Hematopoietic stem cell quiescence maintained by p21cip1/waf1

Development of a Model of the Acute and Delayed Effects of High Dose Radiation Exposure in Jackson Diversity Outbred Mice; Comparison to Inbred C57BL/6 Mice

Development of medical countermeasures against radiation relies on robust animal models for efficacy testing. Mouse models have advantages over larger species due to economics, ease of conducting aging studies, existence of historical databases, and research tools allowing for sophisticated mechanistic studies. However, the radiation dose-response relationship of inbred strains is inherently steep and sensitive to experimental variables, and inbred models have been criticized for lacking genetic diversity. Jackson Diversity Outbred (JDO) mice are the most genetically diverse strain available, developed by the Collaborative Cross Consortium using eight founder strains, and may represent a more accurate model of humans than inbred strains. Herein, models of the Hematopoietic-Acute Radiation Syndrome and the Delayed Effects of Acute Radiation Exposure were developed in JDO mice and compared to inbred C57BL/6. The dose response relationship curve in JDO mice mirrored the more shallow curves of primates and humans, characteristic of genetic diversity. JDO mice were more radioresistant than C57BL/6 and differed in sensitivity to antibiotic countermeasures. The model was validated with pegylated-G-CSF, which provided significantly enhanced 30-d survival and accelerated blood recovery. Long-term JDO survivors exhibited increased recovery of blood cells and functional bone marrow hematopoietic progenitors compared to C57BL/6. While JDO hematopoietic stem cells declined more in number, they maintained a greater degree of quiescence compared to C57BL/6, which is essential for maintaining function. These JDO radiation models offer many of the advantages of small animals with the genetic diversity of large animals, providing an attractive alternative to currently available radiation animal models.

CircRNA CDR1as promotes hepatoblastoma proliferation and stemness by acting as a miR-7-5p sponge to upregulate KLF4 expression

Hepatoblastoma (HB) is a malignant embryonal tumor of the liver that consists of heterogenous populations of stem/progenitor cells. Although circular RNAs (circRNAs) play an essential role in tumor development, the effects of circRNA on the proliferation of HB cells, especially cancer stem cells (CSCs), remain unclear. We found that the circRNA, CDR1as, was highly expressed in CSC-enriched populations of HB cell lines. Results from flow cytometric and sphere-forming assays revealed that CDR1as knockdown in HB cell lines decreased the proportion of stem cells. The Cell Counting Kit-8 (CCK-8) assay, colony formation experiments, and EdU assay revealed that CDR1as knockdown in HB cell lines decreased cell growth and the colony-forming abilities. Biotin-coupled probe pull-down assays and biotin-coupled microRNA capture were conducted to evaluate the interaction between CDR1as and miR-7-5p. Dual-luciferase reporter assays demonstrated that Kruppel-like factor 4 (KLF4), expression of which is highly correlated with cancer stemness, was a target of miR-7-5p. Overall, the knockdown of CDR1as significantly inhibited the proliferation and stemness of HB cells by reducing the sponge activity on miR-7-5p and subsequently suppressing the interaction between miR-7-5p and KLF4. Results from this study suggest that CDR1as is an oncogene that effects the proliferation and stemness of HBs.

ATP-dependent chromatin remodelers in ageing and age-related disorders

Ageing is characterized by the perturbation in cellular homeostasis associated with genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion and altered intracellular communication. Changes in the epigenome represent one of the crucial mechanisms during ageing and in age-related disorders. The ATP-dependent chromatin remodelers are an evolutionarily conserved family of nucleosome remodelling factors and generally regulate DNA repair, replication, recombination, transcription and cell cycle. Here, we review the chromatin based epigenetic changes that occur in ageing and age-related disorders with a specific reference to chromatin remodelers. We also discuss the link between dietary restriction and chromatin remodelers in regulating age-related processes with a view for consideration in future intervention studies.

m6A RNA Methylation Maintains Hematopoietic Stem Cell Identity and Symmetric Commitment

Stem cells balance cellular fates through asymmetric and symmetric divisions in order to self-renew or to generate downstream progenitors. Symmetric commitment divisions in stem cells are required for rapid regeneration during tissue damage and stress. The control of symmetric commitment remains poorly defined. Using single-cell RNA sequencing (scRNA-seq) in combination with transcriptomic profiling of HSPCs (hematopoietic stem and progenitor cells) from control and m⁶A methyltransferase Mettl3 conditional knockout mice, we found that m⁶A-deficient hematopoietic stem cells (HSCs) fail to symmetrically differentiate. Dividing HSCs are expanded and are blocked in an intermediate state that molecularly and functionally resembles multipotent progenitors. Mechanistically, RNA methylation controls Myc mRNA abundance in differentiating HSCs. We identified MYC as a marker for HSC asymmetric and symmetric commitment. Overall, our results indicate that RNA methylation controls symmetric commitment and cell identity of HSCs and may provide a general mechanism for how stem cells regulate differentiation fate choice.

Cheng et al. uncover RNA methylation as a guardian in hematopoietic stem cell (HSC) fate decisions. m⁶A maintains hematopoietic stem cell symmetric commitment and identity. This study may provide a general mechanism for how RNA methylation controls cellular fate.

A single copy of large tumor suppressor 1 or large tumor suppressor 2 is sufficient for normal hematopoiesis

Background

Hematopoietic stem cells (HSCs) have the ability to differentiate into all subsets of blood cells and self-renew. Large tumor suppressor 1 (LATS1) and large tumor suppressor 2 (LATS2) kinases are essential for cell cycle regulation, organism fitness, genome integrity, and cancer prevention. Here, we investigated whether Lats1 and Lats2 are critical for the maintenance of the self-renewal and quiescence capacities of HSCs in mice.

Methods

Quantitative reverse transcription-polymerase chain reaction was used to determine the expression levels of Lats1 and Lats2 in subsets of progenitor cells and mature bone marrow cells. A clustered regularly interspaced short palindromic repeats system was used to generate Lats1 or Lats2 knockout mice. Complete blood cell counts were used to compare the absolute number of white blood cells, lymphocytes, monocytes, neutrophils, and platelets between Lats1 or Lats2 heterozygotes and littermates. Flow cytometry was used to assess the size of hematopoietic progenitor cells (HPCs) and HSC pools in Lats1 or Lats2 heterozygotes and littermates. The comparison between the two groups was analyzed using Student's t test.

Results

Lats1 and Lats2 were widely expressed in hematopoietic cells with higher expression levels in primitive hematopoietic cells than in mature cells. Lats1 or Lats2 knockout mice were generated, with the homozygotes showing embryonic lethality. The size of the HPC and HSC pools in Lats1 (HPC: wild-type [WT] vs. heterozygote, 220,426.77 ± 54,384.796 vs. 221,149.4 ± 42,688.29, P = 0.988; HSC: WT vs. heterozygote, 2498.932 ± 347.856 vs. 3249.763 ± 370.412, P = 0.105) or Lats2 (HPC: WT vs. heterozygote, 425,540.52 ± 99,721.86 vs. 467,127.8 ± 89,574.48, P = 0.527; HSC: WT vs. heterozygote, 4760.545 ± 1518.01 vs. 5327.437 ± 873.297, P = 0.502) heterozygotes were not impaired. Moreover, the depletion of Lats1 or Lats2 did not affect the overall survival of the heterozygotes (Lats1: P = 0.654; Lats2: P = 0.152).

