Inhibitors of ribonucleotide reductase alter DNA repair in human fibroblasts through specific depletion of purine deoxynucleotide triphosphates

Changes in the Concentrations of Deoxynucleotides in Human Lymphocytes TreatedIn Vitrowith Genotoxic Agents

We studied the toxic and genotoxic effects of UV light and an alkylating agent (methyl methanesulphonate; MMS) in human peripheral blood lymphocytes (PBL) in Goand after PHA stimulation. We measured DNA repair synthesis and replication in treated PBL together with deoxynucleoside triphosphate pool and ATP concentrations. In addition to its genotoxic action, MMS induced a strong toxic effect in cycling cells, as shown by a marked depletion of nucleotide pools.

The depletion of DNA methyltransferase-1 and the epigenetic effects of 5-aza-2'deoxycytidine (decitabine) are differentially regulated by cell cycle progression

5-Aza-2'-deoxycytidine (decitabine) is a drug targeting the epigenetic abnormalities of tumors. The basis for its limited efficacy in solid tumors is unresolved, but may relate to their indolent growth, their p53 genotype or both. We report that the primary molecular mechanism of decitabine-depletion of DNA methyltransferase-1 following its "suicide" inactivation-is not absolutely associated with cell cycle progression in HCT 116 colon cancer cells, but is associated with their p53 genotype. Control experiments affirmed that the secondary molecular effects of decitabine on global and promoter-specific CpG methylation and MAGE-A1 mRNA expression were S-phase dependent, as expected. Secondary changes in CpG methylation occurred only in growing cells ~24-48 h after decitabine treatment; these epigenetic changes coincided with p53 accumulation, an index of DNA damage. Conversely, primary depletion of DNA methyltransferase-1 began immediately after a single exposure to 300 nM decitabine and it progressed to completion within ~8 h, even in confluent cells arrested in G 1 and G 2/M. Our results suggest that DNA repair and remodeling activity in arrested, confluent cells may be sufficient to support the primary molecular action of decitabine, while its secondary, epigenetic effects require cell cycle progression through S-phase.

Activation of caspases and induction of apoptosis by novel ribonucleotide reductase inhibitors amidox and didox

OBJECTIVE

Amidox and didox are two polyhydroxy-substituted benzohydroxamic acid derivatives that belong to a new class of ribonucleotide reductase (RR) inhibitors. RR is the rate-limiting enzyme for de novo deoxyribonucleotide synthesis, and its activity is significantly increased in tumor cells in proportion to the proliferation rate. Therefore, RR is a target for antitumor therapy.

MATERIALS AND METHODS

HL-60 and K562 leukemia cells were treated with increasing doses of amidox and didox. Thereafter, the mode of cytotoxic drug action was determined by Hoechst 33258/propidium iodide (HO/PI) double staining, annexin binding, DNA fragmentation, and caspase activation. This was correlated to the decrease in dNTP levels. Staining with HO/PI and binding of fluorescein isothiocyanate-conjugated annexin V to externalized phosphatidylserine were used to quantify apoptosis.

RESULTS

Low doses of amidox or didox resulted in an increase of apoptotic HL-60 cells within 48 hours. Higher doses (50 microM amidox or 250 microM didox) led to rapid induction of apoptosis, which could be detected as early as 4 hours after treatment. After 48 hours with these concentrations, almost 100% of the HL-60 cells died by apoptosis without an increase in necrosis. K562 cells were found to be resistant to amidox but not to didox. In HL-60 cells, upstream caspase 8 is processed in response to didox, whereas caspases 8 and 9 are processed upon amidox treatment. Didox-induced apoptosis, but not amidox-induced apoptosis, can be correlated with the decrease in dNTP levels. The results suggests that amidox induces several apoptosis mechanisms in HL-60 cells. In contrast, only caspase 9 is activated by didox in K562 cells, and because amidox hardly induces apoptosis in this cell line, no caspase cleavage is observed.

CONCLUSIONS

Didox triggers distinct apoptosis pathways in HL-60 and K562 cells.

