Slow-release pellets of sodium butyrate increase apoptosis in the colon of rats treated with azoxymethane, without affecting aberrant crypt foci and colonic proliferation

Microbial Metabolites as Molecular Mediators of Host-Microbe Symbiosis in Colorectal Cancer

The symbiosis between the gut microbiota and the host has been identified as an integral part of normal human physiology and physiological development. Research in germ-free or gnotobiotic animals has demonstrated the importance of this symbiosis in immune, vascular, hepatic, respiratory and metabolic systems. Disruption of the microbiota can also contribute to disease, and the microbiota has been implicated in numerous intestinal and extra-intestinal pathologies including colorectal cancer. Interactions between host and microbiota can occur either directly or indirectly, via microbial-derived metabolites. In this chapter, we focus on two major products of microbial metabolism, short-chain fatty acids and bile acids, and their role in colorectal cancer. Short-chain fatty acids are the products of microbial fermentation of complex carbohydrates and confer protection against cancer risk, while bile acids are compounds which are endogenous to the host, but undergo microbial modification in the large intestine leading to alterations in their bioactivity. Lastly, we discuss the ability of microbial modulation to mediate cancer risk and the potential to harness this ability as a prophylactic or therapeutic treatment in colorectal cancer.

Calcium butyrate: Anti-inflammatory effect on experimental colitis in rats and antitumor properties

Butyric acid is a physiological component of the colonic environment that possesses anti-inflammatory and antitumor properties, among others. However, little is known regarding its effects following direct application on the colonic surface. This study was conducted to investigate the topical anti-inflammatory effect of calcium butyrate in chemically-induced colitis in rats and to evaluate its antitumor properties in vivo and in vitro. The anti-inflammatory activity of calcium butyrate was evaluated in dinitrobenzene sulfonic acid-induced colitis in rats, following intracolonic instillation for 6 consecutive days and its in vivo antitumor activity was evaluated in F344 rats with the azoxymethane (AOM)-induced aberrant crypt foci (AFC) test, following intracolonic instillation for 4 weeks. The in vitro antiproliferative activity was assessed by incubation for 48 h with the HT29, SW620 and HCT116 intestinal tumour cell lines to evaluate the rate of ³H-thymidine uptake. In dinitrobenzene-induced colitis, the intracolonic instillation of calcium butyrate completely prevented body weight reduction in the animals and counteracted the local noxious effects of the irritant by reducing colon edema (-22.7%, P=0.048) and the area of mucosal damage (-48%, P=0.045). In the AOM-induced AFC test, the intracolonic instillation of calcium butyrate significantly reduced the number of AFC in the entire colon (-22.7%, P<0.05). Calcium butyrate, following incubation with the HT29, SW620 and HCT116 tumour cell lines, induced a significant antiproliferative, dose-dependent effect (P=0.046 to P=0.002) in all three strains, as measured by the reduction in ³H-thymidine uptake. Calcium butyrate directly applied to the mucosa of the rat colon was able to ameliorate colonic inflammation, suggesting a possible beneficial role in the treatment of inflammatory colon diseases. Moreover, calcium butyrate exhibited notable antitumor effects in vivo and in vitro; however, their clinical relevance requires confirmation by additional clinical investigations.

Effect of sodium butyrate on DMH-induced small intestinal and large intestinal tumors in rats

AIM: To investigate whether 1,2-dimethylhydrazine (DMH) can induce small intestinal tumors in rats and to examine the effect of sodium butyrate on DMH-induced small intestinal and large intestinal tumors.

METHODS: Eighty male Wistar rats were randomly divided into four groups: DMH group, DMH + NaBt group, NaBt group, and control group. After 30-32 weeks, rats were euthanized with an overdose of intravenous pentobarbital (200 mg/kg). After laparotomy, the small intestine and large intestine were dissected. The location, number, shape and size of intestinal tumors were examined and recorded. All tissues were subjected to hematoxylin and eosin (H&E) staining to observe histological changes.

