DNA methylation based biomarkers in non-invasive cancer screening

DNA methylation plays a critical role in the regulation of gene expression, differentiation and in the development of cancer and other diseases. Hypermethylation of CpG islands located in the promoter regions of tumor suppressor genes is now firmly established as the most frequent mechanism for gene inactivation in cancers. Feasibility of using DNA methylation based biomarkers for early detection of cancer has been shown. Potential of using DNA methylation for prediction of therapeutic outcome and patient survival has also been shown. DNA originated from cancer cells has been routinely detected in clinical specimens (ex. Plasma/serum, sputum, urine etc.) from cancer patients. Presence of methylated DNA sequences in clinical specimens and potential of using them as biomarkers have been recognized. Novel methylation based biomarkers that can be used in clinical specimens, obtained non-invasively from cancer patients, offer significant practical advantages. More resources need to be committed to this area of biomarker research. Thus, we review recent findings on DNA methylation based cancer biomarkers with particular focus on these applicable to the clinical specimens obtained non-invasively from cancer patients.

Evidence against a role for simian virus 40 in malignant mesothelioma in Japan

10040

Background: Malignant mesothelioma is a highly aggressive tumor arising from serosal membranes, most commonly the pleura. Worldwide incidence is increasing because of widespread exposure to asbestos, the major causal agent. Incidence of this disease also is increasing dramatically in Japan. Association of simian virus 40 (SV40) with malignant mesothelioma has been reported, suggesting that SV40 plays an important role in the origin of a subset of these tumors. Most recently, evidence against any role for SV40 in this disease has been also reported. The role of SV40 in malignant mesothelioma remains still controversial. In addition, it has been argued that differences in the reported frequency of SV40 detection in malignant mesothelioma may be related to geographic variation in populations exposed to the virus. Whether SV40 is associated with malignant mesothelioma in Japan therefore is an important issue. However, no study concerning SV40 in malignant mesothelioma has been reported from Japan. Methods: To address this, we studied 35 malignant mesotheliomas including 32 men and 3 women with a median age of 61 years (ranges 34 to 85) and examined the presence of SV40 large T antigen DNA with real time PCR based on TaqMan technology using primers that PCR amplified a specific 156-bp region of the large Tag of SV40 as well as its expression with immnohistological methods using anti-SV40 large T antigen antibody (pAb101, Santa Cruz Biotechnology, Inc., CA). Results: Two of 35 mesotheliomas were considered positive for the presence of SV40 large T antigen DNA, showing ratios of 36.0 and 4.9. The ratio in the positive control was 199.0. The two positive cases consisted of one epithelioid tumor and one biphasic tumor. In addition, none of 35 malignant mesothelioma specimens were positve for staining with SV40 large T antigen antibody; in contrast, diffuse staining for SV40 large T antigen was observed in the cytoplasm and on the cell membranes in the positive control. Conclusions: Taken together, these findings strongly argue against any role of SV40 in the etiology of the majority of malignant mesothelioma in Japan.

No significant financial relationships to disclose.

DNA methylation-associated inactivation of TGFbeta-related genes DRM/Gremlin, RUNX3, and HPP1 in human cancers

The transforming growth factor β (TGFβ)-signalling pathway is deregulated in many cancers. We examined the role of gene silencing via aberrant methylation of DRM/Gremlin and HPP1, which inhibit TGFβ signalling, and RUNX3, which facilitates TGFβ-signalling, of all genes that are thought to be tumour suppressors, are aberrantly expressed, and are thus thought to have important role in human cancers. We examined DRM/Gremlin mRNA expression in 44 cell lines and the promoter methylation status of DRM/Gremlin, HPP1, and RUNX3 in 44 cell lines and 511 primary tumours. The loss of DRM/Gremlin mRNA expression in human cancer cell lines is associated with DNA methylation, and treatment with the methylation inhibitor-reactivated mRNA expression (n=13). Methylation percentages of the three genes ranged from 0–83% in adult tumours and 0–50% in paediatric tumours. Methylation of DRM/Gremlin was more frequent in lung tumours in smokers, and methylation of all three genes was inversely correlated with prognosis in patients with bladder or prostate cancer. Our results provide strong evidence that these TGFβ-related genes are frequently deregulated through aberrant methylation in many human malignancies.

Aberrant methylation of SPARC in human lung cancers

SPARC (secreted protein acidic and rich in cysteine) is an extracellular Ca²⁺-binding matricellular glycoprotein associated with the regulation of cell adhesion and growth. We investigated loss of expression of SPARC gene and promoter methylation in lung cancers and correlated the data with clinicopathological features. We observed loss of SPARC expression in 12 of 20 (60%) lung cancer cell lines. Treatment of expression-negative cell lines with a demethylating agent restored expression in all cases. Methylation frequencies of SPARC gene were 55% in 20 lung cancer cell lines. Primary tumours had methylation at a rate of 69% (119 of 173), while nonmalignant lung tissues (n=60) had very low rates (3%). In lung adenocarcinomas, SPARC methylation correlated with a negative prognosis (P=0.0021; relative risk 4.65, 95% confidence interval 1.75–12.35, multivariate Cox's proportional-hazard model). Immunostaining revealed protein expression in bronchial epithelium (weak intensity) and in juxtatumoral stromal tissues (strong intensity) accompanied by frequent loss in cancer cells that correlated with the presence of methylation (P<0.001). Our findings are of biological interest and potentially of clinical importance in human lung cancers.

Aberrant methylation and simian virus 40 tag sequences in malignant mesothelioma

Aberrant promoter methylation and resultant silencing of several genes plays an important role in the pathogenesis of many tumor types. We compared the methylation profile of 66 malignant mesotheliomas (MMs) and 40 lung adenocarcinomas using methylation-specific PCR for seven genes frequently methylated in lung cancer. We also compared the methylation frequencies of these genes as well as the methylation index, a reflection of all of the gene frequencies, with the presence of SV40 large T-antigen (Tag) sequences, histological subtype, and patient survival. Our major findings are: (a) with the exception of the RASSF1A promoter of the RASSF1 gene, frequencies of aberrant methylation were significantly lower in MMs than in adenocarcinomas; (b) the frequency of RASSF1A aberrant methylation and the value of the methylation index were significantly higher in SV40 sequence positive MM than in negative MM; and (c) the methylation index was higher in epithelial MM than in sarcomatous/mixed MM. Our results demonstrate a relationship between SV40 and aberrant methylation in MMs.

