Cytochrome c: a non-invasive biomarker of drug-induced liver injury

The Role of Mechanistic Biomarkers in Understanding Acetaminophen Hepatotoxicity in Humans

Our understanding of the fundamental molecular mechanisms of acetaminophen (APAP) hepatotoxicity began in 1973 to 1974, when investigators at the US National Institutes of Health published seminal studies demonstrating conversion of APAP to a reactive metabolite that depletes glutathione and binds to proteins in the liver in mice after overdose. Since then, additional groundbreaking experiments have demonstrated critical roles for mitochondrial damage, oxidative stress, nuclear DNA fragmentation, and necrotic cell death as well. Over the years, some investigators have also attempted to translate these mechanisms to humans using human specimens from APAP overdose patients. This review presents those studies and summarizes what we have learned about APAP hepatotoxicity in humans so far. Overall, the mechanisms of APAP hepatotoxicity in humans strongly resemble those discovered in experimental mouse and cultured hepatocyte models, and emerging biomarkers also suggest similarities in liver repair. The data not only validate the first mechanistic studies of APAP-induced liver injury performed 50 years ago but also demonstrate the human relevance of numerous studies conducted since then. SIGNIFICANCE STATEMENT: Human studies using novel translational, mechanistic biomarkers have confirmed that the fundamental mechanisms of acetaminophen (APAP) hepatotoxicity discovered in rodent models since 1973 are the same in humans. Importantly, these findings have guided the development and understanding of treatments such as N-acetyl-l-cysteine and 4-methylpyrazole over the years. Additional research may improve not only our understanding of APAP overdose pathophysiology in humans but also our ability to predict and treat serious liver injury in patients.

pH-Stimulated Response Gating for Mimic Cytochrome C Transport on Biomimetic Asymmetric Nanochannels

Proteins are vital components in cells, biological tissues, and organs, playing a pivotal role in growth and developmental processes in living organisms. Cytochrome C (Cyt C) is a class of heme proteins found in almost all life and is involved in cellular energy metabolic processes such as respiration, mainly as electron carriers or terminal reductases. It binds cardiolipin in the inner mitochondrial membrane, leading to apoptosis. It is a challenge to design a simple and effective artificial system to mimic the complex Cyt C biological transport process. In this paper, an asymmetric biomimetic pH-driven protein gate is described by introducing arginine (Arg) at one end of an hourglass-shaped nanochannel. The nanochannel shows a sensitive protonation-driven protein gate that can be "off" at pH = 7 and "on" at pH = 2. Further studies show that differences in the binding of Arg and Cyt C at different levels of protonation lead to different switching behaviors within the nanochannels, which in turn lead to different surface charges within the nanochannels. It can be used for detecting Cyt C and as an excellent and robust gate for developing integrated circuits and nanoelectronic logic devices.

Hypoosmosis alters hepatocyte mitochondrial morphology and induces selective release of carbamoyl phosphate synthetase 1

