Kinetics of excretion of 2-acetylaminofluorene in normal and xenobiotic-treated rats and in rats with hepatocyte nodules

This study was designed to explore further the hypothesis that the special resistance phenotype seen in hepatocyte nodules during liver carcinogenesis could have a physiologic correlate in the manner with which a carcinogenic xenobiotic is handled. Hepatocyte nodules were induced in male rats by continuous or intermittent exposure to dietary 2-acetylaminofluorene over a 25-week period. Two or 5 weeks after the exposure, the animals were given a single dose of 9-14C-2-acetylaminofluorene. The amounts and rates of excretion of unconjugated compound and derivatives and of the glucuronic acid metabolites in the bile and urine and the amounts in the blood and liver were measured over a period of 180 minutes. For comparison, animals fed the basal diet alone, animals injected with phenobarbital or 3-methylcholanthrene, animals receiving a single dose of cobalt heme and animals fed the 2-acetylaminofluorene for only 2 weeks were studied. These groups were used as controls for different patterns of drug metabolism, especially relating to the cytochromes P-450. The nodule-bearing animals showed a pattern of handling of the carcinogen that is quite different than that of the animals of any other group. They excreted in the bile plus urine from 20 to 30% less. However, relatively much more was in the urine. The free and glucuronide-conjugated metabolic products of the carcinogen were assessed by high performance liquid chromatography. The nodule-bearing animals and the animals treated with 3-methylcholanthrene excreted much more glucuronic acid esters. The pattern of distribution of labeled 2-acetylaminofluorene is different in the nodule-bearing rats than in other animals in which variations in phase I and phase II drug-metabolizing enzymes were induced by treatment with cobalt heme, phenobarbital, 3-methylcholanthrene or short-term exposure to dietary 2-acetylaminofluorene.

Inverse relationship between epidermal growth factor induced proliferation and expression of high affinity surface epidermal growth factor receptors in rat hepatocytes

Rat hepatocytes express large numbers of high and low affinity surface membrane receptors (EGFR) for epidermal growth factor (EGF) but the roles of EGF and EGFRs in hepatocyte proliferation in vivo are unclear. F344 rat hepatocytes in primary culture proliferated maximally in response to continuous serum-free culture with 3.3 nM (20 ng/ml) EGF, as quantified by cumulative [3H]thymidine labeling index. However, serum concentrations of EGF in rats with normal livers or induced hepatocyte proliferation due to partial hepatectomy, carbon tetrachloride-induced necrosis, or hepatic neoplasia were consistently below 0.1 nM. The 3- or 6-hour pulse exposures to EGF (1.7 nM) between 0 to 16 hours had minimal effect on labeling index at 48 hours, but these pulse exposures at 24 or 32 hours were equivalent to continuous exposure. At 24 and 32 hours, the total specific surface binding of [125I]EGF to hepatocytes cultured free of EGF decreased to 43 and 24% of the initial values, respectively. Scatchard analysis of EGF binding indicated that hepatocytes lost all high affinity EGFRs (Kd of 0.08 nM) by 24 hours. Low affinity [125I]EGF binding at 0 hour (Kd 0.8 nM) was further reduced at 24 hours (Kd = 3.9 nM) and corresponded more closely to mitogenic concentrations of EGF in culture. These studies demonstrate that proliferative responsiveness of hepatocytes to EGF increases during culture by a process that involves prior loss of constitutive high affinity EGFRs. These results suggest that constitutive high affinity EGFRs do not elicit the proliferative response to EGF.

Decreased cytosolic levels of the heme binding Z protein in rat hepatocyte nodules and hepatocellular carcinomas

Hepatocyte nodules and hepatocellular carcinomas were induced in male Fischer rats using the resistant hepatocyte model. The immunoreactive cytosolic levels of the heme-binding Z protein (HBP) were reduced by 56% (P less than 0.001; 2-tailed t-test) in early hepatocyte nodules (25 weeks) and hepatocellular carcinomas (10-12 months). This finding is in accordance with the previously reported reduced heme content of hepatocyte nodules and is consistent with the postulated role for HBP in intracellular heme transport and distribution. The immunoreactive levels of the glutathione S-transferase isozymes (GST) which like HBP bind heme, were elevated 2-fold (P less than 0.01) in early and late hepatocyte nodules and were unchanged in hepatocellular carcinomas.

