Drug-induced toxicity on mitochondria and lipid metabolism: mechanistic diversity and deleterious consequences for the liver

Metabonomics and toxicology

Being an emerging field of "omics" research, metabonomics has been increasingly used in toxicological studies mostly because this technology has the ability to provide more detailed information to elucidate mechanism of toxicity. As an interdisciplinary field of science, metabonomics combines analytical chemistry, bioinformatics, statistics, and biochemistry. When applied to toxicology, metabonomics also includes aspects of patho-biochemistry, systems biology, and molecular diagnostics. During a toxicological study, the metabolic changes over time and dose after chemical treatment can be monitored. Therefore, the most important use of this emerging technology is the identification of signatures of toxicity-patterns of metabolic changes predictive of a hazard manifestation. This chapter summarizes the current state of metabonomics technology and its applications in various areas of toxicological studies.

Comparative analysis of lipotoxicity induced by endocrine, pharmacological, and innate immune stimuli in rat basophilic leukemia cells

Cellular lipotoxicity manifests as the steatotic accumulation of lipid droplets or lipid bodies, and/or induction of phospholipidosis. Lipotoxicity can be induced by hyperinsulinemia/nutrient overload, cationic amphiphilic drugs (CAD), and innate immunological stimuli, all of which are stimuli relevant to mast cell physiology. Hyper-accumulation of mast cell lipid bodies in response to hyperinsulinemia has been documented, but lipotoxicity in response to CAD or innate immunologic stimuli has not been analysed comparatively. Moreover, gaps in our understanding of this steatosis remain, specifically as to whether hyperinsulinemia-driven steatosis in these cells attains lipotoxic levels or is accompanied by phospholipidosis. To compare endocrine, pharmacological, and innate immunological stimuli for their ability to induce steatosis and phospholipidosis in a rat basophilic leukemia mast cell model (RBL2H3), differential fluorescence microscopy staining and quantitation of phospholipidosis and steatosis in the RBL2H3 cell line was examined. The three classes of stimuli differentially induced phospholipidosis and steatosis. PPARγ up-regulation was not uniformly associated with the expansion of the lipid body population. Fluorescence imaging of lipid-enriched structures generated in response to lipotoxic cationic amphiphilic drugs, chronic insulin exposure, and TLR2/4 ligands revealed differential staining patterns when visualized using lipophilic dyes. It is concluded that lipotoxicity-inducing pathways in this model mast cell system are diverse, and include steatotic responses to an endocrine stimulus, as well as phospholipidosis responses to cationic lipophilic drugs not previously described in this cell type.

Whole genome transcript profiling of drug induced steatosis in rats reveals a gene signature predictive of outcome

Drug induced steatosis (DIS) is characterised by excess triglyceride accumulation in the form of lipid droplets (LD) in liver cells. To explore mechanisms underlying DIS we interrogated the publically available microarray data from the Japanese Toxicogenomics Project (TGP) to study comprehensively whole genome gene expression changes in the liver of treated rats. For this purpose a total of 17 and 12 drugs which are diverse in molecular structure and mode of action were considered based on their ability to cause either steatosis or phospholipidosis, respectively, while 7 drugs served as negative controls. In our efforts we focused on 200 genes which are considered to be mechanistically relevant in the process of lipid droplet biogenesis in hepatocytes as recently published (Sahini and Borlak, 2014). Based on mechanistic considerations we identified 19 genes which displayed dose dependent responses while 10 genes showed time dependency. Importantly, the present study defined 9 genes (ANGPTL4, FABP7, FADS1, FGF21, GOT1, LDLR, GK, STAT3, and PKLR) as signature genes to predict DIS. Moreover, cross tabulation revealed 9 genes to be regulated ≥10 times amongst the various conditions and included genes linked to glucose metabolism, lipid transport and lipogenesis as well as signalling events. Additionally, a comparison between drugs causing phospholipidosis and/or steatosis revealed 26 genes to be regulated in common including 4 signature genes to predict DIS (PKLR, GK, FABP7 and FADS1). Furthermore, a comparison between in vivo single dose (3, 6, 9 and 24 h) and findings from rat hepatocyte studies (2 h, 8 h, 24 h) identified 10 genes which are regulated in common and contained 2 DIS signature genes (FABP7, FGF21). Altogether, our studies provide comprehensive information on mechanistically linked gene expression changes of a range of drugs causing steatosis and phospholipidosis and encourage the screening of DIS signature genes at the preclinical stage.

Impact of the superoxide dismutase 2 Val16Ala polymorphism on the relationship between valproic acid exposure and elevation of γ-glutamyltransferase in patients with epilepsy: a population pharmacokinetic-pharmacodynamic analysis

BACKGROUND

There has been accumulating evidence that there are associations among γ-glutamyltransferase (γ-GT) elevation and all-cause mortality, cardiovascular diseases and metabolic diseases, including nonalcoholic fatty liver disease. The primary objective of this study was to evaluate the impact of the most common and potentially functional polymorphisms of antioxidant enzyme genes, i.e. superoxide dismutase 2 (SOD2), glutathione S-transferase M1 and glutathione S-transferase T1, on the γ-GT elevation during valproic acid (VPA) therapy.

METHODS AND FINDINGS

This retrospective study included 237 and 169 VPA-treated Japanese patients with epilepsy for population pharmacokinetic and pharmacokinetic-pharmacodynamic analyses, respectively. A nonlinear mixed-effect model represented the pharmacokinetics of VPA and the relationships between VPA exposure and γ-GT elevation. A one-compartment model of the pharmacokinetic parameters of VPA adequately described the data; while the model for the probability of the γ-GT elevation was fitted using a logistic regression model, in which the logit function of the probability was a linear function of VPA exposure. The SOD2 Val16Ala polymorphism and complication with intellectual disability were found to be significant covariates influencing the intercept of the logit function for the probability of an elevated γ-GT level. The predicted mean percentages of the subjects with γ-GT elevation were about 2- to 3-fold, 3- to 4-fold and 4- to 8-fold greater in patients with the SOD2 Val/Val genotype but without any intellectual disability, those with the SOD2 Val/Ala or Ala/Ala genotype and intellectual disability and those with the SOD2 Val/Val genotype and intellectual disability, respectively, compared to those with the SOD2 Val/Ala or Ala/Ala genotype without intellectual disability.

