Inhibition of cytochrome P450s enhances (+)-usnic acid cytotoxicity in primary cultured rat hepatocytes

Correlation of Vanillin-Induced Cytotoxicity with CYP3A-Mediated Metabolic Activation

Vanillin (VAN) is a common flavoring agent that can cause liver damage when ingested in large amounts. Nevertheless, the precise processes responsible for its toxicity remain obscure. The present research aimed to examine the metabolic activation of VAN and establish a potential correlation between its reactive metabolites and its cytotoxicity. In rat liver microsomes incubated with VAN, reduced glutathione/N-acetylcysteine (GSH/NAC), and nicotinamide adenine dinucleotide phosphate (NADPH), two conjugates formed from GSH and one conjugate derived from NAC were identified. We also discovered one GSH conjugate in both the bile obtained from rats and the rat primary hepatocytes that were subjected to VAN exposure. Additionally, the NAC conjugate exerted in the urine of VAN-treated rats was observed. These results indicate that a quinone intermediate was produced from VAN both in vitro and in vivo. Next, we identified CYP3A as the main enzyme that initiated the bioactive pathway of VAN. After the activity of CYP3A was selectively inhibited by ketoconazole (KTZ), the generation of the GSH conjugate declined in hepatocytes exposed to VAN. Furthermore, the vulnerability to VAN-induced toxicity was alleviated by KTZ in hepatocytes. Thus, we propose that the cytotoxicity of VAN may derive from metabolic activation triggered by CYP3A.

Oxidative DNA damage contributes to usnic acid-induced toxicity in human induced pluripotent stem cell-derived hepatocytes

Dietary supplements containing usnic acid have been increasingly marketed for weight loss over the past decades, even though incidences of severe hepatotoxicity and acute liver failure due to their overuse have been reported. To date, the toxic mechanism of usnic acid-induced liver injury at the molecular level still remains to be fully elucidated. Here, we conducted a transcriptomic study on usnic acid using a novel in vitro hepatotoxicity model employing human induced pluripotent stem cell (iPSC)-derived hepatocytes. Treatment with 20 μM usnic acid for 24 h caused 4272 differentially expressed genes (DEGs) in the cells. Ingenuity Pathway Analysis (IPA) based on the DEGs and gene set enrichment analysis (GSEA) using the whole transcriptome expression data concordantly revealed several signaling pathways and biological processes that, when taken together, suggest that usnic acid caused oxidative stress and DNA damage in the cells, which further led to cell cycle arrest and eventually resulted in cell death through apoptosis. These transcriptomic findings were subsequently corroborated by a variety of cellular assays, including reactive oxygen species (ROS) generation and glutathione (GSH) depletion, DNA damage (pH2AX detection and 8-hydroxy-2'-deoxyguanosine [8-OH-dg] assay), cell cycle analysis, and caspase 3/7 activity. Collectively, the results of the current study accord with previous in vivo and in vitro findings, provide further evidence that oxidative stress-caused DNA damage contributes to usnic acid-induced hepatotoxicity, shed new light on molecular mechanisms of usnic acid-induced hepatotoxicity, and demonstrate the usefulness of iPSC-derived hepatocytes as an in vitro model for hepatotoxicity testing and prediction.

Hepatotoxicity of usnic acid and underlying mechanisms

Since usnic acid was first isolated in 1844 as a prominent secondary lichen metabolite, it has been used for various purposes worldwide. Usnic acid has been claimed to possess numerous therapeutic properties, including antimicrobial, anti-inflammatory, antiviral, anti-proliferative, and antipyretic activities. Approximately two decades ago, crude extracts of usnic acid or pure usnic acid were marketed in the United States as dietary supplements for aiding in weight loss as a "fat-burner" and gained popularity in the bodybuilding community; however, hepatotoxicity was documented for some usnic acid containing products. The US Food and Drug Administration (FDA) received numerous reports of liver toxicity associated with the use of dietary supplements containing usnic acid, leading the FDA to issue a warning letter in 2001 on a product, LipoKinetix. The FDA also sent a recommendation letter to the manufacturer of LipoKinetix, resulting in the withdrawal of LipoKinetix from the market. These events triggered investigations into the hepatotoxicity of usnic acid and its mechanisms. In 2008, we published a review article titled "Usnic Acid and Usnea Barbata Toxicity". This review is an updated version of our previous review article and incorporates additional data published since 2008. The purpose of this review is to provide a comprehensive summary of the understanding of the liver toxicity associated with usnic acid, with a particular focus on the current understanding of the putative mechanisms of usnic acid-related hepatotoxicity.

