Co-expression of human cytochrome P4501A1 (CYP1A1) variants and human NADPH-cytochrome P450 reductase in the baculovirus/insect cell system

Identification of CYP 2A6 inhibitors in an effort to mitigate the harmful effects of the phytochemical nicotine

AIM

In this study, our goal was to study the inhibition of nicotine metabolism by P450 2A6, as a means for reduction in tobacco use and consequently the prevention of smoking-related cancers. Nicotine, a phytochemical, is an addictive stimulant, responsible for the tobacco-dependence in smokers. Many of the other phytochemicals in tobacco, including polycyclic aromatic hydrocarbons, N-nitrosamines, and aromatic amines, are potent systemic carcinogens. Tobacco smoking causes about one of every five deaths in the United States annually. Nicotine plasma concentration is maintained by the smokers' smoking behavior within a small range. Nicotine is metabolized by cytochrome P450s 2A6 and 2A13 to cotinine. This metabolism causes a decrease in nicotine plasma levels, which in turn leads to increased tobacco smoking, and increased exposure to the tobacco carcinogens.

METHODS

Using the phytochemical nicotine as a lead structure, and taking its interactions with the P450 2A6 binding pocket into consideration, new pyridine derivatives were designed and synthesized as potential selective mechanism-based inhibitors for this enzyme.

RESULTS

The design and synthesis of two series of novel pyridine-based compounds, with varying substituents and substitution locations on the pyridine ring, as well as their inhibitory activities on cytochrome P450 2A6 and their interactions with its active site are discussed here. Substitutions at position 3 of the pyridine ring with an imidazole or propargyl ether containing group showed the most optimal interactions with the P4502A6 active site.

CONCLUSION

The pyridine compounds with an imidazole or propargyl ether containing substituent on position 3 were found to be promising lead compounds for further development. Hydrogen-bonding interactions were determined to be crucial for effective binding of these molecules within the P450 2A6 active site.

Development and Uses of Offline and Web-Searchable Metabolism Databases - The Case of Benzo[a]pyrene

BACKGROUND

The present work describes development of offline and web-searchable metabolism databases for drugs, other chemicals, and physiological compounds using human and model species, prompted by the large amount of data published after year 1990. The intent was to provide a rapid and accurate approach to published data to be applied both in science and to assist therapy.

METHODS

Searches for the data were done using the Pub Med database, accessing the Medline database of references and abstracts. In addition, data presented at scientific conferences (e.g., ISSX conferences) are included covering the publishing period beginning with the year 1976.

RESULTS

Application of the data is illustrated by the properties of benzo[a]pyrene (B[a]P) and its metabolites. Analysis show higher activity of P450 1A1 for activation of the (-)- isomer of trans-B[a]P-7,8-diol, while P4501B1 exerts higher activity for the (+)- isomer. P450 1A2 showed equally low activity in the metabolic activation of both isomers.

CONCLUSION

The information collected in the databases is applicable in prediction of metabolic drug-drug and/or drug-chemical interactions in clinical and environmental studies. The data on the metabolism of searched compound (exemplified by benzo[a]pyrene and its metabolites) also indicate toxicological properties of the products of specific reactions. The offline and web-searchable databases had wide range of applications (e.g. computer assisted drug design and development, optimization of clinical therapy, toxicological applications) and adjustment in everyday life styles.

