The p53 inhibitor MDM2 facilitates Sonic Hedgehog-mediated tumorigenesis and influences cerebellar foliation

Medulloblastoma and the DNA Damage Response

Medulloblastoma (MB) is the most common malignant brain tumor in children with standard of care consisting of surgery, radiation, and chemotherapy. Recent molecular profiling led to the identification of four molecularly distinct MB subgroups – Wingless (WNT), Sonic Hedgehog (SHH), Group 3, and Group 4. Despite genomic MB characterization and subsequent tumor stratification, clinical treatment paradigms are still largely driven by histology, degree of surgical resection, and presence or absence of metastasis rather than molecular profile. Patients usually undergo resection of their tumor followed by craniospinal radiation (CSI) and a 6 month to one-year multi-agent chemotherapeutic regimen. While there is clearly a need for development of targeted agents specific to the molecular alterations of each patient, targeting proteins responsible for DNA damage repair could have a broader impact regardless of molecular subgrouping. DNA damage response (DDR) protein inhibitors have recently emerged as targeted agents with potent activity as monotherapy or in combination in different cancers. Here we discuss the molecular underpinnings of genomic instability in MB and potential avenues for exploitation through DNA damage response inhibition.

Acupuncture combined with moxibustion promote the recovery of spinal cord injury in correlation with Shh/Gli-1 signaling pathway

Objective: Acupuncture combined with moxibustion (AM) therapy has been applied to treat spinal cord injury (SCI), but the underlying mechanism is unclear. The present study aimed to confirm the effect and mechanism of AM treatment on the recovery of SCI.Design: Male Sprague-Dawley rats were used to establish the SCI model by impact method. SCI rat models were subjected to AM treatment at Dazhui (GV14) and Jiaji points (T7-T12), Yaoyangguan (GV3), Zusanli (ST36) and Ciliao (BL32).Outcome measures: Motor function and cell apoptosis in rats after SCI. The mRNA and protein expression levels of Shh and Gli-1 were determined by real-time quantitative polymerase chain reaction, western blot and immunohistochemistry.Results: After AM treatment, the hindlimb motor function of SCI rats was significantly increased than the SCI group at 7, 9, 11, 14 days (P < 0.05). AM treatment 7 d and 14 d significantly preserved the nissl-stained positive neurons and significantly decreased number of apoptotic cells, compared to that of SCI 7 and 14 d groups (P < 0.05). AM treatment improved the mRNA protein levels of Shh and Gli-1 after 7 and 14 days treatment compared to the SCI group (P < 0.05).Conclusion: AM could improve the expression of Shh and Gli-1 in injured spinal cord of rats. That could be part of underlying mechanisms of AM treatment including recover motor function and preserve the neuron cells and alleviate the apoptosis of nerve cells in rats after SCI.

Cyclosporin A impairs neurogenesis and cognitive abilities in brain development via the IFN-γ-Shh-BDNF pathway

A wealth of evidence indicate that the peripheral immune activation alters brain development. However, it is still largely unclear whether and how peripheral immunosuppression affects neurodevelopment. Here, we found that the immunosuppressant cyclosporin A (CsA) decreased the number of BrdU+, BrdU+/DCX+, BrdU+/NeuN + cells in the hippocampus, impaired learning and memory and inhibited protein levels of the shh signaling pathway, including Shh, Smo and Gli1. However, the shh pathway receptor agonist SAG could block the impairment of cognitive ability and the decrease of hippocampal neurogenesis and brain-derived neurotrophic factor (BDNF) level induced by CsA. We also found that CsA decreased the level of interferon-gamma (IFN-γ), while up-regulation of IFN-γ altered the inhibitory effect of the shh signaling pathway and the decrease of BDNF induced by CsA. Collectively, these data indicate that peripheral CsA impairs neurogenesis and cognition in brain development through downregulating the IFN-γ-Shh-BDNF pathway. The present study guides us to correctly apply immunomodulatory drugs in early life and suggests that the IFN-γ-Shh-BDNF pathway may represent a novel protective target for neurodevelopment under the condition of immunosuppression.

Phosphatase magnesium-dependent 1 δ (PPM1D), serine/threonine protein phosphatase and novel pharmacological target in cancer

Aberrations in DNA damage response genes are recognized mediators of tumorigenesis and resistance to chemo- and radiotherapy. While protein phosphatase magnesium-dependent 1 δ (PPM1D), located on the long arm of chromosome 17 at 17q22-23, is a key regulator of cellular responses to DNA damage, amplification, overexpression, or mutation of this gene is important in a wide range of pathologic processes. In this review, we describe the physiologic function of PPM1D, as well as its role in diverse processes, including fertility, development, stemness, immunity, tumorigenesis, and treatment responsiveness. We highlight both the advances and limitations of current approaches to targeting malignant processes mediated by pathogenic alterations in PPM1D with the goal of providing rationale for continued research and development of clinically viable treatment approaches for PPM1D-associated diseases.

