ATM-dependent activation of p53 involves dephosphorylation and association with 14-3-3 proteins

p53 in neuronal apoptosis

The tumor suppressor and transcription factor p53 is a key modulator of cellular stress responses, and activation of p53 can trigger apoptosis in many cell types including neurons. Apoptosis is a form of programmed cell death that occurs in neurons during development of the nervous system and may also be responsible for neuronal deaths that occur in neurological disorders such as stroke, and Alzheimer's and Parkinson's diseases. p53 production is rapidly increased in neurons in response to a range of insults including DNA damage, oxidative stress, metabolic compromise, and cellular calcium overload. Target genes induced by p53 in neurons include those encoding the pro-apoptotic proteins Bax and the BH3-only proteins PUMA and Noxa. In addition to such transcriptional control of the cell death machinery, p53 may more directly trigger apoptosis by acting at the level of mitochondria, a process that can occur in synapses (synaptic apoptosis). Preclinical data suggest that agents that inhibit p53 may be effective therapeutics for several neurodegenerative conditions.

Evidence for distinct pathomechanisms in genetic subgroups of chronic lymphocytic leukemia revealed by quantitative expression analysis of cell cycle, activation, and apoptosis-associated genes

PURPOSE

In patients with chronic lymphocytic leukemia (CLL), the VH mutation status and genomic aberrations (13q-, +12q, 11q-, 17p-) identify distinct prognostic subgroups. The aim was to elucidate biologic mechanisms through which these genetic markers may exert their pathogenic influence.

PATIENTS AND METHODS

Twenty-four genes involved in apoptosis, cell cycle, B-cell activation, and B-cell receptor (BCR) signaling were analyzed by real-time quantitative reverse transcription polymerase chain reaction (RQ-PCR) in 82 CLL cases constituting prototypic genetic CLL subgroups as defined by the VH mutation status and the genomic aberrations 13q-, +12, 11q-, and 17p-.

RESULTS

The VH mutation subgroups were characterized by a differential expression of the BCR associated genes ZAP70 and PI3K. Among the subgroups defined by genomic aberrations, there was a deregulation of candidate genes from the affected critical genomic regions such as CDK4 (up), ATM (down), and TP53 (down) in the groups +12, 11q-, and 17p-, respectively. Additionally, the genomic subgroups were characterized by a significant deregulation of cell cycle and apoptosis regulators: AKT (up) in 13q, E2F1 (up) in +12, MYC (up) and BCL-2 (down) in 17p-, and CCND3 (down) in 11q- as well as 17p-. The 17p- subgroup showed an additional down-regulation of BCR-associated genes such as SYK and PI3K.

CONCLUSION

The characteristic gene expression patterns observed implicate a differential regulation of cell cycle, apoptosis, and BCR signaling in the genetic subgroups illustrating distinct pathomechanisms and are evidence for a gene dosage effect being operative in CLL. These findings link the biologic diversity and clinical heterogeneity of CLL.

p53 C-terminal phosphorylation by CHK1 and CHK2 participates in the regulation of DNA-damage-induced C-terminal acetylation

The tumor suppressor protein p53 mediates stress-induced growth arrest or apoptosis and plays a major role in safeguarding genome integrity. In response to DNA damage, p53 can be modified at multiple sites by phosphorylation and acetylation. We report on the characterization of p53 C-terminal phosphorylation by CHK1 and CHK2, two serine/threonine (Ser/Thr) protein kinases, previously implicated in the phosphorylation of the p53 N terminus. Using tryptic phosphopeptide mapping, we have identified six additional CHK1 and CHK2 sites residing in the final 100 amino acids of p53. Phosphorylation of at least three of these sites, Ser366, Ser378, and Thr387, was induced by DNA damage, and the induction at Ser366 and Thr387 was abrogated by small interfering RNA targeting chk1 and chk2. Furthermore, mutation of these phosphorylation sites has a different impact on p53 C-terminal acetylation and on the activation of p53-targeted promoters. Our results demonstrate a possible interplay between p53 C-terminal phosphorylation and acetylation, and they provide an additional mechanism for the control of the activity of p53 by CHK1 and CHK2.

Redox factor 1 (Ref-1) enhances specific DNA binding of p53 by promoting p53 tetramerization

Sequence-specific DNA binding is a major activity of the tumor suppressor p53 and a prerequisite for the transactivating potential of the protein. p53 interaction with target DNA is tightly regulated by various mechanisms, including binding of different components of the transcription machinery, post-translational modifications, and interactions with other factors that modulate p53 transactivation in a cell context- and promoter-specific manner. The bi-functional redox factor 1 (Ref-1/APE1) has been identified as one of the factors, which can stimulate p53 DNA binding by redox-dependent as well as redox-independent mechanisms. Whereas stimulation of p53 DNA binding by the redox activities of Ref-1 is understood quite well, little is known about mechanisms that underlie the redox-independent effects of Ref-1. We report in this study a previously unknown activity of Ref-1 as a factor promoting tetramerization of p53. We demonstrate that Ref-1 promotes association of dimers into tetramers, and de-stacking of higher oligomeric forms into the tetrameric form in vitro, thereby enhancing p53 binding to target DNA.

p53: traffic cop at the crossroads of DNA repair and recombination

p53 mutants that lack DNA-binding activities, and therefore, transcriptional activities, are among the most common mutations in human cancer. Recently, a new role for p53 has come to light, as the tumour suppressor also functions in DNA repair and recombination. In cooperation with its function in transcription, the transcription-independent roles of p53 contribute to the control and efficiency of DNA repair and recombination.

Protein kinase CK1delta phosphorylates key sites in the acidic domain of murine double-minute clone 2 protein (MDM2) that regulate p53 turnover

Murine double-minute clone 2 protein (MDM2) is an E3 ubiquitin ligase that regulates the turnover of several cellular factors including the p53 tumor suppressor protein. As part of the mechanism of p53 induction in response to DNA damage, a cluster of serine residues within the central acidic domain of MDM2 become hypophosphorylated, leading to attenuation of MDM2-mediated p53 destruction. In the present study, we identify the protein kinase CK1delta as a major cellular activity that phosphorylates MDM2. Amino acid substitution, coupled with phosphopeptide analyses, indicates that several serine residues in the acidic domain, including Ser-240, Ser-242, and Ser-246, as well as Ser-383 in the C-terminal region, are phosphorylated by CK1delta in vitro. We also show, through expression of a dominant negative mutant of CK1delta or treatment of cells with IC261, a CK1delta-selective inhibitor, that MDM2 is phosphorylated by CK1delta in cultured cells. These data establish the identity of a key signaling molecule that promotes the phosphorylation of a major regulatory region in MDM2 under normal growth conditions.

