Development of a comprehensive panel of antibodies against the major xenobiotic metabolising forms of cytochrome P450 in humans

Defining the Contribution of CYP1A1 and CYP1A2 to Drug Metabolism Using Humanized CYP1A1/1A2 and Cyp1a1/Cyp1a2 Knockout Mice

Cytochrome P450s CYP1A1 and CYP1A2 can metabolize a broad range of foreign compounds and drugs. However, these enzymes have significantly overlapping substrate specificities. To establish their relative contribution to drug metabolism in vivo, we used a combination of mice humanized for CYP1A1 and CYP1A2 together with mice nulled at the Cyp1a1 and Cyp1a2 gene loci. CYP1A2 was constitutively expressed in the liver, and both proteins were highly inducible by 2,3,7,8-tetrachlorodibenzodioxin (TCDD) in a number of tissues, including the liver, lung, kidney, and small intestine. Using the differential inhibition of the human enzymes by quinidine, we developed a method to distinguish the relative contribution of CYP1A1 or CYP1A2 in the metabolism of drugs and foreign compounds. Both enzymes made a significant contribution to the hepatic metabolism of the probe compounds 7-methoxy and 7-ehthoxyresorufin in microsomal fractions from animals treated with TCDD. This enzyme kinetic approach allows modeling of the CYP1A1, CYP1A2, and non-CYP1A contribution to the metabolism of any substrate at any substrate, inhibitor, or enzyme concentration and, as a consequence, can be integrated into a physiologically based pharmacokinetics model. The validity of the model can then be tested in humanized mice in vivo. SIGNIFICANCE STATEMENT: Human CYP1A1 and CYP1A2 are important in defining the efficacy and toxicity/carcinogenicity of drugs and foreign compounds. In light of differences in substrate specificity and sensitivity to inhibitors, it is of central importance to understand their relative role in foreign compound metabolism. To address this issue, we have generated mice humanized or nulled at the Cyp1a gene locus and, through the use of these mouse lines and selective inhibitors, developed an enzyme kinetic-based model to enable more accurate prediction of the fate of new chemicals in humans and which can be validated in vivo using mice humanized for cytochrome P450-mediated metabolism.

Effect of Naltrexone Hydrochloride on Cytochrome P450 1A2, 2C9, 2D6, and 3A4 Activity in Human Liver Microsomes

BACKGROUND AND OBJECTIVE

Cytochrome P450 (CYP) 1A2, 2C9, 2D6, and 3A4 are the most important phase I drug-metabolizing enzymes in the liver, but there is a dearth of literature available on the effects of naltrexone hydrochloride on these major enzymes present in the human liver. Thus, in the present study, the effect of naltrexone hydrochloride on the activity of CYP1A2, 2C9, 2D6, and 3A4 using human liver microsomes (HLM) was investigated.

METHODS

A selective probe for CYP1A2, 2C9, 2D6, and 3A4 was incubated with HLM with or without naltrexone hydrochloride. Phenacetin O-deethylation, tolbutamide 4-hydroxylation, dextromethorphan O-demethylation, and testosterone 6β-hydroxylation reactions were monitored for enzyme activity.

RESULTS

The activity of all the studied CYP enzymes except 1A2 was significantly inhibited by naltrexone hydrochloride 1 µM. Furthermore, 1 µM naltrexone hydrochloride inhibited CYP3A4 enzyme activity, the most by 37.9% followed by CYP2C9 (36.5%) and CYP2D6 (31.8%). The CYP2C9 and CYP2D6 metabolic activities were greatly affected by naltrexone hydrochloride, which even at the lowest concentration of naltrexone hydrochloride (0.01 µM) significantly decreased the metabolic activity by 34.9 and 16.0%, respectively. The half maximal inhibition concentration (IC₅₀) values for CYP2C9 and CYP2D6 inhibition were 3.40 ± 1.78 and 5.92 ± 1.58 µM, respectively.

CONCLUSION

These outcomes advocate that there is a great possibility of drug interactions resulting from the concurrent administration of naltrexone hydrochloride with actives that are metabolized by these CYP enzymes, particularly CYP2C9 and CYP2D6. Nevertheless, further clarification is needed through detailed in vivo pharmacokinetic studies.

Paracetamol metabolism, hepatotoxicity, biomarkers and therapeutic interventions: a perspective

After over 60 years of therapeutic use in the UK, paracetamol (acetaminophen, N-acetyl-p-aminophenol, APAP) remains the subject of considerable research into both its mode of action and toxicity. The pharmacological properties of APAP are the focus of some activity, with the role of the metabolite N-arachidonoylaminophenol (AM404) still a topic of debate. However, that the hepatotoxicity of APAP results from the production of the reactive metabolite N-acetyl-p-benzoquinoneimine (NAPQI/NABQI) that can deplete glutathione, react with cellular macromolecules, and initiate cell death, is now beyond dispute. The disruption of cellular pathways that results from the production of NAPQI provides a source of potential biomarkers of the severity of the damage. Research in this area has provided new diagnostic markers such as the microRNA miR-122 as well as mechanistic biomarkers associated with apoptosis, mitochondrial dysfunction, inflammation and tissue regeneration. Additionally, biomarkers of, and systems biology models for, glutathione depletion have been developed. Furthermore, there have been significant advances in determining the role of both the innate immune system and genetic factors that might predispose individuals to APAP-mediated toxicity. This perspective highlights some of the progress in current APAP-related research.

Gene structure and comparative and phylogenetic analyses of Catla catla CYP1A full-length cDNA and its responsiveness to benzo(a)pyrene and copper sulphate at early developmental stages

In the present study, full-length CYP1A cDNA from Catla catla (Catla) has been identified, and its real-time quantitative reverse transcription PCR (qRT-PCR) expression has been evaluated in different tissues, developmental stages (0, 3, 6, 12 and 24 h and 5, 7 and 9 days post-fertilization) and copper sulphate and benzo(a)pyrene (BaP)-treated 5-day post-fertilization (dpf) larvae (6 to 6.5 mm). Various structural, comparative and phylogenetic analyses of the deduced amino acid sequence revealed that the identified gene of Catla belongs to the CYP1A1 subfamily. Among different tissues of Catla, the highest CYP1A expression was observed in the kidney followed by the liver, muscle, gill, intestine and brain. CYP1A mRNA expression was detected during all the larval developmental stages, including the unfertilized egg with the highest expression on 9 dpf. BaP (3.5 ppb) and copper sulphate (sublethal dose 0.516 ppm) challenge test for 96 h to Catla larvae revealed the highest CYP1A1 expression at 48 h post-challenge. CYP1A1 transcript also showed a concentration-dependent increase in expression following exposure at 1.75 and 3.5 ppb of BaP for 48 h. Its expression profiling indicates that it is functional at early developmental stages. It can also be used to develop a specific biomarker tool for monitoring environmental pollution.

