CREB-induced LINC00473 promotes chemoresistance to TMZ in glioblastoma by regulating O6-methylguanine-DNA-methyltransferase expression via CEBPα binding

Temozolomide (TMZ) offers substantial therapeutic benefits for glioblastoma (GB), yet its efficacy is hindered the development of chemoresistance. The role of long non-coding RNAs (lncRNAs) in tumorigenesis and chemoresistance has garnered great attention in studies on TMZ resistance. This study aimed to reveal the role of LINC00473 in TMZ chemoresistance and the underlying mechanism in GB. The expression of LINC00473 in TMZ-resistant and TMZ-sensitive GB cells was investigated using qPCR analysis. The role of LINC00473 in regulating TMZ resistance in GB cells was analyzed using the CCK-8 assay, colony formation assay, and flow cytometry. The next steps included assessing if LINC00473 is regulated by CREB and whether LINC00473 promotes chemoresistance through MGMT regulation via CEBPα. Further, chemoresistance delivery between cells via exosomal LINC00473 was validated in vitro and in vivo. Results showed that LINC00473 levels were elevated in TMZ-resistant cells upon CREB activation, and the lncRNA promoted the chemoresistance of GB cells through the upregulation of MGMT expression. Mechanistically, LINC00473 regulated the MGMT expression by binding to CEBPα. The highly-expressed LINC00473 packaged in exosomes transferred chemoresistance to the adjacent TMZ-sensitive GB cells. In conclusion, a novel CREB/LINC00473/CEBPα/MGMT pathway was revealed in the GB TMZ-resistance formation. In addition, an exosome-based mechanism of chemoresistance transmission was revealed, suggesting that LINC00473 could be used as a novel therapeutic target for GB.

A modified nucleoside O6-methyl-2'-deoxyguanosine-5'-triphosphate exhibits anti-glioblastoma activity in a caspase-independent manner

Resistance to temozolomide (TMZ), the frontline chemotherapeutic agent for glioblastoma (GBM), has emerged as a formidable obstacle, underscoring the imperative to identify alternative therapeutic strategies to improve patient outcomes. In this study, we comprehensively evaluated a novel agent, O6-methyl-2'-deoxyguanosine-5'-triphosphate (O6-methyl-dGTP) for its anti-GBM activity both in vitro and in vivo. Notably, O6-methyl-dGTP exhibited pronounced cytotoxicity against GBM cells, including those resistant to TMZ and overexpressing O6-methylguanine-DNA methyltransferase (MGMT). Mechanistic investigations revealed that O6-methyl-dGTP could be incorporated into genomic DNA, disrupting nucleotide pools balance, and inducing replication stress, resulting in S-phase arrest and DNA damage. The compound exerted its anti-tumor properties through the activation of AIF-mediated apoptosis and the parthanatos pathway. In vivo studies using U251 and Ln229 cell xenografts supported the robust tumor-inhibitory capacity of O6-methyl-dGTP. In an orthotopic transplantation model with U87MG cells, O6-methyl-dGTP showcased marginally superior tumor-suppressive activity compared to TMZ. In summary, our research, for the first time, underscores the potential of O6-methyl-dGTP as an effective candidate against GBM, laying a robust scientific groundwork for its potential clinical adoption in GBM treatment regimens.

EPIC-0307-mediated selective disruption of PRADX-EZH2 interaction and enhancement of temozolomide sensitivity to glioblastoma via inhibiting DNA repair and MGMT

BACKGROUND

Temozolomide (TMZ) treatment efficacy in glioblastoma (GBM) has been limited by resistance. The level of O-6-methylguanine-DNA methyltransferase (MGMT) and intrinsic DNA damage repair factors are important for the TMZ response in patients. Here, we reported a novel compound, called EPIC-0307, that increased TMZ sensitivity by inhibiting specific DNA damage repair proteins and MGMT expression.

