Localization of cytochromes P450 in human tissues: implications for chemical toxicity

The involvement of CYP1A2 in biodegradation of dioxins in pigs

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is one of the most harmful chemicals showing resistance to biodegradation. The majority of TCDD effects is mediated by the aryl hydrocarbon receptor (AhR) pathway. TCDD binding to AhR results in the activation of cytochrome P450 enzymes (CYP1A1, CYP1A2, CYP1B1) involved in dioxin biodegradation. The goal of the study was to explore the potentialrole of CYP1A2 in the metabolism of TCDD. We investigated a molecular structure of CYP1A2 and the binding selectivity and affinity between the pig CYP1A2 and: 1/ DiCDD or TCDD (dioxins differing in toxicity and biodegradability) or 2/ their selected metabolites. pCYP1A2 demonstrated higher affinity towards DiCDD and TCDD than other pCYP1 enzymes. All dioxin-pCYP1A2 complexes were found to be stabilized by hydrophobic interactions. The calculated distances between the heme oxygen and the dioxin carbon nearest to the oxygen, reflecting the hydroxylating potential of CYP1A2, were higher than in other pCYP1 enzymes. The distances between the heme iron and the nearest dioxin carbon exceeded 5 Å, a distance sufficient to allow the metabolites to leave the active site. However, the molecular dynamics simulations revealed that two access channels of CYP1A2 were closed upon binding the majority of the examined dioxins. Moreover, the binding of dioxin metabolites did not promote opening of channel S–an exit for hydroxylated products. It appears that the undesired changes in the behavior of access channels prevail over the hydroxylating potential of CYP1A2 towards TCDD and the favorable distances, ultimately trapping the metabolites at the enzyme’s active site.

Preclinical models for colonic absorption, application to controlled release formulation development

Oral controlled release (CR) formulations have many benefits and have become a valuable resource for the local and systemic administration of drugs. The most important characteristic of these pharmaceutical products is that drug absorption occurs mainly in the colon. Therefore, this review analyses the physiological and physicochemical features that may affect an orally administered CR product, as well as the different strategies to develop a CR dosage form and the methods used to evaluate the formulation efficacy. The models available to study the intestinal permeability and their applicability to colonic permeability determinations are also discussed.

Synthesis and evaluation of mesoporous carbon/lipid bilayer nanocomposites for improved oral delivery of the poorly water-soluble drug, nimodipine

PURPOSE

A novel mesoporous carbon/lipid bilayer nanocomposite (MCLN) with a core-shell structure was synthesized and characterized as an oral drug delivery system for poorly water-soluble drugs. The objective of this study was to investigate the potential of MCLN-based formulation to modulate the in vitro release and in vivo absorption of a model drug, nimodipine (NIM).

METHODS

NIM-loaded MCLN was prepared by a procedure involving a combination of thin-film hydration and lyophilization. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), specific surface area analysis, differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were employed to characterize the NIM-loaded MCLN formulation. The effect of MCLN on cell viability was assessed using the MTT assay. In addition, the oral bioavailability of NIM-loaded MCLN in beagle dogs was compared with that of the immediate-release formulation, Nimotop®.

RESULTS

Our results demonstrate that the NIM-loaded MCLN formulation exhibited a typical sustained release pattern. The NIM-loaded MCLN formulation achieved a greater degree of absorption and longer lasting plasma drug levels compared with the commercial formulation. The relative bioavailability of NIM for NIM-loaded MCLN was 214%. MCLN exhibited negligible toxicity.

CONCLUSION

The data reported herein suggest that the MCLN matrix is a promising carrier for controlling the drug release rate and improving the oral absorption of poorly water-soluble drugs.

Effects of pravastatin on the pharmacokinetic parameters of nimodipine after oral and intravenous administration in rats: possible role of CYP3A4 inhibition by pravastatin

Objective:

The aim of this study was to investigate the effects of pravastatin on the pharmacokinetics of nimodipine in rats.

Materials and Methods:

The effect of pravastatin on P-glycoprotein (P-gp) and cytochrome P450 (CYP) 3A4 activity was evaluated. Nimodipine was administered to rats intravenously (3 mg/kg) and orally (12 mg/kg) with pravastatin (0.3 and 1 mg/kg).

