Induction of apoptosis by wild-type p53 in a human colon tumor-derived cell line

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.

Functional domains of wild-type and mutant p53 proteins involved in transcriptional regulation, transdominant inhibition, and transformation suppression

The wild-type (wt) p53 protein has transcriptional activation functions which may be linked to its tumor suppressor activity. Many mutant p53 proteins expressed in cancers have lost the ability to function as transcriptional activators and furthermore may inhibit wt p53 function. To study the mechanisms by which mutant forms of p53 have lost their transactivation function and can act in a dominant negative manner, a structure-function analysis of both mutant and engineered truncated forms of p53 was carried out. We show that different mutant p53 proteins found in cancers vary in the ability to inhibit the transcriptional transactivation and specific DNA binding activities of wt human p53. This transdominant effect was mediated through the carboxy-terminal oligomerization region. The role of the transactivation activity in transformation suppression by wt p53 was also examined by constructing an N-terminal deletion mutant lacking the transactivation domain. This mutant was unable to transactivate but could bind specifically to DNA. Although it was impaired in its ability to suppress transformation of primary rat embryo fibroblasts by adenovirus E1A plus activated ras, the N-terminal deletion mutant still had some suppression activity, suggesting that additional functions of p53 may contribute to transformation suppression.

The relation between p53 mutation and p53 immunostaining in non-melanoma skin cancer

Extensive study of the p53 gene has established its role as a tumour-suppressor gene, and the involvement of mutant p53 in a wide spectrum of human malignancy. Many mutations of p53 result in a protein product that is abnormally stable, so that it becomes readily detectable by immunocytochemistry. In contrast, under normal conditions, it has been considered that levels of wild-type p53 were too low to be detectable. Although positive immunocytochemistry has been used as a marker of mutation, recent evidence suggests that this assumption may not always be valid. We have carried out both PCR-sequencing of exons 5-8 of the p53 gene in 20 basal cell carcinomas (BCC), and immunocytochemistry of these tumours with the anti-p53 antibody DO7. Twenty cases of Bowen's disease, in which we had previously documented mutations, were also immunostained. We report a low rate of p53 mutation in the BCCs we examined (2/20), and a discrepancy between tumours with positive immunostaining and those with mutation in both Bowen's disease and BCC. Of eight tumours in which we detected mutation, only four were immunopositive: of 19 immunopositive samples, only four showed detectable mutation. We discuss the implications of our results for the use of positive immunostaining in clinical diagnosis, and the involvement of p53 in skin carcinogenesis.

Activation of the cryptic DNA binding function of mutant forms of p53

Wild type p53 assembles into a latent multiprotein complex which can be activated for sequence-specific DNA binding in vitro by proteins targeting the carboxy-terminal domain. Using an optimized system coupling the post-translational modification of wild type p53 to activation of sequence specific DNA binding, we examined the affects of common mutations on the cryptic DNA binding function of p53. Two mutant forms of p53 were shown to be efficiently converted from the latent state by PAb421 and DnaK, but were defective in activation by casein kinase II, indicating that mutant p53 may not be receptive to allosteric regulation by casein kinase II phosphorylation. A reactive sulfhydryl group is absolutely required for DNA binding by wild type and mutant forms of p53 once converted to the activated state. Together, these data show that some mutant forms of p53 harbour the wild-type machinery required to engage in sequence-specific DNA binding and define a signalling pathway whose inactivation may directly result in a loss of p53 function.

