Hepatic Tumorigenesis Research Articles

2-(Allylthio)pyrazine inhibition of aflatoxin B 1-induced hepatocarcinogenesis in rats

2-(Allylthio)pyrazine (2-AP), a synthetic pyrazine derivative with an allylsulfur moiety, has hepatoprotective effects against toxicants. Effect of 2-AP on hepatic tumorigenesis in association with glutathione S-transferase (GST) induction was examined in rats exposed to aflatoxin B 1 (AFB 1). Both AFB 1-DNA adduct formation in the liver and urinary elimination of 8,9-dihydro-8-( N 7-guanyl)-9-hydroxy-aflatoxin B 1 (AFB 1- N 7-guanine) adduct were also determined. Male Sprague–Dawley rats were treated with 2-AP at the daily oral doses of 10, 25 and 50 mg/kg for 16 consecutive days, during which four repeated doses of AFB 1 (1.0 mg/kg) were given to the animals. Rats were then subjected to two-thirds of hepatectomy, followed by administration of phenobarbital (PB). Focal areas of hepatocellular alteration were identified after 44 days and preneoplastic foci expressing the placental form of glutathione S-transferase P (GST-P) were quantified by immunostaining of liver sections. 2-AP reduced the volume of liver occupied by GST-P foci by 65–96%. Under these experimental conditions, 2-AP treatment resulted in significant elevations in GST activity in the liver. Levels of radiolabeled AFB 1 covalently bound to hepatic DNA, RNA and proteins were significantly reduced in rats treated with 2-AP for 5 days. 2-AP pretreatment also caused a 45% reduction in the urinary elimination of AFB 1- N 7-guanine adduct over the 24-h postdosing period. The present findings demonstrated that 2-AP exhibited protective effects against AFB 1-induced hepatocarcinogenesis in rats with a marked decrease in the level of AFB 1-DNA adduct. Reduction of hepatic DNA adducts might result from elevations of activity of GST, which catalyzes detoxification of the carcinogen.

Identification of dithiolethiones with better chemopreventive properties than oltipraz.

Oltipraz and related dithiolethiones are an important class of chemopreventive agents. Studies were undertaken to identify cancer chemopreventive dithiolethiones more active than oltipraz. Largely based upon enzyme induction activities in vitro, 17 dithiolethiones, including oltipraz, were analyzed for their ability to induce hepatic phase II enzyme activities in vivo. Of these compounds, 15 produced greater induction of NAD(P)H:quinone reductase and 11 yielded greater induction of glutathione S-transferase than oltipraz. All 17 dithiolethiones were then tested for their ability to inhibit acute hepatotoxicity by aflatoxin B1 (AFB1), which previously has been shown to be an intermediate predictor of chemopreventive activity. Rats were pretreated with dithiolethiones (0.3 mmol/kg body wt, three times a week per os) and challenged with two acutely toxic doses of AFB1 (0.5 mg/kg body wt, once daily for two successive days per os). Inhibition of hepatotoxicity was measured by changes in body weight gain during AFB1 challenge, reduction in levels of hepatic enzymes in serum and diminution of bile duct cell proliferation. Nine dithiolethiones spanning a range of responses in this toxicity screen were further tested for their ability to prevent AFB1-induced tumorigenicity, as assessed by a reduction in hepatic burden of putative preneoplastic foci. Six dithiolethiones were found to be considerably more effective than oltipraz in preventing AFB1-induced tumorigenesis. In general, dithiolethiones that were very effective in inhibition of acute hepatotoxicity were also found to be effective in prevention of hepatic tumorigenesis.

Chemopreventive effects of 2-(allylthio)pyrazine on hepatic lesion, mutagenesis and tumorigenesis induced by vinyl carbamate or vinyl carbamate epoxide.

