A pathobiologic pathway linking thrombopoietin, GATA-1, and TGF-beta1 in the development of myelofibrosis

Array comparative genomic hybridization and sequencing of 23 genes in 80 patients with myelofibrosis at chronic or acute phase

Myelofibrosis is a myeloproliferative neoplasm that occurs de novo (primary myelofibrosis) or results from the progression of polycythemia vera or essential thrombocytemia (hereafter designated as secondary myelofibrosis or post-polycythemia vera/ essential thrombocythemia myelofibrosis). To progress in the understanding of myelofibrosis and to find molecular prognostic markers we studied 104 samples of primary and secondary myelofibrosis at chronic (n=68) and acute phases (n=12) from 80 patients, by using array-comparative genomic hybridization and sequencing of 23 genes (ASXL1, BMI1, CBL, DNMT3A, EZH2, IDH1/2, JAK2, K/NRAS, LNK, MPL, NF1, PPP1R16B, PTPN11, RCOR1, SF3B1, SOCS2, SRSF2, SUZ12, TET2, TP53, TRPS1). We found copy number aberrations in 54% of samples, often involving genes with a known or potential role in leukemogenesis. We show that cases carrying a del(20q), del(17) or del(12p) evolve in acute myeloid leukemia (P=0.03). We found that 88% of the cases were mutated, mainly in signaling pathway (JAK2 69%, NF1 6%) and epigenetic genes (ASXL1 26%, TET2 14%, EZH2 8%). Overall survival was poor in patients with more than one mutation (P=0.001) and in patients with JAK2/ASXL1 mutations (P=0.02). Our study highlights the heterogeneity of myelofibrosis, and points to several interesting copy number aberrations and genes with diagnostic and prognostic impact.

The expression of lysyl-oxidase gene family members in myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are malignant disorders originating from clonal expansion of a single neoplastic stem cell and characteristically show an increase in bone marrow reticulin fibers. Lysyl oxidases (LOXs) are copper-dependent amine oxidases that play a critical role in the biogenesis of connective tissue by crosslinking extracellular matrix proteins, collagen and elastin. Expression of LOX gene family members is increased in disorders associated with increased fibrosis. To evaluate involvement of LOX gene family in various MPNs. In-situ hybridization was used to detect Lysyl-Oxidase family members in bone marrow biopsies from patients with different MPNs. We compared normal bone marrows and those from patients with polycythemia vera, essential thrombocythemia, chronic myeloid leukemia, and primary myelofibrosis (PMF). Serum levels of lysyl-oxidase from patients with PMF and healthy controls were also examined. LOX gene family was not detected in normal bone marrows. All members of the LOX gene family were over expressed in PMF. In other MPNs a differential pattern of expression was observed. Differences in gene expression were statistically significant (P < 0.010). The medianserum LOX levels in normal controls was 28.4 ± 2.5 ng\ml and 44.6 ± 9.44 ng\ml in PMF (P = 0.02). The varying pattern of expression of LOX genes may reflect differences in the pathophysiology of bone marrow fibrosis in these MPNs. These observations could be used as the basis for future targeted therapy directed against bone marrow fibrosis.

Characterization of the TGF-β1 signaling abnormalities in the Gata1low mouse model of myelofibrosis

Primary myelofibrosis (PMF) is characterized by fibrosis, ineffective hematopoiesis in marrow, and hematopoiesis in extramedullary sites and is associated with abnormal megakaryocyte (MK) development and increased transforming growth factor (TGF)-β1 release. To clarify the role of TGF-β1 in the pathogenesis of this disease, the TGF-β1 signaling pathway of marrow and spleen of the Gata1(low) mouse model of myelofibrosis (MF) was profiled and the consequences of inhibition of TGF-β1 signaling on disease manifestations determined. The expression of 20 genes in marrow and 36 genes in spleen of Gata1(low) mice was altered. David-pathway analyses identified alterations of TGF-β1, Hedgehog, and p53 signaling in marrow and spleen and of mammalian target of rapamycin (mTOR) in spleen only and predicted that these alterations would induce consequences consistent with the Gata1(low) phenotype (increased apoptosis and G1 arrest both in marrow and spleen and increased osteoblast differentiation and reduced ubiquitin-mediated proteolysis in marrow only). Inhibition of TGF-β1 signaling normalized the expression of p53-related genes, restoring hematopoiesis and MK development and reducing fibrosis, neovascularization, and osteogenesis in marrow. It also normalized p53/mTOR/Hedgehog-related genes in spleen, reducing extramedullary hematopoiesis. These data identify altered expression signatures of TGF-β1 signaling that may be responsible for MF in Gata1(low) mice and may represent additional targets for therapeutic intervention in PMF.

Stromal SPARC contributes to the detrimental fibrotic changes associated with myeloproliferation whereas its deficiency favors myeloid cell expansion

In myeloid malignancies, the neoplastic clone outgrows normal hematopoietic cells toward BM failure. This event is also sustained by detrimental stromal changes, such as BM fibrosis and osteosclerosis, whose occurrence is harbinger of a dismal prognosis. We show that the matricellular protein SPARC contributes to the BM stromal response to myeloproliferation. The degree of SPARC expression in BM stromal elements, including CD146(+) mesenchymal stromal cells, correlates with the degree of stromal changes, and the severity of BM failure characterizing the prototypical myeloproliferative neoplasm primary myelofibrosis. Using Sparc(-/-) mice and BM chimeras, we demonstrate that SPARC contributes to the development of significant stromal fibrosis in a model of thrombopoietin-induced myelofibrosis. We found that SPARC deficiency in the radioresistant BM stroma compartment impairs myelofibrosis but, at the same time, associates with an enhanced reactive myeloproliferative response to thrombopoietin. The link betwen SPARC stromal deficiency and enhanced myeloid cell expansion under a myeloproliferative spur is also supported by the myeloproliferative phenotype resulting from the transplantation of defective Apc(min) mutant hematopoietic cells into Sparc(-/-) but not WT recipient BM stroma. Our results highlight a complex influence of SPARC over the stromal and hematopoietic BM response in myeloproliferative conditions.

Megakaryocyte pathology and bone marrow fibrosis: the lysyl oxidase connection

Megakaryocytes (MKs), the platelet precursors, are capable of accumulating DNA greater than a diploid content as part of their cell cycle. MKs have been recognized as mediating fibrosis in a subset of hematologic malignancies, including acute megakaryoblastic leukemia and a subset of myeloproliferative neoplasms. The mechanisms responsible for fibrosis remain only partially understood. Past studies highlighted the role of growth factors in such pathologies, and recently, the protein lysyl oxidase (LOX) has been implicated in proliferation of MKs, ploidy and deposition of fibers. LOX was initially characterized as a protein responsible for the intermolecular cross-linking of elastin and collagen, and in recent years it has been identified as regulator of various pathologies, such as cancer and inflammation. Here, we review recent advances in the understanding of the contribution of MKs to the progression of myelofibrosis, highlighting the newly identified role of LOX.

