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

Aloe-emodin relieves zidovudine-induced injury in neonatal rat ventricular myocytes by regulating the p90rsk/p-bad/bcl-2 signaling pathway

BACKGROUND/AIMS

Zidovudine (3'-azido-2',3'-deoxythymidine; AZT) is a first-line drug for treatment of human immunodeficiency virus infection (HIV). However, its application is limited by cardiotoxicity due to cardiomyocyte injury. This study investigated whether Aloe-emodin (AE), an anthraquinone compound, protects against AZT-induced cardiomyocyte toxicity.

METHODS

MTT, JC-1 assays and TUNEL were examined to verify the protective effect of AE against AZT-induced cardiomyocyte injury. Western blotting was performed to explore the anti-apoptotic effect of AE using anti-apoptotic proteins p90rsk, p-bad, and bcl-2 and pro-apoptotic proteins apaf-1, cleaved-caspase-3, and cytochrome c.

RESULTS

We observed a protective effect of AE against cell viability decrease and TUNEL positive cells increase induced by AZT, which was counteracted by BI-D1870. Western blot analysis found that AE significantly inhibited cardiomyocyte apoptosis by activating p90rsk/p-bad/bcl-2 signaling pathway. Furthermore, BI-D1870 counteracted the anti-apoptotic effect of AE.

CONCLUSIONS

Taken together, these results indicate that AE attenuated AZT-induced cardiomyocyte apoptosis by activating p90rsk.

Genotoxic and Cytotoxic Effects of Antiretroviral Combinations in Mice Bone Marrow

Commonly used guidelines for the management of human immunodeficiency virus (HIV) infection (highly active antiretroviral therapy, HAART) include drug combinations such as tenofovir disoproxil fumarate (TDF) + lamivudine (3TC) and combivir [zidovudine (AZT) + 3TC] + efavirenz (EFV). These combinations may enhance the genotoxic effects induced by such drugs individually, since the therapy requires lifelong adherence and the drugs have unknown effects during treatment. Thus, the evaluation of the benefits and risks of HAART is of great importance. In order to assess the cytotoxic and genotoxic potential of three concentrations of each of the antiretroviral combinations TDF + 3TC (800 + 400, 1600 + 800, and 3200 + 1600 mg/kg body weight, BW) and combivir + EFV (200 + 100 + 400, 400 + 200 + 800, and 800 + 400 + 1600 mg/kg BW) after two exposure periods (24 h and 48 h), in the present study the in vivo comet assay (single-cell gel electrophoresis) and the mouse bone marrow micronucleus test were used. Neither TDF + 3TC nor combivir + EFV induced DNA damage at any concentrations tested after 24 h or 48 h using the comet assay. After 24 h, both combinations increased the micronucleus frequency at all concentrations tested. After 48 h, combivir + EFV increased the micronucleated polychromatic erythrocyte (MNPCE) frequency at the two highest concentrations tested. Polychromatic erythrocytes (PCE)/normochromatic erythrocytes (NCE) ratio was high for both combinations, suggesting that they can be mitogenic. Since genotoxicity may be related to carcinogenesis, it is necessary to conduct further studies to verify the long-term mutagenic effects of these drugs.

Kras, Egfr, and Tp53 Mutations in B6C3F1/N Mouse and F344/NTac Rat Alveolar/Bronchiolar Carcinomas Resulting from Chronic Inhalation Exposure to Cobalt Metal

Rodent lung tumors are morphologically similar to a subtype of human lung adenocarcinomas. The objective of this study was to evaluate Kirsten rat sarcoma oncogene homolog (Kras), epidermal growth factor receptor (Egfr), and tumor protein 53 (Tp53) mutations, which are relevant to human lung cancer, in cobalt metal dust (CMD)-induced alveolar/bronchiolar tumors of B6C3F1/N mice and F344/NTac rats. Kras mutations were detected in 67% (mice) and 31% (rats) of CMD-induced lung tumors and were predominantly exon 1 codon 12 G to T transversions (80% in mice and 57% in rats). Egfr mutations were detected in 17% (both mice and rats) of CMD-induced lung tumors and were predominantly in exon 20 with 50% G to A transitions (mice and rats). Tp53 mutations were detected in 19% (mice) and 23% (rats) of CMD-induced lung tumors and were predominant in exon 5 (mice, 69% transversions) and exon 6 (rats, all transitions). No mutations were observed for these genes in spontaneous lung tumors or normal lungs from untreated controls. Ames assay indicated that CMD is mutagenic in the absence but not in the presence of S9 mix. Thus, the mutation data (G to T transversions) and Ames assay results suggest that oxidative damage to DNA may be a contributing factor in CMD-induced pulmonary carcinogenesis in rodents.

Effect of antiretroviral drugs on the DNA damage in mice

In order to investigate the effects of two non-nucleoside reverse transcriptase inhibitors (NNRTIs) on the DNA damage in vivo, nevirapine (NVP; 3.3 mg/kg), efavirenz (EFV; 10 mg/kg) or saline were administered orally. Acute effects were analyzed 24 h after the administration of a single NNRTI dose, and subchronic effects 24 h after the last dose. Peripheral blood, brain, heart and liver samples were subjected to genotoxicity analyses and polychromatic erythrocytes from the bone marrow to micronucleus test. The micronucleus test did not reveal any significant differences between animals from the acute or subchronic groups. Comet assay showed that acute and subchronic NNRTI treatment did not cause any significant DNA damage in heart, liver or peripheral blood cells. However, increased damage indexes and frequencies were observed in the brain of mice, subchronically treated with EFV. This result suggests for the first time that this drug might induce genotoxicity in the brain.

Metabolic and mitochondrial effects of antiretroviral drug exposure in pregnancy and postpartum: implications for fetal and future health

Antiretroviral drugs (ARVs) are indispensable in the treatment and prevention of human immunodeficiency virus infection. Although their use before, during and after pregnancy is considered safe for mother and child, there are still lingering concerns about their long-term health consequences and the ramifications of their effects on lipid, glucose, intermediary and mitochondrial metabolism. This article reviews the known effects of ARVs on macromolecular and mitochondrial metabolism as well as the potential maternal, fetal, neonatal and adult health risks associated with abnormal energy metabolism during gestation. Recommendations about enhanced monitoring for these risks in affected populations are being provided.

