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

The diversity of p53 mutations among human brain tumors and their functional consequences

The p53 tumor suppressor is implicated in cell cycle control, DNA repair, replicative senescence and programmed cell death. Inactivation of the p53 contributes to the wide range of human tumors, including glial neoplasms. In this review, we describe the regulation and biochemical properties of p53 protein that may explain its ability to activate various genetic programs underlying cellular responses to stress conditions. The overall spectrum of p53 mutations is rather shared between tumor types indicating that these mutations are not tumor type-specific. However, there is one example of germ-line mutation of p53 gene (the deletion of the codon 236) that is associated with a familiar brain tumor syndrome. We compare the frequency and type of most common mutations among various brain tumours (focusing on glioblastomas) and their consequences on protein functions. Furthermore, we discuss the most promising approaches of potential brain tumor therapy, including an adenovirus-mediated p53 gene transfer. Human glioblastomas are highly sensitive to the effects of p53 activity when the wild-type p53 is introduced ectopically. It suggests that the genetic or pharmacological modulation of the p53 pathway is potentially important strategy in the treatment of human cancers.

The onset of p53-dependent DNA repair or apoptosis is determined by the level of accumulated damaged DNA

The p53 tumor suppressor gene plays an important role in both apoptosis and DNA repair pathways that are pivotal for genomic stability. Here we show that the treatment of cells with low doses of gamma-irradiation or cisplatin resulted in an immediate enhancement of p53-dependent DNA repair, measured by base excision repair (BER) activity. However, treatment of cells with high doses of DNA damaging agents resulted in a reduction in p53-dependent DNA repair and in the induction of p53-dependent apoptosis. Analysis of p53 upstream molecular events suggested that regulation of p53-associated DNA repair is ATM-dependent. Furthermore, we observed that while dephosphorylation of Ser376 at the C-terminus of the p53 protein was associated with enhancement in DNA repair, phosphorylation at the N-terminal Ser15 resulted in the reduction in DNA repair. The latter is also in correlation with an enhancement in the specific DNA binding activity and in the induction of apoptosis. Treatment of cells with a caspase inhibitor, prior to the damaging agent-blocked apoptosis, had no effect on the DNA repair pattern. Taken together, this suggests that the decision of cells to induce a p53-dependent DNA repair or apoptosis is most probably controlled by the level of genotoxic agent introduced to cells.

Refolding and structural characterization of the human p53 tumor suppressor protein

The human tumor suppressor p53 is a conformationally flexible and functionally complex protein that is only partially understood on a structural level. We expressed full-length p53 in the cytosol of Escherichia coli as inclusion bodies. To obtain active, recombinant p53, we varied renaturation conditions using DNA binding activity and oligomeric state as criteria for successful refolding. The optimized renaturation protocol allows the refolding of active, DNA binding p53 with correct quaternary structure and domain contact interfaces. The purified protein could be allosterically activated for DNA binding by addition of a C-terminally binding antibody. Analytical gelfiltration and chemical cross-linking confirmed the tetrameric quaternary structure and the spectroscopic analysis of renatured p53 by fluorescence and circular dichroism, suggested that native p53 is partially unstructured.

Physical interaction of p73 with c-Myc and MM1, a c-Myc-binding protein, and modulation of the p73 function

p73 shares high sequence homology with the tumor suppressor p53. Like p53, ectopic overexpression of p73 induces cell cycle arrest and/or apoptosis, and these biological activities are linked to its sequence-specific transactivation function. The COOH-terminal region of p73 is unique and has a function to modulate DNA-binding ability and transactivation activity. To identify and characterize cellular proteins that interact with the COOH-terminal region of p73 alpha and regulate its activity, we employed a yeast-based two-hybrid screen with a human fetal brain cDNA library. We found MM1, a nuclear c-Myc-binding protein, was associated with p73 alpha in both yeast two-hybrid and in vitro pull-down assays. In mammalian cells, MM1 co-immunoprecipitated with p73 alpha, whereas p73 beta and tumor suppressor p53 did not interact with MM1. Overexpression of MM1 in p53-deficient osteosarcoma SAOS-2 cells enhanced the p73 alpha-dependent transcription from the p53/p73-responsive Bax and PG13 promoters, whereas p73 beta- and p53-mediated transcriptional activation was unaffected in the presence of MM1. MM1 also stimulated the p73 alpha-mediated growth suppression in SAOS-2 cells. More importantly, we found that c-Myc was physically associated with p73 alpha and significantly impaired the transcriptional activity of p73 alpha on Bax and p21(waf1) promoters. Expression of MM1 strongly reduced the c-Myc-mediated inhibitory activity on p73 alpha. These results suggest that MM1 may act as a molecular partner for p73 to prevent the c-Myc-mediated inhibitory effect on its activity.

Cdk5 phosphorylates p53 and regulates its activity

Cyclin dependent kinase 5 (Cdk5) is a proline-direct protein kinase that is most active in the CNS, and has been implicated as a contributing factor in certain neurodegenerative diseases. Further, there is evidence to suggest that Cdk5 may facilitate the progression of apoptosis. However, the mechanisms involved have not been elucidated. The tumor suppressor protein p53, a transcription factor that is regulated by phosphorylation, increases the expression of genes that control growth arrest or cell death. To understand how Cdk5 could facilitate apoptosis, the effects of Cdk5 on p53 activity were examined. In the present study it is shown that in apoptotic PC12 cells the levels of p53 and Cdk5 increase concomitantly. Further, Cdk5/p25 effectively phosphorylates recombinant p53 in vitro. Transient transfection of Cdk5/p25 into cells results in an increase in p53 levels, as well as the expression of the p53-responsive genes p21 and Bax. Furthermore, evidence is provided that increased Cdk5 activity increases p53 transcriptional activity significantly, suggesting that p53 is modulated in situ by Cdk5. This is the first demonstration that p53 is a substrate of Cdk5, and that Cdk5 can modulate p53 levels and activity.

