Human plasma thrombopoietin levels are regulated by binding to platelet thrombopoietin receptors in vivo

The GPIb-IX complex on platelets: insight into its novel physiological functions affecting immune surveillance, hepatic thrombopoietin generation, platelet clearance and its relevance for cancer development and metastasis

The glycoprotein (GP) Ib-IX complex is a platelet receptor that mediates the initial interaction with subendothelial von Willebrand factor (VWF) causing platelet arrest at sites of vascular injury even under conditions of high shear. GPIb-IX dysfunction or deficiency is the reason for the rare but severe Bernard-Soulier syndrome (BSS), a congenital bleeding disorder. Although knowledge on GPIb-IX structure, its basic functions, ligands, and intracellular signaling cascades have been well established, several advances in GPIb-IX biology have been made in the recent years. Thus, two mechanosensitive domains and a trigger sequence in GPIb were characterized and its role as a thrombin receptor was deciphered. Furthermore, it became clear that GPIb-IX is involved in the regulation of platelet production, clearance and thrombopoietin secretion. GPIb is deemed to contribute to liver cancer development and metastasis. This review recapitulates these novel findings highlighting GPIb-IX in its multiple functions as a key for immune regulation, host defense, and liver cancer development.

Treatment Options for Thrombocytopenia in Patients With Chronic Liver Disease Undergoing a Scheduled Procedure

Thrombocytopenia is a consequence of portal hypertension and is the most common hematological manifestation of chronic liver disease (CLD) (ie, cirrhosis). Data indicates the rates of CLD are increasing and, as a result, so will the incidence of this complication. Although bleeding risks are only relevant when elective procedures are performed, this is a frequent concern as these procedures are commonly part of the spectrum of care for patients with cirrhosis. As such, thrombocytopenia remains a pertinent issue. Fortunately, we now have effective and accurate treatment modalities to raise platelet counts before scheduled procedures, known as thrombopoietin receptor agonists. Two drugs in this therapeutic class (avatrombopag and lusutrombopag) are now approved for the treatment of thrombocytopenia in adults with CLD undergoing a procedure and have revolutionized how this is managed. Although there is progress in the field, peer-reviewed literature and expert guidance are lacking. Recognizing these unmet needs, a group of expert hepatologists comprised this review, which summarizes the most current and relevant peer-reviewed literature on thrombocytopenia in CLD and provides clinical expertise on this timely topic.

Circulating thrombopoietin levels in normal healthy blood donors and in aplastic anemia patients in relation to disease severity

Background:

Thrombopoietin (TPO) is the key hematopoietic growth factor regulating the production of platelets from bone marrow megakaryocytes and maintaining platelet hemostasis. This study was done to find any relationship between the levels of thrombopoietin and the severity of disease in patients with aplastic anemia.

Materials and Methods:

Serum samples were collected from 52 patients with a confirmed diagnosis of aplastic anemia and 45 normal healthy blood donors of both sexes over a period of 2 years, and TPO was estimated by using commercially available TPO-specific-enzyme-linked immunosorbent assay.

Results:

The median TPO level of 1190 pg/ml (range 625-7651 pg/ml) in aplastic anemia patients was significantly higher than the median TPO level of 121.1 pg/ml (81.25-237.7 pg/ml) in normal healthy blood donors (P = 0.000). No significant difference was observed in TPO levels of male and female patients (P = 0.453). The median TPO concentrations observed in very severe aplastic anemia, severe aplastic anemia, and nonsevere aplastic anemia were 2765 pg/ml (range 625-6451 pg/ml), 1190 pg/ml (range 672.1-7651 pg/ml), and 1111.5 pg/ml (range 761.1-2289.2 pg/ml), respectively. TPO in patients of very severe aplastic anemia was significantly higher than patients of nonsevere aplastic anemia (P = 0.043), with no significant relation among rest of the groups.

Discussion:

TPO levels in aplastic anemia patients were significantly higher than in healthy blood donors; however, in aplastic anemia patients TPO levels were significantly higher only in patients with very severe disease.

Severe Romiplostim-Induced Rebound Thrombocytopenia After Splenectomy for Refractory ITP