Nonclinical safety assessment of a synthetic peptide thrombopoietin agonist: effects on platelets, bone homeostasis, and immunogenicity and the implications for clinical safety monitoring of adverse bone effects

RWJ-800088 is a novel, potent polyethylene glycol (PEG)-conjugated thrombopoietin (TPO) mimetic that increases platelet levels and protects against thrombocytopenia. A nonclinical safety program was customized for this peptide that takes into account its protein-like structure, synthetic chemical nature, agonist pharmacologic activity, and mode of administration. In repeat-dose toxicity studies, the salient findings were dose-related increases in circulating platelet counts, mean platelet volume, and megakaryocytes in the bone marrow with no antibody formation. Reversible myelofibrosis and hyperostosis were observed in rats, but not dogs, when the circulating platelet levels exceeded 3× those of vehicle controls. The bone effects were due to the exaggerated pharmacologic effect and excessive stimulation and elevation of megakaryocytes by TPO, which results in intramedullary proliferation of fibroblasts and mesenchymal cells followed by osseous metaplasia. These findings support the use of platelet elevations of >3× as a stopping criterion to prevent potential adverse bone-related effects in humans.

In vitro megakaryocyte differentiation and proplatelet formation in Ph-negative classical myeloproliferative neoplasms: distinct patterns in the different clinical phenotypes

BACKGROUND

Ph-negative myeloproliferative neoplasms (MPNs) are clonal disorders that include primary myelofibrosis (PMF), polycythemia vera (PV) and essential thrombocythemia (ET). Although the pathogenesis of MPNs is still incompletely understood, an involvement of the megakaryocyte lineage is a distinctive feature.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed the in vitro megakaryocyte differentiation and proplatelet formation in 30 PMF, 8 ET, 8 PV patients, and 17 healthy controls (CTRL). Megakaryocytes were differentiated from peripheral blood CD34(+) or CD45(+) cells in the presence of thrombopoietin. Megakaryocyte output was higher in MPN patients than in CTRL with no correlation with the JAK2 V617F mutation. PMF-derived megakaryocytes displayed nuclei with a bulbous appearance, were smaller than ET- or PV-derived megakaryocytes and formed proplatelets that presented several structural alterations. In contrast, ET- and PV-derived megakaryocytes produced more proplatelets with a striking increase in bifurcations and tips compared to both control and PMF. Proplatelets formation was correlated with platelet counts in patient peripheral blood. Patients with pre-fibrotic PMF had a pattern of megakaryocyte proliferation and proplatelet formation that was similar to that of fibrotic PMF and different from that of ET.

CONCLUSIONS/SIGNIFICANCE

In conclusion, MPNs are associated with high megakaryocyte proliferative potential. Profound differences in megakaryocyte morphology and proplatelet formation distinguish PMF, both fibrotic and prefibrotic, from ET and PV.

Control of megakaryocyte expansion and bone marrow fibrosis by lysyl oxidase

Lysyl oxidase (LOX), a matrix cross-linking protein, is known to be selectively expressed and to enhance a fibrotic phenotype. A recent study of ours showed that LOX oxidizes the PDGF receptor-β (PDGFR-β), leading to amplified downstream signaling. Here, we examined the expression and functions of LOX in megakaryocytes (MKs), the platelet precursors. Cells committed to the MK lineage undergo mitotic proliferation to yield diploid cells, followed by endomitosis and acquisition of polyploidy. Intriguingly, LOX expression is detected in diploid-tetraploid MKs, but scarce in polyploid MKs. PDGFR-BB is an inducer of mitotic proliferation in MKs. LOX inhibition with β-aminopropionitrile reduces PDGFR-BB binding to cells and downstream signaling, as well as its proliferative effect on the MK lineage. Inhibition of LOX activity has no influence on MK polyploidy. We next rationalized that, in a system with an abundance of low ploidy MKs, LOX could be highly expressed and with functional significance. Thus, we resorted to GATA-1(low) mice, where there is an increase in low ploidy MKs, augmented levels of PDGF-BB, and an extensive matrix of fibers. MKs from these mice display high expression of LOX, compared with control mice. Importantly, treatment of GATA-1(low) mice with β-aminopropionitrile significantly improves the bone marrow fibrotic phenotype, and MK number in the spleen. Thus, our in vitro and in vivo data support a novel role for LOX in regulating MK expansion by PDGF-BB and suggest LOX as a new potential therapeutic target for myelofibrosis.

The role of Smad signaling in hematopoiesis and translational hematology

Hematopoietic stem cells (HSCs) reside in the bone marrow (BM) of adult individuals and function to produce and regenerate the entire blood and immune system over the course of an individual's lifetime. Historically, HSCs are among the most thoroughly characterized tissue-specific stem cells. Despite this, the regulation of fate options, such as self-renewal and differentiation, has remained elusive, partly because of the expansive plethora of factors and signaling cues that govern HSC behavior in vivo. In the BM, HSCs are housed in specialized niches that dovetail the behavior of HSCs with the need of the organism. The Smad-signaling pathway, which operates downstream of the transforming growth factor-β (TGF-β) superfamily of ligands, regulates a diverse set of biological processes, including proliferation, differentiation and apoptosis, in many different organ systems. Much of the function of Smad signaling in hematopoiesis has remained nebulous due to early embryonic lethality of most knockout mouse models. However, recently new data have been uncovered, suggesting that the Smad-signaling circuitry is intimately linked to HSC regulation. In this review, we bring the Smad-signaling pathway into focus, chronicling key concepts and recent advances with respect to TGF-β-superfamily signaling in normal and leukemic hematopoiesis.

Bone marrow fibrosis in myeloproliferative neoplasms-associated myelofibrosis: deconstructing a myth?

The dominant pathophysiological and clinical features of myeloproliferative neoplasm (MPN)-associated myelofibrosis are caused by bone marrow fibrosis. It is widely believed this fibrosis is a reaction to the MPN-clone implying the cells causing fibrosis are polyclonal and genetically unrelated to the MPN-clone. We cite recent data illustrating the complexity of cell types comprising the bone marrow micro-environment and showing that at least some of the cells responsible for bone marrow fibrosis are clonal and genetically related to the MPN-clone.

