Acetaminophen heptotoxicity and transplacental carcinogenesis. Humanized model to study acetaminophen-induced hepatotoxicity: Acetaminophen (APAP), is the most common nonprescription analgesic widely used for pain relief and antipyresis. However, APAP has also been extensively characterized as a representative compound that causes liver and kidney toxicities upon overdose. The reactive metabolite, N-acetyl-p-benzoquinone imine (NAPQ1), is generated from APAP oxidized by hepatic cytochromes P450, which triggers hepatic toxicity through mitochondria injury, apoptosis and cell necrosis possibly through covalently binding with nucleophilic macromolecules such as DNA or proteins and/or by elevating oxidative stress. Multiple CYPs can convert APAP to NAPQ1. Compared to comprehensive investigations of the effects of CYP2E1 and CYP1A2 on APAP toxicity have been performed, the influence of CYP3A4 on APAP metabolism has remained largely unstudied. Since CYP3A4 can represent up to 60% of the total hepatic CYP content and is responsible for hepatic metabolism of 50% of drugs, its role in APAP metabolism and toxicity in humans could be significant. CYP3A4 is under control of the pregnane X receptor (PXR). Upon ligand binding, and formation of a heterodimer with retinoid X receptor, human PXR activates genes encoding CYP3As, drug transporters and other xenobiotic-metabolizing enzymes through binding to PXR response elements usually located in the 5'-flanking region of target genes. Murine PXR plays a critical role in APAP-induced hepatic toxicity, probably through inducing Cyp3a11 expression and a reduction of APAP hepatotoxicity was found in Pxr-null. However, due to species difference between human PXR and rodent PXR, rodent models cannot accurately predict inducers and potential drug-drug interactions mediated by human PXR because of different responses to PXR ligands. Therefore, it is necessary to investigate the potential roles of CYP3A4 and human PXR on APAP-induced toxicity. The relationship between human PXR-regulated CYP3A4 function and risk of APAP-induced hepatotoxicity, was examined using the human PXR and CYP3A4 double transgenic (TgCYP3A4/hPXR) mouse line and rifampicin, a bactericidal antibiotic drug which is an effective ligand and agonist of hPXR. Human PXR activation and CYP3A4 induction enhanced APAP-induced hepatotoxicity as revealed by hepatic alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities elevated in serum, and hepatic necrosis after coadministration of rifampicin and APAP, compared with APAP administration alone. In contrast, hPXR mice, wild-type mice, and Pxr-null mice exhibited significantly lower ALT/AST levels compared with TgCYP3A4/hPXR mice after APAP administration. Toxicity was coincident with depletion of hepatic glutathione and increased production of hydrogen peroxide, suggesting increased oxidative stress upon hPXR activation. Moreover, mRNA analysis demonstrated that CYP3A4 and other PXR target genes were significantly induced by rifampicin treatment. Urinary metabolomic analysis indicated that cysteine-APAP and its metabolite were the major contributors to the toxic phenotype. Plasma APAP metabolites indicated that the APAP dimer formed coincident with increased oxidative stress. These findings demonstrated that human PXR is involved in regulation of APAP-induced toxicity through CYP3A4-mediated hepatic metabolism of APAP in the presence of PXR ligands. Mechanism of APAP-induced hepatotoxicity: Metabolic bioactivation, glutathione depletion, and covalent binding are the early hallmark events after APAP overdose. However, the subsequent metabolic consequences contributing to APAP-induced hepatic necrosis and apoptosis have not been fully elucidated. In this study, serum metabolomes of control and APAP-treated wild-type and Cyp2e1-null mice were examined by liquid chromatography-mass spectrometry and multivariate data analysis. A dose-response study showed that the accumulation of long-chain acylcarnitines in serum contributes to the separation of wild-type mice undergoing APAP-induced hepatotoxicity from other mouse groups in a multivariate model. This observation, in conjunction with the increase of triglycerides and free fatty acids in the serum of APAP-treated wild-type mice, suggested that APAP treatment can disrupt fatty acid beta-oxidation. Both wild-type and Cyp2e1-null mice had serum acylcarnitine levels markedly elevated within the early hours of APAP treatment. While remaining high in wild-type mice, serum acylcarnitine levels gradually returned to normal in null mice. Distinct from serum aminotransferase activity and hepatic glutathione levels, the pattern of serum acylcarnitine accumulation suggested that acylcarnitines can function as complementary biomarkers for monitoring the APAP-induced hepatotoxicity. An essential role for peroxisome proliferator-activated receptor alpha (PPARalpha) in the regulation of serum acylcarnitine levels was established by comparing the metabolomic responses of wild-type and Ppara-null mice to a fasting challenge. The upregulation of PPARalpha activity following APAP treatment was transient in wild-type mice but was much more prolonged in Cyp2e1-null mice. Metabolomics revealed that the CYP2E1-mediated metabolic activation and oxidative stress following APAP treatment can cause irreversible inhibition of fatty acid oxidation, potentially through suppression of PPARalpha-regulated pathways. Fetal mouse Cyp1b1 and transplacental carcinogenesis from maternal exposure to dibenzo(a,l)pyrene (DBP): The fetus and infant are at increased risk, relative to adults, upon exposure to many environmental chemicals. Yet, only exposures to ionizing radiation and diethylstilbesterol to pregnant women have been sufficiently well documented as causing cancer in their children. However, an increasing number of carcinogens have been demonstrated to be effective transplacentally in animal models. Human epidemiological studies correlated maternal chemical exposure with cancer in children, thus indicating the importance of understanding the mechanism(s) of transplacental carcinogenesis. A mouse model of transplacental carcinogenesis in which maternal exposure to DBP resultes in high mortality in offspring at a relatively young age from a T-cell lymphoma. At 10 months, the offspring exhibited a 100% incidence of lung adenomas and carcinomas and most of the males had liver lesions as well. Mouse Cyp1b1 and human CYP1B1 have the highest activity toward conversion of DBP to DBPDE, a fjord region diol-epoxide thought to be responsible for the high mutagenic and carcinogenic potency of DBP. In vivo evidence also shows that disruption of the Cyp1b1 gene protected adult mice from DBP-induced cancer. DBP administration to pregnant mice resulted in high mortality of offspring from aggressive T-cell lymphoma. Mice heterozygous for the disrupted Cyp1b1 allele were used to examine the effect of Cyp1b1 gene dosage on DBP transplacental carcinogenesis. Dams were treated with DBP or benzo(a)pyrene. Importantly, Cyp1b1-null offspring did not develop lymphoma, whereas wild-type and heterozygous siblings, born to dams given high doses of DBP, exhibited significant mortalities between 10 and 30 weeks of age. At 10 months, all groups had lung adenomas or carcinomas and Cyp1b1 status did not alter benzo(a)pyrene-dependent carcinogenesis. At low DBP doses, Cyp1b1 status altered the incidence of lung tumors for Cyp1b1-nulls, heterozygous, and wild-type, resp [summary truncated at 7800 characters]
Showing the most recent 10 out of 187 publications