An unusual phenotypic and genotypic expression in F2 generation following one stage zidovudine exposure during pregnancy and lactation- an experiment in mice

Zidovudine (3'-Azido-2', 3'-dideoxythymidine, AZT, ZDV) is routinely used as one of the component of antiretroviral therapy to prevent transmission of the HIV infection from mother to child. The drug, when given during pregnancy can give rise to myriad toxicities as reported in previous studies on human, animal and in-vitro experiments. The present study was an attempt to explore the Zidovudine teratogenesis in F1 and F2 generation of mice following initial maternal exposure to Zidovudine during pregnancy, through delivery and lactation. The F1 generation actually would have got the exposure during embryonic development and infant stages. Pregnant Swiss mice were treated orally with ZDV 50 mg/kg/day or distilled water (control), from day eighth of gestation, through delivery and continued for first ten days of lactation. The F1 generation litters were raised and mated to produce F2 generation mice. An interesting phenotype of "healthy" and "sick" was noted in F2 generation but not in the F1 generation. In F2 generation 35% died on different postnatal day during 120 days of follow up period. Chromosomal study from bone marrow of F1 and F2 showed various chromosomal aberrations. Lipodystrophy and hepatotoxicity was observed in "sick" mice. The study generated a hypothesis of recessive mutation and concludes that Zidovudine is a transplacental genotoxic agent. The result of present study therefore suggests the need to study the effect of zidovudine in human subjects for a longer period of time to rule out similar genotoxic effect.

XPC is essential for nucleotide excision repair of zidovudine-induced DNA damage in human hepatoma cells

Zidovudine (3'-azido-3'-dexoythymidine, AZT), a nucleoside reverse transcriptase inhibitor, can be incorporated into DNA and cause DNA damage. The mechanisms underlying the repair of AZT-induced DNA damage are unknown. To investigate the pathways involved in the recognition and repair of AZT-induced DNA damage, human hepatoma HepG2 cells were incubated with AZT for 2 weeks and the expression of DNA damage signaling pathways was determined using a pathway-based real-time PCR array. Compared to control cultures, damaged DNA binding and nucleotide excision repair (NER) pathways showed significantly increased gene expression. Further analysis indicated that AZT treatment increased the expression of genes associated with NER, including XPC, XPA, RPA1, GTF2H1, and ERCC1. Western blot analysis demonstrated that the protein levels of XPC and GTF2H1 were also significantly up-regulated. To explore further the function of XPC in the repair of AZT-induced DNA damage, XPC expression was stably knocked down by 71% using short hairpin RNA interference. In the XPC knocked-down cells, 100 μM AZT treatment significantly increased [³H]AZT incorporation into DNA, decreased the total number of viable cells, increased the release of lactate dehydrogenase, induced apoptosis, and caused a more extensive G2/M cell cycle arrest when compared to non-transfected HepG2 cells or HepG2 cells transfected with a scrambled short hairpin RNA sequence. Overall, these data indicate that XPC plays an essential role in the NER repair of AZT-induced DNA damage.

Metabolic complications associated with antiretroviral therapy in HIV-infected and HIV-exposed uninfected paediatric patients

IMPORTANCE OF THE FIELD

HIV-infection has become a chronic disease in paediatric patients with the potential for long-term survival and exposure to antiretroviral (ARV) therapies for 2 decades longer than HIV-infected adults. On the other hand, the administration of ARV to HIV-infected pregnant women has greatly increased both treatment of HIV infection and prevention of perinatal HIV transmission. Therefore, researches aiming to evaluate the safety of ARV therapies in HIV-infected children as well as in HIV-uninfected infants born to HIV-infected mothers are emerging as a new challenge and urgent priority.

AREAS COVERED IN THIS REVIEW

The purpose of this review is to describe some of the more concerning metabolic complications associated with ARV in paediatric population: hyperlactataemia (HLA) syndromes, body shape abnormalities, disorders of glucose homeostasis and dyslipidaemia in HIV-infected children and adolescents. Frequency, risk factors, clinical findings, prevention and intervention strategies of the previously described abnormalities are discussed in depth.

WHAT THE READER WILL GAIN

This review covers our current understanding of HLA syndromes in ARV-exposed uninfected infants born to HIV-infected mothers.

TAKE HOME MESSAGE

Prevention of these metabolic complications should assume prominence and future researches should address several of the existing treatment gaps.

Absence of mitochondrial toxicity in hearts of transgenic mice treated with abacavir

Abacavir (ABC) is a guanosine nucleoside reverse transcriptase inhibitor (NRTI) with potent antiretroviral activity. Since NRTIs exhibit tissue-specific inhibition of mitochondrial DNA (mtDNA) synthesis, the ability of ABC to inhibit mtDNA synthesis in vivo was evaluated. Inbred wild-type (WT) and transgenic mice (TG) treated with ABC (3.125 mg/d p. o., 35 days) were used to define mitochondrial oxidative stress and cardiac function. Chosen TGs exhibited overexpression of HIV-1 viral proteins (NL4-3Deltagag/pol, non-replication competent), hemizygous depletion or overexpression of mitochondrial superoxide dismutase (SOD2(+/-) knock-out (KO) or MnSOD OX, respectively), overexpression of mitochondrially targeted catalase (MCAT), or double "knockout" deletion of aldehyde dehydrogenase activity (ALDH2 KO). Impact on mtDNA synthesis was assessed by comparing changes in mtDNA abundance between ABC-treated and vehicle-treated WTs and TGs. No changes in mtDNA abundance occurred from ABC treatment in any mice, suggesting no inhibition of mtDNA synthesis. Left ventricle (LV) mass and LV end-diastolic dimension (LVEDD) were defined echocardiographically and remained unchanged as well. These results indicate that treatment with ABC has no visible cardiotoxicity in these adult mice exposed for 5 weeks compared to findings with other antiretroviral NRTI studies and support some claims for its relative safety.

Nucleoside reverse transcriptase inhibitor toxicity and mitochondrial DNA

IMPORTANCE OF THE FIELD

HIV/AIDS is a worldwide epidemic. While there remains no cure for the HIV-1 infection, nucleoside reverse transcriptase inhibitors (NRTIs) have helped transform the HIV-1 infection from a lethal disease into a chronic illness. Though NRTIs inhibit HIV-1 replication, they exhibit side effects in human tissues that appear to result from NRTI inhibition of human mitochondrial polymerase γ (pol γ).

