Activation of signaling pathways ensuring cell growth is essential for the proliferative competence of human papillomavirus (HPV)-infected cells. Tyrosine kinases and phosphatases are key regulators ...of cellular growth control pathways. A recently identified potential cellular target of HPV E7 is the cytoplasmic protein tyrosine phosphatase PTPN14, which is a potential tumor suppressor and is linked to the control of the Hippo and Wnt/beta-catenin signaling pathways. In this study, we show that the E7 proteins of both high-risk and low-risk mucosal HPV types can interact with PTPN14. This interaction is independent of retinoblastoma protein (pRb) and involves residues in the carboxy-terminal region of E7. We also show that high-risk E7 induces proteasome-mediated degradation of PTPN14 in cells derived from cervical tumors. This degradation appears to be independent of cullin-1 or cullin-2 but most likely involves the UBR4/p600 ubiquitin ligase. The degree to which E7 downregulates PTPN14 would suggest that this interaction is important for the viral life cycle and potentially also for the development of malignancy. In support of this we find that overexpression of PTPN14 decreases the ability of HPV-16 E7 to cooperate with activated EJ-ras in primary cell transformation assays.
This study links HPV E7 to the deregulation of protein tyrosine phosphatase signaling pathways. PTPN14 is classified as a potential tumor suppressor protein, and here we show that it is very susceptible to HPV E7-induced proteasome-mediated degradation. Intriguingly, this appears to use a mechanism that is different from that employed by E7 to target pRb. Therefore, this study has important implications for our understanding of the molecular basis for E7 function and also sheds important light on the potential role of PTPN14 as a tumor suppressor.
Approximately 200 human papillomaviruses (HPVs) infect human epithelial cells, of which the alpha and beta types have been the most extensively studied. Alpha HPV types mainly infect mucosal ...epithelia and a small group of these causes over 600,000 cancers per year worldwide at various anatomical sites, especially anogenital and head-and-neck cancers. Of these the most important is cervical cancer, which is the leading cause of cancer-related death in women in many parts of the world. Beta HPV types infect cutaneous epithelia and may contribute towards the initiation of non-melanoma skin cancers. HPVs encode two oncoproteins, E6 and E7, which are directly responsible for the development of HPV-induced carcinogenesis. They do this cooperatively by targeting diverse cellular pathways involved in the regulation of cell cycle control, of apoptosis and of cell polarity control networks. In this review, the biological consequences of papillomavirus targeting of various cellular substrates at diverse anatomical sites in the development of HPV-induced malignancies are highlighted.
Human papillomaviruses (HPVs), which are small, double-stranded, circular DNA viruses infecting human epithelial cells, are associated with various benign and malignant lesions of mucosa and skin. ...Intensive research on the oncogenic potential of HPVs started in the 1970s and spread across Europe, including Croatia, and worldwide. Nowadays, the causative role of a subset of oncogenic or high-risk (HR) HPV types, led by HPV-16 and HPV-18, of different anogenital and head and neck cancers is well accepted. Two major viral oncoproteins, E6 and E7, are directly involved in the development of HPV-related malignancies by targeting synergistically various cellular pathways involved in the regulation of cell cycle control, apoptosis, and cell polarity control networks as well as host immune response. This review is aimed at describing the key elements in HPV-related carcinogenesis and the advances in cancer prevention with reference to past and on-going research in Croatia.
Cancer-causing HPV E6 oncoproteins are characterized by the presence of a PDZ binding motif (PBM) at their extreme carboxy terminus. It was long thought that this region of E6 had a sole function to ...confer interaction with a defined set of cellular substrates. However, more recent studies have shown that the E6 PBM has a complex pattern of regulation, whereby phosphorylation within the PBM can regulate interaction with two classes of cellular proteins: those containing PDZ domains and the members of the 14-3-3 family of proteins. In this review, we explore the roles that the PBM and its ligands play in the virus life cycle, and subsequently how these can inadvertently contribute towards the development of malignancy. We also explore how subtle alterations in cellular signal transduction pathways might result in aberrant E6 phosphorylation, which in turn might contribute towards disease progression.
A subset of high-risk Human Papillomaviruses (HPVs) are the causative agents of a large number of human cancers, of which cervical is the most common. Two viral oncoproteins, E6 and E7, contribute ...directly towards the development and maintenance of malignancy. A characteristic feature of the E6 oncoproteins from cancer-causing HPV types is the presence of a PDZ binding motif (PBM) at its C-terminus, which confers interaction with cellular proteins harbouring PDZ domains. Here we show that this motif allows E6 interaction with Sorting Nexin 27 (SNX27), an essential component of endosomal recycling pathways. This interaction is highly conserved across E6 proteins from multiple high-risk HPV types and is mediated by a classical PBM-PDZ interaction but unlike many E6 targets, SNX27 is not targeted for degradation by E6. Rather, in HPV-18 positive cell lines the association of SNX27 with components of the retromer complex and the endocytic transport machinery is altered in an E6 PBM-dependent manner. Analysis of a SNX27 cargo, the glucose transporter GLUT1, reveals an E6-dependent maintenance of GLUT1 expression and alteration in its association with components of the endocytic transport machinery. Furthermore, knockdown of E6 in HPV-18 positive cervical cancer cells phenocopies the loss of SNX27, both in terms of GLUT1 expression levels and its vesicular localization, with a concomitant marked reduction in glucose uptake, whilst loss of SNX27 results in slower cell proliferation in low nutrient conditions. These results demonstrate that E6 interaction with SNX27 can alter the recycling of cargo molecules, one consequence of which is modulation of nutrient availability in HPV transformed tumour cells.
