Molecular basis of a high affinity murine interleukin-5 receptor.

The mouse interleukin-5 receptor (mIL-5R) consists of two components one of which, the mIL-5R alpha-chain, binds mIL-5 with low affinity. Recently we demonstrated that monoclonal antibodies (Mabs) recognizing the second mIL-5R beta-chain, immunoprecipitate a p130-140 protein doublet which corresponds to the mIL-3R and the mIL-3R-like protein, the latter chain for which so far no ligand has been identified. In this study we show that a high affinity mIL-5R can be reconstituted on COS1 cells by co-expression of the mIL-5R alpha-chain with the mIL-3R-like protein (beta-chain). Cross-linking of 125I-labeled mIL-5 to the COS1 cells co-transfected with both cDNAs revealed the same pattern as in B13 cells, i.e. two proteins of 60 and 130 kd which correspond to the low affinity mIL-5R alpha-chain and the mIL-3R-like protein, respectively. The dissociation rate of mIL-5 from this reconstituted high affinity site was lower than that of the low affinity site, whereas the association rate was unchanged. Nonetheless, the apparent dissociation constant (Kd) for this reconstituted receptor was still 10-fold higher than the Kd observed for B13 cells. Although the mIL-3R is greater than 90% homologous to the mIL-3R-like protein, no increase in affinity for mIL-5 was detected on COS1 cells co-transfected with the cDNAs for the mIL-5R alpha-chain and the mIL-3R protein.     

Specific binding of radioiodinated granulocyte-macrophage colony-stimulating factor to hemopoietic cells

The hemopoietic growth factor granulocyte-macrophage colony-stimulating factor, GM-CSF, specifically controls the production of granulocytes and macrophages. This report describes the binding of biologically-active 125I-labeled murine GM-CSF to a range of hemopoietic cells. Specific binding was restricted to murine cells and neither rat nor human bone marrow cells appeared to have surface receptors for 125I-labeled GM-CSF. 125I-Labeled GM-CSF only appeared to bind specifically to cells in the myelomonocytic lineage. The binding of 125I-labeled GM-CSF to both bone marrow cells and WEHI-3B(D+) was rapid (50% maximum binding was attained within 5 min at both 20 degrees C and 37 degrees C). Unlabeled GM-CSF was the only polypeptide hormone which completely inhibited the binding of 125I-labeled GM-CSF to bone marrow cells, however, multi-CSF (also called IL-3) and G-CSF partially reduced the binding of 125I-labeled GM-CSF to bone marrow cells. Interestingly, the binding of 125I-labeled GM-CSF to a myelomonocytic cell line, WEHI-3B(D+), was inhibited by unlabeled GM-CSF but not by multi-CSF or G-CSF. Scatchard analysis of the binding of 125I-labeled GM-CSF to WEHI-3B(D+) cells, bone marrow cells and peritoneal neutrophils indicated that there were two classes of binding sites: one of high affinity (Kd1 = 20 pM) and one of low affinity (Kd2 = 0.8-1.2 nM). Multi-CSF only inhibited the binding of 125I-labeled GM-CSF to the high affinity receptor on bone marrow cells: this inhibition appeared to be a result of down regulation or modification of the GM-CSF receptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

The IL-2 receptor alpha-chain alters the binding of IL-2 to the beta-chain

The binding of IL-2 to its high affinity receptor results in the formation of the ternary complex consisting of IL-2, alpha-chain (p55, Tac) and beta-chain (p75). We studied the role of alpha-chain in IL-2 binding to the high affinity receptor using IL-2 analog Lys20 which was made by the substitution of Lys for Asp20 of wild-type rIL-2. Lys20 bound to MT-1 cells solely expressing alpha-chain at low affinity, but did not bind to YT-2C2 cells which solely expressed beta-chain. However, direct binding of radiolabeled Lys20 to ED515-D cells, an HTLV-I-infected and IL-2-dependent T cell line, revealed both high affinity and low affinity binding although the Kd value of high affinity binding was 50 to 100 times higher than that of the high affinity binding of wild-type rIL-2. High affinity binding of Lys20 was completely blocked by 2R-B mAb recognizing IL-2R beta-chain. Anti-Tac mAb recognizing IL-2R alpha-chain abolished all of the specific Lys20 bindings. In contrast to the replacement of cell bound 2R-B mAb with wild-type rIL-2 at 37 degrees C, the addition of an excess of Lys20 did not cause the detachment of cell-bound radiolabeled or FITC-labeled 2R-B mAb. Consistent with the results of binding studies, Lys20 induced the proliferation of ED515-D cells, but not large granular lymphocyte leukemic cells. The growth of ED-515D cells was completely suppressed by either anti-Tac mAb or 2R-B mAb. These results strongly suggest that coexpression of the IL-2R alpha- and beta-chains alters the binding affinity of Lys20 and that the interaction between IL-2 and the alpha-chain is a key event in the formation of the IL-2/IL-2R ternary complex.     

