Dietary n-3 PUFA increases the apoptotic response to 1,2-dimethylhydrazine, reduces mitosis and suppresses the induction of carcinogenesis in the rat colon

The effect of dietary fish oil on colonic crypt cell apoptosis and proliferation was examined in male Wistar rats, 24 and 48 h after administration of 1,2-dimethylhydrazine (DMH), and its influence on the induction of aberrant crypt foci (ACF) in the distal colon was assessed. Rats (125-150 g) fed a high-fat semi-synthetic diet containing corn oil (CO) were given DMH (30 mg/kg body wt) or a sham injection of EDTA/NaCl. Animals were then fed either the CO diet or a diet in which fish oil (EPA 18.7%; DHA 8%) was substituted for corn oil. Subgroups of rats (n = 5) were killed after 24 and 48 h, and crypt cell apoptosis and proliferation were quantified by morphological criteria in isolated intact crypts from the mid and distal colon. Consumption of the fish oil diet (FO) was associated with increased apoptotic cell death (P < 0.001) and suppression of proliferation (P < 0.05) in colonic crypts both 24 and 48 h after DMH. In a second experiment, animals were given three injections of DMH or sham injections of carrier at weekly intervals. For 48 h after each injection animals were fed either the CO or FO diet, but otherwise maintained on the CO throughout. The number and crypt multiplicity of ACF in the distal colon were determined after 18 weeks, and animals given the FO diet for the 48 h period following carcinogen administration were found to have significantly fewer ACF than rats fed the CO diet (P < 0.05). The data demonstrate that the fatty acid composition of the diet is an important determinant in the induction of carcinogenesis by DMH. The proliferative and apoptotic response of the colonic crypt to carcinogen and fish oil, coupled with the reduced incidence of ACF, suggest n-3 PUFA can protect against the carcinogenic effects of DMH by mediating changes in the balance proliferation and cell death.     

Resistance of aberrant crypt foci to apoptosis induced by azoxymethane in rats chronically fed cholic acid

We have previously shown that chronic feeding of cholic acidto carcinogen treated rats reduces the number of putativepreneoplasticlesions of colonic cancer, aberrant crypt foci(ACF), but enhancesthe growth of remaining ACF and the incidence of colonic tumors.The following study was conducted to further explore the effectsof cholic acid on ACF growth by determining if ACF in cholicacid-fed animals display resistance to apoptotic cell death.ACF were induced in male Sprague- Dawley rats with two injectionsof azoxy-methane (20 mg/kg body wt). Rats were divided intotwo groups and fed either the control AIN-76 diet or the AIN-76diet containing 0.2% cholic acid. After 18 weeks, colonic apoptoticcelldeath was induced with an acute low dose of azoxymethane(10 mg/kg body wt). The number of cells, apoptotic bodies andbromodeoxyuridine (BUdR)-labeled cells were determined in coloniccrypts comprising ACF and surrounding normal crypts in ratsfrom each diet group. The number of apoptotic bodies per 100cells was lower in ACF crypts than in normal-appearing crypts(P = 0.0034). Both normal and ACF crypts from rats fed the cholicacid diet had fewer apoptotic bodies per 100 cells than cryptsfrom rats fed the control diet (P =0.0102). These data suggestthat ACF harbor resistanceto induction of apoptosis. Chronicfeeding of a diet containing 0.2% cholic acid results in thedevelopment of increased resistance to apoptosis. The lowerrate of cell death in ACF may contribute to the enhanced growthof ACF and higher tumor incidence previously observed in cholicacid-fed animals.     

