The Spi1/PU.1 transcription factor accelerates replication fork progression by increasing PP1 phosphatase in leukemia

Oncogenes trigger replicative stress that can lead to genetic instability, which participates in cancer progression. Thus, determining how cells cope with replicative stress can help our understanding of oncogenesis and lead to the identification of new antitumor treatment targets. We previously showed that constitutive overexpression of the oncogenic transcription factor Spi1/PU.1 leads to pre-leukemic cells that have a shortened S phase duration with an increased replication fork speed and increased mutability in the absence of DNA breaks. Here, we demonstrate that the S phase checkpoint protein CHK1 is maintained in a low phosphorylation state in Spi1/PU.1-overexpressing cells and provide evidence that this is not due to negative control of its primary kinase ATR. Notably, we found that the expression of the CHK1 phosphatase PP1α is increased in Spi1/PU.1-overexpressing cells. By exogenously modulating its activity, we demonstrate that PP1α is required to maintain CHK1 in a dephosphorylated state and, more importantly, that it is responsible for the accelerated replication fork progression in Spi1/PU.1-overexpressing cells. These results identify a novel pathway by which an oncogene influences replication in the absence of DNA damage.

FOXO3 Transcription Factor Is Essential for Protecting Hematopoietic Stem and Progenitor Cells from Oxidative DNA Damage

Accumulation of damaged DNA in hematopoietic stem cells (HSC) is associated with chromosomal abnormalities, genomic instability, and HSC aging and might promote hematological malignancies with age. Despite this, the regulatory pathways implicated in the HSC DNA damage response have not been fully elucidated. One of the sources of DNA damage is reactive oxygen species (ROS) generated by both exogenous and endogenous insults. Balancing ROS levels in HSC requires FOXO3, which is an essential transcription factor for HSC maintenance implicated in HSC aging. Elevated ROS levels result in defective Foxo3^-/- HSC cycling, among many other deficiencies. Here, we show that loss of FOXO3 leads to the accumulation of DNA damage in primitive hematopoietic stem and progenitor cells (HSPC), associated specifically with reduced expression of genes implicated in the repair of oxidative DNA damage. We provide further evidence that Foxo3^-/- HSPC are defective in DNA damage repair. Specifically, we show that the base excision repair pathway, the main pathway utilized for the repair of oxidative DNA damage, is compromised in Foxo3^-/- primitive hematopoietic cells. Treating mice in vivo with N-acetylcysteine reduces ROS levels, rescues HSC cycling defects, and partially mitigates HSPC DNA damage. These results indicate that DNA damage accrued as a result of elevated ROS in Foxo3^-/- mutant HSPC is at least partially reversible. Collectively, our findings suggest that FOXO3 serves as a protector of HSC genomic stability and health.

Evidence for AKT-independent regulation of FOXO1 and FOXO3 in haematopoietic stem and progenitor cells

Transcription factors FOXOs (1, 3, 4) are essential for the maintenance of haematopoietic stem cells. FOXOs are evolutionary conserved substrates of the AKT serine threonine protein kinase that are also phosphorylated by several kinases other than AKT. Specifically, phosphorylation by AKT is known to result in the cytosolic localization of FOXO and subsequent inhibition of FOXO transcriptional activity. In addition to phosphorylation, FOXOs are regulated by a number of other post-translational modifications including acetylation, methylation, redox modulation, and ubiquitination that altogether determine these factors' output. Cumulating evidence raises the possibility that in stem cells, including in haematopoietic stem cells, AKT may not be the dominant regulator of FOXO. To address this question in more detail, we examined gene expression, subcellular localization, and response to AKT inhibition of FOXO1 and FOXO3, the main FOXO expressed in HSPCs (haematopoietic stem and progenitor cells). Here we show that while FOXO1 and FOXO3 transcripts are expressed at similar levels, endogenous FOXO3 protein is mostly nuclear compared to the cytoplasmic localization of FOXO1 in HSPCs. Furthermore, inhibition of AKT does not enhance nuclear localization of FOXO1 nor FOXO3. Nonetheless AKT inhibition in the context of loss of NAD-dependent SIRT1 deacetylase modulates FOXO3 localization in HSPCs. Together, these data suggest that FOXO3 is more active than FOXO1 in primitive haematopoietic stem and multipotent progenitor cells. In addition, they indicate that upstream regulators other than AKT, such as SIRT1, maintain nuclear FOXO localization and activity in HSPCs.

