Poly (ADP-ribose) polymerase inhibitor increases apoptosis and reduces necrosis induced by a DNA minor groove binding methyl sulfonate ester

Anti-cancer activity of benzoxazinone derivatives via targeting c-Myc G-quadruplex structure

AIMS

This study aimed to analyze the impact of four synthesized benzoxazinone derivatives as screening drugs on c-Myc-overexpressed cancer cells (H7402, HeLa, SK-RC-42, SGC7901, and A549) and to explore their interaction mechanisms in detail.

MATERIALS AND METHODS

Using morphological analysis, real-time cytotoxicity analysis, wound healing assay, reverse transcription PCR, electrophoretic mobility shift assay, and circular dichroism spectroscopy techniques.

KEY FINDINGS

Results revealed that these four compounds could inhibit proliferation of SK-RC-42, SGC7901, and A549 cells in five cancer cell lines to varying degrees and significantly hinder migration. More importantly, the RT-PCR assay showed that the compounds could surprisingly downregulate the expression of c-Myc mRNA in a dose-dependent manner in the five cancer cells, which may be one of the causes of cancer cell proliferation in vitro inhibition. Further EMSA assays demonstrated that at the molecular level of DNA, four compounds can induce the formation of G-quadruplexes (G4-DNAs) in the c-Myc gene promoter. In addition, the CD result of compound 1 clearly indicates that it specifically induces a c-Myc GC-rich 36mer double-stranded DNA in the c-Myc promoter to form a G-quadruplex hybrid configuration. In conclusion, the compounds studied could dose-dependently inhibit the growth and migration of the cancer cells being investigated. This is positively associated with the reduction of overexpression of the c-Myc gene, which may be significantly regulated by the association of compounds with the G-quadruplexes produced in the c-Myc gene promoter region.

SIGNIFICANCE

We conclude that three compounds merit further study, particularly against non-small-cell lung cancer, as leading compounds of anticancer drugs.

Minor Groove 3-Deaza-Adenosine Analogues: Synthesis and Bypass in Translesion DNA Synthesis

Anticancer drugs that alkylate DNA in the minor groove may give rise to 3-alkyl-adenosine adducts that interfere with replication, inducing apoptosis in rapidly dividing cancer cells. However, translesion DNA synthesis (TLS) by polymerase enzymes (Pols) with the capacity to bypass DNA adducts may contribute to damage tolerance and drug resistance. 3-Alkyl-adenosine adducts are unstable and depurinate, which is a barrier to addressing chemical and enzymatic aspects of how they impact the progress of DNA Pols. To characterize structure-based relationships of 3-adenine alkylation relevant to cancer drugs on duplex stability and DNA Pol-catalyzed DNA synthesis, we synthesized stable 3-deaza-3-alkyl-adenosine analogues, including 3-deaza-3-phenethyl-adenosine and 3-deaza-3-methoxynaphthylethyl-adenosine, and incorporated them into oligonucleotides. A moderate reduction of duplex stability was observed on the basis of thermal denaturation data. Replication studies using purified Y-family human DNA Pols hPol η, κ, and ι indicated that these enzymes can perform TLS over the modified bases. hPol η had higher misincorporation rates when synthesizing opposite the modified bases compared with adenine, whereas hPol κ and ι maintained high fidelity. These results provide insight into how alterations in chemical structure reduce bypass of minor-groove adducts, and provide novel chemical probes for evaluating minor-groove DNA alkylation.

