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

Functional consequences of APO-1/Fas (CD95) antigen expression by normal and neoplastic hematopoietic cells

Murine monoclonal antibody (mAb) 7C11 binds to the same cell surface epitope as anti-APO-1 and anti-Fas and reacts specifically with cells transfected with a cDNA encoding the human Fas antigen. Furthermore, incubation with 7C11 causes death of hematopoietic cell lines that express APO-1/Fas but not APO-1/Fas-negative cell lines. 7C11 therefore recognizes the human APO-1/Fas (CD95) antigen, a 40 to 50 kDa cell surface glycoprotein that can trigger apoptosis or programmed cell death. Expression of APO-1/Fas antigen by normal and neoplastic hematopoietic cells was determined by flow cytometry using 7C11. APO-1/Fas is expressed by approximately 30 to 40% of resting peripheral blood T cells, B cells, and monocytes and by approximately 5% of resting NK cells and thymocytes. It was not detected on granulocytes, erythrocytes, or platelets. Approximately 80 to 90% of activated T cells, B cells, and thymocytes express APO-1/Fas, as do the majority of activated NK cells. Perturbation of APO-1/Fas by 7C11 does not affect the viability of resting lymphocytes or monocytes. In contrast, activated T cells and NK cells undergo apoptosis within 3 hours of exposure to 7C11. Other mAb that stimulate T cells or NK cells do not cause rapid induction of programmed cell death. APO-1/Fas antigen is expressed by many cell lines of lymphoid and myeloid lineage. However, this antigen was detected on neoplastic cells from only one of 69 patients with acute myeloid leukemia, acute lymphoblastic leukemia, chronic myelogenous leukemia, chronic lymphocytic leukemia, or multiple myeloma. Only 3 out of 25 tumor samples from patients with non-Hodgkin's lymphoma were found to express APO-1/Fas. All three of these lymphomas harbored the bcl-2-Ig fusion gene associated with the chromosomal translocation t (14;18). Conversely, only 27% of lymphomas that possessed the bcl-2-Ig gene were found to express the APO-1/Fas antigen. Like normal activated lymphocytes, leukemia and lymphoma cells that expressed APO-1/Fas antigen were found to undergo apoptosis in vitro after incubation with 7C11. The APO-1/Fas antigen appears to regulate the growth of normal hematopoietic cells, and the marked upregulation of this antigen on activated normal lymphocytes contrasts sharply with the absence of APO-1/Fas on neoplastic cells of hematopoietic lineage. Defects in the apoptotic signal delivered through this antigen might contribute to the pathogenesis of hematopoietic neoplasms. Thus, the gene encoding APO-1/Fas can be considered a novel type of tumor suppressor gene, just as bcl-2 can be considered a cellular proto-oncogene.

The p53-mediated G1 checkpoint is retained in tumorigenic rat embryo fibroblast clones transformed by the human papillomavirus type 16 E7 gene and EJ-ras

Rat embryo fibroblast clones transformed with the human papillomavirus type 16 E7 gene and the H-ras oncogene (ER clones) fall into two groups on the basis of endogenous p53 genotype, wild type or mutant. We have compared these clones with the aim of indentifying physiological differences that could be attributed to p53 protein function. We show that all ER clones, regardless of p53 gene status, are tumorigenic and metastatic in severe combined immunodeficiency mice. We demonstrate that only the wild-type p53 protein expressed in ER clones is functional on the basis of its site-specific double-stranded DNA-binding activity and its ability to confer a G1 delay on cells following treatment with ionizing radiation. These data indicate that disruption of the p53 growth-regulatory pathway is not a prerequisite for the malignant conversion of rat embryo fibroblasts expressing the E7 gene and mutant ras. Differences in phenotype that were correlated with loss of p53 protein function included the following: serum-independent growth of ER clones in culture, decreased tumor doubling time in vivo, and increased radioresistance. In addition, we demonstrate the p53-dependent G1 checkpoint alone does not determine radiosensitivity.

Multiple expression patterns of biopathological markers in primary invasive breast carcinoma: a useful tool for elucidating its biological behaviour

BACKGROUND

Commonly used clinical and morphologic criteria have been reported to be of limited value in predicting the outcome of malignant tumours of the breast. Integrated information from the quantitative analysis in tumour tissue of biological parameters such as oestrogen and progesterone receptors (ER and PGR), proliferative activity, and proto-oncogene p53, c-erB2, and bcl-2 expression, may be useful for defining the biology of growth of breast carcinoma and to plan effective therapeutic strategies.

PATIENTS AND METHODS

Immunohistochemistry with antibodies recognizing ER, PGR, Ki-67, and the p53, c-erbB2, and bcl-2 encoded proteins was performed on 291 primary breast carcinomas. Results were integrated with clinico-pathological indicators and examined with multivariate statistical procedures and modeling.

RESULTS

P53, c-erbB2, and bcl-2 gene products were detected, respectively, in 30.6%, 31.6%, and 85.9% of the examined invasive breast carcinomas, revealing variable associations with cellular differentiation and proliferation as defined by ER/PGR status, Ki-67, tumour mass and histologic and nuclear grading. A multivariate graphical display on a subset of the most informative cases revealed that bcl-2 expression parallels ER/PGR status and is of importance in separating tumour clusters with different degrees of aggressiveness.

CONCLUSIONS

The results of this study indicate that multivariate explorative analyses conducted on biological and clinico-pathological parameters might constitute an integrated approach to data analysis useful for distinguishing different biological behaviours and therapeutic groups in breast carcinoma. Our findings also suggest that bcl-2 expression may play a pivotal role in tumours lacking ER-mediated growth regulation.