Results:

Pravastatin inhibited CYP3A4 enzyme activity in a concentration-dependent manner with a 50% inhibition concentration (IC₅₀) of 14 µM. Compared with the oral control group, the area under the plasma concentration-time curve (AUC_0-∞) of nimodipine was increased significantly. Consequently, the absolute bioavailability (AB) of nimodipine with pravastatin (1 mg/kg) was 31.1%, which was significantly enhanced compared with the oral control group. Moreover, the relative bioavailability (RB) of nimodipine was 1.12- to 1.31-fold greater than that of the control group.

Conclusions:

The enhanced oral bioavailability of nimodipine might be mainly due to inhibition of the CYP3A-mediated metabolism of nimodipine in the small intestine and/or in the liver and due to reduction of the total body clearance rather than both to inhibition of the P-gp efflux transporter in the small intestine and reduction of renal elimination of nimodipine by pravastatin. The increase in the oral bioavailability of nimodipine with pravastatin should be taken into consideration of potential drug interactions between nimodipine and pravastatin.

Expression of cytochrome P450 1A1/2 and 3A4 in liver tissues of hepatocellular carcinoma cases and controls from Taiwan and their relationship to hepatitis B virus and aflatoxin B1-and 4-aminobiphenyl-DNA adducts

Cytochrome P450 enzymes play a major role in the metabolism of several of the chemical carcinogens involved in the development of hepatocellular carcinoma (HCC). To investigate by immunohistochemistry interindividual differences in these enzymes, polyclonal antisera and immunoperoxidase staining were used to detect the expression of CYP1A1/2 and 3A4 in 37 surgical control tissues and 105 tumour and adjacent nontumour tissues of HCC cases from Taiwan. There was variability in the expression and staining pattern for both CYP1A1/2 and 3A4 in all tissue types. In tissues from controls, there was no correlation between P450 expression and smoking history or hepatitis B virus antigen status. Since these samples had been previously analysed for the DNA adducts of aflatoxin B1 (AFB1), a dietary mould contaminant, and 4-aminobiphenyl (4-ABP), a component of cigarette smoke, we also investigated the relationship between P450 levels and DNA adducts. 4-ABP-DNA adducts were higher in tissues with elevated levels of CYP1A1/2 (p = 0.02). Overall there was no relationship between CYP1A1/2 or CYP3A4 and AFB1-DNA adducts in control tissues. Staining intensity for CYP1A1/2 and 3A4 followed the order: tumour tissues < control tissues < adjacent non-tumour tissues. CYP1A1/2 levels tended to be lower in tumour and adjacent non-tumour tissues than for CYP3A4. In HCC cases, 4-ABP-DNA adducts were higher in subjects with higher levels of CYP1A1/2, stratified by tissue type, but these differences were not significant. For CYP3A4, in contrast to control tissues, there was a significant association with AFB1-DNA adducts in tumour and adjacent non-tumour tissue of HCC cases. These results suggest that one factor influencing carcinogen-DNA adducts is levels of specific P450 enzymes. However, adduct formation in vivo is a complex processes dependent upon numerous genetic and environmental factors.

Supersaturated self-nanoemulsifying drug delivery systems (Super-SNEDDS) enhance the bioavailability of the poorly water-soluble drug simvastatin in dogs

This study investigates the potential of supersaturated self-nanoemulsifying drug delivery systems (super-SNEDDS) to improve the bioavailability of poorly water-soluble drugs compared to conventional SNEDDS. Conventional SNEDDS contained simvastatin (SIM) at 75% of the equilibrium solubility (S (eq)). Super-SNEDDS containing SIM at 150 and 200% of S (eq) were produced by subjecting the SNEDDS preconcentrates to a heating and cooling cycle. The super-SNEDDS were physically stable over 10 months. During in vitro lipolysis of SNEDDS and super-SNEDDS the SIM concentration in the aqueous phase increased for the first 30 min almost proportional to the drug loads and amounts of preconcentrate employed. The 200% drug-loaded super-SNEDDS generated an amorphous SIM precipitate at the end of in vitro lipolysis. In vivo, the relative bioavailability of SIM from super-SEDDDS increased significantly to 180 ± 53.3% (p = 0.014) compared to the dosing of two capsules of (dose equivalent) 75% drug-loaded SNEDDS. A significant increase in the terminal half-life of elimination was observed for super-SNEDDS (2.3 ± 0.6 h) compared to conventional SNEDDS (1.4 ± 0.3 h) as well as a decreased area under the curve ratio of the SIM metabolite simvastatin acid to the parent compound (0.57 ± 0.20 and 0.90 ± 0.3), possibly due to a combination of saturation effects on presystemic metabolising enzymes and prolonged absorption along the small intestine. In summary, this study demonstrated that super-SNEDDS are a viable formulation option to enhance the bioavailability of poorly water-soluble drugs such as simvastatin while reducing the pill burden by an increased drug load of SNEDDS.

