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

The MDM2 oncoprotein targets the p53 tumor suppressor protein for degradation when the two proteins are expressed in cells. The regulation of p53 levels by MDM2 requires the ability of MDM2 to be exported from the nucleus by utilizing its nuclear export signal (NES). The drug leptomycin B (LMB) blocks the formation of nuclear export complexes consisting of CRM1, RanGTP, and NES-containing proteins. It is predicted that LMB should inhibit nuclear-cytoplasmic shuttling by MDM2 and subsequently stabilize p53. This communication demonstrates that LMB treatment of various cell lines led to an increase in the steady-state levels of the p53 protein as a result of an increase in its stability. The stabilized p53 protein localized to the nucleus and was an active transcription factor. These results indicate that the low steady-state levels of p53 in the absence of DNA damage result from p53's nuclear export for cytoplasmic degradation. LMB also led to p53 stabilization in cell lines that contain human papillomavirus (HPV) DNA and express HPV E6, a protein that targets p53 for degradation. MDM2 is not necessary for E6-dependent degradation of p53, as evidenced by the observation that E6 promoted p53 degradation in cells lacking endogenous MDM2. In addition, LMB reduced E6's ability to degrade p53 in the absence of MDM2, demonstrating that complete degradation of p53 by E6 requires nuclear export and therefore likely occurs in cytoplasmic proteasomes. These data suggest that the nuclear export of p53 to the cytoplasm for degradation is a general mechanism for regulating p53 levels.
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PMID:Nuclear export is required for degradation of endogenous p53 by MDM2 and human papillomavirus E6. 981 15

Leptomycin B is a cytotoxin which directly interacts with and inhibits the action of CRM1, an essential mediator of the nuclear exit of proteins containing nuclear export signals (NES) of the HIV1 REV type. We show that addition of leptomycin B to human primary fibroblasts increased the levels of the p53 tumor suppressor protein. This was accompanied by the induction of p53-dependent transcriptional activity in cultured cells and an increase in the levels of the products of two p53-responsive genes, the p21(CIP1/WAF1) and HDM2 proteins. Leptomycin B induced the accumulation of p53 and HDM2 in the nucleus and the appearance of discrete nuclear aggregates containing both proteins. It has been reported that the transcriptional activity of p53 is modulated by its interaction with the HDM2 protein which also targets p53 for rapid degradation. Using a model cell line conditionally expressing MDM2, the murine analogue of HDM2, we present evidence indicating that leptomycin B abrogates MDM2's role in p53 degradation and that the accumulation of p53 in distinct nuclear bodies is mediated by MDM2. Since HDM2 has recently been shown to contain a functional NES of the REV type, the most likely explanation for our results is that the effect of leptomycin B on HDM2 and p53 is due to the inhibition of nuclear export. The ability to visualize sites where p53 and HDM2 colocalize provides a new approach to study the association between the two proteins in vivo. These p53/HDM2-positive nuclear foci were found to also contain the U1A snRNP A and to be juxtaposed to the PML oncogenic domains.
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PMID:An inhibitor of nuclear export activates the p53 response and induces the localization of HDM2 and p53 to U1A-positive nuclear bodies associated with the PODs. 1022 37

Trichostatin A (TSA), an inhibitor of the eukaryotic cell cycle and an inducer of morphological reversion of transformed cells, inhibits histone deacetylase (HDAC) at nanomolar concentrations. Recently, trapoxin, oxamflatin, and FR901228, antitumor agents structurally unrelated to TSA, were found to be potent HDAC inhibitors. These inhibitors activate expression of p21Waf1 and 16INK4A in a p53-independent manner. Changes in the expression of these cell cycle regulators by an increase in histone acetylation may be responsible for cell cycle arrest and antitumor activity by HDAC inhibitors. The target molecule of leptomycin B (LMB), a potent antitumor agent, was genetically and biochemically identified as CRM1, a protein reported as being required for chromosome structure control. We showed that CRM1 was a receptor for the nuclear export signal (NES) and that LMB inhibited nuclear export of proteins. Using LMB, we identified a novel NES in fission yeast transcription factor Pap1, the function of which is abolished by oxidative stress in a manner conserved in eukaryotes.
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PMID:Trichostatin and leptomycin. Inhibition of histone deacetylation and signal-dependent nuclear export. 1066