DNA damage induced by the drinking water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX) in mammalian cells in vitro and in mice

3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX) formed during chlorination of water containing natural organic substances, is a very potent bacterial mutagen. Recently, tumours at multiple sites were reported in rats given MX-containing drinking water. We have investigated the genotoxicity of MX in mammalian cells exposed in vitro and in vivo using alkaline filter elution to detect DNA single-strand breaks and/or alkali-labile sites (SSBs). Concentrations as high as 100 and 300 microM MX were required to induce detectable levels of SSBs in the HL-60 cells. If MX treatment was carried out in the presence of DNA repair inhibitors (AraC plus hydroxyurea), the sensitivity of the assay to detect MX-induced SSBs was increased by a factor of 100. The presence of serum proteins during exposure resulted in a minor reduction of the MX-induced DNA damage in HL-60 cells at the lowest MX concentrations. In primary cultures of testicular cells as well as in resting human peripheral blood mononuclear cells (PBMC), a slightly increased level of SSBs was observed at MX-concentrations above 30 microM, this effect was not further increased by repair inhibitors. In LLC-PK1 renal proximal tubular epithelial cells and in growth stimulated human peripheral PBMC, increased SSBs were detected at MX concentrations as low as low as 3-10 microM and higher using repair inhibitors, and at 10 times higher concentrations without repair inhibitors. No dose dependent DNA damage was detected in the liver, kidney, spleen and colon of male B6C3F1 mice administrated high doses of MX (40 and 80 mg kg-1). Moderately increased and dose dependent SSBs were detected in the liver and kidney in the presence of DNA repair inhibitors during MX treatment, but no such increase was observed in the spleen and colon.

The enzyme ribonucleotide reductase: target for antitumor and anti-HIV therapy

Ribonucleotide reductase is the rate-limiting enzyme of DNA synthesis, and it has been shown to be linked with malignant transformation and tumor cell proliferation. It was therefore considered as an excellent target for cancer chemotherapy. This article reviews the in vitro and in vivo effects of hydroxyurea the first inhibitor of the enzyme, which is currently used in general clinical practice. In addition, we summarize the results obtained with other inhibitors of the enzyme; for instance, polyhydroxy-substituted benzohydroxamic acid derivatives, a promising group of inhibitors of ribonucleotide reductase that was synthesized by Bart van'T Riet and investigated by our group. In vitro as well as animal data and pharmacokinetic results are reviewed and possible implications for an improvement in the management of various patient groups are outlined.

DNA damage induced by 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX) in HL-60 cells and purified DNA in vitro

Chlorinated tap water often contains 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), which is a potent directly acting bacterial mutagen. We have investigated the induction of DNA damage by MX in a promyelocytic human leukaemia cell line (HL-60 cells). Exposure of HL-60 cells to 100-300 microM MX resulted in increased levels of DNA single-strand breaks and/or alkali-labile sites (SSBs) as detected by alkaline filter elution. When adding inhibitors of DNA break repair (AraC plus hydroxyurea), increased levels of DNA SSBs were observed at very low concentrations (1-3 microM) of MX, as observed by both alkaline filter elution and the single-cell gel electrophoresis assay. Increased DNA SSBs could also be observed if DNA repair inhibitors were added immediately after exposure to 10 microM MX, indicating that low concentrations of MX cause a relatively stable modification of DNA that may be recognized and incised by DNA repair enzyme activities. Further studies with DNA break repair inhibitors indicated that HL-60 cells exposed to 10 microM MX for 1 h repaired 50% of their initial DNA damage during a 2-h period and the repair appeared to be complete at 22 h. Analysis of MX-treated DNA by sequencing methods indicated that MX preferentially reacts with guanines in DNA.

