3'-Azido-3'-deoxythmidine uptake into isolated rat liver mitochondria and impairment of ADP/ATP translocator

Unlocking mitochondrial drug targets: The importance of mitochondrial transport proteins

A disturbed mitochondrial function contributes to the pathology of many common diseases. These organelles are therefore important therapeutic targets. On the contrary, many adverse effects of drugs can be explained by a mitochondrial off-target effect, in particular, due to an interaction with carrier proteins in the inner membrane. Yet this class of transport proteins remains underappreciated and understudied. The aim of this review is to provide a deeper understanding of the role of mitochondrial carriers in health and disease and their significance as drug targets. We present literature-based evidence that mitochondrial carrier proteins are associated with prevalent diseases and emphasize their potential as drug (off-)target sites by summarizing known mitochondrial drug-transporter interactions. Studying these carriers will enhance our knowledge of mitochondrial drug on- and off-targets and provide opportunities to further improve the efficacy and safety of drugs.

Impaired Mitochondrial Function in T-Lymphocytes as a Result of Exposure to HIV and ART

Mitochondrial dysfunction is a described phenomenon for a number of chronic and infectious diseases. At the same time, the question remains open: is this condition a consequence or a cause of the progression of the disease? In this review, we consider the role of the development of mitochondrial dysfunction in the progression of HIV (human immunodeficiency viruses) infection and the onset of AIDS (acquired immunodeficiency syndrome), as well as the direct impact of HIV on mitochondria. In addition, we will touch upon such an important issue as the effect of ART (Antiretroviral Therapy) drugs on mitochondria, since ART is currently the only effective way to curb the progression of HIV in infected patients, and because the identification of potential side effects can help to more consciously approach the development of new drugs in the treatment of HIV infection.

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.

A Critical Review of the Biochemical Mechanisms and Epigenetic Modifications in HIV- and Antiretroviral-Induced Metabolic Syndrome

Metabolic syndrome (MetS) is a non-communicable disease characterised by a cluster of metabolic irregularities. Alarmingly, the prevalence of MetS in people living with Human Immunodeficiency Virus (HIV) and antiretroviral (ARV) usage is increasing rapidly. This study aimed to look at biochemical mechanisms and epigenetic modifications associated with HIV, ARVs, and MetS. More specifically, emphasis was placed on mitochondrial dysfunction, insulin resistance, inflammation, lipodystrophy, and dyslipidaemia. We found that mitochondrial dysfunction was the most common mechanism that induced metabolic complications. Our findings suggest that protease inhibitors (PIs) are more commonly implicated in MetS-related effects than other classes of ARVs. Furthermore, we highlight epigenetic studies linking HIV and ARV usage to MetS and stress the need for more studies, as the current literature remains limited despite the advancement in and popularity of epigenetics.

A Walk in the Memory, from the First Functional Approach up to Its Regulatory Role of Mitochondrial Bioenergetic Flow in Health and Disease: Focus on the Adenine Nucleotide Translocator

The mitochondrial adenine nucleotide translocator (ANT) plays the fundamental role of gatekeeper of cellular energy flow, carrying out the reversible exchange of ADP for ATP across the inner mitochondrial membrane. ADP enters the mitochondria where, through the oxidative phosphorylation process, it is the substrate of Fo-F1 ATP synthase, producing ATP that is dispatched from the mitochondrion to the cytoplasm of the host cell, where it can be used as energy currency for the metabolic needs of the cell that require energy. Long ago, we performed a method that allowed us to monitor the activity of ANT by continuously detecting the ATP gradually produced inside the mitochondria and exported in the extramitochondrial phase in exchange with externally added ADP, under conditions quite close to a physiological state, i.e., when oxidative phosphorylation takes place. More than 30 years after the development of the method, here we aim to put the spotlight on it and to emphasize its versatile applicability in the most varied pathophysiological conditions, reviewing all the studies, in which we were able to observe what really happened in the cell thanks to the use of the "ATP detecting system" allowing the functional activity of the ANT-mediated ADP/ATP exchange to be measured.

Characterization of drug-induced human mitochondrial ADP/ATP carrier inhibition

An increasing number of commonly prescribed drugs are known to interfere with mitochondrial function, causing cellular toxicity, but the underlying mechanisms are largely unknown. Although often not considered, mitochondrial transport proteins form a significant class of potential mitochondrial off-targets. So far, most drug interactions have been reported for the mitochondrial ADP/ATP carrier (AAC), which exchanges cytosolic ADP for mitochondrial ATP. Here, we show inhibition of cellular respiratory capacity by only a subset of the 18 published AAC inhibitors, which questions whether all compound do indeed inhibit such a central metabolic process. This could be explained by the lack of a simple, direct model system to evaluate and compare drug-induced AAC inhibition.

Methods: For its development, we have expressed and purified human AAC1 (hAAC1) and applied two approaches. In the first, thermostability shift assays were carried out to investigate the binding of these compounds to human AAC1. In the second, the effect of these compounds on transport was assessed in proteoliposomes with reconstituted human AAC1, enabling characterization of their inhibition kinetics.

Results: Of the proposed inhibitors, chebulinic acid, CD-437 and suramin are the most potent with IC₅₀-values in the low micromolar range, whereas another six are effective at a concentration of 100 μM. Remarkably, half of all previously published AAC inhibitors do not show significant inhibition in our assays, indicating that they are false positives. Finally, we show that inhibitor strength correlates with a negatively charged surface area of the inhibitor, matching the positively charged surface of the substrate binding site.

Conclusion: Consequently, we have provided a straightforward model system to investigate AAC inhibition and have gained new insights into the chemical compound features important for inhibition. Better evaluation methods of drug-induced inhibition of mitochondrial transport proteins will contribute to the development of drugs with an enhanced safety profile.

