Normal and leukemic hematopoiesis: are leukemias a stem cell disorder or a reacquisition of stem cell characteristics?

Sumoylation is tumor-suppressive and confers proliferative quiescence to hematopoietic progenitors in Drosophila melanogaster larvae

How cell-intrinsic regulation of the cell cycle and the extrinsic influence of the niche converge to provide proliferative quiescence, safeguard tissue integrity, and provide avenues to stop stem cells from giving rise to tumors is a major challenge in gene therapy and tissue engineering. We explore this question in sumoylation-deficient mutants of Drosophila. In wild type third instar larval lymph glands, a group of hematopoietic stem/progenitor cells acquires quiescence; a multicellular niche supports their undifferentiated state. However, how proliferative quiescence is instilled in this population is not understood. We show that Ubc9 protein is nuclear in this population. Loss of the SUMO-activating E1 enzyme, Aos1/Uba2, the conjugating E2 enzyme, Ubc9, or the E3 SUMO ligase, PIAS, results in a failure of progenitors to quiesce; progenitors become hyperplastic, misdifferentiate, and develop into microtumors that eventually detach from the dorsal vessel. Significantly, dysplasia and lethality of Ubc9 mutants are rescued when Ubc9(wt) is provided specifically in the progenitor populations, but not when it is provided in the niche or in the differentiated cortex. While normal progenitors express high levels of the Drosophila cyclin-dependent kinase inhibitor p21 homolog, Dacapo, the corresponding overgrown mutant population exhibits a marked reduction in Dacapo. Forced expression of either Dacapo or human p21 in progenitors shrinks this population. The selective expression of either protein in mutant progenitor cells, but not in other hematopoietic populations, limits overgrowth, blocks tumorogenesis, and restores organ integrity. We discuss an essential and complex role for sumoylation in preserving the hematopoietic progenitor states for stress response and in the context of normal development of the fly.

Overexpression of wild-type or mutants forms of CEBPA alter normal human hematopoiesis

C/EBPα (CEBPA) is mutated in approximately 8 % of AML in both familial and sporadic AML and, with FLT3 and NPM1, has received most attention as a predictive marker of outcome in patients with normal karyotype disease. Mutations clustering to either the N- or C-terminal (N-and C-ter) portions of the protein have different consequences on the protein function. In familial cases the N-ter form is inherited with patients exhibiting long latency period before the onset of overt disease, typically with the acquisition of a C-ter mutation. Despite the essential insights murine models provide the functional consequences of wild-type C/EBPα in human hematopoiesis and how different mutations are involved in AML development have received less attention. Our data underline the critical role of C/EBPα in human hematopoiesis and demonstrate that C/EBPα mutations (alone or in combination) are insufficient to convert normal human hematopoietic stem/progenitors (HSC/HPCs) into leukemic initiating cells, although individually each altered normal hematopoiesis. It provides the first insight into the effects of N- and C-terminal mutations acting alone and to the combined effects of N/C double mutants. Our results mimicked closely what happens in CEBPA mutated patients.

ARTS-based anticancer therapy: taking aim at cancer stem cells

Apoptosis related protein in TGF-β signaling pathway (ARTS/septin 4 isoform 2) hereforth referred to as ARTS, was originally found to promote apoptosis induced by TGF-β, but later was shown to promote apoptosis induced by a wide variety of apoptotic stimuli. In vivo and in vitro studies revealed that ARTS-induced apoptosis is mainly executed through direct binding and antagonizing XIAP. High levels of XIAP are found in many types of cancers and often correlate with poor prognosis. ARTS was shown to function as a tumor-suppressor protein in human patients and mouse-tumor models. In particular, Septin 4/ARTS-deficient mice have increased tumor susceptibility and contain increased numbers of stem cells (SCs) and progenitor cells, apparently owing to their resistance towards apoptosis. Based on these results we propose that loss of proapoptotic ARTS may act as the 'first hit' initiating tumorigenesis in two distinct ways. First, loss of ARTS-mediated apoptosis leads to increased numbers of normal SCs. Elevated numbers of normal SCs may lead to increased cancer risk due to higher numbers of cellular targets available for transforming mutations. Second, after these SCs acquire additional transforming mutations and become cancer SC (CSCs), they are more likely to survive in the absence of ARTS owing to increased resistance toward apoptosis. A combination of these two mechanisms, over time, is expected to significantly increase tumor risk. Because CSCs appear to share phenotypic markers with normal SCs, targeting the signaling pathways that affect normal SC development and maintenance can serve as a useful approach towards true eradication of cancer. In this article we describe the role of ARTS in apoptosis and cancer, with focus on its potential role as a CSC marker and as a potential target for anticancer and anti-CSC therapy.

Regulation of cell growth by Notch signaling and its differential requirement in normal vs. tumor-forming stem cells in Drosophila

Cancer stem cells (CSCs) are postulated to be a small subset of tumor cells with tumor-initiating ability that shares features with normal tissue-specific stem cells. The origin of CSCs and the mechanisms underlying their genesis are poorly understood, and it is uncertain whether it is possible to obliterate CSCs without inadvertently damaging normal stem cells. Here we show that a functional reduction of eukaryotic translation initiation factor 4E (eIF4E) in Drosophila specifically eliminates CSC-like cells in the brain and ovary without having discernable effects on normal stem cells. Brain CSC-like cells can arise from dedifferentiation of transit-amplifying progenitors upon Notch hyperactivation. eIF4E is up-regulated in these dedifferentiating progenitors, where it forms a feedback regulatory loop with the growth regulator dMyc to promote cell growth, particularly nucleolar growth, and subsequent ectopic neural stem cell (NSC) formation. Cell growth regulation is also a critical component of the mechanism by which Notch signaling regulates the self-renewal of normal NSCs. Our findings highlight the importance of Notch-regulated cell growth in stem cell maintenance and reveal a stronger dependence on eIF4E function and cell growth by CSCs, which might be exploited therapeutically.