Variation in the Response of Clozapine Biotransformation Pathways in Human Hepatic Microsomes to CYP1A2- and CYP3A4-selective Inhibitors

The atypical antipsychotic agent clozapine (CLZ) is effective in many patients who are resistant to conventional antipsychotic drugs. Cytochromes P450 (CYPs) 1A2 and 3A4 oxidize CLZ to norCLZ and CLZ N-oxide in human liver. Concurrent treatment with inducers and inhibitors of CYP1A2 modulates CLZ elimination that disrupts therapy. Drug-drug interactions involving CYP3A4 are also significant but less predictable. To further characterize the factors underlying these interactions, we used samples from a cohort of human livers to assess variation in CLZ oxidation pathways in relation to intrinsic CYP3A4 and CYP1A2 activities and the effects of the corresponding selective inhibitors ketoconazole (0.2 and 2 μM) and fluvoxamine (1 and 10 μM). The CYP3A4-selective inhibitor ketoconazole (2 μM) impaired CLZ N-oxide formation in all 14 of the livers used in inhibition studies (≥50% inhibition) while the CYP1A2-selective inhibitor fluvoxamine (10 μM) decreased norCLZ formation in nine. Ketoconazole effectively inhibited CLZ metabolism in five of seven livers that catalysed CYP3A4-dependent testosterone 6β-hydroxylation at or above the median rate and in four other livers with lower intrinsic CYP3A4 activity. Similarly, fluvoxamine (10 μM) readily inhibited CLZ oxidation in seven livers with high CYP1A2-mediated 7-ethoxyresorufin O-deethylation activity (at or above the median) and three livers with lower intrinsic CYP1A2 activity. In three livers, CLZ biotransformation was impaired by both ketoconazole and fluvoxamine, consistent with a major role for both CYPs. These findings suggest that the intrinsic activities of CYPs 1A2 and 3A4 are unrelated to the response to CYP-selective inhibitors and that assessment of the activities in vivo may not assist the prediction of drug-drug interactions.

Characterization of xenobiotic metabolizing enzymes of a reconstructed human epidermal model from adult hair follicles

In this study, a comprehensive characterization of xenobiotic metabolizing enzymes (XMEs) based on gene expression and enzyme functionality was made in a reconstructed skin epidermal model derived from the outer root sheath (ORS) of hair follicles (ORS-RHE). The ORS-RHE model XME gene profile was consistent with native human skin. Cytochromes P450 (CYPs) consistently reported to be detected in native human skin were also present at the gene level in the ORS-RHE model. The highest Phase I XME gene expression levels were observed for alcohol/aldehyde dehydrogenases and (carboxyl) esterases. The model was responsive to the CYP inducers, 3-methylcholanthrene (3-MC) and β-naphthoflavone (βNF) after topical and systemic applications, evident at the gene and enzyme activity level. Phase II XME levels were generally higher than those of Phase I XMEs, the highest levels were GSTs and transferases, including NAT1. The presence of functional CYPs, UGTs and SULTs was confirmed by incubating the models with 7-ethoxycoumarin, testosterone, benzo(a)pyrene and 3-MC, all of which were rapidly metabolized within 24h after topical application. The extent of metabolism was dependent on saturable and non-saturable metabolism by the XMEs and on the residence time within the model. In conclusion, the ORS-RHE model expresses a number of Phase I and II XMEs, some of which may be induced by AhR ligands. Functional XME activities were also demonstrated using systemic or topical application routes, supporting their use in cutaneous metabolism studies. Such a reproducible model will be of interest when evaluating the cutaneous metabolism and potential toxicity of innovative dermo-cosmetic ingredients.

The impact of individual cytochrome P450 enzymes on oxidative metabolism of benzo[a]pyrene in human livers

Benzo[a]pyrene (BaP) is a human carcinogen that covalently binds to DNA after metabolic activation by cytochrome P450 (CYP) enzymes. In this study human recombinant CYPs (CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C19, 2E1, 3A4, and 3A5) were expressed in Supersomes™ together with their reductases, NADPH:CYP oxidoreductase, epoxide hydrolase and cytochrome b5 , to investigate BaP metabolism. Human CYPs produced up to eight BaP metabolites. Among these, BaP-7,8-dihydrodiol and BaP-9-ol, which are intermediates in BaP-derived DNA adduct formation, were mainly formed by CYP1A1 and 1B1, and to a lesser extent by CYP2C19 and 3A4. BaP-3-ol, a metabolite that is a 'detoxified' product of BaP, was formed by most human CYPs tested, although CYP1A1 and 1B1 produced it the most efficiently. Based on the amounts of the individual BaP metabolites formed by these CYPs and their expression levels in human liver, we determined their contributions to BaP metabolite formation in this organ. Our results indicate that hepatic CYP1A1 and CYP2C19 are most important in the activation of BaP to BaP-7,8-dihydrodiol, whereas CYP2C19, 3A4, and 1A1 are the major enzymes contributing to the formation of BaP-9-ol. BaP-3-ol is predominantly formed by hepatic CYP3A4, while CYP1A1 and 2C19 are less active.

The stress response of human proximal tubule cells to cadmium involves up-regulation of haemoxygenase 1 and metallothionein but not cytochrome P450 enzymes

Enzymes of the cytochrome P450 (CYP) super-family are implicated in cadmium (Cd) -induced nephrotoxicity, however, direct evidence is lacking. This study investigated the endogenous expression of various CYP proteins together with the stress-response proteins, heme oxygenase-1 (HO-1) and metallothionein (MT) in human kidney sections and in cadmium-exposed primary cultures of human proximal tubular epithelial cells (PTC). By immunohistochemistry, the CYP members 2B6, 4A11 and 4F2 were prominently expressed in the cortical proximal tubular cells and to a lesser extent in distal tubular cells. Low levels of CYPs 2E1 and 3A4 were also detected. In PTC, in the absence of Cd, CYP2E1, CYP3A4, CYP4F2 and MT were expressed, but HO-1, CYP2B6 and CYP4A11 were not detected. A range of cadmium concentrations (0-100μM) were utilized to induce stress conditions. MT protein was further induced by as little as 0.5μM cadmium, reaching a 6-fold induction at 20μM, whereas for HO-1, a 5μM cadmium concentration was required for initial induction and at 20μM cadmium reached a 15-fold induction. The expression of CYP2E1, CYP3A4, and CYP4F2 were not altered by any cadmium concentrations tested at 48h. Cadmium caused a reduction in cell viability at concentrations above 10μM. In conclusion although cultured PTC, do express CYP proteins, (CYP2E1, CYP3A4, and CYP4F2), Cd-induced cell stress as indicted by induction of HO-1 and MT does not alter expression of these CYP proteins at 48h.