Modulation of experimental colon tumorigenesis by types and amounts of dietary fatty acids

Epidemiological studies and laboratory animal model assays suggest that a high intake of dietary fat promotes colorectal cancer. Several in vivo and in vitro studies support the hypothesis that omega-6 fatty acids promote colon tumorigenesis, whereas omega-3 fatty acids lack promoting activity. Fat intake in the United States traditionally includes high amounts (30% of total caloric intake) of saturated fat rather than omega-6 fatty acids. Therefore, the present study was designed to compare the modulatory effects of a high-fat diet containing mixed lipids (HFML), a diet rich in saturated fatty acids (the average American diet), a diet with fish oil (HFFO) that is rich in omega-3 fatty acids, and a low-fat corn oil diet (LFCO) on the formation of chemically induced colonic aberrant crypt foci (ACF) and tumors, cyclooxygenase (COX)-2 activity, and apoptosis during experimental colon carcinogenesis. At 5 weeks of age, groups of male F344 rats were fed a 5% corn oil diet (LFCO). At 7 weeks of age, rats intended for carcinogen treatment received s.c. injections of azoxymethane at a dose level of 15 mg/kg of body weight once weekly for 2 weeks. Beginning 1 day after the carcinogen treatment, groups of rats were then maintained on experimental diets containing 20% HFML or 20% HFFO. Rats were killed at 8, 23, or 38 weeks after azoxymethane treatment. Colonic ACF and tumors were evaluated histopathologically, and apoptosis was evaluated by the terminal deoxynucleotidyl transferase-mediated nick end labeling method. Colonic mucosae and tumor samples harvested at week 38 were analyzed for COX-2 synthetic activity and expression. The rats fed the HFML diet showed significantly increased total colonic ACF (P < 0.001-0.0001) with a multiplicity of > or = 4 aberrant crypts/focus (P < 0.0001) compared with the effects of the HFFO or LFCO diets at week 8, 23, and 38. Interestingly, there was a 2- to 3-fold increase (> or = 4) in multicrypt foci in rats given the HFML diet as compared with such foci in rats fed the HFFO or LFCO diets. By week 23, the HFML diet had significantly increased the incidence of colonic tumors (30-60%) and their multiplicity (100-141%) when compared with the effects of the LFCO or HFFO diets. At week 38, the HFML diet had induced 100% colon tumor incidence and a 4-fold multiplicity of adenocarcinomas compared with the LFCO and HFFO diets. At weeks 23 and 38, a significantly lower percentage of apoptotic colonic epithelial cells were observed in the tumors of animals fed the HFML diet as compared with those fed the HFFO diet. The HFML diet caused significantly increased levels of COX-2 activity in colon tumors (P < 0.05-0.01), and these tumors had enhanced levels of COX-2 expression as compared with those in assays with LFCO or HFFO diets. These observations demonstrate for the first time that HFML diets containing high levels of saturated fatty acids (such as those in Western diets) promote colon carcinogenesis. Although the mechanisms involved in colon tumor promotion by a HFML diet are not fully known, our results indicate that the modulation of eicosanoid production via the influence on COX activity and the suppression of apoptosis may play a key role in HFML diet-induced colon tumorigenesis.     

Effect of vegetable and carotenoid consumption on aberrant crypt multiplicity, a surrogate end-point marker for colorectal cancer in azoxymethane-induced rats

Epidemiological studies indicate that increased vegetable consumption reduces the risk of colorectal cancer mortality. In the present study we have investigated the effect of consumption of standard diets supplemented with freeze-dried vegetables (peas, spinach, sprouts and broccoli) and carotenoids (all-trans beta-carotene and palm oil carotenoid extract) on surrogate end-point markers for colorectal cancer in an azoxymethane-induced rat model. Mean aberrant crypt multiplicity was reduced (19%) by the pea-supplemented diet only (P < 0.05). The vegetable-induced effect was more apparent in aberrant crypt foci with higher multiplicity. Intervention with diets supplemented with peas, spinach, sprouts and a mix of all vegetables reduced the number of foci with >2 aberrant crypts/focus by 37, 26, 23 and 26%, respectively (P < 0.05). Even more pronounced effects were observed in foci with >3 aberrant crypts/focus, with reductions of approximately 50% in the pea and spinach intervention groups. All-trans beta-carotene and palm oil-derived carotenoids, supplied at similar doses to those expected in the vegetable diets, inhibited ACM only marginally. Aberrant crypt foci formation in groups fed a sprout-supplemented diet prior to or following azoxymethane treatment was similar, indicating that this effect is due to inhibition of promotion rather than initiation of colorectal carcinogenesis. Vegetable and carotenoid consumption did not affect in situ proliferation of colonic crypt cells, as assessed by semi-automated image analysis of bromodeoxyuridine (BrdU)-positive nuclei. BrdU-negative nuclei of colonic crypt cells were reduced slightly in the combined vegetable groups, as compared with the control (P < 0.05). These data: (i) are in line with epidemiological evidence regarding beneficial effects of vegetable consumption on colorectal carcinogenesis; (ii) indicate that consumption of several types of vegetables inhibits early post-initiation events in colorectal carcinogenesis; (iii) suggest that the vegetable-induced effect is more pronounced in advanced lesions; (iv) indicate that the carotenoid content of the vegetables (alpha- and beta-carotene) contributes only marginally to the vegetable-induced effects.     