2-(Allylthio)pyrazine (2-AP), synthesized for its possible use as a hepatoprotective agent, has been found to selectively inhibit rat hepatic cytochrome P450 2E1 (Kim et al., Biochem. Pharmacol., 53, 261-269, 1997), while it enhances the activities of phase II detoxification enzymes such as glutathione S-transferase and epoxide hydrolase. As part of a program in evaluating the chemopreventive potential of 2-AP, we have determined its effects on hepatotoxicity, mutagenicity and tumorigenicity of vinyl carbamate (VC), a prototypic hepatocarcinogen preferentially activated by P450 2E1 to the ultimate carcinogenic metabolite vinyl carbamate epoxide (VCO), which undergoes detoxification by glutathione conjugation and oxirane hydrolysis. Administration of 2-AP (100 mg/kg body wt) to male Sprague-Dawley rats by gavage, 2 days, 1 day and 4 h prior to VC or VCO, markedly ameliorated the hepatotoxicity of these compounds as determined by decreased serum aspartate aminotransferase and alanine aminotransferase activities. Furthermore, 2-AP pre-treatment significantly suppressed the VC-induced hepatocarcinogenesis in infant male B6C3F1 mice. In a separate experiment, the multiplicities of skin tumors formed in female ICR mice treated with 5.8 micromol of VC or VCO were inhibited 58 and 70%, respectively, by pre-treatment with 2-AP by oral administration. The mutational spectrum of ras-oncogene in papillomas was not altered by 2-AP pre-treatment. 2-AP also inhibited the mutagenicity of VC in the Salmonella-microsome assay. Taken together, these findings suggest that 2-AP is a potential chemopreventive agent.

Phenobarbital does not promote hepatic tumorigenesis in a twenty-six-week bioassay in p53 heterozygous mice.

The tumorigenic potential of phenobarbital was examined in a 26-wk carcinogenesis bioassay using p53 heterozygous mice and wild-type controls. Fifteen mice/sex/genotype were exposed to either 500 or 1,000 ppm phenobarbital in the diet. Dietary administration of 3,750 ppm p-cresidine, a transspecies mutagenic carcinogen, to both heterozygous and wild-type mice served as a positive control. Phenobarbital treatment caused increases in liver:body weight ratios and histologic evidence of centrilobular hepatocellular hypertrophy. No tumors were observed in any phenobarbital-treated mice. Mice given p-cresidine exhibited a moderate reduction in body weight gain over the course of the study. Heterozygous mice treated with p-cresidine exhibited a high incidence of urinary bladder tumors. Similar tumors were also present in a small number of p-cresidine-treated wild-type mice. Our results demonstrate the lack of a hepatic tumor response to phenobarbital, a compound that is a potent and potent and prototypic hepatic microsomal enzyme inducer, a nongenotoxic rodent carcinogen, and a human noncarcinogen. This finding supports the continued utility of this model as an alternative to the mouse bioassay for human carcinogenic safety assessment of potentially genotoxic carcinogenes because it did not produce a false-positive response to this potent nongenotoxic agent.

Chemoprevention by inducers of carcinogen detoxication enzymes.

