Characterization of the soluble human granulocyte-macrophage colony-stimulating factor receptor complex

G- and GM-CSF in oncology and oncological haematology

Administration of G- and GM-CSF increases the neutrophil counts in a number of clinical situations. GM-CSF shows the additional effect of increasing the number of monocytes and eosinophil granulocytes. Both G- and GM-CSF affect of neutrophil functions, in the case of GM-CSF there are some potentially negative effects on neutrophil migration and adhesiveness. The clinical relevance of the various effects on mature haematopoietic cells is not fully understood. Clinical data with G-CSF treatment indicate that increased levels of neutrophil granulocytes following cytotoxic chemotherapy may translate into clinical benefit such as a decreased rate of neutropenic infection and an increased cytotoxic chemotherapy dose even though the data are conflicting and the risk of "laboratory cosmetics" is apparent. Regarding treatment with GM-CSF following chemotherapy, the clinical benefit is unclear. The clinical benefit of GM-CSF-induced monocytes and eosinophils is unknown. G- and GM-CSF accelerates neutrophil recovery following autologous or allogeneic BMT. The influence on neutropenic infections is, however, less impressive. Pretreatment with G- or GM-CSF increases the yield of peripheral stem cell harvest, thereby reducing the number of leukaphereses needed. Transplantation of G- and GM-CSF primed autologous peripheral stem cells tends to reduce the period of post-transplant cytopenia, particularly thrombocytopenia, in comparison with traditional ABMT. In patients with MDS, G- and GM-CSF appear to increase the number of neutrophil granulocytes and there is some evidence that patients with severe infectious problems will benefit from this treatment. However, little influence was seen on the main clinical problems with these patients, which are anaemia and thrombocytopenia. In conclusion, G- and GM-CSF are two different proteins with different properties in vivo and in vitro. GM-CSF has, compared with G-CSF, more complex pharmacological effects and a more trouble-some side-effect profile. Early clinical development indicates that both compounds have a substantial influence on the levels of certain blood cells. Whether the increases in different blood cells translate into long-term clinical benefit for greater patient groups is the focus of ongoing research. The effects of G- and GM-CSF may be potentiated by other cytokines, an area which is presently being explored.

Developmental expression of Magmas in murine tissues and its co-expression with the GM-CSF receptor

Magmas is a protein that is involved in GM-CSF signaling in a myeloid cell line. Its precise role in the signal transduction process is unclear. To accurately characterize Magmas expression in a variety of cells, mouse embryos and adult murine tissues were analyzed for both mRNA and protein content. Magmas expression was detected as early as the day 6.5 embryo. The level of expression was developmentally regulated. During embryogenesis, elevated Magmas was observed in several structures, including heart, liver, notochord, choroid plexus, cervical ganglion, and nasal mucosa. Muscle, pancreas, intestinal mucosa, and testes were among the adult tissues with high Magmas expression. Most cell types, including hepatocytes and skeletal, smooth, and cardiac myocytes, also expressed the GM-CSF receptor (GMR) but the relative tissue levels of GMR were not always proportional to Magmas. The expression patterns suggest that Magmas has a role in both developing and mature tissues.

Molecular characterization of a granulocyte macrophage–colony-stimulating factor receptor α subunit-associated protein, GRAP

