Tumor-initiating function of nucleostemin-enriched mammary tumor cells

Mechanisms of Breast Cancer Stem Cell Specification and Self-Renewal Mediated by Hypoxia-Inducible Factor 1

Many advanced human cancers contain regions of intratumoral hypoxia, with O2 gradients extending to anoxia. Hypoxia-inducible factors (HIFs) are activated in hypoxic cancer cells and drive metabolic reprogramming, vascularization, invasion, and metastasis. Hypoxia induces breast cancer stem cell (BCSC) specification by inducing the expression and/or activity of the pluripotency factors KLF4, NANOG, OCT4, and SOX2. Recent studies have identified HIF-1-dependent expression of PLXNB3, NARF, and TERT in hypoxic breast cancer cells. PLXNB3 binds to and activates the MET receptor tyrosine kinase, leading to activation of the SRC non-receptor tyrosine kinase and subsequently focal adhesion kinase, which promotes cancer cell migration and invasion. PLXNB3-MET-SRC signaling also activates STAT3, a transcription factor that mediates increased NANOG gene expression. Hypoxia-induced NARF binds to OCT4 and serves as a coactivator by stabilizing OCT4 binding to the KLF4, NANOG, and SOX2 genes and by stabilizing the interaction of OCT4 with KDM6A, a histone demethylase that erases repressive trimethylation of histone H3 at lysine 27, thereby increasing KLF4, NANOG, and SOX2 gene expression. In addition to increasing pluripotency factor expression by these mechanisms, HIF-1 directly activates expression of the TERT gene encoding telomerase, the enzyme required for maintenance of telomeres, which is required for the unlimited self-renewal of BCSCs. HIF-1 binds to the TERT gene and recruits NANOG, which serves as a coactivator by promoting the subsequent recruitment of USP9X, a deubiquitinase that inhibits HIF-1α degradation, and p300, a histone acetyltransferase that mediates acetylation of H3K27, which is required for transcriptional activation.

The role of hypoxia-inducible factors in breast cancer stem cell specification

Breast tumor is heterogeneous cancer with high morbidity and mortality rates, particularly in developing countries. Despite new efforts to reduce the breast cancer implications, the number of newly diagnosed cases is increasing worldwide. It is believed that cancer stem cells (CSCs) are responsible for the implication of cancers including breast cancer. Although CSCs compose a small population in tumor bulks, they play a crucial role in tumor initiation, progression, metastasis, and chemotherapeutic resistance. These events are mediated by the hypoxia-inducible factor (HIF) pathway which regulates the transcription of genes involved in CSC maintenance and tumorigenesis. In this review, we discussed the mechanisms by which hypoxia- or chemotherapy-induced HIFs promote breast CSC specification.

Designing a Mucoadhesive ChemoPatch to Ablate Oral Dysplasia for Cancer Prevention

Oral cancer has a high mortality rate, and its treatment often causes debilitating complications. More than 90% of oral cancers are oral squamous cell carcinomas (OSCCs) that may develop from clinically recognizable oral premalignant lesions (OPLs). To eradicate OPLs before they turn into cancers, a non-invasive topical formulation is developed based on a novel combination of synergistically acting oxaliplatin (OXP) and mycophenolate (MPS) embedded in a controlled-release mucoadhesive patch fabricated by computer-aided 3D printing. After multiple rounds of testing and optimization, a v6.4 ChemoPatch is designed, which shows sustained release of OXP and MPS in vitro, minimal side leakage of drugs, an average elastic modulus of 2.38 MPa, and suitable drug stability at 4 °C or below for up to 12 months. In vivo analyses show almost all patches adhere to the dorsal tongue surface for 4 hours, and display a sustained release of OXP and MPS to tongue tissue for 3-4 hours. When applied in the 4-nitroquinoline-1-oxide-induced OPL rat model, the OXP-MPS patch significantly ablates dysplastic lesions with no damage to normal epithelial cells and minimal systemic absorption and side effects. This study reports the design of a novel mucoadhesive ChemoPatch as a noninvasive therapy to treat OPLs.

GNL3 Regulates SIRT1 Transcription and Promotes Hepatocellular Carcinoma Stem Cell-Like Features and Metastasis

The expression of GNL3 in hepatocellular carcinoma was detected, and its effect on the proliferation and metastasis of hepatocellular carcinoma cells was investigated. Hepatocellular carcinoma and adjacent tissues were collected. The mRNA and protein expression levels of GNL3 were detected by qRT-PCR, Western blot, and immunohistochemistry. The relationship between GNL3 and the prognosis of liver cancer was analysed using public databases. A GNL3 interfering plasmid was constructed, and the effects of GNL3 on the proliferation of HepG2 and PLC-PRF-5 hepatoma cells were detected by the CCK-8 method. Transwell chamber assays were used to detect the effects of GNL3 on the migration and invasion of hepatocellular carcinoma cells. The effects of GNL3 on SIRT1 expression and stem cell markers were analysed. The effect of GNL3 on the proliferation of hepatocellular carcinoma was detected in a subcutaneous tumor-bearing animal model. The results showed that the mRNA and protein levels of GNL3 were higher than those of adjacent tissues. The overall survival (OS) of HCC patients with high GNL3 expression was worse. In vivo and in vitro experiments confirmed that silencing GNL3 could inhibit the proliferation, migration, and invasion of hepatocellular carcinoma cells. Mechanistic studies have shown that GNL3 regulates SIRT1 expression. GNL3 mediates the stem cell-like properties of HCC cells through SIRT1. In conclusion, this study found that GNL3 increased expression in hepatocellular carcinoma, which promoted the malignant biological behavior of hepatocellular carcinoma cells and was related to the cell dry phenotype. This study has certain significance in evaluating the prognosis of HCC patients.

