p53 levels, functional domains, and DNA damage determine the extent of the apoptotic response of tumor cells

Identification of a novel p53 functional domain that is necessary for mediating apoptosis

The ability of p53 to induce apoptosis requires its sequence-specific DNA binding activity; however, the transactivation-deficient p53(Gln22-Ser23) can still induce apoptosis. Previously, we have shown that the region between residues 23 and 97 in p53 is necessary for such activity. In an effort to more precisely map a domain necessary for apoptosis within the N terminus, we found that deletion of the N-terminal 23 amino acids compromises, but does not abolish, p53 induction of apoptosis. Surprisingly, p53(Delta1-42), which lacks the N-terminal 42 amino acids and the previously defined activation domain, retains the ability to induce apoptosis to an even higher level than wild-type p53. A more extensive deletion, which eliminates the N-terminal 63 amino acids, renders p53 completely inert in mediating apoptosis. In addition, we found that both p53(Delta1-42) and p53(Gln22-Ser23) can activate a subset of cellular p53 targets. Furthermore, we showed that residues 53 and 54 are critical for the apoptotic and transcriptional activities of both p53(Delta1-42) and p53(Gln22-Ser23). Taken together, these data suggest that within residues 43-63 lie an apoptotic domain as well as another transcriptional activation domain. We therefore postulate that the apoptotic activity in p53(Gln22-Ser23) and p53(Delta1-42) is still transcription-dependent.

Why is p53 protein stabilized in neoplasia? Some answers but many more questions?

The p53 pathway provides a physiological system for integrating signals from diverse insults and eliciting adaptive cellular responses that include (but importantly are not restricted to) growth arrest and apoptosis. Defects in the pathway are prevalent in cancer, most notably being associated with mis-sense mutations in p53 itself. This leads to the inability of p53 to act as a transcription factor and thus to the non-occurrence of downstream events. Recent data indicate that the stability (and hence level) of p53 protein in cells is regulated by its interaction with mdm2: this results in enhanced p53 degradation by ubiquitin-mediated events. Since mdm2 is itself regulated by p53, loss of function of p53 leads to lack of mdm2 and thus to p53 protein accumulation. This provides a mechanistic explanation for the observation that p53 accumulation is associated with neoplasia. It may be that accumulation of p53 in the absence of p53 mutation can occur as a consequence of mdm2 defects, as well as being a physiological response in many situations. Another recent development is the recognition of p53 homologues (p73 alpha, p73 beta, and KET) which have many sequence and probable structural features in common with p53. It seems likely that this will reveal another layer of complexity in the control and regulation of p53 and its role in physiology and pathology.

Apoptosis caused by oxidized LDL is manganese superoxide dismutase and p53 dependent

Oxidized low density lipoprotein (oxLDL) induces apoptosis in human macrophages (Mphi), a significant feature in atherogenesis. We found that induction of apoptosis in Mphi by oxLDL, C2-ceramide, tumor necrosis factor alpha (TNF-alpha), and hydrogen peroxide (H2O2) was associated with enhanced expression of manganese superoxide dismutase (MnSOD) and p53. Treatment of cells with p53 or MnSOD antisense oligonucleotides prior to stimulation with oxLDL, C2-ceramide, TNF-alpha, or H2O2 caused an inhibition of the expression of the respective protein together with a marked reduction of apoptosis. Exposure to N-acetylcysteine before treatment with oxLDL, C2-ceramide, TNF-alpha, or H2O2 reversed a decrease in cellular glutathione concentrations as well as the enhanced production of p53 and MnSOD mRNA and protein. In apoptotic macrophages of human atherosclerotic plaques, colocalization of MnSOD and p53 immunoreactivity was found. These results indicate that in oxLDL-induced apoptosis, a concomitant induction of p53 and MnSOD is critical, and suggest that it is at least in part due to an enhancement of the sphingomyelin/ceramide pathway.

Bax involvement in p53-mediated neuronal cell death

The tumor suppressor gene p53 has been implicated in the loss of neuronal viability, but the signaling events associated with p53-mediated cell death in cortical and hippocampal neurons are not understood. Previous work has shown that adenovirus-mediated delivery of the p53 gene causes cortical and hippocampal neuronal cell death with some features typical of apoptosis. In the present study we determined whether p53-initiated changes in neuronal viability were dependent on members of the Bcl-2 family of cell death regulators. Primary cultures of cortical neurons were derived from animals containing Bax (+/+ and +/-) or those deficient in Bax (-/-). Cell damage was assessed by direct cell counting and by measurements of MTT activity. Neurons containing at least one copy of the Bax gene were damaged severely by exposure to excitotoxins or by the induction of DNA damage. In contrast, Bax-deficient neurons (-/-) exhibited significant protection from both types of injury. Bax protein expression was elevated significantly by glutamate exposure, but not by camptothecin-induced DNA damage in wild-type neurons. The glutamate-induced increase in Bax protein was dependent on the presence of the p53 gene. However, increased p53 expression, using adenovirus-mediated transduction, was not sufficient by itself to elevate Bax protein levels. These results demonstrate that Bax is required for neuronal cell death in response to some forms of cytotoxic injury and further support the key role for p53 activation in response to excitotoxic and genotoxic injury.