Stem cell pathologies and neurological disease

The presence of stem and progenitor cells in the adult human brain suggests a putative and persistent role in reparative behaviors following neurological injury and neurological disease. Too few stem/progenitor cells (as in the case of Parkinson's disease) or too many of these cells (as in the case of Huntington's disease and glioma) could contribute to and even signal brain pathology. We address here critical issues faced by the field of stem cell biology and regenerative medicine, arguing from well-documented as well as speculative perspectives for a potential role for stem cells in the pathology of many human neurological diseases. Although stem cell responses may result in regenerative failure, in many cases they may help in the establishment or re-establishment of a functional neural circuitry (eg, after stroke). Therefore, we would argue that stem cells have a crucial-either positive or negative-role in the pathology of many neurological diseases.

Acute myeloid leukemia stem cells and CD33-targeted immunotherapy

Although the identification of cancer stem cells as therapeutic targets is now actively being pursued in many human malignancies, the leukemic stem cells in acute myeloid leukemia (AML) are a paradigm of such a strategy. Heterogeneity of these cells was suggested by clonal analyses indicating the existence of both leukemias resulting from transformed multipotent CD33(-) stem cells as well others arising from, or predominantly involving, committed CD33(+) myeloid precursors. The latter leukemias, which may be associated with an intrinsically better prognosis, offer a particularly attractive target for stem cell-directed therapies. Targeting the CD33 differentiation antigen with gemtuzumab ozogamicin was the first attempt of such an approach. Emerging clinical data indicate that gemtuzumab ozogamicin is efficacious not only for acute promyelocytic leukemia but, in combination with conventional chemotherapy, also for other favorable- and intermediate-risk AMLs, providing the first proof-of-principle evidence for the validity of this strategy. Herein, we review studies on the nature of stem cells in AML, discuss clinical data on the effectiveness of CD33-directed therapy, and consider the mechanistic basis for success and failure in various AML subsets.

Understanding cancer stem cell heterogeneity and plasticity

Heterogeneity is an omnipresent feature of mammalian cells in vitro and in vivo. It has been recently realized that even mouse and human embryonic stem cells under the best culture conditions are heterogeneous containing pluripotent as well as partially committed cells. Somatic stem cells in adult organs are also heterogeneous, containing many subpopulations of self-renewing cells with distinct regenerative capacity. The differentiated progeny of adult stem cells also retain significant developmental plasticity that can be induced by a wide variety of experimental approaches. Like normal stem cells, recent data suggest that cancer stem cells (CSCs) similarly display significant phenotypic and functional heterogeneity, and that the CSC progeny can manifest diverse plasticity. Here, I discuss CSC heterogeneity and plasticity in the context of tumor development and progression, and by comparing with normal stem cell development. Appreciation of cancer cell plasticity entails a revision to the earlier concept that only the tumorigenic subset in the tumor needs to be targeted. By understanding the interrelationship between CSCs and their differentiated progeny, we can hope to develop better therapeutic regimens that can prevent the emergence of tumor cell variants that are able to found a new tumor and distant metastases.

A novel murine model of myeloproliferative disorders generated by overexpression of the transcription factor NF-E2

Mice expressing a transgene encoding the transcription factor NF-E2 in hematopoietic cells exhibit features of myeloproliferative neoplasms, including thrombocytosis, Epo-independent colony formation, stem and progenitor cell overabundance, leukocytosis, and progression to acute myeloid leukemia.

The molecular pathophysiology of myeloproliferative neoplasms (MPNs) remains poorly understood. Based on the observation that the transcription factor NF-E2 is often overexpressed in MPN patients, independent of the presence of other molecular aberrations, we generated mice expressing an NF-E2 transgene in hematopoietic cells. These mice exhibit many features of MPNs, including thrombocytosis, leukocytosis, Epo-independent colony formation, characteristic bone marrow histology, expansion of stem and progenitor compartments, and spontaneous transformation to acute myeloid leukemia. The MPN phenotype is transplantable to secondary recipient mice. NF-E2 can alter histone modifications, and NF-E2 transgenic mice show hypoacetylation of histone H3. Treatment of mice with the histone deacetylase inhibitor (HDAC-I) vorinostat restored physiological levels of histone H3 acetylation, decreased NF-E2 expression, and normalized platelet numbers. Similarly, MPN patients treated with an HDAC-I exhibited a decrease in NF-E2 expression. These data establish a role for NF-E2 in the pathophysiology of MPNs and provide a molecular rationale for investigating epigenetic alterations as novel targets for rationally designed MPN therapies.

Sumoylation is tumor-suppressive and confers proliferative quiescence to hematopoietic progenitors in Drosophila melanogaster larvae

How cell-intrinsic regulation of the cell cycle and the extrinsic influence of the niche converge to provide proliferative quiescence, safeguard tissue integrity, and provide avenues to stop stem cells from giving rise to tumors is a major challenge in gene therapy and tissue engineering. We explore this question in sumoylation-deficient mutants of Drosophila. In wild type third instar larval lymph glands, a group of hematopoietic stem/progenitor cells acquires quiescence; a multicellular niche supports their undifferentiated state. However, how proliferative quiescence is instilled in this population is not understood. We show that Ubc9 protein is nuclear in this population. Loss of the SUMO-activating E1 enzyme, Aos1/Uba2, the conjugating E2 enzyme, Ubc9, or the E3 SUMO ligase, PIAS, results in a failure of progenitors to quiesce; progenitors become hyperplastic, misdifferentiate, and develop into microtumors that eventually detach from the dorsal vessel. Significantly, dysplasia and lethality of Ubc9 mutants are rescued when Ubc9wt is provided specifically in the progenitor populations, but not when it is provided in the niche or in the differentiated cortex. While normal progenitors express high levels of the Drosophila cyclin-dependent kinase inhibitor p21 homolog, Dacapo, the corresponding overgrown mutant population exhibits a marked reduction in Dacapo. Forced expression of either Dacapo or human p21 in progenitors shrinks this population. The selective expression of either protein in mutant progenitor cells, but not in other hematopoietic populations, limits overgrowth, blocks tumorogenesis, and restores organ integrity. We discuss an essential and complex role for sumoylation in preserving the hematopoietic progenitor states for stress response and in the context of normal development of the fly.

