Cardiac mitochondrial DNA polymerase-gamma is inhibited competitively and noncompetitively by phosphorylated zidovudine

Assessing Drug-Induced Mitochondrial Toxicity in Cardiomyocytes: Implications for Preclinical Cardiac Safety Evaluation

Drug-induced cardiotoxicity not only leads to the attrition of drugs during development, but also contributes to the high morbidity and mortality rates of cardiovascular diseases. Comprehensive testing for proarrhythmic risks of drugs has been applied in preclinical cardiac safety assessment for over 15 years. However, other mechanisms of cardiac toxicity have not received such attention. Of them, mitochondrial impairment is a common form of cardiotoxicity and is known to account for over half of cardiovascular adverse-event-related black box warnings imposed by the U.S. Food and Drug Administration. Although it has been studied in great depth, mitochondrial toxicity assessment has not yet been incorporated into routine safety tests for cardiotoxicity at the preclinical stage. This review discusses the main characteristics of mitochondria in cardiomyocytes, drug-induced mitochondrial toxicities, and high-throughput screening strategies for cardiomyocytes, as well as their proposed integration into preclinical safety pharmacology. We emphasize the advantages of using adult human primary cardiomyocytes for the evaluation of mitochondrial morphology and function, and the need for a novel cardiac safety testing platform integrating mitochondrial toxicity and proarrhythmic risk assessments in cardiac safety evaluation.

Mitochondria as the Target of Hepatotoxicity and Drug-Induced Liver Injury: Molecular Mechanisms and Detection Methods

One of the major mechanisms of drug-induced liver injury includes mitochondrial perturbation and dysfunction. This is not a surprise, given that mitochondria are essential organelles in most cells, which are responsible for energy homeostasis and the regulation of cellular metabolism. Drug-induced mitochondrial dysfunction can be influenced by various factors and conditions, such as genetic predisposition, the presence of metabolic disorders and obesity, viral infections, as well as drugs. Despite the fact that many methods have been developed for studying mitochondrial function, there is still a need for advanced and integrative models and approaches more closely resembling liver physiology, which would take into account predisposing factors. This could reduce the costs of drug development by the early prediction of potential mitochondrial toxicity during pre-clinical tests and, especially, prevent serious complications observed in clinical settings.

Remdesivir induces persistent mitochondrial and structural damage in human induced pluripotent stem cell derived cardiomyocytes

AIMS

Remdesivir is a prodrug of an adenosine triphosphate analogue and is currently the only drug formally approved for the treatment of hospitalised COVID-19 patients. Nucleoside/nucleotide analogues have been shown to induce mitochondrial damage and cardiotoxicity, and this may be exacerbated by hypoxia, which frequently occurs in severe COVID-19 patients. Although there have been few reports of adverse cardiovascular events associated with remdesivir, clinical data are limited. Here, we investigated whether remdesivir induced cardiotoxicity using an in vitro human cardiac model.

METHODS AND RESULTS

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were exposed to remdesivir under normoxic and hypoxic conditions to simulate mild and severe COVID-19 respectively. Remdesivir induced mitochondrial fragmentation, reduced redox potential and suppressed mitochondrial respiration at levels below the estimated plasma concentration under both normoxic and hypoxic conditions. Non-mitochondrial damage such as electrophysiological alterations and sarcomere disarray were also observed. Importantly, some of these changes persisted after the cessation of treatment, culminating in increased cell death. Mechanistically, we found that inhibition of DRP1, a regulator of mitochondrial fission, ameliorated the cardiotoxic effects of remdesivir, showing that remdesivir-induced cardiotoxicity was preventable and excessive mitochondrial fission might contribute to this phenotype.

CONCLUSIONS

Using an in vitro model, we demonstrated that remdesivir can induce cardiotoxicity in hiPSC-CMs at clinically relevant concentrations. These results reveal previously unknown potential side-effects of remdesivir and highlight the importance of further investigations with in vivo animal models and active clinical monitoring to prevent lasting cardiac damage to patients.

TRANSLATIONAL PERSPECTIVE

Adult cardiomyocytes have limited ability to regenerate, thus treatment-induced cardiotoxicity can potentially cause irreparable harm. Remdesivir is currently the only FDA approved treatment for COVID-19 but clinical safety data are limited. Using human pluripotent stem cell-derived cardiomyocytes, we revealed that remdesivir induced persistent mitochondrial and structural abnormalities at clinically relevant concentrations. We advise confirmatory experiments in in vivo animal models, investigations of cardioprotective strategies, and closer patient monitoring such that treatment-induced cardiotoxicity does not contribute to the long term sequelae of COVID-19 patients.

Coronavirus helicases: attractive and unique targets of antiviral drug-development and therapeutic patents

Introduction: Coronaviruses encode a helicase that is essential for viral replication and represents an excellent antiviral target. However, only a few coronavirus helicase inhibitors have been patented. These patents include drug-like compound SSYA10-001, aryl diketo acids (ADK), and dihydroxychromones. Additionally, adamantane-derived bananins, natural flavonoids, one acrylamide derivative [(E)-3-(furan-2-yl)-N-(4-sulfamoylphenyl)acrylamide], a purine derivative (7-ethyl-8-mercapto-3-methyl-3,7-dihydro-1 H-purine-2,6-dione), and a few bismuth complexes. The IC₅₀ of patented inhibitors ranges between 0.82 μM and 8.95 μM, depending upon the assays used. Considering the urgency of clinical interventions against Coronavirus Disease-19 (COVID-19), it is important to consider developing antiviral portfolios consisting of small molecules.

Areas covered: This review examines coronavirus helicases as antiviral targets, and the potential of previously patented and experimental compounds to inhibit the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) helicase.

Expert opinion: Small molecule coronavirus helicase inhibitors represent attractive pharmacological modalities for the treatment of coronaviruses such as SARS-CoV and SARS-CoV-2. Rightfully so, the current emphasis is focused upon the development of vaccines. However, vaccines may not work for everyone and broad-based adoption of vaccinations is an increasingly challenging societal endeavor. Therefore, it is important to develop additional pharmacological antivirals against the highly conserved coronavirus helicases to broadly protect against this and subsequent coronavirus epidemics.

Etiology and pathophysiology of heart failure in people with HIV

HIV-associated cardiomyopathy is a well-established sequela in people infected with HIV (PHIV). Despite significant advances in HIV management through the use of highly active anti-retroviral therapy (HAART), PHIV on HAART continue to have elevated risk of cardiomyopathy and heart failure, even when accounting for known cardiovascular risk factors. This review article will explore the proposed mechanisms by which chronic HIV infection induces cardiomyopathy and heart failure in the setting of HAART. Evaluation, work-up, and management of cardiomyopathy in PHIV will also be briefly discussed. The advent of HAART has altered the pathophysiology HIV-associated cardiomyopathy from a rapidly progressive cardiomyopathy, often with pericardial involvement, into a chronic process involving inflammation and persistent immune dysregulation. With the significant decrease in AIDS-related deaths, the prevalence of cardiomyopathy and the mortality associated with heart failure in PHIV have increased. Multiple immune-related and inflammatory mechanisms have been proposed, which may provide insight into evaluation and management of cardiomyopathy in PHIV.

Genetic aspects of the oxidative phosphorylation dysfunction in dilated cardiomyopathy

Dilated cardiomyopathy is a frequent and extremely heterogeneous medical condition. Deficits in the oxidative phosphorylation system have been described in patients suffering from dilated cardiomyopathy. Hence, mutations in proteins related to this biochemical pathway could be etiological factors for some of these patients. Here, we review the clinical phenotypes of patients harboring pathological mutations in genes related to the oxidative phosphorylation system, either encoded in the mitochondrial or in the nuclear genome, presenting with dilated cardiomyopathy. In addition to the clinical heterogeneity of these patients, the large genetic heterogeneity has contributed to an improper allocation of pathogenicity for many candidate mutations. We suggest criteria to avoid incorrect assignment of pathogenicity to newly found mutations and discuss possible therapies targeting the oxidative phosphorylation function.

