The stages of initiation and promotion in hepatocarcinogenesis

Synergistic inhibitory effects of the oxyresveratrol and dacarbazine combination against melanoma cells

Various therapies have been developed to target malignant melanoma, which is associated with a high mortality rate worldwide. Although dacarbazine (DTIC) is employed for treating melanoma, it is associated with several side effects. Hence, patients with melanoma are co-treated with additional drugs to mitigate the side effects of DTIC. In the present study, synergistic therapeutic effects of the DTIC/oxyresveratrol (ORT) combination were examined using the human malignant melanoma WM-266-4 cell line. Treatment with ORT and DTIC inhibited the proliferation of WM-266-4 cells. Compared with those in the ORT- and DTIC-treated groups, the proportion of cells arrested at the S phase, as well as apoptotic rates, were increased in the ORT and DTIC co-treatment group. In WM-266-4 cells, synergistic proliferation-inhibitory activities of the ORT/DTIC combination were assessed based on cell viability and migration, antioxidant capacity, cytokine production, cell cycle arrest, apoptotic rate and protein expression through WST-1 assay, wound healing assay, flow cytometry and western blotting. Furthermore, the expression levels of proteins, including NOTCH, involved in the pathogenesis of solid cancers, such as melanoma, were examined. Overall, the ORT/DTIC combination synergistically promoted cell cycle arrest at the S phase and the apoptosis of WM-266-4 cells. Thus, this combination treatment may serve as a novel therapeutic strategy for treating malignant melanoma.

Effect of Hepatitis Viruses on the Nrf2/Keap1-Signaling Pathway and Its Impact on Viral Replication and Pathogenesis

With respect to their genome and their structure, the human hepatitis B virus (HBV) and hepatitis C virus (HCV) are complete different viruses. However, both viruses can cause an acute and chronic infection of the liver that is associated with liver inflammation (hepatitis). For both viruses chronic infection can lead to fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Reactive oxygen species (ROS) play a central role in a variety of chronic inflammatory diseases. In light of this, this review summarizes the impact of both viruses on ROS-generating and ROS-inactivating mechanisms. The focus is on the effect of both viruses on the transcription factor Nrf2 (nuclear factor erythroid 2 (NF-E2)-related factor 2). By binding to its target sequence, the antioxidant response element (ARE), Nrf2 triggers the expression of a variety of cytoprotective genes including ROS-detoxifying enzymes. The review summarizes the literature about the pathways for the modulation of Nrf2 that are deregulated by HBV and HCV and describes the impact of Nrf2 deregulation on the viral life cycle of the respective viruses and the virus-associated pathogenesis.

Inhibitory potential of methanolic extracts of Aristolochia tagala and Curcuma caesia on hepatocellular carcinoma induced by diethylnitrosamine in BALB/c mice

CONTEXT

Aristolochia tagala (AT) and Curcuma caesia (CC) have been used traditionally by local herbal practitioners for cancer treatment and as chief ingredients of many polyherbal formulations for various types of ailments. However, there is void in scientific study to evaluate their anti-cancer property.

AIMS

The aim of this study was to evaluate the anti-carcinogenic properties of the crude methanolic extracts of roots of AT and rhizomes of CC in BALB/c mice exposed to a hepatocarcinogen, diethylnitrosamine (DEN).

SETTINGS AND DESIGN

(I) Toxicity of herbal plant extracts (HPE); (II) Anticancer studies; (III) Histological studies; and (IV) Biochemical studies.

MATERIALS AND METHODS

To evaluate the effects of these two HPE either alone or following DEN exposure, serum transaminases (aspartate aminotransferase [AST], alanine aminotransferase [ALT]), alkaline phosphatase (ALP), and cancer marker enzyme acetylcholine esterase (AChE) were assayed in mice. In addition, histological study was also carried out under similar conditions. The antioxidant potentials of the HPE were evaluated by monitoring the activity of antioxidant enzymes and metabolites, such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione (GSH).

STATISTICAL ANALYSIS USED

Statistical analysis was performed by GraphPad Prism 6 Software using one-way analysis of variance followed by the Tukey's multiple comparisons test. Significance was set at P < 0.05.

RESULTS

Our findings show that DEN administration elevated AST, ALT, ALP, and AChE activities. CC or AT extracts attenuated the increased activities of these marker enzymes. The activities of antioxidant enzymes, which were decreased following DEN administration, were significantly increased in mice treated with CC or AT.

CONCLUSIONS

The present study clearly documents anticarcinogenic and antioxidant properties of AT and CC in DEN-induced mouse liver cancer in vivo.

Pomegranate-mediated chemoprevention of experimental hepatocarcinogenesis involves Nrf2-regulated antioxidant mechanisms

Hepatocellular carcinoma (HCC), one of the most prevalent and lethal cancers, has shown an alarming rise in the USA. Without effective therapy for HCC, novel chemopreventive strategies may effectively circumvent the current morbidity and mortality. Oxidative stress predisposes to hepatocarcinogenesis and is the major driving force of HCC. Pomegranate, an ancient fruit, is gaining tremendous attention due to its powerful antioxidant properties. Here, we examined mechanism-based chemopreventive potential of a pomegranate emulsion (PE) against dietary carcinogen diethylnitrosamine (DENA)-induced rat hepatocarcinogenesis that mimics human HCC. PE treatment (1 or 10 g/kg), started 4 weeks prior to the DENA challenge and continued for 18 weeks thereafter, showed striking chemopreventive activity demonstrated by reduced incidence, number, multiplicity, size and volume of hepatic nodules, precursors of HCC. Both doses of PE significantly attenuated the number and area of γ-glutamyl transpeptidase-positive hepatic foci compared with the DENA control. PE also attenuated DENA-induced hepatic lipid peroxidation and protein oxidation. Mechanistic studies revealed that PE elevated gene expression of an array of hepatic antioxidant and carcinogen detoxifying enzymes in DENA-exposed animals. PE elevated protein and messenger RNA expression of the hepatic nuclear factor E2-related factor 2 (Nrf2). Our results provide substantial evidence, for the first time, that pomegranate constituents afford chemoprevention of hepatocarcinogenesis possibly through potent antioxidant activity achieved by upregulation of several housekeeping genes under the control of Nrf2 without toxicity. The outcome of this study strongly supports the development of pomegranate-derived products in the prevention and treatment of human HCC, which remains a devastating disease.