The granulocyte macrophage–colony-stimulating factor receptor (GM-CSF-R) is a heterodimer composed of 2 subunits,  and β, and ligand binding to the high-affinity receptor leads to signalling for the multiple actions of GM-CSF on target cells. In order to explore the role of the  subunit in signalling, we used a yeast-2-hybrid system to identify proteins interacting with the intracellular domain of the GMR-. A cDNA encoding a predicted protein of 198 amino acids, designated GRAP (GM-CSFreceptor  subunit-associatedprotein), was isolated in experiments using the intracellular portion of GMR- as bait. The interaction between GRAP and GMR- was confirmed by coimmunoprecipitation in mammalian cells. GRAP mRNA is widely expressed in normal human and mouse tissues and in neoplastic human cell lines, but it is not restricted to cells or tissues that express GM-CSF receptors. Three discrete GRAP mRNA species were detected in human tissues and cells, with estimated sizes of 3.3, 3.1, and 1.3 kb. GRAP is highly conserved throughout evolution, and homologues are found in yeast. The GRAP locus in Saccharomyces cerevisiae was disrupted, and mutant yeast cells showed an inappropriate stress response under normal culture conditions, manifested by early accumulation of glycogen during the logarithmic growth phase. GRAP is, therefore, a highly conserved and widely expressed protein that binds to the intracellular domain of GMR-, and it appears to play an important role in cellular metabolism.

Molecular characterization of a granulocyte macrophage–colony-stimulating factor receptor α subunit-associated protein, GRAP

The granulocyte macrophage–colony-stimulating factor receptor (GM-CSF-R) is a heterodimer composed of 2 subunits,  and β, and ligand binding to the high-affinity receptor leads to signalling for the multiple actions of GM-CSF on target cells. In order to explore the role of the  subunit in signalling, we used a yeast-2-hybrid system to identify proteins interacting with the intracellular domain of the GMR-. A cDNA encoding a predicted protein of 198 amino acids, designated GRAP (GM-CSFreceptor  subunit-associatedprotein), was isolated in experiments using the intracellular portion of GMR- as bait. The interaction between GRAP and GMR- was confirmed by coimmunoprecipitation in mammalian cells. GRAP mRNA is widely expressed in normal human and mouse tissues and in neoplastic human cell lines, but it is not restricted to cells or tissues that express GM-CSF receptors. Three discrete GRAP mRNA species were detected in human tissues and cells, with estimated sizes of 3.3, 3.1, and 1.3 kb. GRAP is highly conserved throughout evolution, and homologues are found in yeast. The GRAP locus in Saccharomyces cerevisiae was disrupted, and mutant yeast cells showed an inappropriate stress response under normal culture conditions, manifested by early accumulation of glycogen during the logarithmic growth phase. GRAP is, therefore, a highly conserved and widely expressed protein that binds to the intracellular domain of GMR-, and it appears to play an important role in cellular metabolism.

High-affinity binding to the GM-CSF receptor requires intact N-glycosylation sites in the extracellular domain of the β subunit

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor consists of 2 glycoprotein subunits, GMR and GMRβ. GMR in isolation binds to GM-CSF with low affinity. GMRβ does not bind GM-CSF by itself, but forms a high-affinity receptor in association with GMR. Previously, it was found that N-glycosylation of GMR is essential for ligand binding. The present study investigated the role of N-glycosylation of the β subunit on GM-CSF receptor function. GMRβ has 3 potential N-glycosylation sites in the extracellular domain at Asn58, Asn191, and Asn346. Single mutants and triple mutants were constructed, converting asparagine in the target sites to aspartic acid or alanine. A single mutation at any of the 3 consensus N-glycosylation sites abolished high-affinity GM-CSF binding in transfected COS cells. Immunofluorescence and subcellular fractionation studies demonstrated that all of the GMRβ mutants were faithfully expressed on the cell surface. Reduction of apparent molecular weight of the triple mutant proteins was consistent with loss of N-glycosylation. Intact N-glycosylation sites of GMRβ in the extracellular domain are not required for cell surface targeting but are essential for high-affinity GM-CSF binding.