Sensitization of rat glioblastoma multiforme to cisplatin in vivo following restoration of wild-type p53 function

OBJECT

To study the combined potential of wild-type p53 gene transfer and administration of cisplatin for the treatment of glioblastoma multiforme, the authors used the 9L rat glioblastoma cell line, which expresses a mutant p53.

METHODS

Stable expression of wild-type p53 in 9L cells was achieved by transfection of the cells with a wild-type p53-expressing plasmid (pCEP4p53). The resultant cell line, 9LpCEP4p53, was found to be more sensitive to cisplatin treatment in vitro than control (9LpCEP4) cells. The in vitro growth rates of control cells and wild-type p53-modified cells were similar in the absence of cisplatin. Fischer 344 rats were implanted intracerebrally with 9LpCEP4p53 cells and intraperitoneally administered 4 mg/kg cisplatin weekly for 7 weeks. These animals survived significantly longer than animals that were implanted with 9LpCEP4p53 cells but were given no cisplatin treatment. In contrast, concurrent cisplatin treatment provided no benefit for animals implanted with 9LpCEP4 cells. Tumors that developed in animals that had been implanted with 9LpCEP4p53 cells and treated with cisplatin had lost expression of wild-type p53, indicating a correlation between expression of wild-type p53 and cisplatin sensitivity in vivo.

CONCLUSIONS

The findings of this study suggest that p53-based gene therapy in combination with cisplatin-based chemotherapy may be superior to single-modality treatment in dealing with glioblastoma multiforme.

Induction of DNA synthesis and apoptosis by regulated inactivation of a temperature-sensitive retinoblastoma protein

The retinoblastoma protein, pRb, controls entry into the S phase of the cell cycle and acts as a tumor suppressor in many tissues. Re-introduction of pRb into tumor cells lacking this protein results in growth arrest, due in part to transcriptional repression of genes required for S phase. Several studies suggest that pRb may also be involved in terminal cell cycle exit as a result of the instigation of senescence or differentiation programs. To understand better these multiple growth-inhibitory properties of pRb, a temperature-sensitive mutant of pRb has been produced. This tspRb induces G1 arrest and morphological changes efficiently at the permissive temperature of 32.5 degrees C, but is weakly functional at 37 degrees C. Consistent with this, tspRb is compromised in nuclear association and E2F regulation at the non-permissive temperature, but regains these properties at 32.5 degrees C. Serial activation and inactivation of tspRb in SAOS-2 cells does not allow proliferation, but rather leads to apoptotic cell death. Transient activation of pRb may kill tumor cells by establishing a conflict between persistent proliferation-inhibitory signals and renewed deregulation of pRb targets such as E2F, and may thus be a more potent means of eliminating these cells than through simple re-introduction of the tumor suppressor gene.

Altered DNA repair and dysregulation of p53 in IRF-1 null hepatocytes

The tumor suppressor proteins IRF-1 and p53 are involved in response pathways after DNA damage. In different cell types, IRF-1 and p53 can cooperate to produce cell cycle arrest (embryo fibroblasts) or can independently trigger apoptosis (lymphoid cells). p53 may also regulate DNA repair, but there is no information on IRF-1 and repair. The cell lineage dependency of these effects precludes extrapolation of findings to other tissues of relevance to human cancer. Here, we report the consequences of IRF-1 deficiency for apoptosis, cell cycle arrest, and DNA repair in primary hepatocytes after DNA damage and extend previous work on the role of p53 in hepatocytes. IRF-1-deficient hepatocytes showed reduced DNA repair activity compared with wild-type, as assessed by unscheduled DNA synthesis after UV irradiation (10J/m2) and by host reactivation of a UV-damaged reporter construct. p53-deficient hepatocytes also showed reduced unscheduled DNA synthesis after UV, but there was no impairment of specific repair in host reactivation assays. IRF-1 deficiency did not affect the p53-dependent G1/S arrest after UV irradiation. Hepatocyte apoptosis after UV treatment, previously reported to be independent of p53, was also independent of IRF-1. However, IRF-1 deficiency produced dysregulation of p53, manifested as increased transactivation of a p53-reporter plasmid in undamaged hepatocytes, and accelerated p53 stabilization after DNA damage. Hence, in hepatocytes, IRF-1 is not required for growth arrest or apoptosis after DNA damage, but the results suggest for the first time a role in DNA repair regulation.