METHODS

EPIC-0307 was derived by molecular docking screening. RNA immunoprecipitation (RIP), and chromatin immunoprecipitation by RNA (ChIRP) assays were used to verify the blocking effect. Chromatin immunoprecipitation (ChIP) and co-immunoprecipitation (Co-IP) assays were performed to explore the mechanism of EPIC-0307. A series of in vivo and in vitro experiments were designed to evaluate the efficacy of EPIC-0307 in sensitizing GBM cells to TMZ.

RESULTS

EPIC-0307 selectively disrupted the binding of PRADX to EZH2 and upregulated the expression of P21 and PUMA, leading to cell cycle arrest and apoptosis in GBM cells. EPIC-0307 exhibited a synergistic inhibitory effect on GBM when combined with TMZ by downregulating TMZ-induced DNA damage repair responses and epigenetically silencing MGMT expression through modulating the recruitment of ATF3-pSTAT3-HDAC1 regulatory complex to the MGMT promoter. EPIC-0307 demonstrated significant efficacy in suppressing the tumorigenesis of GBM cells, restoring TMZ sensitivity.

CONCLUSION

This study identified a potential small-molecule inhibitor (SMI) EPIC-0307 that selectively disrupted the PRADX-EZH2 interaction to upregulate expressions of tumor suppressor genes, thereby exerting its antitumor effects on GBM cells. EPIC-0307 treatment also increased the chemotherapeutic efficacy of TMZ by epigenetically downregulating DNA repair-associate genes and MGMT expression in GBM cells.

Iron Oxide Nanoparticles Decorated with Functional Peptides for a Targeted siRNA Delivery to Glioma Cells

Glioma is a deadly form of brain cancer, and the difficulty of treating glioma is exacerbated by the chemotherapeutic resistance developed in the tumor cells over the time of treatment. siRNA can be used to silence the gene responsible for the increased resistance, and sensitize the glioma cells to drugs. Here, iron oxide nanoparticles functionalized with peptides (NP-CTX-R₁₀) were used to deliver siRNA to silence O6-methylguanine-DNA methyltransferase (MGMT) to sensitize tumor cells to alkylating drug, Temozolomide (TMZ). The NP-CTX-R₁₀ could complex with siRNA through electrostatic interactions and was able to deliver the siRNA to different glioma cells. The targeting ligand chlorotoxin and cell penetrating peptide polyarginine (R₁₀) enhanced the transfection capability of siRNA to a level comparable to commercially available Lipofectamine. The NP-siRNA was able to achieve up to 90% gene silencing. Glioma cells transfected with NP-siRNA targeting MGMT showed significantly elevated sensitivity to TMZ treatment. This nanoparticle formulation demonstrates the ability to protect siRNA from degradation and to efficiently deliver the siRNA to induce therapeutic gene knockdown.

Reducing Chemo-/Radioresistance to Boost the Therapeutic Efficacy against Temozolomide-Resistant Glioblastoma

Chemo-/radioresistance is the most important reason for the failure of glioblastoma (GBM) treatment. Reversing the chemo-/radioresistance of GBM for boosting therapeutic efficacy is very challenging. Herein, we report a significant decrease in the chemo-/radioresistance of GBM by the in situ generation of SO₂ within a tumor, which was released on demand from the prodrug 5-amino-1,3-dihydrobenzo[c]thiophene 2,2-dioxide (ATD) loaded on rare-earth-based scintillator nanoparticles (i.e., NaYF₄:Ce@NaLuF₄:Nd@ATD@DSPE-PEG₅₀₀₀, ScNPs) under X-ray irradiation. Our novel X-ray-responsive ScNPs efficiently converted highly penetrating X-rays into ultraviolet rays for controlling the decomposition of ATD to generate SO₂, which effectively damaged the mitochondria of temozolomide-resistant U87 cells to lower the production of ATP and inhibit P-glycoprotein (P-gp) expression to reduce drug efflux. Meanwhile, the O⁶-methylguanine-DNA methyltransferase (MGMT) of drug-resistant tumor cells was also reduced to prevent the repair of damaged DNA and enhance cell apoptosis and the efficacy of chemo-/radiotherapy. The tumor growth was obviously suppressed, and the mice survived significantly longer than untreated temozolomide-resistant GBM-bearing mice. Our work demonstrates the potential of SO₂ in reducing chemo-/radioresistance to improve the therapeutic effect against resistant tumors if it can be well controlled and in situ generated in tumor cells. It also provides insights into the rational design of stimuli-responsive drug delivery systems for the controlled release of drugs.

