Stem cells: Self-renewal writ in blood

Elucidating the cell metabolic heterogeneity during hematopoietic lineage differentiation based on Met-Flow

Cell metabolism is critically involved in the differentiation of the hematopoietic lineage and, therefore, has attracted the attention of researchers, however, in-depth studies on cellular metabolic activity of hematopoietic cells (HCs) require attention. This investigation compared the metabolic activity of HCs at critical lineage differentiation stages, including hematopoietic stem cells (HSCs), hematopoietic progenitor cells (HPCs), and differentiated blood cells, via multiple methods and basic reference values. Primary metabolic processes of HCs, including anabolism, catabolism, phosphate, and glucose metabolism, were analyzed, and their maps were drawn. The data revealed that GLUT1 expression in HSCs was substantially higher than in all progenitor cells and mature myeloid blood cells, indicating their strong glucose uptake capacity. In myeloid differentiation, the ACAC expression of HPC2 was markedly higher than in neutrophils and monocytes. The ACAC, ASS1, ATP5A, and PRDX2 of HPC2 expression in lymphoid differentiation was substantially greater than in B and Natural-killer cells. CLP, CMP, GMP, MEP, and HPC1 inherit increased glucose uptake stem cell properties. In lymphocyte subsets, the expression of ACAC, ASS1, ATP5A, CPT1A, and PRDX2 in CD4+ T subgroups (naive and memory CD4+ T and nTreg) were elevated than in B subgroups (pro-, pre-, immature and mature Bs) and CD8+ T subgroups. Furthermore, leukemia stem cells (LSCs) had increased levels of ACAC, CPT1A, G6PD, IDH2, and PRDX2 than leukemia cells, indicating a stronger metabolic capacity of LSCs than differentiated leukemia cells.

New insights into Human Hematopoietic Stem and Progenitor Cells via Single-Cell Omics

Residing at the apex of the hematopoietic hierarchy, hematopoietic stem and progenitor cells (HSPCs) give rise to all mature blood cells. In the last decade, significant progress has been made in single-cell RNA sequencing as well as multi-omics technologies that have facilitated elucidation of the heterogeneity of previously defined human HSPCs. From the embryonic stage through the adult stage to aging, single-cell studies have enabled us to trace the origins of hematopoietic stem cells (HSCs), demonstrating different hematopoietic differentiation during development, as well as identifying novel cell populations. In both hematological benign diseases and malignancies, single-cell omics technologies have begun to reveal tissue heterogeneity and have permitted mapping of microenvironmental ecosystems and tracking of cell subclones, thereby greatly broadening our understanding of disease development. Furthermore, advances have also been made in elucidating the molecular mechanisms for relapse and identifying therapeutic targets of hematological disorders and other non-hematological diseases. Extensive exploration of hematopoiesis at the single-cell level may thus have great potential for broad clinical applications of HSPCs, as well as disease prognosis.

New paradigms on hematopoietic stem cell differentiation

Ever since hematopoietic stem cells (HSCs) were first identified half a century ago, their differentiation roadmap has been extensively studied. The classical model of hematopoiesis has long held as a dogma that HSCs reside at the top of a hierarchy in which HSCs possess self-renewal capacity and can progressively give rise to all blood lineage cells. However, over the past several years, with advances in single cell technologies, this developmental scheme has been challenged. In this review, we discuss the evidence supporting heterogeneity within HSC and progenitor populations as well as the hierarchical models revised by novel approaches mainly in mouse system. These evolving views provide further understanding of hematopoiesis and highlight the complexity of hematopoietic differentiation.

Identification of High-Risk Aberrant Crypt Foci and Mucin-Depleted Foci in the Human Colon With Study of Colon Cancer Stem Cell Markers

BACKGROUND

During colonoscopic screening, only macroscopic lesions will be identified, and these are usually the result of multiple genetic abnormalities. Magnification endoscopic detection of aberrant crypt foci (ACF), long before they acquire complex genetic abnormalities, is promising. However, the features of high-risk ACF-like lesions need to be identified.

MATERIALS AND METHODS

In the present cross-sectional study, grossly visible normal mucosal flaps were shaved from 152 colectomies, including 96 colorectal cancer (CRC) cases and 56 controls (22 control specimens with disease with malignant potential and 34 without malignant potential). Methylene and Alcian blue stains were performed directly on the unfixed mucosal flaps to identify ACF and mucin-depleted foci (MDF). Detailed topographic analyses, with immunohistochemical staining for β-catenin and cancer stem cell (CSC) markers (CD44, CD24, and CD166) were performed.

RESULTS

ACF, MDF, and β-catenin-accumulated crypts were detected more in specimens with adjacent CRC. The left colon had ACF with a larger diameter and greater crypt multiplicity, density, and gyriform pit pattern and were considered the high-risk ACF group. MDF, more commonly associated with dysplasia, is also a marker of possible carcinogenesis. The CD44 CSC marker was significantly upregulated in ACF specimens compared with normal controls. Our 3-tier ACF-only pit pattern classification system showed better linearity with mucosal dysplasia than did the 6-tier Kudo classification.

CONCLUSION

High-risk ACF, when detected during chromoendoscopic screening, should be followed up. CSCs might play an important role in pathogenesis. Larger studies and genotypic risk stratification for definite identification of high-risk ACF are needed.

Role of Ultrasound in Complications of Hematopoetic Stem Cell Transplantation

Hematopoetic stem cell transplantation (HSCT) is an established therapeutic option for both malignant and nonmalignant indications, whose incidence has continued to increase in recent years. Because of its lower cost and lack of radiation exposure, ultrasound examination is often the first-line imaging modality in evaluating patients both before and after HSCT. It is important for radiologists to be aware of sonographic manifestations of the complications that may arise from HSCT. In this study, we will review the basics of HSCT, the role of imaging, and ultrasound examination findings in common and uncommon complications arising from HSCT.

