Immortalization of inbred rabbit keratinocytes from a Shope papilloma and tumorigenic transformation of the cells by EJ-ras

An immortalized cell line of keratinocytes, named SPG1-3,was established from a papilloma induced from cottontail rabbit papillomavirus (CRPV)-infected inbred rabbit skin. The cells have reached 60 passages in culture and are still growing well, but they are not tumorigenic in athymic mice. Although CRPV DNA was present as extrachromosomal episomes in the papilloma from which the cell line was derived from a single colony of keratinocytes, there was no CRPV DNA detectable in the cells. Three sub-cell lines of SPG1-3EJ, SPG1-3EJ1 and SPG1-3EJ2 were then established from the EJ-ras transfected SPG1-3 cells. All of the three sub-lines contained both EJ-ras DNA and a 1.2 kb transcript of EJ-ras, and they are malignantly tumorigenic in athymic mice. These data indicate that CRPV genome and its expression might be essential for the initiation and maintenance of neoplasia, but not for the maintenance of immortalization of the tumor-derived cells. In addition, some oncogenes such as EJ-ras may play an essential role in tumorigenic and malignant conversion of the immortalized cells. These cell lines derived from inbred rabbit skin may provide a useful in vitro system for better understanding of the oncogenic processes of papillomavirus-involved neoplastic progression by transfecting the cells with CRPV genes and serial transplantation to the inbred rabbits for studying host immune responses to the viral oncogenic potential.