Unlocking the code of 14-3-3

One of the most striking 'rags to riches' stories in the protein world is that of 14-3-3, originally identified in 1967 as merely an abundant brain protein. The first clues that 14-3-3 would play an important role in cell biology came almost 25 years later when it was found to interact with various proto-oncogene proteins and signaling proteins. The subsequent identification of 14-3-3 as a phosphoserine/phosphothreonine-binding protein firmly established its importance in cell signaling. 14-3-3 family members are found in all eukaryotes - from plants to mammals - and more than 100 binding partners have been identified to date. The targets of 14-3-3 are found in all subcellular compartments and their functional diversity is overwhelming - they include transcription factors, biosynthetic enzymes, cytoskeletal proteins, signaling molecules, apoptosis factors and tumor suppressors. 14-3-3 binding can alter the localization, stability, phosphorylation state, activity and/or molecular interactions of a target protein. Recent studies now indicate that the serine/threonine protein phosphatases PP1 and PP2A are important regulators of 14-3-3 binding interactions, and demonstrate a role for 14-3-3 in controlling the translocation of certain proteins from the cytoplasmic and endoplasmic reticulum to the plasma membrane. New reports also link 14-3-3 to several neoplastic and neurological disorders, where it might contribute to the pathogenesis and progression of these diseases.

Dynamic interactions between 14-3-3 proteins and phosphoproteins regulate diverse cellular processes

14-3-3 proteins exert an extraordinarily widespread influence on cellular processes in all eukaryotes. They operate by binding to specific phosphorylated sites on diverse target proteins, thereby forcing conformational changes or influencing interactions between their targets and other molecules. In these ways, 14-3-3s 'finish the job' when phosphorylation alone lacks the power to drive changes in the activities of intracellular proteins. By interacting dynamically with phosphorylated proteins, 14-3-3s often trigger events that promote cell survival--in situations from preventing metabolic imbalances caused by sudden darkness in leaves to mammalian cell-survival responses to growth factors. Recent work linking specific 14-3-3 isoforms to genetic disorders and cancers, and the cellular effects of 14-3-3 agonists and antagonists, indicate that the cellular complement of 14-3-3 proteins may integrate the specificity and strength of signalling through to different cellular responses.

Stable-isotope dimethyl labeling for quantitative proteomics

In this paper, we report a novel, stable-isotope labeling strategy for quantitative proteomics that uses a simple reagent, formaldehyde, to globally label the N-terminus and epsilon-amino group of Lys through reductive amination. This labeling strategy produces peaks differing by 28 mass units for each derivatized site relative to its nonderivatized counterpart and 4 mass units for each derivatized isotopic pair. This labeling reaction is fast (less than 5 min) and complete without any detectable byproducts based on the analysis of MALDI and LC/ESI-MS/MS spectra of both derivatized and nonderivatized peptide standards and tryptic peptides of hemoglobin molecules. The intensity of the a(1) and y(n-1) ions produced, which were not detectable from most of the nonderivatized fragments, was substantially enhanced upon labeling. We further tested the method based on the analysis of an isotopic pair of peptide standards and a pair of defined protein mixtures with known H/D ratios. Using LC/MS for quantification and LC/MS/MS for peptide sequencing, the results show a negligible isotopic effect, a good mass resolution between the isotopic pair, and a good correlation between the experimental and theoretical data (errors 0-4%). The relative standard deviation of H/D values calculated from peptides deduced from the same protein are less than 13%. The applicability of the method for quantitative protein profiling was also explored by analyzing changes in nuclear protein abundance in an immortalized E7 cell with and without arsenic treatment.

A role for 14-3-3 tau in E2F1 stabilization and DNA damage-induced apoptosis

Genotoxic stress triggers apoptosis through multiple signaling pathways. Recent studies have demonstrated a specific induction of E2F1 accumulation and a role for E2F1 in apoptosis upon DNA damage. Induction of E2F1 is mediated by phosphorylation events that are dependent on DNA damage-responsive protein kinases, such as ATM. How ATM phosphorylation leads to E2F1 stabilization is unknown. We now show that 14-3-3 tau, a phosphoserine-binding protein, mediates E2F1 stabilization. 14-3-3 tau interacts with ATM-phosphorylated E2F1 during DNA damage and inhibits E2F1 ubiquitination. Depletion of 14-3-3 tau or E2F1, but not E2F2 or E2F3, blocks adriamycin-induced apoptosis. 14-3-3 tau is also required for expression and induction of E2F1 apoptotic targets, such as p73, Apaf-1, and caspases, during DNA damage. Together, these data demonstrate a novel function for 14-3-3 tau in the regulation of E2F1 protein stability and apoptosis during DNA damage.

Post-translational modification of p53 in tumorigenesis

Interest in the tumour suppressor p53 has generated much information regarding the complexity of its function and regulation in carcinogenesis. However, gaps still exist in our knowledge regarding the role of p53 post-translational modifications in carcinogenesis and cancer prevention. A thorough understanding of p53 will be extremely useful in the development of new strategies for treating and preventing cancer, including restoration of p53 function and selective killing of tumours with mutant TP53.

14-3-3 Proteins—a focus on cancer and human disease

14-3-3 Proteins are a ubiquitous family of molecules that participate in protein kinase signaling pathways within all eukaryotic cells. Functioning as phosphoserine/phosphothreonine-binding modules, 14-3-3 proteins participate in phosphorylation-dependent protein-protein interactions that control progression through the cell cycle, initiation and maintenance of DNA damage checkpoints, activation of MAP kinases, prevention of apoptosis, and coordination of integrin signaling and cytoskeletal dynamics. In this review, we discuss the regulation of 14-3-3 structure and ligand binding, with a focus on the role of 14-3-3 proteins in human disease, particularly cancer. We discuss the latest data on the role of different 14-3-3 isotypes, the interaction of 14-3-3 proteins with Raf, Cdc25, and various integrin family members, and the likelihood that 14-3-3 proteins could be useful therapeutic targets in the treatment of human disease.

14-3-3σ Mediation of Cell Cycle Progression Is p53-independent in Response to Insulin-like Growth Factor-I Receptor Activation

We investigated the role of 14-3-3sigma protein in insulin-like growth factor-I (IGF-I) receptor signaling. It has been previously shown that 14-3-3sigma negatively regulates cell cycle especially in response to p53-sensitive DNA damage. In this study we demonstrated that 14-3-3sigma is a positive mediator of IGF-I receptor-induced cell proliferation. Treatment with IGF-I increased 14-3-3sigma mRNA and protein levels about 4-fold, in a time-dependent manner in MCF-7 breast cancer cells. Preincubation with the phosphoinositide 3'-kinase inhibitor LY294002 significantly reduced the effects of IGF-I on 14-3-3sigma gene expression in these cells, suggesting that this effect of IGF-I occurs via the phosphoinositide 3'-kinase pathway. 14-3-3sigma is induced by IGF-I in MCF-7 cells, which express wild-type p53, as well as in MCF-7 cells transfected with a small interference RNA targeting duplex that reduced p53 expression levels. These results suggest that IGF-I induces 14-3-3sigma expression in a manner that is independent of p53. Using the small interference RNA strategy, we demonstrated that a 70-75% reduction of 14-3-3sigma mRNA levels resulted in a similar decrease in the effects of IGF-I on cell cycle progression and proliferation in MCF-7 cells. This effect was also associated with a reduction in IGF-I-induced cyclin D1 expression. Taken together, these results suggest that 14-3-3sigma positively mediates IGF-I-induced cell cycle progression.

