Mitochondria: important target for drug toxicity?

A human liver organoid screening platform for DILI risk prediction

BACKGROUND & AIMS

Drug-induced liver injury (DILI), both intrinsic and idiosyncratic, causes frequent morbidity, mortality, clinical trial failures and post-approval withdrawal. This suggests an unmet need for improved in vitro models for DILI risk prediction that can account for diverse host genetics and other clinical factors. In this study, we evaluated the utility of human liver organoids (HLOs) for high-throughput DILI risk prediction and in an organ-on-chip system.

METHODS

HLOs were derived from three separate iPSC lines and benchmarked on two platforms for their ability to model in vitro liver function and identify hepatotoxic compounds using biochemical assays for albumin, ALT, AST, microscopy-based morphological profiling, and single-cell transcriptomics: i) HLOs dispersed in 384-well-formatted plates and exposed to a library of compounds; ii) HLOs adapted to a liver-on-chip system.

RESULTS

Dispersed HLOs derived from the three iPSC lines had similar DILI predictive capacity as intact HLOs in a high-throughput screening format, allowing for measurable IC50 values of compound cytotoxicity. Distinct morphological differences were observed in cells treated with drugs exerting differing mechanisms of toxicity. On-chip HLOs significantly increased albumin production, CYP450 expression, and ALT/AST release when treated with known hepatoxic drugs compared to dispersed HLOs and primary human hepatocytes. On-chip HLOs were able to predict the synergistic hepatotoxicity of tenofovir-inarigivir and displayed steatosis and mitochondrial perturbation, via phenotypic and transcriptomic analysis, on exposure to fialuridine and acetaminophen, respectively.

CONCLUSIONS

The high-throughput and liver-on-chip systems exhibit enhanced in vivo-like functions and demonstrate the potential utility of these platforms for DILI risk assessment. Tenofovir-inarigivr-associated hepatotoxicity was observed and correlates with the clinical manifestation of DILI observed in patients.

IMPACT AND IMPLICATIONS

Idiosyncratic (spontaneous, patient-specific) drug-induced liver injury (DILI) is difficult to study due to the lack of liver models that function as human liver tissue and are adaptable for large-scale drug screening. Human liver organoids grown from patient stem cells respond to known DILI-causing drugs in both a high-throughput and on a physiological "chip" culture system. These platforms show promise for researchers in their use as predictive models for novel drugs before entering clinical trials and as a potential in vitro diagnostic tool. Our findings support further development of patient-derived liver organoid lines and their use in the context of DILI research.

Bicalutamide Exhibits Potential to Damage Kidney via Destroying Complex I and Affecting Mitochondrial Dynamics

Bicalutamide (Bic) is an androgen deprivation therapy (ADT) for treating prostate cancer, while ADT is potentially associated with acute kidney injury. Previously, we recognized Bic induced renal mitochondria dysfunction in vitro and in vivo via the ROS -HIF1α pathway. Whether OXPHOS complex, as well as mitochondrial dynamics, can be influenced by Bic via modulation of peroxisome proliferator-activated receptor coactivator 1α (PGC1α), NADPH oxidase 4 (Nox4), mitofusins 1/2 (MFN 1/2), optic atrophy 1 (OPA1), and sirtuins (SIRTs) has not been documented. Renal mesangial cell line was treated with Bic (30~60 μM) for the indicated time. SIRTs, complex I, mitochondrial dynamics- and oxidative stress-related proteins were analyzed. Bic dose-dependently reduced mitochondrial potential, but dose- and time-dependently suppressed translocase of the outer mitochondrial membrane member 20 (Tomm 20), complex I activity. Nox4 and glutathione lead to decreased NAD⁺/NADH ratio, with upregulated superoxide dismutase 2. SIRT1 was initially stimulated and then suppressed, while SIRT3 was time- and dose-dependently downregulated. PGC1α, MFN2, and OPA1 were all upregulated, with MFN1 and pro-fission dynamin-related protein I downregulated. Bic exhibits potential to damage mitochondria via destroying complex I, complex I activity, and mitochondrial dynamics. Long-term treatment with Bic should be carefully followed up.

Mitochondrial dysfunction and apoptosis underlie the hepatotoxicity of perhexiline

Perhexiline is an anti-anginal drug developed in the late 1960s. Despite its therapeutic success, it caused severe hepatoxicity in selective patients, which resulted in its withdrawal from the market. In the current study we explored the molecular mechanisms underlying the cytotoxicity of perhexiline. In primary human hepatocytes, HepaRG cells, and HepG2 cells, perhexiline induced cell death in a concentration- and time-dependent manner. Perhexiline treatment also caused a significant increase in caspase 3/7 activity at 2 h and 4 h. Pretreatment with specific caspase inhibitors suggested that both intrinsic and extrinsic apoptotic pathways contributed to perhexiline-induced cytotoxicity, which was confirmed by increased expression of TNF-α, cleavage of caspase 3 and 9 upon perhexiline treatment. Moreover, perhexiline caused mitochondrial dysfunction, demonstrated by the classic glucose-galactose assay at 4 h and 24 h. Results from JC-1 staining suggested perhexiline caused loss of mitochondrial potential. Blocking mitochondrial permeability transition pore using inhibitor bongkrekic acid attenuated the cytotoxicity of perhexiline. Western blotting analysis also showed decreased expression level of pro-survival proteins Bcl-2 and Mcl-1, and increased expression of pro-apoptotic protein Bad. Direct measurement of the activity of individual components of the mitochondrial respiratory complex demonstrated that perhexiline strongly inhibited Complex IV and Complex V and moderately inhibited Complex II and Complex II + III. Overall, our data demonstrated that both mitochondrial dysfunction and apoptosis underlies perhexiline-induced hepatotoxicity.