Osteoblastic and vascular endothelial niches, their control on normal hematopoietic stem cells, and their consequences on the development of leukemia

Stem cell self-renewal is regulated by intrinsic mechanisms and extrinsic signals mediated via specialized microenvironments called “niches.” The best-characterized stem cell is the hematopoietic stem cell (HSC). Self-renewal and differentiation ability of HSC are regulated by two major elements: endosteal and vascular regulatory elements. The osteoblastic niche localized at the inner surface of the bone cavity might serve as a reservoir for long-term HSC storage in a quiescent state. Whereas the vascular niche, which consists of sinusoidal endothelial cell lining blood vessel, provides an environment for short-term HSC proliferation and differentiation. Both niches act together to maintain hematopoietic homeostasis. In this paper, we provide some principles applying to the hematopoietic niches, which will be useful in the study and understanding of other stem cell niches. We will discuss altered microenvironment signaling leading to myeloid lineage disease. And finally, we will review some data on the development of acute myeloid leukemia from a subpopulation called leukemia-initiating cells (LIC), and we will discuss on the emerging evidences supporting the influence of the microenvironment on chemotherapy resistance.

Cancer stem cell markers in breast cancer: pathological, clinical and prognostic significance

INTRODUCTION

The cancer stem cell (CSC) hypothesis states that tumours consist of a cellular hierarchy with CSCs at the apex driving tumour recurrence and metastasis. Hence, CSCs are potentially of profound clinical importance. We set out to establish the clinical relevance of breast CSC markers by profiling a large cohort of breast tumours in tissue microarrays (TMAs) using immunohistochemistry (IHC).

METHODS

We included 4, 125 patients enrolled in the SEARCH population-based study with tumours represented in TMAs and classified into molecular subtype according to a validated IHC-based five-marker scheme. IHC was used to detect CD44/CD24, ALDH1A1, aldehyde dehydrogenase family 1 member A3 (ALDH1A3) and integrin alpha-6 (ITGA6). A 'Total CSC' score representing expression of all four CSC markers was also investigated. Association with breast cancer specific survival (BCSS) at 10 years was assessed using a Cox proportional-hazards model. This study was complied with REMARK criteria.

RESULTS

In ER negative cases, multivariate analysis showed that ITGA6 was an independent prognostic factor with a time-dependent effect restricted to the first two years of follow-up (hazard ratio (HR) for 0 to 2 years follow-up, 2.4; 95% confidence interval (95% CI), 1.2 to 4.8; P = 0.009). The composite 'Total CSC' score carried independent prognostic significance in ER negative cases for the first four years of follow-up (HR for 0 to 4 years follow-up, 1.3; 95% CI, 1.1 to 1.6; P = 0.006).

CONCLUSIONS

Breast CSC markers do not identify identical subpopulations in primary tumours. Both ITGA6 and a composite Total CSC score show independent prognostic significance in ER negative disease. The use of multiple markers to identify tumours enriched for CSCs has the greatest prognostic value. In the absence of more specific markers, we propose that the effective translation of the CSC hypothesis into patient benefit will necessitate the use of a panel of markers to robustly identify tumours enriched for CSCs.

Evolutionary dynamics of feedback escape and the development of stem-cell-driven cancers

Cancers are thought to arise in tissue stem cells, and similar to healthy tissue, are thought to be maintained by a small population of tumor stem or initiating cells, whereas the majority of tumor cells are more differentiated with limited replicative potential. Healthy tissue homeostasis is achieved by feedback loops, and particular importance has been attached to signals secreted from differentiated cells that inhibit stem-cell division and stem-cell self-renewal, as documented in the olfactory epithelium and other tissues. Therefore, a key event in carcinogenesis must be escape from these feedback loops, which is studied here using evolutionary computational models. We find that out of all potential evolutionary pathways, only one unique sequence of phenotypic transitions can lead to complete escape in stem-cell-driven tumors, even though the required mutations for these transitions are certainly tissue specific. This insight, supported by data, facilitates the search for driver mutations and for therapeutic targets. Different growth patterns can result from feedback escape, which we call "inhibited," "uninhibited," and "sigmoidal," and which are found in published data. The finding of inhibited growth patterns in data indicates that besides architecture, the regulatory mechanisms of healthy tissue continue to operate to a degree in tumors.

p53, Stem Cells, and Reprogramming: Tumor Suppression beyond Guarding the Genome

p53 is well recognized as a potent tumor suppressor. In its classic role, p53 responds to genotoxic insults by inducing cell cycle exit or programmed cell death to limit the propagation of cells with corrupted genomes. p53 is also implicated in a variety of other cellular processes in which its involvement is less well understood including self-renewal, differentiation, and reprogramming. These activities represent an emerging area of intense interest for cancer biologists, as they provide potential mechanistic links between p53 loss and the stem cell-like cellular plasticity that has been suggested to contribute to tumor cell heterogeneity and to drive tumor progression. Despite accumulating evidence linking p53 loss to stem-like phenotypes in cancer, it is not yet understood how p53 contributes to acquisition of "stemness" at the molecular level. Whether and how stem-like cells confer survival advantages to propagate the tumor also remain to be resolved. Furthermore, although it seems reasonable that the combination of p53 deficiency and the stem-like state could contribute to the genesis of cancers that are refractory to treatment, direct linkages and mechanistic underpinnings remain under investigation. Here, we discuss recent findings supporting the connection between p53 loss and the emergence of tumor cells bearing functional and molecular similarities to stem cells. We address several potential molecular and cellular mechanisms that may contribute to this link, and we discuss implications of these findings for the way we think about cancer progression.

