Cellular NAD+ and ATP levels in alkylation-induced cytotoxicity enhanced by an inhibitor of poly(ADP-ribose) synthesis

Alkylating agents cause a marked depletion of cellular NAD+ levels by activating nuclear ADP-ribosyl transferase (ADPRT), which utilizes NAD+ as a substrate in the synthesis of poly(ADP-ribose). As a consequence of NAD+ depletion, it is possible that cellular ATP pools could be depleted. Because of this, exogenously supplied NAD+ had been proposed as a way to counteract some of the effects of an alkylator. We found that exogenously supplied NAD+ significantly increased intracellular levels of NAD+ in MMS- and MNNG-treated V79 Chinese hamster cells. Cytotoxicity was not changed by the exogenously supplied NAD+, however. 3-Aminobenzamide (3-ABA), an ADPRT inhibitor, prevented the depletion of intracellular NAD+ by MMS or MNNG treatment and potentiated cytotoxicity. As was the case without 3-ABA, exogenously supplied NAD+ plus 3-ABA did not change the cytotoxicity, even though NAD+ levels were increased. Intracellular ATP levels were also measured and were found to be unaffected following MMS treatment, and only slightly depleted following MNNG treatment. Exogenously supplied NAD+ raised these levels above those for their respective controls. Because survival was unaffected by elevated levels of NAD+ and ATP, our results suggest that depletion of cellular NAD+ pools following MMS and MNNG treatment is not a critical factor in determining cytotoxicity for these V79 cells. The energy reserves of V79 cells, at doses of MMS or MNNG which kill 99% of the cells, are apparently adequate to maintain normal levels of ATP.     

Pyridine nucleotide cycling and poly(ADP-ribose) synthesis in resting human lymphocytes

NAD is a critical cofactor for the oxidation of fuel molecules. The exposure of human PBL to agents that cause DNA strand breaks to accumulate can deplete NAD pools by increasing NAD consumption for poly(ADP-ribose) formation. However, the pathways of NAD synthesis and degradation in viable PBL have not been carefully documented. The present experiments have used radioactive labeling techniques to trace the routes of NAD metabolism in resting PBL. The cells could generate NAD from either nicotinamide or nicotinic acid. PBL incubated with [14C]nicotinic acid excreted [14C]nicotinamide into the medium. Approximately 50% of a prelabeled [14C]NAD pool was metabolized during 6 to 8 hr in tissue culture. Basal NAD turnover was prolonged threefold to fourfold by 3-aminobenzamide (3-ABA), an inhibitor of poly(ADP-ribose) synthetase. Supplementation of the medium with 3-ABA also prevented the accelerated NAD degradation that ensued after exposure of PBL to deoxyadenosine plus deoxycoformycin at concentrations previously shown to cause DNA strand break accumulation. These results demonstrate that quiescent human PBL continually produce NAD and utilize the nucleotide for poly(ADP-ribose) synthesis.     

Poly(ADP-ribose) Polymerase inhibitors preserve nicotinamide adenine dinucleotide and adenosine 5'-triphosphate pools in DNA-damaged cells: mechanism of stimulation of unscheduled DNA synthesis

Inhibitors of poly(ADP-ribose) polymerase stimulated the level of DNA, RNA, and protein synthesis in DNA-damaged L1210 cells but had negligible effects in undamaged L1210 cells. The poly(ADP-ribose) polymerase inhibitors stimulated DNA repair synthesis after cells were exposed to high concentrations of N-methyl-N'-nitro-N-nitrosoguanidine (68 and 136 microM) but not after exposure to low concentrations (13.6 and 34 microM). When the L1210 cells were exposed to 136 microM N-methyl-N'-nitro-N-nitrosoguanidine, the activation of poly(ADP-ribose) polymerase resulted in the rapid depletion of oxidized nicotinamide adenine dinucleotide (NAD+) levels and subsequent depletion of adenosine 5'-triphosphate (ATP) pools. After low doses of N-methyl-N'-nitro-N-nitrosoguanidine (13.6 microM), there were only small decreases in NAD+ and ATP. Poly(ADP-ribose) polymerase inhibitors prevented the rapid fall in NAD+ and ATP pools. This preservation of the ATP pool has a permissive effect on energy-dependent functions and accounts for the apparent stimulation of DNA, RNA, and protein synthesis. Thus, the mechanism by which poly(ADP-ribose) polymerase inhibitors stimulate DNA, RNA, and protein synthesis in DNA-damaged cells appears to be mediated by their ability to prevent the drastic depletion of NAD+ pools that occurs in heavily damaged cells, thereby preserving the cells' ability to generate ATP and maintain energy-dependent processes.     

