Enzymatic assay for quantification of deoxynucleoside triphosphates in human cells exposed to antiretroviral 2',3'-dideoxynucleosides

Single-molecule detection of deoxyribonucleoside triphosphates in microdroplets

A new approach to single-molecule DNA sequencing in which dNTPs, released by pyrophosphorolysis from the strand to be sequenced, are captured in microdroplets and read directly could have substantial advantages over current sequence-by-synthesis methods; however, there is no existing method sensitive enough to detect a single nucleotide in a microdroplet. We have developed a method for dNTP detection based on an enzymatic two-stage reaction which produces a robust fluorescent signal that is easy to detect and process. By taking advantage of the inherent specificity of DNA polymerases and ligases, coupled with volume restriction in microdroplets, this method allows us to simultaneously detect the presence of and distinguish between, the four natural dNTPs at the single-molecule level, with negligible cross-talk.

Modulating radiation resistance by inhibiting ribonucleotide reductase in cancers with virally or mutationally silenced p53 protein

Therapeutic ionizing radiation damages DNA, increasing p53-regulated ribonucleotide reductase (RNR) activity required for de novo synthesis of the deoxyribonucleotide triphosphates used during DNA repair. This study investigated the pharmacological inhibition of RNR in cells of virally or mutationally silenced p53 cancer cell lines using 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, Triapine(R), NSC #663249), a chemotherapeutic radiosensitizer that equally inhibits RNR M2 and p53R2 small subunits. The effects of 3-AP on RNR inhibition and resulting radiosensitization were evaluated in cervical (CaSki, HeLa and C33-a) and colon (RKO, RKO-E6) cancer cells. 3-AP treatment significantly enhanced radiation-related cytotoxicity in cervical and colon cancer cells. 3-AP treatment significantly decreased RNR activity, caused prolonged radiation-induced DNA damage, and resulted in an extended G(1)/S-phase cell cycle arrest in all cell lines. Similar effects were observed in both RKO and RKO-E6 cells, suggesting a p53-independent mechanism of radiosensitization. We conclude that inhibition of ribonucleotide reductase by 3-AP enhances radiation-mediated cytotoxicity independent of p53 regulation by impairing repair processes that rely on deoxyribonucleotide production, thereby substantially increasing the radiation sensitivity of human cancers.

Modulation of human UMP/CMP kinase affects activation and cellular sensitivity of deoxycytidine analogs

Deoxycytidine analogs are an important class of clinically active antiviral and anticancer agents. The stepwise phosphorylation of these analogs to triphosphate metabolites is crucial for biological action. Human UMP/CMP kinase (UMP/CMPK; cytidylate kinase; EC 2.7.4.14) is thought to be responsible for phosphorylation of UMP, CMP, and dCMP and may also play an important role in the activation of pyrimidine analogs. However, no evidence has verified this notion in intact cells. In this study we explored the functional roles of UMP/CMPK in natural pyrimidine synthesis and metabolism of deoxycytidine analogs, as well as 5-FU in HeLa S3 and HCT8 cells. The amounts of UMP/CMPK protein in different cell lines correlated with UMP, CMP, and dCMP kinase activities and amounts of UMP/CMPK RNA. Modulation of UMP/CMPK by overexpression or down-regulation had no impact on natural pyrimidine nucleotides and cell growth. However, down-regulating UMP/CMPK expression by siRNA led to a decrease in the formation of the triphosphate metabolites, resulting in cellular resistance to these analogs. More diphosphate and triphosphate metabolites of deoxycytidine analogs were detected and cellular sensitivity to these agents was increased in the UMP/CMPK-overexpressing cells. This study indicates that the second step enzyme (UMP/CMPK) is responsible for the phosphorylation of pyrimidine analogs and also has an impact on cellular sensitivity to these analogs in those cell lines.

A LC-MS/MS method for the analysis of intracellular nucleoside triphosphate levels

PURPOSE

To simultaneously quantify intracellular nucleoside triphosphate (NTP) and deoxynucleoside triphosphate (dNTP) pools and to assess their changes produced by interfering with ribonucleotide reductase (RNR) expression in leukemia cells.

