Molecular mechanism of the short-term cardiotoxicity caused by 2',3'-dideoxycytidine (ddC): modulation of reactive oxygen species levels and ADP-ribosylation reactions

Mitochondrial defects arise from nucleoside/nucleotide reverse transcriptase inhibitors in neurons: Potential contribution to HIV-associated neurocognitive disorders

The cornerstone of current HIV treatment is a class of drugs called nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs). However, patients who receive long term treatment with NRTIs often develop severe side effects, which are related to mitochondrial toxicity. The potential contribution of NRTI-mediated toxicity to HIV-associated neurocognitive disorders (HAND) has not been fully explored. NRTI toxicity is thought to be mediated through mitochondrial DNA polymerase γ (pol γ) inhibition, which impairs mitochondrial DNA (mtDNA) synthesis and leads to various mitochondrial dysfunctions. To evaluate the relationship between NRTI-mediated pol γ inhibition and mitochondrial toxicity in neurons, we systematically investigated mitochondrial regulation in NRTI-treated primary cortical neurons by measuring parameters related to mtDNA content, retrograde signaling responses and mitochondrial homeostasis. The effects of four different NRTIs with variable pol γ inhibitory activity and mitochondrial toxicity were assessed. The strong pol γ inhibitor, ddI, abolished mtDNA synthesis and greatly reduced mtDNA content. However, mtDNA transcription was not as severely affected, and no defects in oxidative phosphorylation were observed. Detrimental effects on mitochondrial respiration and motility were observed after AZT treatment in the absence of mtDNA depletion or inhibition of mtDNA synthesis. The results suggest that individual NRTIs, such as ddI and AZT, have the potential to cause mitochondrial toxicity in neurons. This mitochondrial toxicity would be expected to contribute to neurotoxicity in the central nervous system, and therefore, HAND etiology may be affected by NRTI treatment.

Premature and accelerated aging: HIV or HAART?

Highly active antiretroviral therapy (HAART) has significantly increased life expectancy of the human immunodeficiency virus (HIV)-positive population. Nevertheless, the average lifespan of HIV-patients remains shorter compared to uninfected individuals. Immunosenescence, a current explanation for this difference invokes heavily on viral stimulus despite HAART efficiency in viral suppression. We propose here that the premature and accelerated aging of HIV-patients can also be caused by adverse effects of antiretroviral drugs, specifically those that affect the mitochondria. The nucleoside reverse transcriptase inhibitor (NRTI) antiretroviral drug class for instance, is known to cause depletion of mitochondrial DNA via inhibition of the mitochondrial specific DNA polymerase-γ. Besides NRTIs, other antiretroviral drug classes such as protease inhibitors also cause severe mitochondrial damage by increasing oxidative stress and diminishing mitochondrial function. We also discuss important areas for future research and argue in favor of the use of Caenorhabditis elegans as a novel model system for studying these effects.

Pathogenesis of HIV-associated sensory neuropathy: evidence from in vivo and in vitro experimental models

HIV-associated sensory neuropathy (HIV-SN) is a frequent neurological complication of HIV infection and its treatment with some antiretroviral drugs. We review the pathogenesis of the viral- and drug-induced causes of the neuropathy, and its primary symptom, pain, based on evidence from in vivo and in vitro models of HIV-SN. Viral coat proteins mediate nerve fibre damage and hypernociception through direct and indirect mechanisms. Direct interactions between viral proteins and nerve fibres dominate axonal pathology, while somal pathology is dominated by indirect mechanisms that occur secondary to virus-mediated activation of glia and macrophage infiltration into the dorsal root ganglia. The treatment-induced neuropathy and resulting hypernociception arise primarily from drug-induced mitochondrial dysfunction, but the sequence of events initiated by the mitochondrial dysfunction that leads to the nerve fibre damage and dysfunction are still unclear. Overall, the models that have been developed to study the pathogenesis of HIV-SN, and hypernociception associated with the neuropathy, are reasonable models and have provided useful insights into the pathogenesis of HIV-SN. As new models are developed they may ultimately lead to identification of therapeutic targets for the prevention or treatment of this common neurological complication of HIV infection.