Long-lasting stem cell-like memory CD8+ T cells with a naïve-like profile upon yellow fever vaccination

Efficient and persisting immune memory is essential for long-term protection from infectious and malignant diseases. The yellow fever (YF) vaccine is a live attenuated virus that mediates lifelong protection, with recent studies showing that the CD8(+) T cell response is particularly robust. Yet, limited data exist regarding the long-term CD8(+) T cell response, with no studies beyond 5 years after vaccination. We investigated 41 vaccinees, spanning 0.27 to 35 years after vaccination. YF-specific CD8(+) T cells were readily detected in almost all donors (38 of 41), with frequencies decreasing with time. As previously described, effector cells dominated the response early after vaccination. We detected a population of naïve-like YF-specific CD8(+) T cells that was stably maintained for more than 25 years and was capable of self-renewal ex vivo. In-depth analyses of markers and genome-wide mRNA profiling showed that naïve-like YF-specific CD8(+) T cells in vaccinees (i) were distinct from genuine naïve cells in unvaccinated donors, (ii) resembled the recently described stem cell-like memory subset (Tscm), and (iii) among all differentiated subsets, had profiles closest to naïve cells. Our findings reveal that CD8(+) Tscm are efficiently induced by a vaccine in humans, persist for decades, and preserve a naïveness-like profile. These data support YF vaccination as an optimal mechanistic model for the study of long-lasting memory CD8(+) T cells in humans.

Stem cell-derived exosomes: roles in stromal remodeling, tumor progression, and cancer immunotherapy

Stem cells are known to maintain stemness at least in part through secreted factors that promote stem-like phenotypes in resident cells. Accumulating evidence has clarified that stem cells release nano-vesicles, known as exosomes, which may serve as mediators of cell-to-cell communication and may potentially transmit stem cell phenotypes to recipient cells, facilitating stem cell maintenance, differentiation, self-renewal, and repair. It has become apparent that stem cell-derived exosomes mediate interactions among stromal elements, promote genetic instability in recipient cells, and induce malignant transformation. This review will therefore discuss the potential of stem cell-derived exosomes in the context of stromal remodeling and their ability to generate cancer-initiating cells in a tumor niche by inducing morphologic and functional differentiation of fibroblasts into tumor-initiating fibroblasts. In addition, the immunosuppressive potential of stem cell-derived exosomes in cancer immunotherapy and their prospective applications in cell-free therapies in future translational medicine is discussed.

Targeting acute myeloid leukemia stem cells: a review and principles for the development of clinical trials

Despite an increasingly rich understanding of its pathogenesis, acute myeloid leukemia remains a disease with poor outcomes, overwhelmingly due to disease relapse. In recent years, work to characterize the leukemia stem cell population, the disease compartment most difficult to eliminate with conventional therapy and most responsible for relapse, has been undertaken. This, in conjunction with advances in drug development that have allowed for increasingly targeted therapies to be engineered, raises the hope that we are entering an era in which the leukemia stem cell population can be eliminated, resulting in therapeutic cures for acute myeloid leukemia patients. For these therapies to become available, they must be tested in the setting of clinical trials. A long-established clinical trials infrastructure has been employed to shepherd new therapies from proof-of-concept to approval. However, due to the unique features of leukemia stem cells, drugs that are designed to specifically eliminate this population may not be adequately tested when applied to this model. Therefore, in this review article, we seek to identify the relevant features of acute myeloid leukemia stem cells for clinical trialists, discuss potential strategies to target leukemia stem cells, and propose a set of guidelines outlining the necessary elements of clinical trials to allow for the successful testing of stem cell-directed therapies.

Identification of cancer stem cells and a strategy for their elimination

It has been established previously that up to 40% of mouse CD34⁺ hematopoietic stem cells are capable of internalizing exogenous dsDNA fragments both in vivo and ex vivo. Importantly, when mice are treated with a combination of cyclophosphamide and dsDNA, the repair of interstrand crosslinks in hematopoietic progenitors is attenuated, and their pluripotency is altered. Here we show for the first time that among various actively proliferating mammalian cell populations there are subpopulations capable of internalizing dsDNA fragments. In the context of cancer, such dsDNA-internalizing cell subpopulations display cancer stem cell-like phenotype. Furthermore, using Krebs-2 ascites cells as a model, we found that upon combined treatment with cyclophosphamide and dsDNA, engrafted material loses its tumor-initiating properties which we attribute to the elimination of tumor-initiating stem cell subpopulation or loss of its tumorigenic potential.

BMI‑1 is important in bufalin‑induced apoptosis of K562 cells

The purpose of this study was to analyze the effects of bufalin on the gene expression of K562 cells and on the expression of BMI‑1 pathway constituents in K562 cell apoptosis. K562 cells were treated with bufalin, and the inhibition rate and apoptosis were detected by an MTT assay, flow cytometry and a microarray assay. BMI‑1, p16INK4a and p14ARF were examined by quantitative polymerase chain reaction (qPCR). Bufalin induced significant changes in the gene expression of the K562 cells; 4296 genes were differentially expressed, 2185 were upregulated and 2111 were downregulated. The most upregulated genes were associated with transcription regulation, while the most downregulated genes were associated with the non-coding RNA metabolic processes and DNA repair. qPCR analysis demonstrated that BMI‑1 was overexpressed in the K562 cells. Bufalin is able to downregulate BMI‑1 expression levels in K562 cells prematurely and cause an increase in the expression levels of p16INK4a and p14ARF. Moreover, bufalin downregulated BCR/ABL expression levels in a time‑dependent manner, and the expression of BCR/ABL was not associated with the upregulation or downregulation of BMI‑1 expression. Bufalin may induce K562 cell apoptosis by downregulating BMI‑1 expression levels and accordingly upregulating the expression levels of p16INK4a and p14ARF. Bufalin may also induce K562 cell apoptosis via downregulating BCR/ABL expression levels, and this pathway may be independent of the BMI‑1 pathway.