CONCLUSION

Our results showed that the SOD2 Val16Ala polymorphism has an impact on the relationship between VPA exposure and γ-GT elevation in patients with epilepsy. These results suggest that determining the SOD2 genotype could be helpful for preventing the VPA-induced γ-GT elevation.

Burn after feeding. An old uncoupler of oxidative phosphorylation is redesigned for the treatment of nonalcoholic fatty liver disease

Uncoupling of oxidative phosphorylation (OXPHOS) in brown adipose tissue can be used by hibernating animals to produce heat at the expense of their fat mass. In a recent work, Dr Shulman et al. generated a liver-targeted derivative of the prototypical OXPHOS uncoupler 2,4-dinitrophenol that alleviated steatosis, hypertriglyceridemia and insulin resistance in several models of nonalcoholic fatty liver disease and type 2 diabetes.

Characterization of ritonavir-mediated inactivation of cytochrome P450 3A4

Ritonavir is a human immunodeficiency virus (HIV) protease inhibitor and an inhibitor of cytochrome P450 3A4, the major human hepatic drug-metabolizing enzyme. Given the potent inhibition of CYP3A4 by ritonavir, subtherapeutic doses of ritonavir are used to increase plasma concentrations of other HIV drugs oxidized by CYP3A4, thereby extending their clinical efficacy. However, the mechanism of inhibition of CYP3A4 by ritonavir remains unclear. To date, data suggests multiple types of inhibition by ritonavir, including mechanism-based inactivation by metabolic-intermediate complex formation, competitive inhibition, irreversible type II coordination to the heme iron, and more recently heme destruction. The results presented here demonstrate that inhibition of CYP3A4 by ritonavir occurs by CYP3A4-mediated activation and subsequent formation of a covalent bond to the apoprotein. Incubations of [(3)H]ritonavir with reconstituted CYP3A4 and human liver microsomes resulted in a covalent binding stoichiometry equal to 0.93 ± 0.04 moles of ritonavir bound per mole of inactivated CYP3A4. The metabolism of [(3)H]ritonavir by CYP3A4 leads to the formation of a covalent adduct specifically to CYP3A4, confirmed by radiometric liquid chromatography-trace and whole-protein mass spectrometry. Tryptic digestion of the CYP3A4-[(3)H]ritonavir incubations exhibited an adducted peptide (255-RM K: ESRLEDTQKHR-268) associated with a radiochromatic peak and a mass consistent with ritonavir plus 16 Da, in agreement with the whole-protein mass spectrometry. Additionally, nucleophilic trapping agents and scavengers of free oxygen species did not prevent inactivation of CYP3A4 by ritonavir. In conclusion, ritonavir exhibited potent time-dependent inactivation of CYP3A, with the mechanism of inactivation occurring though a covalent bond to Lys257 of the CYP3A4 apoprotein.

Curcumol induces HSC-T6 cell death through suppression of Bcl-2: involvement of PI3K and NF-κB pathways

The major feature in the molecular pathogenesis of hepatic fibrosis requires maintenance of the activated hepatic stellate cells (HSCs) phenotype by both proliferation and inhibition of apoptosis. Thus, the induction of activated HSCs apoptosis has been proposed as an antifibrotic treatment strategy. Curcumol has pro-apoptotic activity in a number of cancer cell types. The aim of this study is to test the hypothesis that the interruption of the phosphatidylinositol 3 kinase (PI3K)/nuclear factor-κB (NF-κB) signaling pathway by curcumol might induce apoptosis of activated HSCs. Our results indicated that curcumol-induced growth inhibition correlated with apoptosis induction as evidenced by Annexin V staining, and cleavage of caspase-3 and poly (ADP-ribose) polymerase (PARP) in HSC-T6. Importantly, we show that the apoptotic effect of curcumol was specific to the activated HSCs (HSC-T6). Suppression of the NF-κB translocation via inhibition of IκB-α phosphorylation by the curcumol led to the inhibition of expression of NF-κB-regulated gene, e.g. Bcl-xL and Bcl-2, in a PI3K-dependent manner, which is upstream of NF-κB activation. Also, curcumol-mediated apoptosis of HSC-T6 were reversed by LY294002 and Bay 11-7082. Taken together, our findings perfectly support the hypothesis and demonstrate that the inhibition of PI3K/NF-κB pathway by curcumol lead to HSC-T6 apoptosis. Thus, our study indicates that curcumol is a potential candidate for further preclinical study aimed at the treatment of liver fibrosis.

L-carnitine prevents progression of non-alcoholic steatohepatitis in a mouse model with upregulation of mitochondrial pathway

Non-alcoholic steatohepatitis (NASH) is a severe form of non-alcoholic fatty liver disease characterized by lobular inflammation, hepatocellular ballooning, and fibrosis with an inherent risk for progression to cirrhosis and hepatocellular carcinoma (HCC). Mitochondrial dysfunction appears to play a role in the progression from simple steatosis to NASH. L-carnitine (L-b-hydroxy-g-N-trimethylaminobutyric acid), an essential nutrient that converts fat into energy in mitochondria, has been shown to ameliorate liver damage. The aim of the present study was to explore the preventive and therapeutic effect of L-carnitine in NASH model mice. Eight-week-old male STAM mice, a NASH-cirrhosis-hepatocarcinogenic model, were divided into 3 experimental groups and fed as follows: 1) high-fat diet (HFD) (control group); 2) HFD mixed with 0.28% L-carnitine (L-carnitine group); and 3) HFD mixed with 0.01% α-tocopherol (α-tocopherol group). After 4 or 8 weeks, mice were sacrificed. Blood samples and livers were collected, and hepatic tumors were counted and measured. Livers were subjected to histological study, immunohistochemical staining of 4-hydroxynonenal and ferritin, determination of 8-OHdG levels, mRNA and protein expressions for multiple genes, and metabolomic analysis. The intestinal microbiome was also analyzed. L-carnitine increased hepatic expression of genes related to long-chain fatty acid transport, mitochondrial β-oxidation, and antioxidant enzymes following suppression of hepatic oxidative stress markers and inflammatory cytokines in NASH, and mice treated with L-carnitine developed fewer liver tumors. Although α-tocopherol resulted in NASH improvement in the same manner as L-carnitine, it increased periodontitis-related microbiotic changes and hepatic iron transport-related gene expression and led to less effective for anti-hepatocarcinogenesis.

