The phosphoprotein p53 is down-regulated post-transcriptionally during embryogenesis in vertebrates

Cockayne Syndrome Patient iPSC-Derived Brain Organoids and Neurospheres Show Early Transcriptional Dysregulation of Biological Processes Associated with Brain Development and Metabolism

Cockayne syndrome (CS) is a rare hereditary autosomal recessive disorder primarily caused by mutations in Cockayne syndrome protein A (CSA) or B (CSB). While many of the functions of CSB have been at least partially elucidated, little is known about the actual developmental dysregulation in this devasting disorder. Of particular interest is the regulation of cerebral development as the most debilitating symptoms are of neurological nature. We generated neurospheres and cerebral organoids utilizing Cockayne syndrome B protein (CSB)-deficient induced pluripotent stem cells derived from two patients with distinct severity levels of CS and healthy controls. The transcriptome of both developmental timepoints was explored using RNA-Seq and bioinformatic analysis to identify dysregulated biological processes common to both patients with CS in comparison to the control. CSB-deficient neurospheres displayed upregulation of the VEGFA-VEGFR2 signalling pathway, vesicle-mediated transport and head development. CSB-deficient cerebral organoids exhibited downregulation of brain development, neuron projection development and synaptic signalling. We further identified the upregulation of steroid biosynthesis as common to both timepoints, in particular the upregulation of the cholesterol biosynthesis branch. Our results provide insights into the neurodevelopmental dysregulation in patients with CS and strengthen the theory that CS is not only a neurodegenerative but also a neurodevelopmental disorder.

Characterization of p53/p63/p73 and Myc expressions during embryogenesis of the sea urchin

BACKGROUND

Some marine invertebrate organisms are considered not to develop tumors due to unknown mechanisms. To gain an initial insight into how tumor-related genes may be expressed and function during marine invertebrate development, we here leverage sea urchin embryos as a model system and characterize the expressions of Myc and p53/p63/p73 which are reported to function synergistically in mammalian models as an oncogene and tumor suppressor, respectively.

RESULTS

During sea urchin embryogenesis, a combo gene of p53/p63/p73 is found to be maternally loaded and decrease after fertilization both in transcript and protein, while Myc transcript and protein are zygotically expressed. p53/p63/p73 and Myc proteins are observed in the cytoplasm and nucleus of every blastomere, respectively, throughout embryogenesis. Both p53/p63/p73 and Myc overexpression results in compromised development with increased DNA damage after the blastula stage. p53/p63/p73 increases the expression of parp1, a DNA repair/cell death marker gene, and suppresses endomesoderm gene expressions. In contrast, Myc does not alter the expression of specification genes or oncogenes yet induces disorganized morphology.

CONCLUSIONS

p53/p63/p73 appears to be important for controlling cell differentiation, while Myc induces disorganized morphology yet not through conventional oncogene regulations or apoptotic pathways during embryogenesis of the sea urchin.

Resveratrol and p53: How are they involved in CRC plasticity and apoptosis?

BACKGROUND

Colorectal cancer (CRC), which is mainly caused by epigenetic and lifestyle factors, is very often associated with functional plasticity during its development. In addition, the malignant plasticity of CRC cells underscores one of their survival abilities to functionally adapt to specific stresses, including inflammation, that occur during carcinogenesis. This leads to the generation of various subsets of cancer cells with phenotypic diversity and promotes epithelial-mesenchymal transition (EMT), formation of cancer cell stem cells (CSCs) and metabolic reprogramming. This can enhance cancer cell differentiation and facilitate tumorigenic potential, drug resistance and metastasis.

AIM OF REVIEW

The tumor protein p53 acts as one of the central suppressors of carcinogenesis by regulating its target genes, whose proteins are involved in the plasticity of cancer cells, autophagy, cell cycle, apoptosis, DNA repair. The aim of this review is to summarize the latest published research on resveratrol's effect in the prevention of CRC, its regulatory actions, specifically on the p53 pathway, and its treatment options.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Resveratrol, a naturally occurring polyphenol, is a potent inducer of a variety of tumor-controlling. However, the underlying mechanisms linking the p53 signaling pathway to the functional anti-plasticity effect of resveratrol in CRC are still poorly understood. Therefore, this review discusses novel relationships between anti-cellular plasticity/heterogeneity, pro-apoptosis and modulation of tumor protein p53 signaling in CRC oncogenesis, as one of the crucial mechanisms by which resveratrol prevents malignant phenotypic changes leading to cell migration and drug resistance, thus improving the ongoing treatment of CRC.

Unraveling the Mechanisms of Clinical Drugs-Induced Neural Tube Defects Based on Network Pharmacology and Molecular Docking Analysis

Chemotherapeutic agents such as methotrexate (MTX), raltitrexed (RTX), 5-fluorouracil (5-FU), hydroxyurea (HU), and retinoic acid (RA), and valproic acid (VPA), an antiepileptic drug, all can cause malformations in the developing central nervous system (CNS), such as neural tube defects (NTDs). However, the common pathogenic mechanisms remain unclear. This study aimed to explore the mechanisms of NTDs caused by MTX, RTX, 5-FU, HU, RA, and VPA (MRFHRV), based on network pharmacology and molecular biology experiments. The MRFHRV targets were integrated with disease targets, to find the potential molecules related to MRFHRV-induced NTDs. Protein-protein interaction analysis and molecular docking were performed to analyze these common targets. Utilizing the kyoto encyclopedia of genes and genomes (KEGG) signaling pathways, we analyzed and searched the possible causative pathogenic mechanisms by crucial targets and the signaling pathway. Results showed that MRFHRV induced NTDs through several key targets (including TP53, MAPK1, HSP90AA1, ESR1, GRB2, HDAC1, EGFR, PIK3CA, RXRA, and FYN) and multiple signaling pathways such as PI3K/Akt pathway, suggesting that abnormal proliferation and differentiation could be critical pathogenic contributors in NTDs induced by MRFHRV. These results were further validated by CCK8 assay in mouse embryonic stem cells and GFAP staining in embryonic brain tissue. This study indicated that chemotherapeutic and antiepileptic agents induced NTDs might through predicted targets TP53, MAPK1, GRB2, HDAC1, EGFR, PIK3CA, RXRA, and FYN and multiple signaling pathways. More caution was required for the clinical administration for women with childbearing potential and pregnant.

