Use of precision-cut liver slices for studies of unscheduled DNA synthesis

Use of precision cut human liver slices for studying the metabolism and genotoxic potential of xenobiotics by means of the (32)P-postlabelling technique: steps towards method validation using testosterone and 2-aminofluorene

In the present study, a new in vitro model combining the short-term incubation of precision-cut human liver slices with DNA-adduct analysis by the (32)P-postlabelling technique is proposed for investigation of the genotoxic potential of xenobiotics. For method validation, the metabolic turnover of testosterone (TES) and the DNA-adduct inducing potential of 2-aminofluorene (2-AF) were used. Precision-cut human liver slices were prepared from a total of 12 human liver samples which were freshly obtained as parts of resectates from liver surgery. The slices were incubated as submersion cultures with TES and 2-AF for up to 6 h in 12-well tissue culture plates at concentrations of 10-50 and 0.06-28 μM, respectively. Slices recovered from the slicing procedure in the 4 °C cold Krebs-Henseleit buffer as indicated by intracellular potassium concentrations which increased for 2 h and then remained stable until the end of the incubation. TES was extensively metabolized by human liver slices with a similar metabolite pattern as observed in vivo. Almost 90% of the metabolites were conjugates. Major phase-I metabolites were androstendione, 6β-OH-androstendione, 6β-OH-TES, and 15β-OHTES. After incubation with 2-AF, substance related DNA-adducts were detected which increased dose-dependently from 12 to 1146 adducts per 10(9) nucleotides. The adduct pattern consisted of one major adduct spot, A, representing 80-90% of the total adduct level and up to four minor adduct spots, B-E. In summary, the present data demonstrate that precision-cut liver slices are a valuable alternative in vitro system for DNA-adduct determination to screen chemicals for potential genotoxicity in humans.

Detection of xenobiotic-induced DNA damage by the comet assay applied to human and rat precision-cut liver slices

The comet assay is a simple and sensitive method for measuring DNA damage at the level of individual cells and is extensively used in genotoxicity studies. It is commonly applied to cultured cells. The aim of this study was to apply the comet assay for use in fresh liver tissue, where metabolic activity, all cell types and tissue architecture are preserved. The response of liver slices to genotoxic agents was tested with the reactive oxygen species generating tert-butyl hydroperoxide (tBOOH, 0.1-2 mM), [corrected] and the pro-carcinogens 2-amino-3-methylimidazo[4,5-f]quinoline (IQ, 0.5-2 mM) and benzo(a)pyrene (BaP, 10-100 microM). Dose-dependent DNA damage was observed and compared to HepG2 cells. At non-cytotoxic concentrations of carcinogens, human liver slices were more sensitive to tBOOH than rat liver slices, while no significant difference was found for BaP and IQ. Human liver slices were more sensitive to IQ than HepG2 cells, equally sensitive to BaP and less to tBOOH. Control slices showed low levels of DNA damage, which did not increase during 24 h preservation (0 degrees C) or 48 h culturing (37 degrees C). In conclusion, the comet assay that we applied for measuring DNA damage in precision-cut liver slices is an useful tool to study genotoxic effects induced by various potential genotoxicants, allowing for detection of species differences in susceptibility to carcinogens.

Lack of effect of furfural on unscheduled DNA synthesis in the in vivo rat and mouse hepatocyte DNA repair assays and in precision-cut human liver slices

