Thrombopoietin, the Mp1 ligand, is essential for full megakaryocyte development

Thrombopoietin modulates the proliferation, migration and cytokine profile of decidual cell subsets during early gestation

During early gestation, a considerable increase in different leukocyte subsets can be observed in the decidualized endometrium concomitantly to the invasion of cytotrophoblast cells (CTB). To date, it is still in question which factors induce this accumulation of immune cells and whether it is evoked by an in situ proliferation or by a migratory process. Studies on hepatoblastoma cells identified thrombopoietin (TPO) as a novel factor, which elicits dose-dependent chemotactic and chemokinetic effects. However, the impact and function of TPO on decidual cells has not been clarified yet. This study analyses the expression and function of TPO and its receptor c-Mpl in decidua during early gestation. Applying western blot analysis, we detected that TPO is expressed by decidual immune cells (uNK cells and CD14+ monocytes) as well as CTB and decidual stromal cells (DSCs). Expression of the different isoforms of c-Mpl was found in uNK cells, CD14+ monocytes and DSC. Studying the signalling pathway proteins in the uNK cells, an activation of STAT3/Tyr by TPO, was detected. The investigation of the proliferative effects of TPO on the decidual cell subsets revealed that TPO enhances the proliferation of uNK cells and CTB. No change of the proliferative activity after TPO incubation was found in DSC and even a decrease in CD14+ monocytes. In addition, TPO was observed to induce significantly the migratory activity of uNK cells, CD14+ monocytes and CTB. Investigating the effects of TPO on the cytokine profile of the isolated decidual cells, we observed a decrease in the secretion of IL-8, IL-10 and IL-1β of isolated uNK cells, CD14+ monocytes and CTB, although these changes did not reach statistical significance. Thus, we here identified TPO as a novel factor modulating the proliferation, migration and possibly cytokine secretion of decidual cell subsets.

Myelodysplastic syndrome hematopoietic stem cell

Myelodysplastic syndromes (MDSs) are clonal hematopoietic stem cell (HSC) malignancies that are characterized by ineffective hematopoiesis and frequent progression to acute myeloid leukemia (AML). Thus far, few treatments can actually alter the natural history of this disease. Allogeneic stem-cell transplantation for high-risk MDS is becoming the only curative therapy probably because of the improvement of bone marrow transplant procedures. The lack of other options underscores the urgent need to develop new therapy. The prevailing model suggests that genetic and/or epigenetic alterations that occur in HSCs or HSC niche compromise HSC function, resulting in MDS; therefore, MDS HSCs are likely the ideal targets for MDS treatment. Recent encouraging advances--capturing a molecular portrait of the whole genome of MDS CD34(+) cells, including identifying altered signaling pathways and altered microRNAs--have improved our understanding of MDS pathogenesis and provided novel potential clinical targets for MDS. Here, I will briefly review the characteristics of MDS HSCs and discuss the therapeutic promise of targeting MDS HSCs.

Cytokines, platelet production and hemostasis

A number of hematopoietic growth factors and other cytokines are capable of altering platelet production and function. Enhancement of these processes may be exploited to ameliorate bleeding propensity in thrombocytopenic patients. Under certain circumstances, cytokines may have adverse effects on the hemostatic system, potentially involved in thrombogenesis and atherogenesis. Inhibition of those cytokines may prove to be a useful experimental approach to investigate their potential pathophysiologic role.

Thrombopoietin stimulates proliferation and megakaryocytic differentiation of mouse pro-B cell line BF-TE22

We have isolated and characterized a thrombopoietin (TPO)-dependent BF-TE22 cell line endogenously expressing murine Mpl, which is a subclone of murine pro-B Ba/F3 cells. TPO stimulated the proliferation of BF-TE22 cells in a dose-dependent manner, and also induced the expression of megakaryocyte lineage-specific AP-51 and CD61 cell surface antigens. The results indicate that the murine Mpl on BF-TE22 cells can transmit both proliferation and megakaryocyte lineage-specific differentiation signals to cells. Furthermore, it was shown that IL-3 inhibits the TPO-induced differentiation signals of BF-TE22 cells. These results suggest that the signals mediated by IL-3 predominate over those of TPO in BF-TE22 cells. Thus, BF-TE22 cells will be useful for the biological and biochemical studies of the TPO-Mpl signal transduction mechanism.

Low or undetectable TPO receptor expression in malignant tissue and cell lines derived from breast, lung, and ovarian tumors

BACKGROUND

Numerous efficacious chemotherapy regimens may cause thrombocytopenia. Thrombopoietin receptor (TPO-R) agonists, such as eltrombopag, represent a novel approach for the treatment of chemotherapy-induced thrombocytopenia. The TPO-R MPL is expressed on megakaryocytes and megakaryocyte precursors, although little is known about its expression on other tissues.

METHODS

Breast, lung, and ovarian tumor samples were analyzed for MPL expression by microarray and/or quantitative reverse transcription-polymerase chain reaction (qRT-PCR), and for TPO-R protein expression by immunohistochemistry (IHC). Cell line proliferation assays were used to analyze the in vitro effect of eltrombopag on breast, lung, and ovarian tumor cell proliferation. The lung carcinoma cell lines were also analyzed for TPO-R protein expression by Western blot.

RESULTS

MPL mRNA was not detectable in 118 breast tumors and was detectable at only very low levels in 48% of 29 lung tumors studied by microarray analysis. By qRT-PCR, low but detectable levels of MPL mRNA were detectable in some normal (14-43%) and malignant (3-17%) breast, lung, and ovarian tissues. A comparison of MPL to EPOR, ERBB2, and IGF1R mRNA demonstrates that MPL mRNA levels were far lower than those of EPOR and ERBB2 mRNA in the same tissues. IHC analysis showed negligible TPO-R protein expression in tumor tissues, confirming mRNA analysis. Culture of breast, lung, and ovarian carcinoma cell lines showed no increase, and in fact, showed a decrease in proliferation following incubation with eltrombopag. Western blot analyses revealed no detectable TPO-R protein expression in the lung carcinoma cell lines.

CONCLUSIONS

Multiple analyses of breast, lung, and ovarian tumor samples and/or cell lines show no evidence of MPL mRNA or TPO-R protein expression. Eltrombopag does not stimulate growth of breast, lung, or ovarian tumor cell lines at doses likely to exert their actions on megakaryocytes and megakaryocyte precursors.

Herbal Cocktail Ka-Mi-Kae-Kyuk-Tang Stimulates Mouse Bone Marrow Stem Cell Hematopoiesis and Janus-Activated Kinase 2/Signal Transducer and Activator of Transcription 5 Pathway

Ka-mi-kae-kyuk-tang (KMKKT) is an Oriental herbal medicinal cocktail. Our collaborative team has shown that it has potent anti-angiogenic, anti-cancer and anti-metastatic activities in vivo without observable side effects. We have documented evidence for KMKKT to alleviate drug-induced hematotoxicity in vivo. In the present study, we investigated the mechanistic and signaling events through which KMKKT enhances hematopoiesis, using hematopoietic stem cells (HSCs) isolated from the bone marrow of 8–12 week-old C57BL/6 mice. Our results show that KMKKT significantly increased the expression of the hematopoietic cytokines interleukin (IL)-3, stem cell factor (SCF), granulocyte-macrophage-colony stimulating factor (GM-CSF), thrombopoietin (TPO) and erythropoietin (EPO) at the level of mRNA and secretion in HSCs. KMKKT also increased the expression of c-Kit, a cytokine receptor expressed in HSCs. In addition, KMKKT enhanced phosphorylation of Janus kinase 2 (JAK2) and signal transducer and activator of transcription 5 (STAT5), and increased the binding activity of STAT5 to gamma interferon activated sites (GAS) that mediate JAK2 downstream signaling. Furthermore, we found that KMKKT significantly enhanced the growth rate of colony-forming unit granulocyte erythrocyte monocyte macrophages (CFU-GEMM) and burst forming unit erythroid (BFU-E) of mouse HSCs (mHSCs) stimulated by IL-3/EPO. Overall, our results demonstrated that KMKKT alleviated drug-induced side effects through enhanced hematopoiesis, at least in part through cytokine-mediated JAK2/STAT5 signaling.

