Abstract
To cast light on the relationship between the development of pulmonary fibrosis and lung cancer, female Syrian golden hamsters were given 5 sc injections of 0.6 mg/animal of N-methyl- N-nitrosourethane (MNUR) at 2-wk intervals and then maintained without any treatment for 26 wk. Bronchiolo-alveolar cell tumors and hyperplasias, which were recognized from week 4 after termination of treatment, were each subdivided morphologically into 3 types. Bronchiolo-alveolar cell tumors included papillary tumors consisting of basophilic cells, papillary tumors consisting of clear cells, and papillary/solid tumors consisting of pleomorphic cells, with papillary tumors consisting of basophilic cells predominating. Bronchiolo-alveolar cell hyperplasias encountered were glandular metaplasia, simple hyperplasia, and papillary hyperplasia. Glandular metaplasia was the most common of the hyperplastic lesions. To some extent, all the proliferative lesions were associated with inflammatory cell infiltration, but this varied greatly. The rate for papillary tumors consisting of basophilic cells with connective tissue proliferation was 35%. Among the hyperplastic lesions, the cell prolif erative activity of papillary hyperplasia (12.5%) was significantly higher than in other hyperplastic lesions, suggesting that this lesion might be a preneoplastic change. None of the lung proliferative lesions showed any unequivocal immunoreactivity for the p53 protein. The present study suggests that most lung tumors may develop from pulmonary inflammatory lesions induced by MNUR, but the possibility that DNA injuries to the respiratory epithelial cells by MNUR may cause lung tumors cannot be precluded.