The leukemia-specific fusion gene ETV6/RUNX1 perturbs distinct key biological functions primarily by gene repression

BACKGROUND

ETV6/RUNX1 (E/R) (also known as TEL/AML1) is the most frequent gene fusion in childhood acute lymphoblastic leukemia (ALL) and also most likely the crucial factor for disease initiation; its role in leukemia propagation and maintenance, however, remains largely elusive. To address this issue we performed a shRNA-mediated knock-down (KD) of the E/R fusion gene and investigated the ensuing consequences on genome-wide gene expression patterns and deducible regulatory functions in two E/R-positive leukemic cell lines.

FINDINGS

Microarray analyses identified 777 genes whose expression was substantially altered. Although approximately equal proportions were either up- (KD-UP) or down-regulated (KD-DOWN), the effects on biological processes and pathways differed considerably. The E/R KD-UP set was significantly enriched for genes included in the "cell activation", "immune response", "apoptosis", "signal transduction" and "development and differentiation" categories, whereas in the E/R KD-DOWN set only the "PI3K/AKT/mTOR signaling" and "hematopoietic stem cells" categories became evident. Comparable expression signatures obtained from primary E/R-positive ALL samples underline the relevance of these pathways and molecular functions. We also validated six differentially expressed genes representing the categories "stem cell properties", "B-cell differentiation", "immune response", "cell adhesion" and "DNA damage" with RT-qPCR.

CONCLUSION

Our analyses provide the first preliminary evidence that the continuous expression of the E/R fusion gene interferes with key regulatory functions that shape the biology of this leukemia subtype. E/R may thus indeed constitute the essential driving force for the propagation and maintenance of the leukemic process irrespective of potential consequences of associated secondary changes. Finally, these findings may also provide a valuable source of potentially attractive therapeutic targets.

The ups and downs of p53 regulation in hematopoietic stem cells

Hematopoietic stem cells provide an indispensible source for replenishing the blood with all its constituents throughout the organism's lifetime. Mice with a compromised hematopoietic stem cell compartment cannot survive. p53, a major tumor suppressor gene, has been implicated in regulation of hematopoiesis. In particular, p53 plays a role in homeostasis by regulating HSC quiescence and self renewal. We recently utilized a hypomorphic p53(515C) allele in conjunction with Mdm2, a negative regulator of p53 to gain insights into the role of p53 in hematopoietic regulation. Our analyses revealed that p53(515C/515C) Mdm2(-/-) double mutant mice die soon after birth due to hematopoietic failure. Further mechanistic studies revealed that in the absence of Mdm2, ROS induced postnatal p53 activity depletes hematopoietic stem cells, progenitors and differentiated cells.

Inhibitors of Glioma Growth that Reveal the Tumour to the Immune System

Treated glioblastoma patients survive from 6 to 14 months. In the first part of this review, we describe glioma origins, cancer stem cells and the genomic alterations that generate dysregulated cell division, with enhanced proliferation and diverse response to radiation and chemotherapy. We review the pathways that mediate tumour cell proliferation, neo-angiogenesis, tumor cell invasion, as well as necrotic and apoptotic cell death. Then, we examine the ability of gliomas to evade and suppress the host immune system, exhibited at the levels of antigen recognition and immune activation, limiting the effective signaling between glioma and host immune cells.The second part of the review presents current therapies and their drawbacks. This is followed by a summary of the work of our laboratory during the past 20 years, on oligosaccharide and glycosphingolipid inhibitors of astroblast and astrocytoma division. Neurostatins, the O-acetylated forms of gangliosides GD1b and GT1b naturally present in mammalian brain, are cytostatic for normal astroblasts, but cytotoxic for rat C6 glioma cells and human astrocytoma grades III and IV, with ID50 values ranging from 200 to 450 nM. The inhibitors do not affect neurons or fibroblasts up to concentrations of 4 μM or higher.At least four different neurostatin-activated, cell-mediated antitumoral processes, lead to tumor destruction: (i) inhibition of tumor neovascularization; (ii) activation of microglia; (iii) activation of natural killer (NK) cells; (iv) activation of cytotoxic lymphocytes (CTL). The enhanced antigenicity of neurostatin-treated glioma cells, could be related to their increased expression of connexin 43. Because neurostatins and their analogues show specific activity and no toxicity for normal cells, a clinical trial would be the logical next step.

Antigenic and Genotypic Similarity between Primary Glioblastomas and Their Derived Neurospheres

Formation of neurospheres (NS) in cultures of glioblastomas (GBMs), with self-renewal, clonogenic capacities, and tumorigenicity following transplantation into immunodeficient mice, may denounce the existence of brain tumor stem cells (BTSCs) in vivo. In sixteen cell lines from resected primary glioblastomas, NS showed the same genetic alterations as primary tumors and the expression of stemness antigens. Adherent cells (AC), after adding 10% of fetal bovine serum (FBS) to the culture, were genetically different from NS and prevailingly expressed differentiation antigens. NS developed from a highly malignant tumor phenotype with proliferation, circumscribed necrosis, and high vessel density. Beside originating from transformed neural stem cells (NSCs), BTSCs may be contained within or correspond to dedifferentiated cells after mutation accumulation, which reacquire the expression of stemness antigens.

Cell of origin in AML: susceptibility to MN1-induced transformation is regulated by the MEIS1/AbdB-like HOX protein complex

Pathways defining susceptibility of normal cells to oncogenic transformation may be valuable therapeutic targets. We characterized the cell of origin and its critical pathways in MN1-induced leukemias. Common myeloid (CMP) but not granulocyte-macrophage progenitors (GMP) could be transformed by MN1. Complementation studies of CMP-signature genes in GMPs demonstrated that MN1-leukemogenicity required the MEIS1/AbdB-like HOX-protein complex. ChIP-sequencing identified common target genes of MN1 and MEIS1 and demonstrated identical binding sites for a large proportion of their chromatin targets. Transcriptional repression of MEIS1 targets in established MN1 leukemias demonstrated antileukemic activity. As MN1 relies on but cannot activate expression of MEIS1/AbdB-like HOX proteins, transcriptional activity of these genes determines cellular susceptibility to MN1-induced transformation and may represent a promising therapeutic target.

