Localization and Regulation of Thrombopoietin mRNA Expression in Human Kidney, Liver, Bone Marrow, and Spleen Using In Situ Hybridization

The last intron of the human thrombopoietin gene enhances expression in milk of transgenic mice

Introns can enhance gene expression levels. This effect is known as intron-mediated enhancement, which is different from that of enhancers or promoters. In our previous study, under the control of the cytomegalovirus or goat β-casein promoter, the vector containing intron V-TPOcDNA expressed the highest thrombopoietin (TPO) level, whereas the vector containing TPOgDNA expressed the lowest level. In order to verify whether intron V also improves TPO expression in the milk of transgenic mice, rat whey acidic protein promoter was used as regulatory element to construct mammary gland expression vectors including pTPOWA (containing TPOcDNA), pTPOWB (containing intron V-TPOcDNA), and pTPOWC (containing TPOgDNA). These vectors were transfected into HC-11 cells and the supernatants were analyzed at 48 h. The highest TPO level was found in pTPOWB (795 pg/mL) and the lowest level in pTPOWC (193 pg/mL). Then, corresponding vectors were microinjected into fertilized mice zygotes. Transgenic mice were identified by polymerase chain reaction and Southern blot. Enzyme-linked immunosorbent assay was performed to measure TPO levels in the milk of lactating transgenic mice. The highest and lowest TPO levels were found in transgenic mice carrying intron V-TPOcDNA (2,307 pg/mL) and in transgenic mice carrying TPOgDNA (242 pg/mL), respectively. Thus, intron V remarkably improved TPO expression in transgenic mice.

Exome sequencing reveals a thrombopoietin ligand mutation in a Micronesian family with autosomal recessive aplastic anemia

We recently identified 2 siblings afflicted with idiopathic, autosomal recessive aplastic anemia. Whole-exome sequencing identified a novel homozygous missense mutation in thrombopoietin (THPO, c.112C>T) in both affected siblings. This mutation encodes an arginine to cysteine substitution at residue 38 or residue 17 excluding the 21-amino acid signal peptide of THPO receptor binding domain (RBD). THPO has 4 conserved cysteines in its RBD that form 2 disulfide bonds. Our in silico modeling predicts that introduction of a fifth cysteine may disrupt normal disulfide bonding to cause poor receptor binding. In functional assays, the mutant-THPO-containing media shows two- to threefold reduced ability to sustain UT7-TPO cells, which require THPO for proliferation. Both parents and a sibling with heterozygous R17C change have reduced platelet counts, whereas a sibling with wild-type sequence has normal platelet count. Thus, the R17C partial loss-of-function allele results in aplastic anemia in the homozygous state and mild thrombocytopenia in the heterozygous state in our family. Together with the recent identification of THPO receptor (MPL) mutations and the effects of THPO agonists in aplastic anemia, our results have clinical implications in the diagnosis and treatment of patients with aplastic anemia and highlight a role for the THPO-MPL pathway in hematopoiesis in vivo.

Effects of thrombopoietin on growth of hepatocellular carcinoma: Is thrombopoietin therapy for liver disease safe or not?

AIM

Liver cirrhosis (LC) is the end stage of chronic liver disease. No definitive pharmacological treatment is currently available. We previously reported that thrombopoietin (TPO) promoted liver regeneration and improved liver cirrhosis by increasing platelet count. TPO is therefore considered to be a therapeutic agent for LC; however, it is unclear whether TPO has proliferative effects on hepatocellular carcinoma (HCC), which arises frequently in cirrhotic livers. In this study, we examined the effects of TPO on growth of HCC.

METHODS

Expression of the TPO receptor, myeloproliferative leukemia virus oncogene (MPL) was examined in various liver tumor cell lines and liver cell types. In an in vitro study, the effects of TPO on signal transduction, cell proliferation, migration and invasion were examined in Huh7 cells, in which MPL is highly expressed. In an in vivo study, we subcutaneously transplanted Huh7 cells into nude mice that were divided into a TPO-treated group and a control group, and the tumor volume of each group was measured.

RESULTS

MPL was expressed strongly in hepatocytes but not in other cell types. Among liver tumor cell lines, Huh7 showed the highest expression of MPL. In Huh7, the addition of TPO activated Akt phosphorylation but not cell proliferation, migration or invasion. In the mouse experiment, there was no significant difference in tumor volume between the two groups.

CONCLUSION

TPO had no proliferative effect on HCC in vitro or in vivo, and could therefore be useful in the treatment of liver cirrhosis.