Mitochondrial metabolism in hematopoietic stem cells requires functional FOXO3

Hematopoietic stem cells (HSC) are primarily dormant but have the potential to become highly active on demand to reconstitute blood. This requires a swift metabolic switch from glycolysis to mitochondrial oxidative phosphorylation. Maintenance of low levels of reactive oxygen species (ROS), a by-product of mitochondrial metabolism, is also necessary for sustaining HSC dormancy. Little is known about mechanisms that integrate energy metabolism with hematopoietic stem cell homeostasis. Here, we identify the transcription factor FOXO3 as a new regulator of metabolic adaptation of HSC. ROS are elevated in Foxo3(-/-) HSC that are defective in their activity. We show that Foxo3(-/-) HSC are impaired in mitochondrial metabolism independent of ROS levels. These defects are associated with altered expression of mitochondrial/metabolic genes in Foxo3(-/-) hematopoietic stem and progenitor cells (HSPC). We further show that defects of Foxo3(-/-) HSC long-term repopulation activity are independent of ROS or mTOR signaling. Our results point to FOXO3 as a potential node that couples mitochondrial metabolism with HSC homeostasis. These findings have critical implications for mechanisms that promote malignant transformation and aging of blood stem and progenitor cells.

Resveratrol increases the bone marrow hematopoietic stem and progenitor cell capacity

Resveratrol is a plant-derived polyphenol that has shown protective effects against many disorders including, several types of cancers and other age-associated diseases as well as blood disorders in cultured cells and/or animal models. However, whether resveratrol has any impact specifically on normal blood stem cells remains unknown. Here, we show that a 3-week treatment of resveratrol increases the frequency and total numbers of normal bone marrow hematopoietic stem cells (HSC) without any impact on their competitive repopulation capacity. In addition, we show that resveratrol enhances the bone marrow multipotent progenitor capacity in vivo. These results have therapeutic value for disorders of hematopoietic stem and progenitor cells (HSPC) as well as for bone marrow transplantation settings.

FOXO3-mTOR metabolic cooperation in the regulation of erythroid cell maturation and homeostasis

Ineffective erythropoiesis is observed in many erythroid disorders including β-thalassemia and anemia of chronic disease in which increased production of erythroblasts that fail to mature exacerbate the underlying anemias. As loss of the transcription factor FOXO3 results in erythroblast abnormalities similar to the ones observed in ineffective erythropoiesis, we investigated the underlying mechanisms of the defective Foxo3(-/-) erythroblast cell cycle and maturation. Here we show that loss of Foxo3 results in overactivation of the JAK2/AKT/mTOR signaling pathway in primary bone marrow erythroblasts partly mediated by redox modulation. We further show that hyperactivation of mTOR signaling interferes with cell cycle progression in Foxo3 mutant erythroblasts. Importantly, inhibition of mTOR signaling, in vivo or in vitro enhances significantly Foxo3 mutant erythroid cell maturation. Similarly, in vivo inhibition of mTOR remarkably improves erythroid cell maturation and anemia in a model of β-thalassemia. Finally we show that FOXO3 and mTOR are likely part of a larger metabolic network in erythroblasts as together they control the expression of an array of metabolic genes some of which are implicated in erythroid disorders. These combined findings indicate that a metabolism-mediated regulatory network centered by FOXO3 and mTOR control the balanced production and maturation of erythroid cells. They also highlight physiological interactions between these proteins in regulating erythroblast energy. Our results indicate that alteration in the function of this network might be implicated in the pathogenesis of ineffective erythropoiesis.

