Thrombopoietin: understanding and manipulating platelet production

Efficacy of Recombinant Human Thrombopoietin for the Treatment of Secondary Failure of Platelet Recovery After Allogeneic HSCT

Secondary failure of platelet recovery (SFPR) is a life-threatening complication that may affect up to 20% of patients after allogeneic hematopoietic stem cell transplantation (HSCT). In this study, to evaluate the efficacy of recombinant human thrombopoietin (rhTPO), we retrospectively analyzed 29 patients who received continuous rhTPO for the treatment of SFPR. Overall response and complete response were observed in 24 (82.8%) patients and 10 (34.5%) patients, at a median time of 21.5 days (range, 3-41 days) and 39.5 days (range, 7-53 days) after initiation of rhTPO treatment, respectively. Among the responders, the probability of keeping overall response and complete response at 1 year after response was 77.3% and 80.0%, respectively. In multivariate analysis, higher CD34⁺ cells (≥3 × 10⁶/kg) infused during HSCT (HR: 7.22, 95% CI: 1.53-34.04, P = 0.01) and decreased ferritin after rhTPO treatment (HR: 6.16, 95% CI: 1.18-32.15, P = 0.03) were indicated to associate with complete response to rhTPO. Importantly, rhTPO was well tolerated in all patients without side effects urging withdrawal and clinical intervention. The results of this study suggest that rhTPO may be a safe and effective treatment for SFPR.

rhTPO combined with chemotherapy and G-CSF for autologous peripheral blood stem cells in patients with refractory/relapsed non-Hodgkin's lymphoma

Objective

The mobilization and collection of sufficient autologous peripheral blood stem cells (APBSCs) are important for the fast and sustained reconstruction of hematopoietic function after autologous transplantation. This study aims to evaluate the mobilization effect and safety of thrombopoietin (TPO) combined with chemotherapy + G-CSF for APBSCs in patients with refractory/relapsed non-Hodgkin’s lymphoma.

Methods

A total of 78 patients were included in the present study. After receiving mobilization chemotherapy, all patients were randomly divided into two groups: TPO group (n=40), patients were given subcutaneous injection of rhTPO + G-CSF, and control group (n=38), patients were given subcutaneous injection of G-CSF. The primary endpoint was the total number of obtained CD34+ cells. The secondary endpoints were the mononuclear cell count, the proportion of target and minimum mobilization, the engraftment time of neutrophils and platelets after APBSCT, the number of platelet and red blood cell infusions, the incidence of infectious fever and fever duration, and TPO-related side effects in patients.

Results

TPO participation significantly increased the total CD34+ cell count. A higher proportion of patients in the TPO group achieved the minimum and target CD34+ cells, when compared to the control group. TPO-related adverse events were not observed in either of these groups. In addition, there were no significant differences in engraftment time, the number of platelet and red blood cell transfusions, the incidence of infectious fever, and fever duration between these two groups.

Conclusion

TPO combined with chemotherapy + G-CSF can safely and effectively enhance the mobilization effect for APBSCs in patients with refractory/relapsed non-Hodgkin’s lymphoma.

Crosstalk between Platelet and Bacteria: A Therapeutic Prospect

Platelets are typically recognized for their roles in the maintenance of hemostasis and vascular wall repair to reduce blood loss. Beyond hemostasis, platelets also play a critical role in pathophysiological conditions like atherosclerosis, stroke, thrombosis, and infections. During infection, platelets interact directly and indirectly with bacteria through a wide range of cellular and molecular mechanisms. Platelet surface receptors such as GPIbα, FcγRIIA, GPIIbIIIa, and TLRs, etc. facilitate direct interaction with bacterial cells. Besides, the indirect interaction between platelet and bacteria involves host plasma proteins such as von Willebrand Factor (vWF), fibronectin, IgG, and fibrinogen. Bacterial cells induce platelet activation, aggregation, and thrombus formation in the microvasculature. The activated platelets induce the Neutrophil Extracellular Traps (NETs) formation, which further contribute to thrombosis. Thus, platelets are extensively anticipated as vital immune modulator cells during infection, which may further lead to cardiovascular complications. In this review, we cover the interaction mechanisms between platelets and bacteria that may lead to the development of thrombotic disorders. Platelet receptors and other host molecules involved in such interactions can be used to develop new therapeutic strategies to combat against infection-induced cardiovascular complications. In addition, we highlight other receptor and enzyme targets that may further reduce infection-induced platelet activation and various pathological conditions.

Recombinant Human Thrombopoietin Promotes Platelet Engraftment and Improves Prognosis of Patients with Myelodysplastic Syndromes and Aplastic Anemia after Allogeneic Hematopoietic Stem Cell Transplantation

Poor platelet graft function (PPGF) is a significant complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT). However, no optimal treatment has been recommended. This study investigated aspects of platelet recovery after allo-HSCT, including prognostic value and the effect of recombinant human thrombopoietin (rhTPO). We retrospectively analyzed 275 patients who received allo-HSCT in our center. Of them, 135 (49.1%) patients had good platelet graft function (GPGF) and 140 (50.9%) had PPGF. The latter included 59 (21.5%) patients with primary PPGF and 81 (29.4%) with secondary PPGF. Multivariate analysis showed that male gender (P = .024), lower CD34⁺ cell count (P = .04), and no use of rhTPO (P <.001) were associated with PPGF. The 3-year overall survival rate of patients with PPGF (58%) was significantly less than that of patients with GPGF (82%; P <.001). We further analyzed the effect of rhTPO on prognosis of patients after allo-HSCT. Although no advantage was apparent when analyzing the entire cohort, for patients with myelodysplastic syndromes and aplastic anemia, rhTPO was associated with a significant survival advantage (P = .014).

