Mutational analysis of transcriptional activation by the bovine papillomavirus type 1 E6

Papillomaviruses: a systematic review

In the last decades, a group of viruses has received great attention due to its relationship with cancer development and its wide distribution throughout the vertebrates: the papillomaviruses. In this article, we aim to review some of the most relevant reports concerning the use of bovines as an experimental model for studies related to papillomaviruses. Moreover, the obtained data contributes to the development of strategies against the clinical consequences of bovine papillomaviruses (BPV) that have led to drastic hazards to the herds. To overcome the problem, the vaccines that we have been developing involve recombinant DNA technology, aiming at prophylactic and therapeutic procedures. It is important to point out that these strategies can be used as models for innovative procedures against HPV, as this virus is the main causal agent of cervical cancer, the second most fatal cancer in women.

Mutagenic Potential ofBos taurus Papillomavirus Type 1 E6 Recombinant Protein: First Description

Bovine papillomavirus (BPV) is considered a useful model to study HPV oncogenic process. BPV interacts with the host chromatin, resulting in DNA damage, which is attributed to E5, E6, and E7 viral oncoproteins activity. However, the oncogenic mechanisms of BPV E6 oncoprotein per se remain unknown. This study aimed to evaluate the mutagenic potential of Bos taurus papillomavirus type 1 (BPV-1) E6 recombinant oncoprotein by the cytokinesis-block micronucleus assay (CBMNA) and comet assay (CA). Peripheral blood samples of five calves were collected. Samples were subjected to molecular diagnosis, which did not reveal presence of BPV sequences. Samples were treated with 1 μg/mL of BPV-1 E6 oncoprotein and 50 μg/mL of cyclophosphamide (positive control). Negative controls were not submitted to any treatment. The samples were submitted to the CBMNA and CA. The results showed that BPV E6 oncoprotein induces clastogenesis per se, which is indicative of genomic instability. These results allowed better understanding the mechanism of cancer promotion associated with the BPV E6 oncoprotein and revealed that this oncoprotein can induce carcinogenesis per se. E6 recombinant oncoprotein has been suggested as a possible vaccine candidate. Results pointed out that BPV E6 recombinant oncoprotein modifications are required to use it as vaccine.

A mitotic kinase scaffold depleted in testicular seminomas impacts spindle orientation in germ line stem cells

Correct orientation of the mitotic spindle in stem cells underlies organogenesis. Spindle abnormalities correlate with cancer progression in germ line-derived tumors. We discover a macromolecular complex between the scaffolding protein Gravin/AKAP12 and the mitotic kinases, Aurora A and Plk1, that is down regulated in human seminoma. Depletion of Gravin correlates with an increased mitotic index and disorganization of seminiferous tubules. Biochemical, super-resolution imaging, and enzymology approaches establish that this Gravin scaffold accumulates at the mother spindle pole during metaphase. Manipulating elements of the Gravin-Aurora A-Plk1 axis prompts mitotic delay and prevents appropriate assembly of astral microtubules to promote spindle misorientation. These pathological responses are conserved in seminiferous tubules from Gravin(-/-) mice where an overabundance of Oct3/4 positive germ line stem cells displays randomized orientation of mitotic spindles. Thus, we propose that Gravin-mediated recruitment of Aurora A and Plk1 to the mother (oldest) spindle pole contributes to the fidelity of symmetric cell division.

Bovine papillomavirus E7 transformation function correlates with cellular p600 protein binding

The E7 oncoprotein of bovine papillomavirus type 1 (BPV-1) is required for the full transformation activity of the virus. However, the mechanism by which E7 contributes to cellular transformation is unknown. To address this question, we used the proteomic approach of tandem affinity purification to identify cellular proteins that are in complex with E7, and identified the 600-kDa protein, p600, as a binding partner of E7. The ability of E7 to complex with p600 correlated with its ability to enhance anchorage independence of BPV-1 E6-expressing cells. Furthermore, E7 mutant proteins impaired in their ability to bind p600 were transformation defective. Additionally, knockdown of p600 reduced transformation of cells expressing both BPV-1 E6 and E7, as well as E6 alone, suggesting that the ability of E7 to transformed cells is mediated, at least in part, through its ability to bind p600. These data complement work that shows that HPV16 E7 also interacts with p600, and that this interaction correlates with the ability of HPV16 E7 to transform cells. These studies thus identify p600 as a shared target of the E7 proteins of multiple papillomaviruses.