The roles of resveratrol on cardiac mitochondrial function in cardiac diseases

Left ventricular (LV) dysfunction is commonly associated with a variety of health conditions including acute myocardial infarction and obesity/diabetes. In addition, administration of several pharmacological agents such as anticancer, antiviral, and immunosuppressive drugs has been shown to be related with LV dysfunction. The molecular mechanism responsible for LV dysfunction has been extensively studied, and it has been proposed that the overproduction of reactive oxygen species (ROS) plays a crucial role in the regulation of this function. Mitochondria require the balance between ROS production and antioxidants to maintain their appropriate function and to prevent excessive ROS production. Thus, the excessive production of ROS and the reduced scavenging process under any pathological conditions could disrupt mitochondrial function, leading to energy depletion with subsequent cell death. Therefore, maintenance of the balance between oxidative stress and antioxidants is essential. Resveratrol, a stilbene, has been investigated extensively, and potentially used to treat or prevent various cardiovascular diseases. Resveratrol directly upregulates antioxidative capacity by increasing antioxidant genes such as heme oxygenase-1, superoxide dismutase, catalase, and glutathione. In this review, accumulated data from in vitro, ex vivo, and in vivo studies regarding the effects of resveratrol on cardiac mitochondrial function in cardiac pathologies are comprehensively summarized and discussed. Since there is no conclusive available clinical study regarding the effects of resveratrol on cardiac mitochondrial function, this review also aims to encourage more clinical investigations to confirm findings from basic research. This comprehensive review will provide insight regarding the potential mechanistic roles of resveratrol in preventing and/or treating patients with cardiovascular diseases to improve LV function and their health status.

Prelamin A mediates myocardial inflammation in dilated and HIV-associated cardiomyopathies

Cardiomyopathies are complex heart muscle diseases that can be inherited or acquired. Dilated cardiomyopathy can result from mutations in LMNA, encoding the nuclear intermediate filament proteins lamin A/C. Some LMNA mutations lead to accumulation of the lamin A precursor, prelamin A, which is disease causing in a number of tissues, yet its impact upon the heart is unknown. Here, we discovered myocardial prelamin A accumulation occurred in a case of dilated cardiomyopathy, and we show that a potentially novel mouse model of cardiac-specific prelamin A accumulation exhibited a phenotype consistent with inflammatory cardiomyopathy, which we observed to be similar to HIV-associated cardiomyopathy, an acquired disease state. Numerous HIV protease therapies are known to inhibit ZMPSTE24, the enzyme responsible for prelamin A processing, and we confirmed that accumulation of prelamin A occurred in HIV+ patient cardiac biopsies. These findings (a) confirm a unifying pathological role for prelamin A common to genetic and acquired cardiomyopathies; (b) have implications for the management of HIV patients with cardiac disease, suggesting protease inhibitors should be replaced with alternative therapies (i.e., nonnucleoside reverse transcriptase inhibitors); and (c) suggest that targeting inflammation may be a useful treatment strategy for certain forms of inherited cardiomyopathy.

Heart health in the age of highly active antiretroviral therapy: a review of HIV cardiomyopathy

PURPOSE OF REVIEW

Advancements in highly active antiretroviral therapy (HAART) for human immunodeficiency virus (HIV) has led to increased survival, approaching that of the general population. These patients are at an increased risk for heart disease, specifically HIV-associated cardiomyopathy.

RECENT FINDINGS

Initially, HIV-associated cardiomyopathy was predominantly noted as systolic dysfunction, but diastolic dysfunction has become more common with increased use of HAART. The pathogenesis of HIV-associated cardiomyopathy involves direct viral infection, cytokine activity, focal myocarditis, HAART side effects, immune system dysregulation, and/or ischemia. If cardiomyopathy is diagnosed, an HIV patient should be started, or continued, on HAART and initiated on guideline-directed medical therapy (GDMT) per the ACC/AHA guidelines. The safety and efficacy of mechanical circulatory support and heart transplant in this population has been repeatedly demonstrated, but it remains less available compared with the general advanced heart failure population.

SUMMARY

The widespread use of HAART has led to prolonged survival in HIV patients to the point that cardiac disease has become the most common cause of death. Patients should be treated with HAART for HIV and GDMT for heart failure. Patients progressing to end-stage heart failure should be referred for advanced therapies to centers with experience treating HIV patients.

Targeting the Viral Polymerase of Diarrhea-Causing Viruses as a Strategy to Develop a Single Broad-Spectrum Antiviral Therapy

Viral gastroenteritis is an important cause of morbidity and mortality worldwide, being particularly severe for children under the age of five. The most common viral agents of gastroenteritis are noroviruses, rotaviruses, sapoviruses, astroviruses and adenoviruses, however, no specific antiviral treatment exists today against any of these pathogens. We here discuss the feasibility of developing a broad-spectrum antiviral treatment against these diarrhea-causing viruses. This review focuses on the viral polymerase as an antiviral target, as this is the most conserved viral protein among the diverse viral families to which these viruses belong to. We describe the functional and structural similarities of the different viral polymerases, the antiviral effect of reported polymerase inhibitors and highlight common features that might be exploited in an attempt of designing such pan-polymerase inhibitor.

Mechanisms of Cardiovascular Disease in the Setting of HIV Infection

Although the initial reports of increased cardiovascular (CV) disease in the setting of advanced AIDS were reported approximately 30 years ago, advances in antiretroviral therapy and immediate initiation of therapy on diagnosis have transformed what was once a deadly infectious disease into a chronic health condition. Accordingly, the types of CV diseases occurring in HIV have shifted from pericardial effusions and dilated cardiomyopathy to atherosclerosis and heart failure. The underlying pathophysiology of HIV-associated CV disease remains poorly understood, partly because of the rapidly evolving nature of HIV treatment and because clinical endpoints take many years to develop. The gut plays an important role in the early pathogenesis of HIV infection as HIV preferentially infects CD4+ T cells, 80% of which are located in gut mucosa. The loss of these T cells damages gut mucosa resulting in increased gut permeability and microbial translocation, which incites chronic inflammation and immune activation. Antiretroviral therapy does not cure HIV infection and immune abnormalities persist. These abnormalities correlate with mortality and CV events. The effects of antiretroviral therapy on CV risk are complex; treatment reduces inflammation and other markers of CV risk but induces lipid abnormalities, most commonly hypertriglyceridemia. On a molecular level, monocytes/macrophages, platelet reactivity, and immune cell activation, which play a role in the general population, may be heightened in the setting of HIV and contribute to HIV-associated atherosclerosis. Chronic inflammation represents an inviting therapeutic target in HIV, as it does in uninfected persons with atherosclerosis.

Dysregulation of autophagy in rat liver with mitochondrial DNA depletion induced by the nucleoside analogue zidovudine

The nucleoside reverse transcriptase inhibitor zidovudine (AZT), used in HIV infection treatment, induces mitochondrial DNA (mtDNA) depletion. A cause-effect relationship between mtDNA status alterations and autophagy has been reported. Both events are common in several liver diseases, including hepatocellular carcinoma. Here, we have studied autophagy activation in rat liver with mtDNA depletion induced by AZT administration in drinking water for 35 days. AZT at a concentration of 1 mg/ml, but not 0.5 mg/ml in the drinking water, decreased mtDNA levels in rat liver and extrahepatic tissues. In liver, mtDNA-encoded cytochrome c oxidase 1 protein levels were decreased. Although serum biomarkers of liver and kidney toxicity remained unaltered, β-hydroxybutyrate levels were increased in liver of AZT-treated rats. Moreover, autophagy was dysregulated at two levels: (i) decreased induction signalling of this process as indicated by increases in autophagy inhibitors activity (AKT/mTOR), and absence of changes (Beclin-1, Atg5, Atg7) or decreases (AMPK/ULK1) in the expression/activity of pro-autophagy proteins; and (ii) reduced autophagosome degradation as indicated by decreases in the lysosome abundance (LAMP2 marker) and the transcription factor TFEB controlling lysosome biogenesis. This resulted in increased autophagosome abundance (LC3-II marker) and accumulation of the protein selectively degraded by autophagy p62, and the transcription factor Nrf2 in liver of AZT-treated rats. Nrf2 was activated as indicated by the up-regulation of antioxidant target genes Nqo1 and Hmox-1. In conclusion, rat liver with AZT-induced mtDNA depletion presented dysregulations in autophagosome formation and degradation balance, which results in accumulation of these structures in parenchymal liver cells, favouring hepatocarcinogenesis.

