The effect of age on inducibility of various types of rat liver cytochrome P-450

Age-Specific Regulation of Drug-Processing Genes in Mouse Liver by Ligands of Xenobiotic-Sensing Transcription Factors

The xenobiotic-sensing transcription factors (xeno-sensors) AhR, CAR, and PXR upregulate the expression of many drug-processing genes (DPGs) in liver. Previous studies have unveiled profound changes in the basal expression of DPGs during development; however, knowledge on the ontogeny of the inducibility of DPGs in response to pharmacological activation of xeno-sensors is still limited. The goal of this study was to investigate the age-specific regulation of DPGs by prototypical xeno-sensor ligands: 2,3,7,8-tetrachlorodibenzodioxin (TCDD) for AhR; 1,4-bis [2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP) for CAR; and pregnane-16α-carbonitrile (PCN) for PXR during mouse liver development. The basal mRNAs of most DPGs were low during neonatal age, but gradually increased to adult levels, whereas some DPGs (Cyp1a2, Cyp2b10, Cyp3a11, Gstm2, Gstm3, Papss2, and Oatp1a4) exhibited an adolescent-predominant expression pattern. The inducibility of DPGs was age-specific: 1) during neonatal age, the highest fold increase in the mRNA expression was observed for Cyp1a2, Sult5a1, and Ugt1a9 by TCDD; Cyp3a11 and Mrp2 by TCPOBOP; as well as Gstm2 and Gstm3 by PCN; 2) during adolescent age, the highest fold increase in the mRNA expression was observed for Ugt1a6 and Mrp4 by TCDD, Cyp2b10, Ugt2b34, and Ugt2b35 by TCPOBOP, as well as Gsta1, Gsta4, Sult1e1, Ugt1a1, Mrp3, and Mrp4 by PCN; 3) in adults, the highest fold increase in the mRNA expression was observed for Aldh1a1, Aldh1a7, and Ugt2b36 by TCPOBOP, as well as Papss2 and Oatp1a4 by PCN. In conclusion, the inducibility of hepatic DPGs following the pharmacological activation of xeno-sensors is age specific.

Ontogeny of mammalian metabolizing enzymes in humans and animals used in toxicological studies

It is well recognized that expression of enzymes varies during development and growth. However, an in-depth review of this acquired knowledge is needed to translate the understanding of enzyme expression and activity into the prediction of change in effects (e.g. kinetics and toxicity) of xenobiotics with age. Age-related changes in metabolic capacity are critical for understanding and predicting the potential differences resulting from exposure. Such information may be especially useful in the evaluation of the risk of exposure to very low (µg/kg/day or ng/kg/day) levels of environmental chemicals. This review is to better understand the ontogeny of metabolizing enzymes in converting chemicals to either less-toxic metabolite(s) or more toxic products (e.g. reactive intermediate[s]) during stages before birth and during early development (neonate/infant/child). In this review, we evaluated the ontogeny of major "phase I" and "phase II" metabolizing enzymes in humans and commonly used experimental animals (e.g. mouse, rat, and others) in order to fill the information gap.

Phenacetin and chlorzoxazone biotransformation in aging male Fischer 344 rats

We evaluated the role of specific isoforms in the biotransformation of phenacetin and chlorzoxazone and examined the effect of age on these reactions using liver microsomes from Fischer 344 rats between 3 and 26 months of age. Using rat cDNA-expressed cytochrome P450 (CYP) enzymes, we found that phenacetin biotransformation was primarily mediated by CYP2C6 and CYP1A isoforms, while chlorzoxazone biotransformation was largely mediated by CYP2E1 and CYP1A1. Incubations with liver microsomes prepared from rats of varying ages demonstrated that both phenacetin and chlorzoxazone biotransformation declined with age. Metabolite formation rates in the old rats (25–26 months) were reduced by approximately 60–70% for these reactions. This study suggests that the activity of CYP2E and CYP1A enzymes decline with age in the rat liver. Also, the relative specificity of the index substrates phenacetin (for CYP1A2) and chlorzoxazone (for CYP2E1) in man appears not to be applicable in rats.