Conclusion

L-carnitine prevents progression of non-alcoholic steatohepatitis in a mouse model by upregulating the mitochondrial β-oxidation and redox system.

Determination of liver specific toxicities in rat hepatocytes by high content imaging during 2-week multiple treatment

DILI is a major safety issue during drug development and one of the leading causes for market withdrawal. Despite many efforts made in the past, the prediction of DILI using in vitro models remains very unreliable. In the present study, the well-established hepatocyte Collagen I-Matrigel™ sandwich culture was used, mimicking chronic drug treatment after multiple incubations for 14 days. Ten drugs associated with different types of specific preclinical and clinical liver injury were evaluated at non-cytotoxic concentrations. Mrp2-mediated transport, intracellular accumulation of neutral lipids and phospholipids were selected as functional endpoints by using Cellomics™ Arrayscan® technology and assessed at five timepoints (day 1, 3, 7, 10, 14). Liver specific functional impairments after drug treatment were enhanced over time and could be monitored by HCI already after few days and before cytotoxicity. Phospholipidosis-inducing drugs Chlorpromazine and Amiodarone displayed the same response as in vivo. Cyclosporin A, Chlorpromazine, and Troglitazone inhibited Mrp2-mediated biliary transport, correlating with in vivo findings. Steatosis remained difficult to be reproduced under the current in vitro testing conditions, resulting into false negative and positive responses. The present results suggest that the repeated long-term treatment of rat hepatocytes in the Collagen I-Matrigel™ sandwich configuration might be a suitable tool for safety profiling of the potential to induce phospholipidosis and impair Mrp2-mediated transport processes, but not to predict steatosis.

Assessment of fatty acid beta oxidation in cells and isolated mitochondria

Fatty acid beta oxidation is a major pathway of energy metabolism and occurs primarily in mitochondria. Drug-induced modulation of this pathway can cause adverse effects such as liver injury, or be beneficial for treating heart failure, type 2 diabetes, and obesity. Hence, in vitro assays that are able to identify compounds that affect fatty acid oxidation are of value for toxicity assessments, as well as for efficacy assessments. Here, we describe two high-throughput assays, one for assessing fatty acid oxidation in cells and the other for assessing fatty acid oxidation in isolated rat liver mitochondria. Both assays measure fatty acid-driven oxygen consumption and can be used for rapid and robust screening of compounds that modulate fatty acid oxidation.

Mitochondrial dysfunction in early life resulted from perinatal bisphenol A exposure contributes to hepatic steatosis in rat offspring

An emerging literature suggests that bisphenol A (BPA), a widespread endocrine disrupting chemical, when exposure occurs in early life, may increase the risk of metabolic syndrome. In this study, we investigated the hypothesis that perinatal exposure to BPA predisposed offspring to fatty liver disease: the hepatic manifestation of metabolic syndrome, and its possible mechanism. Pregnant Wistar rats were administered with BPA (40μg/kg/day) or vehicle during gestation and lactation. Liver histology, biochemical analysis, transcriptome, and mitochondrial function were examined in male offspring at postnatal 3, 15 and 26 weeks. At 3 weeks of age, abnormal liver morphology and function were not observed in the BPA-exposed offspring, but a decrease in mitochondrial respiratory complex (MRC) activity (I and III) and significant changes in gene expression involved in mitochondrial fatty acid metabolism were observed compared with controls. At 15 weeks, micro-vesicular steatosis in liver, up-regulated genes involved in lipogenesis pathways, increased ROS generation and Cytc release were observed in the BPA-exposed offspring. Then, extensive fatty accumulation in liver and elevated serum ALT were observed in BPA-exposed offspring at 26 weeks. In the longitudinal observation, hepatic mitochondrial function including MRC activity, ATP production, ROS generation and mitochondrial membrane potential were progressively worsened in the BPA-exposed offspring. Perinatal BPA exposure contributes to the development of hepatic steatosis in the offspring of rats, which may be mediated through impaired hepatic mitochondrial function and up-regulated hepatic lipid metabolism.

Chronic administration of fluoxetine or clozapine induces oxidative stress in rat liver: a histopathological study

Chronic exposure to stress contributes to the etiology of mood disorders, and the liver as a target organ of antidepressant and antipsychotic drug metabolism is vulnerable to drug-induced toxicity. We investigated the effects of chronic administration of fluoxetine (15mg/kg/day) or clozapine (20mg/kg/day) on liver injury via the measurement of liver enzymes, oxidative stress and histopathology in rats exposed to chronic social isolation (21days), an animal model of depression, and controls. The activity of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST), the liver content of carbonyl groups, malonyldialdehyde (MDA), reduced glutathione (GSH), cytosolic glutathione S-transferase (GST) and nitric oxide (NO) metabolites were determined. We also characterized nuclear factor-κB (NF-κB), cyclooxygenase-2 (COX-2) and CuZn-superoxide dismutase (CuZnSOD) protein expression as well as histopathological changes. Increased serum ALT activity in chronically-isolated and control animals treated with both drugs was found while increased AST activity was observed only in fluoxetine-treated rats (chronically-isolated and controls). Increased carbonyl content, MDA, GST activity and decreased GSH levels in drug-treated controls/chronically-isolated animals suggest a link between drugs and hepatic oxidative stress. Increased NO levels associated with NF-κB activation and the concomitant increased COX-2 expression together with compromised CuZnSOD expression in clozapine-treated chronically-isolated rats likely reinforce oxidative stress, observed by increased lipid peroxidation and GSH depletion. In contrast, fluoxetine reduced NO levels in chronically-isolated rats. Isolation induced oxidative stress but histological changes were similar to those observed in vehicle-treated controls. Chronic administration of fluoxetine in both chronically-isolated and control animals resulted in more or less normal hepatic architecture, while clozapine in both groups resulted in liver injury. These data suggest that clozapine appears to have a higher potential to induce liver toxicity than fluoxetine.

PKCε contributes to chronic ethanol-induced steatosis in mice but not inflammation and necrosis

BACKGROUND

Protein kinase C epsilon (PKCε) has been shown to play a role in experimental steatosis by acute alcohol. The "two-hit" hypothesis implies that preventing steatosis should blunt more advanced liver damage (e.g., inflammation and necrosis). However, the role of PKCε in these pathologies is not yet known. The goal of this current work was to address this question in a model of chronic alcohol exposure using antisense oligonucleotides (ASO) against PKCε.

