Combined effects of adenovirus-mediated wild-type p53 transduction, temozolomide and poly (ADP-ribose) polymerase inhibitor in mismatch repair deficient and non-proliferating tumor cells

Unveiling the vulnerabilities of synthetic lethality in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is the most invasive molecular subtype of breast cancer (BC), accounting for about nearly 15% of all BC cases reported annually. The absence of the three major BC hormone receptors, Estrogen (ER), Progesterone (PR), and Human Epidermal Growth Factor 2 (HER2) receptor, accounts for the characteristic "Triple negative" phraseology. The absence of these marked receptors makes this cancer insensitive to classical endocrine therapeutic approaches. Hence, the available treatment options remain solemnly limited to only conventional realms of chemotherapy and radiation therapy. Moreover, these therapeutic regimes are often accompanied by numerous treatment side-effects that account for early distant metastasis, relapse, and shorter overall survival in TNBC patients. The rigorous ongoing research in the field of clinical oncology has identified certain gene-based selective tumor-targeting susceptibilities, which are known to account for the molecular fallacies and mutation-based genetic alterations that develop the progression of TNBC. One such promising approach is synthetic lethality, which identifies novel drug targets of cancer, from undruggable oncogenes or tumor-suppressor genes, which cannot be otherwise clasped by the conventional approaches of mutational analysis. Herein, a holistic scientific review is presented, to undermine the mechanisms of synthetic lethal (SL) interactions in TNBC, the epigenetic crosstalks encountered, the role of Poly (ADP-ribose) polymerase inhibitors (PARPi) in inducing SL interactions, and the limitations faced by the lethal interactors. Thus, the future predicament of synthetic lethal interactions in the advancement of modern translational TNBC research is assessed with specific emphasis on patient-specific personalized medicine.

Pharmacological inhibition of poly(ADP-ribose) polymerase-1 modulates resistance of human glioblastoma stem cells to temozolomide

BACKGROUND

Chemoresistance of glioblastoma multiforme (GBM) has been attributed to the presence within the tumor of cancer stem cells (GSCs). The standard therapy for GBM consists of surgery followed by radiotherapy and the chemotherapeutic agent temozolomide (TMZ). However, TMZ efficacy is limited by O6-methylguanine-DNA-methyltransferase (MGMT) and Mismatch Repair (MMR) functions. Strategies to counteract TMZ resistance include its combination with poly(ADP-ribose) polymerase inhibitors (PARPi), which hamper the repair of N-methylpurines. PARPi are also investigated as monotherapy for tumors with deficiency of homologous recombination (HR). We have investigated whether PARPi may restore GSC sensitivity to TMZ or may be effective as monotherapy.

METHODS

Ten human GSC lines were assayed for MMR proteins, MGMT and PARP-1 expression/activity, MGMT promoter methylation and sensitivity to TMZ or PARPi, alone and in combination. Since PTEN defects are frequently detected in GBM and may cause HR dysfunction, PTEN expression was also analyzed. The statistical analysis of the differences in drug sensitivity among the cell lines was performed using the ANOVA and Bonferroni's post-test or the non-parametric Kruskal-Wallis analysis and Dunn's post-test for multiple comparisons. Synergism between TMZ and PARPi was analyzed by the median-effect method of Chou and Talalay. Correlation analyses were done using the Spearman's rank test.

RESULTS

All GSCs were MMR-proficient and resistance to TMZ was mainly associated with high MGMT activity or low proliferation rate. MGMT promoter hypermethylation of GSCs correlated both with low MGMT activity/expression (Spearman's test, P = 0.004 and P = 0.01) and with longer overall survival of GBM patients (P = 0.02). Sensitivity of each GSC line to PARPi as single agent did not correlate with PARP-1 or PTEN expression. Notably, PARPi and TMZ combination exerted synergistic antitumor effects in eight out of ten GSC lines and the TMZ dose reduction achieved significantly correlated with the sensitivity of each cell line to PARPi as single agent (P = 0.01).

CONCLUSIONS

The combination of TMZ with PARPi may represent a valuable strategy to reverse GSC chemoresistance.