One of the major mechanisms of chemical protection against carcinogenesis, mutagenesis, and other forms of toxicity mediated by electrophiles is the induction of enzymes involved in their metabolism, particularly phase 2 enzymes such as glutathione S-transferases (GSTs), uridine diphosphate-glucuronosyltransferases, and NAD(P)H:quinone reductase. Furthermore, induction of phase 2 enzymes appears to be a sufficient condition for obtaining chemoprevention and can be achieved in many target tissues by administering any of a diverse array of naturally occurring and synthetic chemical agents. One class of chemopreventive agents, 1,2-dithiole-3-thiones, was developed on the basis of their potent activity in rodent tissues as inducers of GSTs. A substituted dithiolethione, oltipraz [4-methyl-5-(2-pyrazinyl)-1,2-dithiole-3-thione], is an effective inhibitor of aflatoxin B1-mediated hepatocarcinogenesis in the rat. Oltipraz produces dramatic decreases in the levels of aflatoxin-DNA adducts in the liver as well as in the urinary levels of the depurination product aflatoxin-N7-guanine. Corresponding increases are seen in the biliary elimination of aflatoxin-glutathione conjugates. Administration of oltipraz results in 3- to 4-fold increases in hepatic cytosolic GST activities and mRNA levels for some alpha, mu and pi isoforms. Nuclear run-on assays have indicated that oltipraz treatment elevates rates of transcription of some GST subunits. In the rat, induction of phase 2 enzymes by oltipraz is mediated, at least in part, through the antioxidant response element in the 5' flanking region of these genes. Although oltipraz has a very short plasma half-life, elevations in the levels of some GST isoforms can persist up to 1 week after dosing with oltipraz. Concordantly, intermittent dosing schedules (i.e., once a week) are nearly as effective as daily interventions for inhibition of aflatoxin-mediated hepatic tumorigenesis. The protective efficacy of daily and weekly administration of oltipraz to people in Qidong, People's Republic of China, who are at high risk for aflatoxin exposure and subsequent development of hepetocellular carcinoma, is currently under evaluation.

P-417 Androgen supplement for castrated TGF? transgenic male mice restored hepatic tumorigenesis

P-417 Androgen supplement for castrated TGF? transgenic male mice restored hepatic tumorigenesis

Regulation by vascular endothelial growth factor of human colon cancer tumorigenesis in a mouse model of experimental liver metastasis.

To investigate the relationship between angiogenesis and hepatic tumorigenesis, we examined the expression of vascular endothelial growth factor (VEGF) in 8 human colon carcinoma cell lines and in 30 human colorectal cancer liver metastases. Abundant message for VEGF was found in all tumors, localized to the malignant cells within each neoplasm. Two receptors for VEGF, KDR and flt1, were also demonstrated in most of the tumors examined. KDR and flt1 mRNA were limited to tumor endothelial cells and were more strongly expressed in the hepatic metastases than in the sinusoidal endothelium of the surrounding liver parenchyma. VEGF monoclonal antibody administration in tumor-bearing athymic mice led to a dose- and time-dependent inhibition of growth of subcutaneous xenografts and to a marked reduction in the number and size of experimental liver metastases. In hepatic metastases of VEGF antibody-treated mice, neither blood vessels nor expression of the mouse KDR homologue flk-1 could be demonstrated. These data indicate that VEGF is a commonly expressed angiogenic factor in human colorectal cancer metastases, that VEGF receptors are up-regulated as a concomitant of hepatic tumorigenesis, and that modulation of VEGF gene expression or activity may represent a potentially effective antineoplastic therapy in colorectal cancer.

Androgen replacement for castrated TGFα transgenic male mice recovered hepatic tumorigenesis: [formula omitted], S. Saitoh, R. Shimoda, K. Takehara, T. Nagamine, M. Mori, T. Takeuchi∗, G. Merlino∗∗ and H. Takagi. The First Dept. of Internal Medicine and Molecular Endocrinology∗, Gunma University School of Medicine, Maebashi, Japan. ∗∗Laboratory of Molecular Biology, NCI/NIH, Bethesda MD, USA

Androgen replacement for castrated TGFα transgenic male mice recovered hepatic tumorigenesis: [formula omitted], S. Saitoh, R. Shimoda, K. Takehara, T. Nagamine, M. Mori, T. Takeuchi∗, G. Merlino∗∗ and H. Takagi. The First Dept. of Internal Medicine and Molecular Endocrinology∗, Gunma University School of Medicine, Maebashi, Japan. ∗∗Laboratory of Molecular Biology, NCI/NIH, Bethesda MD, USA

Oltipraz: a laboratory and clinical review.

Oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione; RP 35972] is a synthetic, substituted 1,2-dithiole-3-thione previously used in humans as an antischistosomal agent. Cruciferous vegetables (e.g., Brussels sprouts, cabbage) contain several agents, including dithiolethiones, which appear to inhibit carcinogenesis; however, it is unclear which dietary compounds produce the protective effects. Animal studies have demonstrated that oltipraz is a potent inducer of Phase II detoxification enzymes, most notably glutathione-S-transferase (GST). Laboratory evaluations have shown that dietary concentrations of oltipraz produce marked inhibition of aflatoxin B1-induced hepatic tumorigenesis in rats. Levels of hepatic aflatoxin-DNA adducts, urinary aflatoxin-N7-guanine, and serum aflatoxin-albumin adducts decreased when biliary elimination of aflatoxin-glutathione conjugants increased, thus providing predictive biomarkers that measured a chemopreventive effect. In other animal experiments, oltipraz was found to inhibit chemically induced carcinogenesis in bladder, colon, breast, stomach, and skin cancer models. In addition, oltipraz has been shown to be non-mutagenic, a radioprotector, and a chemoprotective agent against carbon tetrachloride and acetaminophen toxicity. More recent studies in rats suggest that unsubstituted 1,2-dithiole-3-thiones may more effectively inhibit aflatoxin-induced hepatic tumorigenesis and induce electrophile detoxification enzymes. Multiple human clinical trials have been conducted using 1.0-4.5 gram doses of oltipraz over 1-3 days for the treatment of schistosomiasis. Phototoxicity has precluded its use in tropical areas. More recently, a 6 month Phase I trial was completed in which patients with resected colon polyps, or females with first degree relatives with breast cancer, were given oral daily doses of oltipraz at 125 mg or 250 mg. The maximum tolerated dose of oltipraz was < or = 125 mg daily. Grade I/II toxicities included photosensitivity/heat intolerance, GI and neurologic toxicity. Peak plasma concentrations were analyzed by HPLC with wide variability. In another Phase I study, a single oral dose of oltipraz was given to normal volunteers at dose levels of 125, 250, 375, and 500 mg. There was no significant difference in half-life (t1/2) between the four dose levels nor in clearance at the 125 and 250 mg levels. Peak oltipraz levels > or = 1.0 microgram/mL were achievable with marked interpatient variability. A series of small trials evaluating single oral doses of oltipraz for up to 28 days (dosing range 1 mg/kg-3 mg/kg/day) also showed a short t1/2 (4.1-5.3 hours), a sustained steady state without variation after a loading dose, and increased serum and urine concentrations with consumption of a high-fat diet.(ABSTRACT TRUNCATED AT 400 WORDS)

Aspirin shares a short-term effect, inhibition of pyruvate kinase activity with tumor-promoting agents, but fails to promote rat liver carcinogenesis.

Since aspirin was found to cause a persistent decrease in pyruvate kinase activity, which is a prominent characteristic of the hepatic promoters, the effect of aspirin on hepatic tumorigenesis in the rat has been studied. Male Wistar rats were initiated with a diet containing 0.06% 3'-methyl-4-dimethylamino-azobenzene for 4 weeks. They were divided into 3 groups and each group was fed one of the following diets containing either 0, 0.5 or 1% aspirin. At the end of a 58-week experimental period, no differences were observed in the incidence of hepatic lesions and hepatocellular carcinomas among the groups. Thus, it was concluded that aspirin does not promote nor inhibit the postinitiation step of hepatic tumorigenesis in the rat.

Evaluation of the post-initiation effects of oltipraz on aflatoxin B1-induced preneoplastic foci in a rat model of hepatic tumorigenesis.