The switch from survival responses to apoptosis after chromosomal breaks

Eukaryotic cells have evolved highly sophisticated cellular responses to DNA double strand breaks (DSBs) that increase the likelihood of survival. However, cells can also respond to DSBs by undergoing programmed cell death. The mechanisms underlying the cellular decision on whether to repair and survive or to die are not well understood but may be related to the efficiency of repair or the extent of the damage. Presumably, a few easily reparable DSBs will not result in cell death in most cell types. However, abundant complex DSBs will present a severe challenge to the repair machineries with repeated attempts at repair likely to result in genome instability. For multicellular eukaryotes at least, struggling to complete repair is folly, whereas removal of severely damaged cells is a more sensible strategy. Here we discuss recent evidence linking DSBs to a highly regulated form of cell death termed, apoptosis. In particular, we focus on the roles of the tumour suppressor, p53 and a recently discovered role for an isotype of the linker histone H1. We present a hypothesis that the elevated levels of ssDNA produced during ongoing attempts at DSB repair may be involved in the switch from repair to apoptosis.

14-3-3 proteins: a number of functions for a numbered protein

Many signal transduction events are orchestrated by specific interactions of proteins mediated through discrete phosphopeptide-binding motifs. Although several phosphospecific-binding domains are now known, 14-3-3s were the first proteins recognized to specifically bind a discrete phosphoserine or phosphothreonine motif. The 14-3-3 proteins are a family of ubiquitously expressed, exclusively eukaryotic proteins with an astonishingly large number of binding partners. Consequently, 14-3-3s modulate an enormous and diverse group of cellular processes. The effects of 14-3-3 proteins on their targets can be broadly defined using three categories: (i) conformational change; (ii) physical occlusion of sequence-specific or structural protein features; and (iii) scaffolding. This review will describe the current state of knowledge on 14-3-3 proteins, highlighting several important advances, and will attempt to provide a framework by which 14-3-3 functions can be understood.

Activation of the DNA-binding ability of latent p53 protein by protein kinase C is abolished by protein kinase CK2

p53 is one of the most important regulators of cell proliferation and differentiation and of programmed cell death, triggering growth arrest and/or apoptosis in response to different cellular stress signals. The sequence-specific DNA-binding function of p53 protein can be activated by several different stimuli that modulate the C-terminal domain of this protein. The predominant mechanism of activation of p53 sequence-specific DNA binding is phosphorylation at specific sites. For example, phosphorylation of p53 by PKC (protein kinase C) occurs in undamaged cells, resulting in masking of the epitope recognized by monoclonal antibody PAb421, and presumably promotes steady-state levels of p53 activity in cycling cells. In contrast, phosphorylation by cdk2 (cyclin-dependent kinase 2)/cyclin A and by the protein kinase CK2 are both enhanced in DNA-damaged cells. We determined whether one mechanism to account for this mutually exclusive phosphorylation may be that each phosphorylation event prevents modification by the other kinase. We used non-radioactive electrophoretic mobility shift assays to show that C-terminal phosphorylation of p53 protein by cdk2/cyclin A on Ser315 or by PKC on Ser378 can efficiently stimulate p53 binding to DNA in vitro, as well as binding of the monoclonal antibody Bp53-10, which recognizes residues 371-380 in the C-terminus of p53. Phosphorylation of p53 by CK2 on Ser392 induces its DNA-binding activity to a much lower extent than phosphorylation by cdk2/cyclin A or PKC. In addition, phosphorylation by CK2 strongly inhibits PKC-induced activation of p53 DNA binding, while the activation of p53 by cdk2/cyclin A is not affected by CK2. The presence of CK2-mediated phosphorylation promotes PKC binding to its docking site within the p53 oligomerization domain, but decreases phosphorylation by PKC, suggesting that competition between CK2 and PKC does not rely on the inhibition of PKC-p53 complex formation. These results indicate the crucial role of p53 C-terminal phosphorylation in the regulation of its DNA-binding activity, but also suggest that antagonistic relationships exist between different stress signalling pathways.

Phosphorylation on Thr-55 by TAF1 mediates degradation of p53: a role for TAF1 in cell G1 progression

The largest subunit of TFIID, TAF1, possesses an intrinsic protein kinase activity and is important for cell G1 progression and apoptosis. Since p53 functions by inducing cell G1 arrest and apoptosis, we investigated the link between TAF1 and p53. We found that TAF1 induces G1 progression in a p53-dependent manner. TAF1 interacts with and phosphorylates p53 at Thr-55 in vivo. Substitution of Thr-55 with an alanine residue (T55A) stabilizes p53 and impairs the ability of TAF1 to induce G1 progression. Furthermore, both RNAi-mediated TAF1 ablation and apigenin-mediated inhibition of the kinase activity of TAF1 markedly reduced Thr-55 phosphorylation. Thus, phosphorylation and the resultant degradation of p53 provide a mechanism for regulation of the cell cycle by TAF1. Significantly, the Thr-55 phosphorylation was reduced following DNA damage, suggesting that this phosphorylation contributes to the stabilization of p53 in response to DNA damage.

Rep68 protein of adeno-associated virus type 2 interacts with 14-3-3 proteins depending on phosphorylation at serine 535

Rep78/68 proteins of adeno-associated virus type 2 (AAV-2) are involved in many aspects of the viral life cycle, including replication, gene expression, and site-specific integration. To understand the molecular mechanisms of the actions of Rep proteins, we searched for Rep68-interacting cellular proteins by utilizing a one-step affinity purification technique and identified two members of 14-3-3 proteins (14-3-3 epsilon and gamma). We found that phosphorylation of 535Ser at the carboxy terminus of Rep68 was critical for its association with 14-3-3. The association of 14-3-3 proteins to Rep68 resulted in reduction of the affinity of Rep68 for DNA. Furthermore, genome DNA replication of a recombinant mutant virus carrying a phosphorylation-deficient Rep68 (Ser535Ala) was more efficient than that of the wild-type virus. These results suggest that phosphorylation of Rep68 and subsequent association with 14-3-3 proteins regulates Rep-mediated functions during the AAV life cycle.

