Drosophila p53 binds a damage response element at the reaper locus

Cell surface Death Receptor signaling in normal and cancer cells

The extrinsic cell death pathway is initiated upon ligand-receptor interactions at the cell surface including FAS ligand-FAS/APO1, TNF-TNF receptors, and TRAIL-TRAIL receptors. Abnormalities of various components of these pathways have been identified in human cancer including loss of FAS expression, deletion or loss of TRAIL receptor DR4, mutation of TRAIL receptor DR5, overexpression of TRAIL decoy TRID or overexpression of Fas decoy, as well as overexpression of the caspase activation inhibitor, FLIP. Death ligands have been explored as potential therapeutics in cancer therapy with some limitations in the case of FAS and TNF due to toxicities. TRAIL remains promising as a therapeutic and has potential for combination with chemo- or radio-therapy. The death receptor signaling pathways include cross-talk with the mitochondrial pathway and can in some cases be influenced by mitochondrial membrane potential changes or NF-kappaB. FLIP and BCL-XL expression may reduce sensitivity of cancer cells to combination therapies.

Distinct pathways mediate UV-induced apoptosis in Drosophila embryos

Cell death in Drosophila is regulated by many of the same signals that control apoptosis in mammalian systems. For all the three major cell death pathways that have been described in humans, homologous components have been identified in Drosophila. Here we report that distinct pathways mediate UV-induced apoptosis at different developmental stages in the Drosophila embryo. In midstage embryos, UVC irradiation induces reaper expression and cell death through a mei-41(dATM)-dependent pathway; UVB does not have the same effect. In contrast, in pregastrulation embryos, both UVB and UVC promote apoptosis via transcriptional induction of the Drosophila Apaf-1/ced-4 homolog. This early UV response requires E2F but not mei-41 function and appears to be independent of DNA damage.

Ehp53, an Entamoeba histolytica protein, ancestor of the mammalian tumour suppressor p53

This paper reports the identification of Ehp53, a p53-like Entamoeba histolytica protein, which binds to the human p53 DNA consensus sequence (oli-p53). Monoclonal antibodies against p53 (Ab-1 and Ab-2) recognized a single 53 kDa spot in two-dimensional gels and inhibited the formation of complexes produced by E. histolytica nuclear extracts and oli-p53. Additionally, E. histolytica gene promoter sequences with high homology to oli-p53 formed complexes with nuclear proteins that were abolished by oli-p53. Ehp53 protein levels increased in UV-irradiated trophozoites. This protein was also detected in Entamoeba moshkovskii and Entamoeba invadens. By confocal microscopy, Ehp53 was located in the nuclei, EhkO organelles and cytoplasm. The Ehp53-encoding gene was cloned and its predicted amino acid sequence showed 30-54 % and 50-57 % homology with important domains of the human and the Drosophila melanogaster p53 proteins, respectively. This homology included the tetramerization domain, the nuclear export signal and a nuclear localization signal. Ehp53 also contains seven of the eight DNA-binding residues and two of the four Zn(2+)-binding sites described for p53. A recombinant Ehp53 was recognized by Ab-2. Ehp53 is believed to be the first p53-like protein found in protozoa and may be the evolutionary ancestor of the mammalian p53.

Genome-wide analyses of steroid- and radiation-triggered programmed cell death in Drosophila

Apoptosis and autophagy are two forms of programmed cell death that play important roles in the removal of unneeded and abnormal cells during animal development. While these two forms of programmed cell death are morphologically distinct, recent studies indicate that apoptotic and autophagic cell death utilize some common regulatory mechanisms. To identify genes that are associated with apoptotic and autophagic cell death, we monitored changes in gene transcription by using microarrays representing nearly the entire Drosophila genome. Analyses of steroid-triggered autophagic cell death identified 932 gene transcripts that changed 5-fold or greater in RNA level. In contrast, radiation-activated apoptosis resulted in 34 gene transcripts that exhibited a similar magnitude of change. Analyses of these data enabled us to identify genes that are common and unique to steroid- and radiation-induced cell death. Mutants that prevent autophagic cell death exhibit altered levels of gene transcription, including genes encoding caspases, non-caspase proteases, and proteins that are similar to yeast autophagy proteins. This study also identifies numerous novel genes as candidate cell death regulators and suggests new links between apoptosis and autophagic cell death.

Reaper is regulated by IAP-mediated ubiquitination

In most cases, apoptotic cell death culminates in the activation of the caspase family of cysteine proteases, leading to the orderly dismantling and elimination of the cell. The IAPs (inhibitors of apoptosis) comprise a family of proteins that oppose caspases and thus act to raise the apoptotic threshold. Disruption of IAP-mediated caspase inhibition has been shown to be an important activity for pro-apoptotic proteins in Drosophila (Reaper, HID, and Grim) and in mammalian cells (Smac/DIABLO and Omi/HtrA2). In addition, in the case of the fly, these proteins are able to stimulate the ubiquitination and degradation of IAPs by a mechanism involving the ubiquitin ligase activity of the IAP itself. In this report, we show that the Drosophila RHG proteins (Reaper, HID, and Grim) are themselves substrates for IAP-mediated ubiquitination. This ubiquitination of Reaper requires IAP ubiquitin-ligase activity and a stable interaction between Reaper and the IAP. Additionally, degradation of Reaper can be blocked by mutating its potential ubiquitination sites. Most importantly, we also show that regulation of Reaper by ubiquitination is a significant factor in determining its biological activity. These data demonstrate a novel function for IAPs and suggest that IAPs and Reaper-like proteins mutually control each other's abundance.

