Biomedical subjects

D Pim

Publications and source records attributed to D Pim.

At least 19 recordsLinked to original sources

PubMed2001

HPV E6 and MAGUK protein interactions: determination of the molecular basis for specific protein recognition and degradation.

It has recently been shown that the high-risk human papillomavirus (HPV) E6 proteins can target the PDZ-domain containing proteins, Dlg, MUPP-1, MAGI-1 and hScrib for proteasome-mediated degradation. However, the E6 proteins from HPV-16 and HPV-18 (the two most common high-risk virus types) differ in their ability to target these proteins in a manner that correlates with their malignant potential. To investigate the underlying mechanisms for this, we have mutated HPV-16 and HPV-18 E6s to give each protein the other's PDZ-binding motif. Analysis of these mutants shows that the greater ability of HPV-18 E6 to bind to these proteins and to target them for degradation is indeed due to a single amino acid difference. Using a number of assays, we show that the E6 proteins interact specifically with only one of the five PDZ domains of MAGI-1, and this is the first interaction described for this particular PDZ domain. We also show that the guanylate kinase homology domain and the regions of MAGI-1 downstream of amino acid 733 are not required for the degradation of MAGI-1. Finally, in a series of comparative analyses, we show that the degradation of MAGI-1 occurs through a different mechanism from that used by the E6 protein to induce the degradation of Dlg and p53.

Amino Acid Motifs↗

PubMed2001

Regulation of the human papillomavirus oncoproteins by differential phosphorylation.

Human papillomaviruses (HPVs) are intimately associated with the development of cervical cancer. The virus encodes two oncoproteins, E6 and E7, that are primarily responsible for inducing malignant transformation. The last few years have seen significant progress in elucidating the mechanisms by which these two viral proteins bring about cell transformation. Both proteins interact with a large number of cellular targets, many of which are involved in regulating diverse functions such as cell cycle regulation, transcription, differentiation and apoptosis. However both E6 and E7 are normally present at low levels within the virally infected cell, and how all these interactions are achieved and regulated has, until recently, been unclear. We have found that both E6 and E7 are subject to differential phosphorylation, the net results of which regulate their abilities to interact with some of their respective target proteins. In the case of E6, phosphorylation by Protein Kinase A (PKA) negatively regulates its ability to interact with the Discs Large (Dlg) tumour suppressor. In the case of E7, phosphorylation by Casein Kinase II (CKII) significantly increases its ability to interact with the TATA Box Binding Protein (TBP). Further, CKII regulation of E7 appears to vary during the cell cycle, therefore this provides a means of specifically targeting E7 to a given substrate at a given point within the cell cycle. This differential regulation of E6 and E7 by phosphorylation thus provides specificity to a diverse set of protein-protein interactions.

Animals↗

PubMed2000

Allosteric activation of acid alpha-glucosidase by the human papillomavirus E7 protein.

Changes in the cellular carbohydrate metabolism are a hallmark of malignant transformation and represent one of the earliest discernible events in tumorigenesis. In the early stages of certain epithelial cancers, a metabolic switch is regularly observed, in which slowly growing glycogenotic cells are converted to highly proliferating basophilic cells. This step is accompanied by a rapid depletion of the intracellular glycogen stores, which in liver carcinogenesis results from the activation of the enzyme acid alpha-glucosidase by an as yet unknown mechanism. We show here that acid alpha-glucosidase is a target for the E7 protein encoded by human papillomavirus type 16, a human tumor virus that plays a key role in the genesis of cervical carcinoma. We show that expression of E7 induces the catalytic activity of acid alpha-glucosidase in vivo and wild type E7, but not transformation-deficient mutants bind directly to acid alpha-glucosidase and increase the catalytic activity of the enzyme in vitro. The data suggest that the E7 protein encoded by human papillomavirus type 16 can act as an allosteric activator of acid alpha-glucosidase.

Allosteric Regulation↗

PubMed2000

HPV E6 targeted degradation of the discs large protein: evidence for the involvement of a novel ubiquitin ligase.