AREAS COVERED IN THIS REVIEW

this review discusses the current knowledge of NRTI-induced toxicity, specifically the inhibition of pol γ and the mitochondrial toxicity from incorporation of NRTIs into mitochondrial DNA. Details are discussed about general mechanisms of NRTI toxicity and how specific tissue toxicities in mitochondria relate to clinical manifestation.

WHAT THE READER WILL GAIN

a detailed knowledge of the mitochondrial toxicity resulting from NRTI-inclusive therapies and related tissue toxicities are provided. This review presents both the molecular effects of NRTI usage on mitochondrial genetic homeostasis and energy metabolism as well as the clinical manifestations associated with NRTI toxicities.

TAKE HOME MESSAGE

NRTIs remain a critical component of current HIV-1 treatment regimens. Future NRTIs should provide higher specificity for HIV-RT and lower incorporation by pol γ to minimize mitochondrial toxicity. Alternatively, therapeutic interventions to prevent or alleviate mitochondrial toxicity should be addressed.

Transgenic mitochondrial superoxide dismutase and mitochondrially targeted catalase prevent antiretroviral-induced oxidative stress and cardiomyopathy

Transgenic mice (TG) were used to define mitochondrial oxidative stress and cardiomyopathy (CM) induced by zidovudine (AZT), an antiretroviral used to treat HIV/AIDS. Genetically engineered mice either depleted or overexpressed mitochondrial superoxide dismutase (SOD2(+/-) KOs and SOD2-OX, respectively) or expressed mitochondrially targeted catalase (mCAT). TGs and wild-type (WT) littermates were treated (oral AZT, 35 days). Cardiac mitochondrial H(2)O(2), aconitase activity, histology and ultrastructure were analyzed. Left ventricle (LV) mass and LV end-diastolic dimension were determined echocardiographically. AZT induced cardiac oxidative stress and LV dysfunction in WTs. Cardiac mitochondrial H(2)O(2) increased and aconitase was inactivated in SOD2(+/-) KOs, and cardiac dysfunction was worsened by AZT. Conversely, the cardiac function in SOD2-OX and mCAT hearts was protected. In SOD2-OX and mCAT TG hearts, mitochondrial H(2)O(2), LV mass and LV cavity volume resembled corresponding values from vehicle-treated WTs. AZT worsens cardiac dysfunction and increases mitochondrial H(2)O(2) in SOD2(+/-) KO. Conversely, both SOD2-OX and mCAT TGs prevent or attenuate AZT-induced cardiac oxidative stress and LV dysfunction. As dysfunctional changes are ameliorated by decreasing and worsened by increasing H(2)O(2) abundance, oxidative stress from H(2)O(2) is crucial pathogenetically in AZT-induced mitochondrial CM.

Mitochondrial function, morphology and metabolic parameters improve after switching from stavudine to a tenofovir-containing regimen

OBJECTIVES

HIV-associated lipoatrophy has been associated with mitochondrial dysfunction induced by nucleoside reverse transcriptase inhibitor therapy. We hypothesize that lipid profiles and markers of mitochondrial function will improve in HIV-lipoatrophic patients switched to the nucleotide analogue tenofovir.

METHODS

Ten patients receiving stavudine, lamivudine and lopinavir/ritonavir (Kaletra(R)) for over 6 years were switched from stavudine to tenofovir for 48 weeks. Subcutaneous fat tissue biopsies, fasting metabolic tests, HIV RNA, CD4 cell count and whole body dual energy X-ray absorptiometry (DEXA) scans were obtained at study entry and week 48. Mitochondrial DNA (mtDNA) copies/cell and mitochondrial morphology were assessed in adipose tissue biopsies, mtDNA 8-oxo-deoxyguanine in peripheral blood mononuclear cells, and glutathione (GSH) and F2-isoprostane in plasma.

RESULTS

There was no change in limb fat mass by DEXA; however, trunk fat mass increased by 18.9% (P = 0.01). Fasting total cholesterol decreased by 33 mg/dL (P = 0.005) and serum glucose decreased by 4 mg/dL (P = 0.039). mtDNA copies/cell increased from 386 to 1537 (P < 0.001). Transmission electron microscopy showed that mitochondrial cristae were lacking or poorly defined at study entry, whereas mitochondrial inner structures were more well defined and outer membranes were intact at 48 weeks. Oxidative damage decreased in 8/10 patients, GSH increased and F2-isoprostane decreased.

CONCLUSIONS

The results from this study demonstrate that systemic and peripheral fat mitochondria improve in patients switched to tenofovir following long-term exposure to stavudine, while continuing protease inhibitor therapy.

Mutational analysis of the mitochondrial tRNA genes and flanking regions in umbilical cord tissue from uninfected infants receiving AZT-based therapies for prophylaxis of HIV-1

A sensitive vertical denaturing gradient gel electrophoresis (DGGE) method, using 13 unipolar psoralen-clamped PCR primer pairs, was developed for detecting sequence variants in the 22 tRNA genes and flanking regions (together spanning approximately 21%) of the human mitochondrial genome. A study was conducted to determine (i) if mitochondrial DNA (mtDNA) polymorphisms and/or mutations were detectable in healthy newborns and (ii) if prepartum 3'-azido-2',3'-dideoxythymidine (AZT) based HIV-1 prophylaxis was associated with significant increases in mtDNA mutations and changes in the degree of heteroplasmy of sequence variants in uninfected infants born to HIV-1-infected mothers. DGGE analysis of umbilical cord tissue (where vascular endothelium and smooth muscle cells are the major source of mtDNA) showed that mtDNA sequence variants were significantly elevated by threefold in AZT-treated infants compared with unexposed controls (P < 0.001), with 24 changes observed in 19/52 (37%) treated newborns (averaging 0.46 changes/subject) versus only eight changes found in 7/55 (13%) unexposed newborns (averaging 0.15 changes/subject). Six distinct sequence variants occurring in unexposed controls were predominately synonymous and homoplasmic, representing previously reported polymorphisms. Uninfected infants exposed to a combination of AZT and 2',3'-dideoxy-3'-thiacytidine and "maternal HIV-1" had a significant shift in the spectrum of mutations (P = 0.04) driven by increases in nonsynonymous heteroplasmic sequence variants at polymorphic sites (10 distinct variants) and novel sites (four distinct variants). While the weight of evidence suggests that prepartum AZT-based prophylaxis produces mtDNA mutations, additional research is needed to determine the degree to which fetal responses to maternal HIV-1 infection, in the absence of antiretroviral treatment, contribute to prenatal mtDNA mutagenesis.

