Thrombopoietin Cells Research Articles

Effects of Ziyin Liangxue Formula Combined with Prednisone on Immune Function and ST2/IL-33 Pathway in Mice with Immune Thrombocytopenia

To investigate the effects of Ziyin Liangxue formula combined with prednisone on immune function and the ST2/IL-33 pathway in mice with immune thrombocytopenia. In 40 BALB/c mice, 32 were constructed as immune thrombocytopenia mouse models by antiplatelet serum injection. After successful modeling, the mice were randomly divided into model group, Ziyin Liangxue formula group (0.2 ml/10 g), prednisone group (0.2 ml/10 g), and Ziyin Liangxue formula + prednisone group (0.2 ml/10 g), 8 mice in each group, and the other 8 mice were set as control group. The drugs were administered by gavage at the dose, and the model group and control group were given equal amounts of saline by gavage once a day for 2 weeks of continuous intervention. Blood samples and spleen tissues were collected, the peripheral platelet count was measured by automatic hematology analyzer, the pathological changes in spleen tissue was observed by HE staining, the levels of serum transforming growth factor (TGF)-β, interleukin (IL)-17, and peripheral blood thrombopoietin (TPO) were detected by enzyme-linked immunosorbent assay (ELISA), the expression of IL-33, sST2, and ST2 in spleen tissue was detected by Western blot, and the cell counts of peripheral blood Th17 and Treg were detected by flow cytometry. Compared with the control group, the number of platelets, the level of TPO, TGF-β, and Treg cells were significantly decreased (P <0.05), while the level of IL-17, Th17 cells, and the expression of IL-33, sST2, and ST2 protein were significantly increased in the model group (P <0.01). Compared with the model group, the number of platelets, the level of TPO, TGF-β, and Treg cells were significantly increased (P <0.05), while the level of IL-17, Th17 cells, and the expression of IL-33, sST2, and ST2 protein were significantly decreased in the Ziyin Liangxue formula + prednisone group (P <0.01). Ziyin Liangxue formula + prednisone can effectively regulate Th17/Treg balance, thus effectively improve immune thrombocytopenia, and the mechanism may be related to the regulation of ST2/IL-33 signaling pathway.

Activation of the thrombopoietin receptor by mutant calreticulin in CALR-mutant myeloproliferative neoplasms

AbstractRecurrent somatic mutations of calreticulin (CALR) have been identified in patients harboring myeloproliferative neoplasms; however, their role in tumorigenesis remains elusive. Here, we found that the expression of mutant but not wild-type CALR induces the thrombopoietin (TPO)-independent growth of UT-7/TPO cells. We demonstrated that c-MPL, the TPO receptor, is required for this cytokine-independent growth of UT-7/TPO cells. Mutant CALR preferentially associates with c-MPL that is bound to Janus kinase 2 (JAK2) over the wild-type protein. Furthermore, we demonstrated that the mutant-specific carboxyl terminus portion of CALR interferes with the P-domain of CALR to allow the N-domain to interact with c-MPL, providing an explanation for the gain-of-function property of mutant CALR. We showed that mutant CALR induces the phosphorylation of JAK2 and its downstream signaling molecules in UT-7/TPO cells and that this induction was blocked by JAK2 inhibitor treatment. Finally, we demonstrated that c-MPL is required for TPO-independent megakaryopoiesis in induced pluripotent stem cell–derived hematopoietic stem cells harboring the CALR mutation. These findings imply that mutant CALR activates the JAK2 downstream pathway via its association with c-MPL. Considering these results, we propose that mutant CALR promotes myeloproliferative neoplasm development by activating c-MPL and its downstream pathway.

Platelet-derived growth factor receptors form complexes with neuropilin-1 during megakaryocytic differentiation of thrombopoietin-dependent UT-7/TPO cells

Neuropilin-1 (NRP-1) is involved in angiogenesis, but the role of NRP-1 in megakaryocytopoiesis is not yet fully understood. In this study, we investigated whether thrombopoietin (TPO) regulates the expression of platelet-derived growth factor (PDGF) and its receptors (PDGFRs) on TPO-dependent UT-7/TPO cells and whether PDGFRs and NRP-1 on UT-7/TPO cells form complexes during megakaryocytic differentiation. When UT-7/TPO cells were starved of TPO for 24 h and then stimulated with 5 ng/ml TPO, the expression of PDGF-B, PDGFRα, and PDGFRβ were significantly up-regulated after the addition of TPO. TPO also induced tyrosine phosphorylation of PDGFRα but not PDGFRβ, and promoted the formation of PDGFRαβ heterodimer complexes. Furthermore, megakaryocytic differentiation of UT-7/TPO cells on treatment with phorbol myristate acetate (PMA) was accompanied by a marked up-regulation of PDGFRβ and NRP-1 protein expression, complex formation between PDGFRs and NRP-1, PDGFRαβ heterodimer complexes, and an increase in PDGF-BB-binding activity. Immunocytochemistry confirmed complex formation between PDGFRs and NRP-1 and PDGFRαβ heterodimer complexes in PMA-differentiated UT-7/TPO cells. Our observations suggest that NRP-1 is involved in megakaryocytopoiesis through complex formation with PDGFRs, and that NRP-1-PDGFR-complexes may contribute to effective cellular functions mediated by TPO and PDGF in megakaryocytic cells.

