p53 coordinates cranial neural crest cell growth and epithelial-mesenchymal transition/delamination processes

Neural crest development involves epithelial-mesenchymal transition (EMT), during which epithelial cells are converted into individual migratory cells. Notably, the same signaling pathways regulate EMT function during both development and tumor metastasis. p53 plays multiple roles in the prevention of tumor development; however, its precise roles during embryogenesis are less clear. We have investigated the role of p53 in early cranial neural crest (CNC) development in chick and mouse embryos. In the mouse, p53 knockout embryos displayed broad craniofacial defects in skeletal, neuronal and muscle tissues. In the chick, p53 is expressed in CNC progenitors and its expression decreases with their delamination from the neural tube. Stabilization of p53 protein using a pharmacological inhibitor of its negative regulator, MDM2, resulted in reduced SNAIL2 (SLUG) and ETS1 expression, fewer migrating CNC cells and in craniofacial defects. By contrast, electroporation of a dominant-negative p53 construct increased PAX7(+) SOX9(+) CNC progenitors and EMT/delamination of CNC from the neural tube, although the migration of these cells to the periphery was impaired. Investigating the underlying molecular mechanisms revealed that p53 coordinates CNC cell growth and EMT/delamination processes by affecting cell cycle gene expression and proliferation at discrete developmental stages; disruption of these processes can lead to craniofacial defects.     

DNA damage induced activation of Cygb stabilizes p53 and mediates G1 arrest

Cytoglobin (Cygb) is an emerging tumor suppressor gene silenced by promoter hypermethylation in many human tumors. So far, the precise molecular mechanism underlying its tumor suppressive function remains poorly understood. Here, we identified Cygb as a genotoxic stress-responsive hemoprotein upregulated upon sensing cellular DNA damage. Our studies demonstrated that Cygb physically associates with and stabilizes p53, a key cellular DNA damage signaling factor. We provide evidence that Cygb extends the half-life of p53 by blocking its ubiquitination and subsequent degradation. We show that, upon DNA damage, cells overexpressing Cygb displayed proliferation defect by rapid accumulation of p53 and its target gene p21, while Cygb knockdown cells failed to efficiently arrest in G1 phase in response to DNA insult. These results suggest a possible involvement of Cygb in mediating cellular response to DNA damage and thereby contributing in the maintenance of genomic integrity. Our study thus presents a novel insight into the mechanistic role of Cygb in tumor suppression.     

Zebrafish Models of p53 Functions

Zebrafish models have significantly contributed to our understanding of vertebrate development and, more recently, human disease. The growing number of genetic tools available in zebrafish research has resulted in the identification of many genes involved in developmental and disease processes. In particular, studies in the zebrafish have clarified roles of the p53 tumor suppressor in the formation of specific tumor types, as well as roles of p53 family members during embryonic development. The zebrafish has also been instrumental in identifying novel mechanisms of p53 regulation and highlighting the importance of these mechanisms in vivo. This article will summarize how zebrafish models have been used to reveal numerous, important aspects of p53 function.The zebrafish, Danio rerio, is a small model organism that has long been used to study vertebrate development. Zebrafish embryos are optically clear and develop externally to the mother, facilitating the study of early developmental processes. In addition, zebrafish have increasingly been used in modeling human diseases, including a number of cancers. The availability of forward and reverse genetic tools in the zebrafish has resulted in the identification and characterization of many genes involved in development and disease. One gene that has been extensively studied is the p53 tumor suppressor gene, which is structurally and functionally conserved in the zebrafish. This article will discuss how studies in the zebrafish have increased our understanding of how p53 contributes to the formation of specific tumor types, resulted in the identification of novel mechanisms of p53 regulation, and showed how p53 and p53 family members are involved in embryonic development.     

Involvement of stromal p53 in tumor-stroma interactions

p53 is a major tumor-suppressor gene, inactivated by mutations in about half of all human cancer cases, and probably incapacitated by other means in most other cases. Most research regarding the role of p53 in cancer has focused on its ability to elicit apoptosis or growth arrest of cells that are prone to become malignant owing to DNA damage or oncogene activation, i.e. cell-autonomous activities of p53. However, p53 activation within a cell can also exert a variety of effects upon neighboring cells, through secreted factors and paracrine and endocrine mechanisms. Of note, p53 within cancer stromal cells can inhibit tumor growth and malignant progression. Cancer cells that evolve under this inhibitory influence acquire mechanisms to silence stromal p53, either by direct inhibition of p53 within stromal cells, or through pressure for selection of stromal cells with compromised p53 function. Hence, activation of stromal p53 by chemotherapy or radiotherapy might be part of the mechanisms by which these treatments cause cancer regression. However, in certain circumstances, activation of stromal p53 by cytotoxic anti-cancer agents might actually promote treatment resistance, probably through stromal p53-mediated growth arrest of the cancer cells or through protection of the tumor vasculature. Better understanding of the underlying molecular mechanisms is thus required. Hopefully, this will allow their manipulation towards better inhibition of cancer initiation, progression and metastasis.     

