Nuclear bud formation: a novel manifestation of Zidovudine genotoxicity

Normal diploid somatic mammalian cell division generates 2 daughter cells as a result of a strict and well-controlled mitotic process. However, some defects during the progression of that process could generate an unbalanced distribution of chromosomes, aneuploidy and eventually, a malignant phenotype. Previous observations using a transgenic mouse model with diminished DNA repair capacity revealed the presence of nuclear buds (NBs) induced in vitro by the nucleoside analog zidovudine (Retrovir(R), 3'-azido-3'-deoxythymidine, AZT). Here we used bone marrow mesenchymal cells, taken from mice with the Xpa(-/-)Trp53(+/-) genotype, that were cultured and exposed to 0 and 100 muM AZT for 24 hours. Fixed and denatured cells were processed by fluorescence in situ hybridization (FISH) with whole chromosome painting probes used to identify chromosomes in cells growing on glass chamber slides (2 probes/slide). A variety of sizes and shapes of NBs were observed. Some NBs had a large connection with the main nucleus (>(1/4) of the NB diameter), others hada smaller connection (<(1/4) of the NB diameter), some were circular and positioned close to the nucleus, while some resided in the cytoplasm separated from the nucleus or connected by a thin chromatin strand. We had hypothesized that NBs would progress in the process of budding until separation occurred, but this was not proven by time-lapse photography studies performed for 20 hours. From 1,126 cells scored in the unexposed cultures, 10.39 % of cells carried NBs, while from 1,108 cells scored in the AZT-exposed cultures 29.16% of cells carried NBs (p = 0.001). In AZT-exposed cells there were a total of 322 NBs scored; 46.6% or 150 NBs contained positive signals for one or both probes used, while 53% or 172 NBs had no probe signal. In addition, FISH analysis showed no preferential localization of any chromosome within the NBs. Among the NBs that carried no probe signal, the presence of positive signals with inversion of DAPI imaging demonstrated centromeric content. It has been hypothesized that NBs occur as a result of expulsion of amplified DNA from the main nucleus; however, this data demonstrates that NBs may contain any chromosome, suggesting that NBs do not consist of just amplified DNA.

Plasma drug levels compared with DNA incorporation of 3'-azido-3'-deoxythymidine (AZT) in adult cynomolgus (Macaca fascicularis) monkeys

Zidovudine (3'-azido-3'-deoxythymidine, AZT), widely used for the therapy of the Human Immunodeficiency Virus-1 (HIV-1), is a nucleoside analog of thymidine that becomes phosphorylated and incorporated into nuclear and mitochondrial DNA. Levels of AZT incorporation into DNA of humans, monkeys, and mice are highly variable and suggest interindividual variability in phosphorylation pathways. In addition, studies in rhesus monkeys (1) have shown a lack of correlation between levels of unbound AZT in plasma and tissue AZT-DNA. However, the correlation between plasma AZT and tissue AZT-DNA has not been previously examined in the same primate. Here we examine the relationship between AZT-DNA incorporation in leukocytes and multiple organs, and levels of the drug circulating in plasma of adult female cynomolgus (Macaca fascicularis) monkeys. Three monkeys were dosed with 40.0 mg of AZT/day for 30 days by naso-gastric intubation. The average daily dose of 9.9 mg of AZT/kg/body wt was similar to the approximately 8.6 mg of AZT/kg/body wt (600 mg/day) given to adult HIV-1-infected patients. In all three monkeys, at the time of sampling, values for AZT concentrations in plasma were similar and values for AZT incorporation into leukocyte DNA (86.1, 100.0, and 114.1 molecules of AZT/10(6) nucleotides) were also similar. AZT-DNA incorporation was detected in liver, uterus, spleen, and kidney from the three AZT-exposed animals, with values for positive samples ranging from 5.8 to 97.4 molecules of AZT/10(6) nucleotides. In brain cortex and lung DNA from AZT-exposed animals, AZT incorporation was undetectable. The data suggest that organ-specific differences in AZT uptake and/or metabolism may contribute to AZT phosphorylation and subsequent drug incorporation into DNA. In addition, AZT-DNA levels in monkey organs were similar to or lower than values observed in peripheral leukocytes of adult AIDS patients.

Chronic in vitro exposure to 3'-azido-2', 3'-dideoxythymidine induces senescence and apoptosis and reduces tumorigenicity of metastatic mouse mammary tumor cells

Normal cells in culture divide a certain amount of times and undergo a process termed replicative senescence. Telomere loss is thought to control entry into senescence. Activation of telomerase in tumors bypasses cellular senescence and is thus a requirement for tumor progression. We reported previously the preferential incorporation of 3'-azido-2', 3'-dideoxythymidine (AZT) in telomeric sequences of immortalized cells in culture. In this work, we have investigated the effects of chronic in vitro AZT exposure on F3II mouse mammary carcinoma cells. We demonstrate, for the first time, that AZT-treated tumor cells have a reduced tumorigenicity in syngeneic BALB/c mice. Tumor incidence was reduced and survival was prolonged in animals inoculated with AZT-treated cells when comparing with control counterparts. The number and size of spontaneous metastases were also decreased in animals inoculated with AZT-treated cells. In addition, we present evidence of morphological and biochemical signs of senescence, as shown by the staining for senescence associated beta-galactosidase activity, and induction of programmed cell death, as demonstrated by an increase of caspase-3 activity, in tumor cells exposed to AZT. These data indicate that chronic exposure of mammary carcinoma cells to AZT may be sufficient to induce a senescent phenotype and to reduce tumorigenicity.

Zidovudine-didanosine coexposure potentiates DNA incorporation of zidovudine and mutagenesis in human cells

Drug combinations that include nucleoside reverse transcriptase inhibitors (NRTIs) are remarkably effective in preventing maternal-viral transmission of HIV during pregnancy. However, there may be potential long-term risks for children exposed in utero. Examination of the genotoxic and mutagenic effects of two NRTIs, zidovudine [AZT (3'-azido-3'-deoxythymidine)] and didanosine [ddI (2',3'-dideoxyinosine)], in cultured human lymphoblastoid cells revealed multiplicative synergistic enhancement of AZT-DNA incorporation and mutant frequency induction in response to the combined drug exposure, as compared with single-drug exposures. Dose-related increases in DNA incorporation of AZT (as measured by a competitive RIA) and mutagenicity at the HPRT and TK loci (as assessed by cell-cloning assays) were observed in cells exposed in culture to AZT, or equimolar combinations of AZT + ddI, at exposure concentrations ranging from 3 to 30 times the maximum plasma levels found in humans. Because mutagenesis is strongly associated with tumor induction in experimental models, children exposed transplacentally to combinations of NRTIs may be at risk for cancer development later in life.

