Heterodimerization of the  and β Chains of the Interleukin-3 (IL-3) Receptor Is Necessary and Sufficient for IL-3–Induced Mitogenesis

Neutralizing monoclonal antibodies can potentiate IL-5 signaling

IL-5 is a major determinant in the survival, differentiation and effector-functions of eosinophils. It mediates its effect upon binding and activation of a membrane bound receptor (R), composed of a ligand-specific alpha-chain and a beta-chain, shared with the receptors for IL-3 and granulocyte-macrophage colony-stimulating factor. We have generated and mapped the epitopes of three monoclonal antibodies (mAb) directed against this cytokine: the strong neutralizing mAb 5A5 and 1E1, and the very weak neutralizing mAb H30. We found that H30 as well as 5A5 can increase proliferation above the level induced by human (h)IL-5 alone, in a JAK-2-dependent manner, and at every sub-optimal hIL-5 concentration analyzed. This effect is dependent on mAb-mediated cross-linking of IL-5R complexes, and is only observed on cell lines expressing a hybrid human/mouse IL-5Ralpha-chain. We discuss these findings in view of the stoichiometric and topological requirements for an activated IL-5R. Since humanized anti-IL-5 mAb are currently in clinical testing, our findings imply that such mAb should be carefully evaluated for their potentiating effects.

GM-CSF binding to its receptor induces oligomerisation of the common beta-subunit

The stoichiometry of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor complex is still unresolved. We have utilised a sensitive, functional assay for receptor homodimerisation to show that GM-CSF induces dimerisation of the common signalling subunit, hbeta(c). We generated a chimeric cytokine receptor in which the extracellular and transmembrane domains of hbeta(c)are fused to the cytoplasmic domain of erythropoietin receptor (EPO-R). Given that to induce EPO-R activation and mitogenic signalling there is a requirement for formation of a specific homodimeric complex, we reasoned that the cytoplasmic domain of EPO-R could be utilised as a highly sensitive reporter for functional homodimer formation. We show that, in the presence of a cytoplasmically truncated GM-CSF alpha-subunit, the hbetac-EPO receptor chimera transduces a mitogenic signal in BaF-B03 in response to GM-CSF. This is consistent with formation of a hbeta(c)homodimer following GM-CSF binding and implies that ligand stimulation induces formation of a higher order complex that contains the hbeta(c)homodimer.

Characterization of diphtheria fusion proteins targeted to the human interleukin-3 receptor

Diphtheria fusion proteins are chimeric proteins consisting of the catalytic and translocation domains of diphtheria toxin (DT(388)) linked through an amide bond to one of a variety of peptide ligands. The ligand targets the molecule to cells and the toxin enters the cell, inactivates protein synthesis and induces cell death. Diphtheria fusion proteins directed to human myeloid leukemic blasts are a novel class of therapeutics for patients with chemotherapy refractory myeloid leukemia. Because of the presence of interleukin-3 (IL3) receptors on myeloid leukemic progenitors and its absence from mature myeloid cells, we synthesized four bacterial expression vectors encoding DT(388) fused to human IL3. Different molecules were engineered to assess the effects of modifications on yield, purity and potency of product. The constructs differed in the size of the linker peptide between the DT(388) and IL3 domains and in the presence or absence of an oligohistidine tag on the N- or C-terminus. Escherichia coli were transformed and recombinant protein induced and purified from inclusion bodies. Similar final yields of 3-6 mg of purified protein per liter of bacterial culture were obtained with each of the four molecules. Purity ranged from 70 to 90% after partial purification by anion-exchange, size-exclusion chromatography and/or nickel affinity chromatography. Proteins were soluble and stable at 4 degrees C and -80 degrees C in phosphate-buffered saline at 0.03-0.5 mg/ml. The fusion proteins showed predicted molecular weights by SDS-PAGE, HPLC and tandem mass spectrometry and had full ADP-ribosylating activities. Each was immunoreactive with antibodies to DT(388) and IL3. Each of the fusion proteins with the exception of the one with an N-terminal oligohistidine tag showed full IL3 receptor binding affinity (K:(d) = 3 nM) and potent and selective cytotoxicity to IL3 receptor positive human myeloid leukemia cell lines (IC(50) = 5-10 pM). In contrast, the N-terminal histidine-tagged fusion protein bound IL3 receptor with a 10-fold lower affinity and was 10-fold less cytotoxic to IL3 receptor positive blasts. Thus, we report a series of novel, biologically active DT(388)IL3 fusion proteins for potential therapy of patients with receptor positive myeloid leukemias.