Anesthetic Considerations for Adult Cardiac Surgery Patients with Congenital Heart Disease

The number of adults with congenital heart disease surviving into adulthood is increasing. The proportion of adults undergoing revision of a previous repair is increasing in comparison to those that present for a palliative or curative operation. At the Toronto Congenital Cardiac Centre for Adults, 528 patients underwent cardiac surgery between January 1, 1992 and December 31, 2001. The anesthetic management of the surgical correction of simple and complex congenital heart lesions includes general physiologic considerations such as dysrhythmias, hypoxemia, polycythemia, and pulmonary hypertension. Palliative shunts from early childhood have anatomical and physiologic implications for the adult. Preparation for the operating room and postoperative care are natural extensions of the anesthetic management of the surgical correction of the congenital heart lesions. Anesthetic management of septal lesions in the interventional suite and operating room is discussed. Complex lesions such as tetralogy of Fallot, transposition of the great arteries, Glenn anastomosis, and the Fontan operation are reviewed. The anesthetic management of these patients is rewarding but impossible without an integrated team approach involving cardiologists, surgeons, perfusionists, and nursing staff.

Developing a co-culture system for effective megakaryo/thrombopoiesis from umbilical cord blood hematopoietic stem/progenitor cells

BACKGROUND AIMS

Platelet transfusion can be a life-saving procedure in different medical settings. Thus, there is an increasing demand for platelets, of which shelf-life is only 5 days. The efficient ex vivo biomanufacturing of platelets would allow overcoming the shortages of donated platelets.

METHODS

We exploited a two-stage culture protocol aiming to study the effect of different parameters on the megakaryo/thrombopoiesis ex vivo. In the expansion stage, human umbilical cord blood (UCB)-derived CD34(+)-enriched cells were expanded in co-culture with human bone marrow mesenchymal stromal cells (BM-MSCs). The megakaryocytic commitment and platelet generation were studied, considering the impact of exogenous addition of thrombopoietin (TPO) in the expansion stage and a cytokine cocktail (Cyt) including TPO and interleukin-3 in the differentiation stage, with the use of different culture medium formulations, and in the presence/absence of BM-MSCs (direct versus non-direct cell-cell contact).

RESULTS

Our results suggest that an early megakaryocytic commitment, driven by TPO addition during the expansion stage, further enhanced megakaryopoiesis. Importantly, the results suggest that co-culture with BM-MSCs under serum-free conditions combined with Cyt addition, in the differentiation stage, significantly improved the efficiency yield of megakaryo/thrombopoiesis as well as increasing %CD41, %CD42b and polyploid content; in particular, direct contact of expanded cells with BM-MSCs, in the differentiation stage, enhanced the efficiency yield of megakaryo/thrombopoiesis, despite inhibiting their maturation.

CONCLUSIONS

The present study established an in vitro model for the hematopoietic niche that combines different biological factors, namely, the presence of stromal/accessory cells and biochemical cues, which mimics the BM niche and enhances an efficient megakaryo/thrombopoiesis process ex vivo.

Dynamical modelling of haematopoiesis: an integrated view over the system in homeostasis and under perturbation

A very high number of different types of blood cells must be generated daily through a process called haematopoiesis in order to meet the physiological requirements of the organism. All blood cells originate from a population of relatively few haematopoietic stem cells residing in the bone marrow, which give rise to specific progenitors through different lineages. Steady-state dynamics are governed by cell division and commitment rates as well as by population sizes, while feedback components guarantee the restoration of steady-state conditions. In this study, all parameters governing these processes were estimated in a computational model to describe the haematopoietic hierarchy in adult mice. The model consisted of ordinary differential equations and included negative feedback regulation. A combination of literature data, a novel divide et impera approach for steady-state calculations and stochastic optimization allowed one to reduce possible configurations of the system. The model was able to recapitulate the fundamental steady-state features of haematopoiesis and simulate the re-establishment of steady-state conditions after haemorrhage and bone marrow transplantation. This computational approach to the haematopoietic system is novel and provides insight into the dynamics and the nature of possible solutions, with potential applications in both fundamental and clinical research.

Tryptophan at the transmembrane-cytosolic junction modulates thrombopoietin receptor dimerization and activation

Dimerization of single-pass membrane receptors is essential for activation. In the human thrombopoietin receptor (TpoR), a unique amphipathic RWQFP motif separates the transmembrane (TM) and intracellular domains. Using a combination of mutagenesis, spectroscopy, and biochemical assays, we show that W515 of this motif impairs dimerization of the upstream TpoR TM helix. TpoR is unusual in that a specific residue is required for this inhibitory function, which prevents receptor self-activation. Mutations as diverse as W515K and W515L cause oncogenic activation of TpoR and lead to human myeloproliferative neoplasms. Two lines of evidence support a general mechanism in which W515 at the intracellular juxtamembrane boundary inhibits dimerization of the TpoR TM helix by increasing the helix tilt angle relative to the membrane bilayer normal, which prevents the formation of stabilizing TM dimer contacts. First, measurements using polarized infrared spectroscopy show that the isolated TM domain of the active W515K mutant has a helix tilt angle closer to the bilayer normal than that of the wild-type receptor. Second, we identify second-site R514W and Q516W mutations that reverse dimerization and tilt angle changes induced by the W515K and W515L mutations. The second-site mutations prevent constitutive activation of TpoR W515K/L, while preserving ligand-induced signaling. The ability of tryptophan to influence the angle and dimerization of the TM helix in wild-type TpoR and in the second-site revertants is likely associated with its strong preference to be buried in the headgroup region of membrane bilayers.

