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

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.

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.

Diet Restriction as a Protective Mechanism in Noncancer Toxicity Outcomes: A Review

It is well documented that diet restriction (DR) increases life expectancy, slows aging, and decreases the incidence of a variety of age-associated diseases including cancer and chemical-induced carcinogenesis. With regard to chemical toxicity, very few studies have attempted to investigate the effects of DR on noncancer toxicity outcomes. This review summarizes the findings of how DR influences acute toxicity outcomes and mechanisms. DR-induced protection in ozone lung inflammation, acute toxicity of isoproterenol, ganciclovir-, aspirin-and acidified ethanol-induced gastric injury are discussed. Because similar physiologic mechanisms exist in humans, DR, if practiced as a fife-style option, may improve quality of life in addition to accruing savings in burgeoning health care costs. Finally, these studies may be of value in boosting research in the area of pharmacology and therapeutics in developing potential therapeutic and safety assessment tools in human and veterinary medicine.

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.

A nutrigenomic approach to detect nutritional stress from gene expression in blood samples drawn from Steller sea lions

Gene expression profiles are increasingly being used as biomarkers to detect the physiological responses of a number of species to disease, nutrition, and other stressors. However, little attention has been given to using gene expression to assess the stressors and physiological status of marine mammals. We sought to develop and validate a nutrigenomic approach to quantify nutritional stress in Steller sea lions (Eumetopias jubatus). We subjected 4 female Steller sea lions to 3 feeding regimes over 70-day trials (unrestricted food intake, acute nutritional stress, and chronic nutritional stress), and drew blood samples from each animal at the end of each feeding regime. We then extracted the RNA of white blood cells and measured the response of 8 genes known to react to diet restriction in terrestrial mammals. Overall, we found that the genomic response of Steller sea lions experiencing nutritional stress was consistent with how terrestrial mammals respond to dietary restrictions. Our nutritionally stressed sea lions down-regulated some cellular processes involved in immune response and oxidative stress, and up-regulated pro-inflammatory responses and metabolic processes. Nutrigenomics appears to be a promising means to monitor nutritional status and contribute to mitigation measures needed to assist in the recovery of Steller sea lions and other at-risk species of marine mammals.

Fasting in mood disorders: neurobiology and effectiveness. A review of the literature

Clinicians have found that fasting was frequently accompanied by an increased level of vigilance and a mood improvement, a subjective feeling of well-being, and sometimes of euphoria. Therapeutic fasting, following an established protocol, is safe and well tolerated. We aim in this article to explore the biological mechanisms activated during fasting that could have an effect on brain function with particular focus on mood (we do not discuss here the mechanisms regulating eating behavior) and to provide a comprehensive review on the potential positive impact of therapeutic fasting on mood. We explored Medline, Web of Science and PsycInfo according to the PRISMA criteria (Preferred Reporting Items for Systematic reviews and Meta-Analysis). The initial research paradigm was: [(fasting OR caloric restriction) AND (mental health OR depressive disorders OR mood OR anxiety)]. Many neurobiological mechanisms have been proposed to explain fasting effects on mood, such as changes in neurotransmitters, quality of sleep, and synthesis of neurotrophic factors. Many clinical observations relate an early (between day 2 and day 7) effect of fasting on depressive symptoms with an improvement in mood, alertness and a sense of tranquility reported by patients. The persistence of mood improvement over time remains to be determined.

NRF2, cancer and calorie restriction

The transcription factor NF-E2-related factor (NRF2) is a key regulator of several enzymatic pathways, including cytoprotective enzymes in highly metabolic organs. In this review, we summarize the ongoing research related to NRF2 activity in cancer development, focusing on in vivo studies using NRF2 knockout (KO) mice, which have helped in defining the crucial role of NRF2 in chemoprevention. The lower cancer protection observed in NRF2 KO mice under calorie restriction (CR) suggests that most of the beneficial effects of CR on the carcinogenesis process are likely mediated by NRF2. We propose that future interventions in cancer treatment would be carried out through the activation of NRF2 in somatic cells, which will lead to a delay or prevention of the onset of some forms of human cancers, and subsequently an extension of health- and lifespan.

