Human Megakaryoblastic Cell Line MEG-01 Research Articles

Platelet WDR1 suppresses platelet activity and is associated with cardiovascular disease

AbstractPlatelet activity plays a major role in hemostasis with increased platelet activity likely contributing to the pathogenesis of atherothrombosis. We sought to identify associations between platelet activity variability and platelet-related genes in healthy controls. Transcriptional profiling of platelets revealed that WD-40 repeat domain 1 (WDR1), an enhancer of actin-depolymerizing factor activity, is downregulated in platelet messenger RNA (mRNA) from subjects with a hyperreactive platelet phenotype. We used the human megakaryoblastic cell line MEG-01 as an in vitro model for human megakaryocytes and platelets. Stimulation of MEG-01 with thrombin reduced levels of WDR1 transcripts and protein. WDR1 knockdown (KD) in MEG-01 cells increased adhesion and spreading in both the basal and activated states, increased F-actin content, and increased the basal intracellular calcium concentration. Platelet-like particles (PLPs) produced by WDR1 KD cells were fewer in number but larger than PLPs produced from unmodified MEG-01 cells, and had significantly increased adhesion in the basal state and upon thrombin activation. In contrast, WDR1 overexpression reversed the WDR1 KD phenotype of megakaryocytes and PLPs. To translate the clinical significance of these findings, WDR1 expression was measured in platelet RNA from subjects with established cardiovascular disease (n = 27) and age- and sex-matched controls (n = 10). The WDR1 mRNA and protein level was significantly lower in subjects with cardiovascular disease. These data suggest that WDR1 plays an important role in suppressing platelet activity, where it alters the actin cytoskeleton dynamics, and downregulation of WDR1 may contribute to the platelet-mediated pathogenesis of cardiovascular disease.

Deciphering the molecular and biologic processes that mediate histone deacetylase inhibitor–induced thrombocytopenia

AbstractHistone deacetylase inhibitor (HDACI)–induced thrombocytopenia (TCP) is a major dose-limiting toxicity of this new class of drugs. Using preclinical models to study the molecular and biologic events that underpin this effect of HDACI, we found that C57BL/6 mice treated with both the HDAC1/2-selective HDACI romidepsin and the pan-HDACI panobinostat developed significant TCP. HDACI-induced TCP was not due to myelosuppression or reduced platelet lifespan, but to decreased platelet release from megakaryocytes. Cultured primary murine megakaryocytes showed reductions in proplatelet extensions after HDACI exposure and a dose-dependent increase in the phosphorylation of myosin light chain 2 (MLC2). Phosphorylation of MLC to phospho-MLC (pMLC) and subsequent proplatelet formation in megakaryocytes is regulated by the Rho-GTPase proteins Rac1, CDC42, and RhoA. Primary mouse megakaryocytes and the human megakaryoblastic cell line Meg-01 showed reductions in Rac1, CDC42, and RhoA protein levels after treatment with HDACIs. We were able to overcome HDACI-induced TCP by administering the mouse-specific thrombopoietin (TPO) mimetic AMP-4, which improved platelet numbers to levels similar to untreated controls. Our report provides the first detailed account of the molecular and biologic processes involved in HDACI-mediated TCP. Moreover, our preclinical studies provide evidence that dose-limiting TCP induced by HDACIs may be circumvented using a TPO mimetic.

Cholinergic drugs inhibit in vitro megakaryopoiesis via the alpha7-nicotinic acetylcholine receptor

Besides thrombopoietin several additional factors (i.e neurotransmitters and receptors) are known to be involved in the regulation of megakaryopoiesis at different stages. Recently, we identified functional α7 nicotinic acetylcholine receptors (nAChRα7) on platelets and megakaryocytic precursors. In platelets nAChRα7 form functional Ca2+ channels and are involved in fibrinogen receptor activation and aggregation. Here, we investigated the impact of nAChRα7 on the differentiation of the human megakaryoblastic cell line MEG-01. In vitro differentiation of MEG-01 cells was induced by the phorbol ester TPA for 5 days in the absence or presence of nicotine or the nAChRα7-selective antagonist methyllycaconitine (MLA), and this was monitored by the expression of the megakaryocytic antigens CD41 and CD61. In the presence of the cholinergic drugs (nicotine or MLA) CD41 and CD61 expression was significantly reduced, both at RNA and protein level. We postulate that the nAChRα7 receptor is involved in megakaryopoietic signal transduction and gene regulation. This could affect the generation of platelets in vivo and contribute to the development of novel therapeutic drugs that regulate platelet formation.

