Evidence for a selective inhibitory effect of thrombin on megakaryocyte progenitor growth mediated by the thrombin receptor

Hirudin and heparin enable efficient megakaryocyte differentiation of mouse bone marrow progenitors

Hematopoietic progenitors from murine fetal liver efficiently differentiate in culture into proplatelet-producing megakaryocytes and have proved valuable to study platelet biogenesis. In contrast, megakaryocyte maturation is far less efficient in cultured bone marrow progenitors, which hampers studies in adult animals. It is shown here that addition of hirudin to media containing thrombopoietin and serum yielded a proportion of proplatelet-forming megakaryocytes similar to that in fetal liver cultures (approximately 50%) with well developed extensions and increased the release of platelet particles in the media. The effect of hirudin was maximal at 100 U/ml, and was more pronounced when it was added in the early stages of differentiation. Hirugen, which targets the thrombin anion binding exosite I, and argatroban, a selective active site blocker, also promoted proplatelet formation albeit less efficiently than hirudin. Heparin, an indirect thrombin blocker, and OTR1500, a stable heparin-like synthetic glycosaminoglycan generated proplatelets at levels comparable to hirudin. Heparin with low affinity for antithrombin was equally as effective as standard heparin, which indicates antithrombin independent effects. Use of hirudin and heparin compounds should lead to improved culture conditions and facilitate studies of platelet biogenesis in adult mice.

Regulation of the human protein S gene promoter by liver enriched transcription factors

Protein S is expressed in a number of tissue types, one of the most physiologically relevant being the liver. However, transcriptional control of protein S gene expression is poorly understood. We have characterised a 638 bp area in the 5' flanking region of the human protein S gene, spanning all 10 previously reported transcription initiation sites, which demonstrates promoter activity in the human liver-derived cell line HepG2. More refined reporter gene analysis of this region enabled the identification of three transcription initiation sites whose absence is associated with significantly reduced promoter activity, together with a number of positively and negatively acting transcriptional regulatory elements. Consistent with these findings, DNaseI footprinting analysis identified eleven sites (I-XI) from within this 638 bp region that show evidence of binding nuclear proteins. We present evidence to show that the liver-specific factors hepatocyte nuclear factor 1 (HNF1) and HNF4 bind regions of the protein S promoter, which lie within the identified protein binding sites V and VIII, respectively, and that HNF4 activates the protein S promoter. Reporter gene analysis suggests that members of the CCAAT/enhancer binding protein (C/EBP) family of transcription factors are potent activators of protein S gene transcription in HepG2 cells.

Erythropoietin– and Stem Cell Factor–Induced DNA Synthesis in Normal Human Erythroid Progenitor Cells Requires Activation of Protein Kinase C and Is Strongly Inhibited by Thrombin

Proliferation, differentiation, and survival of erythroid progenitor cells are mainly regulated by stem cell factor (SCF) and erythropoietin (Epo). Using normal human progenitors, we analyzed the role of Ca2+-sensitive protein kinase C (PKC) subtypes and of G-protein–coupled receptor ligands on growth factor–dependent DNA synthesis. We show that stimulation of DNA synthesis by the two growth factors requires activation of PKC. Inhibitors of Ca2+-activated PKC subtypes blocked the growth factor–induced 3H-thymidine incorporation. SCF and Epo caused no significant translocation of PKC into the membrane, but treatment of intact cells with either of the two cytokines resulted in enhanced activity of immunoprecipitated cytosolic PKC. Stimulation of PKC with the phorbol ester PMA mimicked the cytokine effect on DNA synthesis. Epo-, SCF-, and PMA-induced thymidine incorporation was potently inhibited by thrombin (half-maximal inhibition with 0.1 U/mL). This effect was mediated via the G-protein-coupled thrombin receptor and the Rho guanosine triphosphatase. Adenosine diphosphate caused a modest Ca2+-dependent stimulation of DNA synthesis in the absence of cytokines and specifically enhanced the effect of SCF. Cyclic 3′,5′-adenosine monophosphate exerted a selective inhibitory effect on Epo-stimulated thymidine incorporation. Our results define PKC as major intermediate effector of cytokine signaling and suggest a role for thrombin in controlling erythroid progenitor proliferation.