Predominant modifier of extreme liver cancer susceptibility in C57BR/cdJ female mice localized to 6 Mb on chromosome 17

Sex hormones influence the susceptibility of inbred mice to liver cancer. C57BR/cdJ (BR) females are extremely susceptible to spontaneous and chemically induced liver tumors, in part due to a lack of protection against hepatocarcinogenesis normally offered by ovarian hormones. BR males are also moderately susceptible, and the susceptibility of both sexes of BR mice to liver tumors induced with N,N-diethylnitrosamine relative to the resistant C57BL/6J (B6) strain is caused by two loci designated Hcf1 and Hcf2 (hepatocarcinogenesis in females) located on chromosomes 17 and 1, respectively. The Hcf1 locus on chromosome 17 is the predominant modifier of liver cancer in BR mice. To validate the existence of this locus and investigate its potential interaction with Hcf2, congenic mice for each region were generated. Homozygosity for the B6.BR(D17Mit164-D17Mit2) region resulted in a 4-fold increase in liver tumor multiplicity in females and a 4.5-fold increase in males compared with B6 controls. A series of 16 recombinants covering the entire congenic region was developed to further narrow the area containing Hcf1. Susceptible heterozygous recombinants demonstrated a 3- to 7-fold effect in females and a 1.5- to 2-fold effect in males compared with B6 siblings. The effect in susceptible lines completely recapitulated the susceptibility of heterozygous full-length chromosome 17 congenics and furthermore narrowed the location of the Hcf1 locus to a single region of the chromosome from 30.05 to 35.83 Mb.

Increased oxidative stress is associated with balanced increases in hepatocyte apoptosis and proliferation in glycerol-3-phosphate acyltransferase-1 deficient mice

The absence of mouse mitochondrial glycerol-3-phosphate acyltransferase-1 (Gpat1-/-) increases the amount of arachidonate in liver phospholipids and increases beta-hydroxybutyrate and acyl-carnitines, suggesting an elevated rate of liver fatty acid oxidation. We asked whether these alterations might increase reactive oxygen species (ROS), apoptosis, or hepatocyte proliferation. Compared to wildtype controls, liver mitochondria from Gpat1-/- mice showed a 20% increase in the rate of ROS production and a markedly increased sensitivity to the induction of the mitochondrial permeability transition. Mitochondrial phosphatidylethanolamine and phosphatidylcholine from Gpat1-/- liver contained 21% and 67% more arachidonate, respectively, than wildtype controls, and higher amounts of 4-hydroxynonenal, a product of arachidonate peroxidation. Oxidative stress was associated with an increase in apoptosis, and with 3-fold and 15-fold higher TUNEL positive cells in liver from young and old Gpat1-/- mice, respectively, compared to age-matched controls. Compared to controls, bromodeoxyuridine labeling was 50% and 7-fold higher in livers from young and old Gpat1-/- mice, respectively, but fewer glutathione-S-transferase positive cells were present. Thus, Gpat1-/- liver exhibits increased oxidative stress and sensitivity of the mitochondrial permeability transition pore, and a balanced increase in apoptosis and proliferation.

TCDD exposure-response analysis and risk assessment

We examined the relation between cancer mortality and time-dependent cumulative exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) estimated from a concentration- and age-dependent kinetic model of elimination, and we estimated incremental cancer risks at age 75. Data from the National Institute for Occupational Safety and Health study of 3,538 workers with occupational exposure to TCDD were analyzed using standardized mortality ratios and Cox regression procedures. Analyses adjusted for potential confounding by age, year of birth, and race and considered exposure lag periods of 0, 10, or 15 years. Other potential confounders including smoking and other occupational exposures were evaluated indirectly. To explore the influence of extreme values of cumulative TCDD ppt-years, we restricted the analysis to observations with exposure below the 95th percentile or used logarithmic (ln) transformed exposure values. We applied penalized smoothing splines to examine variation in the exposure-response relation across the exposure range. TCDD was not statistically significantly associated with cancer mortality using the full data set, regardless of the lag period. When we restricted the analysis to observations with exposure below the 95th percentile, TCDD was associated positively with cancer mortality, particularly when a 15-year lag was applied (untransformed exposure data: regression coefficient , standard error (s.e.) = 1.4 x 10(-6), p < 0.05; ln-transformed exposure data: , s.e. = 2.9 x 10(-2), p < 0.05). The estimated incremental lifetime risk of mortality at age 75 from all cancers was about 6 to more than 10 times lower than previous estimates derived from this cohort using exposure models that did not consider the age and concentration dependence of TCDD elimination.

Chemopreventive Effects of Melatonin on Diethylnitrosamine and Phenobarbital-Induced Hepatocarcinogenesis in Male F344 Rats

The antitumor effects of melatonin on diethylnitrosamine (DEN)-initiated and/or phenobarbital (PB)-promoted hepatocarcinogenesis were investigated in male F344 rats. Five-week-old male F344 rats were divided into eight groups. Rats in groups 1-5 were given DEN (100 mg/kg body weight, i.p.) once a week for 3 wk, whereas those in groups 6-8 received vehicle treatment. Groups 1-3 and 7 were given 500 ppm PB in drinking water for 20 wk after DEN or vehicle treatment. Group 2 was given 400 ppm melatonin-containing diet during the initiation phase. Groups 3 and 5 were fed melatonin-containing diet for 20 wk, starting 1 wk after the last dosing of DEN. Group 6 was given melatonin-containing diet alone throughout the experiment (24 wk). Group 8 was treated with vehicle alone. Liver neoplasms were recognized only in DEN-treated groups. The incidences and multiplicities of hepatocellular adenoma and hepatocellular carcinoma (HCC) in group 3 were significantly smaller when compared with group 1 (P < 0.001 or P < 0.002). The average and unit areas of glutathione S-transferase placental form (GST-P)-positive foci of groups 2 and 3 were significantly smaller than those of group 1 (P < 0.001 or P < 0.01). The density and average area of these preneoplastic lesions of group 5 were also smaller than those of group 4 (P < 0.001 or P < 0.005). In addition, the ornithine decarboxylase activity in nonneoplastic liver tissue was reduced by melatonin treatment in both the initiation and postinitiation phases. These results suggest that melatonin has an antitumor-promoting ability in DEN-initiated and PB-promoted hepatocarcinogenesis in rats.