Structural and functional analyses of glycosylation on the distinct molecules of human GM-CSF receptors.

We have previously demonstrated that granulocyte-macrophage colony-stimulating factor (GM-CSF) receptors are composed of at least two molecules of 80 and 135 kDa, which were denoted alpha- and beta-chains, respectively [Chiba, S., Shibuya, K., Piao, Y.-F., Tojo, A., Sasaki, N., Matsuki, S., Miyagawa, K., Miyazono, K. & Takaku, F. (1990) Cell Regul. 1, 327-335]. In this paper, we describe an investigation of the biochemical disparity noted between the alpha- and beta-chains of GM-CSF receptors using proteolytic and deglycosidic enzymes, and further demonstrate the potential importance of carbohydrate structures of the GM-CSF receptors using different lectins and glycoprotein synthesis inhibitors. Cross-linked alpha- and beta-chains with 125I-GM-CSF were digested by Staphylococcus aureus V8 protease and gave a different pattern. Furthermore, the size of the alpha-chain was reduced by 25 kDa by the removal of the N-linked oligosaccharides with peptidase: N-glycosidase F treatment, whereas that of the beta-chain remained unmodified by the enzyme. These results suggest that the alpha-chain of GM-CSF receptors agrees with the recently cloned low-affinity GM-CSF receptor [Gearing, D.P., King, J.A., Gough, N. M. & Nicola, N.A. (1989) EMBO J. 8, 3667-3676] having approximately 30% N-linked oligosaccharides and is biochemically different from the alpha beta-chain. By analyses using lectins, some of the oligosaccharides in the alpha-chain seem to be the complex-type and/or hybrid-type, because wheat germ agglutinin and leukoagglutinating phytohemagglutinin inhibited both GM-CSF-induced proliferation and GM-CSF binding to its receptors. Further analyses using glycoprotein synthesis inhibitors showed that N-linked processing of the alpha-chain, especially glucose removal by glucosidase I and II (whose activities are inhibited by deoxynojirimycin), appeared to be required for the expression onto the cell surface although the beta-chain expression was little affected by their inhibitors. Thus the beta-chain, probably located near the alpha-chain on the cell surface, was associated with a high-affinity class of GM-CSF receptors.     

Affinity purification of human granulocyte macrophage colony-stimulating factor receptor alpha-chain. Demonstration of binding by photoaffinity labeling

The human granulocyte macrophage colony-stimulating factor (GM-CSF) receptor alpha-chain, a low affinity component of the receptor, was solubilized and affinity-purified from human placenta using biotinylated GM-CSF. Scatchard analysis of 125I-GM-CSF binding to the placental membrane extract disclosed that the GM-CSF receptor had a dissociation constant (Kd) of 0.5-0.8 nM, corresponding to the Kd value of the GM-CSF receptor alpha-chain on the intact placental membrane. Affinity labeling of the solubilized protein using a photoreactive cross-linking agent, N-hydroxysuccinimidyl-4-azidobenzoate (HSAB), demonstrated a single specific band of 70-95 kDa representing a ligand-receptor complex. Approximately 2 g of the placental membrane extract was subjected to a biotinylated GM-CSF-fixed streptavidin-agarose column, resulting in a single major band at 70 kDa on a silver-stained sodium dodecyl sulfate gel. The radioiodination for the purified material disclosed that the purified protein had an approximate molecular mass of 70 kDa and a pI of 6.6. Binding activity of the purified material was demonstrated by photoaffinity labeling using HSAB-125I-GM-CSF, producing a similar specific band at 70-95 kDa as was demonstrated for the crude protein.     