RESULTS: The mortality rate of rats was 60.00% (18/30) in the DMH group and 48.00% (12/25) in the DMH + NaBt group. The intestinal tumor incidence was 66. 67% (8/12) in the DMH group with four small intestinal tumors and twelve large intestinal tumors observed. Four rats beared a single tumor while other four rats had multiple tumors. The mean number of beared tumors was 1.33. The intestinal tumor incidence was 84.62% (11/13) in the DMH + NaBt group with three small intestinal tumors and sixty large intestinal tumors observed. Six rats beared a single tumor while other five rats had multiple tumors. The mean number of beared tumors was 1.46. There were no significant differences in tumor incidence and mean tumor number between te DMH + NaBt group and DMH group. The large intestinal tumor incidence was significantly higher than the small intestinal tumor incidence in both the DMH group and DMH + NaBt group (75.00% vs 25.00%, P < 0.05; 84.21% vs 15.79%, P < 0.01). There were significant differences in average tumor volume (37.50% vs 73.68%, P < 0.05) and tumor infiltration depth (43.75% vs 10.53%, P < 0.05) between the DMH group and DMH + NaBt group.

CONCLUSION: These results suggest that small intestinal tumors can also be induced by DMH. Sodium butyrate can increase tumor malignancy by increasing tumor volume and infiltration depth.

Butyrate delivered by butyrylated starch increases distal colonic epithelial apoptosis in carcinogen-treated rats

Animal studies show that increasing large bowel butyrate concentration through ingestion of butyrylated or resistant starches opposes carcinogen-induced tumorigenesis, which is consistent with population data linking greater fiber consumption with lowered colorectal cancer (CRC) risk. Butyrate has been shown to regulate the apoptotic response to DNA damage. This study examined the impact of increasing large bowel butyrate concentration by dietary butyrylated starch on the colonic epithelium of rats treated with the genotoxic carcinogen azoxymethane (AOM). Four groups of 10 male rats were fed AIN-93G based-diets containing either low amylose maize starch (LAMS), LAMS with 3% tributyrin, 10% high amylose maize starch (HAMS) or 10% butyrylated HAMS (HAMSB). HAMS and HAMSB starches were cooked by heating in water. After 4 weeks, rats were injected once with AOM and killed 6 h later. Rates of apoptosis and proliferation were measured in colonic epithelium. Short-chain fatty acid concentrations in large bowel digesta and hepatic portal venous plasma were higher in HAMSB than all other groups. Apoptotic rates in the distal colon were increased by HAMSB and correlated with luminal butyrate concentrations but cellular proliferation rates were unaffected by diet. The increase in apoptosis was most marked in the base and proliferative zone of the crypt. Regulation of luminal butyrate using HAMSB increases the rates of apoptotic deletion of DNA-damaged colonocytes. We propose this pro-apoptotic function of butyrate plays a major role reducing tumour formation in the AOM-treated rat and that these data support a potential protective role of butyrate in CRC.

Fecal butyrate levels vary widely among individuals but are usually increased by a diet high in resistant starch

Butyrate and other SCFA produced by bacterial fermentation of resistant starch (RS) or nonstarch polysaccharides (NSP) promote human colonic health. To examine variation in fecal variables, especially butyrate, among individuals and the response to these fibers, a randomized cross-over study was conducted that compared the effects of foods supplying 25 g of NSP or 25 g of NSP plus 22 g of RS/d over 4 wk in 46 healthy adults (16 males, 30 females; age 31-66 y). Fecal SCFA levels varied widely among participants at entry (butyrate concentrations: 3.5-32.6 mmol/kg; butyrate excretions: 0.3-18.2 mmol/48 h). BMI explained 27% of inter-individual butyrate variation, whereas protein, starch, carbohydrate, fiber, and fat intake explained up to 16, 6, 2, 4, and 2% of butyrate variation, respectively. Overall, acetate, butyrate, and total SCFA concentrations were higher when participants consumed RS compared with entry and NSP diets, but individual responses varied. Individual and total fecal SCFA excretion, weight, and moisture were higher than those for habitual diets when either fiber diet was consumed. SCFA concentrations (except butyrate) and excretions were higher for males than for females. Butyrate levels increased in response to RS in most individuals but often decreased when entry levels were high. Fecal butyrate and ammonia excretions were positively associated ((2) = 0.76; P < 0.001). In conclusion, fecal butyrate levels vary widely among individuals but consuming a diet high in RS usually increases levels and may help maintain colorectal health.