METHODS

A HPLC-MS/MS system was used to quantify intracellular NTP and dNTP pools.

RESULTS

The assay was linear between 50 nM, the lower limit of quantification (LLOQ), and 10 muM in cell lysate. The within-day coefficients of variation (CVs, n = 5) were found to be 12.0-18.0% at the LLOQ and 3.0-9.0% between 500 and 5,000 nM for dNTPs and 8.0-15.0% and 2.0-6.0% for NTPs. The between-day CVs (n = 5) were 9.0-13.0% and 3.0-11.0% for dNTPs and 9.0-13.0% and 3.0-6.0% for NTPs. The within-day accuracy values were 93.0-119.0% for both NTPs and dNTPs. ATP overlapped with dGTP and they were analyzed as a composite. This method was applied to measure basal intracellular dNTPs/NTPs in five leukemia cell lines exposed to the RNR antisense GTI-2040. Following drug treatment, dCTP and dATP levels were found to decrease significantly in MV4-11 and K562 cells. Additionally, perturbation of dNTP/NTP levels in bone marrow sample of a patient treated with GTI-2040 was detected.

CONCLUSIONS

This method provides a practical tool to measure intracellular dNTP/NTP levels in cells and clinical samples.

A phase I study of the novel ribonucleotide reductase inhibitor 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, Triapine) in combination with the nucleoside analog fludarabine for patients with refractory acute leukemias and aggressive myeloproliferative disorders

Triapine is a potent ribonucleotide reductase (RR) inhibitor that depletes intracellular deoxyribonculeotide pools, especially dATP. We designed a Phase I trial of Triapine followed by the adenosine analog fludarabine in adults with refractory acute leukemias and aggressive myeloproliferative disorders (MPD). Two schedules were examined: (A) Triapine 105 mg/m(2)/day over 4 h followed by fludarabine daily x 5 (24 patients, fludarabine 15-30 mg/m(2)/dose); (B) Triapine 200 mg/m(2) over 24h followed by 5 days of fludarabine 30 mg/m(2)/day (9 patients). Complete and partial responses (CR, PR) occurred in Schedule A (5/24, 21%), with CR occurring at the 2 highest fludarabine doses (2/12, 17%). In contrast, no CR or PR occurred in Schedule B. Four of the 5 responses occurred in patients with underlying MPD (4/14, 29%). Drug-related toxicities included fever and metabolic acidosis. Triapine 105 mg/m(2) followed by fludarabine 30 mg/m2 daily x 5 is active in refractory myeloid malignancies and warrants continuing study for patients with aggressive MPD.

Resistance to 9-beta-D-arabinofuranosyl-2-fluoroadenine due to reduced incorporation into DNA from competition by excess deoxyadenosine triphosphate: implications for different sensitivities to nucleoside analogues

The cytotoxic action of the deoxyadenosine analogue 9-beta-D-arabinofuranosyl-2-fluoroadenine (F-ara-A) depends on the incorporation into DNA after being phosphorylated to F-ara-A triphosphate (F-ara-ATP) by deoxycytidine kinase (dCK). The mechanisms of resistance to F-ara-A were investigated in a newly established variant of L1210 mouse leukemia cells (L1210/F). L1210/F was more than 41-fold more resistant to F-ara-A than the parental cell line and had a 55% lower dCK activity. Interestingly, L1210/F showed a modest level of cross-resistance to deoxycytidine analogues phosphorylated by dCK, for instance, 1-beta-D-arabinofuranosylcytosine (ara-C). The comparative study of F-ara-A and ara-C demonstrated that the difference in the accumulation of their respective triphosphates was minor. In contrast, the incorporation of F-ara-A into DNA was strikingly suppressed compared with that of ara-C. In general, the high natural triphosphate levels interfere with corresponding analogue incorporation into DNA. The deoxyadenosine triphosphate (dATP) and deoxycytidine triphosphate pool sizes in L1210/F cells were increased by 4.9-fold and 1.9-fold, respectively, compared with the parental cells. Treatment with hydroxyurea increased the ratio of F-ara-ATP to dATP 2.1-fold and enhanced the action of F-ara-A in L1210/F. This is the first cell line to show that the profoundly defective incorporation of F-ara-A into DNA during competition with excess dATP confers a high degree of resistance to F-ara-A.