CXCR4-independent rescue of the myeloproliferative defect of the Gata1low myelofibrosis mouse model by Aplidin

The discovery of JAK2 mutations in Philadelphia-negative myeloproliferative neoplasms has prompted investigators to evaluate mutation-targeted treatments to restore hematopoietic cell functions in these diseases. However, the results of the first clinical trials with JAK2 inhibitors are not as promising as expected, prompting a search for additional drugable targets to treat these disorders. In this paper, we used the hypomorphic Gata1(low) mouse model of primary myelofibrosis (PMF), the most severe of these neoplasms, to test the hypothesis that defective marrow hemopoiesis and development of extramedullary hematopoiesis in myelofibrosis is due to insufficient p27(Kip1) activity and is treatable by Aplidin, a cyclic depsipeptide that activates p27(Kip1) in several cancer cells. Aplidin restored expression of Gata1 and p27(Kip1) in Gata1(low) hematopoietic cells, proliferation of marrow progenitor cells in vitro and maturation of megakaryocytes in vivo (reducing TGF-beta/VEGF levels released in the microenvironment by immature Gata1(low) megakaryocytes). Microvessel density, fibrosis, bone growth, and marrow cellularity were normal in Aplidin-treated mice and extramedullary hematopoiesis did not develop in liver although CXCR4 expression in Gata1(low) progenitor cells remained low. These results indicate that Aplidin effectively alters the natural history of myelofibrosis in Gata1(low) mice and suggest this drug as candidate for clinical evaluation in PMF.

Differential localization of P-selectin and von Willebrand factor during megakaryocyte maturation

An important step in megakaryocyte maturation is the appropriate assembly of at least two distinct subsets of alpha-granules. The mechanism that sorts the alpha-granule components into distinct structures and mediates their release in response to specific stimuli is now emerging. P-selectin and von Willebrand factor are two proteins present in the alpha-granules that recognize P-selectin glycoprotein ligand on neutrophils and collagen in the subendothelial matrix. These proteins may play an important role in determining the differential release of the alpha-granule contents in response to external stimuli. If P-selectin and von Willebrand factor are localized in the same or different alpha-granules is not known. To clarify this question, we analyzed by immunoelectron microscopy the localization of von Willebrand factor and P-selectin during the maturation of wild-type and Gata1(low) megakaryocytes induced in vivo by treating animals with thrombopoietin. Gata1(low) is a hypomorphic mutation that blocks megakaryocyte maturation, reduces the levels of von Willebrand factor expression and displaces P-selectin on the demarcation membrane system. The maturation block induced by this mutation is partially rescued by treatment in vivo with thrombopoietin. In immature megakaryocytes, both wild-type and Gata1(low), the two receptors were co-localized in the same cytoplasmic structures. By contrast, the two proteins were segregated to separate alpha-granule subsets as the megakaryocytes matured. These observations support the hypothesis that P-selectin and von Willebrand factor may ensure differential release of the alpha-granule content in response to external stimuli.

Outcome of transplantation for myelofibrosis

Myelofibrosis is a myeloproliferative disorder incurable with conventional strategies. Several small series have reported long-term disease-free survival (DSF) after allogeneic hematopoietic cell transplantation (HCT). In this study, we analyze the outcomes of 289 patients receiving allogeneic transplantation for primary myelofibrosis between 1989 and 2002, from the database of the Center for International Bone Marrow Transplant Research (CIBMTR). The median age was 47 years (range: 18-73 years). Donors were HLA identical siblings in 162 patients, unrelated individuals in 101 patients, and HLA nonidentical family members in 26 patients. Patients were treated with a variety of conditioning regimens and graft-versus-host disease (GVHD) prophylaxis regimens. Splenectomy was performed in 65 patients prior to transplantation. The 100-day treatment-related mortality was 18% for HLA identical sibling transplants, 35% for unrelated transplants, and 19% for transplants from alternative related donors. Corresponding 5-year overall survival (OS) rates were 37%, 30%, and 40%, respectively. DFS rates were 33%, 27%, and 22%, respectively. DFS for patients receiving reduced-intensity transplants was comparable: 39% for HLA identical sibling donors and 17% for unrelated donors at 3 years. In this large retrospective series, allogeneic transplantation for myelofibrosis resulted in long-term relapse-free survival (RFS) in about one-third of patients.

Increased differentiation of dermal mast cells in mice lacking the Mpl gene

Thrombopoietin interactions with its receptor, Mpl, play an important role in the regulation of hematopoietic stem/progenitor cell proliferation and differentiation. In this study, we report that the mast cell restricted progenitor cells (MCP) and the mast cell precursors in the bone marrow of wild-type mice express Mpl on their surface. Furthermore, targeted deletion of the Mpl gene in mice decreases the number of MCP while increasing the number of mast cell precursors present in the marrow and spleen. It also increases the number of mast cells present in the dermis, in the peritoneal cavity, and in the gut of the mice. In addition, serosal mast cells from Mpl(null) mice have a distinctive differentiation profile similar to that expressed by wild-type dermal mast cells. These results suggest that not only does ligation of thrombopoietin with the Mpl receptor exert an effect at the mast cell restricted progenitor cell level, but also plays an unexpected yet important role in mast cell maturation.

Ultrastructural megakaryocyte modifications after vanadium inhalation in spleen and bone marrow

Previous reports from our laboratory informed in mice an increase in platelets in blood, and megakaryocytes in spleen and bone marrow after vanadium inhalation. This element has become important in recent years because of its increased presence as an air pollutant. With this precedent, we evaluate the ultrastructural modifications in MKs from the spleen and bone marrow in our mouse experimental model. Mice inhaled 0.02 M V(2)O(5) 1 h twice a week for 12 weeks. Tissues were processed for transmission electron microscopy. Results indicate an increase in the size and cytoplasmic granular content, as well as nuclear changes in MKs of exposed mice, changes which correlate with the time of exposure. Modifications in MKs described here suggest that inhaled vanadium induce megakaryocytic maturation, a raise in its granules content and demarcation membrane systems, which may lead to a rise in circulating platelet production and an increased risk for thromboembolic events.

A novel role for the marrow microenvironment in initiating and sustaining hematopoietic disease

BACKGROUND

The marrow microenvironment is composed of a complex network of cells and extra cellular matrix that cooperate to regulate normal hematopoiesis. There is growing evidence that microenvironmental defects can contribute to the pathogenesis of hematological malignancies.