The Impact of HIV- and ART-Induced Mitochondrial Dysfunction in Cellular Senescence and Aging

According to the WHO, 38 million individuals were living with human immunodeficiency virus (HIV), 25.4 million of which were using antiretroviral therapy (ART) at the end of 2019. Despite ART-mediated suppression of viral replication, ART is not a cure and is associated with viral persistence, residual inflammation, and metabolic disturbances. Indeed, due to the presence of viral reservoirs, lifelong ART therapy is required to control viremia and prevent disease progression into acquired immune deficiency syndrome (AIDS). Successful ART treatment allows people living with HIV (PLHIV) to achieve a similar life expectancy to uninfected individuals. However, recent studies have illustrated the presence of increased comorbidities, such as accelerated, premature immune aging, in ART-controlled PLHIV compared to uninfected individuals. Studies suggest that both HIV-infection and ART-treatment lead to mitochondrial dysfunction, ultimately resulting in cellular exhaustion, senescence, and apoptosis. Since mitochondria are essential cellular organelles for energy homeostasis and cellular metabolism, their compromise leads to decreased oxidative phosphorylation (OXPHOS), ATP synthesis, gluconeogenesis, and beta-oxidation, abnormal cell homeostasis, increased oxidative stress, depolarization of the mitochondrial membrane potential, and upregulation of mitochondrial DNA mutations and cellular apoptosis. The progressive mitochondrial damage induced by HIV-infection and ART-treatment likely contributes to accelerated aging, senescence, and cellular dysfunction in PLHIV. This review discusses the connections between mitochondrial compromise and cellular dysfunction associated with HIV- and ART-induced toxicities, providing new insights into how HIV and current ART directly impact mitochondrial functions and contribute to cellular senescence and aging in PLHIV. Identifying this nexus and potential mechanisms may be beneficial in developing improved therapeutics for treating PLHIV.

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.

Endothelial mitochondrial senescence accelerates cardiovascular disease in antiretroviral-receiving HIV patients

Combination antiretroviral therapy (cART) has been hugely successful in reducing the mortality associated with human immunodeficiency virus (HIV) infection, resulting in a growing population of people living with HIV (PLWH). Since PLWH now have a longer life expectancy, chronic comorbidities have become the focus of the clinical management of HIV. For example, cardiovascular complications are now one of the most prevalent causes of death in PLWH. Numerous epidemiological studies show that antiretroviral treatment increases cardiovascular disease (CVD) risk and early onset of CVD in PLWH. Nucleoside reverse transcriptase inhibitors (NRTIs) are the backbone of cART, and two NRTIs are typically used in combination with one drug from another drug class, e.g., a fusion inhibitor. NRTIs are known to induce mitochondrial dysfunction, contributing to toxicity in numerous tissues, such as myopathy, lipoatrophy, neuropathy, and nephropathy. In in vitro studies, short-term NRTI treatment induces an endothelial dysfunction with an increased reactive oxygen species (ROS) production; long-term NRTI treatment decreases cell replication capacity, while increasing mtROS production and senescent cell accumulation. These findings suggest that a mitochondrial oxidative stress is involved in the pathogenesis of NRTI-induced endothelial dysfunction and premature senescence. Mitochondrial dysfunction, defined by a compromised mitochondrial quality control via biogenesis and mitophagy, has a causal role in premature endothelial senescence and can potentially initiate early cardiovascular disease (CVD) development in PLWH. In this review, we explore the hypothesis and present literature supporting that long-term NRTI treatment induces vascular dysfunction by interfering with endothelial mitochondrial homeostasis and provoking mitochondrial genomic instability, resulting in premature endothelial senescence.

DNA polymerase-γ hypothesis in nucleoside reverse transcriptase-induced mitochondrial toxicity revisited: A potentially protective role for citrus fruit-derived naringenin?

Nucleoside reverse transcriptase inhibitors (NRTIs) form the backbone in combination antiretroviral therapy (cARVs). They halt chain elongation of the viral cDNA by acting as false substrates in counterfeit incorporation mechanism to viral RNA-dependent DNA polymerase. In the process genomic DNA polymerase as well as mitochondrial DNA (mtDNA) polymerase-γ (which has a much higher affinity for these drugs at therapeutic doses) are also impaired. This leads to mitochondrial toxicity that manifests clinically as mitochondrial myopathy, peripheral neuropathy, hyperlactatemia or lactic acidosis and lipoatrophy. This has led to the revision of clinical guidelines by World Health Organization to remove stavudine from first-line listing in the treatment of HIV infections. Recent reports have implicated oxidative stress besides mtDNA polymerase-γ hypothesis in NRTI-induced metabolic complications. Reduced plasma antioxidant concentrations have been reported in HIV positive patients on cARVs but clinical intervention with antioxidant supplements have not been successful either due to low efficacy or poor experimental designs. Citrus fruit-derived naringenin has previously been demonstrated to possess antioxidant and free radical scavenging properties which could prevent mitochondrial toxicity associated with these drugs. This review revisits the controversy surrounding mtDNA polymerase-γ hypothesis and evaluates the potential benefits of naringenin as a potent anti-oxidant and free radical scavenger which as a nutritional supplement or therapeutic adjunct could mitigate the development of mitochondrial toxicity associated with these drugs.

Absence of effect of the antiretrovirals Duovir and Viraday on mitochondrial bioenergetics

Most toxicity associated with antiretroviral drugs is thought to result from disruption of mitochondrial function. Unfortunately, there are no validated laboratory markers for clinically assessing the onset of mitochondrial toxicity associated with antiretroviral therapy. In a previous study on mitochondrial hepatocytes, the protease inhibitor lopimune was shown to induce mitochondrial toxicity by increasing reactive oxygen species (ROS) production and decreasing respiratory control ratio (RCR) reflecting compromised mitochondrial efficiency in adenosine triphosphate production. Mitochondrial dysfunction and ROS production were directly correlated with the expression of uncoupling protein 2 (UCP2). In the current study we aim to determine the toxicity of nucleoside or nucleotide and nonnucleoside reverse-transcriptase inhibitors, Duovir and Viraday on liver mitochondria isolated from treated mice by monitoring UCP2 expression. Our results showed that both Duovir and Viraday had no effect on mitochondrial respiration states 2, 3, 4, and on RCR. In addition, ROS generation and UCP2 expression were not affected. In conclusion, our results indicate the difference in the mechanism of action of distinct classes of antiretroviral drugs on mitochondrial functions and may associate UCP2 expression with subclinical mitochondrial damage as marker of cellular oxidative stress.

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.

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.

