RNA Adduction Resulting from the Metabolic Activation of Myristicin by P450 Enzymes and Sulfotransferases.

Myristicin (MYR) mainly occurs in nutmeg and belongs to alkoxy-substituted allylbenzenes, a class of potentially toxic natural chemicals. RNA interaction with MYR metabolites in vitro and in vivo has been investigated in order to gain a better understanding of MYR toxicities. We detected two guanosine adducts (GA1 and GA2), two adenosine adducts (AA1 and AA2), and two cytosine adducts (CA1 and CA2) by LC-MS/MS analysis of total RNA extracts from cultured primary mouse hepatocytes and liver tissues of mice after exposure to MYR. An order of nucleoside adductions was found to be GAs > AAs > CAs, and the result of density functional theory calculations was in agreement with that detected by the LC-MS/MS-based approach. In vitro and in vivo studies have shown that MYR was oxidized by cytochrome P450 enzymes to 1'-hydroxyl and 3'-hydroxyl metabolites, which were then sulfated by sulfotransferases (SULTs) to form sulfate esters. The resulting sulfates would react with the nucleosides by SN1 and/or SN2 reactions, resulting in RNA adduction. The modification may alter the biochemical properties of RNA and disrupt RNA functions, perhaps partially contributing to the toxicities of MYR.

Identification of an Epoxide Metabolite of Amitriptyline In Vitro and In Vivo.

Amitriptyline (ATL), a tricyclic antidepressant, has been reported to cause various adverse effects, particularly hepatotoxicity. The mechanisms of ATL-induced hepatotoxicity remain unknown. The study was performed to identify the olefin epoxidation metabolite of ATL and determine the possible toxicity mechanism. Two glutathione (GSH) conjugates (M1 and M2) and two N-acetylcysteine (NAC) conjugates (M3 and M4) were detected in rat liver microsomal incubations supplemented with GSH and NAC, respectively. Moreover, M1/M2 and M3/M4 were respectively found in ATL-treated rat primary hepatocytes and in bile and urine of rats given ATL. Recombinant P450 enzyme incubations demonstrated that CYP3A4 was the primary enzyme involved in the olefin epoxidation of ATL. Treatment of hepatocytes with ATL resulted in significant cell death. Inhibition of CYP3A attenuated the susceptibility to the observed cytotoxicity of ATL. The metabolic activation of ATL most likely participates in the cytotoxicity of ATL.

Metabolic Activation and Cytotoxicity of Gramine Mediated by CYP3A in Rats.

Gramine (GRM), which occurs in Gramineae plants, has been developed to be a biological insecticide. Exposure to GRM was reported to induce elevations of serum ALT and AST in rats, but the mechanisms of the observed hepatotoxicity have not been elucidated. The present study aimed to identify reactive metabolites that potentially participate in the toxicity. In rat liver microsomal incubations fortified with glutathione or N-acetylcysteine, one oxidative metabolite (M1), one glutathione conjugate (M2), and one N-acetylcysteine conjugate (M3) were detected after exposure to GRM. The corresponding conjugates were detected in the bile and urine of rats after GRM administration. CYP3A was the main enzyme mediating the metabolic activation of GRM. The detected GSH and NAC conjugates suggest that GRM was metabolized to a quinone imine intermediate. Both GRM and M1 showed significant toxicity to rat primary hepatocytes.

Dihydrotanshinone I-Induced CYP1 Enzyme Inhibition and Alteration of Estradiol Metabolism.

Dihydrotanshinone I (DHTI) is a pharmacologically active component occurring in the roots of the herbal medicine Salvia miltiorrhiza Bunge. This study investigated DHTI-induced inhibition of CYP1A1, CYP1A2, and CYP1B1 with the aim to determine the potential effects of DHTI on the bioactivation of estradiol (E2), possibly related to preventive/therapeutic strategy for E2-associated breast cancer. Ethoxyresorufin as a specific substrate for CYP1s was incubated with human recombinant CYP1A1, CYP1A2, or CYP1B1 in the presence of DHTI at various concentrations. Enzymatic inhibition and kinetic behaviors were examined by monitoring the formation of the corresponding product. Molecular docking was further conducted to define the interactions between DHTI and the three CYP1s. The same method and procedure were employed to examine the DHTI-induced alteration of E2 metabolism. DHTI showed significant inhibition of ethoxyresorufin O-deethylation activity catalyzed by CYP1A1, CYP1A2 and CYP1B1 in a concentration-dependent manner (IC50 = 0.56, 0.44, and 0.11 µM, respectively). Kinetic analysis showed that DHTI acted as a competitive type of inhibitor of CYP1A1 and CYP1B1, whereas it noncompetitively inhibited CYP1A2. The observed enzyme inhibition was independent of NADPH and time. Molecular docking analysis revealed hydrogen bonding interactions between DHTI and Asp-326 of CYP1B1. Moreover, DHTI displayed preferential activity to inhibit 4-hydroxylation of E2 (a genotoxic pathway) mediated by CYP1B1. Exposure to DHTI could reduce the risk of genotoxicity induced by E2. SIGNIFICANCE STATEMENT: CYP1A1, CYP1A2, and CYP1B1 enzymes are involved in the conversion of estradiol (E2) into 2-hydroxyestradiol (2-OHE2) and 4-hydroxyestradiol (4-OHE2) through oxidation. 2-OHE2 is negatively correlated with breast cancer risk, and 4-OHE2 may be a significant initiator and promoter of breast cancer. The present study revealed that dihydrotanshinone I (DHTI) competitively inhibits CYP1A1/CYP1B1 and noncompetitively inhibits CYP1A2. DHTI exhibits a preference for inhibiting the genotoxicity associated with E2 4-hydroxylation pathway mediated by CYP1B1, potentially reducing the risk of 4-OHE2-induced genotoxicity.