Nucleostemin upregulation and STAT3 activation as early events in oral epithelial dysplasia progression to squamous cell carcinoma

Most low-grade oral epithelial dysplasia remains static or regress, but a significant minority of them (4-11%) advances to oral squamous cell carcinoma (OSCC) within a few years. To monitor the progression of epithelial dysplasia for early cancer detection, we investigated the expression profiles of nucleostemin (NS) and phospho-STAT3 (p-STAT3) in rodent and human samples of dysplasia and OSCCs. In a 4NQO-induced rat oral carcinogenesis model, the number and distribution of NS and p-STAT3-positive cells increased in hyperplastic, dysplastic, and neoplastic lesions compared to normal epithelium. In human samples, the NS signal significantly increased in high-grade dysplasia and poorly differentiated OSCC, whereas p-STAT3 was more ubiquitously expressed than NS and showed increased intensity in high-grade dysplasia and both well and poorly differentiated OSCC. Analyses of human dysplastic samples with longitudinally followed outcomes revealed that cells with prominent nucleolar NS signals were more abundant in low-grade dysplasia that advanced to OSCC in 2 or 3 years than those remaining static for 7-14 years. These results suggest that NS upregulation and STAT3 activation are early events in the progression of low-grade dysplasia to OSCC.

HIF-1 recruits NANOG as a coactivator for TERT gene transcription in hypoxic breast cancer stem cells

Breast cancer stem cells (BCSCs) play essential roles in tumor formation, drug resistance, relapse, and metastasis. NANOG is a protein required for stem cell self-renewal, but the mechanisms by which it performs this function are poorly understood. Here, we show that hypoxia-inducible factor 1α (HIF-1α) is required for NANOG-mediated BCSC enrichment. Mechanistically, NANOG is recruited by HIF-1 to cooperatively activate transcription of the TERT gene encoding the telomerase reverse transcriptase that maintains telomere length, which is required for stem cell self-renewal. NANOG stimulates HIF-1 transcriptional activity by recruitment of the deubiquitinase USP9X, which inhibits HIF-1α protein degradation, and by stabilizing HIF-1α interaction with the coactivator p300, which mediates histone acetylation. Our results delineate a cooperative transcriptional mechanism by which HIF-1 and NANOG mediate BCSC self-renewal.

Nucleostemin Modulates Outcomes of Hepatocellular Carcinoma via a Tumor Adaptive Mechanism to Genomic Stress

Hepatocellular carcinomas (HCC) are adapted to survive extreme genomic stress conditions imposed by hyperactive DNA replication and genotoxic drug treatment. The underlying mechanisms remain unclear, but may involve intensified DNA damage response/repair programs. Here, we investigate a new role of nucleostemin (NS) in allowing HCC to survive its own malignancy, as NS was previously shown to promote liver regeneration via a damage repair mechanism. We first established that a higher NS transcript level correlates with high-HCC grades and poor prognostic signatures, and is an independent predictor of shorter overall and progression-free survival specifically for HCC and kidney cancer but not for others. Immunostaining confirmed that NS is most abundantly expressed in high-grade and metastatic HCCs. Genome-wide analyses revealed that NS is coenriched with MYC target and homologous recombination (HR) repair genes in human HCC samples and functionally intersects with those involved in replication stress response and HR repair in yeasts. In support, NS-high HCCs are more reliant on the replicative/oxidative stress response pathways, whereas NS-low HCCs depend more on the mTOR pathway. Perturbation studies showed NS function in protecting human HCC cells from replication- and drug-induced DNA damage. Notably, NS depletion in HCC cells increases the amounts of physical DNA damage and cytosolic double-stranded DNA, leading to a reactive increase of cytokines and PD-L1. This study shows that NS provides an essential mechanism for HCC to adapt to high genomic stress for oncogenic maintenance and propagation. NS deficiency sensitizes HCC cells to chemotherapy but also triggers tumor immune responses. IMPLICATIONS: HCC employs a novel, nucleostemin (NS)-mediated-mediated adaptive mechanism to survive high genomic stress conditions, a deficiency of which sensitizes HCC cells to chemotherapy but also triggers tumor immune responses.

Nucleostemin expression in breast cancer is a marker of more aggressive phenotype and unfavorable patients' outcome: A STROBE-compliant article

Cancer stem cells (CSCs) are postulated to play significant role in the pathogenesis, progression as well as drug resistance of breast cancer. Nucleostemin (NS) is thought to be a key molecule for stemness, and the clinical impact of NS immunoreactivity in breast cancer can indicate its actual role and future therapeutic potentials.The current study is an observational study with an attempt to evaluate the correlation between NS expression (protein and gene expression levels) and different clinicopathological attributes of invasive breast cancer. For that reason, we investigated NS immunohistochemistry expression on commercial tissue microarray (TMA) of 102 patients and 51 archival specimens from patients admitted to Saqr Hospital, Ras Al Khaimah and diagnosed in Al Baraha Hospital, Dubai, UAE. In addition, the association between NS (GNL3) gene expression and different prognostic parameters as well as patient outcome was also evaluated using 2 large publicly available databases.Interestingly, we found NS expression to be associated with less differentiated and more advance stage. In addition, NS expression was significantly higher in larger size (P = .001) and LN-positive tumors (P = .007). Notably, NS expression was significantly correlated to P53 positive (P = .037) status. Furthermore, NS was found to be more expressed in the highly aggressive breast cancer subtypes including human epidermal growth factor receptor 2 (HER-2) and triple negative breast cancer (TNBC) subtypes. Moreover, our results also showed that high GNL3 gene expression to be associated with poor patient outcome and higher chances of tumor recurrence.Our results highlight NS expression as a marker of aggressive phenotype and poor outcome and indicate its possible use as a potential target for CSC-associated breast cancer management.

Nucleostemin reveals a dichotomous nature of genome maintenance in mammary tumor progression

A defective homologous recombination (HR) repair program increases tumor incidence as well as providing a survival advantage in patients with breast and ovarian cancers. Here, we hypothesize that the tumor-promoting side of genome maintenance programs may be contributed by a self-renewal protein, nucleostemin (NS). To address this issue, we established its functional importance in mammary tumor progression in mice and showed that mammary tumor cells become highly susceptible to replicative DNA damage following NS depletion and are protected from hydroxyurea-induced damage by NS overexpression. Breast cancer cells with basal-like characters display more reliance on NS for genome maintenance than those with luminal characters. Mechanistically, NS-deficient cells demonstrate a significantly reduced HR repair activity. TCGA analyses of human breast cancers revealed that NS is co-enriched positively with HR repair proteins and that high NS expression correlates with low HR defects and predicts poor progression-free survival and resistance to knockdown of cell cycle checkpoint genes in triple-negative/basal-like breast cancers. This work indicates that NS constitutes a tumor-promoting genome maintenance program required for mammary tumor progression.