Constitutive expression of the c-H-ras oncogene inhibits doxorubicin-induced apoptosis and promotes cell survival in a rhabdomyosarcoma cell line

Drugs used in anti-cancer chemotherapy are thought to exert their cytotoxic action by induction of apoptosis. Genes have been identified which can mediate or modulate this drug-induced apoptosis, among which are c-myc, p53 and bcl-2. Since expression of oncogenic ras genes is a frequent observation in human cancer, we investigated the effects of the c-H-ras oncogene on anti-cancer drug-induced apoptosis. Apoptosis induced by a 2 h doxorubicin exposure was measured by in situ nick translation and flow cytometry in a rat cell line (R2T24) stably transfected with the c-H-ras oncogene and in a control cell line (R2NEO) transfected only with the antibiotic resistance gene neo. Both cell lines (R2T24 and R2NEO) had nearly identical growth characteristics, including cell doubling time, distribution over the cell cycle phases and plating efficiency in soft agar. Doxorubicin exposure of the R2NEO cells led to massive induction of apoptosis. In contrast, R2T24 cells, expressing the c-H-ras oncogene, showed significantly less apoptosis after doxorubicin incubation. Doxorubicin induced approximately 3- to 5-fold less cytotoxicity in the R2T24 cells than in the R2NEO cells, as determined by clonogenic assay in soft agar. No difference was observed in intracellular doxorubicin accumulation between the two cell lines, indicating that the classical, P-glycoprotein-mediated multidrug resistance phenotype is not involved in the observed differences in drug sensitivity. In conclusion, our data show that constitutive expression of the c-H-ras oncogene suppresses doxorubicin-induced apoptosis and promotes cell survival, suggesting that human tumours with ras oncogene expression might be less susceptible to doxorubicin treatment.

Genetic dissection of susceptibility to radiation-induced apoptosis of thymocytes and mapping of Rapop1, a novel susceptibility gene

Genetic dissection of susceptibility to radiation-induced apoptosis of thymocytes was performed by counting dead cells in histologically processed thymuses after 0.5 Gy of whole-body X-irradiation, using recombinant congenic (CcS/Dem) strains derived from inbred mouse strains BALB/cHeA (susceptible) and STS/A (resistant). A high (8/20) number of strains with lower dead cell scores than BALB/cHeA among CcS/Dem recombinant congenic strains (RCS), which contain 12.5% of STS/A genome in the genetic background of BALB/cHeA strain, indicates that the difference between BALB/cHeA and STS/A is caused by several genes and that susceptibility probably requires BALB/cHeA alleles at more than one locus. Similar results were obtained with CXS/Hg recombinant inbred (CXS/Hg) strains. Analysis of F2 hybrids between BALB/cHeA and CcS-7, one of the CcS/Dem strains that showed lower dead cell scores than BALB/cHeA, demonstrated that a novel gene (Rapop1, radiation-induced apoptosis 1) controlling susceptibility to radiation-induced apoptosis in the thymus is located in the proximal region of mouse chromosome 16.

Inhibition by differentiation-inducing agents of wild-type p53-dependent apoptosis in HL-60 cells

The product of the p53 tumor-suppressor gene has been shown to function in apoptosis and cell cycle regulation. However, there is little information regarding the regulation of apoptosis in cell differentiation. We investigated the relationship between p53-dependent apoptosis and differentiation induction using human promyelocytic leukemia HL-60 cells transfected with pMAMneo expression vectors containing dexamethasone-inducible wild-type p53 (wt-p53) cDNA inserts. Continuous exposure of the pMAMneo/wt-p53 transfectants to 1 microM dexamethasone for more than 24 h caused overexpression of wt-p53 followed by cell death with morphological changes typical of apoptosis. Using the wt-p53-inducible HL-60 cells, we examined the effects of differentiation inducers on the wt-p53-dependent apoptosis. All-trans retinoic acid (all-trans RA) at 1 nM or granulocyte macrophage colony-stimulating factor (GM-CSF) at 35 pM inhibited the wt-p53-induced apoptosis over a 42-h treatment. The apoptosis inhibition by GM-CSF, but not all-trans RA, was abolished by specific inhibitors of protein kinase C. These results suggest that extracellular signals involved in the differentiation induction could modulate the wt-p53-dependent apoptosis through protein kinase C-dependent and independent pathways.

Safety evaluation of Ad5CMV-p53 in vitro and in vivo

In preparation for a clinical trial of the recombinant p53 adenovirus Ad5CMV-p53 for the treatment of lung cancer, the potential adverse effects of Ad5CMV-p53 were assessed in vitro and in vivo. No infectious replication of Ad5CMV-p53 was detectable in HeLa cells infected with extracts from HeLa cells previously infected with Ad5CMV-p53. No Ad5CMV-p53 DNA replication was detected by 32Pi labeling in lung cancer cells infected with Ad5CMV-p53 at multiplicities of infection (moi) up to 1,000 pfu/cell (total of 5 x 10(9) pfu viruses). The infectivity and cytotoxicity of Ad5CMV-p53 were examined in vitro in normal human bronchial epithelial (NHBE) cells. At a moi of 50 pfu/cell, Ad5CMV-p53 infection and expression were detectable in 80% of the treated cells. The exogenous p53 protein was first detected by western blotting at 8 hr and peaked at 48 hr after infection. Growth of NHBE cells was not affected by Ad5CMV-p53 infection at a moi of 100 pfu/cell. The pathogenicity of Ad5CMV-p53 was assessed in BALB/c mice. The virus was given to four groups of mice by intratracheal injection at dosages from 10(7) to 10(10) pfu; a fifth group received phosphate-buffered saline alone. None of the viral injections proved to be lethal. Mild to moderate peribronchiolar and perivascular infiltration by mononuclear cells and lymphocytes, with patches of pneumonitis, was the most acute toxic effect detected by histologic analysis in the two high-dose groups. Immunohistochemical analysis of the same paraffin-embedded sections showed that infectivity and level of expression of p53 in lung tissue were dose-dependent. Our results demonstrate that Ad5CMV-p53 is a replication-defective virus that yields a relatively low degree of acute toxicity in mice; these data document a safety profile encouraging for clinical trials of Ad5CMV-p53 in the therapy of lung cancer.

