Molecular analysis of mutations at the HPRT and TK loci of human lymphoblastoid cells after combined treatments with 3'-azido-3'-deoxythymidine and 2',3'-dideoxyinosinedagger

The stress response resolution assay. I. Quantitative assessment of environmental agent/condition effects on cellular stress resolution outcomes in epithelium

The events or factors that lead from normal cell function to conditions and diseases such as aging or cancer reflect complex interactions between cells and their environment. Cellular stress responses, a group of processes involved in homeostasis and adaptation to environmental change, contribute to cell survival under stress and can be resolved with damage avoidance or damage tolerance outcomes. To investigate the impact of environmental agents/conditions upon cellular stress response outcomes in epithelium, a novel quantitative assay, the "stress response resolution" (SRR) assay, was developed. The SRR assay consists of pretreatment with a test agent or vehicle followed later by a calibrated stress conditions exposure step (here, using 6-thioguanine). Pilot studies conducted with a spontaneously-immortalized murine mammary epithelial cell line pretreated with vehicle or 20 µg N-ethyl-N-nitrososurea/ml medium for 1 hr, or two hTERT-immortalized human bronchial epithelial cell lines pretreated with vehicle or 100 µM zidovudine/lamivudine for 12 days, found minimal alterations in cell morphology, survival, or cell function through 2 weeks post-exposure. However, when these pretreatments were followed 2 weeks later by exposure to calibrated stress conditions of limited duration (for 4 days), significant alterations in stress resolution were observed in pretreated cells compared with vehicle-treated control cells, with decreased damage avoidance survival outcomes in all cell lines and increased damage tolerance outcomes in two of three cell lines. These pilot study results suggest that sub-cytotoxic pretreatments with chemical mutagens have long-term adverse impact upon the ability of cells to resolve subsequent exposure to environmental stressors.

Long-term AZT exposure alters the metabolic capacity of cultured human lymphoblastoid cells

The antiretroviral efficacy of 3′-azido-3′-deoxythymidine (AZT) is dependent upon intracellular mono-, di-, and triphosphorylation and incorporation into DNA in place of thymidine. Thymidine kinase 1 (TK-1) catalyzes the first step of this pathway. MOLT-3, human lymphoblastoid cells, were exposed to AZT continuously for 14 passages (P₁–P₁₄) and cultured for an additional 14 passages (P₁₅–P₂₈) without AZT. Progressive and irreversible depletion of the enzymatically active form of the TK-1 24-kDa monomer with loss of active protein was demonstrated during P₁–P₅ of AZT exposure. From P₁₅ to P₂₈, both the 24- and the 48-kDa forms of TK-1 were undetectable and a tetrameric 96-kDa form was present. AZT-DNA incorporation was observed with values of 150, 133, and 108 molecules of AZT/10⁶ nucleotides at the 10μM plasma-equivalent AZT dose at P₁, P₅, and P₁₄, respectively. An exposure-related increase in the frequency of micronuclei (MN) was observed in cells exposed to either 10 or 800μM AZT during P₁–P₁₄. Analysis of the cell cycle profile revealed an accumulation of S-phase cells and a decrease in G₁-phase cells during exposure to 800μM AZT for 14 passages. When MOLT-3 cells were grown in AZT-free media (P₁₅–P₂₉), there was a reduction in AZT-DNA incorporation and MN formation; however, TK-1 depletion and the persistence of S-phase delay were unchanged. These data suggest that in addition to known mutagenic mechanisms, cells may become resistant to AZT partially through inactivation of TK-1 and through modulation of cell cycle components.

A review of the molecular mechanisms of chemically induced neoplasia in rat and mouse models in National Toxicology Program bioassays and their relevance to human cancer

Tumor response in the B6C3F1 mouse, F344 rat, and other animal models following exposure to various compounds provides evidence that people exposed to these or similar compounds may be at risk for developing cancer. Although tumors in rodents and humans are often morphologically similar, underlying mechanisms of tumorigenesis are often unknown and may be different between the species. Therefore, the relevance of an animal tumor response to human health would be better determined if the molecular pathogenesis were understood. The underlying molecular mechanisms leading to carcinogenesis are complex and involve multiple genetic and epigenetic events and other factors. To address the molecular pathogenesis of environmental carcinogens, the authors examine rodent tumors (e.g., lung, colon, mammary gland, skin, brain, mesothelioma) for alterations in cancer genes and epigenetic events that are associated with human cancer. National Toxicology Program (NTP) studies have identified several genetic alterations in chemically induced rodent neoplasms that are important in human cancer. Identification of such alterations in rodent models of chemical carcinogenesis caused by exposure to environmental contaminants, occupational chemicals, and other compounds lends further support that they are of potential human health risk. These studies also emphasize the importance of molecular evaluation of chemically induced rodent tumors for providing greater public health significance for NTP evaluated compounds.