K-ras cancer gene mutations in lung tumors from female Swiss (CD-1) mice exposed transplacentally to 3'-azido-3'-deoxythymidine

A transplacental carcinogenicity study was conducted by exposing pregnant Swiss (CD-1) mice to 0, 50, 100, 200, or 300 mg 3'-azido-3'-deoxythymidine (AZT)/kg body weight (BW) daily for the duration of gestation (18-19 days) [National Toxicology Program,2006]. The incidence of alveolar/bronchiolar adenomas and carcinomas in the 200 and 300 mg/kg groups was significantly higher (P = 0.027 and 0.007, respectively) in male offspring, but not in females (P = 0.338 and 0.315, respectively). The purpose of the present study was to evaluate K-ras mutation status in lung tumors from the female offspring in AZT exposed groups and to determine whether at the molecular level there were signature K-ras mutations in lung tumors that were different from spontaneous tumors. K-ras mutation was detected by cycle sequencing of polymerase chain reaction (PCR)-amplified DNA, isolated from formalin-fixed, paraffin-embedded lung tumors. K-ras mutations were detected in 17 of 28 (61%) lung tumors from the female offspring in AZT exposed groups. No K-ras mutations were detected in the 8 tumors examined from the female control group. The predominant mutations were Codon 12 G-->T transversions in the 50, 100, and 300 mg/kg groups, and Codon 12 G-->C transversions in the 200 and 300 mg/kg groups. K-ras Codon 12 G-->T transversions (TGT mutations) may be induced by oxidative DNA damage and 8-oxoguanine (8-oxoG), while K-ras Codon 12 G-->C transversions (CGT mutations) may be due to further oxidative lesions of guanine and 8-oxoG.

Use of the alkaline comet assay for the detection of transplacental genotoxins in newborn mice

Several lines of evidence show that in utero exposure to different toxicants has greater consequences than their exposure during adult life. This may be due to involvement of critical developmental stages, physiological immaturity and the long later-life span over which disease may initiate, develop and progress. The in vivo alkaline comet (single-cell gel electrophoresis) assay has been favoured by the scientific community for the evaluation of genotoxins. The objective of this study was to demonstrate the suitability of alkaline comet assay in detecting transplacental genotoxins using newborn mice. Here, we report the successful use of the comet assay in detecting multi-organ genotoxicity of known transplacental genotoxins in newborn mice. Three well known transplacental genotoxic agents, cyclophosphamide (CP), mitomycin-C (MMC) and zidovudine (AZT) were tested in pregnant Swiss mice. These compounds were administered in the late gestational period (16-20th days of pregnancy) and the comet assay was performed with lymphocytes, bone marrow, liver and kidney cells of newborn mice. Significant DNA damage was observed in all the tissues with tested transplacental genotoxins. The results of the comet assay were confirmed by the micronucleus (MN) assay of the peripheral blood of newborn mice. The results of this study provide sufficient evidence that the comet assay can be applied successfully for the detection of transplacental genotoxins in newborn mice.

Mitochondrial disease in the offspring as a result of antiretroviral therapy

Nucleoside analogue reverse transcriptase inhibitors (NRTIs) have substantially lowered the risk of the mother-to-child transmission of HIV. Evidence of mitochondrial toxicity in vitro, in animal models and in adult HIV-infected patients, has raised concern about the perinatal safety of these antiretrovirals. In zidovudine-exposed, but HIV-uninfected infants, transient anaemia and additional long-term blood abnormalities (neutropenia, thrombopenia and lymphopenia) and hyperlactatemia have been documented. The overall risk of mortality and congenital abnormalities does not appear to be increased, but rare mitochondrial events cannot be excluded for lack of statistical power. French data suggest an above background incidence of mitochondrial symptomatology. Preclinical data demonstrate zidovudine also to be a carcinogen. Long-term systematic follow-up of exposed babies in large cohorts is needed, as are randomised trials with NRTIs carrying a lower risk of mitochondrial toxicity.

Metabolism and pharmacokinetics of the combination Zidovudine plus Lamivudine in the adult Erythrocebus patas monkey determined by liquid chromatography-tandem mass spectrometric analysis

Because of their similarity to humans, non-human primates constitute useful preclinical models in which to examine potential human drug toxicities. Antiretroviral nucleoside reverse transcriptase inhibitor (NRTI) toxicity is currently under investigation in Erythrocebus patas monkeys, and whereas NRTI pharmacokinetics have been studied in other monkey species, pharmacokinetics for Zidovudine plus Lamivudine (AZT/3TC) dosing have not been reported in the patas. Here we present 24 h serum pharmacokinetic parameters after a single oral exposure to the combination of AZT (40 mg) and 3TC (24 mg), doses equivalent to a human daily dose of Combivir. The patas (n=3) AZT/3TC pharmacokinetic profiles were similar to those seen in other primate species. Average maximum serum concentrations (Cmax) for AZT and 3TC were 2.35 and 2.65 microg/ml, respectively, and were observed at 0.83 h (Tmax). Cmax was 13.34 microg/ml for the AZT-glucuronide (AZT-G) and was 0.023 microg/ml for the potentially toxic minor metabolite 3'-amino-3'-deoxythymidine (AMT), both occurring at about 1 h after dosing. Similar elimination half-times, 0.70 and 0.68 h(-1), were found for AZT and AZT-G, respectively, while 3TC was eliminated about half as fast (0.33 h(-1)) resulting in AUC(0-infinity) values of 6.97 microg/ml h for 3TC, 2.99 microg/ml h for AZT, 20.5 microg/ml h for AZT-G and 0.002 for AMT 6.97 microg/ml h. This study shows similar metabolism and pharmacokinetics for oral administration of AZT/3TC in the adult patas monkey, other primate species and humans. The data validate the use of the patas monkey for studies of NRTI toxicity.

