Abstract
Deregulation of proliferation and differentiation-dependent signalling pathways is a hallmark of human papillomavirus (HPV) infection. Although the manipulation of these pathways by E6 and E7 has been extensively studied, controversies surround the role of the E5 oncoprotein during a productive virus life cycle. By integrating primary keratinocytes harbouring wild type or E5 knockout HPV18 genomes with pharmacological and gain/loss of function models, this study aimed to provide molecular information about the role of E5 in epithelial proliferation and differentiation. We show that E5 contributes to cell cycle progression and unscheduled host DNA synthesis in differentiating keratinocytes. E5 function correlates with increased EGFR activation in differentiating cells and blockade of this pathway impairs differentiation-dependent cell cycle progression of HPV18 containing cells. Our findings provide a functional requirement of enhanced EGFR signalling for suprabasal cellular DNA synthesis during the virus life cycle. They also reveal an unrecognised contribution of E5 towards the impaired keratinocyte differentiation observed during a productive HPV infection. E5 suppresses a signalling axis consisting of the keratinocyte growth factor receptor (KGFR) pathway. Inhibition of this pathway compensates for the loss of E5 in knockout cells and re-instates the delay in differentiation. The negative regulation of KGFR involves suppression by the EGFR pathway. Thus our data reveal an unappreciated role for E5-mediated EGFR signalling in orchestrating the balance between proliferation and differentiation in suprabasal cells.
Highlights
Human papillomaviruses (HPV) infect the squamous epithelial cells at a number of body sites [1, 2]
Wild type (WT) and E5 knock-out (E5KO) (KO) HPV18 genomes were transfected into low passage neonate normal human keratinocytes (NHK) obtained from two individual foreskin donors
This study provides a comprehensive analysis of E5 function during productive infection by HPV18; the second most prevalent high-risk type
Summary
Human papillomaviruses (HPV) infect the squamous epithelial cells at a number of body sites [1, 2]. In addition to increasing proliferation, EGFR www.impactjournals.com/oncotarget signalling suppresses keratinocyte differentiation [6]. This is required to maintain homeostasis between self-renewing and committed keratinocytes in the basal proliferative compartment of the epidermis, whereas in the upper layers this pathway is down regulated. Whilst there is not a complete understanding of the pathways that orchestrate differentiation, a number of critical regulators of this process are recognized including the keratinocyte growth factor receptor (KGFR/FGFR2IIIb). This is expressed in suprabasal layers and induces gene expression changes associated with terminal differentiation. KGFR overexpression in basal cells induces premature expression of spinous layer associated markers, whilst ablation, by genetic or chemical means, results in a hyperproliferative epidermis [8, 9]
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