Nano-Codelivery of Temozolomide and siPD-L1 to Reprogram the Drug-Resistant and Immunosuppressive Microenvironment in Orthotopic Glioblastoma

Glioblastoma (GBM) is an invasive cancer with high mortality in central nervous system. Resistance to temozolomide (TMZ) and immunosuppressive microenvironment lead to low outcome of the standardized treatment for GBM. In this study, a 2-deoxy-d-glucose modified lipid polymer nanoparticle loaded with TMZ and siPD-L1 (TMZ/siPD-L1@GLPN/dsb) was prepared to reprogram the TMZ-resistant and immunosuppressive microenvironment in orthotopic GBM. TMZ/siPD-L1@GLPN/dsb simultaneously delivered a large amount of TMZ and siPD-L1 to the deep area of the orthotopic TMZ-resistant GBM tissue. By inhibiting PD-L1 protein expression, TMZ/siPD-L1@GLPN/dsb markedly augmented the percentage of CD3⁺CD8⁺IFN-γ⁺ cells (T_eff cells) and reduced the percentage of CD4⁺CD25⁺FoxP3⁺ cells (T_reg cells) in orthotopic TMZ-resistant GBM tissue, which enhanced T-cell mediated cytotoxicity on orthotopic TMZ-resistant GBM. Moreover, TMZ/siPD-L1@GLPN/dsb obviously augmented the sensitivity of orthotopic TMZ-resistant GBM to TMZ through decreasing the protein expression of O⁶-methyl-guanine-DNA methyltransferase (MGMT) in TMZ-resistant GBM cells. Thus, TMZ/siPD-L1@GLPN/dsb markedly restrained the growth of orthotopic TMZ-resistant GBM and extended the survival time of orthotopic GBM rats through reversing a TMZ-resistant and immunosuppressive microenvironment. TMZ/siPD-L1@GLPN/dsb shows potential application to treat orthotopic TMZ-resistant GBM.

mTOR target NDRG1 confers MGMT-dependent resistance to alkylating chemotherapy

A hypoxic microenvironment induces resistance to alkylating agents by activating targets in the mammalian target of rapamycin (mTOR) pathway. The molecular mechanisms involved in this mTOR-mediated hypoxia-induced chemoresistance, however, are unclear. Here we identify the mTOR target N-myc downstream regulated gene 1 (NDRG1) as a key determinant of resistance toward alkylating chemotherapy, driven by hypoxia but also by therapeutic measures such as irradiation, corticosteroids, and chronic exposure to alkylating agents via distinct molecular routes involving hypoxia-inducible factor (HIF)-1alpha, p53, and the mTOR complex 2 (mTORC2)/serum glucocorticoid-induced protein kinase 1 (SGK1) pathway. Resistance toward alkylating chemotherapy but not radiotherapy was dependent on NDRG1 expression and activity. In posttreatment tumor tissue of patients with malignant gliomas, NDRG1 was induced and predictive of poor response to alkylating chemotherapy. On a molecular level, NDRG1 bound and stabilized methyltransferases, chiefly O(6)-methylguanine-DNA methyltransferase (MGMT), a key enzyme for resistance to alkylating agents in glioblastoma patients. In patients with glioblastoma, MGMT promoter methylation in tumor tissue was not more predictive for response to alkylating chemotherapy in patients who received concomitant corticosteroids.

Genomic and molecular profiling predicts response to temozolomide in melanoma

PURPOSE

Despite objective response rates of only approximately 13%, temozolomide remains one of the most effective single chemotherapy agents against metastatic melanoma, second only to dacarbazine, the current standard of care for systemic treatment of melanoma. The goal of this study was to identify molecular and/or genetic markers that correlate with, and could be used to predict, response to temozolomide-based treatment regimens and that reflect the intrinsic properties of a patient's tumor.

