Thrombopoietin: a tool for understanding thrombopoiesis

Safety and efficacy of eltrombopag in patients with aplastic anemia: a systematic review and meta-analysis of randomized controlled trials

OBJECTIVE

This article conducts a systematic review of eltrombopag combined with immunosuppressive therapy for the treatment of aplastic anemia (AA), to demonstrate the effectiveness and safety of eltrombopag.

METHODS

PubMed, Cochrane Library, Embase, OVID, Web of Science, China National Knowledge Infrastructure, and Wanfang databases were searched. Studies that met the inclusion criteria were collected, ranging from the establishment of the database to August 2023. Two reviewers performed meta-analyses using the Cochrane systematic review method and RevMan 5.3 software.

RESULTS

This meta-analysis enrolled 5 studies with a total of 542 AA patients, including 274 in the experimental group and 268 in the control group. Meta-analyses were performed for efficacy and adverse reactions. The endpoint of effects included 6-month complete response (CR), 6-month partial response (PR), and 6-month overall response (OR). Eltrombopag combined with immunotherapy showed significant improvements in 6-month CR (OR: 2.20; 95% CI;1.54-3.12; P < 0.0001) and 6-month OR (OR = 3.66, 95% CI 2.39-5.61, P < 0.001)compared to immunosuppressive therapy for AA patients. In terms of safety, eltrombopag combined with immunosuppressive therapy showed significantly increased pigment deposition and abnormal liver function compared to immunosuppressive therapy alone.

CONCLUSION

Compared to immunosuppressive therapy alone, eltrombopag combined with immunosuppressive therapy showed significant improvements in 6-month CR and 6-month OR. However, it also resulted in increased pigment deposition and abnormal liver function in terms of safety.

RepID represses megakaryocytic differentiation by recruiting CRL4A-JARID1A at DAB2 promoter

BACKGROUND

Megakaryocytes (MKs) are platelet precursors, which arise from hematopoietic stem cells (HSCs). While MK lineage commitment and differentiation are accompanied by changes in gene expression, many factors that modulate megakaryopoiesis remain to be uncovered. Replication initiation determinant protein (RepID) which has multiple histone-code reader including bromodomain, cryptic Tudor domain and WD40 domains and Cullin 4-RING E3 ubiquitin ligase complex (CRL4) recruited to chromatin mediated by RepID have potential roles in gene expression changes via epigenetic regulations. We aimed to investigate whether RepID-CRL4 participates in transcriptional changes required for MK differentiation.

METHODS

The PCR array was performed using cDNAs derived from RepID-proficient or RepID-deficient K562 erythroleukemia cell lines. Correlation between RepID and DAB2 expression was examined in the Cancer Cell Line Encyclopedia (CCLE) through the CellMinerCDB portal. The acceleration of MK differentiation in RepID-deficient K562 cells was determined by estimating cell sizes as well as counting multinucleated cells known as MK phenotypes, and by qRT-PCR analysis to validate transcripts of MK markers using phorbol 12-myristate 13-acetate (PMA)-mediated MK differentiation condition. Interaction between CRL4 and histone methylation modifying enzymes were investigated using BioGRID database, immunoprecipitation and proximity ligation assay. Alterations of expression and chromatin binding affinities of RepID, CRL4 and histone methylation modifying enzymes were investigated using subcellular fractionation followed by immunoblotting. RepID-CRL4-JARID1A-based epigenetic changes on DAB2 promoter were analyzed by chromatin-immunoprecipitation and qPCR analysis.

RESULTS

RepID-deficient K562 cells highly expressing MK markers showed accelerated MKs differentiation exhibiting increases in cell size, lobulated nuclei together with reaching maximum levels of MK marker expression earlier than RepID-proficient K562 cells. Recovery of WD40 domain-containing RepID constructs in RepID-deficient background repressed DAB2 expression. CRL4A formed complex with histone H3K4 demethylase JARID1A in soluble nucleus and loaded to the DAB2 promoter in a RepID-dependent manner during proliferation condition. RepID, CRL4A, and JARID1A were dissociated from the chromatin during MK differentiation, leading to euchromatinization of the DAB2 promoter.

CONCLUSION

This study uncovered a role for the RepID-CRL4A-JARID1A pathway in the regulation of gene expression for MK differentiation, which can form the basis for the new therapeutic approaches to induce platelet production. Video Abstract.

Identification of novel clusters of co-expressing cytokines in a diagnostic cytokine multiplex test

Introduction

Cytokines are mediators of the immune system that are essential for the maintenance, development and resolution of immune responses. Beneficial immune responses depend on complex, interdependent networks of signaling and regulatory events in which individual cytokines influence the production and release of others. Since disruptions in these signaling networks are associated with a wide spectrum of diseases, cytokines have gained considerable interest as diagnostic, prognostic and precision therapy-relevant biomarkers. However, currently individual cytokines testing has limited value because the wider immune response context is often overlooked. The aim of this study was to identify specific cytokine signaling patterns associated with different diseases.

Methods

Unbiased clustering analyses were performed on a clinical cytokine multiplex test using a cohort of human plasma specimens drawn from individuals with known or suspected diseases for which cytokine profiling was considered clinically indicated by the attending physician.

Results and discussion

Seven clusters of co-expressing cytokines were identified, representing common patterns of immune activation. Common expression profiles of the cytokine clusters and preliminary associations of these profiles with specific diseases or disease categories were also identified. These findings increase our understanding of the immune environments underlying the clinical presentations of patients of inflammatory, autoimmune and neoplastic diseases, which could then improve diagnoses and the identification of evidence-based treatment targets.

RepID represses megakaryocytic differentiation by recruiting CRL4A-JARID1A at DAB2 promoter

Background Megakaryocytes (MKs) are platelet precursors, which arise from hematopoietic stem cells (HSCs). While MK lineage commitment and differentiation are accompanied by changes in gene expression, many factors that modulate megakaryopoiesis remain to be uncovered. Replication origin binding protein (RepID) which has multiple histone-code reader including bromodomain, cryptic Tudor domain and WD40 domains and Cullin 4-RING ubiquitin ligase complex (CRL4) recruited to chromatin mediated by RepID have potential roles in gene expression changes via epigenetic regulations. We aimed to investigate whether RepID-CRL4 participates in transcriptional changes required for MK differentiation. Methods The PCR array was performed using cDNAs derived from RepID-proficient or RepID-deficient K562 erythroleukemia cell lines. Correlation between RepID and DAB2 expression was examined in the Cancer Cell Line Encyclopedia (CCLE) through the CellMinerCDB portal. The acceleration of MK differentiation in RepID-deficient K562 cells was determined by estimating cell sizes as well as counting multinucleated cells known as MK phenotypes, and by qRT-PCR analysis to validate transcripts of MK markers using phorbol 12-myristate 13-acetate (PMA)-mediated MK differentiation condition. Interaction between CRL4 and histone methylation modifying enzymes were investigated using BioGRID database, immunoprecipitation and proximity ligation assay. Alterations of expression and chromatin binding affinities of RepID, CRL4 and histone methylation modifying enzymes were investigated using subcellular fractionation followed by immunoblotting. RepID-CRL4-JARID1A-based epigenetic changes on DAB2 promoter were analyzed by chromatin-immunoprecipitation and qPCR analysis. Results RepID-deficient K562 cells highly expressing MK markers showed accelerated MKs differentiation exhibiting increases in cell size, lobulated nuclei together with reaching maximum levels of MK marker expression earlier than RepID-proficient K562 cells. Recovery of WD40 domain-containing RepID constructs in RepID-deficient background repressed DAB2 expression. CRL4A formed complex with histone H3K4 demethylase JARID1A in soluble nucleus and loaded to the DAB2 promoter in a RepID-dependent manner during proliferation condition. RepID, CRL4A, and JARID1A were dissociated from the chromatin during MK differentiation, leading to euchromatinization of the DAB2 promoter. Conclusion This study uncovered a role for the RepID-CRL4A-JARID1A pathway in the regulation of gene expression for MK differentiation, which can form the basis for the new therapeutic approaches to induce platelet production.