Differential regulation of cell death programs in males and females by Poly (ADP-Ribose) Polymerase-1 and 17β estradiol

Cell death can be divided into the anti-inflammatory process of apoptosis and the pro-inflammatory process of necrosis. Necrosis, as apoptosis, is a regulated form of cell death, and Poly-(ADP-Ribose) Polymerase-1 (PARP-1) and Receptor-Interacting Protein (RIP) 1/3 are major mediators. We previously showed that absence or inhibition of PARP-1 protects mice from nephritis, however only the male mice. We therefore hypothesized that there is an inherent difference in the cell death program between the sexes. We show here that in an immune-mediated nephritis model, female mice show increased apoptosis compared to male mice. Treatment of the male mice with estrogens induced apoptosis to levels similar to that in female mice and inhibited necrosis. Although PARP-1 was activated in both male and female mice, PARP-1 inhibition reduced necrosis only in the male mice. We also show that deletion of RIP-3 did not have a sex bias. We demonstrate here that male and female mice are prone to different types of cell death. Our data also suggest that estrogens and PARP-1 are two of the mediators of the sex-bias in cell death. We therefore propose that targeting cell death based on sex will lead to tailored and better treatments for each gender.

PARP-1: Friend or Foe of DNA Damage and Repair in Tumorigenesis?

Oxidative stress induced by reactive oxygen species can result in DNA damage within cells and subsequently increase risk for carcinogenesis. This may be averted by repair of DNA damage through the base or nucleotide excision repair (BER/NER) pathways. PARP, a BER protein, is known for its role in DNA-repair. However, multiple lesions can occur within a small range of DNA, known as oxidative clustered DNA lesions (OCDLs), which are difficult to repair and may lead to the more severe DNA double-strand break (DSB). Inefficient DSB repair can then result in increased mutagenesis and neoplastic transformation. OCDLs occur more frequently within a variety of tumor tissues. Interestingly, PARP is highly expressed in several human cancers. Additionally, chronic inflammation may contribute to tumorigenesis through ROS-induced DNA damage. Furthermore, PARP can modulate inflammation through interaction with NFκB and regulating the expression of inflammatory signaling molecules. Thus, the upregulation of PARP may present a double-edged sword. PARP is needed to repair ROS-induced DNA lesions, but PARP expression may lead to increased inflammation via upregulation of NFκB signaling. Here, we discuss the role of PARP in the repair of oxidative damage versus the formation of OCDLs and speculate on the feasibility of PARP inhibition for the treatment and prevention of cancers by exploiting its role in inflammation.

Poly(ADP-ribose) signaling in cell death

Poly(ADP-ribosyl)ation (PARylation) is a reversible protein modification carried out by the concerted actions of poly(ADP-ribose) polymerase (PARP) enzymes and poly(ADP-ribose) (PAR) decomposing enzymes such as PAR glycohydrolase (PARG) and ADP-ribosyl hydrolase 3 (ARH3). Reversible PARylation is a pleiotropic regulator of various cellular functions but uncontrolled PARP activation may also lead to cell death. The cellular demise pathway mediated by PARylation in oxidatively stressed cells has been described almost thirty years ago. However, the underlying molecular mechanisms have only begun to emerge relatively recently. PARylation has been implicated in necroptosis, autophagic cell death but its role in extrinsic and intrinsic apoptosis appears to be less predominant and depends largely on the cellular model used. Currently, three major pathways have been made responsible for PARP-mediated necroptotic cell death: (1) compromised cellular energetics mainly due to depletion of NAD, the substrate of PARPs; (2) PAR mediated translocation of apoptosis inducing factor (AIF) from mitochondria to nucleus (parthanatos) and (3) a mostly elusive crosstalk between PARylation and cell death/survival kinases and phosphatases. Here we review how these PARP-mediated necroptotic pathways are intertwined, how PARylation may contribute to extrinsic and intrinsic apoptosis and discuss recent developments on the role of PARylation in autophagy and autophagic cell death.

Small-molecule inhibitors of DNA damage-repair pathways: an approach to overcome tumor resistance to alkylating anticancer drugs

A major challenge in the future development of cancer therapeutics is the identification of biological targets and pathways, and the subsequent design of molecules to combat the drug-resistant cells hiding in virtually all cancers. This therapeutic approach is justified based upon the limited advances in cancer cures over the past 30 years, despite the development of many novel chemotherapies and earlier detection, which often fail due to drug resistance. Among the various targets to overcome tumor resistance are the DNA repair systems that can reverse the cytotoxicity of many clinically used DNA-damaging agents. Some progress has already been made but much remains to be done. We explore some components of the DNA-repair process, which are involved in repair of alkylation damage of DNA, as targets for the development of novel and effective molecules designed to improve the efficacy of existing anticancer drugs.