Neuronal Migration During Development of the Cerebellum

Neuronal migration is a fundamental process in central nervous system (CNS) development. The assembly of functioning neuronal circuits relies on neuronal migration occurring in the appropriate spatio-temporal pattern. A defect in the neuronal migration may result in a neurological disorder. The cerebellum, as a part of the CNS, plays a pivotal role in motor coordination and non-motor functions such as emotion, cognition and language. The excitatory and inhibitory neurons within the cerebellum originate from different distinct germinal zones and migrate through complex routes to assemble in a well-defined neuronal organization in the cerebellar cortex and nuclei. In this review article, the neuronal migration modes and pathways from germinal zones to the final position in the cerebellar cortex and nuclei will be described. The cellular and molecular mechanisms involved in cerebellar neuronal migration during development will also be reviewed. Finally, some diseases and animal models associated with defects in neuronal migration will be presented.

Prenatal assessment of cerebellar vermian lobulation: fetal MRI with 3-Tesla postmortem validation

OBJECTIVES

To optimize the imaging assessment of fetal hindbrain malformations, this observational magnetic resonance imaging (MRI) study aimed to assess whether fetal vermian lobulation can be quantified accurately and whether the relative growth of vermian lobules is uniform.

METHODS

This retrospective study included singleton fetuses which underwent T2-weighted MRI in vivo with a 1.5-Tesla (T) scanner or within 24 h postmortem with a 3-T scanner between January 2007 and November 2016 at the Medical University of Vienna. We included only those showing normal structural brain development on ultrasound and MRI and which had image quality appropriate for quantitative analysis, i.e. good image quality and a precise midsagittal slice. Fetal brains were segmented and, for all discernible vermian lobules, we determined the mean relative area contribution (MRAC, the proportion of the lobule relative to the total vermian area, in terms of number of voxels). Inter- and intrarater measurement variability of a representative selection (21 cases) was determined by intraclass correlation coefficient (ICC) for voxel-based differences. A linear regression model was used to assess the correlation between the relative size of each vermian lobule (i.e. MRAC) and gestational age.

RESULTS

A total of 78 fetuses scanned in vivo aged 18-32 gestational weeks and seven fetuses scanned postmortem aged 16-30 weeks had a precise midsagittal slice and image quality sufficient for quantitative analysis. After 22 weeks of gestation, seven of the nine known vermian lobules could be discriminated reliably. The MRAC showed a mean ± SD difference of only 2.89 ± 3.01% between in-vivo and postmortem measurements. The ICC of voxel-based interrater differences was mean ± SD, 0.91 ± 0.05 and the intrarater ICC was 0.95 ± 0.03. Growth of cerebellar lobules was non-uniform: the MRAC of culmen and DFT (declive + folium + tuber) increased with gestational age, whereas that of lingula, centralis, pyramis and nodulus decreased. The growth of the uvula showed no significant correlation with gestational age.

CONCLUSIONS

Fetal vermian lobulation can be assessed accurately and reliably after 22 weeks on precise midsagittal sequences with 1.5-T T2-weighted MRI. Fetal vermian lobules show non-uniform growth, with expansion of DFT and culmen at the expense of the other vermian lobules. Evaluation and elucidation of vermian lobulation in normal fetuses should enable better characterization of fetuses with hindbrain malformations. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

p53 Function Is Compromised by Inhibitor 2 of Phosphatase 2A in Sonic Hedgehog Medulloblastoma

Medulloblastomas, the most common malignant pediatric brain tumors, have been genetically defined into four subclasses, namely WNT-activated, Sonic Hedgehog (SHH)-activated, Group 3, and Group 4. Approximately 30% of medulloblastomas have aberrant SHH signaling and thus are referred to as SHH-activated medulloblastoma. The tumor suppressor gene TP53 has been recently recognized as a prognostic marker for patients with SHH-activated medulloblastoma; patients with mutant TP53 have a significantly worse outcome than those with wild-type TP53. It remains unknown whether p53 activity is impaired in SHH-activated, wild-type TP53 medulloblastoma, which is about 80% of the SHH-activated medulloblastomas. Utilizing the homozygous NeuroD2:SmoA1 mouse model with wild-type Trp53, which recapitulates human SHH-activated medulloblastoma, it was discovered that the endogenous Inhibitor 2 of Protein Phosphatase 2A (SET/I2PP2A) suppresses p53 function by promoting accumulation of phospho-MDM2 (S166), an active form of MDM2 that negatively regulates p53. Knockdown of I2PP2A in SmoA1 primary medulloblastoma cells reduced viability and proliferation in a p53-dependent manner, indicating the oncogenic role of I2PP2A. Importantly, this mechanism is conserved in the human medulloblastoma cell line ONS76 with wild-type TP53. Taken together, these findings indicate that p53 activity is inhibited by I2PP2A upstream of PP2A in SHH-activated and TP53-wildtype medulloblastomas. IMPLICATIONS: This study suggests that I2PP2A represents a novel therapeutic option and its targeting could improve the effectiveness of current therapeutic regimens for SHH-activated or other subclasses of medulloblastoma with wild-type TP53.

