Drug toxicity mechanisms in human hepatoma HepG2 cells: cyclosporin A and tamoxifen

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.

Combination of siRNA-directed gene silencing with epigallocatechin-3-gallate (EGCG) reverses drug resistance in human breast cancer cells

Elevated expression of NF-E2-related factor 2 (Nrf2), a nuclear transcription factor, is a frequent genetic abnormality seen in this malignancy and is an important contributor to chemoresistance in cancer therapy. In the present study, we investigated if Nrf2 was associated with drug resistance in tamoxifen-resistant MCF-7 (MCF-7/TAM) cells, and whether EGCG, major flavonoid isolated from green tea, could reverse drug resistance in MCF-7/TAM cells. Our results showed that the endogenous expression of Nrf2 as well as its target proteins heme oxygenase-1, NADP (H):quinone oxidoreductase in MCF-7/TAM cells was higher than that in MCF-7 cells. Epicatechin gallate (EGCG) significantly sensitizes MCF-7/TAM cells to tamoxifen and dramatically reduced Nrf2 expression at both the messenger RNA and protein, leading to a reduction of Nrf2-downstream genes. In addition, using siRNA technique, we found that the intracellular Nrf2 protein level was significantly decreased in MCF-7/TAM cells and tamoxifen resistance was partially reversed by Nrf2 siRNA. Combination of siRNA-directed gene silencing with EGCG downregulated the Nrf2-dependent response and partly reversed tamoxifen resistance in MCF-7/TAM cells in a synergic manner. These results suggested that combining the chemotherapeutic effect of EGCG with siRNA-mediated Nrf2 knock-down results in the feasibility of using Nrf2 inhibitors to increase efficacy of chemotherapeutic drugs.

Antioxidant Activity of Porcelainberry (Ampelopsis brevipedunculata(Maxim.) Trautv.)

The stem and root of Porcelainberry (Ampelopsis brevipedunculata (Maxim.) Trautv.) (AB) was traditionally used as an anti-inflammatory, diuretic and anti-hepatotoxic agent in folk medicine. In this study, cell-free and cell culture systems were employed to investigate the antioxidant activity of the methanol extract of AB (MEAB). The cell-free system showed that the MEAB exhibited dose-dependent antioxidant activities against linoleic acid peroxidation and plasmid DNA oxidation. We also demonstrated that the MEAB possessed strong reducing power and scavenging effects of hydroxyl radicals and DPPH free radicals.

The harmful effects of reactive oxygen metabolites on HepG2 cells and the possible antioxidant effects of the MEAB were also investigated. Pretreatment or cotreatment of HepG2 cells with the MEAB could significantly protect cells from H 2 O 2-induced oxidative stress. This implies that the antioxidant effects in cell culture may result from the direct interaction between the MEAB and exogenous oxidant sources, as these occur in cell free systems, as well as from the induction of cellular stress gene expression. The antioxidant activity of the MEAB may partially explain its anti-inflammatory and anti-hepatotoxic effects.

Cytotoxicity of chloroacetanilide herbicide alachlor in HepG2 cells independent of CYP3A4 and CYP3A7

Alachlor is cytotoxic to human hepatoblastoma HepG2s, a cell line that expresses constitutive CYP3A7 and dexamethasone (DEX)-inducible CYP3A4 and CYP3A7. CYP3A4 catalyzes alachlor N-dealkylation to 2-chloro-N-(2,6-diethylphenyl)acetamide (CDEPA), precursor of 2,6-diethylbenzoquinoneimine, putative reactive metabolite for rat nasal carcinogenicity. We hypothesized that HepG2 alachlor cytotoxicity would be mediated by CYP3A4/7 and increased with DEX. Here, we report time-dependent alachlor cytotoxicity (EC(50) approximately 500 microM and 264+/-17 microM at 6 and 24h, respectively) as assessed by lactate dehydrogenase leakage. DEX pretreatment (25 microM, 48 h) significantly increased CYP3A7-catalyzed luciferin 6' benzylether O-debenzylation, but had no effect on alachlor toxicity. Further, CYP3A4/7 inhibitor triacetyloleandomycin did not prevent, but rather potentiated, alachlor cytotoxicity. In agreement, CDEPA was less toxic than parent alachlor. HepG2 CYP3A4 activity was unaffected by 48 h DEX pretreatment; therefore, studies were done in DPX-2 cells, a HepG2 derivative engineered to overexpress pregnane-X receptor (PXR) that exhibits rifampicin (RIF)-inducible endogenous CYP3A4. Alachlor cytotoxicity in DPX-2 cells occurred over a concentration range equivalent to that in HepG2. CYP3A4 activity of DPX-2 cells treated with RIF (10 microM, 48 h) was twice that of untreated cells, but RIF did not increase alachlor toxicity. These results demonstrate that neither CYP3A4 nor CYP3A7 initiate a pathway leading to a toxic alachlor metabolite.