A spectrin-like protein in retinal rod outer segments

Biochemical and immunochemical studies indicate that rod outer segments (ROS) of bovine photoreceptor cells contain a Mr 240,000 polypeptide related to the alpha-subunit of red blood cell (RBC) spectrin. With the use of sodium dodecyl sulfate gel electrophoresis in conjunction with the immunoblotting technique, monoclonal antibody 4B2 was found to bind to a Mr 240,000 polypeptide in ROS that is distinct from the prominent Mr 220,000 concanavalin A binding glycoprotein. The Mr 240,000 polypeptide is highly susceptible to degradation by endogenous proteases. It does not appear to be an integral membrane protein but is tightly membrane associated since it can be partially extracted from ROS membranes with urea in the absence of detergent. The 4B2 antibody cross-reacted with RBC ghosts and bovine brain microsomal membranes. Radioimmune assays and immunoblotting analysis of purified bovine RBC spectrin further revealed that the 4B2 antibody predominantly labeled the alpha-chain of RBC spectrin having an apparent molecular weight of 240,000. Polyclonal anti-spectrin antibody that bound to both the alpha- and beta-chain of RBC spectrin predominantly labeled a Mr 240,000 polypeptide of ROS membranes. Two faintly labeled bands in the molecular weight range of 210,000-220,000 were also observed. These components may represent variants of the beta-chain of spectrin that are weakly cross-reacting or present in smaller quantities than the alpha-chain.(ABSTRACT TRUNCATED AT 250 WORDS)     

IL-3 specifically inhibits GM-CSF binding to the higher affinity receptor

The inhibition of binding between human granulocyte-macrophage colony-stimulating factor (GM-CSF) and its receptor by human interleukin-3 (IL-3) was observed in myelogenous leukemia cell line KG-1 which bore the receptors both for GM-CSF and IL-3. In contrast, this phenomenon was not observed in histiocytic lymphoma cell line U-937 or in gastric carcinoma cell line KATO III, both of which have apparent GM-CSF receptor but an undetectable IL-3 receptor. In KG-1 cells, the cross-inhibition was preferentially observed when the binding of GM-CSF was performed under the high-affinity binding condition; i.e., a low concentration of ¹²⁵I-GM-CSF was incubated. Scatchard analysis of ¹²⁵I-GM-CSF binding to KG-1 cells in the absence and in the presence of unlabeled IL-3 demonstrated that IL-3 inhibited GM-CSF binding to the higher-affinity component of GM-CSF receptor on KG-1 cells. Moreover, a chemical cross-linking study has revealed that the cross-inhibition of the GM-CSF binding observed in KG-1 cells is specific for the β-chain, M_r 135,000 binding protein which has been identified as a component forming the high-affinity GM-CSF receptor existng specifically on hemopoietic cells.     

Binding, internalization and degradation of radio-iodinated interleukin 3 and granulocyte-macrophage colony stimulating factor by various hemopoietic cells

The proliferation and differentiation of hemopoietic committed progenitor cells depend on colony stimulating factors (CSF). However, isolated mouse granulocyte-macrophage progenitor cells can still undergo limited proliferation in serum-free cultures after CSF deprivation. To test whether this is due to an accumulated pool of internalized factor, we examined the binding, internalization and degradation of radiolabelled interleukin 3 (IL-3) and granulocyte-macrophage colony stimulating factor (GM-CSF) in various hemopoietic cells. We found 20,000 high affinity IL-3 receptors on cells of two IL-3-dependent hemopoietic cell lines, FDC-P1 and FDC-P2 (Kd = 85 and 129 pM). FDC-P1 cells, which also respond to GM-CSF, possess 600 high-affinity GM-CSF receptors (Kd = 64 pM). Cells of both lines internalize IL-3, but only FDC-P1 cells release degraded IL-3 at a rapid rate. Both cell lines have similar dose-response curves for IL-3 and survival kinetics after factor removal. All other cells tested behave like FDC-P1, suggesting that the metabolism of IL-3 by FDC-P2 is exceptional. Our study indicates that transient proliferation of committed progenitor cells in the absence of added factors is apparently not due to a stable pool of internalized CSF but merely represents an intrinsic capability of these cells.     

