Role of the alveolar type II cell in the development and progression of pulmonary tumors induced by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone in the A/J mouse

Abstract

The role of the type II cell in the development of pulmonary tumors induced in the adult A/J mouse (6 weeks of age) by treatment with a single dose (100 mg/kg, i.p.) of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) was investigated. Twenty-four h following treatment with NNK, the concentration of O6-methylguanine was similar in Clara and type II cells. However, hyperplasias were detected only along the alveolar septa in lungs 14 weeks after carcinogen treatment. Examination of the ultrastructure of several hyperplasias revealed that the proliferating cells resembled type II pneumocytes. The proliferating cells were cuboidal in shape, with centrally localized ovoid nuclei characterized by minor indentations. Lamellar bodies, one of the major hallmarks of the type II cell, were present in the cytoplasm. The progression of pulmonary lesions was followed by sacrificing mice at 4-week intervals from 14 to 54 weeks after treatment with NNK. From 34 to 42 weeks after treatment, progression to neoplasia was demonstrated by a decline in the frequency of hyperplasias and an increase in the frequency of adenomas. Approximately 50% of the adenomas were observed arising within hyperplasias. Carcinomas appeared to increase in frequency 34 weeks after carcinogen treatment and comprised greater than 50% of the pulmonary lesions by 54 weeks. Approximately 30% of the carcinomas were observed arising within adenomas. The growth pattern of carcinomas began to change from solid to mixed (solid and papillary) 42 weeks after NNK. Moreover, electron micrographic analysis demonstrated that, within a hyperplasia, proliferating type II cells could change from cuboidal to columnar in shape and could also exhibit nuclear indentations, both characteristics displayed by the Clara cell. Thus, this divergence of the type II cell from its well characterized morphological features indicates that the selective growth advantage which these initiated cells possess can result in changes to the normal ultrastructure of this cell as it progresses toward malignancy. DNA was isolated from 20 hyperplasias and screened for the presence of an activated K-ras gene. This gene was activated in 17 of 20 lesions, with 85% of the mutations involving a GC to AT transition within codon 12 (GGT to GAT), a mutation consistent with base mispairing produced by the formation of the O6-methylguanine adduct. This specificity for activation of the K-ras gene was identical to that observed previously in adenocarcinomas induced by NNK.(ABSTRACT TRUNCATED AT 400 WORDS)