Tamoxifen induces oxidative stress and mitochondrial apoptosis via stimulating mitochondrial nitric oxide synthase

Tamoxifen is an anticancer drug that induces oxidative stress and apoptosis via mitochondria-dependent and nitric oxide (NO)-dependent pathways. The present report shows that tamoxifen increases intramitochondrial ionized Ca(2+) concentration and stimulates mitochondrial NO synthase (mtNOS) activity in the mitochondria from rat liver and human breast cancer MCF-7 cells. By stimulating mtNOS, tamoxifen hampers mitochondrial respiration, releases cytochrome c, elevates mitochondrial lipid peroxidation, increases protein tyrosine nitration of certain mitochondrial proteins, decreases the catalytic activity of succinyl-CoA:3-oxoacid CoA-transferase, and induces aggregation of mitochondria. The present report suggests a critical role for mtNOS in apoptosis induced by tamoxifen.

Hydroxytamoxifen protects against oxidative stress in brain mitochondria

This study evaluated the effect of hydroxytamoxifen, the major active metabolite of tamoxifen (synthetic, nonsteroidal antiestrogen drug), on the function of brain mitochondria. We observed that only high concentrations of hydroxytamoxifen (60 nmol/mg protein) induced a significant decrease in RCR, while ADP/O ratio remained statistically unchanged. Similarly, only the highest concentration of hydroxytamoxifen (60 nmol/mg protein) affected the phosphorylative capacity of brain mitochondria, characterized by a decrease in the repolarization level and an increase in the repolarization lag phase. We observed that all the concentrations of hydroxytamoxifen tested (7.5, 15 and 30 nmol/mg protein) prevented lipid peroxidation induced by the oxidant pair ADP/Fe(2+). Furthermore, through the analyses of calcium fluxes and mitochondrial transmembrane potential parameters, we observed that hydroxytamoxifen (30 nmol/mg protein) exerted some protection against pore opening, although in a less extension than that promoted by cyclosporin A, the specific inhibitor of the mitochondrial permeability transition pore. However, in the presence of hydroxytamoxifen plus cyclosporin A, the protection observed was significantly higher when compared with that induced by both agents alone. These results support the idea that hydroxytamoxifen protects lipid peroxidation and inhibits the mitochondrial permeability transition pore in brain. Since numerous neurodegenerative diseases are intimately related with mitochondrial dysfunction resulting from lipid peroxidation and induction of mitochondrial permeability transition, among other factors, future therapeutical strategies could be designed taking in account this neuroprotective role of hydroxytamoxifen, which is pharmacologically much more potent and less toxic than its promoter tamoxifen.

Tamoxifen cytotoxicity in hepatoblastoma cells stably transfected with human CYP3A4

Tamoxifen can exert its effects through the competitive inhibition of estrogen receptors or other mechanisms. HepG2 cells lacking estrogen receptors and engineered to overexpress CYP3A4, the most important CYP to metabolize the drug, appear to be a good model to study the effects of tamoxifen metabolites. Tamoxifen altered cell cycle of transduced HepG2 cells, decreased G0/G1 cell numbers, diminished proliferation index and induced cell death mostly in cells overexpressing CYP3A4 but was without significant effect on cytotoxicity or proliferation of cells engineered to overexpress CYP2E1 or on empty vector transfected cells. Tamoxifen did not change MDR1 levels irrespectively on CYP450s expression, but inhibited by approximately 50% p-gp functions in all cell types. Drug treatment significantly increased dehydroepiandrosterone sulfotransferase activity and sulfotransferase inhibition significantly decreased tamoxifen cytotoxicity. Our results support the view that metabolic activation of tamoxifen in liver cells may proceed via CYP450-mediated metabolism and subsequent sulfotransferase-mediated activation and point to the role of CYP3A4 and dehydroepiandrosterone sulfotransferase in adverse tamoxifen effects.