The Discs Large (DLG) tumour suppressor protein is targeted for ubiquitin mediated degradation by the high risk human papillomavirus (HPV) E6 proteins. In this study we have used a mutational analysis of E6 in order to investigate the mechanism by which this occurs. We first show that the differences in the affinities of HPV-16 and of HPV-18 E6 proteins for binding DLG is reflected in their respective abilities to target DLG for degradation. A mutational analysis of HPV-18 E6 has enabled us to define regions within the carboxy terminal half of the protein which are essential for the ability of E6 to direct the degradation of DLG. Mutants within the amino terminal portion of E6 which have lost the ability to bind the E6-AP ubiquitin ligase, as measured by their ability to degrade p53, nonetheless retain the ability to degrade DLG. Significant levels of DLG degradation are also obtained using wheat germ extracts which lack E6-AP. Finally, we show that the transfer of the DLG binding domain onto the low risk HPV-6 E6 confers DLG binding activity to that protein and, most significantly, allows HPV-6 E6 to target DLG for degradation. These results indicate that E6 mediated degradation of DLG does not involve the E6-AP ubiquitin ligase and, in addition, shows that the high and low risk HPV E6 proteins most likely share a common cellular intermediary in the ubiquitin pathway.

Adaptor Proteins, Signal Transducing↗

PubMed1999

Interaction between the HPV-16 E2 transcriptional activator and p53.

The HPV-16 E2 protein is a major regulator of viral DNA replication and gene expression. Through interactions with the viral origin binding protein, E1, it localizes E1 to the origin of replication and stimulates the initiation of viral DNA replication. However, several recent reports have described a number of diverse activities of E2 relating to the induction of apoptosis through both p53 dependent and independent mechanisms, and to induction of growth arrest in both the G1 and G2M phases of the cell cycle. Recent studies have also shown that p53 can specifically inhibit HPV DNA replication, albeit through an unknown mechanism. Since p53 has been described in the replication centres of Herpes Viruses, Adenovirus and SV40 we decided to investigate whether any of the above activities of E2 may be related to an association with p53. We show, in a series of in vitro assays, specific interaction between p53 and HPV-16 E2 via residues in the carboxy terminal half of the E2 protein. Mutational analysis of p53 indicates that sequences in both the DNA binding and oligomerization domains are essential for the interaction, and a mutant of p53 which is unable to bind E2 is also unable to inhibit HPV DNA replication. Finally, using an inducible system of p53 expression we also show that E2 will complex with p53 in vivo. These results raise the intriguing possibility that p53 may also be involved in HPV DNA replication centres, and also provides explanations for some of the diverse activities reported for the HPV E2 proteins.

Amino Acid Sequence↗

PubMed1999

The role of the E6-p53 interaction in the molecular pathogenesis of HPV.

Human papillomaviruses (HPVs) are associated with a number of clinical conditions, of which the most serious is cervical carcinoma. The E6 protein of the oncogenic, mucosal-specific HPV types has been shown to complex with p53 and, as a result, target it for rapid proteasome-mediated degradation. As a consequence, p53's growth-arrest and apoptosis-inducing activities are abrogated. Since p53 is frequently wild type in cervical cancers, unlike other cancers in which it is often mutated, the notion has arisen that E6's activity with respect to p53 is equivalent to an inactivating mutation of p53. In addition, several studies have shown that the pathways both upstream and downstream of p53 are intact in cervical cancers; this suggests the potential importance of the E6 - p53 interaction for therapeutic intervention. However, like all viral oncoproteins, E6 is a multifunctional protein and a plethora of other cellular targets has been identified. Indeed, E6's interactions with some of these additional targets appear to be equally important in the pathogenesis of HPV, and may also represent valid targets for therapeutic intervention.

Alternative Splicing↗

PubMed1999

HPV-18 E6*I protein modulates the E6-directed degradation of p53 by binding to full-length HPV-18 E6.

We have previously demonstrated that ectopic expression of the HPV-18 E6*I protein has an antiproliferative effect in cells derived from HPV-containing cervical tumours. This effect correlated with the ability of E6*I to inhibit the E6-mediated degradation of p53 both in vitro and in vivo and with an increase in p53 transcriptional trans-activation. The observation that the E6*I protein can interact with both full-length HPV-18 E6 and E6-AP proteins in vitro indicated the mechanism by which this activity was mediated. In this study we describe a mutational strategy to attempt to differentiate between the E6-AP and full-length HPV-18 E6 interactions, with respect to the biological function of E6*I. We identify regions of the E6*I protein essential for its interaction with full-length E6 and important for its interaction with E6-AP. We show that a mutant of E6*I which is unable to bind to full-length HPV-18 E6 protein is unable to inhibit the E6-directed degradation of p53 and is also unable to inhibit the proliferation of a cervical tumour-derived cell line. Finally, we show that inhibition of transformed cell growth by E6*I protein correlates with its ability to induce apoptosis in a p53-dependent manner. These results raise the intriguing possibility of using E6*I as a basis for therapeutic intervention in HPV-associated tumours.

Amino Acid Sequence↗

PubMed1999

Two polymorphic variants of wild-type p53 differ biochemically and biologically.

