Metabolism of 2-acetylaminofluorene in primary rat hepatocyte cultures

High-Affinity Low-Capacity and Low-Affinity High-Capacity N-Acetyl-2-Aminofluorene (AAF) Macromolecular Binding Sites Are Revealed During the Growth Cycle of Adult Rat Hepatocytes in Primary Culture

Long-term cultures of primary adult rat hepatocytes were used to study the effects of N-acetyl-2-aminofluorene (AAF) on hepatocyte proliferation during the growth cycle; on the initiation of hepatocyte DNA synthesis in quiescent cultures; and, on hepatocyte DNA replication following the initiation of DNA synthesis. Scatchard analyses were used to identify the pharmacologic properties of radiolabeled AAF metabolite binding to hepatocyte macromolecules. Two classes of growth cycle-dependent AAF metabolite binding sites-a high-affinity low-capacity site (designated Site I) and a low-affinity high-capacity site (designated Site II)-associated with two spatially distinct classes of macromolecular targets, were revealed. Based upon radiolabeled AAF metabolite binding to purified hepatocyte genomic DNA or to DNA, RNA, proteins, and lipids from isolated nuclei, Site IDAY 4 targets (KD[APPARENT] ≈ 2-4×10-6 M and BMAX[APPARENT] ≈ 6 pmol/106 cells/24 h) were consistent with genomic DNA; and with AAF metabolized by a nuclear cytochrome P450. Based upon radiolabeled AAF binding to total cellular lysates, Site IIDAY 4 targets (KD[APPARENT] ≈ 1.5×10-3 M and BMAX[APPARENT] ≈ 350 pmol/106 cells/24 h) were consistent with cytoplasmic proteins; and with AAF metabolized by cytoplasmic cytochrome P450s. DNA synthesis was not inhibited by concentrations of AAF that saturated DNA binding in the neighborhood of the Site I KD. Instead, hepatocyte DNA synthesis inhibition required higher concentrations of AAF approaching the Site II KD. These observations raise the possibility that carcinogenic DNA adducts derived from AAF metabolites form below concentrations of AAF that inhibit replicative and repair DNA synthesis.

N-Acetyl-2-Aminofluorene (AAF) Processing in Adult Rat Hepatocytes in Primary Culture Occurs by High-Affinity Low-Velocity and Low-Affinity High-Velocity AAF Metabolite-Forming Systems

N-acetyl-2-aminofluorene (AAF) is a procarcinogen used widely in physiological investigations of chemical hepatocarcinogenesis. Its metabolic pathways have been described extensively, yet little is known about its biochemical processing, growth cycle expression, and pharmacological properties inside living hepatocytes-the principal cellular targets of this hepatocarcinogen. In this report, primary monolayer adult rat hepatocyte cultures and high specific-activity [ring G-3 H]-N-acetyl-2-aminofluorene were used to extend previous observations of metabolic activation of AAF by highly differentiated, proliferation-competent hepatocytes in long-term cultures. AAF metabolism proceeded by zero-order kinetics. Hepatocytes processed significant amounts of procarcinogen (≈12 μg AAF/106 cells/day). Five ring-hydroxylated and one deacetylated species of AAF were secreted into the culture media. Extracellular metabolite levels varied during the growth cycle (days 0-13), but their rank quantitative order was time invariant: 5-OH-AAF > 7-OH-AAF > 3-OH-AAF > N-OH-AAF > aminofluorene (AF) > 1-OH-AAF. Lineweaver-Burk analyses revealed two principal classes of metabolism: System I (high-affinity and low-velocity), Km[APPARENT] = 1.64 × 10-7  M and VMAX[APPARENT] = 0.1 nmol/106 cells/day and System II (low-affinity and high-velocity), Km[APPARENT] = 3.25 × 10-5  M and VMAX[APPARENT] = 1000 nmol/106 cells/day. A third system of metabolism of AAF to AF, with Km[APPARENT] and VMAX[APPARENT] constants of 9.6 × 10-5  M and 4.7 nmol/106 cells/day, was also observed. Evidence provided in this report and its companion paper suggests selective roles and intracellular locations for System I- and System II-mediated AAF metabolite formation during hepatocarcinogenesis, although some of the molecules and mechanisms responsible for multi-system processing remain to be fully defined.

Gas chromatography-mass spectrometry analysis of tert.-butyldimethylsilyl derivatives of 2-acetylaminofluorene and metabolites in isolated rat hepatocytes

A new technique for the conversion of 2-acetylaminofluorene and several ring-hydroxylated metabolites to mono- and di-tert.-butyldimethylsilyl derivatives was developed to permit their analysis by gas chromatography-mass spectrometry in order to quantify the metabolism of 2-acetylaminofluorene incubated in freshly isolated rat hepatocytes. This new gas chromatography-mass spectrometry method allowed the separation, identification and quantitation of seven known metabolites comprising five arylhydroxylated compounds, 2-aminofluorene and N-hydroxy-2-acetylaminofluorene.

Sex differences in the biotransformation of 2-acetylaminofluorene in cultured rat hepatocytes

Sex-related differences in susceptibility to 2-acetylaminofluorene (2-AAF) hepatocarcinogenicity and in vivo biotransformation of 2-AAF have been observed. In order to determine the contribution of hepatocytes to these differences, 2-AAF biotransformation was investigated in monolayer cultures of hepatocytes freshly isolated from male and female F-344 rats. In cultured hepatocytes from both sexes, ring and N-hydroxylated, deacetylated and conjugated metabolites were formed. The half-life of 2-AAF was similar at concentrations of 5 x 10(-6) and 10(-5) M; however, at 10(-4) M a slower rate was observed in cultures from males. Although the total formation of aqueous metabolites was similar, the ratio of sulfate to glucuronide conjugates of 2-AAF formed by hepatocytes from male and female rats differed. Sulfate conjugates predominated in hepatocytes from male rats, whereas in females, glucuronides predominated. The demonstration of sex-dependent variations in the rate of metabolism at a high concentration of 2-AAF and in conjugation provides evidence that in vivo differences are a function, at least in part, of the biotransformation characteristics of hepatocytes.