Conclusion

These results indicate that a single allele of Lats1 or Lats2 may be sufficient for normal hematopoiesis.

Overexpression of p21 Has Inhibitory Effect on Human Hematopoiesis by Blocking Generation of CD43＋ Cells via Cell-Cycle Regulation

Background and Objectives

p21, an important member of the Cip/Kip family, is involved in inhibitory effects of RUNX1b overexpression during the early stage of human hematopoiesis.

Methods and Results

We established a human embryonic stem cell (hESC) line with inducible expression of p21 (p21/hESCs). Overexpression of p21 did not influence either mesoderm induction or emergence of CD34＋ cells, but it significantly decreased the production of CD43＋ cells and changed the expression profile of hematopoiesis-related factors, leading to the negative effects of p21 on hematopoiesis.

Conclusions

In RUNX1b/hESC co-cultures when RUNX1b was induced from D0, perturbation of the cell cycle caused by upregulation of p21 probably prevented the appearance of CD43＋ cells, but not CD34＋ cells. The mechanisms via which CD34＋ cells are blocked by RUNX1b overexpression remain to be elucidated.

A Single Radioprotective Dose of Prostaglandin E2 Blocks Irradiation-Induced Apoptotic Signaling and Early Cycling of Hematopoietic Stem Cells

Ionizing radiation exposure results in acute and delayed bone marrow suppression. Treatment of mice with 16,16-dimethyl prostaglandin E₂ (dmPGE₂) prior to lethal ionizing radiation (IR) facilitates survival, but the cellular and molecular mechanisms are unclear. In this study we show that dmPGE₂ attenuates loss and enhances recovery of bone marrow cellularity, corresponding to a less severe hematopoietic stem cell nadir, and significantly preserves long-term repopulation capacity and progenitor cell function. Mechanistically, dmPGE₂ suppressed hematopoietic stem cell (HSC) proliferation through 24 h post IR, which correlated with fewer DNA double-strand breaks and attenuation of apoptosis, mitochondrial compromise, oxidative stress, and senescence. RNA sequencing of HSCs at 1 h and 24 h post IR identified a predominant interference with IR-induced p53-downstream gene expression at 1 h, and confirmed the suppression of IR-induced cell-cycle genes at 24 h. These data identify mechanisms of dmPGE₂ radioprotection and its potential role as a medical countermeasure against radiation exposure.

Chromatin remodeler Znhit1 preserves hematopoietic stem cell quiescence by determining the accessibility of distal enhancers

Hematopoietic stem cell (HSC) utilizes its quiescence feature to combat exhaustion for lifetime blood cell supply. To date, how certain chromatin architecture and subsequent transcription profile permit HSC quiescence remains unclear. Here, we show an essential role of chromatin remodeler zinc finger HIT-type containing 1 (Znhit1) in maintaining HSC quiescence. We find that loss of Znhit1 leads to exhaustion of stem cell pool and impairment of hematopoietic function. Mechanically, Znhit1 determines the chromatin accessibility at distal enhancers of HSC quiescence genes, including Pten, Fstl1, and Klf4, for sustained transcription and consequent PI3K-Akt signaling inhibition. Moreover, Znhit1-Pten-PI3K-Akt axis also participates in controlling myeloid expansion and B-lymphoid specification. Our findings therefore identify a dominant role of Znhit1-mediated chromatin remodeling in preserving HSC function for hematopoietic homeostasis.

Zinc finger and BTB domain-containing protein 46 is essential for survival and proliferation of acute myeloid leukemia cell line but dispensable for normal hematopoiesis

Supplemental Digital Content is available in the text

Background

Zinc finger and BTB domain-containing protein 46 (Zbtb46) is a transcription factor identified in classical dendritic cells, and maintains dendritic cell quiescence in a steady state. Zbtb46 has been reported to be a negative indicator of acute myeloid leukemia (AML). We found that Zbtb46 was expressed at a relatively higher level in hematopoietic stem and progenitor cells (HSPCs) compared to mature cells, and higher in AML cells compared to normal bone marrow (BM) cells. However, the role of Zbtb46 in HSPCs and AML cells remains unclear. Therefore, we sought to elucidate the effect of Zbtb46 in normal hematopoiesis and AML cells.

Methods

We generated Zbtb46^fl/fl and Zbtb46^fl/flMx1-Cre mice. The deletion of Zbtb46 in Zbtb46^fl/flMx1-Cre mice was induced by intraperitoneal injection of double-stranded poly (I). poly (C) (poly(I:C)), and referred as Zbtb46 cKO. After confirming the deletion of Zbtb46, the frequency and numbers of HSPCs and mature blood cells were analyzed by flow cytometry. Serial intraperitoneal injection of 5-fluorouracil was administrated to determine the repopulation ability of HSCs from Zbtb46^fl/fl and Zbtb46 cKO mice. The correlation between Zbtb46 expression and prognosis was analyzed using the data from the Cancer Genome Atlas. To investigate the role of Zbtb46 in AML cells, we knocked down the expression of Zbtb46 in THP-1 cells using lentiviral vectors expressing small hairpin RNAs targeting Zbtb46. Cell proliferation rate was determined by cell count assay. Cell apoptosis and bromodeoxyuridine incorporation were determined by flow cytometry.

Results

The percentages and absolute numbers of HSPCs and mature blood cells were comparable in Zbtb46 cKO mice and its Zbtb46^fl/fl littermates (Zbtb46^fl/flvs. Zbtb46 cKO, HPC: 801,310 ± 84,282 vs. 907,202 ± 97,403, t = 0.82, P = 0.46; LSK: 86,895 ± 7802 vs. 102,210 ± 5025, t = 1.65, P = 0.17; HSC: 19,753 ± 3116 vs. 17,608 ± 3508, t = 0.46, P = 0.67). The repopulation ability of HSCs from Zbtb46^fl/flMx1-Cre mice was similar to those from Zbtb46^fl/fl control (P = 0.26). Zbtb46 had elevated expression in AML cells compared to total BM cells from normal control. Knockdown of Zbtb46 in THP-1 cells led to a significant increase in cell apoptosis and reduced cell growth and proliferation.

Conclusion

Collectively, our data indicate that Zbtb46 is essential for survival and proliferation of AML cells, but dispensable for normal hematopoiesis.

p53 balances between tissue hierarchy and anarchy

Normal tissues are organized in a hierarchical model, whereas at the apex of these hierarchies reside stem cells (SCs) capable of self-renewal and of producing differentiated cellular progenies, leading to normal development and homeostasis. Alike, tumors are organized in a hierarchical manner, with cancer SCs residing at the apex, contributing to the development and nourishment of tumors. p53, the well-known ‘guardian of the genome’, possesses various roles in embryonic development as well as in adult SC life and serves as the ‘guardian of tissue hierarchy’. Moreover, p53 serves as a barrier for dedifferentiation and reprogramming by constraining the cells to a somatic state and preventing their conversion to SCs. On the contrary, the mutant forms of p53 that lost their tumor suppressor activity and gain oncogenic functions serve as ‘inducers of tissue anarchy’ and promote cancer development. In this review, we discuss these two sides of the p53 token that sentence a tissue either to an ordered hierarchy and life or to anarchy and death. A better understanding of these processes may open new horizons for the development of new cancer therapies.