Reduction of colonic inflammation in HLA-B27 transgenic rats by feeding Marie Ménard apples, rich in polyphenols

Inflammatory bowel diseases (IBD) are immunomediated ailments affecting millions of individuals. Although diet is regarded as an important factor influencing IBD, there are no accepted dietary recommendations presently available. We administered 7.6 % lyophilised apples obtained from two cultivars (Golden Delicious and Marie Ménard, low and high in polyphenols, respectively) to HLA-B27 transgenic rats which develop spontaneous IBD. After 3 months feeding, rats fed Marie Ménard apples had reduced myeloperoxidase activity (3.6 (sem 0.3) v. 2.2 (sem 0.2) U/g tissue; P < 0.05) and reduced cyclo-oxygenase-2 (P < 0.05) and inducible NO synthase gene expression (P < 0.01) in the colon mucosa and significantly less diarrhoea (P < 0.05), compared with control rats. Cell proliferation in the colon mucosa was reduced significantly by feeding Golden Delicious apples, with a borderline effect of Marie Ménard apples. Gene expression profiling of the colon mucosa, analysed using the Whole Rat Genome 4 x 44 K Agilent Arrays, revealed a down-regulation of the pathways of PG synthesis, mitogen-activated protein kinase (MAPK) signalling and TNFalpha-NF-kappaB in Marie Ménard-fed rats. In the stools of the animals of this group we also measured a significant reduction of bacteria of the Bacteriodes fragilis group. In conclusion, the administration of Marie Ménard apples, rich in polyphenols and used at present only in the manufacturing of cider, ameliorates colon inflammation in transgenic rats developing spontaneous intestinal inflammation, suggesting the possible use of these and other apple varieties to control inflammation in IBD patients.

3,3'-diindolylmethane enhances the efficacy of butyrate in colon cancer prevention through down-regulation of survivin

Butyrate is an inhibitor of histone deacetylase (HDAC) and has been extensively evaluated as a chemoprevention agent for colon cancer. We recently showed that mutations in the adenomatous polyposis coli (APC) gene confer resistance to HDAC inhibitor-induced apoptosis in colon cancers. Here, we show that APC mutation rendered colon cancer cells resistant to butyrate-induced apoptosis due to the failure of butyrate to down-regulate survivin in these cells. Another cancer-preventive agent, 3,3'-diindolylmethane (DIM), was identified to be able to down-regulate survivin in colon cancers expressing mutant APC. DIM inhibited survivin mRNA expression and promoted survivin protein degradation through inhibition of p34(cdc2)-cyclin B1-mediated survivin Thr(34) phosphorylation. Pretreatment with DIM enhanced butyrate-induced apoptosis in colon cancer cells expressing mutant APC. DIM/butyrate combination treatment induced the expression of proapoptotic Bax and Bak proteins, triggered Bax dimerization/activation, and caused release of cytochrome c and Smac proteins from mitochondria. Whereas overexpression of survivin blocked DIM/butyrate-induced apoptosis, knocking down of survivin by small interfering RNA increased butyrate-induced apoptosis in colon cancer cells. We further showed that DIM was able to down-regulate survivin and enhance the effects of butyrate in apoptosis induction and prevention of familial adenomatous polyposis in APC(min/+) mice. Thus, the combination of DIM and butyrate is potentially an effective strategy for the prevention of colon cancer.

Chemoprevention of rat hepatocarcinogenesis with histone deacetylase inhibitors: efficacy of tributyrin, a butyric acid prodrug

Hepatocellular carcinoma (HCC) ranks in prevalence and mortality among top 10 cancers worldwide. Butyric acid (BA), a member of histone deacetylase inhibitors (HDACi) has been proposed as an anticarcinogenic agent. However, its short half-life is a therapeutical limitation. This problem could be circumvented with tributyrin (TB), a proposed BA prodrug. To investigate TB effectiveness for chemoprevention, rats were treated with the compound during initial phases of "resistant hepatocyte" model of hepatocarcinogenesis, and cellular and molecular parameters were evaluated. TB inhibited (p < 0.05) development of hepatic preneoplastic lesions (PNL) including persistent ones considered HCC progression sites. TB increased (p < 0.05) PNL remodeling, a process whereby they tend to disappear. TB did not inhibit cell proliferation in PNL, but induced (p < 0.05) apoptosis in remodeling ones. Compared to controls, rats treated with TB presented increased (p < 0.05) hepatic levels of BA indicating its effectiveness as a prodrug. Molecular mechanisms of TB-induced hepatocarcinogenesis chemoprevention were investigated. TB increased (p < 0.05) hepatic nuclear histone H3K9 hyperacetylation specifically in PNL and p21 protein expression, which could be associated with inhibitory HDAC effects. Moreover, it reduced (p < 0.05) the frequency of persistent PNL with aberrant cytoplasmic p53 accumulation, an alteration associated with increased malignancy. Original data observed in our study support the effectiveness of TB as a prodrug of BA and as an HDACi in hepatocarcinogenesis chemoprevention. Besides histone acetylation and p21 restored expression, molecular mechanisms involved with TB anticarcinogenic actions could also be related to modulation of p53 pathways.