X-gene product of hepatitis B virus induces apoptosis in liver cells

Hepatitis B virus is a causative agent of hepatocellular carcinoma, and in the course of tumorigenesis, the X-gene product (HBx) is known to play important roles. Here, we investigated the transforming potential of HBx by conventional focus formation assay in NIH3T3 cells. Cells were cotransfected with the HBx expression plasmid along with other oncogenes including Ha-ras, v-src, v-myc, v-fos, and E1a. Unexpectedly, the introduction of HBx completely abrogated the focus-forming ability of all five tested oncogenes. In addition, the cotransfection of Bcl-2, an apoptosis inhibitor, reversed the HBx-mediated inhibition of focus formation, suggesting that the observed repression of focus formation by HBx is through the induction of apoptosis. Next, to test unequivocally whether HBx induces apoptosis in liver cells, we established stable Chang liver cell lines expressing HBx under the control of a tetracycline-inducible promoter. Induction of HBx in these cells in the presence of 1% calf serum resulted in typical apoptosis phenomena such as DNA fragmentation, nuclear condensation, and fragmentation. Based on these results, we propose that HBx sensitizes liver cells to apoptosis upon hepatitis B virus infection, contributing to the development of hepatitis and the subsequent generation of hepatocellular carcinoma.

Apoptotic response of spermatogenic cells to the germ cell mutagens etoposide, adriamycin, and diepoxybutane

In testis, apoptosis is a way to eliminate damaged germ cells during their development. In this study, we evaluated the ability of three germ cell mutagens to induce apoptosis (or programmed cell death) at specific stages of rat seminiferous epithelial cycle. These chemicals include the cancer chemotherapy drugs etoposide and adriamycin and the butadiene metabolite diepoxybutane. According to our results, etoposide is a very potent inducer of apoptosis in male rat germ cells and the cell types most sensitive to it include all types of spermatogonia, zygotene, and early pachytene spermatocytes and meiotically dividing spermatocytes. Also, adriamycin causes an increase in apoptosis at specific stages of seminiferous epithelial cycle and the most sensitive cell types are type A3-4 spermatogonia, preleptotene, zygotene, and early pachytene spermatocytes. Diepoxybutane does not cause any significant increase in the frequency of apoptosis in rat testis. In addition, we studied whether p53 is taking part in the apoptotic response of spermatogenic cells by studying the levels of p53 protein in testis before and after chemical treatment. No accumulation of p53 in testis was seen after treatment with these three chemicals. The expression of two p53-regulated genes, p21WAF1 and mdm2, was also studied but no increase in the levels of mRNA of these genes was observed after treatment. The results indicate that apoptosis should be taken into consideration when the genotoxic effects of chemicals are evaluated in germ cells.

Role of UEV-1, an inactive variant of the E2 ubiquitin-conjugating enzymes, in in vitro differentiation and cell cycle behavior of HT-29-M6 intestinal mucosecretory cells

By means of differential RNA display, we have isolated a cDNA corresponding to transcripts that are down-regulated upon differentiation of the goblet cell-like HT-29-M6 human colon carcinoma cell line. These transcripts encode proteins originally identified as CROC-1 on the basis of their capacity to activate transcription of c-fos. We show that these proteins are similar in sequence, and in predicted secondary and tertiary structure, to the ubiquitin-conjugating enzymes, also known as E2. Despite the similarities, these proteins lack a critical cysteine residue essential for the catalytic activity of E2 enzymes and, in vitro, they do not conjugate or transfer ubiquitin to protein substrates. These proteins constitute a distinct subfamily within the E2 protein family and are highly conserved in phylogeny from yeasts to mammals. Therefore, we have designated them UEV (ubiquitin-conjugating E2 enzyme variant) proteins, defined as proteins similar in sequence and structure to the E2 ubiquitin-conjugating enzymes but lacking their enzymatic activity (HW/GDB-approved gene symbol, UBE2V). At least two human genes code for UEV proteins, and one of them, located on chromosome 20q13.2, is expressed as at least four isoforms, generated by alternative splicing. All human cell types analyzed expressed at least one of these isoforms. Constitutive expression of exogenous human UEV in HT-29-M6 cells inhibited their capacity to differentiate upon confluence and caused both the entry of a larger proportion of cells in the division cycle and an accumulation in G2-M. This was accompanied with a profound inhibition of the mitotic kinase, cdk1. These results suggest that UEV proteins are involved in the control of differentiation and could exert their effects by altering cell cycle distribution.