BAI1 Suppresses Medulloblastoma Formation by Protecting p53 from Mdm2-Mediated Degradation

Adhesion G protein-coupled receptors (ADGRs) encompass 33 human transmembrane proteins with long N termini involved in cell-cell and cell-matrix interactions. We show the ADGRB1 gene, which encodes Brain-specific angiogenesis inhibitor 1 (BAI1), is epigenetically silenced in medulloblastomas (MBs) through a methyl-CpG binding protein MBD2-dependent mechanism. Knockout of Adgrb1 in mice augments proliferation of cerebellar granule neuron precursors, and leads to accelerated tumor growth in the Ptch1+/- transgenic MB mouse model. BAI1 prevents Mdm2-mediated p53 polyubiquitination, and its loss substantially reduces p53 levels. Reactivation of BAI1/p53 signaling axis by a brain-permeable MBD2 pathway inhibitor suppresses MB growth in vivo. Altogether, our data define BAI1's physiological role in tumorigenesis and directly couple an ADGR to cancer formation.

Genipin suppresses colorectal cancer cells by inhibiting the Sonic Hedgehog pathway

Genipin, a major component of Gardenia jasminoides Ellis fruit, has been shown to inhibit the growth of gastric, prostate, and breast cancers. However, the anti-proliferative activity of genipin in colorectal cancer (CRC) has not been characterized. Herein, we demonstrated that genipin inhibits the proliferation of CRC cells and that genipin suppressed the Hedgehog pathway. Further investigation showed that p53 and NOXA protein levels were increased during inhibition of Hedgehog pathway-mediated apoptosis in CRC cells. We also showed that p53 modulated the expression of NOXA during genipin-induced apoptosis, and suppression via SMO also played a role in this process. Subsequently, GLI1 was ubiquitinated by the E3 ligase PCAF. In a xenograft tumor model, genipin suppressed tumor growth, which was also associated with Hedgehog inactivation. Taken together, these results suggest that genipin induces apoptosis through the Hedgehog signaling pathway by suppressing p53. These findings reveal a novel regulatory mechanism involving Hedgehog/p53/NOXA signaling in the modulation of CRC cell apoptosis and tumor-forming defects.

Intracranial undifferentiated malign neuroglial tumor in Smith-Lemli-Opitz syndrome: A theory of a possible predisposing factor for primary brain tumors via a case report

BACKGROUND

Smith-Lemli-Opitz Syndrome (SLOS) is a rare hereditary autosomal recessive disorder with broken cholesterol synthesis causing by 7-dehydrocholesterol reductase deficiency. Although the clinical features and pathogenesis is well-defined, it is unknown whether there is a relationship between SLOS and neoplastic processes, especially brain neoplasms.

PURPOSE

We aimed to attract the attentions to any possibility of relation between SLOS and intracranial tumor development via a pediatric case with both intracranial high-grade neuroglial tumor and SLOS, and thus to contribute an additional data to the literature on togetherness of these two clinical conditions.

METHOD

In our clinic, we experienced an interesting case of a 10-year-old child with both SLOS and huge brain tumor as rarely seen. Here, we reviewed the features and pathophysiology of SLOS and brain tumors via this case.

RESULTS

The patient was operated in our clinic, after, his brain tumor had been diagnosed, and his histopathology was resulted in undifferentiated malignant neuroglial WHO grade 4 tumor.

CONCLUSION

According to current literature, our case is the first report on coexisting of SLOS and intracranial undifferentiated malignant neuroglial tumor. Common pathways like impaired sonic hedgehog (Shh) signaling pathway may be considered for pathogenesis of a probable link between SLOS and brain tumors in further studies.

Inhibition of the CyclinD1 promoter in response to sonic hedgehog signaling pathway transduction is mediated by Gli1