Incorporation of zidovudine into cord blood DNA of infants and peripheral blood DNA of their HIV-1-positive mothers

The nucleoside analogue 3'-azido-3'-deoxythymidine (AZT) is a weak carcinogen in adult female mice and a moderately strong carcinogen in the offspring of female mice given the drug during gestation. In addition, incorporation of AZT into DNA was observed in multiple organs of transplacentally exposed newborn mice. Here we investigate the incorporation of AZT into peripheral leukocyte DNA of HIV-1-positive adult pregnant women given AZT for variable times during gestation and cord blood of infants exposed to AZT in utero. The length of treatment varied between 10 days and 9 months. High molecular weight DNA was extracted from maternal peripheral blood mononuclear cells (PBMC) and infant cord blood. A specific AZT-DNA radioimmunoassay was used to determine the amount of AZT incorporated into leukocyte DNA. Incorporation of AZT into DNA ranged up to 183.3 and 344.5 molecules of AZT/10(6) nucleotides in the mothers and infants, respectively, and was detected in about 70% of samples. Therefore, AZT-induced mutagenic events are possible in the majority of adults and infants. No correlation was found between level of incorporation and length of AZT treatment, suggesting that the differences observed among the individuals arise from variability in AZT metabolism. These data support previous observations that a high degree of inter-individual variability in AZT phosphorylation occurs in primates.

Association between GSTM1*0 and squamous dysplasia of the esophagus in the high risk region of Linxian, China

Individuals with specific phase I and phase II enzyme polymorphisms may be at increased risk for squamous cell carcinoma of the esophagus. However, to our knowledge there has been only one previous report that evaluates a potential role for these polymorphisms in increasing risk for preneoplastic squamous lesions of the esophagus. To explore this further, we examined polymorphisms in CYP1A1, CYP2E1, GSTM1 and GSTT1, both independently and in combination, for potential associations with the risk of biopsy-proven squamous dysplasia of the esophagus in asymptomatic adults from Linxian, a high risk region in China. Cases consisted of 56 individuals from an esophageal cancer screening study with an endoscopic biopsy diagnosis of mild or moderate squamous dysplasia. Each case was matched on age (+/- 1 year) and gender to a control. Controls were defined as screening study participants with an endoscopic biopsy diagnosis of normal mucosa or esophagitis. DNA was extracted from frozen cell samples obtained by cytologic balloon examination and genotyped using standard methods. Individuals who were GSTM1 null (homozygous for GSTM1*0) were found to have a tendency for an increased risk of esophageal squamous dysplasia (odds ratio=2.6, 95% CI, 0.9-7.4). No excess risks were observed for inheritance of other putative at risk genotypes CYP1A1*2B, CYP2E1*6 or GSTT1*0. The risk associated with the inheritance of combined genotypes was not significantly different than the risk estimates from the univariate analysis. These results are consistent with the notion that exposure to environmental carcinogens that are detoxified by GSTM1, such as polycyclic aromatic hydrocarbons, may contribute to the etiology of esophageal cancer in Linxian.

Oxidative DNA damage in fetal tissues after transplacental exposure to 3'-azido-3'-deoxythymidine (AZT)

The nucleoside analogue 3'-azido-3'-deoxythymidine (AZT) has been used successfully to reduce the incidence of transplacental and perinatal transmission of the HIV virus. However, prolonged treatment with high doses of AZT is utilized in this therapy, and AZT has been found to be a perinatal carcinogen in mice. Any possible perinatal carcinogenic side effects in the human can best be managed if the mechanism is understood. AZT targets mitochondria and might cause increased intracellular production of reactive oxygen species (ROS). We tested whether transplacental AZT may cause oxidative damage in nuclear DNA of fetal tissues. CD-1 Swiss pregnant mice were treated with the transplacental carcinogenesis regimen (25 mg/day AZT, for gestation days 12-18) and tissues collected on the day of birth. Significant increases in 8-oxo-2'-deoxyguano- sine (8-oxo-dG) were found in the livers, a target tissue for transplacental carcinogenesis, and in the kidneys. A non-significant increase occurred in brain, with no change in lung. Tissues were also obtained from fetal patas monkeys (Erythrocebus patas), whose mothers had received 10 mg AZT/day during the last half of gestation. Although limited numbers of samples were available, possible increases in 8-oxo-dG were noted, relative to controls, for placenta and for fetal lung and brain (P = 0.055 for treatment-related increases in these tissues). These results suggest that an increase in reactive oxygen species could contribute to the mechanism of transplacental carcinogenesis by AZT in mice, and that this may also occur in primates.

Absence of structural or functional alterations in male and female reproductive organs of F1 and F2 generations derived from female mice exposed to 3'-azido-3'-deoxythymidine during pregnancy

To investigate the effects of in utero exposure to 3'-azido-3'-deoxythymidine (AZT) on male and female reproductive system development, pregnant CD-1 mice were given daily intragastric doses of 25.0 mg AZT during days 12 through 18 of gestation. The offspring were examined at birth, as well as at pubertal, young adult and adult stages of development, for reproductive organ endpoints including anogenital distance, onset of testicular descent, latency to vaginal opening, and proportion of time for each of the stages of estrous cycle. These reproductive endpoints remained mostly unchanged in AZT-treated offspring as compared to the controls. Males and females exposed in utero to AZT (F1 generation) were fertile when mated to untreated females and males, respectively, and their liveborn F2 offspring showed no adverse effects for any of the reproductive parameters tested. Thus, no evidence of developmental reproductive toxicity was noted either in the F1 mice exposed to AZT during the critical period of male and female reproductive system development, or in the F2 mice born of matings between the AZT-exposed F1 mice and unexposed animals.