Genetic alterations in K-ras and p53 cancer genes in lung neoplasms from Swiss (CD-1) male mice exposed transplacentally to AZT

A transplacental carcinogenicity study was conducted by exposing pregnant Swiss (CD-1) mice to 0, 50, 100, 200, or 300 mg of 3'-azido-3'-deoxythymidine (AZT)/kg bw/day, through a 18 to 19-day gestation [National Toxicology Program, NIH Pub. No. 04-4458, 2004]. The incidences of alveolar/bronchiolar adenomas and carcinomas, in the 200 and 300 mg/kg male treatment groups, were significantly greater than that of the controls. In the present study, we evaluated the benign and malignant lung neoplasms from this bioassay for point mutations, in the K-ras and p53 cancer genes that are often mutated in human lung tumors. K-ras and p53 mutations were detected by cycle sequencing of polymerase chain reaction-amplified DNA, isolated from formalin-fixed, paraffin-embedded neoplasms. K-ras mutations were detected in 25 of 38 (66%) of the AZT-induced lung tumors, and the predominant mutations were codon 12 G-->T transversions. p53 mutations were detected in 32 of 38 (84%) of the AZT-induced lung tumors, with the predominant mutations being exon 8, codon 285 A-->T transversions, and exon 6, codon 198 T-->A transversions. No K-ras or p53 mutations were detected in five tumors, examined from control mice. The patterns of mutations identified in the lung tumors suggest that incorporation of AZT or its metabolites into DNA, oxidative stress, and genomic instability may be the contributing factors to the mutation profile and development of lung cancer in these mice.

Relative mutagenic potencies of several nucleoside analogs, alone or in drug pairs, at the HPRT and TK loci of human TK6 lymphoblastoid cells

Experiments were performed to investigate the impact of didanosine (ddI), lamivudine (3TC), and stavudine (d4T) on cell survival and mutagenicity in two reporter genes, hypoxanthine-guanine phosphoribosyltransferase (HPRT) and thymidine kinase (TK), using a cell cloning assay for assessing the effects of individual nucleoside analogs (NRTIs)/drug combinations in human TK6 B-lymphoblastoid cells. Three-day treatments with 0, 33, 100, or 300 microM ddI, 3TC, or ddI-3TC produced positive trends for increased HPRT and TK mutant frequencies. While dose-related trends were too small to reach significance after treatments with d4T or d4T-3TC, pairwise comparisons with control cells indicated that exposure to 100 microM d4T or d4T-3TC caused significant elevations in HPRT mutants. Measurements of mutagenicity in cells exposed to d4T (or d4T-3TC) were complicated by the cytotoxicity of this NRTI. Enhanced increases in mutagenic responses to combined NRTI treatments, compared with single drug treatments, occurred as additive to synergistic effects in the HPRT gene of cells exposed to 100 microM ddI-3TC or 100 microM d4T-3TC, and in the TK gene of cells exposed to 100 or 300 microM ddI-3TC. Comparisons of these data to mutagenicity studies of other NRTIs in the same system (Meng Q et al. [2000c]: Proc Natl Acad Sci USA 97:12667-126671; Torres SM et al. [2007]: Environ Mol Mutagen) indicate that the relative mutagenic potencies for all drugs tested to date are: AZT-ddI > ddI-3TC > AZT-3TC congruent with AZT-3TC-ABC (abacavir) > AZT >/=ddI > d4T-3TC > 3TC > d4T >/= ABC. These collective data suggest that all NRTIs with antiviral activity against HIV-1 may cause host cell DNA damage and mutations, and impose a cancer risk.

