Alterations in deoxynucleoside triphosphate metabolism in DNA damaged cells: identification and consequences of poly(ADP-ribose) polymerase dependent and independent processes

Nicotinamide metabolism alterations in bladder cancer: Preliminary studies

OBJECTIVES

Cancer is one of the main cause of death in Western countries. Inflammation plays an important role in the pathogenesis of cancer. Nicotinamide (NA) - known for its anti-inflammatory properties - participates in the processes related to the cell cycle regulation and DNA repair, which are relevant in cancer development. This study aimed to investigate the nicotinamide metabolism alterations in bladder cancer.

METHODS

Blood and plasma samples of patients with bladder cancer were collected. Blood pyridine and adenine nucleotides concentration were measured using high performance liquid chromatography (HPLC). Plasma nicotinamide metabolites concentration were determined using high performance liquid chromatography - mass spectrometry (LC/MS).

RESULTS

Our results indicated that the development of bladder cancer caused significant decrease in the concentration of N-methylnicotinamide (MetNA) (0.07 ± 0.02 vs 0.1 ± 0.03 µmol/l) and an increase in the concentration of N-methyl-2-pyridone-5-carboxamide (Met2PY) - one of the final nicotinamide metabolites: (1.1 ± 0.15 vs 0.7 ± 0.07 µmol/l) in comparison to the control. The association between the stage of cancer and the increase in both, Met2PY and Met4PY levels was observed. Blood ATP and NAD levels were significantly decreased in bladder cancer patients as compared to the control (970.8 ± 77.84 vs 1165.00 ± 57.76 µmol/l; 45.86 ± 2.98 vs 53.06 ± 2.28 µmol/l respectively).

CONCLUSIONS

Bladder cancer development caused substantial changes in nicotinamide metabolism, such as decreased plasma MetNA and increased Met2PY concentration. Analysis of the nicotinamide and its metabolites concentrations - as new biomarkers - may allow to track the course of pathological processes in cancer.

The involvement of ATP produced via (ADP-Ribose)n in the maintenance of DNA replication apparatus during DNA repair

The formation of ATP produced from poly(ADP-ribose) [(ADP-R)n] has been suggested to be required to repair damaged DNA. Here we investigate whether this ATP is involved in DNA replication processes during DNA repair. Poly(ADP-ribosyl)ated mid-S phase cell nuclei, which were isolated from synchronized HeLa S3 cells followed by the treatment with a DNA damaging agent, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), were revealed to retain DNA replication synthesizing activity during preincubation for de-poly(ADP-ribosyl)ation only in the presence of pyrophosphate (PPi) before DNA synthesis was started by adding 3 mM ATP. This DNA replication activity was not maintained in the presence of a potent and specific inhibitor of poly(ADP-ribose) glycohydrolase (PARG), Oenothein B (Oen B) during the preincubation with PPi. In the preincubation with PPi, muM orders of ATP was produced from (ADP-R)n. These results point to an important function of ATP generated from (ADP-R)n in nuclei for the maintenance of replication apparatus during DNA repair.

Induction of ROS formation, poly(ADP-ribose) polymerase-1 activation, and cell death by PCB126 and PCB153 in human T47D and MDA-MB-231 breast cancer cells