Relationships between DNA incorporation, mutant frequency, and loss of heterozygosity at the TK locus in human lymphoblastoid cells exposed to 3'-azido-3'-deoxythymidine

3'-Azido-3'-deoxythymidine (AZT), a thymidine analogue widely used in the treatment of AIDS patients and for prevention of the onset of AIDS in HIV-seropositive individuals, causes tumors in mice exposed as adults or in utero. The purpose of this study was to investigate the potential mechanisms of AZT mutagenicity and carcinogenicity by quantifying the incorporation of AZT into cellular DNA, measuring AZT-induced thymidine kinase (TK) mutant frequencies (Mfs), and determining the percentage of loss of heterozygosity (LOH) in spontaneous or AZT-induced TK mutants in the human lymphoblastoid cell line, TK6. Cells were exposed to 300 microM AZT for 0, 1, 3, or 6 days, or to 0, 33, 100, 300, or 900 microM AZT for 3 days (n = 5 flasks/group). The effects of exposure concentration on incorporation of AZT into cellular DNA were evaluated by an AZT radioimmunoassay, and the effects of duration and concentration of AZT exposure on the TK Mfs were assessed by a cell-cloning assay. AZT was incorporated into DNA in a dose-related manner at concentrations up to 300 microM, above which no further increase was observed. TK Mf increased with the extended duration and with incremental concentrations of AZT exposure. There was a positive correlation (P = 0.036, coefficient = 0.903) between AZT-DNA incorporation and AZT-induced TK Mfs, suggesting that AZT incorporation into cellular DNA has a direct role in the genotoxicity of AZT. Southern blot analyses indicated that 84% (6.2 x 10(-6)/7.4 x 10(-6)) of AZT-induced mutants were attributable to LOH, consistent with the known mechanism of AZT as a DNA chain terminator. Considering the importance of LOH in human carcinogenesis, AZT-induced LOH warrants further study.

Incorporation of 3'-azido-3'-deoxythymidine (AZT) into fetal DNA and fetal tissue distribution of drug after infusion of pregnant late-term rhesus macaques with a human-equivalent AZT dose

In the United States, the nucleoside analogue drug 3'-azido-3'deoxythymidine (AZT; also called zidovudine or ZDV) is given to most pregnant women who produce a positive test result for HIV-1. To investigate transplacental distribution and genotoxicity of AZT, near-term pregnant rhesus (Macaca mulatta) monkeys and their fetuses were studied. Four pregnant monkeys were continuously infused with 8 mg AZT/kg body weight for the 4 hours just prior to hysterotomy at term. This short-term AZT exposure resulted in AZT incorporation into DNA of fetal liver, lung, heart, skeletal muscle, brain, testis, and placenta, which varied between 29 and 1944 molecules of AZT/10(6) nucleotides. In contrast, values for AZT and combined metabolites, determined by radioactivity, varied between 0.94 and 5.20 microg AZT equivalents/g tissue. A fifth animal, (H076), was infused with 17.3 mg AZT/kg body weight for approximately 3 hours, followed by 1 hour without drug before hysterotomy. Similar to the 4 other monkeys, variable levels of AZT (16-147 molecules of AZT/10(6) nucleotides) were incorporated into organ DNA of H076, whereas organ tissues contained less-variable levels of AZT and metabolites (0.86-2.05 microg AZT equivalents/g tissue). For H076, at hysterotomy 1 hour after discontinuation of drug, values for AZT and the 3'-azido-3'-deoxythymidine-beta-D-glucuronide (AZTG) in fetal blood and amniotic fluid were twofold and threefold higher than those in maternal blood. Most AZT pharmacokinetic parameters in the fifth monkey were similar to those previously reported for the first 4 monkeys and those observed in a similar study of pregnant women. These data show that a short-term AZT infusion in pregnant rhesus monkeys, which have similar AZT pharmacokinetics to those present in a pregnant human, results in incorporation of drug into the DNA of placenta and most fetal organs. Data imply that the human fetus may also be subject to incorporation of AZT into DNA even after short-term AZT infusion to the mother just before delivery.

Multiorgan transplacental and neonatal carcinogenicity of 3'-azido-3'-deoxythymidine in mice

The anti-HIV drug 3'-azido-3'-deoxythymidine (AZT) is used successfully for reduction of perinatal viral transmission. However toxic side effects including carcinogenesis are possible. To test this, pregnant CD-1 Swiss mice were given 25.0 or 12.5 mg AZT on gestation days 12-18. Previously we reported an increase in lung, liver, and female reproductive system tumors in offspring euthanized at 1 year (Olivero et al., J. Natl. Cancer Inst. 89, 1602-1608, 1997). Findings for all remaining offspring up to 2 years old are reported here. AZT effects were most prominent in female offspring, with a significant threefold increase in lung tumors, a reduction in lymphoblastic and follicle center cell lymphomas, and a significant increase in histiocytic sarcomas (0 in controls, 3% after low-dose AZT, and 8% after high-dose AZT, p = 0.022). Dose-dependent incidences of mammary gland, ovarian, and seminal vesicle tumors were low but significant: 0/106 controls, 3/105 low-dose, and 8/105 high-dose mice presented one of these neoplasms (p = 0.0025). Incidences of females showing any clearly AZT-related neoplasm, in lung, liver, ovary, or mammary gland or histiocytic sarcoma, in the second year, were 12/32 after the low dose and 14/27 after the high dose vs 3/23 controls (p = 0.0045). Also, the sensitivity of neonatal mice was assessed by administration of 25, 50, 100, or 200 mg/kg AZT on postnatal days 1 through 8. The effects at 2 years were similar to those seen after transplacental exposure, with significant increases in lung, liver, and mammary tumors in females. The results confirm that AZT is a moderately effective perinatal carcinogen in mice, targeting several tissue types.