Medulloblastoma (MB) is the most common malignant tumor of the central nervous system in children. Accumulating evidence suggests a major role for the activation of the sonic hedgehog (SHH) signaling pathway in the development of MB cells; however, the mechanisms underlying the effect of this pathway on tumor survival and growth remain poorly understood. The Gli family zinc finger 1 (Gli1) transcription factor is considered as a mediator of the SHH signaling pathway in MB cells. Therefore, the present study investigated whether the SHH signaling pathway promotes the apoptosis of MB cells via downregulation of Gli1. GANT61, a novel Gli1 inhibitor, is known to have an in vitro activity against tumors. In the current study, Daoy cells were treated with different concentrations of GANT61 for 24 h, and the effect on cell proliferation was assayed by cell counting kit-8 assay. In addition, the cell cycle progression and apoptosis were assayed by flow cytometry analysis and hematoxylin-eosin (HE) staining. The effects of GANT61 treatment on SHH signaling pathway at the mRNA level were assayed by polymerase chain reaction (PCR). To further elucidate the inhibitory effects of GANT61 on the expression of Gli1 and CyclinD1, their protein levels were examined by western blot and immunofluorescence. The results indicated that GANT61 significantly inhibited the proliferation of Daoy cells in a dose-dependent manner, compared with the control group (P<0.05). HE staining revealed that cells had increasingly abnormal protuberance with increasing GANT61 concentration. Flow cytometry analysis also demonstrated that GANT61 induced G1/S arrest and apoptosis of Daoy cells in a dose-dependent manner (P<0.05). Gli1 and CyclinD1 mRNA expression levels were downregulated by GANT61 treatment (P<0.05); similarly, their protein levels were downregulated by GANT61 treatment in a dose-dependent manner (P<0.05). In conclusion, Gli1 expression was significantly associated with CyclinD1 expression in MB. These data demonstrated that Gli1 is an important mediator of the SHH pathway activity in MB, and may be a novel agent for use in combined chemotherapeutic regimens.

Norrin/Frizzled4 signalling in the preneoplastic niche blocks medulloblastoma initiation

The tumor microenvironment is a critical modulator of carcinogenesis; however, in many tumor types, the influence of the stroma during preneoplastic stages is unknown. Here we explored the relationship between pre-tumor cells and their surrounding stroma in malignant progression of the cerebellar tumor medulloblastoma (MB). We show that activation of the vascular regulatory signalling axis mediated by Norrin (an atypical Wnt)/Frizzled4 (Fzd4) inhibits MB initiation in the Ptch^+/− mouse model. Loss of Norrin/Fzd4-mediated signalling in endothelial cells, either genetically or by short-term blockade, increases the frequency of pre-tumor lesions and creates a tumor-permissive microenvironment at the earliest, preneoplastic stages of MB. This pro-tumor stroma, characterized by angiogenic remodelling, is associated with an accelerated transition from preneoplasia to malignancy. These data expose a stromal component that regulates the earliest stages of tumorigenesis in the cerebellum, and a novel role for the Norrin/Fzd4 axis as an endogenous anti-tumor signal in the preneoplastic niche.

DOI:http://dx.doi.org/10.7554/eLife.16764.001

Differential timing of granule cell production during cerebellum development underlies generation of the foliation pattern

Background

The mouse cerebellum (Cb) has a remarkably complex foliated three-dimensional (3D) structure, but a stereotypical cytoarchitecture and local circuitry. Little is known of the cellular behaviors and genes that function during development to determine the foliation pattern. In the anteroposterior axis the mammalian cerebellum is divided by lobules with distinct sizes, and the foliation pattern differs along the mediolateral axis defining a medial vermis and two lateral hemispheres. In the vermis, lobules are further grouped into four anteroposterior zones (anterior, central, posterior and nodular zones) based on genetic criteria, and each has distinct lobules. Since each cerebellar afferent group projects to particular lobules and zones, it is critical to understand how the 3D structure of the Cb is acquired. During cerebellar development, the production of granule cells (gcs), the most numerous cell type in the brain, is required for foliation. We hypothesized that the timing of gc accumulation is different in the four vermal zones during development and contributes to the distinct lobule morphologies.

Methods and Results

In order to test this idea, we used genetic inducible fate mapping to quantify accumulation of gcs in each lobule during the first two postnatal weeks in mice. The timing of gc production was found to be particular to each lobule, and delayed in the central zone lobules relative to the other zones. Quantification of gc proliferation and differentiation at three time-points in lobules representing different zones, revealed the delay involves a later onset of maximum differentiation and prolonged proliferation of gc progenitors in the central zone. Similar experiments in Engrailed mutants (En1^−/+;En2^−/−), which have a smaller Cb and altered foliation pattern preferentially outside the central zone, showed that gc production, proliferation and differentiation are altered such that the differences between zones are attenuated compared to wild-type mice.

Conclusions

Our results reveal that gc production is differentially regulated in each zone of the cerebellar vermis, and our mutant analysis indicates that the dynamics of gc production plays a role in determining the 3D structure of the Cb.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-016-0072-z) contains supplementary material, which is available to authorized users.

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.

