Inhibition by acetylsalicylic acid, a cyclo-oxygenase inhibitor, and p-bromophenacylbromide, a phospholipase A2 inhibitor, of both cirrhosis and enzyme-altered nodules caused by a choline-deficient, L-amino acid-defined diet in rats

Effects of inhibitors of arachidonic acid (AA) metabolism onthe development of fatty liver, cirrhosis, glutathione-S-transferaseplacental form (GST-P)-positive nodules and the generation of8-hydroxydeoxyguanosine (8-OHdG) and thiobarbituric acid-reactivesubstances (TBARS), caused by a choline-deficient, L-amino acid-defined(CDAA) diet, were examined in male Fischer 344 rats by feedingCDAA diets supplemented with the inhibitors for 12 and 30 weeks.Acetylsalicylic acid (ASA) (at doses of 0.1 and 0.2%) and p-bromophenacylbromide(BPB) (0.1 and 0.2%) were used as inhibitors of, respectively,cyclo-oxygenase and phospholipase A₂, and quercetin (QU) (0.75and 1.5%) and nordihydroguaiaretic acid (NDGA) (0.1 and 0.2%)as inhibitors of lipoxygenase. None of the inhibitors affectedthe development of fatty liver caused by the CDAA diet. ASAat a dose of 0.2% almost completely prevented the appearanceof cirrhosis, GST-P-positive nodules, 8-OHdG and TBARS in sevenout of 11 (63.7%) rats. BPB at a dose of 0.2% also exerted inhibitoryeffects on all of these lesions but to a lesser extent thanASA. QU and NDGA exerted inhibitory effects limited to the GST-P-positivenodule case. The results indicate that a perturbed AA metabolism,particularly of the cyclo-oxygenase pathway, derived secondarilyfrom depletion of labile methyl groups or phosphatidylcholine,might play key roles in the cirrhosis, hepatocarcinogenesisand oxidative stress caused by a CDAA diet. The results alsoindicated a possible involvement of the lipoxygenase pathwayin hepatocarcinogenic processes.     

Enhanced preneoplastic liver lesion development under 'selection pressure' conditions after administration of deoxycholic or lithocholic acid in the initiation phase in rats

The initiating potential of the secondary bile acids, deoxycholic acid (DCA) and lithocholic acid (LCA), for rat hepatocarcinogenesis was investigated using the development of hyperplastic nodules and/or glutathione S-transferase placental form (GST-P)-positive liver foci as the end point. Five week old male F344 rats were given either basal diet, or diets containing 0.5% DCA or 0.5% LCA for 3 weeks in conjunction with partial hepatectomy performed midway, followed by a selection regimen consisting of 2 weeks feeding of 0.02% 2-acetylaminofluorene diet and a single gastric intubation of carbon tetrachloride. The animals were then placed on either basal diet or a diet containing 0.05% phenobarbital (PB) for 52 weeks. Significantly higher numbers of hyperplastic liver nodules developed in the DCA-treated rats irrespective of PB promotion as compared with the respective control groups. No such increase was evident in the LCA-treated rats. In contrast, both DCA and LCA treatments enhanced the development of GST-P-positive liver foci with or without subsequent PB promotion. Only one hepatocellular carcinoma was diagnosed in a control group animal. The present data indicate that a short period of feeding of DCA and LCA in the initiation stage in conjunction with partial hepatectomy results in enhanced development of preneoplastic liver lesions under selection pressure conditions with or without subsequent PB promotion. They thus confirm and extend our previous finding of enhanced gamma-glutamyltranspeptidase-positive liver foci development in a short-term assay of DCA and LCA, and suggest that these secondary bile acids either possess possible initiating activity or some other priming effect for rat hepatocarcinogenesis.     