A Mechanism of O-Demethylation of Aristolochic Acid I by Cytochromes P450 and Their Contributions to This Reaction in Human and Rat Livers: Experimental and Theoretical Approaches

Aristolochic acid I (AAI) is a plant alkaloid causing aristolochic acid nephropathy, Balkan endemic nephropathy and their associated urothelial malignancies. AAI is detoxified by cytochrome P450 (CYP)-mediated O-demethylation to 8-hydroxyaristolochic acid I (aristolochic acid Ia, AAIa). We previously investigated the efficiencies of human and rat CYPs in the presence of two other components of the mixed-functions-oxidase system, NADPH:CYP oxidoreductase and cytochrome b₅, to oxidize AAI. Human and rat CYP1A are the major enzymes oxidizing AAI. Other CYPs such as CYP2C, 3A4, 2D6, 2E1, and 1B1, also form AAIa, but with much lower efficiency than CYP1A. Based on velocities of AAIa formation by examined CYPs and their expression levels in human and rat livers, here we determined the contributions of individual CYPs to AAI oxidation in these organs. Human CYP1A2 followed by CYP2C9, 3A4 and 1A1 were the major enzymes contributing to AAI oxidation in human liver, while CYP2C and 1A were most important in rat liver. We employed flexible in silico docking methods to explain the differences in AAI oxidation in the liver by human CYP1A1, 1A2, 2C9, and 3A4, the enzymes that all O-demethylate AAI, but with different effectiveness. We found that the binding orientations of the methoxy group of AAI in binding centers of the CYP enzymes and the energies of AAI binding to the CYP active sites dictate the efficiency of AAI oxidation. Our results indicate that utilization of experimental and theoretical methods is an appropriate study design to examine the CYP-catalyzed reaction mechanisms of AAI oxidation and contributions of human hepatic CYPs to this metabolism.

Whole-exome sequencing reveals defective CYP3A4 variants predictive of paclitaxel dose-limiting neuropathy

PURPOSE

Paclitaxel, a widely used chemotherapeutic drug, can cause peripheral neuropathies leading to dose reductions and treatment suspensions and decreasing the quality of life of patients. It has been suggested that genetic variants altering paclitaxel pharmacokinetics increase neuropathy risk, but the major causes of interindividual differences in susceptibility to paclitaxel toxicity remain unexplained. We carried out a whole-exome sequencing (WES) study to identify genetic susceptibility variants associated with paclitaxel neuropathy.

EXPERIMENTAL DESIGN

Blood samples from 8 patients with severe paclitaxel-induced peripheral neuropathy were selected for WES. An independent cohort of 228 cancer patients with complete paclitaxel neuropathy data was used for variant screening by DHPLC and association analysis. HEK293 cells were used for heterologous expression and characterization of two novel CYP3A4 enzymes.

RESULTS

WES revealed 2 patients with rare CYP3A4 variants, a premature stop codon (CYP3A4*20 allele) and a novel missense variant (CYP3A4*25, p.P389S) causing reduced enzyme expression. Screening for CYP3A4 variants in the independent cohort revealed three additional CYP3A4*20 carriers, and two patients with missense variants exhibiting diminished enzyme activity (CYP3A4*8 and the novel CYP3A4*27 allele, p.L475V). Relative to CYP3A4 wild-type patients, those carrying CYP3A4 defective variants had more severe neuropathy (2- and 1.3-fold higher risk of neuropathy for loss-of-function and missense variants, respectively, P = 0.045) and higher probability of neuropathy-induced paclitaxel treatment modifications (7- and 3-fold higher risk for loss-of-function and missense variants, respectively, P = 5.9 × 10(-5)).

CONCLUSION

This is the first description of a genetic marker associated with paclitaxel treatment modifications caused by neuropathy. CYP3A4 defective variants may provide a basis for paclitaxel treatment individualization.

Interleukin-6 mediated upregulation of CYP1B1 and CYP2E1 in colorectal cancer involves DNA methylation, miR27b and STAT3

Background:

The pro-inflammatory cytokine interleukin-6 (IL6) promotes colorectal cancer (CRC) development. It is also known to regulate cytochrome P450 (CYP450) enzymes, which are involved in CRC tumour initiation and promotion via activation of chemical carcinogens. Here, IL6 regulation of CYP450 expression was investigated in CRC.

Methods:

The effect of IL6 on CYP 1A1, 1B1 and 2E1 expression was determined in vitro using CRC cell lines HCT116 and SW480, and CYP450 expression was determined by immunohistochemistry in CRC tissues previously shown to have increased levels of IL6.

Results:

In mechanistic studies, IL6 treatment significantly induced CYP1B1 and CYP2E1, but not CYP1A1, gene expression in HCT116 and SW480 cells. CYP2E1 expression regulation occurred via a transcriptional mechanism involving STAT3. For CYP1B1 regulation, IL6 downregulated the CYP1B1-targeting microRNA miR27b through a mechanism involving DNA methylation. In clinical samples, the expression of CYP1B1 and CYP2E1, but not CYP1A1, was significantly increased in malignant tissue overexpressing IL6 compared with matched adjacent normal tissue.

Conclusions:

Colonic inflammation with the presence of IL6 associated with neoplastic tissue can alter metabolic competency of epithelial cells by manipulating CYP2E1 and CYP1B1 expression through transcriptional and epigenetic mechanisms. This can lead to increased activation of dietary carcinogens and DNA damage, thus promoting colorectal carcinogenesis.

Mitochondrial targeting of cytochrome P450 (CYP) 1B1 and its role in polycyclic aromatic hydrocarbon-induced mitochondrial dysfunction

We report that polycyclic aromatic hydrocarbon (PAH)-inducible CYP1B1 is targeted to mitochondria by sequence-specific cleavage at the N terminus by a cytosolic Ser protease (polyserase 1) to activate the cryptic internal signal. Site-directed mutagenesis, COS-7 cell transfection, and in vitro import studies in isolated mitochondria showed that a positively charged domain at residues 41-48 of human CYP1B1 is part of the mitochondrial (mt) import signal. Ala scanning mutations showed that the Ser protease cleavage site resides between residues 37 and 41 of human CYP1B1. Benzo[a]pyrene (BaP) treatment induced oxidative stress, mitochondrial respiratory defects, and mtDNA damage that was attenuated by a CYP1B1-specific inhibitor, 2,3,4,5-tetramethoxystilbene. In support, the mitochondrial CYP1B1 supported by mitochondrial ferredoxin (adrenodoxin) and ferredoxin reductase showed high aryl hydrocarbon hydroxylase activity. Administration of benzo[a]pyrene or 2,3,7,8-tetrachlorodibenzodioxin induced similar mitochondrial functional abnormalities and oxidative stress in the lungs of wild-type mice and Cyp1a1/1a2-null mice, but the effects were markedly blunted in Cyp1b1-null mice. These results confirm a role for CYP1B1 in inducing PAH-mediated mitochondrial dysfunction. The role of mitochondrial CYP1B1 was assessed using A549 lung epithelial cells stably expressing shRNA against NADPH-cytochrome P450 oxidoreductase or mitochondrial adrenodoxin. Our results not only show conservation of the endoprotease cleavage mechanism for mitochondrial import of family 1 CYPs but also reveal a direct role for mitochondrial CYP1B1 in PAH-mediated oxidative and chemical damage to mitochondria.