Inhibition of DNA synthesis by paracetamol in different tissues of the rat in vivo

DNA synthesis in the spleen, testis, thymus, stomach, small intestine and bone marrow was inhibited by 70-90% at 1 h following an oral dose of paracetamol (1 g/kg). This inhibitory effect was still apparent using a lower dose of 125 mg/kg paracetamol, but not when the dose was reduced to 60 mg/kg. In contrast, the liver was resistant to the inhibitory action of paracetamol on DNA synthesis, there being no significant inhibition of DNA synthesis at 500 mg/kg or 1 g/kg paracetamol. These doses and the associated plasma levels are in the range found in human overdose. Tissue levels of paracetamol in the liver, spleen, thymus, kidney and testis were essentially the same as the plasma level. However the apparent paracetamol tissue levels in the stomach wall and duodenum were orders of a magnitude higher than the plasma level. The tissue levels of paracetamol did not explain the differences between tissues in the degree of inhibition of DNA synthesis, in particular the high levels of paracetamol in the tissue of the stomach and duodenum did not result in higher levels of inhibition in these tissues. This study also shows that the inhibitory effect of paracetamol on DNA synthesis is transient. All the tissues, except the spleen, no longer showed inhibition of DNA synthesis by 4 h post paracetamol dosing.

The influence of high dose hydroxyurea on the incorporation of 5-iodo-2-deoxyuridine (IUdR) by human bone marrow and tumour cells in vivo

Resistance to cytotoxics precludes the successful treatment of many solid tumours. Inhibition of DNA synthesis in normal tissues with antimetabolites such as hydroxyurea (HU) may be a useful means of improving the selective uptake of toxic thymidine analogues by the relatively resistant tumour cells. HU also inhibits DNA repair by the critical depletion of intracellular deoxyribonucleotides. Twenty-five patients with various malignancies received 5-iodo-2-deoxyuridine (IUdR) 100 mg m-2 as a 20 min i.v. infusion and the uptake of IUdR was determined 1 h later immunocytochemically. Of these patients, 14 received IUdR 23 h from the start of a continuous i.v. infusion of HU (36 g over 36 h). Uptake of IUdR was equally suppressed in bone marrow and tumour aspirates, 0.1% (+/- 0.2%) of marrow precursor cells and 0.5% (+/- 0.4%) of tumour cells respectively, in patients who received HU compared to the uptake of IUdR in 11 patients who were not given HU 6.8% (+/- 1.1%) and 12.2% (+/- 1.8%) respectively. Mean HU plasma concentrations at the time of IUdR administration was 1.7 +/- 0.2 mM. The growth fraction of tumour cells (using Ki67 labelling) was not changed after treatment with HU. It is concluded that (1) since DNA synthesis is effectively inhibited by HU in tumour cells, differential uptake of radiolabelled IUdR by those cells will not be feasible using the current schedule of HU administration, (2) HU may be used as an inhibitor of DNA repair in vivo since the degree of inhibition correlates with that required to inhibit repair experimentally and that (3) Ki67 labelling index is not useful in studying cell kinetics in patients treated with HU.

5-Fluorouracil, leucovorin, hydroxyurea, and escalating doses of continuous-infusion cisplatin with concomitant radiotherapy: a clinical and pharmacologic study