Genotoxicity of 3'-azido-3'-deoxythymidine in the human lymphoblastoid cell line, TK6: relationships between DNA incorporation, mutant frequency, and spectrum of deletion mutations in HPRT

Perinatal treatment with 3'-azido-3'-deoxythymidine (AZT) has been found to reduce the rate of maternal-infant transmission of HIV; however, AZT is genotoxic in mammalian cells in vitro and induces tumors in the offspring of mice treated in utero. The purpose of the present study was to investigate the relationships between incorporation of AZT into DNA, and the frequency and spectrum of mutations at the HPRT locus of the human lymphoblastoid cell line, TK6, following in vitro exposures to AZT. Cells were cultured in medium containing 0 or 300 microM AZT for 1, 3, or 6 day(s) (n = 5/group). The effects of exposure duration on incorporation of AZT into DNA and HPRT mutant frequency were determined using an AZT radioimmunoassay and a cell cloning assay, respectively. AZT accumulated in DNA in a supralinear manner, approaching a plateau at 6 days of treatment (101.9 +/- 14.7 molecules AZT/10(6) nucleotides). After 3 days of AZT exposure, HPRT mutant frequency was significantly increased (1.8-fold, p = 0.016) compared to background (mutant frequency = 3.78 x 10(-6)). Multiplex PCR amplification of genomic DNA was used to determine the frequency of exon deletions in HPRT mutant clones from untreated cells versus AZT-treated cells. Molecular analyses of AZT-induced mutations revealed a significant difference in the frequency of total gene deletions (44/120 vs. 18/114 in controls, p = 0.004 by the Mann-Whitney U-statistic). In fact, the Chi-square test of homogeneity demonstrate that the differences between the control and AZT-treatment groups is attributed mainly to this increase in total gene deletion mutations (p = 0.00001). These data indicate that the primary mechanism of AZT mutagenicity in human TK6 cells is through the production of large deletions which occur as a result of AZT incorporation into DNA and subsequent chain termination. The data imply that perinatal chemoprophylaxis with AZT may put children of HIV-infected women at potential risk for genetic damage.

3'-azido-3'-deoxythymidine (AZT) transplacental perfusion kinetics and DNA incorporation in normal human placentas perfused with AZT

Vertical transmission of the human immunodeficiency virus 1 (HIV-1) is reduced from approximately 25% to approximately 7% as a result of 3'-azido-3'-deoxythymidine (AZT) therapy given during pregnancy; however, the consequences of transplacental AZT exposure to the fetus remain unknown. To address the extent and kinetics of AZT transfer across the human placenta, perfusion studies have been performed with fresh uninfected human placentas perfused with 0.5, 1. 0 and 5.0 mg AZT/ml for 2 h using a dual recirculating single cotyledon perfusion apparatus [T.I. Ala-Kokko, P. Pienimaki, R. Herva, A.I. Hollmen, O. Pelkonen, K. Vähäkangas, Transfer of lidocaine and bupivacaine across the isolated perfused human placenta, Pharmacol. Toxicol. 77 (1995) 142-148]. For two placentas, samples of perfusion effluent were taken every 15 min from the maternal and fetal sides of the apparatus and AZT levels were determined by AZT radioimmunoassay (RIA). At the end of the perfusion, AZT-DNA incorporation into placental DNA was determined by AZT-RIA. The concentration of AZT in the fetal perfusate increased with time, along with a concomitant slow decrease in the concentration of AZT in the maternal perfusates. For three different placentas, at 2 h after the start of perfusion, AZT-DNA incorporation values (molecules of AZT/10(6) nucleotides) were 11.8 for the 0.5 mg AZT/ml perfusate, 13.7 for the 1.0 mg AZT/ml perfusion, and 42.0 for the 5 mg AZT/ml perfusion. An additional placenta perfused with 1 mg AZT/ml did not have detectable values of AZT incorporated into DNA (data not shown). The data show that AZT crosses the human placenta and becomes rapidly incorporated into DNA of placental tissue in a dose-dependent fashion, suggesting that even short exposures to this drug might induce fetal genotoxicity and might also inhibit maternal-fetal viral transmission.

Incorporation of zidovudine into leukocyte DNA from HIV-1-positive adults and pregnant women, and cord blood from infants exposed in utero

OBJECTIVE

The nucleoside analog 3'-azido-3'-deoxythymidine (ZDV) has widespread clinical use but also is carcinogenic in newborn mice exposed to the drug in utero and becomes incorporated into newborn mouse DNA. This pilot study was designed to determine ZDV incorporation into human blood cell DNA from adults and newborn infants.

DESIGN

In this prospective cohort study, peripheral blood mononuclear cells (PBMC) were obtained from 28 non-pregnant adults and 12 pregnant women given ZDV therapy, six non-pregnant adults with no exposure to ZDV, and six non-pregnant adults who last received ZDV > or = 6 months previously. In addition, cord blood leukocytes were obtained from 22 infants of HIV-1-positive, ZDV-exposed women and from 12 infants unexposed to ZDV. There were 11 mother-infant pairs involving HIV-1 -positive women.

METHODS

DNA was extracted from PBMC obtained from non-pregnant HIV-1-positive adults taking ZDV, pregnant HIV-1-positive women given ZDV during pregnancy, and from adults not taking ZDV. Cord blood leukocytes were examined from infants exposed to ZDV in utero and from unexposed controls. DNA samples were assayed for ZDV incorporation by anti-ZDV radioimmunoassay (RIA).

RESULTS

The majority (76%) of samples from ZDV-exposed individuals, pregnant women (8 of 12), non-pregnant adults (24 of 28), or infants at delivery (15 of 22), had detectable ZDV-DNA levels. The range of positive values for ZDV-treated adults and infants was 25-544 and 22-452 molecules ZDV/10(6) nucleotides, respectively. Analysis of 11 mother-infant pairs showed variable ZDV-DNA incorporation in both, with no correlation by pair or by duration of drug treatment during pregnancy. Two of the 24 samples from individuals designated as controls were positive by anti-ZDV RIA. The 20-fold range for ZDV-DNA values in both adults and infants suggested large interindividual differences in ZDV phosphorylation.

