The role of microRNA in nutritional control

MicroRNAs (miRNAs) are one of a growing class of noncoding RNAs that are involved in the regulation of a wide range of metabolic processes including cellular differentiation, cell proliferation and apoptosis. The generation of miRNA is regulated in complex ways, for example by small interfering RNAs (small nucleolar and nuclear RNAs) and various other metabolites. This complexity of control is likely to explain how a relatively small part of the DNA that codes for proteins has enabled the evolution of such complex organisms as mammals. Non-protein-coding DNA is therefore thought to carry the memory of early evolutionary steps that led to progressively complex metabolic controls. Clinically, miRNAs are becoming increasingly important following the recognition that some congenital abnormalities can be traced to defects in miRNA processing. The potential for manipulating metabolism and affecting disease processes by the pharmaceutical or biological targeting of specific miRNA pathways is now being tested. miRNAs are also released into the extracellular milieu after packaging by cells into nano-sized extracellular vesicles. Such vesicles can be taken up by adjacent and possibly more distant cells, thereby allowing coordinated intercellular communication in specific tissues. Extracellular miRNAs found in the blood stream may also serve as novel biomarkers for both diagnosing specific forms of cancer and assessing the likelihood of metastasis, and as powerful prognostic indices for various cancers. Here, we discuss the role of intracellular and extracellular miRNAs in nutritional control of various (patho)physiological processes. In this review, we provide an update of the presentations from the 25th Marabou Symposium (Stockholm, 14-16 June 2013) entitled 'Role of miRNA in health and nutrition', attended by 50 international experts

Global histone H4 acetylation and HDAC2 expression in colon adenoma and carcinoma

Chromatin remodeling and activation of transcription are important aspects of gene regulation, but these often go awry in disease progression, including during colon cancer development. We investigated the status of global histone acetylation (by measuring H3, H4 acetylation of lysine residues, which also occur over large regions of chromatin including coding regions and non-promoter sequences) and expression of histone deacetylase 2 (HDAC2) in colorectal cancer (CRC) tissue microarrays using immunohistochemical staining. Specifically, HDAC2 and the acetylation of histones H4K12 and H3K18 were evaluated in 134 colonic adenomas, 55 moderate to well differentiated carcinomas, and 4 poorly differentiated carcinomas compared to matched normal tissue. In addition, the correlation between expression of these epigenetic biomarkers and various clinicopathological factors including, age, location, and stage of the disease were analyzed. HDAC2 nuclear expression was detected at high levels in 81.9%, 62.1%, and 53.1% of CRC, adenomas, and normal tissue, respectively (P = 0.002). The corresponding nuclear global expression levels in moderate to well differentiated tumors for H4K12 and H3K18 acetylation were increased while these levels were decreased in poorly differentiated tumors (P = 0.02). HDAC2 expression was correlated significantly with progression of adenoma to carcinoma (P = 0.002), with a discriminative power of 0.74, when comparing cancer and non-cancer cases. These results suggest HDAC2 expression is significantly associated with CRC progression.

Bcl-2 overexpression in PhIP-induced colon tumors: cloning of the rat Bcl-2 promoter and characterization of a pathway involving beta-catenin, c-Myc and E2F1

Beta-catenin/T-cell factor (Tcf) signaling is constitutively active in the majority of human colorectal cancers, and there are accompanying changes in Bcl-2 expression. Similarly, 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP)-induced colon tumors in the rat have increased beta-catenin and elevated Bcl-2. To examine the possible direct transcriptional regulation of rat Bcl-2 by beta-catenin/Tcf, we cloned and characterized the corresponding promoter region and found 70.1% similarity with its human counterpart, BCL2. Bcl-2 promoter activity was increased in response to LiCl and exogenous beta-catenin, including oncogenic mutants of beta-catenin found in PhIP-induced colon tumors. Protein/DNA arrays identified E2F1, but not beta-catenin/Tcf, as interacting most strongly with the rat Bcl-2 promoter. Exogenous E2F1 increased the promoter activity of rat Bcl-2, except in mutants lacking the E2F1 sites. As expected, beta-catenin induced its downstream target c-Myc, as well as E2F1 and Bcl-2, and this was blocked by siRNA to c-Myc or E2F1. These findings suggest an indirect pathway for Bcl-2 over-expression in PhIP-induced colon tumors involving beta-catenin, c-Myc and E2F1.

The disposition and metabolism of 2-amino-3-methylimidazo-[4,5-f]quinoline in the F344 rat at high versus low doses of indole-3-carbinol

Indole-3-carbinol (I3C), a compound found in cruciferous vegetables, inhibits the formation of DNA adducts, colonic aberrant crypts, and tumors in rats given heterocyclic amines, such as 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). Previous mechanism studies indicated that I3C induces cytochromes P4501A1 (CYP1A1) and CYP1A2, as well as phase 2 pathways, leading to enhanced metabolism and excretion of IQ. However, the chemopreventive activity is dependent on the dose of I3C, and at low doses which do not induce CYP1A activity, there is evidence for increased IQ-DNA adduct formation in vivo. The present study examined the fate of IQ in the rat and the profile of urinary metabolites across a broad range of I3C doses. Male F344 rats were given a single injection of I3C by oral gavage, at a dose equivalent to that received from a single daily exposure to 0, 5, 10, 25, 50, 100, 200, 500 or 1000 ppm I3C in the diet, or they were given the 1000-ppm-equivalent dose of I3C for 14 consecutive days. Subsequently, each rat was given 14C-labeled IQ (5 mg/kg; 0.1 mCi/kg) and the animal was sacrificed 8 h later. With increasing I3C, there was a dose-dependent decrease in IQ-associated radiolabel in several systemic tissues, and an increase in the radiolabel eliminated via the feces. In the urine, there was a dose-dependent increase in IQ-5-O-glucuronide and IQ-5-O-sulfate metabolites, and a concomitant decrease in the IQ-sulfamate at intermediate and high doses of I3C. However, 5- and 10 ppm-equivalent doses of I3C enhanced the levels of IQ-sulfamate compared with controls, possibly due to the high ratio of hepatic CYP1A2 versus CYP1A1 activities at these I3C doses. The possible significance of the low versus high dose effects are discussed in the context of ongoing clinical trials with I3C and the reported chemopreventive mechanisms in vivo.

Potent antimutagenic activity of white tea in comparison with green tea in the Salmonella assay

There is growing interest in the potential health benefits of tea, including the antimutagenic properties. Four varieties of white tea, which represent the least processed form of tea, were shown to have marked antimutagenic activity in the Salmonella assay, particularly in the presence of S9. The most active of these teas, Exotica China white tea, was significantly more effective than Premium green tea (Dragonwell special grade) against 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and four other heterocyclic amine mutagens, namely 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-3,4,8-trimethyl-3H-imidazo[4,5-f]quinoxaline (4,8-DiMeIQx), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), and 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2). Mechanism studies were performed using rat liver S9 in assays for methoxyresorufin O-demethylase (MROD), a marker for the enzyme cytochrome P4501A2 that activates heterocyclic amines, as well as Salmonella assays with the direct-acting mutagen 2-hydroxyamino-3-methylimidazo[4,5-f]quinoline (N-hydroxy-IQ). White tea at low concentrations in the assay inhibited MROD activity, and attenuated the mutagenic activity of N-hydroxy-IQ in the absence of S9. Nine of the major constituents found in green tea also were detected in white tea, including high levels of epigallocatechin-3-gallate (EGCG) and several other polyphenols. When these major constituents were mixed to produce "artificial" teas, according to their relative levels in white and green teas, the complete tea exhibited higher antimutagenic potency compared with the corresponding artificial tea. The results suggest that the greater inhibitory potency of white versus green tea in the Salmonella assay might be related to the relative levels of the nine major constituents, perhaps acting synergistically with other (minor) constituents, to inhibit mutagen activation as well as "scavenging" the reactive intermediate(s).

