Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UNIPROT:P04637 (p53)
 77,613 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

One of the mechanisms in which anticancer therapies function is to induce cell death by apoptosis. In this regard, the biological activity of p53 may be critical since the presence of p53 appears to play a role in apoptosis following genotoxic stress. In experimental systems using in vitro transformed primary cells, there is a direct correlation between the presence of p53 and apoptosis. For example, E1A/ras-transformed primary mouse fibroblasts are less viable and undergo apoptosis following genotoxic stress if these cells contain a wild type p53. In comparison, similarly transformed cells which are p53-deficient are more viable and will not undergo apoptosis under these conditions. Although these are important observations, it remains to be established whether there exists a similar relationship between the presence of wild type p53 and loss of cell viability following therapy in human tumour cells. One way to address this question is to target wild type p53 in human tumour cells using human papillomavirus E6 which mediates the degradation of wild type p53 through the ubiquitin pathway. In this manner, E6 engineered p53-deficient and parental p53-containing human tumour cells provides an appropriate experimental system in which to determine whether wild type p53 in tumour cells has influence on cell viability following genotoxic anticancer treatments. In the present study, the wild type p53 protein in human fibrosarcoma HT1080 cells were targeted with HPV-18 E6 and the viability of these cells in response to treatment with adriamycin, u.v.-irradiation and gamma-irradiation was examined. Data is presented which shows that p53-containing and p53-deficient cells were equally sensitive to these treatments. These data argue that the wild type p53 in these tumour cells does not cause these cells to be less viable when treated with anticancer agents or u.v.-irradiation. Therefore, the status of p53 alone in tumour cells may not be an indicator of response to anticancer treatments.
 ...
PMID:Viability of wild type p53-containing and p53-deficient tumor cells following anticancer treatment: the use of human papillomavirus E6 to target p53. 762 52

Programmed cell death (apoptosis) is a normal physiological process, which could in principle be manipulated to play an important role in cancer therapy. The key importance of p53 expression in the apoptotic response to DNA-damaging agents has been stressed because mutant or deleted p53 is so common in most kinds of cancer. An important strategy, therefore, is to find ways to induce apoptosis in the absence of wild-type p53. In this paper, we compare apoptosis in normal human mammary epithelial cells, in cells immortalized with human papilloma virus (HPV), and in mammary carcinoma cell lines expressing wild-type p53, mutant p53, or no p53 protein. Apoptosis was induced with mitomycin C (MMC), a DNA cross-linking and damaging agent, or with staurosporine (SSP), a protein kinase inhibitor. The normal and HPV-transfected cells responded more strongly to SSP than did the tumor cells. After exposure to MMC, cells expressing wild-type p53 underwent extensive apoptosis, whereas cells carrying mutated p53 responded weakly. Primary breast cancer cell lines null for p53 protein were resistant to MMC. In contrast, two HPV immortalized cell lines in which p53 protein was destroyed by E6-modulated ubiquitinylation were highly sensitive to apoptosis induced by MMC. Neither p53 mRNA nor protein was induced in the HPV immortalized cells after MMC treatment, although p53 protein was elevated by MMC in cells with wild-type p53. Importantly, MMC induced p21 mRNA but not p21 protein expression in the HPV immortalized cells. Thus, HPV 16E6 can sensitize mammary epithelial cells to MMC-induced apoptosis via a p53- and p21-independent pathway. We propose that the HPV 16E6 protein modulates ubiquitin-mediated degradation not only of p53 but also of p21 and perhaps other proteins involved in apoptosis.
 ...
PMID:The human papilloma virus 16E6 gene sensitizes human mammary epithelial cells to apoptosis induced by DNA damage. 764