Biosynthesis of eicosanoids and transcellular metabolism of leukotrienes in murine bone marrow cells

Leukotriene B(4) (LTB(4)) biosynthesis by polymorphonuclear leukocytes (PMNs) is an important factor of inflammatory responses. PMNs also release LTA(4), an unstable intermediate that can be taken up by neighboring cells and metabolized into LTC(4). Most studies of LT synthesis have been carried out using human PMNs, but very little information is available about mouse PMNs. Mouse bone marrow PMNs were found to synthesize eicosanoids upon stimulation with A23187, fMLP, or zymosan. The major eicosanoids produced are LTB(4) and 5-hydroxyeicosatetraenoic acid, with some nonenzymatic products of LTA(4) hydrolysis. No cysteinyl leukotrienes were produced, in contrast to what was observed with human blood neutrophil preparations. Human megakaryoblast-like MEG-01 cells synthesized thromboxane B(2) and prostaglandin E(2) in response to A23187 but produced no 5-lipoxygenase (5-LO)-derived eicosanoids. When mouse bone marrow cells (mBMCs) and MEG-01 cells were stimulated during coincubation, LTC(4) and LTD(4) were produced. Mouse peritoneal macrophages from 5-LO-deficient mice were able to synthesize LTC(4) when incubated with mBMCs from wild-type mice, demonstrating transcellular exchange of LTA(4) from mBMCs into murine peritoneal macrophages. These data demonstrate that murine bone marrow PMNs are a valid model for the study of LT biosynthesis, which now offers the possibility to investigate specific biochemical pathways through the use of transgenic mice.

A critical role for the transcription factor Scl in platelet production during stress thrombopoiesis

AbstractThe generation of platelets from megakaryocytes in the steady state is regulated by a variety of cytokines and transcription factors, including thrombopoietin (TPO), GATA-1, and NF-E2. Less is known about platelet production in the setting of stress thrombopoiesis, a pivotal event in the context of cytotoxic chemotherapy. Here we show in mice that the transcription factor Scl is critical for platelet production after chemotherapy and in thrombopoiesis induced by administration of TPO. Megakaryocytes from these mice showed appropriate increases in number and ploidy but failed to shed platelets. Ultrastructural examination of Scl-null megakaryocytes revealed a disorganized demarcation membrane and reduction in platelet granules. Quantitative real-time polymerase chain reaction showed that Scl-null platelets lacked NF-E2, and chromatin immunoprecipitation analysis demonstrated Scl binding to the NF-E2 promoter in the human megakaryoblastic-cell line Meg-01, along with its binding partners E47, Lmo2, and the cofactors Ldb1 and GATA-2. These findings suggest that Scl acts up-stream of NF-E2 expression to control megakaryocyte development and platelet release in settings of thrombopoietic stress.

Sp1/3 and NF-1 mediate basal transcription of the human P2X1 gene in megakaryoblastic MEG-01 cells.

