Advantage of serum type-I arginase and ornithine carbamoyltransferase in the evaluation of acute and chronic liver damage induced by thioacetamide in rats

Biomarkers of Hepatic Toxicity: An Overview

Background

Hepatotoxicity is the foremost issue for clinicians and the primary reason for pharmaceutical product recalls. A biomarker is a measurable and quantifiable attribute used to evaluate the efficacy of a treatment or to diagnose a disease. There are various biomarkers which are used for the detection of liver disease and the intent of liver damage.

Objective

This review aims to investigate the current state of hepatotoxicity biomarkers and their utility in clinical settings. Using hepatic biomarkers, the presence of liver injury, its severity, prognosis, causative agent, and type of hepatotoxicity can all be determined.

Methods

Relevant published articles up to 2022 were systematically retrieved from MEDLINE/PubMed, SCOPUS, EMBASE, and WOS databases using keywords such as drug toxicity, hepatotoxicity biomarkers, biochemical parameters, and nonalcoholic fatty liver disease.

Results

In clinical trials and everyday practice, biomarkers of drug-induced liver injury are essential for spotting the most severe cases of hepatotoxicity. Hence, developing novel biomarker approaches to enhance hepatotoxicity diagnosis will increase specificity and/or identify the person at risk. Importantly, early clinical studies on patients with liver illness have proved that some biomarkers such as aminotransferase, bilirubin, albumin, and bile acids are even therapeutically beneficial.

Conclusions

By assessing the unique signs of liver injury, health care professionals can rapidly and accurately detect liver damage and evaluate its severity. These measures contribute to ensuring prompt and effective medical intervention, hence reducing the risk of long-term liver damage and other major health concerns.

Molecular and Pathological Analyses of IARS1-Deficient Mice: An IARS Disorder Model

Most mitochondrial diseases are hereditary and highly heterogeneous. Cattle born with the V79L mutation in the isoleucyl-tRNA synthetase 1 (IARS1) protein exhibit weak calf syndrome. Recent human genomic studies about pediatric mitochondrial diseases also identified mutations in the IARS1 gene. Although severe prenatal-onset growth retardation and infantile hepatopathy have been reported in such patients, the relationship between IARS mutations and the symptoms is unknown. In this study, we generated hypomorphic IARS1^V79L mutant mice to develop an animal model of IARS mutation-related disorders. We found that compared to wild-type mice, IARS^V79L mutant mice showed a significant increase in hepatic triglyceride and serum ornithine carbamoyltransferase levels, indicating that IARS1^V79L mice suffer from mitochondrial hepatopathy. In addition, siRNA knockdown of the IARS1 gene decreased mitochondrial membrane potential and increased reactive oxygen species in the hepatocarcinoma-derived cell line HepG2. Furthermore, proteomic analysis revealed decreased levels of the mitochondrial function-associated protein NME4 (mitochondrial nucleoside diphosphate kinase). Concisely, our mutant mice model can be used to study IARS mutation-related disorders.

Serum Amino Acids Imbalance in Canine Chronic Hepatitis: Results in 16 Dogs

Simple Summary

Human chronic liver disease is reported to be associated with alterations in amino acids metabolism, with a decrease in serum branched-chain amino acids and an increase in aromatic amino acids. A decreased Fischer ratio (branched to aromatic amino acids ratio) has showed prognostic significance and is a therapeutic target in human cirrhosis. In dogs, few studies have been performed, and the Fischer ratio seems to be reduced in animals with congenital portosystemic shunts. The aim of this study was to evaluate serum amino acids in dogs with chronic hepatic inflammation compared with healthy dogs. The serum amino acids in dogs with chronic hepatitis were also evaluated in relation to their histological severity. Eighteen amino acidic metabolites were measured using the leftover serum samples of 16 dogs with histological chronic hepatitis and 25 healthy dogs. Several amino acid concentrations were significantly different between dogs diagnosed with chronic hepatitis and healthy controls. In human medicine, aromatic amino acids seem to increase during chronic hepatitis, whereas isoleucine decreases. The Fischer ratio was significantly reduced if higher grades of fibrosis were present. Even if total serum proteins did not significantly differ between groups, we observed qualitative imbalances in serum amino acids among dogs presenting with chronic hepatitis.