Cisplatin (CDDP), 5-fluorouracil (5-FU), and hydroxyurea (HU) have individually demonstrated activity against several solid tumors, act synergistically with each other in vitro, and may act as radiation sensitizers. Therefore, we designed a phase I study to determine the maximally tolerated dose of cisplatin as given in addition to our previously described combination of 5-FU, HU, and concomitant radiotherapy (XRT). Patients exhibiting advanced solid tumors requiring palliative XRT were eligible. The regimen consisted of 1 g HU given p.o.b.i.d. on days 1-5, 600 mg/m2 5-FU given i.v. daily by continuous infusion (c.i.) on days 1-5, escalating doses of cisplatin starting at 10 mg/m2 daily given by c.i. on days 1-5, and involved-field XRT carried out on days 1-5. The cycle was repeated every 14 days until the target XRT dose had been reached. In all, 19 patients were entered at the first dose level, and cumulative grade 3-4 myelosuppression was seen in 16 subjects. As no dose escalation was feasible, the chemotherapy was subsequently altered by using the above regimen for cycles 1, 3, 5, and 7 and substituting the less myelosuppressive regimen of 1 g HU given p.o.b.i.d. on days 1-5, 400 mg/m2 5-FU given i.v. daily by c.i., and 100 mg leucovorin given p.o.4 h on days 1-5 for cycles 2, 4, and 6. On this alternating program, 28 patients were treated with escalating doses of CDDP. The dose-limiting toxicity was again myelosuppression, which was prohibitive at a CDDP dose of 20 mg/m2 daily. In the final phase of the protocol, 30 subjects were treated with the above alternating-cycle regimen at a CDDP dose of 20 mg/m2 daily and a decreased HU dose of 500 mg p.o.b.i.d. in an attempt to circumvent the myelosuppression associated with this dose of CDDP. Although severe acute toxicity (cycles 1 and 2) was observed less frequently, cumulative toxicity (all cycles) remained pronounced. The other major toxicity encountered was mucositis, which was particularly pronounced in patients receiving radiation to the head and neck and following leucovorin-containing cycles. Plasma concentrations of free platinum did not correlate with the CDDP dose, possibly due to the narrow range of doses given. Pharmacodynamic modeling demonstrated that the CDDP dose and the HU dose were associated with leukopenia. Antitumor activity was demonstrated in a number of solid tumors particularly non-small-cell lung cancer and head and neck cancer.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased frequency of sister-chromatid exchange and chromatid breaks in lymphocytes after treatment of human volunteers with therapeutic doses of paracetamol

Paracetamol was given to 10 healthy human volunteers in 3 doses of 1 g each during a period of 8 h. Blood samples for lymphocyte cultures were taken before and 24 h after paracetamol administration. A small but significant increase was found in the frequency of sister-chromatid exchanges (SCE) after intake of paracetamol (0.187 +/- 0.030 per chromosome before and 0.208 +/- 0.024 per chromosome after). After exposure the mean frequency of chromatid breaks per 100 cells was significantly increased (2.16 +/- 1.33 versus 0.33 +/- 0.50 before exposure). Exposure of human lymphocytes in vitro showed that concentrations of paracetamol above 0.1 mM induced inhibition of replicative DNA synthesis. Increased SCE was found in lymphocytes exposed to 1-10 mM paracetamol for 2 h. Furthermore, 0.75-1.5 mM paracetamol exposure for 24 h increased the frequency of chromatid and chromosome breaks in the lymphocytes. The paracetamol-induced SCE and chromosome aberrations may be secondary effects of paracetamol-induced inhibition of DNA synthesis or due to covalent binding of paracetamol metabolite(s) to DNA.

DNA repair induced by various mutagens in rat hepatocyte primary cultures measured in the presence of hydroxyurea, guanazole or aphidicolin

Guanazole and aphidicolin were chosen as candidates in the search for a selective, non-genotoxic inhibitor of DNA replication which could be used instead of hydroxyurea to measure DNA repair synthesis in rat hepatocyte primary cultures by liquid scintillation counting. The genotoxicity of these 3 chemicals was studied using the Salmonella/liver homogenate assay and the autoradiographic UDS test in hepatocytes. Hydroxyurea was positive in both of these assays. Guanazole and aphidicolin did not induce DNA repair in hepatocytes. Aphidicolin was not mutagenic for Salmonella typhimurium, whereas guanazole increased the revertant numbers of strain TA102 slightly. The incorporation of [3H]thymidine was measured by liquid scintillation to determine DNA repair induced by 2-acetylaminofluorene (2-AAF), aflatoxin B1, benzo[a]pyrene, cyclophosphamide, H2O2, 6-hydroxydopamine, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), methylnitrosourea (MNU), 4-nitroquinoline-N-oxide and UV irradiation in the presence of either 10 mM hydroxyurea, 15 mM guanazole or 0.015 mM aphidicolin. Aphidicolin had an inhibitory effect on DNA repair. Except for the 3 chemicals mentioned below, the sensitivity of the DNA repair measurement was the same, no matter whether hydroxyurea or guanazole was used to inhibit replicative DNA synthesis. In the presence of hydroxyurea, DNA repair synthesis was found at lower concentrations in the case of aflatoxin B1, due to differences in the solvent control values, and in the case of H2O2, possibly due to a synergistic effect between hydroxyurea and H2O2. Guanazole allowed the detection of DNA repair induced by MNNG at lower concentrations, probably because of an antagonistic effect between hydroxyurea and MNNG. Based on these results, it was concluded that guanazole, but not aphidicolin, could be used instead of hydroxyurea to measure DNA repair synthesis by liquid scintillation in rat hepatocyte primary cultures. Although guanazole does not completely fulfill the criteria for an ideal DNA replication inhibitor, it has the advantage of being less genotoxic than hydroxyurea, and also appears to have a smaller potential to falsify the results by interacting with the test compounds.