Differential expression of nucleostemin in the cytoplasm and nuclei of normal and cancerous cell lines

Studies conducted in the past decade have reported nucleostemin (NS) as a nucleolar protein that has a role in self-renewal and cell cycle regulation in cancer/stem cells, but is absent in differentiated cells. The localization and expression patterns of NS have always been disputed, as reports indicate its varied levels among tissues and cells. This study evaluates the expression and localization pattern of NS in normal cells, cancer cell lines, and stem cells. Our findings revealed that the expression of NS was high in cancers originating from the skin and liver compared to the normal cell lines. NS knockdown effects the proliferation of normal cell lines, similar to cancerous cell lines. The localization pattern of NS was analyzed by immunofluorescence, which showed that NS was localized in the nuclei of normal cell lines but is present both in the nucleus and the cytoplasm of cancerous/stem cell lines. Interestingly, we observed that siNS cancerous cell lines had lower NS in the cytoplasm, which did not salvage the reduction in proliferation caused by siNS. We postulate that the loss of NS in the nucleus inhibits the proliferative ability of both normal and cancerous cells at similar rates, although the role of NS in the cytoplasm apart from proliferation needs to be further explored.

Nucleostemin silencing induces differentiation and potentiates all-trans-retinoic acid effects in human acute promyelocytic leukemia NB4 cells via autophagy

Here, we report that targeting Nucleostemin (NS), a recently discovered stem cells-enriched gene, by a specific small interference RNA (siNS), decreases the rate of proliferation of acute promyelocytic leukemia (APL) NB4 cells and induces differentiation and autophagy. In addition, NS silencing promotes the effects of all-trans-retinoic acid (ATRA)-based differentiation therapy in NB4 cells. Autophagy inhibitors 3-methyladenine and bafilomycin block the effect of NS targeting on differentiation, indicating a new functional link between NS and autophagy as an important regulator of differentiation in NB4 cells. The capability of NS in modulating autophagy and differentiation, alone or in combination with ATRA, may help to broaden the range of treatment options available to treat leukemia.

High OCT4A levels drive tumorigenicity and metastatic potential of medulloblastoma cells

Medulloblastoma is a highly aggressive pediatric brain tumor, in which sporadic expression of the pluripotency factor OCT4 has been recently correlated with poor patient survival. However the contribution of specific OCT4 isoforms to tumor aggressiveness is still poorly understood. Here, we report that medulloblastoma cells stably overexpressing the OCT4A isoform displayed enhanced clonogenic, tumorsphere generation, and invasion capabilities. Moreover, in an orthotopic metastatic model of medulloblastoma, OCT4A overexpressing cells generated more developed, aggressive and infiltrative tumors, with tumor-bearing mice attaining advanced metastatic disease and shorter survival rates. Pro-oncogenic OCT4A effects were expression-level dependent and accompanied by distinct chromosomal aberrations. OCT4A overexpression in medulloblastoma cells also induced a marked differential expression of non-coding RNAs, including poorly characterized long non-coding RNAs and small nucleolar RNAs. Altogether, our findings support the relevance of pluripotency-related factors in the aggravation of medulloblastoma traits classically associated with poor clinical outcome, and underscore the prognostic and therapeutic value of OCT4A in this challenging type of pediatric brain cancer.

Knockdown of Nucleostemin in an ovarian cancer SKOV-3 cell line and its effects on cell malignancy

Nucleostemin plays an essential role in the proliferation of some stem cells and cancer cells, but no research has been conducted to assess the link between Nucleostemin and ovarian cancer. To investigate the role of Nucleostemin in ovarian cancer tumorigenesis, we generated short hairpin (sh)RNA to knockdown the expression of the Nucleostemin gene in an ovarian cancer SKOV-3 cell line. We found that knockdown of this gene led to cell-cycle arrest, as well as to an increase in apoptosis. In addition, migration and invasion demonstrated significant inhibition in Nucleostemin-deficient cells. Furthermore, the knockdown of Nucleostemin dramatically suppressed xenograft progression in BALB/c nude mice. Our findings suggest that Nucleostemin is associated with malignancy in an ovarian cancer SKOV-3 cell line.

Balancing self-renewal against genome preservation in stem cells: How do they manage to have the cake and eat it too?

Stem cells are endowed with the awesome power of self-renewal and multi-lineage differentiation that allows them to be major contributors to tissue homeostasis. Owing to their longevity and self-renewal capacity, they are also faced with a higher risk of genomic damage compared to differentiated cells. Damage on the genome, if not prevented or repaired properly, will threaten the survival of stem cells and culminate in organ failure, premature aging, or cancer formation. It is therefore of paramount importance that stem cells remain genomically stable throughout life. Given their unique biological and functional requirement, stem cells are thought to manage genotoxic stress somewhat differently from non-stem cells. The focus of this article is to review the current knowledge on how stem cells escape the barrage of oxidative and replicative DNA damage to stay in self-renewal. A clear statement on this subject should help us better understand tissue regeneration, aging, and cancer.

p53 Configures the G2/M Arrest Response of Nucleostemin-Deficient Cells

Nucleostemin (NS) protects the genome from replication-induced DNA damage and has an indispensable role in maintaining the continuous proliferation of both p53-wild-type and mutant cells. Yet, some outcomes of NS-deficient cells appear to be shaped by their p53 status, which stimulates conflicting claims on the role of p53 in executing the NS function. This disparity was conveniently attributed to the usual suspect of cell-type variations. To provide a definitive resolution, we investigated the interplay between NS and p53 in two pairs of isogenic cells, that is, genetically modified mouse embryonic fibroblast (MEF) cells and HCT116 human colon cancer cells. In MEF cells, p53 deletion further compromises rather than rescues the proliferative potential of NS-depleted cells without changing their G2/M arrest fate before prophase entry. The detrimental effect of p53 loss in NS-depleted MEF cells correlates with a dramatic increase of polyploid giant cells (PGCs) (up to 24%), which indicates aberrant mitosis. To determine how p53 shapes the response of cells to NS depletion at the molecular level, we showed that p53 turns on the expression of reprimo and MDM2 in NS-deficient MEF cells. In absence of p53, NS-deficient MEF cells exhibit increased levels of phosphorylated cdc2 (Y15) protein and cyclin B1. In cancer (HCT116) cells, NS loss leads to G2/M arrest under both p53wt and p53ko conditions and increases phosphorylated cdc2 more in p53ko than in p53wt cells, as it does in MEF cells. Unlike its effect in MEF cells, NS depletion decreases tumor growth and increases the expression of reprimo and cyclin B1 in a p53-independent manner in HCT116 cells. Our data indicate that the p53 status of NS-deficient cells orchestrates how they respond to G2/M arrest in a normal versus cancer cell distinct fashion.

