RUNX1 suppression induces megakaryocytic differentiation of UT-7/GM cells

Megakaryopoiesis and platelet production: insight into hematopoietic stem cell proliferation and differentiation

Hematopoietic stem cells (HSCs) undergo successive lineage commitment steps to generate megakaryocytes (MKs) in a process referred to as megakaryopoiesis. MKs undergo a unique differentiation process involving endomitosis to eventually produce platelets. Many transcription factors participate in the regulation of this complex progress. Chemokines and other factors in the microenvironment where megakaryopoiesis and platelet production occur play vital roles in the regulation of HSC lineage commitment and MK maturation; among these factors, thrombopoietin (TPO) is the most important. Endomitosis is a vital process of MK maturation, and granules that are formed in MKs are important for platelet function. Proplatelets are firstly generated from mature MKs and then become platelets. The proplatelet production process was verified by novel studies that revealed that the mechanism is partially regulated by the invaginated membrane system (IMS), microtubules and Rho GTPases. The extracellular matrices (ECMs) and shear stress also affect and regulate the process while the mature MKs migrate from the marrow to the sub-endothelium region near the venous sinusoids leading to the release of platelets into the circulation. This review describes the entire process of megakaryopoiesis in detail, illustrates both the transcriptional and microenvironmental regulation of MKs and provides insight into platelet biogenesis.

RUNX1, but not its familial platelet disorder mutants, synergistically activates PF4 gene expression in combination with ETS family proteins

BACKGROUND

Familial platelet disorder (FPD) is a rare autosomal dominant disease characterized by thrombocytopenia and abnormal platelet function. Causal mutations have been identified in the gene encoding runt-related transcription factor 1 (RUNX1) of FPD patients.

OBJECTIVES

To elucidate the role of RUNX1 in the regulation of expression of platelet factor 4 (PF4) and to propose a plausible mechanism underlying RUNX1-mediated induction of the FPD phenotype.

METHODS

We assessed whether RUNX1 and its mutants, in combination with E26 transformation-specific-1 (ETS-1), Core-binding factor subunit beta (CBFβ), and Friend leukemia virus integration 1 (FLI-1), cooperatively regulate PF4 expression during megakaryocytic differentiation. In an embryonic stem cell differentiation system, expression levels of endogenous and exogenous RUNX1 and PF4 were determined by real-time RT-PCR. Promoter activation by the transcription factors were evaluated by reporter gene assays with HepG2 cells. DNA binding activity and protein interaction were analyzed by electrophoretic mobility shift assay and immunoprecipitation assay with Cos-7 cells, respectively. Protein localization was analyzed by immunocytochemistry and Western blotting with Cos-7 cells.

RESULTS

We demonstrated that RUNX1 activates endogenous PF4 expression in megakaryocytic differentiation. RUNX1, but not its mutants, in combination with ETS-1 and CBFβ, or FLI-1, synergistically activated the PF4 promoter. Each RUNX1 mutant harbors various functional abnormalities, including loss of DNA-binding activity, abnormal subcellular localization, and/or alterations of binding affinities for ETS-1, CBFβ, and FLI-1.

CONCLUSIONS

RUNX1, but not its mutants, strongly and synergistically activates PF4 expression along with ETS family proteins. Furthermore, loss of the RUNX1 transcriptional activation function is induced by various functional abnormalities.

PEAR1 attenuates megakaryopoiesis via control of the PI3K/PTEN pathway

Platelet endothelial aggregation receptor-1 (PEAR1) participates in platelet aggregation via sustaining αIIbβ3 activation. To investigate the role of PEAR1 in platelet formation, we monitored and manipulated PEAR1 expression in vitro in differentiating human CD34(+) hematopoietic stem cells and in vivo in zebrafish embryos. PEAR1 expression rose during CD34(+) cell differentiation up to megakaryocyte (MK) maturation. Two different lentiviral short hairpin knockdowns of PEAR1 did not affect erythropoiesis in CD34(+) cells, but increased colony-forming unit MK cell numbers twofold vs control in clonogenic assays, without substantially modifying MK maturation. The PEAR1 knockdown resulted in a twofold reduction of the phosphatase and TENsin homolog (PTEN) phosphatase expression and modulated gene expression of several phosphatidylinositol 3-kinase (PI3K)-Akt and Notch pathway genes. In zebrafish, Pear1 expression increased progressively during the first 3 days of embryo development. Both ATG and splice-blocking PEAR1 morpholinos enhanced thrombopoiesis, without affecting erythropoiesis. Western blots of 3-day-old Pear1 knockdown zebrafish revealed elevated Akt phosphorylation, coupled to transcriptional downregulation of the PTEN isoform Ptena. Neutralization by morpholinos of Ptena, but not of Ptenb, phenocopied the Pear1 zebrafish knockdown and triggered enhanced Akt phosphorylation and thrombocyte formation. In summary, this is the first demonstration that PEAR1 influences the PI3K/PTEN pathway, a critical determinant of Akt phosphorylation, itself controlling megakaryopoiesis and thrombopoiesis.