The primary purpose of this research is to investigate whether exposure to polychlorinated biphenyls (PCBs), i.e. PCB153 and PCB126, is associated with induction of reactive oxygen species (ROS), poly(ADP-ribose) polymerase-1 (PARP-1) activation, and cell death in human T47D and MDA-MB-231 breast cancer cells. Results indicated that PCB153 and PCB126 induced concentration- and time-dependent increases in cytotoxic response and ROS formation in both T47D and MDA-MB-231 cells. At non-cytotoxic concentrations both PCB153 and PCB126 induced decreases in intracellular NAD(P)H and NAD+ in T47D and MDA-MB-231 cells where T47D cells were more resistant to PCB-induced reduction in intracellular NAD(P)H than MDA-MB-231 cells. Further investigation indicated that three specific PARP inhibitors completely blocked PCB-induced decreases in intracellular NAD(P)H in both T47D and MDA-MB-231 cells. These results imply that decreases in intracellular NAD(P)H in PCB-treated cells may be, in part, due to depletion of intracellular NAD+ pool mediated by PARP-1 activation through formation of DNA strand breaks. Overall, the extent of cytotoxic response, ROS formation, and PARP-1 activation generated in T47D and MDA-MB-231 cells was greater for PCB153 than for PCB126. In addition, the cytotoxicity induced by PCB153 and PCB126 in both T47D and MDA-MB-231 cells was completely blocked by co-treatment of catalase, dimethylsulfoxide, cupper (I)-/iron (II)-specific chelators, and CYP1A/2B inhibitors. This evidence suggests the involvement of ROS, Cu(I), Fe(II), and CYP1A/2B enzymes in mediating the induction of cell death by PCB153 and PCB126. Further, antagonism was observed between PCB126 and PCB153 for effects on cytotoxic response and ROS formation in T47D and MDA-MB-231 cells. Antagonism was also observed between PCB153 and PCB126 in the induction of NAD(P)H depletion at lower concentration (<10 microM) in T47D cells, but not in MDA-MB-231 cells. In conclusions, results from our investigation suggest that ROS formation induced by PCBs is a significant determinant factor in mediating the DNA damage and cell death in human breast cancer cells. The data also suggests that the status of estrogen receptor alpha may play a role in modulating the PCB-induced oxidative DNA damage and cell death in human breast cancer cells.

Cancer chemotherapy by deoxynucleotide depletion and E2F-1 elevation

We propose that the lethality of commonly used anticancer drugs, e.g., methotrexate and cis-platinum are due, at least in part, to an increase of the E2F-1-mediated apoptotic cascade. The drugs directly or indirectly decrease deoxynucleoside triphosphates. The E2F family acts to provide control of S phase by transcribing genes required for deoxynucleoside triphosphate and DNA synthesis. Thus, a mechanism for control of E2F-1 is essential, a signal safeguarding against aberrant or uncontrolled cell proliferation. We have proposed a feedback control by NTPs that down-regulates E2F-1. Here, we provide evidence in support of this hypothesis.

Active poly(ADPribose) metabolism in DNAase- and salt-resistant rat testis chromatin with high transcriptional activity/competence

A chromatin fraction, named pP fraction, was prepared from rat testis nuclei, which had been digested with nuclease in order to separate soluble and insoluble chromatin. This fraction resembled nuclear matrix as it was highly resistant to DNAase digestion, had a high content of proteins compared to the low DNA percentage, and a noticeable transcriptional activity. Moreover, poly(ADPribosyl)ation system (i.e., poly(ADPR)polymerase, poly(ADPribose), and acceptor proteins) was still present at high levels. In order to study whether it might be identified as the protein support surrounding chromatin loops, this pP fraction was further analyzed after 3 M NaCl extraction. The 3 M NaCl extract and the highly insoluble pellet, named Nuclear Matrix Pellet, were characterized as it regards DNA, newly synthesized RNA and proteins. Furthermore, poly(ADPribose) metabolism was analyzed by measuring both poly(ADPribose) polymerase and poly(ADPribose) glycohydrolase activities, poly(ADPribose) distribution and by identifying protein acceptors. The final pellet had features of nuclear matrix containing less than 10% DNA and high percentage of proteins; 28% of newly synthesized RNA was still associated with this fraction. Long and branched polyADPribose were found in the nuclear matrix-like pellet, although ADPribose acceptors (mainly H1 and core histones) appeared to be modified mostly with short ADPribose oligomers. Longest and branched polymers were retained on the top of protein gel, likely bound to automodified poly(ADPribose) polymerase.

Characterization of recombinant human nicotinamide mononucleotide adenylyl transferase (NMNAT), a nuclear enzyme essential for NAD synthesis