Endoplasmic reticulum stress induces p53 cytoplasmic localization and prevents p53-dependent apoptosis by a pathway involving glycogen synthase kinase-3beta

The tumor suppressor p53, a sensor of multiple forms of cellular stress, is regulated by post-translational mechanisms to induce cell-cycle arrest, senescence, or apoptosis. We demonstrate that endoplasmic reticulum (ER) stress inhibits p53-mediated apoptosis. The mechanism of inhibition involves the increased cytoplasmic localization of p53 due to phosphorylation at serine 315 and serine 376, which is mediated by glycogen synthase kinase-3 beta (GSK-3beta). ER stress induces GSK-3beta binding to p53 in the nucleus and enhances the cytoplasmic localization of the tumor suppressor. Inhibition of apoptosis caused by ER stress requires GSK-3beta and does not occur in cells expressing p53 with mutation(s) of serine 315 and/or serine 376 to alanine(s). As a result of the increased cytoplasmic localization, ER stress prevents p53 stabilization and p53-mediated apoptosis upon DNA damage. It is concluded that inactivation of p53 is a protective mechanism utilized by cells to adapt to ER stress.

ATM and ATR: Sensing DNA damage

Cellular response to genotoxic stress is a very complex process, and it usually starts with the “sensing” or “detection” of the DNA damage, followed by a series of events that include signal transduction and activation of transcription factors. The activated transcription factors induce expressions of many genes which are involved in cellular functions such as DNA repair, cell cycle arrest, and cell death. There have been extensive studies from multiple disciplines exploring the mechanisms of cellular genotoxic responses, which have resulted in the identification of many cellular components involved in this process, including the mitogen-activated protein kinases (MAPKs) cascade. Although the initial activation of protein kinase cascade is not fully understood, human protein kinases ATM (ataxia-telangiectasia, mutated) and ATR (ATM and Rad3-related) are emerging as potential sensors of DNA damage. Current progresses in ATM/ATR research and related signaling pathways are discussed in this review, in an effort to facilitate a better understanding of genotoxic stress response.

The Centrosome in Higher Organisms: Structure, Composition, and Duplication

The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.

14-3-3 sigma positively regulates p53 and suppresses tumor growth

The 14-3-3 sigma (sigma) protein, a negative regulator of the cell cycle, is a human mammary epithelium-specific marker that is downregulated in transformed mammary carcinoma cells. It has also been identified as a p53-inducible gene product involved in cell cycle checkpoint control after DNA damage. Although 14-3-3 sigma is linked to p53-regulated cell cycle checkpoint control, detailed mechanisms of how cell cycle regulation occurs remain unclear. Decreased expression of 14-3-3 sigma was recently reported in several types of carcinomas, further suggesting that the negative regulatory role of 14-3-3 sigma in the cell cycle is compromised during tumorigenesis. However, this possible tumor-suppressive role of 14-3-3 sigma has not yet been characterized. Here, we studied the link between 14-3-3 sigma activities and p53 regulation. We found that 14-3-3 sigma interacted with p53 in response to the DNA-damaging agent adriamycin. Importantly, 14-3-3 sigma expression led to stabilized expression of p53. In studying the molecular mechanism of this increased stabilization of p53, we found that 14-3-3 sigma antagonized the biological functions of Mdm2 by blocking Mdm2-mediated p53 ubiquitination and nuclear export. In addition, we found that 14-3-3 sigma facilitated the oligomerization of p53 and enhanced p53's transcriptional activity. As a target gene of p53, 14-3-3 sigma appears to have a positive feedback effect on p53 activity. Significantly, we also showed that overexpression of 14-3-3 sigma inhibited oncogene-activated tumorigenicity in a tetracycline-regulated 14-3-3 sigma system. These results defined an important p53 regulatory loop and suggested that 14-3-3 sigma expression can be considered for therapeutic intervention in cancers.

Phosphorylation site interdependence of human p53 post-translational modifications in response to stress

Modification-specific antibodies were used to characterize the phosphorylation and acetylation of human p53 in response to genotoxic (UV, IR, and adriamycin) and non-genotoxic (PALA, taxol, nocodazole) stress in cultured human cells at 14 known modification sites. In A549 cells, phosphorylation or acetylation was induced at most sites by the three DNA damage-inducing agents, but significant differences between agents were observed. IR-induced phosphorylation reached a maximum 2 h after treatment and returned to near pretreatment levels by 72 h; UV light and adriamycin induced a less rapid but more robust and prolonged p53 phosphorylation, which reached a maximum between 8 and 24 h, but persisted (UV) even 96 h after treatment. Ser33, Ser37, Ser46, and Ser392 were more efficiently phosphorylated after exposure to UV light than after IR. The non-genotoxic agents PALA, taxol and nocodazole induced p53 accumulation and phosphorylation at Ser6, Ser33, Ser46, and Ser392. Some phosphorylation at Ser15 also was observed. Modifications occurred similarly in the HCT116 human colon carcinoma cell line. Analysis of single site mutant p53s indicated clear interdependences between N-terminal phosphorylation sites, which could be classified in four clusters: Ser6 and Ser9; Ser9, Ser15, Thr18 and Ser20; Ser33 and Ser37; and Ser46. We suggest that p53 phosphorylation is regulated through a double cascade involving both the activation of secondary, effector protein kinases as well as intermolecular phosphorylation site interdependencies that check inappropriate p53 inactivation while allowing for signal amplification and the integration of signals from multiple stress pathways.

Transcription factors activated in mammalian cells after clinically relevant doses of ionizing radiation

Over the past 15 years, a wealth of information has been published on transcripts and proteins 'induced' (requiring new protein synthesis) in mammalian cells after ionizing radiation (IR) exposure. Many of these studies have also attempted to elucidate the transcription factors that are 'activated' (i.e., not requiring de novo synthesis) in specific cells by IR. Unfortunately, all too often this information has been obtained using supralethal doses of IR, with investigators assuming that induction of these proteins, or activation of corresponding transcription factors, can be 'extrapolated' to low-dose IR exposures. This review focuses on what is known at the molecular level about transcription factors induced at clinically relevant (< or =2 Gy) doses of IR. A review of the literature demonstrates that extrapolation from high doses of IR to low doses of IR is inaccurate for most transcription factors and most IR-inducible transcripts/proteins, and that induction of transactivating proteins at low doses must be empirically derived. The signal transduction pathways stimulated after high versus low doses of IR, which act to transactivate certain transcription factors in the cell, will be discussed. To date, only three transcription factors appear to be responsive (i.e. activated) after physiological doses (doses wherein cells survive or recover) of IR. These are p53, nuclear factor kappa B(NF-kappaB), and the SP1-related retinoblastoma control proteins (RCPs). Clearly, more information on transcription factors and proteins induced in mammalian cells at clinically or environmentally relevant doses of IR is needed to understand the role of these stress responses in cancer susceptibility/resistance and radio-sensitivity/resistance mechanisms.