Volatile isoprenoid constituents of fruits, vegetables and herbs cumulatively suppress the proliferation of murine B16 melanoma and human HL-60 leukemia cells.

Substantial evidence from epidemiological studies supports the inverse association between the intake of fruits, vegetables and other plant products and cancer incidence. Cancer-preventive constituents of fruits and vegetables may inhibit carcinogen activation, enhance carcinogen detoxification, prevent carcinogens from interacting with critical target sites, or impede tumor progression. These activities, however, are achievable only when levels of individual bioactive constituents reach beyond those attainable from a normal balanced diet. Isoprenoids, a broad class of mevalonate-derived phytochemicals ubiquitous in the plant kingdom, suppress the proliferation of tumor cells and the growth of implanted tumors. A search for volatile isoprenoid constituents of food products spanning seven plant families identified 179 isoprenoids. Of these, 41 purchased from commercial sources were screened for efficacy in suppressing the proliferation of murine B16 melanoma cells. Individual isoprenoids suppressed the proliferation of B16 and HL-60 promyelocytic leukemia cells with varying degrees of potency. Cell cycle arrest at the G(0)-G(1) phase and apoptosis account, at least in part, for the suppression. Blends of isoprenoids suppressed B16 and HL-60 cell proliferation with efficacies equal to the sum of the individual impacts. These findings suggest that the cancer-protective property of fruits, vegetables, and related products is partly conferred by the cumulative impact of volatile isoprenoid constituents.     

The effects of dietary selenomethionine on polyamines and azoxymethane-induced aberrant crypts

We evaluated the effects of dietary selenomethionine supplementation on colonic polyamine levels and the ability of L-selenomethionine supplementation to modulate the carcinogenic activity of azoxymethane (AOM) in the rat colon. Four-week-old male F344 rats were treated with 15 mg/kg body weight of AOM once a week for 2 weeks. Dietary selenomethionine at a concentration of either 1 or 2 ppm was administered in AIN-76A rodent diet to AOM-treated animals for 16 weeks. Aberrant crypt foci (ACF), precursor lesions of colon cancer, were investigated after the 16 week treatment course. Selenomethionine given in the diet at 2 ppm markedly reduced the number of aberrant crypt foci. The multiplicity of ACFs (i.e. the number of aberrant crypts/focus) and the percentage of microadenomas were also affected by selenomethionine in a dose dependent manner. However, evaluation of the colonic tissue polyamine levels between control and treated groups showed no significant difference. These results demonstrate that selenomethionine can modulate the development of AOM-induced premalignant lesions through a polyamine-independent mechanism.     

The anti-obesity agent Orlistat is associated to increase in colonic preneoplastic markers in rats treated with a chemical carcinogen

Orlistat is an anti-obesity agent that increases the fecal fat excretion, which promotes colon carcinogenesis. Therefore, the present study was designed to verify the effects of Orlistat on the formation of rat colonic aberrant crypt foci (ACF) and cell proliferation evaluated by the PCNA method. Male Wistar rats received either a standard diet or a high fat diet (HFD), supplemented or not with Orlistat (200 mg/kg chow) and two doses of the carcinogen dimethyl-hydrazine (25 mg/Kg). After 30 days, Orlistat was associated to a significant increase in the number of colonic ACFs and cell proliferation in DMH-treated animals, independently of the HFD.     

