Taxol biosynthesis and molecular genetics

Biosynthesis of the anticancer drug Taxol in Taxus (yew) species involves 19 steps from the universal diterpenoid progenitor geranylgeranyl diphosphate derived by the plastidial methyl erythritol phosphate pathway for isoprenoid precursor supply. Following the committed cyclization to the taxane skeleton, eight cytochrome P450-mediated oxygenations, three CoA-dependent acyl/aroyl transfers, an oxidation at C9, and oxetane (D-ring) formation yield the intermediate baccatin III, to which the functionally important C13-side chain is appended in five additional steps. To gain further insight about Taxol biosynthesis relevant to the improved production of this drug, and to draw inferences about the organization, regulation, and origins of this complex natural product pathway, Taxus suspension cells (induced for taxoid biosynthesis by methyl jasmonate) were used for feeding studies, as the foundation for cell-free enzymology and as the source of transcripts for cDNA library construction and a variety of cloning strategies. This approach has led to the elucidation of early and late pathway segments, the isolation and characterization of over half of the pathway enzymes and their corresponding genes, and the identification of candidate cDNAs for the remaining pathway steps, and it has provided many promising targets for genetically engineering more efficient biosynthetic production of Taxol and its precursors.

Cytochrome P450 oxygenases of Taxol biosynthesis

Cytochrome P450 monooxygenases play a prominent role in the biosynthesis of the diterpenoid anticancer drug Taxol, as they appear to constitute about half of the 19 enzymatic steps of the pathway in yew (Taxus) species. A combination of classical biochemical and molecular methods, including cell-free enzyme studies and differential-display of mRNA-reverse transcription polymerase chain reaction (RT-PCR) combined with a homology-based searching and random sequencing of a cDNA library from induced T. cuspidata cells, led to the discovery of six novel cytochrome P450 taxoid (taxane diterpenoid) hydroxylases. These genes show unusually high sequence similarity with each other (>70%) but low similarity (<30%) to, and significant evolutionary distance from, other plant P450s. Despite their high similarity, functional analysis of these hydroxylases demonstrated distinctive substrate specificities responsible for an early bifurcation in the biosynthetic pathway after the initial hydroxylation of the taxane core at C5, leading into a biosynthetic network of competing, but interconnected, branches. The use of surrogate substrates, in cases where the predicted taxoid precursors were not available, led to the discovery of two core oxygenases, the 2α- and the 7β-hydroxylase. This general approach could accelerate the functional analysis of candidate cDNAs from the extant family of P450 genes to identify the remaining oxygenation steps of this complex pathway.

Identifying a selective substrate and inhibitor pair for the evaluation of CYP2J2 activity

CYP2J2, an arachidonic acid epoxygenase, is recognized for its role in the first-pass metabolism of astemizole and ebastine. To fully assess the role of CYP2J2 in drug metabolism, a selective substrate and potent specific chemical inhibitor are essential. In this study, we report amiodarone 4-hydoxylation as a specific CYP2J2-catalyzed reaction with no CYP3A4, or other drug-metabolizing enzyme, involvement. Amiodarone 4-hydroxylation enabled the determination of liver relative activity factor and intersystem extrapolation factor for CYP2J2. Amiodarone 4-hydroxylation correlated with astemizole O-demethylation but not with CYP2J2 protein content in a sample of human liver microsomes. To identify a specific CYP2J2 inhibitor, 138 drugs were screened using terfenadine and astemizole as probe substrates with recombinant CYP2J2. Forty-two drugs inhibited CYP2J2 activity by ≥50% at 30 μM, but inhibition was substrate-dependent. Of these, danazol was a potent inhibitor of both hydroxylation of terfenadine (IC(50) = 77 nM) and O-demethylation of astemizole (K(i) = 20 nM), and inhibition was mostly competitive. Danazol inhibited CYP2C9, CYP2C8, and CYP2D6 with IC(50) values of 1.44, 1.95, and 2.74 μM, respectively. Amiodarone or astemizole were included in a seven-probe cocktail for cytochrome P450 (P450) drug-interaction screening potential, and astemizole demonstrated a better profile because it did not appreciably interact with other P450 probes. Thus, danazol, amiodarone, and astemizole will facilitate the ability to determine the metabolic role of CYP2J2 in hepatic and extrahepatic tissues.