The ability of furfural to induce unscheduled DNA synthesis (UDS) in hepatocytes of male and female B6C3F(1) mice and male F344 rats after in vivo administration and in vitro in precision-cut human liver slices has been studied. Preliminary toxicity studies established the maximum tolerated dose (MTD) of furfural to be 320 and 50 mg/kg in the mouse and rat, respectively. Furfural was dosed by gavage at levels of 0 (control), 50, 175 and 320 mg/kg to male and female mice and 0, 5, 16.7 and 50 mg/kg to male rats. Hepatocytes were isolated by liver perfusion either 2-4 h or 12-16 h after treatment, cultured in medium containing [3H]thymidine for 4 h and assessed for UDS by grain counting of autoradiographs. Furfural treatment did not produce any statistically significant increase or any dose-related effects on UDS in mouse and rat hepatocytes either 2-4 h or 12-16 h after dosing. In contrast, UDS was markedly induced in mice and rats 2-4 h after treatment with 20 mg/kg dimethylnitrosamine and 12-16 h after treatment of mice and rats with 200 mg/kg o-aminoazotoluene and 50 mg/kg 2-acetylaminofluorene (2-AAF), respectively. Precision-cut human liver slices from four donors were cultured for 24 h in medium containing [3H]thymidine and 0-10 mM furfural. Small increases in the net grain count (i.e. nuclear grain count less mean cytoplasmic grain count) observed with 2-10 mM furfural were not due to any increase in the nuclear grain count. Rather, it was the result of concentration-dependent decreases in the mean cytoplasmic grain counts and to a lesser extent in nuclear grain counts, due to furfural-induced cytotoxicity. In contrast, marked increases in UDS (both net grain and nuclear grain counts) were observed in human liver slices treated with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM aflatoxin B(1) and 0.005 and 0.05 mM 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. This study demonstrates that furfural does not induce UDS in the hepatocytes of male and female B6C3F(1) mice and male F344 rats after oral treatment at doses up to the MTDs. Moreover, human liver slice studies suggest that furfural is also not a genotoxic agent in human liver.

Inhibition of xenobiotic-induced genotoxicity in cultured precision-cut human and rat liver slices

In this study precision-cut liver slices have been used to evaluate the effects of the flavone tangeretin, the flavonoid glycoside naringin and the flavanone naringenin (the aglycone derived from naringin) on xenobiotic-induced genotoxicity. Liver slices were cultured for 24 h in medium containing [3H]thymidine and the test compounds and then processed for autoradiographic determination of unscheduled DNA synthesis (UDS). The cooked food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) markedly induced UDS in cultured human liver slices and both 2-acetylaminofluorene (2-AAF) and aflatoxin B1 (AFB1) induced UDS in cultured rat liver slices. Tangeretin (20 and 50 microM) was found to be a potent inhibitor of 5 and 50 microM PhIP-induced UDS in human liver slices, whereas 20 and 50 microM naringenin was ineffective and naringin only inhibited genotoxicity at a concentration of 1000 microM. In rat liver slices 50 microM tangeretin inhibited 10 and 50 microM 2-AAF-induced UDS, whereas 50 microM naringenin and 100 and 1000 microM naringin were ineffective. None of the three flavonoids examined inhibited 5 microM AFB1-induced UDS in rat liver slices. The inhibition of PhIP- and 2-AAF-induced UDS by tangeretin is probably attributable to the inhibition of the human and rat cytochrome P-450 isoforms which are responsible for the bioactivation of these two genotoxins. Although flavonoids can modulate xenobiotic-induced genotoxicity in human and rat liver slices, any protective effect is dependent on the particular combination of genotoxin and flavonoid examined. These results demonstrate that cultured precision-cut liver slices may be utilised as an in vitro model system to examine the modulation of xenobiotic-induced genotoxicity by flavonoids and other dietary components.

Lack of effect of coumarin on unscheduled DNA synthesis in precision-cut human liver slices

In this study the effect of coumarin on unscheduled DNA synthesis (UDS) in precision-cut human liver slices has been examined. Liver slices from tissue samples from four donors were cultured for 24 hr in medium containing [3H]thymidine and 0-5.0 mM coumarin using a dynamic organ culture system and processed for autoradiographic evaluation of UDS. As positive controls liver slices were also cultured with three known genotoxic agents, namely 0.02 and 0.05 mM 2-acetylaminofluorene (2-AAF), 0.002 and 0.02 mM aflatoxin B1 (AFB1) and 0.005 and 0.05 mM 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with more than five net grains. Compared with control liver slice cultures, treatment with 0.05-5.0 mM coumarin had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in the net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced significant increases in the number of centrilobular hepatocyte nuclei with more than five net grains. At the concentrations examined neither coumarin. 2-AAF, AFB1 nor PhIP had any significant effect on replicative DNA synthesis in 24 hr cultured human liver slices. These results demonstrate that coumarin does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. The results of this study suggest that coumarin is not a genotoxic agent in human liver.

