Abelson murine leukemia virus-transformed cells that lack p53 protein synthesis express aberrant p53 mRNA species

Overexpression of p53 protein in human tumors

According to the current concept of carcinogenesis, neoplastic transformation consists of multistep accumulations of adverse genetic and epigenetic events. p53 is a transcription factor that regulates cellular response to diverse forms of stress through a complex network which monitors genome integrity and cell homeostasis. Mutant p53 loss-of-function, dominant-negative, and gain-of-function properties have been implicated in the development of a wide variety of human cancers, and it is generally accepted that p53 is a component in biochemical pathways central to human carcinogenesis. Study of p53 has come to the forefront of cancer research, and detection of its abnormalities during the development of tumors may have diagnostic, prognostic, and therapeutic implications. In this review, we focus on recent research on overexpression of mutant p53 in human cancer, with an emphasis on mutant p53 regulation, gain of function of mutant p53 in transcriptional effects, and the diagnostic, prognostic, and predictive value of p53 overexpression in human cancer.

p53/p63/p73 isoforms: an orchestra of isoforms to harmonise cell differentiation and response to stress

p63, p73 and p53 compose a family of transcription factors involved in cell response to stress and development. p53 is the most frequently mutated gene in cancer (50%) and loss of p53 activity is considered to be ubiquitous to all cancers. Recent publications may have a profound impact on our understanding of p53 tumour suppressor activity. p63, p73 and p53 genes have a dual gene structure conserved in drosophila, zebrafish and man. They encode for multiple p63, p73 or p53 proteins containing different protein domains (isoforms) due to multiple splicing, alternative promoter and alternative initiation of translation. In this review, we describe the different isoforms of p63, p73, p53 and their roles in development and cancer. The changes in the interactions between p53, p63 and p73 isoforms are likely to be fundamental to our understanding in the transition between normal cell cycling and the onset of tumour formation.

The regional integration of retroviral sequences into the mosaic genomes of mammals

We have reviewed here three sets of data concerning the integration of retroviral sequences in the mammalian genome: (i) our experimental localization of a number of proviruses integrated in isochores characterized by different GC levels; (ii) results from other laboratories on the localization of retroviral sequences in open chromatin regions and/or next to CpG islands; and (iii) our compositional analysis of genes located in the neighborhood of integrated retroviral sequences. The three sets of data have provided a very consistent picture in that a compartmentalized, isopycnic integration of expressed proviruses appears to be the rule ('isopycnic' refers to the compositional match between viral and host sequences around the integration site). The results reviewed here suggest that: (i) integration of proviral sequences is targeted initially towards 'open chromatin regions'; while these exist in both GC-rich and GC-poor isochores, the 'open chromatin regions' of GC-rich isochores are the main targets for integration of retroviral sequences because of their much greater abundance; (ii) isopycnicity is associated with stability of integration; indeed, even non-expressed integrated retroviral sequences tend to show an isopycnic localization in the genome; (iii) transcription of integrated viral sequences (like transcription of host genes) appears to be associated, as a rule, with an isopycnic localization, as indicated by transcribed sequences that show an isopycnic integration and act in trans; (iv) selection plays a role in the choice of specific sites within an isopycnic region; in exceptional cases [such as mouse mammary tumor virus (MMTV) activating GC-rich oncogenes], selection may override isopycnicity.