Complex splicing control of the human Thrombopoietin gene by intronic G runs

The human thrombopoietin (THPO) gene displays a series of alternative splicing events that provide valuable models for studying splicing mechanisms. The THPO region spanning exon 1–4 presents both alternative splicing of exon 2 and partial intron 2 (IVS2) retention following the activation of a cryptic 3′ splice site 85 nt upstream of the authentic acceptor site. IVS2 is particularly rich in stretches of 3–5 guanosines (namely, G1–G10) and we have characterized the role of these elements in the processing of this intron. In vivo studies show that runs G7–G10 work in a combinatorial way to control the selection of the proper 3′ splice site. In particular, the G7 element behaves as the splicing hub of intron 2 and its interaction with hnRNP H1 is critical for the splicing process. Removal of hnRNP H1 by RNA interference promoted the usage of the cryptic 3′ splice site so providing functional evidence that this factor is involved in the selection of the authentic 3′ splice site of THPO IVS2.

Comparison between anagrelide and hydroxycarbamide in their activities against haematopoietic progenitor cell growth and differentiation: selectivity of anagrelide for the megakaryocytic lineage

Anagrelide (ANA) and hydroxycarbamide (HC) are two distinct pharmacological agents used to treat thrombocythaemia associated with myeloproliferative disorders. Although both drugs have been in clinical use for a number of years, comparative studies of their selectivity and mode of action are still lacking. Here, we have evaluated the activities of ANA and HC on the growth and differentiation of human haematopoietic progenitor cells in liquid culture. Both drugs inhibited thrombopoietin-induced megakaryocytopoiesis in a dose-dependent manner, but with strikingly different potencies (IC(50)=26 nM for ANA and 30 muM for HC) and modes of action. Whereas HC inhibited cell proliferation, ANA acted primarily on the differentiation process. At doses that abrogated megakaryocytopoiesis, HC also inhibited the expansion of CD34(+) cells stimulated by stem cell factor, interleukin-3 and Flt-3 ligand and also induced apoptosis. Furthermore, HC inhibited erythroid and myelomonocytic cell growth, induced by erythropoietin or granulocyte-macrophage colony-stimulating factor, respectively. In contrast, ANA showed none of these additional effects. Taken together, these results demonstrate that ANA is a potent and selective inhibitor of megakaryocytopoiesis, having no significant activity against haematopoietic progenitor cell expansion or differentiation into other lineages. In contrast, the anti-megakaryocytopoietic activity of HC cannot be dissociated from its more general cytoreductive and cytotoxic actions.

Two candidate genes for low platelet count identified in an Asian Indian kindred by genome-wide linkage analysis: glycoprotein IX and thrombopoietin

A genome-wide linkage analysis of platelet count was carried out in a large Asian Indian kindred. Linkage analysis showed one marker (D3S1309) on chromosome 3q with a lod score of 3.26 and another (D3S1282) approximately 30 cM centromeric, with a lod score of 2.52. Multipoint analysis of chromosome 3q identified two peaks with maximum multipoint lod scores of 3.52 and 4.11 under markers D3S1309 and D3S1282, respectively. Two strong candidate genes for platelet variation were identified in the linked region; thrombopoietin (THPO) and glycoprotein IX (GPIX). Resequencing of four individuals revealed five single-nucleotide polymorphisms (SNPs) in THPO and one mutation in the transmembrane region of GPIX. Analysis of variance showed that the GPIX mutation and one THPO SNP accounted for 6 and 4% of the variation in platelet count, respectively. The THPO SNP lies in the 3' untranslated region of the gene and has not been previously reported. The G to A transition at nucleotide 653 resulted in an Ala 156 (GCC) to Thr (ACC) replacement in the GPIX protein. The GPIX mutation was recently identified in a Chinese patient with Bernard-Soulier syndrome (BSS), a rare recessive bleeding disorder characterized by thrombocytopenia and giant platelets. One copy of the GPIX mutation was found in 300 European individuals with platelet counts within the normal range. The results suggest that two QTLs on chromosome 3q influence platelet count variation in the Asian Indian kindred, with the GPIX transmembrane mutation and the 3' UTR SNP in THPO being strong candidates.

Megakaryocyte biology and related disorders

Platelets, derived from megakaryocytes, have an essential role in thrombosis and hemostasis. Over the past 10 years, a great deal of new information has been obtained concerning the various aspects of hematopoiesis necessary to maintain a steady-state platelet level to support physiologic hemostasis. Here we discuss the differentiation of HSCs into megakaryocytes, with emphasis on the key cytokine signaling pathways and hematopoietic transcription factors. Recent insight into these processes elucidates the molecular bases of numerous acquired and inherited hematologic disorders. It is anticipated that the growing knowledge in these areas may be exploited for new therapeutic strategies to modulate both platelet numbers and their thrombogenicity.

