Metabolism of the carcinogen 1,2-dimethylhydrazine by isolated human colon microsomes and human colon tumor cells in culture

Discovery of specific inhibitors for intestinal E. coli β-glucuronidase through in silico virtual screening

Glucuronidation is a major metabolism process of detoxification for carcinogens, 4-(methylnitrosamino)-1-(3-pyridy)-1-butanone (NNK) and 1,2-dimethylhydrazine (DMH), of reactive oxygen species (ROS). However, intestinal E. coli    β-glucuronidase (eβG) has been considered pivotal to colorectal carcinogenesis. Specific inhibition of eβG may prevent reactivating the glucuronide-carcinogen and protect the intestine from ROS-mediated carcinogenesis. In order to develop specific eβG inhibitors, we found that 59 candidate compounds obtained from the initial virtual screening had high inhibition specificity against eβG but not human βG. In particular, we found that compounds 7145 and 4041 with naphthalenylidene-benzenesulfonamide (NYBS) are highly effective and selective to inhibit eβG activity. Compound 4041  (IC₅₀ = 2.8 μM) shows a higher inhibiting ability than compound 7145  (IC₅₀ = 31.6 μM) against eβG. Furthermore, the molecular docking analysis indicates that compound 4041 has two hydrophobic contacts to residues L361 and I363 in the bacterial loop, but 7145 has one contact to L361. Only compound 4041 can bind to key residue (E413) at active site of eβG via hydrogen-bonding interactions. These novel NYBS-based eβG specific inhibitors may provide as novel candidate compounds, which specifically inhibit eβG to reduce eβG-based carcinogenesis and intestinal injury.

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

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

Plant tannins inhibit the induction of aberrant crypt foci and colonic tumors by 1,2-dimethylhydrazine in mice

We have shown that naturally occurring tannins possess antitumor promotion activity in mouse skin. In the present investigation, we studied the ability of a hydrolyzable tannin, gallotannin (GT), and a condensed tannin extracted from red alder (RA) bark to inhibit 1,2-dimethylhydrazine (DMH)-induced colonic aberrant crypt foci (ACF) and tumors in Balb/c mice. In addition, we determined the ability of GT to inhibit the proliferation and to induce apoptosis in a human colon cancer cell line (T-84). Mice were given tannins by intraperitoneal injections, by gavage, or in drinking water before treatment with DMH for 24 weeks. Alternatively, mice were given tannins by intraperitoneal injection or gavage for only 2 weeks before DMH administration, then tannin administration was discontinued and mice were treated with DMH for 24 weeks. The multiplicity, size, and distribution of ACF and tumors were significantly inhibited by GT and RA in the above treatment regimens. The most effective treatments included GT by gavage, RA bark extract by intraperitoneal injection, and either tannin dissolved in drinking water. Extent of inhibition of ACF and tumors was gender independent. In cell culture experiments, GT treatment for three days inhibited the growth of T-84 cells, with a concentration resulting in half-maximal inhibition estimated to be 20 micrograms/ml. The treatment was not cytotoxic to cells at 1-40 micrograms/ml. Interestingly, at 10 micrograms/ml, GT induced apoptosis in T-84 cells as determined by the Hoechst DNA staining technique. Collectively, these findings support a potential role for tannins as chemopreventive agents against colon cancer.

Human health perspective on environmental exposure to hydrazines: a review

Hydrazines are colorless liquid compounds that have been found at various Department of Defense hazardous waste sites. They are designated as environmental contaminants causing adverse effects to public health and have been identified at many National Priorities List (NPL) hazardous waste sites and federal facilities sites in the United States. Three chemically similar hydrazines-hydrazine, 1,1-dimethylhydrazine, and 1,2-dimethylhydrazine--occur in the environment and cause adverse health effects to persons living near hazardous waste sites. Humans are exposed to hydrazines by drinking contaminated, water, by inhaling contaminated air, or by swallowing or touching contaminated dust. Human occupational data and studies in laboratory animals suggest that people exposed to hydrazines may develop adverse systemic health effects or cancer. Hydrazines have caused cancer in animals following acute- or intermediate- duration exposure by the oral and inhalation routes. The U.S. Environmental Protection Agency, the U.S. Department of Health and Human Services, the International Agency for Research on Cancer, and the World Health Organization have classified hydrazines as possible cancer-causing environmental contaminants.

