Thrombopoietin induces tyrosine phosphorylation of a common beta subunit of GM-CSF receptor and its association with Stat5 in TF-1/TPO cells

Lusutrombopag increases hematocytes in a compensated liver cirrhosis patient

A 56-year-old Japanese man with liver cirrhosis (LC) due to hepatitis C virus was admitted to our hospital for radiofrequency ablation of residual tumor following lusutrombopag administration. Laboratory tests revealed thrombocytopenia and leukopenia. The patient's LC was managed, and he was classified as Child-Pugh A. After admission, lusutrombopag was administered for 7 days. The platelet count increased to over 50,000/mm³ after 7-14 days and returned to previous levels 50 days after administration. Leukocyte and erythrocyte counts also increased in response to the treatment and stayed elevated for over 120 days. Lusutrombopag acts selectively on human thrombopoietin (TPO) receptors and activates signaling pathways that promote the proliferation and differentiation of bone marrow progenitor cells into megakaryocytes, consequently increasing the blood platelet count. However, the patient treated with lusutrombopag in our case study showed increased blood leukocyte and erythrocyte counts as well. Given that TPO receptors are reportedly expressed in not only megakaryocyte progenitor cells but also hematopoietic progenitors, lusutrombopag may potentially improve pancytopenia caused by LC and can be used for the recovery of blood counts before other treatments.

STAT5 in hematopoietic stem cell biology and transplantation

Signal transducer and activator of transcription 5 (STAT5) regulates normal lympho-myeloid development through activation downstream of early-acting cytokines, their receptors, and Janus kinases (JAKs). Despite a general understanding of the role of STAT5 in hematopoietic stem cell (HSC) proliferation, survival, and self-renewal, the transcriptional targets and mechanisms of gene regulation that control multi-lineage engraftment following transplantation for the most part remain to be understood. In this review, we focus on the role of STAT5 in HSC transplantation and recent developments toward identifying the relevant downstream target genes and their role as part of a pleiotropic STAT5 mediated signaling response.

Hematopoietic-repopulating defects from STAT5-deficient bone marrow are not fully accounted for by loss of thrombopoietin responsiveness

Signal transducer and activator of transcription-5 (STAT5) plays an important role in repopulating activity of hematopoietic stem cells (HSCs). However, the relationship of STAT5 activation with early acting cytokine receptors is not well established. We have directly compared bone marrow (BM) from mice mutant for STAT5a and STAT5b (STAT5ab(-/-)) with that from mice lacking c-Mpl (c-Mpl(-/-)), the thrombopoietin receptor. Both STAT5 and c-Mpl deficiency only mildly affected committed myeloid progenitors assayed in vitro, but STAT5ab(-/-) BM showed lower Gr-1+ (4.4-fold), B220+ (23-fold), CD4+ (20-fold), and Ter119+ (17-fold) peripheral blood repopulating activity than c-Mpl(-/-) BM against wild-type competitor in long-term repopulating assays in vivo. Direct head-to-head competitions of STAT5ab(-/-) BM and c-Mpl(-/-) BM showed up to a 25-fold reduction in STAT5ab(-/-) contribution. Differences affecting reconstitution of primitive c-Kit+Lin-Sca-1+ multipotent progenitor (MPP)/HSC (1.8-fold) and c-Kit+Lin-Sca-1- oligopotent progenitor BM fractions (3.3-fold) were more modest. In serial transplantation experiments, STAT5ab(-/-) and c-Mpl(-/-) BM both failed to provide consistent engraftment in tertiary hosts and could not radioprotect lethally irradiated quaternary recipients. These results indicate substantial overlap in c-Mpl-STAT5 signaling defects at the MPP/HSC level but indicate that STAT5 is activated independent of c-Mpl to promote multilineage hematopoietic differentiation.

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

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

Reduced lymphomyeloid repopulating activity from adult bone marrow and fetal liver of mice lacking expression of STAT5

Signal transducers and activators of transcription (STATs) are intracellular mediators of cytokine receptor signals. Because many early-acting growth factors have been implicated in STAT5 activation, this study sought to investigate whether STAT5 may be a transcriptional regulator of hematopoietic stem cell (HSC) long-term repopulating activity. To test this possibility, bone marrow (BM) and fetal liver (FL) cells from mice containing homozygous deletions of both STAT5a and STAT5b genes (STAT5ab(-/-)) were characterized for hematopoietic repopulating activities. BM and FL grafts were capable of repopulating lymphoid and myeloid lineages of lethally irradiated primary and secondary hosts, with defects observed primarily in T-lymphocyte engraftment. Because only a fraction of normal HSC function is required to reconstitute hematopoiesis, competitive repopulation assays of adult BM or FL cells were used against wild type adult BM or FL cells to quantitate stem cell function. In these analyses, average 25-, 28-, 45-, and 68-fold decreases in normal repopulating activity were evident in granulocyte (Gr-1(+)), macrophage (Mac-1(+)), erythroid progenitor (Ter119(+)), and B-lymphocyte (B220(+)) populations, respectively, with T lymphocytes (CD4(+)) always undetectable from the STAT5ab(-/-) graft. Consistent with previous reports of divergence between stem cell phenotype and function in cases of perturbed hematopoiesis, the absolute number of cells within Sca-1(+)c-kit(+)lin(-) or lin(-) Hoechst 33342 side population fractions was not significantly different between wild type and STAT5ab(-/-) BM or FL cells. These results demonstrate that a significant proportion of the growth factor signals required for multilineage reconstitution potential of HSCs is STAT5 dependent.