Protein kinase Cα protects against multidrug resistance in human colon cancer cells

Multidrug resistance is the phenomenon by which, after exposure to a single chemotherapeutic agent, cancer cells evade the agent's cytotoxic effects as well as become resistant to several classes of diverse drugs. ATP-binding cassette (ABC) transporters are a family of transporter proteins that contribute to drug resistance via a n ATP - dependent drug efflux pump. P-glycoprotein (P-gp) is a prominent ABC superfamily protein encoded by the mdr gene which has the ability to mediate the cellular extrusion of xenobiotics and anticancer drugs from tumor cells. Exclusively expressed P-gp cells from the human colon cancer HCT15/DOX line showed resistance to doxorubicin while parental HCT15 cells treated with doxorubicin displayed typical signs of apoptosis. In order to verify the hypothesis that expression of MDR is controlled in part, by protein kinase C (PKC), expression patterns of different PKC isoforms were examined in both cell lines. Of the PKC isoforms evaluated, the membrane translocation and expression levels of PKCα were strikingly increased in HCT15/DOX cells. PKCα reversed doxorubicin-induced apoptosis through the scavenging of ROS as well as inhibition of PARP cleavage. In addition, inhibition of PKCα with Go6976, a specific inhibitor of classical PKC, led to reduced MDR expression and increased doxorubicin-induced apoptosis. Knockdown of PKCα by siRNA diminished the protective effects of PKCα for doxorubicin-induced apoptosis. These results suggested that over-expression and activity of PKCα is closely associated with the regulation of the MDR phenotype in human colon cancer HCT15 cells and provided insight into a new strategy for inhibiting doxorubicin resistance in human cancers.

Chemical regulation of epigenetic modifications: opportunities for new cancer therapy

Epigenetics is concerned about heritable changes in gene expression without alteration of the coding sequence. Epigenetic modification of chromatin includes methylation of genomic DNA as well as post-translational modification of chromatin-associated proteins, in particular, histones. The spectrum of histone and non-histone modifications ranges from the addition of relatively small groups such as methyl, acetyl and phosphoryl groups to the attachment of larger moieties such as poly(ADP-ribose) and small proteins ubiquitin or small ubiquitin-like modifier (SUMO). The combinatorial nature of DNA methylation and histone modifications constitutes a significant pathway of epigenetic regulation and considerably extends the information potential of the genetic code. Chromatin modification has emerged as a new fundamental mechanism for gene transcriptional activity control associated with many cellular processes like proliferation, growth, and differentiation. Also it is increasingly recognized that epigenetic modifications constitute important regulatory mechanisms for the pathogenesis of malignant transformations. We review here the recent progress in the development of chemical inhibitors/activators that target different chromatin modifying enzymes. Such potent natural or synthetic modulators can be utilized to establish the quantitative contributions of epigenetic modifications in DNA regulated pathways including transcription, replication, recombination and repair, as well as provide leads for developing new cancer therapeutics.

Targeting base excision repair to improve cancer therapies

Most commonly used cancer therapies, particularly ionizing radiation and certain classes of cytotoxic chemotherapies, cause cell death by damaging DNA. Base excision repair (BER) is the major system responsible for the removal of corrupt DNA bases and repair of DNA single strand breaks generated spontaneously and induced by exogenous DNA damaging factors such as certain cancer therapies. In this review, the physico-chemical properties of the proteins involved in BER are discussed with particular emphasis on molecular mechanisms coordinating repair processes. The aim of this review is to apply extensive knowledge that currently exists regarding the biochemical mechanisms involved in human BER to the molecular biology of current therapies for cancer. It is anticipated that the application of this knowledge will translate into the development of novel effective therapies for improving existing treatments such as radiation therapy and oxaliplatin chemotherapy.