Role of Müller cell cytochrome P450 2c44 in murine retinal angiogenesis

Polyunsaturated fatty acids (PUFA) and their cytochrome P450 (CYP450) metabolites have been linked to angiogenesis and vessel homeostasis. However, the role of individual CYP isoforms and their endogenous metabolites in those processes are not clear. Here, we focused on the role of Cyp2c44 in postnatal retinal angiogenesis and report that Cyp2c44 is highly expressed in Müller glial cells in the retina. The constitutive as well as inducible postnatal genetic deletion of Cyp2c44 resulted in an increased vessel network density without affecting vessel radial expansion during the first postnatal week. This phenotype was associated with an increased endothelial cell proliferation and attenuated Notch signaling. LC-MS/MS analyses revealed that levels of hydroxydocosahexaenoic acids (HDHA), i.e., 10-, 17- and 20-HDHA were significantly elevated in retinas from 5day old Cyp2c44^-/- mice compared to their wild-type littermates. Enzymatic activity assays revealed that HDHAs were potential substrates for Cyp2c44 which could account for the increased levels of HDHAs in retinas from Cyp2c44^-/- mice. These data indicate that Cyp2c44 is expressed in the murine retina and, like the soluble epoxide hydrolase, is expressed in Müller glia cells. The enhanced endothelial cell proliferation and Notch inhibition seen in retinas from Cyp2c44-deficient mice indicate a role for Cyp2c44-derived lipid mediators in physiological angiogenesis.

The function of two P450s, CYP9M10 and CYP6AA7, in the permethrin resistance of Culex quinquefasciatus

Cytochrome P450 monooxygenases play a critical role in insecticide resistance by allowing resistant insects to metabolize insecticides. Previous studies revealed that two P450 genes, CYP9M10 and CYP6AA7, are not only up-regulated but also induced in resistant Culex mosquitoes. In this study, CYP9M10 and CYP6AA7 were separately co-expressed with cytochrome P450 reductase (CPR) in insect Spodoptera frugiperda (Sf9) cells using a baculovirus-mediated expression system and the enzymatic activity and metabolic ability of CYP9M10/CPR and CYP6AA7/CPR to permethrin and its metabolites, including 3-phenoxybenzoic alcohol (PBOH) and 3-phenoxybenzaldehyde (PBCHO), characterized. PBOH and PBCHO, both of which are toxic to Culex mosquito larvae, can be further metabolized by CYP9M10/CPR and CYP6AA7/CPR, with the ultimate metabolite identified here as PBCOOH, which is considerably less toxic to mosquito larvae. A cell-based MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) cytotoxicity assay revealed that Sf9 cells expressing CYP9M10/CPR or CYP6AA7/CPR increased the cell line's tolerance to permethrin, PBOH, and PBCHO. This study confirms the important role played by CYP9M10 and CYP6AA7 in the detoxification of permethrin and its metabolites PBOH and PBCHO.

CYP1A1 I462V polymorphism is associated with reduced genotoxicity in yeast despite positive association with increased cancer risk

CYP1A1 functions in detoxifying xenobiotics but occasionally converts compounds into potent genotoxins. CYP1A1 activates polyaromatic hydrocarbons, such as benzo[a]pyrene 7,8 dihydrodiol (BaP-DHD), rendering them genotoxic. Particular alleles of CYP1A1, such as CYP1A1 I462V have been correlated with a higher incidence of breast and lung cancer, but it is unknown whether these variants express enzymes in vivo that are more potent in generating genotoxins. We individually expressed CYP1A1 (CYP1A1.1), CYP1A1 T461N (CYP1A1.4) and I462V (CYP1A1.2) alleles in wild-type and DNA repair deficient mutant strains of Saccharomyces cerevisiae (budding yeast) and asked which yeast strains exhibited the highest levels of carcinogen-associated genotoxicity after exposure to BaP-DHD, aflatoxin B1 (AFB₁), and heterocyclic aromatic amines (HAAs). We measured carcinogen-associated recombination, Rad51 foci, and carcinogen-associated toxicity in a DNA repair mutant deficient in both nucleotide excision repair and recombinational repair. CYP1A1 activity was confirmed by measuring ethoxyresorufin-O-deethylation (EROD) activities. Our data indicate that CYP1A1 I462V allele confers the least carcinogen-associated genotoxicity, compared to CYP1A1; however, results vary depending on the chemical carcinogen and the genotoxic endpoint. We speculate that the cancer-associated risk of CYP1A1 I462V may be caused by exposure to other xenobiotics.