BackgroundP2X1 receptors play an important role in platelet function as they can induce shape change, granule centralization and are also involved in thrombus formation. As platelets have no nuclei, the level of P2X1 expression depends on transcriptional regulation in megakaryocytes, the platelet precursor cell. Since nothing is known about the molecular mechanisms regulating megakaryocytic P2X1 expression, this study aimed to identify and functionally characterize the P2X1 core promoter utilized in the human megakaryoblastic cell line MEG-01.ResultsIn order to identify cis-acting elements involved in the transcriptional regulation of P2X1 expression, the ability of 4.7 kb P2X1 upstream sequence to drive luciferase reporter gene expression was tested. Low promoter activity was detected in proliferating MEG-01 cells. This activity increased 20-fold after phorbol-12-myristate-13-acetate (PMA) induced differentiation. A transcription start site was detected 365 bp upstream of the start codon by primer extension. Deletion analysis of reporter constructs indicated a core promoter located within the region -68 to +149 bp that contained two Sp1 sites (named Sp1a and Sp1b) and an NF-1 site. Individual mutations of Sp1b or NF-1 binding sites severely reduced promoter activity whereas triple mutation of Sp1a, Sp1b and NF-1 sites completely abolished promoter activity in both untreated and PMA treated cells. Sp1/3 and NF-1 proteins were shown to bind their respective sites by EMSA and interaction of Sp1/3, NF-1 and TFIIB with the endogenous P2X1 core promoter in MEG-01 cells was demonstrated by chromatin immunoprecipitation. Alignment of P2X1 genes from human, chimp, rat, mouse and dog revealed consensus Sp1a, Sp1b and NF-1 binding sites in equivalent positions thereby demonstrating evolutionary conservation of these functionally important sites.ConclusionThis study has identified and characterized the P2X1 promoter utilized in MEG-01 cells and shown that binding of Sp1/3 and NF-1 to elements in the direct vicinity of the transcription start site is essential for basal transcription. Targeting the function of these transcription factors in megakaryocytes may therefore provide a basis for the future therapeutic manipulation of platelet P2X1 function.

A fluorescence microscopy method for quantifying levels of prostaglandin endoperoxide H synthase-1 and CD-41 in MEG-01 cells.

In platelets, PGHS-1-dependant formation of thromboxane A2 is an important modulator of platelet function and a target for pharmacological inhibition of platelet function by aspirin. Since platelets are a-nucleated cells, we have used the immortalized human megakaryoblastic cell line MEG-01 which can be induced to differentiate into platelet-like structures upon addition of TPA as a model system to study PGHS-1 gene expression. Using a specific antibody to PGHS-1 we have developed a technique utilizing immunofluorescence microscopy and analysis of multiple digital images to monitor PGHS-1 protein levels as MEG-01 cells were induced to differentiate by a single addition of TPA (1.6 x 10-8 M) over a period of 8 days. The method represents a rapid and economical alternative to flow cytometry. Using this technique we observed that TPA induced adherence of MEG-01 cells, and only the non-adherent TPA-stimulated cells demonstrated compromised viability. The differentiation of MEG-01 cells was evaluated by the expression of the platelet-specific cell surface antigen, CD-41. The latter was expressed in MEG-01 cells at the later stages of differentiation. We demonstrated a good correlation between PGHS-1 levels and the overall level of cellular differentiation of MEG-01 cells. Furthermore, PGHS-1 protein level, which shows a consistent increase over the entire course of differentiation, can be used as an additional and better index by which to monitor megakaryocyte differentiation.

Characterization of prostaglandin endoperoxide H synthase-1 enzyme expression during differentiation of the megakaryocytic cell line MEG-01