EXPERIMENTAL DESIGN

Using a panel of 26 human melanoma-derived cell lines, we determined in vitro temozolomide sensitivity, O(6)-methylguanine-DNA methyltransferase (MGMT) activity, MGMT protein expression and promoter methylation status, and mismatch repair proficiency, as well as the expression profile of 38,000 genes using an oligonucleotide-based microarray platform.

RESULTS

The results showed a broad spectrum of temozolomide sensitivity across the panel of cell lines, with IC(50) values ranging from 100 micromol/L to 1 mmol/L. There was a significant correlation between measured temozolomide sensitivity and a gene expression signature-derived prediction of temozolomide sensitivity (P < 0.005). Notably, MGMT alone showed a significant correlation with temozolomide sensitivity (MGMT activity, P < 0.0001; MGMT expression, P <or= 0.0001). The promoter methylation status of the MGMT gene, however, was not consistent with MGMT gene expression or temozolomide sensitivity.

CONCLUSIONS

These results show that melanoma resistance to temozolomide is conferred predominantly by MGMT activity and suggest that MGMT expression could potentially be a useful tool for predicting the response of melanoma patients to temozolomide therapy.

Role of O6-methylguanine-DNA methyltransferase in protecting from alkylating agent-induced toxicity and mutations in mice

The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) protects from toxicity and mutations incurred following alkylating agents by removing O(6)-alkylguanine lesions. Using Mgmt-/- mice, we examined MGMT's role in protecting from in vivo mutations induced by three different alkylating agents, temozolomide (TMZ), 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) and cyclophosphamide. Mutant frequencies were determined in the hypoxanthine-guanine phosphoribosyltransferase gene of splenic T-lymphocytes from C57BL/6 mice (Mgmt+/+ and Mgmt-/-) following TMZ, BCNU or cyclophosphamide. Following TMZ, the mutation frequency was significantly greater in Mgmt-/- mice (5.5 and 9.8 x 10(-6) for 7 and 10 mg/kg TMZ, respectively) compared with vehicle-treated mice (1.0 x 10(-6), P <or= 0.05). In contrast, TMZ-induced mutations were not increased over vehicle in Mgmt+/+ mice. The mutation frequency of mice treated with BCNU (7.5 mg/kg) was the same regardless of Mgmt status. Similarly, pretreatment of Mgmt+/+ mice with 30 mg/kg O(6)-benzylguanine, a potent inactivator of MGMT, prior to BCNU (15 mg/kg) did not result in significantly more mutations than mice treated with BCNU alone. Following cyclophosphamide, mutation frequencies significantly increased from 1.8 x 10(-6) in control-treated mice to 12.9 x 10(-6) in Mgmt+/+ and 18.1 x 10(-6) in Mgmt-/- mice, although the difference in Mgmt-/- compared with Mgmt+/+ was not significant. Acrolein and chloroacetaldehyde, metabolites of cyclophosphamide, were not mutagenic in Mgmt+/+ and Mgmt-/- mice. These results demonstrate that MGMT significantly protects against in vivo TMZ-induced mutations and that MGMT deficiency does not result in greater mutation frequency following cyclophosphamide or BCNU compared with wild-type mice.

Evolving the substrate specificity of O6-alkylguanine-DNA alkyltransferase through loop insertion for applications in molecular imaging

We introduce a strategy for evolving protein substrate specificity by the insertion of random amino acid loops into the protein backbone. Application of this strategy to human O6-alkylguanine-DNA alkyltransferase (AGT) led to the isolation of mutants that react with the non-natural substrate O6-propargylguanine. Libraries generated by conventional random or targeted saturation mutagenesis, by contrast, did not yield any mutants with activity towards this new substrate. The strategy of loop insertion to alter enzyme specificity should be general and applicable to other classes of proteins. An important application of the isolated AGT mutant is in molecular imaging, where the mutant and parental AGTs are used to label two different AGT fusion proteins with different fluorophores in the same living cell or in vitro . This allowed the establishment of fluorescence-based assays to detect protein-protein interactions and measure enzymatic activities.