Vitrification transiently alters Oct-4, Bcl2 and P53 expression in mouse morulae but does not affect embryo development in vitro

This study was conducted to determine the impact of vitrification on the expression of genes regulating pluripotency and apoptosis in mouse morulae. The morulae were randomly allocated into three groups: (1) untreated (control), (2) exposed to vitrification solution without freezing (toxicity), or (3) vitrified by open-pulled straw method (vitrification). In vitro development was evaluated by morphology and assessed by the blastocyst rate and the blastocyst total cell number. Gene expression in morulae and blastocysts was assessed by quantitative Real Time-PCR (qRT-PCR) and western blot. The results showed that at morulae stage, the POU class 5 homeobox1 (Oct-4) and B-cell lymphoma2 (Bcl2) mRNA levels of vitrification group were significantly lower (P < 0.05) than those of control. Strikingly, the p53 mRNA level was significantly higher in vitrification group. However, the Oct-4, Bcl2 and p53 mRNA levels in mouse blastocysts were not statistically different. Furthermore, western blot results showed that there was no significant difference in Oct-4, Bcl2 and p53 expression at protein level in mouse morulae among three groups. Additionally, the blastocyst rate (96.67%-100.00%) and the average cell number of blastocysts (89.67-92.33) were similar between all groups. The data demonstrate that vitrification transiently changes the mRNA expression of several key genes in mouse morulae regulating early embryo development but does not affect embryo developmental potential in vitro.

Evasion of cell senescence in SHH medulloblastoma

The mechanisms leading to brain tumor formation are poorly understood. Using Ptch1^+/- mice as a medulloblastoma model, sequential mutations were found to shape tumor evolution. Initially, medulloblastoma preneoplastic lesions display loss of heterozygosity of the Ptch1 wild-type allele, an event associated with cell senescence in preneoplasia. Subsequently, p53 mutations lead to senescence evasion and progression from preneoplasia to medulloblastoma. These findings are consistent with a model where high levels of Hedgehog signaling caused by the loss of the tumor suppressor Ptch1 lead to oncogene-induced senescence and drive p53 mutations. Thus, cell senescence is an important characteristic of a subset of SHH medulloblastoma and might explain the acquisition of somatic TP53 mutations in human medulloblastoma. This mode of medulloblastoma formation contrasts with the one characterizing Li-Fraumeni patients with medulloblastoma, where TP53 germ-line mutations cause chromothriptic genomic instability and lead to mutations in Hedgehog signaling genes, which drive medulloblastoma growth. Here we discuss in detail these 2 alternative mechanisms leading to medulloblastoma tumorigenesis.

The MDM2-p53 pathway: multiple roles in kidney development

The molecular basis of nephron progenitor cell renewal and differentiation into nascent epithelial nephrons is an area of intense investigation. Defects in these early stages of nephrogenesis lead to renal hypoplasia, and eventually hypertension and chronic kidney disease. Terminal nephron differentiation, the process by which renal epithelial precursor cells exit the cell cycle and acquire physiological functions is equally important. Failure of terminal epithelial cell differentiation results in renal dysplasia and cystogenesis. Thus, a better understanding of the transcriptional frameworks that regulate early and late renal cell differentiation is of great clinical significance. In this review, we will discuss evidence implicating the MDM2-p53 pathway in cell fate determination during development. The emerging central theme from loss- and gain-of-function studies is that tight regulation of p53 levels and transcriptional activity is absolutely required for nephrogenesis. We will also discuss how post-translational modifications of p53 (e.g., acetylation and phosphorylation) alter the spatiotemporal and functional properties of p53 and thus cell fate during kidney development. Mutations and polymorphisms in the MDM2-p53 pathway are present in more than 50 % of cancers in humans. This raises the question of whether sequence variants in the MDM2-p53 pathway increase the susceptibility to renal dysgenesis, hypertension or chronic kidney disease. With the advent of whole exome sequencing and other high throughput technologies, this hypothesis is testable in cohorts of children with renal dysgenesis.

Driving apoptosis-relevant proteins toward neural differentiation

Emerging evidence suggests that apoptosis regulators and executioners may control cell fate, without involving cell death per se. Indeed, several conserved elements of apoptosis are integral components of terminal differentiation, which must be restrictively activated to assure differentiation efficiency, and carefully regulated to avoid cell loss. A better understanding of the molecular mechanisms underlying key checkpoints responsible for neural differentiation, as an alternative to cell death will surely make stem cells more suitable for neuro-replacement therapies. In this review, we summarize recent studies on the mechanisms underlying the non-apoptotic function of p53, caspases, and Bcl-2 family members during neural differentiation. In addition, we discuss how apoptosis-regulatory proteins control the decision between differentiation, self-renewal, and cell death in neural stem cells, and how activity is restrained to prevent cell loss.

Mechanisms of p53 activation and physiological relevance in the developing kidney

The tumor suppressor protein p53 is a short-lived transcription factor due to Mdm2-mediated proteosomal degradation. In response to genotoxic stress, p53 is stabilized via posttranslational modifications which prevent Mdm2 binding. p53 activation results in cell cycle arrest and apoptosis. We previously reported that tight regulation of p53 activity is an absolute requirement for normal nephron differentiation (Hilliard S, Aboudehen K, Yao X, El-Dahr SS Dev Biol 353: 354-366, 2011). However, the mechanisms of p53 activation in the developing kidney are unknown. We show here that metanephric p53 is phosphorylated and acetylated on key serine and lysine residues, respectively, in a temporal profile which correlates with the maturational changes in total p53 levels and DNA-binding activity. Site-directed mutagenesis revealed a differential role for these posttranslational modifications in mediating p53 stability and transcriptional regulation of renal function genes (RFGs). Section immunofluorescence also revealed that p53 modifications confer the protein with specific spatiotemporal expression patterns. For example, phos-p53(S392) is enriched in maturing proximal tubular epithelial cells, whereas acetyl-p53(K373/K382/K386) are expressed in nephron progenitors. Functionally, p53 occupancy of RFG promoters is enhanced at the onset of tubular differentiation, and p53 loss or gain of function indicates that p53 is necessary but not sufficient for RFG expression. We conclude that posttranslational modifications are important determinants of p53 stability and physiological functions in the developing kidney. We speculate that the stress/hypoxia of the embryonic microenvironment may provide the stimulus for p53 activation in the developing kidney.