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.

Application of prodrugs to inflammatory diseases of the gut

Oral delivery is the most common and preferred route of drug administration although the digestive tract exhibits several obstacles to drug delivery including motility and intraluminal pH profiles. The gut milieu represents the largest mucosal surface exposed to microorganisms with 10^10-12 colony forming bacteria/g of colonic content. Approximately, one third of fecal dry matter is made of bacteria/ bacterial components. Indeed, the normal gut microbiota is responsible for healthy digestion of dietary fibers (polysaccharides) and fermentation of short chain fatty acids such as acetate and butyrate that provide carbon sources (fuel) for these bacteria. Inflammatory bowel disease (IBD) results in breakage of the mucosal barrier, an altered microbiota and dysregulated gut immunity. Prodrugs that are chemically constructed to target colonic release or are degraded specifically by colonic bacteria, can be useful in the treatment of IBD. This review describes the progress in digestive tract prodrug design and delivery in light of gut metabolic activities.

Bacterial population dynamics and faecal short-chain fatty acid (SCFA) concentrations in healthy humans

Fermentation products, SCFA, particularly butyrate, are considered a sign of 'good' bowel health but the influence of bacterial population composition and diet on inter-individual difference in metabolites and colonic health is poorly understood. Faecal specimens were collected weekly from eight healthy human volunteers over 12 weeks. Dietary intake was self-reported and ten macronutrient factors were analysed at selected weekly periods. Faecal weight, pH and moisture were recorded, and SCFA concentrations were measured in all samples. From each specimen, DNA was prepared and eubacterial 16S rRNA gene PCR performed. Bacterial population profiles were captured by denaturing gradient gel electrophoresis (DGGE) of PCR products, and multivariate statistical analysis was performed. Faecal weight, pH and moisture varied widely within and between individuals. Average total SCFA concentrations over 12 weeks ranged from 36.9 to 144.4 mmol/kg in 48 h specimens and faecal butyrate concentrations ranged from 1.8 to 48.5 mmol/kg. Two individuals with butyrate concentrations below 10 mmol/kg were considered to be 'low butyrate types' and may represent an at-risk population for bowel health. Dietary fat, sugar and carbohydrate showed weak correlation with SCFA (R - 0.612, P = 0.015; R 0.607, P = 0.016; R 0.610, P = 0.016, respectively) and butyrate concentrations (R - 0.593, P = 0.02; R 0.504, P = 0.054; R 0.528, P = 0.043, respectively). Multivariate analysis of DGGE bacterial profiles demonstrated concise and repeated grouping of intra-individual samples, but these were combined with distinct inter-individual differences (analysis of similarities P < 0.001, R > or = 0.99) The exact relationship of these SCFA values to the overall bacterial profiles and SCFA-producer bacterial groups was not direct nor linear.

Wholegrain foods made from a novel high-amylose barley variety (Himalaya 292) improve indices of bowel health in human subjects

Himalaya 292 (Hordeum vulgare var. Himalaya 292) is a novel hull-less barley variety lacking activity of a key enzyme of starch synthesis giving a grain containing less total starch, more amylose and higher total dietary fibre. Animal trials have shown that Himalaya 292 contains more resistant starch and has greater positive impact on biomarkers of large-bowel health than comparable wholegrain cereal products. The present study compared the effects of foods made from wholegrain Himalaya 292 with those made from wholegrain wheat on faecal biomarkers of bowel health in human subjects. Seventeen male and female volunteers aged 31-66 years consumed similar quantities of Himalaya 292, whole-wheat or refined cereal foods daily for 4 weeks in a randomised cross-over design. Total dietary fibre intakes from weighed food records were 45, 32 and 21 g/d for the Himalaya 292, whole-wheat and refined cereal periods, respectively. Compared with the refined cereal foods, consumption of Himalaya 292 foods resulted in 33 % higher faecal weight, a lowering of faecal pH from 7.24 to 6.98, a 42 % higher faecal concentration and a 91 % higher excretion of butyrate, a 57 % higher faecal total SCFA excretion and a 33 % lower faecal p-cresol concentration. pH and butyrate concentration and excretion were also significantly different compared with wholemeal wheat. It is concluded that consumption of a diet that included foods made from Himalaya 292 supplied more fibre and improved indices of bowel health compared with refined cereal foods and, for some indices, similar wholemeal wheat foods at equivalent levels of intake.

Does butyrate protect from colorectal cancer?