iASPP oncoprotein is a key inhibitor of p53 conserved from worm to human

We have previously shown that ASPP1 and ASPP2 are specific activators of p53; one mechanism by which wild-type p53 is tolerated in human breast carcinomas is through loss of ASPP activity. We have further shown that 53BP2, which corresponds to a C-terminal fragment of ASPP2, acts as a dominant negative inhibitor of p53 (ref. 1). Hence, an inhibitory form of ASPP resembling 53BP2 could allow cells to bypass the tumor-suppressor functions of p53 and the ASPP proteins. Here, we characterize such a protein, iASPP (inhibitory member of the ASPP family), encoded by PPP1R13L in humans and ape-1 in Caenorhabditis elegans. iASPP is an evolutionarily conserved inhibitor of p53; inhibition of iASPP by RNA-mediated interference or antisense RNA in C. elegans or human cells, respectively, induces p53-dependent apoptosis. Moreover, iASPP is an oncoprotein that cooperates with Ras, E1A and E7, but not mutant p53, to transform cells in vitro. Increased expression of iASPP also confers resistance to ultraviolet radiation and to cisplatin-induced apoptosis. iASPP expression is upregulated in human breast carcinomas expressing wild-type p53 and normal levels of ASPP. Inhibition of iASPP could provide an important new strategy for treating tumors expressing wild-type p53.

Signal-induced transcriptional activation by Dif requires the dTRAP80 mediator module

The Mediator complex is the major multiprotein transcriptional coactivator complex in Drosophila melanogaster. Mediator components interact with diverse sets of transcriptional activator proteins to elicit the sophisticated regulation of gene expression. The distinct phenotypes associated with certain mutations in some of the Mediator genes and the specific in vitro interactions of Mediator gene products with transcriptional activator proteins suggest the presence of activator-specific binding subunits within the Mediator complex. However, the physiological relevance of these selective in vitro interactions has not been addressed. Therefore, we analyzed dTRAP80, one of the putative activator-binding subunits of the Mediator, for specificity of binding to a number of natural transcriptional activators from Drosophila. Among the group of activator proteins that requires the Mediator complex for transcriptional activation, only a subset of these proteins interacted with dTRAP80 in vitro and only these dTRAP80-interacting activators were defective for activation under dTRAP80-deficient in vivo conditions. In particular, activation of Drosophila antimicrobial peptide drosomycin gene expression by the NF-kappa B-like transcription factor Dif during induction of the Toll signaling pathway was dependent on the dTRAP80 module. These results, and the indirect support from the dTRAP80 artificial recruitment assay, indicate that dTRAP80 serves as a genuine activator-binding target responsible for a distinct group of activators.

CIAP1 and the serine protease HTRA2 are involved in a novel p53-dependent apoptosis pathway in mammals

Recently a Drosophila p53 protein has been identified that mediates apoptosis via a novel pathway involving the activation of the Reaper gene and subsequent inhibition of the inhibitors of apoptosis (IAPs). The present study found that CIAP1, a major mammalian homolog of Drosophila IAPs, is irreversibly inhibited (cleaved) during p53-dependent apoptosis and this cleavage is mediated by a serine protease. Serine protease inhibitors that block CIAP1 cleavage inhibit p53-dependent apoptosis. Furthermore, activation of the p53 protein increases the transcription of the HTRA2 gene, which encodes a serine protease that interacts with CIAP1 and potentiates apoptosis. These results demonstrate that the mammalian p53 protein may activate apoptosis through a novel pathway functionally similar to that in Drosophila, which involves HTRA2 and subsequent inhibition of CIAP1 by cleavage.

p53-dependent cell death signaling in neurons

The p53 tumor suppressor gene is a sequence-specific transcription factor that activates the expression of genes engaged in promoting growth arrest or cell death in response to multiple forms of cellular stress. p53 expression is elevated in damaged neurons in acute models of injury such as ischemia and epilepsy and in brain tissue samples derived from animal models and patients with chronic neurodegenerative diseases. p53 deficiency or p53 inhibition protects neurons from a wide variety of acute toxic insults. Signal transduction pathways associated with p53-induced neuronal cell death are being characterized, suggesting that intervention may prove effective in maintaining neuronal viability and restoring function following neural injury and disease.

Molecular strategies in Metazoan genomic evolution

During the evolution of Metazoa, the mitochondrial genome has remained practically unchanged as indicated by its small size and constant gene content in each lineage, although several evolutionary processes have taken place. In contrast, the nuclear genome of Metazoa has undergone dramatic changes, as is demonstrated by some entire genomes completely sequenced so far. The expansion of gene families and the appearance of new functions and new regulatory circuits are among the most prominent features. Here, we report a description of the evolution of the p53 gene family as an example of expansion of both genetic complexity and gene expression of the nuclear genome.

Activation of a meiotic checkpoint during Drosophila oogenesis regulates the translation of Gurken through Chk2/Mnk

BACKGROUND

During Drosophila oogenesis, unrepaired double-strand DNA breaks activate a mei-41-dependent meiotic checkpoint, which couples the progression through meiosis to specific developmental processes. This checkpoint affects the accumulation of Gurken protein, a transforming growth factor alpha-like signaling molecule, as well as the morphology of the oocyte nucleus. However, the components of this checkpoint in flies have not been completely elucidated.

RESULTS

We show that a mutation in the Drosophila Chk2 homolog (DmChk2/Mnk) suppresses the defects in the translation of gurken mRNA and also the defects in oocyte nuclear morphology. We also found that DmChk2 is phosphorylated in a mei-41-dependent pathway. Analysis of the meiotic cell cycle progression shows that the Drosophila Chk2 homolog is not required during early meiotic prophase, as has been observed for Chk2 in C. elegans. We demonstrate that the activation of the meiotic checkpoint affects Dwee1 localization and is associated with DmChk2-dependent posttranslational modification of Dwee1. We suggest that Dwee1 has a role in the meiotic checkpoint that regulates the meiotic cell cycle, but not the translation of gurken mRNA. In addition, we found that p53 and mus304, the Drosophila ATR-IP homolog, are not required for the patterning defects caused by the meiotic DNA repair mutations.