"Control" laboratory rodents are metabolically morbid: why it matters

Failure to recognize that many standard control rats and mice used in biomedical research are sedentary, obese, glucose intolerant, and on a trajectory to premature death may confound data interpretation and outcomes of human studies. Fundamental aspects of cellular physiology, vulnerability to oxidative stress, inflammation, and associated diseases are among the many biological processes affected by dietary energy intake and exercise. Although overfed sedentary rodents may be reasonable models for the study of obesity in humans, treatments shown to be efficacious in these animal models may prove ineffective or exhibit novel side effects in active, normal-weight subjects.

Obesity-related promotion of aberrant crypt foci in DMH-treated obese Zucker rats correlates with dyslipidemia rather than hyperinsulinemia

BACKGROUND

Obesity and energy restriction modulate the development of precancerous aberrant crypt foci (ACF) in animal models of colon cancer.

AIM

Investigation of the major obesity-associated determinants for ACF-development and underlying mechanisms leading to ACF-modulation, such as changes in DNA damage or colonocytes hyperproliferation.

METHODS

Lean and obese Zucker rats fed ad libitum (a.l.) or obese pair fed (p.f.) were induced with 1,2-dimethylhydrazine (DMH) for colon cancer. Multiple regression analyses were performed to identify major metabolic factors correlated with ACF number and size (aberrant crypts/ACF). DNA damage is analyzed by the comet-assay, epithelial proliferation by immunohistochemistry.

RESULTS

Aberrant crypt foci number was significantly elevated in Zucker obese a.l. (205.7+/-65.4 vs. lean 9.5+/-6.3, P<0.05) and is reduced by pair feeding in Zucker obese rats (81.4+/-28.5 vs. obese a.l., P<0.05). Compared to lean the ACF size was higher in Zucker obese a.l. (2.1+/-0.3 vs. lean 1.3+/-0.2., P<0.05) but is not reduced by pair feeding (1.7+/-0.2; P>0.05). While ACF number and size were modulated by genotype and/or pair feeding the DMH-induced DNA damage and hyperproliferation in colonocytes did not differ significantly between groups. Regression analysis showed that plasma parameters associated with lipid-metabolism (triglycerides, cholesterol, malondialdehyde) significantly correlated with the ACF number and size while parameters linked to carbohydrate-metabolism (glucose, insulin) were weaker determinants.

CONCLUSION

Obesity or pair feeding-associated modulation of ACF correlate with parameters related to lipid-metabolism but is not accompanied by changes in DNA damage and proliferation.

Role of CYP2E1 and saturation kinetics in the bioactivation of thioacetamide: Effects of diet restriction and phenobarbital

Thioacetamide (TA) undergoes saturation toxicokinetics in ad libitum (AL) fed rats. Diet restriction (DR) protects rats from lethal dose of TA despite increased bioactivation-mediated liver injury via CYP2E1 induction. While a low dose (50 mg TA/kg) produces 6-fold higher initial injury, a 12-fold higher dose produces delayed and mere 2.5-fold higher injury. The primary objective was to determine if this less-than-expected increase in injury is due to saturation toxicokinetics. Rats on AL and DR for 21 days received either 50 or 600 mg TA/kg i.p. T(1/2) and AUCs for TA and TA-S-oxide were consistent with saturable kinetics. Covalent binding of (14)C-TA-derived-radiolabel to liver macromolecules after low dose was 2-fold higher in DR than AL rats. However, following lethal dose, no differences were found between AL and DR. This lack of dose-dependent response appears to be due to saturation of bioactivation at the higher dose. The second objective was to investigate the effect of phenobarbital pretreatment (PB) on TA-initiated injury following a sub-lethal dose (500 mg/kg). PB induced CYP2B1/2 approximately 350-fold, but did not increase covalent binding of (14)C-TA, TA-induced liver injury and mortality, suggesting that CYP2B1/2 has no major role in TA bioactivation. The third objective was to investigate the role of CYP2E1 using cyp2e1 knockout mice (KO). Injury was assessed over time (0-48 h) in wild type (WT) and KO mice after LD(100) dose (500 mg/kg) in WT. While WT mice exhibited robust injury which progressed to death, KO mice exhibited neither initiation nor progression of injury. These findings confirm that CYP2E1 is responsible for TA bioactivation.

