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)

Normal somatic cells of higher organisms do not divide indefinitely. After a finite number of divisions, normal cells irreversibly cease proliferation by a process termed replicative or cellular senescence. Replicative senescence is controlled by multiple, dominant-acting genes about which very little is known. The only genes known to reactivate DNA synthesis in senescent cells are viral oncogenes encoding proteins that bind and inactivate the p53 and retinoblastoma (pRb) tumor suppressor proteins. SV40 T antigen is the best studied of these viral oncoproteins. T[K1] is a T antigen point mutant that selectively is defective in binding pRb and the pRb-related proteins p107 and p130. We show that T[K1] stimulated quiescent human fibroblasts to synthesize DNA nearly as well as wild-type T but was incapable of stimulating senescent cells. We tested several growth regulatory genes that are repressed in senescent cells for ability to restore activity to T[K1]. These included c-fos, c-jun, Id-1, Id-2, E2F-1, and cdc2. Only the helix-loop-helix (HLH) protein, Id-1, restored the ability of T[K1] to reactivate DNA synthesis in senescent cells. This activity of Id-1 was not shared by Id-2, a related protein, and depended on an intact HLH domain. It did not appear that Id-1 interacted directly with pRb or p107. Constitutive Id-1 expression failed to rescue proliferating cells from growth inhibition by pRb, p107, or p130, and failed to interact with pRb in the yeast two hybrid system. Because Id proteins negatively regulate basic-HLH (bHLH) transcription factors, we suggest that senescent cells express one or more bHLH factor that cooperates with pRb, or pRb-related proteins, to suppress proliferation.
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PMID:The helix-loop-helix protein Id-1 and a retinoblastoma protein binding mutant of SV40 T antigen synergize to reactivate DNA synthesis in senescent human fibroblasts. 893 78

The Mdm2 oncoprotein forms a complex with the p53 tumor suppressor protein and inhibits p53-mediated regulation of heterologous gene expression. Recently, Mdm2 has been found to bind several other proteins that function to regulate cell cycle progression, including the E2F-1/DP1 transcription factor complex and the retinoblastoma tumor-suppressor protein. To determine whether Mdm2 plays a role in cell cycle control or tumorigenesis that is distinct from its ability to modulate p53 function, we have examined and compared both the in vitro growth characteristics of p53-deficient and Mdm2/p53-deficient fibroblasts, and the rate and spectrum of tumor formation in p53-deficient and Mdm2/p53-deficient mice. We find no difference between p53-deficient fibroblasts and Mdm2/p53-deficient fibroblasts either in their rate of proliferation in culture or in their survival frequency when treated with various genotoxic agents. Cell cycle studies indicate no difference in the ability of the two cell populations to enter S phase when treated with DNA-damaging agents or nucleotide antimetabolites, and p53-deficient fibroblasts and Mdm2/p53-deficient fibroblasts exhibit the same rate of spontaneous immortalization following long-term passage in culture. Finally, p53-deficient mice and Mdm2/p53-deficient mice display the same incidence and spectrum of spontaneous tumor formation in vivo. These results demonstrate that deletion of Mdm2 has no additional effect on cell proliferation, cell cycle control, or tumorigenesis when p53 is absent.
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PMID:The tumorigenic potential and cell growth characteristics of p53-deficient cells are equivalent in the presence or absence of Mdm2. 894 68

Mutations in the retinoblastoma (pRb) tumor suppressor pathway including its cyclin-cdk regulatory kinases, or cdk inhibitors, are a hallmark of most cancers and allow unrestrained E2F-1 transcription factor activity, which leads to unregulated G1-to-S-phase cell cycle progression. Moderate levels of E2F-1 overexpression are tolerated in interleukin 3 (IL-3)-dependent 32D.3 myeloid progenitor cells, yet this induces apoptosis when these cells are deprived of IL-3. However, when E2F activity is augmented by coexpression of its heterodimeric partner, DP-1, the effects of survival factors are abrogated. To determine whether enforced E2F-1 expression selectively sensitizes cells to cytotoxic agents, we examined the effects of chemotherapeutic agents and radiation used in cancer therapy. E2F-1 overexpression in the myeloid cells preferentially sensitized cells to apoptosis when they were treated with the topoisomerase II inhibitor etoposide. Although E2F-1 alone induces moderate levels of p53 and treatment with drugs markedly increased p53, the deleterious effects of etoposide in E2F-1-overexpressing cells were independent of p53 accumulation. Coexpression of Bcl-2 and E2F-1 in 32D.3 cells protected them from etoposide-mediated apoptosis. However, Bcl-2 also prevented apoptosis of these cells upon exposure to 5-fluorouracil and doxorubicin, which were also cytotoxic for control cells. Pretreating E2F-1-expressing cells with ICRF-193, a second topoisomerase II inhibitor that does not damage DNA, protected the cells from etoposide-induced apoptosis. However, ICRF-193 cooperated with DNA-damaging agents to induce apoptosis. Therefore, topoisomerase II inhibition and DNA damage can cooperate to selectively induce p53-independent apoptosis in cells that have unregulated E2F-1 activity resulting from mutations in the pRb pathway.
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PMID:E2F-1 cooperates with topoisomerase II inhibition and DNA damage to selectively augment p53-independent apoptosis. 903 31