A morphometric and immunohistochemical study to assess the benefit of a sustained virological response in hepatitis C virus patients with cirrhosis

Although annular fibrosis is the hallmark of cirrhosis, other microscopic changes that affect liver function such as sinusoid capillarization or loss of metabolic zonation are common. A sustained virological response (SVR) may halt fibrosis deposition in hepatitis C virus (HCV)-infected patients, but its impact on the other cirrhosis-associated lesions is unknown. The aim of this study was to assess the impact of an SVR on cirrhosis-related histopathological features. Paired pre- and posttreatment liver biopsies from 38 HCV patients with cirrhosis with an SVR were analyzed. Fibrosis was staged using the METAVIR scoring system, and the area of fibrosis was measured using morphometry. Ductular proliferation, metabolic zonation, sinusoid capillarization, and hepatic stellate cell activation were assessed by anti-cytokeratin-7, anti-glutamine synthetase (GS), anti-cytochrome P4502E1 (CYP2E1), anti-CD34, and anti α-smooth muscle actin (αSMA). After 61 months from an SVR, cirrhosis regression was observed in 61%, and the collagen content decreased in 89%. Although periportal and lobular necroinflammation vanished, portal inflammation persisted in 66%. Ductular proliferation decreased in 92%. Before treatment, metabolic zonation was lost, as shown by GS and CYP2E1, in 71% and 88%, respectively, with normalization in 79% and 73%, after an SVR. Conversely, no changes in sinusoidal capillarization were observed after treatment, as assessed by CD34 (P = 0.41) and αSMA (P = 0.95). Finally, no differences in all the immunohistochemical scores emerged whether or not cirrhosis persisted.

CONCLUSION

Cirrhosis regression and decreased fibrosis are frequently observed among HCV patients with cirrhosis with an SVR. Despite ductular proliferation vanishing and lobular zonation restoration, portal inflammation and sinusoidal capillarization may not regress after viral eradication.

Hydrogen sulfide-releasing aspirin modulates xenobiotic metabolizing enzymes in vitro and in vivo

The balance between phase-I carcinogen-activating and phase-II detoxifying xenobiotic metabolizing enzymes is critical to determining an individual's risk for cancer. We evaluated the effect of Hydrogen sulfide-releasing aspirin (HS-ASA) on xenobiotic metabolizing enzymes in HT-29 human colon and Hepa 1c1c7 mouse liver adenocarcinoma cells and in Wistar rats. HS-ASA inhibited the growth of HT-29 and Hepa 1c1c7 cells, with an IC(50) of 3.2 ± 0.3 μM and 4.2 ± 0.4 μM, respectively. The IC(50) for ASA in both cell lines was greater than 5000 μM at 24h. In these cell lines, HS-ASA caused a dose-dependent increase in activity and expression of the phase-II enzymes glutathione S-transferase (GST) and NAD(P)H:quinoneoxireductase (NQO1). It also caused an increase in UDP-glucuronosyltransferase (UGT) expression. The levels of CYP 1A1 a phase-I enzyme was increased by HS-ASA in both cell lines. Pretreatment of cells with NaF, an esterase inhibitor, abrogated the HS-ASA-mediated increases in NQO1 enzyme activity. HS-ASA increased the protein levels of the transcription factor Nrf2, which is a regulator of the phase-II enzymes. In vivo, HS-ASA at 100mg/kg/day had no effect on rat's weights; it induced a 3.4-fold and 1.4-fold increase in hepatic GST and NQO1 enzyme activities, respectively. GST and NQO1 protein levels were also increased. In contrast to that in cultured cells, CYP 1A1 protein levels were not altered in vivo. Therefore, HS-ASA induces phase-II enzymes, at least in part, through the action of H(2)S and by modulating Nrf2; these effects may be part of its mechanism of action against carcinogenesis.