Effects of harman and norharman on the metabolism and genotoxicity of 2-acetylaminofluorene in cultured rat hepatocytes

Monolayers of rat hepatocytes metabolize 0.25 mM 2-acetylaminofluorene (AAF) to various ether-extractable, water-soluble as well as covalently bound products. The major ether-extractable metabolite formed is 2-aminofluorene (AF), followed by 7-OH-AAF and 9-OH-AAF. Pretreatment of rats with the inducer Aroclor 1254 (PCB) increased the metabolism of AAF and caused an increased DNA repair synthesis in hepatocytes exposed to AAF or AF. With N-OH-AAF, a decreased genotoxic response in PCB-treated cells compared to control cells was seen. The addition of harman and norharman decreased the metabolism of AAF to ether-extractable metabolites, water-soluble metabolites and metabolites covalently bound to macromolecules. In contrast, the DNA-repair synthesis caused by the same concentrations of AAF was increased by harman. One explanation for this apparent discrepancy could be that the aromatic amines changed the metabolism of harman and norharman in such a way that these compounds were converted into genotoxic metabolites.

Effect of incubation and activation conditions on the hepatocyte-mediated plate incorporation and preincubation Salmonella typhimurium mutagenesis assays

Primary cell-mediated microbial mutagenesis assays have been shown to be useful in detecting specific target organ genotoxic activity. The lack of a standard protocol for these assays, however, makes interlaboratory comparisons difficult. In order to standardize the hepatocyte-mediated Salmonella typhimurium mutagenesis assay, incubation and activation conditions for the plate incorporation and preincubation assays were examined using two aromatic amines, 2-aminofluorene (AF) and 2-acetylaminofluorene (AAF). Direct comparison of two preincubation protocols demonstrated the necessity for the hepatocytes to be present during the two- to three-day plate incubation period. An examination of various preincubation times showed relatively minor differences between 15 and 90 minutes. The preincubation and plate incorporation protocols were directly compared using both hamster and rat hepatocytes. For both preincubation and plate incorporation, the optimum concentration of hepatocytes was shown to be 1 X 10(6)/plate. Direct evaluation of various hepatocyte-mediated bacterial protocols should facilitate future interlaboratory comparisons using a more standardized procedure.

The comparative metabolism and toxic potency of aflatoxin B1 and aflatoxin M1 in primary cultures of adult-rat hepatocytes

Both aflatoxin B1 (AFB1) and a hydroxylated metabolite, aflatoxin M1 (AFM1), were potent cytotoxins and genotoxins to primary cultures of rat hepatocytes. However, AFB1 stimulated the release of lactate dehydrogenase into the culture medium and the loss of viable cells from the monolayer at lower doses than did AFM1. The lowest toxic doses of AFB1 and AFM1 were 0.05-01 and 0.6 microgram/culture, respectively. Genotoxicity, determined by an assay for stimulation of DNA repair, was apparent at lower doses than was cytotoxicity. AFB1 was again more potent than AFM1, stimulating DNA repair at 0.025 microgram/culture, compared to the lowest genotoxic dose of AFM1 of 0.05 microgram/culture. At higher doses (1.2-2.4 microgram/culture) the responses due to both aflatoxins in the cytotoxicity and DNA-repair assays were approximately equal. The metabolism of a low dose (c. 0.17 microgram/culture) of [14C]AFB1 and [3H]AFM1 by cultured hepatocytes differed significantly. After 1 hr, 50% of the [14C]AFB1 remained unchanged in the culture medium, whereas about 18 hr were required for the same amount of [3H]AFM1 metabolism to occur [14C]AFB1 was metabolized to AFM1, to polar metabolites recovered in the aqueous phase after chloroform extraction, and to metabolites covalently bound to hepatocyte macromolecules. [3H]AFM1 was also metabolized to polar metabolites and to forms bound to macromolecules. The degree of covalent binding of the aflatoxins correlated with their cytotoxicity and genotoxicity at lower doses. After a 24-hr incubation, 12.5% of the dose of [14C]AFB1 was covalently bound to macromolecules compared to 1.5% of [3H]AFM1. Although AFM1 was less potent than AFB1 in cytotoxicity, DNA-repair and covalent-binding assays using primary cultures of hepatocytes, AFM1 was still active at relatively low doses and therefore is probably a potent hepatotoxin in vivo.

Isolation and culture of adult hepatocytes from liver biopsies

Hepatocytes were isolated from liver biopsies of rats, guinea pigs, rabbits, dogs, and humans. The procedure is based on cannulation of large veins in the cut face of the biopsy, followed by collagenase perfusion. Yields averaged 19 x 10(6) viable hepatocytes/g liver. Viability averaged 84%, as determined by trypan blue dye exclusion. Cultures were prepared from the isolated hepatocytes and were found to be comparable in morphology and N-demethylase activity to hepatocyte cultures prepared by the in situ perfusion of the liver. The development of this method should facilitate comparative studies of the cytotoxicity, genotoxicity, and metabolism of foreign chemicals in primary hepatocyte cultures.