The mechanism of carcinogenic action of 1,2-dimethylhydrazine (SDMH) in rats

Loss of Peroxiredoxin IV Protects Mice from Azoxymethane/Dextran Sulfate Sodium-Induced Colorectal Cancer Development

Peroxiredoxin IV (Prx4), a typical two-cysteine-containing member of the peroxidase family, functions as an antioxidant to maintain cellular redox homeostasis through the reduction of reactive oxygen species (ROS) via cycles of oxidation-reduction reactions. Under oxidative stress, all Prxs including Prx4 are inactivated as their catalytic cysteines undergo hyperoxidation, and hyperoxidized two-cysteine Prxs can be exclusively repaired and revitalized through the reduction cycle catalyzed by sulfiredoxin (Srx). Previously, we showed that Prx4 is a preferred substrate of Srx, and knockout of Srx in mice leads to resistance to azoxymethane/dextran sulfate sodium (AOM/DSS)-induced colon carcinogenesis. To further understand the significance of the Srx/Prx4 axis in colorectal cancer development, Prx4-/- mice were established and subjected to standard AOM/DSS protocol. Compared with wildtype littermates, mice with Prx4-/- genotype had significantly fewer and smaller tumors. Histopathological analysis revealed that loss of Prx4 leads to increased cell death through lipid peroxidation and lower infiltration of inflammatory cells in the knockout tumors compared to wildtype. Treatment with DSS alone also showed decreased infiltration of macrophages and lymphocytes in the colon of knockout mice, suggesting a role for Prx4 in inflammatory response. In addition, loss of Prx4 caused alterations in plasma cytokines and chemokines after DSS and AOM/DSS treatments. These findings suggest that loss of Prx4 protects mice from AOM/DSS-induced colon tumorigenesis. Thus, targeting Prx4 may provide novel strategies for colon cancer prevention and treatment.

Antioxidant and Anticancer Potentials of the Olive and Sesame Mixture against Dimethylhydrazine-Induced Colorectal Cancer in Wistar Rats

Cancer is one of the leading causes of death worldwide, and natural agents have shown some promise in fighting it. Thus, the present study tried to evaluate the healing potential of an equal combination of olive and sesame extract (MOS) against the colorectal cancerous lesions that were induced by dimethylhydrazine (DMH) in male rats and also compare the anticarcinogenic potential of the MOS and vitamin E with each other. Therefore, the mixture of equal olive and sesame extract (MOS) was used as the main treatment, alongside vitamin E as a parallel treatment. This study examined the red blood cell (RBC) and white blood cell (WBC) levels, biochemical indices, lactate dehydrogenase (LDH), C-reactive protein (CRP), total protein (TP), creatine kinase (CPK), albumin, and the colon tissue pathology, as well as the level of protein expression of the adenomatous polyposis coli (APC), proliferating cell nuclear antigen (PCNA), carcinoembryonic antigen (CEA), and platelet-derived growth factor (PDGF). Also, the tissue stress markers including total antioxidant capacity (TAC), malondialdehyde (MDA), and superoxide dismutase (SOD) were analyzed. Overall, the results represented a significant reduction in the congestion, mitotic index, inflammation, and cell destruction in the MOS group compared to the DMH group. In terms of the oxidative stress level, a significant increase was observed in the DMH group in comparison with the DMH-MOS group (P < 0.05), and the MOS significantly increased TAC level (P < 0.05). Furthermore, the DMH+MOS-exposed group exhibited a significantly lower expression of the PCNA, CEA, and PDGF proteins than those of the DMH group. Overall, the MOS showed that it can effectively prevent DMH-induced colon lesions. This mixture, as a strong antioxidant agent, can be clinically applied for preventing and treating colorectal cancer, the effectiveness of which is higher than that of vitamin E.