In situ hybridization and immunolocalization of concentrative and equilibrative nucleoside transporters in the human intestine, liver, kidneys, and placenta

To better understand the role of human equilibrative (hENTs) and concentrative (hCNTs) nucleoside transporters in physiology and pharmacology, we investigated the regional, cellular, and spatial distribution of two hCNTs (hCNT1 and hCNT2) and two hENTs (hENT1 and hENT2) in four human tissues. Using in situ hybridization and immunohistochemical techniques, we found that the duodenum expressed hCNT1 and hCNT2 mRNAs in enterocytes and hENT1 and hENT2 mRNAs in crypt cells. In these cells, the hCNT and hENT proteins were predominantly localized in the apical and lateral membrane, respectively. Hepatocytes expressed higher levels of mRNAs of hENT1, hCNT1, and hENT2 than of hCNT2 and expressed all these proteins at hepatocyte cell borders and in the cytoplasm. While the kidney expressed hCNT1 and hCNT2 mRNAs in the proximal tubules, hENT1 and hENT2 mRNAs were present in the distal tubules, glomeruli, endothelial cells, and vascular smooth muscle cells. Proximal tubules adjacent to corticomedullary junctions expressed hENT1, hCNT1, and hCNT2 mRNA. Immunolocalization studies revealed predominant localization of hCNTs in the brush-border membrane of the proximal tubular epithelial cells and hENTs in the basolateral membrane of the distal tubular epithelial cells. Chorionic villi sections of human term placenta expressed mRNAs and proteins for hENT1 and hENT2 but only mRNA for hCNT2. Immunolocalization studies showed presence of hENT1 in the brush-border membrane of the syncytiotrophoblasts. These data are critical for a better understanding of the role of nucleoside transporters in the physiological and pharmacological effects of nucleosides and nucleoside drugs, respectively.

In utero-initiated cancer: the role of reactive oxygen species

It is becoming more evident that not only can drugs and environmental chemicals interfere with normal fetal development by causing structural malformations, such as limb defects, but that xenobiotic exposure during development can also cause biochemical and functional abnormalities that may ultimately lead to cancer later on in life. Fetal toxicity may be partly mediated by the embryonic bioactivation of xenobiotics to free radical intermediates that can lead to oxidative stress and potentially lead, in some cases, to carcinogenesis. Using a number of examples, this review will focus on the role of reactive oxygen species (ROS) in the mechanisms pertaining to in utero initiated cancers.

Frequency of Hprt mutant lymphocytes and micronucleated erythrocytes in p53-haplodeficient mice treated perinatally with AZT and AZT in combination with 3TC

Azidothymidine (AZT) is a nucleoside reverse transcriptase inhibitor (NRTI) that is used for reducing mother-to-child transmission of human immunodeficiency virus I. Combinations of AZT and 3'-thiacytidine (3TC) are even more effective than AZT alone. AZT, however, is a mutagen and carcinogen in rodent models and 3TC can increase the genotoxicity of AZT. Since p53 plays a key role in human and mouse tumorigenesis, p53-haplodeficient mice are currently being evaluated as a model for assessing the carcinogenicity of perinatal exposure to NRTIs. In the present study, male C57BL/6 p53(+/+) and p53(-/-) mice were mated with C3H p53(+/+) females; the pregnant females were treated on gestation day 12 through parturition with 40, 80, and 160 mg/kg of AZT or a combination of 160 mg/kg AZT and 100 mg/kg 3TC (AZT-3TC); the p53(+/+) and p53(+/-) offspring were treated daily after birth through postnatal day (PND) 28. The frequencies of micronucleated reticulocytes (MN-RETs) and micronucleated normochromatic erythrocytes (MN-NCEs) were determined on PND1, PND10, and PND28; the frequency of Hprt mutant lymphocytes was measured on PND28. The frequencies of MN-RETs and MN-NCEs were increased in treated animals at all time points; there were no differences in the responses of p53(+/+) and p53(+/-) animals treated with identical doses of NRTIs. After correction for clonal expansion, both AZT and AZT-3TC treatments induced small but significant increases in the frequency of Hprt mutant lymphocytes in p53(+/-) mice, but not in p53(+/+) mice. The data indicate that p53 haplodeficiency affects the genotoxicity of NRTIs; thus, p53(+/-) mice may be a sensitive model for evaluating the carcinogenicity of perinatal exposure to NRTIs.

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.

Transplacental drug transfer and frequency of Tk and Hprt lymphocyte mutants and peripheral blood micronuclei in mice treated transplacentally with zidovudine and lamivudine

In previous studies, we have shown that zidovudine (3'-azido-3'-deoxythymidine; AZT), but not lamivudine [(-)2',3'-dideoxy-3'-thiacytidine; 3TC], is genotoxic when administered to neonatal mice, and that 3TC when coadministered with AZT does not alter the responses observed with AZT alone (Von Tungeln et al. [2002] Carcinogenesis 23:1427-1432). We now have investigated the transplacental transfer of these drugs and the induction of mutants and micronuclei in the neonatal offspring. From gestational day 12 until parturition, female C57BL/6N and C57BL/6N/Tk(+/-) mice, which had been mated to male C3H/HeNMTV mice, were treated daily by gavage with AZT, 3TC, or a combination of AZT and 3TC. In both dams and fetuses, AZT was found at much higher levels than its metabolites, AZT 5'-glucuronide and 3'-azido-3'-deoxythymidine. In the neonates, AZT and the mixture of AZT and 3TC caused a decrease in the percentage of reticulocytes (RETs) and an increase in the percentage of micronucleated RETs and micronucleated normochromatic erythrocytes. When assessed 3 weeks after birth, AZT and the combination of AZT and 3TC increased the thymidine kinase (Tk) mutant frequency in male mice; at 5 weeks, 3TC increased the Tk mutant frequency in female mice. The increase in Tk mutants in mice treated with AZT and the mixture of AZT and 3TC was associated with loss of the wild-type (Tk(+)) allele (loss of heterozygosity; LOH) and a pattern of discontinuous LOH. These data indicate that AZT, 3TC, and the combination of AZT and 3TC are transplacental mutagens and that the increase in mutants resulting from AZT is due mainly to large-scale genetic alterations.

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.