Regulation of reactive oxygen species generation in cell signaling

Reactive oxygen species (ROS) including superoxide anion and hydrogen peroxide (H(2)O(2)) are thought to be byproducts of aerobic respiration with damaging effects on DNA, protein, and lipid. A growing body of evidence indicates, however, that ROS are involved in the maintenance of redox homeostasis and various cellular signaling pathways. ROS are generated from diverse sources including mitochondrial respiratory chain, enzymatic activation of cytochrome p450, and NADPH oxidases further suggesting involvement in a complex array of cellular processes. This review summarizes the production and function of ROS. In particular, how cytosolic and membrane proteins regulate ROS generation for intracellular redox signaling will be detailed.

Pulmonary cytochrome P450 enzymes belonging to the CYP4B subfamily from an Australian marsupial, the tammar wallaby (Macropus eugenii)

Cytochromes P450 (CYPs) are critically important in the oxidative metabolism of a diverse array of xenobiotics and endogenous substrates. We have previously reported the cloning and characterisation of the koala CYP4A15, the first reported member of the CYP4 family from marsupials, and have demonstrated important species differences in CYP4A activity and tissue expression. In the present study, the cloning of CYP4B1 in the wallaby (Macropus eugenii) and their expression across marsupials is described. Rabbit anti-mouse CYP4B1 antibody detected immunoreactive proteins in lung and liver microsomes from all test marsupials, with relative weak signal detected from the koala, suggesting a species-specific expression. Microsomal 2-aminofluorene bio-activation (a CYP4B1 marker) in wallaby lung was comparable to that of rabbit, with significant higher activities detected in wallaby liver and kidneys compared to rabbit. A 1548bp wallaby lung CYP4B complete cDNA, designated CYP4B1, which encodes a protein of 510 amino acids and shares 72% nucleotide and 69% amino acid sequence identity to human CYP4B1, was cloned by polymerase chain reaction approaches. The results demonstrate the presence of wallaby CYP4B1 that shares several common features with other published CYP4Bs; however the wallaby CYP4B1 cDNA contains four extra amino acid residues at the NH₂-terminal, a fundamentally conserved transmembrane anchor of all eukaryote CYPs.

Cytochrome 3A and 2E1 in human liver tissue: Individual variations among normal Japanese subjects

AIMS

The metabolism of drugs, xenobiotic compounds, and other endogenous/exogenous substrates generally begins with their oxidation through cytochrome P450 (CYP). The results of recent pharmacogenetic analyses have demonstrated CYP's polymorphisms to be related to individual differences in metabolism, but only a limited number of CYP3A4 and CYP2E1 variant alleles influence enzymatic activities. Therefore, CYP gene expression profiling of both normal and pathological human livers should provide critical information for an evaluation of the biological significance of CYPs.

MAIN METHODS

In our present study, we first characterized the individual differences in CYP3A4 and CYP2E1 expression levels among Japanese normal or non-pathological liver tissue obtained from autopsy or surgery using immunohistochemistry and quantitative RT-PCR array of phase I metabolic enzymes with combined laser capture microscopy and qPCR analysis.

KEY FINDINGS

Both CYP3A4 and CYP2E1 mRNA and proteins were predominantly detected in hepatocytes surrounding central veins in normal liver, but there were marked individual differences in both CYP3A4 and CYP2E1 mRNA and proteins among the 23 Japanese subjects examined. Individual differences in CYP3A and CYP2E1 subtypes were also detected in the livers obtained from monozygotic neonatal Japanese female twins with different survival periods. CYP3A and CYP2E1-positive cells were decreased in number in non-pathological hepatocytes of diseased livers compared to those in disease-free livers from autopsy.

