Identification of a characteristic cytosolic polypeptide of rat preneoplastic hepatocyte nodules as placental glutathione S-transferase

Evidence is presented that a distinctive type of glutathione-S-transferase (GSHTase-P), and a cytosolic polypeptide of Mr 52,000 (P-52), each appearing in greatly increased amounts in hepatocyte nodules during liver carcinogenesis in the rat, are so far indistinguishable. The probable identity of the two polypeptides was established with the use of SDS polyacrylamide gel electrophoresis and Western blot techniques with purified GSHTase-P and P-52 and their respective antibodies and by comparison of the sequence of the first 26 N-terminal amino acids. Since the enzyme and the polypeptide are each considered to be the best available early markers for hepatocyte nodules, as putative precancerous lesions, their probable identity makes them attractive cellular components for in depth studies on their transcriptional and translational regulation and their use in new approaches to the sequential analysis of liver carcinogenesis.

Pathogenesis of experimental liver cancer: comparison with humans

The multi-step nature of cancer development in the liver with chemical carcinogens is very well developed in the rat and is highly probable in humans and in mice. Since this step-by-step analysis of hepatocarcinogenesis is much more advanced in the rat than in the other two species, the pathogenesis in the rat will be briefly reviewed. Initiation steps I and II, promotion steps III and IV and progression steps V and VI will be highlighted together with further unknown steps in progression to cancer. The focal proliferation of hepatocytes, as represented by hepatocyte nodules, appears to play a central and critical role as a precursor for cancer. In the rat, nodules have a very characteristic phenotype biologically, histologically and biochemically. Some of the options available to nodules and their significance will be discussed. The possible analogy between rat on the one hand and mice and humans on the other will be presented. A critical assessment of the relevance of pathological changes seen in rats and mice to the possible carcinogenicity of agents in humans will be discussed. The urgent need for more step-by-step studies of hepatocarcinogenesis in mice is very obvious and must be stressed.

Allogeneic hepatocyte transplantation in the rat spleen under cyclosporine immunosuppression

An innovative approach for stimulating the rapid growth of allogeneic hepatocytes implanted into splenic tissue with maintenance of the structural integrity is described. Single cell suspensions of hepatocytes from normal male ACI-strain rats (RTIa) were injected (2 X 10(6) cells) into the spleen of allogeneic male Fischer (RTI1) recipient rats. A 70% partial hepatectomy (PH) was performed at the same time as hepatocyte transplantation. Animals were treated for 4 days prior to, and 1 day after, transplantation with a feeding regimen containing 0.05% 2-acetylaminofluorene (AAF) to inhibit regeneration of the residual host liver. Animals received cyclosporine (CsA) 3 mg/kg/day s.c. posttransplantation. Histological examination of a standard longitudinal section of the recipient spleen two days posttransplant revealed an approximately 0.54-mm2 area replaced by hepatocytes. By 7 days this had increased to 0.97 +/- .15 mm2. Without CsA administration, hepatocytes were undetected at 7 days. Both PH and AAF treatment were necessary for successful colonization and sustained proliferation. Withdrawal of CsA treatment at 10 days after transplantation resulted in rapid rejection of established hepatocytes. This study demonstrates that rapid colonization of the rat spleen with allogeneic hepatocytes can be achieved, and that the viability and structural integrity of these transplanted cells can be maintained for at least 14 days using cyclosporine immunosuppression.

A choline-devoid diet, carcinogenic in the rat, induces DNA damage and repair

A diet deficient in choline when fed to rats for as few as 3 days resulted in liver DNA damage. The damage could be monitored as alkali-labile lesions using alkaline-sucrose gradients as well as alkaline elution technique. The DNA damage disappeared when the choline-deficient diet was replaced by a choline-supplemented diet suggesting the alkali-labile lesions were repairable. The DNA damage was detected at a time when no liver cell death was detectable. The induction of DNA damage in a proliferating liver by the choline-deficient diet may be an early important event leading to initiation of liver carcinogenesis.