Promegapoietin, a family of chimeric growth factors, supports megakaryocyte development through activation of IL-3 and c-Mpl ligand signaling pathways

OBJECTIVE

The signaling pathways induced by promegapoietin (PMP), a family of chimeric growth factors that activate the human IL-3 and c-Mpl receptors, were investigated.

METHODS

The biological activity of PMP was examined by receptor binding, cell proliferation, ex vivo expansion of hematopoietic progenitor cells, and in vivo production of platelets. The activation of signaling pathways was examined by Western blot and Northern blot analyses.

RESULTS

Two PMP molecules, PMP-1 and PMP-1a, induced proliferation of cells expressing the IL-3 receptor, c-Mpl, or both receptors and bound to the IL-3 receptor and c-Mpl with high affinity. Ex vivo expansion assays using human bone marrow CD34(+) cells suggested that PMP-1 induced greater total cellular expansion as well as expansion of CD41(+) megakaryocytic precursor cells than IL-3 or c-Mpl ligand alone. Subcutaneous administration of 50 microg/kg of PMP-1 for 10 days to rhesus monkeys resulted in increased platelet production in vivo from a baseline of 357 +/- 45 x 10(3) cells/mL to 1376 +/- 151 x 10(3) cell/mL. PMP-1 induced phosphorylation of the beta(c) subunit of IL-3 receptor and c-Mpl, JAK2, and STAT5b, but not STAT3. PMP-1 induced greater expression of Pim-1, c-Myc, and cyclin D2 than did either an IL-3 receptor agonist or c-Mpl receptor agonist alone. The magnitude of induction of early response genes was similar for PMP and the coaddition of IL-3 receptor agonist and c-Mpl receptor agonist.

CONCLUSION

PMP combines the biological activities of IL-3 and c-Mpl ligand in a single molecule that can simultaneously activate signaling pathways induced by both these ligands.

Membrane localization is not required for Mpl function in normal hematopoietic cells

Cellular trafficking of growth factor receptors, including cross-talk among receptors at the cell surface, may be important for signal transduction in normal hematopoietic cells. To test this idea, the signaling domain of Mpl (the thrombopoietin receptor) was targeted to the plasma membrane, or to the cytoplasm of murine marrow cells, and the ability of the cells to proliferate and differentiate in response to Mpl dimerized at the plasma membrane or free in the cytoplasm was assessed. Constructs encoding the signaling domain of Mpl linked to an FK506 binding protein domain (to permit dimerization by the membrane-permeable ligand AP20187) with or without a myristylation sequence (to target the receptor to the plasma membrane) and a hemagglutinin epitope tag were generated and introduced into murine marrow cells using a murine stem cell virus (MSCV)-based retroviral vector. Both populations of transduced marrow cells proliferated in Iscoves modified Dulbecco medium-10% FCS-100 nM AP20187 without exogenous growth factors for more than 100 days and achieved greater than a 10(7)-fold expansion of cells by day 50 (n = 4 transductions). Growth was dimerizer dependent, and myeloid, erythroid, and megakaryocytic progenitors were generated. Activation of Mpl either at the plasma membrane or in the cytoplasm allowed for the terminal maturation of transduced progenitor cells. Introduction of membrane-targeted or cytoplasmic Mpl into fetal liver cells from homozygous JAK2 knock-out mice or wild-type littermates demonstrated that both forms of Mpl require JAK2 for signaling. These data show that the activation of Mpl independent of its normal plasma membrane location can support production of the full range of normal hematopoietic progenitor cells in vitro.

Expression of interleukin 3 and granulocyte-macrophage colony-stimulating factor receptor common chain betac, betaIT in normal haematopoiesis: lineage specificity and proliferation-independent induction

Interleukin 3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 5 (IL-5) exert their biological activities through interaction with cell-surface receptors that consist of two subunits, a specific alpha subunit and a common beta transducing subunit (betac). We have evaluated the expression of betac on purified haematopoietic progenitor cells (HPCs) induced to unilineage differentiation/maturation through the erythroid (E), granulocytic (G), megakaryocytic (Mk) or monocytic (Mo) lineage. HPCs displayed low betac expression, which increased during the initial stages of HPC differentiation along the E, G, Mo or Mk lineages. At later stages of differentiation, betac chain expression increased in both G and Mo lineages, was expressed at low levels in the Mk lineage and declined to undetectable levels in the E lineage. Analysis of the full-length betac and intracytoplasmically truncated betac (betaIT) mRNAs showed that the former was predominant in the G and Mo lineages, whereas the latter was prevalent in the E and Mk lineages. The betac induction takes place even in the absence of cell cycling. Thus, incubation of HPCs with graded amounts of IL-3 showed that the initial induction of betac expression is unrelated to cell proliferation. Furthermore, circulating monocytes and granulocytes exhibit a low level of betac expression that is greatly stimulated following incubation with either IL-3 or GM-CSF.