Different effects of guanine nucleotides (GDP and GTP) on protein-mediated mitochondrial proton leak

In this study, we compared the influence of GDP and GTP on isolated mitochondria respiring under conditions favoring oxidative phosphorylation (OXPHOS) and under conditions excluding this process, i.e., in the presence of carboxyatractyloside, an adenine nucleotide translocase inhibitor, and/or oligomycin, an FOF1-ATP synthase inhibitor. Using mitochondria isolated from rat kidney and human endothelial cells, we found that the action of GDP and GTP can differ diametrically depending on the conditions. Namely, under conditions favoring OXPHOS, both in the absence and presence of linoleic acid, an activator of uncoupling proteins (UCPs), the addition of 1 mM GDP resulted in the state 4 (non-phosphorylating respiration)-state 3 (phosphorylating respiration) transition, which is characteristic of ADP oxidative phosphorylation. In contrast, the addition of 1 mM GTP resulted in a decrease in the respiratory rate and an increase in the membrane potential, which is characteristic of UCP inhibition. The stimulatory effect of GDP, but not GTP, was also observed in inside-out submitochondrial particles prepared from rat kidney mitochondria. However, the effects of GDP and GTP were more similar in the presence of OXPHOS inhibitors. The importance of these observations in connection with the action of UCPs, adenine nucleotide translocase (or other carboxyatractyloside-sensitive carriers), carboxyatractyloside- and purine nucleotide-insensitive carriers, as well as nucleoside-diphosphate kinase (NDPK) are considered. Because the measurements favoring oxidative phosphorylation better reflect in vivo conditions, our study strongly supports the idea that GDP cannot be considered a significant physiological inhibitor of UCP. Moreover, it appears that, under native conditions, GTP functions as a more efficient UCP inhibitor than GDP and ATP.

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.

Human FAD synthase (isoform 2): a component of the machinery that delivers FAD to apo-flavoproteins

A soluble form of human FAD synthase (isoform 2; hFADS2) was produced and purified to homogeneity as a recombinant His-tagged protein. The enzyme binds 1 mole of the FAD product very tightly, although noncovalently. Complete release of FAD from the 'as isolated' protein requires extensive denaturation. A 75 : 25 mixture of apo/holoprotein could be prepared by treatment with mild chaotropes, allowing estimatation of the contribution made by bound FAD to the protein stability and evaluatation of whether structural rearrangements may be required for FAD release. Under turnover conditions, the enzyme catalyzes FAD assembly from ATP and FMN and, at a much lower rate, the pyrophosphorolytic hydrolysis of FAD. Several mechanistic features of both reactions were investigated in detail, along with their dependence on environmental conditions (pH, temperature, dependence on metals). Our data indicate that FAD release may represent the rate-limiting step of the whole catalytic cycle and that the process leading to FAD synthesis, and delivery to client apoproteins may be tightly controlled.

Nucleoside analogues improve the short-term and long-term prognosis of patients with hepatitis B virus-related acute-on-chronic liver failure

Hepatitis B virus (HBV) infection is a major public health problem, and HBV-related acute-on-chronic liver failure (ACLF) has an extremely poor prognosis. There is no standard approach for managing ACLF. Nucleos(t)ide analogue has been proven effective in suppressing viral replication, improving histology and biochemical, and decreasing the inflammatory response in patients with chronic hepatitis B. This study was designed to evaluate the short-term and long-term efficacy of nucleoside analogue treatment of patients with HBV-related ACLF. One hundred and six consecutive subjects were recruited from 2,308 patients with elevated alanine aminotransferase activity. Forty-two patients were treated with 0.5 mg entecavir (ETV) daily (ETV group); 30 patients received 100 mg lamivudine (LAM) daily (LAM group); 34 patients did not take any nucleos(t)ide analogues (non-NAs group). All eligible patients were given standard medical treatment. All the patients were followed up until death or until October 2010. The HBV DNA levels and the short-term and long-term efficacy of the drugs were evaluated. After 3 weeks of nucleoside analogue treatment and/or supportive therapy, HBV DNA levels were decreased when compared with the baseline level in the ETV group (7.04 ± 1.58 log(10) IU/mL vs. 4.03 ± 2.04 log(10) IU/mL, P = 0.001), the LAM group (7.25 ± 0.89 log(10) IU/mL vs. 4.33 ± 2.48 log(10) IU/mL, P = 0.01), and the non-NAs group (5.73 ± 0.96 log(10) IU/mL vs. 4.21 ± 1.47 log(10) IU/mL, P = 0.01). The ETV and LAM groups showed a similar accumulative mortality in the first 3 months of treatment (33.3% vs. 40%, χ(2) = 0.568, P = 0.374). The non-NAs group had a significantly high mortality, compared with the ETV group (64.7% vs. 33.3%, χ(2) = 7.163, P = 0.007), the LAM group (64.7% vs. 40%, χ(2) = 3.906, P = 0.042), and the nucleoside analogue group (ETV group + LAM group) (64.7% vs. 36.2%, χ(2) = 7.443, P = 0.006). All the 56 patients survived were followed up to October 2010. The median follow-up period was 7.3 months. Recurrence was observed in a total of 6 patients (10.72%), of whom 4 patients (33.33%) were from the non-NAs group, 2 (11.11%) from the LAM group after cessation LAM therapy by patients himself, and 0 from the ETV group (P = 0.003). Nucleoside analogue may improve the short-term and long-term prognosis of patients with HBV-related ACLF.

Central role of mitochondria in drug-induced liver injury

A frequent mechanism for drug-induced liver injury (DILI) is the formation of reactive metabolites that trigger hepatitis through direct toxicity or immune reactions. Both events cause mitochondrial membrane disruption. Genetic or acquired factors predispose to metabolite-mediated hepatitis by increasing the formation of the reactive metabolite, decreasing its detoxification, or by the presence of critical human leukocyte antigen molecule(s). In other instances, the parent drug itself triggers mitochondrial membrane disruption or inhibits mitochondrial function through different mechanisms. Drugs can sequester coenzyme A or can inhibit mitochondrial β-oxidation enzymes, the transfer of electrons along the respiratory chain, or adenosine triphosphate (ATP) synthase. Drugs can also destroy mitochondrial DNA, inhibit its replication, decrease mitochondrial transcripts, or hamper mitochondrial protein synthesis. Quite often, a single drug has many different effects on mitochondrial function. A severe impairment of oxidative phosphorylation decreases hepatic ATP, leading to cell dysfunction or necrosis; it can also secondarily inhibit ß-oxidation, thus causing steatosis, and can also inhibit pyruvate catabolism, leading to lactic acidosis. A severe impairment of β-oxidation can cause a fatty liver; further, decreased gluconeogenesis and increased utilization of glucose to compensate for the inability to oxidize fatty acids, together with the mitochondrial toxicity of accumulated free fatty acids and lipid peroxidation products, may impair energy production, possibly leading to coma and death. Susceptibility to parent drug-mediated mitochondrial dysfunction can be increased by factors impairing the removal of the toxic parent compound or by the presence of other medical condition(s) impairing mitochondrial function. New drug molecules should be screened for possible mitochondrial effects.