Carbamoyl phosphate synthetase 1 (CPS1) is the most abundant hepatocyte mitochondrial matrix protein. Hypoosmotic stress increases CPS1 release in isolated mouse hepatocytes without cell death. We hypothesized that increased CPS1 release during hypoosmosis is selective and associates with altered mitochondrial morphology. Both ex vivo and in vivo models were assessed. Mouse hepatocytes and livers were challenged with isotonic or hypoosmotic (35 mosM) buffer. Mice were injected intraperitoneally with water (10% body weight) with or without an antidiuretic. Mitochondrial and cytosolic fractions were isolated using differential centrifugation, then analyzed by immunoblotting to assess subcellular redistribution of four mitochondrial proteins: CPS1, ornithine transcarbamylase (OTC), pyrroline-5-carboxylate reductase 1 (PYCR1), and cytochrome c. Mitochondrial morphology alterations were examined using electron microscopy. Hypoosmotic treatment of whole livers or hepatocytes led to preferential or increased mitochondrial release, respectively, of CPS1 as compared with two mitochondrial matrix proteins (OTC/PYCR1) and with the intermembrane space protein, cytochrome c. Mitochondrial apoptosis-induced channel opening using staurosporine in hepatocytes led to preferential CPS1 and cytochrome c release. The CPS1-selective changes were accompanied by dramatic alterations in ultrastructural mitochondrial morphology. In mice, hypoosmosis/hyponatremia led to increased liver vascular congestion and increased CPS1 in bile but not blood, coupled with mitochondrial structural alterations. In contrast, isotonic increase of intravascular volume led to a decrease in mitochondrial size with limited change in bile CPS1 compared with hypoosmotic conditions and absence of the hypoosmosis-associated histological alterations. Taken together, hepatocyte CPS1 is selectively released in response to hypoosmosis/hyponatremia and provides a unique biomarker of mitochondrial injury.NEW & NOTEWORTHY Exposure of isolated mouse livers, primary cultured hepatocytes, or mice to hypoosmosis/hyponatremia conditions induces significant mitochondrial shape alterations accompanied by preferential release of the mitochondrial matrix protein CPS1, a urea cycle enzyme. In contrast, the intermembrane space protein, cytochrome c, and two other matrix proteins, including the urea cycle enzyme ornithine transcarbamylase, remain preferentially retained in mitochondria. Therefore, hepatocyte CPS1 manifests unique mitochondrial stress response compartmentalization and is a sensitive sensor of mitochondrial hypoosmotic/hyponatremic injury.

Comparing the effectiveness of L-carnitine and paraffin oil in acute aluminum phosphide poisoning using predictive biomarkers and scores: A randomized controlled clinical trial

Aluminum phosphide (AlP) poisoning is a serious medical emergency with a high mortality rate. The absence of an exact antidote for AlP poisoning necessitates the quest for alternative treatment options. The study sought to assess the efficacy of adding L-carnitine or medicated paraffin oil to the conventional approach of treatment employed in cases of acute AlP poisoning. We conducted a 1 year, randomized, controlled, parallel-group, single-blind clinical study. 96 individuals with acute AlP poisoning were randomly assigned to one of three groups. The standard AlP therapy was administered to all groups according to the Poison Control Center guidelines at the Ain-Shams University hospitals. All patients underwent a medical history review, clinical examination, and laboratory tests. The outcomes were assessed. The participants in the study groups had mean ages ranging from 25.6 to 26.3 years. The cases analyzed were evenly distributed between genders, with the majority originating from rural areas. The average delay time varied from 2.9 to 4.2 h. All patients in the study reported ingesting AlP during suicide attempts. 12 hours after admission, many clinical and biochemical data improved in both intervention groups including cytochrome c oxidase, caspase-3, caspase-9, catalase, and superoxide dismutase. The intervention groups required significantly less mechanical ventilation and had a lower mortality rate than the control group. Decontamination with paraffin oil could be advantageous for reducing the severity of AlP poisoning, improving prognosis, and lowering the mortality rate.

Molecularly imprinted electrochemical sensor for the ultrasensitive detection of cytochrome c

Cytochrome c (Cyt c) is an important biomarker for the early stage of apoptosis that plays a role in the diagnosis and therapy of several diseases including cancer. Here, an electrochemical sensor based on molecularly imprinted polymer (MIP) for the ultrasensitive detection of Cyt c is studied. It is prepared by electropolymerization of o-phenylenediamine in the presence of Cyt c as template, followed by solvent extraction, resulting in the formation of Cyt c recognition sites. The MIP is characterised by cyclic voltammetry and differential pulse voltammetry, using ferrocenecarboxylic acid as redox probe. Voltammetric data indicates that the MIP-sensor behaves as an electrode with partially blocked surface. The partition isotherm obtained fits the Langmuir model, indicating a high affinity for Cyt c, with an association constant K_a = 5 × 10 ¹¹ M^-1. DPV measurements allow to achieve extremely high analytical sensitivity and low detection limit, in the femtomolar range, with negligible unspecific adsorption. Satisfactory analytical recovery tests performed in the presence of possible interfering proteins and in diluted human serum confirmed the selectivity of the MIP-sensor as well as its potential applicability for real samples analysis.

