Intracellular localization and function of DNA repair methyltransferase in human cells

Protein sensing in living cells by molecular rotor-based fluorescence-switchable chemical probes

In this paper, we introduce a general design to construct fluorescence-switching probes by using conjugates of a fluorescent molecular rotor and protein specific ligands for the selective protein detection and real-time tracking of protein degradation in living cells. Upon the interaction of the ligand with the protein ligand-binding domain, the crowded surroundings restrict the bond rotation of the fluorescent molecular rotor to trigger the emission of a strong fluorescence signal, which is reduced upon the addition of a competitive ligand or after protein degradation. With this probe design, two fluorescent probes for MGMT and hCAII proteins were constructed and applied for detecting the endogenous proteins in living cells. In addition, real-time degradation kinetics of the alkylated-MGMT at the single living cell level were revealed for the first time. We believe that this fluorescence-switching probe design can possibly be extended for the analysis of other proteins, for which there are still no effective tools to visualize them in living cells.

The ZEB1 pathway links glioblastoma initiation, invasion and chemoresistance

Glioblastoma remains one of the most lethal types of cancer, and is the most common brain tumour in adults. In particular, tumour recurrence after surgical resection and radiation invariably occurs regardless of aggressive chemotherapy. Here, we provide evidence that the transcription factor ZEB1 (zinc finger E-box binding homeobox 1) exerts simultaneous influence over invasion, chemoresistance and tumourigenesis in glioblastoma. ZEB1 is preferentially expressed in invasive glioblastoma cells, where the ZEB1-miR-200 feedback loop interconnects these processes through the downstream effectors ROBO1, c-MYB and MGMT. Moreover, ZEB1 expression in glioblastoma patients is predictive of shorter survival and poor Temozolomide response. Our findings indicate that this regulator of epithelial-mesenchymal transition orchestrates key features of cancer stem cells in malignant glioma and identify ROBO1, OLIG2, CD133 and MGMT as novel targets of the ZEB1 pathway. Thus, ZEB1 is an important candidate molecule for glioblastoma recurrence, a marker of invasive tumour cells and a potential therapeutic target, along with its downstream effectors.

Expression of O-Alkylguanine-DNA Alkyltransferase in Normal and Malignant Bladder Tissue of Egyptian Patients

Bladder tumour tissues and corresponding uninvolved mucosa (normal tissue) of Egyptian bladder cancer patients were assessed for O⁶-alkylguanine-DNA-alkyltransferase (MGMT) activity by functional assay of tissue extracts (36 paired samples), and distribution by immunofluorescence (IF) microscopy of fixed material (24 paired samples). MGMT varied widely from 42–253 fmoles/mg protein and from 3.2–40 fmoles/μg DNA in normal and 58–468 fmoles/mg protein and 2.5–49.5 fmoles/mg protein, in the tumour tissues; only one tumour had undetectable activity. Pairwise comparison of MGMT activity in tumour and adjacent normal tissue showed no significant difference based on DNA content but was 1.75-fold higher in tumour (P < .01) based on protein. There was no effect of gender or bilharzia infection status. IF showed that in tumours, both the mean percentage of positive nuclei (57.3 ± 20.3%) and mean integrated IF (5.47 ± 3.66) were significantly higher than those in uninvolved tissues (42.8 ± 13.5% P = .04) and (1.89 ± 1.42; P < .01), respectively. These observations suggest that, overall, MGMT levels are increased during human bladder carcinogenesis and that MGMT downregulation is not a common feature of bladder cancers. Based on this, bladder cancers would be expected to be relatively resistant to chemotherapy which involved O⁶-guanine alkylating antitumour agents.

Targeting of O6-MeG DNA methyltransferase (MGMT) to mitochondria protects against alkylation induced cell death

Mitochondrial DNA (mtDNA) mutations are implicated in pathogenesis of human diseases including cancer. To prevent mutations cells have developed repair systems to counteract harmful genetic changes caused by DNA damaging agents. One such DNA repair protein is the O(6)-Methylguanine-DNA methyltransferase (MGMT) that prevents certain types of alkylation damage. Yet, the role of MGMT in preventing alkylation induced DNA damage in mtDNA is unclear. We explored the idea of increasing cell survival after alkylation damage by overexpressing MGMT in mitochondria. We show that overexpression of this repair protein in mitochondria increases cell survival after treatment with the DNA damaging agent MNNG.

Significance of the O6-methylguanine-DNA methyltransferase and glutathione S-transferase activity in the sera of patients with malignant and benign ovarian tumors

OBJECTIVE

To demonstrate O6-methylguanine-DNA methyltransferase (MGMT) and glutathione S-transferase (GST) activities by analyzing the sera separately obtained from patients with malignant ovarian tumors, benign ovarian tumors, and healthy individuals.