Cytochrome b5 and epoxide hydrolase contribute to benzo[a]pyrene-DNA adduct formation catalyzed by cytochrome P450 1A1 under low NADPH:P450 oxidoreductase conditions

In previous studies we had administered benzo[a]pyrene (BaP) to genetically engineered mice (HRN) which do not express NADPH:cytochrome P450 oxidoreductase (POR) in hepatocytes and observed higher DNA adduct levels in livers of these mice than in wild-type mice. To elucidate the reason for this unexpected finding we have used two different settings for in vitro incubations; hepatic microsomes from control and BaP-pretreated HRN mice and reconstituted systems with cytochrome P450 1A1 (CYP1A1), POR, cytochrome b5, and epoxide hydrolase (mEH) in different ratios. In microsomes from BaP-pretreated mice, in which Cyp1a1 was induced, higher levels of BaP metabolites were formed, mainly of BaP-7,8-dihydrodiol. At a low POR:CYP1A1 ratio of 0.05:1 in the reconstituted system, the amounts of BaP diones and BaP-9-ol formed were essentially the same as at an equimolar ratio, but formation of BaP-3-ol was ∼ 1.6-fold higher. Only after addition of mEH were BaP dihydrodiols found. Two BaP-DNA adducts were formed in the presence of mEH, but only one when CYP1A1 and POR were present alone. At a ratio of POR:CYP1A1 of 0.05:1, addition of cytochrome b5 increased CYP1A1-mediated BaP oxidation to most of its metabolites indicating that cytochrome b5 participates in the electron transfer from NADPH to CYP1A1 required for enzyme activity of this CYP. BaP-9-ol was formed even by CYP1A1 reconstituted with cytochrome b5 without POR. Our results suggest that in livers of HRN mice Cyp1a1, cytochrome b5 and mEH can effectively activate BaP to DNA binding species, even in the presence of very low amounts of POR.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Co-expression vs. co-infection using baculovirus expression vectors in insect cell culture: Benefits and drawbacks

The baculovirus expression vector system (BEVS) is a versatile and powerful platform for protein expression in insect cells. With the ability to approach similar post-translational modifications as in mammalian cells, the BEVS offers a number of advantages including high levels of expression as well as an inherent safety during manufacture and of the final product. Many BEVS products include proteins and protein complexes that require expression from more than one gene. This review examines the expression strategies that have been used to this end and focuses on the distinguishing features between those that make use of single polycistronic baculovirus (co-expression) and those that use multiple monocistronic baculoviruses (co-infection). Three major areas in which researchers have been able to take advantage of co-expression/co-infection are addressed, including compound structure-function studies, insect cell functionality augmentation, and VLP production. The core of the review discusses the parameters of interest for co-infection and co-expression with time of infection (TOI) and multiplicity of infection (MOI) highlighted for the former and the choice of promoter for the latter. In addition, an overview of modeling approaches is presented, with a suggested trajectory for future exploration. The review concludes with an examination of the gaps that still remain in co-expression/co-infection knowledge and practice.

Association of human cytochrome P450 1A1 (CYP1A1) and sulfotransferase 1A1 (SULT1A1) polymorphisms with differential metabolism and cytotoxicity of aminoflavone