Mitochondrial involvement in drug-induced liver injury

Mitochondrial dysfunction is a major mechanism of liver injury. A parent drug or its reactive metabolite can trigger outer mitochondrial membrane permeabilization or rupture due to mitochondrial permeability transition. The latter can severely deplete ATP and cause liver cell necrosis, or it can instead lead to apoptosis by releasing cytochrome c, which activates caspases in the cytosol. Necrosis and apoptosis can trigger cytolytic hepatitis resulting in lethal fulminant hepatitis in some patients. Other drugs severely inhibit mitochondrial function and trigger extensive microvesicular steatosis, hypoglycaemia, coma, and death. Milder and more prolonged forms of drug-induced mitochondrial dysfunction can also cause macrovacuolar steatosis. Although this is a benign liver lesion in the short-term, it can progress to steatohepatitis and then to cirrhosis. Patient susceptibility to drug-induced mitochondrial dysfunction and liver injury can sometimes be explained by genetic or acquired variations in drug metabolism and/or elimination that increase the concentration of the toxic species (parent drug or metabolite). Susceptibility may also be increased by the presence of another condition, which also impairs mitochondrial function, such as an inborn mitochondrial cytopathy, beta-oxidation defect, certain viral infections, pregnancy, or the obesity-associated metabolic syndrome. Liver injury due to mitochondrial dysfunction can have important consequences for pharmaceutical companies. It has led to the interruption of clinical trials, the recall of several drugs after marketing, or the introduction of severe black box warnings by drug agencies. Pharmaceutical companies should systematically investigate mitochondrial effects during lead selection or preclinical safety studies.

Increased mtDNA levels without change in mitochondrial enzymes in peripheral blood mononuclear cells of infants born to HIV-infected mothers on antiretroviral therapy

BACKGROUND

The effects of gestational nucleoside reverse transcriptase inhibitors (NRTIs) on mitochondrial DNA (mtDNA) are controversial. The effects of mtDNA depletion on mitochondrial function have not been assessed.

METHOD

In peripheral blood mononuclear cells (PBMCs) from infants born to HIV-infected women and infants born to HIV-1-uninfected women, mtDNA copy numbers were determined by quantitative PCR; nuclear (COXIV)- and mitochondrial (COXII)-encoded polypeptides of the oxidative phosphorylation enzyme cytochrome c-oxidase (COX or complex IV) were quantified by Western blot.

RESULTS

Overall, 86 infants born to HIV-infected women and 50 controls were studied. HIV-infected mothers had a median CD4 count of 506 cells/microL; 59% had HIV RNA 50 copies/mL. No infant had clinical evidence of mitochondrial disease. The birth weight was lower (p = .016) and the body length higher (p = .002) in the HIV-exposed newborns. Eighty-one HIV-infected women had received gestational NRTIs (median duration 162 days). Median mtDNA copies/PBMC in the HIV-exposed infants were 505 (range, 120-1365) vs. 213 (27-426) in controls (p < .001). COX II/IV ratios were similar in both groups. Although mtDNA levels correlated inversely with maternal lactate, mitochondrial indices did not correlate with maternal CD4+ count, HIV RNA, smoking, or alcohol consumption.

CONCLUSION

We found elevated mtDNA copy numbers in PBMC of infants born to HIV-infected women, the majority of whom received NRTI-based therapy, when compared to those born to healthy HIV-negative controls, but there was no difference in mtDNA-encoded respiratory chain protein. The clinical consequence of these findings is unknown and requires further investigations.

The role of mitochondrial and plasma membrane nucleoside transporters in drug toxicity

Many anticancer and antiviral drugs are nucleoside analogues, which interfere with nucleotide metabolism and DNA replication to produce pharmacological effects. Clinical efficacy and toxicity of nucleoside drugs are closely associated with nucleoside transporters because they mediate the transport of nucleoside drugs across biological membranes. Two families of human nucleoside transporters (equilibrative nucleoside transporters and concentrative nucleoside transporters) have been extensively studied for several decades. They are widely distributed, from the plasma membrane to membranes of organelles such as mitochondria, and the distribution differs in different tissues. In addition, they have different specificities to nucleoside drugs. The characteristics of equilibrative and concentrative nucleoside transporters affect the therapeutic outcomes achieved with anticancer and antiviral nucleoside drugs. In this review, an overview of the role of mitochondrial and plasma membrane nucleoside transporters in nucleoside drug toxicity is provided. Rational design and therapeutic application of nucleoside analogues are also discussed.

Mitochondrial Toxicity of Tenofovir, Emtricitabine and Abacavir Alone and in Combination with Additional Nucleoside Reverse Transcriptase Inhibitors

Background

Some nucleoside/nucleotide reverse transcriptase inhibitor (NRTI) combinations cause additive or synergistic interactions in vitro and in vivo.

Methods

We evaluated the mitochondrial toxicity of tenofovir (TFV), emtricitabine (FTC) and abacavir as carbovir (CBV) alone, with each other, and in combination with additional NRTIs. HepG2 human hepatoma cells were incubated with TFV, FTC, CBV, didanosine (ddI), stavudine (d4T), lamivudine (3TC) and zidovudine (AZT) at concentrations equivalent to 1 and 10x clinical steady-state peak plasma levels (Cmax). NRTIs were also used in double and triple combinations. Cell growth, lactate production, intracellular lipids, mtDNA and the mtDNA-encoded respiratory chain subunit II of cytochrome c oxidase (COXII) were monitored for 25 days.

Results

TFV and 3TC had no or minimal toxicity. FTC moderately reduced hepatocyte proliferation independent of effects on mtDNA. ddI and d4T induced a time- and dose-dependent loss of mtDNA and COXII, decreased cell growth and increased levels of lactate and intracellular lipids. CBV and AZT strongly impaired hepatocyte proliferation and increased lactate and lipid production, but did not induce mtDNA depletion. The dual combination of TFV plus 3TC had only minimal toxicity; TFV plus FTC slightly reduced cell proliferation without affecting mitochondrial parameters. All other combinations exhibited more pronounced adverse effects on mitochondrial endpoints. Toxic effects on mitochondrial parameters were observed in all combinations with ddI, d4T, AZT or CBV. TFV and 3TC both attenuated ddI-related cytotoxicity, but worsened the effects of CBV and AZT.

Conclusions

The data demonstrate unpredicted interactions between NRTIs with respect to toxicological endpoints and provide an argument against the liberal use of NRTI cocktails without first obtaining data from clinical trials.

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.

