Induction of Aryl Hydrocarbon (Benzo[a]pyrene) Hydroxylase in Liver Cell Culture by Temporary Inhibition of Protein Synthesis

Induction of CYP1A by carbofuran in primary culture of fish hepatocytes

Carbofuran is a nematicide used in agricultural fields throughout the world. Indiscriminate use of this pesticide poses severe detrimental effects on our ecosystem. We have shown that it induces the CYP1A (cytochrome P4501A) monooxygenase enzyme system in cultured hepatocytes from Indian catfish, Heteropneustes fossilis (Bloch). We have quantified this induction by measuring the activity of the enzyme 7-ethoxyresorufin-O-deethylase (EROD), synthesized from CYP1A1 gene. The induction followed a dose-dependent relationship with carbofuran. The dose-dependent curve of EROD using carbofuran was very much similar with beta-napthoflavone, which is a known inducer of CYP1A1. Coexposure of these compounds to the culture media showed a synergistic effect on the enzyme activity. A blocker of aromatic hydrocarbon receptor, alpha-napthoflavone, blocked carbofuran-induced EROD activity in a dose-dependent manner. All these findings suggest that metabolism of carbofuran might be mediated by the CYP1A monooxygenase system through binding of the aromatic hydrocarbon receptor. We have also studied the superinduction phenomenon, which is a typical characteristic of the CYP1A gene in our system.

Activation of Cyp1a1 and Cyp1a2 genes in adult mouse hepatocytes in primary culture

Expression of Cyp1a1 and Cyp1a2 genes was investigated in adult C57BL/6NCrj mouse hepatocytes in primary culture for up to 5 days. When the cells were cultivated as monolayers on collagen-coated dishes, CYP1A1 mRNA species were prominently induced after treatment with 3-methylcholanthrene (MCA) throughout the observation period. Substantial induction of CYP1A2 mRNA by MCA was also observed at day 1 of cultivation, followed by a decrease to very low levels thereafter. In contrast, when cultivated on non-coated dishes, the hepatocytes formed multicellular aggregates (spheroids) and prominent induction of both mRNA species was found for up to 5 days. Constitutive expression of CYP1A2 mRNA in spheroid culture was maintained throughout the observation period, whereas that in monolayer culture decreased rapidly. The time-course of the induced CYP1A2 mRNA amounts after the treatment with MCA or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) followed the same pattern as that of CYP1A1 mRNA. Expressed amounts of CYP1A1 or CYP1A2 mRNA in spheroid culture were higher than or similar to the levels in the case of in vivo production, respectively. Induction of both mRNA species was also observed in hepatocytes from nonresponsive DBA/2NCrj mouse in spheroid culture, but the expressed amount after MCA treatment was far smaller than for C57BL/6NCrj cells, despite equivalent expression in the two strains after TCDD. Activities of aryl hydrocarbon hydroxylase (AHH) and acetanilide 4-hydroxylase (AAH) were elevated with either type of cultivation after treatment with MCA or TCDD. Ratios of AAH to AHH were not changed between the two cultures after 24 h treatment. However, the ratios in spheroid culture after 48 h treatment increased, whereas they did not change in monolayer culture. The present observations indicate that the spheroid culture is more suitable than the monolayer system for studying the mechanism of Cyp1a2 gene expression in adult mouse hepatocytes.

Cell-density-dependent expression of Cyp1a2 gene in monolayer-cultured adult mouse hepatocytes

Expression of Cyp1a1 and Cyp1a2 genes was investigated in adult C57BL/6NCrj mouse hepatocytes for up to 5 days after transfer to monolayer culture. CYP1A1 mRNA was substantially induced by treatment with 3-methylcholanthrene during the observation period, independently of the seeded cell density. However, expression of CYP1A2 mRNA was dependent on cell density and was higher in cells cultivated at lower density. With increasing culture period the expression was decreased, so that only negligible levels were evident by day 5, and reduced expression of constitutive and induced CYP-1A2 mRNA became apparent earlier in more densely seeded cells. This was not related to differences in numbers of inducer molecules per cell. While mouse hepatocytes incorporated tritium-labeled thymidine under the given culture conditions, induction of expression of the two Cyp1a genes did not show any direct relationship with DNA synthesizing activity. These observations suggest some role for Cyp1a2 during changes in physiological state.