Concentration of Nucleosides and Related Compounds in Cerebral and Cerebellar Cortical Areas and White Matter of the Human Brain

1. Nucleosides potentially participate in the neuronal functions of the brain. However, their distribution and changes in their concentrations in the human brain is not known. For better understanding of nucleoside functions, changes of nucleoside concentrations by age and a complete map of nucleoside levels in the human brain are actual requirements. 2. We used post mortem human brain samples in the experiments and applied a recently modified HPLC method for the measurement of nucleosides. To estimate concentrations and patterns of nucleosides in alive human brain we used a recently developed reverse extrapolation method and multivariate statistical analyses. 3. We analyzed four nucleosides and three nucleobases in human cerebellar, cerebral cortices and in white matter in young and old adults. Average concentrations of the 308 samples investigated (mean+/-SEM) were the following (pmol/mg wet tissue weight): adenosine 10.3+/-0.6, inosine 69.5+/-1.7, guanosine 13.5+/-0.4, uridine 52.4+/-1.2, uracil 8.4+/-0.3, hypoxanthine 108.6+/-2.0 and xanthine 54.8+/-1.3. We also demonstrated that concentrations of inosine and adenosine in the cerebral cortex and guanosine in the cerebral white matter are age-dependent. 4. Using multivariate statistical analyses and degradation coefficients, we present an uneven regional distribution of nucleosides in the human brain. The methods presented here allow to creation of a nucleoside map of the human brain by measuring the concentration of nucleosides in microdissected tissue samples. Our data support a functional role for nucleosides in the brain.

Nature of DNA lesions induced in human hepatoma cells, human colonic cells and human embryonic lung fibroblasts by the antiretroviral drug 3'-azido-3'-deoxythymidine

This study tried to clarify the question if nuclear genotoxicity played a role in 3'-azido-3'-deoxythymidine (AZT) toxicity. We investigated cytotoxic and DNA-damaging effects of AZT on human hepatoma HepG2 and human colonic CaCo-2 cells as well as on human diploid lung fibroblasts HEL. The amount of induced DNA damage was measured by standard alkaline single cell gel electrophoresis (SCGE). The nature of induced DNA lesions was evaluated (1) by modified SCGE, which includes treatment of lysed cells with DNA repair enzymes Endo III and Fpg and enables to recognize oxidized bases of DNA, and (2) by SCGE processed in parallel at pH 13.0 (standard technique) and pH 12.1, which enables to recognize alkali labile DNA lesions and direct DNA strand breaks. Cytotoxicity of AZT was evaluated by the trypan blue exclusion technique. Our findings showed that 3-h treatment of cells with AZT decreased the viability of all cell lines studied. SCGE performed in the presence of DNA repair enzymes proved that AZT induced oxidative lesions to DNA in all cell types. In hepatoma HepG2 cells and embryonic lung fibroblasts HEL the majority of AZT-induced DNA strand breaks were pH-independent, i.e. they were identified at both pH values (12.1 and 13.0). These DNA lesions represented direct DNA breaks. In colonic Caco-2 cells DNA lesions were converted to DNA strand breaks particularly under strong alkaline conditions (pH>13.0), which is characteristic for alkali-labile sites of DNA. DNA strand break rejoining was investigated by the standard comet assay technique during 48 h of post-AZT-treatment in HepG2 and Caco-2 cells. The kinetics of DNA rejoining, considered an indicator of DNA repair, revealed that AZT-induced DNA breaks were repaired in both cell types slowly, though HepG2 cells seemed to be more repair proficient with respect to AZT-induced DNA lesions.

AZT binds RNA at multiple sites

Azidothymidine (AZT) is a widely used inhibitor of type I human immunodeficiency virus (HIV) reverse transcriptase that act as a DNA chain terminator. Studies have shown primer unblocking and rescue of DNA synthesis AZT-resistant HIV-1 reverse transcriptase on DNA and RNA templates. Our recent study showed AZT bindings to the G-C, A-T base pairs and the backbone phosphate group of DNA duplex resulting in partial DNA conformational changes. This study was designed to examine the interaction of AZT with RNA in aqueous solution at physiological condition, using different drug/RNA (phosphate) molar ratios of 1/800 to 1/2 and constant RNA concentration of 1.25 or 12.5 mM (phosphate). Capillary electrophoresis, FTIR, and UV-visible difference spectroscopic methods and molecular modeling were used to determine the drug binding sites, binding constants, and the effects of AZT complexation on RNA conformation. Structural analysis showed that AZT binds RNA through G-C and A-U bases with two binding constants of K1=7.3 x 10(5) M(-1) and K2=1.90 x 10(5) M(-1). The drug distributions were 54% with G-C, 36% A-U, and 10% with the backbone phosphate group. RNA remains in A-family structure and drug sugar pucker in the C2'-endo/anti conformation in the AZT-RNA complexes. Molecular modeling studies show hydrogen bondings between RNA and AZT donor groups.

Prenatal Environmental Tobacco Smoke Exposure Promotes Adult Atherogenesis and Mitochondrial Damage in Apolipoprotein E −/− Mice Fed a Chow Diet

Background— Environmental tobacco smoke (ETS) exposure is recognized as a cardiovascular disease risk factor; however, the impact of prenatal ETS exposure on adult atherogenesis has not been examined. We hypothesized that in utero ETS exposure promotes adult atherosclerotic lesion formation and mitochondrial damage.

Methods and Results— Atherosclerotic lesion formation, mitochondrial DNA damage, antioxidant activity, and oxidant load were determined in cardiovascular tissues from adult apolipoprotein E −/− mice exposed to either filtered air or ETS in utero and fed a standard chow diet (4.5% fat) from weaning until euthanasia. All parameters were significantly altered in male mice exposed in utero to ETS.

Conclusions— These data support the hypothesis that prenatal ETS exposure is sufficient to promote adult cardiovascular disease development.

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.