Drug Resistant Breast Cancer Cell Line Displays Cancer Stem Cell Phenotype and Responds Sensitively to Epigenetic Drug SAHA

Cancer stem cell (CSC) population in solid human breast tumor was identified by CD44(+)/CD24(-) phenotype, characterized by high tumorigenicity, invasiveness and drug resistance. In this study, we characterized drug resistant breast cancer cell line-MCF-7/Adr and a number of breast cancer cell lines using flow cytometry, immunofluorescence, mammosphere formation assay and migration assay, examining their CSC immunophenotypes, presence of CSC proteins, tumorigenicity in vitro and migratory rates, respectively. Our results show that MCF-7/Adr cells uniformly display CSC characteristics yet retain low migratory rate. They are also able to self-renew and differentiate under floating culture conditions. Furthermore, MCF-7/Adr is selectively sensitive to epigenetic drug, suberoylanilide hydroxamic acid (SAHA), losing drug resistance and changes morphology yet retaining CSC immunophenotypes. In conclusion, we show that resistant breast cancer cell line MCF-7/Adr demonstrates uniform CSC like characteristics and are sensitive to epigenetic drug treatment.

The differential expression pattern of the BMI-1, SALL4 and ABCA3 genes in myeloid leukemia

Background and methods

In order to characterize the expression pattern of SALL4, BMI-1 and ABCA3 genes in patients with myeloid leukemia and those who achieved complete remission (CR) after chemotherapy. Real-time PCR was used to determine the expression level of these genes in peripheral blood mononuclear cells from 24 patients with AML, eight patients with AML-CR, 13 patients with CML in the chronic phase (CML-CP), 12 patients with CML in blast crisis (CML-BC), 13 patients with CML-CR and 11 healthy individuals (HI).

Results

Overexpression of the BMI-1 gene was found in the AML, CML-CP and CML-BC groups as compared with HI group, while the BMI-1 expression level was lower in patients who achieved CR. In contrast, significantly increased SALL4 expression was only found in AML group, additionally, SALL4 expression was lower in the CML-CP and CML-CR groups compared with the HI group, while the SALL4 expression level in the CML-BC group was higher and significantly greater than that in the CML-CP and CML-CR groups. Moreover, a positive correlation between the expression of SALL4 and BMI-1 genes was found in samples from most groups. There was no significant difference of ABCA3 expression level in AML and CML-BC group in comparison with HI group. Interestingly, the ABCA3 expression level was significantly decreased in the CML-CP, AML-CR and CML-CR in comparison with the HI group. Moreover, the ABCA3 expression level in all of the CR groups was lower than that in their corresponding groups.

Conclusions

These results describe the altered SALL4, ABCA3 and BMI-1 expression pattern in different phases of myeloid leukemia, which may relate to the development and progression to different diseases. SALL4 expression was strongly correlated with BMI-1 in most of the myeloid leukemia patient groups, providing a potential link between SALL4 and BMI-1 in leukemogenesis.

ID1 and ID3 regulate the self-renewal capacity of human colon cancer-initiating cells through p21

There is increasing evidence that some cancers are hierarchically organized, sustained by a relatively rare population of cancer-initiating cells (C-ICs). Although the capacity to initiate tumors upon serial transplantation is a hallmark of all C-ICs, little is known about the genes that control this process. Here, we establish that ID1 and ID3 function together to govern colon cancer-initiating cell (CC-IC) self-renewal through cell-cycle restriction driven by the cell-cycle inhibitor p21. Regulation of p21 by ID1 and ID3 is a central mechanism preventing the accumulation of excess DNA damage and subsequent functional exhaustion of CC-ICs. Additionally, silencing of ID1 and ID3 increases sensitivity of CC-ICs to the chemotherapeutic agent oxaliplatin, linking tumor initiation function with chemotherapy resistance.

25 years of epidermal stem cell research

This is a chronicle of concepts in the field of epidermal stem cell biology and a historic look at their development over time. The past 25 years have seen the evolution of epidermal stem cell science, from first fundamental studies to a sophisticated science. The study of epithelial stem cell biology was aided by the ability to visualize the distribution of stem cells and their progeny through lineage analysis studies. The excellent progress we have made in understanding epidermal stem cell biology is discussed in this article. The challenges we still face in understanding epidermal stem cells include defining molecular markers for stem and progenitor sub-populations, determining the locations and contributions of the different stem cell niches, and mapping regulatory pathways of epidermal stem cell proliferation and differentiation. However, our rapidly evolving understanding of epidermal stem cells has many potential uses that promise to translate into improved patient therapy.

The biology of cancer stem cells and its clinical implication in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a highly malignant tumor with limited treatment options in its advanced state. The molecular mechanisms underlying HCC remain unclear because of the complexity of its multi-step development process. Cancer stem cells (CSCs) are defined as a small population of cells within a tumor that possess the capability for self-renewal and the generation of heterogeneous lineages of cancer cells. To date, there have been two theories concerning the mechanism of carcinogenesis, i.e., the stochastic (clonal evolution) model and the hierarchical (cancer stem cell-driven) model. The concept of the CSC has been established over the past decade, and the roles of CSCs in the carcinogenic processes of various cancers, including HCC, have been emphasized. Previous experimental and clinical evidence indicated the existence of liver CSCs; however, the potential mechanistic links between liver CSCs and the development of HCC in humans are not fully understood. Although definitive cell surface markers for liver CSCs have not yet been found, several putative markers have been identified, which allow the prospective isolation of CSCs from HCC. The identification and characterization of CSCs in HCC is essential for a better understanding of tumor initiation or progression in relation to signaling pathways. These markers could be used along with clinical parameters for the prediction of chemoresistance, radioresistance, metastasis and survival and may represent potential targets for the development of new molecular therapies against HCC. This review describes the current evidence for the existence and function of liver CSCs and discuss the clinical implications of CSCs in patients demonstrating resistance to conventional anti-cancer therapies, as well as clinical outcomes. Such data may provide a future perspective for targeted therapy in HCC.