The miR 302-367 cluster drastically affects self-renewal and infiltration properties of glioma-initiating cells through CXCR4 repression and consequent disruption of the SHH-GLI-NANOG network

Glioblastoma multiforme (GBM) is the most common form of primary brain tumor in adults, often characterized by poor survival. Glioma-initiating cells (GiCs) are defined by their extensive self-renewal, differentiation, and tumor initiation properties. GiCs are known to be involved in tumor growth and recurrence, and in resistance to conventional treatments. One strategy to efficiently target GiCs in GBM consists in suppressing their stemness and consequently their tumorigenic properties. In this study, we show that the miR-302-367 cluster is strongly induced during serum-mediated stemness suppression. Stable miR-302-367 cluster expression is sufficient to suppress the stemness signature, self-renewal, and cell infiltration within a host brain tissue, through inhibition of the CXCR4 pathway. Furthermore, inhibition of CXCR4 leads to the disruption of the sonic hedgehog (SHH)-GLI-NANOG network, which is involved in self-renewal and expression of the embryonic stem cell-like signature. In conclusion, we demonstrated that the miR-302-367 cluster is able to efficiently trigger a cascade of inhibitory events leading to the disruption of GiCs stem-like and tumorigenic properties.

Notch signaling in intestinal homeostasis across species: the cases of Drosophila, Zebrafish and the mouse

Notch signaling has been recently shown to have a fundamental role in stem cell maintenance and control of proper homeostasis in the intestine of different species. Here, we briefly review the current literature on Notch signals in the intestine of Drosophila, Zebrafish and the mouse, and try to highlight conserved and divergent Notch functions across species. Notch signals show a remarkably conserved role in skewing cell fate choices in intestinal lineages throughout evolution. Genetic analysis demonstrates that loss of Notch signaling invariably leads to increased numbers of secretory cells and loss of enterocytes, while gain of Notch function will completely block secretory cell differentiation. Finally, we discuss the potential contribution of Notch signaling to the initiation of colorectal cancer by controlling the maintenance of the undifferentiated state of intestinal neoplastic cells and speculate on the therapeutic consequences of affecting cancer stem cells.

Stem cell niches and other factors that influence the sensitivity of bone marrow to radiation-induced bone cancer and leukaemia in children and adults

PURPOSE

This paper reviews and reassesses the internationally accepted niches or 'targets' in bone marrow that are sensitive to the induction of leukaemia and primary bone cancer by radiation.

CONCLUSIONS

The hypoxic conditions of the 10 μm thick endosteal/osteoblastic niche where preleukemic stem cells and hematopoietic stem cells (HSC) reside provides a radioprotective microenvironment that is 2- to 3-fold less radiosensitive than vascular niches. This supports partitioning the whole marrow target between the low haematological cancer risk of irradiating HSC in the endosteum and the vascular niches within central marrow. There is a greater risk of induced bone cancer when irradiating a 50 μm thick peripheral marrow adjacent to the remodelling/reforming portion of the trabecular bone surface, rather than marrow next to the quiescent bone surface. This choice of partitioned bone cancer target is substantiated by the greater radiosensitivity of: (i) Bone with high remodelling rates, (ii) the young, (iii) individuals with hypermetabolic benign diseases of bone, and (iv) the epidemiology of alpha-emitting exposures. Evidence is given to show that the absence of excess bone-cancer in atomic-bomb survivors may be partially related to the extremely low prevalence among Japanese of Paget's disease of bone. Radiation-induced fibrosis and the wound healing response may be implicated in not only radiogenic bone cancers but also leukaemia. A novel biological mechanism for adaptive response, and possibility of dynamic targets, is advocated whereby stem cells migrate from vascular niches to stress-mitigated, hypoxic niches.

Mdm2 is required for survival of hematopoietic stem cells/progenitors via dampening of ROS-induced p53 activity

Mdm2 is an E3 ubiquitin ligase that targets p53 for degradation. p53(515C) (encoding p53R172P) is a hypomorphic allele of p53 that rescues the embryonic lethality of Mdm2(-/-) mice. Mdm2(-/-) p53(515C/515C) mice, however, die by postnatal day 13 resulting from hematopoietic failure. Hematopoietic stem cells and progenitors of Mdm2(-/-) p53(515C/515C) mice were normal in fetal livers but were depleted in postnatal bone marrows. After birth, these mice had elevated reactive oxygen species (ROS) thus activating p53R172P. In the absence of Mdm2, stable p53R172P induced ROS and cell cycle arrest, senescence, and cell death in the hematopoietic compartment. This phenotype was partially rescued with antioxidant treatment and upon culturing of hematopoietic cells in methycellulose at 3% oxygen. p16 was also stabilized because of ROS, and its loss increased cell cycling and partially rescued hematopoiesis and survival. Thus, Mdm2 is required to control ROS-induced p53 levels for sustainable hematopoiesis.

Cancer stem cells and cancer therapy

Cancer stem cells (CSCs) are a subpopulation of tumour cells that possess the stem cell properties of self-renewal and differentiation. Stem cells might be the target cells responsible for malignant transformation, and tumour formation may be a disorder of stem cell self-renewal pathway. Epigenetic alterations and mutations of genes involved in signal transmissions may promote the formation of CSCs. These cells have been identified in many solid tumours including breast, brain, lung, prostate, testis, ovary, colon, skin, liver, and also in acute myeloid leukaemia. The CSC theory clarifies not only the issue of tumour initiation, development, metastasis and relapse, but also the ineffectiveness of conventional cancer therapies. Treatments directed against the bulk of the cancer cells may produce striking responses but they are unlikely to result in long-term remissions if the rare CSCs are not targeted. In this review, we consider the properties of CSCs and possible strategies for controlling the viability and tumourigenecity of these cells, including therapeutic models for selective elimination of CSCs and induction of their proper differentiation.

