4,4′-Methylenedianiline-Induced Hepatotoxicity Is Modified by N-Acetyltransferase 2 (NAT2) Acetylator Polymorphism in the Rat

Identification of Specific MicroRNA Biomarkers in Early Stages of Hepatocellular Injury, Cholestasis, and Steatosis in Rats

Recently, studies on circulating microRNAs (miRNAs) as potential biomarkers of drug-induced liver injury (DILI) have received increasing attention. It has been demonstrated that miR-122 and miR-192, which are liver enriched, could be potential biomarkers of DILI; however, these miRNAs cannot discern types of injuries. In the present study, we comprehensively analyzed time-dependent plasma miRNA profiles in rats with drug- or chemical-induced hepatocellular injury, cholestasis, and steatosis with high-throughput miRNA sequencing. To enable the comparison of miRNA expression levels between DILI models with different severity and peak time of injuries, the stages of injury were defined as early, middle, and late, according to cluster patterns of miRNA expression profiles. Through differential analysis, we characterized miRNAs that were specifically up- or down-regulated in each DILI model. Several miRNAs were dramatically changed earlier than traditional biomarkers such as alanine aminotransferase (ALT) and aspartate aminotransferase (AST). For example, in an acetaminophen (APAP)-induced hepatocellular injury model, plasma let-7b-5p was up-regulated as early as 3 h after dosing, whereas a significant change in ALT level was observed at 12 h. We then focused on the DILI type-specific miRNAs in plasma that were up-regulated at the early stage of injury. RT-qPCR analysis validated that let-7b-5p and miR-1-3p for hepatocellular injury, miR-143-3p and miR-218a-5p for cholestasis, and miR-320-3p for steatosis models showed significant increases in the early stage of the injuries. The present study suggests the utility of miRNAs as specific biomarkers for the early detection of DILI.

Role of the human N-acetyltransferase 2 genetic polymorphism in metabolism and genotoxicity of 4, 4'-methylenedianiline

4, 4'-Methylenedianiline (MDA) is used extensively as a curing agent in the production of elastomers and is classified as reasonably anticipated to be a human carcinogen based on sufficient evidence in animal experiments. Human N-acetyltransferase 1 (NAT1) and 2 (NAT2) catalyze the N-acetylation of aromatic amines and NAT2 is subjected to a common genetic polymorphism in human populations separating individuals into rapid, intermediate, and slow acetylator phenotypes. Although MDA is known to undergo N-acetylation to mono- and di-acetyl metabolites, very little is known regarding whether this metabolism is subject to the NAT2 genetic polymorphism. We investigated the N-acetylation of MDA by recombinant human NAT1, NAT2, genetic variants of NAT2, and cryoplateable human hepatocytes obtained from rapid, intermediate and slow acetylators. MDA N-acetylation was catalyzed by both recombinant human NAT1 and NAT2 exhibiting a fivefold higher affinity for human NAT2. N-acetylation of MDA was acetylator genotype dependent as evidenced via its N-acetylation by recombinant human NAT2 genetic variants or by cryoplateable human hepatocytes. MDA N-acetylation to the mono-acetyl or di-acetyl-MDA was highest in rapid, lower in intermediate, and lowest in slow acetylator human hepatocytes. MDA-induced DNA damage in the human hepatocytes was dose-dependent and also acetylator genotype dependent with highest levels of DNA damage in rapid, lower in intermediate, and lowest in slow acetylator human hepatocytes under the same MDA exposure level. In summary, the N-acetylation of MDA by recombinant human NAT2 and cryopreserved human hepatocytes support an important role for the NAT2 genetic polymorphism in modifying MDA metabolism and genotoxicity and potentially carcinogenic risk.

Utility of Serum miR-122, Liver Enzymes, and Hepatic Histopathology in Response to Hepatotoxicants in Sprague-Dawley Rats