Background gene expression in rat kidney: influence of strain, gender, and diet

In order to gain better insight into factors (strain, gender, and diet) influencing background variability in kidney gene expression, we examined the transcriptomes of male and female Crl:CD(SD)IGSBR (Sprague-Dawley [SD]) and CDF(Fischer 344)/CrlBR rats maintained for 19 days on three different diets (ad libitum [AL], diet restriction-75% of AL, and casein-based phytoestrogen-free diet). Kidney RNA was analyzed using Agilent Rat oligo microarrays (approximately 20,000 genes). Principal component analysis demonstrated that strain and gender have the most impact on the variability in gene expression, while diet had a lesser effect. The majority of the affected genes differed by a magnitude of four-fold or less between strains/gender, with some previously known to be sex-hormone regulated (SLC22A7 and SLC21A1). One gene of particular interest was ornithine decarboxylase, a significant marker of cell proliferation and tumor promotion, which was expressed at an 18-fold greater level in SD rats. Further analysis revealed that the difference in expression was due to the use of an alternate polyadenylation signal resulting in the production of two different sizes of transcripts. These results demonstrate that gender and strain have significant influence on gene expression which could be a confounder when comparing results, especially when it involves predictive fingerprint/patterns.

Stress resistance and aging: influence of genes and nutrition

Previous studies have shown that dermal fibroblast cell lines derived from young adult mice of the long-lived Snell dwarf (dw/dw), Ames dwarf (df/df) and growth hormone receptor knockout (GHR-KO) mouse stocks are resistant, in vitro, to the cytotoxic effects of hydrogen peroxide, cadmium, ultraviolet light, paraquat, and heat. Here we show that, in contrast, fibroblasts from mice on low-calorie (CR) or low methionine (Meth-R) diets are not stress resistant in culture, despite the longevity induced by both dietary regimes. A second approach, involving induction of liver cell death in live animals using acetaminophen (APAP), documented hepatotoxin resistance in the CR and Meth-R mice, but dw/dw and GHR-KO mutant mice were not resistant to this agent, and were in fact more susceptible than littermate controls to the toxic effects of APAP. These data thus suggest that while resistance to stress is a common characteristic of experimental life span extension in mice, the cell types showing resistance may differ among the various models of delayed or decelerated aging.

Effect of strain and diet upon constitutive and chemically induced activities of several xenobiotic-metabolizing enzymes in rats

The response of animals in toxicity studies reflects a complex interaction of a number of variables, some intrinsic to a particular study design and others resulting from the treatment itself. The influences of strain and diet upon constitutive and benzo(a)pyrene (B(a)P) induced activities of several hepatic Phase I and II enzymes were studied in a multifactoral design. Male and female CDF and Crl:CD rats were fed a standard rodent diet ad libitum, a 75% of ad libitum restricted feeding regimen or a phytoestrogen-free diet for approximately 3 weeks. During the last five days of the study, rats were administered either corn oil (vehicle) or 15 mg/kg/day B(a)P via oral gavage. The constitutive activities of hepatic CYP1A1, CYP1A2, CYP2B1/2, and mixed isoforms of UDP-glucuronosyl transferase, sulfotransferase, and glutathione-S-transferase varied significantly by feeding regimen and strain. Responses to B(a)P administration were also observed to be influenced by diet and strain in a manner similar to that observed for constitutive activities. These findings point out the potentially significant interactions of relatively commonly encountered variables that may affect results of hazard testing, especially when employing near metabolically saturating dosages of test chemicals.

Overweight, obesity, and cancer risk

Over the past few decades the proportion of people with excess body weight has been increasing in both developed and less developed countries. About 50% of men and 35% of women in Europe are currently estimated to be overweight or obese. In addition to an increase in the risk of cardiovascular disease and type II diabetes, the evidence summarised here shows that excess body weight is directly associated with risk of cancer at several organ sites, including colon, breast (in postmenopausal women), endometrium, oesophagus, and kidney. In part, these associations with cancer risk may be explained by alterations in the metabolism of endogenous hormones-including sex steroids, insulin, and insulin-like growth factors-which can lead to distortion of the normal balance between cell proliferation, differentiation, and apoptosis. Avoidance of weight gain thus seems to be an important factor for cancer prevention.

Unexpected tumour findings in lifetime rodent bioassay studies--what to do?