In vitro and in vivo metabolic activation and hepatotoxicity of chlorzoxazone mediated by CYP3A

Chlorzoxazone (CZX), a benzoxazolone derivative, has been approved for the treatment of musculoskeletal disorders to relieve localized muscle spasm. However, its idiosyncratic toxicity reported in patients brought attention, particularly for hepatotoxicity. The present study for the first time aimed at the relationship between CZX-induced hepatotoxicity and identification of oxirane intermediate resulting from metabolic activation of CZX. Two N-acetylcysteine (NAC) conjugates (namely M1 and M2) and two glutathione (GSH) conjugates (namely M3 and M4) were detected in rat & human microsomal incubations with CZX (200 μM) fortified with NAC or GSH, respectively. The formation of M1-M4 was NADPH-dependent and these metabolites were also observed in urine or bile of SD rats given CZX intragastrically at 10 mg/kg or 25 mg/kg. NAC was found to attach at C-6' of the benzo group of M1 by sufficient NMR data. CYPs3A4 and 3A5 dominated the metabolic activation of CZX. The two GSH conjugates were also observed in cultured rat primary hepatocytes after exposure to CZX. Inhibition of CYP3A attenuated the susceptibility of hepatocytes to the cytotoxicity of CZX (10-400 μM). The in vitro and in vivo studies provided solid evidence for the formation of oxirane intermediate of CZX. This would facilitate the understanding of the underlying mechanisms of toxic action of CZX.

Metabolism and toxicity of usnic acid and barbatic acid based on microsomes, S9 fraction, and 3T3 fibroblasts in vitro combined with a UPLC-Q-TOF-MS method

Introduction: Usnic acid (UA) and barbatic acid (BA), two typical dibenzofurans and depsides in lichen, have a wide range of pharmacological activities and hepatotoxicity concerns. This study aimed to clarify the metabolic pathway of UA and BA and illuminate the relationship between metabolism and toxicity. Methods: An UPLC-Q-TOF-MS method was developed for metabolite identification of UA and BA in human liver microsomes (HLMs), rat liver microsomes (RLMs), and S9 fraction (RS9). The key metabolic enzymes responsible for UA and BA were identified by enzyme inhibitors combined with recombinant human cytochrome P450 (CYP450) enzymes. The cytotoxicity and metabolic toxicity mechanism of UA and BA were determined by the combination model of human primary hepatocytes and mouse 3T3 fibroblasts. Results: The hydroxylation, methylation, and glucuronidation reactions were involved in the metabolic profiles of UA and BA in RLMs, HLMs, and RS9. CYP2C9, CYP3A4, CYP2C8, and UGT1A1 are key metabolic enzymes responsible for metabolites of UA and CYP2C8, CYP2C9, CYP2C19, CYP1A1, UGT1A1, UGT1A3, UGT1A7, UGT1A8, UGT1A9, and UGT1A10 for metabolites of BA. UA and BA did not display evident cytotoxicity in human primary hepatocytes at concentrations of 0.01-25 and 0.01-100 µM, respectively, but showed potential cytotoxicity to mouse 3T3 fibroblasts with 50% inhibitory concentration values of 7.40 and 60.2 µM. Discussion: In conclusion, the attenuated cytotoxicity of BA is associated with metabolism, and UGTs may be the key metabolic detoxification enzymes. The cytotoxicity of UA may be associated with chronic toxicity. The present results provide important insights into the understanding of the biotransformation behavior and metabolic detoxification of UA and BA.

In Vitro and In Vivo Metabolic Activation of Tolterodine Mediated by CYP3A

Tolterodine (TOL) is an antimuscarinic drug used for the treatment of patients with overactive bladder presenting urinary frequency, urgency, and urge incontinence. During the clinical use of TOL, adverse events such as liver injury took place. The present study aimed at the investigation of the metabolic activation of TOL possibly associated with its hepatotoxicity. One GSH conjugate, two NAC conjugates, and two cysteine conjugates were found in both mouse and human liver microsomal incubations supplemented with TOL, GSH/NAC/cysteine, and NADPH. The detected conjugates suggest the production of a quinone methide intermediate. The same GSH conjugate was also observed in mouse primary hepatocytes and in the bile of rats receiving TOL. One of the urinary NAC conjugates was observed in rats administered TOL. One of the cysteine conjugates was found in a digestion mixture containing hepatic proteins from animals administered TOL. The observed protein modification was dose-dependent. CYP3A primarily catalyzes the metabolic activation of TOL. Ketoconazole (KTC) pretreatment reduced the generation of the GSH conjugate in mouse liver and cultured primary hepatocytes after TOL treatment. In addition, KTC reduced the susceptibility of primary hepatocytes to TOL cytotoxicity. The quinone methide metabolite may be involved in TOL-induced hepatotoxicity and cytotoxicity.