CONCLUSIONS

DmChk2 is a transducer of the meiotic checkpoint in flies that is activated by unrepaired double-strand DNA breaks. Activation of DmChk2 in this specific checkpoint affects a cell cycle regulator as well as mRNA translation.

A method for assaying the sensitivity of Drosophila replication checkpoint mutants to anti-cancer and DNA-damaging drugs

In multi-cellular organisms, failure to properly regulate cell-cycle progression can result in inappropriate cell death or uncontrolled cell division leading to tumor formation. To guard against such events, conserved regulatory mechanisms called "checkpoints" block progression into mitosis in response to DNA damage and incomplete replication, as well as in response to other signals. Checkpoint mutants in organisms as diverse as yeast and humans are sensitive to various chemical agents that inhibit DNA replication or cause DNA damage. This phenomenon is the primary rationale for chemotherapy, which uses drugs that preferentially target tumor cells with compromised checkpoints. In this study, we demonstrate the use of Drosophila checkpoint mutants as a system for assaying the effects of various DNA-damaging and anti-cancer agents in a developing multicellular organism. Dwee1, grp and mei-41 are genes that encode kinases that function in the DNA replication checkpoint. We tested zygotic mutants of each gene for sensitivity to the DNA replication inhibitor hydroxyurea (HU), methyl methanosulfonate (MMS), ara-C, cisplatin, and the oxygen radical generating compound paraquat. The mutants show distinct differences in their sensitivity to each of the drugs tested, suggesting an underlying complexity in the responses of individual checkpoint genes to genotoxic stress.

reaper and bax initiate two different apoptotic pathways affecting mitochondria and antagonized by bcl-2 in Drosophila

bcl-2 was the first regulator of apoptosis shown to be involved in oncogenesis. Subsequent studies in mammals, in the nematode and in Drosophila revealed wide evolutionary conservation of the regulation of apoptosis. Although dbok/debcl, a member of the bcl-2 gene family described in Drosophila, shows pro-apoptotic activities, no anti-apoptotic bcl-2 family gene has been studied in Drosophila. We have previously reported that the human anti-apoptotic gene bcl-2 is functional in Drosophila, suggesting that the fruit fly shares regulatory mechanisms with vertebrates and the nematode, involving anti-apoptotic members of the bcl-2 family. We now report that bcl-2 suppresses rpr-induced apoptosis in Drosophila. Additionally, we have compared features of bax- and rpr-induced apoptosis. Flow cytometry analysis of wing disc cells demonstrate that both killers trigger mitochondrial defects. Interestingly, bcl-2 suppresses both bax- and rpr-induced mitochondrial defects while the caspase-inhibitor p35 is specific to the rpr pathway. Finally, we show that the inhibition of apoptosis by bcl-2 is associated with the down-regulation of rpr expression.

DNA repair and transcriptional effects of mutations in TFIIH in Drosophila development

Mutations in XPB and XPD TFIIH helicases have been related with three hereditary human disorders: xeroderma pigmentosum, Cockayne syndrome, and trichothiodystrophy. The dual role of TFIIH in DNA repair and transcription makes it difficult to discern which of the mutant TFIIH phenotypes is due to defects in any of these different processes. We used haywire (hay), the Drosophila XPB homolog, to dissect this problem. Our results show that when hay dosage is affected, the fly shows defects in structures that require high levels of transcription. We found a genetic interaction between hay and cdk7, and we propose that some of these phenotypes are due to transcriptional deficiencies. We also found more apoptotic cells in imaginal discs and in the CNS of hay mutant flies than in wild-type flies. Because this abnormal level of apoptosis was not detected in cdk7 flies, this phenotype could be related to defects in DNA repair. In addition the apoptosis induced by p53 Drosophila homolog (Dmp53) is suppressed in heterozygous hay flies.

Identification of X-Linked Genes Required for Migration and Programmed Cell Death of Drosophila melanogaster Germ Cells

Drosophila germ cells form at the posterior pole of the embryo and migrate to the somatic gonad. Approximately 50% of the germ cells that form reach their target. The errant cells within the embryo undergo developmentally regulated cell death. Prior studies have identified some autosomal genes that regulate germ cell migration, but the genes that control germ cell death are not known. To identify X-linked genes required for germ cell migration and/or death, we performed a screen for mutations that disrupt these processes. Here we report the identification of scattershot and outsiders, two genes that regulate the programmed death of germ cells. The scattershot gene is defined by a mutation that disrupts both germ cell migration and the death of germ cells ectopic to the gonad. Maternal and zygotic expression of scattershot is required, but the migration and cell death functions can be genetically uncoupled. Zygotic expression of wild-type scattershot rescues germ cell pathfinding, but does not restore the programmed death of errant cells. The outsiders gene is required zygotically. In outsiders mutant embryos, the appropriate number of germ cells is incorporated into the gonad, but germ cells ectopic to the gonad persist.

A tormentor in the quest for plant p53-like proteins

Over the past few years the presence of p53-like proteins in plants was frequently reported, by using the monoclonal antibody Pab240. By means of protein purification and screening a cDNA library, a Pab240 cross-reacting protein and a cDNA clone were isolated from barley. Peptide- and DNA-sequence analysis identified one and the same protein: 2-oxoglutarate dehydrogenase. Sequence analysis of 2-oxoglutarate dehydrogenase revealed that the protein contains a perfect Pab240 epitope. In barley, the 110 kDa oxoglutarate dehydrogenase was degraded during isolation to a 53 kDa Pab240 cross-reacting polypeptide, thereby mimicking curiously p53-like properties.