Markedly enhanced micronucleus induction by 1,2-dimethylhydrazine dihydrochloride in colonic cells of rats with bacterial colonization in the intestine

To investigate how intestinal bacteria affect host cytogenetic alterations in the early initiation step of colon carcinogenesis, we conducted a comet assay and micronucleus (MN) test using germ-free (GF) and conventionalized (Cvd) rats after a single subcutaneous injection of the carcinogen, 1,2-dimethylhydrazine dihydrochloride (DMH). DNA damage was also determined in the liver in comet assays, as DMH is metabolized and activated in this organ. The time-response patterns of DNA damage in the liver and colon were similar in both rats, and maximum values were observed at 3 h after the treatment. In contrast, the maximum frequency of micronucleated (MNed) colonic cells was markedly higher in the Cvd rats than in the GF rats and was observed after 72 h and 120 h, respectively. The frequency of MNed cells in non-treated animals was similar in the GF and Cvd rats. In addition, we determined time-responses in the incidence of apoptosis and cell proliferation indices, i.e., the numbers of BrdU-labeled cells, mitotic cells in the crypts, and crypt column heights, using histological sections of the colons in these rats. Maximal incidence of apoptosis was observed at 6 and 24 h in the Cvd and GF rats, respectively. The value in the Cvd rats tended to be higher than that in the GF rats. Cell proliferation was suppressed until 24 and 48 h in the Cvd and GF rats, respectively, and increased subsequently. The rebound response of cell proliferation was more pronounced and occurred earlier in the Cvd rats than that in the GF rats. We demonstrated that cytogenetic alterations other than DNA damage, particularly the MNed colonic cell induction by DMH, were markedly enhanced in rats with bacterial colonization in the intestine compared to those in GF rats.

Increased paracellular permeability of tumor-adjacent areas in 1,2-dimethylhydrazine-induced colon carcinogenesis in rats

Objective:

The morphology and functions of the proximal and distal large intestine are not the same. The incidence of colorectal cancer in these regions is also different, as tumors more often appear in the descending colon than in the ascending colon. Inflammatory bowel disease and colorectal cancer can increase transepithelial permeability, which is a sign of reduced intestinal barrier function. However, there is not enough evidence to establish a connection between the difference in colorectal cancer incidence in the proximal and distal colon and intestinal permeability or the effects of carcinogenesis on the barrier properties in various areas of the colon. The aim of the study was to assess the permeability of different segments of the large intestine according to a developed mapping methodology in healthy rats and rats with 1,2-dimethylhydrazine (DMH)-induced colon adenocarcinoma.

Methods:

The short circuit current, the transepithelial electrical resistance and the paracellular permeability to fluorescein of large intestine wall of male Wistar rats were examined in the Ussing chambers. The optical density of the solution from the serosa side to assess the concentration of the diffused fluorescein from mucosa to serosa was analyzed by spectrophotometry. The morphometric and histological studies were performed by optical microscopy.

Results:

Rats with DMH-induced colon adenocarcinomas showed elevated transepithelial electrical resistance in the areas of neoplasm development. In contrast, there was no change in the electrophysiological properties of tumor adjacent areas, however, the paracellular permeability of these areas to fluorescein was increased compared to the control rats and was characterized by sharply reduced barrier function.

Conclusions:

The barrier properties of the colon vary depending on tumor location. The tumors were less permeable than the intact intestinal wall and probably have a negative influence on tumor-adjacent tissues by disrupting their barrier function.

Detection of early primary colorectal cancer with upconversion luminescent NP-based molecular probes

Early detection and diagnosis of cancers is extremely beneficial for improving the survival rate of cancer patients and molecular imaging techniques are believed to be relevant for offering clinical solutions. Towards early cancer detection, we developed a primary animal colorectal cancer model and constructed a tumor-specific imaging probe by using biocompatible NaGdF4:Yb,Er@NaGdF4 upconversion luminescent NPs for establishing a sensitive early tumor imaging method. The primary animal tumor model, which can better mimic the human colorectal cancer, was built upon continual administration of 1,2-dimethylhydrazine in Kunming mice and the tumor development was carefully monitored through histopathological and immunohistochemical analyses to reveal the pathophysiological processes and molecular features of the cancer microenvironment. The upconversion imaging probe was constructed through covalent coupling of PEGylated core-shell NPs with folic acid whose receptor is highly expressed in the primary tumors. Upon 980 nm laser excitation, the primary colorectal tumors in the complex abdominal environment were sensitively imaged owing to the ultralow background of the upconversion luminescence and the high tumor-targeting specificity of the nanoprobe. We believe that the current studies provide a highly effective and potential approach for early colorectal cancer diagnosis and tumor surgical navigation.