The galactosyltransferase activity of hepatic nodules during rat liver carcinogenesis

Galactosyltransferase has been isolated from putative preneoplastic hepatocyte nodules generated in the resistant hepatocyte model by the procedure of Solt et al. (Am. J. Pathol., 88 (1977) 595-609). The following observations have resulted from these studies: (a) the specific activity of galactosyltransferase isolated from hepatocyte nodules by affinity chromatography was reduced to about 1/3 that of the enzyme in control and in liver tissue surrounding the nodules; (b) the galactosyltransferase activity from normal rat serum eluted from the alpha-lactalbumin affinity column as a single peak (spec. act. = 1.57 nmol/min per mg) while that from the serum of nodule-bearing rats eluted in two distinct peaks (spec. act. = 2.49 and 0.49 nmol/min per mg protein); (c) the elution profile of the enzyme from hepatocyte nodules was broad compared to that from normal liver, surrounding liver or serum; (d) the Km for N-acetyl-D-glucosamine (GlcNAc) was lower in all four independent batches of nodules compared to the Km for GlcNAc from control and surrounding liver; (e) the Km for uridine diphosphogalactose (UDP-Gal) was higher for the enzyme from nodules compared to that from control tissue. These data suggest that the hepatocyte nodule produces several glycoforms of galactosyltransferase the kinetic properties of which differ from those of the enzyme from control liver.

Cancer development and its natural history. A cancer prevention perspective

In the design of new approaches to cancer prevention, it is important to realize that most cancers develop stepwise over a long period of time with nonmalignant precursor lesions that only slowly evolve toward cancer. With many chemicals and some radiations, as well as some viruses (DNA and some retroviruses), cancer development can be divided into 3 major stages or periods, initiation, promotion and progression. Initiation is frequently associated with a more or less permanent change in the phenotype of a rare target cell, presumably due to a change in base composition in DNA or to gene rearrangements. During promotion, these rare cells expand by proliferation to generate focal proliferations that resemble benign neoplasms. These in turn exercise at least one of two options, regression to normal appearing tissue or slow evolution to cancer. Progression is self generating but can be modulated by dietary manipulations or by other drugs or xenobiotics. The prolonged nature of the promotion-progression stages in most tissues and its modulatability indicate that these stages are vulnerable sites for the development of dietary and other ways to prevent the progression to cancer. This overall pattern is known to occur in the liver, skin and urinary bladder and is probable in several other tissues or organs including the colon, breast and pancreas. What we know about the human suggests that the patterns may be very similar for cancer development in many sites. The best worked out is melanoma. The phenotypic pattern of the precursor lesions in the experimental animals is remarkably similar in any single organ. For example, the hepatocyte nodules are very similar to each other with many different carcinogens and promoting environments even though the ultimate cancers are quite heterogeneous and diverse. The diversity and heterogeneity appears to be an acquisition that is quite late in the step-by-step development of cancer. Although its exact step has not been delineated as yet, it appears to be acquired as malignancy is. Unlike the cancers, the commonality or homogeneity in the precursor lesions offers many opportunities for interrupting the process and thus in preventing cancer. The experience to date in experimental systems with some hormones, drugs and dietary manipulations indicates that inhibition of the development of cancer may be most readily achieved by affecting the promotion and progression sequences in carcinogenesis.

Glutathione and enzymes related to free radical metabolism in liver of rats fed a choline-devoid low-methionine diet

Fischer F-344 male rats, fed a choline-devoid diet that leads to a highly reproducible sequence of biochemical and biological changes with an ultimate development of hepatocellular carcinoma, show elevated levels of glutathione in the liver at 3, 6 and 8 days. Several enzymes related to the metabolism of free radicals, including superoxide dismutase, catalase, glutathione peroxidase, glutathione S-transferase and DT-diaphorase show neither increased nor decreased activity as measured between 12 h and 8 days on the diet. Thus, of several known cellular components related to the possible scavenger of free radicals in the liver, only glutathione responded to the feeding of the CD diet. It is tentatively concluded that a decrease in the levels of possible scavengers for free radicals is not a major basis for the nuclear and mitochondrial lipid peroxidation seen early in rats fed a choline-devoid diet.

