Multistage Friend erythroleukemia: independent origin of tumor clones with normal or rearranged p53 cellular oncogenes

Transcription regulation by mutant p53

In addition to the loss of wild-type p53 activity, a high percentage of tumor cells accumulate mutant p53 protein isoforms. Whereas the hallmark of the wild-type p53 is its tumor suppressor activities, tumor-associated mutant p53 proteins acquire novel functions enabling them to promote a large spectrum of cancer phenotypes. During the last years, it became clear that tumor-associated mutant p53 proteins are not only distinct from the wild-type p53, but they also represent a heterogeneous population of proteins with a variety of structure-function features. One of the major mechanisms underlying mutant p53 gain of function is the ability to regulate gene expression. Although a large number of specific target genes were identified, the molecular basis for this regulation is not fully elucidated. This review describes the present knowledge about the transcriptional activities of mutant p53 and the mechanisms that might underlie its target gene specificity.

Loss of p53 tumor suppressor function is required for in vivo progression of Friend erythroleukemia

A role for p53 in the in vivo progression of Friend virus-induced erythroleukemia has been suggested but not clearly defined. We developed a Friend virus-sensitive, p53-deficient mouse model to directly address the role of p53 in Friend erythroleukemia. When infected with the polycythemia-inducing strain of Friend virus (FVP), p53 null mice exhibited accelerated progression to erythroleukemia and accelerated death following diagnosis when compared to wild type mice. Confirmation that p53 mutations were required for disease progression was provided by sequence analysis of p53 transcripts in leukemic wild type and heterozygous mice. All transcripts evaluated had point mutations, deletions or insertions in the p53 gene. The ability to grow tumor colonies in vitro and derive cell lines was enhanced in FVP-infected p53 null animals. Although PU.1 oncogene overexpression is a common mutation observed in cell lines derived from Friend virus-infected p53 wild type mice, it was not a universal finding in cell lines derived from p53 null animals. Our data conclusively demonstrate that loss of p53 function is a requirement for progression of Friend erythroleukemia in vivo. Further, the data demonstrate that erythroleukemias arising in Friend virus-infected p53 null mice are biologically and genetically distinct from those that occur in wild type animals, suggesting that the temporal order of PU.1 and p53 mutations is an important parameter in the pathogenesis of leukemic development.

Bcl-2 expression in F-MuLV-induced erythroleukemias: a role for the anti-apoptotic action of Bcl-2 during tumor progression

Erythroleukemias induced by various strains of Friend virus are multistage malignancies that result from the accumulation of genetic mutations, including the activation of proto-oncogenes and the inactivation of tumor suppressor genes. In this study, we demonstrate that Bcl-2 expression is activated in the majority of F-MuLV-induced erythroleukemia cell lines. In contrast, Bcl-2 was not expressed in any of the FV-P-induced erythroleukemia cell lines and protein levels were low or negligible in FV-A-induced erythroleukemia cell lines examined. In vivo, Bcl-2 expression levels gradually increased in F-MuLV-induced erythroleukemic cells prior to adaptation to culture. High expression of Bcl-2 in F-MuLV-induced erythroleukemic cells was shown to proceed the emergence of p53 mutation suggesting that Bcl-2 expression may delay p53 mutation in the leukemic cells. This is further supported by the demonstration that the majority of F-MuLV-induced erythroleukemia cell lines established from primary tumors induced in p53 mutant mice express low to negligible levels of Bcl-2. We have shown that the high levels of Bcl-2 expression in FV-P-induced erythroleukemic cells inhibited apoptosis induced by etoposide, low serum and p53 expression. Similarly, ectopic Bcl-2 expression within these cells also provided protection from apoptosis induced by etoposide and growth in low serum. These results suggest that the anti-apoptotic action of Bcl-2 may confer a selective in vivo and in vitro growth advantage to F-MuLV-induced erythroleukemic cells, which is not shared by FV-P/FV-A-induced erythroleukemic cells. The observed induction of Bcl-2 expression in vivo constitutes a novel but late oncogenic event associated with the progression of F-MuLV-induced erythroleukemias.

p45(NFE2) is a negative regulator of erythroid proliferation which contributes to the progression of Friend virus-induced erythroleukemias

In previous studies, we identified a common site of retroviral integration designated Fli-2 in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia cell lines. Insertion of F-MuLV at the Fli-2 locus, which was associated with the loss of the second allele, resulted in the inactivation of the erythroid cell- and megakaryocyte-specific gene p45(NFE2). Frequent disruption of p45(NFE2) due to proviral insertion suggests a role for this transcription factor in the progression of Friend virus-induced erythroleukemias. To assess this possibility, erythroleukemia was induced by F-MuLV in p45(NFE2) mutant mice. Since p45(NFE2) homozygous mice mostly die at birth, erythroleukemia was induced in +/- and +/+ mice. We demonstrate that +/- mice succumb to the disease moderately but significantly faster than +/+ mice. In addition, the spleens of +/- mice were significantly larger than those of +/+ mice. Of the 37 tumors generated from the +/- and +/+ mice, 10 gave rise to cell lines, all of which were derived from +/- mice. Establishment in culture was associated with the loss of the remaining wild-type p45(NFE2) allele in 9 of 10 of these cell lines. The loss of a functional p45(NFE2) in these cell lines was associated with a marked reduction in globin gene expression. Expression of wild-type p45(NFE2) in the nonproducer erythroleukemic cells resulted in reduced cell growth and restored the expression of globin genes. Similarly, the expression of p45(NFE2) in these cells also slows tumor growth in vivo. These results indicate that p45(NFE2) functions as an inhibitor of erythroid cell growth and that perturbation of its expression contributes to the progression of Friend erythroleukemia.

