The purification of megapoietin: a physiological regulator of megakaryocyte growth and platelet production

The role of platelet α-granular proteins in the regulation of thrombopoietin messenger RNA expression in human bone marrow stromal cells

Thrombopoietin (TPO), the specific cytokine that regulates platelet production, is expressed in human bone marrow (BM), kidney, and liver. There appears to be no regulation of TPO in the kidney and liver, but TPO messenger RNA (mRNA) expression can be modulated in the stromal cells of the BM. In this study, we used primary human BM stromal cells as a model to study the regulation of TPO mRNA expression in response to various platelet -granular proteins. We showed that platelet-derived growth factor (PDGF) BB and fibroblast growth factor (FGF) 2 stimulated TPO mRNA expression in both a dose-dependent and time-dependent manner. The addition of 50 ng/mL of PDGF and 20 ng/mL of FGF resulted in maximal induction of TPO mRNA expression in 4 hours. We also found that platelet factor 4 (PF4), thrombospondin (TSP), and transforming growth factor-beta (TGF-β) are negative modulators of megakaryocytopoiesis. We observed suppression in TPO mRNA expression with 1 μg/mL of both PF4 and TSP and 50 ng/mL of TGF-β, with maximal suppression occurring 4 hours after the addition of these proteins. Finally, the addition of whole-platelet lysate produced a dose-dependent inhibition of TPO expression. On the basis of these findings, we propose that the platelet -granular proteins studied may regulate TPO gene expression in BM stromal cells by means of a feedback mechanism.

Role of thrombopoietin in hematopoietic stem cell and progenitor regulation

Thrombopoietin performs an essential role during hematopoiesis by regulating the expansion and maturation of megakaryocytes. In keeping with this function, megakaryocytes, platelets, and their precursors all express the thrombopoietin receptor, Mpl, on their cell surface. However, Mpl is also expressed on primitive, pluripotent hematopoietic progenitors and plays an important role in the regulation of lineages other than megakaryocytes as well as primitive progenitors. Recently, the ability of thrombopoietin to maintain and expand repopulating stem cells has been demonstrated. Thus, thrombopoietin is unique among the hematopoietic cytokines because it is necessary both for terminal maturation and regulation of lineage-specific megakaryocytes and also for maintenance of the most primitive hematopoietic stem cells. Many new strategies are evolving to exploit the activity of thrombopoietin on primitive progenitors. This may lead to faster hematopoietic recovery from marrow-suppressive therapy, effective methods of ex vivo expansion of hematopoietic stem cells, and retroviral transduction of stem cells to facilitate gene therapy.

Functional analysis of the C-terminal region of recombinant human thrombopoietin. C-terminal region of thrombopoietin is a "shuttle" peptide to help secretion

Thrombopoietin (TPO) is a cytokine that primarily stimulates megakaryocytopoiesis and thrombopoiesis. TPO has a unique C-terminal tail peptide of about 160 amino acids that consists mostly of hydrophilic residues and contains six N-linked sugar chains. In order to investigate the biological function of the C-terminal domain, two series of mutations were performed. One is systematic truncation from the C terminus. Another is elimination of N-glycosylation sites in the C-terminal domain by Asn to Gln mutations. After the mutant proteins were expressed by mammalian cells, it was found that the elimination of the N-linked sugar sites did not affect the biological activity, whereas truncation of the C-terminal domain resulted in elevation of in vitro activity up to 4-fold. The C-terminal peptide itself was found to inhibit the in vitro activity. Moreover, both the C-terminal truncation and the elimination of the N-glycosylation sites decreased the secretion level progressively down to (1)/(10) that of wild type, and the amount of the mutant left in the cell increased. The N-glycosylation in the C-terminal region was found to be important for secretion of TPO. Among six N-glycosylation sites in the C-terminal region, two locations, Asn-213 and Asn-234, were found to be critical for secretion, and two other locations, Asn-319 and Asn-327, did not affect the secretion.

Effects of megakaryocyte growth and development factor on platelet production, platelet life span, and platelet function in healthy human volunteers

The effects of thrombopoietic stimulation on megakaryocytopoiesis, platelet production, and platelet viability and function were examined in normal volunteers randomized to receive single bolus subcutaneous injections of 3 μg/kg pegylated recombinant megakaryocyte growth and development factor (PEG-rHuMGDF) or placebo in a 3:1 ratio. PEG-rHuMGDF transiently doubled circulating platelet counts, from 237 ± 41 × 103/μL to 522 ± 90 × 103/μL (P< .0001), peaking on day 12. Baseline and day-12 samples showed no differences in responsiveness of platelets to adenosine diphosphate or thrombin receptor agonist peptide (P > .4 in all cases); expression of platelet ligand-induced binding sites or annexin V binding sites (P > .6 in both cases); or density of platelet TPO-receptors (P > .5). Platelet counts normalized by day 28. The life span of autologous 111In-labeled platelets increased from 205 ± 18 hours (baseline) to 226 ± 22 hours (P < .01) on day 8. Platelet life span decreased from 226 ± 22 hours (day 8) to 178 ± 53 hours (P < .05) on day 18. The theoretical basis for senescent changes in mean platelet life span was illustrated by biomathematical modeling. Platelet turnover increased from 43.9 ± 11.9 × 103 platelets/μL/d (baseline) to 101 ± 27.6 × 103 platelets/μL/d (P = .0009), and marrow megakaryocyte mass expanded from 37.4 ± 18.5 fL/kg to 62 ± 17 × 1010 fL/kg (P = .015). Although PEG-rHuMGDF initially increased megakaryocyte volume and ploidy, subsequently ploidy showed a transient reciprocal decrease when the platelet counts exceeded placebo values. In healthy human volunteers PEG-rHuMGDF transiently increases megakaryocytopoiesis 2-fold. Additionally, peripheral platelets expand correspondingly and exhibit normal function and viability during the ensuing 10 days. The induced perturbation in steady state thrombopoiesis resolves by 4 weeks.

Effects of megakaryocyte growth and development factor on platelet production, platelet life span, and platelet function in healthy human volunteers

The effects of thrombopoietic stimulation on megakaryocytopoiesis, platelet production, and platelet viability and function were examined in normal volunteers randomized to receive single bolus subcutaneous injections of 3 μg/kg pegylated recombinant megakaryocyte growth and development factor (PEG-rHuMGDF) or placebo in a 3:1 ratio. PEG-rHuMGDF transiently doubled circulating platelet counts, from 237 ± 41 × 103/μL to 522 ± 90 × 103/μL (P&lt; .0001), peaking on day 12. Baseline and day-12 samples showed no differences in responsiveness of platelets to adenosine diphosphate or thrombin receptor agonist peptide (P &gt; .4 in all cases); expression of platelet ligand-induced binding sites or annexin V binding sites (P &gt; .6 in both cases); or density of platelet TPO-receptors (P &gt; .5). Platelet counts normalized by day 28. The life span of autologous 111In-labeled platelets increased from 205 ± 18 hours (baseline) to 226 ± 22 hours (P &lt; .01) on day 8. Platelet life span decreased from 226 ± 22 hours (day 8) to 178 ± 53 hours (P &lt; .05) on day 18. The theoretical basis for senescent changes in mean platelet life span was illustrated by biomathematical modeling. Platelet turnover increased from 43.9 ± 11.9 × 103 platelets/μL/d (baseline) to 101 ± 27.6 × 103 platelets/μL/d (P = .0009), and marrow megakaryocyte mass expanded from 37.4 ± 18.5 fL/kg to 62 ± 17 × 1010 fL/kg (P = .015). Although PEG-rHuMGDF initially increased megakaryocyte volume and ploidy, subsequently ploidy showed a transient reciprocal decrease when the platelet counts exceeded placebo values. In healthy human volunteers PEG-rHuMGDF transiently increases megakaryocytopoiesis 2-fold. Additionally, peripheral platelets expand correspondingly and exhibit normal function and viability during the ensuing 10 days. The induced perturbation in steady state thrombopoiesis resolves by 4 weeks.