Advances in Research on Bioactivity, Toxicity, Metabolism, and Pharmacokinetics of Usnic Acid In Vitro and In Vivo

Lichens are among the most widely distributed plants on earth and have the longest growth cycle. Usnic acid is an abundant characteristic secondary metabolite of lichens and the earliest lichen compound used commercially. It has diverse pharmacological activities, such as anti-inflammatory, antibacterial, antiviral, anticancer, antioxidant, and photoprotective effects, and promotes wound healing. It is widely used in dietary supplements, daily chemical products (fodder, dyes, food, perfumery, and cosmetics), and medicine. However, some studies have found that usnic acid can cause allergic dermatitis and drug-induced liver injury. In this paper, the bioactivity, toxicity, in vivo and in vitro metabolism, and pharmacokinetics of usnic acid were summarized. The aims were to develop and utilize usnic acid and provide reference for its future research.

Toxicity of Usnic Acid: A Narrative Review

Usnic acid (UA) is a dibenzofuran derivative naturally present in lichens, organisms resulting from the symbiosis between a fungus and a cyanobacterium, or an alga. UA shows antimicrobial, antitumor, antioxidant, analgesic, anti-inflammatory as well as UV-protective activities. Its use as pharmacological agent is widely described in traditional medicine, and in the past few years, the product has been marketed as a food supplement for the induction of weight loss. However, the development of severe hepatotoxicity in a limited number of subjects prompted the FDA to issue a warning letter, which led to the withdrawal of the product from the market in November 2001. Data published in literature on UA toxicology, genotoxicity, mutagenesis, and teratogenicity have been reviewed, as well as the case reports of subjects who developed hepatotoxicity following oral administration of UA as a slimming agent. Finally, we reviewed the most recent studies on the topical use of UA, as well as studies aimed at improving UA pharmacologic activity and reducing toxicity. Indeed, advancements in this field of research could open the possibility to reintroduce the use of UA as therapeutical agent.

Antioxidant, Cytotoxic, and Rheological Properties of Canola Oil Extract of Usnea barbata (L.) Weber ex F.H. Wigg from Călimani Mountains, Romania

Usnea genus (Parmeliaceae, lichenized Ascomycetes) is a potent phytomedicine, due to phenolic secondary metabolites, with various pharmacological effects. Therefore, our study aimed to explore the antioxidant, cytotoxic, and rheological properties of Usnea barbata (L.) Weber ex F.H. Wigg (U. barbata) extract in canola oil (UBO) compared to cold-pressed canola seed oil (CNO), as a green solvent used for lichen extraction, which has phytoconstituents. The antiradical activity (AA) of UBO and CNO was investigated using UV-Vis spectrophotometry. Their cytotoxicity was examined in vivo through a brine shrimp lethality (BSL) test after Artemia salina (A. salina) larvae exposure for 6 h to previously emulsified UBO and CNO. The rheological properties of both oil samples (flow behavior, thixotropy, and temperature-dependent viscosity variation) were comparatively analyzed. The obtained results showed that UBO (IC₅₀ = 0.942 ± 0.004 mg/mL) had a higher 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity than CNO (IC₅₀ = 1.361 ± 0.008 mg/mL). Both UBO and CNO emulsions induced different and progressive morphological changes to A. salina larvae, incompatible with their survival; UBO cytotoxicity was higher than that of CNO. Finally, in the temperature range of 32-37 °C, the UBO and CNO viscosity and viscoelastic behavior indicated a clear weakening of the intermolecular bond when temperature increases, leading to a more liquid state, appropriate for possible pharmaceutical formulations. All quantified parameters were highly intercorrelated. Moreover, their significant correlation with trace/heavy minerals and phenolic compounds can be observed. All data obtained also suggest a possible synergism between lichen secondary metabolites, minerals, and canola oil phytoconstituents.

