Introduction of the v-Ha-ras gene into primary epidermal keratinocytes, followed by grafting of these cells to animals, leads to the formation of benign epidermal
tumors that resemble
papillomas induced chemically by a two-stage
carcinogenesis protocol. In this study, we investigated v-Ha-ras-induced
papillomas for aberrant expression of
type I keratin K13, previously described in 7,12-dimethylbenz[a]
anthracene/12-O-tetradecanoylphorbol-13-
acetate (DMBA/TPA)-induced mouse epidermal
tumors.
Papillomas produced from three independent
infection series were removed 3 wk after grafting concomitant with control grafts originating from mock-, neo-, and v-fos-infected primary keratinocytes. Combined analysis of the grafts by western blotting of extracted
keratins and immunofluorescence studies of frozen sections with a K13-monospecific antibody revealed K13 expression in all v-Ha-ras-induced
papillomas and absence of this
keratin in all control grafts. K13-positive cells in
papillomas were restricted to the suprabasal cell layers of the lesions and, at this stage of
papilloma development, occurred as foci of varying extensions. Analysis of genomic
DNA from v-Ha-ras-induced
papillomas for the methylation state of a CpG dinucleotide in the distant promoter region of the K13 gene revealed the occurrence of unmethylated
DNA copies that were generated at the expense of methylated
DNA copies ubiquitously present in normal epidermis. The ratio of unmethylated to methylated
DNA copies correlated with the extent of suprabasal K13
protein expression. Thus, all features of aberrant K13 expression previously described in DMBA/TPA-induced
papillomas were shared by v-Ha-ras-induced
papillomas.