Abstract
While the role of high-risk HPV E6 and E7 in targeting p53 and Rb has been intensively studied, how E6 and E7 manipulate cellular signaling cascades to promote the viral life cycle and cancer development is less understood. We have shown that HPV-16, specifically 16E7, attenuates pAKT, enhances IRES-dependent translation of several cellular proteins, inhibits keratinocyte differentiation and translocates YAP to the nucleus. Attenuation of pAKT was ablated by a missense mutation in the E7 carboxy-terminus, H73E, thereby defining a novel structure-function phenotype for E7. Coinciding with the pAKT attenuation, phosphorylation of S6K and 4E-BP1 was also attenuated and correlated with an increase in IRES-dependent translation of cellular proteins, including cMYC. This shift in cellular translation could lead to the enhanced expression of other cellular proteins that are essential for the viral life cycle or a novel mechanism to express other HPV proteins, like E5. The attenuation of pAKT could also represent another mechanism by which E7 induces dysplastic phenotypes typically associated with cervical cancer, as Rb degradation alone does not induce invasive cervical cancers. Keratinocytes containing the HPV-16 genome showed severe dysplasia and impaired differentiation in organotypic raft cultures, while HPV-16 genomes harboring the 16E7 H73E mutation had enhanced differentiation and markedly reduced cellular dysplasia. This demonstrates that the ability of E7 to reduce AKT activation correlates with increased dysplasia and reduced keratinocyte differentiation. The block in keratinocyte differentiation could also promote the viral life cycle, as viral amplification is dependent upon terminally differentiated keratinocytes re-entering the cell cycle. Inhibition or delay of keratinocyte differentiation through attenuation of pAKT could promote a more “basal-like” phenotype, which is ideal for the amplification of the viral genome. We have also shown that 16E7 promotes the nuclear accumulation of YAP, where YAP acts as a transcriptional co-activator to promote the expression of genes controlling cell growth. Nuclear localization and activation of YAP have been associated with many cancers and thus, YAP has been termed an oncogene. We hypothesize that E7 could be mediating both the attenuation of pAKT and the nuclear localization of YAP through similar mechanisms involving PP2A or PDK1, both of which are involved in AKT and Hippo signaling. The attenuation of pAKT by 16E7 could be to promote a cellular environment that is ideal for viral amplification, by promoting cell cycle re-entry through inhibition of keratinocyte differentiation and activating YAP to enhance the expression of genes involved in cell proliferation.
