Cytochrome P450 1A1/2, 2B6 and 3A4 HepaRG Cell-Based Biosensors to Monitor Hepatocyte Differentiation, Drug Metabolism and Toxicity.

Human hepatoma HepaRG cells express most drug metabolizing enzymes and constitute a pertinent in vitro alternative cell system to primary cultures of human hepatocytes in order to determine drug metabolism and evaluate the toxicity of xenobiotics. In this work, we established novel transgenic HepaRG cells transduced with lentiviruses encoding the reporter green fluorescent protein (GFP) transcriptionally regulated by promoter sequences of cytochromes P450 (CYP) 1A1/2, 2B6 and 3A4 genes. Here, we demonstrated that GFP-biosensor transgenes shared similar expression patterns with the corresponding endogenous CYP genes during proliferation and differentiation in HepaRG cells. Interestingly, differentiated hepatocyte-like HepaRG cells expressed GFP at higher levels than cholangiocyte-like cells. Despite weaker inductions of GFP expression compared to the strong increases in mRNA levels of endogenous genes, we also demonstrated that the biosensor transgenes were induced by prototypical drug inducers benzo(a)pyrene and phenobarbital. In addition, we used the differentiated biosensor HepaRG cells to evidence that pesticide mancozeb triggered selective cytotoxicity of hepatocyte-like cells. Our data demonstrate that these new biosensor HepaRG cells have potential applications in the field of chemicals safety evaluation and the assessment of drug hepatotoxicity.

Concurrent cooperativity and substrate inhibition in the epoxidation of carbamazepine by cytochrome P450 3A4 active site mutants inspired by molecular dynamics simulations.

Cytochrome P450 3A4 (CYP3A4) is the major human P450 responsible for the metabolism of carbamazepine (CBZ). To explore the mechanisms of interactions of CYP3A4 with this anticonvulsive drug, we carried out multiple molecular dynamics (MD) simulations, starting with the complex of CYP3A4 manually docked with CBZ. On the basis of these simulations, we engineered CYP3A4 mutants I369F, I369L, A370V, and A370L, in which the productive binding orientation was expected to be stabilized, thus leading to increased turnover of CBZ to the 10,11-epoxide product. In addition, we generated CYP3A4 mutant S119A as a control construct with putative destabilization of the productive binding pose. Evaluation of the kinetics profiles of CBZ epoxidation demonstrate that CYP3A4-containing bacterial membranes (bactosomes) as well as purified CYP3A4 (wild-type and mutants I369L/F) exhibit substrate inhibition in reconstituted systems. In contrast, mutants S119A and A370V/L exhibit S-shaped profiles that are indicative of homotropic cooperativity. MD simulations with two to four CBZ molecules provide evidence that the substrate-binding pocket of CYP3A4 can accommodate more than one molecule of CBZ. Analysis of the kinetics profiles of CBZ metabolism with a model that combines the formalism of the Hill equation with an allowance for substrate inhibition demonstrates that the mechanism of interactions of CBZ with CYP3A4 involves multiple substrate-binding events (most likely three). Despite the retention of the multisite binding mechanism in the mutants, functional manifestations reveal an exquisite sensitivity to even minor structural changes in the binding pocket that are introduced by conservative substitutions such as I369F, I369L, and A370V.

Identification of the rat liver cytochrome P450 enzymes involved in the metabolism of the calcium channel blocker dipfluzine hydrochloride.

This study aimed to identify the specific cytochrome P450 (CYP450) enzymes involved in the metabolism of dipfluzine hydrochloride using the combination of a chemical inhibition study, a correlation analysis and a panel of recombinant rat CYP450 enzymes. The incubation of Dip with rat liver microsomes yielded four metabolites, which were identified by liquid chromatography-coupled tandem mass spectrometry (LC/MS/MS). The results from the assays involving eight selective inhibitors indicated that CYP3A and CYP2A1 contributed most to the metabolism of Dip, followed by CYP2C11, CYP2E1 and CYP1A2; however, CYP2B1, CYP2C6 and CYP2D1 did not contribute to the formation of the metabolites. The results of the correlation analysis and the assays involving the recombinant CYP450 enzymes further confirmed the above results and concluded that CYP3A2 contributed more than CYP3A1. The results will be valuable in understanding drug-drug interactions when Dip is coadministered with other drugs.