Pre-eminence and persistence of immature natural killer cells in acute myeloid leukemia patients in first complete remission

Despite substantial progress in the treatment of AML, a proportion of patients do not achieve first complete remission (1(st) CR) with the induction chemotherapy, and, among patients achieving it, a majority is expected to relapse within three years. As allogeneic hematopoietic stem cell transplantation has been established as the most effective form of antileukemic therapy in patients with AML in remission, many studies have focused on the reconstitution and the functionality of the innate immune system in this context, especially regarding cytotoxic effectors such as natural killer (NK) cells. On the contrary, very few data are available concerning the innate immune system of patients in 1st CR. Herein we investigated the phenotype of autologous NK cells of AML patients in 1st CR. We showed that immature NK cells were pre-eminent in the blood of these patients and that this immature phenotype was persistent during the first months after 1st CR.

Analysis of epithelial and mesenchymal markers in ovarian cancer reveals phenotypic heterogeneity and plasticity

In our studies of ovarian cancer cells we have identified subpopulations of cells that are in a transitory E/M hybrid stage, i.e. cells that simultaneously express epithelial and mesenchymal markers. E/M cells are not homogenous but, in vitro and in vivo, contain subsets that can be distinguished based on a number of phenotypic features, including the subcellular localization of E-cadherin, and the expression levels of Tie2, CD133, and CD44. A cellular subset (E/M-MP) (membrane E-cadherin(low)/cytoplasmic E-cadherin(high)/CD133(high), CD44(high), Tie2(low)) is highly enriched for tumor-forming cells and displays features which are generally associated with cancer stem cells. Our data suggest that E/M-MP cells are able to differentiate into different lineages under certain conditions, and have the capacity for self-renewal, i.e. to maintain a subset of undifferentiated E/M-MP cells during differentiation. Trans-differentiation of E/M-MP cells into mesenchymal or epithelial cells is associated with a loss of stem cell markers and tumorigenicity. In vivo xenograft tumor growth is driven by E/M-MP cells, which give rise to epithelial ovarian cancer cells. In contrast, in vitro, we found that E/M-MP cells differentiate into mesenchymal cells, in a process that involves pathways associated with an epithelial-to-mesenchymal transition. We also detected phenotypic plasticity that was dependent on external factors such as stress created by starvation or contact with either epithelial or mesenchymal cells in co-cultures. Our study provides a better understanding of the phenotypic complexity of ovarian cancer and has implications for ovarian cancer therapy.

Stem cells in cancer: instigators and propagators?

There is growing realization that many - if not all - cancer-cell populations contain a subpopulation of self-renewing stem cells known as cancer stem cells (CSCs). Unlike normal adult stem cells that remain constant in number, CSCs can increase in number as tumours grow, and give rise to progeny that can be both locally invasive and colonise distant sites - the two hallmarks of malignancy. Immunodeficient mouse models in which human tumours can be xenografted provide persuasive evidence that CSCs are present in human leukaemias and many types of solid tumour. In addition, many studies have found similar subpopulations in mouse tumours that show enhanced tumorigenic properties when they are transplanted into histocompatible mice. In this Commentary, we refer to CSCs as tumour-propagating cells (TPCs), a term that reflects the assays that are currently employed to identify them. We first discuss evidence that cancer can originate from normal stem cells or closely related descendants. We then outline the attributes of TPCs and review studies in which they have been identified in various cancers. Finally, we discuss the implications of these findings for successful cancer therapies.

Sept4/ARTS is required for stem cell apoptosis and tumor suppression

Inhibitor of Apoptosis Proteins (IAPs) are frequently overexpressed in tumors and have become promising targets for developing anti-cancer drugs. IAPs can be inhibited by natural antagonists, but a physiological requirement of mammalian IAP antagonists remains to be established. Here we show that deletion of the mouse Sept4 gene, which encodes the IAP antagonist ARTS, promotes tumor development. Sept4-null mice have increased numbers of hematopoietic stem and progenitor cells, elevated XIAP protein, increased resistance to cell death, and accelerated tumor development in an Eμ-Myc background. These phenotypes are partially suppressed by inactivation of XIAP. Our results suggest that apoptosis plays an important role as a frontline defense against cancer by restricting the number of normal stem cells.

[Mechanisms of leucemogenesis]

The genetic origins of the development of malignant haematological disorders have been established at the beginning of the 80ies. Systematic characterization of chromosomal structural abnormalities and, more recently by DNA microarray approaches and sequencing of tumour genomes have allowed the identification of a large number of genes that are mutated during malignant transformation in humans. Functional studies of these human oncogenes have shown that most of them were not able to transform a haematologic progenitor when acting alone and that cooperation with other oncogenic events was required. The present challenges are the evaluation of the role of each mutation in malignant transformation and the definition of the chronology of their emergence. From these data, the development of efficient therapeutic approaches will be possible by targeting the early oncogenic events which are at the origin of the malignant transformation.

Cancer stem cell markers: what is their diagnostic value?

IMPORTANCE OF THE FIELD

Cancer resistance to conventional therapies has been attributed to cancer stem cells (CSCs). Although a variety of markers have been reported, a universal marker has not yet been found to identify CSCs. Better identification of these CSCs may lead to new therapies that selectively target these cells and thereby result in more effective treatment. This article categorizes the types of marker that have been identified and explores their potential diagnostic and therapeutic value.

AREAS COVERED IN THIS REVIEW

A focused literature review of studies relating to CSCs and their identification was conducted. Databases evaluated include MEDLINE and Web of Science through 2009.

WHAT THE READER WILL GAIN

The ideal identification method needs to be effective and practical in terms of application. The measurement of aldehyde dehydrogenase activity is simple to accomplish compared with other reported identification methods; however, cell surface antigens have been studied most frequently in the therapeutic targeting of CSCs.

TAKE HOME MESSAGE

Although specific targeting methods have been reported for various cancers, there does not appear to be a proven universal marker for CSCs that would apply to all cancers. Each particular identification method appears to have advantages and disadvantages. From a therapeutic standpoint, targeting of these CSCs should improve prognosis.

Cancer stem cells: problems for therapy?