Abstract

In humans, chronic liver disease may cause alterations in amino acids (AAs) metabolism, with serum branched-chain AAs (BCAAs) decreasing and aromatic AAs (AAAs) increasing. A reduced Fischer ratio (BCAAs/AAAs) has been found to be associated with hepatic fibrosis and is useful for assessing prognosis in human patients. In veterinary medicine, few studies have been performed, and in contrast to human patients, dogs with different kinds of hepatopathy tend to show both increased AAAs and BCAAs. In dogs, the association between histological scores and serum AAs has not been previously investigated. The aim of this study was to evaluate serum AAs in dogs with chronic hepatitis (CH) compared with a healthy control group (C) and, among CH dogs, in relation to their histological fibrosis and necroinflammatory activity scores. Leftover serum samples of 16 dogs with histological CH and 25 healthy dogs were employed. Serum AAs were measured by high performance liquid chromatography. Proline and the AAAs phenylalaine and tyrosine progressively increased with the histological severity. In contrast, cysteine, tryptophan and BCAA isoleucine progressively reduced. Lysine and the BCAAs leucine and valine showed a non-linear trend with the histological findings. The BCAAs/AAAs ratio was significatively reduced if higher grades of liver fibrosis were present.

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

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

Biomarker-Guided Individualization of Antibiotic Therapy

Treatment failure of antibiotic therapy due to insufficient efficacy or occurrence of toxicity is a major clinical challenge, and is expected to become even more urgent with the global rise of antibiotic resistance. Strategies to optimize treatment in individual patients are therefore of crucial importance. Currently, therapeutic drug monitoring plays an important role in optimizing antibiotic exposure to reduce treatment failure and toxicity. Biomarker-based strategies may be a powerful tool to further quantify and monitor antibiotic treatment response, and reduce variation in treatment response between patients. Host response biomarkers, such as CRP, procalcitonin, IL-6, and presepsin, could potentially carry significant information to be utilized for treatment individualization. To achieve this, the complex interactions among immune system, pathogen, drug, and biomarker need to be better understood and characterized. The purpose of this tutorial is to discuss the use and evidence of currently available biomarker-based approaches to inform antibiotic treatment. To this end, we also included a discussion on how treatment response biomarker data from preclinical, healthy volunteer, and patient-based studies can be further characterized using pharmacometric and system pharmacology based modeling approaches. As an illustrative example of how such modeling strategies can be used, we describe a case study in which we quantitatively characterize procalcitonin dynamics in relation to antibiotic treatments in patients with sepsis.

Exogenous L-carnitine ameliorates burn-induced cellular and mitochondrial injury of hepatocytes by restoring CPT1 activity

Background

Impaired hepatic fatty acid metabolism and persistent mitochondrial dysfunction are phenomena commonly associated with liver failure. Decreased serum levels of L-carnitine, a amino acid derivative involved in fatty-acid and energy metabolism, have been reported in severe burn patients. The current study aimed to evaluate the effects of L-carnitine supplementation on mitochondrial damage and other hepatocyte injuries following severe burns and the related mechanisms.

Methods

Serum carnitine and other indicators of hepatocytic injury, including AST, ALT, LDH, TG, and OCT, were analyzed in severe burn patients and healthy controls. A burn model was established on the back skin of rats; thereafter, carnitine was administered, and serum levels of the above indicators were evaluated along with Oil Red O and TUNEL staining, transmission electron microscopy, and assessment of mitochondrial membrane potential and carnitine palmitoyltransferase 1 (CPT1) activity and expression levels in the liver. HepG2 cells pretreated with the CPT1 inhibitor etomoxir were treated with or without carnitine for 24 h. Next, the above indicators were examined, and apoptotic cells were analyzed via flow cytometry. High-throughput sequencing of rat liver tissues identified several differentially expressed genes (Fabp4, Acacb, Acsm5, and Pnpla3) were confirmed using RT-qPCR.

Results

Substantially decreased serum levels of carnitine and increased levels of AST, ALT, LDH, and OCT were detected in severe burn patients and the burn model rats. Accumulation of TG, evident mitochondrial shrinkage, altered mitochondrial membrane potential, decreased ketogenesis, and reduced CPT1 activity were detected in the liver tissue of the burned rats. Carnitine administration recovered CPT1 activity and improved all indicators related to cellular and fatty acid metabolism and mitochondrial injury. Inhibition of CPT1 activity with etomoxir induced hepatocyte injuries similar to those in burn patients and burned rats; carnitine supplementation restored CPT1 activity and ameliorated these injuries. The expression levels of the differentially expressed genes Fabp4, Acacb, Acsm5, and Pnpla3 in the liver tissue from burned rats and etomoxir-treated hepatocytes were also restored by treatment with exogenous carnitine.

Conclusion

Exogenous carnitine exerts protective effects against severe burn-induced cellular, fatty-acid metabolism, and mitochondrial dysfunction of hepatocytes by restoring CPT1 activity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12986-021-00592-x.