Premature aging of circulating T cells predicts all-cause mortality in hemodialysis patients

Background

Patients with end-stage renal disease (ESRD) exhibit a premature aging phenotype of immune system, which is recently concerned as a significant factor for increased risk of various morbidities. Nevertheless, there are few dates explicating the relevancy of T cell senescence to mortality. In this study, we prospectively studied the predictive value of T cell senescence for mortality in hemodialysis patients.

Methods

Patients who had been on hemodialysis treatment for at least 6 months were enrolled. T cell senescence determined by differentiation status was evaluated by flow cytometry. Survival outcomes were estimated using the Kaplan-Meier method. Univariate and multivariate analyses were performed to evaluate the prognostic impact of T cell premature aging and other clinical factors on all-cause mortality.

Results

A total of 466 patients (277 man and 169 women) were enrolled in this study. Decreased number of naïve T cell, as the most prominent feature of T cell senescence, did not change in parallel with age in these patients. Decreased absolute count of T cell, naïve T cell, CD4⁺ naïve T cell were independently associated with all-cause mortality. Decreased percentage of T cell and increased percentage of CD8⁺central-memory T cell were also independently associated with all-cause mortality. After including all the T cell parameters in one regression model, only decreased count of naïve T cell was significantly associated with increased mortality in these patients.

Conclusions

Aging-associated T cell changes are aggravated in ESRD patients. For the first time, our study demonstrates that naïve T cell depletion is a strong predictor of all-cause mortality in HD patients.

Multifaceted p21 in carcinogenesis, stemness of tumor and tumor therapy

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

Knock-out of Hopx disrupts stemness and quiescence of hematopoietic stem cells in mice

HOPX is a stem cell marker in hair follicles and intestines. It was shown critical for primitive hematopoiesis. We previously showed an association between higher HOPX expression and clinical characteristics related to stemness and quiescence of leukemic cells in acute myeloid leukemia (AML) patients. To further explore its physiologic functions in hematopoietic system, we generated a mouse model with hematopoietic cell-specific knockout of Hopx (Hopx^-/-). In young Hopx^-/- mice, the hematopoietic stem cells (HSC) showed decreased reconstitution ability after serial transplantation. Further transcriptomic study revealed decreased HSC signatures in long-term HSCs from the Hopx^-/- mice. At 18 months of age, half of the Hopx^-/- mice developed cytopenia and splenomegaly. Bone marrow (BM) from the sick mice showed myeloid hyperplasia with predominant mature neutrophils, and decreased progenitor cells and lymphocytes. These phenotypes suggested critical functions of Hopx in maintaining HSC quiescence. Transcriptomic study of the Hopx^-/- marrow cells showed significant downregulation of the Cxcl12-Cxcr4 axis, which is critical for maintenance of HSC quiescence. We next examined the role of Hopx in AML by using the MN1 overexpression murine leukemia model. Mice transplanted with MN1-overexpressed Hopx^-/- BM cells developed AML with more aggressive phenotypes compared with those transplanted with MN1-overexpressed Hopx-wild cells. Hopx^-/- MN1-overexpressed leukemia cells showed higher proliferation rate and downregulation of Cxcl12 and Cxcr4. Furthermore, in human AML, BM plasma CXCL12 levels were lower in patients with lower HOPX expression. In conclusion, our study highlights the roles of Hopx in maintenance of quiescence of the hematopoietic stem cells through CXCL12 pathway in vivo and provides implication of this protein in normal and malignant hematopoiesis.

Emerging roles of metazoan cell cycle regulators as coordinators of the cell cycle and differentiation

In multicellular organisms, cell proliferation must be tightly coordinated with other developmental processes to form functional tissues and organs. Despite significant advances in our understanding of how the cell cycle is controlled by conserved cell-cycle regulators (CCRs), how the cell cycle is coordinated with cell differentiation in metazoan organisms and how CCRs contribute to this process remain poorly understood. Here, we review the emerging roles of metazoan CCRs as intracellular proliferation-differentiation coordinators in multicellular organisms. We illustrate how major CCRs regulate cellular events that are required for cell fate acquisition and subsequent differentiation. To this end, CCRs employ diverse mechanisms, some of which are separable from those underpinning the conventional cell-cycle-regulatory functions of CCRs. By controlling cell-type-specific specification/differentiation processes alongside the progression of the cell cycle, CCRs enable spatiotemporal coupling between differentiation and cell proliferation in various developmental contexts in vivo. We discuss the significance and implications of this underappreciated role of metazoan CCRs for development, disease and evolution.

Modifiable Cardiovascular Risk, Hematopoiesis, and Innate Immunity

Unhealthy diet, lack of exercise, psychosocial stress, and insufficient sleep are increasingly prevalent modifiable risk factors for cardiovascular disease. Accumulating evidence indicates that these risk factors may fuel chronic inflammatory processes that are active in atherosclerosis and lead to myocardial infarction and stroke. In concert with hyperlipidemia, maladaptive immune system activities can contribute to disease progression and increase the probability of adverse events. In this review, we discuss recent insight into how the above modifiable risk factors influence innate immunity. Specifically, we focus on pathways that raise systemic myeloid cell numbers and modulate immune cell phenotypes, reviewing hematopoiesis, leukocyte trafficking, and innate immune cell accumulation in cardiovascular organs. Often, relevant mechanisms that begin with lifestyle choices and lead to cardiovascular events span multiple organ systems, including the central nervous, endocrine, metabolic, hematopoietic, immune and, finally, the cardiovascular system. We argue that deciphering such pathways provides not only support for preventive interventions but also opportunities to develop biomimetic immunomodulatory therapeutics that mitigate cardiovascular inflammation.

N-Cadherin-Expressing Bone and Marrow Stromal Progenitor Cells Maintain Reserve Hematopoietic Stem Cells

Regulation of hematopoietic stem cells (HSCs) by bone marrow (BM) niches has been extensively studied; however, whether and how HSC subpopulations are distinctively regulated by BM niches remain unclear. Here, we functionally distinguished reserve HSCs (rHSCs) from primed HSCs (pHSCs) based on their response to chemotherapy and examined how they are dichotomously regulated by BM niches. Both pHSCs and rHSCs supported long-term hematopoiesis in homeostasis; however, pHSCs were sensitive but rHSCs were resistant to chemotherapy. Surviving rHSCs restored the HSC pool and supported hematopoietic regeneration after chemotherapy. The rHSCs were preferentially maintained in the endosteal region that enriches N-cadherin⁺ (N-cad⁺) bone-lining cells in homeostasis and post-chemotherapy. N-cad⁺ cells were functional bone and marrow stromal progenitor cells (BMSPCs), giving rise to osteoblasts, adipocytes, and chondrocytes in vitro and in vivo. Finally, ablation of N-cad⁺ niche cells or deletion of SCF from N-cad⁺ niche cells impaired rHSC maintenance during homeostasis and regeneration.