CONCLUSIONS

Incorporation of ZDV into DNA was detected in most of the samples from ZDV-exposed adults and infants. Therefore, the biologic significance of ZDV-DNA damage and potential subsequent events, such as mutagenicity, should be

Skin tumorigenesis and Ki-ras and Ha-ras mutations in tumors from adult mice exposed in utero to 3'-azido-2',3'-dideoxythymidine

This study was designed to evaluate the potential initiating effects of transplacental 3'-azido-2',3'-dideoxythymine (AZT) and the role of ras mutational activation in skin tumors induced in a two-stage mouse skin model. In addition, mouse liver and lung tumors from a transplacental AZT tumorigenicity study reported elsewhere (Olivero et al., J Natl Cancer Inst 89:1602-1608, 1997) were examined for evidence of ras activation. For both tumor studies, pregnant CD-1 mice were given either vehicle or 25 mg of AZT daily on days 12-18 of gestation. In the 1997 study, the offspring were given no further exposure and were killed at 1 yr of age. For the skin tumor study, all mice received twice-weekly topical 12-O-tetradecanoyl-phorbol-13-acetate (TPA) treatment from weeks 5-35; half of the mice had been exposed to AZT in utero. At weeks 16-18, 30, 31, and 34-41, the skin tumor incidences in mice given AZT and TPA were significantly higher than in mice given TPA alone (P < or = 0.05). At week 41, the average numbers of tumors per mouse were 1.44+/-0.36 (mean +/- standard error of the mean) and 0.57+/-0.13 for mice given AZT plus TPA and TPA alone, respectively (P = 0.006). Mutagenesis in ras exons I and II was determined by polymerase chain reaction (PCR) and dye-terminator cycling sequencing of PCR products. Ha-ras exon I codons 12 and 13 were mutated in 11 of 19 tumors (58%) from mice given AZT and TPA and in one of 15 tumors (7%) from mice given TPA alone (P= 0.004). The only mutation in Ha-ras codon 12 (four in four tumors examined) was a G-->A transition in the second base, and the major mutation in codon 13 (six in seven tumors examined) was a G-->T transversion in the second base. In skin tumors, AZT exposure did not increase the number of Ha-ras codon 61 mutations, and no Ki-ras mutations were observed. Analysis of ras mutations in liver and lung tumors from mice exposed to AZT in utero (Olivero et al., J Natl Cancer Inst 89:16021608, 1997) with no TPA promotion showed no significant AZT-related increases.

Irreversible telomere shortening by 3'-azido-2',3'-dideoxythymidine (AZT) treatment

Telomeres shorten by 30 to 50 bp with each cell division. Germ line, tumor and stem cells overcome progressive shortening by elongating their telomeres with telomerase. Previously we demonstrated that 3'-azido-2',3'-dideoxythymidine (AZT), incorporates into telomeric DNA. To determine if telomeric AZT incorporation was a telomerase mediated phenomenon, we subjected tumor cells to long-term AZT exposure. Here we report the shortening of the telomeric sequences of HeLa cells cultured with 800 microM AZT for 15 passages. Southern blots of HeLa DNA cultured with AZT and digested with SAU 3AI, Alu I, and Rsa I revealed a progressive shortening of the telomeric repeats when probed with a human biotinylated telomeric probe. The shortened telomeric repeats did not elongate after culturing without AZT for an additional 25 passages. No evidence of senescence could be detected.

Transplacental effects of 3'-azido-2',3'-dideoxythymidine (AZT): tumorigenicity in mice and genotoxicity in mice and monkeys

BACKGROUND

When given during pregnancy, the drug 3'-azido-2',3'-dideoxythymidine (AZT) substantially reduces maternal-fetal transmission of human immunodeficiency virus type 1 (HIV-1). However, AZT has been shown to be carcinogenic in adult mice after lifetime oral administration. In this study, we assessed the transplacental tumorigenic and genotoxic effects of AZT in the offspring of CD-1 mice and Erythrocebus patas monkeys given AZT orally during pregnancy.

METHODS

Pregnant mice were given daily doses of either 12.5 or 25.0 mg AZT on days 12 through 18 of gestation (last 37% of gestation period). Pregnant monkeys were given a daily dose of 10.0 mg AZT 5 days a week for the last 9.5-10 weeks of gestation (final 41%-43% of gestation period). AZT incorporation into nuclear and mitochondrial DNA and the length of chromosomal end (telomere) DNA were examined in multiple tissues of newborn mice and fetal monkeys. Additional mice were followed from birth and received no further treatment until subjected to necropsy and complete pathologic examination at 1 year of age. An anti-AZT radioimmunoassay was used to monitor AZT incorporation into DNA.

RESULTS

At 1 year of age, the offspring of AZT-treated mice exhibited statistically significant, dose-dependent increases in tumor incidence and tumor multiplicity in the lungs, liver, and female reproductive organs. AZT incorporation into nuclear and mitochondrial DNA was detected in multiple organs of transplacentally exposed mice and monkeys. Shorter chromosomal telomeres were detected in liver and brain tissues from most AZT-exposed newborn mice but not in tissues from fetal monkeys.

CONCLUSIONS

AZT is genotoxic in fetal mice and monkeys and is a moderately strong transplacental carcinogen in mice examined at 1 year of age. Careful long-term follow-up of AZT-exposed children would seem to be appropriate.

Preferential formation and decreased removal of cisplatin-DNA adducts in Chinese hamster ovary cell mitochondrial DNA as compared to nuclear DNA

Levels of DNA adducts in Chinese hamster ovary (CHO) cells exposed to cis-diamminedichloroplatinum(II) (cisplatin) for 24 h, have been shown to be 4- to 6-fold higher in mitochondrial (mt) DNA as compared to nuclear (n) DNA (Olivero et al., Mutation Res., 346 (1995) 221). The aim of the present study was to understand if the preferential cisplatin binding in mtDNA is partially caused by lack of adduct removal in the mitochondria. Chinese hamster ovary cells were exposed for 6 h to 50 microM cisplatin, followed by incubation for 24 and 48 h in cisplatin-free medium. At the 30-h time point (6 h with cisplatin, 24 h without cisplatin), half of the cells from each plate were harvested and the remainder were cultured and harvested at 54 h (6 h with cisplatin, 48 h without cisplatin). The 30- and 54-h time points are called 'T30' and 'T54', respectively. Cisplatin-DNA adducts were measured in DNA from nuclear and mitochondrial fractions by dissociation-enhanced lanthanide fluoroimmunoassay (DELFIA), a sensitive competitive microtiter-based immunoassay utilizing antiserum elicited against cisplatin-modified DNA. An initial higher level of cisplatin-DNA adducts was observed in mtDNA when compared to nDNA, at T30. In addition, a lack of removal of adducts in mtDNA was demonstrated in cells at T54. Dilution of DNA adducts by DNA replication was documented in pulse-chase experiments that employed [3H]thymidine incorporation. Adduct removal by repair-related mechanisms was considered to comprise the difference between total DNA adduct removal and adduct removal related to DNA replication. The final results demonstrated that both, higher initial binding and lack of removal of cisplatin-DNA adducts appear to contribute to the preferential cisplatin-mtDNA binding observed in CHO cells.