DNA site-specific N3-adenine methylation targeted to estrogen receptor-positive cells

A compound that can target cells expressing the estrogen receptor (ER), and produce predominantly 3-MeA adducts in those cells has been designed and synthesized. This compound produces mainly the 3-MeA adduct upon reaction with calf thymus DNA, and binds to the ER with a relative binding affinity of 51% (estradiol = 100%). The compound is toxic to ER-expressing MCF-7 breast cancer cells, and pre-treatment with the ER antagonist fulvestrant abrogates the toxicity. Pre-treatment of MCF-7 cells with netropsin, which inhibits N3-adenine methylation by the compound, resulted in a threefold decrease in the toxicity. These results demonstrate the feasibility of this strategy for producing 3-MeA adducts in targeted cells.

Poly(ADP-ribose) polymerase plays a differential role in DNA damage-response and cell death pathways in Trypanosoma cruzi

Poly(ADP-ribosyl)ation is a post-translational modification of proteins. Poly(ADP-ribose) polymerase (PARP) and poly(ADP-ribose) glycohydrolase (PARG) are the enzymes responsible for poly(ADP-ribose) (PAR) polymer metabolism and are present in most higher eukaryotes. The best understood role of PARP is the maintenance of genomic integrity either via promotion of DNA repair at low levels of genotoxic stress or via promotion of cell death at higher levels of damage. The unicellular eukaryote Trypanosoma cruzi, as opposed to humans and other organisms, has only one PARP (TcPARP) and one PARG (TcPARG). In the present study we show that under different DNA-damaging agents (H(2)O(2) or UV-C radiation) TcPARP is activated and translocated from the cytosol to the nucleus, while TcPARG always shows a nuclear localisation. Parasites in the presence of PARP or PARG inhibitors, as well as parasites over-expressing either TcPARP or TcPARG, suggested that PAR metabolism could be involved in different phases of cell growth, even in the absence of DNA damage. We also believe that we provide the first reported evidence that different proteins could be poly(ADP-ribosyl)ated in response to different stimuli, leading to different cell death pathways.

XRCC1 deficiency influences the cytotoxicity and the genomic instability induced by Me-lex, a specific inducer of N3-methyladenine

Me-lex is a sequence-specific alkylating agent synthesized to preferentially (>90%) generate N3-methyladenine (3-mA) in the minor groove of double-strand DNA, in A-T rich regions. In this paper we investigated the effect of XRCC1 deficiency in the processing of 3-mA adducts generated by Me-lex, through the molecular analysis of the Hprt mutations and the evaluation of cytogenetic end points such as sister chromatid exchanges (SCEs), micronuclei (MN) and nucleus fragmentation. EM-C11 cells, deficient in XRCC1 activity, showed a 2.5-fold higher sensitivity to the toxicity of Me-lex compared to the DNA repair proficient parental CHO-9 cells, but were not hyper mutable. The spontaneous mutation spectrum at the Hprt locus generated in EM-C11 cells revealed a high percentage of genomic deletions. After Me-lex treatment, the percentage of genomic deletions did not increase, but a class of mutations which appeared to target regulatory regions of the gene significantly increased (p=0.0277), suggesting that non-coding Hprt genomic sequences represent a strong target for the rare mutations induced by Me-lex. The number of SCEs per chromosome increased 3-fold above background in 50mucapital EM, Cyrillic Me-lex treated CHO-9 cells, while at higher Me-lex concentrations a sharp increase in the percentage of MN and fragmented nuclei was observed. In EM-C11 cells the background level of SCEs (0.939+/-0.182) was approximately 10-fold higher than in CHO-9 (0.129+/-0.027) and higher levels of multinucleated cells and MN were also found. In EM-C11, even low doses of Me-lex (25microM) led to a significant increase in genomic damage. These results indicate that XRCC1 deficiency can lead to genomic instability even in the absence of an exogenous genotoxic insult and low levels of Me-lex-induced lesions, i.e., 3-mA and/or a BER intermediate, can exacerbate this instability.