Thrombopoietin participates in platelet activation in COVID-19 patients

BACKGROUND

The pathogenesis of coronavirus disease 2019 (COVID-19) is characterized by enhanced platelet activation and diffuse hemostatic alterations, which may contribute to immunothrombosis/thromboinflammation and subsequent development of target-organ damage. Thrombopoietin (THPO), a growth factor essential to megakariocyte proliferation, is known to prime platelet activation and leukocyte-platelet interaction. In addition, THPO concentrations increase in several critical diseases, such as acute cardiac ischemia and sepsis, thus representing a potential diagnostic and prognostic biomarker. Furthermore, several data suggest that interleukin (IL)-6 is one of the most important inflammatory mediators involved in these phenomena, which led to explore the potential therapeutic role of IL-6 inhibitors. In this prospective cohort study, we aimed to study THPO and IL-6 concentrations in COVID-19 patients at the time of first clinical evaluation in the Emergency Department (ED), and to investigate their potential use as diagnostic and prognostic biomarkers. In addition, we sought to explore the role of THPO contained in plasma samples obtained from COVID-19 patients in priming in vitro platelet activation and leukocyte-platelet interaction.

METHODS

We enrolled 66 patients presenting to the ED with symptoms suggestive of COVID-19, including 47 with confirmed COVID-19 and 19 in whom COVID-19 was excluded (Non-COVID-19 patients). As controls, we also recruited 18 healthy subjects. In vitro, we reproduced the effects of increased circulating THPO on platelet function by adding plasma from COVID-19 patients or controls to platelet-rich plasma or whole blood obtained by healthy donors, and we indirectly studied the effect of THPO on platelet activation by blocking its biological activity.

FINDINGS

THPO levels were higher in COVID-19 patients than in both Non-COVID-19 patients and healthy subjects. Studying THPO as diagnostic marker for the diagnosis of COVID-19 by receiver-operating-characteristic (ROC) statistics, we found an area under the curve (AUC) of 0.73, with an optimal cut-off value of 42.60 pg/mL. IL-6 was higher in COVID-19 patients than in healthy subjects, but did not differ between COVID-19 and Non-COVID-19 patients. THPO concentrations measured at the time of diagnosis in the ED were also higher in COVID-19 patients subsequently developing a severe disease than in those with mild disease. Evaluating THPO as biomarker for severe COVID-19 using ROC analysis, we found an AUC of 0.71, with an optimal cut-off value of 57.11 pg/mL. IL-6 was also higher in severe than in mild COVID-19 patients, with an AUC for severe COVID-19 of 0.83 and an optimal cut-off value of 23 pg/ml. THPO concentrations correlated with those of IL-6 (r=0.2963; p=0.043), and decreased 24 h after the administration of tocilizumab, an IL-6 receptor blocking antibody, showing that the increase of THPO levels depends on IL-6-stimulated hepatic synthesis. In vitro, plasma obtained from COVID-19 patients, but not from healthy subjects, primed platelet aggregation and leukocyte-platelet binding, and these effects were reduced by inhibiting THPO activity.

INTERPRETATION

Increased THPO may be proposed as an early biomarker for the diagnosis of COVID-19 and for the identification of patients at risk of developing critical illness. Elevated THPO may contribute to enhance platelet activation and leukocyte-platelet interaction in COVID-19 patients, thus potentially participating in immunothrombosis/thromboinflammation.

FUNDING

This work was supported by Ministero dell'Università e della Ricerca Scientifica e Tecnologica (MURST) ex 60% to GM and EL.

Insights Into Platelet-Derived MicroRNAs in Cardiovascular and Oncologic Diseases: Potential Predictor and Therapeutic Target

Non-communicable diseases (NCDs), represented by cardiovascular diseases and cancer, have been the leading cause of death globally. Improvements in mortality from cardiovascular (CV) diseases (decrease of 14%/100,000, United States) or cancers (increase 7.5%/100,000, United States) seem unsatisfactory during the past two decades, and so the search for innovative and accurate biomarkers of early diagnosis and prevention, and novel treatment strategies is a valuable clinical and economic endeavor. Both tumors and cardiovascular system are rich in angiological systems that maintain material exchange, signal transduction and distant regulation. This pattern determines that they are strongly influenced by circulating substances, such as glycolipid metabolism, inflammatory homeostasis and cyclic non-coding RNA and so forth. Platelets, a group of small anucleated cells, inherit many mature proteins, mRNAs, and non-coding RNAs from their parent megakaryocytes during gradual formation and manifest important roles in inflammation, angiogenesis, atherosclerosis, stroke, myocardial infarction, diabetes, cancer, and many other diseases apart from its classical function in hemostasis. MicroRNAs (miRNAs) are a class of non-coding RNAs containing ∼22 nucleotides that participate in many key cellular processes by pairing with mRNAs at partially complementary binding sites for post-transcriptional regulation of gene expression. Platelets contain fully functional miRNA processors in their microvesicles and are able to transport their miRNAs to neighboring cells and regulate their gene expression. Therefore, the importance of platelet-derived miRNAs for the human health is of increasing interest. Here, we will elaborate systematically the roles of platelet-derived miRNAs in cardiovascular disease and cancer in the hope of providing clinicians with new ideas for early diagnosis and therapeutic strategies.

Thrombopoietin and collagen in low doses cooperatively induce human platelet activation

Aim

In acute medicine, we occasionally treat life‐threatening conditions such as sepsis and trauma, which cause severe thrombocytopenia. Serum thrombopoietin levels have been reported to increase under the condition of thrombocytopenia related to severity. Collagen is a crucial activator of platelets, and Rho family members, such as Rho/Rho‐kinase and Rac, play roles as active molecules involved in the intracellular signaling pathways in platelet activation. The present study aimed to elucidate the effects of thrombopoietin (TPO) on subthreshold low‐dose collagen‐stimulated human platelets in terms of Rho/Rho‐kinase and Rac.

Methods

Platelet‐rich plasma donated from healthy volunteers was stimulated by the subthreshold low‐dose of collagen after pretreatment with TPO and/or NSC23766, an inhibitor of the Rac‐guanine nucleotide exchange factor interaction, or Y27632, an inhibitor of Rho‐kinase. Platelet aggregation was measured using an aggregometer based on laser‐scattering methods. Proteins involved in intracellular signaling were analyzed using western blotting, and the secretion of platelet‐derived growth factor‐AB from activated platelets was determined using an enzyme‐linked immunosorbent assay.

Results

Under the existence of TPO, the low dose of collagen remarkably elicited the aggregation and platelet‐derived growth factor‐AB secretion of platelets, which were suppressed by NSC23766 and Y27632. The combination of TPO and collagen considerably induced a transient increase of guanosine triphosphate (GTP)‐binding Rac and GTP‐binding Rho followed by an increase of phosphorylated cofilin, a Rho‐kinase substrate.

Conclusion

These results strongly suggest that TPO and collagen in low doses cooperatively potentiate human platelet activation through both Rac and Rho/Rho‐kinase mediated pathways.

