MDM2 expression during mouse embryogenesis and the requirement of p53

ZEB1 serves as an oncogene in acute myeloid leukaemia via regulating the PTEN/PI3K/AKT signalling pathway by combining with P53

Acute myeloid leukaemia is a complex, highly aggressive hematopoietic disorder. Currently, in spite of great advances in radiotherapy and chemotherapy, the prognosis for AML patients with initial treatment failure is still poor. Therefore, the need for novel and efficient therapies to improve AML treatment outcome has become desperately urgent. In this study, we identified the expression of ZEB1 (a transcription factor) and focused on its possible role and mechanisms in the progression of AML. According to the data provided by the Gene Expression Profiling Interactive Analysis (GEPIA), high expression of ZEB1 closely correlates with poor prognosis in AML patients. Additionally, the overexpression of ZEB1 was observed in both AML patients and cell lines. Further functional experiments showed that ZEB1 depletion can induce AML differentiation and inhibit AML proliferation in vitro and in vivo. Moreover, ZEB1 expression was negatively correlated with tumour suppressor P53 expression and ZEB1 can directly bind to P53. Our results also revealed that ZEB1 can regulate PTEN/PI3K/AKT signalling pathway. The inhibitory effect of ZEB1 silencing on PTEN/PI3K/AKT signalling pathway could be significantly reversed by P53 small interfering RNA treatment. Overall, the present data indicated that ZEB1 may be a promising therapeutic target for AML treatment or a potential biomarker for diagnosis and prognosis.

Regulation of p53 by E3s

More than 40 years of research on p53 have given us tremendous knowledge about this protein. Today we know that p53 plays a role in different biological processes such as proliferation, invasion, pluripotency, metabolism, cell cycle control, ROS (reactive oxygen species) production, apoptosis, inflammation and autophagy. In the nucleus, p53 functions as a bona-fide transcription factor which activates and represses transcription of a number of target genes. In the cytoplasm, p53 can interact with proteins of the apoptotic machinery and by this also induces cell death. Despite being so important for the fate of the cell, expression levels of p53 are kept low in unstressed cells and the protein is largely inactive. The reason for the low expression level is that p53 is efficiently degraded by the ubiquitin-proteasome system and the vast inactivity of the tumor suppressor protein under normal growth conditions is due to the absence of activating and the presence of inactivating posttranslational modifications. E3s are important enzymes for these processes as they decorate p53 with ubiquitin and small ubiquitin-like proteins and by this control p53 degradation, stability and its subcellular localization. In this review, we provide an overview about E3s that target p53 and discuss the connection between p53, E3s and tumorigenesis.

Mdm2 Promotes Odontoblast-like Differentiation by Ubiquitinating Dlx3 and p53

Dentin is an important structural component of the tooth. Odontoblast differentiation is an essential biological process that guarantees normal dentin formation, which is precisely regulated by various proteins. Murine double minute 2 (Mdm2) is an E3 ubiquitin ligase, and it plays a pivotal role in the differentiation of different cell types, such as osteoblasts and myoblasts. However, whether Mdm2 plays a role in odontoblast differentiation remains unknown. Here, we investigated the spatiotemporal expression of Mdm2 by immunostaining and found that Mdm2 was highly expressed in the odontoblasts and slightly in the dental papilla cells of mouse incisors and molars. Gene knockdown and overexpression experiments verified that Mdm2 promoted the odontoblast-like differentiation of mouse dental papilla cells (mDPCs). Intranuclear colocalization and physical interaction between Mdm2 and distal-less 3 (Dlx3), a transcription factor important for odontoblast differentiation, was found during the odontoblast-like differentiation of mDPCs by double immunofluorescence and immunoprecipitation. Mdm2 was proved to monoubiquitinate Dlx3, which enhanced the expression of Dlx3 target gene Dspp. In addition, p53, the canonical substrate of Mdm2, was validated to be also ubiquitinated but degraded by Mdm2 during the odontoblast-like differentiation of mDPCs. Gene knockdown experiments confirmed that p53 inhibited the odontoblast-like differentiation of mDPCs. p53 and Mdm2 double knockdown partially rescued the reduced odontoblast-like differentiation by knockdown of Mdm2 alone. Taken together, our study revealed that Mdm2 promoted the odontoblast-like differentiation of mDPCs by ubiquitinating both Dlx3 and p53. On one hand, the monoubiquitination of Dlx3 by Mdm2 led to upregulation of Dspp, which is a marker of the odontoblast differentiation. On the other hand, ubiquitination of p53 by Mdm2 resulted in its degradation, which eliminated the inhibitory effect of p53 on the odontoblast-like differentiation of mDPCs.