High frequency of genomic deletions induced by Me-lex, a sequence selective N3-adenine methylating agent, at the Hprt locus in Chinese hamster ovary cells

We have investigated the mutagenicity induced at the Hprt locus in Chinese hamster ovary (CHO) cells treated with increasing concentrations of Me-lex, a minor groove selective methylating agent that efficiently generates more than 90-95% of 3-MeA DNA adducts. Me-lex treatment was cytotoxic but weakly mutagenic, resulting in up to 7-fold induction above background in the Hprt mutation frequency. The molecular nature of 43 Hprt mutations induced by Me-lex was determined by sequence analysis of the Hprt cDNA and genomic analysis of the gene locus. Base pair substitutions represented about 25% of Me-lex induced mutations. The mutation spectrum revealed a high percentage of genomic deletions (51%) comprising single/multiple exon(s) and even the loss of the complete locus. When the distribution of mutations among different classes was considered, the difference between the spontaneous and Me-lex induced CHO spectra was statistically significant (p<0.012), indicating that the sites where mutations occurred were Me-lex specific. Based upon these results we hypothesize that a large proportion of mutations may result from the processing of 3-MeA, the main adduct induced by Me-lex, within A/T rich sequences in non-coding regions of the Hprt gene. The processing of these lesions by DNA polymerases could result in recombination and genomic deletions, thus representing a severe threat for genome integrity.

Dual role of poly(ADP-ribose) glycohydrolase in the regulation of cell death in oxidatively stressed A549 cells

Activation of poly(ADP-ribose) polymerase-1 (PARP1) has been shown to mediate cell death induced by genotoxic stimuli. The role of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for polymer degradation, has been largely unexplored in the regulation of cell death. Using lentiviral gene silencing we generated A549 lung adenocarcinoma cell lines with stably suppressed PARG and PARP1 expression (shPARG and shPARP1 cell lines, respectively) and determined parameters of apoptotic and necrotic cell death following hydrogen peroxide exposure. shPARG cells accumulated large amounts of poly(ADP-ribosyl)ated proteins and exhibited reduced PARP activation. Hydrogen peroxide-induced cell death is regulated by PARG in a dual fashion. Whereas the shPARG cell line (similarly to shPARP1 cells) was resistant to the necrotic effect of high concentrations of hydrogen peroxide, these cells exhibited stronger apoptotic response. Both shPARP1 and especially shPARG cells displayed a delayed repair of DNA breaks and exhibited reduced clonogenic survival following hydrogen peroxide treatment. Translocation of apoptosis-inducing factor could not be observed, but cells could be saved by methyl pyruvate and alpha-ketoglutarate, indicating that energy failure may mediate cytotoxicity in our model. These data indicate that PARG is a survival factor at mild oxidative damage but contributes to the apoptosis-necrosis switch in severely damaged cells.

Dual role of poly(ADP-ribose) glycohydrolase in the regulation of cell death in oxidatively stressed A549 cells

Activation of poly(ADP-ribose) polymerase-1 (PARP1) has been shown to mediate cell death induced by genotoxic stimuli. The role of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for polymer degradation, has been largely unexplored in the regulation of cell death. Using lentiviral gene silencing we generated A549 lung adenocarcinoma cell lines with stably suppressed PARG and PARP1 expression (shPARG and shPARP1 cell lines, respectively) and determined parameters of apoptotic and necrotic cell death following hydrogen peroxide exposure. shPARG cells accumulated large amounts of poly(ADP-ribosyl)ated proteins and exhibited reduced PARP activation. Hydrogen peroxide-induced cell death is regulated by PARG in a dual fashion. Whereas the shPARG cell line (similarly to shPARP1 cells) was resistant to the necrotic effect of high concentrations of hydrogen peroxide, these cells exhibited stronger apoptotic response. Both shPARP1 and especially shPARG cells displayed a delayed repair of DNA breaks and exhibited reduced clonogenic survival following hydrogen peroxide treatment. Translocation of apoptosis-inducing factor could not be observed, but cells could be saved by methyl pyruvate and alpha-ketoglutarate, indicating that energy failure may mediate cytotoxicity in our model. These data indicate that PARG is a survival factor at mild oxidative damage but contributes to the apoptosis-necrosis switch in severely damaged cells.