Romiplostim dose response in patients with immune thrombocytopenia

A pharmacodynamic model was developed for platelet counts in 52 patients with immune thrombocytopenia (ITP) receiving subcutaneous romiplostim in 3 phase I/II studies (dose range, 0.2-10 µg/kg). The model consisted of a drug-sensitive progenitor cell compartment linked to a peripheral blood compartment through 4 transition compartments. The baseline platelet count, mean transit time, and kinetics of drug effect constant were 11.1 × 10(9)/L, 170 hours, and 0.6 day(-1), respectively. The ITP patients had a shorter platelet life span and lower progenitor cell production rates than healthy volunteers. Romiplostim response was described for 2 subpopulations. The romiplostim stimulatory effect in ITP patients was 351%/100 µg/wk and 12%/100 µg/wk in 68% and 32% of patients, respectively. Visual and numerical predictive checks suggested accurate prediction of platelet time course and durable response rate in ITP patients. Model-based simulations confirmed the effectiveness of dose reduction to prevent platelet counts >400 × 10(9)/L.

Manipulation of oxygenation and flow-induced shear stress can increase the in vitro yield of platelets from cord blood

A method to produce clinically useful platelets in vitro would help overcome the frequent shortages, donor deferrals, disease transmission, and alloimmunization with volunteer donor-derived platelets. Using CD34 positively selected cord blood cells, we investigated ways to increase platelet quality and yield in a three-dimensional modular perfusion bioreactor system. We found a two- to threefold increase in platelet numbers produced only when the early phases of the culture process were carried out at 5% oxygen, versus when 20% oxygen was used throughout the culture period (p<0.05), and much more than when 5% oxygen was used throughout. When the medium was routed through the cell-scaffold construct, versus when it flowed under and over the construct, or just intermittent feeding was used, the number of platelets increased two- to threefold (p<0.05), and enhanced collagen-induced aggregation. The 5% oxygen early in the culture process mimics the marrow adjacent to the bone where early progenitors proliferate. Flow through the cell-scaffold construct creates shear forces that mimic the flow in central venous sinuses of the marrow and enhances platelet production from proplatelets. The use of altered oxygen levels and cross flow enhanced platelet numbers and quality, and will contribute to eventual in vitro platelet production for clinical use.

The genetics of common variation affecting platelet development, function and pharmaceutical targeting

Common variant effects on human platelet function and response to anti-platelet treatment have traditionally been studied using candidate gene approaches involving a limited number of variants and genes. These studies have often been undertaken in clinically defined cohorts. More recently, studies have applied genome-wide scans in larger population samples than prior candidate studies, in some cases scanning relatively healthy individuals. These studies demonstrate synergy with some prior candidate gene findings (e.g., GP6, ADRA2A) but also uncover novel loci involved in platelet function. Here, I summarise findings on common genetic variation influencing platelet development, function and therapeutics. Taken together, candidate gene and genome-wide studies begin to account for common variation in platelet function and provide information that may ultimately be useful in pharmacogenetic applications in the clinic. More than 50 loci have been identified with consistent associations with platelet phenotypes in ≥ 2 populations. Several variants are under further study in clinical trials relating to anti-platelet therapies. In order to have useful clinical applications, variants must have large effects on a modifiable outcome. Regardless of clinical applications, studies of common genetic influences, even of small effect, offer additional insights into platelet biology including the importance of intracellular signalling and novel receptors. Understanding of common platelet-related genetics remains behind parallel fields (e.g., lipids, blood pressure) due to challenges in phenotype ascertainment. Further work is necessary to discover and characterise loci for platelet function, and to assess whether these loci contribute to disease aetiologies or response to therapeutics.

Developmental differences in megakaryocytopoiesis are associated with up-regulated TPO signaling through mTOR and elevated GATA-1 levels in neonatal megakaryocytes

Multiple observations support the existence of developmental differences in megakaryocytopoiesis. We have previously shown that neonatal megakaryocyte (MK) progenitors are hyperproliferative and give rise to MKs smaller and of lower ploidy than adult MKs. Based on these characteristics, neonatal MKs have been considered immature. The molecular mechanisms underlying these differences are unclear, but contribute to the pathogenesis of disorders of neonatal megakaryocytopoiesis. In the present study, we demonstrate that low-ploidy neonatal MKs, contrary to traditional belief, are more mature than adult low-ploidy MKs. These mature MKs are generated at a 10-fold higher rate than adult MKs, and result from a developmental uncoupling of proliferation, polyploidization, and terminal differentiation. This pattern is associated with up-regulated thrombopoietin (TPO) signaling through mammalian target of rapamycin (mTOR) and elevated levels of full-length GATA-1 and its targets. Blocking of mTOR with rapamycin suppressed the maturation of neonatal MKs without affecting ploidy, in contrast to the synchronous inhibition of polyploidization and cytoplasmic maturation in adult MKs. We propose that these mechanisms allow fetuses/neonates to populate their rapidly expanding bone marrow and intravascular spaces while maintaining normal platelet counts, but also set the stage for disorders restricted to fetal/neonatal MK progenitors, including the Down syndrome-transient myeloproliferative disorder and the thrombocytopenia absent radius syndrome.

Mesenchymal stem cells from bone marrow show a stronger stimulating effect on megakaryocyte progenitor expansion than those from non-hematopoietic tissues

In order to evaluate whether mesenchymal stem cells (MSCs) from non-hematopoietic tissues are able to regulate megakaryocytopoiesis, we identified human MSCs from adult bone marrow (ABM), fetal pancreas (FPan) and umbilical cord (UC), and their abilities to support megakaryocyte (MK) differentiation from CD34(+) hematopoietic progenitor cells (HPCs) were comparatively studied. First, MSCs were isolated from ABM, FPan and UC then their growth kinetics, molecular characterization and mesodermal differentiation capacity were determined. ABM-MSCs, FPan-MSCs and UC-MSCs were irradiated and cocultured with human umbilical cord blood (UCB) CD34(+) cells, and the expansion efficiency of MK progenitor cells and MK formation were analysed and compared. Finally, SCF, IL-6 and GM-CSF expression by the three types of MSCs were also examined. Our results showed that FPan-MSCs and UC-MSCs shared most of the characteristic of ABM-MSCs, including morphology, immunophenotype, adipogenic and osteogenic differentiation potentials. Compared with ABM-MSCs, fetal MSCs had higher proliferative capacity. After 7 days' coculture, the maximal production of CD34(+)/CD41a(+) cells was obtained in a group of CD34(+) HPCs + ABM-MSCs. Furthermore, this group produced more MK colonies than other groups (p < 0.05). Surface antigen and ploidy analysis morphological observation demonstrated that a proportion of expanded cells in each group differentiated into mature MKs. ABM-MSCs, FPan-MSCs and UC-MSCs were revealed to express SCF, IL-6 and GM-CSF at mRNA level. We conclude that FPan-MSCs and UC-MSCs have the ability to promote megakaryocytopoiesis, while ABM-MSCs expand more MK progenitor cells from CD34(+) HPCs than MSCs from non-hematopoietic tissues and CD34(+) cells alone.

Platelets prevent acute hepatitis induced by anti-fas antibody

BACKGROUND AND AIM

Platelets provide many functions in the body, especially to the liver. The purpose of this study is to investigate the effect of thrombocytosis with acute hepatitis induced by anti-Fas antibody and its mechanism.

METHODS

Acute hepatitis was induced by administration of anti-Fas antibody in normal and thrombocytotic C57BL6J mice. For thrombocytosis, thrombopoietin; PEG-rHuMGDF was injected 5 days before and just prior to administration of anti-Fas antibody. To investigate the mechanisms, hepatocyte cell line (AML12) and sinusoidal endothelial cell line (M1) were induced apoptosis by staurosporine. They were cultured with platelets or thrombopoietin. Examination items were as follows: platelet number, alanine aminotransferase (ALT), histological findings, TUNEL (TdT-mediated dUTP-biotin Nick End Labeling) staining, and the expression of proteins associated with apoptosis in vivo and in vitro.

