Abstract 3697: Molecular link that associates high fat diet and prostate tumor growth

Prostate Cancer (PCa) is one of the most common invasive tumors in men. Epidemiological studies indicate that diet and overweight are important factors implicated in prostate carcinogenesis. Obesity is associated with PCa aggressiveness, poorer prognosis and increased mortality. Breast cancer susceptibility gene 1 (BRCA1) interacts with several transcriptional regulators to modulate the androgen receptor (AR) signaling in PCa cell lines. Germline mutations in this gene increase breast cancer risk and are associated with high grade PCa. Previously, it had been reported that C-terminal Binding Protein 1 (CtBP1) acts as a switch to control BRCA1 transcription in response to the metabolic status of the cells. The release of CtBP1 from BRCA1 promoter through estrogen induction and high NAD+/NADH ratio (similar to high caloric intake) increases BRCA1 transcription in breast cancer cells.

The aim of this work was to assess the effect of androgens and/or high fat diet over the BRCA1/CtBP1 axis and PCa tumor growth.

We found that BRCA1 and CtBP1 proteins associate to BRCA1 proximal promoter region in PC3 cells and suppress BRCA1 transcription. Testosterone stimulation released these factors from BRCA1 promoter increasing its transcription. To assess whether this activation is mediated by testosterone or the estrogen, synthesized from testosterone by the aromatase (CYP19A1), we investigated this mechanism in the presence of letrozol (LTZ), an aromatase inhibitor. We found that LTZ abolished BRCA1 induction by testosterone, suggesting that BRCA1 activation is mediated by estrogen in these cells.

Furthermore, we generated PC3 cell lines transfected with pcDNA3-CtBP1 (PC3-CtBP1) or shRNA-CtBP1 (PC3-shCtBP1) plasmids, to overexpress or knock down CtBP1 expression, respectively. CtBP1 induction decreased BRCA1 expression in these cells and this effect was reverted by CtBP1 depletion. In addition, PC3-CtBP1 cells showed increased clonogenic capacity and proliferation compared to PC3-shCtBP1 cells.

Moreover, we developed an in vivo model to investigate the effect of high caloric diet on PCa growth after CtBP1 modulated-expression. High fat or control diet fed male nude mice were inoculated with PC3-CtBP1 and PC3-shCtBP1 stable cells. We found that CtBP1 depleted cells growing as xenografts in high fat diet fed mice dramatically decreased prostate tumor growth. Molecular analysis of tumors by RT-qPCR showed that CtBP1 depletion correlated with high BRCA1 expression. In addition, serum from high fat fed mice significantly induced PC3-CtBP1 cell proliferation in vitro.

These results strongly suggest that the potential oncogenic role of CtBP1 is dependent on the caloric diet intake. Hence, BRCA1 regulation by CtBP1 provides an important molecular link between caloric intake and tumor suppressor expression.

Citation Format: Cristian P. Moiola, Paola De Luca, Florencia Zalazar, Javier Cotignola, Estefania Labanca, Roberto Meiss, Elba S. Vazquez, Kevin Gardner, Adriana De Siervi. Molecular link that associates high fat diet and prostate tumor growth. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3697. doi:10.1158/1538-7445.AM2013-3697

A systemic transcriptome analysis reveals the regulation of neural stem cell maintenance by an E2F1-miRNA feedback loop

Stem cell fate decisions are controlled by a molecular network in which transcription factors and miRNAs are of key importance. To systemically investigate their impact on neural stem cell (NSC) maintenance and neuronal commitment, we performed a high-throughput mRNA and miRNA profiling and isolated functional interaction networks of involved mechanisms. Thereby, we identified an E2F1–miRNA feedback loop as important regulator of NSC fate decisions. Although E2F1 supports NSC proliferation and represses transcription of miRNAs from the miR-17∼92 and miR-106a∼363 clusters, these miRNAs are transiently up-regulated at early stages of neuronal differentiation. In these early committed cells, increased miRNAs expression levels directly repress E2F1 mRNA levels and inhibit cellular proliferation. In mice, we demonstrated that these miRNAs are expressed in the neurogenic areas and that E2F1 inhibition represses NSC proliferation. The here presented data suggest a novel interaction mechanism between E2F1 and miR-17∼92 / miR-106a∼363 miRNAs in controlling NSC proliferation and neuronal differentiation.

