Characterization of the high-risk human papillomavirus 18 genome replication and transcription in different cellular models

Abstract

Human papillomavirus (HPV) is responsible for approximately 30% of infection-caused cancers worldwide. The development of antiviral drugs requires a thorough understanding of the viral life cycle and its interactions with host cells. Replication of HPV genomes is restricted to a very limited number of human cell lines, and the reasons for this limitation are unknown. Human primary keratinocytes and osteosarcoma U2OS cells are widely used cellular model systems in HPV-related research. Conversely, it has been established that viral proteins, namely helicase E1 and transcription factor E2, can trigger the replication from the origin of replication if expressed from separate plasmids in almost all other cell types studied so far. This suggests that a deficiency in at least one of these viral factors may be a reason for the lack of replication of the viral genome in most cell lines. To test this hypothesis, the transcription, translation, and replication of the oncogenic HPV type 18 genome were studied in two different types of cells: embryonic kidney cells 293FT, which are not permissive for HPV replication, and well-described HPV host U2OS cells. The results obtained in the present thesis show that expression levels of all early viral genes, including E1 and E2, are very low in 293FT cells, and the absence of the E1 and E2 proteins causes the lack of viral replication. Additionally, it has been shown that in epithelial cells, such as keratinocytes HaCaT and squamous cervical cancer C33A cells, replication of HPV18 may be restored in the absence of the E8^E2 viral transcriptional repressor, which is expressed from an alternative viral promoter and counteracts E2 activities. However, this effect was not observed in 293FT cells and only exogenously added E1 protein was able to trigger replication of the HPV18 genome in these cells. These results suggest that different cell type-specific mechanisms may be employed to restrict replication of the HPV genome.