Deletions of chromosome 4 occur early during the pathogenesis of colorectal carcinoma

Allelic losses at one or both arms of chromosome 4 are frequent in several tumor types, but information about colorectal carcinoma is limited. We have previously defined 4 nonoverlapping regions of frequent deletions in several tumor types. In an effort to more precisely locate the putative tumor suppressor gene(s) on chromosome 4 involved in the multistage pathogenesis of colorectal carcinomas, we performed loss of heterozygosity (LOH) studies using 19 polymorphic microsatellite markers. After precise microdissection of archival surgical cases, we determined LOH in DNA obtained from 23 colorectal adenocarcinomas, 20 colorectal adenomas, and from corresponding histologically normal-appearing colonic epithelial samples adjacent to the tumors and at the resection margins. We observed localized deletions of chromosome 4 at multiple regions in both carcinomas and adenomas. We identified deletions at 4 previously identified regions: R1 at 4q33-34 (18%-33%), R2 at 4q25-26 (45%-65%), R3 at 4p15.1-15.3 (35%-47%), and R4 at 4p16.3 (40%-49%). Six of fifteen (40%) cases examined with deletions of chromosome 4 in either adenocarcinomas or adenomas had loss of the same parental alleles in adjacent histologically normal epithelium but not in epithelial samples from the surgical resection margins. The deletions, which commenced on the short arm of chromosome 4 (regions R3 and/or R4), were more extensive in adenocarcinomas, intermediate in length in adenomas, and least extensive in histologically normal epithelium. Our results suggest that there may be multiple putative tumor suppressor genes located on both arms of chromosome 4 whose inactivation are important early events in the pathogenesis of colorectal carcinoma.

Chromosome 4 deletions are frequent in invasive cervical cancer and differ between histologic variants

OBJECTIVE

Patterns of discontinuous deletion of chromosome 4 have been described in histologic variants of lung carcinomas and may represent different "hotspot" targets for gene-environment interactions. Since similar environmental risks exist for cervical cancer, we investigated patterns of discontinuous deletion in two major histologic variants.

METHODS

Thirteen archival cases of squamous cell cancer (SCCA) and 11 cases of adenocarcinoma (AC) were precisely microdissected. Matched normal and tumor DNA were used for polymerase chain reaction (PCR) based loss of heterozygosity (LOH) analyses using 19 polymorphic markers spanning chromosome 4. Human papillomavirus (HPV) detection was determined by PCR using general and type-specific primers (HPV 16, 18). Differences in LOH between histologic tumor types and chromosomal regions were determined using Fisher's exact test.

RESULTS

Loss at any chromosome 4 locus occurred in 92% of all tumors studied, with the majority of deletions occurring on the long arm of the chromosome. Four discrete minimal regions of discontinuous deletion (R) were identified. For these regions, LOH frequencies were 76% (R1, 4q34-q35), 48% (R2, 4q25-q26), 36% (R3, 4p15.1-p15.3), and 26% (R4, 4p16). Loss in SCCA predominated at 4q (4q34-q35; 83%) and in AC at 4p (4p15.3; 50%). Overall LOH on the p arm was significant in AC (82%) compared to SCCA (31%) (P = 0.02). HPV detection was similar in SCCA (85%) and AC (73%), and HPV 16/18 subtypes were similarly represented in both histologies.

CONCLUSIONS

Chromosome 4 deletions are frequent in cervical carcinomas. Different patterns of deletion between SCCA and AC may represent gene regions targeted by different gene-environment interactions in these tumor subtypes.

Comparative genomic hybridization reveals complex genetic changes in primary breast cancer tumors and their cell lines

DNA copy number changes were characterized by comparative genomic hybridization (CGH) in 18 breast cancer cell lines. In 5 of these, the results were comparable with those from the primary tumors of which the cell lines were established. All of the cell lines showed extensive DNA copy number changes, with a mean of 16.3 +/- 1.1 aberrations per sample (range 7-26). All of the cell lines had a gain at 8q22-qter. Other common gains of DNA sequences occurred at 1q31-32 (89%), 20q12-q13.2 (83%), 8q13 (72%), 3q26.1-qter (67%), 17q21-qter (67%) 5p14 (61%), 6p22 (56%), and 22pter-qter (50%). High-level amplifications were observed in all cell lines; the most frequent minimal common regions were 8q24.1 (89%), 20q12 (61%), 1q41 (39%), and 20p11.2 (28%). Losses were observed less frequently than gains and the minimal common regions of the most frequent losses were Xq11-q12 (56%), Xp11.2-pter (50%), 13q21 (50%), 8p12-pter (44%), 4p13-p14 (39%), 6q15-q22 (39%), and 18q11.2-qter (33%). Although the cell lines showed more DNA copy number changes than the primary tumors, all aberrations, except one found in a primary tumor, were always present in the corresponding cell line. High-level amplifications found both in primary tumors and cell lines were at 1q, 8q, 17q, and 20q. The DNA copy number changes detected in these cell lines can be valuable in investigation of tumor progression in vitro and for a more detailed mapping and isolation of genes implicated in breast cancer.

Presence of simian virus 40 sequences in malignant mesotheliomas and mesothelial cell proliferations

Malignant mesotheliomas (MMs) are pleural-, pericardial-, or peritoneal-based neoplasms usually associated with asbestos exposure. Mesothelial cells are biphasic and may give rise to epithelial and sarcomatous MMs. In addition, benign or atypical proliferations of mesothelial cells may occur in response to many stimuli. There have been recent reports of simian virus 40 (SV40) DNA large T antigen (Tag) sequences in pleural MMs. To further understand the relationship between SV40, MMs, and mesothelial proliferations, we studied 118 MMs from multiple sites in Germany and North America, including 93 epithelial pleural, 14 sarcomatous or mixed pleural MMs, and 11 peritoneal MMs. In 12 pleural MMs, adjacent noninvasive tumor foci were identified and studied separately. Information about asbestos exposure (detailed history and/or microscopic examination for asbestos bodies) was available from 43 German patients. In addition, 13 examples of reactive mesothelium and 20 lung cancers from the United States were tested. DNA was extracted from frozen tumor and adjacent nontumorous tissues or after microdissection of archival formalin-fixed, paraffin-embedded microslides. Two rounds of PCR were performed with primers SVFor 3 and SVRev, which amplify a 105 bp region specific for SV40 Tag. The specificity of the PCR product was confirmed in some cases by sequencing. Our major findings were: 1) Specific SV40 viral sequences were present in 57% of epithelial invasive MMs, of both pleural and peritoneal origin. No significant geographic differences were found, and frozen and paraffin-embedded tissues were equally suitable for analysis. 2) There was no apparent relationship between the presence of SV40 sequences and asbestos exposure. 3) SV40 sequences were present in the surface (noninvasive) components of epithelial MMs. 4) SV40 sequences were not detected in MMs of sarcomatous or mixed histologies. 5) Viral sequences were present in two of 13 samples (15%) of reactive mesothelium. 6) Lung cancers lacked SV40 sequences, as did non-malignant tissues adjacent to MMs. Our findings demonstrate the presence of SV40 sequences in epithelial MMs of pleural and peritoneal origin and their absence in tumors with a sarcomatous component. Viral sequences may be present in reactive and malignant mesothelial cells, but they are absent in adjacent tissues and lung cancers.

