Human Papillomavirus 16 E6 and E7 Synergistically Repress Innate Immune Gene Transcription

Abstract

Human papillomaviruses (HPV) are causative agents in 5% of all cancers, including the majority of anogenital and oropharyngeal cancers. Downregulation of innate immune genes (IIGs) by HPV to promote the viral life cycle is well documented; E6 and E7 are known repressors of these genes. More recently, we demonstrated that E2 could also repress IIGs. These studies have been carried out in cells overexpressing the viral proteins, and to further investigate the role of individual viral proteins in this repression, we introduced stop codons into E6 and/or E7 in the entire HPV16 genome and generated N/Tert-1 cells stably maintaining the HPV16 genomes. We demonstrate that E6 or E7 individually is not sufficient to repress IIG expression in the context of the entire HPV16 genome; both are required for a synergistic repression. The DNA damage response (DDR) is activated by HPV16 irrespective of E6 and E7 expression, presumably due to viral replication; E1 is a known activator of the DDR. In addition, replication stress was apparent in HPV16-positive cells lacking E6 and E7, manifested by attenuated cellular growth and activation of replication stress genes. These studies led us to the following model. Viral replication per se can activate the DDR following infection, and this activation is a known inducer of IIG expression, which may induce cellular senescence. To combat this, E6 and E7 synergistically combine to manipulate the DDR and actively repress innate immune gene expression promoting cellular growth; neither protein by itself is able to do this. IMPORTANCE The role of human papillomavirus 16 (HPV16) in human cancers is well established; however, to date there are no antiviral therapeutics that are available for combatting these cancers. To identify such targets, we must enhance the understanding of the viral life cycle. Innate immune genes (IIGs) are repressed by HPV16, and we have reported that this repression persists through to cancer. Reversal of this repression would boost the immune response to HPV16-positive tumors, an area that is becoming more important given the advances in immunotherapy. This report demonstrates that E6 and E7 synergistically repress IIG expression in the context of the entire HPV16 genome. Removal of either protein activates the expression of IIGs by HPV16. Therefore, gaining a precise understanding of how the viral oncogenes repress IIG expression represents an opportunity to reverse this repression and boost the immune response to HPV16 infections for therapeutic gain.KEYWORDS DNA damage response, E6, E7, innate immunity, papillomavirus H uman papillomavirus (HPV) is the most common sexually transmitted infection in the United States, with an estimated 80% of sexually active adults acquiring an HPV infection in their lifetime (1). Of the high-risk HPVs known to be causative in the development of cancer, HPV16 is the most prevalent genotype (2). HPV16 is causative in around 50% of cervical cancers and nearly 90% of HPV-related head and neck RESULTSEstablishment and characterization of...

Vesicular trafficking permits evasion of cGAS/STING surveillance during initial human papillomavirus infection

“…Indeed, IFN responses are detrimental to persistent HPV infections, reducing cellular proliferation and causing apoptosis, episome loss, mutation, and/or integration [73][74][75][76][77]. Likewise, it is well known that some HPV early genes-E5, E6, and E7 -counteract these detrimental antiviral IFN and IFN-stimulated gene (ISG) responses through a variety of mechanisms [78][79][80][81].…”

Manipulation of JAK/STAT Signalling by High-Risk HPVs: Potential Therapeutic Targets for HPV-Associated Malignancies

“…This results in the decreased expression of ISGs, such as IFIT1, MX1 and OAS1. Furthermore, the authors demonstrated that IFNβ treatment of HPV16 containing N/Tert1s restores expression of the ISGF3 complex, but not ISG expression, suggesting that HPV16 can also regulate ISG expression independently of the ISGF3 complex [128]. In this study, the viral genes E2, E6 and E7 were responsible for the downregulation of the ISGF3 complex and downstream ISG expression.…”

“…Importantly, they also showed that IFNβ treatment activated STAT1 in HPV-containing keratinocytes and reduced genome amplification, suggesting that downregulation of STAT1 is an essential, conserved function of HR-HPV E6 and E7 during the viral life cycle. Additionally, the persistence of HPV16 viral genomes also resulted in the suppression of ISGs, IFNs and STAT1 [127,128].…”

“…HPV16 can target the ISGF3 complex through additional mechanisms, most notably by downregulating the mRNA expression of STAT2 and IRF9 in immortalised keratinocytes (N/Tert-1 cells) [128]. This results in the decreased expression of ISGs, such as IFIT1, MX1 and OAS1.…”

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“…Viruses that encode anti-apoptotic protease are likely to inhibit the apoptosis process to facilities its replication and spreading 26 . Several similar viruses encode Bcl-2 homologs, which inhibit activation of apoptotic caspases and block the death of the host cell 27 as well as interfering with other proapoptotic molecules, for example papillomavirus E7 and HHV-8 that encodes several versions of the IRFs, which interferes with normal IRF function 28 . Many viruses take advantage of apoptosis to facilitate its replication 27; the major protein involved in the apoptosis process is a family of cysteine-dependent aspartate-directed proteinases called caspases 29 .…”