Induction of the DNA repair gene O6-methylguanine—DNA methyltransferase by dexamethasone in glioblastomas

Object. The DNA repair enzyme O6-methylguanine—DNA methyltransferase (MGMT) inhibits the cytotoxic effect of alkylating agents on tumor cells. The presence of two nonconsensus glucocorticoid-responsive elements in the human MGMT promoter region indicates the potential regulation of MGMT expression by glucocorticoid agents. This study was performed to elucidate whether dexamethasone affects the expression of MGMT in glioblastoma multiforme (GBM) cells, thereby limiting the benefit of chemotherapeutic alkylating agents.

Methods. Four GBM cell lines (A172, T98G, U138MG, and U87MG) were exposed to the alkylating agent 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) with or without dexamethasone. The expression levels of MGMT were correlated with the cytotoxic effects of ACNU in GBM cells. In the presence of ACNU alone, dexamethasone alone, and the combination of both agents, messenger RNA expression of MGMT was induced to varying degrees with the highest increases seen in the later conditions. This dexamethasone-dependent induction of the MGMT gene was even observed in U87MG cells in which the promoter is methylated, although the absolute expression of MGMT mRNA was the lowest in that cell line. The induction of MGMT by dexamethasone was associated with an increased resistance of these cells to ACNU.

Conclusions. These results indicate that dexamethasone-mediated upregulation of MGMT limits the efficiency of alkylating agents in the treatment of malignant gliomas.

Sensitization of a human ovarian cancer cell line to temozolomide by simultaneous attenuation of the Bcl-2 antiapoptotic protein and DNA repair by O6-alkylguanine-DNA alkyltransferase

Temozolomide is an alkylating agent that mediates its cytotoxic effects via O(6)-methylguanine (O(6)-meG) adducts in DNA. O(6)-alkylguanine-DNA-alkyltransferase (MGMT) can repair such adducts and therefore constitutes a major resistance mechanism to the drug. MGMT activity can be attenuated in vitro and in vivo by the pseudosubstrate O(6)-(4-bromothenyl)guanine (PaTrin-2, Patrin, Lomeguatrib), which in clinical trials is in combination with temozolomide. Resistance to cytotoxic agents can also be mediated by the Bcl-2 protein, which inhibits apoptosis and is frequently up-regulated in tumor cells. Attenuation of Bcl-2 expression can be affected by treatment of cells with the antisense oligonucleotide, oblimersen sodium (Genasense), currently in phase III clinical trials in combination with the methylating agent dacarbazine. Using a human ovarian cancer cell line (A2780) that expresses both Bcl-2 and MGMT, we show that cells treated with active dose levels of either oblimersen (but not control reverse sequence or mismatch oligonucleotides) or PaTrin-2 are substantially sensitized to temozolomide. Furthermore, the exposure of oblimersen-pretreated cells to PaTrin-2 leads to an even greater sensitization of these cells to temozolomide. Thus, growth of cells treated only with temozolomide (5 microg/mL) was 91% of control growth, whereas additional exposure to PaTrin-2 alone (10 micromol/L) or oblimersen alone (33 nmol/L) reduced this to 81% and 66%, respectively, and the combination of PaTrin-2 (10 micromol/L) and oblimersen (33 nmol/L) reduced growth to 25% of control. These results suggest that targeting both Bcl-2 with oblimersen and MGMT with PaTrin-2 would markedly enhance the antitumor activity of temozolomide and merits testing in clinical trials.

Exon 5 polymorphisms in the O6-alkylguanine DNA alkyltransferase gene and lung cancer risk in non-smokers exposed to second-hand smoke

PURPOSE

The objective of the study was to examine the association of three exon 5 variants in the O(6)-alkylguanine DNA alkyltransferase (AGT) gene involved in the repair of the mutagenic DNA lesion O(6)-alkylguanine formed by nitrosamines, with lung cancer risk in never-smokers.

EXPERIMENTAL DESIGN

Exon 5 of the AGT gene was sequenced in genomic DNA from 136 cases and 133 hospital- or population-based controls for whom questionnaire information on second-hand smoke and diet was available to determine the frequencies of the Gly(160)Arg, Ile(143)Val, and Lys(178)Arg variant alleles.