Colon Cancer Stem Cells: Bench-to-Bedside-New Therapeutical Approaches in Clinical Oncology for Disease Breakdown

It is widely accepted by the scientific community that cancer, including colon cancer, is a “stem cell disease”. Until a few years ago, common opinion was that all neoplastic cells within a tumor contained tumorigenic growth capacity, but recent evidences hint to the possibility that such a feature is confined to a small subset of cancer-initiating cells, also called cancer stem cells (CSCs). Thus, malignant tumors are organized in a hierarchical fashion in which CSCs give rise to more differentiated tumor cells. CSCs possess high levels of ATP-binding cassette (ABC) transporters and anti-apoptotic molecules, active DNA-repair, slow replication capacities and they produce growth factors that confer refractoriness to antineoplastic treatments. The inefficacy of conventional therapies towards the stem cell population might explain cancer chemoresistance and the high frequency of relapse shown by the majority of tumors. Nowadays, in fact all the therapies available are not sufficient to cure patients with advanced forms of colon cancer since they target differentiated cancer cells which constitute most of the tumor mass and spare CSCs. Since CSCs are the entities responsible for the development of the tumor and represent the only cell population able to sustain tumor growth and progression, these cells represent the elective target for innovative therapies.

Characteristics of leukemia stem cells of murine myeloproliferative disease involving the liver

We studied surface phenotype of tumor cells and characterized leukemia stem cells in various cell populations with phenotypes of stem and committed precursor cells in the hierarchy of hemopoietic stem cells. Transplantable murine leukemia was used as the model. Bone marrow and liver cells from mice in the terminal phase of the disease were stained with antibodies to various surface markers and analyzed on a flow cytofuorometer. The cells were sorted by various differentiation markers using a system of magnetic separation Miltenyi Biotec MACS and then transplanted to syngeneic recipients. In some cases, limiting dilutions were used for measuring the concentration of leukemia-initiating cells. All transplanted cell populations caused death of recipients: c-kit- CD45- over 23.9 days, c-kit+ over 22.2 days, c-kit- CD45+ over 15.4 days, Ter119+ over 18.2 days, and Ter119- over 17.7 days. The concentrations of leukemia cells determined by the method of limiting dilutions was 1 per 37,000 c-kit+ bone marrow cells and 1 per 45 unsorted liver cells from sick animals. Thus, leukemia stem cells retain hierarchic organization in the studied model and can differentiate at least into myeloid and erythroid cells without loosing self-maintenance capacity. This model can be used for the study of regulation of self-maintenance mechanisms in various hierarchic populations of leukemia stem cells.

Targeting breast cancer stem cells

The cancer stem cell (CSC) hypothesis postulates that tumors are maintained by a self-renewing CSC population that is also capable of differentiating into non-self-renewing cell populations that constitute the bulk of the tumor. Although, the CSC hypothesis does not directly address the cell of origin of cancer, it is postulated that tissue-resident stem or progenitor cells are the most common targets of transformation. Clinically, CSCs are predicted to mediate tumor recurrence after chemo- and radiation-therapy due to the relative inability of these modalities to effectively target CSCs. If this is the case, then CSC must be efficiently targeted to achieve a true cure. Similarities between normal and malignant stem cells, at the levels of cell-surface proteins, molecular pathways, cell cycle quiescence, and microRNA signaling present challenges in developing CSC-specific therapeutics. Approaches to targeting CSCs include the development of agents targeting known stem cell regulatory pathways as well as unbiased high-throughput siRNA or small molecule screening. Based on studies of pathways present in normal stem cells, recent work has identified potential "Achilles heals" of CSC, whereas unbiased screening provides opportunities to identify new pathways utilized by CSC as well as develop potential therapeutic agents. Here, we review both approaches and their potential to effectively target breast CSC.

Cancer stem cells and self-renewal

The cancer stem cell (CSC) or cancer-initiating cancer (C-IC) model has garnered considerable attention over the past several years since Dick and colleagues published a seminal report showing that a hierarchy exists among leukemic cells. In more recent years, a similar hierarchical organization, at the apex of which exists the CSC, has been identified in a variety of solid tumors. Human CSCs are defined by their ability to: (i) generate a xenograft that histologically resembles the parent tumor from which it was derived, (ii) be serially transplanted in a xenograft assay thereby showing the ability to self-renew (regenerate), and (iii) generate daughter cells that possess some proliferative capacity but are unable to initiate or maintain the cancer because they lack intrinsic regenerative potential. The emerging complexity of the CSC phenotype and function is at times daunting and has led to some confusion in the field. However, at its core, the CSC model is about identifying and characterizing the cancer cells that possess the greatest capacity to regenerate all aspects of the tumor. It is becoming clear that cancer cells evolve as a result of their ability to hijack normal self-renewal pathways, a process that can drive malignant transformation. Studying self-renewal in the context of cancer and CSC maintenance will lead to a better understanding of the mechanisms driving tumor growth. This review will address some of the main controversies in the CSC field and emphasize the importance of focusing first and foremost on the defining feature of CSCs: dysregulated self-renewal capacity.

Activation of Ras-dependent Elk-1 activity by MLL-AF4 family fusion oncoproteins

OBJECTIVE

Mixed lineage leukemia (MLL) gene rearrangement is commonly observed in human leukemias. Many of the resultant MLL fusion proteins are found correlated with Ras signaling. Nevertheless, Ras mutations have only been reported in a small subset of MLL-rearranged leukemia. With the potential of developing new therapeutic regimens targeting Ras signaling pathway, we studied the role of MLL-AF4 family fusions and MLL-septin family fusions in the activation of Ras signaling in leukemogenesis.

MATERIALS AND METHODS

Elk-1-driven luciferase reporter system was used to study the role of MLL-AF4, MLL-AF5q31, MLL-LAF4, MLL-CDCrel, MLL-MSF, and MLL-Septin 6 in the activation of Ras signaling. Dominant negative Ras S17N mutant and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) inhibitor U0126 were employed to demonstrate the involvement of Ras and MEK in this transactivation event. The activation of endogenous Ras/MEK signaling pathway by MLL fusion proteins in leukemia cell lines was also addressed by immunoblot analysis and small interfering RNA knockdown approach.