Karyotypic abnormalities associated with haemopoietic lineage switching are not linked with mutations to p53

Leukemic cells can undergo lineage switching to display the phenotypic features of another haemopoietic pathway, as exemplified by B lymphoma and erythroleukemic cell lines generating variants with a monocytic appearance. Unlike the diploid parental lines, the vast majority of myeloid derivative lines examined (12 of 13 lines) were aneuploid. As p53 is involved in the maintenance of chromosomal stability, we investigated the role of p53 in the emergence of abnormal karyotypes in cells which had undergone lineage switching. Single strand conformation polymorphism and sequence analysis of cDNA, together with protein immunoprecipitations, were used to assess the p53 status of parental and variant cell lines. Unexpectedly, four or five monocytic lines with chromosomal alterations contained wild type p53. Conversely, a p53 point mutation found in one aneuploid monocytic line was also present in the diploid parental pre-B cell. These results provide strong evidence that mechanisms other than p53 mutations are responsible for karyotypic abnormalities seen in cells that have undergone lineage switching.

Loss of p53 in F-MuLV induced-erythroleukemias accelerates the acquisition of mutational events that confers immortality and growth factor independence

Erythroleukemias induced by Friend Murine Leukemia Virus (F-MuLV) involve the insertional activation of the proto-oncogene Fli-1, and the inactivation of the p53 tumor suppressor gene. While the activation of Fli-1 is an early, primary transforming event, p53 mutations are correlated with the immortalization of erythroleukemic cells in culture. In this study we have further analysed the role of p53 loss in F-MuLV induced erythroleukemias by examining the progression of this disease in p53 deficient mice. We found that p53-/- mice succumb to the disease more rapidly than p53+/+ littermates. Additionally, of the 112 tumors generated, 19 gave rise to immortal cell lines, eight of which were derived from p53-/- mice, and ten of which were from p53+/- mice. The ability of these primary tumor cells to grow in culture was associated with the complete loss of wild-type p53 in these cell lines. However, cells from many of the tumors induced in p53-/- hosts did not survive in vitro. These results suggest that the loss of p53 does not directly immortalize tumor cells. Instead, we have evidence to suggest that the loss of p53 promotes the accumulation of mutations that are required for survival in culture and that are capable of accelerating tumor progression in vivo. Indeed, mutations causing expression of the growth factor gene erythropoietin (Epo), were detected in two of seven Epo-independent cell lines from p53 deficient primary erythroleukemias. Moreover, the mechanism of activation of the Epo gene in one of these two Epo-independent cell lines involved genomic rearrangement, that is a hallmark of genetic instability. We propose that, in F-MuLV induced-erythroleukemias, p53 loss may encourage the accumulation of further mutations, subsequently conferring a growth advantage and immortality to the transformed erythroblasts.

Fli-1, an Ets-related transcription factor, regulates erythropoietin-induced erythroid proliferation and differentiation: evidence for direct transcriptional repression of the Rb gene during differentiation

Erythropoietin (Epo) is a major regulator of erythropoiesis that alters the survival, proliferation, and differentiation of erythroid progenitor cells. The mechanism by which these events are regulated has not yet been determined. Using HB60, a newly established erythroblastic cell line, we show here that Epo-induced terminal erythroid differentiation is associated with a transient downregulation in the expression of the Ets-related transcription factor Fli-1. Constitutive expression of Fli-1 in HB60 cells, similar to retroviral insertional activation of Fli-1 observed in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia, blocks Epo-induced differentiation while promoting Epo-induced proliferation. These results suggest that Fli-1 modulates the response of erythroid cells to Epo. To understand the mechanism by which Fli-1 regulates erythropoiesis, we searched for downstream target genes whose expression is regulated by this transcription factor. Here we show that the retinoblastoma (Rb) gene, which was previously shown to be involved in the development of mature erythrocytes, contains a Fli-1 consensus binding site within its promoter. Fli-1 binds to this cryptic Ets consensus site within the Rb promoter and transcriptionally represses Rb expression. Both the expression level and the phosphorylation status of Rb are consistent with the response of HB60 cells to Epo-induced terminal differentiation. We suggest that the negative regulation of Rb by Fli-1 could be one of the critical determinants in erythroid progenitor cell differentiation that is specifically deregulated during F-MuLV-induced erythroleukemia.

Role of PU.1 in hematopoiesis

The ETS-family transcription factor PU.1 is expressed in hematopoietic tissues, with significant levels of expression in the monocytic and B lymphocytic lineages. PU.1 is identical to the Spi-1 proto-oncogene which is associated with the generation of spleen focus-forming virus-induced erythroleukemias. An extensive body of in vitro gene regulatory studies has implicated PU.1 as an important, versatile regulator of B lymphoid- and myeloid-specific genes. The first half of the review is designed to coalesce data generated from studies examining the two PU.1 "knockout" animals, which have prompted a reevaluation of the proposed function of PU.1 during hematopoiesis. During hematopoiesis, PU.1 is required for development along the lymphoid and myeloid lineages but needs to be downregulated during erythropoiesis. These unique functional characteristics of PU.1 will be exemplified by contrasting the function of PU.1 with other transcription factors required during fetal hematopoiesis. The second half of this review will reexamine the functional characteristics of PU.1 deduced from traditional biochemical and transactivation assays in light of recent experiments examining the functional behavior of PU.1 in an embryonic stem cell in vitro differentiation system. Working models of how PU.1 regulates promoter and enhancer regions in the B cell and myeloid lineage will be presented and discussed.