Evaluation of conventional and non-conventional biomarkers of liver toxicity in greenhouse workers occupationally exposed to pesticides

The liver plays a prominent role in maintenance of homeostasis and is the major organ for xenobiotic metabolism, including pesticides. Conventional liver function tests are widely used to assess hepatocellular and biliary system dysfunction by measuring serum levels of aminotransferases (ALT, AST) and cholestasis enzymes (alkaline phosphatase -ALP- and γ-glutamyl transferase -GGT-), respectively. Although these tests are not entirely specific for liver damage, their specificity increases when measured concurrently, but still have limited usefulness to predict early liver dysfunction. Hence, non-conventional biomarkers may have a better performance for the early detection of biochemical hepatotoxicity with a greater specificity and sensitivity. A cross-sectional study with a follow-up component was conducted on 175 greenhouse workers regularly exposed to pesticides under integrated production system, and 91 controls living in the same geographical area. All individuals were evaluated for conventional (ALT, AST, ALP, GGT) and non-conventional biomarkers of hepatotoxicity (ornithine transcarbamylase (-OTC-), Arginase-1 -ARG1- and glutathione S-transferase alpha -GSTα-) over two periods of the same crop season, one of high pesticide exposure and other of low exposure. A slight increase in AST was observed in greenhouse workers relative to controls, suggestive of subtle hepatocellular toxicity. Although ALP, ARG1 and GST-α levels were decreased in greenhouse workers, this might be related to a potential homeostatic mechanism that regulates their expression. Altogether, these findings do not represent unambiguous evidence of liver dysfunction (e.g., hepatocellular or biliary system impairment) but may be the result of the low-toxicity pesticides used by greenhouse workers.

Sidr honey abrogates the oxidative stress and downregulates the hyaluronic acid concentration and gene expression of TGF-β1 and COL1a1 in rat model of thioacetamide-induced hepatic fibrosis

Liver fibrosis is a major health concern, which might progress to cirrhosis. To date, treatment trials rely mainly on the removal of the causative factor. The current study investigated the potential ameliorative role of sidr honey on thioacetamide (TAA)-induced liver fibrosis in rats. Forty-eight Wistar albino rats were equally allocated into four groups: control; sidr honey (5g/kg body weight (BW), orally); TAA (200 mg/kg BW, IP three times weekly/15 weeks); and sidr honey plus TAA at the same dose and administration rout. Rats co-treated with sidr honey plus TAA revealed significant reduction in hepatic malondialdehyde, hyaluronic acid (HA), alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, gamma glutamyl transferase, direct bilirubin, and hepatic mRNA expression of transforming growth factor (TGF)-β1 and collagen type I alpha 1 chain (COL1a1) compared to TAA-exposed rats. In addition, the hepatoprotective potential of sidr honey was indicated via improvement of histopathologic picture of hepatocytes and upregulation of total antioxidant capacity, reduced glutathione, catalase, glutathione peroxidase, superoxide dismutase, total protein, and albumin compared to TAA-treated rats. In conclusion, daily administration of sidr honey (5 g/kg BW) is a promising natural antioxidant and fibrosuppressive agent that could ameliorate liver fibrosis via downregulation of fibrosis genes including TGF-β1 and COL1a1 and HA and via enhancement of antioxidant system.

Clinical prospects of biomarkers for the early detection and/or prediction of organ injury associated with pharmacotherapy

Several biomarkers are used to monitor organ damage caused by drug toxicity. Traditional markers of kidney function, such as serum creatinine and blood urea nitrogen are commonly used to estimate glomerular filtration rate. However, these markers have several limitations including poor specificity and sensitivity. A number of serum and urine biomarkers have recently been described to detect kidney damage caused by drugs such as cisplatin, gentamicin, vancomycin, and tacrolimus. Neutrophil gelatinase-associated lipocalin (NGAL), liver-type fatty acid-binding protein (L-FABP), kidney injury molecule-1 (KIM-1), monocyte chemotactic protein-1 (MCP-1), and cystatin C have been identified as biomarkers for early kidney damage. Hy's Law is widely used as to predict a high risk of severe drug-induced liver injury caused by drugs such as acetaminophen. Recent reports have indicated that glutamate dehydrogenase (GLDH), high-mobility group box 1 (HMGB-1), Keratin-18 (k18), MicroRNA-122 and ornithine carbamoyltransferase (OCT) are more sensitive markers of hepatotoxicity compared to the traditional markers including the blood levels of amiotransferases and total bilirubin. Additionally, the rapid development of proteomic technologies in biofluids and tissue provides a new multi-marker panel, leading to the discovery of more sensitive biomarkers. In this review, an update topics of biomarkers for the detection of kidney or liver injury associated with pharmacotherapy.

