[Mutagenic activation and carcinogenicity of aminoazo dyes of ortho-aminoazotoluene and 3'-methyl-4-dimethylaminoazobenzene in experiments on suckling mice]

It is found that after administration of 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB,) which was hepatocarcinogenic to rats, in suckling mice, the number of neoplastic lesions in the liver of mice was 3 times higher than after analogous administration of equimolar dose of ortho-aminoazotoluene (OAT)). However, in the Ames test (TA-98 strain of Salmonella typhimurium) with activation by hepatic enzymes (S-9 fraction) of both intact and Aroclor-1254-induced mice and rats OAT contributed by an order of magnitude to revertant colonies compared to 3'-Me-DAB. In vivo inhibition of sulfotransferase activity, the enzyme which catalyzes the final stage of the mutagenic activation of aminoazo dyes, had no effect on carcinogenicity of 3'-Me-DAB but more than 4 times elevated that of OAT. It was concluded that the mechanism of carcinogenic action of aminoazo dyes studied is not genotoxic and that the carcinogenic potential of OAT is lost in the process of mutagenic activation.

Resistance of DRH strain rats to chemical carcinogenesis of liver: genetic analysis of later progression stage

The inbred DRH rats are highly resistant to the induction of hepatocellular carcinoma (HCC) by feeding of 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB). Previously, we found that two quantitative trait loci (QTLs), Drh1 and Drh2, significantly reduced the number, size and area of glutathione S-transferase-placental form (GST-P)-positive foci and GST-P mRNA levels in (F344xDRH)F(2) rat livers induced by feeding 3'-Me-DAB for 8 weeks. It is unclear, however, whether these QTLs affecting pre-neoplastic lesions are also the determinants of the later stage hepatocarcinogenesis, and whether there are any additional QTLs affecting hepatocarcinogenesis in the progression stage. To answer these questions, we analyzed QTL parameters for liver tumors in 99 (F344xDRH)F(2) rats induced by feeding 3'-Me-DAB for 20 weeks. The QTL parameters examined were GST-P mRNA, ornithine decarboxylase activity, and the number and total area of HCC/nodules macroscopically detectable on the liver surface. In composite interval mapping, we observed two major QTL peaks overlapping on the map positions of Drh1 on rat chromosome 1 (RNO1) and Drh2 on RNO4, respectively. The newly mapped QTL on RNO1 affected the GST-P mRNA level at 20 weeks of 3'-Me-DAB feeding, but did not affect the number and size of tumors. The primary effect of Drh1 is, therefore, to inhibit GST-P induction and to prevent enzyme altered foci (EAF) formation. On the other hand, the QTLs on RNO4, co-mapped to Drh2, affected all parameters of liver tumors examined except for the level of GST-P mRNA. The latter QTLs influenced not only the induction of GST-P and formation of EAF but also the progression of tumors in the later stage of hepatocarcinogenesis. The GST-P induction is differentially controlled by stages of hepatocarcinogenesis and the DRH resistance to carcinogenesis is principally attributed to the QTLs on RNO4 out of two resistance QTLs identified in the pre-neoplastic stage.

Immunohistochemical localization of transforming growth factor alpha in chemically induced rat hepatocellular carcinomas with reference to differentiation and proliferation

Hepatocellular carcinomas (HCCs) were induced in male Fischer 344 rats with dietary 3'-methyl-4-(dimethylamino)-azobenzene treatment and were classified into solid, glandular (well- or poorly differentiated), and trabecular types. Investigation of cell proliferation kinetics and immunohistochemical localization of transforming growth factor alpha (TGF-alpha) demonstrated all solid (n = 24) and poorly differentiated glandular type (n = 6) HCCs to have TGF-alpha-positive nuclei. Nuclear staining of TGF-alpha was also observed in 13 of 28 (46%) trabecular-type HCCs, whereas 12 (43%) exhibited cytoplasmic staining, and 3 (11%) were negative. As for well-differentiated glandular HCCs, 7 of 20 (35%) were positively stained in their nucleus, another 7 (35%) demonstrated antibody binding in the cytoplasm, and 6 (30%) were negative. The order for growth rate evaluated by bromodeoxyuridine (BrdU) labeling was solid (38.22%), poorly differentiated glandular (26.82%), trabecular (7.98%), and well-differentiated glandular (2.57%) types. For trabecular HCCs with nuclear, cytoplasmic, or negative TGF reactions, values were 13.39% (n = 13), 3.61% (n = 12), and 2.01% (n = 3), respectively. Likewise, BrdU-labeling indices for the counterpart groups of well-differentiated glandular type HCCs were 4.53, 1.91, and 1.29%, respectively. The results indicate that TGF-alpha expression might be linked to histopathological differentiation and cell proliferation in rat HCCs.

Changes in concentrations of methylglyoxal, D-lactate and glyoxalase activities in liver and plasma of rats fed a 3'-methyl-4-dimethylaminoazobenzene-rich diet

Donryu male albino rats were fed a diet containing 0.064% 3'-methyl-4-dimethylaminoazobenzene (MDAB) for 21 weeks. During the ensuing rat liver carcinogenesis, changes in the concentrations of methylglyoxal, D-lactate and glutathione as well as activities of glyoxalase I and II in liver and plasma were examined. After the start of the diet, hepatic contents of methylglyoxal and D-lactate increased to about 7 and 3 times that of the control, respectively. However, after 21 weeks the D-lactate content decreased from the elevated level, but remained at a higher level of 1.4 times the control. The hepatic glyoxalase I activity increased 1.2 to 1.7 times over the control during carcinogenesis, while glyoxalase II activity increased 160% during the precancerous state and decreased to 55% of control at 21 weeks. the hepatic level of reduced glutathione (GSH) increased and peaked after 4 weeks of the MDAB diet and decreased thereafter to 57% of the control level after 21 weeks. Both pyruvate and L-lactate levels increased in the liver and plasma of MDAB-fed rats when rats had obvious symptoms of hepatoma.

Changes of expressions of phosphotyrosine phosphatases in rat hepatocellular carcinoma induced by 3'-methyl-4-dimethylamino-azobenzene

Using rats with hepatocellular carcinoma induced by 3'-methyl-4-dimethylamino-azobenzene (3'-MeDAB), we evaluated the expression of a protein tyrosine phosphatase (PTPase) in the tumor region, non-tumorous region and control rat liver. The expression of SHPTP2 increased 4.1 fold (p < 0.05) in mRNA, 2.1 fold (p < 0.01) in cytosol fraction, and 5.1 fold (p < 0.05) in membrane fraction, respectively, at a protein level in the tumor region compared with control liver. The expression of other phosphatases, LAR, LRP, and PTPase1B, did not change significantly. SHPTP2 phosphatase activity in the tumor region from rats also increased compared with control, suggesting that an increase of this activity may parallel the expression of SHPTP2. This increase of expression of SHPTP2 may contribute to the progression of hepatocellular carcinoma in this rat model.