Hematopoietic Stem Cells and Their Roles in Tissue Regeneration

Hematopoietic stem cells (HSCs) are regarded as one of essential cell sources for treating regenerative diseases. Among many stem cells, the feasibility of using adult-derived hematopoietic stem cells in therapeutic approaches is very diverse, and is unarguably regarded as an important cell source in stem cell biology. So far, many investigators are exploring HSCs and modified HSCs for use in clinical and basic science. In the present review, we briefly summarized HSCs and their application in pathophysiologic conditions, including non-hematopoietic tissue regeneration as well as blood disorders. HSCs and HSCs-derived progenitors are promising cell sources in regenerative medicine and their contributions can be properly applied to treat pathophysiologic conditions. Among many adult stem cells, HSCs are a powerful tool to treat patients with diseases such as hematologic malignancies and liver disease. Since HSCs can be differentiated into diverse progenitors including endothelial progenitors, they may be useful for constructing strategies for effective therapy.

Haematopoietic stem cell activity and interactions with the niche

The haematopoietic stem cell (HSC) microenvironment in the bone marrow, termed the niche, ensures haematopoietic homeostasis by controlling the proliferation, self-renewal, differentiation and migration of HSCs and progenitor cells at steady state and in response to emergencies and injury. Improved methods for HSC isolation, driven by advances in single-cell and molecular technologies, have led to a better understanding of their behaviour, heterogeneity and lineage fate and of the niche cells and signals that regulate their function. Niche regulatory signals can be in the form of cell-bound or secreted factors and other local physical cues. A combination of technological advances in bone marrow imaging and genetic manipulation of crucial regulatory factors has enabled the identification of several candidate cell types regulating the niche, including both non-haematopoietic (for example, perivascular mesenchymal stem and endothelial cells) and HSC-derived (for example, megakaryocytes, macrophages and regulatory T cells), with better topographical understanding of HSC localization in the bone marrow. Here, we review advances in our understanding of HSC regulation by niches during homeostasis, ageing and cancer, and we discuss their implications for the development of therapies to rejuvenate aged HSCs or niches or to disrupt self-reinforcing malignant niches.

Hypercholesterolemia Accelerates the Aging Phenotypes of Hematopoietic Stem Cells by a Tet1-Dependent Pathway

Hypercholesterolemia accelerates the phenotypes of aging in hematopoietic stem cells (HSCs). As yet, little is known about the underlying mechanism. We found that hypercholesterolemia downregulates Ten eleven translocation 1 (Tet1) in HSCs. The total HSC population was increased, while the long-term (LT) population, side population and reconstitution capacity of HSCs were significantly decreased in Tet1^−/− mice. Expression of the Tet1 catalytic domain in HSCs effectively restored the LT population and reconstitution capacity of HSCs isolated from Tet1^−/− mice. While Tet1 deficiency upregulated the expression of p19 and p21 in HSCs by decreasing the H3K27me3 modification, the restoration of Tet1 activity reduced the expression of p19, p21 and p27 by restoring the H3K27me3 and H3K36me3 modifications on these genes. These results indicate that Tet1 plays a critical role in maintaining the quiescence and reconstitution capacity of HSCs and that hypercholesterolemia accelerates HSC aging phenotypes by decreasing Tet1 expression in HSCs.

Integrative analysis of transcription factors and microRNAs in ovarian cancer cell spheroids

BACKGROUND

Cancer stem cells (CSCs) can self-renew, proliferate into differentiated cells, or enter a quiescent state and are regarded to cause chemoresistance and recurrence. An integrative analysis of transcription factors (TF) and miRNAs was performed in ovarian CSC-enriched spheroid-forming cells (SFCs) to identify factors relevant to ovarian CSCs.

METHODS

Fresh tumor cells from three ovarian cancer patients were cultured in standard and in selective medium. The mRNAs and miRNAs that exhibited significant differential expression between SFCs and adherent cells were identified using mRNA and miRNAs microarrays. Target genes of miRNAs were further selected if predicted with TargetScan by half of the miRNAs or more. Gene enrichment analysis was performed on over- or under-expressed mRNAs and target genes of miRNAs using DAVID tools. Complex regulatory networks were combined from TF-genes and miRNA-genes interactions using the MAGIA webtool.

RESULTS

A total of 1245 mRNA and 55 miRNAs were differentially expressed (p-value< 0.05, paired t-test). Elevation of transcription-related processes and suppression of focal adhesion pathway were noted in SFCs, according to the enrichment analyses. Transcriptional hyperactivity is a known characteristic of the stem cell transcriptome. The integrative network suggested that cell cycle was arrested in SFCs where over-expressed EGR1 and under-expressed MYC and miR-130a-3p had multiple connections with target genes.

CONCLUSIONS

MYC, EGR1, and miR-130a-3p were hubs in our integrative analysis of ovarian CSC-enriched SFCs, suggesting that ovarian cancer SFCs display a stem cell identity with the quiescent phenotype where adhesion- and cell cycle-related genes were suppressed.

Genetic Pathways of Aging and Their Relevance in the Dog as a Natural Model of Human Aging

Aging research has experienced a burst of scientific efforts in the last decades as the growing ratio of elderly people has begun to pose an increased burden on the healthcare and pension systems of developed countries. Although many breakthroughs have been reported in understanding the cellular mechanisms of aging, the intrinsic and extrinsic factors that contribute to senescence on higher biological levels are still barely understood. The dog, Canis familiaris, has already served as a valuable model of human physiology and disease. The possible role the dog could play in aging research is still an open question, although utilization of dogs may hold great promises as they naturally develop age-related cognitive decline, with behavioral and histological characteristics very similar to those of humans. In this regard, family dogs may possess unmatched potentials as models for investigations on the complex interactions between environmental, behavioral, and genetic factors that determine the course of aging. In this review, we summarize the known genetic pathways in aging and their relevance in dogs, putting emphasis on the yet barely described nature of certain aging pathways in canines. Reasons for highlighting the dog as a future aging and gerontology model are also discussed, ranging from its unique evolutionary path shared with humans, its social skills, and the fact that family dogs live together with their owners, and are being exposed to the same environmental effects.

NKAP Regulates Senescence and Cell Death Pathways in Hematopoietic Progenitors

NKAP is a multi-functional nuclear protein that has been shown to be essential for hematopoiesis. Deletion of NKAP in hematopoietic stem cells (HSCs) was previously found to result in rapid lethality and hematopoietic failure. NKAP deficient cells also exhibited diminished proliferation and increased expression of the cyclin dependent kinase inhibitors (CDKIs) p19 Ink4d and p21 Cip1. To determine how dysregulation of CDKI expression contributes to the effects of NKAP deficiency, NKAP was deleted in mice also deficient in p19 Ink4d or p21 Cip1 using poly-IC treatment to induce Mx1-cre. Hematopoietic failure and lethality were not prevented by deficiency in either CDKI when NKAP was deleted. Inducible deletion of NKAP in cultured hematopoietic progenitors ex vivo resulted in a senescent phenotype and altered expression of numerous cell cycle regulators including the CDKI p16 INK4a. Interestingly, while combined deficiency in p16 INK4a and p21 Cip1 did not reverse the effect of NKAP deficiency on hematopoiesis in vivo, it did shift the consequence of NKAP deficiency from senescence to apoptosis in ex vivo cultures. These results suggest that NKAP may limit cellular stress that can trigger cell cycle withdrawal or cell death, a role critical for the maintenance of a viable pool of hematopoietic progenitors.