Cerebellum Development and Tumorigenesis: A p53-Centric Perspective

The p53 protein has been extensively studied for its role in suppressing tumorigenesis, in part through surveillance and maintenance of genomic stability. p53 has been associated with the induction of a variety of cellular outcomes including cell cycle arrest, senescence, and apoptosis. This occurs primarily, but not exclusively, through transcriptional activation of specific target genes. By contrast, the participation of p53 in normal developmental processes has been largely understudied. This review focuses on possible functions of p53 in cerebellar development. It can be argued that a better understanding of such mechanisms will provide needed insight into the genesis of certain embryonic cancers including medulloblastomas, and thus lead to more effective therapies.

Evasion of Cell Senescence Leads to Medulloblastoma Progression

How brain tumors progress from precancerous lesions to advanced cancers is not well understood. Using Ptch1(+/-) mice to study medulloblastoma progression, we found that Ptch1 loss of heterozygosity (LOH) is an early event that is associated with high levels of cell senescence in preneoplasia. In contrast, advanced tumors have evaded senescence. Remarkably, we discovered that the majority of advanced medulloblastomas display either spontaneous, somatic p53 mutations or Cdkn2a locus inactivation. Consistent with senescence evasion, these p53 mutations are always subsequent to Ptch1 LOH. Introduction of a p53 mutation prevents senescence, accelerates tumor formation, and increases medulloblastoma incidence. Altogether, our results show that evasion of senescence associated with Ptch1 LOH allows progression to advanced tumors.

Targeting the Hedgehog Pathway in Pediatric Medulloblastoma

Medulloblastoma (MB), a primitive neuroectomal tumor of the cerebellum, is the most common malignant pediatric brain tumor. The cause of MB is largely unknown, but aberrant activation of Hedgehog (Hh) pathway is responsible for ~30% of MB. Despite aggressive treatment with surgical resection, radiation and chemotherapy, 70%–80% of pediatric medulloblastoma cases can be controlled, but most treated patients suffer devastating side effects. Therefore, developing a new effective treatment strategy is urgently needed. Hh signaling controls transcription of target genes by regulating activities of the three Glioma-associated oncogene (Gli1-3) transcription factors. In this review, we will focus on current clinical treatment options of MB and discuss mechanisms of drug resistance. In addition, we will describe current known molecular pathways which crosstalk with the Hedgehog pathway both in the context of medulloblastoma and non-medulloblastoma cancer development. Finally, we will introduce post-translational modifications that modulate Gli1 activity and summarize the positive and negative regulations of the Hh/Gli1 pathway. Towards developing novel combination therapies for medulloblastoma treatment, current information on interacting pathways and direct regulation of Hh signaling should prove critical.

Sonic hedgehog from pharyngeal arch 1 epithelium is necessary for early mandibular arch cell survival and later cartilage condensation differentiation

BACKGROUND

Morphogenesis of vertebrate craniofacial skeletal elements is dependent on a key cell population, the cranial neural crest cells (NCC). Cranial NCC are formed dorsally in the cranial neural tube and migrate ventrally to form craniofacial skeletal elements as well as other tissues. Multiple extracellular signaling pathways regulate the migration, survival, proliferation, and differentiation of NCC.

RESULTS

In this study, we demonstrate that Shh expression in the oral ectoderm and pharyngeal endoderm is essential for mandibular development. We show that a loss of Shh in these domains results in increased mesenchymal cell death in pharyngeal arch 1 (PA1) after NCC migration. This increased cell death can be rescued in utero by pharmacological inhibition of p53. Furthermore, we show that epithelial SHH is necessary for the early differentiation of mandibular cartilage condensations and, therefore, the subsequent development of Meckel's cartilage, around which the dentary bone forms. Nonetheless, a rescue of the cell death phenotype does not rescue the defect in cartilage condensation formation.

CONCLUSIONS

Our results show that SHH produced by the PA1 epithelium is necessary for the survival of post-migratory NCC, and suggests a key role in the subsequent differentiation of chondrocytes to form Meckel's cartilage.

p63 Sustains self-renewal of mammary cancer stem cells through regulation of Sonic Hedgehog signaling

The predominant p63 isoform, ΔNp63, is a master regulator of normal epithelial stem cell (SC) maintenance. However, in vivo evidence of the regulation of cancer stem cell (CSC) properties by p63 is still limited. Here, we exploit the transgenic MMTV-ErbB2 (v-erb-b2 avian erythroblastic leukemia viral oncogene homolog 2) mouse model of carcinogenesis to dissect the role of p63 in the regulation of mammary CSC self-renewal and breast tumorigenesis. ErbB2 tumor cells enriched for SC-like properties display increased levels of ΔNp63 expression compared with normal mammary progenitors. Down-regulation of p63 in ErbB2 mammospheres markedly restricts self-renewal and expansion of CSCs, and this action is fully independent of p53. Furthermore, transplantation of ErbB2 progenitors expressing shRNAs against p63 into the mammary fat pads of syngeneic mice delays tumor growth in vivo. p63 knockdown in ErbB2 progenitors diminishes the expression of genes encoding components of the Sonic Hedgehog (Hh) signaling pathway, a driver of mammary SC self-renewal. Remarkably, p63 regulates the expression of Sonic Hedgehog (Shh), GLI family zinc finger 2 (Gli2), and Patched1 (Ptch1) genes by directly binding to their gene regulatory regions, and eventually contributes to pathway activation. Collectively, these studies highlight the importance of p63 in maintaining the self-renewal potential of mammary CSCs via a positive modulation of the Hh signaling pathway.

