Tumor Promotion Studies in Rat Tracheal Epithelium

Abstract

The tracheal epithelium of the Fischer 344 rat is histologically very similar to that of the human bronchus. Also, carcinomas of tracheal origin in F-344 rats are similar in morphology to human bronchogenic carcinomas. Tumor promotion in rat tracheal epithelium was studied by using two model systems. The first is a heterotopic transplant system in which rat tracheas are implanted subcutaneously on the backs of isogenic recipents. In the first system, the epithelium was topically exposed to pellets containing 7,12-dimethylbenz(a)anthracene (DMBA), used as the initiating agent, followed by pellets containing the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), the promoting agent. After 98 weeks, a three- to fourfold increase in the percentage of tracheas having malignant tumors was seen in tracheal transplants receiving both DMBA and TPA compared to DMBA alone. Exposure of the tracheal grafts to TPA alone resulted in epithelial hyperplasia and inflammation, but no dysplastic lesions. The second system is an organ culture-cell culture system in which small pieces of trachea are grown in organ culture, then epithelial cells are grown from these pieces as primary cell cultures. The organ cultures were exposed to the direct alkylating agent, N-methyl-N′-nitro-N -nitrosoguanidine (MNNG) used as the initiator, then multiple short exposures to TPA were used to promote. Primary cell cultures and cell lines were then established from these explants. After 52 weeks, a five-fold increase in the percentage of explants producing tumorigenic cell lines was observed when MNNG + TPA-exposed explants were compared to MNNG-exposed explants. Tracheal explants exposed to TPA alone produced many cell lines but none tested were tumorigenic. These two systems provide a means to study tumor promotion in respiratory epithelium. The evidence more importantly suggests that airborne promoting substances may play a key role in the development of bronchogenic carcinoma.