The ability of human papillomavirus E6 proteins to target p53 for degradation in vivo correlates with their ability to abrogate actinomycin D-induced growth arrest.

Functional p53 protein is associated with the ability of cells to arrest in G1 after DNA damage. The E6 protein of cancer-associated human papillomavirus type 16 (HPV-16) binds to p53 and targets its degradation through the ubiquitin pathway. To determine whether the ability of E6 to interact with p53 leads to a disruption of cell cycle control, mutated E6 proteins were tested for p53 binding and p53 degradation targeting in vitro, the ability to reduce intracellular p53 levels in vivo, and the ability to abrogate actinomycin D-induced growth arrest in human keratinocytes. Mutations scattered throughout the amino terminus, either zinc finger or the central region but not the carboxy terminus, severely reduced the ability of E6 to interact with p53. Expression of HPV-16 E6 or mutated E6 proteins that bound and targeted p53 for degradation in vitro sharply reduced the level of intracellular p53 induced by actinomycin D in human keratinocytes. A perfect correlation between the ability of E6 proteins to reduce the level of intracellular p53 and their ability to block actinomycin D-induced cellular growth arrest was observed. These results suggest that interaction with p53 is important for the ability of HPV E6 proteins to circumvent growth arrest.     

Human papillomavirus E6 proteins mediate resistance to interferon-induced growth arrest through inhibition of p53 acetylation

The high-risk human papillomavirus (HPV) E6 and E7 proteins act cooperatively to mediate multiple activities in viral pathogenesis. For instance, E7 acts to increase p53 levels while E6 accelerates its rate of turnover through the binding of the cellular ubiquitin ligase E6AP. Interferons are important antiviral agents that modulate both the initial and persistent phases of viral infection. The expression of HPV type 16 E7 was found to sensitize keratinocytes to the growth-inhibitory effects of interferon, while coexpression of E6 abrogates this inhibition. Treatment of E7-expressing cells with interferon ultimately resulted in cellular senescence through a process that is dependent upon acetylation of p53 by p300/CBP at lysine 382. Cells expressing mutant forms of E6 that are unable to bind p300/CBP or bind p53 failed to block acetylation of p53 at lysine 382 and were sensitive to growth arrest by interferon. In contrast, mutant forms of E6 that are unable to bind E6AP remain resistant to the effects of interferon, demonstrating that the absolute levels of p53 are not the major determinants of this activity. Finally, p53 acetylation at lysine 382 was found not to be an essential determinant of other types of senescence such as that induced by overexpression of Ras in human fibroblasts. This study identifies an important physiological role for E6 binding to p300/CBP in blocking growth arrest of human keratinocytes in the presence of interferon and so contributes to the persistence of HPV-infected cells.     

Involvement of nuclear export in human papillomavirus type 18 E6-mediated ubiquitination and degradation of p53

The E6 protein from high-risk human papillomaviruses (HPVs) targets the p53 tumor suppressor for degradation by the proteasome pathway. This ability contributes to the oncogenic potential of these viruses. However, several aspects concerning the mechanism of E6-mediated p53 degradation at the cellular level remain to be clarified. This study therefore examined the role of cell localization and ubiquitination in the E6-mediated degradation of p53. As demonstrated within, following coexpression both p53 and high-risk HPV type 18 (HPV-18) E6 (18E6) shuttle from the nucleus to the cytoplasm. Mutation of the C-terminal nuclear export signal (NES) of p53 or treatment with leptomycin B inhibited the 18E6-mediated nuclear export of p53. Impairment of nuclear export resulted in only a partial reduction in 18E6-mediated degradation, suggesting that both nuclear and cytoplasmic proteasomes can target p53 for degradation. This was also consistent with the observation that 18E6 mediated the accumulation of polyubiquitinated p53 in the nucleus. In comparison, a p53 isoform that localizes predominantly to the cytoplasm was not targeted for degradation by 18E6 in vivo but could be degraded in vitro, arguing that nuclear p53 is the target for E6-mediated degradation. This study supports a model in which (i) E6 mediates the accumulation of polyubiquitinated p53 in the nucleus, (ii) E6 is coexported with p53 from the nucleus to the cytoplasm via a CRM1 nuclear export mechanism involving the C-terminal NES of p53, and (iii) E6-mediated p53 degradation can be mediated by both nuclear and cytoplasmic proteasomes.     

Human papillomavirus type 16 E6 increases the degradation rate of p53 in human keratinocytes.