Thrombopoietin Contributes to Enhanced Platelet Activation in Patients with Type 1 Diabetes Mellitus

Atherosclerotic cardiovascular disease is the major cause of morbidity and mortality in patients with type 1 diabetes mellitus (T1DM). Enhanced platelet reactivity is considered a main determinant of the increased atherothrombotic risk of diabetic patients. Thrombopoietin (THPO), a humoral growth factor able to stimulate megakaryocyte proliferation and differentiation, also modulates the response of mature platelets by enhancing both activation and binding to leukocytes in response to different agonists. Increased THPO levels have been reported in different clinical conditions characterized by a generalized pro-thrombotic state, from acute coronary syndromes to sepsis/septic shock, and associated with elevated indices of platelet activation. To investigate the potential contribution of elevated THPO levels in platelet activation in T1DM patients, we studied 28 T1DM patients and 28 healthy subjects. We measured plasma levels of THPO, as well as platelet-leukocyte binding, P-selectin, and THPO receptor (THPOR) platelet expression. The priming activity of plasma from diabetic patients or healthy subjects on platelet–leukocyte binding and the role of THPO on this effect was also studied in vitro. T1DM patients had higher circulating THPO levels and increased platelet–monocyte and platelet–granulocyte binding, as well as platelet P-selectin expression, compared to healthy subjects, whereas platelet expression of THPOR did not differ between the two groups. THPO concentrations correlated with platelet–leukocyte binding, as well as with fasting glucose and Hb1Ac. In vitro, plasma from diabetic patients, but not from healthy subjects, primed platelet–leukocyte binding and platelet P-selectin expression. Blocking THPO biological activity using a specific inhibitor prevented the priming effect induced by plasma from diabetic patients. In conclusion, augmented THPO may enhance platelet activation in patients with T1DM, potentially participating in increasing atherosclerotic risk.

Cytokine-mediated communication: a quantitative appraisal of immune complexity

Intercellular communication mediated by cytokines is the main mechanism by which cells of the immune system talk to each other. Many aspects of cytokine signalling in the immune system have been explored in great detail at the structural, biophysical, biochemical and cellular levels. However, a systematic understanding of the quantitative rules that govern cytokine-mediated cell-to-cell communication is still lacking. Here, we discuss recent efforts in the field of systems immunology to bring about a quantitative understanding of cytokine-mediated communication between leukocytes and to provide novel insights into the orchestration of immune responses and inflammation.

The BRISC deubiquitinating enzyme complex limits hematopoietic stem cell expansion by regulating JAK2 K63-ubiquitination

Hematopoietic stem cell (HSC) homeostasis is controlled by cytokine receptor-mediated Janus kinase 2 (JAK2) signaling. We previously found that JAK2 is promptly ubiquitinated upon cytokine stimulation. Whether a competing JAK2 deubiquitination activity exists is unknown. LNK is an essential adaptor protein that constrains HSC expansion through dampening thrombopoietin (TPO)-induced JAK2 signaling. We show here that a LNK-associated lysine-63 (K63)-deubiquitinating enzyme complex, Brcc36 isopeptidase complex (BRISC), attenuates HSC expansion through control of JAK2 signaling. We pinpoint a direct interaction between the LNK SH2 domain and a phosphorylated tyrosine residue in KIAA0157 (Abraxas2), a unique and defining BRISC component. Kiaa0157 deficiency in mice led to an expansion of phenotypic and functional HSCs. Endogenous JAK2 and phospho-JAK2 were rapidly K63-ubiquitinated upon TPO stimulation, and this action was augmented in cells depleted of the BRISC core components KIAA0157, MERIT40, or BRCC36. This increase in JAK2 ubiquitination after BRISC knockdown was associated with increased TPO-mediated JAK2 activation and protein levels, and increased MPL receptor presence at the cell surface. In addition, BRISC depletion promoted membrane proximal association between the MPL receptor and pJAK2/JAK2, thus enhancing activated JAK2/MPL at the cell membrane. These findings define a novel pathway by which K63-ubiquitination promotes JAK2 stability and activation in a proteasome-independent manner. Moreover, mutations in BRCC36 are found in clonal hematopoiesis in humans. This research may shed light on the mechanistic understanding of a potential role of BRCC36 in human HSCs.

Lnk/Sh2b3 deficiency restores hematopoietic stem cell function and genome integrity in Fancd2 deficient Fanconi anemia

Fanconi anemia (FA) is a bone marrow failure (BMF) syndrome that arises from mutations in a network of FA genes essential for DNA interstrand crosslink (ICL) repair and replication stress tolerance. While allogeneic stem cell transplantation can replace defective HSCs, interventions to mitigate HSC defects in FA do not exist. Remarkably, we reveal here that Lnk (Sh2b3) deficiency restores HSC function in Fancd2^−/− mice. Lnk deficiency does not impact ICL repair, but instead stabilizes stalled replication forks in a manner, in part, dependent upon alleviating blocks to cytokine−mediated JAK2 signaling. Lnk deficiency restores proliferation and survival of Fancd2^−/− HSCs, while reducing replication stress and genomic instability. Furthermore, deletion of LNK in human FA-like HSCs promotes clonogenic growth. These findings highlight a new role for cytokine/JAK signaling in promoting replication fork stability, illuminate replication stress as a major underlying origin of BMF in FA, and have strong therapeutic implications.

Loss of Fancd2 leads to replication stress intolerance and Fanconi Anemia, where haematopoietic stem cell (HSC) function is compromised. Here, the authors show that Lnk/Sh2b3 loss restores HSC proliferation and survival in Fancd2 knockout mice and ameliorates replication stress in a cytokine/JAK2 signaling dependent manner.

Thrombopoietin knock-in augments platelet generation from human embryonic stem cells

Background

Refinement of therapeutic-scale platelet production in vitro will provide a new source for transfusion in patients undergoing chemotherapy or radiotherapy. However, procedures for cost-effective and scalable platelet generation remain to be established.

Methods

In this study, we established human embryonic stem cell (hESC) lines containing knock-in of thrombopoietin (TPO) via CRISPR/Cas9-mediated genome editing. The expression and secretion of TPO was detected by western blotting and enzyme-linked immunosorbent assay. Then, we tested the potency for hematopoietic differentiation by coculturing the cells with mAGM-S3 cells and measured the generation of CD43⁺ and CD45⁺ hematopoietic progenitor cells (HPCs). The potency for megakaryocytic differentiation and platelet generation of TPO knock-in hESCs were further detected by measuring the expression of CD41a and CD42b. The morphology and function of platelets were analyzed with electronic microscopy and aggregation assay.

Results

The TPO gene was successfully inserted into the AAVS1 locus of the hESC genome and two cell lines with stable TPO expression and secretion were established. TPO knock-in exerts minimal effects on pluripotency but enhances early hematopoiesis and generation of more HPCs. More importantly, upon its knock-in, TPO augments megakaryocytic differentiation and platelet generation. In addition, the platelets derived from hESCs in vitro are functionally and morphologically comparable to those found in peripheral blood. Furthermore, TPO knock-in can partially replace the large quantities of extrinsic TPO necessary for megakaryocytic differentiation and platelet generation.

Conclusions

Our results demonstrate that autonomous production of cytokines in hESCs may become a powerful approach for cost-effective and large-scale platelet generation in translational medicine.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0926-x) contains supplementary material, which is available to authorized users.

CBL family E3 ubiquitin ligases control JAK2 ubiquitination and stability in hematopoietic stem cells and myeloid malignancies

Here, Lv et al. report that the CBL family E3 ubiquitin ligases down-regulate JAK2 stability and signaling via the adaptor protein LNK/SH2B3. Their results reveal a novel signaling axis that regulates JAK2 in normal and malignant HSPCs and suggest new therapeutic strategies for treating CBL^mut myeloid malignancies.

