A proline-rich motif in p53 is required for transactivation-independent growth arrest as induced by Gas1

Targeting p53 pathways: mechanisms, structures, and advances in therapy

The TP53 tumor suppressor is the most frequently altered gene in human cancers, and has been a major focus of oncology research. The p53 protein is a transcription factor that can activate the expression of multiple target genes and plays critical roles in regulating cell cycle, apoptosis, and genomic stability, and is widely regarded as the "guardian of the genome". Accumulating evidence has shown that p53 also regulates cell metabolism, ferroptosis, tumor microenvironment, autophagy and so on, all of which contribute to tumor suppression. Mutations in TP53 not only impair its tumor suppressor function, but also confer oncogenic properties to p53 mutants. Since p53 is mutated and inactivated in most malignant tumors, it has been a very attractive target for developing new anti-cancer drugs. However, until recently, p53 was considered an "undruggable" target and little progress has been made with p53-targeted therapies. Here, we provide a systematic review of the diverse molecular mechanisms of the p53 signaling pathway and how TP53 mutations impact tumor progression. We also discuss key structural features of the p53 protein and its inactivation by oncogenic mutations. In addition, we review the efforts that have been made in p53-targeted therapies, and discuss the challenges that have been encountered in clinical development.

Tumor suppressor p53: Biology, signaling pathways, and therapeutic targeting

TP53 is the most commonly mutated gene in human cancer with over 100,000 literature citations in PubMed. This is a heavily studied pathway in cancer biology and oncology with a history that dates back to 1979 when p53 was discovered. The p53 pathway is a complex cellular stress response network with multiple diverse inputs and downstream outputs relevant to its role as a tumor suppressor pathway. While inroads have been made in understanding the biology and signaling in the p53 pathway, the p53 family, transcriptional readouts, and effects of an array of mutants, the pathway remains challenging in the realm of clinical translation. While the role of mutant p53 as a prognostic factor is recognized, the therapeutic modulation of its wild-type or mutant activities remain a work-in-progress. This review covers current knowledge about the biology, signaling mechanisms in the p53 pathway and summarizes advances in therapeutic development.

Induction of Rod-Shaped Structures by Herpes Simplex Virus Glycoprotein I

The envelope glycoprotein I (gI) of herpes simplex virus 1 (HSV-1) is a critical mediator of virus-induced cell-to-cell spread and cell-cell fusion. Here, we report a previously unrecognized property of this molecule. In transfected cells, the HSV-1 gI was discovered to induce rod-shaped structures that were uniform in width but variable in length. Moreover, the gI within these structures was conformationally different from the typical form of gI, as a previously used monoclonal antibody mAb3104 and a newly made peptide antibody to the gI extracellular domain (ECD) (amino acids [aa] 110 to 202) both failed to stain the long rod-shaped structures, suggesting the formation of a higher-order form. Consistent with this observation, we found that gI could self-interact and that the rod-shaped structures failed to recognize glycoprotein E, the well-known binding partner of gI. Further analyses by deletion mutagenesis and construction of chimeric mutants between gI and gD revealed that the gI ECD is the critical determinant, whereas the transmembrane domain served merely as an anchor. The critical amino acids were subsequently mapped to proline residues 184 and 188 within a conserved PXXXP motif. Reverse genetics analyses showed that the ability to induce a rod-shaped structure was not required for viral replication and spread in cell culture but rather correlated positively with the capability of the virus to induce cell fusion in the UL24syn background. Together, this work discovered a novel feature of HSV-1 gI that may have important implications in understanding gI function in viral spread and pathogenesis.IMPORTANCE The HSV-1 gI is required for viral cell-to-cell spread within the host, but the molecular mechanisms of how gI exactly works have remained poorly understood. Here, we report a novel property of this molecule, namely, induction of rod-shaped structures, which appeared to represent a higher-order form of gI. We further mapped the critical residues and showed that the ability of gI to induce rod-shaped structures correlated well with the capability of HSV-1 to induce cell fusion in the UL24syn background, suggesting that the two events may have an intrinsic link. Our results shed light on the biological properties of HSV-1 gI and may have important implications in understanding viral pathogenesis.

How the Other Half Lives: What p53 Does When It Is Not Being a Transcription Factor

It has been four decades since the discovery of p53, the designated ‘Guardian of the Genome’. P53 is primarily known as a master transcription factor and critical tumor suppressor, with countless studies detailing the mechanisms by which it regulates a host of gene targets and their consequent signaling pathways. However, transcription-independent functions of p53 also strongly define its tumor-suppressive capabilities and recent findings shed light on the molecular mechanisms hinted at by earlier efforts. This review highlights the transcription-independent mechanisms by which p53 influences the cellular response to genomic instability (in the form of replication stress, centrosome homeostasis, and transposition) and cell death. We also pinpoint areas for further investigation in order to better understand the context dependency of p53 transcription-independent functions and how these are perturbed when TP53 is mutated in human cancer.

Mutant and wild-type p53 form complexes with p73 upon phosphorylation by the kinase JNK

The transcription factors p53 and p73 are critical to the induction of apoptotic cell death, particularly in response to cell stress that activates c-Jun N-terminal kinase (JNK). Mutations in the DNA-binding domain of p53, which are commonly seen in cancers, result in conformational changes that enable p53 to interact with and inhibit p73, thereby suppressing apoptosis. In contrast, wild-type p53 reportedly does not interact with p73. We found that JNK-mediated phosphorylation of Thr⁸¹ in the proline-rich domain (PRD) of p53 enabled wild-type p53, as well as mutant p53, to form a complex with p73. Structural algorithms predicted that phosphorylation of Thr⁸¹ exposes the DNA-binding domain in p53 to enable its binding to p73. The dimerization of wild-type p53 with p73 facilitated the expression of apoptotic target genes [such as those encoding p53-up-regulated modulator of apoptosis (PUMA) and Bcl-2-associated X protein (BAX)] and, subsequently, the induction of apoptosis in response to JNK activation by cell stress in various cells. Thus, JNK phosphorylation of mutant and wild-type p53 promotes the formation of a p53/p73 complex that determines cell fate: apoptosis in the context of wild-type p53 or cell survival in the context of the mutant. These findings refine our current understanding of both the mechanistic links between p53 and p73 and the functional role for Thr⁸¹ phosphorylation.

Gas1 up-regulation is inducible and contributes to cell apoptosis in reactive astrocytes in the substantia nigra of LPS and MPTP models

Background

Reactive astrogliosis is a remarkable pathogenetic hallmark of the brains of Parkinson’s disease (PD) patients, but its progressive fate and regulation mechanisms are poorly understood. In this study, growth arrest specific 1 (Gas1), a tumor growth suppressor oncogene, was identified as a novel modulator of the cell apoptosis of reactive astrocytes in primary culture and the injured substantia nigra.

Methods

Animal models and cell cultures were utilized in the present study. Lipopolysaccharide (LPS)- and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated animal models were used to detect Gas1 expression in the brain via immunohistochemistry and western blot. Cell cultures were performed to analyze Gas1 functions in the viability and apoptosis of reactive astrocytes and SH-SY5Y cells by double labeling, CCK-8, LDH, TUNEL, flow cytometry, and siRNA knockdown methods.