Metabolic Activation of Perampanel Mediated by CYP1A2

Perampanel (PRP), a noncompetitive α-amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid (AMPA) receptor antagonist with high selectivity, has been used as a new adjuvant for the treatment of fractional seizures with or without primary generalized tonic-clonic seizures and secondary generalized seizures in epilepsy patients over the age of 12. Adverse events such as liver injury have been reported during the clinical application of PRP. The purpose of the study is to explore the in vitro and in vivo metabolic activation of PRP. Two GSH conjugates were detected in rat liver microsomal incubations containing PRP, GSH, and NADPH. The two GSH conjugates were both obtained from the bile of rats and rat primary hepatocytes after exposure to PRP. Similar microsomal incubations complemented with N-acetylcysteine (NAC) in place of GSH offered two NAC conjugates. As expected, the NAC conjugates were detected in the urine of PRP-treated rats. One of the two NAC conjugates was identified as NAC conjugate 12 verified by chemical synthesis. The individual human recombinant P450 enzyme incubation assay demonstrated that CYP1A2 dominated the catalysis for the metabolic activation of PRP. Pretreatment with α-naphthoflavone (NTF) decreased the formation of PRP-derived GSH conjugates in both livers of rats and cultured primary hepatocytes after being treated with PRP. Additionally, NTF was found to decrease the susceptibility of primary hepatocytes to the cytotoxicity of PRP. The findings indicate that PRP was metabolized to the corresponding epoxide, which could participate in PRP-induced cytotoxicity.

Evaluation of the utility of the Beta Human Liver Emulation System (BHLES) for CFSAN's regulatory toxicology program

Microphysiological systems (MPS), such as organ-on-a-chip platforms, are an emerging alternative model that may be useful for predicting human physiology and/or toxicity. Due to the interest in these platforms, the Center for Food Safety and Applied Nutrition partnered with Emulate to evaluate the utility of the Beta Human Liver Emulation System (BHLES) for its regulatory science program. Using known hepatotoxic compounds (usnic acid, benzbromarone, tamoxifen, and acetaminophen) and compounds that have no reported human cases of liver toxicity (dimethyl sulfoxide, theophylline, and aminohippurate) the platforms' performance was evaluated. Chemical toxicity was assessed by albumin secretion, urea and LDH release, nuclei number, mitochondrial membrane potential, and apoptosis. System/platform performance was evaluated in terms of sensitivity and specificity, power, and variability and repeatability. Chemical interactions with the Chip material were also assessed. Preliminary findings suggested that for the model test compounds selected, the BHLES was able to accurately predict toxicity, demonstrated high sensitivity and specificity, high power, and low variability. However, some compounds interacted with the Chip material indicating variable exposure levels that should be accounted for when planning experimentation. The details of the evaluation are presented herein.

PORIMIN: The key to (+)-Usnic acid-induced liver toxicity and oncotic cell death in normal human L02 liver cells

ETHNOPHARMACOLOGICAL RELEVANCE

Usnic acid (UA) is one of the well-known lichen metabolites that induces liver injury. It is mainly extracted from Usnea longissima and U. diffracta in China or from other lichens in other countries. U. longissima has been used as traditional Chinese medicine for treatment of cough, pain, indigestion, wound healing and infection. More than 20 incidences with hepatitis and liver failure have been reported by the US Food and Drug Administration since 2000. UA is an uncoupler of oxidative phosphorylation causing glutathione and ATP depletion. Previous histological studies observed extensive cell and organelle swellings accompanied with hydrotropic vacuolization of hepatocytes.

AIM OF THE STUDY

This study was to investigate the mechanism of UA-induced liver toxicity in normal human L02 liver cells and ICR mice using various techniques, such as immunoblotting and siRNA transfection.

MATERIALS AND METHODS

Assays were performed to evaluate the oxidative stress and levels of GSH, MDA and SOD. Double flouresencence staining was used for the detection of apoptotic cell death. The protein expressions, such as glutathione S transferase, glutathione reductase, glutathione peroxidase 4, catalase, c-Jun N-terminal protein kinase, caspases, gastamin-D and porimin were detected by Western blotting. Comparisons between transfected and non-transfected cells were applied for the elucidation of the role of porimin in UA-induced hepatotoxicity. Histopathological examination of mice liver tissue, serum total bilirubin and hepatic enzymes of alanine aminotransferase and aspatate aminotransferase were also studied.