RESULTS

No codon (160)Arg variant alleles were found in the study population. The codon (143)Val and (178)Arg variant alleles, present at allele frequencies of 0.07, showed 100% linkage. The odds ratio (OR) of lung cancer for these variant carriers was 2.05 [95% confidence interval (CI) 1.03-4.07]. The risk varied between the different lung cancer pathologies with an increased risk for adenocarcinoma (OR 2.67, 95% CI 1.21-5.87) or small cell carcinoma (OR 4.83, 95% CI 0.91-25.7) but not for squamous cell carcinoma (OR 1.07, 95% CI 0.27-4.18). Compared with individuals carrying the mutant alleles unexposed to second-hand smoke, the OR for exposed variant carriers was 1.95 (95% CI 0.53-1.15); a similar interaction, although not significative, was observed for low consumption of cruciferous vegetables and for green vegetables and tomatoes.

CONCLUSIONS

These results point toward a role of AGT polymorphisms in lung cancer susceptibility among never-smokers, in particular among subjects exposed to environmental carcinogens.

Human-yeast chimeric repair protein protects mammalian cells against alkylating agents: enhancement of MGMT protection

Chemotherapeutic DNA alkylating agents are common weapons employed to fight both pediatric and adult cancers. In addition to cancerous cells, nontarget tissues are subjected to the cytotoxicity of these agents, and dose-limiting toxicity in the form of myelosuppression is a frequent result of treatment. One approach to prevent myelosuppression that results from the use of chemotherapeutic agents is to increase the levels of DNA repair proteins in bone marrow cells. Here we report our second successful attempt to create a fusion protein that possesses both direct reversal and base excision repair pathway DNA repair activities. The chimeric protein is composed of the human O(6)-Methylguanine-DNA Methyltransferase (MGMT) and the yeast Apn1 proteins and retains both MGMT and AP endonuclease activities as determined by biochemical analysis. We have also demonstrated that the chimeric protein is able to protect mammalian cells from the DNA alkylating agents 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) and methyl methanesulfonate (MMS). The protection by the chimeric protein against BCNU is even greater than MGMT alone, which has potential translational significance given that MGMT is currently in clinical trials. Additionally, we show that the chimeric MGMT-Apn1 protein can protect mammalian cells from dual treatments of BCNU and MMS and that this effect is greater than that provided by MGMT alone. The data support our previous finding that a protein with multiple DNA repair activities can be constructed and that this and other constructs may play an important clinical role in guarding against dose-limiting effects of chemotherapy, particularly in situations of multiple drug use.

Drug selection of mutant methylguanine methyltransferase from different oncoretroviral backbones results in multilineage hematopoietic transgene expression in primary and secondary recipients

Optimized hematopoietic gene therapy requires vectors with strong expression in the desired target cell population and the ability to select for the expressing transduced cells. In the context of drug resistance selection of repopulating hematopoietic stem cells in the mouse, we examined tissue expression after transduced marrow transplantation of the drug selection gene, G156A mutant O6-methylguanine-DNA methyltransferase (G156A MGMT). To gain more experience with the rigor of the impact of selection on tissue-specific gene expression, we also asked whether there are expression differences between three different onco-retroviral backbones--MPSV, SF, and MFG. MGMT expression was compared after O6-benzylguanine (BG) and 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) drug selection in vivo. After mice were transplanted with cells transduced with MPSV, MFG, or SF retroviral vectors expressing G156A MGMT and drug treated, nearly complete replacement by transduced progenitors was observed in the marrow. Each backbone supported MGMT expression in all four hematopoietic lineages in vivo indicating that MGMT-mediated selection is indeed robust. Expression in marrow, spleen, and thymus was very similar between the vectors and differences were most likely due to differences in gene copy number per selected cell. In primary and secondary recipients, the highest expression was observed in MFG and this was the vector that transduced at the greatest proviral copy number per cell. These data indicate that strong selection pressure using the MGMT gene to protect primary and secondary repopulating murine stem cells from the toxicity of BCNU. Regardless of the vector backbone used, multiorgan expression was observed without evidence of gene silencing. These data help establish mutant, BG-resistant MGMT as a potent selection gene for stem cell selection in vivo.