Results

Gas1 expressions were significantly elevated in the majority of the reactive astrocytes of the brains with LPS or MPTP insults. In the injured substantia nigras, GFAP-positive astrocytes exhibited higher levels of cleaved caspase-3. In cell culture, the up-regulated Gas1 expression induced apoptosis of reactive astrocytes that were insulted by LPS in combination with interferon-γ and tumor necrosis factor-a. This effect was confirmed through siRNA knockdown of Gas1 gene expression. Finally and interestingly, the potential underlying signaling pathways were evidently related to an increase in the Bax/Bcl-2 ratio, the abundant generation of reactive oxygen species and the activation of cleaved caspase-3.

Conclusions

This study demonstrated that the up-regulation of inducible Gas1 contributed to the apoptosis of reactive astrocytes in the injured nigra. Gas1 signaling may function as a novel regulator of astrogliosis and is thus a potential intervention target for inflammatory events in PD conditions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0643-2) contains supplementary material, which is available to authorized users.

The adapter protein CD2AP binds to p53 protein in the cytoplasm and can discriminate its polymorphic variants P72R

Proline-rich motifs are widely distributed in eukaryotic proteomes and are usually involved in the assembly of functional complexes through interaction with specific binding modules. The tumour-suppressor p53 protein presents a proline-rich region that is crucial for regulating apoptosis by connecting the p53 with a complex protein network. In humans, a common polymorphism determines the identity of residue 72, either proline or arginine, and affects the features of the motifs present in the polyproline domain. The two isoforms have different biochemical properties and markedly influence cancer onset and progression. In this article, we analyse the binding of the p53 proline-rich region with a pool of selected polyproline binding domains (i.e. SH3 and WW), and we present the first demonstration that the purified SH3 domains of the CD2AP/Cin85 protein family are able to directly bind the p53 protein, and to discriminate between the two polymorphic variants P72R.

RUNX Family Participates in the Regulation of p53-Dependent DNA Damage Response

A proper DNA damage response (DDR), which monitors and maintains the genomic integrity, has been considered to be a critical barrier against genetic alterations to prevent tumor initiation and progression. The representative tumor suppressor p53 plays an important role in the regulation of DNA damage response. When cells receive DNA damage, p53 is quickly activated and induces cell cycle arrest and/or apoptotic cell death through transactivating its target genes implicated in the promotion of cell cycle arrest and/or apoptotic cell death such as p21^WAF1, BAX, and PUMA. Accumulating evidence strongly suggests that DNA damage-mediated activation as well as induction of p53 is regulated by posttranslational modifications and also by protein-protein interaction. Loss of p53 activity confers growth advantage and ensures survival in cancer cells by inhibiting apoptotic response required for tumor suppression. RUNX family, which is composed of RUNX1, RUNX2, and RUNX3, is a sequence-specific transcription factor and is closely involved in a variety of cellular processes including development, differentiation, and/or tumorigenesis. In this review, we describe a background of p53 and a functional collaboration between p53 and RUNX family in response to DNA damage.

SUMOylation of mouse p53b by SUMO-1 promotes its pro-apoptotic function in ovarian granulosa cells

Follicular atresia is a process of spontaneous degradation of follicles, hindering growth and development in the mammalian ovary. Previous studies showed that follicular atresia was caused by apoptosis of granulosa cells, for which a number of apoptosis-related genes have already been identified. The roles of p53 in apoptosis of mouse granulosa cells and its post-translational modification are still unclear. The main objective of this study was to explore the roles of p53 in mouse granulosa cells. We found that mouse p53b, but not p53a, could be SUMOylated by SUMO-1 at lysine 375, which was essential for the protein stability of p53b in a dose-dependent manner. Immunofluorescent staining showed that wild p53b was located in the nucleus of granulosa cells, while its mutation of SUMOylated site (K375R) was localized in both nucleus and cytoplasm, implying that SUMOylation was necessary for the nuclear localization of p53b in granulosa cells. Overexpression of wild-type p53b, but not the mutation of SUMOylation site (K375R), significantly induced the expression of apoptosis-related gene, Bax, and increased the level of apoptosis in granulosa cells. This suggested that SUMO-1 modification of p53b was essential for inducing apoptosis in granulosa cells. Our results provide strong evidences that modification of p53b by SUMO-1 at lysine 375 was necessary for its activity to induce apoptosis in mouse granulosa cells, and it was involved in the regulation of p53b protein stability and nuclear localization. This implies that modification of p53b by SUMO-1 might regulate follicular atresia by inducing the apoptosis of ovarian granulosa cells in mice.

Extrinsic and intrinsic control of germ cell proliferation in Caenorhabditis elegans

The germ cells of Caenorhabditis elegans serve as a useful model to study the balance between proliferation and differentiation within the context of development and changing environmental signals experienced by the animal. Germ cells adjacent to a stem cell niche in the distal region of the gonad retain the capacity to divide during adulthood, making them unique from other cells in the organism. We will highlight recent advances in our understanding of mechanisms that control proliferation, as well as the signaling pathways involved in promoting mitosis at the expense of differentiation.

HNCA-TOCSY-CANH experiments with alternate (13)C- (12)C labeling: a set of 3D experiment with unique supra-sequential information for mainchain resonance assignment

Described here is a set of three-dimensional (3D) NMR experiments that rely on CACA-TOCSY magnetization transfer via the weak ³J(CαCα) coupling. These pulse sequences, which resemble recently described (13)C detected CACA-TOCSY (Takeuchi et al. 2010) experiments, are recorded in (1)H(2)O, and use (1)H excitation and detection. These experiments require alternate (13)C-(12)C labeling together with perdeuteration, which allows utilizing the small ³J(CαCα) scalar coupling that is otherwise masked by the stronger (1)J(CC) couplings in uniformly (13)C labeled samples. These new experiments provide a unique assignment ladder-mark that yields bidirectional supra-sequential information and can readily straddle proline residues. Unlike the conventional HNCA experiment, which contains only sequential information to the ¹³C(α) of the preceding residue, the 3D hnCA-TOCSY-caNH experiment can yield sequential correlations to alpha carbons in positions i-1, i + 1 and i-2. Furthermore, the 3D hNca-TOCSY-caNH and Hnca-TOCSY-caNH experiments, which share the same magnetization pathway but use a different chemical shift encoding, directly couple the (15)N-(1)H spin pair of residue i to adjacent amide protons and nitrogens at positions i-2, i-1, i + 1 and i + 2, respectively. These new experimental features make protein backbone assignments more robust by reducing the degeneracy problem associated with the conventional 3D NMR experiments.

