Nanog-induced dedifferentiation of p53-deficient mouse astrocytes into brain cancer stem-like cells

Self-renewal, differentiation, and tumorigenicity characterize cancer stem cells (CSCs), which are rare and maintained by specific cell fate regulators. CSCs are isolated from glioblastoma multiforme (GBM) and may be responsible for the lethality of incurable brain tumors. Brain CSCs may arise from the transformation of undifferentiated, nestin-positive neural stem or progenitor cells and GFAP-expressing astrocytes. Here, we report a role of Nanog in the genesis of cancer stem-like cells. Using primary murine p53-knockout astrocytes (p53^−/− astrocytes), we provide evidence that enforced Nanog expression can increase the cellular growth rate and transform phenotypes in vitro and in vivo. In addition, Nanog drives p53^−/− astrocytes toward a dedifferentiated, CSC-like phenotype with characteristic neural stem cell/progenitor marker expression, neurosphere formation, self-renewal activity, and tumor development. These findings suggest that Nanog promotes dedifferentiation of p53-deficient mouse astrocytes into cancer stem-like cells by changing the cell fate and transforming cell properties.     

The emerging functions of the p53-miRNA network in stem cell biology

The p53 pathway plays an essential role in tumor suppression, regulating multiple cellular processes coordinately to maintain genome integrity in both somatic cells and stem cells. Despite decades of research dedicated to p53 function in differentiated somatic cells, we are just starting to understand the complexity of the p53 pathway in the biology of pluripotent stem cells and tissue stem cells. Recent studies have demonstrated that p53 suppresses proliferation, promotes differentiation of embryonic stem (ES) cells and constitutes an important barrier to somatic reprogramming. In addition, emerging evidence reveals the role of the p53 network in the self-renewal, proliferation and genomic integrity of adult stem cells. Interestingly, non-coding RNAs, and microRNAs in particular, are integral components of the p53 network, regulating multiple p53-controlled biological processes to modulate the self-renewal and differentiation potential of a variety of stem cells. Thus, elucidation of the p53-miRNA axis in stem cell biology may generate profound insights into the mechanistic overlap between malignant transformation and stem cell biology.     

The emerging functions of the p53-miRNA network in stem cell biology

The p53 pathway plays an essential role in tumor suppression, regulating multiple cellular processes coordinately to maintain genome integrity in both somatic cells and stem cells. Despite decades of research dedicated to p53 function in differentiated somatic cells, we are just starting to understand the complexity of the p53 pathway in the biology of pluripotent stem cells and tissue stem cells. Recent studies have demonstrated that p53 suppresses proliferation, promotes differentiation of embryonic stem (ES) cells and constitutes an important barrier to somatic reprogramming. In addition, emerging evidence reveals the role of the p53 network in the self-renewal, proliferation and genomic integrity of adult stem cells. Interestingly, non-coding RNAs, and microRNAs in particular, are integral components of the p53 network, regulating multiple p53-controlled biological processes to modulate the self-renewal and differentiation potential of a variety of stem cells. Thus, elucidation of the p53-miRNA axis in stem cell biology may generate profound insights into the mechanistic overlap between malignant transformation and stem cell biology.     

p53 in stem cells

p53 is well known as a "guardian of the genome" for differentiated cells,in which it induces cell cycle arrest and cell death after DNA damage and thus contributes to the maintenance of genomic stability.In addition to this tumor suppressor function for differentiated cells,p53 also plays an important role in stem cells.In this cell type,p53 not only ensures genomic integrity after genotoxic insults but also controls their proliferation and differentiation.Additionally,p53 provides an effective barrier for the generation of pluripotent stem celllike cells from terminally differentiated cells.In this review,we summarize our current knowledge about p53 activities in embryonic,adult and induced pluripotent stem cells.     

p63 in prostate biology and pathology

The identification of stem cells and differentiation programs regulating the development and maintenance of the normal prostate epithelium is essential for the identification of the cell type(s) and molecular alterations involved in the development and propagation of prostate cancer (CaP). The p53-homologue p63 is highly expressed in normal prostate basal cells and is a clinically useful biomarker for the diagnosis of CaP. Importantly, p63 has been shown to play a critical role in prostate development. Recent experimental evidence also suggests that this gene is essential for normal stem cell function in the prostate as well as other epithelial organs. Future studies aimed at better defining the role of p63 in the renewal of the adult prostate epithelium are likely to shed new light on the mechanisms involved in prostate carcinogenesis.     

Combinations of genetic mutations in the adult neural stem cell compartment determine brain tumour phenotypes

It has been suggested that intrinsic brain tumours originate from a neural stem/progenitor cell population in the subventricular zone of the post‐natal brain. However, the influence of the initial genetic mutation on the phenotype as well as the contribution of mature astrocytes to the formation of brain tumours is still not understood. We deleted Rb/p53, Rb/p53/PTEN or PTEN/p53 in adult subventricular stem cells; in ectopically neurografted stem cells; in mature parenchymal astrocytes and in transplanted astrocytes. We found that only stem cells, but not astrocytes, gave rise to brain tumours, independent of their location. This suggests a cell autonomous mechanism that enables stem cells to generate brain tumours, whereas mature astrocytes do not form brain tumours in adults. Recombination of PTEN/p53 gave rise to gliomas whereas deletion of Rb/p53 or Rb/p53/PTEN generated primitive neuroectodermal tumours (PNET), indicating an important role of an initial Rb loss in driving the PNET phenotype. Our study underlines an important role of stem cells and the relevance of initial genetic mutations in the pathogenesis and phenotype of brain tumours.     