Morphometric evaluation of hepatocellular proliferative lesions in the rat liver

Classification of rat hepatocellular proliferative lesions can vary between pathologists as the many qualitative histologic criteria have not been satisfactorily evaluated and ranked for prognostic value. Computer-assisted morphometry offers an objective method to evaluate certain cellular features. The Solt-Farber resistant hepatocyte model was used in this study to produce populations of rats with a full range of hepatocellular proliferative lesions. Cellular features within the lesions were then measured morphometrically and the data were analyzed by animal age and by subjective lesion diagnosis. The nuclear/cytoplasmic ratio followed by the cell area and nuclear area were found to be the most important parameters for separating microscopic foci and islands of cellular alteration, an early hyperplastic lesion, from other hepatocellular proliferative lesions. The coefficient of variation, as a relative measure of heterogeneity, increased in a linear manner for cell, nuclear and nucleolar areas as the animals aged and was significantly higher for cell and nuclear area in hepatocellular carcinoma compared to other hepatocellular proliferative lesions. Hepatocyte nodules (representing primarily late hyperplastic lesions) and persistent hepatocyte nodules (lesions with similarities to hepatocellular adenoma) could not be satisfactorily separated within the limits of this study. As these borderline lesions show a continuum of cytologic change, other features, such as architectural change, are necessary to satisfactorily classify them on a subjective basis. An alternative approach is to use discriminant functions derived from morphometric studies.

Purification and characterization of P-52 (glutathione S-transferase-P or 7-7) from normal liver and putative preneoplastic liver nodules

A previous study from our laboratory (L.C. Eriksson et al., Biochem. Biophys. Res. Commun., 117: 740-745, 1983) revealed that a cytosolic polypeptide of approximate Mr 21,000 (designated P-21) was markedly elevated in amount in hepatocyte nodules induced by six different regimens. The molecular weight of this polypeptide, subsequently revised to approximately 26,000, was redesignated P-26 and was identified (T.H. Rushmore et al., Biochem. Biophys. Res. Commun., 143: 98-103, 1987) as a subunit of a placental form of glutathione S-transferase (K. Sato et al., Gann 75: 199-202, 1984), also named glutathione S-transferase 7-7 (H. Jensson et al., FEBS Lett., 187: 115-120, 1985). We describe here a convenient method for purifying relatively large amounts of P-26 from hepatocyte nodules involving the sequential use of affinity chromatography on S-hexyl glutathione-Sepharose 4B, CM-Sephadex, and DEAE-Sephacel. Evidence is presented that P-26 exists as a dimer of approximate Mr 52,000 (P-52). Analyses by two-dimensional electrophoresis have indicated that the subunits of Mr 26,000 may consist of five separate charged isomers. Investigations using appropriate antisera and analysis by amino acid sequencing have provided additional confirmation that P-52 is probably identical to rat placental glutathione S-transferase. Antibodies to P-52 are proving to be useful as a marker of new cell populations that appear regularly during hepatocarcinogenesis.

Decreased expression of cytochrome P-452 in the resistance phenotype characteristic of putative preneoplastic hepatocyte nodules during hepatocarcinogenesis

Hepatocyte nodules, a characteristic early step in the development of liver cancer in rats, has a distinctive resistance phenotype including a large decrease in total cytochromes P-450 and in two isozymes induced by phenobarbital and two by 3-methylcholanthrene. In this study, it has been observed that the nodules show a large decrease in an additional cytochrome P-450, cytochrome P-452, which is very active in the hydroxylation of lauric acid at C-11 and C-12. The decrease in activity of this microsomal cytochrome P-452 is of the same order of magnitude as the decreases in the other cytochrome P-450 components. These observations are consistent with the hypothesis that there is some more basic alteration in the synthesis or availability of heme and that the changes in the activities of the cytochromes P-450 are secondary.

