Irreversible depression in the ratio of tetraploid:diploid liver nuclei in rats treated with 3'-methyl-4-dimethylaminoazobenzene (3'M)

Accelerated growth of hepatocytes in association with Up-regulation of cyclin E in transgenic mice expressing the dominant negative form of retinoic acid receptor

Retinoids play an important role in pathogenesis of liver diseases. To clarify the functional role of retinoic acid (RA) in liver, we developed transgenic mice (Tg) which express the dominant negative form of retinoic acid receptor (RARE) in liver. Here, we report that proliferation of hepatocytes in RARE Tg is greatly enhanced and that cyclin E is up-regulated in RARE Tg. Liver weight, liver/body weight, and proliferating cell nuclear antigen (PCNA) labeling index in RARE Tg were significantly increased, compared to those in wild-type mice (P < 0.01, each). Cell cycle analysis showed that 2N DNA content cells and aneuploid area between 2N and 4N DNA, reflecting S phase cells, were significantly increased in RARE Tg, compared to wild-type mice (P < 0.01, each). Of G1 phase-related proteins including cyclins, cyclin-dependent protein kinases (CDKs) and cyclin-dependent protein kinase inhibitors (CKIs), cyclin E mRNA and protein was up-regulated in liver from RARE Tg by reverse transcription polymerase chain reaction and Western blot analysis. Furthermore, the immunoprecipitation with anti-cdk2 antibody, followed by Western blot analysis with anti-cyclin E antibody indicated that cyclin E/cdk2 complex is increased in liver of RARE Tg. The results of the present study suggest that cyclin E in association with cdk2 governs cell cycle progression through G1 in hepatocytes where function of RA is inhibited.

Thyroid hormone regulation of rat hepatocyte proliferation and polyploidization

The liver of adult mammals contains various classes of polyploid hepatocytes produced by a process that is partially regulated by hormones. However, it is not well understood how the hormones affect the rate of hepatocyte proliferation under physiological conditions. Here we have studied the specific roles of 3,5,3'-triiodothyronine (T3), growth hormone (GH), and sex steroids on the percentage of diploid nuclei in S phase and on the population of liver tetraploid (4C) cell nuclei in several rat model systems. Gonadal steroids had no effect on the S phase but account for gender differences in the 4C nuclei. Hypophysectomy in adult male rats produced a moderate decrease in 4C nuclei that was reversed by treatment with 25 micrograms T3. kg-1. day-1, whereas treatment with 200 micrograms human recombinant GH (hGH). kg-1. day-1 was ineffective. Rats made hypothyroid by methimazole treatment of dams and pups until death showed a low S phase and only 5% of 4C nuclei at 70 days of age. T3 significantly increased the S phase 24 h after administration and restored the adult normal level of 4C nuclei after 10 days of treatment. hGH did not affect the 4C nuclei or the S phase in the hypothyroid rats. These results suggest that the processes of hepatocyte proliferation and polyploidization of the rat liver are under endocrine control, with thyroid hormones playing the essential regulatory role.

Deferoxamine arrests in vitro the proliferation of porcine hepatocyte in G1 phase of the cell cycle

Iron is required for cell proliferation of all living species. Moreover, iron excess may be involved in the development of hepatocellular carcinoma. In this study we analyzed the effects of deferoxamine, an iron chelator, on normal porcine hepatocyte proliferation. We confirmed that hepatocytes isolated from young pigs proliferate in the presence of insulin and fetal calf serum as shown by [3H] methyl-thymidine incorporation, presence of mitotic figures and increase in cell number. This was paralleled by nuclear expression of p34cdc2 and its associated histone H1 kinase activity. In the presence of deferoxamine, [3H] methyl-thymidine incorporation, expression of nuclear proteins (p34cdc2 and PCNA) and H1 kinase activity were drastically reduced. In addition, in contrast with control cultures, cells in S-phase were not detected by flow cytometry. These data suggest that iron chelation by deferoxamine can arrest the progression of porcine hepatocytes in the G1 phase of the cell cycle.

Iron may induce both DNA synthesis and repair in rat hepatocytes stimulated by EGF/pyruvate

BACKGROUND/AIMS

Hepatocellular carcinoma develops frequently in the course of genetic hemochromatosis, and a role of iron overload in hepatic carcinogenesis is strongly suggested.

METHODS

The aim of our study was to investigate the effect of iron exposure on DNA synthesis of adult rat hepatocytes maintained in primary culture stimulated or not by EGF/pyruvate and exposed to iron-citrate complex.