A potential role for apoptosis in neurodegeneration and Alzheimer's disease

Previous studies have shown that beta-amyloid (A beta) peptides are neurotoxic. Recent data suggest that neurons undergoing A beta-induced cell death exhibit characteristics that correspond to the classical features of apoptosis, suggesting that these cells may initiate a program of cell death. This chapter explores the criteria and precautions that must be applied to evaluate mechanisms of cell death in vitro and in vivo, discusses the evidence supporting an apoptotic mechanism of cell death in response to A beta in cultured neurons, and describes potential correlations for these findings in the Alzheimer's disease brain. In addition, cellular signaling pathways that may be associated with apoptosis in response to A beta are examined, and support for apoptosis as a mechanism of cell death for other neurodegeneration-inducing stimuli (e.g., oxidative injury) is described. The connection of multiple stimuli that induce neuronal cell death to an apoptotic mechanism suggests that apoptosis could play a central role in neurodegeneration in the brain.

Nucleotide sequence of exons 5 to 9 of the p53 tumour-suppressor gene of the donkey (Equus asinus)

The evolutionary conserved region of the equine homologue of the p53 gene from the donkey genome was PCR amplified and cloned. The 1380 bp fragment consisted of exons 5 to 9 and the intervening introns. The exonic and intronic DNA sequences showed a variable but high level of homology with previously published human sequences. The aminoacid sequences corresponding to the evolutionary conserved domains II, III, and V were identical to the human regions, whilst domain IV was 96% homologous.

Mutation and expression of TP53 in malignant melanomas

Mutations of the TP53 gene are the most common genetic alterations in human malignancies. Overexpression of the p53 protein has been reported in high frequencies in all types of skin cancer. To determine the role of TP53 in the pathogenesis of malignant melanoma, we investigated the expression of p53 in 12 cell lines and 145 primary and metastatic lesions by immunohistochemistry. Overexpression of p53 was predominantly detected in the cytoplasm of the cells in 96 (66%) tumor and 12 (93%) cell lines. In contrast to findings in other tumor types, in melanomas immunoreactive cells were found in clusters or as scattered single cells. In primary melanomas, the frequency of p53 overexpression did not correlate with tumor thickness. Nucleotide sequencing of TP53 genes of 24 melanoma tumors/cell lines demonstrated point mutations in seven samples, all coding for mutant p53 protein species. The frequency of TP53 alterations of 20%-30% is lower than in other skin tumor types. Notably, immunohistochemistry was not a suitable method to distinguish overexpression of wild-type p53 from mutant species, since cell lines/tumors with TP53 mutations did not show distinctive staining patterns. The mutation pattern in six out of seven lesions was similar to that caused by ultraviolet light damage. This finding may be regarded a further indication for a pathogenetic role of UV light damage in at least a subgroup of malignant melanomas.

Regulation of transcription functions of the p53 tumor suppressor by the mdm-2 oncogene

BACKGROUND

Mdm-2, a zinc finger protein, negatively regulates the p53 tumor suppressor gene product by binding to it and preventing transcriptional activation (16).

MATERIALS AND METHODS

Assays for p53 mediated transcription, repression and activation by mutant and wild-type p53 proteins were used to measure the ability of mdm-2 to block each activity.

RESULTS

Mdm-2 was able to inhibit all three functions of the wild-type and mutant p53 activities; transcriptional activation by the wild-type protein, transcriptional activation by the mutant p53 protein, and repression by the wild-type protein.

CONCLUSIONS

The mdm protein binds to the amino terminal portion of the p53 protein and, in so doing, blocks the ability of p53 to interact with the transcriptional machinery of the cell (23). The mdm-2 protein binds to both leucine-tryptophan residues at amino acids 22 and 23, from the amino terminal end of the protein, and in so doing, prevents all p53 functions. The ability of a mutant p53 protein to transactivate a multidrug resistance-1 gene promoter is blocked by mdm-2 and the ability of the wild-type p53 protein to repress transcription of some genes is also blocked by the mdm-2 protein. Thus, all three functions of the p53 protein-transcriptional activation, repression and mutant protein activation-require the p53 amino terminal domain functions and are regulated by the mdm-2 protein in a cell. When mdm-2 is overproduced, resulting in a tumor or transformation of a cell, all of the p53 activities are inactivated.

Nucleotide sequence of the ovine P53 tumor-suppressor cDNA and its genomic organization

A 2155 bp cDNA clone corresponding to the ovine p53 tumor suppressor gene has been isolated from a cDNA library made from the BLV-transformed YR2 cell line RNA. After the sequencing analysis, it appeared that this clone contains the entire p53 coding region (including 126 bp upstream of the ATG initiation codon). The nucleotide sequence shows a high degree of homology with the human (73%), murine (74%) and rat (66%) cDNAs. The encoded ovine p53 protein is 382 amino acids long with an apparent molecular weight of 50 kDa and shares 79% and 72% amino acid homology with the human and the mouse p53 respectively. Furthermore, the homology is not equally distributed along the molecule but is mainly located within five highly conserved regions. As its mouse and human counterparts, the ovine p53 contains a high proportion of proline residues, an acidic N-terminal domain and a basic C-terminal domain. We also report the structure of the ovine p53 gene that is similar to those already defined from other species.

Alternatively spliced forms in the carboxy-terminal domain of the p53 protein regulate its ability to promote annealing of complementary single strands of nucleic acids

The carboxy-terminal domain of the p53 protein comprising amino acid residues 311 to 393 is able to promote the reassociation of single-stranded RNA or DNA into duplex hybrids. This domain is as efficient as the intact p53 protein in both the rate and the extent of the double-stranded product produced in this reaction. Both wild-type and mutant p53 proteins from cancerous cells carry out this reaction. The monoclonal antibody PAb421, which detects an epitope between residues 370 and 378, blocks the ability of p53 to reassociate single strands of RNA or DNA. Similarly, the alternative splice form of the murine p53 protein, which removes amino acid residues 364 to 390 and replaces them with 17 new amino acids, does not carry out the reassociation reaction with RNA or DNA. This is the first indication of functionally distinct properties of the alternative splice forms of p53. These results suggest that this splice alternative can regulate a p53-mediated reaction that may be related to the functions of this protein.