RESULTS

The protein expressions of glutathione reductase, glutathione S transferase and glutathione peroxidase-4 were increased significantly in normal human L02 liver cells. Catalase expression was diminished in dose-dependent manner. Moreover, (+)-UA did not induce the activation of caspase-3, caspase-1 or gasdermin-D. No evidence showed the occurrence of pyroptosis. However, the porimin expressions were increased significantly. In addition, (+)-UA caused no cytotoxicity in the porimin silencing L02 cells.

CONCLUSIONS

In conclusion, (+)-UA induces oncotic L02 cell death via increasing protein porimin and the formation of irreversible membrane pores. This may be the potential research area for future investigation in different aspects especially bioactivity and toxicology.

Metabolic Activation of Aegeline Mediated by CYP2C19

1. Aegeline (AGL) is a natural alkaloidal amide mainly isolated from the leaves and fruits of tropical plant Aegle marmelos, with multiple pharmacological activities.2. As one component of several dietary supplements, AGL caused a series of acute and chronic liver injuries. Nevertheless, the mechanisms of AGL-induced hepatotoxicity remain unclear. This study was conducted to identify reactive metabolite(s), to determine related metabolic pathways, and define the possible association of the bioactivation with AGL cytotoxicity.3. A demethylation metabolite (M1) and a GSH conjugate (M2) were detected in rat liver microsomal incubations containing AGL and GSH. The two metabolites were both found in bile of rats and rat primary hepatocytes after AGL administration.4. Recombinant P450 enzyme incubations showed that CYP2C19 was the principal enzyme catalyzing this metabolic activation.5. Ticlopidine, a selective inhibitor of CYP2C19, decreased the formation of M1 and M2 in hepatocytes and attenuated the susceptibility of hepatocytes to the cytotoxicity of AGL. The results suggest that AGL was metabolized to a p-quinone methide intermediate which could in part participate in AGL-induced cytotoxicity.

Review: Usnic acid-induced hepatotoxicity and cell death

Increasing prevalence of herbal and dietary supplement-induced hepatotoxicity has been reported worldwide. Usnic acid (UA) is a well-known hepatotoxin derived from lichens. Since 2000, more than 20 incident reports have been received by the US Food and Drug Administration after intake of UA containing dietary supplement resulting in severe complications. Scientists and clinicians have been studying the cause, prevention and treatment of UA-induced hepatotoxicity. It is now known that UA decouples oxidative phosphorylation, induces adenosine triphosphate (ATP) depletion, decreases glutathione (GSH), and induces oxidative stress markedly leading to lipid peroxidation and organelle stress. In addition, experimental rat liver tissues have shown massive vacuolization associated with cellular swellings. Additionally, various signaling pathways, such as c-JNK N-terminal kinase (JNK), store-operated calcium entry, nuclear erythroid 2-related factor 2 (Nrf2), and protein kinase B/mammalian target of rapamycin (Akt/mTOR) pathways are stimulated by UA causing beneficial or harmful effects. Nevertheless, there are controversial issues, such as UA-induced inflammatory or anti-inflammatory responses, cytochrome P450 detoxifying UA into non-toxic or transforming UA into reactive metabolites, and unknown mechanism of the formation of vacuolization and membrane pore. This article focused on the previous and latest comprehensive putative mechanistic findings of UA-induced hepatotoxicity and cell death. New insights on controversial issues and future perspectives are also discussed and summarized.

Dr. Daniel Acosta and In Vitro toxicology at the U.S. Food and Drug Administration's National Center for Toxicological Research

For the past five years, Dr. Daniel Acosta has served as the Deputy Director of Research at the National Center for Toxicological Research (NCTR), a principle research laboratory of the U.S. Food and Drug Administration (FDA). Over his career at NCTR, Dr. Acosta has had a major impact on developing and promoting the use of in vitro assays in regulatory toxicity and product safety assessments. As Dr. Acosta nears his retirement we have dedicated this paper to his many accomplishments at the NCTR. Described within this paper are some of the in vitro studies that have been conducted under Dr. Acosta's leadership. These studies include toxicological assessments involving developmental effects, and the development and application of in vitro reproductive, heart, liver, neurological and airway cell and tissue models.