Expression of functional alpha beta T cell receptor gene rearrangements in suppressor T cell hybridomas correlates with antigen binding, but not with suppressor cell function

We have examined the expression of TCR genes in 4-hydroxy-3-nitrophenyl-acetyl (NP)-specific Ts cell hybridomas. Each of three independently isolated hybridomas expressed in-frame TCR alpha-chain rearrangements derived from the original suppressor Ts cell. Different V alpha and J alpha gene segments were rearranged and expressed in each Ts cell line. The only TCR beta-chain expressed in these cells was derived from the BW5147 fusion partner. Expression of the BW5147 beta-chain was found to correlate with cell surface Ag binding, inasmuch as subclones derived from one of the original Ts lines expressed greatly reduced levels of beta-chain mRNA and no longer bound to NP-coupled RBC. Subclones that continued to express beta-chain mRNA did bind to NP-coupled RBC. This suggests that the Ag receptor on Ts hybridomas is a TCR-alpha beta dimer composed of a unique alpha-chain and the BW5147 beta-chain. Ag binding could be modulated by preincubation of Ts hybridoma cells with anti-TCR-alpha beta antibody, thereby supporting this conclusion. Suppressor factor activity was measured in the conditioned media of Ts subclones that differed by 250-fold in levels of beta-chain mRNA expression. No difference in suppressor factor activity was found; conditioned media from these subclones suppressed both plaque-forming cell responses and delayed-type hypersensitivity responses at approximately equivalent dilutions. Suppressor factor activity in the conditioned media of both a beta-chain negative subclone and a beta-chain positive subclone could be absorbed with an antibody that recognizes the TCR alpha-chain, but not with an antibody that recognizes the TCR beta-chain. We conclude that suppressor factor activity in the conditioned media of these Ts hybridomas is not derived from surface TCR-alpha beta receptors, although it does share TCR alpha-chain determinants.     

Characterization of carbohydrates in the alpha 2-macroglobulin receptor.

Carbohydrates were characterized in the human placental alpha 2-macroglobulin receptor and its associated protein. Carbohydrates, largely N-linked, contributed to about 18% of the size of the receptor alpha-chain and to about 25% of the beta-chain. The 40 kDa receptor-associated protein also contained carbohydrate. The alpha- and beta-chains contained a wide variety of carbohydrates as judged by binding of lectins. Monosaccharide-competing inhibition of alpha 2M-methylamine binding by WGA suggested a functional significance of sugars in binding of ligands to the alpha-chain.     

Characterization of the cell surface receptor for granulocyte-macrophage colony-stimulating factor

125I-labeled recombinant murine granulocyte-macrophage colony-stimulating factor (GM-CSF) was used to characterize receptors specific for this lymphokine on the surface of cells of both myelomonocytic and T-cell origin. GM-CSF binding to these cells was specific and saturable. Equilibrium binding studies revealed that on all cell types examined, GM-CSF bound to a single class of high affinity receptor (1000-5000 receptors/cell) with a Ka of 10(8)-10(9) M-1. More extensive characterization with P388D1 cells showed that binding of GM-CSF was rapid at 37 degrees C with a slow subsequent dissociation rate. Among a panel of lymphokines and growth hormones, only unlabeled natural or recombinant GM-CSF were able to compete for the binding of 125I-GM-CSF to these cells. Affinity cross-linking experiments with the homobifunctional cross-linking reagents disuccinimidyl suberate, disuccinimidyl tartrate, and dithiobis(succinimidyl propionate) resulted in the identification of a receptor protein with a Mr of 130,000 on five out of the seven cell types examined. This protein was extremely sensitive to proteolysis and in the absence of protease inhibitors was degraded to a form with an approximate Mr of 70,000. A receptor protein of Mr 180,000, in addition to the Mr 70,000 protein, was found on bone marrow cells and on P815 cells. The potential tissue-specific molecular heterogeneity associated with the GM-CSF receptor may help to explain some of the diverse biological effects associated with this growth and differentiation factor.     