Role of hepatic and intestinal p450 enzymes in the metabolic activation of the colon carcinogen azoxymethane in mice

P450-mediated bioactivation of azoxymethane (AOM), a colon carcinogen, leads to the formation of DNA adducts, of which O⁶-methylguanine (O⁶-mG) is the most mutagenic and contributes to colon tumorigenesis. To determine whether P450 enzymes of the liver and intestine both contribute to AOM bioactivation in vivo, we compared tissue levels of AOM-induced DNA adducts, microsomal AOM metabolic activities, and incidences of colonic aberrant crypt foci (ACF) among wild-type (WT), liver-specific P450 reductase (Cpr)-null (LCN), and intestinal epithelium-specific Cpr-null (IECN) mice. At 6 h following AOM treatment (at 14 mg/kg, s.c.), O⁶-mG and N⁷-mG levels were highest in the liver, followed by the colon, and then small intestine in WT mice. As expected, hepatic adduct levels were significantly lower (by >60%) in LCN mice but unchanged in IECN mice, whereas small-intestinal adduct levels were unchanged or increased in LCN mice but lower (by >50%) in IECN mice compared to that in WT mice. However, colonic adduct levels were unchanged in IECN mice compared to that in WT mice and increased in LCN mice (by 1.5–2.9-fold). The tissue-specific impact of the CPR loss in IECN and LCN mice on microsomal AOM metabolic activity was confirmed by rates of formation of formaldehyde and N⁷-mG in vitro. Furthermore, the incidence of ACF, a lesion preceding colon cancer, was similar in the three mouse strains. Thus, AOM-induced colonic DNA damage and ACF formation is not solely dependent on either hepatic or intestinal microsomal P450 enzymes. P450 enzymes in both the liver and intestine likely contribute to AOM-induced colon carcinogenesis.

Effects of High Amylose Maize Starch andClostridium butyricumon Metabolism in Colonic Microbiota and Formation of Azoxymethane-Induced Aberrant Crypt Foci in the Rat Colon

High amylose maize starch (HAS) is not digested in the small intestine and most of it reaches the large intestine. In the large intestine, HAS is fermented by intestinal bacteria, resulting in production of short-chain fatty acids (SCFA), particularly butyrate. Clostridium butyricum can utilize HAS and produce butyrate and acetate. It has been proposed that butyrate inhibits carcinogenesis in the colon. In this study, we examined the inhibitory effects of HAS and C. butyricum strain MIYAIRI588 (CBM588) on azoxymethane-induced aberrant crypt foci (ACF) formation in rats. In the group of rats administered only CBM588 spores, the concentration of butyrate in the cecum increased, but there was no decrease in the number of ACF. In the group of rats fed an HAS diet, a decrease in the number of ACF was observed, and in the group of rats administered HAS and CBM588, the number of ACF decreased significantly. In these two groups, the concentrations of acetate and propionate in intestinal contents significantly increased, but the concentration of butyrate did not change. It was found that the beta-glucuronidase activity level of colonic contents decreased significantly in the two groups of rats fed HAS. This study showed that HAS and CBM588 changed the metabolism of colonic microbiota and decreased the level of beta-glucuronidase activity, phenomena that may play a role in the inhibition of ACF formation in the rat colon.

Assessment of the mode of action underlying development of rodent small intestinal tumors following oral exposure to hexavalent chromium and relevance to humans