Two domains of p53 interact with the TATA-binding protein, and the adenovirus 13S E1A protein disrupts the association, relieving p53-mediated transcriptional repression

The tumor suppressor gene product p53 can activate and repress transcription. Both transcriptional activation and repression are thought to involve the direct interaction of p53 with the basal transcriptional machinery. Previous work has demonstrated an in vitro interaction between p53 and the TATA-binding protein that requires amino acids 20 to 57 of p53 and amino acids 220 to 271 of the TATA-binding protein. The present results show that a 75-amino-acid segment from the carboxy terminus of p53 also can bind to the TATA-binding protein in vitro, and this interaction requires amino acids 217 to 268 of the TATA-binding protein, essentially the same domain that is required for interaction with the amino-terminal domain of p53. A carboxy-terminal segment of p53 can mediate repression when bound to DNA as a GAL4-p53 fusion protein. The amino- and carboxy-terminal p53 interactions occur within the domain on the TATA-binding protein to which the adenovirus 13S E1A oncoprotein has previously been shown to bind. The 13S E1A oncoprotein can dissociate the complex formed between the carboxy-terminal domain of p53 and the TATA-binding protein and relieve p53-mediated transcriptional repression. These results demonstrate that two independent domains of p53 can potentially interact with the TATA-binding protein, and they define a mechanism--relief of repression--by which the 13S E1A oncoprotein can activate transcription through the TATA motif.

Detection of apoptosis and expression of apoptosis-related proteins during human intrahepatic bile duct development

We investigated apoptosis by nick end labeling and the expression of apoptosis-related proteins by immunohistochemistry in fetal development of human intrahepatic bile ducts and hepatocytes. During intrahepatic bile duct development, apoptosis was present at all stages, and its positive ratio was high in the remodeling ductal plate, moderate in the ductal plate, and relatively low in remodeled ducts. The cell proliferative activity as determined by proliferating cell nuclear antigen was also high in the remodeling ductal plate, and relatively low in the ductal plate and remodeled ducts. fas antigen and c-myc protein were constantly positive in the ductal plate, remodeling ductal plate and remodeled ducts. Bcl-2 protein was negative or faintly positive in the ductal plate and remodeling ductal plate, but was apparently positive in remodeled ducts. Lewisy as detected by the BM-1 antibody was present in the ductal plate, remodeling ductal plate, and remodeled ducts. p53 protein was not found in any cell types in the liver development. During hepatocyte development, many apoptotic and proliferating cell nuclear antigen-positive hepatocytes were noted. The developing hepatocytes expressed c-myc protein and fas antigen. Bcl-2 protein and Lewisy antigen were also weakly positive in the developing hepatocytes. These findings showed that balanced cell proliferation and apoptosis are involved in the normal development of intrahepatic bile ducts and hepatocytes, and suggest that c-myc protein, fas antigen, Bcl-2 protein, and Lewisy antigen modulate apoptosis of fetal intrahepatic biliary cells and hepatocytes, probably by stimulative (c-myc protein and fas and Lewisy antigens) or inhibitory (Bcl-2 protein) effects.

Tumor suppressor p53 mutations and breast cancer: a critical analysis

Alterations in the tumor suppressor gene p53 are the most commonly identified changes in cancer, including neoplasia of the breast. The activity of p53 is regulated post-translationally. Phosphorylation state, subcellular localization, and interaction with any of a number of cellular proteins are likely to influence the function of p53. The exact effect of p53-mediated growth suppression seems to be cell-type specific but appears to be directly related to the ability of p53 to act as a specific transcriptional activator. The role that transcriptional repression plays in the function of WT p53 is less clear. It is also possible that p53 has a more direct activity in DNA replication and repair. Most documented p53 mutations result in single amino acid substitutions which may confer one or more of a spectrum of transforming abilities on the protein. Mutation may lead to nuclear accumulation of p53 protein; however, inactivation of p53 by nuclear exclusion and interaction with the mdm2 protein also appear to be important in tumorigenesis. Used in conjunction with other established factors, accumulation of cellular p53 may be a useful prognostic indicator in breast cancer. A syngeneic mouse model system yielded evidence that p53 mutations are important in the early, preneoplastic stages of mammary tumorigenesis. This murine system may provide the ability to investigate the functions of p53 in the early stages of breast cancer which are technically difficult to examine in the human system.

Nucleotide sequence of the bovine P53 tumor-suppressor cDNA

The bovine p53 open reading frame was cloned and characterized using a reverse transcription-polymerase chain reaction amplification (RT-PCR) method. After sequencing, it appeared that this cDNA is able to code for a 386 amino acids protein with high degree of homology with the ovine p53. Furthermore, the homology is not equally distributed along the molecule since the highest divergence is located within the exon 4. The bovine p53 shares 93% and 91% homology with the ovine p53 at the DNA and protein level respectively.

Probing the molecular program of apoptosis by cancer chemopreventive agents

This paper provides a rational molecular basis for studies intended to clarify the interactions between cancer chemopreventive agents and apoptosis, one of the natural forms of cell death that overlaps molecular mechanisms with other forms such as programmed cell death and specialized forms of physiological cell death. Molecular details of the process show the existence of distinct molecular pathways leading to the activation of critical effector elements (apoptosis gene products) functioning under the control of a network of negative regulatory elements. Dysregulation of either apoptosis or anti-apoptosis genes has a significant role in multistage carcinogenesis. Inhibition of apoptosis is one of the underlying mechanisms of the action of tumor promoters. The network of apoptosis and anti-apoptosis gene products provides multiple targets for compounds with cancer chemopreventive potential. Many data in the literature show initiating, potentiating or inhibitory effects of such compounds on apoptosis. However, the molecular mechanism of these effects is largely unknown. We initiated a series of studies using mouse thymocytes which undergo apoptosis through distinct molecular mechanisms after T-cell receptor activation (TCR pathway), following the addition of glucocorticoids (DEX pathway) or DNA damaging agents (p53 pathway). All trans-and 9-cis-retinoic acid induced apoptosis, elicited through the DEX pathway, inhibited the TCR pathway, and did not affect p53- initiated apoptosis. N-acetylcysteine can inhibit all forms. Sodium salicylate enhanced spontaneous cell death, decreased p53-dependent apoptosis, and did not affect the DEX and TCR pathways. These preliminary results, which show differential effects of the studied compounds on distinct molecular pathways of apoptosis, warrant further investigations in the effort to utilize the molecular elements of apoptosis in proper cancer chemoprevention, and find biochemical targets for apoptosis-related surrogate endpoint biomarker assays of chemoprevention.