Biomarkers associated with binaprofen‑induced liver injury

Drug‑induced liver injury (DILI) is a common hepatic disease. The identification of biomarkers for DILI prediction is critical for rational drug use. The aim of the present study was to investigate liver injury caused by binaprofen and identify proteins that may serve as early biomarkers to predict DILI. For in vivo DILI assays, zebrafish were exposed to acetaminophen (APAP) and binaprofen for 12‑96 h before lethal concentration 50 (LC50), histopathological analysis, conventional and non‑conventional biomarker measurements were conducted. In vitro assays were performed in cultured liver cells; after 6‑24 h treatment with APAP and binaprofen the same measurements were conducted as aforementioned. The in vivo assays indicated that the LC50 of APAP was 5.2 mM, whereas the LC50 of binaprofen was 1.2 mM; 12‑48 h post‑treatment, liver cells exhibited mild to moderate vacuolization in a time‑ and concentration‑dependent manner in response to both drugs. During this time, conventional and non‑conventional biomarkers were also altered in a time‑ and concentration‑dependent manner; however, alterations in the levels of non‑conventional biomarkers occurred at an earlier time point compared with conventional biomarkers. The in vitro assays indicated that the half maximal inhibitory concentration (IC50) of APAP was 16.2 mM, whereas the IC50 of binaprofen was 5.3 mM; 12‑48 h post‑treatment, cultured liver cells exhibited mild to moderate swelling in a time‑ and concentration‑dependent manner. Alterations in the levels of conventional and non‑conventional biomarkers were similar to those observed in the in vivo assays. As a non‑steroidal anti‑inflammatory drug, binaprofen exhibited expected levels of liver toxicity in in vitro and in vivo assays, which were similar to APAP. Total bile acid and argininosuccinate lyase were identified as early biomarkers, which could accurately predict onset of binaprofen‑induced liver injury.

Characterization of release profile of ornithine carbamoyltransferase from primary rat hepatocytes treated with hepatotoxic drugs: Implications for its unique potential as a drug-induced liver injury biomarker

Ornithine carbamoyltransferase (OCT) is a mitochondrial protein expressed primarily in the liver. It has been shown that, like alanine aminotransferase (ALT), OCT is released from damaged hepatocytes in rats and humans, which has given rise to the possibility that OCT might provide a diagnostic biomarker of various forms of liver damage, including drug-induced liver injury (DILI). However, OCT release characteristics in DILI, as well as their diagnostic advantages, remain elusive. Therefore, this study aimed at clarifying whether and how OCT is released from rat primary hepatocytes in vitro using seven potentially hepatotoxic drugs. The results showed that OCT releases from damaged hepatocytes were observed for all tested drugs, and that those releases were not associated with mitochondrial membrane proteins. It should be underscored that the release dynamics were significantly larger than those of ALT. Furthermore, unlike ALT, the maximum OCT release levels showed differences depending on the drug being tested, suggesting that OCT release was susceptible to toxicity mechanisms. Taken together, these unique release characteristics highlight the possibility that OCT could provide a promising DILI biomarker that might contribute not only to diagnostic accuracy improvements, but also to a better understanding of toxicity types in clinical and drug development settings.

Serum fetuin-A and arginase-1 in human obesity model: Is there any interaction between inflammatory status and arginine metabolism?

Obesity is a major risk factor for many chronic metabolic diseases such as inflammation, insulin resistance (IR) and fatty liver injury. It was reported that obesity causes some variations on the serum levels of fetuin-A and is associated with arginine metabolism, especially arginase-1 levels. The aim of our study was to evaluate, the interaction and possible changes of these liver over produced proteins, fetuin-A and arginase-1 levels in obesity-related inflammatory status. Study groups were composed of individuals aged between 19 and 63 (n = 62). The control group included healthy subjects with BMI < 25, obese group included obese patients with BMI > 30 and with no other chronic disease. Biochemical markers were determined by an auto-analyzer. Adiponectin, fetuin-A, arginase-1, asymmetric dimethylarginine (ADMA), arginine, Hexanoyl-lysine (HEL) and leptin levels were measured with commercial ELISA immunoassay kits. Nitrite and nitrate were determined with colorimetric assay kit in serum samples. High sensitive C-reactive protein (hsCRP) levels and liver function enzymes activities were higher in the obese group in respect to the control group. Serum fetuin-A, arginase-1 and leptin levels were increased but adiponectin levels were decreased in obese subjects. Fetuin-A levels showed significant correlations with arginase-1 and HOMA-IR. Consequently, we carried out an investigation about higher serum fetuin-A and arginase-1 levels may have an important role in obesity and obesity-related liver damage.