Nucleotide pools and mutagenic effects of alkylating agents in wild-type and APRT-deficient Friend erythroleukaemia cells

Wild-type Friend mouse erythroleukaemia cells (clone 707) were compared with adenine phosphoribosyltransferase (APRT)-deficient mutant subclones (707DAP8 and 707DAP10) for sensitivity to cell killing and mutagenesis by ethyl methanesulphonate (EMS) and methyl methanesulphonate (MMS). Cells were exposed to 0-300 micrograms/ml EMS and to 0-20 micrograms/ml MMS for a period of 16 h. A slight difference was found between wild-type cells and the two APRT-deficient subclones in terms of sensitivity to cell killing by both mutagens. The APRT-deficient subclones were, however, significantly more sensitive than wild-type cells to mutagenesis to 5-bromo-2-deoxyuridine resistance and 6-thioguanine resistance by EMS and MMS. The APRT-deficient subclones were found to have significantly decreased levels of dATP and dTTP nucleotides and decreased levels of all four ribonucleoside triphosphates (ATP, GTP, CTP and UTP) relative to wild-type cells. Wild-type Friend cells were found to have insignificant levels O6-methylguanine-DNA methyl transferase and it is suggested that the increased mutagen sensitivity of APRT-deficient cells may be due to imbalance of deoxyribonucleoside triphosphate pools during DNA excision-repair processes, or more probably due to deficiency of ATP for ATP-dependent DNA excision-repair enzymes.     

Induction and disappearance of DNA strand breaks in human peripheral blood lymphocytes and fibroblasts treated with methyl methanesulfonate

The induction and disappearance of DNA single-strand breaks (SSB) in human peripheral blood lymphocytes (PBL) and fibroblasts exposed to methyl methanesulfonate (MMS) were investigated by using the alkaline filter elution assay. In the two cell types, identical amounts of SSB were induced during a 45-min treatment with a given dose of MMS. In quiescent PBL only 9 +/- 4% (mean +/- SD) of the induced SSB had disappeared at 1 h after exposure, whereas in phytohemagglutinin-stimulated PBL, 23 +/- 12% disappeared within the same repair period. The percentage SSB disappearance in confluent fibroblasts was 25 +/- 2% at 1 h after exposure. As in PBL, the percentage SSB disappearance in fibroblasts appeared to be proliferation-dependent; actively dividing fibroblasts removed 50 +/- 12% of the MMS-induced SSB during the 1-h repair period. The accumulation of SSB in PBL, but not in fibroblasts, during MMS exposure in the presence of the excision-repair inhibitor 1-beta-D-arabinofuranosylcytosine indicated the utilization of different repair pathways in these two cell types. The generally lower rate of disappearance of MMS-induced SSB in PBL as compared to fibroblasts correlated with an increased loss of cell viability, measured by determining the incorporation of [3H]thymidine.     

Synergistic DNA damaging effects of 4-nitroquinoline-1-oxide and non-effective concentrations of methyl methanesulfonate in human fibroblasts

DNA damage and DNA repair in human fibroblasts induced by the combination mixture of the genotoxic agents methyl methanesulfonate (MMS) and 4-nitroquinoline-1-oxide (4-NQO) were studied using the comet assay and the unscheduled DNA synthesis (UDS), respectively. Cells were simultaneously treated for 1h with the no observed effect concentration (noec) of MMS and increasing concentrations of 4-NQO or vice versa. Different results were obtained with the two types of mixtures. When the noec of 4-NQO was combined with increasing concentrations of MMS, no combination effects were observed. However, in experiments with increasing concentrations of 4-NQO and the noec of MMS, an increase in DNA damage and repair (and an enhancement of cytotoxicity) was demonstrated. Quantitative analysis of the effects by the isobologram method confirmed synergistic responses in both tests. We are proposing interactive actions between 4-NQO and MMS, whereby 4-NQO facilitates the attack of MMS on the DNA bases.     

Nonpermissivity of human peripheral blood lymphocytes to adenovirus type 2 infection.