More than 80,000 chemicals are in commercial use worldwide. Hepatic metabolism to toxic intermediates is often a key mechanism leading to tissue damage and organ dysfunction. Effective treatment requires prompt detection of hepatotoxicity, ideally with rapid, minimally invasive diagnostic assays. In this study, archetypal histologic features of chemically induced hepatic injury were compared with clinical chemistries (including liver enzymes) and serum concentrations of microRNA-122 (miR-122, the processed form miR-122-5p), a biomarker of liver injury. The hepatotoxicants 4,4'-methylenedianiline (4,4'-MDA), allyl alcohol (AA), or carbon tetrachloride (CCl₄) were orally administered to male Sprague-Dawley rats for 1, 5, 14, or 28 days to induce liver damage. Formalin-fixed, paraffin-embedded liver sections were evaluated histologically for inflammation, fibrosis, necrosis, and lipid accumulation. Liver enzymes were measured in serum, and serum miR-122 concentrations were assessed by quantitative polymerase chain reaction (qPCR). Histologic features of hepatic injury dose-dependently increased in both severity and frequency. Increases in liver enzymes and bilirubin were more pronounced in response to AA or 4,4'-MDA than to CCl₄ at early time points. Elevated serum miR-122 levels in animals administered CCl₄, AA, or 4,4'-MDA were more strongly associated with degree of hepatic histopathology than with dosage. Given this sensitive expression pattern postexposure, liver-specific miR-122 may improve the diagnostic accuracy of early hepatic injury.

Synthesis and characterization of a high temperature thermosetting polyimide oligomer derived from a non-toxic, sustainable bisaniline

A high temperature, hydrophobic thermosetting polyimide oligomer has been prepared from a non-toxic, bio-based bisaniline derived from p-cymene.

Xenobiotic-induced liver injury and fibrosis

INTRODUCTION

Many different drugs and xenobiotics (chemical compounds foreign to an organism) can injure the bile duct epithelium and cause inflammatory bile duct diseases (cholangiopathies) ranging from transient cholestasis to vanishing bile duct syndrome, sclerosing cholangitis with development of biliary fibrosis and cirrhosis. Animal models of xenobiotic-induced liver injury have provided major mechanistic insights into the molecular mechanisms of xenobiotic-induced cholangiopathies and biliary fibrosis including primary biliary cirrhosis and primary sclerosing cholangitis.

AREAS COVERED

In this review, the authors discuss the basic principles of xenobiotic-induced liver and bile duct injury and biliary fibrosis with emphasis on animal models. A PubMed search was performed using the search terms "xenobiotic," "liver injury," "cholestasis," and "biliary fibrosis." Reference lists of retrieved articles were also searched for relevant literature.

EXPERT OPINION

Xenobiotic-induced cholangiopathies are underestimated and frequently overlooked medical conditions due to their often transient nature. However, biliary disease may progress to vanishing bile duct syndrome, biliary fibrosis, and cirrhosis. Moreover, xenobiotics may prime the liver for subsequent liver disease by other agents and may also contribute to the development of hepatobiliary cancer though interaction with resident stem cells.

4,4'-Methylenedianiline-induced hepatitis in an industrial worker: case report and review of the literature

4,4'-Methylenedianiline (MDA) is a chemical used in manufacturing and insulation processes and is a well-known hepatotoxin. We report the case of a 42-year-old construction-site worker who was accidentally exposed to large amounts of MDA and developed acute liver damage. The clinical course is described, with particular emphasis on the timely identification of the underlying cause and prompt management that led to an uneventful recovery. We review the relevant literature and discuss the safety measures necessary to minimize similar occupational hazards in industrial workers.

N-acetyltransferase 2 (Nat2) polymorphism in the sand rat Psammomys obesus

Human arylamine N-acetyltransferase 1 (NAT1) and its homologue in rodents (Nat2) are polymorphic xenobiotic metabolizing enzymes and also seem to play a role in endogenous metabolism. NAT1 and Nat2 polymorphism was associated to cancers under xenobiotic procarcinogens metabolism as well as under endogenous substrate metabolism. This study investigated the p-aminobenzoic acid (PABA) -Nat2 catalytic activity and its polymorphism in liver homogenates of adult sand rats Psammomys obesus Cretzschmar, 1828. These Saharian sand rats develop high incidence of spontaneous cancers under standard laboratory diet. The average value of PABA-Nat2 specific activity tested in nine sand rats was significant (2.96 ± 2.16 nmoles/min/mg). The N-acetylation exhibited a bimodal distribution. There was a significant difference (p<0.01) between PABA-Nat2 activity in the fast acetylators group (4.10 ± 1.67 nmol/min/mg) and slow acetylators group (0.7 ± 0.27 nmol/min/mg). The percentage of the fast acetylator group was 66.66%. These results support the presence of Nat2 polymorphism in the liver of the strain sand rats Psammomys obesus. This strain is useful for investigating the role of Nat2 polymorphisms in susceptibility to cancers related to arylamine carcinogen exposures as well as to endogenous substrate metabolism.