Cisplatin with high-dose infusions of hydroxyurea to inhibit DNA repair. A phase II study in non-small-cell lung cancer

A total of 45 patients with locally advanced and/or metastatic non-small-cell lung cancer (NSCLC) were treated in a phase II trial with high-dose i.v. infusions of 24 g hydroxyurea over 24 h, with 50 mg/m2 i.v. cisplatin 8 h after the start of hydroxyurea infusion. Hydroxyurea, a cell-cycle-specific inhibitor of ribonucleotide reductase, inhibits DNA repair by depleting nucleotide pools. We gave hydroxyurea to achieve steady-state levels of greater than or equal to 1 mM and to potentiate therapy by inhibiting repair of DNA damage produced by cisplatin. Among 21 patients with squamous cell lung cancer, there were 1 complete response (CR), 2 partial responses (PR) and 3 minor responses (MR). Of 13 patients with adenocarcinoma of the lung, 2 had MRs; of 11 patients with large-cell anaplastic lung cancer, none responded. The dominant toxicity was nausea and vomiting, which was manageable and mainly related to cisplatin. The response rate in squamous cell lung cancer was similar to responses obtained with cisplatin alone. The relative ineffectiveness of high-dose 24-h infusions of hydroxyurea in inhibiting repair of DNA damage produced by cisplatin may be due to the low growth fraction of human NSCLC. The high-dose hydroxyurea approach may be more applicable in tumours with a high growth fraction.

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.

Consequences of the depletion of cellular deoxynucleoside triphosphate pools on the excision-repair process in cultured human fibroblasts

DNA excision repair requires the insertion of bases into gaps in the DNA which arise during the removal of damaged sites from the chromatin. The number of bases required is dependent on the amount of damage and the patch size of repair in response to the particular type of damage. In cells in which the ability to synthesize deoxynucleoside triphosphates (dNTPs) has been compromised, repair cannot proceed to completion following doses of DNA-damaging agents which induce repair that requires greater than the steady-state level of dNTPs. Repair is thus not equally sensitive to depletion of dNTPs when measured in rapidly cycling cells with relatively high dNTP pools or in non-cycling cells with significantly smaller pools. Critical depletion of dNTPs results in the production of long-lived DNA strand breaks at repairing sites and reduction in the number of sites initiating repair. On the other hand, elevation of dNTP pools to 10-50-fold normal levels did not inhibit repair. This indicates that dNTP pool depletion but not general pool-imbalance affects DNA excision repair.

Evaluation of putative inhibitors of DNA excision repair in cultured human cells by the rapid nick translation assay

A human fibroblast nick translation assay has been applied to an examination of 48 diverse chemical agents to assess their ability to specifically interfere with the DNA excision-repair process following ultraviolet irradiation. Certain inhibitors of DNA polymerase, ribonucleotide reductase and purine and pyrimidine biosynthesis are shown to inhibit the resynthesis step of repair while DNA intercalators and inhibitors of DNA topoisomerases appear to inhibit the incision step. A variety of other agents previously implicated as inhibitors of DNA repair was also examined and found to have no such effect. This type of analysis should prove useful in the rapid identification of new classes of compounds that antagonize normal cellular repair functions and that might, therefore, act as comutagens or cocarcinogens.