Abstract Chronic exposure to high concentrations of hexavalent chromium (Cr(VI)) in drinking water causes intestinal adenomas and carcinomas in mice, but not in rats. Cr(VI) causes damage to intestinal villi and crypt hyperplasia in mice after only one week of exposure. After two years of exposure, intestinal damage and crypt hyperplasia are evident in mice (but not rats), as are intestinal tumors. Although Cr(VI) has genotoxic properties, these findings suggest that intestinal tumors in mice arise as a result of chronic mucosal injury. To better understand the mode of action (MOA) of Cr(VI) in the intestine, a 90-day drinking water study was conducted to collect histological, biochemical, toxicogenomic and pharmacokinetic data in intestinal tissues. Using MOA analyses and human relevance frameworks proposed by national and international regulatory agencies, the weight of evidence supports a cytotoxic MOA with the following key events: (a) absorption of Cr(VI) from the intestinal lumen, (b) toxicity to intestinal villi, (c) crypt regenerative hyperplasia and (d) clonal expansion of mutations within the crypt stem cells, resulting in late onset tumorigenesis. This article summarizes the data supporting each key event in the MOA, as well as data that argue against a mutagenic MOA for Cr(VI)-induced intestinal tumors.

Effects of lycopene, synbiotic and their association on early biomarkers of rat colon carcinogenesis

This study evaluated whether a synergy exists for the combined treatment with lycopene and synbiotic on early biomarkers of colon carcinogenesis. Male Wistar rats received a diet containing 300 mg/kg of lycopene and/or synbiotic (Bifidobacterium lactisplus oligofructose/inulin) or their combination 2 weeks before and during carcinogen treatment with 1,2-dimethylhydrazine (DMH). Twenty-four hours after the last DMH application, the colons were processed for immunohistochemical analysis of proliferating cell nuclear antigen (PCNA), p53 protein, hematoxylin-eosin staining for apoptosis analysis and genotoxicity of fecal water by comet assay. Eight weeks after the last DMH application, the colons were analyzed for development of classical aberrant crypt foci (ACF) and mucin-negative ACF. Treatment with lycopene, synbiotic or their combination significantly increased apoptosis, reduced the PCNA and p53 labeling indexes and the development of classical ACF and mucin-negative ACF. Furthermore, a lower genotoxicity of fecal water was also detected in the groups treated with the chemopreventive agents. An additive/synergistic effect of the combined treatment with lycopene/synbiotic was observed only for the fecal water genotoxicity and mucin-negative ACF parameters. These results indicate that an additive/synergistic of the combination of chemopreventive agents on the initiation phase of colon carcinogenesis can be detected using selective early biomarkers.

Prolonged chronic inflammation progresses to dysplasia in a novel rat model of colitis-associated colon cancer

Inflammatory bowel disease (IBD) is a gastrointestinal disorder of unknown etiology or cure. One complication of IBD is an increased risk for development of colon cancer. The aims of this study were to use a previously established rat model of colitis to develop a new model of colitis-associated colon cancer and ascertain the involvement of three cancer-related genes: K-ras, adenomatous polyposis coli (APC), and p53. Four groups of rats were used: reactivated 1,2-dimethylhydrazine [DMH; trinitrobenzene sulfonic acid (TNBS) was used to induce colitis followed by a weekly s.c. dose of DMH], prolonged reactivation (inflammation was induced with TNBS, then maintained twice a week), saline-DMH (animals received saline instead of TNBS followed by a weekly dose of DMH), and normal (received no treatment). Animals were sacrificed at 5, 10, or 15 weeks, and colon samples were taken for pathologic analysis and gene mutation detection. No dysplasia was found in the normal group. The highest incidences of dysplasia were as follows: prolonged reactivation group at 5 weeks (60%), reactivated DMH group at 10 weeks (83%), and saline-DMH group at 15 weeks (67%). Carcinoma was found in both the prolonged reactivation and saline-DMH groups. No mutations were found in the K-ras oncogene; however 62% of the APC samples (exon 15 at nucleotide 2778) and 76% of p53 (exon 6 at nucleotide 1327) showed substitutions. The prolonged reactivation group may be considered a new model of colitis-associated colon cancer, offering the potential to study cancer prevention strategies for patients with IBD.