A comparison of whole wheat, refined wheat and wheat bran as inhibitors of heterocyclic amines in the Salmonella mutagenicity assay and in the rat colonic aberrant crypt focus assay

Refined wheat, unrefined whole wheat, and wheat bran were studied for their ability to protect against heterocyclic amines (HCAs) in vitro and in vivo. Wheat bran, which binds HCAs in vitro, as well as refined wheat and unrefined whole wheat, inhibited the mutagenic activities of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) when they were co-incubated and the supernatant (minus grain) was added to the Salmonella assay. The water-soluble fraction alone from refined and unrefined wheat, but not bran, also inhibited against these mutagens in vitro. In vivo, AIN-93G diets containing refined wheat or unrefined wheat were examined for their ability to inhibit IQ-induced colonic aberrant crypt foci (ACF) in the Fischer 344 rat. A slight increase in the number of AC/ACF (aberrant crypts/ACF) was seen after 16 weeks in rats treated post-initiation with refined wheat (P < 0.05), and fewer foci with two or three aberrant crypts (ACF-2) were found in rats given unrefined whole wheat post-initiation compared with animals treated with the same diet during the initiation phase (P < 0.05). There was no significant difference in the profile of IQ urinary metabolites or excretion of promutagens 0-48 h after carcinogen dosing, and grains had no effect on hepatic cytochrome P4501A1 (CYP1A1), CYP1A2, aryl sulfotransferase or N-acetyltransferase activities; however, a slightly higher UDP-glucuronosyl transferase activity was observed in rats fed unrefined wheat compared with refined wheat diets (P < 0.05). Thus, despite their antimutagenic activities in vitro, only marginal effects were seen with refined and unrefined wheat in vivo with respect to hepatic enzyme activities, carcinogen metabolism and IQ-induced ACF in the rat colon.

Colonic cell proliferation, apoptosis and aberrant crypt foci development in rats given 2-amino-3-methylimidaz

Sodium-copper chlorophyllin (CHL) inhibits the formation of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ)- and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced colonic aberrant crypt foci (ACF) and tumours in the F344 rat when it is given simultaneously with either carcinogen. However, CHL reportedly increased the incidence of dimethylhydrazine (DMH)-induced colon tumours in the same species when administered post-initiation. In the present study, rats were given IQ (130 mg/kg body weight, by oral gavages on alternating days) for 2 weeks, starting in experiment week 3, and one week after the final IQ dose rats received CHL treatment until the study was terminated at 16 weeks. Compared with animals given carcinogen alone, the mean number of IQ-induced ACF per colon was reduced significantly by 1% (w/v) CHL in the drinking water (P < 0.05), whereas 0.1% and 0.01% CHL had no effect. These CHL concentrations increased in a dose-related manner both the terminal deoxynucleotidyl transferase-mediated nick end labelling (TUNEL) and bromodeoxyuridine (BrdU) labelling indices in the distal colon. However, the lowest concentration tested, 0.001% CHL, increased the mean number of IQ-induced ACF per colon (P < 0.05), and increased the BrdU labelling index without a concomitant change in TUNEL. These studies indicated that 0.001% CHL promoted IQ-ACF due to deregulation of the homeostatic balance between cell birth and apoptosis in the colonic mucosa, whereas higher concentrations of CHL had either no effect or protected against IQ-induced ACF by causing dose-related increases in the overall rate of cell turnover in the colon.

beta-Catenin mutation in rat colon tumors initiated by 1,2-dimethylhydrazine and 2-amino-3-methylimidazo[4,5-f]quinoline, and the effect of post-initiation treatment with chlorophyllin and indole-3-carbinol

Carcinogens 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and 1,2-dimethylhydrazine (DMH) induce colon tumors in the rat that contain mutations in beta-catenin, but the pattern of mutation differs from that found in human colon cancers. In both species, mutations affect the glycogen synthase kinase-3beta consensus region of beta-catenin, but whereas they directly substitute critical Ser/Thr phosphorylation sites in human colon cancers, the majority of mutations cluster around Ser33 in the rat tumors. Two dietary phytochemicals, chlorophyllin and indole-3-carbinol, given post-initiation, shifted the pattern of beta-catenin mutations in rat colon tumors induced by IQ and DMH. Specifically, 17/39 (44%) of the beta-catenin mutations in groups given carcinogen plus modulator were in codons 37, 41 and 45, and substituted critical Ser/Thr residues directly, as seen in human colon cancers. None of the tumors from groups given carcinogen alone had mutations in these codons. Interestingly, many of the mutations that substituted critical Ser/Thr residues in beta-catenin were from a single group given DMH and 0.001% chlorophyllin, in which a statistically significant increase in colon tumor multiplicity was observed compared with the group given DMH only. These tumors had marked over-expression of cyclin D1, c-myc and c-jun mRNA and c-Myc and c-Jun proteins were strongly elevated compared with tumors containing wild-type beta-catenin. The results indicate that the pattern of beta-catenin mutations in rat colon tumors can be influenced by exposure to dietary phytochemicals administered post-initiation, and that the mechanism might involve the altered expression of beta-catenin/Tcf/Lef target genes.

Post-initiation effects of chlorophyllin and indole-3-carbinol in rats given 1,2-dimethylhydrazine or 2-amino-3-methyl- imidazo

Chlorophyllin (CHL) is a water-soluble derivative of chlorophyll, the ubiquitous pigment in green and leafy vegetables, whereas indole-3-carbinol (I3C) is present in cruciferous vegetables such as cabbage, broccoli and cauliflower. In rats initiated with 1,2-dimethylhydrazine (DMH), CHL and I3C reportedly promoted or enhanced the incidence of colon tumors when they were administered after, or during and after the carcinogen exposure, respectively. The same compounds given post-initiation inhibited the formation of colonic aberrant crypts induced by heterocyclic amines, such as 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), but tumor suppression was not examined in the latter studies. In the present investigation, male F344 rats were treated with IQ or DMH during the first 5 weeks of a 1 year study; IQ was given in the diet (0.03%), whereas DMH was administered once a week by s.c. injection (20 mg/kg body wt). Beginning 1 week after the last dose of IQ or DMH until sacrifice, rats received 0.001, 0.01 or 0.1% (w/v) CHL in the drinking water or 0.001, 0.01 or 0.1% I3C in the diet. Compared with controls given carcinogen alone, 0.1% I3C treatment suppressed the multiplicity of IQ-induced colon tumors, and CHL inhibited in a dose-related manner the incidence of IQ-induced liver tumors. However, 0.001% CHL increased significantly the multiplicity of DMH-induced colon tumors while having no effect on the colon tumors induced by IQ. These results indicate that both the choice of carcinogen as well as the dose of the tumor modulator can be important determinants of the events that occur during post-initiation exposure to CHL or I3C. Based on the present findings and data in the literature, it is possible for CHL and I3C to act as tumor promoters or anticarcinogens, depending upon the test species, initiating agent and exposure protocol.

Inhibition by white tea of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-induced colonic aberrant crypts in the F344 rat

There is growing interest in the potential health benefits of tea, including the anticarcinogenic properties. We report here that white tea, the least processed form of tea, is a potent inhibitor of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced colonic aberrant crypts in the rat. Male Fischer 344 rats were treated for 8 wk with white tea (2% wt/vol) or drinking water alone, and on alternating days in experimental Weeks 3 and 4 the animals were given PhIP (150 mg/kg body wt p.o.) or vehicle alone. At the end of the study there were 5.65 +/- 0.81 and 1.31 +/- 0.27 (SD) aberrant crypt foci per colon in groups given PhIP and PhIP + white tea, respectively (n = 12, P < 0.05). No changes were detected in N-acetyltransferase or arylsulfotransferase activities compared with controls, but there was marked induction of ethoxyresorufin O-deethylase, methoxyresorufin O-demethylase, and UDP-glucuronosyltransferase after treatment with white tea. Western blot revealed corresponding increases in cytochrome P-450 1A1 and 1A2 proteins. Enzyme assays and Western blot also revealed induction of glutathione S-transferase by white tea. There was less parent compound and 4'-hydroxy-PhIP but more PhIP-4'-O-glucuronide and PhIP-4'-O-sulfate in the urine from rats given PhIP + white tea than in urine from animals given carcinogen + drinking water. The results indicate that white tea inhibits PhIP-induced aberrant crypt foci by altering the expression of carcinogen-metabolizing enzymes, such that there is increased ring hydroxylation at the 4' position coupled with enhanced phase 2 conjugation.