Nitrogen-detected CAN and CON experiments as alternative experiments for main chain NMR resonance assignments

Heteronuclear direct-detection experiments, which utilize the slower relaxation properties of low gamma nuclei, such as (13)C have recently been proposed for sequence-specific assignment and structural analyses of large, unstructured, and/or paramagnetic proteins. Here we present two novel (15)N direct-detection experiments. The CAN experiment sequentially connects amide (15)N resonances using (13)C(alpha) chemical shift matching, and the CON experiment connects the preceding (13)C' nuclei. When starting from the same carbon polarization, the intensities of nitrogen signals detected in the CAN or CON experiments would be expected four times lower than those of carbon resonances observed in the corresponding (13)C-detecting experiment, NCA-DIPAP or NCO-IPAP (Bermel et al. 2006b; Takeuchi et al. 2008). However, the disadvantage due to the lower gamma is counteracted by the slower (15)N transverse relaxation during detection, the possibility for more efficient decoupling in both dimensions, and relaxation optimized properties of the pulse sequences. As a result, the median S/N in the (15)N observe CAN experiment is 16% higher than in the (13)C observe NCA-DIPAP experiment. In addition, significantly higher sensitivity was observed for those residues that are hard to detect in the NCA-DIPAP experiment, such as Gly, Ser and residues with high-field C(alpha) resonances. Both CAN and CON experiments are able to detect Pro resonances that would not be observed in conventional proton-detected experiments. In addition, those experiments are free from problems of incomplete deuterium-to-proton back exchange in amide positions of perdeuterated proteins expressed in D(2)O. Thus, these features and the superior resolution of (15)N-detected experiments provide an attractive alternative for main chain assignments. The experiments are demonstrated with the small model protein GB1 at conditions simulating a 150 kDa protein, and the 52 kDa glutathione S-transferase dimer, GST.

p53-mediated apoptosis prevents the accumulation of progenitor B cells and B-cell tumors

We propose that the apoptotic function of p53 has an important role in B-cell homeostasis, which is important for the prevention of B-cell lymphomas. We created a mouse model (mDeltapro) that lacked residues 58-88 of the proline-rich domain of p53. mDeltapro is defective for apoptosis, but is able to arrest cell-cycle progression in hematopoietic tissues. mDeltapro develops late-onset B-cell lymphoma, but not the thymic T-cell tumors found in p53-null mice. Interestingly, mDeltapro lymphomas comprised incorrectly differentiated B cells. B-cell irregularities were also detected in mDeltapro before tumor onset, in which aged mice showed an increased population of inappropriately differentiated B cells in the bone marrow and spleen. We predict that by keeping B-cell populations in check, p53-dependent apoptosis prevents irregular B cells from eventuating in lymphomas.

The effects of Ankrd2 alteration indicate its involvement in cell cycle regulation during muscle differentiation

Ankrd2 is a member of the Muscle Ankyrin Repeat Protein family (MARPs), consisting of sarcomere-associated proteins that can also localize in the nucleus. There are indications that MARPs might function as shuttle proteins between the cytoplasm and nucleus, likely sending information to the nucleus concerning the changes in the structure or function of the contractile machinery. Even though recent findings suggest that the MARP gene family is not essential for the basal functioning of skeletal muscle, its influence on the gene expression program of skeletal muscle cells was highlighted. To investigate this regulatory role we produced and examined both morphological and functional features of myocytes stable overexpressing or silencing the Ankrd2 protein. The transcriptional profiles of the myocytes revealed that the molecular pathways perturbed by changes in Ankrd2 protein level are congruent with the morpho-physiological and biochemical data obtained in Ankrd2-modified myoblasts induced to differentiate. Our results suggest that Ankrd2 gives an important contribution to the coordination of proliferation and apoptosis during myogenic differentiation in vitro, mainly through the p53 network.

Role of PACAP in the physiology and pathology of the sympathoadrenal system

Sympathetic neurons and chromaffin cells derive from common sympathoadrenal precursors which arise from the neural crest. Cells from this lineage migrate to their final destination and differentiate by acquiring a catecholaminergic phenotype in response to different environmental factors. It has been shown that the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) and its PAC1 receptor are expressed at early stages of sympathetic development, and participate to the control of neuroblast proliferation and differentiation. PACAP also acts as a neurotransmitter to stimulate catecholamine and neuropeptide biosynthesis and release from sympathetic neurons and chromaffin cells, during development and in adulthood. In addition, PACAP and its receptors have been described in neuroblastoma and pheochromocytoma, and the neuropeptide regulates the differentiation and activity of sympathoadrenal-derived tumoral cell lines, suggestive of an important role in the pathophysiology of the sympathoadrenal lineage. Transcriptome studies uncovered genes and pathways of known and unknown roles that underlie the effects of PACAP in the sympathoadrenal system.

Targeted-simultaneous expression of Gas1 and p53 using a bicistronic adenoviral vector in gliomas

The targeted expression of transgenes is one of the principal goals of gene therapy, and it is particularly relevant for the treatment of brain tumors. In this study, we examined the effect of the overexpression of human gas1 (growth arrest specific 1) and human p53 cDNAs, both under the transcriptional control of a promoter of the human glial fibrillary acidic protein (gfa2), employing adenoviral expression vectors, in glioma cells. We showed that the targeted overexpression of gas1 and p53 (AdSGas1 and AdSp53, respectively) in rat glioma cells (C6) reduced the number of viable cells and induced apoptosis. Moreover, the adenovirally targeted expression of these genes also reduced tumor growth in vivo. Unexpectedly, there was no additive effect when both gas1 and p53 were simultaneously expressed in the same cells using a bicistronic adenoviral vector. We suggest that Gas1 does not act in combination with p53 in the C6 and U373 glioma cell lines, inducing apoptosis and cell cycle arrest. Our results indicate that the targeted expression of tumor suppressor genes (gas1 and p53) regulated by the gfa2 promoter, together with adenoviral vectors may provide an interesting approach for adjuvant selective glioma gene therapy.

Regulation of developmental rate and germ cell proliferation in Caenorhabditis elegans by the p53 gene network

Caenorhabditis elegans CEP-1 activates germline apoptosis in response to genotoxic stress, similar to its mammalian counterpart, tumor suppressor p53. In mammals, there are three p53 family members (p53, p63, and p73) that activate and repress many distinct and overlapping sets of genes, revealing a complex transcriptional regulatory network. Because CEP-1 is the sole p53 family member in C. elegans, analysis of this network is greatly simplified in this organism. We found that CEP-1 functions during normal development in the absence of stress to repress many (331) genes and activate only a few (28) genes. In response to genotoxic stress, 1394 genes are activated and 942 are repressed, many of which contain p53-binding sites. Comparison of the CEP-1 transcriptional network with transcriptional targets of the human p53 family reveals considerable overlap between CEP-1-regulated genes and homologues regulated by human p63 and p53, suggesting a composite p53/p63 action for CEP-1. We found that phg-1, the C. elegans Gas1 (growth arrest-specific 1) homologue, is activated by CEP-1 and is a negative regulator of cell proliferation in the germline in response to genotoxic stress. Further, we find that CEP-1 and PHG-1 mediate the decreased developmental rate and embryonic viability of mutations in the clk-2/TEL2 gene, which regulates lifespan and checkpoint responses.