OBJECTIVE/METHODS

We review the role of the microenvironment in inducing and sustaining hematological malignancies.

RESULTS/CONCLUSIONS

Two basic mechanisms could explain the role of microenvironmental defects in the evolution of hematopoietic neoplasms. There is significant data to support the first mechanism, in which the malignant hematopoietic clone induces reversible functional changes in the microenvironment that result in improved growth conditions for the malignant cells. More recent studies from mouse models have indicated that a second mechanism involving primary microenvironmental defects can also result in malignancy.

Megakaryocyte-bone cell interactions

Emerging data show that megakaryocytes (MKs) play a role in the replication and development of bone cells. Both in vivo and in vitro evidence now show that MKs can have significant effects on cells of the osteoclast (OC) and osteoblast (OB) lineage, with obvious manifestations on bone phenotype, and probable significance for human pathology.There are currently four mouse models in which increases in MK number lead to a specific bone pathology of markedly increased bone volume. While these models all achieve megakaryocytosis by different mechanisms, the resultant osteosclerotic phenotype observed is consistent across all models.In vitro data suggest that MKs play a role in OC and OB proliferation and differentiation. While MKs express receptor activator of nuclear factor kappa B ligand (RANKL), a prerequisite for osteoclastogenesis, they also express many factors known to inhibit OC development, and co-cultures of MKs with OCs show a significant decrease in osteoclastogenesis. In contrast, MKs express several proteins with a known critical role in osteoblastogenesis and bone formation, and co-cultures of these two lineages result in up to a six-fold increase in OB proliferation and alterations in OB differentiation.This research demonstrates the complex regulatory interactions at play between MKs and bone cells, and opens up potential targets for therapeutic intervention.

Aberrant signal transduction pathways in myeloproliferative neoplasms

The BCR-ABL-negative myeloproliferative neoplasms (MPNs), polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF), entered the spotlight in 2005 when the unique somatic acquired JAK2 V617F mutation was described in >95% of PV and in 50% of ET and PMF patients. For the very rare PV patients who do not harbor the JAK2 V617F mutation, exon 12 JAK2 mutants were discovered also to result in activated forms of JAK2. A minority of ET and PMF patients harbor mutations that constitutively activate the thrombopoietin receptor (TpoR). In bone marrow reconstitution models based on retroviral transduction, the phenotype induced by JAK2 V617F is less severe and different from the rapid fatal myelofibrosis induced by TpoR W515L. The reasons for these differences are unknown. Exactly by which mechanism(s) one acquired somatic mutation, JAK2 V617F, can promote three different diseases remains a mystery, although gene dosage and host genetic variation might have important functions. We review the recent progress made in deciphering signaling anomalies in PV, ET and PMF, with an emphasis on the relationship between JAK2 V617F and cytokine receptor signaling and on cross-talk with several other signaling pathways.

Osteoimmunology: interactions of the bone and immune system

Bone and the immune system are both complex tissues that respectively regulate the skeleton and the body's response to invading pathogens. It has now become clear that these organ systems often interact in their function. This is particularly true for the development of immune cells in the bone marrow and for the function of bone cells in health and disease. Because these two disciplines developed independently, investigators in each don't always fully appreciate the significance that the other system has on the function of the tissue they are studying. This review is meant to provide a broad overview of the many ways that bone and immune cells interact so that a better understanding of the role that each plays in the development and function of the other can develop. It is hoped that an appreciation of the interactions of these two organ systems will lead to better therapeutics for diseases that affect either or both.

Hypermethylation of CXCR4 promoter in CD34+ cells from patients with primary myelofibrosis

Constitutive mobilization of CD34(+) cells in patients with primary myelofibrosis (PMF) has been attributed to proteolytic disruption of the CXCR4/SDF-1 axis and reduced CXCR4 expression. We document here that the number of circulating CD34(+)/CXCR4(+) cells in PMF patients, as well as the cellular CXCR4 expression, was directly related to CXCR4 mRNA level and that reduced CXCR4 mRNA level was not due to SDF-1-induced downregulation. To address whether epigenetic regulation contributes to defective CXCR4 expression, we studied the methylation status of the CXCR4 promoter using methylation-specific polymerase chain reaction and methylation-specific sequencing in the JAK2V617F-positive HEL cell line and in CD34(+) cells. We found that CD34(+) cells from PMF patients, unlike those from normal subjects, presented hypermethylation of CXCR4 promoter CpG island 1. Following incubation with the demethylating agent 5-Aza-2'-deoxycytidine (5-AzaD), the percentage of PMF CD34(+) cells expressing CXCR4 increased 3-10 times, whereas CXCR4 mRNA level increased approximately 4 times. 5-AzaD-treated PMF CD34(+) cells displayed almost complete reversal of CpG1 island 1 hypermethylation and showed enhanced migration in vitro in response to SDF-1. These data point to abnormal methylation of the CXCR4 promoter as a mechanism contributing to constitutive migration of CD34(+) cells in PMF. Disclosure of potential conflicts of interest is found at the end of this article.

TPO, but not soluble-IL-6 receptor, levels increase after anagrelide treatment of thrombocythemia in chronic myeloproliferative disorders

Anagrelide is often used in the treatment of thrombocythemia in myeloproliferative disease (MPD), but information concerning effects of treatment on cytokines involved in regulation of blood platelet levels is limited. Here, we investigated serum levels of thrombopoietin (TPO) and soluble IL-6 receptor (sIL-6R) in relation to response to treatment with and plasma concentrations of anagrelide. Samples from 45 patients with thrombocythemia due to MPD (ET=31, PV=14), being treated with anagrelide for 6 months, were analyzed for TPO, sIL-6R and anagrelide levels. The mean baseline platelet count was 983x10(9)/L. A reduction of platelets to <600 in asymptomatic or <400 x 10(9)/L in symptomatic patients was defined as a complete remission (CR), a reduction with >50% of baseline as partial remission, and <50% reduction as failure. At 6 months, 35 patients were in CR, 1 had a partial remission and 9 were treatment failures. For all patients, there was an increase in TPO of 44% from baseline; this change was more pronounced for patients with partial remission and failure. sIL-6R levels did not change significantly. There was no correlation between levels of anagrelide and cytokine levels at 6 months, and changes of cytokine levels did not relate to changes of platelet counts. Thus, a pronounced increase of TPO levels after 6 months of anagrelide treatment indicated that this treatment affected a major regulatory mechanism for megakaryocyte and platelet formation in MPD.