Increased susceptibility of adult rats to azoxymethane-induced aberrant crypt foci

The purpose of this study was to compare azoxymethane-induced aberrant crypt foci development in the colons of young and adult rats. Young (4 weeks of age) and adult (50 weeks of age) Sprague-Dawley rats were treated with two weekly injections of azoxymethane or saline. Rats were killed either 6 or 14 weeks following the first injection, and the number, size and location of aberrant crypt foci were determined. At both the 6- and 14-week time points, the number of aberrant crypt foci in older rats was significantly greater than in young rats (P<0.01). A higher percentage of aberrant crypt foci were found in the region from the mid-colon to the cecum in older rats as compared to young rats. Colonic cell proliferation was evaluated using bromodeoxyuridine immunohistochemistry. Colonic cell proliferation indices in the rectal, mid-colon and cecal regions of young and older rats were similar in young compared to adult rats. Ten large ACF from each group were screened for mutations in the beta-catenin gene (Ctnnb1) by PCR single strand conformation polymorphism. No mutations were detected. These results demonstrate that older female rats are more susceptible to the induction of aberrant crypt foci by azoxymethane than young female rats. Differences in colonic cell proliferation or beta-catenin mutations in these two age groups do not appear to be responsible for differences in aberrant crypt foci development.

Effect of carcinogen dose fractionation, diet and source of F344 rat on the induction of colonic aberrant crypts by 2-amino-3-methylimidazo[4,5-f]quinoline

Carcinogen dose fractionation, diet and source of laboratory animal were examined as variables in the induction of colonic aberrant crypt foci (ACF) by the heterocyclic amine 2-amino-3-methylimidazo [4, 5-f]quinoline (IQ). In the first experiment, male F344 rats from the National Cancer Institute (NCI rats) were fed AIN-93G diet and, starting in the third week, IQ was given by gavage on alternating days, the total carcinogen dose of 105 mg being fractionated proportionally over 2, 4, 8 or 14 weeks. Only the high dose (2 week) treatment with IQ was effective for the induction of ACF at 16 weeks, producing on average 3.8 ACF/colon versus 0.5 ACF/colon in all other groups (P < 0.05). The 2 week IQ dosing protocol was used in a second experiment in which male F344 rats from Simonsen Laboratories (SN) or NCI were fed AIN-93G, AIN-76A or chow diet. On average, SN rats on chow diet had twice the number of aberrant crypts compared with NCI rats given the same diet and three to four times as many aberrant crypts as NCI rats fed AIN diets. Hepatic cytochrome P4501A1 (CYP1A1) levels were essentially unaffected by diet, but methoxyresorufin O-demethylase activities and CYP1A2 protein levels were increased 2- to 3-fold in animals fed chow versus AIN diets. During the 2 week period of carcinogen administration, IQ markedly induced CYP1A proteins and negated the differences among groups related to diet. No consistent diet-related changes were detected in the activities of aryl sulfotransferase or N-acetyltransferase, but UDP-glucuronosyltransferase activities were elevated 2- to 3-fold in rats given chow versus AIN diets. In summary, high dose treatment with IQ was required for the induction of ACF, rats on the chow diet had more aberrant crypts than those given AIN diets and male F344 rats purchased from different vendors and fed chow diet differed with respect to their sensitivity to induction of ACF.

Chemoprevention studies of heterocyclic amine-induced colon carcinogenesis

The cooking of meat and fish produces heterocyclic amine mutagens, including 2-amino-1-methyl-6-phenylimidazo[4,5b]pyridine (PhIP) and 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). Chronic administration of PhIP or IQ to the F344 rat induces tumors at several sites, including adenocarcinomas of the colon, and short-term treatment leads to the formation of colonic aberrant crypt foci (ACF). We have used these end-points to identify potential chemopreventive agents that might be effective against heterocyclic amine colon carcinogens. Typically, IQ or PhIP were administered to groups of 10-15 rats by oral gavage on alternating days in weeks 3 and 4, and ACF were scored after 8, 12, or 16 weeks or tumors were detected at 52 weeks. To distinguish between 'blocking' and 'suppressing' agents, potential inhibitors were administered during the initiation or post-initiation phases, respectively, and subsequent studies focused on the inhibitory mechanisms. Among the most effective inhibitors identified to date, and their major mechanisms, were the following: chlorophyllin (molecular complex formation); indole-3-carbinol (inhibition and induction of cytochromes P450 and phase II enzymes); green and black tea catechins (induction of UDP-glucuronosyl transferase, inhibition of NADPH-cytochrome P450 reductase, scavenging of reactive intermediates); and conjugated linoleic acids (inhibition of cytochrome P450 and prostaglandin H synthase).

Cancer chemopreventive mechanisms of tea against heterocyclic amine mutagens from cooked meat

Cooking meat and fish under normal conditions produces heterocyclic amine mutagens, several of which have been shown to induce colon tumors in experimental animals. In our search for natural dietary components that might protect against these mutagens, it was found that green tea and black tea inhibit the formation of heterocyclic amine-induced colonic aberrant crypt foci (ACF) in the rat. Since ACF are considered to be putative preneoplastic lesions, we examined the inhibitory mechanisms of tea against the heterocyclic amines. In the initial studies using the Salmonella mutagenicity assay, green tea and black tea inhibited according to the concentration of tea leaves during brewing and the time of brewing; a 2-3-min brew of 5% green tea (w/v) was sufficient for >90% antimutagenic activity. N-hydroxylated heterocyclic amines, which are direct-acting mutagens in Salmonella, were inhibited by complete tea beverage and by individual components of tea, such as epigallocatechin-3-gallate (EGCG). Inhibition did not involve enhanced mutagen degradation, and EGCG and other catechins complexed only weakly with the mutagens, suggesting electrophile scavenging as an alternative mechanism. Enzymes that contribute to the metabolic activation of heterocyclic amines, namely microsomal NADPH-cytochrome P450 reductase and N, O-acetyltransferase, were inhibited by tea in vitro. Studies in vivo established that tea also induces cytochromes P450 and Phase II enzymes in a manner consistent with the rapid metabolism and excretion of heterocyclic amines. Collectively, the results indicate that tea possesses anticarcinogenic activity in the colon, and this most likely involves multiple inhibitory mechanisms.

Early detection and prevention of colorectal cancer (review)

Colorectal cancer is a leading cause of cancer-related deaths, and the two most important considerations for avoidance of this disease are early detection and prevention. If metastasis has occurred to distant sites, such as the liver and lung, the 5-year survival rate for colorectal cancer is below 10%, but this increases to greater than 90% when the cancer is found early. Early detection can be facilitated by use of the digital rectal exam, fecal occult blood test, sigmoidoscopy, and colonoscopy, but these methods might be supplemented in the future by other screening assays using intermediate biomarkers. One interesting biomarker, the aberrant crypt focus (ACF), has been observed in resected human colons, and is the earliest detectable morphological change in the colons of experimental animals treated with carcinogens such as the cooked meat heterocyclic amines. The ACF can also be used as an end-point to screen for potential inhibitors of colorectal cancer; using this approach, we identified conjugated linoleic acids, indole-3-carbinol, chlorophyllin, and tea polyphenols as promising inhibitors in the colon. These compounds can be added to a growing list of natural and synthetic agents that might be effective against colorectal cancer, including selenium, calcium, and nonsteroidal anti-inflammatory agents. However, results from human clinical trials with several of these compounds have highlighted the need for detailed mechanism data before recommendations can be made for wide-scale use in humans. In the meantime, the best approach to reducing the risk of colorectal cancer would be to increase the dietary intake of fruits, vegetables and cereals, while reducing the overall intake of fat, particularly from animal sources.