Previous studies have demonstrated that ingestion of 5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione (oltipraz) during the aflatoxin B1 (AFB1) treatment phase completely prevented hepatic cancer. In this study we evaluated the effect of feeding oltipraz during the post-AFB1 treatment phase. Fifty-five male F344 rats were divided into five groups. All rats were gavaged with 25 micrograms AFB1/rat, five times a week for two successive weeks. The rats were fed the oltipraz-supplemented diet according to three different feeding regimes: during the AFB1 treatment phase (1 week prior to, during and 1 week after the last gavage with AFB1); during the post-treatment phase; or throughout the entire time of the experiment. Phenobarbital-supplemented diet was fed during post-treatment phase to one group and this was used as a positive control for the promotion of AFB1-induced focal growth. The burden of putative, preneoplastic, hepatic glutathione S-transferase P-positive foci was evaluated at 13 weeks after the AFB1 treatment phase. As seen previously, oltipraz fed during the AFB1 treatment phase significantly inhibited focal development, i.e. the volume percent of the liver occupied with foci was reduced by 87%. Oltipraz when fed during the post-treatment phase neither inhibited nor enhanced focal development.

Molecular dosimetry of urinary aflatoxin-N7-guanine and serum aflatoxin-albumin adducts predicts chemoprotection by l,2-dithiole-3-thione in rats

Hepatocellular carcinoma has one of the poorest 5 year survival rates of any human cancer. Preventive measures offer the best possibility of ameliorating this disease and chemoprotective agents are being developed for this purpose. The dithiolethiones, including oltipraz and the unsubstituted molecule 1,2-dithiole-3-thione, have been shown to be potent inhibitors of aflatoxin-induced hepatic tumorigenesis in rats. However, subsequent evaluation of dithiolethiones or other chemoprotective agents in human clinical trials will require the development of intermediate, non-invasive biomarkers to evaluate the efficacy of these interventions. In this study, levels of molecular dosimetry biomarkers for determining genotoxic damage caused by aflatoxin B1 have been measured in a chronic exposure model with male F344 rats wherein half the animals were fed a diet supplemented with 0.03% 1,2-dithiole-3-thione to lower their risk for tumors and the other half were fed unsupplemented AIN-76A diet and were at high risk for tumor development. Levels of hepatic aflatoxin-DNA adducts, serum aflatoxin-albumin adducts and excreted aflatoxin-N7-guanine adducts in urine were determined following multiple administrations of 250 micrograms aflatoxin B1/kg body wt on days 0-4 and 7-11 to assess the use of the serum and urinary biomarkers as indices of chemoprotective efficacy. In the rats fed 1,2-dithiole-3-thione, the overall diminutions in the levels of hepatic DNA adducts, urinary aflatoxin-N7-guanine and serum aflatoxin-albumin adducts over the 2 week exposure period were 76, 62 and 66% respectively. This parallelism in reductions of levels of biomarkers relative to target organ DNA adduct burden suggests that these biomarkers are predictive short-term, non-invasive measures for assessing the efficacy of chemoprotective interventions in experimental studies and can be applied to human clinical trials directed at populations at high risk for aflatoxin exposure and primary hepatocellular carcinoma.

Potent inhibition of aflatoxin-induced hepatic tumorigenesis by the monofunctional enzyme inducer 1,2-dithiole-3-thione.