Epoxyeicosatrienoic acids: formation, metabolism and potential role in tissue physiology and pathophysiology

BACKGROUND

CYP enzymes from the CYP2C and CYP2J subfamilies metabolize arachidonic acid in a regiospecific and stereoselective manner to eight epoxyeicosatrienoic acids (EETs). Various EETs have been detected in the liver, as well as in many extrahepatic tissues, and have been implicated in numerous physiological functions from cell signaling to vasodilation and angiogenesis.

OBJECTIVE

This report reviews the sites of expression and activity of arachidonic acid epoxygenase CYP isoforms, as well as the physiological role and metabolism of EETs in various extrahepatic tissues. Possible functions of EETs in tissue pathophysiology and implications as potential drug targets are also discussed.

METHODS

The most recent primary research literature on EET forming enzymes and the new physiological functions of EETs in various tissues were reviewed.

RESULTS/CONCLUSIONS

Epoxyeicosatrienoic acids are important in maintaining the homeostasis and in responding to stress in various extra hepatic tissues. It is not clear whether these effects are owing to EETs acting on a universal receptor or through a mechanism involving a second messenger. A better understanding of the regulation of EET levels and their mechanism of action on various receptors will accelerate research aiming at developing therapeutic agents that target EET formation or metabolism pathways.

Activity, inhibition, and induction of cytochrome P450 2J2 in adult human primary cardiomyocytes

Cytochrome P450 2J2 plays a significant role in the epoxidation of arachidonic acid to signaling molecules important in cardiovascular events. CYP2J2 also contributes to drug metabolism and is responsible for the intestinal clearance of ebastine. However, the interaction between arachidonic acid metabolism and drug metabolism in cardiac tissue, the main expression site of CYP2J2, has not been examined. Here we investigate an adult-derived human primary cardiac cell line as a suitable model to study metabolic drug interactions (inhibition and induction) of CYP2J2 in cardiac tissue. The primary human cardiomyocyte cell line demonstrated similar mRNA-expression profiles of P450 enzymes to adult human ventricular tissue. CYP2J2 was the dominant isozyme with minor contributions from CYP2D6 and CYP2E1. Both terfenadine and astemizole oxidation were observed in this cell line, whereas midazolam was not metabolized suggesting lack of CYP3A activity. Compared with recombinant CYP2J2, terfenadine was hydroxylated in cardiomyocytes at a similar K(m) value of 1.5 μM. The V(max) of terfenadine hydroxylation in recombinant enzyme was found to be 29.4 pmol/pmol P450 per minute and in the cells 6.0 pmol/pmol P450 per minute. CYP2J2 activity in the cell line was inhibited by danazol, astemizole, and ketoconazole in submicromolar range, but also by xenobiotics known to cause cardiac adverse effects. Of the 14 compounds tested for CYP2J2 induction, only rosiglitazone increased mRNA expression, by 1.8-fold. This cell model can be a useful in vitro model to investigate the role of CYP2J2-mediated drug metabolism, arachidonic acid metabolism, and their association to drug induced cardiotoxicity.