The proto-oncogene function of Mdm2 in bone

Mouse double minute 2 (Mdm2) is a multifaceted oncoprotein that is highly regulated with distinct domains capable of cellular transformation. Loss of Mdm2 is embryonically lethal, making it difficult to study in a mouse model without additional genetic alterations. Global overexpression through increased Mdm2 gene copy number (Mdm2^Tg ) results in the development of hematopoietic neoplasms and sarcomas in adult animals. In these mice, we found an increase in osteoblastogenesis, differentiation, and a high bone mass phenotype. Since it was difficult to discern the cell lineage that generated this phenotype, we generated osteoblast-specific Mdm2 overexpressing (Mdm2^TgOb ) mice in 2 different strains, C57BL/6 and DBA. These mice did not develop malignancies; however, these animals and the MG63 human osteosarcoma cell line with high levels of Mdm2 showed an increase in bone mineralization. Importantly, overexpression of Mdm2 corrected age-related bone loss in mice, providing a role for the proto-oncogenic activity of Mdm2 in bone health of adult animals.

Mdm2 Splice isoforms regulate the p53/Mdm2/Mdm4 regulatory circuit via RING domain-mediated ubiquitination of p53 and Mdm4

p53 is regulated by heterodimer E3 ligase Mdm2-Mdm4 via RING domain interaction. Mdm2 transcripts undergo alternative splicing, and Mdm2 splice isoforms are increased in cancer and induced by DNA damage. Although 2 major Mdm2 splice isoforms that do not bind to p53 were reported to impact the p53 pathway, the underlying biochemical mechanisms were not understood. Here, we show that these Mdm2 splice isoforms ubiquitinate Mdm2 and Mdm4 in vivo and regulate the activity of Mdm2-Mdm4 E3 complex in cells. The Mdm2 isoforms are capable of promoting p53 ubiquitination in the absence of Mdm2 or Mdm4. The 2 isoforms stimulate Mdm2 or Mdm4 activity for p53 ubiquitination in vivo and promote degradation of p53 and Mdm4 in cells. However, the Mdm2 isoforms have opposing effects on the steady-state p53 levels depending on the stoichiometric ratios of Mdm2, Mdm4 and the isoforms, causing either decreased or increased p53 levels in cells. Our data indicate that the Mdm2 splice isoforms can act as independent E3 ligases for p53 when Mdm2 and Mdm4 are absent, form potent heterodimer E3 ligases with either Mdm2 or Mdm4 for targeting p53 degradation, or act as inhibitory regulators of Mdm2-Mdm4 E3 ligase activity by downregulating Mdm4. These findings suggest that Mdm2 splice isoforms may play critical roles in the regulatory loop of p53/Mdm2-Mdm4 via a RING domain-mediated biochemical mechanism.

Genome-scale functional analysis of the human genes modulating p53 activity by regulating MDM2 expression in a p53-independent manner

MDM2, a critical negative regulator of p53, is often overexpressed in leukemia, but few p53 mutations are found, suggesting that p53-independent MDM2 expression occurs due to alterations in MDM2 upstream regulators. In this study, a high MDM2 transcription level was observed (41.17%) regardless of p53 expression in patient with acute myeloid leukemia (AML). Therefore, we performed genome-scale functional screening of the human genes modulating MDM2 expression in a p53-independent manner. We searched co-expression profiles of genes showing a positive or negative pattern with MDM2 expression in a DNA microarray database, selected1089 links, and composed a screening library of 368 genes. Using MDM2 P1 and P2 promoter-reporter systems, we screened clones regulating MDM2 transcriptions in a p53-independent manner by overexpression. Nine clones from the screening library showed enhanced MDM2 promoter activity and MDM2 expression in p53-deficient HCT116 cells. Among them, six clones, including NTRK2, GNA15, SFRS2, EIF5A, ELAVL1, and YWHAB mediated MAPK signaling for expressing MDM2. These results indicate that p53-independent upregulation of MDM2 by increasing selected clones may lead to oncogenesis in AML and that MDM2-modulating genes are novel potential targets for AML treatment.

Clinical significance of expression of murine double minute 2 and ribosomal protein L23 in gastric cancer

AIM: To detect the expression of murine double minute 2 (MDM2) and ribosomal protein L23 (RPL23) in gastric cancer and explore their biological significance in the development of gastric cancer.