The Drosophila Hox gene deformed sculpts head morphology via direct regulation of the apoptosis activator reaper

Hox proteins control morphological diversity along the anterior-posterior body axis of animals, but the cellular processes they directly regulate are poorly understood. We show that during early Drosophila development, the Hox protein Deformed (Dfd) maintains the boundary between the maxillary and mandibular head lobes by activating localized apoptosis. Dfd accomplishes this by directly activating the cell death promoting gene reaper (rpr). One other Hox gene, Abdominal-B (Abd-B), also regulates segment boundaries through the regional activation of apoptosis. Thus, one mechanism used by Drosophila Hox genes to modulate segmental morphology is to regulate programmed cell death, which literally sculpts segments into distinct shapes. This and other emerging evidence suggests that Hox proteins may often regulate the maintenance of segment boundaries.

Chk2 regulates irradiation-induced, p53-mediated apoptosis in Drosophila

The tumor suppressor function of p53 has been attributed to its ability to regulate apoptosis and the cell cycle. In mammals, DNA damage, aberrant growth signals, chemotherapeutic agents, and UV irradiation activate p53, a process that is regulated by several posttranslational modifications. In Drosophila melanogaster, however, the regulation modes of p53 are still unknown. Overexpression of D. melanogaster p53 (Dmp53) in the eye induced apoptosis, resulting in a small eye phenotype. This phenotype was markedly enhanced by coexpression with D. melanogaster Chk2 (DmChk2) and was almost fully rescued by coexpression with a dominant-negative (DN), kinase-dead form of DmChk2. DN DmChk2 also inhibited Dmp53-mediated apoptosis in response to DNA damage, whereas overexpression of Grapes (Grp), the Drosophila Chk1-homolog, and its DN mutant had no effect on Dmp53-induced phenotypes. DmChk2 also activated the Dmp53 transactivation activity in cultured cells. Mutagenesis of Dmp53 amino terminal Ser residues revealed that Ser-4 is critical for its responsiveness toward DmChk2. DmChk2 activates the apoptotic activity of Dmp53 and Ser-4 is required for this effect. Contrary to results in mammals, Grapes, the Drosophila Chk1-homolog, is not involved in regulating Dmp53. Chk2 may be the ancestral regulator of p53 function.

p53 induces the expression of its antagonist p73 Delta N, establishing an autoregulatory feedback loop

The p53 tumor suppressor protein activates transcription and induces cell death. A close homologue of p53, termed p73, is expressed in transactivating (TA) forms that induce growth arrest and apoptosis much like p53. However, the p73 gene contains a second promoter, giving rise to the expression of p73 Delta N, a species of p73 proteins that lack the N-terminal transactivation domain. We show here that the expression of p73 Delta N is induced by p53 on the mRNA and protein level. The promoter that regulates p73 Delta N expression in human cells was cloned and found to be activated by p53, as well as by p73TA, directly through a specific DNA element. The p73 Delta N proteins, that are thereby expressed, bound to p53-responsive promoter DNA, competed with p53 for DNA binding, antagonized the activation of transcription by p53, and prevented p53-induced cell death. In addition, a transcriptional repressor domain was identified within the splicing variant p73 Delta Nalpha. The combination of p73DeltaNalpha and mdm2 antagonized p53 more strongly than either p73Nalpha or mdm2 alone. Blocking endogenous p73 Delta N by a trans dominant fragment, or its removal by siRNA, increased the activity of a p53-responsive promoter in cells that contain a wild type p53 gene. Thus, the induction of p73 Delta N expression by p53 establishes an autoregulatory feedback loop that keeps the trigger of cell death under tight control.

GH3, a novel proapoptotic domain in Drosophila Grim, promotes a mitochondrial death pathway

Grim encodes a protein required for programmed cell death in Drosophila. The Grim N-terminus induces apoptosis by disrupting IAP blockage of caspases; however, N-terminally-deleted Grim retains pro apoptotic activity. We describe GH3, a 15 amino acid internal Grim domain absolutely required for its proapoptotic activity and sufficient to induce cell death when fused to heterologous carrier proteins. A GH3 homology region is present in the Drosophila proapoptotic proteins Reaper and Sickle. The GH3 domain and the homologous regions in Reaper and Sickle are predicted to be structured as amphipathic alpha-helixes. During apoptosis induction, Grim colocalizes with mitochondria and cytochrome c in a GH3-dependent but N-terminal- and caspase activity-independent manner. When Grim is overexpressed in vivo, both the N-terminal and the GH3 domains are equally necessary, and cooperate for apoptosis induction. The N-terminal and GH3 Grim domains thus activate independent apoptotic pathways that synergize to induce programmed cell death efficiently.

Death to flies: Drosophila as a model system to study programmed cell death

Programmed cell death (PCD) is essential for the removal of unwanted cells and is critical for both restricting cell numbers and for tissue patterning during development. Components of the cell death machinery are remarkably conserved through evolution, from worms to mammals. Central to the PCD process is the family of cysteine proteases, known as caspases, which are activated by death-inducing signals. Comparisons between C. elegans and mammalian PCD have shown that there is additional complexity in the regulation of PCD in mammals. The fruitfly, Drosophila melanogaster, is proving an ideal genetically tractable model organism, of intermediary complexity between C. elegans and mammals, in which to study the intricacies of PCD. Here, we review the literature on PCD during Drosophila development, highlighting the methods used in these studies.

