N-nitrosodiethylamine and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone induced morphological transformation of C3H/10T1/2CL8 cells expressing human cytochrome P450 2A6

Malignant transformation of human bronchial epithelial cells with the tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

Sufficient evidence has demonstrated that cigarette smoking is causally associated with various types of human cancers. In the United States, about 90% of deaths from lung cancer among men and 79% of those among women are associated with smoking. Tobacco-specific nitrosamines are formed from nicotine and related tobacco alkaloids and are the most carcinogenic compounds of tobacco smoke. The most potent N-nitrosamine contained in tobacco smoke is 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, NNK. In the our study, the oncogenic transforming effects of graded doses of NNK were examined using papillomavirus-immortalized human bronchial epithelial cells. Growth kinetics, saturation density, resistance to serum-induced terminal differentiation, anchorage independent growth and tumorigenicity in nude mice were used to investigate the various stages of transformation in bronchial epithelial cells. We show here that immortalized human bronchial epithelial cells in culture can be malignantly transformed by treatment with NNK (100 microg/ml or 400 microg/ml) for 7 days. Transformed cells produced progressively growing subcutaneous tumors upon inoculation into nude mice. Immunofluorescence staining for keratin expression confirmed the epithelial nature of the tumor cells. Increased expression of p16, beta-catenin and PCNA in the established cell lines were detected by immunofluorescence staining and quantified by confocal microscopy. These data suggested that NNK can induce malignant transformation of human bronchial epithelial cells, and the tumor cell lines established are useful models in investigating the carcinogenic mechanism(s) of NNK.

Genetic variation in CYP2A6-mediated nicotine metabolism alters smoking behavior

Approximately 50% of the initiation of tobacco dependence is genetically influenced, whereas maintenance of dependent smoking behavior and amount smoked have approximately 70% genetic contribution (1-5). Determining the variation in nicotine's inactivation is important because of nicotine's role in producing tobacco dependence and regulating smoking patterns (6-11). The genetically polymorphic CYP2A6 enzyme is responsible for the majority of the metabolic inactivation of nicotine to cotinine (12-14). Both in vitro and in vivo studies have demonstrated considerable interindividual variation in CYP2A6 activity (15-17). CYP2A6 is genetically polymorphic, individuals carrying inactive CYP2A6 alleles have decreased nicotine metabolism, are less likely to become smokers and if they do, they smoke fewer cigarettes per day (13,18,19). The decrease in smoking behavior was confirmed by measuring carbon monoxide (CO, a measure of smoke inhalation) levels, plasma and urine nicotine and cotinine levels, and cigarette counts (13,18,19). A duplication variant in the CYP2A6 gene locus has been identified which increases nicotine inactivation and increases smoking (19). CYP2A6 can also activate tobacco smoke procarcinogens (e.g. NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone); current studies are investigating the role of CYP2A6 in risk for lung cancer. Based on these epidemiologic data it was postulated that inhibition of CYP2A6 activity might be useful in a therapeutic context. Kinetic studies in humans indicated that selective CYP2A6 inhibitors decrease the metabolic removal of nicotine. It was also shown that inhibiting CYP2A6 in vivo (phenocopying, or mimicking the genetic defect) in smokers results in decreased smoking, making nicotine orally bioavailable, and the rerouting of procarcinogens to detoxifying pathways (20-22).

Molecular epidemiology of lung cancer

Lung cancer occurs through a complex multistage process that results from the combination of carcinogen exposure and genetic susceptibilities. The primary etiology of lung cancer is tobacco smoking, but an understanding of why some smokers develop lung cancer, and others do not, remains unclear. Current studies focus on genetic susceptibilities to lung cancer, and how they modify the effects of tobacco smoke carcinogens. New assays are being developed to study other contributors to cancer risk, such as interindividual differences in DNA repair. There is current evidence to suggest that the risk of lung cancer for women, compared to men, is higher for the same level of smoking. Several biological differences for the types of lung cancer have been observed in women and men. Also, there appear to be differences in lung cancer between Caucasians and African-Americans. Molecular epidemiology tools are uniquely suited to study these biological differences. These studies will improve cancer risk assessments and focus cancer prevention strategies. Other studies also are focusing on tobacco addiction, in order to lead to improved smoking cessation strategies.