RESULTS

Platelets were significantly increased in the thrombocytotic group (P < 0.01). Serum ALT levels were significantly reduced by thrombocytosis at 6, 24 and 72 h after the administration (P < 0.05). In histological findings, hemorrhagic necrosis was observed in the normal group, but not observed in the thrombocytotic group. TUNEL-positive hepatocytes were reduced and the expression of cleaved caspase-3 was significantly decreased in the thrombocytotic group. The phosphorylation of Akt, the increment of Bcl-xL and the decrease of cleaved caspase-3 were observed in AML12 cells cultured with platelets, but were not observed cultured with thrombopoietin. Platelets and thrombopoietin had no anti-apoptotic effect on M1 cells.

CONCLUSION

Increase of platelets has a preventative effect against acute hepatitis induced by the anti-Fas antibody. It is suggested that platelets have a direct protective effect against apoptosis of hepatocytes.

Thrombopoietin receptor levels in tumor cell lines and primary tumors

Thrombopoietin (TPO) receptor agonists represent a new approach for the treatment of thrombocytopenia, which may develop as a consequence of immune thrombocytopenia, chemotherapy treatment, chronic hepatitis C infection, or myelodysplastic syndromes. There are concerns that use of certain growth factors can hasten disease progression in some types of hematologic malignancies and solid tumors. In this study, expression of MPL (TPO-R) mRNA was examined in tumor cell lines, patient tumor samples (renal cell carcinoma, prostatic carcinoma, soft tissue and bony/cartilage sarcoma, colon cancer, and lymphoma), and normal tissues using microarray analysis and qRT-PCR. MPL mRNA is expressed at very low or undetectable levels compared with erythropoietin receptor (EPOR), human epidermal growth factor (ERBB2; HER2), and insulin-like growth factor-1 receptor (IGF1R) in these patient samples. These data suggest TPO-R agonists will likely preferentially stimulate proliferation and differentiation of cells of megakaryocytic lineage, potentially demonstrating their utility for correcting thrombocytopenia in clinical settings.

Platelet growth factors suppress ex vivo expansion and enhance differentiation of umbilical cord blood CD133+ stem cells to megakaryocyte progenitor cells

BACKGROUND AND OBJECTIVES

Umbilical cord blood (UCB) is a rich source of hematopoietic cells. Here, for the first time, we surveyed the effects of different concentrations of platelet growth factors and cytokine cocktail (CC) on the expansion and differentiation of UCB CD133(+) stem cells into megakaryocyte progenitors.

MATERIALS AND METHODS

UCB CD133(+) cells were separated by magnetic cell sorting and cultured in different concentrations of platelet growth factors in combination with a CC containing interleukins 3 and 6, stem cell factor, and thrombopoietin. Cell expansion and differentiation were assessed using mononuclear cell count and flow cytometry.

RESULTS

The results show that either activated platelet-rich plasma or the platelet supernatant, when added in the first day of culture, significantly suppress the expansion of CD133(+) cells after 7 days in culture (p < 0.05). By contrast, the expression of CD41, CD61, and CD42b markers in the presence of all platelet growth factors increased compared with that of the control (p < 0.05).

CONCLUSION

Taken together, platelet growth factors in the presence of CC suppress ex vivo expansion of UCB CD133(+) cells and enhance their differentiation into megakaryocytic progenitor cells in a dose- and time-dependent manner.

JAK2/STAT5 signaling pathway mediates Bojungbangdocktang enhanced hematopoiesis

Bojungbangdocktang (BJBDT) is a medicinal herbal cocktail that has been used for cancer prevention and treatment in traditional Korean medicine. In the current study, BJBDT was demonstrated to regulate hematopoiesis. BJBDT significantly increased the expression of hematopoietic cytokines interleukin (IL)-3, stem cell factor (SCF), granulocyte-macrophage-colony stimulating factor (GM-CSF), thrombopoietin (TPO) and erythropoietin (EPO) at the level of mRNA and secretion in hematopoietic stem cells (HSCs). Additionally, BJBDT enhanced the phosphorylation of Janus activated kinase 2 (JAK2) and signal transducer and activator of transcription 5 (STAT5) and STAT binding to gamma interferon activated sites (GAS) in HSCs. Furthermore, BJBDT significantly enhanced the growth rate of granulocyte erythrocyte monocyte macrophage colony-forming units (CFU-GEMM) and erythroid burst forming units (BFU-E) in vitro. Moreover, BJBDT increased the level of EPO at mRNA in kidney and plasma, and the numbers of erythroid-specific antigen Ter-119(+) erythroid cells in mice with aplastic anemia induced by 20% benzene. Consistently, histochemical staining revealed BJBDT increased the bone marrow and stromal cells as well as decreased macrophages and adipocytes in bone marrow tissues of mice with aplastic anemia. Taken together, the results suggest that BJBDT can enhance hematopoiesis via hematopoietic cytokine-mediated JAK2/STAT5 pathway as a potent hematopoietic candidate. Copyright © 2010 John Wiley & Sons, Ltd.

In vitro megakaryocyte production and platelet biogenesis: state of the art

The exciting and extraordinary capabilities of stem cells to proliferate and differentiate into numerous cell types not only offers promises for changing how diseases are treated but may also impact how transfusion medicine may be practiced in the future. The possibility of growing platelets in the laboratory to some day supplement and/or replace standard platelet products has clear advantages for blood centers and patients. Because of the high utilization of platelets by patients undergoing chemotherapy or receiving stem cell transplants, platelet transfusions have steadily increased over the past decades. This trend is likely to continue as the number of adult and pediatric patients receiving stem cell transplants is also continuously rising. As a result of increased demand, coupled with the short shelf-life of platelet concentrates, providing platelets to patients can stretch the resources of most blood centers and drive donor recruitment efforts, and on occasion, platelet shortages can compromise the care of thrombocytopenic patients.

Deregulated gene expression pathways in myelodysplastic syndrome hematopoietic stem cells

To gain insight into the molecular pathogenesis of the myelodysplastic syndromes (MDS), we performed global gene expression profiling and pathway analysis on the hematopoietic stem cells (HSC) of 183 MDS patients as compared with the HSC of 17 healthy controls. The most significantly deregulated pathways in MDS include interferon signaling, thrombopoietin signaling and the Wnt pathways. Among the most significantly deregulated gene pathways in early MDS are immunodeficiency, apoptosis and chemokine signaling, whereas advanced MDS is characterized by deregulation of DNA damage response and checkpoint pathways. We have identified distinct gene expression profiles and deregulated gene pathways in patients with del(5q), trisomy 8 or -7/del(7q). Patients with trisomy 8 are characterized by deregulation of pathways involved in the immune response, patients with -7/del(7q) by pathways involved in cell survival, whereas patients with del(5q) show deregulation of integrin signaling and cell cycle regulation pathways. This is the first study to determine deregulated gene pathways and ontology groups in the HSC of a large group of MDS patients. The deregulated pathways identified are likely to be critical to the MDS HSC phenotype and give new insights into the molecular pathogenesis of this disorder, thereby providing new targets for therapeutic intervention.

Critical role for ERK1/2 in bone marrow and fetal liver-derived primary megakaryocyte differentiation, motility, and proplatelet formation

Objective

Megakaryopoiesis and platelet formation is a multistep process through which hematopoietic progenitor cells develop into mature megakaryocytes (MKs) and form proplatelets. The present study investigates the regulation of different steps of megakaryopoiesis (i.e., differentiation, migration, and proplatelet formation) by extracellar signal-regulated kinase (ERK)1/2 and p38 mitogen-activated protein kinase (MAPK) in two models of primary murine MKs derived from bone marrow (BM) cells and fetal liver (FL) cells.