Frequent mutations of the rat beta-catenin gene in colon cancers induced by methylazoxymethanol acetate plus 1-hydroxyanthraquinone

Recent evidence suggests that the beta-catenin gene (CTNNB1) acts as an oncogene, and some human colon tumors with an intact APC gene have activating mutations in CTNNB1. In this study, mutations in the region corresponding to N-terminal phosphorylation sites (codons 1-51) of the rat Ctnnb1 gene were investigated in 20 colon tumors associated with ulcerative colitis and induced with methylazoxymethanol acetate and 1-hydroxyanthraquinone. Ninety percent (18 of 20) of the tumors induced in male F344 rats harbored mutations, which were detected in three of four adenomas (75%) and 15 of 16 adenocarcinomas (94%). Of 18 total missense mutations, 13 (72%) were G-->A transitions at position 101, three were G-->A transitions at position 94, and two were C-->T transitions at position 122, resulting in the amino acid substitutions Gly34-->Glu, Asp32-->Asn, and Thr41-->Ile, respectively. Although there were no mutations in the Apc gene, as we previously reported in the same tumor samples, the results obtained in this study strongly implicate the Apc-beta-catenin-T-cell factor (Tcf) signaling pathway in methylazoxymethanol acetate, 1-hydroxyanthraquinone-induced colon carcinogenesis.

Early detection and prevention of colorectal cancer (review)

Colorectal cancer is a leading cause of cancer-related deaths, and the two most important considerations for avoidance of this disease are early detection and prevention. If metastasis has occurred to distant sites, such as the liver and lung, the 5-year survival rate for colorectal cancer is below 10%, but this increases to greater than 90% when the cancer is found early. Early detection can be facilitated by use of the digital rectal exam, fecal occult blood test, sigmoidoscopy, and colonoscopy, but these methods might be supplemented in the future by other screening assays using intermediate biomarkers. One interesting biomarker, the aberrant crypt focus (ACF), has been observed in resected human colons, and is the earliest detectable morphological change in the colons of experimental animals treated with carcinogens such as the cooked meat heterocyclic amines. The ACF can also be used as an end-point to screen for potential inhibitors of colorectal cancer; using this approach, we identified conjugated linoleic acids, indole-3-carbinol, chlorophyllin, and tea polyphenols as promising inhibitors in the colon. These compounds can be added to a growing list of natural and synthetic agents that might be effective against colorectal cancer, including selenium, calcium, and nonsteroidal anti-inflammatory agents. However, results from human clinical trials with several of these compounds have highlighted the need for detailed mechanism data before recommendations can be made for wide-scale use in humans. In the meantime, the best approach to reducing the risk of colorectal cancer would be to increase the dietary intake of fruits, vegetables and cereals, while reducing the overall intake of fat, particularly from animal sources.

Antimutagenic activity of tea towards 2-hydroxyamino-3-methylimidazo[4,5-f]quinoline: effect of tea concentration and brew time on electrophile scavenging

Green tea and black tea inhibit colon carcinogenesis in rats exposed to the cooked meat-derived mutagen 2-amino-3-methylimidazo[4, 5-f]quinoline (IQ). The present study compared the inhibitory activities of green tea and black tea towards a direct-acting mutagenic metabolite of IQ, namely 2-hydroxyamino-3-methylimidazo[4, 5-f]quinoline (N-hydroxy-IQ), under various brewing conditions. The following observations were made: (a) green tea (Sencha midoriiro) and black tea (English Breakfast tea) brewed at concentrations of 1. 25%, 2.5% or 5.0% (w/v) dose-relatedly inhibited the mutagenic activity of N-hydroxy-IQ in the Salmonella assay, (b) most of the antimutagenic components were released from the teas within 1-2 min of brewing, (c) under identical brewing conditions, green tea was significantly more effective than black tea, and (d) fractionation of green tea by HPLC revealed that most of the antimutagenic activity co-eluted with the compounds epigallocatechin (EGC) and epigallocatechin-3-gallate (EGCG), both of which are known for their anti-oxidant properties. These results suggest that catechins in tea might protect against such diverse reactive intermediates as free radicals and electrophiles formed during the metabolic activation of carcinogens and mutagens.

High frequency of beta-catenin (ctnnb1) mutations in the colon tumors induced by two heterocyclic amines in the F344 rat

Activating mutations in the beta-catenin (CTNNB1) gene corresponding to N-terminal phosphorylation sites in the protein have been implicated in the development of human colon cancer. To determine the possible involvement of such mutations during chemically induced colon carcinogenesis, we examined the corresponding region of Ctnnb1 in colon tumors induced in the F344 rat by two cooked meat heterocyclic amines, 2-amino-3-methylimidazo[4,5-f]quinoline and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). All of the colon tumors induced by 2-amino-3-methylimidazo[4,5-f]quinoline that were examined (5 of 5) and 4 of 7 PhIP-induced colon tumors had mutations within or flanking codons corresponding to important phosphorylation sites in beta-catenin. None of the colon tumors bearing Ctnnb1 mutations had genetic changes in the Apc gene, and those that contained wild-type Ctnnb1 were known from our previous work to contain Apc mutations. The results provide evidence for a major role of the beta-catenin/Apc pathway in the development of heterocyclic amine-induced colon tumors and give further weight to the view that regulation of beta-catenin is critical to the tumor suppressive effects of Apc during colon carcinogenesis. In contrast, Ctnnb1 mutations were completely absent in 23 PhIP-induced mammary tumors, in accordance with recent work showing that human breast carcinomas lack mutations in CTNNB1.

Indole-3-carbinol: anticarcinogen or tumor promoter in brassica vegetables?

Indole-3-carbinol (I3C) is one of several compounds in brassica vegetables that demonstrate anticarcinogenic effects in experimental animals. A review of Medline and CancerLit databases indicated that interest in I3C, as a cancer chemopreventive agent, has increased significantly in the past 5-10 years. Whereas most studies report inhibitory or protective effects of I3C in vivo, a few provide clear evidence for promotion or enhancement of carcinogenesis, depending upon the initiator, exposure protocol and species. In the absence of detailed information on the inhibitory and in particular, promotional mechanisms, it would seem advisable to proceed with caution before including I3C in extensive human clinical trials.

Inhibition of 2-amino-3-methylimidazo[4,5-f]quinoline-DNA adducts by indole-3-carbinol: dose-response studies in the rat colon

Indole-3-carbinol (I3C) inhibits the formation of colonic aberrant crypt foci and DNA adducts in rats given heterocyclic amine colon carcinogens, such as 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). Mechanism studies indicate that I3C induces cytochromes P4501A1 and 1A2 (CYP1A1 and CYP1A2), isozymes that respectively metabolize IQ via ring hydroxylation or activate the carcinogen by N-hydroxylation. The present study examined the dose-response for induction of CYP1A1 versus CYP1A2 by I3C, and compared the profiles of induction with the dose-response for inhibition of IQ-DNA adducts in the colon of the F344 rat. Dietary equivalent doses of I3C in the range 100-1000 p.p.m. increased in a dose-related manner both ethoxyresorufin O-deethylase (EROD) and methoxyresorufin O-demethylase (MROD) activities in the liver and colonic mucosa, and Western blots showed a corresponding induction of CYP1A1 and CYP1A2 proteins. However, dietary equivalent doses of I3C in the range 10-25 p.p.m. (i) reduced hepatic EROD and MROD activities and CYP1A protein levels compared with controls, (ii) increased the ratio of CYP1A2 versus CYP1A1, and (iii) activated IQ to a more potent mutagen when liver microsomes from rats given I3C were used for metabolic activation in the Salmonella assay. Rats given a single oral dose of I3C shortly before administering IQ (5 mg/kg body wt, p.o.) exhibited dose-related inhibition of colonic IQ-DNA adducts in the range 25-100 p.p.m. I3C, reaching 95% inhibition at doses > or = 100 p.p.m. I3C, but IQ-DNA adducts were elevated slightly at the lowest I3C dose as compared with the controls. The possible significance of the low versus high dose effects of I3C are discussed in the context of human dietary exposures to I3C and the reported chemopreventive mechanisms of I3C in vivo.