Differences in susceptibility to colonic stem cell somatic mutation in three strains of mice

Different species and different strains of animals commonly show very different sensitivities to carcinogenic regimes, which are often unexplained. A major possible contributory factor is variation in susceptibility to mutation, but this has not been directly demonstrated. This study therefore quantified the colonic stem cell mutation frequency in three strains of mice using two carcinogens. Stem cell mutations were identified using loss of function of glucose 6-phosphate dehydrogenase (G6PD) in individual crypts, a technique validated by several previous studies. The carcinogens dimethylhydrazine (DMH) and ethyl nitrosurea (ENU) were given to Balb/C, C57BL/6J, and C3H mice. In response to DMH, Balb/C mice were most susceptible, with approximately double the stem cell mutation frequency found in C3H and more than ten-fold that found in C57BL/6J (3.3+/-0.71 vs. 1.5+/-0.52 vs. 0.28+/-0.8x10(-4)). In response to ENU, Balb/C mice and C3H mice were equally susceptible, showing a stem cell mutation frequency approximately twice that of C57BL/6J (3.1+/-0.4 vs. 3.1+/-0.65 vs. 1.63+/-0.28x10(-4)). The observed differences among the strains with respect to somatic mutation following DMH treatment are likely to be due to the previously documented differences in metabolic conversion to the active metabolite. However, as ENU is a directly acting, rapidly inactivated mutagen, strain differences in response to ENU are unlikely to be due to strain-dependent metabolism of the mutagen and are likely to reflect differences in DNA repair efficiency, or possibly in stem cell kinetics among the strains studied. Susceptibility to the induction of colonic stem cell mutation is an important factor in susceptibility to carcinogens, whether due to differences in DNA repair or to other factors. Direct quantification of stem cell mutation frequency allows the separate identification of this component of the carcinogenic cascade and shows that it can make a major contribution to the differing susceptibility of different mouse strains.

Effects of beta-glucuronidase-deficient and lycopene-producing Escherichia coli strains on formation of azoxymethane-induced aberrant crypt foci in the rat colon

We tried to inhibit the formation of azoxymethane-induced aberrant crypt foci (ACF) in the rat intestine by feeding a culture of a beta-glucuronidase-deficient Escherichia coli strain or a cell suspension of a lycopene-producing E. coli strain. Feeding of the former culture to F344 rats did not decrease fecal beta-glucuronidase activity or the number of ACF compared with the control beta-glucuronidase-proficient groups. However, a significant positive correlation between the fecal beta-glucuronidase activity and the ACF number was observed among groups treated with cultures of beta-glucuronidase-proficient and -deficient strains. In the group treated with lycopene-producing cells, the number of ACF was significantly lower than that in the control group. A vegetable juice containing a larger amount of lycopene than a cell suspension of the lycopene-producing E. coli also decreased the number of ACF to the same extent as a cell suspension of the lycopene-producing bacteria. These results suggest that feeding of the beta-glucuronidase-deficient E. coli is not very effective in preventing colon carcinogenesis, although activity of the fecal beta-glucuronidase is associated with AOM-induced ACF formation, and that lycopene-producing intestinal bacteria can effectively prevent colon carcinogenesis.

Experimental models of colorectal cancer

PURPOSE

A review of in vivo and in vitro models of colorectal cancer is presented.

METHODS

A retrospective literature review was performed with reference to CD-ROM Medline and Index Medicus.

RESULTS

A comparison of the advantages and disadvantages of the models is presented in addition to a summary of individual model methodology and applications.

CONCLUSIONS

Such models are a useful adjunct for surgical research in colorectal oncology.

Effect of intestinal bacteria on formation of azoxymethane-induced aberrant crypt foci in the rat colon

The effect of intestinal bacteria on formation of azoxymethane (AOM)-induced aberrant crypt foci (ACF) and DNA adducts in the rat colon was investigated. Male Sprague-Dawley rats were administered cultures of Lactobacillus acidophilus, Bifidobacterium adolescentis, Bacteroides fragilis, Escherichia coli and Clostridium perfringens for five weeks and given injections of AOM at 15 mg/kg body weight at the first and second weeks. The number of ACF five weeks after the start of the experiment was decreased in the rats treated with the cultures or culture supernatants of L. acidophilus and C. perfringens. The half-life of O6-methylguanine (O6-meG) in the L. acidophilus group was shorter than that in the GAM broth group. The half-life of 7-methylguanine did not differ among the groups. These results suggest that the metabolite(s) of L. acidophilus and C. perfringens inhibit(s) the ACF formation in rats treated with AOM and that the inhibitory effect of L. acidophilus is due to the enhanced removal of O6-meG from the colon mucosal DNA.