Altered SDF-1/CXCR4 axis in patients with primary myelofibrosis and in the Gata1 low mouse model of the disease

OBJECTIVE

To assess whether alterations in the stromal cell-derived factor-1 (SDF-1)/CXCR4 occur in patients with primary myelofibrosis (PMF) and in Gata1 low mice, an animal model for myelofibrosis, and whether these abnormalities might account for increased stem/progenitor cell trafficking.

MATERIALS AND METHODS

In the mouse, SDF-1 mRNA levels were assayed in liver, spleen, and marrow. SDF-1 protein levels were quantified in plasma and marrow and CXCR4 mRNA and protein levels were evaluated on stem/progenitor cells and megakaryocytes purified from the marrow. SDF-1 protein levels were also evaluated in plasma and in marrow biopsy specimens obtained from normal donors and PMF patients.

RESULTS

In Gata1 low mice, the plasma SDF-1 protein was five times higher than normal in younger animals. Furthermore, SDF-1 immunostaining of marrow sections progressively increased with age. Similar abnormalities were observed in PMF patients. In fact, plasma SDF-1 levels in PMF patients were significantly higher (by twofold) than normal (p < 0.01) and SDF-1 immunostaining of marrow biopsy specimens demonstrated increased SDF-1 deposition in specific areas. In two of the patients, SDF-1 deposition was normalized by curative therapy with allogenic stem cell transplantation. Similar to what already has been reported for PMF patients, the marrow from Gata1 low mice contained fewer CXCR4 pos CD117 pos cells and these cells expressed low levels of CXCR4 mRNA and protein.

CONCLUSION

Similar abnormalities in the SDF-1/CXCR4 axis are observed in PMF patients and in the Gata1 low mice model of myelofibrosis. We suggest that these abnormalities contribute to the increased stem/progenitor cell trafficking observed in this mouse model as well as patients with PMF.

Thrombopoietin inhibits murine mast cell differentiation

We have recently shown that Mpl, the thrombopoietin receptor, is expressed on murine mast cells and on their precursors and that targeted deletion of the Mpl gene increases mast cell differentiation in mice. Here we report that treatment of mice with thrombopoietin or addition of this growth factor to bone marrow-derived mast cell cultures severely hampers the generation of mature cells from their precursors by inducing apoptosis. Analysis of the expression profiling of mast cells obtained in the presence of thrombopoietin suggests that thrombopoietin induces apoptosis of mast cells by reducing expression of the transcription factor Mitf and its target antiapoptotic gene Bcl2.

Bone marrow pathology in essential thrombocythemia: interobserver reliability and utility for identifying disease subtypes

The role of histopathology in the diagnosis of essential thrombocythemia (ET) is controversial, and there has been little attempt to quantitate interobserver variability. Diagnostic bone marrow trephine biopsy specimens from 370 patients with ET by Polycythemia Vera Study Group (PVSG) criteria were assessed by 3 experienced hematopathologists for 16 different morphologic features and overall diagnosis according to the World Health Organization (WHO) classification. Our results show substantial interobserver variability, particularly for overall diagnosis and individual cellular characteristics such as megakaryocyte morphology. Reticulin grade was the dominant independent predictor of WHO diagnostic category for all 3 hematopathologists. Factor analysis identified 3 independent factors likely to reflect underlying biologic processes. One factor related to overall and lineage-specific cellularity and was significantly associated with JAK2 V617F status (P < .001), a second factor related to megakaryocyte clustering, and a third was associated with the fibrotic process. No differences could be discerned between patients labeled as having "prefibrotic myelofibrosis" or "true ET" in clinical and laboratory features at presentation, JAK2 status, survival, thrombosis, major hemorrhage, or myelofibrotic transformation. These results show that histologic criteria described in the WHO classification are difficult to apply reproducibly and question the validity of distinguishing true ET from prefibrotic myelofibrosis on the basis of subjective morphologic criteria. This study was registered at http://isrctn.org as #72251782 and at http://eudract.emea.europa.eu/ as #2004-000245-38.

Role of thrombopoietin in mast cell differentiation

Mast cells are important elements of the body response to foreign antigens, being those represented either by small molecules (allergic response) or harbored by foreign microorganisms (response to parasite infection). These cells derive from hematopoietic stem/progenitor cells present in the marrow. However, in contrast with most of the other hematopoietic lineages, mast cells do not differentiate in the marrow but in highly vascularized extramedullary sites, such as the skin or the gut. Mast cell differentiation in the marrow is activated as part of the body response to parasites. We will review here the mast cell differentiation pathway and what is known of its major intrinsic and extrinsic control mechanisms. It will also be described that thrombopoietin, the ligand for the Mpl receptor, in addition to its pivotal rule in the control of thrombocytopoiesis and of hematopoietic stem/progenitor cell proliferation, exerts a regulatory function in mast cell differentiation. Some of the possible implications of this newly described biological activity of thrombopoietin will be discussed.

Expression of bone morphogenetic proteins and their receptors in the bone marrow megakaryocytes of GATA-1(low) mice: a possible role in osteosclerosis

The mechanism of osteosclerosis associated with myelofibrosis in megakaryocyte (MK)-related myeloproliferative disorders is largely unknown. However, growth factors released from the bone marrow cells, including from MKs, have been implicated in myelofibrosis, osteosclerosis, and angiogenesis. GATA-1 is a transcription factor required for normal MK development. GATA-1 deficiency in mice (GATA-1(low)) leads to increased megakaryocytic proliferation, followed by osteosclerosis and myelofibrosis. In this study we investigated the expression of bone morphogenetic proteins (BMPs) and BMP receptors and their possible role in the development of osteosclerosis in the MKs of 12-month-old GATA-1(low) mice by immunohistochemistry, cytomorphometry, and quantitative real-time PCR. Marrow MKs from both wild-type and GATA-1(low) mice showed moderate to intense staining for BMP-2, -4, and -6 and BMPR-IA and BMPR-II, whereas splenic MKs showed no BMP immunostaining. Presence of BMP protein in the bone marrow of GATA-1(low) mice was more than that seen in controls, owing to an increased number of MKs and osteoblasts. The osteosclerosis seen in GATA-1(low) mice appeared not to be due to a reduced number of functional osteoclasts because the number of tartrate-resistant acid phosphatase-positive osteoclasts was greater in GATA-1(low) mice than in controls. Our findings demonstrate the presence of significant amounts of BMP-2, -4, and -6 along with their receptors in bone marrow MKs of WT and GATA-1(low) mice. The increased levels of BMPs appear to be a result of increased numbers of MKs in GATA-1(low) mice and may, in part, account for the stimulation of osteoblastic activity and resulting osteosclerosis.