Iatrogenic mitochondriopathies: a recent lesson from nucleoside/nucleotide reverse transcriptase inhibitors

The use of nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) has revolutionized the treatment of infection by human immunodeficiency virus (HIV) and hepatitis-B virus. NRTIs can suppress viral replication in the long-term, but possess significant toxicity that can seriously compromise treatment effectiveness. The major toxicity of NRTIs is mitochondrial toxicity. This manifests as serious side effects such as myopathy, peripheral neuropathy and lactic acidosis. In general, it is believed that the mitochondrial pathogenesis is closely related to the effect of NRTIs on mitochondrial DNA polymerase-γ. Depletion and mutation of mitochondrial DNA during chronic NRTI therapy may lead to cellular respiratory dysfunction and release of reactive oxidative species, resulting in cellular damage. It is now apparent that the etiology is far more complex than originally thought. It appears to involve multiple mechanisms as well as host factors such as HIV per se, inborn mitochondrial mutation, and sex. Management of mitochondrial toxicity during NRTI therapy remains a challenge. Interruption of NRTI therapy and substitution of the causative agents with alternative better-tolerated NRTIs represents the mainstay of management for mitochondrial toxicity and its clinical manifestations. A range of pharmacological approaches has been proposed as treatments and prophylaxes.

Role of Drp1, a key mitochondrial fission protein, in neuropathic pain

While oxidative stress has been implicated in small-fiber painful peripheral neuropathies, antioxidants are only partially effective to treat patients. We have tested the hypothesis that Drp1 (dynamin-related protein 1), a GTPase that catalyzes the process of mitochondrial fission, which is a mechanism central for the effect and production of reactive oxygen species (ROS), plays a central role in these neuropathic pain syndromes. Intrathecal administration of oligodeoxynucleotide antisense against Drp1 produced a decrease in its expression in peripheral nerve and markedly attenuated neuropathic mechanical hyperalgesia caused by HIV/AIDS antiretroviral [ddC (2',3'-dideoxycytidine)] and anticancer (oxaliplatin) chemotherapy in male Sprague Dawley rats. To confirm the role of Drp1 in these models of neuropathic pain, as well as to demonstrate its contribution at the site of sensory transduction, we injected a highly selective Drp1 inhibitor, mdivi-1, at the site of nociceptive testing on the dorsum of the rat's hindpaw. mdivi-1 attenuated both forms of neuropathic pain. To evaluate the role of Drp1 in hyperalgesia induced by ROS, we demonstrated that intradermal hydrogen peroxide produced dose-dependent hyperalgesia that was inhibited by mdivi-1. Finally, mechanical hyperalgesia induced by diverse pronociceptive mediators involved in inflammatory and neuropathic pain-tumor necrosis factor α, glial-derived neurotrophic factor, and nitric oxide-was also inhibited by mdivi-1. These studies provide support for a substantial role of mitochondrial fission in preclinical models of inflammatory and neuropathic pain.

Exploiting the nucleotide substrate specificity of repair DNA polymerases to develop novel anticancer agents

The genome is constantly exposed to mutations that can originate during replication or as a result of the action of both endogenous and/or exogenous damaging agents [such as reactive oxygen species (ROS), UV light, genotoxic environmental compounds, etc.]. Cells have developed a set of specialized mechanisms to counteract this mutational burden. Many cancer cells have defects in one or more DNA repair pathways, hence they rely on a narrower set of specialized DNA repair mechanisms than normal cells. Inhibiting one of these pathways in the context of an already DNA repair-deficient genetic background, will be more toxic to cancer cells than to normal cells, a concept recently exploited in cancer chemotherapy by the synthetic lethality approach. Essential to all DNA repair pathways are the DNA pols. Thus, these enzymes are being regarded as attractive targets for the development of specific inhibitors of DNA repair in cancer cells. In this review we examine the current state-of-the-art in the development of nucleotide analogs as inhibitors of repair DNA polymerases.

Pyrimidine deoxynucleoside and nucleoside reverse transcriptase inhibitor metabolism in the perfused heart and isolated mitochondria

BACKGROUND

The metabolism of pyrimidine deoxynucleosides and nucleoside reverse transcriptase inhibitors has been studied in growing cells. However, many of these drugs are associated with mitochondrial toxicities observed in non-replicating tissues, such as in the heart, where their metabolism has not been investigated.

METHODS

The aims of this study were twofold. The first was to investigate the metabolism of the thymidine analogues 3'-azido-3'deoxythymidine (AZT) and 2',3'-didehydrodideoxy-thymidine (d4T), and the deoxycytidine (dCyd) analogues 2'-deoxy-3'-thiacytidine (3TC) and 2',3'-dideoxycytidine (ddC) with regard to phosphorylation and breakdown. The second was to investigate their potential effects, singly or in combination with AZT, on metabolism of the naturally occurring deoxynucleosides in the perfused rat heart and in isolated heart mitochondria.