Megakaryocyte polyploidy is inhibited by lysyl oxidase propeptide

Megakaryocytes (MKs), the platelet precursors, undergo an endomitotic cell cycle that leads to polyploidy. Lysyl oxidase propeptide (LOX-PP) is generated from lysyl oxidase (LOX) pro-enzyme after proteolytical cleavage. We recently reported that LOX, a known matrix cross-linking enzyme, contributes to MK lineage expansion. In addition, LOX expression levels are ploidy-dependent, with polyploidy MKs having minimal levels. This led us to test the effects of LOX-PP on the number and ploidy of primary MKs. LOX-PP significantly decreases mouse bone marrow MK ploidy coupled with a reduction in MK size. MK number is unchanged upon LOX-PP treatment. Analysis of LOX-PP- or vehicle-treated MKs by western blotting revealed a reduction in ERK1/2 phosphorylation and in the levels of its downstream targets, cyclin D3 and cyclin E, which are known to play a central role in MK endomitosis. Pull-down assays and immunochemistry staining indicated that LOX-PP interacts with α-tubulin and the mictotubules, which can contribute to decreased MK ploidy. Thus, our findings defined a role for LOX-PP in reducing MK ploidy. This suggests that high-level expression of LOX in aberrantly proliferating MKs could play a part in inhibiting their polyploidization via LOX-PP.

Oxidases and reactive oxygen species during hematopoiesis: a focus on megakaryocytes

Reactive oxygen species (ROS), generated as a result of various reactions, control an array of cellular processes. The role of ROS during megakaryocyte (MK) development has been a subject of interest and research. The bone marrow niche is a site of MK differentiation and maturation. In this environment, a gradient of oxygen tension, from normoxia to hypoxia results in different levels of ROS, impacting cellular physiology. This article provides an overview of major sources of ROS, their implication in different signaling pathways, and their effect on cellular physiology, with a focus on megakaryopoiesis. The importance of ROS-generating oxidases in MK biology and pathology, including myelofibrosis, is also described.

The circulating platelet count is not dictated by the liver, but may be determined in part by the bone marrow: analyses from human liver and stem cell transplantations

BACKGROUND

The platelet count varies considerably between individuals, but within an individual the platelet count is remarkably stable over time. Mechanisms controlling the platelet count are not yet established.

OBJECTIVE

In the present study, we tested the hypothesis that the liver is important in controlling the circulating platelet count, as the liver is the main producer of thrombopoietin.

METHODS

We compared the platelet count prior to and after liver transplantation in >250 patients transplanted for familial amyloidotic polyneuropathy (FAP). In contrast to most patients undergoing liver transplantation, patients with FAP have normal liver function before transplantation. Furthermore, we compared platelet counts in 89 living liver donors with the platelet count in the recipients of these grafts. Finally we compared platelet counts in donor-recipient pairs of hematopoietic stem cells.

RESULTS AND CONCLUSIONS

The platelet count prior to transplantation correlated with the platelet count at 3 or 12 months after transplantation in patients with FAP (r=0.48, P<0.0001 at 3 months, r=0.39, P<0.0001 at 12 months), whereas the platelet count in a living liver donor did not correlate with the platelet count in the recipient at 3 or 12 months after transplantation (r=0.16, P=0.26 at 3 months, r=0.11, P=0.30 at 12 months). The platelet count of related donors of hematopoietic stem cells correlated with the platelet count in the recipient after transplantation (r=0.25, P=0.011).

CONCLUSIONS

These results suggest that the liver, in spite of being the prime producer of thrombopoietin, does not dictate the circulating platelet count, whereas the bone marrow does appear to play a role.

Thrombopoietin as biomarker and mediator of cardiovascular damage in critical diseases

Thrombopoietin (TPO) is a humoral growth factor originally identified for its ability to stimulate the proliferation and differentiation of megakaryocytes. In addition to its actions on thrombopoiesis, TPO directly modulates the homeostatic potential of mature platelets by influencing their response to several stimuli. In particular, TPO does not induce platelet aggregation per se but is able to enhance platelet aggregation in response to different agonists (“priming effect”). Our research group was actively involved, in the last years, in characterizing the effects of TPO in several human critical diseases. In particular, we found that TPO enhances platelet activation and monocyte-platelet interaction in patients with unstable angina, chronic cigarette smokers, and patients with burn injury and burn injury complicated with sepsis. Moreover, we showed that TPO negatively modulates myocardial contractility by stimulating its receptor c-Mpl on cardiomyocytes and the subsequent production of NO, and it mediates the cardiodepressant activity exerted in vitro by serum of septic shock patients by cooperating with TNF-α and IL-1β. This paper will summarize the most recent results obtained by our research group on the pathogenic role of elevated TPO levels in these diseases and discuss them together with other recently published important studies on this topic.

Patients with liver cirrhosis suffer from primary haemostatic defects? Fact or fiction?