Expression, induction and regulation of the cytochrome P450 monooxygenase system in the rat glioma C6 cell line

The cytochrome P450 monooxygenase system consists of NADPH-cytochrome P450 reductase (P450 reductase) and cytochromes P450, which can catalyze the oxidation of a wide variety of endogenous and exogenous compounds. P450 reductase transfers reducing equivalents from NADPH to P450, which in turn catalyzes metabolic reactions. In previous studies, we have used the rat glioma C6 cell line as an in vitro model system and identified the presence of P450 reductase and of cytochrome P450 1A1, 1A2, 2A1, 2B1/2, 2C7, 2D1-5 and 2E1 by reverse transcription followed by polymerase chain reaction (RT-PCR). In C6 cells, the induction of P450 1A1 and 2B1/2 at mRNA level after BA (benzo(a)anthracene) or PB (phenobarbital) treatments was detected. In this study, analysis of microsomal preparations of glioma C6 cells was utilized to demonstrate the presence of P450 2B and P450 reductase at the protein level. ELISAs showed that PB induced P450 2B proteins 12-fold. These experiments further establish that the rat glioma C6 cell line contains an active cytochrome P450 monooxygenase system that can be induced by P450 inducers. We also found that the mRNAs of P450 1A1 and 2B1/2 from glioma C6 cells do not bind to the oligo(dT)-based separation techniques efficiently, suggesting that they may have very short poly(A) tails. The half-lives of P450 1A1 and 2B1/2 mRNA in glioma C6 cells are 1/10 and 1/3 of that in liver, respectively. This may partly contribute to the low expression level of P450s in glial cells. The induction of P450s by BA or PB did not change their mRNA half-lives, indicating the induction may be due to transcriptional regulation. In summary of this study, we believe the presence of the cytochrome P450 monooxygenase system in glial cells of the brain may be important in chemotherapy and carcinogenesis of brain tumors.

Genotoxic activation of hydrazine, two dialkylhydrazines, thiourea and ethylene thiourea in the somatic w/w + assay of Drosophila melanogaster

Genotoxic activation of hydrazine (HZ), two symmetrical dialkylhydrazines, namely, 1,2dimethylhydrazine and 1,2-diethylhydrazine (SDMH and SDEH), thiourea (TU) and ethylene thiourea (ETU) has been evaluated by means of the w/w + somatic assay of Drosophila. Both low bioactivation insecticide-susceptible (IS) and high biotransformation insecticide-resistant (IR) strains were used. The combined application of insecticide-susceptible and insecticide-resistant strains should, in principle, detect somatic cell recombinagens in the Drosophila melanogaster in vivo w/w + assay. The IS strain was more susceptible to toxicity induced by the test chemicals than the IR stocks. Its performance in the biotransformation of the chemicals tested was rather poor. TU was inactive in all strains. With the active compounds, spot frequencies increased approximately linearly with dose for each spot type. SDEH gave a strong positive result in all three female genotypes exposed. HZ, ETU and SDMH were overall weakly positive in the IR strain Haag-79 (HG-R). Interestingly, ETU was clearly positive in the IR Hikone-R (HK-R) strain. A comparison of the recombinagenic potencies between the active and the weakly positive compounds, and among strains, showed pronounced genotype-dependent differences between the low and the high bioactivation strains. The ability of Drosophila to express several procarcinogens in relation to insecticide-resistance after activation catalyzed by CYP450 enzymes is discussed.