METHODS: The expression of MDM2 and RPL23 was detected by immunohistochemistry in 90 human gastric carcinoma specimens and 30 normal gastric tissue specimens. The correlation of MDM2 and RPL23 expression with the clinicopathologic features of gastric carcinoma was analyzed statistically.

RESULTS: The positive expression rate of MDM2 in gastric cancer tissues was significantly higher than that in the control group (62.2% vs 40%, P < 0.05), while the positive rate of RPL23 expression was significantly lower in gastric cancer tissues (30% vs 63.3%, P < 0.05). The expression of MDM2 and RPL23 in gastric cancer was negatively correlated (r = -0.23, P = 0.029). Multivariate analysis showed that overexpression of MDM2, low expression of RPL23, lymph node metastasis, depth of invasion and tumor size were significant prognostic factors.

CONCLUSION: MDM2 and RPL23 expression may be associated with the development of gastric cancer, and they may be used as prognostic markers and new therapeutic targets in gastric cancer.

Screening for E3-ubiquitin ligase inhibitors: challenges and opportunities

The ubiquitin proteasome system (UPS) plays a role in the regulation of most cellular pathways, and its deregulation has been implicated in a wide range of human pathologies that include cancer, neurodegenerative and immunological disorders and viral infections. Targeting the UPS by small molecular regulators thus provides an opportunity for the development of therapeutics for the treatment of several diseases. The proteasome inhibitor Bortezomib was approved for treatment of hematologic malignancies by the FDA in 2003, becoming the first drug targeting the ubiquitin proteasome system in the clinic. Development of drugs targeting specific components of the ubiquitin proteasome system, however, has lagged behind, mainly due to the complexity of the ubiquitination reaction and its outcomes. However, significant advances have been made in recent years in understanding the molecular nature of the ubiquitination system and the vast variety of cellular signals that it produces. Additionally, improvement of screening methods, both in vitro and in silico, have led to the discovery of a number of compounds targeting components of the ubiquitin proteasome system, and some of these have now entered clinical trials. Here, we discuss the current state of drug discovery targeting E3 ligases and the opportunities and challenges that it provides.

The p53-Mdm2 loop: a critical juncture of stress response

The presence of a functional p53 protein is a key factor for the proper suppression of cancer development. A loss of p53 activity, by mutations or inhibition, is often associated with human malignancies. The p53 protein integrates various stress signals into a growth restrictive cellular response. In this way, p53 eliminates cells with a potential to become cancerous. Being a powerful decision maker, it is imperative that p53 will be activated properly, efficiently and temporarily in response to stress. Equally important is that p53 activation will be extinguished upon recovery from stress, and that improper activation of p53 will be avoided. Failure to achieve these aims is likely to have catastrophic consequences for the organism. The machinery that governs this tight regulation is largely based on the major inhibitor of p53, Mdm2, which both blocks p53 activities and promotes its destabilization. The interplay between p53 and Mdm2 involves a complex network of positive and negative feedback loops. Relief from Mdm2 suppression is required for p53 to be stabilized and activated in response to stress. Protection from Mdm2 entails a concerted action of modifying enzymes and partner proteins. The association of p53 with the PML-nuclear bodies may provide an infrastructure in which this complex regulatory network can be orchestrated. In this chapter we use examples to illustrate the regulatory machinery that drives this network.

HDAC6 promotes hepatocellular carcinoma progression by inhibiting P53 transcriptional activity

Hepatocellular carcinoma (HCC) is the most common type of liver cancer. HDAC6 is a transcriptional regulator of the histone deacetylase family, subfamily 2. Previous studies have shown that HDAC6 plays critical roles in transcription regulation, cell cycle progression and developmental events. However, its biological roles in the development of HCC remain largely unexplored. In the present study, we found that mRNA and protein levels of HDAC6 were up-regulated in HCC tissues and cell lines. The proinflammatory cytokines, which were up-regulated in the human HCC microenvironment, increased HDAC6 expression through a proximal NF-kappaB binding site on the HDAC6 gene promoter. Furthermore, overexpression of HDAC6 could promote cell proliferation in HCC cell lines. In contrast, HDAC6 knockdown using small interfering RNA inhibited cell proliferation. At the molecular level, we demonstrated that HDAC6 could interact with p53 and attenuate its transcriptional activity through promotion of its degradation. Therefore, our results suggest a previously unknown HDAC6-p53 molecular network controlling HCC development.