High warfarin sensitivity in carriers of CYP2C9*35 is determined by the impaired interaction with P450 oxidoreductase

Cytochrome P450 2C9 (CYP2C9) metabolizes many clinically important drugs including warfarin and diclofenac. We have recently reported a new allelic variant, CYP2C9*35, found in a warfarin hypersensitive patient with Arg125Leu and Arg144Cys mutations. Here, we have investigated the molecular basis for the functional consequences of these polymorphic changes. CYP2C9.1 and CYP2C9-Arg144Cys expressed in human embryonic kidney 293 cells effectively metabolized both S-warfarin and diclofenac in NADPH-dependent reactions, whereas CYP2C9-Arg125Leu or CYP2C9.35 were catalytically silent. However, when NADPH was replaced by a direct electron donor to CYPs, cumene hydroperoxide, hereby bypassing the CYP oxidoreductase (POR), all variant enzymes were active, indicating unproductive interactions between CYP2C9.35 and POR. In silico analysis revealed a decrease of the electrostatic potential of CYP2C9-Arg125Leu-POR interacting surface and the loss of stabilizing salt bridges between these proteins. In conclusion, our data strongly suggest that the Arg125Leu mutation in CYP2C9.35 prevents CYP2C9-POR interactions resulting in the absence of NADPH-dependent CYP2C9-catalyzed activity in vivo, thus influencing the warfarin sensitivity in the carriers of this allele.

Elucidation of xenobiotic metabolism pathways in human skin and human skin models by proteomic profiling

Background

Human skin has the capacity to metabolise foreign chemicals (xenobiotics), but knowledge of the various enzymes involved is incomplete. A broad-based unbiased proteomics approach was used to describe the profile of xenobiotic metabolising enzymes present in human skin and hence indicate principal routes of metabolism of xenobiotic compounds. Several in vitro models of human skin have been developed for the purpose of safety assessment of chemicals. The suitability of these epidermal models for studies involving biotransformation was assessed by comparing their profiles of xenobiotic metabolising enzymes with those of human skin.

Methodology/Principal Findings

Label-free proteomic analysis of whole human skin (10 donors) was applied and analysed using custom-built PROTSIFT software. The results showed the presence of enzymes with a capacity for the metabolism of alcohols through dehydrogenation, aldehydes through dehydrogenation and oxidation, amines through oxidation, carbonyls through reduction, epoxides and carboxylesters through hydrolysis and, of many compounds, by conjugation to glutathione. Whereas protein levels of these enzymes in skin were mostly just 4–10 fold lower than those in liver and sufficient to support metabolism, the levels of cytochrome P450 enzymes were at least 300-fold lower indicating they play no significant role. Four epidermal models of human skin had profiles very similar to one another and these overlapped substantially with that of whole skin.

Conclusions/Significance

The proteomics profiling approach was successful in producing a comprehensive analysis of the biotransformation characteristics of whole human skin and various in vitro skin models. The results show that skin contains a range of defined enzymes capable of metabolising different classes of chemicals. The degree of similarity of the profiles of the in vitro models indicates their suitability for epidermal toxicity testing. Overall, these results provide a rational basis for explaining the fate of xenobiotics in skin and will aid chemical safety testing programmes.

Impaired irinotecan biotransformation in hepatic microsomal fractions from patients with chronic liver disease

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

The anticancer agent irinotecan is a prodrug that is hydrolyzed by hepatic carboxylesterase to its active and toxic metabolite SN-38 and oxidized by CYP3A4 to its inactive metabolite APC. Irinotecan therapy is complicated by co-administered drugs that inhibit CYP3A4 and decrease APC formation and that indirectly increase SN-38 formation. Dose adjustment in cancer patients with liver disease has been recommended.

WHAT THIS STUDY ADDS

In microsomal fractions from patients with severe hepatic dysfunction both APC and SN-38 formation were decreased due to down-regulation of CYP3A4 and carboxylesterase enzymes. Thus relative SN-38 : APC formation was preserved. In some fractions the SN-38:APC ratio was increased, thus providing a possible explanation for clinical reports of increased SN-38 exposure in some patients with liver dysfunction. Close monitoring of SN-38 formation in patients with severe liver disease is warranted.

AIMS

Dose modification with the anticancer agent irinotecan is recommended in patients with severe liver dysfunction. This study evaluated the impact of liver disease on the relative formation of phase I products of irinotecan biotransformation in human microsomes in vitro.

METHODS

Microsomes from subjects with normal liver function and liver dysfunction (n=20) were assessed for irinotecan biotransformation and the expression of cytochrome P450 (CYP) 3A4 and carboxylesterase (CES) enzymes.

RESULTS

Liver disease down-regulated CYP3A4 expression (median 33% of control, range 0-126%, P<0.05) and impaired CYP3A4-dependent oxidation of irinotecan to the inactive 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino]carbonyloxycamptothecin (APC) (median 0.2, range 0-1.21 pmol mg protein(-1) min(-1) compared with median 0.66, range 0-2.35 in control, P<0.01). CES-mediated hydrolysis of irinotecan to 7-ethyl-10-hydroxycamptothecin (SN-38) was also impaired in liver disease (median 8.38, range 0-20.7 pmol mg protein(-1) min(-1) compared with median 13.3, range 0-28.9 in control, P<0.05). In seven of 20 liver disease microsomes neither metabolite was detected but in three the SN-38:APC ratio was high (41-68) compared with the remaining 10 samples (ratio 11-36).

CONCLUSIONS

Down-regulation of CYP3A4 in liver disease decreased APC formation from irinotecan. SN-38 production was decreased and CES1 and 2 were down-regulated in most samples. However, in a subset of disease samples SN-38 production was relatively high because CYP3A4 activity was markedly impaired. This may account for clinical reports of increased SN-38 exposure in some patients with liver disease. Dose adjustments in cancer patients with liver disease who receive irinotecan are important and circulating SN-38 concentrations should be monitored closely.

In vitro and in vivo evaluation of the inhibition potential of risperidone toward clozapine biotransformation

AIMS

To study the impact of risperidone (RISP) on clozapine (CLZ) biotransformation in vitro in microsomal fractions containing varying expression of CYP oxidases and in vivo in patients.

METHODS

Human liver microsomes (n= 11) were assessed for expression of CYPs 1A2, 2D6 and 3A4, because these enzymes mediate RISP and CLZ oxidation. Inhibition of CLZ oxidation by RISP was assessed. Plasma CLZ elimination was estimated in patients with schizophrenia who received either CLZ alone or the CLZ-RISP combination (n= 10 per group).

RESULTS

(i) The CYP3A4 and CYP1A2 inhibitors ketoconazole and fluvoxamine inhibited CLZ oxidation to varying extents in individual microsomal fractions. (ii) RISP did not inhibit CLZ oxidation, regardless of variations in CYP expression. (iii) RISP co-administration did not impair CLZ clearance.

CONCLUSIONS

No evidence was found for CYP-mediated inhibitory or pharmacokinetic interactions between RISP and CLZ. Occasional literature reports of such interactions may involve other pathways that participate in CLZ disposition.