Dynamic coregulatory complex containing BRCA1, E2F1 and CtIP controls ATM transcription

Chromosomal instability is a key feature in cancer progression. Recently we have reported that BRCA1 regulates the transcription of several genes in prostate cancer, including ATM (ataxia telangiectasia mutated). Although it is well accepted that ATM is a pivotal mediator in genotoxic stress, it is unknown whether ATM transcription is regulated during the molecular response to DNA damage. Here we investigate ATM transcription regulation in human prostate tumor PC3 cell line. We have found that doxorubicin and mitoxantrone repress ATM transcription in PC3 cells but etoposide and methotrexate do not affect ATM expression. We have demonstrated that BRCA1 binds to ATM promoter and after doxorubicin exposure, it is released. BRCA1 overexpression increases ATM transcription and this enhancement is abolished by BRCA1 depletion. Moreover, BRCA1-BRCT domain loss impairs the ability of BRCA1 to regulate ATM promoter activity, strongly suggesting that BRCT domain is essential for ATM regulation by BRCA1. BRCA1-overexpressing PC3 cells exposed to KU55933 ATM kinase inhibitor showed significant decreased ATM promoter activity compared to untreated cells, suggesting that ATM transcriptional regulation by BRCA1 is partially mediated by the ATM kinase activity. In addition, we have demonstrated E2F1 binding to ATM promoter before and after doxorubicin exposure. E2F1 overexpression diminishes ATM transcription after doxorubicin exposure which is impaired by E2F1 dominant negative mutants. Finally, the co-regulator of transcription CtIP increases ATM transcription. CtIP increases ATM transcription. Altogether, BRCA1/E2F1/CtIP binding to ATM promoter activates ATM transcription. Doxorubicin exposure releases BRCA1 and CtIP from ATM promoter still keeping E2F1 recruited and, in turn, represses ATM expression.

Abstract 1312: ATM transcriptional regulation mediated by BRCA1/E2F1 axis controls DNA damage response in prostate cancer

Prostate cancer (PCa) is the second leading cause of cancer-related death affecting men worldwide. Chromosomal instability is a key feature in PCa progression. Using genome-wide screen for factor binding in combination with expression profiles, we previously reported BRCA1 binding to several promoters involved in cell cycle regulation and DNA damage response. We also generated a BRCA1 depleted xenograft model in order to study the role of BRCA1 role in DNA damage response in vivo. We demonstrated that BRCA1 expression status plays a central role in doxorubicin resistance in PCa. One of the BRCA1 targets that emerged from these analyses was ATM (ataxia telangiectasia mutated).Although it is well accepted that the kinase protein ATM is a pivotal mediator in genotoxic stress, it is unknown if its transcriptional regulation plays a role in the DNA damage response. Our goal was to investigate ATM transcription regulation in PCa under different genotoxic insults. Here we exposed PC3 cells to different genotoxic agents and the ATM promoter activity was determined by a luciferase reporter assay. We found that the topoisomerase II inhibitors doxorubicin and mitoxantrone repressed ATM transcription; however etoposide and methotrexate did not show significant changes. Using BRCA1-overexpressing PC3 cell lines, we found that BRCA1 increases ATM mRNA and promoter activity. Accordingly, BRCA1 depletion by shRNA abolished ATM transcription induction. Furthermore, BRCT domain loss (BRCA1ΔBRCT) impaired the ability of BRCA1 to regulate ATM promoter activity, strongly suggesting that BRCT domain is essential for ATM regulation. Xenograft tumors generated by BRCA1 depleted PC3 cells injected in nu/nu mice demonstrated that BRCA1 knock-down abolished ATM transcriptional induction. Considering ATM phosphorylates BRCA1, we investigated BRCA's ability to activate ATM promoter after inhibition of the ATM kinase activity by KU55933. BRCA1 overexpressing PC3 cells exposed to KU55933 showed significant decreased ATM promoter activity compared to control cells suggesting ATM regulation by BRCA1 is not mediated by ATM kinase activity. In addition, we performed BRCA1-ChIP-qPCR using primers spanning every 500bp along ATM promoter; we found that BRCA1 binds at 500bp upstream of the ATM transcription start site which was disrupted by doxorubicin. We identified one E2F1 putative DNA binding site at this region suggesting that BRCA1 association to ATM promoter could be E2F1 mediated. Finally, E2F1 transfection in PC3 cells significant decreased ATM transcription which was impaired by E2F1 dominant negative (E1-363). In summary, BRCA1/E2F1 complex binds and induces ATM transcription. After genotoxic stress BRCA1 protein is displaced from the ATM promoter and E2F1 downregulates ATM transcription. Thus, BRCA1/E2F1 axis controls DNA damage response in PCa through ATM transcriptional regulation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1312. doi:1538-7445.AM2012-1312