Hypersensitivity phenotypes associated with genetic and synthetic inhibitor-induced base excision repair deficiency

Single-base lesions in DNA are repaired predominantly by base excision repair (BER). DNA polymerase beta (pol beta) is the polymerase of choice in the preferred single-nucleotide BER pathway. The characteristic phenotype of mouse fibroblasts with a deletion of the pol beta gene is moderate hypersensitivity to monofunctional alkylating agents, e.g., methyl methanesulfonate (MMS). Increased sensitivity to MMS is also seen in the absence of pol beta partner proteins XRCC1 and PARP-1, and under conditions where BER efficiency is reduced by synthetic inhibitors. PARP activity plays a major role in protection against MMS-induced cytotoxicity, and cells treated with a combination of non-toxic concentrations of MMS and a PARP inhibitor undergo cell cycle arrest and die by a Chk1-dependent apoptotic pathway. Since BER-deficient cells and tumors are similarly hypersensitive to the clinically used chemotherapeutic methylating agent temozolomide, modulation of DNA damage-induced cell signaling pathways, as well as BER, are attractive targets for potentiating chemotherapy.

Should concomitant and adjuvant treatment with temozolomide be used as standard therapy in patients with anaplastic glioma?

Malignant gliomas are devastating tumors associated with poor prognosis. Standard treatment has been surgery followed by radiotherapy while the role of chemotherapy has remained controversial. Concomitant and adjuvant treatment with temozolomide has recently been shown to improve survival in patients with glioblastoma. While it seems intuitive to apply this regimen to patients with anaplastic gliomas which have traditionally been considered more chemosensitive, chemotherapy has not been shown to prolong life in patients with anaplastic gliomas. Despite promising preclinical and early clinical results, there is currently not enough level 1 evidence to justify concomitant and adjuvant temozolomide as standard therapy for patients with newly diagnosed anaplastic gliomas. Further investigation is needed to better define the role of chemotherapy in patients with anaplastic gliomas. Trials evaluating chemoradiotherapy as well as targeted therapeutic agents are the subject of further research.

Poly(ADP-ribose)polymerase inhibition decreases angiogenesis

Inhibitors of poly(ADP-ribose)polymerase (PARP), a nuclear enzyme involved in regulating cell death and cellular responses to DNA repair, show considerable promise in the treatment of cancer both in monotherapy as well as in combination with chemotherapeutic agents and radiation. We have recently demonstrated that PARP inhibition with 3-aminobenzamide or PJ-34 reduced vascular endothelial growth factor (VEGF)-induced proliferation, migration, and tube formation of human umbilical vein endothelial cells (HUVECs) in vitro. Here, we show dose-dependent reduction of VEGF- and basic fibroblast growth factor (bFGF)-induced proliferation, migration, and tube formation of HUVECs in vitro by two potent PARP inhibitors 5-aminoisoquinolinone-hydrochloride (5-AIQ) and 1,5-isoquinolinediol (IQD). Moreover, PARP inhibitors prevented the sprouting of rat aortic ring explants in an ex vivo assay of angiogenesis. These results establish the novel concept that PARP inhibitors have antiangiogenic effects, which may have tremendous clinical implications for the treatment of various cancers, tumor metastases, and certain retinopathies.

Pharmacological inhibition of poly(ADP-ribose) polymerase inhibits angiogenesis

Poly(ADP-ribose) polymerase (PARP) is a nuclear enzyme which plays an important role in regulating cell death and cellular responses to DNA repair. Pharmacological inhibitors of PARP are being considered as treatment for cancer both in monotherapy as well as in combination with chemotherapeutic agents and radiation, and were also reported to be protective against untoward effects exerted by certain anticancer drugs. Here we show that pharmacological inhibition of PARP with 3-aminobenzamide or PJ-34 dose-dependently reduces VEGF-induced proliferation, migration, and tube formation of human umbilical vein endothelial cells in vitro. These results suggest that treatment with PARP inhibitors may exert additional benefits in various cancers and retinopathies by decreasing angiogenesis.

O6-methylguanine DNA methyltransferase and p53 status predict temozolomide sensitivity in human malignant glioma cells

