Loss of heterozygosity and mutational alterations of the p53 gene in skin tumours of interspecific hybrid mice.

Functional alterations or loss of tumor-suppressor genes are an important feature of neoplastic progression in humans. The employment of suitable animal model systems would greatly facilitate the detection and manipulation of such genes. We describe here an experimental approach to this problem based on the analysis of skin tumors induced in F1 hybrids between Mus musculus and Mus spretus mice. The results show that loss of heterozygosity on chromosome 11 occurred in 4/13 mouse skin carcinomas, but not in premalignant papillomas. Since the murine p53 gene is located on this chromosome, immunoprecipitation and DNA-sequencing studies were carried out on tumorigenic cell lines and primary tumor DNA respectively to determine the status of p53 alleles. These studies revealed the presence of p53 mutations, both frameshifts and missense, some of which are identical to those found in human tumors. Loss of normal p53 function is found in well-differentiated squamous-cell carcinomas and thus does not appear to be directly responsible for further progression to an undifferentiated spindle cell phenotype.

Aberrant expression of the p53 oncoprotein is a common feature of a wide spectrum of human malignancies.

Accumulation of the p53 protein was analysed in 212 human malignant lesions. Immunohistochemical staining with new polyclonal (CM-1) and monoclonal antibodies (BP 53-12 and BP53-24) to p53 on methacarn-fixed paraffin sections showed positive staining in 161 (76%). The positive tumours were found across a wide range of human malignancies including breast, colon, stomach, bladder and testis carcinomas, soft-tissue sarcomas and melanomas. The staining was always confined to the malignant lesion. Immunoprecipitation and quantitative ELISA assays established that the positive staining was associated with accumulation of the protein and that the protein was frequently in a mutant conformation. Accumulation of mutant p53 protein is therefore a common feature of human malignant disease.

Protein synthesis required to anchor a mutant p53 protein which is temperature-sensitive for nuclear transport.

The p53 protein is rendered temperature-sensitive by a point mutation. Rat cells transformed by this mutant p53 and an activated ras oncogene grow well at 37 degrees C but cease DNA synthesis and cell division when shifted to 32 degrees C. Immunostaining demonstrates that the mutant p53 protein is in the nucleus of the arrested cells at 32 degrees C but in the cytoplasm of the growing cells at 37 degrees C. This is the first example of a protein which is temperature-sensitive for nuclear transport. The translocation from cytoplasm to nucleus and vice versa occurs 6 h after temperature shift and is coincident with the inhibition of DNA synthesis; transport from cytoplasm to nucleus does not require protein synthesis. Remarkably, inhibition of protein synthesis at 37 degrees C also results in the rapid appearance of mutant p53 in the cell nucleus. These results suggest the presence of a short-lived protein responsible for holding p53 in the cytoplasm at 37 degrees C but not at 32 degrees C. Analysis of a non-temperature-sensitive mutant p53 protein shows that its cytoplasmic location is sensitive to protein synthesis inhibitors but not to temperature.

p53 and DNA polymerase alpha compete for binding to SV40 T antigen.

The large T antigen (T) of simian virus 40 is a multifunctional protein required for both viral DNA replication and cellular transformation. T antigen forms specific protein complexes with the host protein p53 in both virus-infected and transformed cells. p53 has recently been shown to be an oncogene, but its normal function is not clear. We previously established a radioimmunoassay to study the newly described complex between T antigen and DNA polymerase alpha, and have noted a similarity between the antigenic changes induced in T by the binding of both p53 and polymerase. We now extend this analysis to a larger collection of anti-T antibodies and formally establish that p53 and DNA polymerase alpha can compete for binding to the SV40 T antigen. At a critical concentration of the three components it is possible to detect a trimeric complex of T, p53 and DNA polymerase alpha. Our observations have important implications for the control by these nuclear oncogenes of viral and cellular DNA synthesis and viral host range in both normal and transformed cells. We present a model for the action of p53 in growth control.

Interactions between SV40 T antigen and DNA polymerase alpha.