Redox Control of the Immune Response in the Hepatic Progenitor Cell Niche

The liver commonly self-regenerates by a proliferation of mature cell types. Nevertheless, in case of severe or protracted damage, the organ renewal is mediated by the hepatic progenitor cells (HPCs), adult progenitors capable of differentiating toward the biliary and the hepatocyte lineages. This regeneration process is determined by the formation of a stereotypical niche surrounding the emerging progenitors. The organization of the HPC niche microenvironment is crucial to drive biliary or hepatocyte regeneration. Furthermore, this is the site of a complex immunological activity mediated by several immune and non-immune cells. Indeed, several cytokines produced by monocytes, macrophages and T-lymphocytes may promote the activation of HPCs in the niche. On the other side, HPCs may produce pro-inflammatory cytokines induced by liver inflammation. The inflamed liver is characterized by high generation of reactive oxygen and nitrogen species, which in turn lead to the oxidation of macromolecules and the alteration of signaling pathways. Reactive species and redox signaling are involved in both the immunological and the adult stem cell regeneration processes. It is then conceivable that redox balance may finely regulate the immune response in the HPC niche, modulating the regeneration process and the immune activity of HPCs. In this perspective article, we summarize the current knowledge on the role of reactive species in the regulation of hepatic immunity, suggesting future research directions for the study of redox signaling on the immunomodulatory properties of HPCs.

Molecular Toxicological Mechanisms of Synthetic Cathinones on C2C12 Myoblasts

Synthetic cathinones are popular psychoactive substances that may cause skeletal muscle damage. In addition to indirect sympathomimetic myotoxicity, these substances could be directly myotoxic. Since studies in myocytes are currently lacking, the aim of the present study was to investigate potential toxicological effects by synthetic cathinones on C2C12 myoblasts (mouse skeletal muscle cell line). We exposed C2C12 myoblasts to 3-methylmethcathinone, 4-methylmethcathinone (mephedrone), 3,4-methylenedioxymethcathinone (methylone), 3,4-methylenedioxypyrovalerone (MDPV), alpha-pyrrolidinovalerophenone (α-PVP), and naphthylpyrovalerone (naphyrone) for 1 or 24 h before cell membrane integrity, ATP content, mitochondrial oxygen consumption, and mitochondrial superoxide production was measured. 3,4-Methylenedioxymethamphetamine (MDMA) was included as a reference compound. All investigated synthetic cathinones, as well as MDMA, impaired cell membrane integrity, depleted ATP levels, and increased mitochondrial superoxide concentrations in a concentration-dependent manner in the range of 50–2000 μM. The two pyrovalerone derivatives α-PVP and naphyrone, and MDMA, additionally impaired basal and maximal cellular respiration, suggesting mitochondrial dysfunction. Alpha-PVP inhibited complex I, naphyrone complex II, and MDMA complex I and III, whereas complex IV was not affected. We conclude that, in addition to sympathetic nervous system effects and strenuous muscle exercise, direct effects of some cathinones on skeletal muscle mitochondria may contribute to myotoxicity in susceptible synthetic cathinone drugs users.

Redox cell signaling and hepatic progenitor cells

Hepatic diseases are widespread in the world and organ transplantation is currently the only treatment for liver failure. New cell-based approaches have been considered, since stem cells may represent a possible source to treat liver diseases. Acute and chronic liver diseases are characterized by high production of reactive oxygen and nitrogen species, with consequent oxidative modifications of cellular macromolecules and alteration of signaling pathways, metabolism and cell cycle. Although considered harmful molecules, reactive species are involved in cell growth and differentiation processes, modulating the activity of transcription factors, which take part in stemness/proliferation. It is conceivable that redox balance may regulate the development of hepatic progenitor cells, function and survival in synchrony with metabolism during chronic liver diseases. This review aims to summarize diverse redox-sensitive signaling pathways involved in stem cell fate, highlighting the important role of hepatic progenitor cells as a possible source to treat end-stage liver disease for organ regeneration.

Mechanisms of hepatocellular toxicity associated with new psychoactive synthetic cathinones

Synthetic cathinones are a new class of psychostimulant substances. Rarely, they can cause liver injury but associated mechanisms are not completely elucidated. In order to increase our knowledge about mechanisms of hepatotoxicity, we investigated the effect of five frequently used cathinones on two human cell lines. Bupropion was included as structurally related drug used therapeutically. In HepG2 cells, bupropion, MDPV, mephedrone and naphyrone depleted the cellular ATP content at lower concentrations (0.2-1mM) than cytotoxicity occurred (0.5-2mM), suggesting mitochondrial toxicity. In comparison, methedrone and methylone depleted the cellular ATP pool and induced cytotoxicity at similar concentrations (≥2mM). In HepaRG cells, cytotoxicity and ATP depletion could also be demonstrated, but cytochrome P450 induction did not increase the toxicity of the compounds investigated. The mitochondrial membrane potential was decreased in HepG2 cells by bupropion, MDPV and naphyrone, confirming mitochondrial toxicity. Bupropion, but not the other compounds, uncoupled oxidative phosphorylation. Bupropion, MDPV, mephedrone and naphyrone inhibited complex I and II of the electron transport chain, naphyrone also complex III. All four mitochondrial toxicants were associated with increased mitochondrial ROS and increased lactate production, which was accompanied by a decrease in the cellular total GSH pool for naphyrone and MDPV. In conclusion, bupropion, MDPV, mephedrone and naphyrone are mitochondrial toxicants impairing the function of the electron transport chain and depleting cellular ATP stores. Since liver injury is rare in users of these drugs, affected persons must have susceptibility factors rendering them more sensitive for these drugs.