RESULTS

We demonstrated that MLL-AF4, MLL-AF5q31, and MLL-LAF4 activated Elk-1 transcription factor, one of the major downstream effectors of Ras. This activation was abolished in the presence of dominant negative Ras or MEK inhibitor U0126, indicating the requirements of Ras and MEK. We further showed that endogenous MEK is phosphorylated in a MLL-AF4-expressing leukemia cell line, whereas depletion of MLL-AF4 by small interfering RNA reduced the phospho-MEK level.

CONCLUSION

Our findings suggest that MLL-AF4 family fusion oncoproteins can activate Elk-1 through Ras/MEK/extracellular signal-regulated kinase (ERK) pathway and strongly support the role of Ras signaling in the pathogenesis of MLL-rearranged leukemia.

Reduced c-Myb activity compromises HSCs and leads to a myeloproliferation with a novel stem cell basis

Murine haematopoietic stem cells (HSCs) are contained in the Kit+Sca1+Lin(-) (KSL) population of bone marrow and are able to repopulate lethally irradiated mice. Myeloproliferative disorders (MPDs) are thought to be clonogenic diseases arising at the level of the HSC. Here, we show that mice expressing low levels of the transcription factor c-Myb, as the result of genetic knockdown, develop a transplantable myeloproliferative phenotype that closely resembles the human disease essential thrombocythaemia (ET). Unlike wild-type cells, the KSL population in c-myb knockdown bone marrow cannot repopulate irradiated mice and does not transfer the disease. Instead, cells positive for Kit and expressing low to medium levels of CD11b acquire self-renewing stem cell properties and are responsible for the perpetuation of the myeloproliferative phenotype.

"Stemness" genomics law governs clinical behavior of human cancer: implications for decision making in disease management

One of the most significant accomplishments of translational oncogenomics is a realistic promise of efficient diagnostic tests that would facilitate implementation of the concept of individualized cancer therapies. Recent discovery of the BMI1 pathway rule indicates that gene expression signatures (GESs) associated with the "stemness" state of a cell might be informative as molecular predictors of cancer therapy outcome. We illustrate a potential clinical utility of this concept using GESs derived from genomic analysis of embryonic stem cells (ESCs) during transition from self-renewing, pluripotent state to differentiated phenotypes. Signatures of multiple stemness pathways (signatures of BMI1, Nanog/Sox2/Oct4, EED, and Suz12 pathways; transposon exclusion zones and ESC pattern 3 signatures; signatures of Polycomb-bound and bivalent chromatin domain transcription factors) seem informative in stratification of cancer patients into low- and high-intensity treatment groups on the basis of prediction of the long-term therapy outcome. A stemness cancer therapy outcome predictor (CTOP) algorithm combining scores of nine stemness signatures outperforms individual signatures and demonstrates a superior prognostic accuracy in retrospective supervised analysis of large cohorts of breast, prostate, lung, and ovarian cancer patients. Our analysis suggests that stemness genomics law governs clinical behavior of human malignancies and defines epigenetic boundaries of therapy-resistant and -sensitive tumors within distinct stemness/differentiation programs. One of the main conclusions of our analysis is that near-term progress in practical implementation of the concept of personalized cancer therapies would depend on timely delivery to practicing physicians of relevant scientific information regarding the outcome of prospective trials validating prognostic performance of CTOP tests in a clinical setting.

PI-103, a dual inhibitor of Class IA phosphatidylinositide 3-kinase and mTOR, has antileukemic activity in AML

The phosphatidylinositol 3-kinase (PI3K)/Akt and mammalian target of rapamycin complex 1 (mTORC1) signaling pathways are frequently activated in acute myelogenous leukemia (AML). mTORC1 inhibition with RAD001 induces PI3K/Akt activation and both pathways are activated independently, providing a rationale for dual inhibition of both pathways. PI-103 is a new potent PI3K/Akt and mTOR inhibitor. In human leukemic cell lines and in primary blast cells from AML patients, PI-103 inhibited constitutive and growth factor-induced PI3K/Akt and mTORC1 activation. PI-103 was essentially cytostatic for cell lines and induced cell cycle arrest in the G1 phase. In blast cells, PI-103 inhibited leukemic proliferation, the clonogenicity of leukemic progenitors and induced mitochondrial apoptosis, especially in the compartment containing leukemic stem cells. In contrast, apoptosis was not induced with RAD001 and IC87114 association, which specifically inhibits mTORC1 and p110delta activity, respectively. PI-103 had additive proapoptotic effects with etoposide in blast cells and in immature leukemic cells. Interestingly, PI-103 did not induce apoptosis in normal CD34(+) cells and had moderate effects on their clonogenic and proliferative properties. Here, we demonstrate that multitargeted therapy against PI3K/Akt and mTOR with PI-103 may be of therapeutic value in AML.

SALL4 is a key regulator of survival and apoptosis in human leukemic cells

Increasing studies suggest that SALL4 may play vital roles in leukemogenesis and stem cell phenotypes. We have mapped the global gene targets of SALL4 using chromatin immunoprecipitation followed by microarray hybridization and identified more than 2000 high-confidence, SALL4-binding genes in the human acute promyelocytic leukemic cell line, NB4. Analysis of SALL4-binding sites reveals that genes involved in cell death, cancer, DNA replication/repair, and cell cycle were highly enriched (P < .05). These genes include 38 important apoptosis-inducing genes (TNF, TP53, PTEN, CARD9, CARD11, CYCS, LTA) and apoptosis-inhibiting genes (Bmi-1, BCL2, XIAP, DAD1, TEGT). Real-time polymerase chain reaction has shown that expression levels of these genes changed significantly after SALL4 knockdown, which ubiquitously led to cell apoptosis. Flow cytometry revealed that reduction of SALL4 expression in NB4 and other leukemia cell lines dramatically increased caspase-3, annexin V, and DNA fragmentation activity. Bromodeoxyuridine-incorporation assays showed decreased numbers of S-phase cells and increased numbers of G1- and G2-phase cells indicating reduced DNA synthesis, consistent with results from cell proliferation assays. In addition, NB4 cells that express low levels of SALL4 have significantly decreased tumorigenecity in immunodeficient mice. Our studies provide a foundation in the development of leukemia stem cell-specific therapy by targeting SALL4.