High-affinity binding to the GM-CSF receptor requires intact N-glycosylation sites in the extracellular domain of the β subunit

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor consists of 2 glycoprotein subunits, GMR and GMRβ. GMR in isolation binds to GM-CSF with low affinity. GMRβ does not bind GM-CSF by itself, but forms a high-affinity receptor in association with GMR. Previously, it was found that N-glycosylation of GMR is essential for ligand binding. The present study investigated the role of N-glycosylation of the β subunit on GM-CSF receptor function. GMRβ has 3 potential N-glycosylation sites in the extracellular domain at Asn58, Asn191, and Asn346. Single mutants and triple mutants were constructed, converting asparagine in the target sites to aspartic acid or alanine. A single mutation at any of the 3 consensus N-glycosylation sites abolished high-affinity GM-CSF binding in transfected COS cells. Immunofluorescence and subcellular fractionation studies demonstrated that all of the GMRβ mutants were faithfully expressed on the cell surface. Reduction of apparent molecular weight of the triple mutant proteins was consistent with loss of N-glycosylation. Intact N-glycosylation sites of GMRβ in the extracellular domain are not required for cell surface targeting but are essential for high-affinity GM-CSF binding.

Identification of the soluble granulocyte-macrophage colony stimulating factor receptor protein in vivo

On the basis of the finding of alternatively spliced mRNAs, the -subunit of the receptor for GM-CSF is thought to exist in both a membrane spanning (tmGMR) and a soluble form (solGMR). However, only limited data has been available to support that the solGMR protein product exists in vivo. We hypothesized that hematopoietic cells bearing tmGMR would have the potential to also produce solGMR. To test this hypothesis we examined media conditioned by candidate cells using functional, biochemical, and immunologic means. Three human leukemic cell lines that express tmGMR (HL60, U937, THP1) were shown to secrete GM-CSF binding activity and a solGMR-specific band by Western blot, whereas a tmGMR-negative cell line (K562) did not. By the same analyses, leukapheresis products collected for autologous and allogeneic stem cell transplants and media conditioned by freshly isolated human neutrophils also contained solGMR. The solGMR protein in vivo displayed the same dissociation constant (Kd = 2-5 nmol) as that of recombinant solGMR. A human solGMR ELISA was developed that confirmed the presence of solGMR in supernatant conditioned by the tmGMR-positive leukemic cell lines, hematopoietic progenitor cells, and neutrophils. Furthermore, the ELISA demonstrated a steady state level of solGMR in normal human plasma (36 ± 17 pmol) and provided data suggesting that plasma solGMR levels can be elevated in acute myeloid leukemias.

Construction and binding kinetics of a soluble granulocyte-macrophage colony-stimulating factor receptor alpha-chain-Fc fusion protein

Granulocyte-macrophage colony-stimulating factor (GM-CSF) activity is mediated by a cellular receptor (GM-CSFR) that is comprised of an alpha-chain (GM-CSFRalpha), which specifically binds GM-CSF, and a beta-chain (betac), shared with the interleukin-3 and interleukin-5 receptors. GM-CSFRalpha exists in both a transmembrane (tmGM-CSFRalpha) and a soluble form (sGM-CSFRalpha). We designed an sGM-CSFRalpha-Fc fusion protein to study GM-CSF interactions with the GM-CSFRalpha. The construct was prepared by fusing the coding region of the sGM-CSFRalpha with the CH2-CH3 regions of murine IgG2a. Purified sGM-CSFRalpha-Fc ran as a monomer of 60 kDa on reducing SDS-polyacrylamide gel electrophoresis but formed a trimer of 160-200 kDa under nonreducing conditions. The sGM-CSFRalpha-Fc bound specifically to GM-CSF as demonstrated by standard and competitive immunoassays, as well as by radioligand assay with 125I-GM-CSF. The sGM-CSFRalpha-Fc also inhibited GM-CSF-dependent cell growth and therein is a functional antagonist. Kinetics of sGM-CSFRalpha-Fc binding to GM-CSF were evaluated using an IAsys biosensor (Affinity Sensors, Paramus, NJ) with two assay systems. In the first, the sGM-CSFRalpha-Fc was bound to immobilized staphylococcal protein A on the biosensor surface, and binding kinetics of GM-CSF in solution were determined. This revealed a rapid koff of 2.43 x 10(-2)/s. A second set of experiments was performed with GM-CSF immobilized to the sensor surface and the sGM-CSFRalpha-Fc in solution. The dissociation rate constant (koff) for the sGM-CSFRalpha-Fc trimer from GM-CSF was 1.57 x 10(-3)/s, attributable to the higher avidity of binding in this assay. These data indicate rapid dissociation of GM-CSF from the sGM-CSFRalpha-Fc and suggest that in vivo, sGM-CSFRalpha may need to be present in the local environment of a responsive cell to exert its antagonist activity.