Patients with cirrhosis can have abnormalities in laboratory tests reflecting changes in primary haemostasis, including bleeding time, platelet function tests, markers of platelet activation, and platelet count. Such changes have been considered particularly relevant in the bleeding complications that occur in cirrhosis. However, several studies have shown that routine diagnostic tests, such as platelet count, bleeding time, PFA-100, thromboelastography are not clinically useful to stratify bleeding risk in patients with cirrhosis. Moreover, treatments used to increase platelet count or to modulate platelet function could potentially do harm. Consequently the optimal management of bleeding complications is still a matter of discussion. Moreover, in the last two decades there has been an increased recognition that not only bleeding but also thrombosis complicates the clinical course of cirrhosis. Thus, we performed a literature search looking at publications studying both qualitative and quantitative aspects of platelet function to verify which primary haemostasis defects occur in cirrhosis. In addition, we evaluated the contribution of qualitative and quantitative aspects of platelet function to the clinical outcome in cirrhosis and their therapeutic management according to the data available in the literature. From the detailed analysis of the literature, it appears clear that primary haemostasis may not be defective in cirrhosis, and a low platelet count should not necessarily be considered as an automatic index of an increased risk of bleeding. Conversely, caution should be observed in patients with severe thrombocytopenia where its correction is advised if bleeding occurs and before invasive diagnostic and therapeutic procedures.

Eltrombopag for the treatment of idiopathic thrombocytopenic purpura

Strategies aimed at stimulating platelet production are a rational approach to the treatment of patients with primary immune thrombocytopenia, as, for many of them, the low platelet count is a consequence of ineffective megakaryopoiesis. Recently, intense clinical trial activity in immune thrombocytopenia has been reported for second-generation thrombopoietic agents. These novel molecules bear no structural resemblance to thrombopoietin, but still bind and activate the thrombopoietin receptor. One of these agents is eltrombopag (formerly SB497115), an orally available, small organic compound. Randomized trials have shown the short-term efficacy of eltrombopag in elevating the platelet count of most adult patients with immune thrombocytopenia unresponsive to at least one standard treatment. No significant adverse events were observed, but long-term safety data are still lacking. Ongoing studies will reveal the potential of this agent in the management of immune thrombocytopenia for long-term maintenance therapy, as well as its relative benefit compared with standard-of-care treatment.

Thrombopoietin factors

Megakaryopoiesis and thrombopoiesis are the central biological processes of platelet generation. Severe thrombocytopenia is a major morbidity and mortality factor in several diseases and represents a significant unmet medical need. Since the discovery of thrombopoietin (TPO) as the primary physiological regulator of megakaryopoiesis, a number of therapeutics have been developed for thrombocytopenia and been tested in preclinical models and human clinical trials. The TPO mimetics romiplostim (Nplate(®) or AMG531) and eltrombopag (Promacta(®)) have recently been approved for the treatment of adult chronic idiopathic (immune) thrombocytopenic purpura (ITP) and are successful examples of these endeavors. This chapter will review scientific progress over the last 20 years on various thrombopoietic factors with an emphasis on the biology, physiology, and pharmacology of TPO, its cognate receptor, c-Mpl, and various TPO mimetics.

Pharmacodynamics-mediated drug disposition (PDMDD) and precursor pool lifespan model for single dose of romiplostim in healthy subjects

The objective of this study was to characterize the pharmacokinetics and pharmacodynamics (PK-PD) of romiplostim after single-dose administration in healthy subjects. The mean serum romiplostim concentrations (PK data) and mean platelet counts (PD data) collected from 32 subjects receiving a single intravenous (0.3, 1 and 10 μg/kg) or subcutaneous (0.1, 0.3, 1, and 2 μg/kg) dose were fitted simultaneously to a mechanistic PK-PD model based on pharmacodynamics-mediated drug disposition (PDMDD) and a precursor pool lifespan concept. The two-compartment PK model incorporated receptor-mediated endocytosis and linear mechanisms as parallel elimination pathways. The maximal concentration of receptors (assumed to be proportional to the platelet count), the equilibrium dissociation constant, and the first-order internalization rate constant for endocytosis of the drug-receptor complex were 0.022 fg/platelet, 0.131 ng/mL, and 0.173 h⁻¹, respectively. Romiplostim concentration stimulates the production of platelet precursors via the Hill function, where the SC₅₀ was 0.052 ng/mL and S (max) was 11.2. The estimated precursor cell and platelet lifespans were 5.9 and 10.5 days, respectively. Model-based simulations revealed that the romiplostim exposure and the platelet response are both dependent on the dose administered and the baseline platelet counts. Also, weekly dosing produced a sustained PD response while dosing intervals ≥2 weeks resulted in fluctuating platelet counts. Thus, the mechanistic PK-PD model was suitable for describing the romiplostim PK-PD interplay (PDMDD), the dose-dependent platelet stimulation, and the lifespans of thrombopoietic cell populations.

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.