Detection of Anti-Thrombopoietin Antibodies in Patients with Immune Thrombocytopenia

Background: Immune thrombocytopenia (ITP) is an autoimmune disease characterized by the presence of autoantibodies against platelet membrane glycoproteins, which cause the autoantibody-mediated destruction of platelets and impaired platelet production. Thrombopoietin (TPO) binds to its receptor on the surface of hematopoietic stem cells and megakaryocytes and induces their maturation and proliferation. Patients with thrombocytopenia due to aplastic anemia have drastically elevated plasma levels of TPO, whereas patients with ITP have normal or slightly elevated plasma levels of TPO despite their low platelet count. Furthermore, based on the existence of a multitude of autoantibody reactivities in ITP, including antibodies against platelets and TPO receptors, the presence of anti-TPO antibodies in patients with ITP may be suspected.Objective: We developed assay systems to detect plasma anti-TPO antibodies and screen patients with ITP. We examined the clinical characteristics associated with anti-TPO antibodies and their pathogenic roles in patients with ITP.Methods: Plasma anti-TPO antibodies from 101 patients with ITP and 72 healthy controls were measured by enzyme-linked immunosorbent assay (ELISA) using recombinant human TPO (rhTPO) as an antigen. The specificity of anti-TPO antibody reactivity was confirmed by ELISA competition assay. The presence of anti-TPO antibodies was further examined using immunoprecipitation and immunoblotting using rhTPO. To investigate whether anti-TPO antibodies inhibited functional interactions between TPO and TPO receptors, we examined extracellular signal-regulated kinases (ERKs), downstream signals induced by TPO. The binding of TPO to TPO receptors induced the phosphorylation of ERK in TPO receptor-expressing UT-7/TPO cells.Results: The level of anti-TPO antibodies measured by ELISA was significantly greater in the samples from patients with ITP than in those from healthy controls (2.91 ± 3.64 units versus 1.45 ± 0.67 units, P < 0.001). Samples were classified as positive or negative for anti-TPO antibody, as determined by immunoprecipitation and immunoblotting. Thus, the ELISA positive-cutoff value was considered to be the mean plus 3.5 standard deviation (SD) of 72 healthy control plasma samples. Plasma anti-TPO antibodies were detected in twenty-four ITP patients (23.8%), but in none of the healthy controls. By ELISA competition assay, anti-TPO antibody reactivity was inhibited dose-dependently by preincubation of patient plasma with rhTPO. In addition, anti-TPO antibody-positive plasma samples inhibited the phosphorylation of ERK in UT-7/TPO cells. In contrast, healthy control plasma had no inhibitory effect. Furthermore, the number of megakaryocytes was decreased relatively in the anti-TPO antibody-positive ITP patients. There was no difference in the TPO levels in plasma between ITP patients with anti-TPO antibodies and patients without anti-TPO antibodies (63.6 ± 79.7 pg/ml versus 45.2 ± 49.3 pg/ml).Conclusion: Our results have thus demonstrated the presence of anti-TPO autoantibodies in patients with ITP. The ELISA using rhTPO was specific for the detection of anti-TPO antibodies and thus allows their easy and rapid measurement in clinical settings. These findings suggest that functional anti-TPO antibodies cause impaired megakaryocyte proliferation and platelet production in patients with ITP. DisclosuresHigashihara:Bristol-Myers Squibb: Research Funding; Baxter: Research Funding; Teijin: Research Funding; Pfizer: Research Funding; Astellas: Research Funding; Yakurt: Honoraria; KyowaHakkoKirin: Honoraria, Research Funding; Chugai: Honoraria, Research Funding; Eisai: Honoraria; GlaxoSmithKline: Honoraria, Research Funding; Nippon Shinyaku: Research Funding; Shionogi: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Celgene: Honoraria; Takeda: Honoraria; Janssen pharma: Honoraria, Research Funding; Alexion: Honoraria; Dainippon Sumitomo: Research Funding; Taisho Tomiyama: Research Funding.

PI3Kδ Inhibitor Idelalisib Inhibits AKT Signaling In Myelofibrosis Patients On Chronic JAK Inhibitor Therapy