PPM1K Regulates Hematopoiesis and Leukemogenesis through CDC20-Mediated Ubiquitination of MEIS1 and p21

In addition to acting as building blocks for biosynthesis, amino acids might serve as signaling regulators in various physiological and pathological processes. However, it remains unknown whether amino acid levels affect the activities of hematopoietic stem cells (HSCs). By using a genetically encoded fluorescent sensor of the intracellular levels of branched-chain amino acids (BCAAs), we could monitor the dynamics of BCAA metabolism in HSCs. A mitochondrial-targeted 2C-type Ser/Thr protein phosphatase (PPM1K) promotes the catabolism of BCAAs to maintain MEIS1 and p21 levels by decreasing the ubiquitination-mediated degradation controlled by the E3 ubiquitin ligase CDC20. PPM1K deficiency led to a notable decrease in MEIS1/p21 signaling to reduce the glycolysis and quiescence of HSCs, followed by a severe impairment in repopulation activities. Moreover, the deletion of Ppm1k dramatically extended survival in a murine leukemia model. These findings will enhance the current understanding of nutrient signaling in metabolism and function of stem cells.

The paradox of metabolism in quiescent stem cells

The shift between a proliferating and a nonproliferating state is associated with significant changes in metabolic needs. Proliferating cells tend to have higher metabolic rates, and their metabolic profiles facilitate biosynthesis, as compared to those of nondividing cells of the same sort. Recent studies have elucidated specific molecules that control metabolic changes while cells shift between proliferation and quiescence. Embryonic stem cells, which are rapidly proliferating, tend to have metabolic patterns that are similar to those of nonstem cells in a proliferative state. Moreover, although adult stem cells tend to be quiescent, their metabolic profiles have been reported in multiple organs to more closely resemble those of proliferating than those of nondividing cells in some respects. The findings raise questions about whether there are metabolic profiles that are required for stemness, and whether these profiles relate to the metabolic properties that may be required for quiescence. Here, we review the literature on how metabolism changes upon commitment to proliferation and compare the proliferating and nonproliferating metabolic states of differentiated cells and embryonic and adult stem cells.

The cell cycle in stem cell proliferation, pluripotency and differentiation

Cyclins, cyclin-dependent kinases and other components of the core cell cycle machinery drive cell division. Growing evidence indicates that this machinery operates in a distinct fashion in some mammalian stem cell types, such as pluripotent embryonic stem cells. In this Review, we discuss our current knowledge of how cell cycle proteins mechanistically link cell proliferation, pluripotency and cell fate specification. We focus on embryonic stem cells, induced pluripotent stem cells and embryonic neural stem/progenitor cells.

The Multifaceted p21 (Cip1/Waf1/CDKN1A) in Cell Differentiation, Migration and Cancer Therapy

Loss of cell cycle control is characteristic of tumorigenesis. The protein p21 is the founding member of cyclin-dependent kinase inhibitors and an important versatile cell cycle protein. p21 is transcriptionally controlled by p53 and p53-independent pathways. Its expression is increased in response to various intra- and extracellular stimuli to arrest the cell cycle ensuring genomic stability. Apart from its roles in cell cycle regulation including mitosis, p21 is involved in differentiation, cell migration, cytoskeletal dynamics, apoptosis, transcription, DNA repair, reprogramming of induced pluripotent stem cells, autophagy and the onset of senescence. p21 acts either as a tumor suppressor or as an oncogene depending largely on the cellular context, its subcellular localization and posttranslational modifications. In the present review, we briefly mention the general functions of p21 and summarize its roles in differentiation, migration and invasion in detail. Finally, regarding its dual role as tumor suppressor and oncogene, we highlight the potential, difficulties and risks of using p21 as a biomarker as well as a therapeutic target.

p21 in Cancer Research

p21 functions as a cell cycle inhibitor and anti-proliferative effector in normal cells, and is dysregulated in some cancers. Earlier observations on p21 knockout models emphasized the role of this protein in cell cycle arrest under the p53 transcription factor activity. Although tumor-suppressor function of p21 is the most studied aspect of this protein in cancer, the role of p21 in phenotypic plasticity and its oncogenic/anti-apoptotic function, depending on p21 subcellular localization and p53 status, have been under scrutiny recently. Basic science and translational studies use precision gene editing to manipulate p21 itself, and proteins that interact with it; these studies have led to regulatory/functional/drug sensitivity discoveries as well as therapeutic approaches in cancer field. In this review, we will focus on targeting p21 in cancer research and its potential in providing novel therapies.

IL-1Ra protects hematopoietic cells from chemotoxicity through p53-induced quiescence

The protection of constantly proliferating gut epithelia and hematopoietic tissues from cytotoxicity could improve conventional chemotherapy efficacy and widen its therapeutic window. Previously, we reported that, in mouse models, pretreatment of recombinant human IL-1 receptor antagonist (rhIL-1Ra) protected both types of vulnerable tissues from chemotherapeutics. Here, we showed that rhIL-1Ra treatment up-regulated the protein levels of phosphorylated p38, p53, and p21 and induced transient hematopoietic stem/progenitor cell (HS/PC) quiescence. Knockout of IL-1 receptor I (IL-1RI), p53, or p21 alleles and pharmacological inactivation of p38 mapped the rhIL-1Ra pathway in the induction of HS/PC quiescence. Therefore, rhIL-1Ra administration before but not after chemotherapy alleviated 5-fluorouracil-induced neutropenia. In addition, in vivo and in vitro cell proliferation assays revealed that the rhIL-1Ra treatment did not affect cancer cell proliferation or chemosensitivity. Lastly, we propose an IL-1/IL-1Ra pathway (IL-1RI → p38 → p53 → p21), which regulates HS/PC quiescence. The rhIL-1Ra may provide a new route for p53-based cyclotherapy, which spares normal cells but kills cancer cells during chemotherapy.-Ye, H., Qian, L., Zhu, S., Deng, S., Wang, X., Zhu, J., Chan, G. L., Yu, Y., Han, W. IL-1Ra protects hematopoietic cells from chemotoxicity through p53-induced quiescence.

p21cip1/waf1 Coordinate Autophagy, Proliferation and Apoptosis in Response to Metabolic Stress

Cancer cells possess metabolic properties that are different from benign cells. These unique characteristics have become attractive targets that are being actively investigated for cancer therapy. p21cip1/waf1, also known as Cyclin-Dependent Kinase inhibitor 1A, is encoded by the CDKN1A gene. It is a major p53 target gene involved in cell cycle progression that has been extensively evaluated. To date, p21 has been reported to regulate various cell functions, both dependent and independent of p53. Besides regulating the cell cycle, p21 also modulates apoptosis, induces senescence, and maintains cellular quiescence in response to various stimuli. p21 transcription is induced in response to stresses, including those from oxidative and chemotherapeutic treatment. A recent study has shown that in response to metabolic stresses such as nutrient and energy depletion, p21 expression is induced to regulate various cell functions. Despite the biological significance, the mechanism of p21 regulation in cancer adaptation to metabolic stress is underexplored and thus represents an exciting field. This review focuses on the recent development of p21 regulation in response to metabolic stress and its impact in inducing cell cycle arrest and death in cancer cells.