Cord blood is better than bone marrow for generating megakaryocytic progenitor cells

Thrombocytopenia remains an important problem for patients post high-dose chemotherapy and hematopoietic stem cell transplantation. The study of megakaryocytes, the direct precursors of platelets, has been hampered by their relatively low frequency in hematopoietic tissues. In an attempt to obtain a large number of functional megakaryocytic cells, we established a serum-free culture system to grow megakaryocytic progenitor cells derived from normal human bone marrow (BM) and cord blood (CB). Highly purified (purity >95%) CD34+ cells were obtained using magnetic cell sorting (MACS) followed by fluorescence activated cell sorting (FACS). The cells were cultured in a serum-free culture system for 3 weeks in the presence of a single dose of MGDF (50 ng/mL). On days 0, 5, 8, 12, 14, 18, and 21 of culture, the cellularity and morphology were examined. Megakaryocytic cells were monitored by detecting the expression of GPIIIa (CD61), GPIIb/IIIa (CD41) and GPIb (CD42b), and the distribution of megakaryocyte (MK) ploidy was analyzed by two-color flow cytometry. MGDF alone induced maximal nucleated cell expansion at day 14, resulting in a 38.20+/-10.47-fold increase in cell number for CB and a 5.08+/-1.30-fold increase in cell number for BM. On day 14 of the culture, the percentage of CD41-/CD14- cells derived from CB reached 73.54%+/-6.01% giving an absolute number of CD41+/CD14- cells of 27.25+/-2.23 x 10(4)/mL (27,250-fold increase), whilst the percentage of CD41+/CD14- cells derived from BM was only 29.21%+/-5.63% with an absolute number of 1.36+/-0.26 x 10(4)/mL (680-fold increase). Increased expression of GPIIIa occurred the earliest in culture, followed by GPIIb/IIIa, and then GPIb. The majority (81.6%-92.6%) of megakaryocytes (CD41+ cells) on day 14 of culture were 2N, although we did detect some 4N, 8N and greater ploidy cells. In conclusion, CD34+ cells stimulated by MGDF alone generated highly enriched MK progenitor cells at day 14 of serum-free culture. CB stem and progenitor cells have a greater proliferative response to MGDF alone than those derived from BM and may, therefore, prove to be a better source of cells for MK expansion.

The human thrombin receptor gene and the 5q-syndrome

The human thrombin receptor gene has been localized to band q13 of chromosome 5, a site that is at or contiguous to the common proximal breakpoint found in the majority of patients with interstitial deletions involving 5q (5q- syndrome; refractory anemia with dysmegakaryocytopoiesis). Recent evidence suggests that the thrombin receptor may represent the prototype of an emerging family of proteolytically activated receptors that may be clustered within this region of the human genome. The phenotypic heterogeneity evident in patients with the 5q- syndrome may be explained by two (or more) distinct molecular defects-one associated with megakaryocytic dyspoiesis and the other dysregulated myeloid growth potentially related to development of leukemogenesis. Because the thrombin receptor is known to mediate proliferative effects on diverse cells including vascular smooth muscle cells, endothelial cells and megakaryocytes, we have studied the role of this receptor in the pathogenesis of this syndrome using fluorescent in situ hybridization (FISH) analysis. Dual-label FISH using a q12-specific genomic fragment and the TR gene was completed using interphase and metaphase analysis from seven patients with a del(5)(q13q33). These data unequivocally demonstrate that the thrombin receptor gene is located centromeric to the common proximal breakpoint, and is grossly present in the seven patients containing this specific interstitial deletion. Additionally, one patient demonstrated a small proximal rearrangement, most likely representing a paracentric inversion, which has not previously been described within the intact region centromeric to the breakpoint. The biological properties of proteolytically activated receptors are reviewed in more detail, with a focus on the molecular genetics of the thrombin receptor and its potential role in megakaryoctyopoiesis.

Positive and negative regulation of megakaryocytopoiesis

The recently cloned physiological regulator of megakaryocytopoiesis, known as Mpl ligand, thrombopoietin (TPO), megakaryocyte growth and development factor (MGDF) or megapoietin, is undergoing preclinical and clinical trials. This factor is an extremely potent thrombocytopoietic agent in vivo in normal animals, and accelerates platelet recovery in some but not all models of myelosuppression. Together with its apparent lack of adverse effects, the preclinical data suggest that TPO might permit the use of higher doses of chemotherapy in dose-intensive regimens and be useful in patients with ineffective platelet production or production abnormalities. The direct effects of TPO on primitive and various myeloid committed haematopoietic progenitor cells predict that TPO may be used in combination with other cytokines in a variety of clinical disorders. Along with the progress made in the understanding of the positive regulation of megakaryocytopoiesis, accumulating data demonstrate that platelet production is also controlled by negative regulators with potential clinical applications. Some of these regulators are effective in the treatment of essential thrombocythaemia and myeloproliferative disorders, while others seem capable of protecting progenitor cells from the cytotoxicity of chemotherapeutic drugs.