High Doses of Stavudine Induce Fat Wasting and Mild Liver Damage without Impairing Mitochondrial Respiration in Mice

Objective

Stavudine (d4T), a nucleoside reverse-transcriptase inhibitor (NRTI), can induce lipoatrophy, fatty liver, hyperlactataemia and abnormal liver tests. NRTI toxicity is usually ascribed to mitochondrial DNA (mtDNA) depletion and impaired mitochondrial respiration. However, NRTIs could have effects unrelated to mtDNA. Recently, we reported that 100 mg/kg/day of d4T stimulated fatty acid oxidation (FAO) in mouse liver, and reduced body fatness without depleting white adipose tissue (WAT) mtDNA. We hypothesized that higher d4T doses could further reduce adiposity, while inhibiting hepatic FAO.

Methods

Mice were treated for 2 weeks with d4T (500 mg/kg/day), L-carnitine (200 mg/kg/day) or both drugs concomitantly. Body fatness was assessed by dual energy X-ray absorptiometry, and investigations were performed in plasma, liver, muscle and WAT.

Results

D4T reduced the gain of body adiposity, WAT leptin, whole body FAO and plasma ketone bodies, and increased liver triglycerides and plasma aminotransferases with mild ultrastructural abnormalities in hepatocytes. Plasma lactate and respiratory chain activities in tissues were unchanged. Stearoyl-CoA desaturase (SCD-1), an enzyme negatively regulated by leptin, was overexpressed in liver. High doses of β-aminoisobutyric acid (BAIBA), a d4T catabolite, increased plasma ketone bodies. Although L-carnitine did not correct body adiposity, it prevented d4T-induced impairment of FAO and liver abnormalities.

Conclusions

D4T overdosage triggers fat wasting, leptin insufficiency and mild liver damage, without causing respiratory chain dysfunction. Overexpression of SCD-1 reduces fatty acid oxidation and overcomes the stimulating effect of BAIBA on hepatic FAO. L-carnitine does not correct leptin insufficiency but prevents d4T-induced impairment of FAO and liver damage.

Tratamiento antirretroviral y toxicidad mitocondrial

The introduction of highly active antiretroviral therapy for the treatment of human immunodeficiency virus (HIV) infection has led to substantial reduction in morbidity and near-complete suppression of HIV-1 replication. This progress has been tempered by a growing number of new adverse effects. Mitochondrial toxicity is one aspect of these long-term toxicities of antiretroviral drugs, with the role of nucleoside analogs being particularly underlined. Some cases of impaired mitochondrial function have been clearly identified, such as pancreatitis, neuropathy, miopathy and lactic acidosis. Beyond the inhibition of DNA polymerase-g using nucleoside analogs, it appears that several physiopathologic mechanisms interact to explain the observed toxicity. At present there is no reliable method to detect subclinical mitochondrial toxicity. There is no proven effective therapy for antiretroviral therapy-associated mitochondrial toxicity other than ceasing the implicated agent, and even with this strategy, resolution of symptoms may be incomplete. Therefore, investigation of mitochondrial toxicity of new compounds or new combinations is of growing interest for the clinical application of antiretroviral agents.

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.

A computational model of mitochondrial AZT metabolism

The mechanisms of the mitochondrial toxicity of AZT (azidothymidine; zidovudine) are not clear. The two main contenders are the incorporation of phosphorylated AZT into the mtDNA (mitochondrial DNA) and the competitive inhibition of natural deoxynucleotide metabolism. We have built a computational model of AZT metabolism in mitochondria in order to better understand these toxicity mechanisms. The model includes the transport of non-phosphorylated and phosphorylated forms of AZT into mitochondria, phosphorylation, and incorporation into mtDNA. The model also includes the mitochondrial metabolism of the natural deoxynucleotides. We define three simulated cell types, i.e. rapidly dividing, slowly dividing and postmitotic cells. Our standard simulation indicates that incorporation of AZT into mtDNA is highest in rapidly dividing cells because of the higher mitochondrial AZTTP (3'-azidothymidine-5'-triphosphate)/dTTP ratio in this cell type. However, under these standard conditions the rate of incorporation into mtDNA is too low to be a major cause of toxicity. These simulations relied on the assumption that phosphorylated AZT is transported with the same kinetics as phosphorylated thymidine. In simulations with mitochondria set to have a limited ability to transport phosphorylated AZT, AZTTP accumulates to toxic levels in the mitochondria of postmitotic cells, while low levels are maintained in mitochondria from rapidly dividing cells. This result is more consistent with the tissue toxicities observed in patients. Our model also predicts that inhibition by AZT of mitochondrial deoxycytidine phosphorylation by thymidine kinase 2 may contribute to the mitochondrial toxicity, since in simulations using a typical peak plasma AZT level the mtDNA replication rate is decreased by 30% in postmitotic cell simulations.

Mitochondrial dysfunction in NASH: causes, consequences and possible means to prevent it

Calorie-enriched diet and lack of exercise are causing a worldwide surge of obesity, insulin resistance and lipid accretion in liver (i.e. hepatic steatosis), which can lead to steatohepatitis. Steatosis and nonalcoholic steatohepatitis (NASH) can also be induced by drugs such as amiodarone, tamoxifen and some antiretroviral drugs, including stavudine and zidovudine. There is accumulating evidence that mitochondrial dysfunction (more particularly respiratory chain deficiency) plays a key role in the physiopathology of NASH whatever its initial cause. In contrast, the mitochondrial beta-oxidation of fatty acids can be either increased (as in insulin resistance-associated NASH) or decreased (as in drug-induced NASH). However, in both circumstances, generation of reactive oxygen species (ROS) by the damaged respiratory chain can be augmented. ROS generation in an environment enriched in lipids in turn induces lipid peroxidation which releases highly reactive aldehydic derivatives (e.g. malondialdehyde) that have diverse detrimental effects on hepatocytes and other hepatic cells. In hepatocytes, ROS, reactive nitrogen species and lipid peroxidation products further impair the respiratory chain, either directly or indirectly through oxidative damage to the mitochondrial genome. This consequently leads to the generation of more ROS and a vicious cycle occurs. Mitochondrial dysfunction can also lead to apoptosis or necrosis depending on the energy status of the cell. ROS and lipid peroxidation products also increase the generation of several cytokines (TNF-alpha, TGF-beta, Fas ligand) playing a key role in cell death, inflammation and fibrosis. Recent investigations have shown that some genetic polymorphisms can significantly increase the risk of steatohepatitis and that several drugs can prevent or even reverse NASH. Interestingly, most of these drugs could exert their beneficial effects by improving directly or indirectly mitochondrial function in liver. Finding a drug, which could fully prevent oxidative stress and mitochondrial dysfunction in NASH is a major challenge for the next decade.