BackgroundMyelofibrosis (MF) is characterized by activation of the JAK-STAT pathway, with the JAK2 V617F mutation found in 50-60% of patients. Although JAK inhibitors, such as FDA-approved ruxolitinib, have been effective in reducing splenomegaly and mitigating symptoms, patients uniformly exhibit “disease persistence” which is equated with a lack of hematologic or molecular remissions, or with loss of clinical improvement over time. Prior studies using cell lines or primary patient samples have shown that the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway is dysregulated in MPNs and is a potential therapeutic target (Kamishimoto et al, Cell Signal 2011; Huang et al, ASH 2009 Abstract 1896; Vannucchi et al, ASH 2011 Abstract 3835; Khan et al, Leukemia 2013). In CLL and other B-cell malignancies, the PI3K pathway is constitutively upregulated and is dependent on PI3Kδ. Idelalisib is a δ-isoform-specific PI3K inhibitor that is efficacious in patients with CLL and indolent NHL. Herein, the specific aims of our study were: 1) to determine whether the PI3Kδ isoform is expressed in progenitor cells from MF patients, and 2) to evaluate the inhibitory effects of idelalisib on basal and thrombopoietin (TPO)-stimulated AKT/S6RP phosphorylation (p-AKT/p-S6RP) in cell lines and in primary samples from MF patients who were either on chronic ruxolitinib (RUX) therapy or were not exposed to ruxolitinib (RUX-naïve or off-therapy at the time of sample collection). MethodsTo evaluate isoform expression, CD34+ cells from the peripheral blood of MF patients were sorted by FACSAria and cell lysates were analyzed by Simple Western using Peggy (ProteinSimple) with recombinant protein as a positive control. For cell line studies, BaF3/MPL W515L and UT-7/TPO cells were stimulated with recombinant human TPO and incubated with idelalisib. Whole cell lysates were analyzed by Western blot to quantify the % of p-AKT and p-S6RP levels compared to idelalisib-untreated cells. For MF patient samples, PBMCs were isolated from the whole blood of MF patients who were either RUX-naïve or on chronic RUX therapy and treated for 2 hours with idelalisib. Antibodies specific to p-AKT Ser473 and pS6RP Ser235/236 were used to quantify the proportion of p-AKT and pS6RP in basal and TPO-stimulated CD34+/CD3-/CD14-/CD19-/CD66- gated cells. ResultsThe PI3Kδ isoform was found to be the predominant isoform expressed in 3 of 3 RUX-naïve and 4 of 4 chronic RUX patients tested; PI3Kβ was expressed at lower levels and no PI3Kα or γ was detected (Figure 1). In BaF3/MPL cells, p-AKT levels decreased by 51%, 64% and 67%, with 0.1, 1.0, 2.0 µM idelalisib, respectively, when compared to idelalisib-untreated cells; p-S6RP levels decreased by 24%, 27%, and 41%, respectively. Similarly, for UT-7/TPO cells, p-AKT decreased by 11%, 44%, and 55%, and p-S6 decreased by 13%, 28% and 48%, respectively. In CD34+ cells from RUX-naïve patients (n=3), p-AKT and p-S6RP levels decreased with increasing concentrations of idelalisib (0.02, 0.2, 2 µM). All patients on chronic RUX treatment demonstrated decreased p-AKT (n=3) and p-S6RP (n=4 basal, n=3 TPO-induced; patient 4 was only tested for basal) levels with increasing concentrations of idelalisib in both basal (Figure 2A) and TPO-stimulated (Figure 2B) assays. All 4 chronic RUX and 2 of 3 RUX-naïve patients tested carried the JAK2 V617F mutation. [Display omitted] [Display omitted] ConclusionsThe PI3Kδ isoform was identified as the predominant isoform expressed in CD34+ cells from MF patients. In both cell lines and patient samples, idelalisib inhibits the PI3K/AKT pathway, with a dose-dependent decrease of p-AKT and p-S6RP. Inhibition was observed for both RUX-naïve and chronic RUX-treated patients. Studies are underway to evaluate the effects of idelalisib on progenitor colony formation and induction of cell cycle arrest and apoptosis.* Meadows and Nguyen are first co-authors Disclosures:Meadows:Gilead: Employment, Equity Ownership. Queva:Gilead: Employment, Equity Ownership. Lannutti:Gilead, Acetra, Effector: Consultancy, Employment, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Kashishian:Gilead: Employment, Equity Ownership. Jun:Gilead: Employment, Equity Ownership. Coutre:Gilead: Research Funding. Dansey:Gilead: Employment, Equity Ownership. Gotlib:Gilead: Consultancy, Research Funding.

Neuropilin-1 forms complexes with vascular endothelial growth factor receptor-2 during megakaryocytic differentiation of UT-7/TPO cells

We investigated whether the gene expression of vascular endothelial growth factor (VEGF) and its receptors (VEGFR and neuropilin-1 [NRP-1]) could be specifically regulated during the megakaryocytic differentiation of human thrombopoietin (TPO)-dependent UT-7/TPO cells. Undifferentiated UT-7/TPO cells expressed a functional VEGFR-2, leading to VEGF binding and VEGF 165-induced tyrosine phosphorylation, cell proliferation, and apoptosis inhibition. The megakaryocytic differentiation of UT-7/TPO cells on treatment with phorbol myristate acetate (PMA) was accompanied by a marked up-regulation of NRP-1 mRNA and protein expression and by an increase in VEGF-binding activity, which was mainly mediated by VEGFR-2. VEGF 165 promoted the formation of complexes containing NRP-1 and VEGFR-2 in undifferentiated UT-7/TPO cells in a dose-dependent manner. Unlike human umbilical vein endothelial cells, PMA-differentiated UT-7/TPO cells exhibited complex formation between NRP-1 and VEGFR-2 even in the absence of VEGF 165. These findings suggest that NRP-1–VEGFR-2-complex formation may contribute to effective cellular functions mediated by VEGF 165 in megakaryocytic cells.