A screening study of drug-drug interactions in cerivastatin users: an adverse effect of clopidogrel

An analysis of a case-control study of rhabdomyolysis was conducted to screen for previously unrecognized cytochrome P450 enzyme (CYP) 2C8 inhibitors that may cause other clinically important drug-drug interactions. Medication use in cases of rhabdomyolysis using cerivastatin (n = 72) was compared with that in controls using atorvastatin (n = 287) for the period 1998-2001. The use of clopidogrel was strongly associated with rhabdomyolysis (odds ratio (OR) 29.6; 95% confidence interval (CI), 6.1-143). In a replication effort that used the US Food and Drug Administration (FDA) Adverse Event Reporting System (AERS), it was found that clopidogrel was used more commonly in patients with rhabdomyolysis receiving cerivastatin (17%) than in those receiving atorvastatin (0%, OR infinity; 95% CI = 5.2-infinity). Several medications were tested in vitro for their potential to cause drug-drug interactions. Clopidogrel, rosiglitazone, and montelukast were the most potent inhibitors of cerivastatin metabolism. Clopidogrel and its metabolites also inhibited cerivastatin metabolism in human hepatocytes. These epidemiological and in vitro findings suggest that clopidogrel may cause clinically important, dose-dependent drug-drug interactions with other medications metabolized by CYP2C8.

Investigating the contribution of CYP2J2 to ritonavir metabolism in vitro and in vivo

Ritonavir, an HIV protease inhibitor, is successfully used for the prevention and treatment of HIV infections. Ritonavir pharmacokinetics are complicated by inhibition, induction and pharmacogenetics of cytochrome P450 (CYP) enzymes mediating its clearance. This investigation revealed that CYP2J2, along with CYP3A4/5 and CYP2D6, efficiently metabolizes ritonavir, and to a CYP2J2-specific (minor) metabolite. Chemical inhibition of ritonavir metabolism, clearance, KI/kinact and abundance of CYP2J2 in liver microsomes were evaluated and then applied to an in vitro-in vivo static scaling model to estimate the contribution of each isozyme, as a function of CYP abundance, activity, and genotype. Disposition of the CYP2J2-specific metabolite was also evaluated in vivo. In plasma, metabolite abundance was well above previously reported levels with circulating concentrations measured at 2 μM for the main hydroxylisopropyl metabolite. Ritonavir and metabolite plasma profiles were simulated using Simcyp(®). A modest (2-6%) contribution of CYP2J2 to ritonavir clearance is predicted which increases to more than 20% in subjects carrying CYP2D6 poor metabolizer polymorphisms and CYP3A4 irreversible inhibition. These results indicate that minor drug metabolizing enzymes could become quantitatively important in RTV clearance if main metabolic pathways are impeded.

Bioconversion of (+)-valencene in submerged cultures of the ascomycete Chaetomium globosum

Submerged cultures of the ascomycete Chaetomium globosum oxidised the exogenous sesquiterpene (+)-valencene to nootkatone via the stereoselective generation of alpha-nootkatol. Inhibition experiments suggested that the first introduction of oxygen, the rate-limiting step of the bioconversion, may have been catalysed by a cytochrome-P450-monooxygenase. However, nootkatone was not the final metabolite: further flavour-active and inactive, non-volatile oxidation products were identified. (+)-Valencene and the flavour-active mono-oxyfunctionalised transformation products, alpha-nootkatol, nootkatone, and valencene-11,12-epoxide accumulated preferably inside the fungal cells. Di- and poly-oxygenated products, such as nootkatone-11,12-epoxide, were found solely in the culture medium, indicating an active transport of these metabolites into the extracellular compartment during (+)-valencene detoxification. These metabolic properties may have contributed to the high tolerance of the fungus towards the exogenous hydrocarbon.

Drug metabolism by CYP2C8.3 is determined by substrate dependent interactions with cytochrome P450 reductase and cytochrome b5