The emerging picture of the mouse mammary stem cell

The isolation and characterisation of mammary stem cells is an important step towards elucidating the hierarchy of epithelial cell development in the mammary gland and identifying cells that are targets of breast carcinogenesis. Mammary stem cells have recently been prospectively isolated through the identification of specific cell surface markers and in vivo transplantation into cleared fat pads. These cells were demonstrated to reconstitute an entire mammary gland comprising all mature epithelial cell types and to be capable of self-renewal on serial transplantation, thus possessing the defining features of stem cells. Notably, mouse mammary stem cells were found to share the hallmark properties of the basal subtype of breast cancer. This review will summarize the strategy used in the identification of mouse mammary stem cells and their characterisation.

Cancer stem cells: the lessons from pre-cancerous stem cells

How a cancer is initiated and established remains elusive despite all the advances in decades of cancer research. Recently the cancer stem cell (CSC) hypothesis has been revived, challenging the long-standing model of "clonal evolution" for cancer development and implicating the dawning of a potential cure for cancer [1]. The recent identification of precancerous stem cells (pCSCs) in cancer, an early stage of CSC development, however, implicates that the "clonal evolution" is not contradictory to the CSC hypothesis, but is rather an aspect of the process of CSC development [2]. The discovery of pCSC has revealed and will continue to reveal the volatile properties of CSC with respects to their phenotype, differentiation and tumorigenic capacity during initiation and progression. Both pCSC and CSC might also serve as precursors of tumor stromal components such as tumor vasculogenic stem/progenitor cells (TVPCs). Thus, the CSC hypothesis covers the developing process of tumor-initiating cells (TIC) --> pCSC --> CSC --> cancer, a cellular process that should parallel the histological process of hyperplasia/metaplasia (TIC) --> precancerous lesions (pCSC) --> malignant lesions (CSC --> cancer). The embryonic stem (ES) cell and germline stem (GS) cell genes are subverted in pCSCs. Especially the GS cell protein piwil2 may play an important role during the development of TIC --> pCSC --> CSC, and this protein may be used as a common biomarker for early detection, prevention, and treatment of cancer. As cancer stem cell research is yet in its infancy, definitive conclusions regarding the role of pCSC can not be made at this time. However this review will discuss what we have learned from pCSC and how this has led to innovative ideas that may eventually have major impacts on the understanding and treatment of cancer.

Ontogenic growth of the haemopoietic stem cell pool in humans

Recently, the size of the active stem cell pool has been predicted to scale allometrically with the adult mass of mammalian species with a 3/4 power exponent, similar to what has been found to occur for the resting metabolic rate across species. Here we investigate the allometric scaling of human haemopoietic stem cells (HSCs) during ontogenic growth and predict a linear scaling with body mass. We also investigate the allometric scaling of resting metabolic rate during growth in humans and find a linear scaling with mass similar to that of the haemopoietic stem cell pool. Our findings suggest a common underlying organizational principle determining the linear scaling of both the stem cell pool and resting metabolic rate with mass during ontogenic growth within the human species, combined with a 3/4 scaling with adult mass across mammalian species. It is possible that such common principles remain valid for haemopoiesis in other mammalian species.

Stem cell origin of death-from-cancer phenotypes of human prostate and breast cancers

In clinical terms, all human cancers diagnosed in individuals can be divided in two major categories: malignant tumors that will be cured with the existing cancer therapies and tumors that have therapy-resistant phenotypes and will return after initial treatment as incurable metastatic disease. These tumors manifesting clinically lethal death-from-cancer phenotypes represent the most formidable challenge of experimental, translational, and clinical cancer research. Clinical genomics data demonstrate that gene expression signatures associated with the "stemness" state of a cell are informative as molecular predictors of cancer therapy outcome and can help to identify cancer patients with therapy-resistant tumors. Here, we present experimental and clinical evidence in support of the BMI1 pathway rule indicating a genetic link between the stemness state and therapy-resistant death-from-cancer phenotypes. Our analysis demonstrates that therapy-resistant and therapy-responsive cancer phenotypes manifest distinct patterns of association with stemness/differentiation pathways, suggesting that therapy-resistant and therapy-responsive tumors develop within genetically distinct stemness/differentiation programs. These differences can be exploited for development of prognostic and therapy selection genetic tests utilizing a microarray-based cancer therapy outcome predictor algorithm. One of the major regulatory pathways manifesting distinct patterns of association with therapy-resistant and therapy-responsive cancer phenotypes is the Polycomb group proteins chromatin silencing pathway.

Bmi-1 is a target gene for SALL4 in hematopoietic and leukemic cells

Bmi-1 and SALL4 are putative oncogenes that modulate stem cell pluripotency and play a role in leukemogenesis. Murine Sall4 also has been shown to play an essential role in maintaining the properties of ES cells and governing the fate of the primitive inner cell mass. Here, we demonstrate that transcription from the Bmi-1 promoter is strikingly activated by SALL4 in a dose-dependent manner by using a luciferase reporter gene assay. Both promoter deletion construct studies and ChIP from a myeloid stem cell line, 32D, demonstrate that SALL4 binds to a specific region of the Bmi-1 promoter. Deletion of one copy of Sall4 by gene targeting in mouse bone marrow significantly reduced Bmi-1 expression. Reducing SALL4 expression by siRNA in the HL-60 leukemia cell line also results in significant down-regulation of Bmi-1. Furthermore, Bmi-1 expression is up-regulated in transgenic mice that constitutively overexpress human SALL4, and the levels of Bmi-1 in these mice increase as they progress from normal to preleukemic (myelodysplastic syndrome) and leukemic (acute myeloid leukemia) stages. High levels of H3-K4 trimethylation and H3-K79 dimethylation were observed in the SALL4 binding region of the Bmi-1 promoter. These findings suggest a novel link between SALL4 and Bmi-1 in regulating self-renewal of normal and leukemic stem cells. An increase in histone H3-K4 and H3-K79 methylation within the Bmi-1 promoter provides an epigenetic mechanism for histone modifications in SALL4-mediated Bmi-1 gene deregulation.

