Thrombopoietin serum concentration in patients with reactive and myeloproliferative thrombocytosis

Cell-autonomous megakaryopoiesis associated with polyclonal hematopoiesis in triple-negative essential thrombocythemia

A subset of essential thrombocythemia (ET) cases are negative for disease-defining mutations on JAK2, MPL, and CALR and defined as triple negative (TN). The lack of recurrent mutations in TN-ET patients makes its pathogenesis ambiguous. Here, we screened 483 patients with suspected ET in a single institution, centrally reviewed bone marrow specimens, and identified 23 TN-ET patients. Analysis of clinical records revealed that TN-ET patients were mostly young female, without a history of thrombosis or progression to secondary myelofibrosis and leukemia. Sequencing analysis and human androgen receptor assays revealed that the majority of TN-ET patients exhibited polyclonal hematopoiesis, suggesting a possibility of reactive thrombocytosis in TN-ET. However, the serum levels of thrombopoietin (TPO) and interleukin-6 in TN-ET patients were not significantly different from those in ET patients with canonical mutations and healthy individuals. Rather, CD34-positive cells from TN-ET patients showed a capacity to form megakaryocytic colonies, even in the absence of TPO. No signs of thrombocytosis were observed before TN-ET development, denying the possibility of hereditary thrombocytosis in TN-ET. Overall, these findings indicate that TN-ET is a distinctive disease entity associated with polyclonal hematopoiesis and is paradoxically caused by hematopoietic stem cells harboring a capacity for cell-autonomous megakaryopoiesis.

Clinical and laboratory assessment of a patient with thrombocytosis

Elevated platelet counts are frequently encountered in hospital medicine and arise from both physiological and pathological mechanisms. Thrombocytosis may be secondary, reflecting an inflammatory state, iron deficiency, recent surgery or point towards an underlying neoplasm. Thrombocytosis may be the presenting sign of solid tumours and haematological conditions. The discovery of the activating mutations affecting thrombopoiesis led to greater understanding of the pathobiology of essential thrombocythaemia and other myeloproliferative neoplasms. The investigation of suspected primary thrombocytosis has evolved to include testing for these disease-associated mutations. Therapy for patients with essential thrombocythaemia aims to reduce their risk of thrombotic complications by addressing cardiovascular risk factors, and using antiplatelet agents and, in selected patients, cytoreductive therapy. This article provides a logical approach to distinguishing reactive or secondary thrombocytosis from thrombocytosis associated with an underlying myeloproliferative neoplasm and gives an overview of the management of essential thrombocythaemia.

Platelets and cancer angiogenesis nexus

There has been remarkable insight into the importance of platelets in a wide range of pathophysiologic events, including inflammation and cancer progression. Thrombocytosis in cancer patients is a common finding. Tumor cells induce platelet activation and subsequent aggregation through direct and indirect mechanisms. Platelets are recognized to contribute to metastatic dissemination. There is plenty of evidence that components of the hemostatic system contribute to the process of angiogenesis. Furthermore, there are accumulated data on the substantial influence of blood platelets in the process of blood vessel formation during malignancy. Platelets appear to be the main physiologic transporters of proangiogenic and antiangiogenic factors. Moreover, they influence the process of angiogenesis through platelet-derived microparticles, microRNA, lipids, and variety of surface receptors. Platelets contribute to early and late stages of angiogenesis. Available data support the overall stimulatory effect of platelets on tumor angiogenesis. It raises the possibility that interfering with platelet function may be an effective antineoplastic treatment strategy.

Erythropoietin-induced cytoprotection in intestinal epithelial cells is linked to system Xc<sup/>

Necrotizing enterocolitis is a common but serious complication among premature babies. Currently, there are limited treatment options. These include intensive supportive care and surgical intervention. In this study, we hypothesize that erythropoietin (Epo) could be protective against cell necrosis by increasing the levels of glutathione. This can be regulated by increasing the activity of system xC^-. This was demonstrated using intestinal epithelial cells (IEC-6) as a model system. S4-CPG and sulfasalazine pharmacologically inhibit xCT, which induced cell death. Our data showed a dose dependent decrease in cell viability when treated with both inhibitors. In addition, the IEC-6 cells displayed a dose dependent increase when treated with Epo. In conclusion, Epo can be protective against cell death and ultimately be considered as a treatment option for intestinal epithelial cell death.