The Fbw7 and betaTRCP E3 ubiquitin ligases and their roles in tumorigenesis

The Ubiquitin Proteasome System (UPS) is a major regulator of protein abundance in the cell. The UPS influences the functions of multiple biological processes by targeting key regulators for destruction. E3 ubiquitin ligases are a vital component of the UPS machinery, working with E1 and E2 enzymes to bind substrates and facilitate the transfer of ubiquitin molecules onto the target protein. This poly-ubiquitination, in turn, directs the modified proteins for proteolysis by the 26S proteasome. As the UPS regulates the degradation of multiple oncogenes and tumor suppressors, the dysregulation of this pathway is known to promote various diseases including cancer. While E1 and E2 enzymes have only been minimally linked to cancer development, burgeoning amounts of evidence have implicated loss or gain of E3 function as a key factor in cancer initiation and progression. This review will examine the literature on two SCF-type E3 ligases, SCFFbw7 and SCFbeta-TRCP. In particular, we will highlight novel substrates recently identified for these two E3 ligases, and further discuss how UPS regulation of these targets may promote carcinogenesis.

USP22 antagonizes p53 transcriptional activation by deubiquitinating Sirt1 to suppress cell apoptosis and is required for mouse embryonic development

The NAD-dependent histone deacetylase Sirt1 antagonizes p53 transcriptional activity to regulate cell-cycle progression and apoptosis. We have identified a ubiquitin-specific peptidase, USP22, one of the 11 death-from-cancer signature genes that are critical in controlling cell growth and death, as a positive regulator of Sirt1. USP22 interacts with and stabilizes Sirt1 by removing polyubiquitin chains conjugated onto Sirt1. The USP22-mediated stabilization of Sirt1 leads to decreasing levels of p53 acetylation and suppression of p53-mediated functions. In contrast, depletion of endogenous USP22 by RNA interference destabilizes Sirt1, inhibits Sirt1-mediated deacetylation of p53 and elevates p53-dependent apoptosis. Genetic deletion of the usp22 gene results in Sirt1 instability, elevated p53 transcriptional activity and early embryonic lethality in mice. Our study elucidates a molecular mechanism in suppression of cell apoptosis by stabilizing Sirt1 in response to DNA damage and reveals a critical physiological function of USP22 in mouse embryonic development.

Desferrioxamine attenuates doxorubicin-induced acute cardiotoxicity through TFG-β/Smad p53 pathway in rat model

Interaction of doxorubicin DOX with iron and the consequent generation of reactive oxygen species (ROS) is a major player in DOX-induced cardiomyopathy. Accordingly, this study has been initiated to investigate the preventive effect of the iron chelator, desferrioxamine (DFX), against DOX-induced acute cardiotoxicity in rats. Male Wistar albino rats were divided into four groups and were injected intraperitoneally (I.P.) with normal saline, a single dose of DOX (15 mg/kg), a single dose of DFX (250 mg/kg) and a combined treatment with DFX (250 mg/kg) 30 min prior to a single dose of DOX, (15 mg/kg). A single dose of DOX significantly increased mRNA expression of TGF-β, Smad2, Smad4, CDKN2A and p53 and significantly decreased Samd7 and Mdm2 mRNA expression levels. Administration of DFX prior to DOX resulted in a complete reversal of DOX-induced alteration in cardiac enzymes and gene expression to normal levels. Data from this study suggest that (1) DOX induces its acute cardiotoxicity secondary to increasing genes expression of TGF-β/Smad pathway. (2) DOX increases apoptosis through upregulation of CDKN2A and p53 and downregulation of Mdm2 gene expression. (3) The preventive effect of DFX against DOX-induced cardiotoxicity is mediated via the TGF-β1/Smad pathway.

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.

Expression analysis of proline rich 15 (Prr15) in mouse and human gastrointestinal tumors

Proline rich 15 (Prr15), which encodes a protein of unknown function, is expressed almost exclusively in postmitotic cells both during fetal development and in adult tissues, such as the intestinal epithelium and the testis. To determine if this specific expression is lost in intestinal neoplasias, we examined Prr15 expression by in situ hybridization (ISH) on mouse intestinal tumors caused by different gene mutations, and on human colorectal cancer (CRC) samples. Prr15/PRR15 expression was consistently observed in mouse gastrointestinal (GI) tumors caused by mutations in the Apc gene, as well as in several advanced stage human CRCs. In contrast, no Prr15 expression was detected in intestinal tumors derived from mice carrying mutations in the Smad3, Smad4, or Cdkn1b genes. These findings, combined with the fact that a majority of sporadic human CRCs carry APC mutations, strongly suggest that the expression of Prr15/PRR15 in mouse and human GI tumors is linked, directly or indirectly, to the absence of the APC protein or, more generally, to the disruption of the Wnt signaling pathway.