1,2-Dithiole-3-thiones are five-membered cyclic sulfur-containing compounds with antioxidant, chemotherapeutic, radioprotective and chemoprotective properties. Several substituted 1,2-dithiole-3-thiones are used medicinally and one of these, oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione], has been recently shown to be an inhibitor of aflatoxin B1 (AFB1) hepatocarcinogenesis in the rat. Structure-activity studies have been undertaken to probe the mechanisms by which dithiolethiones inhibit carcinogenesis. Such studies revealed that unsubstituted 1,2-dithiole-3-thione was more effective than oltipraz at inhibiting aflatoxin-DNA adduct formation in vivo and at inducing electrophile detoxication enzymes in cell culture. In the present studies the effects of dietary administration of 1,2-dithiole-3-thione on the induction of xenobiotic metabolizing enzymes and inhibition of aflatoxin-induced hepatic tumorigenesis were examined. Male F344 rats were fed graded doses of 1,2-dithiole-3-thione (0.001-0.03%) for 4 weeks. During the second and third weeks of 1,2-dithiole-3-thione feeding, rats were dosed by gavage with 250 micrograms of AFB1/kg five times a week. Rats were then restored to control AIN-76A diet 1 week after cessation of AFB1 dosing. At 4 months, focal areas of hepatocellular alteration were identified and quantified by staining sections of liver for gamma-glutamyltranspeptidase (GGT) activity and glutathione S-transferase P (GST-P) expression. Treatment with 1,2-dithiole-3-thione at the lowest dose (0.001%) reduced by greater than 80% the volume of liver occupied by GGT or GST-P foci; higher dietary concentrations provided greater than 98% reductions in the volume per cent of these markers for presumptive preneoplastic lesions. All dietary concentrations of 1,2-dithiole-3-thione resulted in significant elevations in hepatic GST activities. In accord with the protective effects against tumorigenesis, 4- to 6-fold increases in the specific activities of aflatoxin-glutathione conjugation were observed in cytosols prepared from livers of animals fed 1,2-dithiole-3-thione. By contrast, 1,2-dithiole-3-thione did not have any detectable inductive effects on hepatic microsomal cytochrome P450 levels or activities. Dietary administration of 1,2-dithiole-3-thione also elevated activities of GSTs and other phase II enzymes in several extrahepatic organs. This broad pattern of induction of detoxication enzymes by 1,2-dithiole-3-thione supports the potential widespread use of this compound as a protective agent against chemical carcinogenesis and other forms of electrophile toxicity.

Coordinate activation of multidrug‐resistance (P‐glycoprotein) genes mdr2 and mdr3 during mouse liver regeneration

We previously reported that only one of the three mouse multidrug-resistance (mdr) genes, mdr3, is activated in hepatocellular carcinomas (HCCs). The present study examined the expression of mdr family members during mouse liver regeneration after partial hepatectomy to determine whether the regeneration that occurs during hepatic tumorigenesis is responsible for mdr3 elevation in HCC. We demonstrated that in both C3H/HeN and B6C3/F1 mice strains, the levels of both mdr2 and mdr3 mRNAs coordinately increased five- to sevenfold 24 h after partial hepatectomy, whereas the levels of mdr1 mRNA were not statistically different from those in the controls. Forty-eight hours after partial hepatectomy, mdr mRNA levels decreased and in most cases returned to normal levels after 72 h. These results indicate that mdr3 induction during hepatocarcinogenesis is not due to liver regeneration alone.

Phenobarbital promotion in diethylnitrosamine-initiated infant B6C3F1 mice: influence of gender.

Phenobarbital (PB) promotes hepatic tumorigenesis when chronically administered to male B6C3F1 mice after initiation with diethylnitrosamine (DENA) at 30 days of age. In contrast, when male B6C3F1 mice were initiated with DENA at 15 days of age, an inhibition of hepatic tumorigenesis occurred. The present study was undertaken to evaluate the influence of gender on the inhibiting ability of PB in the 15 day old DENA-initiated B6C3F1 mouse. Mice were injected with either DENA (5 micrograms/g) or saline at 15 days of age. At weaning mice were given either PB (500 p.p.m.) containing drinking water or deionized drinking water for 24 weeks. Male mice treated with DENA and PB demonstrated a significant decrease in the number of hepatocellular adenomas compared to males receiving DENA only. In contrast, females exposed to DENA and PB exhibited an enhancement of hepatic adenoma number compared to those receiving only DENA. In an additional experiment, individual preneoplastic foci from male and female B6C3F1 mice initiated with DENA at 15 days of age were examined for their responsiveness to the mitogenic stimuli of PB. Mice were exposed to either PB-containing or PB-free drinking water for 7 days. In non-PB treated males and females, preneoplastic hepatocytes demonstrated higher rates of DNA synthetic labelling compared to normal hepatocytes with no gender difference noted. Males exposed to PB exhibited increased levels of DNA synthesis in normal cells but not in preneoplastic foci. Females treated with PB, however, demonstrated significant increases in DNA synthesis in both preneoplastic and normal hepatocytes compared to non-PB treated females and PB-treated males. These findings suggest that in male mice initiated with DENA at 15 days of age, the preneoplastic foci are refractory to the proliferative effects of PB which may account for the observed inhibition of hepatic tumorigenesis by PB in this mouse strain.