In most cases, the transcriptional factor NF-kappa B is a heterodimer consisting of two subunits, p50 and p65, which are encoded by two distinct genes of the Rel family. p50 is translated as a precursor of 105 kDa. The C-terminal domain of the precursor is rapidly degraded, forming the mature p50 subunit consisted of the N-terminal region of the molecule. The mechanism of generation of p50 is not known. It has been suggested that the ubiquitin-proteasome system is involved in the process; however, the specific enzymes involved and the mechanism of limited proteolysis, in which half of the molecule is spared, have been obscure. Palombella and colleagues (Palombella, V. J., Rando, O. J., Goldberg, A. L., and Maniatis, T. (1994) Cell 78, 773-785) have shown that ubiquitin is required for the processing in a cell-free system of a truncated, artificially constructed, 60-kDa precursor. They have also shown that proteasome inhibitors block the processing both in vitro and in vivo. In this study, we demonstrate reconstitution of a cell-free processing system and demonstrate directly that: (a) the ubiquitin-proteasome system is involved in processing of the intact p105 precursor, (b) conjugation of ubiquitin to the precursor is an essential intermediate step in the processing, (c) the recently discovered novel species of the ubiquitin-carrier protein, E2-F1, that is involved in the conjugation and degradation of p53, is also required for the limited processing of the p105 precursor, and (d) a novel, approximately 320-kDa species of ubiquitin-protein ligase, is involved in the process. This novel enzyme is distinct from E6-AP, the p53-conjugating ligase, and from E3 alpha, the "N-end rule" ligase.
 ...
PMID:Ubiquitin-mediated processing of NF-kappa B transcriptional activator precursor p105. Reconstitution of a cell-free system and identification of the ubiquitin-carrier protein, E2, and a novel ubiquitin-protein ligase, E3, involved in conjugation. 766 88

E6-AP is a 100-kDa cellular protein that interacts with the E6 protein of the cancer-associated human papillomavirus types 16 and 18. The E6/E6-AP complex binds to and targets the p53 tumor-suppressor protein for ubiquitin-mediated proteolysis. E6-AP is an E3 ubiquitin-protein ligase which accepts ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfers the ubiquitin to targeted substrates. The amino acid sequence of E6-AP shows similarity to a number of protein sequences over an approximately 350-aa region corresponding to the carboxyl termini of both E6-AP and the E6-AP-related proteins. Of particular note is a conserved cysteine residue within the last 32-34 aa, which in E6-AP is likely to be the site of ubiquitin thioester formation. Two of the E6-AP-related proteins, a rat 100-kDa protein and a yeast 95-kDa protein (RSP5), both of previously unknown function, are shown here to form thioesters with ubiquitin. Mutation of the conserved cysteine residue of these proteins destroys their ability to accept ubiquitin. These data strongly suggest that the rat 100-kDa protein and RSP5, as well as the other E6-AP-related proteins, belong to a class of functionally related E3 ubiquitin-protein ligases, defined by a domain homologous to the E6-AP carboxyl terminus (hect domain).
 ...
PMID:A family of proteins structurally and functionally related to the E6-AP ubiquitin-protein ligase. 776 80

The E6 protein of the high-risk human papillomaviruses inactivates the tumor suppressor protein p53 by stimulating its ubiquitinylation and subsequent degradation. Ubiquitinylation is a multistep process involving a ubiquitin-activating enzyme, one of many distinct ubiquitin-conjugating enzymes, and in certain cases, a ubiquitin ligase. In human papillomavirus-infected cells, E6 and the E6-associated protein are thought to act as a ubiquitin-protein ligase in the ubiquitinylation of p53. Here we describe the cloning of a human ubiquitin-conjugating enzyme that specifically ubiquitinylates E6-associated protein. Furthermore, we define the biochemical pathway of p53 ubiquitinylation and demonstrate that in vivo inhibition of various components in the pathway leads to an inhibition of E6-stimulated p53 degradation.
 ...
PMID:Reconstitution of p53-ubiquitinylation reactions from purified components: the role of human ubiquitin-conjugating enzyme UBC4 and E6-associated protein (E6AP). 772 50

The tumour suppressor p53 is a transcription factor with high affinity for specific DNA target sequences. Wild type p53 has a very short half life in normal cells but the protein shows transient accumulation in response to DNA damage, accompanied by up-regulation of target genes such as p21 and induction of growth arrest in G1 of the cell cycle. The rapid turnover of p53 may involve the ubiquitin-dependent proteolytic pathway. In order to investigate p53 turnover we have employed an in vitro system with rabbit reticulocyte lysate, in which ubiquitin-dependent degradation of p53 is mediated by the oncoprotein E6 of human papilloma virus type 16 (HPV-16). Using this system we have previously shown that E6-mediated degradation is preferential for p53 in the 1620+ conformation (reactive with the monoclonal antibody PAb1620). p53-1620+ is a pre-requisite for specific DNA binding and we have now asked if p53 in complex with DNA remains susceptible to ubiquitin-dependent proteolysis in the presence of E6. Our results indicate that p53-DNA complexes are resistant to degradation, whereas the 'free' protein is completely degraded within 20 min. Moreover, E6 did not complex with p53-DNA, possibly due to masking of sites recognised either by E6 or by the E6-associated protein (E6-AP) which facilitates E6-p53 interaction. Preincubation with E6 inhibited the DNA binding capacity of p53 and this effect could be explained, at least in part, by ubiquitination of the p53 protein.
 ...
PMID:p53 in complex with DNA is resistant to ubiquitin-dependent proteolysis in the presence of HPV-16 E6. 775 60

