S-alkylthiolation of O6-methylguanine-DNA-methyltransferase (MGMT) to sensitize cancer cells to anticancer therapy

O6-methylguanine DNA methyltransferase/O6-alkylguanine DNA alkyltransferase (MGMT/AGT) removes alkyl adducts from the O6-position of guanine in DNA. Expression of MGMT in human cancers has been associated with resistance to therapies using alkylating agents. MGMT promoter methylation regulates its expression and response to alkylating agents. A combination of O6-benzylguanine-based inhibitors of MGMT with alkylating agents improved the efficacy. However, this is associated with enhanced cytotoxicity and the induction of GC to AT transition mutations presumably also in progenitor/stem cells. A few recent studies have described analogs of O6-benzylguanine targeting defined pathways of cancer cells that can be used to improve the selectivity of O6-benzylguanine-based inhibitors for cancer cells. Therefore, MGMT inhibitor targeting represents a reliable strategy for improving cancer therapy with alkylating agents.     

S-Alkylthiolation of O6-methylguanine-DNA-methyltransferase (MGMT) to sensitize cancer cells to anticancer therapy

O⁶-Methylguanine DNA methyltransferase/O⁶-alkylguanine DNA alkyltransferase (MGMT/AGT) removes alkyl adducts from the O⁶-position of guanine in DNA. Expression of MGMT in human cancers has been associated with resistance to therapies using alkylating agents. MGMT promoter methylation regulates its expression and response to alkylating agents. A combination of O⁶-benzylguanine-based inhibitors of MGMT with alkylating agents improved the efficacy. However, this is associated with enhanced cytotoxicity and the induction of GC to AT transition mutations presumably also in progenitor/stem cells. A few recent studies have described analogs of O⁶-benzylguanine targeting defined pathways of cancer cells that can be used to improve the selectivity of O⁶-benzylguanine-based inhibitors for cancer cells. Therefore, MGMT inhibitor targeting represents a reliable strategy for improving cancer therapy with alkylating agents.     

The L84F and the I143V polymorphisms in the O6-methylguanine-DNA-methyltransferase (MGMT) gene increase human sensitivity to the genotoxic effects of the tobacco-specific nitrosamine carcinogen NNK

O-Methylguanine-DNA-methyltransferase (MGMT) is a direct-reversal DNA repair protein that removes DNA adducts formed by alkylating mutagens found in tobacco smoke. Several coding single nucleotide polymorphisms (cSNPs) in the MGMT gene have been reported. However, their effect on the levels and types of genetic damage induced by specific environmental carcinogens remains to be fully elucidated. We developed two novel genotyping techniques and used them, in conjunction with the mutagen-sensitivity assay, to test the hypothesis that the L84F and I143V cSNPs in the MGMT gene confer increased sensitivity to genetic damage induced by the alkylating tobacco-specific nitrosamine carcinogen NNK. Lymphocytes from 114 healthy volunteers were exposed in vitro to NNK, and the genotoxic response was assessed by measuring chromosome aberration (CA) frequencies. A significant (P<0.02) increase in NNK-induced CA was observed in cells from individuals with the 84F polymorphism compared to cells from individuals homozygous for the referent L84 allele. A significant positive interaction between this cSNP and smoking, gender and age was observed (P<0.03). In subjects with the variant 143V allele, significantly higher levels of NNK-induced CA were observed in males and in young subjects (<43 years old) compared to subjects homozygous for the referent I143 allele (P<0.02). Individuals who inherited two cSNPs had significantly higher levels of NNK-induced CA compared to individuals with none or with one cSNP (P<0.002). These new data suggest that the 84F and 143V cSNPs may alter the function characteristics of the MGMT protein, resulting in suboptimal repair of genetic damage induced by NNK.     