Materials and Methods

A preparation of MKs was generated from BM obtained from femora and tibiae of C57BL6 mice. FL-derived MKs were obtained from the liver of mouse fetuses aged 13 to 15 days.

Results

For both cell populations, activation of MEK-ERK1/2 pathway by thrombopoietin was found to have a critical role in MK differentiation, regulating polyploidy and surface expression of CD34, GPIIb, and GPIb. The MEK-ERK1/2 pathway plays a major role in migration of BM-derived MKs toward a stromal-cell−derived factor 1α (SDF1α) gradient, whereas unexpectedly, FL-derived cells fail to migrate in response to the chemokine due to negligible expression of its receptor, CXCR4. The MEK-ERK1/2 pathway also plays a critical role in the generation of proplatelets. In contrast, p38MAPK pathway was not involved in any of these processes.

Conclusion

This report demonstrates a critical role of MEK-ERK1/2 pathway in MK differentiation, motility, and proplatelet formation. This study highlights several differences between BM- and FL-derived MKs, which are discussed.

c-Myc-mediated control of cell fate in megakaryocyte-erythrocyte progenitors

It has been found that c-Myc protein plays a critical role in controlling self-renewal versus differentiation in hematopoietic stem cells. We report that c-Myc also controls the fate of megakaryocyte-erythrocyte progenitors through regulating the differentiation of erythroid and megakaryocytic progenitors. In addition to the significant reduction of granulocytes/macrophages and B and T lymphocytes because of the reduction of their corresponding progenitors, we found significantly increased numbers of megakaryocytic progenitors and mature megakaryocytes in bone marrow and spleens of c-Myc-knockout (c-Myc(-/-)) mice. Differentiation of erythrocytes was blocked at the erythroid progenitor stage. This increased megakaryocytopoiesis is a cell-intrinsic defect of c-Myc-mutant hematopoietic stem cells, as shown by transplantation studies. Furthermore, we found that c-Myc is required for polyploidy formation but not for cytoplasmic maturation of megakaryocytes. Megakaryocytes from c-Myc(-/-) mice are significantly smaller in size and lower in ploidy than those of control mice; however, because of the dramatic increase in megakaryocyte number, although fewer platelets are produced by each megakaryocyte, a greater than 3-fold increase in platelet number was consistently observed in c-Myc(-/-) mice. Thus, c-Myc(-/-) mice develop a syndrome of severe thrombocytosis-anemia-leukopenia because of significant increases in megakaryocytopoiesis and concomitant blockage of erythrocyte differentiation and reductions in myelolymphopoiesis.

Pharmacokinetic and pharmacodynamic modeling of pegylated thrombopoietin mimetic peptide (PEG-TPOm) after single intravenous dose administration in healthy subjects

Pegylated thrombopoietin mimetic peptide (PEG-TPOm) is a novel, potent thrombopoietin receptor agonist with low immunotoxicity potential that protects against chemotherapy-induced thrombocytopenia in preclinical animal models. The aim of this study was to develop a population pharmacokinetic and pharmacodynamic model of PEG-TPOm following single intravenous doses in healthy subjects. Data were obtained from a double-blind, randomized, placebo-controlled study. A model based on target-mediated drug disposition and precursor pool life spans was applied. Model evaluation was performed through predictive checks and bootstrap analysis. The half-life of PEG-TPOm ranged between 18 and 36 hours, and the estimated distributional volume was 5 L. The increase in platelet counts was observed after a 4-day delay, consistent with the megakaryocyte cell life span. The platelet life span was estimated to be 5 days. After maximum platelets counts were achieved on day 9, platelets returned back to baseline on day 29. Model-based simulations were undertaken to explore pharmacodynamic effects after multiple dosing. Weekly dosing produced a sustained pharmacodynamic response, whereas an interdosing interval >or=2 weeks resulted in fluctuating pharmacodynamic profiles. Thus, the mechanistic pharmacokinetic/pharmacodynamic model was suitable for describing the complex PEG-TPOm pharmacokinetics/pharmacodynamics, including target-mediated disposition, dose-dependent platelet stimulation, and mean life spans of thrombopoietic cell populations.

An ENU-induced recessive mutation in Mpl leads to thrombocytopenia with overdominance

OBJECTIVE

The aim of this study was to identify and characterize the causative mutation in the thrombocytopenic mouse strain HLB219 that was generated at the Jackson Laboratory as part of a large-scale N-ethyl-N-nitrosourea mutagenesis screen.

MATERIALS AND METHODS

The HLB219 mutation was identified by interval mapping of F2 mice generated from intercross breeding of HLB219 to both BALB/cByJ (BALB) and 129/SvImJ (129/Sv). Mpl was identified as a candidate gene and sequenced. The mutation was characterized in vivo in mouse hematopoietic stem/progenitor cell assays and in cell culture by expression in Ba/F3 cells.

RESULTS

A novel mutation in the thrombopoietin (TPO) receptor Mpl in HLB219 mice caused a Cys-->Arg substitution at codon 40 in the extracellular region of the receptor. Mice homozygous for the Mpl(hlb219) mutation had an 80% decrease in the number of platelets in comparison to the wild-type C57BL/6J strain, low numbers of bone marrow megakaryocytes, high TPO levels, and decreased competitive repopulating ability, consistent with a loss-of-function mutation in the receptor. Mice heterozygous for Mpl(hlb219) however, showed an overdominance effect with a significant increase in platelet number. Functional analysis in vitro demonstrated that Ba/F3 cells expressing the mutant MPL(hlb219) protein failed to activate extracellular signal-regulated kinase and signal transducers and activators of transcription 5, but proliferated in the absence of TPO and required constitutive phosphorylation of RAC-alpha serine/threonine protein kinase (AKT) for cytokine-independent growth.

CONCLUSION

Thrombocytopenia in HLB219 mice is caused by a recessive mutation in Mpl that abrogates mitogen-activated protein kinase-extracellular signal regulated kinase and janus kinase-signal transducers and activators of transcription signaling.

Prolonged continuous in vitro human platelet production using three-dimensional scaffolds

OBJECTIVE

Methods producing human platelets using growth on plastic, on feeder layers, or in suspension have been described. We hypothesized that growth of hematopoietic progenitors in a three-dimensional (3D) scaffold would enhance platelet production sans feeder layer.

MATERIALS AND METHODS

We grew CD34 positively selected human cord blood cells in surgical-grade woven polyester fabric or purpose-built hydrogel scaffolds using a cocktail of cytokines.

RESULTS

We found production of functional platelets over 10 days with two-dimensional (2D), 24 days with 3D scaffolds in wells, and more than 32 days in a single-pass 3D perfusion bioreactor system. Platelet numbers produced daily were higher in 3D than 2D, and much higher in the 3D perfusion bioreactor system. Platelet output increased in hydrogel scaffolds coated with thrombopoietin and/or fibronectin, although this effect was largely obviated with markedly increased production caused by changes in added cytokines. In response to thrombin, the platelets produced aggregated and displayed increased surface CD62 and CD63.

CONCLUSION

Use of 3D scaffolds, especially in a bioreactor-maintained milieu, may allow construction of devices for clinical platelet production without cellular feeder layers.