Expression, purification, and characterization of human recombinant thrombopoietin in Chinese hamster ovary cells

Thrombopoietin (TPO) is a primary regulator of megakaryocytopoiesis, a process through which megakaryocytes proliferate and mature into platelets. Recombinant human TPO (rhTPO) was expressed in Chinese hamster ovary (CHO) cells and purified from the culture medium. The cDNA encoding full-length TPO, including the native signal peptide sequence, was amplified by PCR from a human fetal liver cDNA library. The product was cloned into a mammalian expression vector under the control of the SV40 early promoter and enhancer. Secreted rhTPO was purified in three conventional chromatography steps. It migrates on SDS-PAGE as a broad band, characteristic of a heavily glycosylated protein, with an average molecular mass of 85 kDa. rhTPO expressed in CHO cells is biologically active in vitro as demonstrated by its ability to stimulate the proliferation of a megakaryocytic cell line and to trigger the JAK/STAT signal transduction pathway. rhTPO also shows activity in vivo as judged by the elevation of platelet count in treated mice.

Expression of thrombopoietin and its receptor (c-mpl) in chronic myelogenous leukemia: correlation with disease progression and response to therapy

BACKGROUND

Chronic myelogenous leukemia (CML) represents a paradigm of the stepwise increment in disease aggressiveness, resistance to therapy, and transformation. Thrombopoietin (TPO) and its receptor, c-mpl, support the proliferation of multiple types of immature hematopoietic progenitor cells, and induce clonal growth of leukemic cells. The authors investigated whether TPO or c-mpl overexpression might correlate with progression of CML, disease aggressiveness, or response to therapy.

METHODS

Expression of c-mpl and TPO was measured in bone marrow samples from 208 patients with CML by Western blot analysis and solid-phase plate radioimmunoassay (used for quantification). Samples obtained from individuals without evidence of hematologic abnormalities were used as controls.

RESULTS

There were no significant differences in TPO or c-mpl expression among patients in different phases of CML or between patients with Philadelphia chromosome positive and negative CML. When TPO and c-mpl levels were analyzed in relation to prognostically important host and disease characteristics in early chronic phase CML, platelet and white blood cell counts demonstrated significant differences in both TPO and c-mpl expression, but age and spleen size demonstrated significant differences in TPO expression only. Responses to interferon (INF)-alpha-based therapy and survival were not influenced by TPO or c-mpl levels.

CONCLUSIONS

TPO or c-mpl overexpression did not correlate with different CML phases, suggesting that they were not involved in CML progression from early to advanced phase. Neither TPO nor c-mpl overexpression was particularly evident in any risk group, suggesting lack of correlation between their expression and disease aggressiveness. This was supported by the finding of similar response to IFN-alpha-based therapy and survival regardless of the level of TPO or c-mpl expression.

Role of the distal half of the c-Mpl intracellular domain in control of platelet production by thrombopoietin in vivo

The cytokine thrombopoietin (TPO) controls the formation of megakaryocytes and platelets from hematopoietic stem cells. TPO exerts its effect through activation of the c-Mpl receptor and of multiple downstream signal transduction pathways. While the membrane-proximal half of the cytoplasmic domain appears to be required for the activation of signaling molecules that drive proliferation, the distal half and activation of the mitogen-activated protein kinase pathway have been implicated in mediating megakaryocyte maturation in vitro. To investigate the contribution of these two regions of c-Mpl and the signaling pathways they direct in mediating the function of TPO in vivo, we used a knock-in (KI) approach to delete the carboxy-terminal 60 amino acids of the c-Mpl receptor intracellular domain. Mice lacking the C-terminal 60 amino acids of c-Mpl (Delta60 mice) have normal platelet and megakaryocyte counts compared to wild-type mice. Furthermore, platelets in the KI mice are functionally normal, indicating that activation of signaling pathways connected to the C-terminal half of the receptor is not required for megakaryocyte differentiation or platelet production. However, Delta60 mice have an impaired response to exogenous TPO stimulation and display slower recovery from myelosuppressive treatment, suggesting that combinatorial signaling by both ends of the receptor intracellular domain is necessary for an appropriate acute response to TPO.

Effects of Recombinant Human Megakaryocyte Growth and Development Factor (rHuMGDF) on Platelet Production, Platelet Aggregation, and Thrombosis

Recombinant human megakaryocyte growth and development factor (rHuMGDF) is a c-mpl ligand that promotes the differentiation of CD34+ precursor cells into megakaryocyte, and then platelets. In experimental animals, injection of this and other c-mpl ligands leads to profound increases in the circulating platelet count in a matter of days. However, c-mpl ligands have also been shown to sensitize platelets to aggregating agents in vitro, raising the possibility that c-mpI ligands may have prothrombotic effects in vivo. Therefore, characterizing rHuMGDF in an in vivo model of thrombosis is a necessary and critical step in defining the in vivo pharmacology of this novel and important hernatopoietic factor, a pegylated form of which is currently in clinical trials. To determine the biologically effective doses in the rabbit, daily subcutaneous injections of rHuMGDF at 0.1, 1.0, or 10 µg/kg were administered ever 7 days. Daily injection of 10 µ/kg produced an approximate fourfold increase in platelet count and 1.0 µ/kg doubled platelet count over the injection period, both of which were statistically significant. The serum concentrations of rHuMGDF were determined 10 minutes following a single intravenous injection with 0.1, 1.0, and 10 µ/kg, and were 0.05 +/- 0.02, 0.98 +/- 0.07, and 21.32 +/- 21.35 ng/ml. To determine whether rHuMGDF can sensitize platelets in vivo, platelet aggregometry was performed on platelets isolated from animals immediately before and 10 minutes after they had been injected intravenously with rHuMGDF (0.1, 1.0, and 10 µ/kg). Intravenous injection of 10 µ/kg produced measurable changes in platelet aggregometry ex vivo, as evidenced by an increased sensitivity of platelets to adenosine diphosphate (ADP). To assess. the in vivo prothrombotic potential of rHuMGDF, a rabbit carotid artery model of cyclic flow reduction (CFR) was used to measure the effect of intravenous rHuMGDF administration on the rate of thrombus formation as assessed by CFR slope and frequency. Intravenous administration of rHuMGDF had no effect on CFR slope or frequency when administered in doses ranging from 0.1 to 10 µ/kg. Control experiments demonstrated that CFR slope and frequency can be enhanced by intravenous infusion of epinephrine and can be abolished by the combined administration of aspirin and ketanserin, indicating that potentially prothrombotic and antithrombotic agents can be identified in this model. We conclude that biologically active doses of rHuMGDF used in this study (1.0 and 10 µ/kg) produce measurable serum levels, induce a thrombopoietic effect, and sensitize platelets in vivo, as determined by ex vivo aggregometry, at 10 µ/kg. Despite the sensitization of platelets to aggregation induced by ADP, it is clear that rHuMGDF does not alter the pattern of CFRs observed in the rabbit carotid artery, whereas agents known to sensitize platelets (epinepherine) and to inhibit platelets (aspirin and ketanserin) readily affected the CFR pattern. These findings indicate that intravenous rHuMIGDF administration, while capable of sensitizing pIatelets, does not enhance platelet-dependent thrombosis in vivo.

PEG-rHuMGDF promotes multilineage hematopoietic recovery in myelosuppressed mice

PEG-rHuMGDF administered to normal mice is a lineage-specific growth factor for megakaryocytes and platelets as judged by morphologic examination of hematologic cells in marrow and peripheral blood smears. The purpose of this study was to document that PEG-rHuMGDF in myelosuppressed mice promotes multilineage hematopoietic recovery. High-dose 5-fluorouracil (5-FU) in mice results in profound myelosuppression and 0-30% survival. Mice receiving a single dose of PEG-rHuMGDF (1000 microg/kg) 1 day after 5-FU (225 mg/kg) demonstrate an increased survival (76% vs 27% in control mice at 14 days). Compared to surviving controls, PEG-rHuMGDF-treated mice not only show the expected higher platelet counts, but also increased marrow colony-forming unit granulocyte-macrophage, increased multilineage marrow cellularity, and increased neutrophil, monocyte, and lymphocyte counts in peripheral blood. PEG-rHuMGDF- and vehicle-treated mice both develop hepatic abscesses after 5-FU treatment, but the abscesses in the PEG-rHuMGDF-treated mice contain more neutrophils, suggesting that myeloid reconstitution contributes to their survival. Furthermore, survival in 5-FU-treated mice is significantly improved by granulocyte colony-stimulating factor and antibiotics, suggesting that infection rather than thrombocytopenia is the predominant cause of death. PEG-rHuMGDF after 5-FU promotes survival accompanied by accelerated lymphohematopoietic repopulation, suggesting that PEG-rHuMGDF, a lineage-specific thrombopoietic factor in normal mice, promotes multilineage hematopoietic recovery in myelosuppressed mice.