Aging-like phenotype and defective lineage specification in SIRT1-deleted hematopoietic stem and progenitor cells

Aging hematopoietic stem cells (HSCs) exhibit defective lineage specification that is thought to be central to increased incidence of myeloid malignancies and compromised immune competence in the elderly. Mechanisms underlying these age-related defects remain largely unknown. We show that the deacetylase Sirtuin (SIRT)1 is required for homeostatic HSC maintenance. Differentiation of young SIRT1-deleted HSCs is skewed toward myeloid lineage associated with a significant decline in the lymphoid compartment, anemia, and altered expression of associated genes. Combined with HSC accumulation of damaged DNA and expression patterns of age-linked molecules, these have striking overlaps with aged HSCs. We further show that SIRT1 controls HSC homeostasis via the longevity transcription factor FOXO3. These findings suggest that SIRT1 is essential for HSC homeostasis and lineage specification. They also indicate that SIRT1 might contribute to delaying HSC aging.

Resveratrol accelerates erythroid maturation by activation of FoxO3 and ameliorates anemia in beta-thalassemic mice

Resveratrol, a polyphenolic-stilbene, has received increased attention in the last decade due to its wide range of biological activities. Beta(β)-thalassemias are inherited red cell disorders, found worldwide, characterized by ineffective erythropoiesis and red cell oxidative damage with reduced survival. We evaluated the effects of low-dose-resveratrol (5 μM) on in vitro human erythroid differentiation of CD34(+) from normal and β-thalassemic subjects. We found that resveratrol induces accelerated erythroid-maturation, resulting in the reduction of colony-forming units of erythroid cells and increased intermediate and late erythroblasts. In sorted colony-forming units of erythroid cells resveratrol activates Forkhead-box-class-O3, decreases Akt activity and up-regulates anti-oxidant enzymes as catalase. In an in vivo murine model for β-thalassemia, resveratrol (2.4 mg/kg) reduces ineffective erythropoiesis, increases hemoglobin levels, reduces reticulocyte count and ameliorates red cell survival. In both wild-type and β-thalassemic mice, resveratrol up-regulates scavenging enzymes such as catalase and peroxiredoxin-2 through Forkhead-box-class-O3 activation. These data indicate that resveratrol inhibits Akt resulting in FoxO3 activation with upregulation of cytoprotective systems enabling the pathological erythroid precursors to resist the oxidative damage and continue to differentiate. Our data suggest that the dual effect of resveratrol on erythropoiesis through activation of FoxO3 transcriptional factor combined with the amelioration of oxidative stress in circulating red cells may be considered as a potential novel therapeutic strategy in treating β-thalassemia.

Spi-1/PU.1 activates transcription through clustered DNA occupancy in erythroleukemia

Acute leukemias are characterized by deregulation of transcriptional networks that control the lineage specificity of gene expression. The aberrant overexpression of the Spi-1/PU.1 transcription factor leads to erythroleukemia. To determine how Spi-1 mechanistically influences the transcriptional program, we combined a ChIP-seq analysis with transcriptional profiling in cells from an erythroleukemic mouse model. We show that Spi-1 displays a selective DNA-binding that does not often cause transcriptional modulation. We report that Spi-1 controls transcriptional activation and repression partially through distinct Spi-1 recruitment to chromatin. We revealed several parameters impacting on Spi-1-mediated transcriptional activation. Gene activation is facilitated by Spi-1 occupancy close to transcriptional starting site of genes devoid of CGIs. Moreover, in those regions Spi-1 acts by binding to multiple motifs tightly clustered and with similar orientation. Finally, in contrast to the myeloid and lymphoid B cells in which Spi-1 exerts a physiological activity, in the erythroleukemic cells, lineage-specific cooperating factors do not play a prevalent role in Spi-1-mediated transcriptional activation. Thus, our work describes a new mechanism of gene activation through clustered site occupancy of Spi-1 particularly relevant in regard to the strong expression of Spi-1 in the erythroleukemic cells.