Hepatic Effects of Pharmacological Doses of Hydroxy-Cobalamin[c-lactam] in Mice

The vitamin B₁₂ analog hydroxy-cobalamin[c-lactam] (HCCL) impairs hepatic mitochondrial protein synthesis and function of the electron transport chain in rats. We aimed to establish an in vivo model for mitochondrial dysfunction in mice, which could be used to investigate hepatotoxicity of mitochondrial toxicants. In a first step, we performed a dose-finding study in mice treated with HCCL 0.4 mg/kg and 4 mg/kg i.p. for two to four weeks. The plasma methylmalonate concentration was strongly increased at 4 mg/kg starting at three weeks of treatment. We subsequently treated mice daily with 4 mg/kg HCCL i.p. for three weeks and characterized liver function and histology as well as liver mitochondrial function. We found an increase in liver weight in HCCL-treated mice, which was paralleled by hepatocellular accumulation of triglycerides. In liver homogenate of HCCL-treated mice, the complex I activity of the electron transport chain was reduced, most likely explaining hepatocellular triglyceride accumulation. The activity of CPT1 was not affected by methylmalonyl-CoA in isolated liver mitochondria. Despite impaired complex I activity, mitochondrial superoxide anion production was not increased and the hepatocellular glutathione (GSH) pool was maintained. Finally, the mitochondrial DNA content was not altered with HCCL treatment. In conclusion, treatment of mice with HCCL is associated with increased liver weight explained by hepatocellular triglyceride accumulation. Hepatocellular fat accumulation is most likely a consequence of impaired activity of the mitochondrial electron transport chain. The impairment of complex I activity is not strong enough to result in ROS accumulation and reduction of the GSH stores.

Potential hepatic toxicity of buprofezin at sublethal concentrations: ROS-mediated conversion of energy metabolism

Buprofezin is known for its broad-spectrum action and environmental safety. The popularity of buprofezin has raised concerns about its potentially adverse effects on human health and risk to the environment. In this study, we first identified the liver as one of the major organs in which buprofezin accumulated, and we detected a severe oxidative stress response. Next, we demonstrated that sublethal concentrations of buprofezin promoted the conversion of energy metabolism from the aerobic tricarboxylic acid (TCA) cycle and oxidative phosphorylation to anaerobic glycolysis. Importantly, reactive oxygen species (ROS) generation partially accounted for the shunting of the energy metabolism through the buprofezin-mediated inhibition of cytochrome c oxidase activity. ROS directly perturbed the activities of several key TCA cycle enzymes, stimulated glycolysis, and indirectly disturbed the activity of the respiratory chain complex by altering mitochondrial DNA (mtDNA). These findings clarify the potential mechanisms of buprofezin toxicity and provide biomarkers for buprofezin-mediated hepatotoxicity at sublethal concentrations.

Oxidative Stress and Liver Morphology in Experimental Cyclosporine A-Induced Hepatotoxicity

Cyclosporine A is an immunosuppressive drug used after organ's transplantation. The adverse effects on such organs as kidney or liver may limit its use. Oxidative stress is proposed as one of the mechanisms of organs injury. The study was designed to elucidate CsA-induced changes in liver function, morphology, oxidative stress parameters, and mitochondria in rat's hepatocytes. Male Wistar rats were used: group A (control) receiving physiological saline, group B cyclosporine A in a dose of 15 mg/kg/day subcutaneously, and group C the CsA-vehicle (olive oil). On the 28th day rats were anesthetized. The following biochemical changes were observed in CsA-treated animals: increased levels of ALT, AST, and bilirubin in the serum, statistically significant changes in oxidative stress parameters, and lipid peroxidation products in the liver supernatants: MDA+4HAE, GSH, GSSG, caspase 3 activity, and ADP/ATP, NAD(+)/NADH, and NADP(+)/NADPH ratios. Microscopy of the liver revealed congestion, sinusoidal dilatation, and focal hepatocytes necrosis with mononuclear cell infiltration. Electron microscope revealed marked mitochondrial damage. Biochemical studies indicated that CsA treatment impairs liver function and triggers oxidative stress and redox imbalance in rats hepatocytes. Changes of oxidative stress markers parallel with mitochondrial damage suggest that these mechanisms play a crucial role in the course of CsA hepatotoxicity.

Development of an in Silico Profiler for Mitochondrial Toxicity

This study outlines the analysis of mitochondrial toxicity for a variety of pharmaceutical drugs extracted from Zhang et al. ((2009) Toxicol. In Vitro, 23, 134-140). These chemicals were grouped into categories based upon structural similarity. Subsequently, mechanistic analysis was undertaken for each category to identify the molecular initiating event driving mitochondrial toxicity. The mechanistic information elucidated during the analysis enabled mechanism-based structural alerts to be developed and combined together to form an in silico profiler. This profiler is envisaged to be used to develop chemical categories based upon similar mechanisms as part of the adverse outcome pathway paradigm. Additionally, the profiler could be utilized in screening large data sets in order to identify chemicals with the potential to induce mitochondrial toxicity.