Li-Fraumeni syndrome and the role of the p53 tumor suppressor gene in cancer susceptibility

Mutation of the tumor suppressor gene p53 is a molecular genetic event frequently observed in human cancer, and inactivating missense mutations usually are accompanied by the resultant overexpression of mutant p53 protein. In gynecologic cancers, p53 is also often altered; the frequency varies depending on types of cancers and where they develop. Further, human papillomavirus oncoproteins that inactivate p53 and Rb proteins play important roles in the development of several gynecologic cancers. Individuals who are heterozygous for germline mutations of the p53 gene are strongly predisposed to a variety of cancers. The identification of these individuals may have profound value in the future when therapies or chemopreventive agents specific for the p53 alteration are available. The role of p53 tumor suppressor gene in gynecologic cancers and heritable cancer susceptibility syndromes including Li-Fraumeni and Lynch II syndromes is an active and important area of study.

p53 antisense oligonucleotide inhibits growth of human colon tumor and normal cell lines

We examined the relationship between the expression of mutant p53 proteins and tumor cell growth using a p53 antisense oligonucleotide (5'-CCCTGCTCCCCCCTGGCTCC-3'). The oligonucleotide inhibited the growth of three human colon tumor cell lines (DLD-a, SW620 and WiDr), which produce only mutant p53 proteins with different mutation sites. Treatment of DLD-1 cells with the p53 antisense oligonucleotide caused a decrease in the level of p53 mutant protein. Synthesis of DNA in DLD-1 and SW620 cells was inhibited more potently than that of RNA or protein after antisense treatment. Furthermore, these cells were accumulated in the S phase when DNA synthesis was inhibited. Meanwhile, the antisense oligonucleotide also inhibited the growth of three human normal cell lines (WI-38, TIG-1 and Intestine 407). While treatment of WI-38 and TIG-1 cells with the antisense oligonucleotide inhibited synthesis of DNA more potently than that of RNA or protein, these normal cells were accumulated in the G0/G1 phase. These results suggest that p53 proteins, either with or without mutation, play a pivotal role in the growth of tumor and normal cells, but that mutant and wild-type p53 proteins may function differently in cell growth.

Solution structure of the tetrameric minimum transforming domain of p53

We report the solution structure of the minimum transforming domain (residues 303-366) of human p53 (p53tet) determined by multidimensional NMR spectroscopy. This domain contains a number of important functions associated with p53 activity including transformation, oligomerization, nuclear localization and a phosphorylation site for p34/cdc2 kinase. p53tet forms a symmetric dimer of dimers that is significantly different from a recent structure reported for a shorter construct of this domain. Phosphorylation of Ser 315 has only minor structural consequences, as this region of the protein is unstructured. Modelling based on the p53tet structure suggests possible modes of interaction between adjacent domains in full-length p53 as well as modes of interaction with DNA.

Gain-of-function mutations of the p53 gene induce lymphohematopoietic metastatic potential and tissue invasiveness

Leukemia cell infiltration and the induction of lethal hematopoietic disease in immune-deficient SCID mice transplanted with human T cell acute lymphoblastic T leukemia (T-ALL) cells occurred only when the cells possessed mutant p53 genes and lacked a wild-type allele or when T-ALL cells lacking p53 protein were infected with specific mutant p53 genes. A series of six mutant p53 genes were cloned from relapse T-ALL-derived cell lines and were constructed into defective retroviral expression vectors. Viruses encoding mutant p53 proteins were used to infect relapse T-ALL cells in a study designed to compare their pathogenic potency. The mutant p53 genes possessed a distinct hierarchy in vivo and in vitro: mutants inducing the greatest increase in proliferation of different T-ALL lines in vitro and colony formation in methylcellulose cultures also induced tissue invasiveness of infected T-ALL cells in vivo. Mutant p53 gene transfer to a cell line lacking p53 protein showed that the more potent p53 mutants possessed a distinctive dominant oncogenic activity in vitro and in vivo. The dominant oncogenic activity of these mutant p53 proteins was not dependent on the presence of and on complex formation with wild-type p53 protein. These "hot" p53 mutations thus represent bona fide gain-of-function mutations. Infection of p53-negative T-ALL cells with viruses encoding gain-of-function mutant p53 genes resulted in the acquisition of metastatic potential and tissue invasiveness. Taken together, our results suggest that specific mutant p53 genes play a role in the generation of lymphohematopoietic metastatic potential and tissue invasiveness as assayed in SCID mice, whereas the expression of wild-type p53 is capable of keeping this metastatic potential in check.