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.

Heart failure in patients with human immunodeficiency virus: a review of the literature

Coronary artery disease represents the leading cause of death for HIV patients treated with highly active antiretroviral treatment. Besides this, an extensive amount of data related to the risk of overt heart failure and consequently of atrial fibrillation and sudden cardiac death (SCD) in this population has been reported. It seems that persistent deregulation of immunity in HIV-infected patients is a common pathway related to both of these adverse clinical outcomes. Despite the fact that atrial fibrillation and heart failure are relatively common in HIV, few data are reported about screening, diagnosis, and potential treatment of these conditions.

Mitochondrial polymerase gamma dysfunction and aging cause cardiac nuclear DNA methylation changes

Cardiomyopathy (CM) is an intrinsic weakening of myocardium with contractile dysfunction and congestive heart failure (CHF). CHF has been postulated to result from decreased mitochondrial energy production and oxidative stress. Effects of decreased mitochondrial oxygen consumption also can accelerate with aging. We previously showed DNA methylation changes in human hearts with CM. This was associated with mitochondrial DNA depletion, being another molecular marker of CM. We examined the relationship between mitochondrial dysfunction and cardiac epigenetic DNA methylation changes in both young and old mice. We used genetically engineered C57Bl/6 mice transgenic for a cardiac-specific mutant of the mitochondrial polymerase-γ (termed Y955C). Y955C mice undergo left ventricular hypertrophy (LVH) at a young age (∼ 94 days old), and LVH decompensated to CHF at old age (∼ 255 days old). Results found 95 genes differentially expressed as a result of Y955C expression, while 4,452 genes were differentially expressed as a result of aging hearts. Moreover, cardiac DNA methylation patterns differed between Y955C (4,506 peaks with 68.5% hypomethylation) and aged hearts (73,286 peaks with 80.2% hypomethylated). Correlatively, of the 95 Y955C-dependent differentially expressed genes, 30 genes (31.6%) also displayed differential DNA methylation; in the 4,452 age-dependent differentially expressed genes, 342 genes (7.7%) displayed associated DNA methylation changes. Both Y955C and aging demonstrated significant enrichment of CACGTG-associated E-box motifs in differentially methylated regions. Cardiac mitochondrial polymerase dysfunction alters nuclear DNA methylation. Furthermore, aging causes a robust change in cardiac DNA methylation that is partially associated with mitochondrial polymerase dysfunction.

Effects of Zidovudine Treatment on Heart mRNA Expression and Mitochondrial DNA Copy Number Associated with Alterations in Deoxynucleoside Triphosphate Composition in a Neonatal Rat Model

The prevention of mother-to-child transmission (MTCT) of HIV is a crucial component in HIV therapy. Nucleoside reverse transcriptase inhibitors (NRTIs), primarily 3'-azido-3'-thymidine (AZT [zidovudine]), have been used to treat both mothers and neonates. While AZT is being replaced with less toxic drugs in treating mothers in MTCT prevention, it is still commonly used to treat neonates. Problems related to mitochondrial toxicity and potential mutagenesis associated with AZT treatment have been reported in treated cohorts. Yet little is known concerning the metabolism and potential toxicity of AZT on embryonic and neonatal tissues, especially considering that the enzymes of nucleoside metabolism change dramatically as many tissues convert from hyperplastic to hypertrophic growth during this period. AZT is known to inhibit thymidine phosphorylation and potentially alter deoxynucleoside triphosphate (dNTP) pools in adults. This study examines the effects of AZT on dNTP pools, mRNA expression of deoxynucleoside/deoxynucleotide metabolic enzymes, and mitochondrial DNA levels in a neonatal rat model. Results show that AZT treatment dramatically altered dNTP pools in the first 7 days of life after birth, which normalized to age-matched controls in the second and third weeks. Additionally, AZT treatment dramatically increased the mRNA levels of many enzymes involved in deoxynucleotide synthesis and mitochondrial biogenesis during the first week of life, which normalized to age-matched controls by the third week. These results were correlated with depletion of mitochondrial DNA noted in the second week. Taken together, results demonstrated that AZT treatment has a powerful effect on the deoxynucleotide synthesis pathways that may be associated with toxicity and mutagenesis.

Drug-induced mitochondrial dysfunction and cardiotoxicity

Mitochondria has an essential role in myocardial tissue homeostasis; thus deterioration in mitochondrial function eventually leads to cardiomyocyte and endothelial cell death and consequent cardiovascular dysfunction. Several chemical compounds and drugs have been known to directly or indirectly modulate cardiac mitochondrial function, which can account both for the toxicological and pharmacological properties of these substances. In many cases, toxicity problems appear only in the presence of additional cardiovascular disease conditions or develop months/years following the exposure, making the diagnosis difficult. Cardiotoxic agents affecting mitochondria include several widely used anticancer drugs [anthracyclines (Doxorubicin/Adriamycin), cisplatin, trastuzumab (Herceptin), arsenic trioxide (Trisenox), mitoxantrone (Novantrone), imatinib (Gleevec), bevacizumab (Avastin), sunitinib (Sutent), and sorafenib (Nevaxar)], antiviral compound azidothymidine (AZT, Zidovudine) and several oral antidiabetics [e.g., rosiglitazone (Avandia)]. Illicit drugs such as alcohol, cocaine, methamphetamine, ecstasy, and synthetic cannabinoids (spice, K2) may also induce mitochondria-related cardiotoxicity. Mitochondrial toxicity develops due to various mechanisms involving interference with the mitochondrial respiratory chain (e.g., uncoupling) or inhibition of the important mitochondrial enzymes (oxidative phosphorylation, Szent-Györgyi-Krebs cycle, mitochondrial DNA replication, ADP/ATP translocator). The final phase of mitochondrial dysfunction induces loss of mitochondrial membrane potential and an increase in mitochondrial oxidative/nitrative stress, eventually culminating into cell death. This review aims to discuss the mechanisms of mitochondrion-mediated cardiotoxicity of commonly used drugs and some potential cardioprotective strategies to prevent these toxicities.

Lower Preprandial Insulin and Altered Fuel Use in HIV/Antiretroviral-Exposed Infants in Cameroon

CONTEXT

Intrauterine HIV/antiretroviral (ARV) and postnatal ARVs are known to perturb energy metabolism and could have permanent effects on future metabolic health. Such maladaptive effects could be mediated by changes in mitochondrial function and intermediary metabolism due to fetal and early-life ARV exposure in HIV/ARV-exposed uninfected (HEU) infants.

OBJECTIVE

The objective of the study was to understand the relationship(s) between mitochondrial fuel use (assessed via acylcarnitines and branched chain amino acids) and preprandial insulin in infants exposed to in utero HIV/ARV plus postnatal zidovudine or nevirapine compared with HIV/ARV-unexposed uninfected (HUU) infants.

DESIGN

This was a prospective cohort study with the following three groups: 1) intrauterine HIV/ARV/postnatal zidovudine-exposed (HEU-A), 2) intrauterine HIV/ARV/postnatal nevirapine-exposed (HEU-N), and 3) HUU infants. Principal component analysis and linear regression modeling were performed to assess the association between in utero HIV/ARV exposure and infant insulin.

SETTING

The study was conducted at Cameroonian urban antenatal centers.

PARTICIPANTS

HIV-infected and -uninfected pregnant woman/infant dyads participated in the study.

MAIN OUTCOME

Preprandial insulin was the main outcome measured.