The clinicopathological significance and relationship of Gli1, MDM2 and p53 expression in resectable pancreatic cancer

AIMS

To study the expression of Gli1, MDM2 and p53 for clinical significance in pancreatic cancer (PC), and their functional relationship in regulating the biological behaviour of PC cells.

METHODS AND RESULTS

Immunohistochemistry showed that the expression of Gli1, MDM2 and p53 was much higher in 57 cases of PC than in paired normal pancreatic tissues, and was positively associated with tumour UICC stage and T stage (P < 0.05). Patients with expression of Gli1 only or coexpression of Gli1 and MDM2 had significantly worse overall survival than patients with negative expression (P < 0.05). RNA interference showed that p53 knockdown increased the protein level of Gli1 but decreased the level of MDM2, and enhanced cell invasion and migration in wild-type p53 Capan-2 cells; whereas Gli1 or MDM2 knockdown did not change p53 expression, but decreased the protein level of MDM2 or Gli1, respectively, and inhibited cell invasion and migration in mutant p53 PANC-1 cells.

CONCLUSIONS

Overexpression of Gli1, MDM2 and mutant p53 contributes to the development and progression of PC, and plays an important role in predicting PC patients' prognosis. Moreover, we report a positive association between Gli1 and MDM2 in PC cells, but their relationship with p53 is dependent on wild-type or mutant p53 status.

The C terminus of p53 regulates gene expression by multiple mechanisms in a target- and tissue-specific manner in vivo

The C terminus of the tumor suppressor p53 is subjected to multiple post-translational modifications, suggesting that differing sets of modifications determine distinct cellular outcomes. Hamard et al. address this question by generating a Trp53 mutant mouse that constitutively expresses truncated p53. Intriguingly, the C terminus acts via three distinct mechanisms to control p53-dependent gene expression depending on the tissue. This study reconciles contradictory reports and delineates how regulation of target gene selectivity by p53 leads to alternate cellular outcomes.

The p53 tumor suppressor is a transcription factor that mediates varied cellular responses. The C terminus of p53 is subjected to multiple and diverse post-translational modifications. An attractive hypothesis is that differing sets of combinatorial modifications therein determine distinct cellular outcomes. To address this in vivo, a Trp53^ΔCTD/ΔCTD mouse was generated in which the endogenous p53 is targeted and replaced with a truncated mutant lacking the C-terminal 24 amino acids. These Trp53^ΔCTD/ΔCTD mice die within 2 wk post-partum with hematopoietic failure and impaired cerebellar development. Intriguingly, the C terminus acts via three distinct mechanisms to control p53-dependent gene expression depending on the tissue. First, in the bone marrow and thymus, the C terminus dampens p53 activity. Increased senescence in the Trp53^ΔCTD/ΔCTD bone marrow is accompanied by up-regulation of Cdkn1 (p21). In the thymus, the C-terminal domain negatively regulates p53-dependent gene expression by inhibiting promoter occupancy. Here, the hyperactive p53^ΔCTD induces apoptosis via enhanced expression of the proapoptotic Bbc3 (Puma) and Pmaip1 (Noxa). In the liver, a second mechanism prevails, since p53^ΔCTD has wild-type DNA binding but impaired gene expression. Thus, the C terminus of p53 is needed in liver cells at a step subsequent to DNA binding. Finally, in the spleen, the C terminus controls p53 protein levels, with the overexpressed p53^ΔCTD showing hyperactivity for gene expression. Thus, the C terminus of p53 regulates gene expression via multiple mechanisms depending on the tissue and target, and this leads to specific phenotypic effects in vivo.

PCAF ubiquitin ligase activity inhibits Hedgehog/Gli1 signaling in p53-dependent response to genotoxic stress