The Mdm2-p53 relationship evolves: Mdm2 swings both ways as an oncogene and a tumor suppressor

Mdm2 has been well characterized as a negative regulator of the tumor suppressor p53. Recent studies have shown that Mdm2 is activated in response to a variety of oncogenic pathways independent of p53. Although its role as an oncogene via suppression of p53 function remains clear, growing evidence argues for p53-independent effects, as well as the remarkable possibility that Mdm2 has tumor suppressor functions in the appropriate context. Hence, Mdm2 is proving to be a key player in human cancer in its own right, and thus an important target for therapeutic intervention.

Transcriptional control of terminal nephron differentiation

Terminal differentiation of epithelial cells into more specialized cell types is a critical step in organogenesis. Throughout the process of terminal differentiation, epithelial progenitors acquire or upregulate expression of renal function genes and cease to proliferate, while expression of embryonic genes is repressed. This exquisite coordination of gene expression is accomplished by signaling networks and transcription factors which couple the external environment with the new functional demands of the cell. While there has been much progress in understanding the early steps involved in renal epithelial cell differentiation, a major gap remains in our knowledge of the factors that control the steps of terminal differentiation. A number of signaling molecules and transcription factors have been recently implicated in determining segmental nephron identity and functional differentiation. While some of these factors (the p53 gene family, hepatocyte nuclear factor-1beta) promote the terminal epithelial differentiation fate, others (Notch, Brn-1, IRX, KLF4, and Foxi1) tend to regulate differentiation of specific nephron segments and individual cell types. This review summarizes current knowledge related to these transcription factors and discusses how diverse cellular signals are integrated to generate a transcriptional output during the process of terminal differentiation. Since these transcriptional processes are accompanied by profound changes in nuclear chromatin structure involving the genes responsible for creating and maintaining the differentiated cell phenotype, future studies should focus on identifying the nature of these epigenetic events and factors, how they are regulated temporally and spatially, and the chromatin environment they eventually reside in.

Mdm4 and Mdm2 cooperate to inhibit p53 activity in proliferating and quiescent cells in vivo

The Mdm2 and Mdm4 oncoproteins are key negative regulators of the p53 tumor suppressor. However, their physiological contributions to the regulation of p53 stability and activity remain highly controversial. Here, we combined a p53 knock-in allele, in which p53 is silenced by a transcriptional stop element flanked by loxP sites, with the mdm2- and mdm4-null alleles. This approach allows Cre-mediated conditional p53 expression in tissues in vivo and cells in vitro lacking Mdm2, Mdm4, or both. Using this strategy, we show that Mdm2 and Mdm4 are essential in a nonredundant manner for preventing p53 activity in the same cell type, irrespective of the proliferation/differentiation status of the cells. Although Mdm2 prevents accumulation of the p53 protein, Mdm4 contributes to the overall inhibition of p53 activity independent of Mdm2. We propose a model in which Mdm2 is critical for the regulation of p53 levels and Mdm4 is critical for the fine-tuning of p53 transcriptional activity, both proteins acting synergistically to keep p53 in check.

Synergistic roles of Mdm2 and Mdm4 for p53 inhibition in central nervous system development

Loss of Mdm2 or Mdm4 leads to embryo lethal phenotypes that are p53-dependent. To determine whether Mdm2 and Mdm4 inhibit p53 function redundantly in a more restricted cell type, conditional alleles were crossed to a neuronal specific Cre transgene to delete Mdm2 and Mdm4 in the CNS. Mice lacking Mdm2 in the CNS developed hydranencephaly at embryonic day 12.5 due to apoptosis, whereas Mdm4 deletion showed a proencephaly phenotype at embryonic day 17.5 because of cell cycle arrest and apoptosis. The deletion of both genes, strikingly, contributed to an even earlier and more severe CNS phenotype. Additionally, Mdm2 and Mdm4 had a gene dosage effect, because loss of three of the four Mdm alleles also showed a more accelerated CNS phenotype than deletion of either gene alone. All phenotypes were rescued by deletion of p53. Thus, these in vivo data demonstrate the importance of Mdm4 independent of Mdm2 in inhibition of p53.