New treatments for idiopathic thrombocytopenic purpura: rethinking old hypotheses

BACKGROUND

The efficacy of thrombopoietin (TPO) mimetics in patients with idiopathic thrombocytopenic purpura (ITP) reaffirms that impaired platelet production is an important mechanism. New strategies to reduce platelet destruction, like rituximab, are also effective.

OBJECTIVES

To describe the efficacy and safety of rituximab and the TPO mimetics, romiplostim and eltrombopag, and how they relate to ITP pathogenesis.

METHODS

Narrative review summarizing full publications and meeting abstracts.

RESULTS/CONCLUSIONS

A 4-week course of rituximab is associated with a platelet count response in 60% of patients with ITP, and durable responses have been observed. Subtle increases in infection have been reported. Romiplostim and eltrombopag are each associated with a 60 - 85% response while on treatment. Transient bone marrow reticulin with romiplostim and elevated liver enzymes with eltrombopag are rare side effects. The application of these agents in non-splenectomized patients requires further study.

New-generation drugs that stimulate platelet production in chronic immune thrombocytopenic purpura

Idiopathic thrombocytopenic purpura is an acquired disease characterised by a low platelet count. Development of autoantibodies is a main cause of the disease. Although many patients have few symptoms, life-threatening bleeding can arise and hence, when platelet counts fall to unacceptable values treatment should be initiated. However, conventional immunosuppressive approaches can fail, perhaps because of the heterogeneous nature of the disease. Newly developed agents that increase platelet production by stimulating megakaryocytes-such as drugs that bind to the thrombopoietin receptor c-MPL-offer an alternative treatment strategy. Although initial thrombopoietin analogues caused adverse immune reactions, second-generation thrombopoietin-receptor agonists that are in late-stage clinical development seem promising. In particular, eltrombopag and romiplostim safely increase and maintain platelet production in patients with refractory disease. However, long-term side-effects are being assessed and the exact role of these agents in the overall treatment strategy of chronic idiopathic thrombocytopenic purpura remains to be established.

Determinants of platelet number and regulation of thrombopoiesis

Our understanding of thrombopoiesis has improved greatly in the last two decades with the availability of in vitro assays of megakaryocyte progenitor cell growth, with the cloning and characterization of stem cell factor (SCF) and thrombopoietin (Tpo), the latter the primary humoral regulator of this process, and with the generation of genetically altered murine models of thrombopoietic failure and excess. While SCF affects developmentally early aspects of megakaryocyte growth, Tpo affects nearly all aspects of platelet production, from hematopoietic stem cell (HSC) self-renewal and expansion, through stimulation of megakaryocyte progenitor cell proliferation, to supporting their maturation into platelet-producing cells. The molecular and cellular mechanisms through which the marrow microenvironment and humoral mediators affect platelet production provide new insights into the interplay between intrinsic and extrinsic influences on hematopoiesis, and highlight new opportunities to translate basic biology into clinical advances.

Single administration of thrombopoietin prevents progression of liver fibrosis and promotes liver regeneration after partial hepatectomy in cirrhotic rats

OBJECTIVE

To evaluate the effect of thrombopoietin on liver regeneration after hepatectomy and antifibrosis under conditions of liver cirrhosis in rats.

SUMMARY BACKGROUND DATA

We revealed that platelets induced by thrombopoietin administration promote liver regeneration after hepatectomy in the normal liver.

METHODS

Seventy percent hepatectomy was carried out in rats, which were subsequently divided into 4 groups: (1) normal group without any treatment, (2) liver cirrhosis (LC) group, (3) combined thrombopoietin and liver cirrhosis (LC+TPO) group, and (4) combined thrombopoietin, antiplatelet serum and liver cirrhosis (LC+TPO+APS) group. Growth kinetics in the liver regeneration and growth factors were analyzed. Liver fibrotic area and activation of hepatic stellate cells were also investigated.

RESULTS

In LC group, liver regeneration was significantly delayed compared with normal group 24 hours after hepatectomy. On the other hand, liver regeneration of LC+TPO group increased significantly compared with LC group, to a level that was the same as that recorded in normal group. In LC group, liver fibrotic area before hepatectomy was significantly higher compared with the normal group. Liver fibrosis of LC+TPO group was significantly reduced compared with LC group. The antifibrotic and liver regeneration promoting effects of LC+TPO group were inhibited by antiplatelet serum in LC+TPO+APS group.

CONCLUSION

The administration of thrombopoietin reduces liver fibrosis and stimulates regeneration after hepatectomy through increment and accumulation of platelets in the cirrhotic liver. This could be a potentially useful treatment for liver cirrhosis.