Inosine-5'-monophosphate dehydrogenase is a rate-determining factor for p53-dependent growth regulation

We have proposed that reduced activity of inosine-5'-monophosphate dehydrogenase (IMPD; IMP:NAD oxidoreductase, EC 1.2.1.14), the rate-limiting enzyme for guanine nucleotide biosynthesis, in response to wild-type p53 expression, is essential for p53-dependent growth suppression. A gene transfer strategy was used to demonstrate that under physiological conditions constitutive IMPD expression prevents p53-dependent growth suppression. In these studies, expression of bax and waf1, genes implicated in p53-dependent growth suppression in response to DNA damage, remains elevated in response to p53. These findings indicate that under physiological conditions IMPD is a rate-determining factor for p53-dependent growth regulation. In addition, they suggest that the impd gene may be epistatic to bax and waf1 in growth suppression. Because of the role of IMPD in the production and balance of GTP and ATP, essential nucleotides for signal transduction, these results suggest that p53 controls cell division signals by regulating purine ribonucleotide metabolism.

Effects of p21(Cip1/Waf1) at both the G1/S and the G2/M cell cycle transitions: pRb is a critical determinant in blocking DNA replication and in preventing endoreduplication

It has been proposed that the functions of the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27Kip1 are limited to cell cycle control at the G1/S-phase transition and in the maintenance of cellular quiescence. To test the validity of this hypothesis, p21 was expressed in a diverse panel of cell lines, thus isolating the effects of p21 activity from the pleiotropic effects of upstream signaling pathways that normally induce p21 expression. The data show that at physiological levels of accumulation, p21, in addition to its role in negatively regulating the G1/S transition, contributes to regulation of the G2/M transition. Both G1- and G2-arrested cells were observed in all cell types, with different preponderances. Preponderant G1 arrest in response to p21 expression correlated with the presence of functional pRb. G2 arrest was more prominent in pRb-negative cells. The arrest distribution did not correlate with the p53 status, and proliferating-cell nuclear antigen (PCNA) binding activity of p21 did not appear to be involved, since p27, which lacks a PCNA binding domain, produced similar arrest distributions [corrected], DNA endoreduplication occurred in pRb-negative but not in pRb-positive cells, suggesting that functional pRb is necessary to prevent DNA replication in p21 G2-arrested cells. These results suggest that the primary target of the Cip/Kip family of inhibitors leading to efficient G1 arrest as well as to blockade of DNA replication from either G1 or G2 phase is the pRb regulatory system. Finally, the tendency of Rb-negative cells to undergo endoreduplication cycles when p21 is expressed may have negative implications in the therapy of Rb-negative cancers with genotoxic agents that activate the p53/p21 pathway.

Hepatitis B virus X protein and p53 tumor suppressor interactions in the modulation of apoptosis

We have reported previously that the hepatitis B virus oncoprotein, HBx, can bind to the C terminus of p53 and inhibit several critical p53-mediated cellular processes, including DNA sequence-specific binding, transcriptional transactivation, and apoptosis. Recognizing the importance of p53-mediated apoptosis for maintaining homeostasis and preventing neoplastic transformation, here we further examine the physical interaction between HBx and p53 as well as the functional consequences of this association. In vitro binding studies indicate that the ayw and adr viral subtypes of HBx bind similar amounts of glutathione S-transferase-p53 with the distal C terminus of HBx (from residues 111 to 154) being critical for this interaction. Using a microinjection technique, we show that this same C-terminal region of HBx is necessary for sequestering p53 in the cytoplasm and abrogating p53-mediated apoptosis. The transcriptional transactivation domain of HBx also maps to its C terminus; however, a comparison of the ability of full-length and truncated HBx protein to abrogate p53-induced apoptosis versus transactivate simian virus 40- or human nitric oxide synthase-2 promoter-driven reporter constructs indicates that these two functional properties are distinct and thus may contribute to hepatocarcinogenesis differently. Collectively, our data indicate that the distal C-terminal domain of HBx, independent of its transactivation activity, complexes with p53 in the cytoplasm, partially preventing its nuclear entry and ability to induce apoptosis. These pathobiological effects of HBx may contribute to the early stages of hepatocellular carcinogenesis.