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.

Metabolomic analysis of arginine metabolism in acute hepatic injury in rats

To clarify the relationship between arginine metabolism and hepatic injury, metabolomic analysis was performed in rats treated with 3 representative hepatotoxicants, monocrotaline (MCT), concanavalin A (ConA), and α-naphthyl isothiocyanate (ANIT); or a myotoxicant, tetramethyl-p-phenylenediamine (TMPD). A single dose of MCT, ConA, or ANIT dose-dependently induced hepatocellular necrosis accompanied by decreased blood arginine and increased blood alanine aminotransferase (ALT) and arginase. A close correlation was detected between arginine and ALT (r = -0.746, -0.795, -0.787 for MCT, ConA, ANIT, respectively) or between arginine and arginase (r = -0.605, -0.808, -0.672 for MCT, ConA, ANIT, respectively) in all three hepatic injury models. In contrast, neither hepatocellular necrosis nor alterations in arginine were found in the skeletal muscle injury model, although ALT was slightly increased. An in vitro assay revealed that blood samples obtained from ConA-treated rats transformed external arginine to ornithine, and the reaction was totally inhibited by an arginase inhibitor. These results suggest that blood arginase plays a crucial role in arginine metabolism associated with hepatic injury. In metabolomic analysis, nearly 450 endogenous metabolites were identified in blood obtained from all the models. Among the 13 metabolites involved in arginine metabolism, decreased arginine and increased ornithine occurred in common in the hepatic injury models, whereas citrulline and other metabolites were not altered. These results indicate that arginine metabolism, especially the arginine-to-ornithine pathway, is altered in association with acute hepatic injury. Furthermore, blood arginine and ornithine are possibly specific biomarkers for hepatic injury.

Hepatoprotective, antioxidant, and ameliorative effects of ginger (Zingiber officinale Roscoe) and vitamin E in acetaminophen treated rats

Ginger is a remedy known to possess a number of pharmacological properties. This study investigated efficacy of ginger pretreatment in alleviating acetaminophen-induced acute hepatotoxicity in rats. Rats were divided into six groups; negative control, acetaminophen (APAP) (600 mg/kg single intraperitoneal injection); vitamin E (75 mg/kg), ginger (100 mg/kg), vitamin E + APAP, and ginger + APAP. Administration of APAP elicited significant liver injury that was manifested by remarkable increase in plasma alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), arginase activities, and total bilirubin concentration. Meanwhile, APAP significantly decreased plasma total proteins and albumin levels. APAP administration resulted in substantial increase in each of plasma triacylglycerols (TAGs), malondialdhyde (MDA) levels, and total antioxidant capacity (TAC). However, ginger or vitamin E treatment prior to APAP showed significant hepatoprotective effect by lowering the hepatic marker enzymes (AST, ALT, ALP, and arginase) and total bilirubin in plasma. In addition, they remarkably ameliorated the APAP-induced oxidative stress by inhibiting lipid peroxidation (MDA). Pretreatment by ginger or vitamin E significantly restored TAGs, and total protein levels. Histopathological examination of APAP treated rats showed alterations in normal hepatic histoarchitecture, with necrosis and vacuolization of cells. These alterations were substantially decreased by ginger or vitamin E. Our results demonstrated that ginger can prevent hepatic injuries, alleviating oxidative stress in a manner comparable to that of vitamin E. Combination therapy of ginger and APAP is recommended especially in cases with hepatic disorders or when high doses of APAP are required.

Identification of novel liver-specific mRNAs in plasma for biomarkers of drug-induced liver injury and quantitative evaluation in rats treated with various hepatotoxic compounds