CABLES1 Deficiency Impairs Quiescence and Stress Responses of Hematopoietic Stem Cells in Intrinsic and Extrinsic Manners

Bone marrow (BM) niche cells help to keep adult hematopoietic stem cells (HSCs) in a quiescent state via secreted factors and induction of cell-cycle inhibitors. Here, we demonstrate that the adapter protein CABLES1 is a key regulator of long-term hematopoietic homeostasis during stress and aging. Young mice lacking Cables1 displayed hyperproliferation of hematopoietic progenitor cells. This defect was cell intrinsic, since it was reproduced in BM transplantation assays using wild-type animals as recipients. Overexpression and short hairpin RNA-mediated depletion of CABLES1 protein resulted in p21^Cip/waf up- and downregulation, respectively. Aged mice lacking Cables1 displayed abnormalities in peripheral blood cell counts accompanied by a significant reduction in HSC compartment, concomitant with an increased mobilization of progenitor cells. In addition, Cables1^−/− mice displayed increased sensitivity to the chemotherapeutic agent 5-fluorouracil due to an abnormal microenvironment. Altogether, our findings uncover a key role for CABLES1 in HSC homeostasis and stress hematopoiesis.

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CABLES1 is expressed in immature hematopoietic progenitor cells and niche cells

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CABLES1 in an intrinsic negative cell-cycle regulator of hematopoietic progenitor cells

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CABLES1 regulates p21^Cip/waf protein levels

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The abnormal stress responses of Cables1^−/− HSC during aging are niche cell dependent

Control of Hematopoietic Stem and Progenitor Cell Function through Epigenetic Regulation of Energy Metabolism and Genome Integrity

It remains largely unclear how stem cells regulate bioenergetics and genome integrity to ensure tissue homeostasis. Here, our integrative gene analyses suggest that metabolic and genotoxic stresses may underlie the common functional defects of both fetal and adult hematopoietic stem and progenitor cells (HSPCs) upon loss of DPY30, an epigenetic modulator that facilitates H3K4 methylation. DPY30 directly regulates expression of several key glycolytic genes, and its loss in HSPCs critically impaired energy metabolism, including both glycolytic and mitochondrial pathways. We also found significant increase in DNA breaks as a result of impaired DNA repair upon DPY30 loss, and inhibition of DNA damage response partially rescued clonogenicity of the DPY30-deficient HSPCs. Moreover, CDK inhibitor p21 was upregulated in DPY30-deficient HSPCs, and p21 deletion alleviated their functional defect. These results demonstrate that epigenetic mechanisms by H3K4 methylation play a crucial role in HSPC function through control of energy metabolism and protecting genome integrity.

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DPY30-deficient fetal and adult HSCs are defective in maintenance and differentiation

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Glycolytic and oxidative metabolism are dysregulated in DPY30-deficient HSCs

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Increase in DNA damage response contributes to dysfunction of DPY30-deficient HSPCs

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P21 increase partially mediates dysfunction of DPY30-deficient HSPCs

Quiescence Entry, Maintenance, and Exit in Adult Stem Cells

Cells of unicellular and multicellular eukaryotes can respond to certain environmental cues by arresting the cell cycle and entering a reversible state of quiescence. Quiescent cells do not divide, but can re-enter the cell cycle and resume proliferation if exposed to some signals from the environment. Quiescent cells in mammals and humans include adult stem cells. These cells exhibit improved stress resistance and enhanced survival ability. In response to certain extrinsic signals, adult stem cells can self-renew by dividing asymmetrically. Such asymmetric divisions not only allow the maintenance of a population of quiescent cells, but also yield daughter progenitor cells. A multistep process of the controlled proliferation of these progenitor cells leads to the formation of one or more types of fully differentiated cells. An age-related decline in the ability of adult stem cells to balance quiescence maintenance and regulated proliferation has been implicated in many aging-associated diseases. In this review, we describe many traits shared by different types of quiescent adult stem cells. We discuss how these traits contribute to the quiescence, self-renewal, and proliferation of adult stem cells. We examine the cell-intrinsic mechanisms that allow establishing and sustaining the characteristic traits of adult stem cells, thereby regulating quiescence entry, maintenance, and exit.

Autophagy mediates serum starvation-induced quiescence in nucleus pulposus stem cells by the regulation of P27

Background

Adult stem cells exist in a quiescent state (G0) within the in vivo niche; the loss of quiescence often leads to a decrease in the number and function of adult stem cells, impairing tissue regeneration and repair. Endogenous repair by nucleus pulposus-derived stem cells has recently shown promising regenerative potential for the treatment of intervertebral disc degeneration (IDD). However, the number and function of nucleus pulposus stem cells (NPSCs) declined throughout the process of IDD. This effect may have a specific relationship with quiescence. However, the biology of the quiescent NPSCs has not been reported.

Methods

First, we established an in vitro model for NPSC quiescence with serum starvation. The induction of G0 was confirmed by flow cytometry analyses of dual staining with Hoechst 33342 and Pyronin Y, immunofluorescent staining with Ki67 and Western blot analysis of P27 expression. NPSCs were cultured under serum starvation conditions for a long time period (21 days). To examine the functional phenotype of quiescent NPSCs, the cells were reactivated with 10% serum and differentiated into osteogenic and chondrogenic lineages in vitro. The number of colony-forming units was also estimated. To elucidate the role of autophagy in the quiescence of NPSCs, we activated and inhibited autophagy in starved cells with rapamycin and chloroquine, respectively. Then, the expression of P27 was evaluated by Western blot analysis, and the immunofluorescence of Ki67 was assessed. Finally, we assessed the role of P27 siRNA in NPSC quiescence by flow cytometry analyses and 5-ethynyl-20-deoxyuridine incorporation assays under normal and serum-starved conditions.

Results

NPSC quiescence was induced by 48 h of serum starvation, and they maintained quiescence for up to 21 days. Upon reactivation with serum, the quiescent NPSCs re-entered the cell cycle and exhibited enhanced clonogenic self-renewal, osteogenic differentiation and chondrogenic differentiation potentials compared to control NPSCs under normal culture conditions. We also found that autophagy underlay serum starvation-induced NPSC quiescence. Further study demonstrated that autophagy mediated the quiescence of NPSCs by regulating P27.

Conclusions

Serum starvation efficiently induces quiescence in NPSCs. Quiescent NPSCs maintain stem cell properties. Our study reveals that autophagy plays a role in maintaining NPSC quiescence and that autophagy mediates the quiescence of NPSCs by regulating P27. We conclude that the induction of quiescence in cultured NPSCs provides a useful model for the analysis of mechanisms that might be relevant to the biology of NPSCs in vivo.

p21CIP1 Promotes Mammary Cancer-Initiating Cells via Activation of Wnt/TCF1/CyclinD1 Signaling

Cancer stem cells (CSC) generate and sustain tumors due to tumor-initiating potential, resulting in recurrence or metastasis. We showed that knockout of the cell-cycle inhibitor, p21CIP1, in the PyMT mammary tumor model inhibits metastasis; however the mechanism remained unknown. Here, we show a pivotal role for p21 in potentiating a cancer stem-like phenotype. p21 knockout in PyMT mammary tumor cells caused dramatic suppression of CSC properties involving tumorsphere formation, ALDH1 activity, and tumor-initiating potential, which were in turn rescued by p21 overexpression into PyMT/p21 knockout cells. Interestingly, p21 knockout dramatically suppresses Wnt/β-catenin signaling activity, leading to striking inhibition of LEF1 and TCF1 expression. TCF1 knockdown in PyMT cells suppressed tumorsphere formation due to Cyclin D1 attenuation. These data demonstrate that p21 promotes a CSC-like phenotype via activation of Wnt/TCF1/Cyclin D1 signaling. IMPLICATIONS: p21 is a strong promoter of mammary CSCs.