The Hedgehog (Hh) signaling regulates tissue development, and its aberrant activation is a leading cause of malignancies, including medulloblastoma (Mb). Hh-dependent tumorigenesis often occurs in synergy with other mechanisms, such as loss of p53, the master regulator of the DNA damage response. To date, little is known about mechanisms connecting DNA-damaging events to morphogen-dependent processes. Here, we show that genotoxic stress triggers a cascade of signals, culminating with inhibition of the activity of Gli1, the final transcriptional effector of Hh signaling. This inhibition is dependent on the p53-mediated elevation of the acetyltransferase p300/CBP-associated factor (PCAF). Notably, we identify PCAF as a novel E3 ubiquitin ligase of Gli1. Indeed PCAF, but not a mutant with a deletion of its ubiquitination domain, represses Hh signaling in response to DNA damage by promoting Gli1 ubiquitination and its proteasome-dependent degradation. Restoring Gli1 levels rescues the growth arrest and apoptosis effect triggered by genotoxic drugs. Consistently, DNA-damaging agents fail to inhibit Gli1 activity in the absence of either p53 or PCAF. Finally, Mb samples from p53-null mice display low levels of PCAF and upregulation of Gli1 in vivo, suggesting PCAF as potential therapeutic target in Hh-dependent tumors. Together, our data define a mechanism of inactivation of a morphogenic signaling in response to genotoxic stress and unveil a p53/PCAF/Gli1 circuitry centered on PCAF that limits Gli1-enhanced mitogenic and prosurvival response.

Mutations in Lyar and p53 are synergistically lethal in female mice

BACKGROUND

Ly-1 antibody reactive clone (LYAR) is a nucleolar zinc finger protein that has been implicated in cell growth, self-renewal of embryonic stem cells, and medulloblastoma. To test whether LYAR is critical for cell growth and development, we generated Lyar mutant mice.

METHODS

Mice carrying the mutant Lyar(gt) allele were generated from embryonic stem cells that contained a gene-trap insertion in the Lyar gene. Phenotypic analyses were performed on Lyar mutant mice and mouse embryonic fibroblasts. Lyar(gt/gt) mice were crossed to mice lacking the p53 tumor suppressor protein and Lyar/p53 compound mutants scored for external abnormalities.

RESULTS

Lyar(gt/gt) homozygotes are viable, fertile, and indistinguishable from wild type littermates. However, the growth of Lyar(+/gt) and Lyar(gt/gt) mouse embryonic fibroblasts (MEFs) was impaired, coincident with an increase in the steady-state level of p53 and a key p53 effector of growth arrest, p21, suggesting that a cellular stress response is triggered in the absence of a wild type level of LYAR. Remarkably, the majority of Lyar(+/gt) and Lyar(gt/gt) female mice lacking p53 mice failed to survive. The neural tube defect (NTD) exencephaly was observed in ≈26% and ≈61% of female Lyar(+/gt;) p53(-/-) and Lyar(gt/gt;) p53(-/-) embryos, respectively.

CONCLUSIONS

Lyar/p53 mutant mice represent a new digenic model of NTDs. Furthermore, these studies identify Lyar as a novel candidate gene for a role in human NTDs. These results provide new data to support the idea that loss of a p53-mediated developmental checkpoint may increase the risk of NTDs owing to some germline mutations.

Pharmacological activation of the p53 pathway by nutlin-3 exerts anti-tumoral effects in medulloblastomas

Medulloblastomas account for 20% of pediatric brain tumors. With an overall survival of 40%-70%, their treatment is still a challenge. The majority of medulloblastomas lack p53 mutations, but even in cancers retaining wild-type p53, the tumor surveillance function of p53 is inhibited by the oncoprotein MDM2. Deregulation of the MDM2/p53 balance leads to malignant transformation. Here, we analyzed MDM2 mRNA and protein expression in primary medulloblastomas and normal cerebellum and assessed the mutational status of p53 and MDM2 expression in 6 medulloblastoma cell lines. MDM2 expression was elevated in medulloblastomas, compared with cerebellum. Four of 6 medulloblastoma cell lines expressed wild-type p53 and high levels of MDM2. The tumor-promoting p53-MDM2 interaction can be inhibited by the small molecule, nutlin-3, restoring p53 function. Targeting the p53-MDM2 axis using nutlin-3 significantly reduced cell viability and induced either cell cycle arrest or apoptosis and expression of the p53 target gene p21 in these 4 cell lines. In contrast, DAOY and UW-228 cells harboring TP53 mutations were almost unaffected by nutlin-3 treatment. MDM2 knockdown in medulloblastoma cells by siRNA mimicked nutlin-3 treatment, whereas expression of dominant negative p53 abrogated nutlin-3 effects. Oral nutlin-3 treatment of mice with established medulloblastoma xenografts inhibited tumor growth and significantly increased survival. Thus, nutlin-3 reduced medulloblastoma cell viability in vitro and in vivo by re-activating p53 function. We suggest that inhibition of the MDM2-p53 interaction with nutlin-3 is a promising therapeutic option for medulloblastomas with functional p53 that should be further evaluated in clinical trials.