FOXO1 is an essential regulator of pluripotency in human embryonic stem cells

Pluripotency of embryonic stem cells (ESCs) is defined by their ability to differentiate into three germ layers and derivative cell types and is established by an interactive network of proteins including OCT4 (also known as POU5F1; ref. 4), NANOG (refs 5, 6), SOX2 (ref. 7) and their binding partners. The forkhead box O (FoxO) transcription factors are evolutionarily conserved regulators of longevity and stress response whose function is inhibited by AKT protein kinase. FoxO proteins are required for the maintenance of somatic and cancer stem cells; however, their function in ESCs is unknown. We show that FOXO1 is essential for the maintenance of human ESC pluripotency, and that an orthologue of FOXO1 (Foxo1) exerts a similar function in mouse ESCs. This function is probably mediated through direct control by FOXO1 of OCT4 and SOX2 gene expression through occupation and activation of their respective promoters. Finally, AKT is not the predominant regulator of FOXO1 in human ESCs. Together these results indicate that FOXO1 is a component of the circuitry of human ESC pluripotency. These findings have critical implications for stem cell biology, development, longevity and reprogramming, with potentially important ramifications for therapy.

Spi-1/PU.1 oncogene accelerates DNA replication fork elongation and promotes genetic instability in the absence of DNA breakage

The multistage process of cancer formation is driven by the progressive acquisition of somatic mutations. Replication stress creates genomic instability in mammals. Using a well-defined multistep leukemia model driven by Spi-1/PU.1 overexpression in the mouse and Spi-1/PU.1-overexpressing human leukemic cells, we investigated the relationship between DNA replication and cancer progression. Here, using DNA molecular combing and flow cytometry methods, we show that Spi-1 increases the speed of replication by acting specifically on elongation rather than enhancing origin firing. This shortens the S-phase duration. Combining data from Spi-1 knockdown in murine cells with Spi-1 overexpression in human cells, we provide evidence that inappropriate Spi-1 expression is directly responsible for the replication alteration observed. Importantly, the acceleration of replication progression coincides with an increase in the frequency of genomic mutations without inducing DNA breakage. Thus, we propose that the hitherto unsuspected role for spi-1 oncogene in promoting replication elongation and genomic mutation promotes blastic progression during leukemic development.

Spi-1/PU.1 participates in erythroleukemogenesis by inhibiting apoptosis in cooperation with Epo signaling and by blocking erythroid differentiation

Overexpression of the transcription factor Spi-1/PU.1 in mice leads to acute erythroleukemia characterized by a differentiation block at the proerythroblastic stage. In this study, we made use of a new cellular system allowing us to reach graded expression of Spi-1 in preleukemic cells to dissect mechanisms of Spi-1/ PU-1 in erythroleukemogenesis. This system is based on conditional production of 1 or 2 spi-1-interfering RNAs stably inserted into spi-1 transgenic proerythroblasts. We show that Spi-1 knock-down was sufficient to reinstate the erythroid differentiation program. This differentiation process was associated with an exit from the cell cycle. Evidence is provided that in the presence of erythropoietin (Epo), Spi-1 displays an antiapoptotic role that is independent of its function in blocking erythroid differentiation. Apoptosis inhibited by Spi-1 did not involve activation of the Fas/FasL signaling pathway nor a failure to activate Epo receptor (EpoR). Furthermore, we found that reducing the Spi-1 level yields to ERK dephosphorylation and increased phosphorylation of AKT and STAT5, suggesting that Spi-1 may affect major signaling pathways downstream of the EpoR in erythroid cells. These findings reveal 2 distinct roles for Spi-1 during erythroleukemogenesis: Spi-1 blocks the erythroid differentiation program and acts to impair apoptotic death in cooperation with an Epo signaling.