Specific Signals Generated by the Cytoplasmic Domain of the Granulocyte Colony-Stimulating Factor (G-CSF) Receptor Are Not Required for G-CSF–Dependent Granulocytic Differentiation

Expression of a homodimeric type I cytokine receptor is required for JAK2V617F-mediated transformation

A recurrent somatic activating mutation in the nonreceptor tyrosine kinase JAK2 (JAK2V617F) occurs in the majority of patients with the myeloproliferative disorders polycythemia vera, essential thrombocythemia, myelofibrosis with myeloid metaplasia, and, less commonly, chronic myelomonocytic leukemia. We do not understand the basis for the specificity of the JAK2V617F mutation in clonal disorders of the myeloid, but not lymphoid, lineage, nor has the basis for the pleiotropic phenotype of JAK2V617F-associated myeloproliferative disorders been delineated. However, the presence of the identical mutation in patients with related, but clinicopathologically distinct, myeloid disorders suggests that interactions between the JAK2V617F kinase and other signaling molecules may influence the phenotype of hematopoietic progenitors expressing JAK2V617F. Here, we show that coexpression of the JAK2V617F mutant kinase with a homodimeric Type I cytokine receptor, the erythropoietin receptor (EpoR), the thrombopoietin receptor, or the granulocyte colony-stimulating-factor receptor, is necessary for transformation of hematopoietic cells to growth-factor independence and for hormone-independent activation of JAK-STAT signaling. Furthermore, EpoR mutations that impair erythropoietin-mediated JAK2 or STAT5 activation also impair transformation mediated by the JAK2V617F kinase, indicating that JAK2V617F requires a cytokine receptor scaffold for its transforming and signaling activities. Our results reveal the molecular basis for the prevalence of JAK2V617F in diseases of myeloid lineage cells that express these Type I cytokine receptors but not in lymphoid lineage cells that do not.

Cytokine signals through STAT3 promote expression of granulocyte secondary granule proteins in 32D cells

OBJECTIVE

In a previous study, we showed that activation of a transfected human erythropoietin receptor (EPOR) in the murine myeloid cell line 32D resulted in the development of morphologic features of granulocytic differentiation and expression of the neutrophil primary granule protein myeloperoxidase. We now studied if EPOR signaling could also mediate secondary granule protein gene expression and investigated the signal transduction requirements for induction of secondary granule gene expression in 32D cells.

MATERIALS AND METHODS

Wild-type and variant 32D cells expressing normal or chimeric EPORs or receptors for granulocyte colony-stimulating factor (G-CSFRs) were stimulated with EPO or G-CSF and the expression of granulocyte-specific genes was analyzed by Northern blot analysis. To determine the signaling mechanisms required for secondary granule protein gene induction, the activation of STAT pathways following growth factor stimulation was studied by Western blot analysis.

RESULTS

We found that EPO treatment of 32D cells engineered to express EPOR did not result in induction of the secondary granule protein genes encoding lactoferrin and 24p3 lipocalin, the mouse homolog of human N-Gal, or the myeloid transcription factor C/EBPepsilon. Replacement of the intracellular domain of EPOR with the intracellular domain of G-CSFR in a chimeric receptor was associated with EPO-mediated induction of lactoferrin, 24p3 lipocalin, and C/EBPepsilon genes. We found that STAT3 phosphorylation was mediated by the intracellular domain of G-CSFR, but not EPOR. Replacement of one or two of the STAT5 binding sites in the intracytoplasmic domain of the EPOR with STAT3 binding sites resulted in EPO-mediated STAT3 activation and a marked increase in the expression of the 24p3 lipocalin gene. Knockdown of STAT3 protein levels with siRNA caused significant decrease in 24p3 lipocalin gene induction.

CONCLUSION

These results indicate that EPOR signaling cannot substitute for G-CSFR signaling to stimulate secondary granule protein gene expression in 32D cells. In addition, STAT3 is a critical mediator of 24p3 lipocalin gene expression in these cells.

Critical role for granulocyte colony-stimulating factor in inflammatory arthritis

Granulocyte colony-stimulating factor (G-CSF) is a well known regulator of granulopoiesis, but the role of endogenous G-CSF in inflammatory joint disease has not been explored. We studied the response of G-CSF-deficient mice in experimental models of joint inflammation. We show that G-CSF deficiency protects mice from acute and chronic arthritis. Reduced severity was associated with blunted mobilization of granulocytic cells from the bone marrow and less cellular infiltrate and cellular activation in inflamed joints. We also demonstrate that G-CSF blockade in established collagen-induced arthritis in WT mice markedly reduces disease manifestations and is as effective as tumor necrosis factor blockade. Our results reveal a critical role for G-CSF in driving joint inflammation and highlight G-CSF as a potential therapeutic target in inflammatory joint diseases, such as rheumatoid arthritis.