RESULTS

The analogue d4T was not metabolized in perfused heart or in isolated mitochondria, and had no effect on either thymidine or dCyd metabolism. The dCyd analogues were both phosphorylated in perfused heart to the triphosphate, but only at the limit of detection and they were not phosphorylated in isolated mitochondria. Neither ddC nor 3TC had any effect on thymidine or dCyd metabolism in either perfused heart or in isolated mitochondria. AZT has been previously shown to inhibit thymidine phosphorylation. When d4T, 3TC or ddC were given with AZT, only ddC caused a significant further decrease in thymidine phosphorylation.

CONCLUSIONS

These results indicate that with the exception of the competition between AZT and thymidine, there was little competition for phosphorylation among and between these other nucleoside reverse transcriptase inhibitors and the naturally occurring deoxynucleosides in cardiac tissue and isolated heart mitochondria.

Adenosine 3',5'-cyclic monophosphate (cAMP)-dependent phosphoregulation of mitochondrial complex I is inhibited by nucleoside reverse transcriptase inhibitors

Nucleoside analog reverse transcriptase inhibitors (NRTIs) are known to directly inhibit mitochondrial complex I activity as well as various mitochondrial kinases. Recent observations that complex I activity and superoxide production are modulated through cAMP-dependent phosphorylation suggests a mechanism through which NRTIs may affect mitochondrial respiration via kinase-dependent protein phosphorylation. In the current study, we examine the potential for NRTIs to inhibit the cAMP-dependent phosphorylation of complex I and the associated NADH:CoQ oxidoreductase activities and rates of superoxide production using HepG2 cells. Phosphoprotein staining of immunocaptured complex I revealed that 3'-azido-3'-deoxythymidine (AZT; 10 and 50 microM), AZT monophosphate (150 microM), and 2',3'-dideoxycytidine (ddC; 1 microM) prevented the phosphorylation of the NDUFB11 subunit of complex I. This was associated with a decrease in complex I activity with AZT and AZT monophosphate only. In the presence of succinate, superoxide production was increased with 2',3'-dideoxyinosine (ddI; 10 microM) and ddC (1 microM). In the presence of succinate+cAMP, AZT showed an inverse dose-dependent effect on superoxide production. None of the NRTIs examined inhibit PKA activity suggesting that the observed effects are due to a direct interaction with complex I. These data demonstrate a direct effect of NRTIs on cAMP-dependent regulation of mitochondrial bioenergetics independent of DNA polymerase-gamma activity; in the case of AZT, these observations may provide a mechanism for the observed long-term toxicity with this drug.

Nucleoside reverse transcriptase inhibitors and human immunodeficiency virus proteins cause axonal injury in human dorsal root ganglia cultures

Distal symmetric polyneuropathy (DSP) has emerged as the most common complication of human immunodeficiency virus (HIV) infection, which is associated with neuronal injury in the dorsal root ganglion (DRG). With the advent of highly active antiretroviral therapy, especially nucleoside analogs, patients are living longer. Some of the antiretroviral drugs used to treat HIV infection have been associated with neuropathies. The pathogenesis of these neuropathies remains poorly understood. Utilizing a human fetal DRG model of predominantly nociceptive fibers, the authors investigated the effects of HIV gp120 and Tat(1-72), alone or in combination with nucleoside analogs on both morphological and ultra-structural changes in DRG neurons. Nucleoside analogs and HIV proteins both caused a significant decrease in the mean axonal length. However, ddI was the most potent, followed by ddC, d4T, and AZT. Despite the combined exposure to toxic dosages of HIV proteins and nucleoside analogs, there appeared to be a ceiling effect on the amount of axonal retraction, indicating that the proximal and distal axon are differentially regulated. In conclusion, both HIV proteins and nucleoside reverse transcriptase inhibitors (NRTIs) cause axonal damage by inducing mitochondrial injury and rearrangement of microtubules.

Absence of a universal mechanism of mitochondrial toxicity by nucleoside analogs

Nucleoside analogs are associated with various mitochondrial toxicities, and it is becoming increasingly difficult to accommodate these differences solely in the context of DNA polymerase gamma inhibition. Therefore, we examined the toxicities of zidovudine (AZT) (10 and 50 microM; 2.7 and 13.4 microg/ml), didanosine (ddI) (10 and 50 microM; 2.4 and 11.8 microg/ml), and zalcitabine (ddC) (1 and 5 microM; 0.21 and 1.1 microg/ml) in HepG2 and H9c2 cells without the presumption of mitochondrial DNA (mtDNA) depletion. Ethidium bromide (EtBr) (0.5 microg/ml; 1.3 microM) was used as a positive control. AZT treatment resulted in metabolic disruption (increased lactate and superoxide) and increased cell mortality with decreased proliferation, while mtDNA remained unchanged or increased (HepG2 cells; 50 microM AZT). ddC caused pronounced mtDNA depletion in HepG2 cells but not in H9c2 cells and increased mortality in HepG2 cells, but no significant metabolic disruption in either cell type. ddI caused a moderate depletion of mtDNA in both cell types but showed no other effects. EtBr exposure resulted in metabolic disruption, increased cell mortality with decreased cell proliferation, and mtDNA depletion in both cell types. We conclude that nucleoside analogs display unique toxicities within and between culture models, and therefore, care should be taken when generalizing about the mechanisms of nucleoside reverse transcriptase inhibitor toxicity. Additionally, mtDNA abundance does not necessarily correlate with metabolic disruption, especially in cell culture; careful discernment is recommended in this regard.