HIF-1 Prevents Hematopoietic Cells from Cell Damage by Overproduction of Mitochondrial ROS after Cytokine Stimulation through Induction of PDK-1

Reactive oxygen species (ROS) play numerous functions in both physiological and pathological hematopoiesis. Although ROS are generated in multiple cellular compartments and by multiple enzymes, the majority is derived from mitochondria coupled with glucose metabolism. To detoxify the cytotoxicity of ROS, cells have several ROS scavenging mechanisms. In addition, recent studies have revealed that hypoxia inducible factor-1 (HIF-1) contributes to the reduction of mitochondrial ROS production through the induction of pyruvate dehydrogenase kinase-1 (PDK-1) in response to hypoxia. PDK-1 phosphorylates and inactivates pyruvate dehydrogenase (PDH) E1α, which converts pyruvate to acetylcoenzyme A. Thus, the induction of PDK-1 by HIF-1 blocks the entry of pyruvate into the tricarboxylic acid (TCA) cycle and subsequently suppresses mitochondrial ROS production under hypoxic conditions. In addition to hypoxia, mitochondrial ROS production is also enhanced by cytokine stimulation. Previously, we showed that thrombopoietin (TPO) induced the generation of mitochondrial ROS and that mitochondrial ROS induced by TPO promoted the elevation of HIF-1α, a subunit of HIF-1, both in UT-7/TPO cells and in primary mouse progenitor cells. Based on these observations, we speculated that TPO-activated HIF-1 works as a feedback mechanism to block overproduction of ROS, which may be harmful to hematopoietic cells, by controlling PDK-1 expression. To investigate this notion, we first analyzed ROS production kinetics after TPO stimulation using UT-7/TPO cells. ROS production was gradually increased, peaking at 48 hr, and then decreasing by 72 hr after TPO stimulation. PDK-1 expression increased 48 hr after TPO treatment before ROS dropped. Phosphorylation of PDH-E1α was enhanced by TPO in the same fashion. To confirm that PDK-1 induction by TPO contributes to reduction of ROS, we treated UT-7/TPO cells with a PDK-1 inhibitor, dichloroacetate (DCA). As expected, DCA blocked the phosphorylation of PDH-E1α and induced sustained ROS production. Furthermore, DCA treatment resulted in increased apoptotic cell ratio after TPO stimulation. These results support our hypothesis that PDK-1 works to prevent overproduction of ROS after TPO stimulation and also suggest that sustained production of ROS after cytokine stimulation might be toxic to hematopoietic cells. Next, we analyzed whether HIF-1 is required for this process. Echinomycin, a specific inhibitor of HIF-1, blocked PDK-1 elevation by TPO in a dose-dependent manner. For further study, we established a stable HIF-1α knockdown system in UT-7/TPO cells using siRNA. TPO failed to induce PDK-1 expression in these clones. Concomitantly, ROS levels in HIF-1α knockdown cells remained high, compared to parental cells stimulated with TPO for 72 hr. We also found sustained activation of p38 MAPK and JNK in HIF-1α knockdown clones after TPO stimulation. Taken together, our observations suggest that TPO-induced activation of HIF-1 and subsequent induction of PDK-1 is an important mechanism to prevent overproduction of mitochondrial ROS, a secondary product of glucose metabolism. This feedback mechanism may be critical for protecting hematopoietic cells from DNA damage by ROS production after cytokine stimulation.

Thrombopoietin (TPO) regulates HIF-1α levels through generation of mitochondrial reactive oxygen species

Hypoxia inducible factor (HIF)-1 is a master transcriptional regulator mediating the cellular adaptation to hypoxia. In addition, HIF-1 is also vital for the development of hematopoietic stem cells (HSCs). In a previous study we found that thrombopoietin (TPO), an important and non-redundant cytokine for HSC maintenance and expansion, induces HIF-1alpha expression in HSCs by enhancing the stability of HIF-1alpha under normoxic conditions. However, the molecular mechanisms of these effects are not yet fully understood. In this study, we explored the mechanisms and found that TPO-induced mitochondrial reactive oxygen species (ROS) played a crucial role in stabilization of HIF-1. Both ROS scavengers and inhibitors of mitochondrial electron transport completely blocked HIF-1alpha induction by TPO in UT-7/TPO cells and in primary immature mouse bone marrow cells. We also found that TPO-induced HIF-1alpha induction was tightly coupled with glucose metabolism. Inhibition of glucose transporter or glycolytic enzyme blocked HIF-1alpha elevation of TPO. These results indicate that TPO induces HIF-1alpha expression in a manner very similar to that of hypoxia.

Expression of miR-188 and 362 Induced by Erythropoietin Stimulation in a Human Erythrocytic Leukemia Cell Line.

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the post-transcriptional level and participate in a lot of biological processes. Several studies indicated that miRNAs play an important role in hematopoietic system. It has not been elucidated well about miRNAs involved in cytokine-regulated molecular mechanisms in cellular proliferation and differentiation. We screened specific miRNAs expressed in human leukemia cell lines, UT-7 and its sublines UT-7/GM, UT-7/EPO and UT-7/TPO, which exhibit hematopoietic lineage properties depending on cytokine stimuli by GM-CSF, erythropoietin (EPO), or thrombopoietin (TPO), respectively. The expression profiles of 470 miRNAs in all were analyzed using LNA-based microarray between UT-7/GM, UT-7/EPO and UT-7/TPO cells. The initial microarray data were validated, and specific expressions of several miRNAs were confirmed using quantitative real-time PCR. Especially, the expression levels of miR-188 and 362 clustered on X chromosome were significantly higher in UT-7/EPO cells as compared with those in UT-7, UT-7/GM and UT-7/TPO cells. To assess the effect of EPO on the expression of miR-188 and 362, UT-7/EPO cells were cultured under various concentration of EPO (0 to 10 U/ml) after cytokine withdrawal for 24 hours. Quantification of miRNAs expression by real-time PCR showed that miR-188 and 362 were increased 2- to 20-fold by EPO stimuli, but both expressions were not in dose-dependent manner. Furthermore, the addition of EPO (1 ng/ml) to growth factor-derived UT-7 cells which were cultured for 1 month caused a 4- to 50-fold increase of miR-362 expression; while miR-188 expression was not increased. Similarly, EPO induced the expression of miR-362 in erythroleukemic cell line TF-1. The expression levels of miR-188 and miR-362 were significantly higher in UT-7/EPO with erythrocytic features than UT-7/GM and UT-7/TPO cells, in addition, expressions of both miRNAs were induced by EPO stimuli in UT-7/EPO cells, suggesting that miR-188 and miR-362 are involved in lineage specific molecular mechanisms of erythropoiesis.