Effects of recombinant thrombopoietin on the growth of murine primitive and committed hematopoietic progenitors in serum-free culture

BACKGROUND

The aim of the present study was to determine the effect of thrombopoietin (Tpo) in combination with other cytokines on the growth of murine megakaryocytic, granulocyte-macrophage, erythroid and primitive hematopoietic progenitor cells, excluding possibilities of synergistic effects by serum factor(s).

METHODS

Serum-free clonogenic assay systems were used for assay of colony forming units in megakaryocytes (CFU-Mk), colony forming units in granulocytes-macrophages (CFU-GM), burst-forming units in erythrocytes (BFU-E) in marrow of normal mice and of high proliferative potential colony forming cells in 5-fluorouracil-treated marrow.

RESULTS

Serum-free culture enabled megakaryocyte colony growth by recombinant murine (rm) Tpo and this was synergistically supported with rm interleukin (IL)-3, rm stem cell factor (SCF) or recombinant human (rh) erythropoietin (Epo). Recombinant human IL-6, rhIL-11 and rm granulocyte-macrophage colony stimulating factor (GM-CSF) showed synergistic effects with rmTpo only in the presence of serum. Recombinant murine IL-3 or rmSCF increased the large colonies and mixed-type colonies containing other populations, suggesting that these cytokines promoted the proliferation of immature populations of CFU-Mk. The rmTpo enhanced the growth of granulocyte-macrophage (GM) colonies stimulated by rmGM-CSF or rmIL-3 and erythroid bursts by rhEpo, with or without rmIL-3. The rmTpo also significantly increased HPP colonies in synergy with rmIL-3 only in the presence of serum or rmSCF.

CONCLUSION

Serum-free culture is valuable for evaluating synergistic effects of cytokines and Tpo acts not only on megakaryocytic progenitors but also on granulocyte-macrophage, erythroid and primitive progenitor cells in combination with other cytokines, such as rmIL-3 and rmSCF. However, serum or SCF was required for enhancement of the colony growth of primitive progenitors by Tpo.

Plasma thrombopoietin levels in liver cirrhosis and kidney failure

BACKGROUND

Recently, c-Mpl ligand (thrombopoietin, TPO) has been cloned by several groups and found to be a primary regulator of thrombopoiesis. Its mRNA expression has been detected in several organs including kidneys, bone marrow stroma cells, muscles, and is very strongly expressed in the liver.

OBJECTIVE

To clarify thrombopoiesis and the regulation of TPO in severe liver and renal failure.

DESIGN

We analysed plasma TPO levels in patients with biopsy verified liver cirrhosis (n = 18; mean platelet count 115 +/- 54 x 109 L-1), in patients on chronic haemodialysis as a result of end-stage renal failure (n = 20; mean platelet count 295 +/- 94 x 109 L-1), and in healthy individuals (n = 20; mean platelet count 250 +/- 40 x 109 L-1). Plasma was prepared from EDTA-anticoagulated whole blood and a commercially available ELISA kit was used for the analysis.

RESULTS

The mean plasma TPO concentration amongst the normal individuals was 50 +/- 14 pg mL-1. In the patients with liver cirrhosis and in patients on haemodialysis the mean TPO levels were 62 +/- 19 pg mL-1 and 46 +/- 17 pg mL-1, respectively. The mean plasma TPO concentration for the cirrhotic patients was significantly higher than the mean recorded for the healthy volunteers (P = 0.031), whereas no statistically significant differences in plasma TPO were seen between the group of end-stage renal failure and normals.

CONCLUSION

Our results suggest that TPO production is maintained in liver cirrhosis and in renal failure, and that the thrombocytopenia in liver cirrhosis is not due to an impaired TPO production.

Thrombopoietin and the c-Mpl receptor: insights from gene targeting

The availability of thrombopoietin (TPO) in recombinant form has revolutionized the study of megakaryocytopoiesis and provided an exciting new reagent for clinical evaluation. Through the application of gene targeting technology, the production of mice lacking TPO or its receptor c-Mpl has provided valuable insights into the physiological roles of TPO signalling. The near identical phenotype of c-mpl-/- and TPO-/- mice provides strong biological evidence that TPO is the sole c-Mpl ligand and uses no other additional receptor itself. TPO-/- and mpl-/- mice are severely thrombocytopenic indicating that TPO is the primary physiological regulator of platelet production in vivo. The physiological basis for this platelet deficiency has been further defined by analysis of megakaryocytes and committed progenitor cells, the numbers of which are also reduced in these mutants. The platelets that are produced in the absence of TPO signalling are morphologically and functionally normal and residual production is sufficient to prevent bleeding and allow a normal lifespan. Thus, TPO-/- and mpl-/- mice also reveal that important TPO-independent mechanisms exist that control platelet production in vivo, and these mice are ideal models to explore the nature of these alternative regulators. The mechanisms regulating the circulating levels of TPO have also been elucidated in these mice, highlighting the central role of c-Mpl mediated internalisation and degradation. The unexpected observation that progenitor cells of all hemopoietic lineages are produced in reduced numbers in TPO-/- and mpl-/- mice has also led to studies that uncovered a central role for TPO signalling in hemopoietic stem cell regulation.

Production of Thrombopoietin by Human Carcinomas and Its Novel Isoforms

Thrombocytosis is occasionally seen in patients with carcinomas and has been assumed to be attributable to interleukin-6 or granulocyte-macrophage colony-stimulating factor produced by carcinoma cells. In this study, we clarified whether thrombopoietin (TPO) is involved in carcinoma-associated thrombocytosis. Expression of TPO mRNA was observed in the majority of 27 carcinoma cell lines as determined by reverse transcriptase-polymerase chain reaction (RT-PCR). There were 6 PCR products differing in size; sequence analysis showed the full-length TPO mRNA (TPO-1), 12- and 116-bp deleted variants (TPO-2 and TPO-3, respectively), and 3 novel isoforms (197- and 128-bp deleted forms and a 60-bp insert form of TPO-3; named TPO-4, TPO-5, and TPO-6, respectively). Of 27 lines, 24 expressed TPO-1 mRNA with various other isoforms. Culture supernatants of COS-1 cells transfected with TPO-5 or TPO-6 cDNA did not promote the proliferation of TPO-responsive cells, whereas Western blot analysis on the cell lysates demonstrated TPO-5 but not TPO-6 protein, suggesting poor extracellular secretion (TPO-5) or poor protein synthesis (TPO-6). TPO protein was detected in 10-fold concentrated culture supernatants of cells of these carcinoma lines, with a median concentration of 0.38 fmol/mL as evaluated by enzyme-linked immunosorbent assay. High blood TPO levels were observed with a median value of 3.46 fmol/mL (range, 0.34 to 8.67 fmol/mL) in patients with advanced carcinomas associated with thrombocytosis. These results indicate that thrombocytosis in patients with carcinomas might be caused, at least in part, by TPO produced by carcinoma cells.

Production of Thrombopoietin by Human Carcinomas and Its Novel Isoforms

Thrombocytosis is occasionally seen in patients with carcinomas and has been assumed to be attributable to interleukin-6 or granulocyte-macrophage colony-stimulating factor produced by carcinoma cells. In this study, we clarified whether thrombopoietin (TPO) is involved in carcinoma-associated thrombocytosis. Expression of TPO mRNA was observed in the majority of 27 carcinoma cell lines as determined by reverse transcriptase-polymerase chain reaction (RT-PCR). There were 6 PCR products differing in size; sequence analysis showed the full-length TPO mRNA (TPO-1), 12- and 116-bp deleted variants (TPO-2 and TPO-3, respectively), and 3 novel isoforms (197- and 128-bp deleted forms and a 60-bp insert form of TPO-3; named TPO-4, TPO-5, and TPO-6, respectively). Of 27 lines, 24 expressed TPO-1 mRNA with various other isoforms. Culture supernatants of COS-1 cells transfected with TPO-5 or TPO-6 cDNA did not promote the proliferation of TPO-responsive cells, whereas Western blot analysis on the cell lysates demonstrated TPO-5 but not TPO-6 protein, suggesting poor extracellular secretion (TPO-5) or poor protein synthesis (TPO-6). TPO protein was detected in 10-fold concentrated culture supernatants of cells of these carcinoma lines, with a median concentration of 0.38 fmol/mL as evaluated by enzyme-linked immunosorbent assay. High blood TPO levels were observed with a median value of 3.46 fmol/mL (range, 0.34 to 8.67 fmol/mL) in patients with advanced carcinomas associated with thrombocytosis. These results indicate that thrombocytosis in patients with carcinomas might be caused, at least in part, by TPO produced by carcinoma cells.