Janus kinase 2 (JAK2) is a central kinase in hematopoietic stem/progenitor cells (HSPCs), and its uncontrolled activation is a prominent oncogenic driver of hematopoietic neoplasms. However, molecular mechanisms underlying the regulation of JAK2 have remained elusive. Here we report that the Casitas B-cell lymphoma (CBL) family E3 ubiquitin ligases down-regulate JAK2 stability and signaling via the adaptor protein LNK/SH2B3. We demonstrated that depletion of CBL/CBL-B or LNK abrogated JAK2 ubiquitination, extended JAK2 half-life, and enhanced JAK2 signaling and cell growth in human cell lines as well as primary murine HSPCs. Built on these findings, we showed that JAK inhibitor (JAKi) significantly reduced aberrant HSPCs and mitigated leukemia development in a mouse model of aggressive myeloid leukemia driven by loss of Cbl and Cbl-b. Importantly, primary human CBL mutated (CBL^mut) leukemias exhibited increased JAK2 protein levels and signaling and were hypersensitive to JAKi. Loss-of-function mutations in CBL E3 ubiquitin ligases are found in a wide range of myeloid malignancies, which are diseases without effective treatment options. Hence, our studies reveal a novel signaling axis that regulates JAK2 in normal and malignant HSPCs and suggest new therapeutic strategies for treating CBL^mut myeloid malignancies.

Thrombopoietin as Early Biomarker of Disease Severity in Patients With Acute Pancreatitis

OBJECTIVES

To study the concentrations of thrombopoietin (TPO), a growth factor recently involved in the pathogenesis of experimental acute pancreatitis (AP), and its potential role as an early diagnostic and prognostic biomarker in patients with AP.

METHODS

Thrombopoietin was measured in 44 AP patients, 18 patients with nonpancreatic acute abdominal pain, and 18 healthy volunteers. Acute pancreatitis severity was classified on the basis of the 2012 International Atlanta Symposium on Acute Pancreatitis criteria.

RESULTS

Thrombopoietin levels did not differ between AP patients and control subjects, whereas these were higher in patients with moderately severe or severe AP compared with those with mild AP. Receiver operating characteristic curve analysis of TPO for severe AP diagnosis showed an area under the curve of 0.80. A cutoff value of 31.48 pg/mL showed the highest sensitivity, allowing to rule out severe AP when TPO was lower, whereas TPO higher than 98.23 pg/mL was associated with severe AP with high specificity (93.5%). Furthermore, TPO levels were greater in AP patients developing organ dysfunction or sepsis and in nonsurvivors compared with survivors.

CONCLUSIONS

Our data provide the first evidence for TPO as potential early prognostic biomarker in AP patients. High TPO levels at hospital admission may predict organ dysfunction, sepsis, and fatal outcome in AP patients.

Blockade of Thrombopoietin Reduces Organ Damage in Experimental Endotoxemia and Polymicrobial Sepsis

Background and Purpose

Thrombopoietin (TPO), a growth factor primarily involved in thrombopoiesis may also have a role in the pathophysiology of sepsis. In patients with sepsis, indeed, TPO levels are markedly increased, with disease severity being the major independent determinant of TPO concentrations. Moreover, TPO increases and correlates with ex vivo indices of platelet activation in patients with burn injury upon sepsis development, and may contribute to depress cardiac contractility in septic shock. Still, the role of TPO in sepsis pathophysiology remains controversial, given the protective role of TPO in other experimental disease models, for instance in doxorubicin-induced cardiotoxicity and myocardial ischemia/reperfusion injury. The aim of our study was to define the contribution of TPO in the development of organ damage induced by endotoxemia or sepsis, and to investigate the effects of inhibiting TPO in these conditions.

Methods

We synthesized a chimeric protein able to inhibit TPO, mTPOR-MBP, and studied its effect in two murine experimental models, acute endotoxemia and cecal ligation and puncture (CLP) model.

Results

In both models, TPO levels markedly increased, from 289.80±27.87 pg/mL to 465.60±45.92 pg/mL at 3 hours in the LPS model (P<0.01), and from 265.00±26.02 pg/mL to 373.70±26.20 pg/mL in the CLP model (P<0.05), respectively. Paralleling TPO levels, also platelet-monocyte aggregates increased, from 32.86±2.48% to 46.13±1.39% at 3 hours in the LPS model (P<0.01), and from 43.68±1.69% to 56.52±4.66% in the CLP model (P<0.05). Blockade of TPO by mTPOR-MBP administration reduced histological damage in target organs, namely lung, liver, and gut. In particular, neutrophil infiltration and lung septal thickening were reduced from a score of 1.86±0.34 to 0.60±0.27 (P<0.01) and from 1.43±0.37 to 0.40±0.16 (P<0.05), respectively, in the LPS model at 3 hours, and from a score of 1.75±0.37 to 0.38±0.18 (P<0.01) and from 1.25±0.31 to 0.13±0.13 (P<0.001), respectively, in the CLP model. Similarly, the number of hepatic microabscesses was decreased from 14.14±1.41 to 3.64±0.56 in the LPS model at 3 hours (P<0.001), and from 1.71±0.29 to 0.13±0.13 in the CLP model (P<0.001). Finally, the diameter of intestinal villi decreased from 90.69±3.95 μm to 70.74±3.60 μm in the LPS model at 3 hours (P<0.01), and from 74.29±4.29 μm to 57.50±1.89 μm in the CLP model (P<0.01). This protective effect was associated with the blunting of the increase in platelet-monocyte adhesion, and, on the contrary, with increased platelet-neutrophil aggregates in the circulation, which may be related to decreased neutrophil sequestration into the inflamed tissues. Conversely, circulating cytokine levels were not significantly changed, in both models, by mTPOR-MBP administration.

Conclusion

Our results demonstrate that TPO participates in the development of organ damage induced by experimental endotoxemia or polymicrobial sepsis via a mechanism involving increased platelet-leukocyte adhesion, but not cytokine release, and suggest that blocking TPO may be useful in preventing organ damage in patients affected by systemic inflammatory response or sepsis.

IL-1α induces thrombopoiesis through megakaryocyte rupture in response to acute platelet needs

An alternative pathway triggering enhanced platelet release from bone marrow megakaryocytes via a rupture-based mechanism is regulated by IL-1α in response to acute platelet requirements.

Intravital visualization of thrombopoiesis revealed that formation of proplatelets, which are cytoplasmic protrusions in bone marrow megakaryocytes (MKs), is dominant in the steady state. However, it was unclear whether this is the only path to platelet biogenesis. We have identified an alternative MK rupture, which entails rapid cytoplasmic fragmentation and release of much larger numbers of platelets, primarily into blood vessels, which is morphologically and temporally different than typical FasL-induced apoptosis. Serum levels of the inflammatory cytokine IL-1α were acutely elevated after platelet loss or administration of an inflammatory stimulus to mice, whereas the MK-regulator thrombopoietin (TPO) was not elevated. Moreover, IL-1α administration rapidly induced MK rupture–dependent thrombopoiesis and increased platelet counts. IL-1α–IL-1R1 signaling activated caspase-3, which reduced plasma membrane stability and appeared to inhibit regulated tubulin expression and proplatelet formation, and ultimately led to MK rupture. Collectively, it appears the balance between TPO and IL-1α determines the MK cellular programming for thrombopoiesis in response to acute and chronic platelet needs.

MERIT40 deficiency expands hematopoietic stem cell pools by regulating thrombopoietin receptor signaling

Hematopoietic stem cell (HSC) self-renewal and multilineage reconstitution are controlled by positive and negative signaling cues with perturbations leading to disease. Lnk is an essential signaling adaptor protein that dampens signaling by the cytokine thrombopoietin (Tpo) to limit HSC expansion. Here, we show that MERIT40 (Mediator of RAP80 Interactions and Targeting 40 kDa [M40]), a core subunit of an Lnk-associated Lys63 deubiquitinating (DUB) complex, attenuates HSC expansion. M40 deficiency increases the size of phenotypic and functional HSC pools. M40(-/-) HSCs are more resistant to cytoablative stress, and exhibit superior repopulating ability and self-renewal upon serial transplantation. M40(-/-) HSCs display increased quiescence and decelerated cell cycle kinetics accompanied by downregulation of gene sets associated with cell division. Mechanistically, M40 deficiency triggers hypersensitivity to Tpo stimulation and the stem cell phenotypes are abrogated on a background null for the Tpo receptor Mpl. These results establish M40-containing DUB complexes as novel HSC regulators of HSC expansion, implicate Lys63 ubiquitination in HSC signaling, and point to DUB-specific inhibitors as reagents to expand stem cell populations.