Mitochondrial haplogroups and peripheral neuropathy during antiretroviral therapy: an adult AIDS clinical trials group study

OBJECTIVE

HIV nucleoside reverse transcriptase inhibitors (NRTI) can cause peripheral neuropathy that is a result of mitochondrial injury. Polymorphisms in the mitochondrial genome define haplogroups that may have functional implications. The objective of this study was to determine if NRTI-associated peripheral neuropathy is associated with European mitochondrial haplogroups.

DESIGN

Case-control study of Adult AIDS Clinical Trials Group (ACTG) study 384 and ACTG Human DNA Repository participants.

METHODS

ACTG study 384 was a treatment strategy trial of antiretroviral therapy with didanosine (ddI) plus stavudine (d4T) or zidovudine plus lamivudine given with efavirenz, nelfinavir, or both. Subjects were followed for up to 3 years. Peripheral neuropathy was ascertained based on signs and symptoms. For this analysis, polymorphisms that define European mitochondrial haplogroups were characterized in a majority of ACTG 384 participants, and associations with peripheral neuropathy were assessed using logistic regression.

RESULTS

A total of 509 subjects were included in this analysis of whom 250 (49%) were self-identified as white, non-Hispanic. Mitochondrial haplogroup T was more frequent in subjects who developed peripheral neuropathy. Among 137 white subjects randomized to receive ddI plus d4T, 20.8% of those who developed peripheral neuropathy belonged to mitochondrial haplogroup T compared to 4.5% of control subjects (odds ratio, 5.4; 95% confidence interval, 1.4-25.1; P = 0.009). Independent predictors of peripheral neuropathy were randomization to receive ddI plus d4T, older age, and mitochondrial haplogroup T.

CONCLUSIONS

A common European mitochondrial haplogroup may predict NRTI-associated peripheral neuropathy. Future studies should validate this relationship, and evaluate non-European mitochondrial haplogroups and other NRTI toxicities.

Cardioprotective effects of poly(ADP-ribose) polymerase inhibition

Free radical and oxidant production in cardiac myocytes during ischemia/reperfusion, cardiomyopathy, cardiotoxic drug exposure and ageing leads to DNA strand-breakage which activates the nuclear enzyme poly(ADP-ribose) polymerase (PARP) and initiates an energy consuming, inefficient cellular metabolic cycle with transfer of the ADP-ribosyl moiety of NAD+ to protein acceptors. These processes lead to the functional impairment of the myocytes and promote myocyte death. During the last decade a growing number of experimental studies demonstrated the beneficial effects of PARP inhibition in cell cultures through rodent models and more recently in pre-clinical large animal models of regional and global ischemia/reperfusion injury and various forms of heart failure. The current article provides an overview of the experimental evidence implicating PARP as a pathophysiological modulator of cardiac myocyte injury in vitro and in vivo.

Possible Ways Nucleoside Analogues Can Affect Mitochondrial Dna Content and Gene Expression during HIV Therapy

In recent years, research into nucleoside reverse transcriptase inhibitor (NRTI)-related mitochondrial (mt) toxicity in HIV therapy has led to conflicting results and many unanswered questions regarding the molecular mechanisms that lead to such toxicity. From the early hypothesis that inhibition of the human mt polymerase γ by NRTIs was responsible for the drugs’ mt toxicity, an increasingly complex picture is emerging that probably involves multiple mt pathways. Results have been presented suggesting that NRTIs affect not only mtDNA but also mtRNA, nucleotide phosphorylation and the mt respiratory chain. Based on the current level of knowledge, this overview addresses some of the potential mechanisms through which NRTIs could affect mitochondria and ultimately cause the toxicity symptoms observed in HIV patients receiving NRTI-containing antiretroviral therapy.

Modification of the incidence of drug-associated symmetrical peripheral neuropathy by host and disease factors in the HIV outpatient study cohort

BACKGROUND

We sought to identify factors associated with the clinical diagnosis of symmetrical peripheral neuropathy (SPN) during the era of highly active antiretroviral therapy (HAART) in a retrospective, longitudinal cohort analysis.