The E6AP ubiquitin ligase is required for transactivation of the hTERT promoter by the human papillomavirus E6 oncoprotein

Most human cancer cells display increased telomerase activity that appears to be critical for continued cell proliferation and tumor formation. The E6 protein of malignancy-associated human papillomaviruses increases cellular telomerase in primary human keratinocytes at least partly via transcriptional activation of the telomerase catalytic subunit, hTERT. In the present study, we investigated whether E6AP, a ubiquitin ligase well known for binding and mediating some of the activities of the E6 oncoprotein, participated in the transactivation of the hTERT promoter. Our results demonstrate that E6 mutants that fail to bind E6AP are also defective for increasing telomerase activity and transactivating the hTERT promoter. More importantly, E6AP knock-out mouse cells and small interfering RNA techniques demonstrated that E6AP was required for hTERT promoter transactivation in both mouse and human cells. Neither E6 nor E6AP bound to the hTERT promoter or activated the promoter in the absence of the partner protein. With all transactivation-competent E6 proteins, induction of the hTERT promoter was dependent upon E box elements in the core promoter. It appears, therefore, that E6-mediated activation of the hTERT promoter requires a complex of E6-E6AP to engage the hTERT promoter and that activation is dependent upon Myc binding sites in the promoter. The recruitment of a cellular ubiquitin ligase to the hTERT promoter during E6-mediated transcriptional activation suggests a role for the local ubiquitination (and potential degradation) of promoter-associated regulatory proteins, including the Myc protein.

Interaction of oncogenic papillomavirus E6 proteins with fibulin-1

Human papillomavirus (HPV) infection is the primary risk factor for the development of cervical cancer. The papillomavirus E6 gene is essential for virus-induced cellular transformation and the viral life cycle. Important insight into the mechanism of E6 function came from the discovery that cancer-related HPV E6 proteins promote the degradation of the tumor suppressor p53. However, mounting evidence indicates that interaction with p53 does not mediate all E6 activities. To explore the p53-independent functions of E6, we performed a yeast two-hybrid screen and identified fibulin-1 as an E6 binding protein. Fibulin-1 is a calcium-binding plasma and extracellular matrix protein that has been implicated in cellular transformation and tumor invasion. The interaction between E6 and fibulin-1 was demonstrated by both in vitro and in vivo assays. Fibulin-1 is associated specifically with cancer-related HPV E6s and the transforming bovine papillomavirus type 1 E6. Significantly, overexpression of fibulin-1 specifically inhibited E6-mediated transformation. These results suggest that fibulin-1 plays an important role in the biological activities of E6.

Bovine papillomavirus type 1 E6-induced sensitization to apoptosis is distinct from its transforming activity

The bovine papillomavirus type 1 (BPV-1) E6 oncoprotein induces tumorigenic transformation of murine C127 cells and stimulates transcription when targeted to a promoter. We have previously shown that C127 cells expressing BPV-1 E6 exhibited increased tumor necrosis factor alpha (TNF)-mediated apoptosis. To understand the mechanisms by which BPV-1 E6 sensitizes cells to apoptosis and to investigate the relevance of E6-enhanced apoptosis to its other biological activities, we analyzed a BPV-1 E6 mutant (491, with four amino acids deleted at the C-terminus) for its ability to sensitize C127 cells to apoptosis. The result was then compared with the E6 mutant's ability to transform cells, to activate transcription, and to associate with known cellular binding proteins. Our data indicated that the transcriptional activation function of BPV-1 E6 correlated with sensitization of cells to TNF-mediated apoptosis. Moreover, functions required for BPV-1 E6-mediated sensitization of cells to apoptosis are distinct from those required for transformation. A potential role of paxillin in E6 sensitization of cells to apoptosis is implicated. These results thus indicate that sensitization of cells to TNF-induced apoptosis represents a novel function of BPV-1 E6.