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.

Recent studies on mechanisms of bioactivation and detoxification of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific lung carcinogen

This article reviews recent advances in the biochemistry and molecular biology of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific pulmonary carcinogen believed to be involved in the induction of lung cancer in smokers. Several aspects of NNK bioactivation are addressed, including identification of its metabolites in laboratory animals and humans, cytochrome P450 enzyme involvement in its metabolic activation, DNA and protein adduct formation, biological significance of the major DNA adducts formed, and mutations in oncogenes from tumors induced by NNK. Collectively, the presently available data provide a reasonably clear picture of NNK bioactivation in rodents, although there are still important gaps in our mechanistic understanding of NNK-induced tumorigenesis. The studies in rodents and primates have facilitated development of methods to assess NNK bioactivation in humans, which will be applicable to studies of lung cancer susceptibility and prevention.

4-Ipomeanol and 2-aminoanthracene cytotoxicity in C3H/10T1/2 cells expressing rabbit cytochrome P450 4B1

In the present study, retroviral vectors were used to stably transfer and express the cDNA encoding rabbit CYP4B1 in mouse C3H/10T1/2 cells. The replication defective retroviral vector was packaged in the ecotropic packaging cell line, GP+E-86, with infectious titer of approximately 1 x 10(6) cfu/mL. Infection, followed by selection with G418, showed an infection efficiency of approximately 70% for the recipient C3H/10T1/2 cells. Analysis of ten G418 resistant clones showed that the number of vector inserts ranged from 4 to 13 copies per cell genome. Each clone was positive for microsomal CYP4B1 protein as determined by immunoblotting. Cytochrome P450 4B1 activity was assessed by the cytotoxicity of 4-ipomeanol, a known substrate for P450 4B1 and a model compound for chemical-induced injury to the lung. The initial clonigenic assays showed that 100% toxicity occurred in all the clones after a 96-hr exposure to 250 microM 4-ipomeanol. Parental C3H/10T1/2 cells were resistant to 4-ipomeanol at concentrations as high as 1 mM. Two clones, designated No. 2 and No. 19, differing in levels of P450 4B1 protein, were characterized further for 4-ipomeanol and other chemical toxicities. A concentration-response study indicated 50% cytotoxicity at 4-ipomeanol concentrations of 1.5 micrograms/mL for clone No. 2 and 2.5 micrograms/mL for clone No. 19. A panel of agents representing the aromatic amines, some of which are known or suspected P450 4B1 substrates, were tested for cytotoxicity in clone No. 2. These agents included 2-aminoanthracene, 2-aminonaphthalene, 2-aminofluorene, 2-acetylaminofluorene and 4-aminobiphenyl. Only 2-aminoanthracene gave a clear cytotoxic response reducing the survival fraction of clone No. 2 to 50% at 0.2 micrograms/mL while affecting parental cells minimally. In vitro expression of CYP4B1 provides a new experimental system for further elucidating the cytotoxic and mutagenic effects of P450 4B1 substrates.

Alcohol-related cancer risk: a toxicokinetic hypothesis

Consumption of alcoholic beverages is an accepted social custom world-wide. This makes its involvement in events contributing to human cancer risk very important. Although it is neither tumorigenic nor genotoxic in animals, ethanol can potentiate the carcinogenic risk associated with certain environmentally present agents. The reasons for such a synergistic action are speculative, but among theories postulated may be ethanol's ability to modify the toxicokinetics/dynamics of carcinogen metabolism. Experiments conducted with rodents and primates support this hypothesis, demonstrating increased exposure of posthepatic organs to nitrosamines when given in combination with ethanol, followed by enhancement of DNA adduct formation and, at least in rodents, of tumor development. In addition, ethanol may induce enzymes responsible for carcinogen activation, including hepatic cytochrome P450 2E1 in rodents and humans, and in lung, kidney, and brain in rodents. Studies have also shown that these effects can extend to the next generation via maternal and in utero fetal exposure. What impact such ethanol-induced modulations have on tumorigenesis during childhood and later stages of life needs to be investigated further.