Multiple regions of chromosome 4 demonstrating allelic losses in breast carcinomas

Allelotyping studies suggest that allelic losses at one or both arms of chromosome 4 are frequent in several tumor types, but information about breast cancer is scant. A recent comparative genomic hybridization analysis revealed frequent losses of chromosome 4 in breast carcinomas. In an effort to more precisely locate the putative tumor suppressor gene(s) on chromosome 4 involved in the pathogenesis of breast carcinomas, we performed loss of heterozygosity studies using 19 polymorphic microsatellite markers. After precise microdissection of archival surgical cases, we analyzed DNA obtained from 44 breast carcinomas for loss of heterozygosity. In addition, DNA from tumor cell lines derived from 14 of these 44 breast carcinomas were also analyzed. We observed deletions of chromosome 4 at multiple sites in both tumor cell lines and breast carcinomas. The deletions in cell lines and their corresponding tumors were extensive in nature, whereas they were more localized in noncultured breast carcinomas. The localized deletions in the noncultured breast carcinomas clearly defined four nonoverlapping regions of frequent deletions: 4q33-34 (76%); 4q25-26 (63%); 4p15.1-15.3 (57%); and 4p16.3 (50%). Our results suggest that there may be multiple putative tumor suppressor genes, located on both arms of chromosome 4, whose inactivation is important in the pathogenesis of breast cancer.

Deletions of chromosome 4 at multiple sites are frequent in malignant mesothelioma and small cell lung carcinoma

Recent allelotyping studies suggest that allelic losses at one or both arms of chromosome 4 are frequent in several tumor types. Cytogenetic studies of malignant mesothelioma (MM) and comparative genomic hybridization analyses of small cell lung carcinoma (SCLC) suggest that chromosome 4 deletions may also play a role in these tumor types, although these results have not been confirmed by allelotyping. In an effort to more precisely identify and map the locations of putative tumor suppressor gene(s) on chromosome 4 involved in the pathogenesis of these tumors, we performed loss of heterozygosity studies using 16 polymorphic microsatellite markers. After precise microdissection of archival surgical cases, we studied DNA obtained from 20 MMs, 21 SCLCs, and 20 non-SCLCs (NSCLCs). In addition, DNA from 14 SCLC and 17 NSCLC cell lines and corresponding B lymphoblastoid lines were studied. In MM and SCLC, we observed frequent losses at three nonoverlapping regions: (a) 4q33-34 (region R1; >80%); (b) 4q25-26 (region R2; >60%); and (c) 4p15.1-15.3 (region R3; >50%). Losses at these sites occurred at lower frequencies in NSCLC (>20-30%). Data from tumors and cell lines were similar. In MM and SCLC, the most frequently observed pattern was loss at all three regions. However, in NSCLC, the most frequent pattern was loss at R3 alone. Our study has delineated three nonoverlapping regions of frequent deletions on chromosome 4 in MM and SCLC, suggesting that there may be three putative suppressor genes on chromosome 4, the inactivation of which may be important in the pathogenesis of these tumor types.

Suppression of intestinal polyp development by low-fat and high-fiber diet in Apc(delta716) knockout mice

Most epidemiological and animal studies show a positive correlation of the dietary intake of fat with the incidence of colon cancer, whereas an inverse correlation of the dietary intake of fiber. In rats fed a diet low in fat and high in wheat bran fiber and calcium, a significant decrease was reported in the number of azoxymethane-induced aberrant crypt foci compared with those fed a high-fat, low-fiber and low-calcium diet. Mutations in the human APC gene play a key role, not only in familial adenomatous polyposis, but also in many sporadic cancers of the entire digestive tract. We previously constructed a mouse strain Apc(delta716), carrying a truncation mutation at codon 716 of the Apc gene, the homolog of human APC (10). The heterozygous mice developed numerous intestinal polyps, and all microadenomas dissected from the earliest polyps had already lost the wild-type allele, indicating the loss of heterozygosity. Using these Apc(delta716) knockout mice, we have investigated the effect of a low-fat and high-fiber diet (LRD for 'low-risk' diet) on intestinal polyposis, and compared it with that of a high-fat and low-fiber diet (HRD for 'high-risk' diet). The mice were fed either diet for 7 weeks, and the number and size of intestinal polyps were scored. The LRD-fed mice had fewer polyps than the HRD-fed mice, by 36% in the small intestine and by 64% in the colon. As for the polyp size distribution, there was no significant difference between the HRD- and LRD-fed mice. These results indicate that LRD can suppress intestinal polyposis compared with HRD which does not, and suggest that its suppression is at the initiation of polyp formation. This is likely to be due to a decreased frequency of loss of heterozygosity, rather than a retarded growth of the polyp adenomas.

Inhibition by wheat bran cereals of the development of aberrant crypt foci and colon tumours

As variation in both type of fibre and its physical properties can influence physiological effects, the effects of different dietary levels (1, 4, 8%, w/w) of unprocessed wheat bran (WB) were compared with those of two of its processed commercial formulations used in breakfast cereals, on the formation of aberrant crypt foci (ACF) and colon tumours in Fischer 344 rats following azoxymethane (AOM) administration. All diets were high in fat (20 g/100 g) and low in calcium (0.2%, w/w). The rats were fed the experimental diets for 2 wk before receiving two sc injections of AOM (15 mg/kg body weight/wk). 8 wk following the first injection of AOM, five rats per group were killed and the formation of ACF was measured. 23 wk following the first injection of AOM, 12 rats per group were killed and the colon tumour incidence in different dietary groups was measured. The results showed that increasing the dietary concentration of fibre from 1 to 8% (w/w), using all the wheat bran formulations, significantly reduced the number of ACF per rat. None of the diets showed any significant effect on the normal growth of rats. No statistically significant differences were observed between the protective properties of WB and the two commercial formulations under investigation in terms of the reduction of the number of ACF, or in terms of the reduction of the colon adenocarcinoma incidence. The results suggest that wheat bran and its two commercial formulations can offer protection against colon cancer even when they are consumed with a high-fat/low-calcium diet. The addition of any of these formulations of wheat bran fibre is likely to be equally effective in the prevention of colon cancer in human populations that habitually consume high-fat/low-fibre Western-style diets.