Application of key events analysis to chemical carcinogens and noncarcinogens

The existence of thresholds for toxicants is a matter of debate in chemical risk assessment and regulation. Current risk assessment methods are based on the assumption that, in the absence of sufficient data, carcinogenesis does not have a threshold, while noncarcinogenic endpoints are assumed to be thresholded. Advances in our fundamental understanding of the events that underlie toxicity are providing opportunities to address these assumptions about thresholds. A key events dose-response analytic framework was used to evaluate three aspects of toxicity. The first section illustrates how a fundamental understanding of the mode of action for the hepatic toxicity and the hepatocarcinogenicity of chloroform in rodents can replace the assumption of low-dose linearity. The second section describes how advances in our understanding of the molecular aspects of carcinogenesis allow us to consider the critical steps in genotoxic carcinogenesis in a key events framework. The third section deals with the case of endocrine disrupters, where the most significant question regarding thresholds is the possible additivity to an endogenous background of hormonal activity. Each of the examples suggests that current assumptions about thresholds can be refined. Understanding inter-individual variability in the events involved in toxicological effects may enable a true population threshold(s) to be identified.

Antibodies as a probe in cytochrome P450 research

Cytochrome P450 (P450) is the superfamily of enzymes responsible for biotransformation of endobiotics and xenobiotics. However, their large isoform multiplicity, inducibility, diverse structure, widespread distribution, polymorphic expression, and broad overlapping substrate specificity make it difficult to measure the precise role of each individual P450 to the metabolism of drugs (or carcinogens) and hamper the understanding of the relationship between the genetic/environmental factors that regulate P450 phenotype and the responses of the individual P450s to drugs. The antibodies against P450s have been useful tools for the quantitative determination of expression level and contribution of the epitope-specific P450 to the metabolism of a drug or carcinogen substrate in tissues containing multiple P450 isoforms and for implications in pharmacogenetics and human risk assessment. In particular, the inhibitory antibodies are uniquely suited for reaction phenotyping that helps to predict human pharmacokinetics for clinical drug-drug interaction potential in drug discovery and development.

Expression and transcriptional regulation of ABC transporters and cytochromes P450 in hCMEC/D3 human cerebral microvascular endothelial cells

We investigated the expression of genes encoding ABC transporters, cytochromes P450 (CYPs) and some transcription factors in the hCMEC/D3 immortalized human cerebral microvascular endothelial cell line, a promising in vitro model of the human BBB, and we compared these expressions to a non-brain endothelial cell line (HUVEC) and freshly human brain microvessels. qRT-PCR showed that the MDR1, BCRP, MRP1, MRP3, MRP4 and MRP5 genes were expressed and that the main CYP gene was CYP2U1 in hCMEC/D3. The pattern of ABC and CYPs gene expression in hCMEC/D3 differed from HUVEC which did not express MDR1. Moreover, expression of P-gp and BCRP was lower in hCMEC/D3 than in human brain microvessels but remain functional as shown by rhodamine 123 efflux assay. The gene encoding the aryl hydrocarbon receptor (AhR), a transcription factor that regulates the expression of some ABC and CYPs was highly expressed in hCMEC/D3 and HUVEC, while the pregnane-X-receptor (PXR) and the constitutive androstane receptor (CAR) were barely detected. We investigated the function of the AhR-mediated regulatory pathway in hCMEC/D3 by treating them with the AhR agonist TCDD. The expressions of two AhR-target genes, CYP1A1 and CYP1B1, were increased 26-fold and 28-fold. But the expressions of ABC transporter genes were not significantly altered. We have thus determined the pattern of expression of the genes encoding ABC transporters, CYPs and three transcription factors in hCMEC/D3 and shown that the AhR pathway might afford an original functional transport and metabolic pattern in cerebral endothelial cells that is different from other peripheral endothelial cells.

The Endoplasmic Reticulum in Xenobiotic Toxicity

The endoplasmic reticulum (ER) is involved in an array of cellular functions that play important roles in xenobiotic toxicity. The ER contains the majority of cytochrome P450 enzymes involved in xenobiotic metabolism, as well as a number of conjugating enzymes. In addition to its role in drug bioactivation and detoxification, the ER can be a target for damage by reactive intermediates leading to cell death or immune-mediated toxicity. The ER contains a set of luminal proteins referred to as ER stress proteins (including GRP78, GRP94, protein disulfide isomerase, and calreticulin). These proteins help regulate protein processing and folding of membrane and secretory proteins in the ER, calcium homeostasis, and ER-associated apoptotic pathways. They are induced in response to ER stress. This review discusses the importance of the ER in molecular events leading to cell death following xenobiotic exposure. Data showing that the ER is important in both renal and hepatic toxicity will be discussed.

Interindividual variation in relative CYP1A2/3A4 phenotype influences susceptibility of clozapine oxidation to cytochrome P450-specific inhibition in human hepatic microsomes

The atypical antipsychotic drug clozapine (CLZ) is effective in a substantial number of patients who exhibit treatment-resistance to conventional agents. CYP1A2 is generally considered to be the major enzyme involved in the biotransformation of CLZ to its N-demethylated (norCLZ) and N-oxygenated (CLZ N-oxide) metabolites in liver, but several studies have also implicated CYP3A4. The present study assessed the interplay between these cytochrome P450s (P450s) in CLZ biotransformation in a panel of hepatic microsomal fractions from 14 individuals. The relative activity of P450s 1A2 and 3A4 in microsomes was found to be a major determinant of the relative susceptibility of norCLZ formation to inhibition by the P450-selective inhibitors fluvoxamine and ketoconazole. In contrast, the activity of CYP3A4 alone was correlated with the susceptibility of CLZ N-oxide formation to inhibition by these agents. These findings suggest that both P450s may be dominant CLZ oxidases in patients and that the relative activities of these enzymes may determine clearance pathways. In vivo assessment of CYP1A2 and CYP3A4 activities, perhaps by phenotyping approaches, could assist the optimization of CLZ dosage and minimize pharmacokinetic interactions with coadministered drugs.

Expression of CYP4F2 in human liver and kidney: assessment using targeted peptide antibodies

P450 enzymes comprising the human CYP4F gene subfamily are catalysts of eicosanoid (e.g., 20-HETE and leukotriene B4) formation and degradation, although the role that individual CYP4F proteins play in these metabolic processes is not well defined. Thus, we developed antibodies to assess the tissue-specific expression and function of CYP4F2, one of four CYP4F P450s found in human liver and kidney. Peptide antibodies elicited in rabbits to CYP4F2 amino acid residues 61-74 (WGHQGMVNPTEEG) and 65-77 (GMVNPTEEGMRVL) recognized on immunoblots only CYP4F2 and not CYP4F3b, CYP4F11 or CYP4F12. Immunoquantitation with anti-CYP4F2 peptide IgG showed highly variable CYP4F2 expression in liver (16.4+/-18.6pmol/mg microsomal protein; n=29) and kidney cortex (3.9+/-3.8 pmol/mg; n=10), with two subjects lacking the hepatic or renal enzyme entirely. CYP4F2 content in liver microsomes was significantly correlated (r> or =0.63; p<0.05) with leukotriene B4 and arachidonate omega-hydroxylase activities, which are both CYP4F2-catalyzed. Our study provides the first example of a peptide antibody that recognizes a single CYP4F P450 expressed in human liver and kidney, namely CYP4F2. Immunoquantitation and correlation analyses performed with this antibody suggest that CYP4F2 functions as a predominant LTB4 and arachidonate omega-hydroxylase in human liver.