Suppression of altered hepatic foci development by a high fish oil diet compared with a high corn oil diet in rats

Effects of low corn oil, high corn oil, and high fish oil diets on altered hepatic foci development in female Sprague-Dawley rats were investigated. Rats assigned to Groups 1-4 were initiated with saline as the control and those assigned to Groups 5-7 were initiated with diethylnitrosamine (DEN 15 mg/kg) at 24 hours of age. After weaning, all rats, except those in Group 1, received 500 ppm phenobarbital (PB) in their diet as tumor promoter for three months. Altered hepatic foci development was significantly lower in DEN-initiated rats fed the high fish oil + PB diet than in DEN-initiated rats fed the high corn oil + PB diets. Liver weight and relative liver weight were significantly greater in rats fed the high fish oil + PB diet than in rats fed the other diets, and hepatic biotransformation/detoxification enzyme activities were greater in rats fed the fish oil + PB diets than in rats fed the other diets. These results suggest that the effect of a high fish oil diet on altered hepatic foci may occur through regulation of hepatic biotransformation/detoxification enzyme activities, leading to alteration in the tumor-promoting action of PB. Dietary lipid significantly affected the hepatic phospholipid fatty acid composition of rats. n-3 polyunsaturated fatty acids were incorporated into membrane phospholipid at the expense of n-6 polyunsaturated fatty acids. A high fish oil diet caused greater oxidative stress in rats, as measured by plasma vitamin E level, red blood cell glutathione status, liver lipid peroxidation, and hepatic glutathione reductase activity. Pearson's correlation analysis indicated that the foci number was negatively correlated to the liver thiobarbituric acid-reactive substance and 7-pentoxyresorufin O-dealkylase activity, and the foci area was negatively correlated to the liver thiobarbituric acid-reactive substance activity (p < 0.05) in rats of groups that developed foci. These results suggest that the type of dietary lipid is the more important determinant for gamma-glutamyl transpeptidase-positive foci development than the amount of dietary lipid when rats consumed approximately the same amount of calories in all the dietary groups, and the underlying mechanisms may be partially ascribed to the antioxidant/oxidation status and biotransformation/detoxification system of rats.

Marker Enzymes of Rat Chemical Hepatocarcinogenesis in Human Liver Tumors

Reduced glucose-6-phosphatase, increased GGT activity and reduction of cytochrome P-450 content are considered to be markers of chemical hepatocarcinogenesis in rats. The significance of these changes were studied in certain human liver lesions; adenoma, focal nodular hyperplasia and hepatocellular carcinoma all developed in non-cirrhotic livers. Enzymes showed normal values in 4 out of 5 adenomas, in 2/13 FNH and in 4/18 HCC samples. The decreased cP-450 content in HCC proved to be the most consistent alteration (12/18). Only 3 HCC samples possessed changes off all enzymes. These data suggest that at least those enzymes which are used as markers in rat chemical hepatocarcinogenesis have little or no biological significance in human liver tumors, primarily due to the intertumoral heterogeneity of enzyme activity. Such heterogeneity was observed in the peritumoral "normal" liver tissue, too.

Morphology, proliferation and apoptosis of mouse liver epithelial cells cultured as spheroids

The MLEC10 is an epithelial cell line derived from an untreated, normal C3H/HeN mouse liver. We previously demonstrated that tumorigenic variants from this cell line produced moderately differentiated hepatocellular carcinomas in nude mice. However, it has remained unclear whether the parental MLEC10 cells represent immortalized hepatocytes or so-called oval cells, both of which may serve as precursors for hepatocellular neoplasms. In this study, we performed 3-dimensional, spheroid culture of the MLEC100 cells in order to facilitate histological assessment of their lineage. Spheroidal aggregates were formalin-fixed and embedded in paraffin for routine light-microscopic observation of hematoxylin and eosin-stained sections. Histopathologically, the MLEC10 cells were indistinguishable from immature hepatocytes and distinct from oval cells. At the electron-microscopic level, their hepatocytic nature was evidenced by bile canaliculus structures and glycogen storage. Intriguingly, the spheroids contained fragmentary material reminiscent of Councilman bodies, implying apoptosis of the hepatocytes. Although the cells significantly proliferated during the first three days of culture, apoptotic death then resulted in a 75 % decrease in viable cell number. Thereafter, both apoptosis and cell division appeared silent, the numbers being unchanged. Expression of the p53 tumor suppressor gene became gradually elevated, correlating positively with growth arrest, but negatively with apoptosis, suggesting that the cell death occurred independently of p53. Our results indicate that at least some liver epithelial cell lines derived from untreated murine livers exhibit a hepatocytic morphology in spheroid culture. Also, the present culture system provides a useful tool for investigating biological phenomena, e.g. apoptosis, specifically involving liver cells, under 3-dimensional conditions.

CHART (continuous, hyperfractionated, accelerated radiotherapy): a tale of two disciplines

Zingiberaceae rhizomes commonly used in the Malaysian traditional medicine were screened for anti-tumour promoter activity using the short-term assay of inhibition of 12-O-tetradecanoyl phorbol-13-acetate (TPA)-induced Epstein-Barr virus early antigen (EBV-EA) in Raji cells. The inhibition of TPA-induced EBV-EA was detected using the indirect immunofluorescence assay (IFA) and Western blot technique. The indirect IFA detected the expression/inhibition of EBV-EA-D (diffused EA antigen), whereas the Western blot technique detected the expression/inhibition of both EBV-EA-D and EA-R (restricted EA antigen). Seven rhizomes were found to possess inhibitory activity towards EBV activation, induced by TPA; they are: Curcuma domestica, C. xanthorrhiza, Kaempferia galanga, Zingiber cassumunar, Z. officinale, Z. officinale (red variety), and Z. zerumbet. A cytotoxicity assay was carried out to determine the toxicity of the Zingiberaceae rhizome extracts. The rhizome extracts that exhibited EBV activation inhibitory activity had no cytotoxicity effect in Raji cells. Therefore, the present study shows that several Zingiberaceae species used in Malaysian traditional medicine contain naturally occurring non-toxic compounds that inhibit the EBV activation, which, if further investigated, could contribute in the development of cancer prevention methods at the tumour-promoting stage. © 1999 Cancer Research Campaign

