Albertus T. J. Wierenga

The p38 MAP kinase inhibitor SB203580 enhances nuclear factor-kappa B transcriptional activity by a non-specific effect upon the ERK pathway.

In the present study we investigated a possible role for the p38 mitogen‐activated protein (MAP) kinase pathway in mediating nuclear factor‐kappa B (NF‐κB) transcriptional activity in the erythroleukaemic cell line TF‐1. TF‐1 cells stimulated with the phosphatase inhibitor okadaic acid (OA) demonstrated enhanced NF‐κB and GAL4p65‐regulated transcriptional activity which was associated with elevated p38 phosphorylation. However, pretreatment with the p38 MAPK specific inhibitor SB203580 (1 μM) or overexpression of kinase‐deficient mutants of MKK3 or MKK6 did not affect OA‐enhanced NF‐κB transcriptional potency, as determined in transient transfection assays. In fact, 5 and 10 μM SB203580 enhanced rather than inhibited NF‐κB‐mediated promoter activity by 2 fold, which was independent of phosphorylation of the p65 subunit. The SB203580‐mediated increase in NF‐κB transcriptional activity was associated with enhanced phosphorylation of extracellular signal‐regulated kinase (ERK)1/2 and c‐Jun N‐terminal kinase (JNK), but not p38 kinase. Overexpression of kinase‐deficient mutants belonging to the ERK1/2, JNK, and p38 pathways showed that only dominant‐negative Raf‐1 abrogated SB203580‐enhanced NF‐κB activity. This would implicate the involvement of the ERK1/2 pathway in the enhancing effects of SB203580 on NF‐κB‐mediated gene transcription. This study demonstrates that the p38 MAP kinase pathway is not involved in the OA‐induced activation of NF‐κB. SB203580 at higher concentrations activates the ERK pathway, which subsequently enhances NF‐κB transcriptional activity.

Maximal STAT5-Induced Proliferation and Self-Renewal at Intermediate STAT5 Activity Levels

ABSTRACT The level of transcription factor activity critically regulates cell fate decisions, such as hematopoietic stem cell (HSC) self-renewal and differentiation. We introduced STAT5A transcriptional activity into human HSCs/progenitor cells in a dose-dependent manner by overexpression of a tamoxifen-inducible STAT5A(1*6)-estrogen receptor fusion protein. Induction of STAT5A activity in CD34+ cells resulted in impaired myelopoiesis and induction of erythropoiesis, which was most pronounced at the highest STAT5A transactivation levels. In contrast, intermediate STAT5A activity levels resulted in the most pronounced proliferative advantage of CD34+ cells. This coincided with increased cobblestone area-forming cell and long-term-culture-initiating cell frequencies, which were predominantly elevated at intermediate STAT5A activity levels but not at high STAT5A levels. Self-renewal of progenitors was addressed by serial replating of CFU, and only progenitors containing intermediate STAT5A activity levels contained self-renewal capacity. By extensive gene expression profiling we could identify gene expression patterns of STAT5 target genes that predominantly associated with a self-renewal and long-term expansion phenotype versus those that identified a predominant differentiation phenotype.

Erythropoietin-induced serine 727 phosphorylation of STAT3 in erythroid cells is mediated by a MEK-, ERK-, and MSK1-dependent pathway

OBJECTIVE Erythropoietin (EPO) is a key regulator of erythropoiesis, playing a role in both the proliferation and differentiation of erythroid cells. One of the signal transduction molecules activated upon EPO stimulation is signal transducer and activator of transcription (STAT) 3. Besides tyrosine 705 phosphorylation of STAT3, serine 727 phosphorylation has been described upon EPO stimulation. In the present study, we investigated which molecular pathways mediate the STAT3 serine 727 phosphorylation and the functional implications of this phosphorylation. METHODS The EPO-dependent erythroid cell line ASE2 was used to investigate which signaling routes were involved in the STAT3 serine 727 phosphorylation. Western blotting using phosphospecific antibodies was used to assess the phosphorylation status of STAT3 molecules. Transfection analysis was performed to investigate the transactivational potential of STAT3, and quantitative RT-PCR was used to study the in vivo gene expression of STAT3-responsive genes. RESULTS Western blotting of extracts of cells exposed to various chemical inhibitors revealed that the MEK inhibitors PD98059 and U0126 abrogated the EPO-mediated STAT3 serine 727 phosphorylation without an effect on tyrosine phosphorylation. Further analysis showed that MSK1 is activated downstream of ERK, and retroviral transductions with kinase-inactive MSK1 revealed that MSK1 is necessary for STAT3 serine phosphorylation. Furthermore, the STAT3-mediated transactivation was reduced by blocking the STAT3 serine phosphorylation with the MEK inhibitor U0126 or by expression of kinase-inactive MSK1. CONCLUSIONS The EPO-induced STAT3 serine 727 phosphorylation is mediated by a pathway involving MEK, ERK, and MSK1. Furthermore, serine phosphorylation of STAT3 augments the transactivational potential of STAT3.

