PU.1 promotes cell cycle exit in the murine myeloid lineage associated with downregulation of E2F1

The CSN5/HSF/SPI1/PU.1 Axis Regulates Cell Proliferation in Hypocellular Myelodysplastic Syndrome Patients

OBJECTIVE

This study explored the relationship between the activation of the jak/stat3 signaling pathway and the CSN5 gene transcript and protein expression levels in the hematopoietic stem cells of patients with myelodysplastic syndromes (MDSs). This study also aimed to investigate the correlation between the expression level of CSN5 and the deubiquitination of HSF1, as well as the transcript level of the spi1/pu.1 genes to explore the pathogenesis of MDS.

MATERIALS AND METHODS

We isolated cells from normal individuals and MDS patients, and the mRNA and protein expression levels of spi1/pu.1 in cd34+ cells (hematopoietic stem cells) were measured by PCR and western blotting, respectively. A ChIP assay was used to detect the binding of HSF1 to the spi1/pu.1 promoter in cd34+ cells. The ubiquitination of HSF1 in cd34+ cells was detected by CO-IP. The binding of HSF1 and Fbxw7α was detected in in cd34+ cells by CO-IP. The binding of HSF1 and CSN5 was evaluated. A luciferase reporter assay was used to detect the effect of STAT3 on CSN5 promoter activation in cd34+ cells. Western blotting was used to detect the phosphorylation of STAT3 in cd34+ cells of MDS patients. The binding of STAT3 and C/EBP beta in cd34+ cells was detected by CO-IP.

RESULTS

Inhibition of SPI1/PU.1 expression was observed in MDS samples with low proliferation ability. Further experiments proved that phosphorylation of STAT3 affected CSN5 function and mediated the ubiquitination of HSF, the upstream regulator of SPI1/PU.1 transcription, which led to the inhibition of SPI1/PU.1 expression. Restoration of CSN5 rescued the inhibition of HSF1 ubiquitination, causing SPI1/PU.1 transcription to resume and increasing SPI1/PU.1 expression, promoting the recovery of cell proliferation in hypocellular MDS.

CONCLUSIONS

Our research revealed the regulatory role of the CSN5/HSF/SPI1/PU.1 axis in hypocellular MDS, providing a probable target for clinical intervention.

PU.1 enforces quiescence and limits hematopoietic stem cell expansion during inflammatory stress

Hematopoietic stem cells (HSCs) are capable of entering the cell cycle to replenish the blood system in response to inflammatory cues; however, excessive proliferation in response to chronic inflammation can lead to either HSC attrition or expansion. The mechanism(s) that limit HSC proliferation and expansion triggered by inflammatory signals are poorly defined. Here, we show that long-term HSCs (HSC^LT) rapidly repress protein synthesis and cell cycle genes following treatment with the proinflammatory cytokine interleukin (IL)-1. This gene program is associated with activation of the transcription factor PU.1 and direct PU.1 binding at repressed target genes. Notably, PU.1 is required to repress cell cycle and protein synthesis genes, and IL-1 exposure triggers aberrant protein synthesis and cell cycle activity in PU.1-deficient HSCs. These features are associated with expansion of phenotypic PU.1-deficient HSCs. Thus, we identify a PU.1-dependent mechanism triggered by innate immune stimulation that limits HSC proliferation and pool size. These findings provide insight into how HSCs maintain homeostasis during inflammatory stress.

A role for ATP Citrate Lyase in cell cycle regulation during myeloid differentiation

Differentiation of myeloid progenitor cells into macrophages is accompanied by increased PU.1 concentration and increasing cell cycle length, culminating in cell cycle arrest. Induction of PU.1 expression in a cultured myeloid cell line expressing low PU.1 concentration results in decreased levels of mRNA encoding ATP-Citrate Lyase (ACL) and cell cycle arrest. ACL is an essential enzyme for generating acetyl-CoA, a key metabolite for the first step in fatty acid synthesis and for histone acetylation. We hypothesized that ACL may play a role in cell cycle regulation in the myeloid lineage. In this study, we found that acetyl-CoA or acetate supplementation was sufficient to rescue cell cycle progression in cultured BN cells treated with an ACL inhibitor or induced for PU.1 expression. Acetyl-CoA supplementation was also sufficient to rescue cell cycle progression in BN cells treated with a fatty acid synthase (FASN) inhibitor. We demonstrated that acetyl-CoA was utilized in both fatty acid synthesis and histone acetylation pathways to promote proliferation. Finally, we found that Acly mRNA transcript levels decrease during normal macrophage differentiation from bone marrow precursors. Our results suggest that regulation of ACL activity is a potentially important point of control for cell cycle regulation in the myeloid lineage.