Pathobiology of secondary immune thrombocytopenia

Primary immune thrombocytopenic purpura (ITP) remains a diagnosis of exclusion both from nonimmune causes of thrombocytopenia and immune thrombocytopenia that develops in the context of other disorders (secondary immune thrombocytopenia). The pathobiology, natural history, and response to therapy of the diverse causes of secondary ITP differ from each other and from primary ITP, so accurate diagnosis is essential. Immune thrombocytopenia can be secondary to medications or to a concurrent disease, such as an autoimmune condition (eg, systemic lupus erythematosus [SLE], antiphospholipid antibody syndrome [APS], immune thyroid disease, or Evans syndrome), a lymphoproliferative disease (eg, chronic lymphocytic leukemia or large granular T-lymphocyte lymphocytic leukemia), or chronic infection, eg, with Helicobacter pylori, human immunodeficiency virus (HIV), or hepatitis C virus (HCV). Response to infection may generate antibodies that cross-react with platelet antigens (HIV, H pylori) or immune complexes that bind to platelet Fcgamma receptors (HCV), and platelet production may be impaired by infection of megakaryocyte (MK) bone marrow-dependent progenitor cells (HCV and HIV), decreased production of thrombopoietin (TPO), and splenic sequestration of platelets secondary to portal hypertension (HCV). Sudden and severe onset of thrombocytopenia has been observed in children after vaccination for measles, mumps, and rubella or natural viral infections, including Epstein-Barr virus, cytomegalovirus, and varicella zoster virus. This thrombocytopenia may be caused by cross-reacting antibodies and closely mimics acute ITP of childhood. Proper diagnosis and treatment of the underlying disorder, where necessary, play an important role in patient management.

Effects of sepsis on neonatal thrombopoiesis

We serially evaluated the effects of sepsis and/or necrotizing enterocolitis (NEC) on neonatal thrombopoiesis, using a panel of tests that included platelet counts, thrombopoietin concentrations (Tpo), circulating megakaryocyte progenitor concentrations (CMPs), and reticulated platelets (RPs). Variables analyzed included sepsis type, time after onset of sepsis, platelet counts, and gestational (GA) and postconceptional ages (PCA). Twenty neonates were enrolled. Ten had Gram-negative, six had Gram-positive, and four had presumed sepsis. Four neonates had NEC stage II or higher, and six developed thrombocytopenia. Overall, septic neonates had significantly elevated Tpo concentrations and circulating megakaryocyte progenitors. The highest Tpo levels were associated with Gram-negative or presumed sepsis. RP percentages were increased only in neonates with low platelet counts, while RP counts (RP% x platelet count) were elevated in neonates with high platelet counts. Our findings suggest that septic neonates up-regulate Tpo production, leading to increased megakaryocytopoiesis and platelet release, although the degree of upregulation is moderate. The changes in RP% and RP count most likely reflect increased thrombopoiesis with variable degrees of platelet consumption. In addition, our findings suggest that different factors, likely including level of illness and/or specific platelet or bacterial products, can down-regulate the magnitude of the thrombopoietic response.

Thrombopoietin and its splicing variants: structure and functions in thrombopoiesis and beyond

Since its cloning in 1994, several studies have reported that thrombopoietin (THPO) presents several alternative splicing products that differ from the full-length protein in its 5' UTR, N- or C-terminal regions. Most of these splice variants are evolutionarily conserved and have been detected in different tissues as well as in cell lines. Although the possible functions of the THPO isoforms are still elusive, different clues link them to the peculiar mechanism that regulates THPO production. Moreover, novel fields to explore possible roles of the THPO variants are opened by observations that this hormone can influence the formation of hematopoietic progenitors and its expression occurs in some tumors as well as in tissues not directly related to the thrombopoiesis. In this review, we summarize the structure and functions of THPO through the published evidence on its splicing isoforms and discuss about their involvement with physiopathologic phenomena.

Interferon-alpha 2b-induced thrombocytopenia is caused by inhibition of platelet production but not proliferation and endomitosis in human megakaryocytes

Human interferon (IFN)-alpha is the standard therapy for chronic hepatitis C to prevent its progression to liver cirrhosis and hepatocellular carcinoma. Thrombocytopenia is one of the major adverse effects of IFN-alpha and often leads to dose reduction or treatment discontinuation. However, there is little information on how IFN-alpha inhibits human megakaryopoiesis. In this study, we demonstrated that IFN-alpha did not inhibit colony formation of megakaryocytes from human CD34(+) hematopoietic stem cells. IFN-alpha did not inhibit endomitosis but did inhibit cytoplasmic maturation of megakaryocytes and platelet production in vitro. IFN-alpha suppressed the expression of transcription factors regulating late-stage megakaryopoiesis, such as GATA-1, p45(NF-E2), MafG. IFN-alpha also significantly reduced the number of human platelets but not megakaryocytes, and did not inhibit endomitosis of human megakaryocytes in immunodeficient NOD/Shi-scid/IL-2R gamma(null) (NOG) mice transplanted with human CD34(+) cells (hu-NOG). We also demonstrated that a novel thrombopoietin mimetic, NIP-004, was effective for treating IFN-alpha-induced thrombocytopenia in hu-NOG mice. From ultrastructural study, IFN-alpha inhibited the maturation of demarcation membranes in megakaryocytes, although NIP-004 prevented the inhibitory effects of IFN-alpha. These results defined the pathogenesis of IFN-alpha-induced thrombocytopenia and suggested possible future clinical applications for thrombopoietin mimetics.

Review article: pharmacological approaches for the treatment of thrombocytopenia in patients with chronic liver disease and hepatitis C infection

BACKGROUND

Patients with chronic liver disease and hepatitis C virus (HCV) frequently experience thrombocytopenia that complicates the management of their disease. Traditional therapy for thrombocytopenia consists of platelet transfusion, which can be associated with significant safety and economic issues. Consequently, efforts have been directed toward developing novel approaches for the treatment of thrombocytopenia.

AIM

To summarize the available data on the limitations of traditional therapies and the effects of novel therapies currently in clinical development for the treatment of thrombocytopenia.

RESULTS

Recent research has begun to reveal the complex mechanisms that regulate thrombopoiesis. Cytokines and growth factors, such as interleukin-11 and thrombopoietin (TPO), play a key role in the production of platelets. A number of recent clinical studies have provided evidence that pharmacologic agents that target megakaryocyte precursors and stimulate thrombopoiesis can effectively reverse thrombocytopenia. Here, we review the regulation of thrombopoiesis, the role of TPO, and a number of novel compounds that stimulate platelet production by acting through the TPO receptor. Agents that stimulate TPO include the orally available nonpeptidic agonists eltrombopag and AKR-501, peptidic agonists AMG-531 and Peg-TPOmp, and small engineered antibodies.

CONCLUSION

Results from clinical trials with these agents in healthy subjects confirm that activation of thrombopoiesis via the TPO pathway is an effective method of stimulating platelet production. This approach may provide safer, more effective treatment for thrombocytopenia in patients with chronic liver disease. Several of these agents are currently being tested in large scale trials.

Sites and kinetics of donor thrombopoiesis following transplantation of whole bone marrow and progenitor subsets

INTRODUCTION

Little is known about the sites and kinetics of thrombopoiesis following bone marrow transplant. The spleen is a site of hematopoiesis in a healthy mouse, and hematopoietic activity increases in response to stress. We hypothesized that the spleen is a major site of early post-transplant thrombopoiesis.

METHODS

We transplanted whole bone marrow (WBM) or lineage depleted progenitor subsets fractionated based on expression of c-kit and Sca-1 from transgenic mice expressing green fluorescent protein into lethally irradiated C57BL/6 recipients. We also transplanted whole bone marrow cells into healthy and splenectomized mice. Post-transplant megakaryopoiesis was assessed by measuring circulating platelet number, percent donor-derived platelets, bone marrow cellularity, splenic weight, megakaryocyte size, and megakaryocyte concentration from hour 3 to day 28 post transplant.

RESULTS

Following transplant, circulating donor-derived platelets were derived only from c-kit expressing subsets. Donor-derived platelets first appeared on post-transplant day five. Splenectomy reduced the number of these earliest circulating platelets. Splenic megakaryopoiesis increased dramatically from day 7-14 post-transplant. However, splenectomy accelerated platelet engraftment during this time frame.

CONCLUSION

Overall, these results demonstrate that the first platelets are produced by c-kit expressing megakaryocyte progenitors in the bone marrow and spleen. After post-transplant day 5, the net effect of the spleen on thrombopoiesis is to slow engraftment due to immune effects or hypersplenism.