Butyrate, the four-carbon fatty acid, is formed in the human colon by bacterial fermentation of carbohydrates (including dietary fiber), and putatively suppresses colorectal cancer (CRC). Butyrate has diverse and apparently paradoxical effects on cellular proliferation, apoptosis and differentiation that may be either pro-neoplastic or anti-neoplastic, depending upon factors such as the level of exposure, availability of other metabolic substrate and the intracellular milieu. In humans, the relationship between luminal butyrate exposure and CRC has been examined only indirectly in case-control studies, by measuring fecal butyrate concentrations, although this may not accurately reflect effective butyrate exposure during carcinogenesis. Perhaps not surprisingly, results of these investigations have been mutually contradictory. The direct effect of butyrate on tumorigenesis has been assessed in a number of in vivo animal models, which have also yielded conflicting results. In part, this may be explained by methodological differences in the amount and route of butyrate administration, which are likely to significantly influence delivery of butyrate to the distal colon. Nonetheless, there appears to be some evidence that delivery of an adequate amount of butyrate to the appropriate site protects against early tumorigenic events. Future study of the relationship between butyrate and CRC in humans needs to focus on risk stratification and the development of feasible strategies for butyrate delivery.

The influence of specific luminal factors on the colonic epithelium: high-dose butyrate and physical changes suppress early carcinogenic events in rats

INTRODUCTION

Although luminal delivery of butyrate is one putative mechanism by which biology of the colonic epithelium might be influenced by changes in luminal contents, there is a paucity of supportive cause-effect evidence. This study aimed to directly establish whether distal colonic butyrate delivery is able to alter the response of the distal colonic epithelium to a carcinogen.

METHODS

Groups of male Sprague-Dawley rats with chronically intubated colons received infusions of 80 mM butyrate or 0.9 percent saline into distal colon two or five times daily. Three weeks after exposure to azoxymethane (15 mg/kg subcutaneously), the density of aberrant crypts was quantified in distal colon.

RESULTS

Infusions of 0.5 ml twice daily, whether containing saline or butyrate, decreased the number of aberrant crypt foci by 45 percent compared with rats receiving no infusions (P = 0.004, analysis of variance). Similar results were obtained when infusions were restricted to the post-initiation phase. When infusions were increased to 1 ml five times daily, saline infusions similarly suppressed aberrant crypt formation (38 percent), but butyrate infusions suppressed it to a greater degree (by 64 percent; P = 0.02 compared with saline infusion, t-test).

CONCLUSIONS

High levels of butyrate delivery to the distal colonic lumen alter the epithelial response to a carcinogen in otherwise healthy rats. This finding directly supports the notion that the effects of butyrate on cells in vitro do occur in vivo provided a sufficient dose is delivered. The effect of infusion of liquid per se on the epithelial response highlights the potential impact physical changes alone can have on the colonic epithelium.

Acetylated, propionylated or butyrylated starches raise large bowel short-chain fatty acids preferentially when fed to rats

Maize starch was acylated with acetic, propionic or butyric anhydride to produce the corresponding acylated starch. In the first experiment, butyrylated starch at a degree of substitution (DS) of 0.25 (i.e., 1 acyl unit per 4 glucosyl units) was fed to rats for 3 d. Cecal and distal colonic SCFA concentrations were 170 and 78% higher, respectively, in rats fed the butyrylated starch. However, the greatest increase was in butyrate with corresponding increases of 460 and 212%. Subsequently, acetylated, propionylated or butyrylated starches with DS of approximately 0.18 were prepared on a larger scale. Body weight gain did not differ between rats fed these acylated starches or a control starch for 14 d. Large bowel pH was significantly lower and digesta mass significantly higher throughout the large bowel in rats fed the acylated starches. Cecal + distal colonic starch averaged 12 mg in rats fed the control starch and 103, 134 and 135 (pooled SEM = 6) mg in rats fed acetylated, propionylated or butyrylated starch, respectively. Large bowel SCFA concentrations and pools were significantly higher in rats fed the three acylated starches and were disproportionately greater in the SCFA that had been esterified to the starch. In the cecum, acetate, propionate and butyrate pools were 280, 690 and 1060% higher, respectively, in rats fed the corresponding acylated starch than in those fed the control diet. In the distal colon, the corresponding increases were 320, 940 and 1370%. These data indicate that acylated starches are resistant starch (RS) and raise large bowel SCFA, apparently through bacterial release of the esterified fatty acid and fermentation of the residual starch.