Mpl expression on megakaryocytes and platelets is dispensable for thrombopoiesis but essential to prevent myeloproliferation

Thrombopoietin (TPO) acting via its receptor, the cellular homologue of the myeloproliferative leukemia virus oncogene (Mpl), is the major cytokine regulator of platelet number. To precisely define the role of specific hematopoietic cells in TPO-dependent hematopoiesis, we generated mice that express the Mpl receptor normally on stem/progenitor cells but lack expression on megakaryocytes and platelets (Mpl(PF4cre/PF4cre)). Mpl(PF4cre/PF4cre) mice displayed profound megakaryocytosis and thrombocytosis with a remarkable expansion of megakaryocyte-committed and multipotential progenitor cells, the latter displaying biological responses and a gene expression signature indicative of chronic TPO overstimulation as the underlying causative mechanism, despite a normal circulating TPO level. Thus, TPO signaling in megakaryocytes is dispensable for platelet production; its key role in control of platelet number is via generation and stimulation of the bipotential megakaryocyte precursors. Nevertheless, Mpl expression on megakaryocytes and platelets is essential to prevent megakaryocytosis and myeloproliferation by restricting the amount of TPO available to stimulate the production of megakaryocytes from the progenitor cell pool.

Characterization of ENU-induced Mutations in Red Blood Cell Structural Proteins

Murine models with modified gene function as a result of N-ethyl-N-nitrosourea (ENU) mutagenesis have been used to study phenotypes resulting from genetic change. This study investigated genetic factors associated with red blood cell (RBC) physiology and structural integrity that may impact on blood component storage and transfusion outcome. Forward and reverse genetic approaches were employed with pedigrees of ENU-treated mice using a homozygous recessive breeding strategy. In a "forward genetic" approach, pedigree selection was based upon identification of an altered phenotype followed by exome sequencing to identify a causative mutation. In a second strategy, a "reverse genetic" approach based on selection of pedigrees with mutations in genes of interest was utilised and, following breeding to homozygosity, phenotype assessed. Thirty-three pedigrees were screened by the forward genetic approach. One pedigree demonstrated reticulocytosis, microcytic anaemia and thrombocytosis. Exome sequencing revealed a novel single nucleotide variation (SNV) in Ank1 encoding the RBC structural protein ankyrin-1 and the pedigree was designated Ank1(EX34). The reticulocytosis and microcytic anaemia observed in the Ank1(EX34) pedigree were similar to clinical features of hereditary spherocytosis in humans. For the reverse genetic approach three pedigrees with different point mutations in Spnb1 encoding RBC protein spectrin-1β, and one pedigree with a mutation in Epb4.1, encoding band 4.1 were selected for study. When bred to homozygosity two of the spectrin-1β pedigrees (a, b) demonstrated increased RBC count, haemoglobin (Hb) and haematocrit (HCT). The third Spnb1 mutation (spectrin-1β c) and mutation in Epb4.1 (band 4.1) did not significantly affect the haematological phenotype, despite these two mutations having a PolyPhen score predicting the mutation may be damaging. Exome sequencing allows rapid identification of causative mutations and development of databases of mutations predicted to be disruptive. These tools require further refinement but provide new approaches to the study of genetically defined changes that may impact on blood component storage and transfusion outcome.

Thrombocytosis: diagnostic evaluation, thrombotic risk stratification, and risk-based management strategies

Thrombocytosis is a commonly encountered clinical scenario, with a large proportion of cases discovered incidentally. The differential diagnosis for thrombocytosis is broad and the diagnostic process can be challenging. Thrombocytosis can be spurious, attributed to a reactive process or due to clonal disorder. This distinction is important as it carries implications for evaluation, prognosis, and treatment. Clonal thrombocytosis associated with the myeloproliferative neoplasms, especially essential thrombocythemia and polycythemia vera, carries a unique prognostic profile, with a markedly increased risk of thrombosis. This risk is the driving factor behind treatment strategies in these disorders. Clinical trials utilizing targeted therapies in thrombocytosis are ongoing with new therapeutic targets waiting to be explored. This paper will outline the mechanisms underlying thrombocytosis, the diagnostic evaluation of thrombocytosis, complications of thrombocytosis with a special focus on thrombotic risk as well as treatment options for clonal processes leading to thrombocytosis, including essential thrombocythemia and polycythemia vera.