SIGNIFICANCE

The above results suggest that individual differences in CYP3A4 and CYP2E1 exist among normal human liver tissues and in non-pathological hepatocytes between diseased and normal liver, and these differences may be important in evaluating the pharmacodynamics of various substances.

Toward an increased understanding of the barriers to colonic drug absorption in humans: implications for early controlled release candidate assessment

The purpose of this study was to increase the understanding of in vivo colonic drug absorption in humans by summarizing and evaluating all regional in vivo human absorption data with focus on the interpretation of the colonic absorption data in relation to intestinal permeability and solubility. In addition, the usefulness of the Biopharmaceutics Classification System (BCS) in early assessment of the in vivo colonic absorption potential of controlled release drug candidates was investigated. Clinical regional absorption data (Cmax, Tmax, and AUC) of 42 drugs were collected from journal articles, abstracts, and internal reports, and the relative bioavailability in the colon (Frel(colon)) was obtained directly or calculated. Bioavailability, fraction dose absorbed, and information if the compounds were substrates for P-glycoprotein (P-gp) or cytochrome P450 3A (CYP3A) were also obtained. The BCS I drugs were well absorbed in the colon (Frel(colon) > 70%), although some drugs had lower values due to bacterial degradation in the colon. The low permeability drugs (BCS III/IV) had a lower degree of absorption in the colon (Frel(colon) < 50%). There was a clear correlation between in vitro Caco-2 permeability and Frel(colon), and atenolol and metoprolol may function as permeability markers for low and high colonic absorption, respectively. No obvious effect of P-gp on the colonic absorption of the drugs in this study was detected. There was insufficient data available to fully assess the impact of low solubility and slow dissolution rate. The estimated in vivo fractions dissolved of the only two compounds administered to the colon as both a solution and as solid particles were 55% and 92%, respectively. In conclusion, permeability and solubility are important barriers to colonic absorption in humans, and in vitro testing of these properties is recommended in early assessment of colonic absorption potential.

Pharmacokinetics of the CYP 3A substrate simvastatin following administration of delayed versus immediate release oral dosage forms

PURPOSE

The study was designed to evaluate the effect of delayed release (DR) on absorption and bioavailability of intestinally metabolized drugs after oral dosing, using the HMG-CoA reductase inhibitor simvastatin, a CYP3A substrate, as a model drug.

MATERIALS AND METHODS

To target drug release and to assess regional gastrointestinal absorption of the CYP 3A substrate simvastatin from the distal parts of the intestine, delayed release film coated tableted oral dosage forms were developed. Simvastatin delayed release tablet, simvastatin immediate release capsule and simvastatin immediate release tablet Zocor were administered as single doses (20 mg) to fasting healthy volunteers in a crossover design.

RESULTS

Simvastatin bioavailability was increased by a factor of three, as compared to the reference formulation Zocor. The overall metabolite levels from the immediate release capsules tended to be higher throughout the period studied than the metabolite levels following administration of Zocor and simvastatin delayed release dosage form.

CONCLUSIONS

The interplay between gastrointestinal physiology (lower CYP 3A expression in the distal ileum and the colon) and formulation design (zero-order controlled release after a predetermined lag-time) resulted in successful absorption and bioavailability improvement and represent a viable strategy to reduce the dose of CYP 3A drugs.

CYP4B1: an enigmatic P450 at the interface between xenobiotic and endobiotic metabolism

CYP4B1 belongs to the mammalian CYP4 enzyme family that also includes CYP4A, 4F, 4V, 4X, and 4Z subfamilies. CYP4B1 shares with other CYP4 proteins a capacity to omega-hydroxylate medium-chain fatty acids, which may be related to an endogenous role for the enzyme. CYP4B1 also participates in the metabolism of certain xenobiotics that are protoxic, including valproic acid, 3-methylindole, 4-ipomeanol, 3-methoxy-4-aminoazobenzene, and numerous aromatic amines. Although these compounds have little in common structurally or chemically, their metabolism by CYP4B1 leads to tissue-specific toxicities in several experimental animals. The bioactivation capabilities of rabbit CYP4B1 have also attracted attention in the cancer community and form the basis of a potential therapeutic strategy involving prodrug activation by the CYP4B1 transgene. The metabolic capabilities of human CYP4B1 are less clear due to difficulties in heterologous expression and existence of alternatively spliced products. Also, many CYP4B1 enzymes covalently bind their heme, a posttranslational modification unique to the CYP4 family of P450s, but common to the mammalian peroxidases. These varied characteristics render CYP4B1 an interesting and enigmatic investigational target.

