Effect of caloric restriction on the metabolic activation of xenobiotics

Dietary intervention improves health metrics and life expectancy of the genetically obese Titan mouse

Suitable animal models are essential for translational research, especially in the case of complex, multifactorial conditions, such as obesity. The non-inbred mouse (Mus musculus) line Titan, also known as DU6, is one of the world’s longest selection experiments for high body mass and was previously described as a model for metabolic healthy (benign) obesity. The present study further characterizes the geno- and phenotypes of this non-inbred mouse line and tests its suitability as an interventional obesity model. In contrast to previous findings, our data suggest that Titan mice are metabolically unhealthy obese and short-lived. Line-specific patterns of genetic invariability are in accordance with observed phenotypic traits. Titan mice also show modifications in the liver transcriptome, proteome, and epigenome linked to metabolic (dys)regulations. Importantly, dietary intervention partially reversed the metabolic phenotype in Titan mice and significantly extended their life expectancy. Therefore, the Titan mouse line is a valuable resource for translational and interventional obesity research.

This study further characterizes the non-inbred Titan (also known as DU6) mouse line, which could be a useful model for obesity research.

Daytime Restricted Feeding Modifies the Temporal Expression of CYP1A1 and Attenuated Damage Induced by Benzo[a]pyrene in Rat Liver When Administered before CYP1A1 Acrophase

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that heterodimerizes with the AhR nuclear translocator (ARNT) to modulate CYP1A1 expression, a gene involved in the biotransformation of benzo[a]pyrene (BaP). The AhR pathway shows daily variations under the control of the circadian timing system. Daytime restricted feeding (DRF) entrains the expression of genes involved in the processing of nutrients and xenobiotics to food availability. Therefore, we evaluate if temporal AhR, ARNT, and CYP1A1 hepatic expression in rats are due to light/dark cycles or fasting/feeding cycles promoted by DRF. Our results show that AhR oscillates throughout the 24 h period in DRF and ad libitum feeding rats (ALF), showing maximum expression at the same time points. DRF modified the peak of ARNT expression at ZT5; meanwhile, ALF animals showed a peak of maximum expression at ZT17. An increased expression of CYP1A1 was linked to the meal time in both groups of animals. Although a high CYP1A1 expression has been previously associated with BaP genotoxicity, our results show that, compared with the ALF group, DRF attenuated the BaP-CYP1A1 induction potency, the liver DNA-BaP adducts, the liver concentration of unmetabolized BaP, and the blood aspartate aminotransferase and alanine aminotransferase activities when BaP is administered prior to the acrophase of CYP1A1 expression. These results demonstrate that DRF modifies the ARNT and CYP1A1 expression and protects from BaP toxicity.

Caloric restriction attenuates C57BL/6 J mouse lung injury and extra-pulmonary toxicity induced by real ambient particulate matter exposure

Background

Caloric restriction (CR) is known to improve health and extend lifespan in human beings. The effects of CR on adverse health outcomes in response to particulate matter (PM) exposure and the underlying mechanisms have yet to be defined.

Results

Male C57BL/6 J mice were fed with a CR diet or ad libitum (AL) and exposed to PM for 4 weeks in a real-ambient PM exposure system located at Shijiazhuang, China, with a daily mean concentration (95.77 μg/m³) of PM_2.5. Compared to AL-fed mice, CR-fed mice showed attenuated PM-induced pulmonary injury and extra-pulmonary toxicity characterized by reduction in oxidative stress, DNA damage and inflammation. RNA sequence analysis revealed that several pulmonary pathways that were involved in production of reactive oxygen species (ROS), cytokine production, and inflammatory cell activation were inactivated, while those mediating antioxidant generation and DNA repair were activated in CR-fed mice upon PM exposure. In addition, transcriptome analysis of murine livers revealed that CR led to induction of xenobiotic metabolism and detoxification pathways, corroborated by increased levels of urinary metabolites of polycyclic aromatic hydrocarbons (PAHs) and decreased cytotoxicity measured in an ex vivo assay.