RESULTS

Of 366 infants, 38 were HEU-A, 118 HEU-N. Forty intermediary metabolites were consolidated into seven principal components. In a multivariate analysis, both HEU-A (β = -.116, P= .012) and HEU-N (β = -.070, P= .022) demonstrated lower insulin compared with HUU infants. However, at high levels of plasma metabolites, HEU-A (β = .027, P= .050) exhibited higher insulin levels than HEU-N or HUU infants. A unique array of short-chain acylcarnitines (β = .044, P= .001) and branched-chain amino acids (β = .033, P= .012) was associated with insulin.

CONCLUSION

HEU-A and HEU-N infants have lower preprandial insulin levels at 6 weeks of age and appear to use metabolic fuel substrates differently than HUU infants. Future studies are warranted to determine whether observed differences have lasting metabolic implications, such as later insulin resistance.

AZT-induced mitochondrial toxicity: an epigenetic paradigm for dysregulation of gene expression through mitochondrial oxidative stress

Mitochondrial dysfunction causes oxidative stress and cardiomyopathy. Oxidative stress also is a side effect of dideoxynucleoside antiretrovirals (NRTI) and is observed in NRTI-induced cardiomyopathy. We show here that treatment with the NRTI AZT {1-[(2R,4S,5S)-4-azido-5-(hydroxymethyl)oxolan-2-yl]-5-methylpyrimidine-2,4-dione} modulates cardiac gene expression epigenetically through production of mitochondrially derived reactive oxygen species. Transgenic mice with ubiquitous expression of mitochondrially targeted catalase (MCAT) and C57Bl/6 wild-type mice littermates (WT) were administered AZT (0.22 mg/day po, 35 days), and cardiac DNA and mRNA were isolated. In AZT-treated WT, 95 cardiac genes were differentially expressed compared with vehicle-treated WTs. When MCAT mice were treated with AZT, each of those 95 genes reverted toward the expression of vehicle-treated WTs. In AZT-treated WT hearts, Mthfr [5,10-methylenetetrahydrofolate reductase; a critical enzyme in synthesis of methionine cycle intermediates including S-adenosylmethionine (SAM)], was overexpressed. Steady-state abundance of SAM in cardiac extracts from AZT-treated MCAT mice increased 60% above that of vehicle-treated MCAT. No such change occurred in WT. AZT caused hypermethylation (47%) and hypomethylation (53%) of differentially methylated DNA regions in WT cardiac DNA. AZT-treated MCAT heart DNA exhibited greater hypermethylation (91%) and less hypomethylation (9%) compared with vehicle-treated MCAT controls. The gene encoding protein kinase C-α displayed multifocal epigenetic regulation caused by oxidative stress. Results show that mitochondrially derived oxidative stress in the heart hinders cardiac DNA methylation, alters steady-state abundance of SAM, alters cardiac gene expression, and promotes characteristic pathophysiological changes of cardiomyopathy. This mechanism for NRTI toxicity offers insight into long-term side effects from these commonly used antiviral agents.

The changes in bone organic and inorganic matrix in newborn rats after maternal application of antiretroviral agents: Indinavir and zidovudine

This work presents results concerning influence of indinavir (protease inhibitor, PI(1)) and zidovudine (nucleoside and nucleotide inhibitor of reverse transcriptase, NRTI) administered to pregnant Wistar rat females on organic and mineral constituents of bones and teeth (mandibles, skulls, tibiae, femurs, and incisors) of their offspring at the age of: 7, 14, and 28 days studied by means of induced laser and X-ray fluorescence spectroscopy supported by digital radiography. Influence of indinavir administered to pregnant female rats on bone of their offspring revealed mainly in changes of mineral concentration: lowered Ca concentration and disturbances of trace elements. Zidovudine influenced organic matter more than inorganic matrix which was seen in enhancement of LIF fluorescence. However, there was also an unexpected increase of bone density for rats from zidovudine group, unlike indinavir group, observed. Our studies suggest that studied antiretroviral agents given to pregnant women, may have different destructive impact on bone state of their offspring in the first period of life. Maternal administration of zidovudine may delay development of organic matrix, while indinavir may have adverse effects on inorganic structure.

Neurotoxic effects of AZT on developing and adult neurogenesis

Azidothymidine (AZT) is a synthetic, chain-terminating nucleoside analog used to treat HIV-1 infection. While AZT is not actively transported across the blood brain barrier, it does accumulate at high levels in cerebrospinal fluid, and subsequently diffuses into the overlying parenchyma. Due to the close anatomical proximity of the neurogenic niches to the ventricular system, we hypothesize that diffusion from CSF exposes neural stem/progenitor cells and their progeny to biologically relevant levels of AZT sufficient to perturb normal cell functions. We employed in vitro and in vivo models of mouse neurogenesis in order to assess the effects of AZT on developing and adult neurogenesis. Using in vitro assays we show that AZT reduces the population expansion potential of neural stem/progenitor cells by inducing senescence. Additionally, in a model of in vitro neurogenesis AZT severely attenuates neuroblast production. These effects are mirrored in vivo by clinically-relevant animal models. We show that in utero AZT exposure perturbs both population expansion and neurogenesis among neural stem/progenitor cells. Additionally, a short-term AZT regimen in adult mice suppresses subependymal zone neurogenesis. These data reveal novel negative effects of AZT on neural stem cell biology. Given that the sequelae of HIV infection often include neurologic deficits—subsumed under AIDS Dementia Complex (Brew, 1999)—it is important to determine to what extent AZT negatively affects neurological function in ways that contribute to, or exacerbate, ADC in order to avoid attributing iatrogenic drug effects to the underlying disease process, and thereby skewing the risk/benefit analysis of AZT therapy.

Metabolic complications of in utero maternal HIV and antiretroviral exposure in HIV-exposed infants

BACKGROUND

Despite a wide body of literature supporting the use of antenatal antiretrovirals (ARV) for the prevention of mother-to-child transmission, there remains a need for continued monitoring as the intrauterine interval is a critical period during which fetal programming influences the future health and development of the child.

METHODS

We conducted a systematic review of the current literature addressing potential metabolic complications of in utero HIV and ARV exposure. We describe studies evaluating metabolic outcomes such as intrauterine and early postnatal growth, bone health and mitochondrial toxicity.

RESULTS

Overall, infants exposed to HIV/ARV do not appear to exhibit vastly compromised intrauterine or early postnatal growth. However, some studies on the effect of combination antiretroviral therapy on small for gestational age and low birth weight outcomes in low-middle income countries show a risk for small for gestational age/low birth weight while those in the United States do not. Postnatal growth to 1 year does not appear to be affected by intrauterine tenofovir exposure in African studies, but a US study found statistically significant differences in length for age z scores (LAZ) at 1 year. Little data exists on long-term bone health. Mitochondrial toxicity including abnormal mitochondrial morphology and DNA content, as well as neurologic deficits and death, have been demonstrated in HIV/ARV-exposed infants.

CONCLUSION

Although gross measures of metabolic well-being appear to be reassuring, careful vigilance of even small risks for potential serious adverse effects to infants exposed to intrauterine HIV/ARVs is warranted as intrauterine fetal metabolic programming may substantially impact the future health of the child.