Aminoflavone (AF), a clinically investigational novel anticancer agent, requires sequential metabolic activation by CYP1A1 and SULT1A1 to exert its antitumor activities. The purpose of this study was to determine the functional significance of common polymorphisms of human CYP1A1 and SULT1A1 on the metabolism and cytotoxicity of AF. To this end, Chinese Hamster V79 cells were genetically engineered to stably express human CYP1A1*1 (wild-type), CYP1A1*2C (I462V), or CYP1A1*4 (T461N) and coexpress human CYP1A1*1 with human SULT1A1*1 (wild-type), SULT1A1*2 (R213H), or SULT1A1*3 (M223V). The metabolism and cytotoxicity of AF were evaluated in these cellular models. All common variants of CYP1A1 and SULT1A1 were actively involved in the metabolic activation of AF, but with a varying degree of activity. Whereas CYP1A1 I462V variant exhibited a superior activity (mainly caused by a significantly higher V(max)) for hydroxylations of AF, expression of different CYP1A1 variants did not confer cell differential sensitivity to AF. The cells coexpressing CYP1A1*1 with SULT1A1*1, SULT1A1*2, or SULT1A1*3 displayed SULT1A1 allele-specific sensitivity to AF: SULT1A1*3 exhibited the highest sensitivity (IC(50), 0.01 μmol/L), followed by SULT1A1*1 (IC(50), 0.5 μmol/L), and SULT1A1*2 showed the lowest sensitivity (IC(50), 4.4 μmol/L). These data suggest that the presence of low-activity SULT1A1*2 may predict poor response to AF, whereas the presence of high-activity CYP1A1/SULT1A1 alleles, especially combination of CYP1A1*2C and SULT1A1*3 or SULT1A1*1, may be beneficial to patients receiving AF. The present study provides a foundation for future clinical investigations of potential genetic biomarkers that may enable selection of patients for the greatest potential benefit from AF treatment.

Effects of heme precursors on CYP1A2 and POR expression in the baculovirus/Spodoptera frugiperda system

OBJECTIVE

CYP1A2 and NADPH-CYP450 oxidoreductase (POR) were expressed in the baculovirus/Spodoptera frugiperda (sf9) system. The aim of this study was to investigate the effects of heme precursors on the expression of CYP1A2 and POR.

METHODS

The heme precursors [δ-Aminolaevulinic Acid (5-ALA), Fe(3+) and hemin] were introduced into the system to evaluate their effects on the expression of CYP1A2, POR and their co-expression. All the proteins were identified using immunoblotting, CO-difference spectroscopy, or cytochrome c assay.

RESULTS

In the present study, functional CYP1A2 and POR were successfully expressed in the baculovirus/sf9 system, and both of them showed high activities. Co-addition of 5-ALA and Fe(3+) significantly improved expression of CYP1A2 by about 50% compared with the addition of 5-ALA, Fe(3+) or hemin alone. Either co-addition of 5-ALA and Fe(3+) or addition of 5-ALA or Fe(3+) alone improved the POR expression level 2 fold and its activity 7-10 fold compared with control (no addition). However, unlike CYP1A2, there was no difference between the co-addition and addition of these heme precursors alone. Different ratios of BvCYP1A2 to BvPOR also affected the co-expression of CYP1A2 and POR, with a 3:1 ratio of BvCYP1A2 / BvPOR significantly increasing their co-expression. Surprisingly, the addition of 0.1 mM 5-ALA or Fe(3+) alone, but not their co-addition, could significantly improve the CYP1A2 and POR co-expression (P < 0.05).

CONCLUSION

5-ALA and Fe(3+) increased the expression of CYP1A2 and POR in a baculovirus/sf9 system, but the pattern of their expression was different between their expression alone and co-expression.

Nitric oxide sensitive guanylyl cyclase activity decreases during cerebral postnatal development because of a reduction in heterodimerization

Soluble guanylyl cyclase (sGC) is the major physiological receptor for nitric oxide (NO) throughout the central nervous system. Three different subunits form the alpha(1)/beta(1) and alpha(2)/beta(1) heterodimeric enzymes that catalyze the reaction of GTP to the second messenger cGMP. Both forms contain a prosthetic heme group which binds NO and mediates activation by NO. A number of studies have shown that NO/cGMP signaling plays a major role in neuronal cell differentiation during development of the central nervous system. In the present work, we studied regulation and expression of sGC in brain of rats during postnatal development using biochemical methods. We consistently observed a surprising decrease in cerebral NO sensitive enzyme activity in adult animals in spite of stable expression of sGC subunits. Total hemoprotein heme content was decreased in cerebrum of adult animals, likely because of an increase in heme oxygenase activity. But the loss of sGC activity was not simply because of heme loss in intact heterodimeric enzymes. This was shown by enzyme activity determinations with cinaciguat which can be used to test heme occupancy in intact heterodimers. A reduction in heterodimerization in cerebrum of adult animals was demonstrated by co-precipitation analysis of sGC subunits. This explained the observed decrease in NO sensitive guanylyl cyclase activity in cerebrum of adult animals. We conclude that differing efficiencies in heterodimer formation may be an important reason for the lack of correlation between sGC protein expression and sGC activity that has been described previously. We suggest that heterodimerization of sGC is a regulated process that changes during cerebral postnatal development because of still unknown signaling mechanisms.