Ptpn21 Controls Hematopoietic Stem Cell Homeostasis and Biomechanics

Hematopoietic stem cell (HSC) quiescence is a tightly regulated process crucial for hematopoietic regeneration, which requires a healthy and supportive microenvironmental niche within the bone marrow (BM). Here, we show that deletion of Ptpn21, a protein tyrosine phosphatase highly expressed in HSCs, induces stem cell egress from the niche due to impaired retention within the BM. Ptpn21^-/- HSCs exhibit enhanced mobility, decreased quiescence, increased apoptosis, and defective reconstitution capacity. Ptpn21 deletion also decreased HSC stiffness and increased physical deformability, in part by dephosphorylating Spetin1 (Tyr²⁴⁶), a poorly described component of the cytoskeleton. Elevated phosphorylation of Spetin1 in Ptpn21^-/- cells impaired cytoskeletal remodeling, contributed to cortical instability, and decreased cell rigidity. Collectively, these findings show that Ptpn21 maintains cellular mechanics, which is correlated with its important functions in HSC niche retention and preservation of hematopoietic regeneration capacity.

Metastases in Prostate Cancer

Prostate cancer (PCa) prognosis and clinical outcome is directly dependent on metastatic occurrence. The bone microenvironment is a favorable metastatic niche. Different biological processes have been suggested to contribute to the osteotropism of PCa such as hemodynamics, bone-specific signaling interactions, and the "seed and soil" hypothesis. However, prevalence of disseminating tumor cells in the bone is not proportional to the actual occurrence of metastases, as not all patients will develop bone metastases. The fate and tumor-reforming ability of a metastatic cell is greatly influenced by the microenvironment. In this review, the molecular mechanisms of bone and soft-tissue metastasis in PCa are discussed. Specific attention is dedicated to the residual disease, novel approaches, and animal models used in oncological translational research are illustrated.

Stem Cell Quiescence: Dynamism, Restraint, and Cellular Idling

Stem cells can reside in a state of reversible growth arrest, or quiescence, for prolonged periods of time. Although quiescence has long been viewed as a dormant, low-activity state, increasing evidence suggests that quiescence represents states of poised potential and active restraint, as stem cells "idle" in anticipation of activation, proliferation, and differentiation. Improved understanding of quiescent stem cell dynamics is leading to novel approaches to enhance maintenance and repair of aged or diseased tissues. In this Review, we discuss recent advances in our understanding of stem cell quiescence and techniques enabling more refined analyses of quiescence in vivo.

Cohesin-mediated NF-κB signaling limits hematopoietic stem cell self-renewal in aging and inflammation

Organism aging is characterized by increased inflammation and decreased stem cell function, yet the relationship between these factors remains incompletely understood. This study shows that aged hematopoietic stem and progenitor cells (HSPCs) exhibit increased ground-stage NF-κB activity, which enhances their responsiveness to undergo differentiation and loss of self-renewal in response to inflammation. The study identifies Rad21/cohesin as a critical mediator of NF-κB signaling, which increases chromatin accessibility in the vicinity of NF-κB target genes in response to inflammation. Rad21 is required for normal differentiation, but limits self-renewal of hematopoietic stem cells (HSCs) during aging and inflammation in an NF-κB-dependent manner. HSCs from aged mice fail to down-regulate Rad21/cohesin and inflammation/differentiation signals in the resolution phase of inflammation. Inhibition of cohesin/NF-κB reverts hypersensitivity of aged HSPCs to inflammation-induced differentiation and myeloid-biased HSCs with disrupted/reduced expression of Rad21/cohesin are increasingly selected during aging. Together, Rad21/cohesin-mediated NF-κB signaling limits HSPC function during aging and selects for cohesin-deficient HSCs with myeloid-skewed differentiation.

Constitutive Activation of NF-κB Pathway in Hematopoietic Stem Cells Causes Loss of Quiescence and Deregulated Transcription Factor Networks

Identifying physiological roles of specific signaling pathways that regulate hematopoietic stem cell (HSC) functions may lead to new treatment strategies and therapeutic interventions for hematologic disorders. Here, we provide genetic evidence that constitutive activation of NF-κB in HSCs results in reduced pool size, repopulation capacities, and quiescence of HSCs. Global transcriptional profiling and bioinformatics studies identified loss of ‘stemness’ and ‘quiescence’ signatures in HSCs with deregulated NF-κB activation. In particular, gene set enrichment analysis identified upregulation of cyclin dependent kinase- Ccnd1 and down regulation of cyclin dependent kinase inhibitor p57^kip2. Interestingly, constitutive activation of NF-κB is sufficient to alter the regulatory circuits of transcription factors (TFs) that are critical to HSC self-renewal and functions. Molecular studies identified Junb, as one of the direct targets of NF-κB in hematopoietic cells. In essence, these studies demonstrate that aberrant activation of NF-κB signals impairs HSC quiescence and functions and alters the ‘TF networks’ in HSCs.

The Senescence-Stemness Alliance - A Cancer-Hijacked Regeneration Principle

Activated oncogenes or anticancer therapies evoke senescent cell-cycle arrest in (pre-)malignant cells, thereby interrupting tumor formation or progression. Physiologically, cellular senescence contributes to embryonic development and tissue regeneration. These observations and the overlap of numerous gene products in senescence and stem cell signaling prompted investigations into whether epigenetic establishment of the senescent state may concomitantly reprogram the cell into a latent stem-like condition, whose functional impact becomes evident when arrested cells resume proliferation. We review here recent discoveries underscoring the unexpected senescence-stemness alliance, elucidate underlying molecular mechanisms, and discuss its fundamentally different implications in normal tissue repair - to replenish the exhausted repopulation capacity - as compared to cancer biology, where usurpation of this natural principle accounts for particularly aggressive tumor behavior.

Targeting the Extracellular Signal-Regulated Kinase 5 Pathway to Suppress Human Chronic Myeloid Leukemia Stem Cells

Tyrosine kinase inhibitors (TKi) are effective against chronic myeloid leukemia (CML), but their inefficacy on leukemia stem cells (LSCs) may lead to relapse. To identify new druggable targets alternative to BCR/ABL, we investigated the role of the MEK5/ERK5 pathway in LSC maintenance in low oxygen, a feature of bone marrow stem cell niches. We found that MEK5/ERK5 pathway inhibition reduced the growth of CML patient-derived cells and cell lines in vitro and the number of leukemic cells in vivo. Treatment in vitro of primary CML cells with MEK5/ERK5 inhibitors, but not TKi, strikingly reduced culture repopulation ability (CRA), serial colony formation ability, long-term culture-initiating cells (LTC-ICs), and CD26-expressing cells. Importantly, MEK5/ERK5 inhibition was effective on CML cells regardless of the presence or absence of imatinib, and did not reduce CRA or LTC-ICs of normal CD34+ cells. Thus, targeting MEK/ERK5 may represent an innovative therapeutic approach to suppress CML progenitor/stem cells.