METHODS

Patients infected with human immunodeficiency virus type 1 were evaluated for clinical signs of SPN and its association with immunologic, virologic, clinical, and drug treatment factors by means of univariate and multivariate logistic regression analyses.

RESULTS

Of 2515 patients, 329 (13.1%) received a diagnosis of SPN. In the logistic regression analysis, statistically significant non-drug-based risk factors for SPN were age >40 years (adjusted odds ratio [aOR], 1.17), diabetes mellitus (aOR, 1.79), white race (aOR, 1.33), nadir CD4(+) T lymphocyte count <50 cells/mm(3) (aOR, 1.64), CD4(+) T lymphocyte count 50-199 cells/mm(3) (aOR, 1.40), and viral load >10,000 copies/mL at first measurement (aOR, 1.44). Although initial use of didanosine, stavudine (40 mg b.i.d.), nevirapine, or 4 protease inhibitors was associated with SPN (ORs for all 4 treatments, >1.41), the strength of association decreased with continued use of all medications studied.

CONCLUSION

Since HAART was introduced, the incidence of SPN has decreased. Host factors and signs of increased disease severity were associated with an increased risk of developing SPN during the initial period of exposure to drug therapy. Immunity improved and the risk of SPN decreased with continued use of HAART. Delaying the initiation of therapy may select those individuals who will be more likely to develop SPN, and earlier initiation of HAART may decrease the risk of developing this common problem, as well as increase the therapeutic effects and decrease the toxic effects of the drugs.

Modulation of the activation of extracellular signal-regulated kinase (ERK) and the production of inflammatory mediators by ADP-ribosylation inhibitors

ADP-ribosylation is involved in nuclear factor kappaB (NF-kappaB)-dependent gene expression induced by lipopolysaccharide in murine macrophages. Here we have investigated the mechanism by which ADP-ribosylation inhibitors block signaling pathways induced in macrophages. In RAW264.7 macrophages the inducers of NF-kappaB activate the production of reactive oxygen species and three mitogen-activated protein kinases (MAPK), the extracellular signal regulated kinase (ERK), the c-jun N-terminal kinase/stress-activated protein kinase (JNK), and p38. We demonstrate that ADP-ribosylation inhibitors specifically inhibit ERK MAPK activation and reduce the release of inflammatory mediators such as tumor necrosis factor alpha (TNF-alpha), IL-6 and nitrite.

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.

Azidothymidine promotes free radical generation by activated macrophages and hydrogen peroxide-iron-mediated oxidation in a cell-free system

Azidothymidine (AZT) and AZT monophosphate (AZT-MP) in concentrations as low as 10 and 50 microM, respectively, promote oxidation of chemically deacetylated 2',7'-dichlorodihydrofluorescein (DCDHF) to 2',7'-dichlorofluorescein (DCF) by rat peritoneal macrophages activated with latex. Cells were incubated with AZT and AZT-MP for 18 h, washed out from residual AZT or AZT-MP and activated with latex for 30 or 60 min in the presence of DCDHF. Latex-activated cells oxidize DCDHF extracellularly due to release of hydrogen peroxide and low-molecular iron complexes, which is verified using catalase, desferal and the peroxidase inhibitor sodium azide. AZT and AZT-MP increase DCDHF oxidation due to additional release of hydrogen peroxide as demonstrated by catalase inhibition of DCDHF oxidation and direct H(2)O(2) measurement. Thymidine and thymidine phosphates did not show any effect on macrophage activation. In separate experiments we evaluated the in vitro prooxidant activity of AZT, AZT-MP, AZT triphosphate (AZT-TP), AZT glucuronide (GAZT) and 3'-amino-3'-deoxythymidine (AMT) in a cell-free system using the hydrogen peroxide-iron-mediated oxidation of DCDHF. Under these conditions, AZT and AZT phosphates exhibit a prooxidant effect in concentrations as low as 100 microM. Furthermore, GAZT is a less effective prooxidant and AMT acts like an antioxidant. Thymidine did not show any effect.

Mitochondrion-mediated apoptosis in HIV-1 infection

Acquired immunodeficiency syndrome (AIDS), which is caused by human immunodeficiency virus (HIV-1), involves the apoptotic destruction of lymphocytes and, in the context of AIDS-associated pathologies, of neurons and myocytes. Several proteins encoded by HIV-1 trigger apoptosis by inducing permeabilization of the mitochondrial membrane. Several nucleoside analogs used clinically in the treatment of HIV-1 inhibit the replication of mitochondrial DNA (mtDNA) and/or increase the frequency of mtDNA mutations. These cause severe mitochondriopathy and might contribute to lipodystrophy, the complication associated with HIV-1 therapy. HIV-1 protease inhibitors can inhibit apoptosis at the mitochondrial level, which might help to alleviate lymphopenia. Thus, it appears that the pathogenesis of AIDS, and the pharmacological interventions and complications associated with this disease, affect the mitochondrial regulation of apoptosis, which, therefore, largely determines the outcome of HIV-1 infection.