肺癌患者O6-甲基鸟嘌呤-DNA甲基转移酶基因多态性研究

目的 检测人类O^6－甲基鸟嘌呤－DNA甲基转移酶（MGMT）基因5个外显子的单核苷酸多态（SNPs）及其与肺癌的关系。采用聚合酶链式反应（PCR）－单链构象多态性（SSCP）－DNA直接测序法对100例正常人和60例肺癌患者外周血白细胞进行MGMT基因SNPs研究。结果 在正常人群和肺癌患者中检测到MGMT基因的第3、4外子上存在着SNPs，即第3外显子上第250位碱基C被T所取代，导致错义突变（CTT→TTT，使Leu84Pphe）；第4外显子第280位碱基C被T取代，导致同义突变（TTC→TTT，均编码Phe）。这两个位点的SNPs在两组人群间的检出率差异无显著性（P＞0．05）。结论 MGMT基因第3、4外显子上存在着SNPs，其侈态改变与肺癌形成的关系较弱。     

The anti-cancer actions of O6-methylguanine-DNA-methyltransferase in relation to colon polyps

BackgroundGenetic variability in DNA repair genes may contribute to differences in DNA repair capacity and susceptibility to colon polyps and cancer. In this study, we examined the role of MGMT polymorphisms in colon polyps formation.MethodsPCR–SSCP analysis was performed included 254 patients with colon polyps and 330 controls.ResultsThe homozygous F84F genotype was significantly more prevalent in study group than in controls. The polymorphic allele 84F was more frequent appeared in group of older patients and in group of smoking patients. On the other hand, there were no association between 84F and gender, size of polyps, cancer family history.ConclusionsWe concluded that high frequency of 84F allele in the group of patients may suggest the role of the MGMT variant in colon polyps etiology.     

Synthesis of 131I-labeled glucose-conjugated inhibitors of O6-methylguanine-DNA methyltransferase (MGMT) and comparison with nonconjugated inhibitors as potential tools for in vivo MGMT imaging

O(6)-Substituted guanine derivatives are powerful agents used for tumor cell sensitization by inhibition of the DNA repair enzyme O(6)-methylguanine-DNA methyltransferase (MGMT). To provide targeted accumulation of MGMT inhibitors in tumor tissue as well as tools for in vivo imaging, we synthesized iodinated C(8)-alkyl-linked glucose conjugates of 2-amino-6-(5-iodothenyl)-9H-purine (O(6)-(5-iodothenyl) guanine, ITG) and 2-amino-6-(3-iodobenzyloxy)-9H-purine (O(6)-(5-iodobenzyl) guanine, IBG). These compounds have MGMT inhibitor constants (IC(50) values) of 0.8 and 0.45 microM for ITGG and IBGG, respectively, as determined in HeLa S3 cells after 2-h incubation with inhibitor. To substantiate that the (131)I-(hetero)arylmethylene group at the O(6)-position of guanine is transferred to MGMT, both the glucose conjugated inhibitors ITGG and IBGG and the corresponding nonglucose conjugated compounds ITG and IBG were labeled with iodine-131. The radioiodinations of all compounds with [(131)I]I(-) were performed with radiochemical yields of >70% for the destannylation of the corresponding tri-n-butylstannylated precursors. The binding ability of [(131)I]ITGG, [(131)]IBGG, [(131)I]ITG, and [(131)I]IBG to purified MGMT was tested. All radioactive compounds were substrates for MGMT, as demonstrated using a competitive repair assay. The newly synthesized radioactive inhibitors were utilized to study ex vivo biodistribution in mice, and the tumor-to-blood ratio of tissue uptake of [(131)I]IBG and [(131)I]IBGG was determined to be 0.24 and 0.76 after 0.5 h, respectively.     

Monosaccharide-linked inhibitors of O(6)-methylguanine-DNA methyltransferase (MGMT): synthesis, molecular modeling, and structure-activity relationships.

A series of potential inhibitors of the human DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) were synthesized, characterized in detail by NMR, and tested for their ability to deplete MGMT activity in vitro. The new compounds, omega-[O(6)-R-guan-9-yl]-(CH(2))(n)-beta-d-glucosides with R = benzyl or 4-bromothenyl and omega = n = 2, 4,. 12, were compared with the established inhibitors O(6)-benzylguanine (O(6)-BG), 8-aza-O(6)-benzylguanine (8-aza-BG), and O(6)-(4-bromothenyl)guanine (4-BTG), which exhibit in an in vitro assay IC(50) values of 0.62, 0.038, and 0.009 microM, respectively. Potential advantages of the glucosides are improved water solubility and selective uptake in tumor cells. The 4-BTG glucosides with n = 2, 4, 6 show moderate inhibition with an IC(50) of ca. 0.5 microM, while glucosides derived from BG and 8-aza-BG showed significantly poorer inhibition compared to the parent compounds. The 4-BTG glucosides with n = 8, 10, 12 were effective inhibitors with IC(50) values of ca. 0.03 microM. To understand this behavior, extensive molecular modeling studies were performed using the published crystal structure of MGMT (PDB entry: ). The inhibitor molecules were docked into the BG binding pocket, and molecular dynamics simulations with explicit water molecules were carried out. Stabilization energies for the interactions of specific regions of the inhibitor and individual amino acid residues were calculated. The alkyl spacer is located in a cleft along helix 6 of MGMT. With increasing spacer length there is increasing interaction with several amino acid residues which play an important role in the proposed nucleotide flipping mechanism required for DNA repair.     