Intracellular substrates for the primer-unblocking reaction by human immunodeficiency virus type 1 reverse transcriptase: detection and quantitation in extracts from quiescent- and activated-lymphocyte subpopulations

Treatment of human immunodeficiency virus type 1 (HIV-1)-infected patients with 3'-azido-3'-deoxythymidine (AZT) selects for mutant forms of viral reverse transcriptase (RT) with increased ability to remove chain-terminating nucleotides from blocked DNA chains. We tested various cell extracts for the presence of endogenous acceptor substrates for this reaction. Cell extracts incubated with HIV-1 RT and [(32)P]ddAMP-terminated DNA primer/template gave rise to (32)P-labeled adenosine 2',3'-dideoxyadenosine 5',5'''-P(1),P(4)-tetraphosphate (Ap(4)ddA), ddATP, Gp(4)ddA, and Ap(3)ddA, corresponding to the transfer of [(32)P]ddAMP to ATP, PP(i), GTP, and ADP, respectively. Incubation with [(32)P]AZT monophosphate (AZTMP)-terminated primer/template gave rise to the analogous (32)P-labeled AZT derivatives. Based on the rates of formation of the specific excision products, ATP and PP(i) levels were determined: ATP was present at 1.3 to 2.2 mM in H9 cells, macrophages, and unstimulated CD4(+) or CD8(+) T cells, while PP(i) was present at 7 to 15 microM. Under these conditions, the ATP-dependent reaction predominated, and excision by the AZT-resistant mutant RT was more efficient than wild type RT. Activated CD4(+) or CD8(+) T cells contained 1.4 to 2.7 mM ATP and 55 to 79 microM PP(i). These cellular PP(i) concentrations are lower than previously reported; nonetheless, the PP(i)-dependent reaction predominated in extracts from activated T cells, and excision by mutant and wild-type RT occurred with similar efficiency. While PP(i)-dependent excision may contribute to AZT resistance in vivo, it is likely that selection of AZT-resistant mutants occurs primarily in an environment where the ATP-dependent reaction predominates.

Stable suppression of the R2 subunit of ribonucleotide reductase by R2-targeted short interference RNA sensitizes p53(-/-) HCT-116 colon cancer cells to DNA-damaging agents and ribonucleotide reductase inhibitors

Ribonucleotide reductase catalyzes the production of deoxyribonucleoside diphosphates, the precursors of deoxyribonucleoside triphosphates for DNA synthesis. Mammalian ribonucleotide reductase (RNR) is a tetramer consisting of two non-identical homodimers, R1 and either R2 or p53R2, which are considered to be involved in DNA replication and repair, respectively. We have demonstrated that DNA damage by doxorubicin and cisplatin caused a steady elevation of the R2 protein in p53(-/-) HCT-116 human colon carcinoma cells but induced degradation of the protein in p53(+/+) cells. To evaluate the involvement of R2 in response to DNA damage, p53(-/-) HCT-116 cells were stably transfected with an expression vector transcribing short hairpin/short interference RNA directed against R2 mRNA. Stably transfected clones exhibited a pronounced reduction of the R2 protein with no change in the cellular growth rate. Furthermore, short interference RNA-mediated reduction of the R2 protein caused a marked increase in sensitivity to the DNA-damaging agent cisplatin as well as to the RNR inhibitors Triapine and hydroxyurea. Ectopic expression of p53R2 partially reversed the cytotoxicity of cisplatin but not that of RNR inhibitors to R2 knockdown cells. The increase in sensitivity to cisplatin and RNR inhibitors was correlated with the suppression of dATP and dGTP levels caused by stable expression of R2-targeted short interference RNA. These results indicated that DNA damage resulted in elevated levels of the R2 protein and dNTPs and, consequently, enhanced the survival of p53(-/-) HCT-116 cells. The findings provide evidence that R2-RNR can be employed to supply dNTPs for the repair of DNA damage in cells with an impaired p53-dependent induction of p53R2.