SN2 DNA-alkylating agent-induced phosphorylation of p53 and activation of p21 gene expression

p53 is an important player in the cellular response to genotoxic stress whose functions are regulated by phosphorylation of a number of serine and threonine residues. Phosphorylation of p53 influences its DNA-binding and gene regulation activities. This study examines p53 phosphorylation in HCT-116 (MMR-deficient) and HCT-116+ch3 (MMR-proficient) human colon cancer cells treated with a S(N)2 DNA-alkylating agent, methylmethane sulfonate (MMS). MMS induces phosphorylation of p53 on Ser15 and Ser392 in a dose- and time-dependent manner. MMS-induced p53 phosphorylation is independent of DNA mismatch repair (MMR) activity. Nuclear extracts from MMS-treated HCT-116 cells had higher p21WAF1/Cip1 (p21) promoter DNA-binding activity in vitro opposed to untreated cells. After MMS treatment, the activation of the cloned p21 promoter in a transient transfection assay and endogenous p21 mRNA levels in HCT-116(p53+/+) versus HCT-116(p53-/-) cells increased, which correlates with an increased levels of phospho-p53(Ser15) and phospho-p53(Ser392). These results suggest that SN2 DNA-alkylating agent-induced phosphorylation of p53 on Ser15 and Ser392 increases its DNA-binding properties to cause an increased expression of p21 that may play a role in cell cycle arrest and/or apoptosis of HCT-116 cells.

Activation of the p53 tumor suppressor protein

The p53 tumor suppressor gene plays an important role in preventing cancer development, by arresting or killing potential tumor cells. Mutations within the p53 gene, leading to the loss of p53 activity, are found in about half of all human cancers, while many of the tumors that retain wild type p53 carry mutations in the pathways that allow full activation of p53. In either case, the result is a defect in the ability to induce a p53 response in cells undergoing oncogenic stress. Significant advances have been made recently in our understanding of the molecular pathways through which p53 activity is regulated, bringing with them fresh possibilities for the design of cancer therapies based on reactivation of the p53 response.

A role for p53 in the frequency and mechanism of mutation

The tumor suppressor protein, p53, is often referred to as the guardian of the genome. When p53 function is impaired, its ability to preserve genomic integrity is compromised. This may result in an increase in mutation on both a molecular and chromosomal level and contribute to the progression to a malignant phenotype. In order to study the effect of p53 function on the acquisition of mutation, in vitro and in vivo models have been developed in which both the frequency and mechanism of mutation can be analyzed. In human lymphoblastoid cells in which p53 function was impaired, both the spontaneous and induced mutant frequency increased at the autosomal thymidine kinase (TK) locus. The mutant frequency increased to a greater extent in cell lines in which p53 harbored a point mutation than in those lines in which a "null" mutation had been introduced by molecular targeting or by viral degradation indicating a possible "gain-of-function" associated with the mutant protein. Further, molecular analysis revealed that the loss of p53 function was associated with a greater tendency towards loss-of-heterozygosity (LOH) within the TK gene that was due to non-homologous recombination than that found in wild-type cells. Most data obtained from the in vivo models uses the LacI reporter gene that does not efficiently detect mutation that results in LOH. However, studies that have examined the effect of p53 status on mutation in the adenine phosphoribosyl transferase (APRT) gene in transgenic mice also suggest that loss of p53 function results in an increase in mutation resulting from non-homologous recombination. The results of these studies provide clear and convincing evidence that p53 plays a role in modulating the mutant frequency and the mechanism of mutation. In addition, the types of mutation that occur within the p53 gene are also of importance in determining the mutant frequency and the pathways leading to mutation.

How do 14-3-3 proteins work?-- Gatekeeper phosphorylation and the molecular anvil hypothesis

14-3-3 proteins were the first signaling molecules to be identified as discrete phosphoserine/threonine binding modules. This family of proteins, which includes seven isotypes in human cells and up to 15 in plants, plays critical roles in cell signaling events that control progress through the cell cycle, transcriptional alterations in response to environmental cues, and programmed cell death. Despite over 30 years of research, distinct roles for most isotypes remain unknown. Though 14-3-3 proteins perform different functions for different ligands, general mechanisms of 14-3-3 action include changes in activity of bound ligands, altered association of bound ligands with other cellular components, and changes in intracellular localization of 14-3-3-bound cargo. We present a speculative model where binding of 14-3-3 to multiple sites on some ligands results in global ligand conformational changes that mediate their biological effects. For these multi-site ligands, one binding site is likely to function as a 'gatekeeper' whose phosphorylation is necessary for 14-3-3 binding but may not always be sufficient for full biological activity. If correct, then 14-3-3 may prove to be a bona fide phosphodependent signaling chaperone.

ATM-dependent dissociation of B55 regulatory subunit from nuclear PP2A in response to ionizing radiation

Ionizing radiation (IR) is known to activate multiple cell cycle checkpoints that are thought to enhance the ability of cells to respond to DNA damage. Protein phosphatase 2A (PP2A) has been implicated in IR-induced activation of checkpoints; therefore, Jurkat cells were exposed to an activating dose of IR or sham treatment as control, and nuclear extracts were analyzed for PP2A by Mono Q anion exchange chromatography and microcystin affinity chromatography. PP2A exists in eukaryotic cells both as a heterodimer consisting of a 65-kDa scaffolding subunit (A) plus a 36-kDa catalytic subunit (C) and as ABC heterotrimers, containing one of a variety of regulatory (B) subunits. Here we show that IR produces a transient and reversible reduction in the amount of nuclear AB55C heterotrimer without affecting the AB'C heterotrimer or AC heterodimer. In ataxia telangiectasia-mutated (ATM)-deficient cells the amount of nuclear PP2A heterotrimer relative to heterodimer was not reduced by radiation, but the radiation response was restored by transfection of these cells with plasmids encoding ATM. Wortmannin, an inhibitor of kinases such as phosphatidylinositol 3-kinase, also prevented the IR-induced reduction in nuclear PP2A heterotrimer. The changes in nuclear PP2A occurred without a noticeable difference in the carboxyl-terminal methylation of the C subunit, which is known to influence association with B subunits. We conclude a novel ATM-dependent mechanism is regulating association of B55 subunits with nuclear PP2A in response to IR.