DNA hypomethylation in ethionine-induced rat preneoplastic hepatocyte nodules

DNA from hepatocyte nodules induced in rats with dietary DL-ethionine and from the surrounding non-nodular liver contained less 5-methyldeoxycytidine per deoxycytidine when compared with that from normal adult liver. The degree of apparent hypomethylation, 37% in nodules and 20% in the surrounding liver, decreased somewhat (29% and 16% respectively) at 2 weeks after terminating the exposure to ethionine. Nodules and surrounding liver, like normal liver, responded to partial hepatectomy with a decrease in the 5-methyldeoxycytidine level at 24 hrs and a return to the level at the time of partial hepatectomy by 38 hrs. These findings indicate the need for careful control of cell proliferation in comparing the levels of a post-replicative DNA modification, methylation, in proliferating and non-proliferating cell populations. These findings also suggest that a portion of the hypomethylation in preneoplastic nodules may be due to a bona fide decrease in the level of cytosine methylation in the parental strand of DNA. This hypomethylation could be one basis for the altered gene expression in hepatocyte nodules, possible precursors for liver cancer.

Initiators, promoters and the uncertainty principle

The development of cancer in humans and in animals including experimental animals is almost always a multistep process. With carcinogenic chemicals and probably with some other etiological agents as well (e.g. radiations, DNA viruses), the first few steps require exogenous stimuli to induce the requisite tissue changes. However, the known agents are now being shown to have different and even antagonistic or contradictory effects depending upon when and how they are used. This uncertainty principle in regard to agents in cancer development, although complicating the analysis, may facilitate a more rational scientific analysis of the fundamental nature of the different steps as cancer develops and could lead to new approaches to cancer prevention and/or therapy.

Probable free radical effects on rat liver nuclei during early hepatocarcinogenesis with a choline-devoid low methionine diet

Fischer-344 rats fed a choline-devoid diet show lipid peroxidation in the liver nuclei, beginning at 1 day, reaching a peak at 3 days, and subsequently declining by 35 days. Lipid peroxidation in the mitochondria was seen first at 3 days, increased to a maximum at 28 days, and decreased after 35 days to undetectable values at 49 days. Lipid peroxidation was found in both nuclear and mitochondrial fractions both before and after stripping of their outer membranes. No microsomal lipid peroxidation could be detected at any time up to 63 days. The animals fed the same diet supplemented with choline showed no lipid peroxidation in any liver fraction. Animals given CCl4 showed the expected lipid peroxidation in the microsomes but not in the nuclear fraction. The administration of the free radical trapping agent, N-tert-butyl-alpha-phenylnitrone, prevented completely or almost so, microsomal lipid peroxidation induced by CCl4 and nuclear lipid peroxidation in the animals fed the choline-devoid, low methionine diet. The genesis of free radicals in the livers of rats fed a choline-devoid diet is considered as a likely hypothesis for the observed lipid peroxidation. The lipid peroxidation in turn is considered to be closely related to the induction of liver cell death and to the production of alterations in DNA. The DNA alterations coupled with regenerative liver cell proliferation suggest an attractive hypothesis for the initiation of hepatocarcinogenesis in rats fed a choline-devoid diet.

Alternative methods of selecting rat hepatocellular nodules resistant to 2-acetylaminofluorene

Dietary 2-acetylaminofluorene (2-AAF) coupled with a stimulus for cell proliferation such as a 2/3 partial hepatectomy (PH) or a necrotizing dose of carbon tetrachloride is frequently employed to generate nodules of resistant ("initiated") rat hepatocytes. This regimen is a useful model for experimental analysis of alterations in hepatocytes during carcinogenesis, and also as an assay for initiation by various carcinogens. Because of the decreasing availability of carcinogen-containing diets from commercial sources, we have developed alternative methods of 2-AAF administration to generate nodules in rats initiated with N-nitrosodiethylamine. This study compared the nodule-selecting and cancer-promoting efficacy of 2-AAF administered by the Solt-Farber procedure (0.02% in diet for 2 weeks) with 2-AAF administered by gavage, as a suspension in 1% aqueous carboxymethyl-cellulose (CMC). Three or 4 daily administrations of 2-AAF by gavage (20 mg/kg/day) followed by PH on day 4 were equivalent to the dietary regimen in generating early resistant nodules, late persistent nodules and hepatocellular carcinomas. These regimens were similar to the dietary regimen of 2-AAF in inhibiting virtually all normal hepatocyte proliferation. These regimens permit control over the duration and level of 2-AAF exposure and the resulting size of selected nodules.