Targeted mutagenesis by homologous recombination in D. melanogaster

We used a recently developed method to produce mutant alleles of five endogenous Drosophila genes, including the homolog of the p53 tumor suppressor. Transgenic expression of the FLP site-specific recombinase and the I-SceI endonuclease generates extrachromosomal linear DNA molecules in vivo. These molecules undergo homologous recombination with the corresponding chromosomal locus to generate targeted alterations of the host genome. The results address several questions about the general utility of this technique. We show that genes not near telomeres can be efficiently targeted; that no knowledge of the mutant phenotype is needed for targeting; and that insertional mutations and allelic substitutions can be easily produced.

B56-associated protein phosphatase 2A is required for survival and protects from apoptosis in Drosophila melanogaster

Protein phosphorylation and specific protein kinases can initiate signal transduction pathways leading to programmed cell death. The specific protein phosphatases regulating apoptosis have been more elusive. Using double-stranded RNA-mediated interference (RNAi), the role of protein phosphatase 2A (PP2A) in cellular signaling was investigated. Knockdown of A or C subunits individually or of combined B subunits led to concurrent loss of nontargeted PP2A subunits, suggesting that PP2A is an obligate heterotrimer in vivo. Global knockdown of PP2A activity or specific loss of redundant B56 regulatory subunits caused cell death with the morphological and biochemical changes characteristic of apoptosis in cultured S2 cells. B56:PP2A-regulated apoptosis required caspases and the upstream regulators dark, reaper, head involution defective, and dp53. In Drosophila embryos, knockdown of B56-regulated PP2A activity resulted in apoptosis and failure of gastrulation, an effect that was blocked by concurrent RNAi of the caspase DRICE: B56-regulated PP2A activity appears to be required upstream of dp53 to maintain a critical proapoptotic substrate in a dephosphorylated, inactive state, thereby preventing apoptosis in Drosophila S2 cells.

Redox state of tumor suppressor p53 regulates its sequence-specific DNA binding in DNA-damaged cells by cysteine 277

Using a bio-oligo pull-down DNA-binding assay we investigated the binding capacity of endogenous, DNA damage-induced p53 in human diploid fibroblasts to several p53-responsive elements (REs) present in p53-regulated genes. During the course of p53 accumulation, we observed a decrease in p53 binding to the GADD45 but not to the p21(WAF1/CIP1) RE. Using mutated GADD45 sequences we show that this change is dependent on the presence of cytosines at position 3 in RE pentamers and on the p53 redox state. Site-directed mutagenesis experiments demonstrated that Cys277 (a residue directly contacting base 3 in a RE pentamer) is critical for differential regulation of GADD45 in DNA-damaged cells. These data represent a novel mechanism for differential affinity of p53 to distinct REs.

Study of external low irradiation dose effects on induction of chromosome aberrations in Pisum sativum root tip meristem

The effects of low doses of ionizing radiation have been a matter of important debate over the last few years. The point of discussion concerns the validity of the linear dose-response extrapolation for low doses, used by international organizations, to establish radio-protection norms. Here, we contributed to this discussion by investigating the induction of chromosome aberrations by low to moderate doses ranging from 0 to 10 Gy in root meristem cells of 6-day-old Pisum plantlets. After acute irradiation of plantlets by a (60)Co source, the percentage of root tip meristem cells displaying chromosome aberrations was estimated immediately after irradiation and after 20 h recovery time. The dose-effect curves show non-linear responses, especially in the low dose range (0- 1 Gy), which is of particular interest. After 20 h of recovery, a steep increase of aberrations was observed for cells exposed to 0.4 Gy, followed by a plateau for doses until 1 Gy. There was an irradiation effect on plant growth during the first and second generations, showing the persistence of cell division anomalies as a long term effect of acute irradiation. This result suggests the induction of a genomic instability. Our results, in agreement with some obtained in animals, show rather non-linear dose-effect responses, with notably higher biological effects of low doses than expected.

Programmed cell death takes flight: genetic and genomic approaches to gene discovery in Drosophila

Programmed cell death (PCD) is an essential and wide-spread physiological process that results in the elimination of cells. Genes required to carry out this process have been identified, and many of these remain the subjects of intense investigation. Here, we describe PCD, its functions, and some of the consequences when it goes awry. We review PCD in the model system, the fruit fly, Drosophila melanogaster, with a particular emphasis on cell death gene discovery resulting from both genetics and genomics-based approaches.

Unrestrained caspase-dependent cell death caused by loss of Diap1 function requires the Drosophila Apaf-1 homolog, Dark

In mammals and Drosophila, apoptotic caspases are under positive control via the CED-4/Apaf-1/Dark adaptors and negative control via IAPs (inhibitor of apoptosis proteins). However, the in vivo genetic relationship between these opposing regulators is not known. In this study, we demonstrate that a dark mutation reverses catastrophic defects seen in Diap1 mutants and rescues cells specified for Diap1- regulated cell death in development and in response to genotoxic stress. We also find that dark function is required for hyperactivation of caspases which occurs in the absence of Diap1. Since the action of dark is epistatic to that of Diap1, these findings demonstrate that caspase-dependent cell death requires concurrent positive input through Apaf-1-like proteins together with disruption of IAP-caspase complexes.