PCB accumulation in osprey exposed to local sources in lake sediment

We examined the accumulation of PCBs in ospreys (Pandion haleaetus) that were exposed to local sediment sources. Eggs, chick plasma, and sediment samples were collected over a range of 14 km (0.2-14.2 km) from a PCB source in Sturgeon Lake, ON. Sum PCB concentrations declined in chick plasma (range 422.5-58.3 ng/g) as distance from the PCB source increased, but there was a poor relationship with sum PCBs in eggs. Both tissues indicated an Aroclor 1248/1254 source. Aroclor 1254 comprised an average of 66.9% of sum PCBs in chick plasma from Sturgeon Lake, but comprised only from 27.0 to 44.4% in plasma from other Great Lake colonies. Dietary differences among osprey colonies were not sufficient to explain the PCB patterns observed. There was weak evidence that the ability to metabolize PCBs may differ between juveniles and adults, based upon the PCB profile in eggs and chick plasma.

Metabolism of a Heterocyclic Amine Colon Carcinogen in Young and Old Rats

The incidence of colon cancer increases with age, and this may be related to altered metabolism and disposition of carcinogens. One such carcinogen implicated in colon cancer is the heterocyclic amine found in well done meat, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). The purpose of these studies was to determine whether the disposition and metabolism of IQ changes with age, comparing young (3-month) and old (22- to 24-month) male F344 rats. Animals were treated with vehicle or beta-naphthoflavone (BNF), an inducer of drug-metabolizing cytochromes P450. Disposition and metabolism of IQ were determined after i.p. injection of radiolabeled IQ. Urinary IQ metabolites were identified and quantitated by high-performance liquid chromatography and mass spectroscopy. In BNF-treated animals, total radiolabeled IQ excretion by old rats was less than half that of young rats. Binding of radiolabeled IQ metabolites by the old kidney was 10 times higher than that of the young. There were no age differences in intestinal and hepatic binding. There was a significant age-related increase in IQ conjugation to glucuronic acid and a decrease in conjugation to sulfate regardless of treatment. The induction of renal CYP1A1, a major P450 involved in IQ metabolism, by BNF did not change with age. Changes in IQ metabolism with age along with altered renal function may contribute to the decreased urinary excretion and increased renal binding of IQ and/or its metabolites seen in the old animals.

Constitutive and inducible hepatic cytochrome P450 isoforms in senescent male and female rats and response to low-dose phenobarbital

Numerous studies, usually limited to male rodents, have reported an inverse relationship between the age of the animal and the activities of various multi-cytochrome P450-dependent drug-metabolizing enzymes. It has been suggested that the aging-induced decline in hepatic drug-metabolizing capacity is solely a male phenomenon. That is, whereas the levels of male-specific isoforms of P450 decline with senescence, the female-dependent isoforms remain unchanged in females and even increase in male liver. In addition to their baseline activities, induction levels of hepatic monooxygenases have also been reported to decrease with aging. To examine aging- and sex-dependent effects on drug metabolism at a more molecular level, we measured the expression (mRNA, protein, and/or catalytic activity) of a near dozen constitutive and inducible isoforms of P450 in 5-and 23-month-old male and female Sprague-Dawley rats. Moreover, we investigated the induction effects of low concentrations of phenobarbital known to reveal gender differences and the threshold sensitivities of both constitutive and inducible isoforms. With the exception of male-specific CYP2C11 (whose expression declined approximately 70% in aged male rats), we observed little senescence-associated reduction in either preinduction or induction levels of CYP2B1, CYP2B2, CYP3A1, CYP3A2, CYP2C6, CYP2C7, CYP2C12, and CYP2C13 in either male or female rats. Moreover, the sexually dimorphic expression levels apparent at 5 months of age persisted in the old rats.

