Evidence for the hormone dependency of hepatic hyperplastic nodules: inhibition of malignant transformation after exogenous 17 beta-estradiol and tamoxifen

Hepatic hyperplastic nodules (HHNs) in rats were studied as an experimental prototype of oral contraceptive-related hepatic tumors. We have found cytoplasmic estrogen receptors in HHNs produced by acetylaminofluorene (AAF) (four cycles of 0.02% in diet). Rats with AAF-induced HHNs were randomized into four groups: (i) AAF-treated control; (ii) estrogen alone (estradiol-17 beta); (iii) tamoxifen alone, and (iv) estrogen + tamoxifen. After 8 months of treatment with estrogen (estradiol-17 beta) in combination with tamoxifen, there was regression of nodular involvement and no evidence of malignant transformation. Decreased nodular proliferation also occurred after 2 and 4 months treatment with estradiol-17 beta and after 8 months of tamoxifen administration. The incidence of hepatocellular carcinoma after 8 months of treatment was significantly less after treatment with estrogen (40%) or tamoxifen (42.9%) when compared to AAF-treated controls (87.5%). The number of gamma-glutamyltranspeptidase-positive foci were reduced in all treatment groups after 2, 4, and 8 months of treatment; these changes were most pronounced in the estrogen-treated group and did not directly correlate with the per cent inhibition of malignant transformation. Our results suggest that the malignant transformation of estrogen receptor-positive HHNs is hormone dependent.

Distinctive biochemical pattern associated with resistance of hepatocytes in hepatocyte nodules during liver carcinogenesis

Hepatocyte ("hyperplastic") nodules induced in the liver by initiation with diethylnitrosamine and selected by dietary 2-acetylaminofluorene plus partial hepatectomy ("resistant hepatocyte model") have a special pattern of biochemical behavior and metabolic activity different than that seen acutely with many xenobiotics including many promoting agents and carcinogens. The nodule cells show a very low uptake of 2-acetylaminofluorene, relative to surrounding and normal liver, low levels of activity in the cytochromes P-450 and aryl hydrocarbon hydroxylase, high levels of activity in gamma-glutamyltransferase, microsomal epoxide hydrolase, soluble glutathione-S-transferase and soluble UDP-glucuronyltransferase (UDP-GT(1)) and elevated levels of glutathione. This metabolic pattern appears to maximize the resistance of the nodules to xenobiotics generally, such as 2-acetylaminofluorene, and thereby may account for the resistant behavior of nodule hepatocytes to the inhibition of cell proliferation and the cytotoxicity by 2-acetylaminofluorene and other carcinogens. The possible importance of this seemingly new metabolic program in carcinogenesis is discussed briefly.

An experimental method for rapid growth of liver in spleen. The survival and proliferation of chemically induced preneoplastic hepatocytes in spleen

Cellular suspensions (2 x 10(6) cells) of isolated preneoplastic liver cells, obtained from carcinogen-treated rats, were injected in the spleens of syngeneic rats divided into groups on the basis of no treatment, partial hepatectomy (PH), and/or feeding regimens including 2-acetylaminofluorene (AAF). Recipient rats undergoing both PH and AAF showed significantly more rapid proliferation of the preneoplastic liver cell implant, compared with other treatment groups and control. The theoretic basis for this observation, supported by a large body of data derived from hepatocarcinogenesis, is as follows: The phenotype of the donor cells has been altered by chemical carcinogens such that the liver cells develop resistance to growth-inhibiting agents such as AAF. The recipient receives PH and AAF, the former creating a strong proliferative stimulus for hepatocytes, while the latter inhibits regeneration of normal liver cells but not those resistant to the mito-inhibitory effect of AAF, ie, the carcinogen-altered donor cells. These manipulations in donors and recipients thus create a selective environment in which the implant undergoes rapid proliferation. This model of resistance induction followed by selective proliferation, built upon the principles of carcinogenesis and applied to isolated liver cell transplantation, provides an experimental basis for achieving rapid liver growth of the splenic implant.

Sequential analysis of hepatic carcinogenesis. Regeneration of liver after carbon tetrachloride-induced liver necrosis when hepatocyte proliferation is inhibited by 2-acetylaminofluorene

The effect of inhibition of hepatocyte proliferation by dietary 2-acetylaminofluorene (2-AAF) on the restoration of liver in rats after a necrogenic dose of carbon tetrachloride has been studied. The liver weights remained low during the entire feeding period of the 2-AAF, and virtually no hepatocyte proliferation was seen, as determined autoradiographically after thymidine incorporation and by the absence of mitotic figures. Oval cell proliferation was extensive. Morphometric analysis showed (a) equal and maximum liver cell necrosis by 24 hours in both the experimental and control groups, (b) similar kinetics of removal of dead liver cells, and (c) similar values for the mean liver cell area. The distance between the portal triad and terminal hepatic vein in animals on the dietary 2-AAF was considerably reduced. Massive hepatocyte proliferation began after termination of the 2-AAF diet, and the liver returned to normal appearance within 14 days. The oval cells disappeared during this period of liver cell restoration. A new hypothesis for oval cell proliferation based on differential inhibition of hepatocyte proliferation resulting in unbalanced growth of ductular cells is presented.