Statistical determination of optimal baculovirus infection condition for recombinant protein production in Drosophila S2 cells

Insect Drosophila melanogaster S2 cell was developed as plasmid-based and, therefore, a nonlytic expression system for functional foreign proteins. To achieve multiple protein expressions, it was suggested that baculovirus be used on S2 cell system because baculovirus can infect S2 cells but cannot replicate inside the cells. Therefore, establishment of baculovirus infection conditions is the first important step and this should be properly optimized for production yield. We used statistical methodology to optimize the baculovirus infection conditions using green fluorescent protein (GFP) as a reporter protein. Consequently, we arrived at optimal infection conditions through a statistical regression method. The secreted GFP yield from vMT-GFP baculovirus-infected wild-type S2 cells under optimal infection conditions was >15-fold higher than that under nonoptimal conditions and comparable to that from stably transfected recombinant S2 cells.

Recombinant baculovirus-based multiple protein expression platform for Drosophila S2 cell culture

A platform for selective and controllable expression of multiple foreign protein types was developed in insect cell culture. Based on the fact that baculovirus cannot replicate in nonpermissive Drosophila melanogaster Schneider line 2 (S2) cells, S2 cells that stably express human erythropoietin (hEPO) under the control of the S2-derived inducible metallothionein (MT) promoter were infected with three types of recombinant baculoviruses, each of which expressed a different fluorescent protein gene under the control of MT promoter. Addition of copper sulfate as an inducer to infected, stably transfected S2 cells resulted in simultaneous expression of hEPO and three fluorescent proteins. Expression profiles and levels of the three induced fluorescent proteins were similar in all single infected cells. Importantly, expression profiles and levels of hEPO were similar in both non-infected and infected cells, indicating that baculovirus expressed recombinant proteins do not adversely affect expression of host cell recombinant proteins. Expressions of the three fluorescent proteins were able to be selectively regulated by altering combination ratios of the three types of recombinant baculoviruses. Collectively, these data indicate that the baculovirus/stably transfected S2 cell system can be successfully used to express multiple foreign proteins in a controlled and selective manner without the burden of additional selection markers. Such a system would be expected to be attractive as a multiple protein expression platform for engineering metabolic or glycosylation pathways.

Recalling P446. P4501A1 (CYP1A1) opting for clinical application

This review goes back to spectral studies [see Hildebrandt et al., 1968]. The findings of apparent absolute spectra of two interconvertible forms of microsomal mixed function oxidases are looked back on to recall whether their impact sustained scrutiny or are rather remembered as of sentimental value only. The second part summarizes studies on the clinical relevance of CYP1A1 with special reference to our investigations. The impact of genetic variability of CYP1A1 on cancer susceptibility, differential effects of polyphenols and hyperforin on toxification and detoxification pathways of benzo[a]pyrene, and differential metabolite patterns of 17 beta-estradiol, estrone, and eicosapentaenoic acid are presented.

Polymorphic variation of Cyp1A1 is associated with the risk of gastric cardia cancer: a prospective case-cohort study of cytochrome P-450 1A1 and GST enzymes

OBJECTIVE

To determine if genetic polymorphisms of CYP1A1, GSTM1, GSTP1, or GSTT1 are associated with an increased risk of developing esophageal squamous cell carcinoma (ESCC), gastric cardia cancer (GCC), or either in a high-risk Asian population.