Protection by green tea, black tea, and indole-3-carbinol against 2-amino-3-methylimidazo[4,5-f]quinoline-induced DNA adducts and colonic aberrant crypts in the F344 rat

Male F344 rats were exposed for 8 weeks to extracts of green tea (2% w/v) or black tea (1% w/v), or to 0.1% dietary indole-3-carbinol (I3C). In weeks 3 and 4 of the study, rats were given 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) every other day by oral gavage (50 mg/kg body wt) in order to induce aberrant crypt foci (ACF) in the colon. Compared with controls given IQ alone, all three inhibitors reduced the number of total aberrant crypts per colon, and green tea and I3C inhibited significantly the mean number of ACF (P < 0.05). Rats pre-treated with green tea, black tea, or I3C and given a single p.o. injection of 50 mg IQ/kg body wt 24-48 h before sacrifice had reduced levels of IQ-DNA adducts in the liver, and excreted lower amounts of IQ and other promutagens in the urine and feces. Inhibitors also reduced the excretion of IQ-sulfamate in the urine, but increased the relative amounts of IQ-5-O-sulfate and IQ-5-O-glucuronide. Western blotting together with assays for 7-ethoxyresorufin O-deethylase and methoxyresorufin O-demethylase established that I3C preferentially induced cytochrome P4501A1 over 1A2, consistent with the altered profile of urinary metabolites. However, both teas caused slight induction of cytochrome P4501A2 versus 1A1, which would be predicted to enhance the activation of IQ. Thus, green tea and black tea are likely to protect against IQ-DNA adducts and ACF by mechanisms other than induction of cytochromes P450, such as inhibition of enzymes which activate IQ or the scavenging of reactive intermediates.

Protection by chlorophyllin and indole-3-carbinol against 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP)-induced DNA adducts and colonic aberrant crypts in the F344 rat

The most abundant heterocyclic amine in fried ground beef, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), induces colon carcinomas in the male F344 rat. The potential chemopreventive effects of two compounds, namely, the 'interceptor molecule' chlorophyllin (CHL) and a modulator of carcinogen activation, indole-3-carbinol (I3C), were examined in a PhIP colon carcinogenesis model. During weeks 3 and 4 of a 16-week study, F344 rats were given PhIP by oral gavage (50 mg/kg body weight, alternating days). Inhibitors were given either before and during PhIP exposure, after PhIP treatment, or continuously for 16 weeks. Treatment of rats with 0.1% CHL in the drinking water inhibited the formation of aberrant crypt foci (ACF) with > or = 4 crypts/focus, from 1.4 +/- 0.9 in controls to 0.7 +/- 0.3 following post-initiation CHL treatment, and to 0.3 +/- 0.5 in rats given CHL continuously for 16 weeks (mean +/- SD; P < 0.05). Potent inhibition of PhIP-induced ACF occurred following initiation, post-initiation and continuous exposure to 0.1% I3C in the diet. Using the initiation protocol, I3C completely inhibited the induction of the ACF with > or = 4 crypts/focus. In a separate experiment, rats were given 0.1% CHL in the drinking water or 0.1% I3C in the diet for 4 weeks. At the end of week 3, animals received 50 mg PhIP/kg body weight by single oral gavage and PhIP-DNA adducts were quantified in the colon and several other tissues by 32P-postlabeling analysis. In addition, the urine and feces were collected to study the effects of inhibitor treatment on PhIP metabolism and excretion. No significant protection against PhIP-DNA adduct formation was detected in the colon after CHL dosing, nor was a consistent pattern of CHL inhibition observed in several other tissues. In contrast, I3C shifted the time-course of adducts in all tissue; compared with controls, adducts were increased by I3C at 6 h but decreased at 24 h and 7 days following PhIP treatment. Analysis of urine metabolites revealed that I3C and CHL decreased the excretion of unmetabolized PhIP and 4'-hydroxy- << PhIP but increased the phase II detoxification products PhIP-4'-O-glucuronide and PhIP-4'- sulfate. In the feces, the elimination of unmetabolized PhIP was increased from 54.5% in controls to approximately 67% in CHL-treated rats and decreased to 28% in rats given I3C (P < 0.05). These results support a protective role for CHL and I3C against PhIP-induced colon carcinogenesis through mechanisms which alter the uptake or metabolism of the carcinogen, and by suppression in the post-initiation phase.

Protection of conjugated linoleic acids against 2-amino-3- methylimidazo[4,5-f]quinoline-induced colon carcinogenesis in the F344 rat: a study of inhibitory mechanisms

Grilled ground beef contains a number of heterocyclic amine carcinogens, such as 2-amino-3-methylimidazo[4,5-f] quinoline (IQ), as well as anticarcinogenic conjugated linoleic acids (CLA). In the present study, CLA was administered to male F344 rats by gavage on alternating days in weeks 1-4, while IQ was given by gavage every other day in weeks 3 and 4 (100 mg/kg body wt). Rats were killed 6 h after the final carcinogen dose 16 in order to score colonic aberrant crypt foci (ACF). In the ACF study, CLA had no effect on the size of the foci, but inhibited significantly (P < 0.05) the number of ACF/colon, from 4.3 +/- 2.4 in controls to 1.1 +/- 1.3 in CLA-treated rats (mean +/- SD, n = 10). Rats given CLA also had significantly lower IQ-DNA adducts in the colon as determined by 32P-postlabeling analysis; relative adduct labeling levels (RAL x 10(7) for the major adduct were 9.13 +/- 2.6 in controls versus 5.42 +/- 1.8 in CLA-treated animals (P < 0.05). Mechanism studies indicated that CLA and other fatty acids interact with certain heterocyclic amines in a manner consistent with substrate-ligand binding. However, no such interaction occurred with IQ, and CLA failed to inhibit significantly the mutagenicity of N-hydroxy-IQ in the Salmonella assay. Liver microsomes from CLA-treated rats exhibited lower activities for dealkylation of 7-ethoxyresorufin and methoxyresorufin and activated IQ to DNA binding species less effectively than microsomes from control animals. Direct addition of CLA to the in vitro incubation inhibited IQ-DNA binding and was associated with increased recovery of unmetabolized parent compound. In the Salmonella assay, CLA inhibited the mutagenic activity of IQ in the presence of S9 or ram seminal vesicle microsomes. Collectively, these results support a mechanism involving inhibition of carcinogen activation by CLA, as opposed to direct interaction with the procarcinogen, scavenging of electrophiles or selective induction of phase I detoxification pathways.

Mechanisms of indole-3-carbinol (I3C) anticarcinogenesis: inhibition of aflatoxin B1-DNA adduction and mutagenesis by I3C acid condensation products

Possible inhibitory mechanisms of indole-3-carbinol (I3C) against aflatoxin B1 (AFB1), a potent hepatocarcinogen, were examined in rainbow trout. In the Salmonella assay using a trout post-mitochondrial activation system, I3C itself was not an antimutagen against AFB1. The study also evaluated: the antimutagenic ability of I3C oligomers; an acid reaction mixture (RXM) of I3C, generated at low pH to simulate I3C products formed under acidic conditions of the stomach; 3,3-diindolylmethane (I33'), the major derivative of I3C found in trout liver; and 5,6,11,12,17,18- hexahydrocyclononal [1,2-b:4,5-b':7,8-b"]triindole , the cyclic trimer of I3C (CT), a derivative of I3C in liver and one of the major components of RXM. Concentrations of 3.5 microM and greater of I33', CT or RXM showed about 80% inhibition compared with the control. Higher concentrations (70 microM) of the various I3C oligomers also inhibited (to a maximum of 55%) mutagenesis of synthetic AFB1-8,9-epoxide added to the Salmonella assay, in the absence of activating enzymes. I33' inhibited total microsome catalysed AFB1-DNA binding in vitro in an apparently non-competitive manner (Kis = 27.6 +/- 9.4 microM, Kii = 37.5 +/- 32.2 microM). These results suggest that the anticarcinogenic effect of I3C against AFB1 in rainbow trout, and perhaps other species, is due in part to inhibition of AFB1 bioactivation enzymes and to scavenging of the activated AFB1-8,9-epoxide.