Expression and Induction of Cytochromes P450 in Rat White Adipose Tissue

Lipophilic environmental pollutants are often stored in adipose tissues after exposure. These compounds have been well studied in terms of their cell toxicity in organs such as liver and kidney, and their xenoestrogenic action on reproductive tissues as endocrine disruptors. However, the effects of these chemicals on the depot, adipose tissue, have not been studied, although adipose tissue is an important endocrine tissue secreting obesity/diabetes-related hormones and cytokines. In this study, we identified the expression of cytochromes P450 in rat white adipose tissues and investigated the effects of typical lipophilic cytochrome P450 inducers, namely phenobarbital, dexamethasone, and beta-naphthoflavone. The results showed that beta-naphthoflavone was a strong CYP1A inducer in adipose tissue as well as in liver. It increased CYP1A1 mRNA, protein, and its related activity, ethoxyresorufin O-deethylase. Phenobarbital and dexamethasone also induced both the mRNA and protein of CYP2Bs and CYP3As, respectively, in adipose tissue, although significant interindividual differences were observed. Furthermore, we demonstrated that 48 h of fasting was as effective in adipose tissue as in the liver in the induction of CYP2E1 mRNA and protein. These results suggest that the mechanisms by which cytochrome P450 genes are regulated in the liver are also functional in rat adipose tissues. This has raised the possibility that lipophilic environmental contaminants accumulated in adipose tissue may dysregulate the gene expression profile.

Etiopathogenesis of pediatric Crohn's disease. Biologic pathways based on interactions between genetic and environmental factors

The incidence and prevalence of Crohn's disease (CD) among children in developed countries is extremely high. Although, dietary factors have long been implicated, to date no single causative element has been identified. Risks or benefits for CD from dietary elements would depend on their appropriate metabolism within the gut by xenobiotic metabolising enzymes (XME). The metabolising capacity among individuals differs and is determined by the level and/or activity of the specific XME. As the latter is the function of the DNA variants that code them, we propose that certain individuals exposed to dietary elements are likely to be more or less susceptible for CD. We highlight this hypothesis by describing gene-environment interaction (GxE) mechanisms that could determine susceptibility for CD. We suggest that investigating these mechanisms will be paramount for the appropriate identification of susceptible populations, such that preventive and/or therapeutic interventions could be adequately targeted.

Genotyping and site-directed mutagenesis of a cytochrome P450 meander Pro-X-Arg motif critical to CYP4B1 catalysis

CYP4B1 isoforms from rodents and other common laboratory animals are involved in the bioactivation of a range of protoxins, including 2-aminofluorene, 4-ipomeanol, and valproic acid. However, an earlier study provided evidence for a human allele encoding a nonfunctional CYP4B1 enzyme due to a Pro427Ser transversion in the meander region of the protein. In the present study, the CYP4B1 gene from several racial groups, Caucasians, African-Americans, and Hispanics, and from six nonhuman primate species was genotyped using a PCR-Hinf1 restriction enzyme fragment length polymorphism assay or by direct sequencing. All human populations examined were found to possess only the Ser allele at codon 427 ((1279)TCT) and all of the nonhuman primate species possessed only the Pro (CCT) allele. Therefore, an inactivating (1279)C-->T mutation in the human CYP4B1 gene likely arose following divergence of the Homo and Pan clades. Amino acid sequence alignments revealed further that this key Pro residue is located two amino acid residues N-terminal to the distal Arg of a Glu-Arg-Arg triad thought to participate in heme binding and/or redox partner interactions. Mutation of the corresponding Arg424 residue in rabbit CYP4B1 to Leu, but not His, resulted in a loss of lauric acid hydroxylase activity and ability to generate a reduced-CO binding spectrum. These data provide additional evidence for the importance of this meander region Pro-X-Arg motif in CYP4B1 heme binding and catalytic function.