FK506 is neuroprotective in a model of antiretroviral toxic neuropathy

Antiretroviral toxic neuropathy is the most common neurological complication of human immunodeficiency virus infection. This painful neuropathy not only affects the quality of life of human immunodeficiency virus-infected patients but also severely limits viral suppression strategies. We have developed an in vitro model of this toxic neuropathy to better understand the mechanism of neurotoxicity and to test potential neuroprotective compounds. We show that among the dideoxynucleosides, ddC appears to be the most neurotoxic, followed by ddI and then d4T. This reflects their potency in causing neuropathy. AZT, which does not cause a peripheral neuropathy in patients, does not cause significant neurotoxicity in our model. Furthermore, in this model, we show that the immunophilin ligand FK506 but not cyclosporin A prevents the development of neurotoxicity by ddC, as judged by amelioration of ddC-induced "neuritic pruning," neuronal mitochondrial depolarization, and neuronal necrotic death. This finding suggests a calcineurin-independent mechanism of neuroprotection. As calcineurin inhibition underlies the immunosuppressive properties of these clinically used immunophilin ligands, this holds promise for the neuroprotective efficacy of nonimmunosuppressive analogs of FK506 in the prevention or treatment of antiretroviral toxic neuropathy.

Nucleoside reverse transcriptase inhibitors impair endothelium-dependent relaxation by increasing superoxide

Nucleoside reverse transcriptase inhibitors (NRTIs) have been used successfully to reduce acquired immunodeficiency syndrome mortality. However, the use of these compounds is associated with numerous tissue toxicities, including cardiomyopathy. These studies address the effects of NRTIs on vascular function. Functional assays of contraction and relaxation were performed on isolated mouse aorta segments obtained from FVB/n mice exposed to zidovudine (AZT), stavudine, or water for 35 days. AZT and stavudine treatment impaired sensitivity to endothelium-dependent relaxation by acetylcholine. Dihydroethidium staining revealed that AZT treatment was associated with an increase in superoxide levels. Pretreatment of AZT-treated vessels with tiron (1 mM), a free radical scavenger, restored endothelium-dependent relaxation in mice. In cellular preparations, electron spin resonance measurements revealed elevated superoxide in cultured endothelial cells exposed to AZT; elevation was dependent on the length of exposure. These results indicate that NRTIs impair endothelium-dependent relaxation by increasing superoxide levels and suggest that NRTI therapy contributes to cardiovascular complications in acquired immunodeficiency syndrome.

Influence of mitochondrial control of apoptosis on the pathogenesis, complications and treatment of HIV infection

HIV infection is inexorably linked with disordered regulation of apoptosis, and consequent alterations in mitochondrial homeostasis, resulting in CD4 T cell death and enhanced susceptibility to opportunistic infections and malignancies. Effective treatment of HIV reverses the changes in mitochondrial homeostasis and apoptosis, and enhances immunocompetence. This review will summarize current knowledge of: i) the associations of apoptosis with HIV disease progression; ii) mechanisms of enhanced apoptosis in HIV infection; iii) putative role of apoptosis in HIV complications; iv) direct effects of HIV therapies on mitochondria and apoptosis; and finally v) treatment strategies for HIV based upon modifying the apoptotic response.

Drugs and cardiotoxicity in HIV and AIDS

The advent of potent antiretroviral drugs in recent years has had an impressive impact on mortality and disease progression in HIV-infected patients, so that issues related to long-term effects of drugs are of growing importance. Hyperlipidemia, hyperglycemia, and lipodystrophy are increasingly described adverse effects of highly active antiretroviral therapy (HAART), in particular when protease inhibitors are used. Hyperlipidemia is strikingly associated with the use of most available protease inhibitors, with an estimated prevalence of up to 50%. Because of the short observation period and the small number of cardiovascular events, epidemiological evidence for an increased risk of coronary heart disease in HIV-infected patients treated with HAART is not adequate at present; however, it is likely that shortly more data will accumulate to quantify this risk. Before starting HAART and during treatment it is reasonable to evaluate all patients for traditional coronary risk factors, including lipid profile. Among the drugs that are currently used in HIV+ patients, antibacterials, antifungals, psychotropic drugs and anti-histamines have been associated with QT prolongation or torsade de pointe, a life-threatening ventricular arrhythmia. Among the risk factors that may precipitate an asymptomatic electrocardiographic abnormality into a dangerous arrhythmia is the concomitant use of drugs that share the CYP3A metabolic pathway. Since most protease inhibitors are potent inhibitors of CYP3A, clinicians should be aware of this potentially dangerous effect of HAART. Anthracyclines are potent cytotoxic antibiotics that have been widely used for the treatment of HIV-related neoplasms. Their cardiotoxicity is well known, ranging from benign and reversible arrhythmias to progressive severe cardiomyopathy. The increased survival and quality of life of HIV+ patients emphasize the importance of a high awareness of adverse drug-related cardiac effects.