Effect of Human Thrombopoietin-Modified Bone Marrow Mesenchymal Stem Cells Mediated by Recombinant Adeno-Associated Virus on Megakaryocytopoiesis

Recently, the research of recombinant thrombopoietin (TPO) and its subsequent use in treating thrombocytopenia following radiation therapy and chemotherapy have become more important in clinics. Our study was to determine the feasibility of recombinant adeno-associated virus (rAAV)-mediated TPO gene transfer into bone marrow-derived mesenchymal stem cells (MSCs) and to evaluate the conditioned medium (CM) obtained from TPO-transduced human (h) hMSCs for promoting the process of megakaryocytopoiesis. We constructed recombinant adeno-associated viruses expressing TPO successfully, and TPO mRNA and protein were both strongly expressed in TPO-transduced hMSCs. There was no decrease in green fluorescent protein (GFP) fluorescence expression of the transduced cells with continuous passaged culturing in vitro. The CM of TPO-transduced hMSCs has been shown to enhance the number of CD41(+) cells and megakaryocytic progenitors (colony-forming unit-megakaryocyte) significantly as compared to the nontransduced control. In this study, a novel safe and efficient method of promoting the megakaryocytopoiesis was established following the TPO-transduced hMSCs. These results provide a basis for the future studies on hematopoietic regulation by hMSCs transfected with TPO.

Megakaryocyte development and platelet production

Megakaryocytopoiesis involves the commitment of haematopoietic stem cells, and the proliferation, maturation and terminal differentiation of the megakaryocytic progenitors. Circulating levels of thrombopoietin (TPO), the primary growth-factor for the megakaryocyte (MK) lineage, induce concentration-dependent proliferation and maturation of MK progenitors by binding to the c-Mpl receptor and signalling induction. Decreased platelet turnover rates results in increased concentration of free TPO, enabling the compensatory response of marrow MKs to increased platelet production. C-Mpl activity is orchestrated by a complex cascade of signalling molecules that induces the action of specific transcription factors to drive MK proliferation and maturation. Mature MKs form proplatelet projections that are fragmented into circulating particles. Newly developed thrombopoietic agents operating via c-Mpl receptor may prove useful in supporting platelet production in thrombocytopenic state. Herein, we review the regulation of megakaryocytopoiesis and platelet production in normal and disease state, and the new approaches to thrombopoietic therapy.

Clinical approaches involving thrombopoietin to shorten the period of thrombocytopenia after high-dose chemotherapy

High-dose chemotherapy followed by a peripheral blood stem cell transplant is successfully used for a wide variety of malignancies. A major drawback, however, is the delay in platelet recovery. Several clinical strategies using thrombopoietin (Tpo) have been developed in an attempt to speed up platelet repopulation. In contrast to its success in immune thrombocytopenia and in low-dose toxic chemotherapeutic regimens, Tpo appears less effective in the case of high-dose chemotherapy and peripheral blood stem cell transplant. To develop a successful therapeutic approach, more knowledge is needed on several aspects of megakaryocyte (progenitor) biology, such as homing to the bone marrow, endomitosis, and platelet formation. Interactions of the megakaryocytes with the marrow vasculature and the microvascular microenvironment are other key factors for optimal thrombocytopoiesis. The present report reviews the background of the inefficiency of Tpo after intensive chemotherapy and describes possible strategies that might lead to successful therapies to treat chemotherapy-induced thrombocytopenia.

Differential effects of recombinant thrombopoietin and bone marrow stromal-conditioned media on neonatal versus adult megakaryocytes

Umbilical cord blood (CB) is a valuable source of stem cells for transplantation, but CB transplantations are frequently complicated by delayed platelet engraftment. The reasons underlying this are unclear. We hypothesized that CB- and peripheral-blood (PB)-derived megakaryocytes (MKs) respond differently to the adult hematopoietic microenvironment and to thrombopoietin (Tpo). To test this, we cultured CB- and PB-CD34(+) cells in adult bone marrow stromal conditioned media (CM) or unconditioned media (UCM) with increasing concentrations of recombinant Tpo and compared the effects of these conditions on CB-versus PB-MKs. PB-MKs reached highest ploidy in response to UCM + 100 ng/mL rTpo, and the addition of CM inhibited their maturation. In contrast, CB-MKs reached highest ploidy in CM without rTpo, and high rTpo concentrations (> 0.1 ng/mL) inhibited their maturation. This is the first evidence that human neonatal and adult MKs have substantially different biologic responses to Tpo and potentially to other cytokines.

The role of megakaryocytes in skeletal homeostasis and rheumatoid arthritis

PURPOSE OF REVIEW

This review provides an update on the role of megakaryocytes in skeletal homeostasis, and discusses these findings in the context of rheumatoid arthritis.

RECENT FINDINGS

Thrombocytosis is a common complication of rheumatoid arthritis, and is presumably caused by an up-regulation in megakaryocytopoiesis. In general, patients with rheumatoid arthritis exhibit localized joint bone erosion with systemic bone loss, and rheumatoid arthritis patients with thrombocytosis tend to have more severe disease. Interestingly, in addition to their role in rheumatoid arthritis with thrombocytosis, it has been demonstrated recently that megakaryocytes play a dual role in regulating skeletal mass by inhibiting bone resorption while simultaneously stimulating bone formation. This seeming contradiction in the putative role of megakaryocytes in skeletal regulation and rheumatoid arthritis is the focus of this review.

SUMMARY

In rheumatoid arthritis there are substantial increases in the levels of several pro-inflammatory pleiotropic cytokines. As would be expected, in addition to their role in inflammation, these cytokines play a critical role in the megakaryocytopoiesis seen in patients who develop reactive thrombocytosis, and these cytokines also are known to regulate osteoclastogenesis. Thus, it appears that in rheumatoid arthritis with reactive thrombocytosis, the ability of the cytokines to enhance osteoclastogenesis outweighs the ability of megakaryocytes to inhibit osteoclastogenesis.

Inhibition of TPO-induced MEK or mTOR activity induces opposite effects on the ploidy of human differentiating megakaryocytes

The megakaryocyte is a paradigm for mammalian polyploid cells. However, the mechanisms underlying megakaryocytic polyploidization have not been elucidated. In this study, we investigated the role of Shc-Ras-MAPK and PI3K-AKT-mTOR pathways in promoting megakaryocytic differentiation, maturation and polyploidization. CD34+ cells, purified from human peripheral blood, were induced in serum-free liquid suspension culture supplemented with thrombopoietin (TPO) to differentiate into a virtually pure megakaryocytic progeny (97-99% CD61+/CD41+ cells). The early and repeated addition to cell cultures of low concentrations of PD98059, an inhibitor of MEK1/2 activation, gave rise to a population of large megakaryocytes showing an increase in DNA content and polylobated nuclei (from 45% to 70% in control and treated cultures, respectively). Conversely, treatment with the mTOR inhibitor rapamycin strongly inhibited cell polyploidization, as compared with control cultures. Western blot analysis of PD98059-treated progenitor cells compared with the control showed a downmodulation of phospho-ERK 1 and phospho-ERK 2 and a minimal influence on p70S6K activation; by contrast, p70S6K activation was completely inhibited in rapamycin-treated cells. Interestingly, the cyclin D3 localization was nuclear in PD98059-induced polyploid megakaryocytes, whereas it was completely cytoplasmic in those treated with rapamycin. Altogether, our results are in line with a model in which binding of TPO to the TPO receptor (mpl) could activate the rapamycin-sensitive PI3K-AKT-mTOR-p70S6K pathway and its downstream targets in promoting megakaryocytic cell polyploidization.

The molecular mechanisms that control thrombopoiesis

Our understanding of thrombopoiesis--the formation of blood platelets--has improved greatly in the last decade, with the cloning and characterization of thrombopoietin, the primary regulator of this process. Thrombopoietin affects nearly all aspects of platelet production, from self-renewal and expansion of HSCs, through stimulation of the proliferation of megakaryocyte progenitor cells, to support of the maturation of these cells into platelet-producing cells. The molecular and cellular mechanisms through which thrombopoietin affects platelet production provide new insights into the interplay between intrinsic and extrinsic influences on hematopoiesis and highlight new opportunities to translate basic biology into clinical advances.