Impaired mitochondrial function in HepG2 cells treated with hydroxy-cobalamin[c-lactam]: A cell model for idiosyncratic toxicity

The vitamin B12 analog hydroxy-cobalamin[c-lactam] (HCCL) impairs mitochondrial protein synthesis and the function of the electron transport chain. Our goal was to establish an in vitro model for mitochondrial dysfunction in human hepatoma cells (HepG2), which can be used to investigate hepatotoxicity of idiosyncratic mitochondrial toxicants. For that, HepG2 cells were treated with HCCL, which inhibits the function of methylmalonyl-CoA mutase and impairs mitochondrial protein synthesis. Secondary, cells were incubated with propionate that served as source of propionyl-CoA, a percursor of methylmalonyl-CoA. Dose-finding experiments were conducted to evaluate the optimal dose and treatment time of HCCL and propionate for experiments on mitochondrial function. 50 μM HCCL was cytotoxic after exposure of HepG2 cells for 2d and 10 and 50 μM HCCL enhanced the cytotoxicity of 100 or 1000 μM propionate. Co-treatment with HCCL (10 μM) and propionate (1000 μM) dissipated the mitochondrial membrane potential and impaired the activity of enzyme complex IV of the electron transport chain. Treatment with HCCL decreased the mRNA content of mitochondrially encoded proteins, whereas the mtDNA content remained unchanged. We observed mitochondrial ROS accumulation and decreased mitochondrial SOD2 expression. Moreover, electron microscopy showed mitochondrial swelling. Finally, HepG2 cells pretreated with a non-cytotoxic combination of HCCL (10 μM) and propionate (100 μM) were more sensitive to the mitochondrial toxicants dronedarone, benzbromarone, and ketoconazole than untreated cells. In conclusion, we established and characterized a cell model, which could be used for testing drugs with idiosyncratic mitochondrial toxicity.

Hepatocellular toxicity of benzbromarone: effects on mitochondrial function and structure

Benzbromarone is an uricosuric structurally related to amiodarone and a known mitochondrial toxicant. The aim of the current study was to improve our understanding in the molecular mechanisms of benzbromarone-associated hepatic mitochondrial toxicity. In HepG2 cells and primary human hepatocytes, ATP levels started to decrease in the presence of 25-50μM benzbromarone for 24-48h, whereas cytotoxicity was observed only at 100μM. In HepG2 cells, benzbromarone decreased the mitochondrial membrane potential starting at 50μM following incubation for 24h. Additionally, in HepG2 cells, 50μM benzbromarone for 24h induced mitochondrial uncoupling,and decreased mitochondrial ATP turnover and maximal respiration. This was accompanied by an increased lactate concentration in the cell culture supernatant, reflecting increased glycolysis as a compensatory mechanism to maintain cellular ATP. Investigation of the electron transport chain revealed a decreased activity of all relevant enzyme complexes. Furthermore, treatment with benzbromarone was associated with increased cellular ROS production, which could be located specifically to mitochondria. In HepG2 cells and in isolated mouse liver mitochondria, benzbromarone also reduced palmitic acid metabolism due to an inhibition of the long-chain acyl CoA synthetase. In HepG2 cells, benzbromarone disrupted the mitochondrial network, leading to mitochondrial fragmentation and a decreased mitochondrial volume per cell. Cell death occurred by both apoptosis and necrosis. The study demonstrates that benzbromarone not only affects the function of mitochondria in HepG2 cells and human hepatocytes, but is also associated with profound changes in mitochondrial structure which may be associated with apoptosis.

Effects of tramadol, clonazepam, and their combination on brain mitochondrial complexes

The present study is an unsubstantiated qualitative assessment of the abused drugs—tramadol and clonazepam. The aim of this study is to evaluate whether the effects of tramadol, clonazepam, and their combination on mitochondrial electron transport chain (ETC) complexes were influential at therapeutic or at progressively increasing doses. The study comprised of a total of 70 healthy male rats, aged 3 months. According to the drug intake regimen, animals were divided into seven groups: control, tramadol therapeutic, clonazepam therapeutic, combination therapeutic, tramadol abuse, clonazepam abuse, and combination abuse group. At the end of the experiment, brain mitochondrial ETC complexes (I, II, III, and IV) were evaluated. Histopathological examinations were also performed on brain tissues. The results showed that groups that received tramadol (therapeutic and abuse) suffered from weight loss. Tramadol abuse group and combination abuse group showed significant decrease in the activities of I, III, and IV complexes but not in the activity of complex II. In conclusion, tramadol but not clonazepam has been found to partially inhibit the activities of respiratory chain complexes I, III, and IV but not the activity of complex II and such inhibition occurred only at doses that exceeded the maximum recommended adult human daily therapeutic doses. This result explains the clinical and histopathological effects of tramadol, such as seizures and red neurons (marker for apoptosis), respectively.

Mitochondrial superoxide dismutase and glutathione peroxidase in idiosyncratic drug-induced liver injury

Drug-induced liver injury (DILI) susceptibility has a potential genetic basis. We have evaluated possible associations between the risk of developing DILI and common genetic variants of the manganese superoxide dismutase (SOD2 Val16Ala) and glutathione peroxidase (GPX1 Pro200Leu) genes, which are involved in mitochondrial oxidative stress management. Genomic DNA from 185 DILI patients assessed by the Council for International Organizations of Medical Science scale and 270 sex- and age-matched controls were analyzed. The SOD2 and GPX1 genotyping was performed using polymerase chain reaction restriction fragment length polymorphism and TaqMan probed quantitative polymerase chain reaction, respectively. The statistical power to detect the effect of variant alleles with the observed odds ratio (OR) was 98.2% and 99.7% for bilateral association of SOD2 and GPX1, respectively. The SOD2 Ala/Ala genotype was associated with cholestatic/mixed damage (OR = 2.3; 95% confidence interval [CI] = 1.4-3.8; corrected P [Pc] = 0.0058), whereas the GPX1 Leu/Leu genotype was associated with cholestatic injury (OR = 5.1; 95%CI = 1.6-16.0; Pc = 0.0112). The presence of two or more combined risk alleles (SOD2 Ala and GPX1 Leu) was more frequent in DILI patients (OR = 2.1; 95%CI = 1.4-3.0; Pc = 0.0006). Patients with cholestatic/mixed injury induced by mitochondria hazardous drugs were more prone to have the SOD2 Ala/Ala genotype (OR = 3.6; 95%CI = 1.4-9.3; Pc = 0.02). This genotype was also more frequent in cholestatic/mixed DILI induced by pharmaceuticals producing quinone-like or epoxide metabolites (OR = 3.0; 95%CI = 1.7-5.5; Pc = 0.0008) and S-oxides, diazines, nitroanion radicals, or iminium ions (OR = 16.0; 95%CI = 1.8-146.1; Pc = 0.009).