Nile Red binding to HepG2 cells: an improved assay for in vitro studies of hepatosteatosis

Nile Red is a fluorescent dye used extensively to study fat accumulation in many types of cells; unfortunately protocols that work well for most cells are not effective for studying drug-induced lipid accumulation in cultured liver cells and hepatocyte-derived cell lines. Using human hepatoma (HepG2) cells, we have developed a simple Nile Red binding assay as a screen for steatosis-inducing compounds. Increases in Nile Red binding in response to known hepatotoxic compounds were observed after incubating treated cells with 1 microM Nile Red for several hours, washing away free Nile Red, and then allowing redistribution, and/or clearance of the lipid-indicator dye. Several compounds known to cause hepatic fat accumulation in vivo were examined and most robustly increased Nile Red binding in HepG2 cells. These include estrogen and other steroids, ethionine, cyclosporin A, and valproic acid. Required concentrations for increased Nile Red binding were generally three-fold or more lower than the cytotoxic concentration determined by a resazurin reduction assay in the same cells. Qualitatively similar Nile Red binding results were obtained when primary canine or rat hepatocytes were used. Morphological differences in Nile Red staining were observed by confocal fluorescence microscopy in HepG2 cells after treatment with different compounds and likely reflect distinct toxicological mechanisms.

Cyclosporine directly causes oxidative stress and promotes Epstein-Barr virus transformation of human B cells

BACKGROUND

We have previously shown that oxidative stress alone can promote transformation of human B cells infected with Epstein-Barr virus (EBV) in vitro, an accepted model mimicking posttransplant lymphoproliferative disorders (PTLDs). Our laboratory has investigated the direct effects of cyclosporine A (CyA) as an oxidant promoting B-cell transformation and we have proposed that CyA directly promotes B-cell transformation and that this effect can be blocked by antioxidants.

METHODS

Human splenocytes were prepared by centrifugation and plating technique to provide a greater than 80% pure preparation of B cells that was used for the direct oxidative stress experiments. These cells were cocultured with CyA (500 ng/ml) and hydrogen peroxide (H(2)O(2), 0.15 mM) with or without antioxidant vitamin E (40 microM). Oxidative stress was evaluated by using a commercial lipid hydroperoxide (LPO) assay kit. In another set of three separate experiments, human B lymphocytes infected with EBV were cultured with CyA (500 ng/ml), H(2)O(2) (0.15 mM), and vitamin E (40 microM). B-Cell transformation by EBV was evaluated by counting colony number and [(3)H]-thymidine incorporation.

RESULTS

At therapeutic concentrations, CyA (500 ng/mL) had an oxidative effect on human splenocytes in vitro, similar to the effect of H(2)O(2) (90 and 97% increases, respectively in LPO production over control P < 0.005), which was abrogated by the addition of vitamin E. Similarly, both CyA and H(2)O(2) promoted transformation of B cells infected with EBV(75 and 108% increases respectively in colony counts over control, P < 0.005). This effect was also blocked by vitamin E.

CONCLUSIONS

Both CyA and H(2)O(2) have a direct oxidative effect on human B cells and cause promotion of EBV-induced transformation of B cells. These effects are blocked by the antioxidant vitamin E. These findings may have future therapeutic implications for PTLDs.

Molecular and pharmacological aspects of antiestrogen resistance

Endocrine therapy is effective in approximately one-third of all breast cancers and up to 80% of tumors that express both estrogen and progesterone receptors. Despite the low toxicity, good overall response rates, and additional benefits associated with its partial agonist activity, most Tamoxifen-responsive breast cancers acquire resistance. The development of new antiestrogens, both steroidal and non-steroidal, provides the opportunity for the development of non-cross-resistant therapies and the identification of additional mechanisms of action and resistance. Drug-specific pharmacologic mechanisms may confer a resistance phenotype, reflecting the complexities of both tumor biology/pharmacology and the molecular endocrinology of steroid hormone action. However, since all antiestrogens will be effective only in cells that express estrogen receptors (ER), many mechanisms will likely be directly related to ER expression and signaling. For example, loss of ER expression/function is likely to confer a cross-resistance phenotype across all structural classes of antiestrogens. Altered expression of ERalpha and ERbeta, and/or signaling from transcription complexes driven by these receptors, may produce drug-specific resistance phenotypes. We have begun to study the possible changes in gene expression that may occur as cells acquire resistance to steroidal and non-steroidal antiestrogens. Our preliminary studies implicate the altered expression of several estrogen-regulated genes. However, resistance to antiestrogens is likely to be a multigene phenomenon, involving a network of interrelated signaling pathways. The way in which this network is adapted by cells may vary among tumors, consistent with the existence of a highly plastic and adaptable genotype within breast cancer cells.