Gomisin A, a lignan component of Schizandora fruits, inhibits development of preneoplastic lesions in rat liver by 3'-methyl-4-dimethylamino-azobenzene

The effects of gomisin A, a lignan component of Schizandra fruits, on development of preneoplastic lesions in the liver after a short-term (3 weeks) feeding of 3'-methyl-4-dimethyl-aminoazobenzene (3'-MeDAB) to male Donryu rats were investigated, and compared with the effects of phenobarbital. Gomisin A inhibited both increases of the level of glutathione-S-transferase placental form (GST-P) and the number and size of GST-P positive foci in the liver increased after treatment with 3'-MeDAB. Moreover, although the population of diploid nuclei was increased and that of tetraploid nuclei was decreased by pretreatment with 3'-MeDAB, gomisin A returned this to near the normal ploidy pattern. But phenobarbital increased the level of GST-P and the number and size of GST-P positive foci with little affect on the ploidy population changed by 3'-MeDAB. Thus, the effect of gomisin A on hepatocarcinogenesis was inhibitory in contrast with that of phenobarbital. This study suggests that gomisin A is a candidate for a chemopreventive drug inhibiting the promotion process in hepatocarcinogenesis.

Expression of non-hepatic-type S-adenosylmethionine synthetase isozyme in rat hepatomas induced by 3'-methyl-4-dimethylaminoazobenzene

It is known that a high incidence of hepatocellular carcinoma in rat liver can be induced by such azo dye carcinogens as 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB). Mammalian S-adenosylmethionine (AdoMet) synthetase exists as two isozymes, non-hepatic-type and liver-type enzymes, which are the products of two different genes. We have examined the expression of two AdoMet synthetase isozyme proteins and mRNAs in rat hepatomas induced by 3'-Me-DAB. The levels of non-hepatic-type enzyme protein and mRNA are clearly induced by 3'-Me-DAB feeding. On the other hand, the levels of liver-type enzyme protein and mRNA are nearly the same or slightly decreased during hepatocarcinogenesis. These results indicate that the expression of the non-hepatic-type isozyme gene is obviously influenced with the progression of carcinogenesis and that the non-hepatic-type isozyme is useful as a oncodevelopmental marker in the liver.

Marked increases of two kinds of two-exon-skipped albumin mRNAs with aging and their further increase by treatment with 3'-methyl-4-dimethylaminoazobenzene in Nagase analbuminemic rats

Nagase analbuminemic rats (NARs) have a 7-base-pair deletion at the 5' splice site of the HI intron of the albumin gene. The level of immunohistochemically albumin-positive hepatocytes is about 1 per 10(5) cells in neonatal NARs, increases with age, and further increases with chronic oral treatment with 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB). The mechanisms involved in the increase in albumin-positive hepatocytes during aging of NARs and their treatment with 3'-MeDAB were analyzed. NARs were found to have four species of albumin mRNA: intact mRNA and those lacking the regions corresponding to exon H, exon G-H, and exon H-I. In 4-week-old NARs, the level of intact albumin mRNA was about 1/4000 of that in normal rats and mRNA lacking the exon H sequence was the major species. In aged and 3'-MeDAB-treated aged NARs, all four species of mRNA increased and the relative proportion of mRNAs lacking two exon sequences to mRNAs lacking one exon sequence was greatly increased, suggesting that aging is associated with changes of the splicing pattern and that 3'-MeDAB treatment enhanced these changes. In aged NARs and 3'-MeDAB-treated aged NARs, there was an increase in the amount of aberrant 60-kDa albumin. The 60-kDa protein could be a translation product of mRNAs lacking two exons, the amount of which increases in aged NARs and 3'-MeDAB-treated NARs.

Low frequency of ras activation in 2-acetylaminofluorene- and 3'-methyl-4-(dimethylamino)azobenzene-induced rat hepatocellular carcinomas

Activation of ras protooncogenes in 11 2-acetylaminofluorene (AAF)-induced hepatocellular carcinomas (HCCs) or liver cell lines and 9 3'-methyl-(dimethylamino)azobenzene (3'-Me-DAB)-induced HCCs in rats was examined using the NIH3T3 cell transfection assay and oligonucleotide hybridization analysis. Only one cell line established from a AAF-treated rat liver demonstrated transforming activity with a point mutation and ATTA transversion at the second position of H-ras codon 61. The rates of ras activation were thus very low for both AAF- and 3'-Me-DAB-induced rat HCCs, the results thus extending and confirming the findings indicating that ras activation in rat HCCs induced by various type of carcinogens is infrequent.

Analysis of cytological changes in hepatocytes from rats dosed with 3'-methyl-4-dimethylaminoazobenzene: initial response appears to involve cytokinesis of binucleated cells

The reduction in the ratio of tetraploid (4N + 2 X 2N) to diploid (2N) hepatocytes in the adult rat after treatment with the hepatocarcinogen 3'-methyl-4-dimethylaminoazobenzene (3'M) has been investigated. Analysis of isolated hepatocytes 18-28 days after treatment has confirmed that initially some of the 2 X 2N hepatocytes are converted into 2N cells by cytokinesis, and that there is no DNA synthesis during this process. Shortly afterwards nonpolyploidizing growth commences by proliferation of some 2N cells.

Is macronodular liver cirrhosis an irreversible lesion? Changes in the intrahepatic micro-vasculature treated with estradiol benzoate in rats

Estradiol benzoate (EB), a synthetic estrogen, was injected to macronodular cirrhotic rats, and the reversibility of the lesion was investigated. Macronodular liver cirrhosis (MNLC) was produced by use of carbon tetrachloride plus 3'-Me-DAB. In animals with no treatment, regenerative nodules remained unchanged during an eighteen-week observation period. Microangiograms showed markedly deranged intrahepatic microvasculatures, including an irregularly contoured vascular tree and rarefaction in the peripheral area. On the other hand, EB treatment exerted conspicuous efficacy on the above-mentioned intrahepatic microcirculatory disorder and on sinusoidal dilatation, which led the livers with MNLC to revert to almost normal appearance. It is concluded that EB provides a salutary effect on the intrahepatic microcirculatory system, by means of which MNLC can be shown to be a reversible lesion.