Stable phenotypic expression of glutathione S-transferase placental type and unstable phenotypic expression of gamma-glutamyltransferase in rat liver preneoplastic and neoplastic lesions

Using immunohistochemical demonstration of glutathione S-transferase placental type (GST-P) and histochemical demonstration of gamma-glutamyltransferase (gamma-GT), the long-term development of preneoplastic and neoplastic lesions was followed in rats over a 50-week period. Rats were given a single i.p. injection of 200 mg/kg body weight of diethylnitrosamine (DEN), and then 2 weeks later were administered 0.02% 2-acetylaminofluorene (2-AAF) (group 1), 0.05% phenobarbital (PB) (group 2), 2.0% butylated hydroxyanisole (BHA) (group 3) or no supplement (group 4) in their diet for 6 weeks, all rats being subjected to partial hepatectomy at week 3. Hepatocellular proliferated lesions were classified as foci, nodules and hepatocellular carcinomas. Development of foci, nodules and hepatocellular carcinomas was enhanced strongly by 2-AAF and weakly by PB, and inhibited by BHA. Almost all foci and nodules were GST-P positive, although 5-10% of the GST-P-positive foci were gamma-GT negative. The areas of GST-P-positive foci and nodules increased with time in all groups. In contrast, while the areas of gamma-GT-positive lesions also increased with time in groups 2-4, they decreased from week 12 in group 1. As the percentage gamma-GT-positive area in GST-P-positive foci significantly decreased with time in all groups, the rate of phenotypic reversion of gamma-GT in foci in group 1 was revealed to be larger than the focus growing rate, whereas that in groups 2-4 was smaller. Gamma-GT-negative and GST-P-positive micro-nodules of altered morphology appeared within gamma-GT- and GST-P-positive nodules in later stages. All hepatocellular carcinomas found in this experiment consisted of GST-P-positive cells. In contrast, 37% (13/35) of the hepatocellular carcinomas were negative for gamma-GT. The results indicate GST-P to be the most accurate marker enzyme for detection of initiated cells during liver carcinogenesis and gamma-GT to be more appropriate for indicating changes of phenotypic expression in each lesion type.     

Lack of modifying effects of 6-mercaptopurine in a medium term bioassay system for liver carcinogenesis using male F344 rats.

Carcinogenic and modification potential of 6-mercaptopurine (6-MP) was studied in a medium-term bioassay system for rat liver carcinogenesis. F344 male rats were initiated with a single dose (200 mg/kg body wt.) of diethylnitrosamine (DEN) i.p. and fed diets containing either 0.005% or 0.02% 6-MP with or without 0.05% phenobarbital (PB) for 6 weeks. Quantitative data revealed that 6-MP did not enhance the appearance of enzyme-altered preneoplastic foci and nodules even when administered at the highest dose (0.02%) despite showing an immunosuppressive effect and slight liver cell damage. Neither of the doses of 6-MP exerted any significant influence on the enhancing effect of PB when administered simultaneously in the medium-term-bioassay.     

Expression of the c-myc, c-fos and c-rasHa protooncogenes during sex-differentiated rat liver carcinogenesis in the resistant hepatocyte model

The expression of the protooncogenes c-myc, c-fos and c-rasHa has been studied in rats treated according to the resistant hepatocyte model. Protooncogene expression was studied in male and female rat liver during the selection phase, when the outgrowth of putative preneoplastic foci/nodules is markedly faster in males, and compared with the expression in advanced nodules and hepatocellular carcinomas in males. During the first 16 h after partial hepatectomy the expression of c-fos and c-myc showed transient, 2- to 3-fold, increases in both sexes, both in initiated and in 'control' animals, receiving the selection/promotion regiment but no diethylnitrosamine, with a maximum at 0.5 and 2-4 h respectively. c-rasHa exhibited a moderate increase (1.5-fold) at 16-24 h in all groups. A second increase in c-myc expression (2-fold) started 24 h after partial hepatectomy and lasted over the entire selection period in initiated males, while it was unchanged in females and uninitiated males. The c-fos expression also showed a short-lived increase 24 h post partial hepatectomy in initiated males. The expression of c-myc and c-fos was increased 2- to 4-fold in both preneoplastic nodules and hepatocellular carcinomas, whereas c-rasHa expression was unchanged. In conclusion, sex differences were observed in the expression of c-myc and c-fos during the early outgrowth of preneoplastic lesions, possibly reflecting a connection between the expression of these genes and the sex differentiated response to promotion in the resistant hepatocyte model. Furthermore, an overexpression also in later stages of liver carcinogenesis might indicate that expression of the protooncogenes in question is related to the entire process of multistep carcinogenesis in this model.     

Loss of glutathione S-transferases in pollution-associated liver neoplasms in white suckers (Catostomus commersoni) from Lake Ontario.