Redox Activation of Mitochondrial DAMPs and the Metabolic Consequences for Development of Autoimmunity

Significance: Reactive oxygen species (ROS) are well known to promote innate immune responses during and in the absence of microbial infections. However, excessive or prolonged exposure to ROS provokes innate immune signaling dysfunction and contributes to the pathogenesis of many autoimmune diseases. The relatively high basal expression of pattern recognition receptors (PRRs) in innate immune cells renders them prone to activation in response to minor intrinsic or extrinsic ROS misbalances in the absence of pathogens. Critical Issues: A prominent source of ROS are mitochondria, which are also major inter-organelle hubs for innate immunity activation, since most PRRs and downstream receptor molecules are directly located either at mitochondria or at mitochondria-associated membranes. Due to their ancestral bacterial origin, mitochondria can also act as quasi-intrinsic self-microbes that mimic a pathogen invasion and become a source of danger-associated molecular patterns (DAMPs) that triggers innate immunity from within. Recent Advances: The release of mitochondrial DAMPs correlates with mitochondrial metabolism changes and increased generation of ROS, which can lead to the oxidative modification of DAMPs. Recent studies suggest that ROS-modified mitochondrial DAMPs possess increased, persistent immunogenicity. Future Directions: Herein, we discuss how mitochondrial DAMP release and oxidation activates PRRs, changes cellular metabolism, and causes innate immune response dysfunction by promoting systemic inflammation, thereby contributing to the onset or progression of autoimmune diseases. The future goal is to understand what the tipping point for DAMPs is to become oxidized, and whether this is a road without return. Antioxid. Redox Signal. 36, 441-461.

Analysis of Cytochrome c Expression on Liver Histology of Hepatitis Rats after Administration of Tin and Olive Leaf Ethanol Extract

<b>Background and Objective:</b> Hepatitis is a liver illness caused by a viral infection, autoimmune conditions or the use of certain medicines. In molecular hepatitis treatment, cytochrome c can be used as a potential predictor of the severity of liver impairment. In Asia, particularly in Indonesia, antioxidant-rich plants include <i>Ficus</i> <i>carica</i> and <i>Olea europaea.</i> This study aimed to see what impact cytochrome c in hepatitis after these two botanicals were administered. <b>Materials and Methods:</b> Rats were grouped as follows: Normal rats with no additions or herbs (G₀), the physiological solution group (G₁), the intravenous administration of the quercetin-copper (II) (G₂), Olive leaf extract or OLE (300 mg kg¹ b.wt.) (G₃) and Tin leaf extract or TLE (100 mg kg¹ b.wt.) (G₄). For an animal model of hepatitis, the rats were given thioacetamide 280 mg kg¹ b.wt., 8 days later. The rats were dissected and blood and liver samples were collected for enzyme and immunohistochemistry examination. <b>Results:</b> Malondialdehyde (MDA), superoxide dismutase (SOD) and cytochrome c expression levels differed significantly (p<0.05) across treatment groups in rat's models of hepatitis. Hepatocytes first displayed symptoms of lipid degradation, inflammatory and necrosis cells. When administered quercetin and the two herbs, necrosis and inflammatory cells were reduced, demonstrating that OLE and TLE can enhance liver histology and lower cytochrome c expression in a mouse model of hepatitis. <b>Conclusion:</b> Administration of Olive leaf extract (OLE) and Tin leaf extract (TLE) can improve liver histology in hepatitis model rats while decreasing cytochrome c expression, which is a mechanism for hepatocyte cell death.