Gatekeeper between quiescence and differentiation: p53 in axonal outgrowth and neurogenesis

The transcription factor and tumor suppressor gene p53 regulates a wide range of cellular processes including DNA damage/repair, cell cycle progression, apoptosis, and cell metabolism. In the past several years, a specific novel role for p53 in neuronal biology has emerged. p53 orchestrates the polarity of self-renewing divisions in neural stem cells both during embryonic development and in adulthood and coordinates the timing for cell fate specification. In postmitotic neurons, p53 regulates neurite outgrowth and postinjury axonal regeneration via neurotrophin-dependent and -independent signaling by both transcriptional and posttranslational control of growth cone remodeling. This review provides an insight into the molecular mechanisms upstream and downstream p53 both during neural development and following axonal injury. Their understanding may provide therapeutic targets to enhance neuroregeneration following nervous system injury.

Gene and microRNA expression in p53-deficient day 8.5 mouse embryos

BACKGROUND

Neural tube defects (NTDs) are one of the most common human birth defects, with a prevalence of approximately 1 in 1000 live births in the United States. In animal studies, deletion of p53 leads to a significant increase in embryos that exhibit exencephaly. Whereas several studies have closely investigated the morphologic changes of p53-deficient embryos, no study has reported the molecular-level alteration in p53-deficient embryos. Here we attempt to identify genes and microRNAs (miRNAs) modified by deletion of p53 in day 8.5 mouse embryos.

METHODS

Mouse embryos from p53 heterozygous crosses were collected, genotyped, and embryos of similar genotype (+/+; +/-; -/-) were pooled. RNA from the pooled samples was isolated to determine mRNA and miRNA expression levels using Whole Genome Bioarrays and Low Density Arrays, respectively.

RESULTS

In p53 -/- embryos, 388 genes showed statistically significant alteration in gene expression of more than twofold compared to p53 +/+ embryos. Expression of p53 and well known p53 target genes, such as p21 and cyclin G1, were significantly down-regulated in p53 -/- embryos. In contrast, expression of other p53 target genes, such as Mdm2, Noxa, and Puma, were unchanged. We also identified six genes (Csk, Itga3, Jarid2, Prkaca, Rarg, and Sall4), known to cause NTDs when deleted, that are also down-regulated in p53 -/- embryos. Finally, five miRNAs (mir-1, mir-30e-3p, mir-142-3p, mir-301, and mir-331) also showed statistically significant alterations in expression levels in p53 -/- embryos compared to p53 +/+ embryos. Combined analysis of the experimental data using stepwise regression model and two publicly available algorithms identified putative target genes of these miRNAs.

CONCLUSIONS

Our data have identified genes and miRNAs that may be involved in the mechanisms underlining NTDs and begin to define the developmental role of p53 in the etiology of NTDs.

The roles of p53 and p21 in normal development and hyperthermia-induced malformations

BACKGROUND

Hyperthermia (HS) is a well-studied teratogen that induces serious malformations, including neural tube defects. Our previous studies have shown that HS induces apoptosis by activating the mitochondrial apoptotic pathway. Prior to activation of the mitochondrial apoptotic pathway, HS also activates p53 and its target genes. In the present study, we determine whether p53 and/or p21 play a role as teratogen suppressors or inducers of HS-induced malformations.

METHODS

Pregnant mice carrying all three p53 or p21 genotype embryos were exposed to HS on day 8.5. Subsequently, fetuses were collected on day 15.5, and genotyped. In addition to genotype, we also determined the number of resorptions and dead fetuses as well as the number and types of external malformations.

RESULTS

In the absence of HS exposure, fetuses exhibiting exencephaly and spina bifida were observed in approximately 11% of p53 -/- fetuses, whereas no malformations were observed among p21 -/- fetuses. Exposure to HS resulted in an increase in exencephaly and polydactyly in fetuses of all three p53 genotypes. However, the incidence of these malformations was statistically significantly higher in p53 -/- compared to p53 +/- and p53 +/+ fetuses. Exencephaly was the only malformation observed in p21 fetuses exposed to HS, with an approximately 2-fold increase among p21 +/- and a 3-fold increase among p21 -/- compared to p21 +/+ fetuses.

CONCLUSIONS

Our study confirms that p53 plays a role in normal development and has shown, for the first time that p53 and p21 function to suppress HS-induced malformations.

The non-apoptotic role of p53 in neuronal biology: enlightening the dark side of the moon

The transcription factor p53 protects neurons from transformation and DNA damage through the induction of cell-cycle arrest, DNA repair and apoptosis in a range of in vitro and in vivo conditions. Indeed, p53 has a crucial role in eliciting neuronal cell death during development and in adult organisms after exposure to a range of stressors and/or DNA damage. Nevertheless, accumulating evidence challenges this one-sided view of the role of p53 in the nervous system. Here, we discuss how-unexpectedly-p53 can regulate the proliferation and differentiation of neural progenitor cells independently of its role in apoptosis, and p53 post-translational modifications might promote neuronal maturation, as well as axon outgrowth and regeneration, following neuronal injury. We hope to encourage a more comprehensive view of the non-apoptotic functions of p53 during neural development, and to warn against oversimplifications regarding its role in neurons. In addition, we discuss how further insight into the p53-dependent modulation of these mechanisms is necessary to elucidate the decision-making processes between neuronal cell death and differentiation during development, and between neuronal degeneration and axonal regeneration after injury.

NGF-mediated transcriptional targets of p53 in PC12 neuronal differentiation

BACKGROUND

p53 is recognized as a critical regulator of the cell cycle and apoptosis. Mounting evidence also suggests a role for p53 in differentiation of cells including neuronal precursors. We studied the transcriptional role of p53 during nerve growth factor-induced differentiation of the PC12 line into neuron-like cells. We hypothesized that p53 contributed to PC12 differentiation through the regulation of gene targets distinct from its known transcriptional targets for apoptosis or DNA repair.

RESULTS

Using a genome-wide chromatin immunoprecipitation cloning technique, we identified and validated 14 novel p53-regulated genes following NGF treatment. The data show p53 protein was transcriptionally activated and contributed to NGF-mediated neurite outgrowth during differentiation of PC12 cells. Furthermore, we describe stimulus-specific regulation of a subset of these target genes by p53. The most salient differentiation-relevant target genes included wnt7b involved in dendritic extension and the tfcp2l4/grhl3 grainyhead homolog implicated in ectodermal development. Additional targets included brk, sdk2, sesn3, txnl2, dusp5, pon3, lect1, pkcbpb15 and other genes.