Genotoxicity and mitochondrial damage in human lymphocytic cells chronically exposed to 3'-azido-2',3'-dideoxythymidine

AZT (3'-azido-2',3'-dideoxythymidine), the first nucleoside analog approved for the treatment of AIDS (acquired immunodeficiency syndrome), induces significant toxic effects in humans exposed to therapeutic doses. As an inhibitor of the HIV-1 (human immunodeficiency virus 1) reverse transcriptase, AZT blocks the incorporation of nucleotides into the host's newly synthesized DNA. Incorporation of AZT into mammalian DNA as well as specific localization of the drug into telomeric DNA, has been previously documented by immunohistochemistry. As with other nucleoside analogs, AZT has affinity for polymerase-gamma, the enzyme responsible for the replication of mitochondrial DNA. In order to examine the mechanisms of toxic events induced by long-term AZT exposure, human T-lymphocytic H9 cells were cultured with 25 microM AZT for 7 months. In the resulting H9-AZT cells, incorporation of AZT into DNA was demonstrated by radioimmunoassay and immunohistochemistry, chromosomal aberrations and micronuclei were scored and intracellular lipid distribution was determined. Two pmol of AZT per microgram of DNA were detected by radioimmunoassay in H9-AZT cells. Control cells showed negative values in the radioimmunoassay. Cytogenetic observations on H9-AZT cells showed an increase in chromosomal aberrations and nuclear fragmentation when compared with unexposed H9 cells. Electron microscopy revealed mitochondrial damage and an elevated accumulation of neutral intracellular lipid deposits probably as a consequence of a distortion in the beta-oxidation of fatty acids normally carried out by this organelle. The toxicities explored here suggest that the mechanisms of AZT induced cytotoxicity in bone marrow of the patients chronically exposed to the drug in vivo may involve both chromosomal and mitochondrial DNA damage.

Transplacental cisplatin exposure induces persistent fetal mitochondrial and genomic DNA damage in patas monkeys

A previous attempt to model transplacental cisplatin exposure and genotoxicity employed several pregnant Erythrocebus patas monkeys; most of the animals were exposed near the end of gestation and cisplatin-DNA adduct analyses included only genomic DNA. Here, both genomic and mitochondrial DNA adduct formation have been determined in fetuses from two pregnant monkeys exposed at the end of the second trimester of gestation. Multiple fetal tissues were obtained after doses of 0.315 mg cisplatin/kg body weight (5.3 mg/m2 total) on days 101 and 106 of gestation. Cesarean sections were performed 24 h after exposure and 27 d after exposure. Cisplatin genomic (g)-DNA adducts were observed in fetal adrenal, brain, heart, kidney, liver, skin, spleen, and thymus. When placentas from the two animals were divided into four concentric regions at increasing distances from the umbilical cord, and g-DNA was assayed, cisplatin DNA adduct levels were similar in all four regions. Mitochondrial (mt)-DNA adducts were higher than g-DNA adducts in maternal liver and fetal liver, brain and kidney, suggesting that the mitochondria may constitute a particular target for cisplatin genotoxicity. The study demonstrates significant fetal genotoxicity in g-DNA and mt-DNA of patas monkeys exposed to cisplatin in utero, suggesting that similarly exposed human fetuses may also sustain drug-induced DNA damage.

Elevated mitochondrial cisplatin-DNA adduct levels in rat tissues after transplacental cisplatin exposure

Although there is evidence that the toxic effects of cis-diamminedichloroplatinum(II) (cisplatin) include morphologically abnormal mitochondria, direct demonstrations of mitochondrial DNA damage by this chemotherapeutic agent have rarely been reported. Here we show that, in rats exposed to a single dose of cisplatin during gestation, cisplatin-DNA binding levels in both maternal and fetal liver and brain mitochondrial DNA are higher than those observed in genomic DNA. Pregnant F344/NCr rats were injected i.p. with either 5 or 15 mg cisplatin/kg body wt at 18 days of gestation and killed 24 h later. Cisplatin-DNA adducts were determined by dissociation-enhanced lanthanide fluoroimmunoassay using a cisplatin-DNA standard modified in the same range as the biological samples. Values for genomic cisplatin-DNA adducts in multiple maternal and fetal tissues have been presented elsewhere. Here, genomic DNA adduct levels for liver, brain, kidney and placenta are reported again for comparison with mitochondrial DNA adduct levels in the same tissues. In maternal and fetal brain, mitochondrial DNA adduct levels were approximately 7- to 50-fold higher than genomic DNA adduct levels, and in fetal liver they were approximately 2- to 16-fold higher than genomic DNA adduct levels. These studies demonstrate extensive cisplatin-DNA adduct formation in brain and liver mitochondria of fetal rats exposed transplacentally and suggest that mitochondrial DNA in some organs may be a particular target for cisplatin genotoxicity.