The wild-type p53 protein exhibits a common polymorphism at amino acid 72, resulting in either a proline residue (p53Pro) or an arginine residue (p53Arg) at this position. Despite the difference that this change makes in the primary structure of the protein resulting in a difference in migration during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, no differences in the biochemical or biological characteristics of these wild-type p53 variants have been reported. We have recently shown that p53Arg is significantly more susceptible than p53Pro to the degradation induced by human papillomavirus (HPV) E6 protein. Moreover, this may result in an increased susceptibility to HPV-induced tumors in homozygous p53Arg individuals. In further investigating the characteristics of these p53 variants, we now show that both forms are morphologically wild type and do not differ in their ability to bind to DNA in a sequence-specific manner. However, there are a number of differences between the p53 variants in their abilities to bind components of the transcriptional machinery, to activate transcription, to induce apoptosis, and to repress the transformation of primary cells. These observations may have implications for the development of cancers which harbor wild-type p53 sequences and possibly for the ability of such tumors to respond to therapy, depending on their p53 genotype.

Animals↗

PubMed1997

Alternatively spliced HPV-18 E6* protein inhibits E6 mediated degradation of p53 and suppresses transformed cell growth.

The E6 proteins originating from the tumour-associated Human Papillomavirus (HPV) types 16 and 18 have been shown to bind to and target the tumour suppressor protein, p53, for ubiquitin-mediated degradation. However, in cell lines derived from cervical neoplasias, the predominant early region transcripts are spliced and encode truncated forms of E6, termed E6*. We report here that HPV-18 E6* protein will interact both with the full-length E6 proteins from HPV-16 and HPV-18 and also with E6-AP, and subsequently blocks the association of full length E6 protein with p53. We also show that, as a result of this block, E6* can inhibit E6-mediated degradation of p53 both in vitro and in vivo. The biological consequences of this are increased transcriptional activity on p53-responsive promoters and an inhibition of cell growth in cells transfected with E6*. This is the first report of a potential biological function for this polypeptide and may represent a means by which HPV is able to modulate the activity of the full-length E6 protein with respect to p53 during viral infection.

Alternative Splicing↗

PubMed1997

Human papillomavirus type 16 E7 binds to the conserved carboxy-terminal region of the TATA box binding protein and this contributes to E7 transforming activity.

We have previously shown that the human papillomavirus E7 proteins bind to the cellular TATA box binding protein (TBP). In this paper we show that the HPV-18 E6 and the HPV-16 E2 proteins will also bind TBP in vitro. This feature of virus proteins is conserved across many viral types and we were interested in determining whether these HPV proteins interacted with the same conserved region of the TBP molecule. A series of deletions was introduced into the TBP protein and its binding to these HPV proteins was measured. The previously well-characterized interaction between p53 and TBP was used for comparison. All four proteins were found to interact with the carboxy-terminal domain of the TBP protein, although the precise residues involved and the relative strengths of association differed between the different HPV proteins. Mutational analysis of HPV-16 E7 protein identified a stretch of four amino acids responsible for the binding to TBP. This mutant E7 protein possessed wild-type levels of transcriptional activity on the adenovirus E2 promoter but exhibited reduced transforming activity in cooperation with EJ-ras. These results demonstrate that the mechanisms of interaction between diverse viral proteins and TBP are similar and that, in the case of E7, this interaction may contribute to its transforming activity.

Animals↗

PubMed1996

Induction of apoptosis by p53 is independent of its oligomeric state and can be abolished by HPV-18 E6 through ubiquitin mediated degradation.

Inhibition of p53 function is a common feature of many DNA tumour viruses. Human papillomavirus (HPV) E6 proteins from the oncogenic HPVs inhibit p53 function either by blocking its ability to bind DNA or by labelling newly synthesised p53 as a target for ubiquitin mediated degradation. In this study we have investigated the role of the degradation function of E6 with respect to p53 function. Using a panel of previously characterised p53 mutant proteins we have been able to establish a series of assays which separates p53 growth suppression from transformation suppression and from induction of apoptosis. Only wild type p53 inhibits the growth of p53 null 10(1) cells, whereas wild type, dimeric and monomeric mutants of p53 suppress transformed cell growth of both Saos-2 cells and baby rat kidney cells. Cells expressing the different oligomeric forms of p53 all retain the ability to induce apoptosis upon u.v. treatment. Using HPV E6 and E7 we have been able to show that E7 will overcome p53 growth suppressor activity with an efficiency similar to that observed with E6. However, in contrast to E6, E7 has no effect on the ability of p53 to suppress transformed cell growth. Finally, we show that the ability of E6 to label p53 for ubiquitin mediated degradation is prerequisite for its ability to overcome p53 inhibition of transformed cell growth and induction of apoptosis. These observations argue that E6 inhibits p53 mediated apoptosis and suppression of transformation while E7 inhibits p53 suppression of cell proliferation.