The relative contribution of human cytochrome P450 isoforms to the four caffeine oxidation pathways: an in vitro comparative study with cDNA-expressed P450s including CYP2C isoforms

The aim of the present study was to estimate the relative contribution of cytochrome P450 isoforms (P450s), including P450s of the CYP2C subfamily, to the metabolism of caffeine in human liver. The experiments were carried out in vitro using cDNA-expressed P450s, liver microsomes and specific P450 inhibitors. The obtained results show that (1) apart from the 3-N-demethylation of caffeine - a CYP1A2 marker reaction and the main oxidation pathway of caffeine in man - 1-N-demethylation is also specifically catalyzed by CYP1A2 (not reported previously); (2) 7-N-demethylation is catalyzed non-specifically, mainly by CYP1A2 and, to a smaller extent, by CYP2C8/9 and CYP3A4 (and not by CYP2E1, as suggested previously); (3) C-8-hydroxylation preferentially involves CYP1A2 and CYP3A4 and, to a smaller degree, CYP2C8/9 and CYP2E1 (and not only CYP3A, as suggested previously) at a concentration of 100 microM corresponding to the maximum therapeutic concentration in humans. At a higher caffeine concentration, the contribution of CYP1A2 to this reaction decreases in favour of CYP2C8/9. The obtained data show for the first time the contribution of CYP2C isoforms to the metabolism of caffeine in human liver and suggest that apart from 3-N-demethylation, 1-N-demethylation may also be used for testing CYP1A2 activity. Moreover, they indicate that the C-8-hydroxylation is not exclusively catalyzed by CYP3A4.

The ontogeny of drug metabolism enzymes and implications for adverse drug events

Profound changes in drug metabolizing enzyme (DME) expression occurs during development that impacts the risk of adverse drug events in the fetus and child. A review of our current knowledge suggests individual hepatic DME ontogeny can be categorized into one of three groups. Some enzymes, e.g., CYP3A7, are expressed at their highest level during the first trimester and either remain at high concentrations or decrease during gestation, but are silenced or expressed at low levels within one to two years after birth. SULT1A1 is an example of the second group of DME. These enzymes are expressed at relatively constant levels throughout gestation and minimal changes are observed postnatally. ADH1C is typical of the third DME group that are not expressed or are expressed at low levels in the fetus, usually during the second or third trimester. Substantial increases in enzyme levels are observed within the first one to two years after birth. Combined with our knowledge of other physiological factors during early life stages, knowledge regarding DME ontogeny has permitted the development of robust physiological based pharmacokinetic models and an improved capability to predict drug disposition in pediatric patients. This review will provide an overview of DME developmental expression patterns and discuss some implications of the data with regards to drug therapy. Common themes emerging from our current knowledge also will be discussed. Finally, the review will highlight gaps in knowledge that will be important to advance this field.

Influence of cytochrome P450 polymorphisms on drug therapies: pharmacogenetic, pharmacoepigenetic and clinical aspects

The polymorphic nature of the cytochrome P450 (CYP) genes affects individual drug response and adverse reactions to a great extent. This variation includes copy number variants (CNV), missense mutations, insertions and deletions, and mutations affecting gene expression and activity of mainly CYP2A6, CYP2B6, CYP2C9, CYP2C19 and CYP2D6, which have been extensively studied and well characterized. CYP1A2 and CYP3A4 expression varies significantly, and the cause has been suggested to be mainly of genetic origin but the exact molecular basis remains unknown. We present a review of the major polymorphic CYP alleles and conclude that this variability is of greatest importance for treatment with several antidepressants, antipsychotics, antiulcer drugs, anti-HIV drugs, anticoagulants, antidiabetics and the anticancer drug tamoxifen. We also present tables illustrating the relative importance of specific common CYP alleles for the extent of enzyme functionality. The field of pharmacoepigenetics has just opened, and we present recent examples wherein gene methylation influences the expression of CYP. In addition microRNA (miRNA) regulation of P450 has been described. Furthermore, this review updates the field with respect to regulatory initiatives and experience of predictive pharmacogenetic investigations in the clinics. It is concluded that the pharmacogenetic knowledge regarding CYP polymorphism now developed to a stage where it can be implemented in drug development and in clinical routine for specific drug treatments, thereby improving the drug response and reducing costs for drug treatment.

Disparity in holoprotein/apoprotein ratios of different standards used for immunoquantification of hepatic cytochrome P450 enzymes

An analysis of reported hepatic abundances of CYP3A4 and 3A5 indicated that values determined by immunoquantification using commercially available, unpurified recombinant enzymes as standards are significantly lower than those determined using purified enzymes or human liver microsomes characterized with lysosomal peptides (CYP3A4: mean 45 versus 121 pmol/mg protein, p < 0.01; CYP3A5: mean 28 versus 83 pmol/mg protein, p < 0.05). When immunoquantifying cytochromes P450 (P450s), it is assumed that the holoprotein (holo)/apoprotein ratio is the same in the samples and the standard. Estimates of holo/apoprotein ratios from data reported for a range of P450s purified from human liver and non-commercial recombinant systems indicated less than complete and variable heme coupling dependent on enzyme and system.

Molecular and functional characterization of drug-metabolizing enzymes and transporter expression in the novel spontaneously immortalized human hepatocyte line HC-04

In toxicological research, immortalized human hepatocytes provide a useful alternative to primary hepatocytes because interindividual variability in the expression of drug-metabolizing enzymes and drug transporters can largely be eliminated. However, it is essential that the cell line retain the original phenotype. The purpose of this study was to characterize a novel spontaneously immortalized human hepatocyte cell line, HC-04, with respect to the transcript and functional protein expression profile for the major drug-metabolizing enzymes and transmembrane transporters. HC-04 cells retained hepatocyte-specific function including albumin production and ornithine transcarbamoylase and glucose-6-phosphatase activity. Most of the major CYP forms were expressed at basal levels and responsive to inducing agents. In particular, CYP3A4 was expressed abundantly, and HC-04 cells were able to metabolize the CYP3A4 probe, midazolam, at a rate similar to primary human hepatocytes. Furthermore, the major human sulfotransferase and UDP-glucuronosyltransferase forms, as well as members of the ABC and SLC transporter superfamilies, nuclear receptors, and hepatic transcription factors were also expressed. HC-04 cells readily responded to standard hepatotoxicants that are dependent on CYP-mediated bioactivation, while another, tumor-derived cell line remained refractory to the drug challenge. Collectively, HC-04 cells provide a reliable, stable, and reproducible model for biomechanistic studies in drug toxicology.