Ubiquitination of proteins involves the concerted action of the E1 ubiquitin-activating enzyme, E2 ubiquitin-conjugating enzymes and E3 ubiquitin-protein ligases. It has been proposed that E3s function as 'docking proteins', specifically binding substrate proteins and specific E2s, and that ubiquitin is then transferred directly from E2s to substrates. We show here that formation of a ubiquitin thioester on E6-AP, an E3 involved in the human papillomavirus E6-induced ubiquitination of p53 (refs 6-10), is an intermediate step in E6-AP-dependent ubiquitination. The order of ubiquitin transfer is from E1 to E2, from E2 to E6-AP, and finally from E6-AP to a substrate. This cascade of ubiquitin thioester complexes suggests that E3s have a defined enzymatic activity and do not function simply as docking proteins. The cysteine residue of E6-AP responsible for ubiquitin thioester formation was mapped to a region that is highly conserved among several proteins of unknown function, suggesting that these proteins share the ability to form thioesters with ubiquitin.
 ...
PMID:Protein ubiquitination involving an E1-E2-E3 enzyme ubiquitin thioester cascade. 780 44

A key activity of the p53 protein during suppression of cell growth is its ability to stimulate transcription from promoters of cellular genes which contain a p53 responsive element. The E6 proteins from the oncogenic associated Human Papillomaviruses (HPVs) have been shown to inhibit specifically the p53 transcriptional activation and this has been proposed as a mechanism whereby the virus prevents the suppression of cell cycle progression and the induction of apoptosis. However, the mechanism by which E6 exercises this function is unknown, as is the ability of E6 to associate with different oligomeric forms of p53. In this study we demonstrate that E6 induces changes within the p53 protein which result both in inhibition of DNA binding and in dissociation of p53 protein previously bound to the DNA. These activities correlate exactly with the ability of E6 to inhibit p53 transcriptional activation and are independent of the ability of E6 to direct the degradation of the p53 protein. Further, we show that E6 labels wild type tetrameric and dimeric forms of p53 proteins for ubiquitin mediated degradation more readily than monomeric forms of the protein. However, in vivo analyses indicate that E6 is capable of inhibiting the transcriptional activation induced by the tetrameric, dimeric and monomeric forms of p53.
 ...
PMID:HPV-18 E6 mediated inhibition of p53 DNA binding activity is independent of E6 induced degradation. 783 26

The wild-type tumor suppressor protein p53 is a short-lived protein that plays important roles in regulation of cell cycle, differentiation, and survival. Mutations that inactivate or alter the tumor suppressor activity of the protein seem to be the most common genetic change in human cancer and are frequently associated with changes in its stability. The ubiquitin system has been implicated in the degradation of p53 both in vivo and in vitro. A mutant cell line that harbors a thermolabile ubiquitin-activating enzyme, E1, fails to degrade p53 at the nonpermissive temperature. Studies in cell-free extracts have shown that covalent attachment of ubiquitin to the protein requires the three conjugating enzymes: E1, a novel species of ubiquitin-carrier protein (ubiquitin-conjugating enzyme; UBC),E2-F1, and an ubiquitin-protein ligase, E3. Recognition of p53 by the ligase is facilitated by formation of a complex between the protein and the human papillomavirus (HPV) oncoprotein E6. Therefore, the ligase has been designated E6-associated protein (E6-AP). However, these in vitro studies have not demonstrated that the conjugates serve as essential intermediates in the proteolytic process. In fact, in many cases, conjugation of ubiquitin to the target protein does not signal its degradation. Thus, it is essential to demonstrate that p53-ubiquitin adducts serve as essential proteolytic intermediates and are recognized and degraded by the 26S protease complex, the proteolytic arm of the ubiquitin pathway. In this study, we demonstrate that conjugates of p53 generated in the presence of purified, E1, E2, E6-AP, E6, ubiquitin and ATP, are specifically recognized by the 26S protease complex and degraded. In contrast, unconjugated p53 remains stable. The ability to reconstitute the system from purified components will enable detailed analysis of the recognition process and the structural motifs involved in targeting the protein for degradation.
 ...
PMID:Complete reconstitution of conjugation and subsequent degradation of the tumor suppressor protein p53 by purified components of the ubiquitin proteolytic system. 803 27

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.
 ...
PMID: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. 805 51


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>