The p53 gene and its role in human brain tumors

Mutation of the p53 gene is among the most common lesions in a variety of human tumors, including those of the central nervous system. In most instances, mutation of one p53 allele is followed by loss of the remaining wild-type allele, resulting in cells with a complete absence of functional wild-type p53 protein. However, in some situations, such as at initiation of spontaneously arising gliomas or as the germline configuration of patients with the Li-Fraumeni syndrome, cells clearly carry both wild-type and mutant p53 alleles. These observations lead to the hypothesis that p53 mutations can give rise to loss of tumor suppressor functions as well as to gain of oncogenic transformation capabilities. In this review, we define the types of mutations that occur in the p53 gene in various glial tumors, contrast that with the spectra described in other human tumor types, and discuss the biochemistry and physiology of the p53 protein and its ability to regulate and be regulated by other gene products. We use this information to propose roles for p53 in the initiation and progression of human gliomas.

The Ets-related transcription factor PU.1 immortalizes erythroblasts

In vivo studies of Friend virus erythroleukemia have implied that proviral integrations adjacent to the gene for the Ets-related transcription factor PU.1 may inhibit the commitment of erythroblasts to differentiate and cause their capability for indefinite transplantation (C. Spiro, B. Gliniak, and D. Kabat, J. Virol. 62:4129-4135, 1988; R. Paul, S. Schuetze, S. L. Kozak, C. Kozak, and D. Kabat, J. Virol. 65:464-467, 1991). To test this hypothesis, we ligated PU.1 cDNA into a retroviral vector and studied its effects on cultured cells. Infection of fibroblasts with PU.1-encoding retrovirus resulted in PU.1 synthesis followed by nuclear pyknosis, cell rounding, and degeneration. In contrast, in long-term bone marrow cultures, erythroblasts were efficiently and rapidly immortalized. The resulting cell lines were polyclonal populations that contained PU.1, were morphologically blast-like, required erythropoietin and bone marrow stromal cells for survival and proliferation, and spontaneously differentiated at low frequency to synthesize hemoglobin. After 9 months in culture, erythroblasts became stroma independent, and they then grew as clonal cell lines. We conclude that PU.1 perturbs the pathway(s) that controls potential for indefinite proliferation and that it can be used to generate permanent erythroblast cell lines.

The Ets-related transcription factor PU.1 immortalizes erythroblasts

In vivo studies of Friend virus erythroleukemia have implied that proviral integrations adjacent to the gene for the Ets-related transcription factor PU.1 may inhibit the commitment of erythroblasts to differentiate and cause their capability for indefinite transplantation (C. Spiro, B. Gliniak, and D. Kabat, J. Virol. 62:4129-4135, 1988; R. Paul, S. Schuetze, S. L. Kozak, C. Kozak, and D. Kabat, J. Virol. 65:464-467, 1991). To test this hypothesis, we ligated PU.1 cDNA into a retroviral vector and studied its effects on cultured cells. Infection of fibroblasts with PU.1-encoding retrovirus resulted in PU.1 synthesis followed by nuclear pyknosis, cell rounding, and degeneration. In contrast, in long-term bone marrow cultures, erythroblasts were efficiently and rapidly immortalized. The resulting cell lines were polyclonal populations that contained PU.1, were morphologically blast-like, required erythropoietin and bone marrow stromal cells for survival and proliferation, and spontaneously differentiated at low frequency to synthesize hemoglobin. After 9 months in culture, erythroblasts became stroma independent, and they then grew as clonal cell lines. We conclude that PU.1 perturbs the pathway(s) that controls potential for indefinite proliferation and that it can be used to generate permanent erythroblast cell lines.

Site-specific binding of wild-type p53 to cellular DNA is inhibited by SV40 T antigen and mutant p53

Wild-type p53 protein was shown to bind specifically to DNA sequences within SV40 (Bargonetti et al. 1991), the human ribosomal gene cluster (RGC) (Kern et al. 1991a), and the murine muscle creatine kinase gene (MCK) (Zambetti et al. 1992). However, a direct comparison of these three sites was not performed. Here we demonstrate, by filter binding and gel mobility-shift assays, that wild-type p53 binds with similar affinities to MCK and RGC sites but less tightly to the SV40 site. We examined the effects of two candidate regulators of p53 function, SV40 large T antigen and oncogenic mutant p53, on the binding of wild-type p53 to RGC DNA. We show that wild-type T antigen prevents p53 from binding to the RGC site under all conditions tested. Moreover, two temperature-sensitive mutant SV40 T antigens, which fail to transform cells at the nonpermissive temperature, prevent p53 from binding to the RGC site at the permissive, but not at the restrictive, temperature. The ability of complexes containing wild-type p53 and tumor-derived mutant p53 proteins to bind to RGC DNA varies according to the position of the mutation. Complexes containing wild-type and either his175 or his273 mutant p53 proteins are completely unable to bind to the RGC DNA sequence. Interestingly, a complex containing wild-type p53 and the trp248 mutant p53 characteristic of Li-Fraumeni syndrome patients displays nearly wild-type levels of binding. Perhaps this mutant allele can be tolerated in these individuals because the wild-type mutant p53 complex maintains the ability to bind to DNA. Our data indicate that the oncogenic potential of both T antigen and some mutant p53 proteins is the result of their ability to block binding of wild-type p53 to DNA.

Role of the PU.1 transcription factor in controlling differentiation of Friend erythroleukemia cells