Megakaryocyte proliferation and ploidy regulated by the cytoplasmic tail of glycoprotein Ibα

We have investigated the ability of glycoprotein (GP) Ibα, a megakaryocytic gene product, to sequester the signal transduction protein 14-3-3ξ and to influence megakaryocytopoiesis. Using a Gp1ba–/– mouse colony, we compared the rescued phenotypes produced by a wild-type human GP Ibα allele or a similar allele containing a 6-residue cytoplasmic tail truncation that abrogates binding to 14-3-3ξ. The observed phenotypes illustrate an involvement for GP Ibα in thrombopoietin-mediated events of megakaryocyte proliferation, polyploidization, and the expression of apoptotic markers in maturing megakaryocytes. We developed a hypothesis for the involvement of a GP Ibα/14-3-3ξ/PI-3 kinase complex in regulating thrombopoietin-mediated responses. An observed increase in thrombopoietin-mediated Akt phosphorylation in the truncated variant supported the hypothesis and led to the development of a model in which the GP Ibα cytoplasmic tail sequestered signaling proteins during megakaryocytopoiesis and, as such, became a critical regulator in the temporal sequence of events that led to normal megakaryocyte maturation.

Recombinant human thrombopoietin augments mobilization of peripheral blood progenitor cells for autologous transplantation

This study assessed the ability of various schedules of recombinant human thrombopoietin (rhTPO) to enhance mobilization of peripheral blood progenitor cells (PBPCs) in 134 patients with cancer undergoing high-dose chemotherapy and autologous PBPC transplantation. Patients received the study drug on days 1, 3, and 5 before initiation of granulocyte colony-stimulating factor (G-CSF) 10 microg/kg/day on day 5 and pheresis starting on day 9. Randomly assigned treatments on days 1, 3, and 5 were: group 1 (n=27) placebo, placebo, rhTPO 1.5 microg/kg; group 2 (n=27) rhTPO 1.5 microg/kg, placebo, placebo; groups 3 (n=28) and 4 (n=22) rhTPO 0.5 microg/kg on all 3 treatment days; and group 5 (n=30) placebo on all 3 treatment days. After high-dose chemotherapy and PBPC transplantation, groups 1 through 4 received rhTPO 1.5 microg/kg days 0, +2, +4, and +6 with either G-CSF 5 microg/kg/day (groups 1-3) or granulocyte-macrophage colony-stimulating factor 250 microg/m(2)/day (group 4). Group 5 received placebo plus G-CSF 5 microg/kg/day. The addition of rhTPO to G-CSF increased median CD34+ cell yield/pheresis in cohorts in which rhTPO was started before day 5, with higher yields in groups 2 (2.67 x 10(6)/kg) and groups 3 and 4 (3.10 x 10(6)/kg) than in group 1 (1.86 x 10(6)/kg) or group 5 (1.65 x 10(6)/kg) (P=.006 across groups). Comparing rhTPO to placebo, higher percentages of patients achieved the minimum yield of CD34+ > or =2 x 10(6)/kg (92% v 75%; P=.050) as well as the target yield of CD34+ > or =5 x 10(6)/kg (73% v 46%; P= .041). rhTPO-treated patients required fewer phereses to achieve minimum (P= .011) and target (P= .015) CD34+ cell values. rhTPO given after transplantation did not speed platelet recovery. No neutralizing antibodies were observed. We conclude that rhTPO can safely enhance mobilization of PBPC, reduce the number of leukapheresis, and allow more patients to meet minimal cell yield requirements to receive high-dose chemotherapy with PBPC transplantation.

Thrombopoietin promotes mixed lineage and megakaryocytic colony-forming cell growth but inhibits primitive and definitive erythropoiesis in cells isolated from early murine yolk sacs

The role of thrombopoietin (Tpo) in promoting hematopoiesis has been extensively studied in late fetal, neonatal, and adult mice. However, the effects of Tpo on early yolk sac hematopoiesis have been largely unexplored. We examined whole embryos or the cells isolated from embryo proper and yolk sacs and identified both Tpo and c-mpl (Tpo receptor) mRNA transcripts in tissues as early as embryonic day 6.5 (E6.5). Presomite whole embryos and somite-staged yolk sac and embryo proper cells were plated in methylcellulose cultures and treated with selected hematopoietic growth factors in the presence or absence of Tpo. Tpo alone failed to promote colony-forming unit (CFU) formation. However, in the presence of other growth factors, Tpo caused a substantial dose-dependent reduction in primitive and definitive erythroid CFU growth in cultures containing E7.5 and E8.0 whole embryos and E8.25 to 9.5 yolk sac-derived cells. Meanwhile, Tpo treatment resulted in a substantial dose-dependent increase in CFU-mixed lineage (CFU-Mix) and CFU-megakaryocyte (CFU-Meg) formation in cultures containing cells from similar staged tissues. Addition of Tpo to cultures of sorted E9.5 yolk sac c-Kit(+)CD34(+) hematopoietic progenitors also inhibited erythroid CFU growth but augmented CFU-Mix and CFU-Meg activity. Effects of Tpo on CFU growth were blocked in the presence of a monoclonal antibody with Tpo-neutralizing activity but not with control antibody. Thus, under certain growth factor conditions, Tpo directly inhibits early yolk sac erythroid CFU growth but facilitates megakaryocyte and mixed lineage colony formation.