Pharmacokinetic/Pharmacodynamic modeling of abexinostat-induced thrombocytopenia across different patient populations: application for the determination of the maximum tolerated doses in both lymphoma and solid tumour patients

BACKGROUND

In the clinical development of oncology drugs, the recommended dose is usually determined using a 3 + 3 dose-escalation study design. However, this phase I design does not always adequately describe dose-toxicity relationships.

METHODS

125 patients, with either solid tumours or lymphoma, were included in the study and 1217 platelet counts were available over three treatment cycles. The data was used to build a population pharmacokinetic/pharmacodynamic (PKPD) model using a sequential modeling approach. Model-derived Recommended Doses (MDRD) of abexinostat (a Histone Deacetylase Inhibitor) were determined from simulations of different administration schedules, and the higher bound for the probability of reaching these MDRD with a 3 + 3 design were obtained.

RESULTS

The PKPD model developed adequately described platelet kinetics in both patient populations with the inclusion of two platelet baseline counts and a disease progression component for patients with lymphoma. Simulation results demonstrated that abexinostat administration during the first 4 days of each week in a 3-week cycle led to a higher MDRD compared to the other administration schedules tested, with a maximum probability of 40 % of reaching these MDRDs using a 3 + 3 design.

CONCLUSIONS

The PKPD model was able to predict thrombocytopenia following abexinostat administration in both patient populations. A model-based approach to determine the recommended dose in phase I trials is preferable due to the imprecision of the 3 + 3 design.

Pathological interactions between hematopoietic stem cells and their niche revealed by mouse models of primary myelofibrosis

Primary myelofibrosis (PMF) belongs to the Philadelphia-negative myeloproliferative neoplasms and is a hematological disorder caused by abnormal function of the hematopoietic stem cells. The disease manifests itself with a plethora of alterations, including anemia, splenomegaly and extramedullary hematopoiesis. Its hallmarks are progressive marrow fibrosis and atypical megakaryocytic hyperplasia, two distinctive features used to clinically monitor disease progression. In an attempt to investigate the role of abnormal megakaryocytopoiesis in the pathogenesis of PMF, several transgenic mouse models have been generated. These models are based either on mutations that interfere with the extrinsic (thrombopoietin and its receptor, MPL) and intrinsic (the GATA1 transcription factor) control of normal megakaryocytopoiesis, or on known genetic lesions associated with the human disease. Here we provide an up-to-date review on the insights into the pathobiology of human PMF achieved by studying these animal models, with particular emphasis on results obtained with Gata1(low) mice.

Promoting Effects of Heparin on ex vivo Expansion of Megakaryocytopoiesis from Human Cord Blood CD34+ Cells

INTRODUCTION

Transfusion of ex vivo expanded megakaryocytes (MKs) has been proposed to sustain platelet recovery after cord blood (CB) hematopoietic stem cell transplantation. In this study, we investigated the effects of heparin on ex vivo colony forming unit-megakaryocytes (CFU-MKs) and MKs expansion from CB CD34+ cells.

METHODS

CB CD34+ cells were stimulated by a combination of thrombopoietin (TPO), stem cell factor (SCF), Flt3-Ligand (FL), IL-6, and IL-11 supplemented with autologous serum and heparin during 14 days. Expanded cells were analyzed by flow cytometry and cultured in a CFU-MK assay.

RESULTS

Compared to control cultures, the 5-factor combination with heparin induced significantly (p ≤ 0.05) higher numbers of: CFU-MKs and CD41+ cells on days 7 and 14; CD41+ cells displaying hyperploidy levels (≥8N) on day 14; platelets on day 14. The culture-derived platelets were activated upon collagen stimulation.

CONCLUSION

Heparin can significantly enhance the stimulating effects of a combination of TPO, SCF, FL, IL-6, and IL-11 supplemented with autologous serum on CFU-MK, MK, and platelet production from CB CD34+ cells. This expansion system could represent a promising method to generate CFU-MKs and MKs cells for transfusion to sustain platelet reconstitution following CB transplantation.

Recombinant human thrombopoietin alleviates infection-associated thrombocytopenia: a retrospective study in senile patients

OBJECTIVE

To examine the efficacy and adverse events of recombinant human thrombopoietin (rhTPO) in the treatment of infection-associated thrombocytopenia in senile patients.

METHODS

The current study is a retrospective analysis of the patients receiving rhTPO for infection-associated thrombocytopenia in our hospital.

RESULTS

Forty-nine cases were included in the analysis as rhTPO group. The absolute platelet count after treatment, increase in platelet count, and the overall response rate were considerably higher in the rhTPO group than that in the control group. Improvement in bleeding score was higher in the rhTPO treatment group than that in the control group (2.1 ± 5.4 vs 0.4 ± 1.7). Bleeding event was stopped in 68.2% of the patients after rhTPO treatment and in 35% of the patients in the control group (P = .032). A stratified analysis indicated that the therapeutic efficacy is much better in patients without organ failure.

CONCLUSION

Recombinant human TPO is effective in alleviating infection-associated thrombocytopenia and hemorrhage in senile patients, particularly if given prior to the emergence of organ failure.

14-3-3 regulates the LNK/JAK2 pathway in mouse hematopoietic stem and progenitor cells

Hematopoietic stem and progenitor cell (HSPC) functions are governed by intricate signaling networks. The tyrosine kinase JAK2 plays an essential role in cytokine signaling during hematopoiesis. The adaptor protein LNK is a critical determinant of this process through its inhibitory interaction with JAK2, thereby limiting HSPC self-renewal. LNK deficiency promotes myeloproliferative neoplasm (MPN) development in mice, and LNK loss-of-function mutations are found in human MPNs, emphasizing its pivotal role in normal and malignant HSPCs. Here, we report the identification of 14-3-3 proteins as LNK binding partners. 14-3-3 interfered with the LNK-JAK2 interaction, thereby alleviating LNK inhibition of JAK2 signaling and cell proliferation. Binding of 14-3-3 required 2 previously unappreciated serine phosphorylation sites in LNK, and we found that their phosphorylation is mediated by glycogen synthase kinase 3 and PKA kinases. Mutations of these residues abrogated the interaction and augmented the growth inhibitory function of LNK. Conversely, forced 14-3-3 binding constrained LNK function. Furthermore, interaction with 14-3-3 sequestered LNK in the cytoplasm away from the plasma membrane-proximal JAK2. Importantly, bone marrow transplantation studies revealed an essential role for 14-3-3 in HSPC reconstitution that can be partially mitigated by LNK deficiency. We believe that, together, this work implicates 14-3-3 proteins as novel and positive HSPC regulators by impinging on the LNK/JAK2 pathway.

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.

A novel role of thrombopoietin as a physiological modulator of coronary flow

Thrombopoietin (TPO) is known for its ability to stimulate platelet production. However, little is currently known whether TPO plays a physiological function in the heart. The potential vasodilatory role of TPO was tested on the isolated rat heart. The expression of TPO receptor (c-mpl) and the TPO-dependent eNOS phosphorylation (P(Ser1179)) were studied on Cardiac-derived normal Human Micro Vascular Endothelial Cells (HMVEC-C) by Western blot analysis. While TPO (10-200 pg/mL) did not modify coronary flow (CF) under basal conditions, it reduced the coronary constriction caused by endothelin-1 (ET-1; 10nM) in a dose-dependent manner. This effect was blocked by both Wortmannin (100 nM) and L-NAME (100 nM); on HMVEC-C, TPO induced eNOS phosphorylation through a Wortmannin sensitive mechanism. Taken together, our data suggest a potential role of TPO as a physiological regulator of CF. By acting on specific receptors present on endothelial cells, TPO may induce PI3K/Akt-dependent eNOS phosphorylation and NO release.