Atm selectively regulates distinct p53-dependent cell-cycle checkpoint and apoptotic pathways

Atm is part of a pathway that responds to DNA damage from ionizing radiation (IR). This pathway involves p53, as Atm-deficient cell lines and mice are defective in p53 induction after IR. p53 is a multi-functional protein that simultaneously regulates distinct downstream pathways controlling cell-cycle progression and apoptosis. However, the mechanisms by which p53 differentially activates downstream pathways are unknown. To determine the relationship between Atm and p53, we examined cell-cycle and apoptotic responses in Atm-, p53-(ref. 8) and p21-deficient mice after IR in the whole animal. As expected, p53 protein levels were not induced by IR in thymus of Atm-deficient mice. IR-induced cell-cycle checkpoint function was also defective, and induction of p21 was attenuated in thymus from Atm-deficient mice. However, IR-induced apoptosis and Bax induction were completely normal; both of which are mediated by p53. IR-induced thymic apoptosis was suppressed in Atm/p53 double-mutant mice but not in Atm/p21 double mutants, demonstrating p53 dependence and Atm independence. Thus, Atm deficiency results in lack of p53 induction by IR, but only selective disruption of p53-dependent functions. Our results support a model in which upstream effectors such as Atm selectively activate p53 to regulate specific downstream pathways, providing a mechanism for controlling distinct cell-cycle and apoptotic responses.

Modulation of p53 expression in cultured colonic adenoma cell lines by the naturally occurring lumenal factors butyrate and deoxycholate

The high incidence of colorectal cancer in Western society is believed to be strongly related to diet. Mutation of the p53 gene is a late event in colorectal carcinogenesis, and thus, the majority of pre-malignant adenomas express wild-type p53. As loss of p53 protein function is an important step in colorectal carcinogenesis, we investigated whether naturally occurring lumenal factors can modulate the expression of p53 in non-tumorigenic human colonic adenoma cell lines. Levels of p53 protein and mRNA were measured in adherent cells which had been incubated with growth-inhibitory concentrations of sodium butyrate (a by-product of dietary fibre fermentation) or sodium deoxycholate (a bile acid) for up to 48 hr. We report that both butyrate and deoxycholate can down-regulate the expression of wild-type and mutant p53. In contrast, incubation for 48 hr with the endogenous inhibitory growth factor TGFbeta1 did not alter p53 protein expression. Thus, in addition to cellular mechanisms which regulate p53 function, such as post-translational stabilisation, nuclear exclusion, negative feedback inhibition of p53 mRNA translation or binding of p53 by cellular proteins, p53 protein levels also may be regulated by changes in the level of p53 gene transcription. Furthermore, we show that lumenal factors are able to affect directly the expression of p53 protein in colonic epithelial cells.

Regulation of ES cell differentiation by functional and conformational modulation of p53

Embryonic stem (ES) cell lines were used to examine the role of p53 during in vitro differentiation. Undifferentiated ES cells express high levels of p53 exclusively in the wild-type conformation, immunoprecipitable by monoclonal antibody PAb246, and p53 was found to be functionally active as determined by its ability to bind DNA specifically and to activate transcription of target genes. Differentiation in vitro resulted in a decrease in the levels of p53 and in a shift in its conformational status to the mutant form, detectable by monoclonal antibody PAb240, with a concomitant loss of functional activity. The presence of functional p53 in the undifferentiated ES cells renders them hypersensitive to UV irradiation, whereas the differentiated cells were resistant to UV treatment. ES cells lacking p53 exhibit enhanced proliferation in both the undifferentiated and differentiated state, and apoptosis accompanying differentiation was found to be reduced. Furthermore, wild-type ES cells undergoing apoptosis expressed functional p53. Expression of the temperature-sensitive p53val135 mutant in wild-type ES cells resulted in a reduction of apoptosis accompanying differentiation when it adopted a mutant conformation at 39 degrees C. These data demonstrate that functional inactivation of p53 allows differentiating cells to escape from apoptosis, and suggest that the conformational switch could regulate the inactivation process.

Induced N- and C-terminal cleavage of p53: a core fragment of p53, generated by interaction with damaged DNA, promotes cleavage of the N-terminus of full-length p53, whereas ssDNA induces C-terminal cleavage of p53

p53 is able to recognize and bind sites of DNA damage and, in some way, damage to cellular DNA activates a p53 response leading to G1 arrest or apoptosis. We have previously shown that 'damaged DNA' induces N-terminal cleavage of p53 to generate p40(DeltaN) and p35 (core) protein products. We now show that the p35 product has protease activity and is able to cleave between residues 23 and 24 of full-length p53 to generate a novel product, p50(DeltaN23). This activity was inhibited by bestatin, an aminopeptidase inhibitor. Residues 23 and 24 lie within the mdm-2 binding domain of p53 and the possibility that p50(DeltaN23) may be resistant to feedback regulation by mdm-2 is discussed. Unexpectedly, interaction with ssDNA induced two further cleavage products of p53, generated by C-terminal cleavage and designated p50(DeltaC) and p40(DeltaC). In vivo generation of a C-terminal cleavage product of endogenous p53 similar in size to p50(DeltaC) correlated with up-regulation of p21 expression in ML-1 cells exposed to either adriamycin or cisplatin. The possible significance of the various p53 cleavage products in relation to the cellular response to DNA damage is discussed.