Sequential alterations in growth control and cell dynamics of rat hepatocytes in early precancerous steps in hepatocarcinogenesis

This set of experiments is the second of a series designed to explore alterations in cell dynamics and growth control of new populations of hepatocytes that appear to play a role in the carcinogenic process induced in the liver by chemical carcinogens. This is part of an ongoing study of the biochemical and molecular basis for cancer development. A rat model for hepatocarcinogenesis, the resistant hepatocyte model, was chosen with its synchrony of several steps in the process. Carcinogenesis was initiated by the administration of a single necrogenic dose of diethylnitrosamine. Resistant hepatocytes so induced were stimulated to proliferate rapidly to form nodules by a mitogenic stimulus in the presence of a brief exposure to dietary 2-acetylaminofluorene sufficient to inhibit the proliferation of the majority of uninitiated hepatocytes, the nonresistant population. A small subset of these hepatocyte nodules, the persistent nodules, was examined at 2, 4, and 6 mo postinitiation. Duration of phases of the cell cycle, growth fraction, doubling time, cell death, and cell loss and the responses and subsequent recovery after the application of a strong mitogenic stimulus, partial hepatectomy, were measured. The first precancerous hepatocyte nodule, at 2 mo, showed a "normal" duration of phases of the cell cycle. The growth fractions were about 4,4, and 8% at 2, 4, and 6 mo, respectively, as compared to 0.4% in the surrounding hepatocytes. Accompanying the increased growth fractions were considerable levels of cell loss, measuring about 3% at 2 mo and 7% at 6 mo. At 6 mo, the hepatocyte nodule population, unlike the hepatocytes in the surrounding liver, shows a failure to return to its base-line level after stimulation of cell proliferation by partial hepatectomy. The results of this study have identified two new steps in the early precancerous phase of hepatocarcinogenesis relating to alterations in the control of cell proliferation and are consistent with the hypothesis that new and evolving cell populations may play an important role in the step-by-step carcinogenic process. These new populations appear to acquire alterations in growth control in a seriatim fashion, with retention of some "normal" properties.

The pattern of metabolism of 2-acetylaminofluorene in carcinogen-induced hepatocyte nodules in comparison to normal liver

This study was designed to test further the hypothesis that the special biochemical pattern seen in hepatocyte nodules during liver carcinogenesis could be of fundamental importance in their selective metabolism of one carcinogenic xenobiotic, 2-AAF, as related to their resistance to xenobiotics. Nodules of a certain stage were induced using the resistant hepatocyte model. The metabolism of a single small dose of 2-AAF in hepatocyte nodules in comparison to normal liver was studied at different time intervals up to 30 h. The levels of free 2-AAF in nodules and in normal liver were approximately the same over the whole time period. However, the nodules showed a large decrease in the binding of 2-AAF to DNA, RNA and proteins as well as in the metabolic conversion to hydroxylated forms, both free and conjugated with glucuronic acid. The patterns of metabolic conversion to metabolites and of conjugation of the metabolites are in harmony with the known biochemical patterns in nodules, a decrease in phase I components involved in the metabolism of carcinogens and other xenobiotics and an increase in most phase II components involved in conjugation and detoxification.

Influences of different polychlorinated biphenyls on cytocidal, mitoinhibitory, and nodule-selecting activities of N-2-fluorenylacetamide in rat liver

The influences of different polychlorinated biphenyl (PCB) isomers and congeners on distinct hepatotoxic responses to the hepatocarcinogen N-2-fluorenylacetamide [(2-FAA) CAS: 53-96-3] were examined in F344 rats. Cytocidal toxicity of 2-FAA (25-400 microM), determined by lactate dehydrogenase release during 20 hours in primary monolayer cultures of isolated rat hepatocytes, was reduced by in vivo pretreatment with either phenobarbitone [(PB) CAS: 50-06-6] or 2,2',4,4',5,5'-hexachlorobiphenyl (HCBP), a PB-type PCB inducer. However, cytocidal toxicity of 2-FAA was substantially potentiated by either 3-methylcholanthrene [(MCA) CAS: 56-49-5] or 3,3',4,4'-tetrachlorobiphenyl [(TCBP) CAS: 32598-13-3], an MCA-type PCB. In the same cell culture assays, all four pretreatments similarly reduced cytocidal toxicity of N-hydroxy-N-2-fluorenylacetamide (0.32-32 microM; CAS: 53-95-2). By comparison, pretreatments with either the PB-type or MCA-type PCB's (50-200 mumol/kg) diminished mitoinhibitory toxicity of 2-FAA in vivo, as measured by hepatic regenerative growth and hepatocyte labeling indices 7 days after partial hepatectomy (PH) in rats given 3 consecutive daily doses of 2-FAA (20/mg/kg/day) before PH. This regimen of 2-FAA and PH promoted rapid selective growth of gamma-glutamyltranspeptidase-positive (gamma-GT+) nodules at 2 and 4 weeks after PH in rats previously given an initiating hepatocarcinogen, diethylnitrosamine [(DENA) CAS: 55-18-5]. However, various PCB's, including 2,2',4,4',5,5'-HCBP, 3,3',4,4'-TCBP, 2,2',4,4'-TCBP, 2,2',5,5'-TCBP, and the commercial mixture Aroclor 1254, each given as a single dose of 50 mumol/kg by gavage 10 days after DENA and 7 days before 2-FAA, all reduced the size of 2-FAA-selected gamma-GT+ nodules during the 4-week period after PH. These results indicate that, in spite of predictable inducer-specific opposite influences of different types of PCB's on cytocidal toxicity of 2-FAA, all PCB's similarly reduce nodule selection by 2-FAA in initiated livers. Reduced growth of 2-FAA-selected nodules correlated with the consistent ability of all PCB's to enhance regeneration of liver mass after 2-FAA and PH.

