Research progress on metabolic activation of indirect carcinogens by CYP1A1 / 中国职业医学

Cytochrome P450 enzyme 1A1 (CYP 1A1) is one of the main members of CYP1A subfamily, which participates in metabolizing and activating a variety of indirect carcinogens. CYP1A1 can induce carcinogenesis by participating in activating exogenous compounds to produce intermediates or active metabolites that bind to specific biomolecules. CYP1A1 plays a critical role in the metabolic activation of benzo(a)pyrene e ［B(a)P］, and plays a key role in activating the toxic and carcinogenic effects of B(a)P. CYP1A1 involves in the metabolic activation of 7,12-dimethyl benzanthracene and 2-amino-1-methyl-6-phenylimidazo［4,5-b］pyridine (PhIP), and plays an important role in PhIP-induced genotoxicity. CYP1A1 is the main enzyme to metabolize and activate 7H-dibenzo［c,g］carbazole (DBC), a key factor in the carcinogenic effect of DBC. CYP1A1 is also associated with metabolic activation of indirect carcinogens such as aflatoxin B1, 3-nitrobenzene, and naphthalene. Inhibition of the catalytic activity of CYP1A1 can decrease the CYP1A1-mediated activity of carcinogens, thus playing a role in the prevention and treatment of malignant tumors.

The protective effect of ritonavir against Gynura japonica-induced liver injury in rats / 药学学报

Numerous in vitro studies have shown that most pyrrolizidine alkaloids (PAs) are hepatotoxic after being metabolically activated by cytochrome P450 (CYP) 3A4. However, the key role of CYP3A4 has not been confirmed in vivo. Therefore, the CYP3A4 chemical inhibitor ritonavir was employed in this work and the effect of ritonavir on Gynura japonica-induced liver injury in rats was investigated. All experiments were approved by the Animal Research Committee of Shanghai University of Traditional Chinese Medicine. Animal welfare and the animal experimental protocols were strictly consistent with related ethics regulations of Shanghai University of Traditional Chinese Medicine. Acute liver injury was induced by a single gavage of Gynura japonica extracts (GJE, 8 g·kg-1); rats in the protection group were gavaged with ritonavir (RIT, 30 mg·kg-1) 1 h before GJE treatment. The results show that RIT could significantly attenuate GJE-induced liver injury in rats. Rats in the protection group showed decreased serum activities for alanine aminotransferase and aspartate aminotransferase, as well as lower total bile acids. In addition, the infiltration of inflammatory cells, sinusoidal hemorrhage, and hepatic necrosis in GJE-treated rats were markedly attenuated in the protection group. The content of pyrrole-protein adducts (PPAs), a recommended biomarker for PA-induced hepatotoxicity in clinics, was determined at 10 min to 24 h after GJE treatment. The content of 13 bile acids was also quantified. RIT treatment reduced the content of PPAs in serum dramatically and restored the impaired bile acid homeostasis caused by GJE. These studies indicate that RIT attenuated Gynura japonica-induced liver injury in rats, which was closely related to the inhibition of the metabolic activation of PAs and the regulation of bile acid metabolism. These results provide a better understanding of the relationship between CYP3A4 and PA-induced toxicity. This work will also be helpful in developing effective treatments for PA-induced liver injury and making a reasonable evaluation of the safety of drugs containing PAs in clinic.

An important mechanism of herb-induced hepatotoxicity: To produce RMs based on active functional groups-containing ingredients from phytomedicine by binding CYP450s / 中草药·英文版

Reactive metabolites (RMs) generated by hepatic metabolism are thought to play an important role in the pathogenesis of drug-induced liver injury (DILI). Like many synthetic drugs undergoing metabolic activation to form RMs which are often associated with drug toxicity, it is recognized that some herbal components may be also converted to toxic, or even mutagenetic and carcinogenic metabolites by cytochrome P450s (CYP450s). This review focuses on the metabolic activation of herbal components and its liver toxicological implications. By summarizing references, we found that hepatotoxic herbal components via producing RMs have some certain structural dependence. There is a correlation between the generation of RMs and the structures, which provides a good chance for the early discovery of toxic ingredients in Traditional Chinese medicines (TCMs): i) A potential hepatotoxic component information database based on active functional groups can be built, which might provide an early information for the basic research of hepatotoxic substances in TCMs; ii) RMs can combine with CYP450s to form a complete antigen, which eventually leads to an antigen-specific immune response. RMs-CYP450 protein complete antigen can be set up, and the potential idiosyncratic liver toxicity might be predicted by testing RMs-CYP450 protein antibody in plasma.

