E7 PROTEINS OF 4 GROUPS OF HUMAN PAPILLOMAVIRUSES, IRRESPECTIVE OF THEIR TISSUE TROPISM OR CANCER ASSOCIATION, POSSESS THE ABILITY TO TRANSACTIVATE TRANSCRIPTIONAL PROMOTERS E2F SITE DEPENDENTLY

In an experimental system in which an expression vector including the E7 gene of a given human papillomavirus (HPV), together with a lucifer ase reporter plasmid including the adenovirus E2 (Ad E2) promoter, was transiently transfected into cultured mouse NIH3T3 fibroblastic cells , we obtained the signal indicating that E7 proteins of HPV type 5, 12 , 14, 20, 21, 25, and 47, which are associated with epidermodysplasia verruciformis (EV), can transactivate the Ad E2 promoter, as previousl y reported for E7 proteins of other HPVs. Because the underlying mecha nism of the transactivation had not been analyzed, except for transact ivation by E7 gene of cervical cancer-associated HPV-16, we compared t he E7 genes of representatives of three other groups of HPVs (HPV-1, - 11, and -47) with that of HPV-16 with regard to their transactivating activity toward artificially constructed promoters. The experiment wit h a shortened AdE2 promoter carrying only the E2F sites and TATA box p rovided evidence that all four E7 proteins can transactivate the short ened promoter and that this phenomenon is E2F site dependent. Further experiments with the reporter gene constructs carrying basal promoters or more complex forms with or without linked E2F sites, (a) confirmed previous finding by others that in cells producing no transactivator, the transcriptional level from promoters linked to E2F sites is rathe r repressed in comparison with the level of the corresponding promoter s that are not linked to the E2F sites, and (b) demonstrated, for the first time, that in cells expected to produce the E7 protein of any on e of the four HPVs, transcription from the promoter linked to the E2F sites was released from repression. In other words, the present result s reveal that E7 proteins of any of the four HPVs can remove the E2F s ite-dependent repression, probably by modulating E2F complexes from re pressing forms to activating ones.