Temozolomide (TMZ) is a methylating agent which prolongs survival when administered during and after radiotherapy in the first-line treatment of glioblastoma and which also has significant activity in recurrent disease. O6-methylguanine DNA methyltransferase (MGMT) is a DNA repair enzyme attributed a role in cancer cell resistance to O6-alkylating agent-based chemotherapy. Using a panel of 12 human glioma cell lines, we here defined the sensitivity to TMZ in acute cytotoxicity and clonogenic survival assays in relation to MGMT, mismatch repair and p53 status and its modulation by dexamethasone, irradiation and BCL-X(L). We found that the levels of MGMT expression were a major predictor of TMZ sensitivity in human glioma cells. MGMT activity and clonogenic survival after TMZ exposure are highly correlated (p < 0.0001, r2 = 0.92). In contrast, clonogenic survival after TMZ exposure does not correlate with the expression levels of the mismatch repair proteins mutS homologue 2, mutS homologue 6 or post-meiotic segregation increased 2. The MGMT inhibitor O6-benzylguanine sensitizes MGMT-positive glioma cells to TMZ whereas MGMT gene transfer into MGMT-negative cells confers protection. The antiapoptotic BCL-X(L) protein attenuates TMZ cytotoxicity in MGMT-negative LNT-229 but not in MGMT-positive LN-18 cells. Neither ionizing radiation (4 Gy) nor clinically relevant concentrations of dexamethasone modulate MGMT activity or TMZ sensitivity. Abrogation of p53 wild-type function strongly attenuates TMZ cytotoxicity. Conversely, p53 mimetic agents designed to stabilize the wild-type conformation of p53 sensitize glioma cells for TMZ cytotoxicity. Collectively, these results suggest that the determination of MGMT expression and p53 status will help to identify glioma patients who will or will not respond to TMZ.

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.

Clinical perspectives of PARP inhibitors

Poly(ADP-ribose) polymerase (PARP) activation plays a role in the pathogenesis of various cardiovascular and inflammatory diseases. At the same time, PARP activation is also relevant for the ability of cells to repair injured DNA. Thus, depending on the circumstances, pharmacological inhibitors of PARP may be able to attenuate ischemic and inflammatory cell and organ injury or may be able to enhance the cytotoxicity of antitumor agents. Both aspects of the "double-edged sword" role of PARP can be exploited for the experimental therapy of disease. As several classes of PARP inhibitors move towards clinical development, or have already entered the stage of clinical trials, we expect that in the upcoming few years, clinical proof of PARP inhibitors' therapeutic effect will be obtained in human disease. In the current short overview, we summarize the pros and cons and challenges with respect to the clinical use of PARP inhibitors, the expected clinical outcomes and potential risks. It appears that on the cytoprotective aspect of PARP, acute, life-threatening diseases (myocardial infarction, cardiopulmonary bypass in high-risk patients, and other, severe forms of ischemia-reperfusion to other organs including stroke and thoracoabdominal aneurysm repair) may represent some of the prime development indications. In the context of inhibition of DNA repair, combination of PARP inhibitors with certain antitumor agents (for example temozolomide) in patients with tumors with extremely poor prognosis are expected to provide the initial clinical results. Development of PARP inhibitors for additional indications (e.g. chronic use for the therapy of neurodegeneration and neuroinflammation, or diabetic complications) may be more challenging because of the unknown potential long-term side effects of PARP inhibitors.

Poly(ADP-ribose) polymerase and the therapeutic effects of its inhibitors

Poly(ADP-ribose) polymerases (PARPs) are involved in the regulation of many cellular functions. Three consequences of the activation of PARP1, which is the main isoform of the PARP family, are particularly important for drug development: first, its role in DNA repair; second, its capacity to deplete cellular energetic pools, which culminates in cell dysfunction and necrosis; and third, its capacity to promote the transcription of pro-inflammatory genes. Consequently, pharmacological inhibitors of PARP have the potential to enhance the cytotoxicity of certain DNA-damaging anticancer drugs, reduce parenchymal cell necrosis (for example, in stroke or myocardial infarction) and downregulate multiple simultaneous pathways of inflammation and tissue injury (for example, in circulatory shock, colitis or diabetic complications). The first ultrapotent novel PARP inhibitors have now entered human clinical trials. This article presents an overview of the principal pathophysiological pathways and mechanisms that are governed by PARP, followed by the main structures and therapeutic actions of various classes of novel PARP inhibitors.

Temozolomide in the treatment of solid tumours: current results and rationale for dosing/scheduling

This review examines the current evidence for the use of temozolomide in the treatment of solid tumours. The possible molecular and clinical advantages of temozolomide are identified and the molecular mechanism of temozolomide resistance is explored. Attempts to maximise efficacy have led to manipulation of both dosage and drug scheduling and the evidence for the various strategies is reviewed. Finally, the potential role of combination therapy is considered.