Effect of fibroblast growth factor 2 on stromal cell-derived factor 1 production by bone marrow stromal cells and hematopoiesis

BACKGROUND

Reduction of intramedullary hematopoiesis and the development of myelofibrosis and splenic hematopoiesis are frequent complications of clonal myeloid disorders that cause severe morbidity and death and present a therapeutic challenge. However, the pathogenesis of these complications is still unknown. We evaluated the effect of fibroblast growth factor 2 (FGF-2), the level of which is elevated in patients with clonal myeloid disorders, on bone marrow stromal cell expression of stromal cell-derived factor 1 (SDF-1), a chemokine that is essential for normal hematopoiesis.

METHODS

Reverse transcription-polymerase chain reaction analysis, immunoblot analysis, and enzyme-linked immunosorbent assays were used to examine effects of human recombinant FGF-2 exposure on SDF-1 expression in mouse stromal MS-5 and S-17 cells. Cocultures of human CD34-positive peripheral blood stem cells or mouse pre-B DW34 cells with mouse stromal cells were used to characterize the functional relevance of the effects of FGF-2 on SDF-1 expression. The in vivo hematologic effects of FGF-2 were determined by systemic administration to mice (n = 10). All statistical tests were two-sided.

RESULTS

FGF-2 reduced constitutive SDF-1 mRNA expression and secretion in stromal cells (SDF-1 levels in supernatants: MS-5 cells cultured for 3 days in medium only versus in medium with FGF-2, 95.4 ng/mL versus 22.2 ng/mL, difference = 73.2 ng/mL, 95% confidence interval [CI] = 60.52 to 85.87 ng/mL; P = .002, two-sided Student's t test; S-17 cultured in medium only versus in medium with FGF-2, 203.53 ng/mL versus 32.36 ng/mL, difference = 171.17 ng/mL, 95% CI = 161.8 to 180.6 ng/mL; P<.001). These effects of FGF-2 were reversible. FGF-2 compromised stromal cell support of the growth and survival of pre-B DW34 and myeloid lineage cells, and these effects were reversed in part by exogenous recombinant SDF-1alpha (rSDF-1alpha) (DW34 pre-B cells recovery on S-17 stromal cells, expressed as a percentage of DW34 cells recovered from medium only: with FGF-2 versus without FGF-2, 27.6% versus 100%, difference = 72.4%, 95% CI = 45.34% to 99.51%, P = .008; with FGF-2 plus rSDF1 versus with FGF-2 only, 60.3% versus 27.6%, difference = 32.7%, 95% CI = 9.35% to 56.08%, P = .034; fold increase in number of myeloid lineage cells after culture on S-17 stromal cells: with FGF-2 versus without FGF-2, 0.25-fold versus 3.8-fold, difference = 3.55-fold, 95% CI = 2.66- to 4.44-fold, P<.001; recovery of myeloid cells on S-17 stromal cells, expressed as a percentage of myeloid cells recovered from medium only: FGF-2 plus rSDF-1alpha versus FGF-2 only, 76.5% versus 32.4%, difference = 44.1%, 95% CI = 32.58% to 55.68%, P<.001). Administration of FGF-2 to mice reversibly reduced bone marrow levels of SDF-1 and cellularity and induced immature myeloid cell mobilization, extramedullary hematopoiesis, and splenomegaly.

CONCLUSIONS

Systemic administration of FGF-2 in mice disrupts normal bone marrow hematopoiesis in part through reduced expression of SDF-1. Thus, endogenous FGF-2 may represent a potential therapeutic target in clonal myeloid disorders characterized by bone marrow failure.

Experimental therapy in myelofibrosis with myeloid metaplasia

Myelofibrosis with myeloid metaplasia (MMM) is a Philadelphia chromosome-negative myeloproliferative disorder that is characterised by constitutional symptoms, progressive anaemia and extramedullary haematopoiesis. There are no curative therapies available for patients with MMM apart from stem cell transplantation, which is associated with significant morbidity and mortality, and for which most patients are not suitable candidates. Traditional pharmacological therapy of MMM has focused on the palliation of symptoms associated with myeloproliferation and correction of cytopoenias. Recently, new findings regarding the molecular basis of MMM and the pathogenesis of the associated bone marrow stromal reaction have provided both basic and clinical researchers with invaluable tools to develop effective targeted therapies for patients with MMM. Several novel treatment strategies are being investigated including antiangiogenic agents, signal transduction inhibitors, inhibitors of fibrogenesis and small-molecule inhibitors of the JAK2(V617F )mutation. This article reviews the current status of experimental novel therapies for MMM.

Adenoviral-mediated TGF-β1 inhibition in a mouse model of myelofibrosis inhibit bone marrow fibrosis development

Myelofibrosis is characterized by excessive deposits of extracellular matrix proteins, which occur as a marrow microenvironment reactive response to cytokines released from the clonal malignant myeloproliferation. The observation that mice exposed to high systemic levels of thrombopoietin (TPO) invariably developing myelofibrosis has allowed demonstration of the crucial role of transforming growth factor (TGF)-beta1 released by hematopoietic cells in the onset of myelofibrosis. The purpose of this study was to investigate whether TGF-beta1 inhibition could directly inhibit fibrosis development in a curative approach of this mice model. An adenovirus encoding for TGF-beta1 soluble receptor (TGF-beta-RII-Fc) was injected either shortly after transplantation (preventive) or 30 days post-transplantation (curative). Mice were transplanted with syngenic bone marrow cells transduced with a retrovirus encoding for murine TPO. All mice developed a myeloproliferative syndrome. TGF-beta-RII-Fc was detected in the blood of all treated mice, leading to a dramatic decrease in TGF-beta1 level. Histological analysis show that the two approaches (curative or preventive) were successful enough to inhibit bone marrow and spleen fibrosis development in this model. However, lethality of TPO overexpression was not decreased after treatment, indicating that in this mice model, myeloproliferation rather than fibrosis was probably responsible for the lethality induced by the disorder.