IL-3 signaling and the role of Src kinases, JAKs and STATs: a covert liaison unveiled

Hematopoiesis is the cumulative result of intricately regulated signal transduction cascades that are mediated by cytokines and their cognate receptors. Proper culmination of these diverse signaling pathways forms the basis for an orderly generation of different cell types and aberrations in these pathways is an underlying cause for diseases such as cancer. Over the past several years, downstream events initiated upon cytokine/growth factor stimulation have been a major focus of biomedical research. As a result, several key concepts have emerged allowing a better understanding of the complex signaling processes. A group of novel transcription factors, termed signal transducers and activators of transcription (STATs) appear to orchestrate the downstream events propagated by cytokine/growth factor interactions with their cognate receptors. Until recently, the JAK proteins were considered to be the tyrosine kinases, which dictated the levels of phosphorylation and activation of STAT proteins, forming the basis of the JAK-STAT model. However, over the past few years, increasing evidence has accumulated which indicates that at least some of the STAT protein activation may be mediated by members of the Src gene family following cytokine/growth factor stimulation. Studies have demonstrated that the Src-family of tyrosine kinases can phosphorylate and activate certain STAT proteins, in lieu of JAK kinases. In such a scenario, JAK kinases may be more crucial to phosphorylation of the cytokine/growth factor receptors and in the process create docking sites on the receptors for binding of SH2-containing proteins such as STATs, Src-kinases and other signaling intermediates. Tyrosine phosphorylation and activation of STAT proteins can be achieved either by JAKs or Src-kinases depending on the nature of STAT that is being activated. This forms the basis for the JAK-Src-STAT model proposed in this review. The concerted action of JAK kinases, members of the Src-kinase family and STAT proteins, leads to cell proliferation and cell survival, the end-point of the cytokine/growth factor stimulus. Oncogene (2000).

A model for assembly and activation of the GM-CSF, IL-3 and IL-5 receptors: insights from activated mutants of the common beta subunit

Granulocyte-macrophage colony stimulating factor (GM-CSF), Interleukin-3 (IL-3) and Interleukin-5 (IL-5) have overlapping, pleiotropic effects on hematopoietic cells, including neutrophils, eosinophils, monocytes and early progenitor cells. The high-affinity receptors for human GM-CSF, IL-3, and IL-5 share a common beta-subunit (hbeta(c)), which is essential for signalling and plays a major role in recruiting intracellular signalling molecules. While activation of the cytoplasmic tyrosine kinase JAK2 appears to be the initiating event for signalling, the immediate events that trigger this are still unclear. We have isolated a number of activated mutants of hbeta(c), which can be grouped into classes defined by their state of receptor phosphorylation, their requirement for alpha subunit as a cofactor, and their activities in primary cells and cell lines. We discuss these findings with regard to the stoichiometry, activation, and signalling of the normal GM-CSF/IL-3/IL-5 receptor complexes. Specifically, this work has implications for the role of the ligand-specific alpha-subunits in initiating the signalling through the beta-subunit, the role of beta subunit dimerization as a receptor trigger, and the function of receptor tyrosine phosphorylation in generating growth and survival signals. Based on the properties of the activated mutants and the recent structures of erythropoietin receptor (Epo-R) complexes, we propose a model in which (1) activation of hbeta(c) can occur via alternative states that differ with respect to stoichiometry and subunit assembly, but which all mediate proliferative responses, and (2) each of the different classes of activated mutants mimics one of these alternative states.

CYRL, a novel cytokine receptor-like protein expressed in testis, lung, and spleen

The interleukin-3 receptor is composed of a ligand-specific alpha subunit (IL-3Ralpha) and a beta subunit (beta(c) or beta(IL3)). Here we report the cloning of a rat brain cDNA transcript with significant homology to IL-3Ralpha, which we have termed CYRL, for CYtokine Receptor-Like protein. A number of conserved motifs identify CYRL as a member of the alpha family of cytokine receptor subunits, but the extracellular domain was too divergent from the mouse IL-3Ralpha sequence to suggest that CYRL is the rat ortholog of IL-3Ralpha. CYRL mRNA expression by Northern blotting was highest in the testis, intermediate in the lung, and modest in spleen, brain, and heart. Antibodies generated against the extracellular domain of CYRL specifically labeled a broad immunoreactive band of M(r) approximately 50,000 in membrane fractions of testis, lung, and spleen. CYRL appears to be a novel cytokine receptor alpha-subunit of unknown function and with no defined ligands.