Expanding the Role of the Dynein Regulatory Complex to Non-Axonemal Functions: Association of GAS11 with the Golgi Apparatus

The mammalian GAS11 gene is a candidate tumor suppressor of unknown function that was previously identified as one of several genes upregulated upon growth arrest. Interestingly, although GAS11 homologs in Trypanosoma brucei (trypanin) and Chlamydomonas reinhardtii (PF2) are integral components of the flagellar axoneme and are necessary for regulating flagellar beat, the GAS11 gene was discovered based on its expression in cells that do not assemble a motile cilium. This suggests that GAS11 function might not be restricted to the cilium. To investigate this possibility, we generated GAS11-specific antibodies and demonstrate here that GAS11 is expressed in a variety of mammalian cells that lack a motile cilium. In COS7 cells, GAS11 is associated with the detergent-insoluble cytoskeleton and exhibits a juxtanuclear localization that overlaps with the pericentrosomal Golgi apparatus. This localization is dependent upon intact microtubules and is cell-cycle regulated, such that GAS11 is dispersed throughout the cytoplasm as cells progress through mitosis. GAS11 remains associated with Golgi fragments following depolymerization of cytoplasmic microtubules but is dispersed upon disruption of the Golgi with brefeldin A. These data suggest that GAS11 is associated with the Golgi apparatus. In support of this, recombinant GAS11 binds Golgi membranes in vitro. In growth-arrested mIMCD3 cells, GAS11 co-localizes with gamma-tubulin at the base of the primary cilium. The pericentrosomal Golgi apparatus and base of the cilium both represent convergence points for microtubule minus ends and correspond to sites where dynein regulation is required. The algal GAS11 homolog functions as part of a dynein regulatory complex (DRC) in the axoneme (Rupp and Porter. J Cell Biol 2003;162:47-57) and our findings suggest that components of this axonemal dynein regulatory system have been adapted in mammalian cells to participate in non-axonemal functions.

Retinoic acid induces CDK inhibitors and growth arrest specific (Gas) genes in neural crest cells

Retinoic acid (RA), the active metabolite of vitamin A, regulates cellular growth and differentiation during embryonic development. In excess, this vitamin is also highly teratogenic to animals and humans. The neural crest is particularly sensitive to RA, and high levels adversely affect migration, proliferation and cell death. We investigated potential gene targets of RA associated with neural crest proliferation by determining RA-mediated changes in gene expression over time, using microarrays. Statistical analysis of the top ranked RA-regulated genes identified modest changes in multiple genes previously associated with cell cycle control and proliferation including the cyclin-dependent kinase inhibitors Cdkn1a (p21), Cdkn2b (p15(INK4b)), and Gas3/PMP22. The expression of p21 and p15(INK4b) contribute to decreased proliferation by blocking cell cycle progression at G1-S. This checkpoint is pivotal to decisions regulating proliferation, apoptosis, or differentiation. We have also confirmed the overexpression of Gas3/PMP22 in RA-treated neural crests, which is associated with cytoskeletal changes and increased apoptosis. Our results suggest that increases in multiple components of diverse regulatory pathways have an overall cumulative effect on cellular decisions. This heterogeneity contributes to the pleiotropic effects of RA, specifically those affecting proliferation and cell death.

Direct Interaction of the N-terminal Domain of Focal Adhesion Kinase with the N-terminal Transactivation Domain of p53

Focal adhesion kinase (FAK) is a nonreceptor kinase that is overexpressed in many types of tumors and associates with multiple cell surface receptors and intracellular signaling proteins through which it can play an important role in survival signaling. A link between FAK and p53 in survival signaling has been reported, although the molecular basis of these events has not been described. In the present study, we report that FAK physically and specifically interacts with p53 as demonstrated by pull-down, immunoprecipitation, and co-localization analyses. Using different constructs of N-terminal, central, and C-terminal fragments of FAK and p53 proteins, we determined that the N-terminal fragment of FAK directly interacts with the N-terminal transactivation domain of p53. Inhibition of p53 with small interfering p53 RNA resulted in a decreased complex of FAK and p53 proteins in 293 cells, and induction of p53 with doxorubicin in normal human fibroblasts caused an increase of FAK and p53 interaction. Introduction of the FAK plasmid into p53-null SAOS-2 cells was able to rescue these cells from apoptosis induced by expression of wild type p53. In HCT 116 colon cancer cells, co-transfection of FAK plasmid with p21, MDM-2, and BAX luciferase plasmids resulted in significant inhibition of p53-responsive luciferase activities, demonstrating that FAK can reduce transcriptional activity of p53. The results of the FAK and p53 interaction study strongly support the conclusion that FAK can suppress p53-mediated apoptosis and inhibit transcriptional activity of p53. This provides a novel mechanism for FAK-p53-mediated survival/apoptotic signaling.

p53 mutation heterogeneity in cancer

The p53 gene is inactivated in about 50% of human cancers and the p53 protein is an essential component of the cell response induced by genotoxic stresses such as those generated by radiotherapy or chemotherapy. It is therefore highly likely that these alterations are an important component in tumor resistance to therapy. The particular characteristics of these alterations, 80% of which are missense mutations leading to functionally heterogeneous proteins, make p53 a unique gene in the class of tumor suppressor genes. A considerable number of mutant p53 proteins probably have an oncogenic activity per se and therefore actively participate in cell transformation. The fact that the apoptotic and antiproliferative functions of p53 can be dissociated in certain mutants also suggests another level of complexity in the relationships between p53 inactivation and neoplasia.

Del (9q) AML: clinical and cytological characteristics and prognostic implications

Del (9q) is a recurrent cytogenetic abnormality in acute myeloid leukaemia (AML). We report an analysis of 81 patients with del(9q) as a diagnostic karyotypic abnormality entered into the Medical Research Council AML trials 10, 11 and 12. Patients were divided into three groups: (i) Sole del (9q), 21 patients; (ii) Del(9q) in association with t(8;21), 29 patients; (iii) Del(9q) in association with other cytogenetic abnormalities, 31 patients. Sole del(9q) was associated with a characteristic bone marrow phenotype at diagnosis: a single Auer rod was found in all cases examined. There was also an association with erythroid dysplasia (74%) and granylocytic lineage vacuolation (90%). The incidence of all three of these features was significantly higher (P < 0.05) in the sole del(9q) group compared with control cases lacking del(9q). The overall survival (OS) of all 81 patients was compared with a control group of 1738 patients with normal cytogenetics entered in the same trials over the period of investigation. The 5-year OS for patients with del(9q) was 45%, compared with 35% for the control group (P = 0.09). Patients with del(9q) in association with t(8;21) had a 5-year OS of 75%, which was significantly better than the groups with either sole del(9q) (40%) and del(9q) with other abnormalities (26%; P = 0.008). Karyotyping indicated a common area of deletion in the region 9q21-22, which was present in 94% of cases. It is likely that the deletion of single or multiple tumour suppressor genes located in this region may underlie the pathogenesis of del (9q) AML.