Erythropoietin-induced neuroprotection requires cystine glutamate exchanger activity

Erythropoietin (Epo) has been used for many years in neonates for the treatment of anemia of prematurity. Epo has also been proposed for treatment of neonatal brain injury, as mounting evidence suggests neuroprotective properties for Epo. However, Epo's neuroprotective mechanism of action is poorly understood. In this study we hypothesized that Epo may confer neuroprotection by enhancing cellular redox defense brought about by cellular glutathione (GSH). This was examined in cultures of differentiated cortical neural stem cells and using the B104 cell line as model systems. Our data shows that Epo causes a time- and dose-dependent increase in expression and activity of system Xc(-), the transporter responsible for uptake of cystine for the production of glutathione. Cystine uptake increases 3-5 fold in differentiated neural stem cells and B104 cells treated with Epo. Exposure of cells to 100 microM kainate suppressed cellular GSH and caused excitotoxicity, but GSH levels and cell viability were completely restored by Epo in the continued presence of kainate. This rescue effect of Epo vanished if system Xc(-) was inhibited pharmacologically using S4-CPG in the presence of Epo leading to marked cell death of B104 cells and cultured mouse cortical neural stem cells. This could also be achieved using xCT siRNA to decrease xCT expression. This data suggests that system Xc(-) activity and protein expression are positively regulated by Epo directly explaining its neuroprotective effect.

High platelet count associated with venous thromboembolism in cancer patients: results from the Vienna Cancer and Thrombosis Study (CATS)

BACKGROUND

In cancer patients, laboratory parameters that predict venous thromboembolism (VTE) are scarce. Increased platelet count has been found to be a risk factor for VTE in cancer patients receiving chemotherapy (CHT). We have assessed high platelet count as a risk predictor for VTE in patients with cancer undergoing discriminative anti-cancer treatments and investigated whether platelet count correlates with thrombopoietin (TPO) levels.

DESIGN AND METHODS

The Cancer and Thrombosis Study (CATS) is an ongoing prospective observational study of patients with newly diagnosed cancer or progression of disease, which started in October 2003. Occurrence of VTE and information on the patients' anti-cancer treatment during follow-up were recorded.

RESULTS

Between October 2003 and February 2008, 665 patients with solid tumors were included (314 female/351 male, mean age 62 years). VTE occurred in 44 patients (18 female/26 male, mean age 62 years). The cumulative probability of VTE after 1 year was 34.3% in patients with a platelet count (PC) above the 95th percentile representing 443 x 10(9)/L compared with 5.9% in those below 443 x 10(9)/L. High platelet count [hazard ratio (HR): 3.50, 95% confidence interval (CI): 1.52-8.06, P = 0.0032], soluble P-selectin [HR: 2.66, 95% CI: 1.42-4.96, P = 0.0021] and surgery [HR: 4.05, 95% CI: 1.74-9.46, P = 0.0012] were statistically significant risk factors for VTE in multivariable analysis along with leucocyte count, age, gender, radio- and CHT. We found no correlation between platelet count and TPO levels.

CONCLUSIONS

High PC is a clinically important, independent risk predictor for VTE in cancer patients. PC was not found to be associated with TPO levels.

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.

Characteristic changes in platelet-large cell ratio, lactate dehydrogenase and C-reactive protein in thrombocytosis-related diseases

We examined the clinical usefulness of 3 parameters of routine laboratory tests [platelet-large cell ratio (P-LCR), lactate dehydrogenase (LDH) and C-reactive protein (CRP)] in 84 patients with thrombocytosis-related diseases (reactive thrombocytosis, chronic myeloid leukemia, essential thrombocythemia and polycythemia vera). These thrombocytosis-related diseases were characterized using the 3 parameters P-LCR, LDH and CRP as follows: high P-LCR and high LDH in chronic myeloid leukemia; high CRP in reactive thrombocytosis; slightly high P-LCR and high LDH in essential thrombocythemia and polycythemia vera. For essential thrombocythemia and polycythemia vera, levels of P-LCR and CRP were nearly identical, but the LDH level in essential thrombocythemia was significantly higher than in polycythemia vera. These characteristics of P-LCR, LDH and CRP may be useful for simple and very rough differentiation of the thrombocytosis-related disease mentioned above.