Both viral and cellular genes have been directly implicated in pathogenesis of Friend viral erythroleukemia. The virus-encoded gp55 glycoprotein binds to erythropoietin receptors to cause mitogenesis and differentiation of erythroblasts. However, if the provirus integrates adjacent to the gene for the PU.1 transcription factor, the cell loses its commitment to terminally differentiate and becomes immortal, as indicated by its transplantability and by its potential for indefinite growth in culture (C. Spiro, B. Gliniak, and D. Kabat, J. Virol. 63:4434-4437, 1989; R. Paul, S. Schuetze, S. L. Kozak, and D. Kabat, J. Virol. 65:464-467, 1991). To test the implications of these results, we produced polyclonal antiserum to bacterially synthesized PU.1, and we used it to analyze PU.1 expression throughout leukemic progression and during chemically induced differentiation of Friend erythroleukemia (F-MEL) cell lines. This antiserum identified three electrophoretically distinct PU.1 components in extracts of F-MEL cells and demonstrated their nuclear localization. Although PU.1 proteins are abundant in F-MEL cells, they are absent or present in only trace amounts in normal erythroblasts or in differentiating erythroblasts from the preleukemic stage of Friend disease. Furthermore, chemicals (dimethyl sulfoxide or N,N'-hexamethylenebisacetamide) that overcome the blocked differentiation of F-MEL cells induce rapid declines of PU.1 mRNA and PU.1 proteins. The elimination of PU.1 proteins coincides with recommitment to the program of erythroid differentiation and with loss of immortality. These results support the hypothesis that PU.1 interferes with the commitment of erythroblasts to differentiate and that chemicals that reduce PU.1 expression reinstate the erythropoietic program.

What the papers say: p53 Loss of Function: Implications for the Processes of Immortalization and Tumorigenesis

The complex process of cell immortalization and transformation is likely to involve the inactivation of growth regulatory genes. Mutations (deletions, missense mutations) in the p53 gene are the most frequently observed genetic alteration in human tumors, making p53 a candidate for a cellular protein involved in the control of cell growth. Two recent studies have examined the role of p53 in immortalization and tumorigenesis. In the first study, p53 expression was examined in both mortal and immortal chick embryo fibroblasts. All mortal clones expressed p53 but the loss of wild-type p53 expression was observed in every immortal cell line examined. In the second study, a line of mice carrying two null p53 alleles has been created and characterized. Although these mice develop normally, they show a predisposition to develop a variety of neoplasms at an early age (< 6 months). Although it is unclear whether p53 regulates the same, different, or overlapping pathways in the two experimental systems, these data demonstrate that p53 function is critical for the maintenance of normal growth control and support the current classification of p53 as a growth suppressive or tumor suppressor gene.

Role of the PU.1 transcription factor in controlling differentiation of Friend erythroleukemia cells

Both viral and cellular genes have been directly implicated in pathogenesis of Friend viral erythroleukemia. The virus-encoded gp55 glycoprotein binds to erythropoietin receptors to cause mitogenesis and differentiation of erythroblasts. However, if the provirus integrates adjacent to the gene for the PU.1 transcription factor, the cell loses its commitment to terminally differentiate and becomes immortal, as indicated by its transplantability and by its potential for indefinite growth in culture (C. Spiro, B. Gliniak, and D. Kabat, J. Virol. 63:4434-4437, 1989; R. Paul, S. Schuetze, S. L. Kozak, and D. Kabat, J. Virol. 65:464-467, 1991). To test the implications of these results, we produced polyclonal antiserum to bacterially synthesized PU.1, and we used it to analyze PU.1 expression throughout leukemic progression and during chemically induced differentiation of Friend erythroleukemia (F-MEL) cell lines. This antiserum identified three electrophoretically distinct PU.1 components in extracts of F-MEL cells and demonstrated their nuclear localization. Although PU.1 proteins are abundant in F-MEL cells, they are absent or present in only trace amounts in normal erythroblasts or in differentiating erythroblasts from the preleukemic stage of Friend disease. Furthermore, chemicals (dimethyl sulfoxide or N,N'-hexamethylenebisacetamide) that overcome the blocked differentiation of F-MEL cells induce rapid declines of PU.1 mRNA and PU.1 proteins. The elimination of PU.1 proteins coincides with recommitment to the program of erythroid differentiation and with loss of immortality. These results support the hypothesis that PU.1 interferes with the commitment of erythroblasts to differentiate and that chemicals that reduce PU.1 expression reinstate the erythropoietic program.

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.

An activating mutation in the murine erythropoietin receptor induces erythroleukemia in mice: a cytokine receptor superfamily oncogene

A point mutation at codon 129 of the murine erythropoietin receptor (cEpoR) results in constitutive activation. We have generated a recombinant spleen focus-forming retrovirus in which the env gene is replaced by the cEpoR cDNA. Mice infected with this virus (but not by viruses expressing the wild-type EpoR) develop erythrocytosis and splenomegaly. From the spleen of infected animals we have isolated clonal, growth factor-independent, proerythroblast cell lines that express cEpoR, do not express the putative oncogene spi-1, and have rearranged and inactivated expression of the p53 suppressor oncogene. These cells induce erythroleukemia upon injection into mice. This demonstrates that oncogenic point mutations exist in a member of the cytokine receptor superfamily. The activated erythropoietin receptor does not transform cultured fibroblasts, suggesting why oncogenic mutations in other members of this receptor superfamily have not been detected.

Unregulated expression of the erythropoietin receptor gene caused by insertion of spleen focus-forming virus long terminal repeat in a murine erythroleukemia cell line

A murine erythroleukemia (MEL) cell line, F5-5, expressed 10,000 binding sites for erythropoietin (EPO) per cell, 10-fold more than was expressed by other murine erythroleukemia cell lines and normal erythroid progenitors. Northern (RNA) and Southern blot analyses revealed overexpression of mRNA for the EPO receptor (EPOR) and rearrangement of one of the EPOR gene alleles in F5-5 cells, respectively. Molecular cloning of F5-5-derived cDNA encoding EPOR revealed that the 5' noncoding region of the EPOR cDNA corresponds to the 3' long terminal repeat sequence of the polycythemic strain of Friend spleen focus-forming virus (F-SFFVP). The aberrant EPOR transcripts containing the 3' long terminal repeat sequence were mainly expressed in F5-5 cells. The same integration upstream of the EPOR gene was also observed in other subclones and the parent cell line. It is possible that overexpression of EPOR by viral promoter insertion will confer growth advantage to an F-SFFVP-infected erythroid progenitor cell, leading to positive clonal selection through further leukemogenic steps.