RESULTS

In EGF/pyruvate-stimulated cultures, the level of [3H] methyl thymidine incorporation was strongly increased as compared to unstimulated cultures. The addition of iron to stimulated cultures increased [3H] methyl thymidine incorporation. The mitotic index was also significantly higher at 72 h. However, the number of cells found in the cell layer was not significantly different from iron-citrate free culture. By flow cytometry, no difference in cell ploidy was found between iron-treated and untreated EGF/pyruvate-stimulated cultures. A significant increase in LDH leakage reflecting a toxic effect of iron was found in the cell medium 48 h after cell seeding. In addition, [3H] methyl thymidine incorporation in the presence of hydroxyurea was increased in iron-treated compared to untreated cultures.

CONCLUSIONS

Our results show that DNA synthesis is increased in the presence of iron in rat hepatocyte cultures stimulated by EGF/pyruvate, and they suggest that DNA synthesis is likely to be related both to cell proliferation and to DNA repair. These observations may allow better understanding of the role of iron overload in the development of hepatocellular carcinoma.

Flow cytometric analysis of neoplastic nodules and hepatocellular carcinomas induced by ciprofibrate in the rat

Alterations in DNA ploidy accompany hepatocellular carcinoma (HCC). However, changes in DNA content are also seen in regenerating liver and with increasing age. Thus, to investigate the role of DNA ploidy changes in development of HCC, flow cytometric DNA content determinations were done in a rat model system of peroxisome proliferator-induced HCC. Paraffin blocks of liver isolated from 18 Fisher 344 male rats fed ciprofibrate for 20 weeks (4), 40 weeks (4) or 20 months (10) were examined. Livers from age-matched control rats were also examined. From the 20 month ciprofibrate group, nine neoplastic nodules (NNs), 27 HCCs and four non-tumorous surrounding tissue controls (NTCs) were examined. Significant DNA tetraploid populations were seen in both the NNs and NTCs. A significant increase in the percentage of DNA diploid cells was observed in the NN samples. No significant difference in the percentage S-phase cells was seen. Emergence of cell populations with new DNA ploidy classes (8c or DNA aneuploid) as compared with NTCs was only seen in HCCs (7 of 27), and five of these seven were DNA aneuploid, as distinct from DNA tetraploid, populations. A total of 16 of 24 HCC samples that were adequate for cell cycle analysis had average percent S-phase greater than the mean of the NTCs plus three standard deviations. Although a direct role cannot be inferred, these results support the hypothesis that increases in the fraction of diploid cells is an important early event in the development of rat HCC and that further alterations in DNA ploidy and increased proliferative fraction accompany the development of HCC.

Hepatocyte growth factor in transgenic mice: effects on hepatocyte growth, liver regeneration and gene expression

Attention has recently been focused on hepatocyte growth factor as a major candidate factor in liver regeneration because it is the most potent known mitogen for hepatocytes in vitro. However, hepatocyte growth factor also displays diverse activities in vitro as scatter factor, as an epithelial morphogen, as a pluripotent mitogen and as a growth inhibitor. Consequently, we developed transgenic mice that expressed hepatocyte growth factor under the control of albumin regulatory sequences to examine its in vivo role in hepatocyte growth. Hepatocytes of these mice expressed increased levels of hepatocyte growth factor as an autocrine growth factor. Hepatocyte growth factor was a potent stimulus for liver repair; the livers of hepatocyte growth factor-transgenic mice recovered completely in half the time needed for their normal siblings after partial hepatectomy. This transgenic model also enabled us to study the chronic effects of hepatocyte growth factor expression. During several months of observation, the labeling index of hepatocytes in albumin-hepatocyte growth factor mice was doubled, and liver DNA content was increased compared with that in wild-type mice. To identify intermediate signaling pathways for hepatocyte growth factor that might regulate this increased growth response, we examined transgenic mice for changes in expression of genes that are known to be regulated during liver regeneration. We found that levels of c-myc and c-jun mRNA were increased in the hepatocyte growth factor-transgenic mice. In additional experiments the increased c-myc expression was the consequence of increased transcription rates as seen in nuclear run-on and myc-CAT reporter gene experiments. We conclude that hepatocyte growth factor increases growth and repair processes when expressed for long periods in the liver and that c-myc and c-jun may be important intermediaries in the hepatocyte growth response caused by hepatocyte growth factor.

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.