Polycythemia vera: a comprehensive review and clinical recommendations

More than a century has elapsed since the appearance of the modern descriptions of polycythemia vera (PV). During this time, much has been learned regarding disease pathogenesis and PV-associated molecular aberrations. New information has allowed amendments to traditional diagnostic criteria. Phlebotomy remains the cornerstone treatment of PV, whereas myelosuppressive agents may augment the benefit of using phlebotomy for thrombosis prevention in high-risk patients. Excessive aspirin use is contraindicated in PV, although the use of lower-dose aspirin has been shown to be safe and effective in alleviating microvascular symptoms including erythromelalgia and headaches. Recent studies have shown the utility of selective serotonin receptor antagonists for treating PV-associated pruritus. Nevertheless, many questions remain unanswered. What is the specific genetic mutation or altered molecular pathway that is causally related to the disease? In the absence of a specific molecular marker, how is a working diagnosis of PV made? What evidence supports current practice in the management of PV? This article summarizes both old and new information on PV; proposes a modern diagnostic algorithm to formulate a working diagnosis; and provides recommendations for patient management, relying whenever possible on an evidence-based approach.

Serum proinflammatory cytokines and its relationship to clinical parameters in lung cancer patients with reactive thrombocytosis

Proinflammatory cytokines Interleukin-1 beta (IL-1 beta) and Interleukin-6 (IL-6) play a significant role in the pathogenetic processes related to various malignant and inflammatory conditions. Leukocytosis, thrombocytosis and increased acute phase protein levels are part of a systemic inflammatory response. In this study, we measured the concentrations of IL-1 beta, IL-6 and ferritin as well as hemoglobin, lactate dehydrogenase (LDH), C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) in 23 patients (male 15, female 8, median age 68 years) with lung cancer and reactive thrombocytosis (LCRT), in 27 (male 18, female 9, median age 64 years) with benign inflammatory lung disorder (BILD) and 18 (male 10, female 8, median age 62 years) lung cancer patients with a normal platelet count (LCNP). IL-1 beta levels were significantly higher in the three patient groups in comparison with control subjects (P < 0.001) but without significant difference among the three patient groups. IL-6 was higher in all three patients groups but only in the BILD group it was significantly higher than the control group (P < 0.05). However, no significant difference in IL-6 serum levels was found between the two lung cancer groups. CRP and LDH were significantly higher in the LCRT group in comparison with the other two patient groups (P < 0.01 and 0.001, respectively), while ferritin was higher in both lung cancer groups in comparison with the BILD group (P < 0.001). Our data suggest that in lung cancer patients, reactive thrombocytosis is part of the systemic inflammatory reaction for which IL-1 beta and IL-6 may be intermediate but not independent mediators.

The molecular and cellular biology of thrombopoietin: the primary regulator of platelet production

The term thrombopoietin (TPO) was first coined in 1958 and used to describe the humoral substance responsible for causing the platelet count to rise in response to thrombocytopenic stimuli. Despite much progress during the 1980s in the purification and characterization of the humoral regulators of lymphocyte, erythrocyte, monocyte and granulocyte production, the successful search to purify and molecularly clone thrombopoietin did not begin until the oncogene v-mpl was discovered in 1990. Since that time the proto-oncogene c-mpl was identified and, based on homology arguments, believed to encode a hematopoietic cytokine receptor, a hypothesis later proven when the cytoplasmic domain was linked to the ligand binding domain of the IL-4 receptor and shown to support the IL-4 induced growth of hematopoietic cells (Skoda et al., 1993). Finally, two different strategies using c-mpl lead to the identification of a novel ligand for the receptor in 1994 (de Sauvage et al., 1994; Lok et al., 1994; Bartley et al., 1994), a protein that displays all the biologic properties of TPO. This review attempts to distill what has been learned of the molecular and cellular biology of TPO and its receptor during the past several years, and links this information to several new insights into human disease and its treatment.