Conclusion

These novel results demonstrate, for the first time, that CR in mice confers resistance against pulmonary injuries and extra-pulmonary toxicity induced by PM exposure. CR led to activation of xenobiotic metabolism and enhanced detoxification of PM-bound chemicals. These findings provide evidence that dietary intervention may afford therapeutic means to reduce the health risk associated with PM exposure.

A Mechanistic Basis for the Beneficial Effects of Caloric Restriction On Longevity and Disease: Consequences for the Interpretation of Rodent Toxicity Studies

Caloric restriction in rodents has been repeatedly shown to increase life span while reducing the severity and retarding the onset of both spontaneous and chemically induced neoplasms. These effects of caloric restriction are associated with a spectrum of biochemical and physiological changes that characterize the organism's adaptation to reduced caloric intake and provide the mechanistic basis for caloric restriction's effect on longevity. Here, we review evidence suggesting that the primary adaptation appears to be a rhythmic hypercorticism in the absence of elevated adrenocorticotropin (ACTH) levels. This characteristic hypercorticism evokes a spectrum of responses, including reduced body temperature and increased metabolic efficiency, decreased mitogenic response coupled with increased rates of apoptosis, reduced inflammatory response, reduced oxidative damage to proteins and DNA, reduced reproductive capacity, and altered drug-metabolizing enzyme expression. The net effect of these changes is to (1) decrease growth and metabolism in peripheral tissues to spare energy for central functions, and (2) increase the organism's capacity to withstand stress and chemical toxicity. Thus, caloric restriction research has uncovered an evolutionary mechanism that provides rodents with an adaptive advantage in conditions of fluctuating food supply. During periods of abundance, body growth and fecundity are favored over endurance and longevity. Conversely, during periods of famine, reproductive performance and growth are sacrificed to ensure survival of individuals to breed in better times. This phenomena can be observed in rodent populations that are used in toxicity testing. Improvements over the last 30 years in animal husbandry and nutrition, coupled with selective breeding for growth and fecundity, have resulted in several strains now exhibiting larger animals with reduced survival and increased incidence of background lesions. The mechanistic data from caloric restriction studies suggest that these large animals will also be more susceptible to chemically induced toxicity. This creates a problem in comparing tests performed on animals of different weights and comparing data generated today with the historical database. The rational use of caloric restriction to control body weight to within preset guidelines is a possible way of alleviating this problem.

Effect of various diets on the expression of phase-I drug-metabolizing enzymes in livers of mice

1. Previous studies have shown that diets can alter the metabolism of drugs; however, it is difficult to compare the effects of multiple diets on drug metabolism among different experimental settings. Phase-I-related genes play a major role in the biotransformation of pro-drugs and drugs. 2. In the current study, effects of nine diets on the mRNA expression of phase-I drug metabolizing enzymes in livers of mice were simultaneously investigated. Compared to the AIN-93M purified diet (control), 73 of the 132 critical phase-I drug-metabolizing genes were differentially regulated by at least one diet. Diet restriction produced the largest number of changed genes (51), followed by the atherogenic diet (27), high-fat diet (25), standard rodent chow (21), western diet (20), high-fructose diet (5), EFA deficient diet (3) and low n-3 FA diet (1). The mRNAs of the Fmo family changed most, followed by Cyp2b and 4a subfamilies, as well as Por (from 1121- to 21-fold increase of theses mRNAs). There were 59 genes not altered by any of these diets. 3. The present results may improve the interpretation of studies with mice and aid in determining effective and safe doses for individuals with different nutritional diets.

