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

Mechanisms of telomerase inhibition by oxidized and therapeutic dNTPs

Telomerase is a specialized reverse transcriptase that adds GGTTAG repeats to chromosome ends and is upregulated in most human cancers to enable limitless proliferation. Here, we uncover two distinct mechanisms by which naturally occurring oxidized dNTPs and therapeutic dNTPs inhibit telomerase-mediated telomere elongation. We conduct a series of direct telomerase extension assays in the presence of modified dNTPs on various telomeric substrates. We provide direct evidence that telomerase can add the nucleotide reverse transcriptase inhibitors ddITP and AZT-TP to the telomeric end, causing chain termination. In contrast, telomerase continues elongation after inserting oxidized 2-OH-dATP or therapeutic 6-thio-dGTP, but insertion disrupts translocation and inhibits further repeat addition. Kinetics reveal that telomerase poorly selects against 6-thio-dGTP, inserting with similar catalytic efficiency as dGTP. Furthermore, telomerase processivity factor POT1-TPP1 fails to restore processive elongation in the presence of inhibitory dNTPs. These findings reveal mechanisms for targeting telomerase with modified dNTPs in cancer therapy.

Stilbene Compounds Inhibit Tumor Growth by the Induction of Cellular Senescence and the Inhibition of Telomerase Activity

Cellular senescence is a state of cell cycle arrest characterized by a distinct morphology, gene expression pattern, and secretory phenotype. It can be triggered by multiple mechanisms, including those involved in telomere shortening, the accumulation of DNA damage, epigenetic pathways, and the senescence-associated secretory phenotype (SASP), and so on. In current cancer therapy, cellular senescence has emerged as a potent tumor suppression mechanism that restrains proliferation in cells at risk for malignant transformation. Therefore, compounds that stimulate the growth inhibition effects of senescence while limiting its detrimental effects are believed to have great clinical potential. In this review article, we first review the current knowledge of the pro- and antitumorigeneic functions of senescence and summarize the key roles of telomerase in the regulation of senescence in tumors. Second, we review the current literature regarding the anticancer effects of stilbene compounds that are mediated by the targeting of telomerase and cell senescence. Finally, we provide future perspectives on the clinical utilization of stilbene compounds, especially resveratrol and pterostilbene, as novel cancer therapeutic remedies. We conclude and propose that stilbene compounds may induce senescence and may potentially be used as the therapeutic or adjuvant agents for cancers with high telomerase activity.

Telomerase as a Cancer Target. Development of New Molecules

Telomeres are the terminal part of the chromosome containing a long repetitive and non-codifying sequence that has as function protecting the chromosomes. In normal cells, telomeres lost part of such repetitive sequence in each mitosis, until telomeres reach a critical point, triggering at that time senescence and cell death. However, in most of tumor cells in each cell division a part of the telomere is lost, however the appearance of an enzyme called telomerase synthetize the segment that just has been lost, therefore conferring to tumor cells the immortality hallmark. Telomerase is significantly overexpressed in 80–95% of all malignant tumors, being present at low levels in few normal cells, mostly stem cells. Due to these characteristics, telomerase has become an attractive target for new and more effective anticancer agents. The capability of inhibiting telomerase in tumor cells should lead to telomere shortening, senescence and apoptosis. In this work, we analyze the different strategies for telomerase inhibition, either in development, preclinical or clinical stages taking into account their strong points and their caveats. We covered strategies such as nucleosides analogs, oligonucleotides, small molecule inhibitors, G-quadruplex stabilizers, immunotherapy, gene therapy, molecules that affect the telomere/telomerase associated proteins, agents from microbial sources, among others, providing a balanced evaluation of the status of the inhibitors of this powerful target together with an analysis of the challenges ahead.

Foetal loss and enhanced fertility observed in mice treated with Zidovudine or Nevirapine

BACKGROUND

Health concerns for HIV-infected persons on antiretroviral therapy (ART) have moved from morbidity to the challenges of long-term ART. We investigated the effect of Zidovudine or Nevirapine on reproductive capacity across two mouse generations.

