Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P43146 (tumour suppressor)
 5,935 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The tumour suppressor protein p53 is expressed at very low levels in normal cells but accumulates in response to DNA damaging agents such as u.v. irradiation. This increase is accompanied by transcriptional upregulation of the expression of a number of proteins including Mdm2 which can in turn inhibit p53 dependent transcriptional activation, creating a feedback loop resulting in down-regulation of p53 activity. Mutant p53 proteins are however frequently detected at constitutively high levels in many tumours and tumour cell lines, indeed this phenomenon has been used in several studies to diagnose p53 mutation in patient tumours. We show here that expression of mouse mutant p53 in tumour cell lines of this type results in high levels of both the endogenous p53 protein and the exogenously expressed mutant mouse protein, whereas the human tumour line MCF7 does not exhibit high levels of either endogenous human or exogenously expressed mouse mutant p53 unless stabilisation is induced by DNA damage. This suggests that the stability of mutant p53 is not intrinsic to mutant p53 protein structure but may vary in different cell backgrounds. We present evidence that p53 protein stability in tumour cell lines is determined by association with the Mdm2 tumour suppressor protein, and that p53 mutants which are unable to bind Mdm2 are stable in MCF7 cells. We propose that tumour lines which express high levels of transcriptionally inactive mutant p53 are unable to induce the expression of the Mdm2 protein which would normally provide a feedback mechanism down-regulating p53 protein levels in the absence of DNA damage signals. MCF7 cells however express a transcriptionally active p53 and retain the feedback regulation of p53 protein levels by Mdm2.
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PMID:p53 protein stability in tumour cells is not determined by mutation but is dependent on Mdm2 binding. 929 11

The p53 protein is activated in response to physiological stress resulting in either a G1 arrest of cells or apoptosis. As such, p53 must be tightly regulated, and the MDM2 oncoprotein plays a central role in that regulatory process. The transcription of the Mdm2 oncogene is induced by the p53 protein after DNA damage, and the MDM2 protein then binds to p53 and blocks its activities as a tumour suppressor and promotes its degradation. These two proteins thus form an autoregulatory feedback loop in which p53 positively regulates MDM2 levels and MDM2 negatively regulates p53 levels and activity. Immediately after ultraviolet (UV) irradiation MDM2 messenger RNA and protein levels fall in a p53-independent fashion, resulting in increased p53 levels. The p53 protein is then activated as a transcription factor by posttranslational modification permitting p53 to initiate its cell-cycle arrest or apoptotic (programmed cell death) functions. At later times, after the repair of DNA, MDM2 levels increase in a p53-dependent fashion. This induction of MDM2 results in the inhibition of p53 transcriptional activity and the degradation of p53 protein. MDM2-p53 complexes in the nucleus are transported to the cytoplasm via signals present in the MDM2 protein, where p53 is degraded in the proteasome. Thus MDM2 acts as a nuclear-cytoplasmic shuttle for the p53 protein. There are many levels at which this process is regulated, and as such there are many places for chemotherapeutic interventions. The amino-terminal domain of the MDM2 protein is all that is required to bind the p53 protein. The MDM2 protein has additional domains and therefore may have additional functions. Any of these MDM2 domains may contribute to MDM2's activities as an oncogene independent of its inhibition of the tumour suppressor functions of p53. Thus MDM2 itself could be a target for cancer therapeutic intervention.
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PMID:Functions of the MDM2 oncoprotein. 1006 55

Levels of the tumour suppressor protein p53 are increased in response to a variety of DNA damaging agents. DNA damage-induced phosphorylation of p53 occurs at serine-15 in vivo. Phosphorylation of p53 at serine-15 leads to a stabilization of the polypeptide by inhibiting its interaction with Mdm2, a protein that targets p53 for ubiquitin-dependent degradation. However, the mechanisms by which DNA damage is signalled to p53 remain unclear. Here, we report the identification of a novel DNA-activated protein kinase that phosphorylates p53 on serine-15. Fractionation of HeLa nuclear extracts and biochemical analyses indicate that this kinase is distinct from the DNA-dependent protein kinase (DNA-PK) and corresponds to the human cell cycle checkpoint protein ATR. Immunoprecipitation studies of recombinant ATR reveal that catalytic activity of this polypeptide is required for DNA-stimulated phosphorylation of p53 on serine-15. These data suggest that ATR may function upstream of p53 in a signal transduction cascade initiated upon DNA damage and provide a biochemical assay system for ATR activity.
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PMID:The ataxia-telangiectasia related protein ATR mediates DNA-dependent phosphorylation of p53. 1043 22