Weight control and cancer preventive mechanisms: role of insulin growth factor-1-mediated signaling pathways

Overweight and obese not only increase the risk of cardiovascular disease and type-2 diabetes mellitus, but are also now known risk factors for a variety of cancers. Weight control, via dietary calorie restriction and/or exercise, has been demonstrated to be beneficial for cancer prevention in various experimental models, but the underlying mechanisms are still not well defined. Recent studies conducted in a mouse skin carcinogenesis model show that weight loss induced a significant reduction of the circulating levels of insulin growth factor (IGF)-1 and other hormones, including insulin and leptin, resulting in reduced IGF-1-dependent signaling pathways, i.e. Ras-MAPK proliferation and protein kinase B-phosphoinositide 3-kinase (Akt-PI3K) antiapoptosis. Selective targeting IGF-1 to Akt/mammalian target of rapamycin and AMP-activated protein kinase pathways, via negative energy balance, might inactivate cell cycle progression and ultimately suppress tumor development. This review highlights the current studies focused on the major role of reducing IGF-1-activated signaling via weight control as a potential cancer preventive mechanism.

Can we intervene in human ageing?

Ageing is a progressive failure of defence and repair processes that produces physiological frailty (the loss of organ reserve with age), loss of homeostasis and eventual death. Over the past ten years exceptional progress has been made in understanding both why the ageing process happens and the mechanisms that are responsible for it. The study of natural mutants that accelerate some, but not all, of the features of the human ageing process has now progressed to a degree that drug trials are either taking place or can be envisaged. Simultaneously, a series of mutations have been identified in different species that confer extended healthy life, indicating that the ageing process is much more malleable than might have been expected and that single interventions have the potential to delay the onset of multiple age-associated conditions. Data generated using these organisms have led to the formulation of a powerful new hypothesis, the ‘green theory’ of ageing. This proposes that a finite capacity to carry out broad-spectrum detoxification and recycling is the primary mechanistic limit on organismal lifespan. This is turn suggests important new experimental approaches and potential interventions designed to increase healthy lifespan.

Cardiovascular disease could be contained based on currently available data!

Largely due to better control of infectious diseases and significant advances in biomedical research, life expectancy worldwide has increased dramatically in the last three decades. However, as the average age of the population has risen, the incidence of chronic age-related diseases such as arthritis, Alzheimer's, Parkinson's, cardiovascular disease, cancer, osteoporosis, benign prostatic hyperplasia, and late-onset diabetes have increased and have become serious public health problem, as well. The etiology of these disorders is still incompletely understood, therefore, neither preventive strategies nor long-term effective treatment modalities are available for these disorders. In keeping with the aforementioned, the ultimate goal in cardiovascular research is to prevent the onset of cardiovascular episodes and thereby allow successful ageing without morbidity and cognitive decline. Herein, I argue that cardiovascular episodes could be contained with relatively simple approaches. Cardiovascular disorder is characterized by cellular and molecular changes that are commonplace in age-related diseases in other organ system, such alterations include increased level of oxidative stress, perturbed energy metabolism, and "horror autotoxicus" largely brought about by the perturbation of ubiquitin -proteasome system, and excessive oxidative stress damage to the cardiac muscle cells and tissues, and cross-reactions of specific antibodies against human heat shock protein 60 with that of mycobacterial heat shock protein 65. "Horror autotoxicus", a Latin expression, is a term coined by Paul Ehrlich at the turn of the last century to describe autoimmunity to self, or the attack of "self" by immune system, which ultimately results to autoimmune condition. Based on the currently available data, the risk of cardiovascular episodes and several other age-related disorders, including cancer, Alzheimer's disease and diabetes, is known to be influenced by the nature and level of food intake. Now, a wealth of scientific data from studies of rodents and monkeys has documented the significant beneficial effects of calorie restriction (CR) or dietary restriction (DR), and multiple antioxidant agents in extending life span and reducing the incidence of progeroid-related diseases. Reduced levels of cellular oxidative stress, protection of genome from deleterious damage, detoxification of toxic molecules, and enhancement of energy homeostasis, contribute to the beneficial effects of dietary restriction and multiple antioxidant agents. Recent findings suggest that employment of DR and multiple antioxidant agents (including, catalase, glutathione peroxidase, CuZn superoxide dismutase, and Mn superoxide dismutase = enzymes forming the primary defense against oxygen toxicity), and ozone therapy may mount an effective resistance to pathogenic factors relevant to the pathogenesis of cardiovascular episodes. Hence, while further studies will be needed to establish the extent to which CR and multiple antioxidant agents will reduce incidence of cardiovascular episodes in humans, it would seem prudent to recommend CR and multiple antioxidant agents as widely applicable preventive approach for cardiovascular disorders and other progeroid-related disorders.