Expression and induction of cytochrome P-450 1A1 and P-450 2D subfamily in the rat glioma C6 cell line

The cytochrome P-450 (P-450) monooxygenase system can catalyze the oxidation of a wide variety of endogenous and exogenous compounds, including steroid hormones, fatty acids, drugs and pollutants. The functions of this system are as diverse as the substrates. Though this enzyme system has the highest level of activity in the liver, it is present in other tissues, including brain. In this study, we have established the rat glioma C6 cell line as an in vitro model system to examine the expression and induction of P-450 1A1 and the P-450 2D subfamily. Rat glioma C6 cells were treated with P-450 inducers phenobarbital (PB) or benzo[a]anthracene (BA). The presence of P-450 1A1 and 2D1-5 was detected by reverse transcription followed by polymerase chain reaction (RT-PCR) and confirmed by restriction enzyme digestion. The induction of P-450 1A1 and 2D1-5 was quantified using competitive PCR. Although P-450 2D1-5 do not seem to be affected by PB or BA treatment, tenfold induction of P-450 1A1 mRNA after BA treatment was detected. Western blot analysis of microsomal preparations of glioma C6 cells demonstrated the presence of P-450 1A1 at the protein level. ELISAs showed that BA induces P-450 1A1 proteins 7.3-fold. These experiments provide further evidence that the rat glioma C6 cell line contains an active cytochrome P-450 monooxygenase system which can be induced by P-450 inducers. In summary, we believe the presence of the cytochrome P-450 monooxygenase system in glial cells of the brain may be important in chemotherapy and carcinogenesis of brain tumors.

Flow-cytometric analysis of the DNA-content in paraffin-embedded tissue from colorectal carcinomas and its prognostic significance

The nuclear DNA content of 163 colorectal carcinomas was determined by flow-cytometry (FCM) on formalin-fixed, paraffin-embedded tissue. DNA-aneuploidy was found in 97 cases (59.5%), in which no statistically significant correlations with sex, mean age, tumour stage (Dukes and pTNM) and tumour grade were noted. The frequency of aneuploidy was significantly higher in patients less than 70 years of age (p less than 0.01) and in tumours localized in the left colon and rectum (p less than 0.002), irrespective of their stage. The tumours in which different areas could be analysed (n = 80) showed a heterogeneous DNA-ploidy pattern in 18%. Comparison of the DNA content in primary tumours and in lymph node metastases (n = 49) showed a difference in DNA-ploidy in 38% of the DNA-aneuploid tumours, but in only 6% of the DNA-diploid carcinomas (p less than 0.02). DNA-aneuploid carcinomas tended to show a higher rate of local recurrence and were associated with an unfavourable prognosis (p = 0.04) in those patients in which complete resection of their tumours was possible (n = 72). The significantly higher mortality of patients with DNA-aneuploid carcinomas of stage pT3, as well as those with Dukes stage A and B tumours indicates that DNA-aneuploidy may be a stage-independent additional risk factor in colorectal cancer.

Expression of two cytochromes P450 involved in carcinogen activation in a human colon cell line

Cytochrome P450 is known to cause carcinogen activation and correspondingly increased cancer risk in animal models. In order to determine whether P450 in the colon may be involved in cancer development in the human, the human colon cell line LS174T was examined for the presence of various cytochromes P450. Two isozymes of P450 were identified in the human cell line. Expression of P450IA1 or IA2 was increased by treatment of the cell line with benzanthracene; the induction was demonstrated by an increase in RNA hybridizing to a probe for P450IA1 and by ethoxyresorufin deethylation activity. Western analysis of microsomes isolated from human colon tissue also demonstrated the presence of P450IA1, as well as a form which cross-reacted to an antibody to human P450IIC9. Another isozyme, P450IIE1, was identified by polymerase chain reaction amplification of RNA from LS174T cells. These results underscore the presence of cytochromes P450 in colonic tissue and provide a basis for the involvement of isozyme-specific P450 mediated reactions in carcinogenesis of the colon.