Carcinogenesis induced by a single administration of 1,2-diethylhydrazine in female rats of various ages

Three- or fourteen-month-old female L10 rats were exposed to a single intravenous injection of 1,2-diethylhydrazine (SDEH) at 150 mg/kg of body weight. At the 95th week after carcinogen treatment when the experiment was stopped, 30.7% and 4.5% of rats from the younger and older groups survived, respectively. Total tumor incidences were 68.8% and 84.6%, respectively, in rats treated with SDEH at the age of 3 or 14 months vs. 18.2% and 34.5% in corresponding young and old controls (P < 0.01). Leukemias, thyroid adenomas, uterine tumors and mammary malignancies developed more frequently in animals exposed to carcinogens than in control groups. No age-related differences in tumor incidence or localization between rats exposed to SDEH at various ages were observed, but tumors developed earlier in older groups than in younger groups. The results supported the suggestion that the accumulation of initiated cells in some tissues during natural aging is a cause of the age-related increase in cancer incidence.

A quantitative study of epithelial alterations during the early stages of experimental colonic tumorigenesis in mice

The weekly administration of 1,2-dimethyl-hydrazine (DMH) by subcutaneous injection for a period of 16-20 weeks is a well known procedure for producing colonic tumors in mice and rats. Quantitative histomorphological, histochemical and kinetic studies, as well as investigation of the significance of epithelial cell density were carried out in mice between the 7th and the 91st day after the first DMH injection. These studies showed that between the 28th and the 35th day, several simultaneous alterations in the colonic epithelium involving modification of glandular form, decreased mucus secretion, an increase in epithelial cell density and an increase in the number of S phase cells (BrdU labeling index: LI). Around the 35th day, the glands tended to expand and from the 35th to the 63rd day, they were stretched and displayed compartments of dedifferentiated and non-mucinous crypts (DNMC). In these crypts the cell density became very high, reaching twice the control value on the 91st day. This feature was accompanied by alteration in cell morphology and by an increase in the available basement membrane area. A decrease in mucus secretion was apparent from the 14th day and by the 63rd day, mucus secretion was only about 60% of the control value in all crypts. The LI was increased until the 35th day following which a paradoxical and progressive decrease occurred in all glandular compartments.

Mutagenicity of bile and duodenal adenomas in familial adenomatous polyposis

Duodenal adenomas occur almost inevitably in patients with familial adenomatous polyposis (FAP) whereas gastric adenomas are rare. FAP patients are also at high risk of duodenal cancer. Within the duodenum, adenomas cluster around the ampulla of Vater, as do the majority of duodenal cancers, suggesting that bile plays a role in tumour development. We therefore tested duodenal bile from 29 postcolectomy FAP patients (27 of whom had duodenal adenomas) and 24 non-FAP patients for mutagenicity, using techniques that detect point mutations in bacteria. Results which appeared to show that FAP bile was more mutagenic than control bile could be accounted for by a feeding effect, elimination of which also eliminated 'mutagenicity'. Under the conditions of our assays we conclude that if bile is an important factor in genesis of duodenal tumours, it does not act by inducing point mutation.

Colorectal cancer in animal models--a review

Colorectal cancer remains the second leading cause of cancer death in the western hemisphere. During the past several decades information regarding epidemiology, etiology and associated factors regarding colo-rectal cancer in humans has been collected through study of experimental colonic tumors in animal models. Much of this work has been influenced by the use of 1,2-dimethylhydrazine as the inducing carcinogen in susceptible populations of animals, although other specific carcinogens have been used. Through application of this experimental model, knowledge of dietary, immunologic, and bacterial factors has been realized in the etiology of colo-rectal cancer. This review details methodology and results of developing experimental models as they pertain to human colo-rectal cancer.