METHODS

A prospective mouse study with drugs administered through one spermatogenic cycle. Mouse groups (16 males and 10 females) were given Zidovudine or Nevirapine for 56 days. Males were mated to untreated virgin females to determine dominant lethal effects. Twenty females (10 treated and 10 untreated) mated with the treated males per dose and gave birth to the F1 generation. Parental mice were withdrawn from drugs for one spermatogenic cycle and mated to the same dams to ascertain if effects are reversible. The F1 generation were exposed for another 56 days and mated to produce the F2 generation.

RESULTS

Foetal loss was indicated in the dominant lethal assay as early as four weeks into drug administration to the males. At the first mating of the parental generation to produce the F1 generation, births from 10 dams/dose when the 'father-only' was exposed to Zidovudine (10, 100 and 250 mg/kg) was 3, 2 and 1 while it was 7, 1 and 4 respectively when 'both-parents' were exposed. Similarly births from the parental generation first mating when the 'father-only' was exposed to Nevirapine (5, 50 and 150 mg/kg) was 2, 2 and 0 while it was 6, 5 and 9 respectively when 'both-parents' were exposed. However, fertility was not significantly different neither by dose nor by the parental exposure. The F1 mice mated to produce the F2 generation recorded only one birth.

CONCLUSION

The dominant lethal analysis showed foetal loss occurred when the "fathers-only" were treated while fertility was enhanced when "both-parents" were on therapy at the time of mating.

AZT as a telomerase inhibitor

Telomerase is a highly specialized reverse transcriptase (RT) and the maintenance of telomeric length is determined by this specific enzyme. The human holoenzyme telomerase is a ribonucleoprotein composed by a catalytic subunit, hTERT, an RNA component, hTR, and a group of associated proteins. Telomerase is normally expressed in embryonic cells and is repressed during adulthood. The enzyme is reexpressed in around 85% of solid tumors. This observation makes it a potential target for developing drugs that could be developed for therapeutic purposes. The identification of the hTERT as a functional catalytic RT prompted studies of inhibiting telomerase with the HIV RT inhibitor azidothymidine (AZT). Previously, we have demonstrated that AZT binds preferentially to telomeres, inhibits telomerase and enhances tumor cell senescence, and apoptosis after AZT treatment in breast mammary adenocarcinoma cells. Since then, several studies have considered AZT for telomerase inhibition and have led to potential clinical strategies for anticancer therapy. This review covers present thinking of the inhibition of telomerase by AZT and future treatment protocols using the drug.

Anticancer conjugates and cocktails based on methotrexate and nucleoside synergism

Conjugates of methotrexate (MTX) and the nucleoside analogs 3-azidodeoxythymidine (AZT), iododeoxyuridine (IUdR) and dideoxycytidine (ddC) linked using poly(ethyleneglycol) are presented. In vitro cytotoxicity assays of the conjugates against drug resistant ovarian cell line A2780/AD are preformed and comparisons made to such assays performed for unconjugated (cocktail) systems. All systems tested were inactive, or had low activity, at 24 h. After 72 hr incubation however, the cocktails of MTX and AZT, IUdR or ddC showed high cytotoxicity in the low nanomolar range. The conjugates were only very moderately active with IC(50) values in the [0.1 to 1.0 mM] range. Conjugation of the antifolate to the nucleoside analogs has it seems reduced the activity significantly when compared to a cocktail of the components, indicating a conjugate approach is unlikely to translate into success in vivo. The positive note comes from the observation that by combining two of the new conjugates, namely those based on MTX with IUdR or AZT, an IC50 at 24 hours of ~ [180 muM] was produced.