CONCLUSION

Patients homozygous for the SOD2 Ala allele and the GPX1 Leu allele are at higher risk of developing cholestatic DILI. SOD2 Ala homozygotes may be more prone to suffer DILI from drugs that are mitochondria hazardous or produce reactive intermediates.

Biochemical mechanisms in drug-induced liver injury: Certainties and doubts

Drug-induced liver injury is a significant and still unresolved clinical problem. Limitations to knowledge about the mechanisms of toxicity render incomplete the detection of hepatotoxic potential during preclinical development. Several xenobiotics are lipophilic substances and their transformation into hydrophilic compounds by the cytochrome P-450 system results in production of toxic metabolites. Aging, preexisting liver disease, enzyme induction or inhibition, genetic variances, local O₂ supply and, above all, the intrinsic molecular properties of the drug may affect this process. Necrotic death follows antioxidant consumption and oxidation of intracellular proteins, which determine increased permeability of mitochondrial membranes, loss of potential, decreased ATP synthesis, inhibition of Ca²⁺-dependent ATPase, reduced capability to sequester Ca²⁺ within mitochondria, and membrane bleb formation. Conversely, activation of nucleases and energetic participation of mitochondria are the main intracellular mechanisms that lead to apoptosis. Non-parenchymal hepatic cells are inducers of hepatocellular injury and targets for damage. Activation of the immune system promotes idiosyncratic reactions that result in hepatic necrosis or cholestasis, in which different HLA genotypes might play a major role. This review focuses on current knowledge of the mechanisms of drug-induced liver injury and recent advances on newly discovered mechanisms of liver damage. Future perspectives including new frontiers for research are discussed.

Acute liver and renal failure during treatment with buprenorphine at therapeutic dose

Buprenorphine is a semi-synthetic opioid derivative commonly used in the treatment of heroin addiction. Life-threatening complications have been described following overdoses while few cases of hepatotoxicity due to drug use at therapeutic doses have been recently described in hepatitis C virus carriers. In these cases, however, histological assessment was not exhaustive and no extra-hepatic organ failure was observed. We describe herein a case of acute liver and kidney failure in a patient with previously latent hepatitis C virus chronic infection following recommended doses of buprenorphine. Histology did not demonstrate any feature compatible with hepatitis C virus reactivation or liver cirrhosis and suspension of the treatment led to the resolution of both liver and kidney failure. Causality criteria fulfillment indicates a high probability of buprenorphine-induced liver toxicity. No signs of pre-existant kidney impairment or of pre- or post-renal causes were observed. Since buprenorphine is metabolized through cytochrome P450 3A4, we genotyped six genetic polymorphisms previously described in poor metabolizers but could not confirm these pharmacogenetic bases in this case. In conclusion, we surmise that buprenorphine at suggested doses can induce liver and kidney failure in susceptible individuals, possibly through direct mitochondrial toxicity.

Mitochondrial off targets of drug therapy

The bioenergetic features of mitochondria have long been exploited in the design of pharmacological agents suited to accomplish a desired physiological effect; uncoupling of oxidative phosphorylation to induce weight loss, for example. However, more recent experience demonstrates mitochondria to be unintended off targets of other drug therapies and responsible, at least in part, for the dose-limiting adverse events associated with a large array of pharmaceuticals. Review of the fundamentals of mitochondrial molecular biology and bioenergetics reveals a multiplicity of off targets that can be invoked to explain drug-induced mitochondrial failure. It is this redundancy of mitochondrial off targets that complicates identification of discrete mechanisms of toxicity and confounds QSAR-based design of new small molecules devoid of this potential for mitochondrial toxicity. The present review article briefly reviews the molecular biology and biophysics of mitochondrial bioenergetics, which then serves as a platform for identifying the various potential off targets for drug-induced mitochondrial toxicity.

Prediction of drug-induced liver injury in humans by using in vitro methods: the case of ximelagatran

OBJECTIVE

To investigate the possible mechanisms underlying the liver enzyme elevations seen during clinical studies of long-term treatment (>35 days) with ximelagatran, and investigate the usefulness of pre-clinical in vitro systems to predict drug-induced liver effects.

METHODS

Ximelagatran and its metabolites were tested for effects on cell viability, mitochondrial function, formation of reactive metabolites and reactive oxygen species, protein binding, and induction of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) gene expression or nuclear orphan receptors. Experimental systems included fresh and cryopreserved hepatocytes, human hepatoma cell lines (HepG2 and HuH-7) and subcellular human liver fractions.

RESULTS

Loss of cell viability was only seen in HepG2 cells at ximelagatran concentrations 100 microM and in cryopreserved human hepatocytes at 300 microM, while HuH-7 cells were not affected by 24 h exposure at up to 300 microM ximelagatran. Calcium homeostasis was not affected in HepG2 cells exposed to ximelagatran up to 300 microM for 15 min. There was no evidence for the formation of reactive metabolites when cell systems were exposed to ximelagatran. ALT and AST expression in human hepatoma cell lines were also unchanged by ximelagatran. Mitochondrial functions such as respiration, opening of the transition pore, mitochondrial membrane depolarization and beta-oxidation were not affected by ximelagatran or its metabolites.