Liquid chromatography-tandem mass spectrometry assays for intracellular deoxyribonucleotide triphosphate competitors of nucleoside antiretrovirals

This study was aimed to apply an LC-MS-MS method previously developed for intracellular nucleoside reverse transcriptase inhibitors-triphosphate (NRTI-TPs) to the determination of natural deoxyribonucleotides (dNTPs) in human peripheral blood mononuclear cells. The LC-MS-MS method was directly used in assay of dATP and dTTP. Interferences by ribonucleotides (rNTPs) prevented direct application to the two other analytes: dGTP and dCTP. A periodate oxidation procedure was therefore optimized to remove rNTPs from the cell medium in order to quantitate dCTP and dGTP. The determination of the intracellular ratio of NRTI-TP/dNTP in HIV-infected patients now involves use of the same chromatographic system for simultaneous assay of several analytes.

Ribonucleotide reductase and thymidine phosphorylation: two potential targets of azodicarbonamide

Azodicarbonamide tested as an anti-HIV agent was reported to expulse zinc from viral zinc-cysteine factors and to inhibit calcium mobilization machinery. It has structural analogy with hydroxyurea that inhibits ribonucleotide reductase and could also act on this target. Azodicarbonamide was therefore tested for its capacity to modulate deoxyribonucleotides triphosphate pools alone or in combination with other agents in the lymphoblastic SUP-T1 cell line susceptible to HIV infection. The deoxyribonucleotides triphosphate were evaluated by an enzymatic assay using sequenase. Two hours exposure of SUP-T1 cells to 100 microM azodicarbonamide induced a 50% reduction of each deoxyribonucleotide triphosphate. Among other inhibitors of nucleotide metabolism (hydroxyurea, methotrexate and thymidine), hydroxyurea only reproduces the effect of azodicarbonamide. This suggests, but does not demonstrate directly, that azodicarbonamide inhibits ribonucleotide reductase activity. The combination of azodicarbonamide with each of these inhibitors affected particularly the dCTP pool. During this study it was also suggested that azodicarbonamide could interfere with thymidine phosphorylation. Thymidine phosphorylating activity was measured with 3H-thymidine as substrate. In acellular preparations, azodicarbonamide also non-competitively inhibits thymidine phosphorylating activity. This effect was not reproduced by hydroxyurea. Thus, in vitro azodicarbonamide decreases the intracellular pool of deoxyribonucleotide and thymidine phosphorylation.

A direct determination of thymidine triphosphate concentrations without dephosphorylation in peripheral blood mononuclear cells by LC/MS/MS

A rapid, sensitive and specific analytical method with minimal sample preparation for the measurement of thymidine triphosphate (TTP) in peripheral blood mononuclear cells (PBMC) by LC/MS/MS has been developed. PBMC were separated from whole blood or buffy coat. The analyte and internal standard were extracted from PBMC with 70% methanol (pH 7.2). These extracts after centrifugation were directly injected onto LC/MS/MS without need for any further sample preparation. The calibration curve was linear over the range 0.8-800 ng/ml. Mean inter- and intra-assay coefficients of variation (CVs) over the range of the standard curve were less than 10%. The overall recovery of TTP was 103.5%.

Simultaneous quantitation of the 5'-triphosphate metabolites of zidovudine, lamivudine, and stavudine in peripheral mononuclear blood cells of HIV infected patients by high-performance liquid chromatography tandem mass spectrometry