Abstract 2340: CPS49 and Flavopiridol: a new selective drug combination for advanced prostate cancer

Prostate cancer (PCa) still ranks as the second most commonly diagnosed cancer and metastatic castration-resistant prostate cancer (CRPC) is a leading cause of cancer death in men around the world. The common treatment for patients with CRPC is chemotherapy based on docetaxel. While there are currently seven agents approved for CRPC and four regimens have shown some survival benefit, those survival prolongations have been modest and unfortunately all patients will eventually progress. Thus, there is a need for new agents and regimens for this disease. 2-(2,4-Difluoro-phenyl)-4,5,6,7-tetrafluoro-1H-isoindole-1,3(2H)-dione (CPS49) is a member of a class of redox-reactive thalidomide analogs that show selective killing of leukemic cells by increasing intracellular reactive oxygen species (ROS) and targeting multiple transcriptional pathways. Flavopiridol is a semisynthetic flavonoid that inhibits cyclin dependent kinases and also provokes lethality against leukemic cells. We previously found that CPS49 and flavopiridol combination induced selective cytotoxicity associated with mitochondrial dysfunction and elevations of ROS in leukemic cells ranging from additive to synergistic activity at low micromolar concentrations. The goal of this study was to investigate the selectivity and efficacy of CPS49-flavopiridol combination in prostate cancer preclinical models. Our results showed that flavopiridol enhanced CPS49 cytotoxicity in all human prostate tumor cell lines analyzed (PC3, C4-2, LNCaP and 22RV1); however non tumor cell lines (293HEK and MCF10) were resistant to the tested doses. Furthermore, it was previously reported, high doses of flavopiridol (10 μM) or CPS49 (12 μM) were needed to inhibit tumor growth in PC3 xenograft mice compared with vehicle treated mice. Here, we demonstrated that combining these two agents, antitumor activity was synergistically enhanced with low doses. Injecting subcutaneously PC3 cells in nu/nu mice, we found that CPS49-flavopiridol administration reduced tumor volume approximately 83% after 2 weeks of co-treatment and 54% after 1 week of low dose flavopiridol pretreatment and 2 weeks of drug combination. In addition, we performed RT-qPCR array containing 26 genes from PC3 cells exposed to CPS49 and flavopiridol combination. We determined that this treatment shut down the expression of several genes involved in cell cycle, DNA damage and tumor progression. Histological analysis of xenograft PC3 tumor samples showed extensive areas of necrosis induced by the treatment. Furthermore, we assessed the efficacy of CPS49 in combination with paclitaxel (docetaxel analog). All the prostate tumor cell lines tested were highly sensitive to this combination. However, this combination did not reduce the tumor volume in PC3 xenografts. These results indicate that the CPS49 and flavopiridol is a promising new alternative for the treatment of CRPC.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2340. doi:1538-7445.AM2012-2340