Mesenchymal stem cells feeder layer from human umbilical cord blood for ex vivo expanded growth and proliferation of hematopoietic progenitor cells

Ex vivo expansion of hematopoietic stem cells was suggested as the best way of overcoming problems caused by limited hematopoietic cell number for cord blood transplantation. In this study, we quantified and characterized an ex vivo expansion capacity of umbilical cord blood (UCB)-derived mesenchymal stem cells (MSCs) as a cell feeder layer for support of UCB-derived committed hematopoietic progenitor cells (HPCs) in the absence or presence of recombinant cytokines. The UCB-derived MSCs used in the study differentiated into osteoblast, chondrocytes, and adipocytes under proper conditions. Frequencies in colony forming unit-granulocyte, macrophage, colony forming unit-granulocyte, erythrocyte, macrophage, megakaryocyte, burst forming unit-erythrocyte, and colony forming unit-erythrocyte increased to 3.46-, 9.85-, 3.64-, and 2.03-folds, respectively, only in culture supplemented by UCB-derived MSCs as a cell feeder layer without recombinant cytokines (culture condition C). Identified expansion kinetics in all kinds of committed HPCs showed plateaus at 7 culture days, suggesting some consumable components were required for the expansion. Physiological importance and different roles for different committed HPCs of UCB-derived MSCs as a cell feeder layer were revealed by a distinguished expansion capacity for colony forming unit-megakaryocyte. The preferred maintenance of CD33(-)CD34(+) in culture condition C was also identified. The presence of cobblestone-like areas as hematopoietic microenvironment and various cell feeder layer-originated hematopoietic cytokines including interleukin-1beta and granulocyte, macrophage-colony stimulating factor were suggested as underlying mechanisms for the identified expansion capacity. The present numeric and biological information about intrinsic expansion capacity for UCB-derived committed HPCs will increase further biological and clinical applications of UCB-derived MSCs.

A preliminary investigation into the action of anagrelide: Thrombopoietin–c-Mpl receptor interactions

OBJECTIVE

Many studies have validated the clinical efficacy of anagrelide to reduce platelet counts in thrombocythemic conditions. With the ability to support human megakaryopoiesis in vitro using thrombopoietin (TPO), specific investigation of changes in platelet levels can be carried out in human systems. Using CD34(+) stem cells and murine BaF3 cells transfected with the human or murine TPO receptor, c-Mpl (BaF3mpl), the effect of anagrelide on cell differentiation, proliferation, and signaling was examined in the presence of TPO.

METHODS

Inhibition of TPO-mediated cell differentiation by anagrelide was evaluated by fluorescein-activated cell sorting analysis. Cell proliferation was monitored by 3-(4,5-dimethylthiazol-2-yl)-5-3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assays. Effect of anagrelide on TPO-mediated phosphotyrosine (pTyr) activity was examined by Western analysis of whole cell lysates.

RESULTS

In the presence of TPO, anagrelide reduced the number of CD41(+) cells without a reduction in the total mononuclear cell number in a dose-dependent manner. Growth inhibition was also observed in BaF3 cells transfected with human c-Mpl. Anagrelide also reduced TPO-specific pTyr activity in a species-specific manner. No inhibitory effect could be demonstrated with interleukin-3 stimulation.

CONCLUSION

Parallel dose-response effects were found in both CD41(+) number and TPO-specific pTyr activity. These results suggest that anagrelide reduces TPO-mediated megakaryocyte proliferation of CD34(+) cells through a mechanism that leads to inhibition of intracellular signaling events. Furthermore, data also suggest that it is a species-specific effect, with no inhibitory activity against the murine receptor. Because there is a less than 10% difference in DNA sequence homology between human and murine receptors, the difference in sequence-specific activity must reside in these amino acid differences.

Pegylated recombinant human megakaryocyte growth and development factor suppresses the development of megakaryoblastic leukemia in mice

We examined the effects of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) on the development of L-8057, a murine megakaryoblastic leukemia that expresses the thrombopoietin receptor c-Mpl, in mice. PEG-rHuMGDF administration prolonged survival of L-8057 leukemic mice, in which L-8057 cell growth in the spleen was decreased. L-8057 cells harvested from PEG-rHuMGDF-treated leukemic mice had decreased ability to generate leukemic colonies in vitro as well as to induce leukemia in vivo. PEG-rHuMGDF administration also resulted in prolonged survival of mice transplanted with a c-Mpl-expressing erythroleukemia, but had no effect on survival of mice transplanted with a myeloblastic leukemia that does not possess c-Mpl. Thus, PEG-rHuMGDF suppresses the development of c-Mpl-expressing leukemia in vivo in mice.

Induction of megakaryocytopoiesis and thrombocytopoiesis by JTZ-132, a novel small molecule with thrombopoietin mimetic activities

We report in this paper that a novel small molecule, JTZ-132, induced growth and differentiation of megakaryocytic progenitor cells and improved thrombocytopenia in myelosuppressed mice. JTZ-132 stimulated proliferation of UT-7/TPO cells, a cell line highly sensitive to thrombopoietin (TPO), and exhibited full efficacy comparable to TPO with an approximate EC50 (median effective concentration) value of 0.43 μM, whereas little proliferation was observed in a TPO-insensitive cell line, UT-7/EPO, and human carcinoma cell line, HCT116. Signal transduction studies revealed that JTZ-132 induced tyrosine phosphorylation of c-Mpl, Janus kinase-2 (JAK2), and signal transducers and activators of transcription 5 (STAT5) in UT-7/TPO cells as well as TPO. JTZ-132 increased the number of megakaryocyte-specific marker, CD61+ and CD41+, cells in cultures of mouse and human bone marrow cells, respectively, and the colonyforming unit megakaryocytes in mouse bone marrow cells. In vivo experiments in x-ray irradiation– or busulfan injection–induced myelosuppressed mice demonstrated that subcutaneously injected JTZ-132 at 30 mg/kg showed significantly higher platelet number at nadir and accelerated platelet recovery without affecting white blood cell number. These data suggest that JTZ-132 is a novel stimulator of megakaryocytopoiesis and thrombocytopoiesis in vitro and in vivo with TPO mimetic activities and that it is useful for the treatment of thrombocytopenia.

Pharmacokinetic model of target-mediated disposition of thrombopoietin

Thrombopoietin, TPO, a 353 amino acid cytokine, is a primary regulator of platelet production that was cloned recently. A target-mediated (platelet receptors) pharmacokinetic model was developed to characterize the disposition of TPO. Receptor-mediated endocytosis was assigned as the major elimination pathway in the model. A nonspecific binding compartment was also incorporated into the model. TPO concentration vs time profiles from a published phase 1 and 2 clinical trial were used to apply this model. Noncompartmental analysis demonstrated that TPO exhibits nonlinear kinetics. The proposed model captured the concentration-time profiles relatively well. The first-order internalization rate constant was estimated as 0.1 h(-1). The endogenous binding capacity was estimated as 164.0 pM. The second-order binding association constant (k(on)) was 0.055 h(-1).pM(-1) and the first-order dissociation constant (k(off)) was estimated as 2.5 h(-1), rendering the equilibrium dissociation constant K(d) as 45.5 pM. This model may be relevant to other therapeutic agents with receptor-mediated endocytotic disposition.

Evaluation and treatment of severe and prolonged thrombocytopenia in neonates

Thrombocytopenia is one of the most common hematologic problems in the neonatal intensive care unit (NICU). Despite its prevalence,several basic pathophysiologic questions remain unanswered. For instance, there is a lack of evidence-based guidelines for treatment,and the kinetic mechanisms (decreased platelet production,increased platelet consumption, or sequestration) responsible for most varieties of neonatal thrombocytopenia are not well defined.Moreover, a clear correlation between degree of thrombocytopenia and the resulting bleeding risk has not been demonstrated, and no transfusion-trigger studies have been conducted in neonates. As a consequence of these deficiencies in knowledge, there is great variability in platelet transfusion practices among NICUs. This article presents an overview of the evaluation of a neonate with severe thrombocytopenia and a review of current and projected therapeutic options.