Inhibition of poly(ADP-ribose) polymerase (PARP) influences the mode of sulfur mustard (SM)-induced cell death in HaCaT cells

Sulfur mustard (SM) is a bifunctional alkylating agent. Its primary toxic consequence is severe skin damage with blisters, occurring after skin contact. These vesicant properties of SM have been linked to cell death of proliferating keratinocytes in the basal layer of the skin. Catalytic activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP-1) has been demonstrated to be a major event in response to high levels of DNA damage, and PARP-1 activation may be part of apoptotic signaling. In other contexts, overstimulation of PARP-1 triggers necrotic cell death because of rapid consumption of its substrate, beta-nicotinamide adenine dinucleotide (NAD+) and the consequent depletion of ATP. These findings prompted us to evaluate whether SM induces apoptosis in keratinocytes like HaCaT cells and to determine whether blocking of PARP enzyme activity with 3-aminobenzamide (3AB) can influence the mode of cell death. HaCaT cells were exposed to SM (10-1,000 microM; 30 min) and then cultivated in SM-free medium with or without 3AB for up to 48 h. This treatment resulted in a time and SM dose-dependent increase of apoptotic cell death characterized by PARP-1 cleavage and DNA fragmentation during the experimental period. After just 45 min of exposure to 1 mM SM, we observed a significant increase in PARP-1 activity in HaCaT cells. About 6 h after exposure, intracellular ATP levels were diminished by 22%, which seemed to be completely prevented by the addition of 3AB directly after exposure. However, 18 h later, this 3AB effect on the SM concentration-dependent loss of ATP was no longer detectable. Interestingly, the effect of SM on total cell viability was not changed by 3AB. However, the mode of cell death was influenced by 3AB exhibiting an increase of apoptotic cells and a concomitant decrease of necrotic HaCaT cells during the first 24 h after SM exposure. Our results indicate that SM concentrations of 1 mM or higher induce a prominent PARP activation leading to ATP depletion and necrosis. In contrast, lower concentrations of SM cause minor PARP activation and, especially, PARP-1 cleavage by caspase 3 without ATP depletion. Because ATP is required for apoptosis, we suggest that ATP acts as an early molecular switch from apoptotic to necrotic modes of SM-induced cell death, at least at high concentrations (> or =1 mM). Thus, the observed early proapoptotic effect of 3AB at lower SM concentrations may point to the influence of ATP-independent cell-death regulating mechanisms.

beta4GalT-II increases cisplatin-induced apoptosis in HeLa cells depending on its Golgi localization

beta1,4-Galactosyltransferase II (beta4GalT-II) is one of the enzymes transferring galactose to the terminal N-acetylglucosamine of complex-type N-glycans and its expression is significantly altered during oncogenesis with unknown functions. Here, we reported for the first time the pro-apoptotic role of beta4GalT-II in tumor cells. The level of beta4GalT-II mRNA expression was obviously decreased during HeLa cell apoptosis induced by cisplatin. Interestingly, the ectopic expression of beta4GalT-II in HeLa cells markedly increased apoptosis and cleavage of PARP induced by cisplatin as well as the expression of pro-apoptotic protein Bax. Furthermore, deletion of Golgi localization domain abolished the apoptotic role of beta4GalT-II in HeLa cells. Collectively, these results suggest that beta4GalT-II increases HeLa cell apoptosis induced by cisplatin depending on its Golgi localization, which indicates that beta4GalT-II might contribute to the therapeutic efficiency of cisplatin for cervix cancer.