Dephosphorylation of p53 during cell death by N-alpha-tosyl-L-phenylalanyl chloromethyl ketone

The apoptotic function of N-alpha-tosyl-L-phenylalanyl chloromethyl ketone (TPCK) was investigated in cultured human colorectal carcinoma cells (HCT116). TPCK-induced apoptosis was shown to be p53-dependent in HCT116 cells during the early stage of incubation. The function of p53 was required for TPCK-induced activation of caspase-3 and caspase-7. TPCK promoted dephosphorylation of p53 on serine residues at 6, 9, 46, 376, and 378 in parallel with the activation of p53 transcriptional activity. HCT116 p53-/- cells expressing p53 mutant, in which serine residues at 6, 9, 46, 376, and 378 were replaced by aspartic acids, were resistant to TPCK-induced apoptosis suggesting the requirement of dephosphorylation of p53 on serine residues during TPCK-induced apoptosis.

Function of p73, not of p53, is inhibited by the physical interaction with RACK1 and its inhibitory effect is counteracted by pRB

The newly identified p53-related gene, p73, encodes a nuclear transcription factor. Unlike p53, p73 has various isoforms with different NH(2)- and COOH-terminal tails. p73alpha with the longest COOH-terminal extension is most abundantly expressed in many tissues and cells among those splicing isoforms of p73 and the COOH-terminal region appears to have an autoregulatory function. To isolate and characterize the cellular protein(s) that interacts with the unique COOH-terminal region of p73alpha, we employed a yeast two-hybrid screen with a human fetal brain and 293 cell cDNA libraries. We identified the receptor for activated C kinase (RACK1) as a new member of p73alpha-binding proteins. The interaction was confirmed by coimmunoprecipitation experiments, whereas RACK1 did not interact with p53 or p73beta. Ectopic overexpression of RACK1 in SAOS-2 cells reduced the p73alpha-mediated transcription from the p53/p73-responsive promoters, and inhibited the p73alpha-dependent apoptosis. On the other hand, the p53-dependent transcriptional activation as well as apoptosis was unaffected in the presence of RACK1. Furthermore, we found that pRB physically bound to RACK1, and repressed the RACK1-dependent inhibition of p73alpha. Taken together, our observations suggest that pRB diminishes the RACK1-mediated inhibition of p73alpha activity through the interaction with RACK1.

Cell cycle regulation and p53 activation by protein phosphatase 2C alpha

Protein phosphatase 2C (PP2C) dephosphorylates a broad range of substrates, regulating stress response and growth-related pathways in both prokaryotes and eukaryotes. We now demonstrate that PP2C alpha, a major mammalian isoform, inhibits cell growth and activates the p53 pathway. In 293 cell clones, in which PP2C alpha expression is regulated by a tetracycline-inducible promoter, PP2C alpha overexpression led to G(2)/M cell cycle arrest and apoptosis. Furthermore, PP2C alpha induced the expression of endogenous p53 and the p53-responsive gene p21. Activation of the p53 pathway by PP2C alpha took place both in cells harboring endogenous p53, as well as in p53-null cells transfected with exogenous p53. Induction of PP2C alpha resulted in an increase in the overall levels of p53 protein as well as an augmentation of p53 transcription activity. The dephosphorylation activity of PP2C alpha is essential to the described phenomena, as none of these effects was detected when an enzymatically inactive PP2C alpha mutant was overexpressed. p53 plays an important role in PP2C alpha-directed cell cycle arrest and apoptosis because perturbation of p53 expression in human 293 cells by human papillomavirus E6 led to a significant increase in cell survival. The role of PP2C alpha in p53 activation is discussed.

TERT suppresses apoptotis at a premitochondrial step by a mechanism requiring reverse transcriptase activity and 14-3-3 protein-binding ability

The catalytic subunit of telomerase (TERT) is a reverse transcriptase (RT) that adds a six-base DNA repeat onto chromosome ends and prevents their shortening during successive cell divisions. Telomerase is associated with cell immortality and cancer, which may by related to the ability of TERT to prevent apoptosis by stabilizing telomeres. However, fundamental information concerning the antiapoptotic function of TERT is lacking, including whether RT activity and/or nuclear localization are required and where telomerase acts to suppress the cell death process. Here, we show that overexpression of wild-type human TERT in HeLa cells, and in a cells lacking TERT but containing the telomerase RNA template, increases their resistance to apoptosis induced by the DNA damaging agent etoposide or the bacterial alkaloid staurosporine. In contrast, TERT mutants with disruptions of either the RT domain or a 14-3-3 binding domain fail to protect cells against apoptosis, and overexpression of TERT in cells lacking the telomerase RNA template is also ineffective in preventing apoptosis. Additional findings show that TERT suppresses apoptosis at an early step before release of cytochrome c and apoptosis-inducing factor from mitochondria. We conclude that both RT activity and 14-3-3 protein binding ability are required for the antiapoptotic function of TERT in tumor cells and that TERT can suppress a nuclear signal(s) that is an essential component of apoptotic cascades triggered by diverse stimuli.

Type III intermediate filament proteins interact with four-way junction DNA and facilitate its cleavage by the junction-resolving enzyme T7 endonuclease I

The isolation from proliferating mouse and human embryo fibroblasts of SDS-stable crosslinkage products of vimentin with DNA fragments containing inverted repeats capable of cruciform formation under superhelical stress and the competitive effect of a synthetic Holliday junction on the binding of cytoplasmic intermediate filament (cIF) proteins to supercoiled DNA prompted a detailed investigation of the proteins' capacity to associate with four-way junction DNA and to influence its processing by junction-resolving endonucleases. Electrophoretic mobility shift analysis of reaction products obtained from vimentin and Holliday junctions under varying ionic conditions revealed efficient complex formation of the filament protein not only with the unstacked, square-planar configuration of the junctions but also with their coaxially stacked X-conformation. Glial fibrillary acidic protein (GFAP) was less efficient and desmin virtually inactive in complex formation. Electron microscopy showed binding of vimentin tetramers or octamers almost exclusively to the branchpoint of the Holliday junctions under physiological ionic conditions. Even at several hundredfold molar excess, sequence-related single- and double-stranded DNAs were unable to chase Holliday junctions from their complexes with vimentin. Vimentin also stimulated bacteriophage T7 endonuclease I in introducing single-strand cuts diametrically across the branchpoint and thus in the resolution of the Holliday junctions. This effect is very likely due to vimentin-induced structural distortion of the branchpoint, as suggested by the results of hydroxyl radical footprinting of Holliday junctions in the absence and the presence of vimentin. Moreover, vimentin, and to a lesser extent GFAP and desmin, interacted with the cruciform structures of inverted repeats inserted into a supercoiled vector plasmid, thereby changing their configuration via branch migration and sensibilizing them to processing by T7 endonuclease I. This refers to both plasmid relaxation caused by unilateral scission and, particularly, linearization via bilateral scission at primary and cIF protein-induced secondary cruciform branchpoints that were identified by T7 endonuclease I footprinting. cIF proteins share these activities with a variety of other architectural proteins interacting with and structurally modulating four-way DNA junctions. In view of the known and hypothetical functions of four-way DNA junctions and associated protein factors in DNA metabolism, cIF proteins as complementary nuclear matrix proteins may play important roles in such nuclear matrix-associated processes as DNA replication, recombination, repair, and transcription, with special emphasis on both the preservation and evolution of the genome.