Southwestern Internal Medicine Conference: clinical use of hematopoietic growth factors

The hematopoietic growth factors, granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF), have been cloned, produced in bacteria and yeast, and approved for clinical use in the treatment of neutropenia. Both factors stimulate the proliferation and maturation of neutrophil progenitors and enhance the effector functions of mature cells by interaction with specific receptors on the cell surface. Serum levels of G-CSF correlate inversely with the neutrophil count, suggesting that G-CSF may be the normal homeostatic regulator of the neutrophil count, while GM-CSF is generally undetectable in the serum and appears under normal physiologic conditions to act locally at inflammatory sites. Phase I and II clinical trials with these factors demonstrated minimal toxicity for G-CSF and mild to moderate dose-dependent toxicity for GM-CSF. Recent clinical trials, including double-blind, randomized studies, support a role for these growth factors in the treatment of chronic neutropenias, such as Kostmann's syndrome, acquired immune deficiency syndrome (AIDS), aplastic anemia, and myelodysplasia, as well as in acute neutropenias, such as cyclic neutropenia, chemotherapy-induced neutropenia, and bone marrow transplantation.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF): receptor biology, signal transduction, and neutrophil activation

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF) are two of the growing number of recognized cytokines involved in the regulation of hematopoiesis. The purification of these factors and the subsequent cloning of the cDNAs which encode these proteins have led to their widespread clinical use in the setting of therapy or disease-induced myelosuppression. Although originally purified on the basis of their colony-stimulating properties, GM-CSF and G-CSF may also play important roles in the regulation of effector cell function. The mechanisms underlying progenitor cell proliferation and effector cell stimulation remain poorly understood. However, the characterization of the GM-CSF and G-CSF receptors and recent work in signal transduction are helping to elucidate these mechanisms. This paper will review the biology of the GM-CSF and G-CSF receptors, the mechanisms of post-receptor signal transduction, and the resultant effects on neutrophil function. In addition, the current and potential clinical uses of these factors will be examined in light of their ability to activate and perhaps enhance the function of neutrophils.

Affinity conversion of receptors for colony stimulating factors: properties of solubilized receptors

Murine receptors for granulocyte-macrophage colony-stimulating factor (GM-CSF) and Multi-CSF (interleukin-3) can exist in both high- and low-affinity forms and demonstrate trans-modulation by several different ligands. In contrast the recently cloned human GM-CSF receptor and murine interleukin-3 (IL-3) receptor display only low-affinity binding. To begin to understand the molecular basis of the formation of high- and low-affinity receptors and their trans-modulation we have developed methods for the solubilization and assay of GM-CSF and interleukin-3 receptors so that their binding characteristics can be studied in cell-free solution. Both receptors displayed a single class of high-affinity binding on intact FDC-P1 cells and IL-3 receptors had unaltered binding characteristics in cells, membranes and in detergent solution. However, GM-CSF receptors were converted to a single class of low-affinity binding in detergent solution while both high- and low-affinity forms were evident in membranes. The basis of affinity conversion of GM-CSF receptors was exclusively a change in the kinetic dissociation rate of ligand. Cross-linking experiments suggested that high-affinity receptors for GM-CSF and IL-3 might consist of two different protein species and, if this is so, the data suggest that this association is more stable for IL-3 than for GM-CSF receptors.