Detection of carcinogenic exposures by urinalysis: induction of cell-mediated immunity

Studies were undertaken to determine whether urine collected from rats during the 24 h following exposure to the colon carcinogen 1,2-dimethylhydrazine (DMH) contained substances which would induce specific antitumor immunity. The evaluation for such components was carried out by administering the test and control urine to young male adult Fisher F344 inbred rats and then establishing the animals' antitumor cell-mediated immunity (CMI) at a 14-day post-exposure interval. The CMI was measured by determining the injury and destruction inflicted upon cultured X-ray-induced rat small bowel adenocarcinoma target cells by peripheral blood lymphoid cells obtained from the animals exposed to the urine. A significantly increased CMI was found to be induced by the urine collected from the chemically exposed rats, thus suggesting the presence of mutagenic/carcinogenic components. In addition, exposure through the levels of CMI that were induced, with an approximate threshold detection limit of 100 microgram (1.7 mumol)/kg body weight (100 ppb) to the chemical. These preliminary findings suggest that such immune measurements might serve as a basis for the development of a rapid and inexpensive bioassay for monitoring population exposures to carcinogenic substances.

Bile as a source of potential reactive metabolites

Many toxic substances are excreted via the bile as chemically inert metabolites. In some cases, however, highly toxic, mutagenic and carcinogenic products can be derived from bile, either directly or after further metabolism by intestinal microflora or intestinal cells. Potential reactive substances may thus reach biliary tract and intestinal cells via this route and may undergo subsequent enterohepatic circulation. The possible role of biliary reactants in carcinogenesis of the biliary tract and intestine is discussed.

Study of kinetics of epithelial cell populations in normal tissues of the rat's intestines and in carcinogenesis. III. Changes in kinetics of enterocyte populations in the course of experimental intestinal tumour induction in rats

A stage-by-stage study of disturbances in enterocyte proliferation in the ileum and descending colon in the course of tumour induction by treatment with 1,2-dimethylhydrazine was performed. Even at early stages, an expansion of the zone of epithelial cell proliferation in the crypts and migration of dividing cells as far as to the crypt mouth, which is a manifestation of enterocyte differentiation disturbances, were observed. Enterocytes of the crypts chiefly proliferated through a short cycle, the mean duration of which was slightly greater than in normal intestinal tissue. The reduced cell loss in the epithelium and resultant disturbances of its steady state led to the accumulation of great numbers of atypical cells in the superficial layers of the crypts and formation of carcinomas in situ in the descending colon. The microscopically unaltered sections of the mucosa, prior to development of overt neoplastic changes carcinomas in situ, superficial cancers and small-size adenocarcinomas revealed a simplified structure of enterocyte population, as compared with normal epithelium. As tumours progressed, the heterogeneity of its component cell subpopulations increased, and several subpopulations, differing in mean duration of the mitotic cycle, were formed. Pathologic mitoses made up a greater portion (50-60 per cent) of the dividing cells of the descending colon, as compared with ordinary 4 per cent at all stages of experimental tumour induction.

In vivo covalent binding of organic chemicals to DNA as a quantitative indicator in the process of chemical carcinogenesis

The covalent binding of chemical carcinogens to DNA of mammalian organs is expressed per unit dose, and a 'Covalent-Binding Index', CBI, is defined. CBI for various carcinogens span over 6 orders of magnitude. A similar range is observed for the carcinogenic potency in long-term bioassays on carcinogenicity. For the assessment of a risk from exposure to a carcinogen, the total DNA dmaage can be estimated if the actual dose is also accounted for. A detailed description is given for planning and performing a DNA-binding assay. A complete literature survey on DNA binding in vivo (83 compounds) is given with a calculation of CBI, where possible, 153 compounds are listed where a covalent binding to any biological macromolecule has been shown in vivo or in vitro. Recent, so far unpublished findings with aflatoxin M1, macromolecule-bound aflatoxin B1, diethylstilbestrol, and 1,2-epithiobutyronitrile are included. A comparison of CBI for rat-liver DNA with hepatocarcinogenic potency reveals a surprisingly good quantitative correlation. Refinements for a DNA-binding assay are proposed. Possibilites and limitations in the use of DNA binding in chemical carcinogenesis are discussed extensively.