Pharmacodynamic modeling of thrombopoietin, platelet, and megakaryocyte dynamics in patients with acute myeloid leukemia undergoing dose intensive chemotherapy

A proposed model of thrombopoietin (TPO) regulation is that of a constitutive production of TPO with circulating levels being predominately regulated by changes in platelet and megackaryocyte mass. Using a pharmacodynamic (PD) approach, the authors examined the validity of this model for patients with acute myeloid leukemia (AML) undergoing dose-intensive postinduction chemotherapy (HDT). TPO and platelet values were assayed weekly in AML patients undergoing HDT. A parsimonious dynamic model was then applied to these experimental data. The results (1) support the proposed model of TPO regulation, (2) model and quantify the effects of HDT on the megakaryocyte compartment, (3) characterize variables not amenable to direct measurement, and (4) have clinical utility as this model predicted that TPO given after HDT would not have a significant effect on platelet recovery, a finding borne out in clinical trials. This model provides information relevant to the interpretation of clinical trials of hematopoietic growth factors.

Subcellular localization of thrombopoietin in human blood platelets and its release upon thrombin stimulation

Thrombopoietin (TPO) is a major regulator of platelet production. The concentration of circulating TPO seems to be determined by its binding and internalization by megakaryocytes and platelets. To elucidate the platelet compartments involved in TPO metabolism, we investigated intraplatelet TPO by post-embedding immunoelectron microscopy, incubated platelets with recombinant human (rh)TPO coupled to colloidal gold and visualized the TPO uptake using electron microscopy. TPO concentrations were measured in 12 platelet concentrates (PC) before and after stimulation with thrombin and after disruption of platelets by freezing-thawing. In resting platelets, immunogold labelling revealed a prevailing cytoplasmic localization of TPO antigen and minor labelling within the surface-connected canalicular system (SCCS); storage granules were devoid of labelling. In tracer experiments, TPO-gold was observed on the plasma and SCCS membranes and within the cytoplasm. Upon thrombin stimulation, endogenous TPO was still detected within the cytoplasm by immunolabelling, and tracer experiments revealed TPO-gold within the cytoplasm and on fibrin fibres. After thrombin stimulation of PC, the plasma TPO levels increased to an average of 535%, and after platelet lysis to an average of 1625% compared with plasma values in unstimulated PC. We conclude that platelets contain releasable immunoreactive TPO within the SCCS and within their cytoplasm, but not within granular compartments. Stored immunoreactive TPO is released upon thrombin stimulation, but only to a minor degree.

Cloning and functional characterization of a novel c-mpl variant expressed in human CD34 cells and platelets

The thrombopoietin receptor, c-mpl, is a crucial element not only in thrombopoietin (TPO)-initiated signaling pathways but also in the regulation of the circulating amount of TPO. We have identified a new c-mpl isoform, called c-mpl-del, that lacks 72 bp (24 amino acids) in the extracellular region of c-mpl and arises as a consequence of alternative RNA splicing between exons 8 and 9. c-mpl-del is expressed along with c-mpl-wt in blood mononuclear cells, CD34(+)cells, megakaryocytes, and platelets prepared from either normal donors or ET patients, although its relative expression appears to increase with megakaryocyte differentiation. The c-mpl-del-transfected cells expressed greater amounts of c-mpl-del RNA and protein than the comparable c-mpl-wt-transfected cells, however flow cytometry analysis could not detect any c-mpl receptor on the surface of the c-mpl-del-transfected cells. Further evidence for the absence of surface c-mpl-del was that in contrast to cells transfected with c-mpl-wt, those transfected with c-mpl-del did not grow in response to TPO, failed to undergo tyrosine phosphorylation of TPO-specific signal molecules, and did not bind(125)I-rHuTPO. Taken together, these results demonstrate that c-mpl-del, a naturally occurring variant of c-mpl, fails to be incorporated into the cell membrane but might serve as a mechanism to decrease the overall expression of functional c-mpl late in megakaryocyte differentiation.

Interleukin-6 increases thrombopoietin production in human hepatoma cells HepG2 and Hep3B

The concentration of circulating thrombopoietin (TPO) is relatively high in patients with thrombocytosis reactive to inflammatory diseases. We investigated whether immunomodulatory cytokines stimulate TPO synthesis in cultured human hepatoma cells (lines HepG2 and Hep3B), renal proximal tubular cells, and bone marrow fibroblasts. The effects of interleukins (IL) IL-1beta, IL-6, and IL-11 and of tumor necrosis factor-a (TNF-alpha) on the rate of TPO secretion were measured by ELISA. TPO mRNA levels were quantitated by competitive reverse transcription PCR. HepG2 and Hep3B cells produced significant amounts of TPO mRNA and TPO protein. Renal tubular cells synthesized less TPO, and in bone marrow fibroblasts, neither TPO mRNA nor TPO protein was detected. Only IL-6 affected TPO protein secretion, causing a 1.5-fold stimulation in HepG2 and Hep3B cells in 24-h incubation periods. The TPO mRNA content in these cells was doubled by IL-6 after 2, 6, or 24 h of stimulation. Thus, IL-6 could cause thrombocytosis in inflammatory disease partly by increasing hepatic TPO production.

Thrombopoietin in the treatment of acute myeloid leukemia and in stem-cell transplantation

Recent studies indicate that thrombopoietin (TPO) may be highly effective in mobilizing autologous peripheral blood stem cells (PBSCs) for transplantation in patients undergoing intensive chemotherapy. The yield of CD34+ progenitor cells can be increased as can the percentage of patients achieving adequate grafts for use in transplantation. However, the effect of TPO in patients with hematologic malignancies undergoing induction or postremission chemotherapy or in the stem-cell transplantation setting has not been demonstrated. Further study is warranted for better definition of the role of TPO in the treatment of severe thrombocytopenia in these settings.