GNL3 and SKA3 are novel prostate cancer metastasis susceptibility genes

Prostate cancer (PC) is very common in developed countries. However, the molecular determinants of PC metastasis are unclear. Previously, we reported that germline variation influences metastasis in the C57BL/6-Tg(TRAMP)8247Ng/J (TRAMP) mouse model of PC. These mice develop prostate tumors similar to a subset of poor outcome, treatment-associated human PC tumors. Here, we used TRAMP mice to nominate candidate genes and validate their role in aggressive human PC in PC datasets and cell lines. Candidate metastasis susceptibility genes were identified through quantitative trait locus (QTL) mapping in 201 (TRAMP × PWK/PhJ) F2 males. Two metastasis-associated QTLs were identified; one on chromosome 12 (LOD = 5.86), and one on chromosome 14 (LOD = 4.41). Correlation analysis using microarray data from (TRAMP × PWK/PhJ) F2 prostate tumors identified 35 metastasis-associated transcripts within the two loci. The role of these genes in susceptibility to aggressive human PC was determined through in silico analysis using multiple datasets. First, analysis of candidate gene expression in two human PC datasets demonstrated that five candidate genes were associated with an increased risk of aggressive disease and lower disease-free survival. Second, four of these genes (GNL3, MAT1A, SKA3, and ZMYM5) harbored SNPs associated with aggressive tumorigenesis in the PLCO/CGEMS GWAS of 1172 PC patients. Finally, over-expression of GNL3 and SKA3 in the PC-3 human PC cell line decreased in vitro cell migration and invasion. This novel approach demonstrates how mouse models can be used to identify metastasis susceptibility genes, and gives new insight into the molecular mechanisms of fatal PC.

FAK activity protects nucleostemin in facilitating breast cancer spheroid and tumor growth

Introduction

Focal adhesion kinase (FAK) controls cell growth and survival downstream of integrin-matrix receptors. Upon adhesion loss or FAK inhibition, FAK can translocate to the nucleus. The nucleolus is a non-membrane nuclear structure that regulates ribosome biogenesis and cell proliferation. Nucleostemin (NS), a nucleolar-localized protein, modulates cell cycle progression, stemness, and three-dimensional tumor spheroid formation. The signaling pathways that regulate NS levels in tumors remain undefined.

Methods

Human breast carcinoma cells were evaluated for growth in culture (adherent and anchorage-independent spheroid) and as orthotopic tumors. FAK signaling was evaluated by pharmacological FAK inhibitor addition (PF-271, IC50 ~ 0.1 μM) and by small hairpin RNA (shRNA) knockdown followed by re-expression of FAK wildtype (WT) or a kinase-dead (KD, K454R) FAK point mutant. Immunoblotting was used to evaluate FAK, NS, nucleolar phosphoprotein B23, and nucleolin levels. Total and phosphospecific antibody imunoblotting were used to detect changes in FAK, Akt kinase (Akt also known as protein kinase B), and 4E-binding protein 1 (4E-BP1) phosphorylation, a translation repressor protein and target of the mammalian target of rapamycin (mTOR) complex. Immunohistochemical, co-immunoprecipitation, and cellular fractionation analyses were used to evaluate FAK association with nucleoli.

Results

Pharmacological (0.1 μM PF-271) or genetic inhibition of FAK activity prevents MDA-MB-231 and 4T1L breast carcinoma growth as spheroids and as orthotopic tumors. FAK inhibition triggers proteasome-mediated decreased NS levels but no changes in other nucleolar proteins such as B23 (nucleophosmin) or nucleolin. Active FAK was associated with purified nucleoli of anchorage-independent cells and present within nucleoli of human invasive ductal carcinoma tumor samples. FAK co-immunoprecipitated with B23 that binds NS and a complex between FAK, NS, Akt, and mTOR was detected. Constitutively-active Akt kinase promoted tumor spheroid growth, stabilized NS levels, and promoted pS65 4E-BP1 phosphorylation in the presence of inhibited FAK. Rapamycin lowered NS levels and inhibited pS65 4E-BP1 phosphorylation in cells with activated Akt-mTOR signaling.

Conclusions

FAK signaling occurs in the nucleolus, active FAK protects NS, and Akt-mTOR pathway regulates NS protein stability needed for breast carcinoma spheroid and tumor growth.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-015-0551-x) contains supplementary material, which is available to authorized users.

Eribulin mesylate targets human telomerase reverse transcriptase in ovarian cancer cells

Treatment of advanced ovarian cancer involves platinum-based chemotherapy. However, chemoresistance is a major obstacle. Cancer stem cells (CSCs) are thought to be one of the causes of chemoresistance, but the underlying mechanism remains elusive. Recently, human telomerase reverse transcriptase (hTERT) has been reported to promote CSC-like traits. In this study, we found that a mitotic inhibitor, eribulin mesylate (eribulin), effectively inhibited growth of platinum-resistant ovarian cancer cell lines. Eribulin-sensitive cells showed a higher efficiency for sphere formation, suggesting that these cells possess an enhanced CSC-like phenotype. Moreover, these cells expressed a higher level of hTERT, and suppression of hTERT expression by siRNA resulted in decreased sensitivity to eribulin, suggesting that hTERT may be a target for eribulin. Indeed, we found that eribulin directly inhibited RNA-dependent RNA polymerase (RdRP) activity, but not telomerase activity of hTERT in vitro. We propose that eribulin targets the RdRP activity of hTERT and may be an effective therapeutic option for CSCs. Furthermore, hTERT may be a useful biomarker to predict clinical responses to eribulin.