Monomeric 14-3-3 protein is sufficient to modulate the activity of the Drosophila slowpoke calcium-dependent potassium channel

Drosophila 14-3-3zeta (D14-3-3zeta) modulates the activity of the Slowpoke calcium-dependent potassium channel (dSlo) by interacting with the dSlo binding protein, Slob. We show here that D14-3-3zeta forms dimers in vitro. Site-directed mutations in its putative dimerization interface result in a dimerization-deficient form of D14-3-3zeta. Both the wild-type and dimerization-deficient forms of D14-3-3zeta bind to Slob with similar affinity and form complexes with dSlo. When dSlo and Slob are expressed in mammalian cells, the dSlo channel activity is similarly modulated by co-expression of either the wild-type or the dimerization-deficient form of D14-3-3zeta. In addition, dSlo is still modulated by wild-type D14-3-3zeta in the presence of a 14-3-3 mutant, which does not itself bind to Slob but forms heterodimers with the wild-type 14-3-3. These data, taken together, suggest that monomeric D14-3-3zeta is capable of modulating dSlo channel activity in this regulatory complex.

14-3-3sigma is a cruciform DNA binding protein and associates in vivo with origins of DNA replication

A human cruciform binding protein (CBP) was previously shown to bind to cruciform DNA in a structure-specific manner and be a member of the 14-3-3 protein family. CBP had been found to contain the 14-3-3 isoforms beta, gamma, epsilon, and zeta. Here, we show by Western blot analysis that the CBP-cruciform DNA complex eluted from band-shift polyacrylamide gels also contains the 14-3-3sigma isoform, which is present in HeLa cell nuclear extracts. An antibody specific for the 14-3-3sigma isoform was able to interfere with the formation of the CBP-cruciform DNA complex. The effect of the same anti-14-3-3sigma antibody in the in vitro replication of p186, a plasmid containing the minimal replication origin of the monkey origin ors8, was also analyzed. Pre-incubation of total HeLa cell extracts with this antibody decreased p186 in vitro replication to approximately 30% of control levels, while non-specific antibodies had no effect. 14-3-3sigma was found to associate in vivo with the monkey origins of DNA replication ors8 and ors12 in a cell cycle-dependent manner, as assayed by a chromatin immunoprecipitation (ChIP) assay that involved formaldehyde cross-linking, followed by immunoprecipitation with anti-14-3-3sigma antibody and quantitative PCR. The association of 14-3-3sigma with the replication origins was maximal at the G(1)/S phase. The results indicate that 14-3-3sigma is an origin binding protein involved in the regulation of DNA replication via cruciform DNA binding.

Paradoxical effects of a stress signal on pro- and anti-apoptotic machinery in HTLV-1 Tax expressing cells

Adult T-cell leukemia (ATL) and HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) are associated with Human T-cell lymphotropic virus type 1 (HTLV-1) infection. The viral transactivator, Tax is able to mediate the cell cycle progression by targeting key regulators of the cell cycle such as p21/waf1, p16/ink4a, p53, cyclins D1-3/cdk complexes, and the mitotic spindle checkpoint MAD apparatus, thereby deregulating cellular DNA damage and checkpoint control. Genome expression profiling of infected cells exemplified by the development of DNA microarrays represents a major advance in genome-wide functional analysis. Utilizing cDNA microarray analysis, we have observed an apparent opposing and paradoxical regulatory network of host cell gene expression upon the introduction of DNA damage stress signal. We find the apparent induction of cell cycle inhibitors, and pro- as well as anti-apoptotic gene expression is directly linked to whether cells are at either G1, S, or G2/M phases of the cell cycle. Specifically, a G1/S block is induced by p21/waf1 and p16/ink4a, while pro-apoptotic expression at S, and G2/M is associated with caspase activation, and anti-apoptotic gene expression is associated with up regulation of Bcl-2 family member, namely bfl-1 gene. Therefore, the microarray results indicating expression of both pro- and anti-apoptotic genes could easily be explained by the particular stage of the cell cycle. Mechanism and the functional outcome of induction for both pathways are discussed.

ATM and related protein kinases: safeguarding genome integrity

Maintenance of genome stability is essential for avoiding the passage to neoplasia. The DNA-damage response--a cornerstone of genome stability--occurs by a swift transduction of the DNA-damage signal to many cellular pathways. A prime example is the cellular response to DNA double-strand breaks, which activate the ATM protein kinase that, in turn, modulates numerous signalling pathways. ATM mutations lead to the cancer-predisposing genetic disorder ataxia-telangiectasia (A-T). Understanding ATM's mode of action provides new insights into the association between defective responses to DNA damage and cancer, and brings us closer to resolving the issue of cancer predisposition in some A-T carriers.

Multisite phosphorylation provides sophisticated regulation of transcription factors

Reversible phosphorylation is a prevalent mechanism by which the activity of eukaryotic transcription factors is regulated rapidly in response to changes in the cellular environment. Accumulated evidence has expanded the concept of phosphorylation to a process that provides dynamic and precise tuning of the transactivating potential of a factor, rather than being a static on/off switch. In the case of transcription factors such as heat shock factor 1 (HSF1), p53 and nuclear factor of activated T cells (NFAT), multisite phosphorylation enables several effects to operate within a single factor, thereby functioning as a key to signal integration. Studies on these transcription factors illustrate recent progress in solving the dynamic nature of transcriptional regulation by multisite phosphorylation.

Recombinational DNA repair and human disease

We review the genes and proteins related to the homologous recombinational repair (HRR) pathway that are implicated in cancer through either genetic disorders that predispose to cancer through chromosome instability or the occurrence of somatic mutations that contribute to carcinogenesis. Ataxia telangiectasia (AT), Nijmegen breakage syndrome (NBS), and an ataxia-like disorder (ATLD), are chromosome instability disorders that are defective in the ataxia telangiectasia mutated (ATM), NBS, and Mre11 genes, respectively. These genes are critical in maintaining cellular resistance to ionizing radiation (IR), which kills largely by the production of double-strand breaks (DSBs). Bloom syndrome involves a defect in the BLM helicase, which seems to play a role in restarting DNA replication forks that are blocked at lesions, thereby promoting chromosome stability. The Werner syndrome gene (WRN) helicase, another member of the RecQ family like BLM, has very recently been found to help mediate homologous recombination. Fanconi anemia (FA) is a genetically complex chromosomal instability disorder involving seven or more genes, one of which is BRCA2. FA may be at least partially caused by the aberrant production of reactive oxidative species. The breast cancer-associated BRCA1 and BRCA2 proteins are strongly implicated in HRR; BRCA2 associates with Rad51 and appears to regulate its activity. We discuss in detail the phenotypes of the various mutant cell lines and the signaling pathways mediated by the ATM kinase. ATM's phosphorylation targets can be grouped into oxidative stress-mediated transcriptional changes, cell cycle checkpoints, and recombinational repair. We present the DNA damage response pathways by using the DSB as the prototype lesion, whose incorrect repair can initiate and augment karyotypic abnormalities.