Expression and characterization of a functional human insulin-like growth factor I receptor

Stable transfectants of Chinese hamster ovary (CHO) cells were developed that expressed the protein encoded by a human insulin-like growth factor I (IGF-I) receptor cDNA. The transfected cells expressed approximately 25,000 high affinity receptors for IGF-I (apparent Kd of 1.5 X 10(-9) M), whereas the parental CHO cells expressed only 5,000 receptors per cell (apparent Kd of 1.3 X 10(-9) M). A monoclonal antibody specific for the human IGF-I receptor inhibited IGF-I binding to the expressed receptor and immunoprecipitated polypeptides of apparent Mr values approximately 135,000 and 95,000 from metabolically labeled lysates of the transfected cells but not control cells. The expressed receptor was also capable of binding IGF-II with high affinity (Kd approximately 3 nM) and weakly recognized insulin (with about 1% the potency of IGF-I). The human IGF-I receptor expressed in these cells was capable of IGF-I-stimulated autophosphorylation and phosphorylation of endogenous substrates in the intact cell. This receptor also mediated IGF-I-stimulated glucose uptake, glycogen synthesis, and DNA synthesis. The extent of these responses was comparable to the stimulation by insulin of the same biological responses in CHO cells expressing the human insulin receptor. These results indicate that the isolated cDNA encodes a functional IGF-I receptor and that there are no inherent differences in the abilities of the insulin and IGF-I receptors to mediate rapid and long term biological responses when expressed in the same cell type. The high affinity of this receptor for IGF-II also suggests that it may be important in mediating biological responses to IGF-II as well as IGF-I.     

Purification and characterization of the receptor for murine granulocyte colony-stimulating factor.

A receptor for mouse granulocyte colony-stimulating factor (G-CSF) has been found on the cell surface of mouse myeloid leukemia cell line NFS-60. Chemical cross-linking of the receptor with radioiodinated G-CSF, followed by gel electrophoresis in the presence of sodium dodecyl sulfate, has revealed that the G-CSF receptor in the NFS-60 cells is a single polypeptide of Mr approximately 100,000-130,000. The receptor in the membrane fraction of NFS-60 cells were solubilized in an active form with 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonic acid. The solubilized receptor was purified approximately 100,000-fold to near homogeneity using a G-CSF affinity gel and gel filtration on a Superose 12 column, as measured by the selective precipitation of the 125I-G-CSF-receptor complex by polyethylene glycol. The purified G-CSF receptor has two classes of binding characteristics, one with an equilibrium dissociation constant (Kd) of 120-360 pM which is comparable with the Kd value for the cell-surface receptor, and the other with a higher Kd value of 2.6-4.2 nM. Analyses of the purified receptor by ligand blotting and sucrose density gradient centrifugation indicated that the low-affinity receptor is the monomer of the Mr 100,000-130,000 protein, whereas the high-affinity receptor consists of oligomers of the protein.     

Purification and characterization of the bombesin/gastrin-releasing peptide receptor from Swiss 3T3 cells

The bombesin/gastrin-releasing peptide (GRP) receptor was solubilized from Swiss mouse 3T3 cell membranes in an active form and was purified about 90,000-fold to near homogeneity by a combination of wheat germ agglutinin-agarose and ligand affinity chromatography. The purified receptor displayed a single diffuse band with a Mr of 75,000-100,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. After treatment of the receptor with N-glycanase, removing N-linked oligosaccharide moieties, the protein yielded a Mr = 38,000 band. These results agree with the Mr value estimated for the GRP receptor that was labeled on Swiss 3T3 cells by cross-linking to 125I-GRP1-27. GRP1-27 bound to the purified receptor with a Kd of 0.038 +/- 0.019 nM. By comparison, the soluble receptor in unfractionated extracts and intact membranes displayed a Kd for GRP1-27 of 0.036 +/- 0.003 nM and 0.13 +/- 0.04 nM, respectively. The relative potencies of a series of GRP analogs for the soluble receptor and intact membranes indicated that the extraction procedure did not significantly alter the receptor's ligand binding specificity. However coupling of the receptor to its guanyl nucleotide regulatory protein was not maintained in the soluble extract, and a G-protein did not co-purify with the receptor. Physiological concentrations of NaCl greatly inhibited the binding of some GRP analogs to the receptor, while the binding of other analogs was not affected. A domain on the GRP molecule involving Lys-13 or Arg-17 was identified which promoted binding to the GRP receptor under conditions of low ionic strength. These findings aided the development of an effective ligand affinity resin for the purification of the GRP receptor.     