DNA repair enzymes and cytotoxic effects of temozolomide: comparative studies between tumor cells and normal cells of the immune system

O6-alkylguanine-DNA alkyltransferase (OGAT) and the mismatch repair system (MRS) play a crucial role in the susceptibility of tumor cells to the cytotoxic effects of agents that generate O6-methylguanine in DNA, including the triazene compound temozolomide (TMZ). Studies performed with peripheral blood mononuclear cells (MNC) showed that TMZ was scarcely active on lymphocyte functions not dependent on cell proliferation (e.g. NK activity and cytokine-mediated induction of CD1b molecule in adherent MNC). In contrast, TMZ depressed proliferation and lymphokine activated killer (LAK) cell generation in response to IL-2. In this case, a reasonably good inverse relationship was found between OGAT levels of MNC and their susceptibility to TMZ. This study also analyzed the ratio of the toxic effect of TMZ on MNC and on tumor cells (i.e. "Tumor-Immune Function Toxicity Index", TIFTI). A particularly favorable TIFTI can be obtained when OGAT levels are extremely high in MNC and markedly low in tumor cells. This holds true for MRS-proficient neoplastic cells, but not for MRS-deficient tumors. In conclusion, strategies aimed at modulating OGAT and MRS may improve the clinical response to TMZ. However, the use of OGAT inhibitors to potentiate the antitumor activity of TMZ might result in a concomitant increase of the immunosuppressive effects of the drug, thus reducing the relative TIFTI.

O6-Alkylguanine-DNA-alkyltransferase activity in peripheral leukocytes, smoking and risk of lung cancer

The level of activity of O6-alkylguanine-DNA-alkyltransferase (AGT), a DNA repair enzyme, in blood lymphocytes may be a marker of susceptibility to lung cancer. We measured the AGT activity level, expressed as pmoles of repaired bases/mg protein, in leukocytes of 153 lung cancer cases (of whom 80 were never smokers) and 106 controls (76 never smokers) enrolled in eight centres from seven countries. Subjects were interviewed with respect to active smoking and exposure to environmental tobacco smoke (ETS). Among never smokers, the odds ratios (ORs) of lung cancer were 1.3 (95% confidence interval 0.5-3.9), 1.5 (0.6-4.1) and 1.4 (0.5-3.8) in quartiles of decreasing AGT activity level, as compared to the upper quartile (P value of test for linear trend 0.6). Corresponding ORs among smokers were 3.4 (0.9-13), 2.0 (0.5-8.3) and 0.4 (0.1-1.6) (P value of test for linear trend 0.4). No interaction was suggested between AGT activity level and either cumulative smoking or exposure to ETS. Reduced AGT activity was not clearly associated with increased lung cancer risk in either smokers or non-smokers. However, the small size of our study argues for a prudent interpretation of our results.

The sensitivity of MCF10A breast epithelial cells to alkylating drugs is enhanced by the inhibition of O6-methylguanine-DNA methyltransferase transcription with a synthetic double strand DNA oligonucleotide

Cytoxicity of alkylating chemotherapeutic drugs is affected by the cellular content of the enzyme O6_ methylguanine-DNA methyl transferase (MGMT). Since high levels of the enzyme confer the efficient repair of DNA alkylation, the chemotherapeutic potential of alkylating chemicals can be maintained either increasing drug dosage or reducing the amount of endogenous MGMT. This study strives to the latter end by competing away a transcriptional activator of the MGMT gene from its native enhancer sequence using a synthetic double strand DNA oligonucleotide (MEBP-ODN). MEBP-ODN was administered in culture medium to MCF10A human breast epithelial cells expressing high level of MGMT. Reverse transcription-polymerase chain reaction and western blotting analyses showed decrease in both MGMT mRNA and protein content. Concomitantly, MEBP-ObN exposed cells were more sensitive to the alkylating drug mitozolomide than their controls, which were not exposed to MEBP-ODN. These results indicate that the cis-acting MEBP-ODN can efficiently deplete MGMT protein by working as decoy binding site for the transcriptional activator MEBP. This approach represents a successful strategy to counteract the protective role of MGMT repair enzyme during an alkylating drug based chemotherapeutic regimen.