Difference in hepatotoxicity of furan-containing components in cortex dictamni correlates the efficiency of their metabolic activation.

BACKGROUND: Cortex Dictamni (CD) has been associated with an increased risk of liver injury, which may be attributable to the metabolic activation of its furan-containing components (FCC). However, the hepatotoxic potencies of these FCCs and the mechanisms behind the differences in their toxicity intensity remain unknown. METHODS: The constituents of CD extract were determined by LC-MS/MS. Potentially toxic FCCs were screened by a previously published method. Hepatotoxicity of potentially toxic FCCs was evaluated in cultured mouse primary hepatocytes and mice. The ability to deplete hepatic glutathione (GSH), along with the formation of the corresponding GSH conjugates, resulting from the metabolic activation was determined ex vivo in mice. Intrinsic clearance rates (CLint,Vmax/Km) were assessed by a microsome-bases assay. RESULTS: A total of 18 FCCs were detected in CD extract. Among them, four FCCs, including rutaevin (RUT), limonin (LIM), obacunone (OBA) and fraxinellone (FRA) were found to be bioactivated in microsomal incubations. Only FRA displayed significant hepatotoxicity in vitro and in vivo. Similarly, FRA caused GSH depletion and GSH conjugation the most in vivo. The order of CLint for the four FCCs was FRA>>OBA>LIM>RUT. CONCLUSION: FRA is the major toxic FCC component of hepatotoxic CD extract. The hepatotoxicity of FCCs is closely related to the efficiency of their metabolic activation.

Immunochemical and LC-MS/MS Detection of Protein Adduction Resulting from Metabolic Activation of Columbin In Vitro and In Vivo.

Columbin (CLB) is a diterpenoid furanolactone compound occurring in some herbal medicines. Administration of CLB has been reported to induce liver injury. The reported CLB hepatotoxicity is suggested to require metabolism to a cis-enedial intermediate. We successfully detected hepatic protein adduction resulting from the metabolic activation of CLB and found that the intermediate reacted with lysine residues or lysine/cysteine residues to produce the corresponding pyrroline derivative or pyrrole derivative. The detection was achieved by proteolysis- and liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based methods. Furthermore, we prepared a polyclonal antibody approach which allowed us to detect the protein adduction in the forms of protein immunoblot as well as tissue- and cell-based immunostaining. The antibody technique verified the protein adduction detected by LC-MS/MS.

Mechanistic Study of Xanthotoxin-Mediated Inactivation of CYP1A2 and Related Drug-Drug Interaction with Tacrine.

Xanthotoxin (XTT) is a biologically active furanocoumarin widely present in foods and plants. The present study is designed to systematically investigate the enzymatic interaction of XTT with CYP1A2, along with pharmacokinetic alteration of tacrine resulting from the co-administration of XTT. The results showed that XTT induced a time-, concentration-, and NADPH-dependent inhibition of CYP1A2, and the inhibition was irreversible. Co-incubation of glutathione (GSH) and catalase/superoxide dismutase was unable to prevent enzyme inactivation. Nevertheless, competitive inhibitor fluvoxamine exhibited a concentration-dependent protective effect against the XTT-induced CYP1A2 inactivation. A GSH trapping experiment provided strong evidence for the production of epoxide or/and γ-ketoenal intermediates resulting from the metabolic activation of XTT. Furthermore, pretreatment of rats with XTT was found to significantly increase the Cmax and area under the curve of plasma tacrine relative to those of tacrine administration alone.