Intestinal responses to xenobiotics

The gastrointestinal tract represents the first barrier met by the exogenous compounds of food or orally delivered drugs. To be transferred to the whole body, drugs and xenobiotics have first to pass through the intestinal epithelium, where detoxification systems have to minimize the potential of damage from toxic xenobiotics. However, most studies on xenobiotic-metabolizing enzymes have focused on liver enzymes. Such a situation may be explained by the fact that this organ is the site of toxification/detoxification for both endogenous and exogenous compounds, and also because adequate in vitro hepatocytes models have been available for a long time. By contrast, normal cellular models for the in vitro study of the intestinal processes of biotransformation still remain difficult to obtain. In the present report we will thus focus on the most commonly used models, which are Caco-2 cells and their derivative clones, and we will report recent procedures that allow the isolation of normal enterocytes which maintain their functions and integrity for several hours or even several days. Their respective performance and advantages for the study of the induction of the drug-metabolizing enzymes will be discussed.

Cytochromes P450 2A6, 2E1, and 3A and production of protein-aldehyde adducts in the liver of patients with alcoholic and non-alcoholic liver diseases

BACKGROUND/AIMS

Interaction between CYP2E1, ethanol metabolites, and enhanced lipid peroxidation is linked to the pathogenesis of alcoholic liver disease. This study was conducted to compare the expression of various cytochrome enzymes and the appearance of aldehyde adducts in humans.

METHODS

Acetaldehyde- and lipid peroxidation-derived protein adducts and CYP2A6, 2E1, and 3A4/5 were examined immunohistochemically from liver specimens of 12 alcohol abusers with either mild (n=7) or severe (n=5) liver disease, and from nine non-drinking patients with non-alcoholic steatosis (n=4), or hepatitis (n=5).

RESULTS

Ethanol-inducible CYP2E1 was present in all alcoholic livers. While CYP2A6 in zone 3 hepatocytes was also abundant in the alcoholic patients with various degrees of liver disease, CYP3A415 was most prominent in alcoholic cirrhosis. The sites of CYP2E1 and CYP2A6 immunoreactivity co-localized with fatty deposits, and with the sites of acetaldehyde and lipid peroxidation-derived protein adducts. The CYP enzymes were also abundant in the centrilobular hepatocytes of patients with fatty liver due to obesity or diabetes.

CONCLUSIONS

Alcohol-induced liver damage is associated with a generalized induction of CYP2A6, CYP2E1 and CYP3A4 and generation of acetaldehyde and lipid peroxidation-derived protein-aldehyde adducts. However, CYP induction also occurred in patients with non-alcoholic steatosis.

Induction of cytochrome P450 enzymes and generation of protein-aldehyde adducts are associated with sex-dependent sensitivity to alcohol-induced liver disease in micropigs

To assess possible links between ethanol-induced oxidant stress, expression of hepatic cytochrome P450 (CYP) enzymes, and sex steroid status, we used immunohistochemical methods to compare the generation of protein adducts of acetaldehyde (AA), malondialdehyde (MDA), and 4-hydroxynonenal (4-HNE) with the amounts of CYP2E1, CYP2A, and CYP3A in the livers of castrated and noncastrated male micropigs fed ethanol for 12 months. In castrated micropigs, ethanol feeding resulted in accumulation of fat, hepatocellular necrosis, inflammation, and centrilobular fibrosis, whereas only minimal histopathology was observed in their noncastrated counterparts. CYP2A and CYP3A were more prominent in the castrated animals than in the noncastrated micropigs. Ethanol feeding increased the hepatic content of all CYP forms. The most significant increases occurred in CYP2E1 and CYP3A in the noncastrated animals and in CYP2E1 and CYP2A in the castrated animals. Ethanol-fed castrated animals also showed the greatest abundance of perivenular adducts of AA, MDA, and HNE. In the noncastrated ethanol-fed micropigs a low expression of each CYP form was associated with scant evidence of aldehyde-protein adducts. Significant correlations emerged between the levels of different CYP forms, protein adducts, and plasma levels of sex steroids. The present findings indicate that the generation of protein-aldehyde adducts is associated with the induction of several cytochrome enzymes in a sex steroid-dependent manner. It appears that the premature, juvenile, metabolic phenotype, as induced by castration, favors liver damage. The present findings should be implicated in studies on the gender differences on the adverse effects of ethanol in the liver.