Ethionine in the analysis of the possible separate roles of methionine and choline deficiencies in carcinogenesis

The importance of ethionine, the ethyl analogue of methionine, as a metabolic probe to study the possible roles of methionine and choline in liver carcinogenesis has been briefly reviewed. Ethionine-induced liver carcinogenesis is similar in many aspects, including initiation, promotion, and progression, to carcinogenesis with other agents. However, the special role of methionine in preventing virtually all metabolic and pathologic effects of ethionine, including liver cancer, places ethionine in a special position. On the basis of these observations and our current knowledge about choline deficiency in the genesis of liver cancer, we proposed that choline and methionine play separate but overlapping roles in the initiation and promotion of liver carcinogenesis.

1,2-Dibromoethane initiation of hepatic nodules in Sprague-Dawley rats selected with Solt-Farber system

Initiating effects of the fumigant 1,2-dibromoethane (DBE) for liver were examined using the Solt-Farber selection system. Male Sprague Dawley rats (230-260 g) were given one oral dose of DBE (75 mg/kg), a two-thirds partial hepatectomy 4 h later, five oral doses of 2-acetylamidofluorene (AAF) (25 mg/kg) on days 17-21, CCl4 (2 ml/kg) on day 22, a booster dose of AAF (10 mg/kg) on day 32, and were sacrificed on day 82. Nodules and appreciable gamma-glutamyl transpeptidase foci were found in the livers of four of six animals given DBE, but not in any animals of the control group.

Initiation and selection of resistant hepatocyte nodules in rats given the pyrrolizidine alkaloids lasiocarpine and senecionine

The biological mechanisms by which pyrrolizidine alkaloids contribute to initiation and nodule selection (promotion) steps in hepatic carcinogenesis were studied in male Fischer 344 rats. Lasiocarpine at single or double dosages (up to 80 mumol/kg) delayed hepatic regeneration for at least 8 weeks after partial hepatectomy (PH). This regimen of lasiocarpine and PH had a strong selective influence on the growth of gamma-glutamyltranspeptidase (gamma-GT)-positive hepatocyte nodules in rats previously initiated with diethylnitrosamine. However, both lasiocarpine (up to 80 mumol/kg) and senecionine (up to 160 mumol/kg) were inactive as initiators of gamma-GT-positive nodules in rats exposed to a similar selection regimen consisting of 2-acetylaminofluorene and PH. When lasiocarpine or senecionine was given 12 h after PH, very few nodules were initiated. Lasiocarpine pretreatments reduced the initiating activity of diethylnitrosamine and N-nitrosomethylurea in rats subsequently selected with 2-acetylaminofluorene and PH. Resistant nodules selected with lasiocarpine had the typical resistant nodule phenotype (positive for gamma-GT and epoxide hydrolase) and also lacked pyrrolizidine alkaloid-induced megalocytosis. Lasiocarpine treatment also resulted in small regenerative nodular proliferations of hepatocytes that were distinct from resistant nodules because they were negative for gamma-GT and epoxide hydrolase and unrelated to diethylnitrosamine pretreatments. These studies suggest that the hepatocarcinogenicity of pyrrolizidine alkaloids can be better explained by their strong selection (promotion) influence on initiated hepatocytes, rather than by their very weak initiating activity.