New approaches to the role of diet in the prevention of cancers of the alimentary tract

Cancers of the alimentary tract are, collectively, amongst the major causes of morbidity and deaths from cancer across the world today. Of the 10 million new cases of cancer diagnosed in 2000, about 2.3 million were cancers of the pharynx, oesophagus, stomach or colorectum. Nevertheless, epidemiological studies indicate that cancers of the digestive organs are also amongst the most susceptible to modification by dietary factors. International variations in incidence suggest that round three quarters of all sporadic colorectal cancers are attributable to diet. Even within the relatively uniform environment of the European Union, there are variations in the incidence of colorectal and oesophageal cancers of about two- and six-fold, respectively. Carcinomas of the alimentary tract arise from epithelial cells via distinct sequences of neoplastic change, which require a large fraction of an individual's lifespan. The best characterised of these is the adenoma-carcinoma sequence of colorectal carcinogenesis, in which progressive loss of differentiation and normal morphology in a growing lesion is associated with the acquisition of somatic mutations, and of aberrant methylation of CpG-islands, leading to gene silencing. These molecular events are accompanied by functional changes, including increased mitosis and evasion of apoptosis. There is little evidence that diet exerts its effects primarily through food-borne carcinogens that can be identified and eliminated from the food-chain. It is far more probable that the adverse effects of diet are caused largely by over-consumption of energy, coupled with inadequate intakes of protective substances, including micronutrients, dietary fibre and a variety of phytochemicals. The latter are biologically active secondary plant metabolites, many of which modify cell proliferation and induce apoptosis in vitro. There is growing evidence that such effects also occur in vivo, and that they can suppress the progress of neoplasia. Carcinomas of the oesophagus, stomach and colon all appear to be partially preventable by diets rich in fruits and vegetables. Plant foods contain a variety of components including micronutrients, polyunsaturated fatty acids, and secondary metabolites such as glucosinolates and flavonoids, many of which can inhibit cell proliferation and induce apoptosis, and which may well act synergistically when combined in the human diet. The future challenge is to fully characterise and evaluate these effects at the cellular and molecular level, so at to exploit their full potential as protective mechanisms for the population as a whole.

Microbial degradation products influence colon cancer risk: the butyrate controversy

All dietary fiber, by definition, escapes digestion in the small intestine and thus arrives relatively intact in the large intestine. Its fate in the large intestine depends upon the type of fiber and the colonic microflora. Highly fermentable fibers result in short chain fatty acids including butyrate, which is thought by some to be protective against colon cancer. However, not all studies support a chemopreventive effect for butyrate and the lack of agreement (particularly between in vivo and in vitro studies) on butyrate and colon cancer has been termed the "butyrate paradox." There are a number of reasons for this discrepant effect including differences between the in vitro and in vivo environments, the timing of butyrate administration, the amount of butyrate administered, the source of butyrate (usually dietary fiber) as a potential confounder, and an interaction with dietary fat. Collectively, the studies suggest that the chemopreventive benefits of butyrate depend in part on amount, time of exposure with respect to the tumorigenic process, and the type of fat in the diet.

Different molecular events account for butyrate-induced apoptosis in two human colon cancer cell lines

We studied the molecular events underlying butyrate-induced apoptosis in two different colon cancer cell lines: Caco-2, a well defined cancer cell and RSB, a cell line obtained from a colonic tumor of an ulcerative colitis patient. Caco-2 and RSB cells were exposed to 2, 5 and 10 mmol/L butyrate for 48 h. Caspase-1 was cleaved in Caco-2-cells at all butyrate concentrations, whereas in RSB-cells caspase-1 expression was undetectable. In RSB cells, butyrate dose-dependently induced caspase-3 cleavage, whereas in Caco-2-cells, butyrate up-regulated expression of the caspase-3 active subunit. Caspase-3-specific activity, cytoplasmic nucleosome concentration and growth were directly correlated with butyrate doses in both cell lines; however, the response was more pronounced in Caco-2 than in RSB cells. Expression of the cleaved poly(ADP-ribose) polymerase (PARP) product was elevated in both cell lines at the highest butyrate concentration. Bak expression gradually increased as a function of butyrate concentrations in both cell lines. At 10 mmol/L butyrate, expression increased by fivefold and sevenfold in Caco-2 and RSB cells, respectively. The highest expression of Bcl-2 was observed in control Caco-2 cells, and expression decreased with increasing butyrate concentration. This effect was not observed in RSB cells. Inactivation of caspase-1 with Z-YVAD-FMK abrogated butyrate-induced apoptosis in Caco-2 but not in RSB cells. Inactivation of caspase-3 with Z-DVED-FMK completely inhibited butyrate-induced apoptosis in RSB cells whereas this effect was less pronounced in Caco-2 cells. Our data demonstrate that butyrate-induced apoptosis is activated via different apoptotic pathways in diversely stratified colon cancers.