A New Set of Monoclonal Antibodies Directed to Proline-Rich and Central Regions of p53

The p53 protein can adopt several conformations in cells--"latent," "active," or mutant--depending on cellular stress or mutations of the TP53 gene. Today, only a few antibodies discriminating these conformations are available. We produced three new anti-p53 monoclonal antibodies (MAbs) directed against epitopes of human p53. The H53C1 MAb recognizes an epitope located at the N-terminal part of the central region of p53 and can discriminate mutant from wild-type conformation. The H53C2 and H53C3 MAbs are against different epitopes within the proline-rich region of p53. Moreover, the H53C2 epitope is located in the second negative regulatory domain of p53 between residues 80 and 93. These MAbs can be used as new tools to study and modulate the cellular functions of p53.

The proline-rich region of mouse p53 influences transactivation and apoptosis but is largely dispensable for these functions

The N-terminal proline-rich domain of human p53 has been shown to be important for the induction of apoptosis. However, the corresponding region in mouse and other species is not highly conserved and has been less well studied. In this paper, we have characterized mutants with deletions in this region of mouse p53. Our results demonstrate that deletions in the proline-rich domain have varying effects on function ranging from no effect to severe impairment of cell death activity, depending on precisely which residues are deleted. We also show that the mutants differ in their ability to transactivate different p53 target promoters. Although we have been able to obtain mutants selectively impaired for apoptosis, our data are not generally consistent with this region being a functional domain. The data are more consistent with the interpretation that the region influences function by altering local protein structure which may affect promoter discrimination.

Characterization of p73 functional domains necessary for transactivation and growth suppression

p73, a p53 family member, is highly similar to p53 in both structure and function. Like p53, the p73 protein contains an N-terminal activation domain, a DNA-binding domain, a tetramerization domain, and several PXXP motifs. Previously, we and others have shown that some functional domains in p53, such as the DNA-binding and tetramerization domains, are required for inducing both cell cycle arrest and apoptosis whereas others, such as the second activation domain, the proline-rich domain, and the C-terminal basic domain, are only required for inducing apoptosis. To determine the activity of p73 functional domains, we have generated stable inducible cell lines that express p73beta and various mutants deficient in one or more functional domains. We found that in addition to the DNA-binding domain, p73-mediated growth suppression requires the N-terminal activation domain and the tetramerization domain. However, unlike p53, p73-mediated apoptosis does not require the region adjacent to the activation domain or the entire C-terminal region. Interestingly, while the N- or the C-terminal PXXP motifs are dispensable for p73 function, deletion of both the N- and the C-terminal PXXP motifs renders p73 inactive in transactivation. In addition, we found that substitution of two conserved tandem hydrophobic residues with two hydrophilic ones, which can abrogate the activity of the first activation domain in p53, has no effect on p73 transcriptional activity. Together, we showed that the p73 protein has its own unique determinants for transactivation and growth suppression.

Microarray and suppression subtractive hybridization analyses of gene expression in pheochromocytoma cells reveal pleiotropic effects of pituitary adenylate cyclase-activating polypeptide on cell proliferation, survival, and adhesion

Pituitary adenylate cyclase-activating polypeptide (PACAP) exerts trophic effects on several neuronal, neuroendocrine, and endocrine cells. To gain insight into the pattern of the transcriptional modifications induced by PACAP during cell differentiation, we studied the effects of this neuropeptide on rat pheochromocytoma PC12 cells. We first analyzed the transcriptome of PC12 cells in comparison to that of terminally differentiated rat adrenomedullary chromaffin cells, using a high-density microarray, to identify genes associated with the proliferative phenotype that are possible targets of PACAP during differentiation of sympathoadrenal normal and tumoral cells. We then studied global gene expression in PC12 cells after 48 h of exposure to PACAP, using both cDNA microarray and suppression subtractive hybridization technologies. These complementary approaches resulted in the identification of 75 up-regulated and 70 down-regulated genes in PACAP-treated PC12 cells. Among the genes whose expression is modified in differentiated cells, a vast majority are involved in cell proliferation, survival, and adhesion/motility. Expression changes of most of these genes have been associated with progression of several neoplasms. A kinetic study of the effects of PACAP on some of the identified genes showed that the neuropeptide likely exerts early as well as late actions to achieve the gene expression program necessary for cell differentiation. In conclusion, the results of the present study underscore the pleiotropic role of PACAP in cell differentiation and provide important information on novel targets that could mediate the effects of this neuropeptide in normal and tumoral neuroendocrine cells.

Growth arrest specific gene 1: a fuel for driving growth in the cerebellum

Cell proliferation is an essential force to build up the size, shape, and function of an organ. This force is particularly prominent in the production of the cerebellar granule neurons, which represent 80% of all brain neurons. Extensive cell biological and tissue transplantation studies have uncovered both long-range diffusible and local cell-cell, contact-dependent growth cues for the granular neurons. The assignment of specific gene products to their contributions to the genesis of the granular neurons is greatly facilitated by in vitro culture assays and knock-out mouse analyses. Among them, the Growth arrest specific gene 1 (Gas1), a known negative regulator of the cell cycle, was shown to have profound influence on the production of the granule cells. Our aim here is to review the contributions of Gas1 and a few other selected genes and put them into a more comprehensive framework, though it may be speculative at times, of granule cell proliferation regulation.

Suppression of the STK15 oncogenic activity requires a transactivation-independent p53 function

Using a transactivation-defective p53 derivative as bait, STK15, a centrosome-associated oncogenic serine/threonine kinase, was isolated as a p53 partner. The p53-STK15 interaction was confirmed further by co-immunoprecipitation and GST pull-down studies. In co-transfection experiments, p53 suppressed STK15-induced centrosome amplification and cellular transformation in a transactivation-independent manner. The suppression of STK15 oncogenic activity by p53 might be explained in part by the finding that p53 inhibited STK15 kinase activity via direct interaction with the latter's Aurora box. Taken together, these findings revealed a novel mechanism for the tumor suppressor function of p53.

Identification of genes regulating colorectal carcinogenesis by using the algorithm for diagnosing malignant state method

We studied the expression profiles of various stages of colorectal tumors (adenoma (AD), seven samples; carcinoma (CA), 16 samples) by using cDNA microarrays and developed ADMS (algorithm for diagnosing malignant state) method, selecting 335 clones characteristic of CA state. We, then, applied ADMS to 12 additional samples (five from primary lesions with metastasis and seven metastases); all 16 CAs and 12 metastatic tumors were diagnosed correctly as cancerous states. Although three of the seven ADs were diagnosed as "cancerous," the large size of two of these tumors suggested their potential malignancy. Our strategy for selecting clones characteristic of the malignant state is widely applicable to diagnosis and for predicting the stage of progression during multistep carcinogenesis. Of the 335 clones we selected, 135 were known genes. Included in the 135 genes were tumor suppressor and growth factor-related genes and were consistent with the literature. ADMS is a reliable means for identifying genes useful for the diagnosis of cancer.