The E1B-55K and E4orf6 oncoproteins of adenovirus type 5 are involved in the export of viral mRNAs. Previously, it was suggested that a complex composed of E1B-55K and E4orf6 serves as a nucleocytoplasmic transporter for viral mRNAs in which the E4orf6 protein directs both nuclear import and export. We now demonstrate that the E1B-55K protein itself shuttles efficiently in the absence of E4orf6. In addition, E1B-55K trafficking was independent of the defined shuttle proteins Mdm2 or p53, which interacts with E1B-55K. The identified N-terminal E1B-55K leucine-rich nuclear-export signal (NES) conferred rapid nuclear export even in a heterologous system in contrast to the postulated E4orf6NES. Interestingly, although shuttling was blocked by inhibitors of the CRM1 mediated export pathway, E1B-55K inhibited neither the activity nor the trafficking of the retroviral shuttle proteins HIV-1 Rev and HTLV-1 Rex. In contrast, Rev or Rex blocked the nuclear export of E1B-55K, most likely by competing for essential export factors. Our results provide new insights into the regulation of the adenovirus mRNA export system and the processes of adenovirus mediated transformation. Oncogene (2000) 19, 850 - 857.
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PMID:The adenovirus type 5 E1B-55K oncoprotein is a highly active shuttle protein and shuttling is independent of E4orf6, p53 and Mdm2. 1070 93

Nuclear export sequences (NESs) have been identified in many cellular proteins, but it remains unclear how different NESs compare in activity. We describe a sensitive new in vivo export assay which we have used to assess the relative export activity of different types of NES. The most common type of export sequence resembles that first identified in the HIV-1 Rev protein and typically comprises a core of large hydrophobic amino acids that specify recognition by the CRM1 export receptor. We compared 10 previously identified Rev-type NESs in our assay, and whereas all were functional, the relative export activities of these signals varied considerably. We further identified 3 new Rev-type NESs from a computer database search, and each export signal was assigned a score of 1 to 9 and ranked in order of activity (e.g., PKI > c-ABL > Ran-BP1 > FMRP > PML > IkappaB-alpha > hdm2). The weakest NESs were found in the p53 tumor suppressor and the p53-regulated proteins p21 and hdm2, which are all normally localized to the nucleus. All of the Rev-type NESs were inactivated by mutation of key hydrophobic residues and by treatment with the CRM1-specific export inhibitor, leptomycin B. In contrast, a different type of export signal, the KNS shuttling element derived from hnRNP K, exhibited a modest export activity that was insensitive to leptomycin B treatment. KNS thus appears to mediate export via a CRM1-independent pathway. Mutagenesis of the KNS sequence identified, for the first time, specific serines and acidic residues necessary for its export activity, thereby distinguishing KNS from other types of nuclear transport signal. We have shown that different nuclear export signals can vary profoundly in activity and therefore conclude that the nuclear export rate of a specific shuttling protein largely depends on both the strength and the accessibility of its NES.
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PMID:A comparison of the activity, sequence specificity, and CRM1-dependence of different nuclear export signals. 1073 68

Bile acids, principally deoxycholic acid (DCA), have been implicated in the promotion of colon tumorigenesis in both animals and humans. Increasing evidence suggests that bile acids may exert their tumor promoting activity by modulating intracellular signaling and altering gene expression. In this study we have investigated the effect of bile acids on the tumor suppressor p53 using the human colon tumor cell line HCT116, which retains the wild-type p53 gene and functional p53 signaling in response to DNA damage. We found that exposure of the cells to elevated concentrations of DCA suppressed accumulation of p53 protein as well as p53 transactivation and impaired the p53 response of the cells to DNA damaging agents, such as ionizing radiation. Neither ursodeoxycholic acid, a putative chemopreventive agent, nor cholic acid, which is biologically inert, had any effect on p53 protein level and transactivation activity. Further examination revealed that instead of inhibition, DCA induced p53 mRNA in a dose-dependent manner, indicating that the inhibitory effect of DCA on p53 protein is mediated by a post-transcriptional mechanism. Both lactacystin, a specific inhibitor of the 26S proteasome, and leptomycin B, a specific inhibitor of the nuclear export protein CRM1, could block the effect that DCA had on p53 protein levels, suggesting that DCA suppressed p53 by stimulating the process of proteasome-mediated degradation of p53. Significantly, blocking extracellular signal-regulated kinase (ERK) signaling, but not protein kinase C (PKC), blunted suppression by DCA of p53 protein levels and transactivation activity, suggesting that DCA suppressed p53, in part, by stimulating the ERK signaling pathway. Both ERK and PKC signaling have been previously demonstrated to be stimulated by DCA. These results suggest a novel signaling mechanism of bile acids that may play an important role in colon tumor promotion mediated by bile acids.
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PMID:Deoxycholic acid suppresses p53 by stimulating proteasome-mediated p53 protein degradation. 1137 5