K-ras and p53 mutations in aberrant crypt foci and colonic tumors from colon cancer patients

Aberrant crypt foci (ACF) are microscopic lesions which can be detected, after methylene blue staining, in the overtly normal looking colonic mucosa of cancer patients. ACF have been postulated to be precursor lesions which develop into colorectal cancer. Mutations of K-ras and p53 are two important genetic events implicated in colon carcinogenesis. Mutations in K-ras are detectable at earlier stages, while mutations in p53 are detectable at later stages of colon carcinogenesis. Our objective was to compare the nature of genetic alterations in K-ras (codon 12 and 13) and in p53 (exon 4-9) between ACF and corresponding colonic tumors from cancer patients. ACF with > or =20 crypts/focus were harvested from overtly normal looking colonic mucosa of cancer patients at a distance of (approx.) 5 cm from the site of colonic tumors. The colonic tumors and ACF samples were compared for K-ras codon 12 and 13 base pair sequence, using DNA sequencing and for p53 (exon 5-9) allelic types, using PCR-SSCP and DNA sequencing. The results demonstrated a perfect correlation in terms of the type of K-ras allele (wild or mutated) between the ACF (> or =20 crypts/focus) and corresponding colonic tumors in 11/13 cancer patients. Analyses of p53 mutations demonstrated the presence of p53 mutations in colonic carcinomas from 10/13 patients. However, p53 mutations could be detected in an ACF from only 1/13 patient. The results provides further evidence to the role of ACF as precursor to colon cancer. The presence of an identical K-ras as well as p53 mutation in an ACF and the corresponding colonic carcinoma in a patient suggests the possibility of existence of ACF that may be at a more advanced stage in the sequence of colonic tumorigenesis than others. In conclusion, the results suggest that a subset of ACF with higher multiplicity might be considered more likely to progress to more advanced lesions and should be explored as markers of colon cancer risk.

Absence of p53 gene mutations in rat colon carcinomas induced through the synergistic interaction between methylazoxymethanol and X-irradiation

p53 is one the most frequently mutated genes found in human colonic tumors. Because colonic neoplasms induced in rats by certain chemical carcinogens are similar to human colonic tumors in their histological features and proliferation characteristics, the rat has been used as an experimental model to study the pathogenesis of colon cancer. However, p53 mutations were not detected in the chemically induced colonic tumors analyzed for p53 mutations. X-irradiation has also been shown to induce colonic neoplasms in rats that resemble human colonic tumors histopathologically. Because the incidence of colonic tumors induced by methylazoxymethanol (MAM) in rats was shown to be enhanced by X-irradiation, we immunohistochemically analyzed these colonic carcinomas for the presence of p53 gene mutations. The immunohistochemical analyses clearly showed the absence of nuclear immunoreactivity in all ten tumors examined. The results from the present study indicate that point mutations in p53, at least in the coding region, are not involved in the development of colon cancer induced by the combination of MAM and X-irradiation. Our observations, together with the data from previous studies, further suggest that rat colon carcinogenesis, unlike human colon cancer, may not involve p53 mutation as an obligatory event.

Microsatellite instability in aberrant crypt foci from human colons

Aberrant crypt foci (ACF) are distinct microscopic lesions of the colon thought to be the earliest identifiable precursors of colon cancer. As precursors of colon cancer, ACF may contain mutations in genes that are altered early in colorectal tumorigenesis. Candidates for these genes include APC, K-Ras, and those of the DNA mismatch repair system. Some colon cancers with mutations in DNA mismatch repair genes are characterized by genomic instability at simple repeated sequences, also known as microsatellite instability. In this study, we analyzed 19 ACF (> or = 20 crypts/focus) and adjoining, microscopically normal colonic mucosa from 10 colon cancer patients for the presence of microsatellite instability. DNA from two ACF from two different patients displayed microsatellite instability. None of the DNA samples from normal mucosa displayed microsatellite instability. These observations support the role of ACF as a precursor to colon cancer and provide some evidence that mutations in DNA mismatch repair genes are early somatic events in colon cancer.

A rapid dual organ rat carcinogenesis bioassay for evaluating the chemoprevention of breast and colon cancer

In this study we evaluated the effect of dietary administration of a high fat, low fiber diet (HRD) with or without 2% phytic acid (PA) on the development of mammary cancer and/or colon cancer in rats exposed to methylnitrosourea (MNU), azoxymethane (AOM) or MNU + AOM. The rats were fed a HRD alone or a HRD + 2% PA. At the end of week 2, the rats were given either a s.c. injection of MNU (50 mg/kg body wt) or one of normal saline (vehicle). At the end of weeks 3 and 4, the rats were given either a s.c. injection of AOM (15 mg/kg body wt per week) or one of normal saline (vehicle). Nine weeks after the injection of MNU or saline, 10 rats from each group were sacrificed and the mammary tumor incidence and the number of colonic aberrant crypt foci (ACF) were compared between different groups. The administration of different diets was continued for an additional 21 weeks and the mammary tumor and colon tumor incidence between different groups were compared. Results showed that rats injected with MNU alone did not develop ACF or colon tumors while those injected with AOM alone did not develop mammary tumors. Linear regression analysis of the number of ACF at 11 weeks versus colonic tumor incidence at 32 weeks, and the linear regression analysis of mammary tumor incidence at 11 weeks versus mammary tumor incidence at 32 weeks, both showed good linear correlation. These results demonstrate the potential value of the short term dual organ carcinogenesis bioassay for screening chemopreventive agents for their relative ability to inhibit the development of mammary cancer and/or colon cancer while on high risk diet.