Most effective colon cancer chemopreventive agents in rats: a systematic review of aberrant crypt foci and tumor data, ranked by potency

Potential chemopreventive agents for colorectal cancer are assessed in rodents. We speculated that the magnitude of the effect is meaningful and ranked all published agents according to their potency. Data were gathered systematically from 137 articles with the aberrant crypt foci (ACF) end point and from 146 articles with the tumor end point. The potency of each agent to reduce the number of ACF is listed in one table and the potency of each agent to reduce the tumor incidence in another table. Both tables are shown in this review and on a website with sorting abilities (http://www.inra.fr/reseau-nacre/sci-memb/corpet/indexan.html). Potency was estimated as the ratio of the value in control rats to the value in treated rats. From each article, only the most potent agent was kept, except in articles reporting the effect of more than seven agents. Among the 186 agents in the ACF table, the median agent reduced the number of ACF by one-half. The most potent agents to reduce azoxymethane-induced ACF were Pluronic, polyethylene glycol, perilla oil with beta-carotene, and sulindac sulfide. Among the 160 agents in the tumor table, the median agent reduced the tumor incidence in rats by one-half. The most potent agents to reduce the incidence of azoxymethane-induced tumors were celecoxib, a protease inhibitor from soy, difluoromethylornithine with piroxicam, polyethylene glycol, and a thiosulfonate. For the 57 agents present in both tables, a significant correlation (r) was found between the potencies against ACF and tumors (r = 0.45, P < 0.001); without celecoxib, a major outlying point in the correlation, r = 0.68 (P < 0.001, n = 56). In conclusion, this review gathers most known chemopreventive agents, ranks the most promising agents against colon carcinogenesis in rats or mice, and further supports the use of ACF as a surrogate end point for tumors in rats.

Dietary fiber and colorectal neoplasia

PURPOSE

Dietary fiber has been implicated in colorectal neoplasia, despite conflicting evidence. This is a review of the currently available data on the role of dietary fiber in colorectal carcinogenesis.

METHODS

A literature search was conducted using the MEDLINE database. All case-control, longitudinal, and randomized, controlled studies published in English between 1988 and 2000 were identified, as were animal model studies in the period 1986 to 2000. Data from the various studies were tabulated and systematically analyzed, with particular emphasis on the effect of dietary fiber on tumor incidence and luminal parameters such as short chain fatty acids.

RESULTS

Epidemiologic correlation studies show a high intake of dietary fiber to be associated with a lower risk of colorectal neoplasia. Thirteen of the 24 case-control studies reviewed demonstrated a protective effect of dietary fiber against colorectal neoplasia, and 16 showed a protective effect of vegetables or vegetable fiber. On the other hand, of 13 longitudinal studies in various cohorts, only 3 demonstrated a protective effect of fiber and 4 a protective effect of vegetables or vegetable fiber. The five published randomized, controlled trials all investigated the effect of increased fiber intake on short-term adenoma recurrence; however, none showed any significant protective effect. Among 19 experimental studies in animal models, 15 showed a protective effect of fiber against tumor induction compared with controls. Animal studies also showed that poorly fermentable fibers (e.g., wheat bran and cellulose) were more protective than soluble fibers (e.g., guar gum and oat bran), which sometimes enhanced carcinogenesis. No clear correlation was found between luminal pH or short chain fatty acids and tumor induction.

CONCLUSIONS

On the basis of current data, there is little evidence to support the use of dietary fiber supplements to reduce the risk of colorectal neoplasia. Lifelong and early exposure may be important but are difficult to study. Other risk factors interact with the effects of dietary fiber.

Short-chain fatty acids and human colonic function: roles of resistant starch and nonstarch polysaccharides

Resistant starch (RS) is starch and products of its small intestinal digestion that enter the large bowel. It occurs for various reasons including chemical structure, cooking of food, chemical modification, and food mastication. Human colonic bacteria ferment RS and nonstarch polysaccharides (NSP; major components of dietary fiber) to short-chain fatty acids (SCFA), mainly acetate, propionate, and butyrate. SCFA stimulate colonic blood flow and fluid and electrolyte uptake. Butyrate is a preferred substrate for colonocytes and appears to promote a normal phenotype in these cells. Fermentation of some RS types favors butyrate production. Measurement of colonic fermentation in humans is difficult, and indirect measures (e.g., fecal samples) or animal models have been used. Of the latter, rodents appear to be of limited value, and pigs or dogs are preferable. RS is less effective than NSP in stool bulking, but epidemiological data suggest that it is more protective against colorectal cancer, possibly via butyrate. RS is a prebiotic, but knowledge of its other interactions with the microflora is limited. The contribution of RS to fermentation and colonic physiology seems to be greater than that of NSP. However, the lack of a generally accepted analytical procedure that accommodates the major influences on RS means this is yet to be established.

