Experimental carcinogenesis has led to a concept that defines two discrete stages in the development of skin tumors: (i) initiation, which is accomplished by using a mutagen that presumably activates a protooncogene, and (ii) promotion, which is a reversible process brought about most commonly by repeated application of phorbol esters. We have created a transgenic mouse strain that carries the activated v-Ha-ras oncogene fused to the promoter of the mouse embryonic a-like, {-globin gene. Unexpectedly, these animals developed papillomas at areas of epidermal abrasion and, because abrasion can also serve as a tumor-promoting event in mutagentreated mouse skin, we tested these mice for their ability to respond to phorbol ester application. Within 6 weeks virtually all treated carrier mice had developed multiple papillomas, some of which went on to develop squamous cell carcinomas and, more frequently, underlying sarcomas. We conclude that the oncogene "preinitiates" carrier mice, replacing the initiation/mutagenesis step and immediately sensitizing them to the action of tumor promoters. In addition, treatment of the mice with retinoic acid dramatically delays, reduces, and often completely inhibits the appearance of promoter-induced papillomas. This strain has use in screening tumor promoters and for assessing antitumor and antiproliferative agents.The two-stage mouse epidermal model for tumorigenesis provides compelling evidence that some tumors can pass through several stages of increasing virulence before emerging as a lethal malignancy (1)(2)(3)(4)(5). In this system, tumor development depends upon two discrete steps, initiation and promotion, which differ from one another in very fundamental ways. Initiation, generally induced by a potent mutagen, is an irreversible process that constitutes the first step. This step can be followed days, weeks, or even months later by a reversible step involving multiple applications of one of several tumor-promoting compounds, most commonly the ester phorbol 12-myristate 13-acetate (PMA). This treatment, delivered over several weeks, finally leads to the formation of papillomas, some of which progress to form carcinomas. Interestingly, abrasion or mechanical disruption of the skin can also promote papilloma and carcinoma formation in mutagen-treated mice, presumably by inducing regenerative epidermal hyperplasia.The initiation step is generally held to create cryptic carcinogenic mutations in epidermal stem cells that remain dormant pending some epigenetic intervention, called promotion, which induces papilloma formation. Initiation is highly correlated with activation of the c-Ha-ras oncogene; for example, v-Ha-ras-bearing retroviruses can serve in place of mutagens as initiators (6-9). The role of the promoter is less well understood, but the activating effect of PMA on protein kinase C (10-12) suggests that promotion might operate through this kinase, which, among other actions, induces transcription of an array of genes, including the protooncogenes c-myc, c-fos, ...