Inhibition by chlorophyllin of 2-amino-3-methylimidazo-[4,5-f]quinoline-induced tumorigenesis in the male F344 rat

This report describes a 1-year carcinogenicity bioassay in which male F344 rats received 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) by gavage 2-3 times per week throughout the first 35 weeks of study, while 0.1% sodium/copper chlorophyllin (CHL) was given concomitantly in the drinking water. The carcinogen dose was calculated to provide an average exposure of 4.2 mg IQ/rat per day, equivalent to that reported in previous carcinogenicity study in which rats were given 0.03% IQ in the diet. Adjusted for length of follow-up, inhibition was highly significant for total tumor burden (P < 0.001), and for specific sites was significant in the small intestine and Zymbal's gland (P < or = 0.1), and in the liver (P < 0.01). In rats given CHL, the first onset of tumors in the Zymbal's gland was delayed from week 20 to 36, but was shortened from week 31 to 26 in the skin (P < 0.05). These results support a chemopreventive role for CHL in several of the major target organs for IQ tumorigenesis in the rat, but raise concerns over possible deleterious effects in the skin.

Regulation of hepatic cytochrome P4501A by indole-3-carbinol: transient induction with continuous feeding in rainbow trout

This study investigated the kinetics of hepatic cytochrome P-4501A (CYP1A) induction in rainbow trout by indole-3-carbinol (I3C), a natural tumour modulator from cruciferous vegetables, and its low pH reaction products 3,3'-diindolylmethane (I33'), 5,6,11,12,17,18-hexahydrocyclononal[1,2-b:4,5-b':7,8-b"]triindo le cyclic trimer (CT), and the unresolved I3C acid reaction mixture (RXM). RXM, CT and I33' were potent inducers of total embryonic CYP1A following direct microinjection, and of fingerling hepatic CYP1A following ip exposure, whereas I3C itself produced only a transient and relatively weak induction. It is also reported for the first time that dietary I3C induced hepatic CYP1A and its associated ethoxyresorufin O-deethylase (EROD) activity in trout but, again, the induction was weak and transient even with continuous I3C feeding. Mechanism studies and mixed exposures with the Ah agonist beta-naphthoflavone indicated that transient induction by I3C was not due to diet ageing, but appears to involve inactivation of the Ah inductive pathway and irreversible inactivation of CYP1A-mediated EROD activity by I3C-derived metabolites. Thus, I3C derivatives exhibit dual capacities for CYP1A induction and inhibition in trout.

Anticarcinogenic activity of indole-3-carbinol acid products: ultrasensitive bioassay by trout embryo microinjection

The relative contribution of indole-3-carbinol (I3C) and its acid condensation products to the anticarcinogenic activity of this crucifer phytochemical has been studied using trout embryo microinjection. I3C was treated with 0.07 N HCl to give a reaction mixture (RXM) comprising < 0.5% parent compound and over 20 products, the most prevalent being the dimer 3,3'-diindolylmethane (I33') and a related cyclic trimer (CT). RXM, I33' or CT was injected into embryos with [3H]aflatoxin B1 (AFB1) and total embryonic DNA was isolated 1, 3, or 10 days postinjection. Compared with controls given AFB1 alone, I3C failed to inhibit carcinogen-DNA binding at any time point. In contrast I33', CT, and RXM inhibited AFB1-DNA binding by an average of 37, 51, and 65%, respectively. Coinjection of AFB1 and 350 microM I3C, RXM, or I33' into trout embryos reduced AFB1-induced hepatocarcinogenesis after 1 year from 43.4% in positive controls to 36.0, 12.2 (P < 0.05), and 24.6% (P < 0.05), respectively. No tumor data were obtained in the AFB1 plus CT group due to poor survival of the embryos posthatching. These results indicate that acid condensation products, not the parent compound, represent the anticarcinogenic species in trout and that their formation in the stomach is a likely prerequisite for I3C anticarcinogenesis.

Carcinogenicity, metabolism and Ki-ras proto-oncogene activation by 7,12-dimethylbenz[a]anthracene in rainbow trout embryos

Field studies suggest that recent epizootics of hepatic neoplasms in some feral fish populations are associated with polycyclic aromatic hydrocarbon (PAH) exposure, but attempts to induce liver tumors in these species under laboratory conditions have been unsuccessful. Several studies have shown hepatic neoplasma to be inducible in laboratory fish species following PAH exposure at the free-swimming life stage. However, neither the susceptibility of the fish embryonic life stage to tumor induction by PAHs nor the potential of these carcinogens to induce oncogenic point mutations analogous to those reported in feral fish hepatic tumors have been clearly established. To address this, rainbow trout embryos were exposed by passive water uptake to 7,12-dimethylbenz[a]anthracene (DMBA), a potent model PAH in many mammalian tumor protocols. DMBA was rapidly absorbed by trout eggs and metabolized. The major non-polar metabolites identified were 12-hydroxymethyl-7-methylbenz[a]anthracene and 3,4-dihydroxy-3,4-dihydro-DMBA, whereas approximately 25% of the water soluble metabolites were identified as glucuronides by beta-glucuronidase treatment. Embryonic DNA adduction increased with time of DMBA exposure (2.2 +/- 0.3 pmol DMBA-equivalents/mg DNA at 24 h). Liver tumor incidence nine months after DMBA treatment was found to increase with DMBA concentration and exposure period (3.8% at 1 p.p.m./2 h; 23% at 5 p.p.m./2 h; 85% at 5 p.p.m./24 h). Stomach adenomas and nephroblastomas also were observed at low incidence in the DMBA-treated trout. Among 11 hepatic tumors examined, nine carried Ki-ras alleles with activating point mutations in codon 12 (4/11 GGA-->AGA; 4/11 GGA-->GTA) or codon 61 (1/11 CAG-->CTG). This spectrum differs substantially from those reported for DMBA-initiated mouse skin papillomas or hepatic tumors. These results may have important environmental implications because they suggest that even a brief exposure to PAHs during a sensitive stage of development may adversely affect some fish populations. They also indicate considerable variation in DMBA ras gene mutations among species and target organs.

Protection by chlorophyllin against the covalent binding of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) to rat liver DNA

Chlorophyllin (CHL), a sodium/copper salt of chlorophyll used in the treatment of geriatric patients, exhibits potent antimutagenic activity in a range of assays in vitro and in vivo. The protective effects of CHL were studied in Sprague-Dawley rats using inhibition of carcinogen-DNA binding as an end-point. Animals were administered CHL (150 mg/kg body wt) and [2-14C]2-amino-3-methylimidazo[4,5-f]quinoline (IQ, 50 mg/kg body wt) by single oral gavage. Covalent IQ-DNA binding in liver was determined 8, 24 and 48 h after dosing; CHL inhibited binding at these times by 58, 56 and 46% respectively, compared with rats given IQ alone. The total liver burden of IQ-derived radioactivity was reduced in CHL-treated rats, as was the total amount of radiolabel eliminated in the urine and bile. However, elimination via the feces was increased in rats given CHL, both in terms of total radiolabel eliminated and amount of unmetabolized IQ in dichloromethane extracts of feces. Finally, pretreatment with CHL in the drinking water, or injection of CHL into isolated loops of intestine in situ, reduced the absorption of IQ from the gut. Collectively, these findings indicate that, when administered simultaneously with the carcinogen, CHL attenuates IQ-DNA binding in rat liver by interacting with IQ in the gut and reducing carcinogen uptake, distribution and metabolism. The results suggest that further studies should be conducted with respect to the protective mechanisms and possible anti-carcinogenic properties of CHL.