Currently, the majority of substances tested in lifetime bioassays in rodents are not mutagenic and, therefore, at the most weakly carcinogenic, generally by epigenetic mechanisms. It thus appears obvious that only marginal increases of tumour incidences can be expected in lifetime bioassays and that, therefore, every aspect of a potential carcinogenic effect must be thoroughly evaluated. This paper describes a series of key factors, which should be looked at in order to exclude that the lifetime bioassay in question is flawed for design, technical or qualification reasons. It also provides some hints whether there is indeed a real effect and not just a variation of the spontaneous tumour incidences. Tumour findings must be seen in the context of the animal model, the pharmcokinetics and pharmcodynamics of the test substance, as well as any other observation in the present or other studies with the test substance, including non-tumour findings and--in particular--potential precursor lesions and effects on feed intake and survival. The possibility that the observed carcinogenic effects may be species-specific and not relevant for man is discussed. It is also important to check what findings are reported with similar substances or substances with the same pharmacological effect. Data from additional investigations on material of the same study and/or mechanistic studies are often needed to support the final risk assessment.

Downregulation of hepatic cytochrome P450 in chronic renal failure

Chronic renal failure (CRF) is associated with a decrease in drug metabolism. The mechanism remains poorly understood. The present study investigated the repercussions of CRF on liver cytochrome P450 (CYP450). Three groups of rats were defined: control, control paired-fed, and CRF. Total CYP450 activity, protein expression of several CYP450 isoforms as well as their mRNA, and the in vitro N-demethylation of erythromycin were assessed in liver microsomes. The regulation of liver CYP450 by dexamethasone and phenobarbital was assessed in CRF rats. Compared with control and control paired-fed rats, creatinine clearance was reduced by 60% (P: < 0.01) in CRF rats. Weight was reduced by 30% (P: < 0.01) in control paired-fed and CRF rats, compared with control animals. There was no difference in the CYP450 parameters between control and control paired-fed. Compared with control paired-fed rats, total CYP450 was reduced by 47% (P: < 0.001) in CRF rats. Protein expression of CYP2C11, CYP3A1, and CYP3A2 were considerably reduced (>40%, P: < 0.001) in rats with CRF. The levels of CYP1A2, CYP2C6, CYP2D, and CYP2E1 were the same in the three groups. Northern blot analysis revealed a marked downregulation in gene expression of CYP2C11, 3A1, and 3A2 in CRF rats. Although liver CYP450 was reduced in CRF, its induction by dexamethasone and phenobarbital was present. N-demethylation of erythromycin was decreased by 50% in CRF rats compared with control (P: < 0.001). In conclusion, CRF in rats is associated with a decrease in liver cytochrome P450 activity (mainly in CYP2C11, CYP3A1, and 3A2), secondary to reduced gene expression.

Differential in vivo effects of alpha-naphthoflavone and beta-naphthoflavone on CYP1A1 and CYP2E1 in rat liver, lung, heart, and kidney

Male Sprague-Dawley rats were treated intraperitoneally with corn oil, the aryl hydrocarbon receptor (AHR) agonist beta-naphthoflavone (betaNF), or the relatively weak AHR agonist alpha-naphthoflavone (alphaNF). Animals treated with betaNF experienced a significant loss (12%) of total body mass over 5 days and a dramatic elevation of CYP1A1 mRNA in all of the organs studied. Treatment with alphaNF had no significant effect on body mass after 5 days and caused only minor increases of liver, kidney, and heart CYP1A1 mRNA. In contrast, lung CYP1A1 mRNA was increased by alphaNF treatment to levels comparable to that seen with betaNF treatment. CYP2E1 mRNA levels were also elevated in liver, lung, kidney, and heart in response to betaNF treatment, whereas alphaNF was without effect. Large increases of CYP1Al-dependent 7-ethoxyresorufin O-deethylation (EROD) activity occurred with microsomes prepared from the tissues of betaNF-treated animals. Comparatively small changes were associated with alphaNF treatment, with the exception of lung, where EROD activity was increased to approximately 60% of that with betaNF treatment. CYP2E1-dependent p-nitrophenol hydroxylase (PNP) activity was also increased by betaNF treatment in microsomes prepared from kidney (3.1-fold), whereas alphaNF was without effect. In contrast, alphaNF or betaNF treatment caused significant decreases of lung microsomal PNP (72% and 27% of corn oil control, respectively) and 7-pentoxyresorufin O-deethylation (48% and 17% of corn oil control, respectively) activities, indicating that PNP activity may be catalyzed by P450 isoforms other than CYP2E1 in rat lung. We conclude that betaNF and alphaNF have differential effects on the expression and catalytic activity of CYP1A1 and CYP2E1, depending upon the organ studied. These changes most likely occur as a result of the direct actions of these compounds as AHR agonists, in addition to secondary effects associated with AHR-mediated toxicity.