Cytochrome P450 expression in rat gastric epithelium with intestinal metaplasia induced by high dietary NaCl levels

Drug metabolizing enzymes like cytochrome P450 (CYP) play an important role in determining the susceptibility of organs or tissue to the toxic effects of drugs or other xenobiotics. There is some evidence indicating that individual isoforms of CYPs are over-expressed in different types of malignant tumors including that of oesophagus, pancreas, breast, lung, colon and stomach. Nevertheless, it is not clear if this change in expression is previous or after the appearance of malignancy. This is important in order to clarify the possible role of xenobiotics in the development of gastric cancer. On the other hand, it has been reported that a high salt ingestion leads to histological changes in rat stomach mucosa including enhanced cell proliferation, lipid peroxidation and intestinal metaplasia. The aim of this study is to explore the expression and activity of CYP families involved in the metabolism of carcinogens in normal rat stomach mucosa and intestinal metaplasia induced by high NaCl ingestion. Male Wistar rats were exposed to diets containing different NaCl concentrations (0.6% control group, 6%, 12%, 18% and 24%) for 12 weeks and histological changes as well as CYP modulation were monitored in gastric mucosa. Chronic gastritis, regenerative hyperplasia and focal metaplasia were noted in animals receiving the 12%, 18% and 24% NaCl diets. In the same groups, induction of CYP1A1 and CYP3A2 was produced, mainly in areas of metaplasia. The expression of xenobiotic metabolizing enzymes in the gastric mucosa might contribute to chemical activation in the stomach, metabolizing both exogenous and endogenous compounds implicated in the development of gastric cancer.

The effect of dietary restriction on PhIP-induced mutation in the distal colon and B[a]P- and ENU-induced mutation in the liver of the rat

A reduction in dietary intake has been shown to significantly increase the lifespan of rodents, lower the incidence of tumors, and reduce DNA damage. The objective of this study was to determine whether dietary restriction (DR) reduced the frequency of mutation induced by two environmentally relevant metabolically activated mutagens and one direct-acting mutagen in the lacI transgene of male and female Big BlueR rats. Both male and female rats were maintained on either an ad libitum (AL) or a 40%-reduced diet for 22 wk. The mutagenicity of a 100-mg/kg intraperitoneal injection with 2-amino-1-methyl-6-pheny-imidazo[4,5-b] pyridine (PhIP), benzo[a]pyrene (B[a]P), and N-ethyl-N- nitrosourea (ENU) was determined in the colon or liver. The results indicated that DR did not significantly alter the PhIP-induced mutant frequency in male or female colons. DR completely prevented mutagenicity induced by B[a]P in the female liver (2.6 +/- 0.6 10(-5) vs 10.9 +/- 5.8 10(-5) in AL females), yet increased the induced frequency in male livers (16.3 +/- 3.7 10(-5) vs 10.6 +/- 1.5 10(-5) in AL male livers). Although there was no difference in mutation frequency in the liver between AL and DR females treated with ENU, there was approximately a 40% decrease in induced frequency in DR males compared with AL males. These results indicate that a reduction in dietary intake has no preventive effect against PhIP-induced mutation in the colon, but has sex-dependent protective effects against B[a]P- and ENU-induced mutation in the liver.