O(6)-methylguanine DNA-methyltransferase (MGMT) overexpression in melanoma cells induces resistance to nitrosoureas and temozolomide but sensitizes to mitomycin C

Alkylating agents play an important role in the chemotherapy of malignant melanomas. The activity of alkylating agents depends on their capacity to form alkyl adducts with DNA, in some cases causing cross-linking of DNA strands. However, the use of these agents is limited by cellular resistance induced by the DNA repair enzyme O(6)-methylguanine DNA-methyltransferase (MGMT) which removes alkyl groups from alkylated DNA strands. To determine to what extent the expression of MGMT in melanoma cells induces resistance to alkylating agents, the human cell line CAL77 Mer- (i.e., MGMT deficient) were transfected with pcMGMT vector containing human MGMT cDNA. Several clones expressing MGMT at a high level were selected to determine their sensitivity to chemotherapeutic drugs. Melanoma-transfected cells were found to be significantly less sensitive to nitrosoureas (carmustine, fotemustine, streptozotocin) and temozolomide with an increase of IC(50) values between 3 and 14 when compared to parent cells. No difference in cell survival rates between MGMT-proficient and -deficient cells was observed for melphalan, chlorambucil, busulphan, thiotepa and cisplatin which preferentially induce N(7) guanine lesions. Surprisingly, MGMT overexpression increased the sensitivity of CAL77 cells to mitomycin C by approximately 10-fold. Treatment of clonal cell lines with buthionine-[S,R]-sulfoximine (BSO), an inhibitor of gamma-glutamylcysteine synthetase which depletes cellular glutathione, completely reversed this unexpected increase in sensitivity to mitomycin C. This observation suggests that glutathione is involved in the sensitivity of MGMT-transfected cells to mitomycin C and may act synergistically with MGMT via an unknown mechanism.     

Mutagenesis by O(6)-methyl-, O(6)-ethyl-, and O(6)-benzylguanine and O(4)-methylthymine in human cells: effects of O(6)-alkylguanine-DNA alkyltransferase and mismatch repair.

Double-stranded and gapped shuttle vectors were used to study mutagenesis in human cells by O(6)-methyl (m(6)G)-, O(6)-ethyl (e(6)G)-, and O(6)-benzylguanine (b(6)G), and O(4)-methylthymine (m(4)T) when these bases were incorporated site-specifically in the ATG initiation codon of a lacZ' gene. Vectors were transfected into either human kidney cells (293) or colon tumor cells (SO) or into mismatch repair defective human colon tumor cells (H6 and LoVo). Cellular O(6)-alkylguanine-DNA alkyltransferase (alkyltransferase) was optionally inactivated by treating cells with O(6)-benzylguanine prior to transfection. In alkyltransferase competent cells, the mutagenicity of all the modified bases was substantially higher in gapped plasmids than in double-stranded plasmids. Alkyltransferase inactivation increased mutagenesis by the three O(6)-substituted guanines in both double-stranded and gapped plasmids but did not affect m(4)T mutagenesis. In the absence of alkyltransferase, mutagenesis by m(6)G and to a lesser extent e(6)G in double-stranded vectors was higher in the mismatch repair defective H6 and LoVo cells than in SO or 293 cells indicating that e(6)G as well as m(6)G were subject to mismatch repair processing in these cells. The level of mutagenesis by m(4)T and b(6)G was not affected by mismatch repair status. When incorporated in gapped plasmids and in the absence of alkyltransferase, the order of mutagenicity for the modified bases was m(4)T > e(6)G congruent with m(6)G > b(6)G. The O(6)-substituted guanines primarily produced G-->A transitions while m(4)T primarily produced T-->C transitions. However, m(4)T also produced a significant number of T-->A transversion mutations in addition to T-->C transitions in mismatch repair deficient LoVo cells.     