Differential effect of ik3-1/cables on p53- and p73-induced cell death

ik3-1/Cables is associated with cdk3 in self-replicating cells. In postmitotic neurons, it may serve as an adaptor molecule, functionally connecting c-abl and cdk5, and supporting neurite growth. Here we report that ik3-1 binds to p53 and p73 in vivo. Ectopically expressed ik3-1 potentiates p53-induced cell death but not p73-induced cell death in U2OS cells. On the contrary, coexpression of ik3-1-DeltaC, an ik3-1 deletion mutant lacking the C-terminal 139 [corrected] amino acids (corresponding to the cyclin box-homologous region), inhibits p73-induced cell death but not p53-induced cell death. ik3-1-DeltaC-mediated inhibition of p73-induced cell death are partially attenuated by overexpression of ik3-1. These data indicate that ik3-1 is not only a regulator for p53-induced cell death but also an essential regulator for p73-induced cell death, and ik3-1-DeltaC competes with ik3-1 only in p73-induced cell death. Furthermore, functional domains of p53 responsible for its interaction with ik3-1 are partially different from those of p73. In conclusion, we found that ik3-1, a putative component of cell cycle regulation, is functionally connected with p53 and p73, but in distinct fashions.

Hetero-oligomerization does not compromise 'gain of function' of tumor-derived p53 mutants

Tumor-derived p53 mutants activate transcription from promoters of various growth-related genes. We tested whether this transactivation function of the mutant protein is sufficient to induce tumorigenesis ('gain of function'). Tumor-derived mutant p53-281G transactivates the promoters of human epidermal growth factor receptor (EGFR) and human multiple drug resistance gene (MDR-1). To determine whether the C-terminal domain functions only as an oligomerization domain in mutant p53-mediated transactivation, we have replaced the tetramerization domain of p53 by a heterologous tetramerization domain; although this mutant protein formed tetramers in solution, it failed to transactivate significantly. Therefore, for successful mutant p53-mediated transactivation, sequences near the C-terminus of mutant p53 are required to perform functions in addition to tetramerization. We also demonstrate that co-expression of a deletion mutant of p53 (p53 del 1-293), which retains the p53 oligomerization domain, inhibits this transactivation. p53 del 1-293 co-immunoprecipitates with p53-281G suggesting that hetero-oligomers of p53-281G and p53 del 1-293 are defective in transactivation. We also show that a cell line stably transfected with p53-281G expresses higher levels of endogenous NF-kappaB and proliferating cell nuclear antigen (PCNA) compared to that transfected with vector alone. On co-expression, p53 del 1-293 lowered the levels of NF-kappaB and PCNA in p53-281G-expressing cells. However, on co-expression, p53 del 1-293 did not inhibit the tumorigenicity and colony forming ability of p53-281G expressing cells. Our earlier work showed that a deletion of the C-terminal sequences of p53-281G overlapping the oligomerization domain obliterates 'gain of function'. Taken together, the above information suggests that the C-terminal sequences have some critical role in 'gain of function' in addition to transactivation.

p53-dependent apoptosis pathways

The p53 tumor suppressor limits cellular proliferation by inducing cell cycle arrest and apoptosis in response to cellular stresses such as DNA damage, hypoxia, and oncogene activation. Many apoptosis-related genes that are transcriptionally regulated by p53 have been identified. These are candidates for implementing p53 effector functions. In response to oncogene activation, p53 mediates apoptosis through a linear pathway involving bax transactivation, Bax translocation from the cytosol to membranes, cytochrome c release from mitochondria, and caspase-9 activation, followed by the activation of caspase-3, -6, and -7. p53-mediated apoptosis can be blocked at multiple death checkpoints, by inhibiting p53 activity directly, by Bcl-2 family members regulating mitochondrial function, by E1B 19K blocking caspase-9 activation, and by caspase inhibitors. Understanding the mechanisms by which p53 induces apoptosis, and the reasons why cell death is bypassed in transformed cells, is of fundamental importance in cancer research, and has great implications in the design of anticancer therapeutics.

Camptothecin and Zeocin can increase p53 levels during all cell cycle stages

The ability of DNA damage to stabilize p53 in all cell cycle stages has not been examined in actively growing cells. The chemotherapeutic drug camptothecin is a topoisomerase I poison. Zeocin is a member of the bleomycin/phleomycin family of antibiotics, known to bind DNA. Both increase the level of p53 albeit by different mechanisms. We have utilized centrifugal elutriation to separate exponentially growing ML-1 cells (containing wild-type p53) into cell cycle fractions and have subsequently treated these cells with the two drugs. We provide evidence that both drugs can mediate an increase in p53 protein levels independent of the cell cycle stage. The p53 induced by both drugs was able to bind to DNA; however, only the p53 induced by camptothecin was phosphorylated at serine-392. This is the first demonstration that camptothecin and Zeocin can differentially signal for increased levels of modified p53 during all stages of the cell cycle.

Activation and activities of the p53 tumour suppressor protein

The p53 tumour suppressor protein inhibits malignant progression by mediating cell cycle arrest, apoptosis or repair following cellular stress. One of the major regulators of p53 function is the MDM2 protein, and multiple forms of cellular stress activate p53 by inhibiting the MDM2-mediated degradation of p53. Mutations in p53, or disruption of the pathways that allow activation of p53, seem to be a general feature of all cancers. Here we review recent advances in our understanding of the pathways that regulate p53 and the pathways that are induced by p53, as well as their implications for cancer therapy.

Genotoxic and non-genotoxic pathways of p53 induction

Since the initial concept of p53 as a sensor of DNA-damage, the picture of the role of p53 has widened to include the sensing of much more diverse forms of stress, including hypoxia and constitutive activation of growth-promoting cascades. The pathways by which these processes regulate p53 are partially overlapping, but imply different patterns of post-translational modifications. In this review, we summarize current knowledge on post-translational modifications of p53, and we discuss how hypoxia and oncogene activation stresses may induce p53 independently of DNA damage.

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.

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.

14-3-3 proteins; bringing new definitions to scaffolding

The 14-3-3 proteins are a part of an emerging family of proteins and protein domains that bind to serine/threonine-phosphorylated residues in a context specific manner, analogous to the Src homology 2 (SH2) and phospho-tyrosine binding (PTB) domains. 14-3-3 proteins bind and regulate key proteins involved in various physiological processes such as intracellular signaling (e.g. Raf, MLK, MEKK, PI-3 kinase, IRS-1), cell cycling (e.g. Cdc25, Wee1, CDK2, centrosome), apoptosis (e.g. BAD, ASK-1) and transcription regulation (e.g. FKHRL1, DAF-16, p53, TAZ, TLX-2, histone deacetylase). In contrast to SH2 and PTB domains, which serve mainly to mediate protein-protein interactions, 14-3-3 proteins in many cases alter the function of the target protein, thus allowing them to serve as direct regulators of their targets. This review focuses on the various mechanisms employed by the 14-3-3 proteins in the regulation of their diverse targets, the structural basis for 14-3-3-target protein interaction with emphasis on the role of 14-3-3 dimerization in target protein binding and regulation and provides an insight on 14-3-3 regulation itself.