Overexpression of nucleostemin contributes to an advanced malignant phenotype and a poor prognosis in oral squamous cell carcinoma

BACKGROUND

Nucleostemin (NS) is essential for the maintenance of stem cell properties, the functions of which remain poorly understood in cancer cells. The purpose of this study was to explore the impact of NS on malignancy and its clinical significance in oral squamous cell carcinoma (OSCC) patients.

METHODS

We investigated the effects of NS on the proliferation and invasion of OSCC using NS-overexpressing or -knockdown OSCC cells. We assessed the activation of the STAT3 (signal transducer and activator of transcription 3) signalling pathway and the downstream targets in the cells with different expression levels of NS. An immunohistochemical analysis of NS was also performed in 54 OSCC patients who were treated with preoperative chemoradiotherapy and surgery.

RESULTS

The overexpression of NS significantly enhanced the proliferation and invasive potential of OSCC cells. On the other hand, downregulation of NS suppressed the invasiveness of the cells. The alterations of these malignant phenotypes were associated with the activation of STAT3 signalling and its downstream targets. An immunohistochemical analysis demonstrated that a high NS tumour expression level significantly correlated with an advanced T-stage and N-stage. Furthermore, a Cox regression analysis revealed that the NS status (hazard ratio, 9.09; P=0.002) was a significant progression factor for OSCC patients.

CONCLUSIONS

Our results suggest that targeting NS may provide a promising treatment for highly malignant OSCC.

Essential role of the cancer stem/progenitor cell marker nucleostemin for indole-3-carbinol anti-proliferative responsiveness in human breast cancer cells

BACKGROUND

Nucleostemin is a nucleolus residing GTPase that is considered to be an important cancer stem/progenitor cell marker protein due to its high expression levels in breast cancer stem cells and its role in tumor-initiation of human mammary tumor cells. It has been proposed that nucleostemin may represent a valuable therapeutic target for breast cancer; however, to date evidence supporting the cellular mechanism has not been elucidated.

RESULTS

Expression of exogenous HER2, a member of the EGF receptor gene family, in the human MCF-10AT preneoplastic mammary epithelial cell line formed a new breast cancer cell line, 10AT-Her2, which is highly enriched in cells with stem/progenitor cell-like character. 10AT-Her2 cells display a CD44+/CD24-/low phenotype with high levels of the cancer stem/progenitor cell marker proteins nucleostemin, and active aldehyde dehydrogenase-1. The overall expression pattern of HER2 protein and the stem/progenitor cell marker proteins in the 10AT-Her2 cell population is similar to that of the luminal HER2+ SKBR3 human breast cancer cell line, whereas, both MCF-7 and MDA-MB-231 cells display reduced levels of nucleostemin and no detectable expression of ALDH-1. Importantly, in contrast to the other well-established human breast cancer cell lines, 10AT-Her2 cells efficiently form tumorspheres in suspension cultures and initiate tumor xenograft formation in athymic mice at low cell numbers. Furthermore, 10AT-Her2 cells are highly sensitive to the anti-proliferative apoptotic effects of indole-3-carbinol (I3C), a natural anti-cancer indolecarbinol from cruciferous vegetables of the Brassica genus such as broccoli and cabbage. I3C promotes the interaction of nucleostemin with MDM2 (Murine Double Mutant 2), an inhibitor of the p53 tumor suppressor, and disrupts the MDM2 interaction with p53. I3C also induced nucleostemin to sequester MDM2 in a nucleolus compartment, thereby freeing p53 to mediate its apoptotic activity. siRNA knockdown of nucleostemin functionally documented that nucleostemin is required for I3C to trigger its cellular anti-proliferative responses, inhibit tumorsphere formation, and disrupt MDM2-p53 protein-protein interactions. Furthermore, expression of an I3C-resistant form of elastase, the only known target protein for I3C, prevented I3C anti-proliferative responses in cells and in tumor xenografts in vivo, as well as disrupt the I3C stimulated nucleostemin-MDM2 interactions.

CONCLUSIONS

Our results provide the first evidence that a natural anti-cancer compound mediates its cellular and in vivo tumor anti-proliferative responses by selectively stimulating cellular interactions of the stem/progenitor cell marker nucleostemin with MDM2, which frees p53 to trigger its apoptotic response. Furthermore, our study provides a new mechanistic template that can be potentially exploited for the development of cancer stem/progenitor cell targeted therapeutic strategies.

Turning a new page on nucleostemin and self-renewal

A quintessential trait of stem cells is embedded in their ability to self-renew without incurring DNA damage as a result of genome replication. One key self-renewal factor is the nucleolar GTP-binding protein nucleostemin (also known as guanine-nucleotide-binding protein-like 3, GNL3, in invertebrate species). Several studies have recently pointed to an unexpected role of nucleostemin in safeguarding the genome integrity of stem and cancer cells. Since its discovery, the predominant presence of nucleostemin in the nucleolus has led to the notion that it might function in the card-carrying event of the nucleolus--the biogenesis of ribosomes. As tantalizing as this might be, a ribosomal role of nucleostemin is refuted by evidence from recent studies, which argues that nucleostemin depletion triggers a primary event of DNA damage in S phase cells that then leads to ribosomal perturbation. Furthermore, there have been conflicting reports regarding the p53 dependency of nucleostemin activity and the cell cycle arrest profile of nucleostemin-depleted cells. In this Commentary, I propose a model that explains how the many contradictory observations surrounding nucleostemin can be reconciled and suggest that this protein might not be as multi-tasking as has been previously perceived. The story of nucleostemin highlights the complexity of the underlying molecular events associated with the appearance of any cell biological phenotype and also signifies a new understanding of the genome maintenance program in stem cells.