Involvement of poly(ADP-ribose) polymerase activity in regulating Chk1-dependent apoptotic cell death

The activity of poly(ADP-ribose) polymerase (PARP) is highly stimulated following DNA damage resulting in formation of DNA nicks and strand breaks. This leads to modification of numerous proteins, including itself, using NAD(+) as substrate and to exhaustion of intracellular ATP. A highly cytotoxic concentration of the DNA methylating agent methyl methanesulfonate (MMS) results in cellular ATP depletion and cell death primarily by necrosis in both wild-type and DNA polymerase beta null mouse fibroblasts. The loss of ATP can be prevented by the PARP inhibitor 4-amino-1,8-naphthalimide (4-AN), and now cells die by an energy-dependent apoptotic pathway. We find that inhibition of PARP activity transforms a sub-lethal exposure to MMS into a highly cytotoxic event. Under this condition, ATP is not depleted and cell death is by apoptosis. The caspase inhibitor, Z-VAD, shifts the mechanism of cell death to necrosis indicating a caspase-dependent component of the apoptotic cell death. Co-exposure to the Chk1 inhibitor UCN-01 also produces a decrease in apoptotic cell death, but now there is an increase in viable cells and an enhancement in long-term survival. Taken together, our results suggest that inhibition of PARP activity, induced as a result of low dose MMS exposure, signals via a Chk1-dependent pathway for cell death by apoptosis.

Poly(ADP-ribose) glycohydrolase inhibitor as chemosensitiser of malignant melanoma for temozolomide

Disruption of poly(ADP-ribose) polymerase (PARP) pathways by inhibitors of PARP catalytic domain has been shown to increase the anti-tumour activity of temozolomide (TMZ). Since PARP is inhibited by poly(ADP)ribosylation, herein we tested whether inhibition of poly(ADP-ribose) glycohydrolase (PARG) might enhance TMZ efficacy. The PARG inhibitor N-bis-(3-phenyl-propyl)9-oxo-fluorene-2,7-diamide (GPI 16552) was administered in combination with TMZ to mice injected subcutaneously or intracranially with B16 melanoma cells. The ability of treatment to reduce melanoma metastatic spreading and invasion of the extracellular matrix was also tested. The results indicated that combined treatment with GPI 16552 and TMZ significantly reduced melanoma growth, increased life-span of mice bearing tumour at the CNS site, and decreased the ability of melanoma cells to form lung metastases and to invade the extracellular matrix. In conclusion, PARG inhibition represents an alternative strategy to enhance TMZ efficacy against melanoma in peripheral as well as at CNS site.

Chemopotentiation by PARP inhibitors in cancer therapy

Poly(ADP-ribose) polymerases (PARP) constitute a family of enzymes involved in the regulation of many cellular processes such as DNA repair, gene transcription, cell cycle progression, cell death, chromatin functions and genomic stability. Among the 18 members identified so far, PARP-1 and PARP-2 are the only proteins stimulated by DNA strand breaks and implicated in the repair of DNA injury. Therefore, these molecules have been exploited as potential targets for the development of pharmacological strategies to increase the antitumor efficacy of chemotherapeutic agents, which induce DNA damage. PARP inhibitors have been shown to restore sensitivity of resistant tumors to methylating agents or topoisomerase I inhibitors, drugs presently used for the treatment of primary and secondary brain tumors or malignancies refractory to standard chemotherapy. Interestingly, PARP inhibitors may also provide protection from the untoward effects exerted by certain anticancer drugs, which cause oxidative stress and consequent PARP overactivation. The aim of this article is to provide a brief overview of the recent literature on preclinical studies with the specific and potent inhibitors newly synthesized.