Overexpression of mouse Mdm2 induces developmental phenotypes in Drosophila

The Mdm2 proto-oncogene is amplified and over-expressed in a variety of tumors. One of the major functions of Mdm2 described to date is its ability to modulate the levels and activity of the tumor suppressor protein p53. Mdm2 binds to the N-terminus of p53 and, through its action as an E3 ubiquitin ligase, targets p53 for rapid proteasomal degradation. Mdm2 can also bind to other cellular proteins such as hNumb, E2F1, Rb and Akt; however, the biological significance of these interactions is less clear. To gain insight into the function of Mdm2 in vivo, we have generated a transgenic Drosophila strain bearing the mouse Mdm2 gene. Ectopic expression of Mdm2, using the UAS/GAL4 system, causes eye and wing phenotypes in the fly. Analysis of wing imaginal discs from third instar larvae showed that expression of Mdm2 induces apoptosis. Crosses did not reveal genetic interactions between Mdm2 and the Drosophila homolog of E2F, Numb and Akt. These transgenic flies may provide a unique experimental model for exploring the molecular interactions of Mdm2 in a developmental context.

reaper is required for neuroblast apoptosis during Drosophila development

Developmentally regulated apoptosis in Drosophila requires the activity of the reaper (rpr), grim and head involution defective (hid) genes. The expression of these genes is differentially regulated, suggesting that there are distinct requirements for their proapoptotic activity in response to diverse developmental and environmental inputs. To examine this hypothesis, a mutation that removes the rpr gene was generated. In flies that lack rpr function, most developmental apoptosis was unaffected. However, the central nervous systems of rpr null flies were very enlarged. This was due to the inappropriate survival of both larval neurons and neuroblasts. Importantly, neuroblasts rescued from apoptosis remained functional, continuing to proliferate and generating many extra neurons. Males mutant for rpr exhibited behavioral defects resulting in sterility. Although both the ecdysone hormone receptor complex and p53 directly regulate rpr transcription, rpr was found to play a limited role in inducing apoptosis in response to either of these signals.

On the shoulders of giants: p63, p73 and the rise of p53

The discoveries of the p53 homologs, p63 and p73, have both fueled new insights and exposed enigmas in our understanding of the iconic p53 tumor suppressor. Although the pivotal role of p53 in cancer pathways remains unchallenged, because p63 and p73 are now implicated in stem cell identity, neurogenesis, natural immunity and homeostatic control. Despite their seemingly separate tasks, there are hints that the p53 family members both collaborate and interfere with one another. The question remains, therefore, as to whether these genes evolved to function independently or whether their familial ties still bind them in pathways of cell proliferation, death and tumorigenesis.

The damage-responsive Drosophila gene sickle encodes a novel IAP binding protein similar to but distinct from reaper, grim, and hid

In flies and mammals, critical regulators of cell death function by antagonizing Inhibitor of Apoptosis Proteins (IAPs), which themselves directly block caspase action. The three currently known IAP antagonists in Drosophila map to the H99 genomic interval required for all programmed cell death. Here we describe a fourth member of this genetic group, sickle (skl), which maps just outside of the H99 deletion. At its N terminus, Skl shares residues in common with other IAP antagonists in flies (Rpr, Grim, and Hid) and in mammals (Smac/DIABLO and Omi/Htra2). Like other activators of apoptosis mapping in the Reaper region, full-length skl induced apoptosis when overexpressed, and the N terminus of this protein specifically bound to the BIR2 domain of DIAP1. However, unlike the N termini of Grim, Hid, and Rpr, the N terminus of Skl did not induce apoptosis. skl transcripts accumulate in cells that are fated to die in some but not all regions of the embryo. Genotoxic stimuli induced skl expression, but skl was not responsive to all signals that trigger premature apoptosis. skl is potentially a fourth IAP antagonist in the "Reaper region" and a new candidate transducer of apoptotic damage signaling in Drosophila.

The evolution of diverse biological responses to DNA damage: insights from yeast and p53

The cellular response to ionizing radiation provides a conceptual framework for understanding how a yeast checkpoint system, designed to make binary decisions between arrest and cycling, evolved in a way as to allow reversible arrest, senescence or apoptosis in mammals. We propose that the diversity of responses to ionizing radiation in mammalian cells is possible because of the addition of a new regulatory control module involving the tumour-suppressor gene p53. We review the complex mechanisms controlling p53 activity and discuss how the p53 regulatory module enables cells to grow, arrest or die by integrating DNA damage checkpoint signals with the response to normal mitogenic signalling and the aberrant signalling engendered by oncogene activation.

Drosophila Chk2 is required for DNA damage-mediated cell cycle arrest and apoptosis

Chk2 is a major target of ataxia telangiectasia-mutated (ATM) and ATM- and Rad3-related (ATR). Germline mutations in Chk2 have been identified in a subset of patients with Li-Fraumeni syndrome, suggesting that Chk2 is a tumor suppressor gene. To investigate the role of Chk2 in multicellular organisms, a Drosophila chk2 (Dmchk2) mutant was generated. Dmchk2 mutants are viable but show defects in maintaining genome stability and are highly sensitive to ionizing radiation. Interestingly, mutating Dmchk2 completely blocks DNA damage-induced apoptosis and partially blocks DNA damage-induced cell cycle arrest. These results indicate that Chk2 protein plays a crucial role in the DNA damage response pathway mediating cell cycle arrest and apoptosis, and that the ATM-Chk2 pathway is likely conserved in Drosophila.