Kainic acid induces 14-3-3 zeta expression in distinct regions of rat brain

Areas of the limbic system of adult male Wistar rats were screened for kainic-acid-induced gene expression. Polymerase-chain-reaction-based differential display identified a 147-bp cDNA fragment, which represented an mRNA that was upregulated in the entorhinal cortex and hippocampus in the kainic-acid-treated animals. The sequence was 97.8% homologous to rat 14-3-3 zeta isoform mRNA. Detailed Northern analysis revealed increased mRNA levels in the entorhinal cortex 1 h after kainic acid exposure and continued elevation 24 h post-injection in both the entorhinal cortex and hippocampus. Western blot analyses confirmed that the protein product of this gene was also present in increased amounts over the same time period. Immunohistochemistry and terminal transferase-mediated dUTP nick end labelling (TUNEL) detected expression of 14-3-3 zeta protein exclusively in the entorhinal cortex and hippocampus, and only in TUNEL-positive neuronal cells. Expression of the tumor suppressor protein, p53 was also induced by kainate injection, and was co-localized with 14-3-3 zeta protein in selected cells only in the affected brain regions. The increase gene expression of 14-3-3 zeta represents a transcription-mediated response associated with region selective neuronal damage induced by kainic acid.

Ionizing radiation activates nuclear protein phosphatase-1 by ATM-dependent dephosphorylation

Ionizing radiation (IR) is known to activate multiple signaling pathways, resulting in diverse stress responses including apoptosis, cell cycle arrest, and gene induction. IR-activated cell cycle checkpoints are regulated by Ser/Thr phosphorylation, so we tested to see if protein phosphatases were targets of an IR-activated damage-sensing pathway. Jurkat cells were subjected to IR or sham radiation followed by brief (32)P metabolic labeling. Nuclear extracts were subjected to microcystin affinity chromatography to recover phosphatases, and the proteins were analyzed by two-dimensional gel electrophoresis. Protein sequencing revealed that the microcystin-bound proteins with the greatest reduction in (32)P intensity following IR were the alpha and delta isoforms of protein phosphatase 1 (PP1). Both of these PP1 isoforms contain an Arg-Pro-Ile/Val-Thr-Pro-Pro-Arg sequence near the C terminus, a known site of phosphorylation by Cdc/Cdk kinases, and phosphorylation attenuates phosphatase activity. In wild-type Jurkat cells or ataxia telangiectasia (AT) cells that are stably transfected with full-length ATM kinase, IR resulted in net dephosphorylation of this site in PP1 and produced activation of PP1. However, in AT cells that are deficient in ATM, IR failed to induce dephosphorylation or activation of PP1. IR-induced PP1 activation in the nucleus may be a critical component in an ATM-mediated pathway controlling checkpoint activation.

MADM, a novel adaptor protein that mediates phosphorylation of the 14-3-3 binding site of myeloid leukemia factor 1

A yeast two-hybrid screen was conducted to identify binding partners of Mlf1, an oncoprotein recently identified in a translocation with nucleophosmin that causes acute myeloid leukemia. Two proteins isolated in this screen were 14-3-3zeta and a novel adaptor, Madm. Mlf1 contains a classic RSXSXP sequence for 14-3-3 binding and is associated with 14-3-3zeta via this phosphorylated motif. Madm co-immunoprecipitated with Mlf1 and co-localized in the cytoplasm. In addition, Madm recruited a serine kinase, which phosphorylated both Madm and Mlf1 including the RSXSXP motif. In contrast to wild-type Mlf1, the oncogenic fusion protein nucleophosmin (NPM)-MLF1 did not bind 14-3-3zeta, had altered Madm binding, and localized exclusively in the nucleus. Ectopic expression of Madm in M1 myeloid cells suppressed cytokine-induced differentiation unlike Mlf1, which promotes maturation. Because the Mlf1 binding region of Madm and its own dimerization domain overlapped, the levels of Madm and Mlf1 may affect complex formation and regulate differentiation. In summary, this study has identified two partner proteins of Mlf1 that may influence its subcellular localization and biological function.

Activation of Bcl-2-associated death protein and counter-response of Akt within cell populations during seizure-induced neuronal death

Bcl-2 family gene products are critical to the integration of cell death stimuli that target the mitochondrion. Proapoptotic BAD (Bcl-2-associated death protein) has been shown to dissociate from its sequestered site with the molecular chaperone protein 14-3-3 and displace proapoptotic BAX (Bcl-2-associated X protein) from antiapoptotic BCL-Xl. BAX subsequently translocates to the mitochondrion and induces cytochrome c release and caspase activation. Herein we report the response of the key members of this proposed pathway after seizures. Seizures evoked by microinjection of kainic acid into the amygdala of the rat induced unilateral CA3 pyramidal neuron death with features of apoptosis. In control hippocampus and cortex, BAD was found constitutively bound to 14-3-3, whereas BCL-Xl bound BAX. Within damaged hippocampus, seizures induced the dissociation of BAD from 14-3-3 and the subsequent dimerization of BAD with BCL-Xl as determined by immunoprecipitation and immunohistochemical colocalization. 14-3-3 was found to translocate to the nucleus of degenerating neurons, whereas BAX accumulated at mitochondrial membranes. In contrast, the primarily uninjured cortex exhibited increased phosphorylation of Akt (protein kinase B), which may phosphorylate and inhibit BAD, and no altered binding of BAD to BCL-Xl. Finally, administration of an inhibitor of phosphatidylinositol 3-kinase (LY294002), thought to be an upstream activator of Akt, exacerbated cortical apoptosis after seizures. These data suggest that seizures elicit divergent cell death and survival responses within neuronal populations and that the BAD cell death pathway may perform an instigator or reinforcement role in seizure-induced neuronal death.