METHODS

We conducted a case-cohort analysis with 5 years of prospective follow-up. The analytical cohort contained 642 individuals who participated in either the Dysplasia Trial (DT) or the General Population (GPT) of the Nutrition Intervention Trials conducted in Linxian, China, and included 131 cases of ESCC and 90 cases of GCC. Genotyping analysis was performed on DNA extracted from red blood cells using a PureGene kit (Gentra Systems, Inc., Minneapolis, MN) and real-time PCR analysis amplification (Taq-Man). Relative risks and 95% confidence intervals were estimated using the case - cohort estimator for the Cox proportional hazards models. p-values from nested models with genotyping variables came from score tests.

RESULTS

The relative risks for developing ESCC, GCC, or either cancer were calculated in the entire analytic cohort for GSTM1, P1*B (A313G), and T1 and CYP1A1*2A (T3801C) and *2C (A2455G) genotypes, and no significant associations were identified. However, because of the difference in cancer risks between the DT (9.3 cases per 1000 person years) and the GPT (5.3 cases), the analytical cohort was stratified by trial; the DT participants who were heterozygous or homozygous for the variant-allele at CYP1A1*2A had a reduced risk for developing GCC (adjusted RR (95% CI) 0.47 (0.23-1.00) p = 0.037).

CONCLUSIONS

This study found an association for the CYP1A1*2A variant allele and a reduced risk of GCC in people at high risk for development of this disease. This finding is consistent with previous studies suggesting that substrates for the cytochrome P-450 1A1 metabolic pathway, such as polycyclic aromatic hydrocarbons, may be etiologically significant in this high-risk region.

Carcinogenic polycyclic aromatic hydrocarbon-DNA adducts and mechanism of action

Polycyclic aromatic hydrocarbons (PAHs) are a class of widespread environmental carcinogens. Most of our knowledge of their mechanisms of metabolic activation to DNA-binding "ultimate carcinogenic" metabolites has come from analysis of the DNA interaction products formed by these highly reactive intermediates. Studies of their role in forming DNA-binding intermediates identical to those formed in vivo from the PAH itself have also allowed identification of the particular cytochrome P450 enzymes involved in activating various structural classes of carcinogenic PAHs. It has been established that PAHs, after metabolic activation in vivo, are capable of inducing mutations in oncogenes and, by inducing multiple mutations, may result in tumors. PAHs also cause changes in cellular gap-junction communication similar to those caused by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate. Thus, PAHs may also act through a promotional mechanism in addition to serving as tumor initiators. Previous studies on these mechanisms are described and summarized.

FUNCTIONAL CHARACTERIZATION OF FIVE NOVEL CYP2C8 VARIANTS, G171S, R186X, R186G, K247R, AND K383N, FOUND IN A JAPANESE POPULATION

Cytochrome P450 2C8 is one of the primary enzymes responsible for the metabolism of a wide range of drugs such as paclitaxel, cerivastatin, and amiodarone. We have sequenced the CYP2C8 gene from 201 Japanese subjects and found five novel nonsynonymous single nucleotide polymorphisms (SNPs): 511G>A (G171S), 556C>T (R186X; X represents the translational stop codon), 556C>G (R186G), 740A>G (K247R), and 1149G>T (K383N), with the allele frequency of 0.0025. The CYP2C8 variants were heterologously expressed in COS-1 cells and functionally characterized in terms of expression level, paclitaxel 6alpha-hydroxylase activity, and intracellular localization. The prematurely terminated R186X variant was undetectable by Western blotting and inactive toward paclitaxel 6alpha-hydroxylation. The G171S, K247R, and K383N variants exhibited properties similar to those of the wild-type CYP2C8. Paclitaxel 6alpha-hydroxylase activity of the R186G transfectant was only 10 to 20% that of wild-type CYP2C8. Furthermore, the R186G variant displayed a lower level of protein expression in comparison to the wild type, which was restored by the addition of a proteasome inhibitor (MG-132; Z-Leu-Leu-Leu-aldehyde). The reduced CO-difference spectral analysis using recombinant proteins from an insect cell/baculovirus system revealed that the R186G variant has a minor peak at 420 nm in addition to the characteristic Soret peak at 450 nm, suggesting the existence of improperly folded protein. These results indicate that the novel CYP2C8 SNPs, 556C>T (R186X) and 556C>G (R186G), could influence the metabolism of CYP2C8 substrates such as paclitaxel and cerivastatin.