Changes in number of alpha-adrenergic receptor subtypes in hepatocytes from rats fed 3'-methyl-4-dimethylaminoazobenzene

Changes in numbers of alpha 1- and alpha 2-adrenergic receptors in the plasma membranes of hepatocytes from female Donryu rats given feed containing 0.06% of the carcinogen 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB), were examined. alpha 1-Adrenergic receptors, measured in terms of [3H]prazosin binding, decreased to half of the control 2 weeks after the start of this diet, then gradually decreased for the next 22 weeks. alpha 2-Adrenergic receptors, measured in terms of [3H]clonidine binding, transiently increased 3-fold over the control at 2 weeks. These changes in the early period of the 3'-MeDAB diet intake may be related to hepatocarcinogenesis.

Synergistic effects of low-dose hepatocarcinogens in induction of glutathione S-transferase P-positive foci in the rat liver

The effects of combined administration of hepatocarcinogens at low doses on the development of glutathione S-transferase P-form (GST-P)-positive foci of rat liver were examined utilizing a bioassay model which consists of a single injection of diethylnitrosamine (DEN, 200 mg/kg, ip), two-thirds partial hepatectomy at week 3 and a 6-week administration of test compounds. The chemicals used, 2-acetylaminofluorene (2-AAF), 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB), phenobarbital (PB), thioacetamide (TAA), N-ethyl-N-hydroxyethylnitrosamine (EHEN), benzo[a]pyrene (B[a]P), carbazole, and alpha-hexachlorocyclohexane (alpha-HCH) were incorporated in the diet, except for EHEN which was dissolved in the drinking water, at levels of 1/6 of the doses usually used. The combinations were: I) 2-AAF, 3'-Me-DAB, PB, TAA, EHEN and B[a]P, II) 2-AAF, 3'-Me-DAB and PB, III) TAA, EHEN and B[a]P, IV) 2-AAF, 3'-Me-DAB, carbazole, TAA, EHEN and alpha-HCH, V) 2-AAF, 3'-Me-DAB and carbazole, and VI) TAA, EHEN and alpha-HCH. All combinations, except for II, caused an increase in the area of the foci as evaluated by the ratios of areas in the combined administration groups to the sum totals of 3 or 6 individual data: I) 1.75, II) 0.81, III) 2.01, IV) 3.62, V) 1.34 and VI) 2.91. The non-synergistic effect in combination II might be related to PB induction of hepatic microsomal enzymes leading to enhanced enzymatical detoxification of 2-AAF and 3'-Me-DAB. The present results indicate that exposure to several chemicals of similar organotropism, even at doses lower than the apparent carcinogenic levels, might be critical to the carcinogenic process.

Transformation of cultured fetal rat liver cells by MDAB and phenobarbital. Morphological, biochemical and immunocytochemical characterization of cell lines

Fetal rat liver cells derived from 19-day gestation rats were exposed in culture to the carcinogen, 3'-methyl-4-dimethyl-aminoazobenzene (MDAB) for 3 days and then maintained in medium supplemented with the tumor promoter, phenobarbital (PB). Tumors developed in immunodeficient mice inoculated with cells derived from cultures which had been maintained for more than 8 weeks. Histologically, three types of tumors could be distinguished. One contained epithelial-like cells, which resembled what has previously been described as 'clear' epithelial cells. The second contained cells which were more basophilic, with prominent nuclei and closely resembled the hepatoma cell line Mc-A-R-777. The third group of tumors possessed cells of both varieties. Cell lines derived from these tumors were then characterized by determining their capacity to synthesize and secrete alpha-fetoprotein, albumin and transferrin by measuring the incorporation of 35S-methionine into immunoprecipitates obtained by reaction with the respective specific antibodies and the content of the respective mRNAs were determined by hybridization to cDNAs. The activity of gamma-glutamyl-transpeptidase (GGT) and the liver specific enzyme tyrosine aminotransferase (TAT), as well as the induction of TAT by dexamethasone was also evaluated. The presence of these markers in some of the cell lines strongly suggests that they are derived from parenchymal cells. In contrast, other cell lines which morphologically resemble 'clear' epithelial cells are negative, suggesting that they may be derived from non-parenchymal epithelial cells which exist in the original culture. However, some epithelial-like cell lines derived from tumors of mixed morphology appear different to those established from tumors which contained only epithelial-like cells. These express low levels of transferrin and tyrosine aminotransferase suggesting that they may be more closely related to hepatocytes than those cells which are derived from tumors which originally comprised only epithelial cells. The absence or presence of liver markers correlates with the morphology of the respective cell lines since transferrin and TAT are only present in significant levels in those lines which comprise cells with a morphology resembling hepatoma cell lines. In cell lines which show mixed morphology, immunocytochemistry reveals that significant amounts of transferrin are only present in the parenchymal-like population. Growth rate measurements show that the faster growing cell lines generally possessed lower levels of transferrin and TAT expression. It can be concluded from these studies that it is possible to transform cells derived from fetal rat liver in culture using a hepatocarcinogen.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative electron microscopy of carcinogen-induced alterations in hepatocyte rough endoplasmic reticulum. II. Modulation of the effects of 3'MeDAB by adrenalectomy and adrenal corticosteroids

A quantitative electron microscope study was made of the effects of bilateral adrenalectomy (ADX) and deoxycorticosterone acetate (DOCA) on the state of aggregation of hepatocyte rough ER cisternae in both untreated and 3'MeDAB-fed Leeds strain rats. Disaggregation of hepatocellular rough ER appears to be a common response of the rat liver to hepatocarcinogenic insult, while ADX and DOCA treatment are known to inhibit the chemical induction of neoplasia in this species. In untreated animals both ADX and DOCA significantly increased the mean number of cisternae per array or stack, while an even more pronounced effect was obtained from a combination of the two. The carcinogen 3'MeDAB induced a highly significant loss of aggregation, which was prevented by the combination of ADX and DOCA. In a separate experiment, a single dose of cortisone acetate was also found to partially reverse 3'MeDAB-induced rough ER disaggregation. In the rat hepatocyte, rough ER stacking or aggregation appears to be at least partially under hormonal control.