Effect of doxycycline‐regulated protein disulfide isomerase expression on the specific productivity of recombinant CHO cells: Thrombopoietin and antibody

Protein disulfide isomerase (PDI), one of the ER-resident molecular chaperones, forms and isomerizes disulfide bonds. This study attempts to investigate the effect of PDI expression level on specific productivity (q) of recombinant Chinese hamster ovary (rCHO) cells producing thrombopoietin (TPO) and antibody (Ab). To regulate the PDI expression level, the Tet-Off system was introduced in TPO and Ab producing CHO cells, and stable Tet-Off cells (TPO-Tet-Off and Ab-Tet-Off) were screened using the luciferase assay. The doxycycline-regulated PDI expression system in Tet-Off rCHO cells (Tet-TPO-PDI and Tet-Ab-PDI) was established by the cotransfection of pTRE-PDI and pTK-Hyg expression vector into TPO-Tet-Off and Ab-Tet-Off cells, respectively. Subsequent screening was done by Western blot analysis of PDI and an enzyme-linked immunosorbent assay of the secreted TPO and antibody. We cultured two Tet-TPO-PDI and two Tet-Ab-PDI clones, and all these clones showed an average of 2.5-fold increase in PDI expression when compared to the basal level. In both these cell lines the PDI expression was tightly controlled by various concentrations of doxycycline. The q of TPO (q(TPO)) was unaffected but that of antibody producing cells was increased by 15-27% due to the PDI expression level.

Continuous expression of Bcl-xL protein during megakaryopoiesis is post-translationally regulated by thrombopoietin-mediated Akt activation, which prevents the cleavage of Bcl-xL.

One of the important biological activities of thrombopoietin (TPO) is to prevent the apoptosis of megakaryocytes. As the antiapoptotic protein Bcl-xL, which has been proven to be indispensable for erythroid differentiation, is also abundantly expressed in megakaryocytes, it is assumed that Bcl-xL plays an important role in megakaryopoiesis. We investigated the expression of Bcl-xL during megakaryopoiesis and the underlying regulatory mechanism. In stem cell-derived megakaryocytes, expression of Bcl-xL increased in the early and mid-stages of the differentiation. Both in vitro in cell culture and in vivo in an animal model, expression of Bcl-xL protein was maintained until the platelet-producing stage. TPO depletion caused significant decrease in Bcl-xL protein level without affecting its mRNA in both megakaryocytes and TPO-dependent megakaryocytic UT-7/TPO cells, suggesting that Bcl-xL protein level in TPO-dependent cells is post-translationally regulated. In agreement with this finding, we recognized the appearance of a 12-kD fragment of Bcl-xL upon TPO depletion. This cleavage of Bcl-xL was inhibited by a caspase-3-specific inhibitor. Furthermore, pretreatment of UT-7/TPO with a phosphatidylinositol 3-kinase (PI3 K) inhibitor resulted in the cleavage of Bcl-xL even in the presence of TPO. We thus hypothesized that PI3 K inhibits the activation of caspase-3 and consequent cleavage of Bcl-xL. To prove this, we prepared UT-7/TPO cells transfected with constitutively active Akt-1. When TPO was depleted, the transfectant was significantly less liable to caspase-3 activation and Bcl-xL cleavage. Bcl-xL protein is expressed throughout megakaryopoiesis until platelets are produced, and its expression level is at least in part post-translationally regulated through TPO-mediated Akt activation.

32D TPO, a TPO-Dependent Subclone of the 32d Cell Line, Expresses CXCR4 and Proliferates in Response to SDF-1.