Promotion of hepatocarcinogenesis by hexachlorobenzene in energy-restricted rats

The interaction between dietary energy restriction and low dose of the fungicide hexachlorobenzene (HCB) was evaluated in a rat liver medium-term bioassay for carcinogenesis. Male Wistar rats were fed a control or a 50% energy-restricted diet, both added or not with 50 ppm HCB, for 6 weeks. HCB exposure or energy restriction separately did not exert any influence on the development of glutathione S-transferase placental form (GST-P(+)) foci of hepatocytes. Simultaneous HCB exposure and energy restriction induced a significant increase in liver centrilobular hypertrophy and GST-P(+) foci development. Our findings suggest that energy restriction increases liver response to low dose of HCB, unmasking the promoting potential of this fungicide.

Suppression of rat hepatic cytochrome P450s by protein-calorie malnutrition: complete or partial restoration by cysteine or methionine supplementation

Pharmacokinetic profiles of therapeutic agents are altered by protein-calorie malnutrition (PCM). The current study was designed to determine the expression of hepatic cytochrome P450s in rats after protein restriction and to investigate its molecular basis. Western blot analysis revealed that rats with protein restriction for 4 weeks exhibited marked suppression in the hepatic P450 1A2, 2C11, 2E1, and 3A1/2 levels. Northern blot analysis showed that hepatic P450 1A2, 2C11, and 3A1/2 mRNAs were significantly decreased in the state of PCM. The P450 2E1 mRNA level was slightly decreased in PCM rats, suggesting the possibility that expression of P450 2E1 affected by PCM might result from the transcriptional and/or posttranscriptional regulation. PCM-induced changes in most P450 expression completely or partially returned to control levels by a week of cysteine supplementation. Cysteine also prevented decreases in P450 1A2, 2C11, 2E1, and 3A1/2 mRNA levels by PCM. Methionine was minimally active in restoring the P450 expression. A metabolic change in hepatic ethoxyresorufin dealkylase activity in PCM rats was consistent with the P450 apoprotein and mRNA levels. Although the plasma concentrations of azosemide, a loop diuretic, primarily metabolized by cytochrome P450 1A, increased in protein-deprived rats, cysteine supplementation significantly reduced the increased plasma concentrations of the drug. The altered pharmacokinetic parameters of azosemide in PCM rats returned to those of control after cysteine supplementation, corroborating the conclusion that cysteine was effective in restoring cytochrome P450 expression and metabolic activities.

Circadian expression of the steroid 15 alpha-hydroxylase (Cyp2a4) and coumarin 7-hydroxylase (Cyp2a5) genes in mouse liver is regulated by the PAR leucine zipper transcription factor DBP

To study the molecular mechanisms of circadian gene expression, we have sought to identify genes whose expression in mouse liver is regulated by the transcription factor DBP (albumin D-site-binding protein). This PAR basic leucine zipper protein accumulates according to a robust circadian rhythm in nuclei of hepatocytes and other cell types. Here, we report that the Cyp2a4 gene, encoding the cytochrome P450 steroid 15alpha-hydroxylase, is a novel circadian expression gene. This enzyme catalyzes one of the hydroxylation reactions leading to further metabolism of the sex hormones testosterone and estradiol in the liver. Accumulation of CYP2A4 mRNA in mouse liver displays circadian kinetics indistinguishable from those of the highly related CYP2A5 gene. Proteins encoded by both the Cyp2a4 and Cyp2a5 genes also display daily variation in accumulation, though this is more dramatic for CYP2A4 than for CYP2A5. Biochemical evidence, including in vitro DNase I footprinting on the Cyp2a4 and Cyp2a5 promoters and cotransfection experiments with the human hepatoma cell line HepG2, suggests that the Cyp2a4 and Cyp2a5 genes are indeed regulated by DBP. These conclusions are corroborated by genetic studies, in which the circadian amplitude of CYP2A4 and CYP2A5 mRNAs and protein expression in the liver was significantly impaired in a mutant mouse strain homozygous for a dbp null allele. These experiments strongly suggest that DBP is a major factor controlling circadian expression of the Cyp2a4 and Cyp2a5 genes in the mouse liver.