Flow cytometric analysis of neoplastic nodules and hepatocellular carcinomas induced by ciprofibrate in the rat

Alterations in DNA ploidy accompany hepatocellular carcinoma (HCC). However, changes in DNA content are also seen in regenerating liver and with increasing age. Thus, to investigate the role of DNA ploidy changes in development of HCC, flow cytometric DNA content determinations were done in a rat model system of peroxisome proliferator-induced HCC. Paraffin blocks of liver isolated from 18 Fisher 344 male rats fed ciprofibrate for 20 weeks (4), 40 weeks (4) or 20 months (10) were examined. Livers from age-matched control rats were also examined. From the 20 month ciprofibrate group, nine neoplastic nodules (NNs), 27 HCCs and four non-tumorous surrounding tissue controls (NTCs) were examined. Significant DNA tetraploid populations were seen in both the NNs and NTCs. A significant increase in the percentage of DNA diploid cells was observed in the NN samples. No significant difference in the percentage S-phase cells was seen. Emergence of cell populations with new DNA ploidy classes (8c or DNA aneuploid) as compared with NTCs was only seen in HCCs (7 of 27), and five of these seven were DNA aneuploid, as distinct from DNA tetraploid, populations. A total of 16 of 24 HCC samples that were adequate for cell cycle analysis had average percent S-phase greater than the mean of the NTCs plus three standard deviations. Although a direct role cannot be inferred, these results support the hypothesis that increases in the fraction of diploid cells is an important early event in the development of rat HCC and that further alterations in DNA ploidy and increased proliferative fraction accompany the development of HCC.

Inhibitory effect of vanadium on rat liver carcinogenesis initiated with diethylnitrosamine and promoted by phenobarbital

The chemoprotective effect of vanadium, a dietary micronutrient, against chemically induced hepatocarcinogenesis in rats was investigated. Initiation was performed by a single intraperitoneal injection of diethylnitrosamine (DENA; 200 mg kg-1) followed by promotion with phenobarbital (0.05%) in the diet. Supplementary vanadium (0.5 p.p.m.) in the drinking water was provided ad libitum throughout the experiment, before the initiation or during the promotion period. At the end of the study (20 weeks), vanadium supplementation throughout the experiment reduced the incidence (P < 0.01), total number and multiplicity (P < 0.001) and altered the size distribution of visible persistent nodules (PNs) as compared with DENA control animals. Mean nodular volume (P < 0.05) and nodular volume as a percentage of liver volume (P < 0.01) were also attenuated following long-term vanadium treatment. It also caused a large decrease in the number (P < 0.001) and surface area (P < 0.01) of gamma-glutamyltranspeptidase (GGT)-positive hepatocyte foci and in the labelling index (P < 0.001) of focal cells, coupled with increased (P < 0.01) remodelling. The activity of GGT, measured quantitatively, was found to be significantly less in the PNs (P < 0.001) and non-nodular surrounding parenchyma (P < 0.01) of vanadium-supplemented rats. The anticarcinogenic effect of vanadium was also reflected in the histopathological analysis of liver sections that showed a well-maintained hepatocellular architecture as compared with DENA control. Similar results were observed when vanadium was given only before the initiation. However, supplementation of vanadium during the promotion period did not result in significant alterations of these parameters. Our results, thus, strongly suggest that vanadium may have a unique anti-tumour potential which is primarily exerted on the initiation phase and only secondarily on the promotion stage.

The importance of biological realism in dioxin risk assessment models

Mechanistic mathematical models of hepatocarcinogenesis in the female rat were constructed to investigate possible relationships among the Ah, estrogen, and EGF receptors in TCDD hepatocarcinogenicity. Each model generates dose-response curves for the expression of biomarker liver proteins CYP1A1, CYP1A2, and residual plasma membrane EGF receptor consequent to exposure to TCDD. The shapes of the response curves were strongly dependent on the assumed mechanisms of constitutive expression of these proteins. Assuming a constant level of the hepatic Ah receptor, a sigmoidal dose-response of hepatic CYP1A1 to total liver TCDD was computed. However, inclusion of induction of the Ah receptor by TCDD in a physiologically realistic dosimetric model produced a linear low-dose response of CYP1A1. This behavior was computed to arise from the net effect of sublinear response of CYP1A1 mRNA to the concentration of the Ah-TCDD complex and supralinear response of the protein concentration to the mRNA level, illustrating the importance of biological realism in dose-response modeling. The dosimetric model also computed effects of TCDD on the hepatic estradiol concentration and consequent effects on the binding capacity of the EGF receptor and suggests plausible mechanisms for tumor promotion by TCDD. Setting circulating estradiol levels in the model to values typical of the male rat indicated possible sources of the differences in the responses of the EGF receptor and in development of tumors in the two sexes.

Phenobarbital induction of AP-1 binding activity mediates activation of glutathione S-transferase and quinone reductase gene expression

Phenobarbital is an inducer of xenobiotic-metabolizing enzymes, such as cytochrome P-450, glutathione S-transferases (GSTs) and NAD(P)H:quinone reductase, as well as being a promoter of hepatocarcinogenesis. The molecular mechanisms regulating these biological activities are, however, unknown. In this paper we show that induction by phenobarbital of GST Ya and quinone reductase gene expression is mediated by regulatory elements, EpRE and ARE respectively, which are composed of two adjacent AP-1-like binding sites. EpRE was recently found to be activated by a Fos/Jun heterodimeric complex (AP-1). Here we show that phenobarbital induces an increase in AP-1 binding activity in nuclear extracts of cultured hepatoma cells. Furthermore, we observe that the induction of chloramphenicol acetyltransferase (CAT) activity from an EpRE Ya-cat gene construct and of AP-1 binding activity by phenobarbital is inhibited by the thiol compounds N-acetyl-L-cysteine and glutathione. These results suggest that the phenobarbital induction of AP-1 activity, leading to the AP-1-mediated transcriptional activation of the GST Ya and quinone reductase genes, may involve production of reactive oxygen species and an increase in intracellular oxidant levels, which is prevented by thiol compounds. In view of the involvement of AP-1 in the control of cell proliferation and transformation, the induction by phenobarbital of AP-1 binding activity observed here provides a possible molecular mechanism for the tumour-promoting activity of this drug.