Use of transgenic mice reveals cell-specific transformation by a simian virus 40 T-antigen amino-terminal mutant

We have used the multifunctional transforming protein, simian virus 40 T antigen, as a probe to study the mechanisms of cell growth regulation in the intact organism. T antigen appears to perturb cell growth, at least in part, by stably interacting with specific cellular proteins that function to maintain normal cell growth properties. Experiments in cultured cells indicate that at least three distinct regions of simian virus 40 T antigen have roles in transformation. Two regions correlate with the binding of known cellular proteins, p53, pRB, and p107. A third activity, located near the amino terminus, has been defined genetically but not biochemically. By targeting expression of wild-type and mutant forms of T antigen to distinct cell types in transgenic mice, we have begun to systematically determine which activities play a role in tumorigenesis of each cell type. In this study, we sought to determine the role of the amino-terminal transformation function with such an analysis of the T-antigen mutant dl1135. This protein, which lacks amino acids 17 to 27, retains the p53-, pRB-, and p107-binding activities yet fails to transform cells in culture. To direct expression in transgenic mice, we used the lymphotropic papovavirus transcriptional signals that are specific for B and T lymphocytes and the choroid plexus epithelium of the brain. We show here that although defective in cell culture, dl1135 specifically induced the development of thymic lymphomas in the mouse. Expression of the protein was routinely observed in B- and T-lymphoid cells, although B-cell abnormalities were not observed. Choroid plexus tumors were observed only infrequently; however, dl1135 was not consistently expressed in this tissue. Within a given transgenic line, the penetrance of T-cell tumorigenesis was 100% but appeared to require secondary events, as judged from the clonal nature of the tumors. These experiments suggest that the amino-terminal region of T antigen has a role in the transformation of certain cell types (such as fibroblasts in culture and B lymphocytes) but is dispensable for the transformation of T lymphocytes.

Mutation and expression of the p53 gene in malignant melanoma cell lines

Three monoclonal antibodies (MAbs) against p53 protein (PAb 24o, DO-I and PAb1801) were used to define the immunophenotype of 13 melanoma cell lines. Immunoreactions could be detected in 12 out of 13 cell lines by using the indirect immunofluorescence technique. In 7 of these the majority of cells displayed cytoplasmic staining whereas positive nuclei were detected in only a few cells. Two cell lines had predominantly nuclear reactivity, while the remaining 3 cell lines showed signals in both locations. Despite identical nuclear staining patterns, the 3 MAbs produced qualitatively distinct cytoplasmic immunoreactions. PAb240 and DO-1, which showed similar staining frequencies, appeared more sensitive in the detection of p53 protein than did PAb1801. Immunoprecipitations of lysates from each of the cell lines, with MAbs DO-1 and 1801 (which bind to both wild-type and mutant p53 species) detected 53-kDa proteins, whereas PAb240 (which recognizes the mutant conformation of the protein in this type of assay) detected 53-kDa proteins in only 4 cell lines. Nucleotide sequencing of exons 5 to 9 of TP53 in these latter cell lines showed that each has homozygous point mutations in the locus, whereas in the others no TP53 alterations were found. Three of the 4 mutations were C-to-T transversions, alterations possibly caused by damage from UV-light. Our findings indicate that immunostaining with p53 antibodies, although common in malignant melanoma, results from the presence of mutant p53 protein in about 30% of the cases tested. Neither immunostaining with PAb240 nor the patterns of intracellular distributions of the signals are sufficient to detect TP53 mutations.

Use of transgenic mice reveals cell-specific transformation by a simian virus 40 T-antigen amino-terminal mutant

We have used the multifunctional transforming protein, simian virus 40 T antigen, as a probe to study the mechanisms of cell growth regulation in the intact organism. T antigen appears to perturb cell growth, at least in part, by stably interacting with specific cellular proteins that function to maintain normal cell growth properties. Experiments in cultured cells indicate that at least three distinct regions of simian virus 40 T antigen have roles in transformation. Two regions correlate with the binding of known cellular proteins, p53, pRB, and p107. A third activity, located near the amino terminus, has been defined genetically but not biochemically. By targeting expression of wild-type and mutant forms of T antigen to distinct cell types in transgenic mice, we have begun to systematically determine which activities play a role in tumorigenesis of each cell type. In this study, we sought to determine the role of the amino-terminal transformation function with such an analysis of the T-antigen mutant dl1135. This protein, which lacks amino acids 17 to 27, retains the p53-, pRB-, and p107-binding activities yet fails to transform cells in culture. To direct expression in transgenic mice, we used the lymphotropic papovavirus transcriptional signals that are specific for B and T lymphocytes and the choroid plexus epithelium of the brain. We show here that although defective in cell culture, dl1135 specifically induced the development of thymic lymphomas in the mouse. Expression of the protein was routinely observed in B- and T-lymphoid cells, although B-cell abnormalities were not observed. Choroid plexus tumors were observed only infrequently; however, dl1135 was not consistently expressed in this tissue. Within a given transgenic line, the penetrance of T-cell tumorigenesis was 100% but appeared to require secondary events, as judged from the clonal nature of the tumors. These experiments suggest that the amino-terminal region of T antigen has a role in the transformation of certain cell types (such as fibroblasts in culture and B lymphocytes) but is dispensable for the transformation of T lymphocytes.