Expression of Cytochromes P450 3A and P-Glycoprotein in Human Large Intestinse in Paired Tumour and Normal Samples

Our objective was to investigate the expression of different cytochromes P450 3A (CYP3A4, CYP3A5, and CYP3A7) and P-glycoprotein (ABCB1) genes along the human large intestine in paired tumour and normal samples. Real-time reverse transcriptase-polymerase chain reaction was used to measure CYP3A4-, CYP3A5-, CYP3A7- and ABCB1-specific mRNA expression, and Western blot analysis was used to measure membrane protein levels of CYP3A4/7, CYP3A5 and P-glycoprotein. Levels of mRNA and membrane protein fractions in the large intestine were compared with those of normal human liver. The mRNA expressions of CYP3A4, CYP3A5, CYP3A7 and ABCB1 in the large intestine were found to be highly variable, but overall the levels were significantly lower than those measured in liver (P < 0.0001, P < 0.001, P < 0.0001 and P < 0.01, respectively). At the membrane protein level, CYP3A4/7 was detected in all large intestine samples examined and the levels were substantially higher than those of the liver (P < 0.01). Although expression of CYP3A5 was detected in all large intestine samples, in most the levels were too low to allow quantification. P-glycoprotein was readily detected at levels slightly higher than those of liver (P < 0.05). Comparison between paired samples of normal and tumour in large intestine showed no significant differences in either the mRNA or membrane protein levels of these genes. In conclusion, this work suggests a potential role of the large intestine in the absorption and metabolism of xenobiotics and nutrients and no difference in the CYP3A and P-glycoprotein membrane protein fractions and mRNA expression between normal and tumour tissues.

Evaluation of 227 drugs for in vitro inhibition of cytochrome P450 2B6

Cytochrome P450 2B6 (CYP2B6) is involved in the metabolism of drugs such as bupropion, efavirenz, propofol, and selegiline, among others. More than 200 commonly prescribed drugs and other xenobiotics were examined for their ability to inhibit CYP2B6-mediated bupropion hydroxylase activity. Thirty compounds were found exhibiting greater than 50% inhibition at 30 microM. Inhibitors of CYP2B6 were identified from a wide variety of therapeutic classes. The 2 platelet aggregation inhibitors, clopidogrel and ticlopidine, were both identified as potent inhibitors (IC50 = 0.0206 and 0.149 microM, respectively). Other inhibitors (IC50 < 1 microM) included clotrimazole, itraconazole, sertraline, and raloxifene. These in vitro data were used along with clinical pharmacokinetic information in the prediction of potential drug-drug interactions that could occur by inhibition of CYP2B6. Although few drugs tested are expected to cause drug interactions, clopidogrel and ticlopidine were identified as being of concern as potential inhibitors of clinical relevance. These findings are discussed in context to potential drug interactions that could be observed between these agents and drugs for which CYP2B6 is involved in metabolism.

Metabolic and Efflux Properties of Caco-2 Cells Stably Transfected with Nuclear Receptors

PURPOSE

To characterise in detail the patterns of expression and functional activities of CYP and efflux pump genes in Caco-2 cells stably transfected with human Pregnane X Receptor or murine Constitutive Androstane Receptor.

MATERIALS AND METHODS

Cell lines transfected with nuclear receptors were treated with established ligands, and gene expression of CYP and efflux pump genes were quantified by qRT-PCR and Western blot. P-glycoprotein activity was assessed by measuring calcein-AM accumulation and bidirectional permeability coefficients of digoxin and quinidine. CYP activities were measured with both fluorescent and non-fluorescent substrates.

RESULTS

hPXR and mCAR upregulated some CYP and efflux pump genes ligand dependently. P-glycoprotein level was increased, but CYP3A4 protein remained below the limit of detection. P-glycoprotein activity was markedly elevated in Caco/mCAR cells and more modestly in Caco/hPXR cells. CYP3A4 activity remained lower than that in vitamin D-treated Caco-2 cells.

CONCLUSIONS

Nuclear receptors can modulate the expression of metabolic genes in Caco-2 cells, but the overall level of metabolism could not be efficiently controlled. P-glycoprotein activity increased, but CYP activities remained very low.

Cytochrome P450 1B1: a novel anticancer therapeutic target

Cytochrome P450 (CYP)1B1 is overexpressed in tumor cells and is also recognized as a biomarker of the tumor phenotype. This review highlights the tremendous potential of this enzyme as a novel cancer therapeutic target. The range of therapeutic strategies including immunotherapeutics, CYP1B1-activated prodrugs and CYP1B1 inhibitors, that are currently being developed to exploit the presence and activity of CYP1B1 in tumor cells is outlined. The therapeutic strategy, which is at the most advanced stage of development, is a CYP1B1-based vaccine which has already successfully completed a Phase I clinical trial.

Diazinon, chlorpyrifos and parathion are metabolised by multiple cytochromes P450 in human liver

This research describes both the activation and detoxification of diazinon, chlorpyrifos and parathion by recombinant P450 isozymes and by human liver microsomes that had been characterised for P450 marker activities. Wide variations in activity were found for diazinon (50 microM; 500 microM) activation to diazoxon, chlorpyrifos (100 microM) to chlorpyrifos oxon and parathion (5 microM, 20 microM and 200 microM) to paraoxon in NADPH-dependent reactions. In parallel, the dearylated metabolites pyrimidinol (IHMP), trichloro-2-pyridinol (TCP) and p-nitrophenol (PNP) were produced from diazinon, chlorpyrifos and parathion, respectively, with similarly wide variations in activity. There were significant correlations between diazoxon formation from diazinon (50 microM; 500 microM) with the three CYP3A4/5 marker reactions, while IHMP formation correlated significantly with the three CYP3A4/5 reactions, the CYP2C8 marker reaction (p<0.05) and the CYP2C19 marker (p<0.01). Chlorpyrifos oxon formation from chlorpyrifos did not correlate with any of the P450 markers but TCP formation correlated with one of the CYP3A4/5 reactions (p<0.01) and CYP2C8 (p<0.01), CYP2C19 (p<0.01) and CYP1A2 (p<0.01) mediated reactions. There were significant relationships between paraoxon formation from parathion (5 microM, 20 microM and 200 microM) and the CYP3A4/5, CYP2C8 and CYP1A2 mediated reactions, although only the latter two isoforms correlated significantly with the lowest parathion concentration. Recombinant CYPs 2D6, 2C19, 3A5, 3A4 were most efficient in producing diazoxon and IHMP from diazinon; CYPs 2D6, 3A5, 2B6 and 3A4 were best at producing chlorpyrifos-oxon and CYPs 2C19, 2D6, 3A5 and 3A4 at producing TCP from chlorpyrifos (100 microM). These data strongly suggest that CYPs 3A4/5, 2C8, 1A2, 2C19 and 2D6 are primarily involved in the metabolism of all three OPs, although the profile of participating isoforms was different for each of the pesticides suggesting that chemical structure influences which P450s mediate the reaction. The marked inter-individual variation in expression of the various P450 isozymes may result in sub-populations of individuals that produce higher systemic oxon levels with increased susceptibility to OP toxicity.

Determination of a Human Hepatic Microsomal Scaling Factor for Predicting in Vivo Drug Clearance

PURPOSE

To determine a microsomal scaling factor for human liver suitable for prediction of in vivo drug clearance from in vitro data and to explore the role of inter-liver variability in this factor on the reported underprediction from microsomal parameters.