Nicotinamide mononucleotide adenylyl transferase (NMNAT) is an essential enzyme in all organisms, because it catalyzes a key step of NAD synthesis. However, little is known about the structure and regulation of this enzyme. In this study we established the primary structure of human NMNAT. The human sequence represents the first report of the primary structure of this enzyme for an organism higher than yeast. The enzyme was purified from human placenta and internal peptide sequences determined. Analysis of human DNA sequence data then permitted the cloning of a cDNA encoding this enzyme. Recombinant NMNAT exhibited catalytic properties similar to the originally purified enzyme. Human NMNAT (molecular weight 31932) consists of 279 amino acids and exhibits substantial structural differences to the enzymes from lower organisms. A putative nuclear localization signal was confirmed by immunofluorescence studies. NMNAT strongly inhibited recombinant human poly(ADP-ribose) polymerase 1, however, NMNAT was not modified by poly(ADP-ribose). NMNAT appears to be a substrate of nuclear kinases and contains at least three potential phosphorylation sites. Endogenous and recombinant NMNAT were phosphorylated in nuclear extracts in the presence of [gamma-(32)P]ATP. We propose that NMNAT's activity or interaction with nuclear proteins are likely to be modulated by phosphorylation.

ATP for the DNA ligation step in base excision repair is generated from poly(ADP-ribose)

In mammalian cells, the base excision repair (BER) pathway is the main route to counteract the mutagenic effects of DNA lesions. DNA nicks induce, among others, DNA polymerase activities and the synthesis of poly(ADP-ribose). It is shown here that poly(ADP-ribose) serves as an energy source for the final and rate-limiting step of BER, DNA ligation. This conclusion was drawn from experiments in which the fate of [(32)P]poly(ADP-ribose) or [(32)P]NAD added to HeLa nuclear extracts was systematically followed. ATP was synthesized from poly(ADP-ribose) in a pathway that strictly depended on nick-induced DNA synthesis. NAD was used for the synthesis of poly(ADP-ribose), which, in turn, was converted to ATP by pyrophosphorylytic cleavage utilizing the pyrophosphate generated from dNTPs during DNA synthesis. The adenylyl moiety was then preferentially used to adenylate DNA ligase III, from which it was transferred to the 5'-phosphoryl end of the nicked DNA. Finally, ligation to the 3'-OH end resulted in the release of AMP. When using NAD, but not poly(ADP-ribose), in the presence of 3-aminobenzamide, the entire process was blocked, confirming poly(ADP-ribosyl)ation to be the essential initial step. Thus, poly(ADP-ribose) polymerase-1, DNA polymerase beta, and ligase III interact with x-ray repair cross-complementing protein-1 within the BER complex, which ensures that ATP is generated and specifically used for DNA ligation.

Poly(ADP-ribosyl)ation reactions in the regulation of nuclear functions

Poly(ADP-ribosyl)ation is a post-translational modification of proteins. During this process, molecules of ADP-ribose are added successively on to acceptor proteins to form branched polymers. This modification is transient but very extensive in vivo, as polymer chains can reach more than 200 units on protein acceptors. The existence of the poly(ADP-ribose) polymer was first reported nearly 40 years ago. Since then, the importance of poly(ADP-ribose) synthesis has been established in many cellular processes. However, a clear and unified picture of the physiological role of poly(ADP-ribosyl)ation still remains to be established. The total dependence of poly(ADP-ribose) synthesis on DNA strand breaks strongly suggests that this post-translational modification is involved in the metabolism of nucleic acids. This view is also supported by the identification of direct protein-protein interactions involving poly(ADP-ribose) polymerase (113 kDa PARP), an enzyme catalysing the formation of poly(ADP-ribose), and key effectors of DNA repair, replication and transcription reactions. The presence of PARP in these multiprotein complexes, in addition to the actual poly(ADP-ribosyl)ation of some components of these complexes, clearly supports an important role for poly(ADP-ribosyl)ation reactions in DNA transactions. Accordingly, inhibition of poly(ADP-ribose) synthesis by any of several approaches and the analysis of PARP-deficient cells has revealed that the absence of poly(ADP-ribosyl)ation strongly affects DNA metabolism, most notably DNA repair. The recent identification of new poly(ADP-ribosyl)ating enzymes with distinct (non-standard) structures in eukaryotes and archaea has revealed a novel level of complexity in the regulation of poly(ADP-ribose) metabolism.