Age-related changes in biotransformation of azoxymethane and methylazoxymethanol in vitro

1. Age-related changes in hepatic hydroxylation of azoxymethane (AZO) to methylazoxymethanol (MAM), as well as colonic phase I metabolism of MAM by alcohol dehydrogenase (ADH) were examined in young (2-4 months), middle-aged (12-14 months), and old (22-24 months) male Fischer 344 rats. In addition, the possibility that colonic glucuronyltransferase might be involved in the biotransformation of MAM was also investigated. 2. A significant decrease in hepatic conversion of AZO to MAM was found in old vs young rats, concomitant with a decrease in hepatic cytochrome P-450 content, while no age-related difference was found in the colonic metabolism of MAM by ADH. MAM inhibition of colonic 4-methylumbelliferone glucuronyltransferase was non-competitive, suggesting indirectly that colonic glucuronyltransferase is not involved in conjugation of MAM. 3. It is concluded that ageing in the male Fischer 344 rat results in alternations of AZO and MAM biotransformation which indicate that AZO may be less carcinogenic in older rats.

Human colon tumor cell line LS174T drug metabolizing system

The effects of culture variables on the specific content and activity of various enzymes of the drug metabolizing system were assessed in colon tumor cell line LS174T. The NADH reduced cytochrome b5 (cyt b5)4 spectrum of these cells was similar to rat liver cyt b5. When released from the membrane by trypsin and concentrated, the cyt b5 was found to cross react with rabbit antibody to rat liver cyt b5 and human liver cyt b5. The enzyme activities were found stable over limited cell passages with control values of 0.03 and 0.13 mumol/min/mg protein for NADPH and NADH cytochrome c (cyt c) reducing activity, 0.05 nmol cyt b5 and 0.013 nmol cytochrome P450 per milligram of microsomal protein. Phenobarbital/hydrocortisone showed a consistent, but not always significant increase in the NADPH and NADH cyt c reduction and benzanthracene an increase in the NADH cyt c reducing activity and cyt b5 content. Griseofulvin lowered the NADH cyt c reducing activity. Delta-aminolevulinic acid (0.5 mM) caused a significant decrease in the specific activity of all enzymes, as judged by a student's t test, with a p less than 0.001.

Cytochrome P450 activity and distribution in the human colon mucosa

Enzymatic activity associated with the mixed-function oxidase system was determined in microsomes prepared from the mucosal cells extracted from normal human colons. A high activity toward nitrogen oxidation reactions was observed. 1,2-Dimethylhydrazine, a colon-specific carcinogen, was metabolized at a higher rate in vitro by human colon microsomes as compared with the rat, and exhibited a km ten-fold lower, 1.03 mmol/l versus 9.68 mmol/l, respectively. This activity was inhibited by classic cytochrome P450 inhibitors; 70% inhibition was achieved using 70 mmol/l metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone), 20 mmol/l; SKF-525A (diethylaminoethyl-2,-2-diphenylvalerate HCl), or 350 mumol/l n-octylamine. These data suggest the presence of a stable, active mixed-function oxidase system in the human colon mucosa which has a preferential activity toward nitrogenous compounds and provides a mechanism for the activation of carcinogens. Its distribution in the colon appears to parallel the reported incidence of human colonic carcinomas.

Production of ethane by rats treated with the colon carcinogen, 1,2-dimethylhydrazine

