Activation of the ras Ha Gene in Urethane-Initiated Papillomas Induced by Protocols with High and Low Frequencies of Malignant Conversion

Abstract

The number of papillomas that develop in mice initiated with a single exposure to urethane and promoted by repeated applications of 12-O-tetradecanoylphorbol-13-acetate (TPA) is increased greater than 4-fold by pretreating the skin once with TPA 24 hr before administration of urethane. In contrast, the carcinoma incidence was increased only 2-fold by the TPA pretreatment. Individual papillomas developed from the two protocols, differing in the potential for conversion to malignancy, were compared for activation of the rasHa gene. An activated oncogene that transformed 3T3 cells was found in the DNA of four of five papillomas from urethane-initiated, TPA-promoted mice and in eight of eleven papillomas from similarly promoted mice exposed to TPA before urethane initiation. Southern analysis of DNA from tumors or 3T3 foci demonstrated that the rasHa gene was activated by an A----T transversion at the second base of codon 61 in all mutated alleles. Thus, tumors induced by the two protocols did not differ in rasHa activation.