p21 controls patterning but not homologous recombination in RPE development

p21/WAF1/CIP1/MDA6 is a key cell cycle regulator. Cell cycle regulation is an important part of development, differentiation, DNA repair and apoptosis. Following DNA damage, p53 dependent expression of p21 results in a rapid cell cycle arrest. p21 also appears to be important for the development of melanocytes, promoting their differentiation and melanogenesis. Here, we examine the effect of p21 deficiency on the development of another pigmented tissue, the retinal pigment epithelium. The murine mutation pink-eyed unstable (p(un)) spontaneously reverts to a wild-type allele by homologous recombination. In a retinal pigment epithelium cell this results in pigmentation, which can be observed in the adult eye. The clonal expansion of such cells during development has provided insight into the pattern of retinal pigment epithelium development. In contrast to previous results with Atm, p53 and Gadd45, p(un) reversion events in p21 deficient mice did not show any significant change. These results suggest that p21 does not play any role in maintaining overall genomic stability by regulating homologous recombination frequencies during development. However, the absence of p21 caused a distinct change in the positions of the reversion events within the retinal pigment epithelium. Those events that would normally arrest to produce single cell events continued to proliferate uncovering a cell cycle dysregulation phenotype. It is likely that p21 is involved in controlling the developmental pattern of the retinal pigment. We also found a C57BL/6J specific p21 dependent ocular defect in retinal folding, similar to those reported in the absence of p53.

Characterization of Progenitor-Cell-Specific Genes Identified by Subtractive Suppression Hybridization

We have utilized subtractive suppression hybridization (SSH) to identify differentially expressed genes present in either neuroepithelial (NEP) cells or glial restricted precursor (GRP) cells. Eighteen clones enriched in GRP cells and 28 in NEP cells were identified. Five of the GRP-specific clones (tenascin C, cystatin C, GABA transporter 3, extracellular matrix molecule 2 and H2-4) were characterized further, and their glial specificity was confirmed by RT-PCR, in situ hybridization and immunocytochemistry. H2-4 (an expressed sequence tag) was shown to be part of chondroitin sulfate proteoglycan 3. Overall, our results show that SSH can be used to identify lineage- and stage-specific markers and that extracellular matrix molecules likely play important roles in the migration and differentiation of GRPs.

Mdm2 in the Response to Radiation

Murine double minute 2 (Mdm2) is a critical component of the responses to both ionizing and UV radiation. The level of Mdm2 expression determines the extent to which radiation induces an increase in the activity of the p53 tumor suppressor. Mdm2 acts as a survival factor in many cell types by limiting the apoptotic function of p53. In addition, expression of mdm2 is induced in response to DNA damage, and the resulting high levels of Mdm2 protein are thought to shorten the length of the cell cycle arrest established by p53 in the radiation response. Increased levels of Mdm2 appear to ensure that the activity of p53 returns to its low basal levels in surviving cells. Decreased levels of Mdm2 sensitize cells to ionizing radiation. Thus, Mdm2 is a potential target for therapeutic intervention because its inhibition may radiosensitize the subset of human tumors expressing wild-type p53 such that radiotherapy is more efficacious.

mdm2 Is critical for inhibition of p53 during lymphopoiesis and the response to ionizing irradiation

The function of the p53 tumor suppressor protein must be highly regulated because p53 can cause cell death and prevent tumorigenesis. In cultured cells, the p90MDM2 protein blocks the transcriptional activation domain of p53 and also stimulates the degradation of p53. Here we provide the first conclusive demonstration that p90MDM2 constitutively regulates p53 activity in homeostatic tissues. Mice with a hypomorphic allele of mdm2 revealed a heretofore unknown role for mdm2 in lymphopoiesis and epithelial cell survival. Phenotypic analyses revealed that both the transcriptional activation and apoptotic functions of p53 were increased in these mice. However, the level of p53 protein was not coordinately increased, suggesting that p90MDM2 can inhibit the transcriptional activation and apoptotic functions of p53 in a manner independent of degradation. Cre-mediated deletion of mdm2 caused a greater accumulation of p53, demonstrating that p90MDM2 constitutively regulates both the activity and the level of p53 in homeostatic tissues. The observation that only a subset of tissues with activated p53 underwent apoptosis indicates that factors other than p90(MDM2) determine the physiological consequences of p53 activation. Furthermore, reduction of mdm2 in vivo resulted in radiosensitivity, highlighting the importance of mdm2 as a potential target for adjuvant cancer therapies.

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.