The capacity of freshly explanted human peripheral blood lymphocytes (PBL) to support the replication of human adenovirus type 2 (Ad2) was investigated. Unlike other types of human cells, PBL were found to be highly nonpermissive. Ad2 adsorbed 30 to 40% of both T and non-T cells. Virus uncoating was very slow and inefficient, resulting in a 40-fold reduction compared with HEp-2 cells. On a population basis, viral DNA synthesis was reduced 460-fold and infectious virus production was reduced 10(6)-fold. Only 0.35% of PBL produced infectious centers, yielding 0.8 PFU per infected cell. Phytohemagglutinin stimulation increased DNA synthesis 23-fold, infectious centers 11-fold, and virus yield 14-fold. We conclude that resting human PBL are highly nonpermissive to Ad2 infection and that phytohemagglutinin can only marginally lift this nonpermissiveness.     

T cell receptor activation induces rapid phosphorylation of prosolin, which mediates down-regulation of DNA synthesis in proliferating peripheral lymphocytes

Prosolin is a major cytosolic phosphoprotein expressed prominently in rapidly proliferating human peripheral lymphocytes but produced at very low levels in resting (G0) PBL. It undergoes rapid phosphorylation upon treatment of growing cells with tumor-producing phorbol esters (TPA) and this phosphorylation event is correlated with a rapid down-regulation of DNA synthesis. In the present report we have studied various agents that, like TPA, act as partial or complete mitogens for G0 PBL and have determined their effect on phosphorylation of prosolin and on DNA synthesis in rapidly proliferating (IL-2-dependent) human PBL. Agents that activate the TCR (OKT3 and PHA), as well as agents that by-pass the receptor but activate biochemical pathways associated with TCR activation (TPA and Ca2(+)-ionophore), all produced rapid phosphorylation of prosolin and prompt down-regulation of DNA synthesis. Four phosphorylated forms of prosolin were produced, indicating activation of a complex phosphorylation pathway. Down-regulation of DNA synthesis did not lead to cell death or to permanent arrest, but was reversed after 24 to 48 h, and was not associated with any reduction in overall protein synthesis. Agents that bind to determinants closely connected to the TCR but without activating it (OKT4 and OKT8) had no effect on either prosolin phosphorylation or DNA synthesis. The results indicate that prosolin is an early target of the protein kinase activities induced by activation of the TCR in proliferating PBL, and suggest that its phosphorylation mediates the TCR signal, transmitting it into a biochemical pathway leading specifically to down-regulation of DNA synthesis. In G0 PBL, in which the negligible expression of prosolin precludes significant production of phosphorylated species, this inhibitory pathway is effectively blocked.     

Metabolic consequences of DNA damage: alteration in purine metabolism following poly(ADP ribosyl)ation in human T-lymphoblasts

The effect of DNA damage caused by N-methyl-N'-nitro-nitrosoguanidine (MNNG) on poly(ADP-ribose) synthesis, NAD levels, and purine nucleotide metabolism was studied in human T-lymphoblasts. Excessive DNA breaks caused by MNNG activated poly(ADP-ribose) polymerase and rapidly consumed intracellular NAD. NAD depletion was followed by rapid catabolism of ATP as well as induction of total purine nucleotide catabolism leading to excretion of purine catabolic products. MNNG-treated cells were not able to replenish the intracellular nucleotide pools due to the depletion of intracellular ATP and phosphoribosylpyrophosphate pools which are required for de novo purine biosynthesis. Inhibition of poly(ADP-ribose) polymerase by 3-aminobenzamide prevented both the depletion of NAD pools and the associated changes in purine nucleotide metabolism.     

Suppression of DNA synthesis in normal, but not neoplastic, nuclei by cytoplasmic extracts from resting cells

We have previously shown that a heat-stable protein in cytoplasmic extracts from human quiescent peripheral blood lymphocytes (PBL) is capable of inhibiting the induction of DNA synthesis in isolated resting nuclei. We now report that these cytoplasmic extracts are also capable of suppressing DNA synthetic activity in replicative nuclei isolated from mitogen-activated PBL. PBL extracts had little or no inhibitory effect, however, on replicative nuclei derived from several transformed lymphoblastoid cell lines. These results suggest that the growth of normal lymphocytes may be negatively controlled by cytoplasmic inhibitory factors. Furthermore, the relative resistance of tumor cell nuclei to these inhibitory signals provides a possible explanation for the loss of growth control in neoplastic cells.     