METHODS

Accordingly, PKCε ASO- and saline-treated mice were fed high-fat control or ethanol (EtOH)-containing enteral diets for 4 weeks.

RESULTS

Chronic EtOH exposure significantly elevated hepatic lipid pools as well as activated PKCε. The PKCε ASO partially blunted the increases in hepatic lipids caused by EtOH. Administration of PKCε ASO also completely prevented the increase in the expression of fatty acid synthase, and tumor necrosis factor α caused by EtOH. Despite these protective effects, the PKCε ASO was unable to prevent the increases in inflammation and necrosis caused by chronic EtOH. These latter results correlated with an inability of the PKCε ASO to blunt the up-regulation of plasminogen activator inhibitor-1 (PAI-1) and the accumulation of fibrin. Importantly, PAI-1 has been previously shown to more robustly mediate inflammation and necrosis (vs. steatosis) after chronic EtOH exposure.

CONCLUSIONS

This study identifies a novel potential mechanism where EtOH, independent of steatosis, can contribute to liver damage. These results also suggest that PAI-1 and fibrin accumulation may be at the center of this PKCε-independent pathway.

A simple transcriptomic signature able to predict drug-induced hepatic steatosis

It is estimated that only a few marketed drugs are able to directly induce liver steatosis. However, many other drugs may exacerbate or precipitate fatty liver in the presence of other risk factors or in patients prone to non-alcoholic fatty liver disease. On the other hand, current in vitro tests for drug-induced steatosis in preclinical research are scarce and not very sensitive or reproducible. In the present study, we have investigated the effect of well-characterized steatotic drugs on the expression profile of 47 transcription factors (TFs) in human hepatoma HepG2 cells and found that these drugs are able to up- and down-regulate a substantial number of these factors. Multivariate data analysis revealed a common TF signature for steatotic drugs, which consistently and significantly repressed FOXA1, HEX and SREBP1C in cultured cells. This signature was also observed in the livers of rats and in cultured human hepatocytes. Therefore, we selected these three TFs as predictive biomarkers for iatrogenic steatosis. With these biomarkers, a logistic regression analysis yielded a predictive model, which was able to correctly classify 92 % of drugs. The developed algorithm also predicted that ibuprofen, nifedipine and irinotecan are potential steatotic drugs, whereas troglitazone is not. In summary, this is a sensitive, specific and simple RT-PCR test that can be easily implemented in preclinical drug development to predict drug-induced steatosis. Our results also indicate that steatotic drugs affect expression of both common and specific subsets of TF and lipid metabolism genes, thus generating complex transcriptomic responses that cause or contribute to steatosis in hepatocytes.

Uridine prevents fenofibrate-induced fatty liver

Uridine, a pyrimidine nucleoside, can modulate liver lipid metabolism although its specific acting targets have not been identified. Using mice with fenofibrate-induced fatty liver as a model system, the effects of uridine on liver lipid metabolism are examined. At a daily dosage of 400 mg/kg, fenofibrate treatment causes reduction of liver NAD(+)/NADH ratio, induces hyper-acetylation of peroxisomal bifunctional enzyme (ECHD) and acyl-CoA oxidase 1 (ACOX1), and induces excessive accumulation of long chain fatty acids (LCFA) and very long chain fatty acids (VLCFA). Uridine co-administration at a daily dosage of 400 mg/kg raises NAD(+)/NADH ratio, inhibits fenofibrate-induced hyper-acetylation of ECHD, ACOX1, and reduces accumulation of LCFA and VLCFA. Our data indicates a therapeutic potential for uridine co-administration to prevent fenofibrate-induced fatty liver.

The Role of Non-alcoholic Fatty Liver Disease in Cardiovascular Disease

Non-alcoholic fatty liver disease (NAFLD) has become the most prevalent chronic liver disease in western countries and is closely related to the metabolic syndrome. When NAFLD is associated with hepatocellular damage and inflammation (non-alcoholic steatohepatitis [NASH]) it can lead to severe liver disease. However, it has become clear that NAFLD is also associated with an increased risk of cardiovascular disease (CVD), independently of classical known risk factors for the latter. In the current review we briefly summarise the current clinical evidence on the role of NAFLD in CVD and discuss the potential mechanisms by which NAFLD can be linked to the pathophysiology of CVD.

Predictivity of dog co-culture model, primary human hepatocytes and HepG2 cells for the detection of hepatotoxic drugs in humans

Drug induced liver injury (DILI) is a major cause of attrition during early and late stage drug development. Consequently, there is a need to develop better in vitro primary hepatocyte models from different species for predicting hepatotoxicity in both animals and humans early in drug development. Dog is often chosen as the non-rodent species for toxicology studies. Unfortunately, dog in vitro models allowing long term cultures are not available. The objective of the present manuscript is to describe the development of a co-culture dog model for predicting hepatotoxic drugs in humans and to compare the predictivity of the canine model along with primary human hepatocytes and HepG2 cells. After rigorous optimization, the dog co-culture model displayed metabolic capacities that were maintained up to 2 weeks which indicates that such model could be also used for long term metabolism studies. Most of the human hepatotoxic drugs were detected with a sensitivity of approximately 80% (n=40) for the three cellular models. Nevertheless, the specificity was low approximately 40% for the HepG2 cells and hepatocytes compared to 72.7% for the canine model (n=11). Furthermore, the dog co-culture model showed a higher superiority for the classification of 5 pairs of close structural analogs with different DILI concerns in comparison to both human cellular models. Finally, the reproducibility of the canine system was also satisfactory with a coefficient of correlation of 75.2% (n=14). Overall, the present manuscript indicates that the dog co-culture model may represent a relevant tool to perform chronic hepatotoxicity and metabolism studies.

Fat accretion in a subpopulation of hepatocytes as a strategy to protect the whole liver against oxidative stress and lipotoxicity

Fatty liver can be induced by obesity, some drugs and alcohol intoxication. In this liver lesion, lipid accumulation can involve only some hepatocytes but the significance of this cell-to-cell heterogeneity is unknown. In a recent work, Dr Pol et al. propose that high-fat hepatocytes could protect the cell population against oxidative stress and lipotoxicity.

