The HPV8 E6 protein targets the Hippo and Wnt signaling pathways as part of its arsenal to restrain keratinocyte differentiation

Abstract

Infections with {beta}-genus HPVs cause hyperplastic cutaneous lesions. In individuals with the rare hereditary skin disease, epidermodysplasia verruciformis, such lesions can progress to cutaneous squamous cell carcinomas (cSCC). {beta}-HPV infections may also underlie cSCC development in chronically immunosuppressed individuals. Despite their prevalence and disease association, these viruses are not as well studied as the cancer-associated high-risk -genus HPVs. HPV-associated lesions are characterized by a marked expansion of dividing, basal-like, poorly differentiated viral cells that contain viral genomes. This reflects the ability of HPVs to inhibit epithelial cell differentiation which is likely driven by the need to establish and maintain long-term viral infections in basal-like epithelial cells. Remarkably, the {beta}-HPVs accomplish this by targeting different cellular effectors than the -genus HPVs. It was previously reported that the HPV8 E6 protein restrains epithelial cell differentiation by inhibiting Notch and TGF-{beta} signaling. Here we report that the HPV8 E6 protein can subvert Hippo signaling by activating TEAD transcriptional programs that inhibit the expression of keratinocyte differentiation markers. Moreover, we determined that HPV8 E6 can interfere with gene expression programs triggered by Wnt signaling by binding to the {beta}-catenin-associated transcriptional co-activator BCL9L and that this also serves to restrain the expression of epithelial differentiation markers. Hence the HPV8 E6 protein has evolved a remarkably large array of mechanisms to subvert the differentiation program of the infected epithelial cells. IMPORTANCEHuman papillomaviruses (HPVs) infect basal epithelial cells and cause a dramatic expansion of basal-like, proliferative cells. This reflects the ability of papillomaviruses to delay keratinocyte differentiation thereby maintaining aspects of the basal cell identity of persistently infected cells. This may enable papillomaviruses to establish and maintain long-term infections in squamous epithelial tissues. Previous work has revealed that the ability of {beta}-HPV8 E6 protein to inhibit NOTCH and TGF-{beta} signaling importantly contributes to this activity. Here we present evidence that HPV8 E6 also subverts Hippo and Wnt signaling and that these activities also aid in restraining keratinocyte differentiation.