[Cloning and expression of MGMT cDNA and analysis of the DNA repair activity of the recombinant protein]

A cDNA encoding human O6-methylguanine-DNA methyltranferase (MGMT) was cloned from Hela S3 cells and the sequence is identical with the published data. The MGMT cDNA was inserted into the expression vector pET-21a and transformed into E. coli BL21 (DE3). A 24 kD protein was expressed after IPTG induction. Essays using lethal dose of alkylating agents indicate that expression of MGMT protein can repair the DNA mutations of the recombinant bacteria produced by alkylating agents.

Reduction of BCNU toxicity to lung cells by high-level expression of O(6)-methylguanine-DNA methyltransferase

1,3-Bis(2-chloroethyl)-1-nitrosourea (BCNU) is an important cause of pulmonary toxicity. BCNU alkylates DNA at the O(6) position of guanine. O(6)-methylguanine-DNA methyltransferase (MGMT) is a DNA repair protein that removes alkyl groups from the O(6) position of guanine. To determine whether overexpression of MGMT in a lung cell reduces BCNU toxicity, the MGMT gene was transfected into A549 cells, a lung epithelial cell line. Transfected A549 cell populations demonstrated high levels of MGMT RNA, MGMT protein, and DNA repair activity. The overexpression of MGMT in lung epithelial cells provided protection from the cytotoxic effects of BCNU. Control A549 cells incubated with 100 microM BCNU had a cell survival rate of 12.5 +/- 1.2%; however, A549 cells overexpressing MGMT had a survival rate of 71.8 +/- 2.7% (P < 0.001). We also demonstrated successful transfection of MGMT into human pulmonary artery endothelial cells and a primary culture of rat type II alveolar epithelial cells with overexpression of MGMT, resulting in significant protection from BCNU toxicity. These data suggest that overexpression of DNA repair proteins such as MGMT in lung cells may protect the lung cells from cytotoxic effects of cancer chemotherapy drugs such as BCNU.

MGMT expression in murine bone marrow is a major determinant of animal survival after alkylating agent exposure

Myelosuppression is commonly observed after alkylating agent chemotherapy due to low levels of O(6)-alkylguanine DNA alkyltransferase protein (AGT) in hematopoietic progenitors. Mice that lack AGT in all organs, O(6)-methylguanine-DNA methyltransferase gene knockout (MGMT(-/-)) mice are extremely hypersensitive to the methylating agent N-methyl-N-nitrosourea (MNU) and exhibit a 10-fold reduction in the LD(90). To determine whether bone marrow damage was the cause of the increased lethality, we transplanted 1 x 10(6) wild-type marrow into MGMT(-/-) mice and MGMT(-/-) marrow into wild-type mice and observed survival after MNU. Lethally irradiated MGMT(-/-) mice given > or = 25 mg/kg MNU 3 weeks after transplant of wild-type cells survived > 30 days (n = 11), whereas this dose was lethal to control MGMT(-/-) mice 9-12 days post treatment (n = 5). Conversely, lethally irradiated wild-type mice transplanted with MGMT(-/-) cells died after only 20-60 mg/kg MNU within 8-12 days (n = 6). No significant toxicities were found in other organs. Additionally, in an in vivo post transplant competition model, wild-type long-term repopulating cells had a > 200-fold competitive survival advantage over MGMT(-/-) cells, and after MNU treatment completely repopulated the mouse when transplanted at only one-tenth the cell number. We also observed a strong selection for transplanted marrow-derived wild-type stromal elements in the MGMT(-/-) background after drug treatment. These data indicate that alkylating agent hypersensitivity of MGMT(-/-) mice results from hematopoietic damage at the stem level. Thus, DNA repair involving AGT in hematopoietic cells is required for normal host survival following exposure to methylating and chloroethylating agents.