Expression of osteoprotegerin mRNA and protein in murine megakaryocytes

OBJECTIVE

Osteoprotegerin (OPG) is a soluble member of the tumor necrosis factor receptor superfamily critically involved in the regulation of bone resorption. Within the bone microenvironment, OPG is abundantly produced by osteoblast/stromal cells, and its expression is regulated by transforming growth factor-beta(1) (TGF-beta(1)). However, OPG expression and regulation in primary hematopoietic cells have not been fully investigated.

MATERIALS AND METHODS

Opg mRNA was studied in murine hematopoietic cells by semiquantitative reverse transcriptase-polymerase chain reaction. The OPG protein was identified by immunofluorescence labeling and secretion was assessed by enzyme-linked immunosorbent assay.

RESULTS

Opg transcripts were detected in platelets, megakaryocytes (MK), monocytes, and B lymphocytes, but not in erythroblasts, neutrophils, and T lymphocytes. Mature MK and proplatelets exhibited strong immunostaining for OPG outside the storage alpha-granules, and secretion was detected in the conditioned medium. To analyze whether opg transcription in MK was influenced by TGF-beta(1), the opg/GpIIb mRNA ratio was compared in cultured MK derived from TGF-beta(1) null mutants and wild-type littermates without or after the addition of bioactive TGF-beta(1). No difference was seen, indicating that opg expression in MK was not modulated by TGF-beta(1). However, mRNA levels were increased when thrombopoietin was present in the culture medium, suggesting that MK maturation was correlated with enhanced opg expression.

CONCLUSIONS

With these results we document for the first time that murine MK and platelets express OPG. This suggests a novel role for MK in bone homeostasis, in addition to its role in vascular homeostasis.

NMDA receptor-mediated regulation of human megakaryocytopoiesis

Identification of the regulatory inputs that direct megakaryocytopoiesis and platelet production is essential for the development of novel therapeutic strategies for the treatment of thrombosis and related hematologic disorders. We have previously shown that primary human megakaryocytes express the N-methyl-d-aspartate acid (NMDA) receptor 1 (NR1) subunit of NMDA-type glutamate receptors, which appear to be pharmacologically similar to those identified at neuronal synapses, responsible for mediating excitatory neurotransmission in the central nervous system. However, the functional role of NMDA receptor signaling in megakaryocytopoiesis remains unclear. Here we provide evidence that demonstrates the fundamental importance of this signaling pathway during human megakaryocyte maturation in vitro. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of RNA extracted from CD34+-derived megakaryocytes identified expression of NR2A and NR2D receptor subunits in these cells, as well as the NMDA receptor accessory proteins, Yotiao and postsynaptic density protein 95 (PSD-95). In functional studies, addition of a selective NMDA receptor antagonist, MK-801 inhibited proplatelet formation, without affecting proliferation or apoptosis. Exposure of CD34+ cells to MK-801 cultured for 14 days in the presence of thrombopoietin induced a decrease in expression of the megakaryocyte cell surface markers CD61, CD41a, and CD42a compared with controls. At an ultrastructural level, MK-801-treated cells lacked alpha-granules, demarcated membranes, and multilobed nuclei, which were prominent in untreated mature megakaryocyte controls. Using immunohistochemistry on sections of whole tibiae from c-Mpl knockout mice we demonstrated that megakaryocytic NMDA receptor expression was maintained following c-Mpl ablation. These data support a fundamental role for glutamate signaling in megakaryocytopoiesis and platelet production, which is likely to be independent of thrombopoietin-mediated effects.

Internalization of the thrombopoietin receptor is regulated by 2 cytoplasmic motifs

Receptor-mediated internalization appears to be the primary mean of regulating the plasma level of thrombopoietin (TPO). However, the processes that regulate Mpl internalization have not previously been described. Using the cytokine-dependent cell line BaF3, we have identified 2 distinct motifs within the cytoplasmic domain of Mpl that underlie ligand-dependent internalization. Removal of the fourth cytoplasmic tyrosine residue by deletion or truncation results in a significant decrease in maximal internalization. The remaining receptor internalization is abrogated by deletion of cytoplasmic residues 54-69, which include the core box2 region (L54L55E56I57L58) and the only dileucine motifs (L54L55 and I57L58) within the cytoplasmic domain of Mpl. Receptor internalization mediated by this latter subdomain does not require Jak2 activation. Furthermore, TPO-stimulated cellular proliferation appears to be directly correlated with receptor internalization, indicating that internalization of the TPO/Mpl complex may be essential for normal signal transduction. Finally, we have demonstrated that upon removal of TPO from the supernatant, Mpl promptly reappears on the cell surface, suggesting that a pool of intracellular Mpl can be rapidly recycled to the cell surface. These data help identify the receptor motifs involved in TPO-induced internalization of Mpl and suggest that Mpl translocation may be necessary for normal cellular proliferation.

The 4830C>A polymorphism within intron 5 affects the pattern of alternative splicing occurring within exon 6 of the thrombopoietin gene

OBJECTIVE

A common variant in intron 5 of the thrombopoietin (TPO) gene (4830C>A) has been associated with risk of myocardial infarction (MI). To explore the molecular mechanism of this association, the ability of the intron to act as a transcription enhancer and to influence mRNA splicing was tested.

METHOD AND RESULTS

In HepG2 cells the presence of intron 5 upstream of the TPO promoter decreased promoter activity to between 60% and 30%. This effect was orientation dependent; in the reverse orientation, intron 5 caused a twofold greater decrease in promoter activity compared to the forward orientation. However, the effects were similar with either the C or the 4830A allele. An in vitro exon trapping system was used to study the effect of the polymorphism on splicing events in exon 6. The full-length (TPO-1) and three previously reported splice variants (TPO-2, TPO-3, and TPO-5) were identified. The 4830A allele resulted in a small but statistically significant increase in production of the TPO-3 splice variant relative to the full-length transcript (10.6%+/-0.6%) compared to the 4830C allele (8.3%+/-0.6%) (p=0.02). Generation of TPO-5 was also slightly increased, but this did not reach significance.

CONCLUSION

The identification of a potential "silencer" sequence in intron 5 of the TPO gene demonstrates the complexity of control of expression of the gene. Although the precise role of the different splice variants is not known, the finding that the 4830C>A sequence change alters their relative amounts, suggests a possible molecular mechanism whereby TPO genotype may influence the risk of MI.

Thrombopoietin increases platelet adhesion under flow and decreases rolling

Thrombopoietin (TPO) is known to sensitize platelets to other agonists at 20 ng/ml, and above 100 ng/ml it is an independent activator of aggregation and secretion. In studies with a perfusion chamber, TPO, between 0.01 ng/ml and 1 ng/ml, increased platelet adhesion to surface-coated fibrinogen, fibronectin and von Willebrand Factor (VWF) but not to a collagen-coated surface. Increased adhesion was observed at shear rates of 300/s and 800/s in perfusions with whole blood as well as in suspensions of platelets and red blood cells reconstituted in plasma. The by the cyclooxygenase inhibitor, indomethacin, and the thromboxane A2-receptor blocker, SQ30741, abolished the stimulation by TPO. The effect of TPO was mimicked by a very low concentration (10 nmol/l) of the thromboxane TxA2 analogue, U46619. Real-time studies of platelet adhesion to a VWF-coated surface at a shear of 1000/s showed that about 20% of the platelets were in a rolling phase before they became firmly attached. TPO (1 ng/ml) pretreatment reduced this number to < 5%, an effect again abolished by indomethacin. Thus, TPO potentiates the direct and firm attachment of platelets to surface-coated ligands for alphaIIbbeta3, possibly by increasing the ligand affinity of the integrin.