| Human papillomaviruses (HPVs) are small circular DNA viruses that persistently infect stratified squamous epithelia. HPVs are implicated in the etiology of a wide variety of hyperproliferative lesions, and particularly in cancer of the cervix. Although much effort has been made to understand the mechanisms by which these agents can lead to the development of malignant carcinomas, much less is understood about the normal life cycle of HPVs. It is clear that all of the major life cycle events of HPV, including transcription, splicing, translation, replication, virion morphogenesis, and shedding are tied to the differentiation of keratinocyte host cells. As a step towards understanding the normal life cycle of HPVs, we have undertaken studies of the differentiation-dependent "late" promoter, p742. Transcripts from this promoter become very abundant upon differentiation of the host cell. We have mapped the core p742 promoter to a 150 base pair region in the E7 ORF and demonstrated that E6/E7 ORF region contains several positive and negative transcriptional elements, including at least one element that acts on the upstream promoter p99. We have found that several differentiation-related transcription factors, including CDP, Skn-1a, and Tst-1 can repress transcriptional activity from p742. The major differentiation response elements appear to reside in the area of the multiple start sites, and enhancer elements in the viral upstream regulatory region contribute to both basal and differentiation-dependent p742 activity. The viral transcription factor E2 does not appear to regulate this promoter. Using several techniques, we have discovered that productive viral genome replication, which is also differentiation dependent, is neither necessary nor sufficient for p742 activation. Finally, we showed that p742 requires the PKCdelta signaling pathway, and viral genome amplification requires tyrosine kinases and several of the PKC isoforms. These studies shed light on the elements in the virus genome responsible for late promoter activity, the interaction between differentiation-dependent viral processes, and the signaling pathways to which the virus responds in a differentiating cell. |