Loss of MDM2 expression in human head and neck squamous cell carcinomas and clinical significance

The transforming potential of the MDM2 oncogene has been attributed to the overproduction of the protein. In order to investigate regulation of MDM2 expression in head and neck squamous cell carcinomas, we analysed MDM2 gene amplification, and mRNA and protein expression in tumour specimens from 62 patients, in cell lines, and in normal epithelium adjacent to tumours or obtained from healthy patients. Additionally, TP53-induced MDM2-P2 transcription was evaluated and compared with TP53 status. MDM2 gene amplification and mRNA over-expression is infrequent, 7 and 9%, respectively. The predominant transcript codes for full-length MDM2 protein (90kD) and the level of alternatively spliced forms is not significant. We show that only 47% of tumours exhibit MDM2 immunostaining in more than one third of the neoplastic cells, and thus more than half of the tumours display no or low levels of MDM2 protein. In contrast, MDM2 protein is always detectable in basal and parabasal cells of morphologically normal epithelium outside the invasively growing tumour, as well as in a normal uvula sample. Similarly, the total amount of MDM2 transcripts analysed by reverse transcriptase-polymerase chain reaction is reduced in tumour samples compared to normal tissues, essentially due to a decrease in P2 transcript levels. The relationship between mutated p53 status and low levels of MDM2 found in cell lines is also observed to a certain extent in primary tumour samples. Overall, there is a high frequency of TP53 mutation and under-expression of MDM2 in the head and neck tumours. Moreover, a significant association of decreased MDM2 expression is observed with advanced tumour stage and 3 years survival.

Defining the molecular basis of Arf and Hdm2 interactions

Understanding the interaction of Arf and Hdm2 has recently become a central issue in cancer biology. In response to hyperproliferative signals, p14(Arf) stabilizes p53 by binding to Hdm2 and inhibits the ubiquitination and subsequent proteosome-dependent degradation of p53. The medical importance of the Arf-Hdm2-p53 regulatory system is highlighted by the finding that either p53 or p14(Arf) are lost or modified in virtually all human cancers. Isolated Arf and Hdm2 domains are dynamically disordered in solution, yet they retain the ability to interact in vitro and in cellular assays. Upon binding, domains of both Arf and Hdm2 undergo a dramatic transition from disordered conformations to extended structures comprised of beta-strands. The presence of domains from both proteins are necessary and sufficient for the formation of the highly stable extended beta structures. We have mapped sites within Arf and Hdm2 that interact at a resolution of five amino acid residues using surface plasmon resonance. Surface plasmon resonance and circular dichroism spectropolarimetry confirm the presence of multiple interaction domains within each protein. Both p14(Arf) (human) and p19(Arf) (mouse) interact with Hdm2 through two short motifs present in their N termini. The Arf interacting region of Hdm2 is also composed of two short sequences located in the central acidic domain, between residues 235-264 and 270-289. The binding-induced structural transition is also induced by short peptides, 15 amino acids in length, that contain the binding motifs. Micro-injection and live cell imaging of proteins tagged with fluorescent labels was used to confirm the in vivo function of the interaction domains. Arf and Hdm2 thus appear to interact through a novel mechanism that exerts control over the cell division cycle. The novel molecular mechanism of interaction and the limited size of the protein domains involved provide opportunities for the development of anticancer therapeutics.

Preferential expression of Mdm2 oncogene during the development of neural crest and its derivatives in mouse early embryogenesis

The Mdm2 oncoprotein acts as the principal negative regulator of p53 activities and is essential for its control during mouse early development, at least before implantation. We analyzed Mdm2 expression between 7.5 and 9 days post-coitum (dpc) by whole-mount in situ hybridization and report here a novel expression pattern during neural crest development. At 7.5 dpc Mdm2 becomes preferentially expressed at the top of the neural folds. Between 8 and 9 dpc, this preferential expression is also observed in neural crest cells migrating from the closing brain towards craniofacial regions and the first three branchial arches. It persists in the craniofacial mesenchyme and the first branchial arch in 9 dpc embryos. Migrating neural crest cells in the tail region are also preferentially labeled at this stage. At day 9.5 Mdm2 becomes more ubiquitously expressed throughout the embryo as reported before.

MDM2 induces hyperplasia and premalignant lesions when expressed in the basal layer of the epidermis

The MDM2 oncogene is overexpressed in 5-10% of human tumours. Its major physiological role is to inhibit the tumour suppressor p53. However, MDM2 has p53-independent effects on differentiation and does not predispose to tumorigenesis when it is expressed in the granular layer of the epidermis. These unexpected properties of MDM2 could be tissue specific or could depend on the differentiation state of the cells. Strikingly, we found that MDM2 has p53-dependent effects on differentiation, proliferation and apoptosis when it is expressed in the less differentiated basal layer cells. MDM2 inhibits UV induction of p53, the cell cycle inhibitor p21(WAF1/CIP1) and apoptosis ('sunburn cells'). Importantly, MDM2 increases papilloma formation induced by chemical carcinogenesis and predisposes to the appearance of premalignant lesions and squamous cell carcinomas. p53 has a natural role in the protection against UV damage in the basal layer of the epidermis. Our results show that MDM2 predisposes to tumorigenesis when expressed at an early stage of differentiation, and provide a mouse model of MDM2 tumorigenesis relevant to p53's tumour suppressor functions.