Drugs targeting mitochondrial functions to control tumor cell growth

Mitochondria, the power houses of the cell, are at the cross-road of many cellular pathways. They play a central role in energy metabolism, regulate calcium flux and are implicated in apoptosis. Mitochondrial dysfunctions have been associated with various physiopathological disorders, especially neurodegenerative diseases and cancer. Structurally diverse pharmacological agents have shown direct effects on mitochondria ultra-structures and functions, either at the DNA level or upon targeting proteins located in the inner or outer mitochondrial membranes. The brief review deals with the molecular targets and mechanisms of action of chemically diverse small molecules acting on specific mitochondrial loci, such as the respiratory chain, DNA biogenesis, potassium channels, the Bcl-2 protein and the permeability transition pores (PTP). Drugs, which specifically compromise the structural and functional integrity of mitochondria, may provide novel opportunities to combat cancer cell proliferation, providing that these molecules can be selectively delivered to tumor sites. Different examples reported here show that mitochondrial insult or failure can rapidly lead to inhibition of cell survival and proliferation. Mitochondrial impairment may be a successful anti-cancer strategy.

Reverse Transcriptase Inhibitors Alter Uncoupling Protein-1 and Mitochondrial Biogenesis in Brown Adipocytes

Objective

Human adipose depots contain remnant brown adipocytes interspersed among white adipocytes, and disturbances of brown with respect to white adipocyte biology have been implicated in highly active antiretroviral therapy (HAART)-induced lipo matosis. Brown adipocytes express the uncoupling protein-1 (UCP1) and contain a large number of mitochondria, potential targets of HAART toxicity. The aim of this study was to evaluate the effects of reverse transcriptase inhibitors (RTIs) on primary brown adipocytes differentiated in culture.

Design and methods

We analysed the effects of RTIs, nucleoside analogues (NRTIs: stavudine, zidovudine, didanosine and lamivudine) and non-nucleoside analogues (NNRTIs: nevirapine and efavirenz), on differentiation, mitochondrial biogenesis and gene expression in brown adipocytes.

Results

None of the NRTIs altered brown adipocyte differentiation whereas NNTRIs had differing effects. Efavirenz blocked lipid deposition and expression of adipose marker genes but nevirapine induced lipid accumulation and adipose gene expression, promoted mitochondrial biogenesis and increased UCP1. Stavudine, zidovudine and didanosine reduced mitochondrial DNA (mtDNA) content. However, mitochondrial genome expression was only impaired in didanosine-treated adipocytes. Stavudine, but not zidovudine, induced expression of the mitochondrial transcription factors and this may explain compensatory mechanisms for the depletion of mtDNA by up-regulating mtDNA transcription. Stavudine caused a specific induction of UCP1 gene expression through direct interaction with a retinoic acid-dependent pathway.

Conclusions

Specific disturbances in brown adipocytes in adipose depots may contribute to HAART-induced lipomatosis. Mitochondrial depletion does not appear to be the only mechanism explaining adverse effects in brown adipocytes because there is evidence of compensatory mechanisms that maintain mtDNA expression, and the expression of the UCP1 gene is specifically altered.

Ex vivo zidovudine (AZT) treatment of CD34+ bone marrow progenitors causes decreased steady state mitochondrial DNA (mtDNA) and increased lactate production

Haematopoietic suppression is one of the dose-limiting side effects of chronic zidovudine (AZT) therapy. We tested the hypothesis that AZT would reduce mitochondrial DNA (mtDNA) content in haematopoietic progenitors causing impaired haematopoiesis and mitochondrial dysfunction. We studied the effects of AZT 0-50 microM in vitro, on normal human CD34+ haematopoietic progenitor cells cultured ex vivo for up to 12 days. The mean AZT IC50 for granulocyte (phenotype CD15+/CD14-) and erythroid (phenotype glycophorin+/CD45-) cell proliferation was 2.5 microM (SD+/-0.7) and 0.023 microM (SD+/-0.005), respectively. In myeloid-rich cell cultures, the mean lactate content of the media, compared to untreated controls, increased by 86% (SD+/-23) at 10 microM AZT and in erythroid-rich cultures it increased by 134% (SD+/-24) in the presence of 0.5 microM AZT. In myeloid-rich cultures the AZT IC50 for the reduction in the mitochondrial/nuclear DNA content ratio was 5.6 microM, whereas in erythroid rich cultures this AZT IC50 was < 0.0005 microM. AZT produced concentration-dependent inhibition of CD34+ progenitor proliferation into both myeloid and erythroid lineages; erythropoiesis was more sensitive than myelopoiesis. Concurrently, AZT reduced steady state mtDNA content, while increasing lactate production. These findings support the hypothesis that mtDNA is one of the intracellular targets involved in the pathogenesis of AZT-associated bone marrow progenitor cell toxicity.

Mitochondrial Studies in Haart-Related Lipodystrophy: From Experimental Hypothesis to Clinical Findings

Chronic use of antiretrovirals (ARVs) to treat HIV infection, along with more prolonged patient survival, has been associated with an increase in adverse drug effects in HIV-infected patients on treatment. It has been proposed that some of these adverse effects (including myopathy, cardiomyopathy, anaemia, hyperlactataemia/ lactic acidosis, pancreatitis, polyneuritis and lipodystrophy) could be mediated by mitochondrial (mt) toxicity. From the experimental data, it has been proposed that nucleoside analogue reverse transcriptase inhibitors (NRTIs) also inhibit γ-polymerase, the enzyme devoted to replicate (and, to a lesser extent, repair) mtDNA. It is now widely accepted that the use of most NRTIs in HIV-infected patients is associated with mtDNA depletion. Although cross-sectional studies suggest that certain ARVs, especially stavudine, are more toxic to mitochondria, the differences among different highly active ARV therapy (HAART) schedules detected in the analysis of longitudinal studies are not so clear. These types of study in previously untreated individuals suggest that the greatest mtDNA loss appears at the beginning of the treatment. Conversely, in ARV-experienced patients, the potential beneficial effects of HAART changes in terms of mtDNA content remain controversial and must be further investigated. Functional studies accompanying genetic investigations are needed for the correct pathogenic interpretation of the mtDNA abnormalities.