Inhibition of DNA polymerase activity by methyl methanesulfonate

Methyl methanesulfonate (MMS) inhibits both thymidine incorporation into DNA in mitogen-activated human lymphocytes and deoxythymidine triphosphate incorporation into template DNA by DNA polymerase-alpha in a cell-free system. When MMS-modified DNA was used as the template for DNA synthesis utilizing unmodified DNA polymerase-alpha, nucleotide incorporation into template DNA was not inhibited. When unmodified DNA was used as the template for DNA synthesis utilizing MMS-modified DNA polymerase-alpha, nucleotide incorporation was differentially inhibited dependent on the MMS concentration. An analysis of the kinetics of DNA polymerase-alpha inhibition showed that incorporation of all 4 deoxynucleoside triphosphates into DNA template was noncompetitively inhibited by MMS, which is consistent with nonspecific MMS modification of the enzyme. These data indicate that MMS modification of DNA polymerase-alpha alone is sufficient to inhibit the incorporation of deoxynucleoside triphosphates into template DNA in vitro. The data further indicate that alkylation of both DNA polymerase-alpha and DNA template synergistically increases inhibition of DNA synthesis.     

Laboratory calibrations of the [h]adenine technique for measuring rates of RNA and DNA synthesis in marine microorganisms

The nucleic acid synthesis rates of several marine phytoplankton and bacteria grown in chemostat and batch cultures were measured by using [H]adenine. The [H]adenine synthesis rates showed excellent agreement with the known rates of synthesis estimated from chemical RNA and DNA data. Under certain conditions, RNA turnover and ATP pool compartmentalization produce inaccuracies in synthesis measurements made with [H]adenine. However, accurate measurements of the rates of microbial RNA and DNA synthesis can be made in any environmental situation provided a few simple precautions are observed. First, time course experiments are recommended. Second, experiments should be conducted for periods long enough to avoid problems arising from disequilibria of internal ATP pools. Finally, exogenous [H]adenine should remain in the medium over the length of the time course.     

The modifying action of methylmethane sulfonate on unscheduled DNA synthesis in the UV irradiation of human peripheral blood lymphocytes

The value of the unscheduled DNA synthesis after the combined effect of UV radiation and methyl methanesulfonate (MMS) was considerably lower than that upon exposure to UV radiation alone and after two-hour incubation of the culture. These differences were insignificant after 26 h incubation. The result can be attributed to the alkylating effect of MMS on the repair DNA polymerase. With MMS delivered prior to UV irradiation there was an even larger decrease in the unscheduled DNA synthesis with both 2- and 26-hour incubation. The data obtained can be explained by the fact that MMS inhibits an excision endonuclease.     

The recovery of mammalian cells treated with methyl methanesulfonate, nitrogen mustard or UV light. I. The effect of alkylation products on DNA replication.

CHO cells were synchronized in G1 phase and treated with MMS or HN2. The subsequent rate of DNA replication was found to be reduced in a dose-dependent manner. In addition, 2 X 10(-3 M and 3 X 10(-3) M MMS resulted in a 3--4 h delay prior to the initiation of S phase. If the cells were held for 8 h in hydroxyurea after MMS treatment, no subsequent lag in DNA synthesis was seen after removal of the hydroxyurea. The entry of confluent cells into S phase was found to be delayed 7 h upon trypsinizing and replating. Treatment of these cells with MMS resulted in a reduced rate of DNA replication, but no further delay in its initiation. Repair replication was found to continue at a constant rate for at least 12 h following MMS treatment of cells under all of these conditions. At the concentrations used in these experiments MMS severely inhibited the rate of protein synthesis, but HN2 had little effect. By comparing both the kinetics of repair replication and recovery of protein synthesis with the rate of DNA replication, it was concluded that the initial, severe reduction in rate following MMS treatment was probably due to an inhibition of protein synthesis.     

Adenine metabolism of Ehrlich mouse ascites cells in proliferating and resting phase of tumor growth.

The incorporation of 14C from [U-14C]adenine into the pools of purine nucleotides, nucleosides and bases in Ehrlich mouse ascites cells (EMAC1) during the proliferating and resting phases of tumor growth was compared. In the proliferating phase the total 14C incorporation into purine pools is much faster than in the resting phase. The ATP turnover as well as the purine breakdown to hypoxanthine and uric acid are increased in the proliferating phase. That corresponds to previous findings on higher nucleotide pool sizes and higher ATP yield and ATP-consuming processes in this growth period.     