Dietary fiber and the chemopreventive modelation of colon carcinogenesis

Comparative international epidemiological data indicate that the difference between the highest and lowest colon cancer incidence is approximately 10-fold. This suggests that the dominant causes of colon cancer are environmental rather than genetic in origin, with the dominant environmental cause being the typical diet of Western industrialized countries. Many epidemiological and experimental studies have suggested an important role for dietary fiber in the prevention of colon cancer. Using the Fischer-344 rat as the experimental model, data clearly demonstrate a strong protective effect of a diet that is low in fat, high in fiber and high in calcium (low-risk diet). Such a diet prevents the development of both preneoplastic aberrant crypt foci (ACF) and colon tumors. Recent experiments have also demonstrated a direct relationship between a ras point mutation in ACF at different stages of rat colon carcinogenesis, and a ras point mutation that is subsequently present in colon tumors. Using wheat bran as the model dietary fiber source, its effects were compared to the effects of psyllium, phytic acid, vitamin E, beta-carotene, folic acid, alone or in combination, for their ability to prevent colon cancer in rats on high-risk Western-style diets. Our studies clearly demonstrated the ability of wheat bran to reduce ACF and colon tumors in rats that consumed high-fat, Western-style diets. Although phytic acid, which is a constituent of wheat bran, alone demonstrated strong cancer-preventive potential, our experiments provided evidence for the cancer-preventive effect of the crude fiber fraction that is independent of the effect of phytic acid. The synergistic combination of wheat bran with the soluble fiber psyllium led to enhanced protection; while the combination of wheat bran with beta-carotene showed only an additive effect. Beta-carotene appeared to show higher protection than wheat bran at an intake level that is nutritionally relevant to humans, suggesting the possibility of using beta-carotene to enhance the effects of dietary fiber in high-risk Western populations. Using ACF as an intermediate endpoint, it was also shown that vitamin E and beta-carotene appear to inhibit progression of ACF to colon cancer, while wheat bran and folic acid appeared to have weak cancer-preventive potential at this late stage of carcinogenesis. In conclusion, wheat bran alone, or in combination with psyllium, appears to have greater potential to inhibit earlier phases of carcinogenesis, while beta-carotene and vitamin E may also inhibit later stages of carcinogenesis. Despite considerable epidemiological and experimental evidence that increasing the fiber and lowering the fat content of the Western diet could substantially reduce the risk of cancer and heart disease, the real challenge is to find effective ways to educate and motivate people to overcome their intrinsic cultural resistance to such changes in their eating habits.

The effect of dietary fat on the promotion of mammary and colon cancer in a dual-organ rat carcinogenesis model

Female Sprague-Dawley rats (n = 270) were randomly divided into nine subgroups (30 rats/group) and fed diets with fat contents ranging from 5% to 45% of total energy, in which fat was isocalorically substituted for carbohydrates. At Week 3, the rats were given a single injection of methylnitrosourea (MNU, 50 mg/kg body wt). At Weeks 4 and 5, rats were given an injection of azoxymethane (AOM, 15 mg/kg body wt sc). The rats were maintained on their respective experimental diets until they were sacrificed. Nine weeks after the injection of MNU, 10 rats from each group were killed and their mammary tissue was examined for tumors and their colons for aberrant crypt foci (ACF) formation. The results showed no significant increase in the number of colonic ACF or mammary tumors at 11 weeks among individual groups consuming 5-25% energy from fat or among groups consuming 30-45% energy from fat. Data analysis indicated that the effects of the two diets (5-25% energy from fat and 30-45% energy from fat) on ACF formation as well mammary tumor development were statistically different. Continuation of the diets for 32 weeks led to the development of colonic and mammary tumors. Analysis of the combined group tumor incidence data clearly showed that the colonic and mammary tumor incidence in groups fed 30-45% of energy from fat was significantly higher than that observed among groups fed 5-25% of energy from fat. Linear regression analysis of the colonic ACF at 11 weeks, colonic tumor incidence at 32 weeks, and mammary tumor incidence at 11 and 32 weeks showed good linear correlation, demonstrating the potential value of ACF and mammary tumors at 11 weeks for evaluating the carcinogenic risk associated with different diets. The main conclusion of the experiment is that the incidence of colon and mammary cancers increased rapidly when the dietary levels of fat were increased from 15% to 30% of calories and that there was no significant influence on the colon and mammary cancer risk beyond 30% of calories from fat.

Absence of p53 gene mutations in rat colon carcinomas induced by azoxymethane

The K-ras and p53 genes are two of the most frequently mutated genes found in the human colonic tumors. Since azoxymethane (AOM) induced rat colonic neoplasms are similar to human colonic tumors in their histological features and proliferation characteristics, the rat has been used as an experimental model to study the pathogenesis of colon cancer in humans. Although the presence of K-ras point mutations has been reported in AOM induced rat colonic tumors, there are no reports describing the frequency for mutation of the p53 gene in these tumors. In this study, colon adenocarcinomas induced in rats by AOM were examined for the presence of point mutations in exons 5-8 of the p53 gene, using a combination of single strand conformation (SSCP) analysis, immunohistochemistry and direct DNA sequencing. SSCP analysis showed no differences in banding patterns between the normal mucosa and any of the 20 adenocarcinomas analyzed. Nuclear p53 immunoreactivity was absent in all tumors examined. Since p53 point mutations predominate in malignant colonic tumors, five adenocarcinomas with the greatest local invasiveness were analyzed by direct DNA sequencing of exons 5-8 of the p53 gene. Direct DNA sequencing did not reveal mutations in any of the adenocarcinomas analyzed, within the coding region of p53 gene that were sequenced. The results from the present study indicate that point mutations in the p53 gene, at least in the coding region (exons 5-8) are not involved in the development of colon cancer induced by AOM in the rat.

Inhibition of progression of aberrant crypt foci and colon tumor development by vitamin E and beta-carotene in rats on a high-risk diet

In this study we evaluated the effect of dietary administration of a high-fat, low-fiber diet (HRD) supplemented with Vitamin E, beta-carotene or folic acid and wheat bran on the growth of pre-existing aberrant crypt foci (ACF) that had been induced in Fischer-344 rats exposed to azoxymethane (AOM) and a HRD for 10 weeks. The rats (25 rats/dietary group) were fed a HRD for 2 weeks and were then given 2 subcutaneous injections of AOM (15 mg/kg body weight) while the rats continued on the HRD. After 6 weeks, rats were either maintained on the HRD (control) or crossed over to a HRD containing non-toxic levels of either Vitamin E, beta-carotene, folic acid or wheat bran. At 10, 14 and 18 weeks after the initiation of the experiment, 5 rats from each group were killed and the number of aberrant crypt foci (ACF) with different multiplicities were compared between groups. The dietary intervention was continued for 30 weeks to determine whether the inhibitory effect on the growth of ACF influenced the subsequent development of colonic tumors. The results revealed that vitamin E and beta-carotene caused a significant decrease in the number of ACF of different multiplicities when compared to the effect of the HRD alone. The decrease in the number of ACF due to folic acid and wheat bran appeared to be much smaller and in most cases was not significant. However, there was also a significant decrease in the incidence of colonic tumors and tumor multiplicity in both the vitamin E and beta-carotene groups that was not seen in the control group. The reports clearly demonstrates the ability of vitamin E and beta-carotene to inhibit the growth of colonic ACF, even in the presence of the strong promoting effect of high levels of dietary fat, using a post-initiation experimental design.