Aging and HIV/AIDS: pathogenetic role of therapeutic side effects

The intersection of aging and HIV/AIDS is a looming 'epidemic within an epidemic.' This paper reviews how HIV/AIDS and its therapy cause premature aging or contribute mechanistically to HIV-associated non-AIDS illnesses (HANA). Survival with HIV/AIDS has markedly improved by therapy combinations containing nucleoside reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors, and protease inhibitors (PIs) called HAART (highly active antiretroviral therapy). Because NRTIs and PIs together prevent or attenuate HIV-1 replication, and prolong life, the population of aging patients with HIV/AIDS increases accordingly. However, illnesses frequently associated with aging in the absence of HIV/AIDS appear to occur prematurely in HIV/AIDS patients. Theories that help to explain biological aging include oxidative stress (where mitochondrial oxidative injury exceeds antioxidant defense), chromosome telomere shortening with associated cellular senescence, and accumulation of lamin A precursors (a nuclear envelop protein). Each of these has the potential to be enhanced or caused by HIV/AIDS, antiretroviral therapy, or both. Antiretroviral therapy has been shown to enhance events seen in biological aging. Specifically, antiretroviral NRTIs cause mitochondrial dysfunction, oxidative stress, and mitochondrial DNA defects that resemble features of both HANA and aging. More recent clinical evidence points to telomere shortening caused by NRTI triphosphate-induced inhibition of telomerase, suggesting telomerase reverse transcriptase (TERT) inhibition as being a pathogenetic contributor to premature aging in HIV/AIDS. PIs may also have a role in premature aging in HIV/AIDS as they cause prelamin A accumulation. Overall, toxic side effects of HAART may both resemble and promote events of aging and are worthy of mechanistic studies.

Metabolism of deoxypyrimidines and deoxypyrimidine antiviral analogs in isolated brain mitochondria

The goal of this project was to characterize deoxypyrimidine salvage pathways used to maintain deoxynucleoside triphosphate pools in isolated brain mitochondria and to determine the extent that antiviral pyrimidine analogs utilize or affect these pathways. Mitochondria from rat brains were incubated in media with labeled and unlabeled deoxynucleosides and deoxynucleoside analogs. Products were analyzed by HPLC coupled to an inline UV monitor and liquid scintillation counter. Isolated mitochondria transported thymidine and deoxycytidine into the matrix, and readily phosphorylated both of these to mono-, di-, and tri-phosphate nucleotides. Rates of phosphorylation were much higher than rates observed in mitochondria from heart and liver. Deoxyuridine was phosphorylated much more slowly than thymidine and only to dUMP. 3'-azido-3'-deoxythymidine, zidovudine (AZT), an antiviral thymidine analog, was phosphorylated to AZT-MP as readily as thymidine was phosphorylated to TMP, but little if any AZT-DP or AZT-TP was observed. AZT at 5.5 ± 1.7 μM was shown to inhibit thymidine phosphorylation by 50%, but was not observed to inhibit deoxycytidine phosphorylation except at levels > 100 μM. Stavudine and lamivudine were inert when incubated with isolated brain mitochondria. The kinetics of phosphorylation of thymidine, dC, and AZT were significantly different in brain mitochondria compared to mitochondria from liver and heart.

Human immunodeficiency virus/acquired immunodeficiency syndrome and infertility: emerging problems in the era of highly active antiretrovirals

OBJECTIVE

To review the effects of human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) in terms of its associated comorbid conditions and the side effects of antiretroviral treatment on fertility.

DESIGN

PubMed computer search to identify relevant articles.

SETTING

Research institution.

PATIENT(S)

None.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

None.

RESULT(S)

Biological alterations in reproductive physiology may account for subfertility in patients infected with HIV. Psychosocial factors in patients with HIV infection may affect their reproductive desires and outcomes. Antiretroviral medications may have direct toxicity on gametes and embryos. Available evidence indicates that fertility treatments can be a safe option for couples with HIV-discordant infection status, although the potential risk of viral transmission cannot be completely eliminated.

CONCLUSION(S)

Because their potential reproductive desires are increasingly becoming a concern in the health care of young HIV-infected patients, additional data are needed to address the effect of HIV and its treatments on their fertility and reproductive outcomes.

Association of thymidylate synthase gene polymorphisms with stavudine triphosphate intracellular levels and lipodystrophy

The antiviral activity and toxicity of stavudine (d4T) depend on its triphosphate metabolite, stavudine triphosphate (d4T-TP). Therefore, modifications in intracellular levels of d4T-TP may change the toxicity profile of stavudine. d4T-TP intracellular levels in peripheral blood mononuclear cells were determined with a prominence liquid chromatograph connected to a triple-quadruple mass spectrometer. Polymorphisms in the thymidylate synthase (TS), methylenetetrahydrofolate reductase (MTHFR), dihydrofolate reductase (DHFR), reduced folate carrier 1 (RFC1; SLC19A1), and cyclin D1 (CCND1) genes were determined by direct sequencing using an ABI Prism 3100 genetic analyzer or Fluidigm's Biomark system. The Mann-Whitney test, rank analysis of variance (with Bonferroni's adjusted post hoc comparisons), and logistic regression were used for the inferential analyses. Thirty-three stavudine-treated patients were enrolled in this cross-sectional study. d4T-TP intracellular levels were 11.50 fmol/10(6) cells (interquartile range [IQR] = 8.12 to 13.87 fmol/10(6) cells) in patients with a high-expression TS genotype (2/3G, 3C/3G, and 3G/3G), whereas in those with a low-expression TS genotype (2/2, 2/3C, and 3C/3C), they were 21.40 fmol/10(6) cells (IQR = 18.90 to 27.0 fmol/10(6) cells) (P < 0.0001). Polymorphisms in the MTHFR, DHFR, RFC1, and CCND1 genes did not influence the intracellular concentration of d4T-TP. d4T-TP levels were independently associated with the TS genotype (low versus high expression; odds ratio [OR] = 86.22; 95% confidence interval [CI] = 8.48 to nonestimable; P = 0.0023). The low-expression TS genotype was associated with the development of HIV/highly active antiretroviral therapy-associated lypodystrophy syndrome (HALS) (OR = 14.0; 95% CI = 2.09 to 108.0; P = 0.0032). Our preliminary data show that polymorphisms in the thymidylate synthase gene are strongly associated with d4T-TP intracellular levels and with development of HALS.

Resveratrol attenuates azidothymidine-induced cardiotoxicity by decreasing mitochondrial reactive oxygen species generation in human cardiomyocytes

Nucleotide reverse transcriptase inhibitors, such as zidovudine (azidothymidine, AZT) and stavudine, represent a class of approved antiretroviral agents for highly active antiretroviral therapy, which prolongs the life expectancy of patients infected with human-immunodeficiency virus. Unfortunately, the use of these drugs is associated with known toxicities in the liver, skeletal muscle, heart and other organs, which may involve increased reactive oxygen species (ROS) generation, among other mechanisms. Resveratrol is a polyphenolic plant-derived antioxidant abundantly found in certain grapes, roots, berries, peanuts and red wine. This study, using primary human cardiomyocytes, evaluated the effects of AZT and pre-treatment with resveratrol on mitochondrial ROS generation and the cell death pathways. AZT induced concentration-dependent cell death, involving both caspase-3 and -7 and poly(ADP-ribose) polymerase activation, coupled with increased mitochondrial ROS generation in human cardiomyocytes. These effects of AZT on mitochondrial ROS generation and cell death may be attenuated by resveratrol pre-treatment. The results demonstrate that mitochondrial ROS generation plays a pivotal role in the cardiotoxicity of AZT in human cardiomyocytes, and resveratrol may provide a potential strategy to attenuate these pathological alterations, which are associated with widely used antiretroviral therapy.

Myocarditis in CD8-depleted SIV-infected rhesus macaques after short-term dual therapy with nucleoside and nucleotide reverse transcriptase inhibitors

BACKGROUND

Although highly active antiretroviral therapy (HAART) has dramatically reduced the morbidity and mortality associated with HIV infection, a number of antiretroviral toxicities have been described, including myocardial toxicity resulting from the use of nucleotide and nucleoside reverse transcriptase inhibitors (NRTIs). Current treatment guidelines recommend the use of HAART regimens containing two NRTIs for initial therapy of HIV-1 positive individuals; however, potential cardiotoxicity resulting from treatment with multiple NRTIs has not been addressed.