Chemopreventive properties of chlorophyllin: inhibition of aflatoxin B1 (AFB1)-DNA binding in vivo and anti-mutagenic activity against AFB1 and two heterocyclic amines in the Salmonella mutagenicity assay

Chlorophyllin (CHL), a sodium/copper derivative of chlorophyll, has been used to treat a number of human conditions with no toxic effects being reported. Recent studies have described the anti-mutagenic activity of CHL in several short-term genotoxicity assays; however, this compound has not been reported to inhibit carcinogen--DNA binding in vivo, and it has yet to be evaluated as an anti-carcinogen in any species. The chemopreventive properties of CHL were studied in trout using inhibition of aflatoxin B1 (AFB1)--DNA binding as an end-point. Chlorophyllin and AFB1 were coadministered in the diet, and carcinogen--DNA binding levels were determined in liver after 1, 3, 5 and 7 days. Linear increases in AFB1--DNA binding occurred with time of treatment at each CHL dose level (0, 500, 1000 and 2000 p.p.m.). Each increase in CHL dose produced a concomitant decrease in AFB1--DNA binding, resulting in a series of curves of decreasing slope. At the highest CHL dose level of 2000 p.p.m., AFB1--DNA binding was inhibited by 70%. These results suggest that CHL should be a potent inhibitor of AFB1-induced hepatocarcinogenesis in this model. In the Salmonella assay, CHL exhibited potent anti-mutagenic activity against AFB1 and two heterocyclic amines when incubated in the presence of trout liver activation systems. CHL also inhibited the mutagenic activity of AFB1-8,9-epoxide in the absence of a metabolic activation system. Dietary CHL substantially inhibited liver AFB1-DNA binding in vivo, even when AFB1 was given by i.p. injection to avoid direct AFB1--CHL interaction in the diet or gut. Collectively, these studies support a CHL inhibitory mechanism involving complex formation with the carcinogen in the gut coupled with electrophile scavenging or further complexing in the target organ.

Promotion of aflatoxin B1 carcinogenesis by the natural tumor modulator indole-3-carbinol: influence of dose, duration, and intermittent exposure on indole-3-carbinol promotional potency

Indole-3-carbinol (13C), a secondary metabolite from cruciferous vegetables, inhibits aflatoxin B1 (AFB1) hepatocarcinogenesis in trout (Bailey et al., J. Natl. Cancer Inst., 78: 931-934, 1987) and rats (Selivonchick et al., unpublished results) when given prior to and with carcinogen but promotes carcinogenesis in both species when given continuously following AFB1 initiation. Since human 13C intake may not be continuous, and the promotional stimulation may be reversible, we have assessed 13C promotion using delayed and discontinuous exposure protocols. Following initiation with AFB1, 13C was fed to trout for varying periods of time, with varying lengths of delay after initiation and continuous or intermittent patterns of 13C treatment. Promotional enhancement of tumor incidence by 13C was found to be significant when 13C treatment was delayed for several weeks or months after the initial AFB1 challenge. Promotion also was found to increase with length of exposure to 13C treatment and to be decreased but still evident when 13C was given in alternating months or weeks, or twice per week only. These results do not support the idea that promotional stimulation in hepatocarcinogenesis is a reversible phenomenon. To quantify 13C promotional potency in terms of its dietary concentration, a series of AFB1 tumor dose-response curves was established, each with a different level of 13C fed continuously following AFB1 initiation. The resultant tumor dose-response curves, plotted as logit percentage of incidence versus log AFB1 dose, were displaced parallel toward lower AFB1 50% tumor take (TD50) values with increasing 13C concentration. The level of 13C that halves the AFB1 dose for 50% tumor incidence was calculated to be approximately 1000 ppm 13C, fed continuously, with no substantial threshold for promotion. By comparison, 13, when fed before and with AFB1, shows a 50% inhibitory value (13C concentration that doubles the dose of AFB1 for 50% tumor incidence) in trout of 1400 ppm 13C [Dashwood et al., Carcinogenesis (Lond.), 10: 175-181, 1989]. Thus the potential for 13C as a dietary additive to promote prior hepatic initiation events when fed continuously is approximately as great as its potential to inhibit concurrent AFB1 initiation.

Tumor dose-response studies with aflatoxin B1 and the ambivalent modulator indole-3-carbinol: inhibitory versus promotional potency

Indole-3-carbinol (I3C), a natural compound from cruciferous vegetables, inhibits aflatoxin B1 (AFB1) carcinogenesis in trout when administered prior to and during carcinogen exposure, but also promotes it in the same species when given after AFB1 initiation. To provide quantitative potency information for these opposing activities, detailed tumor dose-response studies were performed with AFB1 (10-400 ppb) and I3C (0-4,000 ppm). In a plot of (logit) percent tumor response vs log AFB1 exposure, the results generated a series of parallel AFB1 dose-response curves. Increasing I3C doses displaced these curves, respectively, toward higher and lower AFB1 doses in the inhibition and promotion studies. Similar potencies were observed over the dose range 0-1,500 ppm I3C; the 50% promotion and inhibition (P50 and I50) values were 1,000 vs 1,400 ppm I3C, respectively. Differences in the protocols used in the two studies suggest that the inhibitory activity of I3C is more likely to supersede promotion under human exposure conditions.

In vivo disposition of the natural anti-carcinogen indole-3-carbinol after po administration to rainbow trout

Indole-3-carbinol (I3C), a compound found naturally as a glucosinolate in cruciferous vegetables such as broccoli and cabbage, has been shown to modulate the carcinogenic process in a number of animal species. The lack of detailed information on the disposition of I3C in vivo provided the main impetus for the study reported here, in which the distribution and metabolic fate of I3C was assessed in selected tissues and excreta after po administration to rainbow trout (Salmo gairdneri). Animals were fasted for 3 days and given [5-3H]I3C either in the diet or by single oral gavage (40 mg/kg body weight; 15 muCi/kg body weight). Following administration, 75% of the initial 3H-dose was detected within the stomach between 0.5 and 12 hr, after which it was released to distal regions of the gut for subsequent uptake, distribution and elimination. At the end of the study (72 hr) 25% of the administered dose was recovered from the water which reflected excretion through the gills and urinary tract. Significant excretion also occurred in the bile, with approximately 5% of the initial 3H-dose recovered from the bile sacs at 72 hr. Further analyses of the radioactive components in the bile indicated that one or more derivatives of I3C, but not the parent compound itself, are excreted as glucuronide conjugates using this route. Radioactivity accumulated in the liver throughout most of the study, reaching levels of 1-1.5% between 48 and 72 hr of the administered dose. High-performance liquid chromatography analyses indicated the presence of four main radiolabelled species in these livers, one of which co-eluted with the parent compound, I3C. The major radiolabelled species recovered from the liver was tentatively identified as the dimer, 3,3'-diindolylmethane (I33'), which comprised 40% of the total hepatic radiolabel. This dimer, I33', was also found to accumulate in the diet containing I3C, which reflected a time-dependent dimerization of the parent compound in vitro. These findings are discussed in view of recent postulates of a role for I3C condensation products such as I33' in the mechanism of I3C anti-carcinogenesis.

Quantitative inter-relationships between aflatoxin B1 carcinogen dose, indole-3-carbinol anti-carcinogen dose, target organ DNA adduction and final tumor response

A number of recent studies have described inhibitor-mediated reductions in the covalent DNA binding and tumorigenicity of various carcinogens, in species such as rats, mice and rainbow trout (Salmo gairdneri). Since inhibitory effects have, in most cases, been reported after testing at one carcinogen and one inhibitor level only, the detailed relationships between carcinogen dose, inhibitor dose, in vivo DNA binding and final tumor response are not well understood in any species. To determine these relationships we have employed the trout model in a combined DNA binding/tumor dose-response protocol using approximately 10,000 animals. Trout were pretreated with one of five different dose-levels of indole-3-carbinol (I3C), a naturally occurring anti-carcinogen found in cruciferous vegetables such as broccoli and cabbage. After 4 weeks, animals received the same dietary level of I3C for a further 2 weeks together with [3H]aflatoxin B1 (AFB1) in the dose-range 10-320 p.p.b. From tanks containing 150 animals (three tanks per I3C-AFB1 dose-point), 15 fish were selected at random in order to assess hepatic AFB1-DNA binding levels. Remaining animals were returned to control diet for determination of tumor response at 12 months. Linear increases in DNA binding occurred with dose of AFB1 at each I3C dose-level. Successive increases in I3C dose gave dose-related decreases in AFB1-DNA binding, resulting in a series of curves of decreasing slope. Shifts in DNA-binding slopes were compared quantitatively with horizontal displacements towards higher carcinogen dose in corresponding tumor dose-response curves. At I3C doses of less than or equal to 2000 p.p.m., the inhibitor-altered tumor response was predicted precisely by changes in dose received (DNA adducts formed) in the target organ. These data constitute the first direct evidence of pure anti-initiating activity by a natural anti-carcinogen found in human diet, where all animals were treated at the same time and under identical conditions of exposure in both DNA binding and tumor studies. The data are discussed further in view of (i) their implications for DNA binding-carcinogenicity correlations and the concept of 'molecular dosimetry', and (ii) limitations in the current database on anti-carcinogenesis as regards in vivo potency information, particularly for 'ambivalent modulators' which exhibit both inhibitory and promotional activity.