Genotoxicity of formaldehyde and an evaluation of its effects on the DNA repair process in human diploid fibroblasts

Formaldehyde treatment of human fibroblasts gave rise to DNA damage detected by a nick translation assay. This damage was not repaired by typical 'long-patch'-type excision repair as evidenced by the failure of DNA repair inhibitor post-treatment to elevate the amount of DNA strand breakage. In addition, the effects of formaldehyde on DNA repair were examined in light of a recent report suggesting that formaldehyde inhibited the repair of X-ray-induced strand breaks and UV- and benzo [a]pyrene diol epoxide-induced unscheduled DNA synthesis in human bronchial cells. We report that formaldehyde (1) was ineffective at inhibiting the sealing of X-ray- or bleomycin-induced DNA strand breaks, (2) did not inhibit the removal of pyrimidine dimers from cellular DNA at short treatment times, and (3) that the previously observed inhibition of unscheduled DNA synthesis was most likely due to the inhibition of uptake of labeled precursor into formaldehyde-treated cells. Thus, our findings are not consistent with the notion that formaldehyde inhibits the repair process in human fibroblasts. Finally, formaldehyde was shown to elevate the level of misincorporation of bases into synthetic polynucleotides catalyzed by E. coli DNA polymerase I, indicating that the mutagenicity of formaldehyde may be due to covalent alteration of DNA bases.

Elevation of dCTP pools in xeroderma pigmentosum variant human fibroblasts alters the effects of DNA repair arrest by arabinofuranosyl cytosine

DNA excision repair inhibition by arabinofuranosyl cytosine (ara-C) or by ara-C/hydroxyurea (HU) was measured in log phase and confluent cultures of normal and xeroderma pigmentosium (XP)-variant human fibroblasts following insult by ultraviolet (UV) light (20 J/m2). Repair inhibition was determined by measuring the accumulation of DNA single-strand breaks/10(8) daltons following cell culture exposure to ara-C or ara-C/HU in a series of 3 hr. pulses up ro 24 hr. after UV insult. Both normal and XP-variant derived cells showed a wide range of sensitivity to ara-C in log phase cells (0.2-9.4 breaks/10(8) daltons DNA), although strand break accumulation was constant for each specific cell line. The same cells were more sensitive to ara-C/HU with a 2-14 fold increase in DNA strand breaks depending upon the cell line assayed. In confluent cultures of normal cells, maximum sensitivity to ara-C and ara-C/HU was achieved with similar levels of repair inhibition observed (16.1 and 16.5 breaks/10(8) daltons, respectively). The same level of repair inhibition was observed in confluent XP-variants receiving ara-C/HU, but was reduced by 62-68% in cells treated with ara-C alone. Ara-C repair arrest was more rapidly reversed by competing concentrations of exogenous deoxycytidine (dCyd) in XP-variant compared to normal cells, especially in confluent cell cultures. In ara-C/HU treated cells, the level of dCyd reversal was reduced in the XP-variant when compared to cells exposed to ara-C alone. However, the same addition of HU had relatively little effect on dCyd reversal in normal cells. The measurements of dNTP levels indicate an elevated level of intracellular deoxycytosine triphosphate in XP-variant vs normal cells. The implications of these results are discussed as they relate to possible excision repair anomalies in the XP-variant.

3-Aminobenzamide does not alter DNA repair in human fibroblasts through modulation of deoxynucleoside triphosphate pools

3-Aminobenzamide does not deplete cellular purine deoxynucleoside triphosphate pools as do the structurally-related ribonucleotide reductase inhibitors, the hydroxy- and amino-substituted benzohydroxamic acids. Thus, the previously reported ability of 3-aminobenzamide to inhibit de novo synthesis of DNA purines does not appear to be due to a direct effect on pools via inhibition of ribonucleotide reductase. The enhancement rather than inhibition by 3-aminobenzamide of DNA repair in the present studies, however, leaves open the possibility that pool modulation may play a role in cell systems where repair inhibitory effects are seen.