Multiple pathways control cell growth and transformation: overlapping and independent activities of p53 and p21Cip1/WAF1/Sdi1

Many tumour therapies act by inducing a cellular damage response pathway mediated by the tumour suppressor protein p53. Alternative outcomes of p53 induction include apoptosis or transient cell-cycle arrest, both thought to require the transcriptional activity of wild-type p53. Current research highlights the action of a p53-activated gene, p21Cip1/WAF1/Sdi1, which encodes a cyclin-kinase inhibitor important in mediating p53-dependent cell-cycle arrest, while programmed cell death in response to DNA damage requires transcriptionally active p53 but not activation of p21Cip1/WAF1/Sdi1. This review examines the roles of p53 and p21Cip1/WAF1/Sdi1 in controlling cell proliferation, in the light of a new study on expression of p53 and p21Cip1/WAF1/Sdi1 in squamous cell carcinoma of the larynx.

p53 activity is essential for normal development in Xenopus

BACKGROUND

The tumor suppressor p53 plays a key role in regulating the cell cycle and apoptosis in differentiated cells. Mutant mice lacking functional p53 develop normally but die from multiple neoplasms shortly after birth. There have been hints that p53 is involved in morphogenesis, but given the relatively normal development of p53 null mice, the significance of these data has been difficult to evaluate. To examine the role of p53 in vertebrate development, we have determined the results of blocking its activity in embryos of the frog Xenopus laevis.

RESULTS

Two different methods have been used to block p53 protein activity in developing Xenopus embryos--ectopic expression of dominant-negative forms of human p53 and ectopic expression of the p53 negative regulator, Xenopus dm-2. In both instances, inhibition of p53 activity blocked the ability of Xenopus early blastomeres to undergo differentiation and resulted in the formation of large cellular masses reminiscent of tumors. The ability of mutant p53 to induce such developmental tumors was suppressed by co-injection with wild-type human or wild-type Xenopus p53. Cells expressing mutant p53 activated zygotic gene expression and underwent the mid-blastula transition normally. Such cells continued to divide at approximately normal rates but did not form normal embryonic tissues and never underwent terminal differentiation, remaining as large, yolk-filled cell masses that were often associated with the neural tube or epidermis.

CONCLUSIONS

In Xenopus, the maternal stockpile of p53 mRNA and protein seems to be essential for normal development. Inhibiting p53 function results in an early block to differentiation. Although it is possible that mutant human p53 proteins have a dominant gain-of-function or neomorphic activity in Xenopus, and that this is responsible for the development of tumors, most of the evidence indicates that this is not the case. Whatever the basis of the block to differentiation, these results indicate that Xenopus embryos are a sensitive system in which to explore the role of p53 in normal development and in developmental tumors.

Apoptosis and other effects of radiation in normal human urothelial cells

In this paper, an attempt is made to identify endpoints that might be of potential use in the quantification of radiation effects in human tissues. Irradiated cultures of cells that are not selected for clonogenic survival but are left in situ to grow after irradiation show a wide variety of morphological and biochemical abnormalities. These include nuclear fragmentation and other evidence of programmed cell death, but they also include a considerable amount of lysis, necrosis, and persistent abnormal growth and function, which are expressed in the progeny of irradiated cells. Induction of proteins associated with stress or shock responses, growth and cell cycle control, and control of apoptosis are also seen and may persist. The dose dependence of these various responses is documented, because it probably determines to a large extent the outcome of radiation exposure in terms of whether a cell dies, divides normally, or develops genomic instability, mutation, and ultimate carcinogenic progression of the progeny. Clearly, a cell that dies presents no further threat to the organism, nor does a fully repaired cell. Therefore, a major challenge facing radiation protection research is to define the population at risk of surviving with damage. The results show that there is a variation in response to radiation between different patient cultures that is detectable in an explant culture system of primary normal human urothelium. The growth pattern and protein expression postirradiation is consistent with apoptosis being a major determinant of low dose response to radiation. This form of death appears to be suppressed at higher doses and, in the majority of subjects, results in the presence of a highly abnormal population of cells, even though the population size is the same whether their progenitors were irradiated or not.