Review: toxicometabolomics

Metabolomics use in toxicology is rapidly increasing, particularly owing to advances in mass spectroscopy, which is widely used in the life sciences for phenotyping disease states. Toxicology has the advantage of having the disease agent, the toxicant, available for experimental induction of metabolomics changes monitored over time and dose. This review summarizes the different technologies employed and gives examples of their use in various areas of toxicology. A prominent use of metabolomics is the identification of signatures of toxicity - patterns of metabolite changes predictive of a hazard manifestation. Increasingly, such signatures indicative of a certain hazard manifestation are identified, suggesting that certain modes of action result in specific derangements of the metabolism. This might enable the deduction of underlying pathways of toxicity, which, in their entirety, form the Human Toxome, a key concept for implementing the vision of Toxicity Testing for the 21st century. This review summarizes the current state of metabolomics technologies and principles, their uses in toxicology and gives a thorough overview on metabolomics bioinformatics, pathway identification and quality assurance. In addition, this review lays out the prospects for further metabolomics application also in a regulatory context.

Locked nucleic acid antisense inhibitor targeting apolipoprotein C-III efficiently and preferentially removes triglyceride from large very low-density lipoprotein particles in murine plasma

A 20-mer phosphorothioate antisense oligodeoxyribonucleotide having locked nucleic acids (LNA-AON) was used to reduce elevated serum triglyceride levels in mice. We repeatedly administered LNA-AON, which targets murine apolipoprotein C-III mRNA, to high-fat-fed C57Bl/6J male mice for 2 weeks. The LNA-AON showed efficient dose-dependent reductions in hepatic apolipoprotein C-III mRNA and decreased serum apolipoprotein C-III protein concentrations, along with efficient dose-dependent reductions in serum triglyceride concentrations and attenuation of fat accumulation in the liver. Through precise lipoprotein profiling analysis of sera, we found that serum reductions in triglyceride and cholesterol levels were largely a result of decreased serum very low-density lipoprotein (VLDL)-triglycerides and -cholesterol. It is noteworthy that larger VLDL particles were more susceptible to removal from blood than smaller particles, resulting in a shift in particle size distribution to smaller diameters. Histopathologically, fatty changes were markedly reduced in antisense-treated mice, while moderate granular degeneration was frequently seen the highest dose of LNA-AON. The observed granular degeneration of hepatocytes may be associated with moderate elevation in the levels of serum transaminases. In conclusion, we developed an LNA-based selective inhibitor of apolipoprotein C-III. Although it remains necessary to eliminate its potential hepatotoxicity, the present LNA-AON will be helpful for further elucidating the molecular biology of apolipoprotein C-III.

Nonalcoholic fatty liver disease: molecular pathways and therapeutic strategies

Along with rising numbers of patients with metabolic syndrome, the prevalence of nonalcoholic fatty liver disease (NAFLD) has increased in proportion with the obesity epidemic. While there are no established treatments for NAFLD, current research is targeting new molecular mechanisms that underlie NAFLD and associated metabolic disorders. This review discusses some of these emerging molecular mechanisms and their therapeutic implications for the treatment of NAFLD. The basic research that has identified potential molecular targets for pharmacotherapy will be outlined.

Highly expressed MT-ND3 positively associated with histological severity of hepatic steatosis

Hepatic steatosis is the accumulation of an excess amount of triglycerides and other fats inside liver cells resulting from abnormal hepatic lipid metabolism. Mitochondrial structural and molecular defects are involved in the progression of hepatic steatosis pathogenesis. Hepatic methylation and transcriptional activity of the mitochondrial-encoded NADH dehydrogenase (MT-ND) play a critical role in the progression of non-alcoholic fatty liver disease (NAFLD). However, the expression of MT-ND3 in hepatic steatosis has not been extensively studied. In this study, liver specimens were collected from different patients, and were subjected to immunohistochemistry. Primary hepatocytes were treated with oxidative stress, hypoxia, and lipotoxicity to investigate the respective roles of these factors on MT-ND3 expression and cell apoptosis by western blotting and flow cytometry, respectively. We found that increased MT-ND3 expression in human hepatic steatosis was positively associated with histological severity of hepatic steatosis. Hypoxia, H2O2 , and saturated fatty acid treatment induced cell apoptosis mediated by mitochondria. These three factors all had effects on MT-ND3 expression in cultured hepatocytes. Taken together, MT-ND3 may play important roles in hepatic steatosis progress. Hypoxia, oxidative stress, and lipotoxicity could all influence expression of MT-ND3 and thus may play a role in the progression of hepatic steatosis.

Mitochondria as a pharmacological target: magnum overview

Mitochondria, responsible for energy metabolism within the cell, act as signaling organelles. Mitochondrial dysfunction may lead to cell death and oxidative stress and may disturb calcium metabolism. Additionally, mitochondria play a pivotal role in cardioprotective phenomena and a variety of neurodegenerative disorders ranging from Parkinson's to Alzheimer's disease. Mitochondrial DNA mutations may lead to impaired respiration. Hence, targeting the mitochondria with drugs offers great potential for new therapeutic approaches. The purpose of this overview is to present the recent state of knowledge concerning the interactions of various substances with mitochondria.

Dynamic regulation of hepatic lipid droplet properties by diet

Cytoplasmic lipid droplets (CLD) are organelle-like structures that function in neutral lipid storage, transport and metabolism through the actions of specific surface-associated proteins. Although diet and metabolism influence hepatic CLD levels, how they affect CLD protein composition is largely unknown. We used non-biased, shotgun, proteomics in combination with metabolic analysis, quantitative immunoblotting, electron microscopy and confocal imaging to define the effects of low- and high-fat diets on CLD properties in fasted-refed mice. We found that the hepatic CLD proteome is distinct from that of CLD from other mammalian tissues, containing enzymes from multiple metabolic pathways. The hepatic CLD proteome is also differentially affected by dietary fat content and hepatic metabolic status. High fat feeding markedly increased the CLD surface density of perilipin-2, a critical regulator of hepatic neutral lipid storage, whereas it reduced CLD levels of betaine-homocysteine S-methyltransferase, an enzyme regulator of homocysteine levels linked to fatty liver disease and hepatocellular carcinoma. Collectively our data demonstrate that the hepatic CLD proteome is enriched in metabolic enzymes, and that it is qualitatively and quantitatively regulated by diet and metabolism. These findings implicate CLD in the regulation of hepatic metabolic processes, and suggest that their properties undergo reorganization in response to hepatic metabolic demands.