METHODOLOGY/PRINCIPAL FINDINGS

We examined myocardial tissue from twelve CD8 lymphocyte-depleted adult rhesus macaques, including eight animals infected with simian immunodeficiency virus, four of which received combined antiretroviral therapy (CART) consisting of two NRTIs [(9-R-2-Phosphonomethoxypropyl Adenine) (PMPA) and (+/-)-beta-2',3'-dideoxy-5-fluoro-3'-thiacytidine (RCV)] for 28 days. Multifocal infiltrates of mononuclear inflammatory cells were present in the myocardium of all macaques that received CART, but not untreated SIV-positive animals or SIV-negative controls. Macrophages were the predominant inflammatory cells within lesions, as shown by immunoreactivity for the macrophage markers Iba1 and CD68. Heart specimens from monkeys that received CART had significantly lower virus burdens than untreated animals (p<0.05), but significantly greater quantities of TNF-α mRNA than either SIV-positive untreated animals or uninfected controls (p<0.05). Interferon-γ (IFN-γ), IL-1β and CXCL11 mRNA were upregulated in heart tissue from SIV-positive monkeys, independent of antiretroviral treatment, but CXCL9 mRNA was only upregulated in heart tissue from macaques that received CART.

CONCLUSIONS/SIGNIFICANCE

These results suggest that short-term treatment with multiple NRTIs may be associated with myocarditis, and demonstrate that the CD8-depleted SIV-positive rhesus monkey is a useful model for studying the cardiotoxic effects of combined antiretroviral therapy in the setting of immunodeficiency virus infection.

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.

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.

Partial selective inhibition of HIV-1 reverse transcriptase and human DNA polymerases γ and β by thiated 3'-fluorothymidine analogue 5'-triphosphates

3'-Deoxy-3'-fluorothymidine (FLT, alovudine(®)) belongs to the most potent agents inhibiting HIV-1 replication. Its 5'-triphosphate (FLTTP) is a potent inhibitor of HIV-1 reverse transcriptase (HIV RT). Unfortunately, FLT exerts substantial hematologic toxicity both in vitro and in vivo. It was suggested that this toxicity may be related to inhibition of human DNA polymerases, especially mitochondrial DNA polymerase γ, by nucleoside analogue 5'-triphosphates leading to termination of DNA synthesis and mitochondrial dysfunction. To decrease the toxicity of FLT, its thiated analogues, 4-SFLT and 2-SFLT, were previously synthesized and shown to be potent inhibitors of HIV-1 with low in vitro cytotoxicity. To explain this phenomenon in the present study the synthesis of 5'-triphosphates of thiated FLT analogues was undertaken and their interaction with recombinant HIV-1 RT and human DNA polymerases γ (pol γ) and β (pol β) was investigated. It was shown that 3'-deoxy-3'-fluoro-4-thiothymidine 5'-triphosphate (4-SFLTTP) and 3'-deoxy-3'-fluoro-2-thiothymidine 5'-triphosphate (2-SFLTTP) were, similarly to FLTTP, potent competitive inhibitors of HIV-1 RT, with K(i)(app) values of 0.091 and 0.022 μM respectively. It is of interest that 2-SFLTTP, a compound in an unusual syn conformation around the glycosidic bond was an uncompetitive inhibitor of human mitochondrial DNA pol γ with K(i)(app) of 0.174 μM, while 4-SFLTTP in anti conformation inhibited this enzyme similarly to FLTTP, i.e., non-competitively, with K(i)(app) of 0.055 μM. Both 4-SFLTTP and 2-SFLTTP were competitive inhibitors of human DNA pol β, with K(i)(app) values of 16.84 and 4.04 μM, respectively. The results point to partially selective inhibition of HIV RT by thiated 3'-fluorothymidine 5'-triphosphate analogues. Of special interest is that 2-SFLTTP, showing syn conformation, is a less potent inhibitor of human mitochondrial pol γ than 4-SFLTTP and FLTTP, both in the anti conformation, and has a higher inhibitory activity against HIV-1 RT than 4-SFLTTP. Moreover, the parent nucleoside 2-SFLT possessing the syn conformation shows a more potent anti-HIV-1 activity and a better selectivity index than its 4-thio isomer in the anti conformation (Matthes et al., 1989; Poopeiko et al., 1995), 2-SFLT is a potent and selective anti-HIV-1 agent with the selectivity index 4-fold higher than that of FLT. Findings regarding the mechanisms of antiviral and cytotoxic activities of FLT and its thioanalogues are discussed.

A transient kinetic approach to investigate nucleoside inhibitors of mitochondrial DNA polymerase gamma

Nucleoside analogs play an essential role in treating human immunodeficiency virus (HIV) infection since the beginning of the AIDS epidemic and work by inhibition of HIV-1 reverse transcriptase (RT), a viral polymerase essential for DNA replication. Today, over 90% of all regimens for HIV treatment contain at least one nucleoside. Long-term use of nucleoside analogs has been associated with adverse effects including mitochondrial toxicity due to inhibition of the mitochondrial polymerase, DNA polymerase gamma (mtDNA pol gamma). In this review, we describe our efforts to delineate the molecular mechanism of nucleoside inhibition of HIV-1 RT and mtDNA pol gamma based upon a transient kinetic approach using rapid chemical quench methodology. Using transient kinetic methods, the maximum rate of polymerization (k(pol)), the dissociation constant for the ground state binding (K(d)), and the incorporation efficiency (k(pol)/K(d)) can be determined for the nucleoside analogs and their natural substrates. This analysis allowed us to develop an understanding of the structure activity relationships that allow correlation between the structural and stereochemical features of the nucleoside analog drugs with their mechanistic behavior toward the viral polymerase, RT, and the host cell polymerase, mtDNA pol gamma. An in-depth understanding of the mechanisms of inhibition of these enzymes is imperative in overcoming problems associated with toxicity.

DNA polymerases as potential therapeutic targets for cancers deficient in the DNA mismatch repair proteins MSH2 or MLH1

Synthetic sickness/lethality (SSL) can be exploited to develop therapeutic strategies for cancer. Deficiencies in the tumor suppressor proteins MLH1 and MSH2 have been implicated in cancer. Here we demonstrate that deficiency in MSH2 is SSL with inhibition of the DNA polymerase POLB, whereas deficiency in MLH1 is SSL with DNA polymerase POLG inhibition. Both SSLs led to the accumulation of 8-oxoG oxidative DNA lesions. MSH2/POLB SSL caused nuclear 8-oxoG accumulation, whereas MLH1/POLG SSL led to a rise in mitochondrial 8-oxoG levels. Both SSLs were rescued by silencing the adenine glycosylase MUTYH, suggesting that lethality could be caused by the formation of lethal DNA breaks upon 8-oxoG accumulation. These data suggest targeted, mechanism-based therapeutic approaches.

Measurement of kinetic parameters of human platelet DNA polymerase gamma

Synthesis of mitochondrial DNA is performed by DNA polymerase gamma. Mutations in POLG, the gene encoding the catalytic subunit of DNA polymerase gamma, are a major cause of neurological disease. A large proportion of patients carry rare nucleotide substitutions leading to single amino acid changes. Confirming that these replacements are pathogenic can be problematic without biochemical evidence. Here, we provide a hands-on protocol for an in vitro kinetic assay of DNA polymerase gamma which allows assessment of the K(m) and V(max) for the incoming nucleotide of the polymerization reaction. To avoid measurement of contaminating nuclear DNA polymerases, platelet extracts are used since platelets do not contain a nucleus. Moreover, platelets have the advantage of being obtainable relatively non-invasively. Polymerization activity is determined by measurement of the incorporation of radioactive thymidine 5'-triphosphate (dTTP) on the homopolymeric RNA substrate poly(rA).oligo(dT)(12-18). To further minimize nuclear DNA polymerase activity, aphidicolin, an inhibitor of most nuclear DNA polymerases, is included in the reaction. In addition, reactions are carried out in the absence and presence of the competitive inhibitor of DNA polymerase gamma, 2',3'-dideoxythymidine 5'-triphosphate (ddTTP), to allow calculation of the ddTTP-sensitive incorporation. With this method, platelets from healthy control subjects extracted with 3% Triton X-100 showed a K(m) for dTTP of 1.42 microM and a V(max) of 0.83 pmol min(-1)mg(-1).