Overexpression of genes encoding E2F transcription factors can transform some cultured cell lines and cause apoptosis of others. Apoptosis due to E2F overexpression requires the presence of wild-type p53. Cell lines in which stable E2F overexpression is possible might be expected, therefore, to contain mutant p53. In this report, it was asked whether endogenous p53 was mutant or wild-type in four established fibroblast cell lines that this laboratory previously showed stably overexpressed and were transformed by exogenous E2F-1. Unexpectedly, it was found that the p53 in these cells was wild-type by the criteria of immunoprecipitation with conformation-specific, p53 monoclonal antibodies and by transactivation of a p53-dependent reporter gene construct in transient transfection assays. These data indicate that stable overexpression of E2F-1 is possible in the presence of wild-type p53 and may result in cell transformation.
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PMID:Established cell lines transformed by E2F-1 overexpression contain wild-type p53. 910 15

In a previous study, we found that treatment of HCT-8 cells with ZD1694, a specific antifolate-based thymidylate synthase inhibitor, resulted in DNA fragmentation. In this study, we have demonstrated the dose- and time-dependent induction of DNA fragmentation accompanied by elevation of p53 and WAF1 protein expression by ZD1694. WAF1 mRNA showed a time-dependent increase, whereas p53 mRNA was not found to be significantly overexpressed. The initial increase in WAF1 mRNA was detected at 4 hr, but increased WAF1 protein expression was detected 8-24 hr after a 2-hr exposure. The amount of total and hypophosphorylated pRb seems to be rising greatly after ZD1694 exposure. The effects of ZD1694 on the expression of E2F1 and formation of the E2F1-Rb complex were investigated after a 2-hr drug exposure (IC90). The results showed a time-dependent decrease in E2F1 mRNA and protein expression; an increase in the abundance of the E2F-Rb complex could be demonstrated beginning 4 hr after drug exposure by a gel shift assay. Kinetic analysis showed increased availability of hypophosphorylated pRb for inhibition of E2F, which could indirectly result from WAF1-induced inhibition cyclin-dependent kinase activity. Whereas thymidylate synthase inhibition by ZD1694 was rapid in onset and maintained for at least 24 hr after drug treatment, drug-induced cellular growth inhibition was significant 24 hr after drug exposure. The increased abundance of hypophosphorylated pRb and binding to transcription factor E2F-1 is consistent with ZD1694-induced cell growth inhibition in HCT-8 cells. Therefore, the observed effect on downstream events after effective inhibition of thymidylate synthase may offer the critical determinants of response to ZD1694.
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PMID:p53 and WAF1 are induced and Rb protein is hypophosphorylated during cell growth inhibition by the thymidylate synthase inhibitor ZD1694 (Tomudex). 910 28

The effect of overexpression of p21waf1 on drug sensitivity was studied in an osteosarcoma cell line (SaOs-2) lacking both p53 and functional retinoblastoma protein using a tetracycline (TC)-inducible expression system. p21waf1 expression was barely detectable in SaOS-2 cells incubated in the presence of TC. After TC withdrawal, high levels of p21waf1 were induced in these cells. These p21waf1-induced cells showed increased sensitivity to doxorubicin, tomudex, and methotrexate as compared to uninduced cells; this condition is associated with increased apoptosis. Expression of p21waf1 reduced cyclin A-associated kinase activity and, surprisingly, resulted in inhibition of phosphorylation of E2F-1 and increased E2F-1 binding activity. An S-G2 cell cycle arrest/delay and an increase in expression of E2F-responsive genes (dihydrofolate reductase and thymidylate synthase) was correspondingly observed. Overexpression of p21waf1 in cells lacking functional retinoblastoma protein may mediate sensitivity to anticancer drugs by inhibiting E2F-1 phosphorylation, which may contribute to increased S-G2 cell cycle delay and increased cell susceptibility to apoptosis.
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PMID:Overexpression of p21waf1 leads to increased inhibition of E2F-1 phosphorylation and sensitivity to anticancer drugs in retinoblastoma-negative human sarcoma cells. 918 20