Effects of fasting and intermittent fasting on rat hepatocarcinogenesis induced by diethylnitrosamine

The influences of fasting on DEN-initiation and of intermittent fasting (IF) on the rat liver chemical carcinogenesis process were evaluated in a 52-week long assay. Three groups of adult male Wistar rats were used: Groups 1 to 3 were treated with a single i.p. injection of 200 mg/kg of diethylnitrosamine (DEN). Group 2 was submitted to 48 h fasting prior to DEN treatment. After the 4th week, Group 3 was submitted to IF, established as 48 h weekly fasting during 48 weeks, while Groups 1 and 2 were fed ad libitum until the 52nd week. All animals were submitted to 70% partial hepatectomy and sacrificed at the 3rd and 52nd weeks, respectively. Fasting prior to DEN-initiation did not influence the development of altered foci of hepatocytes (AFHs) and of hepatic nodules (Group 2 vs. Group G1). IF inhibited the development of preneoplastic lesions, since this dietary regimen decreased the number and the size of glutathione S-transferase (GST-P) positive foci and the number and size of liver nodules (Group G3 vs. Group G1). The inhibitory effect of IF was also reflected in the development of clear and basophilic cell foci. These results indicate that long-term IF regimen exerts an anti-promoting effect on rat hepatocarcinogenesis induced by DEN.

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.

Food restriction reduces aflatoxin B1 (AFB1)-DNA adduct formation, AFB1-glutathione conjugation, and DNA damage in AFB1-treated male F344 rats and B6C3F1 mice

The objective of this study was to examine effects of food restriction (FR) on the metabolic activation of aflatoxin B1 (AFB1) in rats and mice, which are AFB1-sensitive and -resistant rodent species, respectively. Forty percent FR [60% of ad libitum (AL) food consumption] reduced the metabolic activation of AFB1 in both rats and mice, causing formation of hepatic AFB1-DNA adducts to be 43% and 31% lower, respectively. The AFB1-DNA adduct 8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 (AFB1-N7-Gua) was predominantly formed in rat liver DNA; the formation of the ring-open analogue of AFB1-N7-Gua, AFB1-formamidopyrimidine (AFB1-FAP), was predominantly found in mouse liver DNA. In contrast to the in vivo results, the in vitro AFB1-DNA adduct formation mediated by the microsomes of liver, kidney or lung from FR-mice was greater than the formation of AFB1-DNA adducts mediated by the tissue microsomes from the AL-mice. Food restriction induced hepatic glutathione S-transferase (GST) activity, as measured by the formation of AFB1-glutathione conjugates (AFB1-SG), in both rats and mice; AFB1-SG was also formed in mouse kidney. Food restriction-induced GST activity assayed in an in vitro system, using [3H]AFB1-8,9-epoxide and glutathione (GSH) as substrates, was also found when mouse kidney and lung cytosolic fractions were used. Food restriction inhibited the AFB1-induced DNA double strand breaks in mouse kidney. The reduction of levels of AFB1-DNA adduct formation in mouse kidney was comparable to the degree of AFB1-induced DNA strand breakages. The results of this study indicate that the metabolic activation of AFB1 can be modulated by FR through the alteration of the formation of AFB1-DNA adducts and AFB1-SG conjugation. However, species and tissue specificities exist regarding the metabolic activation of AFB1.

Effect of dietary restriction on benzo[a]pyrene (BaP) metabolic activation and pulmonary BaP-DNA adduct formation in mouse