Coordination of Myeloid Differentiation with Reduced Cell Cycle Progression by PU.1 Induction of MicroRNAs Targeting Cell Cycle Regulators and Lipid Anabolism

During macrophage development, myeloid progenitor cells undergo terminal differentiation coordinated with reduced cell cycle progression. Differentiation of macrophages from myeloid progenitors is accompanied by increased expression of the E26 transformation-specific transcription factor PU.1. Reduced PU.1 expression leads to increased proliferation and impaired differentiation of myeloid progenitor cells. It is not understood how PU.1 coordinates macrophage differentiation with reduced cell cycle progression. In this study, we utilized cultured PU.1-inducible myeloid cells to perform genome-wide chromatin immunoprecipitation sequencing (ChIP-seq) analysis coupled with gene expression analysis to determine targets of PU.1 that may be involved in regulating cell cycle progression. We found that genes encoding cell cycle regulators and enzymes involved in lipid anabolism were directly and inducibly bound by PU.1 although their steady-state mRNA transcript levels were reduced. Inhibition of lipid anabolism was sufficient to reduce cell cycle progression in these cells. Induction of PU.1 reduced expression of E2f1, an important activator of genes involved in cell cycle and lipid anabolism, indirectly through microRNA 223. Next-generation sequencing identified microRNAs validated as targeting cell cycle and lipid anabolism for downregulation. These results suggest that PU.1 coordinates cell cycle progression with differentiation through induction of microRNAs targeting cell cycle regulators and lipid anabolism.

PU.1 Regulates Ig Light Chain Transcription and Rearrangement in Pre-B Cells during B Cell Development

B cell development and Ig rearrangement are governed by cell type- and developmental stage-specific transcription factors. PU.1 and Spi-B are E26-transformation-specific transcription factors that are critical for B cell differentiation. To determine whether PU.1 and Spi-B are required for B cell development in the bone marrow, Spi1 (encoding PU.1) was conditionally deleted in B cells by Cre recombinase under control of the Mb1 gene in Spib (encoding Spi-B)-deficient mice. Combined deletion of Spi1 and Spib resulted in a lack of mature B cells in the spleen and a block in B cell development in the bone marrow at the small pre-B cell stage. To determine target genes of PU.1 that could explain this block, we applied a gain-of-function approach using a PU.1/Spi-B-deficient pro-B cell line in which PU.1 can be induced by doxycycline. PU.1-induced genes were identified by integration of chromatin immunoprecipitation-sequencing and RNA-sequencing data. We found that PU.1 interacted with multiple sites in the Igκ locus, including Vκ promoters and regions located downstream of Vκ second exons. Induction of PU.1 induced Igκ transcription and rearrangement. Upregulation of Igκ transcription was impaired in small pre-B cells from PU.1/Spi-B-deficient bone marrow. These studies reveal an important role for PU.1 in the regulation of Igκ transcription and rearrangement and a requirement for PU.1 and Spi-B in B cell development.

Reprogramming of human peripheral blood monocytes to erythroid lineage by blocking of the PU-1 gene expression

In hematopoietic system development, PU.1 and GATA-1 as lineage-specific transcription factors (TF) are expressed in common myeloid progenitors. The cross antagonism between them ascertains gene expression programs of monocytic and erythroid cells, respectively. This concept in transdifferentiation approaches has not been well considered yet, especially in intralineage conversion systems. To demonstrate whether PU.1 suppression induces monocyte lineage conversion into red blood cells, a combination of three PU.1-specific siRNAs was implemented to knock down PU.1 gene expression and generate the balance in favor of GATA-1 expression to induce erythroid differentiation. For this purpose, monocytes were isolated from human peripheral blood and transfected by PU.1 siRNAs. In transfected monocytes, the rate of PU.1 expression in mRNA level was significantly decreased until 0.38 ± 0.118 when compared to untreated monocytes at 72 h (p value ≤0.05) which resulted in significant overexpression of GATA1 of 16.1 ± 0.343-fold compared to the untreated group (p value ≤0.01). Subsequently, overexpression of hemoglobin (α 13.26 ± 1.34-fold; p value≤0.0001) and β-globin (37.55 ± 16.56-fold; p value≤0.0001) was observed when compared to control groups. The results of western immunoblotting confirm those findings too. While, reduced expression of monocyte, CD14 gene, was observed in qRT-PCR and flow cytometry results. Our results suggest that manipulating the ratio of the two TFs in bifurcation differentiation pathways via applying siRNA technology can possibly change the cells' fate as a safe way for therapeutics application.

