Experimental studies on tumor promotion by nitrogen dioxide

Abstract

The effects of nitrogen dioxide (NO 2) on promotion of lung tumorigenesis induced by N-bis(2-hydroxypropyl) nitrosamine (BHPN) were investigated in male Wistar rats. In a preliminary study, the highest non-effective dose of BHPN was found to be 0.5 g per kg body weight. Rats were given a single intraperitoneal injection of BHPN at a dose of 0.5 g per kg body weight or saline at 6 weeks of age, and then exposed to clean air, 0.04 ppm, 0.4 ppm or 4 ppm of NO 2 for 17 months, respectively. The incidence of pulmonary tumors in rats exposed to BHPN plus 4 ppm of NO 2 was 12.5%; the tumors were adenomas and adenocarcinomas. Adenomas were found in 4 out of 40 rats (10%) and adenocarcinomas were found in 1 out of 40 rats (2.5%). The tumor incidence in the lungs of rats kept in BHPN plus clean air and BHPN plus 0.04 ppm of NO 2 was 2.5% (1/40). In both groups adenomas were found. There was no significant difference in tumor incidence between animals exposed to BHPN plus clean air and to BHPN plus 4 ppm of NO 2. No lung tumors were found in the group of BHPN plus 0.4 ppm NO 2 and in animals exposed to NO 2 without BHPN treatment. A high incidence of alveolar cell hyperplasia was observed in the lungs of rats injected with BHPN, and the effect of NO 2 on development of alveolar cell hyperplasia was slight. On the other hand, marked bronchiolar mucosal hyperplasia was found in 17 out of 40 rats (42.5%) in the group of BHPN plus 4 ppm of NO 2, and in 1 out of 40 rats (2.5%) in each of the group exposed to clean air, 0.04 ppm or 0.4 ppm of NO 2 with BHPN treatment, respectively. The hyperplasia in lungs of rats exposed to 4 ppm of NO 2 without BHPN treatment was slighter than that in lung of rat exposed to 4 ppm of NO 2 with BHPN treatment. On the other hand, tumor incidence in the nasal cavity of rats in each of group exposed to clean air and NO 2 with BHPN treatment was 97–100%. Incidence of tumors in other organs in the groups exposed to clean air and NO 2 with and without BHPN treatment was very low, and NO 2 had no effect on tumor development in the nasal cavity and other organs whether animals were treated with BHPN or not. Tumors found in the lungs were not metastases of tumors that had developed in the nasal cavity or in the other organs. These results suggest that NO 2 can promote the development of lung tumors initiated by BHPN, the NO 2 concentration which has promoting activity is 4 ppm or higher.