Brain cancer stem-like cell genesis from p53-deficient mouse astrocytes by oncogenic Ras

Here, we show that H-ras^V12 causes the p53-knockout mouse astrocytes (p53−/− astrocytes) to be transformed into brain cancer stem-like cells. H-ras^V12 triggers the p53−/− astrocytes to express a Nestin and a Cd133, which are expressed in normal and cancer neural stem cells. H-ras^V12 also induces the formation of a single cell-derived neurosphere under neural stem cell culture conditions. Furthermore, H-ras^V12-overexpressing p53−/− astrocytes (p53−/−ast-H-ras^V12) possess an in vitro self-renewal capacity, and are aberrantly differentiated into Tuj1-positve neurons both in vitro and in vivo. Amongst a variety of Ras-mediated canonical signaling pathways, we demonstrated that the MEK/ERK signaling pathway is responsible for neurosphere formation in p53-deficient astrocytes, whereas the PI3K/AKT signaling pathway is involved in oncogenic transformation in these cells. These findings suggest that the activation of Ras signaling pathways promotes the generation of brain cancer stem-like cells from p53-deficient mouse astrocytes by changing cell fate and transforming cell properties.     

Multifaceted p21 in carcinogenesis,stemness of tumor and tumor therapy

Cancer cells possess metabolic properties that are different from those of benign cells. p21, encoded by CDKN1A gene, also named p21^Cip1/WAF1, was first identified as a cyclin-dependent kinase regulator that suppresses cell cycle G1/S phase and retinoblastoma protein phosphorylation. CDKN1A (p21) acts as the downstream target gene of TP53 (p53), and its expression is induced by wild-type p53 and it is not associated with mutant p53. p21 has been characterized as a vital regulator that involves multiple cell functions, including G1/S cell cycle progression, cell growth, DNA damage, and cell stemness. In 1994, p21 was found as a tumor suppressor in brain, lung and colon cancer by targeting p53 and was associated with tumorigenesis and metastasis. Notably, p21 plays a significant role in tumor development through p53-dependent and p53-independent pathways. In addition, expression of p21 is closely related to the resting state or terminal differentiation of cells. p21 is also associated with cancer stem cells and acts as a biomarker for such cells. In cancer therapy, given the importance of p21 in regulating the G1/S and G2 check points, it is not surprising that p21 is implicated in response to many cancer treatments and p21 promotes the effect of oncolytic virotherapy.     

Loss of interactions between p53 and survivin gene in mesenchymal stem cells after spontaneous transformation in vitro

Mesenchymal stem cells (MSC) from various animals undergo a spontaneous transformation in long-term culture. The transformed MSCs are highly tumorigenic and are likely to be the tumor-initiating cells of sarcoma. To explain why the transformed MSCs become tumorigenic, the present study investigated the characteristics of rat MSCs before and after spontaneous transformation. It was shown that although the transformed MSCs maintained typical surface markers of MSC, they exhibited some cancer stem cell-like characteristics such as loss of contact inhibition and multi-potency to mesenchymal lineages, and acquirement of ability of anchorage-independent growth. The expression of a key senescence regulator p16 almost disappeared, but the other one, p53 abnormally increased in the transformed MSCs. ChIP assay demonstrated that a normal negative regulation of p53 on survivin gene disappeared in the transformed cells due to a lack of p53 binding to the promoter of survivin gene. DNA sequencing revealed that the p53 gene in transformed MSCs was not a wild-type, but a 942C > T mutant with the mutation located in the sequence coding p53 protein’s DNA-binding domain. These findings indicate that the transformed MSCs express high levels of a p53 mutant that loses the ability to bind survivin gene, leading to an abnormally upregulated expression of survivin, which is a key reason for the cell’s unlimited proliferation.     

Rat bone marrow mesenchymal stem cells undergo malignant transformation via indirect co‐cultured with tumour cells

Mesenchymal stem cells (MSCs) have potential applications in regenerative medicine and tissue engineering as well as being potential carriers for tumour therapy. However, the safety of using MSCs in tumours is unknown. Herein, we analyse malignant transformation of MSCs in the tumour microenvironment. Rat bone marrow MSCs were cultured with malignant rat glioma C6 cells without direct cell–cell contact. After 7 days, the cells were assessed for transformation using flow cytometry, real‐time quantitative PCR, immunofluorescence and chromosomal analysis. In addition, wild‐type (WT) p53, mutant p53 and mdm2 was determined using Western blotting. Almost all MSCs became phenotypically malignant cells, with significantly decreased WT p53 expression and increased expression of mutant p53 and mdm2, along with an aneuploid karyotype. To evaluate tumorigenesis in vivo, the MSCs indirect co‐cultured with C6 cells for 7 days were transplanted subcutaneously into immuno‐deficient mice. The cells developed into a large tumour at the injection site within 8 weeks, with systemic symptoms including cachexia and scoliosis. Pathological and cytological analysis revealed poorly differentiated pleomorphic cells with a dense vascular network and aggressive invasion into the adjacent muscle. These data demonstrate that MSCs became malignant cancer cells when exposed to the tumour microenvironment and suggest that factors released from the cancer cells have a critical role in the malignant transformation of MSCs. Copyright © 2012 John Wiley & Sons, Ltd.     