Carcinogen-induced mdr overexpression is associated with xenobiotic resistance in rat preneoplastic liver nodules and hepatocellular carcinomas

We have previously reported the isolation of a human breast cancer cell line resistant to doxorubicin (adriamycin; AdrR MCF-7 cells) that has also developed the phenotype of multidrug resistance (MDR). MDR in this cell line is associated with increased expression of mdr (P glycoprotein) gene sequences. The development of MDR in AdrR MCF-7 cells is also associated with changes in the expression of several phase I and phase II drug-detoxifying enzymes. These changes are remarkably similar to those associated with development of xenobiotic resistance in rat hyperplastic liver nodules, a well-studied model system of chemical carcinogenesis. Using an mdr-encoded cDNA sequence isolated from AdrR MCF-7 cells, we have examined the expression of mdr sequences in rat livers under a variety of experimental conditions. The expression of mdr increased 3-fold in regenerating liver. It was also elevated (3- to 12-fold) in several different samples of rat hyperplastic nodules and in four of five hepatomas that developed in this system. This suggests that overexpression of mdr, a gene previously associated with resistance to antineoplastic agents, may also be involved in the development of resistance to xenobiotics in rat hyperplastic nodules. In addition, although the acute administration of 2-acetylaminofluorene induced an 8-fold increase in hepatic mdr-encoded RNA, performance of a partial hepatectomy either before or after administration of 2-acetylaminofluorene resulted in a greater than 80-fold increase in mdr gene expression over that in normal untreated livers. This represents an important in vivo model system in which to study the acute regulation of this drug resistance gene.

Mitogenic activity in platelet-poor plasma from rats with persistent liver nodules or liver cancer

Platelet-poor plasma (PPP) from F-344 rats with chemically-induced preneoplastic liver nodules or hepatocellular carcinoma stimulated S-phase DNA synthesis in monolayer cultures of normal rat hepatocytes. Similar mitogenic activity was detected in PPP 6 hrs to 1 week after partial hepatectomy (PH) or after necrotizing doses of CCl4 or diethylnitrosamine (DENA). Very little activity was found in PPP4 from control rats. The mitogenic activity in PPP from animals with nodules was non-dialyzable (greater than 14 kd) and bound to a heparin-sepharose affinity column. None of the mitogenic PPPs competed with [125I] epidermal growth factor (EGF) for binding sites on A431 cells or normal rat hepatocytes. These studies indicate that persistent proliferation of preneoplastic and neoplastic hepatocytes is associated with increased circulating levels of mitogenic hepatocyte growth factor.

Progression in hepatocarcinogenesis: differences in growth and behavior of transplants of early and later hepatocyte nodules in the rat spleen

Hepatocyte nodules were induced in Fischer 344 rats using the resistant hepatocyte model. Nodules harvested at 5, 24, or 25 weeks after initiation were isolated, diced, and transplanted into the spleen of normal rats and observed for periods up to 104 weeks. In the first experiment, 50% of the animals developed hepatocellular carcinoma, some with invasion and metastasis, by 70 to 104 weeks. In the second experiment, transplants of 5-week nodules grew very slowly and diffusely in the spleen, as did normal liver, but retained at least some of their phenotypic properties. In contrast, transplants of 25-week nodules grew into nodules up to 2.5 cm in diameter by 70 weeks. Two of the larger nodules had smaller nodules within resembling trabecular carcinoma. Transplants from the liver surrounding the 25-week nodules did not grow and produced no nodules by 70 weeks after transplantation. The implications of these observations in the study of progression in hepatocarcinogenesis are discussed briefly.

Initiation of carcinogenesis by a dietary deficiency of choline in the absence of added carcinogens

The feeding for 10 or 11 weeks of young male Fischer-344 rats, a diet devoid of choline and low in methionine, leads to the appearance of gamma-glutamyltransferase-positive foci of altered hepatocytes in the liver and to the induction of initiated resistant hepatocytes. The latter are known to contain the primary precursor cells for the ultimate development of hepatocellular carcinoma. This initiation of carcinogenesis with the choline-devoid diet is prevented by added choline. These observations indicate that a dietary deficiency may, by itself, without known contaminating or added carcinogens, initiate the carcinogenic process.