Synthesis, characterization, and in vitro assay of folic acid conjugates of 3'-azido-3'-deoxythymidine (AZT): toward targeted AZT based anticancer therapeutics

Conjugates of three components namely folic acid, poly(ethyleneglycol) and 3 '-azido-3 '-deoxythymidine (AZT) are presented. Folate-PEG units were coupled to AZT to facilitate delivery of the nucleoside into the cell. A convenient separation of the polydisperse PEGylated-folic acid regioisomers produced upon conjugation is described. This is to select for the active gamma-regioisomer over the inactive alpha-regioisomer. In vitro cytotoxicity assays were conducted against an ovarian cell line (A2780/AD) that overexpresses the folate receptor (FR) and compared to a FR free control cell line. Compared to AZT a approximately 20-fold greater potency against the resistant ovarian line was observed for the conjugates.

Relevance of experimental models for investigation of genotoxicity induced by antiretroviral therapy during human pregnancy

The current incidence of human immunodeficiency virus (HIV-1)/AIDS affects around 7000 pregnant women in the United States. When given during pregnancy, the nucleoside analog 3'-azido-3'-deoxythymidine (AZT) significantly reduces maternal-fetal transmission. It has been previously shown that AZT is incorporated into DNA, where it causes mutations in the HPRT and TK genes. It also changes cell cycle gene expression, and induces S-phase arrest, micronuclei, chromosomal aberrations, sister chromatid exchanges, telomeric attrition, and other genotoxic effects in cultured cells. A predicted consequence of these events is genomic instability that together, with clastogenicity may contribute to the carcinogenic potency of AZT. Various aspects of genotoxicity are explored in this contribution seeking to understand the multiple effects of this antiretroviral agent in animal models and humans. This mini-review describes some of the experimental models used to elucidate the genotoxicity induced by antiretroviral therapy during human pregnancy. The use of diverse methods to detect biomarkers of exposure, such as an AZT-specific radioimmunoassay, micronuclei bearing intact chromosomes, and telomeric DNA attrition highlight the role of in vitro models to elucidate exposure and risk. The relevance of the in vitro models is followed by the introduction of the role of the nucleoside analogs in transplacental carcinogenesis along with the description of a transplacental perfusion model and a transplacental carcinogenesis rodent model. In a more direct clinical application the use of AZT-DNA incorporation as a biomarker of exposure, in experiments conducted in vivo in Erythrocebus patas monkeys and in humans, addresses the possibility of elucidation of potential cancer risk in those infants exposed in utero. Two relevant aspects of this contribution are the potential application of some of the models described in this mini-review, as diagnostic tools in antiretroviral-exposed populations, and the use of these models to understand the nature of the genotoxicities and minimize the undesirable side effects of the antiretroviral therapy.

Perinatal genotoxicity and carcinogenicity of anti-retroviral nucleoside analog drugs

The current worldwide spread of the human immunodeficiency virus-1 (HIV-1) to the heterosexual population has resulted in approximately 800,000 children born yearly to HIV-1-infected mothers. In the absence of anti-retroviral intervention, about 25% of the approximately 7,000 children born yearly to HIV-1-infected women in the United States are HIV-1 infected. Administration of zidovudine (AZT) prophylaxis during pregnancy reduces the rate of infant HIV-1 infection to approximately 7%, and further reductions are achieved with the addition of lamivudine (3TC) in the clinical formulation Combivir. Whereas clinically this is a remarkable achievement, AZT and 3TC are DNA replication chain terminators known to induce various types of genotoxicity. Studies in rodents have demonstrated AZT-DNA incorporation, HPRT mutagenesis, telomere shortening, and tumorigenicity in organs of fetal mice exposed transplacentally to AZT. In monkeys, both AZT and 3TC become incorporated into the DNA from multiple fetal organs taken at birth after administration of human-equivalent protocols to pregnant dams during gestation, and telomere shortening has been found in monkey fetuses exposed to both drugs. In human infants, AZT-DNA and 3TC-DNA incorporation as well as HPRT and GPA mutagenesis have been documented in cord blood from infants exposed in utero to Combivir. In infants of mice, monkeys, and humans, levels of AZT-DNA incorporation were remarkably similar, and in newborn mice and humans, mutation frequencies were also very similar. Given the risk-benefit ratio, these highly successful drugs will continue to be used for prevention of vertical viral transmission, however evidence of genotoxicity in mouse and monkey models and in the infants themselves would suggest that exposed children should be followed well past adolescence for early detection of potential cancer hazard.