Inhibitory effects of inhibitors of arachidonic acid metabolism on the evolution of rat liver preneoplastic foci into nodules and hepatocellular carcinomas with or without phenobarbital exposure

Effects of inhibitors of arachidonic acid (AA) metabolism on the evolution of preneoplastic foci into nodules and of nodules into hepatocellular carcinomas were examined in F344 male rat livers with or without phenobarbital (PB) exposure. p-Bromophenacyl bromide (BPB), acetylsalicylic acid (ASA), and quercetin (QU) were used as inhibitors of phospholipase A2, cyclooxygenase and lipoxygenase, respectively. Preneoplastic liver foci were induced by initiation with N-nitrosodiethylamine (200 mg/kg, i.p.) followed by selection using the procedure of Cayama et al. For the nodule experiment, starting 1 week after completion of the selection procedure, animals bearing foci were given diets containing 0.05% PB plus 0.75, 1, or 1.5% of one of the inhibitors, 0.05% PB alone, or 0.75, 1 or 1.5% of inhibitor alone, or basal diet for 9 weeks. For the carcinoma experiment, 3 weeks after completion of the selection procedure, animals bearing nodules were given the same diets mentioned above for 29 weeks. BPB, ASA and QU either with or without PB accelerated the remodeling of preneoplastic foci, significantly decreasing the numbers of persistent nodules and hyperplastic nodules. ASA either with or without PB significantly decreased the number of hepatocellular carcinomas per rat. BPB and QU, however, significantly decreased the numbers of hepatocellular carcinomas with but not without PB. The results suggested an involvement of AA metabolism in the process of evolution of preneoplastic foci into nodules and hepatocellular carcinomas in rat liver with or without PB exposure.

Quantitative comparison of initiation and mutation phenotypes in hepatocytes of the analbuminemic rat

The potential relationship between mutagenesis and carcinogenesis has been examined in the Nagase analbuminemic rat treated with a single dose of benzo[a]pyrene, an incomplete liver carcinogen. The apparent mutation rate at the albumin locus was calculated by determining the number of hepatocytes expressing a cross-reactive product of albumin in analbuminemic rats treated with benzo[a]pyrene. The rate of initiation, the first stage in carcinogenesis, was determined by assessing the number of hepatocytes expressing the placental isozyme of glutathione S-transferase (PGST) after administration of benzo[a]pyrene. Since the expression of PGST may represent hepatocellular changes independent of initiation, promotion with phenobarbital was employed to clonally expand those putatively initiated hepatocytes expressing PGST. With immunohistochemical measures to assess changes in albumin expression, a threefold increase in the number of hepatocytes expressing albumin was detected after administration of benzo[a]pyrene in Nagase analbuminemic rats. A more than five-fold increase in altered hepatic foci (AHF) exhibiting increased PGST expression was observed in animals given benzo[a]pyrene treatment followed by phenobarbital, compared with those given benzo[a]pyrene alone. The number of albumin-expressing single hepatocytes detected was of the same order of magnitude as the number of individual hepatocytes and AHF expressing PGST, suggesting that similar events may be involved in their formation. Since 3 x 10(6) single hepatocytes expressing albumin were found in the analbuminemic rat liver after a single administration of benzo[a]pyrene, while less than 2 x 10(4) AHF expressing PGST were observed, formation of individual hepatocytes expressing albumin was a far more frequent event than clonal expansion of initiated hepatocytes in the Nagase analbuminemic rat. However, the number of loci of PGST expression including AHF and single hepatocytes is comparable to that of single hepatocytes expressing albumin.

Inhibitory effect of dietary iron deficiency on the induction of putative preneoplastic foci in rat liver initiated with diethylnitrosamine and promoted by phenobarbital

The effects of dietary iron deficiency on induction of putative preneoplastic, gamma-glutamyltransferase (GGT)-positive hepatocyte focal lesions in the liver of rats treated with diethylnitrosamine (DEN) followed by phenobarbital (PB) were investigated. Male Fischer 344 rats of 4 weeks old were placed on an iron deficient (ID) diet containing less than 5 p.p.m. of iron or an iron supplemented (IS) diet containing 180 p.p.m. of iron throughout experimental period of 12 weeks. Both groups of rats were administered 200 mg kg-1 body weight of DEN by a single intraperitoneal injection at Week 4 followed by PB mixed into each diet at a concentration of 0.05% from Week 6 to the final sacrifice at Week 12 when induction of GGT-positive foci was quantitatively analysed. On the ID and IS diets, respective numbers of GGT-positive foci were 6.3 and 14.2 cm-2. The sizes of foci were not altered by the iron content of the diet. The present results indicate that iron plays a role in the development of preneoplastic foci in the livers of rats initiated with DEN and promoted by PB especially in the initiation phase.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 19. The classification of carcinogens identified in the rodent bioassay as potential risks to humans: what type of substance should be tested next?

The relevance of rodent cancer bioassay data to humans is discussed in relation to the needs of regulatory agencies. The usefulness of in vivo and in vitro genotoxicity testing in this connection is also discussed. In the case of rodent carcinogens that do not elicit genotoxicity, it is suggested that homeostatic imbalance, cell proliferation, and other processes may play a major role in tumor development and its importance to the possible ability of the test agent to induce human cancer. These possibilities need to be evaluated on a case by case basis. The methods by which chemicals are selected for the rodent cancer bioassay are also discussed and it is pointed out that naturally-occurring constituents of human foods should in future receive greater priority as a consequence of anticipated changes resulting from biotechnology.