Feasibility of a simple double-layered coculture system incorporating metabolic processes of the intestine and liver tissue: application to the analysis of benzo[a]pyrene toxicity

A simple double-layered coculture system using Caco-2 cell and Hep G2 cell, which mimic metabolic processes occurring in humans such as absorption through the intestine and cytochrome P450 1A1/2 involving biotransformation in both the intestine and liver cells, was used to investigate the toxicity of model chemical, benzo[a]pyrene (B[a]P). It was found that both Caco-2 and Hep G2 cells can metabolize B[a]P to toxic metabolites including B[a]P-7,8-hydrodiol (7,8-diol), an immediate precursor to the highly-reactive ultimate toxicant of B[a]P, B[a]P-7,8-hydrodiol-9,10-epoxide (BPDE), possibly mediated by cytochrome P450 1A1/2 activity. However, in a double-layered coculture system, no significant reduction of Hep G2 cell viability was found, although an approximately 50% reduction in viability was observed in pure Hep G2 cells. HPLC analysis showed that Caco-2 cells transfer B[a]P and its toxic metabolites back to the apical side, thus decreasing the concentrations of toxic metabolites including B[a]P-7,8-hydrodiol (7,8-diol) in cocultured Hep G2 cells. These results appear to be correlated with in vivo data on the effects of orally administered B[a]P, that is, low (10%) bioavailability in the rats and almost no acute lethal toxicity in rats or mice. As such, the simple double-layered coculture system can provide more accurate information regarding the toxic actions of the hazardous chemicals in humans than a pure culture system, as it also gives the final toxicity as a result of many complicated phenomena such as selective permeation in the intestine and biotransformation in the intestine and liver.

Linking exposure to environmental pollutants with biological effects

Exposure to ambient air pollution has been associated with cancer. Ambient air contains a complex mixture of toxics, including particulate matter (PM) and benzene. Carcinogenic effects of PM may relate both to the content of PAH and to oxidative DNA damage generated by transition metals, benzene, metabolism and inflammation. By means of personal monitoring and biomarkers of internal dose, biologically effective dose and susceptibility, it should be possible to characterize individual exposure and identify air pollution sources with relevant biological effects. In a series of studies, individual exposure to PM(2.5), nitrogen dioxide (NO(2)) and benzene has been measured in groups of 40-50 subjects. Measured biomarkers included 1-hydroxypyrene, benzene metabolites (phenylmercapturic acid (PMA) and trans-trans-muconic acid (ttMA)), 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in urine, DNA strand breaks, base oxidation, 8-oxodG and PAH bulky adducts in lymphocytes, markers of oxidative stress in plasma and genotypes of glutathione transferases (GSTs) and NADPH:quinone reductase (NQO1). With respect to benzene, the main result indicates that DNA base oxidation is correlated with PMA excretion. With respect to exposure to PM, biomarkers of oxidative damage showed significant positive association with the individual exposure. Thus, 8-oxodG in lymphocyte DNA and markers of oxidative damage to lipids and protein in plasma associated with PM(2.5) exposure. Several types of DNA damage showed seasonal variation. PAH adduct levels, DNA strand breaks and 8-oxodG in lymphocytes increased significantly in the summer period, requiring control of confounders. Similar seasonal effects on strand breaks and expression of the relevant DNA repair genes ERCC1 and OGG1 have been reported. In the present setting, biological effects of air pollutants appear mainly related to oxidative stress via personal exposure and not to urban background levels. Future developments include personal time-resolved monitors for exposure to ultrafine PM and PM(2.5,) use of GPS, as well as genomics and proteomics based biomarkers.