Repression of A TAF(II)32 isoform as part of a program of genes regulated during mpl ligand-induced megakaryocyte differentiation

Circulating platelets, essential for thrombosis and hemostasis, originate from megakaryocytes. Megakaryocyte growth, differentiation and survival processes are regulated by the c-Mpl receptor ligand. In the current study we used differential display to identify part of the program of genes regulated during Mpl ligand-induced murine megakaryocyte differentiation. Several of the genes, including the retinoblastoma binding protein p84, were found to be induced, while others were repressed. One such repressed gene was identified as a TATA-binding protein (TBP)-Associated Factor (TAF) family member, TAF(II)32, previously reported to be upregulated during apoptosis. Our analysis of various cell types suggested that the previously identified species homologs, human TAF(II)32 and murine TAF(II)32, are in fact different isoforms, which we propose to re-name TAF(II)32alpha and TAF(II)32beta, respectively. Only the TAF(II)32beta isoform is regulated during Mpl ligand-induced megakaryocyte differentiation, which suggests individual roles for the two forms.

Thrombopoietin and interleukin 11 have different modulatory effects on cell cycle and programmed cell death in primary acute myeloid leukemia cells

The c-mpl ligand, thrombopoietin (TPO), is a physiologic regulator of platelet and megakaryocytic production, acting synergistically on thrombopoiesis with the growth factors interleukin 11 (IL-11), stem cell factor, interleukin 3 (IL-3), interleukin 6 (IL-6), and granulocyte-macrophage colony-stimulating factor. Because some of these growth factors, especially TPO and IL-11, are now being evaluated clinically to reduce chemotherapy-associated thrombocytopenia in cancer patients, we evaluated 25 acute myeloid leukemia (AML) samples to test whether TPO, IL-11, and other early-acting megakaryocyte growth factors can affect leukemic cell proliferation, cell cycle activation, and programmed cell death (PCD) protection. TPO induced proliferation in the majority of AML samples from an overall mean proportion of S-phase cells of 7.8% +/-1.5% to 14.5% +/- 2.1% (p = 0.0006). Concurrent G0 cell depletion was found in 47.3% of AML samples. TPO-supported leukemic cell precursor (CFU-L) proliferation was reported in 5 of 17 (29.4%) of the samples with a mean colony number of 21.4 +/- 9.6 x 10(5) cells plated. In 13 of 19 samples, a significant protection from PCD (from an overall mean value of 13% +/-0.7% to 8.8% +/- 1.8%;p = 0.05) was detected after TPO exposure. Conversely, IL-11-induced cell cycle changes (recruitment from G0 to S phase) were detected in only 2 of 14 samples (14.2%). In addition, IL-11 showed little, if any, effect on CFU-L growth (mean colony number = 17.5 9.5) or apoptosis. Combination of TPO with IL-11 resulted in only a slight increase in the number of CFU-L, whereas IL-3 and stem cell factor significantly raised the mean colony numbers up to 119.2 +/- 68.3 and 52.9 +/- 22.1 x 10(5) cells plated, respectively. We conclude that TPO induces cell cycle activation in a significant proportion of cases and generally protects the majority of AML blast cells from PCD. On the other hand, IL-11 has little effect on the cell cycle or PCD. Combination of both TPO and IL-11 is rarely synergistic in stimulating AML clonogenic growth. These findings may be useful for designing clinical studies aimed at reducing chemotherapy-associated thrombocytopenia in AML patients.

The influence of automated plateletpheresis on systemic levels of hematopoietic growth factors

BACKGROUND

Megakaryocytopoiesis and platelet production are regulated by several hematopoietic growth factors. The present study focuses on the effects of automated plateletpheresis on systemic levels of different hematopoietic growth factors.

STUDY DESIGN AND METHODS

Platelet count, mean platelet volume, and serum levels of thrombopoietin, erythropoietin, interleukin-1beta, interleukin-6, and stem cell factor in 21 healthy donors were measured before platelet collection, after the first half of the apheresis procedure, at the end of apheresis, and on Days 1, 2, and 7 thereafter.

RESULTS

Thrombopoietin levels (initial level: 49.5 +/- 25.5 pg/mL) showed a significant increase between measurements taken at the end of apheresis and Day 1 (56.9 +/- 26.7 pg/mL; p = 0.01). There was a highly significant decrease in stem cell factor levels during apheresis (p<0.0005), reaching preapheresis values (1679 +/- 210 pg/mL) on Day 1. A highly significant increase in erythropoietin levels (initial level: 7.5 +/- 4.0 U/L) was seen after apheresis (p<0.0005 on Days 1 and 2). The level remained significantly elevated until Day 7 (p = 0.004). Interleukin-1beta and interleukin-6 levels (before donation: 1.4 +/- 1.8 pg/mL and 1.1 +/- 0.7 pg/mL, respectively) did not change during the observation period. Thrombopoietin levels correlated consistently and inversely with stem cell factor levels after apheresis (Day 1, r = -0.46, p = 0.035; Day 2, r = -0.50, p = 0.02; Day 7, r = -0.50, p = 0.02).

CONCLUSION

The data show a coordinated response of the hematopoietic system to platelet loss. It is suggested that the decrease in serum stem cell factor levels during apheresis reflects the consumption of stem cell factor by early hematopoietic progenitors that expand to initiate early megakaryocytopoiesis. The temporary increase in thrombopoietin is the result of platelet loss and serves as a stimulus for subsequent thrombopoiesis. The pronounced elevation of erythropoietin after apheresis suggests a role for this primarily erythropoietic cytokine in thrombopoiesis, too.

Interaction of thrombopoietin with the platelet c-mpl receptor in plasma: binding, internalization, stability and pharmacokinetics

Thrombopoietin (TPO) is the primary regulator of platelet production and acts through binding its receptor, c-mpl, found on megakaryocyte progenitor cells, megakaryocytes and platelets. Circulating levels of TPO are regulated primarily by the clearance of TPO after it binds to c-mpl receptors on circulating platelets. In this study the interaction of TPO with the platelet c-mpl receptor has been analysed under physiological conditions using radiochemical and pharmacokinetic approaches. 125I-rHuTPO was prepared using a novel method of gentle iodination that preserved its biological activity and used to demonstrate that platelets, but not endothelial cells, have a single class of binding sites (56 +/- 17 binding sites/platelet) with high affinity (Kd = 163 +/- 31 pM). Cross-linking experiments confirmed that TPO, but not erythropoietin (EPO), specifically associated with the 95 kD platelet c-mpl receptor. Upon addition of TPO to platelets, 80% of the TPO binding sites were internalized within an hour and were not recycled. TPO that was not bound by platelets was stable for up to 6 d in both platelet-poor and platelet-rich plasma. Using unlabelled recombinant human TPO (rHuTPO), standard pharmacokinetic analysis demonstrated that platelets have an average TPO clearance of 1.24 +/- 0.38 ml/h/109 platelets and that TPO clearance was reduced by low temperature but not by a number of drugs or metabolic inhibitors. The maximal amount of TPO removed by platelets in vitro was identical to that predicted by the total number of TPO binding sites. These results provide a biochemical and pharmacokinetic basis for the clinical use of TPO and for understanding possible disorders of platelet production.

Physiology and clinical applications of platelet growth factors

Recombinant human thrombopoietin, or pegylated recombinant human megakaryocyte growth and development factor, produces log-linear increases in megakaryocytopiesis and platelet production. Recent clinical studies investigating the efficacy, safety, and cost benefit of administering platelet growth factors indicate that appropriate dosing of pegylated recombinant human megakaryocyte growth and development factor, or recombinant human thrombopoietin, during chemotherapeutic marrow suppression produces these increases in four ways. First, these factors improve ensuant thrombocytopenia without producing platelet-dependent thrombo-occlusive complications. Second, recombinant human thrombopoietin or pegylated recombinant human megakaryocyte growth and development factor mobilize hematopoietic progenitor cells into the peripheral blood. Third, chronic dosing of HIV-infected thrombocytopenic patients with pegylated recombinant human megakaryocyte growth and development factor normalizes peripheral platelet counts without antibody formation. Fourth, the administration of pegylated recombinant human megakaryocyte growth and development factor to normal human volunteer platelet donors increases platelet yields nearly fourfold, with corresponding increases in peripheral platelet counts when transfused into thrombocytopenic recipients. The clinical application of platelet growth factors is continuing to be defined.

