Cytoplasmic p53 polypeptide is associated with ribosomes.

Our previous finding that the tumor suppressor p53 is covalently linked to 5.8S rRNA suggested functional association of p53 polypeptide with ribosomes. p53 polypeptide is expressed at low basal levels in the cytoplasm of normal growing cells in the G1 phase of the cell cycle. We report here that cytoplasmic wild-type p53 polypeptide from both rat embryo fibroblasts and MCF7 cells and the A135V transforming mutant p53 polypeptide were found associated with ribosomes to various extents. Treatment of cytoplasmic extracts with RNase or puromycin in the presence of high salt, both of which are known to disrupt ribosomal function, dissociated p53 polypeptide from the ribosomes. In immunoprecipitates of p53 polypeptide-associated ribosomes, 5.8S rRNA was detectable only after proteinase K treatment, indicating all of the 5.8S rRNA in p53-associated ribosomes is covalently linked to protein. While 5.8S rRNA linked to protein was found in the immunoprecipitates of either wild-type or A135V mutant p53 polypeptide associated with ribosomes, little 5.8S rRNA was found in the immunoprecipitates of the slowly sedimenting p53 polypeptide, which was not associated with ribosomes. In contrast, 5.8S rRNA was liberated from bulk ribosomes by 1% sodium dodecyl sulfate, without digestion with proteinase K, indicating that these ribosomes contain 5.8S rRNA, which is not linked to protein. Immunoprecipitation of p53 polypeptide coprecipitated a small fraction of ribosomes. p53 mRNA immunoprecipitated with cytoplasmic p53 polypeptide, while GAPDH mRNA did not. These results show that cytoplasmic p53 polypeptide is associated with a subset of ribosomes, having covalently modified 5.8S rRNA.     

p53 is associated with p34cdc2 in transformed cells.

The normal functioning of p53 is thought to involve p53 target proteins. We have previously identified a cellular 35 kd protein associated with p53 and now report evidence identifying this 35 kd protein as p34cdc2, product of the cell cycle control cdc2 gene. The association between p53 and p34cdc2 was detected in SV3T3 and T3T3 cell lines, both expressing the wild-type p53 phenotype, and in 3T3tx cells, expressing 'mutant' p53 phenotype. Binding of the mutant p53 phenotype with p34cdc2 was greatly reduced relative to wild-type. Complexes of p53-p34cdc2 may represent inactivation or activation of either component. The p34cdc2 kinase functions at cell cycle control points and is necessary for entry and passage through mitosis. It also operates in G1 and is involved in the commitment of cells into the proliferative cycle. Since we were unable to detect p53-p34cdc2 complexes in mitotic cells we propose that the interaction between p53 and p34cdc2 may be functional in cell growth control, possibly to promote or to suppress cell proliferation.     

Low-dose radiation hypersensitivity is associated with p53-dependent apoptosis

Exposure to environmental radiation and the application of new clinical modalities, such as radioimmunotherapy, have heightened the need to understand cellular responses to low dose and low-dose rate ionizing radiation. Many tumor cell lines have been observed to exhibit a hypersensitivity to radiation doses <50 cGy, which manifests as a significant deviation from the clonogenic survival response predicted by a linear-quadratic fit to higher doses. However, the underlying processes for this phenomenon remain unclear. Using a gel microdrop/flow cytometry assay to monitor single cell proliferation at early times postirradiation, we examined the response of human A549 lung carcinoma, T98G glioma, and MCF7 breast carcinoma cell lines exposed to gamma radiation doses from 0 to 200 cGy delivered at 0.18 and 22 cGy/min. The A549 and T98G cells, but not MCF7 cells, showed the marked hypersensitivity at doses <50 cGy. To further characterize the low-dose hypersensitivity, we examined the influence of low-dose radiation on cell cycle status and apoptosis by assays for active caspase-3 and phosphatidylserine translocation (Annexin V binding). We observed that caspase-3 activation and Annexin V binding mirrored the proliferation curves for the cell lines. Furthermore, the low-dose hypersensitivity and Annexin V binding to irradiated A549 and T98G cells were eliminated by treating the cells with pifithrin, an inhibitor of p53. When p53-inactive cell lines (2800T skin fibroblasts and HCT116 colorectal carcinoma cells) were examined for similar patterns, we found that there was no hyperradiosensitivity and apoptosis was not detectable by Annexin V or caspase-3 assays. Our data therefore suggest that low-dose hypersensitivity is associated with p53-dependent apoptosis.     

Nucleosome rotational setting is associated with transcriptional regulation in promoters of tissue-specific human genes

Background  

The position of a nucleosome, both translational along the DNA molecule and rotational between the histone core and the DNA, is controlled by many factors, including the regular occurrence of specific dinucleotides with a period of approximately 10 bp, important for the rotational setting of the DNA around the histone octamer.     

Immunohistochemical expression of p53 in breast carcinoma is associated with the intron 1 BglII polymorphism of the p53 gene

Breast carcinoma is a public health problem worldwide. It is known that both genetic and environmental factors are important for breast carcinogenesis and that structural and/or functional alterations at p53 gene are commonly observed in breast tumors. In addition, polymorphisms of several genes in either their coding or non-coding sequences have been found related to cancer risk and/or clinicopathological characteristics of tumors. In this study we have evaluated the intron 1 BglII polymorphism of the p53 gene with a PCR-based approach in 117 cases of breast cancer and 102 healthy women and its association with the immunohistochemical expression of p53 in the tumors. The results showed that the presence of the polymorphism (allele 2) is highly associated with the tumor expression of p53 (p<0.0001) and that there is a trend for increased frequency of allele 2 in cases than in controls (p=0.2376). These data suggest that the germ-line variation in the intron 1 of the p53 gene could produce functional or structural changes of the protein that is reflected by its abnormal expression.