PU.1 opposes IL-7-dependent proliferation of developing B cells with involvement of the direct target gene bruton tyrosine kinase

Deletion of genes encoding the E26 transformation-specific transcription factors PU.1 and Spi-B in B cells (CD19-CreΔPB mice) leads to impaired B cell development, followed by B cell acute lymphoblastic leukemia at 100% incidence and with a median survival of 21 wk. However, little is known about the target genes that explain leukemogenesis in these mice. In this study we found that immature B cells were altered in frequency in the bone marrow of preleukemic CD19-CreΔPB mice. Enriched pro-B cells from CD19-CreΔPB mice induced disease upon transplantation, suggesting that these were leukemia-initiating cells. Bone marrow cells from preleukemic CD19-CreΔPB mice had increased responsiveness to IL-7 and could proliferate indefinitely in response to this cytokine. Bruton tyrosine kinase (BTK), a negative regulator of IL-7 signaling, was reduced in preleukemic and leukemic CD19-CreΔPB cells compared with controls. Induction of PU.1 expression in cultured CD19-CreΔPB pro-B cell lines induced Btk expression, followed by reduced STAT5 phosphorylation and early apoptosis. PU.1 and Spi-B regulated Btk directly as shown by chromatin immunoprecipitation analysis. Ectopic expression of BTK was sufficient to induce apoptosis in cultured pro-B cells. In summary, these results suggest that PU.1 and Spi-B activate Btk to oppose IL-7 responsiveness in developing B cells.

Concurrence of B-lymphoblastic leukemia and myeloproliferative neoplasm with copy neutral loss of heterozygosity at chromosome 1p harboring a MPL W515S mutation

B-lymphoblastic leukemia (B-ALL) is a neoplasm of precursors committed to B-cell lineage, whereas myeloproliferative neoplasm (MPN) is a clonal proliferation derived from myeloid stem cells. Concurrent B-ALL with MPN is uncommon except in the presence of abnormalities of the PDGFRA, PDGFRB, or FGFR1 genes or the BCR-ABL1 fusion gene. Herein, we describe a rare concurrence, B-ALL with MPN without the aforementioned genetic aberrations, in a 64-year-old male patient. The patient was initially diagnosed with B-ALL with normal karyotype and responded well to aggressive chemotherapy but had sustained leukocytosis and splenomegaly. The posttreatment restaging bone marrow was free of B-ALL but remained hypercellular with myeloid predominance. Using a single nucleotide polymorphism microarray study, we identified a copy neutral loss of heterozygosity at the terminus of 1p in the bone marrow samples taken at diagnosis and again at remission, 49% and 100%, respectively. Several additional genetic abnormalities were present in the initial marrow sample but not in the remission marrow samples. Retrospective molecular studies detected a MPL W515S homozygous mutation in both the initial and remission marrows for B-ALL, at 30-40% and 80% dosage effect, respectively. In summary, we present a case of concurrent B-ALL and MPN and demonstrate a stepwise cytogenetic and molecular approach to the final diagnosis.

New answers to old questions from genome-wide maps of DNA methylation in hematopoietic cells

DNA methylation is a well-studied epigenetic modification essential for efficient cellular differentiation. Aberrant DNA methylation patterns are a characteristic feature of cancer, including myeloid malignancies such as acute myeloid leukemia. Recurrent mutations in DNA-modifying enzymes were identified in acute myeloid leukemia and linked to distinct DNA methylation signatures. In addition, discovery of Tet enzymes provided new mechanisms for the reversal of DNA methylation. Advances in base-resolution profiling of DNA methylation have enabled a more comprehensive understanding of the methylome landscape in the genome. This review will summarize and discuss the key questions in the function of DNA methylation in the hematopoietic system, including where and how DNA methylation regulates diverse biological processes in the genome as elucidated by recent studies.

The genome-wide molecular signature of transcription factors in leukemia

Transcription factors control expression of genes essential for the normal functioning of the hematopoietic system and regulate development of distinct blood cell types. During leukemogenesis, aberrant regulation of transcription factors such as RUNX1, CBFβ, MLL, C/EBPα, SPI1, GATA, and TAL1 is central to the disease. Here, we will discuss the mechanisms of transcription factor deregulation in leukemia and how in recent years next-generation sequencing approaches have helped to elucidate the molecular role of many of these aberrantly expressed transcription factors. We will focus on the complexes in which these factors reside, the role of posttranslational modification of these factors, their involvement in setting up higher order chromatin structures, and their influence on the local epigenetic environment. We suggest that only comprehensive knowledge on all these aspects will increase our understanding of aberrant gene expression in leukemia as well as open new entry points for therapeutic intervention.