Identification of HIF2 alpha as an important STAT5 target gene in human hematopoietic stem cells

The transcription factor signal transducer and activator of transcription 5 (STAT5) fulfills essential roles in self-renewal in mouse and human hematopoietic stem cells (HSCs), and its persistent activation contributes to leukemic transformation, although little molecular insight into the underlying mechanisms has been obtained. In the present study, we show that STAT5 can impose long-term expansion exclusively on human HSCs, not on progenitors. This was associated with an enhanced cobblestone formation under bone marrow stromal cells of STAT5-transduced HSCs. Hypoxia-induced factor 2α (HIF2α) was identified as a STAT5 target gene in HSCs, and chromatin immunoprecipitation studies revealed STAT5 binding to a site 344 base pairs upstream of the start codon of HIF2α. Lentiviral RNA interference (RNAi)-mediated down-modulation of HIF2α impaired STAT5-induced long-term expansion and HSC frequencies, whereas differentiation was not affected. Glucose uptake was elevated in STAT5-activated HSCs, and several genes associated with glucose metabolism were up-regulated by STAT5 in an HIF2α-dependent manner. Our studies indicate that pathways normally activated under hypoxia might be used by STAT5 under higher oxygen conditions to maintain and/or impose HSC self-renewal properties.

Single-Cell STAT5 Signal Transduction Profiling in Normal and Leukemic Stem and Progenitor Cell Populations Reveals Highly Distinct Cytokine Responses.

Background Signal Transducer and Activator of Transcription 5 (STAT5) plays critical roles in normal and leukemic hematopoiesis. However, the manner in which STAT5 responds to early-acting and lineage-restricted cytokines, particularly in leukemic stem/progenitor cells, is largely unknown. Methodology/Principal Findings We optimized a multiparametric flow cytometry protocol to analyze STAT5 phosphorylation upon cytokine stimulation in stem and progenitor cell compartments at a single-cell level. In normal cord blood (CB) cells, STAT5 phosphorylation was efficiently induced by TPO, IL-3 and GM-CSF within CD34+CD38− hematopoietic stem cells (HSCs). EPO- and SCF-induced STAT5 phosphorylation was largely restricted to the megakaryocyte-erythroid progenitor (MEP) compartment, while G-CSF as well IL-3 and GM-CSF were most efficient in inducing STAT5 phosphorylation in the myeloid progenitor compartments. Strikingly, mobilized adult peripheral blood (PB) CD34+ cells responded much less efficiently to cytokine-induced STAT5 activation, with the exception of TPO. In leukemic stem and progenitor cells, highly distinct cytokine responses were observed, differing significantly from their normal counterparts. These responses could not be predicted by the expression level of cytokine receptors. Also, heterogeneity existed in cytokine requirements for long-term expansion of AML CD34+ cells on stroma. Conclusions/Significance In conclusion, our optimized multiparametric flow cytometry protocols allow the analysis of signal transduction at the single cell level in normal and leukemic stem and progenitor cells. Our study demonstrates highly distinctive cytokine responses in STAT5 phosphorylation in both normal and leukemic stem/progenitor cells.

Convergence of hypoxia and TGFβ pathways on cell cycle regulation in human hematopoietic stem/progenitor cells.

Although it has been shown that HIF1 and 2 fulfill essential roles within the hematopoietic system and in the regulation of HSC fate, little is currently known about the specific mechanisms that are involved. We identified transcriptome changes induced by hypoxia, constitutively active HIF1(P402/564) and HIF2(P405/531) in human cord blood CD34+ cells. Thus, we were able to identify common hypoxia-HIF1-HIF2 gene signatures, but we also identified specific target genes that were exclusively regulated by HIF1, HIF2 or hypoxia. Geneset enrichment analysis (GSEA) revealed that, besides known pathways associated with “hypoxia-induced signaling”, also significant enrichment for the Transforming Growth Factor beta (TGFβ) pathway was observed within the hypoxia/HIF1/HIF2 transcriptomes. One of the most significantly upregulated genes in both gene sets was the cyclin dependent kinase inhibitor CDKN1C (p57kip2). Combined hypoxia treatment or HIF overexpression together with TGFβ stimulation resulted in enhanced expression of CDKN1C and enhanced cell cycle arrest within the CD34+/CD38− stem cell compartment. Interestingly, we observed that CD34+ cells cultured under hypoxic conditions secreted high levels of latent TGFβ, suggesting an auto- or paracrine role of TGFβ in the regulation of quiescence of these cells. However, knockdown of SMAD4 could not rescue the hypoxia induced cell cycle arrest, arguing against direct effects of hypoxia-induced secreted TGFβ. Finally, the Gα-coupled receptor GTPase RGS1 was identified as a HIF-dependent hypoxia target that dampens SDF1-induced migration and signal transduction in human CD34+ stem/progenitor cells.