Mitochondrial oxidative stress in human hepatoma cells exposed to stavudine

The toxicity of nucleoside reverse transcriptase inhibitors (NRTIs) is linked to altered mitochondrial DNA (mtDNA) replication and subsequent disruption of cellular energetics. This manifests clinically as elevated concentrations of lactate in plasma. The mechanism(s) underlying how the changes in mtDNA replication lead to lactic acidosis remains unclear. It is hypothesized that mitochondrial oxidative stress links the changes in mtDNA replication to mitochondrial dysfunction and ensuing NRTIs toxicity. To test this hypothesis, changes in mitochondrial function, mtDNA amplification efficiency, and oxidative stress were assessed in HepG2-cultured human hepatoblasts treated with the NRTI stavudine (2',3'-didehydro-2',3'-deoxythymidine or d4T) for 48 h. d4T produced significant mitochondrial dysfunction with a 1.5-fold increase in cellular lactate to pyruvate ratios. In addition, d4T caused a dose-dependent decrease in mtDNA amplification and a correlative increase in abundance of markers of mitochondrial oxidative stress. Manganese (III) meso-tetrakis (4-benzoic acid) porphyrin, MnTBAP, a catalytic antioxidant, ameliorated or reversed d4T-induced changes in cell injury, energetics, mtDNA amplification, and mitochondrial oxidative stress. In conclusion, d4T treatment elevates mitochondrial reactive oxygen species (ROS), enhances mitochondrial oxidative stress, and contributes mechanistically to NRTI-induced toxicity. These deleterious events may be potentiated in acquired immunodeficiency syndrome (AIDS) by human immunodeficiency virus (HIV) infection itself, coinfection (e.g., viral hepatitis), aging, substance, and alcohol use.

AZT-DNA interaction

Oxidative DNA damage has been reported in fetal tissues by exposure to 3'-azido-3'-deoxythymidine (AZT). AZT has been used effectively for the treatment of human immunodeficiency virus-1 (HIV-1) and AIDS. It showed in vitro to block the nucleoside-binding site of the viral reverse transcriptase and to inhibit DNA replication by chain termination. It incorporates into both nuclear and mitochondrial DNA and is shown to cause cancer in vivo and in vitro. This study was designed to examine the interaction of AZT with DNA in aqueous solution at physiological condition, using different drug/DNA (phosphate) molar ratios (r) of 1/80 to 1/2 and constant DNA concentration of 12.5 mM (phosphate). Capillary electrophoresis, FTIR, and UV-visible difference spectroscopic methods and molecular modeling were used to determine the drug binding sites, the binding constants and the effects of the AZT complexation on DNA conformation. Structural analysis showed that AZT binds to DNA through G-C and A-T base pairs and the backbone PO(2) groups with two binding constants of K(1) = 2.60 x 10(5) M(-1) and K(2) = 1.20 x 10(5) M(-1). The drug distributions are 50% with G-C, 15% with A-T and 35% with the backbone phosphate group. AZT-DNA interaction is associated with a partial B- to A-DNA conformational transition.

Mitochondrial toxicity of NRTI antiviral drugs: an integrated cellular perspective

Highly active antiretroviral therapy (HAART) regimes based on nucleoside reverse transcriptase inhibitors (NRTIs) have revolutionized the treatment of AIDS in recent years. Although HAART can successfully suppress viral replication in the long term, it is not without significant toxicity, which can seriously compromise treatment effectiveness. A major toxicity that has been recognized for more than a decade is NRTI-related mitochondrial toxicity, which manifests as serious side effects such as hepatic failure and lactic acidosis. However, a lack of understanding of the mechanisms underlying mitochondrial toxicity has hampered efforts to develop novel drugs with better side-effect profiles. This review characterizes the pharmacological mechanisms and pathways that are involved in mitochondrial dysfunction caused by NRTIs, and suggests opportunities for future pharmacological research.

Ontogenic differences in human liver 4-hydroxynonenal detoxification are associated with in vitro injury to fetal hematopoietic stem cells

4-hydroxynonenal (4HNE) is a highly mutagenic and cytotoxic alpha,beta-unsaturated aldehyde that can be produced in utero during transplacental exposure to prooxidant compounds. Cellular protection against 4HNE injury is provided by alcohol dehydrogenases (ADH), aldehyde reductases (ALRD), aldehyde dehydrogenases (ALDH), and glutathione S-transferases (GST). In the present study, we examined the comparative detoxification of 4HNE by aldehyde-metabolizing enzymes in a panel of adult and second-trimester prenatal liver tissues and report the toxicological ramifications of ontogenic 4HNE detoxification in vitro. The initial rates of 4HNE oxidation and reduction were two- to fivefold lower in prenatal liver subcellular fractions as compared to adult liver, and the rates of GST conjugation of 4HNE were not detectable in either prenatal or adult cytosolic fractions. GSH-affinity purification of hepatic cytosol yielded detectable and roughly equivalent rates of GST-4HNE conjugation for the two age groups. Consistent with the inefficient oxidative and reductive metabolism of 4HNE in prenatal liver, cytosolic fractions prepared from prenatal liver exhibited a decreased ability to protect against 4HNE-protein adduct formation relative to adults. Prenatal liver hematopoietic stem cells (HSC), which constitute a significant percentage of prenatal liver cell populations, exhibited ALDH activities toward 4HNE, but little reductive or conjugative capacity toward 4HNE through ALRD, ADH, and GST. Cultured HSC exposed to 5 microM 4HNE exhibited a loss in viability and readily formed one or more high molecular weight 4HNE-protein adduct(s). Collectively, our results indicate that second trimester prenatal liver has a lower ability to detoxify 4HNE relative to adults, and that the inefficient detoxification of 4HNE underlies an increased susceptibility to 4HNE injury in sensitive prenatal hepatic cell targets.

Oxidative DNA damage induced by photodegradation products of 3(')-azido-3(')-deoxythymidine

3(')-Azido-3(')-deoxythymidine (AZT) is carcinogenic to experimental animals and can cause the formation of 8-oxo-7,8-dihydro-2(')-deoxyguanosine (8-oxodG) in humans and animals. To clarify the mechanism of carcinogenesis by AZT, we investigated DNA damage induced by its photodegradation products, using 32P-5(')-end-labeled DNA fragments obtained from human genes. Following exposure to UVB, AZT induced DNA damage in the presence of Cu(II). Catalase inhibited DNA damage, indicating the involvement of H(2)O(2). UVB-exposed AZT plus Cu(II) induced 8-oxodG formation in a dose-dependent manner. Mass spectrum of UVB-exposed AZT demonstrated the generation of a hydroxylamine derivative. The colorimetric determination suggested that AZT was converted into the hydroxylamine derivative depending on UVB doses. UVB-exposed AZT induced double base damage at the 5(')-ACG-3(') sequence, complementary to a hot spot of the p53 gene. The basic compound, hydroxylamine, showed similar site specificity. The hydroxylamine derivative produced by photodegradation and/or possible metabolism of AZT induces oxidative DNA damage, which may participate in carcinogenesis.