Altered regulation of Cyp1a-1 gene expression during cultivation of mouse hepatocytes in primary culture

Alterations in Cyp1a-1 gene expression in adult C57BL/6 mouse hepatocytes were followed after transferring them to primary culture during the initial 5 days. Changing the medium to a fresh one was associated with considerable amounts of Cyp1a-1 gene mRNA with a peak at around 6 hr after the medium change, followed by a decrease to negligible levels 24 hr later. Treatment of hepatocytes with cycloheximide increased the medium change-associated mRNA expression, the levels being equivalent to those observed after treatment with 3.2-25.6 nM 3-methylcholanthrene plus cycloheximide. With increasing length of culture period, cycloheximide-aided enhancement of the medium change-associated mRNA transcription increased. Although the chemical alone did not induce Cyp1a-1 gene transcripts in hepatocytes at day 1 or 2 of cultivation, for which medium had been changed 24 hr previously, prominent induction of transcripts was evident at later periods, the levels being elevated in accordance with length of time in culture. To examine whether or not the mRNA transcribed under these culture conditions was translatable, the cells were treated with actinomycin D after washing out the cycloheximide, in order to inhibit degradation of the generated mRNA (Nemoto N and Sakurai J, Carcinogenesis 12: 2115-2121, 1991). After these procedures significant elevation of aryl hydrocarbon hydroxylase activity was observed in hepatocytes, the rise being well correlated with elevated levels of mRNA transcripts. The observations suggest that the Cyp1a-1 gene might be expressed at low levels during the initial phase of cultivation of mouse hepatocytes in primary culture. Whether this expression might be essential for mouse hepatocytes to adapt to culture conditions is unclear. The findings do suggest, however, that superinducibility of the gene expression after cycloheximide treatment might be a result of a regulatory mechanism operating after adaptation to culture.

Multidrug resistance gene family and chemical carcinogens

The data discussed in this review indicate that the coordinated induction of both the mdr gene family and a subfamily of the cytochrome P-450 supergene family provide a unified response of the organism to prevent lethal accumulation of xenobiotics. Consequently, a distinct physiological role for the mdr multigene family now exists. Furthermore, recent evidence suggests the existence of multiple receptors with overlapping substrate specificity that are involved in the induction of both mdr and P-4501A gene families. The increased expression of mdr gene(s) in the early stages of liver carcinogenesis and presumably in other tissues is associated with the development of xenobiotic resistance that is observed in the preneoplastic cell populations. These observations may have important clinical implications and may provide an explanation for resistance to chemotherapy of tumors in organs such as liver and colon that are frequently exposed to both environmental and dietary xenobiotics.

New functional cell-culture approach to pulmonary carcinogenesis and toxicology

Modern pulmonary toxicology (including lung carcinogenesis) has, to assist its rapid development, constantly incorporated the knowledge obtained through cell and tissue-culture studies. While this has been carried out in rather a passive manner until quite recently, the currently necessary multi-disciplinary approach increasingly requires more active involvement of cell/tissue-culture techniques in this area. Our understanding in this regard is that one of such requirements is to establish a cell-culture system consisting of a single population of possible target cells for certain classes of hazardous inhalants. In addition, such target cells in culture should be able to function in a manner as closely resembling the situation in vivo as possible. In view of the culture techniques presently available, this requirement is probably too ideal to be met immediately. Nevertheless, efforts have been made in the last decade to achieve functioning cultures of Clara cells, type II pneumocytes or small mucus granule cells (SMGC), using undifferentiated cells obtained from animal and human fetuses. This attempt forms a sharp contrast to the usual approach, in that while the latter tries to keep the functions of adult cells in an already differentiated state, the former aims at inducing functional differentiation in undifferentiated cells by manipulating culture conditions. In carrying out these efforts, we have shown clear evidence that the type II pneumocytes and Clara cells induced in vitro are closely cognate and share a common precursor cell in culture, and that SMGC are at a pre-stage of differentiation to Clara cells. We have also shown an induced capacity for xenobiotic activation and conjugation in SMGC in culture. Our next plan is to prove similar activity (of mixed-function oxidase) in Clara cells and type II pneumocytes induced to differentiate in culture.

