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)

The tumor suppressor p16(INK4a) inhibits cyclin-dependent kinases 4 and 6. This activates the retinoblastoma protein (pRB) and, through incompletely understood events, arrests the cell division cycle. To permit biochemical analysis of the arrest, we generated U2-OS osteogenic sarcoma cell clones in which p16 transcription could be induced. In these clones, binding of p16 to cdk4 and cdk6 abrogated binding of cyclin D1, p27(KIP1), and p21(WAF1/CIP1). Concomitantly, the total cellular level of p21 increased severalfold via a posttranscriptional mechanism. Most cyclin E-cdk2 complexes associated with p21 and became inactive, expression of cyclin A was curtailed, and DNA synthesis was strongly inhibited. Induction of p21 alone, in a sibling clone, to the level observed during p16 induction substantially reproduced these effects. Overexpression of either cyclin E or A prevented p16 from mediating arrest. We then extended these studies to HCT 116 colorectal carcinoma cells and a p21-null clone derived by homologous recombination. In the parental cells, p16 expression also augmented total cellular and cdk2-bound p21. Moreover, p16 strongly inhibited DNA synthesis in the parental cells but not in the p21-null derivative. These findings indicate that p21-mediated inhibition of cdk2 contributes to the cell cycle arrest imposed by p16 and is a potential point of cooperation between the p16/pRB and p14(ARF)/p53 tumor suppressor pathways.
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PMID:Induction of p21(WAF1/CIP1) and inhibition of Cdk2 mediated by the tumor suppressor p16(INK4a). 1020 15

The p19ARF product of the INK4a/ARF locus is induced in response to potentially oncogenic hyperproliferative signals and activates p53 by interfering with its negative regulator, Mdm2. Mice lacking ARF are highly prone to tumor development, and in this study, 80% of these animals spontaneously developed tumors and died within their first year of life. Mice that were heterozygous for ARF also developed tumors after a longer latency, whereas their wild-type littermates did not. In heterozygotes, tumor formation was accompanied by loss of the residual ARF allele and/or lack of ARF mRNA expression, implying that ARF can act as a canonical "two-hit" tumor suppressor gene. Tumors occurred earlier in life in ARF-null animals that were neonatally irradiated or given dimethylbenzanthrene, and several animals treated with carcinogen simultaneously developed multiple forms of malignancy arising from distinct cell lineages. Although p53-null mice primarily develop lymphomas and fibrosarcomas, the frequency of these two tumor types was inverted in ARF-null animals, with undifferentiated sarcomas predominating in a 3:2 ratio; 28% of ARF-null animals developed carcinomas and tumors of the nervous system, which have been rarely observed in untreated p53-null mice. The longer latency of tumor formation in ARF-null versus p53-null mice, therefore, appears to enable a broader spectrum of tumors to emerge.
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PMID:Tumor spectrum in ARF-deficient mice. 1023 11

Abnormalities of the p53 tumour suppressor gene are among the most frequent molecular events in human and animal neoplasia. Moreover, p53 is one of the most studied proteins in the whole of contemporary biology, with more than 12,500 papers so far written! In this review the choice has been deliberately made not to be fully comprehensive in the coverage of the huge p53 literature. Rather attention is focused on a small number of recent developments which are reviewed in the context of modern models of p53 function. Progress in the analysis of signalling to p53 including phosphorylation cascades, and interactions with proteins such as mdm2 and ARF are highlighted. The plethora of protein-protein interactions is discussed, as are the strategies for defining downstream targets of p53. Finally, the emerging biology of p53 homologues is considered. The need for bridging the gap between reductionist, biochemical and biophysical studies and biological and genetic analysis is emphasized. Only this will provide the needed framework for utilizing the information in clinical care.
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PMID:The p53 pathway. 1034 12