Autophagy proteins ATG5 and ATG7 are essential for the maintenance of human CD34(+) hematopoietic stem-progenitor cells

Autophagy is a highly regulated catabolic process that involves sequestration and lysosomal degradation of cytosolic components such as damaged organelles and misfolded proteins. While autophagy can be considered to be a general cellular housekeeping process, it has become clear that it may also play cell type‐dependent functional roles. In this study, we analyzed the functional importance of autophagy in human hematopoietic stem/progenitor cells (HSPCs), and how this is regulated during differentiation. Western blot‐based analysis of LC3‐II and p62 levels, as well as flow cytometry‐based autophagic vesicle quantification, demonstrated that umbilical cord blood‐derived CD34+/CD38− immature hematopoietic progenitors show a higher autophagic flux than CD34+/CD38+ progenitors and more differentiated myeloid and erythroid cells. This high autophagic flux was critical for maintaining stem and progenitor function since knockdown of autophagy genes ATG5 or ATG7 resulted in reduced HSPC frequencies in vitro as well as in vivo. The reduction in HSPCs was not due to impaired differentiation, but at least in part due to reduced cell cycle progression and increased apoptosis. This is accompanied by increased expression of p53, proapoptotic genes BAX and PUMA, and the cell cycle inhibitor p21, as well as increased levels of cleaved caspase‐3 and reactive oxygen species. Taken together, our data demonstrate that autophagy is an important regulatory mechanism for human HSCs and their progeny, reducing cellular stress and promoting survival. Stem Cells 2016;34:1651–1663

Down-regulation of GATA1 uncouples STAT5-induced erythroid differentiation from stem/progenitor cell proliferation

Previously, we have shown that overexpression of an activated mutant of signal transducer and activator of transcription-5 (STAT5) induces erythropoiesis, impaired myelopoiesis, and an increase in long-term proliferation of human hematopoietic stem/progenitor cells. Because GATA1 is a key transcription factor involved in erythropoiesis, the involvement of GATA1 in STAT5-induced phenotypes was studied by shRNA-mediated knockdown of GATA1. CD34(+) cord blood cells were double transduced with a conditionally active STAT5 mutant and a lentiviral vector expressing a short hairpin against GATA1. Erythropoiesis was completely abolished in the absence of GATA1, indicating that STAT5-induced erythropoiesis is GATA1-dependent. Furthermore, the impaired myelopoiesis in STAT5-transduced cells was restored by GATA1 knockdown. Interestingly, early cobblestone formation was only modestly affected, and long-term growth of STAT5-positive cells was increased in the absence of GATA1, whereby high progenitor numbers were maintained. Thus, GATA1 down-regulation allowed the dissection of STAT5-induced differentiation phenotypes from the effects on long-term expansion of stem/progenitor cells. Gene expression profiling allowed the identification of GATA1-dependent and GATA1-independent STAT5 target genes, and these studies revealed that several proliferation-related genes were up-regulated by STAT5 independent of GATA1, whereas several erythroid differentiation-related genes were found to be GATA1 as well as STAT5 dependent.

Tight Control of STAT5 Activity Determines Human CD34-Derived Interstitial Dendritic Cell and Langerhans Cell Development

Despite the crucial function of dendritic cells (DC) in immunity, the molecular mechanisms regulating human DC development remain poorly defined. STAT5 regulates various hematopoietic lineages and is activated by GM-CSF, a critical cytokine in DC development. In this study, we investigated the role of STAT5 during differentiation of human CD34+ hematopoietic progenitors into precursor DC (pre-DC) and their subsequent differentiation toward interstitial DC and Langerhans cells. Inhibiting STAT5 activity by dominant-negative STAT5 promoted Langerhans cell commitment of hematopoietic progenitors but resulted in loss of pre-interstitial DC development, showing subset-specific regulation. Increasing the low endogenous STAT5 activity by ectopic STAT5 activation downregulated expression of the critical DC transcription factor PU.1 and abrogated commitment to either DC lineage. In contrast, high STAT5 activity was beneficial in already committed pre-DC: terminal DC differentiation was associated with increased endogenous STAT5 phosphorylation levels, JAK2–STAT5 inhibition reduced terminal DC differentiation, and conditional STAT5 activation in pre-DC improved development of BDCA-1+, DC-SIGN+, and Langerin+ DC with normal maturation and T cell stimulation. These data show that STAT5 critically regulates human DC development, with specific requirements for the level of STAT5 activation at distinct differentiation stages. By regulating STAT5 activity, cytokines present at specific locations and under different pathophysiological conditions can determine the fate of DC precursors.

STAT5-mediated self-renewal of normal hematopoietic and leukemic stem cells

The level of transcription factor activity critically regulates cell fate decisions such as hematopoietic stem cell self-renewal and differentiation. The balance between hematopoietic stem cell self-renewal and differentiation needs to be tightly controlled, as a shift toward differentiation might exhaust the stem cell pool, while a shift toward self-renewal might mark the onset of leukemic transformation. A number of transcription factors have been proposed to be critically involved in governing stem cell fate and lineage commitment, such as Hox transcription factors, c-Myc, Notch1, β-catenin, C/ebpα, Pu.1 and STAT5. It is therefore no surprise that dysregulation of these transcription factors can also contribute to the development of leukemias. This review will discuss the role of STAT5 in both normal and leukemic hematopoietic stem cells as well as mechanisms by which STAT5 might contribute to the development of human leukemias.