Anagrelide represses GATA-1 and FOG-1 expression without interfering with thrombopoietin receptor signal transduction

BACKGROUND

 Anagrelide is a selective inhibitor of megakaryocytopoiesis used to treat thrombocytosis in patients with chronic myeloproliferative disorders. The effectiveness of anagrelide in lowering platelet counts is firmly established, but its primary mechanism of action remains elusive.

OBJECTIVES AND METHODS

 Here, we have evaluated whether anagrelide interferes with the major signal transduction cascades stimulated by thrombopoietin in the hematopoietic cell line UT-7/mpl and in cultured CD34(+) -derived human hematopoietic cells. In addition, we have used quantitative mRNA expression analysis to assess whether the drug affects the levels of known transcription factors that control megakaryocytopoiesis.

RESULTS

 In UT-7/mpl cells, anagrelide (1μm) did not interfere with MPL-mediated signaling as monitored by its lack of effect on JAK2 phosphorylation. Similarly, the drug did not affect the phosphorylation of STAT3, ERK1/2 or AKT in either UT-7/mpl cells or primary hematopoietic cells. In contrast, during thrombopoietin-induced megakaryocytic differentiation of normal hematopoietic cultures, anagrelide (0.3μm) reduced the rise in the mRNA levels of the transcription factors GATA-1 and FOG-1 as well as those of the downstream genes encoding FLI-1, NF-E2, glycoprotein IIb and MPL. However, the drug showed no effect on GATA-2 or RUNX-1 mRNA expression. Furthermore, anagrelide did not diminish the rise in GATA-1 and FOG-1 expression during erythropoietin-stimulated erythroid differentiation. Cilostamide, an exclusive and equipotent phosphodiesterase III (PDEIII) inhibitor, did not alter the expression of these genes.

CONCLUSIONS

 Anagrelide suppresses megakaryocytopoiesis by reducing the expression levels of GATA-1 and FOG-1 via a PDEIII-independent mechanism that is differentiation context-specific and does not involve inhibition of MPL-mediated early signal transduction events.

Thrombopoietin modulates cardiac contractility in vitro and contributes to myocardial depressing activity of septic shock serum

Thrombopoietin (TPO) is a humoral growth factor that has been shown to increase platelet activation in response to several agonists. Patients with sepsis have increased circulating TPO levels, which may enhance platelet activation, potentially participating to the pathogenesis of multi-organ failure. Aim of this study was to investigate whether TPO affects myocardial contractility and participates to depress cardiac function during sepsis. We showed the expression of the TPO receptor c-Mpl on myocardial cells and tissue by RT-PCR, immunofluorescence and western blotting. We then evaluated the effect of TPO on the contractile function of rat papillary muscle and isolated heart. TPO did not change myocardial contractility in basal conditions, but, when followed by epinephrine (EPI) stimulation, it blunted the enhancement of contractile force induced by EPI both in papillary muscle and isolated heart. An inhibitor of TPO prevented TPO effect on cardiac inotropy. Treatment of papillary muscle with pharmacological inhibitors of phosphatidylinositol 3-kinase, NO synthase, and guanilyl cyclase abolished TPO effect, indicating NO as the final mediator. We finally studied the role of TPO in the negative inotropic effect exerted by human septic shock (HSS) serum and TPO cooperation with TNF-alpha and IL-1beta. Pre-treatment with the TPO inhibitor prevented the decrease in contractile force induced by HSS serum. Moreover, TPO significantly amplified the negative inotropic effect induced by TNF-alpha and IL-1beta in papillary muscle. In conclusion, TPO negatively modulates cardiac inotropy in vitro and contributes to the myocardial depressing activity of septic shock serum.

Elevated thrombopoietin in plasma of burned patients without and with sepsis enhances platelet activation

BACKGROUND

Thrombopoietin (TPO) is a humoral growth factor that does not induce platelet aggregation per se, but enhances platelet activation in response to several agonists. Circulating levels of TPO are increased in patients with sepsis and are mainly related to sepsis severity.

OBJECTIVES

To investigate the potential contribution of elevated TPO levels in platelet activation during burn injury complicated or not by sepsis.

METHODS

We studied 22 burned patients, 10 without and 12 with sepsis, and 10 healthy subjects. We measured plasma levels of TPO, as well as leukocyte-platelet binding and P-selectin expression. The priming activity of plasma from burned patients or healthy subjects on platelet aggregation and leukocyte-platelet binding, and the role of TPO in these effects were also studied in vitro.

RESULTS

Burned patients without and with sepsis showed higher circulating TPO levels and increased monocyte-platelet binding compared with healthy subjects. Moreover, TPO levels, monocyte-platelet binding and P-selectin expression were significantly higher in burned patients with sepsis than in burned patients without sepsis. In vitro, plasma from burned patients without and with sepsis, but not from healthy subjects, primed platelet aggregation, monocyte-platelet binding and platelet P-selectin expression. The effect of plasma from burned patients with sepsis was significantly higher than that of plasma from burned patients without sepsis. An inhibitor of TPO prevented the priming effect of plasma from burned patients.

CONCLUSIONS

Increased TPO levels may enhance platelet activation during burn injury and sepsis, potentially participating in the pathogenesis of multi-organ failure in these diseases.

Lnk controls mouse hematopoietic stem cell self-renewal and quiescence through direct interactions with JAK2

In addition to its role in megakaryocyte production, signaling initiated by thrombopoietin (TPO) activation of its receptor, myeloproliferative leukemia virus protooncogene (c-Mpl, or Mpl), controls HSC homeostasis and self-renewal. Under steady-state conditions, mice lacking the inhibitory adaptor protein Lnk harbor an expanded HSC pool with enhanced self-renewal. We found that HSCs from Lnk-/- mice have an increased quiescent fraction, decelerated cell cycle kinetics, and enhanced resistance to repeat treatments with cytoablative 5-fluorouracil in vivo compared with WT HSCs. We further provide genetic evidence demonstrating that Lnk controls HSC quiescence and self-renewal, predominantly through Mpl. Consistent with this observation, Lnk-/- HSCs displayed potentiated activation of JAK2 specifically in response to TPO. Biochemical experiments revealed that Lnk directly binds to phosphorylated tyrosine residues in JAK2 following TPO stimulation. Of note, the JAK2 V617F mutant, found at high frequencies in myeloproliferative diseases, retains the ability to bind Lnk. Therefore, we identified Lnk as a physiological negative regulator of JAK2 in stem cells and TPO/Mpl/JAK2/Lnk as a major regulatory pathway in controlling stem cell self-renewal and quiescence.

1,25(OH)2-vitamin D3 inhibits proliferation and decreases production of monocyte chemoattractant protein-1, thrombopoietin, VEGF, and angiogenin by human annulus cells in vitro

STUDY DESIGN

Human lumbar anulus tissue and cultured human lumbar anulus cells were used in retrospective studies of the immunocytochemical localization of the vitamin D receptor (VDR) in disc tissue, and of the in vitro effects of the active metabolite of vitamin D, 1,25(OH)2D3, on anulus cell proliferation, cytokine, and proteoglycan (PG) production. 24,25-D3 was also analyzed. Studies were approved by the authors' Human Subjects Institutional Review Board. Discs were obtained from surgical specimens and from control donors.

OBJECTIVES

To determine if human anulus cells express the VDR in vivo, and to test the effect of in vitro exposure to 1,25(OH)2D3 and 24,25-D3 on anulus cell proteoglycan and cytokine production in 3-dimensional culture.

