Xenobiotic-metabolizing cytochromes P450 convert prostaglandin endoperoxide to hydroxyheptadecatrienoic acid and the mutagen, malondialdehyde

Inhibitors of cyclo-oxygenase 2: a new class of anticancer agents?

Experimental studies have shown that cyclo-oxygenase 2 (COX2) is involved in tumour development and progression. Selective inhibitors of COX2 (coxibs) block tumour growth through many mechanisms, especially by antiangiogenic and proapoptotic effects. In experimental models, coxibs potentiate the activity of cytotoxic agents, hormones, and radiotherapy. Large clinical studies have shown chemopreventive activity of coxibs in colorectal cancer. The findings of preclinical studies coupled with the overexpression of COX2 observed in advanced human tumours are the basis for new therapeutic anticancer strategies based on combinations of coxibs with other anticancer treatment modalities. Early clinical studies have documented the feasibility, good tolerability, and promising activity of coxibs combined with chemotherapy in patients with advanced colorectal and non-small-cell lung cancers. Here, we describe the recent findings on the antitumour effects of coxibs with particular focus on the opportunities that have emerged for treatment of cancer.

Protection by quercetin against cooking oil fumes-induced DNA damage in human lung adenocarcinoma CL-3 cells: role of COX-2

Epidemiological studies have shown that the incidence of lung cancer was associated with exposure to cooking oil fumes (COF) in nonsmoking Taiwanese women. We suspect that quercetin may be a potent inhibitor for reduction of COF-induced DNA damage and prevention of lung cancer development. Comet assay was used to evaluate the DNA damage induced by a relatively low dose of COF (100 g/ml) in human lung adenocarcinoma CL-3 cells. To understand whether quercetin has the most potent protective effect on COF-induced DNA damage, the 50% inhibition concentration of quercetin for COF-induced DNA damage (IC50) was compared with IC50 values of alpha-naphthoflavone(alpha-NF), NS-398, and NaN3 (specific inhibitors) or scavengers of cytochrome P-450 1A1, cyclooxygenase-2 (COX-2),and reactive oxygen species. The IC50 of quercetin was only 1/2, 1/3, and 1/35 of IC50 values of alpha-NF, NS-398, and NaN3,respectively. Clearly, quercetin was the most effective inhibitor of COF-induced DNA damage, followed sequentially by kappa-NF, NS-398, and NaN3. To further elucidate whether inhibition of COF-induced DNA damage of quercetin is mediated through the inhibition of COX-2 gene expression by altering the nuclear factor-kappaB pathway, COX-2 mRNA and its protein expressions induced by COF were evaluated by reverse transcription-polymerase chain reaction and Western blot, respectively. Our data showed that COX-2 mRNA and protein levels were significantly repressed by addition of quercetin in a dose-dependent manner. Gel retardation assay showed that nuclear factor-kappaB DNA binding activity induced by COF was significantly inhibited by quercetin. From our previous and present studies, it is revealed that coexpression of COX-2 and cytochrome P-450 1A1 caused by COF may contribute to genomic instability in lung cancer development. Thus quercetin may act as a potent chemopreventive agent of lung cancer for nonsmoking Taiwanese women.

Clinical importance of the cytochromes P450

The human cytochrome P450 (CYP) superfamily comprises 57 genes. These genes code for enzymes that can have a role in: metabolism of drugs, foreign chemicals, arachidonic acid and eicosanoids; cholesterol metabolism and bile-acid biosynthesis; steroid synthesis and metabolism; vitamin D(3) synthesis and metabolism; retinoic acid hydroxylation; and those of still unknown function. Cytochrome P450 was once believed to be mainly a hepatic drug detoxication system, but is now understood to include a myriad of enzymic reactions implicated in important life processes. Mutations in many CYP genes cause inborn errors of metabolism and contribute to many clinically relevant diseases.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Cyclo-oxygenase 2: a pharmacological target for the prevention of cancer