Animals↗

PubMed1996

HPV-16 E7 and adenovirus E1a complex formation with TATA box binding protein is enhanced by casein kinase II phosphorylation.

The major transforming protein of HPV-16 is encoded by the E7 gene. This has been shown to cooperate with EJ-ras in the immortalisation of primary rodent cells and with the viral E6 gene in the immortalisation of primary human keratinocytes. HPV-16 E7 protein has been shown to bind to a number of cellular proteins involved in the control of cell growth; including pRB, p107 and cyclin A. Loss of pRb or p107 binding results in the loss of transforming activity. In this paper we demonstrate that HPV-16 E7 can also complex with the core component of TFIID, the TATA Box Binding Protein (TBP). This interaction is partly dependent upon phosphorylation of the E7 protein by cellular casein kinase II (CKII), since phosphorylation of E7 by CKII increases the affinity with which E7 binds TBP. Similar results are also obtained with the Adenovirus Ela protein, indicating a conservation of function between these two viral oncoproteins. Mutation of the CKII site to two acidic amino acids significantly increases the affinity of E7 for TBP, indicating that the incorporation of two negative charges at this region of E7 is important in regulating the interaction with TBP.

Adenovirus E1A Proteins↗

PubMed1994

Characterization of the human papillomavirus E2 protein: evidence of trans-activation and trans-repression in cervical keratinocytes.

The major regulator of papillomavirus transcription is encoded by the viral E2 gene. The E2 gene has been well characterized in bovine papillomavirus (BPV) where it encodes at least three different polypeptides which differentially affect viral gene expression. In human papillomaviruses (HPVs) the E2 gene product is much less well characterized. In this study we have analysed the mechanism of action of the HPV-16, HPV-18 and BPV-1 E2 proteins in cervical keratinocytes. We show that the full length HPV E2 protein acts as a potent transcriptional activator of viral gene expression in both normal and immortalized keratinocytes. In contrast, the BPV-1 E2 protein produces transcriptional repression under identical conditions. A cDNA encoding the C-terminal half of the HPV-16 E2 protein in these assays weakly repressed viral gene expression. Further, co-transfection of this cDNA with the full length clone progressively abolishes the activation in trans by the full length HPV E2 protein. Gel retardation assays have defined a number of protein complexes between the long and short forms of E2 but with no evidence for preferential DNA binding. These results define two distinct activities for the HPV-16 E2 protein, indicate functional differences with the BPV E2 protein and suggest that splicing of the HPV E2 mRNA is a critical mechanism for controlling viral gene expression.

Animals↗

PubMed1994

Mutational analysis of HPV-18 E6 identifies domains required for p53 degradation in vitro, abolition of p53 transactivation in vivo and immortalisation of primary BMK cells.

The two major transforming proteins of oncogenic human papillomaviruses are encoded by the E6 and E7 oncogenes. Both viral proteins interact specifically with the products of cellular human tumour suppressor genes; E6 with p53 and E7 with Rb. However, the mechanism of action of E6 is still poorly understood in comparison with that of E7. Although extensive in vitro studies have been done with mutant E6 proteins, very little is known about the activities of E6 in vivo. In this study we have analysed the structure-function relationships of HPV-18 E6 in in vitro analyses and we correlate this with in vivo activity. These studies define a number of domains on the E6 molecule which are involved in the ability of E6 to target p53 for degradation in vitro. This analysis demonstrates that domains previously shown to be important in HPV-16 E6 (Crook et al., 1991; Mietz et al., 1992) are also conserved in HPV-18 and also reconciles the differences between these reports. A series of in vivo studies demonstrate that E6 mediated degradation of p53 in vitro is irrelevant both for cell transformation and for the ability of E6 to abolish p53 transcriptional activation. In addition, we show that at least four distinct regions of the E6 protein are involved in the p53 association in vivo.

3T3 Cells↗

PubMed1994

DNA damage induced p53 mediated transcription is inhibited by human papillomavirus type 18 E6.