Characterisation of aberrant crypt foci in carcinogen-treated rats: association with intestinal carcinogenesis.

Carcinogen-treated rats develop foci of aberrant crypts in the colon (ACFs) that have been interpreted as preneoplastic lesions. To characterise ACFs further, we studied in the unsectioned colon of rats the number, multiplicity, some morphological characteristics and the type of mucin production in ACFs. In ACFs observed 115 days after the administration of 50 mg kg-1 1,2-dimethylhydrazine (DMH), crypt multiplicity [number of aberrant crypts (AC) per focus] was positively correlated (P < 0.0001) with the reduction of goblet cells, and with luminal and nuclear alterations in the cells surrounding the lumen of the ACs. We studied mucin production in the unsectioned colon, demonstrating that ACFs producing sulphomucins (like the normal distal rat colon) were progressively reduced when ACF multiplicity increased, whereas ACFs containing sialomucins (correlated with an increased risk of colon cancer) or both sulphomucins and sialomucins increased with crypt multiplicity. We also studied ACFs in the colon and the occurrence of intestinal tumours in rats treated with azoxymethane (AOM; 64 mg kg-1). A significant association was found (P = 0.04) between tumours and the presence of ''large'' ACFs (AC/ACF > 14 crypts) and a borderline significant association (P = 0.057) between the presence of tumours and sialomucin-producing ACFs. We found no association between the number of ACFs, ACF multiplicity and the presence of tumours.     

Effect of a calcium-enriched diet on the colonic epithelial hyperproliferation induced by N-methyl-N'-nitro-N-nitrosoguanidine in rats on a low calcium and fat diet

We examined whether hyperproliferation of colonic crypt epithelium during cancer induction by N-methyl-N-nitro-N-nitrosoguanidine (MNNG), in rats on a low fat and calcium diet could be reduced by added calcium p.o. From the age of 4 weeks, 104 male Sprague-Dawley rats received a low fat (3.5%), low calcium (0.05% calcium ion), and low vitamin D (0.4 IU/g) diet. Sixty-four also had calcium salts, derived from either calcium lactate or solubilized calcium carbonate, added to their drinking water; therefore their total calcium intake was about 1% of daily diet. At age 12 weeks the rats were divided into 4 treatment groups: 8 rats, not receiving added calcium, had rectal saline instillations weekly (saline control group) and were sacrificed after a further 28 weeks; 3 groups of 32 rats each received intrarectal MNNG (1.5 mg) weekly. One group, not receiving added calcium, was the MNNG control group; while the second group also received added calcium lactate, and the third group received calcium carbonate. Groups of 24 were sacrificed periodically until 28 weeks of treatment. Rats were sacrificed and epithelial proliferation was estimated, 1 week after the last intrarectal instillation, by in vivo labeling with tritiated thymidine and measuring the ratio of labeled to total colonic crypt epithelial cells. The mean labeling index of the MNNG treated and added calcium groups were significantly higher (8.7-9.5%) than that of the saline controls (2.8%) only at week 28; however, it was then still significantly less than that of the MNNG controls not having added calcium (17.9%). Hyperproliferation, during induction of colonic cancer by MNNG in rats on a low calcium diet, can be reduced by a calcium enriched diet even in the presence of a low fat intake.     