Genetic polymorphisms in CYP2C8 can influence the metabolism of important therapeutic agents and cause interindividual variation in drug response and toxicity. The significance of the variant CYP2C8*3 has been controversial with reports of higher in vivo but lower in vitro activity compared to CYP2C8*1. In this study, the contribution of the redox partners cytochrome P450 reductase (CPR) and cytochrome b5 to the substrate dependent activity of CYP2C8.3 (R139K, K399R) was investigated in human liver microsomes (HLMs) and Escherichia coli expressed recombinant CYP2C8 proteins using amodiaquine, paclitaxel, rosiglitazone and cerivastatin as probe substrates. For recombinant CYP2C8.3, clearance values were two- to five-fold higher compared to CYP2C8.1. CYP2C8.3's higher k(cat) seems to be dominated by a higher, but substrate specific affinity, towards cytochrome b5 and CPR (K(D) and K(m,red)) which resulted in increased reaction coupling. A stronger binding affinity of ligands to CYP2C8.3, based on a two site binding model, in conjunction with a five fold increase in amplitude of heme spin change during binding of ligands and redox partners could potentially contribute to a higher k(cat). In HLMs, carriers of the CYP2C8*1/*3 genotype were as active as CYP2C8*1/*1 towards the CYP2C8 specific reaction amodiaquine N-deethylation. Large excess of cytochrome b5 compared to CYP2C8 in recombinant systems and HLMs inhibited metabolic clearance, diminishing the difference in k(cat) between the two enzymes, and may provide an explanation for the discrepancy to in vivo data. In silico studies illustrate the genetic differences between wild type and variant on the molecular level.

Discovery of potent KIFC1 inhibitors using a method of integrated high-throughput synthesis and screening

KIFC1 (HSET), a member of the kinesin-14 family of motor proteins, plays an essential role in centrosomal bundling in cancer cells, but its function is not required for normal diploid cell division. To explore the potential of KIFC1 as a therapeutic target for human cancers, a series of potent KIFC1 inhibitors featuring a phenylalanine scaffold was developed from hits identified through high-throughput screening (HTS). Optimization of the initial hits combined both design-synthesis-test cycles and an integrated high-throughput synthesis and biochemical screening method. An important aspect of this integrated method was the utilization of DMSO stock solutions of compounds registered in the corporate compound collection as synthetic reactants. Using this method, over 1500 compounds selected for structural diversity were quickly assembled in assay-ready 384-well plates and were directly tested after the necessary dilutions. Our efforts led to the discovery of a potent KIFC1 inhibitor, AZ82, which demonstrated the desired centrosome declustering mode of action in cell studies.

Regio- and stereoselective fungal oxyfunctionalisation of limonenes

Selective transformations of limonene by asco- and basidiomycetes were investigated. On the shake flask scale, Penicillium citrinum hydrated R-(+)-limonene to a-terpineol [83% regioselectivity (rs), more than 80 mg l(-1) product yield], and Gongronella butleri catalysed the terminal oxidation to yield perillyl alcohol (60% rs, 16 mg l(-1)). On the laboratory bioreactor scale, Penicillium digitatum produced a peak concentration of 506 mg a-terpineol l(-1) in the fed-batch mode, equivalent to a theoretical yield of 67%, and no volatile by-products were found. Fusarium proliferatum transformed R-(+)-limonene enantiospecifically to cis-(+)-carveol (98.6% ee, more than 35 mg l(-1) product yield) and S-(-)-limonene predominantly to trans-(-)-carveol (96.3% ee). Pleurotus sapidus selectively dehydrogenised the accumulating trans-(-)-carveol to the corresponding enantiopure R-(-)-carvone. The results show that a careful selection of strain and bioprocess parameters may improve both the yield and the optical purity of a desired product.

Extra-hepatic isozymes from the CYP1 and CYP2 families as potential chemotherapeutic targets

Cytochrome P450 isozymes (CYPs) from the CYP1 and CYP2 families located primarily in extra-hepatic tissues represent ideal candidates for chemotherapeutic drug development because: 1.) They are usually involved in the metabolism of endogenous substrates that are important for cell homeostasis and growth 2.) The over-expression of certain CYPs has been reported in various malignancies 3.) There has been much clinical success with inhibitors of CYPs involved in hormone synthesis. The most ideal candidates for chemotherapeutic drug development will be discussed in terms of their biological importance and relevant substrates. This review will focus on: 1.) CYP1A1 and CYP1B1 from the CYP1 family because of the dual role these enzymes play in the bioactivation of known carcinogens and endogenous compounds. 2.) The targeting of CYPs in hypoxic environments as a therapeutic strategy. 3.) CYP2J2 and its role in the metabolism of arachidonic acid to epoxyeicosatrienoic acids and angiogenesis will also be examined. While much progress has been made towards understanding the role of CYPs in extrahepatic tissue, future studies focused on the development of selective inhibitors coupled with appropriate delivery systems that would target the tumor micro-environments could lead to significant advancement in chemotherapeutic strategies.