Association of a murine leukaemia stem cell gene signature based on nucleostemin promoter activity with prognosis of acute myeloid leukaemia in patients

Acute myeloid leukaemia (AML) is a heterogeneous neoplastic disorder in which a subset of cells function as leukaemia-initiating cells (LICs). In this study, we prospectively evaluated the leukaemia-initiating capacity of AML cells fractionated according to the expression of a nucleolar GTP binding protein, nucleostemin (NS). To monitor NS expression in living AML cells, we generated a mouse AML model in which green fluorescent protein (GFP) is expressed under the control of a region of the NS promoter (NS-GFP). In AML cells, NS-GFP levels were correlated with endogenous NS mRNA. AML cells with the highest expression of NS-GFP were very immature blast-like cells, efficiently formed leukaemia colonies in vitro, and exhibited the highest leukaemia-initiating capacity in vivo. Gene expression profiling analysis revealed that cell cycle regulators and nucleotide metabolism-related genes were highly enriched in a gene set associated with leukaemia-initiating capacity that we termed the 'leukaemia stem cell gene signature'. This gene signature stratified human AML patients into distinct clusters that reflected prognosis, demonstrating that the mouse leukaemia stem cell gene signature is significantly associated with the malignant properties of human AML. Further analyses of gene regulation in leukaemia stem cells could provide novel insights into diagnostic and therapeutic approaches to AML.

Nucleostemin stabilizes ARF by inhibiting the ubiquitin ligase ULF

Up-regulated expression of nucleolar GTPase Nucleostemin (NS) has been associated with increased cellular proliferation potential and tumor malignancy during cancer development. Recent reports attribute the growth regulatory effects of NS protein to its role in facilitating ribosome production. However, the oncogenic potential of NS remains unclear since imbalanced levels of NS have been reported to exert growth inhibitory effect by modulating p53 tumor suppressor activity. It also remains in questions if aberrant NS levels might play a p53-independent role in regulation of cell proliferation and growth. In this study, we performed affinity purification and mass spectrometry analysis to explore protein-protein interactions influencing NS growth regulatory properties independently of p53 tumor suppressor. We identified the Alternative Reading Frame (ARF) protein as a key protein associating with NS and further verified the interaction through in vitro and in vivo assays. We demonstrated that NS is able to regulate cell cycle progression by regulating the stability of the ARF tumor suppressor. Furthermore, overexpression of NS suppressed ARF polyubiquitination by its E3 ligase ULF and elongated its half-life, while knockdown of NS led to the decrease of ARF levels. Also, we found that NS can enhance NPM stabilization of ARF. Thus, we propose that in the absence of p53, ARF can be stabilized by NS and NPM to serve as an alternative tumor suppressor surveillance, preventing potential cellular transformation resulting from the growth inducing effects of NS overexpression.

The acquisition of cancer stem cell-like properties and neoplastic transformation of human keratinocytes induced by arsenite involves epigenetic silencing of let-7c via Ras/NF-κB

Exposure of humans to inorganic arsenic can cause skin cancer. The acquisition of cancer stem cell-like properties is involved in the initiation of some cancers, and there are changes in let-7 levels in some tumors. The mechanisms of action, however, remain obscure. Here, we report that there are decreased levels of let-7a, let-7b, and let-7c in human keratinocyte HaCaT cells during malignant transformation induced by a low concentration (1.0μM) of arsenite. The process by which arsenite reduces the level of let-7c apparently involves methylation, for 5-aza-2'-deoxycytidine, an inhibitor of methyltransferases, prevents arsenite-induced hypermethylation, decreases the level of let-7c, and thereby blocks arsenite-induced activation of the Ras/NF-κB signal pathway. Let-7c is an up-stream regulator of the Ras/NF-κB signal pathway and down-regulates activation of this pathway. In arsenite-transformed HaCaT cells, the acquisition of cancer stem cell-like properties is prevented by over-expression of let-7c, and over-expression of let-7c decreases the malignancy of transformed HaCaT cells. Thus, we conclude that epigenetic silencing of let-7c via Ras/NF-κB is involved in the acquisition of cancer stem cell-like properties and neoplastic transformation of HaCaT cells induced by arsenite, which contribute to the tumorigenesis of arsenite.

Nucleostemin and GNL3L exercise distinct functions in genome protection and ribosome synthesis, respectively

The mammalian nucleolar proteins nucleostemin and GNL3-like (GNL3L) are encoded by paralogous genes that arose from an ancestral invertebrate gene, GNL3. Invertebrate GNL3 has been implicated in ribosome biosynthesis, as has its mammalian descendent, GNL3L. The paralogous mammalian nucleostemin protein has, instead, been implicated in cell renewal. Here, we found that depletion of nucleostemin in a human breast carcinoma cell line triggers prompt and significant DNA damage in S-phase cells without perturbing the initial step of ribosomal (r)RNA synthesis and only mildly affects the total ribosome production. By contrast, GNL3L depletion markedly impairs ribosome production without inducing appreciable DNA damage. These results indicate that, during vertebrate evolution, GNL3L retained the role of the ancestral gene in ribosome biosynthesis, whereas the paralogous nucleostemin acquired a novel genome-protective function. Our results provide a coherent explanation for what had seemed to be contradictory findings about the functions of the invertebrate versus vertebrate genes and are suggestive of how the nucleolus was fine-tuned for a role in genome protection and cell-cycle control as the vertebrates evolved.

A novel role of nucleostemin in maintaining the genome integrity of dividing hepatocytes during mouse liver development and regeneration

During liver development and regeneration, hepatocytes undergo rapid cell division and face an increased risk of DNA damage associated with active DNA replication. The mechanism that protects proliferating hepatocytes from replication-induced DNA damage remains unclear. Nucleostemin (NS) is known to be up-regulated during liver regeneration, and loss of NS is associated with increased DNA damage in cancer cells. To determine whether NS is involved in protecting the genome integrity of proliferating hepatocytes, we created an albumin promoter-driven NS conditional-null (albNS(cko) ) mouse model. Livers of albNS(cko) mice begin to show loss of NS in developing hepatocytes from the first postnatal week and increased DNA damage and hepatocellular injury at 1-2 weeks of age. At 3-4 weeks, albNS(cko) livers develop bile duct hyperplasia and show increased apoptotic cells, necrosis, regenerative nodules, and evidence suggestive of hepatic stem/progenitor cell activation. CCl4 treatment enhances degeneration and DNA damage in NS-deleted hepatocytes and increases biliary hyperplasia and A6(+) cells in albNS(cko) livers. After 70% partial hepatectomy, albNS(cko) livers show increased DNA damage in parallel with a blunted and prolonged regenerative response. The DNA damage in NS-depleted hepatocytes is explained by the impaired recruitment of a core DNA repair enzyme, RAD51, to replication-induced DNA damage foci.