Programmed cell death in development

Although cell death has long been recognized to be a significant element in the process of embryonic morphogenesis, its relationships to differentiation and its mechanisms are only now becoming apparent. This new appreciation has come about not only through advances in the understanding of cell death in parallel immunological and pathological situations, but also through progress in developmental genetics which has revealed the roles played by death in the cell lineages of invertebrate embryos. In this review, we discuss programmed cell death as it is understood in developmental situations, and its relationship to apoptosis. We describe the morphological and biochemical features of apoptosis, and some methods for its detection in tissues. The occurrence of programmed cell death during invertebrate development is reviewed, as well as selected examples in vertebrate development. In particular, we discuss cell death in the early vertebrate embryo, in limb development, and in the nervous system.

Deregulated expression of E2F-1 induces S-phase entry and leads to apoptosis

E2F-1, the first gene product identified among a family of E2F transcription factors, is thought to play a critical role in G1/S progression of the cell cycle. Transcriptional activities of E2F are modulated during the cell cycle, mainly by the formation of complexes between E2F and several key regulators of cell cycle such as the retinoblastoma protein and related proteins. To further understand the roles of E2F in the cell cycle progression, we have overexpressed exogenous E2F-1 by using a tetracycline-controlled expression system. We have found that the induced expression of E2F-1 in Rat-2 fibroblasts promotes S-phase entry and subsequently leads to apoptosis. The apoptosis occurs in an E2F-1 dose-dependent manner. Cells resistant to the induction of apoptosis have lost the ability to express exogenous E2F-1. Cells growing in low serum are more sensitive to the E2F-1-mediated cell death. Overexpression of E2F-1 mutants that impair DNA binding or transactivation does not alter cell cycle progression or induce apoptosis. These results define a novel pathway to apoptosis and demonstrate that premature S-phase entry is associated with apoptotic cell death.

Transactivation of the human p53 tumor suppressor gene by c-Myc/Max contributes to elevated mutant p53 expression in some tumors

Elevated levels of mutant forms of the p53 tumor suppressor are a hallmark of many transformed cells. Multiple mechanisms such as increased stability of the protein and increased transcription of the gene can account for elevated p53 expression. Recent findings indicate that c-Myc/Max heterodimers can bind to an essential CA(C/T)GTG-containing site in the p53 promoter and elevate its expression. We have addressed the possibility that elevated mutant p53 expression is due to deregulated c-Myc expression. Here we demonstrate that the human p53 promoter is transactivated by high c-Myc expression and repressed by high Max expression. In examining the relative levels of c-Myc and p53 in human Burkitt's lymphomas and other B-lymphoid lines, we found that there is a correlation between the levels of c-Myc protein and p53 mRNA expression. In particular, cells that express very low levels of c-Myc protein also express low levels of p53 mRNA, while cells that express high levels of c-Myc tend to express high levels of p53 mRNA. To determine whether the p53 gene can be a target for c-Myc in vivo, we assayed the effects of antisense c-myc RNA on the levels of endogenous p53 mRNA. The results indicate that the presence of antisense c-myc RNA leads to a reduction in the levels of c-Myc protein, p53 mRNA, and expression from the p53 promoter. Taken together, our findings support a direct role for c-Myc in elevating expression of the mutant p53 gene in some tumors.

Altered gene expression in neurons during programmed cell death: identification of c-jun as necessary for neuronal apoptosis

We have examined the hypothesis that neuronal programmed cell death requires a genetic program; we used a model wherein rat sympathetic neurons maintained in vitro are deprived of NGF and subsequently undergo apoptosis. To evaluate gene expression potentially necessary for this process, we used a PCR-based technique and in situ hybridization; patterns of general gene repression and selective gene induction were identified in NGF-deprived neurons. A temporal cascade of induced genes included "immediate early genes," which were remarkable in that their induction occurred hours after the initial stimulus of NGF removal and the synthesis of some required ongoing protein synthesis. The cascade also included the cell cycle gene c-myb and the genes encoding the extracellular matrix proteases transin and collagenase. Concurrent in situ hybridization and nuclear staining revealed that while c-jun was induced in most neurons, c-fos induction was restricted to neurons undergoing chromatin condensation, a hallmark of apoptosis. To evaluate the functional role of the proteins encoded by these genes, neutralizing antibodies were injected into neurons. Antibodies specific for either c-Jun or the Fos family (c-Fos, Fos B, Fra-1, and Fra-2) protected NGF-deprived neurons from apoptosis, whereas antibodies specific for Jun B, Jun D, or three nonimmune antibody preparations had no protective effect. Because these induced genes encode proteins ranging from a transcription factor necessary for death to proteases likely involved in tissue remodeling concurrent with death, these data may outline a genetic program responsible for neuronal programmed cell death.

DNA recombination induced by aflatoxin B1 activated by cytochrome P450 1A enzymes

Mutations in tumor suppressor genes are intricately associated with the etiology of neoplasia. Often, such mutations are followed by the loss of the second, functional alleles of tumor suppressor genes, a phenomenon known as loss of heterozygosity. Loss of heterozygosity may occur by different molecular mechanisms, including mitotic recombination, and it is conceivable that these molecular events are influenced by endogenous as well as exogenous factors. To test whether mitotic recombination is induced by certain carcinogens, we genetically engineered a Saccharomyces cerevisiae tester strain so that it metabolizes two important classes of carcinogens, polycyclic aromatic hydrocarbons and heterocyclic arylamines. This was accomplished by expressing human cDNA's coding for the cytochrome P450 (CYP) enzymes CYP1A1 or CYP1A2 in combination with NADPH-CYP oxidoreductase in a strain heterozygous for two mutations in the trp5 gene. Microsomes isolated from the transformed yeast strains activated various xenobiotics to powerful mutagens that were detected in the Ames test. Of these, the mycotoxin aflatoxin B1, when activated intracellularly in the strains containing either human CYP enzyme, significantly induced mitotic recombination. These results are discussed in light of possible mechanisms that are involved in aflatoxin B1-mediated hepatocarcinogenesis. Similarly, benzo[a]pyrene-trans-7,8-dihydrodiol and 3-amino-1-methyl-5H-pyrido[4,3-b]indole were activated to recombinagenic products, whereas benzo[a]pyrene and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline were negative in this assay. Our results argue that the constructed yeast strains may be a valuable tool for the investigation of drug-induced mitotic recombination.

