Kinetic analysis of the activation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone by heterologously expressed human P450 enzymes and the effect of P450-specific chemical inhibitors on this activation in human liver microsomes

A high-level expression of CYP2A in the lung of the suncus (Suncus murinus) and its role in the activation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

Northern blot analysis of mRNA prepared from the lung of Suncus murinus (suncus), which was classified as an ancestor of primates, revealed that the expression level of cytochrome P450 2A (CYP2A) mRNA was about 100-fold higher than in the lung from rats and mice. To confirm that the pulmonary CYP2A of the suncus had a catalytic activity, the metabolism of a specific substrate for CYP2A6, (+)-cis-3,5-dimethyl-2-(3-pyridyl) thiazolidin-4-one hydrochloride (SM-12502), was determined. The intrinsic clearance for SM-12502 S-oxidation by the suncus lung microsomes was calculated to be 99-fold higher than that by rat liver microsomes. The mutagen-producing activity of a 9,000 g supernatant fraction prepared from suncus lung was examined using 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) as a promutagen. The results showed that the suncus lung possessed 82-fold higher mutagen-producing activity than the rat lung, indicating that NNK was efficiently activated by the CYP2A isoform expressed in the suncus lung and that the suncus was a sensitive animal species to the genotoxicity of NNK contained in tobacco smoke.

Tobacco smoke carcinogens and lung cancer

The complexity of tobacco smoke leads to some confusion about the mechanisms by which it causes lung cancer. Among the multiple components of tobacco smoke, 20 carcinogens convincingly cause lung tumors in laboratory animals or humans and are, therefore, likely to be involved in lung cancer induction. Of these, polycyclic aromatic hydrocarbons and the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone are likely to play major roles. This review focuses on carcinogens in tobacco smoke as a means of simplifying and clarifying the relevant information that provides a mechanistic framework linking nicotine addiction with lung cancer through exposure to such compounds. Included is a discussion of the mechanisms by which tobacco smoke carcinogens interact with DNA and cause genetic changes--mechanisms that are reasonably well understood--and the less well defined relationship between exposure to specific tobacco smoke carcinogens and mutations in oncogenes and tumor suppressor genes. Molecular epidemiologic studies of gene-carcinogen interactions and lung cancer--an approach that has not yet reached its full potential--are also discussed, as are inhalation studies of tobacco smoke in laboratory animals and the potential role of free radicals and oxidative damage in tobacco-associated carcinogenesis. By focusing in this review on several important carcinogens in tobacco smoke, the complexities in understanding tobacco-induced cancer can be reduced, and new approaches for lung cancer prevention can be envisioned.

Characterization of multiple products of cytochrome P450 2A6-catalyzed cotinine metabolism

The primary metabolite of nicotine in smokers is cotinine. Cotinine is further metabolized to trans-3'-hydroxycotinine, the major urinary metabolite of nicotine in tobacco users. It was recently reported that cytochrome P450 2A6 catalyzes the conversion of cotinine to trans-3'-hydroxycotinine. In this work, we report that P450 2A6 metabolizes cotinine not only to trans-3'-hydroxycotinine but also to 5'-hydroxycotinine, norcotinine, and a fourth as yet unidentified metabolite. The products of baculovirus-expressed P450 2A6 [methyl-(3)H]cotinine metabolism were analyzed by radioflow HPLC. Three (3)H-labeled metabolites were detected and were present in approximately equal amounts. The identities of two of the metabolites were confirmed to be 5'-hydroxycotinine and trans-3'-hydroxycotinine by LC/MS/MS and LC/MS analysis and comparison to standards. The third product was not identified. A fourth product of P450 2A6-catalyzed cotinine metabolism was detected by LC/MS. It was identified by cochromatography with a standard and MS and MS/MS data to be norcotinine. An attempt was made to further characterize the unidentified (3)H-labeled metabolite by comparison to the cotinine metabolites generated by hamster liver microsomes. Hamster liver microsomes contain a P450, 2A8, which is closely related to P450 2A6, and have previously been shown to metabolize cotinine to three hydroxylated products, trans-3'-hydroxycotinine, 5'-hydroxycotinine, and N-(hydroxymethyl)norcotinine. We were unable to confirm that N-(hydroxymethyl)norcotinine was the unidentified cotinine metabolite generated by P450 2A6.