Analysis of Gene Expression in 4,4'-Methylenedianiline-induced Acute Hepatotoxicity

4,4′-Methylenedianiline (MDA) is an aromatic amine that is widely used in the industrial synthetic process. Genotoxic MDA forms DNA adducts in the liver and is known to induce liver damage in human and rats. To elucidate the molecular mechanisms associated with MDA-induced hepatotoxicity, we have identified genes differentially expressed by microarray approach. BALB/c male mice were treated once daily with MDA (20 mg/kg) up to 7 days via intraperitoneal injection (i.p.) and hepatic damages were revealed by histopathological observation and elevation of serum marker enzymes such as AST, ALT, ALP, cholesterol, DBIL, and TBIL. Microarray analysis showed that 952 genes were differentially expressed in the liver of MDA-treated mice and their biological functions and canonical pathways were further analyzed using Ingenuity Pathways Analysis (IPA). Toxicological functional analysis showed that genes related to hepatotoxicity such hyperplasia/hyperproliferation (Timpl), necrosis/cell death (Cd14, Mt1f, Timpl, and Pmaipl), hemorrhaging (Mt1f), cholestasis (Akr1c3, Hpx, and Slc10a2), and inflammation (Cd14 and Hpx) were differentially expressed in MDA-treated group. This gene expression profiling should be useful for elucidating the genetic events associated with aromatic amine-induced hepatotoxicity and for discovering the potential biomarkers for hepatotoxicity.

Update on the pharmacogenetics of NATs: structural considerations

The arylamine N-acetyltransferase (NAT) genes encode enzymes that catalyze the N-acetylation of aromatic amines and hydrazines and the O-acetylation of heterocyclic amines. These genes, which play a key role in cellular homeostasis as well as in gene-environment interactions, are subject to marked pharmacogenetic variation, and different combinations of SNPs in the human NAT genes lead to different acetylation phenotypes. Our understanding of the consequences of pharmacogenetic variability in NATs has recently been enhanced by structural studies showing that effects on protein folding, aggregation and turnover, as well as direct changes in active site topology, are involved. These developments pave the way for a better understanding of the role played by NATs in maintaining cellular homeostasis. In addition, the NATs represent a model for studying fundamental processes associated with protein folding and pharmacogenomic effects mediated by inheritance in human populations across a polymorphic region of the genome.

Genetic polymorphism in N-Acetyltransferase (NAT): Population distribution of NAT1 and NAT2 activity

N-Acetyltransferases (NAT) are key enzymes in the conjugation of certain drugs and other xenobiotics with an arylamine structure. Polymorphisms in NAT2 have long been recognized to modulate toxicity produced by the anti-tubercular drug isoniazid, with molecular epidemiologic studies suggesting a link between acetylator phenotype and increased risk for bladder cancer. Recent evidence indicates that the other major NAT isozyme, NAT1, is also polymorphic. The current analysis characterizes the main polymorphisms in both NAT2 and NAT1 in terms of their effect on enzyme activity and frequency in the population. Multiple NAT2 alleles (NAT2*5, *6, *7, and *14) have substantially decreased acetylation activity and are common in Caucasians and populations of African descent. In these groups, most individuals carry at least one copy of a slow acetylator allele, and less than 10% are homozygous for the wild type (fast acetylator) trait. Incorporation of these data into a Monte Carlo modeling framework led to a population distribution of NAT2 activity that was bimodal and associated with considerable variability in each population assessed. The ratio of the median to the first percentile of NAT2 activity ranged from 7 in Caucasians to 18 in the Chinese population. This variability indicates the need for more quantitative approaches (e.g., physiologically based pharmacokinetic [PBPK] modeling) to assess the full distribution of internal dose and adverse responses to aromatic amines and other NAT2 substrates. Polymorphisms in NAT1 are generally associated with relatively minor effects on acetylation function, with Monte Carlo analysis indicating less interindividual variability than seen in NAT2 analysis.