Inhibitory effect of tamoxifen on diethylstilbestrol-promoted hepatic tumorigenesis in male rats and its possible mechanism of action.

Male Sprague‐Dawley rats were given a single intraperitoneal injection of diethylnitrosamine (DEN, 200 mg/kg body weight). Two weeks later the rats were divided into 4 groups; DEN‐C group rats were given no further treatment; DEN‐DES group rats were fed diethylstilbestrol (DES, 0.5 mg/day); DEN‐TMX group rats were given tamoxifen (TMX, 1.0 mg/ day) orally; DEN‐DES TMX group rats were fed both DES and TMX for 8 months. Rats of the DEN‐DES group developed grossly visible hepatic tumors. On the other hand, tumor development was significantly inhibited in rats of the DEN‐DES TMX group. Total area of γ‐glutamyl transpeptidase‐positive lesions and the mean area per lesion were significantly larger in rats of the DEN‐DES group than those of the DEN‐C, DEN‐TMX or DEN‐DES TMX group. Estrogen receptor (ER) content of liver cytosol assayed by enzyme immunoassay (EIA) was significantly greater in rats of the DEN‐DES group than in those of the DEN‐C group and smaller in rats of the DEN‐TMX and DEN‐DES TMX group than in the DEN‐C group. On the contrary, ER content of liver nuclei was significantly greater in rats of the DEN‐TMX and DEN‐DES TMX group than in those of the DEN‐C or DEN‐DES group. These results suggest that the promotive action of DES and the inhibitory action of TMX on DES‐promoted hepatic tumorigenesis are, at least in part, mediated by ER in the rat.

Effect of dietary phenobarbital on spontaneous hepatic tumorigenesis in germfree C3H/He male mice

Phenobarbital (PB) is known to be a promoter of liver tumorigenesis in rats and mice. The present study was designed to examine the effect of PB on liver tumorigenesis in C3H/He germfree (Gf) male mice. Gf and conventional (Cv) animals were given an irradiated (5 Mrad) basal diet containing 200 ppm PB from 6 weeks old until termination of the experiment. When they were 12 months old, the animals were killed under CO 2 inhalation and autopsied for the number and size of tumor nodules. The incidence of liver tumors was significantly higher in PB-fed Gf animals (67%) than in non-treated Gf controls (30%), and higher in PB-fed Cv animals (100%) than in non-treated Cv controls (75%). The number of tumor nodules per mouse was also significantly higher in PB-fed Gf animals (2.0) than in non-treated Gf controls (0.4), and higher in PB-fed Cv animals (4.5) than in non-treated controls (1.3). The present study demonstrated that dietary PB induced increases in number of the tumor nodules and deceases in the tumor size of C3H/He mice.

Effects of dietary sinigrin or indole-3-carbinol on O6-methylguanine-DNA-transmethylase activity and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced DNA methylation and tumorigenicity in F344 rats.