Tamoxifen alone or in combination with estradiol-17 beta inhibits the growth and malignant transformation of hepatic hyperplastic nodules

Hepatic hyperplastic nodules (HHNs) induced by the 'resistant hepatocyte method' of Solt et al. were studied as an experimental prototype of oral contraceptive-related tumors. Cytoplasmic estrogen receptors were present in all HHNs harvested and their concentration was always less than that in normal liver. No specific cytoplasmic progestin receptors could be measured in the above tumor or liver specimens. The long-term administration of estradiol-17 beta (4.8-24.0 micrograms/day) resulted in the death of all but one of 20 animals prior to termination at 10 months. Tamoxifen (0.25-2.5 mg biweekly), which did not lead to excess mortality, decreased HHN grade (proportion of liver slice occupied by HHN) and inhibited malignant transformation. Combination therapy with single-dose estradiol-17 beta (4.8 micrograms/day) and various doses of tamoxifen (0.25-2.5 mg biweekly) in most cases reduced mortality, HHN grade and malignant transformation. Cytoplasmic progestin receptors were absent and estrogen receptors were either undetectable or present in low concentrations in hepatic tumors harvested at the time of termination. Our results indicated that HHNs are hormone-dependent and that malignant transformation can be inhibited by tamoxifen alone or in combination with estradiol-17 beta.

Intestinal metaplasia as a common option of oval cells in relation to cholangiofibrosis in liver of rats exposed to 2-acetylaminofluorene

Oval cell proliferation in the liver was studied in rats exposed to dietary 2-acetylaminofluorene for 2 weeks with partial hepatectomy performed midway at 1 week. Using autoradiography after either continuous or pulse exposure to [3H]thymidine, histochemical staining for leucine aminopeptidase and gamma-glutamyltransferase, and transmission electron microscopy, we found that every animal showed islands of oval cells with small intestinal metaplasia at 3 weeks. Transitions from these islands of metaplasia to cholangiofibrosis were commonly seen. The importance of small intestinal metaplasia of oval cells in the genesis of cholangiofibrosis is presented and discussed.

Concepts of disease, medical research and the challenges to the schools of the healing professions

The study of disease is a significant part of the pattern of funding for medical research in North America and elsewhere. Also, the existence of disease and its importance in all branches of the healing professions is the major justification for separate professional schools of medicine. These considerations should encourage a vigorous exploration and development of concepts of disease as an important part of any medical education. Based on much of the current research activities, concepts of disease, especially chronic disease, seem largely outdated and not intimately reflecting the realization that the development of disease is often a physiological response to perturbations in the internal or external environment and not abnormal or pathological in the etymological sense. The importance of viewing cancer and other chronic diseases from this physiological point of view and not from the point of view of end-stage disease is emphasized by the use of one example, the development of cancer with chemicals. The challenge to the healing professions to develop more modern programs for educating the prospective research worker for the study of disease is discussed briefly.

Tamoxifen alone or in combination with estradiol-17 beta inhibits the growth and malignant transformation of hepatic hyperplastic nodules

Hepatic hyperplastic nodules (HHNs) induced by the 'resistant hepatocyte method' of Solt and Farber were studied as an experimental prototype of oral contraceptive-related tumors. Cytoplasmic estrogen receptors were present in all HHNs harvested and their concentration was always less than that in normal liver. No specific cytoplasmic progestin receptors could be measured in the above tumor or liver specimens. The long-term administration of estradiol-17 beta (4.8-24.0 micrograms/day) resulted in the death of all but one of 20 animals prior to termination at 10 months. Tamoxifen (0.25-2.5 mg biweekly) which did not lead to excess mortality, decreased HHN grade (proportion of liver slice occupied by HHN) and inhibited malignant transformation. Combination therapy with single-dose estradiol-17 beta (4.8 micrograms/day) and various doses of tamoxifen (0.25-2.5 mg biweekly) in most cases reduced mortality, HHN grade and malignant transformation. Cytoplasmic progestin receptors were absent and estrogen receptors were either undetectable or present in low concentration in hepatic tumors harvested at the time of termination. Our results indicate that HHNs are hormone-dependent and that malignant transformation can be inhibited by tamoxifen alone or in combination with estradiol-17 beta.