Resistance to hepatotoxins acquired by hepatocytes during liver regeneration

The appearance of resistance to a number of hepatotoxins in primary cultures of hepatocytes prepared at various time intervals up to 2 weeks after partial hepatectomy is the major focus in this study. Resistance to the cytocidal effect of aflatoxin B1, 2-acetylaminofluorene, N-hydroxy-2-acetylaminofluorene, methotrexate, or methyl methanesulfonate shows a progressive increase until 48 hr and then returns to the resting level of susceptibility by 2 weeks. The genesis of mutagens from 2-acetylaminofluorene and aflatoxin B1 by S-9 liver fractions shows a decrease from and return to control values after partial hepatectomy that parallels the resistance. The levels of total cellular cytochromes P-450 also decrease following partial hepatectomy and remain from 28 to 36% less than those of controls for at least 1 week. The glutathione and total soluble sulfhydryl ("glutathione") content increase following partial hepatectomy, and the pattern is consistent with a partial role for glutathione in the resistance phenomenon as it relates to 2-acetylaminofluorene. The possible relationship between resistance to the cytocidal effects in vitro and the resistance to inhibition of cell proliferation in vivo during liver carcinogenesis is discussed.

The induction of resistant hepatocytes during initiation of liver carcinogenesis with chemicals in rats fed a choline deficient methionine low diet

Feeding a choline deficient methionine low (CLD) diet for two weeks can substitute for partial hepatectomy in the genesis with benzo[a]pyrene (B[a]P) or 1,2-dimethylhydrazine (1,2-DMH) of resistant hepatocytes that can be selected with dietary 2-acetylaminofluorene plus partial hepatectomy. With single doses of B[a]P or 1,2-DMH during the feeding of the CLD diet for 3 weeks, the number of foci of hepatocytes positive for gamma-glutamyl transferase is approximately the same as with the same dose of each carcinogen given after partial hepatectomy. DL-ethionine was now found to be positive when given as a single dose to animals on the CLD diet. Feeding ethionine for 6 or 12 weeks in a similar choline lipotrope deficient diet also induced a significant increase in foci of enzyme altered hepatocytes. The results indicate that dietary imbalance can have an important influence on initiation of chemical carcinogenesis.

Failure of glutathione to prevent liver cancer development in rats initiated with diethylnitrosamine in the resistant hepatocyte model

This study was undertaken to observe whether the administration of reduced glutathione intragastrically to male Fischer 344 rats during the precancerous steps of liver carcinogenesis has any protective effect on the development of hepatocellular carcinoma. Hepatocyte nodules were induced in the liver with a single initiating dose of diethylnitrosamine followed by selection of resistant hepatocytes to generate nodules by a two week exposure to dietary 2-acetylaminofluorene coupled with partial hepatectomy. Animals had hepatocyte ('hyperplastic') nodules when examined by laparotomy at three months. At that time, the animals were divided into two groups. One received daily intragastric glutathione for 8 months while the other received no further treatment. An additional control group received only the selecting (promoting) regimen with no initiator or glutathione. At 12 months, the animals receiving the initiator and promoter regimen had a 65% incidence of hepatocellular carcinoma and those receiving glutathione in addition had a 71% incidence. Under these experimental conditions, the long term administration of glutathione appears to have no observable influence on liver cancer development in this model.

Promotion of liver cancer development by brief exposure to dietary 2-acetylaminofluorene plus partial hepatectomy or carbon tetrachloride

Adult male Fischer rats were exposed to a necrogenic dose (200 mg/kg) of diethylnitrosamine or to nonnecrogenic doses of N-methyl-N-nitrosourea, 1,2-dimethylhydrazine, or benzo(a)pyrene following partial hepatectomy or sham hepatectomy. This treatment by itself led to no hepatocellular carcinomas by 8 to 18 months, except in animals given N-methyl-N-nitrosourea, which showed a 30% incidence by 12 months. With each treatment regimen, exposure to dietary 2-acetylaminofluorene for 2 weeks coupled with partial hepatectomy or the administration of a necrogenic dose of CCl4, was associated with an incidence of 68 to 94% of cancer at 8, 12, or 18 months, depending upon the initiating carcinogen used. Appropriate controls showed either no hepatocellular carcinoma or a much lower incidence. It is concluded that the 2-week exposure to dietary 2-acetylaminofluorene plus partial hepatectomy or the administration of CCl4 has a strong promoting effect on liver carcinogenesis with four different chemical carcinogens.