Many, if not all, tumours contain a sub-population of self-renewing and expanding stem cells known as cancer stem cells (CSCs). The symmetric division of CSCs is one mechanism enabling expansion in their numbers as tumours grow, while epithelial-mesenchymal transition (EMT) is an increasingly recognized mechanism to generate further CSCs endowed with a more invasive and metastatic phenotype. Putative CSCs are prospectively isolated using methods based on either a surface marker or an intracellular enzyme activity and then assessed by a 'sphere-forming' assay in non-adherent culture and/or by their ability to initiate new tumour growth when xenotransplanted into immunocompromised mice-hence, these cells are often referred to as tumour-propagating cells (TPCs). Cell sub-populations enriched for tumour-initiating ability have also been found in murine tumours, countering the argument that xenografting human cells merely select human cells with an ability to grow in mice. Cancer progression can be viewed as an evolutionary process that generates new/multiple clones with a fresh identity; this may be a major obstacle to successful cancer stem cell eradication if treatment targets only a single type of stem cell. In this review, we first briefly discuss evidence that cancer can originate from normal stem cells or closely related descendants. We then outline the attributes of CSCs and review studies in which they have been identified in various cancers. Finally, we discuss the implications of these findings for successful cancer therapies, concentrating on the self-renewal pathways (Wnt, Notch, and Hedgehog), aldehyde dehydrogenase activity, EMT, miRNAs, and other epigenetic modifiers as potential targets for therapeutic manipulation.

Targeting the mechanisms of resistance to chemotherapy and radiotherapy with the cancer stem cell hypothesis

Despite advances in treatment, cancer remains the 2nd most common cause of death in the United States. Poor cure rates may result from the ability of cancer to recur and spread after initial therapies have seemingly eliminated detectable signs of disease. A growing body of evidence supports a role for cancer stem cells (CSCs) in tumor regrowth and spread after initial treatment. Thus, targeting CSCs in combination with traditional induction therapies may improve treatment outcomes and survival rates. Unfortunately, CSCs tend to be resistant to chemo- and radiation therapy, and a better understanding of the mechanisms underlying CSC resistance to treatment is necessary. This paper provides an update on evidence that supports a fundamental role for CSCs in cancer progression, summarizes potential mechanisms of CSC resistance to treatment, and discusses classes of drugs currently in preclinical or clinical testing that show promise at targeting CSCs.

Targeting human clonogenic acute myelogenous leukemia cells via folate conjugated liposomes combined with receptor modulation by all-trans retinoic acid

Our previous data demonstrated that folate receptor β (FR-β) targeted liposomal doxorubicin (FT-L-DOX) showed enhanced cytotoxicity relative to non-targeted liposomal doxorubicin (CON-L-DOX), and the effect was enhanced by selective FR-β upregulation by all-trans retinoic acid (ATRA) in AML blast cells. In this study, the enhanced cytotoxicity was investigated in the proliferating human AML clonogenic cells by combining FT-L-DOX with ATRA. Also, pharmacokinetic properties by pretreatment of ATRA were evaluated using FR-targeted liposomal calcein (FT-L-Calcein). Pharmacokinetic study showed that the area under the concentration curve (AUC) of FT-L-Calcein was decreased and total clearance was increased by pretreatment with ATRA. Meanwhile, the volume of distribution was significantly increased by pretreatment of ATRA. Moreover, calcein level in the liver, spleen and kidney was increased following intravenous administration of FT-L-Calcein by pretreatment of ATRA. In vitro cytotoxicity of FT-L-DOX was higher than that of CON-L-DOX and was increased by pretreatment with ATRA. Colony formation in AML cells was lower due to treatment with FT-L-DOX compared with CON-L-DOX and colony formation further decreased upon pretreatment with ATRA. Moreover, FT-L-DOX was more toxic to AML clonogenic cells than to AML blast cells. The results demonstrate that the efficiency of FR-mediated targeting of FT-L-DOX was preferentially enhanced by ATRA induced FR-β upregulation in AML clonogenic cells.

The t(6;9) associated DEK/CAN fusion protein targets a population of long-term repopulating hematopoietic stem cells for leukemogenic transformation

The t(6;9)-positive acute myeloid leukemia (AML) is classified as a separate clinical entity because of its early onset and poor prognosis. The hallmark of t(6;9) AML is the expression of the DEK/CAN fusion protein. The leukemogenic potential of DEK/CAN has been called into question, because it was shown to be unable to block the differentiation of hematopoietic progenitors. We found that DEK/CAN initiated leukemia from a small subpopulation within the hematopoietic stem cell (HSC) population expressing a surface marker pattern of long-term (LT) HSC. The propagation of established DEK/CAN-positive leukemia was not restricted to the LT-HSC population, but occurred even from more mature and heterogeneous cell populations. This finding indicates that in DEK/CAN-induced leukemia, there is a difference between 'leukemia-initiating cells' (L-ICs) and 'leukemia-maintaining cells' (L-MCs). In contrast to the L-IC cells represented by a very rare subpopulation of LT-HSC, the L-MC seem to be represented by a larger and phenotypically heterogeneous cell population.

p53 loss promotes acute myeloid leukemia by enabling aberrant self-renewal

The p53 tumor suppressor limits proliferation in response to cellular stress through several mechanisms. Here, we test whether the recently described ability of p53 to limit stem cell self-renewal suppresses tumorigenesis in acute myeloid leukemia (AML), an aggressive cancer in which p53 mutations are associated with drug resistance and adverse outcome. Our approach combined mosaic mouse models, Cre-lox technology, and in vivo RNAi to disable p53 and simultaneously activate endogenous Kras(G12D)-a common AML lesion that promotes proliferation but not self-renewal. We show that p53 inactivation strongly cooperates with oncogenic Kras(G12D) to induce aggressive AML, while both lesions on their own induce T-cell malignancies with long latency. This synergy is based on a pivotal role of p53 in limiting aberrant self-renewal of myeloid progenitor cells, such that loss of p53 counters the deleterious effects of oncogenic Kras on these cells and enables them to self-renew indefinitely. Consequently, myeloid progenitor cells expressing oncogenic Kras and lacking p53 become leukemia-initiating cells, resembling cancer stem cells capable of maintaining AML in vivo. Our results establish an efficient new strategy for interrogating oncogene cooperation, and provide strong evidence that the ability of p53 to limit aberrant self-renewal contributes to its tumor suppressor activity.