Modulation of diethylnitrosamine-induced hepatocarcinogenesis and O6-ethylguanine formation in rainbow trout by indole-3-carbinol, beta-naphthoflavone, and Aroclor 1254

Rainbow trout were fed a diet containing indole-3-carbinol (2000 ppm), beta-naphthoflavone (500 ppm), or Aroclor 1254 (100 ppm) for 6 weeks before a single 24-hr exposure to an aqueous solution of 250 ppm diethylnitrosamine (DEN). The fish were killed 42 weeks later to determine the carcinogenic response. DEN exposure produced an 80.2% incidence of liver tumors and an average of 3.47 tumors per tumor-bearing fish, whereas no tumors were detected in the sham-treated control fish. Tumor induction was inhibited by indole-3-carbinol (27.5% incidence, 1.89 tumors per tumor-bearing fish) but enhanced by beta-naphthoflavone (91.8% incidence, 3.60 tumors per tumor-bearing fish). Aroclor 1254 had no effect on DEN-induced hepatocarcinogenesis (80.0% incidence, 3.03 tumors per tumor-bearing fish). The effects of these modulators on O6-ethylguanine and 7-ethylguanine formation (measured by HPLC and fluorescence spectrophotometry) were examined. Liver DNA ethylguanine levels were reduced in indole-3-carbinol-pretreated fish and increased in beta-naphthoflavone-pretreated fish compared to untreated controls after DEN exposure. Aroclor 1254 pretreatment had no significant effect on DNA ethylguanine formation. Similar O6-ethylguanine to 7-ethylguanine ratios were found among the control and treated groups. The results of this study indicate that modulation of DEN hepatocarcinogenesis by indole-3-carbinol and beta-naphthoflavone may be mediated by their effects on O6-ethylguanine formation and, therefore, on the initiation phase of carcinogenesis.

Formation and persistence of ethylguanines in liver DNA of rainbow trout (Salmo gairdneri) treated with diethylnitrosamine by water exposure

Diethylnitrosamine exposure via the water resulted in the formation of 7-ethylguanine and O6-ethylguanine in rainbow trout liver DNA. The modified bases were quantitated by high-pressure liquid chromatography and fluorescence spectrophotometry. In vivo 7-ethylguanine and O6-ethylguanine levels were directly proportional to DEN concentrations between 62.5 and 250 ppm. 7-Ethylguanine and O6-ethylguanine levels were approximately directly proportional to duration of exposure to DEN between 0 and 6 hr under the conditions used, with less than proportionate increases thereafter. Removal of ethylguanines from liver DNA following a 24-hr exposure to 250 ppm DEN (a known carcinogenic regimen) was biphasic; 24% of the O6-ethylguanine and 32% of the 7-ethylguanine found immediately after exposure were removed in 12 hr but no significant decline was found over the period from 12 to 96 hr after exposure. Alkyl acceptor protein activity in trout liver was examined to assess the role of enzymatic repair in the observed loss of O6-ethylguanine in vivo. Although an O6-alkylguanine repair system similar to the alkyltransferase system reported in rodents was found in trout liver, only 4% of the O6-ethylguanine lost from DNA after exposure to 250 ppm DEN can be accounted for by activity of the alkyl acceptor protein. The high incidence of liver tumours observed in DEN-treated rainbow trout may be related to the rapid formation and substantial persistence of the promutagenic O6-ethylguanine in liver DNA.

DNA-binding studies with 6BT and 5I: implications for DNA-binding/carcinogenicity and DNA-binding/mutagenicity correlations

The divergent activities of a reported carcinogen/noncarcinogen pair of monoazo dyes related to the hepatocarcinogen Butter Yellow (DAB) are currently under investigation in our laboratories. As part of these studies we have determined (a) target organ distribution after oral dosing to rats and (b) covalent binding of 14C-labelled compound to DNA. In DNA-binding studies, 3 rat liver-metabolising systems were employed: in vivo (whole liver), isolated intact hepatocytes, and liver subcellular fractions. Distribution studies revealed that comparable levels of both compounds were detected in the liver at similar times after dosing, and these in vivo tissue concentrations were used for in vitro DNA-binding studies. At this 'in vivo equivalent dose', the carcinogen was consistently bound to DNA more effectively, and the difference (ratio of DNA binding) between the 2 compounds was far greater in vivo. In subsequent studies, covalent DNA binding to bacterial (Salmonella) DNA was assessed at the in vivo equivalent dose. In contrast to the afore-mentioned findings in mammalian systems, the carcinogen was bound less effectively to DNA, and gave fewer revertant counts/plate when the 2 compounds were bound to an equivalent extent. These data are discussed in view of their implications for DNA-binding/carcinogenicity correlations, and with respect to the relationship between DNA binding and mutagenicity in the Salmonella assay.

Mechanisms of anti-carcinogenesis by indole-3-carbinol: detailed in vivo DNA binding dose-response studies after dietary administration with aflatoxin B1

Several recent reports have described inhibitor-mediated reductions in the covalent binding of various carcinogens to DNA in vivo. The majority of these studies show inhibitory effects after testing at one inhibitor and one carcinogen dose level only. Consequently, the detailed relationships between inhibitor dose, carcinogen dose, and in vivo inhibitory potency have not been clearly delineated in any species. To systematically determine these relationships in vivo, rainbow trout (Salmo gairdneri) were exposed to a range of carcinogen (aflatoxin B1, AFB1) and inhibitor (indole-3-carbinol, I3C) doses by concomitant dietary exposure. Inhibitory potencies were then assessed using in vivo covalent binding of AFB1 to hepatic DNA as an end-point. Linear increases in DNA binding occurred with increasing dose of AFB1 and with time of inhibitor/carcinogen co-treatment, at each I3C dose level. Successive increases in inhibitor dose resulted in corresponding dose-related decreases in AFB1--DNA binding such that a series of curves of decreasing slope was produced. AFB1--DNA binding was suppressed by almost 95% at the highest I3C dose tested. These studies describe for the first time such a degree of inhibition by I3C on covalent binding of AFB1 to DNA in vivo, where inhibitor and carcinogen are covariables administered repeatedly in the diet. Moreover, the linear inhibitory response observed at low I3C doses indicates the possible absence of any significant threshold for I3C protection against AFB1--DNA binding. Thus, even at low levels I3C may offer some protection against chemically-induced neoplasia.

Mutagenicity to Salmonella of four derivatives of the azo mutagen 5I: some implications for structure--activity databases and the evaluation of combinations of mutagens

A structure--activity study is described in which four new derivatives of the potent bacterial mutagen 5-dimethylaminophenylazoindazole (5I) have been evaluated for mutagenicity to Salmonella. As expected, monodemethylation of the -NMe2 group of 5I increased its mutagenic potency while replacement of the -NMe2 with a cyclic amine reduced it. However, replacement of the aromatic indazole -NH group (of 5I) by an -NMe group (yielding NMe5I) dramatically attenuated mutagenic potency, a reduction which was both unexpected and not reversed in the monomethyl analogue (NMeMA5I). In competition experiments NMe5I had an inhibitory effect on the mutagenic potency of 5I itself and on that of the nonazo mutagen 2-acetylaminofluorene 2AAF. The results illustrate some of the problems associated with evaluating mixtures for mutagenicity and of assuming simple structure--activity relationships in the absence of relevant experimental data.