Elevation of serum thrombopoietin precedes thrombocytosis in acute infections

To clarify the mechanisms underlying thrombocytosis secondary to infections, we longitudinally studied serum levels of thrombopoietin (TPO) and interleukin (IL)-6 in 15 infants and young children with prominent thrombocytosis (platelets >700 x 10(9)/l) following acute infections and 116 age-matched controls using an enzyme-linked immunosorbent assay. The subjects included nine patients with bacterial infections, three with viral infections and three with non-determined pathogens. TPO values in the controls were 2.24 +/- 0.87 fmol/ml (mean +/- SD) with a 95% reference interval of 0.85-4.47 fmol/ml. In the first week of infection, platelet counts were normal, but TPO values increased (approximately 10.73 fmol/ml). TPO levels peaked on day 4 +/- 2 at 6.44 +/- 2.37 fmol/ml and then fell gradually. When platelet counts peaked in the second and third weeks, TPO levels were similar to the controls. IL-6 levels in the first week rose and dropped more rapidly than TPO. Serum TPO values were significantly correlated with C-reactive protein levels (r = 0.688, P < 0.001) and IL-6 levels (r = 0.481, P = 0.027). These results suggest that TPO contributes to thrombocytosis following infections in conjunction with IL-6, arguing for additional regulatory mechanisms of blood TPO levels.

Circulating thrombopoietin in clonal versus reactive thrombocytosis

INTRODUCTION

The aim of this study is to assess circulating thrombopoietin concentrations in patients with both clonal and reactive thrombocytosis (RT), which are two distinct categories of extreme platelet production circumstances. Investigation of the thrombopoietin levels in clonal versus reactive thrombocytosis may help us to understand the interactions of this key regulatory cytokine and the conditions in which abnormally increased platelet formation exist.

MATERIALS AND METHODS

Thrombopoietin levels were measured in patients with platelet counts greater than 500 x1 0(3) microl(-1). The study population consisted of 21 patients with RT (13 with iron deficiency anemia, and 8 with rheumatoid arthritis), 24 patients with clonal thrombocytosis (six with essential thrombocytosis, three with myelofibrosis, eight with chronic myelogenous leukemia, and seven with polycythemia vera (PV)) and 16 healthy subjects were used as controls.

RESULTS

The median plasma thrombopoietin concentration was 100.5 pg ml(-1) in patients with RT, 467 pg ml(-1) in patients with clonal thrombocytosis and 62.65 pg ml(-1) in the control group. The thrombopoietin concentration was found to be higher in the patients with primary thrombocytosis when compared to the control group (p=0.001), as well as in patients with RT (p=0.002). However, there was no statistically significant difference between the patients with RT and the control group (p=0.14). There was no correlation between thrombopoietin levels and the platelet counts in patients with clonal thrombocytosis, including essential thrombocythemia (ET). CONCLUSIION: Increased levels of thrombopoietin were found in patients with clonal thrombocytosis versus patients with RT and control subjects as well. Defective clearance of thrombopoietin by megakaryocytes and platelets due to a reduced number of thrombopoietin receptors may be the causative mechanism behind this. These results indicate that plasma thrombopoietin levels may be helpful in distinguishing between clonal and reactive thrombocytosis.

Early elevation of serum thrombopoietin levels and subsequent thrombocytosis in healthy preterm infants