The combination of N-acetylcysteine and cyclosporin A reduces acetaminophen-induced hepatotoxicity in mice

Acetaminophen (APAP)-induced hepatotoxicity is the most common cause of acute liver failure in worldwide. N-acetyl cysteine (NAC) is used as the APAP antidote. Cyclosporin A (CsA) is suppressed mitochondrial damage by binding cyclophilin, a mitochondrial pore transport component. The study aimed to evaluate the effects of NAC, CsA, and NAC+CsA treatments on APAP-induced hepatotoxicity in mice. Mice were randomly divided into five groups (n = 6). 400 mg/kg/ip/single dose APAP, 1200 mg/kg/i.p/single dose NAC and 50 mg/kg/i.p/single dose CsA were performed. Light and electron microscopic alterations were investigated in liver samples. Levels of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) and liver glutathione (GSH) were analyzed. 3-nitrotyrosine and cytochrome c immunoreactivities were evaluated in liver tissue. Here, we found that APAP leads to histopathological and ultrastructural changes in mice liver. Also, APAP increased cytochrome c and 3-nitrotyrosine immunopositive staining. Besides, a significant decrease in liver GSH and an increase in serum AST and ALT levels were detected in the APAP group. Interestingly, NAC+CsA treatment improved histological alterations, cytochrome c, and 3-nitrotyrosine immunoreactivities and liver GSH, serum AST/ALT levels caused by APAP. We suggest that the combination of NAC and CsA reduces acetaminophen-induced hepatotoxicity in mice.

Mitochondria-Derived Damage-Associated Molecular Patterns in Sepsis: From Bench to Bedside

Sepsis is one of the most serious health hazards. Current research suggests that the pathogenesis of sepsis is mediated by both pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). Mitochondria are among the most important organelles in cells and determine their life and death. A variety of mitochondria-derived DAMPs (mtDAMPs) are similar to bacteria because mitochondria are derived from bacteria according to the mitochondrial endosymbiotic theory. Their activated signaling pathways extensively affect organ functions, the immune system, and metabolic functions in sepsis. In this review, we describe the essential roles of mtDAMPs in sepsis and discuss their research prospects and clinical importance.

Constitutive release of CPS1 in bile and its role as a protective cytokine during acute liver injury

Carbamoyl phosphate synthetase-1 (CPS1) is the major mitochondrial urea cycle enzyme in hepatocytes. It is released into mouse and human blood during acute liver injury, where is has a short half-life. The function of CPS1 in blood and the reason for its short half-life in serum are unknown. We show that CPS1 is released normally into mouse and human bile, and pathologically into blood during acute liver injury. Other cytoplasmic and mitochondrial urea cycle enzymes are also found in normal mouse bile. Serum, bile, and purified CPS1 manifest sedimentation properties that overlap with extracellular vesicles, due to the propensity of CPS1 to aggregate despite being released primarily as a soluble protein. During liver injury, CPS1 in blood is rapidly sequestered by monocytes, leading to monocyte M2-polarization and homing to the liver independent of its enzyme activity. Recombinant CPS1 (rCPS1), but not control r-transferrin, increases hepatic macrophage numbers and phagocytic activity. Notably, rCPS1 does not activate hepatic macrophages directly; rather, it activates bone marrow and circulating monocytes that then home to the liver. rCPS1 administration prevents mouse liver damage induced by Fas ligand or acetaminophen, but this protection is absent in macrophage-deficient mice. Moreover, rCPS1 protects from acetaminophen-induced liver injury even when given therapeutically after injury induction. In summary, CPS1 is normally found in bile but is released by hepatocytes into blood upon liver damage. We demonstrate a nonenzymatic function of CPS1 as an antiinflammatory protective cytokine during acute liver injury.