Thrombin has a bimodal effect on glioma cell growth

Using rat glioma C6 cells as a model, we have found a bimodal effect of alpha-thrombin on cell growth. In C6 cells treated with alpha-thrombin at concentrations from 0.02 nM to 1.0 nM, inhibition of cell proliferation was noted. Because the thrombin receptor agonist peptide TRAP-6 also induced inhibition of cell proliferation and the thrombin receptor antagonist peptide T1 prevented the inhibitory effect of alpha-thrombin on C6 glioma cell growth, thrombin receptor involvement in antiproliferative action of alpha-thrombin in C6 glioma cells is highly likely. However, stimulation of cell proliferation observed when C6 cells were treated with alpha-thrombin at higher doses (> 1.0 nM) seems to be mediated by as yet undefined thrombin receptor-independent biochemical mechanisms.

The thrombin receptor is present in myoblasts and its expression is repressed upon fusion

Cultured myoblasts derived from limb muscle of newborn rats express thrombin receptor immunoreactivity on their surface. Receptor expression is repressed upon myoblast fusion. This is due at least in part to a decrease in the amount of the thrombin receptor mRNA. Addition of thrombin triggers calcium transients only in mono- but not multinucleated muscle cells. Furthermore, thrombin increases the rate of myoblast proliferation that coincides with an activation of mitogen-activated protein kinase. Northern analysis of thrombin receptor mRNA expression in skeletal muscle showed that the transcript is present at a relatively high level at birth, but is almost undetectable in the adult. By in situ hybridization, the mRNA at birth appeared to be present mostly in mononucleated cells grouped in clusters, but not in muscle fibers. Very few nuclei surrounded by a mRNA signal were present on muscle sections of rats 24 days postnatally. These results suggest that the thrombin receptor plays a role in muscle development.

Thrombin-induced increase in endothelial permeability is associated with changes in cell-to-cell junction organization

Thrombin increases endothelial permeability in a rapid and reversible way. This effect requires the catalytic activity of the enzyme and thrombin receptor engagement. Endothelial cell permeability is mostly regulated by intercellular junction organization. In the present study, we investigated whether opening of intercellular gaps after thrombin treatment could be related to changes in adherence-junction molecular organization. By immunofluorescence analysis, we found that thrombin stimulation of endothelial cells caused a marked alteration of the distribution of vascular endothelial (VE)-cadherin and of the associated catenins. These molecules, which are strictly localized at intercellular boundaries in confluent resting cells, were absent in the areas of intercellular retraction. Immunoprecipitation analysis indicated that thrombin disrupted the VE-cadherin/catenin complex. This effect was reversible and correlated with the increase in endothelial permeability. The use of a protein kinase C inhibitor (calphostin C) blocked both thrombin-induced permeability and disassembly of adherence-junction components. We propose that thrombin's effect on endothelial cell junction organization is an important determinant in the increase in endothelial permeability induced by this agent.

Reversal of thrombocytopenia following oral anticoagulation in two patients with primary antiphospholipid syndrome

We report on two patients with the primary antiphospholipid syndrome who had significant thrombocytopenia. In both patients platelet number improved soon after initiation of oral anticoagulation. Possible mechanisms for the resolution of thrombocytopenia are discussed.

Characteristic biological features of human megakaryoblastic leukaemia cell lines

Both normal and leukaemic human megakaryocytopoiesis are stimulated by several cytokines, including stem cell factor, granulocyte-macrophage colony stimulating factor (GM-CSF), interleukin-3, GM-CSF/interleukin-3 fusion protein, interleukin-6, interleukin-11, basic fibroblast growth factor and thrombopoietin, but are inhibited by tumour necrosis factor-alpha, platelet factor 4, beta-thromboglobulin, thrombin, interleukin-4, interferon-alpha and interferon-gamma. Human megakaryoblastic leukaemia cell lines have common biological features, including high expression of the megakaryocytic specific antigen: CD41; high expression of the early myeloid antigens: CD34 and CD33; constitutive expression of interleukin-6 and platelet-derived growth factor; complex karyotype picture; expression of c-kit: the stem cell factor receptor; growth-dependency or -stimulation by stem cell factor, interleukin-3 and/or GM-CSF; megakaryoblastic differentiation by phorbol-myristate-acetate; and in vivo tumorigenicity in mice is associated with marked fibrosis. Only a few agents including phorbol-myristate-acetate; vitamin D3, interferon-alpha, interferon-beta 2, erythropoietin and thrombin have been reported to induce megakaryocytic differentiation in the human megakaryoblastic leukaemia cells.