Kinetics of phenotypic maturation of remodeling of hyperplastic nodules during liver carcinogenesis

Hyperplastic nodules appearing during the preneoplastic phase of liver carcinogenesis were divided into two types, persistent and remodeling, according to the pattern of staining for gamma-glutamyltransferase. In the resistant cell model of liver carcinogenesis used in this study, hyperplastic nodules, uniformly staining for gamma-glutamyltransferase, rapidly emerge by 4 weeks after a single injection of diethylnitrosamine and brief selection by dietary 2-acetylaminofluorene plus partial hepatectomy. By 6 weeks, a majority of nodules (about 75%) show an obvious irregularity and loss of uniformity in staining for gamma-glutamyltransferase while the remaining nodules continue to be uniformly stained. The number of irregularly stained nodules increases over the next 18 weeks until over 95% of nodules show the nonuniform loss of enzyme activity. The progressive loss of enzyme activity is accompanied by architectural remodeling to normal-appearing liver. This is associated with the increasing disappearance of many obvious nodules from the liver as the remodeling ones blend imperceptibly with the surrounding liver. The uniformly stained nodules show the persistence of hepatocyte arrangements in plates two or more cells thick and in acini and of cytoplasmic hypertrophy characteristic of persistent hyperplastic nodules. Labeling indices are much higher in hepatocytes of the persistent uniformly stained nodules than in the remodeling ones. The possibility of exploiting this phase of the model further for in-depth analysis of the nodule-to-carcinoma sequence is discussed.

Effects of delays in the cell cycle on the induction of preneoplastic and neoplastic lesions in rat liver by 1,2-dimethylhydrazine

This study was designed to explore further the relationship between cell proliferation and the induction of early putative preneoplastic lesions by carcinogens. Rats were given a non-necrogenic dose of 1,2-dimethylhydrazine 24 hr before being subjected to partial hepatectomy. Beginning 4 hr later, hydrocortisone was injected 10 times at 4-hr intervals to delay progression through the cell cycle, including inhibition of DNA synthesis by at least 85% for about 40 hr. At the appropriate time thereafter, the putative preneoplastic hepatocytes were selectively stimulated to grow in vivo into gamma-glutamyltransferase-positive focal lesions. Animals given hydrocortisone showed a large decrease (71%) in the number of gamma-glutamyltransferase-positive foci. In contrast, when hydrocortisone was given at 6 days after partial hepatectomy, no inhibition in the induction of hepatic lesions was observed. In the next experiments, rats were treated with 1,2-dimethylhydrazine and were subjected to partial hepatectomy at 12, 24, or 48 hr or 1 week thereafter. A significant number of gamma-glutamyltransferase-positive foci was found when partial hepatectomy was performed at 12 or 24 hr but far fewer were found when the operative partial hepatectomy was delayed to 48 hr or 1 week later. Similarly, in long-term experiments, six of 14 animals developed primary hepatocellular carcinoma 13 months after the time of injection of 1,2-dimethylhydrazine when partial hepatectomy was performed at 12 hr, while none of the animals developed liver cancer when the operation was performed at 48 hr. These results imply that the majority of biochemical lesions induced by 1,2-dimethylhydrazine that are relevant to the induction of liver preneoplasia and neoplasia are short-lived and that their persistence is associated with some cellular activity closely related to the cell cycle.

The effect of portal diversion on the early stages of liver carcinogenesis in the rat

The effect of portal diversion by ameroid constriction was studied in the carcinogenic process in the rat after initiation of foci by diethylnitrosamine and after the establishment of putative preneoplastic nodules. Portal diversion or portal diversion plus partial hepatectomy did not act to promote or select for the development of nodules. Furthermore, portal diversion did not alter the natural history of established nodules. These results suggest that vascular factors are not important at these early stages of the carcinogenic process in the rat liver.

Some conclusions derived from a liver model for carcinogenesis

A new model of liver cancer development with chemicals is described. This model was based on the hypothesis that chemical carcinogens induced as a first step altered hepatocytes that are resistant to the inhibitory effect of a carcinogen, such as N-2-fluorenylacetamide, on cell proliferation. After the administration of a single initiating dose of a carcinogen, the rare resistant hepatocyte is selected by the creation of a special selection pressure, consisting of a stimulus for cell proliferation in the presence of an environment that inhibits normal hepatocyte proliferation. The latter is created by brief exposure to dietary N-2-fluorenylacetamide. With this approach, initiated hepatocytes and large hyperplastic liver nodules can be rapidly induced in a synchronized fashion. A direct material continuity between resistant hepatocytes, foci, and nodules of such cells (hyperplastic nodules) and hepatocellular carcinoma was established with diethylnitrosamine as the initiating carcinogen. The use of the resistant cell model has shown that initiation consisted of at least two steps, the second of which is a compulsory round of cell proliferation. With this model, three mechanisms of promotion in the liver are suggested: differential inhibition, differential stimulation, and differential recovery. The relationship of these early changes to liver cancer development is discussed.