Chemopreventive effect of squalene on colon cancer

Epidemiologic and laboratory studies suggest a cancer protective effect and/or lack of a tumor promoting effect by dietary olive oil as compared with other types of non-marine oils. Squalene, a constituent of olive oil, and a key intermediate in cholesterol synthesis may be regarded as partially responsible for the beneficial effects of olive oil, which include decreased mortality rates among populations with high olive oil consumption. Thus, in this study we have assessed the chemopreventive efficacy of squalene on azoxymethane (AOM)-induced colonic aberrant crypt foci (ACF). In addition, we measured the effect of squalene on serum cholesterol levels in the rats. Male F34 rats (5 weeks old) were fed the control diet (modified AIN-76A) or experimental diets containing 1% squalene or 320 p.p.m. sulindac. Two weeks later, all animals except those in vehicle (normal saline)-treated groups were s.c. injected with AOM (15 mg/kg body wt, once weekly for 2 weeks). At 16 weeks of age, all rats were killed, colons were evaluated for ACF and serum was assayed for the cholesterol levels. As expected, dietary administration of sulindac suppressed ACF development and reduced crypt multiplicity, i.e. number of aberrant crypts/focus. Administration of dietary squalene inhibited total ACF induction and crypt multiplicity by approximately >46% (P < 0.001). Further, squalene at a level of 1% did not show any significant effect on serum cholesterol levels. Our finding that squalene significantly suppresses colonic ACF formation and crypt multiplicity strengthens the hypothesis that squalene possesses chemopreventive activity against colon carcinogenesis.      

Induction of apoptosis by sulindac in azoxymethane-induced possible colonic premalignant lesions in rats.

We have reported that beta-catenin-accumulated crypts (BCAC), which do not have the appearance of aberrant crypt foci (ACF) are possible colonic premalignant lesions in rats. Suppression of the occurrence and advancement of such lesions should have critical relevance to cancer prevention. This study examined whether sulindac, a chemopreventive nonsteroidal anti-inflammatory drug is able to induce apoptosis in such premalignant lesions. At 6 weeks of age, rats groups 1 - 3 were given azoxymethane (AOM) (15 mg/kg-body weight) once weekly for 3 weeks. Two groups were given sulindac in the diet (200 and 400 ppm), starting at 9 weeks of age. The rats were sacrificed at the termination, and the colons were carefully examined. The incidence and crypt multiplicity of BCAC and ACF were significantly less than those of the control group. The effect of sulindac on the expression of BCAC was greater than that on ACF. Exposure to sulindac significantly increased the apoptotic index (terminal deoxynucleotide transferase dUTP nick-end labeling (TUNEL)-positive cells) in BCAC. However, no significant increase of the index was found in the case of ACF. These results suggest that the chemopreventive effect of sulindac in rats is related to the induction of apoptosis in premalignant lesions. Our results also provide additional evidence that BCAC are premalignant lesions in colon carcinogenesis in rodents.     

Effects of soy or rye supplementation of high-fat diets on colon tumour development in azoxymethane-treated rats.

Evidence is accumulating that a diet high in plant-derived foods may be protective against cancer. One class of plant component under increasing investigation is the phytoestrogens of which there are two main groups: the isoflavones, found mainly in soy products, and the lignans, which are more ubiquitous and are found in fruit, vegetables and cereals with high levels being found in flaxseed. In this study, we have used carefully balanced high-fat (40% energy) diets: a control diet (containing low isoflavone soy protein as the sole protein source), a rye diet (the control diet supplemented with rye bran) and a soy diet (containing as protein source a high isoflavone soy protein). The effect of these diets on the development of colonic cancer was studied in F-344 rats treated with the carcinogen, azoxymethane (two doses of 15 mg/kg given 1 week apart). Colons from treated animals were examined for aberrant crypt foci (ACF) and tumours after 12 and 31 weeks. Results after 12 weeks showed no differences in the total number of ACF in the control, soy or rye bran groups. However, the soy group had increased numbers of small ACF (less than four crypts/focus) while the rye group had decreased numbers of large ACF (greater than six crypts/focus). Examination of colons after 31 weeks gave similar low numbers of ACF in each group with no differences in multiplicity. There were no differences in the number of tumours between the control (1.36 tumours/rat) and soy (1.38 tumours/rat) groups. However, there was a significant decrease in the number of tumours in the rye group (0.17 tumours/rat). These results suggest that soy isoflavones have no effect on the frequency of colonic tumours in this model while rye bran supplementation decreases the frequency of colon cancer. This effect is due not to a decrease in early lesions but in their progression to larger multi-crypt ACF. The study also supports the hypothesis that larger ACF are more predictive of subsequent tumorigenicity.     