Highly efficient transactivation by the yeast Kluyveromyces lactis transcription factor LAC9 and its inhibition by the negative regulator GAL80 in mammalian cells

Expression of the LAC9 gene from the yeast Kluyveromyces lactis in HepG2 human hepatoblastoma cells efficiently induced luciferase expression from reporter plasmids containing the four LAC9 binding sites from the K. lactis GAL1-GAL10 gene linked to a basal promoter. Induction was approximately 100fold and was dependent on the presence of the UAS sequence and an intact reading frame in the LAC9 gene. Additional cotransfection of constructs expressing the K. lactis GAL80 gene reduced luciferase activity by up to 98%. This inhibition was not affected by addition of 14mM galactose to the medium. No further yeast-specific factors appear necessary for efficient inhibition of LAC9 by GAL80, but additional gene products may be required for activation by galactose.

p53 is required for radiation-induced apoptosis in mouse thymocytes

The p53 tumour suppressor gene is the most widely mutated gene in human tumorigenesis. p53 encodes a transcriptional activator whose targets may include genes that regulate genomic stability, the cellular response to DNA damage, and cell-cycle progression. Introduction of wild-type p53 into cell lines that have lost endogenous p53 function can cause growth arrest or induce a process of cell death known as apoptosis. During normal development, self-reactive thymocytes undergo negative selection by apoptosis, which can also be induced in immature thymocytes by other stimuli, including exposure to glucocorticoids and ionizing radiation. Although normal negative selection involves signalling through the T-cell receptor, the induction of apoptosis by other stimuli is poorly understood. We have investigated the requirement for p53 during apoptosis in mouse thymocytes. We report here that immature thymocytes lacking p53 die normally when exposed to compounds that may mimic T-cell receptor engagement and to glucocorticoids but are resistant to the lethal effects of ionizing radiation. These results demonstrate that p53 is required for radiation-induced cell death in the thymus but is not necessary for all forms of apoptosis.

Thymocyte apoptosis induced by p53-dependent and independent pathways

Death by apoptosis is characteristic of cells undergoing deletion during embryonic development, T- and B-cell maturation and endocrine-induced atrophy. Apoptosis can be initiated by various agents and may be a result of expression of the oncosuppressor gene p53 (refs 6-8). Here we study the dependence of apoptosis on p53 expression in cells from the thymus cortex. Short-term thymocyte cultures were prepared from mice constitutively heterozygous or homozygous for a deletion in the p53 gene introduced into the germ line after gene targeting. Wild-type thymocytes readily undergo apoptosis after treatment with ionizing radiation, the glucocorticoid methylprednisolone, or etoposide (an inhibitor of topoisomerase II), or after Ca(2+)-dependent activation by phorbol ester and a calcium ionophore. In contrast, homozygous null p53 thymocytes are resistant to induction of apoptosis by radiation or etoposide, but retain normal sensitivity to glucocorticoid and calcium. The time-dependent apoptosis that occurs in untreated cultures is unaffected by p53 status. Cells heterozygous for p53 deletion are partially resistant to radiation and etoposide. Our results show that p53 exerts a significant and dose-dependent effect in the initiation of apoptosis, but only when it is induced by agents that cause DNA-strand breakage.