Molecular determinants of the granulocyte-macrophage colony-stimulating factor receptor complex assembly

The granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GMR) is composed of two chains that belong to the superfamily of cytokine receptors typified by the growth hormone receptor. A common structural element found in cytokine receptors is a module of two fibronectin-like domains, each characterized by seven beta-strands denoted A-G and A'-G', respectively. The alpha-chain (GMRalpha) confers low affinity GM-CSF binding (K(d) = 1-5 nM), whereas the beta-chain (beta(c)) does not bind GM-CSF by itself but confers high affinity binding when associated with alpha (K(d) = 40-100 pM). In the present study, we define the molecular determinants required for ligand recognition and for stabilization of the complex through a convergence of several approaches, including the construction of chimeric receptors, the molecular dynamics of our three-dimensional model of the GM.GMR complex, and site-directed mutagenesis. The functional importance of individual residues was then investigated through ligand binding studies at equilibrium and through determination of the kinetic constants of the GM.GMR complex. Critical to this tripartite complex is the establishment of four noncovalent bonds, three that determine the nature of the ligand recognition process involving residues Arg(280) and Tyr(226) of the alpha-chain and residue Tyr(365) of the beta-chain, since mutations of either one of these residues resulted in a significant decrease in the association rate. Finally, residue Tyr(365) of beta(c) serves a dual function in that it cooperates with another residue of beta(c), Tyr(421) to stabilize the complex since mutation of Tyr(365) and Tyr(421) result in a drastic increase in the dissociation rate (Koff). Interestingly, these four residues are located at the B'-C' and F'-G' loops of GMRalpha and of beta(c), thus establishing a functional symmetry within an apparently asymmetrical heterodimeric structure.     

Secretion of soluble functional insulin receptors by transfected NIH3T3 cells

In order to determine whether the human insulin receptor ectodomain can be expressed as a functional protein, the coding regions for the transmembrane and cytoplasmic domain of a full-length human insulin receptor cDNA were deleted by site-directed mutagenesis, and the resultant construct was inserted into a bovine papilloma virus vector under the control of the mouse metallothionein promoter. After transfection of mouse NIH3T3 cells, a cell line secreting an insulin binding protein was isolated. The insulin binding alpha subunit had an Mr of 138,000 and a beta subunit of Mr 48,000 (compared to 147,000 and 105,000 for the full-length human insulin receptor expressed in NIH3T3 cells). This difference in size of the alpha subunit was due to a difference in glycosylation as N-glycanase digestion reduced the apparent size of the alpha subunits of secreted and normal membrane-bound receptors to identical values. The secreted receptor formed disulfide-linked heterotetrameric structures with an Mr of 280,000. It was synthesized as an Mr 160,000 precursor which was cleaved into mature subunits with a t1/2 of 3 h. Increasing expression of the cDNA by induction with sodium butyrate lead to the appearance of an Mr 180,000 protein in the medium as well as the mature alpha and beta subunits. A Scatchard plot of insulin binding to the secreted receptor was curvilinear with a Kd of 7 X 10(-10) M for the high affinity sites and 10(-7) M for the low affinity site (compared to Kd values of 1.1 X 10(-9) M and 10(-7) M, respectively, for human insulin receptors expressed in these cells.     