A high-performance liquid chromatography (HPLC) method utilizing triple quadrupole mass spectrometry (MS) detection was developed and validated for the simultaneous measurement of the intracellular nucleoside 5'-triphosphate anabolites of zidovudine (ZDV-TP), lamivudine (3TC-TP), and stavudine (d4T-TP). These compounds were extracted from patient peripheral blood mononuclear cells (PBMCs) which are the sites of HIV replication and drug action. Ion-exchange solid phase extraction (SPE) followed by enzymatic digestion with alkaline phosphatase was utilized to yield the measurable nucleoside forms of the nucleotides. Reversed phase C-18 SPE with addition of a nucleoside internal standard, 3'-azido-2',3'-dideoxyuridine (AzdU) allowed for the indirect measurement of the original 5'-triphosphate concentration by HPLC/MS/MS. Quantitation was performed from calibration curves generated from authentic 5'-triphosphate standards spiked in PBMCs from healthy volunteers. Analytical range for the three 5'-triphosphates was equivalent to 50-45,000 pg. Mean interassay accuracies for 3TC-TP, d4T-TP, and ZDV-TP (n > 90) were 99.4%, 100.1%, and 108.0%, respectively. Mean interassay precisions (%C.V.) for 3TC-TP, d4T-TP, and ZDV-TP (n > 90) were 8.8%, 10.4%, and 8.2%, respectively. Recovery of the extraction method was 79.2%, 83.1%, and 98.3% for 3TC-TP, d4T-TP, and ZDV-TP, respectively. This method can be utilized to measure the intracellular 5'-triphosphate levels in HIV infected patients receiving antiretroviral therapy containing the nucleoside reverse transcriptase inhibitors 3TC, d4T, or ZDV.

Potentiation of the anti-HIV activity of zalcitabine and lamivudine by a CTP synthase inhibitor, 3-deazauridine

Low levels of the CTP synthase inhibitor 3-deazauridine (3-DU) strongly potentiated the anti-HIV-1 activity of the 5'-triphosphates of the cytidine-based analogues [-]2'-deoxy-3'-thiacytidine (3TC; lamivudine) and 2',3'-dideoxycytidine (ddC). The potentiation was associated with a 3-DU-induced decrease in dCTP pool size; no changes were seen in cellular pool sizes of dATP, dGTP or dTTP.

Simultaneous determination of pyrimidine or purine deoxyribonucleoside triphosphates using a polymerase assay

In this paper, we describe an improved enzymatic assay for the determination of deoxyribonucleoside triphosphates (dNTPs). This is based on the elongation of 32P 5'-end-labeled oligonucleotide primers annealed to complementary oligonucleotide templates. Incorporation within the primer/template (p/t) was catalyzed by the Klenow fragment of Escherichia coli DNA polymerase I under conditions where the concentration of the dNTP to be analyzed is limiting. Using a combination of two different sized p/t pairs, dCTP and dTTP (or dATP and dGTP) were assayed together. Since the elongated products were clearly separated after electrophoresis on a denaturing 10% polyacrylamide gel, the two dNTPs could be quantified in a single lane. This method allows for the first time the simultaneous determination of two pyrimidine or two purine deoxyribonucleoside triphosphates. Consequently, a large number of biological samples can be tested in a single experiment. The high sensitivity of this method enables the quantification of low concentrations of dNTPs, such as those found in resting nondividing cells. Furthermore, this new protocol is well suited for the determination of dNTPs in cells treated with the antiretroviral ddI, since the Klenow fragment has a low affinity for ddATP, the active form of ddI.

Deoxyribonucleoside triphosphate pool imbalances in vivo are associated with an increased retroviral mutation rate

Deoxyribonucleoside triphosphate (dNTP) pool imbalances are associated with an increase in the rate of misincorporation and hypermutation during in vitro reverse transcription reactions. However, the effects of in vivo dNTP pool imbalances on the accuracy of reverse transcription are unknown. We sought to determine the effects of in vivo dNTP pool imbalances on retroviral mutation rates and to test our hypothesis that 3'-azido-3'-deoxythymidine (AZT) increases the retroviral mutation rates through induction of dNTP pool imbalances. D17 cells were treated with thymidine, hydroxyurea (HU), or AZT, and the effects on in vivo dNTP pools were measured. Thymidine and HU treatments induced significant dNTP pool imbalances. In contrast, AZT treatment had very little effect on the dNTP pools. The effects of in vivo dNTP pool imbalances induced by thymidine and HU treatments on the retroviral mutation rates were also determined. Spleen necrosis virus (SNV)-based and murine leukemia virus (MLV)-based retroviral vectors that expressed the lacZ mutant reporter gene were used. The frequencies of inactivating mutations introduced in the lacZ gene in a single replication cycle provided a measure of the retroviral mutation rates. Treatment of D17 target cells with 500 microM thymidine increased the SNV and MLV mutant frequencies 4.7- and 4-fold, respectively. Treatment of D17 target cells with 2 mM HU increased the SNV and MLV mutant frequencies 2.1- and 2.7-fold, respectively. These results demonstrate that dNTP pool imbalances are associated with an increase in the in vivo retroviral mutation rates, but AZT treatment results in an increase in the retroviral mutation rates by a mechanism not involving alterations in dNTP pools.