Objective Because the prostaglandin endoperoxide H synthase-1 (PGHS-1)-dependent formation of thromboxane A 2 is an important modulator of platelet function, this pathway represents a pharmacologic target for the inhibition of platelet function by aspirin. The objective of our research was to study how PGHS-1 expression is regulated in platelets. Materials and Methods Because platelets are anucleated, their protein content is a consequence of gene expression in precursor cells known as megakaryocytes. We used the immortalized human megakaryoblastic cell line MEG-01 as a model to study the expression of PGHS-1, because MEG-01 cells can be induced to differentiate into platelet-like structures by adding nanomolar concentrations of 12-0-tetradecanoylphorbol-13-acetate (TPA). We determined the expression profiles of PGHS-1 protein and mRNA in the cells comprising the three different populations of MEG-01 cultures: nucleated floating, nucleated attached, and platelet-like structures. Results We determined that PGHS-1 protein levels were higher in the nucleated adherent population than in the nucleated floating population. PGHS-1 protein levels were greatest in the anucleated platelet-like population. In contrast, we found that PGHS-1 mRNA levels were highest in the cells that comprised the nucleated adherent population. Addition of TPA induced the expression of PGHS-1 protein and mRNA in all three populations but did not change the relationship of the amount of PGHS-1 protein or mRNA expressed in a given population relative to the other two fractions. We measured the expression of PGHS-1 protein on a cell-by-cell basis in the nucleated MEG-01 populations. We found that the percentage of MEG-01 cells expressing PGHS-1 protein in the adherent population was greater than in the floating population. We measured a time-dependent increase in the percentage of cells that expressed PGHS-1 over a period of 8 days after singular addition of TPA (1.6 × 10 −8M). Importantly, we observed that TPA treatment stimulated floating MEG-01 to adhere to the surface of the tissue culture vessel and that, after such treatment, only floating MEG-01 cells suffered a compromised viability. We found that a high percentage of control cells expressed glycoprotein IIb/IIIa and that TPA treatment did not significantly alter this percentage. We did not detect glycoprotein Ib in control cells but did measure a slight increase in the percentage of MEG-01 cells that expressed this antigen in the TPA-treated population. Conclusion We established a correlation between the level of PGHS-1 expression and the overall level of differentiation of MEG-01 cells. PGHS-1 protein expression, which increases consistently over the full course of differentiation, now may be used as an additional and perhaps better index by which to survey megakaryocytes.

Characterization of a novel transcript of prostaglandin endoperoxide H synthase 1 with a tissue-specific profile of expression

The enzyme prostaglandin endoperoxide H synthase (PGHS) has a pivotal role in the prostanoid biosynthetic pathway because it catalyses the formation of prostaglandin H(2) (PGH(2)), the common precursor of prostanoids. Two PGHS isoforms have been reported, PGHS-1 and PGHS-2, which have 61% identity (at the amino acid level) and 73% similarity (at the nucleotide level) between the two human enzymes. Transcription of the PGHS-1 gene leads to the formation of two transcripts (2.8 and 5.1 kb); two transcripts of 2.8 and 4.5 kb are produced from the PGHS-2 gene. By Northern blot analysis with the entire coding region of human PGHS-1, 2.8 and 5.1 kb transcripts as well as a novel 4.5 kb transcript were detected in the human megakaryoblastic cell line MEG-01. We designed a strategy to characterize the 4.5 kb PGHS transcript. Probes specific for each PGHS-1 and PGHS-2 were designed on the basis of the 3' untranslated region (3' UTR), where no similarity is present. The 4.5 kb transcript was detected only with the PGHS-1-specific 3' UTR probes and not with the PGHS-2-specific 3' UTR probe. To investigate the origin of the 4.5 kb PGHS-1 transcript, the remaining 947 bp of the 5.1 kb PGHS-1 transcript was generated by 3' rapid amplification of cDNA ends (3' RACE) and sequenced. A non-canonical polyadenylation signal (AAGAAA) located upstream of a potential cleavage site (CA) was found and could generate the 4.5 kb PGHS-1 transcript. Analysis of the sequence also produced several possible G/U-rich elements downstream of the potential cleavage site. An RNA dot-blot with 50 different human tissues was probed with the 4.5 and 5.1 kb PGHS-1-specific probes. A signal for the 4.5 kb PGHS-1 transcript was detected in the bladder and appendix. Signals of lower intensity were detected in the colon, bone marrow, small intestine, uterus, prostate, peripheral leucocyte, lymph node and stomach. In conclusion, our results suggest that the cell line MEG-01, the bladder and the appendix contain a new PGHS-1 transcript of 4.5 kb that can be produced from the PGHS-1 gene and we provide a better strategy for distinguishing PGHS-1 transcripts from PGHS-2.