Simian virus 40 large T antigen is the only viral protein required for SV40 DNA synthesis in vivo and in vitro. This complex protein recruits the cellular DNA replication apparatus to the SV40 origin and provides a good model for the initiation of cellular DNA replication. The interaction between SV40 large T antigen (TAg) and DNA polymerase alpha has been shown previously to be inhibited by murine p53, the nuclear protein product of a cellular anti-oncogene. The murine p53 protein will inhibit SV40 replication both in vivo and in vitro. Using monoclonal antibodies to TAg, p53, and polymerase alpha, we developed immunoassays to measure the complexes formed between TAg and polymerase alpha and between TAg and p53. The assays allowed us to detect the TAg-polymerase alpha and TAg-p53 complexes in lytically infected and transformed cells. The amount of TAg complexed to p53 was far lower in infected cells than in transformed cells. We used a large range of monoclonal antibodies to different sites on T antigen and found that antibodies that inhibited the formation of the TAg-polymerase alpha complex also inhibited the formation of the TAg-p53 complex. Finally, we found that the tsA58 and 5080 point mutations in TAg, previously shown to inhibit the binding of TAg to p53, also inhibit its binding to polymerase alpha. Together these results emphasize the specificity and functional importance of the TAg-polymerase alpha complex. The disruption of this interaction by the cellular anti-oncogene p53 provides an interesting model for the normal action of p53 and the effects of its removal on the regulation of cellular DNA synthesis.

Monoclonal antibody analysis of p53 expression in normal and transformed cells.

The cellular phosphoprotein p53 binds tightly and specifically to simian virus 40 T antigen and the 58,000-molecular-weight adenovirus E1b protein. Many human and murine tumor cell lines contain elevated levels of the p53 protein even in the absence of these associated viral proteins. Recently the cloned p53 gene, linked to strong viral promoters, has been shown to complement activated ras genes in transformation of primary rodent cell cultures. Overexpression of the p53 gene alone rescues some primary rodent cell cultures from senescence. We isolated three new monoclonal antibodies to the p53 protein, designated PAb242, PAb246, and PAb248, and mapped the epitopes they recognized on p53 in comparison with other previously isolated antibodies. At least five sterically separate epitopes were defined on murine p53. One of the antibodies, PAb246, recognizes an epitope on p53 that is unstable in the absence of bound simian virus 40 T antigen. This effect is demonstrable in vivo and in newly developed in vitro assays of T-p53 complex formation. Using the panel of anti-p53 antibodies and sensitive immunocytochemical methods, we found that p53 has a predominantly nuclear location in established but not transformed cells as well as in the vast majority of transformed cell lines. Several monoclonal antibodies to p53 showed cross-reactions with non-p53 components in immunocytochemical staining.

Newly synthesized protein(s) must associate with p34cdc2 to activate MAP kinase and MPF during progesterone-induced maturation of Xenopus oocytes.

The meiotic maturation of Xenopus oocytes triggered by progesterone requires new protein synthesis to activate both maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAP kinase). Injection of mRNA encoding mutant p34cdc2 (K33R) that can bind cyclins but lacks protein kinase activity strongly inhibited progesterone-induced activation of both MPF and MAP kinase in Xenopus oocytes. Similar results were obtained by injection of GST-p34cdc2 K33R protein or by injection of a monoclonal antibody (A17) against p34cdc2 that blocks its activation by cyclins. Both the dominant-negative p34cdc2 and monoclonal antibody A17 blocked the accumulation of p39mos and activation of MAP kinase in response to progesterone, as well as blocking the appearance of MPF, although they did not inhibit the translation of p39mos mRNA. These results suggest that: (i) activation of free p34cdc2 by newly made proteins, probably cyclin(s), is normally required for the activation of both MPF and MAP kinase by progesterone in Xenopus oocytes; (ii) the activation of translation of cyclin mRNA normally precedes, and does not require either MPF or MAP kinase activity; and (iii) de novo synthesis and accumulation of p39mos is probably both necessary and sufficient for the activation of MAP kinase in response to progesterone.

Simian virus 40 large T antigen associates with cyclin A and p33cdk2.

In this paper we provide evidence that a fraction of large T antigen of simian virus 40 (SV40) interacts with cyclin A and p33cdk2 in both virus-infected and stably transformed cells. Immunoprecipitates of SV40 large T antigen from SV40-infected or SV40 large-T-antigen-transformed cells contain cyclin A, p33cdk2, and histone H1 kinase activity. Conversely, immunoprecipitates of cyclin A from these cells contain SV40 large T antigen. In this respect, SV40 large T antigen has properties similar to those of the E1A oncogene of adenoviruses and the E7 oncogene of human papillomaviruses.

Structure and function of SV40 large-T antigen.

The small eukaryotic DNA tumour virus, SV40, has long provided a very useful model for the study of eukaryotic DNA replication and cellular transformation. The viral gene product, large-tumour (large-T) antigen, is essential for the initiation of viral DNA replication and the initiation and maintenance of SV40-virus-mediated cellular transformation. The large-T antigen is a complex multifunctional protein, and to delineate its activity more precisely in viral DNA replication and cellular transformation, small functional domains of the protein have been expressed in Escherichia coli and analysed by using a very extensive library of anti-T monoclonal antibodies.