Light forces the pace: optical manipulation for biophotonics

The biomedical sciences have benefited immensely from photonics technologies in the last 50 years. This includes the application of minute forces that enable the trapping and manipulation of cells and single molecules. In terms of the area of biophotonics, optical manipulation has made a seminal contribution to our understanding of the dynamics of single molecules and the microrheology of cells. Here we present a review of optical manipulation, emphasizing its impact on the areas of single-molecule studies and single-cell biology, and indicating some of the key experiments in the fields.

Innate immune signaling in the myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are heterogeneous clonal hematologic malignancies characterized by cytopenias caused by ineffective hematopoiesis and propensity to progress to acute myeloid leukemia. Innate immunity provides immediate protection against pathogens by coordinating activation of signaling pathways in immune cells. Given the prominent role of the innate immune pathway in regulating hematopoiesis, it is not surprising that aberrant signaling of this pathway is associated with hematologic malignancies. Increased activation of the innate immune pathway may contribute to dysregulated hematopoiesis, dysplasia, and clonal expansion in myelodysplastic syndromes.

Compartmentalized organization: a common and required feature of stem cell niches?

A key question in the stem cell field is how to balance the slow cycling of stem cells with active organ growth. Recent studies of the hair follicle stem cell niche have shown that this can be achieved by organizing the stem cell niche into two compartments: one that engages in immediate, rapid new growth and one that contributes later to long-term growth that fuels hair regeneration. Based on these and other recent findings, we propose that several other adult stem cell niches, including those in the blood, intestine and brain, have a similar bi-compartmental organization and that stem cells might work cooperatively with their progeny to sustain tissue regeneration.

Twist modulates breast cancer stem cells by transcriptional regulation of CD24 expression

The cancer stem cell paradigm postulates that dysregulated tissue-specific stem cells or progenitor cells are precursors for cancer biogenesis. Consequently, identifying cancer stem cells is crucial to our understanding of cancer progression and for the development of novel therapeutic agents. In this study, we demonstrate that the overexpression of Twist in breast cells can promote the generation of a breast cancer stem cell phenotype characterized by the high expression of CD44, little or no expression of CD24, and increased aldehyde dehydrogenase 1 activity, independent of the epithelial-mesenchymal transition. In addition, Twist-overexpressing cells exhibit high efflux of Hoechst 33342 and Rhodamine 123 as a result of increased expression of ABCC1 (MRP1) transporters, a property of cancer stem cells. Moreover, we show that transient expression of Twist can induce the stem cell phenotype in multiple breast cell lines and that decreasing Twist expression by short hairpin RNA in Twist-overexpressing transgenic cell lines MCF-10A/Twist and MCF-7/Twist as well as in MDA-MB-231 partially reverses the stem cell molecular signature. Importantly, we show that inoculums of only 20 cells of the Twist-overexpressing CD44(+)/CD24(-/low) subpopulation are capable of forming tumors in the mammary fat pad of severe combined immunodeficient mice. Finally, with respect to mechanism, we provide data to indicate that Twist transcriptionally regulates CD24 expression in breast cancer cells. Taken together, our data demonstrate the direct involvement of Twist in generating a breast cancer stem cell phenotype through down-regulation of CD24 expression and independent of an epithelial-mesenchymal transition.

Prospective identification of tumorigenic osteosarcoma cancer stem cells in OS99-1 cells based on high aldehyde dehydrogenase activity

High aldehyde dehydrogenase (ALDH) activity has recently been used to identify tumorigenic cell fractions in many cancer types. Herein we hypothesized that a subpopulation of cells with cancer stem cells (CSCs) properties could be identified in established human osteosarcoma cell lines based on high ALDH activity. We previously showed that a subpopulation of cells with high ALDH activity were present in 4 selected human osteosarcoma cell lines, of which a significantly higher ALDH activity was present in the OS99-1 cell line that was originally derived from a highly aggressive primary human osteosarcoma. Using a xenograft model in which OS99-1 cells were grown in NOD/SCID mice, we identified a highly tumorigenic subpopulation of osteosarcoma cells based on their high ALDH activity. Cells with high ALDH activity (ALDH(br) cells) from the OS99-1 xenografts were much less frequent, averaging 3% of the entire tumor population, compared to those isolated directly from the OS99-1 cell line. ALDH(br) cells from the xenograft were enriched with greater tumorigenicity compared to their counterparts with low ALDH activity (ALDH(lo) cells), generating new tumors with as few as 100 cells in vivo. The highly tumorigenic ALDH(br) cells illustrated the stem cell characteristics of self-renewal, the ability to produce differentiated progeny and increased expression of stem cell marker genes OCT3/4A, Nanog and Sox-2. The isolation of osteosarcoma CSCs by their high ALDH activity may provide new insight into the study of osteosarcoma-initiating cells and may potentially have therapeutic implications for human osteosarcoma.

microRNA-29a induces aberrant self-renewal capacity in hematopoietic progenitors, biased myeloid development, and acute myeloid leukemia

The function of microRNAs (miRNAs) in hematopoietic stem cells (HSCs), committed progenitors, and leukemia stem cells (LSCs) is poorly understood. We show that miR-29a is highly expressed in HSC and down-regulated in hematopoietic progenitors. Ectopic expression of miR-29a in mouse HSC/progenitors results in acquisition of self-renewal capacity by myeloid progenitors, biased myeloid differentiation, and the development of a myeloproliferative disorder that progresses to acute myeloid leukemia (AML). miR-29a promotes progenitor proliferation by expediting G1 to S/G2 cell cycle transitions. miR-29a is overexpressed in human AML and, like human LSC, miR-29a-expressing myeloid progenitors serially transplant AML. Our data indicate that miR-29a regulates early hematopoiesis and suggest that miR-29a initiates AML by converting myeloid progenitors into self-renewing LSC.