The Trp53 pathway is induced in vivo by low doses of gamma radiation

The induction of the Trp53 response after very low doses (0.01-1 Gy) of ionizing radiation was studied in the adult mouse using immunochemical and immunohistochemical methods. We found a detectable response at 0.01 Gy and an increased induction of Trp53 with increasing dose in both radiation-resistant and radiation-sensitive tissues. These results suggest that there is no lower threshold for induction. This response was heterogeneous, since cells that received the same dose had different staining intensities, suggesting that the induction of Trp53 is not based simply on dose-dependent responses to DNA damage. These data also demonstrate the exquisite sensitivity of the Trp53 pathway and show that this response is controlled by cell- and tissue-specific factors that have yet to be defined.

The biochemical characterization of the DNA binding activity of pKi67

Ki67 is only expressed in the nucleus of cycling cells. While it is employed as an operational marker of proliferation, little is known of the biochemical properties of this large protein. Using an immunoaffinity strategy for purification of pKi67, this study has shown that it can form higher-order complexes and can bind to DNA cellulose in vitro. No other co-purifying proteins could be identified, strongly suggesting that the DNA binding activity is an inherent property of pKi67. Using an electromobility shift assay, the affinity of pKi67 was shown using a range of different forms of DNA as competitors. Single-stranded DNA was the poorest competitor, followed by double-stranded DNA, with supercoiled DNA being the best competitor. In addition, it was found that purified pKi67 has a preference for AT-rich DNA. The DNA binding domain is mapped to the C-terminal domain of pKi67, and recombinant protein from the terminal 321 residues of pKi67 can bind DNA in vitro. GFP constructs from this domain were used to map regions that could target nucleolar localization and allow DNA binding. Finally, it was found that over-expression of the C-terminal 321 residues in cells induced chromatin disruption and apoptosis. These data provide strong evidence that pKi67 has a novel DNA binding activity within the C-terminal domain and that this protein can influence chromatin structure.

The location of pKi67 in the outer dense fibrillary compartment of the nucleolus points to a role in ribosome biogenesis during the cell division cycle

Although widely used as a marker of cell proliferation, the biochemical properties and function of the Ki67 antigen remain poorly understood. Recent data indicate that it can interact with RNA, DNA, and a number of cellular proteins including elements of the ubiquitin proteolytic pathway and a novel kinase. The evidence for its expression only in cycling cells is extensive and it is not regulated by stress, apoptosis or DNA damage. It was reasoned that a detailed characterization of the localization of pKi67 and analysis of its spatial relationship to other nucleolar proteins may provide insights into its function. Using high-resolution laser scanning confocal microscopy with double and triple labelling, pKi67 expression in MCF7 cells has been defined in relation to the distribution of nucleolin, fibrillarin, p130 (human Nopp 140 homologue), p120 (Nol 1), RH-II/Gu helicase, and topoisomerase II beta. All of these molecules are perichromosomal during mitosis and all but fibrillarin and p130 show extra-nucleolar distribution in early G1. The majority of p120 (Nol 1) and RH-II/Gu helicase co-localize in the diffuse fibrillar centre (DFC) of nucleoli, while there is only partial overlap with nucleolin and fibrillarin. There is no co-localization between p130 and pKi67. These data refine current understanding of the distribution of pKi67 and its physical relationship with functional domains of the nucleolus and place pKi67 in a zone of the DFC associated with late rRNA processing. Taken together with recent biochemical data, these observations allow the proposal of a model of pKi67 function in which it acts as an 'efficiency factor' in ribosome biogenesis during the heavy metabolic demands placed on a cell during the cell division cycle.

Biochemical characterization of pKi67 with the identification of a mitotic-specific form associated with hyperphosphorylation and altered DNA binding

Although widely used as an operational marker of proliferation, the cell cycle-regulated Ki67 protein is of unknown function. pKi67 is found predominantly in the nucleolus in cycling interphase cells and moves to become perichromosomal during mitosis. We have performed a detailed immunochemical analysis of pKi67 in HeLa cells and report the existence of a novel hyperphosphorylated form in mitosis. Two isoforms can be identified on immunoblots as a consequence of the previously described alternative splicing. In extracts from mitotic cells both these isoforms have considerably reduced mobility. Treatment with phosphatase converts the mitotic form to the interphase form. Immunoprecipitated pKi67 can be phosphorylated in vitro both by cdc2/cyclin B and by protein kinase C, and treatment by PKC leads to the full mobility shift. Treatment of nocodazole-arrested mitotic HeLa cells with staurosporine causes a dephosphorylation of pKi67 to the interphase state and a concomitant change in the localization of pKi67 with movement away from the perichromosomal layer to cytoplasmic dots that colocalize with nucleolin. These data indicate that pKi67 localization is regulated by the action of cell cycle-specific kinase(s) and phosphatase(s). The data presented here provide a starting point for the analysis of pKi67 function and regulation.

Expression of the 'dead box' RNA helicase p68 is developmentally and growth regulated and correlates with organ differentiation/maturation in the fetus

The human DEAD box protein p68 is an established RNA-dependent ATPase and RNA helicase, p68 has been highly conserved in evolution and appears to be essential for normal growth, suggesting that this protein plays an important role in the cell. Although the biochemical activities of p68 are fairly well characterized, little is known about its biological function. This report shows that p68 is detectable in quiescent cell lines, but its expression is induced by serum, suggesting that this protein may play a role in cell growth. It is also shown that both p68 mRNA and protein are differentially expressed in adult tissues; in this case, however, the levels do not always correlate with proliferation status, suggesting that the regulation of expression in the animal may be different from that in cell lines. Finally, it is shown that p68 expression is developmentally regulated and appears to correlate with organ differentiation/maturation. These findings suggest that p68 expression may not simply reflect proliferation/differentiation status and that it appears to be regulated in a more complex way.

The p53 response to ionising radiation in adult and developing murine tissues

The induction of the p53 response to ionising radiation has been studied during murine development and in the adult animal. The response has been assessed by precise quantitative assay of p53 protein levels in tissues and by immunohistochemistry. Newly developed transgenic mice in which a lacZ transgene is driven by a p53 response element have also been used to directly assess the transcriptional activity of the induced protein. There is striking developmental control of the p53 response so that in early development all tissues accumulate high levels of p53 following radiation and indeed p53 is present at elevated levels in some unirradiated tissues. Later in development clear heterogeneity of the p53 response becomes apparent, both in terms of the responses of individual tissues and of cell populations, within those tissues. The study of lacZ transgene expression and the occurrence of apoptosis in different tissues that accumulate p53 protein point to a further level of control regulating the nature and degree of the downstream response to elevated levels of p53 in cells. These findings have important implications for the susceptibility of different tissue types to carcinogenic and other insults. The early expression of the p53 response is consistent with novel models of p53 function that suggest it may have evolved principally as a defense against teratogenic insult that permits plasticity of development.