Metabolism of xenobiotic carboxylic acids: focus on coenzyme A conjugation, reactivity, and interference with lipid metabolism

While xenobiotic carboxylic acids (XCAs) have been studied extensively with respect to their enzymatic conversion to potentially reactive acyl glucuronides with implications to drug induced hepatotoxicity, the formation of xenobiotic-S-acyl-CoA thioesters (xenobiotic-CoAs) have been much less studied in spite of data indicating that such conjugates may be equally or more reactive than the corresponding acyl glucuronides. This review addresses enzymes and cell organelles involved in the formation of xenobiotic-CoAs, the reactivity of such conjugates toward biological macromolecules, and in vitro and in vivo methodology to assess consequences of such reactivity. Further, the propensity of xenobiotic-CoAs to interfere with endogenous lipid metabolism, e.g., inhibition of β-oxidation or depletion of the CoA or carnitine pools, adds to the complexity of the potential contribution of XCAs to hepatotoxicity by a number of mechanisms in addition to those in common with the corresponding acyl glucuronides. On the basis of our review of the literature on xenobiotic-CoA conjugates, there appear to be a number of gaps in our understanding of the bioactivation of XCA both with respect to the mechanisms involved and the experimental approaches to distinguish between the role of acyl glucuronides and xenobiotic-CoA conjugates. These aspects are focused upon and described in detail in this review.

Cholesteryl ester storage disease: review of the findings in 135 reported patients with an underdiagnosed disease

Cholesteryl ester storage disease (CESD) is caused by deficient lysosomal acid lipase (LAL) activity, predominantly resulting in cholesteryl ester (CE) accumulation, particularly in the liver, spleen, and macrophages throughout the body. The disease is characterized by microvesicular steatosis leading to liver failure, accelerated atherosclerosis and premature demise. Although CESD is rare, it is likely that many patients are unrecognized or misdiagnosed. Here, the findings in 135 CESD patients described in the literature are reviewed. Diagnoses were based on liver biopsies, LAL deficiency and/or LAL gene (LIPA) mutations. Hepatomegaly was present in 99.3% of patients; 74% also had splenomegaly. When reported, most patients had elevated serum total cholesterol, LDL-cholesterol, triglycerides, and transaminases (AST, ALT, or both), while HDL-cholesterol was decreased. All 112 liver biopsied patients had the characteristic pathology, which is progressive, and includes microvesicular steatosis, which leads to fibrosis, micronodular cirrhosis, and ultimately to liver failure. Pathognomonic birefringent CE crystals or their remnant clefts were observed in hepatic cells. Extrahepatic manifestations included portal hypertension, esophageal varices, and accelerated atherosclerosis. Liver failure in 17 reported patients resulted in liver transplantation and/or death. Genotyping identified 31 LIPA mutations in 55 patients; 61% of mutations were the common exon 8 splice-junction mutation (E8SJM(-1G>A)), for which 18 patients were homozygous. Genotype/phenotype correlations were limited; however, E8SJM(-1G>A) homozygotes typically had early-onset, slowly progressive disease. Supportive treatment included cholestyramine, statins, and, ultimately, liver transplantation. Recombinant LAL replacement was shown to be effective in animal models, and recently, a phase I/II clinical trial demonstrated its safety and indicated its potential metabolic efficacy.

Glycine conjugation: importance in metabolism, the role of glycine N-acyltransferase, and factors that influence interindividual variation

INTRODUCTION

Glycine conjugation of mitochondrial acyl-CoAs, catalyzed by glycine N-acyltransferase (GLYAT, E.C. 2.3.1.13), is an important metabolic pathway responsible for maintaining adequate levels of free coenzyme A (CoASH). However, because of the small number of pharmaceutical drugs that are conjugated to glycine, the pathway has not yet been characterized in detail. Here, we review the causes and possible consequences of interindividual variation in the glycine conjugation pathway.

AREAS COVERED

The authors review the importance of CoASH in metabolism, formation and toxicity of xenobiotic acyl-CoAs, and mechanisms for restoring levels of CoASH. They focus on GLYAT, glycine conjugation, how genetic variation in the GLYAT gene could influence glycine conjugation, and the emerging roles of glycine metabolism in cancer and musculoskeletal development.

EXPERT OPINION

The substrate selectivity of GLYAT and its variants needs to be further characterized, as organic acids can be toxic if the corresponding acyl-CoA is not a substrate for glycine conjugation. GLYAT activity affects mitochondrial ATP production, glycine availability, CoASH availability, and the toxicity of various organic acids. Therefore, variation in the glycine conjugation pathway could influence liver cancer, musculoskeletal development, and mitochondrial energy metabolism.

Mitochondria as a target of environmental toxicants

Enormous strides have recently been made in our understanding of the biology and pathobiology of mitochondria. Many diseases have been identified as caused by mitochondrial dysfunction, and many pharmaceuticals have been identified as previously unrecognized mitochondrial toxicants. A much smaller but growing literature indicates that mitochondria are also targeted by environmental pollutants. We briefly review the importance of mitochondrial function and maintenance for health based on the genetics of mitochondrial diseases and the toxicities resulting from pharmaceutical exposure. We then discuss how the principles of mitochondrial vulnerability illustrated by those fields might apply to environmental contaminants, with particular attention to factors that may modulate vulnerability including genetic differences, epigenetic interactions, tissue characteristics, and developmental stage. Finally, we review the literature related to environmental mitochondrial toxicants, with a particular focus on those toxicants that target mitochondrial DNA. We conclude that the fields of environmental toxicology and environmental health should focus more strongly on mitochondria.