Insertional Mutagenesis by Replication-Deficient Retroviral Vectors Encoding the Large T Oncogene

Insertion sites of replication-deficient retroviral vectors may trigger clonal dominance of hematopoietic cells in vivo. Here, we tested whether this would also be the case when using vectors that express powerful oncogenes, such as the large tumor antigen (TAg) of simian virus 40. TAg inactivates the tumor-suppressor proteins p53 and Rb by virtue of a chaperone-like activity. Primary hematopoietic stem/progenitor cells transduced with retroviral vectors encoding TAg-induced histiocytic sarcoma (HS) or myeloid leukemia (ML) in transplanted mice (average survival of 21 weeks). Retrovirally introducing TAg into pretransformed 32D cells generated a monocytic leukemia, with faster kinetics ( approximately 8 weeks). Leukemic clones showed retroviral insertions in genes contributing to all known TAg cooperation pathways, acting mitogenic and/or modulating apoptosis (such as BclX, Crk, Pim2, Csfr1/Pdgfrb, Osm/Lif, Axl, Fli, Sema4b, Sox4). 32D-derived monocytic leukemias showed hits in Pim2 and Max proto-oncogenes, or the chaperone Hspa4, plus additional signaling genes. Vector-mediated insertional mutagenesis thus revealed a broad spectrum of potential TAg complementation genes. These findings have important implications for the use of retroviral transgenesis in cancer research, and the expression of signaling genes in somatic gene therapy.

Allometric scaling of the active hematopoietic stem cell pool across mammals

Background

Many biological processes are characterized by allometric relations of the type Y = Y₀M^b between an observable Y and body mass M, which pervade at multiple levels of organization. In what regards the hematopoietic stem cell pool, there is experimental evidence that the size of the hematopoietic stem cell pool is conserved in mammals. However, demands for blood cell formation vary across mammals and thus the size of the active stem cell compartment could vary across species.

Methodology/Principle Findings

Here we investigate the allometric scaling of the hematopoietic system in a large group of mammalian species using reticulocyte counts as a marker of the active stem cell pool. Our model predicts that the total number of active stem cells, in an adult mammal, scales with body mass with the exponent ¾.

Conclusion/Significance

The scaling predicted here provides an intuitive justification of the Hayflick hypothesis and supports the current view of a small active stem cell pool supported by a large, quiescent reserve. The present scaling shows excellent agreement with the available (indirect) data for smaller mammals. The small size of the active stem cell pool enhances the role of stochastic effects in the overall dynamics of the hematopoietic system.

Chromosome 5q deletion and epigenetic suppression of the gene encoding α-catenin (CTNNA1) in myeloid cell transformation

Interstitial loss of all or part of the long arm of chromosome 5, or del(5q), is a frequent clonal chromosomal abnormality in human myelodysplastic syndrome (MDS, a preleukemic disorder) and acute myeloid leukemia (AML), and is thought to contribute to the pathogenesis of these diseases by deleting one or more tumor-suppressor genes. Although a major commonly deleted region (CDR) has been delineated on chromosome band 5q31.1 (refs. 3-7), attempts to identify tumor suppressors within this band have been unsuccessful. We focused our analysis of gene expression on RNA from primitive leukemia-initiating cells, which harbor 5q deletions, and analyzed 12 genes within the CDR that are expressed by normal hematopoietic stem cells. Here we show that the gene encoding alpha-catenin (CTNNA1) is expressed at a much lower level in leukemia-initiating stem cells from individuals with AML or MDS with a 5q deletion than in individuals with MDS or AML lacking a 5q deletion or in normal hematopoietic stem cells. Analysis of HL-60 cells, a myeloid leukemia line with deletion of the 5q31 region, showed that the CTNNA1 promoter of the retained allele is suppressed by both methylation and histone deacetylation. Restoration of CTNNA1 expression in HL-60 cells resulted in reduced proliferation and apoptotic cell death. Thus, loss of expression of the alpha-catenin tumor suppressor in hematopoietic stem cells may provide a growth advantage that contributes to human MDS or AML with del(5q).

Cancer stem cells and "stemness" genes in neuro-oncology

The main properties of stem cells include long-term self-renewal and the capacity to give rise to one or more types of differentiated progeny. Recently, much evidence was provided that leukemia and tumor maintenance and growth are sustained by a small proportion of cells exhibiting stem cell properties. In neural tumors, stem cells have been detected in glioblastoma, medulloblastoma and ependymoma. These observations imply that normal stem cells could be the origin of cancer stem cells; alternatively, a more differentiated progeny may revert to a "stem-like" status, and give rise to cancer stem cells. In adult brain residual stem cells are located in the hippocampus, the subventricular zone and possibly the cerebellum. However, evidence for the ability of more differentiated progeny (astroglia, oligodendroglia) to convert into "stem cells" in vitro has also been provided, thus greatly expanding the potential target of oncogenic mutations. In the framework of the cancer stem cell hypothesis, genes originally identified as important for normal neural stem cells may be essential to support cancer stem cells as well. Stem cell genes act in several ways: they stimulate stem cell self-replication, inhibit differentiation, control excessive replication that might lead to "exhaustion" of the stem cell pool. Mutations in man and mouse, in spontaneous or experimental brain tumors, often target stem cell genes or genes lying in their functional pathway, the main examples being the Sonic hedgehog and the Wnt pathways. Interestingly, several stem cell genes are often overexpressed in brain tumors, even if they are not mutated. This suggests that these genes may be important for the generation of cancer stem cells from more differentiated precursors, or for cancer stem cell maintenance. Cancer stem cells partially differentiate in vivo, and in vitro they also give rise to seemingly normal differentiated progeny, like normal stem cells: thus, their main defect, leading to cancer, may lie in the unbalance between self-replication and terminal differentiation of this minority cell population. Knowledge of extrinsic diffusible factors affecting the activity of stem cell genes may help identifying tools for inducing cancer stem cell differentiation, which might be of use in therapy.