Objective: To report a case of severe rebound thrombocytopenia after temporary discontinuation of romiplostim during splenectomy in the context of refractory immune (idiopathic) thrombocytopenic purpura (ITP). Case Summary: A 65-year-old man with a history of severe refractory ITP failing multiple treatments was considered for romiplostim therapy. He was initiated on 1 µg/kg and titrated upward to 4 µg/kg to elevate and stabilize his platelet levels prior to splenectomy. On day 74 of his clinical course, his platelets increased to 434 × 109/L, and his scheduled dose of romiplostim was withheld on day 75 for fear of romiplostim-induced postsplenectomy rebound thrombocytosis. On day 78, his platelets dropped precipitously to 9 × 109/L, and he experienced multiple episodes of epistaxis. He was reinitiated at 5 µg/kg and soon recovered. He later missed a scheduled dose of romiplostim, and his platelets fell to 23 × 109/L. After resuming romiplostim at 8 µg/kg, his platelets continued to recover. Discussion: Romiplostim, a thrombopoietin mimetic is directly regulated by megakaryocytes and existing circulating platelets via a negative feedback mechanism. This explains the theoretical risk of rapid clearance of romiplostim caused by an increased platelet pool. Clinically, alternative causes of his severe postoperative thrombocytopenia were considered and deemed unlikely. The rebound effect was observed after romiplostim was withdrawn on 2 occasions, and platelet counts improved after restarting romiplostim. The Naranjo Adverse Drug Reaction Probability Score of 7 suggests a probable adverse drug reaction. Conclusion: Physicians using romiplostim as a bridge to splenectomy should be cautious about withholding a scheduled dose around the time of surgery.

Thrombopoietin levels in systemic lupus erythematosus are linked to inflammatory cytokines, but unrelated to thrombocytopenia or thrombosis

BACKGROUND

Thrombopoietin (TPO) is a liver-produced protein that drives megakaryocyte maturation. TPO regulates platelet production and can increase platelet and endothelial reactivity. We investigated the relationship between TPO and the occurrence of thrombocytopenia and thrombosis in patients with systemic lupus erythematosus (SLE).

METHODS

We undertook a cohort study of SLE patients (n = 98) with clinical data collected simultaneously with sampling for TPO, inflammatory cytokines and autoantibody detection. TPO levels were measured by sandwich ELISA with patients with rheumatoid arthritis (RA) (n = 100) and controls (n = 79) as comparators. Disease associations were evaluated using non-parametric methods.

RESULTS

TPO levels in SLE (median 8 pg/ml, mean 326, range 8992) were moderately increased compared with RA (median 8 pg/ml, mean 100, range 1659, p = 0.07) and controls (median 8, mean 94, range 2088, p = 0.1). Among SLE patients, TPO levels did not correlate with platelet count or levels of antiphospholipid antibodies. The prevalence of thrombocytopenic episodes, thrombotic events or active disease was not increased in patients with high TPO levels. TPO levels correlated with MIP-1α (Rs 0.56, p < 0.001), IL6 (Rs 0.26, p = 0.02) and IL4 (Rs 0.29, p = 0.01), and inversely correlated to C4 (Rs -0.23, p = 0.04). MIP-1α was the strongest independent predictor of increased TPO levels.

CONCLUSION

TPO levels are elevated in 20% of patients, but are not closely related to the occurrence of thrombocytopenia or thrombosis in SLE. MIP1-alpha is the main factor driving higher TPO levels among patients with SLE, likely through its inhibitory effect on megakaryocyte function.

Advances in megakaryocytopoiesis and thrombopoiesis: from bench to bedside

Megakaryocytopoiesis involves the commitment of haematopoietic stem cells, proliferation and terminal differentiation of megakaryocytic progenitors (MK-p) and maturation of megakaryocytes (MKs) to produce functional platelets. This complex process occurs in specialized niches in the bone marrow where MKs align adjacent to vascular endothelial cells, form proplatelet projections and release platelets into the circulation. Thrombopoietin (THPO, TPO) is the primary growth factor for the MK lineage and necessary at all stages of development. THPO is constitutively produced in the liver, and binds to MPL (c-Mpl) receptor on platelets and MKs. This activates a cascade of signalling molecules, which induce transcription factors to drive MK development and thrombopoiesis. Decreased turnover rate and platelet number result in increased levels of free THPO, which induces a concentration-dependent compensatory response of marrow-MKs to enhance platelet production. Newly developed thrombopoietic agents operating via MPL receptor facilitate platelet production in thrombocytopenic states, primarily immune thrombocytopenia. Other drugs are available for attenuating malignant thrombocytosis. Herein, we review the regulation of megakaryocytopoiesis and platelet production in normal and disease states, and the innovative drugs and therapeutic modalities to stimulate or decrease thrombopoiesis.

Steady-state neutrophil homeostasis is dependent on TLR4/TRIF signaling

Steady-state and emergency granulopoiesis are both dependent on TLR signaling.

Thrombopoietin and platelet production in chronic immune thrombocytopenia

Since 1968, a greater understanding of platelet biology and its regulation by thrombopoietin (TPO) has emerged. It is now recognized that immune thrombocytopenic purpura (ITP) is a disorder of reduced platelet production as well as increased platelet destruction. New therapies for ITP have emerged that have exploited this new pathophysiologic understanding. This article reviews the biology of TPO, the regulation of its circulating level in ITP, the platelet kinetic data supporting inappropriate platelet production in ITP, and the TPO mimetic agents available to treat ITP.

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.