Protein tyrosine nitration of mitochondrial carbamoyl phosphate synthetase 1 and its functional consequences

Mitochondria are the primary locus for the generation of reactive nitrogen species including peroxynitrite and subsequent protein tyrosine nitration. Protein tyrosine nitration may have important functional and biological consequences such as alteration of enzyme catalytic activity. In the present study, mouse liver mitochondria were incubated with peroxynitrite, and the mitochondrial proteins were separated by 1D and 2D gel electrophoresis. Nitrotyrosinylated proteins were detected with an anti-nitrotyrosine antibody. One of the major proteins nitrated by peroxynitrite was carbamoyl phosphate synthetase 1 (CPS1) as identified by LC-MS protein analysis and Western blotting. The band intensity of nitration normalized to CPS1 was increased in a peroxynitrite concentration-dependent manner. In addition, CPS1 activity was decreased by treatment with peroxynitrite in a peroxynitrite concentration- and time-dependent manner. The decreased CPS1 activity was not recovered by treatment with reduced glutathione, suggesting that the decrease of the CPS1 activity is due to tyrosine nitration rather than cysteine oxidation. LC-MS analysis of in-gel digested samples, and a Popitam-based modification search located 5 out of 36 tyrosine residues in CPS1 that were nitrated. Taken together with previous findings regarding CPS1 structure and function, homology modeling of mouse CPS1 suggested that nitration at Y1450 in an α-helix of allosteric domain prevents activation of CPS1 by its activator, N-acetyl-l-glutamate. In conclusion, this study demonstrated the tyrosine nitration of CPS1 by peroxynitrite and its functional consequence. Since CPS1 is responsible for ammonia removal in the urea cycle, nitration of CPS1 with attenuated function might be involved in some diseases and drug-induced toxicities associated with mitochondrial dysfunction.

Synthesis and In Vitro Evaluation of Taxol oxetane ring D precursors

A series of potential taxoid substrates was prepared in radiolabelled form to probe in vitro for the oxirane formation step and subsequent ring expansion step to the oxetane (ring D) presumably involved in the biosynthesis of the anticancer agent Taxol. None of the taxoid test substrates underwent transformation in cell-free systems from Taxus suggesting that these surrogates bore substitution patterns inappropriate for recognition or catalysis by the target enzymes, or that taxoid oxiranes and oxetanes arise by independent biosynthetic pathways.

Cytochrome P450BM-3 reduces aldehydes to alcohols through a direct hydride transfer

Cytochrome P450BM-3 catalyzed the reduction of lipophilic aldehydes to alcohols efficiently. A k(cat) of ∼25 min(-1) was obtained for the reduction of methoxy benzaldehyde with wild type P450BM-3 protein which was higher than in the isolated reductase domain (BMR) alone and increased in specific P450-domain variants. The reduction was caused by a direct hydride transfer from preferentially R-NADP(2)H to the carbonyl moiety of the substrate. Weak substrate-P450-binding of the aldehyde, turnover with the reductase domain alone, a deuterium incorporation in the product from NADP(2)H but not D(2)O, and no inhibition by imidazole suggests the reductase domain of P450BM-3 as the potential catalytic site. However, increased aldehyde reduction by P450 domain variants (P450BM-3 F87A T268A) may involve allosteric or redox mechanistic interactions between heme and reductase domains. This is a novel reduction of aldehydes by P450BM-3 involving a direct hydride transfer and could have implications for the metabolism of endogenous substrates or xenobiotics.