Effects of 31 FDA approved small-molecule kinase inhibitors on isolated rat liver mitochondria

The FDA has approved 31 small-molecule kinase inhibitors (KIs) for human use as of November 2016, with six having black box warnings for hepatotoxicity (BBW-H) in product labeling. The precise mechanisms and risk factors for KI-induced hepatotoxicity are poorly understood. Here, the 31 KIs were tested in isolated rat liver mitochondria, an in vitro system recently proposed to be a useful tool to predict drug-induced hepatotoxicity in humans. The KIs were incubated with mitochondria or submitochondrial particles at concentrations ranging from therapeutic maximal blood concentrations (Cmax) levels to 100-fold Cmax levels. Ten endpoints were measured, including oxygen consumption rate, inner membrane potential, cytochrome c release, swelling, reactive oxygen species, and individual respiratory chain complex (I–V) activities. Of the 31 KIs examined only three including sorafenib, regorafenib and pazopanib, all of which are hepatotoxic, caused significant mitochondrial toxicity at concentrations equal to the Cmax, indicating that mitochondrial toxicity likely contributes to the pathogenesis of hepatotoxicity associated with these KIs. At concentrations equal to 100-fold Cmax, 18 KIs were found to be toxic to mitochondria, and among six KIs with BBW-H, mitochondrial injury was induced by regorafenib, lapatinib, idelalisib, and pazopanib, but not ponatinib, or sunitinib. Mitochondrial liability at 100-fold Cmax had a positive predictive power (PPV) of 72% and negative predictive power (NPV) of 33% in predicting human KI hepatotoxicity as defined by product labeling, with the sensitivity and specificity being 62% and 44%, respectively. Similar predictive power was obtained using the criterion of Cmax ≥1.1 µM or daily dose ≥100 mg. Mitochondrial liability at 1–2.5-fold Cmax showed a 100% PPV and specificity, though the NPV and sensitivity were 32% and 14%, respectively. These data provide novel mechanistic insights into KI hepatotoxicity and indicate that mitochondrial toxicity at therapeutic levels can help identify hepatotoxic KIs.

Cytochrome c as a Potentially Clinical Useful Marker of Mitochondrial and Cellular Damage

Mitochondria are evolutionary endosymbionts derived from bacteria. Thus, they bear molecules, such as mitochondrial DNA (mtDNA) that contains CpG DNA repeats and N-formyl peptides (FPs), found in bacteria. Upon cell necrosis or apoptosis, these molecules are released into the interstitial space and the circulation and recognized by the immune cells through the same receptors that recognize pathogen-associated molecular patterns, leading to inflammation. Other mitochondrial molecules are not of bacterial origin, but they may serve as danger-associated molecular patterns (DAMPs) when due to cell injury are translocated into inappropriate compartments. There they are recognized by pattern recognition receptors of the immune cells. Cytochrome c is such a molecule. In this review, experimental and clinical data are presented that confirms cytochrome c release into the extracellular space in pathological conditions characterized by cell death. This indicates that serum cytochrome c, which can be easily measured, may be a clinically useful marker for diagnosing and assessing the severity of such pathological entities. Reasonably, detection of high cytochrome c level into the circulation means release of various other molecules that serves as DAMPs when found extracellularly, the mtDNA and FPs included. Finally, because the release of this universally found compound into the extracellular space makes cytochrome c an ideal molecule to play the role of a DAMP per se, the available experimental and clinical data that support such a role are provided.

Membrane electrospray ionization for direct ultrasensitive biomarker quantitation in biofluids using mass spectrometry

The ability of rapid biomarker quantitation in raw biological samples would expand the application of mass spectrometry in clinical diagnosis. Up until now, the conventional chromatography-mass spectrometry method is time-consuming in both sample preparation and chromatography separation processes, while ambient ionization methods normally suffer from sensitivity. The membrane electrospray ionization (MESI) introduced in this study could not only achieve sensitive biomolecule quantitation, but also minimize the sample handling process. As a unique feature of MESI, both vertical and horizontal chemical separations could be achieved in real-time. With the capability of mass-selectively minimizing matrix effects from salts, small molecules, and macromolecules, ultrasensitive detection of cytochrome C (>500-fold sensitivity improvement) in raw urine samples was demonstrated in less than 20 min.