Regulation of megakaryocytopoiesis

After 35 years of research, a physiological regulator of platelet production has been identified and the recombinant protein is available. With the discovery of thrombopoietin (TPO), its potential use in a wide variety of clinical megakaryocytic and platelet disorders has been expected and clinical trials have been undertaken. To date, the reported encouraging pre-clinical studies indicate that, as with erythropoietin or G-CSF, minimal toxicity can be expected. A potential limiting side-effect of TPO could be the induction of thrombosis. Nevertheless, it is too early to know whether this cytokine will be of major therapeutic importance for patients with life-threatening thrombocytopenia, such as patients undergoing bone marrow transplantation or subjected to a high dose of chemotherapy. Several experimental and clinical studies are still needed to determine the efficacy of TPO in the prevention or the amelioration of bleeding, which is the ultimate goal for the appropriate use of cytokines with haemostatic benefit. Basic and clinical studies on regulators of megakaryocytopoiesis have rapidly progressed. Now, there is no doubt that some of these regulators are effective in correcting haematopoietic disorders of various aetiologies. Studies on negative regulators not only are important to understand the regulation of megakaryocytopoiesis in normal and pathological states but also have a potential clinical application. Some of these regulators have been shown to be effective in the treatment of essential thrombocythaemia and other myeloproliferative disorders. Platelet factor 4 (PF4) and some other chemokines are also capable of protecting progenitor cells from the cytotoxicity of chemotherapeutic drugs. However, detailed investigations are still required to determine the precise mechanism(s) of action of these regulators and to establish the optimal clinical protocols of negative regulators alone or in association with positive regulators for the treatment of various haematological diseases and cancer.

Elevated serum thrombopoietin and interleukin-6 concentrations in thrombocytosis associated with inflammatory bowel disease

Reactive thrombocytosis is a typical feature in inflammatory bowel disease (IBD). The question arose as to whether the normal negative feedback regulation of the concentration of thrombopoietin (TPO) in blood was altered in IBD patients. We measured serum immunoreactive TPO in 30 patients with active IBD, 29 patients with inactive IBD, and 56 healthy controls. The results were related to platelet and leukocyte counts and to the serum concentration of interleukin 6 (IL-6). Patients with active IBD exhibited significantly increased TPO levels (medians 112 pg/ml vs. 90 pg/ml in controls, p < 0.05) in association with thrombocytosis (428 platelets/nl blood vs. 241 platelets/nl blood in controls), leukocytosis, and increased IL-6 levels (12.9 pg/ml vs. 2.5 pg/nl in controls). In patients with inactive IBD, only platelets (322/nl) and leukocytes were above normal. Although the observation of increased TPO and IL-6 levels provides an explanation for the occurrence of thrombocytosis in IBD, the pathogenetic mechanisms underlying the elevated TPO level still need to be identified.

Soluble Interleukin-6 (IL-6) Receptor With IL-6 Stimulates Megakaryopoiesis From Human CD34+ Cells Through Glycoprotein (gp)130 Signaling

We have recently shown that stimulation of glycoprotein (gp) 130, the membrane-anchored signal transducing receptor component of IL-6, by a complex of human soluble interleukin-6 receptor (sIL-6R) and IL-6 (sIL-6R/IL-6), potently stimulates the ex vivo expansion as well as erythropoiesis of human stem/progenitor cells in the presence of stem cell factor (SCF). Here we show that sIL-6R dose-dependently enhanced the generation of megakaryocytes (Mks) (IIbIIIa-positive cells) from human CD34+ cells in serum-free suspension culture supplemented with IL-6 and SCF. The sIL-6R/IL-6 complex also synergistically acted with IL-3 and thrombopoietin (TPO) on the generation of Mks from CD34+ cells, whereas the synergy of IL-6 alone with TPO was barely detectable. Accordingly, the addition of sIL-6R to the combination of SCF + IL-6 also supported a substantial number of Mk colonies from CD34+ cells in serum-free methylcellulose culture, whereas SCF + IL-6 in the absence of sIL-6R rarely induced Mk colonies. The addition of monoclonal antibodies against gp130 to the suspension and clonal cultures completely abrogated the megakaryopoiesis induced by sIL-6R/IL-6 in the presence of SCF, whereas an anti-TPO antibody did not, indicating that the observed megakaryopoiesis by sIL-6R/IL-6 is a response to gp130 signaling and independent of TPO. Furthermore, human CD34+ cells were subfractionated into two populations of IL-6R–negative (CD34+ IL-6R−) and IL-6R–positive (CD34+ IL-6R+) cells by fluorescence-activated cell sorting. The CD34+IL-6R− cells produced a number of Mks as well as Mk colonies in cultures supplemented with sIL-6R/IL-6 or TPO in the presence of SCF. In contrast, CD34+ IL-6R+cells generated much less Mks and lacked Mk colony forming activity under the same conditions. Collectively, the present results indicate that most of the human Mk progenitors do not express IL-6R, and that sIL-6R confers the responsiveness of human Mk progenitors to IL-6. Together with the presence of functional sIL-6R in human serum and relative unresponsiveness of human Mk progenitors to IL-6 in vitro, current results suggest that the role of IL-6 may be mainly mediated by sIL-6R, and that the gp130 signaling initiated by the sIL-6R/ IL-6 complex is involved in human megakaryopoiesis in vivo.