Evaluation of rat hepatic 2E1 activity in function of age, sex and inducers: choice of an experimental model capable of testing the hepatotoxicity of low molecular weight compounds

The aim of this work on rat hepatic P450 2E1 activity was to seek the most suitable experimental model to study the role of cytochrome P450 2E1 in the metabolism of industrial chemicals. Two sets of experiments were devoted to selecting the age and sex of animals and to estimating the response of male and female rats to different inducers. In the first set, the effect of three inducers (fasting; ethanol; acetone) was studied in male rats aged 5, 7 and 9 weeks. In the second set, the effect of different inducers, namely beta-naphthoflavone (BNF), phenobarbital (PB), ethanol, acetone and pyridine, on PNP and chlorzoxazone (CLZO) hydroxylase activities was studied in 7 week old male and female rats. The results demonstrate firstly that microsomal p-nitrophenol (PNP) hydroxylase activity significantly decreases in control male rats in inverse function of age, and secondly that induction by ethanol decreases with age. The PNP hydroxylase activity level of controls and the significant increases in PNP hydroxylase activity observed in 7 week old male rats show that this is the most suitable age for the second set of experiments. In this second set, it was shown that P450 1A (induced by BNF) is involved in CLZO hydroxylase activity only. PB increased the hydroxylase activities in male and female rats by about 1.5 and 1.7 times those of the controls, respectively. The effects of P450 2E1 inducers in function of sex show that male rats exhibited more significant increases in PNP and CLZO hydroxylase activities than female. The specificity of these two substrates is discussed. Neither of these two reactions was specifically catalysed by P450 2E1, but PNP may be considered as the most specific and the least sensitive substrate. In addition, the linear relationship observed between the two substrates (PNP and CLZO) showed a good correlation between their activities (r = 0.90, P < 0.001). In conclusion, these results suggest the use of the 7 week old male rat as the experimental model to study the role of cytochrome P450 2E1 in the hepatotoxicity of low molecular weight industrial chemicals.

Diet restriction enhances compensatory liver tissue repair and survival following administration of lethal dose of thioacetamide

Diet restriction is known to prevent a plethora of age-associated diseases including cancer. However, the effects of diet restriction on noncancer end points are not known. The objective of this study was to investigate whether diet restriction protects against hepatotoxicity of thioacetamide (TA), and if so, to investigate the underlying mechanism. Male Sprague-Dawley rats (250-275 g) were maintained on 65% of their ad libitum (AL) food consumption for a period of 3 weeks and then treated with a single low dose of 50 mg TA/kg i.p.. Plasma enzymes (ALT and SDH), hepatic glycogen levels, and 3H-thymidine incorporation into hepatocellular nuclear DNA were measured during a time course (0-120 h) after TA administration. Liver sections were examined for histopathology, and cell-cycle progression was assessed by proliferating cell nuclear antigen (PCNA) immunohistochemistry. In AL rats hepatic necrosis was evident at 12 h, peaked at 36 h, persisted up to 72 h, and was resolved by 96 h. In the diet-restricted (DR) group hepatic necrosis was observed at 12 h, peaked at 24 h, persisted till 72 h, and was resolved by 96 h. Maximal injury indicated by enzyme elevation occurred in DR rats and was approximately sixfold greater than that observed in the AL group. Histopathological examination of the liver sections revealed liver injury concordant with plasma enzyme elevations. There was a higher and sustained S-phase synthesis in the DR rats compared to AL group. S-phase stimulation was evident at 36 h, peaked at 48 h, and persisted until 96 h in the DR rats, whereas in the AL rats peak S-phase stimulation occurred at 36 h and subsided by 72 h. PCNA studies revealed a corresponding stimulation of cell-cycle progression indicating highly stimulated compensatory tissue repair. The 14-day lethality experiments (600 mg TA/kg i.p.) indicated 70% survival in the DR rats compared to 10% survival in the AL group. Although diet restriction increases hepatotoxic injury of TA, it protects from the lethal outcome by enhanced liver tissue repair. Comparison of liver injury and tissue repair employing an equitoxic dose (600 mg TA/kg in AL rats yields similar liver injury as observed with 50 mg TA/kg in DR rats) revealed that in spite of near equal injury up to 36 h, tissue repair response in DR rats is much higher. The compensatory tissue repair allows the DR rats to escape death in contrast to much lower compensation in AL rats leading to progression of liver injury culminating in death.

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.