Initiation of eukaryotic DNA replication: regulation and mechanisms

The accurate and timely duplication of the genome is a major task for eukaryotic cells. This process requires the cooperation of multiple factors to ensure the stability of the genetic information of each cell. Mutations, rearrangements, or loss of chromosomes can be detrimental to a single cell as well as to the whole organism, causing failures, disease, or death. Because of the size of eukaryotic genomes, chromosomal duplication is accomplished in a multiparallel process. In human somatic cells between 10,000 and 100,000 parallel synthesis sites are present. This raises fundamental problems for eukaryotic cells to coordinate the start of DNA replication at each origin and to prevent replication of already duplicated DNA regions. Since these general phenomena were recognized in the middle of the 20th century the regulation and mechanisms of the initiation of eukaryotic DNA replication have been intensively investigated. These studies were carried out to find the essential factors involved in the process and to determine their functions during DNA replication. These studies gave rise to a model of the organization and the coordination of DNA replication within the eukaryotic cell. The elegant experiments carried out by Rao and Johnson (1970) (1), who fused cells in different phases of the cell cycle, showed that G1 cells are competent for replication of their chromosomes, but lack a specific diffusible factor required to activate their replicaton machinery and showed that G2 cells are incompetent for DNA replication. These findings suggested that eukaryotic cells exist in two states. In G1 phase, cells are competent to initiate DNA replication, which is subsequently triggered in S phase. After completion of S phase, cells in G2 are no longer able to initiate DNA replication and they require a transition through mitosis to reenable initiation of DNA replication to take place in the next S phase. The Xenopus cell-free replication system has proved a good model system in which to study DNA replication in vitro as well as the mechanism preventing rereplication within a single cell cycle (2). Studies using this system resulted in the development of a model postulating the existence of a replication licensing factor, which binds to chromatin before the G1-S transition and which is displaced during replication (2, 3). These results were supported by genetic and biochemical experiments in Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast) (4, 5). The investigation of cell division cycle mutants and the budding yeast origin of replication resulted in the concept of a prereplicative and a postreplicative complex of initiation proteins (6-9). These three individual concepts have recently started to merge and it has become obvious that initiation in eukaryotes is generally governed by the same ubiquitous mechanisms.

Homogeneous time-resolved fluorescence assay for identifying p53 interactions with its protein partners, directly in a cellular extract

The p53 protein is a tumor suppressor that protects the organism against malignant consequences of DNA damage. Interaction of p53 with numerous cellular or viral proteins regulates its functional activity either positively or negatively. An approach leading to identification of such protein interactions directly in a cell extract could be of help in the development of screening assays to search for drugs acting on p53 in its cellular environment, either by disrupting its association with inhibitory proteins or by increasing its affinity for activating proteins. We show that the homogeneous time-resolved fluorescence (HTRF) assay based on the time-resolved amplified cryptate emission (TRACE) technology allows identification of such an interaction by simply adding a mixture of two labeled monoclonal antibodies, directly in a cellular extract. We validate this assay by studying p53/SV40-LTAg interactions. The antibodies directed against genuine p53 and SV40-LTAg epitopes were labeled with europium cryptate (donor) and XL665, a crosslinked allophycocyanin (acceptor), respectively. We demonstrated that a nonradiative energy transfer occurs between labeled antibodies only when p53 interacts with SV40-LTag, which opens up the possibility of extending this approach to other p53 partners to search for drugs that restore p53 tumor-suppressor activity.

Temperature-sensitive ovarian carcinoma cell line (OvBH-1)

OvBH-1 cells from a patient with ovarian clear cell carcinoma were established and their biochemical status was analyzed. Cells grown at 37 degrees C exhibited normal cell cycle distribution, whereas the cells shifted to 31 degrees C were arrested in the G(2) / M phase of the cell cycle. Immunochemical analysis using anti-p53 antibodies (DO-1, PAb240, PAb421, and PAb1620) revealed that only the DO-1 antibody reacted with p53 with a high and similar percentage at both temperatures. PAb240 reacted with a low percentage of cells at 37 degrees C and no reaction was observed at 31 degrees C. PAb421 antibody stained a significantly lower percentage of cells at 37 degrees C than at 31 degrees C. Cells were not stained with PAb1620 antibody and were negative for antibodies against p21(WAF1) and MDM2 proteins independently of the temperature. Sequencing of all coding exons of the p53 gene demonstrated only a neutral genetic polymorphism, i.e. a G-to-A substitution (GAG to GAA) at nucleotide position 13 432. Thus, the observed temperature sensitivity of OvBH-1 cells cannot be ascribed to a p53 primary structure mutation. Based upon immunochemical analyses, we consider, however, that p53 in nuclei of OvBH-1 cells is in a highly unstable conformation. Furthermore, the N-terminal portion of the p53 protein at Ser20 has not been modified, and Lys373 and / or Ser378 of the C-terminus is acetylated and / or phosphorylated. The nuclear location signal of p53 is preserved. Induction of MDM2 protein is uncoupled from the cell regulatory machinery and the induction of p21(WAF1) by p53 is impaired in OvBH-1 cells.

The processing of Holliday junctions by BLM and WRN helicases is regulated by p53

BLM, WRN, and p53 are involved in the homologous DNA recombination pathway. The DNA structure-specific helicases, BLM and WRN, unwind Holliday junctions (HJ), an activity that could suppress inappropriate homologous recombination during DNA replication. Here, we show that purified, recombinant p53 binds to BLM and WRN helicases and attenuates their ability to unwind synthetic HJ in vitro. The p53 248W mutant reduces abilities of both to bind HJ and inhibit helicase activities, whereas the p53 273H mutant loses these abilities. Moreover, full-length p53 and a C-terminal polypeptide (residues 373-383) inhibit the BLM and WRN helicase activities, but phosphorylation at Ser(376) or Ser(378) completely abolishes this inhibition. Following blockage of DNA replication, Ser(15) phospho-p53, BLM, and RAD51 colocalize in nuclear foci at sites likely to contain DNA replication intermediates in cells. Our results are consistent with a novel mechanism for p53-mediated regulation of DNA recombinational repair that involves p53 post-translational modifications and functional protein-protein interactions with BLM and WRN DNA helicases.

Copper uptake is required for pyrrolidine dithiocarbamate-mediated oxidation and protein level increase of p53 in cells

The p53 tumour-suppressor protein is a transcription factor that activates the expression of genes involved in cell cycle arrest, apoptosis and DNA repair. The p53 protein is vulnerable to oxidation at cysteine thiol groups. The metal-chelating dithiocarbamates, pyrrolidine dithiocarbamate (PDTC), diethyldithiocarbamate, ethylene(bis)dithiocarbamate and H(2)O(2) were tested for their oxidative effects on p53 in cultured human breast cancer cells. Only PDTC oxidized p53, although all oxidants tested increased the p53 level. Inductively coupled plasma MS analysis indicated that the addition of 60 microM PDTC increased the cellular copper concentration by 4-fold, which was the highest level of copper accumulated amongst all the oxidants tested. Bathocuproinedisulphonic acid, a membrane-impermeable Cu(I) chelator inhibited the PDTC-mediated copper accumulation. Bathocuproinedisulphonic acid as well as the hydroxyl radical scavenger d-mannitol inhibited the PDTC-dependent increase in p53 protein and oxidation. Our results show that a low level of copper accumulation in the range of 25-40 microg/g of cellular protein increases the steady-state levels of p53. At copper accumulation levels higher than 60 microg/g of cellular protein, p53 is oxidized. These results suggest that p53 is vulnerable to free radical-mediated oxidation at cysteine residues.