CONCLUSION

Ximelagatran at concentrations considerably higher than that found in plasma following therapeutic dosing had little or no effect on cellular functions studied in vitro. The in vitro studies therefore did not elucidate the mechanism by which ximelagatran induces liver effects in humans, possibly because of limitations in the experimental systems not reflecting characteristics of the human hepatocyte, restricted exposure time, or because the primary mechanism for the observed clinical liver effects is not on the parenchymal liver cell.

Catechin as an antioxidant in liver mitochondrial toxicity: Inhibition of tamoxifen-induced protein oxidation and lipid peroxidation

Tamoxifen (TAM) is a nonsteroidal triphenylethylene antiestrogenic drug widely used in the treatment and prevention of breast cancer. TAM brings about a collapse of the mitochondrial membrane potential. It acts both as an uncoupling agent and as a powerful inhibitor of mitochondrial electron transport chain. The effect of catechin pretreatment on the mitochondrial toxicity of TAM was studied in liver mitochondria of Swiss albino mice. TAM treatment caused a significant increase in the mitochondrial lipid peroxidation (LPO) and the protein carbonyls (PCs). It also caused a significant increase in superoxide radical production. Pretreatment of mice with catechin (40 mg/kg) showed significant protection as demonstrated by marked attenuation of increased oxidative stress parameters such LPO, PCs, and superoxide production. It also restored the decreased nonenzymatic and enzymatic antioxidants of mitochondria. The inhibitory effect of catechin on TAM-:induced oxidative damage suggests that it may have potential benefits in prevention of human diseases where reactive oxygen species have some role as causative agents.

The role of mitochondria in pharmacotoxicology: a reevaluation of an old, newly emerging topic

In addition to their well-known critical role in energy metabolism, mitochondria are now recognized as the location where various catabolic and anabolic processes, calcium fluxes, various oxygen-nitrogen reactive species, and other signal transduction pathways interact to maintain cell homeostasis and to mediate cellular responses to different stimuli. It is important to consider how pharmacological agents affect mitochondrial biochemistry, not only because of toxicological concerns but also because of potential therapeutic applications. Several potential targets could be envisaged at the mitochondrial level that may underlie the toxic effects of some drugs. Recently, antiviral nucleoside analogs have displayed mitochondrial toxicity through the inhibition of DNA polymerase-γ (pol-γ). Other drugs that target different components of mitochondrial channels can disrupt ion homeostasis or interfere with the mitochondrial permeability transition pore. Many known inhibitors of the mitochondrial electron transfer chain act by interfering with one or more of the respiratory chain complexes. Nonsteroidal anti-inflammatory drugs (NSAIDs), for example, may behave as oxidative phosphorylation uncouplers. The mitochondrial toxicity of other drugs seems to depend on free radical production, although the mechanisms have not yet been clarified. Meanwhile, drugs targeting mitochondria have been used to treat mitochondrial dysfunctions. Importantly, drugs that target the mitochondria of cancer cells have been developed recently; such drugs can trigger apoptosis or necrosis of the cancer cells. Thus the aim of this review is to highlight the role of mitochondria in pharmacotoxicology, and to describe whenever possible the main molecular mechanisms underlying unwanted and/or therapeutic effects.

Hepatocytes—the choice to investigate drug metabolism and toxicity in man: In vitro variability as a reflection of in vivo

The pharmaceutical industry is committed to marketing safer drugs with fewer side effects, predictable pharmacokinetic properties and quantifiable drug-drug interactions. Drug metabolism is a major determinant of drug clearance and interindividual pharmacokinetic differences, and an indirect determinant of the clinical efficacy and toxicity of drugs. Progressive advances in the knowledge of metabolic routes and enzymes responsible for drug biotransformation have contributed to understanding the great metabolic variations existing in human beings. Phenotypic as well genotypic differences in the expression of the enzymes involved in drug metabolism are the main causes of this variability. However, only a minor part of phenotypic variability in man is attributable to gene polymorphisms, thus making the definition of a normal liver complex. At present, the use of human in vitro hepatic models at early preclinical stages means that the process of selecting drug candidates is becoming much more rational. Cultured human hepatocytes are considered to be the closest model to human liver. However, the fact that hepatocytes are located in a microenvironment that differs from that of the cell in the liver raises the question: to what extent does drug metabolism variability observed in vitro actually reflect that of the liver in vivo? By comparing the metabolism of a model compound both in vitro and in vivo in the same individual, a good correlation between the in vitro and in vivo relative abundance of oxidized metabolites and the hydrolysis of the compound was observed. Thus, it is reasonable to consider that the variability observed in human hepatocytes reflects the existing phenotypic heterogeneity of the P450 expression in human liver.

Cytosolic heat shock proteins and heme oxygenase-1 are preferentially induced in response to specific and localized intramitochondrial damage by tetrafluoroethylcysteine

Previously, S-(1,1,2,2-tetrafluoroethyl)-l-cysteine (TFEC) was shown to mediate cytotoxicity by covalently modifying a well-defined group of intramitochondrial proteins including aconitase, alpha-ketoglutarate dehydrogenase (alphaKGDH) subunits, heat shock protein 60 (HSP60) and mitochondrial HSP70 (mtHSP70). To investigate the cellular responses to this mitochondrial damage, microarray analysis of TFEC treated murine hepatocytes of the TAMH cell line was carried out. Results of these studies revealed a HSP response that was significantly stronger than other well-characterized hepatotoxicants including acetaminophen, diquat and rotenone. Specifically, cytosolic HSP25, HSP40, HSP70, HSP105 and microsomal HSP32 (HO-1) were strongly upregulated within the first few hours of TFEC treatment, while little change was observed among other HSPs that are predominantly localized in the mitochondria and endoplasmic reticulum (ER). Post-translational modification of HSP25 was also observed with the appearance of a unique DTT-resistant immunoreactive band at about 50kDa, a putative dimer. The biological significance of HSP responses to TFEC-induced toxicity were subsequently demonstrated using the "gain of function" pretreatment: heat shock. Overall, we report an atypical HSP induction profile that does not conform to changes expected of a classical temperature shock. Furthermore, despite a well-defined intramitochondrial origin of toxicity, TFEC rapidly evokes an early and strong upregulation of cytosolic stress proteins. The cytoprotective effects of such HSP responses suggest a plausible role in modulating the progression of TFEC-induced cellular injury.