Receptor for macrophage colony-stimulating factor transduces a signal decreasing erythroid potential in the multipotent hematopoietic EML cell line

OBJECTIVE

To test the hypothesis that hematopoietic growth factors may influence lineage choice in pluripotent progenitor cells, we investigated the effects of macrophage colony-stimulating factor (M-CSF) on erythroid and myeloid potentials of multipotent EML cells ectopically expressing M-CSF receptor (M-CSFR).

METHODS

EML cells are stem cell factor (SCF)-dependent murine cells that give rise spontaneously to pre-B cells, burst-forming unit erythroid (BFU-E), and colony-forming unit granulocyte macrophage (CFU-GM). We determined BFU-E and CFU-GM frequencies among EML cells transduced with murine M-CSFR, human M-CSFR, or chimeric receptors, and cultivated in the presence of SCF, M-CSF, or both growth factors. Effects of specific inhibitors of signaling molecules were investigated.

RESULTS

EML cells transduced with murine M-CSFR proliferated in response to M-CSF but also exhibited a sharp and rapid decrease in BFU-E frequency associated with an increase in CFU-GM frequency. In contrast, EML cells expressing human M-CSFR proliferated in response to M-CSF without any changes in erythroid or myeloid potential. Using chimeric receptors between human and murine M-CSFR, we showed that the effects of M-CSF on EML cell differentiation potential are mediated by a large region in the intracellular domain of murine M-CSFR. Furthermore, phospholipase C (PLC) inhibitor U73122 interfered with the negative effects of ligand-activated murine M-CSFR on EML cell erythroid potential.

CONCLUSION

We propose that signaling pathways activated by tyrosine kinase receptors may regulate erythroid potential and commitment decisions in multipotent progenitor cells and that PLC may play a key role in this process.

Myeloblastin is a granulocyte colony-stimulating factor-responsive gene conferring factor-independent growth to hematopoietic cells

Hematopoiesis depends on a pool of quiescent hematopoietic stem/progenitor cells. When exposed to specific cytokines, a portion of these cells enters the cell cycle to generate an amplified progeny. Myeloblastin (MBN) initially was described as involved in proliferation of human leukemia cells. The granulocyte colony-stimulating factor (G-CSF), which stimulates the proliferation of granulocytic precursors, up-regulates MBN expression. Here we show that constitutive overexpression of MBN confers factor-independent growth to murine bone marrow-derived Ba/F3/G-CSFR cells. Our results point to MBN as a G-CSF responsive gene critical to factor-independent growth and indicate that expression of the G-CSF receptor is a prerequisite to this process. A 91-bp MBN promoter region containing PU.1, C/EBP, and c-Myb binding sites is responsive to G-CSF treatment. Although PU.1, C/EBP, and c-Myb transcription factors all were critical for expression of MBN, its up-regulation by G-CSF was associated mainly with PU.1. These findings suggest that MBN is an important target of PU.1 and a key protease for factor-independent growth of hematopoietic cells.

Maturation rate of mouse neutrophilic granulocytes: acceleration by retardation of proliferation, but no detectable influence from G-CSF or stromal cells

Our purpose was to examine the possible influence of stromal and humoral mediators on granulocytic maturation rates. Sorted immature murine progenitor (Lin-Sca-1+) cells were cultured in peritoneal diffusion chambers (DCs) with or without a confluent layer of irradiated bone marrow stromal cells on one of the micropore membrane walls. In other experiments, 10 microg/kg/d recombinant G-CSF (rhG-CSF) was administered continuously into DC host mice through s.c. implanted osmotic minipumps. Operationally, maturation rate was assessed as the ratio between the number of polymorphonuclear cells (PMN) and proliferative granulocytes (PG) in short-term cultures, based on the differential cell counts, and supported by flow cytometric measurement of a granulocytic differentiation marker; and by the emergence time of PMN in the DCs, obtained by extrapolation. Also, increased maturation is associated with increased cell density, as reflected by the positioning of the granulocytes during centrifugation in a discontinuous Percoll gradient. This method, as well as the conversion rate of 3H-thymidine labeled PG into the heavier non-PG maturational stages, were also used as indicators of maturation rate. After five, six, and seven days of culture in the peritoneal cavity, DC cells were harvested. Their proliferative status, based on measurement of incorporated bromodeoxyuridine, was determined, and their maturation rates were evaluated. Proliferation of immature granulocytic progenitor cells was apparently inhibited by direct contact with bone marrow stromal cells, and stimulated by G-CSF during the early stage of culturing. However, the subsequent maturation rate, which could be accelerated by increasing culture cellularity, thus decreasing PG proliferation rate, was not detectably influenced by either stromal cells or G-CSF.