Ethane was exhaled by rats treated with the colon carcinogen, 1,2-dimethylhydrazine (DMH). At 1 hr, ethane production (mean +/- SD) was 0.2 +/- 0.2 nmol/kg (controls) and 5.2 +/- 1.3, 13.7 +/- 3.4, and 27.7 +/- 9.6, respectively, for DMH injections of 0.15 mmol/kg (20 mg/kg of the dihydrochloride salt), 0.45 mmol/kg, and 1.35 mmol/kg. Rates of ethane evolution tapered off after 2 hr, but persisted for up to 3 hr at the lower dose, and up to 5-6 hr at the higher dose. Although ethane is produced in vivo during lipid peroxidation, experiments with vitamin E, a potent lipid antioxidant, indicated that lipid peroxidation was unlikely to be the source of ethane in DMH-treated rats: pretreatment with vitamin E had no effect on ethane formation from DMH but did suppress ethane production from rats treated with carbon tetrachloride, an inducer of hepatic lipid peroxidation. When rats were injected with 1,2-diethylhydrazine in place of DMH, large amounts of ethane and ethylene were produced (9800 and 5600 nmol/kg/hr). The hydrocarbon gases exhaled by rats may arise from dimerization of methyl radicals (.CH3) generated during the metabolism of DMH, and from ethyl radicals (.CH2CH3) generated during the metabolism of 1,2-diethylhydrazine. Previously, it was shown that methane and ethane are formed from methyl radicals in vitro. Other investigators have observed formation of hydrocarbon gases during the in vitro metabolism of monoalkylhydrazines by microsomes, and ethyl radicals, derived from monoethylhydrazine, have been detected by electron spin-resonance spectroscopy. The results presented here suggest that in vivo metabolism of DMH may produce methyl radicals. Methyl radicals are capable of interacting with biomolecules. Their indiscriminate reaction with tissue constituents may be a contributory factor in DMH-induced carcinogenesis.

Changes in phase I and phase II biotransformation with age in male Fischer 344 rat colon: relationship to colon carcinogenesis

Using male Fischer 344 rats classified as young (2-4 months), middle aged (12-14 months), and old (22-25 months), the activities of several Phase I and Phase II biotransformation pathways in the large intestine were investigated, including benzo[a]pyrene hydroxylase (BPOH), alcohol dehydrogenase (ADH), glutathione S-transferase (GST), glutathione peroxidase (GSH-PX), beta-glucuronidase (BG), and microsomal and nuclear glucuronyltransferase (UDPGT). Levels of oxidized (GSSG) and reduced (GSH) glutathione and uridine 5'-diphosphoglucuronic acid (UDPGA) were also measured. BPOH increased 33% in old rats, while ADH and BG activity remained unchanged with age. Nuclear UDPGT remained unchanged with age, whereas form I of GSH-PX declined slightly in old rats. GST, microsomal UDPGT, and form II of GSH-PX declined by 38, 37 and 44%, respectively, in old rats. The decrease in GST and microsomal UDPGT was also significant in middle aged rats. Levels of colonic GSH, GSSG and UDPGA were found to be unchanged with age. These in vitro data suggest the possibility that if reactive intermediates are generated to the same extent in old rats as in young rats, decreased detoxification mechanisms in the old rat may increase susceptibility of the colon to actions of chemical carcinogens.

Effects of Maillard browned egg albumin on drug-metabolizing enzyme systems in the rat

Adult male Sprague-Dawley rats were fed a purified diet containing 5% Maillard browned egg albumin (EA-B) or browned hydrolysed egg albumin (HEA-B) for 10 wk. Control animals were pair-fed a corresponding isocaloric, isonitrogenous non-browned egg albumin (EA-C) or hydrolysed egg albumin (HEA-C) diet. At the end of 10 wk, the rats were killed and hepatic, small intestinal and colonic microsomes and cytosol fractions were prepared by ultracentrifugation. Animals fed EA-B exhibited significantly (P less than 0.05) increased hepatic benzo[alpha]pyrene hydroxylase activity and significantly (P less than 0.05) decreased colonic aminopyrine N-demethylase activity compared to control (EA-C) animals. HEA-B-fed animals also exhibited a significant (P less than 0.05) decrease in colonic aminopyrine N-demethylase activity compared with HEA-C controls, but no significant differences were detected in hepatic or small intestinal enzyme activities in this group. These data suggest that Maillard browned protein products may modify hepatic and/or colonic drug-metabolizing enzyme system activities, and may thus contribute to alterations in the metabolism of endogenous substrates and of exogenous drugs, precarcinogens and other xenobiotics.