BCL-xL overexpression effectively protects against tetrafluoroethylcysteine-induced intramitochondrial damage and cell death

S-(1,1,2,2-Tetrafluoroethyl)-L-cysteine (TFEC), a major metabolite of the industrial gas tetrafluoroethylene, has been shown to mediate nephrotoxicity by necrosis. TFEC-induced cell death is associated with an early covalent modification of specific intramitochondrial proteins; including aconitase, alpha-ketoglutarate dehydrogenase (KGDH) subunits, HSP60 and HSP70. Previous studies have indicated that the TAMH line accurately models TFEC-induced in vivo cell death with dose- and time-dependent inhibitions of both KGDH and aconitase activities. Here, we show that the molecular pathway leading to TFEC-mediated cell death is associated with an early cytosolic to mitochondrial translocation of BAX, a pro-apoptotic member of the BCL-2 family. Immunoblot analyses indicated movement of BAX (21 kDa) to the mitochondrial fraction after exposure to a cytotoxic concentration of TFEC (250 microM). Subsequent cytochrome c release from mitochondria was also demonstrated, but only a modest increase in caspase activities was observed, suggesting a degeneration of early apoptotic signals into secondary necrosis. Significantly, TAMH cells overexpressing BCL-xL preserved cell viability even to supratoxicological concentrations of TFEC (< or =600 microM), and this cytoprotection was associated with decreased HSP70i upregulation, indicating suppression of TFEC-induced proteotoxicity. Hence, TFEC-induced necrotic cell death in the TAMH cell line is mediated by BAX and antagonized by the anti-apoptotic BCL-2 family member, BCL-xL.

1,1-Dichloroethylene-Induced Mitochondrial Damage Precedes Apoptotic Cell Death of Bronchiolar Epithelial Cells in Murine Lung

1,1-Dichloroethylene (DCE) causes pulmonary injury that is characterized by necrosis of bronchiolar Clara cells. Mitochondria have been identified as an early target in the toxic response. Because mitochondria have been implicated in both necrotic and apoptotic cell death, we have undertaken studies to test the hypothesis that DCE induces apoptosis, in addition to necrosis, in murine lung. A primary objective is to identify the biochemical events associated with pulmonary apoptosis. Groups of female CD-1 mice were treated with DCE (75 mg/kg i.p.) or corn oil. Using an antibody directed against DCE-cysteine conjugates, adducts were detected primarily in association with mitochondria in the apices of bronchiolar Clara cells. Furthermore, morphological studies demonstrated early mitochondrial alterations in Clara cells that included severe swelling and disruption of cristae. Western blotting of lung cytosolic proteins showed greater immunoreactivity for cytochrome c in fractions from mice treated with DCE for 4 h than in controls. Immunohistochemical studies with an antibody to activated caspase-3 and terminal deoxynucleotidyl transferase dUTP nick-end labeling were used to detect apoptotic cells. In both experiments, positive reactivities were observed in the bronchiolar epithelium at 12 and 24 h after DCE treatment, whereas reactivities were absent in tissues from control animals. Finally, bronchiolar epithelial cells showing morphological criteria of apoptosis (chromatin condensation and margination) were observed at 24 h after 75 and 125 mg/kg DCE. Apoptotic-like cells were more abundant in larger bronchioles. These data suggested that DCE produces pulmonary bronchiolar apoptosis by inducing mitochondrial perturbations, causing release of cytochrome c into the cytosol and caspase activation.

Drug-induced increased mitochondrial biogenesis in a liver biopsy

Oncocytic changes seen in hepatocytes in patients receiving highly active antiretroviral therapy (HAART) are a result of mitochondrial damage. This is the first report that provides the electron microscopy illustration of mitochondrial proliferation as a result of the HAART drug Stavudine (Zerit) hepatotoxicity. The drug's effect on mitochondrial DNA replication leads to depleted mitochondrial-encoded proteins and configurational defects of the mitochondrial inner membrane leading to reduced and abnormal cristae, which house the electron transport chain and elementary bodies. This results in a decrease in the NAD/NADH ratio and reduces oxidative phosphorylation. The shift in the NAD/NADH ratio decreases the rate of fatty acid beta oxidation and oxidation of pyruvate by the Krebs cycle. Decreased oxidation of pyruvate drives it into an alternative pathway to form lactate leading to lactic acidosis. This mitochondrial dysfunction results in a compensatory increase in mitochondrial biogenesis, which results in oncocytic changes of the hepatocytes.

Toxicity of low dose azathioprine and 6-mercaptopurine in rat hepatocytes. Roles of xanthine oxidase and mitochondrial injury

BACKGROUND/AIMS

To study effects of pharmacologic concentrations of azathioprine and 6-mercaptopurine (6-MP) on rat hepatocytes.

METHODS

Hepatocytes cultured on matrigel were incubated with azathioprine or 6-MP; effects of putative protective agents were studied. Viability (LDH leakage), reduced (GSH) and oxidized glutathione (GSSG), mitochondrial (mt) GSH, ATP and ultrastructural changes were determined.