Physiological oxygen tension modulates the chemically induced mitogenic response of cultured rat hepatocytes

Freshly isolated rat hepatocytes were cultured at periportal- (13% O2) or perivenous-like (4% O2) oxygen tension and exposed to subtoxic exposure levels of cyproterone acetate (CPA: 10-330 microM), phenobarbital (PB: 0.75-6 mM), and dimethylsulfoxide (DMSO: 0.1-3.3%) from 24-72 h after seeding. Induced alterations in ploidy, in the number of S-phase cells, the degree of binuclearity, and cellular protein content were determined by twin parameter protein/DNA flow cytometry analysis of intact cells and isolated nuclei. CPA and PB increased whereas DMSO decreased dose dependently the total number of S-phase cells. The changes differed within individual ploidy classes and were modulated by the oxygen tension. CPA increased and DMSO decreased the number of S-phase cells preferentially among the diploid hepatocytes at periportal-like oxygen tension. In contrast, PB increased binuclearity and S-phase cells mainly among the tetraploid hepatocytes at perivenous-like oxygen tension. Cellular protein content increased dose dependently after exposure to the hepatomitogens (CPA, PB) and decreased after exposure to DMSO at both oxygen tensions. Comparison with in vitro data proves that chemicals which interact with cells from the progenitor liver compartment (CPA, DMSO) exert their mitogenic activity best in cultures at periportal-like oxygen tension preferentially in diploid hepatocytes, whereas chemicals which affect cells from the functional compartment show a higher activity at perivenous-like oxygen tension. Physiological oxygen tension seems to be an effective modulator of the proliferative response of cultured rat hepatocytes similar to that expected for periportally or perivenously derived hepatocytes.

Cytogenetic changes in hepatocarcinomas from rats treated with chronic exposure to diethylnitrosamine

Cytogenetic analysis of rat hepatocarcinomas obtained after diethylnitrosamine (DEN) exposure showed a wide variety of numerical and structural chromosomal changes: 53 of 86 hepatocellular carcinomas showed at least one recurrent chromosomal aberration. Some of these recurrent changes occurred in several tumors. Chromosomes 1, 3, 11, and 12 were abnormal in more than 30% of the carcinomas; chromosomes 2, 4, 5, and 10 were abnormal in 10%. Moreover, chromosomes 1 and 10 were generally lost or deleted and chromosome 3, 4, and 11 were very often gained. The most frequent anomaly was loss of chromosome 1 which was observed in 35% of the tetraploid cell populations. The occurrence in several tumors of recurrent chromosomal rearrangements as well as various repeated aneuploidies strongly suggests that these anomalies are implicated in the process of rat hepatocarcinogenesis induced by DEN treatment.

Single cell analysis in toxicity testing: the mitogenic activity of thioacetamide in cultured rat hepatocytes analyzed by DNA/protein flow cytometry

Rat hepatocytes were cultured at 4% O2 and 13% O2 and exposed to the nongenotoxic rodent carcinogen thioacetamide (TA) from 24 to 72 h after isolation at exposure levels between 0.01 and 0.33 mM. Hepatocytes and isolated nuclei were analyzed by DNA-protein flow cytometry. An aggregate correction procedure was applied and the proportion of S-phase, diploid, tetraploid or octoploid hepatocytes as well as binucleated cells, were measured or calculated. The proportion of S-phase cells within the diploid hepatocytes increased with increasing concentration of TA up to 3.9-fold, whereas the corresponding increase in S-phase mononucleated tetraploid cells was only 1.8-fold. S-phase binucleate tetraploid cells showed no increase. In the tetraploid hepatocytes, the mitogenic stimuli was detectable only in cultures maintained at 4% O2. The relative contribution of binuclear cells was increased 1.5-fold in the octoploid cells. It is concluded that the mitogenic activity of TA initiates DNA synthesis in diploid hepatocytes in the G1 and in the following G2 cell-cycle phase, omitting karyogenesis. The cellular protein content is not affected which indicates that the mitogenic activity of the chemical is not necessarily associated with an increase in cellular protein content. The results obtained correspond well with data of in vivo studies. The method applied therefore allows the mitogenic activity of nongenotoxic carcinogens to be detected in vitro within 48 h and their mode of action to be elucidated.

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.

Peculiarities of the liver histology of laboratory rodents

Although the general liver morphology is similar in all mammals, there are some structural features in apparently healthy laboratory rodents. These peculiarities are known to be influenced by a great variety of endogenous and exogenous factors. Incidence, intensity, development and disappearance of such elements as extramedullary haemopoiesis, polyploidy, intranuclear and intracytoplasmic inclusions depend markedly upon genetics, age, hygienic condition, hormonal regulation and nutrition of the animals. It is concluded from this short review that the term "normal histology" should only be understood as being relative and that it may only be applied to a given, well defined animal population held under well defined conditions.