Induction of Apoptosis by Sulindac in Azoxymethane-induced Possible Colonic Premalignant Lesions in Rats

We have reported that β-catenin-accumulated crypts (BCAC), which do not have the appearance of aberrant crypt foci (ACF) are possible colonic premalignant lesions in rats. Suppression of the occurrence and advancement of such lesions should have critical relevance to cancer prevention. This study examined whether sulindac, a chemopreventive nonsteroidal anti-inflammatory drug is able to induce apoptosis in such premalignant lesions. At 6 weeks of age, rats groups 1–3 were given azoxymethane (AOM) (15 mg/kg-body weight) once weekly for 3 weeks. Two groups were given sulindac in the diet (200 and 400 ppm), starting at 9 weeks of age. The rats were sacrificed at the termination, and the colons were carefully examined. The incidence and crypt multiplicity of BCAC and ACF were significantly less than those of the control group. The effect of sulindac on the expression of BCAC was greater than that on ACF. Exposure to sulindac significantly increased the apoptotic index (terminal deoxynucleotide transferase dUTP nick-end labeling (TUNEL)-positive cells) in BCAC. However, no significant increase of the index was found in the case of ACF. These results suggest that the chemopreventive effect of sulindac in rats is related to the induction of apoptosis in premalignant lesions. Our results also provide additional evidence that BCAC are premalignant lesions in colon carcinogenesis in rodents.     

Reduction in formation of 2-amino-1-methyl-6-phenylimidazo[4,5- b]pyridine (PhIP)-induced aberrant crypt foci in the rat colon by docosahexaenoic acid (DHA)

Docosahexaenoic acid (DHA), a major component of fish oil, suppresses the formation and growth of aberrant crypt foci induced by 1,2- dimethylhydrazine and azoxymethane. In the present study we examined the effects of intragastric gavage administration of DHA on the yield of rat colonic aberrant crypt foci due to treatment with a heterocyclic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), which induces colon cancer in male F344 rats and is considered to be a possible human colon carcinogen. Male F344 rats were given a standard diet (AIN-76A) and received 10 doses of PhIP (75 mg/kg body wt, by intragastric intubation, on days 1-5 and 8-12) with or without intragastric application of 1 ml DHA 4 h prior to each carcinogen treatment, followed by further DHA dosing. The numbers of PhIP-induced aberrant crypt foci per colon after 4 and 12 weeks DHA administration were significantly reduced to 47 and 38% respectively of the values obtained when PhIP alone was used. The mean number of aberrant crypts per focus was also decreased by DHA treatment. At week 4 the PhIP-DNA adduct levels in the colon of rats from the PhIP+DHA group were approximately two thirds of the PhIP group value. The results thus suggest that DHA exerts a preventive effect on PhIP-induced colon carcinogenesis.      

Cruciferous vegetables: cancer protective mechanisms of glucosinolate hydrolysis products and selenium

Dietetic professionals urge Americans to increase fruit and vegetable intakes. The American Institute of Cancer Research estimates that if the only dietary change made was to increase the daily intake of fruits and vegetables to 5 servings per day, cancer rates could decline by as much as 20%. Among the reasons cited for this health benefit are that fruits and vegetables are excellent sources of fiber, vitamins, and minerals. They also contain nonnutritive components that may provide substantial health benefits beyond basic nutrition. Examples of the latter are the glucosinolate hydrolysis products, sulforaphane, and indole-3-carbinol. Epidemiological studies provide evidence that the consumption of cruciferous vegetables protects against cancer more effectively than the total intake of fruits and vegetables. This review describes the anticarcinogenic bioactivities of glucosinolate hydrolysis products, the mineral selenium derived from crucifers, and the mechanisms by which they protect against cancer. These mechanisms include altered estrogen metabolism, protection against reactive oxygen species, altered detoxification by induction of phase II enzymes, decreased carcinogen activation by inhibition of phase I enzymes, and slowed tumor growth and induction of apoptosis.     

