Cutaneous HPV23 E6 prevents p53 phosphorylation through interaction with HIPK2.

Ultraviolet irradiation (UV) is the major risk factor for the development of skin cancer. Moreover, increasing evidence suggests cutaneotropic human papillomaviruses (HPV) from the beta genus to play a causal role as a co-factor in the development of cutaneous squamous cell carcinoma. Homeodomain-interacting protein kinase 2 (HIPK2) operates as a potential suppressor in skin tumorigenesis and is stabilized by UV-damage. HIPK2 is an important regulator of apoptosis, which forms a complex with the tumor suppressor p53, mediating p53 phosphorylation at Ser 46 and thus promoting pro-apoptotic gene expression. In our study, we demonstrate that cutaneous HPV23 E6 protein directly targets HIPK2 function. Accordingly, HPV23 E6 interacts with HIPK2 both in vitro and in vivo. Furthermore, upon massive UVB-damage HPV23 E6 co-localizes with endogenous HIPK2 at nuclear bodies. Functionally, we demonstrate that HPV23 E6 inhibits HIPK2-mediated p53 Ser 46 phosphorylation through enforcing dissociation of the HIPK2/p53 complex. In addition, HPV23 E6 co-accumulates with endogenous HIPK2 upon UV damage suggesting a mechanism by which HPV23 E6 keeps HIPK2 in check after UV damage. Thus, cutaneous HPV23 E6 prevents HIPK2-mediated p53 Ser 46 phosphorylation, which may favour survival of UV-damaged keratinocytes and skin carcinogenesis by apoptosis evasion.

Epigenetic silencing of interferon-kappa in human papillomavirus type 16-positive cells.

We have investigated interferon-kappa (IFN-kappa) regulation in the context of human papillomavirus (HPV)-induced carcinogenesis using primary human foreskin keratinocytes (HFK), immortalized HFKs encoding individual oncoproteins of HPV16 (E6, E7, and E6/E7), and cervical carcinoma cells. Here, IFN-kappa was suppressed in the presence of E6, whereas its expression was not affected in HFKs or E7-immortalized HFKs. Transcription could be reactivated after DNA demethylation but was decreased again upon drug removal. Partial reactivation could also be accomplished when E6 was knocked down, suggesting a contribution of E6 in IFN-kappa de novo methylation. We identified a single CpG island near the transcriptional start site as being involved in selective IFN-kappa expression. To prove the functional relevance of IFN-kappa in building up an antiviral response, IFN-kappa was ectopically expressed in cervical carcinoma cells where protection against vesicular stomatitis virus-mediated cytolysis could be achieved. Reconstitution of IFN-kappa was accompanied by an increase of p53, MxA, and IFN-regulatory factors, which was reversed by knocking down either IFN-kappa or p53 by small interfering RNA. This suggests the existence of a positive feedback loop between IFN-kappa, p53, and components of IFN signaling pathway to maintain an antiviral state. Our in vitro findings were further corroborated in biopsy samples of cervical cancer patients, in which IFN-kappa was also downregulated when compared with normal donor tissue. This is the first report showing an epigenetic silencing of type I IFN after HPV16 oncogene expression and revealing a novel strategy on how high-risk HPVs can abolish the innate immune response in their genuine host cells.