Relation between membrane fluidity and signal transduction in the human megakaryoblastic cell line MEG-01

The fluidity of the plasma membrane is thought to affect the responsiveness of blood platelets. We measured membrane fluidity in a single cell by Fluorescence Recovery after Photobleaching (FRAP) of the lipophilic probe DiIC 14. Since platelets are too small for this technique, we used the human megakaryoblastic cell-line MEG-O1, which shares many properties with platelets. MEG-01 cells were cultured for 44 h with simvastatin or mevalonate to change the cholesterol content, enabling analysis of signal processing at cholesterol /phospholipid ratios (C/P) between 0.20 and 0.31. The diffusion of DiIC,4 correlated inversely with the C/P ratio with lateral diffusion coefficients (D) of 3.28 · 10 −9 cm 2/s at a low C/P decreasing to 2.55 · 10 −9 cm 2/s at a high C/P ratio. The mobile fraction was 65% and constant at the different C/P ratios. The relation between lipid diffusion and signal processing was measured following stimulation with 10 U/ml thrombin at 22°C There were only little differences in phosphatidylinositol metabolism, Ca 2+ influx or mobilization and prostaglandin I 2-induced formation of cyclic AMP. At 37°C cells with a high C/P ratio showed increased phosphatidylinositol metabolism, but these differences had no major effect on the Ca 2+ responses. These data demonstrate that in megakaryoblasts the lateral diffusion of lipids is inversely correlated with the C/P ratio, but within the range of 0.20–0.31 the influence on signal processing is minor.

Expression of Endothelin and ETB Receptors in the Megakaryoblastic MEG-01 Cell Line

The presence of endothelin (ET) receptors and the nature of the subtype and expression of ET were investigated in the human megakaryoblastic cell line MEG-01. By the RT-PCR procedure, we have shown that both ETA and ETB receptor subtype mRNAs are expressed in the cells. However, binding experiments have shown that the selective ETB receptor antagonist BQ788, but not the selective ETA receptor antagonist BQ123, competes with the specific binding of [125I]ET-1. Using immunocytochemistry, RIA, and RT-PCR Southern blot, we have shown that MEG-01 cells express ET-1. In addition, ET (1-21)-like immunoreactivity was released from the cells into the culture medium, and this release was modulated by thrombin. These data suggest that an ET-1-mediated autocrine loop could occur in the human megakaryoblastic MEG-01 cell line.

Expression of Endothelin and ETB Receptors in the Megakaryoblastic MEG-01 Cell Line

Summary: The presence of endothelin (ET) receptors and the nature of the subtype and expression of ET were investigated in the human megakaryoblastic cell line MEG-01. By the RT-PCR procedure, we have shown that both ETA and ETB receptor subtype mRNAs are expressed in the cells. However, binding experiments have shown that the selective ETB receptor antagonist BQ788, but not the selective ETA receptor antagonist BQ123, competes with the specific binding of [125I]ET-1. Using immunocytochemistry, RIA, and RT-PCR Southern blot, we have shown that MEG-01 cells express ET-1. In addition, ET (1–21)-like immunoreactivity was released from the cells into the culture medium, and this release was modulated by thrombin. These data suggest that an ET-1-mediated autocrine loop could occur in the human megakaryoblastic MEG-01 cell line.

Effects of endothelins on the human megakaryoblastic cell line MEG-01

Some effects of endothelin-1 (ET-1) were studied on the megakaryoblastic cell line MEG-01. ET-1 induced an elevation of the intracellular levels of Ca 2+ ([Ca 2+] 1) as measured with the fluorescent indicator indo-1, which consists of an initial transient increase and an ensuing sustained plateau. The plateau phase was abolished by removal extracellular Ca 2+. In addition, ET-1 induced a rapid (within 5 s) increase in the accumulation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P 1) and more delayed increases in Ins(1,3,4)P 3 and Ins(1,3,4,5)P 4, but did not modify cAMP levels. ET-1 homodogues (ET-2, ET-3, sarafotoxin ϱ 1 and vasointestinal constrictor) also induced biphasic effects on [Ca 2+] 1. The Ca 2+ elevation was concentration dependent, the order of potency being sarafotoxin 6b > ET-1 > ET-2 = vasointestinal constrictor > ET-3. The actions of ET analogs in raising [Ca 2+], were mutually exclusive, suggesting that they act through the same mechanism. These results suggest that cells of the megakaryoblast/megakaryocyte lineage are targets for endothelins.