Human scribble (Vartul) is targeted for ubiquitin-mediated degradation by the high-risk papillomavirus E6 proteins and the E6AP ubiquitin-protein ligase

The high-risk human papillomavirus (HPV) E6 proteins stimulate the ubiquitination and degradation of p53, dependent on the E6AP ubiquitin-protein ligase. Other proteins have also been shown to be targeted for degradation by E6, including hDlg, the human homolog of the Drosophila melanogaster Discs large (Dlg) tumor suppressor. We show here that the human homolog of the Drosophila Scribble (Vartul) (hScrib) tumor suppressor protein is also targeted for ubiquitination by the E6-E6AP complex in vitro and that expression of E6 induces degradation of hScrib in vivo. Characterization of the E6AP-E6-hScrib complex indicated that hScrib binds directly to E6 and that the binding is mediated by the PDZ domains of hScrib and a carboxyl-terminal epitope conserved among the high-risk HPV E6 proteins. Green fluorescent protein-hScrib was localized to the periphery of MDCK cells, where it colocalized with ZO-1, a component of tight junctions. E6 expression resulted in loss of integrity of tight junctions, as measured by ZO-1 localization, and this effect was dependent on the PDZ binding epitope of E6. Thus, the high-risk HPV E6 proteins induce the degradation of the human homologs of two Drosophila PDZ domain-containing tumor suppressor proteins, hDlg and hScrib, both of which are associated with cell junction complexes. The fact that Scrib/Vart and Dlg appear to cooperate in a pathway that controls Drosophila epithelial cell growth suggests that the combined targeting of hScrib and hDlg is an important component of the biologic activity of high-risk HPV E6 proteins.

Interaction with CBP/p300 enables the bovine papillomavirus type 1 E6 oncoprotein to downregulate CBP/p300-mediated transactivation by p53

The E6 oncoprotein of bovine papillomavirus type 1 (BPV-1) can transform cells independently of p53 degradation. The precise mechanisms underlying this transformation are not yet completely understood. Here it is shown that BPV-1 E6 interacts with CBP/p300 in the same way as described for the E6 proteins of oncogenic human papillomaviruses. This interaction results in an inhibition of the transcriptional coactivator function of CBP/p300 required by p53 and probably by other transcription factors. The comparison of the CBP/p300-binding properties of BPV-1 E6 mutants previously characterized in transcription and transformation studies suggests (i) that the E6-CBP/p300 interaction may be necessary, but not sufficient, for cell transformation, and (ii) that the transcriptional activator function, inherent to the E6 protein, is not derived from forming a complex with CBP/p300.

The bovine papillomavirus type 1 E6 oncoprotein sensitizes cells to tumor necrosis factor alpha-induced apoptosis

Expression of viral proteins may result in susceptibility of cells to the cytotoxic effect of Tumor Necrosis Factor Alpha (TNF). While murine C127 cells containing the bovine papillomavirus type 1 (BPV-1) genome were reported to exhibit increased TNF sensitivity, the gene(s) responsible was not identified. The BPV-1 E6 oncoprotein induces tumorigenic transformation of murine C127 cells and stimulates transcription when targeted to a promoter. BPV-1 E6 was introduced into C127 cells (PBE6) by retroviral infection and stable clones were isolated. These cells showed increased apoptosis in response to TNF, as measured by several criteria. TNF-induced apoptosis in PBE6 cells was accompanied by increased release of arachidonic acid, indicating that phospholipase A2 was activated. We also provide evidence that BPV-1 E6 mediated-sensitization of cells to TNF-induced apoptosis can occur in the absence of p53.

The papillomavirus E6 proteins

Specific types of human papillomaviruses (HPV) are strongly associated with the development of cervical cancer. The E6 gene from cancer-related HPVs has exhibited functions in tumorigenesis, regulation of transcription, telomerase, and apoptosis. Cancer-related HPVs E6 proteins bind the tumor suppressor p53 and promotes its degradation through an ubiquitin-dependent pathway. Several additional cellular E6-binding proteins have recently been identified and implicated in playing roles in p53-independent functions of E6.