Mechanisms of clinical resistance by HIV-I variants to zidovudine and the paradox of reverse transcriptase sensitivity

Even with the development of novel nucleoside analog inhibitors, zidovudine (AZT or 3'-azido-3'-deoxythymidine) remains a potent and frequently prescribed antiretroviral therapy for HIV-positive individuals. Failure of AZT in monotherapy due to the emergence of drug-resistant virus has not excluded it from use in most combination therapies with other nucleoside analogs, non-nucleoside reverse transcriptase inhibitors and protease inhibitors. Thus, an understanding of the mechanism of AZT resistance could be the key in predicting the failure of many treatment strategies. In this review, the occurrence, characterization and ramification of AZT resistance in HIV-positive individuals will be discussed in the context of genotypic and phenotypic analyses of AZT-resistant viruses and reverse transcriptases. The mechanisms of resistance to AZT may be distinct from the mechanisms of resistance to other nucleoside analogs.

Palindromic oligonucleotide-directed enzymatic determination of 2'-deoxythymidine 5'-triphosphate and 2'-deoxycytidine 5'-triphosphate in human cells

A new method is presented for the determination of 2'-deoxythymidine 5'-triphosphate and 2'-deoxycytidine 5'-triphosphate concentrations within human cells based on a DNA polymerase reaction directed by a palindromic oligonucleotide precursor. Two 19-mer oligonucleotide precursors are employed that contain a common 8-mer palindromic sequence followed by a sequence-specific insertion site and a 5'-oligodeoxythymidylate tail. To conduct a measurement, two molecules of the 19-mer oligonucleotide precursor are first annealed to form a pair of symmetrical template-primer addition sites at their 3'-termini that are coded for the analyte of interest, present in limiting amounts. The Klenow fragment of Escherichia coli DNA polymerase I then elongates the template-primer by the addition of two molecules of the complementary deoxyribonucleotide analyte. Following the addition of the analyte molecules, the template-primer is extended with a 10-mer oligo(dA) tail in the presence of excess dATP and the Klenow fragment. The result is a 30-mer palindromic oligonucleotide that can be separated from any remaining 19-mer precursor and quantified by paired-ion HPLC using UV detection. Since the molar extinction coefficient of the 30-mer palindromic oligonucleotide is much larger than that of the nucleotide analyte alone, the UV signal is markedly enhanced, thereby increasing sensitivity. Details describing this method and the application of it to measure these analytes in as few as 2.5 x 10(6) human cells are presented.

Point mutations at the purine nucleoside phosphorylase locus impair thymocyte differentiation in the mouse

Three point mutations on the Np(b) allele of the purine nucleoside phosphorylase locus in the mouse have been recovered by male germ cell mutagenesis. The mutants were backcrossed, 12-14 generations, and are designated in increasing order of severity of enzyme deficiency and phenotype: B6-NPE, Met-87 --> Lys; B6-NPF, Ala-228 --> Thr; and B6-NPG, Trp-16 --> Arg. A marked decline in total cell numbers per thymus occurs between 2 and 3 months for the more severe B6-NPF and B6-NPG mutants (35% and 52%, respectively) and by 8 months for the less severe B6-NPE mutation. The thymocyte population is thereafter characterized by a 3- or 8-fold expanded precursor, CD4-CD8- double-negative population and 15% or 55% reduced CD4+CD8+ double-positive cells for the B6-NPF and B6-NPG strains, respectively. Spleen lymphocyte Thy-1+ cells are reduced by 50% and spleen lymphocyte response to T cell mitogen and interleukin 2 is reduced by 80%. Increases of thymocyte dGTP pools of 5- and 2.5-fold for B6-NPF and B6-NPG mutants, respectively, are observed. The purine nucleoside phosphorylase-deficient mouse exhibits age-dependent progressive perturbations in thymocyte differentiation, reduced numbers of thymocytes, and reduced splenic T cell numbers and response. The progressive T cell deficit is similar to the human disorder.