Both Thrombopoietin (TPO) and Stromal Derived Factor 1 (SDF-1) have been reported to regulate megakaryocyte differentiation in vivo and in vitro. Using growth factors [G-CSF, M-CSF, GM-CSF, erythropoietin (EPO) or thrombopoietin (TPO)], as selective pressure, factor-dependent clones were consistently isolated from the murine multipotential IL-3-dependent cell line 32D. The different clones acquired a phenotype consistent with the growth factor for which they were dependent. In particular, the EPO-dependent clones showed erythroblast morphology and expressed high levels of CD71 and TER119, on the other hand, the TPO-dependent clones were megakaryocytes in morphology and expressed CD41 and CD61 (see Figure). The TPO-dependent subclones retain the capacity to grow in IL-3, but respond to neither EPO, G-CSF or GM-CSF. Here it is shown that the TPO-dependent clone, but not its EPO-dependent sister (see figure) or parental 32D cell lines, express robust level of CXCR4 on their surface. 32D TPO cells are also capable to respond to SDF-1 (the CXCR4 ligand). In fact, these cells die in 3 days in the absence of growth factors but proliferate equally well (from 5x104 to 6–9x105 cells/mL) in the presence of TPO (100 ng/mL) and/or SDF-1 (100 ng/mL), retaining a robust megakaryocytic phenotype in both culture conditions. By expression profile 32D TPO cells growing in TPO express the same level of Gata1 (both ~2−ΔCt = 3.6 ± 0.5x10−2 ) and Platelet Factor-4 (both 2−ΔCt = 3.5±1x10−1) more Gata2 (2−ΔCt = 2.3±0.1x10−1 vs 1.3±0.2x10−1, respectively), and Fog1 (2−ΔCt = 4.5±1x10−2 vs 3.1±0.5x10−2, respectively), and less Acetylcholinesterase (2−ΔCt = 4.96±1x10−5 vs 6.68±2x10−5, respectively), and Von Willebrand Factor (2−ΔCt = 2.7±0.3x10−2 vs 3.4±0.5x10−2, respectively), than those growing in SDF-1. These data indicate that a TPO-dependent cell line acquires the ability to respond to SDF-1, supporting the notion that megakaryocytic differentiation can be sustained equally well by TPO or SDF-1. To our knowledge, the 32D TPO cell line is the first megakaryocytic cell line that is dependent for growth on the presence of SDF-1. As such, it might represent a suitable model to compare TPO and SDF-1 signalling in megakaryocytes. [Display omitted]

Thrombopoietin (TPO) Regulates HIF-1α Level through Generation of Mitochondrial Reactive Oxygen Species (ROS).

Hypoxia inducible factor (HIF)-1 is a master transcriptional regulator for adaptation of cells to hypoxia. In addition to hypoxic responses, HIF-1 also plays an important role in the development of hematopoietic stem cells. Genetic deletion of β subunit of HIF-1 causes impairment of hematopoiesis. Culture of hematopoietic stem cells under hypoxic condition induces elevation of HIF-1α , another subunit of HIF-1, and subsequently enhances the growth of these cells. In our previous work we found that thrombopoietin (TPO), an important and non-redundant cytokine required for normal stem cell development, induces HIF-1α elevation in the TPO-dependent human leukemic cell line UT-7/TPO and in Sca-1+/c-kit+/Gr-1- cells (Kirito, K. et.al. Blood 2005). Under normoxic conditions HIF-1α is hydroxylated on proline residues by prolyl hydroxylase (PHD), which leads to its recognition by the von Hippel-Lindau tumor suppressor protein (pVHL), leading to degradation of HIF-1α . Hypoxia inhibits PHD function, blocking ubiquitination of HIF-1α , stabilizing the protein. We found that TPO controls stability of HIF-1α even under normoxic conditions. However, the mechanism by which TPO controls the stability of the protein remains unclear. Recently, several groups have reported that mitochondrial ROS play crucial roles in stabilization of HIF-1α in response to hypoxia. Disruption of mitochondrial function, either by interfering RNA against complex III of the mitochondrial electron transport chain or genetic elimination of cytochrome c, completely abolished the hypoxia-induced HIF-1α response. Based on these findings we hypothesized that ROS might be involved in TPO-induced HIF-1α elevation. To examine our hypothesis, we first tested whether TPO induced ROS production in UT-7/TPO cells using 2′, 7′-dichlorofluorescein diacetate, a redox sensitive fluorescence dye, and found that the hormone clearly induced ROS production in these cells. Next, we analyzed whether TPO-induced ROS generation is required for accumulation of HIF-1α . Pre-treatment of UT-7/TPO cells with the ROS scavenger catalase completely blocked HIF-1α elevation after TPO treatment. Furthermore, diphenylene iodinium (DPI), an inhibitor for ROS generating flavoenzymes including mitochondrial respiratory complexes, also inhibited the effects of TPO on HIF-1α levels. These results indicate that TPO induced HIF-1α activation is mediated by ROS production. To study the molecular pathway(s) by which TPO affects ROS, we tested the effects of ROS blockade on several known TPO-responsive signaling molecules; neither DPI nor catalase affected the activation of JAK2, STAT5, p38-MAPK or p42/p44-ERK induced by TPO, although AKT activation was blocked. Moreover, LY294002, an inhibitor of PI3-kinase and its activation of AKT also blocked of the HIF-1α response to TPO. Finally, inhibition of mitochondrial function in UT-7/TPO cells with rotenone or oligomycin also inhibited TPO-dependent accumulation of HIF-1α without affecting Jak2 activation. In conclusion, we found that TPO regulates HIF-1α levels through activation of ROS generation within mitochondrial respiratory complexes. We speculate that TPO mimics hypoxia by induction of ROS generation at mitochondria and subsequent elevation of HIF-1α , and regulates important genes for metabolisms and survival of hematopoietic stem cells.

PI3K/Akt/FOXO3a Pathway Contributes to Thrombopoietin-Induced Proliferation of Primary Megakaryocytes In Vitro and In Vivo Via Modulation of p27Kip1.