STUDY DESIGN

Fourty-nine patients with ovarian cancer, nine patients with benign tumors, and 22 healthy women were included in this study. Blood samples were obtained from all the subjects in the malignant-tumor, benign-tumor, and control groups. Patients with malignant tumors underwent second and third phlebotomies one week following the surgery and after the chemotherapy regimen, respectively. MGMT, GST, and protein levels were measured for each serum sample. GST activity of the samples was measured by the method of Habig et al. using l-chloro-2-4 dinitrobenzene (CDNB) as substrate. MGMT activity was measured by the transfer of radio labelled methyl groups from a prepared MG-DNA substrate to the enzyme fraction of serum. Protein concentration was measured by biuret method.

RESULTS

Our work demonstrated that untreated patients with malignant ovarian tumors revealed significantly greater MGMT and GST activities in their sera than did both healthy individuals and patients with benign ovarian tumors, while no significant difference was found between the healthy group and the patients with benign ovarian tumors with respect to their sera MGMT and GST activities. GST activity following chemotherapy was significantly lower than the postoperative values preceding chemotherapy. A relationship between sera MGMT and GST activities, tumor histology and pathology was not found in this study.

CONCLUSION

Our work suggests the fact that detection of sera MGMT and GST activities is important in diagnostic and therapeutic approaches during the course of ovarian cancer.

A comparative immunohistochemistry of O6-methylguanine-DNA methyltransferase and p53 in diffusely infiltrating astrocytomas

The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) removes mutagenic adducts from the O6 position of guanine, thereby protecting the genome against guanine : cytosine to adenine : thymine transition and, meanwhile, conferring tumor resistance to many anti-cancer alkylating agents commonly used in the treatment of malignant gliomas. Studies on the involvement of p53 protein in expression of the MGMT gene have provided conflicting results regarding the relation between p53 protein and MGMT gene expression. To examine the potential immunostaining pattern of MGMT expression and to evaluate the possible relationship between p53 and MGMT regulation, we assessed MGMT and p53 accumulation on 35 cases of diffusely infiltrating astrocytomas. With a few cases showing cytoplasmic staining, MGMT accumulation was mainly nuclear. The percentage of labeled tumor cells was lower in high-grade astrocytomas than in low-grade astrocytomas (P < 0.05). Additionally, p53-immunopositive tumor cells were usually immunonegative to MGMT. Thus, it is suggested that MGMT expression is reduced during malignant transformation of diffusely infiltrating astrocytomas, and that mutant p53 protein might be associated with down regulation of the MGMT expression.

Deficient expression of O(6)-methylguanine-DNA methyltransferase combined with mismatch-repair proteins hMLH1 and hMSH2 is related to poor prognosis in human biliary tract carcinoma

BACKGROUND

O(6)-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme that transfers methyl groups from O(6)-methylguanine to itself. Alkylation of DNA at the O(6) position of guanine is an important step in the induction of mutations in the organism by alkylating agents. The O(6)-methyl G:T mismatch is recognized by the mismatch-repair (MMR) pathway. The biliary duct is highly exposed to alkylating agents because of its anatomical location.

METHODS

We examined 39 surgically resected gallbladder carcinomas and 35 extrahepatic bile duct carcinomas and evaluated the expression of MGMT and MMR protein (hMLH1 and hMSH2) by immunohistochemical staining.

RESULTS

MGMT-negative staining was detected in 59.0% of gallbladder carcinoma specimens and 60.0% of extrahepatic bile duct carcinoma specimens. In gallbladder carcinoma, hMLH1- and hMSH2-negative staining was observed in 51.3% and 59.0%, respectively, whereas in extrahepatic bile duct carcinoma, the respective values were 57.1% and 65.7%. MGMT-negative staining correlated with hepatic invasion in gallbladder carcinoma and with poor prognosis in both types of tumor. Furthermore, a combined MGMT and MMR status was shown to be a more significant prognostic biomarker in both tumor types.

CONCLUSIONS

Combined MGMT and MMR is a possible prognostic marker that probably reflects an accumulation of genetic mutations.

Expression and prognostic significance of O6-methylguanine-DNA methyltransferase in hepatocellular, gastric, and breast cancers

BACKGROUND

O6-Methylguanine-DNA methyltransferase (MGMT) is an enzyme that repairs O6-methylguanine, a promutagenic DNA base damaged by endogenous and environmental alkylating agents. There are few reports that describe whether or not abnormal MGMT expression correlates with the prognosis in human solid cancers.

