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Query: UNIPROT:P06889 (Mol)
 630,302 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The p53 gene is compromised in most human cancers by point mutation. Evidence is accumulating that these alterations frequently do not result in a complete loss of the sequence-specific transcriptional regulatory function of p53. Here, we describe the transcriptional activity of the p53 mutant C277Y isolated from a Ewing's sarcoma with high constitutive pig3 expression. Transient transfection of this mutant into a p53 null cell line resulted in activation not only of the pig3 but also of the MDM2 gene compatible with the presence of constitutively expressed MDM2 transcripts initiated from the P2 promoter in the p53-C277Y hemizygous Ewing's sarcoma cell line. Expression of endogenous pig3 and MDM2 genes was further enhanced on irradiation of this cell line. Here, suppression of p53-C277Y by RNAi reduced pig3 promoter activity, RNA, and protein expression. Reporter gene assays revealed that the potential of p53-C277Y to up-regulate MDM2 expression was similar to wild-type p53, whereas activation of the pig3 promoter was at least 5-fold increased over wild-type p53. The pentanucleotide microsatellite sequence present in exon 1 of the pig3 gene was found to be responsible for p53-C277Y-mediated activation. In concordance with a role of PIG3 protein for cell death, we showed residual apoptotic activity of p53-C277Y to which the described Ewing's sarcoma cell line was found to be resistant. p53-C277Y has previously been reported to bind to DNA with altered sequence specificity and to be unable to activate generic p53 target genes in yeast-based functional assays. Our results, therefore, show that a p53 mutant may behave differently when tested in its authentic cellular context.
Mol Cancer Res 2004 May
PMID:Constitutive and DNA damage inducible activation of pig3 and MDM2 genes by tumor-derived p53 mutant C277Y. 1519 23

Phospholipase D (PLD) has been reported to generate survival signals that prevent apoptosis induced by serum withdrawal. We have now found that elevated expression of PLD also suppresses DNA damage-induced apoptosis. Since DNA damage-induced apoptosis is often mediated by p53, we examined the effect of elevated PLD expression on the regulation of p53 stabilization. We report here that PLD suppresses DNA damage-induced increases in p53 stabilization in cells where PLD has been shown to provide a survival signal. Elevated expression of PLD also led to increased expression of the p53 E3 ubiquitin ligase MDM2 and increased turnover of p53. PLD1-stimulated increases in MDM2 expression and suppression of p53 activation were blocked by inhibition of mTOR and the mitogen-activated protein kinase pathway. Although PLD did not activate the phosphatidylinositol 3-kinase (PI3K)/Akt survival pathway activate the basal levels of PI3K activity were partially required for PLD1-induced increases in MDM2. These data provide evidence that survival signals generated by PLD involve suppression of the p53 response pathway.
Mol Cell Biol 2004 Jul
PMID:Phospholipase D elevates the level of MDM2 and suppresses DNA damage-induced increases in p53. 1519 26

The p53-MDM2 feedback loop is vital for cell growth control and is subjected to multiple regulations in response to various stress signals. Here we report another regulator of this loop. Using an immunoaffinity method, we purified an MDM2-associated protein complex that contains the ribosomal protein L23. L23 interacted with MDM2, forming a complex independent of the 80S ribosome and polysome. The interaction of L23 with MDM2 was enhanced by treatment with actinomycin D but not by gamma-irradiation, leading to p53 activation. This activation was inhibited by small interfering RNA against L23. Ectopic expression of L23 reduced MDM2-mediated p53 ubiquitination and also induced p53 activity and G(1) arrest in p53-proficient U2OS cells but not in p53-deficient Saos-2 cells. These results reveal that L23 is another regulator of the p53-MDM2 feedback regulation.
Mol Cell Biol 2004 Sep
PMID:Ribosomal protein L23 activates p53 by inhibiting MDM2 function in response to ribosomal perturbation but not to translation inhibition. 1531 73

The ARF/MDM2/p53 pathway is a principal defense mechanism to protect the organism from uncontrolled effects of deregulated oncogenes. Oncogenes activate ARF, which interacts with and inhibits the ubiquitin ligase MDM2, resulting in p53 stabilization and activation. Once stabilized and activated, p53 can either induce or repress a wide array of different gene targets, which in turn can regulate cell cycle, DNA repair, and a number of apoptosis-related genes. Here we show that, unlike p53, p63, a member of the p53 family, directly interacts with p14(ARF). Through this interaction ARF inhibits p63-mediated transactivation and transrepression. In p63-transfected cells, ARF, which normally localizes into nucleoli, accumulates in the nucleoplasm. Based on these observations, we suggest that stimuli inducing p14(ARF) expression can, at the same time, activate p53 and impair p63 transcriptional activity, altering the pattern of p53 target gene expression. Here we show, for the first time, a physical and functional link between the p14(ARF) tumor suppressor protein and p63, a member of the p53 family.
Mol Cell Biol 2004 Oct
PMID:Inhibition of p63 transcriptional activity by p14ARF: functional and physical link between human ARF tumor suppressor and a member of the p53 family. 1536 73