Synergy between 3'-azido-3'-deoxythymidine and paclitaxel in human pharynx FaDu cells

PURPOSE

We recently demonstrated simultaneous targeting of telomere and telomerase as a novel cancer therapeutic approach, and that telomerase inhibitors such as 3'-azido-3'-deoxythymidine (AZT) significantly enhanced the antitumor activity of paclitaxel, which causes telomere erosion, in telomerase-positive human pharynx FaDu tumors in vitro and in vivo. The present study evaluated the synergy between AZT and paclitaxel to identify optimal combinations for future clinical evaluation.

METHODS

FaDu cells were incubated with or without AZT for 24 h and then treated with AZT with or without paclitaxel for an additional 48 h. Under these conditions, single agent paclitaxel produced a 60% maximum reduction of cell number (IC50) was 7.3 nM), and single agent AZT produced a 97% reduction (IC50 was 5.6 microM). Synergy was evaluated using fixed-concentration and fixed-ratio methods, and data were analyzed by the combination index method.

RESULTS

The results indicate a concentration-dependent synergy between the two drugs; the synergy was higher for combinations containing greater paclitaxel-to-AZT concentration ratios and increased with the level of drug effect. For example, in combinations containing 1 microM AZT, synergy was 1.3-fold at the 20% effect level and 3.1-fold at the 60% effect level. Because the major antitumor activity, determined by comparing the posttreatment cell number to the pretreatment cell number, was antiproliferation at the 20% effect level and cell kill at the 60% effect level, our results suggest that AZT mainly enhances the cell kill effect of paclitaxel.

CONCLUSION

In summary, the present study demonstrates a synergistic interaction between paclitaxel and AZT and supports a combination using a low and nontoxic AZT dose in combination with a therapeutically active dose of paclitaxel.

Long-term mitochondrial toxicity in HIV-uninfected infants born to HIV-infected mothers

Although children born to HIV-infected (HIV+) women receiving antiretroviral therapy during pregnancy show virtually no adverse clinical effects at birth, the antiretroviral nucleoside analog drugs are known to damage nuclear and mitochondrial DNA. In this study, biomarkers of mitochondrial toxicity and genotoxicity have been examined in a well-characterized sample set consisting of infants born to HIV-uninfected (HIV-) mothers (n = 30), and HIV- infants (n = 20) born to HIV-infected (HIV+) mothers who received either no antiretroviral therapy (n = 10) or zidovudine (3'-azido-3'-deoxythymidine [AZT]) during pregnancy (n = 10). DNA from cord blood leukocytes and peripheral blood leukocytes taken at 1 and 2 years of age was examined for loss of mitochondrial DNA (mtDNA) and telomere integrity. Telomere length, a measure of nuclear DNA damage, was the same in all infants at birth and at age 1 year. The quantity of mtDNA was assessed relative to nuclear DNA using a polymerase chain reaction-based chemiluminescence detection (PCR-CID) method that determined mitochondrial D Loop gene copies relative to nuclear 18S RNA gene copies by comparison with a standard curve. MtDNA quantity was expressed as a ratio of gene copy numbers. In infants of uninfected mothers (AZT-/HIV-) at the three time points, the ratios were 442 to 515, whereas in infants of untreated AZT-/HIV+ mothers the ratios were 261 to 297, and in infants of AZT-treated (AZT+/HIV+) mothers the ratios were 146 to 203. At all three time points, differences between the AZT-/HIV- group and the two HIV+ groups were statistically significant (p <.05), and differences between the AZT-/HIV+ and AZT+/HIV+ groups were also statistically significant (p <.05), demonstrating that AZT exposure causes a persistent depletion of mtDNA. The study shows that children of HIV+ mothers are at risk for mitochondrial damage that is further increased in infants of mothers receiving AZT during pregnancy.