Mitochondrial toxicity in hearts of CD-1 mice following perinatal exposure to AZT, 3TC, or AZT/3TC in combination

Antiretroviral therapies based on nucleoside reverse transcriptase inhibitors (NRTIs), like zidovudine (3'-azido-3'-deoxythymidine; AZT) and lamivudine ((-)2',3'-dideoxy-3'-thiacytidine; 3TC), markedly reduce mother-to-child transmission of the human immunodeficiency virus (HIV). However, AZT induces damage in nuclear DNA of mice exposed in utero and postnatally, and mitochondrial DNA (mtDNA) damage has been observed in both human and mouse neonates following perinatal exposure to AZT and AZT/3TC in combination. To provide animal data modeling the NRTI-induced heart damage reported in human infants, we treated pregnant CD-1 mice throughout gestation and treated their pups by direct gavage from postnatal day (PND) 4 through PND 28 with daily doses of 150 mg/kg body weight (bw)/day AZT, 75 mg/kg bw/day 3TC, 125/62.5 mg/kg bw/day AZT/3TC, or the vehicle control. Half the pups were euthanized on PND 28; the remainder received no further dosing, and were euthanized at week 10. Heart tissue was collected, total DNA was extracted, and mtDNA copy number relative to nuclear DNA copy number, mtDNA damage, and mtDNA mutation assays were performed using PCR-based methods. Analyses revealed increases in mtDNA lesions in 4-week-old males and females treated with AZT or 3TC, but not in 10-week-old mice, suggesting that the damage resolved after treatment ceased. Interestingly, 10-week-old females treated with AZT/3TC had significant increases in mtDNA damage. Point mutations were elevated in 10-week-old females treated with AZT or AZT/3TC, but not 3TC; no increases in mutations were seen in either gender at 4 weeks of age. Our data suggest that AZT/3TC combination treatment produces greater mtDNA damage than either agent individually, and that female mice are more sensitive than males to AZT/3TC-induced mtDNA damage.

Genotoxicity assessed by the comet and GPA assays following in vitro exposure of human lymphoblastoid cells (H9) or perinatal exposure of mother-child pairs to AZT or AZT-3TC

The genotoxicity of zidovudine (AZT) based treatments was investigated in human H9 lymphoblastoid cells in an in vitro study and in red blood cells (RBCs) from perinatally exposed HIV-1-infected mothers and their infants in an observational cohort study. Exposure of H9 cells for 24 hr to AZT produced dose-dependent increases in Comet assay tail moment (TM) when electrophoresed at pH 13.0, but not at pH 12.1 or pH 8.0, suggesting that DNA damage was via alkali-labile lesions and not double-stranded DNA strand breaks. The TM dose response at pH 13.0 correlated directly with AZT-DNA incorporation determined by AZT-radioimmunoassay. Levels of DNA damage in utero, measured by Comet assay TM, were similar in cord blood mononuclear cells of nucleoside analog-exposed newborns (n = 43) and unexposed controls (n = 40). In contrast, the glycophorin A (GPA) somatic cell mutation assay (which screens for large-scale DNA damage in RBCs) showed clear evidence that GPA N/N variants, arising from chromosome loss and duplication, somatic recombination, and gene conversion, were significantly elevated in mother-child pairs receiving prepartum AZT plus lamivudine (3TC). Cord blood from newborns exposed to AZT-3TC had GPA N/N variant frequencies of 4.7 +/- 0.7 (mean +/- SE) x 10(-6) RBCs (n = 26 infants) compared with 2.2 +/- 0.3 x 10(-6) RBCs for unexposed controls (n = 30 infants; P < 0.001). Elevations in GPA N/N variants generally persisted through 1 year of age in nucleoside analog-exposed children. Overall, the mutagenic effects found in mother-child pairs receiving AZT-based treatments justify their surveillance for long-term genotoxic consequences.