Activating mutations in p53 produce a common conformational effect. A monoclonal antibody specific for the mutant form.

Point mutations in the p53 gene are the most frequently identified genetic change in human cancer. They convert murine p53 from a tumour suppressor gene into a dominant transforming oncogene able to immortalize primary cells and bring about full transformation in combination with an activated ras gene. In both the human and murine systems the mutations lie in regions of p53 conserved from man to Xenopus. We have developed a monoclonal antibody to p53 designated PAb240 which does not immunoprecipitate wild type p53. A series of different p53 mutants all react more strongly with PAb240 than with PAb246. The PAb240 reactive form of p53 cannot bind to SV40 large T antigen but does bind to HSP70. In contrast, the PAb246 form binds to T antigen but not to HSP70. PAb240 recognizes all forms of p53 when they are denatured. It reacts with all mammalian p53 and chicken p53 in immunoblots. We propose that immunoprecipitation of p53 by PAb240 is diagnostic of mutation in both murine and human systems and suggest that the different point mutations which convert p53 from a recessive to a dominant oncogene exert a common conformational effect on the protein. This conformational change abolishes T antigen binding and promotes self-oligomerization. These results are consistent with a dominant negative model where mutant p53 protein binds to and neutralizes the activity of p53 in the wild type conformation.

A measure of the mitotic index: studies of the abundance and half-life of p34cdc2 in cultured cells and normal and neoplastic tissues.

BACKGROUND: The cdc2 gene encodes a protein kinase, p34cdc2, that is essential for mitosis, and is present at high levels in dividing cells. Classical studies of the levels of this protein in dividing and resting cells used antibodies that cross-react with other members of the CDK family, in particular with CDK2. We have therefore re-examined the abundance of p34cdc2 in a variety of tissues and cell lines, using a highly specific, epitope-mapped monoclonal antibody that does not react with CDK2. RESULTS: We observed high levels of p34cdc2 in proliferating cells, especially those in neoplastic tissues. Cells that have withdrawn from the cell cycle have low or undetectable levels. At the end of mitosis, the level of p34cdc2 declines, with simple first-order kinetics, with a half-life which is never less than 6h and is more typically about 18h. The persistence of p34cdc2 after the last cell division is comparable to that of PCNA, a commonly used marker of proliferation. CONCLUSIONS: The immunochemical detection of p34cdc2 provides an accurate, reliable and meaningful measure of the proliferative activity of cells in tissues. We suggest that p34cdc2 should be considered as the most authentic molecular marker of the mitotic index.

Host proteins that bind to or mimic SV40 large T antigen: using antibodies to look at protein interactions and their significance.

The papovavirus SV40 is able to induce tumours in susceptible hosts and will transform cells in vitro. Its major early protein, large T antigen, is required for viral DNA synthesis, both in vivo and in vitro, and is also responsible for the oncogenic action of the virus. We have made use of an extensive library of anti-T monoclonal antibodies to investigate the cellular effects of T. Large T shares an antigenic determinant with a growth-regulated host protein, p68, which is a member of an expanding super-family of helicases with particular homology to the translation initiation factor elF-4A. We have also studied the binding and interaction of large T with two particular host components: the replicative enzyme DNA polymerase alpha and the proto-oncogene p53. These two proteins bind to similar regions of T and exert similar effects on its antigenic structure. We found that p53 can block the binding of DNA polymerase alpha to T as well as co-existing with DNA polymerase alpha in a trimeric complex with T. This suggests that these interactions may be important in the oncogenic and replicative action of large T.

p53 mutations in colorectal cancer.

Immunohistological staining of primary colorectal carcinomas with antibodies specific to p53 demonstrated gross overexpression of the protein in approximately 50% of the malignant tumors examined. Benign adenomas were all negative for p53 overexpression. To determine the molecular basis for this overexpression we examined p53 protein expression in 10 colorectal cancer cell lines. Six of the cell lines expressed high levels of p53 in ELISA, cell-staining, and immunoprecipitation studies. Direct sequencing and chemical-mismatch-cleavage analysis of p53 cDNA by using the polymerase chain reaction in these cell lines showed that all cell lines that expressed high levels of p53 were synthesizing mRNAs that encoded mutant p53 proteins. In two of those four cell lines where p53 expression was lower, point mutations were still detected. Thus, we conclude that overexpression of p53 is synonymous with mutation, but some mutations would not be detected by a simple immunohistochemical analysis. Mutation of the p53 gene is one of the commonest genetic changes in the development of human colorectal cancer.