Circulating mitochondrial biomarkers for drug-induced liver injury

Liver mitochondria affected by drugs can be released into circulation and serve as biomarkers for drug-induced liver injury (DILI). The tissue specificity of ALT was improved by differentiating cytosolic ALT1 and mitochondrial ALT2 isoforms released in circulation. Prior to ALT elevation, mitochondrial cytochrome c, OCT, GLDH, CPS1 and DNA were increased in circulation following DILI. The baseline expression of mt-Nd6 was predictive of individual DILI susceptibility in animals. As mitochondrial DILI biomarkers appeared to be drug or species dependent, they might have value in clinical scenarios when culprit drugs are established, but may not be ideal tools to assess DILI potentials of new drugs.

Association of circulating cytochrome c with clinical manifestations of antiretroviral-induced toxicity

Diagnosis of antiretroviral therapy (ART) toxicity is complicated. Apoptosis has been implicated in ART toxicity. Cytochrome c (Cyt-C) is a mitochondrial protein found in plasma during pro-apoptotic states. We conducted a study of HIV-infected individuals on ART with (cases, n=21) and without (controls, n=21) clinical evidence of toxicity to determine if elevated plasma Cyt-C is associated with ART toxicity. When corrected for CD4 count, viral load, and duration of HIV infection, cases are 7.86 times more likely than controls to have plasma Cyt-C>0.216 ng/mL. Cyt-C could be a useful clinical tool to guide treatment decisions in this population.

Cytochrome c: potential as a noninvasive biomarker of drug-induced acute kidney injury

INTRODUCTION

Acute kidney injury (AKI) in critically ill patients is closely associated with increased morbidity and mortality, yet there remains continued reliance on increased serum creatinine and blood urea nitrogen to diagnose AKI. These biomarkers increase only after significant renal structural damage has occurred. Recent research efforts have focused on discovery and validation of novel serum and urine biomarkers to detect AKI prior to extensive structural damage. Cytochrome c is best known as an indicator of cell death burden in any organ or tissue. It is released during mitochondrial damage that is associated with processing of apoptosis, cell lysis during necrosis and even reversible mitochondrial and cell injury.

AREAS COVERED

This article reviews the current literature on the potential for cytochrome c as an early biomarker of AKI. The article is based on PubMed searches, using the terms 'acute kidney injury,' 'renal failure,' 'biomarker,' 'toxicity' and 'cytochrome c', with a focus on experimental and clinical data.

EXPERT OPINION

Cytochrome c, as a biomarker, has the potential to improve outcome for AKI patients. Its release indicates mitochondrial damage, one of the earliest changes in cell injury and death. New mitochondrial-targeted therapeutics may be designed around this molecule. Its disadvantages include only transient increase at expression levels that are easily measurable and nonspecificity for kidney injury. The appropriate and optimal utilization of cytochrome c as a biomarker for AKI will be realized only after its complete characterization in experimental and clinical arenas.

Serum UPLC-MS/MS metabolic profiling in an experimental model for acute-liver injury reveals potential biomarkers for hepatotoxicity

A key interest in clinical diagnosis and pharmaceutical industry is to have a repertoire of noninvasive biomarkers to-individually or in combination-be able to infer or predict the degree of liver injury caused by pathological conditions or drugs. Metabolomics-a comprehensive study of global metabolites-has become a highly sensitive and powerful tool for biomarker discovery thanks to recent technological advances. An ultra-performance liquid chromatography/time-of-flight tandem mass spectrometry (UPLC/TOF MS/MS)-based metabolomics approach was employed to investigate sera from galactosamine-treated rats to find potential biomarkers for acute liver injury. Hepatic damage was quantified by determining serum transaminase activity and in situ liver histological lesions. Principal component analysis in combination with coefficient of correlation analysis was used for biomarker selection and identification. According to the data, serum levels of several metabolites including glucose, amino acids, and membrane lipids were significantly modified, some of them showing a high correlation with the degree of liver damage determined by histological examination of the livers. In conclusion, this study supports that UPLC-MS/MS based serum metabolomics in experimental animal models could be a powerful approach to search for biomarkers for drug- or disease-induced liver injury.