14-3-3 proteins: key regulators of cell division, signalling and apoptosis

The 14-3-3 proteins constitute a family of conserved proteins present in all eukaryotic organisms so far investigated. These proteins have attracted interest because they are involved in important cellular processes such as signal transduction, cell-cycle control, apoptosis, stress response and malignant transformation and because at least 100 different binding partners for the 14-3-3 proteins have been reported. Although the exact function of 14-3-3 proteins is still unknown, they are known to (1) act as adaptor molecules stimulating protein-protein interactions, (2) regulate the subcellular localisation of proteins and (3) activate or inhibit enzymes. In this review, we discuss the role of the 14-3-3 proteins in three cellular processes: cell cycle control, signal transduction and apoptosis. These processes are regulated by the 14-3-3 proteins at multiple steps. The 14-3-3 proteins have an overall inhibitory effect on cell cycle progression and apoptosis, whereas in signal transduction they may act as stimulatory or inhibitory factors. This article contains supplementary material which may be viewed at the BioEssays website at http://www.interscience.wiley.com/jpages/0265-9247/Suppmat/23/v23_10.936.

Ataxia telangiectasia mutated-dependent apoptosis after genotoxic stress in the developing nervous system is determined by cellular differentiation status

Ataxia-telangiectasia (A-T) is a neurodegenerative syndrome resulting from dysfunction of ATM (ataxia telangiectasia mutated). The molecular details of ATM function in the nervous system are unclear, although the neurological lesions in A-T are probably developmental because they appear during childhood. The nervous systems of Atm-null mice show a pronounced defect in apoptosis that is induced by DNA damage, suggesting that ATM may function to eliminate DNA-damaged neurons. Here we show that Atm-dependent apoptosis occurs at discrete stages of neurogenesis. Analysis of gamma-irradiated mouse embryos showed that Atm-dependent apoptosis occurred only in the postmitotic populations that were present in the neuroepithelial subventricular zone of the developing nervous system. Notably, Atm deficiency did not prevent radiation-induced apoptosis in multipotent precursor cells residing in the proliferating ventricular zone. Atm-dependent apoptosis required p53 and coincided with the specific phosphorylation of p53 and caspase-3 activation. Thus, these data show that Atm functions early in neurogenesis and underscore the selective requirement for Atm in eliminating damaged postmitotic neural cells. Furthermore, these data demonstrate that the differentiation status of neural cells is a critical determinant in the activation of certain apoptotic pathways.

Regulation of ubiquitination and degradation of p53 in unstressed cells through C-terminal phosphorylation

Previously, we found that the protein kinase C (PKC) inhibitor H7 stimulates p53 to accumulate in a form incapable of inducing transcription from p53-dependent promoters. We concluded that H7 inhibits constitutive C-terminal phosphorylation of p53, which regulates its turnover in unstressed cells. We now show that p53 and its inhibitor MDM2 (HDM2 in human cells) are together in the nuclei of H7-treated cells and can be co-immunoprecipitated. Despite this association of p53 with the ubiquitin ligase MDM2, ubiquitinated p53 was not detected in H7-treated cells. Furthermore, co-treatment with H7 and the proteosome inhibitor LLnL prevented the accumulation of ubiquitinated p53 that was observed in cells treated solely with LLnL. In addition, treatment of cells with the PKC activator phorbol ester stimulated the ubiquitination of p53 and reduced its ability to accumulate after stress. H7 did not induce the phosphorylation of human p53 on Ser-15 (Ser-18 in mouse protein), a modification that occurs in response to DNA damage and leads to the release of MDM2 and to transactivation by p53. We conclude that phosphorylation of the C-terminal domain of p53 by PKC increases its ubiquitination and degradation in unstressed cells.

Post-translational modifications and activation of p53 by genotoxic stresses

In unstressed cells, the tumor suppressor protein p53 is present in a latent state and is maintained at low levels through targeted degradation. A variety of genotoxic stresses initiate signaling pathways that transiently stabilize the p53 protein, cause it to accumulate in the nucleus, and activate it as a transcription factor. Activation leads either to growth arrest at the G1/S or G2/M transitions of the cell cycle or to apoptosis. Recent studies point to roles for multiple post-translational modifications in mediating these events in response to genotoxic stresses through several potentially interacting but distinct pathways. The approximately 100 amino-acid N-terminal and approximately 90 amino-acid C-terminal domains are highly modified by post-translational modifications. The N-terminus is heavily phosphorylated while the C-terminus contains phosphorylated, acetylated and sumoylated residues. Antibodies that recognize p53 only when it has been modified at specific sites have been developed, and studies with these reagents show that most known post-translational modifications are induced when cells are exposed to genotoxic stresses. These recent results, coupled with biochemical and genetic studies, suggest that N-terminal phosphorylations are important for stabilizing p53 and are crucial for acetylation of C-terminal sites, which in combination lead to the full p53-mediated response to genotoxic stresses. Modifications to the C-terminus inhibit the ability of this domain to negatively regulate sequence-specific DNA binding; additionally, they modulate the stability, the oligomerization state, the nuclear import/export process and the degree of ubiquitination of p53.

A decade of site- and phosphorylation state-specific antibodies: recent advances in studies of spatiotemporal protein phosphorylation

From 1990 to 2001, numerous site- and phosphorylation state-specific antibodies have been developed and many are now commercially available. These antibodies have facilitated understanding of the cytoskeletal organization, signal transduction and transcriptional mechanisms as well as clinical diseases. This review is an attempt to cover all these aspects.

p53 dysfunction in B-cell chronic lymphocytic leukemia: inactivation of ATM as an alternative to TP53 mutation

