The cell biology of human papillomavirus transformed cells

Viruses are becoming increasingly recognized as a major etiological agent in the development of numerous forms of human cancer. Human papillomaviruses (HPVs) have been associated with a number of neoplastic lesions, most notably cervical cancer which is one of the major forms of cancer world wide. Of the over 50 types of identified HPVs, HPV types 16, 18, 31 and 33 are the types most commonly associated with malignant carcinomas. These viruses contain double stranded DNA which code for about eight gene products, some of which are oncogenic when introduces into cultured rodent or human cells. In particular, both the E6 and E7 gene products have different oncogenic capabilities and these genes are selectively retained within the genome of cervical carcinoma derived cells. The E7 gene product has immortalizing capabilities in primary cells and is able to cooperate with an activated ras oncogene to fully transform primary rodent cells. The E7 gene product from HPV type 16 is also capable of complexing in vitro to the anti-oncogene product, Rb. Similar complexes occur with Adenovirus E1A and SV40 large T proteins which may suggest a shared mechanism of transformation used by HPV type 16, Adenovirus and SV40. Transformation studies using primary human cells and nontumorigenic HeLa/fibroblast hybrid cells have also suggested that chromosome 11 may be important in suppressing the HPV transformed phenotype. The transformed phenotype may therefore also involve an impaired intracellular control of persisting HPV oncogenic sequences. Although there exists no solid evidence that a cytotoxic T-lymphocyte reaction is mounted against HPV transformed cells, there is evidence that both NK cells and activated macrophages can preferentially kill HPV transformed cells in vitro. Future studies are required to identify possible targets present on HPV transformed cells which are absent on normal cells.