A reciprocal regulatory interaction between megakaryocytes, bone cells, and hematopoietic stem cells

A growing body of evidence suggests that megakaryocytes (MK) or their growth factors play a role in skeletal homeostasis. MK have been shown to express and/or secrete several bone-related proteins including osteocalcin, osteonectin, bone sialoprotein, osteopontin, bone morphogenetic proteins, and osteoprotegerin. In addition, at least 3 mouse models have been described in which MK number was significantly elevated with an accompanying marked increase in bone mineral density. Mice overexpressing thrombopoietin, the major MK growth factor, have an osteosclerotic bone phenotype. Mice deficient in transcription factors GATA-1 and NF-E2, which are required for the differentiation of MK, exhibited a strikingly increased bone mass. Importantly, recent studies have demonstrated that MK can stimulate osteoblast (OB) proliferation and differentiation in vitro and that they can also inhibit osteoclast (OC) formation in vitro. These findings suggest that MK play a dual role in skeletal homeostasis by stimulating formation while simultaneously inhibiting resorption. Conversely, cells of the osteoblast lineage support hematopoiesis, including megakaryopoiesis. Postnatal hematopoiesis occurs almost solely in the bone marrow (BM), close to or on endosteal surfaces. This finding, in conjunction with the observed contact of OB with hematopoietic cells, has lead investigators to explore the molecular and cellular interactions between hematopoietic cells and cells of the OB lineage. Importantly, it has been shown that many of the cytokines that are critical for normal hematopoiesis and megakaryopoiesis are produced by OB. Indeed, culturing osteoblasts with CD34+ BM cells significantly enhances hematopoietic cell number by both enhancing the proliferation of long-term culture initiating cells and the proliferation and differentiation of MK. These data are consistent with cells in the OB lineage playing a critical role in the hematopoietic niche. Overall, these observations demonstrate the importance of MK-bone cell interactions in both skeletal homeostasis and hematopoiesis.

Pathogenesis of myelofibrosis with myeloid metaplasia: Insight from mouse models

Myelofibrosis with myeloid metaplasia or idiopathic myelofibrosis is a myeloproliferative disease. It is known to be a stem-cell disorder that leads to a secondary and reactive stromal reaction in the bone marrow microenvironment that is responsible for impaired haematopoiesis. Although progress has been made in the elucidation of the pathogenesis of idiopathic myelofibrosis, lack of suitable models has limited our understanding of the pathology. The aim of this chapter is to address recent inferred new insights in mouse models into the pathogenesis of osteomyelofibrosis. These insights outline the role of transforming growth factor-beta1 and osteoprotegerin in the promotion of myelofibrosis and osteosclerosis, respectively, paying special regard to the role of abnormal megakaryocyte proliferation and maturation.

Myelofibrosis with myeloid metaplasia: disease overview and non-transplant treatment options

Myelofibrosis with myeloid metaplasia (MMM) is currently classified as a classic (i.e. not yet molecularly defined) myeloproliferative disorder (MPD), along with essential thrombocythemia (ET) and polycythemia vera (PV). All three MPDs represent stem-cell-derived clonal myeloproliferation that, in the case of MMM, is accompanied by an intense bone marrow stromal reaction that includes collagen fibrosis, osteosclerosis, and angiogenesis. To date, both the molecular basis of the primary clonal process and the pathogenetic mechanisms that underlie the secondary histological changes remain elusive. Clinically, MMM is characterized by anemia, multi-organ extramedullary hematopoiesis that often involves the spleen and liver, constitutional symptoms, and premature death from either leukemic transformation or other disease complications. Current diagnosis is based on characteristic but not diagnostic bone marrow histological features. Modern therapy remains palliative but allogeneic stem cell transplantation might be curative to a selected group of patients. This chapter reviews both the old and the new therapy with regard to non-transplant treatment options for MMM.

Chronic idiopathic myelofibrosis: clinicopathologic features, pathogenesis, and prognosis

CONTEXT

Chronic idiopathic myelofibrosis (CIMF) is a clonal myeloproliferative disease characterized by panmyelosis with intact maturation, progressive bone marrow fibrosis, and multiorgan extramedullary hematopoiesis.

OBJECTIVE

This review article aims to summarize the recent updates regarding the clinicopathologic features, molecular pathogenesis, cytogenetic abnormalities, diagnostic criteria, new diagnostic ancillary tests, and prognostic factors of CIMF.

DATA SOURCES

Important relevant articles indexed in PubMed/MEDLINE (National Library of Medicine) through the end of 2005 and referenced medical texts.

CONCLUSIONS

Because CIMF has a variety of clinical presentations, diagnosis may be challenging; the prefibrotic stage of CIMF has always been a challenging disease for pathologists to diagnose accurately. The recently proposed European Clinical and Pathological criteria can be helpful in the diagnosis of CIMF, especially in its prefibrotic stage. The enumeration of CD34-positive cells in the peripheral blood and the presence of circulating endothelial progenitor cells are the new important ancillary tests for the diagnosis of a small subset of patients with CIMF with atypical presentation. The recent discovery of the new mutation affecting the Janus tyrosine kinase 2 (JAK2V617F), more frequently observed in patients with polycythemia vera, is seen in approximately 35% to 57% of patients with CIMF. This mutation can serve as another diagnostic tool. Important factors affecting prognosis in CIMF are anemia, age of the patient, white blood cell count, degree of fibrosis, and number of blasts in the peripheral blood.

Monocyte/macrophage dysfunctions do not impair the promotion of myelofibrosis by high levels of thrombopoietin

Several lines of evidence indicate that the megakaryocyte/platelet lineage is crucial in myelofibrosis induction. The demonstration that NOD/SCID mice with functionally deficient monocytes do not develop fibrotic changes when exposed to thrombopoietin (TPO) also suggests an important role for monocyte/macrophages. However, in this animal model, the development of myelofibrosis is dependent on the level of TPO. This study was conducted to investigate whether NOD/SCID mice exposed to high TPO levels mediated by a retroviral vector would be refractory to the development of bone marrow fibrosis. We show that TPO and TGF-beta1 in plasma from NOD/SCID and SCID mice engrafted with TPO-overexpressing hemopoietic cells reach levels similar to the ones reached in immunocompetent mice, and all animals develop a myeloproliferative disease associated with a dense myelofibrosis at 8 wk posttransplantation. Monocytes in NOD/SCID mice are functionally deficient to secrete cytokines such as IL-1alpha in response to stimuli, even under TPO expression. Surprisingly, the plasma of these mice displays high levels of IL-alpha, which was demonstrated to originate from platelets. Together, these data suggest that completely functional monocytes are not required to develop myelofibrosis and that platelets are able, under TPO stimulation, to synthesize inflammatory cytokines, which may be involved in the pathogenesis of myelofibrosis and osteosclerosis.