The well-established association between TP53 mutations and adverse clinical outcome in a range of human cancers reflects the importance of p53 protein in regulating tumor-cell growth and survival. Although it is theoretically possible for p53 dysfunction to arise through mechanisms that do not involve TP53 mutation, such a phenomenon has not previously been demonstrated in a sporadic tumor. Here, we show that p53 dysfunction in B-cell chronic lymphocytic leukemia (CLL) can occur in the absence of TP53 mutation and that such dysfunction is associated with mutation of the gene encoding ATM, a kinase implicated in p53 activation. Forty-three patients with CLL were examined for p53 dysfunction, as detected by impaired up-regulation of p53 and of the p53-dependent protein p21(CIP1/WAF1) after exposure to ionizing radiation (IR). Thirty (70%) patients had normal p53 responses and underwent progressive IR-induced apoptosis. In 13 (30%) patients, p21 up-regulation was markedly impaired, indicating p53 dysfunction. Six (14%) of these patients with p53 dysfunction had increased baseline levels of p53, were found to have TP53 mutations, and were completely resistant to IR-induced apoptosis. In the other 7 (16%) patients with p53 dysfunction, IR-induced p53 up-regulation and apoptosis were markedly impaired, but baseline levels of p53 were not increased, and no TP53 mutations were detected. Each of these patients was found to have at least one ATM mutation, and a variable reduction in ATM protein was detected in all 4 patients examined. This is the first study to provide a direct demonstration that p53 dysfunction can arise in a sporadic tumor by a mechanism that does not involve TP53 mutation. (Blood. 2001;98:814-822)

Preparation and application of antibodies to phosphoamino acid sequences

In this review, we describe methods to generate and characterize sequence-specific phosphoamino acid antibodies. Several of the early contributions regarding the utility of such antibodies are summarized. Three antiphosphopeptide antibodies derived from sequences of the Bcr protein are described. They are anti-Bcr pSer-354, anti-Bcr pTyr-328, and anti-Bcr pTyr-360. These anti-Bcr phosphopeptide antibodies are directed toward phosphorylated sequences encoded by the first exon of the BCR gene, which is the critical portion of the Bcr sequence present in the Bcr-Abl oncoprotein. Using these antibodies, we established/confirmed the in vivo phosphorylation of Ser-354, Tyr-328, and Tyr-360 in Bcr and Bcr-Abl proteins. The cross-reactivity of these antibodies, which is a common problem with antipeptide antibodies, was also investigated and discussed.

PEAZ-1: a new human prostate neoplastic epithelial cell line

BACKGROUND

The generation of prostatic cell lines provides in vitro models for experimental studies of the pathogenesis of prostate carcinoma. Therefore, we established and characterized a new human prostate epithelial cell line, PEAZ-1 (prostate epithelial Arizona-1).

METHODS

The PEAZ-1 cells were grown from a primary human prostate carcinoma specimen obtained from radical prostatectomy. The isolated cells were characterized by immunobiochemistry, immunohistochemistry, and tumorigenicity studies.

RESULTS

PEAZ-1 cells are near diploid, tumorigenic, and androgen independent for cell growth. PEAZ-1 cells express N-cadherin, alpha- and beta-catenins, and p120 at cell-cell contacts, cytoplasmic laminin 5, vinculin, paxillin, and phosphotyrosine at focal adhesions, vimentin, and cytokeratins 8 and 18. They do not express plakoglobin, E-cadherin, and PSA, and do not form desmosomes and hemidesomomes. PEAZ-1 respond to ocadaic acid, a pro-apoptotic agent, by expression of p53.

CONCLUSIONS

PEAZ-1 cells is a human prostate cancer cell line that has a number of mesenchymal characteristics.

Distinct pattern of p53 phosphorylation in human tumors

The protein product of the tumor suppressor gene p53 is phosphorylated on multiple residues by several protein kinases. Using a battery of 10 antibodies developed against different phosphorylated and acetylated residues of p53, we compared the pattern of p53 phosphorylation and acetylation in tumor-derived cell lines, tumor samples, and non-neoplastic cells. Irrespective of tumor types or the presence of p53 mutation, phosphorylation and acetylation of p53 was substantially higher in samples obtained from tumor tissues than those found in non-transformed samples. Among the 10 sites analysed, phosphorylation of residues 15, 81, 392, and acetylation were among the more frequent modifications. Analysis of two of the more abundant phosphorylation or acetylation sites on p53 is sufficient to detect 72% of tumor-derived p53 proteins. The distinct pattern of p53 phosphorylation and acetylation in human tumors may offer a new means to monitor the status and activity of p53 in the course of tumor development and progression.

Homologous recombinational repair of DNA ensures mammalian chromosome stability

The process of homologous recombinational repair (HRR) is a major DNA repair pathway that acts on double-strand breaks and interstrand crosslinks, and probably to a lesser extent on other kinds of DNA damage. HRR provides a mechanism for the error-free removal of damage present in DNA that has replicated (S and G2 phases). Thus, HRR acts in a critical way, in coordination with the S and G2 checkpoint machinery, to eliminate chromosomal breaks before the cell division occurs. Many of the human HRR genes, including five Rad51 paralogs, have been identified, and knockout mutants for most of these genes are available in chicken DT40 cells. In the mouse, most of the knockout mutations cause embryonic lethality. The Brca1 and Brca2 breast cancer susceptibility genes appear to be intimately involved in HRR, but the mechanistic basis is unknown. Biochemical studies with purified proteins and cell extracts, combined with cytological studies of nuclear foci, have begun to establish an outline of the steps in mammalian HRR. This pathway is subject to complex regulatory controls from the checkpoint machinery and other processes, and there is increasing evidence that loss of HRR gene function can contribute to tumor development. This review article is meant to be an update of our previous review [Biochimie 81 (1999) 87].

Functional analysis and intracellular localization of p53 modified by SUMO-1

p53 tumor suppressor is a subject of several post-translational modifications, including phosphorylation, ubiquitination and acetylation, which regulate p53 function. A new covalent modification of p53 at lysine 386 by SUMO-1 was recently identified. To elucidate the function of sumoylated p53, we compared the properties of wild type p53 and sumoylation-deficient p53 mutant, K386R. No differences were found between wild type p53 and K386R mutant of p53 in transactivation or growth suppression assays. Moreover, overexpression of SUMO-1 has no effect on p53-regulated transcription. Biochemical fractionation showed that sumoylated p53 is localized in the nucleus and is tightly bound to chromatin structures. p53 and SUMO-1 co-localized in PML nuclear bodies in 293 cells and the nucleoli in MCF7 and HT1080 cells. However, sumoylation-deficient p53 mutant showed a similar pattern of intranuclear localization, suggesting that SUMO-1 does not target p53 to subnuclear structures. These data indicate that SUMO-1 modification of p53 at lysine 386 may not be essential for p53's cellular localization, transcriptional activation, or growth regulation.