DNA damage and up-regulation of PARP-1 induced by columbin in vitro and in vivo.

Columbin (CLB) is the most abundant (>1.0%) furan-containing diterpenoid lactone in herbal medicine Tinospora sagittate (Oliv.) Gagnep. The furano-terpenoid was found to be hepatotoxic, but the exact mechanisms remain unknown. The present study demonstrated that administration of CLB at 50 mg/kg induced hepatotoxicity, DNA damage and up-regulation of PARP-1 in vivo. Exposure to CLB (10 µM) induced GSH depletion, over-production of ROS, DNA damage, up-regulation of PARP-1 and cell death in cultured mouse primary hepatocytes in vitro. Co-treatment of mouse primary hepatocytes with ketoconazole (10 µM) or glutathione ethyl ester (200 µM) attenuated the GSH depletion, over-production of ROS, DNA damage, up-regulation of PARP-1, and cell death induced by CLB, while co-exposure to L-buthionine sulfoximine (BSO, 1000 µM) intensified such adverse effects resulting from CLB exposure. These results suggest that the metabolic activation of CLB by CYP3A resulted in the depletion of GSH and increase of ROS formation. The resultant over-production of ROS subsequently disrupted the DNA integrity and up-regulated the expression of PARP-1 in response to DNA damage, and ROS-induced DNA damage was involved in the hepatotoxicity of CLB.

Formation of RNA adducts resulting from metabolic activation of spice ingredient safrole mediated by P450 enzymes and sulfotransferases.

Safrole (SFL) is an IARC class 2B carcinogen. To better understand the mechanism involved in SFL toxicity, we explored the potential interactions between SFL metabolites and RNA. Three guanosine adducts (G1-G3), two adenosine adducts (A1-A2), and two cytosine adducts (C1-C2) were detected by LC-MS/MS in mouse liver S9 incubations, cultured mouse primary hepatocytes, and liver tissues of mice after exposure to SFL. These adducts were chemically synthesized, and one of the guanosine adducts was structurally characterized by 1H-NMR. Studies in vitro and in vivo showed that SFL was oxidized by cytochrome P450 enzymes to the corresponding 1'-hydroxyl metabolite which was further metabolized by sulfotransferases to form allylic sulfate esters. The formed reactive intermediate(s) subsequently reacted with bases of RNA, leading to RNA adduction, which could play a partial role in the toxicities of SFL through the alteration of RNA biochemical properties and interruption of RNA functions.

DNA damage by reactive oxygen species resulting from metabolic activation of 8-epidiosbulbin E acetate in vitro and in vivo.

8-Epidiosbulbin E acetate (EEA), a furan-containing diterpenoid lactone, is one of main component of Dioscorea bulbifera L. (DBL). It has been reported that EEA induces severe hepatotoxicity in mice and that its hepatotoxicity is associated with metabolic activation. The present study demonstrated that exposure to EEA (50, 100 or 200 µM) induced DNA damage, including significant DNA fragmentation, increases of tail DNA and olive tail moment, H2AX phosphorylation and PARP-1 activation, in cultured mouse primary hepatocytes. Similar observation was obtained in mice administered EEA at 50, 100 or 200 mg/kg. Pre-treatment with 10 µM ketoconazole (KTC), 200 µM vitamin C (VC), or 200 µM glutathione ethyl ester (GSH-OEt) reversed the over-production of reactive oxygen species (ROS) induced by EEA and attenuated susceptibility of hepatocytes to EEA-induced cytotoxicity and DNA damage in mouse primary hepatocytes. In contrast, pre-treatment with 1.0 mM L-buthionine sulfoximine (BSO) potentiated over-production of ROS, cytotoxicity and DNA damage induced by EEA. In summary, EEA induced DNA damage in cultured primary hepatocytes and the liver of mice. ROS, possibly along with DNA alkylation, participated in the observed DNA damage.

Metabolic activation of 3-aminodibenzofuran mediated by P450 enzymes and sulfotransferases.

3-Aminodibenzofuran (3-ADBF) is a potent bladder carcinogen. This study aimed to identify reactive metabolites and the metabolic pathways of 3-ADBF. The in vitro and in vivo studies demonstrated that 3-ADBF was oxidized to the corresponding hydroxylamine by cytochrome P450 enzymes, followed by sulfation of the hydroxyl group mediated by sulfotransferases. The resulting sulfate conjugate was chemically reactive to GSH and cysteine residues of hepatic protein to form the corresponding GSH conjugate and protein adduction. Exposure of 3-ADBF to primary hepatocytes caused protein covalent binding and decreased cell viability. The resultant protein adduction was found to correlate the observed cytotoxicity of 3-ADBF.