Understanding the mechanisms underlying carcinogenesis provides insights that are necessary for the development of therapeutic strategies to prevent cancer. Chemoprevention--the use of drugs or natural substances to inhibit carcinogenesis - is an important and rapidly evolving aspect of cancer research. We discuss evidence that cyclooxygenase 2 (COX 2), an inducible form of the enzyme, is a potential pharmacological target to prevent cancer. Key data implicating a causal relation between increased activity of COX 2 and carcinogenesis and possible mechanisms of action of COX 2 in this context are covered. Importantly, selective COX 2 inhibitors appear to be safe enough in human beings to allow large-scale clinical testing in healthy people. Several chemoprevention trials using selective COX 2 inhibitors are underway.

Oxidation of prostaglandin H(2) and prostaglandin H(2) analogues by human cytochromes P450: analysis of omega-side chain hydroxy metabolites and four steroisomers of 5-hydroxyprostaglandin I(1) by mass spectrometry

The objective was to examine the NADPH-dependent oxygenation of prostaglandin H(2) (PGH(2)) and three PGH(2) analogues, 9,11-diazo-15-deoxy-PGH(2) (U51605), 9,11-epoxymethano-PGH(2) (U44069), and 11,9-epoxymethano-PGH(2) (U46619), by cytochromes P450, and to characterize the metabolites by mass spectrometry. CYP2C19, CYP4A11, CYP4F8, and liver and renal cortical microsomes oxidized the omega-side chain of U44069, U46619, and U51605, whereas only CYP4F8 oxidized the omega-side chain of PGH(2). PGH(2) was transformed to four stereoisomers of 5-hydroxy-PGI(1) by recombinant cytochromes P450. CYP4F8 formed the 5-hydroxy-PGI(1) isomers in small amounts compared to the 19-hydroxy metabolites of PGH(2). Isomers of 5-hydroxy-PGI(1) and 6-keto-PGF(1 alpha) were detectable when PGH(2) decomposed in the presence of hemin, hemoglobin, or heat-inactivated microsomes. 5-Hydroxy-PGI(1) is likely formed from PGH(2) in a pseudo-enzymatic reaction involving homolytic scission of the endoperoxide and formation of an ether between C-9 and C-6 and a carbon-centered radical at C-5, which reacts with molecular oxygen. CYP4F8 catalyzes 19-hydroxylation of PGH(2), but the absolute configuration of the 19-hydroxy group is unknown, whereas human seminal fluid contains (19R)-hydroxy-PGE(2). CYP4F8 was found to metabolize U51605 to 90% of the (19R)-hydroxy metabolite, providing further evidence in favor of a role of CYP4F8 in biosynthesis of (19R)-hydroxy PGE in human seminal vesicles. We conclude that omega-side chain hydroxylation of PGH(2) analogues may be catalyzed by many different cytochromes P450, but only CYP4F8 oxidizes the omega-side chain of PGH(2) efficiently.

COX-2 expression and cell cycle progression in human fibroblasts

Cyclooxygenase-2 (COX-2) is continuously expressed in most cancerous cells where it appears to modulate cellular proliferation and apoptosis. However, little is known about the contribution of transient COX-2 induction to cell cycle progression or programmed cell death in primary cells. In this study we determined whether COX-2 regulates proliferation or apoptosis in human fibroblasts. COX-2 mRNA, protein, and prostaglandin E2 (PGE2) were not detected in quiescent cells but were expressed during the G0/G1 phase of the cell cycle induced by serum. Inhibition of COX-2 did not alter G0/G1 to S phase transition or induce apoptosis at concentrations that diminished PGE2. Addition of interleukin-1beta to serum enhanced COX-2 expression and PGE2 synthesis over that by serum alone but had no effect on the progression of these cells into S phase. Furthermore, platelet-derived growth factor drove the G0 fibroblasts into the cell cycle without inducing detectable levels of COX-2 or PGE2. Collectively, these data show that transient COX-2 expression in primary human fibroblasts does not influence cell cycle progression.