3-aminobenzamide protects the mouse thymocytes in vitro from dexamethasone-mediated apoptotic cell death and cytolysis without changing DNA strand breakage

Exposure of mouse thymocytes to 1 microM dexamethasone (Dex) resulted in a dramatic increase in the degree of DNA strand breakage up to 80% between 4 to 6 h postincubation. During incubation a marked decrease in the number of total and viable cells as well as an increase in the release of lactate dehydrogenase into medium were detectable, indicating a strong cytotoxicity of Dex on the mouse thymocytes. Agarose gel electrophoresis of DNA from cells exposed to Dex for 6 h clearly demonstrated an increased laddering of DNA fragments multiple of approx. 200 base pairs as a characteristic feature of an apoptosis or programmed cell death. The cytotoxicity of Dex, as judged by the decrease in the viability and increase in the cell lysis, was effectively prevented by 3-aminobenzamide, a potent inhibitor of poly(ADP-ribose) synthesis. Furthermore, the lowering of intracellular NAD levels, which was observable in the present study most probably as a result of activation of poly(ADP-ribose) synthesis due to Dex-mediated DNA strand breakage, was also specifically prevented by the inhibitor, although the DNA strand breakage itself was not affected under these conditions. Our present results indicate that the Dex-mediated thymocyte death and cytolysis and probably intrathymic apoptotic thymocytolysis could be attributable primarily to the loss of intracellular NAD.

Cockayne's syndrome fibroblasts are characterized by hypersensitivity to deoxyguanosine and abnormal DNA precursor pool metabolism in response to deoxyguanosine or ultraviolet light

New cellular traits of Cockayne's syndrome (CS) associated with DNA precursor metabolism have been identified, namely, hypersensitivity to the toxicity of low concentrations of deoxyguanosine (dG) and abnormal changes in deoxyribonucleotide (dNTP) pools in response to dG or UV. dG treatment results in similar ribonucleotide pool changes in wild-type and CS cells, i.e., GTP levels increase at least twofold. However, the changes in the pool size of the purine deoxyribonucleotides are significantly different; in wild-type cells dATP and dGTP pools increase threefold, but remain unchanged in CS. The mechanism by which dG kills CS cells is not clear, but unlike the inherited purine nucleoside phosphorylase deficiency disease, the toxicity of dG is not due to the accumulation of dGTP and the consequent feedback inhibition of ribonucleotide reductase. UV induces different dNTP pool changes in CS and wild-type cells. In wild-type cells dTTP, dCTP, and dATP pools increase three- to fivefold within 4 h of irradiation, while the dGTP pool contracts. In CS cells, only the dGTP pool expands (four- to sixfold), while the other three contract. Each of these new phenotypic traits, together with UV sensitivity, is coordinately corrected in the complementing proliferating CSA x CSB hybrid cells.

Mutant cells defective in poly(ADP-ribose) synthesis due to stable alterations in enzyme activity or substrate availability

We used two different approaches to develop cell lines deficient in poly(ADP-ribose) synthesis to help determine the role of this reaction in cellular functions. One approach to this problem was to develop cell lines deficient in enzyme activity; the other approach was to develop cell lines capable of growing with such low nicotinamide adenine dinucleotide (NAD) levels so as to effectively limit substrate availability for poly(ADP-ribose) synthesis. The selection strategy for obtaining cells deficient in activity of poly(ADP-ribose) polymerase was based on the ability of this enzyme to deplete cellular NAD in response to high levels of DNA damage. Using this approach, we first obtained cell lines having 37-82% enzyme activity compared to their parental cells. We now report the development and characterization of two cell lines which were obtained from cells having 37% enzyme activity by two additional rounds of further mutagenization and selection procedures. These new cell lines contain 5-11% enzyme activity compared to the parental V79 cells. In pursuit of the second strategy, to obtain cells which limit poly(ADP-ribose) synthesis by substrate restriction, we have now isolated spontaneous mutants from V79 cells which can grow stably in the absence of free nicotinamide or any of its analogs. These cell lines maintain NAD levels in the range of 1.5-3% of that found in their parental V79 cells grown in complete medium. The pathway of NAD biosynthesis in these NAD-deficient cells is not yet known. Further characterization of these lines showed that under conditions that restricted poly(ADP-ribose) synthesis, they all had prolonged doubling times and increased frequencies of sister chromatid exchanges.