Thrombopoietin (TPO), the primary regulator of megakaryocyte (MK) and platelet formation, modulates the activity of multiple signal transduction molecules, including those in the Jak/STAT, p42/p44 MAPK, and phosphatidylinositol 3-kinase (PI3K)/Akt pathways. In the previous study, we reported that PI3K and Akt are necessary for TPO-induced cell cycle progression of primary MK progenitors. The absence of PI3K activity results in a block of transition from G1 to S phase in these cells (Geddis AE et al. JBC 2001 276:34473–34479). However, the molecular events secondary to the activation of PI3K/Akt responsible for MK proliferation remain unclear. In this study we show that FOXO3a and its downstream target p27Kip1 play an important role in TPO-induced proliferation of MK progenitors. TPO induces phosphorylation of Akt and FOXO3a in both UT-7/TPO, a megakaryocytic cell line, and primary murine MKs in a PI3K dependent fashion. Cell cycle progression of UT-7/TPO cells is blocked in G1 phase by inhibition of PI3K. We found that TPO down-modulates p27Kip1 expression at both the mRNA and protein levels in UT-7/TPO cells and primary MKs in a PI3K dependent fashion. UT-7/TPO stably expressing constitutively active Akt or a dominant-negative form of FOXO3a failed to induce p27Kip1 expression after TPO withdrawal. Induced expression of an active form of FOXO3a resulted in increased p27Kip1 expression in this cell line. In an attempt to assess whether FOXO3a has an effect of MK proliferation in vivo, we compared the number of MKs in Foxo3a-deficient mice and in wild type controls. Although peripheral blood cell counts of erythrocytes, neutrophils, monocytes and platelets were normal in the Foxo3a-deficient mice, total nucleated marrow cell count of Foxo3a-deficient mice were 60% increased compared with wild type controls. In addition, the increase of MKs was more profound than that of total nucleated marrow cells; CD41+ MKs from Foxo3a-deficient mice increased 2.1-fold, and mature MKs with 8N and greater ploidy increased 2.5-fold, compared with wild type controls. Taken together with the previous observation that p27Kip1-deficient mice also display increased numbers of MK progenitors, our findings strongly suggest that the effect of TPO on MK proliferation is mediated by PI3K/Akt-induced FOXO3a inactivation and subsequent p27Kip1 down-regulation in vitro and in vivo.

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

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

IFN Regulatory Factor-2 Cooperates with STAT1 to Regulate Transporter Associated with Antigen Processing-1 Promoter Activity

Class I MHC complexes (MHC(I)) are essential in mediating immune response. The transport of antigenic peptides (TAP) to MHC(I) and the stable expression of MHC(I) on the cell surface require the presence of a dedicated TAP. In this study we report that IFN-gamma and thrombopoietin (TPO) strongly increase TAP1 protein expression in megakaryocytes, followed by an enhanced expression of MHC(I) on the cell surface. This expression parallels the enhanced TAP1 promoter activity and TAP1 mRNA expression, which are independent of protein synthesis. We also show that this cytokine-dependent expression of TAP1 transcripts depends on STAT1 and IFN regulatory factor-2 (IRF-2), but not on IRF-1, and provide evidence that IRF-2 constitutively binds to the TAP1 gene promoter and enhances TAP1 promoter activity. We show that IRF-2 forms a complex with STAT1 and the cytokine-responsive region of the TAP1 promoter in any TPO or IFN-gamma target cells tested. Interaction of IRF-2 and STAT1 on the promoter depends on the DNA-binding domain of IRF-2. Overall, our data indicate that TPO and IFN-gamma activate the expression of TAP1 via a new mechanism that involves functional cooperation between STAT1 and IRF-2 on the TAP1 promoter.

Thrombopoietin induces HOXA9 nuclear transport in immature hematopoietic cells: potential mechanism by which the hormone favorably affects hematopoietic stem cells.

Members of the homeobox family of transcription factors are major regulators of hematopoiesis. Overexpression of either HOXB4 or HOXA9 in primitive marrow cells enhances the expansion of hematopoietic stem cells (HSCs). However, little is known of how expression or function of these proteins is regulated during hematopoiesis under physiological conditions. In our previous studies we demonstrated that thrombopoietin (TPO) enhances levels of HOXB4 mRNA in primitive hematopoietic cells (K. Kirito, N. Fox, and K. Kaushansky, Blood 102:3172-3178, 2003). To extend our studies, we investigated the effects of TPO on HOXA9 in this same cell population. Although overall levels of the transcription factor were not affected, we found that TPO induced the nuclear import of HOXA9 both in UT-7/TPO cells and in primitive Sca-1(+)/c-kit(+)/Gr-1(-) hematopoietic cells in a mitogen-activated protein kinase-dependent fashion. TPO also controlled MEIS1 expression at mRNA levels, at least in part due to phosphatidylinositol 3-kinase activation. Collectively, TPO modulates the function of HOXA9 by leading to its nuclear translocation, likely mediated by effects on its partner protein MEIS1, and potentially due to two newly identified nuclear localization signals. Our data suggest that TPO controls HSC development through the regulation of multiple members of the Hox family of transcription factors through multiple mechanisms.

Thrombopoietin upregulates nucleolin mRNA and protein in thrombopoietin-dependent megakaryocytic cell line, UT-7/TPO.