Acute myeloid leukemia stem cells

A fundamental problem in cancer research is identification of the cells within a tumor that sustain the growth of the neoplastic clone. The concept that only a subpopulation of rare cancer stem cells (CSCs) is responsible for maintenance of the neoplasm emerged nearly 50 years ago; however, conclusive proof for the existence of a CSC was obtained only relatively recently. The evidence for the existence of CSCs was first derived from the study of human acute myeloid leukemia (AML), largely because of the availability of quantitative stem cell assays for the leukemic stem cell (LSC). These studies showed that only rare cells within the leukemic clone had the capacity to initiate AML growth after transplant into NOD/SCID mice, establishing the hierarchical organization of AML. Recent clonal-tracking studies showed that the LSC compartment is composed of different classes of LSCs, which can be distinguished on the basis of self-renewal potential. These findings have important implications for our understanding of the leukemogenic process as well as the design of more effective therapies to eliminate AML based on eradication of the LSCs. These studies are briefly reviewed here.

Leukemia stem cells

Cancer is a multifaceted disease in which cell proliferation is no longer under normal growth control. Accumulating data have suggested the existence of cancer stem cells, a minor population of tumor cells that possess the stem cell property of self-renewal and that are responsible for the initiation and maintenance of cancer. The knowledge of cancer stem cell biology is most advanced in research on the hematopoietic cancer, leukemia. With the identification of leukemia stem cells (LSCs) capable of repopulating nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, this body of research has led to conclusive proof for cancer stem cells. This review focuses on the biological characterization of LSCs for each type of leukemia, which has provided key insights into leukemogenic pathology and LSC-targeted therapies.

Clonal analysis of thymus-repopulating cells presents direct evidence for self-renewal division of human hematopoietic stem cells

To elucidate the in vivo kinetics of human hematopoietic stem cells (HSCs), CD34+CD38– cells were infected with lentivirus vector and transplanted into immunodeficient mice. We analyzed the multilineage differentiation and self-renewal abilities of individual thymus-repopulating clones in primary recipients, and their descending clones in paired secondary recipients, by tracing lentivirus gene integration sites in each lymphomyeloid progeny using a linear amplification-mediated polymerase chain reaction (PCR) strategy. Our clonal analysis revealed that a single human thymus-repopulating cell had the ability to produce lymphoid and myeloid lineage cells in the primary recipient and each secondary recipient, indicating that individual human HSCs expand clonally by self-renewal division. Furthermore, we found that the proportion of HSC clones present in the CD34+ cell population decreased as HSCs replicated during extensive repopulation and also as the differentiation capacity of the HSC clones became limited. This indicates the restriction of the ability of individual HSCs despite the expansion of total HSC population. We also demonstrated that the extensive self-renewal potential was confined in the relatively small proportion of HSC clones. We conclude that our clonal tracking studies clearly demonstrated that heterogeneity in the self-renewal capacity of HSC clones underlies the differences in clonal longevity in the CD34+ stem cell pool.

In search of the elusive epidermal stem cell

Recent studies are beginning to reveal that our basic concepts of epidermal stem cell biology may be based on somewhat tenuous ground. For example, it is often assumed that colony-forming cells represent epidermal stem cells, although this has not proved to be the case in hematopoietic cell lineages. In addition, although most stem cells are not cycling, label-retaining cells are used as a primary measure of epidermal stem cells. Moreover, the locations of stem cell niches in epidermis are still being debated. Finally, while putative stem cell markers abound, the most effective isolation procedure for stem cells has not been determined, and the relative efficiency of various methods of stem cell isolation remains unknown. With a functional assay for epidermal stem cells (analogous to the in-vivo competitive assay used for hematopoiesis), we appear to be in a better position to more clearly define the molecular signature of the true long-term repopulating cell/stem cell of the epidermis. Nonetheless, significant progress has been made in regenerative therapy of the epidermis for ulcer and burn treatment, and for corrective gene therapy for inherited skin diseases.

Mutagenesis and oncogenesis by chromosomal insertion of gene transfer vectors

Increasing evidence reveals that random insertion of gene transfer vectors into the genome of repopulating hematopoietic cells may alter their fate in vivo. Although most insertional mutations are expected to have few if any consequences for cellular survival, clonal dominance caused by retroviral vector insertions in (or in the vicinity of) proto-oncogenes or other signaling genes has been described for both normal and malignant hematopoiesis. Important insights into these side effects were initially obtained in murine models. Results from ongoing clinical studies have revealed that similar adverse events may also occur in human gene therapy. However, it remains unknown to what extent the outcome of insertional mutagenesis induced by gene vectors is related to (1) the architecture and type of vector used, (2) intrinsic properties of the target cell, and (3) extrinsic and potentially disease-specific factors influencing clonal competition in vivo. This review discusses reports addressing these questions, underlining the need for models that demonstrate and quantify the functional consequences of insertional mutagenesis. Improving vector design appears to be the most straightforward approach to increase safety, provided all relevant cofactors are considered.

Eradication of leukemia stem cells as a new goal of therapy in leukemia

Leukemias have traditionally been classified and treated on the basis of phenotypic characteristics, such as morphology and cell-surface markers, and, more recently, cytogenetic aberrations. These classification systems are flawed because they do not take into account cellular function. The leukemia cell population is functionally heterogeneous: it consists of leukemia stem cells (LSC) and mature leukemia cells that differentiate abnormally to varying extents. Like normal hematopoietic stem cells, LSCs are quiescent and have self-renewal and clonogenic capacity. Because they are quiescent, LSCs do not respond to cell cycle-specific cytotoxic agents used to treat leukemia and so contribute to treatment failure. These cells may undergo mutations and epigenetic changes, further leading to drug resistance and relapse. Recent data suggest that mature leukemia cells may acquire LSC characteristics, thereby evading chemotherapeutic treatment and sustaining the disease. Ongoing research is likely to reveal the molecular mechanisms responsible for LSC characteristics and lead to novel strategies for eradicating leukemia.