Telomerase inhibition as cancer therapy

A number of different approaches have been developed to inhibit telomerase activity in human cancer cells. Different components and types of inhibitors targeting various regulatory levels have been regarded as useful for telomerase inhibition. Most methods, however, rely on successive telomere shortening. This process is very slow and causes a long time lag between the onset of inhibition and the occurrence of senescence or apoptosis as a reversal of the immortal phenotype. Many telomerase inhibitors seem to be most efficient when combined with conventional chemotherapeutics. There are some promising approaches that seem to circumvent the slow way of telomere shortening and induce fast apoptosis in treated tumor cells. It has been demonstrated that telomerase may be involved in triggering apoptosis, but the underlying molecular mechanism remains unclear.

Telomerase and telomeres: from basic biology to cancer treatment

The limited capacity to divide is one of the major differences between normal somatic cells and cancerous cells. This 'finite life span' of somatic cells is closely linked to loss of telomeric DNA at telomeres, the 'chromosome caps' consisting of repeated (7TAGGG) sequences., In more than 85% of advanced cancers, this telomeric attrition is compensated by telomerase, 'the immortality enzyme', implying that telomerase inhibition may restore mortality in tumor cells. This review discusses the progress in research on the structure and function of telomeres and the telomerase holoenzyme. In addition, new developments in telomere/telomerase targeting compounds such as antisense oligonucleotides and G-quadruplex stabilizing substances, but also new telomerase expression-related strategies such as telomerase promoter-driven suicide gene therapy and telomerase immunotherapy will be presented. It will be discussed how these data can be implemented in telomerase-directed therapies.

Toxicity of antiretroviral nucleoside and nucleotide analogues: is mitochondrial toxicity the only mechanism?

Nucleoside analogues represent the cornerstones of antiretroviral regimens. A range of drug- or tissue-specific toxicities, such as peripheral neuropathy, myopathy, pancreatitis and lactic acidosis with hepatic steatosis, has been documented with these agents. The fat atrophy seen on long term antiretroviral therapy may also be related to nucleoside analogues. The mechanisms by which nucleoside analogues cause toxicity are not clearly established. In vitro, the triphosphates of these agents are weak to modest substrates for human DNA polymerases, showing the greatest affinity for mitochondrial DNA polymerase gamma. Short term exposure in vitro to some nucleoside analogues has been demonstrated to cause increased lactate production or falls in mitochondrial DNA suggestive of mitochondrial toxicity. However, stavudine and to a lesser extent zidovudine are poor substrates for mitochondrial thymidine kinase type 2, the predominant form in cells that are not actively mitotic such as neurons, myocytes and adipocytes. These are the cell types where the proposed mitochondrial toxicities neuropathy, myopathy and lipoatrophy are observed. Thus, active concentrations of phosphorylated products of stavudine and zidovudine may not be present in mitochondria. The familial mitochondrial diseases do not have identical presentations to nucleoside analogue toxicities. These disorders most commonly involve the CNS, typically with seizures or dementia, and occasionally the kidneys. Although nucleoside analogues are known to penetrate the CNS and are commonly renally excreted unchanged, mitochondrial toxicities at these sites have not been documented. Furthermore, toxicity caused by nucleoside or nucleotide analogues does not always appear to arise through the mitochondrial route. Cidofovir appears to cause renal tubular dysfunction via a toxic intracellular metabolite, and zidovudine-related anaemia appears to be related to decreased globin RNA synthesis. In vitro or animal models suggest that zidovudine myopathy, stavudine-related (but not zalcitabine- or didanosine-related) neuropathy and didanosine-related pancreatitis may all be not related, or not exclusively related, to mitochondrial dysfunction. The integration of nucleoside analogues into nuclear DNA, best documented with zidovudine but likely to occur with other agents, represents an alternative but potentially delayed pathway to cytotoxicity and cell apoptosis. This is the mechanism of cell death during therapy with antineoplastic nucleoside analogues, and may have contributed to the multisystem toxicities observed with the anti-hepatitis B drug fialuridine. New research evaluating the effects of long term exposure of cell lines is required to address the possibility that nuclear genotoxicity plays a role in long term nucleoside analogue toxicity.