White suckers (Catostomus commersoni) are one of two species of bottom-feeding fish in which various liver neoplasms are more prevalent in urban/industrial sites in western Lake Ontario than in less polluted sites in the Great Lakes. Previous studies indicate that white suckers excrete metabolites of various polycyclic aromatic hydrocarbons (PAHs) in bile, and that glutathione transferase (GST)-mediated conjugation is a major detoxification pathway for the PAH benzo[alpha]pyrene. To determine whether hepatocarcinogenesis in these wild fish is associated with induced GST-dependent resistance to carcinogens, we examined the expression of immunoreactive GSTs in liver neoplasms and putatively preneoplastic altered hepatocellular foci from white suckers collected from several polluted sites in western Lake Ontario. Histological sections of liver with altered hepatocellular foci, hepatocellular adenomas, hepatocellular carcinomas, bile duct adenomas and bile duct carcinomas were examined for GST immunoreactivity by the peroxidase-antiperoxidase (PAP) technique with polyclonal antiserum specific for all major GST isoenzyme subunits found in normal liver of white suckers. All bile duct adenomas, bile duct carcinomas and hepatocellular carcinomas were markedly or completely deficient in immunoreactive GST in comparison with surrounding normal hepatocytes. The majority of the hepatocellular adenomas were also deficient. Most altered hepatocellular foci had normal GST staining, but several GST-deficient altered hepatocellular foci were observed. However, none of the preneoplastic or advanced liver neoplasms expressed induced GST, suggesting that carcinogenesis is not associated with selection for GST-dependent resistance. Loss of hepatocellular GSTs may be incidental to neoplastic progression in these fish, or might be important in increasing susceptibility of some preneoplastic populations of hepatocytes to further DNA damage by environmental or endogenous chemicals that are normally detoxified by GSTs.     

Inhibition by d-limonene of experimental hepatocarcinogenesis in Sprague-Dawley rats does not involve p21(ras) plasma membrane association.

The effects of d-limonene on hepatocarcinogenesis induced by N-nitrosomorpholine (NNM) and on membrane-associated p21(ras) and labeling and apoptotic indices of the liver were investigated in male Sprague-Dawley rats. Rats were given drinking water containing NNM for 8 weeks, and from the beginning of experimental week 9, they received chow pellets containing 1% or 2% limonene. The preneoplastic and neoplastic liver lesions (cellular alteration foci, neoplastic nodules and hepatocellular carcinomas), and hepatic foci staining positive for glutathione-S-transferase, placental type (GST-P) were examined microscopically and histochemically. At week 16, quantitative histologic analysis showed that oral administration of 1% or 2% limonene resulted in significant reductions in the number and mean area of GST-P-positive hepatic foci and the number of cellular alteration foci, neoplastic nodules and hepatocellular carcinomas. Limonene, at both doses, also caused significant decreases in the labeling indices and significant increases in the apoptotic indices of cellular alteration foci, neoplastic nodules, hepatocellular carcinomas and adjacent liver. However, limonene, at both doses, had no significant influence on the production of membrane-associated p21(ras) in the visible liver white nodules. These findings indicate that limonene inhibits hepatocarcinogenesis and suggest that this effect may be clearly related to its effect in inhibiting cell proliferation and in enhancing apoptosis, but not through ras oncoprotein plasma membrane association.     

Increased susceptibility of aged rats to hepatocarcinogenesis by the peroxisome proliferator nafenopin and the possible involvement of altered liver foci occurring spontaneously

