Loss of the E6AP Ubiquitin Ligase Induces p53-Dependent Phosphorylation of Human Papillomavirus 18 E6 in Cells Derived from Cervical Cancer.

Cancer-causing human papillomavirus (HPV) E6 oncoproteins contain a well-characterized phosphoacceptor site within the PDZ (PSD-95/Dlg/ZO-1) binding motif (PBM) at the C terminus of the protein. Previous studies have shown that the threonine or serine residue in the E6 PBM is subject to phosphorylation by several stress-responsive cellular kinases upon the induction of DNA damage in cervical cancer-derived cells. However, there is little information about the regulation of E6 phosphorylation in the absence of DNA damage and whether there may be other pathways by which E6 is phosphorylated. In this study, we demonstrate that loss of E6AP results in a dramatic increase in the levels of phosphorylated E6 (pE6) despite the expected overall reduction in total E6 protein levels. Furthermore, phosphorylation of E6 requires transcriptionally active p53 and occurs in a manner that is dependent upon DNA-dependent protein kinase (DNA PK). These results identify a novel feedback loop, where loss of E6AP results in upregulation of p53, leading to increased levels of E6 phosphorylation, which in turn correlates with increased association with 14-3-3 and inhibition of p53 transcriptional activity. IMPORTANCE This study demonstrates that the knockdown of E6AP from cervical cancer-derived cells leads to an increase in phosphorylation of the E6 oncoprotein. We show that this phosphorylation of E6 requires p53 transcriptional activity and the enzyme DNA PK. This study therefore defines a feedback loop whereby activation of p53 can induce phosphorylation of E6 and which in turn can inhibit p53 transcriptional activity independently of E6's ability to target p53 for degradation.

Regulation of HPV E7 Stability by E6-Associated Protein (E6AP).

High-risk human papillomaviruses (HPVs) are responsible for most human cervical cancers, and uncontrolled expression of the two key viral oncoproteins, E6 and E7, stimulates the induction of carcinogenesis. Previous studies have shown that both E6 and E7 are closely associated with different components of the ubiquitin proteasome pathway, including several ubiquitin ligases. Most often these are utilized to target cellular substrates for proteasome-mediated degradation, but in the case of E6, the E6AP ubiquitin ligase plays a critical role in controlling E6 stability. We now show that knockdown of E6AP in HPV-positive cervical cancer-derived cells causes a marked decrease in E7 protein levels. This is due to a decrease in the E7 half-life and occurs in a proteasome-dependent manner. In an attempt to define the underlying mechanism, we show that E7 can also associate with E6AP, albeit in a manner different from that of E6. In addition, we show that E6AP-dependent stabilization of E7 also leads to an increase in the degradation of E7's cellular target substrates. Interestingly, ectopic overexpression of E6 oncoprotein results in lower levels of E7 protein through sequestration of E6AP. We also show that increased E7 stability in the presence of E6AP increases the proliferation of the cervical cancer-derived cell lines. These results demonstrate a surprising interplay between E6 and E7, in a manner which is mediated by the E6AP ubiquitin ligase. IMPORTANCE This is the first demonstration that E6AP can directly help stabilize the HPV E7 oncoprotein, in a manner similar to that observed with HPV E6. This redefines how E6 and E7 can cooperate and potentially modulate each other's activity and further highlights the essential role played by E6AP in the viral life cycle and malignancy.

Identification of E6AP-independent degradation targets of HPV E6.

The high-risk Human Papillomavirus (HPV) E6 oncoprotein is known to contribute to human malignancy by targeting several of its cellular substrates through the ubiquitin-mediated degradation pathway. Previous studies have revealed that E6 interacts with the E6AP ubiquitin-protein ligase and directs its ubiquitylation activity toward several specific cellular proteins, one of the most important of which is p53. However, the role of E6AP in the degradation of many other E6 substrates is still ambiguous because loss of E6AP also induces a loss of E6 expression. To examine this further, we used CRISPR-edited E6AP knockout cells to perform E6 degradation assays in the presence of a catalytically inactive mutant form of E6AP, thus ensuring the stabilization of E6 but with the ligase itself being functionally inactive. Using this system, we found that E6 can mediate the degradation of several PDZ domain-containing proteins independently of E6AP ubiquitin ligase activity. This study thus opens up ways to investigate other possible components of the cellular ubiquitin proteasome pathway that E6 might utilize to target these substrates.

Molecular dissection of the E6 PBM identifies essential residues regulating Chk1 phosphorylation and subsequent 14-3-3 recognition.

