The early genes E6 and E7 of cancer associated human papilloma viruses as targets of tumor suppression?

We have transplanted HPV 18 positive nontumorigenic hybrid cells and tumorigenic hybrid segregants as well as nontumorigenic and tumorigenic human keratinocytes immortalized by HPV 16-DNA transfection (HPK cells and HPK-ras cells) into nude mice and grown them under in vivo conditions for different periods of time. By analyzing gene expression at the mRNA level of the early viral genes E6/E7 and a number of cytoskeletal cellular genes we wanted to test the hypothesis that the nontumorigenic phenotype of these cells in vivo may be determined by the specific downregulation of expression of the oncogenic E6/E7 genes by the product(s) of tumor suppressor gene(s). The results obtained showed that: 1) The nontumorigenic hybrid cells (in contrast to tumorigenic segregants) stopped to proliferate about 3 days after transplantation. At this time E6/E7 gene expression was already drastically reduced, whereas at day 2 expression was still high. This suppression specifically affected the HPV 18 E6/E7 genes and preceded cell death by at least 10 days. 2) The same specific suppression of HPV E6/E7 gene expression occurred during in vivo growth of the nontumorigenic HPK cells. These cells gave rise to transiently growing cysts, but had retained the in vivo differentiation potential of normal keratinocytes (orderly expression of cytokeratins 1 and 10, involucrin and filaggrin). HPV 16 E6/E7 gene expression was very low and clearly restricted to a small subset of basal cells, thus showing an inverse relationship to terminal differentiation. In contrast, in the tumors induced by HPK-ras cells E6/E7 gene expression extended from the basal cells into suprabasal cells and terminal differentiation was retarded. These results support the hypothesis that during in vivo growth tumor suppressor gene(s) negatively regulated HPV E6/E7 gene expression resulting in the stop of proliferation of the nontumorigenic cells.