Elevated frequencies of micronucleated erythrocytes in infants exposed to zidovudine in utero and postpartum to prevent mother-to-child transmission of HIV

Zidovudine-based antiretroviral therapies (ARTs) for treatment of HIV-infected pregnant women have markedly reduced mother-to-child transmission of the human immunodeficiency virus (HIV-1) from approximately 25% to <1%. However, zidovudine (ZDV; AZT), a nucleoside analogue, induces chromosomal damage, gene mutations, and cancer in animals following direct or transplacental exposure. To determine if chromosomal damage is induced by ZDV in infants exposed transplacentally, we evaluated micronucleated reticulocyte frequencies (%MN-RET) in 16 HIV-infected ART-treated mother-infant pairs. Thirteen women received prenatal ART containing ZDV; three received ART without ZDV. All infants received ZDV for 6 weeks postpartum. Venous blood was obtained from women at delivery and from infants at 1-3 days, 4-6 weeks, and 4-6 months of life; cord blood was collected immediately after delivery. Ten cord blood samples (controls) were obtained from infants of HIV-uninfected women who did not receive ART. %MN-RET was measured using a single laser 3-color flow cytometric system. Tenfold increases in %MN-RET were seen in women and infants who received ZDV-containing ART prenatally; no increases were detected in three women and infants who received prenatal ART without ZDV. Specifically, mean %MN-RET in cord blood of ZDV-exposed infants was 1.67 +/- 0.34 compared with 0.16 +/- 0.06 in non-ZDV ART-exposed infants (P = 0.006) and 0.12 +/- 0.02 in control cord bloods (P < 0.0001). %MN-RET in ZDV-exposed newborns decreased over the first 6 months of life to levels comparable to cord blood controls. These results demonstrate that transplacentalZDV exposure is genotoxic in humans. Long-term monitoring of HIV-uninfected ZDV-exposed infants is recommended to ensure their continued health.

Mutagenicity of zidovudine, lamivudine, and abacavir following in vitro exposure of human lymphoblastoid cells or in utero exposure of CD-1 mice to single agents or drug combinations

Experiments were performed to investigate the impact of zidovudine (AZT), lamivudine (3TC), and abacavir (ABC) on cell survival and mutagenicity in two reporter genes, hypoxanthine-guanine phosphoribosyltransferase (HPRT) and thymidine kinase (TK), using cell cloning assays for assessing the effects of individual drugs/drug combinations in (1) TK6 human lymphoblastoid cells exposed in vitro and (2) splenic lymphocytes from male CD-1 mice exposed transplacentally on days 12-18 of gestation. In TK6 cells, dose-related increases in HPRT and TK mutant frequencies were found following 3 days of exposure to AZT or 3TC alone (33, 100, or 300 microM), or to equimolar amounts of AZT-3TC. Compared with single drug exposures, AZT-3TC coexposures generally yielded enhanced elevations in HPRT and TK mutant frequencies. Mutagenicity experiments with ABC alone, or in combination with AZT-3TC, were complicated by the extreme cytotoxicity of ABC. Exposure of cells either to relatively high levels of AZT-3TC short-term (100 microM, 3 days), or to peak plasma-equivalent levels of AZT-3TC for an extended period (10 microM, 30 days), resulted in similar drug-induced mutagenic responses. Among sets of mice necropsied on days 13, 15, or 21 postpartum, Hprt mutant frequencies in T-cells were significantly elevated in the AZT-only (200 mg/kg bw/day) and AZT-3TC (200 mg AZT + 100 mg 3TC/kg bw/day) groups at 13 days of age. These results suggest that the mutagenicity by these nucleoside analogs is driven by cumulative dose, and raises the question of whether AZT-3TC has greater mutagenic effects than AZT alone in perinatally exposed children.

3'-azido-3'-deoxythymidine induces deletions in L5178Y mouse lymphoma cells

3'-Azido-3'-deoxythymidine (AZT), a nucleoside analogue used for the treatment of acquired immunodeficiency syndrome (AIDS), induced a significant dose-related increase in the thymidine kinase (Tk) mutant frequency (MF) in L5178Y/Tk(+/-) 3.7.2C mouse lymphoma cells. Treatment with 1 mg/ml (3,742 muM) AZT for 24 hr resulted in a MF of 407 x 10(-6) compared to a control MF of 84 x 10(-6). The MFs of the large and small colony mutants resulting from AZT exposure were 142 x 10(-6) and 265 x 10(-6), respectively. One hundred and fifty mutants from the 1 mg/ml (3,742 muM) AZT-treated culture and sixty-nine mutants from independent untreated cultures were isolated and analyzed. LOH analysis using a heteromorphic microsatellite locus located in the Tk gene was performed to determine the presence or absence of the Tk(+) allele. Eight other microsatellite markers spanning the entire mouse chromosome 11 also were examined for heterozygosity to determine the extent of LOH. In addition, Tk gene dosage analysis was conducted using Real-Time PCR in those mutants showing LOH at the Tk locus. The presence of only one Tk allele based on Real-Time PCR indicated that the mutant resulted from deletion while the presence of two alleles was consistent with a recombination event. More mutants from the AZT-treated culture showed Tk LOH than did independent mutants from the untreated cultures (91% vs. 64%) and the induced mutants also showed distinct chromosome 11 LOH patterns. The mutation spectrum of mutants from AZT-treated cells was also significantly different from that of spontaneous mutants. More deletions and fewer intragenic mutations were observed in the mutants from the AZT-treated culture than independent mutants from the untreated control. Our data indicate that AZT primarily induced LOH mutations in L5178Y mouse lymphoma cells and a large number of LOH mutations resulted from deletions.