Functional and structural profiling of the human thrombopoietin gene promoter

Human thrombopoietin (TPO) is involved in cardiovascular disease as it regulates megakaryocyte development and enhances platelet adhesion/aggregation. The THPO promoter structure is still controversial. By reverse transcription-PCR, we confirm that THPO transcription is cell line-dependently initiated at two alternative promoters, which we newly designated P1a and P1. We subsequently electrophoretically scanned and resequenced these portions in 95 and 57 patients with cardiovascular disease, respectively, and identified seven variants (-1450/del58bp, C-920T [rs2855306], A-622G, C-413T [rs885838], C+5A, G+115A, and C+135T). After subcloning of 1032 bp of THPO P1 in pGL3-basic vector, five molecular haplotypes (MolHaps1-5) were observed: [A(-622)-C(-413)-C(+5)-G(+115); wild type (wt)], [A(-622)-T(-413)-C(+5)-G(+115)], [G(-622)-T(-413)-C(+5)-G(+115)], [A(-622)-C(-413)-A(+5)-G(+115)], [A(-622)-C(-413)-C(+5)-A(+115)], and analyzed in reporter gene assays in HEK293T and HepG2 cells. MolHaps 2, 4, and 5 were significantly more active than wt (all p values < or =0.01) in HEK293T cells, MolHap3 exerted a substantial loss of promoter activity (p < 0.0001 in HEK293T and p < 0.01 in HepG2, compared with wt). Electrophoretic mobility shift assays revealed that A-622G and C-413T individually differed from MolHaps in their DNA-protein interaction patterns. Supershift and chromatin immunoprecipitation assays identified CCAAT/enhancer-binding protein delta as the binding protein exclusively for the -622A allelic portion.

Review article: blood platelet number and function in chronic liver disease and cirrhosis

BACKGROUND

The liver plays a central role in coagulation and fibrinolysis but is also closely intertwined with the function and number of blood platelets.

AIM

To describe and integrate all literature concerning blood platelets and liver disease by performing a thorough literature research.

METHODS

A thorough literature research on 'blood platelets' and 'liver disease' was performed.

RESULTS

Thrombocytopenia is a marked feature of chronic liver disease and cirrhosis. Traditionally, this thrombocytopenia was attributed to passive platelet sequestration in the spleen. More recent insights suggest an increased platelet breakdown and to a lesser extent decreased platelet production plays a more important role. Besides the reduction in number, other studies suggest functional platelet defects. This platelet dysfunction is probably both intrinsic to the platelets and secondary to soluble plasma factors. It reflects not only a decrease in aggregability, but also an activation of the intrinsic inhibitory pathways. The net effect, finally, is a decreased platelet function in the various types of chronic liver diseases and cirrhosis. Finally, recent data suggest that platelets are not only affected by but can also contribute to the liver disease process, as for instance, in viral hepatitis and cholestatic liver disease.

CONCLUSION

Platelet research in liver disease is a growing area of investigation and could provide new pathophysiological insights.

Transcriptional regulation of bone marrow thrombopoietin by platelet proteins

Platelet production is regulated primarily by the cytokine thrombopoietin (TPO). Although TPO is expressed in several different tissues, only in the bone marrow has the level of expression been reported to increase in response to reduced numbers of platelets. In these studies, we demonstrate that platelet granule proteins are able to transcriptionally repress TPO mRNA expression in a marrow stromal cell line as well as in primary bone marrow stromal cell cultures. Like TPO mRNA, secretion of TPO protein was also suppressed by serum treatment. Reporter gene constructs indicate that DNA elements located in an approximately 1.9-kb region between 250-bp upstream of the transcriptional initiation site and the middle of the second intron are able to mediate the transcriptional repression.

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

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

Historical review: megakaryopoiesis and thrombopoiesis

The study of thrombopoiesis has evolved greatly since an era when platelets were termed "the dust of the blood," only about 100 years ago. During this time megakaryocytes were identified as the origin of blood platelets; marrow-derived megakaryocytic progenitor cells were functionally defined and then purified; and the primary regulator of the process, thrombopoietin, was cloned and characterized and therapeutic thrombopoietic agents developed. During this journey we continue to learn that the physiologic mechanisms that drive proplatelet formation can be recapitulated in cell-free systems and their biochemistry evaluated; the molecular underpinnings of endomitosis are being increasingly understood; the intracellular signals sent by engagement of a large number of megakaryocyte surface receptors have been defined; and many of the transcription factors that drive megakaryocytic fate determination have been identified and experimentally manipulated. While some of these biologic processes mimic those seen in other cell types, megakaryocytes and platelets possess enough unique developmental features that we are virtually assured that continued study of thrombopoiesis will yield innumerable clinical and scientific insights for many decades to come.