METHODS

Cytochrome P450 (henceforth P450) content in whole homogenates and microsomes from 38 donor livers was used to determine a microsomal scaling factor. In a subset (n = 20) of these preparations, individual P450 enzymes were examined by Western blotting and selective probe activities were determined.

RESULTS

The scaling factor from 38 livers averaged 40 mg microsomal protein per gram liver with a coefficient of variation of 31%. Western blotting experiments indicated that there was no P450 enzyme-specific trend in the distribution of individual P450 enzymes in liver microsomes relative to whole homogenate. Predictions based on an average scaling factor resulted in a satisfactory prediction of intrinsic clearance of three benzodiazepines similar to that obtained using individual factors for the same livers.

CONCLUSION

A value for human liver microsomal scaling of 40 mg microsomal protein per gram liver has been established. The reason for underprediction previously reported for 52 different drug substrates was not the use of an incorrect value for the scaling factor.

Generation of specific antibodies and their use to characterize sex differences in four rat P450 3A enzymes following vehicle and pregnenolone 16alpha-carbonitrile treatment

The purpose of this study was to identify isozyme-specific antibodies and use them to determine the expression levels of four P450 3A enzymes in the livers of vehicle- and pregnenolone 16alpha-carbonitrile (PCN)-treated rats of both sexes, since previous work on mRNA levels has shown considerable sexual dimorphism. Using Western blot analysis with four isozyme-specific antibodies, we show that P450 3A1, 3A2, and 3A9 were expressed in vehicle-treated adult female rats at very low levels whereas P450 3A18 was not detected. PCN treatment of females strongly induced the expression of P450 3A1 in the livers with protein product increases of 214-, 3-, and 5-fold for P450 3A1, 3A2, and 3A9, respectively, and P450 3A18 was induced to 3.7 pmol/mg protein. In contrast, all four P450 3As were detected in livers of vehicle-treated males, in the order of 3A2 >> 3A18 > 3A9 approximately = 3A1. The protein product increases induced by PCN treatment of male rats were 92-, 3-, 6-, and 16-fold for P450 3A1, 3A2, 3A9, and 3A18, respectively.

Kidney Dysfunction and Hypertension: Role for Cadmium, P450 and Heme Oxygenases?

Cadmium (Cd) is a metal toxin of continuing worldwide concern. Daily intake of Cd, albeit in small quantities, is associated with a number of adverse health effects which are attributable to distinct pathological changes in a variety of tissues and organs. In the present review, we focus on its renal tubular effects in people who have been exposed environmentally to Cd at levels below the provisional tolerable intake level set for the toxin. We highlight the data linking such low-level Cd intake with tubular injury, altered abundance of cytochromes P450 (CYPs) in the kidney and an expression of a hypertensive phenotype. We provide updated knowledge on renal and vascular effects of the eicosanoids 20-hydroxyeicosatetraenoic acid (20-HETE) and eicosatrienoic acids (EETs), which are biologically active metabolites from arachidonate metabolism mediated by certain CYPs in the kidney. We note the ability of Cd to elicit "oxidative stress" and to alter metal homeostasis notably of zinc which may lead to augmentation of the defense mechanisms involving induction of the antioxidant enzyme heme oxygenase-1 (HO-1) and the metal binding protein metallothionein (MT) in the kidney. We hypothesize that renal Cd accumulation triggers the host responses mediated by HO-1 and MT in an attempt to protect the kidney against injurious oxidative stress and to resist a rise in blood pressure levels. This hypothesis predicts that individuals with less active HO-1 (caused by the HO-1 genetic polymorphisms) are more likely to have renal injury and express a hypertensive phenotype following chronic ingestion of low-level Cd, compared with those having more active HO-1. Future analytical and molecular epidemiologic research should pave the way to the utility of induction of heme oxygenases together with dietary antioxidants in reducing the risk of kidney injury and hypertension in susceptible people.

Renal and hepatic accumulation of cadmium and lead in the expression of CYP4F2 and CYP2E1

The present study examined accumulation of the metal toxins cadmium (Cd) and lead (Pb) in relation to the abundance of cytochrome P450 4F2 (CYP4F2), CYP2E1 and concentrations of zinc and copper in liver and kidney samples using immunoblotting coupled with metal analysis. The post mortem liver and kidney cortex samples were from 23 males and 8 females aged 3-89 years. All were Caucasians who had not been exposed to metals in the workplace. Average kidney cortex Cd load of 17.4 microg/g w.w. was 17 times greater than average liver Cd load (1.1 microg/g w.w.). In contrast, average kidney cortex Pb load of 0.09 microg/g w.w. was two times lower than liver Pb load of 0.19 microg/g w.w. Average Zn and Cu concentrations in the kidney cortex samples were 67% and 33% lower than those in the liver. Liver and kidney Cd loads, but not liver or kidney Pb loads, correlated positively with donors' age. After controlling for liver Cd load, an inverse correlation was seen between Zn and age (partial r=-0.39, P=0.02), suggesting reduction in liver Zn levels in old age. Liver CYP2E1 protein abundance correlated with age-adjusted Cd load (partial r=0.37, P=0.02) whereas kidney CYP4F2 protein abundance showed a positive correlation with age-adjusted Cd loads (partial r=0.40, P=0.02). These findings suggest that Cd may be an inducer of renal CYP4F2 and hepatic CYP2E1 and that increased renal CYP4F2 expression may implicate in Cd-linked renal tubular dysfunction and high blood pressure, involving CYP4F2-dependent arachidonic acid metabolism.

CYP3A7 protein expression is high in a fraction of adult human livers and partially associated with the CYP3A7*1C allele

Previously, cytochrome P450 3A7 (CYP3A7), which constitutes the major CYP enzyme in fetal livers, has been considered a fetus-specific enzyme. However, CYP3A7 mRNA has recently been shown to be expressed at significant levels in a subset of adult human livers, several of which carry the CYP3A7*1C allele that contains the proximal PXR/CAR element of CYP3A4. The objective of this study was to investigate CYP3A7 expression at the protein level by developing a CYP3A7-specific antibody to allow its quantification. Based on results from 59 adult human liver samples, we found significant CYP3A7 protein expression in approximately one in 10 adult livers amounting for 24-90 pmol/mg microsomal protein, thereby contributing 9-36% to total CYP3A levels in these livers. CYP3A7 protein was detected in five of seven livers carrying the CYP3A7*1C allele (two of which only had trace amounts), whereas an additional three livers expressing CYP3A7 were apparently homozygous for CYP3A7*1. The mean protein expression level of CYP3A7 was 42 pmol/mg within the group of livers expressing CYP3A7 and 4 pmol/mg in all liver samples. CYP3A7 expression was thus higher than that of the polymorphically expressed CYP3A5 in adult human livers, based on a comparison with a previous study using our CYP3A5 peptide-specific antibody. The relatively high level of CYP3A7 protein expression detected in a subset of adult livers may be relevant with respect to the metabolism of exogenous and endogenous substrates, such as retinoic acid and dehydroepiandrosterone.