Incomplete restoration of Mpl expression in the mpl-/- mouse produces partial correction of the stem cell-repopulating defect and paradoxical thrombocytosis

Expression of Mpl is restricted to hematopoietic cells in the megakaryocyte lineage and to undifferentiated progenitors, where it initiates critical cell survival and proliferation signals after stimulation by its ligand, thrombopoietin (TPO). As a result, a deficiency in Mpl function in patients with congenital amegakaryocytic thrombocytopenia (CAMT) and in mpl(-/-) mice produces profound thrombocytopenia and a severe stem cell-repopulating defect. Gene therapy has the potential to correct the hematopoietic defects of CAMT by ectopic gene expression that restores normal Mpl receptor activity. We rescued the mpl(-/-) mouse with a transgenic vector expressing mpl from the promoter elements of the 2-kb region of DNA just proximal to the natural gene start site. Transgene rescued mice exhibit thrombocytosis but only partial correction of the stem cell defect. Furthermore, they show very low-level expression of Mpl on platelets and megakaryocytes, and the transgene-rescued megakaryocytes exhibit diminished TPO-dependent kinase phosphorylation and reduced platelet production in bone marrow chimeras. Thrombocytosis is an unexpected consequence of reduced Mpl expression and activity. However, impaired TPO homeostasis in the transgene-rescued mice produces elevated plasma TPO levels, which serves as an unchecked stimulus to drive the observed excessive megakaryocytopoiesis.

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.

AMG 531, a thrombopoiesis-stimulating protein, for chronic ITP

BACKGROUND

Most current treatments for chronic immune thrombocytopenic purpura (ITP) act by decreasing platelet destruction. In a phase 1-2 study, we administered a thrombopoiesis-stimulating protein, AMG 531, to patients with ITP.

METHODS

In phase 1, 24 patients who had received at least one treatment for ITP were assigned to escalating-dose cohorts of 4 patients each and given two identical doses of AMG 531 (0.2 to 10 microg per kilogram of body weight). In phase 2, 21 patients were randomly assigned to receive six weekly subcutaneous injections of AMG 531 (1, 3, or 6 microg per kilogram) or placebo. The primary objective was to assess the safety of AMG 531; the secondary objective was to evaluate platelet counts during and after treatment.

RESULTS

No major adverse events that could be attributed directly to AMG 531 occurred during the treatment period; 4 of 41 patients had transient post-treatment worsening of thrombocytopenia. In phase 1, a platelet count that was within the targeted range (50,000 to 450,000 per cubic millimeter) and at least twice the baseline count was achieved in 4 of 12 patients given 3, 6, or 10 mug of AMG 531 per kilogram. Overall, a platelet count of at least 50,000 per cubic millimeter was achieved in 7 of 12 patients, including 3 with counts exceeding 450,000 per cubic millimeter. Increases in the platelet count were dose-dependent; mean peak counts were 163,000, 309,000, and 746,000 per cubic millimeter with 3, 6, and 10 microg of AMG 531 per kilogram [corrected], respectively. In phase 2, the targeted platelet range was achieved in 10 of 16 patients treated with 1 or 3 mug of AMG 531 per kilogram per week for 6 weeks. Mean peak counts were 135,000, 241,000, and 81,000 per cubic millimeter in the groups that received the 1-mug dose, the 3-mug dose, and placebo, respectively.

CONCLUSIONS

AMG 531 caused no major adverse events and increased platelet counts in patients with ITP. (ClinicalTrials.gov number, NCT00111475 [ClinicalTrials.gov].).

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.

Recombinant human thrombopoietin: basic biology and evaluation of clinical studies

Thrombocytopenia is a common medical problem for which the main treatment is platelet transfusion. Given the increasing use of platelets and the declining donor population, identification of a safe and effective platelet growth factor could improve the management of thrombocytopenia. Thrombopoietin (TPO), the c-Mpl ligand, is the primary physiologic regulator of megakaryocyte and platelet development. Since the purification of TPO in 1994, 2 recombinant forms of the c-Mpl ligand--recombinant human thrombopoietin (rhTPO) and pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF)--have undergone extensive clinical investigation. Both have been shown to be potent stimulators of megakaryocyte growth and platelet production and are biologically active in reducing the thrombocytopenia of nonmyeloablative chemotherapy. However, neither TPO has demonstrated benefit in stem cell transplantation or leukemia chemotherapy. Other clinical studies have investigated the use of TPO in treating chronic nonchemotherapy-induced thrombocytopenia associated with myelodysplastic syndromes, idiopathic thrombocytopenic purpura, thrombocytopenia due to human immunodeficiency virus, and liver disease. Based solely on animal studies, TPO may be effective in reducing surgical thrombocytopenia and bleeding, ex vivo expansion of pluripotent stem cells, and as a radioprotectant. Ongoing and future studies will help define the clinical role of recombinant TPO and TPO mimetics in the treatment of chemotherapy- and nonchemotherapy-induced thrombocytopenia.