Common and uncommon cytochrome P450 reactions related to metabolism and chemical toxicity

Cytochrome P450 (P450) enzymes catalyze a variety of reactions and convert chemicals to potentially reactive products as well as make compounds less toxic. Most of the P450 reactions are oxidations. The majority of these can be rationalized in the context of an FeO(3+) intermediate and odd electron abstraction/rebound mechanisms; however, other iron-oxygen complexes are possible and alternate chemistries can be considered. Another issue regarding P450-catalyzed reactions is the delineation of rate-limiting steps in the catalytic cycle and the contribution to reaction selectivity. In addition to the rather classical oxidations, P450s also catalyze less generally discussed reactions including reduction, desaturation, ester cleavage, ring expansion, ring formation, aldehyde scission, dehydration, ipso attack, one-electron oxidation, coupling reactions, rearrangement of fatty acid and prostaglandin hydroperoxides, and phospholipase activity. Most of these reactions are rationalized in the context of high-valent iron-oxygen intermediates and Fe(2+) reductions, but others are not and may involve acid-base catalysis. Some of these transformations are involved in the bioactivation and detoxication of xenobiotic chemicals.

Substrate binding is the rate-limiting step in thromboxane synthase catalysis

Thromboxane synthase (TXAS) is a "non-classical" cytochrome P450. Without any need for an external electron donor, or for a reductase or molecular oxygen, it uses prostaglandin H2 (PGH2) to catalyze either an isomerization reaction to form thromboxane A2 (TXA2) or a fragmentation reaction to form 12-l-hydroxy-5,8,10-heptadecatrienoic acid and malondialdehyde (MDA) at a ratio of 1:1:1 (TXA2:heptadecatrienoic acid:MDA). We report here kinetics of TXAS with heme ligands in binding study and with PGH2 in enzymatic study. We determined that 1) binding of U44069, an oxygen-based ligand, is a two-step process; U44069 first binds TXAS, then ligates the heme-iron with a maximal rate constant of 105-130 s(-1); 2) binding of cyanide, a carbon-based ligand, is a one-step process with k(on) of 2.4 M(-1) s(-1) and k(off) of 0.112 s(-1); and 3) both imidazole and clotrimazole (nitrogen-based ligands) bind TXAS in a two-step process; an initial binding to the heme-iron with on-rate constants of 8.4 x 10(4) M(-1) s(-1) and 1.5 x 10(5) M(-1) s(-1) for imidazole and clotrimazole, respectively, followed by a slow conformational change with off-rate constants of 8.8 s(-1) and 0.53 s(-1), respectively. The results of our binding study indicate that the TXAS active site is hydrophobic and spacious. In addition, steady-state kinetic study revealed that TXAS consumed PGH2 at a rate of 3,800 min(-1) and that the k(cat)/K(m) for PGH2 consumption was 3 x 10(6) M(-1) s(-1). Based on these data, TXAS appears to be a very efficient catalyst. Surprisingly, rapid-scan stopped-flow experiments revealed marginal absorbance changes upon mixing TXAS with PGH2, indicating minimal accumulation of any heme-derived intermediates. Freeze-quench EPR measurements for the same reaction showed minimal change of heme redox state. Further kinetic analysis using a combination of rapid-mixing chemical quench and computer simulation showed that the kinetic parameters of TXAS-catalyzed reaction are: PGH2 bound TXAS at a rate of 1.2-2.0 x 10(7) M(-1) s(-1); the rate of catalytic conversion of PGH2 to TXA2 or MDA was at least 15,000 s(-1) and the lower limit of the rates for products release was 4,000-6,000 s(-1). Given that the cellular PGH2 concentration is quite low, we concluded that under physiological conditions, the substrate-binding step is the rate-limiting step of the TXAS-catalyzed reaction, in sharp contrast with "classical" P450 enzymes.

Selective augmentation of prostacyclin production by combined prostacyclin synthase and cyclooxygenase-1 gene transfer

BACKGROUND

We tested the hypothesis that combined cyclooxygenase-1 (COX-1) and prostacyclin synthase (PGIS) gene transfer selectively augments prostacyclin production without a concurrent overproduction of other prostanoids.