The enigmatic megakaryocyte gradually reveals its secrets

The recent cloning of thrombopoietin has brought many insights into the cellular and molecular mechanisms of megakaryocyte and platelet development. Thrombopoietin was cloned based on its binding to the product of the proto-oncogene c-mpl and was found to affect all aspects of thrombopoiesis. Many of the molecular pathways that mediate thrombopoietin action have been discerned. Upon hormone binding, the megakaryocyte thrombopoietin receptor homodimerizes, activating members of the JAK family of kinases, which, in turn, phosphorylate the receptor, generating docking sites for second messengers that affect multiple signalling pathways. Ultimately, cellular proliferative and anti-apoptotic mechanisms are initiated, increasing megakaryocyte numbers, as are processes that uncouple DNA synthesis from nuclear and cytoplasmic division, generating polyploid cells. As the net result of thrombopoietin action is an expansion of cells that give rise to mature platelets, the availability of the recombinant hormone has provided new opportunities to manipulate blood cell development for therapeutic benefit.

Megakaryocyte ex vivo expansion potential of three hematopoietic sources in serum and serum-free medium

Megakaryocytes (MK) were expanded from purified human CD34+ cells obtained from three sources, bone marrow (BM), mobilized peripheral blood progenitor cells (PB), and umbilical cord (UC) blood. CD34+-selected cells were cultured for 12 days with 10 ng/ml thrombopoietin (TPO), 10 ng/ml IL-3, 10 ng/ml TPO + 10 ng/ml IL-3, or 200 ng/ml promegapoietin (PMP), a chimeric dual agonist of the c-Mpl and human IL-3 receptors. MK production was compared in serum-free versus human serum-supplemented liquid media. PMP and the combination of TPO and IL-3 (TPO + IL-3) increased MK production similarly. Culturing CD34+ cells with PMP in serum-free medium resulted in a twofold increase in MK yield compared with serum-supplemented medium. CD34+ cells from UC proliferated more than those from either BM or PB in liquid culture, resulting in much greater MK production under all conditions. Phenotypic analysis of the uncultured CD34+ cells showed that BM had a higher frequency of CD34+/CD41+ cells than PB or UC. TPO + IL-3 or PMP produced larger and greater numbers of BFU-MK and CFU-MK per seeded CD34+/CD41+ cell from UC than from either BM or PB. Thus, although uncultured CD34+-selected BM cells contained a higher frequency of committed mature MK progenitors, UC CD34+ cells had a greater proliferative capacity and, therefore, were more productive. PMP induced megakaryocytopoietic activity comparable to that achieved with TPO + IL-3 and may be useful for ex vivo expansion of MK for clinical trials.

Thrombopoietin promotes hematopoietic recovery and survival after high-dose whole body irradiation

PURPOSE

The therapeutic potential of thrombopoietin (TPO), the major regulator of platelet production, was evaluated for hematopoietic recovery and survival in mice following lethal and supralethal total body irradiation (TBI).

METHODS AND MATERIALS

Hematopoietic recovery was studied in C57BL6/J mice after 8 Gy TBI (gamma-rays). Survival experiments were performed with C57BL6/J and BCBA F1 mice. Two protocols of TPO administration were evaluated: treatment for 7 consecutive days (7 x 0.3 microg/mice) beginning 2 h after exposure, or a single dose (0.3 microg/mice) administered 2 h after irradiation.

RESULTS

TPO improved the platelet nadir and accelerated the platelet reconstitution of irradiated mice in comparison to placebo-treated mice. Recovery of neutrophils and erythrocytes was stimulated as well. TPO induced an accelerated recovery of hematopoietic progenitors and immature multilineage progenitors in bone marrow and spleen. In addition, TPO administration induced approximately 90% survival of 8 Gy irradiated C57BL6/J mice, a TBI dose which resulted in 100% mortality within 30 days for placebo-treated mice. Single TPO administration was as effective as repeated injections for hematopoietic recovery and prevention of mortality. Dose-effect survival experiments were performed in BCBA F1 mice and demonstrated that TPO shifted the LD50/30 from approximately 9.5 Gy to 10.5 Gy TBI given as a single dose, and from 14 Gy to as high as 17 Gy when TBI was given in three equal doses, each separated by 24 h.

CONCLUSION

These results demonstrate that the multilineage hematopoietic effects of TPO may be advantageously used to protect against lethal bone marrow failure following high dose TBI.

Thrombopoietin

Thrombopoietin (TPO) was first described as an activity in thrombocytopenic serum that could stimulate platelet production upon transfer into other animals. The molecular cloning of TPO has allowed a detailed characterisation of its precise biological activities, both in vitro and in vivo. TPO binds its specific receptor, the c-Mpl protein, which is expressed on the surface of target cells, and induces receptor dimerization and activation of intracellular signalling pathways including the Janus kinase (JAK)-signal transducers and activators of transcription (STAT) and Ras cascades. TPO is a lineage-dominant cytokine, stimulating megakaryocyte production and maturation in vitro and inducing the greatest elevation in platelet numbers of any known cytokine upon administration to humans or laboratory animals. Indeed, gene targeting studies have established that the TPO/c-Mpl signalling system is the major physiological regulator of steady-state megakaryocyte and platelet production. These studies have also revealed a largely unanticipated role for TPO in the regulation of hemopoietic stem cells. Preclinical studies in animal models have indicated the capacity of c-Mpl ligands to overcome the thrombocytopenia associated with chemo/radiotherapy and clinical trials have already demonstrated the safety and efficacy of Mpl-ligands in elevating platelet counts in humans. It is likely that these molecules will lead to new therapeutics that will find utility in a range of clinical contexts.

The prolonged hematologic effects of a single injection of PEG-rHuMGDF in normal and thrombocytopenic mice

A single injection of > or =10 microg/kg PEG-rHuMGDF in mice causes a dose-dependent increase in circulating platelets beginning on day 3 and peaking on days 5-6. The mean platelet volume and platelet distribution width at doses > or =100 microg/kg initially increase in a dose-dependent fashion and later decrease. However, the mean platelet volume does not change when platelets are incubated with PEG-rHuMGDF in vitro. The number of marrow megakaryocytes increases in a dose-dependent fashion as early as day 1 and peaks on day 3. Marrow megakaryocyte colony-forming units (CFU-Meg) do not increase on days 1-3 at a dose of 100 microg/kg (a dose that increases platelet numbers two- to threefold and may be clinically relevant), but the relative frequency of high ploidy megakaryocytes and the proportion of large marrow megakaryocytes (29-50 microm in diameter) increases. After a dose of 1,000 microg/kg the percentage of megakaryocytes in mitosis peaks at 24-48 hours and the percentage of megakaryocytes incorporating BrdU is maximal at 48 hours, the relatively delayed peak of BrdU incorporation most likely representing endomitosis. The relative frequency of type II and III megakaryocytes peaks on days 3 and 4, respectively. Pharmacokinetic analysis of PEG-rHuMGDF shows peak serum concentrations at 2-4 hours and a terminal half-life of 11.4+/-2.5 hours. A single injection of PEG-rHuMGDF ameliorates carboplatin-induced megakaryocytopenia and thrombocytopenia in a dose-response dependent fashion. In conclusion, a single injection of PEG-rHuMGDF increases megakaryocyte and platelet production in normal and myelo-suppressed mice.