Unregulated expression of the erythropoietin receptor gene caused by insertion of spleen focus-forming virus long terminal repeat in a murine erythroleukemia cell line

A murine erythroleukemia (MEL) cell line, F5-5, expressed 10,000 binding sites for erythropoietin (EPO) per cell, 10-fold more than was expressed by other murine erythroleukemia cell lines and normal erythroid progenitors. Northern (RNA) and Southern blot analyses revealed overexpression of mRNA for the EPO receptor (EPOR) and rearrangement of one of the EPOR gene alleles in F5-5 cells, respectively. Molecular cloning of F5-5-derived cDNA encoding EPOR revealed that the 5' noncoding region of the EPOR cDNA corresponds to the 3' long terminal repeat sequence of the polycythemic strain of Friend spleen focus-forming virus (F-SFFVP). The aberrant EPOR transcripts containing the 3' long terminal repeat sequence were mainly expressed in F5-5 cells. The same integration upstream of the EPOR gene was also observed in other subclones and the parent cell line. It is possible that overexpression of EPOR by viral promoter insertion will confer growth advantage to an F-SFFVP-infected erythroid progenitor cell, leading to positive clonal selection through further leukemogenic steps.

Expression of wild-type p53 is not compatible with continued growth of p53-negative tumor cells

Inactivation of the cellular p53 gene is a common feature of Friend virus-induced murine erythroleukemia cell lines and may represent a necessary step in the progression of this disease. As well, frequent loss or mutation of p53 alleles in diverse human tumors is consistent with the view of p53 as a tumor suppressor gene. To examine the significance of p53 gene inactivation in tumorigenesis, we have attempted to express transfected wild-type p53 in three p53-negative tumor cell lines: murine DP16-1 Friend erythroleukemia cells, human K562 cells, and SKOV-3 cells. We found that aberrant p53 proteins, which differ from wild-type p53 by a single amino acid substitution, were expressed stably in these cells, whereas wild-type p53 expression was not tolerated. The inability of p53-negative tumor cell lines to support long-term expression of wild-type p53 protein is consistent with the view that p53 is a tumor suppressor gene.

Expression of wild-type p53 is not compatible with continued growth of p53-negative tumor cells

Inactivation of the cellular p53 gene is a common feature of Friend virus-induced murine erythroleukemia cell lines and may represent a necessary step in the progression of this disease. As well, frequent loss or mutation of p53 alleles in diverse human tumors is consistent with the view of p53 as a tumor suppressor gene. To examine the significance of p53 gene inactivation in tumorigenesis, we have attempted to express transfected wild-type p53 in three p53-negative tumor cell lines: murine DP16-1 Friend erythroleukemia cells, human K562 cells, and SKOV-3 cells. We found that aberrant p53 proteins, which differ from wild-type p53 by a single amino acid substitution, were expressed stably in these cells, whereas wild-type p53 expression was not tolerated. The inability of p53-negative tumor cell lines to support long-term expression of wild-type p53 protein is consistent with the view that p53 is a tumor suppressor gene.

p53 mutations: gains or losses?

Although the case for p53 as a tumor suppressor gene appears very strong, one should still keep an open eye for the possibility that mutations in p53 do not necessarily imply a mere loss of "suppressor" activity. It is still possible that the presence of a p53 mutation in a tumor contributes, in a dominant positive manner, to tumorigenesis. In other words, certain p53 mutants may well be oncogenic in their own right, and carry distinct activities that promote growth deregulation and malignant progression. Elucidating this issue also has practical implications, since the nature of the resident mutations may greatly dictate the consequences of attempts to reintroduce wild-type (wt) p53 into particular types of tumor cells. There are two major obstacles along the road to meaningful answers: the limitations of the experimental systems used for evaluating the biological activities of wt and mutant p53 and a fundamental lack of knowledge about the relevant biochemistry of the p53 protein. These two aspects constitute primary experimental challenges for investigators in the field.

Wild-type, but not mutant, human p53 proteins inhibit the replication activities of simian virus 40 large tumor antigen

Murine p53 blocks many of the replication activities of simian virus 40 (SV40) large tumor antigen (T antigen) in vitro. As murine cells do not replicate SV40 DNA, it was of interest to determine how p53 from permissive human cells functions. Recombinant baculoviruses encoding either the wild-type form of human p53 or a mutant p53 cloned from a human tumor cell line were constructed, and p53 proteins were purified from infected insect cells. Surprisingly, we found that wild-type human p53 was as inhibitory to the ability of T antigen to mediate replication of an SV40 origin-containing (ori DNA) plasmid in vitro as was murine p53. Wild-type human p53 also blocked the DNA unwinding activity of T antigen, as did its murine counterpart. In contrast to murine and wild-type human p53, the mutant human p53 did not block ori DNA replication or DNA unwinding. Murine p53 formed a complex with mutant human p53 in vivo. Furthermore, mutant human p53 reduced the inhibition of SV40 ori DNA replication by murine p53 in vitro. These results provide a model for the way in which mutant p53 proteins can affect normal functions of p53.

Rearrangement and overexpression of the gene coding for tumor antigen p53 in a rat histiocytoma AK-5

The gene coding for the cellular tumor antigen p53 is rearranged and overexpressed in a rat histiocytoma, AK-5. The protein coded by the gene was detected by immunofluorescence and its full size was confirmed by immunoprecipitation using monoclonal antibodies against p53. Southern hybridizations with a full length cDNA probe specific for p53 indicated rearrangement of the gene. Alterations in the upstream region, which probably disrupt the normal regulatory control are suggested by the pattern obtained using a 5'-specific p53 probe in Southern hybridization.