We investigated the mechanism of the hepatocarcinogenic action of nafenopin (NAF), a nongenotoxic peroxisome proliferator. Groups of male rats aged 13 wk (designated "young") or 57 wk (designated "old") were fed NAF for 13 mo; additional groups received a basal diet or a phenobarbital (PB)-containing diet as positive control. The following results were obtained. (a) NAF produced numerous hepatocellular adenomas and carcinomas in old animals but very few in young animals. A similar result, although less pronounced, was seen with PB. Adenomas of PB-treated groups mostly consisted of eosinophilic and glycogen-storing cells. However, adenomas and carcinomas of NAF-treated livers were composed of weakly basophilic cells. (b) Phenotypically altered foci, evaluated in hematoxylin:eosin-stained sections, appeared spontaneously in untreated livers. The majority of these foci was either of the eosinophilic-clear cell or the tigroid cell type. In addition, we identified foci which are characterized by weak, diffuse cytoplasmatic basophilia. Their phenotype was similar to that of adenomas and carcinomas in NAF-treated rats. The number and size of eosinophilic-clear cell and of tigroid cell foci increased considerably with the age of the animals. At the end of the experiment, approximately 2.4% of liver tissue was occupied by focal cells. NAF, but not PB, treatment led to a selective increase in number and size of weakly basophilic foci. This subtype has previously been described as a likely precursor lesion for liver tumors induced by an aflatoxin B1-NAF initiation-promotion regimen (B. Kraupp-Grasl et al., Cancer Res., 50:3701-3708, 1990). These findings suggest that the peroxisome proliferator NAF leads to tumor development in aging rat liver by promotion of spontaneously occurring preneoplastic lesions. The type of lesion appears to be different from that promotable by PB.     

A carcinogenicity assay of Mirex in Charles River CD rats.

The long-term administration of 50 and 100 ppm of Mirex in the diets of male and female Charles River CD rats was associated with a spectrum of liver lesions, from foci or areas of cellular alteration and neoplastic nodules to hepatocellular carcinoma. Statistically significant numbers of neoplastic nodules were observed in the livers of male rats receiving the high dose. Neoplastic nodules and hepatocellular carcinomas were not observed in control rats.     

Possible enhancing effect of the immunosuppressive agent, 6-mercaptopurine(6-MP) on focal lesion development in cirrhotic liver induced by carbon tetrachloride but not furfural in F344 rats.

The modifying effects of an immunosuppressive agent, 6-mercaptopurine (6-MP), on development of focal lesions in liver cirrhosis models induced by carbon tetrachloride (CCl4) or furfural were studied in male F344 rats. Feeding of 6-MP at 50 p.p.m. for 20 weeks to animals with pre-existing liver cirrhosis caused immunosuppression, and significantly enhanced the induction of gamma-glutamyltranspeptidase (GGT)-positive foci and nodules in the CCl4 but not furfural case. Glutathione S-transferase P (GST-P)-positive preneoplastic lesions were not affected. Moreover, phenobarbital (PB) also enhanced the induction of GGT-positive hepatocellular lesions only in the CCl4-induced liver cirrhosis model, no promotion influence being exerted after treatment with the non-carcinogenic furfural. This study, therefore, suggests that 6-MP can enhance the induction of one type of preneoplastic foci and nodules and that essential differences exist between focal lesions arising in cirrhotic livers caused by CCl4 as opposed to furfural.     

Hepatocarcinogenic and promoting action of a choline-devoid diet in the rat

Male Fisher 344 rats were solely fed a choline-supplemented diet for 65 to 105 weeks or a choline-devoid diet for 24 to 102 weeks. Hepatocellular carcinomas developed in the latter animals, beginning at 24 weeks. Other groups of rats were given a single dose of 20 mg diethylnitrosamine/kg, 18 h after a partial hepatectomy and were fed, 4 weeks thereafter, either a choline-supplemented, or a choline-devoid diet for up to 48 weeks. In rats fed the choline-supplemented diet, the only relevant lesion observed was a small transect number of foci of enzyme-altered hepatocytes. On the other hand, a significant number of foci, of preneoplastic nodules, and of hepatocellular tumors developed in rats fed the choline-devoid diet. The results obtained are consistent with those previously reported by others, indicating that diets devoid of choline, or of choline and methionine, are carcinogenic. The diets appear to act as complete carcinogens, since they are also efficient promoters of chemical hepatocarcinogenesis, as shown again, in the present study, by the results obtained in the diethylnitrosamine-pretreated rats.     