Proteolytic cleavage of the mdm2 oncoprotein during apoptosis

The mdm2 oncogene encodes a 90-kDa protein that can bind to the p53 tumor suppressor protein and negatively regulate its functions in transcription, cell cycle arrest, and apoptosis. The mdm2 gene is frequently amplified in human sarcomas, which may be responsible for the malignant transformations. We present evidence that the mdm2 oncoprotein is cleaved by an interleukin 1beta-converting enzyme-like protease (caspase) during p53-mediated apoptosis. The protease that cleaves mdm2 has a specificity similar to that of CPP32 (caspase-3), and recombinant caspase-3 is able to cleave mdm2 in vitro. The protease cleavage site has been mapped to between residue 361 and 362 of human mdm2. The proteolytic cleavage removes the COOH-terminal RING finger domain of mdm2, resulting in the loss of RNA binding activity. The p53 binding and inhibition functions of mdm2 are not affected by the cleavage. The cleavage site sequence of mdm2 is evolutionarily conserved, suggesting that regulation by caspase cleavage during apoptosis is an important feature of mdm2.

Wild-type p53 triggers a rapid senescence program in human tumor cells lacking functional p53

The p53 tumor suppressor gene has been shown to play an important role in determining cell fate. Overexpression of wild-type p53 in tumor cells has been shown to lead to growth arrest or apoptosis. Previous studies in fibroblasts have provided indirect evidence for a link between p53 and senescence. Here we show, using an inducible p53 expression system, that wild-type p53 overexpression in EJ bladder carcinoma cells, which have lost functional p53, triggers the rapid onset of G1 and G2/M growth arrest associated with p21 up-regulation and repression of mitotic cyclins (cyclin A and B) and cdc2. Growth arrest in response to p53 induction became irreversible within 48-72 h, with cells exhibiting morphological features as well as specific biochemical and ultrastructural markers of the senescent phenotype. These findings provide direct evidence that p53 overexpression can activate the rapid onset of senescence in tumor cells.

FIN13, a novel growth factor-inducible serine-threonine phosphatase which can inhibit cell cycle progression

We have identified a novel type 2C serine-threonine phosphatase, FIN13, whose expression is induced by fibroblast growth factor 4 and serum in late G1 phase. The protein encoded by FIN13 cDNA includes N- and C-terminal domains with significant homologies to type 2C phosphatases, a domain homologous to collagen, and an acidic domain. FIN13 expression predominates in proliferating tissues. Bacterially expressed FIN13 and FIN13 expressed in mammalian cells exhibit serine-threonine phosphatase activity, which requires Mn2+ and is insensitive to inhibition by okadaic acid. FIN13 is localized in the nuclei of transiently transfected cells. Cotransfection of FIN13-expressing plasmids with a plasmid that expresses the neomycin resistance gene inhibits the growth of drug-resistant colonies in NIH 3T3, HeLa and Rat-1 cells. In transiently transfected cells, FIN13 inhibits DNA synthesis and results in the accumulation of cells in G1 and early S phases. Similarly, the induction of expression of FIN13 under the control of a tetracycline-regulated promoter in NIH 3T3 cells leads to growth inhibition, with accumulation of cells in G1 and early S phases. Thus, overexpression and/or unregulated expression of FIN13 inhibits cell cycle progression, indicating that the physiological role of this phosphatase may be that of regulating the orderly progression of cells through the mitotic cycle by dephosphorylating specific substrates which are important for cell proliferation.

The p53/IGF-1 receptor axis in the regulation of programmed cell death

The loss or functional inactivation of tumor suppressor genes appears to be one of the most fundamental genetic mechanisms of tumorigenesis, and rational insights into the signaling pathways of tumor suppressor genes have emerged as a successful strategy of identifying novel drug discovery targets downstream of the tumor suppressor protein itself. Elucidation of novel pathways downstream of p53 have established a link between this important tumor suppressor gene and the insulin-like growth factor-1 receptor (IGF-1r), either via direct regulation of IGF-1 receptor levels, or modulation of IGFs via transactivation of the insulin-like growth factor-binding protein 3 (IGF-BP3) gene. Binding of IGF-BP3 to IGFs inhibits both their mitogenic and cell survival functions, highlighting a novel pathway whereby p53 may regulate apoptosis in tumor cells.

A novel p53-inducible gene, PAG608, encodes a nuclear zinc finger protein whose overexpression promotes apoptosis

The biological effects of the p53 tumor suppressor protein are elicited, at least in part, through sequence-specific transactivation of a battery of target genes. The differential display method was employed towards identifying additional p53 target genes, with emphasis on genes whose induction may contribute to p53-mediated apoptosis. We report here the cloning of a novel p53-inducible gene, designated PAG608. PAG608 transcripts are induced by DNA damage in a p53-dependent manner. PAG608 encodes a nuclear zinc finger protein, which appears to localize preferentially to nucleoli when expressed at moderate levels in transfected cells. Transient overexpression of PAG608 in human tumor-derived cells leads to distinctive changes in nuclear morphology, and can promote apoptosis. Together with additional p53 target genes, PAG608 may therefore play a role in mediating the biological activities of p53.