Interaction of mammalian O(6)-alkylguanine-DNA alkyltransferases with O(6)-benzylguanine

Human O(6)-alkylguanine-DNA alkyltransferase (hAGT) activity is a major factor in providing resistance to cancer chemotherapeutic alkylating agents. Inactivation of hAGT by O(6)-benzylguanine (BG) is a promising strategy for overcoming this resistance. Previous studies, which have focused on the region encompassed by residues Pro138 to Gly173, have identified more than 100 individual mutations located at 23 discrete sites at which alterations can render AGT less sensitive to BG. We have now extended the examination of possible sites in hAGT at which alterations might lead to BG resistance to include the residues from Val130 to Asn137, which also make up part of the binding pocket into which BG is postulated to fit. A further 21 mutations located at positions Gly132, Met134, Arg135, and Gly136 were found to lower sensitivity to BG. Mutants R135L, R135Y, and G136P were the most strikingly resistant, with a 50-fold increase in the amount of BG needed to obtain 50% inactivation. These results therefore increase the number of sites at which BG resistance can occur in response to a single amino acid change to 27. Although mammalian AGTs are very similar in amino acid sequence, mouse AGT (mAGT) is significantly less sensitive to BG than rat AGT (rAGT) or hAGT. Construction of chimeric proteins in which portions came from the rAGT and the mAGT indicated that the difference in inactivation resided solely in the amino acids located in the sequence from residues 150 to 188. Individual mutations of the three residues where rAGT and mAGT differ in this region showed that the principal reason for the reduced ability of the mAGT to react with BG was the presence of a histidine residue at position 161, which is occupied by asparagine in rAGT and hAGT. These experiments indicate that many minor changes in amino acids forming all parts of the nucleoside binding pocket of AGT can alter its ability to react with BG and that the possibility that polymorphisms or variants may occur reducing the effectiveness of combination therapy with BG and alkylating agents must be considered.     

Resistance-modifying agents. 8. Inhibition of O(6)-alkylguanine-DNA alkyltransferase by O(6)-alkenyl-, O(6)-cycloalkenyl-, and O(6)-(2-oxoalkyl)guanines and potentiation of temozolomide cytotoxicity in vitro by O(6)-(1-cyclopentenylmethyl)guanine

A series of O(6)-allyl- and O(6)-(2-oxoalkyl)guanines were synthesized and evaluated, in comparison with the corresponding O(6)-alkylguanines, as potential inhibitors of the DNA-repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT). Simple O(6)-alkyl- and O(6)-cycloalkylguanines were weak AGT inactivators compared with O(6)-allylguanine (IC(50) = 8.5 +/- 0.6 microM) with IC(50) values ranging from 100 to 1000 microM. The introduction of substituents at C-2 of the allyl group of O(6)-allylguanine reduced activity compared with the parent compound, while analogous compounds in the O(6)-(2-oxoalkyl)guanine series exhibited very poor activity (150-1000 microM). O(6)-Cycloalkenylguanines proved to be excellent AGT inactivators, with 1-cyclobutenylmethylguanine (IC(50) = 0.55 +/- 0.02 microM) and 1-cyclopentenylmethylguanine (IC(50) = 0.39 +/- 0.04 microM) exhibiting potency approaching that of the benchmark AGT inhibitor O(6)-benzylguanine (IC(50) = 0.18 +/- 0.02 microM). 1-Cyclopentenylmethylguanine also inactivated AGT in intact HT29 human colorectal carcinoma cells (IC(50) = 0.20 +/- 0.07 microM) and potentiated the cytotoxicity of the monomethylating antitumor agent Temozolomide by approximately 3- and 10-fold, respectively, in the HT29 and Colo205 tumor cell lines. The observation that four mutant AGT enzymes resistant to O(6)-benzylguanine also proved strongly cross-resistant to 1-cyclopentenylmethylguanine indicates that the O(6)-substituent of each compound makes similar binding interactions within the active site of AGT.     