Uncertainty in the niches that maintain haematopoietic stem cells

Haematopoietic stem cell (HSC) niches are specialized microenvironments that contain stem cells and regulate their maintenance. Cells at the interface of bone and the bone marrow (the endosteum) contribute to the creation of HSC niches. It remains uncertain whether this interface itself is a niche, or whether endosteal cells secrete factors that diffuse to nearby niches. Vascular and/or perivascular cells may also create niches as many HSCs are observed around sinusoidal blood vessels, and perivascular cells secrete factors that regulate HSC maintenance. Do endosteal and perivascular cells create distinct niches, or do they contribute to a common niche? We discuss a range of niche models consistent with recent evidence.

Thrombopoietin is a growth factor for rat hepatic progenitors

OBJECTIVE

The liver is the primary site of hematopoiesis during fetal development; it has been shown that thrombopoietin (TPO) produced by the liver during fetal development is a major regulator of megakaryocytopoiesis. As maximum liver growth and hematopoiesis occur simultaneously, we hypothesized that TPO may act as a growth factor for hepatic progenitors. Therefore, the influence of TPO on the proliferation of fetal hepatic progenitors in vitro compared with that of adult hepatocytes was analyzed. The expression of the TPO receptor, c-mpl, was investigated in fetal and adult liver.

METHODS

Cell proliferation was measured by bromodeoxyuridine incorporation and total cell counts. TPO and c-mpl gene expression was investigated by reverse transcription polymerase chain reaction. The cell surface expression of c-mpl was analyzed in fetal and adult human liver by immunohistochemistry.

RESULTS

Hepatic progenitors of fetal and adult liver but not hepatocytes expressed the TPO receptor, c-mpl, on the cell surface. Fetal hepatic progenitors expressed mRNA for TPO and its receptor. TPO stimulated cell proliferation and increased cell numbers of cultured rat fetal hepatic progenitors but not adult hepatocytes.

CONCLUSION

We conclude that TPO acts in addition to its known role in megakaryocytopoiesis as a growth factor for hepatic progenitors but not hepatocytes in vitro; thus, TPO represents a growth factor for hepatic progenitors during fetal liver development.

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

BACKGROUND

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

AIM

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

RESULTS

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

CONCLUSION

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

Liver fibrosis is associated with decreased peripheral platelet count in patients with chronic hepatitis B and C

Thrombocytopenia is a common complication of chronic liver diseases, but its pathogenesis is not clear. Although generally attributed to hypersplenism, other factors should also be considered. We investigated the relationship between the peripheral platelet count and the degree of fibrosis in patients with chronic viral hepatitis. In an effort to avoid the effects of hypersplenism, we excluded patients with splenomegaly and/or bi- or pan-cytopenia. Seven hundred eighty-four patients (265 chronic viral hepatitis C and 519 chronic viral hepatitis B) were included in the study. Univariate analysis showed that the peripheral platelet count had a negative correlation with fibrosis score, necroinflammatory activity, and age in both groups. In multivariate analysis, the peripheral platelet count had a similar correlation with the fibrosis score and age, but not with necroinflammatory activity, in both groups. The peripheral platelet count decreased more significantly in females with chronic hepatitis C but not in the chronic hepatitis B group. In conclusion, a decrease in peripheral platelet count may be a sign of an increase in the degree of fibrosis during the course of chronic viral hepatitis B and C and factors other than hypersplenism may play a role in this decrease in the peripheral platelet count.

Thrombopoietin promotes β1-integrin–mediated adhesion in hematopoietic cells via the small GTPase Rap1

OBJECTIVE

The interactions between cells and extracellular matrices in the bone marrow microenvironment are critical for normal hematopoiesis, controlling cell survival, proliferation, differentiation, and motility. A number of hematopoietic growth factors and cytokines can mediate these interactions by changing expression and/or activity of specific integrins, or by changing cell shape. Thrombopoietin (TPO) has previously been shown to stimulate adhesion. in certain hematopoietic cell types, although the exact mechanisms by which adhesion is promoted remain elusive.

MATERIALS AND METHODS

The role of TPO in hematopoietic cell adhesion was determined with fibronectin adhesion and binding assays, flow cytometry, and immunocytochemistry using the hematopoietic cell line UT-7/TPO and bone marrow-derived primary mouse megakaryocytes. The role of Rapl in TPO-mediated adhesion was determined using a RaplGAP overexpressing UT-7/TPO cell line, in which Rapl could not be activated.

RESULTS

We found that TPO promoted hematopoietic cell adhesion by causing cytoskeletal reorganization and not by increasing integrin expression, localization, or affinity, as previously hypothesized. Through studies using the UT-7/TPO-RaplGAP cell line, we found that TPO-mediated cell shape change occurred via activation of Rapl.

CONCLUSIONS

These data demonstrate an important role for TPO in mediating interactions in the bone marrow microenvironment and make a significant contribution to our understanding of how TPO may affect hematopoiesis.