Deregulated expression of E2F-1 induces S-phase entry and leads to apoptosis

E2F-1, the first gene product identified among a family of E2F transcription factors, is thought to play a critical role in G1/S progression of the cell cycle. Transcriptional activities of E2F are modulated during the cell cycle, mainly by the formation of complexes between E2F and several key regulators of cell cycle such as the retinoblastoma protein and related proteins. To further understand the roles of E2F in the cell cycle progression, we have overexpressed exogenous E2F-1 by using a tetracycline-controlled expression system. We have found that the induced expression of E2F-1 in Rat-2 fibroblasts promotes S-phase entry and subsequently leads to apoptosis. The apoptosis occurs in an E2F-1 dose-dependent manner. Cells resistant to the induction of apoptosis have lost the ability to express exogenous E2F-1. Cells growing in low serum are more sensitive to the E2F-1-mediated cell death. Overexpression of E2F-1 mutants that impair DNA binding or transactivation does not alter cell cycle progression or induce apoptosis. These results define a novel pathway to apoptosis and demonstrate that premature S-phase entry is associated with apoptotic cell death.

Transactivation of the human p53 tumor suppressor gene by c-Myc/Max contributes to elevated mutant p53 expression in some tumors

Elevated levels of mutant forms of the p53 tumor suppressor are a hallmark of many transformed cells. Multiple mechanisms such as increased stability of the protein and increased transcription of the gene can account for elevated p53 expression. Recent findings indicate that c-Myc/Max heterodimers can bind to an essential CA(C/T)GTG-containing site in the p53 promoter and elevate its expression. We have addressed the possibility that elevated mutant p53 expression is due to deregulated c-Myc expression. Here we demonstrate that the human p53 promoter is transactivated by high c-Myc expression and repressed by high Max expression. In examining the relative levels of c-Myc and p53 in human Burkitt's lymphomas and other B-lymphoid lines, we found that there is a correlation between the levels of c-Myc protein and p53 mRNA expression. In particular, cells that express very low levels of c-Myc protein also express low levels of p53 mRNA, while cells that express high levels of c-Myc tend to express high levels of p53 mRNA. To determine whether the p53 gene can be a target for c-Myc in vivo, we assayed the effects of antisense c-myc RNA on the levels of endogenous p53 mRNA. The results indicate that the presence of antisense c-myc RNA leads to a reduction in the levels of c-Myc protein, p53 mRNA, and expression from the p53 promoter. Taken together, our findings support a direct role for c-Myc in elevating expression of the mutant p53 gene in some tumors.

Myc-mediated apoptosis requires wild-type p53 in a manner independent of cell cycle arrest and the ability of p53 to induce p21waf1/cip1

Deregulated expression of the c-myc proto-oncogene can lead to apoptosis under certain physiological conditions. By introducing a conditionally active Myc allele into primary embryo fibroblasts null for p53, and into fibroblasts without endogenous p53 expression but ectopically expressing a temperature-sensitive p53 allele, we show that expression of wild-type p53 is required for susceptibility to Myc-mediated apoptosis. Although ectopic expression of wild-type p53 blocked cells in the G1 phase of the cell cycle, G1 arrest by isoleucine starvation, in a manner independent of p53, did not confer susceptibility to apoptosis. Thus, growth arrest per se is not sufficient to induce Myc-mediated apoptosis; instead, a property intrinsic to p53 is specifically required. Moreover, apoptosis did not require induction of p53 target proteins, including the cyclin-dependent kinase inhibitor p21waf1/cip1. Therefore, the role of p53 in apoptosis may be distinct from its role in cell cycle arrest.

Deregulated transcription factor E2F-1 expression leads to S-phase entry and p53-mediated apoptosis

E2F-1 is a transcription factor suspected of activating genes required for S phase and a known target for the action of RB, the retinoblastoma gene product. Its induction in quiescent fibroblasts led to S-phase entry followed by apoptosis. E2F-1-mediated apoptosis was suppressed by coexpression of wild-type RB or a transdominant negative mutant species of p53. In contrast, coexpression of a naturally occurring loss-of-function RB mutant or wild-type p53 did not suppress the induction of apoptosis under these conditions. Thus, deregulated E2F-1 activity gives rise to proliferative and apoptotic signals. p53 appears to participate in the execution of the latter.

Gene regulation by temperature-sensitive p53 mutants: identification of p53 response genes

The ability of the p53 protein to act as a sequence-specific transcriptional activator suggests that genes induced by p53 may encode critical mediators of p53 tumor suppression. Using a tetracycline-regulated p53 expression system and cDNA library subtraction procedure, we identified several p53-induced gene transcripts in human Saos-2 osteosarcoma cells that are novel on the basis of their size, regulation, and low abundance. Wild-type p53-dependent induction of these transcripts was observed in cells that are growth arrested by p53, as well as in cells that undergo apoptosis upon expression of an inducible wild-type p53 transgene. These results show that p53 activates the expression of numerous response genes and suggest that multiple effectors may play a role in mediating cellular functions of p53.