Soluble Interleukin-6 (IL-6) Receptor With IL-6 Stimulates Megakaryopoiesis From Human CD34+ Cells Through Glycoprotein (gp)130 Signaling

We have recently shown that stimulation of glycoprotein (gp) 130, the membrane-anchored signal transducing receptor component of IL-6, by a complex of human soluble interleukin-6 receptor (sIL-6R) and IL-6 (sIL-6R/IL-6), potently stimulates the ex vivo expansion as well as erythropoiesis of human stem/progenitor cells in the presence of stem cell factor (SCF). Here we show that sIL-6R dose-dependently enhanced the generation of megakaryocytes (Mks) (IIbIIIa-positive cells) from human CD34+ cells in serum-free suspension culture supplemented with IL-6 and SCF. The sIL-6R/IL-6 complex also synergistically acted with IL-3 and thrombopoietin (TPO) on the generation of Mks from CD34+ cells, whereas the synergy of IL-6 alone with TPO was barely detectable. Accordingly, the addition of sIL-6R to the combination of SCF + IL-6 also supported a substantial number of Mk colonies from CD34+ cells in serum-free methylcellulose culture, whereas SCF + IL-6 in the absence of sIL-6R rarely induced Mk colonies. The addition of monoclonal antibodies against gp130 to the suspension and clonal cultures completely abrogated the megakaryopoiesis induced by sIL-6R/IL-6 in the presence of SCF, whereas an anti-TPO antibody did not, indicating that the observed megakaryopoiesis by sIL-6R/IL-6 is a response to gp130 signaling and independent of TPO. Furthermore, human CD34+ cells were subfractionated into two populations of IL-6R–negative (CD34+ IL-6R−) and IL-6R–positive (CD34+ IL-6R+) cells by fluorescence-activated cell sorting. The CD34+IL-6R− cells produced a number of Mks as well as Mk colonies in cultures supplemented with sIL-6R/IL-6 or TPO in the presence of SCF. In contrast, CD34+ IL-6R+cells generated much less Mks and lacked Mk colony forming activity under the same conditions. Collectively, the present results indicate that most of the human Mk progenitors do not express IL-6R, and that sIL-6R confers the responsiveness of human Mk progenitors to IL-6. Together with the presence of functional sIL-6R in human serum and relative unresponsiveness of human Mk progenitors to IL-6 in vitro, current results suggest that the role of IL-6 may be mainly mediated by sIL-6R, and that the gp130 signaling initiated by the sIL-6R/ IL-6 complex is involved in human megakaryopoiesis in vivo.

Responses to single-dose thrombopoietin decrease with higher platelet counts in mice

Since the description of human thrombopoietin (TPO) we investigated the thrombocytosis-inducing capacity of human serum samples derived from individuals with altered thrombocytopoiesis. Several times the degree of thrombocytosis developing in recipient mice differed markedly even when applying the same human material. In the last 2 years, we applied single doses of recombinant human TPO (rHuTPO) to random-bred CFLP mice, and the same observation was made. Taken together with previous information (before 1970) it was possible to select cases in which the percent increases in circulating platelet counts inversely correlated with the starting levels. It appears, however, that apart from the known absorbing role of platelets and megakaryocytes, the response to single doses of exogenous rHuTPO in mice depends, at least partially, on an unknown endogenous homeostatic mechanism. Mixing thrombopoietically active human sera with platelet-free normal serum in a 1:1 ratio remarkably reduced the thrombocytosis-inducing capacity. Repeated pharmacological doses of TPO, applied in the majority of the reported trials, however, easily obscure the physiological control mechanism.

Permissive role of thrombopoietin and granulocyte colony-stimulating factor receptors in hematopoietic cell fate decisions in vivo

The question of whether extracellular signals influence hematopoiesis by instructing stem cells to commit to a specific hematopoietic lineage (instructive model) or solely by permitting the survival and proliferation of predetermined progenitors (permissive model) has been controversial since the discovery of lineage-dominant hematopoietic cytokines. To study the potential role of cytokines and their receptors in hematopoietic cell fate decisions, we used homologous recombination to replace the thrombopoietin receptor gene (mpl) with a chimeric construct encoding the extracellular domain of mpl and the cytoplasmic domain of the granulocyte colony-stimulating factor receptor (G-CSFR). This chimeric receptor binds thrombopoietin but signals through the G-CSFR intracellular domain. We found that, despite the absence of a functional mpl signaling domain, homozygous knock-in mice had a normal platelet count, indicating that in vivo the cytoplasmic domain of G-CSFR can functionally replace mpl signaling to support normal megakaryopoiesis and platelet formation. This finding is compatible with the permissive model, according to which cytokine receptors provide a nonspecific survival or proliferation signal, and argues against an instructive role of mpl or G-CSFR in hematopoietic cell fate decisions.

Thrombopoietin Production Is Inhibited by a Translational Mechanism

Thrombopoietin (TPO) is a lineage-dominant hematopoietic cytokine that regulates megakaryopoiesis and platelet production. The major site of TPO biosynthesis is the liver. Despite easily detectable levels of liver TPO mRNA, the circulating TPO serum levels are very low. We have observed that translation of TPO mRNA is inhibited by the presence of inhibitory elements in the 5′-untranslated region (5′-UTR). Alternative promoter usage and differential splicing generate at least three TPO mRNA isoforms that differ in the composition of their 5′-UTR. Using mutational analysis we show that physiologically the translation of these TPO mRNA isoforms is strongly inhibited by the presence of AUG codons, which define several short open reading frames (ORFs) in the 5′-UTR and suppress efficient initiation at the physiologic start site. The two regularly spliced isoforms, which account for 98% of TPO mRNA, were almost completely inhibited, whereas a rare splice variant that lacks exon 2 can be more efficiently translated. Thus, inhibition of translation of the TPO mRNA is an efficient mechanism to prevent overproduction of this highly potent cytokine.