CONCLUSION

This work reveals a novel genome-protective role of NS in developing and regenerating hepatocytes.

Abundant nucleostemin expression supports the undifferentiated properties of germ cell tumors

Nucleostemin (NS) is a nucleolar GTP-binding protein that is involved in ribosomal biogenesis and protection of telomeres. We investigated the expression of NS in human germ cell tumors and its function in a mouse germ cell tumor model. NS was abundantly expressed in undifferentiated, but not differentiated, types of human testicular germ cell tumors. NS was expressed concomitantly with OCT3/4, a critical regulator of the undifferentiated status of pluripotent stem cells in primordial germ cells and embryonal carcinomas. To investigate the roles of NS in tumor growth in vivo, we used a mouse teratoma model. Analysis of teratomas derived from embryonic stem cells in which the NS promoter drives GFP expression showed that cells highly expressing NS were actively proliferating and exhibited the characteristics of tumor-initiating cells, including the ability to initiate and propagate tumor cells in vivo. NS-expressing cells exhibited higher levels of GTP than non-NS-expressing cells. Because NS protein is stabilized by intracellular GTP, metabolic changes may contribute to abundant NS expression in the undifferentiated cells. OCT3/4 deficiency in teratomas led to loss of NS expression, resulting in growth retardation. Finally, we found that teratomas deficient in NS lost their undifferentiated characteristics, resulting in defective tumor proliferation. These data indicate that abundant expression of NS supports the undifferentiated properties of germ cell tumors.

Nucleostemin prevents telomere damage by promoting PML-IV recruitment to SUMOylated TRF1

A novel telomere-protection mechanism relies upon nucleostemin-mediated recruitment of PML-IV to telomeres and subsequent recruitment of RAD51 in both ALT and telomerase-active cells.

Continuously dividing cells must be protected from telomeric and nontelomeric DNA damage in order to maintain their proliferative potential. Here, we report a novel telomere-protecting mechanism regulated by nucleostemin (NS). NS depletion increased the number of telomere damage foci in both telomerase-active (TA⁺) and alternative lengthening of telomere (ALT) cells and decreased the percentage of damaged telomeres associated with ALT-associated PML bodies (APB) and the number of APB in ALT cells. Mechanistically, NS could promote the recruitment of PML-IV to SUMOylated TRF1 in TA⁺ and ALT cells. This event was stimulated by DNA damage. Supporting the importance of NS and PML-IV in telomere protection, we demonstrate that loss of NS or PML-IV increased the frequency of telomere damage and aberration, reduced telomeric length, and perturbed the TRF2^ΔBΔM-induced telomeric recruitment of RAD51. Conversely, overexpression of either NS or PML-IV protected ALT and TA⁺ cells from telomere damage. This work reveals a novel mechanism in telomere protection.

An off-target nucleostemin RNAi inhibits growth in human glioblastoma-derived cancer stem cells

Glioblastomas (GBM) may contain a variable proportion of active cancer stem cells (CSCs) capable of self-renewal, of aggregating into CD133⁺ neurospheres, and to develop intracranial tumors that phenocopy the original ones. We hypothesized that nucleostemin may contribute to cancer stem cell biology as these cells share characteristics with normal stem cells. Here we report that nucleostemin is expressed in GBM-CSCs isolated from patient samples, and that its expression, conversely to what it has been described for ordinary stem cells, does not disappear when cells are differentiated. The significance of nucleostemin expression in CSCs was addressed by targeting the corresponding mRNA using lentivirally transduced short hairpin RNA (shRNA). In doing so, we found an off-target nucleostemin RNAi (shRNA22) that abolishes proliferation and induces apoptosis in GBM-CSCs. Furthermore, in the presence of shRNA22, GBM-CSCs failed to form neurospheres in vitro or grow on soft agar. When these cells are xenotransplanted into the brains of nude rats, tumor development is significantly delayed. Attempts were made to identify the primary target/s of shRNA22, suggesting a transcription factor involved in one of the MAP-kinases signaling-pathways or multiple targets. The use of this shRNA may contribute to develop new therapeutic approaches for this incurable type of brain tumor.

Suppression of Myc oncogenic activity by nucleostemin haploinsufficiency

Nucleostemin (NS), a nucleolar GTPase, is highly expressed in stem/progenitor cells and in most cancer cells. However, little is known about the regulation of its expression. Here, we identify the NS gene as a novel direct transcriptional target of the c-Myc oncoprotein. We show that Myc overexpression enhances NS transcription in cultured cells and in pre-neoplastic B cells from Eμ-myc transgenic mice. Consistent with NS being downstream of Myc, NS expression parallels that of Myc in a large panel of human cancer cell lines. Using chromatin immunoprecipitation we show that c-Myc binds to a well-conserved E-box in the NS promoter. Critically, we show NS haploinsufficiency profoundly delays Myc-induced cancer formation in vivo. NS+/-Eμ-myc transgenic mice have much slower rates of B-cell lymphoma development, with life spans twice that of their wild-type littermates. Moreover, we demonstrate that NS is essential for the proliferation of Myc-overexpressing cells in cultured cells and in vivo: impaired lymphoma development was associated with a drastic decrease of c-Myc-induced proliferation of pre-tumoural B cells. Finally, we provide evidence that in cell culture NS controls cell proliferation independently of p53 and that NS haploinsufficiency significantly delays lymphomagenesis in p53-deficient mice. Together these data indicate that NS functions downstream of Myc as a rate-limiting regulator of cell proliferation and transformation, independently from its putative role within the p53 pathway. Targeting NS is therefore expected to compromise early tumour development irrespectively of the p53 status.

Generation of tumor-initiating cells by exogenous delivery of OCT4 transcription factor

Introduction

Tumor-initiating cells (TIC) are being extensively studied for their role in tumor etiology, maintenance and resistance to treatment. The isolation of TICs has been limited by the scarcity of this population in the tissue of origin and because the molecular signatures that characterize these cells are not well understood. Herein, we describe the generation of TIC-like cell lines by ectopic expression of the OCT4 transcription factor (TF) in primary breast cell preparations.