Uridine in the Prevention and Treatment of Nrti-Related Mitochondrial Toxicity

Long-term side effects of antiretroviral therapy are attributed to the mitochondrial (mt) toxicity of nucleoside analogue reverse transcriptase inhibitors (NRTIs) and their ability to deplete mtDNA. Studies in hepatocytes suggest that uridine is able to prevent and treat mtDNA depletion by pyrimidine NRTIs [zalcitabine (ddC) and stavudine (d4T)] and to fully abrogate hepatocyte death, elevated lactate production and intracellular steatosis. Uridine was also found to improve the liver and haematopoietic toxicities of zidovudine (AZT), which are unrelated to mtDNA depletion, and to prevent neuronal cell death induced by ddC. Most recently, uridine was found to prevent the onset of a lipoatrophic phenotype (reduced intracellular lipids, increased apoptosis, mtDNA depletion and mt depolarization) in adipocytes incubated long-term with d4T and AZT. Various steps of mt nucleoside utilization may be involved in the protective effect, but competition of uridine metabolites with NRTIs at polymerase y or other enzymes is a plausible explanation. Pharmacokinetic studies suggest that uridine serum levels can be safely increased in humans to achieve concentrations which are protective in vitro (50–200 μM). Uridine was not found to interfere with the antiretroviral activity of NRTIs. Mitocnol, a sugar cane extract which effectively increases uridine in human serum, was beneficial in individual HIV patients with mt toxicity and is now being tested in placebo-controlled randomized trials. Until these data become available, the risk-benefit calculation of using uridine should be individualized. The current safety data justify the closely monitored use of uridine in individuals who suffer from mt toxicity but who cannot be switched to less toxic NRTIs.

The ins and outs of mitochondrial dysfunction in NASH

Rich diet and lack of exercise are causing a surge in obesity, insulin resistance and steatosis, which can evolve into steatohepatitis. Steatosis and nonalcoholic steatohepatitis (NASH) can also be induced by drugs such as amiodarone, tamoxifen and some antiretroviral drugs. There is growing evidence that mitochondrial dysfunction, and more specifically respiratory chain deficiency, plays a role in the pathophysiology of NASH whatever its initial cause. In contrast, the B-oxidation of fatty acids can be either increased (as in insulin resistance-associated NASH) or decreased (as in drug-induced NASH). However, in both circumstances, the generation of reactive oxygen species (ROS) by the damaged respiratory chain is augmented, as components of this chain are over-reduced by electrons, which then abnormally react with oxygen to form increased amounts of ROS. Concomitantly, ROS oxidize fat deposits to release lipid peroxidation products that have detrimental effects on hepatocytes and other hepatic cells. In hepatocytes, ROS and lipid peroxidation products further impair the respiratory chain, either directly or indirectly through oxidative damage to the mitochondrial genome. This, in turn, leads to the generation of more ROS and a vicious cycle ensues. Mitochondrial dysfunction can also lead to apoptosis or necrosis depending on the energy status of the cell. ROS and lipid peroxidation products also activate stellate cells, thus resulting in fibrosis. Finally, ROS and lipid peroxidation increase the generation of several cytokines (TNF-alpha, TGF-B, Fas ligand) that play sundry roles in the pathogenesis of NASH. Recent investigations have shown that some genetic polymorphisms can significantly increase the risk of steatohepatitis and that several drugs can prevent or even reverse NASH. For the next decade, reducing the incidence of NASH will be a major challenge for hepatologists.

Mitochondrial and metabolic effects of nucleoside reverse transcriptase inhibitors (NRTIs) in mice receiving one of five single- and three dual-NRTI treatments

Although treatments with nucleoside reverse transcriptase inhibitors (NRTIs) can modify fat metabolism and fat distribution in humans, the mechanisms of these modifications and the roles of diverse NRTIs are unknown. We studied the mitochondrial and metabolic effects of stavudine (d4T), zidovudine (AZT), didanosine (ddI), lamivudine (3TC), zalcitabine (ddC), and three combinations (AZT-3TC, d4T-3TC, and d4T-ddI) in mice treated for 2 weeks with daily doses equivalent to the human dose per body area. Concentrations of AZT and d4T in plasma were lower when these drugs were administered with 3TC or ddI. Whatever the treatment, mitochondrial DNA was not significantly decreased in muscle, heart, brain, or white adipose tissue but was moderately decreased in liver tissue after the administration of AZT, 3TC, or d4T alone. Blood lactate was unchanged, even when NRTIs were administered at supratherapeutic doses. In contrast, the level of plasma ketone bodies increased with the administration of AZT or high doses of d4T but not with ddC, 3TC, or ddI, suggesting that the thymine moiety could be involved. Indeed, the levels of plasma ketone bodies increased in mice treated with beta-aminoisobutyric acid, a thymine catabolite. Treatment with AZT, d4T, or beta-aminoisobutyric acid increased hepatic carnitine palmitoyltransferase I (CPT-I) mRNA expression and the mitochondrial generation of ketone bodies from palmitate. In conclusion, therapeutic doses of NRTIs have no or moderate effects on mitochondrial DNA and no effects on plasma lactate in mice. However, AZT and high doses of d4T increase the levels of hepatic CPT-I, mitochondrial fatty acid beta-oxidation, and ketone bodies, and these catabolic effects are reproduced by beta-aminoisobutyric acid, a thymine metabolite.

Adipocytes Targets and Actors in the Pathogenesis of HIV-Associated Lipodystrophy and Metabolic Alterations

The recent clinical use of potent HIV-1 drugs, including nucleoside reverse transcriptase inhibitors (NRTIs) and non-peptidic viral protease inhibitors (PIs), and their combinations, termed highly active antiretroviral therapy (HAART), has dramatically reduced the infection-related mortality of AIDS patients, but it is associated with severe metabolic adverse events such as lipodystrophy syndrome, dyslipidaemia, insulin resistance and diabetes mellitus. The aetiology of this syndrome and metabolic alterations appear to be multifactorial, including HIV drug inhibitory effects on adipocyte differentiation, alteration of mitochondrial functions in adipocytes and altered leptin, adiponectin and cytokine expression in adipose tissue of patients. Adipose tissue may thus be a central regulator in disorganized lipid metabolism and insulin resistance associated with antiretroviral therapy, and we propose in this review to explore how adipose tissue may be a target, but also an actor, in the aetiopathogenesis of the lipodystrophy syndrome.

