Effects of Vitamin A on 3-Methylcholanthrene-Induced Squamous Metaplasias and Early Tumors in the Respiratory Tract of Rats 2

Abstract

The influence of high levels of vitamin A on the susceptibility of respiratory-tract epithelium to the metaplastogenic and tumorigenic effects of 3-methylcholanthrene (MCA) was tested in rats. Fischer-344 rats were maintained on a vitamin A-free diet and were given either 1740 µg or 2×87 µg retinyl acetate twice a week intragastrically. At 9 weeks of age each rat received 2 intratracheal injections of 5 mg MCA. Groups of 4–5 rats were killed 2, 3, 5, 8, and 12 weeks after carcinogen administration. Vitamin A storage in the liver, assessed at 3 different times, corresponded to the vitamin A dose. The number of microscopic hyperplastic and metaplastic lesions and grossly visible tumor nodules per rat lung was determined. The development of squamous metaplasias and squamous cell tumors was significantly reduced in rats receiving a high dose of retinyl acetate. Our experiments confirm earlier findings suggesting an inhibitory effect of vitamin A on the development of respiratory-tract tumors in hamsters.