CONCLUSION

Within the PC12 neuronal context, putative p53-occupied genomic loci spanned the entire Rattus norvegicus genome upon NGF treatment. We conclude that receptor-mediated p53 transcriptional activity is involved in PC12 differentiation and may suggest a contributory role for p53 in neuronal development.

Ablation of a peptidyl prolyl isomerase Pin1 from p53-null mice accelerated thymic hyperplasia by increasing the level of the intracellular form of Notch1

Tumor suppressor p53 is essential for checkpoint control in response to a variety of genotoxic stresses. DNA damage leads to phosphorylation on the Ser/Thr-Pro motifs of p53, which facilitates interaction with Pin1, a pSer/pThr-Pro-specific peptidyl prolyl isomerase. Pin1 is required for the timely activation of p53, resulting in apoptosis or cell cycle arrest. To investigate the physiological relationship between Pin1 and p53, we created Pin1-/-p53-/- mice. These p53-deficient mice spontaneously developed lymphomas, mainly of thymic origin, as well as generalized lymphoma infiltration into other organs, including the liver, kidneys and lungs. Ablation of Pin1, in addition to p53, accelerated the thymic hyperplasia, but the thymocytes in these Pin1-/-p53-/- mice did not infiltrate other organs. The thymocytes in 12-week-old Pin1-/-p53-/- mice were CD4(-)CD8(-) (double negative) and had significantly higher levels of the intracellular form of Notch1 (NIC) than the thymocytes of p53-/- or wild-type mice. Presenilin-1, a cleavage enzyme for NIC generation from full-length Notch1 was increased in the thymocytes of Pin1-/-p53-/- mice. Pin1 depletion also inhibited the degradation of NIC by proteasomes. These results suggest that both Pin1 and p53 control the normal proliferation and differentiation of thymocytes by regulating the NIC level.

RMCE-ASAP: a gene targeting method for ES and somatic cells to accelerate phenotype analyses

In recent years, tremendous insight has been gained on p53 regulation by targeting mutations at the p53 locus using homologous recombination in ES cells to generate mutant mice. Although informative, this approach is inefficient, slow and expensive. To facilitate targeting at the p53 locus, we developed an improved Recombinase-Mediated Cassette Exchange (RMCE) method. Our approach enables efficient targeting in ES cells to facilitate the production of mutant mice. But more importantly, the approach was Adapted for targeting in Somatic cells to Accelerate Phenotyping (RMCE-ASAP). We provide proof-of-concept for this at the p53 locus, by showing efficient targeting in fibroblasts, and rapid phenotypic read-out of a recessive mutation after a single exchange. RMCE-ASAP combines inverted heterologous recombinase target sites, a positive/negative selection marker that preserves the germline capacity of ES cells, and the power of mouse genetics. These general principles should make RMCE-ASAP applicable to any locus.

Enhanced tumorigenesis in p53 knockout mice exposed in utero to high-dose vitamin E

The limited antioxidative capacity of the embryo and fetus may increase their risk for cancer initiation and/or promotion by reactive oxygen species (ROS)-mediated oxidative DNA damage and/or signaling. To determine if cancer can originate in utero, a high dietary dose of the antioxidant vitamin E (VE) (10% dl-alpha-tocopherol-acetate) was given to cancer-prone p53 knockout mice throughout pregnancy. Although reducing fetal death (P < 0.05), in utero exposure to VE enhanced postnatal tumorigenesis in both +/- (P < 0.04) and -/- (P < 0.0008) p53-deficient offspring. VE did not alter maternal weights, offspring p53 genotypic distribution or tumor spectrum. Constitutive embryonic DNA oxidation in untreated -/- p53 embryos [gestational day (GD) 13] was higher than in +/- and +/+ p53 littermates (P < 0.05). VE reduced DNA oxidation in -/- p53 embryos (P < 0.05) without affecting +/- and +/+ p53 littermates. VE had contrasting, tissue-dependent effects on fetal (GD 19) DNA oxidation, with reductions in -/- and +/- p53-deficient fetal brains (P < 0.01), increases in skin (P < 0.05) and no effect in liver and thymus. The 250-fold increase in dietary VE levels produced only 1.6-6.3-fold, tissue-dependent increases in tissue concentrations. The greatest increase, in fetal skin, correlated with increased DNA oxidation in that tissue in -/- and +/- p53-deficient fetuses and enhanced tumorigenesis in these genotypes. These results show that some cancers may originate in utero and the risk can be enhanced by embryonic and fetal exposure to high dietary levels of VE. The elevated DNA oxidation in some tissues of untreated -/- p53 offspring suggests that ROS may contribute to their higher baseline tumor incidence. The limited and tissue-dependent disposition of VE indicates substantial conceptal regulation. The similarly selective and contrasting effects of VE on DNA oxidation may contribute to its controversial protective efficacy and suggest that its effects on tumorigenesis are cell-specific, possibly in high doses involving a pro-oxidative mechanism.

Cellular lifespan and senescence signaling in embryonic stem cells

Most mammalian cells when placed in culture will undergo a limited number of cell divisions before entering an unresponsive non-proliferating state termed senescence. However, several pathways that are activated singly or in concert can allow cells to bypass senescence at least for limited periods. These include the telomerase pathway required to maintain telomere ends, the p53 and Rb pathways required to direct senescence in response to DNA damage, telomere shortening and mitogenic signals, and the insulin-like growth factor--Akt pathway that may regulate lifespan and cell proliferation. In this review, we summarize recent findings related to these pathways in embryonic stem (ES) cells and suggest that ES cells are immortal because these pathways are tightly regulated.

The European flounder (Platichthys flesus) TP53 functions as a temperature-sensitive transcription factor which inhibits cell growth in yeast

Numerous studies focus on biological roles of the TP53 tumor suppressor gene in mammals but little is known about the actual function of TP53 in lower vertebrates. In this study, we used an in vivo functional assay in yeast to address the transactivation capacity of the flounder TP53 protein. We showed that the flounder TP53 acts as a sequence-specific transcription factor which is able to transactivate various human promoters containing a p53-responsive element (RE). This transcriptional activity was completely abrogated in the Val147Glu TP53 mutant previously identified in two flounder hepatic hyperplasia. In addition, we showed that the wild-type (wt) flounder TP53 but not the Val147Glu mutant inhibits cell growth when expressed in yeast. We finally reported that transcription regulation and growth inhibition by the wild-type flounder TP53 is temperature-dependent. The flounder TP53 optimal temperature appeared lower than those reported for the Xenopus and human homologues.