Protective Effects of Diallyl Sulfide against Thioacetamide-Induced Toxicity: A Possible Role of Cytochrome P450 2E1

Effects of diallyl sulfide (DAS) on thioacetamide-induced hepatotoxicity and immunotoxicity were investigated. When male Sprague-Dawley rats were treated orally with 100, 200 and 400 mg/kg of DAS in corn oil for three consecutive days, the activity of cytochrome P450 (CYP) 2E1-selective p-nitrophenol hydroxylase was dose-dependently suppressed. In addition, the activities of CYP 2B-selective benzyloxyresorufin O-debenzylase and pentoxyresorufin O-depentylase were significantly induced by the treatment with DAS. Western immunoblotting analyses also indicated the suppression of CYP 2E1 protein and/or the induction of CYP 2B protein by DAS. To investigate a possible role of metabolic activation by CYP enzymes in thioacetamide-induced hepatotoxicity, rats were pre-treated with 400 mg/kg of DAS for 3 days, followed by a single intraperitoneal treatment with 100 and 200 mg/kg of thioacetamide in saline for 24 hr. The activities of serum alanine aminotransferase and aspartate aminotransferase significantly elevated by thioacetamide were protected in DAS-pretreated animals. Likewise, the suppressed antibody response to sheep erythrocytes by thioacetamide was protected by DAS pretreatment in female BALB/c mice. Taken together, our present results indicated that thioacetamide might be activated to its toxic metabolite(s) by CYP 2E1, not by CYP 2B, in rats and mice.

Possible Mechanisms for Induction of Oxidative Stress and Suppression of Systemic Nitric Oxide Production Caused by Exposure to Environmental Chemicals

The cytotoxic effects evoked by exposure to environmental chemicals having electrophilic properties are often attributable to covalent attachment to intracellular macromolecules through sulfhydryl groups or enzyme-mediated redox cycling, leading to the generation of reactive oxygen species (ROS). When huge amounts of ROS form they overwhelm antioxidant defenses resulting in the induction of oxidative stress. Nitric oxide (NO) which plays a crucial role in vascular tone, is formed by endothelial NO synthase (eNOS). Since a decrease in systemic NO production is implicated in the pathophysiological actions of vascular diseases, dysfunction of eNOS by environmental chemicals is associated with cardiopulmonary-related diseases and mortality. In this review, we introduce the mechanism-based toxicities (covalent attachment and redox cycling) of electrophiles. Therefore, this review will focus on the possible mechanisms for the induction of oxidative stress and impairment of NO production caused by environmental chemicals.

Possible mechanisms for induction of oxidative stress and suppression of systemic nitric oxide production caused by exposure to environmental chemicals

The cytotoxic effects evoked by exposure to environmental chemicals having electrophilic properties are often attributable to covalent attachment to intracellular macromolecules through sulfhydryl groups or enzyme-mediated redox cycling, leading to the generation of reactive oxygen species (ROS). When huge amounts of ROS form they overwhelm antioxidant defenses resulting in the induction of oxidative stress. Nitric oxide (NO) which plays a crucial role in vascular tone, is formed by endothelial NO synthase (eNOS). Since a decrease in systemic NO production is implicated in the pathophysiological actions of vascular diseases, dysfunction of eNOS by environmental chemicals is associated with cardiopulmonary-related diseases and mortality. In this review, we introduce the mechanism-based toxicities (covalent attachment and redox cycling) of electrophiles. Therefore, this review will focus on the possible mechanisms for the induction of oxidative stress and impairment of NO production caused by environmental chemicals.

Use Of Metabolically Competent Human Hepatoma Cells of The Detection Of Mutagens And Antimutagens.

The human hepatoma cell line (Hep G2) has retained the activities of various phase I and phase II enzymes which play a crucial role in the activation/detoxification of genotoxic procarcinogens and reflects the metabolism of such compounds in vivo better than experimental models with metabolically incompetent cells and exogenous activation mixtures. In recent years, methodologies have been developed which enable the detection of genotoxic effects in Hep G2 cells. Appropriate endpoints are the induction of 6-TGr mutants, of micronuclei and of comets (single cell electrophoresis assays). It has been demonstrated that various classes of environmental carcinogens, such as nitrosamines, aflatoxins, aromatic and heterocyclic amines and polycyclic aromatic hydrocarbons can be detected in genotoxicity assays with Hep G2 cells. Furthermore, it has been shown that these assays can distinguish between structurally related carcinogens and non-carcinogens, and positive results have been obtained with rodent carcinogens (such as safrol and hexamethylphosphoramide) which give false negative results in conventional in vitro assays with rat liver homogenates. Hep G2 cells have also been used in antimutagenicity studies and can identify mechanisms not detected in conventional in vitro systems such as induction of detoxifying enzymes, inactivation of endogenously formed DNA- reactive metabolites and intracellular inhibition of activating enzymes.