Kinetics of p53 binding to promoter sites in vivo

Downstream target genes of p53 are thought to mediate its tumor-suppressive activity, but it is unknown whether differential transactivation of these genes is regulated at the level of p53 binding to their promoters. To address this issue, p53 binding in vivo to consensus sites in the p21(Waf1), MDM2, and PIG3 promoters was investigated in cells exposed to adriamycin (ADR) or ionizing radiation as well as in an inducible p53 cell line. p53-DNA complexes were cross-linked in vivo by treating the cells with formaldehyde and processed by chromatin immunoprecipitation-PCR. This methodology allowed for the analysis of relevant p53-DNA complexes by preventing redistribution of cellular components upon collection of cell extracts. Increased p53 binding to the p21(Waf1), MDM2, and PIG3 promoters occurred within 2 h after p53 activation; however, significant increases in PIG3 transcription did not occur until 15 h after p53 binding. Gel shift analyses indicated that p53 had lower affinity for the consensus binding site in the PIG3 promoters compared to its consensus sites in the p21 and MDM2 genes, which suggests that additional factors may be required to stabilize the interaction of p53 with the PIG3 promoter. Further, acetylated p53 (Lys382) was found in chemically cross-linked complexes at all promoter sites examined after treatment of cells with ADR. In summary, the kinetics of p53 binding in vivo to target gene regulatory regions does not uniformly correlate with target gene mRNA expression for the p53 target genes examined. Our results suggest that target genes with low-affinity p53 binding sites may require additional events and will have delayed kinetics of induction compared to those with high-affinity binding sites.

Phosphatidylinositol 3-kinase signaling inhibits DAF-16 DNA binding and function via 14-3-3-dependent and 14-3-3-independent pathways

In Caenorhabditis elegans, an insulin-like signaling pathway to phosphatidylinositol 3-kinase (PI 3-kinase) and AKT negatively regulates the activity of DAF-16, a Forkhead transcription factor. We show that in mammalian cells, C. elegans DAF-16 is a direct target of AKT and that AKT phosphorylation generates 14-3-3 binding sites and regulates the nuclear/cytoplasmic distribution of DAF-16 as previously shown for its mammalian homologs FKHR and FKHRL1. In vitro, interaction of AKT- phosphorylated DAF-16 with 14-3-3 prevents DAF-16 binding to its target site in the insulin-like growth factor binding protein-1 gene, the insulin response element. In HepG2 cells, insulin signaling to PI 3-kinase/AKT inhibits the ability of a GAL4 DNA binding domain/DAF-16 fusion protein to activate transcription via the insulin-like growth factor binding protein-1-insulin response element, but not the GAL4 DNA binding site, which suggests that insulin inhibits the interaction of DAF-16 with its cognate DNA site. Elimination of the DAF-16/1433 association by mutation of the AKT/14-3-3 sites in DAF-16, prevents 14-3-3 inhibition of DAF-16 DNA binding and insulin inhibition of DAF-16 function. Similarly, inhibition of the DAF-16/14-3-3 association by exposure of cells to the PI 3-kinase inhibitor LY294002, enhances DAF-16 DNA binding and transcription activity. Surprisingly constitutively nuclear DAF-16 mutants that lack AKT/14-3-3 binding sites also show enhanced DNA binding and transcription activity in response to LY294002, pointing to a 14-3-3-independent mode of regulation. Thus, our results demonstrate at least two mechanisms, one 14-3-3-dependent and the other 14-3-3-independent, whereby PI 3-kinase signaling regulates DAF-16 DNA binding and transcription function.

Epidermal growth factor sensitizes cells to ionizing radiation by down-regulating protein mutated in ataxia-telangiectasia

Epidermal growth factor (EGF) has been reported to either sensitize or protect cells against ionizing radiation. We report here that EGF increases radiosensitivity in both human fibroblasts and lymphoblasts and down-regulates both ATM (mutated in ataxia-telangiectasia (A-T)) and the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs). No further radiosensitization was observed in A-T cells after pretreatment with EGF. The down-regulation of ATM occurs at the transcriptional level. Concomitant with the down-regulation of ATM, the DNA binding activity of the transcription factor Sp1 decreased. A causal relationship was established between these observations by demonstrating that up-regulation of Sp1 DNA binding activity by granulocyte/macrophage colony-stimulating factor rapidly reversed the EGF-induced decrease in ATM protein and restored radiosensitivity to normal levels. Failure to radiosensitize EGF-treated cells to the same extent as observed for A-T cells can be explained by induction of ATM protein and kinase activity with time post-irradiation. Although ionizing radiation damage to DNA rapidly activates ATM kinase and cell cycle checkpoints, we have provided evidence for the first time that alteration in the amount of ATM protein occurs in response to both EGF and radiation exposure. Taken together these data support complex control of ATM function that has important repercussions for targeting ATM to improve radiotherapeutic benefit.

Evidence that wild-type p53 in neuroblastoma cells is in a conformation refractory to integration into the transcriptional complex

Neuroblastoma (NB) cells reportedly accumulate wild-type p53 exclusively in the cytoplasm. However, immunofluorescence assays with five different antibodies showed that p53 accumulates in the nucleus of up to 10% of NB cells. PAb1801 detected cytoplasmic 'punctate structures' which were also found in p53-null cells, rendering this antibody unsuitable for p53 detection. A comparison of DO-1 and PAb1801 staining in NB tissue sections confirmed the results obtained with NB cells. Nuclear accumulation of p53 was induced in NB cells using substances which disturb p53's tertiary structure at its zinc finger motif, or by treatment with mitomycin C. Constitutive nuclear accumulation was observed in an SK-N-SH variant, AW-1, which has a point mutation in p53 at Cys176>Ser, disturbing the same motif. Even though p53 showed DNA-binding capability after mitomycin C treatment of NB cells, the target gene products MDM2 and p21(WAF1,CIP1,SDI1) were not synthesized and no p53 transactivating activity measured in a reporter gene assay. Therefore we suggest that p53 in NB cells might be predominantly in a conformation refractory to integration into the transcriptional complex, resulting in at least partial transcriptional inactivity, hyperactive nuclear export and resistance to degradation by exogenously expressed MDM2.