Fibroblast growth factor 2 up regulates telomerase activity in neural precursor cells

During brain development, neuronal and glial cells are generated from neural precursors on a precise schedule involving steps of proliferation, fate commitment and differentiation. We report that telomerase activity is highly expressed during embryonic murine cortical neurogenesis and early steps of gliogenesis and progressively decreases thereafter during cortex maturation to be undetectable in the normal adult brain. We evidenced neural precursor cells (NPC) as the principal telomerase-expressing cells in primary cultures from E15 mouse embryo cortices. Their differentiation either in neurons or in glial cells lead to a down regulation of telomerase activity that was directly correlated to the decrease of telomerase core protein (mTERT) mRNA synthesis. Furthermore, we show that FGF2 (fibroblast growth factor 2), one of the main regulators of CNS development, induces a dose-dependant increase of both the proliferation of NPC and telomerase activity in primary cortical cultures without affecting the mTERT mRNA synthesis compared to that of glyceraldehyde-3-phosphate dehydrogenase (mGAPDH). Finally, we evidenced that AZT (3'-azido-2', 3'-dideoxythymidine), known to inhibit telomerase activity, blocks in a dose dependant manner the FGF2-induced proliferation of NPC. Altogether, our results are in favor of an important role of telomerase activity during brain organogenesis. Oncogene (2000).

Recent advances in the development of telomerase inhibitors for the treatment of cancer

Telomerase is an holoenzyme responsible for the maintenance of telomeres, the protein-nucleic acid structures which exist at the ends of eukaryotic chromosomes that serve to protect chromosomal stability and integrity. Telomerase activity is essential for the sustained proliferation of most immortal cells, including cancer cells. Since the discovery that telomerase activity is expressed in 85 - 90% of all human tumours and tumour-derived cell lines but not in most normal somatic cells, telomerase has become the focus of much attention as a novel and potentially highly-specific target for the development of new anticancer chemotherapeutics. Herein we review recent advances in the development of telomerase inhibitors for the treatment of cancer. To date, these have included antisense strategies, reverse transcriptase inhibitors and compounds capable of interacting with high-order telomeric DNA tetraplex ('G-quadruplex') structures to prevent enzyme access to the necessary linear telomere substrate. In addition, a number of telomerase-inhibitory therapies have been shown to synergistically enhance the effects of clinically-established anticancer drugs. Critical appraisal of each individual approach is provided, together with highlighted areas of likely future development. We also review recent developments in telomere and telomerase biology, of which a more detailed understanding would be essential in order to further develop the present classes of telomerase inhibitors into viable, clinically applicable therapies.

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.

Telomerase targeting in cancer treatment: new developments

Telomerase, a ribonucleoprotein expressed in 85% of advanced cancers but not in most somatic cells, compensates for telomeric DNA erosion and as such stabilizes cell immortality. Telomerase inhibition might restore mortality in tumor cells. Recent progress is illustrated in studies on telomerase and telomere targeting with differentiation induction, reverse transcriptase inhibitors, promoter down regulation, antisense inhibition, and blockage of telomere/telomerase interactions. Also, new developments are described indicating that anti-telomerase treatment can induce apoptosis in tumor cells and can chemosensitize drug-resistant cell lines. Implications of these findings for anti-telomerase-based therapeutic applications, in particular in combination therapies, are discussed. Copyright 1999 Harcourt Publishers Ltd.

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.