Role of transforming growth factor-beta in hematologic malignancies

The transforming growth factor-beta (TGF-beta) signaling pathway is an essential regulator of cellular processes, including proliferation, differentiation, migration, and cell survival. During hematopoiesis, the TGF-beta signaling pathway is a potent negative regulator of proliferation while stimulating differentiation and apoptosis when appropriate. In hematologic malignancies, including leukemias, myeloproliferative disorders, lymphomas, and multiple myeloma, resistance to these homeostatic effects of TGF-beta develops. Mechanisms for this resistance include mutation or deletion of members of the TGF-beta signaling pathway and disruption of the pathway by oncoproteins. These alterations define a tumor suppressor role for the TGF-beta pathway in human hematologic malignancies. On the other hand, elevated levels of TGF-beta can promote myelofibrosis and the pathogenesis of some hematologic malignancies through their effects on the stroma and immune system. Advances in the TGF-beta signaling field should enable targeting of the TGF-beta signaling pathway for the treatment of hematologic malignancies.

Grade of bone marrow fibrosis is associated with relevant hematological findings-a clinicopathological study on 865 patients with chronic idiopathic myelofibrosis

Controversy continues to exist regarding not only the exact definition and grading of myelofibrosis (MF), but also whether, and to what extent, this feature may be correlated with clinical findings. A retrospective study was performed involving 865 bone marrow (BM) biopsies together with the clinical records from patients with chronic idiopathic myelofibrosis (CIMF). Diagnosis was established according to the World Health Organization criteria, and assessment of MF followed a consensus scoring system that included four grades (MF-0 to MF-3). Histopathological and clinical evaluations were carried out in an independent fashion. Prefibrotic and early CIMF (MF-0/-1) were presented by 565 patients showing borderline to mild anemia and no or slight splenomegaly, but frequently, thrombocytosis exceeding 500x10(9)/l was shown. In 300 patients, manifest reticulin and collagen fibrosis (MF-2/-3) were characterized by marked anemia, gross splenomegaly, peripheral blasts, and normal to decreased platelet and leukocyte counts. The latter cohort was consistent with findings generally in keeping with MF with myeloid metaplasia. Regarding the stepwise evolution of disease, sequential BM examinations showed that in 103 patients, prefibrotic and early CIMF transformed into advanced stages accompanied by correspondingly developing clinical and histomorphological features. Survival analysis (univariate calculation) revealed a significantly more favorable prognosis in prefibrotic vs advanced stages of CIMF. On the other hand, higher classes of MF also exerted a higher clinical risk profile (Lille score). In conclusion, the dynamics of the disease process in CIMF are characterized by evolving MF in the BM and closely associated changes of relevant hematological findings.

Biological characteristics of megakaryocytes: Specific lineage commitment and associated disorders

Megakayocytes (Megakaryocytes) are among the rarest type of haematopoietic cells and highly specialized precursors for platelets. Normal mature megakaryocytes are, perhaps, the largest cells in the marrow. In contrast, their annucleated platelets progeny are the smallest subcellular fragments in the circulation, which in spite of their size, play crucial roles in thrombostasis and haemostasis. Megakaryopoiesis involves complicated and multi-step biological processes. Research over the last decade has resulted in certain important new discoveries, such as the specific megakaryocyte-forming haematopoietic stem cell (HSC) subpopulation, thrombopoietin (Tpo), formation and release of platelets, etc. Substantial understanding of the specific lineage commitment, differentiation, and the molecular regulatory mechanisms of megakaryopoiesis has also been achieved. Despite existing controversies and questions, megakaryopoiesis remains an exciting field in biomedical research. Certain recent biological findings as well as future research in megakaryopoiesis are summarised in this article. Certain pathological changes associated with megakaryocytes, such as immune thrombocytopenia purpura (ITP), acute megakayoblastic leukaemia, etc., are also discussed in this article.

Idiopathic Myelofibrosis

Idiopathic myelofibrosis (IMF) is characterized by anemia, progressive splenomegaly, bone marrow fibrosis, and extramedullary hematopoiesis. However, patients with a transitional myeloproliferative disorder (MPD), a prefibrotic form of myelofibrosis, or myelofibrosis with a fatty bone marrow share many features of IMF but have clinical characteristics that deviate from the classical description of IMF. A phenomenon that serves as a unique biomarker of IMF is the constitutive mobilization of hematopoietic progenitor cells (HPCs) and/or endothelial progenitor cells (EPCs) from the bone marrow to the peripheral blood and other extramedullary sites. Using such parameters as hemoglobin level, white blood cell count, and number of blasts in the peripheral blood, prognostic scores can be developed by which to base therapeutic decisions. Androgens, recombinant human erythropoietin (rHuEpo), and thalidomide are effective modalities of treatment of the anemia of IMF. Systemic symptoms of excess myeloproliferation are the primary indication for treatment with chemotherapeutic agents. Hydroxyurea is the most commonly used drug. Ablation of the abnormal hematopoietic clone with high-dose chemotherapy and allogeneic stem cell transplantation offers an opportunity to cure patients with IMF. The use of fully myeloablative conditioning regimens, with or without total body irradiation, is associated with a high transplant-related mortality rate (27% to 48%), especially in patients with advanced disease and in the elderly. The use of reduced intensity conditioning (RIC) regimens has resulted in prolonged survival and lower transplant-related mortality.

Isolation of TPO-dependent subclones from the multipotent 32D cell line

Using thrombopoietin (TPO), as selective pressure, several TPO-dependent clones were isolated from the murine multipotential IL-3-dependent cell line 32D. Four of them were fully characterized. They depended on TPO for survival and proliferation and, although retaining the capacity to grow in IL-3, did not respond to either EPO, G-CSF or GM-CSF. 32D TPO cells were heterogeneous in morphology and ranged from small cells, with a DNA content nearly tetraploid and a modal chromosome no. 66, to cells 50-75 microm in diameter containing multiple (up to 5-6) interconnected nuclei with a clear megakaryocyte (Mk) morphology by electron microscopy. Cell sorter isolation and single cell cloning experiments indicated that the small cells were those capable to proliferate in TPO and to generate the larger ones over time. 32D TPO cells expressed Mk-specific markers by FACS (CD41, CD61 and 2D5) and RT-PCR (acetyl cholinesterase E and platelet factor 4) and their unique profile, by gene array analysis, included expression of urokinase plasminogen activator surface receptor (CD87 or uPAR), plasminogen activator inhibitor and coagulation factor II (thrombin) receptor (Cf2r). In addition, by quantitative RT-PCR, 32D TPO clones expressed levels of Gata1 similar to those expressed by freshly isolated Mks (DeltaCt approximately 4.7 in both cases). In conclusion, the 32D TPO subclones described here are among the few pure Mk cell lines isolated so far and, for their unique properties, may prove themselves as a useful model to study Mk differentiation.