Vitamin A and the susceptibility of respiratory tract tissues to carcinogenic insult.

The influence of vitamin A on the development of chemically induced lung carcinomas in rats was investigated. Rats were maintained on low, "normal" and excess levels of retinyl acetate (RA). Respiratory tract-squamous carcinomas were induced by intratracheal injections of 3-methylcholanthrene (3-MCA). The carcinogen doses used ranged from 1.25 to 10.0 mg of 3-MCA. Serial sacrifices conducted during the first 20 weeks following carcinogen exposure showed that metaplastic lung nodules, presumed to be precursors of later appearing carcinomas, occurred earlier and at higher incidence in rats maintained on low levels of RA than in rats maintained on moderate or high levels of RA. The development of invasive pulmonary carcinomas was enhanced at all four carcinogen doses in rats receiving low levels of RA as compared to rats receiving moderate or high levels of RA. No consistent difference in lung cancer incidence existed between the groups receiving normal and high levels of RA. The data clearly show an increased susceptibility of vitamin A-deficient rats to develop chemically induced lung cancers. Possible mechanisms underlying this effect are discussed.