METHODS

The expression of MGMT was immunohistochemically evaluated in 60, 62, 105, and 46 paraffin-embedded samples from patients with curatively resected hepatocellular, gastric, colorectal, and breast cancers, respectively.

RESULTS

The expression of MGMT was a positive predictive factor for overall survival in hepatocellular (P = .005) and gastric cancers (P < .001) and for relapse-free survival in breast cancers (P < .001). MGMT-positive gastric tumors (n = 42) were correlated with the absence of serosal invasion (P = .045), lymph node metastasis (P = .006), intestinal type (P = .018), and low pathological tumor, node, metastasis stage (P < .001). All breast tumors that recurred locally after operation were MGMT negative (P = .004). The clinicopathologic characteristics of colorectal cancers with respect to MGMT expression did not significantly differ.

CONCLUSIONS

The expression of MGMT is a predictive prognostic marker in patients with hepatocellular, gastric, and breast cancers. These findings may help to establish therapeutic strategies for patients with these types of solid cancer.

Human APE2 protein is mostly localized in the nuclei and to some extent in the mitochondria, while nuclear APE2 is partly associated with proliferating cell nuclear antigen

In human cells APE1 is the major AP endonuclease and it has been reported to have no functional mitochondrial targeting sequence (MTS). We found that APE2 protein possesses a putative MTS. When its N-terminal 15 amino acid residues were fused to the N-terminus of green fluorescent protein and transiently expressed in HeLa cells the fusion protein was localized in the mitochondria. By electron microscopic immunocytochemistry we detected authentic APE2 protein in mitochondria from HeLa cells. Western blotting of the subcellular fraction of HeLa cells revealed most of the APE2 protein to be localized in the nuclei. We found a putative proliferating cell nuclear antigen (PCNA)-binding motif in the C-terminal region of APE2 and showed this motif to be functional by immunoprecipitation and in vitro pull-down binding assays. Laser scanning immunofluorescence microscopy of HeLa cells demonstrated both APE2 and PCNA to form foci in the nucleus and also to be co-localized in some of the foci. The incubation of HeLa cells in HAT medium containing deoxyuridine significantly increased the number of foci in which both molecules were co-localized. Our results suggest that APE2 participates in both nuclear and mitochondrial BER and also that nuclear APE2 functions in the PCNA-dependent BER pathway.

O6-Alkylguanine-DNA alkyltransferase: influence on susceptibility to the genetic effects of alkylating agents

O6-Alkylguanine-DNA alkyltransferase (AGT) repairs DNA containing O6-alkylguanine by a suicide mechanism involving transfer of the alkyl group to its active site. In this way AGT protects cells from the mutagenic and cytotoxic effects of O6-alkylguanine-type lesions such as O6-methylguanine (O6-meG), an observation which has during recent years been confirmed by studies in transgenic animals either over-expressing or completely lacking this activity. While the levels of expression of AGT have been shown to affect strongly the repair of O6-meG after high doses of methylating agents inducing complete and prolonged depletion of the cellular AGT pool, other data suggest that within smaller variations of AGT levels (such as the interindividual variations observed in man or as observed after low or moderate exposures to alkylating agents) the dependence of O6-meG repair is limited. This phenomenon may reflect the intracellular distribution of the repair protein and must be taken into account when assessing the role of AGT in determining susceptibility to alkylating agents of environmental or clinical significance.

The sensitization of cells treated with O6-methylguanine to alkylation damage is affected by the number of O6-methylguanine-DNA methyltransferase molecules escaped from inactivation

O6-Methylguanine (MeG) can bind to the active site of O6-methylguanine-DNA methyltransferase (MGMT) as a free base. The subsequent methyl transfer reaction inactivates the repair protein. Hence, MeG is used to deplete the active MGMT pools in Chinese hamster cell lines (CHO) transfected to express varying amounts of human MGMT. After treatment with the free base, a residual population of active protein molecules remains localized mostly in the cytoplasm. Depleted cells are then challenged with the alkylating drug mitozolomide. Genotoxicity of this agent varied among the cell lines, and the compound sensitivity seemed to be regulated by a steady state equilibrium of residual MGMT molecules between nucleus and cytoplasm.

Expression and subcellular localization of human AP endonuclease 1 (HAP1/Ref-1) protein: a basis for its role in human disease

AIMS

Human AP endonuclease 1 (HAP1) plays a major role in the repair of apurinic/apyrimidinic (AP) sites in cellular DNA by catalysing hydrolytic cleavage of the phosphodiester backbone 5' to the site. HAP1 is also known to be a potent reduction-oxidation (redox) factor, regulating the binding activity of a number of transcription factors. The purpose of the present study was to examine the expression of HAP-1 in a wide range of human tissues.