Ketoconazole-induced apoptosis through P53-dependent pathway in human colorectal and hepatocellular carcinoma cell lines

In this study, we first demonstrated that the widely used oral antifungal drug, ketoconazole (KT), can induce apoptosis in various type of human cancer cells and in a primary culture of rat liver cells. We further investigated the molecular mechanisms of KT-induced apoptosis. It was found that KT induced nuclear accumulation of p53 protein in a dose- and time-dependent manner. The level of p53 protein was elevated approximately three times as much in treated cells 24 h after KT (5 microM) exposure as in cells receiving mock treatment. We found that cells containing wild-type p53 (COLO 205 and Hep G2) were more sensitive to KT exposure. The bax protein was induced and the bcl-2 protein was inhibited by KT in cells containing wild-type p53 (Hep G2, COLO 205) but not in cells without p53 (Hep 3B). The caspase-3 was activated 24 h after KT treatment. The Poly-(ADP ribose) polymerase (PARP) and the lamin A degradation was induced by KT, which promoted nuclear membrane disassembly and eventually caused apoptosis. Our results also indicated that none of the PKC gene family was involved in KT-induced apoptosis.

Adaptation to oxidative stress: effects of vinclozolin and iprodione on the HepG2 cell line

It is well known that the dicarboximide fungicides, vinclozolin and iprodione, induce lipid peroxidation by means of oxygen activation in fungi, but their action on mammalian cells is not yet clear. We therefore investigated the effect of 1- and 24-h treatments with vinclozolin at concentrations of 25, 50, 100 microg/ml and iprodione at concentration of 62.5, 125, 250 microg/ml on malonaldehyde and free radical production and on reduced glutathione levels in the human HepG2 hepatoma cell line. The concentrations were chosen on the basis of neutral red cytotoxicity assays. One-hour treatment with the different concentrations of either vinclozolin or iprodione increased both malonaldehyde and free radical content, and decreased reduced glutathione levels, whereas 24-h treatment decreased malonaldehyde content and free radical production, and increased reduced glutathione concentration. These results suggest that the mammalian cells respond to the initial oxidative damage caused by the two dicarboximide fungicides by means of a characteristic adaptative phenomenon within 24 h. This hypothesis is supported by the antagonized effects caused by treatment with the two dicarboximide fungicides and buthionine sulfoximine 0.5 mM, a specific and irreversible inhibitor of reduced glutathione synthesis. The data confirm that the two dicarboximide fungicides maintain their specific action in mammalian cells, although this action is masked by adaptation.

The effect of dietary flavonoids on DNA damage (strand breaks and oxidised pyrimdines) and growth in human cells

The effects of the flavonoids quercetin, myricetin and silymarin on DNA damage and cytotoxicity in human cells were investigated. DNA strand breaks and oxidised pyrimidines were determined using alkaline single cell gel electrophoresis (the comet assay). Inhibition of cell growth was also measured. Caco-2 (colon), HepG2 (liver), HeLa (epithelial) cells and normal human lymphocytes showed different, dose-dependent susceptibilities (in terms of strand breakage) to the various flavonoids, quercetin being the most damaging. This agreed well with the ability of the flavonoids to inhibit cell growth. None of the flavonoids induced DNA base oxidation above background levels. All of the flavonoids under investigation caused depletion of reduced glutathione, which, in the case of quercetin, occurred prior to cell death. Neither cytotoxicity nor genotoxicity was associated with the antioxidant enzyme capacity (glutathione, glutathione reductase, glutathione peroxidase and catalase) of the cells.

The influence of cell growth, detoxifying enzymes and DNA repair on hydrogen peroxide-mediated DNA damage (measured using the comet assay) in human cells

Single-cell gel electrophoresis (the comet assay) is a sensitive method for detecting strand breaks at the level of individual cells. Cells embedded in agarose are lysed, electrophoresed, and fluorescently stained. Breaks in the DNA release its supercoiling and allow DNA to extend toward the anode, resembling a comet. We have used the comet assay to investigate the influence of growth state, xenobiotic detoxifying enzymes, and DNA repair processes on the response of cultured human cells to oxidative damage. HepG2 and Caco-2 cells are differentiated liver and colon cell lines, respectively. HeLa and GM1899A cells are relatively unspecialized epithelial and lymphoblastoid cells. Substrate-dependent cells showed a cyclical fluctuation of glutathione (GSH) with respect to growth. Enzyme activities (glutathione reductase, glutathione peroxidase, and catalase) varied considerably between cell types and changed with cell growth state. Hydrogen peroxide induced more DNA damage in actively dividing cells than in confluent cultures. Sensitivity to oxidative injury did not correlate with detoxifying enzyme activity. Rather, differences in susceptibility between cells could be correlated with differences in DNA repair capacity. This study highlights the need to standardize experimental conditions if the comet assay is to be employed in the study of genotoxicity.