Anticarcinogenic potential of DNA-repair modulators

Effects of compounds that inhibit repair of DNA lesions in cells have been reported frequently. The consequences include altered incidence of carcinogenicity in vivo, tumorigenic transformation of cultured cells, mutations, and increased lethality as well as sister-chromatid exchanges and chromosome aberrations. This literature is reviewed here, with major emphasis on methylxanthines (caffeine in particular) and nicotinamide analogs. Existing information is also summarized on a novel potent repair inhibitor, beta-lapachone. Compounds that inhibit both DNA replication and repair are not discussed in detail since they have been reviewed often, but miscellaneous inhibitors of repair are summarized in a table. The relatively small number of experiments performed on the anticarcinogenic effects of methyl-xanthines and nicotinamide analogs gave very conflicting results. Some investigators report decreased carcinogenicity of DNA-damaging agents when caffeine was provided, but others obtained the opposite effect. The three studies with nicotinamide analogs all reported enhanced tumorigenicity of carcinogens. The data are too few to enable firm conclusions to be drawn regarding the possibility of using repair inhibitors to prevent cancer in humans. Variations of experimental conditions, carcinogens, cells, etc. have provided conflicting results. The possibility of cancer prevention is, nevertheless, so important that further investigations with DNA-repair inhibitors, particularly with human cells, seem very well justified.

Deoxynucleoside triphosphate pool perturbation is not a general feature in mutagen-treated mammalian cells

Deoxynucleoside triphosphate (dNTP) and ribonucleoside triphosphate (rNTP) pools were analyzed in 4 mammalian cell lines following treatment with UV-C (254 nm), UV-A (365 nm) or the carcinogen, 1-methyl-3-nitro-1-nitrosoguanidine (MNNG). No substantial alterations in dNTP pool levels were observed in any treatment group. However, the cellular conversions of exogenously added deoxycytidine and deoxyguanosine to the corresponding triphosphates were inhibited 30-97% by UV-C and MNNG treatment. In addition, the conversion of dGuo to GTP and deoxyadenosine to ATP were inhibited 25-50% in CHO cells by mutagen treatment. The data do not support the notion that modulation of specific dNTP pools is a general feature of mutagen treatment in mammalian cells, but so suggest a mutagen-sensitivity of deoxynucleoside metabolism.

Strategy for selection of cell variants deficient in poly(ADP-ribose) polymerase

A selection strategy to obtain cells deficient in poly(ADP-ribose) polymerase was developed based on the fact that treatment with high levels of N-methyl-N'-nitro-N-nitrosoguanidine results in sufficient activation of poly(ADP-ribose) polymerase to cause NAD and ATP depletion leading to cessation of all energy-dependent processes and rapid cell death. In contrast, cells with low levels of poly(ADP-ribose) polymerase should not consume their NAD and might therefore be more likely to survive the DNA damage. Using this approach, we have cloned a number of cell lines containing 37-82% enzyme activity. The apparent decrease in poly(ADP-ribose) polymerase activity is not due to increases in NAD glycohydrolase, poly(ADP-ribose) glycohydrolase, or phosphodiesterase activities. Further characterization of the poly(ADP-ribose) polymerase-deficient cells indicates that they have prolonged generation times and increased rates of spontaneous sister chromatid exchanges.

Role of nicotinamide adenine dinucleotide and adenosine triphosphate in glucocorticoid-induced cytotoxicity in susceptible lymphoid cells

The possibility that corticosteroid cytotoxicity could be mediated by activation of poly(ADP-ribose) polymerase and consequent depletion of NAD and ATP was evaluated in steroid-sensitive S49.1 and steroid-resistant S49.143R mouse lymphoma cells and in lymphocytes from a patient with chronic lymphocytic leukemia. All cell types were shown to have the enzyme poly(ADP-ribose) polymerase and to increase activity in response to DNA strand breaks. Incubation of susceptible cells with 1 microM dexamethasone resulted in DNA strand breaks. Susceptible cells also showed a dose-dependent decrease in NAD and ATP that preceded loss of cell viability. These studies suggest that steroid-induced cytotoxicity in susceptible lymphocytes is due to the presence of DNA strand breaks that activate poly(ADP-ribose) polymerase to a sufficient degree to consume cellular pools of NAD with a consequent depletion of ATP and loss of cell viability.