Myc-mediated apoptosis is blocked by ectopic expression of Bcl-2

The product of the c-myc proto-oncogene is an important positive regulator of cell growth and proliferation. Recently, c-Myc has also been demonstrated to be a potent inducer of apoptosis when expressed in the absence of serum or growth factors. To further examine Myc-induced apoptosis, we coexpressed the proto-oncogene bcl2, which has been shown to block apoptosis in other systems, with c-myc in serum-deprived Rat 1a fibroblasts. Here we report that ectopic expression of bcl2 specifically blocks apoptosis induced by constitutive c-myc expression. Constitutive c-myc expression in serum-deprived Rat 1a cells caused a > 15-fold increase in the number of dead cells, accompanied by DNA fragmentation. However, coexpression of bcl2 with c-myc in these cells led to a 10-fold increase in the number of live cells and a significant decrease in DNA fragmentation. Thus, Bcl-2 effectively inhibits Myc-induced apoptosis in serum-deprived Rat 1a fibroblasts without blocking entry into the cell cycle. These results imply that apoptosis serves as a protective mechanism to prevent tumorigenicity elicited by deregulated Myc expression. This protective mechanism is abrogated, however, by Bcl-2 and therefore may explain the synergism between Myc and Bcl-2 observed in certain tumor cells.

p53-mediated cell death: relationship to cell cycle control

M1 clone S6 myeloid leukemic cells do not express detectable p53 protein. When stably transfected with a temperature-sensitive mutant of p53, these cells undergo rapid cell death upon induction of wild-type (wt) p53 activity at the permissive temperature. This process has features of apoptosis. In a number of other cell systems, wt p53 activation has been shown to induce a growth arrest. Yet, wt 53 fails to induce a measurable growth arrest in M1 cells, and cell cycle progression proceeds while viability is being lost. There exists, however, a relationship between the cell cycle and p53-mediated death, and cells in G1 appear to be preferentially susceptible to the death-inducing activity of wt p53. In addition, p53-mediated M1 cell death can be inhibited by interleukin-6. The effect of the cytokine is specific to p53-mediated death, since apoptosis elicited by serum deprivation is refractory to interleukin-6. Our data imply that p53-mediated cell death is not dependent on the induction of a growth arrest but rather may result from mutually incompatible growth-regulatory signals.

Myc-mediated apoptosis is blocked by ectopic expression of Bcl-2

The product of the c-myc proto-oncogene is an important positive regulator of cell growth and proliferation. Recently, c-Myc has also been demonstrated to be a potent inducer of apoptosis when expressed in the absence of serum or growth factors. To further examine Myc-induced apoptosis, we coexpressed the proto-oncogene bcl2, which has been shown to block apoptosis in other systems, with c-myc in serum-deprived Rat 1a fibroblasts. Here we report that ectopic expression of bcl2 specifically blocks apoptosis induced by constitutive c-myc expression. Constitutive c-myc expression in serum-deprived Rat 1a cells caused a > 15-fold increase in the number of dead cells, accompanied by DNA fragmentation. However, coexpression of bcl2 with c-myc in these cells led to a 10-fold increase in the number of live cells and a significant decrease in DNA fragmentation. Thus, Bcl-2 effectively inhibits Myc-induced apoptosis in serum-deprived Rat 1a fibroblasts without blocking entry into the cell cycle. These results imply that apoptosis serves as a protective mechanism to prevent tumorigenicity elicited by deregulated Myc expression. This protective mechanism is abrogated, however, by Bcl-2 and therefore may explain the synergism between Myc and Bcl-2 observed in certain tumor cells.