Induction of DNA synthesis and apoptosis are separable functions of E2F-1

The family of E2F transcription factors have an essential role in mediating cell cycle progression, and recently, one of the E2F protein family, E2F-1, has been shown to participate in the induction of apoptosis. Cooperation between E2F and the p53 tumor suppressor protein in this apoptotic response had led to the suggestion that cell cycle progression induced by E2F-1 expression provides an apoptotic signal when placed in conflict with an arrest to cell cycle progression, such as provided by p53. We show here that although apoptosis is clearly enhanced by p53, E2F-1 can induce significant apoptosis in the absence of p53. Furthermore, this apoptotic function of E2F-1 is separable from the ability to accelerate entry into DNA synthesis. Analysis of E2F-1 mutants indicates that although DNA-binding is required, transcriptional transactivation is not necessary for the induction of apoptosis by E2F-1, suggesting that it may be mediated through alleviation of E2F-dependent transcriptional repression. These results indicate that E2F-1 can show independent cell cycle progression and apoptotic functions, consistent with its putative role as a tumor suppressor.

Binding and modulation of p53 by p300/CBP coactivators

The adenovirus E1A and SV40 large-T-antigen oncoproteins bind to members of the p300/CBP transcriptional coactivator family. Binding of p300/CBP is implicated in the transforming mechanisms of E1A and T-antigen oncoproteins. A common region of the T antigen is critical for binding both p300/CBP and the tumour suppressor p53, suggesting a link between the functions of p53 and p300. Here we report that p300/CBP binds to p53 in the absence of viral oncoproteins, and that p300 and p53 colocalize within the nucleus and coexist in a stable DNA-binding complex. Consistent with its ability to bind to p300, E1A disrupted functions mediated by p53. It reduced p53-mediated activation of the p21 and bax promoters, and suppressed p53-induced cell-cycle arrest and apoptosis. We conclude that members of the p300/CBP family are transcriptional adaptors for p53, modulating its checkpoint function in the G1 phase of the cell cycle and its induction of apoptosis. Disruption of p300/p53-dependent growth control may be part of the mechanism by which E1A induces cell transformation. These results help to explain how p53 mediates growth and checkpoint control, and how members of the p300/CBP family affect progression from G1 to the S phase of the cell cycle.

Identification of a novel antiapoptotic functional domain in simian virus 40 large T antigen

The ability of DNA tumor virus proteins to trigger apoptosis in mammalian cells is well established. For example, transgenic expression of a simian virus 40 (SV40) T-antigen N-terminal fragment (N-termTag) is known to induce apoptosis in choroid plexus epithelial cells. SV40 T-antigen-induced apoptosis has generally been considered to be a p53-dependent event because cell death in the brain is greatly diminished in a p53-/- background strain and is abrogated by expression of wild-type (p53-binding) SV40 T antigen. We now show that while N-termTags triggered apoptosis in rat embryo fibroblasts cultured in low serum, expression of full-length T antigens unable to bind p53 [mut(p53-)Tags] protected against apoptosis without causing transformation. One domain essential for blocking apoptosis by T antigen was mapped to amino acids 525 to 541. This domain has >60% homology with a domain of adenovirus type 5 E1B 19K required to prevent E1A-induced apoptosis. In the context of both wild-type T antigen and mut(p53-)Tags, mutation of two conserved amino acids in this region eliminated T antigen's antiapoptotic activity in REF-52 cells. These data suggest that SV40 T antigen contains a novel functional domain involved in preventing apoptosis independently of inactivation of p53.

Bax suppresses tumorigenesis and stimulates apoptosis in vivo

The protein p53 is a key tumour-suppressor, as evidenced by its frequent inactivation in human cancers. Animal models have indicated that attenuation of p53-dependent cell death (apoptosis) can contribute to both the initiation and progression of cancer, but the molecular mechanisms are unknown. Although p53-mediated transcriptional activation is one possible explanation, none of the known p53-responsive genes has been shown to function in p53-dependent apoptosis. Here we test the role of the death-promoting gene bax in a transgenic mouse brain tumour, a model in which p53-mediated apoptosis attenuates tumour growth. Inactivation of p53 causes a dramatic acceleration of tumour growth owing to a reduction in apoptosis of over ninety per cent. We show that p53-dependent expression of bax is induced in slow-growing apoptotic tumours. Moreover, tumour growth is accelerated and apoptosis drops by fifty per cent in Bax-deficient mice, indicating that it is required for a full p53-mediated response. To our knowledge this is the first demonstration that Bax acts as a tumour suppressor, and our findings indicate that Bax could be a component of the p53-mediated apoptotic response in this system.