To verify pathophysiological mechanisms underlying thrombocytosis in low-birth-weight (LBW) preterm babies, we evaluated kinetic changes in platelet counts and thrombopoietic cytokines including thrombopoietin (TPO), interleukin 6 (IL-6) and IL-11 in 24 uncomplicated preterm infants. Platelet counts in cord blood (CB) (265 +/- 64 x 10(9)/l) were similar to adult levels, increased by d 14 (473 +/- 140 x 10(9)/l), and then remained fairly constant. Thrombocytosis (> 500 x 10(9)/l) was observed in 9/24 (38%) subjects. Mean TPO level in CB was 5.11 +/- 1.51 fmol/ml, peaked at d 2 (7.64 +/- 3.28 fmol/ml), decreased at d 5 (3.93 +/- 1.67 fmol/ml), and thereafter kept fairly constant during the remaining neonatal period. Compared with term infants, mean TPO levels of preterm infants in CB and at d 2 were significantly higher (P < 0.01). There was an inverse correlation between platelet counts and TPO levels (r = 0.45, P < 0.001, n = 88). Preterm neonates with thrombocytosis had significantly higher TPO values in CB than those without thrombocytosis (P < 0.05). There was no significant relationship between platelet counts and IL-6. IL-11 was not detectable. These results suggest that an early elevation of serum TPO levels is related to the subsequent thrombocytosis in LBW preterm infants.

Current opinion in essential thrombocythemia: pathogenesis, diagnosis, and management

A working diagnosis of essential thrombocythemia (ET) is made in the presence of nonreactive thrombocytosis and after the exclusion of another chronic myeloid disorder that may mimic ET in its presentation. Clinically, ET is characterized by vasomotor symptoms, thrombohemorrhagic complications, recurrent fetal loss, and transformation of the disease into either myelofibrosis with myeloid metaplasia or acute myeloid leukemia. Median survival in the majority of patients is close to that of an age-adjusted normal population, and current therapy has not been shown to either retard or hasten leukemic transformation, which is reported to occur in 1% to 20% of patients. The use of hydroxyurea in high-risk patients with ET has reduced the incidence of thrombosis, and recent studies have suggested the value of keeping the platelet count below 400 x 10(9)/L in such cases. The incidence of thrombosis in low-risk patients may not be high enough to warrant the use of cytoreductive therapy. Although effective in controlling vasomotor symptoms, aspirin therapy has not been shown to influence the risk of either recurrent thrombosis or first-trimester miscarriage in ET. Recent laboratory studies have suggested that hematopoiesis in ET may not always be clonal. Similarly, there is substantial heterogeneity in both megakaryocyte/platelet surface expression of the thrombopoietin receptor (c-Mpl) and bone marrow microvessel density. Clinicopathologic correlates to these biologic parameters are currently being defined.

The role of thrombopoietin in post-operative thrombocytosis

Thrombopoietin (Tpo), the main regulator of thrombocytopoiesis, is a probable candidate to play a role in the increase in platelet counts that is frequently seen after surgery. In the current study, serial blood samples of patients that underwent major surgery were analysed with respect to Tpo kinetics, platelet turnover and inflammatory cytokines. Platelet Tpo content and plasma Tpo levels rose before platelet counts increased, suggesting that Tpo was indeed responsible for the elevation in platelet counts. In addition, an increase in interleukin 6 (IL-6) levels, but not in IL-11 and tumour necrosis factor alpha levels, was seen before the rise in Tpo concentration. In vitro, IL-6 was shown to enhance Tpo production by the HepG2 liver cell line. Thus, increased Tpo levels after surgery, possibly resulting from enhanced Tpo production under the influence of IL-6 or other inflammatory cytokines, are involved in an enhanced thrombocytopoiesis.

The role of the liver in the production of thrombopoietin compared with erythropoietin

The liver plays an important role in the production of haemopoietic hormones. It acts as the primary site of synthesis of erythropoietin (EPO) in the fetal stage, and it is the predominant thrombopoietin (TPO)-producing organ for life. In contrast to that of EPO and other liver proteins, the hepatic synthesis of TPO is influenced little by external signals. Hepatocytes express the TPO gene in a constitutive way, i.e. irrespective of the level of platelets in blood. Megakaryocytes and platelets remove the hormone from blood by means of their high-affinity TPO receptors. Normally, the plasma level of TPO is relatively low ( approximately 10(-12) mol/l). However, in thrombocytopenic states due to marrow failure or bleeding, the concentration of circulating TPO may increase greatly. The simple feedback regulation by TPO and its target cells is efficient in maintaining constant platelet numbers in healthy people. Persisting thrombocytopenia develops only in severe liver or marrow failure. On the other hand, an increase in circulating TPO and interleukin 6 (IL-6) may cause reactive thrombocytosis in inflammatory diseases, including cancer. The indications for recombinant human thrombopoietin (rHuTPO) therapy and its impact on transfusion medicine are still under investigation.