Negative growth regulation in a glioblastoma tumor cell line that conditionally expresses human wild-type p53

To investigate the effect that human wild-type p53 (wt-p53) expression has on cell proliferation we constructed a recombinant plasmid, pM47, in which wt-p53 cDNA is under transcriptional control of the hormone-inducible mouse mammary tumor virus promoter linked to the dominant biochemical selection marker gene Eco gpt. The pM47 plasmid was introduced into T98G cells derived from a human glioblastoma multiforme tumor, and a stable clonal cell line, GM47.23, was derived that conditionally expressed wt-p53 following exposure to dexamethasone. We show that induction of wt-p53 expression in exponentially growing cells inhibits cell cycle progression and that the inhibitory effect is reversible upon removal of the inducer or infection with simian virus 40. Moreover, when growth-arrested cells are stimulated to proliferate, induction of wt-p53 expression inhibits G0/G1 progression into S phase and the cells accumulate with a DNA content equivalent to cells arrested in the G0/G1 phase of the cell cycle. Taken together, these studies suggest that wt-p53 may play a negative role in growth regulation.

Inactivation of the cellular p53 gene is a common feature of Friend virus-induced erythroleukemia: relationship of inactivation to dominant transforming alleles

The Friend erythroleukemia virus complex contains no cell-derived oncogene. Transformation by this virus may therefore involve mutations affecting cellular gene expression. We provide evidence that inactivating mutations of the cellular p53 gene are a common feature in Friend virus-induced malignancy, consistent with an antioncogene role for p53 in this disease. We have shown that frequent rearrangements of the p53 gene cause loss of expression or synthesis of truncated proteins, whereas overexpression of p53 protein is seen in other Friend cell lines. We now demonstrate that p53 expression in the latter cells is also abnormal, as a result of missense mutations in regions encoding highly conserved amino acids. Three of these aberrant alleles obtained from cells from different mice were cloned and found to function as dominant oncogenes in gene transfer assays, supporting the view that certain naturally occurring missense mutations in p53 confer a dominant negative phenotype on the encoded protein.

Inactivation of the cellular p53 gene is a common feature of Friend virus-induced erythroleukemia: relationship of inactivation to dominant transforming alleles

The Friend erythroleukemia virus complex contains no cell-derived oncogene. Transformation by this virus may therefore involve mutations affecting cellular gene expression. We provide evidence that inactivating mutations of the cellular p53 gene are a common feature in Friend virus-induced malignancy, consistent with an antioncogene role for p53 in this disease. We have shown that frequent rearrangements of the p53 gene cause loss of expression or synthesis of truncated proteins, whereas overexpression of p53 protein is seen in other Friend cell lines. We now demonstrate that p53 expression in the latter cells is also abnormal, as a result of missense mutations in regions encoding highly conserved amino acids. Three of these aberrant alleles obtained from cells from different mice were cloned and found to function as dominant oncogenes in gene transfer assays, supporting the view that certain naturally occurring missense mutations in p53 confer a dominant negative phenotype on the encoded protein.

Mutation of the serine 312 phosphorylation site does not alter the ability of mouse p53 to inhibit simian virus 40 DNA replication in vivo

Two mutations were introduced into the wild-type mouse p53 gene by oligonucleotide-directed mutagenesis. These mutations substituted alanine or aspartic acid for serine at position 312, which is constitutively phosphorylated. Phosphopeptide mapping of the mutant proteins, expressed in COS cells, confirmed the loss of phosphorylation at position 312. There were no changes in the ability of the mutant p53s to express the conformation-dependent epitope for monoclonal antibody PAb246 or to participate in complexes with the simian virus 40 (SV40) large T antigen. Replication of a plasmid containing the SV40 origin of replication was inhibited in COS cells by wild-type p53 and both of the phosphorylation site mutants with equal efficiency. A transforming mutant of p53, encoding valine at position 135, did not inhibit SV40 DNA replication in COS cells.

Identification and mapping of a common proviral integration site Fli-1 in erythroleukemia cells induced by Friend murine leukemia virus

Friend murine leukemia virus (F-MuLV) induces erythroleukemia when inoculated into newborn BALB/c or NIH/Swiss mice. We have molecularly cloned F-MuLV host cell DNA junction fragments from an erythroleukemia cell line induced by F-MuLV to identify cellular genes involved in the leukemogenic process. One particular proviral integration site, Fli-1, is rearranged in 75% (9/12) of independently isolated erythroleukemia cell lines derived from either BALB/c or NIH/Swiss mice inoculated at birth with F-MuLV. Other hematopoietic neoplasms induced by F-MuLV, including myeloid (granulocytic) and lymphoid tumors, did not show rearrangements of the Fli-1 locus. Similarly, none of 35 erythroleukemia cell lines induced by the Friend virus complexes (FV-A and FV-P) was rearranged at the Fli-1 locus. In contrast, no rearrangements were detected at the Sfpi-1 locus, a preferred site of integration in either FV-P- or FV-A-induced leukemias. Using recombinant inbred mice, the Fli-1 locus was situated on mouse chromosome 9 close to the cellular protooncogene c-ets-1. DNA and RNA analysis suggests, however, that Fli-1 is different from ets-1. Thus, Fli-1 appears to define a distinct locus specifically involved in the induction of erythroid leukemias by F-MuLV.

p53: oncogene or anti-oncogene?