Pathological features of the colonic tumours induced in rats by the administration of 1,2-dimethylhydrazine

The parenteral administration of 1,2-dimethylhydrazine to rats caused the development of colonic neoplasms in about 90% of animals by 24--30 weeks of treatment. Usually there were multiple tumours with a mean of 2.7 per rat. The lesions have been classified histologically into adenomata (26% of all tumours) and carcinomata, the latter showing varying degrees of differentiation. No completely anaplastic tumours were seen, and there were none originating in connective tissue. The distributions of the different tumour types along the length of the colon varied. The more benign lesions were situated predominantly in the distal half of the colon, while the poorly differentiated adenocarcinomata were concentrated in the proximal third of the colon. There was good evidence to suggest that adenomata often progressed to frank malignancy in the distal colon. In the proximal part, however, it appeared that tumours frequently developed de novo as poorly differentiated carcinomata. Perhaps regional variations in the kinetic organisation of the normal colonic mucosa somehow influence the nature of the neoplastic change induced by DMH, thus accounting for the differences in tumor distribution. After 24 weeks of DMH treatment there was only a small increase in the mean number of tumours per rat.

DNA damage in liver, colon, stomach, lung and kidney of BALB/c mice treated with 1,2-dimethylhydrazine

DNA single-strand breaks induced in various organs of BALB/c mice by treatment with a single dose of 1,2-dimethylhydrazine (DMH) were studied by means of the alkaline elution method modified in order to allow the evaluation of DNA damage in vivo with no need of radioactive prelabelling. DNA damage was detected in liver, lung, kidney, stomach and colon mucosa, with the liver showing the greatest amount of damage. Its degree was dependent on the dose and route of administration. A differential effect was evident in colon mucosa from Swiss and C57BL/6 mice which are respectively susceptible and resistant to the induction of bowel tumors by DMH. The higher degree of DNA damage found in liver in comparison with colon mucosa is consistent with the previously reported higher degree of DNA methylation, but does not correlate with the specificity of this carcinogen in inducing tumors of the large intestine in mice given repeated subcutaneous injections.

Kinetics of changes in the crypts of the jejunal mucosa of dimethylhydrazine-treated rats

When symmetrical 1,2 dimethylhydrazine was administered to rats by weekly s.c. injection, 37% of the animals had developed small intestinal carcinomas after 21-27 weeks. These lesions were largely localized to duodenum and upper jejunum. At the same time there was a diffuse crypt hyperplasia in the jejunum which affected all the treated animals, not just those with neoplasms. This marked hyperplasia was preceded by a modest sustained crypt elongation which was seen soon after DMH injections began. In these hyperplastic jejunal crypts the absolute size of the proliferative compartment was increased, but the growth fraction calculated from labelling studies appeared to fall, probably by reduction in relative size of the proliferating population within the proliferative compartment. No convincing alteration in actual cell-cycle time was observed in the abnormal crypts. There was a slight (25%) increase in cell-production rate in the abnormal crypts.

Alkylated purines in the DNA of various rat tissues after administration of 1,2-dimethylhydrazine

[1,2-3H] Dimethylhydrazine (1,2-DMH) (300 mg/kg; 4 mCi/kg) was injected s.c. to BD-VI rats and the degree of DNA alkylation was measured in various tissues 3 h after the administration. The O6-methylguanine/7-methylguanine ratio was approx. 4 times higher in the colon's DNA (0.0565), the principal target organ of this carcinogen, than in the liver's DNA (0.0136). These findings are briefly discussed in relation to the organ specificity of this carcinogen.

Dependence of 1,2-dimethylhydrazine metabolism on its treatment schedule

The content of cytochromes P-450 and b5 in rat liver microsomes, as well as the extent of labeling of nucleic acids and proteins of the liver and kidneys and of mucosa from different intestinal segments, was studied in rats injected daily or once a week subcutaneously with similar total doses of 1,2-dimethyl-hydrazine (SDMH) and in untreated rats. Daily SDMH administrations led to a decrease in cytochrome P-450 activity. Pretreatment of rats with unlabelled SDMH resulted in decreased labeling of DNA, RNA, proteins, and acid-soluble fractions after [3H]SDMH injection. A more pronounced effect was found after the daily treatment.