Methods for genetic modification of megakaryocytes and platelets

During recent decades there have been major advances in the fields of thrombosis and haemostasis, in part through development of powerful molecular and genetic technologies. Nevertheless, genetic modification of megakaryocytes and generation of mutant platelets in vitro remains a highly specialized area of research. Developments are hampered by the low frequency of megakaryocytes and their progenitors, a poor efficiency of transfection and a lack of understanding with regard to the mechanism by which megakaryocytes release platelets. Current methods used in the generation of genetically modified megakaryocytes and platelets include mutant mouse models, cell line studies and use of viruses to transform primary megakaryocytes or haematopoietic precursor cells. This review summarizes the advantages, limitations and technical challenges of such methods, with a particular focus on recent successes and advances in this rapidly progressing field including the potential for use in gene therapy for treatment of patients with platelet disorders.

The symplekin/ZONAB complex inhibits intestinal cell differentiation by the repression of AML1/Runx1

BACKGROUND & AIMS

Symplekin is a ubiquitously expressed protein involved in RNA polyadenylation and transcriptional regulation that localizes at tight junctions in epithelial cells. The association between symplekin and the Y-box transcription factor ZONAB activates proliferation in intestinal and kidney cells. We analyzed symplekin expression in human colonic crypts and investigated its function in differentiation.

METHODS

Expression of differentiation markers and transcription factors was assessed in HT29-Cl.16E cells that expressed inducible symplekin short hairpin RNA or were transfected with ZONAB small interfering RNAs. Intestines of AML1(Delta/Delta) mice were stained with alcian blue and analyzed for expression of AML1/Runx1, GAPDH, KLF-4, and Muc-2. Mobility shift and chromatin immunoprecipitation were used to detect AML1 and ZONAB/DbpA binding to promoter regions of the Krüppel-like factor 4 (KLF4) and acute myeloid leukemia-1 (AML1) genes, respectively.

RESULTS

The gradient of nuclear symplekin expression decreased from the proliferative toward the differentiated compartment of colonic crypts; symplekin down-regulation promoted the differentiation of HT29-Cl.16E colorectal carcinoma cells into goblet cells. Down-regulation of symplekin or ZONAB/Dbpa induced de novo expression of the transcription factor AML1/Runx1, thereby increasing the expression of KLF4 and promoting goblet cell differentiation. Furthermore, increased AML1 expression was required for the induction of goblet cell differentiation after symplekin down-regulation. KLF4 expression and goblet cell numbers were reduced in the intestines of AML1(Delta/Delta) mice, confirming the role of AML1 as a promoter of intestinal differentiation in vivo.

CONCLUSIONS

Symplekin cooperates with ZONAB to negatively regulate intestinal goblet cell differentiation, acting by repression of AML1 and KLF4.

Overexpression or knock-down of runt-related transcription factor 1 affects BCR-ABL-induced proliferation and migration in vitro and leukemogenesis in vivo in mice

BACKGROUND

Runt-related transcription factor 1 (Runx1) plays a crucial role in hematogenesis and its dysfunction may contribute to leukemogenesis. However, it is not clear whether or not abnormal expression of Runx1 will induce leukemia and how the change of Runx1 expression level could affect BCR-ABL-induced leukemogenesis. In the present study, we aimed to analyze if abnormal expression of Runx1 in BaF3 cells alone would induce leukemogenesis. And we also wanted to know if abnormal expression of Runx1 in leukemic cells would affect leukemogenesis. Furthermore, we investigated whether overexpression or knock-down of Runx1 in BaF3 cells would induce leukemogenesis.

METHODS

Plasmids containing full-length Runx1 cDNA were transduced into BaF3 cells and BaF3-P185wt cells (BCR-ABL transformed BaF3 cells) by electroporation. Plasmids containing a short hairpin RNA of Runx1 were transduced into BaF3 cells and BaF3-P185wt cells by electroporation. Runx1 expression level was quantified by Western blotting and quantitative real-time PCR. The effects of overexpression or knock-down of Runx1 on proliferation, apoptosis and migration of cells were detected in vitro. Then, using MSCV-P185wt-EGFP as a control, we transplanted MSCV-P185wt-Runx1 cells or MSCV-P185wt-shRNA cells into Balb/c mice through tail vein and observed tumorgenesis of the different phenotypes.