The onset of p53-dependent apoptosis plays a role in terminal differentiation of human normoblasts

The p53 tumor suppressor gene was found to play a role in the differentiation of several tissue types. We report here that p53-dependent apoptosis plays a role in the final stages of physiological differentiation of normoblasts, resulting in nuclear condensation and expulsion without cell death. Blood samples of healthy newborns, cord blood as well as bone marrow, were analysed for apoptosis by TUNEL and p53 expression by immunostaining. While some samples exhibited simultaneously several distinct patterns of apoptosis, such as perinuclear, diffused nuclear or nuclear apoptotic bodies, others presented a single defined pattern. Overexpression of p53 protein was detected in normoblasts exhibiting either perinuclear or diffused nuclear p53, corresponding to the nuclear apoptotic pattern in the same sample. Similar results were also evident with colonies cultivated for 12-14 days in culture. Differentiated erythroid colonies exhibited overexpression of p53 and positive TUNEL staining only in the normoblasts. We further examined the state of caspase 3/7 and observed a decrease of this activated enzyme during erythroid differentiation in culture. This study suggests a novel role for apoptosis in normoblast differentiation where nuclear degradation occurs with a delay in the actual cell death. A pivotal role for the p53-dependent apoptosis in the erythroid lineage development is implied. However, this apoptotic process is not fully executed because of the exhaustion in caspase 3/7 and thus cells are diverted towards final stages of differentiation.

Ets1 is required for p53 transcriptional activity in UV-induced apoptosis in embryonic stem cells

Embryonic stem (ES) cells contain a p53-dependent apoptosis mechanism to avoid the continued proliferation and differentiation of damaged cells. We show that mouse ES cells lacking Ets1 are deficient in their ability to undergo UV-induced apoptosis, similar to p53 null ES cells. In Ets1(-/-) ES cells, UV induction of the p53 regulated genes mdm2, perp, cyclin G and bax was decreased both at mRNA and protein levels. While p53 protein levels were unaltered in Ets1(-/-) cells, its ability to transactivate genes such as mdm2 and cyclin G was reduced. Furthermore, electrophoretic mobility shift assays and immunoprecipitations demonstrated that the presence of Ets1 was necessary for a CBP/p53 complex to be formed. Chromatin immunoprecipitations demonstrated that Ets1 was required for the formation of a stable p53-DNA complex under physiological conditions and activation of histone acetyltransferase activity. These data demonstrate that Ets1 is an essential component of a UV-responsive p53 transcriptional activation complex in ES cells and suggests that Ets1 may contribute to the specificity of p53-dependent gene transactivation in distinct cellular compartments.

A role for p53 in terminal epithelial cell differentiation

Terminal epithelial cell differentiation is a crucial step in development. In the kidney, failure of terminal differentiation causes dysplasia, cystogenesis, and cancer. The present study provides multiple lines of evidence implicating the tumor suppressor protein p53 in terminal differentiation of the renal epithelium. In the developing kidney, p53 is highly enriched in epithelial cells expressing renal function genes (RFGs), such as receptors for vasoactive hormones, the sodium pump, and epithelial sodium and water channels. In comparison, proliferating renal progenitors express little if any p53 or RFGs. p53 binds to and transactivates the promoters of RFGs. In contrast, expression of a dominant negative mutant form of p53 inhibits endogenous RFG expression. Moreover, binding of endogenous p53 to the promoters of RFGs coincides with the differentiation process and is attenuated once renal epithelial cells are fully differentiated. Finally, p53-null pups exhibit a previously unrecognized aberrant renal phenotype and spatial disorganization of RFGs. Interestingly, the p53-related protein p73 is unable to functionally compensate for the loss of p53 and fails to efficiently activate RFG transcription. We conclude that p53 promotes the biochemical and morphological differentiation of the renal epithelium. Aberrations in p53-mediated terminal differentiation may therefore play a role in the pathogenesis of nephron dysgenesis and dysfunction.

Glutathione and p53 independently mediate responses against oxidative stress in ES cells

We have investigated the roles of the antioxidant glutathione and p53 in the response of embryonic stem (ES) cells to oxidative stress. ES cells express gammaGCS, a critical enzyme in glutathione (GSH) biosynthesis. Treatment with the pro-oxidant menadione led to elevation of GSH, a strong apoptotic response and reduced clonogenic survival. Addition of BSO, a specific gammaGCS inhibitor depleted GSH pools and prevented the menadione-induced increase in GSH, sensitizing cells to oxidative insult. Although p53 status had no bearing on either the basal levels of GSH or the menadione-induced GSH response, the levels of menadione-induced apoptosis were reduced in the absence of p53. We conclude that the pathways involving p53 and GSH act independently to protect against the deleterious effects of oxidative damage. Furthermore, the presence of an intact p53 pathway confers a long-term growth advantage post oxidative stress. Thus, in the absence of p53 ES cells bearing genotoxic damage are less likely to be propagated, suggesting that p53-dependent apoptosis acts to limit the deleterious effects of oxidative stress during early development.

Apoptosis and proliferative activity in mature and immature teratomas of the mediastinum

BACKGROUND

Mediastinal teratomas are the most frequent mediastinal germ cell tumor. Whereas mature teratomas are benign tumors, immature teratomas are malignant. The purpose of this study was to find characteristics that could be used to distinguish between the growth and prognosis of the two teratoma types.

METHODS

Twenty-four mediastinal teratomas (18 mature and 6 immature) were examined for apoptosis by 3'-end labeling of DNA and stained immunohistochemically for proliferating cell nuclear antigen, Bcl-2, Bax, p53 protein, and alpha-fetoprotein (AFP) expression in formalin fixed, paraffin embedded specimens.

RESULTS

AFP was expressed in both immature teratomas and mature teratomas. Whereas p53 protein was expressed by most teratomas, p53 gene mutation was observed in only one patient with an immature teratoma in which the same mutation occurred in all tumor tissue components tested. Bax protein expression was relatively diffuse in mature teratomas but was focally expressed in immature teratomas. Bcl-2 protein was expressed focally in both mature and immature teratomas. Although the proliferative index was significantly higher in immature teratomas compared with mature teratomas (P < 0.001), the apoptotic index (AI) was significantly higher in mature teratomas compared with immature teratomas (P < 0.05). All patients except one in this study remain alive and disease free after undergoing tumor resection.