Circulating liver-specific mRNAs such as albumin (Alb) and α-1-microglobulin/bikunin precursor (Ambp) have been reported to be potential biomarkers for drug-induced liver injury (DILI). We identified novel circulating liver-specific mRNAs and quantified them, together with the two previously reported mRNAs, in plasma from rats treated with various hepatotoxicants to validate circulating liver-specific mRNAs as biomarkers for DILI. Among six genes selected from the database, high liver specificity of apolipoprotein h (Apoh) and group-specific component (Gc) mRNAs were confirmed by reverse transcription (RT)-PCR and the copy numbers of these mRNAs elevated in plasma from rats treated with thioacetamide. Liver-specific mRNAs (Alb, Ambp, Apoh, and Gc) were quantified by real-time RT-PCR in plasma from rats with single dosing of seven hepatotoxicants. There were noticeable interindividual and intercompound variabilities in the severity of liver injury. The levels of four mRNAs increased almost in parallel and correlated with changes in the alanine aminotransferase (ALT) values and the hepatocellular necrosis scores at 24h after dosing. It was noteworthy that the magnitude of the increases in mRNA levels was greater than that in the ALT value. Time course analysis within 24h after dosing revealed that the timing of the increase was different among mRNA species, and the plasma levels of Alb and Gc mRNAs increased substantially earlier than the ALT values, suggesting that patterns of changes in circulating liver-specific mRNAs indicate the progression of liver injury. These results strongly support the reliability and usefulness of the four circulating liver-specific mRNAs as biomarkers for DILI.

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.

Serum ornithine carbamyltransferase reflects hepatic damage in diabetic obese mice

BACKGROUND AND AIM

As ornithine carbamyltransferase (OCT) has proved to be a sensitive serum marker in the detection of hepatotoxicity in several models, it is important to confirm its application to the diagnosis of non-alcoholic fatty liver disease.

METHODS

C57BL/6, KK-Ta and KK-Ay mice were fed a high-fat diet for 8 weeks and serum enzyme markers were examined. Serum OCT and alanine aminotransferase (ALT) were also measured in diabetic obese ob/ob and db/db mice fed a normal diet. Liver damage in these mice was evaluated by the hepatic content of tumor necrosis factor-alpha.

RESULTS

Serum levels of OCT increased in KK-Ay fed a high-fat diet compared with the normal diet-fed group, whereas C57BL/6 and KK-Ta mice were not affected. In ob/ob mice, the relative increase was always greater in OCT than in ALT. In contrast, in db/db mice, the relative increase was always greater in ALT. Hepatic tumor necrosis factor-alpha was significantly elevated in ob/ob mice, but not in db/db mice.

CONCLUSIONS

Serum OCT seemed to reflect tumor necrosis factor-alpha-mediated hepatic damage when compared with ALT in diabetic obese mice and could be useful in the application for non-alcoholic fatty liver disease with features of metabolic syndrome, such as obesity and diabetes.

Marked elevation of serum mitochondrion-derived markers in mild models of non-alcoholic steatohepatitis in rats

BACKGROUND AND AIM

In order to find sensitive serum markers in non-alcoholic steatohepatitis, liver-specific injury markers were thoroughly examined in mild models of NASH in rats.

METHODS

Wistar and Sprague-Dawley rats were fed a choline-deficient diet for 4 weeks, and serum activities of liver-specific enzyme markers were examined. In the drug-induced steatohepatitis model, tetracycline (0.4 mmol/kg) was given i.p. to rats and the course of hepatotoxicity was evaluated with serum markers, together with the accumulation of total lipid and thiobarbituric acid-reactive substances in the liver.

RESULTS

In Wistar rats, serum activities of most enzymes tested were significantly increased. In Sprague-Dawley rats, in contrast, the serum level of ornithine carbamyltransferase and glutamate dehydrogenase were markedly elevated in the choline-deficient diet group compared with the control diet groups, whereas other markers were not significantly increased. In the tetracycline-induced steatohepatitis model, the extent of the increase was much higher in mitochondrial markers and the peak of the increase in these markers corresponded with the increase of hepatic total lipid and thiobarbituric acid-reactive substance.

CONCLUSIONS

These observations show that serum mitochondrial enzyme markers are potent markers for non-alcoholic steatohepatitis in rats and are possibly applicable to humans.

Ornithine carbamyltransferase is a sensitive marker for alcohol-induced liver injury

BACKGROUND

Although mitochondrion-derived markers such as ornithine carbamyltransferase (OCT) and glutamate dehydrogenase (GLDH) have been reported to be good markers for alcohol-induced hepatic injury, their use has been limited due to the notion that mitochondrial markers are less sensitive than cytosol-derived markers. We determined the clinical importance of mitochondrion-derived markers in the evaluation of alcohol-induced hepatotoxicity.

METHODS

Rats were administered alcohol chronically (5-30% ethanol in drinking water with or without high fat diet feeding for 15 weeks) and hepatic damages were evaluated by serum OCT and GLDH, together with other liver enzymes such as alanine aminotransferase and aspartate aminotransferase. Hepatic content of the enzymes was also evaluated in the chronic ethanol feeding model to confirm whether induction of the enzyme in the liver reflects the serum activity.