Effect of beta-carotene and wheat bran fiber on colonic aberrant crypt and tumor formation in rats exposed to azoxymethane and high dietary fat

The typical high-fat, low-fiber American diet promotes colon cancer. An alternative to radical changes in dietary habits is to reinforce the diet with cancer protectors. Experiments to evaluate the effects of beta-carotene in the presence of high fat and low and high dietary levels of wheat bran fiber were designed using the Fischer-344 rat colon cancer model. Rats (20/group), were given either high fat (20% w/w), low wheat bran, fiber (1% w/w) diets, or high fat (20% w/w) high wheat bran fiber (8% w/w) diets, with different levels of beta-carotene. After 2 weeks of adaptation, half were given two weekly s.c. injections of azoxymethane (AOM, 15 mg/kg body wt); and half two weekly s.c. injections of saline. Six weeks later, five rats from each dietary group were killed to evaluate the comparative effect of different dietary regimens on the induction of colon aberrant crypt foci (ACF). The remaining rats were maintained on their respective diets for an additional 20 weeks to examine the effect on colon tumor incidence. The total number of ACF/rat in the low-fiber groups declined from 44.0 +/- 4.18 to 12.8 +/- 1.95 in response to increasing amounts of beta-carotene from 1 to 20 mg/kg diet. A similar progressive reduction in total ACF/rat was also seen in the high-fiber groups (20.8 +/- 2.92 to 9.2 +/- 0.58). ACF did not develop in the saline-exposed groups. Similarly colon tumor incidence declined from 73% to 20% in high-fiber groups and from 27% to 13% in low-fiber groups in response to increasing amounts beta-carotene from 1 to 20 mg/kg diet. The results showed that beta-carotene and wheat bran, individually and when combined, protected the colon in rats consuming high-fat, western-style diets from ACF and benign or malignant tumor formation.

Sequential analysis of K-ras mutations in aberrant crypt foci and colonic tumors induced by azoxymethane in Fischer-344 rats on high-risk diet

Forty Fischer-344 male rats were given a high-risk diet (HRD) that was high in fat, low in fiber and low in calcium. After 4 weeks, the rats were given two weekly s.c. injections of azoxymethane (AOM, 15 mg/kg body wt), and remained on the same diet till death. Eight rats were killed at 12 weeks and again at 20 weeks in order to microdissect aberrant crypt foci (ACF) containing four or more crypts/focus from their colons. The remaining 24 rats were killed at 30 weeks to harvest colonic tumors. The polymerase chain reaction (PCR) was used to amplify specific DNA segments in the K-ras gene from ACF and colonic tumors. The PCR-amplified DNAs were sequenced to identify the point mutations in codons 12 and 13. All the mutations detected in the ACF and colonic tumors were G to A transitions in the second position of codon 12. These mutations were present in the ACF of 2/8 (25%) and 3/8 (37%) rats at 12 and 20 weeks respectively. The mutations were present in colonic tumors of 7/24 (29%) rats. These results provide important evidence for the significance of K-ras mutations in ACF (> 4 crypts/focus) as early markers of malignant potential in the colons of F344 rats exposed to AOM while receiving a high-risk western style diet.

Potential synergism between wheat bran and psyllium: enhanced inhibition of colon cancer

In this study we compared the influence of high fat (20% w/w) diets that combine low levels of calcium (0.18% w/w) and low (1% w/w), medium (4% w/w) and high (8% w/w) levels of dietary fiber from wheat bran (WB), with high (8% w/w) levels of dietary fiber from psyllium (PS) alone or in various combinations with WB, on the induction of colon tumors in Fischer-344 rats following exposure to azoxymethane (AOM). The rats were fed the experimental diets for 2 weeks, and then were given two s.c. injections of AOM (15 mg/kg body wt./week). Twenty-three weeks following the first injection of AOM, the incidence of colon tumors in the different dietary groups (12 rats/group) was compared. The results clearly showed that by increasing the dietary fiber concentration of WB from 1 to 8% significantly reduced the number of colon tumors/group. When the influence of 8% dietary fiber from WB on the development of colon tumors was compared with that of PS (WB:PS = 0:100), no significant difference was observed. However, combinations of WB and PS showed a greater protective effect than either WB or PS alone, at comparable levels of dietary fiber. The 50:50 combination of WB and PS showed maximum protection, while 25:75 and 75:25 combinations both produced intermediate effects. None of the diets showed any significant effect on the normal growth of rats. The results indicate that WB and PS fiber alone, and to a greater degree in combination, can offer protection against colon cancer promoted by high fat, low calcium diets. Diets that include wheat bran in combination with psyllium could be an effective means of reducing colon cancer risk in human populations addicted to high risk western diets.

The effect of high-risk and low-risk diets on aberrant crypt and colonic tumor formation in Fischer-344 rats

Eighty Fischer-344 male rats were divided into eight groups. Half the groups were given a high-risk diet (HRD) that was high in fat, low in fiber and low in calcium, while the remaining groups were given a low-risk diet (LRD) that was low in fat, high in fiber and high in calcium. After 4 weeks, four groups were then given two weekly s.c. injections of azoxymethane (AOM) (15 mg/kg body wt), and four groups were given saline injections. At the end of 6 weeks the rats were either continued on HRD, LRD, or crossed over from HRD to LRD, or LRD to HRD for an additional 6 weeks. The rats were then killed and the influence of different dietary regimens on the induction of foci of aberrant crypts (AC) in their colons was compared. The number of foci of AC was highest in the AOM-injected HRD/HRD dietary group (8.83 +/- 0.98), followed by the AOM-injected LRD/HRD group (5.37 +/- 0.75), the HRD/LRD group (3.32 +/- 0.36) and the LRD/LRD group (2.93 +/- 0.32). Except for the HRD/HRD control group, which developed a small but significant number of foci of AC (1.24 +/- 0.19), the other saline-injected dietary groups showed no significant numbers of foci of AC. Two groups of rats on HRD/HRD (AOM and saline-injected), and two groups of rats on HRD/LRD (AOM and saline-injected) were continued on HRD and LRD respectively for an additional 14 weeks. A continuous HRD in AOM-injected rats induced colon tumors in 92% of rats, while the crossover from HRD to LRD suppressed the tumor incidence to 33%. Neither diet induced any tumors in saline-injected rats. The results clearly show that the influence of HRD and LRD on the induction of foci of AC and their distribution in the colon, correlated with the induction and distribution of tumors in the colon.