Effect of fasting on aggregation of hepatocyte rough endoplasmic reticulum in adrenalectomized and 3'MeDAB-treated rats: quantitative electron microscope study

A method for the quantitative analysis of the degree to which hepatocyte rough endoplasmic reticulum (ER) is arranged into parallel arrays was used to study the effect of fasting on rough ER aggregation in rat liver cells following either bilateral adrenalectomy or administration of the carcinogenic azo dye 3'-methyl-4-dimethylaminoazobenzene (3'MeDAB). One group of male inbred Leeds strain rats was subjected to bilateral adrenalectomy (ADX): after 1 week half of the animals were fasted for 24 h whereupon the whole group was killed. A second group of rats, fed for 4 weeks on a diet containing 0.06% of the carcinogenic azo dye 3'MeDAB, was similarly divided into two groups that were killed either with or without a prior 24-h fast. Untreated control groups of rats, both fasted and unfasted, were also killed. A quantitative electron microscope study was carried out to investigate the effect of each treatment on the degree to which the hepatocyte rough ER was aggregated into parallel arrays. ADX alone was without effect but caused a dramatic fall in rough ER aggregation when combined with fasting. At least as great an effect was induced by 3'MeDAB, with or without fasting, while fasting alone had a significant but much more modest effect than either ADX or the carcinogen. Thus, two disparate treatments induced morphologically identical responses in hepatocyte rough ER. The implications of this are discussed in terms of known interrelations between glucocorticoids and chemical carcinogenesis in the rat liver.

Effects of 3'-methyl-4-dimethylaminoazobenzene on RNA processing in rat liver nuclei

Treatment of rats with the hepatocarcinogen 3'-methyl-4-dimethylaminoazobenzene (3'MeDAB) leads to the accumulation of polyadenylated RNA in rat liver nuclei. This polyadenylated nuclear RNA contains more double-stranded stretches than polyadenylated nuclear RNA isolated from liver of control animals. Treatment with 3'MeDAB also results in an increase of sequences homologous to small chromatin-associated RNA (fr 3-RNA) in the polyadenylated nuclear RNA and in the appearance of these sequences in polyadenylated nuclear RNA which contains some double-stranded stretches. The last mentioned fraction of nuclear RNA from liver of control animals did not hybridize with a DNA probe for fr 3-RNA. It is proposed that, perhaps as a result of inhibition of RNA processing by 3'MeDAB, the transient stages of RNA processing can be observed in animals treated with this compound.

[Increased content of nucleotide sequences in transcriptionally active DNA and poly(A)+-mRNA of the rat liver and a rise in its translation activity as affected by inducers]

Cortisol and 3'-methyl-4-dimethyl-amino-azobenzene induce an increase in the content of repeated sequences (RS) in transcriptionally active (TA) DNA, while the content of respective RS in potentially active DNA fractions enriched with regulatory regions of the genome decreases. RS content in induced poly A+-mRNA also rises, as determined by the nature of hybridization of respective c DNA with total DNA. The translation of induced poly A+-mRNA rises essentially, with the qualitative distinctions in in vitro synthesized protein product spectrum being absent. Inducible RS with unstable chromatine conformation are thought to provide a universal system of rapid response of the genetic apparatus to extreme situations, serving as transcription intensifiers in TA DNA and as translation intensifiers in induced poly A+-mRNA.

Early modifications of gene expression induced in liver by azo-dye diet

The expression and regulation of the phosphoenolpyruvate carboxykinase gene were not grossly modified by feeding rats a 3'-methyl-4-(dimethylamino)azobenzene-containing diet despite maximum expression of the L-type pyruvate kinase gene being dramatically reduced as early as the 24th hour of the carcinogenic diet. Inhibition of aldolase B mRNA synthesis occurred more slowly, being maximum at the 3rd day. After stopping administration of the carcinogen, a very rapid, but transient increase of the L-type pyruvate kinase mRNA was observed at the 24th hour, whereas aldolase B mRNA increased only slowly. The amount of aldolase A mRNA fell quickly after termination of carcinogen administration, levels being normal at the 2nd-3rd day. At this time, the histological structure of the liver was indistinguishable from that of animals still receiving the azo-dye diet. It appears, therefore, that in the rat both administration and withdrawal of the azo-dye carcinogen induce rapid modifications of the expression of some genes, before any cellular modification is distinguishable.

Promotion of 3'-methyl-4-dimethylaminoazobenzene-initiated adult rat liver cells to malignant state by phenobarbital in culture

After treatment with 1.5 mM phenobarbital (PB) for about 2 months, non-tumorigenic rat liver epithelial-type cell lines (6F-7 and 6G-5), which were initiated by 3'-methyl-4-dimethyl-aminoazobenzene (3'-Me-DAB) in primary culture, showed an increase in plating efficiency, growth in soft agar and production of tumors in syngeneic animals. However, the PB-untreated control cells exhibited neither growth in soft agar nor production of tumors. Gamma-glutamyltranspeptidase (GGT)-positive (6F-7-cl-7) and negative (6F-7-cl-8) colonial clones, which were derived from 6F-7 cells, were also treated with 1.5 mM PB for about 2 months. The growth of GGT-positive 6F-7-cl-7 cells was not suppressed but rather stimulated by PB at 1.5 mM, whereas that of GGT-negative 6F-7-cl-8 cells was severely inhibited. Furthermore, the PB-treated 6F-7-cl-7 cells produced tumors in syngeneic animals, but the PB-treated 6F-7-cl-8 cells produced no tumors. The spontaneous cell line 6A-6, which was derived from the 3'-Me-DAB-untreated primary liver cell culture and was GGT-negative, did not exhibit tumorigenicity after treatment with 1.5 mM PB for the same period of time as described above. In conclusion, PB treatment efficiently and rapidly promoted the 3'-Me-DAB-initiated cells to a malignant state.

Azo dye-induced alterations in vitamin B-6 metabolism and in pyridoxal 5'-phosphate-binding proteins in rat liver

The effects of the hepatocarcinogen, 3'-methyl-4-dimethylamino-azobenzene, on vitamin B-6 metabolism in rat liver were studied. The following parameters were measured: pyridoxal 5'-phosphate (PLP) concentrations in plasma, brain, liver and azo dye-induced hepatomas, as well as the activities of pyridoxine (PN) kinase, pyridoxine 5'-phosphate (PNP) oxidase, PNP phosphatase and PLP-dependent ornithine decarboxylase. Hepatomas more closely resembled fetal than normal adult rat liver with respect to their ability to convert vitamer forms such as PN to coenzymatically active PLP. Microtiter plate enzyme-linked immunosorbent analyses revealed that the absence of PNP oxidase activity in a dissectable hepatoma was attributable to the absence of enzyme protein. In addition, monoclonal antibodies to vitamin B-6 were used in a Western immunoblot technique to examine the effects of azo dye ingestion on the pattern of PLP-binding proteins in cytosolic extracts of liver and hepatomas. Nitrocellulose blots of electrophoretically resolved cytosolic extracts probed for PLP-binding proteins showed increasing complexity with development; hepatomas bore a striking resemblance to fetal liver. The data indicate that hepatomas lose the properties of terminally differentiated hepatic tissue and take on the properties of fetal hepatic tissue characterized by lower concentrations of PLP, selective use of the coenzyme, and a lowered-to-absent capability to convert precursor vitamer forms to PLP. Therefore, with respect to vitamin B-6 metabolism and use, it appears likely that azo dye-induced hepatocarcinogenesis involves proliferation of a stem cell type(s) having the phenotypic characteristics of fetal hepatic tissue.