Cloning and functional characterization of a novel c-mpl variant expressed in human CD34 cells and platelets

The thrombopoietin receptor, c-mpl, is a crucial element not only in thrombopoietin (TPO)-initiated signaling pathways but also in the regulation of the circulating amount of TPO. We have identified a new c-mpl isoform, called c-mpl-del, that lacks 72 bp (24 amino acids) in the extracellular region of c-mpl and arises as a consequence of alternative RNA splicing between exons 8 and 9. c-mpl-del is expressed along with c-mpl-wt in blood mononuclear cells, CD34(+)cells, megakaryocytes, and platelets prepared from either normal donors or ET patients, although its relative expression appears to increase with megakaryocyte differentiation. The c-mpl-del-transfected cells expressed greater amounts of c-mpl-del RNA and protein than the comparable c-mpl-wt-transfected cells, however flow cytometry analysis could not detect any c-mpl receptor on the surface of the c-mpl-del-transfected cells. Further evidence for the absence of surface c-mpl-del was that in contrast to cells transfected with c-mpl-wt, those transfected with c-mpl-del did not grow in response to TPO, failed to undergo tyrosine phosphorylation of TPO-specific signal molecules, and did not bind(125)I-rHuTPO. Taken together, these results demonstrate that c-mpl-del, a naturally occurring variant of c-mpl, fails to be incorporated into the cell membrane but might serve as a mechanism to decrease the overall expression of functional c-mpl late in megakaryocyte differentiation.

Thrombopoietin induces the generation of distinct Stat1, Stat3, Stat5a and Stat5b homo- and heterodimeric complexes with different kinetics in human platelets

OBJECTIVE

Thrombopoietin (TPO) is the pivotal regulator of thrombocytopoiesis and megakaryocytopoiesis, and binding to its receptor c-Mpl leads to activation of at least two different signaling pathways: the Jak-Stat pathway and the Ras-MAPK pathway. Our aim was to elucidate which Stat-complexes are formed in TPO signal transduction in human blood platelets.

MATERIALS AND METHODS

We used electrophoretic mobility shift assays (EMSA) in order to analyze the formation of distinct Stat complexes on two distinct oligonucleotide probes. Furthermore, we used immunoprecipitation and Western blotting of protein lysates from TPO-stimulated platelets.

RESULTS

We found homodimers of Stat1alpha, Stat3, Stat5a, and Stat5b, as well as heterodimers of Stat1/Stat3 and Stat5a/Stat5b, but no Stat1/Stat5 or Stat3/Stat5 heterodimers are formed in platelets in response to TPO. Stat5 complexes bound to labeled DNA with a fast kinetic followed by Stat3 and Stat1. The adapter protein CrkL is present in DNA-bound Stat5 complexes and predominantly bound to Stat5b. The kinase ERK2 is also tyrosine phosphorylated after TPO-stimulation of platelets but this activation does not modulate the phosphorylation of the serine residues in the PXSP motif present in Stat1 and Stat3.

CONCLUSION

Our findings thus emphasize the differential regulation of Stat1, Stat3, Stat5a, and Stat5b in platelets and may be an appropriate model of c-Mpl signaling in mega-karyopoiesis.

Dysregulated hematopoiesis and a progressive neurological disorder induced by expression of an activated form of the human common beta chain in transgenic mice

Previously we described activating mutations of hbetac, the common signaling subunit of the receptors for the hematopoietic and inflammatory cytokines, GM-CSF, IL-3, and IL-5. The activated mutant, hbetacFIDelta, is able to confer growth factor-independent proliferation on the murine myeloid cell line FDC-P1, and on primary committed myeloid progenitors. We have used this activating mutation to study the effects of chronic cytokine receptor stimulation. Transgenic mice were produced carrying the hbetacFIDelta cDNA linked to the constitutive promoter derived from the phosphoglycerate kinase gene, PGK-1. Transgene expression was demonstrated in several tissues and functional activity of the mutant receptor was confirmed in hematopoietic tissues by the presence of granulocyte macrophage and macrophage colony-forming cells (CFU-GM and CFU-M) in the absence of added cytokines. All transgenic mice display a myeloproliferative disorder characterized by splenomegaly, erythrocytosis, and granulocytic and megakaryocytic hyperplasia. This disorder resembles the human disease polycythemia vera, suggesting that activating mutations in hbetac may play a role in the pathogenesis of this myeloproliferative disorder. In addition, these transgenic mice develop a sporadic, progressive neurological disease and display bilateral, symmetrical foci of necrosis in the white matter of brain stem associated with an accumulation of macrophages. Thus, chronic hbetac activation has the potential to contribute to pathological events in the central nervous system.