METHODS AND RESULTS

ECV304 cells were transfected with bicistronic pCOX-1/PGIS versus pCOX-1 or pPGIS, and prostanoids were analyzed. Contrary to the high prostaglandin E2 synthesis in pCOX-1 transfected cells, selective prostacyclin formation was noted with bicistronic plasmid transfection. Next, we determined the optimal ratio of Ad-COX-1 to Ad-PGIS by transfecting human umbilical vein endothelial cells with various titers of these 2 adenoviral constructs and determined the level of protein expression and prostanoid synthesis. Our results show that optimal ratios of adenoviral titers to achieve a large prostacyclin augmentation without overproduction of prostaglandin E2 or F2alpha were 50 to 100 plaque forming units (pfu) of Ad-COX-1 to 50 pfu of Ad-PGIS per cell. A higher Ad-PGIS to Ad-COX-1 ratio caused a paradoxical decline in prostacyclin synthesis.

CONCLUSIONS

Prostacyclin synthesis can be selectively augmented by cotransfecting endothelial cells with an optimal ratio of COX-1 to PGIS. Combined COX-1 and PGIS gene transfer has the potential for therapeutic augmentation of prostacyclin.

Cell cycle-dependent expression of cyclooxygenase-2 in human fibroblasts

The purpose of this investigation is to determine whether the levels of cyclooxygenase-2 (COX-2) expression are cell cycle dependent. We used a serum-starved human foreskin fibroblast model to determine changes in COX-2 mRNA, protein, and promoter activity in response to stimulation with interleukin-1b (IL-1b) and phorbol 12-myristate 13-acetate (PMA) at G0, G1, S and G2/M phases of the cell cycle. IL-1b (1 ng/ml) and PMA (100 nM) induced robust COX-2 expression in the G0 cells, and the level of COX-2 expression declined progressively after the cells had entered the cell cycle. The COX-2 mRNA level at G1, S and G2/M phases of the cell cycle was 76%, 46%, and 30% of that at G0, respectively. A 5-flanking promoter fragment of COX-2 constructed into a luciferase expression vector was transfected into cells. The promoter activity in response to PMA stimulation was significantly higher in G0 than in S phase cells. These results imply that G0 cells are the key players in inflammation and other COX-2-dependent pathophysiological processes. When the cells are in the proliferative phase, COX-2 inducibility becomes restrained probably by an endogenous control mechanism to avoid COX-2 mediated oxidative DNA damage.

Reactivity and mutagenicity of endogenous DNA oxopropenylating agents: base propenals, malondialdehyde, and N(epsilon)-oxopropenyllysine

Malondialdehyde (MDA), a mutagenic product of lipid peroxidation, reacts with DNA to form the premutagenic lesion, pyrimido[1, 2-a]purin-10(3H)-one (M(1)G). M(1)G is present in normal human tissues, but the contribution of other endogenously produced MDA analogues is poorly understood. Oxidation of the DNA backbone can cause strand breaks and release base propenals, and MDA condensation with proteins yields N(epsilon)-oxopropenyllysine. Here we compare the M(1)G-forming ability and Salmonella typhimurium mutagenicity of MDA with adenine, thymine, and cytosine propenals and N(alpha)-acetyl-N(epsilon)-oxopropenyllysine methyl ester. Base propenals are 30-150 times more potent than MDA in M(1)G formation and are 30-60 times more mutagenic than MDA. In addition, the Fe-bleomycin complex, which generates base propenals, induced M(1)G, but gamma-radiation, which generates mostly MDA, did not. M(1)G formation by MDA and base propenals was concentration-dependent, time-dependent, and enhanced by acidic conditions. N(alpha)-Acetyl-N(epsilon)-oxopropenyllysine methyl ester was less reactive and less mutagenic than MDA. These differences in potency are consistent with differences in leaving group ability. This work supports a role for other MDA analogues, especially base propenals, in the formation of endogenous M(1)G adducts.