Effect of environmental pollutants on hepatocellular function in rats: 3-methylcholanthrene and Aroclor-1254

Environmental pollutants, Aroclor-1254 (PCB) and 3-methylcholanthrene (MC), were employed in this study to investigate some aspects of the induction of hepatic drug metabolism in rats. PCB and MC treatments increased 7-ethoxyresorufin and 7-ethoxycoumarin O-deethylase activities related to cytochrome P-448. Cytochrome P-450 reductase activity was increased by PCB while no effect was observed by MC treatment. Pretreatment with PCB resulted in approximately 50% increase in the phospholipid content of the microsomes whereas MC caused no change. Liver microsomal cholesterol content was decreased while triglycerides were increased by PCB. The ratio between saturated and unsaturated fatty acids (saturation index) decreased in the total microsomes and phospholipids with PCB treatment, whereas MC did not alter the ratio, except that the major effect of MC was observed in the acyl derivatives of microsomal phosphatidylethanolamine. It is proposed that the uniaxial rotation and mobility of hemoproteins may be restricted by an increase in the saturation index of the membrane, while a decreased index may facilitate contact with reductases for electron transfer by enhanced membrane fluidity. The decreased saturation index after treatment with MC may play a role in carcinogenicity by triggering induction of free radicals.

Feeding melatonin enhances the phase shifting response to triazolam in both young and old golden hamsters

Aging alters many aspects of circadian rhythmicity, including responsivity to phase-shifting stimuli and the amplitude of the rhythm of melatonin secretion. As melatonin is both an output from and an input to the circadian clock, we hypothesized that the decreased melatonin levels exhibited by old hamsters may adversely impact the circadian system as a whole. We enhanced the diurnal rhythm of melatonin by feeding melatonin to young and old hamsters. Animals of both age groups on the melatonin diet showed larger phase shifts than control-fed animals in response to an injection with the benzodiazepine triazolam at a circadian time known to induce phase advances in the activity rhythm of young animals. Thus melatonin treatment can increase the sensitivity of the circadian timing system of young animals to a nonphotic stimulus, and the ability to increase this sensitivity persists into old age, indicating exogenous melatonin might be useful in reversing at least some age-related changes in circadian clock function.

Ontogenesis of hepatocyte respiration processes in relation to rat liver cytochrome P450-dependent monooxygenase system

The study aimed to evaluate the effect of age on the activity of the cytochrome P450-dependent monooxygenase system and on cellular respiration processes in Wistar rats aged 0.5, 1, 2, 4, 8, 12, 20, and 28 months. The following parameters were determined: cytochrome P450 content, cytochrome b5 content, NADPH-cytochrome P450 reductase activity, and NADH-cytochrome b5 reductase activity, succinate dehydrogenase (SDH) activity, and lactate dehydrogenase (LDH) activity. In the study, cytochrome P450 content increased in the first month of life, which was accompanied by increases in SDH and LDH activities. In the subsequent months, SDH activity decreased, whereas LDH activity increased to reach the maximum in month eight and then decreased. Cytochrome b5 content showed a decreasing tendency throughout the experiment. NADH-cytochrome b5 reductase activity showed only slight deviations in individual age groups.

Involvement of CYP3A1, 2B1, and 2E1 in C-8 hydroxylation and CYP 1A2 and flavin-containing monooxygenase in N-demethylation of caffeine; identified by using inducer treated rat liver microsomes that are characterized with testosterone metabolic patterns