The p53 tumour suppressor protein is regulated by ubiquitin-mediated proteasomal degradation. In normal cells p53 is constitutively ubiquitylated by the Mdm2 ubiquitin ligase. When the p53 response is activated by stress signals p53 levels rise due to inhibition of this degradative pathway. Here we show that p53 is modified by the small ubiquitin-like protein SUMO-1 at a single site, K386, in the C-terminus of the protein. Modification in vitro requires only SUMO-1, the SUMO-1 activating enzyme and ubc9. SUMO-1 and ubiquitin modification do not compete for the same lysine acceptor sites in p53. Overexpression of SUMO-1 activates the transcriptional activity of wild-type p53, but not K386R p53 where the SUMO-1 acceptor site has been mutated. The SUMO-1 modification pathway therefore acts as a potential regulator of the p53 response and may represent a novel target for the development of therapeutically useful modulators of the p53 response.
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PMID:SUMO-1 modification activates the transcriptional response of p53. 1056 57

The mdm2 protein interacts with a number of proteins involved in cell growth control. Such interactions favour cell proliferation and may explain the oncogenic potential of mdm2 when over-expressed in cells. Interaction with the tumour suppressor p53 involves the N-terminus of mdm2 and targets p53 for rapid degradation by the ubiquitin pathway. We now describe a novel, highly conserved exon of mdm2 (exon alpha) which includes an in-frame UGA stop codon. Expression of exon alpha disrupts in vitro translation of the p53 binding domain of mdm2. We propose that exon alpha induces translation re-initiation at an internal AUG codon within the mdm2 alpha mRNA isoform. The putative mdm2 alpha protein lacks the N-terminus of mdm2 and shows little, if any, binding capacity for p53. Mdm2 alpha mRNA is expressed in a tissue-specific manner and is observed predominantly in testis and peripheral blood lymphocytes. We propose that mdm2 alpha expression may provide a mechanism for uncoupling mdm2-p53 interaction in certain cell types and/or under specific conditions of cell growth.
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PMID:A novel exon within the mdm2 gene modulates translation initiation in vitro and disrupts the p53-binding domain of mdm2 protein. 1059 3

The p53 tumour suppressor protein is down-regulated by the action of Mdm2, which targets p53 for rapid degradation by the ubiquitin-proteasome pathway. The p14ARF protein is also a potent tumour suppressor that acts by binding to Mdm2 and blocking Mdm2-dependent p53 degradation and transcriptional silencing. We have screened a series of overlapping synthetic peptides derived from the p14ARF protein sequence and found that a peptide corresponding to the first 20 amino acids of ARF (Peptide 3) could bind human Mdm2. The binding site for Peptide 3 on Mdm2 was determined by deletion mapping and lies adjacent to the binding site of the anti-Mdm2 antibody 2A10, which on microinjection into cells can activate p53-dependent transactivation of a reporter plasmid. To determine whether Peptide 3 could similarly activate p53, we expressed a fusion of green fluorescent protein and Peptide 3 in MCF7 and U-2 OS cells and were able to demonstrate induction of p53 protein and p53-dependent transcription. Peptide 3 was able to block in vitro ubiquitination of p53 mediated by Mdm2. Small peptides which are sufficient to block degradation of p53 could provide therapeutic agents able to restore p53-dependent cell death pathways in tumours that retain wild-type p53 expression.
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PMID:An N-terminal p14ARF peptide blocks Mdm2-dependent ubiquitination in vitro and can activate p53 in vivo. 1082 82

Mdm2 has been shown to promote its own ubiquitination and the ubiquitination of the p53 tumour suppressor by virtue of its E3 ubiquitin ligase activity. This modification targets Mdm2 and p53 for degradation by the proteasome. The p14ARF tumour suppressor has been shown to inhibit degradation of p53 mediated by Mdm2. Several models have been proposed to explain this effect of p14ARF. Here we have compared the effects of p14ARF overexpression on the in vivo ubiquitination of p53 and Mdm2. We report that the inhibition of the Mdm2-mediated degradation of p53 by p14ARF is associated with a decrease in the proportion of ubiquitinated p53. The levels of polyubiquitinated p53 decreased preferentially compared to monoubiquitinated species. p14ARF overexpression increased the levels of Mdm2 but it did not reduce the overall levels of ubiquitinated Mdm2 in vivo. This is unexpected because p14ARF has been reported to inhibit the ubiquitination of Mdm2 in vitro. In addition we show that like p14ARF, the proteasome inhibitor MG132 can promote the accumulation of Mdm2 in the nucleolus and that this can occur in the absence of p14ARF expression. We also show that the mutation of the nucleolar localization signal of Mdm2 does not impair the overall ubiquitination of Mdm2 but is necessary for the effective polyubiquitination of p53. These studies reveal important differences in the regulation of the stability of p53 and of Mdm2.
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PMID:Different effects of p14ARF on the levels of ubiquitinated p53 and Mdm2 in vivo. 1152 82