Expression of oncogenes during rat chemical hepatotumorigenesis promoted by estrogen

To elucidate the role of oncogene expression in hepatocarcinogenesis, we examined the expression of 4 cellular oncogenes (c-myc, c-fos, Ha-ras and c-erbA) in liver tissues induced by chemical agents. Four groups of male Sprague-Dawley rats were examined in the present study. Rats of the first and second groups were given a single intraperitoneal injection of diethylnitrosamine (DEN), 200 mg/kg body weight. Two weeks later, these rats were divided into two groups; the DEN-C group received no further medication, whereas the DEN-DES group was given diethylstilbestrol (DES), 0.5 mg/day, for 12 months. The DEN group was given DEN, 100 ppm, in drinking water for five months as the hepatocellular carcinoma (HCC) group. The DES group was given DES, 0.5 mg/day, from the start for 8 months. Rats of the DEN-DES and DEN groups developed grossly visible hepatic tumors. Significantly higher levels of c-myc gene expression were observed in tissues of HCC of the DEN group and in neoplastic nodules of the DEN-DES groups than in the DES and DEN-C group. The increase of c-myc mRNA seemed to begin after 1 month of treatment and became significant at 4 months in the DEN-DES group. On the other hand, no significant differences in mRNA levels of c-fos, Ha-ras and c-erbA were observed among these four groups. Although the significance of increased c-myc gene expression in neoplastic liver is still not known, it is conceivable that the persistent elevation of c-myc gene expression in the DEN and DEN-DES groups might contribute to the development of rat chemical hepatotumorigenesis.

Tumor-promoting activity and cytotoxicity of 3,4,3',4'-tetrachlorobiphenyl on N-nitrosomorpholine-induced murine liver foci

Effects of 3,4,3',4'-tetrachlorobiphenyl (TCB) on glucose-6-phosphatase (G6Pase)-altered hepatic foci of N-nitrosomorpholine (NNM)-treated B6C3F1 mice were investigated. TCB was chosen as a selective 3-methylcholanthrene-type inducer and tumor promoter. To initiate hepatocarcinogenesis, mice were treated with NNM (160 mg/l, in drinking water for 7 weeks), as in previous studies with the rat model. After a treatment-free interval of 22 weeks, TCB was administered (5 x 50 mg/kg, every 3 days), and liver foci were analysed 10 weeks after the start of TCB treatment. Unexpectedly, the number of G6Pase-negative and -positive foci per liver was markedly diminished following TCB treatment (to 32% and 57%, respectively). On the other hand, the mean volume of the remaining G6Pase-altered foci was enhanced, owing to an increase in the percentage of foci of large size (greater than 0.5 mm2). Throughout the experimental period of 39 weeks prolonged liver injury due to NNM and TCB treatment was demonstrated by histology and by elevated serum levels of glutamate-oxaloacetate transaminase. The results suggest that (in contrast to the rat system) TCB exhibited opposing effects on liver foci in the mouse model: (a) moderate tumor-promoting effects and (b) cytotoxic effects in NNM-injured liver, leading to decreased numbers of liver foci.

Profile of drug metabolizing enzymes in the nuclear and microsomal fractions from rat liver nodules and normal liver

The activities of UDP-glucuronyl transferase, DT-diaphorase, epoxide hydrolase, aryl hydrocarbon hydroxylase, gamma-glutamyl transferase and NADPH-cytochrome c reductase were measured in the nuclear and microsomal fractions from normal rat liver and rat liver nodules. Nodules were produced by intermittent feeding of Wistar rats with a standard diet supplemented with 0.05% (w/w) 2-acetylaminofluorene. The nuclear and microsomal fractions were isolated by differential centrifugation. The activities of UDP-glucuronyl transferase, DT-diaphorase, epoxide hydrolase and gamma-glutamyl transferase were significantly increased in the nuclear and microsomal fractions obtained from nodules as compared with normal liver. Aryl hydrocarbon hydroxylase activity was decreased in the microsomal fraction from the pathological tissue but not in the nuclear fraction. NADPH-cytochrome c reductase activity was similar in nodular and normal liver tissue. The nuclear/microsomal ratio for phase I reactions in xenobiotic metabolism was increased over normal more than two fold. Thus the nuclear and microsomal systems for drug metabolism are both changed in liver nodules. The relative enhancement of nuclear activating reactions is remarkable in the light of the increased risk for malignant transformation exhibited by nodular cells.

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 gamma-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.

A method for the comparative study of replicative DNA synthesis in GGT-positive and GGT-negative hepatocytes in primary culture isolated from carcinogen-treated rats

The presence of gamma-glutamyl transpeptidase (GGT) in focal nodules of hepatocytes is a commonly used marker for the identification of preneoplastic cell populations. Female Fischer 344 rats were initiated with a single intragastric administration of 200 mg diethylnitrosamine/kg, altered cells were selected after 0.02% 2-acetylaminofluorene was given in the diet; this was followed by a partial hepatectomy and promotion with dietary sodium phenobarbital for 4 wk. A mixed-cell population of GGT-positive and GGT-negative hepatocytes was obtained after collagenase perfusion and Percoll purification. An enriched population of GGT-positive hepatocytes was obtained by a modified "panning" technique. With quantitative scintillation spectrometry and autoradiography of [3H]thymidine incorporation, replicative DNA synthesis of GGT-positive and GGT-negative rat hepatocytes was observed in both the mixed-cell population and the enriched GGT-positive and GGT-negative cell populations. Under the culture conditions used, GGT-positive cells showed a higher level of replicative DNA synthesis than did GGT-negative cells; this indicates that such altered hepatocytes in the stage of promotion possess an inherently greater capacity for all replication, as previously suggested from studies in vivo.

Generation of high yields of Syrian hamster cholangiocellular carcinomas and hepatocellular nodules by combined nitrite and aminopyrine administration and Opisthorchis viverrini infection

Combined administration of 0.1% nitrite and 0.1% aminopyrine in the drinking water for eight to ten weeks resulted in subsequent development of both hepatocellular nodules and cholangiofibrotic lesions/cholangiocellular carcinomas in Syrian golden hamsters. Additional prior dosing with Opisthorchis viverrini metacercariae (100/animal) induced inflammatory and proliferative changes in the livers of infected hamsters and was associated with a significant increase in yields of hepatocellular and cholangiocellular preneoplastic and neoplastic lesions. Thus, environmental factors thought to be casually related to the high levels of human liver cancer observed in the Northeastern provinces of Thailand were sufficient to bring about development of equivalent tumors in experimental animals. The results indicate that parasite associated liver injury and non-specific compensatory regeneration may play an important role in generation of both hepatocellular and cholangiocellular carcinomas in man.