Stimulation of G2-arrested rat liver and pancreas cells by fasting

The present study complements earlier work that revealed a mitogenic effect of fasting on G2-blocked hepatocytes, demonstrating that the release of hepatocytes from the G2 arrest is maximal at 3 h of fasting and that the phenomenon lasts for 12 h. The mitogenic effect of fasting is not restricted to the liver, as similar results were obtained with the pancreas--suggesting a possible action on other organs as well. A simple physiologic stimulus such as fasting can thus be used in further studies on the properties of G2 cell populations and their possible role in the regulation of normal and abnormal growth processes.

Accumulation of abnormally high ploid nuclei in the liver of LEC rats developing spontaneous hepatitis

Enlarged hepatocytes with huge nuclei were found in LEC rats with hereditary hepatitis. Flow cytometric analysis of the DNA content of nuclei from jaundiced LEC rats revealed the presence of very high polyploids, such as 32n and 64n. At the age of 12 weeks, before the onset of hepatitis, 8n polyploid nuclei were more frequent in LEC rats than in LEA rats, a sibling line of LEC rats. Binucleated hepatocytes were also more frequent in LEC rats than in LEA rats at week 4. Bi-, tri- and tetra-nucleated cells whose nuclei were sometimes different in size were observed when jaundice became manifest. The number of proliferating liver cells, determined by pulse labeling with 5-bromo-2'-deoxyuridine (BrdU), was higher in LEC rats than in LEA rats at 2, 4, 8, 12 and 14 weeks, with a maximum at week 4. A remarkable increase of BrdU uptake was observed at week 16, when jaundice developed. The possible involvement of abnormal cytokinesis and kariokinesis in the manifestation of hepatitis was suggested.

Changes in cellular ploidy and autophagic responsiveness during rat liver carcinogenesis

Liver carcinogenesis was initiated in young rats by diethylnitrosamine/partial hepatectomy and promoted by dietary 2-acetylaminofluorene (for 4 weeks). Eight weeks after initiation, hepatocytes were isolated by means of collagenase perfusion and analyzed by means of flow cytometry. Whereas cells and cell nuclei from normal or hepatectomized livers were predominantly tetraploid, most of the hepatocytes/nuclei from carcinogen-treated rats were diploid. Neoplastic liver nodules and hepatocellular carcinomas also contained almost exclusively diploid nuclei, suggesting that diploidization may be an essential feature of liver carcinogenesis. Two-parametric analysis (simultaneous flow cytometric determination of DNA and protein content within the same cell) revealed that the diploid cells were only half as big as the tetraploid cells. They could therefore be separated from the latter by centrifugal elutriation. Normal, isolated hepatocytes responded to amino acid deprivation by increasing their rates of autophagic sequestration (measured with electroinjected (14C)sucrose as a probe) and endogenous protein degradation, the resulting protein loss eventually leading to cell death. Hepatocytes from carcinogen-treated rats were much less responsive to amino acid deprivation, preserved their protein better, and survived for longer periods of time in culture than did normal cells. The reduced autophagic responsiveness may conceivably give carcinogen-altered cells a survival advantage even in vivo, that could contribute to their outgrowth during carcinogenesis.

Studies in vivo to investigate the status of 4-N-pyrrolidinylazobenzene as a pure cancer initiating agent to the rat liver

4-N-Pyrrolidinylazobenzene (4N) has been described sequentially as a potential rat hepatocarcinogen, a non-hepatocarcinogen to the rat, a possible pure initiating agent to the rat liver and as unlikely to be either a rat hepatocarcinogen or a cancer initiating agent. Given the importance of defining a pure initiating agent to the rodent liver we have conducted experiments to evaluate the status of 4N in this respect. By histopathological criteria 4N is non-hepatotoxic to the rat liver following daily oral gavage for 6 weeks, but it can be detected unbound in the rat liver following a single exposure via oral gavage and methaemoglobin is evident in peripheral blood. In addition, it fails to bind covalently to hepato-proteins or to initiate unscheduled DNA synthesis in the rat liver following oral dosing. Under similar conditions of bioassay, the rat liver carcinogen 3'-methyl-4-dimethylaminoazobenzene (3'M) gave a positive response on each count. Further, no evidence of histopathological change was observed in the rat liver following daily intraperitoneal injection of 4N for 2 weeks. It is concluded that 4N is both non-toxic and non-genotoxic to the rodent liver in vivo in all respects studied, and that it is therefore very unlikely to possess cancer initiating activity in this tissue.