Modulation of basal cell fate during productive and transforming HPV-16 infection is mediated by progressive E6-driven depletion of Notch

Abstract

In stratified epithelia such as the epidermis, homeostasis is maintained by the proliferation of cells in the lower epithelial layers and the concomitant loss of differentiated cells from the epithelial surface. These differentiating keratinocytes progressively stratify and form a self‐regenerating multi‐layered barrier that protects the underlying dermis. In such tissue, the continual loss and replacement of differentiated cells also limits the accumulation of oncogenic mutations within the tissue. Inactivating mutations in key driver genes, such as TP53 and NOTCH1, reduce the proportion of differentiating cells allowing for the long‐term persistence of expanding mutant clones in the tissue. Here we show that through the expression of E6, HPV‐16 prevents the early fate commitment of human keratinocytes towards differentiation and confers a strong growth advantage to human keratinocytes. When E6 is expressed either alone or with E7, it promotes keratinocyte proliferation at high cell densities, through the combined inactivation of p53 and Notch1. In organotypic raft culture, the activity of E6 is restricted to the basal layer of the epithelium and is enhanced during the progression from productive to abortive or transforming HPV‐16 infection. Consistent with this, the expression of p53 and cleaved Notch1 becomes progressively more disrupted, and is associated with increased basal cell density and reduced commitment to differentiation. The expression of cleaved Notch1 is similarly disrupted also in HPV‐16‐positive cervical lesions, depending on neoplastic grade. When taken together, these data depict an important role of high‐risk E6 in promoting the persistence of infected keratinocytes in the basal and parabasal layers through the inactivation of gene products that are commonly mutated in non‐HPV‐associated neoplastic squamous epithelia. © 2017 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Host control of human papillomavirus infection and disease

“…In animal models of disease, lesion formation takes four weeks or so depending on infectious titre, with microlesions being apparent by histology after two weeks, and the first signs of L1 and L2 capsid protein expression being seen at three weeks post-infection. Our current hypothesis suggests that papillomaviruses have a limited ability to increase the growth rate of infected cells during the initial stages of wound healing, and act instead to prevent cell cycle exit and G0-progression as cell density increases (11,16). In an immune competent host, both low and highrisk HPV types can persist for months or years, causing chronic productive lesions that shed virus from their surface layers over a prolonged period of time.…”

“…Given that papillomaviruses are epithelial specialists, it is perhaps not surprising that they interfere with the regulatory pathways that control cell expansion and cell density in the epithelial basal layer, and that they generally do this in a very ordered way that results in a self-limiting infection (16,17). Once a basal cell has become infected, it is not easily lost from the body, even as genetic errors accumulate.…”

Changing Stem Cell Dynamics during Papillomavirus Infection: Potential Roles for Cellular Plasticity in the Viral Lifecycle and Disease

“…The E7 protein of cutaneous HPVs has been shown to lead to the upregulation of several stem cell markers genes in infected keratinocytes and to the formation of cells with stem-like properties—e.g., epithelial cell adhesion molecule (EpCaM) [ 64 ]. High-risk mucosal as well as cutaneous HPVs and murine papillomaviruses also inhibit the Notch pathway which is known to play important roles in commitment to differentiation [ 65 , 66 ]. Of course, the role of such reprogramming to the viral lifecycle and pathogenesis is not well understood.…”

Gene Targeting of HPV18 E6 and E7 Synchronously by Nonviral Transfection of CRISPR/Cas9 System in Cervical Cancer

“…2 The dominant mechanism of E6 action is the degradation of p53, and E7 inactivates retinoblastoma family proteins (pRB) and in-teracts with the downstream protein p21 and other pathways. [3][4][5] In addition, the two driver genes not only induce the immortalization and transformation of cancer cells but also suppress the innate immune response. 6,7 In this regard, knocking out E6 and E7 may be valuable for cervical cancer treatment.…”