SUMMARY OF BACKGROUND DATA

Intragenic polymorphisms in the VDR gene have been associated with disc degeneration. 1,25(OH)2D3 has well-recognized effects on calcium homeostasis and bone mineralization, and is a negative growth regulator of a variety of normal and tumor cells. Its effects on human disc cells, however, are unexplored.

METHODS

Immunocytochemistry was performed on human lumbar disc anulus tissue from 19 subjects; human disc cells were cultured to test the effect of 1,25(OH)2D3 on proliferation of anulus cells from 5 subjects. A paired experimental design was used to determine proteoglycan production in control or 1,25(OH)2D3-treated cells, or in control or 24,25-D3-treated cells using the dimethylmethylene blue assay. A paired experimental design was also used to identify differences in cytokine production in conditioned media from control or 1,25(OH)2D3-treated cells, or in control or 24,25-D3-treated cells using ELISA assays.

RESULTS

Immunocytochemistry documented expression of the VDR in anulus cells. Young donor discs (aged newborn, 15 years) showed positive localization in all cells of the outer anulus, and some inner anulus cells. In adults (mean age, 38.9 years), some, but not all anulus cells, showed positive localization. Exposure to 10M 1,25(OH)2D3 in monolayer significantly reduced cell proliferation in vitro (P = 0.03). PG production in 3-dimensional was unchanged from control in both 1,25(OH)2D3- and 24,25-D3-treated cells. Cytokine production differed, however. 1,25(OH)2D3-treated cells showed significantly decreased production of vascular endothelial growth factor (VEGF) (P = 0.01), monocyte chemoattractant protein-1 (MCP-1) (P = 0.0006), angiogenin (P = 0.002), and thrombopoietin (P = 0.03) compared with controls. 24,25-D3-treated cells showed significantly elevated vascular endothelial growth factor-D (P = 0.01), beta-fibroblast growth factor (0.03), and significantly decreased interleukin-8, interferon-gamma, leptin, MCP-1, and TIMP-2 (tissue inhibitor of metalloproteinases-2) compared with controls (P <or= 0.01).

CONCLUSION

Data suggest that 1,25(OH)2D3 and 24,25-D3 may play roles as regulators of cell proliferation and production of specific cytokines in the lumbar anulus.

Thrombopoietin inhibits nerve growth factor-induced neuronal differentiation and ERK signalling

Thrombopoietin (TPO), a hematopoietic growth factor regulating platelet production, and its receptor (TPOR) were recently shown to be expressed in the brain where they exert proapoptotic activity. Here we used PC12 cells, an established model of neuronal differentiation, to investigate the effects of TPO on neuronal survival and differentiation. These cells expressed TPOR mRNA. TPO increased cell death in neuronally differentiated PC12 cells but had no effect in undifferentiated cells. Surprisingly, TPO inhibited nerve growth factor (NGF)-induced differentiation of PC12 cells in a dose- and time-dependent manner. This inhibition was dependent on the activity of Janus kinase-2 (JAK2). Using phospho-kinase arrays and Western blot we found downregulation of the NGF-stimulated phosphorylation of the extracellular signal-regulated kinase p42ERK by TPO with no effect on phosphorylation of Akt or stress kinases. NGF-induced phosphorylation of ERK-activating kinases, MEK1/2 and C-RAF was also reduced by TPO while NGF-induced RAS activation was not attenuated by TPO treatment. In contrast to its inhibitory effects on NGF signalling, TPO had no effect on epidermal growth factor (EGF)-stimulated ERK phosphorylation or proliferation of PC12 cells. Our data indicate that TPO via activation of its receptor-bound JAK2 delays the NGF-dependent acquisition of neuronal phenotype and decreases neuronal survival by suppressing NGF-induced ERK activity.

Signals emanating from the membrane proximal region of the thrombopoietin receptor (mpl) support hematopoietic stem cell self-renewal

OBJECTIVE

Studies using thrombopoietin -/- (TPO(-/-)) or TPO receptor, mpl(-/-) mice have established a critical role for TPO/mpl signaling in hematopoietic stem cell (HSC) development. In this study, we further dissected mpl signaling in both megakaryopoiesis and HSC function, using mice bearing a truncated mpl receptor lacking the distal 60 amino acids (Delta60). This deletion removes three major signaling tyrosines on the mpl cytoplasmic domain, but retains the membrane proximal Box1 and Box2 domains required for JAK2 activation.

MATERIALS AND METHODS

Competitive bone marrow transplantations (BMT) and serial BMTs were performed to study HSC function. Western blot analysis was used to study TPO-stimulated signaling pathways. BM cell cultures in the presence of TPO were used to study megakaryocyte development.

RESULTS

In agreement with prior findings, we show that Delta60 BM cells cultured in TPO generated normal numbers of megakaryocytes, but with greatly reduced ploidy. As expected from the deletion of three signaling tyrosine residues, freshly isolated Delta60 megakaryocytes showed marked reduction in all known TPO-stimulated signaling pathways tested, including signal transducers and activators of transcription (Stat) 5, Stat3, Akt, and p42/44 mitogen-activated kinase. We found that Delta60 mice displayed normal short-term (ST-HSC) activities and marginally compromised long-term (LT-HSC) stem cell activities in primary transplantation. In addition, Delta60 mice supported HSC self-renewal for at least two serial BMTs.

CONCLUSION

Our data reveal a pivotal role for an unknown signal emanating from the membrane proximal region of the mpl receptor or from JAK2 itself in maintaining stem cell activity and self-renewal, in addition to its role in megakaryocytopoiesis and thrombopoiesis.

Effect of Human Thrombopoietin-Modified Bone Marrow Mesenchymal Stem Cells Mediated by Recombinant Adeno-Associated Virus on Megakaryocytopoiesis

Recently, the research of recombinant thrombopoietin (TPO) and its subsequent use in treating thrombocytopenia following radiation therapy and chemotherapy have become more important in clinics. Our study was to determine the feasibility of recombinant adeno-associated virus (rAAV)-mediated TPO gene transfer into bone marrow-derived mesenchymal stem cells (MSCs) and to evaluate the conditioned medium (CM) obtained from TPO-transduced human (h) hMSCs for promoting the process of megakaryocytopoiesis. We constructed recombinant adeno-associated viruses expressing TPO successfully, and TPO mRNA and protein were both strongly expressed in TPO-transduced hMSCs. There was no decrease in green fluorescent protein (GFP) fluorescence expression of the transduced cells with continuous passaged culturing in vitro. The CM of TPO-transduced hMSCs has been shown to enhance the number of CD41(+) cells and megakaryocytic progenitors (colony-forming unit-megakaryocyte) significantly as compared to the nontransduced control. In this study, a novel safe and efficient method of promoting the megakaryocytopoiesis was established following the TPO-transduced hMSCs. These results provide a basis for the future studies on hematopoietic regulation by hMSCs transfected with TPO.

Thrombopoietin contributes to enhanced platelet activation in patients with unstable angina

OBJECTIVES

We sought to investigate the potential role of elevated levels of thrombopoietin (TPO) in platelet activation during unstable angina (UA).

BACKGROUND

Thrombopoietin is a humoral growth factor that does not induce platelet aggregation per se, but primes platelet activation in response to several agonists. No data concerning its contribution to platelet function abnormalities described in patients with UA are available.

METHODS

We studied 15 patients with UA and, as controls, 15 patients with stable angina (SA) and 15 healthy subjects. We measured TPO and C-reactive protein (CRP), as well as monocyte-platelet binding and the platelet expression of P-selectin and of the TPO receptor, c-Mpl. The priming activity of patient or control plasma on platelet aggregation and monocyte-platelet binding and the role of TPO in this effect also were studied.