Immunohistochemical detection of c-Ha-ras oncogene p21 product in pre-neoplastic and neoplastic lesions during hepatocarcinogenesis in rats

An immunohistochemical study of c-Ha-ras expression was performed on preneoplastic and neoplastic stages of diethylnitrosamine (DENA)-induced hepatocarcinogenesis in rats, using an antibody raised against a peptide sequence of the Ha-ras p21 product. Moderate to high immunostaining intensity was observed in the following hepatocytic lesions: (1) hepatocellular carcinomas (14/14) and associated neoplastic nodules (8/8) and foci of phenotypic alterations (35/40) (after 13-20 months of treatment); (2) neoplastic nodules (6/6) and associated foci (42/50) (after 5-9 months); (3) foci (10/10) (after 2 months); and (4) small, slowly growing foci (26/40) found 9 months after treatment with DENA without prior partial hepatectomy, resulting in low number of nodules and no tumor even after 15 months. No c-Ha-ras p21 was detected immunohistochemically in normal nor in regenerating rat liver. Our results indicate that increased Ha-ras expression is an early and stable event in liver lesions associated with hepatocarcinogenesis. They also imply that increased Ha-ras expression is insufficient (if at all implicated) for inducing fully malignant hepatocyte transformation. It might be indicative of cell populations at an increased transformation risk.     

Increased resistance of peroxisome proliferator-induced hepatic lesions to iron overload in rats

Altered areas (AA), neoplastic nodules (NN) and hepatocellular carcinomas (HCC) induced by chronic administration of ciprofibrate and Wy-14,643 were examined for iron accumulation after systemic iron overload. Eighty percent of AA, 90% NN and 100% HCC showed increased resistance to iron accumulation. However, marked heterogeneity was observed in the amount of iron from area to area within the same lesion with some cells containing no iron, while others showing blue reaction to iron. These findings indicate, that putative preneoplastic and neoplastic hepatic lesions induced by peroxisome proliferators exclude iron in a fashion similar to that of hepatic nodules and carcinomas induced by DNA damaging carcinogens.     

P-450 enzyme induction by 5-ethyl-5-phenylhydantoin and 5,5-diethylhydantoin, analogues of barbiturate tumor promoters phenobarbital and barbital, and promotion of liver and thyroid carcinogenesis initiated by N-nitrosodiethylamine in rats

Male F344/NCr rats, 6 wk old, were fed 500 ppm of phenobarbital (PB) or equimolar doses of either 5-ethyl-5-phenylhydantoin (EPH) or 5,5-diethylhydantoin (EEH) in diet for 2 wk and hepatic cytochrome P-450-mediated alkoxyresorufin O-dealkylase and aminopyrine N-demethylase activities were determined. Both PB and EPH greatly increased P-450-mediated enzyme activities in rat liver while EEH was ineffective. To evaluate the hydantoins as tumor promoters, 5-wk-old male F344 rats were given a single i.p. injection of 75 mg N-nitrosodiethylamine/kg body weight. Beginning 2 wk later, they were placed either on normal diet or diet containing 500 ppm of PB or equimolar doses of EPH or EEH for the remaining experimental period. Control groups received an i.p. injection of saline followed by each of the test diets. Animals were sacrificed at either 52 or 78 wk. PB and EPH significantly enhanced the development of hepatocellular foci and hepatocellular adenomas at 52 wk and hepatocellular carcinomas at 78 wk in N-nitrosodiethylamine-initiated rats. Neither the incidence of hepatocellular neoplasms nor the number and size of hepatocellular foci was significantly increased by EEH. At 78 wk, both PB and EPH enhanced the development of thyroid follicular cell neoplasms in N-nitrosodiethylamine-initiated rats while no such enhancement was observed with EEH. Thus, EPH, a long-acting sedative/anticonvulsant, like the structurally similar PB, promoted hepatocellular and thyroid follicular cell carcinogenesis and induced the PB-inducible form(s) of cytochrome P-450 (P-450b) in rats. In contrast, EEH unlike barbital failed to promote hepatocellular and thyroid follicular cell carcinogenesis and also failed to induce PB-inducible form(s) of cytochrome P-450 in rats.     

Differential Changes in Expression of Gap Junction Proteins Connexin 26 and 32 during Hepatocarcinogenesis in Rats

We examined expressions of the gap junction proteins, connexin 26 (C×26) and 32 (C×32), in preneoplastic and neoplastic lesions during rat hepatocarcinogenesis. A marked reduction in the numher of C×32-positive gap junctions was observed in 17% of the glutathione S-transferase placental form-positive foci, whereas 44% of the foci showed increased expression of C×26. Most hyperplastic nodules exhibited decreased expression of C×32, whereas 16% of the nodules showed increased expression of C×26. In hepatocellular carcinomas, expressions of both C×32 and C×26 were significantly reduced. These results suggest that the expressions of C×32 and 26 are differentially regulated during hepatocarcinogenesis, and that the decrease in C×32 expression occurs earlier, whereas reduction in C×26 expression occurs later in association with promotion and progression of carcinogenesis.     