Aberrant accumulation of p53 associates with Ki67 and mitotic count in benign skin lesions

Sixty-two skin samples from patients with a variety of benign disorders (20 cases of psoriasis, 14 cases of chronic dermatitis, 11 seborrhoeic keratoses, 11 cases of lichen planus), and seven normal skin samples, were stained immunohistochemically with a polyclonal antibody (CM-1) to p53, and a monoclonal antibody to Ki67, using the avidin-biotin complex method. p53-positive keratinocytes could be found in most of these lesions. The percentage of p53-positive cells was, however, far lower than usually seen in p53-positive malignant tumours. No p53 reactivity was observed in the normal skin samples. Variable Ki67 reactivity was observed in all skin samples. Overall, the number of Ki67-positive cells was higher in skin samples in which the proportion of p53-positive cells was high (> 0.5% of total epidermal cell population) (P = 0.004). This also applied separately to psoriatic and non-psoriatic lesions (P = 0.028 and P = 0.033, respectively). In cases with > 10% of Ki67-positive cells, there were significantly more mitoses (P < 0.001). This association applied to both psoriasis and the other lesions studied (P = 0.024 and P < 0.001, respectively). The results show that immunohistochemically detectable accumulation of p53 is a frequent finding in non-neoplastic skin lesions. As p53 positivity was associated with the proliferation marker Ki67, the accumulation of p53 is possibly a response to an increased proliferation rate of the keratinocytes in these skin diseases, or alternatively it may be associated with apoptosis.

Functional complementation of the adenovirus E1B 19-kilodalton protein with Bcl-2 in the inhibition of apoptosis in infected cells

Expression of the adenovirus E1A oncogene induces apoptosis which impedes both the transformation of primary rodent cells and productive adenovirus infection of human cells. Coexpression of E1A with the E1B 19,000-molecular-weight protein (19K protein) or the Bcl-2 protein, both of which have antiapoptotic activity, is necessary for efficient transformation. Induction of apoptosis by E1A in rodent cells is mediated by the p53 tumor suppressor gene, and both the E1B 19K protein and the Bcl-2 protein can overcome this p53-dependent apoptosis. The functional similarity between Bcl-2 and the E1B 19K protein suggested that they may act by similar mechanisms and that Bcl-2 may complement the requirement for E1B 19K expression during productive infection. Infection of human HeLa cells with E1B 19K loss-of-function mutant adenovirus produces apoptosis characterized by enhanced cytopathic effects (cyt phenotype) and degradation of host cell chromosomal DNA and viral DNA (deg phenotype). Failure to inhibit apoptosis results in premature host cell death, which impairs virus yield. HeLa cells express extremely low levels of p53 because of expression of human papillomavirus E6 protein. Levels of p53 were substantially increased by E1A expression during adenovirus infection. Therefore, E1A may induce apoptosis by overriding the E6-induced degradation of p53 and promoting p53 accumulation. Stable Bcl-2 overexpression in HeLa cells infected with the E1B 19K- mutant adenovirus blocked the induction of the cyt and deg phenotypes. Expression of Bcl-2 in HeLa cells also conferred resistance to apoptosis mediated by tumor necrosis factor alpha and Fas antigen, which is also an established function of the E1B 19K protein. A comparison of the amino acid sequences of Bcl-2 family members and that of the E1B 19K protein indicated that there was limited amino acid sequence homology between the central conserved domains of E1B 19K and Bcl-2. This domain of the E1B 19K protein is important in transformation and regulation of apoptosis, as determined by mutational analysis. The limited sequence homology and functional equivalency provided further evidence that the Bcl-2 and E1B 19K proteins may possess related mechanisms of action and that the E1B 19K protein may be the adenovirus equivalent of the cellular Bcl-2 protein.

Loss of APC protein expressed by human colonic epithelial cells and the appearance of a specific low-molecular-weight form is associated with apoptosis in vitro

APC (adenomatous polyposis coli) protein is differentially expressed in the normal colonic crypt and believed to be involved in colonic cell maturation. In this work we investigated whether expression of the APC protein is associated with cell death in colonic epithelial cells. We have previously reported an in vitro system to study apoptosis. Briefly, cells attached to the flask have a low frequency of apoptosis (1-3%), whereas cells that detach from the flask and float in the medium have a high proportion of apoptotic cells (36-96% depending on the cell line). The full-length 300-kDa or truncated APC protein, normally expressed by the attached cells (detected using the FE9 antibody), was found to be lost in the floating apoptotic cells in 8/11 colon tumour cell lines examined. In addition, the APC antibody FE9 detected a 90-kDa protein in the floating apoptotic cells of all cell lines investigated, which was not present in attached cells. Furthermore, loss of full-length APC and gain of the 90-kDa protein was observed in the apoptotic cells of 2 cell lines derived from other tissues: the SV40-transformed fibroblast cell line CMSV40fib and the lymphoblastoid B-cell line BJA-B. In cells repeatedly frozen and thawed, believed to induce necrotic cell death, full-length or truncated APC was also lost, though a 95-kDa protein distinct from that in apoptotic cells was observed. Specific loss of full-length or truncated APC (resulting in a 90-kDa protein in apoptotic cells but a 95-kDa protein in necrotic cells) is therefore associated with cell death. Our findings suggest a possible role for APC in cell survival.

Immunohistochemical detection of p53 protein in human prostatic cancer

Tissue sections from 73 radical-prostatectomy specimens were studied immunohistochemically for the presence of p53 protein. In seven specimens numerous tumor cells showed a strong nuclear immunostaining. An additional 27 revealed a more discrete and focal accumulation of p53 protein. Comparison of the pathologic characteristics of the p53-negative and -positive groups showed that the presence of p53 protein closely correlated with more advanced tumor stages (p < 0.00001), with higher primary (p = 0.0004), combined (p < 0.0001) and worst (p < 0.0001) Gleason grades, and with larger total (p = 0.0001) and high-grade (p < 0.0001) tumor volumes. No staining was found in areas of benign hyperplasia or in well-differentiated tumor zones. Our results suggest that the accumulation of p53 protein to immunohistochemically detectable concentrations is not a feature of low-grade cancer. This finding implies that abnormal p53 accumulation might be involved in the process of prostatic cancer progression.