The E6 proteins of the high-risk human papillomaviruses (HPVs) have been shown to form a complex with and induce the degradation of human p53 in vitro. To determine whether p53 is degraded more rapidly in cells expressing E6 in vivo, the half-life of p53 was determined by pulse-chase analysis in early-passage normal human keratinocytes and fibroblasts, human keratinocytes immortalized with HPV type 16 (HPV16) E6 plus E7, and nonimmortal keratinocytes transfected with E6. The results of these experiments indicate that (i) the half-life of newly synthesized p53 is relatively long (4 h) in early-passage human keratinocytes and fibroblasts but short in keratinocytes expressing E6 (15 to 30 min), (ii) a similar increased rate of p53 degradation was measured in lines immortalized with HPV16 E6 plus E7 and senescent cells expressing E6, indicating that this increase is not simply the result of selection in the immortalized lines, and (iii) very low levels of expression of E6 result in a greatly decreased half-life of p53, suggesting that E6 acts in a catalytic manner.     

Interaction of the human papillomavirus type 16 E6 oncoprotein with wild-type and mutant human p53 proteins.

The E6 oncoproteins encoded by the cancer-associated human papillomaviruses (HPVs) can associate with and promote the degradation of wild-type p53 in vitro. To gain further insight into this process, the ability of HPV-16 E6 to complex with and promote the degradation of mutant forms of p53 was studied. A correlation between binding and the targeted degradation of p53 was established. Mutant p53 proteins that bound HPV-16 E6 were targeted for degradation, whereas those that did not complex HPV-16 E6 were not degraded. Since the HPV-16 E6-promoted degradation involves the ubiquitin-dependent proteolysis pathway, specific mutations were made in the amino terminus of p53 to examine whether the E6 targeted degradation involved the N-end rule pathway. No requirement for destabilizing amino acids at the N terminus of p53 was found, nor was evidence found that HPV-16 E6 could provide this determinant in trans, indicating that the N-terminal rule pathway is not involved in the E6-promoted degradation of p53.     

Human papillomavirus E6 regulates the cytoskeleton dynamics of keratinocytes through targeted degradation of p53

The attachment and spreading of keratinocyte cells result from interactions between integrins and immobilized extracellular matrix molecules. Human papillomavirus type 16 (HPV-16) E6 augmented the kinetics of cell spreading, while E6 genes from HPV-11 or bovine papillomavirus type 1 did not. The ability of E6 to interact with the E6AP ubiquitin ligase and target p53 degradation was required to augment cell-spreading kinetics; dominant negative p53 alleles also enhanced the kinetics of cell spreading and the level of attachment of cells to hydrophobic surfaces. The targeted degradation of p53 by E6 may contribute to the invasive phenotype exhibited by cervical cells that contain high-risk HPV types.     

Non-human primate papillomavirus E6-mediated p53 degradation reveals ancient evolutionary adaptation of carcinogenic phenotype to host niche

Non-human primates (NHPs) are infected with papillomaviruses (PVs) closely related to their human counterparts, but there are few studies on the carcinogenicity of NHP-PVs. Using an in vitro cell co-transfection assay, we systematically screened the biochemical activity of E6 proteins encoded by macaque PVs for their ability to bind and promote degradation of host p53 proteins. A host species barrier exists between HPV16 and MfPV3 with respect to E6-mediated p53 degradation that is reversed when p53 residue 129 is swapped between human and macaque hosts. Systematic investigation found that E6 proteins encoded by most macaque PV types in the high-risk species α12, but not other Alpha-PV clades or Beta-/Gamma-PV genera, can effectively promote monkey p53 degradation. Interestingly, two macaque PV types (MfPV10 and MmPV1) can simultaneously inhibit the expression of human and monkey p53 proteins, revealing complex cross-host interactions between PV oncogenes and host proteomes. Single point-mutant experiments revealed that E6 residue 47 directly interacts with p53 residue 129 for host-specific degradation. These findings suggest an ancient host niche adaptation toward a carcinogenic phenotype in high-risk primate PV ancestors. Following periods of primate host speciation, a loss-of-function mutation model could be responsible for the formation of a host species barrier to E6-mediated p53 degradation between HPVs and NHP-PVs. Our work lays a genetic and functional basis for PV carcinogenicity, which provides important insights into the origin and evolution of specific pathogens in host pathogenesis.     

Localization of the E6-AP regions that direct human papillomavirus E6 binding, association with p53, and ubiquitination of associated proteins.