Upregulation of p21Waf1/Cip1 expression in vivo by butyrate administration can be chemoprotective or chemopromotive depending on the lipid component of the diet

The overall goal of this research was to separate out the effects of butyrate from its fiber source and determine in vivo if it upregulates colonic histone acetylation, p21(Waf1/Cip1) expression (p21) and apoptosis and if this sequela of events is protective against aberrant crypt foci (ACF) formation. Eighty Sprague-Dawley rats were provided defined diets with either corn oil or fish oil as the lipid source, +/- butyrate-containing capsules targeted for release in the colon and +/- azoxymethane (AOM) (10 rats per group). Diets were provided for 11 weeks and at termination colonocyte nuclear histone H4 and p21 expression were determined by immunohistochemistry, apoptosis was measured by the terminal deoxynucleotide transferase biotin-dUTP nick end labeling assay and aberrant crypt numbers and multiplicity were enumerated. Luminal butyrate levels were also quantified. AOM injection repressed p21 expression, which was reversed by butyrate supplementation. Although butyrate enhanced p21 expression with both dietary lipid sources, the increase in p21 resulted in an increase in apoptosis and decrease in ACF with fish oil, but had no effect on apoptosis and increased ACF with corn oil. This significant interaction between fat, butyrate (fiber) and p21 expression with one combination being protective and the other promotive of colon carcinogenesis reinforces the importance of considering diet as a key factor in chemoprevention.     

Chemoprevention of colonic aberrant crypt foci by an inducible nitric oxide synthase-selective inhibitor

Inducible nitric oxide synthase (iNOS) is overexpressed in colonic tumors of humans and also in rats treated with a colon carcinogen. iNOS appear to regulate cyclooxygenase-2 (COX-2) expression and production of proinflammatory prostaglandins, which are known to play a key role in colon tumor development. Experiments were designed to study the inhibitory effects of S,S'-1,4-phenylene-bis(1,2-ethanediyl)bis-isothiourea (PBIT) a selective iNOS-specific inhibitor, measured against formation of azoxymethane (AOM)-induced colonic aberrant crypt foci (ACF). Beginning at 5 weeks of age, male F344 rats were fed experimental diets containing 0 or 50 p.p.m. of PBIT, or 2000 p.p.m. of curcumin (non-specific iNOS inhibitor). One week later, rats were injected s.c. with AOM (15 mg/kg body wt, once weekly for 2 weeks). At 17 weeks of age, all rats were killed, colons were evaluated for ACF formation and colonic mucosa was assayed for isoforms of COX and NOS activities. Both COX and iNOS activities in colonic mucosa of the AOM-treated rats were significantly induced. Importantly, 50 p.p.m. PBIT suppressed AOM-induced colonic ACF formation to 58% (P < 0.0001) and crypt multiplicity containing four or more crypts per focus to 78% (P < 0.0001); it also suppressed AOM-induced iNOS activity. Curcumin inhibited colonic ACF formation by 45% (P < 0.001). These observations suggest that iNOS may play a key regulatory role in colon carcinogenesis. Developing iNOS-specific inhibitors may provide a selective and safe chemopreventive strategy for colon cancer treatment.     

Detoxifying cancer causing agents to prevent cancer

Different vitamins and other micronutrients in vegetables, fruits, and other natural plant products may prevent cancer development (carcinogenesis) by interfering with detrimental actions of mutagens, carcinogens, and tumor promoters. The goal of current studies in cancer prevention is to determine the mechanisms of synergistic action of the natural source compounds known to inhibit one or more stages of carcinogenesis, that is, initiation and promotion/progression. Many natural cancer preventive agents are effective inhibitors of tumor initiation, promotion, and/or progression. The mechanism of action is related to their abilities to prevent critical carcinogen metabolism and to increase detoxification of carcinogens and tumor promoters. The authors review here the potential role of the detoxification system and, in particular, the roles of D-glucaric acid and the enzyme beta-glucuronidase in early detection and prevention of cancer. There is now growing evidence for the possible control of different stages of the cancer induction by inhibiting beta-glucuronidase with D-glucaric acid derivatives, especially with its salts (D-glucarates). D-Glucaric acid has been found in many vegetables and fruits. Therefore, the consumption of fruits and vegetables naturally rich in D-glucaric acid or self-medication with D-glucaric acid derivatives such as calcium D-glucarate offers a promising cancer prevention approach.