Binding characteristics of 4-S proteins from rat and mouse liver. High affinity of ellipticines

The binding characteristics of 4-S components (carcinogen-binding protein) from livers of Sprague-Dawley rats, C57BL/6 and DBA/2 mice have been examined before and after separation from other binding components presents in the cytosol. Competitive potency of 3-methylcholanthrene, benzo[alpha]pyrene, beta-naphthoflavone and 20 ellipticines, a series of compounds differently substituted on the dimethyl-pyrido-carbazole nucleus and deprived of carcinogenic activity, has been determined with [3H]3-methylcholanthrene and/or [3H]benzo[a]pyrene as radioligands. The inducing ability of the same compounds for aryl hydrocarbon hydroxylase and for ethoxyresorufin-O-deethylase has been compared to their affinity for the 4-S protein and the Ah receptor respectively. The main results of this study are as follows. 1. The intrinsic binding characteristics of 4-S proteins were dependent on both the nature of the radioligand used and the presence or absence of other cytosolic binding components. 2. The heterocyclic ellipticines were revealed as strong ligands for the carcinogen-binding protein (stronger than benzo[alpha]pyrene for five derivatives substituted in the A ring of ellipticine), with IC50 values ranging from 0.047 microM (8-hydroxyellipticine) to 5.8 microM (N2-ethyl-9-hydroxyellipticinium). 3. When the affinity of ellipticines was plotted versus their inducing ability of aryl hydrocarbon hydroxylase and ethoxyresorufin-O-deethylase, it appears that a good correlation exists for the Ah receptor but not for the 4-S protein. It is concluded that these data, as well as the lack of enzymatic induction after benzo[alpha]pyrene treatment of DBA/2 mice, which display a high level of 4-S protein, do not support the implication of this binding component in the positive control of cytochrome P-450 induction.

Differential effects of cycloheximide on rat liver cytochrome P-450 gene transcription in the whole animal and hepatoma cell culture

The induction of cytochrome P-450 (c+d) messenger RNAs in rat liver by 3-methyl cholanthrene follows a biphasic pattern. Administration of cycloheximide blocks the induction of cytochrome P-450 (c+d) messenger RNAs by 3-methylcholanthrene as well as cytochrome P-450 (b+e) messenger RNAs by Phenobarbitone. Transcription of these messenger RNAs in isolated nuclei is also blocked by cycloheximide administration. Thus cycloheximide not only fails to mimic the superinduction effects reported in hepatoma cell cultures, but actually blocks the specific transcription process. Exogenous hemin, while counteracting the effects of CoCl2 (heme biosynthetic inhibitor) on cytochrome (c+d) messenger RNA induction by the hydrocarbon, fails to counteract the effects of cycloheximide. It is suggested that a positive labile transcription factor is involved in the regulation of cytochrome P-450 gene expression in vivo.

Sister-chromatid exchange induction by carcinogens in HTC cells: an in vitro system which does not require addition of activating factors

The hepatic tumor cell line (HTC) was tested for the ability to produce sister chromatid exchanges (SCEs) in response to chemical carcinogens which require activation. Without the addition of exogenous microsomal enzyme preparations, cyclophosphamide, N-nitrosodiethylamine (DEN) and aflatoxin B1 (AFB1) induced significant levels of SCEs in these cells. Mitomycin C (MMC) and ultraviolet light, which do not require activation, also produced significant levels of SCEs. The induction of SCEs in HTC cells by AFB1 was shown to be inhibited by estradiol, a known inhibitor of microsomal activating enzymes. For the carcinogens tested, the HTC cell SCE assay was quite sensitive and comparable to other mammalian test systems. Exceptional sensitivity was found in the case of AFB1. SCE analysis of HTC cells offers a simplified system of detecting carcinogens requiring activation. This system also has the potential of investigating interactions between agents such as steroid hormones and carcinogens.

Carcinogenesis

An examination has been made of the major chemicals which are known to be carcinogenic, either in animals or in humans. As a result, a generalized type of chemical property seems to be a prerequisite for the carcinogenic activity. In most cases, this is some kind of reactive electrophilic intermediate produced directly from the carcinogen or from one of its immediate metabolites. This electrophilic reagent is generally a positively charged ion of some kind. In the case of the hydrocarbons it seems to be an incipient carbonium ion; in the case of nitrogen compounds, it is a corresponding nitronium ion. There are a number of possible substrate materials (nucleophilic) in a cells, but the dominant one which has been suspected is the nucleic acid component, either the DNA or the RNA. The bases of these materials all have extra pi electrons susceptible to attach by the electrophiles of the carcinogen. Such a modification, particularly of the DNA, could lead to either major or minor changes in the genetic composition of the cell. Minor changes are usually rectifiable or are not visible, and this would include a large variety of point mutations.