Effect of caloric restriction on aflatoxin B1-induced DNA synthesis, AFB1-DNA binding and cell proliferation in Fischer 344 rats

Effects of food restriction on the expression of genes related to acetaminophen-induced liver toxicity in rats

It is well known that fasting substantially affects the metabolism of drugs and chemicals. Food restriction also affects drug kinetics, such as absorption, metabolism, and excretion, and therefore, it can potentially modulate the onset of chemical toxicity or drug-induced adverse reactions. In the present study, the expression of drug-metabolizing enzyme genes and total glutathione content in the liver, which are related to toxicity induced by overdose of the hepatotoxic drug acetaminophen (N-acetyl-p-aminophenol; APAP), were examined in rats reared under different feeding conditions: ad libitum feeding, 16-h fasting, and food restriction (fed 70% of the average intake of ad libitum feeding for 10 days) conditions. The rats under food restriction conditions as well as fasted rats showed significantly higher expression of Cyp2e1, the gene encoding the enzyme that metabolizes APAP to its toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI). They also had lower levels of liver total glutathione, which detoxifies NAPQI. In contrast, the gene expression of UDP-glucuronosyltransferase 1A6 (Ugt1a6), sulfotransferase 1A1 (Sult1a1), and glutathione S-transferase M1 (Gstm1) was not affected by food restriction or fasting. When APAP was administered (800 mg/kg), histopathological changes were not observed in rats fed ad libitum, while hepatocellular necrosis was observed in most of the rats treated with APAP after fasting or food restriction. Taken together, these results suggest that not only fasting but also food restriction exacerbate APAP-induced acute liver injury, probably by the induction of CYP2E1 and the reduction of liver glutathione contents, in rodents.

Attenuation of bleomycin-induced Hprt mutant frequency in female and male rats by calorie restriction

Calorie restriction modulates spontaneous and chemically induced tumors and increases maximal life span in experimental animals; however, the mechanism by which calorie restriction exerts its ameliorating effects is not fully elucidated, although reduced levels of reactive oxygen species (ROS) by calorie restriction has generated much interest. In the present study, we have determined whether or not calorie restriction would affect the mutagenic response in rats treated with bleomycin (BLM) a radiomimetic drug that is associated with DNA damage by a free radical mechanism. Fourteen weeks after weaning, the rats were divided into two groups; ad libitum (AL)-fed and 40% calorie restriction. Both AL and calorie-restricted animals were injected with 2.5, 5.0 and 10.0 mg BLM/kg, or with phosphate-buffered saline (PBS), and they were killed 4 weeks post drug treatment. Lymphocytes from the spleens were seeded in 96-well microtiter plates to determine mutant frequency in the hypoxantine guanine phosphoribosyl transferase (Hprt) gene. The mutant frequency in the BLM-treated rats was higher in AL males (P=0.001), and AL females (P=0.0174) than in their calorie-restricted counterparts. The difference in mutagenic response relative to AL males and AL females appeared unrelated to a low percent cloning efficiency seen in the males, since the mean absolute number of Hprt mutant clones was higher in the AL males compared to the females. A reduction in animal weight by calorie restriction was significant in both sexes (P<0.001), but the dose effect appeared non-significant. The results indicate that calorie intake of 60% reduced the mutagenic response of BLM, a compound known to induce oxidative DNA damage, and suggest a possible decrease in ROS as a function of calorie restriction.

Dietary restriction modulated carcinogen-DNA adduct formation and the carcinogen-induced DNA strand breaks

Dietary restriction (DR) alters the activities of hepatic drug metabolizing enzymes and modulates the formation of carcinogen-DNA adducts in carcinogen treated animals. Our previous results showed that a 40% restriction of diet (60% of ad libitum (AL) food consumption) reduced the hepatic metabolic activation of aflatoxin B1 (AFB1) but increased the activation of benzo[a]-pyrene (BaP) in both rats and mice. In this study, the focus was directed toward the levels of carcinogen-DNA adducts formation and the carcinogen-induced DNA strand breaks in mouse kidney and liver DNA. DR significantly inhibited both AFB1-DNA adduct formation and AFB1-induced DNA strand breaks in kidney DNA of mice that received a single dose of [3H]AFB1 (5 mg/kg). The levels of AFB1-DNA adduct formation in mouse kidney DNA correlated well with increased AFB1-induced DNA strand breaks. The correlation between the levels of AFB1-DNA-adducts formed and DNA strand breaks in kidney DNA of DR-mice was less linear than between its AL-counterpart suggesting that other factors, such as different rates of DNA repair, may be involved. In addition, DR enhanced hepatic BaP- and 6-nitrochrysene (6-NC)-DNA adduct formation in the mice treated with BaP and 6-NC, respectively. The formation of the specific BaP-adduct, 10-(N2-deoxyguanosinyl)-7,8,9-trihydroxy-7,8,9,10-tetrahydro-BaP (N2-dG-BaP), in mouse liver was proportional to the dose, and was compatible to the BaP-induced DNA strand breaks affected by DR. The enhancement of the total 6-NC-DNA adduct formation in DR-mouse was also in correlation with the increased 6-NC-induced DNA strand breaks. The activity of mouse liver microsomal nitro-reductase increased by 2-fold in response to DR indicating that the nitroreduction may contribute to the increase of the metabolic activation of 6-NC. Our present results indicate that the effect of DR on the carcinogen activation is dependent upon the DR-modulated carcinogen metabolizing enzyme activities.