Hematologic disorders associated with human immunodeficiency virus and AIDS

Nurses encounter patients with human immunodeficiency virus infection at various stages of their infection and in a variety of settings. This article focuses on the most common hematologic disorders associated with human immunodeficiency virus infection and acquired immunodeficiency syndrome, which can precipitate complications and frequently accompany hospitalization. It is important for nurses to have a solid foundation as to the cause of these disorders, their impact on quality of life and outcomes, and management strategies.

Hepatocyte growth factor accelerates thrombopoiesis in transgenic mice

Hepatocyte growth factor (HGF) is one of the potent growth factors for liver regeneration and has a strong effect on epithelial and nonepithelial cells. As one of the pleiotropic functions, HGF acts as a hematopoietic regulator in the proliferation and differentiation of hematopoietic progenitors. However, the effect of HGF on the thrombopoietic function remains unclear. The correlation between HGF and thrombopoiesis was investigated in transgenic (TG) mice overexpressing murine HGF controlled by the murine HGF by the metallothionein promoter. Furthermore, the mechanism of thrombocytosis induced by HGF in vitro was analyzed in hepatoma cell line HepG2. Both the platelet count and the serum thrombopoietin (TPO) concentration were significantly higher in TG than in the wild type (WT) control mice. In the liver and spleen, the expression of TPOmRNA in TG was higher than that in WT by real-time polymerase chain reaction. The expressions of transcriptional factor of TPO, GABP-alpha/beta were more increased in TG liver compared to WT. In an in vitro study, HGF induced TPO and GABP-alpha/beta expression and enhanced TPO promoter activity. Therefore, HGF induced thrombopoiesis accompanied with the overexpression of TPO through GABP stimulation.

Megakaryocyte development and platelet production

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

Impaired megakaryopoiesis in patients with chronic idiopathic neutropenia is associated with increased transforming growth factor β1 production in the bone marrow

Patients with chronic idiopathic neutropenia (CIN) display relatively low peripheral blood platelet counts and hypo-lobulated megakaryocytes in the bone marrow (BM). The underlying pathogenetic mechanismswere probed by studying the reserves and clonogenic potential of BM megakaryocytic progenitor cells using flow-cytometry and a collagen-based clonogenic assay for the identification of megakaryocyte colony-forming units (CFU-Meg). Thrombopoietin (TPO) and transforming growth factor-beta1 (TGFbeta1) levels were also evaluated in long-term BM culture supernatants using an enzyme-linked immunosorbent assay. CIN patients (n = 39) showed a low proportion of BM CD34(+)/CD61(+) megakaryocytic progenitor cells and low frequency of early and mixed CFU-Meg in the BM mononuclear, but not CD34(+), cell fraction, compared with healthy controls (n = 20). TPO and TGFbeta1 levels were significantly higher in patients compared with controls. TPO levels inversely correlated with platelet counts whereas TGFbeta1 values correlated inversely with CD34(+)/CD61(+) and CFU-Meg megakaryocytic progenitor cell numbers and positively with TPO levels. The addition of an anti-TGFbeta1 neutralising antibody significantly increased the numbers of CFU-Meg in CIN patients but not in controls, compared with baseline. These data suggest that increased local production of TGFbeta1 probably affects the BM megakaryocytic progenitor cell growth in CIN whereas the compensatory production of TPO finally balances the TGFbeta1-induced inhibitory effect.

Prolonged thrombocytopenia after allogeneic hematopoietic stem cell transplantation: associations with impaired platelet production and increased platelet turnover

To evaluate the mechanisms underlying prolonged thrombocytopenia after allogeneic hematopoietic stem cell transplantation (SCT), an index for plasma glycocalicin normalized for the individual platelet count (GCI), plasma thrombopoietin (TPO), and circulating B cells producing anti-GPIIb-IIIa antibodies were measured in 50 SCT recipients with or without prolonged thrombocytopenia, 42 patients with idiopathic thrombocytopenic purpura, nine patients with aplastic anemia, and 22 healthy individuals. All three indices were significantly higher in the SCT recipients with thrombocytopenia than in those without (P < 0.01 for all comparisons), and were significantly correlated with the platelet count in SCT recipients. Stepwise multiple regression analysis of the samples from the SCT recipients revealed that GCI and TPO independently pointed to specific mechanisms of thrombocytopenia. The GCI and TPO status in SCT recipients with thrombocytopenia had a pattern similar to that seen in aplastic anemia, suggesting a major role for impaired thrombopoiesis. An antiplatelet antibody response was frequently detected in SCT recipients, but the development of thrombocytopenia is likely to depend on additional factors, such as reticuloendothelial function. In summary, post transplant prolonged thrombocytopenia is associated with complex mechanisms, including impaired thrombopoiesis and increased platelet turnover.

