MGMT inhibitors--The Trinity College-Paterson Institute experience, a chemist's perception

Trps1 is associated with the multidrug resistance of lung cancer cell by regulating MGMT gene expression

Multidrug resistance (MDR) often leads to chemotherapy failure of lung cancer and has been linking to the cellular expression of several DNA transcription- and repair-related genes such as Trps1 and MGMT. However, their roles in the formation of MDR are largely unknown. In this study, overexpression/knockdown, luciferase assay and ChIP assay were performed to study the relationship between Trps1 and MGMT, as well as their roles in MDR formation. Our results demonstrated that Trps1 and MGMT expression both increased in drug-resistant lung cancer cell line (H446/CDDP). Silencing of Trps1 resulted in downregulation of MGMT expression and decrease in the multidrug sensitivity of H446/CDDP cells, while Trps1 overexpression exhibited the opposite effects in H446 cells. Ectopic expression of MGMT had no effect on Trps1 expression, but enhanced the IC50 values of H446 cells or rescued the IC50 values of Trps1-silenced H446/CDDP cells in treatment of multidrug. Our data further showed that, mechanistically, Trps1 acted as a transcription activator that directly induced MGMT transcription by binding to the MGMT promoter. Taken together, we consider that upregulation of Trps1 induces MGMT transcription contributing to the formation of MDR in lung cancer cells. Our findings proved potential targets for reversing MDR in clinical chemotherapy of lung cancer.

Assembly of fully substituted 2,5-dihydrothiophenes via a novel sequential multicomponent reaction

A sequential multicomponent reaction between ketoesters, isothiocyanates and 1,2-diaza-1,3-dienes to create 2,5-dihydrothiophenes that can be converted into thiophenes.

Regulation of DNA Alkylation Damage Repair: Lessons and Therapeutic Opportunities

Alkylation chemotherapy is one of the most widely used systemic therapies for cancer. While somewhat effective, clinical responses and toxicities of these agents are highly variable. A major contributing factor for this variability is the numerous distinct lesions that are created upon alkylation damage. These adducts activate multiple repair pathways. There is mounting evidence that the individual pathways function cooperatively, suggesting that coordinated regulation of alkylation repair is critical to prevent toxicity. Furthermore, some alkylating agents produce adducts that overlap with newly discovered methylation marks, making it difficult to distinguish between bona fide damaged bases and so-called 'epigenetic' adducts. Here, we discuss new efforts aimed at deciphering the mechanisms that regulate these repair pathways, emphasizing their implications for cancer chemotherapy.

Fluorogenic Real-Time Reporters of DNA Repair by MGMT, a Clinical Predictor of Antitumor Drug Response

Common alkylating antitumor drugs, such as temozolomide, trigger their cytotoxicity by methylating the O6-position of guanosine in DNA. However, the therapeutic effect of these drugs is dampened by elevated levels of the DNA repair enzyme, O6-methylguanine DNA methyltransferase (MGMT), which directly reverses this alkylation. As a result, assessing MGMT levels in patient samples provides an important predictor of therapeutic response; however, current methods available to measure this protein are indirect, complex and slow. Here we describe the design and synthesis of fluorescent chemosensors that report directly on MGMT activity in a single step within minutes. The chemosensors incorporate a fluorophore and quencher pair, which become separated by the MGMT dealkylation reaction, yielding light-up responses of up to 55-fold, directly reflecting repair activity. Experiments show that the best-performing probe retains near-native activity at mid-nanomolar concentrations. A nuclease-protected probe, NR-1, was prepared and tested in tumor cell lysates, demonstrating an ability to evaluate relative levels of MGMT repair activity in twenty minutes. In addition, a probe was employed to evaluate inhibitors of MGMT, suggesting utility for discovering new inhibitors in a high-throughput manner. Probe designs such as that of NR-1 may prove valuable to clinicians in selection of patients for alkylating drug therapies and in assessing resistance that arises during treatment.

One pot synthesis of tetrasubstituted thiophenes: [3 + 2] annulation strategy

A simple, efficient and economical synthesis of dimethyl 3-amino-5-(2-oxo-2-arylethyl)thiophene-2,4-dicarboxylates has been reported by ring opening of methyl 3-amino-6-aryl-4-oxo-4H-thieno[3,2-c]pyran-2-carboxylates by alkoxide ions.

Alkyltransferase-like protein (Atl1) distinguishes alkylated guanines for DNA repair using cation-π interactions

Alkyltransferase-like (ATL) proteins in Schizosaccharomyces pombe (Atl1) and Thermus thermophilus (TTHA1564) protect against the adverse effects of DNA alkylation damage by flagging O(6)-alkylguanine lesions for nucleotide excision repair (NER). We show that both ATL proteins bind with high affinity to oligodeoxyribonucleotides containing O(6)-alkylguanines differing in size, polarity, and charge of the alkyl group. However, Atl1 shows a greater ability than TTHA1564 to distinguish between O(6)-alkylguanine and guanine and in an unprecedented mechanism uses Arg69 to probe the electrostatic potential surface of O(6)-alkylguanine, as determined using molecular mechanics calculations. An unexpected consequence of this feature is the recognition of 2,6-diaminopurine and 2-aminopurine, as confirmed in crystal structures of respective Atl1-DNA complexes. O(6)-Alkylguanine and guanine discrimination is diminished for Atl1 R69A and R69F mutants, and S. pombe R69A and R69F mutants are more sensitive toward alkylating agent toxicity, revealing the key role of Arg69 in identifying O(6)-alkylguanines critical for NER recognition.