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ERK5 is constitutively active in chronic myeloid leukemia (CML) cells

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ERK5 pathway inhibition reduces the growth of CML cells in vitro and in vivo

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ERK5 pathway inhibition strikingly reduces CML progenitor/stem cell maintenance

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The combination of ERK5i with imatinib reduces the expression of stem cell proteins

Metabolism as master of hematopoietic stem cell fate

HSCs have a fate choice when they divide; they can self-renew, producing new HSCs, or produce daughter cells that will mature to become committed cells. Technical challenges, however, have long obscured the mechanics of these choices. Advances in flow-sorting have made possible the purification of HSC populations, but available HSC-enriched fractions still include substantial heterogeneity, and single HSCs have proven extremely difficult to track and observe. Advances in single-cell approaches, however, have led to the identification of a highly purified population of hematopoietic stem cells (HSCs) that make a critical contribution to hematopoietic homeostasis through a preference for self-renewing division. Metabolic cues are key regulators of this cell fate choice, and the importance of controlling the population and quality of mitochondria has recently been highlighted to maintain the equilibrium of HSC populations. Leukemic cells also demand tightly regulated metabolism, and shifting the division balance of leukemic cells toward commitment has been considered as a promising therapeutic strategy. A deeper understanding of precisely how specific modes of metabolism control HSC fate is, therefore, of great biological interest, and more importantly will be critical to the development of new therapeutic strategies that target HSC division balance for the treatment of hematological disease.

Restricted cell cycle is essential for clonal evolution and therapeutic resistance of pre-leukemic stem cells

Pre-leukemic stem cells (pre-LSCs) give rise to leukemic stem cells through acquisition of additional gene mutations and are an important source of relapse following chemotherapy. We postulated that cell-cycle kinetics of pre-LSCs may be an important determinant of clonal evolution and therapeutic resistance. Using a doxycycline-inducible H2B-GFP transgene in a mouse model of T-cell acute lymphoblastic leukemia to study cell cycle in vivo, we show that self-renewal, clonal evolution and therapeutic resistance are limited to a rare population of pre-LSCs with restricted cell cycle. We show that proliferative pre-LSCs are unable to return to a cell cycle-restricted state. Cell cycle-restricted pre-LSCs have activation of p53 and its downstream cell-cycle inhibitor p21. Furthermore, absence of p21 leads to proliferation of pre-LSCs, with clonal extinction through loss of asymmetric cell division and terminal differentiation. Thus, inducing proliferation of pre-LSCs represents a promising strategy to increase cure rates for acute leukemia.

Parthenolide and DMAPT induce cell death in primitive CML cells through reactive oxygen species

Tyrosine kinase inhibitors (TKI) have become a first-line treatment for chronic myeloid leuakemia (CML). TKIs efficiently target bulk CML cells; however, they are unable to eliminate the leukaemic stem cell (LSC) population that causes resistance and relapse in CML patients. In this study, we assessed the effects of parthenolide (PTL) and dimethyl amino parthenolide (DMAPT), two potent inhibitors of LSCs in acute myeloid leukaemia (AML), on CML bulk and CML primitive (CD34⁺ lin^- ) cells. We found that both agents induced cell death in CML, while having little effect on the equivalent normal hematopoietic cells. PTL and DMAPT caused an increase in reactive oxygen species (ROS) levels and inhibited NF-κB activation. PTL and DMAPT inhibited cell proliferation and induced cell cycle arrest in G₀ and G₂ phases. Furthermore, we found cell cycle inhibition to correlate with down-regulation of cyclin D1 and cyclin A. In summary, our study shows that PTL and DMAPT have a strong inhibitory effect on CML cells. Given that cell cycle arrest was not dependent on ROS induction, we speculate that this effect could be a direct consequence of NF-κB inhibition and if this mechanism was to be evaded, PTL and DMAPT induced cell death would be potentiated.

Id1 Ablation Protects Hematopoietic Stem Cells from Stress-Induced Exhaustion and Aging

Defining mechanisms that maintain tissue stem cells during homeostasis, stress, and aging is important for improving tissue regeneration and repair and enhancing cancer therapies. Here, we show that Id1 is induced in hematopoietic stem cells (HSCs) by cytokines that promote HSC proliferation and differentiation, suggesting that it functions in stress hematopoiesis. Genetic ablation of Id1 increases HSC self-renewal in serial bone marrow transplantation (BMT) assays, correlating with decreases in HSC proliferation, mitochondrial biogenesis, and reactive oxygen species (ROS) production. Id1-/- HSCs have a quiescent molecular signature and harbor less DNA damage than control HSCs. Cytokines produced in the hematopoietic microenvironment after γ-irradiation induce Id1 expression. Id1-/- HSCs display a blunted proliferative response to such cytokines and other inducers of chronic proliferation including genotoxic and inflammatory stress and aging, protecting them from chronic stress and exhaustion. Thus, targeting Id1 may be therapeutically useful for improving HSC survival and function during BMT, chronic stress, and aging.

Quiescence status of glioblastoma stem-like cells involves remodelling of Ca2+ signalling and mitochondrial shape

Quiescence is a reversible cell-cycle arrest which allows cancer stem-like cells to evade killing following therapies. Here, we show that proliferating glioblastoma stem-like cells (GSLCs) can be induced and maintained in a quiescent state by lowering the extracellular pH. Through RNAseq analysis we identified Ca²⁺ signalling genes differentially expressed between proliferating and quiescent GSLCs. Using the bioluminescent Ca²⁺ reporter EGFP-aequorin we observed that the changes in Ca²⁺ homeostasis occurring during the switch from proliferation to quiescence are controlled through store-operated channels (SOC) since inhibition of SOC drives proliferating GSLCs to quiescence. We showed that this switch is characterized by an increased capacity of GSLCs’ mitochondria to capture Ca²⁺ and by a dramatic and reversible change of mitochondrial morphology from a tubular to a donut shape. Our data suggest that the remodelling of the Ca²⁺ homeostasis and the reshaping of mitochondria might favours quiescent GSLCs’ survival and their aggressiveness in glioblastoma.

Controlling Depth of Cellular Quiescence by an Rb-E2F Network Switch

Quiescence is a non-proliferative cellular state that is critical to tissue repair and regeneration. Although often described as the G0 phase, quiescence is not a single homogeneous state. As cells remain quiescent for longer durations, they move progressively deeper and display a reduced sensitivity to growth signals. Deep quiescent cells, unlike senescent cells, can still re-enter the cell cycle under physiological conditions. Mechanisms controlling quiescence depth are poorly understood, representing a currently underappreciated layer of complexity in growth control. Here, we show that the activation threshold of a Retinoblastoma (Rb)-E2F network switch controls quiescence depth. Particularly, deeper quiescent cells feature a higher E2F-switching threshold and exhibit a delayed traverse through the restriction point (R-point). We further show that different components of the Rb-E2F network can be experimentally perturbed, following computer model predictions, to coarse- or fine-tune the E2F-switching threshold and drive cells into varying quiescence depths.