Preferential binding of cisplatin to mitochondrial DNA of Chinese hamster ovary cells

Some chemical carcinogens localize preferentially in mitochondrial DNA (mtDNA) when compared with genomic DNA (gDNA). Here we compare the ability of cisplatin (cis-diamminedichloroplatinum[II]) to induce DNA adducts in both genomic and mtDNA of Chinese hamster ovary (CHO) cells in culture. Cytotoxicity was examined by cell survival 4, 8 and 24 h after exposure to 50 microM cisplatin. Cisplatin-DNA adducts were measured in DNA from nuclear and mitochondrial fractions by dissociation-enhanced lanthanide fluoroimmunoassay (DELFIA), a sensitive competitive microtiter-based immunoassay utilizing antiserum elicited against cisplatin-modified DNA. An additional comparison of cisplatin-DNA binding in both compartments was performed by immunoelectron microscopy using the cisplatin-DNA antiserum and colloidal gold. DELFIA analysis of cisplatin-DNA adducts in gDNA and mtDNA showed a six-fold higher incorporation of drug into mtDNA as compared to gDNA. Morphometric studies of colloidal gold distribution in photomicrographs of CHO cells showed mtDNA to contain a four-fold higher concentration of cisplatin as compared to nuclear DNA. Therefore, both methods demonstrated a preferential binding of cisplatin to mtDNA versus gDNA.

Vaginal epithelial DNA damage and expression of preneoplastic markers in mice during chronic dosing with tumorigenic levels of 3'-azido-2',3'-dideoxythymidine

3'-Azido-2',3'-dideoxythymidine (AZT, Retrovir, zidovudine), a nucleoside analogue currently used in the therapy of acquired immunodeficiency syndrome, induces papillomas and carcinomas in vaginal epithelium of mice as a result of lifetime drug administration. In this study, female CD-1 mice were administered AZT at doses of 180, 360, and 720 micrograms/ml of drinking water for 28 days to determine whether AZT became incorporated into vaginal DNA and whether this was associated with preneoplastic changes within the target tissue. In addition, bone marrow, a target for AZT-induced cytotoxicity in mice and humans, was examined for chromosomal aberrations. A positive correlation was observed between dose level of AZT, proliferation of cells in the basal layer of vaginal epithelium, and incorporation of AZT into vaginal DNA. Incorporation of AZT into vaginal DNA was originally detected by radioimmunoassay and confirmed by immunohistochemistry. An aberrant pattern for alpha 6 integrin distribution, similar to the pattern described in skin papillomas with high risk for malignant conversion, also increased with dose in mice given AZT. Chromosomal aberrations in bone marrow increased more than 4-fold in AZT-exposed animals. The genotoxicity demonstrated by incorporation of AZT into vaginal DNA and proliferation of vaginal epithelium may play an essential part in the ability of AZT to induce abnormal differentiation in vaginal epithelium and vaginal tumorigenesis in mice.

Preferential incorporation of 3'-azido-2',3'-dideoxythymidine into telomeric DNA and Z-DNA-containing regions of Chinese hamster ovary cells

3'-Azido-2',3'-dideoxythymidine (azidothymidine; AZT) induces bone marrow toxicity in patients chronically given therapeutic doses of drug and is tumorigenic in rodents, inducing squamous cell tumors in vaginal tissues of mice and rats. In the study reported here, we explored the incorporation of AZT into specific regions of mammalian chromosomal DNA. CHO cells were exposed to AZT for 4 h, allowed to complete at least one cell cycle, and then arrested in metaphase with colchicine. Regions of concentrated AZT incorporation were identified in individual metaphase chromosomes by immunohistochemistry using antiserum specific for AZT and a secondary antiserum with a streptavidin--Texas red end point. These studies demonstrated that most of the intensely staining regions were chromosomal ends or telomeres. When 18 metaphases were examined, all telomeres but one (39 of 40) were positive at least once. Using an anti-Z-DNA antibody, chromosomal regions containing DNA in Z conformation were also localized by immunohistochemistry using a rhodamine-conjugated secondary antibody. When metaphase chromosome spreads were stained for either AZT or Z-DNA, ideograms showing localization of AZT (18 metaphases) and DNA in Z configuration (26 metaphases) were drawn for every chromosome of each metaphase examined. These ideograms demonstrated that 60% of the regions that stained positive for AZT were also positive for Z-DNA. Furthermore, slides incubated with both antibodies, using streptavidin--Texas red to identify AZT and fluorescein to identify Z-DNA, confirmed colocalization of the two markers. Additional experiments exploring the induction of chromatin bridges in AZT-treated cells suggest that the analogue may be able to bind to and disrupt the normal functioning of telomeric DNA.

Localization of DNA adducts induced by N-acetoxy-N-2-acetylaminofluorene in Chinese hamster ovary cells using electron microscopy and colloidal gold

DNA adduct induction by N-acetoxy-N-2-acetylaminofluorene (N-Ac-AAF) has been investigated in Chinese hamster ovary (CHO) cells using immunoelectron microscopy. The major RNA and DNA adducts, N-(guanosin-8-yl)-2-aminofluorene (G-C8-AF) and N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF), were localized with a rabbit anti-G-C8-AF antiserum and colloidal gold cytochemistry. Appropriate controls, including incubation of untreated cells with normal rabbit serum and immunogen-absorbed serum, demonstrated that colloidal gold deposits were indicative of the presence of adducts. The localization of gold particles in close association with nuclear chromatin revealed high concentration of adducts in DNA and RNA of nuclei. Morphometric evaluation of adduct formation in organelles of from different carcinogen exposures showed that 85-88% of total adducts were concentrated in nuclei. DNA adducts remaining in nuclei after RNAse treatment appeared to concentrate in heterochromatic areas, and these areas contained 59% of bound gold particles by morphometry. A total of 137-178 particles were found in nuclei of treated cells vs. 15-26 in the surrounding cytoplasm. Treated cells incubated with normal rabbit serum or specific adduct-absorbed serum showed 19-34 particles for all cellular compartments.