Alkylation sensitivity screens reveal a conserved cross-species functionome

To identify genes that contribute to chemotherapy resistance in glioblastoma, we conducted a synthetic lethal screen in a chemotherapy-resistant glioblastoma-derived cell line with the clinical alkylator temozolomide (TMZ) and an siRNA library tailored toward "druggable" targets. Select DNA repair genes in the screen were validated independently, confirming the DNA glycosylases uracil-DNA glycosylase (UNG) and A/G-specific adenine DNA glycosylase (MYH) as well as methylpurine-DNA glycosylase (MPG) to be involved in the response to high dose TMZ. The involvement of UNG and MYH is likely the result of a TMZ-induced burst of reactive oxygen species. We then compared the human TMZ sensitizing genes identified in our screen with those previously identified from alkylator screens conducted in Escherichia coli and Saccharomyces cerevisiae. The conserved biologic processes across all three species compose an alkylation functionome that includes many novel proteins not previously thought to impact alkylator resistance. This high-throughput screen, validation and cross-species analysis was then followed by a mechanistic analysis of two essential nodes: base excision repair (BER) DNA glycosylases (UNG, human and mag1, S. cerevisiae) and protein modification systems, including UBE3B and ICMT in human cells or pby1, lip22, stp22 and aim22 in S. cerevisiae. The conserved processes of BER and protein modification were dual targeted and yielded additive sensitization to alkylators in S. cerevisiae. In contrast, dual targeting of BER and protein modification genes in human cells did not increase sensitivity, suggesting an epistatic relationship. Importantly, these studies provide potential new targets to overcome alkylating agent resistance.

N-methylpurine DNA glycosylase and DNA polymerase beta modulate BER inhibitor potentiation of glioma cells to temozolomide

Temozolomide (TMZ) is the preferred chemotherapeutic agent in the treatment of glioma following surgical resection and/or radiation. Resistance to TMZ is attributed to efficient repair and/or tolerance of TMZ-induced DNA lesions. The majority of the TMZ-induced DNA base adducts are repaired by the base excision repair (BER) pathway and therefore modulation of this pathway can enhance drug sensitivity. N-methylpurine DNA glycosylase (MPG) initiates BER by removing TMZ-induced N3-methyladenine and N7-methylguanine base lesions, leaving abasic sites (AP sites) in DNA for further processing by BER. Using the human glioma cell lines LN428 and T98G, we report here that potentiation of TMZ via BER inhibition [methoxyamine (MX), the PARP inhibitors PJ34 and ABT-888 or depletion (knockdown) of PARG] is greatly enhanced by over-expression of the BER initiating enzyme MPG. We also show that methoxyamine-induced potentiation of TMZ in MPG expressing glioma cells is abrogated by elevated-expression of the rate-limiting BER enzyme DNA polymerase β (Polβ), suggesting that cells proficient for BER readily repair AP sites in the presence of MX. Further, depletion of Polβ increases PARP inhibitor-induced potentiation in the MPG over-expressing glioma cells, suggesting that expression of Polβ modulates the cytotoxic effect of combining increased repair initiation and BER inhibition. This study demonstrates that MPG overexpression, together with inhibition of BER, sensitizes glioma cells to the alkylating agent TMZ in a Polβ-dependent manner, suggesting that the expression level of both MPG and Polβ might be used to predict the effectiveness of MX and PARP-mediated potentiation of TMZ in cancer treatment.

Cell-centric view of apoptosis and apoptotic cell death-inducing antitumoral strategies

Programmed cell death and especially apoptotic cell death, occurs under physiological conditions and is also desirable under pathological circumstances. However, the more we learn about cellular signaling cascades, the less plausible it becomes to find restricted and well-limited signaling pathways. In this context, an extensive description of pathway-connections is necessary in order to point out the main regulatory molecules as well as to select the most appropriate therapeutic targets. On the other hand, irregularities in programmed cell death pathways often lead to tumor development and cancer-related mortality is projected to continue increasing despite the effort to develop more active and selective antitumoral compounds. In fact, tumor cell plasticity represents a major challenge in chemotherapy and improvement on anticancer therapies seems to rely on appropriate drug combinations. An overview of the current status regarding apoptotic pathways as well as available chemotherapeutic compounds provides a new perspective of possible future anticancer strategies.

Correlation of O6-methylguanine methyltransferase (MGMT) promoter methylation with clinical outcomes in glioblastoma and clinical strategies to modulate MGMT activity

Resistance to alkylating agents via direct DNA repair by O(6)-methylguanine methyltransferase (MGMT) remains a significant barrier to the successful treatment of patients with malignant glioma. The relative expression of MGMT in the tumor may determine response to alkylating agents, and epigenetic silencing of the MGMT gene by promoter methylation plays an important role in regulating MGMT expression in gliomas. MGMT promoter methylation is correlated with improved progression-free and overall survival in patients treated with alkylating agents. Strategies to overcome MGMT-mediated chemoresistance are being actively investigated. These include treatment with nontoxic pseudosubstrate inhibitors of MGMT, such as O(6)-benzylguanine, or RNA interference-mediated gene silencing of MGMT. However, systemic application of MGMT inhibitors is limited by an increase in hematologic toxicity. Another strategy is to deplete MGMT activity in tumor tissue using a dose-dense temozolomide schedule. These alternative schedules are well tolerated; however, it remains unclear whether they are more effective than the standard dosing regimen or whether they effectively deplete MGMT activity in tumor tissue. Of note, not all patients with glioblastoma having MGMT promoter methylation respond to alkylating agents, and even those who respond will inevitably experience relapse. Herein we review the data supporting MGMT as a major mechanism of chemotherapy resistance in malignant gliomas and describe ongoing studies that are testing resistance-modulating strategies.