Effect of some cooked food mutagens on unscheduled DNA synthesis in cultured precision-cut rat, mouse and human liver slices

Precision-cut liver slices were prepared from male Fischer 344 rats, female CDF1 mice and humans (both male and female subjects). Liver slices were cultured for 24 hr in medium containing [3H]thymidine and either PhIP, IQ, MeIQ, MeIQx, Glu-P-1 or Trp-P-1, and then processed for auto-radiographic evaluation of unscheduled DNA synthesis (UDS). All six cooked food mutagens examined produced concentration-dependent increases in UDS in human liver slices. PhIP was the most potent compound examined, followed by MeIQx, IQ and then MeIQ, Glu-P-1 and Trp-P-1. Significant increases in UDS were observed with PhIP, IQ and MeIQx at concentrations as low as 5 microM in the culture medium. The same rank order of potency was not apparent in either rat or mouse liver slices. In rat liver slices only MeIQ significantly induced UDS, although positive results were obtained with two other genotoxins, namely 2-acetylaminofluorene and aflatoxin B1. Apart from MeIQx, all the cooked food mutagens produced significant increases in UDS in mouse liver slices. This study demonstrates the usefulness of precision-cut liver slices to evaluate species differences in xenobiotic-induced genotoxicity. Both marked compound and species differences in induction of UDS were observed. The data provide further evidence that dietary cooked food mutagens are potential human carcinogens.

Rat yolk sac explants as a system for studying the regulation of endodermal genes: down-regulation of the alpha-fetoprotein gene by dexamethasone and phorbol ester

The visceral yolk sac is a fetal membrane with essential placental functions. It is the major site of synthesis of alpha-fetoprotein (AFP), the most abundant plasma protein in the fetus. We developed a system of rat yolk sac explants in serum-free culture medium to study the regulation of endodermal gene expression in yolk sac. The explanted yolk sac tissues retained their double-sided morphology for up to 48 hours. The epithelial cells of both layers remained tightly joined on a basement membrane as seen by light and electron microscopy. This probably accounts for the continued expression of several endodermal cell-specific markers. The levels of mRNA encoding AFP, vitamin D-binding protein (DBP), hepatocyte nuclear factor 1alpha and beta transcription factors did not change during the 48-hour culture period. This reflects the stability of the differentiation state of the yolk sac endodermal cells. Dexamethasone and phorbol ester (TPA) specifically reduced the AFP mRNA level without affecting that of DBP. This suggests that these transduction pathways are functional in the yolk sac during this period of gestation and could be involved in the physiological down-regulation of AFP gene expression before birth. All these results show that this serum-free culture of rat yolk sac explants is a valuable system for further investigating the action of natural compounds and pharmacological drugs on endodermal gene expression during the embryonic and fetal periods.

The use of precision-cut rat lung slices for studying PGF(2α) metabolism

The suitability of a dynamic lung slice culture system as an in vitro model for studying pulmonary metabolism of PGF(2α) was assessed. [(3)H]Prostaglandin F(2α) ([(3)H]PGF(2α)), a twenty carbon fatty acid that contains a five-carbon ring and is known to be metabolized by lung in vivo, was incubated with precision-cut rat lung slices in 1.7 ml of Waymouth's buffer fortified with 10% fetal calf serum. At 0, 2, 4 and 8 h after addition of [(3)H]PGF(2α) (1.82 ng/μCi), incubation was stopped and the contents of each vial were analyzed for [(3)H]PGF(2α) and its metabolites using reversed-phase HPLC with radiochemical detection. PGF(2α) was metabolized to 15-keto PGF(2α), 13,14-dihydro-15-keto PGF(2α), and two unknown minor polar metabolites. These results indicate that PGF(2α) was metabolized in lung slices pathways similar to those seen in vivo. Slice viability was assessed by protein synthesis and light microscopic examination of lung slices through 24 h of incubation. Protein synthesis was maintained and no tissue necrosis was observed over the entire 24 h incubation, indicating that the lung slices were viable for at least 24 h. These results indicate that the dynamic lung slice culture system is an appropriate in vitro model for studying the pulmonary metabolism of PGF(2α).