Comparison of drug-metabolizing activities in the livers of carcinogen-sensitive parent rats and carcinogen-resistant descendants

Donryu strain albino rats were maintained on a diet containing 0.06% 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) for nine successive generations. Some rats in the fourth to eighth generations showed marked resistance to the carcinogenic action of 3'-Me-DAB. In the liver where we found tumors, their size and number are smaller than in the corresponding original strain of rats fed on a diet containing 3'-Me-DAB. No significant differences were found in the total cytochrome P-450 contents or epoxide hydrolase activities of the livers of the resistant variant and the original strain, but the benzo(a)pyrene hydroxylase activity which is mainly attributed to cytochrome P-448 and glutathione S-transferase activity of the resistant variant were lower. The inductions of hepatic cytochrome P-488 and benzo(a)pyrene hydroxylase on administration of polychlorinated biphenyls or 3-methylcholanthrene were also lower in the resistant rats. In the mutagenicity test on Salmonella typhimurium TA 98 the liver 9000 X g supernatant fraction from 3'-Me-DAB-resistant F7 rats did not fully induce the mutagenicities of 3'-Me-DAB and several other carcinogens. Thus the resistance of F7 rats to the chemical carcinogen may be related to the lower activities of some drug-metabolizing enzymes and the poor inducibility of cytochrome P-448 in their liver, although selection of resistant rats should be continued for further generations before coming to a definite conclusion on biochemical basis of apparent resistance to 3'-Me-DAB.

Effect of phenobarbital on adult rat liver cells treated with 3'-methyl-4-dimethylaminoazobenzene in primary culture

The effect of phenobarbital (PB) on liver cells treated with 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) was studied using primary cultures of normal adult rat liver cells. Following a 1-day attachment period, primary liver cell cultures were treated with 0.24 mM 3'-Me-DAB for 6 days, and then treated with or without PB at 0.75, 1.5 and 3 mM for 19 days. Similarly, control cultures were treated with 0.5% dimethylsulfoxide (DMSO), a solvent for 3'-Me-DAB, for 6 days, and then treated with or without PB in the same way. Each treatment was done on 8 cultures. Chromosome analysis and cytochemical assay for gamma-glutamyltranspeptidase (GGT) activity were carried out on the carcinogen-treated and control cultures between 1 and 2 months after initiation of primary culture. Chromosomal abnormalities were detected in 23 of 32 carcinogen-treated cultures and also in 2 of 28 control cultures tested. However, GGT positive cells were detected only in the carcinogen-treated cultures at a frequency of 22/32. Of the 23 carcinogen-treated cultures with chromosomal abnormalities, 18 contained GGT positive cells. These results show a good correlation between chromosomal abnormality and acquisition of GGT activity at culture dish level. Furthermore, in the carcinogen-treated cultures, PB treatment caused a dose-dependent increase in the number of GGT positive cultures and in the percentage of GGT positive cells in each culture, and also caused a dose-dependent increase in the number of cultures with chromosomal abnormalities.

Enhancement of hepatocarcinogenesis by sorbitan fatty acid ester, a liver pyruvate kinase activity-reducing substance

Effect on hepatocarcinogenesis of dietary sorbitan fatty acid ester (SorFAE), which had been known to cause decrease in pyruvate kinase (PK) activity, was studied in rats fed a diet containing 0.06% 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) for 6 weeks. The incidence of hyperplastic nodules and/or hepatocellular carcinomas in the rats fed the 3'-Me-DAB diet alone was 45.0% at the end of 51 weeks, whereas the incidence in the rats fed 3'-Me-DAB diet followed by 5 or 10% SorFAE or 0.1% phenobarbital (PB) diet were 76.2, 90.5, and 95.0%, respectively. These incidences were significantly higher compared with the group fed 3'-Me-DAB diet alone (P less than .05). No tumors were observed in rats fed 10% SorFAE diet alone. The results show that SorFAE has an enhancing effect on hepatocarcinogenesis, although the effect was weak compared to that of the effective PB dose. The results seem to confirm our assumption that a chemical that causes decrease in PK activity in rat liver might promote hepatocarcinogenesis.

Long-term survival of functional hepatocytes from adult rat in the presence of phenobarbital in primary culture

In the presence of phenobarbital (PB) at 3 mM, hepatocytes isolated from adult rats by a collagenase-perfusion technique survived well on plastic dishes for at least 49 days after initiation of primary culture. PB at concentrations less than 3 mM was ineffective for the maintenance of hepatocytes, and the maintenance of them was attained only in the continuous presence of 3 mM PB. The hepatocytes surviving in the presence of 3 mM PB were morphologically indistinguishable from the hepatocytes after 1-day attachment period, except for the presence of prominent nucleoli in the former. Although both the albumin secretion and tyrosine aminotransferase (TAT) activities of the cells decreased gradually up to day 7 with time in culture, both were thereafter maintained at relatively high levels at least up to day 35 of primary culture. The addition of 10 microM dexamethasone caused a 3-5-fold induction in TAT activity, and the cells were capable of responding to the hormone in this manner at least up to day 28 of primary culture. Furthermore, the cells also had glucose-6-phosphatase activity, even though the level of this enzyme activity was relatively low as compared with that of TAT activity. Survival of hepatocytes in the presence of 3 mM PB was further enhanced by simultaneous addition of dexamethasone (10 microM) and insulin (10 micrograms/ml). The sensitivity of hepatocytes to 3'-methyl-4-dimethylaminoazobenzene (0.24 mM) was remarkably reduced by treatment with PB at 3 mM. PB treatment decreased efficiently the falling rate of total cytochrome P-450 content, but did not induce P-450PB, which is the specific form of cytochrome P-450 induced by PB, in primary cultured hepatocytes. On the other hand, 3-methylcholanthrene (MC, 10 microM) caused an increase of both contents of total cytochrome P-450 and P-450MC, which is the specific form of cytochrome P-450 induced by MC, in primary cultured hepatocytes. However, MC was ineffective for the maintenance of hepatocytes in primary culture. The possible biological actions of PB on primary cultured hepatocytes are discussed on the basis of the experimental data obtained.

