, indicating that it is a common event during the carcinogenesis of NSCLC.