The mitochondrial DNA polymerase in health and disease

Since mutations in mitochondrial DNA (mtDNA) have been shown to be a cause of many mitochondrial diseases as well as aging, it is important to understand the origin of these mutations and how replication proteins modulate this process. DNA polymerase gamma (pol gamma) is the polymerase that is responsible for replication and repair of mtDNA. Pol gamma has three main roles in mtDNA maintenance and mutagenesis. As the only known DNA polymerase in mitochondria, pol gamma is required for all replication and repair functions and is the main source of errors produced in human mtDNA. Pol gamma is also sensitive to a host of antiviral nucleoside analogs used to treat HIV-1 infections, which can cause an induced mitochondrial toxicity. Finally, the gene for pol gamma, POLG, is a genetic locus for several mitochondrial disease with over 150 genetic mutations currently identified.

Long-term therapy with clevudine for chronic hepatitis B can be associated with myopathy characterized by depletion of mitochondrial DNA

Clevudine (Revovir), a pyrimidine nucleoside analogue, is a recently introduced antiviral drug. Clinical trials have demonstrated potent, sustained antiviral activity against hepatitis B virus without specific adverse events. The lack of cytotoxicity and absence of an effect on mitochondrial function have been considered the reasons for the fewer adverse events. However, it came to our attention that several hepatitis B patients developed myopathy during clevudine therapy. Our study was aimed to analyze the clinical and pathological features of patients with clevudine-induced myopathy with some consideration of its pathogenetic mechanism. Seven hepatitis B patients who developed severe skeletal myopathy during clevudine therapy were examined in this study. The demographic data, clinical features, pathological findings, and molecular studies of these patients were analyzed with speculation about the underlying pathogenic mechanisms. All seven patients were treated with clevudine for more than 8 months (8-13 months). In all, the main symptom was slowly progressive proximal muscular weakness over several months. A markedly elevated creatine kinase level and myopathic patterns on electromyography were found. Muscle biopsies revealed severe myonecrosis associated with numerous ragged red fibers, cytochrome c oxidase-negative fibers, and predominant type II fiber atrophy. Molecular studies using quantitative polymerase chain reaction showed a depletion of the mitochondrial DNA in the patients' skeletal muscle.

CONCLUSION

To the best of our knowledge, this is the first report of myopathy associated with clevudine therapy. This study has clearly shown that long-term clevudine therapy can induce the depletion of mitochondrial DNA and lead to mitochondrial myopathy associated with myonecrosis. Careful clinical and laboratory attention should be paid to patients on long-term clevudine therapy for this skeletal muscle dysfunction.

Cytotoxicological analysis of a gp120 binding aptamer with cross-clade human immunodeficiency virus type 1 entry inhibition properties: comparison to conventional antiretrovirals

The long-term cumulative cytotoxicity of antiretrovirals (ARVs) is among the major causes of treatment failure in patients infected with human immunodeficiency virus (HIV) and patients with AIDS. This calls for the development of novel ARVs with less or no cytotoxicity. In the present study, we compared the cytotoxic effects of a cross-clade HIV type 1-neutralizing aptamer called B40 with those of a panel of nonnucleoside reverse transcriptase inhibitors (NNRTIs), nucleoside reverse transcriptase inhibitors (NRTIs), protease inhibitors (PIs), and the entry inhibitor (EI) T20 in human cardiomyocytes and peripheral blood mononuclear cells. An initial screen in which cell death was used as the end-point measurement revealed that the B40 aptamer and T20 were the only test molecules that had insignificant (0.61 < P < 0.92) effects on the viability of both cell types at the maximum concentration used. PIs were the most toxic class (0.001 < P < 0.00001), followed by NNRTIs and NRTIs (0.1 < P < 0.00001). Further studies revealed that B40 and T20 did not interfere with the cellular activity of the cytochrome P450 3A4 enzyme (0.78 < P < 0.24) or monoamine oxidases A and B (0.83 < P < 0.56) when the activities of the enzymes were compared to those in untreated controls of both cell types. Mitochondrion-initiated cellular toxicity is closely associated with the use of ARVs. Therefore, we used real-time PCR to quantify the relative ratio of mitochondrial DNA to nuclear DNA as a marker of toxicity. The levels of mitochondrial DNA remained unchanged in cells exposed to the B40 aptamer compared to the levels in untreated control cells (0.5 > P > 0.06). These data support the development of B40 and related EI aptamers as new ARVs with no cytotoxicity at the estimated potential therapeutic dose.

Cardiovascular manifestations in human immunodeficiency virus-infected patients

Human immunodeficiency virus (HIV) is now a pandemic. It afflicts multiple organs, including the cardiovascular system. This occurs by direct invasion as well as opportunistic infections complicating acquired immunodeficiency syndrome. The presence of newer highly active antiretroviral therapy has led to longer survival of patients infected with HIV, but the cardiac abnormalities related to HIV have remained less well characterized. It is now evident that cardiac involvement in patients with acquired immunodeficiency syndrome is relatively common. This includes coronary artery disease, dilated cardiomyopathy, pericardial effusion, pulmonary hypertension, and ill effects of highly active antiretroviral therapy in the form of lipodystrophy, lipoatrophy, and dyslipidemia. In fact, HIV can now be viewed as a potential risk factor for coronary artery disease, and the dilemma facing clinicians is how to quantify this risk. Awareness of accelerated coronary artery disease and dilated cardiomyopathy is critical to implement preventive measures early in the course of HIV. However, better guidelines are still needed on the basis of prospective randomized controlled studies involving large populations. In conclusion, this review describes cardiac abnormalities associated with HIV, including possible molecular mechanisms. The co-morbid sequelae, their presentation, and pharmacologic management are also discussed.

Drug-induced mitochondrial dysfunction in cardiac and skeletal muscle injury

BACKGROUND

The list of clinically relevant molecules that affect skeletal and cardiac muscle mitochondria is gradually increasing, which strongly suggest that mitochondrial toxicity should be an important end point during the design and testing of novel pharmaceuticals.

OBJECTIVE

The present review intends to describe mechanisms by which clinically relevant drugs are known to alter mitochondrial function in cardiac and skeletal muscle, which is suggested to be involved in the toxicity associated with those drugs.

METHODS

Literature databases were searched in order to identify clinically relevant drugs with associated mitochondrial muscle toxicity.

CONCLUSION

Mitochondrial function is important in the context of muscle survival, hence, the requirement to identify novel mitochondrial targets and develop new therapies to counteract chemical-induced degeneration of mitochondrial function and muscle performance.

Mitochondrial DNA impairment in nucleoside reverse transcriptase inhibitor-associated cardiomyopathy

Acquired immune deficiency syndrome (AIDS) is a global epidemic that continues to escalate. Recent World Health Organization estimates include over 33 million people currently diagnosed with HIV/AIDS. Another 20 million HIV-infected individuals died over the past quarter century. Antiretrovirals are effective treatments that changed the outcome of HIV infection from a fatal disease to a chronic illness. Cardiomyopathy (CM) is a bona fide component of HIV/AIDS with occurrence that is higher in HIV positive individuals. CM may result from individual or combined effects of HIV, immune reactions, or toxicities of prolonged antiretrovirals. Nucleoside reverse transcriptase inhibitors (NRTIs) are the cornerstone of antiretroviral therapy. Despite pharmacological benefits of NRTIs, NRTI side effects include increased risk for CM. Clinical observations and in vitro and in vivo studies support various mechanisms of CM. This perspective highlights some of the hypotheses and focuses on mitochondrial-associated pathways of NRTI- related CM.