Sterol regulatory element-binding proteins and reactive oxygen species: potential role in highly-active antiretroviral therapy (HAART)-associated lipodystrophy

OBJECTIVES

To summarize the existing pathophysiological concepts and to hypothesize new mechanisms involving sterol regulatory element-binding proteins (SREBP) and reactive oxygen species (ROS), in highly-active antiretroviral therapy (HAART)-associated lipodystrophy.

CONCLUSIONS

The widespread use of HAART has dramatically reduced AIDS-related deaths in the developed world. Unfortunately, long-term HAART has been associated with a unique and unexpected syndrome of lipodystrophy manifested by fat wasting in the subcutaneous adipose tissue of the face and extremities, and accumulation of fat in the viscera and neck, often accompanied by hyperlipidemia and insulin resistance. Despite intensive study of this syndrome over the past three years, the pathophysiologic mechanism(s) underlying HAART-associated lipodystrophy syndrome remains elusive. A continued attempt to elucidate pathophysiological mechanisms involved in HAART-associated lipodystrophy remains critically important to improving the treatment strategies for this epidemic condition. In this review, we suggest two new hypotheses that may explain the pathogenesis and pathophysiology of HAART-associated lipodystrophy that warrant further investigations. First, we hypothesize that upregulation and/or increase in the mature form of SREBP-1 caused by HAART may lead to perturbations in synergistic regulation of genes involved in maintenance of cholesterol homeostasis and synthesis of fatty acids, that may explain the accumulation of fat which is a hallmark of this syndrome. Second, we hypothesize that the generation of reactive oxygen species in adipocytes may be an early and critical event in HAART-associated toxicity leading to cell death, partially explaining the mechanism underlying lipoatrophy.

Direct effect of Taxol on free radical formation and mitochondrial permeability transition

To elucidate the potential role of mitochondria in Taxol-induced cytotoxicity, we studied its direct mitochondrial effects. In Percoll-gradient purified liver mitochondria, Taxol induced large amplitude swelling in a concentration-dependent manner in the microM range. Opening of the permeability pore was also confirmed by the access of mitochondrial matrix enzymes for membrane impermeable substrates in Taxol-treated mitochondria. Taxol induced the dissipation of mitochondrial membrane potential (DeltaPsi) determined by Rhodamine123 release and induced the release of cytochrome c from the intermembrane space. All these effects were inhibited by 2.5 microM cyclosporine A. Taxol significantly increased the formation of reactive oxygen species (ROS) in both the aqueous and the lipid phase as determined by dihydrorhodamine123 and resorufin derivative. Cytochrome oxidase inhibitor CN(-), azide, and NO abrogated the Taxol-induced mitochondrial ROS formation while inhibitors of the other respiratory complexes and cyclosporine A had no effect. We confirmed that the Taxol-induced collapse of DeltaPsi and the induction of ROS production occurs in BRL-3A cells. In conclusion, Taxol-induced adenine nucleotide translocase-cyclophilin complex mediated permeability transition, and cytochrome oxidase mediated ROS production. Because both cytochrome c release and mitochondrial ROS production can induce suicide pathways, the direct mitochondrial effects of Taxol may contribute to its cytotoxicity.

Effect of poly(ADP-ribose) polymerase inhibitors on the ischemia-reperfusion-induced oxidative cell damage and mitochondrial metabolism in Langendorff heart perfusion system

