Life-span inhalation exposure to mainstream cigarette smoke induces lung cancer in B6C3F1 mice through genetic and epigenetic pathways

Abstract

Although cigarette smoke has been epidemiologically associated with lung cancer in humans for many years, animal models of cigarette smoke-induced lung cancer have been lacking. This study demonstrated that life time whole body exposures of female B6C3F1 mice to mainstream cigarette smoke at 250 mg total particulate matter/m(3) for 6 h per day, 5 days a week induces marked increases in the incidence of focal alveolar hyperplasias, pulmonary adenomas, papillomas and adenocarcinomas. Cigarette smoke-exposed mice (n = 330) had a 10-fold increase in the incidence of hyperplastic lesions, and a 4.6-fold (adenomas and papillomas), 7.25-fold (adenocarcinomas) and 5-fold (metastatic pulmonary adenocarcinomas) increase in primary lung neoplasms compared with sham-exposed mice (n = 326). Activating point mutations in codon 12 of the K-ras gene were identified at a similar rate in tumors from sham-exposed mice (47%) and cigarette smoke-exposed mice (60%). The percentages of transversion and transition mutations were similar in both the groups. Hypermethylation of the death associated protein (DAP)-kinase and retinoic acid receptor (RAR)-beta gene promoters was detected in tumors from both sham- and cigarette smoke-exposed mice, with a tendency towards increased frequency of RAR-beta methylation in the tumors from the cigarette smoke-exposed mice. These results emphasize the importance of the activation of K-ras and silencing of DAP-kinase and RAR-beta in lung cancer development, and confirm the relevance of this mouse model for studying lung tumorigenesis.

Gene Promoter Hypermethylation in Mouse Lung Tumors

“…The etiology for lung cancer in the never-smoker may be due to in part to exposure to environmental tobacco smoke and other environmental and occupational carcinogens (e.g., radon). Although these exposures cannot account for spontaneous tumors in the mouse lung, it is interesting that these tumors develop in part through inactivation of genes, such as ER-a, DAPK, RAR-h, and H-cadherin, which are also silenced in adenocarcinomas from the never-smoker (9,20,30,31).…”

“…Female B6C3F1 mice (6-7 weeks old) were exposed to mainstream cigarette smoke for up to 30 months. Details of the exposure system have been described (9). Briefly, two 70-cm 2 puffs/min from research cigarettes (type 2R1, Tobacco Health Research Institute, Lexington, KY) were generated by smoking machines (type 1300, AMESA Electronics, Geneva, Switzerland).…”

“…A robust mouse model for studying tobacco-induced lung cancer had been lacking until a recent study by Hutt et al (9) showed that lifetime, whole-body exposure of B6C3F1 mice to mainstream cigarette smoke induced neoplasms in f45% of mice. The prevalence for methylation of two genes implicated in human lung cancer, retinoic acid receptor-h (RAR-h) and deathassociated protein kinase (DAPK), was determined in tumors induced by cigarette smoke, NNK, vinyl carbamate, and methylene chloride (10,11).…”

Carcinogenic Effects of Mixtures of Chemicals

“…Mixtures, such as d ie s e l e m i s s i on s a nd t ob a c c o s m ok e , h a v e b e e n epidemiologically shown to have carcinogenic potential, but because of the difficulty of establishing appropriate methods to represent the human exposure situation, ie., inhalation through the respiratory tract, no experimental proof of lung carcinogenicity of tobacco smoke in animals has been obtained although carcinogenicity of individual components such as benz[a]pyrene, cadmium, 2-naphthylamine and nitrosamines have been individually confirmed by administration intratracheally, orally or subcutaneously. Hutt et al reported in 2005 the first study to demonstrate a robust increase in lung cancer in mice after a lifetime of inhalation exposure to a mainstream of cigarette smoke 27 . Their report clearly demonstrated that an airborne complex mixture to which human beings are exposed through the respiratory tract can be properly studied in experimental animals even though there are some limitations 28 .…”

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone Induces Retinoic Acid Receptor β Hypermethylation through DNA Methyltransferase 1 Accumulation in Esophageal Squamous Epithelial Cells

“…NNK is a key ingredient of tobacco smoke carcinogens. It is reported that NNK exposure can induce promoter hypermethylation of several tumor suppressor genes (TSGs) in lung and liver carcinomas, such as cyclin-dependent kinase inhibitor 2A and RARβ (Pulling et al, 2001;Hutt et al, 2005). Lin et al (2010) reported that NNK can induce TSGs promoter hypermethylation through the accumulation of DNMT1 in mice and human lung cancer.…”