Pro-inflammatory Cytokines Alter the Immunopeptidome Landscape by Modulation of HLA-B Expression

Antigen presentation on HLA molecules is a major mechanism by which the immune system monitors self and non-self-recognition. Importantly, HLA-I presentation has gained much attention through its role in eliciting anti-tumor immunity. Several determinants controlling the peptides presented on HLA have been uncovered, mainly through the study of model substrates and large-scale immunopeptidome analyses. These determinants include the relative abundances of proteins in the cell, the stability or turnover rate of these proteins and the binding affinities of a given peptide to the HLA haplotypes found in a cell. However, the regulatory principles involved in selection and regulation of specific antigens in response to tumor pro-inflammatory signals remain largely unknown. Here, we chose to examine the effect that TNFα and IFNγ stimulation may exert on the immunopeptidome landscape of lung cancer cells. We show that the expression of many of the proteins involved in the class I antigen presentation pathway are changed by pro-inflammatory cytokines. Further, we could show that increased expression of the HLA-B allomorph drives a significant change in HLA-bound antigens, independently of the significant changes observed in the cellular proteome. Finally, we observed increased HLA-B levels in correlation with tumor infiltration across the TCGA lung cancer cohorts. Taken together, our results suggest that the immunopeptidome landscape should be examined in the context of anti-tumor immunity whereby signals in the microenvironment may be critical in shaping and modulating this important aspect of host-tumor interactions.

Cytotoxicity of 34 FDA approved small-molecule kinase inhibitors in primary rat and human hepatocytes

Of the 34 FDA approved oral small-molecule kinase inhibitors (KI), 23 (68%) have warnings for hepatotoxicity in product labeling. To better understand the mechanisms of KI hepatotoxicity and whether such effects can be predicted, we examined 34 KIs for cytotoxicity in primary rat and human hepatocytes. The hepatocytes were treated with KIs at ten concentrations normalized to maximal therapeutic blood levels (Cmax). At 5 and 24 h post treatment, lactate dehydrogenase or alanine aminotransferase leakage, caspase 3/7 activities and cellular adenosine triphosphate levels were measured. At 1 to 100-fold Cmax, while 5 KIs were neither toxic to human nor rat hepatocytes, 3 KIs showed similar cytotoxicity in both species and 26 KIs showed species-biased cytotoxicity, with 16 KIs being more toxic to human hepatocytes and 10 KIs being more toxic to rat hepatocytes. At concentrations of 1-, 2.5-, 5-, 10-, 100-fold Cmax, the number of cytotoxic KIs in human hepatocytes was 4, 8, 11, 14 and 27, respectively, and the corresponding number in rat hepatocytes was 1, 4, 9, 12 and 27, respectively. When hepatocyte cytotoxicity at 100-fold Cmax was used to predict KI clinical hepatotoxicity reflected in product labeling, the accuracy was 0.65 with human hepatocytes and 0.59 with rat cells. When the criterion of daily dose ≥100 mg or Cmax ≥1.1 μM was used to predict KI hepatotoxicity, the accuracy was 0.56 or 0.47, respectively. These results suggest both indirect and direct drug-induced hepatocyte toxicity may contribute to the mechanisms of KI-induced hepatotoxicity seen clinically and use of primary hepatocytes is a useful in vitro model to help predict such toxicity.

Activation of the Nrf2 signaling pathway in usnic acid-induced toxicity in HepG2 cells

Many usnic acid-containing dietary supplements have been marketed as weight loss agents, although severe hepatotoxicity and acute liver failure have been associated with their overuse. Our previous mechanistic studies revealed that autophagy, disturbance of calcium homeostasis, and ER stress are involved in usnic acid-induced toxicity. In this study, we investigated the role of oxidative stress and the Nrf2 signaling pathway in usnic acid-induced toxicity in HepG2 cells. We found that a 24-h treatment with usnic acid caused DNA damage and S-phase cell cycle arrest in a concentration-dependent manner. Usnic acid also triggered oxidative stress as demonstrated by increased reactive oxygen species generation and glutathione depletion. Short-term treatment (6 h) with usnic acid significantly increased the protein level for Nrf2 (nuclear factor erythroid 2-related factor 2), promoted Nrf2 translocation to the nucleus, up-regulated antioxidant response element (ARE)-luciferase reporter activity, and induced the expression of Nrf2-regulated targets, including glutathione reductase, glutathione S-transferase, and NAD(P)H quinone oxidoreductase-1 (NQO1). Furthermore, knockdown of Nrf2 with shRNA potentiated usnic acid-induced DNA damage and cytotoxicity. Taken together, our results show that usnic acid causes cell cycle dysregulation, DNA damage, and oxidative stress and that the Nrf2 signaling pathway is activated in usnic acid-induced cytotoxicity.