Relief of p53-mediated transcriptional repression by the adenovirus E1B 19-kDa protein or the cellular Bcl-2 protein

The p53 tumor suppressor gene product is a transcriptional regulatory protein. It activates transcription from promoters that contain a p53 DNA binding site but represses many promoters that lack its binding site. High-level expression of wild-type p53 can induce apoptosis in certain cell types, and this activity can be blocked by the adenovirus E1B 19-kDa oncoprotein or by the cellular Bcl-2 oncoprotein. Here we report that p53-mediated repression of promoters that lack a p53 binding site is abrogated by the E1B 19-kDa protein or Bcl-2 oncoprotein. In contrast, transcriptional activation by p53 still occurs in the presence of either protein. The fact that two oncoproteins capable of preventing p53-mediated apoptosis also block transcriptional repression by p53 raises the possibility that p53 might induce apoptosis, at least in part, by repressing transcription.

Expression of Fas antigen on keratinocytes in vivo and induction of apoptosis in cultured keratinocytes

Fas antigen, which belongs to a nerve growth factor/tumor necrosis factor receptor superfamily, is a membrane protein that induces apoptosis. In humans, distribution of Fas antigen has been reported on cell lines and lymphocytes. Immunohistochemical studies revealed Fas antigen on the keratinocytes of lesional epidermis in lichenoid drug eruption, erythema multiforme, contact dermatitis, bullous pemphigoid, pemphigus vulgaris, and herpes zoster; it is co-expressed with intercellular adhesion molecule-1. Cultured keratinocytes expressing Fas antigen increased from 8.4% to 34.6% after stimulation with interferon gamma for 24 h. Treatment of interferon-gamma-stimulated keratinocytes with anti-Fas for 48 h resulted in DNA fragmentation and death of 32% of cells, suggesting that Fas antigen may mediate apoptosis. The expression of Fas antigen on keratinocytes in lesional skin suggests that death via Fas antigen may play an important role in the pathogenesis of keratinocyte cytotoxicity.

p53-dependent apoptosis produced by Rb-deficiency in the developing mouse lens

The retinoblastoma tumour-suppressor gene (RB) has been implicated in negative growth regulation, induction of differentiation, and inhibition of cellular transformation. Homozygous inactivation of the Rb gene in the mouse leads to mid-gestational lethality with defects in erythropoiesis and neurogenesis. Here we describe the effects of the Rb-deficient state on the development of the ocular lens. The regional compartmentalization of growth, differentiation and apoptosis in the developing lens provides an ideal system to examine more closely the relationships of these processes in vivo. We demonstrate that loss of Rb function is associated with unchecked proliferation, impaired expression of differentiation markers, and inappropriate apoptosis in lens fibre cells. In addition, we show that ectopic apoptosis in Rb-deficient lenses is dependent on p53, because embryos doubly null for Rb and p53 show a nearly complete suppression of this effect. This developmental system provides a framework for understanding the consequences of the frequent mutation of both RB and p53 in human cancer.

Development and characterization of recombinant adenoviruses encoding human p53 for gene therapy of cancer

We have constructed recombinant human adenoviruses that express wild-type human p53 under the control of either the Ad 2 major late promoter (MLP) or the human cytomegalovirus (CMV) immediate early gene promoter. Each construct replaces the Ad 5 E1a and E1b coding sequences necessary for viral replication with the p53 cDNA and MLP or CMV promoter. These p53/Ad recombinants are able to express p53 protein in a dose-dependent manner in infected human cancer cells. Tumor suppressor activity of the expressed p53 protein was assayed by several methods. [3H]Thymidine incorporation assays showed that the recombinant adenoviruses were capable of inhibiting DNA synthesis in a p53-specific, dose-dependent fashion. Ex vivo treatment of Saos-2 tumor cells, followed by injection of the treated cells into nude mice, led to complete tumor suppression using the MLP/p53 recombinant. Following a single injection of CMV/p53 recombinant adenovirus into the peritumoral space surrounding an in vivo established tumor derived from a human small cell lung carcinoma cell line (NIH-H69), we were able to detect p53 mRNA in the tumors at 2 and 7 days post-injection. Continued treatment of established H69 tumors with MLP/p53 recombinant led to reduced tumor growth and increased survival time compared to control treated animals. These results indicate that recombinant adenoviruses expressing wild-type p53 may be useful vectors for gene therapy of human cancer.

p53-dependent apoptosis suppresses tumor growth and progression in vivo

To determine the contribution of p53 loss to tumor progression, we have induced abnormal proliferation in the brain choroid plexus epithelium of transgenic mice using a SV40 T antigen fragment that perturbs pRB family function but does not affect p53 function. Tumors induced by this mutant develop slowly compared with those induced by wild-type T antigen. Suppressed tumor growth is directly attributable to p53 function, since rapid tumor development occurs when the T antigen fragment is expressed in p53-null mice. In p53-heterozygous mice, stochastic loss of the wild-type p53 allele results in the focal emergence of aggressive tumor nodules characteristic of tumor progression. In each case, aggressive tumor development in the absence of p53 function corresponds to a decrease in the level of apoptosis. These results provide in vivo evidence that p53-dependent apoptosis, occurring in response to oncogenic events, is a critical regulator of tumorigenesis.

p53 induction is associated with neuronal damage in the central nervous system

The p53 tumor-suppressor gene encodes a growth-regulatory protein that has been implicated in programmed cell death. To investigate the possible role of p53 in neuronal death, we studied p53 expression associated with excitotoxicity in the adult rat brain. Within hours of systemic administration of the glutamate analogue kainic acid, p53 mRNA levels were increased in neurons exhibiting morphological features of damage within kainate-vulnerable brain regions. A similar distribution was found for neurons exhibiting DNA damage as evidenced by in situ end-labeling of fragmented DNA. Pretreatment with the protein synthesis inhibitor cycloheximide prevented both kainate-mediated p53 induction and neuronal damage. The distinctive pattern of excitotoxin-mediated p53 expression suggests that p53 induction is a marker of irreversible injury in postmitotic cells of the central nervous system and could have functional significance in determining selective neuronal vulnerability.