Methods

OCT4 cDNA was over-expressed in four different primary human mammary epithelial (HMEC) breast cell preparations from reduction mammoplasty donors. OCT4-transduced breast cells (OTBCs) generated colonies (frequency ~0.01%) in self-renewal conditions (feeder cultures in human embryonic stem cell media). Differentiation assays, immunofluorescence, immunohistochemistry, and flow cytometry were performed to investigate the cell of origin of OTBCs. Serial dilutions of OTBCs were injected in nude mice to address their tumorigenic capabilities. Gene expression microarrays were performed in OTBCs, and the role of downstream targets of OCT4 in maintaining self-renewal was investigated by knock-down experiments.

Results

OTBCs overcame senescence, overexpressed telomerase, and down-regulated p16^INK4A. In differentiation conditions, OTBCs generated populations of both myoepithelial and luminal cells at low frequency, suggesting that the cell of origin of some OTBCs was a bi-potent stem cell. Injection of OTBCs in nude mice generated poorly differentiated breast carcinomas with colonization capabilities. Gene expression microarrays of OTBC lines revealed a gene signature that was over-represented in the claudin-low molecular subtype of breast cancer. Lastly, siRNA-mediated knockdown of OCT4 or downstream embryonic targets of OCT4, such as NANOG and ZIC1, suppressed the ability of OTBCs to self-renew.

Conclusions

Transduction of OCT4 in normal breast preparations led to the generation of cell lines possessing tumor-initiating and colonization capabilities. These cells developed high-grade, poorly differentiated breast carcinomas in nude mice. Genome-wide analysis of OTBCs outlined an embryonic TF circuitry that could be operative in TICs, resulting in up-regulation of oncogenes and loss of tumor suppressive functions. These OTBCs represent a patient-specific model system for the discovery of novel oncogenic targets in claudin-low tumors.

GTP depletion synergizes the anti-proliferative activity of chemotherapeutic agents in a cell type-dependent manner

Mycophenolic acid (MPA) depletes intracellular GTP by blocking de novo guanine nucleotide synthesis. GTP is used ubiquitously for DNA/RNA synthesis and as a signaling molecule. Here, we made a surprising discovery that the anti-proliferative activity of MPA acts synergistically with specific chemotherapeutic agents in a cell type-dependent manner. In MDA-MB-231 cells, MPA shows an extremely potent synergy with 5-FU but not with doxorubicin or etoposide. The synergy between 5-FU and MPA works most effectively against the highly tumorigenic mammary tumor cells compared to the less tumorigenic ones, and does not work in the non-breast cancer cell types that we tested, with the exception of PC3 cells. On the contrary, MPA shows the highest synergy with paclitaxel but not with 5-FU in SCC-25 cells, derived from oral squamous cell carcinomas. Mechanistically, the synergistic effect of MPA on 5-FU in MDA-MB-231 cells can be recapitulated by inhibiting the RNA polymerase-I activity and requires the expression of nucleostemin. This work reveals that the synergy between MPA and anti-proliferative agents is determined by cell type-dependent factors.

Tumor-initiating cells are enriched in CD44(hi) population in murine salivary gland tumor

Tumor-initiating cells (T-ICs) discovered in various tumors have been widely reported. However, T-IC populations in salivary gland tumors have yet to be elucidated. Using the established Pleomorphic Adenoma Gene-1 (Plag1) transgenic mouse model of a salivary gland tumor, we identified CD44(high) (CD44(hi)) tumor cells, characterized by high levels of CD44 cell surface expression, as the T-ICs for pleomorphic adenomas. These CD44(hi) tumor cells incorporated 5-bromo-2-deoxyuridine (BrdU), at a lower rate than their CD44(negative) (CD44(neg)) counterparts, and also retained BrdU for a long period of time. Cell surface maker analysis revealed that 25% of the CD44(hi) tumor cells co-express other cancer stem cell markers such as CD133 and CD117. As few as 500 CD44(hi) tumor cells were sufficient to initiate pleomorphic adenomas in one third of the wildtype mice, whereas more than 1×10(4) CD44(neg) cells were needed for the same purpose. In NIH 3T3 cells, Plag1 was capable of activating the gene transcription of Egr1, a known upregulator for CD44. Furthermore, deletion of sequence 81-96 in the Egr1 promoter region abolished the effect of Plag1 on Egr1 upregulation. Our results establish the existence of T-ICs in murine salivary gland tumors, and suggest a potential molecular mechanism for CD44 upregulation.

Cancer cells cyclically lose and regain drug-resistant highly tumorigenic features characteristic of a cancer stem-like phenotype

Drug resistance and brisk tumor initiation have traditionally been viewed as preexisting phenotypes present in small subpopulations of neoplastic cells sometimes termed cancer stem cells. However, recent work in cancer cell lines has shown that drug-resistant tumor-initiating features can emerge de novo within fractionated subpopulations of cells initially lacking these phenotypes. In the present study, we asked whether such phenotypic plasticity exists broadly in unperturbed cancer cell lines and tumor xenografts growing spontaneously without interventions such as drug selection or fractionation into subpopulations used in prior studies. To address this question, we used side population (SP) analysis combined with fluorescence labeling to identify a drug-resistant highly tumorigenic subpopulation and to track and analyze its interaction with the larger phenotypically negative population over time. Remarkably, we observed that SP size fluctuated in a cyclical manner: first contracting via differentiation into the non-SP (NSP) and then reexpanding via simultaneous direct conversion of numerous NSP cells back to the SP phenotype both in culture and in tumor xenografts. These findings show for the first time that adaptive, cancer-promoting traits such as drug resistance and brisk tumor initiation arise not only as solitary events under selective pressures but also as highly orchestrated transitions occurring concurrently in large numbers of cells even without specifically induced drug selection, ectopic gene expression, or fractionation into subpopulations. This high level of coordinated phenotypic plasticity bears consideration when using cancer cell lines as experimental models and may have significant implications for therapeutic efforts targeting cancer stem cells, which are marked by a drug-resistant tumor-initiating phenotype.