Mutational inactivation of the p53 gene in the human erythroid leukemic K562 cell line

The K562 human chronic myelogenous leukemia (CML) cell line has attained widespread use as a model for studying hematologic malignancy and erythroid differentiation. Sequencing of the p53 gene in the K562 cell line demonstrated a mutation in exon 5 characterized by a single base insertion (cytosine) between codons 135 and 136. This frameshift mutation leads to an N-terminal truncated protein of 147 amino acids. Only the mutated sequence was present suggesting that the normal allele has been lost. Reverse transcription PCR (RT-PCR) detected a p53 transcript but Western blotting and immunohistochemical staining of cells failed to detect p53 protein. The identification of an inactivation mutation of p53 in the K562 cell line further supports the argument that p53 mutations play a role in myeloid blast transformation of CML.

Gain of function mutations in p53

We report that the expression of murine or human mutant p53 proteins in cells with no endogenous p53 proteins confers new or additional phenotypes upon these cells. Mutant p53 proteins expressed in cell lines lacking p53 resulted in either enhanced tumorigenic potential in nude mice ((10)3 cells) or enhanced plating efficiency in agar cell culture (human SAOS-2 cells). Also, mutant human p53 alleles, unlike the wild-type p53 protein, could also enhance the expression of a test gene regulated by the multi-drug resistance enhancer-promoter element. These data demonstrate a gain of function associated with p53 mutations in addition to the loss of function shown previously to be associated with mutations in this tumour suppressor gene.

The mdm-2 oncogene product forms a complex with the p53 protein and inhibits p53-mediated transactivation

A cellular phosphoprotein with an apparent molecular mass of 90 kd (p90) that forms a complex with both mutant and wild-type p53 protein has been characterized, purified, and identified. The protein was identified as a product of the murine double minute 2 gene (mdm-2). The mdm-2 gene enhances the tumorigenic potential of cells when it is overexpressed and encodes a putative transcription factor. To determine if mdm-2 could modulate p53 transactivation, a p53-responsive element from the muscle creatine kinase gene was employed. A wild-type p53-expressing plasmid enhanced the expression of the p53-responsive element when cotransfected into cells that contain no endogenous p53. When a cosmid expressing mdm-2 was transfected with this p53-expressing plasmid, the transactivation of the p53-responsive element was inhibited. Thus, a product of the mdm-2 oncogene forms a tight complex with the p53 protein, and the mdm-2 oncogene can inhibit p53-mediated transactivation.

Oncoprotein p53 expression in normal, immortalized, and transformed mouse fibroblasts

We have compared the rate of synthesis, half-life, and steady-state content of the oncoprotein p53 in logarithmically growing cultures of (a) primary embryo, (b) immortalized but untransformed, and (c) spontaneously transformed mouse fibroblasts. Steady-state p53 content derived from metabolic labeling and immunoprecipitation data revealed either no change or only a slight decrease (up to 1.5-fold depending on the antibody used) in transformed cells compared with immortal or primary cultures, p53 showed the same short half-life in all cell types. In contrast, immunocytochemical analysis of p53 content in intact cells demonstrated an increase in the proportion of cells with detectable nuclear p53 from approximately 4% in primary and immortal cultures to approximately 10% in fully transformed cells, together with a marked increase in the intensity of nuclear positivity. We suggest that transformation is associated with an increase in the cellular content of p53 in a subcellular pool which was not detectable in detergent for immunoprecipitation. In addition, immunocytochemical analysis demonstrated a marked heterogeneity in p53 content in all cell types which was not related to clonal variation, cell cycle phase, or growth state. These data challenge previous suggestion regarding the role of p53 in growth control.