Nucleoside reverse transcriptase inhibitors (NRTIs)-induced expression profile of mitochondria-related genes in the mouse liver

Mitochondrial dysfunction has been implicated in the adverse effects of nucleoside reverse transcriptase inhibitors (NRTIs) used to treat HIV-1 infections. To gain insight into the mechanism by which NRTIs alter mitochondrial function, the expression level of 542 genes associated with mitochondrial structure and functions was determined in the livers of p53 haplodeficient (+/-) C3B6F1 female mouse pups using mouse mitochondria-specific oligonucleotide microarray. The pups were transplacentally exposed to zidovudine (AZT) at 240 mg/kg bw/day or a combination of AZT and lamivudine (3TC) at 160 and 100mg/kg bw/day, respectively, from gestation day 12 through 18, followed by continuous treatment by oral administration from postnatal day 1-28. In addition, AZT/3TC effect was investigated in wild-type (+/+) C3B6F1 female mice. The genotype did not significantly affect the gene expression profile induced by AZT/3TC treatment. However, the transcriptional level of several genes associated with oxidative phosphorylation, mitochondrial tRNAs, fatty acid oxidation, steroid biosynthesis, and a few transport proteins were significantly altered in pups treated with AZT and AZT/3TC compared to their vehicle counterparts. Interestingly, AZT/3TC altered the expression level of 153 genes with false discovery rate of less than 0.05, in contrast to only 20 genes by AZT alone. These results suggest that NRTI-related effect on expression level of genes associated with mitochondrial functions was much greater in response to AZT/3TC combination treatment than AZT alone.

Isolation, purification and identification of DNA polymerase gamma

AIM: To purify and identify the mitochondrial DNA polymerase gamma (polymerase γ, Pol γ) from HeLa cells.

METHODS: Ion exchange chromatography was used to isolate Pol γ from HeLa cells. Protein concentration was measured using the Bradford method. SDS-PAGE was performed to determine the molecular weights of the subunits of Pol γ. Following the incorporation of α-³²P-dTTP, the activity of Pol γ was counted using a liquid scintillation spectrometer.

RESULTS: Pol γ was purified by 150-fold to apparent homogeneity, with a 6% yield. SDS-PAGE indicated the presence of one subunit of 140 kDa, and Western blotting identified the specificity. Total activity and specific activity of Pol γ were determined to be 4.81 U and 36.17 U/mg, respectively, by chromatography.

CONCLUSION: Pol γ can be purified by ion exchange chromatography. It can then be activated and used as a target to detect the toxicity of some compounds to mitochondria in vitro.

A brief overview of mechanisms of mitochondrial toxicity from NRTIs

Nucleoside reverse transcriptase inhibitors (NRTIs) in combinations with other antiretrovirals (highly active antiretroviral therapy, HAART) are the cornerstones of AIDS therapy, turning HIV infection into a manageable clinical entity. Despite the initial positive impact of NRTIs, therapeutic experience revealed serious side effects that appeared to originate in the mitochondria and which ultimately manifested as dysfunction of that organelle. It may be reasonable to consider that as the AIDS epidemic continues and as survival with HIV infection is prolonged by treatment with HAART, long-term side effects of NRTIs may become increasingly common. This consideration may be underscored in children who are born to HIV-infected mothers who received NRTI therapy in utero during gestation. The long-term effect of that NRTI exposure in utero is not clear yet. This review examines some proposed mechanisms of NRTI mitochondrial toxicity, including genetic predisposition, defects in mitochondria DNA replication, the encompassing "DNA pol-gamma hypothesis," the relationship between mitochondrial nucleotide and NRTI pools, mitochondrial DNA mutation and dysfunction, and oxidative stresses related to HIV infection and NRTIs. Mechanisms of mitochondrial toxicity are reviewed with respect to key cell biological, pathological, and pharmacological events.

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.

Model for intracellular Lamivudine metabolism in peripheral blood mononuclear cells ex vivo and in human immunodeficiency virus type 1-infected adolescents

The pharmacologic variability of nucleoside reverse transcriptase inhibitors such as lamivudine (3TC) includes not only systemic pharmacokinetic variability but also interindividual differences in cellular transport and metabolism. A modeling strategy linking laboratory studies of intracellular 3TC disposition with clinical studies in adolescent patients is described. Data from ex vivo laboratory experiments using peripheral blood mononuclear cells (PBMCs) from uninfected human subjects were first used to determine a model and population parameter estimates for 3TC cellular metabolism. Clinical study data from human immunodeficiency virus type 1-infected adolescents were then used in a Bayesian population analysis, together with the prior information from the ex vivo analysis, to develop a population model for 3TC systemic kinetics and cellular kinetics in PBMCs from patients during chronic therapy. The laboratory results demonstrate that the phosphorylation of 3TC is saturable under clinically relevant concentrations, that there is a rapid equilibrium between 3TC monophosphate and diphosphate and between 3TC diphosphate and triphosphate, and that 3TC triphosphate is recycled to 3TC monophosphate through a 3TC metabolite that remains to be definitively characterized. The resulting population model shows substantial interindividual variability in the cellular kinetics of 3TC with population coefficients of variation for model parameters ranging from 47 to 87%. This two-step ex vivo/clinical modeling approach using Bayesian population modeling of 3TC that links laboratory and clinical data has potential application for other drugs whose intracellular pharmacology is a major determinant of activity and/or toxicity.

Anti-HIV drugs and the mitochondria

Several drugs are currently used that can significantly prolong the course of the infection with the human immunodeficiency virus (HIV), the cause of the acquired immunodeficiency syndrome (AIDS). Among these drugs, the nucleosidic inhibitors of viral reverse transcriptase can alter mitochondrial (mt) function by inhibiting the mitochondrial DNA polymerase gamma (the enzyme responsible for the replication of mtDNA). Decreased mtDNA content provokes a diminished synthesis of respiratory chain enzymes, leading to alterations in mt function. These are in turn responsible for a variety of side effects frequently observed in HIV+ patients, that range from hyperlactatemia and lactic acidosis to lipodystrophy, a pathology characterized by accumulation of visceral fat, breast adiposity, cervical fat-pads, hyperlipidemia, insulin resistance and fat wasting in face and limbs. In this paper, data concerning the effects of different compounds on mitochondria, their role in the pathogenesis of lipodystrophy, and problems related to studies on the mt toxicity of antiviral drugs are reviewed and thoroughly discussed.

Antiretroviral nucleosides, deoxynucleotide carrier and mitochondrial DNA: evidence supporting the DNA pol gamma hypothesis

DESIGN

Nucleoside reverse transcriptase inhibitors (NRTIs) exhibit mitochondrial toxicity. The mitochondrial deoxynucleotide carrier (DNC) transports nucleotide precursors (or phosphorylated NRTIs) into mitochondria for mitochondrial (mt)DNA replication or inhibition of mtDNA replication by NRTIs. Transgenic mice (TG) expressing human DNC targeted to murine myocardium served to define mitochondrial events from NRTIs in vivo and findings were corroborated by biochemical events in vitro.

METHODS

Zidovudine (3'-azido-2',3'-deoxythymidine; ZDV), stavudine (2', 3'-didehydro-2', 3'-deoxythymidine; d4T), or lamivudine ((-)-2'-deoxy-3'-thiacytidine; 3TC) were administered individually to TGs and wild-type (WT) littermates (35 days) at human doses with drug-free vehicle as control. Left ventricle (LV) mass was defined echocardiographically, mitochondrial ultrastructural defects were identified by electron microscopy, the abundance of cardiac mtDNA was quantified by real time polymerase chain reaction, and mtDNA-encoded polypeptides were quantified.

RESULTS

Untreated TGs exhibited normal LV mass with minor mitochondrial damage. NRTI monotherapy (either d4T or ZDV) increased LV mass in TGs and caused significant mitochondrial destruction. Cardiac mtDNA was depleted in ZDV and d4T-treated TG hearts and mtDNA-encoded polypeptides decreased. Changes were absent in 3TC-treated cohorts. In supportive structural observations from molecular modeling, ZDV demonstrated close contacts with K947 and Y951 in the DNA pol gamma active site that were absent in the HIV reverse transcriptase active site.

CONCLUSIONS

NRTIs deplete mtDNA and polypeptides, cause mitochondrial structural and functional defects in vivo, follow inhibition kinetics with DNA pol gamma in vitro, and are corroborated by molecular models. Disrupted pools of nucleotide precursors and inhibition of DNA pol gamma by specific NRTIs are mechanistically important in mitochondrial toxicity.