Systemic functional expression of N-acetyltransferase polymorphism in the F344 Nat2 congenic rat

Rat lines congenic for the rat N-acetyltransferase 2 [(RAT)Nat2] gene were constructed and characterized. F344 (homozygous Nat2 rapid) males were mated to Wistar Kyoto (homozygous Nat2 slow) females to produce heterozygous F1. F1 females were then backcrossed to F344 males. Heterozygous acetylator female progeny from this and each successive backcross were identified by rat Nat2 genotyping and mated with F344 rapid acetylator males. After 10 generations of backcross mating, heterozygous acetylator brother/sister progeny were mated to produce the homozygous rapid and slow acetylator Nat2 congenic rat lines. p-Aminobenzoic acid (selective for rat NAT2) and 4-aminobiphenyl N-acetyltransferase activities were expressed in all tissues examined (liver, lung, esophagus, stomach, small intestine, colon, pancreas, kidney, skin, leukocytes, and urinary bladder in male and female rats and in breast of female and prostate of male rats). NAT2 expression in rat extrahepatic tissues was much higher than that in liver. In each tissue, activities were Nat2-genotype-dependent, with the highest levels in homozygous rapid acetylators, intermediate levels in heterozygous acetylators, and lowest in homozygous slow acetylators. Sulfamethazine (selective for rat NAT1) N-acetyltransferase activities were observed in all tissues examined in both male and female rats except for breast (females), bladder, and leukocytes. In each tissue, the activity was Nat2 genotype-independent, with similar levels in homozygous rapid, heterozygous, and homozygous slow acetylators. These congenic rat lines are useful for investigating the role of NAT2 genetic polymorphisms in susceptibility to cancers related to arylamine carcinogen exposures.

Identification and Characterization of Functional Rat Arylamine N-Acetyltransferase 3: Comparisons with Rat Arylamine N-Acetyltransferases 1 and 2

Arylamine N-acetyltransferases (NATs; EC 2.3.1.5) catalyze both the N-acetylation and O-acetylation of arylamines and N-hydroxyarylamines. Humans possess two functional N-acetyltransferase genes, NAT1 and NAT2, as well as a nonfunctional pseudogene, NATP. Previous studies have identified Nat1 and Nat2 genes in the rat. In this study, we identified and characterized a third rat N-acetyltransferase gene (Nat3) consisting of a single open reading frame of 870 base pairs encoding a 290-amino acid protein, analogous to the previously identified human and rat N-acetyltransferase genes. Rat Nat3 nucleotide sequence was 77.2 and 75.9% identical to human NAT1 and NAT2, respectively. Rat Nat3 amino acid sequence was 68.6 and 67.2% identical to human NAT1 and NAT2, respectively. Rat Nat1, Nat2, and Nat3 were each cloned and recombinantly expressed in Escherichia coli. Recombinant rat Nat3 exhibited thermostability intermediate between recombinant rat Nat1 and Nat2. Recombinant rat Nat3 was functional and catalyzed the N-acetylation of several arylamine substrates, including 3-ethylaniline, 3,5-dimethylaniline, 5-aminosalicylic acid, 4-aminobiphenyl, 4,4'-methylenedianiline, 4,4'-methylenebis(2-chloroaniline), and 2-aminofluorene, and the O-acetylation of N-hydroxy-4-aminobiphenyl. The relative affinities of arylamine carcinogens such as 4-aminobiphenyl, N-hydroxy-4-aminobiphenyl, and 2-aminofluorene for N- and O-acetylation via recombinant rat Nat3 were comparable with recombinant rat Nat1 and higher than for recombinant rat Nat2. This study is the first to report a third arylamine N-acetyltransferase isozyme with significant functional capacity.

N-acetyltransferase 2 genetic polymorphism: effects of carcinogen and haplotype on urinary bladder cancer risk

A role for the N-acetyltransferase 2 (NAT2) genetic polymorphism in cancer risk has been the subject of numerous studies. Although comprehensive reviews of the NAT2 acetylation polymorphism have been published elsewhere, the objective of this paper is to briefly highlight some important features of the NAT2 acetylation polymorphism that are not universally accepted to better understand the role of NAT2 polymorphism in carcinogenic risk assessment. NAT2 slow acetylator phenotype(s) infer a consistent and robust increase in urinary bladder cancer risk following exposures to aromatic amine carcinogens. However, identification of specific carcinogens is important as the effect of NAT2 polymorphism on urinary bladder cancer differs dramatically between monoarylamines and diarylamines. Misclassifications of carcinogen exposure and NAT2 genotype/phenotype confound evidence for a real biological effect. Functional understanding of the effects of NAT2 genetic polymorphisms on metabolism and genotoxicity, tissue-specific expression and the elucidation of the molecular mechanisms responsible are critical for the interpretation of previous and future human molecular epidemiology investigations into the role of NAT2 polymorphism on cancer risk. Although associations have been reported for various cancers, this paper focuses on urinary bladder cancer, a cancer in which a role for NAT2 polymorphism was first proposed and for which evidence is accumulating that the effect is biologically significant with important public health implications.