Stabilization of the p53 tumor suppressor is induced by adenovirus 5 E1A and accompanies apoptosis

Oncogenic transformation by human adenoviruses requires early regions 1A and 1B (E1A and E1B) and provides a model of multistep carcinogenesis. This study shows that the metabolic stabilization of p53 observed in adenovirus 5 (Ad5)-transformed cells can occur in untransformed cells expressing E1A alone. Stabilized p53 was localized to the nucleus and was indistinguishable from wild-type p53 with respect to its interactions with hsc70, PAb420, Ad5 p55E1B, and SV40 large T antigen. Moreover, binding of Ad5 p55E1B or SV40 large T antigen had no additional effect on p53 levels or turnover. Higher levels of p53 were also induced in a variety of cell types within 40 hr after transferring E1A genes. E1A also caused cells to lose viability by a process resembling apoptosis. The apoptosis appeared to involve p53, because p53 levels reverted to normal in surviving cells that had lost E1A, and E1B protected cells from the toxic effects of E1A. These results suggest that (1) the involvement of p53 in tumor suppression and/or apoptosis can be regulated at the level of protein turnover, and (2) a major oncogenic role for E1B is to counter cellular responses to E1A (i.e., stabilization of p53 and associated apoptosis) that preclude transformation by E1A alone. This represents the first physiological setting in which high levels of endogenous p53 are induced in response to an oncogenic challenge, with the apparent consequence of suppressing transformation.

Wild-type p53 mediates apoptosis by E1A, which is inhibited by E1B

Transformation of primary rodent cells by the adenovirus E1A and E1B oncogenes is a two-step process, where E1A-dependent induction of proliferation is coupled to E1B-dependent suppression of programmed cell death (apoptosis). The E1B gene encodes two distinct transforming proteins, the 19K and 55K proteins, both of which independently cooperate with E1A. E1B 19K or 55K protein, or the human Bcl-2 protein, functions to suppress apoptosis and thereby permits transformation with E1A. The E1B 55K protein blocks p53 tumor suppressor protein function, indicating that p53 may mediate apoptosis by E1A. In the mutant conformation, p53 blocked induction of apoptosis by E1A and efficiently cooperated with E1A to transform primary cells. When p53 was returned to the wild-type conformation, E1A+p53 transformants underwent cell death by apoptosis. This induction of apoptosis by conformational shift of p53 from the mutant to the wild-type form was inhibited by expression of the E1B 19K protein. Thus, the p53 protein may function as a tumor suppressor by initiating a cell suicide response to deregulation of growth control by E1A. E1B 19K and 55K proteins provide separate mechanisms that disable the cell suicide pathway of p53.

p53-mediated cell death: relationship to cell cycle control

M1 clone S6 myeloid leukemic cells do not express detectable p53 protein. When stably transfected with a temperature-sensitive mutant of p53, these cells undergo rapid cell death upon induction of wild-type (wt) p53 activity at the permissive temperature. This process has features of apoptosis. In a number of other cell systems, wt p53 activation has been shown to induce a growth arrest. Yet, wt 53 fails to induce a measurable growth arrest in M1 cells, and cell cycle progression proceeds while viability is being lost. There exists, however, a relationship between the cell cycle and p53-mediated death, and cells in G1 appear to be preferentially susceptible to the death-inducing activity of wt p53. In addition, p53-mediated M1 cell death can be inhibited by interleukin-6. The effect of the cytokine is specific to p53-mediated death, since apoptosis elicited by serum deprivation is refractory to interleukin-6. Our data imply that p53-mediated cell death is not dependent on the induction of a growth arrest but rather may result from mutually incompatible growth-regulatory signals.

mdm2 expression is induced by wild type p53 activity

We have recently characterized a 95 kDa protein, p95, which exhibits enhanced binding to temperature-sensitive p53 (ts-p53) when cells are shifted down to 32.5 degrees C, a temperature at which ts-p53 possesses wild-type (wt)-like activities. In the present study we show that p95 is a product of the mdm2 putative proto-oncogene. The enhanced complex formation of mdm2 with ts-p53 in cells maintained at 32.5 degrees C is due to an elevation in total mdm2 protein levels following the temperature shift. We further demonstrate that the induction of mdm2 expression by t p53 activity is at the mRNA level. The induction occurs with very rapid kinetics and does not require de novo protein synthesis, suggesting a direct involvement of p53 in the process. Based on these data and on recent findings implicating p53 as a transcription factor, we suggest that the mdm2 gene is a target for activation by wt p53. In view of the ability of mdm2 to act as a specific antagonist of p53 activity, this induction process may serve to tightly autoregulate p53 activity in living cells.