METHODS AND RESULTS

Using a recently developed specific rabbit polyclonal antibody, we performed immunohistochemistry on paraffin-embedded tissue material. Nuclear staining was detected in crypt cells of the small and large intestine, epithelial cells of breast ducts, basal cells of the skin, alveolar cells of the lung, lymphocytes of the marginal zone of the spleen, in the surface epithelium and stromal cells of the ovary and the transitional epithelium of the bladder. Unexpectedly for a presumed nuclear protein, the staining pattern in some cell populations was mainly cytoplasmic (e.g. superficial cells of gastrointestinal tract, Langerhans cells, Leydig cells and spermatocytes, epithelium of the prostate glands), or both cytoplasmic and nuclear (e.g. epithelial cells of thymus, follicular thyroid cells, parietal cells of the stomach, glandular epithelial cells of the cervix, epithelial cells of exocrine pancreas).

CONCLUSION

This differential expression in a wide spectrum of cells is indicative of a potential multifunctional action of HAP1, not necessarily restricted to a role in the nucleus.

Human AP endonuclease 1 (HAP1) protein expression in breast cancer correlates with lymph node status and angiogenesis

Human AP endonuclease (HAP1) plays a major role in the repair of apurinic/apyrimidinic (AP) sites in cellular DNA. We used immunohistochemistry to examine the expression of HAP1 in normal breast and in 102 primary breast carcinomas. In normal breast epithelium, HAP1 had a uniformly nuclear localization. However, in lactating glandular epithelium, the expression of HAP1 was predominantly cytoplasmic. In carcinomas, both nuclear and cytoplasmic (44%), cytoplasmic (28%) or nuclear staining (24%) were observed. In four cases (4%), no HAP1 expression was detected. All patterns of expression for HAP1 were demonstrated for ductal carcinomas in situ (DCIS), although comedo-type DCIS were usually accompanied by mostly cytoplasmic staining. Similarly, the HAP1 expression in regions of invasive tumour necrosis was cytoplasmic. Pure nuclear HAP1 expression was significantly correlated with low angiogenesis (P = 0.007) and negative lymph node status (P = 0.001). In contrast, cases with cytoplasmic as well as nuclear staining were associated with poor prognostic factors, such as high angiogenesis (P = 0.03) and node positivity (P = 0.03). The pure nuclear staining may be related to better differentiation, as in normal breast, and hence better prognostic features, and cytoplasmic staining to a more metabolically active phenotype with high protein synthesis, as in lactating breast.

Roles of DNA repair methyltransferase in mutagenesis and carcinogenesis

Alkylation of DNA at the O6-position of guanine is one of the most critical events leading to induction of mutation as well as cancer. An enzyme, O6-methylguanine-DNA methyltransferase, is present in various organisms, from bacteria to human cells, and appears to be responsible for preventing the occurrence of such mutations. The enzyme transfers methyl groups from O6-methylguanine and other methylated moieties of the DNA to its own molecule, thereby repairing DNA lesions in a single-step reaction. To elucidate the role of methyltransferase in preventing cancer, animal models with altered levels of enzyme activity were generated. Transgenic mice carrying extra copies of the foreign methyltransferase gene showed a decreased susceptibility to alkylating carcinogens, with regard to tumor formation. By means of gene targeting, mouse lines defective in both alleles of the methyltransferase gene were established. Administration of methylnitrosourea to these gene-targeted mice led to early death while normal mice treated in the same manner showed no untoward effects. Numerous tumors were formed in the gene-defective mice exposed to a low dose of methylnitrosourea, while none or only few tumors were induced in the methyltransferase-proficient mice. It seems apparent that the DNA repair methyltransferase plays an important role in lowering a risk of occurrence of cancer in organisms.

