p53 Gene Dosage Modifies Growth and Malignant Progression of Keratinocytes Expressing the v-ras^Ha Oncogene

Abstract

Epidermal keratinocyte cultures were established from newborn mice expressing a null mutation in the p53 gene to explore the contribution of p53 to epidermal growth regulation and neoplasia. Keratinocytes were initiated by transduction with a replication-defective retrovirus encoding the v-ras^Ha oncogene and grafted onto nude mouse hosts. Tumors arising from keratinocytes heterozygous or null for functional p53 in the presence of v-ras^Ha have growth rates approximately 5-fold higher than those derived from p53(+/+) controls and rapidly form carcinomas, in contrast to the benign phenotype observed in p53(+/+)/v-ras^Ha grafts. In vitro,p53-deficient keratinocytes with and without v-ras^Ha expression display decreased responsiveness to the negative growth regulators transforming growth factors β₁ and β₂. In combination with v-ras^Ha, p53-deficient keratinocytes also exhibit decreased responsiveness to elevated Ca²⁺. These differences between genotypes cannot be attributed to changes in transforming growth factor β receptor types present or altered levels of epidermal growth factor receptor and are independent of c-myc transcript levels. mRNA expression for the p-53 inducible protein WAF1 correlates with p53 gene dosage, but low levels are still detectable in p53(-/-) keratinocytes. The altered responsiveness of p53 deficient keratinocytes to negative growth regulators may provide a growth advantage to such cells in vivo and render them more susceptible to genetic alterations and malignant conversion.