Chemoprevention of rat liver carcinogenesis by S-adenosyl-L-methionine: a long-term study.

Previous work has shown a consistent fall in S-adenosyl-L-methionine (SAM) in the liver of diethylnitrosamine-initiated rats, during the development of preneoplastic lesions, in persistent nodules (PNs), and hepatocellular carcinomas. The injection of SAM into rats causes the reconstitution of the SAM pool, coupled with growth restraint, remodeling, and apoptosis of preneoplastic cells, and inhibits the development of PNs and hepatocellular carcinomas. To evaluate if SAM treatment causes a long-term prevention of preneoplastic and neoplastic liver lesions or merely causes a delay in their development, we evaluated the effect of a relatively short SAM treatment on the development of preneoplastic and neoplastic lesions in a long-term study. Male Wistar rats were subjected to initiation with diethylnitrosamine, followed by selection and then by the administration of phenobarbital for 16 weeks. After selection, the rats were given i.m. injections of a purified SAM preparation (384 mumol/kg/day) for 24 weeks. In SAM-treated rats, a decrease in the incidence of PNs was found 6, 14, and 24-28 months after initiation. At the end of SAM treatment the number of PNs per rat liver, nodule diameter, and labeling and mitotic indices of nodular cells decreased considerably in control rats. Nodule diameter started to increase rapidly again only 8 months after arresting SAM treatment, when complete recovery of DNA synthesis in nodular cells occurred. The majority of nodules present in the liver 6-28 months after initiation belonged to the clear and acidophilic cell types, with lower percentages of mixed cell and basophilic cell types. A decrease in basophilic nodules occurred in SAM-treated rats. Fourteen and 24-28 months after initiation hepatocellular carcinoma incidence was 11 of 12 and 10 of 10 in control rats, respectively, and only 1 of 12 and 3 of 11 in SAM-treated rats. At the 24th-28th month all control rats had tumors identified as 2 poorly differentiated carcinomas, 6 trabecular carcinomas, or 3 adenocarcinomas, while only 2 relatively small trabecular carcinomas and 1 small glandular tumor developed in SAM-treated rats. In 3 of 11 SAM-treated rats, but in none of the control rats, leukemic infiltration of liver occurred 24-28 months after initiation. Leukemic infiltration of the spleen occurred in 5 and 3 control and SAM-treated rats, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)     

Induction of preneoplastic lesions by a low dose of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in the livers of rats treated with carbon tetrachloride.

The multifactorial nature of carcinogenesis in man has impelled us to study the effects of various chemicals and conditions in combination. In the present investigation, we examined the effects of low doses of 2-amino-3,8-dimethyl-imidazo[4,5-f]quinoxaline (MeIQx) in combination with carbon tetrachloride (CCl4) on the formation of glutathione S-transferase placental form (GST-P)-positive foci in rat liver. Administration of diet containing MeIQx at 0.4, 4 or 40 p.p.m., representing one-thousandth, one-hundredth and one-tenth of the dose proved to induce hepatocellular carcinomas (400 p.p.m.), for 8 or 12 weeks did not induce GST-P-positive foci. However, 40 p.p.m. of MeIQx when co-administered with CCl4 (0.7 ml/kg, s.c. twice a week) induced preneoplastic lesions: 7- and 3-fold increases in the numbers and areas of GST-P positive foci in week 8, and 8- and 6-fold increases respectively in week 12, over those with CCl4 alone. Furthermore, a marked increase in the number of hyperplastic nodules was observed in this group of rats in week 12. No significant increases of GST-P-positive foci were observed at doses of 0.4 or 4 p.p.m. MeIQx in combination with CCl4. Thus, it is predicted that chronic exposure to 40 p.p.m. of MeIQx eventually results in induction of hepatocellular carcinomas in injured rat liver.     