Mitochondrial DNA replication, nucleoside reverse-transcriptase inhibitors, and AIDS cardiomyopathy

Nucleoside reverse-transcriptase inhibitors (NRTIs) in combination with other antiretrovirals (HAART) are the cornerstones of current AIDS therapy, but extensive use brought mitochondrial side effects to light. Clinical experience, pharmacological, cell, and molecular biological evidence links altered mitochondrial (mt-) DNA replication to the toxicity of NRTIs in many tissues, and conversely, mtDNA replication defects and mtDNA depletion in target tissues are observed. Organ-specific pathological changes or diverse systemic effects result from and are frequently attributed to HAART in which NRTIs are included. The shared features of mtDNA depletion and energy depletion became key observations and related the clinical and in vivo experimental findings to inhibition of mtDNA replication by NRTI triphosphates in vitro. Subsequent to those findings, other observations suggested that mitochondrial energy deprivation is concomitant with or the result of mitochondrial oxidative stress in AIDS (from HIV, for example) or from NRTI therapy itself.

Mitochondrial dysfunction and antiretroviral nucleoside analog toxicities: what is the evidence?

Mitochondrial dysfunction has been associated with long-term toxicities of human immunodeficiency virus (HIV) therapy, particularly with the nucleoside analog reverse transcriptase inhibitors (NRTIs). Lactic acidosis, hepatic steatosis, myopathies, cardiomyopathies, neuropathies, and lipodystrophy are frequently attributed to mitochondrial toxicity. Since mitochondrial toxicity could pose a major threat to the long-term success of HIV therapy, the scientific evidence underlying an association between mitochondrial toxicity and antiretroviral therapies, must be carefully examined. There is some data to support the association between NRTIs and mitochondria dysfunction. In this review, we examine human, animal, and in vitro data implicating mitochondrial dysfunction as the causal mechanism of NRTI-associated toxicity in HIV-infected patients.

Oxidative stress in the human fetal brain: an immunohistochemical study

Because accumulation of oxidative modification products seems to relate to aging and has not been fully studied in fetal brains, an immunohistochemical examination was performed on nine brains ranging from 22-40 weeks of gestation. These brains did not demonstrate lesions except hypoxic-ischemic changes. Advanced glycation end products and 4-hydroxynonenal are generally reported to be negative in neurons of normal young brains, but, in the present study, distinct positive immunoreaction was observed in neurons of fetal brains. Positive immunoreaction appeared earlier in the medulla oblongata than in the cerebrum, and 4-hydroxynonenal began to accumulate earlier than advanced glycation end products. As for glial cells, advanced glycation end products and 4-hydroxynonenal were positive in reactive astrocytes in mid- to late gestation. Because hypoxic-ischemic changes were observed in most of the patients, it is possible that oxidative stress caused by hypoxic-ischemic may be involved in the accumulation of these products in the fetal brain. 8-Hydroxy-2'-deoxyguanosine was negative even in patients demonstrating positive reaction for advanced glycation end products and 4-hydroxynonenal. In the fetal brain, DNA might be strongly protected from oxidative damage. 4-Hydroxynonenal is generally positive in the cytoplasm but was positive in the nucleus of immature neurons and glial cells in the present study, suggesting a unique metabolism of the fetal brain.

Mitochondria in the pathogenesis of lipodystrophy induced by anti-HIV antiretroviral drugs: actors or bystanders?

Effective therapies are now available that can stop the progression of HIV infection and significantly delay the onset of AIDS. The "highly active antiretroviral therapy" (HAART) is a combination of potent antiretroviral drugs such as viral protease inhibitors or nucleoside-analogue reverse-transcriptase inhibitors, that has a variety of serious side effects, including lipodystrophy, a pathology characterized by accumulation of visceral fat, breast adiposity, cervical fat-pads, hyperlipidemia, insulin resistance as well as fat wasting in face and limbs. There is still an open debate that concerns the precise responsibility of HAART as well as metabolic pathways and mechanisms that are involved in the onset of lipodystrophy. The similarities with multiple symmetric lipomatosis (MSL), in which mitochondria impairment plays a crucial role, lead to the hypothesis that drug-induced damages to mitochondrial DNA are able to alter mitochondria functionality to an extent that is similar to what occurs in MSL. In addition, several evidences indicate that HAART is also linked to a deregulated production of tumour necrosis factor-alpha, which uses mitochondria as intracellular targets. In this paper, we review data concerning the role of mitochondria in the pathogenesis of lipodystrophy, and advance a unifying hypothesis involving either direct or indirect effects of the drugs employed during HAART.

Differential incorporation and removal of antiviral deoxynucleotides by human DNA polymerase gamma

Mitochondrial toxicity can result from antiviral nucleotide analog therapy used to control human immunodeficiency virus type 1 infection. We evaluated the ability of such analogs to inhibit DNA synthesis by the human mitochondrial DNA polymerase (pol gamma) by comparing the insertion and exonucleolytic removal of six antiviral nucleotide analogs. Apparent steady-state K(m) and k(cat) values for insertion of 2',3'-dideoxy-TTP (ddTTP), 3'-azido-TTP (AZT-TP), 2',3'-dideoxy-CTP (ddCTP), 2',3'-didehydro-TTP (D4T-TP), (-)-2',3'-dideoxy-3'-thiacytidine (3TC-TP), and carbocyclic 2',3'-didehydro-ddGTP (CBV-TP) indicated incorporation of all six analogs, albeit with varying efficiencies. Dideoxynucleotides and D4T-TP were utilized by pol gamma in vitro as efficiently as natural deoxynucleotides, whereas AZT-TP, 3TC-TP, and CBV-TP were only moderate inhibitors of DNA chain elongation. Inefficient excision of dideoxynucleotides, D4T, AZT, and CBV from DNA predicts persistence in vivo following successful incorporation. In contrast, removal of 3'-terminal 3TC residues was 50% as efficient as natural 3' termini. Finally, we observed inhibition of exonuclease activity by concentrations of AZT-monophosphate known to occur in cells. Thus, although their greatest inhibitory effects are through incorporation and chain termination, persistence of these analogs in DNA and inhibition of exonucleolytic proofreading may also contribute to mitochondrial toxicity.