Plasma thrombopoietin (TPO) levels and expression of TPO receptor on platelets in patients with myelodysplastic syndromes

Data on endogenous thrombopoietin (TPO) levels and their regulation in myelodysplastic syndromes (MDS) are sparse. We examined the plasma TPO level of 85 MDS patients by a sensitive enzyme immunoassay and the platelet expression of TPO receptor (TPO-R) protein, which metabolizes endogenous TPO, in 19 MDS patients with an equilibrium binding assay using 125I-TPO. The MDS patients had higher plasma TPO levels (7.0 +/- 9.3 fmol/ml) than 52 normal subjects (P < 0.0001). Refractory anaemia (RA) patients (n = 39) had higher plasma TPO levels than patients (n = 28) with RA with excess blasts (RAEB) or RAEB in transformation (RAEB-t) (P = 0.0002), irrespective of similar platelet counts in these groups. The plasma TPO level correlated inversely with the platelet count in RA patients (P = 0.0027) but not in RAEB and RAEB-t patients (P = 0.7865). These data suggest that the physiological pathway for TPO production and metabolism is conserved, at least partially, in RA, but deranged in RAEB/RAEB-t. The number of TPO-R per platelet was significantly smaller in 19 MDS patients (17.5 +/- 13.3) than in normals (P = 0.0014), but similar between RA patients and patients with RAEB and RAEB-t. Further, the bone marrow megakaryocyte count, determined in 31 MDS patients, was quite similar between RA patients and patients with RAEB or RAEB-t. Thus, in addition to thrombocytopenia, a reduced platelet TPO-R number may contribute to elevated plasma TPO levels in MDS, and a regulatory pathway for circulating TPO other than platelet TPO-R and marrow megakaryocytes, such as blasts expressing TPO-R, may operate in RAEB/RAEB-t.

Native thrombopoietin: structure and function

Thrombopoietin (TPO), the c-Mpl ligand, is produced constitutively in liver and other organs, circulates in the bloodstream, and is delivered to bone marrow, where it stimulates the early development of multiple hematopoietic lineages and megakaryocytopoiesis. The concentration of TPO in blood is regulated by c-Mpl mass on platelets and megakaryocytes. In addition to regulation by the number of TPO molecules, including the possible modulation of TPO mRNA abundance in bone marrow, megakaryocytopoiesis and platelet production may be regulated as a result of modulation of TPO activity by proteolytic processing that generates truncated forms of the molecule. Characterization of TPO partially purified from human plasma, however, revealed that the full-length molecule was the predominant form in the blood of both normal individuals and thrombocytopenic patients, although small amounts of truncated species were detected. Thus, truncation of TPO, at least that in the circulation examined, does not appear to contribute to the direct regulation of platelet production in response to increased demand. Given that native TPO isolated from the plasma of thrombocytopenic animals comprises truncated forms, the truncation of TPO is likely of physiological importance in the life history of this molecule.

Thrombopoietins and thrombopoiesis: a clinical perspective

Since the discovery of thrombopoietin four years ago there has been much interest in the clinical use of this growth factor and its impact on platelet transfusions. Two recombinant thrombopoietin molecules are currently under intense clinical investigation. One is a full-length, glycosylated thrombopoietin (rHuTPO) and the other is a non-glycosylated, truncated thrombopoietin coupled to polyethylene glycol (PEG-rHuMGDF). Both bind to the thrombopoietin receptor, c-mpl, and stimulate megakaryocyte growth and platelet production in vitro and in vivo. In early clinical studies these "Mpl ligands" have been effective in reducing thrombocytopenia after non-myeloablative but not after myeloablative chemotherapy. In transfusion medicine, they may serve to increase the yield of stem cell harvests, expand progenitor cells ex vivo and stimulate platelet apheresis donors. Their impact on platelet usage is still unclear but may be less than initially estimated.

Ex vivo expansion of cord blood progenitors

Human umbilical cord blood contains abundant primitive and committed hematopoietic progenitors; in addition, the general availability and the ease of procurement make cord blood a very attractive alternative source of transplantable hematopoietic tissue. However, the major limitation to a widespread use of cord blood for transplantation lays in its limited volume. For such a reason, until now, cord blood transplant has been mainly restricted to children and small size adults. Ex vivo expansion of cord blood stem cells could make the use of cord blood transplant feasible also for adult patients. Recently we developed a stroma-free culture system in which a progressive, increasingly greater production of hemopoietic progenitors belonging to all the hematopoietic lineages was sustained for over six months. A similar sustained and prolonged expansion of the most primitive stem cells that can be detected in vitro (LTC-IC), was also documented. The extremely prolonged maintenance and the massive expansions suggest that extensive self-renewal and little differentiation can be triggered in vitro by FLT3/FLK2 ligand (FL) plus c-mpl ligand (Thrombopoietin) and this could represent a first step towards the implementation of clinical expansion-transplantation strategies.

Thrombopoietin Promotes the Survival of Murine Hematopoietic Long-Term Reconstituting Cells: Comparison With the Effects of FLT3/FLK-2 Ligand and Interleukin-6

The effects of thrombopoietin (TPO; c-mpl ligand), FLT3/FLK-2 ligand (FL), and interleukin-6 (IL-6) on the survival of murine hematopoietic long-term reconstituting cells (LTRC) were studied by using lineage-negative, Sca-1–positive, c-kit–positive (Lin−Sca-1+c-kit+) marrow cells from 5-fluorouracil–treated mice. We tested the ability of these cytokines to maintain the viability of LTRC by transplanting the cultured cells to lethally irradiated Ly-5 congenic mice together with compromised marrow cells. As a single agent, only TPO could maintain the LTRC. Neither IL-6 nor FL was effective by itself, but they acted synergistically to maintain the LTRC. We examined whether the maintenance of LTRC by these cytokines was due to the survival of stem cells or was the result of active cell divisions and self-renewal. To monitor cell division, we used membrane dye PKH26. Enriched cells were stained with PKH26 on day 0 and incubated in suspension culture with TPO or with IL-6 and FL for 7 days. On day 7, PKH26low and PKH26high cells were prepared by sorting and their in vivo reconstituting abilities were tested by transplantation into lethally irradiated Ly-5 congenic mice together with compromised marrow cells. PKH26high populations cultured with both TPO alone and the combination of IL-6 and FL showed greater reconstitution activity than that of PKH26low populations. These data indicate that TPO alone and the combination of IL-6 and FL can support the survival of stem cells without stimulating their active cell proliferation.

Thrombopoietin Promotes the Survival of Murine Hematopoietic Long-Term Reconstituting Cells: Comparison With the Effects of FLT3/FLK-2 Ligand and Interleukin-6

The effects of thrombopoietin (TPO; c-mpl ligand), FLT3/FLK-2 ligand (FL), and interleukin-6 (IL-6) on the survival of murine hematopoietic long-term reconstituting cells (LTRC) were studied by using lineage-negative, Sca-1–positive, c-kit–positive (Lin−Sca-1+c-kit+) marrow cells from 5-fluorouracil–treated mice. We tested the ability of these cytokines to maintain the viability of LTRC by transplanting the cultured cells to lethally irradiated Ly-5 congenic mice together with compromised marrow cells. As a single agent, only TPO could maintain the LTRC. Neither IL-6 nor FL was effective by itself, but they acted synergistically to maintain the LTRC. We examined whether the maintenance of LTRC by these cytokines was due to the survival of stem cells or was the result of active cell divisions and self-renewal. To monitor cell division, we used membrane dye PKH26. Enriched cells were stained with PKH26 on day 0 and incubated in suspension culture with TPO or with IL-6 and FL for 7 days. On day 7, PKH26low and PKH26high cells were prepared by sorting and their in vivo reconstituting abilities were tested by transplantation into lethally irradiated Ly-5 congenic mice together with compromised marrow cells. PKH26high populations cultured with both TPO alone and the combination of IL-6 and FL showed greater reconstitution activity than that of PKH26low populations. These data indicate that TPO alone and the combination of IL-6 and FL can support the survival of stem cells without stimulating their active cell proliferation.

Recombinant human megakaryocyte growth and development factor increases levels of circulating haemopoietic progenitor cells post chemotherapy in patients with acute myeloid leukaemia

By participating in a randomized safety and efficacy study of pegylated-recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) post induction and consolidation chemotherapy for de novo acute myeloid leukaemia, serial determinations of circulating haemopoietic progenitor cells were performed during 18 chemotherapy courses in eight patients (three receiving placebo; one, 2.5; and four, 5.0 microg/kg/d MGDF, respectively). Whereas failure to achieve complete remission (CR) was generally associated with poor progenitor cell increments following chemotherapy, substantial progenitor cell mobilization consistently occurred during haemopoietic recovery in patients entering, or in CR, with significantly higher peak values in patients receiving 5 microg/kg/d of MGDF as compared to controls. The median increases of progenitor cell numbers by chemotherapy alone and chemotherapy plus 5.0 microg/kg/d MGDF over that in normal individuals with steady-state haemopoiesis were 10- and 45-fold for CFU-GM, 3- and 17-fold for BFU-E, and 2- and 18-fold for CFU-mix. CFU-Mk levels were not increased above normal by chemotherapy alone but were 15-fold enhanced by chemotherapy plus MGDF. Recruitment of CD34+ cells post chemotherapy was also potentiated by MGDF. Our results suggest MGDF as a potent agent to augment progenitor cell mobilization after successful induction or consolidation chemotherapy in patients with AML.