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

Human granulocyte-macrophage colony-stimulating factor (GM-CSF) is a cytokine derived from activated T cells, endothelial cells, fibroblasts, and macrophages. It stimulates myeloid and erythroid progenitors to form colonies in semisolid medium in vitro, as well as enhancing multiple differentiated functions of mature neutrophils, macrophages, and eosinophils. We have examined the binding of human GM-CSF to a variety of responsive human cells and cell lines. The most mature myelomonocytic cells, specifically human neutrophils, macrophages, and eosinophils, express the highest numbers of a single class of high affinity receptors (Kd approximately 37 pM, 293-1000 sites/cell). HL-60 and KG-1 cells exhibit an increase in specific binding at high concentrations of GM-CSF; computer analysis of the data is nonetheless consistent with a single class of high affinity binding sites with a Kd approximately 43 pM and 20-450 sites/cell. Dimethyl sulfoxide induces a 3-10-fold increase in high affinity receptors expressed in HL-60 cells, coincident with terminal neutrophilic differentiation. Finally, binding of 125I-GM-CSF to fresh peripheral blood cells from six patients with chronic myelogenous leukemia was analyzed. In three of six cases, binding was similar to the nonsaturable binding observed with HL-60 and KG-1 cells. GM-CSF binding was low, or in some cases, undetectable on myeloblasts obtained from eight patients with acute myelogenous leukemia. The observed affinities of the receptor for GM-CSF are consistent with all known biological activities. Affinity labeling of both normal neutrophils and dimethyl sulfoxide-induced HL-60 cells with unglycosylated 125I-GM-CSF yielded a band of 98 kDa, implying a molecular weight of approximately 84,000 for the human GM-CSF receptor.     

Calcitonin receptors of human osteoclastoma

Osteoclast-rich cultures were prepared by disaggregation of osteoclastomas (giant cell tumour of bone) and settlement onto glass or plastic surfaces. Autoradiography using [125I]-salmon calcitonin ([125I]-sCT) revealed specific binding only to multinucleate giant cells (osteoclasts) and a minor population of mononuclear cells. [125I]-sCT competitive binding studies indicated a Kd of 5 x 10(-10) M and receptor number of approximately 1 million sites/osteoclast. sCT treatment resulted in a dose-dependent rise in cAMP (EC50 10(-10) M). Homogenates of an osteoclastoma also demonstrated specific binding of [125I]-sCT. Chemical cross-linking of a labelled synthetic sCT derivative. [125I]-[Arg11,18,Lys14]-sCT, using disuccinimidyl suberate, resulted in labelling of a receptor component of approximate Mr 85-90,000. The multinucleate giant cells (osteoclasts) of human osteoclastomas possess large number of CT receptors which exhibit the same binding kinetics and apparent Mr as those of other CT target cells.     

Biosynthesis and posttranslational regulation of human IL-12

IL-12 is a heterodimeric proinflammatory cytokine consisting of a light alpha-chain, formerly defined as p35, disulfide-linked to a heavier beta-chain, formerly defined as p40. The beta-chain is also produced in large excess in a free form, and disulfide-linked beta-chain homodimers with anti-inflammatory effects are produced in the mouse. We analyzed the biosynthesis and glycosylation of IL-12 in human monocytes, and in a cell line stably transfected with IL-12 alpha and beta genes (P5-0.1). The IL-12 heterodimer and free beta-chain were immunoprecipitated from supernatants and cell lysates of metabolically labeled cells and resolved in SDS-PAGE. Whereas the beta-chain showed similar pI pattern whether in the free form or associated in the heterodimer, either in the secreted or intracellular form, the alpha-chain in the secreted heterodimer was much more acidic than that present in the intracellular heterodimer. Deglycosylation experiments with neuraminidase and Endo-F combined with two-dimensional PAGE of single bands of the intracellular vs extracellular IL-12 heterodimer revealed that the alpha-chain was extensively modified with sialic acid adducts to N-linked oligosaccharides before secretion. N-glycosylation inhibition by tunicamycin (TM) did not alter free beta-chain secretion, while preventing the IL-12 heterodimer assembling and secretion. Pulse-chase experiments indicated that IL-12 persists intracellularly for a long period as an immature heterodimer, and that glycosylation is the regulatory step that determines its secretion. beta-chain disulfide-linked homodimers were observed in TM-treated P5-0.1 cells, but in neither TM-treated nor untreated monocytes.