Molecular aspects of early stages of breast cancer progression

It is clear that breast cancer progression is associated with inactivation of a number of different recessive oncogenes. The most widely evaluated tumor suppressor gene, p53, is mutated in approximately 30-50% of sporadic breast cancers. Mutations usually occur early in malignant progression. Loss of heterozygosity (LOH) studies have identified numerous chromosomal regions where other recessive oncogenes relevant to breast cancer may be located. Each LOH is seen in a varying proportion of breast cancers and may appear either early or late in progression. High-grade ductal carcinoma in situ (DCIS) and invasive carcinoma have similar genetic lesions, showing that aberrations can occur before invasive disease. Direct evidence that the same aberrations can be acquired later in progression comes from a study of multiple metastases from the same patient; other studies found that primary invasive cancers are characterized by marked intratumor heterogeneity for each lesion examined. The model we propose to account for these results hypothesizes that multiple genetic lesions can accomplish each phenotype required for malignancy (i.e., dysregulated proliferation, invasion, angiogenesis, etc.) and that, for a given tumor, at least one aberrant gene for each phenotypic change is stochastically selected. Biological heterogeneity of breast cancer results from the stochastic acquisition of various genetic aberrations. We further propose that the lymphocytic reaction in high-grade DCIS may select for aggressive tumor subpopulations capable of escaping immune surveillance. Another aspect of tumor heterogeneity may be the multiple mechanisms employed by various tumors to escape immune surveillance.

No point mutation of Ha-ras or p53 genes expressed in preneoplastic-to-neoplastic progression as modeled in mouse JB6 cell variants

Alterations of the p53 gene have been found in many human and animal tumors, and ras mutations have been seen somewhat less frequently. Loss of p53 tumor suppressor activity has been implicated in multistage tumor promotion/progression after initiation by Ha-ras or Ki-ras activation. Whether p53 or ras alterations occur during promotion of neoplastic transformation in JB6 cells has not been reported. Using a series of mouse JB6 variants representing different stages of progression toward the tumor-cell phenotype, we investigated whether mutational activation of Ha-ras and mutational inactivation and altered expression of the p53 gene occurred during progression. We report here that neither point mutations of the Ha-ras or p53 genes nor p53 structural alterations were involved in this process. Although there was no significant difference in steady-state levels of p53 mRNA, an elevated level of immunoprecipitable p53 protein was observed in a subset of transformed cells. We also report the detection of a second 2.2-kb p53-hybridizing transcript. This transcript was not translated into p53 protein and may be a nuclear precursor of the mature message. Both p53-hybridizing transcripts were downregulated by prolonged 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment (24 h) regardless of the stage of progression. We conclude from this study that in JB6 variants (1) mutational activation of Ha-ras and inactivation of p53 are unlikely to be involved in preneoplastic progression; (2) increased amounts of p53 protein may be involved in a subset of transformed cells, possibly reflecting a longer half-life, as demonstrated in other systems; and (3) late downregulation of p53 mRNA but not protein expression may be a secondary response in the TPA-mediated signal transduction pathway.

Mutation of the casein kinase II phosphorylation site abolishes the anti-proliferative activity of p53

The p53 tumour suppressor protein is phosphorylated by several protein kinases, including casein kinase II. In order to understand the functional significance of phosphorylation by casein kinase II, we have introduced mutations at serine 386 in mouse p53, the residue phosphorylated by this kinase, and investigated their effects on the ability of p53 to arrest cell growth. Replacement of serine 386 by alanine led to loss of growth suppressor activity, while aspartic acid at this position partially retained suppressor function. These data suggest that the anti-proliferative activity of p53 is activated by phosphorylation at serine 386, and establish a direct link between the covalent modification of a growth suppressor protein and regulation of its activity in mammalian cells.

A simple and rapid method for detection of apoptosis in human cells

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