Early alcoholic liver injury: formation of protein adducts with acetaldehyde and lipid peroxidation products, and expression of CYP2E1 and CYP3A

The formation of protein adducts with reactive aldehydes resulting from ethanol metabolism and lipid peroxidation has been suggested to play a role in the pathogenesis of alcoholic liver injury. To gain further insight on the contribution of such aldehydes in alcoholic liver disease, we have compared the appearance of acetaldehyde, malondialdehyde, and 4-hydroxynonenal adducts with the expression of cytochrome P-450IIE1, and cytochrome P-4503A enzymes in the liver of rats fed alcohol with a high-fat diet for 2 to 4 weeks according to the Tsukamoto-French procedure and in control rats (high-fat liquid diet or no treatment). Urine alcohol and serum aminotransferase levels were recorded, and the liver pathology was scored from 0 to 10 according to the presence of steatosis, inflammation, necrosis, and fibrosis. The ethanol treatment resulted in the accumulation of fat, mild necrosis and inflammation, and a mean liver pathology score of 3 (range: 1 to 5). Liver specimens from the ethanol-fed animals with early alcohol-induced liver injury were found to contain perivenular, hepatocellular acetaldehyde adducts. Malondialdehyde and 4-hydroxynonenal adducts were also present showing a more diffuse staining pattern with occasional sinusoidal reactions. In the control animals, a faint positive reaction for the hydroxynonenal adduct occurred in some of the animals fed the high fat diet, whereas no specific staining was observed in the livers from the animals receiving no treatment. Expression of both CYP2E1 and CYP3A correlated with the amount of protein adducts in the liver of alcohol-treated rats. Distinct CYP2E1-positive immunohistochemistry was seen in 3 of 7 of the ethanol-fed animals. In 5 of 7 of the ethanol-fed animals, the staining intensities for CYP3A markedly exceeded those obtained from the controls. The present findings indicate that acetaldehyde and lipid peroxidation-derived adducts are generated in the early phase of alcohol-induced liver disease. The formation of protein adducts appears to be accompanied by induction of both CYP2E1 and CYP3A.

Immunohistochemical demonstration of beta-naphthoflavone-inducible cytochrome P450 1A1/1A2 in rat intrahepatic biliary epithelial cells

Although intrahepatic biliary epithelial cells are targets for certain hepatotoxic chemicals, including some procarcinogens, their ability to monooxygenate, and thereby bioactivate and inactivate xenobiotics, remains to be established. Thus, the present study was undertaken to immunohistochemically determine if cytochrome P450 (CYP) 1A1/1A2 is present and can be induced within these nonparenchymal liver cells. Immunoperoxidase and immunofluorescent staining for CYP1A1/1A2 was detected within intrahepatic biliary epithelial cells as well as hepatocytes of control rats and was markedly enhanced in both cell types by beta-naphthoflavone (BNF). Color confocal laser microscopic analyses of dual immunofluorescent staining for CYP1A1/1A2 and cytokeratins 6 and 9 (56 and 64 kd, respectively) provided unequivocal evidence for the presence and induction of CYP1A1/1A2 within intrahepatic bile duct epithelia. Moreover, microdensitometric analyses of immunoperoxidase staining intensities for CYP1A1/1A2 revealed that intrahepatic biliary epithelial cells of control rats contain 44%, 56%, and 58% as much CYP1A1/1A2 as do centrilobular, midzonal, and periportal hepatocytes, respectively. These analyses further revealed that BNF increased the content of CYP1A1/1A2 in biliary epithelial cells by approximately 120%, while CYP1A1/1A2 levels in centrilobular, midzonal, and periportal hepatocytes were increased by 82%, 159%, and 160%, respectively. The results of this study represent the first in situ demonstration that mammalian intrahepatic biliary epithelial cells contain a CYP isoform, and further that CYP1A1/1A2 can be induced in these cells by BNF. These findings therefore indicate that intrahepatic biliary epithelial cells can oxidatively metabolize xenobiotics in situ and that their ability to bioactivate and inactivate xenobiotics can be significantly enhanced by CYP1A1/1A2 induction.