Thymidine in the micromolar range promotes rejoining of UVC-induced DNA strand breaks and prevents azidothymidine from inhibiting the rejoining in quiescent human lymphocytes

The effect and inter-individual variation in the effect of exogenously added deoxynucleosides (2 x 10(-6) M) on rejoining of UVC-induced DNA strand breaks was examined in quiescent human lymphocytes from 25 healthy persons. Thymidine at concentrations below 2 x 10(-6) M, effectively and with statistically extreme significance, increased rejoining of UVC-induced DNA strand breaks in the lymphocytes of every one of the 25 persons tested (p < 0.0001, Wilcoxon's signed ranks test). The mean stimulation after 20 h of postirradiation repair was 48% (range 18-78%) with an inter-individual variation of 30% (coefficient of variation, CV). Deoxyguanosine stimulated rejoining in 16, but inhibited in three of 19 test persons (mean stimulation 28%, range -31 to 71%). The stimulating effect of deoxyguanosine was also extremely significant (p < 0.0004). Deoxycytidine and deoxyadenosine stimulated rejoining in some persons and inhibited it in others, and without statistical significance (p values above 0.5). The stimulating effect of thymidine was significantly inhibited by deoxycytidine (p < 0.05, n = 12) whereas deoxyguanosine neither promoted or inhibited the stimulation by thymidine (p = 1, n = 12). Rejoining of DNA strand breaks induced by methyl methanesulfonate did not appear significantly stimulated or inhibited by any of the four deoxynucleosides. Finally, the inhibiting effect of azidothymidine (AZT) on rejoining of UVC-induced DNA strand breaks was nullified by the addition of thymidine. In three donors examined, 10(-4) M AZT inhibited the rejoining by about 40-50%. The presence of less than 10(-5) M thymidine reduced the level of UVC-induced DNA strand breaks to below the level in control lymphocytes allowed to repair without AZT. These results indicate that among the four deoxynucleoside triphosphates, dTTP has a crucial role on the repair of UVC-induced DNA damage in quiescent lymphocytes. The results also indicate that an expansion of the dTTP pool may counteract the inhibiting effect of AZT on DNA repair in quiescent lymphocytes.

Disparate actions of hydroxyurea in potentiation of purine and pyrimidine 2',3'-dideoxynucleoside activities against replication of human immunodeficiency virus

We and other groups have recently reported the potentiation by ribonucleotide reductase inhibitors such as hydroxyurea of the anti-human immunodeficiency virus type 1 (HIV-1) activity of purine and pyrimidine 2',3'-dideoxynucleosides in both resting and phytohemagglutinin-stimulated peripheral blood mononuclear cells. Little agreement prevails, however, as to the mechanism of the synergistic effects described. We report here that in phytohemagglutinin-stimulated peripheral blood mononuclear cells, two mechanisms exist for the potentiation of the anti-HIV-1 activity by low-dose hydroxyurea of the purine-based dideoxynucleoside 2',3'-dideoxyinosine and the pyrimidine-based dideoxynucleosides 3'-azido-3'-deoxythymidine and 2',3'-dideoxycytidine. For 2',3'-dideoxyinosine, the enhancement arises from a specific depletion of dATP by hydroxyurea, resulting in a favorable shift of the 2',3'-dideoxyadenosine 5'-triphosphate/dATP ratio. For the pyrimidine dideoxynucleosides 3'-azido-3'-deoxythymidine and 2',3'-dideoxycytidine, the more modest anti-HIV enhancement results from hydroxyurea-induced increases of pyrimidine kinase activities in the salvage pathway and, hence, increased 5'-phosphorylation of these drugs, while depletion of the corresponding deoxynucleoside 5'-triphosphates (dTTP and dCTP) plays no significant role.