Role of MmTRA1b/phospholipid scramblase1 gene expression in the induction of differentiation of human myeloid leukemia cells into granulocytes

OBJECTIVE

We previously cloned a human normal counterpart (MmTRA1b/phospholipid scramblase 1) of the mouse leukemogenesis-associated gene MmTRA1a. MmTRA1b gene expression was increased during differentiation of human monoblastic leukemia U937 cells using some differentiation inducers but not 1alpha,25-dihydroxyvitamin D(3) (a typical monocytic differentiation inducer). To further elucidate the role of human MmTRA1b gene expression in the differentiation of myelogenous leukemia cells, we measured MmTRA1b gene expression in several myeloid leukemia cell lines and primary leukemia cells.

MATERIALS AND METHODS

The expression of MmTRA1b mRNA was determined by semiquantitative reverse transcriptase polymerase chain reaction.

RESULTS

Expression of the MmTRA1b gene was markedly induced during granulocytic differentiation of promyelocytic leukemia NB4 and HT93 cells induced by all-trans retinoic acid (ATRA). The level of MmTRA1b mRNA was significantly increased during differentiation toward granulocytes, but not monocytes/macrophages, in bipotential myeloid leukemia HL-60 cells. The level of MmTRA1 mRNA was not increased during erythroid differentiation induced by hemin in erythroid leukemia K562 and HEL cells or during megakaryocytic differentiation induced by 12-O-tetradecanoylphorbol-13-acetate in K562 cells. Expression of the MmTRA1b gene also was not induced when apoptosis of NB4 cells was induced by antileukemic drugs. ATRA-induced differentiation of antisense MmTRA1b-transfected NB4 cells was significantly suppressed. On the other hand, ATRA induced the differentiation of MmTRA1b-transfected NB4 cells more efficiently than that of mock-transfected cells. MmTRA1b mRNA also was clearly induced in ATRA-treated primary acute promyelocytic leukemia cells during granulocytic differentiation.

CONCLUSION

MmTRA1b mRNA was specifically induced during granulocytic differentiation of acute promyelocytic leukemia cells and was associated with induction of their differentiation.

Gas1 is induced during and participates in excitotoxic neuronal death

We have performed differential screening to identify genes participating in NMDA-induced neuronal death. The gas1 (growth arrest-specific gene 1) gene, whose product is known to inhibit cell cycle progression, was induced in cultured corticohippocampal neurons committed to die after a brief exposure to NMDA. Overexpression of Gas1 in cultured hippocampal neurons and in human neuroblastoma NB69 cells produced a marked reduction in the number of viable cells. Furthermore, gas1 antisense oligodeoxynucleotide or antisense mRNA protected hippocampal neurons or NB69 cells from neuronal death. Importantly, Gas1-induced neuronal death was attenuated by coexpression of the human Bcl-2 protein or the baculoviral caspase inhibitor OpIAP2. While Gas1 does not directly interact with Bcl-2, OpIAP2 coimmunoprecipitates with Gas1. In addition, induction of gas1 also occurred in rat brain in two models of excitotoxicity: delayed neuronal death after intraperitoneal kainate injection and neuronal death in hippocampal slices after ischemia. These results indicate that Gas1 is induced by activation of glutamate receptors and is part of the gene expression program directing neuronal death after mild excitotoxic insults.

The EGF-like homeotic protein dlk affects cell growth and interacts with growth-modulating molecules in the yeast two-hybrid system

Levels of dlk, an EGF-like homeotic protein, are critical for several differentiation processes. Because growth and differentiation are, in general, exclusive of each other, and increasing evidence indicates that Dlk1 expression changes in tumorigenic processes, we studied whether dlk could also affect cell growth. We found that, in response to glucocorticoids, Balb/c 3T3 cells with diminished levels of dlk expression develop foci-like cells that have lost contact inhibition, display altered morphology, and grow faster than control cell lines. Balb/c 3T3 cells spontaneously growing more rapidly are also dlk-negative cells. Moreover, screening by the yeast two-hybrid system, using Dlk1 constructs as baits, resulted in the isolation of GAS1 and acrogranin cDNAs. Interestingly, these proteins are cysteine-rich molecules involved in the control of cell growth. Taken together, these observations suggest that dlk may participate in a network of interactions controlling how the cells respond to growth or differentiation signals.

Cloning and characterization of the C. elegans gas1 homolog: phas-1

Among the set of genes expressed during the quiescent G0 phase of the cell cycle (gas genes), gas1 encodes for a GPI anchor protein associated to the plasma membrane, which is able to induce growth arrest when overexpressed in proliferating fibroblasts. In this report we describe the isolation and characterization of a gas1 Caenorhabditis elegans homolog, phas-1, that seems to be transcribed as an operon together with a gene encoding for a protein similar to human acid ceramidases. Phas-1 structure is very similar to its mammalian homolog conserving almost all cysteine residues and it is expressed in the pharynx from its early formation, in the two-fold embryo, until the adult stage. Surprisingly, while phas-1 is expressed in all developmental stages, with the exception of the dauer larva, the ceramidase-like encoding gene, co-expressed in the same operon together with phas-1, is absent in embryos and is very abundantly expressed in the dauer larva. Overexpression of phas-1 in growing NIH3T3 fibroblasts is able to inhibit the S-phase entry in a similar manner as its murine homolog. On the other hand, when phas-1 is overexpressed or ablated in C. elegans, no specific phenotype due to its transcription alteration can be observed, despite its localized expression suggesting a role in pharynx function or development.

Growth, hedgehog and the price of GAS

Embryonic development in a given species is orchestrated by genes regulating growth and differentiation in a stereotyped and conserved manner, resulting in embryos of consistent size and shape. Several signaling pathways, including that of Sonic Hedgehog (SHH), have been implicated in these processes. Recent experiments with Gas1 indicate that it may act as a growth-inducing gene, challenging its previous function as a gene specifically involved in growth arrest. Moreover, GAS1, a GPI-linked membrane protein, can bind SHH, suggesting an interacting link between growth and patterning through SHH and GAS1.

p53 binds the nuclear matrix in normal cells: binding involves the proline-rich domain of p53 and increases following genotoxic stress

The tumour suppressor p53 is a multifunctional protein important for the maintenance of genomic integrity. It is able to form molecular complexes with different DNA targets and also with cellular proteins involved in DNA transcription and DNA repair. In mammalian cells the biochemical processing of DNA occurs on a nuclear sub-structure termed the nuclear matrix. Previously Deppert and co-workers have identified p53 in association with the nuclear matrix in viral- and non-viral transformed cell lines. In the present study we demonstrate, for the first time, that p53 is bound to the nuclear matrix in primary cultures of normal mammalian cells and that this binding increases following DNA damage. Analysis of cell lines expressing structural mutants of p53 revealed that association with the nuclear matrix is independent of the tertiary and quaternary structure of p53. However, the proline-rich domain towards the N-terminus of p53 (residues 67 to 98) appeared important for binding to the nuclear matrix. This was demonstrated by TET-ON regulated expression of p53-derived constructs in p53(-/-) murine embryonic fibroblasts (MEF p53(-/-)). The proline-rich domain of p53 has potential for SH3 protein-protein interaction, and has a role in p53-mediated apoptosis and possibly base excision repair of DNA damage. We discuss our observations in relation to the ability of p53 to facilitate DNA repair and also review evidence indicating that matrix-bound p53 in SV40-transformed cells may facilitate the transforming potential of SV40 large T antigen.