The alternative reading frame product (p19ARF) of the mouse INK4a/ARF locus is induced by oncoproteins such as Myc and E1A as part of a checkpoint response that limits cell cycle progression in response to hyperproliferative signals. ARF binds directly to Mdm2 to prevent down-regulation of p53 and thereby promotes p53-dependent transcription and cell cycle arrest. However, ARF is not required for p53 induction in response to ionizing radiation or other forms of DNA damage. Animals lacking a functional ataxia telangiectasia (Atm) gene are exquisitely sensitive to ionizing radiation; Atm-null mouse embryo fibroblasts (MEFs) undergo premature replicative arrest, which is relieved by the loss of p53. Here we show that the loss of ARF expands the life expectancy of Atm-null MEFs, but alters neither the sensitivity of Atm-null mice to ionizing radiation nor their propensity to develop lymphomas early in life. Therefore, whereas ARF and Atm signal to p53 through distinct pathways, the loss of ARF can modify p53-dependent features of the Atm-null phenotype.
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PMID:Loss of the ARF tumor suppressor reverses premature replicative arrest but not radiation hypersensitivity arising from disabled atm function. 1034 59

The INK4a-ARF locus encodes two distinct tumor suppressors, p16(INK4a) and p19(ARF). Whereas p16(INK4a) restrains cell growth through preventing phosphorylation of the retinoblastoma protein, p19(ARF) acts by attenuating Mdm2-mediated degradation of p53, thereby stabilizing p53. Recent data indicate that Mdm2 shuttles between the nucleus and the cytoplasm and that nucleo-cytoplasmic shuttling of Mdm2 is essential for Mdm2's ability to promote p53 degradation. Therefore, Mdm2 must export p53 from the nucleus to the cytoplasm where it targets p53 for degradation. We show here that coexpression of p19(ARF) blocks the nucleo-cytoplasmic shuttling of Mdm2. Moreover, subnuclear localization of Mdm2 changes from the nucleoplasm to the nucleolus in a shuttling time-dependent manner, whereas p19(ARF) is exclusively located in the nucleolus. In heterokaryons containing Mdm2 and p19(ARF), the longer the Mdm2 shuttling is allowed, the more Mdm2 protein colocalizes with p19(ARF) in the nucleolus, implying that Mdm2 moves from the nucleoplasm to the nucleolus and then associates with p19(ARF) there. Furthermore, whether or not Mdm2 colocalizes with p19(ARF) in the nucleolus, p19(ARF) prevents Mdm2 shuttling. This observation suggests that Mdm2 might be exported through the nucleolus and p19(ARF) could inhibit the nuclear export of Mdm2 by tethering Mdm2 in the nucleolus. Taken together, p19(ARF) could stabilize p53 by inhibiting the nuclear export of Mdm2.
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PMID:P19(ARF) stabilizes p53 by blocking nucleo-cytoplasmic shuttling of Mdm2. 1035 17

The mammalian ARF-INK4a locus uniquely encodes two cell cycle inhibitors by using separate promoters and alternative reading frames. p16INK4a maintains the retinoblastoma protein in its growth suppressive state while ARF stabilizes p53. We report that human ARF protein predominantly localizes to the nucleolus via a sequence within the exon 2-encoded C-terminal domain and is induced to leave the nucleolus by MDM2. ARF forms nuclear bodies with MDM2 and p53 and blocks p53 and MDM2 nuclear export. Tumor-associated mutations in ARF exon 2 disrupt ARF's nucleolus localization and reduce ARF's ability to block p53 nuclear export and to stabilize p53. Our results suggest an ARF-regulated MDM2-dependent p53 stabilization and link the human tumor-associated mutations in ARF with a functional alteration.
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PMID:Mutations in human ARF exon 2 disrupt its nucleolar localization and impair its ability to block nuclear export of MDM2 and p53. 1036 Jan 74

The INK4A and the INK4B genes map to chromosome 9p21, an area frequently deleted in bladder neoplasms. In addition to the p16 protein, the INK4A encodes for a second product, termed p19(ARF). We analyzed tissues from 121 patients with initial Ta and T1 tumors. Deletions of the INK4A gene were observed in 17 of 121 (14.1%) cases. Point mutations were identified in 2 of 64 (3.1%) tumors. The INK4A-exon 1beta and the INK4B gene were codeleted with INK4A in all of the homozygously deleted cases analyzed. The p16 promoter underwent de novo methylation in 7 of 47 (14.9%) evaluable cases. The p16-positive phenotype was observed in 18 of 56 (32%) evaluable cases. p16 negative phenotype correlated with deletion and methylation status. A statistically significant association between INK4A homozygous deletions and tumor size was observed (P = 0.003). Patients bearing tumors with INK4A homozygous deletions had a lower recurrence-free survival (P = 0.040) than those with wild type INK4A. In conclusion, deletions and methylation of the INK4A gene occur frequently in superficial bladder tumors. However, only those deletions that affect both the p16 and the p19(ARF), deregulating both the pRb and p53 pathways, correlated with clinicopathological parameters of worse prognosis.
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PMID:Deletions of the INK4A gene in superficial bladder tumors. Association with recurrence. 1039 43