While a small proportion of high‐risk (HR) alpha (α) human papillomaviruses (HPVs) is associated with numerous human malignancies, of which cervical cancer is the most prevalent, beta (β) HPVs ...predominantly act as co‐factors in skin carcinogenesis. A characteristic feature of both α‐ and β‐E6 oncoproteins is the presence of the LXXLL binding motif, which α‐E6s utilize to form a complex with E6AP and which enables β‐E6s to interact with MAML1. Here we show that multiple α‐E6 oncoproteins bind to MAML1 via the LXXLL binding motif and that this results in increased protein stability. Moreover, β‐E6 oncoprotein stability is also dependent on the interaction with MAML1. Additionally, in the absence of MAML1, endogenous HPV‐8 E6 and HPV‐18 E6 are rapidly degraded at the proteasome. Ablation of both E6AP and MAML1 leads to an even more profound downregulation of α‐E6 protein expression, whereas this is not observed with β‐E6. This highly suggests that there is one cellular pool for most of β‐E6 that interacts solely with MAML1, whereas there are two cellular pools of HR α‐E6, one forming a complex with MAML1 and the other interacting with E6AP. Furthermore, MAML1 induces HPV‐8 E6 shuttling from the nucleus to the cytosolic fraction, while MAML1 interaction with HR E6 induces a drastic nuclear and membrane upregulation of E6. Interestingly, the HR α‐E6/MAML1 complex does not affect targeting of some of the known HR E6 cellular substrates such as p53 and DLG1. However, MAML1 and E6AP joint co‐expression with HR α‐E6 leads to a significant increase in cellular proliferation, whereas silencing MAML1 decreases wound closure in HeLa cells. These results demonstrate that HR α‐E6 interaction with MAML1 results in a stable form of E6, which likely modulates MAML1's normal cellular activities, one consequence of which being an increased proliferative capacity of HPV‐transformed cancer cells. Thus, this study shows a novel function of the α‐E6 oncoprotein and how it's activity might affect HPV‐induced pathogenesis.
Mucosal and skin cancers are associated with infections by human papillomaviruses (HPV). The manner how viral oncoproteins hijack the host cell metabolism to meet their own energy demands and how ...this may contribute to tumorigenesis is poorly understood. We now show that the HPV oncoprotein E7 of HPV8, HPV11 and HPV16 directly interact with the beta subunit of the mitochondrial ATP-synthase (ATP5B), which may therefore represent a conserved feature across different HPV genera. By measuring both glycolytic and mitochondrial activity we observed that the association of E7 with ATP5B was accompanied by reduction of glycolytic activity. Interestingly, there was a drastic increase in spare mitochondrial respiratory capacity in HPV8-E7 and an even more profound increase in HPV16-E7 expressing cells. In addition, we could show that ATP5B levels were unchanged in betaHPV positive skin cancers. However, comparing HPV-positive and HPV-negative oropharyngeal squamous cell carcinomas (OPSCC) we noticed that, while ATP5B expression levels did not correlate with patient overall survival in HPV-negative OPSCC, there was a strong correlation within the HPV16-positive OPSCC patient group. These novel findings provide evidence that HPV targets the host cell energy metabolism important for viral life cycle and HPV-mediated tumorigenesis.
The major causative agents of head and neck squamous cell carcinomas (HNSCCs) are either environmental factors, such as tobacco and alcohol consumption, or infection with oncogenic human ...papillomaviruses (HPVs). An important aspect of HPV-induced oncogenesis is the targeting by the E6 oncoprotein of PDZ domain-containing substrates for proteasomal destruction. Tumor suppressors DLG1 and SCRIB are two of the principal PDZ domain-containing E6 targets. Both have been shown to play critical roles in the regulation of cell growth and polarity and in maintaining the structural integrity of the epithelia. We investigated how modifications in the cellular localization and protein expression of DLG1 and SCRIB in HPV16-positive and HPV-negative histologic oropharyngeal squamous cell carcinomas (OPSCC) might reflect disease progression. HPV presence was determined by p16 staining and HPV genotyping. Whilst DLG1 expression levels did not differ markedly between HPV-negative and HPV16-positive OPSCCs, it appeared to be relocated from cell–cell contacts to the cytoplasm in most samples, regardless of HPV16 positivity. This indicates that alterations in DLG1 distribution could contribute to malignant progression in OPSCCs. Interestingly, SCRIB was also relocated from cell–cell contacts to the cytoplasm in the tumor samples in comparison with normal tissue, regardless of HPV16 status, but in addition there was an obvious reduction in SCRIB expression in higher grade tumors. Strikingly, loss of SCRIB was even more pronounced in HPV16-positive OPSCCs. These alterations in SCRIB levels may contribute to transformation and loss of tissue architecture in the process of carcinogenesis and could potentially serve as markers in the development of OPSCCs.
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