The Mdm2 gene of zebrafish (Danio rerio): preferential expression during development of neural and muscular tissues, and absence of tumor formation after overexpression of its cDNA during early embryogenesis

The Mdm2 protein is most probably the main negative cellular regulator of the p53 tumor-suppressor protein. It was found to be overexpressed in a great number of human tumors and is considered as a potential target for anti-tumor therapies. Mdm2 is an essential gene in mice, yet its role in normal development and tissue differentiation is unknown. In order to study the role of this important protein in an evolutionary perspective, we cloned an Mdm2 cDNA from the fish Danio rerio and analyzed its expression pattern as well as the phenotypic consequences of its overexpression. The main functional domains as well as the interaction between Mdm2 and p53 are conserved in zebrafish. Moreover, we show here that the gene is expressed specifically during early development in neural and muscular tissues. Surprisingly, microinjection of Mdm2 mRNA in two-cell-stage embryos led to inhibition of cellular convergence during gastrulation. The clones derived from Mdm2 microinjected blastomeres were significantly smaller than those derived from control microinjections, and, in contrast to what was observed in Xenopus, did not develop tumors. Our results suggest that Mdm2 expression may be important during the differentiation of neural and muscular tissues of zebrafish. They also point to important differences between phyla in the susceptibility to tumor formation.

The p53 tumor suppressor protein does not regulate expression of its own inhibitor, MDM2, except under conditions of stress

MDM2 is an important regulator of the p53 tumor suppressor protein. MDM2 inhibits p53 by binding to it, physically blocking its ability to transactivate gene expression, and stimulating its degradation. In cultured cells, mdm2 expression can be regulated by p53. Hence, mdm2 and p53 can interact to form an autoregulatory loop in which p53 activates expression of its own inhibitor. The p53/MDM2 autoregulatory loop has been elucidated within cultured cells; however, regulation of mdm2 expression by p53 has not been demonstrated within intact tissues. Here, we examine the role of p53 in regulating mdm2 expression in vivo in order to test the hypothesis that the p53/MDM2 autoregulatory loop is the mechanism by which low levels of p53 are maintained. We demonstrate that basal expression of mdm2 in murine tissues is p53 independent, even in tissues that express functional p53. Transcription of mdm2 is induced in a p53-dependent manner following gamma irradiation, indicating that p53 regulates mdm2 expression in vivo following a stimulus. The requirement for a stimulus to activate p53-dependent regulation of mdm2 expression in vivo appeared to differ from the situation in early-passage mouse embryo fibroblasts, where mdm2 expression is enhanced by the presence of p53. Analysis of mdm2 expression in intact and dispersed embryos revealed that establishment of mouse embryo fibroblasts in culture induces p53-dependent mdm2 expression, suggesting that an unknown stimulus activates p53 function in cultured cells. Together, these results indicate that p53 does not regulate expression of its own inhibitor, except in response to stimuli.

p53 mediated death of cells overexpressing MDM2 by an inhibitor of MDM2 interaction with p53

The p53 tumour suppressor is frequently inactivated in human tumours. One form of inactivation results from overexpression of MDM2, that normally forms a negative auto-regulatory loop with p53 and inhibits its activity through complex formation. We have investigated whether disrupting the MDM2-p53 complex in cells that overexpress MDM2 is sufficient to trigger p53 mediated cell death. We find that expression of a peptide homologue of p53 that binds to MDM2 leads to increased p53 levels and transcriptional activity. The consequences are increased expression of the downstream effectors MDM2 and p21WAF1/CIP1, inhibition of colony formation, cell cycle arrest and cell death. There is also a decrease in E2F activity, that might have been due to the known physical and functional interactions of MDM2 with E2F1/DP1. However, this decrease is p53 dependent, as are also colony formation, cell cycle arrest and cell death. These results show that a peptide homologue of p53 is sufficient to induce p53 dependent cell death in cells overexpressing MDM2, and support the notion that disruption of the p53-MDM2 complex is a target for the development of therapeutic agents.