.HECT类泛素连接酶对p53家族的调控作用

p53家族成员在细胞生长、组织发育及肿瘤形成等方面都具有十分重要的生物学功能,其自身受到严格调控,泛素化修饰就是其中非常重要的方式之一,作为泛素化过程中决定底物特异性的泛素连接酶E3作用则更加突出.泛素连接酶E3可以分为两类：RING（really interesting new gene）类和HECT（homologous to E6AP C-terminus）类E3近年来,HECT类E3对p53家族的调控效应不断得到揭示.本文综述了HECT类E3在调控p53家族转录活性、稳定 性方面的重要作用、分子机制以及其作用对生物体肿瘤形成和生长发育等产生的影响,为进 一步完善p53家族调控网络,揭示HECT类E3在肿瘤发生发展及防治中的作用提供参考.     

Apoptosis of fetal testicular cells is regulated by both p53-dependent and independent mechanisms

A portion of fetal germ cells undergoes apoptosis in the physiological context, but the molecular mechanisms of their apoptosis are largely unknown. Because p53 tumor suppressor gene product promotes apoptosis in various types of cells, we have investigated the expression of p53 in fetal gonads and examined the influence of loss of p53 function in fetal gonad cells using mice deficient in the p53 gene. We found that the expression of p53 protein in fetal testis was induced after 15.5 dpc (days post coitum), while the expression was not detected in fetal ovary. The number of apoptotic cells found in the seminiferous tubules of fetal testes was not significantly different between p53-deficient and wild-type mice until 16.5 dpc. At 17.5 dpc, however, more apoptotic cells were observed in wild-type testes than in the p53-deficient mice. In contrast, a similar number of apoptotic cells was found in fetal ovaries throughout these developmental stages. These observations indicate that p53 promotes apoptosis of fetal testicular cells after 16.5 dpc.     

Mechanisms That Enhance Sustainability of p53 Pulses

The tumor suppressor p53 protein shows various dynamic responses depending on the types and extent of cellular stresses. In particular, in response to DNA damage induced by γ-irradiation, cells generate a series of p53 pulses. Recent research has shown the importance of sustaining repeated p53 pulses for recovery from DNA damage. However, far too little attention has been paid to understanding how cells can sustain p53 pulses given the complexities of genetic heterogeneity and intrinsic noise. Here, we explore potential molecular mechanisms that enhance the sustainability of p53 pulses by developing a new mathematical model of the p53 regulatory system. This model can reproduce many experimental results that describe the dynamics of p53 pulses. By simulating the model both deterministically and stochastically, we found three potential mechanisms that improve the sustainability of p53 pulses: 1) the recently identified positive feedback loop between p53 and Rorα allows cells to sustain p53 pulses with high amplitude over a wide range of conditions, 2) intrinsic noise can often prevent the dampening of p53 pulses even after mutations, and 3) coupling of p53 pulses in neighboring cells via cytochrome-c significantly reduces the chance of failure in sustaining p53 pulses in the presence of heterogeneity among cells. Finally, in light of these results, we propose testable experiments that can reveal important mechanisms underlying p53 dynamics.     

A specific PP2A regulatory subunit, B56gamma, mediates DNA damage-induced dephosphorylation of p53 at Thr55

Protein phosphatase 2A (PP2A) has been implicated to exert its tumor suppressive function via a small subset of regulatory subunits. In this study, we reported that the specific B regulatory subunits of PP2A B56gamma1 and B56gamma3 mediate dephosphorylation of p53 at Thr55. Ablation of the B56gamma protein by RNAi, which abolishes the Thr55 dephosphorylation in response to DNA damage, reduces p53 stabilization, Bax expression and cell apoptosis. To investigate the molecular mechanisms, we have shown that the endogenous B56gamma protein level and association with p53 increase after DNA damage. Finally, we demonstrate that Thr55 dephosphorylation is required for B56gamma3-mediated inhibition of cell proliferation and cell transformation. These results suggest a molecular mechanism for B56gamma-mediated tumor suppression and provide a potential route for regulation of B56gamma-specific PP2A complex function.     

Small-molecule antagonists of p53-MDM2 binding: research tools and potential therapeutics

p53 regulates a key pathway which protects normal tissues from tumor development that may result from diverse forms of stress. In the absence of stress, growth suppressive and proapoptotic activity of p53 is inhibited by MDM2 which binds p53 and negatively regulates its activity and stability. MDM2 antagonists could activate p53 and may offer a novel therapeutic approach to cancer. Recently, we identified the first potent and selective low molecular weight inhibitors of MDM2-p53 binding, the Nutlins. These molecules activate the p53 pathway and suppress tumor growth in vitro and in vivo. They represent valuable new tools for studying the p53 pathway and its defects in cancer. Nutlins induce p53-dependent apoptosis in human cancer cells but appear cytostatic to proliferating normal cells. Their potent activity against osteosarcoma xenografts suggests that MDM2 antagonists may have clinical utility in the treatment of tumors with wild-type p53.