RESULTS

The serum activities of OCT and GLDH increased significantly by chronic ethanol feeding while other markers did not. Although the hepatic content of OCT and GLDH also increased, the serum activities did not correlate with the hepatic activities and the extent of increase in the liver was much less than in serum.

CONCLUSIONS

Mitochondrion-derived markers, especially OCT, appeared superior to cytosol-derived markers in the detection of alcohol-induced liver injury.

Drug-induced liver injury through mitochondrial dysfunction: mechanisms and detection during preclinical safety studies

Mitochondrial dysfunction is a major mechanism whereby drugs can induce liver injury and other serious side effects such as lactic acidosis and rhabdomyolysis in some patients. By severely altering mitochondrial function in the liver, drugs can induce microvesicular steatosis, a potentially severe lesion that can be associated with profound hypoglycaemia and encephalopathy. They can also trigger hepatic necrosis and/or apoptosis, causing cytolytic hepatitis, which can evolve into liver failure. Milder mitochondrial dysfunction, sometimes combined with an inhibition of triglyceride egress from the liver, can induce macrovacuolar steatosis, a benign lesion in the short term. However, in the long term this lesion can evolve in some individuals towards steatohepatitis, which itself can progress to extensive fibrosis and cirrhosis. As liver injury caused by mitochondrial dysfunction can induce the premature end of clinical trials, or drug withdrawal after marketing, it should be detected during the preclinical safety studies. Several in vitro and in vivo investigations can be performed to determine if newly developed drugs disturb mitochondrial fatty acid oxidation (FAO) and the oxidative phosphorylation (OXPHOS) process, deplete hepatic mitochondrial DNA (mtDNA), or trigger the opening of the mitochondrial permeability transition (MPT) pore. As drugs can be deleterious for hepatic mitochondria in some individuals but not in others, it may also be important to use novel animal models with underlying mitochondrial and/or metabolic abnormalities. This could help us to better predict idiosyncratic liver injury caused by drug-induced mitochondrial dysfunction.

Role of mammalian cytosolic molybdenum Fe-S flavin hydroxylases in hepatic injury

The study was designed to investigate the role of molybdenum iron-sulfur flavin hydroxylases in the pathogenesis of liver injuries induced by structurally and mechanistically diverse hepatotoxicants. While carbon tetrachloride (CCl4), thioacetamide (TAA) and chloroform (CHCl3) inflict liver damage by producing free radicals, acetaminophen (AAP) and bromobenzene (BB) exert their effects by severe glutathione depletion. Appropriate doses of these compounds were administered to induce liver injury in rats. The activities of the Mo-Fe-S flavin hydroxylases were measured and correlated with the biochemical markers of hepatic injury. The activity levels of the anti-oxidative enzymes and glutathione redox cycling enzymes were also determined. The treatment of rats with the hepatotoxins that inflict liver injury by generating free radicals (CCl4, TAA, CHCl3) had elevated activity levels of hepatic Mo-Fe-S flavin hydroxylases (p<0.05). Specific inhibition of these hydroxylases by their common inhibitor, sodium tungstate, suppresses biochemical and oxidative stress markers of hepatic tissue damage. On the contrary, Mo-Fe-S flavin hydroxylases did not show any change in animals receiving AAP and BB. Correspondingly, sodium tungstate could not attenuate damage in AAP and BB treated groups of rats. The study concludes that Mo-Fe-S hydroxylases contribute to the hepatic injury inflicted by free radical generating agents and does not play any role in hepatic injury produced by glutathione depleting agents. The study has implication in understanding human liver diseases caused by a variety of agents, and to investigate the efficacy of the inhibitors of Mo-Fe-S flavin hydroxylases as potential therapeutic agents.

The current state of serum biomarkers of hepatotoxicity

The level of serum alanine aminotransferase (ALT) activity reflects damage to hepatocytes and is considered to be a highly sensitive and fairly specific preclinical and clinical biomarker of hepatotoxicity. However, an increase in serum ALT activity level has also been associated with other organ toxicities, thus, indicating that the enzyme has specificity beyond liver in the absence of correlative histomorphologic alteration in liver. Thus, unidentified non-hepatic sources of serum ALT activity may inadvertently influence the decision of whether to continue development of a novel pharmaceutical compound. To assess the risk of false positives due to extraneous sources of serum ALT activity, additional biomarkers are sought with improved specificity for liver function compared to serum ALT activity alone. Current published biomarker candidates are reviewed herein and compared with ALT performance in preclinical and on occasion, clinical studies. An examination of the current state of hepatotoxic biomarkers indicates that serum F protein, arginase I, and glutathione-S-transferase alpha (GSTalpha) levels, all measured by ELISA, may show utility, however, antibody availability and high cost per run may present limitations to widespread applicability in preclinical safety studies. In contrast, the enzymatic markers sorbitol dehydrogenase, glutamate dehydrogenase, paraxonase, malate dehydrogenase, and purine nucleoside phosphorylase are all readily measured by photometric methods and use reagents that work across preclinical species and humans and are commercially available. The published literature suggests that these markers, once examined collectively in a large qualification study, could provide additional information relative to serum ALT and aspartate aminotransferase (AST) values. Since these biomarkers are found in the serum/plasma of treated humans and rats, they have potential to be utilized as bridging markers to monitor acute drug-induced liver injury in early clinical trials.