Identification of the p53 protein domain involved in formation of the simian virus 40 large T-antigen-p53 protein complex

An expression vector utilizing the enhancer and promoter region of the simian virus 40 (SV40) DNA regulating a murine p53 cDNA clone was constructed. The vector produced murine p53 protein in monkey cells identified by five different monoclonal antibodies, three of which were specific for the murine form of p53. The murine p53 produced in monkey cells formed an oligomeric protein complex with the SV40 large tumor antigen. A large number of deletion mutations, in-frame linker insertion mutations, and linker insertion mutations resulting in a frameshift mutation were constructed in the cDNA coding portion of the p53 protein expression vector. The wild-type and mutant p53 cDNA vectors were expressed in monkey cells producing the SV40 large T antigen. The conformation and levels of p53 protein and its ability to form protein complexes with the SV40 T antigen were determined by using five different monoclonal antibodies with quite distinct epitope recognition sites. Insertion mutations between amino acid residues 123 and 215 (of a total of 390 amino acids) eliminated the ability of murine p53 to bind to the SV40 large T antigen. Deletion (at amino acids 11 through 33) and insertion mutations (amino acids 222 through 344) located on either side of this T-antigen-binding protein domain produced a murine p53 protein that bound to the SV40 large T antigen. The same five insertion mutations that failed to bind with the SV40 large T antigen also failed to react with a specific monoclonal antibody, PAb246. In contrast, six additional deletion and insertion mutations that produced p53 protein that did bind with T antigen were each recognized by PAb246. The proposed epitope for PAb246 has been mapped adjacent (amino acids 88 through 109) to the T-antigen-binding domain (amino acids 123 through 215) localized by the mutations mapped in this study. Finally, some insertion mutations that produced a protein that failed to bind to the SV40 T antigen appeared to have an enhanced ability to complex with a 68-kilodalton cellular protein in monkey cells.

Analysis of human p53 proteins and mRNA levels in normal and transformed cells

p53 mRNA and proteins were examined in a variety of human transformed cells and in normal human foreskin fibroblast cells. Both the steady-state and translatable levels of p53 mRNA were the same in normal and transformed human cells. In vitro synthesized p53, programmed by mRNA from normal and transformed human cells, revealed that there was heterogeneity in the primary structure of p53 from these cells. Pulse labeling of cells and immunoprecipitation analysis with a panel of human reactive anti-p53 antibodies demonstrated that the types of p53 synthesized in vitro corresponded to the types made in vivo from SV80 and COLO 320 cells. No p53 was detectable by similar pulse-labeling analysis of HeLa and normal foreskin fibroblast cells. Since it was necessary to use anti-p53 sera from cancer patients to carry out much of the immunoprecipitation analysis in this study we therefore further characterised these sera to determine if they reacted with one or more than one epitope. p53-beta-galactosidase fusion proteins were synthesized in Escherichia coli and used to analyse the anti-p53 antibodies produced by cancer patients. We demonstrate that the antisera contain antibodies directed against epitopes in both the N-terminal and C-terminal regions of the p53 molecule.

Reconstitution of p53 expression in a nonproducer Ab-MuLV-transformed cell line by transfection of a functional p53 gene

L12 are Ab-MuLV-transformed cells that express the abl p120 oncogene product but lack the cellularly encoded p53. The functional p53 gene in these cells has been inactivated by the insertion of Moloney virus-like sequences into the first p53 intron. Transfection of L12 cells with a functional p53 gene, contained in a 16 kb Eco RI genomic cloned fragment gave rise to L12-derived cell lines with novel p53 sequences of various sizes and copy number. A high percentage of L12-derived clones efficiently transcribed p53 mRNA and synthesized the p53 protein. Whereas injection of L12 parental cells into syngeneic mice caused the development of local tumors that later regressed, L12-derived clones that expressed p53 caused lethal tumors in syngeneic mice, thus behaving similarly to other Ab-MuLV-transformed p53-producer cell lines. These results suggest that the expression of p53 is essential for tumor cells to exhibit a fully transformed phenotype, manifested in lethal tumors in syngeneic mice.