Current challenges and controversies in drug-induced liver injury

Current key challenges and controversies encountered in the identification of potentially hepatotoxic drugs and the assessment of drug-induced liver injury (DILI) are covered in this article. There is substantial debate over the classification of DILI itself, including the definition and validity of terms such as 'intrinsic' and 'idiosyncratic'. So-called idiosyncratic DILI is typically rare and requires one or more susceptibility factors in individuals. Consequently, it has been difficult to reproduce in animal models, which has limited the understanding of its underlying mechanisms despite numerous hypotheses. Advances in predictive models would also help to enable preclinical elimination of drug candidates and development of novel biomarkers. A small number of liver laboratory tests have been routinely used to help identify DILI, but their interpretation can be limited and confounded by multiple factors. Improved preclinical and clinical biomarkers are therefore needed to accurately detect early signals of liver injury, distinguish drug hepatotoxicity from other forms of liver injury, and differentiate mild from clinically important liver injury. A range of potentially useful biomarkers are emerging, although so far most have only been used preclinically, with only a few validated and used in the clinic for specific circumstances. Advances in the development of genomic biomarkers will improve the prediction and detection of hepatic injury in future. Establishing a definitive clinical diagnosis of DILI can be difficult, since it is based on circumstantial evidence by excluding other aetiologies and, when possible, identifying a drug-specific signature. DILI signals based on standard liver test abnormalities may be affected by underlying diseases such as hepatitis B and C, HIV and cancer, as well as the concomitant use of hepatotoxic drugs to treat some of these conditions. Therefore, a modified approach to DILI assessment is justified in these special populations and a suggested framework is presented that takes into account underlying disease when evaluating DILI signals in individuals. Detection of idiosyncratic DILI should, in some respects, be easier in the postmarketing setting compared with the clinical development programme, since there is a much larger and more varied patient population exposure over longer timeframes. However, postmarketing safety surveillance is currently limited by the quantity and quality of information available to make an accurate diagnosis, the lack of a control group and the rarity of cases. The pooling of multiple healthcare databases, which could potentially contain different types of patient data, is advised to address some of these deficiencies.

Platycodi Radix attenuates dimethylnitrosamine-induced liver fibrosis in rats by inducing Nrf2-mediated antioxidant enzymes

The purpose of this study was to investigate the anti-fibrotic effects of the aqueous extract of the Platycodi Radix root (Changkil: CK) on dimethylnitrosamine (DMN)-induced liver fibrosis in rats. DMN treatment for 4 weeks led to marked liver fibrosis as assessed by serum biochemistry, histopathological examination, and hepatic lipid peroxidation and collagen content. CK significantly inhibited DMN-induced increases in serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities, fibrosis score, and hepatic malondialdehyde and collagen content. CK also inhibited DMN-induced reductions in rat body and liver weights. Reverse transcription polymerase chain reaction (RT-PCR) and western blot analyses revealed that CK inhibited DMN-induced increases in matrix metalloproteinase-13 (MMP-13), tissue inhibitor of metalloproteinase-1 (TIMP-1), and tumor necrosis factor-α (TNF-α) mRNA, and collagen type I and α-smooth muscle actin protein. DMN-induced cyclooxygenase-2 (COX-2) expression and nuclear factor-kappa B (NF-κB) activation was reduced by CK treatment. Furthermore, CK induced activation of nuclear erythroid 2-related factor 2 (Nrf2)-mediated antioxidant enzymes such as γ-glutamylcysteine synthetase (γ-GCS), heme oxygenase-1 (HO-1), NAD(P)H quinone oxidoreductase 1 (NQO1), and glutathione-S-transferase (GST) in HepG2 cells. These results demonstrated that CK attenuates DMN-induced liver fibrosis through the activation of Nrf2-mediated antioxidant enzymes.

What is the role of adiponectin in obesity related non-alcoholic fatty liver disease?

Non-alcoholic fatty liver disease (NAFLD) is recognized as the most common type of chronic liver disease in Western countries. Insulin resistance is a key factor in the pathogenesis of NAFLD, the latter being considered as the hepatic component of insulin resistance or obesity. Adiponectin is the most abundant adipose-specific adipokine. There is evidence that adiponectin decreases hepatic and systematic insulin resistance, and attenuates liver inflammation and fibrosis. Adiponectin generally predicts steatosis grade and the severity of NAFLD; however, to what extent this is a direct effect or related to the presence of more severe insulin resistance or obesity remains to be addressed. Although there is no proven pharmacotherapy for the treatment of NAFLD, recent therapeutic strategies have focused on the indirect upregulation of adiponectin through the administration of various therapeutic agents and/or lifestyle modifications. In this adiponectin-focused review, the pathogenetic role and the potential therapeutic benefits of adiponectin in NAFLD are analyzed systematically.

Prediction of liver injury induced by chemicals in human with a multiparametric assay on isolated mouse liver mitochondria

Drug-induced liver injury (DILI) in humans is difficult to predict using classical in vitro cytotoxicity screening and regulatory animal studies. This explains why numerous compounds are stopped during clinical trials or withdrawn from the market due to hepatotoxicity. Thus, it is important to improve early prediction of DILI in human. In this study, we hypothesized that this goal could be achieved by investigating drug-induced mitochondrial dysfunction as this toxic effect is a major mechanism of DILI. To this end, we developed a high-throughput screening platform using isolated mouse liver mitochondria. Our broad spectrum multiparametric assay was designed to detect the global mitochondrial membrane permeabilization (swelling), inner membrane permeabilization (transmembrane potential), outer membrane permeabilization (cytochrome c release), and alteration of mitochondrial respiration driven by succinate or malate/glutamate. A pool of 124 chemicals (mainly drugs) was selected, including 87 with documented DILI and 37 without reported clinical hepatotoxicity. Our screening assay revealed an excellent sensitivity for clinical outcome of DILI (94 or 92% depending on cutoff) and a high positive predictive value (89 or 82%). A highly significant relationship between drug-induced mitochondrial toxicity and DILI occurrence in patients was calculated (p < 0.001). Moreover, this multiparametric assay allowed identifying several compounds for which mitochondrial toxicity had never been described before and even helped to clarify mechanisms with some drugs already known to be mitochondriotoxic. Investigation of drug-induced loss of mitochondrial integrity and function with this multiparametric assay should be considered for integration into basic screening processes at early stage to select drug candidates with lower risk of DILI in human. This assay is also a valuable tool for assessing the mitochondrial toxicity profile and investigating the mechanism of action of new compounds and marketed compounds.

Bridging the gap between old and new concepts in drug-induced liver injury

Recent studies have provided important information in the field of drug-induced liver injury (DILI), in particular regarding the pathogenesis of acetaminophen hepatotoxicity. However, these studies have sometimes left aside some old (but seminal) findings. Efforts should be made to bridge the gap between old and new concepts in DILI.