Clonality analysis of hematopoiesis and thrombopoietin levels in patients with essential thrombocythemia

Essential thrombocythemia (ET) is a myeloproliferative disorder, characterized by sustained thrombocytosis. Diagnosis requires the elimination of all known causes of thrombocytosis. ET is believed to be a clonal disorder, and we investigated the frequency of a clonal hematopoiesis in this disease with the aim of using this as a positive diagnostic criterion. However, a non-random inactivation pattern can be encountered in normal females which mimics clonal hematopoiesis. In addition, the percentage of normal females with skewed lyonization seems higher using techniques based on the difference in DNA methylation, compared to G6PD enzyme polymorphism. Recently, new techniques based on transcript analysis have been developed. We report here the results of clonality studies of hematopoiesis in 53 ET patients using two different techniques based on DNA and RNA polymorphisms, and T-lymphocytes as a control tissue of lyonization. The majority of ET patients showed monoclonal hematopoiesis in the presence of polyclonality of T-lymphocytes. Because all ET patients did not show the same clonal pattern of hematopoiesis, we searched for inappropriate secretion of thrombopoietin (TPO) in patients with polyclonal disease. This assay was performed in 48 patients, of whom 9 showed polyclonal hematopoiesis and 27 monoclonal hematopoiesis. We found no difference in TPO levels between ET patients and normal controls, nor between patients with polyclonal hematopoiesis and those with monoclonal hematopoiesis. Our results confirm the high frequency of monoclonal hematopoiesis in ET, the usefulness of RNA markers, and the possibility of using T-lymphocytes as a control tissue for X-chromosome inactivation patterns. On the other hand, TPO levels are not decreased even in ET patients with high platelet counts, suggesting an increased production or decreased clearance of TPO in this disease.

Plasma thrombopoietin levels in thrombocytopenic states: implication for a regulatory role of bone marrow megakaryocytes

To evaluate the diagnostic value of thrombopoietin (TPO, c-mpl ligand) measurements, and clarify the regulatory mechanisms of TPO in normal and in thrombocytopenic conditions, the plasma TPO concentration was determined in normal individuals (n = 20), umbilical cord blood (n = 40), chronic idiopathic thrombocytopenic purpura (ITP; n = 16), in severe aplastic anaemia (SAA; n = 3), chemotherapy-induced bone marrow hypoplasia (n = 10), myelodysplastic syndrome (MDS; n = 11), and sequentially during peripheral blood progenitor cell transplantation (n = 7). A commercially available ELISA and EDTA-plasma samples were used for the analysis. The plasma TPO concentration in the normals and umbilical cord blood were 52 +/- 12 pg/ml and 66 +/- 12 pg/ml, respectively. The corresponding values in patients with SAA and chemotherapy-induced bone marrow hypoplasia were 1514 +/- 336 pg/ml and 1950 +/- 1684 pg/ml, respectively, and the TPO concentration, measured sequentially after myeloablative chemotherapy and peripheral blood progenitor cell transplantation, was inversely related to the platelet count. In contrast, the plasma TPO recorded in patients with ITP (64 +/- 20 pg/ml) and MDS (68 +/- 23 pg/ml) were only slightly higher than normal levels. In conclusion, TPO levels were significantly elevated in patients in which bone marrow megakaryocytes and platelets in circulation were markedly reduced, whereas TPO levels were normal in ITP patients, and only slightly increased in the MDS patients. These latter patients displayed a preserved number of megakaryocytes in bone marrow biopsies. Our data support the suggestion that megakaryocyte mass affects the plasma TPO concentration. In thrombocytopenic patients a substantially increased plasma TPO implies deficient megakaryocyte numbers. However, TPO measurements do not distinguish between ITP and thrombocytopenia due to dysmegakaryopoiesis, as seen in MDS patients.

The thrombopoietic factor, Mpl-ligand

Mpl-ligand (thrombopoietin [TPO] or Megakaryocyte growth and development factor [MGDF]), was identified following the discovery of the oncogene, mpl. This oncogene encodes a truncated receptor belonging to the hemopoietin-receptor superfamily. Mpl-ligand consists of two domains; a carbohydrate-rich domain and an amino-terminal domain which displays homology to erythropoietin and is fully active. Mpl-ligand mRNA is predominantly expressed in liver and kidney and plasma levels are regulated by receptors expressed on megakaryocytes and platelets. Mpl-ligand is the most active single agent capable of promoting proliferation and maturation of cells within the megakaryocyte lineage. In addition to this 'lineage-specific' action, Mpl-ligand also acts on progenitor/stem cells of multiple hemopoietic lineages. Pre-clinical and early clinical studies have demonstrated the ability of Mpl-ligand to increase platelet counts and hasten platelet recovery after chemotherapy. This offers considerable promise for its use in clinical oncology.

Thrombopoietin Requires Additional Megakaryocyte-Active Cytokines for Optimal Ex Vivo Expansion of Megakaryocyte Precursor Cells

Little is known concerning the interaction of thrombopoietin (TPO) with other megakaryocyte-active cytokines in directing the early events of megakaryocyte development. Culture of CD34+ cells in interleukins (IL) -1, -6, -11, plus stem cell factor (SCF; S) results in a 10- to 12-fold expansion in total cell numbers, whereas total CD41+ megakaryocytes are expanded ∼120-fold over input levels. Addition of TPO to IL-1, -6, -11, S generates a biphasic proliferation of CD41+ cells, accelerates their rate of production, and results in an ex vivo expansion of more than 200-fold. The addition of Flt-3 ligand (FL) increases CD41+ cell expansion to ∼380-fold over input levels. In the absence of TPO, ∼95% of the expanded cells show the phenotype of promegakaryoblasts; TPO and/or FL addition increases CD41 antigen density and ploidy in a subpopulation of promegakaryoblasts. A moderate (approximately sevenfold) expansion of megakaryocyte progenitor cells (colony-forming unit-megakaryocyte) occurs in the presence of IL-1, -6, -11, S, and the addition of TPO to this cocktail yields an ∼17-fold expansion. We conclude that early proliferative events in megakaryocyte development in vitro are regulated by multiple cytokines, and that TPO markedly affects these early developmental steps. However, by itself, TPO is neither necessary nor sufficient to generate a full proliferative/maturational in vitro response within the megakaryocyte compartment. TPO clearly affects terminal differentiation and the development of (some) high-ploidy human megakaryocytes. However, its limited in vitro actions on human cell polyploidization suggest that additional megakaryocyte-active cytokines or other signals are essential for the maximal development of human megakaryocytes.

Therapeutic cytokine stimulation of thrombocytopoiesis

Endogenous thrombopoietin (TPO) stimulates platelet production by inducing dose-dependent megakaryocyte development from early marrow hematopoietic progenitors and subsequent proliferation and endoreduplication. Recombinant human (rHu) TPO or pegylated recombinant human megakaryocyte growth and development factor (PEG-rHu MGDF) produce log-linear responses in experimental animals and humans when administered over the dose range of 0.05-25 micrograms/kg/day with respect to peak peripheral platelet counts (or peripheral platelet mass turnover) and marrow megakaryocyte volume, ploidy, number and mass. Other cytokines stimulating megakaryocytopoiesis in various species using different dosing regimens include Interleukin 1 (IL-1), Interleukin 3 (IL-3), Interleukin 6 (IL-6), Interleukin 11 (IL-11), stem cell factor (SCF), granulocytemacrophage colony stimulating factor, (GM-CSF), fusion proteins, such as promegapoietin, and a peptide agonist of TPO receptor. Whereas neither rHuTPO nor PEG-rHuMGDF induce platelet aggregation in vitro, they transiently enhance aggregatory responsiveness of platelets to physiologic agonists from several different experimental animals both in vitro and ex vivo. However, platelet recruitment into forming thrombus is not augmented by these agents when evaluated in quantitative rabbit or baboon models of platelet-dependent thrombus formation, except for the effect of platelet concentration per se. These findings indicate that appropriate dosing of these agents following marrow suppression prevents thrombocytopenia without increasing the risk of platelet-dependent thrombo-occlusive complications.