RESULTS

In vitro analysis revealed that overexpression of Runx1 in P185wt cells could inhibit cell proliferation and slow down cell migration; while knock-down of Runx1 could promote cell proliferation and speed up cell migration. In vivo analysis indicated that mice transplanted with MSCV-P185wt-Runx1 survived longer than controls. In contrast, mice transplanted with MSCV-P185wt-shRNA survived shorter than the control group. Gross pathological analysis revealed that the MSCV-P185wt-Runx1 group had less severe splenomegaly and hepatomegaly compared to the control group, and the MSCV-P185wt-shRNA group had more severe splenomegaly and hepatomegaly. No splenomegaly or hepatomegaly was detected in mice transplanted with MSCV-BaF3-Runx1 cells or MSCV-BaF3-shRNA cells. Both the mice of MSCV-BaF3-Runx1 group and MSCV-BaF3-shRNA group were healthy with no sign of leukemia for up to three months.

CONCLUSIONS

Overexpression or knock-down of Runx1 gene in BaF3 cells alone could not induce leukemogenesis. However, in BaF3-P185wt cells, alteration of Runx1 expression could affect BCR-ABL-induced proliferation and migration in vitro and leukemogenesis in vivo.

EKLF restricts megakaryocytic differentiation at the benefit of erythrocytic differentiation

Previous observations suggested that functional antagonism between FLI-1 and EKLF might be involved in the commitment toward erythrocytic or megakaryocytic differentiation. We show here, using inducible shRNA expression, that EKLF knockdown in mouse erythroleukemia (MEL) cells decreases erythrocytic and increases megakaryocytic as well as Fli-1 gene expression. Chromatin immunoprecipitation analyses revealed that the increase in megakaryocytic gene expression is associated with a marked increase in RNA pol II and FLI-1 occupancy at their promoters, albeit FLI-1 protein levels are only minimally affected. Similarly, we show that human CD34+ progenitors infected with shRNA lentivirus allowing EKLF knockdown generate an increased number of differentiated megakaryocytic cells associated with increased levels of megakaryocytic and Fli-1 gene transcripts. Single-cell progeny analysis of a cell population enriched in bipotent progenitors revealed that EKLF knockdown increases the number of megakaryocytic at the expense of erythrocytic colonies. Taken together, these data indicate that EKLF restricts megakaryocytic differentiation to the benefit of erythrocytic differentiation and suggest that this might be at least partially mediated by the inhibition of FLI-1 recruitment to megakaryocytic and Fli-1 gene promoters.

Delivering new insight into the biology of megakaryopoiesis and thrombopoiesis

PURPOSE OF REVIEW

The aim of this review is to explore the state of the art knowledge on the cell biological and molecular pathways that regulate megakaryopoiesis and lead to platelet production.

RECENT FINDINGS

In the last 2 years there has been considerable progress in the elucidation of molecular mechanisms of megakaryocyte development and platelet biogenesis, driven by the application of modern molecular biology approaches to these specialized and unique cells. Studies have for the first time visualized endomitotic spindle dynamics, characterized the maturation of the demarcation membrane system, and delineated the mechanics of organelle transport and microtubule assembly in living megakaryocytes. The role of specific molecules in platelet production has been elucidated in greater detail by combining molecular studies with genetically engineered mice as well as embryonic cell culture systems.

SUMMARY

This review integrates the latest studies of megakaryocyte development into the molecular pathways that regulate megakaryopoiesis and thrombopoiesis. Decoding the pathways of megakaryopoiesis and platelet production should help revolutionize the management of thrombocytopenia and other platelet disorders.

Segregation of megakaryocytic or erythroid cells from a megakaryocytic leukemia cell line (JAS-R) by adhesion during culture

Adhesion is one of the important biologic characteristics of leukemic cells. We previously reported a new megakaryocytic-erythroid cell line, JAS-R. In this study, JAS-R cells were segregated into two types by the differences of attachment to culture dishes. One type (designated as JAS-RAD cells) adhered to the substratum of the culture dishes, while the other (JAS-REN cells) grew as a single-cell suspension. Adhesion of JAS-RAD was inhibited by treatment with RGDS oligopeptide. Flow cytometric analysis revealed that JAS-RAD cells had high expression of CD41a and CD61 versus low CD235a expression, and JAS-REN showed low expression of CD41a, and CD61, and high CD235a. The two phenotypes were reciprocally exchangeable by selecting adherent or suspended cells from each type of culture. Microarray analysis and RT-PCR revealed that JAS-RAD cells expressed four major alpha-granule genes and JAS-REN cells expressed beta-globin. Interestingly, erythropoietin was only secreted by JAS-RAD cells. With regard to transcription factors, it was shown that GFI1, FLI1 and RUNX1 were strongly expressed in JAS-RAD cells while GATA1, FOG1 and NFE2 were equally expressed by both types. These findings indicate that adhesion via integrins is related to the phenotypic shift of JAS-R cells between megakaryocytic and erythroid lineages.