CONCLUSIONS

The relatively high AI in mature teratomas may be due to the overexpression of the p53 protein. In contrast, immature teratomas exhibited higher proliferative activity and lower rates of apoptosis, which may explain the more aggressive behavior of these tumors. However, patients with immature mediastinal teratomas have a good prognosis if the tumor is resected completely after chemotherapy.

Functional overexpression of wild-type p53 correlates with alveolar cell differentiation in the developing human lung

At 15 weeks after conception (a.c.), the human pulmonary acinus is lined by distal low-columnar and more proximal cuboidal cells that are successive stages in alveolar type II cell differentiation (pseudoglandular period of lung development). From 16 weeks a.c. onward, there are also 'flatter' cells that are intermediate stages in the differentiation of cuboidal type II cells into squamous type I cells (canalicular period). We investigated the role of wild-type p53 protein and the proliferation marker Ki-67 in the differentiation of type II and type I cells in these two periods. Serial sections from fetal lungs (n = 30) were immunoincubated with antibodies against p53 and Ki-67. The presence of prospective type II and type I cells was confirmed using immunohistochemistry for surfactant protein SP-A as a differentiation marker and light and electron microscopy. The p53 and Ki-67 positive nuclei were quantified per alveolar cell phenotype (i.e., low-columnar; cuboidal; flatter). The occurrence of cell apoptosis was studied using propidium iodide (PI) and 4',6'-diamino-2-phenylindol dihydrochloride (DAPI) staining. The combined increase in p53 expression and decrease in Ki-67 expression during alveolar epithelial cell differentiation suggests that wild-type p53 protein plays a role in the differentiation of alveolar type II and type I cells in the human lung, and that this function is mediated through cell cycle arrest. The rare incidence of apoptotic nuclei in alveolar type II cells, together with their absence in alveolar type I cells, supports the view that p53 is involved in the differentiation, rather than the death, of alveolar epithelial cells.

Relative quantitation of p53 and MDM2 gene expression in leiomyosarcoma; real-time semi-quantitative reverse transcription-polymerase chain reaction

To investigate the etiology of leiomyosarcoma, we examined abnormalities of p53 and its regulation in 13 cases of leiomyosarcoma using fresh tumor specimens. We estimated p53 and MDM2 mRNA level and MDM2 gene amplification using a real-time semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) based on the TaqMan fluorescence method. We also used immunohistochemistry (IHC) for p53 and MDM2 protein overexpression, polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) and direct sequencing to detect p53 mutation. Eight of the 13 cases (62%) showed an overexpression of p53 protein on IHC and eight of 13 cases (62%) had p53 gene point mutations. Five of the 13 cases (38%) showed positive staining for MDM2 protein and only one case (7.7%) demonstrated MDM2 gene amplification. The relative p53 mRNA level of the tumors compared with normal tissue ranged from 1.14 to 12.19 arbitrary units (AU), and the MDM2 mRNA level ranged from 1.06 to 17.17 AU. The mRNA level in the p53-positive cases was higher than in the negative cases (positive: 7.70 AU on average; negative: 3.38 AU on average; P=0.0344). However, there was no significant correlation between the MDM2 mRNA level and other factors, such as p53 IHC, p53 mutation status, p53 mRNA level and MDM2 IHC. Our results indicate that p53 abnormalities are major events and that an increasing level of p53 mRNA is associated with an overexpression of p53 protein in leiomyosarcoma and they may play an important role in the tumorigenesis in this tumor.

Lack of ultraviolet-light inducibility of the medakafish (Oryzias latipes) tumor suppressor gene p53

P53 is by far the most frequently altered gene in mammalian tumors. However, so far not a single p53 lesion has been reported in malignancies of cancer model systems in lower vertebrates. For analyzing the function of p53 in lower vertebrates, the gene was cloned from the medakafish (Oryzias latipes). Despite some differences in the genomic organization, the fish p53 amino acid sequence is highly conserved. Contrary to higher vertebrates, the level of p53 mRNA in medaka embryos gradually increases during embryogenesis. High expression of the p53 mRNA was detected in melanoma cells compared to undetectable expression of the gene in embryonic stem cells and fibroblasts. No effect of ultraviolet (UV) irradiation on the expression of p53 in cell cultures as well as in medaka fry was observed, indicating a possible difference in the function of p53 in lower vertebrates.

p53 associates with and targets Delta Np63 into a protein degradation pathway

A human p53 homologue, p63 (p40/p51/p73L/CUSP) that maps to the chromosomal region 3q27-29 was found to produce a variety of transcripts that encode DNA-binding proteins with and without a trans-activation domain (TA- or Delta N-, respectively). The p63 gene locus was found to be amplified in squamous cell carcinoma, and overexpression of Delta Np63 (p40) led to increased growth of transformed cells in vitro and in vivo. Moreover, p63-null mice displayed abnormal epithelial development and germ-line human mutations were found to cause ectodermal dysplasia. We now demonstrate that certain p63 isotypes form complexes with p53. p53 mutations R175H or R248W abolish the association of p53 with p63, whereas V143A or R273H has no effect. Deletion studies suggest that the DNA-binding domains of both p53 and p63 mediate the association. Overexpression of wild type but not mutant (R175H) p53 results in the caspase-dependent degradation of certain Delta Np63 proteins (p40 and Delta Np63 alpha). The association between p53 and Delta Np63 supports a previously unrecognized role for p53 in regulation of Delta Np63 stability. The ability of p53 to mediate Delta Np63 degradation may balance the capacity of Delta Np63 to accelerate tumorigenesis or to induce epithelial proliferation.