Metabolic activation of 6-nitrochrysene in explants of human bronchus and in isolated rat hepatocytes

It has previously been shown that 6-nitrochrysene can be activated to electrophilic species capable of reacting with DNA through metabolic pathways that form N-hydroxy-6-aminochrysene or trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene as critical intermediates. Since the lung is a known target tissue for the carcinogenic action of polycyclic nitroaromatic hydrocarbons, we investigated the metabolism and DNA binding of [3H]6-nitrochrysene in 11 specimens of human bronchus. Analysis of medium from [3H]6-nitrochrysene-treated explants indicated the presence of trans-9,10-dihydroxy-9,10-dihydro-6-nitrochrysene (0.04-330 pmol/mg epithelial DNA), trans-1,2-dihydroxy-1,2-dihydro-6-nitrochrysene (12-1700 pmol/mg epithelial DNA), 6-aminochrysene (1.6-2200 pmol/mg epithelial DNA), and trans-1,2-dihydroxy-1,2-dihydro-6-aminochyrsene (3.6-610 pmol/mg epithelial DNA). Both the levels and the relative proportions of these metabolites varied widely in explants from different individuals. The amount of DNA recovered and the level of DNA modification were sufficient for adduct analysis in eight of the 11 cases for which metabolite data were obtained. Five additional bronchial specimens for which metabolite data were not obtained were also analyzed for carcinogen-DNA adducts. The levels of binding varied from 0.06 to 30.5 pmol [3H]6-nitrochrysene bound/mg DNA (two adducts per 10(8) nucleotides-10 adducts per 10(6) nucleotides). HPLC analyses of enzymatic hydrolysates of the explant DNA indicated that 11 of 13 cases contained adducts with retention times identical to those of adducts derived from trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene or N-hydroxy-6-aminochrysene. The adduct derived from trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene was the major adduct detected in eight of 13 cases. The reasons for the variation in metabolism and adduct formation observed in [3H]6-nitrochrysene-treated explants of bronchus from different donors are not known but may reflect differences in the activities of enzymes responsible for the metabolism of this compound. The influence of induction of drug metabolizing enzymes on the activation pathway of 6-nitrochrysene in an intact cell system was tested using rat hepatocytes. 6-Nitrochrysene was incubated with freshly isolated hepatocytes from rats that were either untreated or pretreated with phenobarbital, 3-methylcholanthrene or Aroclor 1254. Although the levels of adducts were similar in all cases, the pattern of DNA adducts formed in these hepatocytes was dependent on the nature of the pretreatment of the rats. As previously reported, hepatocytes from untreated rats contained adducts derived from N-hydroxy-6-aminochrysene.(ABSTRACT TRUNCATED AT 400 WORDS)

DNA binding of 4,4'-methylene-bis(2-chloroaniline) (MOCA) in explant cultures of human and dog bladder

The binding to DNA of 4,4'-methylene-bis(2-chloroaniline) (MOCA) in explant cultures of human and dog bladder was compared. The DNA binding of MOCA in both human and dog bladder explants increased with the concentration of MOCA in the medium. In both species, there appeared to be a population with high DNA binding activity and another with low DNA binding activity. Furthermore, the binding of MOCA to human bladder DNA appeared to be higher than to dog bladder DNA. The results indicate the potential of MOCA to induce genetic damage in human bladder and suggest caution in the occupational exposure of humans to this chemical.

Hepatic DNA methylation and liver tumor formation in male C3H mice fed methionine- and choline-deficient diets

The effects of the chronic administration of methyl-deficient, amino acid-defined diets on liver tumor formation were examined in male weanling C3H/HeN mice previously treated with a single ip injection of 0 or 150 mg diethylnitrosamine/kg body weight [(DENA) CAS: 55-18-5]. Five diets were used: diet 1, adequate; diet 2, devoid of both methionine and choline; diet 3, devoid of methionine only; diet 4, devoid of choline only; and diet 5, devoid of methionine, choline, folic acid, and vitamin B12. Equimolar homocystine replaced methionine in all methionine-devoid diets. All diets were administered for 1 year. No hepatocellular carcinomas and only 3 liver adenomas were seen among the 129 animals at risk in the 5 groups that had received no DENA. Among the DENA-treated groups fed diets 1-4, the incidence of hepatocellular carcinomas in the mice at risk averaged 40%, with no significant differences noted among groups. A relatively low incidence of liver carcinomas (10%) was seen among DENA-treated mice subsequently fed diet 5; it could be ascribed to the enhanced mortality seen in these animals due to the dietary deficiencies. Lung tumors were seen in 44% of the DENA-treated mice surviving more than 35 experimental weeks and in only 2.5% of the corresponding DENA-untreated animals. Feeding diet 2, deficient in methionine and choline, to male C3H mice for 5-20 weeks decreased the hepatic ratio of S-adenosylmethionine (CAS: 29908-3-0) to S-adenosylhomocysteine (979-92-0) relative to that observed in mice fed the adequate diet 1. The 5-methyldeoxycytidine [(5-MC) CAS: 838-07-3] contents of liver DNA in animals fed diet 2 for 5, 10, and 20 weeks, however, were not significantly different from the corresponding levels in diet 1-fed mice. The results indicate that a methionine- and choline-deficient dietary regimen that lowers the 5-MC levels in DNA and enhances liver tumor formation in male F344 rats does not do so in male C3H mice.