Chromosome site-specific immunohistochemical detection of DNA adducts in N-acetoxy-2-acetylaminofluorene--exposed Chinese hamster ovary cells

In these studies a polyclonal antiserum elicited against a carcinogen-DNA adduct was used to explore the localization of DNA adducts in metaphase chromosomes of cultured cells. Morphological visualization of the adduct N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF) in Chinese hamster ovary (CHO) cells exposed to the direct-acting carcinogen N-acetoxy-2-acetylaminofluorene (N-Ac-AAF) was accomplished by indirect immunofluorescence with an anti-G-C8-AF antiserum. At the same time the pattern of chromosomal DNA replication was determined by replicative incorporation of bromodeoxyuridine (BrdUrd) and chromosomal staining with anti-BrdUrd. Visualization of DNA in chromosomes was accomplished with Hoechst 33258 dye. When synchronized CHO cells were exposed to N-Ac-AAF for 0.5 h during early S phase, the chromosomal pattern of dG-C8-AF adduct formation was not random. Metaphase chromosome spreads from cells exposed to N-Ac-AAF in different experiments contained certain chromosome regions that had a consistently high adduct concentration. The regions of high DNA damage corresponded to the regions active in DNA synthesis when BrdUrd and the carcinogen were given simultaneously in early S phase. In addition, the patterns of high adduct concentration and replicative synthesis shifted when the carcinogen and BrdUrd were given simultaneously during late S phase. Thus, the stage of cell cycle in which adducts are induced is an important factor in the specific location of the highest concentrations of this type of DNA lesion.

Sister-chromatid exchanges and chromosomal aberrations in a population exposed to pesticides

Sister-chromatid exchanges (SCE) and chromosomal aberrations were studied in a population of floriculturists occupationally exposed to organophosphorus, carbamate and organochlorine pesticides. Blood samples from 36 individuals from a community of 154 persons of asiatic origin were obtained. Among the group sampled, 21 individuals exhibited at least one symptom of chronic intoxication. SCE analysis was performed in 14 symptomatic and 13 asymptomatic persons. The asymptomatic group showed a SCE frequency of 5.47 +/- 1.03 and the symptomatic group a frequency of 6.45 +/- 1.19. Comparison between both groups with the Mann-Whitney 'U' test revealed a significant difference (p 0.0409). Case-control analysis of 9 pairs matched by sex and age also showed significant differences between both groups (p 0.0104). In contrast, the frequencies of chromosomal aberrations were not correlated with intoxication symptomatology, though a significant increment of exchange-type aberrations in relation to a group of non floriculturists was observed in the population studied.

Anaphase-telophase analysis of chromosomal damage induced by chemicals

Three main aspects involved in the chemical induction of anaphase-telophase aberrations in the first mitosis after treatment were analyzed: 1) the relationship between the frequency of anaphase-telophase aberrations and the time of fixation after treatment; 2) the dose-response relationships; and 3) the proliferative rate of cells exposed to chemicals which interact with DNA by different mechanisms. Experiments were carried out using Chinese hamster ovary (CHO) cells. The compounds examined were adriamycin (ADR) and mitomycin C (MMC). The frequency of cells with chromatin bridges or with lagging chromosomes as well as the mitotic index was determined in each experiment. The results obtained showed that 1) chromatin bridges and lagging chromosomes are apparently induced during the S period of the previous interphase; 2) the increase in the cytotoxicity index (inferred from the mitotic index) and the frequency of cells with chromatin bridges and lagging chromosomes were proportional to the treatment lapse and to the dose employed; and 3) the effect of ADR on cell growth differs from the effect of MMC. While ADR decreased the mitotic activity of cells in logarithmic growth phase, MMC induced mitotic delay. In accordance with these results, the occurrence of chromatin bridges in anaphase-telophase could be explained by the induction of chromosome stickiness and, to a lesser extent, by the induction of exchange-type aberrations. On the other hand, lagging chromosomes seem to be the result of chromatid or chromosome breaks because the lagging chromosomes observed were primarily, if not all, fragments and not whole chromosomes. Our evaluation of the anaphase-telophase test indicates that it is very sensitive method for the detection of chemical clastogens, but other factors, such as mitotic depression, must be taken into account to avoid false-negative results.

Effect of caffeine on the frequency of chromosome aberrations induced in vivo by triethylenemelamine (TEM) and adriamycin (ADR) in mice

The effect of caffeine on the chromosomal aberrations induced in vivo by TEM and ADR was studied. BALB/c mice were treated intraperitoneally according to the schedule summarized in Table 1. Animals injected only with caffeine exhibited a significant increase in the frequency of gaps with the doses of 100 and 150 mg/kg and in the frequency of breaks with all the doses given. Treatments with TEM induced a significant increment of gaps, breaks and chromatid exchanges. ADR increased the frequency of gaps and breaks and induced centric fusions. Combined treatments with TEM plus caffeine evidenced the potentiating ability of the latter drug. Conversely, in mice injected with ADR and caffeine, only the frequency of centric fusions was increased. These results could be explained by assuming that: (1) the primary lesions induced by ADR are not repaired by post-replication mechanisms; (2) the binding of ADR to the DNA molecule inhibits the unscheduled DNA synthesis necessary to perform the reparation of damaged segments. On the other hand, the increased frequency of biarmed chromosomes in ADR-caffeine treatments probably results from the induction of chromosomal stickiness of the heterochromatin regions.

In vivo dose-response relationship in bone-marrow cells of mice treated with adriamycin

In bone-marrow cells of mice treated with a single dose of adriamycin and killed 6 h later, only chromatid-type aberrations were found. Animals studied 12 h after adriamycin injection showed chromatid- and chromosome-type aberrations, including Robertsonian centric fusions. The frequency of chromatid-type aberrations exhibited a direct-correlation with the dose in mice treated for 6 but not for 12 h. On the other hand, chromosome-type aberrations detected 12 h after injection were directly correlated with the dose of adriamycin. The induction in vivo of chromosome-type aberrations in mice studied 12 h after treatment suggests the existence of bone-marrow cell populations able to carry out 2 mitoses in this time lapse. The induction of Robertsonian fusions can be explained by the preferential induction of chromosomal lesions in pericentromeric heterochromatin followed by reunion of damaged segments of 2 chromosomes. Consequently, the metacentric-like chromosomes induced by adriamycin arise either from translocations involving entire chromosomes arms or from aberrations of the exchange type between 2 short arms of acrocentric chromosomes.

Enhancement by caffeine of the frequency of anaphase-telophase chromatin bridges induced by triethylenemelamine (TEM)

Treatment of BHK cells for 8 h with triethylenemelamine (TEM) followed by caffeine for 4 or 8 h, increased the frequency of anaphase-telophase chromatin bridges in relation to controls and TEM-treated cells. These results indicate that TEM-induced chromosome lesions detected as chromatin bridges at anaphase-telophase could be potentiated by caffeine.