Thrombopoietin Production Is Inhibited by a Translational Mechanism

Thrombopoietin (TPO) is a lineage-dominant hematopoietic cytokine that regulates megakaryopoiesis and platelet production. The major site of TPO biosynthesis is the liver. Despite easily detectable levels of liver TPO mRNA, the circulating TPO serum levels are very low. We have observed that translation of TPO mRNA is inhibited by the presence of inhibitory elements in the 5′-untranslated region (5′-UTR). Alternative promoter usage and differential splicing generate at least three TPO mRNA isoforms that differ in the composition of their 5′-UTR. Using mutational analysis we show that physiologically the translation of these TPO mRNA isoforms is strongly inhibited by the presence of AUG codons, which define several short open reading frames (ORFs) in the 5′-UTR and suppress efficient initiation at the physiologic start site. The two regularly spliced isoforms, which account for 98% of TPO mRNA, were almost completely inhibited, whereas a rare splice variant that lacks exon 2 can be more efficiently translated. Thus, inhibition of translation of the TPO mRNA is an efficient mechanism to prevent overproduction of this highly potent cytokine.

Enhanced Megakaryocyte and Erythroid Development From Normal Human CD34+ Cells: Consequence of Enforced Expression of SCL

The product of the SCL gene is a basic helix-loop-helix (bHLH) transcription factor that is essential for the development of hematopoietic stem cells in both the embryo and the adult. However, once the stem cell compartment is established, the function of SCL in subsequent differentiation and commitment events within normal hematopoietic cells remains undefined. The aim of the current study was to investigate this role using purified normal human hematopoietic CD34+ cells. An SCL retrovirus was used to transduce CD34+ cells isolated from human bone marrow, peripheral blood, and umbilical cord blood. Enforced expression of SCL increased by a median of twofold the number of erythroid colonies, with an increase in both colony size and the rate of hemoglobinization. Unexpectedly, enforced expression in CD34+ cells also significantly increased the number of megakaryocyte colonies, but with no impact on the size of colonies. There was no consistent effect on the number nor size of granulocyte-macrophage (GM) colonies. The proliferative effect of enforced SCL expression on erythroid cells was attributed to a shortened cell cycle time; the self-renewal capacity of erythroid or GM progenitors was unchanged, as was survival of cells within colonies. These results demonstrate a role for SCL in determining erythroid and megakaryocyte differentiation from normal human hematopoietic CD34+ cells.

Enhanced Megakaryocyte and Erythroid Development From Normal Human CD34+ Cells: Consequence of Enforced Expression of SCL

The product of the SCL gene is a basic helix-loop-helix (bHLH) transcription factor that is essential for the development of hematopoietic stem cells in both the embryo and the adult. However, once the stem cell compartment is established, the function of SCL in subsequent differentiation and commitment events within normal hematopoietic cells remains undefined. The aim of the current study was to investigate this role using purified normal human hematopoietic CD34+ cells. An SCL retrovirus was used to transduce CD34+ cells isolated from human bone marrow, peripheral blood, and umbilical cord blood. Enforced expression of SCL increased by a median of twofold the number of erythroid colonies, with an increase in both colony size and the rate of hemoglobinization. Unexpectedly, enforced expression in CD34+ cells also significantly increased the number of megakaryocyte colonies, but with no impact on the size of colonies. There was no consistent effect on the number nor size of granulocyte-macrophage (GM) colonies. The proliferative effect of enforced SCL expression on erythroid cells was attributed to a shortened cell cycle time; the self-renewal capacity of erythroid or GM progenitors was unchanged, as was survival of cells within colonies. These results demonstrate a role for SCL in determining erythroid and megakaryocyte differentiation from normal human hematopoietic CD34+ cells.

An activating splice donor mutation in the thrombopoietin gene causes hereditary thrombocythaemia

Essential thrombocythaemia (ET) is a chronic myeloproliferative syndrome due to sustained proliferation of megakaryocytes, which results in elevated numbers of circulating platelets, thrombotic or haemorrhagic episodes and occasional leukaemic transformation. The cause of ET is unknown. Hereditary thrombocythaemia (HT) with autosomal-dominant transmission has been described with manifestations similar to those of sporadic ET. As the thrombopoietin gene (THPO) encodes a lineage-restricted growth factor with profound stimulatory effects on megakaryopoiesis and platelet production, we tested the hypothesis that HT results from a mutation in the human THPO gene. In a Dutch family with eleven affected individuals, the thrombopoietin protein (TPO) concentrations in serum were consistently elevated in individuals with HT. We derived an intragenic CA marker for the human THPO gene and performed linkage analysis in fourteen informative meioses in this family. This resulted in a lod score of 3.5 at theta=0. A G-->C transversion was found in the splice donor site of intron 3 of the THPO gene in all affected family members. This mutation leads to THPO mRNAs with shortened 5'-untranslated regions (UTR) that are more efficiently translated than the normal THPO transcripts. We conclude that a splice donor mutation in THPO leads to systemic overproduction of TPO and causes thrombocythaemia.