HDM2 promotes WIP1-mediated medulloblastoma growth

Medulloblastoma is the most common malignant childhood brain tumor. The protein phosphatase and oncogene WIP1 is over-expressed or amplified in a significant number of primary human medulloblastomas and cell lines. In the present study, we examine an important mechanism by which WIP1 promotes medulloblastoma growth using in vitro and in vivo models. Human cell lines and intracerebellar xenografted animal models were used to study the role of WIP1 and the major TP53 regulator, HDM2, in medulloblastoma growth. Stable expression of WIP1 enhances growth of TP53 wild-type medulloblastoma cells, compared with cells with stable expression of an empty-vector or mutant WIP1. In an animal model, WIP1 enhances proliferation and reduces the survival of immunodeficient mice bearing intracerebellar xenografted human medulloblastoma cells. Cells with increased WIP1 expression also exhibit increased expression of HDM2. HDM2 knockdown or treatment with the HDM2 inhibitor Nutlin-3a, the active enantomer of Nutlin-3, specifically inhibits the growth of medulloblastoma cells with increased WIP1 expression. Nutlin-3a does not affect growth of medulloblastoma cells with stable expression of an empty vector or of mutant WIP1. Knockdown of WIP1 or treatment with the WIP1 inhibitor CCT007093 results in increased phosphorylation of known WIP1 targets, reduced HDM2 expression, and reduced growth specifically in WIP1 wild-type and high-expressing medulloblastoma cells. Combined WIP1 and HDM2 inhibition is more effective than WIP1 inhibition alone in blocking growth of WIP1 high-expressing medulloblastoma cells. Our preclinical study supports a role for therapies that target WIP1 and HDM2 in the treatment of medulloblastoma.

MDM2 antagonism by nutlin-3 induces death in human medulloblastoma cells

A critical component of the cellular stress response, the p53 tumor suppressor protein must be functional for many cancer therapies to be effective. Adjuvant therapies that augment p53 function are predicted to sensitize tumor cells to cancer therapies that rely upon p53 for their efficacy. Of those strategies currently being explored to enhance p53 function, inhibition of the ubiquitin ligase, MDM2, a negative regulator of p53, has shown promise. Here, we investigated whether MDM2 antagonism might be effective in inducing cell death in human medulloblastoma (MB) cells. Nutlin-3, a small-molecule inhibitor of MDM2, potently induced apoptosis in MB cells with wild-type TP53. Moreover, nutlin-3 potentiated p53 activation and growth impairment of MB cells in combination with the classic DNA-damaging agent doxorubicin. Together, these results support the concept that MDM2 antagonists may be therapeutically beneficial for patients with MB tumors.

Abrogation of Gli3 expression suppresses the growth of colon cancer cells via activation of p53

p53, the major human tumor suppressor, appears to be related to sonic hedgehog (Shh)-Gli-mediated tumorigenesis. However, the role of p53 in tumor progression by the Shh-Gli signaling pathway is poorly understood. Herein we investigated the critical regulation of Gli3-p53 in tumorigenesis of colon cancer cells and the molecular mechanisms underlying these effects. RT-PCR analysis indicated that the mRNA level of Shh and Gli3 in colon tumor tissues was significantly higher than corresponding normal tissues (P<0.001). The inhibition of Gli3 by treatment with Gli3 siRNA resulted in a clear decrease in cell proliferation and enhanced the level of expression of p53 proteins compared to treatment with control siRNA. The half-life of p53 was dramatically increased by treatment with Gli3 siRNA. In addition, treatment with MG132 blocked MDM2-mediated p53 ubiquitination and degradation, and led to accumulation of p53 in Gli3 siRNA-overexpressing cells. Importantly, ectopic expression of p53 siRNA reduced the ability of Gli3 siRNA to suppress proliferation of those cells compared with the cells treated with Gli3 siRNA alone. Moreover, Gli3 siRNA sensitized colon cancer cells to treatment with anti-cancer agents (5-FU and bevacizumab). Taken together, our studies demonstrate that loss of Gli3 signaling leads to disruption of the MDM2-p53 interaction and strongly potentiate p53-dependent cell growth inhibition in colon cancer cells, indicating a basis for the rational use of Gli3 antagonists as a novel treatment option for colon cancer.

p53 in the CNS: Perspectives on Development, Stem Cells, and Cancer

The p53 tumor suppressor potently limits the growth of immature and mature neurons under conditions of cellular stress. Although loss of p53 function contributes to the pathogenesis of central nervous system (CNS) tumors, excessive p53 function is implicated in neural tube defects, embryonic lethality, and neuronal degeneration. Thus, p53 function must be tightly controlled. The anti-proliferative properties of p53 are mediated, in part, through the induction of apoptosis, cell cycle arrest, and senescence. Although there is still much to be learned about the role of p53 in these processes, recent evidence supports exciting new roles for p53 in a wide range of processes, including neural precursor cell self-renewal, differentiation, and cell fate decisions. Understanding the full repertoire of p53 function in CNS development and tumorigenesis may provide us with novel points of therapeutic intervention for human diseases of the CNS.