Caffeine (CA) is oxidized by rat liver microsomal enzymes to theobromine (TB), paraxanthine (PX), and theophylline (TP) by N-demethylation and to trimethylurate (TMU) by C-8 hydroxylation, In order to identify the specific enzymes responsible for productions of these primary CA metabolites, liver microsomes enriched with various isoforms of cytochrome P450 (CYP) and flavin-containing monooxygenase (FMO) are prepared by pretreatment of rats with several inducers. The specific increases in various CYP or FMO activities are identified with the diagnostic testosterone metabolic patterns or the thiobenzamide S-oxidation assay. They are then employed to metabolize the CA. Liver microsomes isolated from rats pretreated with phenobarbital (PB-microsomes) did not have increased FMO activity but had increased activities for hydroxylating the testosterone at 6 beta-(CYP3A1), 16 beta-(CYP2B1), and 2 beta-(CYP3A1) positions. This PB-microsomes had increased activity for TMU production from CA (result of C-8 hydroxylation). Liver microsomes isolated from rats pretreated with acetone (AC-microsomes) had a normal level of FMO activity but had enhanced rates of 6 beta-(CYP3A1) and 2 beta-(CYP3A1) hydroxylations of testosterone. The AC-microsomes again had increased activity for production of TMU. Similarly, the liver microsomes isolated from rats pretreated with dexamethasone (DEX-microsomes) had a normal level of FMO activity but had enhanced rates of forming 6 beta-and 2 beta-hydroxytestosterone (Cyp3A1) as well as androstenedione (CYP3A1). The DEX-microsomes again had increased activity for production of TMU only. Liver microsomes isolated from rats pretreated with 3-methylcholanthrene (MC-microsomes), however, had increased FMO activity and also enhanced rates of forming the 7 alpha-(CYP1A1/2, and 2A1), 6 beta-(CYP3A1), and 2 beta-(CYP3A1) hydroxytestosterone. The MC-microsomes had increased activity for producing all of the four primary metabolites of CA, i.e. the N-demethylation metabolites like TB, PX. and TP, as well as the C-8 hydroxylation metabolite TMU. By the process of association of the obtained results, liver microsomes with increased contents of CYP2B1, 3A1, and 2E1 could catalyze the C-8 hydroxylation at an increased rate producing increased amount of TMU. Increased productions of CA N-demethylation metabolites (TB, PX, and TP) are, however, catalyzed by the increased activities of CYP1A2 and FMO which are associated uniquely with the MC-microsomes.

The influence of aging on dimethylnitrosamine-demethylase enzyme kinetics in rat liver microsomes

To investigate the observed age-related increased susceptibility to chemically-induced carcinogenesis, liver microsomes from 12- or 36-month-old rats either untreated or maximally induced with phenobarbital or isoniazid were used to determine the Vmax and Km for dimethylnitrosamine-demethylase (DMNA-d). A decrease in cytochrome P450 content between young and old animals was observed in the untreated group, but no change was seen in the treated animals. Inducer-related increases were observed after phenobarbital treatment and in the 36-month-old isoniazid-treated group. The Vmax for DMNA-d did not change between 12 and 36 months of age in all experimental groups, but significant changes between the young and old age-group and inducer-related differences were observed in the Km,app for DMNA-d. A high correlation was found between the Cl(int) (= Vmax/Km,app) of DMNA-d and the Vmax of p-nitrophenol-hydroxylation, indicating a major role for CYP2E1 in the metabolism of DMNA-d. The observed changes in the cytochrome-P450 levels and the reduced affinity in DMNA-d metabolism in the untreated group in this study is another indication that aging predominately affects the activity of some constitutive cytochrome P450 enzymes but not the activity of the inducible types of P450.

Acute renal and hepatic effects induced by 3-haloanilines in the Fischer 344 rat

Haloanilines are commonly used as chemical intermediates in the manufacture of a wide range of products. The purpose of this study was to examine the in vivo nephrotoxic and hepatotoxic potentials of the 3-haloanilines. The in vitro effects of the 3-haloanilines on renal function were also examined. In the in vivo experiments, male Fischer 344 rats (four rats/group) were administered a single intraperitoneal (i.p.) injection of an aniline hydrochloride (1.0 or 1.25 mmol kg-1) or vehicle. Renal and hepatic function were monitored at 24 and/or 48 h post-treatment. None of the 3-haloanilines were potent nephrotoxicants at either dose level. The greatest effects on renal function were observed following administration of 3-chloroaniline at a dose of 1.25 mmol kg-1 (oliguria, glucosuria, hematuria, decreased p-aminohippurate accumulation by renal cortical slices and increased blood urea nitrogen concentration). 3-Chloroaniline also was the only aniline compound to increase plasma ALT/GPT activity at 48 h. In the in vitro experiments, the ability of an aniline (10(-5) - 10(-3) M) to decrease organic ion accumulation in renal cortical slices from untreated rats was examined. The decreasing order of in vitro nephrotoxic potential was 3-iodoaniline > 3-bromoaniline > 3-chloroaniline > aniline > 3-fluoroaniline. These results indicate that the 3-haloanilines are not potent nephrotoxicants or hepatotoxicants at sublethal doses. In addition, the reasons why the 3-haloanilines have different orders of nephrotoxic potential in vivo and in vitro are not clear at this time.