Relationship of thrombopoietin and interleukin-11 levels to thrombocytopenia associated with dengue disease

Thrombocytopenia is one of the main clinical findings of dengue. In this work we examined the levels of thrombopoietin (TPO) and interleukin-11 (IL-11), two of the most potent regulators of platelet production, in serum from 28 patients with dengue fever (DF). Patients with DF had increased levels of TPO, compared with healthy individuals (p<0.005). Patients with dengue hemorrhagic fever (DHF, n=7), the more severe form of dengue, had higher TPO levels than patients with DF (p<0.001). Serum TPO levels and platelet counts were inversely correlated in both DF and DHF patients. IL-11 was detectable in neither DF nor DHF patients. Our results demonstrate that thrombocytopenia in dengue disease is associated with changes in the serum levels of TPO, but not IL-11, suggesting that this cytokine could be a potential early clinical marker of the severity of dengue disease.

The Mpl receptor expressed on endothelial cells does not contribute significantly to the regulation of circulating thrombopoietin levels

OBJECTIVE

Blood levels of thrombopoietin (TPO) are regulated in part by cellular degradation following its binding to the cell surface receptor c-mpl. Previous reports have demonstrated that in addition to hematopoietic cells, c-mpl is expressed on and functions in several types of endothelial cells (ECs). We hypothesized that the c-mpl expressed on ECs would contribute to the regulation of circulating TPO levels.

METHODS

To test this hypothesis we transplanted c-mpl-null and wild-type (WT) control mice with WT marrow stem cells, resulting in two groups of posttransplant chimeric animals, one expressing c-mpl on megakaryocytes and platelets only and one in which the receptor is expressed on both hematopoietic and ECs. Should EC c-mpl take up TPO and degrade it, we predicted that c-mpl-null mice reconstituted with WT cells would display increased TPO levels and an increased steady state platelet count compared to the WT recipients.

RESULTS

Contrary to our prediction, for up to 6 months posttransplantation both platelet counts and TPO levels in both groups of transplanted mice were virtually identical.

CONCLUSIONS

Our results indicate that the EC c-mpl receptor does not contribute significantly to the regulation of TPO levels or to steady-state platelet counts. These results also imply that patients with congenital amegakaryocytic thrombocytopenia, lacking the c-mpl receptor, who have successfully been engrafted with normal hematopoietic stem cells should have normal (not elevated) TPO levels and that gene replacement strategies designed to restore c-mpl in these patients do not need to target ECs to establish the normal regulation of TPO.

Thrombopoietin production in wild-type and interleukin-6 knockout mice with acute inflammation

Clinical and laboratory studies indicate that thrombopoietin (TPO) gene expression increases during inflammation. To clarify the role of interleukin 6 (IL-6) in this process, blood cell counts, plasma TPO concentrations, and hepatic and renal TPO mRNA levels were investigated in wild-type and IL-6 knockout mice, with sterile abscesses produced by subcutaneous injection of turpentine oil. Treatment did not cause a change in blood cell counts during the 72 h period of observation. The numbers of thrombocytes and erythrocytes were slightly lower in the IL-6 knockout mice than in the wild-type littermates under all conditions. Plasma IL-6 and TPO concentrations increased on turpentine injection only in the wild-type mice. In addition, turpentine treatment of these caused an increase in hepatic TPO mRNA levels as assessed by competitive polymerase chain reaction (RT-PCR) and real-time PCR, whereas renal TPO mRNA levels were unaltered. TPO mRNA levels did not increase in the livers of IL-6 knockout mice on turpentine treatment. These results support the concept that TPO behaves like an acute-phase protein in that its synthesis is induced by IL-6 in the liver.

Primary and secondary thrombocytosis in childhood

This review summarizes current data on the pathomechanisms and clinical aspects of primary and secondary thrombocytosis in childhood. Primary thrombocytosis is extremely rare in childhood, mostly diagnosed at the beginning of the second decade of life. As in adults, the criteria of the Polycythemia Vera Group are appropriate to diagnose primary thrombocytosis. The pathomechansims of non-familial forms are complex and include spontaneous formation of megakaryopoietic progenitors and increased sensitivity to thrombopoietin (Tpo). Familial forms can be caused by mutations in Tpo or Tpo receptor (c-mpl) genes. These mutations result in overexpression of Tpo, sustained intracellular signalling or disturbed regulation of circulating Tpo. Treatment of primary thrombocytosis is not recommended if platelet counts are <1500/nl and bleeding or thrombosis did not occur in patient's history. In severe cases, decision on treatment should weigh potential risks of treatment options (hydroxyurea, anagrelide) against expected benefits for preventing thrombosis or haemorrhage. Secondary thrombocytosis is frequent in children, in particular in the first decade of life. Hepatic Tpo production is stimulated in acute response reaction to a variety of disorders. Thrombosis prophylaxis is not required, even at platelet counts >1000/nl, except for cases with additional prothrombotic risk factors.