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

Treatment of L1210 cells with increasing concentrations of MNNG produces heterogeneous perturbations of cellular deoxynucleoside triphosphate pools, with the magnitude and direction of the shift depending on the deoxynucleotide and on the concentration and time of exposure of the DNA damaging agent. 5 microM MNNG stimulated an increase in dATP, dCTP and dTTP but dGTP pools remained constant. These increases were not affected by 3-aminobenzamide, indicating that the pool size increases were produced by poly(ADP-ribose) polymerase independent reactions. 30 microM MNNG caused a time dependent decrease in dATP, dGTP, dTTP and dCTP. The dGTP pool was most drastically affected, becoming totally depleted within 3 hours. The fall in all 4 dNTP pools was substantially prevented by 3-aminobenzamide, suggesting that the decrease in dNTPs following DNA damage is mediated by a poly(ADP-ribose) polymerase dependent reaction. Severe depression of dGTP pools consequent to NAD and ATP depletion may provide a metabolic pathway for rapidly stopping DNA synthesis as a consequence of DNA damage and the activation of poly(ADP-ribose) polymerase.     

DNA strand scission and its repair following exposure of cells to inhibitors of oxidative phosphorylation

Mouse L1210 leukemia and HeLa cells exposed to 2,4-dinitrophenol, oligomycin and rotenone under conditions which led to depletion of ATP pools exhibit DNA damage expressed as irreversible DNA strand separation in alkali. Removal of the agents allows both the repletion of ATP pools and repair of DNA damage.     

Cytophotometric determination of unscheduled DNA synthesis

We describe a cytophotometric assay for unscheduled DNA synthesis (UDS) in asynchronously growing cells. Monolayer cultures of human HEp-2 and mouse MCa-11 cells were incubated with the carcinogen methyl-methane sulfonate (MMS), as well as with hydroxyurea and (3H)thymidine. Slides were prepared, and the DNA contents and areas of nuclei were measured by absorption cytophotometry. The labeling of the nuclei, determined on the basis of their DNA content to be in G0/G1, was selectively measured after the preparation of autoradiographs. The labeling of the G0/G1 cells increased with increasing doses of MMS. We also found that the increased nuclear labeling after MMS treatment was not due to induction of replicative DNA synthesis or selective destruction of G0/G1 cells. The results of this assay compared favorably with a standard biochemical method for measuring unscheduled DNA synthesis by benzoylated naphthoylated DEAE cellulose chromatography.     

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.     

Human peripheral lymphocyte growth regulation and response to phorbol esters is linked to synthesis and phosphorylation of the cytosolic protein, prosolin

Prosolin is a major cytosolic protein (Mr 18400, isoelectric point 5.9) first reported in HL-60 promyelocytic leukemia cells. It is rapidly phosphorylated (15 to 30 min) in response to TPA treatment as an early event in a sequence that leads to cessation of cell proliferation and to differentiation of promyelocytes into monocytes. In our study we examined the expression of prosolin in human peripheral lymphocytes and investigated the effects of TPA treatment on prosolin phosphorylation and on lymphocyte proliferation. Prosolin was not expressed in resting PBL but was induced after 24 to 36 h of PHA stimulation, simultaneously with induction of DNA synthesis. In rapidly proliferating (IL-2 dependent) PBL prosolin was a major cytosolic component, comprising 0.5% of total cytosolic protein, of which approximately 28% was phosphorylated. Expression of prosolin decreased again when either mitogen-induced or IL-2-dependent proliferation diminished during extended periods in culture. Thus, expression of prosolin is correlated with periods when PBL are cycling through S-phase. TPA treatment of IL-2-dependent PBL at the peak of their growth caused phosphorylation of about two-thirds of preexisting unphosphorylated prosolin within 1 h. This was accompanied by cessation of cell proliferation, as indicated by measurements of TdR incorporation. Although TPA has well known mitogenic effects in lymphocytes during initial activation, this result shows that it exerts an antiproliferative effect in rapidly dividing PBL. It is suggested that increased phosphorylation of prosolin may be an initiating event in the antiproliferative response to TPA, which would occur only in proliferating lymphocytes expressing prosolin.