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Wild-type p53 can inhibit oncogene-mediated focus formation

Mutant forms of the p53 cellular tumor antigen elicit neoplastic transformation in vitro. Recent evidence indicated that loss of normal p53 expression is a frequent event in certain types of tumors, raising the possibility that such loss provides transformed cells with a selective growth advantage. Thus, it was conceivable that the mutants might contribute to transformation by abrogating normal p53 function. We therefore studied the effect of plasmids encoding wild-type (wt) p53 on the ability of primary rat embryo fibroblasts to be transformed by a combination of mutant p53 and ras. It was found that wt p53 plasmids indeed caused a marked reduction in the number of transformed foci. Furthermore, wt p53 plasmids also suppressed the induction of transformed foci by combinations of bona fide oncogenes, such as myc plus ras or adenovirus E1A plus ras. On the other hand, plasmids carrying mutations in the p53 coding region totally failed to inhibit oncogene-mediated focus induction and often even slightly stimulated it. Hence, such mutations completely abolished the activity of wt p53 that is responsible for the "suppressor" effect. The latter fact is of special interest, since similar mutations in p53 are often observed in human and rodent tumors. The inhibitory effect of p53 was most pronounced when early-passage cells were used as targets, whereas established cell lines were less sensitive. These data support the notions that wt p53 expression may be restrictive to neoplastic progression and that p53 inactivation may play a crucial role in tumorigenesis.

A common site for immortalizing proviral integrations in Friend erythroleukemia: molecular cloning and characterization

By using a tagged derivative of Friend spleen focus-forming virus, we previously obtained evidence that proviral integration(s) in the host genome can cause erythroblast immortality by abrogating the commitment of the cell to differentiate (C. Spiro, B. Gliniak, and D. Kabat, J. Virol. 62:4129-4135, 1988). Exploiting the fact that each leukemia was a single clone that contained one tagged provirus, we have now molecularly cloned and characterized one common genomic site for immortalizing proviral integrations.

High incidence of lung, bone, and lymphoid tumors in transgenic mice overexpressing mutant alleles of the p53 oncogene

We have investigated the role of the p53 gene in oncogenesis in vivo by generating transgenic mice carrying murine p53 genomic fragments isolated from a mouse Friend erythroleukemia cell line or BALB/c mouse liver DNA. Elevated levels of p53 mRNA were detected in several tissues of two transgenic lines tested. Increased levels of p53 protein were also detected in most of the tissues analyzed by Western blotting (immunoblotting). Because both transgenes encoded p53 proteins that were antigenically distinct from wild-type p53, it was possible to demonstrate that overexpression of the p53 protein was mostly, if not entirely, due to the expression of the transgenes. Neoplasms developed in 20% of the transgenic mice, with a high incidence of lung adenocarcinomas, osteosarcomas, and lymphomas. Tissues such as ovaries that expressed the transgene at high levels were not at higher risk of malignant transformation than tissues expressing p53 protein at much lower levels. The long latent period and low penetrance suggest that overexpression of p53 alone is not sufficient to induce malignancies and that additional events are required. These observations provide direct evidence that mutant alleles of the p53 oncogene have oncogenic potential in vivo and that different cell types show intrinsic differences in susceptibility to malignant transformation by p53. Since recent data suggest that p53 may be a recessive oncogene, it is possible that the elevated tumor incidence results from functional inactivation of endogenous p53 by overexpression of the mutant transgene. The high incidence of lung and bone tumors suggests that p53 transgenic mice may provide a useful model to investigate the molecular events that underlie these malignancies in humans.

High incidence of lung, bone, and lymphoid tumors in transgenic mice overexpressing mutant alleles of the p53 oncogene

We have investigated the role of the p53 gene in oncogenesis in vivo by generating transgenic mice carrying murine p53 genomic fragments isolated from a mouse Friend erythroleukemia cell line or BALB/c mouse liver DNA. Elevated levels of p53 mRNA were detected in several tissues of two transgenic lines tested. Increased levels of p53 protein were also detected in most of the tissues analyzed by Western blotting (immunoblotting). Because both transgenes encoded p53 proteins that were antigenically distinct from wild-type p53, it was possible to demonstrate that overexpression of the p53 protein was mostly, if not entirely, due to the expression of the transgenes. Neoplasms developed in 20% of the transgenic mice, with a high incidence of lung adenocarcinomas, osteosarcomas, and lymphomas. Tissues such as ovaries that expressed the transgene at high levels were not at higher risk of malignant transformation than tissues expressing p53 protein at much lower levels. The long latent period and low penetrance suggest that overexpression of p53 alone is not sufficient to induce malignancies and that additional events are required. These observations provide direct evidence that mutant alleles of the p53 oncogene have oncogenic potential in vivo and that different cell types show intrinsic differences in susceptibility to malignant transformation by p53. Since recent data suggest that p53 may be a recessive oncogene, it is possible that the elevated tumor incidence results from functional inactivation of endogenous p53 by overexpression of the mutant transgene. The high incidence of lung and bone tumors suggests that p53 transgenic mice may provide a useful model to investigate the molecular events that underlie these malignancies in humans.

Characterization of the human p53 gene promoter

Transcriptional deregulation of the p53 gene may play an important part in the genesis of some tumors. We report here an accurate determination of the transcriptional start sites of the human p53 gene and show that the majority of p53 mRNA molecules do not contain a postulated stem-loop structure at their 5' ends. Recombinant plasmids of the human p53 promoter-leader region fused to the bacterial chloramphenicol acetyltransferase gene (cat) were constructed. After transfection into rodent or human cells, a 350-base-pair fragment spanning the promoter region conferred 4% of the CAT activity mediated by the simian virus 40 early promoter/enhancer. We monitored the efficiency with which 15 3' and 5' promoter deletion constructs initiated transcription. Our results show that an 85-base-pair fragment, previously thought to have resided in exon 1, is all that is required for full promoter activity.

The p53 proto-oncogene can act as a suppressor of transformation

DNA clones of the wild-type p53 proto-oncogene inhibit the ability of E1A plus ras or mutant p53 plus ras-activated oncogenes to transform primary rat embryo fibroblasts. The rare clones of transformed foci that result from E1A plus ras plus wild-type p53 triple transfections all contain the p53 DNA in their genome, but the great majority fail to express the p53 protein. The three cell lines derived from such foci that express p53 all produce mutant p53 proteins with properties similar or identical to transformation-activated p53 proteins. The p53 mutants selected in this fashion (transformation in vitro) resemble the p53 mutants selected in tumors (in vivo). These results suggest that the p53 proto-oncogene can act negatively to block transformation.