The cytochrome P450 inhibitor SKF-525A disrupts autophagy in primary rat hepatocytes

The cytochrome P450 (CYP) inhibitor SKF-525A is commonly used to study drug metabolism and toxicity, particularly hepatotoxicity. By using Western blot and immunofluorescence staining, we unexpectedly found that SKF-525A at 2-20 μM caused remarkable accumulation of microtubule-associated protein light chain 3 II (LC3-II) in primary rat hepatocytes at 1, 4 and 24 h, indicating that autophagy was disrupted. SKF-525A showed no effects on chloroquine induced LC3-II accumulation, suggesting that autophagic flux was blocked, which is further supported by the increased level of the p62 protein after SKF-525A treatment. SKF-525A did not affect proteasome activities or gene expression of LC3-II or p62. Immunofluorescence of green fluorescent protein fused lysosomal-associated membrane protein 1 (LAMP1, a specific protein marker for lysosomes) and LC3-II showed that co-localization of these two proteins was partially abolished by SKF-525A, indicating that autophagosome-lysosome fusion was blocked. The other five CYP inhibitors, metyrapone, 1-aminobenzotriazole, alpha-naphthoflavone, ticlopidine, and ketoconazole, showed no effects in parallel experiments. These findings provide novel insights into the mechanisms by which various CYP inhibitors differentially affect a same drug's toxicity in hepatocytes. The data also indicate that SKF-525A is not an ideal chemical inhibitor for probing the relation between CYP mediated metabolism and toxicity in primary hepatocytes.

Antitumor activity of DMAKO-05, a novel shikonin derivative, and its metabolism in rat liver microsome

The antitumor activity of shikonin derivatives is largely dependent on the generation of superoxide radicals and the alkylation activity of their naphthoquinone moiety. However, our recent study showed that 1,4-dioxime-5,8-dimethoxynaphthalene (DMAKO-05), a novel shikonin derivative, displayed more potential antitumor activity and less toxicity compared to fluorouracil (5-FU) both in vitro and in vivo, even though the hydroxyl and carbonyl groups of its naphthoquinone structure were shielded. To understand the underlying mechanisms, we investigated the metabolism of DMAKO-05 in rat liver microsomes. The kinetic analysis indicated that DMAKO-05 underwent a biphasic metabolism in rat liver microsomes. The inhibition experiments showed that CYP1A and CYP3A were the major enzymes in the metabolism of DMAKO-05, along with partial contribution from CYP2A. In addition, the structures of eight DMAKO-05 metabolites, which were characterized by accurate mass and MS/MS fragmentograms, implied that DMAKO-05 was mainly metabolized through the oxygenation of its naphthoquinone nucleus and the hydrolysis of its side chain, instead of the oxidation of hydroxyimine to ketone. Therefore, DMAKO-05 should not be considered as a traditional naphthoquinone prodrug.

The role of autophagy in usnic acid-induced toxicity in hepatic cells

The use of usnic acid and usnic acid-containing products is associated with acute liver failure; however, mechanistic studies of hepatotoxicity caused by usnic acid are limited. In this study, we investigated and characterized the possible mechanisms, especially the role of autophagy in usnic acid's toxicity in human HepG2 cells. Usnic acid caused apoptosis as demonstrated by an increased caspase-3/7 activity and an increased subdiploid nucleus formation. Usnic acid-induced autophagy as demonstrated by the conversion of LC3B-I to LC3B-II, degradation of P62, and an increased number of puncta. Inhibition of autophagy by treating cells with autophagy inhibitors (3-methyladenine or chloroquine) or by small interfering RNA against Atg7 aggravated usnic acid-induced apoptosis and decreased cell viability, indicating that autophagy plays a protective role against usnic acid-induced toxicity. Moreover, usnic acid activated the MAPK signaling pathway. Usnic acid-elicited apoptosis was enhanced and autophagy was decreased when JNK was suppressed by a specific inhibitor. Additionally, inhibition of autophagy decreased the activity of JNK. Taken together, our results suggest that usnic acid perturbs various interrelated signaling pathways and that autophagy induction is a defensive mechanism against usnic acid-induced cytotoxicity.