Phosphorylation of serines 15 and 37 is necessary for efficient accumulation of p53 following irradiation with UV

Changes in the phosphorylation state of p53 are important in increasing its half-life and potency as a transcription factor. To investigate their roles, serine residues 15 and 37 were mutated to alanines and the mutated proteins were expressed stably at low basal levels in Li-Fraumeni-derived p53-null human fibroblasts. The accumulation of p53 after DNA damage was analysed quantitatively in multiple clones. Mutation of serine 15, serine 37 or both impaired the accumulation of the protein after exposing the cells to ultraviolet radiation (50-100% increase for the mutant proteins, 500% increase for wild-type p53) but not after treatment with adriamycin. The diminished accumulation of mutant p53 protein is due to a reduction of basal HDM association. Analysis of p53-dependent transcription revealed that phosphorylation of serine 15 is required to maintain basal levels of p21 mRNA. These results provide new evidence for an important function of serine 37 phosphorylation, clearly distinguish the pathways of p53 activation in response to ultraviolet radiation or DNA damage inflicted by adriamycin, and reveal that serine 15 is crucial to support the p53-mediated basal expression of p21.

p53 activation in response to microtubule disruption is mediated by integrin-Erk signaling

The p53 tumor suppressor is activated in response to various stresses driving the cells into growth arrest or apoptosis. We have addressed the question of how disintegration of microtubule system induces activation of p53. Depolymerization of microtubules by colcemid in rat and human quiescent fibroblasts resulted in accumulation of transcriptionally active p53 that caused cell-cycle arrest at the G1/S boundary. The p53 activation correlated with prominent activation of Erk1/2 MAP kinases that resulted from colcemid-stimulated development of focal adhesions. Inhibition of focal contacts development by plating of cells onto poly-L-lysine abrogated both Erk1/2 and p53 activations in colcemid-treated cells, while plating of cells onto fibronectin caused transient up-regulation of p53 even in the absence of colcemid. Pre-treatment of cells with the specific MEK1 inhibitor PD098059 also attenuated colcemid-induced p53 activation and G1 cell cycle arrest. Cell types which either failed to develop focal adhesions in response to colcemid treatment (human MCF-7 epithelial cells), or lacked colcemid-induced sustained Erk activation (primary mouse embryo fibroblasts and 12(1) cells) showed virtually no p53 up-regulation in response to disruption of microtubules during G0/G1. Our results indicate that p53 activation is not triggered by disintegration of microtubule system by itself, but rather originates from some of the consequences of such disintegration, in particular, from the development of focal adhesions leading to activation of Erk signaling pathway.

A DNA damage-induced p53 serine 392 kinase complex contains CK2, hSpt16, and SSRP1

Phosphorylation of the human p53 protein at Ser-392 has been shown to be responsive to UV but not gamma irradiation. Here we describe identification and purification of a mammalian UV-activated protein kinase complex that phosphorylates Ser-392 of p53 in vitro. This kinase complex contains casein kinase 2 (CK2) and the chromatin transcriptional elongation factor FACT (a heterodimer of hSpt16 and SSRP1). In vitro studies show that FACT alters the specificity of CK2 in the complex such that it selectively phosphorylates p53 over other substrates including casein. In addition, phosphorylation by the kinase complex enhances p53 activity. These results thus provide a potential mechanism for p53 activation by UV irradiation.

ATM and ATR: networking cellular responses to DNA damage

Maintenance of genome stability depends on the appropriate response to DNA damage. This response is based on complex networks of signaling pathways that activate numerous processes and lead ultimately to damage repair and cellular survival - or apoptosis. The protein kinases ATM and ATR are master controllers of some of these networks, acting either in concert or separately to orchestrate the responses to specific types of DNA damage or stalled replication. Understanding their mode of action is essential to our understanding of how cells cope with genotoxic stress.

Apoptotic proteins. p53 and c-myc related pathways

c-Myc and p53 are two proteins that have critical roles in the regulation of apoptosis and the cell cycle. The authors review how these two proteins are thought to control the opposing events of proliferation and apoptosis and examine whether their well-documented biological roles in tumorigenesis can be applied to the vascular system.

Strategies for manipulating the p53 pathway in the treatment of human cancer

Human cancer progression is driven in part by the mutation of oncogenes and tumour-suppressor genes which, under selective environmental pressures, give rise to evolving populations of biochemically altered cells with enhanced tumorigenic and metastatic potential. Given that human cancers are biologically and pathologically quite distinct, it has been quite surprising that a common event, perturbation of the p53 pathway, occurs in most if not all types of human cancers. The central role of p53 as a tumour-suppressor protein has fuelled interest in defining its mechanism of function and regulation, determining how its inactivation facilitates cancer progression, and exploring the possibility of restoring p53 function for therapeutic benefit. This review will highlight the key biochemical properties of p53 protein that affect its tumour-suppressor function and the experimental strategies that have been developed for the re-activation of the p53 pathway in cancers.

Increased p53 phosphorylation after microtubule disruption is mediated in a microtubule inhibitor- and cell-specific manner

p53 is present at low levels in unstressed cells. Numerous cellular insults, including DNA damage and microtubule disruption, elevate p53 protein levels. Phosphorylation of p53 is proposed to be important for p53 stabilization and activation after genotoxic stress; however, p53 phosphorylation after microtubule disruption has not been analysed. The goal of the current study was to determine if p53 phosphorylation increases after microtubule disruption, and if so, to identify specific p53 residues necessary for microtubule inhibitor-induced phosphorylation. Two dimensional gel analyses demonstrated that the number of p53 phospho-forms in cells increased after treatment with microtubule inhibitors (MTIs) and that the pattern of p53 phosphorylation was distinct from that observed after DNA damage. p53 phosphorylation also varied in a MTI-dependent manner, as Taxol and Vincristine induced more p53 phospho-forms than nocodazole. Further, MTI treatment increased phosphorylation of p53 on serine-15 in epithelial tumor cells. In contrast, serine-15 phosphorylation of p53 did not increase in MTI-treated primary cultures of human fibroblasts. Analysis of ectopically expressed p53 phospho-mutant proteins from Taxol- and nocodazole-treated cells indicated that multiple p53 amino terminal residues, including serine-15 and threonine-18, were required for Taxol-mediated phosphorylation of p53. Taken together, the results of this study demonstrate that distinct p53 phospho-forms are induced by MTI treatment as compared to DNA damage and that p53 phosphorylation is mediated in a MTI- and cell-specific manner. Oncogene (2001) 20, 113 - 124.