Decreased sensitivity to phalloidin of normal-looking rat hepatocytes after short-term 2-acetylaminofluorene feeding

Male F344/DuCrj rats were fed a diet containing 0.02% 2-acetylaminofluorene (2-AAF) for 1 or 3 weeks, and then fed a basal diet for 2 days, 2 weeks, 8 weeks, 22 weeks or 36 weeks. Hepatocytes were isolated from the liver by collagenase perfusion, and their sensitivity to phalloidin, in terms of the formation of multiple cytoplasmic blebs, was examined. The sensitivity of gamma-glutamyltransferase (GGT)-negative hepatocytes decreased on the 22nd and 36th weeks after withdrawal of 2-AAF feeding, and that of GGT-positive cells decreased on the 36th week. Induction of a small number of foci positive for the placental form of glutathione S-transferase (GSTP) was observed in the liver of all rats on the 8th, 22nd and 36th weeks after the withdrawal of the carcinogen. However, the total area of the foci was estimated to account for less than 0.2% of liver tissues even on the 36th week. Therefore, the decrease in phalloidin sensitivity of hepatocytes, particularly of GGT-negative hepatocytes, on the 22nd and 36th weeks after 2-AAF withdrawal is suggested to be a result of a decrease in the sensitivity of otherwise normal-looking hepatocytes, which may be precursors of the cells forming the preneoplastic foci.

Short-term pathological and proliferative effects of butylated hydroxyanisole and other phenolic antioxidants in the forestomach of Fischer 344 rats

Food grade butylated hydroxyanisole (BHA) when incorporated in the diet and fed to male Fischer 344 rats for 9 or 27 days induced proliferative squamous epithelial changes in the lesser curvature of the forestomach proximate to the glandular stomach. These changes were assessed histopathologically and by [methyl-3H]thymidine radioautography. It was shown that BHA mixed dry into powdered diet, incorporated into the diet in corn oil, or in a pelleted diet, induced similar effects. When levels of 2%, 1%, 0.5%, 0.25%, 0.1% and 0% BHA were incorporated in rat diet for 9 days, the proliferative effect appeared to show a no effect level at 0.25% based on the [methyl-3H]thymidine-labelling index. Other food use antioxidants, namely butylated hydroxytoluene or tertiary butylhydroquinone, induced a lesser response than BHA at the maximum dose employed in the study. Propyl gallate was without effect. Propyl-4-hydroxybenzoate, a food use phenol, on the other hand, induced a less pronounced response than BHA but was more effective than the other antioxidants. Because increased cellular proliferation often provides an optimal milieu for tumor formation, it is suggested that these observations may be relevant to rat forestomach tumors induced by BHA.

Carcinogenicity of diethylnitrosamine in newborn, infant, and adult mice

Modifying effects of age, sex, and mouse strain on diethylnitrosamine (DEN) carcinogenesis have been investigated in C57BL/6Jx C3HeB/FeJ F1 (B6C3F1) and C3HeB/FeJxA/J F1 (C3AF1) hybrid mice. Animals each received four IP injections of 1.5 or 3.0 micrograms DEN/g body weight. The first injections were administered on days 1, 15, or 42 of life. Subsequent treatments were delivered at 3-, 6-, and 6-day intervals, respectively. Mice were kept under observation for the remaining life-span. DEN treatment induced tumors in liver, lungs, and forestomach in descending order of frequency. The majority of the induced liver tumors were hepatocellular carcinomas. Animals treated as newborns and infants developed significantly more liver tumors than animals that were treated as young adults. Newborn and infant females developed liver tumors at a later age (B6C3F1) and with a lower incidence (C3AF1) than similarly treated males. The B6C3F1 mice developed more hepatocellular carcinomas and a higher rate of pulmonary metastases than the C3AF1 mice. In contrast, C3AF1 mice developed lung tumors with a higher incidence and multiplicity than B6C3F1 hybrids. Forestomach tumors were observed also with a slightly but significantly higher incidence in C3AF1 mice.

Sequential cellular changes during chemical carcinogenesis

Phenotypically altered, preneoplastic cell populations were detected by micromorphological and cytochemical methods in a number of tissues treated with various chemical carcinogens. Further cellular analysis of carcinogenesis has shown that different cellular phenotypes follow each other during tumor development. Thus, stages of the neoplastic transformation leading from preneoplastic to early and advanced neoplastic cells can be observed directly. The cellular changes preceding the various tumor types suggest that cytologically different neoplasms have also a different cytogenesis. The identification of putative preneoplastic and early neoplastic cell populations by morphological and cytochemical methods allows for the first time the dissection and subsequent detailed investigation of target cells of chemical carcinogens that are at high risk of becoming cancer cells. Recent results of the cytochemical and biochemical microanalysis of preneoplastic hepatocytes support the concept that the well-known aberration of carbohydrate metabolism in tumor cells might occur in response to a carcinogen-induced metabolic derangement, which frequently appears to be associated with an excessive storage of polysaccharides or lipids persisting for weeks and months until fast-growing tumors develop. The increasing reports on the appearance of hepatic tumors in humans suffering from inborn hepatic glycogenosis agree with this hypothesis. Whereas the cause of the persisting storage phenomena is most probably fixed at the genetic level, epigenetic changes, namely an adaptation of cellular enzymes gradually activating alternative metabolic pathways, might be responsible for the ultimate neoplastic transformation of the cell.

Neoplastic conversion in rat liver by the antihistamine methapyrilene demonstrated by a sequential syncarcinogenic effect with N-2-fluorenylacetamide

A study was performed to determine whether the enhancing effect of the antihistamine methapyrilene (MP) in rat liver carcinogenesis represents promotion or syncarcinogenesis . The effect on hepatocarcinogenicity induced by N-2-fluorenylacetamide (FAA) of sequential administration of MP given either before or after FAA was studied in comparison with diethylnitrosamine (DEN) also given either before or after FAA. MP in either sequence with FAA enhanced liver carcinogenicity as did DEN. Moreover, MP by itself induced liver altered foci, albeit at a high dose for a prolonged interval. A single liver neoplasm occurred with exposure to MP alone. These findings suggest that MP produces neoplastic conversion of liver cells which can be summated with the genotoxic effect of FAA to produce syncarcinogenesis .