BRCA1 loss induces GADD153-mediated doxorubicin resistance in prostate cancer

BRCA1 plays numerous roles in the regulation of genome integrity and chemoresistance. Although BRCA1 interaction with key proteins involved in DNA repair is well known, its role as a coregulator in the transcriptional response to DNA damage remains poorly understood. In this study, we show that BRCA1 plays a central role in the transcriptional response to genotoxic stress in prostate cancer. BRCA1 expression mediates apoptosis, cell-cycle arrest, and decreased viability in response to doxorubicin treatment. Xenograft studies using human prostate carcinoma PC3 cells show that BRCA1 depletion results in increased tumor growth. A focused survey of BRCA1-regulated genes in prostate carcinoma reveals that multiple regulators of genome stability and cell-cycle control, including BLM, FEN1, DDB2, H3F3B, BRCA2, CCNB2, MAD2L1, and GADD153, are direct transcriptional targets of BRCA1. Furthermore, we show that BRCA1 targets GADD153 promoter to increase its transcription in response to DNA damage. Finally, GADD153 depletion significantly abrogates BRCA1 influence on cell-cycle progression and cell death in response to doxorubicin treatment. These findings define a novel transcriptional pathway through which BRCA1 orchestrates cell fate decisions in response to genotoxic insults, and suggest that BRCA1 status should be considered for new chemotherapeutic treatment strategies in prostate cancer.

Cyclin T1 overexpression induces malignant transformation and tumor growth

Human PTE Fb is a protein kinase composed by CDK9 and Cyclin T that controls the elongation phase of RNA Pol II. This complex also affects the activation and differentiation program of lymphoid cells. In this study we found that several head and neck tumor cell lines overexpress PTE Fb. We also established that Cyclin T1 is able to induce transformation in vitro, as we determined by foci and colony formation assays. Nu/nu mice s.c. injected with stable transfected Cyclin T1 cells (NIH 3T3 Cyclin T1) developed tumors faster than animals injected with control cells (NIH 3T3 beta-gal). In vitro, NIH 3T3 Cyclin T1 cells show increased proliferation and CDK4-Rb phosphorylation. Even more, silencing E2F1 expression (shRNA E2F1) in NIH 3T3 cells resulted in a dramatic inhibition of Cyclin T1-induced foci. All these data demonstrate for the first time the Cyclin T1 oncogenic function and suggest a role for this protein in controlling cell cycle probably via Rb/E2F1 pathway.

Transcriptional autoregulation by BRCA1

The BRCA1 gene product plays numerous roles in regulating genome integrity. Its role in assembling supermolecular complexes in response to DNA damage has been extensively studied; however, much less is understood about its role as a transcriptional coregulator. Loss or mutation is associated with hereditary breast and ovarian cancers, whereas altered expression occurs frequently in sporadic forms of breast cancer, suggesting that the control of BRCA1 transcription might be important to tumorigenesis. Here, we provide evidence of a striking linkage between the roles for BRCA1 as a transcriptional coregulator with control of its expression via an autoregulatory transcriptional loop. BRCA1 assembles with complexes containing E2F-1 and RB to form a repressive multicomponent transcriptional complex that inhibits BRCA1 promoter transcription. This complex is disrupted by genotoxic stress, resulting in the displacement of BRCA1 protein from the BRCA1 promoter and subsequent upregulation of BRCA1 transcription. Cells depleted of BRCA1 respond by upregulating BRCA1 transcripts, whereas cells overexpressing BRCA1 respond by downregulating BRCA1 transcripts. Tandem chromatin immmunoprecipitation studies show that BRCA1 is regulated by a dynamic coregulatory complex containing BRCA1, E2F1, and Rb at the BRCA1 promoter that is disrupted by DNA-damaging agents to increase its transcription. These results define a novel transcriptional mechanism of autoregulated homeostasis of BRCA1 that selectively titrates its levels to maintain genome integrity in response to genotoxic insult.

Critical role of endogenous heme oxygenase 1 as a tuner of the invasive potential of prostate cancer cells