Genetic evidence of a role for ATM in functional interaction between human T-cell leukemia virus type 1 Tax and p53

Recent evidence from several investigators suggest that the human T-cell leukemia virus type 1 Tax oncoprotein represses the transcriptional activity of the tumor suppressor protein, p53. An examination of published findings reveals serious controversy as to the mechanism(s) utilized by Tax to inhibit p53 activity and whether the same mechanism is used by Tax in adherent and suspension cells. Here, we have investigated Tax-p53 interaction simultaneously in adherent epithelial (HeLa and Saos) and suspension T-lymphocyte (Jurkat) cells. Our results indicate that Tax activity through the CREB/CREB-binding protein (CBP), but not NF-kappaB, pathway is needed to repress the transcriptional activity of p53 in all tested cell lines. However, we did find that while CBP binding by Tax is necessary, it is not sufficient for inhibiting p53 function. Based on knockout cell studies, we correlated a strong genetic requirement for the ATM, but not protein kinase-dependent DNA, protein in conferring a Tax-p53-repressive phenotype.

Cellular osmoregulation: beyond ion transport and cell volume

All cells are characterized by the expression of osmoregulatory mechanisms, although the degree of this expression is highly variable in different cell types even within a single organism. Cellular osmoregulatory mechanisms constitute a conserved set of adaptations that offset antagonistic effects of altered extracellular osmolality/environmental salinity on cell integrity and function. Cellular osmoregulation includes the regulation of cell volume and ion transport but it does not stop there. We know that organic osmolyte concentration, protein structure, cell turnover, and other cellular parameters are osmoregulated as well. In this brief review two important aspects of cellular osmoregulation are emphasized: 1) maintenance of genomic integrity, and 2) the central role of protein phosphorylation. Novel insight into these two aspects of cellular osmoregulation is illustrated based on two cell models, mammalian kidney inner medullary cells and teleost gill epithelial cells. Both cell types are highly hypertonicity stress-resistant and, therefore, well suited for the investigation of osmoregulatory mechanisms. Damage to the genome is discussed as a newly discovered aspect of hypertonic threat to cells and recent insights on how mammalian kidney cells deal with such threat are presented. Furthermore, the importance of protein phosphorylation as a core mechanism of osmosensory signal transduction is emphasized. In this regard, the potential roles of the 14-3-3 family of phospho-protein adaptor molecules for cellular osmoregulation are highlighted primarily based on work with fish gill epithelial cells. These examples were chosen for the reader to appreciate the numerous and highly specific interactions between stressor-specific and non-specific pathways that form an extensive cellular signaling network giving rise to adaptive compensation of hypertonicity. Furthermore, the example of 14-3-3 proteins illustrates that a single protein may participate in several pathways that are non-specific with regard to the type of stress and, at the same time, in stress-specific pathways to promote cell integrity and function during hypertonicity.

Cisplatinum and taxol induce different patterns of p53 phosphorylation

Posttranslational modifications of p53 induced by two widely used anticancer agents, cisplatinum (DDP) and taxol were investigated in two human cancer cell lines. Although both drugs were able to induce phosphorylation at serine 20 (Ser20), only DDP treatment induced p53 phosphorylation at serine 15 (Ser15). Moreover, both drug treatments were able to increase p53 levels and consequently the transcription of waf1 and mdm-2 genes, although DDP treatment resulted in a stronger inducer of both genes. Using two ataxia telangiectasia mutated (ATM) cell lines, the role of ATM in drug-induced p53 phosphorylations was investigated. No differences in drug-induced p53 phosphorylation could be observed, indicating that ATM is not the kinase involved in these phosphorylation events. In addition, inhibition of DNA-dependent protein kinase activity by wortmannin did not abolish p53 phosphorylation at Ser15 and Ser20, again indicating that DNA-PK is unlikely to be the kinase involved. After both taxol and DDP treatments, an activation of hCHK2 was found and this is likely to be responsible for phosphorylation at Ser20. In contrast, only DDP was able to activate ATR, which is the candidate kinase for phosphorylation of Ser15 by this drug. This data clearly suggests that differential mechanisms are involved in phosphorylation and activation of p53 depending on the drug type.

The N terminus of p53 regulates its dissociation from DNA

It is important to gain insight into p53 DNA binding and how it is regulated. By using electrophoretic mobility shift assays and DNase I footprinting, we show that a region within the N terminus of the protein controls the dissociation of p53 from a p53-binding site. When p53 is bound by a number of N-terminal-specific monoclonal antibodies, its rate of dissociation from DNA is reduced, and its ability to protect a cognate site from DNase I digestion is increased. Moreover, greatly reduced dissociation is observed with p53 protein lacking the N-terminal 96 amino acids. By contrast, deletion of the C terminus does not affect p53 dissociation from DNA or DNase I protection. p53 protein expressed in and purified from bacterial cells displays markedly more instability on its consensus DNA-binding site than does p53 produced in insect cells, suggesting that post-translational modifications may affect the stability of the protein. Our results provide evidence that the N terminus of p53 possesses an auto-inhibitory function that is mechanistically different from the inhibitory region at the C terminus.

TAZ: a novel transcriptional co-activator regulated by interactions with 14-3-3 and PDZ domain proteins

The highly conserved and ubiquitously expressed 14-3-3 proteins regulate differentiation, cell cycle progression and apoptosis by binding intracellular phosphoproteins involved in signal transduction. By screening in vitro translated cDNA pools for the ability to bind 14-3-3, we identified a novel transcriptional co-activator, TAZ (transcriptional co-activator with PDZ-binding motif) as a 14-3-3-binding molecule. TAZ shares homology with Yes-associated protein (YAP), contains a WW domain and functions as a transcriptional co-activator by binding to the PPXY motif present on transcription factors. 14-3-3 binding requires TAZ phosphorylation on a single serine residue, resulting in the inhibition of TAZ transcriptional co-activation through 14-3-3-mediated nuclear export. The C-terminus of TAZ contains a highly conserved PDZ-binding motif that localizes TAZ into discrete nuclear foci and is essential for TAZ-stimulated gene transcription. TAZ uses this same motif to bind the PDZ domain-containing protein NHERF-2, a molecule that tethers plasma membrane ion channels and receptors to cytoskeletal actin. TAZ may link events at the plasma membrane and cytoskeleton to nuclear transcription in a manner that can be regulated by 14-3-3.