Role of acute hepatic necrosis in the induction of early steps in liver carcinogenesis by diethylnitrosamine

Experiments were performed to assess the role of liver cell necrosis in the induction of early steps in liver carcinogenesis with diethylnitrosamine, as measured by the appearance of foci of resistant hepatocytes that stain for gamma-glutamyl transpeptidase and that are presumptive preneoplastic lesions in the rat. With the use of a necrogenic dose of diethylnitrosamine and an assay for the carcinogen-induced early stages or resistant hepatocytes, the number of enzyme-altered foci was decreased to a major extent (up to 62%) by posttreatment with diethyldithiocarbamate, a derivative of disulfiram. Such posttreatment decreased to a large degree (78%) the cumulative labeling index of hepatocytes following an initial exposure to diethylnitrosamine. The performance of partial hepatectomy up to 68 hr after such posttreatment restored the level of induction of the resistant hepatocytes. Nonnecrogenic doses of diethylnitrosamine or dimethylnitrosamine induced virtually no foci of resistant hepatocytes but did so when coupled with cell proliferation. These results establish clearly an important role for liver cell necrosis in the production of early steps in liver carcinogenesis in one model. The mechanism for this effect appears to be by the induction of compensatory liver cell proliferation.

The sequential analysis of cancer induction with chemicals

A critical review of some of our current views on how chemicals induced cancer is presented. Emphasis is placed on pathologic processes, rather than on agents and on the many steps that lie between initial exposure to a carcinogen and the appearance of malignant neoplasia. The nature of some of the early steps in carcinogenesis is outlined including the early appearance during initiation of cells resistant to some of the inhibitory effect of carcinogens. This property is considered to be important in the development of cancer in at least one model of carcinogenesis. The essential characteristic of promotion, differential stimulation of proliferation of initiated cells, is discussed along with at least three possible mechanisms for such differential effects of promoting environments. The multistep hypothesis for cancer development is supported by observations of identifiable biochemical and biological properties of different cell populations at some steps. The implications of this for cancer diagnosis, prevention and control are briefly discussed.

The genesis of cancer with chemicals

In studying the sequence of events during the induction of cancer with chemicals, major emphasis in recent years has been given to the identification of possible carcinogens and to their metabolism and interactions with cellular constituents, such as DNA, in target cells. The repair of relevant chemical lesions seems to be important, and special emphasis can be given to the cellular aspects of chemical carcinogenesis, the biological nature of the initiated cells, and the basis for their growth into focal proliferations during promotion. A new model for the sequential analysis of steps during the earlier periods in cancer development in the liver with chemicals has been developed, delineating the probable role of cell injury as a rate-limiting step in cancer development in organs that are quiescent with respect to cell proliferation, and the possible beneficial role of the early events in chemical carcinogenesis in natural selection.

Purification and partial characterization of a preneoplastic antigen in liver carcinogenesis

A preneoplastic antigen has been isolated from the microsomal fraction of the hyperplastic nodules of rat liver. During the purification, the presence of preneoplastic antigen was monitored by microcomplement fixation test and double immunodiffusion plate following reaction with antiserum specific for this antigen. The antigen was purified by seven steps with a purification factor of 167 and 11% yield. It migrated as a single protein band, both in nondenaturing and denaturing polyacrylamide gel electrophoresis; showed a single sharp peak in an isoelectric focusing column at pH 4.75; and had a single serine amino-terminal amino acid. In the absence of detergent during purification, this protein tends to form molecular aggregates. It appears to be a glycoprotein based on the positive periodic acid-Schiff stain and absorption by concanavalin A:Sepharose affinity column chromatography. Amino acid analysis indicated that the protein is acidic, having acidic amino acids (aspartate and glutamate) and basic amino acids (lysine, arginine, and histidine) in a ratio of 6.8. An estimate of the molecular weight by gel filtration gave a value of 145,000 and a calculated Stokes radius of 41.5 A. Electrophoresis under denaturing conditions on sodium dodecyl sulfate-polyacrylamide gel indicated that this protein is a dimer consisting of two subunits of equal molecular weight (M.W. 74,000). Circular dichroism spectra indicated alpha-helix contents at 298 and 222 nm of 25.5 and 28.3%, respectively. A similar protein is present in the endoplasmic reticulum of normal rat liver, but in a much lower concentration.