Lack of effect of piperonyl butoxide on unscheduled DNA synthesis in precision-cut human liver slices

In this study the effect of piperonyl butoxide (PBO) on unscheduled DNA synthesis in precision-cut human liver slices has been examined. Liver slices prepared from tissue samples from five human donors were cultured in medium containing [3H]thymidine and 0-2.5 mM PBO using a dynamic organ culture system. After 24 h the liver slices were processed for autoradiographic examination of UDS. As positive controls, liver slices were also cultured with three known genotoxic agents, namely 2-acetylaminofluorene (2-AAF), aflatoxin B1 (AFB1) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. Compared to control liver slice cultures PBO had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced increases in the number of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. At the concentrations examined neither PBO, 2-AAF nor PhIP had any significant effect on replicative DNA synthesis in 24 h cultured human liver slices. In cultured liver slices treated with 0.02, but not 0.002, mM AFB1 a significant reduction in the rate of replicative DNA synthesis was observed. These results demonstrate that PBO does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. In conclusion, these results provide further evidence that PBO is a non-genotoxic agent which does not damage DNA in human liver.

Hormone- and endotoxin-modulated gene expression of a long-term organ culture system of adult rat liver

Precision-cut slices of normal adult rat liver maintained in serum-free medium remain hormone- and endotoxin-responsive for at least 48 h. They respond to glucocorticoid (dexamethasone) with the induction of the gluconeogenic enzyme tyrosine aminotransferase (TAT), as determined by enzymatic activity and by the increase in enzyme protein. Furthermore, endotoxin (LPS) induced nitric oxide synthase II (i-NOS), and this induction is repressed, similarly to the in vivo situation, by dexamethasone (DEX). All increases are inhibited by cycloheximide (CHX). The length of the period of responsiveness suggests that this organ culture system might be generally useful for studying the modulation of liver gene expression by physiological and pathological influences.

Use of rat and human liver slices for the detection of steroid hormone-induced DNA-adducts in vitro by means of the (32)P-postlabeling technique

Precision cut liver slices from humans and rats were used to investigate the covalent binding of xenobiotics to the DNA by means of the (32)P-postlabeling technique. Human liver slices were incubated with the structurally related steroid hormones chlormadinone acetate (5 mu g/ml), cyproterone acetate (0.01-5 mu g/ml), megestrol acetate (5 mu g/ml), and the positive control 2-aminofluorene (0.01-5 mu g/ml), which is known for its marked ability to form DNA-adducts in vivo. Rat liver slices were incubated with cyproterone acetate in concentrations of 0.1, 1, and 5 mu g/ml. The functional viability and metabolic activity of the slices were shown to be sufficiently maintained during the incubation time by measurement of the intracellular K(+)-content and the metabolic turnover of the model substrate 7-ethoxycoumarin, respectively. All three test substances and the control induced DNA-adducts in human liver slices, however, with a different adduct pattern. While the total DNA-adduct levels obtained with cyproterone acetate and megestrol acetate were in the same order of magnitude (on average 1000 DNA-adducts/10(9) nucleotides after incubation with 5 mu g /ml), the relative adduct labeling calculated for chlormadinone acetate was about 400. Following in vitro incubation of rat liver slices with cyproterone acetate, the relative adduct labeling values increased proportionally with increasing concentrations and added linearily to in vivo generated DNA-adducts. At the level of liver slices, different DNA-adduct patterns were induced by cyproterone acetate in rat and man. In contrast to the finding of others, using rat hepatocytes, the relative adduct labeling values of cyproterone acetate and megestrol acetate were in the same order of magnitude after incubation with human liver slices. The present study indicates that liver slices are a useful tool to investigate the in vitro DNA-adduct inducing potential of xenobiotics.

Precision-cut tissue slices: applications in pharmacology and toxicology

Almost a decade has passed since the first paper describing the isolation and maintenance of precision-cut liver slices produced using a mechanical tissue slicer was published (1). Although tissue slices of various organs have been employed as an in vitro system for several decades, the lack of reproducibility within the slices and the relatively limited viability of the tissue preparations has prevented a widespread acceptance of the technique. The production of an automated slicer, capable of reproducibly producing relatively thin slices of tissue, as well as the development of a dynamic organ culture system, overcame several of these obstacles. Since that time, significant advances in the methods to produce and culture tissue slices have been made, as well as the application of the technique to several other organs, including kidney, lung and heart. This review will i) summarize the historical use of tissue slices prior to the development of the precision-cut tissue slice system; ii) briefly analyze current methods to produce precision-cut liver, kidney, lung and heart slices; and iii) discuss the applications of this powerful in vitro system to the disciplines of pharmacology and toxicology.