Ischemia-reperfusion induces reactive oxygen species (ROS) formation, and ROS lead to cardiac dysfunction, in part, via the activation of the nuclear poly(ADP-ribose) polymerase (PARP, called also PARS and ADP-RT). ROS and peroxynitrite induce single-strand DNA break formation and PARP activation, resulting in NAD(+) and ATP depletion, which can lead to cell death. Although protection of cardiac muscle by PARP inhibitors can be explained by their attenuating effect on NAD(+) and ATP depletion, there are data indicating that PARP inhibitors also protect mitochondria from oxidant-induced injury. Studying cardiac energy metabolism in Langendorff heart perfusion system by (31)P NMR, we found that PARP inhibitors (3-aminobenzamide, nicotinamide, BGP-15, and 4-hydroxyquinazoline) improved the recovery of high-energy phosphates (ATP, creatine phosphate) and accelerated the reutilization of inorganic phosphate formed during the ischemic period, showing that PARP inhibitors facilitate the faster and more complete recovery of the energy production. Furthermore, PARP inhibitors significantly decrease the ischemia-reperfusion-induced increase of lipid peroxidation, protein oxidation, single-strand DNA breaks, and the inactivation of respiratory complexes, which indicate a decreased mitochondrial ROS production in the reperfusion period. Surprisingly, PARP inhibitors, but not the chemically similar 3-aminobenzoic acid, prevented the H(2)O(2)-induced inactivation of cytochrome oxidase in isolated heart mitochondria, suggesting the presence of an additional mitochondrial target for PARP inhibitors. Therefore, PARP inhibitors, in addition to their important primary effect of decreasing the activity of nuclear PARP and decreasing NAD(+) and ATP consumption, reduce ischemia-reperfusion-induced endogenous ROS production and protect the respiratory complexes from ROS induced inactivation, providing an additional mechanism by which they can protect heart from oxidative damages.

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.

Functional properties of skeletal muscle from transgenic animals with upregulated heat shock protein 70

The influence of inducible heat stress proteins on protecting contracting skeletal muscle against fatigue-induced injury was investigated. A line of transgenic mice overexpressing the inducible form of the 72-kDa heat shock protein (HSP72) in skeletal muscles was used. We examined the relationship between muscle contractility and levels of the constitutive (HSC73) and inducible (HSP72) forms of the 72-kDa heat shock protein in intact, mouse extensor digitorum longus (EDL), soleus (SOL), and the diaphragm (DPH). In all transgenic muscles, HSP72 was expressed at higher levels compared with transgene-negative controls, where HSP72 was below the level of detection. At the same time, HSC73 levels were downregulated in all transgenic muscle types. Shipment-related stress caused an elevation in the levels of HSP72 in all muscles for 1 wk after arrival of the animals. We also found that, although no statistical differences in response to intermittent fatiguing stimulation in the contractile properties of intact transgene-positive muscles compared with their transgene-negative counterparts were observed, the response of intact transgene-positive EDL muscles to caffeine was enhanced. These findings demonstrate that elevated HSP72 does not protect EDL, SOL, or DPH muscles from the effects of intermittent fatiguing stimulation. However, HSP72 may influence the excitation-contraction coupling (ECC) process, either directly or indirectly, in EDL muscle. If the effects on ECC were indirect, then these results would suggest that manipulation of a specific gene might cause functional effects that seem independent of the manipulated gene/protein.

Clinical manifestations and management of antiretroviral nucleoside analog-related mitochondrial toxicity

OBJECTIVES

This article reviews the clinical manifestations of mitochondrial toxicity associated with the use of nucleoside analog reverse transcriptase inhibitors (NRTIs) and outlines strategies to manage these sequelae.

BACKGROUND

NRTIs are the key components of the antiretroviral combinations used in the management of patients infected with HIV. The available NRTIs differ in their convenience of administration, frequency of dosing, resistance profiles, and side-effect profiles. NRTIs act as competitive inhibitors of the RNA/DNA polymerase reverse transcriptase of HIV and cause chain termination in the growing viral DNA chain. Many of the important and treatment-limiting side effects of NRTIs may be related to the effect of these agents on human DNA polymerases, in particular, mitochondrial DNA polymerase gamma. Depletion of mitochondrial DNA during chronic NRTI therapy may lead to cellular respiratory dysfunction and generalized and tissue- and drug-specific toxicities, including myopathy, peripheral neuropathy, and lactic acidosis. Recently, it has been proposed that the fat redistribution syndrome, or lipodystrophy, reported during chronic antiretroviral therapy is a manifestation of the differential impact of at least some NRTIs on peripheral and visceral adipocytes. Management of potential mitochondrial toxicity during NRTI therapy remains a challenge. A range of nutritional supplements, both as treatments and prophylaxes, have been proposed, and some have been investigated in vitro; no in vivo studies have yet been conducted.

METHODS

The information in this review was compiled using MEDLINE and AIDSLINE searches of the literature, including conference abstracts.

CONCLUSIONS

At present, interruption of NRTI therapy or substitution of the probable causative agent with alternative NRTIs that appear to be better tolerated represents the mainstay of management for mitochondrial toxicity and its clinical manifestations.