Mechanisms of genotoxicity of nucleoside reverse transcriptase inhibitors

Nucleoside analogs were first approved by the U.S. Food and Drug Administration for use against HIV-AIDS in 1987. Since then, these agents, now commonly referred to as nucleoside reverse transcriptase inhibitors (NRTIs), have become essential components of the Highly Active Antiretroviral Therapy (HAART) drug combinations used for treatment of Human Immunodeficiency Virus-1 (HIV-1) infections. Their antiretroviral activity is likely two-fold: incorporation of the drug into viral DNA and inhibition of the viral reverse transcriptase. However, incorporation of the drug into host nuclear and mitochondrial DNA may be largely responsible for dose-limiting toxicities. Azidothymidine (AZT, 3'-azido-3'-deoxythymidine, zidovudine), the first NRTI approved for the therapy of HIV-1, is incorporated into DNA, causes mutations in the hypoxanthine-guanine phosphoribosyl-transferase (HPRT) and thymidine kinase (TK) genes, and induces micronuclei, chromosomal aberrations, sister chromatid exchange, shortened telomeres, and other genotoxic effects in cultured cells. Genomic instability would be predicted as a consequence of these events. Metabolic pathways that result in the phosphorylation of AZT play a crucial role in AZT-DNA incorporation, and may be altered after prolonged treatment. For example, thymidine kinase 1, the enzyme responsible for AZT mono-phosphorylation, is down-regulated during long-term exposure and appears to be associated with AZT-induced replication inhibition and the accumulation of cells in S-phase. Detailed information on the mechanisms underlying NRTI-associated antiretroviral efficacy, toxicity, and metabolic resistance were not available when AZT was first approved for use as an antiretroviral agent. Current insights, based on 15 years of research, may lead to intervention strategies to attenuate toxicity without altering drug efficacy.

Mutagenicity of stereochemical configurations of 1,2-epoxybutene and 1,2:3,4-diepoxybutane in human lymphblastoid cells

The carcinogenicity of 1,3-butadiene (BD) is related to its bioactivation to several DNA-reactive metabolites; accumulating evidence suggests that the stereochemistry of these BD intermediates may play a significant role in the mutagenic and carcinogenic actions of the parent compound. The objective of this study was to evaluate the cytotoxicity and mutagenicity of stereochemical forms of 1,2-epoxybutene (EB) and 1,2:3,4-diepoxybutane (DEB), two genotoxic BD metabolites, in a human lymphoblastoid cell line, TK6. Cytotoxicity was measured by comparing cloning efficiencies in chemical-exposed cells versus those in control cells. The hypoxanthine-guanine phosphoribosyltransferase (HPRT) and thymidine kinase (TK) mutant frequencies (MFs) were measured using a cell cloning assay. HPRT mutants collected from cells exposed to the three forms of DEB were analyzed by PCR to characterize large genetic alterations. All the three stereoisomers of DEB caused increased HPRT and TK MFs compared to the concurrent control samples. There were no significant differences in cytotoxicity or mutagenicity among the three isomers of DEB in TK6 cells. Molecular analysis of HPRT mutants revealed similar distributions of types of mutations among the three isomers of DEB. There were also no statistically significant differences in mutagenic efficiencies between the two isomers of EB in TK6 cells. These results were consistent with the in vivo findings that there was little difference in the mutagenic efficiencies of racemic-DEB versus meso-DEB in rodents. Thus, in terms of mutagenic efficiency, stereochemical configurations of EB and DEB are not likely to play a significant role in the mutagenicity and carcinogenicity of BD.