Disruption of uridine homeostasis links liver pyrimidine metabolism to lipid accumulation

We report in this study an intrinsic link between pyrimidine metabolism and liver lipid accumulation utilizing a uridine phosphorylase 1 transgenic mouse model UPase1-TG. Hepatic microvesicular steatosis is induced by disruption of uridine homeostasis through transgenic overexpression of UPase1, an enzyme of the pyrimidine catabolism and salvage pathway. Microvesicular steatosis is also induced by the inhibition of dihydroorotate dehydrogenase (DHODH), an enzyme of the de novo pyrimidine biosynthesis pathway. Interestingly, uridine supplementation completely suppresses microvesicular steatosis in both scenarios. The effective concentration (EC(50)) for uridine to suppress microvesicular steatosis is approximately 20 µM in primary hepatocytes of UPase1-TG mice. We find that uridine does not have any effect on in vitro DHODH enzymatic activity. On the other hand, uridine supplementation alters the liver NAD(+)/NADH and NADP(+)/NADPH ratios and the acetylation profile of metabolic, oxidation-reduction, and antioxidation enzymes. Protein acetylation is emerging as a key regulatory mechanism for cellular metabolism. Therefore, we propose that uridine suppresses fatty liver by modulating the liver protein acetylation profile. Our findings reveal a novel link between uridine homeostasis, pyrimidine metabolism, and liver lipid metabolism.

Curcumin and liver disease

Liver diseases pose a major medical problem worldwide and a wide variety of herbs have been studied for the management of liver-related diseases. In this respect, curcumin has long been used in traditional medicine, and in recent years it has been the object of increasing research interest. In combating liver diseases, it seems clear that curcumin exerts a hypolipidic effect, which prevents the fatty acid accumulation in the hepatocytes that may result from metabolic imbalances, and which may cause nonalcoholic steatohepatitis. Another crucial protective activity of curcumin, not only in the context of chronic liver diseases but also regarding carcinogenesis and other age-related processes, is its potent antioxidant activity, which affects multiple processes and signaling pathways. The effects of curcumin on NF-κβ are crucial to our understanding of the potent hepatoprotective role of this herb-derived micronutrient. Because curcumin is a micronutrient that is closely related to cellular redox balance, its properties and activity give rise to a series of molecular reactions that in every case and biological situation affect the mitochondria.

Toxicogenomic analysis suggests chemical-induced sexual dimorphism in the expression of metabolic genes in zebrafish liver

Differential gene expression in two sexes is widespread throughout the animal kingdom, giving rise to sex-dimorphic gene activities and sex-dependent adaptability to environmental cues, diets, growth and development as well as susceptibility to diseases. Here, we present a study using a toxicogenomic approach to investigate metabolic genes that show sex-dimorphic expression in the zebrafish liver triggered by several chemicals. Our analysis revealed that, besides the known genes for xenobiotic metabolism, many functionally diverse metabolic genes, such as ELOVL fatty acid elongase, DNA-directed RNA polymerase, and hydroxysteroid dehydrogenase, were also sex-dimorphic in their response to chemical treatments. Moreover, sex-dimorphic responses were also observed at the pathway level. Pathways belonging to xenobiotic metabolism, lipid metabolism, and nucleotide metabolism were enriched with sex-dimorphically expressed genes. We also observed temporal differences of the sex-dimorphic responses, suggesting that both genes and pathways are differently correlated during different periods of chemical perturbation. The ubiquity of sex-dimorphic activities at different biological hierarchies indicate the importance and the need of considering the sex factor in many areas of biological researches, especially in toxicology and pathology.

Label-free evaluation of hepatic microvesicular steatosis with multimodal coherent anti-Stokes Raman scattering microscopy

Hepatic microvesicular steatosis is a hallmark of drug-induced hepatotoxicity and early-stage fatty liver disease. Current histopathology techniques are inadequate for the clinical evaluation of hepatic microvesicular steatosis. In this paper, we explore the use of multimodal coherent anti-Stokes Raman scattering (CARS) microscopy for the detection and characterization of hepatic microvesicular steatosis. We show that CARS microscopy is more sensitive than Oil Red O histology for the detection of microvesicular steatosis. Computer-assisted analysis of liver lipid level based on CARS signal intensity is consistent with triglyceride measurement using a standard biochemical assay. Most importantly, in a single measurement procedure on unprocessed and unstained liver tissues, multimodal CARS imaging provides a wealth of critical information including the detection of microvesicular steatosis and quantitation of liver lipid content, number and size of lipid droplets, and lipid unsaturation and packing order of lipid droplets. Such information can only be assessed by multiple different methods on processed and stained liver tissues or tissue extracts using current standard analytical techniques. Multimodal CARS microscopy also permits label-free identification of lipid-rich non-parenchymal cells. In addition, label-free and non-perturbative CARS imaging allow rapid screening of mitochondrial toxins-induced microvesicular steatosis in primary hepatocyte cultures. With its sensitivity and versatility, multimodal CARS microscopy should be a powerful tool for the clinical evaluation of hepatic microvesicular steatosis.

Mechanisms of hepatocellular toxicity associated with dronedarone--a comparison to amiodarone

Dronedarone is a new antiarrhythmic drug with an amiodarone-like benzofuran structure. Shortly after its introduction, dronedarone became implicated in causing severe liver injury. Amiodarone is a well-known mitochondrial toxicant. The aim of our study was to investigate mechanisms of hepatotoxicity of dronedarone in vitro and to compare them with amiodarone. We used isolated rat liver mitochondria, primary human hepatocytes, and the human hepatoma cell line HepG2, which were exposed acutely or up to 24h. After exposure of primary hepatocytes or HepG2 cells for 24h, dronedarone and amiodarone caused cytotoxicity and apoptosis starting at 20 and 50 µM, respectively. The cellular ATP content started to decrease at 20 µM for both drugs, suggesting mitochondrial toxicity. Inhibition of the respiratory chain required concentrations of ~10 µM and was caused by an impairment of complexes I and II for both drugs. In parallel, mitochondrial accumulation of reactive oxygen species (ROS) was observed. In isolated rat liver mitochondria, acute treatment with dronedarone decreased the mitochondrial membrane potential, inhibited complex I, and uncoupled the respiratory chain. Furthermore, in acutely treated rat liver mitochondria and in HepG2 cells exposed for 24h, dronedarone started to inhibit mitochondrial β-oxidation at 10 µM and amiodarone at 20 µM. Similar to amiodarone, dronedarone is an uncoupler and an inhibitor of the mitochondrial respiratory chain and of β-oxidation both acutely and after exposure for 24h. Inhibition of mitochondrial function leads to accumulation of ROS and fatty acids, eventually leading to apoptosis and/or necrosis of hepatocytes. Mitochondrial toxicity may be an explanation for hepatotoxicity of dronedarone in vivo.