A two-year dietary carcinogenicity study of the antiestrogen toremifene in Sprague-Dawley rats

The carcinogenic potential of the nonsteroidal triphenylethylene antiestrogen toremifene (Fareston) was evaluated in a standard 104-week rat dietary carcinogenicity study. The doses were 0, 0.12, 1.2, 5.0 and 12 mg/kg/day and the number of animals 50/sex/dose group. The body weight gain and food consumption were monitored once weekly (study weeks 1-16) or once every four weeks thereafter (study weeks 17-104). Blood samples were taken at weeks 34, 52 and 104 and the plasma concentrations of toremifene, as well as the two main metabolites (deaminohydroxy)toremifene and N-demethyltoremifene, were measured. All doses of toremifene reduced food intake and body weight gain. Toremifene caused a significant reduction in mortality, which was mainly due to reduced incidences of pituitary tumors. This was evident in all dose groups. Drug-related decrease of mammary tumors in females (at all doses) and testicular tumors in male rats (doses > or = 1.2 mg/kg/day) were also evident. The incidence of the preneoplastic foci of basophilic hepatocytes were significantly decreased in treated female groups. Toremifene induced no preneoplastic or neoplastic lesions. Based on histopathology, no obvious toxicity could be observed. Drug-related changes were observed in the genital organs, thyroid, spleen, mammary gland, adrenal, kidney, stomach and lung. These changes were due to hormonal disturbances or as a result of reduced food consumption or reduced incidences of pituitary, mammary or testicular tumors. This study indicates that toremifene is an efficient antiestrogen in long-term treatment, is well tolerated and has no tumorigenic potential in rats.

The beneficial effects of dietary restriction: reduced oxidative damage and enhanced apoptosis

There is compelling evidence for the central role of oxidative damage in the aging process and for the participation of reactive oxygen species in tumor initiation and promotion. Caloric restriction (CR) or energy restriction retards age-associated increases in mitochondrial free-radical production and reduces the accumulation of oxidatively damaged cell components. CR has also been shown to slow down age-related declines in various repair capabilities, including some types of DNA repair. It is proposed that inhibitors of mitochondrial electron transport and/or uncouplers of oxidative phosphorylation (rotenone, amytal, amiodarone, valinomycin, etc.), when used at extremely low doses, could mimic the effects of CR in model systems. The objective is to lower mitochondrial free-radical production by decreasing the fraction of electron carriers in the reduced state. In addition to a variety of other effects, CR has been shown to increase the rate of apoptosis, particularly in preneoplastic cells, and in general, to promote elevated levels of free glucocorticoids (GCs). GCs are known to induce tissue-specific apoptosis and to upregulate gap-junction-mediated intercellular communication (GJIC). Tumor promoters like phorbol esters have the opposite effect, in that they inhibit both the process of apoptosis and GJIC. The enzyme poly (ADP-ribose) polymerase (PARP) is thought to play a central role in apoptosis, in a manner that has been highly conserved in evolution. There is good evidence that the apoptosis-associated Ca/Mg-dependent DNA endonuclease is maintained in a latent form by being poly (ADP-ribosylated). Apoptosis would require the removal of this polymer from the endonuclease, and, most likely, its removal from topoisomerase II and histone H1 as well. The role of poly (ADP-ribose) in apoptosis, carcinogenesis, and aging could be studied by the use of modulators of PARP activity (3-aminobenzamide, 3-nitrosobenzamide, 1% ethanol, etc.), inhibitors of poly ADP-ribose) glycohydrolase activity (ethacridine, 43 degrees C, etc.), and inhibitors of the PARP-specific protease (interleukin-1 beta converting enzyme (ICE)-like protease). Also, it would be of interest to determine if CR can decrease the half-life of poly (ADP-ribose), upregulate GJIC, and modulate the activities of PARP, the glycohydrolase, and the PARP-specific protease, factors potentially important in these processes.

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.

Caloric restriction, aging, and antioxidant enzymes

The basic mechanisms of aging and its retardation by caloric restriction (CR) remain unclear. One suggested means by which CR could retard aging is based on production of mitochondrial free radicals, and efficiency of their subsequent metabolism. Currently, there is little information concerning the influences of age and CR on the rates of in vivo mitochondrial free radical production. However, evidence for CR-induced modulation of free radical detoxification capacities is mounting. The direction of the influence of CR on free radical detoxification is tissue-specific. These effects are broad and appear to provide positive advantage.

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.