Cervical cancer is similar to other human cancers in that it develops through a multistep process. However, infection with oncogenic human papillomaviruses (HPVs) is believed to be essential for the initiation of this disease. Although HPV may play a central role in the early stages of neoplasia, the accumulation of mutations in an assortment of genes precedes the development of malignant cervical carcinoma. The mechanisms by which abnormalities accumulate are various, but it is possible that viral proteins are involved. In particular, the viral E6 oncoprotein has been shown to interact with the cellular tumour suppressor protein p53, which is involved in DNA damage repair pathways. Hence, E6 may contribute to the genomic instability through this interaction with p53. We have tested this hypothesis by monitoring the effects of E6 upon DNA damage induced p53 transcriptional activity. This study shows that HPV-18 E6 inhibits p53 transcriptional activity following genotoxic stress with UV radiation. No effect was observed when a mutant E6 unable to direct the degradation of p53 was included in this assay. These results suggest that continued E6 expression may contribute to the accumulation of DNA damage associated with the progression of cervical cancer.

Amino Acid Sequence↗

PubMed1992

Human papillomavirus type 16 E5 gene stimulates the transforming activity of the epidermal growth factor receptor.

We have until recently made several unsuccessful attempts to assign any activity to the human papillomavirus type 16 (HPV-16) E5 gene product. However, studies with the bovine papilloma virus 1 (BPV-1) E5 protein indicated an interaction with the epidermal growth factor receptor (EGFR). In light of the overall similarity between the HPV and BPV E5 proteins we attempted to determine whether the HPV-16 E5 gene had any common activity. In cells expressing high levels of EGFR plus HPV-16 E5 we found a dramatically increased proliferative activity in soft-agar assays in the presence of EGF. The specificity of this activity was monitored by the addition of other mitogenic agents. The phorbol ester phorbol 12-myristate 13-acetate (PMA) had no effect on the E5-containing cells, although insulin weakly stimulated their growth in soft agar. Further analysis revealed the same number of EGF receptors were present on the E5-containing cells as on the control cells, although the E5 cells were more sensitive to lower concentrations of EGF. These results imply that E5 is amplifying the mitogenic signals from the EGFR in an as yet unknown manner, but which may form the basis of interactions with a variety of growth factor receptors. This report brings to three the number of transforming genes encoded by HPV-16.

3T3 Cells↗

PubMed1992

Lack of immortalizing activity of a human papillomavirus type 16 variant DNA with a mutation in the E2 gene isolated from normal human cervical keratinocytes.

The oncogenic potential of a human papillomavirus type 16 (HPV16) variant cloned from normal human cervical keratinocytes has been tested in vitro using primary rodent epithelial cells and human cervical keratinocytes. The HPV16 variant was able to extend the lifespan of, but failed to immortalize, human keratinocytes. It could however cooperate with an activated ras oncogene to transform primary rodent cells. Radioimmunoprecipitation assays of the rodent cells showed that they expressed the E7 protein. DNA sequence analysis of the URR/E6/E7 and E5 regions of the HPV16 showed them to be fully functional, but a deletion in the viral E2 open reading frame was detected. This truncated E2 only weakly stimulated transcription of the viral regulatory region. Complementation assays using the HPV16 variant and a full-length E2 enabled the cloned variant to immortalize human cervical keratinocytes with wild-type efficiency. These results suggest that other viral gene products in addition to E6/E7 may play an important role in the in vitro immortalization of cervical keratinocytes in HPV16 and the development of cervical cancer.

Amino Acid Sequence↗

PubMed1991

Expression of the human papillomavirus E7 oncogene during cell transformation is sufficient to induce susceptibility to lysis by activated macrophages.

Human papillomaviruses (HPV), and in particular HPV type 16, are etiologic agents in the development of cervical cancer, which is the second most common form of cancer in women worldwide. Mammalian cells are susceptible to transformation in vitro by the E6 and E7 oncogenes derived from the HPV-16 genome. NIH-3T3 cells transfected with the HPV-16 E7 oncogene were found to exhibit cytolytic susceptibility to murine-activated macrophages. In comparison, E6 oncogene-expressing cells were not susceptible to lysis by activated macrophages. The E7 oncoprotein is multifunctional, being capable of complexing with the retinoblastoma tumor suppressor gene (anti-oncogene) product, stimulating DNA synthesis, and causing cell transformation in vitro. Macrophage killing assays performed on cell lines expressing E7 mutants revealed that the ability to complex the retinoblastoma tumor suppressor gene product and stimulate DNA synthesis did not induce susceptibility to activated macrophages, whereas the ability of E7 to cause transformation was required to induce susceptibility to activated macrophages. These data suggest that cell transformation is a more important prerequisite for inducing susceptibility to activated macrophages than is the loss of tumor suppressor gene function. This study also provides an initial link between HPV-16 oncogene expression and the ability of activated macrophages to selectively recognize and destroy HPV-16-associated neoplastic cells.

Amino Acid Sequence↗