Poly(ADP-ribosyl)ation inhibitors: Promising drug candidates for a wide variety of pathophysiologic conditions

Poly(ADP-ribose) polymerases are involved in many aspects of regulation of cellular functions. Using NAD+ as a substrate, they catalyse the covalent transfer of ADP-ribose units onto several acceptor proteins to form a branched ADP-ribose polymer. The best characterised and first discovered member of this multiprotein family is PARP-1. Its catalytic activity is markedly stimulated upon binding to DNA strand interruptions, and the resulting polymer is thought to function in chromatin relaxation as well as in signalling the presence of damage to DNA repair complexes and in regulating enzyme activities. Moderate activation of PARP-1 facilitates the efficient repair of DNA damage arising from monofunctional alkylating agents, reactive oxygen species or ionising radiation, but severe genotoxic stress leads to rapid energy consumption and subsequently to necrotic cell death. The latter aspect of PARP-1 activity has been implicated in the pathogenesis of various clinical conditions such as shock, ischaemia-reperfusion and diabetes. Inhibition of ADP-ribose polymer formation has been shown to be effective, on the one hand, in the treatment of cancer in combination with alkylating agents by suppressing DNA repair and thus driving tumour cells into apoptosis, and on the other hand it appears to be a promising drug target for the treatment of pathologic conditions involving oxidative stress. In view of the existence of several members of the PARP family in mammalian cells, one has to be aware of possible side effects but also of a wide spectrum of potential clinical applications, which calls for the development of more specific inhibitors.

Temozolomide induces apoptosis and senescence in glioma cells cultured as multicellular spheroids

Temozolomide is an alkylating cytostatic drug that finds increasing application in the treatment of melanoma, anaplastic astrocytoma and glioblastoma multiforme. The compound is a prodrug that decomposes spontaneously, independent of an enzymatic activation step. DNA methylation induces futile mismatch repair cycles and depletion of the DNA repair enzyme O(6)-methylguanine-DNA methyltransferase should then initiate programmed cell death. We show drug-dependent inhibition of tumour growth in a three-dimensional cell culture model of the glioma cell lines U87MG and GaMG. Migrational behaviour of the glioblastoma cells remained unaltered. However, coincubation of tumour spheroids with primary brain aggregates showed reduced tumour cell invasion into brain tissue in the presence of temozolomide. This was not achieved by slowing cellular migration, as temozolomide-treated cells displayed no reduced motility. By transferase-mediated dUTP nick-end labelling (TUNEL) of apoptotic nuclei, we found that the drug was able to induce apoptosis throughout the tumour cell spheroids. Apoptosis was highest in the core region of the spheroids. Repetitive application of sublethal doses of temozolomide to multicellular spheroids resulted in the development of drug resistance in GaMG cells. We suggest that temozolomide is a strong initiator of apoptosis in glioblastoma tumour cells in a spheroid cell culture system, when cells are already in a stressful environment.

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.

Combined treatment with temozolomide and poly(ADP-ribose) polymerase inhibitor enhances survival of mice bearing hematologic malignancy at the central nervous system site

Temozolomide (TZM) is a DNA-methylating agent that has recently been introduced into various clinical trials for treatment of solid or hematologic neoplasias, including brain lymphomas. In the current study, we have investigated whether the antitumor activity of TZM could be selectively enhanced at the central nervous system (CNS) site by intracerebral injection of a poly(ADP-ribose) polymerase (PARP) inhibitor. Mice were injected intracranially with lymphoma cells. The PARP inhibitor NU1025 (1 mg/animal) was delivered intracerebrally, whereas TZM was given as a single or a fractionated dose of 200 mg/kg by intraperitoneal administration. Results indicated that this drug combination significantly enhanced the survival of tumor-bearing mice and that this fractionated modality of treatment was the most effective schedule. Increased survival time was related to a marked reduction of tumor growth, as evidenced by histologic studies. Treatment with TZM alone was ineffective. This is the first report exploring in vivo the combination of TZM with PARP inhibitor for intracerebral neoplasias.

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.