Immunochemical detection of cytochrome P450 enzymes in small intestine microsomes of male and female untreated juvenile cynomolgus monkeys.

The expression of small intestinal cytochromes P450 (P450s) has not been systematically measured in cynomolgus monkeys, which are widely used in preclinical drug studies to predict pharmacokinetics and toxicity in humans: therefore, P450 content of small intestine was quantified in 35 cynomolgus monkeys by immunoblotting using 11 selective antibodies. CYP2D, CYP2J2, CYP3A4 and CYP3A5 were detected in all 35 animals, while CYP1A and CYP2C9/19 were detected in 31 and 17 animals, respectively. CYP2C9 and CYP2C19 were detected with the same antibody. CYP1D, CYP2A, CYP2B6, CYP2C76 and CYP2E1 were not detected in any of the 35 animals examined. On analysis of pooled microsomes (35 animals), CYP3A (3A4+3A5) was most abundant (79% of total immunoquantified CYP1-3 proteins), followed by CYP2J2 (13%), CYP2C9/19 (4%), CYP1A (3%) and CYP2D (0.4%). On the analysis of individual microsome samples, each P450 content varied 2-to-6-fold between animals, and no sex differences were observed in any P450 content. These findings should help to increase the understanding of drug metabolism, especially the first-pass effect, in cynomolgus monkey small intestines.

Heterologous expression of human cytochromes P450 2D6 and CYP3A4 in Escherichia coli and their functional characterization.

This study aimed to express two major drug-metabolizing human hepatic cytochromes P450 (CYPs), CYP2D6 and CYP3A4, together with NADPH-cytochrome P450 oxidoreductase (OxR) in Escherichia coli and to evaluate their catalytic activities. Full length cDNA clones of both isoforms in which the N-terminus was modified to incorporate bovine CYP17α sequence were inserted into a pCWori(+) vector. The modified CYP cDNAs were subsequently expressed individually, each together with OxR by means of separate, compatible plasmids with different antibiotic selection markers. The expressed proteins were evaluated by immunoblotting and reduced CO difference spectral scanning. Enzyme activities were examined using high performance liquid chromatography (HPLC) assays with probe substrates dextromethorphan and testosterone for CYP2D6 and CYP3A4, respectively. Results from immunoblotting demonstrated the presence of both CYP proteins in bacterial membranes and reduced CO difference spectra of the cell preparations exhibited the characteristic absorbance peak at 450 nm. Co-expressed OxR also demonstrated an activity level comparable to literature values. Kinetic parameters, K(m) and V(max) values determined from the HPLC assays also agreed well with literature values. As a conclusion, the procedures described in this study provide a relatively convenient and reliable means of producing catalytically active CYP isoforms suitable for drug metabolism and interaction studies.

Kinetics of electron transfer between NADPH-cytochrome P450 reductase and cytochrome P450 3A4.

We have incorporated CYP3A4 (cytochrome P450 3A4) and CPR (NADPH-cytochrome P450 reductase) into liposomes with a high lipid/protein ratio by an improved method. In the purified proteoliposomes, CYP3A4 binds testosterone with Kd (app)=36±6 µM and Hill coefficient=1.5±0.3, and 75±4% of the CYP3A4 can be reduced by NADPH in the presence of testosterone. Transfer of the first electron from CPR to CYP3A4 was measured by stopped-flow, trapping the reduced CYP3A4 as its Fe(II)-CO complex and measuring the characteristic absorbance change. Rapid electron transfer is observed in the presence of testosterone, with the fast phase, representing 90% of the total absorbance change, having a rate of 14±2 s(-1). Measurements of the first electron transfer were performed at various molar ratios of CPR/CYP3A4 in proteoliposomes; the rate was unaffected, consistent with a model in which first electron transfer takes place within a relatively stable CPR-CYP3A4 complex. Steady-state rates of NADPH oxidation and of 6ß-hydroxytestosterone formation were also measured as a function of the molar ratio of CPR/CYP3A4 in the proteoliposomes. These rates increased with increasing CPR/CYP3A4 ratio, showing a hyperbolic dependency indicating a Kd (app) of ~0.4 µM. This suggests that the CPR-CYP3A4 complex can dissociate and reform between the first and second electron transfers.