Growth arrest specific gene 1 is a positive growth regulator for the cerebellum

Postnatal cerebellum development involves the generation of granule cells and Bergmann glias (BGs). The granule cell precursors are located in the external germinal layer (EGL) and the BG precursors are located in the Purkinje layer (PL). BGs extend their glial fibers into the EGL and facilitate granule cells' inward migration to their final location. Growth arrest specific gene 1 (Gas1) has been implicated in inhibiting cell-cycle progression in cell culture studies (G. Del Sal et al., 1992, Cell 70, 595--607). However, its growth regulatory function in the CNS has not been described. To investigate its role in cerebellar growth, we analyzed the Gas1 mutant mice. At birth, wild-type and mutant mice have cerebella of similar size; however, mature mutant cerebella are less than half the size of wild-type cerebella. Molecular and cellular examinations indicate that Gas1 mutant cerebella have a reduced number of granule cells and BG fibers. We provide direct evidence that Gas1 is required for normal levels of proliferation in the EGL and the PL, but not for their differentiation. Furthermore, we show that Gas1 is specifically and coordinately expressed in both the EGL and the BGs postnatally. These results support Gas1 as a common genetic component in coordinating EGL cell and BG cell proliferation, a link which has not been previously appreciated.

Functions of the Growth Arrest Specific 1 Gene in the Development of the Mouse Embryo

The growth arrest specific 1 (gas1) gene is highly expressed in quiescent mammalian cells (Schneider et al., 1988, Cell 54, 787-793). Overexpression of gas1 in normal and some cancer cell lines could inhibit G(0)/G(1) transition. Presently, we have examined the functions of this gene in the developing mouse embryo. The spatial-temporal expression patterns for gas1 were established in 8.5- to 14.5-day-old embryos by immunohistochemical staining and in situ hybridization. Gas1 was found heterogeneously expressed in most organ systems including the brain, heart, kidney, limb, lung, and gonad. The antiproliferative effects of gas1 on 10.5 and 12.5 day limb cells were investigated by flow cytometry. In 10.5 day limbs cells, gas1 overexpression could not prevent G(0)/G(1) progression. It was determined that gas1 could only induce growth arrest if p53 was also coexpressed. In contrast, gas1 overexpression alone was able to induce growth arrest in 12.5 day limb cells. We also examined the cell cycle profile of gas1-expressing and nonexpressing cells by immunochemistry and flow cytometry. For 10.5 day Gas1-expressing heart and limb cells, we did not find these cells preferentially distributed at G0/G1, as compared with Gas1-negative cells. However, in the 12.5 day heart and limb, we did find significantly more Gas1-expressing cells distributed at G0/G1 phase than Gas1-negative cells. These results implied that Gas1 alone, during the early stages of development, could not inhibit cell growth. This inhibition was only established when the embryo grew older. We have overexpressed gas1 in subconfluent embryonic limb cells to determine the ability of gas1 to cross-talk with various response elements of important transduction pathways. Specifically, we have examined the interaction of gas1 with Ap-1, NFkappaB, and c-myc responsive elements tagged with a SEAP reporter. In 10.5 day limb cells, gas1 overexpression had little effect on Ap-1, NFkappaB, and c-myc activities. In contrast, gas1 overexpression in 12.5 day limb cells enhanced AP-1 response while it inhibited NFkappaB and c-myc activities. These responses were directly associated with the ability of gas1 to induce growth arrest in embryonic limb cells. In the 12.5 day hindlimb, gas1 was found strongly expressed in the interdigital tissues. We overexpressed gas1 in these tissues and discovered that it promoted interdigital cell death. Our in situ hybridization studies of limb sections and micromass cultures revealed that, during the early stages of chondrogenesis, only cells surrounding the chondrogenic condensations expressed gas1. The gene was only expressed by chondrocytes after the cartilage started to differentiate. To understand the function of gas1 in chondrogenesis, we overexpressed the gene in limb micromass cultures. It was found that cells overexpressing gas1/GFP could not participate in cartilage formation, unlike cells that just express the GFP reporter. We speculated that the reason gas1 was expressed outside the chondrogenic nodules was to restrict cells from being recruited into the nodules and thereby defining the boundary between chondrogenic and nonchondrogenic forming regions.

A role for the polyproline domain of p53 in its regulation by Mdm2

The p53 protein plays a key role in the cellular response to stress by inducing cell growth arrest or apoptosis. The polyproline region of p53 has been shown to be important for its growth suppression activity. p53 protein lacking the polyproline region has impaired apoptotic activity and altered specificity for certain apoptotic target genes. Here we describe the role of this region in the regulation of p53 by its inhibitor Mdm2. p53 lacking the polyproline region was identified to be more susceptible to inhibition by Mdm2. Furthermore, the absence of this region renders p53 more accessible to ubiquitination, nuclear export, and Mdm2-mediated degradation. This increased sensitivity to Mdm2 results from an enhanced affinity of Mdm2 toward p53 lacking the polyproline region. Our results provide a new explanation for the impaired growth suppression activity of p53 lacking this region. The polyproline region is proposed to be important in the modulation of the inhibitory effects of Mdm2 on p53 activities and stability.

Proapoptotic p53-interacting protein 53BP2 is induced by UV irradiation but suppressed by p53

p53 is an important mediator of the cellular stress response with roles in cell cycle control, DNA repair, and apoptosis. 53BP2, a p53-interacting protein, enhances p53 transactivation, impedes cell cycle progression, and promotes apoptosis through unknown mechanisms. We now demonstrate that endogenous 53BP2 levels increase following UV irradiation induced DNA damage in a p53-independent manner. In contrast, we found that the presence of a wild-type (but not mutant) p53 gene suppressed 53BP2 steady-state levels in cell lines with defined p53 genotypes. Likewise, expression of a tetracycline-regulated wild-type p53 cDNA in p53-null fibroblasts caused a reduction in 53BP2 protein levels. However, 53BP2 levels were not reduced if the tetracycline-regulated p53 cDNA was expressed after UV damage in these cells. This suggests that UV damage activates cellular factors that can relieve the p53-mediated suppression of 53BP2 protein. To address the physiologic significance of 53BP2 induction, we utilized stable cell lines with a ponasterone A-regulated 53BP2 cDNA. Conditional expression of 53BP2 cDNA lowered the apoptotic threshold and decreased clonogenic survival following UV irradiation. Conversely, attenuation of endogenous 53BP2 induction with an antisense oligonucleotide resulted in enhanced clonogenic survival following UV irradiation. These results demonstrate that 53BP2 is a DNA damage-inducible protein that promotes DNA damage-induced apoptosis. Furthermore, 53BP2 expression is highly regulated and involves both p53-dependent and p53-independent mechanisms. Our data provide new insight into 53BP2 function and open new avenues for investigation into the cellular response to genotoxic stress.

p53 binds selectively to the 5' untranslated region of cdk4, an RNA element necessary and sufficient for transforming growth factor beta- and p53-mediated translational inhibition of cdk4

One consequence of transforming growth factor beta (TGF-beta) treatment is inhibition of Cdk4 synthesis, and this is dependent on p53. Here, we show that the 5' untranslated region (UTR) of the cdk4 mRNA is both necessary and sufficient for wild-type p53-dependent TGF-beta-regulated translational inhibition of cdk4. Wild-type p53 bound selectively to the 5' UTR of the cdk4 mRNA and inhibited translation of RNAs that contain this region. RNA binding and translational control are two genetically separable functions of p53, as are specific and nonspecific RNA binding. Moreover, transactivation-defective mutants of p53 retain the ability to regulate cdk4 translation. Our findings suggest that p53 functions as a regulator of translation in response to TGF-beta in vivo.