Hepatic microsomal xenobiotic metabolizing enzyme activities of laboratory animals can be modulated by Dietary restriction (DR). The modulation of xenobiotic metabolizing enzyme activities can affect the metabolic activation of chemical carcinogens. Acute DR (60% of the food consumption of ad libitum (AL)-fed mice for 7 weeks) reduced the body weights of the male B6C3F1 mice, and increased mouse pulmonary cytochrome P4501A1-dependent BaP metabolizing enzyme activity. The effects of DR on the formation of the specific BaP-DNA adduct, 10-(N2-deoxyguanosinyl)-7,8,9-trihydroxy-7,8,9,10-tetrahydro-BaP (BaP-N2-dG) in mouse lung can be detected by using 32P-postlabeling technique. In both AL- and DR-mice total BaP-DNA adduct formation in lung reached a peak at 48 hours after treatment with [3H]BaP and the in vivo formation of BaP-N2-dG was greater in DR mouse lung than in that of AL-animals by 22%. DR increased in vitro BaP-N2-dG formation by 39% when calf-thymus DNA was incubated with BaP using liver microsomes obtained from DR- or AL-mice as the enzyme source. The formation of the specific BaP-N2-dG adducts, measured by 32P-postlabeling, was only 20% of the total [3H]BaP-DNA adducts as determined by liquid scintillation counting. The increase of BaP-DNA adduct formation in mouse lung was correlated to the enhancement of the mouse pulmonary BaP metabolizing enzyme activity. Our results indicated that the effect of DR on the metabolic activation of BaP in mouse lung was dependent upon the mouse lung cytochrome P4501A1-dependent BaP metabolizing enzymes activities which was significantly increased by DR.

Effect of dietary restriction on glutathione S-transferase activity specific toward aflatoxin B1-8,9-epoxide

Dietary restriction (DR) reduced the metabolic activation of aflatoxin B1 (AFB1) in rats. This reduction may be attributed to the decrease of cytochrome P-450-mediated AFB1 epoxidation and/or increase in the detoxification of AFB1 catalyzed by hepatic glutathione S-transferase (GST) and other phase II detoxification enzymes. In this study the effect of DR on male rat liver cytosolic GST activity toward AFB1-8,9-epoxide was studied. The chemically-synthesized AFB1-8,9-epoxide was used as the substrate in this assay, and the formation of AFB1-GSH conjugate was analyzed by HPLC. Male Fischer 344 rats fed DR diets (60% of the food consumption of ad libitum (AL)-fed rats) showed a 2.4-fold increase in GST activity when AFB1-epoxide was used as the substrate. The results from the enzyme kinetic study showed that DR increased Vmax of the liver cytosolic GST but not the Km. Acute DR has little or no impact on GST activity when 1-chloro-2,4-dinitrobenzene and 2,4-dichloronitrobenzene were used as substrates. The mouse liver GST activity toward AFB1-epoxide was 3-fold greater than that of phenobarbital-induced rats, 4.5-fold greater than DR rats, and 14.7-fold greater than the GST activity of AL rats. This direct assay of liver GST activity using AFB1-epoxide as the substrate is useful for studying AFB1-induced biomarkers, such as AFB1-GSH conjugation and AFB1-DNA adducts.

Effect of dietary restriction on partial hepatectomy-induced liver regeneration of aged F344 rats