Effect of methionine and choline on liver tumor promotion by phenobarbital and DDT in diethylnitrosamine-initiated rats

The effect of the chronic feeding of methionine or choline on liver tumor promotion by phenobarbital (PhB) or 1,1 bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) was studied in rats receiving an initiating dose of diethylnitrosamine (DEN). Male weanling rats were injected i.p. with DEN (200 mg/kg body wt). Control rats were injected with saline. Five days after the injection, the rats were placed on different diets containing 0.05% PhB or 0.05% DDT with or without added 1.5% DL-methionine or 1.0% choline chloride. Each diet was administered for 72 weeks, when the animals were placed on the unsupplemented chow diet for an additional 30 weeks. Rats treated with DEN and then fed PhB or DDT developed an 85-100% incidence of hepatocellular carcinomas (HCCs). Single injection of DEN alone produced a 60% incidence of HCCs. Dietary feeding of methionine and choline either alone or in combination with PhB or DDT did not have any significant effect on the incidence of HCC's. Liver tumor formation was negligible in uninitiated rats. Lung metastases developed in 42% and 46% of the DEN + PhB- and DEN + DDT-treated groups, respectively. Supplementation of methionine in the diet lowered the incidence of lung metastases to 14% in the DEN + PhB-treated rats and to 19% in DEN + DDT-treated rats. Choline was not effective in inhibiting the development of lung metastases in either case. The injection of DEN alone produced a 54% incidence of lung tumors. PhB and DDT feeding lowered the DEN-induced lung tumor incidence to 23% and 14% respectively. Further, when the data from different diet groups were combined it was found that single injection of DEN also doubled the incidence of leukemia normally seen in F344 rats. The present report constitutes the first evidence that a single injection of DEN induces lung tumors and enhances the incidence of leukemia in rats.

Hypomethylation of DNA in ethionine-fed rats

Ethionine, the hepatocarcinogenic antimetabolite of methionine, was fed to rats in carcinogenic doses for 1-10 weeks. Levels of 5-methyldeoxycytidine (5-MC) in nuclear DNA and total cellular levels of S-adenosylmethionine (AdoMet) and S-adenosylethionine (AdoEt) were determined at 1, 5 and 10 weeks in livers of control and ethionine-treated animals. The percentage of deoxycytidine residues modified to 5-MC in hepatic DNA of ethionine-fed animals was the same as that in the control animals at 1 week but was 3.6% and 7.6% lower than that observed in control animals at 5 and 10 weeks, respectively. Significant levels of AdoEt, a DNA methylase inhibitor, as well as decreases in the levels of AdoMet were also observed in the livers of ethionine-fed animals. In a second study, the levels of 5-MC, AdoMet and AdoEt were determined in the pancreas, kidneys, testes and thymus of control rats and rats fed ethionine for 10 weeks. Only the testes, an organ known to be susceptible to the toxic effects of ethionine, showed a significant (p less than 0.02) decrease in 5-MC in response to ethionine feeding. AdoEt was present in all tissues studied, except thymus, but at lower levels than those observed in the liver. These results demonstrate that ethionine administration alone under conditions which cause tumors is sufficient for the production of hypomethylated DNA in the target organ and one extrahepatic tissue studied. Hypomethylation of hepatic DNA would appear to result from the accumulation of AdoEt coupled with the decreased levels of AdoMet.

Hypomethylation of hepatic nuclear DNA in rats fed with a carcinogenic methyl-deficient diet

A progressive decrease was observed in the 5-methyldeoxycytidine content of hepatic DNA in male F344 rats fed with a hepatocarcinogenic methyl-deficient diet. The same dietary regimen resulted in altered hepatic contents of S-adenosylmethionine, the methyl-donating species, and S-adenosylhomocysteine, an inhibitor of DNA methylase. The data indicate that this carcinogenic dietary manipulation is sufficient to alter a possible regulatory process, DNA methylation.

Tissue levels of S-adenosylmethionine and S-adenosylhomocysteine in rats fed methyl-deficient, amino acid-defined diets for one to five weeks

The levels of S-adenosylmethionine (AdoMet) and of S-adenosylhomocysteine (AdoHcy) as well as the ratio of AdoMet/AdoHcy were determined in the liver, lungs, testes and kidneys of weanling male rats fed a commercial chow diet or 5 different amino acid-defined diets for 1-5 weeks. The amino acid-defined diets used were as follows: diet 1, supplemented with methionine, choline, folic acid and vitamin B12; diet 2, deficient in methionine and choline; diet 3, deficient in methionine alone; diet 4, deficient in choline alone; diet 5, deficient in methionine, choline, folic acid and vitamin B12. All methionine-deficient diets were supplemented with an equimolar dose of its metabolic precursor, homocystine. The animals were sacrificed after 1, 3 and 5 weeks of treatment. In animals fed either the chow diet or diet 1, liver was the organ found to contain the highest levels of AdoMet and AdoHcy. Similarly, in animals fed diet 1 or chow, the testes and lungs contained the lowest level of AdoMet, while the lungs contained the lowest levels of AdoHcy. In general, the tissue levels of AdoHcy and AdoMet in rats fed diet 1 were very similar to the corresponding values found in chow-fed rats. Diet 1 feeding, however, led to higher hepatic levels of AdoMet than did the administration of the chow diet. The administration of the methyl-deficient diets generally led to decreased hepatic AdoMet contents at 3 and 5 weeks; the methyl-deficient diets also led to increased AdoHcy contents and decreased AdoMet:AdoHcy ratios when compared with diet 1. Linear regression analysis showed a significant direct correlation between the observed hepatic AdoMet levels and the methyl content of the diet as well as an inverse correlation between hepatic AdoHcy levels and dietary methyl contents. Unlike liver, the lung and testes did not show any decrease in AdoMet content following feeding of the methyl-deficient diets. These tissues did show, however, early significant increases in AdoHcy contents and corresponding decreases in the ratios of AdoMet:AdoHcy. These changes were found to be proportional to the dietary methyl content. The renal contents of AdoMet, AdoHcy and the ratio of AdoMet/AdoHcy were unaffected by any of the diets administered except for diet 5. The administration of diet 5 to rats for 5 weeks led to a significant increase in renal AdoHcy. These results provide evidence indicating that dietary methyl insufficiency may exert its role in carcinogenesis through a decreased availability of AdoMet in vivo.

Decreased levels of S-adenosylmethionine in the livers of rats fed phenobarbital and DDT

Chronic feeding of the liver tumour promoters phenobarbital (PB) and 1,1-bis(rho-chlorophenyl)-2,2,2-trichloroethane (DDT) to male weanling rats at levels of 0.05% in the diet for 1, 3 and 5 weeks reduced the hepatic S-adenosylmethionine (AdoMet) contents by approximately 50%. Such reductions were inhibited by the simultaneous feeding of choline chloride and were completely prevented by the dietary administration of methionine. AdoMet levels greater than 100 mug/g liver were noted in rats receiving PB + methionine for 1 week, DDT + methionine for 1 week or methionine alone for 5 weeks. These results are consistent with the hypothesis that the liver tumour promoting activities of PB and DDT may occur in part through a hepatic methyl insufficiency.