Drug metabolism in rat colon: resolution of enzymatic constituents and characterization of activity

A direct demonstration of the basis of mixed function oxidase activity in rat colonic mucosa was achieved by resolution of microsomes into two components, cytochrome P-450 and cytochrome P-450 reductase, which on recombination with phosphatidylcholine catalyzed hydroxylation of benzo[alpha]pyrene and benzphetamine. Reconstitution of hydroxylation activity requires both the cytochrome P-450 component and the cytochrome P-450 reductase component in addition to phospholipid. Omission of either of the protein components or the phospholipid component reduces the activity almost to background levels. The kinetic parameters (Km values) for the reconstituted system suggest that the colonic mucosal system is quite similar to the liver microsomal system in its catalytic capacity as well as in its enzymic composition. The purified colon components substitute for their liver counterparts reasonably well, again consistent with the argument that the colon mucosal mixed function oxidase system is analogous to the liver system.

Detection of free radical intermediates in the oxidative metabolism of carcinogenic hydrazine derivatives

Hydrazine derivatives are widely used in agriculture, in industry, as rocket propellants, and in medicine. Hydrazines also occur naturally in tobacco and mushrooms. Many hydrazines tested in animal studies appear to be carcinogenic and induce tumors in various target tissues in mice, hamsters, and rats. The use of hydrazine derivatives in humans is often complicated by adverse side-effects such as liver injury and rheumatoid arthritis. A number of studies have demonstrated that hydrazine derivatives are activated to reactive intermediates, such as free radicals, through a variety of cellular oxidative metabolic pathways. The aim of this work is to demonstrate the occurrence of free radical intermediates during the metabolic activation of various hydrazine derivatives and to characterize the enzymatic system(s) responsible for the activation to free radical species. The hydrazines studied are acetylhydrazine, isoniazid, isopropylhydrazine, iproniazid, methylhydrazine, 1,1-dimethylhydrazine, and 1,2-dimethylhydrazine. The model systems chosen are those of rat liver microsomes and isolated hepatocytes. Free radical intermediates have been demonstrated by the electron spin resonance spectroscopy coupled to spin trapping technique. The activation mechanism has been characterized using inhibitors of the mixed function oxidase system and of the FAD-dependent oxygenase system. Glutathione was able to scavenge, with high efficiency, the free radicals produced.

In vitro DNA and nuclear proteins alkylation by 1,2-dimethylhydrazine

The methylating carcinogen 1,2-dimethylhydrazine (DMH) CAS 540.73.8 is highly organ-specific and, under certain experimental conditions, produces a high incidence of adenocarcinoma in the colon of rodents. We have tried to assess the possibility that part of the organ-specifity in the carcinogenic effect of DMH could be attributed to its metabolism by specific microsomal enzymes. In particular, we compared the in vitro effects of DMH in the presence of either colon or liver microsomes from animals that had been treated with microsomal enzyme inducers. V79 Chinese hamster cells were used as the target to evaluate the damage to the genetic material, as judged by (1) formation of adducts of DNA bases and (2) amino acid modifications in nuclear proteins using [Me-14C]DMH and appropriate analytical detection systems. Our results tend to support the above postulated hypothesis.