Debenzylation of O(6)-benzyl-8-oxoguanine in human liver: implications for O(6)-benzylguanine metabolism

O(6)-Benzylguanine (BG) effectively inactivates the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase, and enhances the effectiveness of 1,3-bis(2-chloroethyl)-1-nitrosourea in cells in culture and tumor-bearing animals. BG is presently in phase II clinical trials. In humans, BG is converted to O(6)-benzyl-8-oxoguanine (8-oxoBG), a longer-lived, yet equally potent inactivator. We have isolated and identified the debenzylated product, 8-oxoguanine, in plasma and urine of patients following administration of BG. The purpose of this work was to determine the human liver enzymes responsible for the debenzylation of 8-oxoBG. Therefore, 8-oxoBG was incubated with human liver microsomes and cytosol, and the concentration of 8-oxoguanine was determined. No appreciable product was formed in the cytosol; however, increasing amounts of 8-oxoguanine were formed with increasing concentrations of pooled human liver microsomes. The amount of 8-oxoguanine formed increased with time and substrate concentration. Co-incubation of human liver microsomes with 8-oxoBG and various cytochrome P450 isoform-selective inhibitors suggested the possible involvement of CYP1A2, 2E1, and/or 2A6 in this reaction. Incubation of 8-oxoBG with baculovirus cDNA-overexpressed CYP1A2, 2E1, 2A6, and 3A4 demonstrated that formation of 8-oxoguanine was due mainly to CYP1A2. Debenzylation of 8-oxoBG complied with Michaelis-Menten kinetics with K(m) and V(max) values of 35.9 microM and 0.59 pmol/min/pmol of CYP1A2, respectively. CYP1A2 appears to be mainly responsible for the debenzylation of 8-oxoBG in human liver.     

Inactivation of O(6)-alkylguanine-DNA alkyltransferase by folate esters of O(6)-benzyl-2'-deoxyguanosine and of O(6)-[4-(hydroxymethyl)benzyl]guanine

O6-Alkylguanine-DNA alkyltransferase (alkyltransferase) provides an important source of resistance to some cancer chemotherapeutic alkylating agents. Folate ester derivatives of O6-benzyl-2'-deoxyguanosine and of O6-[4-(hydroxymethyl)benzyl]guanine were synthesized and tested for their ability to inactivate human alkyltransferase. Inactivation of alkyltransferase by the gamma-folate ester of O6-[4-(hydroxymethyl)benzyl]guanine was similar to that of the parent base. The gamma-folate esters of O6-benzyl-2'-deoxyguanosine were more potent alkyltransferase inactivators than the parent nucleoside. The 3'-ester was considerably more potent than the 5'-ester and was more than an order of magnitude more active than O6-benzylguanine, which is currently in clinical trials to enhance therapy with alkylating agents. They were also able to sensitize human tumor cells to killing by 1,3-bis(2-chloroethyl)-1-nitrosourea, with O6-benzyl-3'-O-(gamma-folyl)-2'-deoxyguanosine being most active. These compounds provide a new class of highly water-soluble alkyltransferase inactivators and form the basis to construct more tumor-specific and potent compounds targeting this DNA repair protein.     

Exploiting cyclooxygenase-(in)dependent properties of COX-2 inhibitors for malignant glioma therapy

Cyclooxygenase 2 (COX-2) is frequently found up-regulated during pathological conditions and in cancer, where it is thought to support carcinogenesis and tumor angiogenesis. The development of newer-generation non-steroidal anti-inflammatory drugs (NSAIDs) able to more selectively inhibit cyclooxygenase 2 (COX-2) raised expectations that these agents might be beneficial for cancer prevention and therapy. However, while chemopreventive effects of some selective COX-2 inhibitors have been established, it has remained unpersuasive whether these new NSAIDs, such as celecoxib, rofecoxib or etoricoxib, are able to exert cancer therapeutic effects, i.e., whether they would be beneficial for the treatment of advanced cancers that are already grown and established. This issue was further complicated by findings that celecoxib was able to exert pronounced pro-apoptotic effects in vitro and in vivo in the absence of any apparent involvement of COX-2. In fact, newly synthesized close structural analogs of the celecoxib molecule revealed that it was possible to separate COX-2 inhibitory function from the ability to trigger apoptosis; for example, the analog 2,5-dimethyl-celecoxib (DMC) has lost COX-2 inhibitory function, yet exerts increased cytotoxic potency. This review will summarize pertinent results from the exploratory therapeutic use of NSAIDs, in particular celecoxib, in preclinical and clinical studies of malignant glioma. Several COX-2 independent targets will be presented, and it will be discussed how DMC has helped to delineate their relevance for the surmised COX-2 independent tumoricidal effects of celecoxib.