The E6 and E7 proteins from the high-risk human papillomaviruses (HPVs) bind and inactivate the tumor suppressor proteins p53 and Rb, respectively. In HPV-positive cells, expression of E6 proteins from high-risk types results in increased turnover of p53, which leads to an abrogation of p21-mediated G1/S arrest in response to DNA-damaging agents. In contrast, keratinocytes which express E7 alone have increased levels of p53 but, interestingly, also fail to undergo a G1/S arrest. We investigated the mechanism by which E7 bypasses this p21 arrest by using both keratinocytes which stably express E7 as well as U20S cells which stably or transiently express E7. We observed that E7 does not affect the induction of p21 synthesis by p53. While glutathione S-transferase (GST)-E7 bound a low level of in vitro-translated p21, we were unable to detect E7 and p21 in the same complex by GST-E7 binding assays or immunoprecipitations from cell extracts. Furthermore, E7 did not prevent p21-mediated inhibition of cyclin E kinase activity. In keratinocytes expressing E7, increased levels of p53, p21, and cyclin E, as well as increased cyclin E kinase activity, were observed. To determine if this increase in cyclin E activity was necessary for E7's ability to overcome p21-mediated G1/S arrest, we examined U20S cells in which cyclin E levels are not increased in response to E7 expression. U20S cells which stably express E7 were found to initiate DNA synthesis in the presence of DNA-damaging agents despite the inhibition of cyclin E activity by p21. In transient assays, cotransfection of E7 or E2F-1 along with p21 into U20S cells rescued G1 arrest and resulted in S-phase entry, as measured by the ability to incorporate bromodeoxyuridine. These data indicate that E7 is able to overcome G1/S arrest without directly affecting p21 function and likely acts through deregulation of E2F activity.
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PMID:Initiation of DNA synthesis by human papillomavirus E7 oncoproteins is resistant to p21-mediated inhibition of cyclin E-cdk2 activity. 918 31

Cyclin D1 plays a key regulatory role during the G1 phase of the cell cycle and its gene is amplified and overexpressed in many cancers. To address the relationship between cyclin D1 and other cell cycle regulatory proteins, we established human glioma and rodent fibroblast cell lines in which cyclin D1 expression could be regulated ectopically with tetracycline. In both of these cell lines, we found that ectopic expression of cyclin D1 in asynchronously growing cells was accompanied by increased levels of the p53 tumor suppressor protein and the cyclin/cdk inhibitor p21. Despite the induction of these cell cycle inhibitory proteins, cyclin D1-associated cdk kinase remained activated and the cells grew essentially like that of the parent cells. Although growth parameters were unchanged in these cells, morphological changes were clearly identifiable and anchorage independent growth was observed in NIH3T3 cells. In a first step toward elaborating the mechanism for cyclin D1-mediated induction of p21 gene expression we show that co-expression of E2F-1 and DP-1 can specifically transactivate the p21 promoter. In support of these findings and a direct effect of E2F on induction of p21 gene expression a putative E2F binding site was identified within the p21 promoter. In summary, our results demonstrate that ectopic expression of cyclin D1 can induce gene expression of the cdk inhibitor p21 through an E2F mechanism the consequences of which are not to growth arrest cells but possibly to stabilize cyclin D1/cdk function.
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PMID:Regulated ectopic expression of cyclin D1 induces transcriptional activation of the cdk inhibitor p21 gene without altering cell cycle progression. 919 Oct 53

It has been demonstrated that infection of primary human cells with adeno-associated viruses (AAV) leads to a decrease in cellular proliferation and to growth arrest. We analyzed the molecular basis of this phenomenon and observed that infection with AAV type 2 (AAV2) had an effect on several factors engaged in the control of the mammalian cell cycle. In particular, all of the pRB family members, pRB, p107, and p130, which are involved in G1 cell cycle checkpoint control, were affected. After infection, a shift from hyper- to hypophosphorylated forms was observed. Cyclins A and B1, which are required for G1/S transition and progression into mitosis, respectively, were downregulated at the transcriptional level as well as at the protein level, whereas the G1 cyclins D1 and E remained unaffected. In addition, the steady-state levels of cyclin-dependent kinases CDK1 and CDK2 and of transcription factor E2F-1 were diminished. Of all the factors known to be involved in phosphorylation of pRB family proteins, only the CDK inhibitor p21WAF1 exhibited a response to AAV2 infection. p21WAF1 mRNA was quickly and progressively upregulated in a p53-independent manner over at least 72 h. Consistent with the increased p21WAF1 protein levels, cyclin E- and cyclin A-dependent kinase activities declined to low levels and E2F-p130-cyclin-CDK2 complexes were disrupted. From these data, we conclude that the major effect of AAV2 infection on primary human fibroblasts appears to be upregulation of p21WAF1 gene expression and thus cell cycle arrest by the suppression of pRB family protein phosphorylation.
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PMID:Infection of primary cells by adeno-associated virus type 2 results in a modulation of cell cycle-regulating proteins. 922 93

The family of E2F transcription factors have an essential role in mediating cell cycle progression, and recently, one of the E2F protein family, E2F-1, has been shown to participate in the induction of apoptosis. Cooperation between E2F and the p53 tumor suppressor protein in this apoptotic response had led to the suggestion that cell cycle progression induced by E2F-1 expression provides an apoptotic signal when placed in conflict with an arrest to cell cycle progression, such as provided by p53. We show here that although apoptosis is clearly enhanced by p53, E2F-1 can induce significant apoptosis in the absence of p53. Furthermore, this apoptotic function of E2F-1 is separable from the ability to accelerate entry into DNA synthesis. Analysis of E2F-1 mutants indicates that although DNA-binding is required, transcriptional transactivation is not necessary for the induction of apoptosis by E2F-1, suggesting that it may be mediated through alleviation of E2F-dependent transcriptional repression. These results indicate that E2F-1 can show independent cell cycle progression and apoptotic functions, consistent with its putative role as a tumor suppressor.
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PMID:Induction of DNA synthesis and apoptosis are separable functions of E2F-1. 924 92


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