High-risk human papillomaviruses are causally associated with cervical cancer. Two viral oncogenes, E6 and E7, are expressed in most cervical cancers, and these genes cause cancer when expressed in experimental animals. The E6 protein targets the p53 tumor suppressor for degradation, while the E7 protein inactivates the retinoblastoma susceptibility protein (pRb), in part by stimulating its degradation. In contrast, expression of E7 in the absence of E6 leads to stabilization of p53. Here we show that E7 stabilizes p53 in mouse embryo fibroblasts lacking p19(ARF). The stable p53 is active as a transcriptional activator, as evidenced by the increased expression of the p53-responsive mdm2 gene. Normally, MDM2 protein inhibits p53 function in an autoregulatory loop. Regulation of p53 by MDM2 is required for murine development as well as for proliferation of cultured human fibroblasts. However, E7-expressing human fibroblasts continue to divide even though E7 abrogates the ability of MDM2 and p53 to bind. Furthermore, E7-expressing cells are not more sensitive to UV light, an agent that has been reported to induce apoptosis mediated by p53. These results indicate that in addition to inhibiting the ability of MDM2 to regulate p53, E7 must block signaling steps downstream of p53 to allow cell division.
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PMID:The E7 oncoprotein of human papillomavirus type 16 stabilizes p53 through a mechanism independent of p19(ARF). 1043 49

The INK4a/ARF locus on human chromosome 9p resides at the nexus of two critical cell cycle regulatory pathways, the p53 pathway and the retinoblastoma (pRb) gene pathway. Through the use of shared coding regions and alternative reading frames two distinct proteins are produced: INK4a is a cyclin-dependent kinase inhibitor whereas ARF binds the MDM2 proto-oncogene and stabilizes p53. We have examined the expression patterns of the INK4a/ARF locus at the RNA level in normal human and murine tissues to determine if these genes are coordinately regulated. We found that both INK4a and ARF were expressed in most tissues at low levels detectable only by RT-PCR. The pancreas was an exception in that it expressed no detectable ARF mRNA but expressed high levels of INK4a mRNA. Furthermore, human pancreas expressed an additional previously unrecognized splice variant of INK4a, termed p12, through the use of an alternative splice donor site within intron 1. The p12 transcript produced a 12 kD protein composed of INK4a exon 1alpha and a novel intron-derived C-terminus. This novel protein did not interact with cdk4 but was capable of suppressing growth in a pRb-independent manner. The implications of the capacity of the INK4a/ARF locus to encode a third transcript, and for pancreatic cancer, in which the INK4a/ARF locus is nearly always altered, are considered.
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PMID:Tissue-specific alternative splicing in the human INK4a/ARF cell cycle regulatory locus. 1044 44

Oncogene activation increases susceptibility to apoptosis. Thus, tumorigenesis must depend, in part, on compensating mutations that protect from programmed cell death. A functional screen for cDNAs that could counteract the proapoptotic effects of the myc oncogene identified two related bHLH family members, Twist and Dermo1. Both of these proteins inhibited oncogene- and p53-dependent cell death. Twist expression bypassed p53-induced growth arrest. These effects correlated with an ability of Twist to interfere with activation of a p53-dependent reporter and to impair induction of p53 target genes in response to DNA damage. An underlying explanation for this observation may be provided by the ability of Twist to reduce expression of the ARF tumor suppressor. Thus, Twist may affect p53 indirectly through modulation of the ARF/MDM2/p53 pathway. Consistent with a role as a potential oncoprotein, Twist expression promoted colony formation of E1A/ras-transformed mouse embryo fibroblasts (MEFs) in soft agar. Furthermore, Twist was inappropriately expressed in 50% of rhabdomyosarcomas, a tumor that arises from skeletal muscle precursors that fail to differentiate. Twist is known to block myogenic differentiation. Thus, Twist may play multiple roles in the formation of rhabdomyosarcomas, halting terminal differentiation, inhibiting apoptosis, and interfering with the p53 tumor-suppressor pathway.
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PMID:Twist is a potential oncogene that inhibits apoptosis. 1048 44


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