p53 expression and regulation by NMDA receptors in the developing rat brain

The p53 tumor suppressor gene, which is considered the guardian of the genome, encodes a phosphoprotein, which is a sequence-specific transcriptional activator or repressor of target genes. The role of p53 in developmental processes has not been studied extensively, although its expression appears to undergo temporal and spatial changes during prenatal and postnatal development. In the present study, we assessed the levels of p53 mRNA and protein in the developing rat brain and its relation to developmental cell death. Furthermore, we investigated the potential role of n-methyl-d-aspartate (NMDA) receptors in regulating p53 expression, since these receptors are involved in the control of cell death. We found that p53 mRNA and protein were detectable in the rat brain throughout perinatal development. In embryos, p53 immunoreactivity was mainly localized in the nuclei of neuroepithelial cells, with a maximum in staining at embryonic day (E)12. In the neuroepithelium, we also found significant numbers of TdT-mediated dUTP nick end labeling (TUNEL)-positive cells, both in dividing periventricular cells and in migrating neurons. In neonates, immediately after birth there was a reduction in the number of apoptotic cells, which then increased to reach a maximum at postnatal day (P)5. Postnatally, apoptotic as well as p53-positive cells were detected in most brain areas. P53 immunoreactivity was also highest on P5. In most cells, p53 immunoreactivity and the TUNEL signal colocalized. P53 immunoreactivity as well as the number of TUNEL- positive cells were dramatically decreased in the brains of newborns treated with MK-801, an NMDA receptor antagonist. Our results show that p53 is involved in the control of developmental cell death, and that NMDA receptors play a regulatory role in the expression of the p53 gene, and thus in apoptosis occurring in the developing rat brain.

Accentuated apoptosis in normally developing p53 knockout mouse embryos following genotoxic stress

In order to identify the alternative pathways which may substitute for the p53 function during embryogenesis, we have focused our studies on p53 -/- normally developing mouse embryos that survived a genotoxic stress. We assumed that under these conditions p53-independent pathways, which physiologically control genomic stability, are enhanced. We found that while p53 +/+ mouse embryos elicited, as expected, a p53-dependent apoptosis, p53-/- normally developing mice exhibited an accentuated p53-independent apoptotic response. The p53-dependent apoptosis detected in p53+/+ embryos, was an immediate reaction mostly detected in the brain, whereas the p53-independent apoptosis was a delayed reaction with a prominent pattern observed in epithelial cells of most organs in the p53-deficient mice only. These results suggest that in the absence of p53-dependent apoptosis, which is a fast response to damaged DNA, p53-independent apoptotic pathways, with slower kinetics, are turned on to secure genome stability.

Antisense RNA: function and fate of duplex RNA in cells of higher eukaryotes

There is ample evidence that cells of higher eukaryotes express double-stranded RNA molecules (dsRNAs) either naturally or as the result of viral infection or aberrant, bidirectional transcriptional readthrough. These duplex molecules can exist in either the cytoplasmic or nuclear compartments. Cells have evolved distinct ways of responding to dsRNAs, depending on the nature and location of the duplexes. Since dsRNA molecules are not thought to exist naturally within the cytoplasm, dsRNA in this compartment is most often associated with viral infections. Cells have evolved defensive strategies against such molecules, primarily involving the interferon response pathway. Nuclear dsRNA, however, does not induce interferons and may play an important posttranscriptional regulatory role. Nuclear dsRNA appears to be the substrate for enzymes which deaminate adenosine residues to inosine residues within the polynucleotide structure, resulting in partial or full unwinding. Extensively modified RNAs are either rapidly degraded or retained within the nucleus, whereas transcripts with few modifications may be transported to the cytoplasm, where they serve to produce altered proteins. This review summarizes our current knowledge about the function and fate of dsRNA in cells of higher eukaryotes and its potential manipulation as a research and therapeutic tool.

cDNA cloning and expression analysis of flounder p53 tumour suppressor gene

The screening of a flounder cDNA library with a rainbow trout p53 probe allowed the isolation of a 2.8-kb fragment homologous to human (50%) and rainbow trout (57%) p53 coding sequences. The fragment contains a single open reading frame coding for a 366-amino acid protein. The predicted amino acid sequence is relatively divergent from other p53 proteins but it displays the main p53 features: five highly conserved domains, an acidic N-terminus, a hydrophilic and charged C-terminus, a penultimate serine residue and a putative nuclear localization signal. Furthermore, conservation of critical amino acids and comparable distribution of charge and hydrophobicity suggest that flounder p53 properties could be similar to those in mammals. Northern blot analysis revealed a single transcript of about 3 kb in the flounder ovary tissues. In fact, RT-PCR showed an ubiquitous but very low expression of p53 gene in all flounder tissues.

Differences in p53 and Bcl-2 expression in relation to cell proliferation during the development of human embryos

AIMS

To study the patterns of p53 and Bcl-2 expression in relation to cell proliferation during human embryogenesis in order to help elucidate their potential roles in the regulation of cell proliferation and apoptosis during morphogenesis.

METHODS

Immunohistochemistry for p53, Bcl-2, and proliferating cell nuclear cell antigen (PCNA) proteins was performed, using a variety of monoclonal antibodies, on paraffin was embedded sections of tissues from 68 human embryos and fetuses of between 4 and 30 weeks gestation.

RESULTS

Positive relations between sites of proliferative activity (as detected by PCNA expression) and p53 expression were found in the kidney, early developmental stages of intestine and lungs, liver, pancreas, heart, and in embryonic osteoblasts. On the other hand, positive relations between proliferative activity and Bcl-2 expression were found in the gonads, adrenal glands, in the cells of the dental lamina, hair follicles, syncytiotrophoblast, chondrocytes, and more advanced stages of intestinal development. In tissues of the central nervous system, p53 and Bcl-2 were co-expressed at the same sites but there was an inverse relation between p53/Bcl-2 expression and proliferative activity.

CONCLUSIONS

These data suggest that p53 and Bcl-2 have tissue specific and stage specific functions during embryogenesis.

ES cells do not activate p53-dependent stress responses and undergo p53-independent apoptosis in response to DNA damage

BACKGROUND

Embryonic stem (ES) cells can contribute precursors to all adult cell lineages. Consequently, damage to ES cell genomes may cause serious developmental malfunctions. In somatic cells, cell-cycle checkpoints limit DNA damage by preventing DNA replication under conditions that may produce chromosomal aberrations. The tumor suppressor p53 is involved in such checkpoint controls and is also required to avoid a high rate of embryonic malformations. We characterized the cell-cycle and DNA-damage responses of ES cells to elucidate the mechanisms that prevent accumulation or transmission of damaged genomes during development.