Prostate cancer (PCa) is the second leading cause of cancer-associated death in men. Inflammation has been recognized as a risk factor for this disease. Heme oxygenase 1 (HO-1), the inducible isoform of the rate-limiting enzyme in heme degradation, counteracts oxidative and inflammatory damage. Here, we investigated the regulated expression of HO-1 and its functional consequences in PCa. We studied the effect of genetic and pharmacologic disruption of HO-1 in the growth, invasion, and migration in androgen-sensitive (MDA PCa2b and LNCaP) and androgen-insensitive (PC3) PCa cell lines. Our results show that HO-1 levels are markedly decreased in PC3 compared with MDA PCa2b and LNCaP. Hemin treatment increased HO-1 at both protein and mRNA levels in all cell lines and decreased cell proliferation and invasion. Furthermore, overexpression of HO-1 in PC3 resulted in markedly reduced cell proliferation and migration. Accordingly, small interfering RNA-mediated silencing of HO-1 expression in MDA PCa2b cells resulted in increased proliferation and invasion. Using reverse transcription-quantitative PCR-generated gene array, a set of inflammatory and angiogenic genes were upregulated or downregulated in response to HO-1 overexpression identifying matrix metalloprotease 9 (MMP9) as a novel downstream target of HO-1. MMP9 production and activity was downregulated by HO-1 overexpression. Furthermore, PC3 cells stably transfected with HO-1 (PC3HO-1) and controls were injected into nu/nu mice for analysis of in vivo tumor xenograft phenotype. Tumor growth and MMP9 expression was significantly reduced in PC3HO-1 tumors compared with control xenografts. Taken together, these results implicate HO-1 in PCa cell migration and proliferation suggesting its potential role as a therapeutic target in clinical settings.

Identification of new Rel/NFkappaB regulatory networks by focused genome location analysis

NFkappaB is an inducible transcription factor that controls kinetically complex patterns of gene expression. Several studies reveal multiple pathways linking NFkappaB to the promotion and progression of various cancers. Despite extensive interest and characterization, many NFkappaB controlled genes still remain to be identified. We used chromatin immunoprecipitation combined with microarray technology (ChIP/chip) to investigate the dynamic interaction of NFkappaB with the promoter regions of 100 genes known to be expressed in mitogen-induced T-cells. Six previously unrecognized NFkappaB controlled genes (ATM, EP300, TGFbeta, Selectin, MMP-1 and SFN) were identified. Each gene is induced in mitogen-stimulated T-cells, repressed by pharmacological NFkappaB blockade, reduced in cells deficient in the p50 NFkappaB subunit and dramatically repressed by RNAi specifically designed against cRel. A coregulatory role for Ets transcription factors in the expression of the NFkappaB controlled genes was predicted by comparative promoter analysis and confirmed by ChIP and by functional disruption of Ets. NFkappaB deficiency produces a deficit in ATM function and DNA repair indicating an active role for NFkappaB in maintaining DNA integrity. These results define new potential targets and transcriptional networks governed by NFkappaB and provide novel functional insights for the role of NFkappaB in genomic stability, cell cycle control, cell-matrix and cell-cell interactions during tumor progression.

Combinatorial antileukemic disruption of oxidative homeostasis and mitochondrial stability by the redox reactive thalidomide 2-(2,4-difluoro-phenyl)-4,5,6,7-tetrafluoro-1H-isoindole-1,3(2H)-dione (CPS49) and flavopiridol

2-(2,4-Difluoro-phenyl)-4,5,6,7-tetrafluoro-1H-isoindole-1,3(2H)-dione (CPS49) is a member of a recently identified class of redox-reactive thalidomide analogs that show selective killing of leukemic cells by increasing intracellular reactive oxygen species (ROS) and targeting multiple transcriptional pathways. Flavopiridol is a semisynthetic flavonoid that inhibits cyclin-dependent kinases and also shows selective lethality against leukemic cells. The purpose of this study is to explore the efficacy and mechanism of action of the combinatorial use of the redox-reactive thalidomide CPS49 and the cyclin-dependent kinase inhibitor flavopiridol as a selective antileukemic therapeutic strategy. In combination, CPS49 and flavopiridol were found to induce selective cytotoxicity associated with mitochondrial dysfunction and elevations of ROS in leukemic cells ranging from additive to synergistic activity at low micromolar concentrations. Highest synergy was observed at the level of ROS generation with a strong correlation between cell-specific cytotoxicity and reciprocal coupling of drug-induced ROS elevation with glutathione depletion. Examination of the transcriptional targeting of CPS49 and flavopiridol combinations reveals that the drugs act in concert to initiate a cell specific transcriptional program that manipulates nuclear factor-kappaB (NF-kappaB), E2F-1, and p73 activity to promote enhanced mitochondrial instability by simultaneously elevating the expression of the proapoptotic factors BAX, BAD, p73, and PUMA while depressing expression of the antiapoptotic genes MCL1, XIAP, BCL-xL, SURVIVIN, and MDM2. The coadministration of CPS49 and flavopiridol acts through coordinate targeting of transcriptional pathways that enforce selective mitochondrial dysfunction and ROS elevation and is therefore a promising new therapeutic combination that warrants further preclinical exploration.