Cellular stress response and apoptosis in cancer therapy

Anticancer treatment using cytotoxic drugs is considered to mediate cell death by activating key elements of the apoptosis program and the cellular stress response. While proteolytic enzymes (caspases) serve as main effectors of apoptosis, the mechanisms involved in activation of the caspase system are less clear. Two distinct pathways upstream of the caspase cascade have been identified. Death receptors, eg, CD95 (APO-1/Fas), trigger caspase-8, and mitochondria release apoptogenic factors (cytochrome c, Apaf-1, AIF), leading to the activation of caspase-9. The stressed endoplasmic reticulum (ER) contributes to apoptosis by the unfolded protein response pathway, which induces ER chaperones, and by the ER overload response pathway, which produces cytokines via nuclear factor-kappaB. Multiple other stress-inducible molecules, such as p53, JNK, AP-1, NF-kappaB, PKC/MAPK/ERK, and members of the sphingomyelin pathway have a profound influence on apoptosis. Understanding the complex interaction between different cellular programs provides insights into sensitivity or resistance of tumor cells and identifies molecular targets for rational therapeutic intervention strategies.

Insights into cancer therapeutic design based on p53 and TRAIL receptor signaling

Knowledge of the emerging pathways of cell death downstream of the p53 tumor suppressor and the TRAIL death-inducing ligand is suggesting ways to improve therapeutic design in cancer. In contrast to its unique G1 cell cycle arresting mechanism that is maintained by p21(WAF1), there are signals transduced by p53 to multiple apoptotic effectors perhaps due to the importance of apoptosis in suppressing tumors. There is evidence for cytoplasmic as well as mitochondrial activation of caspases downstream of p53, although in some cell lineages the signal ultimately involves the mitochondria. The TRAIL signaling pathway appears promising for therapeutic development despite sharing some similarities with the toxic Fas and TNF pathways, in terms of effector molecules and downstream signals. One of the key findings is the tissue specificity of cell death responses, a feature that could be exploited in strategies to widen the therapeutic window of combination cancer therapies. Efforts continue to develop p53-targeted cancer therapy, and novel clues to enhance or block specific effectors may improve therapeutic design.

Of flies and men--studying human disease in Drosophila

During the past year, the Drosophila genome has been sequenced. More than 60% of genes implicated in human disease have Drosophila orthologues. Developments in RNA-mediated interference and homologous recombination have made 'reverse genetics' feasible in Drosophila. Conventional Drosophila genetics is being used increasingly to place human disease genes of unknown function in the context of functional pathways.

The C. elegans homolog of the p53 tumor suppressor is required for DNA damage-induced apoptosis

In mammals, one of the key regulators necessary for responding to genotoxic stress is the p53 transcription factor. p53 is the single most commonly mutated tumor suppressor gene in human cancers. Here we report the identification of a C. elegans homolog of mammalian p53. Using RNAi and DNA cosuppression technology, we show that C. elegans p53 (cep-1) is required for DNA damage-induced apoptosis in the C. elegans germline. However,cep-1 RNAi does not affect programmed cell death occurring during worm development and physiological (radiation-independent) germ cell death. The DNA binding domain of CEP-1 is related to vertebrate p53 members and possesses the conserved residues most frequently mutated in human tumors. Consistent with this, CEP-1 acts as a transcription factor and is able to activate a transcriptional reporter containing consensus human p53 binding sites. Our data support the notion that p53-mediated transcriptional regulation is part of an ancestral pathway mediating DNA damage-induced apoptosis and reveals C. elegans as a genetically tractable model organism for studying the p53 apoptotic pathway.

Caenorhabditis elegans p53: role in apoptosis, meiosis, and stress resistance

We have identified a homolog of the mammalian p53 tumor suppressor protein in the nematode Caenorhabditis elegans that is expressed ubiquitously in embryos. The gene encoding this protein, cep-1, promotes DNA damage-induced apoptosis and is required for normal meiotic chromosome segregation in the germ line. Moreover, although somatic apoptosis is unaffected, cep-1 mutants show hypersensitivity to hypoxia-induced lethality and decreased longevity in response to starvation-induced stress. Overexpression of CEP-1 promotes widespread caspase-independent cell death, demonstrating the critical importance of regulating p53 function at appropriate levels. These findings show that C. elegans p53 mediates multiple stress responses in the soma, and mediates apoptosis and meiotic chromosome segregation in the germ line.

A "no-hybrids" screen for functional antagonizers of human p53 transactivator function: dominant negativity in fission yeast

We have developed a functional "no-hybrids" screen in the fission yeast Schizosaccharomyces pombe based on the transcription transactivator activity of human p53. The screen can be used to identify antagonizers and modulators of p53 activity. Expression of functional full-length human p53 is conditionally lethal to the screen reporter strains. Co-expression of specific inhibitory proteins promotes cell survival and growth. We have validated the "no-hybrids" system by (a) successful modeling of human wild-type p53 interaction with SV40 large T antigen, Mdm2 and a panel of tumor-derived human p53 mutants, (b) demonstrating the screening system's efficiency through identification of a dominant negative fragment of p53 itself in a library screen context and (c) using Drosophila p53 to demonstrate that the system can detect evolutionarily distant p53 homologues based on their transactivator activity. The "no-hybrids" screen will be of utility in searches for p53 function-modulators of both cellular and viral origin.

Effect of low doses of ionizing radiation on cells cultured from the hematopoietic tissue of the Dublin Bay prawn, Nephrops norvegicus

Explant cultures from the hematopoietic tissue of the Dublin Bay prawn, Nephrops norvegicus, were exposed to low doses of (60)Co gamma radiation. Cells growing from the explants were examined 7 days after irradiation using light and transmission electron microscopy and were also tested for their ability to produce signals indicative of a bystander effect. The exposed cultures displayed pronounced damage and were orders of magnitude more sensitive than the data in the literature would suggest for arthropod cells. The cultures were also more sensitive than mammalian cells that were exposed to similar doses. Cellular abnormalities included damage to cytoplasmic organelles, particularly the cytoskeleton. Abnormal mitochondria were also prominent. At low doses (0.5 Gy), nuclear damage was not apparent in the cultures, but there was evidence of a dose-dependent increase in apoptosis. The irradiated cultures released a factor into the medium that was capable of inducing apoptosis and cell death in unirradiated fish and human cells. This bystander effect was of a similar magnitude to that reported for mammalian cell systems. It is suggested that these crustaceans may be highly sensitive to radiation, unlike terrestrial arthropods and certain other invertebrates, which are generally considered to be radioresistant.