Preclinical safety assessment: current gaps, challenges, and approaches in identifying translatable biomarkers of drug-induced liver injury

Currently, no serum biomarkers, including the biochemical gold standard alanine aminotransferase, can differentiate drug-induced from non-drug-related liver injury, can differentiate liver injury mediated by a specific drug or mechanism, or can accurately predict the progression and outcome of hepatic injury. Efforts have been made by veterinary clinical pathologists, toxicologists, and other scientists to address the gaps in hepatic biomarkers faced during drug development; although there have been no breakthroughs, several novel biomarker candidates have been identified. Efforts to address the gaps in translatable hepatic biomarkers and the challenges and hurdles faced during this process are highlighted in this review.

Emerging role of damage-associated molecular patterns derived from mitochondria in inflammation

Cell death and injury often lead to release or exposure of intracellular molecules called damage-associated molecular patterns (DAMPs) or cell death-associated molecules. These molecules are recognized by the innate immune system by pattern recognition receptors - the same receptors that detect pathogen-associated molecular patterns, thus revealing similarities between pathogen-induced and non-infectious inflammatory responses. Many DAMPs are derived from the plasma membrane, nucleus, endoplasmic reticulum and cytosol. Recently, mitochondria have emerged as other organelles that function as a source of DAMPs. Here, we highlight the significance of mitochondrial DAMPs and discuss their contribution to inflammation and development of human pathologies.

The evolution of bioinformatics in toxicology: advancing toxicogenomics

As one reflects back through the past 50 years of scientific research, a significant accomplishment was the advance into the genomic era. Basic research scientists have uncovered the genetic code and the foundation of the most fundamental building blocks for the molecular activity that supports biological structure and function. Accompanying these structural and functional discoveries is the advance of techniques and technologies to probe molecular events, in time, across environmental and chemical exposures, within individuals, and across species. The field of toxicology has kept pace with advances in molecular study, and the past 50 years recognizes significant growth and explosive understanding of the impact of the compounds and environment to basic cellular and molecular machinery. The advancement of molecular techniques applied in a whole-genomic capacity to the study of toxicant effects, toxicogenomics, is no doubt a significant milestone for toxicological research. Toxicogenomics has also provided an avenue for advancing a joining of multidisciplinary sciences including engineering and informatics in traditional toxicological research. This review will cover the evolution of the field of toxicogenomics in the context of informatics integration its current promise, and limitations.

Genomic indicators in the blood predict drug-induced liver injury

Genomic biomarkers for the detection of drug-induced liver injury (DILI) from blood are urgently needed for monitoring drug safety. We used a unique data set as part of the Food and Drug Administration led MicroArray Quality Control Phase-II (MAQC-II) project consisting of gene expression data from the two tissues (blood and liver) to test cross-tissue predictability of genomic indicators to a form of chemically induced liver injury. We then use the genomic indicators from the blood as biomarkers for prediction of acetaminophen-induced liver injury and show that the cross-tissue predictability of a response to the pharmaceutical agent (accuracy as high as 92.1%) is better than, or at least comparable to, that of non-therapeutic compounds. We provide a database of gene expression for the highly informative predictors, which brings biological context to the possible mechanisms involved in DILI. Pathway-based predictors were associated with inflammation, angiogenesis, Toll-like receptor signaling, apoptosis, and mitochondrial damage. The results show for the first time and support the hypothesis that genomic indicators in the blood can serve as potential diagnostic biomarkers predictive of DILI.

Apoptosis: a clinically useful measure of antiretroviral drug toxicity?