A common biochemical pattern in preneoplastic hepatocyte nodules generated in four different models in the rat

Hepatocyte nodules, structures consistently seen in every model of liver carcinogenesis well before the first appearance of cancer, were examined with respect to some Phase I and Phase II components considered to be important in the metabolism of carcinogens and other xenobiotics. Phase I components are those related to the metabolism of xenobiotics and include microsomal cytochromes P-450 and mixed-function oxygenase activities. Phase II components are those related to the conjugation and detoxification reactions of xenobiotics and their metabolites and include glutathione S-transferases and glutathione. Nodules were induced by the resistant hepatocyte, choline-deficient, methionine-low diet, phenobarbital and orotic acid models of liver carcinogenesis. Also, nodules generated by the resistant hepatocyte model were examined after transplantation to the spleen of syngeneic animals. The hepatocyte nodules show a common biochemical pattern, consisting of decreased microsomal cytochromes P-450, cytochrome b5, and aminopyrine N-demethylase activity and increased glutathione and gamma-glutamyltransferase in whole homogenates and glutathione S-transferase activity in the cytosol. This similarity, appropriate to a resistance phenotype, adds additional support for the hypothesis that hepatocyte nodules may be a common step in liver carcinogenesis in several different models.

Cellular biochemistry of the stepwise development of cancer with chemicals: G. H. A. Clowes memorial lecture

The sequence of possible cellular, tissue, biochemical, and molecular changes that are important during the development of experimental liver cancer with chemicals is reviewed from the viewpoint of the author's experience in carcinogenesis over the past 25 years. The development of a new model for the analysis of liver carcinogenesis, the resistant hepatocyte model, is briefly described, as are the known steps between exposure to an initiating dose of a chemical carcinogen to the appearance of hepatocellular carcinoma. These steps include: (a) the interactions with DNA; (b) the dependence on a round of cell proliferation for initiation; (c) one type of initiated hepatocyte, a resistant hepatocyte; (d) the selection of these new hepatocytes, probably by clonal expansion, to form synchronously the first type of hepatocyte nodules, early nodules; (e) the election of the majority of these nodules to undergo remodeling to normal-appearing liver by differentiation ("redifferentiation"); (f) the election of a minority of early nodules to persist; (g) the slow growth of the few persistent nodules; and (h) the precursor role of persistent nodules in the development of hepatocellular carcinoma. The evidence for such a precursor role includes: (a) the common occurrence in persistent nodules of a subsequent precancerous step, "nodules in nodules"; (b) the occurrence of metastasizing cancer inside nodules without cancer elsewhere in the liver; and (c) the high rate of evolution to cancer of persistent nodules, but not of early nodules, when transplanted to the spleen. Based on the common architecture, organization, blood supply, and biochemical pattern of properties relating to the metabolism of xenobiotics in hepatocyte nodules in six different models of liver carcinogenesis and on the common occurrence of a highly programmed redifferentiation pattern of carcinogen-induced hepatocyte nodules, it is concluded that heterogeneity and diversity seen in many phenotypic properties of cancers, including liver cancers, is preceded by a precursor population that is unusually homogeneous and uniform in phenotype. Thus, the heterogeneity and diversity of cancers are probably late manifestations in carcinogenesis. The available evidence is very suggestive that the hepatocyte nodules are an expression of physiological adaptation to exposure to hazardous xenobiotics and not a form of aberration or mutation. The data also suggest that hepatocyte nodules are an additional pattern of liver differentiation and that liver cancer, to be understood, should be compared with this precursor new state rather than the conventional adult, fetal, or embryonic states.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapid lipid peroxidation in the nuclear fraction of rat liver induced by a diet deficient in choline and methionine

A diet deficient in choline and methionine, known to produce hepatocellular carcinoma in the absence of any added chemical carcinogen, induced lipid peroxidation in the nuclear fraction of the liver when fed to male Fischer 344 rats. This lipid peroxidation was detected within 1 day of feeding the diet by the appearance of diene conjugates and increased progressively up to 3 days. It was prevented completely by the addition of choline chloride to the diet. The close proximity of DNA may make it a possible target for attack by free radicals.