Fourteen weeks-old male F344 rats maintained on a reduced caloric diet (60% of ad libitum (AL) food consumption) for 6 weeks or for 14 months did not affect the hepatic cell proliferation in terms of % S phase population, determined by evaluation of DNA synthesis in hepatocytes isolated from either young (5 months) or aged (18 months) rats. However, hepatic basal cellular DNA synthesis estimated by [3H]thymidine incorporation was reduced through acute dietary restriction (DR) in young rats, but increased in aged animals after 14 months restriction. Partial hepatectomy (PH) on aged rats stimulated hepatocyte regeneration and restored some aging-associated biochemical functions, such as drug metabolizing enzyme-dependent xenobiotic metabolic activation which was determined by measuring the formation of carcinogen-DNA adducts. Forty-eight hours after partial hepatectomy, the % of S phase population and the basal nuclear DNA synthesis of hepatocytes isolated from the partial hepatectomized DR-rats were 4- and 2.8-fold, respectively, greater than those of hepatocytes from AL-animals. DR reduced aflatoxin B1 (AFB1) metabolizing enzyme activity and decreased the AFB1-DNA adduct formation in young rats treated with AFB1. In aged AL-rats, the formation of AFB1-DNA adducts diminished to the same level as that of DR-groups and probably was due to the faster decline of drug metabolizing enzymes in aging AL-rats. However, 48 h after PH, the metabolic activation of AFB1 was restored in AL- and DR-groups which resulted in the increase of AFB1-DNA binding by 4.2 and 1.9-fold, respectively. During the liver regeneration of old PH-rats, DR inhibited the AFB1-DNA adduct formation after the PH-rats received a single dose of AFB1. DR increased benzo[a]pyrene (BaP) metabolic activation in both young and aged rats. Aging also decreased BaP-DNA adduct formation in both DR and AL-rats. The increase of BaP-DNA adduct formation in PH-groups was attributed to the restoration of BaP-metabolizing enzyme activity during liver regeneration. The PH-stimulated BaP-DNA adduct formation in AL- and DR-rats was 3.4- and 2.0-fold greater than control aged rats. Our results indicated that the stimulation of PH-induced liver regeneration by DR in aged animals may be attributed to the retardation of aging by DR and the retention of more active biochemical and enzymological functions in old DR-animals.

Effects of dietary restriction and fasting on selected rat liver enzymes of xenobiotic metabolism and on AOM-induced DNA guanine methylation in rat liver and colon

Using five- to eight-week-old male F344 rats and a high-fat (23.5% corn oil) modified AIN-76A diet, we examined the effects of dietary restriction (a 3-wk 30% reduction of food intake with respect to ad libitum-fed controls) or complete fasting (2 days without food) on the activities of hepatic xenobiotic metabolizing enzymes in vitro and on azoxymethane- (AOM) induced formation of O6-methylguanine and 7-methylguanine in liver and colon DNA in vivo. Compared with ad libitum-fed rats, fasting increased total liver cytochrome P450 by 32%, microsomal aniline hydroxylase by 270%, N-nitrosodimethylamine demethylase by 270%, and azoxymethane hydroxylase by 320%. Liver benzo[a]pyrene (BP) hydroxylase and glutathione-S-transferase were decreased by 39% and 21%, respectively, whereas NADPH cytochrome c reductase and UDP glucuronyltransferase were unchanged. DNA methylation in the livers of fasted animals was 20-31% greater six hours after a 15 mg/kg sc injection of AOM than in ad libitum-fed controls, whereas DNA methylation in the colon was slightly lower. In three-week diet-restricted animals. there were small but not statistically significant changes in the various enzyme activities and in AOM-induced DNA methylation compared with the ad libitum-fed controls, with the exception of BP hydroxylase, which showed a 26% decrease. However, the trends in the increase or decrease of each parameter, although small in magnitude, were similar to those observed in the case of fasting, suggesting that the effects might become significant if the duration of diet restriction were prolonged. The enhancement of AOM metabolism in rat liver by fasting, leading to increased liver DNA methylation, is different from that produced by chemical inducers, such as ethanol, where no increase in liver DNA methylation is observed.

Modulation of toxicity by diet and dietary macronutrient restriction

Restriction of diet and macronutrients has been reported to modulate the toxicity of numerous chemical agents. Of the various forms of restriction studied, using nutritionally adequate diets, food restriction (FR) appears to be most effective, but protein restriction (PR), fat restriction (FtR), carbohydrate restriction (CbR), and excess of dietary fiber (FE) also affect toxicity and the spontaneous diseases that define the background incidence in toxicity tests. The heterogeneity of the dietary macronutrients complicates simple analysis of their effects. Additionally, the interrelationships between these various components in the complex dietary mixture often make experiments difficult to interpret. Despite these complexities, a simple model is presented, which considers the effects of dietary manipulations on the individual steps in the interaction of organism and agent, and puts the varied effects that can occur within an organism into context. Ultimately, many of the effects of dietary modulation on these steps in toxicogenesis can be considered as changing agent exposure and the biologically available dose. The effects of macronutrient restriction are discussed in terms of effects on agent at the interface of organism and toxicant, agent disposition, agent metabolism, and repair of toxicant-induced damage at the level of the genome. After illustrating the influence of these nutritional effects on the chronic bioassay, using mouse liver tumors as an example, the significance of these effects for chronic and short-term testing is discussed. Additionally, methods to address the impact of nutritional factors on toxicity testing are suggested.