Genetically engineered cells stably expressing cytochrome P450 and their application to mutagen assays

Genetically engineered cells transiently and stably expressing cytochrome P450 (P450), a key enzyme for biotransformation of a wide variety of compounds, have provided new tools for investigation of P450 functions such as P450-mediated metabolic activation of chemicals. This review will focus on the development of mammalian cell lines stably expressing P450s and application to toxicology testings. Stable expression systems have an advantage over transient ones in that a series of the process from metabolic activation of test compounds to the appearance of toxicological consequences occurs entirely in the same intact cells. Indeed, many cell lines stably expressing a single form of mammalian P450 have been established so far and applied to cytotoxic or genotoxic assays, the endpoints of which contained mutations at hprt and other gene loci, chromosomal aberrations, sister chromatid exchanges, micronuclei, morphological transformation, and 32P-postlabeling. Analyses of metabolites of toxic substances have also been carried out, using the intact cells or microsomal fractions prepared from the cells. The stable expression systems clearly indicate the form of P450 enzyme capable of activating a certain chemical. More recently, coexpression of P450 together with other components of microsomal electron transfer systems such as NADPH-cytochrome P450 reductase has been successfully performed to increase the metabolic capacity of the heterologously expressed P450. In addition, to reconstruct the entire metabolic activation system for certain heterocyclic amines, cell lines which simultaneously express a form of human P450 and a phase II enzyme, N-acetyltransferase, were established. These cells were highly sensitive to some carcinogenic heterocyclic amines. In genetic toxicology, such a coexpression system for two or more enzymes will provide useful materials which mimic in vivo activation systems.

Tobacco and cancer: approaches using carcinogen biomarkers and chemoprevention

Tobacco products are the leading cause of avoidable cancer death in the U.S., accounting for approximately 30% of all cancer deaths. While avoidance of tobacco and smoking cessation are clearly the best way to decrease tobacco-related cancer, these approaches have not been uniformly successful. Approximately 25% of the U.S. population over 18 years of age smokes cigarettes, while 6% use smokeless tobacco products; these figures have not changed markedly in recent years. Our approach toward the tobacco and cancer problem is based on an understanding of the carcinogens in tobacco smoke. These carcinogens form the link between nicotine addiction and cancer. In this paper, two strategies for cancer prevention--the development of carcinogen-derived biomarkers and chemopreventive agents--are discussed. Carcinogen-derived biomarkers can provide specific information on individual metabolic activation and detoxification of tobacco carcinogens. This information can be used to assess individual risk for cancer development upon exposure to tobacco products. Chemopreventive agents can be targeted against the important carcinogens in tobacco smoke. Isothiocyanates, strong inhibitors of lung cancer development by the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, are discussed as an example of this approach.

Competitive interactions between cytochromes P450 2A6 and 2E1 for NADPH-cytochrome P450 oxidoreductase in the microsomal membranes produced by a baculovirus expression system

The present study investigated the interactions between cytochrome P450 (P450) enzymes and the NADPH:cytochrome oxidoreductase (OR) in the microsomal membrane. Microsomes containing human cytochrome P450 2A6 (h2A6) coexpressed with human OR (hOR) via a baculovirus expression system displayed coumarin hydroxylase activity with apparent Km and Vmax values of 0.41 microM and 4.05 nmol/min/nmol P450, respectively. Incorporation of purified rat liver cytochrome b5 (b5) into the microsomes increased the Vmax 2.5-fold, but did not affect the Km. The N-nitrosodimethylamine (NDMA) demethylase activity of human cytochrome P450 2E1 (h2E1) coexpressed similarly was characterized previously. Coumarin was shown not to be a substrate nor an inhibitor of h2E1, and NDMA was not a substrate nor an inhibitor of h2A6. In microsomes containing h2A6, h2E1, and hOR (M-h2A6-h2E1-hOR) obtained from a triple expression system, the two P450 enzymes were shown to compete with each other for interaction with hOR. In incubations with M-h2A6-h2E1-hOR, the presence of a h2A6 substrate (coumarin) decreased NDMA demethylase activity by a maximum of 47%, and the presence of a h2E1 substrate (NDMA) decreased coumarin hydroxylase activity by a maximum of 19%. This substrate-induced competition between h2A6 and h2E1 was decreased by the addition of purified b5. In the absence of a substrate, the NADPH-dependent H2O2 formation was high in both M-h2A6-h2E1-hOR and M-h2E1-hOR, but low in M-h2A6-hOR. The addition of NDMA had little effect on the H2O2 formation in M-h2A6-h2E1-hOR and M-h2E1-hOR. The addition of coumarin, however, slightly decreased H2O2 formation in M-h2A6-h2E1-hOR, but drastically increased H2O2 formation in M-h2A6-hOR. These results suggest that the presence of a h2A6 substrate decreased the electron flow to h2E1 in M-h2A6-h2E1-hOR. The activities of coumarin hydroxylase and NDMA demethylase of M-h2A6-h2E1-hOR were decreased and increased, respectively, by an increase in ionic strength. The ionic strength, however, did not drastically change the substrate-induced competition between h2A6 and h2E1 for hOR. The results demonstrate the usefulness of the coexpression system for mechanistic studies and illustrate that the interaction of monooxygenase enzymes in the microsomal membrane is regulated by the presence of substrates and b5.