Cellular aging and the importance of energetic factors

The in vitro aging of human fibroblasts has become a classical model for studying cellular aging. This model was lately redefined by showing that these cells represent a stem cell system in which they progressively pass through seven morphotypes. Experimental data showed that external conditions that can be considered as stresses for the cells, can modulate the genome expression by speeding up the passage of the cells from one morphotype to the other. In this article, we will interpret these observations from the point of view of the thermodynamics of far from equilibrium open systems, which shows the importance of the production and the use of energy, both responsible for the generation of a given amount of entropy production. In stable systems like these cell morphotypes, such a production is constant but external stresses can prematurely destabilize the steady state of entropy production and, in doing so, accelerate the process of aging. It is also predicted that cells submitted to a stress will use part of their energy in response to the stress. Some experimental data in favor of such an interpretation have been obtained and more will be presented here that show that both cell death and accelerated cell aging under stress are modulated by the level of energy metabolism. All theoretical and experimental arguments presented in this article will show that cellular aging is related to stress and also to energy production through a very elaborate system of regulatory processes necessary for the cell to survive and to perform specific functions according to its differentiated state. This regulatory system also permits the cell to adapt its response according to the intensity of external as well as internal challenges and one of these responses will influence the cellular aging rate.

Microbial succession and intestinal enzyme activities in the developing rat

The succession of gut bacteria and selected intestinal enzyme activities in developing 7-35-d-old rats was studied. Aerobes and anaerobes were identified as members of four broad major bacterial groups, i.e. Gram-positive rods, Gram-positive cocci, Gram-negative rods and obligate anaerobes. The enzyme activities of nitro and azo reductases, beta-glucuronidase, dechlorinase and dehydrochlorinase were determined by anaerobic incubation of intestinal homogenates with 3,4-dichloronitrobenzene, methyl orange, p-nitrophenyl-beta-D-glucuronide, and p,p'-DDT respectively. Nitroreductase and azo reductase activities increased significantly with the appearance of anaerobes in the large intestine. No increase in either nitroreductase or azo reductase activities in the small intestine was found. The early and high level of beta-glucuronidase activity in the small and large intestines coincided with high numbers of coliforms recovered in 7 and 14 d animals. Dehydrochlorinase activity appeared early but was undetectable at both 21 and 28 d. Its activity increased at 35 d. Dechlorinase activity was variable in development. The rapid changes in the microbial flora and intestinal enzyme activities may influence the susceptibility of pre-pubescent rats to a variety of toxicants. Therefore, age-dependent toxicity may be important in the risk assessment of some environmental chemicals.

Age- and gender-related changes in the hepatic metabolism of 2-methylpropene and relationship to epoxide metabolizing enzymes

The effect of age and gender on the in vitro biotransformation of 2-methylpropene, an alkene metabolized to 2-methyl-1,2-epoxypropane, was studied. The epoxide concentration and the epoxide metabolizing enzymatic activities were investigated in male and female Brown Norway rats of different ages. Liver tissue of senescent rats was exposed to smaller 2-methyl-1,2-epoxypropane concentrations than that of young animals, although changes during ageing were rather modest. With advancing age a feminization of male glutathione S-transferase and cytosolic epoxide hydrolase activities was found, as well as a significant decline of the female microsomal epoxide hydrolase activity and an increase of the cytochrome P-450 content in the oldest female rats.