How good are rodent models of carcinogenesis in predicting efficacy in humans? A systematic review and meta-analysis of colon chemoprevention in rats, mice and men

Both selenoproteins and low molecular weight selenocompounds reduce colon cancer risk in mice with genetically impaired selenoprotein expression

Selenium has cancer protective effects in a variety of experimental systems. Currently, it is not known whether selenoproteins or low molecular weight selenocompounds are responsible for this activity. To evaluate the contribution of selenoproteins to the cancer protective effects of selenium, we used transgenic mice that carry a mutant selenocysteine transfer RNA gene, which causes reduced selenoprotein synthesis. Selenium homeostasis was characterized in liver and colon of wild-type and transgenic mice fed selenium-deficient diets supplemented with 0, 0.1, or 2.0 microg selenium (as selenite)/g diet. (75)Se-labeling, Western blot analysis, and enzymatic activities revealed that transgenic mice have reduced (P < 0.05) liver and colon glutathione peroxidase expression, but conserved thioredoxin reductase expression compared with wild-type mice, regardless of selenium status. Transgenic mice had more (P < 0.05) selenium in the nonprotein fraction of the liver and colon than wild-type mice, indicating a greater amount of low molecular weight selenocompounds. Compared with wild-type mice, transgenic mice had more (P < 0.05) azoxymethane-induced aberrant crypt formation (a preneoplastic lesion for colon cancer). Supplemental selenium decreased (P < 0.05) the number of aberrant crypts and aberrant crypt foci in both wild-type and transgenic mice. These results provide evidence that a lack of selenoprotein activity increases colon cancer susceptibility. Furthermore, low molecular weight selenocompounds reduced preneoplastic lesions independent of the selenoprotein genotype. These results are, to our knowledge, the first to provide evidence that both selenoproteins and low molecular weight selenocompounds are important for the cancer-protective effects of selenium.

Effects of quercetin and beta-carotene supplementation on azoxymethane-induced colon carcinogenesis and inflammatory responses in rats fed with high-fat diet rich in omega-6 fatty acids

Chronic inflammation in gastrointestinal tract has been suggested as a risk factor for tumor formation. The effect of dietary supplementation of quercetin or beta-carotene on colon carcinogenesis and inflammatory response in rats fed with high-fat diet rich in omega-6 fatty acids was assessed. Animals were exposed to two weekly subcutaneous injections of AOM (azoxymethane) at a single dose of 15 mg/kg body weight. A portion of rats from each group was sacrificed at 8 weeks after the last AOM treatment to determine ACF (aberrant crypt foci) formation. Colonic mucosa expression of iNOS (inducible nitric oxide) and COX-2 (cyclooxygenase-2) protein, and blood PGE2 (prostaglandin E2) level were measured. The remaining groups of animals were sacrificed at 33 weeks after the last AOM treatment to examine colon tumor formation. Rats on high-fat diet developed more aberrant crypt foci (P<0.05) compared with those of rats on regular diet. In the same vein, but in contrast to the effect seen with regular diet, the high-fat diet induced a significant up-regulation of iNOS expression. There was no significant change in the extent of COX-2 expression or in the PGE2 levels. Quercetin or beta-carotene supplementation reduced the number of ACF only in animals fed high-fat diet (p<0.05), however, no significant difference in tumor incidence was found. At week 33, the expression of iNOS was reduced by quercetin without a statistical significance, and COX-2 expression was slightly reduced in rats on beta-carotene supplementation. No change in PGE2 levels was observed. Whilst dietary antioxidants are considered as effective suppressors for precancerous lesion formation in colons exposed to high-risk diet, it is clear that elucidating the role of individual antioxidants in colon tumor formation coupled with an understanding of the molecular mechanisms involved would benefit colon cancer prevention strategies.