Thrombopoietin (TPO) is a hematopoietic cytokine that regulates megakaryocytosis and thrombocytosis by binding to its receptor (c-Mpl). The signaling pathways downstream of c-Mpl include the Ras/Raf/MAP kinase and JAK/STAT pathway and are transduced into the regulation of immediate early-, early- and delayed-response genes. How these genes couple c-Mpl activation to the biochemical machinery of cell growth and cell cycle progression in hematopoietic cells is still unclear. UT-7/TPO is a recently characterized TPO-dependent cell line. Using RNA fingerprinting with arbitrarily primed PCR (RAP-PCR) to identify the TPO-regulated genes in this cell line, we found that the mRNA expression of nucleolin was upregulated in the UT-7/TPO cells in response to TPO. Concomitantly, the TPO-stimulated cells expressed an increased amount of full length nucleolin as determined by immunoblot analysis. The TPO-induced upregulation of nucleolin mRNA was not inhibited by the MEK1/2 inhibitor PD98059, suggesting that ERK/MAPK activation is not necessary for elevation of nucleolin gene expression in response to TPO in UT-7/TPO. Nucleolin is a multifunctional nucleolar protein thought to be involved in many cellular processes, including ribosome biogenesis, the processing of ribosomal RNA (rRNA), mRNA stability, transcriptional regulation, and cell proliferation. Thus, these results indicate that the upregulation of nucleolin mRNA and protein may be important for the TPO-induced effects of hematopoietic cells.

EVI1 Is Expressed in Megakaryocyte Cell Lineage and Enforced Expression of EVI1 in UT-7/GM Cells Induces Megakaryocyte Differentiation

3q21q26 syndrome, an acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS) with chromosomal translocations or inversions between the bands 3q21 and 3q26, is frequently associated with dysmegakaryocytopoiesis and increased platelet counts at the initial diagnosis. Since the EVI1 gene at 3q26 is transcriptionally activated in 3q21q26 syndrome, we assessed the role of EVI1 gene expression in the abnormal megakaryocytic differentiation in 3q21q26 syndrome. RT-PCR analysis of various types of hematopoietic cells revealed that the EVI1 gene is expressed specifically in CD34+ cells, megakaryocytes, and platelets. UT-7 is a human immature megakaryoblastic leukemia cell line with dependence for the growth on granulocyte–macrophage colony-stimulating factor (GM-CSF) (designated at UT-7/GM) and with a differentiation capacity to erythroid (UT-7/EPO) and megakaryocytic lineages (UT-7/TPO) by erythropoietin (EPO) and thrombopoietin (TPO), respectively. Among three UT-7 sublines, UT-7/GM, UT-7/EPO, and UT-7/TPO, expression of the EVI1 gene was detected at low levels in UT-7/GM and UT-7/EPO cells, but was detected at a higher level in UT-7/TPO cells. When UT-7/GM cells were cultured with TPO, the level of EVI1 expression was increased, along with increased numbers of polynuclear megakaryocytes and expression of the platelet factor 4 (PF-4) gene. Furthermore, forced expression of the EVI1 gene in UT-7/GM cells changed their morphology to polynuclear megakaryocytes, stopped their growth, and induced cell death within a month. These data indicate that expression of the EVI1 gene is involved in progression of megakaryocytic differentiation and, thus, the dysmegakaryocytopoiesis in 3q21q26 syndrome could be partly due to an enhanced differentiation capacity of leukemia cells and/or megakaryocytes by constitutive expression of the EVI1 gene.

Thrombopoietin Regulates Bcl-xL Gene Expression through Stat5 and Phosphatidylinositol 3-Kinase Activation Pathways

Thrombopoietin (TPO), an essential factor for megakaryopoiesis and thrombopoiesis, works as a survival factor for megakaryocytic lineage cells. However, little is known about the molecular mechanism in detail. We show here that TPO supports the survival of TPO-dependent leukemia cell line UT-7/TPO and normal megakaryocytic progenitors via the induction of Bcl-xL, an anti-apoptotic member of the Bcl-2 family. We further analyzed the signal transduction pathways required for TPO-induced Bcl-xL gene expression. A reporter assay with various lengths of Bcl-x gene promoter revealed that both Stat- and nuclear factor kappa B-binding sites are prerequisites for TPO-induced promoter activity. Consistent with these results, TPO induced the binding of Stat5 and subunits of nuclear factor kappa B, p50, and c-Rel to the Bcl-x gene promoter. AG490, a specific inhibitor for Jak2, and LY294002, a specific inhibitor for phosphatidylinositol (PI) 3-kinase, reduced the protein level of Bcl-xL in UT-7/TPO cells, accompanied by an increase in the ratio of apoptotic cells. Interestingly, LY294002 enhanced the TPO-induced DNA binding activity of Stat5 without affecting the Jak2 activation and tyrosine phosphorylation of Stat5. Concomitantly, confocal microscopy revealed that LY294002 clearly inhibited the nuclear export of Stat5, suggesting that PI 3-kinase regulates the subcellular localization of Stat5. Taken together, our results suggest that both Jak-Stat and PI 3-kinase activation pathways regulate the TPO-induced survival of megakaryocytic cells via Bcl-xL gene expression. In addition, our data suggest possible cross-talk between these two signaling pathways.