Transitional cell carcinomas (TCC) of the urinary bladder develop by a multistep process characterized by various stages of transformation differing in their grade of malignancy and biological behaviour. Since the prospective clinical outcome cannot be reliably predicted on histopathological grounds, we analysed the mRNA expression of the MDM2-p73-P14ARF tumour surveillance pathway in an attempt to detect alterations of gene activity, allowing a better understanding of the mechanisms responsible for conversion of low to high malignant TCC. Expression of the mRNA was determined in 71 TCC of various grades and stages using the real-time quantitative reverse transcription-polymerase chain reaction. The MDM2-p73-P14ARF pathway was dominated by the MDM2 gene, the mRNA expression of which proved to be significantly (5-fold) lower in advanced high-grade, high-stage than in superficial low-grade, low-stage TCC. Conversely, the expression of p73 mRNA increased with increasing tumour grades and stages, while the activity of the P14ARF gene was not substantially altered during early and late phases of urothelial carcinogenesis. Analysing the expression of spliced variants of MDM2 mRNA, we found a heterogeneous pattern including a novel splicing transcript coding for an abnormal protein. Promoter hypermethylation of P14ARF occurred in 10% of the TCC with an under-expression of mRNA. An analysis of the effects of lifestyle and occupational bladder cancer risk factors revealed that TCC of smokers showed a 2-fold elevated expression of MDM2 mRNA and an approximately 2-fold lower expression of P14ARF mRNA, whereas the activity of the p73 gene was unchanged. Heavy coffee consumption was associated with a 2-fold decreased expression level of P14ARF mRNA. Exposure to certain occupational hazards (plastic products, paints and lacquer, polycyclic hydrocarbons, chemical solvents) was observed to modulate the activity of the genes analysed. Our findings suggest that an alteration in the MDM2-p73-P14ARF pathway is involved in the progression of bladder cancer to a more malignant and aggressive form.
Int J Mol Med 2004 Nov
PMID:Alteration of the MDM2-p73-P14ARF pathway related to tumour progression during urinary bladder carcinogenesis. 1549 52

p53 is a critical coordinator of a wide range of stress responses. To facilitate a rapid response to stress, p53 is produced constitutively but is negatively regulated by MDM2. MDM2 can inhibit p53 in multiple independent ways: by binding to its transcription activation domain, inhibiting p53 acetylation, promoting nuclear export, and probably most importantly by promoting proteasomal degradation of p53. The latter is achieved via MDM2's E3 ubiquitin ligase activity harbored within the MDM2 RING finger domain. We have discovered that MTBP promotes MDM2-mediated ubiquitination and degradation of p53 and also MDM2 stabilization in an MDM2 RING finger-dependent manner. Moreover, using small interfering RNA to down-regulate endogenous MTBP in unstressed cells, we have found that MTBP significantly contributes to MDM2-mediated regulation of p53 levels and activity. However, following exposure of cells to UV, but not gamma-irradiation, MTBP is destabilized as part of the coordinated cellular response. Our findings suggest that MTBP differentially regulates the E3 ubiquitin ligase activity of MDM2 towards two of its most critical targets (itself and p53) and in doing so significantly contributes to MDM2-dependent p53 homeostasis in unstressed cells.
Mol Cell Biol 2005 Jan
PMID:Regulation of p53 and MDM2 activity by MTBP. 1563 57

The MDM2 gene is overexpressed in several human tumors and its product may be processed into various isoforms. Recently, alternative splicing forms of MDM2 mRNA have been detected in various types of tumors. In this study, lung tissue from human non small cell lung cancers was examined for MDM2 mRNA splicing variants by nested RT-PCR. Of the 117 lung cancer tissue samples analyzed, a total of 31 (26.5%) had splice variants for the MDM2 gene, while 59 (50.4%) had undetectable levels of MDM2 transcript. Further analysis indicated that the predominant variant for 26 of the 31 samples with alternative MDM2 splicing products was MDM2-657, a splice variant lacking exons 3-11. Significant associations were found between the frequency of alternative splicing and the gender and smoking habits of the patients. Approximately 36% of male patients had alternative splicing of MDM2 compared with only 9.5% of female patients (P = 0.008); 44.2% of the smoker patients had alternative MDM2 splice forms versus 16.2% of nonsmokers (P = 0.003). Furthermore, most normal lung cell lines examined possessed only full-length MDM2 mRNA, while among several lung cancer cell lines, only H1355 and CaLu-1 cells lacked alternatively spliced MDM2 transcripts. When H1355 cells were treated in vitro with the cigarette smoke carcinogen benzo[a]pyrene (B[a]P) or the B[a]P metabolite benzo[a]pyrene diolepoxide (BPDE), three MDM2 splicing products were detected by nested RT-PCR. Finally, with the use of several specific inhibitors, we found that BPDE-induced MDM2 mRNA alternative splicing in H1355 cells may occur through the PI3K or MAPK pathway. Overall, our results suggest that carcinogens present in cigarette smoke increase the risk of alternative MDM2 splicing, which is highly associated with lung cancer.
Environ Mol Mutagen 2005 Jul
PMID:Alternative splicing of MDM2 mRNA in lung carcinomas and lung cell lines. 1571 38