Poly(ADP-ribose) polymerases: homology, structural domains and functions. Novel therapeutical applications

Poly(ADP-ribose) polymerases (PARPs) are a family of enzymes, which show differences in structure, cellular location and functions. However, all these enzymes possess poly(ADP-ribosyl)ation activity. Overactivation of PARP enzymes has been implicated in the pathogenesis of several diseases, including stroke, myocardial infarction, diabetes, shock, neurodegenerative disorder and allergy. The best studied of these enzymes (PARP-1) is involved in the cellular response to DNA damage so that in the event of irreparable DNA damage overactivation of PARP-1 leads to necrotic cell death. Inhibitors of PARP-1 activity in combination with DNA-binding antitumor drugs may constitute a suitable strategy in cancer chemotherapy. In addition, PARP inhibitors may be also useful to restore cellular functions in several pathophysiological states and diseases. This review gives an update of the state-of-the-art concerning PARP enzymes and their exploitation as pharmacological targets in several illnesses.

N3-methyladenine induces early poly(ADP-ribosylation), reduction of nuclear factor-kappa B DNA binding ability, and nuclear up-regulation of telomerase activity

Methylation of N3-adenine represents a novel pharmacological strategy for the treatment of resistant tumors. However, little is known about the biochemical pathways involved in cell death induced by N3-methyladenine. In the present study, we show that MeOSO(2) (CH(2))(2)-lexitropsin (Me-Lex), a compound generating almost exclusively N3-methyladenine (>99%), provoked a burst of poly(ADP-ribosylation) and loss of mitochondrial membrane potential in leukemia cells. These events were followed by a marked decrease in nuclear poly(ADP-ribose) polymerase-1 (PARP-1) expression and nuclear factor-kappaB (NF-kappaB) activity. Moreover, DNA damage generated by N3-methyladenine induced a marked decrease in telomerase in the cytosol that was accompanied by a transient up-regulation of activity in the nucleus, as a consequence of nuclear translocation of telomerase in response to genotoxic damage. PARP-1 inhibition blocked ADP-ribose polymer formation, preserved mitochondrial membrane integrity, and counteracted the reduction of NF-kappaB activity, thus preventing the appearance of necrosis. On the other hand, because PARP-1 is a component of the base excision repair (BER), the combination of Me-Lex + PARP-1 inhibitor triggered apoptosis as a result of disruption of BER process. In conclusion, the present study provides new insight into the cellular response to N3-adenine-selective methylating agents that can be exploited for the treatment of tumors unresponsive to classical wide-spectrum methylating agents. Moreover, the results underline the central and paradoxical role of PARP-1 in cell death induced by N3-methyladenine: effector of necrosis and coordinator of methylpurine repair.

The biological effects of N3-methyladenine

The targeting of damage to DNA remains an attractive strategy to kill tumor cells. One of the serious side effects of alkylating agents is that they create both toxic (desired) and mutagenic (undesired) lesions. The result is that patients successfully treated for a primary cancer are at significant risk to develop cancer related to their therapy. To address this issue we have prepared agents that selectively methylate DNA at the N3-position of adenine. The presence of this lesion in DNA is thought to halt DNA polymerase, and this then initiates a cascade of events including cell death. The toxicity and mutagenicity of the compound, Me-lex, used to generate N3-methyladenine is discussed in bacterial, yeast, and mammalian systems. Mechanisms are proposed to explain the biological activities of N3-methyladenine.

Inhibition of telomerase increases resistance of melanoma cells to temozolomide, but not to temozolomide combined with poly (adp-ribose) polymerase inhibitor

In the present study, we have investigated the influence of telomerase inhibition in chemosensitivity of melanoma cells to temozolomide (TMZ), a methylating agent with promising antitumor activity against metastatic melanoma. In fact, telomerase, a ribonucleoprotein enzyme expressed in the majority of tumors, is presently considered an attractive target for anticancer therapy, with the double aim of reducing tumor growth and increasing chemosensitivity of cancer cells. Susceptibility to TMZ and to other antitumor agents used for treatment of metastatic melanoma was initially assessed in melanoma lines with different basal levels of telomerase activity. Thereafter, chemosensitivity was investigated after inhibition of telomerase by means of stable transfection of a catalytically inactive, dominant-negative mutant of hTERT (DN-hTERT). This study shows for the first time that: a) susceptibility to TMZ of melanoma lines derived from the same patient did not depend on basal telomerase activity; b) inhibition of telomerase by DN-hTERT resulted in reduced growth rate and increased resistance to TMZ and to the chloroethylating agent carmustine, increased sensitivity to cisplatin, and no change in response to tamoxifen or to a selective N3-adenine methylating agent; c) inhibition of poly(ADP-ribose) polymerase (PARP), an enzyme involved in the repair of N-methylpurines, restored sensitivity of DN-hTERT clones to TMZ. These results indicate that a careful selection of the antitumor agent has to be made when antitelomerase therapy is combined with chemotherapy. Moreover, the data presented here suggest that TMZ + PARP inhibitor combination is active against telomerase-suppressed and slowly growing tumors.

