Pulmonary fibrosis caused by N-methyl-N-nitrosourethane inhibits lung tumorigenesis by urethane in transgenic mice carrying the human prototype c-Ha-ras gene.

Male and female transgenic mice carrying the human prototype c-Ha-ras gene (rasH2 mice) and their wild littermates (non-Tg mice) received three subcutaneous injections of 0.3 mg N-methyl-N-nitrosourethane (MNUR) once every 2 weeks for the first 4 weeks followed by a single intraperitoneal injection of 1000 or 0 mg/kg urethane (UR) 2 weeks later. They were then maintained without any other treatment for a further 13 weeks and sacrificed for assessment of pulmonary pathology. Inflammatory lesions, such as macrophage infiltration, alveolar bronchiolization and/or fibrosis, were induced in both rasH2 and non-Tg mice treated with MNUR or MNUR + UR. Lung proliferative lesions were induced in 100% of the UR-treated rasH2 mice but to a significantly lesser extent in the MNUR + UR case. The incidences of lung tumors in non-Tg mice treated with UR or MNUR + UR were relatively low. Point mutations of the transgene were detected in approximately 80% of lung tumors in rasH2 mice treated with UR and MNUR + UR, but murine Ki-ras mutations were rare. No marked difference in the mutation pattern was found between the UR-treated and the MNUR + UR-treated rasH2 mice. In non-Tg mice treated with UR or MNUR + UR, point mutations of the murine c-Ki-ras gene were observed in about 50% of the lung tumors examined. The present study confirmed that rasH2 mice are very sensitive to lung tumor induction by UR and suggested that alveolar epithelial cells in the reparative stage during pulmonary fibrosis are resistant to DNA damage by this carcinogen.