Critical to the inhibitory action of the oncogene product, MDM2, on the tumour suppressor, p53, is association of the N-terminal domain of MDM2 (MDM2N) with the transactivation domain of p53. The structure of MDM2N was previously solved with a p53-derived peptide, or small-molecule ligands, occupying its binding cleft, but no structure of the non-liganded MDM2N (i.e. the apo-form) has been reported. Here, we describe the solution structure and dynamics of apo-MDM2N and thus reveal the nature of the conformational changes in MDM2N that accompany binding of p53. The new structure suggests that p53 effects displacement of an N-terminal segment of apo-MDM2N that occludes access to the shallow end of the p53-binding cleft. MDM2N must also undergo an expansion upon binding, achieved through a rearrangement of its two pseudosymetrically related sub-domains resulting in outward displacements of the secondary structural elements that comprise the walls and floor of the p53-binding cleft. MDM2N becomes more rigid and stable upon binding p53. Conformational plasticity of the binding cleft of apo-MDM2N could allow the parent protein to bind specifically to several different partners, although, to date, all the known liganded structures of MDM2N are highly similar to one another. The results indicate that the more open conformation of the binding cleft of MDM2N observed in structures of complexes with small molecules and peptides is a more suitable one for ligand discovery and optimisation.
J Mol Biol 2005 Jul 15
PMID:Structure of free MDM2 N-terminal domain reveals conformational adjustments that accompany p53-binding. 1595 16

Overexpression or hyperactivation of MDM2 contributes to functional inactivation of wild-type p53 in nearly 50% of tumors. Inhibition of p53 by MDM2 depends on binding between an NH(2)-terminal (residues 16-28) p53 alpha-helical peptide and a hydrophobic pocket on MDM2, presenting an attractive target for development of inhibitors against tumors expressing wild-type p53. Here we report that novel p53 alpha-helical peptide mimics based on a terphenyl scaffold can inhibit MDM2-p53 binding in vitro and activate p53 in vivo. Several active compounds have been identified that inhibit MDM2-p53 binding in an ELISA assay with IC(50) of 10 to 20 micromol/L and induce p53 accumulation and activation in cell culture at 15 to 40 micromol/L. These results suggest that p53 alpha-helical mimetics based on the terphenyl scaffold may be developed into potent p53 activators.
Mol Cancer Ther 2005 Jun
PMID:p53 alpha-Helix mimetics antagonize p53/MDM2 interaction and activate p53. 1595 60

The p53-related p63 gene encodes six isoforms with differing N and C termini. TAp63 isoforms possess a transactivation domain at the N terminus and are able to transactivate a set of genes, including some targets downstream of p53. Accumulating evidence indicates that TAp63 plays an important role in regulation of cell proliferation, differentiation, and apoptosis, whereas transactivation-inert deltaNp63 functions to inhibit p63 and other p53 family members. Mutations in the p63 gene that abolish p63 DNA-binding and transactivation activities cause human diseases, including ectrodactyly ectodermal dysplasia and facial clefting (EEC) syndrome. In this study, we show that mutant p63 proteins with a single amino acid substitution found in EEC syndrome are DNA binding deficient, transactivation inert, and highly stable. We demonstrate that TAp63 protein expression is tightly controlled by its specific DNA-binding and transactivation activities and that p63 is degraded in a proteasome-dependent, MDM2-independent pathway. In addition, the N-terminal transactivation domain of p63 is indispensable for its protein degradation. Furthermore, the wild-type TAp63gamma can act in trans to promote degradation of mutant TAp63gamma defective in DNA binding, and the TA domain deletion mutant of TAp63gamma inhibits transactivation activity and stabilizes the wild-type TAp63 protein. Taken together, these data suggest a feedback loop for p63 regulation, analogous to the p53-MDM2 feedback loop.
Mol Cell Biol 2005 Jul
PMID:DNA-binding and transactivation activities are essential for TAp63 protein degradation. 1598 26


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