Analysis of the domain requirement in Gas1 growth suppressing activity

The product of the growth arrest specific gene, gas1, is a membrane-associated protein which activates a p53-dependent growth suppression signalling pathway. We have shown that Gas1 is linked to the plasma membrane through a glycosyl-phosphatidylinositol (GPI) anchor. Several GPI-anchored protein have been identified as part of receptor complexes either as co-receptors or as membrane bound ligands. In this report, we characterize the Gas1 domains required for its growth suppression function and demonstrate the dispensability of Gas1 GPI anchor.

Tumor-derived mutations within the DNA-binding domain of p53 that phenotypically resemble the deletion of the proline-rich domain

The p53 tumor suppressor protein induces apoptosis through a mechanism that may involve the transcriptional activation of cellular genes, including the PIG3 gene. A p53 protein lacking the proline-rich region (p53delta62-91) induces many p53-responsive genes but not PIG3. In parallel, this mutant induces growth arrest but not apoptosis. We show here that the replacement of the N-terminal (amino acids 1-80) or C-terminal (amino acids 344-393) domains of p53 with heterologous domains does not interfere with transcription from the PIG3 promoter, but these chimeras still require the proline-rich region for PIG3 activation. The p53-homolog p73beta also activated the PIG3 promoter, but in contrast to p53, the proline-rich domain of p73beta (residues 81-113) was dispensable to induce the PIG3 promoter. Some tumor-derived p53-mutants, especially M246I, retained the ability to activate transcription of mdm2 but specifically failed to induce the PIG3 promoter, thus resembling p53delta62-91. Further, p53delta62-91 and p53M246I were defective for induction of apoptosis. Finally, p53delta62-91 and p53M246I both showed reduced binding to the DNA of the PIG3 promoter and also to the DNA of the mdm2 and p21 promoters in vitro. Correspondingly, at low expression levels, p53delta62-91 and p53M246I poorly activated the mdm2 promoter when compared to wild type p53. Our results suggest that the proline-rich domain of p53 affects the ability of the central domain to bind DNA. Moreover, some tumor-derived mutations within the central DNA binding domain of p53 mimic the loss of the proline-rich domain.

p53 is involved in the p120E4F-mediated growth arrest

Control of cell growth and division by the p53 tumor suppressor protein requires its abilities to transactivate and repress specific target genes and to associate in complex with other proteins. Here we demonstrate that p53 binds to the E1A-regulated transcription factor p120E4F, a transcriptional repressor of the adenovirus E4 promoter. The interaction involves carboxy-terminal half of p120E4F and sequences located at the end of the sequence-specific DNA-binding domain of p53. Ectopic expression of p120E4F leads to a block of cell proliferation in several human and murine cell lines and this effect requires the association with wild-type (wt) p53. Although p120E4F can also bind to mutant p53, the growth suppression induced by overexpression of the protein is severely reduced in a cell line that contains mutant p53. These data suggest that p120E4F may represent an important element within the complex network of p53 checkpoint functions.

Phosphorylation of murine p53, but not human p53, by MAP kinase in vitro and in cultured cells highlights species-dependent variation in post-translational modification

The p53 tumour suppressor protein is tightly regulated by protein-protein association, protein turnover and a variety of post-translational modifications. Multisite phosphorylation plays a major role in activating and in finely tuning p53 function. The proline rich domain of murine p53 is a substrate for phosphorylation, in vitro and in cultured cells, by the p42ERK2 and p44ERK1 mitogen-activated protein (MAP) kinases. However, to date there have been no reports of attempts to determine whether p53 from any other species is a substrate for MAP kinase. In this paper we confirm that murine p53 is targeted by recombinant MAP kinase and by MAP kinases in extracts of both murine and human cells. In contrast, human p53 is not a substrate for recombinant MAP kinase nor are there any detectable levels of protein kinase activity in stimulated human cell extracts which phosphorylate the proline rich domain of human p53 in vitro. Finally, although stimulation of murine fibroblasts with o-tetradecanolylphorbol 13-acetate (TPA), an indirect activator of the MAP kinase pathway, leads to site-specific phosphorylation of murine p53, similar treatment of human fibroblasts and epithelial cells showed no significant changes in the phosphorylation pattern. These data are consistent with accumulating evidence that significant species-dependent differences exist in the post-translational modification of p53.

The cellular response to p53: the decision between life and death

The p53 tumor suppressor protein plays a crucial role in regulating cell growth following exposure to various stress stimuli. p53 induces either growth arrest, which prevents the replication of damaged DNA, or programmed cell death (apoptosis), which is important for eliminating defective cells. Whether the cell enters growth arrest or undergoes apoptosis, depends on the final integration of incoming signals with antagonistic effects on cell growth. Many factors affect the cellular response to activated p53. These include the cell type, the oncogenic status of the cell with emphasis on the Rb/E2F balance, the extracellular growth and survival stimuli, the intensity of the stress signals, the level of p53 expression and the interaction of p53 with specific proteins. p53 is regulated both at the levels of protein stability and biochemical activities. This complex regulation is mediated by a range of viral and cellular proteins. This review discusses this intriguing complexity which affects the cell response to p53 activation.

Nucleotide sequence of the porcine p53 cDNA, and the detection of recombinant porcine p53 expressed in vitro with a variety of anti-p53 antibodies

The cDNA of porcine p53 was cloned and sequenced by a reverse transcriptase polymerase chain reaction (RT - PCR) approach with primers based on regions of homology between all known p53 sequences. The p53 cDNA was found to be 87% conserved to human p53 and 86% conserved to bovine p53 at the nucleotide level. The porcine p53 sequence was inserted into an expression vector and recombinant protein expressed in vitro. An approximately 50 kDa protein was detected by Western blotting using both polyclonal and monoclonal anti-p53 antibodies. The sequence data of porcine p53 and the ability to detect expressed protein with various anti-p53 antibodies will allow the p53 status of the pig population, and the role of p53 in porcine tumours, to be assessed. An understanding of tumour development in the pig may be important if pig cells, tissues or organs are to be used in the treatment of humans.