Mitochondrial toxicity of NRTI antiviral drugs: an integrated cellular perspective

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

Mitochondrial expression of the human equilibrative nucleoside transporter 1 (hENT1) results in enhanced mitochondrial toxicity of antiviral drugs

Many antiviral drugs (e.g. fialuridine; FIAU) produce clinically significant mitochondrial toxicity that limits their dose or prevents their use in the clinic. Because the majority of nucleoside drugs is too hydrophilic to cross the highly impermeable mitochondrial membrane, we have hypothesized that they must be transported into the mitochondria to produce their toxicity. To test this hypothesis, we have sought to determine whether the nucleoside transporters, human equilibrative nucleoside transporter 1 (hENT1) or human concentrative nucleoside transporter 1 (hCNT1), when stably expressed in Madin-Darby canine kidney cells as yellow fluorescent fusion protein (YFP), are localized to the mitochondria. By using organelle-selective dyes and confocal microscopy, we have found that hENT1-YFP is localized to the mitochondria as well as the plasma membrane, whereas hCNT1-YFP was found predominantly on the plasma membrane. hENT1-YFP was not localized to the nuclear envelope, endosomes, lysosomes, or Golgi complex. Western blotting confirmed the presence of hENT1-YFP or endogenous hENT1 in mitochondria isolated from hENT1-YFP-expressing cells and human livers, respectively. In agreement with these localization data, [14C]FIAU was efficiently transported into the mitochondria of cells expressing hENT1-YFP but not of cells expressing hCNT1-YFP. The mitochondrial toxicity of FIAU to Madin-Darby canine kidney cells was enhanced by hENT1-YFP, even when hENT1 activity on the plasma membrane was selectively blocked by 10 nm nitrobenzylthioinosine. Moreover, FIAU (50 microm) produced significant mitochondrial toxicity ( approximately 70% decrease in mitochondrial DNA synthesis) when it was directly incubated with mitochondria isolated from hENT1-expressing cells. In conclusion, we have identified for the first time that hENT1 is expressed on the mitochondrial membrane and that this expression enhances the mitochondrial toxicity of nucleoside drugs such as FIAU. Mitochondrial expression of hENTs may explain the clinically significant mitochondrial toxicity caused by the anti-HIV nucleoside drugs such as zidovudine, stavudine, and didanosine.

The selective in vitro cytotoxicity of carcinoma cells by AZT is enhanced by concurrent treatment with delocalized lipophilic cations

This study assessed the selective growth inhibitory effect on cultured carcinoma cells by 3'-azido-3'-deoxythymidine (AZT), as a single agent, and in combination with delocalized lipophilic cations (DLCs) that are known to inhibit mitochondrial function. In cytotoxicity assays, treatment of cells with varying concentrations of AZT induced a dose-dependent inhibition of cell growth of the human carcinoma lines DU-145 (prostate; IC50 at 24 microM), MCF-7 (breast; IC50 at 22 microM), and CX-1 (colon; IC50 at 23 microM), yet caused no significant effect on the growth of the control epithelial cell line CV-1 (monkey kidney) at a concentration as high as 50 microM AZT. Combination treatment employing a constant concentration (1.25 microM) of the DLC dequalinium chloride (DECA) plus varying concentrations of AZT (0-50 microM) enhanced the AZT-induced cytotoxicity of carcinoma cells at least fourfold for MCF-7 and CX-1 cells (IC50 at 5 microM AZT), and twofold for DU-145 cells (IC50 at 11 microM AZT). Similar results were obtained in DU-145 cells using a constant concentration of the DLC MKT-077 (1.0 microM) and varying concentrations of AZT (IC50 at 12.5 microM). As expected, the drug combination of constant DLC and varying AZT had no significant effect on the growth of CV-1 cells. Clonogenic assays demonstrated up to 20-fold enhancement of selective carcinoma cell killing by combination vs. single agent treatment, depending on the specific drug combination and concentrations used. It is hypothesized that the efficacy of the AZT/DLC drug combination in carcinoma cell killing may be based on a dual selectivity involving inhibition of mitochondrial energy metabolism and inhibition of DNA synthesis due to limited deoxythymidine monophosphate availability.

Hyperlactataemia syndromes associated with HIV therapy

Hyperlactataemia is seen in 8-18.3% of HIV-infected patients taking nucleoside-analogue reverse transcriptase inhibitors (NRTIs). Recent epidemiological studies suggest that most episodes are transient and subclinical. However, symptomatic and occasionally life-threatening cases accompanied by metabolic acidosis and hepatic steatosis (ie, lactic acidosis syndrome) have also been described. Though yet to be fully elucidated, the proposed mechanism is NRTI-induced inhibition of mitochondrial DNA polymerase culminating in derangements in oxidative phosphorylation and lactate homeostasis. Signs and symptoms range from mild hyperlactataemia accompanied by nausea, abdominal discomfort, and weight loss to severe, intractable lactic acidosis complicated by coma and multi-organ failure. Significant progress has recently been made with regard to the natural history of NRTI-related hyperlactataemia. However, other important aspects of the disorder, such as its pathogenesis, predisposing conditions, and management, remain poorly understood. This article reviews the current published work on these issues, identifies areas of controversy, and addresses directions for future research.

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.

Increased long-term mitochondrial toxicity in combinations of nucleoside analogue reverse-transcriptase inhibitors

BACKGROUND

Some nucleoside analogue reverse transcriptase inhibitors (NRTI) may cause depletion of mitochondrial (mt) DNA in liver by inhibiting polymerase-gamma. mtDNA depletion may contribute to lactic acidosis, steatohepatitis and liver failure.

OBJECTIVE

To evaluate the long-term mitochondrial toxicity of NRTI combinations.

METHODS

The HepG2 human hepatoma cell line was cultivated in the presence of zalcitabine (ddC), didanosine (ddI), stavudine (d4T), lamivudine (3TC), zidovudine (ZDV) and efavirenz at concentrations equivalent to steady-state peak plasma levels (C ), and also in one-third and 10 times C. The NRTI were added to the medium alone or in combination. Control cells were incubated without any NRTI or with efavirenz. Cell growth, lactate production, intracellular lipid droplets, mtDNA and the mtDNA-encoded respiratory chain subunit COX II were monitored over a period of up to 30 days.

RESULTS

Time- and dose-dependent mtDNA depletion was observed with ddC > ddI > d4T and mtDNA depletion preceded or coincided with a decline in COX II expression, a decrease in cell growth, increased lactate production and increased intracellular lipids. 3TC and efavirenz did not affect any measurement. ZDV increased lactate moderately and cell growth was inhibited, despite normal mtDNA and COX II levels. The negative effects on some measurements were more pronounced in the 3TC-ZDV and ddC-d4T combinations, than in the single-NRTI incubations. The combination of ddI-d4T was not more toxic than ddI alone. Mitochondrial damage by ZDV, d4T, ddI, and ddC did not reach steady-state by day 25. Using a Southern blot technique, mtDNA deletions were never observed.

CONCLUSION

The data indicate additive or synergistic long-term mitochondrial toxicity in some NRTI combinations.