RESULTS

Azathioprine and 6-MP (0.5-5 micromol/l) reduced viability 5-34% at day 1 and 42-92% by day 4. Allopurinol (20 microM) (xanthine oxidase inhibitor) and 2 mM Trolox (vitamin E analog) together provided near complete protection. During culture with azathioprine, GSSG increased before cell death and there was a disproportionate reduction of mtGSH and ATP, together with ultrastructural abnormalities in mitochondria. All changes were prevented by allopurinol and trolox. Discontinuation of 1 micromol/l azathioprine restored ATP levels and arrested cell injury, while culture in glucose-enriched media augmented ATP levels and ameliorated cell death.

CONCLUSIONS

Clinically relevant concentrations of azathioprine and 6-MP are toxic to rat hepatocyte cultures by a mechanism that involves oxidative stress, mitochondrial injury and ATP depletion. This can lead to irreversible de-energization and cell death by oncosis (necrosis).

Frequency, risk factors, and outcome of hyperlactatemia in HIV-positive persons: implications for the management of treated patients

BACKGROUND

The nucleoside reverse transcriptase inhibitors used for the treatment of HIV-positive persons are now clearly associated with metabolic disorders. We determined the prevalence of and risk factors for hyperlactatemia in HIV-positive persons to assess the relevance of lactate venous blood concentrations during antiretroviral therapy.

METHODS

We conducted a prospective cross-sectional study of venous lactate determinations with 282 consecutive HIV-positive persons who, in addition to a physical examination, had blood samples taken every 3-4 months for routine biochemical, immunologic, and viral assessment. The frequencies of hyperlactatemia and lactic acidosis were determined, and the risk factors were analyzed by a multivariate logistic regression model. The effect of modification of antiretroviral therapy in patients with moderate hyperlactatemia was also assessed.

RESULTS

From 782 blood lactate determinations, we identified 65 (23%) patients with moderate hyperlactatemia and 5 (1.8%) with lactate concentrations >5 mmol/L (2 with severe lactic acidosis; 0.7%). Older age, drug regimens containing stavudine [adjusted odds ratio (OR) = 2.5] or a combination of stavudine-didanosine (adjusted OR = 3.1), and the use of buprenorphine (adjusted OR = 14.7) were independent predictors of hyperlactatemia. Among 65 patients with moderate hyperlactatemia, 39 did not have their treatments changed, and 26 had a new combination therapy that was associated with a clinical improvement and a more pronounced decrease in lactate (-1.66 vs -0.99 mmol/L; P <0.05).

CONCLUSIONS

Chronic compensated and moderate hyperlactatemia was common in our population study. Measurement of lactate, under standardized conditions, may be useful in optimizing management of HIV-positive persons on antiretroviral therapy.

Flow cytometric assessment of changes in rat sperm mitochondrial function after treatment with pentachlorophenol

The fluorophore 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl carbocyanine iodide (JC-1) localizes to the mitochondria and is affected by membrane potential, fluorescing bright orange when the membrane potential is high and green when mitochondrial membrane potential is low. The present study used flow cytometric analysis of JC-1 staining patterns of large numbers of spermatozoa to detect chemical-induced alterations of sperm mitochondrial membrane potential. Cauda epididymal rat spermatozoa were incubated with pentachlorophenol (PCP; 0.1 microM or 1.0 microM), a known uncoupler of mitochondrial oxidative phosphorylation. Microscopic evaluation showed that the midpiece (mitochondrial location) of live, highly motile spermatozoa stained bright orange, while the midpiece of live, non-motile spermatozoa stained green. The midpiece of slightly or non-progressively motile spermatozoa stained a faint orange-green. The percentage of spermatozoa stained bright orange and the total percentage of spermatozoa stained orange (bright orange+faint orange) in the control samples of spermatozoa were significantly higher (P<0.001) than in the 0.1 microM and 1.0 microM PCP treated samples. These data indicate that sperm mitochondrial membrane potential is highly sensitive to the uncoupling effects of PCP and that JC-1 staining and flow cytometric analysis may be a sensitive assay to detect the effect of toxicants on rat sperm mitochondrial function.

Early viral clearance and sustained response in chronic hepatitis C: a controlled trial of interferon and ribavirin after high-dose interferon induction

High-dose induction with alpha-interferon induces early viral clearance of hepatitis C and combined with ribavirin enhances sustained response. We assess whether adding ribavirin after viral clearance obtained by alpha-interferon induction increased the rate of viral eradication.Forty-one naïve patients with chronic hepatitis C were randomised to receive, after 4 weeks of 10 mU daily of alpha-interferon (induction), 3 mU daily for 22 weeks and 3 mU thrice weekly for 26 weeks of either interferon alone (monotherapy) or interferon plus 1000-1200 mg daily of ribavirin (combination therapy). At the end of the induction phase, 23 (56%) subjects had cleared HCV-RNA. During therapy, breakthrough was observed in four patients on monotherapy, but never in patients on combination therapy. The rate of clearance of HCV-RNA was different between monotherapy and combination therapy at the end of treatment (40% vs. 76.1%, P=0.02) and at the end of follow-up (5% vs. 57.1%, P=0.001). Twelve of the 23 patients who cleared HCV-RNA during induction, but only one of the 18 still HCV-RNA-positive after 4 weeks of therapy, had a sustained response (52.2% vs. 5.6%, P=0.001). Clearance of HCV-RNA at 1 week had a high positive predictive value for sustained response in combination therapy (PPV=0.75), but not in monotherapy (PPV=0.33). Induction with high daily doses of alpha-interferon obtains suppression of hepatitis C in more than half of patients, but ribavirin is needed to maintain a sustained response. The rate of sustained response is a function of the time to HCV-RNA clearance. In patients not responding to induction therapy addition of ribavirin does not obtain a sustained virological response.