Modification of experimental colon carcinogenesis by dietary fibers

The literature concerning the effect of individual dietary fibers on the experimental induction of colorectal cancer was reviewed. It has become increasingly apparent that the effect of dietary fibers on colon carcinogenesis depends on many factors, including the type and amount of fiber; the other dietary components, particularly fat; animal species, strain, and sex; and the type of carcinogen and its dose and route of administration. Despite such variations in design, most experiments with wheat bran and cellulose have shown evidence of a significant protective effect. In contrast, numerous other fiber supplements have been shown to enhance tumor development. These include pectin, corn bran, undegraded carageenan, agar, Metamucil, and alfalfa. Possible mechanisms by which fibers may inhibit colon tumorigenesis include dilution and adsorption of any carcinogens or promoters contained within the intestinal lumen and faster transit time and therefore less opportunity for carcinogen/promoter interaction with the intestinal epithelium. Modulation of colonic microbial metabolic activity by dietary fibers may also be important in the activation and detoxification of carcinogens and promoters. Dietary fibers produce structural and functional changes in the intestinal epithelium and modify rates of cell proliferation changes in the intestinal epithelium and modify rates of cell proliferation and migration. Evidence suggests that if this stimulus to cell proliferation occurs during the stage of initiation, it may lead to enhancement of the carcinogenic process. Dietary fibers bind not only carcinogens, bile acids, and other potentially toxic agents but also essential nutrients that themselves can modify the carcinogenic process. Fermentation of fibers within the large bowel results in production of volatile fatty acids, which in vitro have been shown to be antineoplastic. Fermentation also produces a lower luminal pH, which in turn affects colonic microbial populations and their metabolic activities. The presence of lignans in higher plants and their bacterial synthesis from precursors present in fiber-rich foods provide an additional source of antineoplastic agents, whose relative importance in colon carcinogenesis is unknown. Because dietary fibers differ in their physiochemical properties, it has been difficult to identify a single mechanism by which fibers prevent or inhibit colon carcinogenesis. Clearly, more investigation is needed regarding the mechanism(s) by which certain fibers inhibit while others enhance experimental colon carcinogenesis.

Aging affects the drug metabolism systems of rat liver, kidney, colon and lung in a differential fashion

Microsomes prepared from the liver, lungs, colon and kidney cortex of Sprague Dawley rats of ages 2, 4, 10, 24 and 78 weeks were assessed for hydroxylation activity with the substrate benzo[alpha]pyrene. Liver microsomal activity declined after reaching a peak of activity at 10 weeks. The hydroxylation of benzo[alpha]pyrene by colon, kidney and lung microsomes, however, either remained the same or decreased only slightly. During the age range examined inducibility of hydroxylation activity by beta-naphthoflavone decreased with age in liver but actually increased with age in the extrahepatic tissues. Although phenobarbital did not elicit any increases in liver, kidney or lung, it increased substantially benzo[alpha]pyrene hydroxylation activity in colon microsomes of 78 week old rats. Total cytochrome P-450 content was induced at all age groups in all tissues by beta-naphthoflavone and in all tissues except lung by phenobarbital. Induction of cytochrome P-450 in kidney by phenobarbital was only observed in 24 and 78 week old rats. These data suggest an increased role for extrahepatic activation of benzo[alpha]pyrene with aging. In contrast to total content of cytochrome P-450, the beta-naphthoflavone inducible amount of Form 5 which has a high turnover number for benzo[alpha]pyrene, declined by 55% in liver between 2 weeks and 78 weeks while it increases dramatically in all extrahepatic tissues (from 80 to 138%).

The effects of fractionated thermally oxidized corn oil on drug-metabolizing enzyme systems in the rat

Corn oil samples were heated, with aeration, to 210 degrees C for a total of 5 hr. Both fresh and oxidized samples were urea-fractionated and the individual fractions were administered to male Sprague-Dawley rats by gastric intubation. The effects of urea adducts, adduct-free fractions, non-saponifiable fractions and unfractioned fresh and thermally oxidized oil samples on hepatic, intestinal and colonic drug-metabolizing enzymes were determined. The treatments had no significant effects on hepatic or intestinal drug-metabolizing or mixed-function oxidase activities. There was a significant (P less than 0.05) increase in colonic UDP-glucuronyltransferase activity in rats treated with thermally oxidized corn oil, while the non-saponifiable fraction of the same sample decreased (P less than 0.1) the activity of this enzyme. There was also a significant increase in the activity of colonic benzo[a]pyrene hydroxylase in rats treated with the non-adduct fraction or with urea adducts of the thermally oxidized corn oil. These data suggest the colon as a possible specific site for the alteration of mixed-function oxidase activities by products of thermally oxidized oils.