Regulation of p53 in embryonic stem cells

Despite an increasing interest in the role of the p53 tumour suppressor protein in embryonic stem cells, not much is known about its regulation in this cell type.We show that the relatively high amount of p53 protein correlates with a higher amount of p53 RNA in ES cells compared to differentiated cells. Moreover, p53 RNA is more stable in embryonic stem cells and the p53 protein is more often transcribed. This is at least partly due to decreased expression of miRNA-125a and 125b in embryonic stem cells. Despite its cytoplasmic localisation, p53 is degraded in 26S proteasomes in embryonic stem cells. This process is controlled by Mdm2, the deubiquitinating enzyme Hausp and Ubc13. In contrast, the E3 ligase PirH2 appears to be less important for the control of p53 in embryonic stem cells. During differentiation, p53 protein and RNA levels are decreased which corresponds to increased expression of miRNA-125a and miRNA-125b.     

Autophagy in stem cells

Autophagy is a highly conserved cellular process by which cytoplasmic components are sequestered in autophagosomes and delivered to lysosomes for degradation. As a major intracellular degradation and recycling pathway, autophagy is crucial for maintaining cellular homeostasis as well as remodeling during normal development, and dysfunctions in autophagy have been associated with a variety of pathologies including cancer, inflammatory bowel disease and neurodegenerative disease. Stem cells are unique in their ability to self-renew and differentiate into various cells in the body, which are important in development, tissue renewal and a range of disease processes. Therefore, it is predicted that autophagy would be crucial for the quality control mechanisms and maintenance of cellular homeostasis in various stem cells given their relatively long life in the organisms. In contrast to the extensive body of knowledge available for somatic cells, the role of autophagy in the maintenance and function of stem cells is only beginning to be revealed as a result of recent studies. Here we provide a comprehensive review of the current understanding of the mechanisms and regulation of autophagy in embryonic stem cells, several tissue stem cells (particularly hematopoietic stem cells), as well as a number of cancer stem cells. We discuss how recent studies of different knockout mice models have defined the roles of various autophagy genes and related pathways in the regulation of the maintenance, expansion and differentiation of various stem cells. We also highlight the many unanswered questions that will help to drive further research at the intersection of autophagy and stem cell biology in the near future.     

Targeting p53 for Novel Anticancer Therapy

Carcinogenesis is a multistage process, involving oncogene activation and tumor suppressor gene inactivation as well as complex interactions between tumor and host tissues, leading ultimately to an aggressive metastatic phenotype. Among many genetic lesions, mutational inactivation of p53 tumor suppressor, the “guardian of the genome,” is the most frequent event found in 50% of human cancers. p53 plays a critical role in tumor suppression mainly by inducing growth arrest, apoptosis, and senescence, as well as by blocking angiogenesis. In addition, p53 generally confers the cancer cell sensitivity to chemoradiation. Thus, p53 becomes the most appealing target for mechanism-driven anticancer drug discovery. This review will focus on the approaches currently undertaken to target p53 and its regulators with an overall goal either to activate p53 in cancer cells for killing or to inactivate p53 temporarily in normal cells for chemoradiation protection. The compounds that activate wild type (wt) p53 would have an application for the treatment of wt p53-containing human cancer. Likewise, the compounds that change p53 conformation from mutant to wt p53 (p53 reactivation) or that kill the cancer cells with mutant p53 using a synthetic lethal mechanism can be used to selectively treat human cancer harboring a mutant p53. The inhibitors of wt p53 can be used on a temporary basis to reduce the normal cell toxicity derived from p53 activation. Thus, successful development of these three classes of p53 modulators, to be used alone or in combination with chemoradiation, will revolutionize current anticancer therapies and benefit cancer patients.     

Increased Arf/p53 activity in stem cells,aging and cancer

Arf/p53 pathway protects the cells against DNA damage induced by acute stress. This characteristic is the responsible for its tumor suppressor activity. Moreover, it regulates the chronic type of stress associated with aging. This is the basis of its anti‐aging activity. Indeed, increased gene dosage of Arf/p53 displays elongated longevity and delayed aging. At a cellular level, it has been recently shown that increased dosage of Arf/p53 delays age‐associated stem cell exhaustion and the subsequent decline in tissue homeostasis and regeneration. However, p53 can also promote aging if constitutively activated. In this context, p53 reduces tissue regeneration, which correlates with premature exhaustion of stem cells. We discuss here the current evidence linking the Arf/p53 pathway to the processes of aging and cancer through stem cell regulation.