Effect of administration of 2-methyl-4-dimethylaminoazobenzene on the half-lives of rat liver mitochondria and cytochrome oxidase

The turnover of total mitochondrial proteins and cytochrome oxidase in the livers of rats administered with 2-methyl-4-dimethylaminoazobenzene (2-Me-DAB) has been determined. The incorporation of [14C]bicarbonate revealed a half-life of 3.1 days in control and 6 to 9 days in azodye administered animals for whole mitochondrial proteins. The incorporation of [35S]methionine yielded t1/2 values of 8.5 days and 15.4 days, respectively. The t1/2 of cytochrome oxidase, 10.8 days for control and 19.3 days for 2-Me-DAB-treated animals, indicated that the delay in the decay of the enzyme was of the same order as that of whole mitochondria. Short term incorporation revealed that the administration of the azodye stimulated the synthesis of the enzyme. Mitochondria isolated from azodye-administered animals appeared less susceptible to lysosomal proteolysis. Also, azodye administration seemed to impair the ability of lysosomes to degrade mitochondria.

Functional changes in rat liver mitochondria on administration of 2-methyl-4-dimethylaminoazobenzene

Administration of 2-methyl-4-dimethylaminobenzene in the diet (0.1%, w/w) for 85-90 days doubled the content of mitochondria in the livers of rats. The azodye was covalently bound to liver proteins, and about 15% of the amount found in liver was associated with the mitochondrial fraction. Mitochondria isolated from the livers of azodye-fed animals showed drastically lowered ability to oxidize NAD+-linked substrates. The inhibited electron-transfer step was the reduction of ubiquinone. The organelles showed a large increase in succinate oxidase activity. The activity of cytochrome oxidase and the content of cytochrome aa3 were substantially higher in these organelles. Azodye-fed animals showed depressed serum cholesterol concentrations. The content of ubiquinone in liver also registered a small increase.

Increased nucleoside triphosphatase activity of rat liver nuclear envelope is associated with hepatocarcinogen exposure

Altered transport of nuclear RNA sequences is an early response to carcinogens. Nuclear envelopes (NE) were isolated and assayed for nucleoside triphosphatase activity (NTPase), on the premise that this enzymatic activity participates in RNA transport. A common feature of the action of five different carcinogens (thioacetamide, 2-acetylaminofluorene, 3'-methyl-4-dimethylaminoazobenzene, dimethylnitrosamine, and aflatoxin B1), at low doses without significant toxicity, was to increase NE NTPase activity and to increase RNA transport, as assessed by the appearance of rapidly labeled RNA in the cytoplasm and by in vitro assay. The increases in NTPase were specific for the NE fraction, and early toxic effects of higher doses initially masked the increases. The induced increases in NE NTPase were long-lived. In contrast, increases in NE NTPase were observed only during the regenerative phase of CCl4 intoxication; the CCl4-induced increase was short-lived and returned promptly to control levels. These changes in NTPase activity were not associated with parallel changes in phosphorylation/dephosphorylation of NE proteins. Increases in NE NTPase and alterations in RNA transport, without attendant nuclear replication, may relate to altered nuclear RNA restriction. This change in a regulatory phenomenon may make these cells more susceptible to further modification, potentially playing a role in the initiation phase of carcinogenesis.

Decrease in the phosphorylation of the proteins associated with heterogeneous nuclear RNA after the application of 3'-methyl-4-dimethylaminoazobenzene

The effect of 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB) on the phosphorylation of the proteins of the nuclear ribonucleoprotein (RNP) particles was studied in liver of rats. Forty eight hours after the application of 4 mg of the hepatocarcinogen per 100 g of body wt. by stomach intubation the particle proteins contained only 7% as much phosphate per mg of protein as the proteins of the same particles isolated from liver of control animals. Determination of the protein kinase and protein phosphatase activities in the total fraction of the non-histone nuclear proteins 48 h after the application of the carcinogen have shown an increase (200% and 159%, respectively) in both enzymatic activities. These results suggest that the hepatocarcinogen could induce the observed high turnover of the phosphates on the proteins of the liver nuclear ribonucleoprotein particles and the resulting dephosphorylation of these particles by stimulation of nuclear protein kinases and phosphatases. Qualitatively the same, but quantitatively much smaller changes were also observed 48 h after the application of the non-carcinogenic p-aminoazobenzene (AB) by stomach intubation and in regenerating liver. After the application of AB phosphorylation of the proteins of rat liver nuclear ribonucleoprotein particles decreased to 70% and in regenerating liver to 61% of the phosphorylation of particle proteins in control liver. Since it is assumed that nuclear RNP particles are involved in the processing and transport of newly synthesized premessenger RNA it is possible that the drastic dephosphorylation of the particle proteins induced by the carcinogen could be connected with the distortion of RNA processing which is observed in liver of animals treated with hepatocarcinogens.

Inhibition of mitochondrial oxidative phosphorylation by 2-methyl-4-dimethylaminoazobenzene

The azodye 2-methyl-4-dimethylaminoazobenzene inhibited oxidation and phosphorylation in tightly coupled rat liver mitochondria. Phosphorylation was more sensitive to the inhibitory action of the azodye than was the oxidation of succinate or ascorbate. The oxidation of NAD+-linked substrate was severely inhibited by the compound. In submitochondrial particles, only NADH oxidation was sensitive. The site of inhibition has been identified to lie between the dehydrogenase flavoprotein and ubiquinone.

Nucleocytoplasmic release of repetitive DNA transcripts in carcinogenesis correlates with a 60 kilodalton cytoplasmic protein

Treatment of rats with the hepatocarcinogen 3'-methyl-4-dimethylamino-azobenzene (3'-MeDAB) causes the appearance in the liver cytosol of a 60 kilodalton oncofetal protein. The appearance of this factor occurs within 40 h of treatment and coincides with the increase in the amount of rapidly labeled RNA released from nuclei in a reconstituted cell-free system. Cross-over experiments show that this increase is due to an enhanced transport capacity of the cytosol. The 60 kilodalton RNA transport factor is also present in the cytosol of tumor cells. Addition of the 60 kilodalton factor to normal liver cytosol causes the transport of repetitive RNA sequences similar to those transported from liver nuclei to tumor cell cytosol and those transported to the tumor cell cytoplasm in vivo. This factor modifies nuclear RNA restriction, at least in part, by eliciting the transport of repetitive RNA normally retained within the nucleus of the normal cell.