E6-AP is a 100-kDa cellular protein that mediates the interaction of the human papillomavirus type 16 and 18 E6 proteins with p53. The association of p53 with E6 and E6-AP promotes the specific ubiquitination and subsequent proteolytic degradation of p53 in vitro. We recently isolated a cDNA encoding E6-AP and have now mapped functional domains of E6-AP involved in binding E6, association with p53, and ubiquitination of p53. The E6 binding domain consists of an 18-amino-acid region within the central portion of the molecule. Deletion of these 18 amino acids from E6-AP results in loss of both E6 and p53 binding activities. The region that directs p53 binding spans the E6 binding domain and consists of approximately 500 amino acids. E6-AP sequences in addition to those required for formation of a stable ternary complex with E6 and p53 are necessary to stimulate the ubiquitination of p53. These sequences lie within the C-terminal 84 amino acids of E6-AP. The entire region required for E6-dependent ubiquitination of p53 is also required for the ubiquitination of an artificial E6 fusion protein.     

Morphologic conversion of a neuroblastoma-derived cell line by E6-mediated p53 degradation.

Neuroblastoma-derived tumor cells, unlike cells from other tumor types, characteristically express a wildtype but cytoplasmically sequestered p53 protein. To ascertain whether the p53 in these cells retained any physiological activity, we inactivated it in SK-N-SH cells, a neuroblastoma-derived cell line, by introducing the human papilloma virus type 16 E6 expression plasmid. Parent SK-N-SH cell cultures are composed of two cell types exhibiting characteristic morphologies designated neuroblastic (N-type) or substrate-adherent fibroblastic (S-type) cells, both of which have been shown to spontaneously transdifferentiate or interconvert. We report here that down-regulation of p53 resulted in conversion of SK-N-SH cells to the substrate-adherent fibroblast-like S-type cells. The morphologic conversion was accompanied by a loss of neurofilament expression, a marker for the neuronal N-type cells, an increase in the expression of vimentin, and a lack of responsiveness to retinoic acid-induced neuronal differentiation. Importantly, we did not observe N-type cells in the E6-transfected cell population, suggesting that they were incapable of transdifferentiating to the N-type morphology. We also tested the ability of these E6-transfected S-type cells to form colonies in soft agar and observed a markedly reduced capacity of these cells to do so when compared with the parent and mutant E6-transfected cells. These results suggest that p53 is required for the maintenance of the neuroblastic tumorigenic phenotype.     

The role of the ubiquitin ligase E6-AP in human papillomavirus E6-mediated degradation of PDZ domain-containing proteins

The E6 oncoprotein of human papillomaviruses associated with cervical cancer targets the tumor suppressor p53 and several other cellular proteins including the human homologs of Dlg and Scribble for degradation via the ubiquitin-proteasome system. Similar to p53 degradation, E6-induced degradation of Scribble is mediated by the ubiquitin ligase E6-AP. In contrast, degradation of Dlg in vitro and within cells has been reported to be independent of E6-AP, suggesting that the E6 oncoprotein has the ability to interact with ubiquitin ligases other than E6-AP. Furthermore, the ability of the E6 oncoprotein to interact with these yet unidentified ubiquitin ligases may be shared by the E6 protein of so-called low risk human papillomaviruses that are not associated with cervical cancer. In this study, we used the RNA interference technology and mouse embryo fibroblasts derived from E6-AP-deficient mice to obtain information about the identity of the ubiquitin ligase(s) involved in E6-mediated degradation of Dlg. We report that, within cells, E6-mediated degradation of Dlg depends on the presence of functional E6-AP and provide evidence that the E6 protein of low risk human papillomaviruses functionally interacts with E6-AP. Based on these data, we propose that, in general, the proteolytic properties of human papillomavirus E6 proteins are mediated by interaction with E6-AP.     

Loss of p53 protein in human papillomavirus type 16 E6-immortalized human mammary epithelial cells.

We have shown previously that introduction of the human papillomavirus type 16 (HPV16) or HPV18 genome into human mammary epithelial cells induces their immortalization. These immortalized cells have reduced growth factor requirements. We report here that transfection with a single HPV16 gene E6 is sufficient to immortalize these cells and reduce their growth factor requirements. The RB protein is normal in these cells, but the p53 protein is sharply reduced, as shown by immunoprecipitation with anti-p53 antibody (pAB 421). We infer that the E6 protein reduces the p53 protein perhaps by signalling its destruction by the ubiquitin system. The HPV-transforming gene E7 was unable to immortalize human mammary epithelial cells. Thus, cell-specific factors may determine which viral oncogene plays a major role in oncogenesis.