Beta Human Papillomavirus E6 Expression Inhibits Stabilization of p53 and Increases Tolerance of Genomic Instability

Abstract

Infections with the beta genus of human papillomaviruses (␤-HPVs) may contribute to the development of nonmelanoma skin cancers. However, ␤-HPV genomes are found at too low a copy number in tumors for the virus to be necessary for tumor maintenance. Instead, they are hypothesized to destabilize the host genome by allowing the persistence of mutations that can drive tumorigenesis independently of the viral genome. Supporting this premise is our previous finding that the expression of some ␤-HPV E6 proteins can attenuate p53 signaling in response to DNA damage. We show that ␤-HPV E6 proteins can prevent the stabilization of p53 in response to two types of genome-destabilizing events, aberrant mitosis and dysregulated centrosome duplication. The inability to stabilize p53 in response to these stimuli allows cells expressing HPV5, HPV8, or HPV38 E6 to remain proliferatively active, leading to further genome deterioration in a proportion of these cells. These phenotypes are lost by the introduction of a mutation into the p300 binding domain of HPV8 E6 or by the transfection of mutated p300 that is resistant to the degradation mediated by HPV5 or HPV8 E6. These findings expand the understanding of the role played by p300 in promoting the faithful resolution of mitotic figures as well as proper centrosome duplication. Finally, we describe a phenomenon by which binucleated cells are resolved via cytokinesis into two cells, each with one nucleus. These data support the hypothesis that ␤-HPV infections may promote tumorigenesis via genome destabilization. IMPORTANCEThe work described in this report provides support for the hypothesis that ␤-HPV infections may contribute to nonmelanoma skin cancer by increasing the likelihood that tumorigenic mutations are introduced into the host cell's genome. We demonstrate that expression of the E6 proteins from some of these viruses increases the tolerance of two genome-destabilizing events, aberrant cell division and dysregulated centrosome duplication. Typically, these mutagenic occurrences elicit the stabilization of the tumor suppressor p53, which prevents further propagation of cells containing these errors. We show that the expression of ␤-HPV E6 restricts this stabilization of p53, leading not only to continued cellular proliferation but also to further accumulation of similar mutagenic events. Finally, in addition to supporting a role for ␤-HPV infections in certain skin cancers, we present studies with a mutated ␤-HPV E6 protein suggesting that the histone acetyltransferase p300 plays a role in promoting genome stability during replication.

Genus Beta Human Papillomavirus E6 Proteins Vary in Their Effects on the Transactivation of p53 Target Genes

“…The genus beta HPV species 1 (HPV8 and -20) E6 proteins in our studies were never able to inhibit the expression of p53 targets, although we note that studies from other groups suggest that these E6s have other, perhaps complementary, effects on DNA repair after UV irradiation (28,31,58). We also note the recent publication of Wallace et al reporting that some beta HPV E6s block the stabilization of p53 after genotoxic stress resulting from multinucleation and abnormal centrosome phenotypes (59). Overall, it is clear that heterogeneity is a characteristic of the p53-dependent response to HPV E6 expression.…”

Atypical Squamous Cells: Cytopathological Findings and Correlation with HPV Genotype and Histopathology

“…Bi/multinucleation was diagnosed by the presence of 2 or more nuclei in the same squamous cell [17]. Corneal pearl was defined as a formation composed of squamous cells arranged concentrically on a keratin nucleus, shaped like an onion, occurring in intermediate and superficial cells [18].…”

Beta Human Papillomavirus and Associated Diseases

“…Furthermore, the E6 protein of β-1 HPVs 5 and 8, and β-2 HPV-38 were shown to attenuate p53 phosphorylation and ubiquitination in response to UVR exposure, thereby leading to a less effective repair of cellular damaged DNA [97]. β-2 HPV-38 E6 and E7 expression in human keratinocytes was also capable of activating telomerase by an E6-AP dependent mechanism [88, 96].…”

“…In addition to the ability to target pRb and cellular proliferation, HPV38 E7 also alters p53 transcriptional functions by inducing ΔNp73 accumulation, a p73 isoform that antagonizes p53 [98]. Binding to p300 was also observed for the E6 protein from β-2 HPV-38 resulting in the inhibition of p53 acetylation, and p53-induced repair in response to UVR damage, which together contribute to the accumulation of mutations, chromosomal abnormalities [97], and cell immortalization [99]. Alternatively, the E6 protein from β-1 HPVs 5 and 8, and β-2 HPV-38 also affect DNA repair by inactivating both ATM and ATR activities [97].…”