Poly(ADP-ribose) polymerase inhibitor increases growth inhibition and reduces G(2)/M cell accumulation induced by temozolomide in malignant glioma cells

Temozolomide (TZM) is a novel methylating agent currently under investigation for treatment of recurrent high-grade gliomas. Although TZM generates a wide spectrum of methyl adducts, its cytotoxicity has been attributed to mismatch repair (MR)-mediated processing of O(6)-methylguanine:T mispairs. N3-methyladenine and N7-methylguanine adducts are promptly repaired by the base excision repair system, unless a poly(ADP-ribose) polymerase (PARP) inhibitor is combined to TZM. In this case, the repair process of N-methylpurines cannot be completed and the deriving DNA strand breaks contribute to cytotoxicity. In this study, we investigated the influence on cell growth and cell cycle of treatment with TZM + PARP inhibitor in glioma cells characterized by different susceptibility to TZM. The results indicated that PARP inhibitor increases growth inhibition induced by TZM in either p53-wild-type or p53-mutant glioblastoma cells, as early as 24 h after drug exposure. The enhancing effect exerted by PARP inhibitor was particularly evident in glioma cells characterized by a defective expression of MR, since these cells are tolerant to O(6)-methylguanine damage and show low sensitivity to TZM. In O(6)-alkylguanine-DNA alkyltransferase (OGAT)-deficient and MR-proficient tumor cells bearing wild-type p53, the drug combination markedly reduced cell accumulation in the G(2)/M phase of cell cycle and induction of the G(2) checkpoint regulator Chk1 kinase. In short-term cultures of glioma cells derived from surgical specimens, PARP inhibitor enhanced chemosensitivity to TZM and this effect was especially evident in OGAT-proficient tumors. Thus, a pharmacological strategy based on the interruption of N-methylpurine repair might represent a novel strategy to restore or increase glioma sensitivity to TZM.

Potential clinical applications of poly(ADP-ribose) polymerase (PARP) inhibitors

Poly(ADP-ribose) polymerases (PARPs) are defined as cell signaling enzymes that catalyze the transfer of ADP-ribose units from NAD(+)to a number of acceptor proteins. PARP-1, the best characterized member of the PARP family, that presently includes six members, is an abundant nuclear enzyme implicated in cellular responses to DNA injury provoked by genotoxic stress (oxygen radicals, ionizing radiations and monofunctional alkylating agents). Due to its involvement either in DNA repair or in cell death, PARP-1 is regarded as a double-edged regulator of cellular functions. In fact, when the DNA damage is moderate, PARP-1 participates in the DNA repair process. Conversely, in the case of massive DNA injury, elevated PARP-1 activation leads to rapid NAD(+)/ATP consumption and cell death by necrosis. Excessive PARP-1 activity has been implicated in the pathogenesis of numerous clinical conditions such as stroke, myocardial infarction, shock, diabetes and neurodegenerative disorders. PARP-1 could therefore be considered as a potential target for the development of pharmacological strategies to enhance the antitumor efficacy of radio- and chemotherapy or to treat a number of clinical conditions characterized by oxidative or NO-induced stress and consequent PARP-1 activation. Moreover, the discovery of novel functions for the multiple members of the PARP family might lead in the future to additional clinical indications for PARP inhibitors.