The induction of liver cancer by dietary deficiency of choline and methionine without added carcinogens

Fischer 344 male rats fed a choline-methionine deficient diet for from 13 to 24 months developed a 100% incidence of putative preneoplastic hepatocyte nodules and a 51% incidence of hepatocellular carcinoma. The addition of 0.8% choline chloride completely prevented the development of both the nodules and the cancer. The diet contained no added known carcinogen. Analysis of the deficient and supplemented diets revealed no detectable volatile nitrosamines or nitrosamides, nitrite, nitrate or malonaldehyde, less than 0.9 p.p.b. aflatoxin B1 and barely detectable levels of Ames positive material with one strain of Salmonella typhimurium. These findings indicate that a dietary deficiency of choline and methionine can be a major rate limiting factor in the development of liver cancer.

Comparative influences of different PB-type and 3-MC-type polychlorinated biphenyl-induced phenotypes on cytocidal hepatotoxicity of bromobenzene and acetaminophen

The influences of in vivo treatment with two pure PCB congeners, 2,2',4,4',5,5'-hexachlorobiphenyl (HCBP) and 3,3',4,4'-tetrachlorobiphenyl (TCBP), on the lethal cytotoxicity of bromobenzene and acetaminophen were examined in short-term primary cultures of isolated rat hepatocytes. Lethal injury was measured by release of lactate dehydrogenase (LDH) into culture medium after 20 hr exposure to the hepatotoxins. The HCBP, a PB-type inducer of cytochrome P-450, resembled phenobarbitone (PB) in its ability to increase susceptibility of hepatocytes to bromobenzene (0.5 to 1.6 mM) and acetaminophen (1 to 16 mM). This induced sensitivity was consistently inhibited by SKF-525-A (10 microM) but not alpha-naphthoflavone (ANF, 10 microM) in culture. The 3,3',4,4'-TCPB, a 3-MC-type inducer of cytochrome P-450, resembled 3-methylcholanthrene (3-MC) in its inability to induce susceptibility to bromobenzene. TCBP and 3-MC each increased (20- to 30-fold) cytotoxicity of acetaminophen by a mechanism substantially inhibitable by ANF but not SKF-525-A. These results demonstrate that categorizing pure PCB isomers and congeners into groups according to their different induction capabilities is predictive for their ability to modulate acute hepatocellular necrosis by bromobenzene and acetaminophen.

Nutrition and cancer

Nutrition has a close association with cancer. Carcinogens in foods, macronutrients, micronutrients such as vitamins A, C and E and selenium, dietary fiber, the modulation of metabolism by diet, food additives and anticarcinogens in foods all affect the development of cancer. In particular, macronutrients (especially meat and animal fat) appear to have a significant effect on the development of cancer of the colon, breast and uterus. This article discusses research on the links between nutrition and cancer, and describes current dietary recommendations to decrease the likelihood of cancer.

Cell cycle kinetics of rat hepatocytes in early putative preneoplastic lesions in hepatocarcinogenesis

This set of experiments is the first of a series designed to explore facets of cell proliferation of hepatocytes during the carcinogenic process induced in liver by chemical carcinogens. A rat model for hepatocarcinogenesis, the resistant hepatocyte model, was used. A major advantage of this model is the unusual degree of synchrony in the early steps. Carcinogenesis was initiated by the administration of a necrogenic dose of diethylnitrosamine. Resistant hepatocytes so induced were stimulated rapidly to proliferate by partial hepatectomy in the presence of a brief exposure to dietary 2-acetylaminofluorene sufficient to inhibit the proliferation of the majority of hepatocytes, the nonresistant population. Cell cycle parameters were measured in the early carcinogen-altered resistant hepatocyte populations and in regenerating hepatocytes. Growth fraction and doubling time were experimentally determined in the altered hepatocytes. The mean cell cycle length of the resistant cells was 38.6 hr, somewhat longer than that of regenerating hepatocytes, which was 33.6 hr. Most of the increase was due to a prolonged S phase which was 13.6 hr in the altered cell population as compared to 7.0 hr in hepatocytes in regenerating control liver. The hepatocytes in normal regenerating liver had a mean duration of 21.6 hr for G1 as compared to 20.4 hr for the altered hepatocytes and a G2 of 3.4 hr as compared to 3.0 hr for carcinogen-altered hepatocyte. M was assumed to be 1.6 hr in both populations. The growth fraction in the altered cell population was determined to be a minimum of 0.83, and the doubling time was about 45 hr. Thus, the resistant hepatocytes which represent an early putative preneoplastic population show, in general, a prolongation of the cell cycle, mostly due to a prolonged S phase.