The growth inhibitory functions of p53 are controlled in unstressed cells by rapid degradation of the p53 protein. One of the principal regulators of p53 stability is MDM2, a RING finger protein that functions as an E3 ligase to ubiquitinate p53. MDM2 promotes p53 nuclear export, and in this study, we show that ubiquitination of the C terminus of p53 by MDM2 contributes to the efficient export of p53 from the nucleus to the cytoplasm. In contrast, MDM2 did not promote nuclear export of the p53-related protein, p73. p53 nuclear export was enhanced by overexpression of the export receptor CRM1, although no significant relocalization of MDM2 was seen in response to CRM1. However, nuclear export driven by CRM1 overexpression did not result in the degradation of p53, and nuclear export was not essential for p53 degradation. These results indicate that MDM2 mediated ubiquitination of p53 contributes to both nuclear export and degradation of p53 but that these activities are not absolutely dependent on each other.
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PMID:C-terminal ubiquitination of p53 contributes to nuclear export. 1171 87

Human keratinocytes respond to UV rays by developing a fast adaptive response that contributes to maintaining their functions and survival. We investigated the role of the mitogen-activated protein kinase pathways in transducing the UV signals in normal human keratinocytes. We found that UVA, UVB or UVC induced a marked and persistent activation of p38, whereas c-Jun N-terminal kinase or extracellular signal-regulated kinase were less or not activated respectively. Inhibition of p38 activity by expression of a dominant-negative mutant of p38 or with SB203580 impaired cell viability and led to an increase in UVB-induced apoptosis. This sensitization to apoptosis was independent of caspase activities. Inhibition of p38 did not sensitize transformed HaCaT keratinocytes to UVB-induced apoptosis. In normal keratinocytes, expression of a dominant-negative mutant of p53 increased UVB-induced cell death, pointing to a role for p53. In these cells, UVB triggered a p38-dependent phosphorylation of p53 on Ser-15. This phosphorylation was associated with an SB203580-sensitive accumulation of p53, even in the presence of a serine phosphatase inhibitor. Accumulated p53 was localized mainly in the cytoplasm, independently of CRM1 nuclear export. In HaCaT cells, p53 was localized exclusively in the nucleus and its distribution and level were not affected by UVB or p38 inhibition. However, UVB induced an SB203580-insensitive phosphorylation on Ser-15 of mutated p53. Overall, our results suggest that, in normal human keratinocytes, protection against UVB depends on p38-mediated phosphorylation and stabilization of p53 and is tightly associated with the cytoplasmic sequestration of wild-type p53. We conclude that the p38/p53 pathway plays a key role in the adaptive response of normal human keratinocytes against UV stress.
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PMID:UVB-mediated activation of p38 mitogen-activated protein kinase enhances resistance of normal human keratinocytes to apoptosis by stabilizing cytoplasmic p53. 1207 47

The p53 tumour suppressor gene is a transcription factor that can induce cell cycle arrest and apoptosis. In response to various stress-inducing signals, p53 level increases and this is accompanied with increased activities of p53. Interestingly, the methylxanthine caffeine can abrogate the p53 accumulation induced by certain DNA-damaging agents by an unknown mechanism. In an effort to understand how different signals induce p53, human tumour cell lines were treated with combinations of various stress-inducing agents and caffeine. Caffeine inhibited the accumulation of p53 induced by leptomycin B (LMB), an inhibitor of CRM1, but not N-acetyl-leu-leu-norleucinal, a proteasome inhibitor. Furthermore, caffeine also inhibited the accumulation of p53 by a variety of stress-inducing agents in vivo, such as 5-fluorouracil, doxorubicin, mitomycin C, camptothecin and roscovitine. However, caffeine failed to affect the accumulation of p53 in hypoxia (HYP)-treated cells. These results suggested that HYP must use a distinct pathway from most DNA-damaging and stress-inducing agents to induce p53.
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PMID:Hypoxia induces p53 through a pathway distinct from most DNA-damaging and stress-inducing agents. 1280 44

Until recently, the accepted model held that p53 degradation occurs exclusively on cytoplasmic proteasomes and, hence, has an absolute requirement for nuclear export of p53 via the CRM1 pathway. However, proteasomes are abundant in both cytosol and nucleus. We recently analyzed HDM2-mediated degradation of endogenous p53 in the presence of various CRM1 blockers. We found that significant HDM2-mediated degradation takes place despite nuclear export blockade, indicating that endogenous p53 degradation occurs locally in the nucleus, in parallel to cytoplasmic degradation. Here, we describe how subcellular fractionation can be used to monitor nuclear and cytoplasmic degradation of endogenous wild-type p53 during the recovery phase after a stress stimulus. The fractions are then analyzed by immunoblotting in a time-dependent fashion. Vimentin and lamin A proteins are used to monitor the purity of the cytosolic and nuclear fractions, respectively, and to control for equal loading.
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PMID:Analysis of nuclear and cytoplasmic degradation of p53 in cells after stress. 1282 34


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