Human CYP1A1 allelic variants: baculovirus expression and purification, hydrodynamic, spectral, and catalytical properties and their potency in the formation of all-trans-retinoic acid

Three human cytochrome P450 1A1 (CYP1A1) allelic variants, namely wild-type (CYP1A1.1), CYP1A1.2 (I462V), and CYP1A1.4 (T461N), were expressed as C-terminal His-tagged fusions including a thrombin cleavage site in Spodoptera frugiperda insect cells by baculovirus infection. The variants were expressed with 30-90 nmol (1.8-5.4 mg) spectrally active cytochrome P450 per one liter of culture and purified to electrophoretic homogeneity by Ni-agarose chromatography. The recombinant variants were structurally characterized by UV/Vis, ultracentrifugation, and EPR. Optical and EPR spectra showed all three variants predominantly in high spin state; moreover, EPR indicated changes in the electronic structure of the heme iron of the two mutant variants. Sedimentation equilibrium experiments demonstrated the purified variants in dimeric state in the presence of 0.2% emulgen+0.05% cholate. Higher detergent concentration, the presence of imidazole, and cleavage of the His-tag led to monomerization. Catalytic activity of all purified variants was reconstituted with purified human NADPH-P450 reductase and dilaurylphosphatidylcholine. Enzyme kinetics of ethoxyresorufin O-deethylation revealed similar K(m) ( approximately 0.4 microM) for all variants but slightly different V(max) values (CYP1A1.1: 4.2, CYP1A1.2: 7.0, and CYP1A1.4: 3.0 nmol/min/nmol CYP1A1). The extended C-terminus influenced the enzymatic activity only slightly. All three variants are able to produce significant amounts of all-trans-retinoic acid from all-trans-retinal with V(max) of 4.0, 3.3, and 5.6 nmol/min/nmol CYP1A1 and K(m) values of 111, 83, and 250 microM for CYP1A1.1, CYP1A1.2, and CYP1A1.4, respectively. Availability of the three purified human CYP1A1 variants should facilitate further characterization of their role in metabolism of endogenous and exogenous compounds as well as structural studies.

Selective association of TRPC channel subunits in rat brain synaptosomes

TRPC genes encode a ubiquitous family of ion channel proteins responsible for Ca(2+) influx following stimulation of G-protein-coupled membrane receptors linked to phospholipase C. These channels may be localized to large multimeric signaling complexes via association with PDZ-containing scaffolding proteins. Based on sequence homology, the TRPC channel family can be divided into two major subgroups: TRPC1, -C4, and -C5 and TRPC3, -C6, and -C7. Although TRPC channels are thought to be tetramers, the actual subunit composition remains unknown. To determine subunit arrangement, individual TRPC channel pairs were heterologously expressed in Sf9 insect cells and immunoprecipitated using affinity-purified rabbit polyclonal antibodies specific for each channel subtype. Reciprocal co-immunoprecipitations showed that TRPC1, -C4, and -C5 co-associate and that TRPC3, -C6, and -C7 co-associate but that cross-association between the two major subgroups does not occur. Additionally, the interaction between each TRPC channel and the PDZ-containing protein, INAD (protein responsible for the inactivation-no-after-potential Drosophila mutant), was examined. TRPC1, -C4, and -C5 co-immunoprecipitated with INAD, whereas TRPC3, -C6, and -C7 did not. To define channel subunit interactions in vivo, immunoprecipitations were performed from isolated rat brain synaptosomal preparations. The results revealed that TRPC1, -C4, and -C5 co-associate and that TRPC3, -C6, and -C7 co-associate in both cortex and cerebellum but that cross-association between the two major subgroups does not occur. These results demonstrate that TRPC channels are present in nerve terminals and provide the first direct evidence for selective assembly of channel subunits in vivo.