The p53 tumour suppressor protein is a short-lived transcription factor that becomes stabilized in response to a wide range of cellular stresses. Ubiquitination and the targeting of p53 for degradation by the proteasome are mediated by Mdm2 (mouse double minute clone 2), a negative regulatory partner of p53. Previous studies have suggested that DNA-damage-induced phosphorylation of p53 at key N-terminal sites has a pivotal role in regulating the interaction with Mdm2 but the precise role of phosphorylation of serines 15 and 20 is still unclear. Here we show that replacement of serine 15 and a range of other key N-terminal phosphorylation sites with alanine, which cannot be phosphorylated, has little effect on the ubiquitination and degradation of full-length human p53. In contrast, replacement of serine 20 makes p53 highly sensitive to Mdm2-mediated turnover. These results define distinct roles for serines 15 and 20, two sites previously demonstrated to be dependent on phosphorylation through mechanisms mediated by DNA damage and ATM (ataxia telangiectasia mutated). We also show that the polyproline region of p53, a domain that has a key role in p53-induced apoptosis, exerts a critical influence over the Mdm2-mediated turnover of p53.
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PMID:Critical roles for the serine 20, but not the serine 15, phosphorylation site and for the polyproline domain in regulating p53 turnover. 1158 95

The p53 tumour suppressor protein plays a central role in maintaining genomic integrity in eukaryotic cells. The most significant biological function of p53 is to act as a sequence-specific DNA-binding transcription factor, which can induce the expression of a variety of target genes in response to diverse stress stimuli. The p53 protein contains six highly conserved regions, one of which, termed Box I, is located in the N-terminal transactivation domain (amino acid residues 13 and 26). The second half of the Box I region is crucial for the interaction with the basal transcription machinery and is thus required for p53's activity as a transcription factor. The same region also binds to Mdm2. Since p53 is targeted by Mdm2 for ubiquitin-mediated proteasome-dependent degradation, this region is also essential for the regulation of p53's stability in response to stress signals. Although the first half of Box I is highly conserved, its biological function is not clearly defined. The aim of this study was to characterise this conserved region and investigate its role in the biological functions of p53. We have generated short deletions and point mutations within this region and analysed their effect on p53 function and regulation. Biochemical analyses demonstrate that deletion of residues 13 to 16 significantly increases both the transcriptional transactivation and G(2) arrest-inducing activities of murine p53. Residues 13 to 16 appear to function as a regulatory element in p53, modulating p53-dependent transcriptional transactivation and cell-cycle arrest, possibly by affecting the structural stability of the core domain of the protein. In support of this, the deletion was found to induce second-site reversion of the Val135 temperature-sensitive mutant of murine p53.
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PMID:Biological significance of a small highly conserved region in the N terminus of the p53 tumour suppressor protein. 1169 99

Several genetic alterations have been implicated in the development of malignant melanoma, but the expression of oncogenes, tumour suppressor, mismatch repair and apoptosis-related genes and their interactions in melanoma have not been completely clarified. We simultaneously examined the expression of p73, c-erbB-2, ras, p53, Mdm2, p27, DCC, hMLH-1, hMSH-2, bcl-2, Bax and NF-kappaB, by immunocytochemistry, in both primary and metastatic melanoma cell lines derived from melanoma patients. p73 was expressed in 7/8 cell lines, but stronger expressed in the metastatic cells than in the primary melanoma cells. c-erbB-2 was detected in all 8 cell lines and ras in 2/5 metastases. p53 was found in all the cell lines and Mdm2 in 1/8 of the cell lines. In the same patient, the intensity of p27 expression was decreased from the primary to the metastatic tumours. bcl-2 was expressed in all the cell lines. Bax was absent in 5/8 cell lines. In the same patient, Bax was weakly expressed in the primary tumour but lacking in the metastases. The data demonstrate that overexpression of p73, c-erbB-2, p53 and bcl-2, and loss of Mdm2 and Bax may interact and play important roles in the development and aggressiveness of human melanoma.
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PMID:Expression of oncogenes, tumour suppressor, mismatch repair and apoptosis-related genes in primary and metastatic melanoma cells. 1171 83


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