Differential influence of butyrate concentration on proximal and distal colonic mucosa in rats born germ-free and associated with a strain of Clostridium paraputrificum

In vivo influence of butyrate in colonic mucosa was studied using a model of gnotobiotic rats monoassociated with a Clostridium paraputrificum. Rats were fed a diet containing increasing amounts of non-digestible carbohydrates, the fermentation of which led to modulated amounts of butyrate in the large intestine. In the proximal colon, the increase in the butyrate concentration alters crypt depth and the number of mucus-containing cells; the increase in butyrate was highly correlated with the number of neutral-mucin-containing cells. Conversely, in the distal colon, no relation was found between the increase in butyrate concentration and crypt depth or number of mucin-containing cells. In both the proximal and distal colon, the mitotic index remained unchanged. In conclusion, in vivo production of physiological quantities of butyrate had a trophic effect on proximal colonic mucosa, but did not influence the distal epithelium.

Effect of diet on colonic-programmed cell death: molecular mechanism of action

Colon cancer evolves from a progressive inhibition of apoptosis and is influenced strongly by diet. Among dietary factors, butyrate (derived from fermentable fibers) may have utility as a chemopreventive agent because of its ability to promote apoptosis. Because CD95 (APO-1/Fas) transduces signals resulting in apoptosis, we tested the hypothesis that butyrate-dependent colonocyte apoptosis is mediated by this death receptor. Treatment of immortalized mouse colon cells with Fas agonistic antibody induced cell death, indicating that Fas in colonocytes is functional. Antagonism of Fas signaling using a soluble Fas:Fc chimera blocked butyrate induction of apoptosis. Therefore, Fas receptor dependent signal transduction is required for butyrate induction of apoptosis in colonic cells.

Aberrant crypt foci in colorectal carcinogenesis. Cell and crypt dynamics

Aberrant crypt foci (ACF) have been identified on the colonic mucosal surface of rodents treated with colon carcinogens and of humans after methylene-blue staining and observation under a light microscope. Several lines of evidence strongly suggest that ACF with certain morphological, histological, cell kinetics, and genetic features are precursor lesions of colon cancer both in rodents and in humans. Thus, ACF represent the earliest step in colorectal carcinogenesis. This paper has the main purpose of reviewing the evidence supporting this view, with particular emphasis on cell and crypt dynamics in ACF. ACF have been used as intermediate biomarkers of cancer development in animal studies aimed at the identification of colon carcinogens and chemopreventive agents. Recently, evidence has also shown that ACF can be effectively employed in chemopreventive studies also in humans.

Antagonism of CD95 signaling blocks butyrate induction of apoptosis in young adult mouse colonic cells

There is great interest in utilizing butyrate as a chemopreventive agent for colon tumorigenesis because of its ability to promote apoptosis in colon tumor cell lines. Because CD95 (APO-1/Fas) transduces signals resulting in apoptosis, we tested the hypothesis that butyrate-dependent colonocyte apoptosis is mediated by this death receptor. Butyrate (1 mM) exposure for 24 h upregulated expression of Fas and its ligand in young adult mouse colon (YAMC) cells. To delineate the proapoptotic effect of butyrate and to avoid the confounding effects of detachment from the extracellular matrix, adherent cell apoptosis was monitored as loss of plasma membrane asymmetry and dissipation of mitochondrial membrane potential (DeltaPsi(mt)) by laser cytometry. Soluble Fas receptor protein (Fas:Fc chimera) and caspase inhibitors (z-VAD-fmk and z-IETD-fmk) blocked butyrate induction of apoptosis. Treatment with Fas agonistic antibody (clone Jo-2) significantly induced cell death, indicating that Fas in colonocytes is functional. In addition, butyrate promoted apoptosis by inducing loss of DeltaPsi(mt) and phospholipid asymmetry of the plasma membrane after 12 and 24 h of exposure, respectively, before cell detachment. Therefore, Fas receptor-dependent signal transduction is involved in butyrate induction of apoptosis in colonocytes.