Recombinant human O6-alkylguanine-DNA alkyltransferase (AGT), Cys145-alkylated AGT and Cys145 --> Met145 mutant AGT: comparison by isoelectric focusing, CD and time-resolved fluorescence spectroscopy

Isoelectric focusing, CD, steady-state and time-resolved fluorescence spectroscopy were used to compare the native recombinant human DNA-repair protein O6-alkylguanine-DNA alkyltransferase (AGT) with AGT derivatives methylated or benzylated on Cys145 or modified by site-directed mutagenesis at the active centre (Met145 mutant). The AGT protein is approximately spherical with highly constrained Trp residues, but is not stabilized by disulphide bridges. In contrast with native AGT, alkylated AGT precipitated at 25 degrees C but remained monomeric at 4 degrees C. As revealed by isoelectric focusing, pI changed from 8.2 (AGT) to 8. 4 (Cys145-methylated AGT) and 8.6 (Cys145-benzylated AGT). The alpha-helical content of the Met145 mutant was decreased by approx. 5% and Trp residues were partially liberated. Although non-covalent binding of O6-benzylguanine did not alter the secondary structure of AGT, its alpha-helical content was increased by approx. 2% on methylation and by approx. 4% on benzylation, altogether indicating a small conformational change in AGT on undergoing alkylation. No signal sequences have been found in AGT that mark it for polyubiquitination. Therefore the signal for AGT degradation remains to be discovered.

Immunohistochemical examination of the expression of O6-methylguanine-DNA methyltransferase in human melanoma metastases

In tumour cell lines, an inverse relationship has been shown between susceptibility to the cytotoxic effects of the O6-alkylating agents and the expression of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT). One of the most effective single agents in chemotherapy of metastatic melanoma is the O6-alkylating drug, dacarbazine. We therefore examined the distribution of MGMT in 37 skin and lymph node melanoma metastases using rabbit antihuman MGMT antiserum. MGMT expression was undetectable in tumours from 2 out of 34 patients and low in 4 further patients. When present, staining was mainly nuclear and showed a marked variation both among tumour cells within the same metastases, between separate metastases in the same patient and between tumours in different patients. MGMT expression determined by immunohistochemistry showed a relation to MGMT activity measurements, but was not related to the number of proliferating cells, as identified by staining with MIB-1 antibody. Tumour cells with moderate to strong immunostaining with MGMT antiserum were significantly more abundant in metastases excised after dacarbazine-based chemotherapy (n = 8) than in those excised before treatment (n = 29).

DNA-repair methyltransferase as a molecular device for preventing mutation and cancer

Alkylation of DNA at the 0(6) position of guanine is regarded as one o f the most critical events leading to induction of mutations and cancers in organisms. Once 0(6)-methylguanine is formed, it can pair with thymine during DNA replication, the result being a conversion of the guanine.cytosine to an adenine.thymine pair in DNA, and such mutations are often found in tumors induced by alkylating agents. To counteract such effects, organisms possess a mechanism to repair 0(6)-methylguanine in DNA. An enzyme, 0(6)-methylguanine-DNA methyltransferase, is present in various organism, from bacteria to human cells, and appears to be responsible for preventing the occurrence of such mutations. The enzyme transfers methyl groups from 0(6)-methylguanine and other methylated moieties of the DNA to its own molecule, thereby repairing DNA lesions in a single-step reaction. To elucidate the role of methyltransferase in preventing cancers, animal models with altered levels of enzyme activity were generated. Transgenic mice carrying the foreign methyltransferase gene with functional promoters had higher levels of methyltransferase activity and showed a decreased susceptibility to N-nitroso compounds in regard to liver carcinogenesis. Mouse lines deficient in the methyltransferase gene, which were established by gene targeting, exhibited an extraordinarily high sensitivity to an alkylating carcinogen.

Intracellular localization of 8-oxo-dGTPase in human cells, with special reference to the role of the enzyme in mitochondria

We examined the intracellular distribution of 8-oxo-dGTPase (8-oxo-7,8-dihydrodeoxyguanosine triphosphatase) encoded by the MTH1 gene, a human mutator homologue. The activity of 8-oxo-dGTPase mainly located in cytosolic and mitochondrial soluble fractions of Jurkat cells, a human T-cell leukemia line. Electron microscopic immunocytochemistry, using a specific antibody against MTH1 protein, showed localization of MTH1 protein in the mitochondrial matrix. Activity in the mitochondria accounted for about 4% of the total activity. The specific activity in the mitochondrial soluble fraction (8093 units/mg protein) was as high as that in the cytosolic fraction (8111 unit/mg protein). The 8-oxo-dGTPase activities in cytosolic and mitochondrial soluble fractions co-eluted with MTH1 protein by anion-exchange chromatography, and the molecular mass of the mitochondrial MTH1 protein was much the same as that of the cytosolic MTH1 protein (about 18 kDa). HeLa cells expressing MTH1 cDNA showed an increased cytoplasmic signal together with a weak signal in the nucleus in in situ immunostaining of MTH1 protein, and the overexpressed MTH1 protein was recovered from both cytosolic and mitochondrial fractions. Thus, the 8-oxo-dGTPase encoded by MTH1 gene is localized in mitochondrial and cytosol.