Determination of ornithine in human plasma by hydrophilic interaction chromatography–tandem mass spectrometry

The amino acid ornithine (Orn) acts as a vital part in the physiologically fundamental urea cycle. As such, it is a main intermediate in the catabolic breakdown as well as in the synthesis of arginine and is involved in many other metabolic pathways with potential clinical implications. We here describe a LC-MS-MS method for the detection of Orn in human plasma which is fast, easy and precise. The sample preparation comprises only protein precipitation and the addition of the isotopic labeled I.S. The analytes are separated by hydrophilic interaction chromatography (HILIC) in less than 4min on a silica column with an isocratic mobile phase consisting of 0.1% trifluoroacetic acid in water and acetonitrile in the ratio of 25:75. Orn and its I.S. are detected and quantified by APCI tandem mass spectrometry. The calibration function is linear from 7.5 to 205 micromol/l and covers the range of concentrations found in patients undergoing different clinical interventions. The quantification results are independent with regard to the biological matrix analyzed. The intra-day and inter-day relative standard deviations are 1.1% and 3.5%, respectively. As an application of the described method in clinical investigations, we report arginine and ornithine plasma concentration results from an arginine supplementation study enrolling healthy volunteers and patients suffering from hypercholesterolemia. After oral dosing of 110 mg/kg arginine, ornithine plasma concentrations rose from 54 to 148 micromol/l after 2h and were back to baseline after 24h. However, arginine to ornithine ratios kept constant during the complete observation time.

Arginine metabolism: boundaries of our knowledge

Arginine has multiple metabolic fates and thus is one of the most versatile amino acids. Not only is it metabolically interconvertible with the amino acids proline and glutamate, but it also serves as a precursor for synthesis of protein, nitric oxide, creatine, polyamines, agmatine, and urea. These processes do not all occur within each cell but are differentially expressed according to cell type, age and developmental stage, diet, and state of health or disease. Arginine metabolism also is modulated by activities of various transporters that move arginine and its metabolites across the plasma and mitochondrial membranes. Moreover, several key enzymes in arginine metabolism are expressed as multiple isozymes whose expression can change rapidly and dramatically in response to a variety of different stimuli in health and disease. As illustrated by the questions raised in this article, we currently have an imperfect and incomplete picture of arginine metabolism for any mammalian species. It has become clear that a more complete understanding of arginine metabolism will require integration of information obtained from multiple approaches, including genomics, proteomics, and metabolomics.

Serum level of ornithine carbamoyltransferase is influenced by the state of Kupffer cells

BACKGROUND

The ratio of ornithine carbamoyltransferase (OCT) to alanine aminotransferase (ALT) or glutamate dehydrogenase (GDH) in serum has been suggested as an indicator for the diagnosis of hepatocellular carcinoma and alcoholic liver disease, respectively. However, the mechanisms responsible for the increase in these ratios are still unclear.

METHODS

Wistar rats were pretreated with lipopolysaccharide (LPS) or gadolinium chloride (GD) before being administered with thioacetamide (TAA, 200 mg/kg, ip). Serum OCT and ALT levels were compared with control values. Half-lives of the enzymes in circulation were evaluated after the intravenous injection of the purified enzymes into rats with or without the pretreatment.

RESULTS

The serum level of OCT at 24 h after the administration of TAA was significantly lower in the LPS-treated group, and not influenced by pretreatment with GD. The half-life of OCT was prolonged from 1.06+/-0.14 to 2.07+/-0.29 h (p<0.05) by the pretreatment with GD, but not influenced by the administration of LPS. No change was observed in the clearance of GDH or ALT among the pretreatments.

CONCLUSIONS

Leakage into and clearance from the circulation of OCT are influenced by whether Kupffer cells are activated or not. OCT alone or in combination with other markers may be a useful indicator for Kupffer cell activation as well as mitochondrial damage in hepatic cells.