The effects of dietary sinigrin and indole-3-carbinol (I3C) on DNA methylation and O6-methylguanine--DNA-transmethylase activity, factors which may be of importance in the induction of tumorigenicity by the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), were investigated. Additionally, the effects of dietary sinigrin on NNK tumorigenicity were assessed in a two-year bioassay in F344 rats. DNA methylation in target tissues of NNK tumorigenesis was examined in F344 rats administered [3H-CH3]NNK (0.6 mg/kg, four doses) s.c. and fed control or experimental diets for two weeks. Dietary sinigrin at a concentration of 3 mumol/g diet decreased 7-methylguanine formation in hepatic DNA, but had no effect on 7-methylguanine levels of lung or nasal mucosa DNA. Dietary I3C at a concentration of 30 mumol/g diet increased 7-methylguanine levels in hepatic DNA, but decreased DNA methylation in lung and nasal mucosa. No effects on O6-methylguanine--DNA-transmethylase activity were observed in tissue extracts derived from the livers, lungs and nasal mucosae of rats fed diets containing sinigrin or I3C. These results suggested that dietary sinigrin might reduce the incidence of NNK-induced hepatic tumors with no effect on NNK tumorigenesis of the lung and nasal cavity, whereas I3C might increase hepatic tumor incidence and reduce NNK tumorigenesis of the lung and nasal cavity. The bioassay results showed that dietary sinigrin had no effect on NNK tumorigenesis in these target tissues. However, dietary sinigrin plus NNK resulted in a significant incidence of pancreatic tumors, a rare occurrence in F344 rats. While the results from DNA methylation studies are in agreement with the bioassay data for lung and nasal cavity, the absence of any inhibitory effect of dietary sinigrin on NNK hepatic tumorigenesis indicates that factors other than DNA methylation and O6-methylguanine repair should be considered in assessing the effects of dietary compounds on NNK hepatic tumorigenesis. The contrary effects on NNK-induced hepatic DNA methylation by sinigrin and I3C, two major components of cruciferous vegetables, demonstrate the complexities of dietary modulation of carcinogenesis.

Role of tryptophan in carcinogenesis.

This paper reviews some of the earlier experimental studies concerning the role that tryptophan plays in enhancing tumorigenesis induced by selected chemical carcinogens. For many years, tryptophan has been implicated in carcinogenesis of the bladder. The evidence regarding tryptophan's effect on hepatic tumorigenesis is conflicting; an enhancing effect has been reported by some investigators, but a reduction in tumorigenesis has been reported by other workers. Some of the unique effects that tryptophan exerts upon the liver are reviewed. Also, experimental studies from our laboratory are reported in which we observed a potentiating effect of increased dietary tryptophan on the induction of gamma-glutamyltranspeptidase-positive foci in liver when rats were fed a choline-supplemented diet but no potentiation was found when rats were fed a choline-deficient diet for 10 weeks. The results suggest that increased dietary tryptophan has a promoting effect on liver carcinogenesis as measured by the induction of gamma-glutamyltranspeptidase-positive foci in the livers of rats exposed to diethylnitrosamine. The possible significance of these findings is reviewed.

Effects of dietary selenium on hepatic and renal tumorigenesis induced in rats by diethylnitrosamine.

Seven groups of female Sprague-Dawley rats (approximately 200 gm initial body weight) were injected i.p. with a single subcarcinogenic dose of diethylnitrosamine (40 mg per kg body weight) between 8 to 10 hr after partial hepatectomy, and after a recovery period of 3 weeks (herein called induction stage) received 0.05% phenobarbital in the diet for the rest of the experiment (promotion stage). The rats were fed a 20% casein-based diet containing 0.16 ppm of selenium or the same diet supplemented with 4 or 6 ppm of selenium as sodium selenite. The effects of these three dietary regimens were tested when administered 9 to 11 days before and during induction, 1 week before and during promotion or during the entire experiment. Pair-feeding conditions were used to minimize influences due to differences in food intake and growth. Despite similarities in food intakes, the growth rates in groups receiving the 6 ppm-selenium diet during promotion or during the entire experiment were in general significantly lower than in rats fed the 4 ppm-selenium diet or the 0.16 ppm-selenium basal diet. Survival rates were also significantly reduced in rats fed the 4 and 6 ppm-selenium diets during promotion or during the entire experiment. In rats killed at the 19th week for interim assessment of the experiment's progress, the stereologically analyzed numerical and volumetric densities of hepatic premalignant hyperplastic nodules did not differ significantly between groups. All the remaining rats were killed at the 46th week.(ABSTRACT TRUNCATED AT 250 WORDS)