Characterization of the human p53 gene promoter

Transcriptional deregulation of the p53 gene may play an important part in the genesis of some tumors. We report here an accurate determination of the transcriptional start sites of the human p53 gene and show that the majority of p53 mRNA molecules do not contain a postulated stem-loop structure at their 5' ends. Recombinant plasmids of the human p53 promoter-leader region fused to the bacterial chloramphenicol acetyltransferase gene (cat) were constructed. After transfection into rodent or human cells, a 350-base-pair fragment spanning the promoter region conferred 4% of the CAT activity mediated by the simian virus 40 early promoter/enhancer. We monitored the efficiency with which 15 3' and 5' promoter deletion constructs initiated transcription. Our results show that an 85-base-pair fragment, previously thought to have resided in exon 1, is all that is required for full promoter activity.

Molecular biology of Friend viral erythroleukemia

Friend virus clearly provides an important model for understanding the molecular biology of cancer. Moreover, the most important aspects of the erythroleukemia can be caused by a single SFFV infection in the absence of any helper virus. The SFFV env gene encodes a membrane glycoprotein, gp55. This glycoprotein, when expressed on erythroblast surfaces, causes a constitutive mitogenesis. However, SFFV infections only rarely increase the cell's self-renewal capability or abrogate its commitment to differentiate. Therefore, the consequence of infection is initially a polyclonal erythroblastosis. This polyclonal proliferation usually leads to cell differentiation and to recovery unless helper virus is present to cause continuing infection of new erythroblasts. Extremely rare SFFV proviral integrations, however, result in abrogation of the cell's commitment to differentiate and in the concomitant acquisition of cell immortality. These immortalizing proviral integrations occur at only a small number of sites in the mouse genome. Therefore, the mitogenic and immortalizing stages of erythroleukemia are now known to be caused by discrete genetic events--the first involving the SFFV env gene and the second involving the rare proviral integration sites. In early investigations of Friend virus, the first stage always preceded the second stage by at least several weeks. Now it is known that this delay in onset of the second stage is caused solely by statistics. Every SFFV-infected erythroblast is mitogenically activated, yet only rarely does the SFFV proviral integration produce immortality. Both steps in leukemogenesis can be caused simultaneously in an erythroblast by a rare single SFFV proviral integration. There has been an explosion of interest in retroviral env gene-mediated pathogenesis. Such pathogenesis has been recently associated with most of the naturally transmitted retroviral diseases including AIDS. Such pathogenesis involves in different viruses immunosuppression, anemia, neuropathy, and leukemia (Mathes et al. 1978; Simon et al. 1984, 1987; Weiss et al. 1985; Lifson et al. 1986; Riedel et al. 1986; Sitbon et al. 1986; Sodroski et al. 1986; Mitani et al. 1987; Schmidt et al. 1987; Klase et al. 1988; Overbaugh et al. 1988a, b). The shuffling and dynamic env gene rearrangements that have been associated with murine retroviral leukemogenesis have also now been seen in FeLV-FAIDS and HIV (Fisher et al. 1988; Overbaugh et al. 1 t88b; Saag et al. 1988; Tersmette et al. 1988). Friend virus provides an important established example of such env gene pathogenesis. Although we still do not understand precisely how gp55 causes erythroblast mitosis, workers in this field have discovered important clues that may lead to answers.(ABSTRACT TRUNCATED AT 400 WORDS)

Oncogene expression in Rauscher murine leukemia virus induced erythroid, myeloid and lymphoid cell lines

A comparative study on the expression of nuclear and cytoplasmic oncogenes was carried out using the Northern blotting technique, in Rauscher virus induced primary leukemias and the more malignant transformed cell lines derived from them. The latter grow permanently in vitro. Hyperplastic spleens obtained from mice recovering from anemia were analysed as controls. In addition to the detection of mRNAs, Southern blotting was carried out to observe whether rearrangement or amplification of oncogenes had occurred. The results show that the nuclear oncogenes c-myc, c-myb and p53 are strongly expressed in leukemic tissue, whereas c-fos transcripts show a much weaker hybridization. The expression of two of these oncogenes, c-myc and c-myb was followed during differentiation in myeloid leukemic cells and showed a gradual decrease when compared with the actin gene, which is constitutively transcribed. A large number of cytoplasmic oncogenes is expressed in the leukemic cells lines, i.e. c-abl, c-fms, c-fes, c-src, c-ros, c-H-ras, c-K-ras and N-ras. Of these, transcripts coding for c-abl and c-src were absent in blast cells of acute erythroid leukemias. Transcripts coding for c-erb, c-mos and c-sis could also not be detected. A number of putative oncogenes which are reported to play a role in Moloney and Friend virus induced leukemias for instance pim-1, fis-1, fim-1 and fim-2 were also used for screening. Only expression of pim-1 in Rauscher virus induced myeloid leukemic cells and in primary acute erythroid leukemias could be observed. At the DNA level no rearrangement or amplification of any of the oncogenes investigated could be detected. The results show that a number of oncogenes are expressed simultaneously in the same leukemic tissue or cell lines. It therefore seems likely that the presence of transcripts of different oncogenes is associated with the progression of leukemia, but is not the primary cause of leukemogenesis or of the transformation of these cells into established cell lines.

Integration of Friend murine leukemia virus into both alleles of the p53 oncogene in an erythroleukemic cell line

The Friend virus-transformed erythroleukemic cell line DP16-9B4 has undergone a complex rearrangement of the p53 oncogene and lacks any detectable expression of the p53 protein. We report here characterization of both p53 alleles in this cell line and identify independent integrations of Friend murine leukemia virus sequences into the coding region of both alleles.