Previous studies have shown that the high-risk HPV E6 oncoprotein PDZ binding motifs (PBMs) can interact with PDZ proteins or members of the 14-3-3 family, depending upon the E6 phosphorylation status. However, different HPV E6 oncoproteins are subjected to phosphorylation by different cellular kinases. We have therefore been interested in determining whether we can dissect E6's PDZ and 14-3-3 interactions at the molecular level. Using HPV-18 E6, we have found that its Chk1 phosphorylation requires residues both upstream and downstream of the phospho-acceptor site, in addition to the Chk1 consensus recognition motif. Furthermore, we demonstrate that different high-risk HPV E6 types are differentially phosphorylated by Chk1 kinases, potentially due to the differences in their carboxy-terminal residues, as they are critical for kinase recognition. Moreover, differences in the E6 phosphorylation levels of different HR HPV types directly link to their ability to interact with different 14-3-3 isoforms, based on their phospho-status. Interestingly, 14-3-3 recognition appears to be less dependent upon the precise sequence constraints of the E6 carboxy terminal region, whilst minor amino acid variations have a major impact upon PDZ recognition. These results demonstrate that changes in E6 phospho-status during the life cycle or during malignant progression will modulate E6 interactions and, potentially, inversely regulate the levels of PDZ and 14-3-3 proteins.

Human papillomavirus E6 and E7: What remains?

Decades of research on the human papillomavirus oncogenes, E6 and E7, have given us huge amounts of data on their expression, functions and structures. We know much about the very many cellular proteins and pathways that they influence in one way or another. However, much of this information is quite discrete, referring to one activity examined under one condition. It is now time to join the dots to try to understand a larger picture: how, where and when do all these interactions occur... and why? Examining these questions will also show how many of the yet obscure cellular processes work together for cellular and tissue homeostasis in health and disease.

Inhibition of kinase IKKß suppresses cellular abnormalities induced by the human papillomavirus oncoprotein HPV 18E6.

Human papillomavirus (HPV) is the leading cause of cervical cancer and has been implicated in several other cancer types including vaginal, vulvar, penile, and oropharyngeal cancers. Despite the recent availability of a vaccine, there are still over 310,000 deaths each year worldwide. Current treatments for HPV-mediated cancers show limited efficacy, and would benefit from improved understanding of disease mechanisms. Recently, we developed a Drosophila 'HPV 18 E6' model that displayed loss of cellular morphology and polarity, junctional disorganization, and degradation of the major E6 target Magi; we further provided evidence that mechanisms underlying HPV E6-induced cellular abnormalities are conserved between humans and flies. Here, we report a functional genetic screen of the Drosophila kinome that identified IKK[Formula: see text]-a regulator of NF-κB-as an enhancer of E6-induced cellular defects. We demonstrate that inhibition of IKK[Formula: see text] reduces Magi degradation and that this effect correlates with hyperphosphorylation of E6. Further, the reduction in IKK[Formula: see text] suppressed the cellular transformation caused by the cooperative action of HPVE6 and the oncogenic Ras. Finally, we demonstrate that the interaction between IKK[Formula: see text] and E6 is conserved in human cells: inhibition of IKK[Formula: see text] blocked the growth of cervical cancer cells, suggesting that IKK[Formula: see text] may serve as a novel therapeutic target for HPV-mediated cancers.

MCV Truncated Large T antigen interacts with BRD4 in tumors.

Among Polyomaviridae family of viruses, Merkel Cell Polyomavirus (MCV) is the only human polyomavirus with convincing data supporting its classification as a direct causative agent of a human skin malignancy, Merkel Cell Carcinoma. Oncogenic transformation by MCV requires the integration of the viral genome into the human genome, truncation of the large T antigen (LT) to render the viral genome replication deficient and expression of small T antigen oncoprotein. The chromatin binding protein BRD4, was recently shown to transcriptionally regulate the expression of virus oncoproteins, thereby enhancing the tumorigenesis of virus-associated cancers, such as HPV associated cervical cancer. Previous work by Wang et al. revealed that BRD4 interacts with MCV full length LT during viral replication. In this study, we demonstrated that MCV truncated tumor LT antigen also interacts with BRD4 protein. We showed that the MCV tumor LT antigen and BRD4 protein complex co-localizes within the nucleus. Furthermore, we tested whether BRD4 protein transcriptionally regulates MCV Non Coding Control Region (NCCR), where we found that though full length LT and sT together, along with the BRD4 protein showed enhanced transcriptional activity whereas tumor truncated LT did not. These findings on the interactions of the MCV tumor truncated LT antigen with the BRD4 protein add to existing knowledge about interactions with LT and its role in tumorigenesis, and assist in efforts to more precisely define new therapy targets for this disease.