Effects of caloric restriction in animals on cellular function, oncogene expression, and DNA methylation in vitro

While the life-extending and disease-modulating effects of caloric restriction (CR) are well documented in whole animal studies and in correlative experiments using cells taken from CR animals, very few studies have used cells in culture after their removal from the CR-fed animal. In using this in vivo-->in vitro approach we have attempted to examine the proposition that the effects of CR can be transferred to individual cells by analyzing the cellular functions of proliferation and transformation, the activation of oncogenes, and the methylation of DNA as a function only of diet. Pancreatic acinar cells excised from CR-fed Brown-Norway rats and placed in rich medium showed different responses compared to cells from ad libitum (AL)-fed controls. CR had the effect of slowing growth rate and protecting against spontaneous and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced transformation over 14 passages of cells in culture. At the molecular level, cells from the CR animals showed reduced c-Ha-ras oncogene expression and mutation as well as reduced mutation of the p53 suppressor gene. CR also increased genomic methylation of ras DNA. We conclude that the effects of CR treatment of the animal are transferred to individual cells and note that these responses (decreased proliferation and transformation; depressed oncogene expression and mutation and decreased suppressor gene mutation; and increased oncogene methylation) are cellular and molecular analogs of in vivo weight loss, life extension, and carcinogenesis modulation, which are hallmarks of CR in the whole animal. The fact that these responses are seen generations after the cells are removed from the CR-treated animal indicates that CR causes a permanent predisposition of pancreatic acinar cells to these modulated responses and shows the value of the in vivo-->in vitro protocol in studies that relate diet to cellular and molecular function.

Effects of caloric restriction on rodent drug and carcinogen metabolizing enzymes: implications for mutagenesis and cancer

Caloric restriction in rodents results in increased longevity and a decreased rate of spontaneous and chemically induced neoplasia. The low rates of spontaneous neoplasia and other pathologies have made calorically restricted rodents attractive for use in chronic bioassays. However, caloric restriction also alters hepatic drug metabolizing enzyme (DME) expression and so may also alter the biotransformation rates of test chemicals. These alterations in DME expression may be divided into two types: (1) those that are the direct result of caloric restriction itself and are detectable from shortly after the restriction is initiated; (2) those which are the result of pathological conditions that are delayed by caloric restriction. These latter alterations do not usually become apparent until late in the life of the organism. In rats, the largest direct effect of caloric restriction on liver DMEs is an apparent de-differentiation of sex-specific enzyme expression. This includes a 40-70% decrease in cytochrome P450 2C11 (CYP2C11) expression in males and a 20-30% reduction of corticosterone sulfotransferase activity in females. Changes in DME activities that occur late in life in calorically restricted rats include a stimulation of CYP2E1-dependent 4-nitrophenol hydroxylase activity and a delay in the disappearance of male-specific enzyme activities in senescent males. It is probable that altered DME expression is associated with altered metabolic activation of chemical carcinogens. For example the relative expression of hepatic CYP2C11 in ad libitum-fed or calorically restricted rats of different ages is closely correlated with the amount of genetic damage in 2-acetylaminofluorene- or aflatoxin B1-pretreated hepatocytes isolated from rats of the same age and caloric intake. This suggests that altered hepatic drug and carcinogen metabolism in calorically restricted rats can influence the carcinogenicity of test chemicals.