Antiretroviral therapy (ART) has improved life expectancy with HIV infection, but long-term toxicities associated with these medications are now a major global disease burden. There is a clear need to develop useful methods for monitoring patients on antiretroviral drugs for early signs of toxicity. Assays with predictive utility -- allowing therapy to be changed before serious end organ damage occurs -- would be ideal. Attempts to develop biochemical methods of monitoring ART toxicity have concentrated on the mitochondrial toxicity of nucleoside analogue reverse transcriptase inhibitors and have not generally lead to assays with widespread clinical applications. For example, plasma lactate and peripheral blood measurements of mitochondrial DNA associate with exposure to potentially toxic nucleoside analogue reverse transcriptase inhibitors but have not reliably predicted clinical toxicity. Better assays are needed, including markers of toxicity from additional drug classes. Apoptosis may be a potential marker of ART toxicity. Increased apoptosis has been demonstrated both in vitro and in vivo in association with various antiretroviral drug classes and a range of clinical toxicities. However, quantifying apoptosis on biopsy specimens of tissue (such as adipose tissue) is impractical for patient monitoring. Novel assays have been described that can quantify apoptosis using minute tissue samples and initial results from clinical samples suggest peripheral blood may have utility in predicting ART toxicities. The limitations and potential of such techniques for monitoring patients for drug side effects will be discussed.

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.

Mechanism-based bioanalysis and biomarkers for hepatic chemical stress

Adverse drug reactions, in particular drug-induced hepatotoxicity, represent a major challenge for clinicians and an impediment to safe drug development. Novel blood or urinary biomarkers of chemically-induced hepatic stress also hold great potential to provide information about pathways leading to cell death within tissues. The earlier pre-clinical identification of potential hepatotoxins and non-invasive diagnosis of susceptible patients, prior to overt liver disease is an important goal. Moreover, the identification, validation and qualification of biomarkers that have in vitro, in vivo and clinical transferability can assist bridging studies and accelerate the pace of drug development. Drug-induced chemical stress is a multi-factorial process, the kinetics of the interaction between the hepatotoxin and the cellular macromolecules are crucially important as different biomarkers will appear over time. The sensitivity of the bioanalytical techniques used to detect biological and chemical biomarkers underpins the usefulness of the marker in question. An integrated analysis of the biochemical, molecular and cellular events provides an understanding of biological (host) factors which ultimately determine the balance between xenobiotic detoxification, adaptation and liver injury. The aim of this review is to summarise the potential of novel mechanism-based biomarkers of hepatic stress which provide information to connect the intracellular events (drug metabolism, organelle, cell and whole organ) ultimately leading to tissue damage (apoptosis, necrosis and inflammation). These biomarkers can provide both the means to inform the pharmacologist and chemist with respect to safe drug design, and provide clinicians with valuable tools for patient monitoring.

Drug-induced liver injury: what was new in 2008?

BACKGROUND

Given the number of publications appearing annually regarding drug-induced liver injury (DILI), there remains a need to concisely summarize each year's new crop of case series and reports as well as the advances in mechanisms of liver injury and in the field of pharmacogenomics relating to DILI.

OBJECTIVE

To present an up-to-date review of the past year's most important clinical studies and reports of DILI, placing them into context of previous publications.

METHODS

A Medline search was conducted of all manuscripts appearing in the fields "hepatotoxicity" and "drug-induced liver injury" during the calendar year 2008. The most clinically relevant English language case reports and studies exploring mechanisms and risk factors for DILI were then chosen for review, and supplemented with older literature where appropriate.

CONCLUSIONS

As in past years, 2008 was replete with publications dealing with virtually all facets of DILI, including updated incidence and prevalence data, as well as the latest information regarding mechanisms of liver injury. Data from the first 300 patients in the National Institute of Health-sponsored DILI Network registry of > 100 non-acetaminophen causes were presented. Antimicrobials and CNS drugs were responsible for > 60% of cases, with herbals and dietary supplements being increasingly reported. Identification of genetic predispositions to DILI is coming of age with the FDA calling for the testing of human leukocyte antigen B(*)5701 before the use of abacavir to reduce the risk of hypersensitivity reactions. Several groups emphasized the pitfalls in utilizing Roussel Uclaf Causality Assessment Method and other causality assessment methodologies, and an updated review appeared on the use of potentially hepatotoxic medications in patients with underlying liver disease.