Comparison between the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and six other compounds on the vitamin A storage, the UDP-glucuronosyltransferase and the aryl hydrocarbon hydroxylase activity in the rat liver

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8- tetrabromodibenzo -p-dioxin ( TBrDD ), 5-chloro-2-(2,4-dichlorophenoxy)phenol (3-Cl- predioxin ) 4,5,6-trichloro-2-(2,4-dichlorophenoxy)phenol (5-Cl- predioxin ), toxaphene, 3-methylcholanthrene (3-MC) and phenobarbital (PB) on the vitamin A storage, UDP-glucuronosyltransferase (UDPGT) and aryl hydrocarbon hydroxylase (AHH) activities in the liver of Sprague-Dawley rats was investigated. Vitamin A was determined as retinol by high pressure liquid chromatography. UDPGT was measured with p-nitrophenol as an aglycone and AHH with 3,4-benzopyrene as a substrate. Both in TCDD- and toxaphene-treated animals a reduced body weight gain was recorded, but no other overt signs of toxicity were seen in this study. Both the concentration and the total amount of hepatic retinol was significantly reduced in TCDD-, 3-MC-, PB- and TBrDD -treated animals. These compounds were also those which gave the most significant enzyme induction as regards the UDPGT and AHH activities. However, the reduction of hepatic retinol caused by these compounds did not correlate with the enzyme activities studied. When compared on a molecular basis, TCDD and TBrDD were in the order of several magnitudes more potent as reducers of hepatic retinol and likewise as enzyme inducers.

O4-ethyldeoxythymidine, but not O6-ethyldeoxyguanosine, accumulates in hepatocyte DNA of rats exposed continuously to diethylnitrosamine

In previous investigations into the mechanisms responsible for cell specificity in hepatocarcinogenesis, we have demonstrated that O6-methylguanine accumulates in the DNA of nonparenchymal cells (NPC) but is efficiently removed from hepatocellular DNA. O6-Alkylguanine may, therefore, be an important promutagenic lesion responsible for the induction of hepatic angiosarcomas after exposure to methylating agents, but other promutagenic DNA alkylation products--i.e., O4-alkylthymine--may be responsible for the initiation of hepatocellular carcinomas. F-344 male rats were provided drinking water containing diethylnitrosamine (DEN) at 40 ppm for 0, 2, 4, 8, 16, 28, 49, or 77 days, a regimen that selectively causes hepatocellular carcinomas. Hepatocytes and NPC were isolated by using low-speed differential centrifugation. DNA was purified by hydroxyapatite chromatography and hydrolyzed enzymatically, and O4-ethyldeoxythymidine (O4-EtdThd) and O6-ethyldeoxyguanosine (O6-EtdGuo) of hepatocyte and NPC DNA were quantitated by competitive radioimmunoassay using high-affinity monoclonal antibodies. O4-EtdThd accumulated in hepatocyte DNA during the first 28 days of DEN exposure, approximating a steady state at an O4-EtdThd-to-deoxythymidine molar ratio of approximately equal to 1 X 10(-5). This O4-EtdThd concentration was maintained from 28 to 77 days of DEN exposure. In contrast, O6-EtdGuo did not accumulate in hepatocyte DNA, its greatest concentration O6-EtdGuo-to-deoxyguanosine ratio (approximately equal to 3.7 X 10(-7) ) being detected after 2 days of exposure to DEN. O6-EtdGuo concentrations in hepatocyte DNA decreased with duration of exposure to DEN to an O6-EtdGuo-to-deoxyguanosine ratio of less than 2 X 10(-7) from 28 to 77 days. The data indicate that O4-EtdThd disappears from the DNA of hepatocytes less than 1/200th as fast as O6-EtdGuo. DNA from NPC contained approximately half as much O4-EtdThd as hepatocytes did, but greater than or equal to 2.5 times more O6-EtdGuo.

Promotion mechanism of phenobarbital and partial hepatectomy in DENA hepatocarcinogenesis cell kinetics effect

Diethylnitrosamine (DENA, 10 mg kg-1 per day) was fed to rats for 2, 4 and 6 weeks. One week after the cessation of DENA, animals were submitted either to partial hepatectomy or to phenobarbital administration. Partial hepatectomy did not promote neoplastic transformation, except after a 6-week DENA treatment. A minimum of phenobarbital was required to reach a significant promoting effect in DENA carcinogenesis. A too-limited treatment was ineffectual but could be compensated for by prolonged DENA administration. The phenobarbital treatment became unnecessary when neoplastic nodules were present. Phenobarbital continuously given after the carcinogen administration promoted neoplastic transformation even after a subcarcinogenic DENA treatment (2 weeks). It accelerated the pathological evolution and increased the tumour incidence. In these conditions, phenobarbital increased the proliferation advantage of preneoplastic cells over normal cells. In the different experimental modalities, the promoting effect was associated with the induction of chronic cell proliferation, the inhibition of the rapid response to the 2/3 partial hepatectomy and the mitotic circadian rhythm normally present during liver regeneration. It is concluded that the promotion mechanism could consist in disturbing the mitotic control in order to maintain, for a long time, a chronic low level of cell proliferation permitting the selective growth of preneoplastic cells and their subsequent transformation.

On the possibility of metabolic control of replicon "misfiring": relationship to emergence of malignant phenotypes in mammalian cell lineages

Constraints of a multireplicon chromosomal organization and of the necessity to maintain constant gene dosages demand that each origin of replication in a eukaryotic cell "fire" (initiate replication) only once per cell cycle. The central idea of this work is that a low probability of an extra ("illegitimate") round of DNA replication (called below "replicon misfiring") within any given chromosomal domain could be increased by certain substances of either intra- or extracellular origin. The term " "firone" is proposed for such a substance. It is shown that existence of firones could greatly speed up evolution of cellular systems under selection pressure, a developing tumor being one example of such a system. Experimentally testable predictions of the firone hypothesis are discussed.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC working paper 2/3: carcinogens and carcinogenesis enhancers

The concept that chemical agents may lead to enhancement of carcinogenesis, rather than to its complete induction, is explored to explain the inexact correlation between carcinogen prescreening tests and the results of whole animal bioassays. It is suggested that carcinogenesis-enhancing agents are non-genotoxic chemicals which are positive in animal carcinogenesis bioassays. The importance of understanding the mechanisms of action of carcinogenesis-enhancing agents is emphasized.