The relation between plasma thrombopoietin and erythropoietin concentrations in polycythaemia vera and essential thrombocythaemia

Plasma thrombopoietin (TPO) was measured, by immunoenzymometric assay, in 39 patients with polycythaemia vera (PV), 33 patients with essential thrombocythaemia (ET) and 10 healthy volunteers. The mean TPO concentration was significantly higher in ET patients than in PV patients (p=0.04) and normals (p<0.001). The 6 untreated ET patients had a significantly lower mean TPO concentration compared to the 27 ET patients who were on myelosuppressive regimens (p=0.01). The mean plasma TPO for the 5 PV patients treated with phlebotomy only did not differ significantly from the corresponding mean for the 34 PV patients treated with myelosuppressive agents. Concomitantly, plasma EPO was measured in 25 of the PV patients and in 30 of the ET patients by an immunoradiometric assay with normal reference interval in adults 3.7-16 IU/L. In the 14 PV patients with EPO <3.7 IU/L mean plasma TPO did not differ significantly from the mean for the 11 PV patients with EPO >or=3.7 IU/L; neither of these two groups had plasma TPO concentrations significantly different from the mean for the control subjects. The 7 ET patients with subnormal plasma EPO had significantly lower mean plasma TPO compared to the ET patients with normal and high plasma EPO concentrations (p=0.03 and p=0.02, respectively). Also, the 16 ET patients with normal plasma EPO had significantly lower plasma TPO compared to the 8 patients with high plasma EPO (p=0.04). The mean plasma TPO for each of these three groups of ET patients was significantly higher than the corresponding mean for the controls (p<0.001 for each group). The results of the present study indicate that a relationship between plasma EPO and TPO concentrations may exist and that myelosuppressive treatment affects the TPO concentration in ET but not in PV patients.

Measurement of thrombopoietic levels: clinical and biological relationships

Platelet production is primarily regulated by the thrombopoietic cytokine thrombopoietin (TPO). In most cases thrombopoietin serum levels are determined by the rate of c-mpl receptor-mediated degradation after TPO uptake into platelets and megakaryocytes. The contribution of increased TPO protein synthesis by a translational mechanism was recently appreciated as the cause for hereditary thrombocythemia and will have to be elucidated in other conditions of thrombocytosis in association with increased TPO levels.

Serum erythropoietin and thrombopoietin levels in patients with essential thrombocythaemia

In 40 patients with essential thrombocythaemia (ET) serum erythropoietin (EPO) and thrombopoietin (TPO) concentrations were determined and compared with the EPO and TPO values of a healthy control group. The mean EPO serum concentration for 24 control patients was 9.4 mU/ml +/- 3.7 (range 2-17.9), for 32 untreated ET patients at diagnosis 6.6 mU/ml +/- 7.6 (range 0.5-44.3) and for 8 ET patients treated with cytoreduction 14.1 mU/ml +/- 8.0 (range 4.5-26.1). Serum EPO levels in untreated ET patients at diagnosis were significantly lower compared with serum EPO levels in healthy control patients (p=0.002). Serum EPO levels in treated ET patients were not different from serum EPO levels in healthy controls (p=0.13) but were significantly higher compared with untreated ET patients (p=0.003). Serum TPO levels were determined in 18 of 40 ET patients, the mean TPO serum concentration was 211 pg/ml +/- 109 (range 62,5-345). The mean TPO serum concentration for 10 untreated ET patients at diagnosis was 162 pg/ml +/- 87 (range 62,5-302) and for 8 ET patients who had received cytoreductive treatment 272 pg/ml +/- 106 (range 96-345), respectively (p=0.04). Both serum TPO levels for treated and untreated ET patients were significantly higher (p<0.001) compared with serum TPO levels for healthy controls. The results of our study suggest a difference in the regulation of serum EPO and TPO in patients with ET. While the mean serum EPO level is decreased in untreated ET patients, the corresponding mean serum TPO level is increased. Treatment with cytoreduction, results in normalisation of the mean serum EPO level, whereas the mean TPO serum level remains elevated.