Carcinogenic activities of hydroxymethyl derivatives of 4-(dimethylamino)azobenzene in the liver of rats, mice, and hamsters

The carcinogenic activity of 3'-hydroxymethyl-N,N-dimethyl-4-aminoazobenzene (3'-CH2OH-DAB) was studied in Sprague-Dawley rats of both sexes, male ddy mice, and male Syrian golden hamsters. The activity of 2'-CH2OH-DAB and 4'-CH2OH-DAB was studied in male Sprague-Dawley rats. Azo dyes were given in the diet at a concentration of 0.064% for 3 months followed by a stock diet for 1 month; animals were sacrificed 4 months after beginning of the experiment. Liver hyperplastic nodules developed in all 8 male rats (100%) and in all 10 female rats (100%) which received 3'-CH2OH-DAB and in 7 out of 12 male rats (58%) which received 2'-CH2OH-DAB. Hepatocellular carcinomas were produced in 1 out of 8 male rats (13%) and in 1 out of 10 female rats (10%) receiving 3'-CH2OH-DAB. No hepatic lesions were observed in rats receiving 4'-CH2OH-DAB and in mice and hamsters receiving 3'-CH2OH-DAB. These results indicate that a carcinogenic effect of 3'-CH2OH-DAB and 2'-CH2OH-DAB could be detected in rats.

Dose and sex dependent effects of 2-acetylaminofluorene, 3'-methyl-4-dimethylaminoazobenzene and DL-ethionine in induction of gamma-glutamyl transpeptidase-positive liver cell foci in rats

The dose-dependent and sex-related effects of 3 hepatocarcinogens were investigated by measuring the number and area of gamma-glutamyl transpeptidase (gamma-GT) positive foci appearing in the liver. For this, three different doses of 2-acetylaminofluorene (2-AAF), 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) and DL-ethionine (ethionine), respectively, were given to F344 rats of both sexes for 6 weeks after a single injection of diethylnitrosamine (DEN). The inductions of gamma-GT positive foci by 2-AAF and 3'-Me-DAB were clearly dose-dependent, but with 2-AAF this was clearer in males than females. Ethionine had less clear dose-dependent effects in both males and females. Under these conditions, the minimal effective doses of 2-AAF, 3'-Me-DAB and ethionine, respectively, were 0.0008%, 0.0024% and 0.05% in males and 0.004%, 0.012% and 0.05% in females. The results indicate that these 3 carcinogens had clear dose-dependent effects in the induction of gamma-GT positive foci when given for a short period in the promotive stage and that male rats were more susceptible than females to 2-AAF.

Purification and characterization of a novel abundant protein in rat bile that binds azo dye metabolites and copper

Following intravenous administration to rats of the azo dye hepatocarcinogen 3'-methyl-N,N-dimethyl-4-aminoazo-[14C]benzene, 60-70% of the injected dose was recovered in bile in 2 h. Approximately 10% of bile radioactivity was trichloroacetic acid-precipitable, not extracted by n-butanol and non-dialyzable. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of bile followed by fluorography revealed two major and several minor proteins to which radiolabelled azo dye metabolites were bound; one of these major proteins (50 kDa) was purified from bile and shown to be homogeneous by SDS-polyacrylamide gel electrophoresis, isoelectric focusing (pI 7) under denaturing conditions and N-terminal analysis. The protein (MBP) comprises 15% of the total bile protein. Amino acid analysis revealed a preponderance of acidic and hydrophobic amino acids. The absorption spectrum of the native protein had a major peak at 280 nm and minor peaks at 345, 400, 600 and 650 nm. The fluorescence spectrum showed a major excitation maxima at 285 and 350 nm and corresponding emission maxima at 345 and 440 nm. Atomic absorption spectroscopy revealed 5 atoms of Cu per mol protein. Approximately 90% of the Cu was EPR silent. MBP did not react with antisera directed against rat serum IgA, albumin or ceruloplasmin; nor did these proteins react against antisera to MBP. Seven distinct peptide bands ranging from 5 to 18 kDa were obtained when MBP was subjected to CNBr cleavage and the digests were analyzed by SDS-polyacrylamide gel electrophoresis. The [14C]-Me-DAB derived radioactivity was present in only two of the peptides, indicating specific binding sites for azodye metabolites.

The alteration of gene expression in rat liver during chemical carcinogenesis

The effect of the carcinogen 3'-methyl-4-dimethylaminoazobenzene (3'-MDAB) on the gene expression of rat liver was studied. Hybridization analysis with enriched neoplastic liver specific cDNA demonstrated a noticeable change in gene expression during liver carcinogenesis. The effect of 3'-MDAB on liver specific gene expression was also studied with albumin and alpha-fetoprotein (AFP) genes as models. Serum AFP levels increased dramatically during hepatocarcinogenesis. Amounts of AFP mRNA present in polysomal poly(A)mRNA were determined by translation and hybridization experiments. AFP mRNA increased in carcinogen-treated liver as the serum level of AFP increased. Increase of AFP mRNA in liver cytoplasmic RNAs can be detected as early as 18 days after feeding rats 3'-MDAB and reached peaks at 30-40 and 185 days of treatment. AFP serum level and AFP mRNA in liver cytosol decreased dramatically between these peaks, with a small amount of AFP mRNA and serum AFP detectable in rats treated with 3'-MDAB for 100 days. The increase in amount of AFP mRNA was due to an increase in transcription of the AFP gene in livers of rats treated with carcinogen. Only a slight change in serum albumin concentration and albumin mRNA in livers of rats treated with 3'-MDAB was noticed.

Cytokeratin and nonhistone protein antigenic changes in rat liver during azo dye but not hepatotoxin feeding

Chromosomal proteins from rat liver have been assessed immunologically for changes in specific cytokeratins or primary tumor nonhistone proteins during treatment with an azo dye hepatocarcinogen (3'-methyl-4-dimethylaminoazobenzene), (3'-Me-DAB) or the hepatotoxin alpha-naphthyl-iso-thiocyanate (alpha-NIT). Fisher rats were fed laboratory diets supplemented with either agent and sacrificed sequentially at various intervals. Chromosomal proteins from these livers were electrophoresed in the presence of sodium dodecyl sulfate, transferred to nitrocellulose sheets and reacted with rabbit antisera to Novikoff hepatoma cytokeratin or 3'-Me-DAB induced primary hepatoma dehistonized chromatin. Livers from carcinogen-fed animals exhibited distinct, sequential changes in antigenic nonhistone proteins until the immunological specificity characteristic of the hepatoma was achieved concomitant with the induction of neoplasia. No antigenic changes were observed to occur in hepatotoxin-fed animals. The rat carcinoma-specific cytokeratin antigens p39 and p49 in Novikoff hepatoma were observed to appear as early as three weeks after the start of carcinogen feeding and were present maximally in 23 week livers with in situ carcinoma. These cytokeratins were not detected in alpha-NIT-fed animals. Our results support the concept that the carcinogenic process can be related to temporal changes in the expression of cell-specific cytokeratins in addition to nonhistone chromosomal proteins. Furthermore, these data suggest the expression of these antigenic species to not be the direct result of changes in liver structure and cellular composition associated with carcinogen toxicity; rather, neoplastic transformation is apparently required.