The Mpl ligand and platelet homeostasis

The classification of Mpl as a cytokine receptor present on cells of the platelet lineage has led to the identification and cloning of its ligand. This has resulted in a rapid accumulation of data advancing the understanding of the processes of megakaryopoiesis and thrombopoiesis and the regulation of endogenous Mpl ligand (thrombopoietin, eTPO). Highlights of in vitro human and non-human primate data will be discussed, as well as preclinical (in vivo) non-human primate studies. Two recombinant forms of Mpl ligands (rTPO) are currently being tested in clinical trials and early results will be reviewed. The preclinical and clinical studies will be summarized with consideration of the observations which provide insights into the biology of the response to exogenous rTPO. Understanding the biology of platelet production and the condition of target cells in treatment populations will facilitate the appropriate use of this potential therapeutic agent.

Thrombopoietin Requires Additional Megakaryocyte-Active Cytokines for Optimal Ex Vivo Expansion of Megakaryocyte Precursor Cells

Little is known concerning the interaction of thrombopoietin (TPO) with other megakaryocyte-active cytokines in directing the early events of megakaryocyte development. Culture of CD34+ cells in interleukins (IL) -1, -6, -11, plus stem cell factor (SCF; S) results in a 10- to 12-fold expansion in total cell numbers, whereas total CD41+ megakaryocytes are expanded ∼120-fold over input levels. Addition of TPO to IL-1, -6, -11, S generates a biphasic proliferation of CD41+ cells, accelerates their rate of production, and results in an ex vivo expansion of more than 200-fold. The addition of Flt-3 ligand (FL) increases CD41+ cell expansion to ∼380-fold over input levels. In the absence of TPO, ∼95% of the expanded cells show the phenotype of promegakaryoblasts; TPO and/or FL addition increases CD41 antigen density and ploidy in a subpopulation of promegakaryoblasts. A moderate (approximately sevenfold) expansion of megakaryocyte progenitor cells (colony-forming unit-megakaryocyte) occurs in the presence of IL-1, -6, -11, S, and the addition of TPO to this cocktail yields an ∼17-fold expansion. We conclude that early proliferative events in megakaryocyte development in vitro are regulated by multiple cytokines, and that TPO markedly affects these early developmental steps. However, by itself, TPO is neither necessary nor sufficient to generate a full proliferative/maturational in vitro response within the megakaryocyte compartment. TPO clearly affects terminal differentiation and the development of (some) high-ploidy human megakaryocytes. However, its limited in vitro actions on human cell polyploidization suggest that additional megakaryocyte-active cytokines or other signals are essential for the maximal development of human megakaryocytes.

A non-peptide compound which can mimic the effect of thrombopoietin via c-Mpl

Thrombopoietin (TPO) is a cytokine which plays a central role in megakaryopoiesis and platelet production by binding to its cell surface receptor, termed c-Mpl. In the present study, two benzodiazepinones that compete with the binding of TPO to the extracellular region of c-Mpl were identified, and one of them stimulated the proliferation of a human TPO-dependent megakaryocytic cell line, UT-7/TPO. It stimulated the activation of signal transducer and activator of transcription 5 in UT-7/TPO cells. These results suggest that a non-peptide compound can mimic the effect of TPO via c-Mpl.

Mpl ligand or thrombopoietin: biological activities

Thrombopoietin (TPO) or Mpl ligand is the primary physiological regulator of platelet production. This cytokine is the most potent stimulator of the proliferation and differentiation of MK progenitor and precursor cells in vitro. It also acts additively or synergistically with several cytokines on progenitor cells from various hematopoietic lineages, including the primitive stem cells. The factor is an extremely potent thrombocytopoietic agent when administrated to normal animals, and it accelerates platelet and erythropoietic recovery in several models of myelosuppression. Phase I/II clinical trials are ongoing with no detectable adverse effects. Mpl ligand does not induce platelet aggregation, but it lowers the platelet sensitivity to physiological dose of agonists. In experimental mouse models, high and chronic dose of Mpl ligand results in myelofibrosis. TPO is constantly produced by the liver and the kidney; its plasmatic clearance occurs by binding to its receptor expressed on megakaryocytes and platelets. However, the full spectrum of the biological effects of this new cytokine is not fully understood, in particular its the role in the terminal stage of platelet production. In the near future, it is likely that new insights will be obtained in the physiopathological mechanisms underlying abnormal platelet production in human.

Quantitative analysis of thrombopoietin receptors on human megakaryocytes

Thrombopoietin (TPO), or c-MPL ligand, is the primary regulator of megakaryocyte and platelet production. TPO receptors expressed on human megakaryocytes derived from peripheral blood (PB) and cord blood (CB) progenitors cultured in the presence of TPO have now been analyzed quantitatively. Like those on human PB platelets, TPO receptors on the cultured megakaryocytes exhibited a molecular mass of approximately 80 kDa. Various characteristics of PB- and CB-derived megakaryocytes indicated that the former were more mature than the latter. Both PB- and CB-derived megakaryocytes expressed a single class of high-affinity TPO receptors, with 1933 +/- 772 (n = 3) and 184 +/- 48 (n = 4) sites per cell, respectively. These data indicate that the number of TPO receptors on human megakaryocytes increases with cell maturation.

Blood thrombopoietin levels in clonal thrombocytosis and reactive thrombocytosis

BACKGROUND

Although the distinction between clonal and reactive thrombocytosis is clinically relevant because clonal thrombocytosis has more thrombohemorragic complications, the differential diagnosis of these two entities can be difficult. Methods such as the detection of unstimulated erythroid or megakaryocyte colony growth are not readily available. Therefore, we measured blood thrombopoietin levels to determine whether these levels can be used to distinguish the two conditions.

PATIENTS AND METHODS

Thrombopoietin levels were measured in 73 patients with thrombocytosis (platelet count > 500,000/microL), including 39 patients with clonal thrombocytosis (20 patients with essential thrombocythemia, 15 with agnogenic myeloid metaplasia, 1 patient with polycythemia vera, and 3 with undefined myeloproliferative disorders) and 34 patients with reactive thrombocytosis (17 with malignant tumors, 11 with inflammatory diseases, 4 with sickle cell disease, and 2 with iron deficiency anemia). Seventeen normal volunteers were used as controls.

RESULTS

Thrombopoietin levels were significantly higher (P < 0.05) in patients with clonal thrombocytosis (mean +/- SD of 555 +/- 585 pg/mL), including the subgroup with essential thrombocythemia (505 +/- 459 pg/mL), than in patients with reactive thrombocytosis (290 +/- 133 pg/mL) who had similar levels as controls (201 +/- 112 pg/mL). Thrombopoietin levels in patients with clonal thrombocytosis, including essential thrombocythemia, were not correlated with platelet counts.

CONCLUSIONS

Thrombopoietin levels may be helpful in distinguishing between clonal thrombocytosis and reactive thrombocytosis. Thrombopoietin is probably responsible for the elevated platelet counts in clonal thrombocytosis including essential thrombocythemia, but not in reactive thrombocytosis. High thrombopoietin levels in patients with clonal thrombocytosis cannot be explained solely by platelet megakaryocyte mass.

Stromal-conditioned medium synergizes with thrombopoietin in stimulating megakaryocytopoiesis

We previously demonstrated that stromal cells release a soluble factor(s) that enhances thrombopoietin (TPO)-induced megakaryocyte (MK) production. We have now further characterized this enhancing activity of human bone marrow stroma and have also analyzed the role of direct contact of stromal cells with human bone marrow CD34+ cells on MK production. The three- to sixfold stromal-enhancing activity of conditioned media is constitutive and is not influenced by the presence of TPO. The addition of interleukins 3, 6 (IL-3, IL-6), stem cell factor (SCF), and TPO at concentrations found in stroma to a stroma-free system only enhanced by 58% the TPO-stimulated MK production. No influence of heparan sulfates on MK production was observed. In co-cultures of CD34+ cells in direct contact with stromal cells, nonadherent and adherent MK, and proplatelet structures were observed, either in the absence or presence of TPO. Direct contact did not further improve the enhancing activity of stromal cells on MK production induced by TPO. We conclude that the stromal enhancement of MK production is neither accounted for by stromal TPO, IL-3, IL-6, and SCF activities nor modified by physical contact.