RESULTS

Patients with UA showed higher circulating TPO levels, as well as increased monocyte-platelet binding, platelet P-selectin expression, and CRP levels, than those with SA and healthy control subjects. The UA patients also showed reduced platelet expression of the TPO receptor, c-Mpl. In vitro, the plasma from UA patients, but not from SA patients or healthy controls, primed platelet aggregation and monocyte-platelet binding, which were both reduced when an inhibitor of TPO was used.

CONCLUSIONS

Thrombopoietin may enhance platelet activation in the early phases of UA, potentially participating in the pathogenesis of acute coronary syndromes.

The Membrane-proximal Region of the Thrombopoietin Receptor Confers Its High Surface Expression by JAK2-dependent and -independent Mechanisms

Janus tyrosine kinase 2 (JAK2) is essential for signaling by the thrombopoietin (TpoR) and erythropoietin (EpoR) receptors. In the absence of JAK2 most EpoR molecules are retained in the endoplasmic reticulum in an Endo H-sensitive form. In contrast, we show that in the absence of JAK2 a large fraction of the TpoR is processed to the mature Endo H-resistant form and reaches the cell surface. By studying chimeras of the TpoR and EpoR we show that high surface expression of the TpoR is entirely conferred by the membrane-proximal region of the intracellular domain that includes the juxtamembrane, Box 1, and Box 2 regions. The TpoR intracellular domain shows similar effects on receptor endocytosis rate as that of the EpoR, but does stabilize the mature receptor isoform from degradation. Co-expression of JAK2 further stabilizes mature TpoR and thus further increases its surface expression. This JAK2 effect depends on the Box 1 region, the only JAK2 interacting site in the TpoR. By contrast, EpoR requires Box 1 as well as the flanking 20 residues on the C-terminal side for JAK2 interaction and JAK2-dependent surface expression. Our study suggests that whereas cell surface expression of type I cytokine receptors requires their cognate JAKs, the mechanisms governing receptor-JAK interactions differ among receptors interacting with the same JAK protein.

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.

Immature Platelet Fraction as a Predictor of Platelet Recovery Following Hematopoietic Progenitor Cell Transplantation

The immature platelet fraction (IPF) as determined by the Sysmex XE-2100 is a rapid automated measure of the least mature component of the platelet population and is thought to correlate with thrombopoietic activity of the marrow. We investigated the ability of IPF to predict platelet recovery following hematopoietic progenitor cell (HPC) transplantation. IPF was compared to standard parameters of hematopoietic recovery, including the immature reticulocyte fraction (IRF), an early predictor of recovery. Fifty patients undergoing peripheral blood HPC transplantation (38 autologous and 12 allogeneic) were followed daily for 11 to 28 days after transplantation with measurement of IPF, IRF, absolute neutrophil counts (ANC) and platelet counts. Mean days to recovery for IPF was 3.1 days less than for platelet count (P <.0001), 3.8 days less than for ANC (P <.0001), and 0.6 days less than for IRF (P = .0477). IPF recovered at least 1 day prior to platelet count in 79% (38 of 48) of patients, and was followed by platelet count recovery within 1 to 12 days (mean, 4.1 days). When autologous and allogeneic patient groups were analyzed separately, IPF recovered significantly earlier than platelet count and ANC in both groups (P <.0001). Thrombopoietin (TPO) levels in 5 patients receiving transplants correlated with IPF; however, this appeared to be secondary to an inverse correlation of both TPO and IPF with platelet count. IPF is comparable to IRF as one of the earliest predictors of hematopoietic recovery following peripheral blood HPC transplantation. IPF could potentially be useful as a predictor of platelet recovery in other bone marrow failure syndromes.

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.

Tetraploidy/aneuploidy and stem cells in cancer promotion: The role of chromosome passenger proteins

While polyploidy, a state of having fully duplicated sets of chromosomes per cell, has been described in normally developing bone marrow megakaryocytes or as an adaptive response in other cell types, aneuploidy is never detected in normal cells. Tetraploidy or aneuploidy can be induced by several signals and it is highly prevalent in different forms of cancers, suggesting a role for this cell cycle state in promoting cellular transformation. Investigations suggested that loss of heterozygosity of cancer-related genes in stem cells might contribute to genetic instability in progeny cells and to subsequent cancer development. Deregulated expression of chromosome passenger proteins, such as Aurora kinases or Survivin, is a hallmark of various cancers, and experimentally induced changes in these regulators can promote tetraploidy or aneuploidy and loss of heterozygosity. Our studies described an induction of tetraploidy/aneuploidy by a stable form of Aurora-B, leading to acquisition of transformation properties. It is intriguing to speculate that in some cancers, tetraploidy/aneuploidy induced by deregulated expression of a mitotic regulator represents a primary event that leads to unbalanced expression of a cluster of crucial genes and to cellular transformation.

The biogenesis of platelets from megakaryocyte proplatelets

Platelets are formed and released into the bloodstream by precursor cells called megakaryocytes that reside within the bone marrow. The production of platelets by megakaryocytes requires an intricate series of remodeling events that result in the release of thousands of platelets from a single megakaryocyte. Abnormalities in this process can result in clinically significant disorders. Thrombocytopenia (platelet counts less than 150,000/microl) can lead to inadequate clot formation and increased risk of bleeding, while thrombocythemia (platelet counts greater than 600,000/microl) can heighten the risk for thrombotic events, including stroke, peripheral ischemia, and myocardial infarction. This Review will describe the process of platelet assembly in detail and discuss several disorders that affect platelet production.

Hematopoietic stem cells

Hematopoietic stem cells (HSCs) have the capacity to self-renew and the potential to differentiate into all of the mature blood cell types. The ability to prospectively identify and isolate HSCs has been the subject of extensive investigation since the first transplantation studies implying their existence almost 50 years ago. Despite significant advances in enrichment protocols, the continuous in vitro propagation of human HSCs has not yet been achieved. This chapter describes current procedures used to phenotypically and functionally characterize candidate human HSCs and initial efforts to derive permanent human HSC lines.

Initial laboratory findings useful for predicting the diagnosis of idiopathic thrombocytopenic purpura

PURPOSE

To identify initial laboratory findings useful for the later diagnosis of idiopathic thrombocytopenic purpura (ITP) in adult patients with thrombocytopenia.

SUBJECTS AND METHODS

We studied 62 consecutive adult patients who had thrombocytopenia and whose peripheral blood film was normal except for thrombocytopenia at presentation. Each patient underwent physical examination and routine laboratory tests and was prospectively followed for 22.5 +/- 9.8 months (range, 8 to 41 months). The frequency of antiglycoprotein (GP) IIb/IIIa antibody-producing B cells, the presence of platelet-associated and plasma anti-GPIIb/IIIa antibodies, the percentage of reticulated platelets, and the plasma thrombopoietin level were examined at the first visit. The final diagnosis was based on the clinical history, physical examination, complete blood test, bone marrow findings, and the clinical course at last observation.

RESULTS

Forty-six patients were diagnosed as having ITP and 16 as having another disorder, including myelodysplastic syndrome, aplastic anemia, amegakaryocytic thrombocytopenia, and reduced platelet production, with or without other cytopenias, and without dysplasia or evidence for destruction. Six initial laboratory findings discriminated ITP from other diagnoses: the absence of anemia, absence of leukocytopenia, increased frequency of anti-GPIIb/IIIa antibody-producing B cells, increased platelet-associated anti-GPIIb/IIIa antibodies, elevated percentage of reticulated platelets, and a normal or slightly increased plasma thrombopoietin level. Three or more of these ITP-associated findings were found at presentation in 44 patients (96%) with thrombocytopenia later diagnosed as ITP, compared with only 1 patient (6%) whose disorder was non-ITP.

CONCLUSION

Initial laboratory findings can well predict future diagnosis of ITP. Further studies prospectively evaluating these same diagnostic criteria on another, independent set of patients are necessary.