Decreased viscosity of rat-liver DNA treated by 3'-methyl-4-dimethylaminoazobenzene, detected with a new viscometric approach

DNA damage induced in vivo by 3'-methyl-4-dimethylaminoazobenzene (3'CH3DAB) was investigated with 2 differently sensitive techniques: the alkaline elution assay and the viscometric measurement of DNA damage. 3'CH3DAB appeared to be falsely negative with the alkaline elution assay, whereas with the viscometric approach, which is about 30-50 times more sensitive, it appeared positive, and the DNA damage was dose-dependent.

The effect of carcinogens on the accumulation of tyrosine aminotransferase by foetal rat hepatocytes in culture

The hepatocarcinogen 3'-methyl-4-dimethyl-aminoazobenzene (MDAB) suppresses the accumulation of tyrosine aminotransferase in cultured foetal hepatocytes. Experiments involving liver derived from foetuses of various ages reveals that a response is only obtained with rats older than 16-day gestation. It has been proposed that the lack of an effect in less mature hepatocytes is due to their inability to activate the carcinogen. Chemically synthesized analogues of MDAB which are considered likely to be activated forms of the procarcinogen are shown to be effective in the less mature cells. This supports the proposal that these cells may be unresponsive because they are unable to activate MDAB. Tests with other carcinogens reveal that the hepatocarcinogen dimethylbenzanthracene is also effective in 19-day gestation hepatocytes. However, the non-hepatocarcinogens azaserine and benz(a)pyrene are ineffective. Treatment with MDAB is shown not to alter the level of steroid receptor and reduce its translocation into the nucleus, suggesting that this is not the mechanism by which TAT is suppressed. The effect of the tumour promoter phorbol-myristate acetate (PMA) administered together with MDAB was shown not to modify the response to the carcinogen alone.

Antigenic changes in nonhistone proteins during azo dye hepatocarcinogenesis

We have investigated the appearance of specific nonhistone proteins during azo dye-induced hepatocarcinogenesis in the rat. Groups of animals fed azo dye-containing diet were sacrificed at approximately 3-week intervals, portions of their livers were examined histologically, and the remaining material was fractionated into chromatin and cytoplasmic fractions. Livers of the azo dye-fed animals exhibited histological changes that have been classically attributed to the course and development of cancer; by 28 to 30 weeks of treatment, nearly all animals had developed hepatomas. Heterogeneous rabbit antisera were prepared to dehistonized chromatin from several azo dye-induced hepatomas. These antisera were then used to assess various chromatins for the appearance of antigens specific for neoplasia during inducing carcinogenesis using immunodetection of antigens separated electrophoretically and transferred to nitrocellulose. Changes in the immunoreactivity of liver chromosomal proteins during carcinogen treatment were evident after 3 weeks, and the antigenic profiles of various chromatin samples gradually assumed the characteristics of the hepatoma. The transformation was accompanied by qualitative changes in chromosomal protein antigens, and although these antigenic species were not directly quantitated, noticeable enrichment of tumor-specific species occurred with treatment time. Immunotransfer assays of cytoplasmic fractions indicated most antigens to be specific for chromatin. Normal tissue chromatin exhibited minimal immunoreactivity, and slightly more antigenic homology was noted with regenerating liver and most transplantable tumor chromatins. Interestingly, the transplantable tumor Walker 256 carcinosarcoma was highly enriched in antigens recognized by antisera to azo dye hepatoma dehistonized chromatin. These studies establish a define chronological correlation between the chemical induction of cancer and sequential changes in the immunological specificity of nonhistone protein antigens.

Preferential inhibition of the B-form of monoamine oxidase in the liver of rats given 3'-methyl-4-dimethylaminoazobenzene in the diet

A and B-form monoamine oxidase (MAO) activities were measured in the liver of rats maintained with a diet containing 0.06% 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB). A-form MAO activity was similar to the control value throughout the feeding periods with serotonin as substrate. In contrast, B-form MAO activity decreased rapidly and the level of MAO activity was maintained at about 30% with beta-phenylethylamine (beta-PEA) as substrate. 3'-Me-DAB feeding did not cause any changes in MAO activity in the brain of rats. A single administration of 3'-Me-DAB (100 mg/kg p.o.) failed to alter A and B-form MAO activities for up to 4 days after its administration. The mechanism of inhibition of B-form MAO activity in rat liver mitochondria by 3'-Me-DAB was investigated. The inhibition of 3'-Me-DAB of B-form MAO activity, in vitro, was competitive and reversible. There was no difference in the apparent Michaelis constant toward beta-PEA between control and 3'-Me-DAB fed rats. B-form MAO in rat liver mitochondria was titrated with (-)deprenyl; this compound is selective to and an irreversible inhibitor of B-form MAO. The content of B-form MAO in liver mitochondria of rats fed 3'-Me-DAB for 3 weeks was decreased to about 60% of the control level.

Inhibition of monoamine oxidase by 3'-methyl-4-dimethylamino azobenzene (3'-me-DAB) in rat liver mitochondria

Effects of 3'-Me-DAB on MAO in rat liver mitochondria, in vitro, were investigated. 3'-Me-DAB at a concentration of 1 x 10(-5) M inhibits MAO activity about 40%, and this inhibition recovered to the control value after dialysis overnight against 0.001 M phosphate buffer. MAO activity was inhibited in an apparently competitive fashion by 3'-Me-DAB. These results indicate that 3'-Me-DAB binds to mitochondrial MAO with a weak affinity in vitro. The Km value toward benzylamine was 220 microM using both the mitochondria from the liver of rats fed a basal diet and those from rats ingesting 3'-Me-DAB. The activity of these enzyme preparations did not revert after dialysis to the control values of rats fed a basal diet. The titration experiment of MAO by pargyline suggests that the decrease of MAO activity, in vitro, is mainly due to the decrease of active MAO molecules in these mitochondrial preparations from livers of rats ingesting 3'-Me-DAB.

Induction of drug-metabolizing enzymes in the rat liver by 3'-methyl-4-(dimethylamino)azobenzene

The influence of 3-methyl-4-(dimethylamino)azobenzene(3'-Me-DAB) on the drug-metabolizing system in the liver was investigated. Feeding of 3-Me-DAB for 3 weeks at 10, 20 and 600 ppm increased the content of hepatic microsomal cytochrome P-450 slightly (up to 27% raise) but significantly. The feeding effects were also demonstrated in S-9 activity (up to 91% raise) when the mutagenicity of 3-methylcholanthrene (3-MC) and 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) was assayed in Ames system using S-9 fraction from 3'-Me-DAB-treated rat livers.