E2F1 inhibits MDM2 expression in a p53-dependent manner

MDM2 expression is down-regulated upon E2F1 over-expression, but the mechanism is not well defined. In the current study, we found that E2F1 inhibits MDM2 expression by suppressing its promoter activity. Although E2F1 binds to the MDM2 promoter, the inhibitory effect of E2F1 on the MDM2 promoter does not require the direct binding. We demonstrate that E2F1 inhibits MDM2 promoter activity in a p53-dependent manner. Knockdown of p53 in U2OS cells impairs the inhibitory effect of E2F1 on the MDM2 promoter. Consistent with this observation, E2F1 does not inhibit MDM2 promoter activity in p53-deficient H1299 cells, and the inhibition is restored when p53 is expressed exogenously. Both E2F1 and p53 are up-regulated after DNA damage stimulation. We show that such stimulation induces E2F1 to inhibit MDM2 promoter activity and promote p53 accumulation. Furthermore, inhibition of MDM2 by E2F1 promotes E2F1 induced apoptosis. These data suggest that E2F1 regulates the MDM2-p53 pathway by inhibiting p53 induced up-regulation of MDM2.     

含p53保守结合位点的microRNA表达载体的构建及应用

目的:构建含p53保守结合位点的microRNA(miRNA)表达载体,促进相关miRNA在具有野生型p53蛋白细胞中的高效表达。方法:改构miRNA表达载体pCMV-miR,在其多克隆位点前插入p53保守结合位点,分别将miR-138、miR-34a和miR-21前体序列pre-miR-138、pre-miR-34a和pre-miR-21插入上述改构的载体pCMV/p53-miR,将构建的pCMV/p53-miR-138、pCMV/p53-miR-34a和pCMV/p53-miR-21表达载体转染具有野生型p53的HeLa细胞和不表达p53的H1299细胞,分析p53对上述miRNA表达调控的影响。结果:转染改构的miRNA表达载体后,HeLa细胞中miR-138、miR-34a和miR-21的表达水平明显提高,它们对应的已知靶基因Cyclin D3、CDK2和PTEN的表达同时被显著下调。结论:在p53转录调控作用下,具有p53保守结合位点的miRNA表达载体能够更加有效地提高miRNA的表达水平;构建的载体不但可用于促进相关miRNA的表达,也能用于miRNA是否受p53调控的检测。     

Activation of p53-dependent responses in tumor cells treated with a PARC-interacting peptide

We tested the activity of a p53 carboxy-terminal peptide containing the PARC-interacting region in cancer cells with wild type cytoplasmic p53. Peptide delivery was achieved by fusing it to the TAT transduction domain (TAT-p53-C-ter peptide). In a two-hybrid assay, the tetramerization domain (TD) of p53 was necessary and sufficient to bind PARC. The TAT-p53-C-ter peptide disrupted the PARC-p53 complex. Peptide treatment caused p53 nuclear relocation, p53-dependent changes in gene expression and enhancement of etoposide-induced apoptosis. These studies suggest that PARC-interacting peptides are promising candidates for the enhancement of p53-dependent apoptosis in tumors with wt cytoplasmic p53.     

Persistent expression of Nqo1 by p62-mediated Nrf2 activation facilitates p53-dependent mitotic catastrophe

Prolonged mitosis due to aberrant chromosome segregation permits cells to enter the G1 phase without cytokinesis and subsequently triggers the p53-dependent cell death program, known as mitotic catastrophe. Cells which fail to go through mitotic catastrophe create aneuploidy, posing a risk of oncogenesis. In the present report, we show that p62-mediated non-canonical activation of Nrf2 leads to the persistent expression of Nqo1, which plays a critical role for p53 stabilization during mitotic catastrophe. With prolonged exposure to nocodazole, a microtubule-depolymerizing agent, p62-deficient HCT116 cells exhibited an accumulation of a polyploid population with a limited appearance of apoptotic cells, which was attributable to the attenuated stabilization of p53. Combinatorial gene manipulation analysis verified that the regulatory cascade with a hierarchy of p62–Keap1–Nrf2–Nqo1 is required for p53 stabilization for mitotic catastrophe. This is consistent with the role of Nqo1 as a gatekeeper for proteasomal degradation of p53. Thus, we demonstrate for the first time the functional connection between the non-canonical Nrf2 pathway and p53-dependent cell death program upon prolonged mitosis.     

A p53 codon 72 polymorphism associated with prostate cancer development and progression in Japanese

An association between the Pro/Pro genotype of p53 codon 72 and a lower risk of prostate cancer in Caucasians was recently reported. However, the association of this polymorphism with prostate cancer risk in a Japanese population has not been clarified. We performed a case-control study consisting of 114 prostate cancer patients and 105 noncancer controls. Sixty-nine percent (79 of 114) of the patients had a positive family history. The genotypic frequencies in the controls were 39.0% for Arg/Arg, 54.3% for Arg/Pro and 6.7% for Pro/Pro; they were in Hardy-Weinberg equilibrium. When a comparison of the distribution of the p53 codon 72 polymorphism was made between patients with a first-degree family history and all control subjects, the adjusted odds ratios (ORs) for prostate cancer associated with the Arg/Arg, Arg/Pro and Pro/Pro genotypes were 1.00, 0.99 [95% confidence interval (CI) 0.53-1.88] and 2.80 (95% CI 1.04-7.53), respectively. When stratification of cases was performed based on clinical stage (localized or metastatic cancer) and pathological grade (a Gleason score of <7 or > or =7), there tended to be a greater number of patients with localized cancers among those patients with the Arg/Pro genotype than among those with the Arg/Arg genotype (overall cases: age-adjusted OR 0.36, 95% CI 0.13-1.00, p = 0.049; positive family history cases: age-adjusted OR 0.25, 95% CI 0.075-0.84, p = 0.025). In addition, there tended to be a greater number of patients with low-grade cancers among those with the Pro/Pro genotype than among those with other genotypes (overall cases: age-adjusted OR 0.41, 95% CI 0.13-1.30, p = 0.13; positive family history cases: age-adjusted OR 0.20, 95% CI 0.004-0.89, p = 0.035). The present findings suggest that the Pro/Pro genotype of p53 codon 72 played a role in prostate cancer susceptibility in a Japanese population. However, the Pro allele did not appear to worsen such clinical parameters as clinical stage or pathological grade.     

Inactivation of Myc-induced p53-dependent apoptosis in human tumors

The Myc family of oncoproteins promote cell growth and are frequently overexpressed in human tumors. However, Myc can also trigger cell death by apoptosis. This is at least in part mediated via the ARF-p53 pathway. Myc activation leads to a selection for inactivation of ARF or p53, allowing cell survival and tumor progression. Restoration of p53-dependent apoptosis by various means is an attractive approach for new cancer therapy.     

Recognition of DNA by p53 core domain and location of intermolecular contacts of cooperative binding

We present an analysis by NMR of a 58 kDa complex of the core domain of the tumour suppressor p53 with DNA that complements and extends the crystal structure analysis. Binding of specific DNA caused significant chemical shifts of residues on the DNA-binding interface that translated into the beta-sheet of the protein. Binding of non-specific DNA caused weak but qualitatively the same shifts, corresponding to weaker binding interactions. The observed chemical shift differences correlate with frequency of cancer-inducing mutations, suggesting that the affected residues contribute to the stability of p53 core domain-DNA complex. We also identified two affected regions on the surface of the protein: helix 1 (residues V173-C182) plus G244 and residues L114-T118, which may represent a dimerisation interface.