Interaction of the ADP-ribosylating enzyme from the hyperthermophilic archaeon S. solfataricus with DNA and ss-oligo deoxy ribonucleotides

The DNA-binding ability of the poly-ADPribose polymerase-like enzyme from the extremely thermophilic archaeon Sulfolobus solfataricus was determined in the presence of genomic DNA or single stranded oligodeoxyribonucleotides. The thermozyme protected homologous DNA against thermal denaturation by lowering the amount of melted DNA and increasing melting temperature. The archaeal protein induced structural changes of the nucleic acid by modifying the dichroic spectra towards a shape typical of condensing DNA. However, enzyme activity was slightly increased by DNA. Competition assays demonstrated that the protein interacted also with heterologous DNA. In order to characterize further the DNA binding properties of the archaeal enzyme, various ss-oligodeoxyribonucleotides of different base composition, lengths (12-mer to 24-mer) and structure (linear and circular) were used for fluorescence titration measurements. Intrinsic fluorescence of the archaeal protein due to tryptophan (excitation at 295 nm) was measured in the presence of each oligomer at 60 degrees C. Changes of tryptophan fluorescence were induced by all compounds in the same range of base number per enzyme molecule, but independently from the structural features of oligonucleotides, although the protein exhibited a slight preference for those adenine-rich and circular. The binding affinities were comparable for all oligomers, with intrinsic association constants of the same order of magnitude (K=10(6) M(-1)) in 0.01 M Na-phosphate buffer, pH 8.0, and accounted for a "non-specific" binding protein. Circular dichroism analysis showed that at 60 degrees C the native protein was better organized in a secondary structure than at 20 degrees C. Upon addition of oligonucleotides, enzyme structure was further stabilized and changed towards a beta-conformation. This effect was more marked with the circular oligomer. The analysed oligodeoxyribonucleotides slightly enhanced enzyme activity with the maximal increase of 50% as compared to the control. No activation was observed with the circular oligomer.

High stability binding of poly(ADPribose) polymerase-like thermozyme from S. solfataricus with circular DNA

The poly(ADPribose) polymerase-like thermozyme from the hyperthermophilic archaeon S. solfataricus was found to bind DNA with high affinity and non-specifically. Binding was independent of base composition and length of the nucleic acid, and the protein showed a slight preference for the circular structure. By using pCMV-Neo-Bam plasmid as experimental model, the behaviour of the thermozyme upon binding with either circular or linear plasmid was analyzed. pCMV-Neo-Bam has a single HindIII site that allows to obtain the linear structure after digestion with the restriction enzyme. Intrinsic tryptophan-dependent fluorescence of poly(ADPribose) polymerase-like thermozyme noticeably changed upon addition of either circular or linear plasmid, showing the same binding affinity (K=2 x 10(9) M-1). However, experiments of protection against temperature and DNase I gave evidence that the thermozyme formed more stable complexes with the circular structure than with the linear pCMV-Neo-Bam. Increasing temperature at various DNA/protein ratios had a double effect to reduce the amount of circular DNA undergoing denaturation and to split the melting point towards higher temperatures. Nil or irrelevant effect was observed with the linear form. Similarly, DNase acted preferentially on the linear plasmid/protein complexes, producing an extensive digestion even at high protein/DNA ratios, whereas the circular plasmid was protected by the thermozyme in a dose-dependent manner. The complexes formed by archaeal poly(ADPribose) polymerase (PARPss) with the circular plasmid were visualized by bandshift experiments both with ethidium bromide staining and by labelling the circular plasmid with 32P. The stability of complexes was tested as a function of enzyme concentration and in the presence of a cold competitor and of 0.1% SDS. From the performed experiments, a number of 3-10 base pairs bound per molecule of enzyme was calculated, indicating a high frequency of binding. The presence of circular DNA was also able to increase by 80% the poly(ADPribose)polymerase-like activity, as compared to 25% activation induced by the linear pCMV-Neo-Bam.