Mixing apples and oranges: Analysis of heterotropic cooperativity in cytochrome P450 3A4.

Heterotropic cooperative phenomena have been documented in studies with cytochrome P450 3A4, with few attempts to quantify this behavior other than to show the apparent stimulatory effect of certain CYP3A4 substrates on the enzyme's catalytic activity for others. Here CYP3A4 solubilized in Nanodiscs is studied for its ability to interact with two substrates, alpha-naphthoflavone and testosterone, which produce transitions in the heme spin state with apparent spectral affinities (corrected for membrane partitioning) of 7 and 38 microM, respectively. Simultaneous addition of both substrates at fixed molar ratios allows for the separation of specific heterotropic cooperative interactions from the simple additive affinities for the given substrate ratios. The absence of any changes in the normalized spectral dissociation constant due to changes in substrate ratio reveals that the observed stimulatory effect is largely due to differences in the relative substrate affinities and the presence of additional substrate in the system, rather than any specific positive heterotropic interactions between the two substrates.

Cloning, expression, and characterization of CYP3A43 cDNA in cynomolgus macaque (Macaca fascicularis).

Cynomolgus macaques are frequently used in drug metabolism studies due to their evolutionary closeness to humans. Despite their importance, genes encoding drug-metabolizing enzymes have not been fully identified in this species. In this study, the cDNA orthologous to human cytochrome P450 3A43 (CYP3A43) was isolated. Deduced amino acids of this cDNA had a high sequence identity ( approximately 95%) to human CYP3A43 cDNA and contained characteristic motifs for CYP3A proteins, heme-binding region and substrate recognition sites. Among 10 tissues analyzed, cynomolgus CYP3A43 was expressed in liver, adrenal gland, and lung, with the highest expression seen in liver. Cynomolgus CYP3A43 protein heterologously expressed in Escherichia coli exhibited metabolic activity toward midazolam 1'-hydroxylation. These results indicated that cynomolgus CYP3A43 was expressed in liver and encoded a functional drug-metabolizing enzyme, and could contribute to overall drug metabolism in cynomolgus macaque liver if expressed as a protein.

Cross-species comparison of conazole fungicide metabolites using rat and rainbow trout (Onchorhynchus mykiss) hepatic microsomes and purified human CYP 3A4.

Ecological risk assessment frequently relies on cross-species extrapolation to predict acute toxicity from chemical exposures. A major concern for environmental risk characterization is the degree of uncertainty in assessing xenobiotic biotransformation processes. Although inherently complex, metabolite identification is critical to risk assessment since the product(s) formed may pose a greater toxicological threat than the parent molecule. This issue is further complicated by differences observed in metabolic transformation pathways among species. Conazoles represent an important class of azole fungicides that are widely used in both pharmaceutical and agricultural applications. The antifungal property of conazoles occurs via complexation with the cytochrome P450 monooxygenases (CYP) responsible for mediating fungal cell wall synthesis. This mode of action has cause for concern regarding the potential adverse impact of conazoles on the broad spectrum of CYP-based processes within mammalian and aquatic species. In this study, in vitro metabolic profiles were determined for thirteen conazole fungicides using rat and rainbow trout (Oncorhynchus mykiss) liver microsomes and purified human CYP 3A4. Results showed that 10 out of the 13 conazoles tested demonstrated identical metabolite profiles among rat and trout microsomes, and these transformations were well conserved via both aromatic and aliphatic hydroxylation and carbonyl reduction processes. Furthermore, nearly all metabolites detected in the rat and trout microsomal assays were detected within the human CYP 3A4 assays. These results indicate a high degree of metabolic conservation among species with an equivalent isozyme activity of human CYP 3A4 being present in both the rat and trout, and provides insight into xenobiotic biotransformations needed for accurate risk assessment.