RESULTS

ES cells derived from wild-type mice did not undergo cell-cycle arrest in response to DNA damage or nucleotide depletion, although they synthesized abundant quantities of p53. The p53 protein in ES cells was cytoplasmic and translocated inefficiently to the nucleus upon nucleotide depletion. Expression of high levels of active p53 from an adenovirus vector could not trigger cell cycle arrest. Instead, ES cells that sustained DNA damage underwent p53-independent apoptosis. The antimetabolite-induced p53-dependent arrest response was restored in ES cells upon differentiation.

CONCLUSIONS

Cell-cycle regulatory pathways in early embryos differ significantly from those in differentiated somatic cells. In undifferentiated ES cells, p53 checkpoint pathways are compromised by factors that affect the nuclear localization of p53 and by the loss of downstream factors that are necessary to induce cell-cycle arrest. A p53-independent programmed cell death pathway is effectively employed to prevent cells with damaged genomes from contributing to the developing organism. The p53-mediated checkpoint controls become important when differentiation occurs.

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

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

The old and the new in p53 functional regulation

The gene termed p53 is one of the most extensively studied for the past 18 years and the amount of literature published on this gene reflects its relevance in the field of molecular oncology; thus, loss or mutation of this oncosuppressor gene is probably the molecular lesion most frequently observed in human tumors. The aim of this minireview is to report, discuss, and interpret some recent observations on this topic: (I) The relationship with the Ataxia-Telangectasia gene and with the signaling enzyme phosphatidylinositol 3-kinase (PI3K). (II) The relationship between DNA damage, p53, and sensitivity to anticancer therapies. (III) The gain of function caused by mutations that transform the oncosuppressor p53 gene into a dominant transforming oncogene and (IV) The phosphorylative regulation of p53 and its relationship with the mitogenic signaling cascade involving protein kinase C and tumor promoters.

Tumor suppressors: a developing role for p53?

An increasing body of evidence indicates that p53, the product of a tumour suppressor gene, has a role in development - could this developmental role have provided the primary driving force in the evolution of a protein best known as a stress-response integrator?

Increased p53 protein does not correlate to p53 gene mutations in microdissected human testicular germ cell tumors

PURPOSE

To determine if primary testicular germ cell tumors that overexpress p53 tumor suppressor gene protein have p53 gene mutations.

MATERIALS AND METHODS

We examined 30 primary testicular tissues from 26 patients representing two groups. Group one consisted of eleven cases (6 nonseminomatous germ cell tumors and 5 seminomas) in which tissue samples for DNA analysis were microdissected from paraffin block regions with elevated immunohistochemical staining for p53 protein. Group two consisted of 19 testis tumor tissues which had been fresh frozen and were chosen to correspond to archival tissue specimens exhibiting elevated levels of p53 protein. The DNA was extracted from these tissues and subjected to exon specific amplification by polymerase chain reaction (PCR) and cold single-strand conformation polymorphism (Cold SSCP) analysis.

RESULTS

In these cases with elevated p53 protein, no p53 gene exon 5-8 mutations were detected except 1 seminoma with a codon 140 silent mutation (no protein alteration).

CONCLUSIONS

Testicular tumors appear to exhibit elevated levels of wild-type p53 protein, the significance of which is yet to be elucidated.

A teratologic suppressor role for p53 in benzo[a]pyrene-treated transgenic p53-deficient mice

DNA damage may mediate birth defects caused by many drugs and environmental chemicals, therefore p53, a tumour suppressor gene that facilitates DNA repair, may be critically embryoprotective. We have studied the effects of the environmental teratogen, benzo[a]pyrene, on pregnant heterozygous p53-deficient mice. Such mice exhibited between 2- to 4-fold higher embryotoxicity and teratogenicity than normal p53-controls. Fetal resorptions reflecting in utero death were genotyped using the polymerase chain reaction and found to be increased 2.6-fold and 3.6-fold respectively with heterozygous and homozygous p53-deficient embryos. These results provide the first direct evidence that p53 may be an important teratological suppressor gene which protects the embryo from DNA-damaging chemicals and developmental oxidative stress.

Tumor suppressor p53 mutations and breast cancer: a critical analysis

Alterations in the tumor suppressor gene p53 are the most commonly identified changes in cancer, including neoplasia of the breast. The activity of p53 is regulated post-translationally. Phosphorylation state, subcellular localization, and interaction with any of a number of cellular proteins are likely to influence the function of p53. The exact effect of p53-mediated growth suppression seems to be cell-type specific but appears to be directly related to the ability of p53 to act as a specific transcriptional activator. The role that transcriptional repression plays in the function of WT p53 is less clear. It is also possible that p53 has a more direct activity in DNA replication and repair. Most documented p53 mutations result in single amino acid substitutions which may confer one or more of a spectrum of transforming abilities on the protein. Mutation may lead to nuclear accumulation of p53 protein; however, inactivation of p53 by nuclear exclusion and interaction with the mdm2 protein also appear to be important in tumorigenesis. Used in conjunction with other established factors, accumulation of cellular p53 may be a useful prognostic indicator in breast cancer. A syngeneic mouse model system yielded evidence that p53 mutations are important in the early, preneoplastic stages of mammary tumorigenesis. This murine system may provide the ability to investigate the functions of p53 in the early stages of breast cancer which are technically difficult to examine in the human system.

C-myc and tumour suppressor gene product expression in developing and term human trophoblast

Proliferation and differentiation of villous trophoblast during placental development, from an early stage to full-term, were investigated in routinely fixed and processed tissues, by means of the immunocytochemical localization of the cell cycle-related proto-oncogene c-myc and the p53 and retinoblastoma susceptibility (Rb) tumour-suppressor gene products. The proliferative activity of the trophoblast was determined using an antibody against proliferating cell nuclear antigen (PCNA) which stains all proliferating cells in paraffin-embedded tissues. Diffuse nuclear immunoreactivity for PCNA, c-myc and Rb gene products was a consistent finding in early cytotrophoblast; c-myc product expression was also detectable in both layers of mid-gestation trophoblast. Only scattered cytotrophoblastic nuclei of early gestational placenta displayed immunostaining for p53 gene product. In full-term placenta c-myc expression was undetectable while Rb gene product and PCNA immunoreactivity declined markedly. These results indicate that the expression of the above genes is spatio-temporally regulated during placental development. A potential involvement of the oncosuppressor gene products p53 and Rb in the control of trophoblastic proliferation and of c-myc in the control of both the proliferative and differentiation pathways of trophoblastic cells is suggested.