Patterns of ligand binding to normal, regenerating, preneoplastic, and neoplastic rat hepatocytes

The binding of epidermal growth factor, asialoorosomucoid, and apoprotein E-rich lipoproteins to isolated hepatocytes was investigated at various time intervals during the step-by-step development of liver cancer in rats. The degree of binding of the three ligands showed a progressive reduction in early persistent and late persistent putative preneoplastic hepatocyte nodules. This was further decreased in hepatocytes isolated from unequivocal hepatocellular carcinomas. Regenerating liver hepatocytes bound lesser amounts of epidermal growth factor and asialoorosomucoid than did hepatocytes from control resting liver but increased amounts of apoprotein E-rich lipoproteins. The progressive decrease in ligand binding during the precancerous phase of hepatocarcinogenesis, the nodule-to-cancer sequence, may render nodules less responsive to the influences of their external environments.

Influences of diethylnitrosamine on longevity of surrounding hepatocytes and progression of transplanted persistent nodules during phenobarbital promotion of hepatocarcinogenesis

The possibility that phenobarbital (PB) selectively promotes liver nodule development by decreasing survival of surrounding hepatocytes previously exposed to diethylnitrosamine (DENA) was evaluated. Livers of F-344 rats were labelled with [3H-methyl]-thymidine (3H-TdR) during developmental or regenerative growth. Neonatal rats given 3H-TdR between days 3 and 12 were subjected at 12 weeks of age to partial hepatectomy (PH) followed 24 hr later by DENA (10 mg/kg) or saline. Subsequent administration of PB (0.1% in drinking water) for 28 weeks reduced total liver label to 46 +/- 10% (saline group) or 40 +/- 4% (DENA group). Adult male rats initiated with DENA (200 mg/kg) and later labelled with 3H-TdR after PH also lost total liver label during 28 weeks' promotion with PB (0.05% in water) at rates similar to those exhibited by noninitiated rats given PB, and by DENA-treated or control rats not given PB. Large persistent (12 weeks) liver nodules generated by DENA in the Solt-Farber model were transplanted as small fragments into the spleens of syngeneic rats previously given 0, 100 or 200 mg/kg of DENA. Subsequent exposure to PB (0.05% in drinking water for 40 weeks) or Aroclor 1254 (6 X 300 mg/kg per month) promoted nodule and cancer development only in livers of DENA-initiated recipients. Surviving transplanted nodules remained as small microscopic clusters even after 40 weeks of promotion. However, PB increased transplant survival (50% vs. 21% in controls) whereas Aroclor reduced it to 8%. These findings indicate that promotion of liver nodules by PB occurs without enhanced mortality of surrounding hepatocytes previously damaged by DENA. They further suggest that promoters such as PB and PCBs do not directly influence the progression of established persistent nodules.

Modulation of calcium by the carcinogenic process in the liver induced by a choline-deficient diet

A diet devoid of choline and low in methionine (CD), without any added carcinogen, has been shown to induce 100% preneoplastic nodules and more than 50% cancer in the rat liver. Attempts to understand the mechanism by which a CD diet induces liver cell cancer revealed that like chemical carcinogens, a CD diet also appears to cause alterations in DNA, perhaps mediated by free radicals. Indeed, a CD diet induces nuclear lipid peroxidation prior to the changes in DNA. The CD diet induced DNA alterations coupled with continuing liver cell proliferation may account for the induction of initiated hepatocytes by the CD diet. To gain insight into the nature of free radicals generated by the CD diet, experiments were designed to determine whether agents that modulate free radical effects influence the CD diet induced changes in the liver. We investigated the effect of Ca2+ in the modulation of CD diet induced alterations in the liver. The results show that extra Ca2+ when added to the CD diet prevented some of the early changes due to choline deficiency, such as nuclear lipid peroxidation and DNA damage, but had little or no effect on the triglyceride accumulation in the liver. Also, the same CD diet with extra Ca2+, when used as a promoter after initiation by diethylnitrosamine, decreased the number and size of early putative preneoplastic foci and nodules.