Regulation of the cell cycle by p53 after DNA damage in an amphibian cell line

In mammalian cells, the p53 protein is a key regulator of the cell cycle following DNA damage. In the present study, we investigated the function of p53 in the A6 amphibian cell line. Using various specific Xenopus p53 monoclonal antibodies, we showed that Xenopus p53 accumulates after DNA damage, including gamma and UV irradiation or treatment with adriamycin. Such accumulation is accompanied by an increase in the apparent molecular weight of the protein. This change was shown to be the result of a phosphorylation event that occurs after DNA damage. Accumulation of Xenopus p53 is parallel to a drastic change in the cell cycle distribution. Brief exposure to adriamycin or gamma irradiation induces reversible growth arrest, whereas long-term exposure to adriamycin leads to apoptosis. Taken together, these results indicate that p53 has a similar behaviour in frog cells and mammalian cells, and that it conserves two activities, cell cycle arrest and apoptosis.

PUMA induces the rapid apoptosis of colorectal cancer cells

Through global profiling of genes that were expressed soon after p53 expression, we identified a novel gene termed PUMA (p53 upregulated modulator of apoptosis). The protein encoded by PUMA was found to be exclusively mitochondrial and to bind to Bcl-2 and Bcl-X(L) through a BH3 domain. Exogenous expression of PUMA resulted in an extremely rapid and profound apoptosis that occurred much earlier than that resulting from exogenous expression of p53. Based on its unique expression patterns, p53 dependence, and biochemical properties, PUMA may be a direct mediator of p53-associated apoptosis.

Expression of homologues for p53 and p73 in the softshell clam (Mya arenaria), a naturally-occurring model for human cancer

Homologues for human p53 (Hsp53) and p73 (Hsp73) genes were cloned and expression patterns for their corresponding proteins analysed in tissues from normal and leukemic softshell clams (Mya arenaria). These are the first structural and functional data for p53 and p73 cDNAs and gene products in a naturally occurring, non-mammalian disease model. Core sequence of the predicted clam p53 (Map53) and p73 (Map73) proteins is virtually identical and includes the following highly conserved regions: the transcriptional activation domain (TAD), MDM2 binding site, ATM phosphorylation site, proline rich domain, DNA binding domains (DBDs) II-V, nuclear import and export signals and the tetramerization domain. The core sequence is a structural mosaic of the corresponding human proteins, with the TAD and DBDs resembling Hsp53 and Hsp73, respectively. This suggests that Map53 and Map73 proteins may function similarly to human proteins. Clam proteins have either a short (Map53) or long (Map73) C-terminal extension. These features suggest that Map53 and Map73 may be alternate splice variants of a p63/p73-like ancestral gene. Map73 is significantly upregulated in hemocytes and adductor muscle from leukemic clams. In leukemic hemocytes, both proteins are absent from the nucleus and sequestered in the cytoplasm. This observation suggests that a non-mutational p53/p73-dependent mechanism may be involved in the clam disease. Further studies of these gene products in clams may reveal p53/p73-related molecular mechanisms that are held in common with Burkitt's lymphoma or other human cancers.

Key targets for the execution of radiation-induced tumor cell apoptosis: the role of p53 and caspases

In many human hematologic and solid malignancies, intrinsic or acquired treatment resistance remains a major obstacle for successful cancer therapy. The molecular understanding of how tumor cells respond to chemotherapy and ionizing radiation is rapidly evolving. Induction of programmed cell death, apoptosis, is one important strategy for successful cancer therapy. This has been shown convincingly for oncogene-transformed normal cells as well as tumor cells of lymphoid origin. However, the relevance of apoptosis in solid human malignancies is less clear. Loss of apoptosis might be linked to specific mutations in the often tissue-specific apoptotic pathways due to aberrations in the stress-related signal transduction cascades. Restoration of a dysfunctional apoptotic program in cancer tissue where apoptosis has been identified as an important mechanism for tissue homeostasis is one rational approach for innovative cancer therapy. In this review, we focus on the relevance of the tumor suppressor p53 for apoptosis-induction and successful cancer therapy outlining the importance of an intact caspase machinery for apoptosis execution. Strategies are discussed to overcome treatment resistance and a high apoptotic threshold in human malignancies where apoptosis is the dominant mode of cell death and the status of p53 is an important determinant for apoptosis induction.

Cell-cycle checkpoint kinases: checking in on the cell cycle

In response to DNA damage, cell-cycle checkpoints integrate cell-cycle control with DNA repair. The idea that checkpoint controls are an integral component of normal cell-cycle progression has arisen as a result of studies in Drosophila and mice. In addition, an appreciation that DNA damage arises as a natural consequence of cellular metabolism, including DNA replication itself, has influenced thinking regarding the nature of checkpoint pathways.

The Drosophila genome

The past year has been a spectacular one for Drosophila research. The sequencing and annotation of the Drosophila melanogaster genome has allowed a comprehensive analysis of the first three eukaryotes to be sequenced-yeast, worm and fly-including an analysis of the fly's influences as a model for the study of human disease. This year has also seen the initiation of a full-length cDNA sequencing project and the first analysis of Drosophila development using high-density DNA microarrays containing several thousand Drosophila genes. For the first time homologous recombination has been demonstrated in flies and targeted gene disruptions may not be far off.