Infrequent activation of K-ras, H-ras, and other oncogenes in hepatocellular neoplasms initiated by methyl(acetoxymethyl)nitrosamine, a methylating agent, and promoted by phenobarbital in F344 rats

Fischer 344/Ncr rats of both sexes were subjected to partial hepatectomy and then initiated 21-24 h later by a single injection of methyl(acetoxymethyl)nitrosamine at 0.1 mmol/kg body weight via the portal vein. Beginning 3 weeks later, development of hepatocellular neoplasms in initiated rats was promoted by feeding 0.05% phenobarbital (PB) in the diet. Not only intrahepatic lesions but also a variety of extrahepatic tumors were induced. High-molecular-weight DNAs were prepared from 67 samples of grossly normal liver containing multiple preneoplastic foci/areas of microscopic dimensions, 137 hepatocellular adenomas (nodules), 93 hepatocellular carcinomas (HCC), 10 cholangiomas, and 25 extrahepatic tumors in 95 rats and tested for transforming activity in the NIH 3T3 transfection assay. DNA preparations from 7 of 93 HCCs, 2 of 10 cholangiomas, 2 of 137 nodules, 1 histiocytic sarcoma, and 1 thyroid carcinoma were positive in the transfection assay. Southern blot analysis showed that NIH 3T3 transformants induced by DNA from 5 HCCs, 1 hepatocellular adenoma, 1 cholangioma, 1 histiocytic sarcoma, and 1 thyroid carcinoma contained an activated K-ras gene of rat origin. Rat-derived H-ras was identified in transformants from 2 additional HCCs and rat c-raf from 1 hepatocellular adenoma. The transforming gene from one cholangioma showed no sequence homology to the ras genes, neu, or c-raf. Immunoprecipitation analysis of ras Mr 21,000 protein in 11 transformants indicated that, based upon protein electrophoretic mobilities, activation of the ras genes consistently resulted from mutations in codon 12 of these genes. Selective oligonucleotide analysis revealed that a G----A transition in the second base of codon 12 of K-ras was present in the 9 K-ras-positive transformants and also in DNAs prepared from the original tumors. In contrast, oligonucleotide hybridization experiments with DNAs from 35 hepatocellular tumors that were negative in transfection assays revealed the presence of mutant K-ras in 1 of 15 HCCs; no mutation could be detected in 20 transfection-negative adenomas. The infrequency of detection of a specific oncogene, more frequent detection of oncogenes in malignant tumors, and failure to observe activated oncogenes in preneoplastic lesions suggest that activation of ras oncogenes may occur as a late and infrequent event in the evolution of some rat hepatocellular neoplasms and that mutation of a specific ras locus is not an obligatory early event in the genesis of these neoplasms.     

Induction of liver tumors in F344 rats by crotonaldehyde

The tumorigenic activities in F344 rats of crotonaldehyde, a representative alpha, beta-unsaturated carbonyl compound, and N-nitrosopyrrolidine, which could produce crotonaldehyde upon metabolism, were compared. Groups of rats were treated with either crotonaldehyde (0.6 mM or 6.0 mM) or N-nitrosopyrrolidine (0.6 mM) in their drinking water for 113 or 84 weeks, respectively. At the 0.6 mM dose, crotonaldehyde induced neoplastic lesions of the liver in 9 of 27 rats; 2 rats had hepatocellular carcinomas, and 9 rats had neoplastic nodules. It also caused altered liver cell foci in 23 of 27 rats. The incidences of tumors and foci were significantly higher than those of the control group. N-Nitrosopyrrolidine induced hepatocellular carcinomas in 20 of 23 rats, liver neoplastic nodules in 16 of 23 rats, and altered liver cell foci in 23 of 23 rats. Thus, crotonaldehyde appears to be a weaker tumorigen than N-nitrosopyrrolidine. At the 6.0 mM dose, crotonaldehyde treatment caused moderate to severe liver damage in 10 of 23 rats. No preneoplastic or neoplastic lesions were observed in these rats. The remaining 13 rats of this group developed altered liver cell foci. The tumorigenicity of crotonaldehyde suggests that alpha, beta-unsaturated carbonyl compounds, which are ubiquitous in the human environment and can be formed endogenously, may be an important class of potential carcinogens.